Lithium-Ion Battery Chemistries: A Primer 0128147784, 9780128147788

Table of contents :
Front Matter
Copyright
List of figures
Preface
Introduction
Batteries and chemistry
Chapter outline
Electrochemistry basics
Electricity
Chemistry
Electrochemistry
Electromotive force (voltage)
Electric current
Discharging the battery
Charging the battery
Efficiency and voltage
Lithium-ion battery operation
Where is the lithium in lithium-ion?
How do the anode and cathode work together?
Reduction/oxidation (redox) process
Intercalation
Cations and anions
Solid electrolyte interphase
What does ``nano´´ mean?
Thermodynamics
Failures modes
Internal short circuit
External short circuit
Thermal runaway
Cascading failure
Impact and effects of temperature on cell aging
Impedance
What happens during overcharge
What happens during overdischarge
Influence of impact, crush, and penetration
Aging mechanisms
Overview and comparison of different lithium-ion chemistries
Performance
Voltage range
Cyclability
Energy density
Power density
Temperature
Cost
The Cathodes
Lithium iron phosphate
Lithium cobalt oxide
Lithium manganese oxide
Lithium nickel manganese cobalt/nickel cobalt manganese
Lithium nickel cobalt aluminum oxide
Other cathodes
The Anodes
Carbons
Graphite
Soft/amorphous carbon
Hard carbon
Graphene
Lithium titanium oxide
Alloys and composite materials
Silicon
Tin
Aluminum
Germanium
Inactive materials
Binders
Electrolytes
Characteristics of electrolytes and salts
Electrolyte additives
Nonaqueous electrolytes
Aqueous electrolytes
Alkaline electrolytes
Neutral electrolytes
Acidic electrolytes
Other types of electrolytes
Polymer electrolytes
Solid electrolytes
Ionic liquids
Water-in-salt electrolyte
Separators
Properties of separators
Ceramic separators
Nonwoven separators
Other separator materials
Current collectors and metal foils
Tabs/terminals
Cases
The materials
Recovering materials through recycling
Where do the different materials come from?
Materials used in lithium-ion batteries
Cobalt
Graphite
Lithium
Manganese
Silicon
Conductive metals
Aluminum
Copper
Nickel
Tin
Titanium
Material coatings/surface treatment
Lithium-ion cell manufacturing
Global capacity
Mixing
Coating
Slitting
Pressing/calendaring
Cutting
Cell assembly
Formation
Next generation and beyond lithium chemistries
Nickel-zinc and nickel-iron
Lithium-based batteries
Lithium-manganese-rich NMC
Lithium metal
Lithium-air
Lithium-sulfur
Lithium-glass
Other ion batteries
Aluminum-ion
Magnesium-ion
Potassium-ion
Sodium-ion
Solid-state batteries
3D Printed batteries
Printed batteries
Painted batteries
Paper batteries
The future and energy storage
Megatrends
Aging population
Urbanization and the megacities
Future mobility and e-transportation
Technology trends
The turning point
Conclusion
USABC goals for advanced electrolytes
USABC goals for advanced separators
Separator requirements
References
Index
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W

Citation preview

Lithium-Ion Battery Chemistries

Lithium-Ion Battery Chemistries

A Primer

John T. Warner

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States # 2019 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-814778-8 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Susan Dennis Acquisition Editor: Kostas Marinakis Editorial Project Manager: Susan Ikeda Production Project Manager: Omer Mukthar Cover Designer: Miles Hitchen Typeset by SPi Global, India

List of figures Fig. 1 Fig. 2

Rate of technology change. Modern battery-specific energy over time.

3 4

Fig. 3 Fig. 4

Lithium-ion development timeline. Scanning electron microscope image of electrodes.

6 10

Fig. 5

Electron flow theory.

19

Fig. 6 Fig. 7

The atom. Lithium atom—3 Protons, 3 neutrons, and 3 electrons.

20 21

Fig. 8 Fig. 9

Nucleus of lithium atom. Copper atom.

22 22

Fig. 10 Lithium atom, molecule, and ion.

24

Fig. 11 Lithium atom and its nucleus make up. Fig. 12 Comparison of the size of different particles.

25 26

Fig. 13 Bohr model lithium atom. Fig. 14 Electron cloud model of lithium atom.

27 27

Fig. 15 Periodic table of elements.

29

Fig. 16 Alkaline earth metals. Fig. 17 Transition metals.

30 30

Fig. 18 Anion and cation. Fig. 19 Crystalline structure of copper molecules.

31 34

Fig. 20 Current path through active material.

38

Fig. 21 Current flow during discharge mode. Fig. 22 Lithium-ion cell in charging mode.

39 40

Fig. 23 Lithium atom. Fig. 24 Active material with lithium-ions inserted.

44 46

Fig. 25 Interfacial areas. Fig. 26 ReDox operations.

47 49

Fig. 27 Lithium-ion rocking chair effect.

51

Fig. 28 Valence electrons and holes in valence shell. Fig. 29 Lithium cation and anion.

52 53

Fig. 30 Cation and anion movement during charging. Fig. 31 Cross section of lithium-ion cell.

54 55

Fig. 32 SEI layer.

56

Fig. 33 Nanotubes on a current collector.

57

ix

x

List of figures

Fig. 34

Nanotubes after lithium insertion.

58

Fig. 35

Fig. 36

Scanning electron microscope image of nanowire structures showing a DWCNT using an SEM microscope in images (A) and (B) and with a TEM microscope in image (C). Safe operating zones for temperature and voltage.

59 61

Fig. 37 Fig. 38

Cell failure types and causes. Catastrophic short circuit events.

62 63

Fig. 39 Fig. 40

Dendrite growth causing internal short circuit. Comparing energy released at different SOC states.

65 66

Fig. 41

Impact of temperature on a Li-ion cell.

68

Fig. 42 Fig. 43

Methods of heat transfer. Periodic table of elements—Transition metals.

69 81

Fig. 44 Fig. 45

Typical voltage discharge curves for different Li-ion chemistries. 83 Operating voltage range of chemistries. 84

Fig. 46

Cycle life at different depth of discharges.

87

Fig. 47 Fig. 48

Energy density of different fuels. Gravimetric and specific energy densities of lithium-ion cells.

88

Fig. 49 Fig. 50

Impact of power on NMC cell capacity. Impact of power on LTO cell capacity.

90 91

Fig. 51

Power versus energy chart.

92

Fig. 52 Fig. 53

Impact of temperature on NMC cell performance. Temperature’s impact on NMC capacity—The area under the curve.

93

89

94

Fig. 54 Fig. 55

Lithium-ion cell costs over time. Crystal structures of Li-ion chemistries.

96 100

Fig. 56 Fig. 57

2015 Cathode market share by chemistry. LiFePO4 crystal structure.

101 103

Fig. 58

Layered crystal structure of LCO.

105

Fig. 59 Fig. 60

Spinel crystal structure of LMO. Layered atomic structure of NMC.

106 108

Fig. 61 Fig. 62

Nickel Manganese Cobalt chemistries. Nickel content vs. energy density in NCM materials.

109 110

Fig. 63

NMC Core shell gradient examples.

111

Fig. 64 Fig. 65

Layered atomic structure of NCA. 2015 market share of different anode materials.

112 118

List of figures

Fig. 66 The structures of eight allotropes of carbon: (A) Diamond, (B) graphite, (C) lonsdaleite, (D) C60 (buckminsterfullerene), (E) C540 fullerene, (F) C70 fullerene, (G) amorphous carbon, and (H) single-walled carbon nanotube. Fig. 67 Hexagonal lattice of graphite.

119 121

Fig. 68 Crystal lattice structure of graphite sheet.

121

Fig. 69 Lithium-ion inserted into graphite structure. Fig. 70 Graphene sheet.

122 124

Fig. 71 Spinel crystal structure of LTO. Fig. 72 Impact of lithium insertion on silicon anode.

126 132

Fig. 73 Silicon microfractures form during lithium insertion.

133

Fig. 74 Pulverized silicon anode particle. Fig. 75 Nonaqueous electrolyte solution.

133 141

Fig. 76 Solvated lithium cation. Fig. 77 Representative LiPF6 molecule.

142 149

Fig. 78 Polyolefin separator types.

156

Fig. 79 Tri-layer separator. Fig. 80 Dry processed separators.

157 159

Fig. 81 Dreamweaver separator courtesy of Dreamweaver International. 161 Fig. 82 Kevlar separator clockwise from top left, TEM image of ANF Kevlar separator; SEM side view; optical photograph and bottom left, SEM top view. 162 Fig. 83 Soteria Innovation Group current collector design. Fig. 84 2-D versus 3-D current collector.

165 166

Fig. 85 Cell form factors. Fig. 86 Aluminum laminate pouch material.

168 169

Fig. 87 Elements in the Earth’s crust.

173

Fig. 88 1924 Estimate of the minerals in the Earth’s crust. Fig. 89 Lithium-ion battery materials as a percent of the Earth’s crust.

174

Fig. 90 Recycling processes. Fig. 91 World reserves of lithium-ion battery elements (2016).

176 177

Fig. 92 Global processing production capacity of lithium-ion materials (2016). Fig. 93 Global processing production capacity of lithium-ion materials excluding aluminum (2016). Fig. 94 Global cobalt reserves.

174

178 179 180

xi

xii

List of figures

Fig. 95

Global cobalt production.

181

Fig. 96 Fig. 97

Global graphite reserves. Global graphite mine production.

184 185

Fig. 98

Global lithium reserves.

189

Fig. 99 Fig. 100

Lithium brine ponds. SQM’s lithium brine production process (courtesy SQM).

190 191

Fig. 101 Fig. 102

Global lithium production. Global lithium producers.

193 194

Fig. 103

Global manganese reserves.

195

Fig. 104 Fig. 105

Global manganese production. Global silicon production.

196 198

Fig. 106 Fig. 107

Global aluminum production. Global copper reserves.

200 203

Fig. 108

Global copper mine production.

203

Fig. 109 Fig. 110

Global nickel reserves. Global nickel mine production.

207 207

Fig. 111 Fig. 112

Global tin reserves. Global tin mine production.

209 209

Fig. 113 Fig. 114

Global tin mine production. Global titanium mine production.

211 212

Fig. 115

Atomic layer deposition.

213

Fig. 116 Fig. 117

ALD process courtesy of Forge Nano. TEM Image Alumina ALD coating on nickel particle courtesy of Forge Nano.

215

Fig. 118 Fig. 119

Lithium-ion cell manufacturing process. Buhler continuous mixers.

220 230

Fig. 120

Planetary mixer and Shear mixer paths.

231

Fig. 121 Fig. 122

Cathode and anode slurry recipes. Coated cathode electrode.

232 233

Fig. 123 Fig. 124

Single-sided electrode coating process. Dual-sided electrode coating process.

234 234

Fig. 125 Fig. 126

Slot die coater head and D€urr MEGTEC slot die coater. Knife blade coating head.

235 236

Fig. 127

Large format electrode coating lanes.

236

Fig. 128 Fig. 129

Intermittent coating of small format electrodes. D€ urr MEGTEC Giga Coater™.

237 238

Fig. 130

Side and top view of electrode slitting process.

239

216

List of figures

Fig. 131 Electrode calendaring process.

241

Fig. 132 Calendaring reduces thickness of active material. Fig. 133 Die cutting electrodes.

241 242

Fig. 134 Large format electrode cutting pattern.

243

Fig. 135 Stacking electrodes in Z-fold separator. Fig. 136 Winding cylindrical jellyroll.

246 246

Fig. 137 Inserting jellyroll into cell container. Fig. 138 Lithium-ion cell formation process.

247 249

Fig. 139 Digatron 1024 circuit formation system.

250

Fig. 140 Sealing prismatic can cell electrolyte fill hole. Fig. 141 Grading cells by capacity.

251 252

Fig. 142 Lead acid, nickel, li-ion, and beyond lithium chemistries. Fig. 143 Battery paradigm shift courtesy visual capitalist.

254 255

Fig. 144 Thomas Edison’s 1901 nickel-iron patent (#678,722).

256

Fig. 145 NMC/LMO core shell gradient compounds. Fig. 146 Lithium-air battery.

259 261

Fig. 147 Four phase Li-sulfur discharge reaction. Fig. 148 Lithium-glass battery.

263 264

Fig. 149 Dual-ion Li-ion and Al-ion battery. Fig. 150 Mg-ion metal battery.

266 267

Fig. 151 Potassium-ion battery (K-ion).

268

Fig. 152 Sodium-ion (Na-ion) battery. Fig. 153 Thin film battery.

269 272

Fig. 154 Semi-solid-state battery. Fig. 155 Comparing traditional Li-ion to solid state.

273 274

Fig. 156 Types of solid-state battery.

275

Fig. 157 Comparing current EVs to future SSB versions. Fig. 158 nano-3D (μ3D) printed battery.

276 278

Fig. 159 3D printed micro lattice. Fig. 160 Simplified example of a screen-printed battery.

279 280

Fig. 161 Painted battery process.

281

Fig. 162 Paper battery. Fig. 163 Paper battery using lithium metal anode.

283 283

Fig. 164 Local motors 3D printed car. Fig. 165 Global megatrends in 2016.

288 290

Fig. 166 Lithium-ion cell price forecasts.

295

Fig. 167 Potential timeline for beyond lithium (Li-S) commercialization. 296

xiii

Preface When the idea of writing a second book came to me, I was trying to get a better understanding of what was really happening inside of the lithium-ion battery. I spent more than a decade working in the sales, marketing, and executive management side of the lithium-ion battery business but I felt that I needed to get a better understanding of what was happening inside the electrochemical cell at the atomic level. In fact, the impetus of this book really comes from me realizing that I really did not know what an ion was. The ion is the core of the lithium-ion battery, but I realized that I had an idea what it was but really was not sure that I really understood what it was or how it worked. It then occurred to me that there could be many other people out there who would benefit from having a clear description of the operations that leads to a deeper understanding of the lithium-ion cell. It was this idea that led to the nearly year and a half I spent researching for this book. I researched the history and the future of each of the components of the lithium-ion battery cell, what were people researching in the past, and what are they researching now. I searched for the best and simplest descriptions for what was happening inside the battery cell. This part was critical as I really needed to break the operation down into its core pieces, which was not always easy as many texts did not agree with each other while others did agree but showed it from different perspectives which I found to be somewhat confusing. At the same time, I realized that I was fielding a lot of calls about the raw materials for lithium-ion batteries. Where did they come from? Would there be enough? How are they processed? Since I was already looking at the functions and chemistries it was a natural fit to expand the scope to include some discussions on the materials which then led to the need to discuss the manufacturing steps for the modern lithiumion battery. I based this chapter on the current processes and those that have been used over the past decade or so. There are many new processes that are being evaluated or beginning to be integrated into current processes but the fact is most of the current capacity is based on the same basic processes. This book may offer a deeper dive than some people need, but I believe that for those of us in the lithium-ion battery industry having a deeper understanding of the fundamental science behind lithium-ion batteries it can help us to ensure we are able to find the right solution to each application. If you are in the battery industry but not a chemist, you may find that this book helps to clarify some of these concepts. I do not claim to make you a chemist once you are done, but you will have a much better understanding of what is happening inside the lithium-ion battery cell. This book would not have been possible without the help of a lot of folks. First and foremost, my wife and children, whose support through the long writing and research process was instrumental in allowing me to complete this book. I have also been very fortunate in having some amazing mentors in the industry. At the top of the list is Bob Galyen, who as I write this is the Chief Technology Officer of the largest lithium-ion company in the world. In addition to his “day job” Bob takes time to

xv

xvi

Preface

work with standards groups, he leads the SAE Vehicle Battery Standards Steering Committee and has served in each of the leadership roles of the industry trade group NAATBatt International, and today holds the role of Chairman Emeritus. But even with all these extracurricular activities he makes time to help mentor a group of executives and leaders to help develop the next generation of leaders in the industry. There are also several chemists and scientists who were kind enough to provide technical guidance and review to make sure I was able to accurately describe the complex reactions. I hope I have done them justice! Without this type of help and mentoring I may not have had the resources to be able to bring this book to completion. I truly hope you enjoy and maybe even learn something new. Keep powering up! John T. Warner

CHAPTER

Introduction

1

Chapter Outline 1.1 Batteries and chemistry ....................................................................................... 9 1.2 Chapter outline ................................................................................................. 12

In Greek mythology Prometheus stole fire from Mount Olympus as a gift to humanity. That gift was to be the spark that enabled technology to allow human civilization to truly take off (Hesiod, 2008). Today, technology continues to be a boon helping to make human life easier. And in fact, it has really become the foundation for modern life. Technology is the undercurrent that makes modern life run, but as it has gotten more complex we find that we really don’t understand much about it or how it works—we just expect it to work. And lithium-ion batteries can be considered the core of much of modern technology—it is the source that powers our life. In fact, it’s not difficult to draw a similarity between Prometheus’ fire and the harnessing of electricity by Ben Franklin, Nikola Tesla, Thomas Edison, and others. But much like the Greeks, few of us really have a good understanding of how these technologies work—lithium-ion batteries fall directly into that category. For the clear majority of people today batteries, much like automobiles, computers, smart phones, and other technologies, just work—until they don’t—we use them, we charge them, but we don’t really understand how they work. We do not entirely understand how they do what they do and usually do not care until they run out of juice. Arthur C. Clarke, futurist and science fiction author wrote “Any sufficiently advanced technology is indistinguishable from magic” (Clarke, 1977, p. 39) and in fact, for many of us today electricity and lithium-ion batteries do seem a lot like magic. Just what magic is involved with bottling lightning? That is the focus of this book and I will do my best to bring the technologies surrounding lithium-ion battery chemistries down to Earth much like Prometheus. This book will break down the “how” of lithium-ion batteries to give the reader a clear and simple understanding of what is happening inside the battery cell. But the faster technology advances and the more mobile it becomes, the more dependent it is on having an adequate energy storage supply. Many current vehicle technologies, such as hybrid and electric vehicles, are enabled by the evolution of higher energy batteries that have allowed them to achieve greater fuel economy, electric drive capability, and reduced or even eliminating emissions. Even nonhybrid vehicles are highly dependent on electrical energy due to the emergence of

Lithium-Ion Battery Chemistries. https://doi.org/10.1016/B978-0-12-814778-8.00001-6 # 2019 Elsevier Inc. All rights reserved.

1

2

CHAPTER 1 Introduction

technologies like lane departure warning, adaptive cruise controls, self-parking capabilities, and infotainment systems. Other new technologies are emerging that are dependent on energy storage, such as autonomous vehicles, drones and unmanned aerial vehicles (UAVs), unmanned or autonomous underwater vehicles (UUVs and AUVs), unmanned delivery vehicles, and household robotics just to name a few. All these technologies are enabled through the advancement of energy storage technologies and lithium-ion batteries are the preferred solution for many of them. And that only covers vehicle and transportation technologies! At the end of the 20th century Bill Gates himself forecast that technology was about to bring humankind to an inflection point where “…change in consumer use becomes sudden and massive” (Gates & Hemingway, 1999, p. xvi) that will create radical change in both our business and personal lives. Of course, Gates was referring to the impact of the internet, digital information, and connectivity which has not only proven to be a very accurate statement but may actually have underestimated the impact. Throughout the rest of our daily lives the addition and embedding of technologies such as the Internet of Things (IoT) (International Telecommunication Union (ITU), 2017) which describes all technologies that are interconnected through a network of communications and are today embedded in every aspect of our life. This includes smart phones, tablet computers, laptop computers, desktop computers, fitbits, 3D printers, smart thermostats, smart refrigerators, smart televisions, and an ever-increasing number of other appliances and tools. These technologies have a couple of things in common. One is that they are all becoming wireless and they can virtually all be connected via a wireless network and second is that many of them need some form of energy storage system, in other words a battery, to provide their power. New medical technologies are being introduced every day due to the increasing capabilities of energy storage technologies. From wireless technology used in hospital equipment to implantable devices to wireless tools that can be used to diagnose and treat patients in remote villages that have no electrical infrastructure. All of these technologies are enabled due to the regular improvements that batteries have experienced over the past few years. And as the use of medical nanobots and microbots increases the next generation of solid-state batteries are being used to power them. Wearable technology is also beginning to emerge in many different types of products, and electronics are even beginning to get embedded in our clothing. Beginning with the evolution of the wristwatch into a device that monitors your health, the number of steps you take, the number of steps you climb and now you can even get your email and your text messages on your wrist. Companies like Google have introduced technologies like their Google Glasses, to allow users to access the internet using eye tracking technologies. This is evolving into a new realm of virtual reality devices that allow us to see the world around us in a different manner. All these factors have pushed the demands on batteries even further. As we have watched technology around us continue to evolve into more complex systems, it seems that our understanding of how things work has diminished at an equal rate, kind of like an inverse Moore’s Law. Think about that for a moment,

CHAPTER 1 Introduction

Consumption spreads faster today Percent of U.S. households 100% Stove 80

Color TV

Refrigerator Electricity 60 Clothes washer 40 Clothes dryer

Telephone Radio 20

Computer Air conditioning Dishwasher Microwave

Auto 1900

1915

1930

1945

1960

1975

VCR Internet Cellphone 1990

2005

FIG. 1 Rate of technology change.

do you really know how your smart phone works? What about your laptop or tablet computer, do you know what the different components are within them, what the software is made up of and how they function? Even your automobile has become a moving computer, 30 years ago it was common place for most people to be able to service their own vehicles but today you just about need to be an electrical engineer to work on a car. There’s not much room for the backyard mechanics any longer. The speed of technology change continues to grow at an increasing rate. Looking at the length of the curves in Fig. 1 (Felton, 2008), we see that newer technologies appear to be seeing higher levels of adoption at a much faster rate than older technologies. For example, while the internet took only 15 years to reach 60% of U.S. households, the automobile took more than 50 years to reach and sustain 60% market penetration. Gordon Moore, cofounder of Intel, coined what came to become described as “Moore’s Law” to describe the rate of technology change in relation to computer processors. Moore’s prediction that the number of components on a chip would double every year ended up being pretty accurate. However, batteries have not experienced the same rate of change as computer processors. For nearly 150 years lead acid batteries were the pillar of battery technology, with very little change in energy density but significant improvements in manufacturability, life, and cost. Even modern nickel metal hydride batteries were not introduced until the early 1980s and not commercialized in large numbers until the early 1990s. The first commercial lithium-ion batteries were not introduced until 1991 but had been in development for more than a decade prior to their commercial introduction. In less than 30 years since their introduction lithium-ion batteries have grown from use in luxury devices to becoming the platform that powers our daily lives. In fact, if we look at the introduction of different secondary (rechargeable) batteries over the past 130 years we see three distinct stages of battery development. But the introduction of these newer chemistries did not displace the traditional lead acid

3

4

CHAPTER 1 Introduction

FIG. 2 Modern battery-specific energy over time.

batteries, in almost every case they ended up in a complementary or new product that could not have been served adequately by lead acid. Lead acid batteries continue to be the mainstay for vehicle starting and many other applications, and while we have seen some premium vehicles moving toward 12-V lithium-ion these prove to be the exception and not the rule but today battery technology is changing quickly. As shown in Fig. 2 when we look at secondary battery energy density over time there appear to be three stages of battery development that have occurred over the past 130 years. The first stage I will call the “Industrial Age” of batteries as these were the earliest batteries used to help power the burgeoning global Industrial Revolution era technologies. These batteries included many variations of lead acid batteries, Thomas Edison’s nickel-zinc and nickel-iron batteries, the sealed nickelcadmium battery, the valve-regulated lead acid battery, and ended with the introduction of the first nickel metal hydride batteries. During this Industrial Age of batteries the manufacturing processes were improved, the costs were reduced, and the cycle life was increased, but there was not much improvement in energy density of secondary batteries during this period. During this nearly 100-year period we saw specific energy density increase on an average of only about 0.2 Wh/kg per year. The next stage of battery development I will call the “Portable Age” as it coincided with the introduction of so many mobile technologies like the laptop computer, cell phone, smart phone, tablet computer, music player, video game, and many other technologies. During this period humanity began demanding that we should be able to bring our technologies with us everywhere we went, which led to the massive growth in mobile energy needs for these technologies. It was during this period that cellular telephones became commonplace and began to replace the land lines that had

CHAPTER 1 Introduction

dominated personal communication for over a century. During this period we also saw the introduction of the laptop computer. Lithium-ion battery technology enabled both of these tools and gave them the power they needed to become everyday household items by making them truly portable. This Portable Age of batteries roughly begins with the introduction of the first commercial lithium-ion battery in 1991 at just under 90 Wh/kg and continues to late 2008/2009 when cell energy densities reached about 180 Wh/kg effectively doubling the energy in a battery cell. Today, there is yet another energy storage shift happening as we move from simply portable to truly mobile. In this third age of batteries, the “Mobile Age,” we see the battery technologies finally achieving high enough energies to be able to fully electrify vehicles. These efforts began in earnest in the mid-2000s but it was the 2009 introduction by the U.S. government of the American Reinvestment and Recovery Act (ARRA) which provided more than $2.3 billion dollars for “…renewable energy generation, energy storage, advanced transmission, energy conservation, renewable fuel refining or blending, plug-in vehicles, and carbon capture and storage” (The White House: Office of the Press Secretary, 2016). This investment in the U.S. battery manufacturing industry led to the emergence of several companies that still continue to be major players in the world market today, these investments included an EnerDel factory in Indiana, and Dow-Kokam, Johnson Controls, A123 and LG Chem factories in Michigan, a Saft plant in Florida, as well as many other investments in battery manufacturing, recycling, and clean energy. These investments helped to accelerate the development of lithium-ion batteries and led to the current technologies that have reached as much as 285 Wh/kg. This equates to a specific energy density increase of an average of 10.5 Wh/kg per year over the past 28 years during the Portable and Mobile ages of batteries. So, comparing the Mobile and Portable Age rates of improvement to the Industrial Age rate of improvement, and then to the previous era of specific energy density the improvement is more than 51.5 times faster and even within the Mobile Age specific energy densities have increased in the range of 200% since the beginning of the Portable Age. Of course, looking at it from the energy density perspective is only one way to look at the evolution of lithium-ion technology. Another is to look at it based on volumes compared to those developments. In Fig. 3 I outlined the commercialization timeline for the modern lithium-ion battery, beginning in the mid-1970s with the earliest laboratory research to about 1979 when Dr. John Goodenough at Oxford and Dr. Ned Godshall at Stanford made their first discoveries that led to the first commercial lithium-ion battery sales in by Sony 1991. From the first lab work to first commercial sales of 18650-type cells was about 16 years. From there the 18650 cell growth exploded as it found ready and willing applications in portable computing, cellular telephones, and similar portable electronic devices. But if we look at the “fit for use” case for automotive electric vehicles, it took another 18 years before the first highvolume production electric vehicles powered by lithium-ion cells hit the streets—the Tesla Roadster, Chevrolet Volt, and Nissan Leaf. It was still nearly 10 more years before the cumulative PHEV and EV sales in the United States reached 1 million units when the 2018 sales reached 370,000 plug-in and fully electric vehicles. This

5

6

CHAPTER 1 Introduction

FIG. 3 Lithium-ion development timeline.

sales volume makes PHEV’s and EV’s combined sales almost 2% of the U.S. new vehicle market share. So just keep in mind when you start getting excited about a new discovery in the laboratory that it took lithium-ion about 45 years from the time it was “discovered” in the labs until it reached market acceptance in the electric vehicle space. But why are all these technology discussions important and what does it have to do with understanding battery chemistry? In other words, what is the “so what” of it all? In short, technology change is now occurring at such a fast rate that we, as a society, have lost touch with how most of those ever-evolving technologies work. When it comes to batteries, most of us, even those of us in the battery industry, still have very little understanding of what makes them work and the forces that are happening inside the battery. One example of this is that batteries do not only store energy, that is a common misunderstanding. What they actually do is store the chemicals that when they are put under an electrical load create a chemical reaction which generates a current flow (energy) thereby allowing energy to be stored in its chemical form. It is this chemical reaction, or more accurately, these chemical reactions that we are concerned with gaining an understanding of in this book. When a load is applied to a cell a current is created as the lithium-ions flow from the anode through the separator and into the cathode. This is all the result of a series of chemical reactions that occur within the battery cell. Batteries represent a combination of intersecting sciences ranging from physics to inorganic chemistry to electrochemistry, thermodynamics, and mechanical engineering. All these sciences must interact in just the right manner to provide energy on demand. That brings us, of course, to the subject of chemistry which will be the focus on much of this book. If you are like me then that word alone can send shivers of fear down your spine. Most people have only a vague and limited understanding of chemistry that came from a high school class. I would not hesitate to guess that chemistry

