Development of Redox Mediators for High-Energy-Density and High-Efficiency Lithium-Oxygen Batteries (Springer Theses) 981162531X, 9789811625312

Table of contents :
Supervisor’s Foreword
Preface
Parts of this thesis have been published in the following documents:
Acknowledgements
Contents
List of Figures
1 Introduction
1.1 Research Objectives
1.2 Introduction to Lithium–Oxygen Battery
1.3 Principle of Redox Mediators
References
2 Exploring a New Redox Mediator Inspired by Biological System
2.1 Research Background
2.2 Experimental Method
2.2.1 Preparation of Cells and Conditions for Electrochemical Tests
2.2.2 Characterization
2.2.3 Quantitative Analysis of Discharge Product
2.3 Result and Discussion
2.4 Concluding Remarks
References
3 Investigation on the Kinetic Property of Redox Mediators
3.1 Research Background
3.2 Experimental Method
3.2.1 Preparation of Cell Components and Assembly of Lithium–Oxygen Cells
3.2.2 Conditions for Electrochemical Tests
3.3 Results and Discussions
3.3.1 Concluding Remarks
References
4 Addressing Shuttle Phenomena: Anchored Redox Mediator for Sustainable Redox Mediation
4.1 Research Background
4.2 Experimental Method
4.2.1 Synthesis of PTMA
4.2.2 Fabrication of Cell Components
4.2.3 Assembly of Lithium–Oxygen Cell
4.2.4 Conditions for Electrochemical Tests and Materials Characterization
4.3 Results and Discussions
4.4 Supplement Notes
4.4.1 Calculations of Nitroxyl Radical Concentration
4.4.2 Electron Diffusion Phenomenon in PTMA
4.5 Concluding Remarks
References
5 Conclusion

Citation preview

Springer Theses Recognizing Outstanding Ph.D. Research

Youngmin Ko

Development of Redox Mediators for High-EnergyDensity and High-Efficiency Lithium-Oxygen Batteries

Springer Theses Recognizing Outstanding Ph.D. Research

Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected for its scientific excellence and the high impact of its contents for the pertinent field of research. For greater accessibility to non-specialists, the published versions include an extended introduction, as well as a foreword by the student’s supervisor explaining the special relevance of the work for the field. As a whole, the series will provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today’s younger generation of scientists.

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More information about this series at http://www.springer.com/series/8790

Youngmin Ko

Development of Redox Mediators for High-Energy-Density and High-Efficiency Lithium-Oxygen Batteries Doctoral Thesis accepted by Seoul National University, Seoul, South Korea

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Author Dr. Youngmin Ko Department of Materials Science and Engineering Seoul National University Seoul, Korea (Republic of)

Supervisor Prof. Dr. Kisuk Kang Advanced Energy Materials Laboratory Department of Materials Science and Engineering Seoul National University Seoul, Korea (Republic of)

ISSN 2190-5053 ISSN 2190-5061 (electronic) Springer Theses ISBN 978-981-16-2531-2 ISBN 978-981-16-2532-9 (eBook) https://doi.org/10.1007/978-981-16-2532-9 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Supervisor’s Foreword

Energy storage systems are the key technology to support sustainable development of human society. They enable the facile exploitation of intermittent renewable power sources such as wind and solar power by chemically storing surplus electricity. They are also expected to make human civilization free from fossil fuels by realizing various portable electric power. Lithium-ion batteries are the state-of-theart commercialized energy storage systems and have occupied majority portion of markets since their first advent in 1991. However, their performance (i.e., energy density) is almost reaching up theoretical limits so that society waits for the breakthrough in next-generation energy storage systems to replace current lithiumion batteries. Among various candidates as next-generation systems, lithium–oxygen batteries are considered most promising systems owing to their extremely high theoretical energy density. In spite of their potentiality, however, the critical challenges mainly originated from intrinsic nature of reaction products of lithium peroxide hinder the realization of practical lithium–oxygen batteries. To address these challenges, researchers have suggested various approaches including the use of catalysts such as redox mediators. This thesis written by Dr. Youngmin Ko addresses the exploitation of redox-mediated catalysts, redox mediators to resolve critical challenges of lithium– oxygen batteries, and thus, improve their performance. The thesis consists of introduction part and following three main parts. The introduction provides the background knowledge essential to understand the content of the thesis such as principles of lithium–oxygen batteries and redox mediators. The main part deals with researches Dr. Youngmin Ko conducted during his doctoral course. Multifaceted researches regarding the development of redox mediators will be introduced, including the exploration of a new material inspired from biological system, the investigation of fundamental aspects such as kinetic property and reaction mechanism of redox mediators, and addressing practical issues raised by redox mediators. As the supervisor, I assure that this thesis will provide broad readers regardless of their background more in-depth understanding on energy

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Supervisor’s Foreword

storage systems, lithium–oxygen batteries, and redox mediators. I also believe that researches of Dr. Youngmin Ko will inspire and motivate the researchers in the field of electrochemistry and catalysis, thereby contributing to the advancement of technology in energy storage systems. Seoul, South Korea March 2021

