M. Wakihara, 0. Yamamoto (Eds.)

Lithium Ion Batteries

Fundamentals and Performance

8 WILEY-VCH Weinheim * Berlin - New York Chichester . KOD-A Brisbane Singapore Toronto

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M. Wakihara, 0. Yamamoto (Eds.)

Lithium Ion Batteries

Fundamentals and Performance

8 KODANSJHA 8 WILEY-VCH

Further Reading from WILEY-VCH

J. 0. Besenhard (Ed.) Handbook of Battery Materials ISBN 3-527-29469-4

K. Kordesch, G. Simader Fuel Cells and Their Applications ISBN 3-521-28579-2

M. Wakihara, 0. Yamamoto (Eds.)

Lithium Ion Batteries

Fundamentals and Performance

8 WILEY-VCH Weinheim * Berlin - New York Chichester . KOD-A Brisbane Singapore Toronto

Masataka Wakihara osamu Yamamoto Professor Professor Department of Chemical Engineering Tokyo Institute of Technology Tokyo 152 Tsu-shi, Mie 514 Japan Japan

Department of Chemistry Mie University

This book was carefully produced. Nevertheless, authors, editors and publisher do not warrant the information contained therein to be free of errors. Readers are advised tp keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

1 .

, .

Published jointly by Kodansha Ltd., Tokyo (Japan), WILEY-VCH Verlag GmbH, Weinheim (Federal Republic of Germany)

Library of Congress Card No. applied for.

A catalogue record for this book is available from the British Library.

Deutsche Bibliothek Cataloguing-in-Publication Data: Lithium Ion Bntteries/eds. by M. Wakihara ; 0. Yamamoto.-Weinheim ; Berlin ; New York ; Chichester ; Brisbane ; Singapore ; Toronto : Wiley-VCH, 1998

ISBN 3-527-29569-0 (WILEY-VCH) ISBN 4-06-208631-X (KODANSHA)

Copyright 0 Kodansha Ltd., Tokyo, 1998 All rights reserved. No part of this book may be reproduced in any form, by photostat, microfilm, retrieval system, or any other means, without the written permission.of Kodansha Ltd. (except in the case of brief quotation for criticism or review).

Printed in Japan

List of Contributors (Numbers in parentheses refer to the pages on which the contributor’s paper begins.)

John B. Goodenough

Masataka Wakihara

Guohua Li

Hiromasa Ikuta

M. Stanley Whittingham

Jun-ichi Yamaki

Nobuyuki Imanishi Yasuo Takeda Osamu Yamamoto Martin Winter

Jurgen 0. Besenhard

Masayuki Morita

Masashi Ishikawa

Yoshiharu Matsuda

Yoshio Nishi Shigeo Kondo

Bruno Scrosati

Center for Materials Science and Engineering, ETC9.102, University of Texas at Austin, USA (1)

Department of Chemical Engineering, Tokyo Institute of Technology, Japan (26)

Department of Chemical Engineering, Tokyo Institute of Technolgy, Japan (26)

Department of Chemical Engineering, Tokyo Institute of Technology, Japan (26)

Chemistry Department and Materials Research Center, State University of New York at Binghamton, USA (49)

NTT, Nippon Telegraph and Telephone Corporation, Japan

Department of Chemistry, Mie University, Japan (98)

Department of Chemistry, Mie University, Japan (98)

Department of Chemistry, Mie University, Japan (98)

Institute of Chemical Technology of Inorganic Materials, Technical University Graz, Austria (127)

Institute of Chemical Technology of Inorganic Materials, Technical University Graz, Austria (127)

Department of Applied Chemistry and Chemical Engineer- ing, Faculty of Engineering, Yamaguchi University, Japan

Department of Applied Chemistry and Chemical Engineer- ing, Faculty of Engineering, Yamaguchi University, Japan

Department of Applied Chemistry, Faculty of Engineering, Kansai University, Japan (156)

Sony Corporation, Japan (181)

Technology Laboratory, Matsushita Battery Industrial Co., Ltd., Japan (199)

Dipartimento di Chimica, UniversitA “La Sapienza”, Italy

(67)

(156)

(156)

(218)

V

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Contents

List of Contributors v Preface xiii

1 General Concepts .................................................................... 1

1.1 Introduction ...................................................................... 1 1.2 Design Considerations ........................................................... 1

1.2.1 Definitions .................................................................. 1 1.2.2 Design Considerations ....................................................... 3

Choosing an Electrode ....................................................... 8

Insertion of Lithium into Structures Containing Polyanions .................. 13 Close-Packed Oxide-Ion Arrays ............................................ 15

1.3.2 NASICON Frameworks .................................................... 18

1.2.3 1.2.4 Anodes .................................................................... 12

1.3.1

1.3.3 Conclusion ................................................................. 24 References ............................................................................ 25