CHAPTER 1 Introduction

may fall right behind public speaking and math, as one of the most frightening and confusing topics for many people. But not to fear, like you I needed a simple and clear explanation of how lithiumion batteries work and this book is the result. Much like my first book, “The Handbook of Lithium-Ion Battery Pack Design” (Warner, 2015), this book is intended to help demystify the topic of battery electrochemistry and how lithium-ion cells work while offering a clear and simple explanation through the use of graphics and images. I am a visual learner so for me to understand something I most often need to show it visually. My home and office whiteboards and my notebooks are often filled with different sketches and drawings of things so that I can really get my head around them. In summarizing his biography on Leonardo Da Vinci, Walter Isaacson described one of the conclusions he reached during his research as being the need to think visually. He wrote, “Too often, when we learn a formula or a rule – even one so simple as the method for multiplying numbers or mixing a paint color – we no longer visualize how it works” (Isaacson, 2017, p. 522). Consequently, I have attempted to create very simple illustrations for many of the concepts, processes, and reactions in order to help simplify them. In fact, I will state now that some of these may be oversimplistic illustrations in order to make them clear. Why do we use lithium-based chemistries? What is lithium and where is it in my battery? How much lithium is in there? Why do different chemistries act differently? What is a SEI layer and why does it form? Why is lithium iron phosphate considered to be a “safer” chemistry than others such as nickel manganese cobalt, nickel cobalt aluminum, or others? What are “side reactions”? Why is heat generated during a battery discharge? What impact does the use of different electrolyte salts have on batteries? Where is the lithium anyway? And, what in the world is the rocking chair effect? Those are just a few of the questions this book will attempt to answer. We will examine the basics of the chemical and electrochemical reactions, but without the confusing formulas and instead using simple graphics intended to help clarify the concepts. I have spent my career in the executive management, sales, marketing, and product management areas, but each of these functions requires a solid understanding of how these cells work. I have spent nearly a decade working in the lithium-ion battery industry, and over two decades in the automotive industry, and have been able to work with some of the most knowledgeable people in the world in these topics. But what qualifies me to write this book? Well, I share Da Vinci’s desire to “seek out knowledge for its own sake” (Isaacson, 2017) which has made me a “life-long learner” studying a wide range of fields, subjects, and topics that may not have an immediate impact on my work life. But it has allowed me to see influences across subjects and make connections that I may not have been able to make otherwise. As I have evolved through my working career I often found myself working closely with these experts in many different fields and because of my natural curiosity and desire to understand how things work, I asked a lot of questions. Why does that work that way? Why did that happen? What causes these results? What does that really mean? How did you calculate that? In short, I found that my own curiosity led me to ask a lot

7

8

CHAPTER 1 Introduction

of questions, which turned out to be the best pathway for me to build my own knowledge and get a better understanding of how things worked. But I didn’t invent anything new here, Socrates was known to use the process of asking questions to help discover, or as he considered it to reveal a priori knowledge. This process later became known as the Socratic method of inquiry. Socrates wrote that, “…I am quite conscious of my ignorance….it seems that I am wiser than he is to this small extent, that I do not think I know what I do not know” (Plato, 1993, p. 42). Throughout his life Leonardo Da Vinci filled up notebook after notebook with, in addition to his many drawings and sketches, lists of questions to ask and people to talk to. Using this curiosity and ability to make analogies across sciences Da Vinci would ask questions like “why is the sky blue?” and would make himself reminders to do things like “describe the tongue of a woodpecker.” Then through experimentation and study he would determine the answers to these questions (Isaacson, 2017). Albert Einstein even discussed the importance of curiosity and questioning during an interview with Life magazine editor William Miller saying, “Then do not stop to think…about the reasons for what you are doing, about why you are questioning. The important thing is not to stop questioning. Curiosity has its own reason for existence” (Miller, 1955, p. 64). Management theorist Peter Drucker wrote that a company, or an individual, should do a “knowledge analysis” to understand whether we have the right knowledge which will in turn lead to asking what he termed as diagnostic questions like “Do we have the right knowledge?.” Drucker also noted that knowledge must continually progress in order to remain knowledge (Drucker, 1996). Taiichi Ohno introduced the concept of asking “The 5 Why’s” into the Toyota Production System as a means of getting to the root cause of an issue (Ohno, 1988). In his book “Great Leaders ask Great Questions” (2014) John Maxwell describes the many benefits of asking questions in relation to becoming a better leader. In other words, we cannot sit on our laurels when it comes to learning and growing, we must continuously seek knowledge. I certainly do not claim to be the smartest guy around and am definitely not in the league of Socrates, Da Vinci, or Einstein, but I do believe in beginning by knowing how much I do not know and using my own curiosity to ask a lot of questions in an attempt to fill in those blanks. So that is where this book, like many of my other endeavors, began—by trying to gain a better understanding of the core engineering principle’s around lithium-ion chemistry and educating myself. This allowed me to start as “an open book” in which new concepts and ideas can be written as I have become a self-educated engineer. I have used some of these same techniques and strategies to increase my own knowledge and understanding of the intricacies and complexities of lithium-ion batteries and how they work. We all know the old saying “the only dumb question is the one that you don’t ask” which still holds true today. Only by asking questions and then asking the right questions are we able to learn and grow. And I asked a lot of questions and not all of them were the right ones, but I would not have known which ones were the right ones had I not asked. In this book I have worked to simplify the

1.1 Batteries and chemistry

explanations of this rather complex topic of lithium-ion battery chemistry. I have attempted to do this without requiring the reader to need to memorize complex chemical formulas like those you may have learned in your high school or college course and more importantly I have worked to express these concepts in very simple terms. In fact, I may oversimplify some of the concepts beyond what may make a chemist or scientist comfortable in order to simplify these concepts for easier understanding by the rest of us who are not engineers or chemists, but I have always been comfortable pushing the limits of comfort.

1.1 Batteries and chemistry As I mentioned at the start, batteries have been around for a long, long time and today they form the basis for our modern life. Think about your smart watch, your smart phone, your tablet, your laptop, handheld gaming device, and more than ever even your vehicles all need a source of power that is small and powerful enough to provide the necessary energy to meet the application’s intended purpose. Because of their mobility and energy density, batteries are this power source. Look around and you will find many applications using these remote energy storage devices. Batteries have become the enabling technology to make our world truly mobile, they are Prometheus’ fire for the modern era. Lithium-ion battery packs are complex systems of interrelated components and subsystems but can be relatively easily understood by most people because the pack is the thing that we can touch, hold, and feel. But understanding the lithium-ion chemistries and the physics and chemical reactions that occur inside those battery cells requires gaining an understanding of an even more complex set of systems and interrelationships that are really well understood only by those few chemists, researchers, and cell engineers who work with them on a daily basis. And there is even a lot that they do not understand about some of the reactions that take place inside the cell. However, even without having done advanced research in chemistry it is possible to achieve a good basic understanding of how these different chemistries work, what the more complex reaction mean, and what happens inside a lithium-ion cell when you use energy from it. This book is centered around trying to create that understanding around the different components and materials that make up the modern battery. The battery cell is a system, made up of an assembly of components that all batteries must have in order to function. The first and most important of these are what are called the active materials. These are the chemical compounds that form the anode and cathode which take and give the lithium-ions and electrons in order to generate energy. For the purpose of this book, we will refer to the anode as the negative terminal of the battery and the cathode the positive terminal. In reality, the positive and negative polarity of the electrodes flip flop during operation as the materials gain and lose electrons and lithium-ions. The active material compounds are coated onto a substrate of some form, most often aluminum and copper foils due to their high electrical conductivity.

9

10

CHAPTER 1 Introduction

Once they are coated on to the foils they are known as the electrodes. The electrodes must be kept separated to prevent a short circuit from occurring. This is done with, you guessed it, a separator that is most often some form of polymer or ceramic-based film. These electrodes are then either stacked or wound into the final electrode configuration, known as the jelly roll if it is rolled. This assembly of electrodes is then inserted into the final cell enclosure which could be a metal can, a laminated aluminum pouch, or a plastic can. Next an electrolytic liquid or gel is added. This electrolyte allows the lithium-ions to travel back and forth between the anode and cathode active materials. Safety devices are added and the cells are sealed. After a formation process that involves performing a specific charge and discharge cycle, they are ready to be used. This book is going to focus on these components with special emphasis on the active materials and the chemical reactions that occur since those are what really differentiate different battery cells. The image in Fig. 4 shows a cross section of a battery electrode pair taken with a scanning electron microscope (SEM) so it is shown at extremely high magnification. While this is a painted battery image, which we will discuss in more detail in Chapter 10, it is a nice image of the components of a lithium-ion battery cell. Within this single electrode pair, we see a copper current collector on the top, anode side, and a carbon nanotube (CNT) current collector on the bottom, cathode side. On the CNT current collector is added a layer of cathode active material, in this case lithium cobalt oxide (LCO). On top of this is added a polymer separator material right in the middle of the image. And the lithium titanate oxide (LTO) anode material is coated on top of that followed by the final copper current collector. Remember that

FIG. 4 Scanning electron microscope image of electrodes. Reprinted with permission from Springer Nature: Singh, N., Galande, C., Miranda, A., Mathkar A., Gao, W., et al. (2012). Paintable battery, Scientific Reports, 2(481).

1.1 Batteries and chemistry

what we are looking at here is a single electrode pair, but within a traditional lithiumion cell there will be multiple sets of these electrode pairs depending on how much capacity the final cell needs. Each of these layers is included in all lithium-ion cells even if they are done in different manners. Earlier I called batteries an “energy storage device” but as I mentioned earlier that is not an entirely accurate description of a battery. Which is why gaining an understanding of the topic of electrochemistry is important, a more accurate description of a battery is that it is a device for storing the chemicals that react to create a direct current flow, in other words batteries convert chemical energy into electrical energy (Crouch, 2012; Linden & Reddy, 2011; U.S. Naval Personnel and Staff of Research & Education Association, 2002). Some batteries work only one time, these are referred to as primary or nonrechargeable batteries. While others can be charged and discharged many times, these are referred to as secondary or rechargeable batteries. But in all cases, the current flow is a result of an electrochemical reaction that occurs when a circuit is completed between the positive and negative poles of the battery and an electrical device. When we complete an electrical circuit with the battery and an electrical device such as an electric motor, the cells generate electricity through a series of electrochemical reactions occurring in the battery cell. The majority of the positively charged lithium-ions, also called cations, are stored in the electrolyte solution but there is also lithium in the anode materials. Under a discharge condition those positively charged lithium-ions are forced out of the anode through an oxidation reaction, they are pushed through the electrolyte and through the separator and combine with the active material in the cathode until all the lithium-ions have been moved. At the same time, this process releases electrons which flow from the anode in the opposite direction as the lithium-ions which cause the current to flow from the anode through the current collectors and to the electrical equipment and then into the cathode. This process will continue until active material and electrolyte run out of fuel (lithium-ions) at which time the cells are said to be in a discharged state (Brain, Bryant, & Pumphrey, 2000). This process creates the electromotive force by pushing electrons through the electrode and into the circuit creating current flow. Ultimately, it is the release and consumption of electrons that create the useful current flow in the battery cell. The process is then repeated but in reverse for the opposite reaction (charging). I mentioned this earlier, but it is worth repeating is that the anode is not always the anode and the cathode is not always the cathode in a rechargeable cell. These terms have become synonymous with the positive and negative electrodes. But in the rechargeable battery the cathode becomes the negative electrode and the anode becomes the positively charged electrode at the end of the discharge cycle. So while we think of the cathode as always being positive, the truth is that it will change polarity as the lithium-ions are released and then again when they are added. But we will review this in more detail in Chapter 3. With a primary battery, a nonrechargeable cell, these electrochemical reactions that discharge the battery are a one-time event. Once the cell has been fully discharged it cannot be recharged again. This is because the electrochemical conversion

11

12

CHAPTER 1 Introduction

process permanently consumes the active materials. In other words, the anode is destroyed as it releases the ions. An example of this type of cell is the traditional “AA,” “AAA,” “C,” “D,” or “9-Volt” cells that are used in many household devices, once it is fully discharged it is impossible to recharge again and must be discarded. While the same general reactions occur in both primary and secondary cells, this book will focus only on the secondary, rechargeable, lithium-ion batteries.

1.2 Chapter outline In the chapters that follow we will review each of these concepts in much greater detail. Chapters 2 and 3 will be the foundational chapters in the book, starting by offering a refresher on the fundamental sciences that make batteries do what they do, including electricity, chemistry, electrochemistry, thermodynamics, and physics. We will build on these subjects by applying them specifically to lithium-ion batteries in an effort to create a clear understanding of the inner workings. Cations and Anions, positive and negative charged ions, will be described as well as electrons, molecules, atoms, and compounds will all be described looking at how they function and do what each of them contribute to the functioning of the battery. The flow of current, electrons, and ions will be reviewed in some detail in order to really understand what happens when you charge and discharge a cell. Building on the foundation created of the previous chapter, Chapter 3 will examine the specifics of lithium-ion cell operations. Where Chapter 2 reviewed fundamental science, Chapter 3 reviews lithium-ion battery fundamentals. In this chapter we will examine the intricacies around current flow, electron and ion flow by applying those fundamental concepts to them. The core reduction and oxidation processes, known as Redox reactions, will be examined. We will also investigate subjects like “nano,” which has become somewhat of a techy catchphrase today, but we will review what it means and how it impacts cell functioning. We will also look at the effect of temperature and thermodynamics and their influence on chemistry function. Finally, we will discuss the different types of failure modes that can happen in a battery cell, how are they spread and how they may be mitigated. In Chapter 4, we will offer a high-level review and comparison of the major lithium-ion chemistries that are in use today. Since there really is no “silver bullet” battery that will do all things for all applications, there is a place for every energy storage option. Therefore we will look at the different value propositions for each chemistry as well as discuss the types of applications they may be best suited for. For each chemistry, the strengths and weaknesses will be reviewed. Chapter 4 will also briefly discuss lithium-ion cell costs. With the amount of change and the speed that costs have dropped in the industry anything that is presented on lithium-ion battery costs will be out of date about as soon as it gets to print. But it is always good to see the roadmap of where the costs have been to understand where they may go as well as understanding what the biggest cost drivers in the cell

1.2 Chapter outline

are. Finally, we will briefly look at some of the criteria that should be considered in selecting lithium-ion cells for a specific application. The next two chapters will delve into the specifics of the individual anode and cathode chemistries, but this chapter will discuss what happens when they are combined. Chapter 5 is the launching off point for looking at different cell chemistries. Each of the major lithium-ion cathode chemistries that are in common use today will be examined and compared from a performance standpoint. Cathode chemistries including lithium iron phosphate (LFP), nickel cobalt manganese (NCM), lithium cobalt oxide (LCO), lithium manganese oxide (LMO), nickel cobalt aluminum (NCA), and lithium manganese phosphate (LMP) will all be reviewed including a description of the differences in function and operation. This chapter will look at the crystal structure of each of these materials in an effort to help understand why they operate in the fashions that they do. Chapter 6 will review the other side of the cell, quite literally, comparing individual anode chemistries starting with the carbon family of anode materials including natural and artificial graphite, graphene, hard carbon, and soft carbon. While the carbons make up the vast majority of anode materials in use today, there are some others that are starting to make their way into the market in big numbers, including titanium, or titanate oxide, and silicon. We will end with a discussion on the strengths, weaknesses, and differences in function and operation of each will be offered. Chapters 7 will review the other materials in the cell, the “inactive materials” that are required for cell operation but are not “active materials.” The electrolytes, separators, current collectors, terminals, and enclosures that are used in modern cells will be discussed, describing their functions, benefits, and challenges of the various components. Separator options ranging from the traditional polyethylene and polypropylene to the more advanced ceramic and woven separators will be reviewed as they play a crucial role in cell operation because they need to balance preventing internal short circuits by separating the anode and cathode, but they also need to be electrically resistive but must allow for ion transport during operation. The general characteristics that a separator must have include having a relatively high strength so that they can be manufactured, they should be very stable in a variety of operating conditions including high voltages and high temperatures, should be stable in solvents, and finally they should include some type of safety mechanism. Chapter 7 will also cover the electrolytes that allow for the ion and electron transport during operation. The most common electrolyte in use today is polyvinylidene fluoride or PVDF for short. Different manufacturers use different solvents and lithium salt additives to achieve different performance characteristics, such as high voltage capability or low temperature capability. The basic characteristics that an electrolyte must be evaluated against include ensuring that it has high ionic conductivity, being stable over a wide voltage range, and having a high rate of ion diffusion. Chapter 7 will also take a look at organic, aqueous, nonaqueous, and ionic liquid electrolytes that are either in use or are being developed today.

13

14

CHAPTER 1 Introduction

In Chapter 8 we will look at the raw materials that are used in lithium-ion cells including a brief survey of the processes that are used to mine and process them. One of the topics that comes up for frequent discussion in respect to material are location and demand, asking where do we get them and how much is available? Both topics will be discussed and reviewed in some detail for all of the major materials used in lithium-ion cells. Chapter 9 will review the current manufacturing processes for lithium-ion battery cells. This first generation of lithium-ion cells is manufactured using processes very similar to that of digital media, but regardless of the manufacturer the general steps are the same regardless of cell type or size. Today these processes are evolving to reduce the cycle time, improve and speed up the drying process through various means, and to reduce the processing costs. Much of the world’s battery manufacturing is also looking to make improvements by looking at new processes and equipment to reduce the time that each step takes. In some instances, some of the most advanced plants in the world have entire floors that are “dark” as they fully automate their process without the need for human support. Chapter 10 will examine the new world of “beyond lithium” chemistries including lithium-ion air, lithium-sulfur, solid-state batteries, and many other new types of cells. How do their functions differ from that of the more mainstream chemistries and what are their benefits and challenges of each? While these new chemistries and cells are in process today and may still some years away from achieving major market share, but some may be closer than you think. For instance, solid-state batteries have already emerged as a very plausible solution to powering near-term consumer electronics and implantable medical devices. They offer the benefit of safety due to their solid electrolytes and high energy density, but currently suffer limitations in the size of the cells that can be manufactured and the rate of ion transfer. This chapter will be somewhat of a futurist’s view of where the next generation of energy storage devices may emerge from. Finally, Chapter 11 will build on the last chapter and wrap up with a discussion on what the future may hold for us. I may overreach a bit, but I like to think of myself as somewhat of a futurist. I am always looking past today, and even past tomorrow, to understand what the future may hold for us and what trends are emerging in society to shape that future. Energy storage development will be largely driven based on technological needs, both those that we know and many that we haven’t even guessed at yet, and on larger social, environmental, and societal trends. Global megatrends such as the explosion of “mega cities” will create an all-new set of demands for energy storage as will the current drive for actively managing building efficiency and the changing models of vehicle ownership, usage, and autonomy. Energy as a resource will become more valuable as the power infrastructures evolve becoming more distributed, more self-generating, and more renewable. Another Megatrend that is influencing energy use is the growth of the Millennial generation, which may actually be larger than the Baby Boomer generation based on some estimates. There appears to be a shift in values and aspirations in many Western countries with the latest generation who seem to have less interest in some of the

1.2 Chapter outline

traditional achievements such as vehicle ownership due to the growing access to various technologies like ride sharing. This means that the entire model of vehicle ownership may have to be reexamined. Questions also abound on the future availability of petroleum-based fuels and the environmental impact that these vehicles, vessels, and power plants will have on our environment and how it will impact our lives. And, of course, entwined with this is the question of availability of the materials used in lithium-ion cell manufacturing. All these factors will affect the development of energy storage solutions to meet these growing and varied needs. We will end the book looking not into the present but into the future to try to understand what the trends that will drive future research and development efforts beyond our current technologies.

15

CHAPTER

Electrochemistry basics

2

Chapter Outline 2.1 2.2 2.3 2.4 2.5

Electricity ......................................................................................................... 18 Chemistry ......................................................................................................... 20 Electrochemistry ............................................................................................... 32 Electromotive force (voltage) ............................................................................. 34 Electric current ................................................................................................. 36 2.5.1 Discharging the battery ....................................................................38 2.5.2 Charging the battery .........................................................................38 2.5.3 Efficiency and voltage ......................................................................39

In very simple terms, chemistry is the science of how things react at the atomic level in a controlled, or even uncontrolled, environment or when a certain set of conditions are applied. There are several fields of interest to us here that are directly applicable to lithium-ion batteries including organic and inorganic chemistry, physics, thermodynamics, and electrochemistry. This chapter will provide a brief and simple introduction to these different sciences in preparation for gaining a better understanding of lithium-ion cell operations. Organic chemistry is the science surrounding a very small group of the atoms including carbon, oxygen, nitrogen, and hydrogen. These are the atoms that are involved in the cellular processes of living beings and are the keys which enable life to exist. In this instance, we are most concerned with carbon and oxygen as they are applied to chemical energy storage in batteries. Most of the work on lithium-ion batteries falls into the category of inorganic chemistry, which is the science of the other 114 atoms in the periodic table of elements and how they interact with each other (Matson & Orbeak, 2013). The science of physics involves studying the motion through both time and space of all types of organic and inorganic matter and introduces and applies the concepts of energy and force. In physics the use of energy and force is relevant in their ability to perform work by applying electromotive force to do things like making electric motors turn. A subset of the science of physics is the field of thermodynamics which is centered around the study of heat and temperature and their relationship to energy and the use of that energy to perform work. Finally, all these different sciences are brought together within the realm of electrochemistry, which is the study of the transformations of matter on the atomic scale which create and manage the flow of electric current (Lefrou, Fabry, & Poignet, 2012). Which ties Lithium-Ion Battery Chemistries. https://doi.org/10.1016/B978-0-12-814778-8.00002-8 # 2019 Elsevier Inc. All rights reserved.

17

18

CHAPTER 2 Electrochemistry basics

us back to lithium-ion batteries as a battery is, in fact, an electrochemical storage device used to create work. We will delve in much greater detail in subsequent chapters into the operation of a lithium-ion cell, but a simple example of the electrochemical reaction within the battery cell occurs when the lithium-ions are forced from the anode into the cathode. These reactions occur after the cathode and anode are assembled together into a cell and an electrolytic medium is added. In the first stages of battery cell assembly no chemical reactions can take place in the cell, this is because until the electrolyte is added and an initial formation process is completed there is no pathway for either ions or electrons to travel. More on this later, but to get us started we need to recognize that the core electrochemical reactions in a lithium-ion battery are the reduction of ions in one element and the oxidation of ions in the other element. This is known as the Reduction-Oxidation (RedOx) reaction and is the reason that batteries operate and can create a current flow that can be used to generate work. The overlapping of each of these sciences is the somewhat mystical product known as electricity, which we will cover next.

2.1 Electricity Even after more than 2000 years of studying it, remember from The Handbook of Lithium-Ion Battery Pack Design (Warner, 2015) that the first known battery is the Bagdad Battery estimated to have been developed over 2000 years ago, we still do not really know what electricity is. We understand many ways to harness and use it, but we really cannot say exactly what it is other than it is the force that flows when electrons flow. Electricity is a natural phenomenon, we see it in nature in the form of lightning and static electricity. But electricity is inherently mysterious thing since it cannot be detected by any of our senses and we need specialized tools to be able to detect it, except for lightning which can be terrifyingly easy to identify. Electricity may be defined as the “…force which moves electrons” (U.S. Naval Personnel and Staff of Research & Education Association, 2002). Electric current flows when positively and negatively charged particles move through some form of medium, which creates an electric current (Gibilisco, 2012). But this only describes the result of that force, not the origin of electricity itself. But fortunately for us the effects of electricity are very predictable and repeatable which has allowed us to harness its power and make modern life possible even if no one can agree on what electricity is. Electron theory states that electric current flow is generated and comprised of the movement of electrons. Therefore when electrons move a current flow is created which is the electricity that we harness to perform work. Atoms, discussed in the next section, are electrical in nature with the protons in the nucleus which are positively charged and electrons circling the nucleus which are negatively charged. It is these opposing forces that keep them together. But this also makes many atoms susceptible to be affected by an electric force. Consequently, when an external electric force such

2.1 Electricity

as an electric motor or generator is applied to a conductor, such as copper or aluminum, some of the electrons that are in the outer orbits of the atoms are forced out of their orbits and driven along the conductor as shown in Fig. 5. These electrons are called free electrons due to their ability to move from atom to atom under an electric force which in turn creates the flow of electric current. This happens when the atom becomes excited which can happen when particles are driven by an electric force causing them to collide with the atoms. Imagine this as being like when pool balls collide, the force on the cue ball pushes the other balls away in different directions. But in this case, imagine that there are very small balls attached to the outside of the pool balls, some of these are what are driven away during this action and are known as free electrons. Through this process the atoms will absorb enough energy to cause their free electrons to jump from atom to atom repeating this process continuously until they reach the end of the conductor (U.S. Naval Personnel and Staff of Research & Education Association, 2002). Another interesting feature of electricity is that it will balance itself. By this I mean that if two atoms with opposite charges come into contact, some of the electrons will easily flow from the negative to the positive atom since opposite charges attract. This may be most easily visible in daily life in the form of static electricity. When you are walking across the carpet with your wool socks on, they are picking up free negative electrons from the carpet and are released through your body when you touch another conductor like a doorknob thereby equalizing them and often surprising us as those electrons are released so quickly. In relation to lithium-ion batteries this may be experienced when connecting cells in series or parallel configurations. For instance, if two cells are connected in series and they are at different voltages, or in parallel with different capacities, the electrons will

FIG. 5 Electron flow theory.

19

20

CHAPTER 2 Electrochemistry basics

flow between them until their voltages or capacities equal without the need to apply any external force. When a pathway for current to flow exists, such as the electrolyte within a battery cell, even without a device trying to pull that electricity, current will flow from the negative anode to the positive anode (U.S. Naval Personnel and Staff of Research & Education Association, 2002). Remember that a battery’s natural state is to be discharged. When we charge it we are forcing the reactions, which is why many batteries have a “self-discharge” when they are not being used. Think of it like a balloon, a balloon’s natural state is to be deflated so even when we blow it up the air is always trying to find a way to escape and release the pressure. This difference in voltages between the negative and positive electrodes, which is referred to as a difference in potential voltages, creates that “pressure” within the cell.

2.2 Chemistry Chemistry is the science of change, specifically change as it occurs at the atomic and subatomic levels between different elements and between the elements and the environment (Matson & Orbeak, 2013, p. 35). Everything around us, all solid material, is composed of atoms, ions, molecules, or some combination of these particles (Hosford, 2013). What are the differences between atoms, molecules, species, and ions? In some instances different terms are seemingly used to describe the same thing. Let’s look at each one individually and define what differences, if any, exist. First, let us start with the atom. An atom is considered a “fundamental” constituent of matter and is the smallest constituent of an element that still contains the characteristics of that element. The best way to visualize the atom is to imagine them as being solid balls of matter as shown in Fig. 6 that, when brought into contact with one

FIG. 6 The atom.

2.2 Chemistry

another, form the basis for all matter we see around us (Hosford, 2013). But the truth in fact is not quite so simple as atoms are not actually solid. The earliest researchers envisioned them as being very much like a planet circling the sun. In Fig. 7 you see a very simplified image that is more akin to what a lithium atom could look like. Its nucleus contains a cluster of three positively charged protons and three neutrons, which carry no charge. I show these in a sphere for visualization purposes only, while the protons and neutrons will form a nucleus to the atom they are not truly solid either being made up of smaller particles called quarks. Finally, orbiting the nucleus of the lithium atom are three negatively charged electrons. I show these as being very small spheres circling the nucleus, when in fact they are very, very, very small spheres circling the nucleus. It is this entire grouping of protons, neutrons, and electrons that form the atom. While the number of protons, neutrons, and electrons differ fro each element this is the same basic structure for the atoms which make up all matter. Atoms are made up of three components—the neutron and the proton, which form the core, or nucleus of the atom (Figs. 7 and 8), and the electrons which are generally described as “orbiting” the nucleus of the atom. The protons are positively charged and the electrons are negatively charged, while neutrons are, not surprisingly, neutral. Since it is made up of only positively charged and neutral components the atom’s nucleus is positively charged. It is this positive force that attracts the negatively charged electrons and keeps them in “orbit” around the nucleus. Elements are differentiated based on the number of protons that an element has, which defines what the atomic element is. For example, all lithium atoms have three protons and all copper atoms have 29 protons (Fig. 9). If it had four protons it would not be lithium, it would be beryllium and if it had only two protons it would be helium. Atoms of an element can exist with the same number of protons but with different numbers of neutrons and electrons, this creates elements with identical properties but with different mass which are known as isotopes. There is, as a general rule, one positively charged proton in the nucleus for every negatively charged electron in an atom

Lithium atom Nucleus (protons & neutrons)

Electrons

FIG. 7 Lithium atom—3 Protons, 3 neutrons, and 3 electrons.

21

22

CHAPTER 2 Electrochemistry basics

FIG. 8 Nucleus of lithium atom.

FIG. 9 Copper atom.