Prof. Dr. Kisuk Kang

Preface

Encounter with energy and environment issue is inevitable for human being to construct a clean and sustainable future. In this regard, the development of energy storage system to facilitate the exploitation of renewable energy resources is essential mission. After continued research during last decades, Li-ion battery is widely employed as the state-of-the-art battery system. However, exploiting heavy transition metal as core component, Li-ion battery possesses clear limitation in terms of energy density enhancement. Scientists have explored various battery chemistry to replace current Li-ion battery such as metal–air, metal–sulfur, and organic battery. Especially, Li-O2 battery has received enormous research interest owing to its extremely high theoretical energy density of 3500 Wh kg-1. Despite its potential, however, limitations stemmed from intrinsic nature of the system still prevent the practical use of Li-O2 battery. In this dissertation, I developed and designed high-performance Li-O2 battery by introducing redox-mediating catalyst, redox mediator to regulate reaction chemistry of Li-O2 battery. With a virtue of redox mediator capable of controlling reaction path, disadvantageous conventional discharge/charge path could be circumvented. Under redirected redox-mediated reaction path, problems originated from conventional path could be successfully released. For the design of better redox mediators, multidisciplinary study from the exploration of a new materials to the understanding on fundamental aspects was conducted. Furthermore, practical issues in cell environment such as shuttle phenomena were also addressed to optimize the use of redox mediators. Chapter 2 introduces the procedure of exploration of a novel redox mediator from biological system. Inspired by the similarity between cell respiration process in biology and redox-mediated oxygen reduction reaction process in Li-O2 battery, the biological catalyst, vitamin K2 was introduced into Li-O2 battery as redox mediator to facilitate oxygen reduction reaction, and it was demonstrated that it successfully works as catalyst and boosts the performance of the cell. This chapter will provide guideline on how similarity can motivate new findings.

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Preface

Chapter 3 describes research approach to understand fundamental aspects of redox mediator from reaction kinetics to reaction mechanism. With comparative study employing various redox mediators, I revealed the factors affecting catalytic kinetics and its significant on cell performance. On oxygen reduction reaction (i.e., discharge) kinetics of redox mediators is governed by steric hindrance around redox center, which reveals that the nature of electron transfer during redox-mediated reaction is inner-sphere electron transfer where intermediate state presents. Further study on its implication on cell performance demonstrated the presence of volcano behavior between kinetics and cell performance. Similarly, kinetics of redox mediator during oxygen evolution reaction (i.e., charge) was also investigated, and it was shown that its kinetic is primarily determined by redox potential of mediator following Marcus theory. Followed study showed that higher kinetics results in better rate capability of Li-O2 cell. This chapter demonstrates the importance of understanding intrinsic properties of redox mediator to design highly performing catalyst. Lastly, Chap. 4 demonstrates strategies to address practical shuttle phenomena of redox mediators. Owing to freely diffusible nature, redox mediator easily diffuses to unwanted reactive anode side causing severe side reactions such as anode degradation and mediator decomposition. By anchoring redox mediator in polymer chain, shuttle phenomenon was suppressed while still maintaining charge-carrying property by polymer in-chain hole diffusion mechanism. As the second approach, a novel Janus liquid electrolyte was employed to fully maintain diffusion behavior and at the same time prevent shuttle phenomena. With the simultaneous use of two liquid electrolytes with distinct solvation capability at two electrodes, the diffusion of redox mediator from cathode to anode side was blocked and thus, shuttle effect was prevented. This chapter demonstrates that appropriate treatment on cell system can suppress shuttle phenomena and enable sustainable use of redox mediators. I believe the multidisciplinary study in this dissertation on redox mediator with three major approaches, such as (i) exploration of a new catalyst, (ii) investigation on fundamentals, and (iii) dealing with practical issues will serve as the solid foundation in developing high-performance redox mediator for Li-O2 battery. In addition, it will provide research motivation and insight to design optimized catalyst for metal–air battery and furthermore, next-generation energy storage system. Seoul, South Korea April 2021

Dr. Youngmin Ko

Parts of this thesis have been published in the following documents: 1. Y. Ko*, H. Park*, B. Kim*, J. S. Kim and K. Kang, “Redox Mediators: A Solution for Advanced Lithium–Oxygen Batteries,” Trends Chem. 1, 349–360 (2019). 2. Y. Ko, H. Park, J. Kim, H. D. Lim, B. Lee, G. Kwon, S. Lee, Y. Bae, S. K. Park and K. Kang. “Biological redox mediation in electron transport chain of bacteria for oxygen reduction reaction catalysts in lithium–oxygen batteries,” Adv. Funct. Mater. 29, 1805623 (2019). 3. Y. Ko, H. Park, B. Lee, Y. Bae, S. K. Park and K. Kang. “A comparative kinetic study of redox mediators for high-power lithium–oxygen batteries,” J. Mater. Chem. A7, 6491–6498 (2019). 4. Y. Ko, H. Park, K. Lee, S. J. Kim, H. Park, Y. Bae, J. Kim, S. Y. Park, J. E. Kwon and K. Kang. “Anchored Mediator Enabling Shuttle‐Free Redox Mediation in Lithium‐Oxygen Batteries,” Angew. Chem.132, 5414–5418 (2020). (*indicates equal contributors)

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Acknowledgements

First of all, I sincerely express my gratitude to Prof. Kisuk Kang who supervised me during the 5 years of doctoral course. It was a great honor to be able to conduct a doctoral course under your supervision. Through your guidance, I could learn how to think, speak, and communicate as a researcher. Once again, I would like to thank Prof. Kisuk Kang for helping me to take the first step as an independent researcher. I also would like to thank the group members of the Advanced Energy Materials Laboratory who shared happy and sad moments during the degree course. In particular, many thanks to members of air battery research team, Dr. Hee-Dae Lim, Dr. Jinsoo Kim, Dr. Byungju Lee, Dr. Youngjoon Bae, Dr. Hyeokjun Park, Mr. Sung Kwan Park, and Mr. Kyungoh Kim. Also, I would like to express my gratitude to Mr. Sehwan, Mr. Giyun, Mr. Kyungho, Mr. Byunghoon, Mr. Sung Kwan, and Mr. Orapa, who started life in graduate school together with basic seminar and are still helping me. In addition, I also feel grateful for Team C members, Mr. Myeonghwan, Dr. Juseong, Dr. Hyunah, Mr. Sung Kwan, Mr. Youngro, Dr. Youngjoon, and Dr. Hyeokjun for giving me many laughs when I felt difficulties in graduate school. Lastly, I would like to express my sincere gratitude to my family, parent, and younger brother Youngtae, who gave me big support emotionally, and financially. Whenever I felt difficulties in graduate school, your support made me overcome them and take a step forward. Without support from my family, I could have not completed the doctoral course. Once again, I am sincerely grateful to all of you. I will keep your help in mind forever and do my best to repay with greater help. Thank you.