1.3

2 Cathode Active Materials with a Three-dimensional Spinel Framework .... 26

2.1 Introduction ..................................................................... 26 Crystal Structure of Spinel Type Phases ....................................... 27

2.3 Synthesis Technique ............................................................ 28

Thermodynamic Function of the Cathode Materials ........................ 30 Phase Transformation During Intercalation Processes ........................ 33

as 4 V-Class Cathode Material ............................................... 34

Doping Effect on Charge-Discharge Behavior of Manganess Spinel ........... 34 OCV and Phase Transformation ............................................ 36

2.6.3 Cycling Performance ....................................................... 38 2.6.4 Structure Aspects .......................................................... 40

2.2

2.4

2.5 2.6 Doped Spinel Phases LiMYMnz- Y 0 4 (M = Co, Cr, Ni)

2.6.1 2.6.2

Relationship between Discharge Voltage and

V i i

viii Contents

2.6.5

2.6.6

The Chemical Diffusion Coefficients of Lithium Ions in LixMyMnz-y04 (M=Co and Cr) ......................................................... 42

Low Temperature Behavior ................................................. 44 2.7 Conclusions ...................................................................... 46 References ............................................................................. 47

3 The Relationship between Structure and Cell Properties of the Cathode for Lithium Batteries .......................................... 49

3.1 Introduction ..................................................................... 49 3.2 Titanium Disulfide and Intercalation Chemistry ............................... 50 3.3 Vanadium Dichalcogenides ..................................................... 53 3.4 Layered Oxides .................................... .., ........................... 53 3.5 Manganese Oxides .............................................................. 55 3.6 Vanadium Oxides ............................................................... 58 3.7 The Future ...................................................................... 63 References ............................................................................ 64

4 Design of the Lithium Anode and Electrolytes in Lithium Secondary Batteries with a Long Cycle Life ....................... 67

4.1 Introduction ..................................................................... 67

4.2 Lithium Metal Anode ........................................................... 67

4.2.1 Protection Films on Lithium Metal Anode .................................. 67 4.2.2 Cycling Efficiency of Lithium Anode ........................................ 69 4.2.3 Morphology of Deposited Lithium .......................................... 72 4.2.4 Mechanism of Lithium Deposition and Dissolution .......................... 73 4.2.5 The Amount of Dead Lithium and Cell Performance ........................ 75 4.2.6 Improvement in the Cycling Efficiency of a Lithium Anode .................. 77

4.3 Safety ............................................................................ 83 4.3.1 Configuration of Prototype Cells ........................................... 84 4.3.2 Cell Performance ................................ i .......................... 84 4.3.3 Heat Generation in a Cell-General Considerations ......................... 86 4.3.4 Incidents During Normal Cycling ........................................... 89 4.3.5 Safety Tests on AA-size Li/a-V205(-P205) Cells ............................. 91

4.4 Conclusion ...................................................................... 94 References .............................................................................. 94

5 Development of the Carbon Anode in Lithium Ion Batteries ................. 98

5.1 Introduction ..................................................................... 98 5.2 Structure of Carbon Materials ................................................. 99 5.3 5.4 5.5 5.6

Development of the Carbon Anode ........................................... 101 Intercalation Mechanism of Graphite ......................................... 105 Electrochemistry of Soft Carbons ............................................. 108 Electrochemistry of Hard Carbons ............................................. 112

Contents ix

5.7 Irreversible Surface Reactions ................................................. 114 5.8 Structural Modifications ...................................................... 116 5.9 Nitrides as New Anode Materials ............................................. 120

5.9.1 Li7MnN4 and LisFeN2 (Antifluorite Structure) ............................. 120 5.9.2 Lis-, Co,N (Li3N Structure) ................................................ 122

5.10 Summary and Conclusions ................................................... 124 References ........................................................................... 125

6 Electrochemical Intercalation of Lithium into Carbonaceous Materials ....................................................... 127

6.1 Introduction .................................................................... 127

6.1.1 Negative Electrodes in Rechargeable Lithium Batteries ..................... 127 6.1.2 Lithium/Carbon Intercalation Compounds ................................ 128 6.1.3 Carbonaceous Host Materials ............................................. 129 Graphitic Carbons as Host for Lithium Intercalation ........................ 131 6.2.1 Lithium/ Graphite Intercalation Compounds ............................... 132 6.2.2 Effects of Electrolyte Composition ......................................... 135 Non-graphitic Carbons as Lithium Intercalation Hosts ...................... 140 6.3.1 “Low Capacity” Non-graphitic Carbons .................................... 140 6.3.2 “High Capacity” Non-graphitic Carbons ................................... 140 Special Carbonaceous Materials as Hosts for Lithium Intercalation ......... 144 6.4.1 Fullerenes ................................................................ 144 6.4.2 Carbonaceous Materials Containing Heteroatoms .......................... 144

