Quick viewing(Text Mode)

Cathodic Materials for Nickel-Metal Hydride Batteries

Cathodic Materials for Nickel-Metal Hydride Batteries

University of Wollongong Theses Collections University of Wollongong Theses Collection

University of Wollongong Year 

Cathodic materials for -metal hydride batteries

Caiyun Wang University of Wollongong

Wang, Caiyun, Cathodic materials for nickel-metal hydride batteries, PhD thesis, In- stitute for Superconducting and Electronic Materials, University of Wollongong, 2003. http://ro.uow.edu.au/theses/192

This paper is posted at Research Online. http://ro.uow.edu.au/theses/192

CATHODIC MATERIALS FOR NICKEL-METAL

HYDRIDE BATTERIES

A thesis submitted in fulfillment of the requirements for the award of the degree

DOCTOR OF PHILOSOPHY

from

UNIVERSITY OF WOLLONGONG

by

CAIYUN WANG, B. Sc., M. Sc.

Institute for Superconducting & Electronic Materials

Faculty of Engineering

December 2003

i

DECLARATION

This is to certify that the work presented in this thesis is original and was carried out by the candidate at the Faculty of Engineering, the University of

Wollongong, New South Wales, Australia, and has not been submitted for a degree to any other university or institution.

Caiyun Wang

ii ABSTRACT

The properties of nickel hydroxide, in particular α-phase nickel hydroxide and spherical nickel hydroxide, were studied. α-phase nickel hydroxide was prepared by an improved chemical co-precipitation method, while a spray drying technique was employed to obtain spherical shaped particles. The possible use of this new type of solid polymer in Ni/MH batteries was also investigated in this work.

The element Al was chosen to stabilize the structure of α-phase nickel hydroxide, that is turbostratic disorder. The content of Al in Al–substituted nickel hydroxide, Ni1-

xAlx(OH)2(CO3)x/2 . nH2O, was x = 0.10, 0.20, 0.25 and 0.33. The as–prepared nickel

hydroxide sample was aged in 6M KOH for 90 days, and the structure was identified

by XRD and TEM techniques. Cyclic Voltammetry, the Tafel Curve and discharge

behaviors were employed to investigate the electrochemical properties, and the

potentials, exchange current density, discharge capacity and voltage are discussed.

The Al–substituted nickel hydroxide samples show superior electrochemical

performance to β-nickel hydroxide.

A new type of α-phase nickel hydroxide was also prepared and investigated in this

study, (Al,Co)–substituted nickel hydroxide. Co together with Al was doped to nickel

hydroxide, Ni1-x(Al+Co)x(OH)2(CO3)x/2 . nH2O, and the content of (Al+Co) was x =

0.25. It was found that Al and Co had entered the structure of nickel hydroxide. A

higher capacity but lower discharge voltage was obtained compared with Al–Ni(OH)2 with 25% Al.

3 The temperature effect on three types of nickel hydroxides, Al–Ni(OH)2 with 25% Al,

(Al,Co)–Ni(OH)2 ((Al+Co) = 0.25, Co/Al = 0.5) and β-nickel hydroxide, was

investigated. The temperatures investigated were –15oC, 0oC, 25oC and 50oC. The

samples’ charge/discharge behavior and cycle life were investigated. It was found that

Al–Ni(OH)2 and (Al,Co)–Ni(OH)2 possess the optimum electrochemical

performance at 0oC, while β-nickel hydroxide shows its best at 25oC. The samples

showed different resistance to the temperature effect, while the highest endurance to

low temperature occurred with Al–Ni(OH)2 and to high temperature with (Al,Co)–

Ni(OH)2.

Spherical agglomerates of nanostructured beta-type Ni(OH)2 were produced by a

spray drying technique. This material features a narrow Gaussian-type particle size

distribution in the range of 0.1 – 10 microns and a high specific surface area of 50 –

200 m2/g. The spray drying technique was also tried for preparing spherical Al–

substituted nickel hydroxide, and the results show that spherical agglomerates could

not be obtained under the same conditions as for β-type. Certain parameters needed to

be changed, the air temperature, the spray speed and the ageing period of the nickel

hydroxide slurry, and spherical agglomerate particles of Al–Ni(OH)2 was obtained.

A new type of solid polymer , tetramethylammonium hydroxide

pentahydrate (TMAH5)–based with addition of poly(tetramethyl ammonium acrylate)

(PTMA) were investigated with a view to its possible use as an electrolyte in Ni/MH

batteries. The contents of PTMA in the electrolytes were 0%, 5% and 15%. It was

found that the Ni/MH cells employing such solid electrolytes were dischargeable, and

4 the electrochemical performance of TMAH5 + 5% PTMA and TMAH5 + 15%

PTMA was improved at 50oC compared with that at 25oC.

5 ACKNOWLEDGMENTS

I would like to express my sincere appreciation to my research supervisors, Professor

H.K. Liu and Professor S. X. Dou for their academic guidance, encouragement and support throughout the whole of the present work. I wish to thank the Australian

Research Council for offering the Australian Postgraduate Award of Industry (APAI) to me. It was this financial support that enabled me to finish my research work and complete this thesis.

Many thanks are also given to all the staff members at ISEM and all the technicians in

Faculty of Engineering, particularly to Dr. M. Ionescu for assistance in the analysis of

XRD patterns, Mr. N. Mackie, Mr. G. Tillman and P. Yao for SEM/TEM analysis,

Mr. M. Lindsay for the assistance in the use of software packages and Dr. K.

Konstantinov for help with the spry drying techniques. Special thanks are given to Dr.

G.X. Wang, X.L Wang, Mrs. Y. Chen, Dr. J.Z. Wang, Mrs. A.H. Li and Dr. Z.P. Guo.

Great appreciation is also given to Dr. T. Silver for helpful comments and advice on this thesis and to Mrs. B. Allen for her help in official matters.

I also would like to think Dr. J. Sun at the School of Physics and Materials

Engineering, Monash University, for providing the solid polymer electrolyte samples.

Finally, I would like to express my deepest thanks and gratitude to Mr. Xiaoming Liu, my husband, and Ruidi, my daughter, for their deep love. Also I would express my deep miss and love to my still-born son, Taotao.

6 Contents

CATHODIC MATERIALS FOR NICKEL-METAL HYDRIDE BATERIES. i

DECLARATION ………………………………………………………….…….. ii

ABSTRACT …………………………………………………………………….... iii

ACKNOWLEDGEMENTS ………………………………………..……………. v

List of Figures ……………………………………………………………………. XI

List of Tables …………………………………………………………………….. XV

Chapter 1 Introduction …………………………………………………………. 1

1.1 Energy ……………………………………………………….…… 1

1.2 Hydrogen Storage in Metal Hydrides ……………………………….……. 1

1.3 Ni/MH ………………………………………………………………. 2 1.3.1 α-Ni(OH)2 ……………………………………………………………. 3

1.4 Aim of the Research ……………………………………………………….… 4

1.5 Structure of Thesis ………………………………………………….....……. 5

Chapter 2 Literature Review ………………………………………………… ……………………………………………………………………………………. 8

2.1 Ni/MH Battery ……………………………………………………………… 8 2.1.1 components of Ni/MH battery ……………………………………..… 8 2.1.2 Electrochemical Reaction in the Ni/MH battery …………………….. 9 2.1.2.1 Fundamental reaction ………………………………………….. 9 2.1.2.2 Phase transformation during the cell cycling ………………….. 9 2.1.2.3 Reaction process of a Ni/MH battery ………………………….. 11 2.1.2.4 Electrochemical reaction during overcharge and overdischarge ………………………………………………..… 12 2.1.3 Advantages of the Ni/MH battery ……………………………………. 13

2.2 Metal Hydrides ……………………………………………………………… 15 2.2.1 Properties of Metal Hydrides (MH) ………………………………….. 15 2.2.1.1 Definition of Metal Hydride ……………………………………. 15 2.2.1.2 Formation of Metal Hydrides …………………………………… 16 2.2.2 Types of Metal Hydrides …………………………………………….. 16 2.2.2.1 AB Hydrides ……………………………………………………. 17 2.2.2.2 A2B Hydrides …………………………………………………… 17 2.2.2.3 AB2 Hydrides …………………………………………………… 18 2.2.2.4 AB5 Hydrides …………………………………………………… 18 2.2.3 Metal Hydrides for application in Ni/MH batteries ………………….. 19 2.2.4 Typical metal hydrides used in Ni/MH batteries …………………….. 20

2.3 Nickel Hydroxide ……………………………………………….………….. 21 2.3.1 Phase transformation of nickel hydroxide …………………………… 21 2.3.2 Structure of nickel hydroxide ……………………………………...… 22 2.3.2.1 Basic structure of nickel hydroxides ………………………...… 22 2.3.2.2 Structure of β-Ni(OH)2 ……………………………………...…. 23 2.3.2.3 Structure of α – Ni(OH)2 …………………………………..…... 23 2.3.2.4 Structure of β-NiOOH ……………………………………..…... 25 2.3.2.5 Structure of γ – NiOOH ……………………………………..… 25 2.3.2.6 Crystalline parameters for Nickel hydroxide ………………..… 26 2.3.3 Dynamic and kinetic mechanisms of nickel hydroxide ..…. 27

2.4 Advantages of α –Ni(OH)2 ………………………………………..….…….. 28

2.5 Development of α -Ni(OH)2 ………………………………………..………. 31 2.5.1 Development History ………………………………………………… 31 2.5.2 Development Trend ………………………………………………….. 34

2.6 Electrolyte ………………………………….……………………………….. 36

Chapter 3 Experimental …………………………………………..…………… 39

3.1 Materials and Chemicals ………………………………….………………. 39

3.2 Experimental Procedures …………………………………..……………… 40

3.3 Nickel Hydroxide Powders ………………………………………...………. 41 3.3.1 Synthesis of nickel hydroxide ………………………………………. 41 3.3.2 Synthesis of spherical nickel hydroxide ……………………………. 41 3.3.3 Ageing of nickel hydroxide samples ……………………………….. 42

3.4 Electrode Fabrication and Cell Construction ……………………………. 43 3.4.1 Fabrication of nickel hydroxide ………….……………… 43 3.4.2 Ni/MH cell construction ……………………………………………. 43

3.5 Electrochemical Characteristics Measurement ………………………….. 44 3.5.1 Charge/discharge behaviour at ambient temperature ………….……. 44 3.5.2 Charge/discharge at different temperatures …………………….…… 44 3.5.3 Cyclic Voltammetry ……………………………………………..…... 45

3.6 Physical Analysis …………………………………………………………… 45

Chapter 4 Al-substituted Nickel Hydroxide ………………………..………… 48

4.1 Introduction ………………………………………………………………… 48

4.2 Physical Properties ……………………………………………….…….…. 50 4.2.1 XRD patterns of as-prepared samples …………………………….… 50 4.2.2 XRD patterns of aged samples ………………………………….…... 53 4.2.3 TEM measurement ……………………………………………….…. 55

4.3 Electrochemical Properties ……………………………….…………….…. 57 4.3.1 Cyclic Voltammetry ………………………………………….….…... 57 4.3.2 Tafel Curves ………………………………………………….….…... 60 4.3.3 Discharge curves …………………………………………….….…… 61 4.3.3.1 Al-Ni(OH)2 with 10% Al ……………………………….….….. 61 4.3.3.2 Al-Ni(OH)2 with 20% Al ……………………………….….….. 64 4.3.3.3 Al-Ni(OH)2 with 25% Al …………………………………..….. 65 4.3.3.4 Al-Ni(OH)2 with 33% Al …………………………………..….. 66 4.3.4 Cycle Life ………………………………………………………..….. 68 4.3.5 XRD patterns after 100 electrochemical cycles ………………..….… 69

4.4 Conclusion …………………………………………………..………….……. 70

Chapter 5 (A1, Co)- substituted Nickel Hydroxide ……………...…….…..…. 73

5.1 Introduction ………………………………………………..………….…..… 73

5.2 Physical Properties …………………………………………………....……. 74 5.2.1 XRD patterns as-prepared samples ………………………...………… 74 5.2.2 XRD patterns of aged samples ……………………………………...... 77 5.2.3 TEM measurement ………………………………………………...…. 79

5.3 Electrochemical Properties ……………………………………………….... 81 5.3.1 (A1,Co)-Ni(OH)2 with 20% Co …………………………………….... 81 5.3.2 (A1, Co)-Ni(OH)2 with 33% Co ………………………………….….. 83 5.3.3 (A1,Co)-Ni(OH)2 with 50% Co …………………………………..….. 84 5.3.4 Cycle life of (A1,Co)-Ni(OH)2 ……………………………………..... 86 5.3.5 XRD Patterns after 100 electrochemical cycles ………………..……. 87

5.4 Conclusion …………………………………………………………………… 88

Chapter 6 Temperature Effects on Nickel Hydroxide Electrode …………… 90

6.1 Introduction ……………………………………………………………….… 90

6.2 Temperature Effects on A1-Ni(OH)2 ……………………………………… 91 6.2.1 Charge and discharge characteristics ………………………………... 91 6.2.2 Cycle life …………………………………………………………..… 95 6.2.3 Physical Properties ……………………………………………….….. 96 6.2.3.1 XRD patterns ………………………………………………...… 96 6.2.3.2 SEM images ……………………………………………..…….. 97

6.3 Temperature Effects on (A1,Co)-substituted nickel Hydroxide Electrode . ………………………………………………………………………………….... 100 6.3.1 Discharge characteristics at different temperatures …………….…… 100 6.3.2 Cycle life ………………………………………………………….…. 104 6.3.3 Physical properties ……………………………………………….….. 105 6.3.3.1 XRD patterns ………………………………………………….. 105 6.3.3.2 SEM images ………………………………………………….... 105

6.4 Temperature Effects on Spherical Nickel Hydroxide Electrode ………… 108 6.4.1 Charge/discharge characteristics ……………………………..……… 108 6.4.2 Cycle life …………………………………………………………..… 112 6.4.3 X-ray diffraction patterns ………………………………………...….. 113

6.5 Conclusions ………………………………………………………………… 114

Chapter 7 Spherical Nickel Hydroxide …………………..…………………… 117

7.1 Introduction ………………………………………………………………… 117

7.2 β-type Nickel Hydroxide …………………………….…………………….. 119 7.2.1 Experimental …………………………………………………………. 119 7.2.2 X-ray diffraction patterns …………………………………………….. 119 7.2.3 Morphological features …………………………………………….…. 120 7.2.4 Particle Size Distribution …………………………………………...… 122

7.3 Alpha – Phase Nickel Hydroxide ………………………………………...… 123 7.3.1 Phase identification ……………………………………………..….... 123 7.3.2 Morphological features …………………………………………..….. 125

7.4 Conclusion …………………………………………………………….……. 128

Chapter 8 Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte ……………………………………………………….…… 129

8.1 Introduction ………………………………………………………………… 129

8.2 Experimental ………………………………………………………………. 131

8.3 Charge and Discharge Behaviors at 25°C …………………………..……. 132

8.4 Charge and Discharge Behaviors at 50°C ……………………………...…. 137

8.5 Conclusion ……………………………………………..……………………. 142

Chapter 9 Summary ……………………………………………………………. 144

Publications ……………………………………………………………………... 149

References ………………………………………………………………………. 151

List of Figures

Figure 2.1 Phase transformation at nickel hydroxide (a) and nickel metal hydride (b) electrodes during Ni/MH cell cycling …………………………………………… 10

Figure 2.2 Schematic diagram of reaction process of a Ni/MH battery …………. 11

Figure 2.3 Schematic representation of the structures of nickel oxyhydroxide and hydroxides ………………………………………………………………………… 22

Figure 3.1 Schematic diagram for experimental procedure ……………………… 40

Figure 3.2 System diagram for Yamato pulvis Mini-Spray instrument (Model GA-32) ……………………………………………………………………………………. 42

Figure 4.1 X-ray diffraction patterns of as-prepared A1-Ni(OH)2 ………………. 51

Figure 4.2 Cell constants (a and c) in A1-Ni(OH)2 samples ……………………... 53

Figure 4.3 X-ray diffraction patterns of A1-Ni(OH)2 after aging treatment ……… 54

Figure 4.4 TEM images (a and c) and halo diffraction patterns (b and d) for as- prepared (a and b) and aged (c and d) A1-Ni(OH)2 with 20% A1 ……………….. 57

Figure 4.5 Cyclic voltammograms of A1-Ni(OH)2 with 0%, 10%, 20% and 33% A1 (mol) ……………………………………………………………………………… 59

Figure 4.6 Tafel curves of nickel hydroxide electrodes with different content of aluminium ………………………………………………………………………… 61

Figure 4.7 Discharge curves of A1-Ni(OH)2 (10% A1) …………………………. 63

Figure 4.8 Discharge curves of A1-Ni(OH)2 with 20% A1 ……………………… 65

Figure 4.9 Discharge curves of A1-Ni(OH)2 (25% A1) …………………………. 66

Figure 4.10 Discharge curves of A1-Ni(OH)2 (33% A1) ………………………… 67

Figure 4.11 cycle life of A1-Ni(OH)2 …………………………………………….. 68

Figure 4.12 X-ray diffraction patterns of A1-Ni(OH)2 electrodes after 100 electrochemical cycles ……………………………………………………………. 69

Figure 5.1 The diffraction patterns of (A1,Co)-Ni(OH)2 …………………………. 76

Figure 5.2 The diffraction patterns of aged samples A1-Ni(OH)2 with addition of Co …………………………………………………………………………………….. 78

Figure 5.3 TEM images (a and c) and halo diffraction patterns (c and d) of (A1,Co)- Ni(OH)2 (TEM image (a) and halo pattern (b) are for the as-prepared sample, (c) and (d) are for the aged sample) ……………………………………………………… 81 Figure 5.4 Discharge curves (A1,Co)-Ni(OH)2 ………………………………….. 82

Figure 5.5 Discharge curves of A1-Ni(OH)2 with 33% Co ……………………… 84

Figure 5.6 Discharge curves of (A1,Co)-Ni(OH)2 with 50% Co ………………… 85

Figure 5.7 Cycle life of (A1,Co)-Ni(OH)2 samples ……………………………… 86

Figure 5.8 XRD patterns for (A1,Co)-Ni(OH)2 electrodes after 100 electrochemical cycles …………………………………………………………………………….. 87

Figure 6.1 Charge and discharge curves at the current rate of 60mA/g for A1-NI(OH)2 samples …………………………………………………………………………... 92

Figure 6.2 Discharge capacities of A1-Ni(OH)2 electrodes under different conditions ……………………………………………………………………………………. 94

Figure 6.3 Discharge midpoint voltages for A1-Ni(OH)2 electrodes under different condition ………………………………………………………………………..... 95

Figure 6.4 Cycle life of A1-Ni(OH)2 electrodes under different conditions …...… 96

Figure 6.5 X-ray diffraction patterns of S1 electrodes after 200 charge/discharge cycles …………………………………………………………………………….. 97

Figure 6.6 SEM images of A1-Ni(OH)2 electrodes before or after electrochemical cycles ……………………………………………………………………………... 99

Figure 6.7 Charge/discharge curves of (A1,Co)-Ni(OH)2 electrodes at different temperatures ……………………………………………………………………… 100

Figure 6.8 Discharge capacities of (A1,Co)-Ni(OH)2 electrode at different temperatures ……………………………………………………………………… 103

Figure 6.9 Discharge midpoint voltage of (A1,Co)-Ni(OH)2 electrode at different temperatures ……………………………………………………………………… 103

Figure 6.10 cycle life of (A1,Co)-Ni(OH)2 electrodes at different temperatures… 104

Figure 6.11 X-ray diffraction patterns of (A1,Co)-Ni(OH)2 electrodes after 200 charge/discharge cycles at different temperatures ……………………………….. 106

Figure 6.12 SEM images of (A1,Co)-Ni(OH)2 electrodes after 200 charge/discharge cycles at different temperatures (all images are at the same magnification 1000x) . 108

Figure6.13 charge/discharge curves of spherical Ni(OH)2 electrodes at different temperatures ……………………………………………………………………….. 109

Figure 6.14 Discharge capacity of spherical Ni(OH)2 electrodes at different temperatures ………………………………………………………………………. 111

Figure 6.15 Discharge plateau voltage of spherical Ni(OH)2 electrodes at different temperature ………………………………………………………………………. 111

Figure 6.16 Cycle life of spherical Ni(OH)2 electrodes at different temperatures .. 112

Figure 6.17 X-ray diffraction patterns of spherical Ni(OH)2 ( β –phase) electrodes before or after 200 charge/discharge cycles at different temperatures …………… 113

Figure 7.1 XRD pattern of spherical Ni(OH)2 prepared by the spray dry method . 120

Figure 7.2 SEM images of nickel hydroxide spherical agglomerates prepared by spray dry method ……………………………………………………………………….. 122

Figure 7.3 Typical particle size distribution graph of spherical Ni(OH)2 prepared by the spray dry method (a) and of commercial spherical Ni(OH)2 (b) …………….. 123

Figure 7.4 X-ray diffraction pattern of A1- substituted nickel hydroxide prepared by spray dry technique ………………………………………………………………. 124

Figure 7.5 Images of A1-substituted nickel hydroxides prepared by spray dry technique …………………………………………………………………………. 127

Figure 8.1 Schematic diagram of TMAH5 ………………………………………. 129

Figure 8.2 Charge curves of Ni/MH cells employing TAMH5-based electrolyte at 25°C ………………………………………………………………………………. 134

Figure 8.3 Discharge curves of Ni/MH cells employing TAMH5-based electrolyte at 25°C ………………………………………………………………………………. 136

Figure 8.4 Relationship between the discharge properties of solid electrolyte and the content of PTMA in them ………………………………………………………… 137

Figure 8.5 Solid-solid transition of Me4NOH . 5H20 …………………………….. 138

Figure 8.6 Charge curves of Ni/MH cells employing TAMH5-based electrolyte at 50°C ……………………………………………………………………………… 139

Figure 8.7 Discharge curves of Ni/MH cells employing TAMH5-based electrolyte at 50°C ……………………………………………………………………………… 141

Figure 8.8 Relationship between discharge properties and percentage of PTMA in TMAH5 ………………………………………………………………………….. 142

List of Tables

Table 1-1 Storage properties of metal hydrides compared with gas and liquid hydrogen …………………………………………………………………………. 2

Table 1.2 Cyclic voltametric data for α – Ni(OH)2 with trivalent cations …………. 4

Table 2.1 Secondary Battery Technology Comparison …………………………... 14

Table 2.2 Typical chemical composition of commercial Ni-MH batteries (wt.%) . 20

Table 2.3 Crystal data of Ni(OH)2 and NiOOH ……………………………….. 26

Table 2.4 Comparison of empirical and nonstoichiometric formulae of the active materials in the nickel hydroxide electrode ………………………………………. 28

Table 3.1 Descriptions of materials and chemicals ………………………………. 39

Table 4.1 Observed d-spacing and calculated unit cell constants for A1-Ni(OH)2 samples ……………………………………………………………………………. 52

Table 4.2 Crystalline size of A1-Ni(OH)2 ………………………………………… 55

Table 4.3 Data obtained from the cyclic voltammorgrams ……………………….. 58

Table 4.4 Exchange current density (io) of different nickel hydroxide samples ….. 60

Table 4.5 Discharge capacity and midpoint voltage of A1-Ni(OH)2 with 10% A1.. 63

Table 4.6 Discharge capacity and midpoint voltage of A1-Ni(OH)2 with 20% A1 64

Table 4.7 Discharge capacity and midpoint voltage of A1-Ni(OH)2 with 25% A1(mol) …………………………………………………………………………… 66

Table 4.8 Discharge capacity and midpoint voltage of A1-Ni(OH)2 with 33% A1(mol) …………………………………………………………………………… 68

Table 5.1 The compositions of the samples prepared …………………………….. 74

Table 5.2 Observed d-spacing for unit cell of (A1,Co)- Ni(OH)2 samples ……….. 76

Table 5.3 FWHM of (A1,Co)- Ni(OH)2 ………………………………………….. 77

Table 5.4 FWHM of aged (A1,Co)- Ni(OH)2 …………………………………….. 79

Table 5.5 discharge capacity and midpoint voltage of (A1,Co)- Ni(OH)2 with 20% Co …………………………………………………………………………………….. 83

Table 5.6 Discharge capacity and midpoint voltage of (A1,Co)- Ni(OH)2 with 33% Co …………………………………………………………………………………….. 84

Table 5.7 Discharge capacity and midpoint voltage of (A1,Co)- Ni(OH)2 with 50% Co …………………………………………………………………………………….. 86

Table 6.1 Nickel hydroxide samples used in the experiment …………………….. 91

Table 6.2 The data obtained from discharge curves for A1- Ni(OH)2 electrode … 93

Table 6.3 Data obtained from the discharge curves for S2 electrode …………….. 101

Table 6.4 Data obtained from the discharge curves for spherical nickel hydroxide electrodes …………………………………………………………………………. 110

Table 7.1 Parameters used in the spray dry process for α - Ni(OH)2 preparation …. 125 Chapter 1 - Introduction

Chapter 1 Introduction

1.1 Hydrogen Energy

Hydrogen is a highly promising source of energy [1], particularly when decreasing non- renewable energy supplies and increasing environmental concern about pollution are taken into consideration. There are abundant hydrogen resources because it can be generated from an almost unlimited raw material (water). Two outstanding advantages make hydrogen energy more attractive:

i. Hydrogen has the highest energy density per unit weight than any other

chemicals.

ii. It does not pollute our atmosphere during combustion because the major by-

product is H2O.