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Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Research Objectives . . . . . . . . . . . . . . . 1.2 Introduction to Lithium-Oxygen Battery . 1.3 Principle of Redox Mediators . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . .

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2 Exploring a New Redox Mediator Inspired by Biological System . 2.1 Research Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Experimental Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Preparation of Cells and Conditions for Electrochemical Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Quantitative Analysis of Discharge Product . . . . . . . . . . 2.3 Result and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 Investigation on the Kinetic Property of Redox Mediators 3.1 Research Background . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Experimental Method . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Preparation of Cell Components and Assembly of Lithium–Oxygen Cells . . . . . . . . . . . . . . . . . 3.2.2 Conditions for Electrochemical Tests . . . . . . . . 3.3 Results and Discussions . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Concluding Remarks . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Addressing Shuttle Phenomena: Anchored for Sustainable Redox Mediation . . . . . . . 4.1 Research Background . . . . . . . . . . . . . 4.2 Experimental Method . . . . . . . . . . . . .

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4.2.1 4.2.2 4.2.3 4.2.4

Synthesis of PTMA . . . . . . . . . . . . . . . . . . . . . . . . Fabrication of Cell Components . . . . . . . . . . . . . . . Assembly of Lithium–Oxygen Cell . . . . . . . . . . . . . Conditions for Electrochemical Tests and Materials Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Results and Discussions . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Supplement Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Calculations of Nitroxyl Radical Concentration . . . . 4.4.2 Electron Diffusion Phenomenon in PTMA . . . . . . . 4.5 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Basic reaction mechanism of Li-O2 battery on discharge and charge process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schematics showing the formation and decomposition of discharge product, Li2O2, with the mediation of redox mediator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The properties of the RM-employing Li-O2 battery critically influenced by the intrinsic characteristics of RM . . . . . . . . . . a Biological system in E. coli. The electron transport coupled with proton shuttling is mediated by vitamin K2 between NADH dehydrogenases and the cytochrome b–c1 complex. b Relative redox potentials of vitamin K2 and its comparison with the redox potential of other redox species in E. coli and lithium-oxygen battery. c ORR process in lithium-oxygen battery mediated by vitamin K2 as potential ORR RM. . . . . . a Cyclic voltammetry profiles of vitamin K2 and Coenzyme Q in 1 M LiTFSI DME electrolyte. Vitamin K2 shows stable profile up to 200 cycles, confirming reversible and stable redox reaction. b Cyclic voltammetry profile of coenzyme Q in the same electrolyte showing irreversible redox reaction . . . . . . . a Cyclic voltammetry profile of vitamin K2 under various scan rates ranging from 50 mV s−1 to 500 mV s−1. b Cyclic voltammetry profiles of vitamin K2 obtained under Ar and oxygen atmosphere confirming the catalytic activity of vitamin K2 as ORR RM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a Galvanostatic discharge profile of the lithium–oxygen cells without and with vitamin K2 at various current densities. The discharge capacity is boosted up to 30 times with the addition of vitamin K2. b Magnified graph of (a) showing the negligible discharge capacity without vitamin K2 . . . . . . .

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List of Figures

The measurement on the oxygen consumption rate during vitamin K2-mediated discharge process showing the 99.85% of oxygen consumption efficiency . . . . . . . . . . . . . . . . . . . . . . Photographs and SEM images of air electrode as pristine (top) and discharged without vitamin K2 (left) and discharged with vitamin K2 (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High magnification SEM images of air electrode a, d pristine electrode, b, e electrode fully discharge without vitamin K2, and c, f electrode fully discharged with vitamin K2. With the addition of vitamin K2, the morphology of discharge product was transformed to large toroidal shape . . . . . . . . . . . . . . . . . X-ray diffraction pattern of electrode discharged with vitamin K2 showing that major discharge product is Li2O2 . . . . . . . . . a Raman spectra and b IR spectra of electrode discharged using vitamin K2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a UV–vis spectra for varying amounts of Li2O2 powder with the purity of 92.8%. b graph showing the linearity between the amount of Li2O2 powder and peak intensity of UV–vis spectra at 407 nm, indicating that the peak intensity can be used as parameter to quantify the amount of Li2O2. Based on the linearity, the Li2O2 yield of the cell without and with vitamin K2 was calculated to be 78% and 86%, respectively. As air electrodes for vitamin K2-free lithium-oxygen cells, commercial carbon paper (P50, Fuelcellearth) was used. . . . . a Galvanostatic discharge profile of the lithium-oxygen cell under the current density of 0.18 mA cm−2 using various ORR RMs, and b bar graph summarizing the obtained discharge capacity in (a). All experiments were conducted with 1 M LiTFSI DME electrolyte containing 0.01 M of each molecule. Vitamin K2 and DBBQ show remarkably high discharge capacity (i.e. catalytic activity) than coenzyme Q and other materials. (Abbreviation for each materials are as follows; BBQ(2-tert-Butyl-1,4-benzoquinone), MBQ(Methyl-pbenzoquinone), MD(Menadion), CoQ(Coenzyme Q)). . . . . . . Schematics comparing the Li2O2 formation mechanism without and with vitamin K2. During conventional discharge without vitamin K2, Li2O2 formation occurs through the generation of surface-absorbed intermediate, LiO2, due to its limited solubility in solvent with low DN. However, during vitamin K2-mediated discharge, Li2O2 formation is mediated by vitamin K2 in the electrolyte solution, during which the disproportionation of a new intermediate species, LiVKO2 results in the formation of toroid-like Li2O2 in electrolyte solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Redox potential and corresponding electron energy diagram related to the vitamin K2-mediated discharge. Due to the higher redox potential of vitamin K2 than O2, vitamin K2 is reduced prior to O2. Subsequently, triggered by the redox potential difference between reduced vitamin K2 and Li2O2, a spontaneous electron transfer from LiVKO2 to Li2O2 occurs leading to the formation of Li2O2 in the solution . . . . . . . . . . Galvanostatic cycling profiles of cells containing vitamin K2 showing that electrochemical charge is hardly possible. . . . . . Cyclic voltammetry profiles of the 1 M LiTFSI DME electrolyte containing vitamin K2 and TEMPO under scan rate of 50 mV s−1 confirming the independent redox reaction of both RMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Galvanostatic cycling profile of lithium-oxygen cells containing 0.01 M of vitamin K2 and 0.1 M of TEMPO (0.1 M). The cell was cycled with the current density of 0.18 mA cm−2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . In-situ analysis on gas evolution during the charging process of lithium-oxygen cells containing both vitamin K2 and TEMPO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SEM image of the electrode of lithium-oxygen cells with both vitamin K2 and TEMPO a after discharge and b charge . . . . a Photographs of the titration solutions prepared by discharged and charged electrodes. The evolution of yellow color indicates the presence of Li2O2 in electrodes. b UV–vis absorption spectra of the solutions in (a). Peak at 407 nm is assigned to the formation [Ti(O2)]2+ complex in the solutions representing the existence of Li2O2 . . . . . . . . . . . . . . . . . . . . . Mechanism of RM-assisted charge consisting of following three steps, step (1) electrochemical RM oxidation, step (2) RM+ and RM diffusion, and (3) chemical lithium peroxide C decomposition by RM+. Step 1 results in an CRMRMþ increase