6.5 Technical Aspects .............................................................. 145 6.6 Outlook ........................................................................ 146 References ........................................................................... 147

6.2

6.3

6.4

7 Organic Electrolytes for Rechargeable Lithium Ion Batteries ............... 156

7.1 Introduction .................................................................... 156 7.2

for Lithium Batteries ........................................................ 157 7.2.1 Organic Solvents .......................................................... 157 7.2.2 Lithium Salts ............................................................. 159 7.2.3 Ionic Conductivity ........................................................ 159 7.2.4 Recent Studies on organic Electrolytes for

Rechargeable Lithium Ion Batteries ...................................... 161 Effects of the Electrolyte Composition on the

with Graphite Structure ..................................................... 163 7.3.1 Voltammetric Behavior in PC- and EC-based Solutions ..................... 163 7.3.2 Charge/Discharge Characteristics of the Carbon Fiber Electrode

in LiCF3SO3 Solutions .................................................. 167 Effects of the Electrolyte Composition on the Electrode Characteristics

of Graphitized Mesocarbon Microbeads (MCMB) ......................... 171

The Basic Concept for Designing Organic Electrolytes

7.3 Electrode Characteristics of Pitch-based Carbon Fiber

7.4

x Contents

7.4.1

7.4.2

Charge/Discharge Performance of MCMB in Electrolyte

Interfacial Characteristics between the MCMB Solutions of Lithium Imide Salt ......................................... 171

Electrode and the Electrolyte Solutions Containing

Effects of Electrolyte Composition on the EQCM Response of Graphite during Cathodic Lithium Intercalation ........................... 174

7.5.1 Effects of Solvent Composition on the First Charging Process .............. 175 7.5.2 Effects on the Structure Changes of Graphite during the

Charge/Discharge Process .............................................. 177 7.5.3 Effects of the Electrolytic Salt on the EQCM Response of Graphite ......... 178

7.6 Concluding Remarks .......................................................... 179 References ........................................................................... 179

Lithium Imide Salt ...................................................... 173 7.5

8 Performance of the First Lithium Ion Battery and Its Process Technology ......................................................... 181

8.2 Anode Materials ............................................................... 182 8.3 Cathode Materials ............................................................. 191 8.4 Other Materials ................................................................ 194

8.4.1 Electrolyte ................................................................ 194 8.4.2 Separator ................................................................. 195 8.4.3 PTCDevices .............................................................. 195

8.5 Cell Structure .................................................................. 196 8.6 Performance of Lithium Ion Secondary Batteries ............................ 197 8.7 Possibility of Further Improvement ........................................... 198 References ........................................................................... 198

8.1 Introduction .................................................................... 181

9 All Solid-state Lithium Secondary Battery with Highly Ion Conductive Glassy Electrolyte ................................................ 199

9.1 Introduction .................................................................... 199 9.2 Lithium Ion Conductive Solid Electrolytes ................................... 200 9.3 Li3PO4-Li2S-SiS2 Glassy Electrolyte .......................................... 202

9.3.1 Synthesis and Glass-Forming Region ...................................... 202 9.3.2 Ionic Conductivity and Structure of the Glass .............................. 203 9.3.3 Electrochemical Stability .................................................. 206

9.4 Solid-state Lithium Batteries .................................................. 208 9.4.1 Brief Review of Solid-state Lithium Batteries in Previous Studies ........... 208 9.4.2 Application of Li~P04-LizS-SiSz Glass to the Solid-state Battery ........... 209

9.5 Prospects for Solid-state Lithium Batteries in the Future .................... 214 9.6 Summary ....................................................................... 216 References ........................................................................... 216

10 Lithium Ion Plastic Batteries ...............................

Contents xi

................ 218

10.1 10.2

10.3 10.4 10.5

10.6

Introduction ................................................................. 218 Polymer Electrolyte Membranes ............................................ 219 10.2.1 Types and Preparation Procedures ..................................... 219 10.2.2 Ionic Conductivity .................................................... 223 10.2.3 Electrochemical Stability Window ..................................... 227 10.2.4 Lithium Ion Transference Number ..................................... 230 Plasticized Electrodes ....................................................... 232 Practical PLI Batteries ...................................................... 235 New Types of PLI Batteries ................................................. 236 10.5.1 PLI Batteries with TiSz Anodes ........................................ 236 10.5.2 The Dion Plastic Battery .............................................. 240 Conclusions ................................................................. 242

References .......................................................................... 243

Index ...................................................................................... 245

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Preface

Recently, rechargeable batteries with high energy density are in great demand as energy sources for various purposes, e.g. handy electronic machines, zero emission electric vehicles, load leveling in electric power. Lithium secondary batteries are the most promising to fulfill such needs because of their intrinsic discharge voltage with relatively light weight.