Moreover, hydrogen has a variety of practical uses, such as in internal combustion engines [2], fuel cells [3], thermal engines [4], etc. However, its storage and transportation remain a big problem for its technical use in either mobile or stationary applications. Hydrogen energy storage has remained a leading research topic to date.

1.2 Hydrogen Storage in Metal Hydrides

It was only after the recognition of the remarkable hydriding properties of AB5 alloys, which were discovered at the Philips Labs about 1969 [5] and were found to have high hydrogen capacity and good reversibility at ambient temperature, that storage of hydrogen in metal hydrides has drawn the attention of scientists. The two basic properties which make metal hydrides attractive are their high and reversible hydrogen storage capacity per mole of compound and the high energy content per unit volume.

The energy density of hydrogen that can be stored in metal hydride may be more than

10 MJ L-1. The storage properties of some metal hydrides are listed in Table 1-1 [6].

1 Chapter 1 - Introduction

Table 1-1 Storage properties of metal hydrides compared with gas and liquid hydrogen

Moreover, storage of hydrogen in metal hydrides has the advantage of smaller pressure and less safety precautions. Metal hydride storage can also be used in periodically working thermal sorption machines (chemical heat pumps, heat transformers).

1.3 Ni/MH Battery

Sealed rechargeable nickel/metal hydride batteries employing metal hydrides as active materials offer significant improvements over conventional rechargeable batteries in terms of performance and environmental friendliness [7-9]. They are environmentally friendly and offer more energy per unit volume or weight than Ni/Cd or lead-acid batteries. Nickel/metal hydride batteries have excellent electrical characteristics and cycle life. Although Li- batteries exhibit several advantages, such as high voltage and high energy density, for most applications Ni/MH batteries may be preferred to lithium batteries as the latter are more expensive and cannot be operated without electronic control for safety reasons.

2 Chapter 1 - Introduction

At present, Ni/MH batteries can be considered among the preferred batteries of the future, at least in the field of small portable energy devices. The high energy density, excellent power density, and long cycle life of Ni/MH batteries also make them a leading technology as the battery power source for electric vehicles (EVs), especially in bipolar designs. Indeed, sealed Ni/MH batteries of bipolar cell design are of great interest for application as a power source in hybrid road vehicles, because increasing specific power (less cell containment), lower cost, easier handling (higher-voltage packages) and a better compromise between individual subcells and series of conventional cells are expected [10].

1.3.1 α-Ni(OH)2

In the design of a Ni/MH battery, the capacity of the positive electrode (nickel hydroxide) is limited [11]. This means that the material properties and utilization of nickel hydroxide dictate the amount of the capacity that can be stored and released during the charging and discharging processes. It is known that there are two polymorphs of nickel hydroxide, denoted as α-Ni(OH)2 and β-Ni(OH)2, and that these will transform into γ-NiOOH and β-NiOOH respectively during the charge/discharge process. It has been found that α-Ni(OH)2 has better electrochemical properties than β-

Ni(OH)2. α-Ni(OH)2 can be oxidized to γ- NiOOH in a lower potential than β-Ni(OH)2 to β-NiOOH, and the α/γ system has a higher discharge capacity than the β/β system

[12]. Moreover, the α/γ system can be transformed reversibly without any mechanical deformation and swelling of the electrode during the cycling process unlike the β/β system [13]. However, α-Ni(OH)2 is unstable in strong alkali solutions and is converted to β-Ni(OH)2. How to stabilize α-Ni(OH)2 has drawn a considerable attention from scientists, and many efforts have been made in this field. Al has been found to be the

3 Chapter 1 - Introduction

optimum element to stabilize α-Ni(OH)2 with Al, it possesses a double layer structure, and has a better electrochemical reversibility, a higher and a higher coulombic efficiency. Cyclic voltammetric (CV) data were obtained for electrosynthesized thin film α-Ni(OH)2 with trivalent cations and listed in Table 1-2

[13, 14] .

Note: ∆ϕa,c/mV, the potential difference between the anodic and cathodic peaks;

ϕrev/mV, average of the anodic and cathodic peak potentials.

1.4 Aim of the Research

Alhough α-Ni(OH)2 has superior electrochemical characteristics to β-phase and is a promising cathode material candidate for the Ni/MH battery, there have been no reports on batteries employing α-phase nickel hydroxide in the Ni/MH battery industry so far.

β-Ni(OH)2 still dominates the cathode active material market. There will be a need for large improvements in order to commercialize α-Ni(OH)2. The aim of this research is to focus on the stabilization of α-Ni(OH)2 with elemental Al or Co and also on the

4 Chapter 1 - Introduction

spherical Al–substituted nickel hydroxide, for improving the performance of nickel hydroxide electrode and leading to the development of a high energy battery.

1.5 Structure of the Thesis

The structure of different types of nickel hydroxides (α, β and γ) has been reviewed in

Chapter 2 to give a clear idea of the differences between them and some understanding of the causes of their different electrochemical properties. The advantages, the disadvantages, and the development history of α-phase nickel hydroxide have been emphasized in particular. The advantages of the Ni/MH battery and its basic working principles and electrochemical reaction mechanism have also been reviewed.

Chapter 3 describes the raw materials and the experimental methods used in this study, as well as the characterization techniques used in the measurements of electrochemical or physical properties.

Elemental Al was chosen to stabilize the turbostratic disorder structure of α-Ni(OH)2 and Ni1-xAlx(OH)2(CO3)x/2.nH2O (x = 0.10, 0.20, 0.33 and 0.50) samples were prepared by a improved chemical co-precipitation method described in Chapter 4. X-ray diffraction patterns (XRD) and transmission microscope (TEM) techniques were employed to identify the structure of Al–Ni(OH)2. The electrochemical characteristics were investigated with CV, Tafel Curves and charge/discharge curves.

Chapter 5 describes the coprecipiation of Co in Al–Ni(OH)2 by the same chemical coprecipitation method described in Chapter 4. The content of (Al + Co) was the same value of 0.25 in Ni1-x(Al+Co)x(OH)2(CO3)x/2.nH2O (x = 0.25), the compound

5 Chapter 1 - Introduction

synthesized, while the ratio of Al/Co varied between 4, 2 and 1 in the samples. The stabilization of the structure of (Al,Co)–substituted nickel hydroxide was identified by

XRD and TEM techniques, and the discharge behaviors and cycle life were also investigated.

The effect of the temperature on three different types of nickel hydroxide electrodes is discussed in Chapter 6. These three types of electrodes are Al–substituted nickel hydroxide with 25% Al; (Al,Co)–substituted nickel hydroxide ((Al + Co) = 0.25; Al/ Co

= 2) nickel hydroxide; and commercial β-phase nickel hydroxide. The temperatures investigated in this work were -15oC, 0oC, 25oC and 50oC. The charge/discharge properties and cyclability of the samples at the different temperatures were measured, and the morphologies of the electrode surface were also investigated before and after the electrochemical cycles.

Spherical β-nickel hydroxide and Al–substituted α-phase nickel hydroxide were prepared by a spray drying method described in Chapter 7. The morphologies of the spherical samples prepared and commercial spherical samples were investigated by scanning electron microscopy (SEM). The surface areas of the spherical commercial and spray dried spherical β-nickel hydroxide samples was measured by the Brauner-

Emmet-Teller (BET) technique, and their electrochemical performances were also investigated.

A new type of solid polymer electrolyte is investigated in Chapter 8.

Tetramethylammonium hydroxide pentahydrate, (CH3)4NOH⋅5H2O (abbreviated to

TMAH5) is the main body of the electrolyte. Poly(tetramethyl ammonium acrylate)

6 Chapter 1 - Introduction

(PTMA) was added to TMAH5–based electrolyte, and the contents of PTMA in the electrolytes were 0%, 5% and 15%. Their discharge behaviors were investigated.

The main results of the present study are summarized in Chapter 9, and the references are also listed. In addition, the author’s publications during Ph. D. study are included.

7 Chapter 2 Literature Review

Chapter 2 Literature Review

2.1 Ni/MH Battery

The Ni/MH battery is classed as an alkaline storage secondary battery due to the use of (KOH) as the electrolyte. Research efforts on rechargeable

Ni/MH batteries have been carried out since the 1950s, but the breakthrough for commercial use was realized only in the 1980s, as a result of the development of cobalt-aluminum modified LaNi5 hydrogen storage alloys [15].

2.1.1 Components of Ni/MH battery

Ni/MH battery consists of two electrodes, the cathode and the anode. In-between the two electrodes there is a and the liquid electrolyte KOH [16]. For the cathode electrode, Co, Zn, or Cd doped Ni hydroxides are used as the active material, while the active material for metal hydride are Ni intermetallic powders (rare earth or Ti-Zr based). Ni plaque, felts, or metal foams act as the substrate for both the cathode and anode electrode. The additives Co, Ni, Co(OH)2 or graphite powders are used in either the cathode or anode electrode to improve the electric conductivity of the material and to give rigidity to the structure to minimize volume changes during cycling [17, 18].

The electrolyte used in Ni/MH batteries is typically 30 wt% (about 5M) KOH, containing LiOH to aid charge acceptance by the cathode.

2.1.2 Electrochemical Reaction in the Ni/MH battery

8 Chapter 2 Literature Review

2.1.2.1 Fundamental reaction

The half-cell reactions involved in the charge-discharge process of nickel/metal hydride elements are the following: during discharge, nickel oxyhydroxide is reduced to nickel hydroxide, and the metal hydride (MH) is oxidized to metal alloy (M) [19,

20], with a potential of 0.490V vs. the standard hydrogen electrode (SHE). The process is reversed during charge. at the positive Ni electrode

- - o [+] Ni(OH)2 + OH ←→ NiOOH + H2O + e E = +0.490V/SHE (2-1) at the negative MH electrode

- - o [-] M + xH2O + xe ←→ MHx + xOH E = -0.828V/SHE (2-2)

The overall cell reaction is as follows

charge xNi(OH) + M ←→ xNiOOH + MH Eo = +1.318V (2-3) 2 discharge x

2.1.2.2 Phase transformation during the cell cycling

The phase transformation appears during the electrochemical cycles. During the charging process, nickel hydroxide (cathode active material) is transformed into nickel oxyhydroxide, and alloy (anode active material) is transformed into metal hydride. The reverse is true for the discharging process. The phase transformation at nickel hydroxide and hydride electrodes during the Ni/MH cell cycling is shown in

Figure 2-1 (a) and (b). In Figure 2-1 (b) the hydrogen storage reactions are accompanied by oxygen and

At the cathode electrode [21]:

9 Chapter 2 Literature Review

At the anode electrode [22]:

Figure 2-1 Phase transformation at nickel hydroxide (a) and nickel metal hydride (b) electrodes

during Ni/MH cell cycling.

10 Chapter 2 Literature Review hydrogen evolution (OER and HER) when cells are driven to overcharge and overdischarge, respectively.

2.1.2.3 Reaction process of a Ni/MH battery

The Ni/MH battery is characterized by the effective migration of hydrogen from the positive to the negative electrode during charging and from the negative to the positive during discharging without change in the electrolyte volume. To show clearly the inner electrochemical reaction process in the Ni/MH battery, its schematic diagram [23] is presented in Figure 2-2.

Figure 2-2 Schematic diagram of reaction process of a Ni/MH battery

11 Chapter 2 Literature Review

During the charge process, the hydrogen atom dissociates from Ni(OH)2 and is absorbed by the MH alloy. For the discharge process, the hydrogen atom dissociates from the MH alloy and joins with NiOOH to form Ni(OH)2.

2.1.2.4 Electrochemical reaction during overcharge and overdischarge.

In the sealed Ni/MH battery, the capacity of the hydride negative electrode is higher than that of the nickel hydroxide electrode (positive-limited cell). If overcharging, an reaction will occur in the electrolyte inside the nickel electrode and oxygen will be generated (OER, Eq. 2-4). Oxygen will then diffuse to the hydride electrode (Eq. 2-5) to combine with the hydrogen to form water (oxygen recombination, Eq. 2-6), eliminating or reducing the internal pressure build-up [24].

This process is called hydrogen-oxygen recombination and plays a key role in battery performance and service life. The reaction rate of hydrogen-oxygen recombination should be large enough to exceed the oxygen evolution rate to ensure that the battery not damaged.

Nickel electrode:

- - 2yOH ↔ yH2O + ½ yO2 + 2ye (2- 4)

Hydride electrode:

- - MHx + 2yH2O + 2ye ↔ MHx+2y + 2yOH (2-5)

½ yO2 + MHx+2y ↔ MHx + yH2O (2-6)

Contrary to oxygen evolution on the nickel electrode, hydrogen will be generated at the nickel electrode (HER) if the cell is overdischarged, and it will diffuse to the hydride electrode and be absorbed by the alloy in the electrode. These reactions are indicated in the following equations [25]:

12 Chapter 2 Literature Review

At the nickel electrode:

- - H2O + e ↔ OH + ½ H2 (2-7)

At the hydride electrode:

- MHx + xOH ↔ M + xH2O + e (2-8)

M + ½ xH2 ↔ MHx (2-9)

However, if overcharge prolonged or at a high current, the hydrogen recombination rate at the hydride electrode is not fast enough to keep recharging the hydride electrode. As a result, the electrode will discharge completely and oxygen evolution will occur at the hydride electrode. Then the internal pressure of the battery will build up and the hydride electrode will be partially oxidized and damaged.

2.1.3 Advantages of the Ni/MH battery

Rechargeable batteries have become more and more important in daily life, especially in applications for consumer electronic devices, such as cellular phones, portable computers, toys, camcorders, power tools, etc [26, 27]. The ideal battery should have these features: easy operation, high energy density, long life, low cost and low toxicity. The Ni/MH battery is the best placed to meet these requirements. The Ni/MH battery is environmentally friendly because it contains no toxic materials such as Pb and Cd [28]. It is also called a green battery because only the electrolyte water is split and formed in consecutive charge/discharge cycles. A technological comparison of secondary batteries is given in Table 2-1 [29].

13 Chapter 2 Literature Review

It can be seen that the Ni/MH battery has many significant advantages over the other rechargeable batteries from Table 2-1. It possesses the unique characteristics of long cycle life, safety, nonhazardous materials and low cost. Its higher energy density, long service life characteristics and lower cost per watt or watt-hour make it even more attractive. Moreover, its electrochemical properties have been improved dramatically to date [30]:

14 Chapter 2 Literature Review

i. High capacity: the capacity of a AAA-sized cell has increased from 500

mAh in 1996 to 750 mAh.

ii. High working potential: the discharge capacity above 1.2V is up to more

than 80% of the total at the 1C-rate.

iii. Excellent rate capacity: a high rate Ni/MH battery can discharge up to the

20C-rate.

iv. Low self discharge rate: the charge retention after 1 month at ambient

temperature is above 80%.

v. Long cycle life. higher than 400 cycles on a 1C-rate charge/discharge

cycle.

The improved Ni/MH battery performance makes it more attractive. Although Li-ion

batteries exhibit several advantages, such as high voltage and high energy density,

for most applications Ni/MH batteries may be preferred to lithium batteries as the

latter are more expensive.

2.2 Metal Hydrides

2.2.1 Properties of Metal Hydrides (MH)

2.2.1.1 Definition of Metal Hydride

The first definition of a metal hydride was given by Gibbs in 1948 [31]. Metal hydride was defined as “a stoichiometric or nonstoichiometric compound in which there is the presence of a metal-to-hydrogen bond”. The type of bonding that is exhibited by the majority of potential hydrogen storage hydrides is metallic in nature. Metal hydrides have a metallic appearance and high thermal and electrical conductivities. They are

15 Chapter 2 Literature Review formed by the reaction of hydrogen with most of the elements of groups IIIA-VIIIA in the Periodic Table.

2.2.1.2 Formation of Metal Hydrides

Metal hydrides are formed in a reaction of a proper metal or an intermetallic compound with hydrogen. In the first phase (α - phase) a few hydrogen molecules are catalytically dissociated on the metal surface and dissolved in the metal. The hydrogen is called the “grid gas” [32, 33]. With increased pressure the hydrogen concentration is increased, and a metal hydride phase (β - phase, which corresponds to an absorption of hydrogen) is formed in an interaction with the hydrogen atoms. The coexistence region of the α- and β-phases is characterized by interdependent plateau pressures and temperatures.

2.2.2 Types of Metal Hydrides

The hydrogen storage alloys in common use are classified in terms of their stoichiometric composition into four categories including AB5-type, AB2-type (Laves- phase type), AB-type and A2B-type [34, 35]. In these alloys, the A component is the one which forms the stable hydride, while the B component performs a catalytic role in enhancing the hydriding/dehydriding characteristics and improving the stability of the alloy [36].

16 Chapter 2 Literature Review

2.2.2.1 AB Hydrides

The first AB type intermetallic hydride reported in detail was ZrNiH3 [37], it has high hysteresis and requires about 300°C for a desorption pressure of 1 atm, so it was not practical for room temperature storage applications. The first practical room temperature AB hydride was TiFe, discovered at Brookhaven National Lab in the US around 1969 [38, 39]. The thermodynamics of TiFe can be modified by ternary and higher order partial substitutions, such as Mn, Ni, Cr, V, etc., and as a result plateau pressures can be adjusted to suit applications [40].

2.2.2.2 A2B Hydrides

The A2B type compounds include Mg2Ni, Ti2Ni and Mg2NiH4 (3.6% wt.% H2). These compounds are formed by direct and reversible reaction with H2 gas around 300°C.

They are not really an interstitial metallic hydrides but rather transition metal complexes [41]. Because of its high hydrogen storage capacity yet high rate of in KOH, Mg2Ni has been a hot research topic to date.

The A2B compound Ti2Ni has been used in a synergistic manner with the AB compound TiNi in the form of a two-phase (Ti2Ni +TiNi) alloy [42]. Ti2NiH2.5 has more capacity than TiNiH but is slow to discharge in KOH and subject to corrosive passivation. The TiNiH phase is much less subject to corrosion and serves as a

“window” to rapidly channel the H to and from the Ti2NiH2.5 phase and the alloy-

KOH reaction interface, as well as contributing its own H to the electrochemical reaction.

17 Chapter 2 Literature Review

2.2.2.3 AB2 Hydrides

For AB2 Laves phase, A is usually a Group IVA element (mostly Ti and/or Zr) and B is one or more first-row transition elements in the range of V and Cu (with special emphasis on V, Mn and Cr). Numerous substitutions can be made for both A and B elements in AB2 hydrides to control absorption/desorption pressures [43-45]. AB2 intermetallics often show a significant range of stoichiometry [46, 47]. The variable stoichiometry allows the control of plateau pressure without ternary substitutions in the composition.

2.2.2.4 AB5 hydrides

AB5 compounds of the general form Mm(Ni, Co, Mn, Al)5 serve as the basis of most of the Ni/MH batteries made commercially today. The remarkable hydriding properties of AB5 were discovered at the Phillips Labs about 1969 [48]. It was found that LaNi5 has high hydrogen capacity and good reversibility at ambient temperature, so that the reaction [2-10] could be run in either direction at pressures less than 2.5 atm absolute pressure.

LaNi5 + 3.35 H2 ↔ LaNi5H6.7 (2-10)

One of the great advantages of the AB5 compounds is the fact that numerous partial substitutions could be made for both the La and Ni sides of LaNi5 [49]. It was found that a higher effective electrochemical capacity could be accomplished by the partial substitution of elements such as Al and Mn for some of the Ni atoms in LaNi5 [50].

Mischmetal (Mm, unrefined rare earth mixture) was selected to substitute for the expensive rare earth element La to reduce the cost [51], which is extensively used in the Ni/MH battery industry at present with the resulting material called mischmetal- based AB5,.

18 Chapter 2 Literature Review

2.2.3 Metal Hydrides for application in Ni/MH batteries

A hydrogen storage material, M can form a hydride, MHx, by an interaction with hydrogen gas/or an electrochemical method as shown in Eqs. (2-11) and (2-12):

M(s) + ½ xH2 (g) ↔ MHx(s) (2-11)

- - M + xH2O + xe ↔ MHx + xOH (2-12)

However, not every hydrogen storage alloy can be charged and discharged electrochemically as given in Eq. (1-12). As the active material of a hydrogen storage electrode, a hydrogen storage alloy has two major roles: as an electrochemical catalyst for the charge/discharge of hydrogen and as a hydrogen storage reservoir/source. A good hydrogen (hydride) electrode material must meet the following criteria [23, 30,

52]:

i. High reversible hydrogen storage capacity, i.e. > 1wt.%. The amount of

hydrogen that the MH material can absorb determines the electrochemical

storage capacity of the battery.

ii. An appropriate desorption pressure (0.1-1 atm). The desorption pressure

should be high enough (close to –0.9324V vs. Hg/HgO) to ensure that a

large amount of hydrogen can be completely released [53].

iii. The materials should exhibit good oxidation and sufficient corrosion

resistance in concentrated alkaline solutions.

iv. The materials should bear high electrocatalytic activity and reversibility of

electrode reactions.

v. Easy to activate. The activation can be taken under moderate conditions

and needs few activation cycles to accomplish.

vi. Long cycle life and low cost.

19 Chapter 2 Literature Review

2.2.4 Typical metal hydrides used in Ni/MH batteries

To meet the requirement for the commercial use of the metal hydride in the battery,

AB5 and AB2 alloys are commonly used in the Ni/MH battery. The typical chemical composition of Ni/MH batteries (wt.%) [18] is listed in Table 1-2.

Table 1-2 Typical chemical composition of commercial Ni-MH batteries (wt.%)

2.3 Nickel Hydroxide

C. Desmazures [54] reported for the first time on the use of nickel hydroxide as an active material in a battery. Now it is used in the positive electrode of alkaline Ni-Fe,

Ni-Zn, Ni-Cd, Ni-H2 and Ni-MH batteries [55]. The first two systems have not been

20 Chapter 2 Literature Review used practically because of their two main drawbacks, bad charge retention and poor cyclability. The nickel hydroxide electrode (NOE) in alkaline cells has remarkable cycling durability due to its highly reversible redox transfer between the active species. Although the nickel oxide electrode has been in use for over a century in alkaline batteries, its behavior is not yet completely understood. A significant improvement in our knowledge of its behavior has been made during the last ten years due to the contribution of solid-state chemists [56].

2.3.1 Phase transformation of nickel hydroxide

The nickel hydroxide electrode constitutes a very complex system involving four structural types of nickel hydroxides identified as α-Ni(OH)2, β-Ni(OH)2, γ-NiOOH and β-NiOOH. There are two polymorphs of nickel hydroxide in the raw material state, denoted as α-Ni(OH)2 and β-Ni(OH)2, and they will transform into γ-NiOOH and β-NiOOH respectively during the electrochemical process. If overcharged, β-

NiOOH can be converted into γ-NiOOH. Discharging occurs via an intermediate phase, α-Ni(OH)2, to β-Ni(OH)2, or directly from γ-NiOOH to β-Ni(OH)2. α-

Ni(OH)2 is unstable in strong alkali and is converted to β-Ni(OH)2. Its phase transformation process is given in Figure 2-1(a).

2.3.2 Structure of nickel hydroxide.

2.3.2.1 Basic structure of nickel hydroxides

The basic structure of nickel hydroxides and oxyhydroxides consists of NiO2 slabs, made of edge-sharing NiO6 octahedra, between which protons, alkali ions, water molecules and even anions can be reversibly intercalated and deintercalated during the

21 Chapter 2 Literature Review electrochemical cycles. The schematic structure of the four types of nickel hydroxides is shown in Figure 2-3.

Figure 2-3 Schematic representation of the structures of nickel oxyhydroxide and hydroxides

(a – crystallized nickel hydroxide; b – turbostratic nickel hydroxide; c - β(III) nickel oxyhydroxide; d -

γ(III) nickel oxyhydroxide [57])

It can be seen that in the β(II) and β(III) phases, there are no water molecules intercalated between the slabs, while in the other two phases water intercalation leads to enlarged interslab distance.

22 Chapter 2 Literature Review

2.3.2.2 Structure of β-Ni(OH)2

β-Ni(OH)2 exhibits a hexagonal close packing (hcp) of oxygen. The space group is

P31m (1 6 2) and the lattice parameters are a = 3.126Å, c = 4.605Å (JCPDS 14-0117)

[58]. Its structure can be described as a hexagonal close-packed structure of hydroxyl ions (AB oxygen packing) with Ni(II) occupying one out of two octahedral interstices.