2.2 Chemistry

and due to this one-to-one positive to negative ratio atoms are electrically neutral by nature. A molecule is simply a group of two or more atoms that are chemically bonded together and since the atoms are electrically neutral, the molecule is also electrically neutral. All molecules are made up of groups of atoms. The molecule is also the smallest part of a particle that still holds the same characteristics of that element. The most common example is the water molecule; a water molecule is made up of two hydrogen atoms (H2) and one oxygen atom (O). If you break those elements apart from that molecule they are no longer water, but instead are only those elements. When you see the subscript number next to an element, such as the two in H2O, this refers to the number of those elements in that particular molecule. Different elements can be bound together to create molecules based on their valence energy levels. Valence energy is the term used to describe an atoms ability to create a chemical bond with other atoms and form molecules (Matson & Orbeak, 2013, p. 33). In the lithium-ion battery the molecules undergo a chemical change, where the molecules are altered such that new molecules result. This is known as the reduction and oxidation (RedOx) processes which will be described later in this chapter. Because most chemical changes involve positive and negative ions, the reactions are electrical in nature (U.S. Naval Personnel and Staff of Research & Education Association, 2002). As ions, atoms, and molecules come together to form solid matter, the structure that they take is either a crystalline structure or an amorphous structure. This can be a bit confusing because the material is not a crystal, but rather the molecules and atoms come together in a well-defined repeating structure which is what defines it as a crystalline element. Most metals have a crystalline structure. An amorphous structure is one when there may be similar structures but not an overall repeating structure and are most commonly seen in glass and plastics (Hosford, 2013). Since the emergence of lithium-ion batteries, the ion has taken front stage in our discussions. But how many people really know what an ion is? I admit that for a long time I really did not have a clear understanding of what it was. In its most simple form, an ion is an atom that has either lost or gained an electron giving it a net positive or negative electric charge. Remember that since the protons and electrons of an atom are equal the atom has a net neutral charge. But the ion is either positively or negatively charged depending on whether it has gained or lost an electron. If the atom has lost one or more negatively charged electrons, then it has a net positive charge because it has more positively charge protons than negatively charged electrons. If it has gained one or more electrons, then it has a net negative charge because it has more negatively charged electrons than positively charge protons. A lithium-ion is a lithium atom that has given up one of its electrons, thereby making it electrically positively charged. A lithium-ion may also be called a cation. When we are talking of lithium-ions we are talking about positively charged ions. Fig. 10 shows an example of a lithium atom, a molecule with two lithium atoms and a lithium-ion. In some of the texts the term “species” is used, in this case the term species is used to represent a specific type of atomic nucleus, atom, molecule, or ion. Or more

23

24

CHAPTER 2 Electrochemistry basics

FIG. 10 Lithium atom, molecule, and ion.

specifically the term species is used when an atom or molecule is identical to another as is seen in the isotopes of an element. The atoms or molecules within a species must share the same set of molecular energy levels when measured on the same time scale of the observation. In much the same way that atoms are made up of protons, neutrons, and electrons, protons and neutrons are made up of even smaller particles called quarks which are about the same size as an electron. Quarks are the smallest particles that form the basis for all matter and they come in six different types: up, down, strange, charm, top, and bottom. Quarks are what we call fundamental particles because they are, as far as we know, the smallest parts of matter, although they are so small that we have never actually seen one and really only have a very general understanding of them (Ford, 2004). There are essentially three types of quarks, baryons, gluons, and mesons and these are the parts that make up the protons and neutrons (Fig. 11). You will note here that I am not including the electrons since they are about the same size as the quarks and so are also fundamental particles. When they are combined baryons, gluons, and mesons form composite particles that are called hadrons. Protons and neutrons are the most stable types of hadrons. However, as we are trying to strive for simplicity we will not cover quantum mechanics in this text in much more detail. The mass of the atom is defined by the protons and neutrons in the nucleus of the atom not by the electrons. This is because the neutrons and protons are about 1860 times heavier than the electrons, but the electrons take up most of the volume of the atom since they “orbit” the nucleus at a distance and sit in different energy spheres. Notice that in all of the images I have shown so far the three electrons of the lithium

2.2 Chemistry

FIG. 11 Lithium atom and its nucleus make up.

atom sit in two distinct circles surrounding the nucleus. These are referred to as energy levels or energy bands and each energy level can only hold so many electrons. Once that level is full, additional electrons go to the next energy level. This is why the electrons take up most of the volume of the atom but not the mass. The nucleus contains about 99.95% of the atom’s mass but makes up less than a trillionth of the volume (Matson & Orbeak, 2013, p. 63). The protons and neutrons in the nucleus are held together by one of the fundamental forces of the universe, known as strong force, while it is electromagnetic forces that keep the negatively charged electrons in “orbit” around the positively charged nucleus. The strong force is about 100 times stronger than the electromagnetic forces which keep the electrons in orbit, hence the rather innovative name strong force. But the strong force has a finite range which is why it does not extend beyond the nucleus whereas electromagnetic force has a much greater range. Fig. 12 shows a visual representation of the atom, proton, neutron, electron, and quark along with their respective relative sizes. When we convert the radius of these components into standard metric numbers, I find it much easier to look at numbers rather than scientific notations, we see a very clear story of the differences in the size of these items. As a point of reference, a human hair is generally between 40 and 120 micrometers (μm) in thickness, that is, a human hair has a radius of between 0.05 and 0.02 mm. In comparison the atom has a radius of 0.0000000003 mm and the quark has a radius of 0.0000000000000000016 mm. It would take nearly 67 million atoms in a straight line to be the equivalent length of the smallest radius of a human hair (Table 1). These extremely small sizes are part of the reason that we

25

26

CHAPTER 2 Electrochemistry basics

FIG. 12 Comparison of the size of different particles.

Table 1 Average radius of different particles Material Atom Proton Electron Quark

Radius in scientific terms Radius Radius Radius Radius

lm to 3  10 of 3  10 of 1.11  10 15 m of 9.1  10 17 m of 1.6  10 19 m 10

Radius in meters 11

m

0.0000000003 0.000000000000111 0.00000000000000091 0.0000000000000000016

do not entirely understand everything that happens during the operation of a lithiumion cell; we simply do not have any tools that will let us see smaller than the atom. As we mentioned before, electrons are negatively charged, but they are also mobile and can move from atom to atom. Atoms are always trying to have just the right number of electrons to remain stable and maintain their neutral charge. This is the process of equalization that we talked about earlier. If a stable atom loses or gains an electron, it becomes reactive and starts a chemical process (Matson & Orbeak, 2013, p. 36). In order for an atom to lose an electron some amount of energy must be applied and this is called the ionization energy. Sounds familiar, right? This is the amount of energy it takes to turn an atom into an ion by releasing an electron or adding an electron, hence ionization energy. Fig. 13 offers what is the more traditional view of the atom, called the Bohr model, and in this model the atom is viewed like the electron is a planet circling the nucleus like a planet circling the sun. While more recent research has moved to a more complex model of the atom, I find this one relatively easy to visualize

2.2 Chemistry

FIG. 13 Bohr model lithium atom.

FIG. 14 Electron cloud model of lithium atom.

and so will use it for the basis for most of our discussions. However, I would be remiss if I did not mention that the more recent work has described the atom with an “electron cloud” circling the nucleus (Fig. 14). This model was driven from the fact that in the traditional model you cannot always find the electrons where they are supposed to be. In the electron cloud model the electrons have regions where they are most likely to be found. This view begins to tie in the science of quantum mechanics into the traditional view.

27

28

CHAPTER 2 Electrochemistry basics

The number of protons in an atom is very important to understand, since the number of protons in an atom is the only thing that differentiates one atom from another. Within an elemental family, various atoms in that family will all have the same number of protons, but they may have different numbers of electrons and these are what are called isotopes. They are all the same atom because they all have the same number of protons but if they have different numbers of electrons however having different numbers of electrons tends to make the atoms unstable. One simple example is carbon-12 (C-12) which is the most common form of carbon making up about 98% of all the carbon in the world. The “12” in C-12 represents the sum of the protons (6) and electrons (6) and it is also the most stable of the other isotopes of carbon. Other isotopes of carbon are C-13, which has six protons and seven electrons and is also stable, and C-14 which also has six protons but has eight electrons but is considered unstable with a half-life of over 5700 years. In total there are 15 isotopes of carbon (Matson & Orbeak, 2013, p. 36). All the known elemental groups are expressed in what is known as the “Periodic Table of Elements” (Fig. 15). The periodic table was created to help us describe each element based on its atomic weight and arranges them into different elemental groups. Remember that the atomic number of an element is equal to the number of protons, while the atomic mass is equal to the combined number of protons and neutrons in the nucleus. The first two columns in the Periodic Table are known as the alkaline earth metals (Fig. 16) and are very reactive, and sometimes explosive, elements. You may recognize these elements from the alkaline batteries you use in your household batteries. These are called alkaline because they typically use elements such as zinc and manganese dioxide in combination with an alkali electrolyte of potassium hydroxide. These alkaline batteries are also most frequently used in nonrechargeable “primary batteries”. These cells are not rechargeable because the active materials are used up during discharging process as the chemical reaction that provides the power dissolves the active materials. The alkali metals are also important because when they are combined into compounds they can create “salts” which are used as additives in the electrolytes for rechargeable lithium-ion batteries. But when it comes to lithium-ion batteries, the most important elements are found in rows three to twelve (Fig. 17). These are what are known as transition metals and are some of the materials we are most concerned with when discussing secondary (rechargeable) batteries. They are called transition metals because they transition from the left of the table to the right. You probably recognize many of these elements already as they are commonly used in many everyday devices ranging from your smart phone to your tablet computer to the wiring in your home. The transition metals tend to have electronic and/or magnetic properties which is why they are so commonly used. One form that an element can take is as an ion; an ion is an atom that has either lost or gained an electron. When we talk about lithium-ion batteries we are referring to batteries that use lithium atoms that have been converted into positively charged ions of lithium. Because it has either more or less electrons than protons an ion will

Periodic table of elements.

2.2 Chemistry

FIG. 15

29

30

CHAPTER 2 Electrochemistry basics

FIG. 16 Alkaline earth metals.

FIG. 17 Transition metals.

have a “net” electrical charge, it will either be positively or negatively charged. If the ion is negatively charged it has more electrons than protons having added electrons in the outer energy shell, if it is positively charged it has lost one or more electrons and now has fewer electrons than protons (Matson & Orbeak, 2013, p. 51). In order to remove or add an electron, it requires a certain amount of energy. This is known as the ionization energy which is the amount of energy necessary to remove an electron from an atom, the higher the ionization energy the stronger the electrons are

2.2 Chemistry

bound to the atom. The amount of energy required to remove the first electron is known as the first ionization energy. Ionization energy is measured in kilo-Joule’s per mole (kJ/mol), where a “mole” (or “mol”) is a unit of measure of molecules. There are 6.022  1023 molecules in a mol of substance. Just to put that in perspective, that is, 602,200,000,000,000,000,000,000 molecules in a mol. A kilo joule, or kJ, represents a thousand joules of energy. After removing one electron, the electron to electron repulsion energy is smaller in the electron cloud surrounding the nucleus which makes the electron cloud contract slightly making the atom (ion) get slightly smaller in size (Fig. 18) and the smaller the atom (ion) the greater the ionization energy is required to remove the next ion (Matson & Orbeak, 2013, p. 65). The further an electron is from the core, or nucleus, the more easily it will interact with other atoms and will be more likely to be impacted by the environment especially when they are not being influenced or hidden by other electrons. In other words, the further out from the nucleus the easier it is to remove the electron (Matson & Orbeak, 2013). Finally, in the field of chemistry we also need to discuss chemical bonding. Bonding describes how the atoms and ions can be “stuck” together to form molecules and compounds. These forces are what holds atoms together to form all solid matter. There are basically four types of forces that hold together matter: metallic, ionic, covalent, and Van der Waals (Hosford, 2013). Ionic bonding occurs when the valence electrons are either gained or lost creating cations and anions. Negatively charged anions and the positively charged cations are held together by an electrostatic force, known as ionic bonding. Metallic bonding occurs when atoms lose their valence electrons forming a negatively charged electron gas which will attract and hold together positive atoms. Covalent bonding occurs when two atoms share

FIG. 18 Anion and cation.

31

32

CHAPTER 2 Electrochemistry basics

valence electrons. Van der Waals is an electrostatic attraction of molecules but it is the weakest of these forms of bonding (Hosford, 2013).

2.3 Electrochemistry The combination of chemistry and electricity gives us the field of electrochemistry which has been defined as involving looking at the interactions between different materials that conduct electricity (Lefrou et al., 2012, p. 22). Electrochemistry tends to focus on the study of the electrical interface zones where two materials with different levels of conduction meet. To put it simply, think of this as the place where the surfaces of different materials touch—where the electrolyte touches the cathode or anode materials, and where the cathode or anode touches the current collectors. These regions are typically only a few nanometers thick, but it is where the magic happens. These are the interface zones where the ions and electrons transition from one material to the other. Electrochemistry is also heavily concerned with the study of the reduction and oxidation transformations that occur on the atomic scale and which we have learned to control, manage, and use through the use of a variety of electronic devices. Remember when we described batteries as devices that store chemicals that can react to store or generate electricity? The redox reactions are the chemical reactions we were referring to there (Lefrou et al., 2012; Salomon, 2011). In simple terms oxidation takes place when an element loses one or more electrons and an element is said to have been reduced when it gains one or more electrons. This terminology can be a bit confusing, reducing a material gains electrons and oxidizing a material loses electrons. In the lithium-ion battery the anode is where the oxidation side of the process takes place, giving up electrons when power is drawn from the battery and the cathode is the reduction side of the process which gains the electrons that the anode gives up during discharge. But something interesting happens here. In the industry we generally think of the anode as being the negative electrode and the cathode the positive electrode and we tend to think of this as being fixed. Well again, that is not entirely correct. We think of the anode as being the negative terminal because it has this excess of negatively charged electrons giving it a negative charge. This also means that the cathode has a net positive charge since it has given up its negatively charged electrons. But once the electrons flow into the cathode, the cathode actually has a net negative charge becomes the negative terminal, while the anode now becomes the positive terminal. Then the process is repeated during the charging cycle where the excess electrons and ions are forced from the cathode back into the anode, making the cathode once again have a positive charge and the anode a negative charge. This is also why we can say that oxidation always takes place at the anode and reduction always takes place at the cathode - the change during cycling. Remember Newton’s Third Law from your secondary school physics class, for every action, there is an equal and opposite reaction (Ewen & Heaton, 1981)? Well lithium-ion batteries also follow the laws of physics. In an electrochemical cell, for

2.3 Electrochemistry

every reduction reaction that takes place there is an oxidation reaction that must also occur. In other words, for every atom that loses an electron an atom in the opposing side must gain an electron (Lefrou et al., 2012, p. 16). We will look at this in a bit more detail in the next chapter. Current is created by the movement of various types of charge carriers. A charge carrier is simply a particle that is free to move and can carry a charge as it moves. There are several types of charge carriers including ions, electrons, and holes, but for our purposes we will limit our discussion to electrons and ions (Lefrou et al., 2012, pp. 17–19). The flow of current within an electrochemical cell is called mass transport kinetics. Mass transport refers to the movement of those molecules and atoms that have mass, usually lithium-ions and electrons in the case of lithium-ion batteries. But as long as the elementary charged species, either the cation, anion, or electron in this case, has mass, then the current flow is tied to the transportation of those materials through the cell. Mass transport kinetics may also be called a limiting diffusion current. This means that the mass containing particles are transported by parameters such as migration, diffusion, and convection and the rate that this can happen is proportional to the concentration of the consumed species because their mass transport limits the reaction rate and therefore the current flow (Lefrou et al., 2012, p. 87). To state this in simple terms the rate at which current can flow within an electrochemical cell is reduced the more the rate of charge transfer is increased. The faster the cell moves cations from the anode back into the cathode the quicker the cell will hit its limit and no longer be able to supply current. Ionic conductors, otherwise known as electrolytes, are the conducting mediums that act as the “super highway” for the ions and charged species to move through to reach either the anode or cathode active materials (Lefrou et al., 2012, p. 29). Without these conductive highways no current would be able to flow between the anode and cathodes. As an example of this, during the cell assembly process there is no voltage or current present in a cell that has been assembled until the electrolyte fill operation—there is simply no way for the electrons or charge carriers to get from one electrode to the other. There are a wide variety of ionic conductors depending on the type of electrochemical cell, including liquid electrolytic solutions, molten salts, solid oxides, and polymer electrolytes. Chapter 7 will discuss the types of electrolytes that are typically used in lithium-ion cells in more detail. Finally, in electrochemistry the flow of current is similar to ionization energy that was described earlier. Current flow is a measure of the rate that the electrons flow through a point over a period of time. Electrochemistry uses the term ampere to measure current flow. One ampere of current is equal to 6,250,000,000,000,000,000 (6.25  1018) electrons flowing past a fixed point in one second. Just to put that into perspective, a typical automotive EV battery cell used in the Chevrolet Bolt is a 60ampere hour (Ah) cell. That means that in one second 375,000,000,000,000,000,000 (3.75  1020) electrons will pass through a point in this cell. That’s a lot of electrons! Which is why using a term like ampere to describe the current flow makes it a lot easier to think about and calculate it.

33

34

CHAPTER 2 Electrochemistry basics

2.4 Electromotive force (voltage) Voltage is sometimes also known as an electromotive force; there are a handful of ways that electromotive force can be created including by friction, pressure, heat, light, magnetism, and chemical action. Voltage created by friction is most easily seen in the form of static electricity. Think of rubbing a balloon on your head. As you do this electrons will flow between your hair to the balloon. Since the voltage that can be created in this manner is very low, it is not usable for industrial applications but can be great fun at children’s birthday parties. However, let me say that we do use a form of static electricity in the double layer capacitors known as ultracapacitors and supercapacitors. The ultracapacitors differ from the lithium-ion battery in that, very simply put, they store electricity only on the surface of their electrodes and not through a chemical reaction. This is why the capacitor has such great power density and cycle life, but such poor energy density. Creating voltage by friction is called piezoelectricity. The term is derived from the Greek word peizein meaning to squeeze or press things together, hence squeezing materials to create electricity. It may sound odd, but this occurs when a pressure is applied to a material compressing it from a noncrystallized to a crystallized form. In materials science the term crystallized simply means an orderly arrangement of atoms. Materials can be crystallized when put under pressure which will cause their molecules to become arranged in a uniform and orderly manner such as is shown in the example in Fig. 19. This example shows an orderly grouping of copper atoms in a “crystal” structure. Diamonds are an example of a crystallized form of carbon.

FIG. 19 Crystalline structure of copper molecules.

2.4 Electromotive force (voltage)

Compressing an element into a crystal structure in some materials can cause the free electrons to be released causing current flow but it is generally not very usable for large-scale industrial applications since the amount of electricity generated is pretty low. Voltage potential can also be created when heat is applied to a conducting material. For instance, if high temperatures are applied to a copper wire it will excite the atoms and the free electrons will flow away from the hot end of the conductor toward the cooler end. One of the most common uses for this form of energy creation in industrial applications is seen in the use of thermocouples or temperature sensors. Heat generated in a battery pack will be identified by these temperature sensors when the materials in them begin to get excited as they heat up thereby sending current and voltage back to the controls system and identifying changes in temperature. But again, energy generated through heat is quite low and in general not usable for large-scale energy generation purposes. Voltage potentials can also be created through the use of light, otherwise known as photovoltaic energy. When the photons, one of the universe’s building block elementary particles, within light hit certain substances they may dislodge some of the free electrons within the atoms of the photovoltaic cells, thereby creating a voltage and electromotive force. The most common application of this today can be seen in the use of solar panels as a source of energy generation. In this case the photons hit the surface of the solar panels and dislodge some of the free electrons sending them through the conductors creating electricity. Much work has been done over the past 20 years or so to improve the efficiency of photovoltaic cells to make them efficient enough to generate usable amounts of electricity cost effectively. Today these photovoltaic cells have become quite commonplace in many regions of the world as a means of self-generating electricity. Perhaps the most commonly used form of electromotive force is through the use of magnetism which is tied directly to electricity. There are several ways that a magnetic field can be created. The first type of magnetic field will occur when the atoms within a ferromagnetic material such as iron are aligned this will create a magnetic field around that object. Think of the common household magnet like the one you may use on a refrigerator to showcase your child’s drawings. In this case the magnetic field is permanent and because the charge in the magnet is opposite of the charge in the material in the refrigerator door, the magnet will stick in place. Another method for creating a magnetic field is through the motion of electrons or ions through a conductor such as in an electric motor or generator. We can use this electromotive force to generate electricity, but in some cases we must also protect other pieces of equipment from it due to the amount of electricity traveling through wires in a battery. In industrial electronics we call that electromagnetic interference (EMI) and often needs to be controlled to protect the electronic controllers from being impacted. Virtually every electrical wire will generate some amount of electromagnetic field. But thanks to the early work by people like Nikola Tesla and Thomas Edison we have also learned how to use this force to create large amounts of electromagnetic forces to create usable energy. When a conductor is moved across a

35

36

CHAPTER 2 Electrochemistry basics

magnetic field it will cause electrons in the conductor to be forced through that conductor creating the electromotive force. This is what happens in an electric motor or generator when they spin. An electromagnetic field is created as the rotor spins that causes the free electrons in the wires in the stator, the stationary part of the machine, to be released thereby creating an electric current. We see this every day in the electric alternators and generator used in our automobiles and on a much larger scale in the dynamos and generators that create the electricity that comes to our homes (Gibilisco, 2012). Finally, electromotive force may be created as the result of a chemical reaction process. This is the case in all batteries where the chemical process of reducing and oxidizing ions creates a current flow within the electrochemical cell. This will be discussed later in this chapter in much more detail (U.S. Naval Personnel and Staff of Research & Education Association, 2002).

2.5 Electric current Earlier we introduced electric current as being related to the flow of electrons. Current flow is measured in amperes, named after French physicist and mathematician Andre-Marie Ampe`re who made significant achievements in the study of electricity in the 18th and 19th centuries including inventing the solenoid and the telegraph. We already defined an ampere is defined as a flow of 6.28  1018 electrons, or 6,280,000,000,000,000,000 electrons, per second flowing past a fixed point on a conductor. When we apply the concept of time to the current flow we end up with a coulomb, which is when 1 A of current (6.28  1018 electrons) flows through a circuit for one second. So remember that amperes measure the number of electrons that pass by a point and coulombs measure how many electrons pass by a point over a fixed period of time (Gibilisco, 2012; U.S. Naval Personnel and Staff of Research & Education Association, 2002). This current flow comes in two types, a current it is either a Direct Current (DC) or an Alternating Current (AC). In its most simple definition, a direct current is one that only flows in a single direction. This is the type of current generated by batteries, fuel cells, ultracapacitors, and solar panels. Think of DC like a one-way street sign, the current and electrons will always flow from the negative electrodes or terminals to the positive electrodes or terminals during discharge. And the current will be the same, so a 50-A discharge will always be 50-A with very little variation. DC current is used in applications where the power demand and generation are at the same voltage. For instance, if the battery in your electric car operates at 355 V then the most efficient solution is to use an electric machine (motor) that operates at the same 355 V. For large power transmissions systems, DC power would be very inefficient as everything on that line would have to operate at the same voltage and the inherent resistance within the power lines would reduce the voltage directly proportional to the length of the power line. Now there are ways that you can get around this like

2.5 Electric current

converting DC to AC using inverters or resistors to reduce or increase the voltage. But most DC systems, think battery operated, are built with the DC voltage matching the demand voltage. An alternating current is one that reverses upon itself from time to time, flowing both forward and then backward. Now “from time to time” is not exactly correct either. An AC current is not random, rather it reverses upon itself at regular intervals. For example, if you are in the United States the power that comes to your house reverses upon itself about 60 times per second (60 Hz) or if you are in Europe it reverses upon itself about 50 times per second (50 Hz). AC current is used in power transmission since it can be transmitted at very high voltages and at very low current with little resistance in the transmission lines and the voltages can easily be reduced anywhere in the system to meet different needs. For example, the voltage in your local power lines is in the thousands of volts, but through the use of step-down transformers in your neighborhood the voltage coming into your house (in the United States) is only about 120 V. Another important topic is where the electrons flow and where they go during operation because there are some confusing things that happen here. First, understand that the reason electrons flow from the anode to the cathode is because there is an excess of negatively charged electrons in the anode. When a power source is attached to the battery to discharge it, the electrons flow out of the anode, through the current collector and through the power supply and into the cathode which has a deficiency of electrons since it gave them all up during the charge. The excess electrons from the anode flow in to fill the open spaces on the cathode. And while the electron flow is believed to be what causes the current to flow, it is important to note that they do not flow in the same direction. As the oxidation process releases electrons they flow through the current collector and through the circuit and back into the other electrode where they are reduced. But the current flows from the reducing electrode into the circuit—in the opposite direction of the electron flow (Figs. 21 and 22). Current flows in the opposite direction of the electron flow

In terms of current flow, one other topic which is worth reviewing is the actual path of the current as it flows through the electrodes. Most images tend to show the electrode active material as being a solid mass; however, when we look at it with an electron microscope we see that the active materials are actually nanoparticles that are pressed together. Since it is not a solid mass, the current cannot flow directly through the material, but instead it can only flow through those particles that are in contact with one another. Fig. 20 shows an example of what this might look like with the arrows representing both the current paths and the electron paths through the material. This is also the reason that electrodes are pressed or calendared, to give them better conductivity.

37

38

CHAPTER 2 Electrochemistry basics

FIG. 20 Current path through active material.

2.5.1 Discharging the battery In an example of an external circuit, the current always flows from the positive pole of the power supply toward its negative pole. This corresponds to discharging mode of the electrochemical battery where the ions flow from anode to cathode through the electrolyte and separator while the current carries the free electrons out through the current collector and to the motor to provide work. During discharge mode, as shown in the example in Fig. 21, the current and electrons flow from the positive terminal to the electric machine using it. While at the same time the negatively charged lithiumions flow from the anode, or negative electrode, to the cathode, or positive electrode carrying electrons with them. Think of this as pulling the energy out of the battery, in its natural state the electrons and ions don’t want to leave the anode. But when we put an electromotive force onto it the ions are pulled out of the anode and into the cathode bringing with them a number of free electrons, which causes the current to flow to power the electric machine. When the electrochemical cell is operating in this manner as a power source (discharging) electrons are being drawn toward the positive electrode. Since there are no free electrons in the electrolyte and because they cannot accumulate at the cathode surface only a reduction reaction can use the electrons arriving at the interface (Lefrou et al., 2012, p. 33).

2.5.2 Charging the battery Charging the battery is the opposite reaction. In order to cause a current to flow in an electrochemical cell during the charging mode of operation, an additional electric energy supply, such as a battery charger or generator, is needed to produce the reactions needed to create the current flow. In this case the electrons leave at the negatively charged cathode, which makes it positively charged again, and are transferred through the current collectors and copper lines to the positively charged anode, which in turn makes the anode once again the negative terminal (Lefrou et al., 2012,

2.5 Electric current

FIG. 21 Current flow during discharge mode.

p. 32). The free electrons flow into the anode from the charging source, causing the reduction reaction to occur at the interface between the anode and electrolyte (Fig. 22). Electrons only flow through the current collector and flow in the opposite direction of the ion flow which creates the current field Ions flow through the electrolyte and separator from one electrode surface to the other

2.5.3 Efficiency and voltage As discussed previously, the voltage of an electrochemical cell calculates the difference between the two electrode potentials and expressed in volts (Lefrou et al., 2012, p. 26). This voltage difference is usually called a “drop” between the electrodes and it exists due to the different potentials of the different materials used in the cell. When talking about voltage drop between two electrodes of a battery cell, it can be split into two parts: the sum of the interfacial voltages and the ohmic drops between the terminals. The interfacial voltages simply refer to the different voltages of the different “interfaces” between the materials within the electrodes. An interface is the area where two surfaces come together. In the case of a lithium-ion battery we start with the interface between the aluminum current collector and the active anode materials, then the interface between the active anode materials and the electrolyte, then interface between the electrolyte and the active cathode materials, and then interface between the active cathode materials and the copper current collector. Each of these

39

40

CHAPTER 2 Electrochemistry basics

FIG. 22 Lithium-ion cell in charging mode.

areas within the cell has slightly different voltages which when added up create the first part of the voltage difference between the terminals. Ohmic drop is the second part of that equation, literally. The ohmic voltage drop is the difference potential between the terminals of a given conductor volume that is part of a chain with a current flowing through it. Ohmic drop is caused by the resistance of the electrolyte, the distance between the electrodes, and the magnitude of the current. The ohmic drop across a conducting volume is always proportional to the amount of current flowing through it and the current density of each charge carrier is proportional to the electric field that is generated (Autolab B.V., 2011; Lefrou et al., 2012, pp. 72–73, 75). What that means is simply that there is a second voltage drop that must be added to the interfacial voltages. It is created due to the resistance of the electrolyte and the distance between the electrodes. The voltage in a lithium-ion cell will increase when the electrochemical cell is in charging mode which triggers the electrochemical redox reactions. In both cases, charging and discharging, there is an energy loss that occurs in the form of heat that is generated during these processes (Lefrou et al., 2012, pp. 79–80). This energy loss is also described as the efficiency of the cells. This energy transformation into heat during the charge and discharge process is due to what is known as the Joule effect which describes the rules around heat generation in relation to the current flowing through a conductor (Lefrou et al., 2012, p. 85). When we talk about lithium-ion battery efficiency energy losses, the conversion to heat is the major factor causing this reduction in battery efficiency. The Joule effect is the heat generated due to internal resistance of the cell, the amount of current, and the duration of time it takes.

2.5 Electric current

So greater internal resistance or higher currents over longer periods of time will generate more heat than lower resistance, lower current, and slower current flow. The chemical reactions within the battery cells continue to occur at very slow rates and cause the lithium to become frozen or locked in place and no longer mobile. This is why many battery cells experience various amounts of temporary and permanent capacity loss and degradation when stored for long periods of time at high levels of charge. In other words, electrochemical cell efficiency describes how much energy gets lost by being converted to heat during the charging and discharging process. But don’t despair, in most lithium-ion batteries this only results in a few percent efficiency loss. Most lithium-ion batteries are very efficient, operating at efficiencies greater than 98%.