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at the surface of electrode and step 2 and 3 lead to CRMRMþ decrease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schematic illustrations voltage profile during galvanostatic charge in a ideal and b actual case . . . . . . . . . . . . . . . . . . . . . Schematics showing the process of RM-assisted lithium peroxide decomposition during RDE experiment following EC mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Profiles of linear sweep voltammetry obtained with a TEMPO, b MPT, c DMPZ, and d TTF. Voltage scans were carried out at a scan rate of 1 mV s−1. 1 M LiTFSI DME electrolyte containing 1 mM RM and excessive amount of dispersed lithium peroxide powder was used . . . . . . . . . . . . . . . . . . . . .

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List of Figures

Linear sweep voltammetry profile using butyl-benzoquinone that is inactive toward lithium peroxide powder. The butylbenzoquinone reduction was employed to study the impact of dispersed lithium peroxide powder in il;a decrease. Approximately 28.6% decrease in il;a was observed with the addition of lithium peroxide powder . . . . . . . . . . . . . a Bar graph showing corrected il;a increase by various redox couple; the corrected il;a increases from redox couples are as follows: TEMPO/TEMPO+, 50.05%; MPT/MPT+, 31.91%; DMPZ/DMPZ+, 23.14%; DMPZ+/DMPZ2+, 59.51%; TTF/TTF+, 27.92%; and TTF+/TTF2+, 25.43%. b Plot of il;a increase versus redox potential of RMs . . . . . . . . . . . . . Cyclic voltammetry profile obtained by using 1 M LiTFSI DME containing 50 mM of a TEMPO, b MPT, c DMPZ, and d TTF under the scan rate of 100 mV s−1. The redox potential of each redox couple was calculated as follows: TEMPO/ TEMPO+, 3.73 V; MPT/MPT+, 3.82 V; DMPZ/DMPZ+, 3.26 V; DMPZ+/DMPZ2+, 3.94 V; TTF/TTF+, 3.42 V; and TTF+/TTF2+, 3.76 V (vs. Li/Li+) . . . . . . . . . . . . . . . . . . . Cyclic voltammetry profile of a TEMPO, b MPT, c DMPZ, and d TTF under the scan rate ranging from 100 to 500 mV s−1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The plot of (scan rate)1/2 versus peak current from cyclic voltammetry profile in the case of a TEMPO, b MPT, c DMPZ, and d TTF for various scan rates ranging from 100 to 500 mV−1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a RM diffusivities calculated by Randles–Sevcik equation. b Plot of RM diffusivity versus il;a increase presenting overall kinetic properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Galvanostatic cycling profiles and their corresponding dQ/dV profiles of RM-containing lithium–oxygen cells. a–d galvanostatic cycling profiles under various current rate. e–f Corresponding dQ/dV profiles . . . . . . . . . . . . . . . . . . . . . a–d Gas analysis result obtained during charging process of lithium–oxygen cells under a current rate of 1.0 mA cm−2. The oxygen and carbon dioxide emission was indicated by the grey and black lines, respectively . . . . . . . . . . . . . . . . Cyclic voltammetry profiles (a and c) and corresponding peak current versus (scan rate)1/2 plot (b and d) obtained by vary scan rate from 100 mV s−1 to 500 mV s−1 . . . . . . . . . . . . . . . Viscosity of three ether solvents, DME (Ref. [19]), DEGDME (Ref. [20]), and TEGDME (Ref. [20]) and TEMPO diffusivity in these ether-based electrolytes . . . . . . . . . . . . . . . . . . . . . . .