It can be visualized as a layered structure, with each layer consisting of an hexagonal planar arrangement of Ni(II) ions with an octahedral coordination of oxygen, three oxygen atoms lying above the nickel plane, and three lying below. The layers are stacked up along the c-axis. The O-H bond is thought to be parallel to the c axis [59].

As determined by IR there are no hydrogen bonds, so only Van der Waals bonding forces exist between the layers [56].

2.3.2.3 Structure of α-Ni(OH)2

The initial structural mode of α-Ni(OH)2 proposed by Bode et al.[21] involves a double-layer structure with an ordered stacking of main (001) type brucite layers and secondary layers consisting of water molecules with a well-defined position between the main layers. They assumed a quasi-compact stacking of OH-H2O-OH planes, and they assigned a fixed definite position for intercalary water molecules which would correspond to the idealized formula 3Ni(OH)2.H2O [21]. Figlarz and Le Bilan proposed a turbostratic structure instead of well-organized structure, based on the X- ray diffraction line profiles of a pure α-type well-defined sample [60]. Its structure consists of parallel and equidistant brucite-type layers randomly oriented along the c- axis. The random orientation along the c-axis was supposed to result from the lability of the water molecules located in the interlamellar space. The intercalary water molecules are bonded to the hydroxyl groups by hydrogen bonds [61].

23 Chapter 2 Literature Review

More recently, P. Genin et al not only confirmed the turbostratic model of α-Ni(OH)2 but also obtained some additional structural information concerning interlamellar species [62]. They believe that anions present in the mother solution are adsorbed in the interlamellar space with water molecules. The adsorption of anions with different ionic radii is responsible for the different intersheet distances, for instance, the intersheet distance around 8.2Å for carbonate or nitrate ions and 13.2Å in the case of adipic solutions. The negative charge excess caused by the presence of anionic species in the interlamellar space is compensated by the hydroxyl vacancies in the nickel hydroxide layers. These anion species ensure the cohesion of the brucite type layers along the c-axis through electrostatic attractions between the positive charged layers and the negatively charged interlamellar spaces despite the large intersheet distance.

A. Delmahaye-Vidal argued that the usual formulation α-Ni(OH)2.nH2O given in the literature is not correct. A better description is given as Ni(OH)2-xAyBz.nH2O where A and B are mono or divalent anions and X = y + 2z. The x, y, and n can be calculated from the results of chemical analysis [57]. The interlamellar water molecules have shown a quasi – zeolitic behavior [63] from thermal studies, suggesting that the hydration equilibrium

Ni(OH)2-x-2y(A)x(B)y.nH2O ↔ Ni(OH) 2-x-2y(A)x(B)y + nH2O (2-13) is a divariant system which depends on both temperature and water partial pressure.

24 Chapter 2 Literature Review

2.3.2.4 Structure of β-NiOOH

This compound was first studied by Glemser and Einerhand [64], who synthesized it by oxidation of nickel nitrate with K2S2O8 in a 1M KOH solution at room temperature. β - NiOOH has a valence lower than 2.9 because the protons occupy the position of Ni and the average valence state of Ni decreases from +3 to +2.9, which has been proved by X-ray absorption near-edge structure (XANES) measurements

[65]. Its structure can be described as NiO2 sheets of edge-sharing NiO6 octahedra, with protons being intercalated between the slabs and oxygen atoms forming a hexagonal (AB) close-packed structure [66]. This phase exhibits a hexagonal lattice, similar to that of β-Ni(OH)2, and the lattice parameters are a = 2.81 Å, and c = 4.84 Å

(JCPDS 0606-0141). The protons are intercalated between the layers. IR spectra indicate that NiOOH is a hydrogen-bonded structure containing no free hydroxyl groups [67].

2.3.2.5 Structure of γ-NiOOH

The “γ” denomination was first given by Glemser and Einerhand [64] to a compound exhibiting a large intersheet distance and a high oxidation state obtained by hydrolysis of the products derived from the melting of NaOH and Na2O2 in a Ni crucible. The γ-

NiOOH phase corresponds to layered structures and rhombohedral cells, which can be described as NiO2 sheets having an octahedral nickel ion environment. The phase has a 2.82 Å Ni-Ni distance, but the interlayer distance is thought to vary over a wide range due to the intercalation of alkali cations (e.g. Li+, K+) and water molecules, and as well as being due to the stacking sequence of the NiO2 layers. A typical value of the interlayer distance is around 7Å [68]. The valence of Ni in γ-NiOOH has been found to exceed 3 due to Ni4+ defects (3.3-3.7) [12].

25 Chapter 2 Literature Review

2.3.2.6 Crystalline parameters for Nickel hydroxide

To elucidate the difference between the four types of nickel hydroxides and oxyhydroxides, the crystalline data is listed in Table 2-3 [69]. It can be seen that a big difference exists in the cell parameter c. A larger c parameter is caused by water and ions intercalation between the layers.

Table 2-3 Crystal data of Ni(OH)2 and NiOOH

2.3.3 Dynamic and kinetic mechanisms of nickel hydroxide electrode

For β-Ni(II)/β-Ni(III) system, the electrochemical process can be easily described, as only one ionic species (H+) is involved. However, the process is much more complicated for the α-Ni(II)/γ-Ni(III) system, since at least four species, including

26 Chapter 2 Literature Review

+ - 2- H+, K , H2O and OH (or CO3 ) must be intercalated or deintercalated during the overall reaction.

The mechanism for the β-Ni(II)/β-Ni(III) system proposed by R. Barnard et al. [12,

70] is commonly accepted. In the charging process, Ni(OH)2 is transformed into

NiOOH at the activation point and proton H+ is released simultaneously (2-14). The proton diffuses to the solid-liquid interface (2-15) and combines with OH- to form water (2-16), which then water diffuses in the solution. The reaction rate is controlled by proton diffusion, namely, the rate determining step is the diffusion of the proton in the hydroxide layer (2-15). The reaction happens in the reverse way for the discharging process, and the mechanism was elucidated as follows: charge Ni(OH) (r) ←→ NiOOH (r) + H+ (r) + e- (2-14) 2 discharge

H+(r) ←→ H+(i) (2-15)

+ - H (i) + OH (i) ←→ H2O(i) (2-16)

H2O (i) ←→ H2O(s) (2-17)

Note: r: reaction; i: interface; s: solution; → : charging process; ← : discharging process.

2.4 Advantages of α-Ni(OH)2

The nickel hydroxide electrode is a very complicated system, involving four types of active materials during electrochemical cycling. Moreover, these materials are in nonstoichiometric formulae except for the ordered β-Ni(OH)2. The empirical and nonstoichiometric formulae of the active materials in the nickel hydroxide electrode are listed in Table 2-4 [71].

27 Chapter 2 Literature Review

Table 2-4 Comparison of empirical and nonstoichiometric formulae of the active materials in the

nickel hydroxide electrode

The average valence state of Ni in α-Ni(OH)2 and β-Ni(OH)2 is 2 for both phases listed in Table 2-4. However, a big difference appears for the charged states, β-

NiOOH and γ-NiOOH. For β-NiOOH, namely Ni0.89(3H)0.08K0.03OOH1.14, 3 protons occupy the vacancies of Ni and the average valence state of Ni decreases from +3 to

4+ 3+ + +2.9. In γ-NiOOH, namely Ni0.05 Ni 0.25K0.25 OOH, only 0.25 Ni vacancies are occupied by K+, and the existence of high valence Ni4+ also causes the average valence state to increase to +3.67 [72]. It can be concluded that the electron transfer number is larger for γ-NiOOH than β-NiOOH when they are reduced to α-Ni(OH)2 and β-Ni(OH)2, respectively. For the α-Ni(OH)2/γ-NiOOH system, it involves 1.7e exchanged per Ni atom during reversible cycling. Only 0.9e exchanged per Ni atom is

28 Chapter 2 Literature Review

involved for β-Ni(OH)2/β-NiOOH. Consequently, the α-Ni(OH)2/γ-NiOOH system has a larger discharge capacity that the latter.

For nickel hydroxide electrodes employing β-Ni(OH)2, the predominant phases during cycling are β(II)/β(III). After an extended overcharge or a long period of flat charge, the γ-phase will be formed as a result of an expansion in the c parameter from the

0.46nm of β- Ni(OH)2 to the 0.7nm of γ-NiOOH caused by the intercalation of water molecules and metallic cations such as K+. This large difference between lattice constants causes mechanical strains. When the low density γ-NiOOH is formed, the active material volume increases, resulting in the expansion of the electrode. As charge and discharge cycles are repeated, the electrode material breaks down due to changes in volume and conductivity significantly deteriorates and the capacity abruptly decreases. The repeated formation of the γ-phase results in irreversible damage to the electrode [73]. Therefore, the formation of γ-NiOOH is thought to be the main cause of lower battery service life. The appearance of low-density γ-NiOOH increases the inner pore void space of the active material, and brings about decreased tapping density, as well as increased electrode thickness [74]. But as for the α-

Ni(OH)2/γ-NiOOH system, both phases have the same turbostratic structure, and

2- inbetween the layers are H2O, CO3 and other ions. The electrochemical cycles occur between the α/γ phase and do not involve any other intermediate phase. α-Ni(OH)2 can be converted to γ-NiOOH reversibly without any mechanical deformation and swelling of the electrode during the cycling process. This means that a nickel electrode employing the α/γ system has a better cycle life than with the β(II)/β(III) system.

29 Chapter 2 Literature Review

In the usual Ni/MH cell, β-Ni(OH)2 is employed as the active material. During the electrochemical charge-discharge cycles, soluble compositions in the rare-earth alloy hydrogen-storage materials may enter the positive electrode, forming unstable α-

Ni(OH)2 and decreasing the cell performance. But for the cycling between α-Ni(OH)2 and γ -NiOOH, the solubility of the negative electrode materials is not a problem [75].

The reaction rate for the nickel electrode is controlled by proton diffusion during the reversible cycling, namely, the rate determining step is the diffusion of protons into the hydroxide layer. α-Ni(OH)2 has a turbostratic structure and a large intersheet distance (about 7Å), moreover, water molecules are inserted between the layers with the anion species. The larger interplanar distance in the α-phase facilitates the easy intercalation and deintercalation of protons and ions between the layers during electrochemical cycling. Thus, a higher diffusion coefficient will be expected for the α/γ system, which makes the reaction between the α/γ phase more reversible during redox cycling and improves its electrochemical properties. The internal resistance of stabilized α-nickel hydroxide electrode is found to be lower than that of a β-nickel hydroxide electrode at all stages of charge, and the charging efficiency of

α-nickel hydroxide electrode is higher for β-nickel hydroxide [76].

The α/γ system has several advantages compared with the β(II)/β(III) system from a theoretical point of review [77]: (1) a larger number of exchanged per nickel atom, (2) a decrease in the mechanical constraints resulting from the suppression of the β(II), (3) a higher diffusion coefficient for protons.

30 Chapter 2 Literature Review

2.5 Development of α-Ni(OH)2

2.5.1 Development History

The structure of α-Ni(OH)2 is turbostratic. Anions, alkali metal ions and water exist inbetween the layers. It is not stable and will transform into β phase in a strong alkaline medium. The intercalated anions are exchanged for hydroxyl ions from the alkaline electrolyte [78], and the highly dehydrating nature of the concentrated alkali extracts the intercalated water from the inter-layer region, thus leading to a transformation of α to β [79]. This transformation has been shown to involve a dissolution–nucleation-growing mechanism [80].

As is well known, the electrochemical properties of the α/γ system are superior to those of β(II)/β(III) system. The properties of nickel hydroxide of the cathode electrode plays an important role in improving Ni/MH battery performance.

Consequently, there has been a concerted effort to stabilize α-Ni(OH)2 in strong alkali and commercialize it as an electrode material in Ni/MH cell. So far, the ways to stabilize the α-nickel hydroxide in strong alkali concentrate on enhancing the strength of anion binding to the layers by increasing the positive charge on them. Compounds with positively charged brucite-type layers of mixed-metal hydroxides are called

Layered Double Hydroxide (LDHs). They were discovered by Feitknecht [81] about

60 years old ago, and their structure was determined by Allman [82] in 1970. Anions located in the interlamellar space compensate for the positive-charge excess of the brucite-type layers. The chemical composition of the brucite-type layers is generally

2+ ’3+ x+ - expressed as [M1-x Mx (OH)2] while that of the interlamellar layers is written [Ay

31 Chapter 2 Literature Review

2- x- ’ Bz .nH2O] with x = y + 2z (M = Mg, Co, Ni, Cu, Zn…; M = Al, Cr, Fe, Se…; A =

Cl, NO3, ClO4…; B = SO4, CO3…).

There are two strategies to increase the positive charge [83] for nickel hydroxide: (i) partial substitution of Ni2+ ions in the hydroxide layers by trivalent cations M3+ ( M =

x+ Al, Cr, Mn) to yield a layered composition [Ni1-xMx(OH)2] in octahedral sites; (ii) incorporation of additional divalent cations M2+ in the tetrahedral sites of the α- hydroxide layer [84]. For instance, a fraction x of the Ni2+ ions residing in the octahedral sites can be removed and replaced by pairs of tetrahedrally coordinated

2+ M ions on either side of the octahedral vacancy, giving a layered composition [Ni1-

octa tetra 2x+ x Zn2x (OH)2] . Thus, the structure can be stabilized by an electrostatic interaction between layers and anions, as well as by a hydrogen bond network among water molecules, interlayer anions and interlayer hydroxyl groups.

The trivalent cations of Co [85, 86], Fe [87], Mn [88] and Al [89, 90] have been studied as substitutes for Ni to stabilize α-Ni(OH)2. It was found that α-Ni(OH)2 can be stabilized by these trivalent cations. However, Co and Mn substitutions were found to decrease the cell voltage, whereas Fe and Al substitution cause an increase. Nickel- iron LDH may prove to be unsuitable as a battery electrode material, as it catalyzes oxygen evolution by 50 mV as compared with pure nickel hydroxide. Therefore, Al is found to be the optimum element to substitute for Ni to stabilize the α-phase, and some research work has been done in this direction.

Kamath et al.[13] chemically prepared Ni/Al-LDHs with a specific discharge capacity of 240 mAh/g and a discharge median potential of 350 mV (vs Hg/HgO) for a 20%

Al-containing compound. Sugimoto et al.[91] obtained a maximum capacity of 381

32 Chapter 2 Literature Review mAh/g for Ni/Al-LDHs that were chemically prepared and found that the utilization and discharge median potential increased as the sample crystallinity was improved.

Chen et al.[92] investigated the effect of zinc and Al on stabilizing Ni-LDHs. The maximum specific discharge capacity of 425 mAh/g of Ni was obtained without zinc.

The maximum discharge median potential of 413 mV and the lowest capacity deterioration rate of < 5% after 398 charge-discharge cycles at the 1C rate were obtained for the 6.4 wt% Zn-containing sample. The LDHs with Cr and Mn were found to be electrochemically active and to deliver capacities of 500 (Ni-Cr), 400 (Ni-

Mn, x = 0.2) and 430 (Ni-Mn, x = 0.1) mAh g-1, respectively [79].

As for bivalent cations to stabilize α-phase nickel hydroxide, the research work has been focused mainly on Zinc. Tessie et al. [93] found that α-phase is stable when the amount of zinc ranges from 20 to 50%. They also found that these compounds present a turbostratic structure with an interslab distance of 8.4 Å and the zinc cations are located in tetrahedral sites. In the interslab space, each of these tetrahedra shares one face with a vacant octahedron of the slab. Carbonate ions were found to be linked by one of their oxygen atoms to nickel or zinc cations. Choy et al [94] investigated the structural phases of Ni/Zn-LDHs and found that brucite-type nickel hydroxide was formed with partial nickel vacancies and the zinc ions stabilized in tetrahedral sites.

Zinc was located below and above the vacant nickel sites in the hydroxide plane with three hydroxide and one acetate ligands.

33 Chapter 2 Literature Review

2.5.2 Development Trend

Nickel hydroxide used for pasted nickel hydroxide electrodes should have high density and a narrow size distribution. Nickel hydroxide appropriate for a battery should have an apparent density of 1.4-1.7 g/cm3, a tap density of 1.8-2.1 g/cm3 and a size range of 5-50 µm [95]. Since the paste made with this kind of nickel hydroxide has excellent fluidity and uniformity, it is possible to fabricate high capacity and uniform electrodes. The use of this kind of nickel hydroxide also improves the utilization of the active material and the discharge capacity of the electrode.

Although α-Ni(OH)2 has higher capacity than β-Ni(OH)2, and the electrochemical cycling in the α/γ system is superior to that in the β/β system, α-Ni(OH)2 has not been used in Ni/MH batteries in practice. All the α-Ni(OH)2 synthesized that has been has a lower tap density and a lower volumetric specific capacity than β-Ni(OH)2. The

3 highest tap density for date for α-Ni(OH)2 is 1.7-1.9 g/cm , obtained by Wang et al

[96]. However, that compound had a mixed structure of α-Ni(OH)2 and β-Ni(OH)2.

How to improve the tap-density is the key problem in commercializing α-Ni(OH)2.

The development of nanocrystalline materials has offered an opportunity to improve the properties of α-Ni(OH)2 and commercialize it. Nanocrystalline materials are defined as single or multi-phase polycrystals, with a crystal size typically within the range of 1-100 nanometers. Because of their ultrafine grain sizes, nanocrystalline materials exhibit a variety of properties that are different and often considerably better than those of conventional coarse-grained polycrystalline materials [97]. These include higher catalytic activity, more uniform corrosion, improved strength/hardness, enhanced diffusivity, higher electrical resistivity and higher thermal expansion

34 Chapter 2 Literature Review

coefficient. Nanocrystalline α-Ni(OH)2 has been synthesized by urea decomposition

[98] or ultrasonic precipitation [99], and its density has been increased to 1.7-1.9

3 g/cm . However, this kind of α-Ni(OH)2 was not the stabilized type, and it will transform into β-Ni(OH)2 in a strong alkaline solution, so that it is of no practical use in Ni/MH batteries. Synthesizing nanocrystalline α-Ni(OH)2 as an active material in

Ni/MH battery is a new promising research topic.

In the traditional chemical precipitation process, the precipitate of nickel hydroxide easily forms a colloidal structure and turns into hard lumps after washing and drying.

The hard lumps must be ground into powders, and the nickel hydroxide particles are converted into an irregular form, with many voids present between the particles. The resulting density is low and it is impossible to further increase the packing density of a three-dimensional porous body. Thus the specific energy of the nickel electrode reaches a limit [100]. In order to produce high density spherical nickel hydroxide, the compounds need to be grown gradually, so the reaction rate must be reduced and controlled. It can be obtained by controlling the concentration, feed rate, temperature and pH of the reaction solution.

Spherical β-Ni(OH)2 is the only active material employed for the cathode electrode in the Ni/MH battery. Spherical nickel hydroxide has the ideal macroscopic parameters

[101] required for practical use in the battery, including pore area, pore volume, pore diameter, pore shape, pore distribution, average particle size, average particle shape, particle size distribution, BET surface area and tap density. If α-Ni(OH)2 can be obtained with similar macroscopic parameters to β-Ni(OH)2, α-Ni(OH)2 will definitely replace β-Ni(OH)2 as the active material in the cathode electrode because it

35 Chapter 2 Literature Review has better electrochemical characteristics, and thus the performance of Ni/MH battery will be greatly improved. However, there has not been such a report to date. It is a new promising area in the Ni/MH battery field.

2.6 Electrolyte

The electrolyte plays an important role in the Ni/MH battery. It is involved in the material transfer to and away from the electrodes by the exchange of both OH- ions and alkali. Moreover, it relates to charge acceptance, charge and discharge potentials, structural changes and electrode ageing. Charge acceptance depends greatly on the oxygen evolution potential, and the higher the oxygen evolution is, the higher the charge acceptance. The characteristics of the electrode redox transfers can be strongly affected by the possibility of alkali intercalation within the crystal lattice [56] also.

The oxygen evolution proves to be much more sensitive to changes in the concentration or the nature of the ionic electrolyte population than the Ni(I)/Ni(III,

IV) system. Small quantities of lithium hydroxide have been added to standard KOH electrolytes to shift the OER (oxygen evolution reaction) to more positive potentials and improve charge acceptance, but this also simultaneously lowers the nickel hydroxide oxidation potential. The use of lithiated electrolytes can increase the capacity of batteries up to 20%, but also increases self-discharge [102]. From an electronic point of view the beneficial effect of LiOH additions to KOH is frequently attributed to p-type semiconduction enhancement in the discharged phase, but it is not good for the charged phases (n-type ) [103, 104].

36 Chapter 2 Literature Review

The addition of lithium hydroxide to the potassium hydroxide has improved some properties of the Ni/MH battery, but also worsens others. Harvivel et al. [105] showed that the γ phase is easily formed on overcharging in KOH and that its formation is promoted by the use of concentrated solutions. The big structural difference between γ-NiOOH and its reduced form β-Ni(OH)2 leads to the deterioration of the active material after a number of charge/discharge cycles. γ phase is thought to be the main cause of lower battery service life. Hence, there has been a strong demand for rechargeable batteries to use a solid or gel electrolyte instead of liquid electrolyte. Nickel metal hydride (Ni/MH) batteries with a solid gel electrolyte would be attractive, especially in terms of their reliability, safety, designing and processibility.

Polymer electrolytes have been extensively studied since Wright’s discovery of poly(ethylene oxide) (PEO) complexes with alkali metal salts [106, 107] and the recognition by Armand, Chabagno and Duclot [108] of their potential use in batteries.

Gel electrolytes have the same function as liquid electrolytes, to carry protons or hydrogen during the electrochemical process. In addition to this, mechanical strength, high ionic conductivity, fast charge transfer at the electrode interfaces and electrochemical stability are also essential for their performance in the battery [109].

There have been some reports on a solid or gel electrolytes for the Ni/MH battery with high ionic conductivity. Mohri et al. [110, 111] first reported Ni/MH related batteries

-3 -1 o using SbO2O5.xH2O (ionic conductivity: 2.6×10 S cm at 20 C) as a solid electrolyte. Kuriyama et al. [112, 113] have investigated tetramethylammonium

-3 -1 hydroxide pentahydrate, (CH3)4NOH.5H2O (ionic conductivity: 4.5×10 S cm at

37 Chapter 2 Literature Review

15oC), as a solid electrolyte with proton conductivity. Very recently Ni/MH batteries with an alkaline solid polymer electrolyte based on poly(ethylene oxide) (PEO), KOH and water (ionic conductivity: 10-3 S cm-1 at room temperature) have been reported by

Vassal et al. [10, 114]. Poly(acrylic acid) (PVA) was investigated as a solid gel electrolyte with KOH and water [115, 116] as well, because it has a high water absorbing capacity, a high water-holding capacity and high gel strength.

However, the batteries with solid gel electrolyte showed lower charge/discharge cycle life under large current densities because of low ionic conductivity of the electrolytes, as compared with aqueous electrolyte-based Ni/MH batteries. A lot of work needs to be done for commercializing the promising solid electrolytes in the Ni/MH battery industry.

38 Chapter 3 - Experimental

Chapter 3 Experimental

3.1 Materials and Chemicals

Most of the materials and chemicals used in this work are supplied by Aldrich Chemical

Company or as specified. The details are given in Table 3-1

Table 3-1 Descriptions of materials and chemicals

Formulae & Weights Materials or Chemicals Purity (wt%) & Shape (Atomic or Molecular)

Nickel(II) nitrate Ni(NO3)2 ⋅ 6H2O (290.81) 98 hexahydrate

Aluminum nitrate Al(NO3)3 ⋅ 9H2O (375.13) 98 nonahydrate

Cobalt Oxalate Co(C2O4)2 ⋅ 2H2O (182.98) 99.5

Sodium carbonate Na2CO3 (105.99) 99.9

Sodium Hydroxide NaOH (40) ≥ 97

Potassium Hydroxide KOH (56.11) ≥ 90

Lithium Hydroxide LiOH (23.95) 98+

99.9+ (spherical) Nickel Ni (58.71) 99.5 (powder < 3µ)

Poly(vinyl alcohol) PVA[-CH2CH(OH)-]n 99 hydrolyzed (50,000)

Carbon black C 99.9 Chapter 3 - Experimental

Si (28.08)

* S1 (CH3)4NOH-5H2O (181.23)

(CH3)4NOH-5H2O + 5% S2* polymer

(CH3)4NOH-5H2O + 15% S3* polymer

*: Provided by Dr. J. Sun, School of Physics and Materials Engineering, Monash University

3.2 Experimental Procedures Nickel hydroxides were synthesized and investigated as cathode materials for Ni/MH

Chemical Chemical Coprecipitation Coprecipitation + Spray dry

Nickel hydroxide synthesis

Structual &

Physical identification

Aging test Electrode Fabrication

Structual & Electrochemical Physical characteristics investigation identification (charge/discharge and CV) Chapter 3 - Experimental

Figure 3-1 Schematic diagram for experimental procedure cells in the present work. Their physical properties were analysed with XRD, SEM and

TEM techniques. Their electrochemical characteristics as cathode electrodes were investigated in an open Ni/MH cell. The overall experimental procedure is schematically illustrated in Figure 3.1.