41

CHAPTER

Lithium-ion battery operation

3

Chapter outline 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9

Where is the lithium in lithium-ion? ................................................................... 44 How do the anode and cathode work together? ................................................... 45 Reduction/oxidation (redox) process ................................................................... 48 Intercalation ..................................................................................................... 51 Cations and anions ............................................................................................ 52 Solid electrolyte interphase ............................................................................... 53 What does “nano” mean? .................................................................................. 56 Thermodynamics ............................................................................................... 59 Failures modes ................................................................................................. 60 3.9.1 Internal short circuit ........................................................................63 3.9.2 External short circuit ........................................................................66 3.9.3 Thermal runaway .............................................................................67 3.9.4 Cascading failure .............................................................................68 3.9.5 Impact and effects of temperature on cell aging .................................70 3.9.6 Impedance ......................................................................................71 3.9.7 What happens during overcharge .......................................................71 3.9.8 What happens during overdischarge ..................................................72 3.9.9 Influence of impact, crush, and penetration .......................................74 3.9.10 Aging mechanisms .........................................................................74

I was recently asked a good question “why lithium”? Lithium is the lightest metal on the periodic table with an atomic number of three, with only the gases helium and hydrogen having lower atomic weights (two and one, respectively). Being one of the alkali earth metals means that lithium is also highly reactive. This combination of low weight and high reactivity means that lithium contains more energy per unit of weight than other metals which is what makes it such a good element to use for energy storage. In comparison, nickel has an atomic number of 28 and lead has an atomic number of 82. Remember that the atomic number represents the number of protons in the nucleus of the element. These higher atomic numbers correspond to the differences in energy density in electrochemical cells of each. Lead acid batteries, with lead’s atomic number of 82, offer on average about 40 Wh/kg cell energy density and nickel metal hydride, with nickel’s atomic number of 28, falls in between lead and lithium with average cell energy density of about 80 Wh/kg. Lithium-Ion Battery Chemistries. https://doi.org/10.1016/B978-0-12-814778-8.00003-X # 2019 Elsevier Inc. All rights reserved.

43

44

CHAPTER 3 Lithium-ion battery operation

FIG. 23 Lithium atom.

Another reason lithium is particularly well suited for electrochemical energy storage is that it has one valence electron. You may remember from Chapter 2 valence electrons are the ones that are “mobile” and can be lost or gained the easiest. The lithium atom contains three electrons that circle around a nucleus containing three protons and three neutrons. These three electrons orbit around the nucleus in two energy bands. The electrons in the innermost band are not mobile, but the electron in the outer band is mobile making it a valence electron. There is also a “hole” in that outer energy band which can accept an electron. By accepting an electron it would have one more electron than proton giving it a negative charge and making it an anion. However, if that single outer electron is removed it has one more proton than electron making it positively charged and a cation (Fig. 23).

3.1 Where is the lithium in lithium-ion? This is another question that comes up quite often, where is the lithium in a lithiumion battery? The question came up a lot right after Canada issued a ban on the air transport of lithium metal batteries in 2016 (Roy, 2016). In this case they were concerned mainly about primary, or nonrechargeable, lithium-ion batteries that used a lithium-metal anode because in its metallic form lithium metal is known to be quite reactive to oxygen and water. Lithium metal has been used in anodes for many years in primary batteries but until recently has been limited in use in secondary batteries because lithium metal is extremely reactive to both oxygen and water. However, in secondary, or rechargeable, lithium-ion batteries there is no lithium metal at least as of today. Future solid-state batteries may use lithium metal as anode

3.2 How do the anode and cathode work together?

electrodes again but today, instead of being used as the metal anode the lithium-ions are mixed into the electrolyte in the form of lithium salts with most of it going into the cathode active materials (Dahn & Ehrlich, 2011). During the initial formation process the lithium is forced out of the cathode and through the electrolyte and separator and into the negative anode during a controlled charge and then back into the cathode during a controlled discharge. It is this process that creates the solid electrolyte interface (SEI) layer, which is essentially an electrochemical process that creates a coating on the anode materials that is made up of lithium-ions that form permanent bonds with the molecules in the anode. This results in some loss of lithium during these first cycles and a permanent loss of initial capacity as the SEI layer is formed. The lithium used in lithium-ion batteries is positively charged cations. During cycling the positively charged lithium-ions are forced by way of the redox process out of the active material on the anode through the electrolyte and separator and are reduced at the cathode. The process is then repeated with the lithium-ions traveling from anode to cathode and back again. Remember that because negative and positive particles are attracted to each other, the lithium cation will always be pulled toward the negative electrode which, as we talked about in the previous chapter, changes during operation. We will review the redox process in more detail in an upcoming section of this chapter. Some cell manufacturers are beginning to use a process called prelithiation wherein lithium in its powder form is added into the anode during electrode processing. This acts to pump up the amount of lithium in the cell and increasing the energy density by storing some active lithium in the negative electrode prior to charge/discharge cycling. This helps to reduce the first cycle loss due to the lithium being consumed in the SEI formation process and lithium that is lost due to the aging processes. And while most prelithiation is mainly being done to the anode, but it is also being done to the cathode side by “over-lithiating” the cathode (Holtstiege, B€armann, N€ olle, Winter, & Placke, 2018). However, there are still some question as to the impact on cycle life when you overlithiate the anodes in this manner.

3.2 How do the anode and cathode work together? The anode and cathode work together like opposite poles of a magnet, sending the lithium-ions back and forth between them each time releasing electrons which create the current flow. In fact, when a metal electrode is immersed in an electrolyte, the electronic charge on the metal attracts ions of the opposite charge and orients the magnetic poles (dipoles) of the elements in the solvent. There is a layer of charge in the metal electrode and a layer of charge in the electrolyte. This charge separation establishes what is commonly known as the “electrical double layer” between the electrodes (Salomon, 2011, p. 2.10). But this small amount of magnetic attraction is only one of the driving forces moving the lithium-ions back and forth and generating current.

45

46

CHAPTER 3 Lithium-ion battery operation

FIG. 24 Active material with lithium-ions inserted.

The basic function of the anode and cathode, the negative and positive electrodes, is to act kind of like a lattice framework to hold the lithium-ions as they pass back and forth. The lithium-ions fill into the gaps within the active materials similar to what is shown in the example in Fig. 24. In this over-simplified example imagine that the large, dark colored spheres represent the “active material” either anode or cathode material. The silver spheres represent the lithium-ions so in this example we see them filling in the voids between the active materials. When the silver colored lithium-ions are moved from anode to cathode during discharging those ions travel from one side of the electrode to the other and attach to the cathode material surface. This process of attracting the ions to the surface of the active material is known as adsorption or intercalation. Adsorption, different from absorption, is the process when atoms or ions get attracted to the outside surface of a material and collect on the surface of the material rather than entering or diffusing into the material (Julita, 2010). In the case of adsorption reaction of a molecule in a lithium-ion cell, the adsorbed species, lithium-ions in this instance, occupy vacant sites on the electrode surface, thus partly blocking it as shown in the example in Fig. 24 (Lefrou, Fabry, & Poignet, 2012, p. 66). While these electrodes may seem to be very simple solid pieces, they are not quite as simple as they would appear at first glance. Each electrode is a composite body made of a combination of active material, binder, performance enhancing additives, and conductive filler and a conductive substrate (Salomon, 2011, p. 2.1).

3.2 How do the anode and cathode work together?

The conductive substrates, most often copper and aluminum, are also coated with a conductive material, usually a form of carbon, that helps bind the active materials to the substrate. As a rule, cell designers prefer to design electrodes with large surface areas since this will act to minimize the energy loss due to the activation and concentration polarizations at the electrode surface that occur as the SEI layer is formed and to increase the electrode efficiency or utilization. This is most often accomplished by using a porous electrode design. A porous electrode consists of porous matrices of solid and void spaces. The electrolyte penetrates the void spaces of the porous matrix. A porous electrode can provide an interfacial area per unit volume several times higher than that of a planar (flat) electrode. What this all means is that a high surface area allows for more electrolyte to penetrate it and it is through the electrolyte that the lithium-ions can travel into the active material thus storing more energy. Reactions at an electrode surface are characterized by both chemical and electrical changes. Electrode reactions can be as simple as the reduction of a metal ion and incorporation of the resultant atom onto or into the electrode structure (Salomon, 2011, p. 2.4). But in this type of active porous mass, the mass transfer condition in combination with the electrochemical reactions that occur at the interface can also be very complicated (Salomon, 2011, p. 2.18). The reactive interface surfaces are those where at least one chemical reaction occurs (as shown in Fig. 25) and the current circulation through an interface involves one or more electrochemical reactions, which tend to be heterogeneous in nature. These chemical reactions include, of course, the reduction and oxidation processes. However, depending on things like rate and temperature other chemical reactions could take place and these are typically called “side reactions” as they are not the intended reactions. Studying the reactions

FIG. 25 Interfacial areas.

47

48

CHAPTER 3 Lithium-ion battery operation

that occur at these interfaces is extremely important in order to ensure high efficiency and long life of a lithium-ion battery. The more efficient the reactions at the interfaces the better the battery performance will be. The movement of the lithium-ions is referred to as mass transport and can be classified into three categories. The first is called migration which is the movement of charged ions due to an electric field. The second mass transport process is diffusion which is the movement of ions that are submitted to a concentration gradient, in this case the ions diffuse from areas of greater concentration to areas of less concentration. A diffusion layer is generally next to the interface layer, which may also see the other two forms of mass transport, migration and convection. The thickness of the diffusion layer is generally only a few nanometers (μm) thick. The third type of mass transport is from convection, which is the movement of the medium when it is in a liquid form (Lefrou et al., 2012, pp. 61–62). These processes that occur during mass transport where cations and anions are transported to and from the electrode surface are perhaps the most important of all processes that occur in a lithium-ion cell because without them, none of the other process can take place (Breitkopf & Swider-Lyons, 2017).

3.3 Reduction/oxidation (redox) process The next process that we must understand in electrochemical lithium-ion cells, after adsorption, is what is referred to as the redox process. This is the reduction and oxidation of ions. Historically the term oxidation referred to an element that reacted with oxygen and reduction referred to a metal that was produced from its mineral ore (Matson & Orbeak, 2013, p. 68) but today these refer to the reduction and oxidation processes that occur as the lithium-ions are transferred from anode to cathode and back. It sounds a bit backwards, but if the atom gains electrons it is said to have been reduced and if the atom loses an electron it is said to have been oxidized. It is important to note that the reduction reaction always takes place at the cathode and the oxidation reaction always takes place at the anode. But to those of us that are not chemists this is a very confusing statement because we think of the cathode as being the positive active material such as NMC or LFP and the anode as being the negative active material, usually graphite. But in actual operation we end up with a cathodic reaction (reduction) that takes place on one electrode and an anodic reaction (oxidation) that takes place on the other. During charging the cathodic reaction takes place in what we refer to as the anode since it oxidizes and releases electrons while the anodic reaction takes place in the cathode since it reduces and gains electrons. When the cells switch to discharging the cathodic reaction takes place at what we would consider the cathode as it is here that the electrons are gained as the electrons follow the circuit from the anode side where an anodic oxidation reaction occurs to release these electrons (Fig. 26). If these reactions did not change between electrodes like this then the cells may not be able to recharge; once the electrons were spent the cell would be dead and quite useless. This gets quite confusing once we start talking

ReDox operations.

3.3 Reduction/oxidation (redox) process

FIG. 26

49

50

CHAPTER 3 Lithium-ion battery operation

about the active materials, so moving forward we will use the term cathode to refer to the electrode with the active cathode materials such as NMC, NCA, LFP, LMO, or LCO and we will use the term anode to refer to the electrode with the active materials of graphite, carbon, silicon, or titanium. Reduction always takes place at the cathode and oxidation always takes place at the anode… but anode and cathode change during charging and discharging When current flows through an electrochemical cell one of the electrodes becomes the anode and the other the cathode as stated above, but the link between the voltage of an electrode and its role as an anode or cathode is not always constant. As we talked about previously, depending on the operating conditions of the system, charging or discharging, an electrode can be either the anode or the cathode and will change its voltage potential accordingly. The current is positive if the electrode is on the oxidation site, the anode. The current is negative if the electrode is on the overall reduction site, the cathode (Lefrou et al., 2012, pp. 27–28). Oxidation occurs when elements combine with other elements that are more electronegative and reduction occurs when an element combines with another element that is less electronegative. Remember that the lithium-ion is a cation, positively charged, so it will always be pulled toward the more electronegative electrode. Oxidation can occur in one of three ways, either gaining oxygen, losing hydrogen, or losing an electron and reduction occurs in the opposite manner, losing oxygen, gaining hydrogen, or gaining an electron. The reduction/oxidation processes are shown together since if an atom loses an electron the other must gain it to ensure the balance is maintained. This is what causes the cathode and anode to “swap” during operation; once all of the free valence electrons have been released from the anode during discharge they flow through the current collector through the electrical machine and back into the other electrode, the cathode where the reduction reaction occurs. Therefore when the free electrons are all released from the anode the cell is discharged. During charging the opposite reactions occur; forcing a current into the cathode releases the electrons which pass through the circuit and into the anode material. These movements of the lithium-ions are also sometimes referred to as the “rocking chair” effect or as “shuttling” of lithium-ions due to the back and forth nature of the reactions. The simple figures later show how the rocking chair picks up the lithium-ions at the anode and then as the chair rocks forward drops off the lithium-ions at the cathode (Fig. 27). The other term that you may hear frequently is that of mass transport, which is simply the movement of any element, atom, or ion that has mass through some form of medium. Mass transport to or from an electrode can occur in one of three different ways: (1) through convection and stirring, (2) through electrical migration in a voltage potential gradient, and (3) through diffusion in a concentration gradient (Salomon, 2011, p. 2.15). Diffusion processes are typically the mass-transfer

3.4 Intercalation

FIG. 27 Lithium-ion rocking chair effect.

processes at work in the majority of battery system where the transport of species to and from reaction sites is required for maintenance of current flow (Salomon, 2011, p. 2.16). It is the redox reaction and that movement of electrons that generates the electric field and forces the current flow in the battery cell. The electric current produced between two electrodes can be determined by looking at how many volts are produced from the transfer of electrons. When you measure this voltage, you can determine how many volts are required to reduce or oxidize an element (Matson & Orbeak, 2013, p. 75). In the lithium-ion battery you want the cathode to have the more positive reduction potential since cell reduction occurs in the cathode and oxidation occurs in the anode. The number of electrons that can be moved is determined by how many “empty holes” are left in the valence shell of the atom/ion. Referring back to Chapter 2 we saw that the lithium atom carries three electrons, one of which is a valence electron. It is the valence electron that determines how many electrons the ion can transport. In the case of an electrically neutral lithium atom, we see that in the valence shell, or outer shell, there is only the one electron; however, there are seven empty holes in that second energy shell that can be filled with electrons during charging and discharging as shown in Fig. 28. The next factor that affects how many electrons an atom can carry is the diameter of the nucleus, the smaller the atomic radius the fewer electrons it has room to hold. Lithium has an atomic radius of 151.9 pico-meters (pm).

3.4 Intercalation At the beginning and end of the RedOx process the lithium-ions get inserted into the anode or cathode in a process called intercalation. This rather confusing looking term that simply means that the lithium-ions are inserted or removed from a host material. Since the lithium-ion is such a small particle it easily intercalates into the much larger

51

52

CHAPTER 3 Lithium-ion battery operation

FIG. 28 Valence electrons and holes in valence shell.

compound materials. This is a reversible process, which allows the repeating charge and discharge of the cell. The lithium-ion must also be inserted without creating significant structural changes in the host materials otherwise it will damage the host material and reduce the cycle life. In the case of lithium-ion batteries the cathode active materials and the anode active materials are the hosts. The metal oxide materials in the cathode have either a layered- or tunnel-type crystal formation which allows the lithium-ions to be inserted in between the layers or into the tunnels. The crystal structure of the carbon-based anodes also has a layered structure which allows the lithium-ions to be inserted in between the layers of active material (Dahn & Ehrlich, 2011).

3.5 Cations and anions Building on the last section and as we discussed in Chapter 2 when an atom carries excess negatively charged electrons the atom becomes a negative ion, otherwise called an anion. When an atom loses its negatively charged electrons it becomes a cation. It is really a lithium cation that is moving from the anode to the cathode and back in the lithium-ion cell. During these reactions, the ions create a temporary bond with the active materials, this is a form of ionic bonding which occurs when atoms donate or receive electrons rather than share them (Matson & Orbeak, 2013, p. 68). Therefore, a lithium-ion is simply a lithium atom that carries either a positive or negative electric charge. Using the lithium atom as an example, we know that it has three protons in the nucleus which are positively charged. We also know that it has three electrons, which are negatively charged. Therefore in its base condition it is

3.6 Solid electrolyte interphase

FIG. 29 Lithium cation and anion.

electrically neutral (3 “P +” and 3 “E ”). It becomes an ion, or a cation, by giving up one of its electrons. Now it has three protons and only two electrons giving it a net positive charge of plus one (3 “P+” and 2 “E ”). On the other side of the equation it becomes an ion, or an anion, by gaining one (or more) electrons. Now it has three protons and four electrons giving it a net negative charge of minus one (3 “P +” and 4 “E ”) as shown in Fig. 29. In the case of the migration form of mass transport discussed earlier, negatively charged anions and positively charged cations move in opposite directions. Positive charges migrate in the same direction as the electric current and negative charges move in the opposite direction as shown in Fig. 30. The directions that the ions move are linked to the direction of the current flow. Simply stated the positively charged cation is always drawn to the more electronegative electrode. In general then we can say that anions migrate toward the cathode and cations migrate toward the anode (Matson & Orbeak, 2013, pp. 43–46). But remember that while I am using a lithium cation and ion in my image here, the lithium-ion is always a cation. But there may be both cations and anions of other species and elements that are in the active materials, electrolyte, and salts.

3.6 Solid electrolyte interphase During the cell formation process most lithium-ion chemistries undergo a chemical reaction that generates a protective layer at the interface of the anode material and the electrolyte which is known as the SEI layer. The growth of the SEI layer is a result of

53

54

CHAPTER 3 Lithium-ion battery operation

FIG. 30 Cation and anion movement during charging.

the chemical reaction between the anode and the electrolyte solvents that is created as lithium-ions from the cathode and the salts effectively get “stuck” in the surface layer of the anode. This is a bit of a simplification as there is much more than just lithium getting stuck at the surface, in fact the SEI layer consists of a combination of materials including reduced or decomposed solvents in the electrolyte, salts used in the electrolytes, lithium-ions, and the impurities in the electrolyte (An et al., 2016). Once formed, the SEI layer “impedes” the flow of lithium-ion between the anode and cathode. The length of time that the cells are in the formation process will determine how thick the SEI layer forms. The thicker the SEI layer the greater the impedance to the lithium-ion flow which ultimately will reduce cycle life. The SEI layer is formed during the first charge/discharge cycle that the cell undergoes during what is known as the formation process. The specific charge rate and amount of charge is different for most cell manufacturers due to the differences in their chemistries and in fact is often a pretty highly guarded secret. However, in most cases the first cycle is generally a very slow charge and equally slow discharge, usually several hours (or at very low C-rates). The speed at which the SEI layer is formed determines the thickness of the layer and will impact the capacity of the cell. In general, a first cycle charge rate of 0.05C up to 0.2C is preferred to create the best SEI layer. Higher C-rates in the first cycle tend to reduce capacity and form a less stable SEI layer. By less stable, I mean that the SEI layer may be thinner and may be susceptible to crack and will then have to recreate new SEI to fill the cracks which

3.6 Solid electrolyte interphase

will consume free lithium and thus reduce the cell capacity. Therefore SEI development is very closely monitored at all cell manufacturers and is part of their “secret sauce” for manufacturing the cells. But it is important to note that in most cases the SEI will continue to develop during subsequent formation cycles at different rates and different temperatures until it is generally fixed. This is the reason many cell manufacturers go through two formation cycles. The first is to create the SEI layer, which is followed by a degassing operation, the second formation cycle occurs after the degassing operation and helps to set the final SEI thickness and determine the capacity, voltage, and impedance of the cell (An et al., 2016). Another important point is that during this first cycle, the cell may lose up to 10% of its original capacity as the lithium is consumed in the creation of the SEI layer. This is referred to as irreversible capacity loss as it can never be regained (An et al., 2016). Most of us tend to use a very simplistic view of the cell materials and view the active materials as a solid mass with the SEI layer forming right on the surface of that mass. The active materials are not really a solid mass as shown in Fig. 31 which shows the different components of the cell, but instead the active material is a composite material made of compressed molecules. The SEI layer is a small layer that forms on the surface of the molecules as shown in Fig. 32. In the following image you see a thin layer that forms on the surface of those molecules that are exposed to the electrolyte interface layer. It does not form throughout the entire material thickness. One of the most unique properties of the SEI layer is that it is both electrically insulating but still permeable enough to be ionically conductive; it should also be no more than a few angstroms thick (a few tenths of a nanometer), have high strength, be tolerant to expansion and contracting during operation; it should be insoluble to the electrolyte after formation; and it should be stable over a wide range of temperatures

FIG. 31 Cross section of lithium-ion cell.

55

56

CHAPTER 3 Lithium-ion battery operation

FIG. 32 SEI layer.

and voltage potentials (An et al., 2016; Huggins, 2009). This means that it provides a level of protection against internal short circuit to some minor extent, but still allows the lithium-ions to pass through it. The SEI must also entirely cover the surface area of the anode material to prevent continuous decomposition of the electrolyte that was not initially protected (An et al., 2016). If not completely covered the cell will continue to lose lithium during the cycling process and gases may continue to form as the lithium is consumed to continue to create SEI—both of which will drive the cell into an early failure. It is also important to note that the during the SEI layer formation process there are frequently some amount of gases that are generated as a by-product of the chemical reactions occurring. This is why lithium-ion cells need to go through a degassing process after the SEI formation period and prior to final sealing of the cells. This process allows these gases to be removed through a vacuum process to prevent them from causing side reactions of reducing the life of the cell.

3.7 What does “nano” mean? The term “nano” is used a lot today, especially when referring to advanced technologies such as batteries. But what does it really mean? The name comes from the ancient Greek word nanos, meaning “dwarf.” But in simple terms it just means something that is really, really small. Nano is short for nanometer, which is a unit of measurement representing one billionth of a meter (0.000000001 m). The typical range of sizes for something to be considered a “nano” is between 1 and 100 nm. When it is used in terms like nanoscience and nanotechnology it is referring to phenomena that occur on the atomic scale. To put it in perspective the deoxyribonucleic acid (DNA) found in the human body has a diameter of about 2.2–2.6 nm while a lithium atom has

3.7 What does “nano” mean?

diameter of about 0.3 nm and the electrons within those atoms have a diameter of only 0.000001 nm. So your DNA is two hundred and sixty million percent (260,000,000%) larger than the electrons we are dealing with in the lithium-ion battery, it is even 867% bigger than the lithium atoms. In his introduction to his book “Nanomaterials for Lithium-ion Batteries” (Yazami, 2014) Rachid Yazami describes the benefits of nanomaterials as offering both increases in energy and power density as compared to micron-scale technologies. These ultra-small materials can provide additional sites for storing lithium due to the high ratio of surface area to volume. Additionally, nanomaterials are believed to be able to fill those sites much faster than other technologies. Both features offer the benefit of increasing energy and power density. Nanomaterials can be applied to the cathode, anode, separator, or other components. Specific to advanced lithium-ion batteries there are many different types of nanomaterials, including nanotubes, nanorods, nanowires, nanoflakes, nanoparticles, nanosheets, nanoshells, nanoribbons, and nanospheres. Researchers are developing many unique form factors of nanomaterials to achieve different characteristics. Nanomaterials fall into two major categories based on the types of materials that are used such as carbon nanotubes (CNT) or silicon nanotubes (SNT). A simplified view of a nanowire may look like grass growing on your lawn as shown in Fig. 33. The nanostructures are typically grown directly on the metal current collector. A couple of important things to note in this image, first is that each of the nanowires is directly connected to the current collector at the bottom of the image which offers several benefits such as allowing for the volume changes in the horizontal direction that occurs during the lithium insertion process as shown in Fig. 34, which is especially helpful with silicon and tin anodes. This direct contact structure also has the potential to eliminate the need for binders and conductive additives that are used in standard anode materials today. You can see in this second image that the nanowires get “fatter” after the lithium-ions are inserted. This is what enables the greater energy storage of the nanowires, the nanowires allow all of the nanoactive materials to join in the charge storage process which increases the energy density, and due to

FIG. 33 Nanotubes on a current collector.

57

58

CHAPTER 3 Lithium-ion battery operation

FIG. 34 Nanotubes after lithium insertion.

the single-dimension electric pathways they allow for good charge transport (Sun & Chang, 2017, p. 866; Yazami, 2014, p. 5). Also keep in mind that due to the nanowire structure all the nanowire surfaces are directly in contact with the electrolyte rather than just the surface which again assists in the charge transport and energy density benefits. The nanotube only differs from the nanowire in that it is grown with a hollow structure such that it looks like a tube. The benefit of this is that it increases the surface area that contacts the electrolyte. Both the inside and the outside surfaces of the tubes contact the electrolyte which improves the ion transport capability. The carbon nanotube can be categorized into several different categories including single-wall carbon nanotubes (SWCNT) and multiwall carbon nanotubes (MWCNT). This differentiation is based on the thickness of the CNT and the number of coaxial layers of the nanotube structure (Goriparti et al., 2014). Due to the extremely small sizes of these materials the only way to get a real image of them is by using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The following images reprinted with permission from Springer Nature in the Nature Nanotechnology journal article “Stable cycling of double-walled silicon nanotube battery anodes through solid–electrolyte interphase control” by Wu, Chan, Choi, Ryu, Yao, McDowell, Lee, Jackson, Yang, Hu & Cui (2012) show good examples of double-walled carbon nanotube structure using a SEM microscope and a TEM microscope (Fig. 35). Nanowires are typically manufactured through several different processes including by a chemical vapor deposition (CVD) process or an electrodeposition (ED) process. From a manufacturing perspective, the biggest benefit of using either CVD or ED is that it simplifies the electrode manufacturing process significantly. Using vapor deposition or electrodeposition eliminates the need to mix the electrode materials with binders and solvents to create a slurry which is then pasted onto the current collector foils. With ED and CVD, the active materials are effectively grown directly on the current collector. These processes may significantly reduce the manufacturing cost as these technologies are scaled up into large-scale production capability (Sun & Chang, 2017). Electrodeposition and similar processes have been used in

3.8 Thermodynamics

FIG. 35 Scanning electron microscope image of nanowire structures showing a DWCNT using an SEM microscope in images (A) and (B) and with a TEM microscope in image (C).

high-volume manufacturing for many years, including in batteries for NiMh electrode coatings. So there is some precedence set for the use of these types of processes in lithium-ion battery manufacturing. But these technologies may still not yet be ready for the high-volume manufacturing required for mass manufacturing in lithium-ion batteries. The cost of making nanomaterials is still relatively high, so there is work to be done to improve the processes and reduce the costs. They also struggle from the perspective of making a thick material as these processes are typically limited to the nanometer-scale thickness. Today what many developers are doing is creating the nanomaterials as either a layer that the active materials are coated onto or as an additive material that can be added in with the active materials to improve their conductivity and performance. One final comment on nanomaterials is that there are some special precautions that need to be taken from a human perspective. Since these materials are so small the traditional personal protective equipment (PPE) will not provide any protection from the ingestion of these materials or from absorption through the skin. Therefore it is important to use specialty PPE when handling and working with nanomaterials.

3.8 Thermodynamics The science of thermodynamics is concerned with accomplishing “work.” But this does not mean that we are engaged in thermodynamics at the office from nine to five every day, rather thermodynamics describes either mechanical work, like the use of a wheel or lever, or in the case of batteries, electrical work. In the electrochemical lithium-ion cell, thermodynamics is specifically concerned with the maximum

59

60

CHAPTER 3 Lithium-ion battery operation

amount of work (power) that a reaction can deliver and how much work (power) is required to make that reaction occur (Breitkopf & Swider-Lyons, 2017). As mentioned earlier, it is the flow of electrons during the redox process that creates the flow of electrical current and it is the external power applied to the circuit that provides the electromotive force to move the ions back and forth between the electrodes. It is important to note that thermodynamic laws apply equally to mechanical power and electrical power. The first law of thermodynamics states that energy can neither be created nor destroyed and only the form in which the energy exists can be changed. The second law of thermodynamics states that heat cannot be completely converted into another form of energy and that all energy transformation processes are irreversible and occur in a preferred direction (Robert Bosch GmbH, 2007). Thermodynamics is an important aspect of the lithium-ion battery operation as it describes the properties of the chemicals and solutions and the impact of temperature and pressure during the reactions. It also helps to define and describe the interaction of the lithium-ion chemistry with the environment. Thermodynamics in batteries also differs somewhat when compared to other chemical reactions due to the electric field and the electrodes. In common chemical reactions outside of the electrochemical battery, energy is delivered as heat while no work is exchanged. Thermodynamics is also useful in defining and determining the losses that occur during the chemical reactions within the cells (Hebecker, 2017). In the lithium-ion battery, the polarization effects that occur as part of the redox process consume part of the energy of the cell as it is converted into waste heat, so not all of the theoretically available energy stored in the electrodes can be fully converted into useful electrical energy (Salomon, 2011, p. 2.1). When energy is no longer able to perform work, such as when electrical energy is converted into heat, it is described as entropy (Robert Bosch GmbH, 2007). One other term that falls within the realm of thermodynamics which I want to briefly discuss is the term adiabatic. This is frequently discussed in terms of looking at the thermal efficiency of a complete battery system. But it applies in cell development as well, especially during testing, cell development, and characterization. Simply stated, a system is described as being adiabatic when heat is neither supplied nor is dissipated (Robert Bosch GmbH, 2007). In measuring a cell or battery’s heat generation capabilities, an adiabatic test might be used wherein the cell is insulated and the test chamber temperature is matched with the cell, or pack, temperature such that the cell does not gain any heat from the environment but also does not release any heat to the environment. This can be a useful way to map the thermal characteristics of a cell.