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List of Figures

Fig. 3.15

Fig. 3.16

Fig. 4.1

Fig. 4.2

Fig. 4.3 Fig. 4.4

Fig. 4.5

Fig. 4.6

Fig. 4.7

Fig. 4.8

Viscosity and ionic conductivity of ether-based electrolytes [19, 20]. The ionic conductivity of 1 M LiTFSI DME, 1 M LiTFSI DEGDME, and 1 M LiTFSI TEGDME was measure to be 13.39, 7.62, and 2.62 mS cm−1 at 300 K, respectively . . . Galvanostatic cycling profiles (a and c) and dQ/dV profiles (b and d) of lithium–oxygen cell containing 50 mM TEMPO in 1 M LiTFSI DEGDME and 1 M LiTFSI TEGDME electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Galvanostatic charge profile using electrolyte of 2 M LiTFSI TEGDME containing 10 mM of TEMPO, showing the ‘over charge’ capacity of TEMPO because of shuttle effect . . . . . . . a Lithium metal photographs before cycling (top) and after 5 times cycling without (left) and with TEMPO (right). b SEM images of lithium metal cycled without (left) and with TEMPO (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UV–vis absorption spectra of electrolyte containing TEMPO before cycling and after 5 times cycling . . . . . . . . . . . . . . . . . a Galvanostatic cycling profiles of RM-free lithium–oxygen cell. b Galvanostatic cycling profiles of cells containing 10 mM of TEMPO, showing that the characteristic voltage plateau from TEMPO redox is shortened with repeated cycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison on cycling performance of RM-free lithium– oxygen cells and TEMPO-containing cells. The cells were operated with a constant current density of 300 mA g−1 . . . . Schematics illustrating working mechanism of TEMPO (left) and PTMA (right). For TEMPO, a typical RM, a detrimental shuttle effect is accompanied with the operation of TEMPO because its mobile nature results in the diffusion of TEMPO toward reactive lithium metal anode. However, PTMA, a polymer-type RM, does not trigger shuttle effect due to its fixation on air electrode by controlled solubility. The electron self-exchange in PTMA polymer replaces the mass diffusion of TEMPO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PTMA synthesis scheme by GTP and RP methods. a Reaction scheme of GTP method to synthesize GTP-PTMA. b Reaction scheme of RP method to synthesize RM-PTMA . . . . . . . . . . . SQUID and EPR spectra of the two synthesized PTMA polymers. Plots of temperature versus v−1 paramagnetic and its Curie–Weiss fittings of a GTP-PTMA and b RP-PTMA. EPR spectra of 10−3 M chloroform solution with c GTP-PTMA and d RP-PTMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Fig. 4.10

Fig. 4.11

Fig. 4.12 Fig. 4.13 Fig. 4.14 Fig. 4.15

Fig. 4.16 Fig. 4.17

Fig. 4.18

Fig. 4.19

Fig. 4.20

Fig. 4.21

List of Figures

Galvanostatic charge profile of GTP-PTMA. GTP-PTMA exhibited a clear voltage plateau at *3.6 V (vs. Li/Li+) and showed the charge capacity close to theoretical value . . . . . . . Photographs of 2 M LiTFSI TEGDME electrolyte where PTMA was added. The dissolution of PTMA was not observed until 1 month . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Linear sweep voltammetry profiles of carbon electrodes containing PTMA and Li2O2, showing PTMA-mediated Li2O2 decomposition occurs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SEM images of a bare carbon electrode and b PTMA-coated electrode. The insets are TEM images of each electrode . . . . . X-ray diffraction patterns of PTMA-coated electrode obtained as pristine, after discharge, and after charge . . . . . . . . . . . . . . SEM images of carbon electrode a before discharge, b after discharge and c after charge . . . . . . . . . . . . . . . . . . . . . . . . . . Galvanostatic cycling profile a first cycle obtained from lithium–oxygen cells without any RM, with TEMPO, and with PTMA. The cells were cycled at a constant current density of 300 mA g−1 carbon using 2 M LiTFSI TEGDME . . . . . . . . . . . . . In-situ gas analysis result during the first charging process of PTMA-containing lithium-oxygen cell . . . . . . . . . . . . . . . . . . UV–vis absorption spectra obtained from electrolyte of PTMA-containing lithium–oxygen cells before and after 5 cycles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photographs of lithium metal anode a before cycling, b after 5 times cycling without PTMA, and c 5 times cycling with PTMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Li2O2 quantification result for Li2O2 pre-loaded electrodes after charged with TEMPO (red dots) and PTMA (orange dots). The standard deviation is shown together . . . . . . . . . . . Cycling performance of lithium–oxygen cells without RM, with TEMPO and with PTMA. The cells were operated under the constant current density of 300 mA g−1 carbon . . . . . . . . . . . . Galvanostatic cycling profile of lithium–oxygen cells containing PTMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 1

Introduction

1.1

Research Objectives

With the prosperity of human technology, society not only takes fruits of it, but also faces accompanied problems such as energy and environmental issues. Among them, the heavy use of fossil fuel is considered the major issue causing severe pollution problem. To this end, the development of sustainable energy resources such as solar and wind energy is the urgent mission for human beings, and therefore, an energy storage system to store electricity is a key technology to realize renewable future energy society. After continued development during the last decades, Li ion battery is currently widely used as the most promising energy storage system [1–4]. However, further enhancement of its energy density is limited because heavy transition metal such as manganese, cobalt, iron, and nickel sets an upper boundary for gravimetric energy density. To develop a new system whose energy density surpasses that of the current Li–ion battery, researchers have explored various chemistry such as metal-air [5–9], metal-sulfur [7, 10–12], and organic battery [13]. Among them, metal-air battery has received intensive attention due to its potentially high energy density attributed to the use of light gas species as redox active materials. Especially, Li–O2 battery exhibits the highest theoretical energy density among metal-air chemistry, and has been developed since its first advent in 1996 [14]. Although its great potentiality for extremely high energy density, however, Li–O2 battery have numerous problems. Most severely, the insulating nature of discharge product, Li2O2, causes integrated problems during both discharge and charge process [5, 6, 8]. For example, electrode passivation by Li2O2 during discharge leads to practically low energy density [15], and low conductivity causes high IR drop during charge leading to low coulombic efficiency [16]. This research aims to develop high-performance Li–O2 battery by addressing limitations induced by the intrinsic nature of discharge product. The Main approach is to drive a new discharge/charge path bypassing conventional reaction path by introducing redox-mediating catalysts. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 Y. Ko, Development of Redox Mediators for High-Energy-Density and High-Efficiency Lithium-Oxygen Batteries, Springer Theses, https://doi.org/10.1007/978-981-16-2532-9_1