3.3 Nickel Hydroxide Powders

3.3.1 Synthesis of nickel hydroxide

The mixed metal nitrate solution (0.1 mol ⋅ dm-3 total concentration) containing Ni and Al or Co in the required stoichiometric ratio was added dropwise to a NaOH solution (0.2 mol

-3 o ⋅ dm ) containing Na2CO3 (3.75g per 100ml NaOH) during constant stirring at 40 C. The pH of the solution after addition was in the range of 10-11. The precipitation suspension was left in the mother liquid at 60oC for 16 hrs, then filtered, and copiously washed with deionized water until the pH was neutral. A blue or green gel was obtained and dried at 65 oC for 18 hrs, then ground into powder and sieved with a 300 mesh sieve. The powders obtained were the final product used in the following experiment.

3.3.2 Synthesis of spherical nickel hydroxide

Spherical nickel hydroxide was synthesized using an automated Yamato Pulvis Mini-Spray instrument (Model GA-32). Its system diagram is shown schematically in Figure 3-2. The blue or green gel obtained by the chemical coprecipitation method (as shown in 3.2.1) was mixed with a predetermined ratio of deionized water, and stirred constantly untill the suspension was homogeneous. The suspension was sprayed into the chamber through the Chapter 3 - Experimental

nozzle under a heated atmosphere. Then the nickel hydroxide particles that were redispersed in deionized water were quickly dried and sucked into the product vessel under vacuum conditions. The powders collected from the product vessel were the final product used in the following experiment. The temperature of the atmosphere and the spray speed were controlled in the process.

Figure 3-2 System diagram for Yamato Pulvis Mini-Spray instrument (Model GA-32)

(1: fluid pump; 2: spray nozzle; 3: needle valve; 4: electromagnetic valve; 5: heater; 6: distributor; 7: drying chamber; 8: cyclone; 9, product vessel; 10: aspirator; 11: inlet temperature sensor; 13: outlet temperature sensor.)

3.3.3 Ageing of nickel hydroxide samples

20 ml of 6M KOH was added to 1 gram of synthesized nickel hydroxide powder. The container was sealed and left at room temperature for 90 days, then filtered and washed Chapter 3 - Experimental

with deionized water till the pH was neutral. The gel obtained was dried at 65oC for 18 hrs in air, then ground into powders and sieved with a 300 mesh sieve. The powders obtained were aged nickel hydroxide.

3.4 Electrode Fabrication and Cell Construction

3.4.1 Fabrication of nickel hydroxide electrodes

Nickel hydroxides electrodes were fabricated by inserting an active paste into a nickel foam matrix (2.5 mm thick and porousity rate ≥ 98%). The paste contained 70 wt% nickel hydroxide and 30 wt% nickel powder as an additive to enhance the electronic conductivity of the electrode. The in the paste was polyvinyl alcohol (PVA, 5 wt%), and it was prepared by mixing pure polyvinyl alcohol with deionized water at a temperature of 900C until a clear solution formed. The nickel foam matrix acted as a current collector for the electrode, and a nickel belt was spot-welded onto it for an electrical connection. These electrodes were dried in air for 36 hrs, then pressed at a pressure of 1000 kg ⋅ cm-2.

3.4.2 Ni/MH cell construction

A Ni/MH cell was assembled to test the electrochemical properties of the nickel hydroxide electrode. In the Ni/MH cells, nickel hydroxide electrodes were employed as cathode electrodes, and a commercial metal hydride (MH) electrode (LaNi5-type) was chosen as the counter electrodes. Each nickel hydroxide electrode was sandwiched between two counter electrodes of the same size in a cell. A non-woven cloth separator was used between the Chapter 3 - Experimental

electrodes. The cell was packed tightly using two stainless steel plates with some pores to allow electrolyte to penetrate.

3.5 Electrochemical Characteristics Measurement

3.5.1 Charge/discharge behavior at ambient temperature

The charge/discharge properties of nickel hydroxide electrodes were measured by use of an automatic battery test unit. Ni/MH cells were immersed in a 6M KOH solution containing

LiOH (5g LiOH per 1L KOH solution) for 1 h to allow the electrolyte to penetrate and reach the inner part of the electrode before starting the test. The electrode being examined was galvanostatically charged at a predetermined rate to 150% of the theoretical capacity

(289 mAh per gram of Ni(OH)2), rested for 0.2 h, then discharged at a predetermined rate vs. 0.9 V to the counter electrode (MH electrode). Cycle tests were conducted under the following scheme: charge at 1.0 C rate for 1.5 h, rest for 0.2 h and discharge at 1.0 C rate to

0.9 V vs. the counter electrode (alloy electrode).

The discharge capacities of nickel hydroxide in the positive electrode were based on the amount of active material (Ni(OH)2) in the positive electrode, not including the weight of the dopants in Ni(OH)2 and the conductive materials (Ni or C) in the electrodes.

3.5.2 Charge/discharge at different temperatures

The electrochemical properties of nickel hydroxide electrodes were investigated at different temperatures. The lowest and highest temperatures in this study were –15oC and 50oC, respectively. The temperature of –15oC was achieved using a Westinghouse Freezer (model Chapter 3 - Experimental

FR121), while a Julabo F10 Water Bath was used to maintain a testing temperature at 50oC.

A temperature of 0oC was maintained in an ice-water bath in an insulated box. The Ni/MH cells were kept in the electrolyte for 2 hrs at the testing temperature to ensure an even temperature in every part of the cells before starting the test. The same scheme was adopted for the measurement of charge/discharge properties and cycle testing as discussed in 3.5.1.

3.5.3 Cyclic Voltammetry

The Cyclic Voltammetry (CV) measurements were carried out with a potentiostat (EG&G

Princeton Applied Research, M362) at a scan rate of 1 mV/s. The scan potential range was

0.1 V to 0.6 V vs. an Hg/HgO/6M KOH electrode. In the test the electrode being examined is nickel hydroxide electrode, the counter electrode is alloy electrode and the reference electrode is an Hg/HgO electrode.

3.6 Physical Analysis

The synthesized cathode materials were thoroughly characterized by a variety of approaches. The phase structure of the nickel hydroxide samples used in this study was identified by X-ray diffraction (XRD). The X-ray diffraction measurements were conducted in a High Temperature Phase X-ray Diffractometer (MacScience Co. Ltd, Japan; MAC =

Material Analysis and Characterization; Model: MO3xHF22) with a Toshiba Cu Tube

(ANACIX Type A – 40Cu). The radiation used for all the experiments was Cu-Kα2 radiation (λ = 1.540562 Å). The voltage and current used were 40 kV and 30 mA, respectively.

Chapter 3 - Experimental

Si powder was chosen as the inner standard to calculate the value of the FWHM (full width at half maximum) and the unit cell constants (a and c) of the nickel hydroxides. The software used was Traces (version 6.3.0). The average grain size of nickel hydroxide was calculated from the X-ray diffraction data using the Scherrer equation [117]:

d = 0.89λ /(βcosθ) (3-1) where λ is the X-ray wavelength, β is the full width at half maximum and θ is the position of the selected Bragg angle. After subtracting the instrumental broadening and microstress broadening, the β value was used in the equation. For cell constants a and c, the peak positions of Si in the X-ray diffraction pattern obtained in the experiment and provided by

JCPDS were chosen to calibrate the peak deviation in the experiment. Then the true peak position of nickel hydroxide was obtained and used to simulate and refine the cell parameters.

The morphology and surface images of the electrode materials were observed by SEM

(Leica/Cambridge Steroscan 440 scanning equipped with energy dispersive spectroscope) and TEM (JEOL 2000FX transmission electron microscope). To enhance the electric conductivity of the nickel hydroxide powders and get clear images, a thin layer of gold was coated on the surface of the samples. The gold coating was applied with a Dynavac Mini Coater (DC Sputter Coater, Model Mini SC). The sputter current was about 60 mA, and the vacuum pressure was 0.15 torr of high purity argon.

The composition of the resulting materials was analysed using an inductively coupled spectrometer (ICP). The analysis was carried out in a Varian Viste MPX Chapter 3 - Experimental

Simultaneous ICP-OES (Axial model; OES: Optical Emission Spectrometer). Nickel hydroxide was dissolved in hydrocholoric acid of Tracepur grade, so that the concentration of Ni in the solution was 30 ppm.

Chapter 4 – A1-substituted Nickel Hydroxide

Chapter 4 A1–substituted Nickel Hydroxide

4.1 Introduction

It was asserted in chapter 2 that the α/γ system has several advantages compared with the

β(II)/β(III) system from a theoretical point of review, and these can be summaried as

[77]: (1) a larger number of electrons exchanged per nickel atom; (2) a decrease in the mechanical constraints resulting from the suppression of the β(II); and (3) a higher diffusion coefficient for the proton. It was also concluded that the commercialization of α

- phase nickel hydroxide will bring a breakthrough in the Ni/MH battery field. However,

α-Ni(OH)2 is not stable and will transform into β-phase in a strong alkali solution or after electrochemical cycles. As a result, many scientific researchers have been attracted to the stabilization of α-phase nickel hydroxide as a major research topic. The most accepted method to date is to enhance the strength of anion binding to the layers by increasing the positive charge on them by partial substitution of Ni2+ ions in the hydroxide layers by trivalent cations M3+ ( M = Al, Cr, Mn and Fe). The element Al has attracted the most attention because of its light atomic weight and low cost.

A structural model describing the nickel-aluminum turbostratic layered double hydroxides has been proposed by Ehlsissen et al [89]. Their view is that the brucite-type layers are non-stoichiometric and contain hydroxide vacancies: their chemical

2+ 3+ composition can be written as [Ni1-x Alx (OH)2-(y+2z-x)] and the interlamellar layers as

- 2- [(NO3 )y(CO3 )z . nH2O] with x ≤ 0.25. The interlamellar anions play two roles: those in

3+ D3h symmetry compensate for the positive-charge excess of the Al ions, while those in Chapter 4 – A1-substituted Nickel Hydroxide

C2v symmetry compensate for that of the hydroxide vacancies. A simple and common formula to describe the Al–substituted nickel hydroxide as Ni1-xAlx(OH)2(CO3)x/2 . nH2O was proposed in Ref. [118, 119]. The authors found that the compounds with compositions where x ≥ 0.2 had prolonged stability in a strong alkaline medium, and that the electrodes containing stabilized α-Ni(OH)2 of x = 0.2 composition were rechargeable with a discharge capacity of 240 (±15) mAh/g. In this work, Al–substituted nickel hydroxide will be described in a simplified way as Al – Ni(OH)2.

The main results on Al–substituted nickel hydroxide are reviewed as follows. Kumar et al. [76] studied the open-circuit potential-time transients of Ni0.8Al0.2(OH)2(CO3)0.1.nH2O and found that the internal resistance of α-nickel hydroxide was lower that that of β- nickel hydroxide. Moreover, their gasometric studies revealed that the charging efficiency of α-nickel hydroxide was also higher. Sugimoto et al [73] studied α-phase nickel hydroxide with 5 – 20% Al substitution, and a maximum capacity of 381 mAh/g was obtained. Dai et al. [120] found that α - phase nickel hydroxide stabilized by 15 mol% Ni substitution with Al was stable during charge/discharge cycling, and the highest specific discharge capacity they obtained was 260 mAh/g. Indira et al. [119] electrosynthesized layered double hydroxides (LDHs) of nickel with aluminum, chromium, manganese and iron and found that nickel-aluminum LDH showed the highest coulombic efficiency. Liu et al. [90] found that α- Ni(OH)2 electrode exhibited a better reversibility of the Ni(OH)2/NiOOH redox reaction and an higher oxygen evolution potential than β-Ni(OH)2 electrode. Chapter 4 – A1-substituted Nickel Hydroxide

There are three main ways to prepare stabilized α-Ni(OH)2 with trivalent metal ions. (1)

Chemical co-precipitation. The common way is to slowly drip the complex nitrate solution of nickel and the substitution element into a sodium hydroxide solution containing the sodium carbonate [13]. The pH is kept between 9 and 11 during the reaction process. Ehlsissen et al. [89] added ammonia solution to the nitrate solution to keep pH < 7.5 during the whole precipitation reaction. (2) or cathodic co-deposition. LDH is synthesized in a one-step deposition process by cathodic reduction of nitrate ions from a mixed-metal nitrate bath that contains nickel and the corresponding trivalent metal [119]. (3) Soft route. The preparation procedure consists of three successive steps [121]: i. building a nickel and additive metal slab by a high temperature solid state reaction; ii. oxidizing hydrolysis of sodium nickelate, leading to layered γ - oxyhydroxide; and iii. reduction by adding a solution in the presence of organic (poly)anions.

An improved chemical co-precipitation method was used to prepare Al-substituted nickel hydroxide in this work. The reaction temperature was raised during the precipitation procedure, and the ageing time and temperature of the mother liquid were also changed.

Al was chosen for stabilizing the turbostratic structure. Al-Ni(OH)2 with 0%, 10%, 20%,

25% and 33% Al (mol) was prepared and its electrochemical properties were investigated. The aging treatment of these samples was also carried out.

Chapter 4 – A1-substituted Nickel Hydroxide

4.2 Physical Properties

4.2.1 XRD patterns of as-prepared samples

The X-ray diffraction patterns of aluminum substituted nickel hydroxide are given in

Figure 4-1, and the observed d-spacing for the samples is shown in are 0%, 10%, 20%,

25% and 33%)

Table 4-1. It can be seen that the patterns are the same as that of unsubstituted α-

Ni(OH)2 reported in the literature [56]. They show a low angle reflection close to 0.8 nm

(d-spacing), followed by another at around 0.4 nm. α-Ni(OH)2 with over 20% Al content shows a broad asymmetric band in the range of 0.22 to 0.26 nm, this is typical of turbostratic structures. For α-Ni(OH)2 with 10% Al content, the broad band in the 0.22 to

0.26 nm region is clearly split into two peaks. This implies that it does not have the perfect structure of α - Ni(OH)2. Moreover, no peaks that can be indexed to Al(OH)3 or other Aluminum compounds have been found, which indicates that aluminum has been incorporated into the inner structure of Ni(OH)2 and occupies the position of nickel in the slabs. So Al – substituted nickel hydroxide can be described as Al-Ni(OH)2. Chapter 4 – A1-substituted Nickel Hydroxide

1 : 0% Al 2 : 10% Al 3 : 20% Al 4 : 25% Al 5 5 : 33% Al

003 4

006 012 015 018 3 110 113 Intendensities(a.u.) 2 001 100 101 110 102 1 111

10 15 20 25 30 35 40 45 50 55 60 65 2-Theta Degree

Figure 4-1 X-ray diffraction patterns of as-prepared Al-Ni(OH)2 samples

(The content of Al in as-prepared samples 1, 2, 3, 4 and 5 are 0%, 10%, 20%, 25% and

33%)

Table 4-1 Observed d-spacing and calculated unit cell constants for Al-Ni(OH)2 samples

Unit Cell Constants Samples 2θ (hkl) d (Å) obs. (Å) 003 7.8441 Al-Ni(OH) 006 3.8568 2 012 2.5708 a = 3.0374 with 015 2.2958

018 1.9585 c = 23.1856 33% Al 110 1.5225 113 1.4907 003 7.9566

006 3.9342 Al-Ni(OH) 2 012 2.5793 a = 3.0522 015 2.3278 with 018 1.9770 c = 23.4037 25% Al 110 1.5284 113 1.5003 Chapter 4 – A1-substituted Nickel Hydroxide

003 8.0724

006 3.9794 Al-Ni(OH) 2 012 2.5894 a = 3.0666 015 2.3348 with 018 1.9976 c = 23.6839 20% Al 110 1.5358 113 1.5047 003 7.9709

006 4.0547 Al-Ni(OH) a = 3.0912 2 012 2.6811 with 015 2.3324 10% Al c = 24.0588 110 1.5526 001 4.6903 Al-Ni(OH)2 100 2.7047 101 2.3394 a = 3.1172 with 102 1.7573 110 1.5631 c = 4.7315 0% Al 111 1.4822

The as-prepared nickel hydroxide samples were mixed with Si powder and investigated with X-ray diffraction. The diffraction pattern of Si was specified as the reference pattern to calculate the lattice parameters of the nickel hydroxide unit cell constants a and c, which are also listed in Table 4-1. The data calculated have also been plotted against the

Al content in nickel hydroxide and are shown in Figure 4-2. It can be seen clearly that cell constant a decreases with increasing Al content, which is caused by the smaller ionic radius of Al (0.54 Å) compared to Ni (0.69 Å) in the lattice of nickel hydroxide. This result also verifies that Al has substituted into the position of nickel in the layers. The cell constant c increases significantly from 4.7315 Å to about 24 Å with the addition of Al in nickel hydroxide. This means that the intersheet distance has been increased with anions and water, which also indicates that the structure of the nickel hydroxide has transformed from brucite type to turbostatic disorder. Chapter 4 – A1-substituted Nickel Hydroxide

3.12 25

3.10 20 Cell Constant (c) Constant Cell

3.08 15

Cell Constant(a) 3.06 10

: cell constant (c) 3.04 5 : cell constant (a)

0.0 0.1 0.2 0.3 0.4 Ratio of Al to Ni (mol)

Figure 4-2 Cell constants (a and c) in Al–Ni(OH)2 samples

4.2.2 XRD patterns of aged samples

XRD patterns of the aged samples (in 6M KOH for 90 days) are shown in Figure 4-3. It can be seen that the sample with 10% Al content shows a distinct tendency to transform into β-phase with the emergence of a reflection at 0.46 nm. The typical broad asymmetric band of turbostratic structure has also disappeared. The results show that 10% Al can not effectively stabilize the α-Ni(OH)2 structure. When the content of aluminum reaches

20% or more, it can be seen that the samples still keep the α-Ni(OH)2 structure.

However, the peaks can be seen to become sharper for the aged samples. This indicates that the material is well crystallized after the ageing treatment in alkali solution. The

(003) peak in the diffraction pattern was chosen to calculate the crystalline size of Al–

Ni(OH)2 with Si as the standard, and the data are listed in Chapter 4 – A1-substituted Nickel Hydroxide

Table 4-2. The data show that the crystalline size of Al–Ni(OH)2 was increased by two times for the samples with 20%, 25% and 33% Al, while three times the original crystalline size was obtained with 10% Al. The enlarged crystalline size means that either the crystallites became larger or the crystallinity increased during the ageing process, which indicates recrystallization during the aging treatment.

100 101 001 10% Al 110 003

006 012 015 018 20% Al 110 113

25% Al Intensity (a.u.)

33% Al

10 20 30 40 50 60 2θ (Degree)

Figure 4-3 X-ray diffraction patterns of Al–Ni(OH)2 after aging treatment

Chapter 4 – A1-substituted Nickel Hydroxide

Table 4-2 Crystalline size of Al–Ni(OH)2

Al content in nickel hydroxide (mol%)

33 25 20 10

As-prepared samples (Å) 6.81 7.25 6.36 2.63

Aged samples (Å) 14.01 15.49 13.58 7.72

4.2.3 TEM measurement

In order to identify the crystallization transformation the as–prepared and aged Al–

Ni(OH)2 with 20% Al were investigated by TEM. TEM images and the halo patterns are shown in Figure 4-4; (a) and (b) are from the as–prepared sample while (c) and (d) are for the aged samples.

It can be clearly seen that Al–Ni(OH)2 (as-prepared) appears as aggregates of thin crumpled sheets, without any definite shape in Figure 4-4 (a), a typical characteristic feature of the turbostratic structure [56]. The centered halo pattern and the diffraction ring in Error! Reference source not found. (b) show that some nanocrystal grains coexist with the amorphous state. However, a regular shape (platelet shape) and homogeneous particle size (about 10 to 30 nm) were detected for the aged sample, as shown in Figure

4-4 (c). The sharper multicrystal diffraction rings in Fig. 4-4 (d) also verify that recrystallization occured during the aging treatment. Chapter 4 – A1-substituted Nickel Hydroxide

The dissolution-recrystallization phenomenon via the electrolyte was discovered by

Delahaye-Vidal et al. [57] in the α → β transformation after ageing α -phase nickel hydroxide in KOH electrolyte. They believed that such important morphological modifications from crumpled films to platelets proved that this crystallization transformation is biphasic and cannot proceed in the solid state. A similar phenomenon has also been observed in Al–Ni(OH)2 samples, so this transformation is thought to be same dissolution-recrystallization process, though α - phase was not transformed into β - phase in this experiment.

(a) Chapter 4 – A1-substituted Nickel Hydroxide

(b)

(c)

(d) Chapter 4 – A1-substituted Nickel Hydroxide

Figure 4-4 TEM images (a and c) and halo diffraction patterns (b and d) for as-prepared (a and b)

and aged (c and d) Al–Ni(OH)2 with 20% Al

4.3 Electrochemical Properties

4.3.1 Cyclic Voltammetry

Typical cyclic voltammograms of Al–Ni(OH)2 with differing aluminum content are shown in Figure 4-5. The anodic oxidation peak corresponds to the Ni(II) oxidation reaction, while the cathodic reduction peak corresponds to the Ni(III) reduction reaction.

In the range of scanning potentials a split anode oxidation peak occurs for the electrodes with 10% and 33% Al prior to oxygen evolution. The cathodic reduction peak shifts to more positive potentials (over 200 mV) for the electrodes compared with 0% Al (199mV,

β-phase nickel hydroxide) when the content of Al reaches or exceeds 20%. In order to compare the characteristics of the electrodes, the data obtained from the CV curves are listed in Table 4-3.

Table 4-3 Data obtained from the cyclic voltammograms

Samples IE/mV ER/mV IR/mA EO/mV IO/mA ∆EO,R/mV

1 0.004 199.2 15.16 621.7 11.97 422.5

2 0.014 195.4 18.05 599.7 18.76 404.3

3 0.028 201.0 17.0 604.4 17.56 403.4

4 0.042 248.7 16.25 603.4 16.36 354.7

5 0.051 298.4 12.53 621.7 11.78 323.3

Chapter 4 – A1-substituted Nickel Hydroxide

Note: The contents of Al in Al-Ni(OH)2 samples 1, 2, 3, 4 and 5 are 0%, 10%, 20%, 25% and 33% (mol), respectively.

EO: Oxidation potential; ER: ; ∆EO,R: difference between the oxidation potential and reduction potential (EO – ER); IE: initial potential, IR: reduction current, Io: oxidation current.

0.020

0.015

0.010

0.005

0.000

-0.005 : 0% Al Current (A) -0.010 : 10% Al : 20% Al -0.015 : 25% Al : 33% Al

-0.020 0.00.20.40.60.8 Voltage (V vs HgO/Hg/6MKOH)

Figure 4-5 Cyclic voltammograms of Al-Ni(OH)2 with 0%, 10%, 20% and 33% Al (mol)

The difference between the anodic and cathodic peak positions, ∆EO,R, is taken as an estimate of the reversibility of the redox reaction [122]. ∆EO,R values of 422.5, 404.3,

403.4 354.7 and 323 mV are obtained for the active materials Al–Ni(OH)2 with 0%, 10%,

20%, 25% and 33% Al (mol), respectively. It can be seen that ∆EO,R values become smaller with the addition of Al, and it means that the charge/discharge processes appears Chapter 4 – A1-substituted Nickel Hydroxide

to be occur more reversibly for Al–Ni(OH)2. It should also be noted that the oxygen evolution shift to a more positive position with the addition of aluminum.

The increase in the oxygen evolution overpotential is beneficial to reduce the internal pressure of the battery.

4.3.2 Tafel Curves

The apparent exchange current density (i0), which is a measure of the catalytic activity of electrodes, was evaluated by a potential sweep method [123]. The polarization tests were conducted using the potentiostat, and the valve of i0 can be calculated from the slope of the polarization curves (Tafel curve) by the following equation,

RT  i  i = ×   o η  F  η→0 where R is the gas constant, T is the temperature, F is the Faraday constant, i is the microkinetic current density and η is the overpotential. The Tafel curves for the nickel hydroxide electrodes with different aluminum contents are shown in Figure 4-6. Their exchange current densities (i0) have also been estimated, and they are listed in Table 4-4.

Table 4-4 Exchange current density (i0) of different nickel hydroxide samples

Electrodes*

1 2 3 4 5

2 -7 -7 -6 -6 -6 i0 (A/m ) 2.34 × 10 6.88 × 10 1.38 × 10 2.73 × 10 2.78 × 10

Chapter 4 – A1-substituted Nickel Hydroxide

* The content of Al in the active materials Al – Ni(OH)2 of electrodes 1, 2, 3, 4, and 5 are

0%, 10%, 20%, 25% and 33%, respectively.