3.9 Failures modes One of the most important aspects of battery design, whether at the cell or pack level, is safety. Safety can come in the form of changes we make to the chemistry within the cell, to the separator used, to the active material selection, and to the electrolytes

3.9 Failures modes

FIG. 36 Safe operating zones for temperature and voltage.

used. But ultimately and in all instances lithium-ion safety is concerned with protecting the cells within a system and working to keep them within their optimal temperature and voltage ranges in order to avoid a catastrophic failure that could cause harm to the people that are using the technology the lithium-ion cells are used in. Fig. 36 shows a simplified example of a safety map from a voltage and temperature perspective; the figure shows where there is a voltage range where the cell will operate without any concerns about safety in the green area. At the cell and chemistry level cell temperature and voltage are two directly measurable characteristics that we can use to determine the state of the battery. As that voltage limit is exceeded, either at high voltages or low voltages, this is where cell failure begins to occur. We’ll talk more about the types of failures that occur at these extremes shortly, but at high voltages we begin to see the cathode materials dissolving into the electrolyte which causes a domino effect of other failures to begin happening. Temperature effectively has a similar set of high and low limits, above and below those limits cell failure begins to occur. High temperatures have similar effect as operating at high voltages, but low temperatures tend to create lithium plating rather than lithium intercalation thereby reducing life and creating the potential for dendrites to form. Therefore looking at the chemistry and determining if there are things we can do to improve the performance at high and low temperatures, such as adding unique salts or additives to the electrolyte, or improving performance at high and low voltages, again possibly by using unique additives to reduce gassing at high voltages, is key to understanding cell and chemistry safety. The specific voltages and temperatures that will begin the failure mode differ depending on the chemistry and design of the cells. Some lithium-ion chemistries operate safely up to 4.2 volts while others max out about 3.0 volts, so trying to charge a lower voltage cell to the higher voltage limit will drive it into a failure mode

61

62

CHAPTER 3 Lithium-ion battery operation

in most cases. Generally, however, most lithium-ion chemistries operate within about the same temperature range, plus or minus a couple of degrees—from 20°C up to about +55°C. As we begin looking at lithium-ion cell failure modes we must start by asking the question what causes a cell to fail? In the end, cell failure comes down to three potential types of failure: internal short circuit, gas generation, or impedance growth. This is a bit of a simplification, as these are all outcomes from other events. For example, a short circuit could be internal, caused by debris generated during the manufacturing process or by the growth of dendrites that end up penetrating the separator and shorting the electrodes, or due to high temperatures melting the separator allowing the electrodes to touch. Or a short circuit could be the result of a foreign body penetrating the cell, like the infamous nail penetration test. But all these lead to a catastrophic failure event due to a short circuit. And pretty much every potential mode of failure ends up in one of these three types of failure. In Fig. 37 we see some of the possible causes of cell failures on the left side of the image, to the right of these we see the results that ultimately lead to cell failure and often thermal runaway event. We see here that either an internal or external short circuit, dissolution of active materials, lithium plating, and SEI growth can all lead to the three types of failures and in fact in many cases they will “stack” upon one another. These of course are meant to be simplified examples as there are a lot of other events that can occur to get the cell to these states. As one example let’s take a look at what happens at high temperatures, usually beginning above 90°C, where some interesting things begin to happen. For example, the traditional LiPF6 electrolyte may begin to break down above this temperature. Charged active electrode materials will react with the nonaqueous electrolytes at

FIG. 37 Cell failure types and causes.

3.9 Failures modes

high temperatures, often violently. As the temperatures rise the chemical breakdown of the cathode continues and the gases that are being generated begin to recombine into hydrogen and oxygen gases. This is one of the reasons lithium-ion cells limit the upper-end operating temperatures. In addition to the gases being generated here the high temperature will begin melting the separator which will cause a short circuit. So you can see both failure methods can happen during the same event. Yet even at room temperatures the lithium salts in the LiPF6 electrolyte have a tendency to react with moisture in the cell which results in the formation of hydrogen fluoride (HF) gas which will interact with the cathode active materials causing first surface erosion then reduction in capacity due to the degradation of the electrodes, electrolyte, and the growth of a new, thick passivation layer (Goriparti et al., 2014). This is also the reason that lithium-ion cell manufacturing must be done in a humidity-controlled environment and the reason that battery manufacturers include moisture limits on the incoming materials.

3.9.1 Internal short circuit There are several ways that a cell can end up with an internal short circuit beginning with the cell reaching high temperatures or due to the growth of a dendrite of lithium metal that penetrates the separator and allows anode and cathode to become electrically connected or even due to debris entering the cell during manufacturing (Fig. 38). The first manner a cell may experience a short circuit is during high temperature events. Most traditional separators are made of either polyethylene or polypropylene or a combination of the two. At temperatures as low as 90°C these separators’ pores

FIG. 38 Catastrophic short circuit events.

63

64

CHAPTER 3 Lithium-ion battery operation

begin closing and about 120°C–130°C the separator will melt and when that happens the polyethylene or polypropylene separators begin to shrink away from the electrodes. The shrinkage of the separator allows the anode and cathode electrodes to come into contact with one another, creating a short circuit within the cell and an internal short circuit. But what is really happening during an internal short circuit event in a lithium-ion cell? Briefly, most of the energy of the cell gets released very rapidly and through a single very small point. Some research has indicated that up to 70% of the energy of the cell may be released in less than a minute (Maleki & Howard, 2009). As most of the current in the cell is pushed through the internal short circuit site the rapid release of energy causes the cell to begin generating heat starting in the localized area around the internal short circuit. If the amount of heat that is generated is greater than the amount of heat that can be removed, the cell will continue to heat up until it reaches thermal runaway onset temperature. An important item to be aware of when it comes to any type of short circuit is the impact of the state of charge of the cell. Cells with a low state of charge will have less energy to release than will cells that are at a high state of charge. So there will be greater heating around the internal short circuit location if the cell is at a high state of charge compared to when it is at a low state of charge (Cai, Wang, Maleiki, Howard, & Lara-Curzio, 2011; Maleki & Howard, 2009). Another area to be aware of is the tendency for the growth of lithium “dendrites” during cycling. A dendrite is formed when the lithium metal begins plating on the anode. What occurs during dendrite growth is that there may be a spot near the negative electrode where there is a depletion of the salts in the electrolyte that may create a place where the chemical potential near the electrode surface is more positive (Huggins, 2009). This causes any bump or imperfection on the surface to become a home for lithium to plate and grow. If this continues it will grow into what appears like a stalagmite growing between the anode and cathode. If the dendrite continues to grow unchecked, it may penetrate through the separator causing direct electrical connection between the anode and the cathode (Fig. 39). This internal short circuit will force all the energy/power in the cell to discharge through that single small point generating heat, cell failure, and depending on the type of electrolyte flame. This failure mode is often seen during high rate charging. It is suspected that one of the causes of the Samsung Galaxy Note 7 fires in 2016 may have been due to dendritic lithium growth. This was not the sole cause of the cell failures but was likely one of the contributing factors that when they all occurred at the same time caused the cells to go into thermal runaway (Hruska, 2017a). The third manner that a cell may experience an internal short circuit is due to debris getting into the cell during the manufacturing process. One example of this happening was back in 2006 when Sony Corporation ended up recalling hundreds of millions of laptop batteries due to a high number of failures and thermal events. In this case the battery cell was a cylindrical 18650-type cell that used a nickel-plated steel can. During the assembly process the lid of the cell was crimped into place. It was found that this crimping process caused tiny particles of nickel to flake off inside

3.9 Failures modes

FIG. 39 Dendrite growth causing internal short circuit.

the cell causing an internal short of the cell (Wong, 2006). This was also one of the contributing factors in the Samsung recall mentioned before. Samsung’s investigation found that during the electrode welding process there were burrs that formed on the cathode which resulted in the burrs breaking through the separator and causing an internal short circuit event to occur (Hruska, 2017a, 2017b). This is one of the reasons that most battery manufacturers do their cell assembly in clean room/dry room environments, with both the humidity and particle size and count closely monitored. Clean rooms are rated according to how much particulate of specific sizes exist per cubic meter within the environment. By assembling the electrodes into the cells in a clean room environment the potential for foreign matter to find its way into a cell is greatly reduced. Finally, there are other types of failures that can get introduced to the cell during the manufacturing process. The Samsung Note 7 cells experienced several different failures including having the tops of the electrodes getting bent over during installation and assembly which caused the electrodes to touch, which in turn created an internal short. This was due to the manner in which the cell was installed in the phone where it created high stress on the cell in several areas and pushed the electrodes together. The other failures they experienced were due to an assembly failure where an insulating tape was not included during the assembly process and the electrode placements were misaligned causing internal stresses (Hruska, 2017a, 2017b). You can see from this example that it is not difficult to create a situation for a cell to create an internal short circuit. These types of manufacturing failures are difficult to identify ahead of time, but can be extremely costly both on the lives of those affected and financially on the companies responsible.

65

66

CHAPTER 3 Lithium-ion battery operation

3.9.2 External short circuit The other type of short circuit event that can occur is called an external short circuit. An external short circuit occurs when a conducting material that is external to the cell either touches the positive and negative terminals at the same time or is inserted into the cell causing an electrical connection with the electrodes to occur. The resulting failure mode of an external short circuit can be the same as an internal short circuit, rapid release of energy at single point, heat generation, gassing, spark, flame, explosion, and often ejection of the jellyroll from the case. In the short circuit event the duration of the event may initially be very rapid, with almost immediate heat generation and current rise in as little as 0.1 seconds. The next stage results in high current and maximum temperatures reached in as little as 120 seconds. From here the current and voltage slowly drop (Chen, Xiong, Tian, Shang, & Lu, 2016; Xia, Chen, & Chris Mi, 2014). If the heat generated during these stages is not removed the cell will continue into thermal runaway as described previously. The ability for a pack to safely manage one of these failure events is strongly dependent on the state of charge and the heat rejection capability of the pack. At higher state of charges, typically above 50%, the amount of energy released is enough to create high enough pressure in the can cell to allow it to begin venting electrolyte. The state of charge has a big influence on whether the cell is driven into thermal runaway or not. For example, if we take a typical NMC type cell as shown in Fig. 40 with a voltage range of 2.5–4.2 V and 15 Ah in capacity and look at it at 10%

FIG. 40 Comparing energy released at different SOC states.

3.9 Failures modes

SOC and at 90% SOC we see that there is a lot more energy being released, in fact about 37% more energy being released, when the cell is at the higher state of charge. For this reason the United Nations and the U.S. Department of Transportation (DOT) require that all lithium-ion cells and packs are discharged to not more than 30% state of charge before they are shipped.

3.9.3 Thermal runaway Thermal runaway occurs when a cell has reached the temperature at which the temperature will continue to increase on its own and it becomes self-sustaining as it creates oxygen which feeds the fire (literally). Once the temperature of the cell reaches about 80°C the SEI layer on the anode begins to decompose and break down in an exothermic reaction (generating heat) due to the reaction of the lithium with the solvents used in the electrolyte. At about 100°C–120°C the electrolyte begins to break down in another exothermic reaction, which in turn generates various gases within the cell. The gases that may be created during this reaction, depending on cell chemistry, include carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), ethane (C2H6), ethylene (C2H4), and hydrogen (H2) (Ohsaki et al., 2005; Wang et al., 2012). As the temperature nears 120°C–130°C the separator finally melts allowing the anode and cathode electrodes to make contact and cause an internal short circuit and generating more heat. As the temperature continues to rise, at about 130°C–150°C, the cathode begins breaking down in another exothermic chemical reaction with the electrolyte which also generates oxygen. It is this release of oxygen along with the carbonate LiPF6 electrolyte that ultimately allows the cell to burn and catch fire. The breakdown of the cathode active material is a highly exothermic reaction generating a lot of heat and continuing to drive the cell toward ultimate failure and fire. When temperatures rise above 150°C–180°C the reaction may become selfsustaining if the cell is not able to rapidly dissipate the heat being generated. At this point the cell is in what is referred to as “thermal runaway” as the oxygen generation makes the fire self-sustaining—at least until all the fuel has been used. If the gases continue to build up within the cell the cell may rupture or vent through a safety valve. The cell may rupture or vent the flammable hydrocarbon gases and hydrofluorocarbon electrolytes at this point and the introduction of a spark could ignite the electrolyte and the gases causing flame, fire, and potentially an explosion. But if the pressure continues to build up it is also possible that the cell will split open and eject the jellyroll from the housing (Wang et al., 2012). An example of a thermal runaway temperature map is shown in Fig. 41, but please note that these temperatures are not exact numbers because different chemistries and additives offer different performance. Through additives in the electrolyte, ceramic coating the separator and other tools the thermal runaway temperature can be raised significantly. In some cases, separators may be able to reach temperatures of 190°C or more without shrinking and causing the short that starts the thermal event. There are even some separators that appear to be able to continue to operate all the way up to 300°C and beyond, such as the Dreamweaver

67

68

CHAPTER 3 Lithium-ion battery operation

FIG. 41 Impact of temperature on a Li-ion cell.

family of separators. In other words, these temperature ranges are dependent on the cell design and chemistry. Some may move into thermal runaway at lower temperatures and others may move into thermal runaway at higher temperatures. During these events a variety of gases may be generated. The exact gases will depend on the specific electrolyte, cathode and anode formulations used. But for a traditional LiPF6 electrolyte with ethylene carbonate (EC) and ethyl methyl carbonate (EMC) salts the gases generated may include, in decreasing order, up to 50% carbon dioxide (CO2), up to 10% ethylene (C2H4), up to 10% hydrogen (H2), with lesser amounts of methane (CH4), ethane (C2H6), carbon monoxide (CO), tetracarbon (C4), ethyl fluoride (C2H5F), cyclopropane (C3H6), and propane (C3H8) (Roth & Orendorff, 2012). As a final note on thermal runaway, you will have recognized that I used the term exothermic at each stage of the process. This is important as an exothermic reaction is one that generates heat. The opposite of which is an endothermic reaction would absorb heat. It is the fact that all of these reactions are exothermic that generates heat during this process, each one adding to the heat generated by the previous reaction until it can no longer be contained.

3.9.4 Cascading failure One cell failing is a problem, but it can lead to a much larger problem if that cell failure drives the cells surrounding it to fail as well. This is referred to as a cascading failure event, when one cell goes into thermal runaway the heat generated may be great enough to drive the cells nearest it to also go into thermal runaway, this

3.9 Failures modes

cascades to the next cell which now adds to the heat being generated and then on to the next cell, and so on until the complete pack is engulfed. Think of it like a line of dominoes, once the first one falls it hits the second one and then continues until all the dominoes have fallen. A cascading failure in a lithium-ion battery pack will act in much the same manner if not controlled. The heat that is generated by a cell in thermal runaway has two major methods by which it can move to the surrounding cells, either through conduction or radiation. Conduction heating is heat transfer through direct contact. In the example image shown (Fig. 42), heat from the cell in thermal runaway is being transferred by conduction through the bus bar at the top of the cell that connects the two cells and to the cooling plate next to the cell. But the heat is also transferred by radiation to the cell next to it. The other method of heat transfer is convection which is done when heat is transferred through a moving liquid (air is considered a liquid in this instance). So as the air between the cells is heated up it can transfer that heat to other cells within the pack, especially if the pack uses an air-cooling methodology wherein the air is circulated within the pack (Warner, 2015). The challenge that is presented during a cascading thermal runaway event is in managing the heat that is being generated during these exothermic reactions. If the system can dispel or disperse the heat being generated quickly enough, then there is a chance of limiting the thermal runaway event to a single cell. However, depending on the size of the cell there can be more heat generated than is possible to remove before impacting other cells in the pack. In one case we studied the failure of a large format

FIG. 42 Methods of heat transfer.

69

70

CHAPTER 3 Lithium-ion battery operation

cell with a capacity of over 70 Ah and calculated the amount of heat generated as being more than a mega-joule of energy. That is roughly equivalent to about 950 BTUs of heat generation for a single cell, so the heat energy in the pack was equivalent to about one-third of a tank of gasoline. With the size of some of the largest lithium-ion applications reaching multiple megawatt hours in capacity you can understand the need to prevent cascading failures.

3.9.5 Impact and effects of temperature on cell aging Temperature has a major impact on performance and life of lithium-ion batteries. At low temperatures the ionic conductivity of the electrolyte gets significantly reduced making it more difficult for the ions to move from anode to cathode. The lower the temperature the lower the conductivity. As temperatures drop below 0°C both discharging and charging become more difficult. At 10°C charging typically must be halted and discharging rates need to be reduced. At 20°C charging may need to be discontinued and discharging may not be possible as the ionic conductivity in most LiPF6-based electrolytes comes down to near zero. At low temperature there is another factor that comes into effect, lithium plating. As the temperature drops the ability of the lithium to properly intercalate into the anode during charging is limited which causes the lithium to plate as lithium metal on the surface of the electrode. This has an immediate effect of reducing the capacity of the cell by reducing the amount of lithium for future intercalation as the plated lithium is no longer mobile and cannot participate in the charge/discharge process. In the long term, the plated lithium may form dendrites which we have already discussed may result in a potential internal short circuit. At high temperatures the separator begins to break down, the electrolyte begins chemically reacting with the cathode creating the forerunners for the thermal runaway events as described earlier. Most standard polyethylene or polypropylene separators begin shrinking at between 90°C and 110°C as was already discussed. This will cause an internal short circuit within the cell and gas production at the interface between the cathode and the electrolyte as the temperature continues to rise. Temperatures above 120°C–130°C cause the dissolution of the cathode into the electrolyte. Once the activity of the dissolved gas exceeds critical solubility levels, gas bubbles are formed (Lefrou et al., 2012, p. 65). This in turn may lead to thermal runaway, fire, and even potential explosion. But even looking at operating a cell at temperatures above the recommended average but below the safety limit will also have negative effects on the cell. Operating a cell regularly at 45°C or 50°C will have the same effects due to cathode dissolution and gassing as noted before, but just at a much slower pace. Operating your cell at these temperatures will cause your cycle life to drop off rapidly. Now temperature can be a bit of a tricky thing because some chemistries, such as many NMCbased cells, will see an increase in capacity up to about 35°C. But this is a short-term effect and will ultimately cause premature failure of the cells if they are operated at this temperature range for too long.

3.9 Failures modes

3.9.6 Impedance During the life of a lithium-ion cell, the cell will slowly increase in internal impedance. Impedance is effectively the resistance to the movement of ions, electrons, and current in a cell. The higher the impedance of a cell the harder it is to push lithiumions through it. This is either from operation at extreme temperatures, either high or low, or as a result of the growth of the SEI layer over time or a combination of these. The impedance of a lithium-ion cell has three parts. The first part is the sum of the ionic resistance of the electrolyte. The second is the electrical resistance of the active materials, such as the current collectors and foils. The third is the contact resistance of the active material and the current collector. The sum of these three types of resistance is the internal impedance of a cell (Dahn & Ehrlich, 2011; Salomon, 2011). Impedance can significantly influence the operation of a cell as it will cause a drop in the voltage as moving lithium-ions in a high impedance cell consumes some of the energy in the form of heat. At low temperatures impedance also increases significantly due to the reduced ionic conductivity of the electrolyte, usually at temperatures below 20°C. Another impact of increased impedance is that over time the SEI layer of most lithium-ion cells will increase in thickness, in some cases by as much as 100% or more. This is generally a result of additional lithium plating at the SEI layer over time which reduces the available surface area for the chemical RedOx reactions to occur as well as consuming some of the free lithium all of which reduces available capacity. As current is continuously moved through the cell this SEI growth slowly reduces the life of the cell. A good example of this is the battery in your tablet computer or smart phone. These types of devices rarely experience temperature variations, nor do they see high power pulses that could drive this impedance growth. Yet over time you begin to notice that the amount of time your battery can operate your laptop or cell phone without being charged gradually get smaller and smaller. This is the result of this impedance growth over time. While an increase in impedance in a cell will have an impact on cell life, it is generally not a driver of catastrophic cell failures.

3.9.7 What happens during overcharge A cell can only be overcharged when it is connected to a power supply. Remember that the natural state of a battery is to discharge so it will naturally lose capacity without an external power source, not gain it. The main reasons a cell (or battery) may be overcharged are either due to a battery management system that is not designed properly or a malfunction of the charging system. However, one other overcharge mechanism may be mentioned here. In a system that includes many cells in parallel and series it is possible that a single cell may have greater, or smaller, internal resistance due to manufacturing variation, location of the cell in the pack, or many other possible causes. This may cause that one cell to be overcharged more than the others around it which could eventually lead it into a failure and thermal runaway event if the overcharging is severe enough (Ye et al., 2016).

71

72

CHAPTER 3 Lithium-ion battery operation

The voltage at which a cell will move into overcharge will differ depending on the chemistry of the cell. For NMC, NCA, LCO, and LMO-type chemistries the top operating voltage is about 4.2 volts. LFP cells have a maximum operating voltage of about 3.6 volts and LTO chemistries have an upper operating voltage of about 2.8 volts. Overcharge begins as those chemistries begin to exceed those upper operating voltage limits. The impact of overcharging a cell will depend on how much it is overcharged and for how long. Most chemistries can accept small amounts of occasional overcharging with minimal impact on the performance or life of the cell. But continuous overcharging will result in a series of failures that can lead to a thermal runaway event. In the first stage of a cell overcharge the voltage and temperature rise slowly as the lithium-ions are permanently forced from the cathode and into the anode, this is an irreversible reaction. In the next stage, temperature and voltage continue rising but much more quickly. The voltage will hit a high peak and then drop off precipitously as the cathode is fully delithiated. During this stage the electrolyte begins to react with the cathode. At this point the electrolyte begins to decompose and the metals in the cathode, such as nickel, manganese, cobalt, and iron, may begin to get dissolved from the cathode material and begin plating on the separator and anode. This causes an increase in internal resistance in the cell by as much as 500%. This increased impedance also causes an increase in the ohmic heating inside the cell which in turn causes the continued dissolution of the cathode and decomposition of the electrolyte and the generation of gases within the cell (Ohsaki et al., 2005; Yuan et al., 2015). As the gases continue to evolve and the temperatures continue to rise the cell will begin to move into a thermal runaway event as the temperatures begin to exceed about 130°C. As temperatures reach 180°C, which is the melting point of lithium, the thermal runaway becomes irreversible. Thermal runaway may be due to a violent reaction between the overcharged anode and the high temperatures of the electrolyte that result in an exothermic reaction between the delithiated cathode and the electrolyte (Ohsaki et al., 2005). At this point the cell is likely to rupture to release the gases and may expel the jellyroll potentially with flame and explosion.

3.9.8 What happens during overdischarge While overcharging a lithium-ion battery is more common than overdischarging the battery, significantly overdischarging a lithium-ion battery has similar undesirable effects on the performance, life, and safety. There are four main methods that a cell (or battery) may be overdischarged. First, a cell (or battery) may be forced to continue discharging due to the controls being inadequate or not having the correct end voltage. Second, a cell (or battery) may overdischarge due to a quiescent current, in other words due to the continuous power drawn from the electronics in the system even when the system is “powered down”. A third method for an overdischarge event could be due to an internal short circuit in the cell, this may start as a filament or

3.9 Failures modes

dendrite growth or it could be due to a manufacturing flaw that introduced a foreign element into the electrodes. The fourth method that a cell (or battery) may be overdischarged is due to the self-discharge that can occur during long-term storage (Maleki & Howard, 2006). Of course, as we often say in the battery world the end result depends on many factors. For a typical NMC cell the operating voltage range is about 2.5 volts at the bottom to 4.2 volts on the top end. Discharging the cells down to 2.5 volts will generally not have any impact on the cell performance. When we are talking about overdischarging of a cell, we are really looking at discharging down below that minimum operating voltage level. In the example of an NMC cell, this means discharging it down to 2.0, 1.5, 1.0 volts all the way down to 0 volts. For LFP chemistries which have an operating range of about 2.5–3.6 volts, overdischarging would begin as the cell is discharged below the 2.5 volts minimum operating voltage. With an LTO-based chemistry the operating voltage is a bit lower still, operating from 1.5 to 2.8 volts, so in this case overdischarging occurs as the cell is discharged below 1.5 volts. Overdischarging a lithium-ion cell will result in a permanent reduction in capacity as well as the potential for internal short circuit as described before. During the overdischarging event the voltage potential of the anode increases which causes the copper current collector to begin to oxidize. At the same time the lithium-ions that were inserted in the anode begin to deintercalate and move into the cathode. As the overdischarging process continues the cathode is affected as its base morphology begins to get changed because of these various “side reaction” that are occurring, including the deintercalation, SEI breakdown, and copper film oxidation. This over deintercalation causes the SEI layer to begin to decompose, which generates gases. When the cell is again recharged a new SEI layer gets formed on the anode surface. These factors cause a permanent reduction of the cell’s capacity (Guo, Lu, Ouyang, & Feng, 2016; Maleki & Howard, 2006). The other impact of overdischarging a cell is the risk of reversing the polarity of the cell. This occurs when you attempt to continue to push current through a cell that is fully discharged. This will cause the polarity of the cell to swap, positive becomes negative and negative becomes positive. In a large system this may have caused this cell to be overtaxed and ultimately drive it into a thermal failure event at worst and at best will significantly reduce the life of the pack as the rest of the cells will have to work much harder to overcome the reversed cell. In addition to the impacts on life, there are some significant impacts on safety as the lithium-ion cell is overdischarged. As mentioned before one of the side reactions that occur is the oxidation and dissolution of the copper current collector. The copper ions that get dissolved into the electrolyte will flow through the separator and begin accumulating on the cathode. As this accumulation increases it begins to form copper dendrites. If these dendrites continue to grow they will penetrate the separators and cause an internal short circuit of the lithium-ion cell (Guo et al., 2016; Maleki & Howard, 2006).

73

74

CHAPTER 3 Lithium-ion battery operation

3.9.9 Influence of impact, crush, and penetration When it comes to physical damage and abuse of lithium-ion cells, the failure mechanisms are those that we have already covered including internal short circuit, external short circuit, and thermal runaway. As I have already said, different cells with different chemistries perform differently. Some cells can withstand an abusive condition such as a penetration without driving it into thermal runaway, fire, and explosion while other chemistries will almost immediately move into a thermal runaway event. During a nail penetration several things occur virtually simultaneously. First, as the nail penetrates the enclosure of the cell and then the electrodes it will cause fracturing of the packaging pulling pieces of enclosure and electrodes into contact with each other causing a short circuit. At this point all the current in the cell is released causing heat, fire, flame, and explosion in many cases. Impact and crush testing either drops a device onto the cell from between 1 to 3 meters in distance or forces a pressure on the cell until it reaches about 25% of its original thickness or thermal runaway onset occurs whichever occurs first (Zhang, Ramadass, & Fang, 2014). In many cases these types of failure modes are being mitigated at the module and pack level. In one study conducted by the MIT Crash Laboratory they evaluated the influence of different types of road debris penetrating the pack enclosure and the impact it had on the cells within the pack. They ultimately recommended using a steel plate to act like a piece of armor on the bottom of the pack to protect the pack from road debris penetrating the pack. This may work to protect the pack but adds significant weight to the pack as well as some additional cost (Xia, Wierzbicki, Sahraei, & Zhang, 2014).