In 1991, the first lithium secondary battery of high quality and revolutionary concept was put on the market by Sony. In the commercial battery, called the “lithium ion battery,” lithium ions swing between the anode and the cathode through an organic liquid elctrolyte dissolving an inorganic lithium salt, like a rocking chair rocking from side to side. The principal concept is based on the intercalation reaction and is rather different from conventional secondary batteries which are based on chemical reactions. The past excellent results of fundamental research and technology have been used to develop the lithium ion battery. The needs of the time, however, do not stop and require further quality and better cost-performance for this type of battery, and given this favorable environment excellent fundamental concepts, idea and technology regarding electrodes and electrolytes in the battery are being published in articles and proceedings. However, the description is not always sufficient for complete understanding due to limited space.

This volume has been conceived keeping in mind selected fundamental topics together with the characteristics of the lithium ion battery on the market. It is thus a comprehensive overview of the new challenges facing the further development of lithium ion batteries with higher quality from the standpoint of both materials science and technology. Fun- damental aspects and ideas on the cathode are discussed in Chapters 1-3. Chapter 4 adresses safety aspects of the lithium ion battery. The various topics on the carbon anode are presented in Chapters 5 and 6. Also the organic electrolytes are discussed in Chapter 7. From the standpoint of application, the performance of the first lithium ion battery is presented in Chapter 8 by Sony and the performance of new all solid battery using inorganic solid electrolyte by Matsushita and a polymer battery based on a new concept is discussed in Chapters 9 and 10.

This volume will be useful for many researchers in chemical companies including battery companies and universities, and also for graduate students who are studying or are planning to start research on the lithium secondary batteries, since it covers important topics from both fundamental and application points of view.

Gratitude is expressed to all the contributers who are actively studying and working in the field of lithium secondary batteries. Our joint publishers, especially Kodansha, were particularly helpful at all stages of the project, and we are very grateful to them for their understanding attitude and support.

December, 1997

xiii

Masataka Wakihara Osamu Yamamoto

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General Concepts

J. B. Goodenough*

1.1 Introduction

A reliable secondary (rechargeable) battery of high energy and power density is needed for a variety of new existing technologies. This introductory chapter is divided into two parts. Section 1.2 (“Design Considerations’) introduces several definitions and the general considerations that make electrochemical cells using Li+ as the working ion the most promising elemental component for a secondary battery of high energy and power density. Section 1.3 (“Insertion of Lithium into Structures Containing Polyanions’) illustrates structural and chemical considerations that may be useful for the design of (search for) a competitive cathode material. It compares lithium insertion into two types of transition-metal oxides: those having a close-packed oxide-ion array and those forming framework structures in which the oxide ions are replaced by the polyanions (so&, (PO&, (AsO& and (PzO~)~-. Of particular interest are (1) tuning of the transition- metal redox energies through the inductive effect, (2) the trade-off between improved Li’ ion diffusion and polaronic electron conduction in open frameworks, (3) the use of mixed phases to buffer aganist overdischarging as exemplified by LiTiz(P04)3 mixed with LixFe2(S04)3, (4) the identification and explanation of a reversible capacity fade at higher current densities, and (5 ) the identification of new cathode materials operating on the Fe3+/ Fez+ redox couple: the olivine LixFeP04 with a flat V,=3.5 V vs. lithium and an Li3+xFe~(P04)3 having the rhombohedral NASICON structure with a closedcircuit voltage centered at V ~ 2 . 8 V and a capacity of 95 mAh-g-’ at a current desity of 1 mA-cm-z.

1.2 Design Considerations

A battery consists of a group of interconnected electrochemical cells. How these are connected and packaged depends upon the specific application for which they are designed. Here we consider only the design of the elemental building block of a battery, viz., the electrochemical cell.

1.2.1 Definitions

An electrochemical cell interfaces the external world through two metallic posts: one

* Center for Materials Science and Engineering, ETC 9.102, University of Texas at Austin, Austin, TX-78712-1063, USA

1

2 1 Generalconcepts

(Oxidant) +- e-

Fig. 1 . 1 Elements of an electrochemical cell; current flows shown occur during discharge.

contacts a negative electrode (the anode) and the other a positive electrode (the cathode) as illustrated schematically in Fig. 1.1. During cell discharge, electrons pass from the anode to the cathode through an external load of resistance RL and ions flow inside the cell to convert chemical energy into electrical energy. The electronic current I delivered by the cell to the external circuit is matched by the ionic current within the cell. Any leakage of electrons from anode to cathode within the cell reduces the current Z delivered by the cell.

During cell charge, electric current is forced in the opposite direction by an externally applied voltage to convert electrical energy back into chemical energy.

The ionic current within an electrochemical cell is carried between the electrodes by an electrolyte, which is ideally an electronic insulator and a good conductor of the working ion of the cell. One measure of the quality of an electrolyte is its transference (or transport) number

ci ai/u (1.1)

where u, is the conductivity of the working ion and the total conductivity u = I: uj + u e is the sum of all the ionic conductivities and the electronic conductivity ue of the electrolyte under the working conditions of the cell.