An exchange current density of 2.34 × 10-7 A/m2 is obtained for β-phase nickel hydroxide electrode 1 with no Al content. The exchange current density increases to 6.88 × 10-7

A/m2 when the content of Aluminum increases from 0 to 10%. This implies that the catalytic activity of nickel hydroxide is increased with the addition of Al. Higher values

-6 2 -6 2 -6 2 of i0 were obtained, 1.38 × 10 A/m , 2.73 × 10 A/m , and 2.78 × 10 A/m , for Al–

Ni(OH)2 with 20%, 25% and 33% Al (mol), respectively. The exchange current density increases with increases in the Al content. This reveals that a high amount of Al is beneficial to the catalytic activity of Al–Ni(OH)2.

-2.0

-2.5

-3.0

-3.5

-4.0

lgi -4.5

-5.0 : 33% Al : 25% Al -5.5 : 20% Al -6.0 : 15% Al : 0% Al -6.5 0.30 0.35 0.40 0.45 0.50 0.55 0.60 η (V vs HgO/Hg/6M KOH)

Figure 4-6 Tafel curves of nickel hydroxide electrodes with different content of aluminum Chapter 4 – A1-substituted Nickel Hydroxide

4.3.3 Discharge curves

4.3.3.1 Al-Ni(OH)2 with 10% Al

The 5th, 25th, 50th and 100th (discharge cycle number) discharge curves of Al– Ni(OH)2 with 10% Al (mol) are shown in

Figure 4-7. The discharge current rate is 60 mA/g in this work. The 5th discharge curve for Al–Ni(OH)2 with 10 % Al can be divided into three stages. The first stage is from the start to the beginning of the discharge plateau. In this process the rate-determining step is proton diffusion in the solid state. It can be seen that the discharge voltage of the cell dropps sharply when it is over 1.30 V. After this one reaches the second stage, the discharge plateau. The discharge curve is smooth, and the rate-determining step is proton diffusion together with charge transfer. In the first part of the process the main rate- determining step is proton diffusion. When the resistance of the charge transfer becomes critical, Ni(OH)2 with poor conductivity is formed between the current collector

(substrate) and the interface of the active material. The third stage occurs from when the discharge voltage drops sharply again to the end of the discharge process.

A second discharge plateau is observed at about 40% of the whole discharge capacity after 10 discharge cycles, and a higher discharge plateau voltage is also seen in the discharge curves. The two distinct discharge plateaus mean that two episodes of reduction occurred during the discharge process. The first plateau probably originates in the transformation from Ni(IV) to Ni(III) and the other is produced by the change from

Ni(III) to Ni(II), however, more evidence is needed to prove this. Voltages of the first discharge plateau of 1.236 V, 1.316 V, 1.353 V and 1.345 V are obtained at the 5th, 25th, Chapter 4 – A1-substituted Nickel Hydroxide

50th and 100th discharge cycle, respectively. The second discharge voltages are 1.232 V,

1.254 V, and 1.271 V at the 25th, 50th and 100th discharge cycles, and they are all much higher than 1.2V (the standard discharge plateau of β - phase nickel hydroxide used to operate appliances at high power efficiency). The discharge plateau voltage is increased with the addition of Al. It can also be seen in

Figure 4-7 that the third stage of the discharge process is very short, even after 100 discharge cycles. It should also be noted that the share of the discharge capacity with the discharge voltage lower than 1.2 V is very small, which indicates that the ‘useless’ capacity is very low and that Ni/MH batteries using Al–Ni(OH)2 as the active material will be very promising in areas such as mobile phones etc., which require a high discharge voltage to work. A discharge capacity of 225 mAh/g has been obtained for Al–

Ni(OH)2 with 10% Al (mol) at the 25th discharge cycle. In order to see the discharge behaviors in the different discharge cycles, the discharge capacity and the midpoint voltage (corresponding to the half maximum discharge capacity) are listed in Table 4-5.

1.5

1.4

1.3

1.2

1.1 : 5th cycle

1.0 : 25th cycle : 50th cycle 0.9 : 100th cycle Voltage (V vs MH (V vs Voltage electrode)

0 50 100 150 200 250 Capacity (mA/g)

Chapter 4 – A1-substituted Nickel Hydroxide

Figure 4-7 Discharge curves of Al–Ni(OH)2 (10% Al)

Table 4-5 Discharge capacity and midpoint voltage of Al–Ni(OH)2 with 10% Al

Discharge cycle 5th 25th 50th 100th

Capacity (mAh/g) 235 225 228 252

Midpoint Voltage (V) 1.236 1.257 1.298 1.296

4.3.3.2. Al-Ni(OH)2 with 20% Al

The discharge curves of Al–Ni(OH)2 with 20% Al (mol) are shown in

Figure 4-8. A similar trend is observed in that the first and third stages of the discharge process are very short. The share of the discharge plateau voltage lower than 1.2V is very small, which indicates that the “useful” capacity of Al–Ni(OH)2 with 20% Al is very high. A second discharge plateau occurs in the discharge curves after 25 discharge cycles.

Compared with that of Al–Ni(OH)2 with 10% Al, the time for its appearance is postponed, and it appeared at about 70% of the whole discharge capacity. The reaction corresponding to the first discharge plateau has been prolonged and the reaction for the second plateau has been restrained, indicating that a high Al content is beneficial to the reaction corresponding to the first plateau. The discharge voltages corresponding to the first plateau are 1.284 V, 1.316 V, 1.304 V and 1.316 V at the 5th, 25th, 50th and 100th discharge cycle, respectively, which are higher than those of Al–Ni(OH)2 with 10% Al except for the 50th discharge cycle. A discharge capacity of 285 mAh/g is obtained at

25th discharge cycle, which is higher that of Al–Ni(OH)2 with 10% Al. The discharge capacity and the mediate voltage obtained and estimated at the different discharge cycles are listed in Table 4-6. It can be seen that Al–Ni(OH)2 with 20% Al has the higher Chapter 4 – A1-substituted Nickel Hydroxide midpoint voltage and discharge capacity compared with nickel hydroxide with 10% Al.

This indicates that the electrochemical properties have been improved with 20%Al.

Table 4-6 Discharge capacity and midpoint voltage of Al–Ni(OH)2 with 20% Al

Discharge cycle 5th 25th 50th 100th

Capacity (mAh/g) 313 285 288 280

Midpoint Voltage (V) 1.285 1.310 1.290 1.307

1.5

1.4

1.3

1.2

1.1

: 20th cycle 1.0 : 5th cycle : 100th cycle 0.9

Voltage ( V ( MHelectrode) vs Voltage : 50th cycle

0.8 0 50 100 150 200 250 300 Capacity (mAh/g)

Figure 4-8 Discharge curves of Al–Ni(OH)2 with 20% Al Chapter 4 – A1-substituted Nickel Hydroxide

4.3.3.3 Al-Ni(OH)2 with 25% Al

A very similar trend in the discharge curves of Al–Ni(OH)2 with 25% Al is shown in

Figure 4-9. The first and third stages of the discharge process are very short, and a second discharge plateau occurrs after 25 discharge cycles. However, the second discharge plateau occurrs at about 80% of the discharge capacity and later than that of Al–Ni(OH)2 with 10% Al (where it occurred at about 40% of the capacity) and 20% Al (where it occurred at about 70% of the capacity). This result also agrees with conclusion drawn in session 4.3.3.2, that the high Al content is beneficial to the reaction corresponding to the first plateau. A discharge capacity of 303 mAh/g was obtained, which is higher than that of the samples with 10% Al (225 mAh/g) and 20% Al (285 mAh/g). This indicates that the discharge capacity of Al–Ni(OH)2 increases with increases in Al content when the amount is lower than 25%. The discharge capacity and midpoint voltage are listed in

Table 4-7. The data show that the electrochemical properties have been improved compared with 20% Al and 10% Al.

Table 4-7 Discharge capacity and midpoint voltage of Al–Ni(OH)2 with 25% Al (mol)

Discharge cycle 5th 25th 50th 100th

Capacity (mAh/g) 275 303 306 296

Midpoint Voltage (V) 1.302 1.304 1.309 1.324

Chapter 4 – A1-substituted Nickel Hydroxide

1.5

1.4

1.3

1.2

1.1

: 5th cycle 1.0 : 25th cycle : 50th cycle

Voltagevs MH ( electrode) : 100th cycle 0.9

0 50 100 150 200 250 300 Capacity (mAh/g)

Figure 4-9 Discharge curves of Al–Ni(OH)2 (25% Al)

4.3.3.4 Al-Ni(OH)2 with 33% Al

The discharge curves for Al–Ni(OH)2 with 33% Al are shown in Figure 4-10. Only one plateau appears in the discharge curve of Al–Ni(OH)2 with 33% Al compared with those of Al-Ni(OH)2 with 5%, 10%, 20% and 25% Al. This reveals that a reduction reaction occurred during the discharge process. Combined with the results and analysis for Al–

Ni(OH)2 with 10%, 20% and 25% Al, it can be considered that this reduction reaction corresponds to the first plateau in those curves. It also verifies that a high amount of Al is beneficial to this reduction reaction. The discharge capacity and midpoint voltage are listed in Table 4-8. Midpoint voltages of 1.307 V, 1.320 V, 1.329 V and 1.349 V are obtained for the 5th, 25th, 50th and 100th discharge cycles, respectively. The data show that the midpoint voltage is increased with increasing Al content. However, the discharge Chapter 4 – A1-substituted Nickel Hydroxide capacity becomes lower. A discharge capacity of 271 mAh/g is obtained at the 25th discharge cycle, and it is lower than that of Al–Ni(OH)2 with 20% and 25% Al. Perhaps such a large amount of Al as 33% has already affected the discharge efficiency of the active material Ni(OH)2 in Al–Ni(OH)2.

1.5

1.4

1.3

1.2

1.1

1.0 : 5th cycle 0.9 : 25th cycle

Voltage (V vs MH electrode) : 100th cycle

0.8 : 50th cycle 0 50 100 150 200 250 300 Capacity ( mAh/g)

Figure 4-10 Discharge curves of Al–Ni(OH)2 (33% Al)

Table 4-8 Discharge capacity and midpoint voltage of Al–Ni(OH)2 with 33% Al (mol)

Discharge cycle 5th 25th 50th 100th

Capacity (mAh/g) 177 271 254 251

Midpoint Voltage (V) 1.307 1.320 1.329 1.349

Chapter 4 – A1-substituted Nickel Hydroxide

4.3.4 Cycle Life

300 280 260 240 220 200 180 160 140 120 100 Content of Al 80 : 25% Al 60 : 10% Al 40 : 20% Al

Discharge Capacity (mAh/g) Capacity Discharge 20 : 33% Al 0 0 20406080100 Cycle Number

Figure 4-11 Cycle life of Al-Ni(OH)2

The discharge capacity variations with the cycle number had been investigated for Al–

Ni(OH)2, and it has been shown in Figure 4-11. It can be seen clearly that all the Al

Ni(OH)2 samples possess the promising cycle life, which the discharge capacity dropped slowly with the cycle number. Al–Ni(OH)2 with 25% Al had the highest discharge capacity, then came with 20%, 33% and 10% Al, which also agrees with the results in the former sections in this chapter.

4.3.5 XRD patterns after 100 electrochemical cycles

The phases of Al–Ni(OH)2 in the electrodes after 100 electrochemical cycles were identified with XRD, and their patterns are shown in Figure 4-12. The peaks can be indexed to β-phase nickel hydroxide for active material Al–Ni(OH)2 with 10% Al, which Chapter 4 – A1-substituted Nickel Hydroxide implies that α-phase nickel hydroxide is not stable and has a tendency to transform into

β-phase during the electrochemical process. However, after the content of Al reaches

20% or more, Al–Ni(OH)2 samples still keep the turbostratic structure of α-phase. It can be concluded that the structure of α-Ni(OH)2 can be stabilized with 20% Al or more, which agrees with the results obtained from the aging treatment experiment.

Ni

Ni

4

3

2 Intensities (a.u.) Intensities

1

20 40 60 2θ(degree)

Figure 4-12 X-ray diffraction patterns of Al–Ni(OH)2 electrodes after 100 electrochemical cycles

(Active materiala of electrodes 1, 2, 3 and 4 are Al–Ni(OH)2 with 10%, 20%, 25% and 33% (mol), respectively)

4.4 Conclusion

Al – substituted nickel hydroxide, Ni1-xAlx(OH)2 (CO3)x/2 . nH2O (x = 0.1, 0.20, 0.25 and

0.33) was prepared by an improved method of chemical co-precipitation method in this work. The X-ray diffraction patterns imply that Aluminum has entered the intersheet regions of the nickel hydroxide and substituted into the position of nickel. Al–Ni(OH)2 Chapter 4 – A1-substituted Nickel Hydroxide has a typical turbostratic structure with over 20% Al. The cell constants a and c were calculated with Si as the standard. Cell constant a decreases with increasing Al content in nickel hydroxide because of the smaller ionic radius of Al, and cell constant c increases from 4.7315 Å to about 24 Å with the addition of Al, which indicates that the structure of nickel hydroxide transforms from brucite type to turbostratic disorder.

The Al–Ni(OH)2 samples were aged in 6M KOH electrolyte to investigate their stability.

X- ray diffraction patterns show that the turbostratic structure of Al–Ni(OH)2 is stable with over 20% Al content after the aging treatment. It was also noted that their peaks became sharper in the pattern, which indicates that the crystallites become larger or crystallinity increases. The data calculated from XRD also show that the crystalline size of Al–Ni(OH)2 has increased by two times for the samples with 20% Al, 25%Al and 33%

Al, and three times for the sample with 10% Al after the ageing treatment.

The crumpled films shape of the as–prepared Al–Ni(OH)2 with 20% Al was found by

TEM to transform into platelets after the aging treatment. The halo diffraction patterns also reveal that a structure of nanocrystal grains coexisting with amorphous phase is transformed into multicrystal. These results show that a crystallization process has occurred, and it is thought to be via the dissolution-recrystallization process.

-7 2 -6 2 -6 2 Exchange current densities (i0) of 6.88 × 10 A/m , 1.38 × 10 A/m , 2.73 × 10 A/m

-6 2 and 2.78 × 10 A/m are obtained from the Tafel curves for Al–Ni(OH)2 with 10%, 20%,

25% and 33% Al (mol), respectively. They are higher than that of β-phase nickel hydroxide, 2.34 × 10-7 A/m2. It can be concluded that the electrochemical catalytic Chapter 4 – A1-substituted Nickel Hydroxide activity has been improved by the addition of Al and that a higher content of Al is more beneficial to its catalytic activity. The cathodic reduction peak and the oxygen evolution overpotentials are found to shift to a more positive position with the addition of aluminum in CV curves. ∆EO,R values of 422.5, 404.3, 403.4, 354.7 and 323.3 mV were obtained for Al–Ni(OH)2 with 0%, 10%, 20%, 25% and 33% Al, respectively. The results from CV indicate that the charge/discharge processes occur more reversibly with the addition of Al and also with an increased content of Al, which agrees with the conclusion from the Tafel curves.

The discharge plateau voltage of Al–Ni(OH)2 is found to be much higher than 1.2 V, the standard discharge plateau voltage of β-phase nickel hydroxide. A second discharge plateau has been observed in the discharge curves for Al–Ni(OH)2 with 10%, 20% and

25% after 25 discharge cycles. This reveals that there are two reduction reactions occurring during this process. The first discharge plateau is prolonged with increases in

Al content, which implies that a high amount of Al is beneficial to the reaction corresponding to the first discharge plateau. No second discharge plateau was observed in the discharge curves of Al–Ni(OH)2 with 33% even after 100 cycles, which also supports this conclusion. The discharge capacities obtained at the 25th discharge cycle are 225 mAh/g, 285mAh/g, 303 mA/g and 271 mAh/g for Al–Ni(OH)2 with 10%, 20%, 25% and

33%Al, respectively. The discharge capacity becomes higher with increasing Al content when it is lower than 25%, and decreases when Al content increases to 33%, perhaps because such a high content of Al as 33% has already affected the discharge properties of the active material Ni(OH)2 in Al–Ni(OH)2. Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

Chapter 5 (Al,Co)–substituted Nickel Hydroxide

5.1 Introduction

The purpose of the use of additives in Ni(OH)2 electrodes is to avoid three major problems [124]: (i) improve the charge efficiency by separating the redox couples

- O2/OH and Ni(II)/Ni(III), (ii) improve mechanical properties, avoiding the so-called γ effect, and (iii) improve the electrical conductivity of the active material, especially in the reduced state. Cobalt is the most popular additives used in Ni(OH)2. It has been added as a metal powder, CoO or Co(OH)2 [125]. Cobalt hydroxide is either blended with the active material during electrode preparation or coprecipitated along with nickel hydroxide synthesis [126]. It has been claimed that cobalt addition enhances the conductivity of the electrode matrix [127], increases the oxygen evolution potential

[128], and delays the mechanical failure of the electrodes [129].

However, there are few reports in the literature about the effect of additives in α-phase nickel hydroxide. In Ref. [130] Al-substituted α-Ni(OH)2 co-doped with the additive

Co2+ was prepared by a complexation-cooperation method. They found that the addition

2+ of Co improves the diffusion coefficient of the proton (DH+) and the stability in alkaline solutions at high temperature. In Ref. [131] Al–Ni(OH)2 co-precipitated with the additive Co2+ was prepared. It was found that (Al,Co)–substituted nickel hydroxide has the structure of α-phase nickel hydroxide, and a discharge capacity of 319 mAh/g has been obtained. Chen. et al. [92] studied the addition of Zn to Al–substituted nickel hydroxide, and the maximum discharge midpoint potential of 413 mV vs. HgO/Hg/6M

KOH electrode and the lowest capacity deterioration rate of < 5% after 398 charge- discharge cycles at the 1C rate were gained for a 6.4 wt% Zn–containing sample.

Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

The effect of the addition of Co co-precipitated in Al–Ni(OH)2 is studied in this work, and the same chemical co-precipitation method was used for preparation. Samples 1, 2 and 3 were prepared, and the content of (Al+Co) in the sample and the ratio of Co/Al are listed in Table 5-1.

Table 5-1The compositions of the samples prepared

Sample 1 Sample 2 Sample 3

Content of (Al+Co) (%) 25 25 25

Co/Al 1:1 1:2 1:4

Co/(Al + Co) (%) 50 33 20

5.2 Physical Properties

5.2.1 XRD patterns of as-prepared samples

The X-ray diffraction patterns for (Al,Co)–substituted nickel hydroxide samples are shown in Figure 5-1, and the observed d-spacing for the main reflections are shown in

Table 5-2. It can be seen that they all show a low angle reflection close to 0.8 nm, followed by another at around 0.4 nm. For sample 1 with 50% Co, its prime peak (about

0.8 nm) is not very strong and its intensity is nearly the same as the peak at about 0.4 nm. Moreover, there are two almost symmetric peaks in the range of 0.23 to 0.26 nm, which indicates that it does not possess the typical structure of turbostratic disorder. Al

–Ni(OH)2 with 25% Al has been identified to have the structure of turbostratic disorder when the content of Co is zero in Chapter 4. This means that the turbostratic disorder has been damaged with the addition of 50% Co. The positive charge has been decreased due to the substitution of trivalent Al with bivalent Co, and the strength of anions binding to the layers in nickel hydroxide has been weakened, with the result that the turbostratic disorder structure cannot be stabilized. Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

When the content of Co decreases or the content of Al increases in nickel hydroxide, it can be seen that the asymmetric characteristics of the two peaks at about 0.23 to 0.26 nm increases. Compared with the X-ray patterns of unsubstituted α-Ni(OH)2 reported in the literature [56], the structure of the (Al,Co)–substituted nickel hydroxide can be attributed to turbostratic disorder with 33% or less Co, which also indicates that

(Al,Co)–substituted nickel hydroxide has the same structure of α-Ni(OH)2. No peaks indexed to cobalt or cobalt compounds have been found in this pattern, meaning that the

Co content has inserted into the interlayer of nickel hydroxide as Al. (Al,Co)– substituted nickel hydroxide will be written into (Al,Co)–Ni(OH)2, and only the content of Co will be mentioned since the content of (Al+Co) in nickel hydroxide is the predetermined value of 25% in this chapter.

3

Intensitites (a.u.) 2

1

20 40 60 2θ (Degree)

Figure 5-1 The diffraction patterns of (Al,Co)–Ni(OH)2

Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

Table 5-2 Observed d-spacing for unit cell of (Al,Co)-Ni(OH)2 samples

Samples hkl 2θ Dobs. (Å)

003 11.20 7.9001

006 22.40 3.9690

Sample 1 012 34.80 2.5780

015 38.64 2.3302

003 11.12 7.9568

006 22.54 3.9447

Sample 2 012 34.82 2.5765

015 28.34 2.3479

003 11.18 7.9142

006 22.48 3.9551

Sample 3 012 34.84 2.5751

015 38.94 2.3129

There are a big differences in the intensity of the prime peak (at about 0.8 nm) for

(Al,Co)–Ni(OH)2. In order to clearly demonstrate the differences, the FWHM (full width at half maximum intensity) of the prime peak is estimated from the pattern and listed in Table 5-3. A smaller FWHM means a sharper peak and also indicates that it is well crystallized. When the content of Co is 50%, its FWHM is 2.990, while a FWHM of 1.7620 and 2.5680 have been obtained for the samples with 33% and 20% Co, respectively. The result shows that the sample with 33% Co has the best crystallization.

Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

Table 5-3 FWHM of (Al,Co)–Ni(OH)2

Samples 1 2 3

FWHM 2.990 1.7620 2.5680

5.2.2 XRD patterns of aged samples

(Al,Co)–Ni(OH)2 samples were aged in 6 M KOH electrolyte for 90 days, and their diffraction patterns are shown in Figure 5-2. It can be seen that the background noise become smaller compared with the as-prepared samples, which indicates that its crystallinity increases after the aging treatment. The prime peak at about 0.8 nm, the peak at 0.4 nm and the asymmetric band at 0.23 nm to 0.26 nm all indicate that the structure of the aged (Al,Co)–Ni(OH)2 samples can be attributed to the typical structure of turbostratic disorder when compared with the X-ray patterns of unsubstituted α-

Ni(OH)2 reported in literature [56], even when the Co content reaches 50%. It can be concluded that the aging treatment in alkali solution is beneficial to the structure of turbostratic disorder for (Al,Co)–Ni(OH)2. This is because bivalent Co(II) is not stable in the strong alkaline solution and thus easy to transform into trivalent Co(III) or higher valence cobalt, enhancing the strength of the forces between the interlayer anions and the intersheet cations, and stabilizing the turbostratic disorder is structure. Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

2

3 Intensities (a.u.) Intensities

1

20 40 60 2θ (Degree)

Figure 5-2 The diffraction patterns of aged samples Al–Ni(OH)2 with addition of Co

The peaks in the patterns of the aged samples become sharper compared with that of the as-prepared samples. This means that (Al,Co)–Ni(OH)2 is well crystallized after the aging treatment in alkali solution, the same result as that obtained for Al–Ni(OH)2 without Co. The FWHM values of the prime peak are estimated and listed in Table 5-4.

It can be seen that the FWHM values become smaller for the aged samples. The FWHM decreases from 2.990 to 1.4060 for sample 1 with 50% Co, and from 2.5680 to 1.1180 for sample 3 with 20% Co. The big difference in FWHM means that a big change has occurred in these two samples’ crystallization. However, not much difference appears for the sample with 33% Co, the FWHM changes from 1.7620 to 1.7060 after the ageing treatment, proving that turbostratic disorder structure of sample 2 is very stable, even after the aging treatment, which also implies that sample 2 has the best crystallization. It is perhaps because that the electrostatic attractions between the positive charged layers caused by Co (III) and Al (III) and the negatively charged Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

interlamellar spaces (anion and water molecules) is the strongest to stabilize the the turbostratic discorder structure of (Al,Co)–Ni(OH)2 with 33% Co.