3.9.10 Aging mechanisms The final failure mode we will review is also a question that is almost always asked at some point in the initial discussions, how long will the lithium-ion battery last? But that is somewhat of a trick question as there are a lot of different factors that determine the life of a battery. When it comes to lithium-ion battery aging there are two major types of aging to be considered. The first is based on evaluating the number of charge and discharge cycles (cycle life) that can be achieved under a variety of depth of discharge and temperature conditions. This one is the most variable as it is entirely dependent on the actual charge/discharge profile that it is being tested under. The second is based on calendar life and measures the capacity of a cell at the beginning and then lets it sit on a shelf at a consistent temperature for months or even years and then measures the capacity at the end both before and after cycling. Some amount of energy will be permanently lost during this testing and some will be able to be recovered through cycling the battery. Cycle life is perhaps the more important of the two, but it is also quite variable in that the cycle life achieved will depend on the application, the temperature, how fast it is charged, how fast it is discharged, and the depth of discharge among many other

3.9 Failures modes

factors. For instance, an all-electric transit bus that charges once a day overnight but operates 7 days a week and 365 days a year for 10 years, it will need to achieve 3650 cycles to reach that 10-year life (365 cycles per year  10 years). In addition, if the battery only uses 80% of the available energy to achieve that 3650 cycles then its 100% cycle life rate might only need to be 1500 or 2000 cycles. But what if the first bus went through the cycles faster? If the operators decided to charge it once during the day and once at night that would increase the cycle usage so now the same bus may only reach a 5-year life using the same set of parameters instead of the 10-year life expected. It will also change if the bus is being operated in Phoenix, Arizona where the ambient temperature is much higher for longer periods of time than if it is operating in Milwaukee, Minnesota. Since temperature has such a strong impact on cycle life, understanding where the application will be used will directly impact the cycle life estimates. This is a relatively common challenge when sizing a battery for an application that may have to operate over a very long time span. Since the way the application is used is likely to change between the first day of its life and the last day of its life. This means that it could face a harsher operating cycle or an easier one at various points during the battery’s life, with either having an impact on overall cycle life. This is one of the reasons that you frequently hear battery professionals answer with “it depends” when asked how long a battery will last! As another example, imagine another all-electric transit bus that wants to fast charge its batteries once an hour for 10 hours or 10times per day. Over the same 10-year life that battery would require 36,500 charge/discharge cycles (10 charges per day  365 days per year  10 years). This battery may need to reduce the amount of usable energy down to 60% DOD to achieve that cycle life, which means that the 100% cycle life may need to be 15,000–18,000 cycles which will also eliminate several chemistries as potentials for this application. Temperature also has a significant impact on the overall life of a lithium-ion battery. As mentioned in my first book (Warner, 2015) batteries are comfortable about the same temperatures that you are. When temperatures drop or rise outside of that comfort zone, the performance of the batteries drop and with repeated exposure will dramatically reduce the life of the battery. Another aspect of cycle life is the power usage during the cycle. A bus operating in a very hilly area would require more power capability than a similar bus operating in a flat route. All these factors influence the life of the battery. But with all that said, what is really happening inside the cell that causes the aging of the cell? While there are a couple of different factors that impact lithium-ion cycle life, the biggest impact is due to the SEI layer. Over time the continuous insertion and deinsertion of the lithium-ions into the graphite layers during cycling may cause the SEI layer to be stretched and potentially destroyed. Once this occurs a new chemical reaction between the electrolyte and the graphite anode will cause a new SEI to be created to repair or replace the damaged area. However, the chemical reaction that creates this new SEI area uses up some of the free lithium in the electrolyte, thereby permanently reducing the overall capacity of the cell. This corrosion of the anode and

75

76

CHAPTER 3 Lithium-ion battery operation

decomposition of the electrolyte continue very slowly over the life of the battery causing the SEI to slowly penetrate into the pores of the anode and even into the separator which results in a reduction in the amount of accessible surface area of the anode, increasing internal resistance, and causing a permanent loss of capacity and power (Barre et al., 2013; Leng, Tan, & Pecht, 2015; Troltzsch, Kanoun, & Trankler, 2006; Vetter et al., 2005). Other aging mechanisms include structural changes to the cathode and anode, the decomposition of the electrolyte, the dissolution of the active material into the electrolyte, and the creation of a SEI-type layer over the electrode surfaces and the current collector surface. On the cathode side of the cell the active materials begin to decompose at voltages above approximately 4.35 volts causing an increase in charge transfer resistance within the cell. At high temperatures the cathode also begins to experience structural and phase changes which reduce the ability to intercalate the lithium-ions during cycling. The structural changes may drive a change in the crystalline structure of the cathode, changing it into a cubic phase or a spinel phase structure both of which reduce the charge transfer rate (Leng et al., 2015). One other impact of the cathode dissolution is that, depending on the chemistry, it may release transition metal (cathode) ions that may get incorporated into the SEI thus reducing the capacity and increasing the thickness of the SEI layer (Vetter et al., 2005). Another mechanism that causes lithium-ion battery aging is due to repeated overcharging and overdischarging the battery. This leads to the decomposition of the electrolyte, reducing ionic conductivity and increasing the internal resistance of the cell. The current collectors may also begin to corrode over time or even begin to be dissolved during operation. This increases the internal resistance of the cell over time (Troltzsch et al., 2006). It is also very important to recognize that all of these aging mechanisms are greatly accelerated at higher temperatures causing rapid deterioration of the cell performance. At high temperatures the active material of the electrodes begins to decompose rapidly. Some studies have found that another SEI layer gets created at the cathode interface with the electrolyte at high temperatures thereby increasing the resistance of the cell and leading to poor reintercalation of the lithium-ions in the cathode. And at elevated temperatures the SEI on the anode may see a replacement of the carbon with an inorganic species in its place, increasing resistance and increasing the thickness of the SEI layer (Leng et al., 2015). At low temperatures the cell may experience lithium metal plating on the SEI layer or lithium dendritic growth on the SEI—both of which reduce the capacity and power due to the permanent loss of the lithium-ions and accelerate the aging of the cells while creating the potential for future safety issues (Vetter et al., 2005). All of these different aging mechanisms are summarized in Table 2 (Vetter et al., 2005) along with their causal factors and the impacts.

3.9 Failures modes

Table 2 Aging mechanisms of lithium-ion batteries Accelerated by

Cause

Effect

Leads to

Reduced by

Electrolyte decomposition (to SEI) (continuous side reaction at a low rate) Solvent cointercalation, gas evolution, and subsequent cracking formation in particles Decrease of accessible surface area due to continuous SEI growth Changes in porosity due to volume changes, SEI formation, and growth Contact loss of active material particles due to volume changes during cycling Decomposition of binder

Loss of lithium, impedance rise

Capacity fade

Stable SEI (additives), rate decreases with time

High temperatures, high SOC (low potential)

Loss of active material (graphite exfoliation), loss of lithium

Capacity fade

Stable SEI (additives), carbon pretreatment

Overcharge

Impedance rise

Power fade

Stable SEI (additives)

High temperatures, high SOC (low potential)

Impedance rise, overpotentials

Power fade

External pressure, stable SEI (additives)

High cycling rate, high SOC (low potential)

Loss of active material

Capacity fade

External pressure

High cycling rate, high DOD

Loss of lithium, loss of mechanical stability Overpotentials, impedance rise, inhomogeneous distribution of current and potentials Loss of lithium, loss of electrolyte

Capacity fade

Proper binder selection

High SOC (low potential), high temperatures

Power fade, enhances other aging mechanisms

Current collector pretreatment

Overdischarge, low SOC (high potential)

Capacity fade, power fade

Narrow potential window

Low temperature, high cycling rates, poor cell balance, geometric misfits

Current collector corrosion

Metallic lithium plating and subsequent electrolyte decomposition by metallic lithium

77

CHAPTER

Overview and comparison of different lithium-ion chemistries

4

Chapter Outline 4.1 Performance ..................................................................................................... 80 4.1.1 Voltage range ..................................................................................80 4.1.2 Cyclability .......................................................................................85 4.1.3 Energy density .................................................................................87 4.1.4 Power density ..................................................................................89 4.1.5 Temperature ....................................................................................92 4.2 Cost ................................................................................................................. 93

In this chapter we will examine some of the key performance characteristics of some of the major lithium-ion chemistries including gravimetric and volumetric energy density, rate capability, cyclability and shelf life, self-discharge, and cost. We will also discuss some of the selection parameters for choosing the right chemistry for an application. When it comes to lithium-ion chemistries there are many different flavors that all fall under the umbrella name of lithium-ion, but in general we will focus on the six major lithium-ion battery chemistries that are in common and high-volume use today. You may also hear the term lithium polymer used but remember that is also just another type of lithium-ion cell and in fact as I have asserted previously it is a bit of a misnomer as it was originally used to indicate a pouch-type cell with a polymer electrolyte which is not overly common today. Today the term appears to be used for all cells that are of the pouch form factor, whether they are truly polymer cells or not. In this book we will not differentiate those lithium-ion-polymer (Li-poly) cells from lithium-ion as they are basically the same thing. The six chemistries that are in general use today include lithium iron phosphate (LFP), lithium manganese oxide (LMO), lithium cobalt oxide (LCO), lithium nickel cobalt aluminum (NCA), lithium nickel cobalt manganese (NCM), and lithium titanate oxide (LTO). Keep in mind that these names refer to the cathode material used in the cell except for the LTO chemistry which refers to the anode material. The raw materials and precursors that are used to create these chemistries will all be reviewed in greater detail in the chapters that follow. But each chemistry has a very distinct and different set of performance characteristics that differentiates it from the others making it a better fit for some applications than others ( Julien & Stoynov, 2000). As scientists have developed these chemistries, the materials that are used were selected based on their combination of electrical conductivity, thermal conductivity, Lithium-Ion Battery Chemistries. https://doi.org/10.1016/B978-0-12-814778-8.00004-1 # 2019 Elsevier Inc. All rights reserved.

79

80

CHAPTER 4 Different lithium-ion chemistries

and stability. For an active material, either cathode or anode, to be considered as a viable candidate for use in lithium-ion batteries it should have several key characteristics. First, it should have reversible capacity without degradation of the materials. In other words, it must be able to both charge and discharge with a minimal amount of capacity loss and do so for a long time, achieving long cycle life. It should have both good ionic and good electrical conductivity. It should have a high rate of lithium diffusion into the active material, which helps drive the energy density. Finally, it should be low cost and environmentally friendly (Goriparti et al., 2014, p. 422). When we look at these criteria this set of materials that are most suitable for lithium-ion batteries mostly fall right in the middle of the transition metals on the periodic table of elements (Fig. 43). And since they all have a good combination of electronic and/or magnetic qualities it makes them very suitable for use in lithium-ion batteries. This group of materials also offers both high electron affinity and ionization energy which are important in the function of a lithium-ion cell. Electron affinity is the amount of energy that an atom releases when an electron is added, and the ionization energy is the energy that is necessary to remove an electron from an atom. Both of these energies increase as you move from left to right and bottom to top in the periodic table (Matson & Orbeak, 2013).

4.1 Performance As mentioned at the start of this chapter, there are several key performance indicators that are looked at when comparing different lithium-ion batteries. Table 3 presents a summary of these six different chemistries and some of their key performance indicators. Keep in mind that these figures are directional only as different manufacturers may achieve different performances with the same chemistries. We will only focus on the six main chemistry combinations but keep in mind that there are many other potential combinations that are in development in various laboratories throughout the world. For instance, the vertically integrated Chinese battery, car and bus company BYD has build their own cells using a lithium manganese iron phosphate (LMFP) blended cathode which is getting some interest recently (Green Car Congress, 2014).

4.1.1 Voltage range One factor that must be considered when evaluating different lithium-ion cell chemistries is the voltage range. Different chemistries have different operating voltage ranges as described previously in Chapter 3. The voltage is effectively the electrical potential difference between the negatively charged anode and the positively charged cathode, which is in turn determined by the specific nature of the different materials used for anode and cathode (Gibilisco, 2012). As I discussed in “The Handbook of Lithium-Ion Battery Pack Design” (Warner, 2015) the voltage is of key importance when designing a large system as it will need many cells in series to

Periodic table of elements—Transition metals.

4.1 Performance

FIG. 43

81

82

Chemistry descriptor Specific energy (Wh/kg) Energy density (Wh/L) Specific power (W/kg) Power density (W/L) Volts (per cell) Cycle life Self-discharge (% per month) Operating temperature range

Lithium iron phosphate

Lithium manganese oxide

Lithium titanium oxide

Lithium cobalt oxide

Lithium nickel cobalt aluminum

Lithium nickel manganese cobalt

LFP 90–120 190–300 4000 10,000 3.3 V 5000–6000 < 1% 20°C to +60°C

LMO 100–150 250–360 4000 10,000 3.7 V 300–700 5% 20°C to +60°C

LTO 60–80 170–230 1000 2000 2.3 V >15,000 + 2–10% 30°C to +75°C

LCO 150–200 400–600 1000 2000 3.6 V 500–1000 1–5% 20°C to +60°C

NCA 200–300 490–675 1000 2000 3.6 V 500 2–10% 20°C to +60° C

NMC 150–280 325 1000–4000 2000–10,000 3.7 V 3000–4000 1% 20°C to +55°C

Based on Warner, J. T. (2015). The handbook of lithium-ion battery pack design: Chemistry, components, types and terminology. Boston: Elsevier, p. 77.

CHAPTER 4 Different lithium-ion chemistries

Table 3 Comparison of lithium-ion chemistries

4.1 Performance

FIG. 44 Typical voltage discharge curves for different Li-ion chemistries.

create high voltages. Chemistries such as the LTO and LFP, with a lower nominal voltage, will require more cells in series to create a high voltage system than when compared to the other chemistries. But as there are many other selection criteria this may not eliminate them from use in large applications. Fig. 44 shows an example of generic discharge curves for the six major lithiumion chemistries overlaid upon one another. The first four chemistries, LMO, LCO, NMC, and NCA, all start with relatively high voltages in the range of 4.1–4.4 volts and then fall at a relatively constant rate until the end voltage is reached at below 3.0 volts. Within these NMC, NCA, and LCO all have similar curves with a “knee” or drop off at the end of the curve, while LMO maintains a relatively constant rate throughout. The other two chemistries, LFP and LTO, start with lower beginning voltages and discharge on an almost flat line until they reach their lower voltage limits when they also hit that knee in the curve. It is important to note the angle, or flatness, of the curves, NMC, NCA, LMO, and LCO all have a relatively steady decline, while LFP and LTO are relatively flat across the entire discharge range. This plays an important role when it comes to developing the battery management systems control algorithms. For the chemistries with flat curves it is slightly more difficult to locate the exact state of charge based on the voltage because the slope is so flat. Whereas with the other chemistries it is easier to identify the state of charge based on the discharge voltage due to the sloping curve.

83

84

CHAPTER 4 Different lithium-ion chemistries

FIG. 45 Operating voltage range of chemistries.

If we take those same curves and overlay the operating voltage range of the six main chemistries on top of them with the typical voltage limits that are used in most lithium-ion batteries, about 80% of the total, by limiting the top 10% of the voltage range and limiting the bottom 10% of the voltage range we find the usable energy available we get a slightly different picture (Fig. 45). The area under the curves now represents the total amount of usable energy of the cells. The 80% depth of discharge is a rule of thumb, but in general is pretty accurate for most chemistries. For accuracy, please keep in mind that when we limit a cells depth of discharge we are really limiting the voltage rather than the capacity but this proved a good visual to overlay the different chemistries. Although some chemistries can use a wider voltage range by operating up to 95% of the total depth of discharge none can fully use 100% in regular operation. There are, of course, other challenges when using wider depth of discharge that emerge especially at the high end of the voltages. At high voltages some chemistries begin to dissolve the cathode materials in the electrolyte which will cause premature aging and ultimately the death of the cell as we have already discussed. For this reason, most cell manufacturers tend to limit the voltage at the top end to 90%–95%. The other reason to limit the voltage on the top end is for safety; if the electronics measurement accuracy tolerance is say 5% then if you tried to charge a cell to 100% you could end up overcharging it and causing serious damage. Using this example if you tried charging a cell to 4.2 volts but the electronics

4.1 Performance

tolerance was off by 5% then you could end up charging the cell to 4.41 volts instead. In a lithium-ion cell this is a huge amount and will impact the life and potentially safety of the cell. Depending on the type of application, power, and energy needed the voltage depth of discharge can be reduced even more in order to attain longer cycle life. There is also a benefit to managing where the operating voltage range stops and starts during operation. In the previous example, we used 80% depth of discharge and limited the cell on the bottom end at 10% state of charge and on the top end 90% state of charge. However, studies have shown that moving away from the top voltage has a net positive effect on the cycle life of the cell. For instance, if we moved that 80% depth of discharge range to start at the 5% position on the bottom and 85% position on the top we reduce the negative electrochemical activities that occur as you get closer to the top voltage limit, but you still get to use 80% depth of discharge you are simply using it at an area with a higher quality of energy. Of course, in this example you need to fully understand the low-end voltage and the knee in the curve to ensure you do not run out of energy prematurely during operation.

4.1.2 Cyclability Cycle life is also a key performance indicator when assessing different batteries. However, there are many differences in the ways that cell manufacturers conduct this testing, so it is important to understand the actual test conditions. The temperatures that are used for “room temperature” cycle life testing range from temperatures 23°C (73°F) up to 30°C (86°F). I would argue that 23°C–25°C falls closest to “room temperature” testing while 30°C falls into the most frequently seen temperatures in actual operation. It is advisable to get a range of cycle life testing data from 20°C up to about 45°C and at different C-rates to really understand how the cells will perform under different operating conditions. With what we now know about the impact of temperature on cell performance and cycle life you can see that it is difficult to get to an “apples to apples” comparison when two nearly identical cells are tested at different temperatures or under different conditions. In fact, you may get different test results by testing the same cells on different equipment. Therefore understanding the test conditions is critical. Additionally, most manufacturers’ internal characterization testing uses a wide variation in charge/discharge rates from one manufacturer to the next. These may range from very slow charge and discharge cycles, as low and 0.05C (C/20) which is a 20-hour discharge followed by a 20-hour charge, up to a 1C rate which is a 1-hour discharge followed by a 1-hour charge, and often up to a 2C charge and discharge which is a 30minute charge and a 30-minute discharge. Again, with this type of variation in the testing you can imagine that two nearly identical chemistries would come out with different performance characteristics. There is not, unfortunately, an industry standard measurement process when it comes to cycle life measurement and characterization, but a 100% depth of discharge measurement at a 1C/1C rate at 23°C is about as close as we can get today to a test that we can get apples to apples comparisons on.

85

86

CHAPTER 4 Different lithium-ion chemistries

As we saw in Table 3 some chemistries offer much higher cycle life capabilities than others. Cells that use an LTO anode can typically achieve over 10,000 full depth of discharge cycles and in some cases have been demonstrated to achieve >20,000 full depth of discharge cycles. I have even seen one manufacturer whose data indicates that they expect to achieve >60,000 full charge/discharge cycles. While cells using an LFP chemistry may achieve >5000 cycles cells with NCA, LMO and LCO chemistries will generally achieve about 1000 dull depth of discharge cycles, while some low nickel content NMC chemistries have been able to achieve 3000–4000 full depth of discharge cycles or more. When it comes to cycle life I believe that there is a point of diminishing returns at which it does not make financial sense to continue pushing for more cycles. Referring back to my earlier example of an all-electric bus that uses one cycle per day 365 days a year for 10 years, at 80% DOD it only needs to achieve 3650 cycles. Which means that its 100% DoD cycles should only need to be in the range of 2000 cycles or less. So would it make sense to pay more for a pack that could achieve 4000 100% DOD cycles? That would mean that it will probably achieve somewhere around 5000 cycles or more at 80% DOD. At this point you have to ask whether it would make sense to use a less expensive cell in order to optimize the performance to the cost rather than paying for a cell that has more cycles than your application will ever need. The answer of course is “it depends” on the application and the product life. With this in mind, it is important to have a good understanding of how the end use application will operate to determine the amount of cycle life that is necessary. When looking at applications and trying to select an appropriate chemistry we can use the 100% DOD cycle life to make a first cut, but we then need to evaluate cycle life at lower depth of discharges to begin to understand how the cells will perform in the application. As a rule of thumb most lithium-ion chemistries use only about 80% of the total available energy for electric vehicle applications. This is due to a couple of reasons, first it is to protect the cell from being overcharged at the top end of the voltage range and second is to take into account the accuracy of the voltage measurement of the battery management system which is usually between 2% and  5% accurate. The third reason is due to the rapid voltage drop off the low end of the voltage, the “knee in the curve” discussed earlier. As a rule of thumb most lithium-ion chemistries use only about 80% of the total available energy Fig. 46 shows an example of the impact on cycle life of reducing the usable voltage range, or depth of discharge, of the cell. By reducing the cell voltage operating range many additional cycles can be gained. Another general rule of thumb, for every 10% DOD that is reduced the cycle life increase is roughly 50% increase over the 100% DOD cycle life. In this typical example we see that by reducing the cell depth of discharge from 100% to 80% we more than double the cycle life. Now this is only a general rule so make sure to ask for the data from the specific cell manufacturer that

4.1 Performance

FIG. 46 Cycle life at different depth of discharges.

you are working with. And of course, this does not consider the other factors, such as temperature, C-rate, and power used during these charge/discharge cycles, which will impact the cycle life of any cell. But the results are consistent across all chemistries, reducing the DOD will increase the cycle life. For every 10% DOD that is reduced, the cycle life increase is roughly 50% over the 100% DOD cycle life

4.1.3 Energy density Depending on the application you are looking at, different energy performance indicators may be more important than others. Gravimetric energy density (Wh/kg), or energy per unit of weight, has become the de facto “standard” that is used to compare chemistries and packs even if it is not necessarily the right measure for a specific application. For instance, if you are looking at an automotive application then volumetric energy density (Wh/L), or energy per unit of volume, may be more important to you as this describes the amount of energy that can be fit in the available space. Yet in an uninterrupted power backup system (UPS) or a large grid energy storage system (GESS) where neither weight nor volume are critical, cost and cycle life may prove to be more important performance measures than weight. However, in aviation and aerospace applications where weight is critical, gravimetric energy density (Wh/kg) is a far more important characteristic due to the costs

87

88

CHAPTER 4 Different lithium-ion chemistries

associated with getting the vessels into space. NASA estimated that it costs about $10,000 to put one pound of payload into Earth orbit back in the days of the space shuttle program, so you can imagine that for these types of applications weight is a critical factor. Just to put this into perspective, when Elon Musk sent his Tesla Roadster into space if we assume that his costs are about the same as the NASA figures, it would have cost them about $9,920,000 just to send the 992-pound battery into space. And recent figures indicate that it may actually currently cost somewhere between $27,000 and $43,000 to send one pound of payload into space (Sarah Kramer, 2016). Which of course would mean that it may have cost between $26,784,000 and $42,656,000 just to send the battery alone of the Tesla Roadster into space. But all this comes back to the importance of energy density to certain markets and applications. But gravimetric energy density is a good comparison tool when relating different energy storage technologies since it equalizes out everything but energy and weight, or volume. Fig. 47 shows a comparison of the energy of different power sources including burning wood, coal, natural gas, ethanol, propane, gas, and diesel fuels with several different battery types. The energy per unit of weight and volume for liquid fuels is extremely high, which is why they continue to be the main fuels used in transportation applications and remain the target for battery development. If we look at the highest gravimetric energy density lithium-ion cell available on the market today, about 285 Wh/kg, and compare it with diesel fuel at over 10,000 Wh/kg, we find that diesel has more than35times greater energy per unit of weight than the best cell on the market today. There is still a very long way to go before battery technology reaches a comparable energy density level to liquid fuels. But there is still quite a wide range of energy densities available from the current lithium-ion chemistries that offer very good performances for today’s electric applications. Fig. 48 shows the volumetric and gravimetric energy density for more

FIG. 47 Energy density of different fuels.

4.1 Performance

FIG. 48 Gravimetric and specific energy densities of lithium-ion cells.

than 288 different production lithium-ion cells that are or have been available over the past 10 years by many different manufacturers. I have attempted to give a general indication of where the different chemistries fall by roughly grouping them together. But recognize that this is not an exact separation as some of the chemistries use blended cathodes and the high end of some chemistries overlap the lower end of other chemistries. There is also a major influence based on the types of anodes that are used. Some of the higher energy density cells shown here begin to use blended anodes with both graphite and silicon to achieve their very high energy density. From this comparison you can see that lithium-ion chemistries range from relatively low energy densities of around 80 Wh/kg for LTO on one end of the spectrum and up to >285 Wh/kg for some NCA chemistries.

4.1.4 Power density On the other hand, if you are looking at a stationary grid-based system that is installed into a shipping container or into a building, then weight and volume may not be as important to you. In this case you may be more interested in how much power you can discharge and how quickly you can do it. Another application that is power biased is the hybrid electric vehicles (HEV), automobiles which range from 12-volt stop/start vehicles to 48-volt mild hybrid and strong hybrids like the Toyota Prius along with some of the plug-in hybrids, which tend to have high power demand. So while an electric vehicle may need high energy density, a hybrid or stop/start type vehicle will need higher power density out of a much smaller battery.

89

90

CHAPTER 4 Different lithium-ion chemistries

We can make the same arguments in respect for power density that we did for energy density that applications with high power demand will require a higher power per unit of weight or volume, but the application may not be limited by weight and volume. Power data may be shown by different manufacturers as power density, shown in watts per liter (W/L) or watts per kilogram (W/kg), or a C-rate capability. C-rate describes the amount of power that can be charged or discharged over a defined period. For example, a 1C rate is equivalent to a 1-hour discharge or charge and a 2C equates to a 30-min charge or discharge. Going the other direction, a 0.5C rate (C/2) is equal to a 1-hour discharge or charge. A large grid system may need to discharge for a period of 30 seconds, which is equivalent to a 120C discharge rate, or for as long as 15 minutes, a 4C discharge rate. The amount of power that is drawn out of or put into a cell has an impact on the overall performance and life of that cell. Fig. 49 shows an example of the impact that power has on the capacity performance of the cell. In this example data the cell discharge power curves for a 5-hour discharge (0.2C), a 2-hour discharge (0.5C), a 1-hour discharge (1C), and a 30-minute discharge (2C) are shown. From this chart we can clearly see that for this particular chemistry up to about a 1C continuous discharge rate there is very little impact on the available energy of the cell, but as we increase to a 30-minute continuous discharge rate (2C) the area below the curve is significantly reduced, indicating lower energy and power capability at the higher discharge rate. This is typical of many lithium-ion chemistries in that if you are using it

FIG. 49 Impact of power on NMC cell capacity.

4.1 Performance

in a high-power discharging manner you will see a reduction in both the available capacity of the cell as well as the life of the cells. There are some chemistries such as LTO and LFP which perform very well under frequent high-power charging and discharging. But in these cases you are trading off energy density for power density, which depending on the application may be perfectly fine. These particular chemistries offer very long cycle life even with continuous high-power cycling. Fig. 49 depicts the impact of C-rate discharge power on cell capacity of a high energy NMC chemistry and Fig. 50 which depicts an example of discharge power capability of a high power LTO cell on cell capacity. If we compare the two, again it seems clear that the energy chemistry quickly begins to lose capacity as the C-rate exceeds 1C. However, the LTO power chemistry continues to provide almost full capacity even up to 8C discharge rates. This is shown from how closely the curves fall on top of each other from 0.2C (1/5C) all the way to 8C. Finally, if we look at energy versus power we find that there is a wide range of performances when we agglomerate all the different chemistries from a wide variety of manufacturers. Using the same data set from the energy density chart before, Fig. 51 shows the range of lithium-ion energy density performances (Wh/kg) on the X-axis to their respective power density (W/kg) performance on the Y-axis. Notice that as the energy density increases to the right, the power density comes down. On the other end if the power density is high, the energy density shifts to the left. This supports the assertion that energy density and power density are inversely related, if a chemistry is high in one it is typically low in the other. Because

FIG. 50 Impact of power on LTO cell capacity.

91

92

CHAPTER 4 Different lithium-ion chemistries

FIG. 51 Power versus energy chart.

of this I often talk about energy biased cells and power biased cells because it is generally always a trade-off between power and energy when selecting lithium-ion cells.

4.1.5 Temperature Temperature is another performance feature that must always be evaluated when comparing different chemistries. Some chemistries are capable of better performance at higher or lower temperatures than others. And in fact temperature has one of the largest impacts on cell performance, life, and safety which was discussed in the previous chapter. In Fig. 52 I show some example data in which we see the impact that temperature has on the performance of an NMC cell chemistry. NMC tends to operate well up to about 35°C (95°F) and can even provide a bit more capacity up to 45°C (113°F); however, this will have a negative impact on the long-term cycle life of the cell. But at colder temperatures beginning at 0°C (32°F) we see the cell capacity begins to drop off quite rapidly. At 10°C (14°F) and 20°C ( 4°F) we see the capacity drops down to 80% and 50% of original capacity, respectively. The other factor that influences how a cell performs at different temperatures is the ionic conductivity of the electrolyte. In many of the standard LiPF6-based electrolytes in use today the ability of the ions to pass back and forth (to conduct) at low temperatures gets reduced significantly. This can be somewhat managed either through the use of various additives to the electrolyte or at the pack level by adding

4.2 Cost

FIG. 52 Impact of temperature on NMC cell performance.

heating to the thermal management system. But this is one of the reasons that there is so much less energy available at low temperatures. Finally, I used the image in Fig. 53 to visualize how much energy is available from the same cell at 45°C and at 20°C. We often hear people talk about the “area under the curve” when they show these types of charts. This is what that means! In this example the larger shaded area is the energy that is available when the NMC cell is operating at an ambient 45°C. You can see here that it is quite a bit more energy than when the cell is operating at 20°C, which is the small area to the lower lefthand corner of the chart. This also shows the impact that the angle of the discharge line has on the area. With the steeper decline at 20°C there is much less energy than if the decline was more of a straight line as it is in the 45°C area.