If a liquid electrolyte is used, a separator is also needed to maintain an even spacing between the electrodes while blocking electronic current and passing the ionic current. A solid electrolyte can act as a separator; in this capacity it can allow the use of different liquid electrolytes at each electrode, a strategy that has yet to be explored. Common separators are porous electronic insulators permeated by a single liquid electrolyte.

The chemical at the anode that is consumed on discharge or produced on charge is the reductant of the chemical reaction; the chemical consumed on discharge or produced on charge at the cathode is the oxidant. The reductant and the oxidant are the two reactants of the cell; the energy of their reaction divided by the electronic charge passed in the reaction gives the maximum discharge voltage available between the positive and negative posts of the cells; it is the minimum voltage required to charge the cell.

A fuel cell operates in the discharge mode; the reductant is a convenient gaseous or liquid fuel that is fed from a reservoir (storage tank) to the anode, the oxidant is dioxygen, 0 2 , in

i

An ideal electrolyte has a ti = 1.

1.2 Design Considerations 3

the air that is fed to the cathode, and the product of the reaction between the two reactants is the exhaust, which is vented at the cathode if the working ion is positive (a cation) and at the anode if the working ion is negative (an anion). An electrolysis cell operates in the charge mode; a product is separated into a reductant (a fuel or some other value-added chemical) that is extracted at the anode and an oxidant that is either vented or extracted at the cathode.

Present-day lithium batteries use a solid reductant as the anode and a solid oxidant as the cathode. On discharge, the metallic anode supplies Li+ ions to the Li+ ion electrolyte and electrons to the external circuit; the cathode is an electronically conducting host into which Li+ ions are inserted from the electrolyte as a guest species and charge-compensated by electrons from the external circuit. The chemical reactions at the anode and cathode of a lithium secondary battery must be reversible; on charge, removal of electrons from the cathode by an external field releases Li+ ions back to the electrolyte to restore the parent host structure and the addition of electrons to the anode by the external field attracts charge-compensating Li+ ions back into the anode to restore it to its original composition. In principle, the anode can be elemental lithium itself; in practice, it has been found necessary to use a reductant host for lithium. Where both the anode and cathode are hosts for the reversible insertion or removal of the working ion into/from the electrolyte, the electrochemical cell is commonly called a “rocking-chair” cell.

1.2.2 Design Considerations

The power output of a battery

P= zv (1.2)

is the product of the electric current Z delivered by the battery and the voltage V across the negative and positive posts. The voltage

is reduced from its opencircuit value (Z=O)

by the voltage drop ZRb due to the internal resistance Rb of the battery. In Eq. (1.4), n is the number of electronic charges carried by the working ion and Pis Faraday’s constant. The magnitude of the opencircuit voltage is constrained to V,< 5 V not only by the attainable difference C(A - pc of the electrochemical potentials of the anode reductant and the cathode oxidant, but also by the energy gap Eg between the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of a liquid electrolyte or by the gap Eg between the top of the valence band and the bottom of the conduction band of a solid electrolyte.

As illustrated in Fig. 1.2, the chemical potential PA, which is the Fermi energy EF of a metallic reductant anode or the HOMO of a gaseous or liquid reductant, must lie below the LUMO of a liquid electrolyte or the conduction band of a solid electrolyte to achieve

4 1 Generalconcepts

(HOMO)

f E I Conduction band I

ridant I

Fig. 1.2 Relative energies of the electrolyte window EE and the electrode electrochemical potentials PA=.% or HOMO of reductant and pc = EF or LUMO of oxidant: (a) solid reactants liquid electrolyte; (b) liquid or gaseous reactants and solid electrolyte.

thermodynamic stability against reduction of the electrolyte by the reductant. Similarly, the chemical potential pc, which is the LUMO of a gaseous or liquid oxidant or the Fermi energy of a metallic oxidant cathode, must lie above the HOMO of a liquid electrolyte or the valence band of a solid electrolyte to achieve thermodynamic stability against oxidation of the electrolyte by the oxidant. Thermodynamic stability thus introduces the constraint

PA- P C ~ Eg (1.5)

as well as the need to match the “window” Eg of the electrolyte to the energies PA and pc of the reactants to maximize V , . Moreover, it follows then from Eqs. (1.2) and (1.3) that realization of a high maximum power

requires, in addition to as high a Voc as possible, a low internal battery resistance

Rb = Re1 + %(A) + Rin(C) -I- &(A) + &(C) (1.7)

The electrolyte resistance

1.2 Design Considerations 5

to the ionic current is proportional to the ratio of the effective thickness L to the geometrical area A of the interelectrode space that is filled with an electrolyte of ionic conductivity ui. Since ions move diffusively, i.e., with a mobility pi = qD/kTwhere the diffusion coefficient D= DO exp(- AGrn/kT) contains a motional free energy AGm, and Ui=niqpi is propor- tional to the density ni of carriers of charge q,

increases with temperature T, and a a i l 0.1 S-cm-' (the maximum ui represents the room temperature protonic conductivity UH in a strong acid) at an operating temperature To, dictates the use of a membrane separator of large geometrical area A and small thickness L.