Table 5-4 FWHM of aged (Al,Co)–Ni(OH)2

Samples 1 2 3

FWHM 1.4060 1.7060 1.1180

5.2.3 TEM measurement

Sample 1 with 50% Co was investigated by TEM. The TEM images and the halo patterns obtained are shown in Figure 5-3, and Figure 5-3 (a) and (b) are for the as- prepared sample and (c) and (d) for the aged sample. It can be clearly seen that the as- prepared (Al,Co)–Ni(OH)2 appears as crumpled sheets or aggregates of thin crumpled sheets, without any definite shape (Figure 5-3 (a)). The centred halo pattern and the diffraction ring in Figure 5-3 (b) show that some nanocrystal grains coexist with the amorphous state. However, the aged samples appear only as aggregates of thin crumpled sheets without any definite shape and as shown in Fig. 5-3 (c), the typical characteristic feature of turbostratic structure [56]. This confirms that (Al,Co)-Ni(OH)2 with 50% Co has the same turbostratic disorder structure as unsubstituted α-Ni(OH)2 after the aging treatment. The halo patterns in Figure 5-3 (d) show that the aged samples have stronger crystallization, and the sharper multicrystal diffraction rings also verify that any amorphous’ structure almost disappeared. This result reveals that a recrystallization process occurred during the aging treatment. More proof needs to be obtained to elucidate the transformation mechanism, whether it is also via the same dissolution-recrystallization process as occurred for Al–Ni(OH)2. Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

(a)

(b)

(c) Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

(d)

Figure 5-3 TEM images (a and c) and halo diffraction patterns (c and d) of (Al,Co)–Ni(OH)2 (TEM image (a) and halo pattern (b) are for the as-prepared sample, (c) and (d) are for the aged sample)

5.3 Electrochemical Properties

5.3.1 (Al,Co)–Ni(OH)2 with 20% Co

The discharge curves of (Al,Co)–Ni(OH)2 with 20% Co are shown in Figure 5-4. There are two discharge plateaus that occur after 25 discharge cycles, as is seen for Al–

Ni(OH)2 with 25% Al. However, the second discharge plateau occurrs at about 50% of the discharge capacity, while it occurs at about 80% of the discharge capacity for Al–

Ni(OH)2. This reveals that the reduction reaction corresponding to the first discharge plateau has been weakened and the second reaction has been enhanced with the addition of 20% Co. It should also be noted that the third stage of the discharge process in Figure

5-4 is longer than that of Al–Ni(OH)2 with 25% Al shown in Figure 4.9, which means that “capacity retention” increases with the addition of Co. Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

1.5

1.4

1.3

1.2

1.1 : 5th : 25th 1.0 Volatge (V vs MH electrode) : 50th :100th

0 50 100 150 200 250 300 Discharge Capacity (mAh/g)

Figure 5-4 Discharge curves of (Al,Co)–Ni(OH)2 with 20% Co

The discharge capacity and the midpoint discharge voltage corresponding to the half maximum discharge capacity are obtained and estimated from the discharge curves, and they are listed in Table 5-5. It can be seen that the discharge capacity has increased by a small amount, but the midpoint voltage has become lower compared with that of Al –

Ni(OH)2 with 25% Al.

Table 5-5 Discharge capacity and midpoint voltage of (Al,Co)–Ni(OH)2 with 20% Co

Discharge cycle 5th 25th 50th 100th

Capacity (mAh/g) 305 309 306 312

Midpoint Voltage (V) 1.267 1.283 1.280 1.274

Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

5.3.2 (Al,Co)–Ni(OH)2 with 33% Co

The discharge curves of (Al,Co)-Ni(OH)2 with 33% Co are shown in Figure 5-5. The second discharge plateau occurrs after 25 discharge cycles. However, it appears at about

40% of the discharge capacity at the 25th discharge cycle, about 50% and 60% of the discharge capacity for the 50th and the 100th discharge cycles, respectively. The result shows that the reduction reaction corresponding to the first discharge plateau has been weakened and the second reaction has been enhanced with the discharge cycle number.

Also, “capacity retention” has been observed in the discharge curves. The discharge capacity and midpoint voltage obtained and estimated from the discharge curves are listed in Table 5-6. It can be seen that a higher discharge capacity has been obtained compared with the result for Al-Ni(OH)2 with 25% Al. It is about 20 mAh/g higher after adding 33% Co. It should also be noted that the midpoint voltage decreases by about 30 mV in the meantime. The result is that the discharge capacity has been improved while the midpoint voltage has been decreased with the addition of 33% Co. The addition of

Co in β-nickel hydroxide has been reported to increases the oxygen evolution potential

[128], and the conductivity by optimizing the lattice imperfections in the active material

[132], thus improving the discharge capacity. The disadvantage of adding Co is that the standard electrode potential E0 of nickel hydroxide is decreased [133]. The results from the addition of Co in Al–Ni(OH)2 is also in agreement with the conclusion about the addition of Co in β-nickel hydroxide.

Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

1.5

1.4

1.3

1.2

1.1 : 5th : 25th : 50th Voltage (V vs MH electrode) 1.0 : 100th

0 50 100 150 200 250 300 350 Discharge Capacity (mAh/g)

Figure 5-5 Discharge curves of Al-Ni(OH)2 with 33% Co

Table 5-6 Discharge capacity and midpoint voltage of (Al,Co)–Ni(OH)2 with 33% Co

Discharge cycle 5th 25th 50th 100th

Capacity (mAh/g) 314 324 321 337

Midpoint Voltage (V) 1.233 1.232 1.255 1.309

5.3.3 (Al,Co)–Ni(OH)2 with 50% Co

The discharge curves of (Al,Co)–Ni(OH)2 with 50% Co for different discharge cycles are shown in Figure 5-6. The second discharge plateau occurred after 25 discharge cycles. It appeared at about 50% of the discharge capacity, the same as the result for

(Al,Co)–Ni(OH)2 with 20% Co. The discharge curves are different from what is seen with 33% Co. “Capacity retention” can also be observed in the discharge curves. The discharge capacity and midpoint voltage obtained and estimated from the discharge curves are listed in Table 5-7. It can be seen that a very similar discharge capacity has been obtained except at the 5th discharge cycle compared with that for 33% Co, and Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

they are all higher than that for Al–Ni(OH)2 with 25% Al. This also confirms that the addition of Cobalt increases the discharge capacity. The midpoint voltages are also found to be lower than that without Co. However, they are all higher than 1.20 V, the standard discharge voltage for β-nickel hydroxide, indicating that a Ni/MH battery employing (Al,Co)–Ni(OH)2 will be more powerful.

1.5

1.4

1.3

1.2

1.1 : 5th : 25th : 50th Volatge (V vs MH electrode) 1.0 : 100th

0 50 100 150 200 250 300 350 Discharge capacity (mAh/g)

Figure 5-6 Discharge curves of (Al,Co)–Ni(OH)2 with 50% Co

Table 5-7 Discharge capacity and midpoint voltage of (Al,Co)–Ni(OH)2 with 50% Co

Discharge cycle 5th 25th 50th 100th

Capacity (mAh/g) 288 323 325 336

Midpoint Voltage (V) 1.249 1.253 1.261 1.251

5.3.4 Cycle life of (Al,Co)–Ni(OH)2

The cycle life of (Al,Co)–Ni(OH)2 samples is shown in Figure 5-7. The trend in the discharge curves shows that the discharge capacity does not deteriorate even after 100 Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

charge/discharge cycles. This indicates that (Al,Co)–Ni(OH)2 samples possess promising endurance in charge/discharge cycles and will have ideal shelf service life.

It also can be seen that the discharge capacity increases sharply with the cycle number until the 8th cycle for (Al,Co)–Ni(OH)2 with 20% Co, while it occurs at the 11th cycle and the 18th cycle for the sample with 33% and 50% Co, respectively. This result shows that it will require more time to activate an electrode with a larger Co content.

350

300

250

Co/(Co+Al): 200 : 20%

Discharge Capacity (mAh/g) Capacity Discharge : 33% : 50%

150 0 50 100 Cycle number

Figure 5-7 Cycle life of (Al,Co)-Ni(OH)2 samples

5.3.5 XRD Patterns after 100 electrochemical cycles

The phase of active material (Al,Co)–Ni(OH)2 in the electrodes after 100 electrochemical cycles had been identified by X-ray diffraction technique, and it is shown in Figure 5-8. Some peaks indexed to β-Ni(OH)2 are clearly seen in sample 3 with

20% Co and sample 1 with 50% Co. All these peaks disappeared and only the peaks indexed to α-Ni(OH)2 are found in the XRD pattern of the sample 1 with 33% Co. It indicates that the turbostratic structure of α-Ni(OH)2 are very stable for the sample with Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

33% Co, which also agrees with the results obtained in section 5.2.2 for these samples after the aging treatment.

Co/(Co+Al)(%): Ni 1, 50 2, 33 3, 20 * : β

2

* * * 1 Intensities (a.u.)

* * * 3

10 20 30 40 50 60 2 θ (degree)

Figure 5-8 XRD patterns for (Al,Co)-Ni(OH)2 electrodes after 100 electrochemical cycles

5.4 Conclusion

The X-ray diffraction patterns of (Al,Co)–Ni(OH)2 show that the samples with 25% and

33% Co have the turbostratic disorder structure, and that this structure is weakened when the content of Co is 50%. This is because the substitution of trivalent Al with bivalent Co weakens the strength between the interlayer anions and intersheet cations.

After the samples are aged in 6 M KOH, the peaks in the X-ray diffraction pattern become sharper, and the patterns reveal that all the aged (Al,Co)–Ni(OH)2 samples have the turbostratic disorder structure. This is because a bivalent Co(II) is easy to transform into trivalent Co(III) or higher valence cobalt in a strong alkali solution. The strength between the anions and intersheet cations is thus enhanced and the structure of turbostratic disorder is stabilized. The results from TEM verify that the sample with Chapter 5 – (A1,Co)-substituted Nickel Hydroxide

50% Co has the turbostratic disorder structure after aging treatment, which agrees with the result from XRD.

The discharge behaviors and cycle life of (Al,Co)–Ni(OH)2 with 20%, 30% and 50% Co are investigated in this work. The results shows that the discharge capacity has been increased but the midpoint voltage has been decreased with the addition of Co, compared with Al–Ni(OH)2 with 25% (employed as precursor). However, the midpoint voltage is still higher than 1.20 V, the standard midpoint plateau for β-nickel hydroxide.

(Al,Co)–Ni(OH)2 samples show a promising cycle life. The discharge capacity obtained does not show deterioration even after 100 charge/discharge cycles. This indicates that

(Al,Co)–Ni(OH)2 samples possess strong endurance under the corrosive electrochemical cycle, and it also implies that (Al,Co)–Ni(OH)2 samples are very stable under these conditions. It can be seen that the discharge capacity increases sharply with the cycle number untill the 8th cycle for (Al,Co)–Ni(OH)2 with 20% Co, while it occurs at the

11th cycle and the 18th cycle for the samples with 33% and 50% Co, respectively. This result shows that more time is needed to activate a (Al,Co)–Ni(OH)2 electrode with an increased Co content. Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

Chapter 6 Temperature Effects on Nickel Hydroxide Electrode

6.1 Introduction

The working temperature plays an important role in the power efficiency and shelf service life of the Ni/MH battery. Although the temperature tolerance of the battery is an important parameter, there have been few papers on temperature effects on nickel hydroxide electrodes. The charge/discharge behaviors of Ni/Co-hydroxide electrodes at

–20oC, 0oC, 22oC, 40oC and 60oC were investigated by Anil K Sood [134]. It was observed that the maximum charging potentials were suppressed at 0oC but increased at

–20oC, although the charge voltage remained lower than at room temperature. At temperatures higher than room temperature, the maximum charging potential decreased increasing temperature. The O2 evolution reaction and nickel oxide transformation reactions over the temperature range from 25oC to 300oC were investigated using steady-state and cyclic voltammetric techniques by Davidson et al. [135]. They found that a change in the valence state of the oxide occurs in the region of oxygen evolution at temperatures above 150oC. A splitting of the anodic and cathodic potentiodynamic

E/I display of nickel hydroxide electrodes between 0oC and 75oC was reported by Meier et al. [136].

However, the effect of temperature on the charge/discharge behaviors of Al–substituted or (Al,Co)–substituted nickel hydroxide electrodes has not been reported to date. In this work, these two kinds of α−phase nickel hydroxides were chosen to be investigated, and the atomic ratio of Al to Ni in Al–substituted nickel hydroxide and (Al + Co) to Ni in

(Al,Co)–substituted nickel hydroxide was chosen to be 0.25. For comparison, commercial spherical nickel hydroxide samples (specified as B3) were also investigated. The samples used in this work are listed in Table 6-1. Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

Table 6-1 Nickel hydroxide samples used in the experiment

Al-Ni(OH)2 (Al,Co)-Ni(OH)2 B3

Content of (Al+Co) (%) 25 25 0

Ratio of (Co/Al) (%) 0 1/2

The temperatures studied in this work were –15oC, 0oC, 25oC and 50oC, and the four discharge current rates of 30, 60, 120 and 300 mA/g (which were based on the amount of nickel hydroxide) were employed, while the charge current rate was 60 mA/g. A current rate of 300 mA/g was used in the charge and discharge process for the cycle life test.

6.2 Temperature Effects on Al–Ni(OH)2 Electrodes

6.2.1 Charge and discharge characteristics

The charge and discharge behaviors of Al–substituted nickel hydroxide electrodes were investigated at different temperatures and discharge current rates. The charge and discharge curves at the current rate of 60 mA/g are shown in Figure 6-1. The discharge curves at other discharge current rates will not be listed here due to their similarity. Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

1.6 -150C

00C 1.5 250C

0 1.4 50 C

1.3

1.2

: -150C 1.1 : 00C

1.0 : 250C

: 500C 0 0 50 C 0 0.9 -15 C 0 C Voltage (Vvs MH electrode) RT

0 100 200 300 400 Capacity (mAh/g)

Figure 6-1 Charge and discharge curves at the current rate of 60 mA/g for Al-Ni(OH)2 samples

Maximum charging voltages of 1.57, 1.52, 1.47 and 1.36V were obtained for nickel hydroxide electrodes at –15oC, 0oC, 25oC and 50oC, respectively. The maximum charging voltages were suppressed when the temperature was elevated, which was caused by the small oxygen overpotential (because the oxygen overpotential of the nickel electrode drop rapidly at high temperature [137]). A second discharge plateau appeared in the discharge curves at –15oC, 0oC and 25oC, and it occurred at an earlier stage of the discharge process when the temperature was –15oC. No second discharge plateau was found in the discharge curves obtained at 50oC, meaning that the reduction reaction corresponding to the first plateau was enhanced with the increase in temperature. The discharge midpoint voltage V (corresponding to the half maximum discharge capacity) and discharge capacity C (based on the amount of nickel hydroxide) obtained from the discharge curves under different conditions are listed in Table 6-2.

Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

Table 6-2 The data obtained from discharge curves for Al-Ni(OH)2 electrode

30 mA/g 60 mA/g 120 mA/g 300 mA/g

V C η V C η V C η V C η mAh mAh mAh mAh Temp. V % V % V % V % g-1 g-1 g-1 g-1

o -15 C 1.289 297 95.8 1.279 292 94.2 1.258 258 83.2 1.200 220 71.0

o 0 C 1.311 321 103 1.305 318 102 1.304 311 100 1.276 303 97.7

o 25 C 1.311 310 100 1.305 308 99.4 1.305 303 97.7 1.288 297 95.8

o 50 C 1.309 208 67.1 1.305 201 64.8 1.305 207 66.8 1.298 191 61.6

Note: V: discharge midpoint voltage; C: discharge capacity; η: discharge efficiency, the ratio of the capacity obtained at a certain discharge current to that at the discharge current rate of 30 mA/g at 25oC.

In order to see clearly the trend for Al-Ni(OH)2 electrode under different conditions, the discharge capacities obtained with different discharge currents are plotted again temperatures and shown in Figure 6-2. It can be clearly seen that at 0oC Al–substituted nickel hydroxide electrodes possess the highest discharge capacity, and the discharge capacity declines with increases or decreases in temperature. The electrodes had the lowest discharge capacity when the temperature was 50oC. The same trend occurred for different discharge current rates, and the discharge capacities (DC) depend on the temperature in the order of: DC (0oC) > DC (25oC) > DC (-15oC) > DC (50oC). It can be also noted that the discharge capacities are all over 290 mAh/g at the discharge current rates from 30 to 300 mA/g when the temperature was 0oC and 25oC. A very high rate capacity (the ratio of the capacity obtained at 300 mA/g to that at 30 mA/g) of over 95% Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

was observed. The results show that Al-Ni(OH)2 electrodes possess the optimum electrochemical characteristics at 0oC and 25oC.

1.32

1.30

1.28

1.26

1.24

1.22 Discharge current rate

V (V vs MH electrode) : 30 mA/g : 60 mA/g 1.20 : 120 mA/g -20 0 20 40 : 300 mA/g 60 Temperature (0C)

Figure 6-2 Discharge capacities of Al-Ni(OH)2 electrodes under different conditions

In Figure 6-3 the curves indicate that discharge midpoint voltages vary with different discharge current rates and temperatures. When the discharge current rates are 30, 60 and 120 mA/g, the midpoint voltages are all over 1.25 V over the temperature range from –15oC to 50oC, higher than 1.20 V (standard discharge voltage for β-nickel hydroxide at room temperature). This confirms that the power efficiency of the Al-

Ni(OH)2 electrode is higher than for β-nickel hydroxide. When the discharge currents are in the range of 30 to 120 mA/g, the discharge voltages (DV) are observed to increase in relation to the temperature in the order: DV (0oC) > DV (25oC) > DV (50oC)

> DV (-15oC). The different trend occurred when the discharge current rate was 300 Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

mA/g, DV (50oC) > DV (25oC) > DV (0oC) > DV (-15oC), with the discharge plateau voltage increasing with increases in temperature.

1.32

1.30

1.28

1.26

1.24

1.22 Discharge current rate V (V vs MH electrode) : 30 mA/g : 60 mA/g 1.20 : 120 mA/g : 300 mA/g

-20 0 20 40 60 Temperature (0C)

Figure 6-3 Discharge midpoint voltages for Al-Ni(OH)2 electrodes under different condition

6.2.2 Cycle life

The discharge capacities obtained as a function of the charge/discharge cycle number are shown in Figure 6-4. A very similar trend and very good cycle life occurred when the temperature was 0oC or 25oC. The discharge capacity declined very slightly with the cycle number, but over 90% of the initial capacity was retained, even after 200 charge/discharge cycles. The discharge capacity dropped very sharply at 50oC, and less than half of the initial capacity was available after 200 cycles. For the cycle ability at –

15oC, a sharp drop appears during the first 100 cycles, and a slight further decline for

o the second 100 cycles. Thus Al-Ni(OH)2 electrodes showed better cycle life at –15 C than at 50oC. Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

300

280

0 260 25 C

0 240 0 C

220

200

180 -150C 160

0 140 : 0 C 0

Discharge Capacity (mAh/g) : -15 C 120 : 250C 0 : 50 C 500C 100 0 50 100 150 200 Cycle Number

Figure 6-4 Cycle life of Al-Ni(OH)2 electrodes under different conditions

6.2.3 Physical properties

6.2.3.1 XRD patterns

XRD patterns of Al-Ni(OH)2 electrodes after 200 charge/discharge cycles are shown in

Figure 6-5, and the pattern of raw material Al–substituted nickel hydroxide is also shown for comparison. All the peaks except for those from the nickel substrate can be indexed to α−phase, and no peak attributed to β–phase was found. This implies that Al-

Ni(OH)2 is very stable in a strong alkali solution (6M KOH) over a temperature range from –15oC to 50oC, even after 200 charge/discharge cycles. It should also be noted that the prime peak (003) becomes broadened with declining temperature. This means that the crystallization of Al–Ni(OH)2 becomes poorer at lower temperatures after the electrochemical cycles. The prime peak for Ni is specified in this work as the standard, with the (003) peak of Al–Ni(OH)2 employed to calculate the relative crystallite size Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

after cycling. Relative crystallite sizes of 8.37, 13.13, 13.53 and 15.14 nm were obtained

o o o o for Al–Ni(OH)2 to Ni at –15 C, 0 C, 25 C and 50 C, respectively. The result clearly shows that the crystallite size becomes smaller with decreases in temperature.

Figure 6-5 X-ray diffraction patterns of S1 electrodes after 200 charge/discharge cycles

6.2.3.2 SEM images

The SEM images of Al–Ni(OH)2 electrodes before or after charge/discharge cycles are shown in Figure 6-6; (a) is the image for the electrode as-prepared, while (b), (c) and (d) are the images at 25oC, 50oC and –15oC after 200 cycles, respectively. A magnification of 1000× was used for these four images. The pulverization of active materials was observed on the electrode surface after the charge/discharge cycles, with definite deterioration at 50oC and –15oC. The pulverization of active materials is thought to be the general cause of cycle life deterioration, which can also explain the cycle life deterioration at 50oC and –15oC. Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

(a)

(b) Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

(c)

(d)

Figure 6-6 SEM images of Al-Ni(OH)2 electrodes before or after electrochemical cycles

(All images are at the same magnification)

Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

6.3 Temperature Effects on (Al,Co)–substituted Nickel Hydroxide Electrode

6.3.1 Discharge characteristics at different temperatures

The charge/discharge curves for (Al,Co)-Ni(OH)2 at a current rate of 60 mA/g at different temperatures are shown in Figure 6-7. It can be seen that the charging voltage is elevated much faster and higher when the temperature is at –15oC, which means that the charge acceptance of the electrode is poor and that it is likely to show poor discharge performance. Maximum charging voltages of 1.67, 1.55, 1.55 and 1.36 V are obtained at –15oC, 0oC, 25oC and 50oC, respectively. Note that the maximum charge voltages were higher than those of the Al-Ni(OH)2 electrode, which means that they will have lower discharge plateau voltage. The electrochemical data on the discharge characteristics obtained from the discharge curves are listed in Table 6-3.

-150C 1.6 250C

00C 1.4 500C

1.2

: -150C

: 00C 1.0 : 250C

0 o : 50 C 0 0 25 C

Voltage (V vs MH (Vvs Voltage electrode) -15 C 50 C 00C 0.8 0 100 200 300 400 Charge Capacity (mAh/g)

Figure 6-7 Charge/discharge curves of (Al,Co)-Ni(OH)2 electrodes at different temperatures

Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

Table 6-3 Data obtained from the discharge curves for S2 electrode

30 mA/g 60 mA/g 120 mA/g 300 mA/g

V C η V C η V C η V C η mAh mAh mAh mAh Temp. V % V % V % V % g-1 g-1 g-1 g-1

0 -15 C 1.220 226 64.0 1.195 198 56.1 1.170 192 54.3 1.074 190 538

0 0 C 1.265 346 98.0 1.259 340 96.3 1.245 336 95.2 1.192 321 90.9

0 25 C 1.271 353 100 1.256 335 94.9 1.249 330 93.5 1.236 323 91.5

0 50 C 1.266 263 73.0 1.256 249 70.5 1.251 242 68.6 1.248 235 66.6

Note: V: discharge midpoint voltage; C: discharge capacity; η: discharge efficiency, the ratio of the capacity obtained at a certain discharge current to that at the discharge current rate of 30 mA/g at 25oC.

It can be clearly seen that very high discharge capacities of more than 300 mAh/g were obtained at the four discharge currents at 0oC and 25oC, higher than those of Al-

o Ni(OH)2 electrodes. A higher discharge capacity is also obtained at 50 C compared with

Al-Ni(OH)2 electrode. However, Al-Ni(OH)2 shows a higher discharge capacity than

o (Al,Co)-Ni(OH)2 electrode at –15 C. Note also that the midpoint discharge voltages of

(Al,Co)-Ni(OH)2 electrodes are lower than those of Al-Ni(OH)2 electrodes within the temperature range from –15oC to 50oC and the discharge current range from 30 to 300 mA/g. It can be concluded that the addition of Co to Al–substituted nickel hydroxides increases the discharge capacity when the temperature is higher than 0oC, but suppresses the discharge plateau voltage, which also agrees with the results in Chapter

5.

Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

The discharge voltage V and discharge capacity C obtained from the discharge curves at different discharge current rates are plotted against temperature and shown in Figure 6-8 and Figure 6-9. A different trend has been observed for (Al,Co)-Ni(OH)2 electrode in

Figure 6-8 in that the discharge capacities depend on the temperature in the order: DC

(0oC) > DC (25oC) > DC(50oC) > DC(-15oC), while an order of DC (0oC) > DC (25oC)

o o > DC(-15 C) > DC(50 C) has been shown for Al-Ni(OH)2 electrode. This means that

o o (Al,Co)-Ni(OH)2 electrode has a higher discharge capacity at 50 C than –15 C, while

o o the discharge obtained at –15 C is higher than that at 50 C for Al-Ni(OH)2 electrode. It also means that the addition of Co to Al-Ni(OH)2 has improved the electrochemical properties at the elevated temperatures. (Al,Co)-Ni(OH)2 electrode also demonstrates its optimum discharge capacity at 0oC and 25oC. In Figure 6-9 it can be seen that (Al,Co)-

o Ni(OH)2 electrode has the lowest discharge midpoint voltage at –15 C compared to Al-

Ni(OH)2 electrode. When the discharge current rate is 300 mA/g, the discharge plateau voltage increases with increasing temperature in Al-Ni(OH)2 electrode. Small differences appear for the discharge midpoint voltage at smaller discharge current rates

(30 – 120 mA/g) when the temperature is higher than 0oC. Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

360

340

320

300

280

260

240

Capacity (mAh/g) 220 Discharge current rate: : 30 mA/g 200 : 60 mA/g : 120 mA/g 180 : 300 mA/g -20 0 20 40 60 Temperature (0C)

Figure 6-8 Discharge capacities of (Al,Co)-Ni(OH)2 electrode at different temperatures

1.30

1.25

1.20

1.15

discharge current rate: 1.10 : 30 mA/g : 60 mA/g

Voltage (V vs MH electrode) : 120 mA/g : 300 mA/g 1.05 -20 0 20 40 60 Temperature0C

Figure 6-9 Discharge midpoint voltage of (Al,Co)-Ni(OH)2 electrode at different temperatures

Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

6.3.2 Cycle life

The discharge capacities obtained at different temperatures have been plotted against the cycle numbers (Figure 6-10). It can be clearly seen that (Al,Co)-Ni(OH)2 electrodes have perfect cycle life behavior at 0oC and 25oC, the discharge capacity does not degrade and over 98% of the initial discharge capacity is retained even after 200 charge/discharge cycles. At 50oC over 80% of the initial capacity has been retained after

200 cycles. Less discharge capacity loss has been found for (Al,Co)-Ni(OH)2 electrode

o o than Al-Ni(OH)2 electrode over the temperature range from 0 C to 50 C. It shows that

Al,Co-Ni(OH)2 electrode posses better cycle life than Al-Ni(OH)2 electrode at the temperature range from 0oC to 50oC, especially at 50oC. It can also be seen that (Al,Co)-

Ni(OH)2 electrode shows worse cycle life and lower discharge capacity than Al–

o Ni(OH)2 electrode at –15 C. The results indicates that the addition of Co in Al–

o o Ni(OH)2 has improved the cycle life over the temperature range from 0 C to 50 C and increased its tolerance to high temperature up to 50oC, but worsened the cycle life at –

15oC.