4.2 Cost The final performance factor that we will discuss is cost. There are many factors that affect lithium-ion cell and battery cost, including the price of raw materials; the cost of converting those materials into cells, modules, and packs; the company overhead, manufacturing throughput, sales volume, and demand. The generally accepted cost measurement for lithium-ion batteries is cost per unit of energy or dollars per watt hour ($/Wh) for cells or dollars per kilowatt hour ($/kWh) for packs and larger systems. While it may not be a perfect measure, it does allow for different chemistries to be equalized to make a direct apples-to-apples cost comparisons. Essentially, it gives

93

94

CHAPTER 4 Different lithium-ion chemistries

FIG. 53 Temperature’s impact on NMC capacity—The area under the curve.

us a standard against which to measure. However, keep in mind that if you are talking about a high power application like a 12 volt stop/start battery, a better measure may be dollars per Watt ($/W) or dollars per kilowatt ($/kW). It is important to take into consideration when comparing costs that the costs provided are all for the same type of product. For instance, some manufacturers and institutions may report cell costs, while others may report the pack costs. And some report costs but do not identify which they are talking about. This tends to create much confusion in the industry, so please make sure to identify which costs you are using. For the purposes of our discussion here we will look only at cell costs which will only include material costs, such as the anode and cathode active materials, electrolyte, separator, housing or enclosure, current collectors, conductive binders, solvents, and safety features. Nonmaterial costs include the cost to convert the raw materials into the cell; the manpower; and the overhead costs such as equipment and facility depreciation, research and development, sales and general administration, along with warranty and profit margin. Table 4 compares three different chemistries all at the same ampere hour capacity to show an example of the cost differences due solely to chemistry and performance. For this example, I used an LTO, an LFP and an NMC chemistry to show the differences and impact that chemistry has on cost. We will assume that the material cost for each of these cells is the same as this example, yet we know in fact that they do vary quite a bit. Next we will assume that in each case the cell is a 20 Ah cell with the standard nominal voltages for those chemistries, from which I calculated the cost per

4.2 Cost

Table 4 Sample costs for different chemistries Material cost Capacity (Ah) Voltage (V) Energy (Wh) Cost/Wh

LTO

LFP

NMC

$22.50 20 2.5 50 $0.45

$22.50 20 3.2 64 $0.35

$22.50 20 3.7 74 $0.30

Wh of energy for each. The resulting calculations show that on a Wh basis, the NMC has the lowest cost per Wh of the three chemistries driven because it has the highest energy. One other methodology for evaluating cell costs that has been gaining some interest recently is comparing cost per Wh per cycle (Table 5). This methodology is used to try to bring in the full lifecycle value of the cell into the spotlight for comparison and not just the initial investment. Building on the previous example we see that while NMC proves to be the lowest cost per Wh, once we apply the average number of cycles that each can achieve LTO comes in with the lowest cost per Wh per cycle due to its ability to provide such high cycle life. Finally, if we take this same example and compare it to the amount of energy that is usable (Table 6) we start getting close to the real cost as it would perform in an application. For this example, we will use 80% as the available energy for each chemistry, but remember that it will vary greatly with different chemistries and applications as discussed previously. Once again due to its very high cycle life the LTO chemistry comes out as the lowest cost per cycle of usable Wh of energy. Again, keep in mind that this example is just for demonstration purposes. If we were to take these three cells and apply actual materials costs, cycle life, and usable depth of discharge we could improve the accuracy significantly. But for our purposes here this serves to describe the effects of adding cycle life and usable energy into the comparison. This type of analysis can give us an idea of the different ways that cell costs can be compared using the same types of data. But when we look at the lithium-ion cell Table 5 Sample lifecycle costs of different lithium-ion chemistries Material Cost Capacity (Ah) Voltage (V) Energy (Wh) Cost/Wh Cycles Cost/Wh/Cycle

LTO

LFP

NMC

$22.50 20 2.5 50 $0.45 10,000 $0.000045

$22.50 20 3.2 64 $0.35 5000 $0.000070

$22.50 20 3.7 74 $0.30 3000 $0.000101

95

96

CHAPTER 4 Different lithium-ion chemistries

Table 6 Sample lifecycle costs of different lithium-ion chemistries at different DOD.

Material cost Capacity (Ah) Voltage (V) Energy (Wh) Cost/Wh Cycles Cost/Wh/cycle Usable energy (%) Usable energy (Wh) Cost/Wh of usable Energy Cost/Wh/cycle of usable energy

LTO

LFP

NMC

$22.50 20 2.5 50 $0.45 10,000 $0.000045 80% 47.5 $0.47 $0.000056

$22.50 20 3.2 64 $0.35 5000 $0.000070 80% 57.6 $0.39 $0.000088

$22.50 20 3.7 74 $0.30 3000 $0.000101 80% 59.2 $0.38 $0.000127

cost trend in dollars per Wh over time, as shown in Fig. 54, we see that there were some significant cost reductions between 2010 and 2015 in the early days of lithiumion growth and as volumes began to rise, manufacturing processes were optimized, and cell chemistries improved we began to see increases in energy density. The chart overlays several different forecasts on top of one another in order to try to get a

FIG. 54 Lithium-ion cell costs over time.

4.2 Cost

consensus or average idea of where the costs are and where they are going. There were some significant differences across all forecasts in those early years due to the amount of uncertainty in the market, but as we look from 2015 into the future we see that the differences in the forecasts quickly begin to erode as costs are driven toward the goals set out by the United States Advanced Battery Consortium (USABC) with cell level targets of $100 per kWh by 2020 for electric vehicles and $75 per kWh by 2023 for a low-cost/fast-charge electric vehicle battery (United States Advanced Battery Consortium, 2017). Similar cost targets were also set out by different organizations and agencies across the globe which also helps to drive these costs down. The costs shown are an aggregation of all chemistries, making them essentially averages of all production chemistries. Different chemistries have different costs per kWh as was shown in the earlier tables due to characteristics like the raw material costs such as the iron phosphate in the LFP cells which is much cheaper than the cobalt and nickel in an NMC cell. But this somewhat equalizes out due to the lower cell voltage of LFP versus NMC cells. Finally, there are a lot of forecasts showing that we will reach cell costs of below $100 per kWh somewhere between 2022 and 2025. But the convergence below $100 per kWh may require the use of new chemistries and/or new cell designs altogether, such as moving to solid-state batteries. But the trend is clear, lithium-ion cells costs are driving toward the point where they can compete with liquid fuels.

97

CHAPTER

The Cathodes

5

Chapter Outline 5.1 5.2 5.3 5.4 5.5 5.6

Lithium iron phosphate .................................................................................... 103 Lithium cobalt oxide ........................................................................................ 105 Lithium manganese oxide ................................................................................ 106 Lithium nickel manganese cobalt/nickel cobalt manganese ............................... 107 Lithium nickel cobalt aluminum oxide .............................................................. 112 Other cathodes ................................................................................................ 113

The cathode is the positively charged electrode of an electrochemical lithium-ion cell where the lithium-ion reduction occurs, or in simpler terms it is the positive side of the battery cell. And as we talked about earlier, that could be either electrode depending on whether it is charged or discharged. But just to remain consistent with the industry understanding and terminology, we will herein consider the cathode to be the electrode that is coated with the NMC, NCA, LFP, LMO, or LCO active materials regardless of its state of charge. The cathode electrode is made up of three separate parts: (1) the current collector; (2) a conductive binder; and (3) the active material, which is the main focus of this chapter. We tend to describe a lithium-ion cell by using the description of the active cathode material in the positive electrode. The main active cathode materials used in lithium-ion cells today were introduced earlier and they include lithium iron phosphate (LFP), lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium nickel cobalt aluminum (NCA), and lithium nickel manganese cobalt (NMC). You will note here that we do not include lithium titanium oxide (LTO) in this list. That is because it is actually an anode material that gets paired with one of these cathode materials, but we will discuss it in the next chapter. For a material to be considered as a viable option for a lithium-ion cathode it must demonstrate a couple of key characteristics. First, the material must have high free energy reaction with lithium which results in a high voltage. It must be able to reversibly incorporate a large number of lithium-ions, which gives it high energy density and rechargeability. It must do this without causing structural changes to the material to deliver long cycle life. Large structural changes will cause the material to degrade, fracture, delaminate from the current collector and literally break apart causing severe reductions in life and performance. A good cathode material should also be able to rapidly intercalate the lithium-ions by offering a high level of lithiumion diffusivity, which also can translate into high power capability. A cathode Lithium-Ion Battery Chemistries. https://doi.org/10.1016/B978-0-12-814778-8.00005-3 # 2019 Elsevier Inc. All rights reserved.

99

100

CHAPTER 5 The Cathodes

material should be a good electronic conductor and yet still be insoluble in the electrolyte, meaning that the electrolyte must not react with the material and cause it to break down. Finally, the material must also be low cost and commercially available to ensure the final cell cost is low (Dahn & Ehrlich, 2011; Xu, Qian, Wang, & Meng, 2012). The active materials are made up of compounds of lithiated metal oxide or lithiated metal phosphate molecules as described in the names of the chemistries, such as lithium iron phosphate or lithium manganese oxide. The cathode is effectively a “host” material capable of allowing the lithium-ions to be inserted and reversibly removed during the charge and discharge intercalation process. Most of the current lithium-ion cathode materials are drawn from the transition metal oxides in the periodic table, since these metals tend to be good materials for energy storage since they have incomplete outer energy shells, which allow the creation of cations when electrons are removed. But they are also excellent conductors of heat and electricity and yet are much less reactive than the Group 1 and 2 metals like lithium. These materials can be further divided up based on their crystalline structure as being either a layered, spinel, or olivine structure, each of which we will discuss in this chapter (Challoner, 2014; Nitta, Wu, Lee, & Yushin, 2015). A crystal structure is created when a material undergoes treatment, usually by bringing it to very high temperatures, that brings the atoms into a repeating, structural alignment. In lithium-ion cathodes the term unit-cell is used to describe the smallest unit of the crystal structure that is made up of the active material compounds. The layered structure is the simplest crystal structure as it consists of layers of the cathode host materials. Fig. 55A shows an example of a layered crystal structure which is seen in the LCO- and NMC-type cathodes. In the layered structure, the lithium-ions are inserted (intercalated) in between the layers of cathode material. The spinel structure is a bit more complex in that it is more of a repeating lattice framework as shown in Fig. 55B where the lithium-ions are inserted into the tunnels that are formed as the material crystalizes. This is the type of structure that an LMO material crystalizes in.

FIG. 55 Crystal structures of Li-ion chemistries.

CHAPTER 5 The Cathodes

Finally, the olivine structure consists of hexagonal close packed array of oxygen atoms in half of the octagonal empty spaces with iron (Fe) and phosphorus (P) in a very complex structure as shown in Fig. 55C. While this one looks a bit more random, it is actually a very repeatable structure wherein the lithium-ions are intercalated into the spaces between the crystalized molecules. LFP is the most common cathode that crystalizes in this form (Birle, Gibbs, Moore, & Smith, 1968; Matson & Orbeak, 2013). If we look at the market penetration of different cathodes from 2015, across all applications the market share by chemistry is pretty evenly split among the top three chemistries with NMC, LCO, and LFP making up about 80% of all lithium-ion cells built today (Fig. 56). However, with the rapid acceptance of electric vehicles and grid energy storage systems the market is shifting. If we were to look at just the electric vehicle segment, we would find that LFP is the most commonly used chemistry in 2015 due to its low cost and because of the rapid growth of all electric vehicles in China. These figures also include lithium-ion batteries for both consumer electronics and large format applications such as electric vehicles. LCO is not used in any large format applications today. However, with much higher energy density targets being set by the U.S. Advanced Battery Consortium of 350 Wh/kg (United States Advanced Battery Consortium, 2017) and the Chinese government target of 350 Wh/kg (Hao, Cheng, Liu, & Zhao, 2017) the product mix is beginning to shift away from LFP in the automotive EV segment and into more NMC chemistries as LFP simply cannot meet these much higher energy targets. For these electric vehicles nickel manganese cobalt (NMC) cathodes with both graphite, and more frequently, and with graphite/ silicon blended anodes will continue to be the “go-to” chemistry for high energy density applications. In fact, many of the traditional LFP manufacturers are either

FIG. 56 2015 Cathode market share by chemistry.

101

102

CHAPTER 5 The Cathodes

developing or have already launched new NMC-based chemistries to remain competitive in these markets. Another interesting approach to developing new and unique materials is the use of blended cathodes. Several manufacturers have already launched lithium-ion cells that use a cathode with a combination of two different cathode materials such as NMC and LMO or NCA and LMO, or LCO and LMO for instance. The goal of blending materials is to get the best attributes from each one to supplement the poorer characteristics of the other and end up with a more balanced performance than with either individually. For instance, LCO which has good energy density but is poor on safety and is high cost may be blended with LMO to help improve the safety without giving up its energy density and reducing the cost (Chikkannanavar, Bernardi, & Liu, 2014). Jouanneau, Patoux, Reynier, and Martinet (2014) describe some of the benefits of blending cathode chemistries as reducing the costs and toxicity while improving the rate performance, energy density, capacity, and stability. When looking at materials and chemistries one thing to keep in mind is that the way that energy density is compared is different at the material level and at the cell level and there is no clear comparison. Chemists tend to think in terms of milliampere hours per gram (mAh/g) but that doesn’t always translate well to the cell level where we tend to think in watt hour per kilogram (Wh/kg). In Table 7 I have attempted to show a rough comparison of material energy density in milliampere hour per gram (mAh/g) and cell energy density in watt hour per kilogram (Wh/kg). Keep in mind that there are a lot of different items that go into calculating the energy density of a material and a cell including the voltage, anode, mix ratio, morphology, electrolytes, salts, binders, and so on. And most importantly the anode material that is selected will ultimately be the “yang” to the cathodes “yin” and will determine the energy density (Linden & Reddy, 2011, pp. 1.10–1.11). Therefore this table can only ever really be directionally correct and used for reference purposes. But it is a question that I have often been asked especially when talking to materials scientists. How does mAh/g relate to Wh/g? In both cases, materials and cells, I have included a range because of the various factors that influence energy density.

Table 7 Comparison of material vs cell energy densities Material energy density (mAh/g)

Chemistry

Cell energy density (Wh/kg)

150–170 140–180 100–120 160–170 175–195 200–220 180–200

LFP LCO LMO NMC 111 NMC 622 NMC 811 NCA

90–120 150–200 100–150 140–180 190–230 220–280 200–300

5.1 Lithium iron phosphate

One last comment on this table, the material energy density that is typically discussed is the theoretically maximum energy density that the material could achieve in a perfect cell with 100% efficiency and zero losses. In reality we know that we cannot achieve these theoretical limits and the cell level energy densities have taken those losses into account. Also note that there is a change in terms from ampere hours per gram to watt hours per gram when we go from materials to cells.

5.1 Lithium iron phosphate Lithium iron phosphate, which is usually abbreviated as either LiFePO4 or simply LFP, is a three-dimensional olivine-type cathode material. It is also called a polyanion material, with the iron (Fe2+) arranged with six atoms symmetrically located around a central atom—this is known as an octahedral. The LFP also has phosphorus (P) arranged with five atoms arranged in a triangular fashion—this is known as a tetrahedral formation as shown in Fig. 57. The phosphate in the LFP also bonds tighter with the oxygen atoms which is what makes the LFP chemistry considered safer than other lithium-ion chemistries. LFP benefits from using low-cost materials, namely, iron, and is thermally stable (American Elements, 2017b) which has given it a leading position in the market as being both low cost and safe. It does not react destructively with the electrolytes until it reaches temperature above 350 °C which is what gives it its excellent thermal stability. The use of iron as the main reactive material also makes it inherently nontoxic and environmentally safe. However, it has a relatively low nominal voltage of about 3.3 volts which gives it lower energy density than other chemistries. LFP energy density typically ranges from 90 Wh/kg to about 120 Wh/kg at the cell level. There have been some claims that have driven the LFP energy density up beyond 120 Wh/kg but

FIG. 57 LiFePO4 crystal structure.

103

104

CHAPTER 5 The Cathodes

these claims have not been the rule and may have actually reached or exceeded the maximum limit that can be achieved by this chemistry (Birle et al., 1968; Dahn & Ehrlich, 2011; Gaberscek, Jamnik, Weichert, Samuelis, & Maier, 2014). Many LFP cathodes also use smaller particle sizes than are used by some of the other transition materials. In most cases LFP particles are in the nanometer size range. This offers a couple of interesting benefits to the chemistry. The smaller particle size allows for faster charging and discharging and better lithium-ion transport than with larger particle sizes. This means faster charging and discharging, and more power with higher charge/discharge rates than in other chemistries that are based on larger particles (Dahn & Ehrlich, 2011; Gaberscek et al., 2014; Nitta et al., 2015). Finally, LFP has a very flat voltage curve when compared to other chemistries with a nominal voltage of between 3.2 volts and 3.3 volts. And when compared with lithium metal, which is a common measure in electrochemistry, it has a voltage of 3.45 volts. Remember this is a measure of the voltage potential difference between the lithium metal and the LFP. LFP has an operating voltage range of 2.0 volts to 3.6 volts. From a system-level perspective, the lower voltage and relatively low volumetric energy density means that a system using LFP cells will require more cells in series to achieve higher voltages when compared to a metal oxide chemistry. There are many variations on the traditional LFP chemistry, but a couple of the most common variations use a blending strategy to create lithium iron magnesium phosphate (LiFeMgPO4), lithium iron manganese phosphate (LiFeMnPO4), and lithium iron cobalt phosphate (LiFeCoPO4). The lithium iron magnesium chemistry (LiFeMgPO4) has been in production from Valence/Lithium Werks for many years now as an off-the-shelf solution to replace lead acid and nickel-based batteries. The lithium iron manganese phosphate material was originally developed by the Dow Chemical Company, but they have stopped work on it some time ago. The LiFeMnPO4 offers the lower cost of LFP with a slightly higher energy density, about 10% over LFP. The LiFeMnPO4 chemistry has a theoretical energy density of about 145–150 mAh/g compared to LFP at about 120 mAh/g. The most popular use of the manganese version appears to have been in the earliest BYD vehicles. However, it appears that they have shifted away from this version in favor of a more traditional LFP chemistry or some other variation such as LiFeCoPO4 (DeMorro, 2014; Dow Energy Materials, n.d.; Lithium Werks, 2018). A couple of other interesting variations are the lithium cobalt phosphate (LiCoPO4) which keeps the olivine structure but eliminates the iron and lithium manganese phosphate (LiMnPO4), which is being worked on by several labs including U.S. Army Research Laboratory and materials manufacturers such as SigmaAldrich. Today, these materials still exhibit poor cycle life and with a material capacity of about 100–125 mAh/g still needs some major development before making it viable for electric vehicle applications. The LiCoPO4 is believed to be able to operate at higher voltages, around 4.8 V nominally, and to have the potential to offer high cell energy densities of around 670 Wh/kg with additional work and development. In early 2018 HydroQuebec and the U.S. Army Research Laboratory were able to show some significant improvements in the traditional LiCoPO4 by substituting

5.2 Lithium cobalt oxide

chromium, iron, and silicon in the chemistry and by coating the materials with carbon. This created a material with more than 167 mAh/g theoretical energy and is capable of operating at a much higher 4.8 V with a very high cell theoretical energy of nearly 800 Wh/kg (Green Car Congress, 2018a). The LiMnPO4 shows higher theoretical energy density of the materials at around 150 mAh/g, but suffers from low ionic and electronic conductivity among other challenges that have yet to be resolved. But these chemistries, while promising, are still not close to being ready for production yet (Oh, Myung, & Sun, 2012; Taniguchi, 2018; Xing et al., 2012).

5.2 Lithium cobalt oxide Lithium Cobalt Oxide, also known as LiCO2 or LCO, was the first commercially available lithium-ion chemistry developed in 1980 and commercialized by Sony in 1991 using a lithiated cobalt oxide active material. LCO is a layered crystal structure with six cobalt atoms arranged in octahedral sites around the oxygen atom and that material in alternating layers of cobalt oxide and lithium as shown in Fig. 58 (Nitta et al., 2015). LCO material is a compound containing at least one oxygen anion and one metallic cobalt cation. It is insoluble in aqueous solutions such as water but is stable which is what makes it so useful in electrochemical applications because of its excellent ionic conductivity. Metal oxide compounds are basic anhydrides which just means that they are created through a process that involves elimination of water and can react with acids and with strong reducing agents in redox reactions (American Elements, 2017a).

FIG. 58 Layered crystal structure of LCO.

105

106

CHAPTER 5 The Cathodes

LCO continues to be one of the major chemistries in use today, especially in small, portable applications such as smart phones, and laptop and tablet computers due to its high energy density, high voltage, and good cycling performance. On the other end of the spectrum, LCO has seen only limited use in large applications due to several major challenges. First, due to the high cost of cobalt, LCO is an expensive cathode material. Second, from a safety point of view LCO is not very thermally stable at high temperatures and can experience the onset of thermal runaway at temperatures around 150 °C and experience full thermal runaway about 200 °C. Finally, LCO tends to have lower cycle life compared to other lithium-ion chemistries due to the distortions that occur in the layered lattice crystal structure during cycling (Nitta et al., 2015). LCO has a nominal voltage of about 3.8 volts and a voltage of 3.9 volts when compared with lithium and has an operating voltage range from 3.0 volts to 4.2 volts. The higher voltage helps to enable the higher energy density of the chemistry. The voltage discharge curve of LCO has a sloping fit much like the other chemistries with layered crystal structures, NMC and NCA.

5.3 Lithium manganese oxide Lithium Manganese Oxide, known as LiMn2O4 or LMO, differs from LFP and LCO in that its crystal form is that of a lattice-type spinel structure. The manganese atoms are in a three-dimensional octahedral site formation with the manganese taking up one-fourth of the location in the lithium layer leaving one quarter of the manganese sites vacant as shown in Fig. 59. In the active materials of this chemistry the oxidized

FIG. 59 Spinel crystal structure of LMO.

5.4 Lithium nickel manganese cobalt/nickel cobalt manganese

manganese structure looks very much like the lattice that surrounds many gardens. Those lattice-type structures then form layers with the lithium-ions filling the “tunnels” within that structure (Julien, 2000; Xu et al., 2012). Like other metal oxides the LMO is a compound containing at least one oxygen anion and one metallic manganese cation. It is insoluble in aqueous solutions such as water but the combination is stable and has good electrical and ionic conductivity which makes it useful in electrochemical applications such as lithium-ion batteries. Much like the LCO materials, the LMO is a metal oxide compound based on anhydrides and can react with acids and with strong reducing agents in redox reactions which is part of what makes them useful in energy storage applications (American Elements, 2017c). Cells with the LMO chemistry are used in applications where cost and stability are key performance factors. Manganese is a cheaper raw material and is much less toxic than cobalt or nickel. Additionally, LMO operates at a higher nominal voltage of about 3.9 volts and it has a midpoint voltage of 4.05 volts versus lithium metal. The voltage discharge curve of LMO has a sloping fit much like the layered crystal chemistries, with an operating voltage range from 2.5 volts to 4.2 volts. However, these benefits may be offset by several challenges including its lower capacity, higher capacity loss during storage and cycling, and poor performance at high temperatures. Additionally, only about half of the lithium can be removed before the oxide begins to lose oxygen or to oxidize the electrolyte. The poor cycling performance is believed to be because the manganese tends to get dissolved into the electrolyte during cycling causing higher impedance and agglomeration of the manganese on the anode SEI layer which may begin forming dendrites and cause safety issues (Dahn & Ehrlich, 2011; Goodenough, 2007; Jouanneau et al., 2014; Nitta et al., 2015). Because of these factors, most companies have moved away from pure LMO but instead are using it as a blending agent with other chemistries. This helps to mitigate some of those challenges by relying on the other chemistry’s strengths.

5.4 Lithium nickel manganese cobalt/nickel cobalt manganese Lithium Nickel Manganese Cobalt Oxide, known as LiNMnCoO2 or NMC, is quickly becoming one of the most frequently used lithium-ion chemistries due to its energy density, relatively low cost, and high voltage. Some manufacturers call it Nickel Cobalt Manganese (NCM) instead of NMC but there is no major difference other than perhaps the order of the mixture with the elements in order of higher percentages. NMC is a layered crystal structure arranged in octahedral sites in alternating layers of nickel and cobalt atoms, manganese and cobalt atoms, and lithium as shown in Fig. 60 (Dahn & Ehrlich, 2011). The NMC material is a thermally stable compound containing at least one oxygen anion and one metallic cation each of cobalt, manganese, and nickel (American Elements, 2017e). The combination of different transition metals together offers different performance benefits than any used alone. For instance, nickel-rich combinations of

107

108

CHAPTER 5 The Cathodes

FIG. 60 Layered atomic structure of NMC.

NMC offer high discharge capacity, while manganese-rich combinations offer better cycle life and thermal stability, and cobalt-rich combinations offer good rate capability. Cells using the NMC chemistry benefit due to its being what I consider a “well-balanced” chemistry, as it is capable of offering very good energy and power density over a wide operating temperature range and can easily be biased toward either energy or power depending on the needs of the application. Additionally, NMC has proven to be able to achieve very high cycle life, in some cases from up to 6000 full depth of discharge cycles have been exhibited which makes it an ideal solution for long-life applications. Of course, with each benefit there is a challenge presented that needs to be overcome. The nickel-rich combinations suffer from structural degradation which reduces the cycle life, manganese-rich combinations suffer from reduced capacity which impacts energy density, and cobalt-rich combinations suffer from both high cost and safety concerns (Sun & Zhao, 2017). Much like other chemistries, NMC chemistries when paired with graphite anodes must also use a reduced depth of discharge to achieve the best performance. In many cases only about 80% of the cell’s capacity is usable in the final application. Traditional NMC chemistries have used nickel, manganese, and cobalt material combinations with a ratio of 1:1:1 (or 3:3:3) which means that the three materials were used in equal amount in the cathode materials 33% nickel, 33% manganese, and 33% cobalt, written as LiNi0.33 Mn0.33Co0.33O2. However, recently much development work has gone into increasing the nickel content as a means of both reducing the amount of cobalt and therefore reducing the cost and increasing the energy density

5.4 Lithium nickel manganese cobalt/nickel cobalt manganese

FIG. 61 Nickel Manganese Cobalt chemistries.

of the chemistry. As shown in the image Fig. 62 below increasing the percentage of nickel in the NMC chemistry can have a significant impact on energy density. Today materials manufacturers have created a wide variety of NMC “flavors” depending on the needs of the cell manufacturer. Many manufacturers have transitioned to mixes of 4:3:3, representing 40% nickel, 30% manganese, and 30% cobalt, written as LiNi0.4 Mn0.3Co0.3O2. and 5:3:2, representing 50% nickel, 30% manganese, and 20% cobalt, written as LiNi0.5 Mn0.3Co0.2O2 which has become the defacto standard mixture. Today, manufacturers are transitioning to 6:2:2 blends, 60% nickel, 20% manganese, and 20% cobalt, written as LiNi0.6 Mn0.2Co0.2O2. Mixes of 6:2:2 are coming into mass production in 2017. Additionally, we are seeing the emergence 7:2:1 and 8:1:1 chemistries with 70% nickel, 20% cobalt, and 10% manganese, written as LiNi0.7 Mn0.2Co0.1O2 and 80% nickel, 10% cobalt, and 10% manganese, written as LiNi0.8 Mn0.1Co0.1O2, respectively. These are expected to move into volume production from multiple manufacturers in 2018. Fig. 61 gives a visual comparison of the materials breakdown in these chemistries showing the change in material quantities with the nickel increasing to become the largest amount of material in the chemistry. The drivers for this change and the evolution of the NMC chemistry mixture is for a couple of reasons. First, NMC has proven to be a very good chemistry for use in lithium-ion batteries, so finding ways to improve something that already works helps to minimize the development risk. Second, the increase in nickel content has a positive effect on energy density. As shown in Fig. 62 increasing the percentage of nickel in the NCM chemistries causes an increase in the energy density of the materials and thereby the cell. The 1:1:1 NCM materials have an energy density of about 158 mAh/g, while the 6:2:2 materials increase energy density to about 180 mAh/g. That is an increase in energy density of about 14%. The 8:1:1 mixture increases the energy density to about 190 mAh/g, an energy density increase of about 20% from the 1:1:1.

109

110

CHAPTER 5 The Cathodes

FIG. 62 Nickel content vs. energy density in NCM materials.