1

2

1.2

1

Introduction

Introduction to Lithium–Oxygen Battery

Li–O2 battery exhibits the highest theoretical energy density among various metal-air chemistry because it exploits the lightest metal, lithium as the anode, and light oxygen gas as redox active species at the cathode side. The electrochemical reactions occurring in Li–O2 battery are as follows. ðAnodeÞ ðCathodeÞ

LiðsÞ ! Li þ þ e

ð1:1Þ

O2 ðgÞ þ 2e ! O2

ð1:2Þ

ðTotalÞ 2LiðsÞ þ O2 ðgÞ ! Li2 O2 ðsÞ

ð1:3Þ

On discharge (i.e. oxygen reduction reaction (ORR)), the oxidation of lithium metal to Li+ occurs at the anode side (Rxn. 1.1). At the same time, the reduction of oxygen gas occurs at the cathode side (Rxn. 1.2) where Li+ and reduced oxygen form Li2O2 as a final discharge product (Rxn. 1.3). On charge (i.e. oxygen evolution reaction (OER)), the reverse reactions (i.e. reduction of Li+ to lithium metal at anode, and decomposition of Li2O2 to Li+ and O2 at cathode) happens. Considering solid discharge product forms from gas phase, the cathode consists of a conductive and porous matrix such as carbon to store discharge product. Since the operation of Li–O2 battery is entirely controlled by the formation and decomposition of a discharge product of Li2O2, its native property plays a critical role in determining the chemistry of Li–O2 battery [8]. The basic reaction process is illustrated in Fig. 1.1. Based on the weight of Li2O2 (MW = 46) and its theoretical formation potential (2.96 V vs. Li/Li+), Li–O2 battery is calculated to have the high theoretical energy density of 3500 Wh kg−1, which much exceeds that of a conventional Li–ion battery. [5] However, the fact that Li2O2 has extremely poor electrical conductivity prevents the realization of high energy density, and also causes further problems regarding system stability, and round-trip efficiency. For example, the formation of insulating Li2O2 on electrode surface during discharge triggers electrode passivation. [15] It leads to premature termination of discharge process and limited discharge capacity. Accordingly, the practical energy density of Li–O2 battery is not as high as 3500 Wh kg−1, and rather strongly dependent on microstructure of cathode (e.g. surface area, and porosity) The insulating nature of Li2O2 is also problematic on charge process during which electrochemical decomposition of Li2O2 occurs. Owing to low electrical conductivity, the amount of IR drop and increased overpotential cannot be ignored. It results in low round-trip efficiency [17]. Moreover, increased charge potential creates an oxidative environment promoting decomposition of cell components such as electrolyte and carbon electrode [18–20]. Consequently, to develop Li–O2 battery with practically high energy density and high round-trip efficiency, the limitation stemmed from insulating nature of Li2O2 must be overcome.

1.2 Introduction to Lithium–Oxygen Battery

3

Oxygen Reduction Reaction (ORR) 2Li+ + 2e- + O2

Li2O2

(Gas → Solid)

Discharge + O2

Li2O2

- O2 Charge Oxygen Evolution Reaction (OER) Li2O2

2Li+ + 2e- + O2 (Solid → Gas)

Fig. 1.1 Basic reaction mechanism of Li–O2 battery on discharge and charge process

1.3

Principle of Redox Mediators

Redox mediator (RM) is the type of catalyst that mediates redox reaction by reduced or oxidized itself [21–25]. How RM can mediate discharge and charge process of Li–O2 battery is expressed as follows. (On discharge, ORR) Li þ þ RM þ e ! LiRM

ð1:4Þ

2LiRM þ O2 ! Li2 O2 þ 2RM

ð1:5Þ

RM ! RM þ þ e

ð1:6Þ

2RM þ þ Li2 O2 ! 2Li þ þ O2 þ 2RM

ð1:7Þ

(On charge, OER)

On discharge (i.e. ORR), RM is reduced instead of O2 to from LiRM, (Rxn. 1.4) followed by the transfer of electron from LiRM to O2 to from Li2O2 (Rxn. 1.5). Here, RM mediates reduction of O2 by itself undergoing redox reaction [22, 24]. Similarly, on charge, RM is first oxidized to RM+ (Rxn. 1.6). Then, it reacts with Li2O2 to decompose into Li+ and O2 [21, 23, 25] (Rxn. 1.7). Although the consequence of discharge and charge process (i.e. formation and decomposition of Li2O2) is identical regardless of the presence of RM, the reaction path is entirely shifted with the introduction of RM. Accordingly, limitations originated from

4

1

Introduction

conventional discharge/charge path, such as electrode passivation and high charge overpotential might be circumvented. The RM-mediated discharge/charge process is schematically shown in Fig. 1.2. It is obvious that the intrinsic property of RM critically affects the performance of itself and RM-employing Li–O2 battery. Firstly, in RM-mediated discharge/charge, the cell potential is directly reflected by redox potential of RM, implying the importance of selecting RM with high/low redox potential for discharge/charge to achieve lower overpotential [23, 24]. Secondly, the kinetics of RM-mediated process determines the rate of Li2O2 formation and decomposition, thereby governing the rate capability of Li–O2 battery [26, 27]. Furthermore, the selectivity of RM toward O2/Li2O2 on discharge/charge also plays an important role affecting coulombic efficiency [25]. Accordingly, understanding the properties of RM is essential to design high-performance RM and it is a key step to develop RM-employing Li–O2 battery with high energy density and high efficiency. The characteristics of RM that influence its performance is schematically illustrated in Fig. 1.3.