The resistance to transport of the working ion across the electrolyte-electrode interfaces is proportional to the ratio of the geometrical and interfacial areas at each electrode:

(1.10) Rin - A/Ain

Where the chemical reaction of the cell involves ionic transport across an interface, relation Eq. (1.10) dictates construction of a porous, small-particle electrode as illustrated in Fig. 1.3. Achievement and retention of a high electrode capacity, ie . , utilization of a high fraction of the electrode material in the reversible reaction, requires the achievement and retention of good electronic contact between particles over many dischargelcharge cycles. If the reversible reaction involves a first-order phase change, the particles may fracture or lose contact with one another on cycling to break a continuous electronic pathway to the current collector.

There may also be a reversible capacity fade. Where there is a two-phase process (or even a sharp guest-species gradient at a diffusion front) without fracture of the particles, the area of the interface (or diffusion front) decreases as the second phase penetrates the electrode particle; at a critical interface area, diffusion across the interface may not be fast enough to sustain the current I, so not all of the particle is accessible. The volume of inaccessible electrode increases with I, which leads to a diffusion-limited reversible capacity fade that increases with I as illustrated in Section 1.3.

Finally, current from the large-area electrodes must be collected to the negative and positive posts, and a clever geometrical design is needed to allow assembly of the cell into a small package that minimizes the current-collector resistance RC of each electrode.

Loss of interparticle electrical contact results in an irreversible loss of capacity.

Current collector

Electrolyte Separator

Fig. 1.3 Schematic of a porous electrode; not shown, individual electrode particles are in electronic contact with each other and the current collector.

6 1 Generalconcepts

V . ) I I

VOC .........,.... ................. .................................... .... I-! \ (ii)

I : (iii)

I

Fig. 1.4 Typical polarization curve.

In Eq. (1.1 l), 1 is the mean distance an electron (or hole) travels through the thickness of the electrode of electronic conductivity ue to reach the current-collector of conductivity a m and d is a geometrical parameter having units of length.

The battery voltage V vs. the current Z delivered across a load is called the polarization curve. The voltage drop ( Voe - V) 5 q(Z) of a typical curve, Fig. 1.4, is a measure of the battery resistance

On charging, q(Z) = (V,,, - Voe) is referred to as an overvoltage. drops saturate in region (i) of Fig. 1.4; therefore in region (ii) the slope of the curve is

The interfacial voltage

dV/dZ- &I + Rc (A) + Rc (C) (1.13)

Region (iii) is diffusion-limited; at the higher currents Z, normal processes do not bring ions to or remove them from the electrode/electrolyte interfaces rapidly enough to sustain an equilibrium reaction.

The battery voltage Vvs. the state of charge, or the time during which a constant current Z has been delivered, is called a discharge curve. Typical curves are shown in Fig. 1.5 for the solid solution reactions

xe- 4- xLi+ 4- TiSz + Li,TiSz (1.14a)

xe- + xLi+ + Li[Mn2]04 + Li1+~[Mnz]O4 ( 1.1 4b)

In the first, illustrated schematically in Fig. 1.6, a complete solid solution 0 5 x 5 1 is obtained, and V(x) is described by the Nernst equation

X In - RT V = VO - -

F 1-x (1.15)

1.2 Design Considerations 7

4.0 v t 1.0 - one-phase, d i d solution

0 I I I -D 0 0.25 0.5 0.75 1.0 x

Fig. 1.5 Typical open-circuit discharge curves V d x ) with respect to a Li anode for two cathodes: (a) LLTiS2 and (b) Li1+4Mn2]04.

Li +

Li+

Li+

Li+ Li electrolyte

Fig. 1.6 Schematic of a TiSz/Li+ electrolyte/Li battery. Current flows represent discharge.

where the standard potential V" for the Ti4+/Ti3+ redox couple is independent of x and corresponds to x= 0.5. It is interesting that Eq. (1.15) is obeyed even though the electrons donated to the TiSz array occupy a narrow Ti-3d band rather than a localized Ti3+: 2Tzg configuration. In the second reaction (1.14b), the insertion of Li into the spinel Li[Mnz]O4 induces a cooperative Jahn-Teller distortion from cubic to tetragonal symmetry that stabilizes the localized Mn3+:5Eg configuration of a cubic octahedral site. The solid solution within the tetragonal phase begins at x-0.8; and in accordance with the Gibbs phase rule, the voltage remains flat, independent of x, over the extended two-phase region 0 <x<0.8.