We should also note that these are not the only combinations that NMC can be made in. Materials manufacturers have been working on a wide variety of different ratios as a means of improving chemistry behavior and cost. In addition to the chemistries mentioned before, there are also 4:2:4 and even 9:1:0 versions that have or are being developed by various materials companies. Therefore with the increase in energy and the decrease in cost, cells using the higher nickel content will see a significant improvement in cost per Wh. That of course does not yet take into account the material manufacturers capital expense as they retool for the new materials, so the actual reductions that a cell manufacturer will see are likely to be somewhat smaller than those shown here and these are only directional materials reductions. The third driver for this change to higher nickel content is that there is a significant decrease in the cost of the material due to the change in material mix. As an example of the difference in the material costs let’s take a comparison based on raw material costs. Today, in October 2018, the London Metal Exchange (LME) lists nickel at $12,620 USD per metric ton, cobalt at $62,000 USD per metric ton, and manganese is listed on the Shanghai Metals Exchange at $2562 USD per metric ton. In the next example we will compare everything to manganese as it is the least expensive metal in the group. Nickel’s cost is about five (5) times that of manganese and cobalt is twenty-four (24) times the price of manganese. If we use those numbers as a base and allocate across the different ratios of NMC chemistries as presented in Table 8, by increasing from 33% nickel to 60% nickel, and decreasing the other two accordingly, there is about a 19% reduction in material cost going from the 1:1:1 (or 3:3:3) to a 6:2:2. When you continue to reduce the cobalt percentage going to the 8:1:1 NMC chemistry the material cost savings nearly doubles to 34% less than

5.4 Lithium nickel manganese cobalt/nickel cobalt manganese

Table 8 NCM material cost comparison. x Mn Nickel Manganese Cobalt

5 1 24

Relative material cost

1:1:1 33 33 33

5:3:2 $165 $33 $792 $990

% Cost reduction from 1:1:1

50 30 20

6:2:2 $250 $30 $480

60 20 20

7:2:1 $300 $20 $480

70 20 10

8:1:1 $350 $20 240

80 10 10

$400 $10 $240

$760

$800

$610

$650

(23%)

(19%)

(38%)

(34%)

the 1:1:1. It is also interesting to note that the 7:2:1 combination actually offers the best material cost reduction from the base 1:1:1 by almost 40%, which is even better than the 8:1:1 blend. You can see from Table 8 that the trend has been to develop combinations that reduce the amount of cobalt in the NMC chemistry. There is no question that it is the most expensive metal used in the chemistry and while we have seen cobalt prices rising steadily since later 2016 it appears that they may have peaked in March 2018 and have now begun to come down again. And it may not make sense to eliminate it completely from the chemistry as it does offer great benefits in the area of electrical and ionic conductivity. So even with the higher costs we may want to leave some amount of cobalt in the chemistry. One reason that NMC chemistries have been so successful is that they operate at relatively higher voltages generally with nominal voltages ranging from 3.60 volts to 3.75 volts depending on the respective chemistry mixture and a voltage of 3.8 volts when compared to lithium metal. The operating range voltages are from a minimum of 2.5 volts up to a maximum of 4.2 volts with a sloping voltage discharge curve which makes it easy to identify state of charge based on the cell voltage.

FIG. 63 NMC Core shell gradient examples.

111

112

CHAPTER 5 The Cathodes

Another area of research for NMC chemistries is in the development of what are called core-shell gradient materials. This is effectively using the same materials but in such a way that it varies the elements from the center of the molecule to the outside edge to achieve different performance characteristics. Fig. 63 shows two different examples of this type of compound structure. The image on the left shows a gradual transition from a more nickel-rich center to a more manganese-rich outside, while the image on the right shows a material with a sharper transition from nickel core to manganese shell. This second image may be more akin to what we may find in a surface coating process such as atomic layer deposition, which will be discussed in more detail in Chapter 8.

5.5 Lithium nickel cobalt aluminum oxide Lithium Nickel Cobalt Aluminum Oxide, known as LiNiCoAlO2 or simply NCA, is also seeing increased use in no small part due to the fact the Tesla uses an NCA chemistry in their electric vehicles. Much like LCO and NMC, NCA is a layered crystal structure arranged in octahedral sites in alternating layers of nickel and cobalt atoms, aluminum and cobalt atoms, and lithium as shown in Fig. 64 (Dahn & Ehrlich, 2011). Much like how with NMC chemistries we saw many different ratios of NMC materials, NCA is typically made of 80% nickel, 15% cobalt, and 5% aluminum and may be written as LiNi0.8Co0.15Al0.05O2 but we may also find different configurations in the future. The reason to add the aluminum is that it was found that by doping the lithium nickel cobalt oxide with aluminum it stabilizes the thermal and charge

FIG. 64 Layered atomic structure of NCA.

5.6 Other cathodes

transfer resistance, thereby making it a highly thermally stable cathode material with high discharge capacity and a long storage life (American Elements, 2017d). NCA offers some advantages compared to some of the other cathode chemistries. First, is the very high energy density that can be achieved especially when combined with a blended graphite and silicon anode. An early concern with cells using the NCA chemistry was cost due to the high cost of cobalt used in the cell. If we compare to the NMC chemistry costs in Table 8 and use the same methodology we would find that the cost of NCA materials would be $960, far more expensive than the high nickel content NMC chemistries. And while that is still a concern, long-term raw material supply agreements and high volume manufacturing of cylindrical cells by Panasonic and Tesla appears to have mitigated the cost risk and in fact has developed a solution that leads the market as one of the lowest price and highest energy density lithiumion batteries available today. However, NCA does suffer from several major challenges. First, at high temperatures it tends to exhibit very high capacity fade because of the rapid SEI growth on the anode at high temperatures. Second, while it is safer than LCO the inclusion of aluminum has been thought to make the NCA chemistry suffer from somewhat lower safety at least in part due to its lower thermal runaway onset temperature of 150 °C. Cycle life is also a concern with NCA as it generally achieves only about 500 full depth of discharge cycles. While it has an excellent energy density it is generally necessary to derate cells in order to achieve adequate usable energy and usable life (Kam & Doeff, 2012). Much like the other layered crystal chemistries NCA has a nominal voltage of about 3.65 volts and about 3.7 volts when compared with lithium metal. The higher voltage helps to enable the higher energy density of the chemistry. NCA has an operating voltage range of 3.0 volts to 4.2 volts, with a voltage curve with a sloping fit much like LCO and NMC the other chemistries with layered crystal structures.

5.6 Other cathodes The chemistries discussed so far make up the most popular commercialized lithiumion cathode chemistries that are on the market today. However, they are certainly not the only ones that have been used or are being investigated. Early work on lithiumion cathode materials also evaluated other transition metals such as nickel and chromium. The work that was done on lithium nickel oxide, known as LiNiO2 or LNO, has formed the basis for many of the current chemistries that use nickel. Lithium chromium manganese oxide, LiCrMnO4, showed some early potential due to its high voltage versus lithium, 4.0 volts versus lithium with a maximum voltage of 5.4 volts. However, due to the side reactions that occurred with the electrolyte at the high charging voltages and the general toxicity of chromium this work was eventually abandoned (Islam, Ammundsen, Jones, & Roziere, 2000; Legallasalle, Vverbaere, Piffard, & Guyomard, 2000).

113

114

CHAPTER 5 The Cathodes

The Chinese company BYD has used a blended chemistry based on LiFeMnPO4 chemistry that adds manganese into the traditional lithium iron phosphate chemistry. In addition, there are some very interesting new cathode chemistries that could see use in the future, including lithium nickel manganese oxide Li[Li0.11Ni0.33Mn0.56]O4 which adds nickel and lithium to the traditional LMO chemistry and may offer energy densities up to nearly 300 mAh/g. Another variation on this LMO is the LiNi0.5Mn0.5O2 variation which attempts to maintain the same energy density as LCO while also achieving lower costs. In the same family is the LiNi0.5Mn1.5O4 which is also based on the LMO chemistry but blends in about 25% nickel. This chemistry looks very interesting as it has a voltage of 4.6 volts versus lithium, meaning that the top end of its operating range will be at or above 5.0 volts. However, for high voltage chemistries such as these new electrolytes will be required in order to operate safely and to achieve high cycle life (Dahn & Ehrlich, 2011; Nitta et al., 2015). There has also been some very interesting work being done by Argonne National Laboratory, the Department of Energy, UC Berkley and others in the area of developing manganese-rich chemistries instead of nickel-rich chemistries. Manganese offers some very interesting potential benefits, not the least of which is that manganese can exchange two electrons instead of just one like some of the other transition metals which means that it should be able to hold more charge and reach higher energy densities. Recent work done at UC Berkley has demonstrated more than 300 mAh/g material capacity which equates to about 1000 Wh/kg at the cell level. And because manganese is both a more abundant material and has low toxicity there is less concern about demand driving up cost or safety (Green Car Congress, 2018b; Lee et al., 2018). Many other chemistries are under development including those from the olivine phosphate family: Lithium cobalt phosphate (LiCoPO4), lithium manganese phosphate (LiMnPO4), lithium nickel cobalt phosphate (LiNiCoPO4), and lithium manganese iron cobalt phosphate (LiMnFeCoPO4). In the layered crystal family: lithium nickel oxide (LiNiO2) and lithium-rich lithium manganese oxide (Li2MnO3). And finally, in a crystal structure family that has not yet been discussed, the tavorite crystal structure work is ongoing on chemistries such as the lithium iron fluorosulfate (LiFeSO4F) (Nitta et al., 2015).

CHAPTER

The Anodes

6

Chapter outline 6.1 Carbons ..........................................................................................................118 6.1.1 Graphite .......................................................................................120 6.1.2 Soft/amorphous carbon ..................................................................122 6.1.3 Hard carbon ..................................................................................123 6.1.4 Graphene ......................................................................................124 6.2 Lithium titanium oxide .....................................................................................125 6.3 Alloys and composite materials ........................................................................129 6.3.1 Silicon ..........................................................................................130 6.3.2 Tin ...............................................................................................135 6.3.3 Aluminum .....................................................................................137 6.3.4 Germanium ...................................................................................138

The anode active material in a lithium-ion cell is the structure that houses the lithiumions once they are forced out of the cathode upon a discharging event. The anode side of an electrochemical cell is the side at which the oxidation half of the redox reaction occurs. And as we learned earlier, the anodic reaction will occur at the most electronegative electrode, but this will swap back and forth during charging and discharging. For our purposes we will refer to the anode as being the electrode that contains the carbon-based active materials. Much like the cathode the anode is made up of a group of molecules held together with a binder that also attaches the active materials to a current collector that is most frequently copper. In simplest terms, the anode is the negative side of the battery equation. For a material to be a good candidate to be used in an anode it should have a couple of specific characteristics. The first is it must have electrochemical values of lithium intercalation and deintercalation that are above that of lithium metal. In other words, it should be able to store large amounts of lithium and be fully reversible, being able to give up the lithium easily and without damaging the anode material. Second, it must have both high electronic and ionic conductivity. Third, the material must have high specific capacity, both gravimetrically (mAh/g) and volumetrically (mAh/cm3). Fourth, the material must have a low working voltage versus lithium. This is different from the cathode materials where you want a large difference with lithium metal, for an anode you want it to be as close to zero against lithium metal as possible. Fifth, a good anode material must exhibit long cycle life with little to no irreversible structural Lithium-Ion Battery Chemistries. https://doi.org/10.1016/B978-0-12-814778-8.00006-5 # 2019 Elsevier Inc. All rights reserved.

115

116

CHAPTER 6 The anodes

changes. The optimal anode material will have very small variation of the electrode structure during lithium storage, in other words it should not destroy the anode material by insertion or removal of the lithium-ions. Sixth and seventh, it must also be chemically and thermally stable with the electrolyte over a wide operating range. Higher thermal stability enables a high degree of safety and high chemical stability means that it will not dissolve in the electrolyte. Finally, the eighth factor is from a cost perspective it should be abundant in nature, easily processed, contain no toxic materials, and should be economically viable (Huang & Li, 2014; Yazami, 2014). The most common anode materials since lithium-ion cells were first introduced have been those from the carbon family. Carbonic materials meet all of the requirements mentioned in the previous paragraph, in one form of the material or another. Carbon is also available in a wide variety of material types that each offer different performance characteristics. The most widely used type of carbon in lithium-ion cells is graphitic carbon, simply called graphite. But in addition to graphite other materials have been used or are being evaluated and developed today. In the carbon family are both natural and artificial graphite, graphene, hard carbon, and porous soft carbon. Other materials that may be used as anode materials include silicon, lithium metal, aluminum, germanium, tin, antimony, and titanium. Some of these materials are commercially available today while others are only in development. In evaluating different anode materials, one of the key criteria is the specific capacity of the material. Interestingly, I was not able to find any single source that compared all of the potential anode materials from this perspective. Therefore I created Table 9 to consolidate the specific capacities and volumetric

Table 9 Specific energy of various anode materials (Goriparti et al., 2014; Huang & Li, 2014, p. 169; Sun & Chang, 2017, p. 867) Element Silicon (Si) Lithium (Li) Germanium (Ge) Aluminum (Al) Tin (Sn) Graphene (C) Antimony (Sb) Hard carbon (C) Natural graphite (C) Artificial graphite (C) Soft carbon (C) Titanium (Ti)

Specific capacity (mAh/g)

Volumetric capacity (mAh/cm23)

4191 3860 1528 993 992 960 660 480 372 342 255 175

9765 2061 8626 2681 7241 4284 4402 553 837 369 293 613

Chapter 6 The anodes

capacities of different anode materials. Natural and artificial graphite have been the standard for lithium-ion batteries and are in the widest use today due to their high specific capacities of 342 mAh/g for synthetic graphite to 372 mAh/g for natural graphite. Today there is perhaps the most interest in using silicon as an anode material as its specific energy is greater than lithium and 11 times that of graphite at more than 4191 mAh/g. However, as will be discussed later in this chapter it also experiences a massive volume change upon insertion and removal of lithium of about 270% which poses a great challenge in achieving acceptable cycle life. Because of this it is not being used in its pure form but rather it is being alloyed and blended with carbon and graphites to help alleviate the cycle life challenges. However, the “gold” standard has always been lithium metal with its very high specific capacity of 3860 mAh/g. However, because of safety challenges with using lithium metal due to its very high reactivity level with both oxygen and water and the challenges associated with reversibility of the metal it has mainly been used for primary batteries until recently. But it is now getting renewed interest due to the development of the solid-state battery. In evaluating specific energy density keep in mind that the anode is typically not going to be the limiting factor when it comes to increasing energy density of a cell. Remember that even the high nickel content NMC cathodes only have a material specific energy density of 200–220 mAh/g while graphite has a specific material energy density of about 372 mAh/g. This means that to increase the energy density of the cell improving the cathode creates big gains, but even so without parallel improvements on the anode side those cathode gains may be lost—you simply cannot do one without the other (Jouanneau, Patoux, Reynier, & Martinet, 2014). In reviewing various pieces of market data from 2015 the most commonly used anode materials were natural and artificial graphite which made up more than 93% of the market (Fig. 65). If we consider all carbon types, we see that they make up about 98% of the market today. However, as we will see in this chapter there is significant growth especially in the areas of silicon and tin anodes, or more specifically in alloyed or blended anode materials using a graphite base and blending in small amounts of silicon or tin. In these cases, the volumetric energy drops to about half of that of pure silicon or pure tin, but still offer much higher capacities than graphite. The composite nature of the materials also helps to improve the volume expansion that these materials experience and helps create a more stable, longer lasting SEI layer. Additionally, another emerging practice has been to coat the silicon or tin materials with an ceramic alumina coating such as Al2O3 which also helps to mitigate some of the volume expansion (An et al., 2016). In this chapter we will review each of these different anode materials in more detail beginning with the materials in the carbon family and then covering titanium, silicon, and tin. I will mention here that lithium metal anodes were omitted from this chapter as they are not in commercial use today, but they will be discussed in more detail in Chapter 10 on Next Generation and Beyond Lithium chemistries.

117

118

CHAPTER 6 The anodes

Anode chemistries 2015 4.1% 1.4%

42.2%

Natural graphite

Artificial graphite

0.7%

51.7%

Amorphous carbon

Silicon

LTO

FIG. 65 2015 market share of different anode materials.

6.1 Carbons

Different types of carbons are the most common materials used in lithium-ion anodes today. Carbon comes in a wide variety of types ranging from the very soft, low grade carbons that are used in products such as pencil leads to the ultra-hard diamonds that are formed from carbon. It may be hard to imagine, but both pencil lead and diamonds are different forms of carbon. Remember from our discussion on atoms in Chapter 2 that a carbon atom is defined as having a fixed number of protons in its nucleus, in the case carbon there are six protons in all carbon atoms. Moreover, there are 15 different isotopes of carbon, which are carbon atoms with different numbers of neutrons but only two of those isotopes are stable ones (Pappas, 2017). There are more known compounds of carbon than any other element, with the only potential exception being hydrogen. Carbon is also the focus of an entire scientific field called organic chemistry, which is effectively the study the organic

6.1 Carbons

compounds that enable life to form with carbon being at its center. Carbon is also one of the most common materials in the universe, estimated at being the fourth most abundant element in the universe. Due to its abundance in the Earth’s crust carbon-based anodes tend to be very low cost. Carbon comes in three different crystalline forms: diamond, graphite, and fullerenes. While they all contain the same carbon atoms, they exhibit very different characteristics. Fig. 66 shows a variety of

FIG. 66 The structures of eight allotropes of carbon: (A) Diamond, (B) graphite, (C) lonsdaleite, (D) C60 (buckminsterfullerene), (E) C540 fullerene, (F) C70 fullerene, (G) amorphous carbon, and (H) single-walled carbon nanotube. € under the GNU Free Documentation License. Image released by Michael Strock

119

120

CHAPTER 6 The anodes

different forms of carbon, known as allotropes. Some of these forms of carbon exist in nature and some are man-made. Lonsdaleite, for instance, is a diamond-like structure that is believed to be more than 58% stronger than diamonds, but is extremely rare as it is made when meteorites with high amounts of graphite collide with another body such as the Earth. The fullerenes are man-made forms of carbon that form a spherical shape. However, fullerenes are not especially important in lithium-ion batteries—at least not today. Yet the single-walled carbon nanotube may play a very large role in future lithium-ion batteries (Chemicool, 2014; Matson & Orbeak, 2013). One of the most important properties of carbons is that they can form long chains of molecules—this is known as catenation. Carbon atoms have four valence electrons meaning that they can easily bind with up to four other atoms. They also form strong and stable atomic bonds and electrochemically, carbons intercalate (insert) lithium-ions in between the 2D graphene layers which translate into good mechanical stability, electrical conductivity, and lithium transport. However, only one lithium atom can be inserted for each six carbon atoms (Nitta, Wu, Lee, & Yushin, 2015). Of course, not all forms of carbon are appropriate for use in batteries. Within the lithium-ion battery carbons used as anode materials generally fall into two categories: graphitic carbons and hard carbons. The differences are due to the crystallinity and carbon atom stacking of the carbons (Goriparti et al., 2014).

6.1.1 Graphite Graphite is the most common form of carbon used in lithium-ion anodes today. Graphite offers low costs and is very abundant and easy to process, it shows good reversibility during intercalation and deintercalation, it also has a voltage potential that is close to 0 volts versus lithium, and it forms a stable SEI layer upon the first charge/discharge formation cycle. So it hits on just about all of the requirements for a good anode material that we discussed earlier ( Jouanneau et al., 2014). Graphite is also the most stable state of all the carbon atom forms; it consists of a collection of carbon atoms that are bonded together in a hexagonal lattice layer. These graphene layers have excellent stacking capability. It is this repeating hexagonal crystal structure that differentiates graphite from other forms of carbon (An et al., 2016). You may frequently see graphite shown in images as overlapping hexagons as shown in Fig. 67 which is the simplest way to visualize graphitic carbon layers. What this image represents is a group of six carbon atoms that have come together in this hexagon-shaped formation. At each intersection there is a carbon atom as shown in Fig. 68 that has formed a bond with the atom next to it. Note that the lines shown there are only simple representations of the shape of the structure based on the connections between the atoms and the size of the atoms has been increased to make it easier to visualize the shape. These bonds between these carbon atoms are formed due to the specific energies in their outermost electron bands, which is the force that keeps the atoms bound together and which forces them into the repeating crystallized hexagonal structure.

6.1 Carbons

FIG. 67 Hexagonal lattice of graphite.

FIG. 68 Crystal lattice structure of graphite sheet.

Due to the size and crystal structure of graphite, there is only room for one lithium-ion to be inserted between the six carbon atoms during the intercalation process (Goriparti et al., 2014, p. 422). Remember that intercalation refers to the process of inserting an ion into a layered structure. I have shown a simplified version of this in Fig. 69 with the graphite atoms in black and the lithium-ion in white. A more accurate perspective would be that there are two layers of the graphite where the lithiumions are inserted between these layers but are bound by the six carbon atoms. Graphite for lithium-ion batteries is either flake shaped or shaped like a sphere or oval, with the spherical form being the most commonly used today. There are many

121

122

CHAPTER 6 The anodes

FIG. 69 Lithium-ion inserted into graphite structure.

commercially available types of graphite that are frequently used in lithium-ion battery anode materials, including mesocarbon microbead (MCMB), meosphase pitchbased carbon fiber (MCF), vapor grown carbon fiber (VGCF), and massive artificial graphite (MAG) (Dahn & Ehrlich, 2011; Goriparti et al., 2014). Finally, in addition to the graphite used as the anode it is becoming more common to see additional forms of carbon being added into the graphite in order to offer different performance increases. For example, carbon nanotubes may get blended into either cathode or anode or both to improve the conductivity and add more spaces for the lithium to be inserted. A soft type of carbon known as carbon black is often used as a conductive additive with both the anode and cathode to improve conductivity and rate capability. At the single atom level, graphene may also be added to provide some of these conductivity and power benefits.

6.1.2 Soft/amorphous carbon In contrast to its name, soft carbons, also known as amorphous carbons, are not so named because of their physical strength but in fact are those that are only capable of being converted into a slightly more ordered crystal structure where the atoms are stacked in the same direction by heating them at very high temperatures. They differ from graphitic carbons in that their crystal structure does not have the same longrange order or repeatability as graphite. Soft carbons are those types of carbon that can be converted into their crystal form by treating at temperatures between 2000°C and 3000°C. Once heat treated the crystal structure of the carbon atoms is arranged in small clusters of graphene with a great deal of imperfections and randomness in their

6.1 Carbons

two-dimensional makeup. Soft carbons can be produced through the pyrolysis, or heating, of materials such as petroleum pitch, which is a product of the petroleum refining process (Huggins, 2009; Yazami, 2014). Coke and carbon black are examples of soft carbons that are sometimes used in lithium-ion batteries due to their high electrical conductivity. Yet, as an anode material for lithium-ion batteries soft carbons are generally not in very wide use as graphitic carbons tend to have superior performance characteristics. Soft carbons find their greatest use in lithium-ion batteries as conductive additives and material coatings. For example, carbon black is frequently used as a conductive additive to both the cathode and anode materials. Carbon black is also used as a primer on the metal foils that the anode and cathodes are coated onto. In this application it provides improved adhesion to the metal foil and improved conductivity. One reason for the use of carbon black is its very low cost, about 10%–20% of that of graphite as well as its wide availability with production volumes that are about 900% greater than that of natural graphite (Ge, Rong, Fang, & Zhou, 2012). Other types of soft carbons are also being used to coat graphite anode materials to improve the performance and reduce irreversible capacity loss that is experienced during the first formation cycle (Nozaki, Nagaoka, Hoshi, Ohta, & Inagaki, 2009). While others are using types of amorphous carbon to coat lithium metal anodes to prevent the rapid oxidation of the lithium metal but also to help prevent the growth of lithium dendrites during cycling (Zheng et al., 2014). So while soft carbons do not currently hold a place as the anode materials, they have found some very useful areas where they can add great benefits into the lithium-ion battery.

6.1.3 Hard carbon Hard carbon crystal structures differ from the graphite and soft carbons. If we looked at a scale from highly ordered to entirely disordered, hard carbons fall after soft and graphitic carbons in that they have an entirely random or disordered alignment of the carbon atoms compared to soft carbons with a semiordered crystal structure and graphites with a highly structured crystal structure. This random alignment of the crystal sheets provides a lot of voids that can accommodate lithium-ions when compared to graphitic. The disorder also leads to a large amount of cross linking between the different layers of hard carbons due to the type of precursors used. The hard carbons also have a smaller number of unit cells, remember the unit cell is the elements’ arrangement of atoms which is unique for each material, which results in a lower number of stacking layers (Yazami, 2014). Hard carbons are produced through the pyrolysis of solid material precursors, but unlike the soft carbons they cannot be made into a more ordered crystal structure even when heating to temperatures over 3000°C. This is due to the presence of noncarbon atoms such as oxygen and sulfur that limit the crystal growth during heat treatment (Huggins, 2009; Yazami, 2014). The biggest advantage of hard carbons is that they have a higher practical capacity than graphite. They also have much less of a tendency for lithium metal dendrites

123

124

CHAPTER 6 The anodes

to grow because the voltage window at which this can occur is very low in hard carbons. And while graphite may experience a volumetric change of up to 12%, hard carbon experiences almost zero volume change, technically it is around 0.5% or less, which allows for longer cycle life than in cells using graphite (Goriparti et al., 2014). However, hard carbons have a couple of weaknesses including low initial coulombic efficiency and low tap density. Hard carbon’s higher practical capacity is offset by the fact that it also experiences a much larger permanent capacity loss on first cycling when compared to graphitic carbons. It also shows a higher irreversible capacity, capacity fading with cycling, low energy density, and requires a higher deintercalation voltage than graphitic carbons. It has also been found that the electrochemical behavior of hard carbons is different from graphitic carbons. Because of the generally disordered structure of the hard carbon the voltage potential varies gradually rather than in the stepwise function of graphitic carbons. This greater potential leads to lower voltage at the cell level, typically maxing out at about 4.1 volts (Huggins, 2009; Yazami, 2014). Hard carbon has been successfully used as an anode material by several manufacturers, but as most cell manufacturers are racing to increase energy density hard carbon is being set aside in place of the graphitic carbons. And now even more so due to the introduction of blended graphite and silicon-type anodes which offer much higher energy density than hard carbon can offer.

6.1.4 Graphene Graphene is a crystalline allotrope of carbon and is the building block for most carbon-based materials. Graphene is simply a sheet of single carbon atoms arranged in a two-dimensional hexagon formation, a honeycomb-shaped network of carbon atoms bonded into two-dimensional sheets of single atom thickness (Fig. 70). So when you think of graphene, think of single carbon atoms bound together. In fact, all of the other types of carbons that we have already discussed are built up of graphene atoms. By itself a single layer of graphene tends to have lower energy density

FIG. 70 Graphene sheet.

6.2 Lithium titanium oxide

than graphite, but when a small number of graphene sheets are used together they have shown energy density of between 780 and 1,116 mAh/g (Goriparti et al., 2014). Graphene also shows tensile strength that is 100 times that of steel, thermal conductivity that is 3 times that of diamond, and electron mobility that is more than 10 times greater than silicon (China Energy Storage Alliance, 2015). As a pure anode material graphene has so far shown pretty low columbic efficiency and low cycle stability so it has not made a good option as an anode material by itself. However, as a compound material graphene may offer some signification advantages. Graphene research has already been strong in the areas of compounds with tin, silicon, and the transition metals. When graphene is used in concert with silicon it offers a reduction in the size of the active material, it prevents agglomeration of nanoparticles, it tends to improve the electrical and ionic performance, and it improves the mechanical stability of the active materials (China Energy Storage Alliance, 2015). In addition to these important benefits, it may have a much larger role to play as a material coating than as a pure anode material. Today there is much research happening in taking active material molecules and coating them with a few layers of graphene or alumina atoms through what is known as an atomic layer deposition (ALD) process (Goriparti et al., 2014). Son et al. (2017) describe creating a graphene ball as a coating material for silicon particles and for high nickel content cathode materials. In this case they start with a silicon oxide particle as the center and build graphene layers around it using a chemical vapor deposition (CVD) method. As this research suggests, graphene is likely to play a much bigger role as a coating or supplement than as a pure anode material at least in the near term. But in addition to working as a coating it is being used as a conductive additive where it has proven to offer much greater cycle life performance and power capability of cells that integrate graphene in this manner (China Energy Storage Alliance, 2015). Other research has focused on creating a graphene shell around the active material compounds in order to protect life and manage expansion.

6.2 Lithium titanium oxide

Lithium titanium, also known as Li4Ti5O12 or simply LTO, has experienced some significant interest over the past 10 years or so in spite of the fact that it has a

125

126

CHAPTER 6 The anodes

relatively low energy density of 175 mAh/g, which is about half of that of graphite. The result of this low energy density is that, at the cell level it ends up with an energy density of only about 70 Wh/kg to 80 Wh/kg compared to cells using graphite anodes which range from 120 Wh/kg for LFP chemistries with graphite anodes to over 280 Wh/kg for NCA chemistries with graphite anodes. So LTO is not a good high energy material, but has some other very interesting properties. The composition of LTO is that of the spinel structure as shown in Fig. 71 and is similar to that of the cathode material LMO discussed in the previous chapter. This structure creates “tunnels” in three dimensions where the lithium-ions are inserted. This tunnel-like structure is what keeps the volume change in LTO at a minimum. During cycling its three-dimensional structure undergoes virtually no volume change (99.99

145

146

CHAPTER 7 Inactive materials

Table 13 USABC LLC requirements for conventional electrolytes #

Parameter

1 2 3

Conductivity at 30°C Li+ transference No. Viscosity 30°C 30°C Water content HF content Components purity

5 6 7

Unit mS/cm – cP

ppm ppm %

Goal >12 >0.35