Fig. 1.2 Schematics showing the formation and decomposition of discharge product, Li2O2, with the mediation of redox mediator

References

5

Fig. 1.3 The properties of the RM-employing Li–O2 battery critically influenced by the intrinsic characteristics of RM

References 1. Kang K, Meng YS, Bréger J, Grey CP, Ceder G (2006) Electrodes with high power and high capacity for rechargeable lithium batteries. Science 311(5763):977–980 2. Armand M, Tarascon J-M (2008) Building better batteries. Nature 451(7179):652–657 3. Tarascon J-M, Armand M (2011) Issues and challenges facing rechargeable lithium batteries. Materials for sustainable energy: a collection of peer-reviewed research and review articles from Nature Publishing Group. World Scientific, pp 171–179 4. Choi JW, Aurbach D (2016) Promise and reality of post-lithium-ion batteries with high energy densities. Nat Rev Mater 1(4):1–16 5. Girishkumar G, McCloskey B, Luntz AC, Swanson S, Wilcke W (2010) Lithium−air battery: promise and challenges. J Phys Chem Lett 1(14):2193–2203 6. Christensen J, Albertus P, Sanchez-Carrera RS, Lohmann T, Kozinsky B, Liedtke R et al (2011) A critical review of Li/air batteries. J Electrochem Soc 159(2):R1 7. Bruce PG, Freunberger SA, Hardwick LJ, Tarascon J-M (2012) Li–O 2 and Li–S batteries with high energy storage. Nat Mater 11(1):19

6

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Introduction

8. Song K, Agyeman DA, Park M, Yang J, Kang YM (2017) High‐energy‐density metal– oxygen batteries: Lithium–Oxygen batteries vs Sodium–Oxygen batteries. Adv Mater 29(48): 1606572 9. Lim H-D, Lee B, Bae Y, Park H, Ko Y, Kim H et al (2017) Reaction chemistry in rechargeable Li–O 2 batteries. Chem Soc Rev 46(10):2873–2888 10. Zhao X, Cheruvally G, Kim C, Cho K-K, Ahn H-J, Kim K-W et al (2016) Lithium/sulfur secondary batteries: a review. J Electrochem Sci Technol 7(2):97–114 11. Fang R, Zhao S, Sun Z, Wang DW, Cheng HM, Li F (2017) More reliable lithium‐sulfur batteries: status, solutions and prospects. Adv Mater 29(48):1606823 12. Kumar D, Rajouria SK, Kuhar SB, Kanchan D (2017) Progress and prospects of sodium-sulfur batteries: a review. Solid State Ionics 312:8–16 13. Lee S, Kwon G, Ku K, Yoon K, Jung SK, Lim HD et al (2018) Recent progress in organic electrodes for Li and Na rechargeable batteries. Adv Mater 30(42):1704682 14. Abraham K, Jiang Z (1996) A polymer electrolyte‐based rechargeable lithium/oxygen battery. J Electrochem Soc 143(1):1 15. Johnson L, Li C, Liu Z, Chen Y, Freunberger SA, Ashok PC et al (2014) The role of LiO2 solubility in O2 reduction in aprotic solvents and its consequences for Li–O2 batteries. Nature 6(12):1091 16. Luntz AC, Viswanathan V, Voss J, Varley JB, Nørskov JK, Scheffler R et al (2013) Tunneling and polaron charge transport through Li2O2 in Li–O2 batteries. J Phys Chem Lett 4(20):3494–3499 17. Ganapathy S, Adams BD, Stenou G, Anastasaki MS, Goubitz K, Miao X-F et al (2014) Nature of Li2O2 oxidation in a Li–O2 battery revealed by operando X-ray diffraction. J Am Chem Soc 136(46):16335–16344 18. Ottakam Thotiyl MM, Freunberger SA, Peng Z, Bruce PG (2013) The carbon electrode in nonaqueous Li–O2 cells. J Am Chem Soc 135(1):494–500 19. Itkis DM, Semenenko DA, Kataev EY, Belova AI, Neudachina VS, Sirotina AP et al (2013) Reactivity of carbon in lithium–oxygen battery positive electrodes. Nano Lett 13(10):4697– 4701 20. Freunberger SA, Chen Y, Drewett NE, Hardwick LJ, Bardé F, Bruce PG (2011) The lithium– oxygen battery with ether-based electrolytes. Angew Chemie Int Ed 50(37):8609–8613 21. Chen Y, Freunberger SA, Peng Z, Fontaine O, Bruce PG (2013) Charging a Li–O2 battery using a redox mediator. Nat Chem 5(6): 489 22. Gao X, Chen Y, Johnson L, Bruce PG (2016) Promoting solution phase discharge in Li–O2 batteries containing weakly solvating electrolyte solutions. Nat Mater 15(8):882–888 23. Ko Y, Park H, Kim B, Kim JS, Kang K (2019) Redox mediators: a solution for advanced Lithium–Oxygen batteries. Trend Chem 1(3):349–360 24. Ko Y, Park H, Kim J, Lim HD, Lee B, Kwon G et al (2019) Biological redox mediation in electron transport chain of bacteria for oxygen reduction reaction catalysts in lithium–oxygen batteries. Adv Funct Mater 29(5):1805623 25. Ko Y, Park H, Lee B, Bae Y, Park SK, Kang K (2019) A comparative kinetic study of redox mediators for high-power lithium–oxygen batteries. J Mater Chem A 7(11):6491–6498 26. Lim H-D, Lee B, Zheng Y, Hong J, Kim J, Gwon H et al (2016) Rational design of redox mediators for advanced Li–O2 batteries. Nat Energy 1(6):1–9 27. Chen Y, Gao X, Johnson LR, Bruce PG (2018) Kinetics of lithium peroxide oxidation by redox mediators and consequences for the lithium–oxygen cell. Nat Commun 9(1):1–6