An implicit additional requirement of a secondary battery is maintenance of the electrode/electrolyte interfaces throughout repeated discharge/ recharge cycles. Since the volumes of the electrodes change as a result of the transfer of atoms from one to another electrode in a reaction, this requirement normally excludes the use of a crystalline or glassy

8 1 Generalconcepts

electrolyte with a solid electrode. Liquid or elastomer electrolytes are used with solid electrodes; a solid electrolyte is preferred with liquid or gaseous reactants.

Protonic conductivity UH 5 0.1 S-cm-' can be achieved in aqueous solutions that are either strongly acidic, e.g., H2S04, or strongly alkaline, e.g., KOH. An acidic solution provides protonic conduction at an H30+/H20 couple, an alkaline solution at an H20/0H- couple.

The HOMO and LUMO for an aqueous solution are, respectively, the 02/H20 and the H+/H2 couples; they are separated by an energy Eg= 1.23 eV. Therefore thermodynamic stability limits the voltage of an aqueous system to 1.23 V. However, a kinetic energy barrier at an electrode/electrolyte interface may allow realization of an aqueous battery having a voltage V> 1.23 V, as in the case of the lead-acid automobile battery with a Vm = 2 V. However, kinetic barriers are not compatible with a long shelf life.

The principal motivation for turning from the proton to Li+ as the working ion is the voltage limitation of a proton cell. Lithium is much more electropositive than hydrogen, which allows the realization of significantly higher voltages. However, to realize the higher voltages possible with a lithium battery, a non-aqueous electrolyte having a large energy-gap window between its HOMO and its LUMO must be used. Practical quantities of very ionic lithium salts such as LiC104, LiBF4 and Lip& can be dissolved in empirically optimized mixtures of propylene carbonate (PC), ethylene carbonate (EC), or dimethyl carbonate (DMC), for example. Voltages approaching 4.5 V have been sustained in an electrolyte of LiPF6 in a 1 : 2 DMC : EC mixture. The (Cl04)- anion is explosive and not suitable for commercial applications. A commonly used electrolyte is LiBF4 in PC, but it is restricted to a Voc <4.2 V vs. Li. An improved Li+ ion electrolyte would be welcome.

Traditional batteries use aqueous electrolytes.

1.2.3 Choosing an Electrode

The working redox couples of a cathode host are commonly transition-metal 3d" energies as in Li,TiSz'). Mixed-valent transition-metal oxides tend to be good electronic conductors, some are even metallic, and, with few exceptions, they are stable against disproportionation reactions like that occurring in HhPbOz. Moreover, oxides allow realization of a higher Voc for a given anode than do sulfides because higher cation valence states are accessible in an oxide than in a sulfide.

To illustrate this latter point, consider the Mn4+/Mn3+ couple used with y-MnO2 cathode in aqueous electrolytes and the spinel Li[Mn2]04 cathode used in lithium batteries. Whereas Mn4+ is accessible in Mn02, the compound MnS2 contains Mn2+ and (S2)2-

polyanions. The top of the S2-: 3p6 valence band of a sulfide lies above the Mn4+/Mn3+ couple; it also lies above the highest occupied molecular orbital of the electrolyte. The top of the 02-: 2p6 valence band of an oxide, on the other hand, lies below both; it is possible to place the energy of the Mn4+/Mn3+ redox couple above the highest occupied molecular orbital of the electrolyte with an oxide cathode, but it is not accessible with a sulfide cathode.

Figure 1.7 illustrates the situation by showing schematically the relevant electron energies for the Mn4+/Mn3+ couple of the spinel Li[Mn2]04. A 50 : 50 mixture of Mn4+ and Mn3+ ions on the octahedral sites of the close-packed oxygen array of an [Mn2]04 framework places the Fermi energy EF at the standard potential Eo =eVo of the Mn4+/Mn3+ couple, which lies above the top of the 02- : 2p6 valence band. The empty Mn2+ : 3d5 level lies an

1.2 Design Considerations 9

A

U, + 4

0 2 - : 2p6

d3

N(4 - Fig. 1.7 Schematic energy diagram for the [Mnz]04 spinel framework of LaMnzIO4.

energy U, above the occupied Mn3+: 3d4 energies. U, is the energy required to add a fifth electron to the empty e orbital of the Mn3+: t3e1 majority-spin state of the high-spin configuration. The empty Mn4+ states of the 3d4 Mn4+/Mn3+ couple are raised above the filled states by a reorganization, i.e., small-polaron, energy because the time 5h for an electron to hop from a Mn3+ to a Mn4+ ion is long compared to the period WE’ of the optical- mode lattice vibration that traps it in a local deformation. If Zh were shorter than WE’, EF

would lie in the middle of a narrow band of one-electron 3d4 states. The occupied Mn4+ : 3d3 states lie an energy U, + A, below the empty Mn4+ states of the 3d4 couple because of the cubic-field splitting A , of the n-bonding t and a-bonding e orbitals of the Mn- 3d manifold. In a sulfide, the top of the S2-: 3p6 valence band would overlap the Mn2+ : 3d5 level to make inaccessible the Mn4+/Mn3+ : 3d4 redox couple; oxidation of the sulfide results in holes in the S2- : 3p6 band that become trapped in S-S disulfide bonds.