350 0 oC

300 25oC

250

50oC

200

: 25oC 150 o o Discharge Capacity (mAh/g) : 0 C -15 C : -15oC : 50oC 100 0 50 100 150 200 Cycle Number

Figure 6-10 Cycle life of (Al,Co)-Ni(OH)2 electrodes at different temperatures Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

6.3.3 Physical properties

6.3.3.1 XRD patterns

X-ray diffraction patterns of (Al,Co)-Ni(OH)2 electrodes after 200 charge/discharge cycles are shown in Figure 6-11, and a pattern for raw active material nickel hydroxide is also shown for comparison. For Al-Ni(OH)2 electrodes, all the peaks can be indexed to α-phase and no peak that can be attributed to β-phase was found. This means that

(Al,Co)–substituted nickel hydroxide is very stable in 6M KOH electrolyte within the temperature range from –15oC to 50oC, even after 200 cycles. Note also that the (003) peak becomes sharper with increasing temperature. As has been discussed for Al-

Ni(OH)2 electrodes, the prime peak from Ni is specified as the standard, and the (003) peak was chosen to calculate the crystallite size of the active material. Relative crystallite sizes of 6.67, 9.93, 17.48 and 23.94 nm are obtained for (Al,Co)-Ni(OH)2 at –

15oC, 0oC, 25oC and 50oC, respectively. The crystallite sizes become smaller with decreasing temperature, which is similar to what occurs with Al-Ni(OH)2 electrode. It can be explained with the percentage of γ phase nickel hydroxide become in larger at because of less transformation of nickel hydroxide

6.3.3.2 SEM images

SEM images of (Al,Co)-Ni(OH)2 electrodes after 200 electrochemical cycles are shown in Figure 6-12. The image of the as-prepared electrode is not shown here because it was nearly exactly the same as that of Al-Ni(OH)2 electrode. Fig. 6-12 (a), (b) and (c) are the SEM images of the electrodes obtained at 25oC, 50oC and –15oC, respectively.

Pulverization of the active material is observed on the electrode surface, and it has

o clearly deteriorated at –15 C, which can explain the bad cycle life of (Al,Co)-Ni(OH)2 electrodes at this temperature. It can be explained by the existence of two different Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

structures, α and γ - phase, in nickel hydroxide caused the pulverization. It has been stated that the structure of nickel hydroxide are different at its charge and discharge state, α and γ - phase, respectively. At the low temperature of –15oC, the less discharge capacity had been obtained. However, the charging capacity is the same as other temperature. That means that more γ - phase had been preserved in nickel hydroxide at its discharge state. The existence of these two different structures in nickel hydroxide caused the pulverization, and it deteriorated for nickel hydroxide at –15oC

Figure 6-11 X-ray diffraction patterns of (Al,Co)-Ni(OH)2 electrodes after 200 charge/discharge

cycles at different temperatures Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

(a)

(b) Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

(c)

Figure 6-12 SEM images of (Al,Co)-Ni(OH)2 electrodes after 200 charge/discharge cycles at different temperatures (All images are at the same magnification 1000×)

6.4 Temperature Effects on Spherical Nickel Hydroxide Electrode

6.4.1 Charge/discharge characteristics

The charge/discharge curves of spherical Ni(OH)2 (β-phase) electrodes at a current rate of 60 mA/g are shown in Figure 6-13. The same trend was observed as for Al-Ni(OH)2 electrode in that the maximum charging voltages decline with increasing temperature.

Maximum charging voltages of 1.68, 1.57, 1.47 and 1.45 V occur at temperatures of –

15oC, 0oC, 25oC and 50oC, respectively, and are higher than the corresponding values for Al-Ni(OH)2 and (Al,Co)-Ni(OH)2 electrode. It means that β-nickel hydroxide has lower charge acceptance than α-nickel hydroxide. The electrochemical data obtained from the discharge curves are listed in Table 6-4. The data demonstrate that the Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

discharge capacities at different discharge currents are much lower than the corresponding discharge capacities of Al-Ni(OH)2 and (Al,Co)-Ni(OH)2 over the

o o temperature range from –15 C to 50 C. Spherical Ni(OH)2 also shows the lowest midpoint discharge voltage compared with Al-Ni(OH)2 and (Al,Co)-Ni(OH)2. The results also confirm that α-phase nickel hydroxide has a higher midpoint discharge voltage and discharge capacity than β- phase, which are stated in Chapters 4 and 5.

-150C

1.6 00C

250C

1.4 500C

1.2

: -150C

: 00C 1.0 : 250C

: 500C o Voltage (V MH vs Voltage electrode) 0 25 C 0 -15 C 500C 0 C 0.8 0 100 200 300 400 Capacity (mAh/g)

Figure 6-13 Charge/discharge curves of spherical Ni(OH)2 electrodes at different temperatures

Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

Table 6-4 Data obtained from the discharge curves for spherical nickel hydroxide electrodes

30 mA/g 60 mA/g 120 mA/g 300 mA/g

V C η V C η V C η V C η mAh mAh mAh mAh Temp. V % V % V % V % g-1 g-1 g-1 g-1

-150C 1.151 231 87.2 1.108 153 57.7 1.044 148 55.8 1.037 140 52.8

00C 1.218 267 101 1.222 265 100 1.220 258 97.4 1.178 240 90.6

250C 1.228 265 100 1.226 255 96.2 1.224 250 94.3 1.208 234 88.3

500C 1.243 231 87.2 1.250 201 75.8 1.238 198 74.7 1.220 200 75.5

Note: V: discharge midpoint voltage; C: discharge capacity; η: discharge efficiency, the ratio of the capacity obtained at a certain discharge current to that at the discharge current rate of 30 mA/g at 25oC.

Figure 6-14 shows that the discharge capacities vary with the temperature and discharge current rate. The same trend as for (Al,Co)-Ni(OH)2 electrode was found in that the discharge capacity depends on the temperature and in the following order: DC (0oC) >

DC (25oC) > DC (50oC) > DC (-15oC). The highest and lowest discharge capacities are obtained when the temperatures are 0oC and –15oC, respectively. However, a very different trend appears for the variation of the discharge midpoint voltage with the temperature and discharge current (Figure 6-15). For spherical nickel hydroxide, the

o o highest discharge midpoint voltage occurs at 50 C instead of 0 C for Al-Ni(OH)2 and

(Al,Co)-Ni(OH)2 electrodes. Note also that differences in the discharge midpoint voltage are very small at temperatures of 0oC to 50oC and discharge current rates of 30 –

120 mA/g. The lowest discharge midpoint voltage occurs at –15oC, the same as for Al-

Ni(OH)2 and (Al,Co)-Ni(OH)2 electrodes. Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

270 260 250 240 230 220 210 200 190 180 Discharge current rate: 170 : 30 mA/g 160 : 60 mA/g : 120 mA/g

Discharge Capacity (mAh/g) 150 : 300 mA/g 140 130 -20 0 20 40 60 Temperature (0C)

Figure 6-14 Discharge capacity of spherical Ni(OH)2 electrodes at different temperatures

1.26 1.24 1.22 1.20 1.18 1.16 1.14 1.12

1.10 Discharge current rate: 1.08 : 30 mA/g : 60 mA/g 1.06

Voltage (V vs MH elecrtode) Voltage (V : 120 mA/g 1.04 : 300 mA/g 1.02 -20 0 20 40 60 Temperature (0C)

Figure 6-15 Discharge plateau voltage of spherical Ni(OH)2 electrodes at different temperature

Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

6.4.2 Cycle life

The performance of spherical nickel hydroxide electrodes over the charge/discharge cycles is shown in Figure 6-16. It can be seen that its cycle life is good, with over 90% of the initial capacity retained after 200 charge/discharge cycles within the temperature range from 0oC to 50oC. Even at –15oC, over 80% of the initial capacity has been retained. The results show that spherical nickel hydroxide has high tolerance to temperature up to 50oC and discharge current rates up to 300 mA/g, although its discharge capacity and midpoint voltage is low.

300

250

25oC 200 0oC 50oC 150

100

: 50oC Discharge Capacity (mAh/g) : 0oC -15oC 50 : 25oC : -15oC 0 50 100 150 200 Cycle Number

Figure 6-16 Cycle life of spherical Ni(OH)2 electrodes at different temperatures

6.4.3 X-ray diffraction patterns

X-ray diffraction patterns of spherical Ni(OH)2 electrodes after 200 charge/discharge cycles are shown in Figure 6-17, and the XRD pattern of the raw material is shown for comparison. It can be seen clearly that (101) peak is broadened and becomes weak at Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

the temperature decreases from 50oC to –15oC. It disappears at –15oC. The (003) peak

o indexed to α-phase had been found at 0 C. The results mean that β-Ni(OH)2 is not stable after the charge/discharge cycles, particularly at low temperature.

Figure 6-17 X-ray diffraction patterns of spherical Ni(OH)2 (β-phase) electrodes before or after 200

charge/discharge cycles at different temperatures

6.5 Conclusions

The charge/discharge characteristics and cycle life of Al–substituted nickel hydroxide

(α-phase) electrodes at different temperatures have been investigated. The results show that Al–substituted nickel hydroxide electrodes have optimum electrochemical characteristics at 0oC and 25oC, including high discharge capacity, high discharge plateau voltage and good cycle life. The cycle life deteriorates at –15oC and 50oC. X-ray diffraction patterns show that Al-substituted nickel hydroxide electrodes are very stable and the turbostratic disorder structure can be stabilized in 6M KOH within the Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

temperature range from –15oC to 50oC even after 200 charge/discharge cycles. The crystalline size has been found to become smaller with decreases in temperature.

Pulverization of the active materials has been observed on the electrode surface after

200 cycles, especially at the temperatures of –15oC and 50oC, which is thought to be the main cause of cycle life deterioration.

The charge/discharge behaviors and cycle life of (Al,Co)–substituted nickel hydroxide at different temperatures have been investigated. The electrochemical data obtained from the discharge curves show that (Al,Co)-Ni(OH)2 electrodes possess higher discharge capacities but a lower discharge midpoint plateau from 0oC to 50oC compared with Al-Ni(OH)2 electrode. The addition of Co to Al–Ni(OH)2 improves the discharge capacity but suppresses the discharge midpoint voltage, which agrees with the result in

Chapter 5. The cycle life shows that the discharge capacity does not degrade, and over

98% of the initial capacity has been retained at 25oC and 0oC, even after 200 electrochemical cycles. Over 80% has been retained at 50oC, indicating (Al,Co)-

Ni(OH)2 electrodes possess better cycle life characteristics than Al-Ni(OH)2 electrodes over temperatures from 0oC to 50oC, especially at 50oC, but poorer cycle life at –15oC.

The results indicate that the addition of Co in Al–Ni(OH)2 also improves the cycle life within the temperature range from 0oC to 50oC and increases its tolerance to high temperature up to 50oC, but worsens the cycle life at –15oC.

XRD patterns show that (Al,Co)–substituted nickel hydroxides (α-phase) are very stable in the electrolyte at temperatures from –15oC to 50oC after 200 charge/discharge cycles, and the structure of turbostratic disorder can be stabilized by Al together with Co under these conditions. The relative crystalline sizes become smaller as the temperature Chapter 6 – Temperature Effects on Nickle Hydroxide Electrode

decreases. Pulverization of the active materials has been observed on the electrode surface, particularly at –15oC, which is thought to be the cause of the bad cycle life of the electrode.

Spherical nickel hydroxide (β-phase) shows a higher maximum charging voltage, a lower discharge capacity and lower midpoint voltage than Al-Ni(OH)2 and (Al,Co)-

o o Ni(OH)2 (α-phase nickel hydroxide) within the temperature range from –15 C to 50 C and for discharge current rates from 30 – 300 mA/g, which means that the electrochemical properties of α-phase nickel hydroxide are superior to those of β-phase nickel hydroxide under these conditions. It also implies that α-phase nickel hydroxide has more higher “useful” capacity, giving it promise as a substitute for β-nickel hydroxide in the Ni/MH battery. The results from the XRD patterns for spherical nickel hydroxide before and after charge/discharge cycles show that the prime (101) peak

o o indexed to β-Ni(OH)2 is broadened and weak at 0 C to 50 C after 200 cycles and nearly

o disappears at –15 C. The (003) peak indexed to α-Ni(OH)2 was found in the pattern at

o 0 C, indicating that β-Ni(OH)2 is not stable after 200 charge/discharge cycles, particularly at low temperature. Chapter 7 – Spherical Nickel Hydroxide

Chapter 7 Spherical Nickel Hydroxide

7.1 Introduction

The positive electrode nowadays used in Ni/MH batteries is a so-called non-sintered type obtained by filling a porous foamed nickel substrate (having a porosity of at least

95 %) with nickel hydroxide particles. To obtain high capacity, spherical Ni(OH)2 particles should have with a diameter ranging from a few microns up to several tens of microns to fill the pores of the substrate, which generally has a pore size of 200 – 500 microns. The production process for improved pure or composite spherical nickel hydroxide has been the object of many studies [100] [138] [139] [140]. Generally, an aqueous Ni salt solution is supplied to a reaction vessel together with an alkali metal hydroxide solution and an ammonium ion donor, with the system maintained at a constant stirring rate and temperature (20-80oC) and a constant pH value (9 – 12).

Under these conditions spherical Ni(OH)2 agglomerates will grow. Different modifications of this method have been tried. In Ref. [141] an amino acid solution was substituted for the ammonium solution in order to reduce its undesirable evaporation. It was also found that ultrasonic precipitation is beneficial for promoting reversible cycling between the beta and gamma phases of Ni(OH)2 during the electrochemical charge/discharge cycle [142]. The introduction of the alkali solution by a spray technique was used in [143], in order to obtain nano-structured materials, which, in some cases, are comprised of spherical agglomerates of nano-particles. Salts of Ni, Co or Cu were dissolved in aqueous ammonia and further converted to an emulsion containing droplets of the solution in a non-aqueous medium by Kazuhiko et al. [144].

This was followed by removing the volatile components (including ammonia) from the droplets, thereby precipitating the hydroxide or carbonate of Ni, Co or Cu in the droplets so that fine spherical particles in the range of 0.1-50 microns were obtained. Chapter 7 – Spherical Nickel Hydroxide

Hiroyuki et al [145] used sophisticated multi stage reaction deposition vessels to obtain high density and uniform complexes of Ni(OH)2 containing large amounts of additional metal salts, e.g. Mn, Al, V, Cr, Fe etc.

The common feature of all these methods is that the formation process producing spherical agglomerates of pure or composite Ni(OH)2 took place by the chemical reaction route, and it took place in the reaction system. The reaction process takes a very long time and it is also difficult to control, because of the need to maintain stable reaction conditions. Any small deviation in the reaction conditions leads to the growth of agglomerates with irregular shapes and a broad range of sizes. The abovementioned problems could be overcome by applying the faster spray-drying process to the formation of spherical agglomerates. Some promising results using such technology have already been reported [146] [147]. The method used includes co-precipitation by simultaneous spraying of Ni-containing and KOH solutions in the spray dry chamber, which integrates all the steps of the process in one place. The dried powder, however, has to be redispersed in water and dried again in order to remove the unreacted KOH particles.

In the present work a modified process is used based on the spray dry method for the formation of spherical agglomerates. Both β-phase and α-phase nickel hydroxide were investigated.

Chapter 7 – Spherical Nickel Hydroxide

7.2 β–type Nickel Hydroxide

7.2.1 Experimental

The first step of the process was co-precipitation of Ni(OH)2 at predetermined pH and temperature. A constant pH was maintained during the process, and reaction temperatures of 60oC and 40oC was used for the co-precipitation of β-and α-type nickel hydroxide. The precipitate consisted of particles with irregular shapes, and it was obtained by filtration and washing to the point of neutral pH. The next step is a spray drying of the washed slurry (re-dispersed precipitate in a particular volume of water).

During this process dried and spherical agglomerates are instantly obtained with the desired diameter, which is mainly controlled by the diameter of the spray nozzle.

7.2.2 X-ray diffraction patterns

An X-ray diffraction pattern of spherical nickel hydroxide prepared by spray dry method is shown in Figure 7-1. All the peaks can be indexed to β-phase, which indicates that the product we obtained is a single-phase beta-type nickel hydroxide. The broad peak width is due to the nanostructured nature of the material, and the primary nanoparticles sizes are calculated to be 10-25 nm from XRD data.

Chapter 7 – Spherical Nickel Hydroxide

Figure 7-1 XRD pattern of spherical Ni(OH)2 prepared by the spray dry method

7.2.3 Morphological features

The morphological features of the spherical materials produced are shown in Figure 7-2

(a). It can be clearly seen that almost all the agglomerates have a spherical shape. The agglomerates consist of very fine nano-scaled particles (b). Their appearance suggests a highly developed surface area, which has been confirmed by BET analysis. The morphological analysis of commercial Ni(OH)2 materials reveals spherical agglomerates, but also the presence of a second fraction of agglomerates without the spherical shape. A typical SEM image of commercial Ni(OH)2 material prepared by the classical solution route is presented in (c). Chapter 7 – Spherical Nickel Hydroxide

(a)

(b) Chapter 7 – Spherical Nickel Hydroxide

(c)

Figure 7-2 SEM images of nickel hydroxide spherical agglomerates prepared by spray dry method

at different magnifications (a, b) and (c) prepared by classical solution route

7.2.4 Particle Size Distribution

The SEM results are also supported by the particle size distributions shown in Figure

7-3. The powders prepared by the spray-drying technique have a mean particle diameter in the range of 1-5 microns, and there is a narrow Gaussian type particle size distribution in the range of 0.1 – 10 microns (a). The industrial material has a submicron fraction (b), which was identified as non-spherical in shape by SEM. The distribution range is much larger, and this is typical for the conventional technology [139]. This is a consequence of the disadvantages of the classical method, which make it unable to precisely control the growth of the agglomerates. This leads to lower reproducibility of the material properties and battery characteristics.

The specific surface area of the materials is determined by using the gas sorption technique. It was found that spray-dried spherical Ni(OH)2 exhibits a high specific Chapter 7 – Spherical Nickel Hydroxide

surface area, which can be controlled in the range of 50-200 m2/g. Conventional nickel hydroxide has a specific surface area in the range of 10-20 m2/g. As is shown in [148], the higher surface area is a beneficial factor for increased capacity of the electrodes.

16

14 (a) Distribution of spray-dry spherical Ni(OH)2

(b) Typical distribution of commercial spherical Ni(OH)2 12

10

8

6

4

2 Volume distribution (%) Volume distribution

0

0.1 1 5102550 Particle Diameter (microns)

Figure 7-3 Typical particle size distribution graph of spherical Ni(OH)2 prepared by the spray dry

method (a) and of commercial spherical Ni(OH)2 (b)

7.3 Alpha – Phase Nickel Hydroxide

7.3.1 Phase identification

Spherical Al–substituted nickel hydroxide produced by the spray-drying technique was also investigated, and a ratio of 0.25 (Al to Ni) was chosen for the sample. The same set of parameters as for the preparation of β-type spherical nickel hydroxide was employed to produce spherical α-Ni(OH)2. The X-ray diffraction pattern of the nickel hydroxide samples obtained by the spray dry technique is shown in Figure 7-4. All the peaks can be indexed to α-phase, which indicates that Al–substituted nickel hydroxide prepared by the spray dry method can also be attributed to the α-type. It reveals that the structure Chapter 7 – Spherical Nickel Hydroxide

of α-type nickel hydroxide can be stabilized by Al even after the high temperature process (usually the inner temperature of the chamber where nickel hydroxide is formed is about 180oC).

003

006 Intensities (a.u.) 012

015 018 110 113

20 40 60 2θ (degree)

Figure 7-4 X-ray diffraction pattern of Al – substituted nickel hydroxide prepared by spray dry

technique

7.3.2 Morphological features

The surface morphology of the spray dry sample was investigated by SEM, and the image is shown in Figure 7-5 (a). It can be clearly seen that the shape of the sample particles obtained is not spherical or solid, but some particles are hollow inside. The results show that spherical α-type nickel hydroxide can not be obtained under the same conditions as for β-type. In order to get a spherical shape, the parameters of the spray dry process (air temperature) and also the ageing time for the nickel hydroxide slurry

(re-dispersed precipitate in water) need to be changed. Two sets of parameters (listed in

Table 7-1), specified for method 1 and method 2, are employed to prepare spherical Al

– substituted nickel hydroxide. Chapter 7 – Spherical Nickel Hydroxide

Table 7-1 Parameters used in spray dry process for α-Ni(OH)2 preparation

Aging time (hrs) Temperature (0oC) Spray speed (g/min)

Method 1 16 180 13

Method 2 48 200 13

XRD patterns of the samples prepared by methods 1 and 2 are not shown here because they are nearly exactly the same as in Figure 7-4. The results show that Al–substituted nickel hydroxide prepared by these two methods can also be classed as α-type nickel hydroxide. The images of the particles are shown in Figure 7-5 (b) and (c). Figure 7-5

(b) shows the image for the sample prepared by method 1 while (c) is the image for the sample prepared by method 2. In Figure 7-5 (b) different shapes of nickel hydroxide particles obtained in method 1 have been observed, with most of them in spherical agglometrates while some are irregular in shape. Note that hollows still exist within the particles. It can be clearly seen in Figure 7-5 (c) that the particles of nickel hydroxide are spherical and there is a distribution of particles of various sizes.

Chapter 7 – Spherical Nickel Hydroxide

(a)

Chapter 7 – Spherical Nickel Hydroxide

(b)

(c)

Figure 7-5 Images of Al–substituted nickel hydroxides prepared by spray dry technique

7.4 Conclusion

Spherical agglomerates of nanostructured beta-type Ni(OH)2 has been produced by a spray drying technique. Compared with the commercial sample, this material features a narrow Gaussian-type particle size distribution in the range of 0.1 – 10 microns. It also has a high specific surface area of 50 – 200 m2/g due to the nanosized dimensions of the primary crystals (10-25 nm). Moreover, the production method used costs less and is faster than the currently used method of controlled crystallization. The results show that this spray-dry technique will be beneficial to produce spherical nickel hydroxide.

The spray dry technique is also used to prepare spherical Al–substituted nickel hydroxide. However, the spherical agglometrates could not be obtained under the same conditions as for β-type. By changing the air temperature to dry the samples in the Chapter 7 – Spherical Nickel Hydroxide

product chamber and altering the spray speed of the slurry together with the ageing period of the nickel hydroxide slurry (precipitate re-dissolved in water), spherical agglometrate particles are obtained. The results indicate that spherical Al–substituted nickel hydroxide can also be produced by the spray dry technique. Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

Chapter 8 Tetramethylammonium Hydroxide Pentahydrate–Based Solid Polymer Electrolyte

8.1 Introduction

Tetramethylammonium hydroxide pentahydrate, (CH3)4NOH⋅5H2O, is abbreviated as

TMAH5, and its schematic formula is shown in Figure 8-1 a [149]. It has a structure of three-dimensionally connected cages (Figure 8-1 b) of hydrogen-bonded water molecules and OH- ions in which tetramethyl ammonium cations are contained. Oxygen atoms occupy the vertices in the cage, while hydrogen atoms exist on the edges shown by solid lines. The broken lines indicate the O-O edges at which hydrogen atoms do not exist. Such a framework has the potential to conduct protons through it by means of proton jumping and reorientation of water molecules [150]. By studying its structure,

Noburiro Kuriyama et al. [112] concluded that water molecules reorientate fast on their lattice sites and protons jump to adjacent water molecules or OH- ions, and that protons are transported in TMAH5 by such motion without diffusion of oxygen atoms.