Chapter 2

Exploring a New Redox Mediator Inspired by Biological System

2.1

Research Background

Lithium–oxygen battery is one of the promising next-generation energy storage systems owing to its high theoretical energy density far exceeding that of conventional Lithium–ion batteries [1–7]. The high energy density of lithium–oxygen battery is attributed to the fact that it exploits light oxygen as a redox-active center However, it is recently revealed that energy density is largely affected by discharge mechanism by which discharge product of Li2O2 is electrochemically formed [8–11]. The evolution of intermediate species of LiO2 and its stability in electrolyte have been reported to determine discharge path and resulting energy density. For example, using electrolyte where LiO2 presents stably (i.e. strongly solvating LiO2) results in the formation of Li2O2 into large toroidal particle following ‘solution rote’, which leads to high energy density. On the other hand, the use of electrolyte that does not stabilize LiO2 (i.e. weakly solvation LiO2) leads to the absorption of LiO2 on electrode surface and growth of Li2O2 into amorphous film-like phase following ‘surface route’, which cause early passivation of electrode surface leading to low energy density. Accordingly, it is accepted that the selection of electrolyte strongly solvating LiO2 and resulting in solution route discharge is essential to achieve a high energy density [8, 9, 11–14]. The nature of the electrolyte is one of the major factors governing solvated LiO2 stability [8–10]. higher donor number electrolyte can stabilize solvated LiO2 due to strong solvating property and, thus, induces the discharge with solution route. However, these electrolytes are also reported to be more prone to cause side reactions because the increased solubility of reactive oxygen radicals (i.e. O2− and LiO2) simultaneously enriches their content leading to the degradation of electrolytes [15–17]. Even though such a ‘trade-off’ issue is considered a major hurdle The essence of this chapter has been published in Advanced Functional Materials. Reproduced with permission from [Ko, Y. et al., Adv. Funct. Mater. 2019, 29, 1805623] Copyright (2019) WILEY–VCH © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 Y. Ko, Development of Redox Mediators for High-Energy-Density and High-Efficiency Lithium-Oxygen Batteries, Springer Theses, https://doi.org/10.1007/978-981-16-2532-9_2

7

8

2 Exploring a New Redox Mediator Inspired by Biological System

for developing lithium–oxygen battery with high energy density, redox mediators (RMs) that function during oxygen reduction reaction (ORR) have been reported as a promising solution. When appropriate ORR RM was introduced in lithium– oxygen cell, the RMs are reduced earlier than O2 due to their higher redox potential. Then, electrons are spontaneously delivered from reduced RM to oxygen, in principle, triggering the formation of Li2O2 with solution route regardless of the type of electrolytes. Furthermore, it reduces the direct formation of risky oxygen radical species. Several materials such as quinones [18] and viologens [19, 20] have been suggested as ORR RMs. For example, it is demonstrated that 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ) can trigger discharge process with solution route even in electrolytes with low donor number (e.g. ethers) by the formation of soluble Li–O2–RM intermediates whose disproportionation leads to the formation of large toroidal Li2O2 and neutral RM [21]. The redox potential of ORR RM determines the activity in lithium–oxygen cell. When redox potential of ORR RM is located between that of O2 and the theoretical formation potential of Li2O2 (2.96 V vs. Li/Li+), sequential chemical electron transport from reduced RM to oxygen in solution phase is enabled. The discovery of an ORR RM promoting the solution route opens a new way to decouple the high energy density and stability, which is inevitable for the development of lithium– oxygen battery with high energy density and sustainability. In this part, we demonstrated that one of well-known biological RMs, vitamin K2 working in the electron transport chain (ETC) in Escherichia coli, also can function as a ORR RM in lithium–oxygen battery environment. The discovery of new ORR RM was conducted by considering appropriate redox potential and reversibility of its redox reaction. The vitamin K2-containing lithium–oxygen battery achieved enormously enhanced energy density by transformed discharge path from surface route to solution route leading to the formation of large toroidal Li2O2 particles.

2.2 2.2.1

Experimental Method Preparation of Cells and Conditions for Electrochemical Tests

To prepare electrolytes, 1 M lithium bis(trifluoromethane)sulfonamide powder (99.95% trace metals basis, Sigma–Aldrich) was added in dimethoxyethane (anhydrous, 99.5%, Sigma–Aldrich). Before use, Lithium bis(trifluoromethane)sulfonamide powder was dried at 180 °C under vacuum for more than a 48 h. Dimethoxyethane was dried before use using an activated molecular sieve (type 3Å, Sigma Aldrich) for more than 48 h. Redox mediator candidates, Menatetrenone (vitamin K2), Menadion (MD), ubiquinone-10 (coenzyme Q), Methyl-p-benzoquinone (MBQ), 2-tert-Butyl-1,4-benzoquinone (BBQ), and

2.2 Experimental Method

9

2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), were purchased from Sigma– Aldrich and they were used as received. RM-containing electrolytes was prepared by dissolving targeted amount of redox mediator candidates in 1 M LiTFSI DME. The water content of the prepared electrolytes was measured lower than 50 ppm (as determined by Karl Fischer titration). Heat-treated (900 °C for 3 h under an Ar:H2 (95:5 volume ratio) atmosphere) gas diffusion layer (H2315, Quintech) was used as an air electrode. Lithium iron phosphate was used at the anode. Lithium iron phosphate electrode was prepared by coating slurry made by mixing Lithium iron phosphate powder, super P carbon, and polyvinylidene fluoride binder in a weight ratio of 90:1.5:8.5. As a separator, two sheets of GF/F glass fiber (Whatman) were used. The cell was assembled using Swagelok-type cells with 300 ll of electrolytes. Cell assembly was conducted in an Ar-filled glove box (O2 level