A. Layered Transition-Metal Oxides Layered Ti& consists of a close-packed-hexagonal sulfide-ion array with Ti4+ in alternate

basal planes of octahedral sites; the empty planes of octahedral sites are between adjacent sulfide-ion planes held together by van der Waals bonding. In an oxide the Coulomb repulsion between oxide-ion planes is greater than any dipole-dipole bonding unless the cations of the “sandwich layer” are displaced from the center of their octahedral interstices to form permanent dipoles as in VzOs.

On the other hand, the cathode NiOOH of the nickel-cadmium battery is a layered oxide with Ni3+ ions occupying every other basal plane of octahedral sites of a close-packedcubic array of oxide ions. In this oxide, protons provide a hydrogen bonding between 0-Ni-0 sandwich layers, and the hydrogen-bond network accommodates one hydrogen bond per oxygen atom in Ni(OH)2. Therefore protons may be inserted reversibly into the hydrogen- bond layers of NiOOH if chargecompensating electrons are introduced into the Ni3+/Ni2+ couple indicated by the reaction

10 1 GeneralConcepts

Fig. 1.8 The structure of layered a-NaFeOz, prototype of LiCoOz and LiNiOz.

3.5- 0 0.2 0.4 0.6 0.8

x in Li, Cooa

D

Fig. 1.9 Open-circuit voltage V d x ) vs. a Li anode for layered LICOO~, O<xl1?) , (Reproduced with permission by K. Mizushma et OL, Maw. &. M.. 15,783 (1980))

NiOOH 4- 2xHz0 -4- 2x e- = Ni01--2+(OH)i+2r -k 2x OH- (1.16)

Replacement of the hydrogen of NiOOH by lithium gives the layered compound LiNiOz with Li+ ions in the alternate basal planes of octahedral sites, Fig. 1.8, which invites

I .2 Design Considerations 1 1

exploration of how much lithium can be extracted to give Li~-~Nioz before the layered structure becomes unstable. The largest solid solubility of Li in such a layered oxide has been found for Lil-,CoOz, and its open-circuit voltage V d x ) curve vs. a lithium anode is shown in Fig. 1.9; a V & w 4 V is to be compared with the V&-2.2V for LixTiS2. A decrease in Voc with increasing x that exceeds prediction from the Nernst equation (1.15) indicates a V&(x) for (1 - x) < 0.7.

B. Framework Transition-Metal Oxides Removal of half the Li from LiNiOz results in a metastable layered structure Lio.sNiO2

that transforms, on heating to 3OO0C, to the cubic spinel Li[Niz]04.3) In the spinel structure, the oxide ions remain close-packed-cubic, but the Li atoms occupy 8a tetrahedral sites forming a diamond-type subarray and the Ni atoms are rearranged into 16d octahedral sites forming tetrahedral clusters in alternate quadrants of the cubic unit cell (Fig. 1.10). The empty 16c octahedral sites form a similar three-dimensional (3D) configuration of edge-shared octahedral sites displaced by half the cubic lattice parameter, so the [Ni2]04 array may be considered a close-packed framework host with Li at tetrahedral sites of a 3D interstitial network of edge-shared octahedra sharing faces with the 8a tetrahedral sites. Whereas the hexagonal layered structure has weak bonding between 0-Ni-0 sandwich layers and a variable c / a ratio that allows 2D Li+ ion transport, the 3 D bonding of the [Ni2]04 framework allows 3 D Li+ ion transport, but constrains the volume of the interstitial space. Although this constraint makes the spinel structure selective for insertion of Li+ ions, it reduces the Li+ ion mobility and hence the Li+ ion conductivity ULi. Nevertheless, the oxospinels Lil*,[Mnz]O4 with 0 5 x 5 1 have a sufficiently high ULi to be used commercially in low-power cells?)

Extraction/insertion of Li from the tetrahedral sites of the Lil,[Mnz]O4 spinel (0 I x 5 1) gives a VOc - 4 V vs. a Li anode with a small step in Voc at x= 0.5 presumably due to the onset of extraction/insertion of Li from the second face-centered-cubic subarray of the diamond-like array of 8a tetrahedral sites. Insertion of Li into Li[Mn2]04 displaces the Li from the tetrahedral 8a to the octahedral 16c sites in Lil+,[Mnz]O4, and the open-circuit

I-%- Fig. 1.10 The spinel structure.