CH3

+

- CH3  N  CH3 OH ⋅ 5H2O

CH3

8-1a 8-1b

Figure 8-1 Schematic diagram of TMAH5

The low activation energy is also a reasonable value for the reorientation-jump mechanism [151].

Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

A high bulk conductivity of 4.5 × 10-3 S cm-1 and low activation energy of 32.3 kJ mol-1 for proton transport at 288K has been determined for TMAH5 [112]. It can be concluded that TMAH5 is one kind of fast proton conductor. As stated in the previous chapter, the Ni/MH battery is characterized by the effective migration of hydrogen ions

(protons) from the positive to the negative electrode during charging and from the negative to the positive during discharging. The electrolyte for Ni/MH batteries must possess the properties to efficiently carry hydrogen ions to and away from the electrodes. Due to its high proton conductivity, TMAH5 may be useful in Ni/MH batteries as a solid polymer electrolyte (SPE). A Ni/MH battery employing TMAH5 as the solid electrolyte has been reported to be dischargeable [112], and the conclusion was that TMAH5 can be used in the Ni/MH battery as a solid electrolyte. There have been no further reports about its use in Ni/MH cells to date.

TMAH5 is a brittle material, so to improve its mechanical properties J. Sun et al. [152] developed solid electrolytes by employing poly(sodium acrylate) (PSA), poly(acrylic acid) (PA) and poly(tetramethyl ammonium acrylate) (PTMA) to mix with TMAH5 to get a new type of solid polymer electrolyte. The alkaline polymer electrolyte system,

PSA/PTMA-TMAH5, was found to have good mechanical properties and high conductivity (10-2 Scm-1 at 40oC). However, its potential as a solid electrolyte in Ni/MH cells has not been investigated yet.

TMAH5–based solid with addition of PTMA were provided by Dr.

J Sun from the School of Physics and Materials Engineering, Monash University. Their potential usage as an electrolyte in Ni/MH sealed cells has been investigated in this Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

work. TMAH5 – based solid polymer electrolyte samples are in solid form, and the percentages of PTMA (%) added to the TMAH5 were 0, 5 and 15.

8.2 Experimental

The electrochemical measurements for Ni/MH cells employing TMAH5–based alkaline solid polymer electrolyte were carried out in sealed cells. In this work commercial spherical nickel hydroxide electrodes were used as the positive electrode to be examined. They were inserted between two counter commercial hydrogen absorbing alloy (AB5-type) electrodes of twice the size (to ensure that the capacity of the metal hydride electrode is much bigger than that of the nickel electrode) and separated by non- woven cloth in the cell. Both the nickel hydroxide and alloy electrodes were activated in

6M KOH solutions in advance, and they were used to assemble the cells in their discharge state. The KOH solution left on the surface of the activated electrodes was absorbed by tissues, then the electrodes were quickly put into a glove box under high purity argon.

To obtain an intimate interface between the polymer electrolyte and the electrodes and hence reduce the resistance caused by grain boundaries of polymer electrolyte powders,

TMAH5 - based polymer electrolytes were melted before use, since TMAH5 melts at

343K without decomposition. The molten polymer electrolyte was used to impregnate all the components (negative and positive electrodes and separators) and then solidified.

All the components were installed between two polyethylene plates with four screws on the corners to tighten them, and epoxy resin was used to seal and isolate the components from air to protect them from moisture, carbon dioxide and oxygen.

Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

The inner pressure will increase in sealed Ni/MH cells when they are charging, especially after the charge capacity reaches 100% of the theoretical capacity [153]. In this work the Ni/MH cells were contained between two polyethylene plates and sealed with epoxy resin. It is difficult to resist high pressure without safety valve, hence, the scheme used in this experiment is as follows: galvanostatically charging to 360 mAh/g of nickel hydroxide in the positive electrode at a predetermined rate, and cut-off voltage of 0.9 V. The experiment was carried out at 25oC and 50oC. The temperature of 50oC was achieved by a thermostatically controlled water bath.

8.3 Charge and Discharge Behaviors at 25oC

The charge and discharge current rates used in this work were based on the amount of nickel hydroxide in the positive electrode in the Ni/MH cells. In Figure 8-2 (a) and

Figure 8-2 (b) the charge current rates were 15 mA/g and 30 mA/g, while the discharge current rates were 15 mA/g and 30 mA/g in Figure 8-3 (a) and Figure 8-3 (b), respectively.

When the charge current rate was 15 mA/g, the final charge voltages (the voltage corresponding to the maximum charge capacity of 360 mAh/g) were 1.449 V, 1.480 V and 1.471 V for Ni/MH cells employing TMAH5 (S1), TMAH5 + 5% PTMA (S2) and

TMAH5 + 15% PTMA (S3) as electrolyte, respectively. To simplify, S1, S2 and S3 stand in for the Ni/MH cells employing them as electrolyte in the following text. A unique charge voltage drop appeared in the charge curve of S3 between the charge capacities of 15 and 55 mAh/g. The charge voltage value of S3 was close to S1 before the charge capacity reached 245 mAh/g, then it increased faster than that of S1. S2 had the highest charge voltage, while S1 had the lowest. The lower the charge voltage is, the Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

higher the charge efficiency and the better the electrochemical characteristics will be. It can be concluded that Ni/MH cells employing S1 possess the best electrochemical properties, which also indicates that sample S1 is the most suitable polymer electrolyte in Ni/MH cells.

Final charge voltages of 1.52 V, 1.58 V and 1.62 V were obtained for S1, S2 and S3 at the charge current rate of 30 mA/g, which were all higher than the corresponding final charge voltages at the charge current rate of 15 mA/g. The higher charge voltage means that the charge efficiency decreases. The final voltage of S2 was lower than that of S3, which contradicted the result obtained when the current rate was 15 mA/g. The reason is unknown and further experiments are needed. It can be seen clearly in Figure 8-2 (b) that the final charge voltage increased with the content of PTMA in TMAH5. Perhaps the interface resistance between the electrode and electrolyte increased with the addition of PTMA and raised the charge voltage, thus reducing the charge efficiency.

1.50 S2 S3 1.45 S1

1.40

1.35

1.30

Voltage (V) : S3 1.25 : S2 : S1 1.20

0 100 200 300 Charge Capacity (mAh/g)

Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

(a)

1.65 S3

1.60 S2

1.55 S1 1.50

1.45

1.40

1.35 Voltage (V) : S2 1.30 : S3 1.25 : S1

1.20 0 100 200 300 Charge capacity (mAh/g)

(b)

Figure 8-2 Charge curves of Ni/MH cells employing TAMH5-based electrolyte at 25oC

(Charge current rate: (a) 15 mA/g; (b) 30 mA/g)

A second discharge plateau appears in the discharge curve of S3 in Figure 8-3 (a) when the discharge capacity reaches 245 mA/g, which agrees with the charge voltage drop in Figure 8-2 (a). Discharge capacities of 195, 154 and 146 mAh/g were obtained for S1, S2 and S3, respectively, when the discharge current was 15 mA/g, while discharge capacities of 200, 134, and 159 mAh/g were obtained for S1, S2 and S3, respectively, at the discharge current rate of 30 mA/g. Their corresponding discharge midpoint voltages (corresponding to half discharge capacity) were 1.18, 1.14 and 1.18 V at the discharge current rate of 15 mA/g, but 1.17, 1.07 and 1.14 V, respectively, at the discharge current rate of 30 mA/g. To see clearly the effect of addition of PTMA on the Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

electrochemical characteristics, the dependence of the discharge capacity and discharge midpoint voltage on the content of PTMA in solid electrolyte is plotted in Figure 8-4.

The same trend between discharge properties and the content of PTMA in the polymer electrolyte appears in Figure 8-4, except for the discharge capacity obtained at the discharge current rate of 15 mA/g. S1 has the highest discharge capacity and plateau voltage. When 5% PTMA was added to TMAH5, both the discharge capacity and plateau voltage decreased, namely, the electrochemical properties became worse with the addition of PTMA. Perhaps the addition of PTMA causes resistance to proton jumping in the TMAH5 structure, so that the proton diffusion coefficient decreases, affecting the electrochemical reaction. However, when the percentage reached 15%, the discharge capacity and plateau voltage increased again. Probably the properties of the polymer will increase further if more than 15% PTMA is added, and then we can find the optimum ratio of PTMA to TMAH5 in the electrolyte. Further experiments are needed to clarify this.

Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

1.5

1.4

1.3

1.2

1.1 Voltage (V)

1.0 : S3 : S2 0.9 : S1 S3 S2 S1 0 20 40 60 80 100 120 140 160 180 200 Discharge Capacity (mAh/g)

(a)

1.4

1.3

1.2

1.1 Voltage (V) Voltage

1.0

: S3 0.9 : S2 S2 S3 S1 : S1 0 50 100 150 200 Discharge Capacity (mAh/g)

(b)

Figure 8-3 Discharge curves of Ni/MH cells employing TAMH5-based electrolyte at 25oC

(Discharge current rate: (a), 15mA/g; (b), 30mA/g) Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

1.18 200

1.16

180

1.14 Voltage (V)

160 1.12

1.10

Discharge Capacity (mAh/g) Discharge Capacity (mAh/g) 140 1.08

: discharge capacity, 15 mA/g : discharge voltage, 15 mA/g : discharge capacity, 30 mA/g : discharge voltage, 30 mA/g 120 1.06 0 2 4 6 8 10121416 Percentage of PTMA in TMAH5 (%)

Figure 8-4 Relationship between the discharge properties of solid electrolyte and the content of

PTMA in them

8.4 Charge and Discharge Behavior at 50°C

TMAH5 melts congruently at 343K and undergoes a solid-solid phase transition at

317K [154]. Its schematic transition from α to β form is shown in Figure 8-5. β-(CH

3)4NOH ⋅ 5H2O has fully four-connected host structures with more hydrogen bonds

[155] than the α form. Moreover, it only exists within a small temperature range, close to its melting point of 42oC. In the following experiment, the electrochemical measurements were carried out at 50oC, which means that the TMAH5 employed the

Ni/MH cells at 50oC was in its β form. The charge and discharge curves for Ni/MH cells employing β form TMAH–based solid polymer electrolyte are shown in Figure 8-6 and Figure 8-7, respectively. In Figure 8-6 (a) and Figure 8-6 (b) the charge current Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

rates were 15 mA/g and 30 mA/g, while in Fig. 8-7 (a) and Fig. 8-7(b) the discharge current rates were 15 mA/g and 30 mA/g, respectively.

315K

α - Me4NOH ⋅ 5H2O β - Me4NOH ⋅ 5H2O

Figure 8-5 Solid-solid transition of Me4NOH . 5H2O

Final charge voltages of 1.422 V, 1.428 V and 1.466 V were obtained for S1, S2 and S3 at the charge current rate of 15 mA/g. Compared with the corresponding final charge voltages of 1.449 V, 1.471 V and 1.480 V at 25oC, the final charge voltage decreased with the increased temperature. It is well known that the conductivity and proton diffusion of TMAH5 will increase when the temperature rises [152]. The higher conductivity and proton diffusion of the electrolyte in the Ni/MH battery improved the electrochemical reaction because the reaction rate is controlled by proton diffusion, namely, the rate determining step is the diffusion of proton in the nickel hydroxide layer

[12]. When the current rate reached 30 mA/g, the final charge voltages were 1.455V,

1.457V and 1.491V for S1, S2 and S3, respectively. The same trend appeared for the final charge voltage, S1 < S2 < S3, which indicates that the final charge voltage increased with increased content of PTMA in TMAH5. Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

1.50

S3 1.45 S2

S1 1.40

1.35

1.30 Voltage (V)

: S3 1.25 : S1 : S2 1.20 0 100 200 300 Charge Capacity (mAh/g)

(a)

1.50 S3

S2 1.45 S1

1.40

1.35 : S3 Voltage (V) : S2 1.30 : S1

1.25 0 100 200 300 Charge Capacity (mAh/g)

(b)

Figure 8-6 Charge curves of Ni/MH cells employing TAMH5-based electrolyte at 50oC

(Charge current rate: (a), 15mA/g; (b), 30mA/g)

Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

A very flat plateau was observed in the discharge curves at 50oC for S2 and S3 in Figure

8-7, which indicates that the electrochemical properties were improved. Discharge capacities of 193, 141 and 166 mAh/g were obtained for S1, S2 and S3 at the discharge current rate of 15 mA/g, while discharge capacities of 172, 130 and 158 mAh/g were obtained for S1, S2 and S3 at the discharge current of 30 mA/g. A big increase in the discharge plateau voltage appears for S2 and S3 when the temperature reaches 50oC compared with that obtained at room temperature. It increases from 1.14 V to 1.18 V for

S2, and 1.18 V to 1.19 V for S3 at the discharge current rate of 15 mA/g, while it increases from 1.07 to 1.17 V for S2 and 1.10 V to 1.17 V for S3 at 30 mA/g. The increased discharge plateau voltage means that the discharge efficiency and capacity usage have been improved in Ni/MH cells employing polymer electrolyte. It also can be concluded that the conductivity and proton coefficient are increased in TMAH5-based electrolyte when the temperature rises, and the resistance to proton diffusion (proton jumping in the TMAH5 structure) caused by PTMA has been minimized or eliminated.

Perhaps the relationship between TMAH5 and PTMA in the solid polymer electrolyte has changed, and the mixture of TMAH5 and PTMA has become a homogeneous compound.

To clearly see the effect of the addition of PTMA on the electrolyte TMAH5, the content of PTMA in the electrolyte has been plotted as a function of the discharge capacity and midpoint voltage is plotted in Figure 8-8. The same trend appears as at

25oC in that the electrolyte with 5% PTMA has the lowest discharge capacity and plateau voltage, which also means that the conductivity and proton coefficient are lowest in this polymer electrolyte. However, when the percentage of PTMA reaches Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

15%, the discharge capacity and plateau voltage increase, very close to that of pure

TMAH5.

1.4

1.3

1.2

1.1 Voltage (V)

1.0 : S3 : S2 : S1 0.9 S3 S2 S1 C 0 50 100 150 200 Discharge capacity (mAh/g)

(a)

1.5

1.4

1.3

1.2

1.1

Voltage (V) : S3 1.0 : S2 : S1 0.9 D S2 S3 S1

0 50 100 150 Discharge capacity (mAh/g)

(b)

Figure 8-7 Discharge curves of Ni/MH cells employing TAMH5-based electrolyte at 50oC Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

(Discharge current: (a), 15 mA/g; (b), 30 mA/g)

200

1.190

180 1.185 Voltage (V)

1.180 160

1.175 Discharge capacity (mAh/g) 140 : discharge capacity, 15 mA/g : discharge capacity, 30 mA/g : discharge voltage, 15 mA/g 1.170 : discharge voltage, 30 mA/g -20246810121416 Percentage of PTMA in TMAH5 (%)

Figure 8-8 Relationship between discharge properties and percentage of PTMA in TMAH5

8.5 Conclusion

The potential application of TMAH5-based alkaline solid polymer with addition of

PTMA as the electrolyte in sealed Ni/MH cells has been investigated in this work. The electrochemical measurements for Ni/MH cells employing polymer electrolyte were carried out at 250C and 500C, which indicates that both α and β form TMAH5–based polymer has been tested. The final charge voltages for these three samples are in the order of S1 < S2 < S3, while their discharge capacities and plateau voltages are in the order of S1 > S3 > S2. The results show that Ni/MH cells employing pure TMAH5 polymer electrolyte has the lowest final charge voltage, and the highest discharge capacity and midpoint voltage, which also indicates that pure TMAH5 is the optimum electrolyte candidate in Ni/MH cells among these three samples. Ni/MH cells Chapter 8 – Tetramethylammonium Hydroxide Pentahydrate-Based Solid Polymer Electrolyte

employing as electrolyte TMAH5 + 5% PTMA had the lowest discharge capacity and discharge plateau voltage. However, when the percentage of PTMA reached 15%, the discharge capacity and discharge plateau voltage increased. At 500C, the electrochemical properties of Ni/MH cells employing TMAH5 + 5% and 15% PTMA were dramatically improved, especially for the latter, very close to that of pure TMAH5.

Perhaps the relationship between PTMA and TMAH5 had changed and they became a homogenous compound instead of a mixture. It can be concluded that both α and β form TMAH5 – based solid polymer with addition of PTMA are dischargeable in

Ni/MH cells, and they have potential usage as an electrolyte in Ni/MH batteries. To commercialize them, considerable further research work needs to be done in the future. Chapter 9 - Summary

Chapter 9 – Summary

Experiments have been conducted on A1substitued nickel hydroxide, A1,Co-

substituted nickel hydroxide and spherical shape nickel hydroxide with the aim to

stabilize their turbostratic disorder structure and improve their properties. The

temperature effect has been investigated and compared. A new type of solid polymer

electrolyte has also been tested for the possible use in the Ni/MH battery. The main results and conclusions from this study can be summarised.

1. A1-substitued nickel hydroxide.

The dopant element A1 has been identified to enter the interlayer spaces of nickel hydroxide and substitute into position of Ni. A1- Ni(OH)2 with over 20% A1 has the

typical structure of α-nickel hydroxide, turbostratic disorder. The cell constant a is

found to decrease with increasing A1 content, while cell constant c increased from

4.7315 A to about 24 A with the addition of A1. The structure of turbostratic disorder

in A1- Ni(OH)2 is found to be very stable with over 20% Aluminum after an aging treatment in 6M KOH for 90 days. The crumpled film shape for as-prepared A1-

Ni(OH)2 with 20% A1 was found to be transformed into platelets after the aging

treatment by the TEM technique. A crystallisation process occurred during the aging

treatment, and it is though to be a via dissolution-recrystallization.

The exchange current density (io) of A1- Ni(OH)2 samples is higher than that of β-

phase nickel hydroxide, and it increases with the A1 content. The electrochemical

catalytic activity is improved by the addition of A1, and a high content of A1 is even

more beneficial. The cathodic reduction peak and the oxygen evolution overpotentials Chapter 9 - Summary

are found to shift to a more positive position in the CV curves with the addition of

aluminium, and the smaller ∆EO.R for A1- Ni(OH)2 indicates that the

charge/discharge process occur more reversibly with the addition of A1 and also with

an increased content of A1. A1- Ni(OH)2 is found to have higher discharge plateau

voltages of over 1.2 V, and a discharge capacity of 303 mA/g has been obtained for

A1- Ni(OH)2 with 24% A1. A second discharge plateau has been observed in the

discharge curves for A1- Ni(OH)2 with 10%, 20% and 25% A1 after 10 discharge cycles, which reveals that two reduction reactions occurred. The first discharge plateau is prolonged and enhanced with increases in A1, so no second discharge plateau was observed in the discharge curves of A1- Ni(OH)2 with 33% A1 even after

100 cycles.

2. (A1,Co)-substituted nickel hydroxide

(A1,Co)- Ni(OH)2 with 25% and 33% Co have the turbostratic disorder structure, which is weakened when the content of Co is 50%. After aging treatment in 6M KOH electrolyte for 90 days, the turbostratic disorder structure is enhanced and stabilized.

This is because the transformation of bivalent Co(II) to trivalent Co(III) or higher valence cobalt in a strong alkali solution has enhanced the strength of the binding between the anions and intersheet cations, and thus the structure of turbostratic disorder has been stabilized.

(A1,Co)0 Ni(OH)2 samples show a higher discharge capacity and lower discharge

voltage compared with A1- Ni(OH)2 with 25% A1. The discharge capacity has been increased but the midpoint voltage has been decreased with the addition of Co. Chapter 9 - Summary

However, the mediate voltage is still higher than 1.20 V, the standard midpoint

plateau for β-nickel hydroxide. (A1,Co)- Ni(OH)2 samples show a promising cycle life. The discharge capacity obtained does not deteriorate even after 100

charge/discharge cycles. This indicates that (A1,Co)- Ni(OH)2 samples possess strong tolerance to the corrosive electrochemical cycle.

3. Temperature effects

The temperature effects on three types of nickel hydroxides, A1-Ni(OH)2 , (A1,Co)-

Ni(OH)2 and β –nickel hydroxide, have been investigated. The optimum electrochemical performance, highest discharge capacity, highest midpoint voltage and best cycle life occur at 0°C for the A1-substitued and (A1, Co)-substituted nickel hydroxides, while β-nickel hydroxide possesses its best properties at 25°C. On comparing their electrochemical performance at the different temperatures, the conclusion can be drawn that at 0°C and 25°C the quality of their performance is in the order of (A1,Co)-substituted nickel hydroxide > A1-substituted nickel hydroxide

> β-nickel hydroxide while at 50°C the order is (A1,Co)-substituted nickel hydroxide

> β-nickel hydroxide > A1-substituted nickel hydroxide. A1-substituted nickel hydroxide. All the samples show the poorest performance at 15°C, and among them that of A1-substituted nickel hydroxide is the best. The results shows that these two α

–phase nickel hydroxides had better electrochemical performance than β-nickel

hydroxide over the temperature range from -15°C to 50°C, and the addition of Co to

A1-Ni(OH)2 improved the electrochemical properties at 50°C.

Chapter 9 - Summary

The results from X-ray diffraction patterns show that the structure of A1-substituted

and (A1,Co)-substituted nickel hydroxide electrodes remained the turbostratic

disorder within the temperature range from -15°C to 50°C, even after 200

charge/discharge cycles. This implies that their turbostratic disorder structure can be

stablised under these conditions. Moreover, their crystalline size has been found to

become smaller with decreasing temperature. The pulverization of active materials

was also observed on the electrode surface after 200 cycles, especially when the

temperatures were -15°C and 50°C. This pulverization is thought to be the main cause

of cycle life deterioration. For β-nickel hydroxide, the XRD patterns show that the

prime (101) peak indexed to β-Ni(OH)2 becomes broadened and weak within the

temperature range from 0°C to 50°C after 200 cycles, and it nearly disappears at -

15°C. The (003) peak indexed to α –Ni(OH)2 is found in the pattern at 0°C. This

means that β-Ni(OH)2 is not stable after 200 charge/discharge cycles, particularly at

low temperatures.

4. Spherical nickel hydroxide

Spherical agglomerates of nanostructured beta-type Ni(OH)2 have been produced by

a spray drying technique. Compared with a commercial sample, this material features

a narrow Gaussian-type particle size distribution in the range of 0.1-10 micorns. It also has a high specific surface area of 50-200m2/g due to the nanosized dimensions of the primary crystals (10-25 nm). Moreover, the production method used costs less and is faster than the currently used method of controlled crystallization.

Chapter 9 - Summary

Spherical A1-substituted nickel hydroxide cannot be produced by the spray dry technique with the same parameters as for β-nickel hydroxide. By changing the air temperature to dry the samples in the product chamber, the spray speed of the slurry, and the ageing period of the nickel hydroxide slurry (precipitate re-dissolved in water), spherical agglometrate particles of A1-substituted nickel hydroxide were obtained. The results indicate that the spray dry technique can be used to produce spherical A1-substituted nickel hydroxide.

5. Solid polymer electrolytes

A new type of TMAH5-based alkaline solid polymer with addition of PTMA has been investigated as an electrolyte in sealed Ni-MH cells. The results show that the Ni/MH cells employing them are all dischargeable and indicate that they have a potential application the Ni/MH battery. The charge/discharge curves show that the pure TMA5 has the lowest final charge voltage, and the highest discharge capacity and plateau voltage, which in turn also indicate that pure TMAH5 is the optimum electrolyte candidate among the three samples studied at 25°C. The addition of PTMA hindered the electrodes’ performance. However, the properties of the solid electrolyte with addition of PTMA are significantly improved at 50°C, in particular for the sample with 15% PTMA with its performance very close to that of pure TMAH5.

References

Publications

Patent:

1. S. Zhong,. K. Konstantinov, C.Wang, S.X. Dou and H.K. Liu “Method for

production of spherical nickel hydroxide for rechargeable nickel-metal hydride

batteries” Australian Innovation Patent No 2002100001 (2002).

Papers:

1. C.Y. Wang, S. Zhong, D.H. Bradhurst, H.K. Liu and S.X. Dou “Ni/Al/Co- substituted α-Ni(OH)2 as electrode materials in the nickel metal hydride cell” J.

Alloys & Compounds 330-332, 802-805 (2002).

2. C.Y. Wang, S. Zhong, K. Konstantinov, G. Walter and H.K. Liu “ Structural study

of Al-substituted nickel hydroxide”, 148, 503-508 (2002).

3. H.K. Liu, B. Bright, C.Y. Wang, M. Lindsay and S. Zhong “ Effect of Zinc-Ion

additive to the positive electrode of rechargeable nickel-metal hydride batteries” J.

New Mat. For Electrochem. Systems 5, 47-52 (2002).

4. K. Konstantinov, S. Zhong, C. Wang, H.K. Liu and S.X. Dou “ Fabrication and

properties of spray-dried nanofeatured spherical Ni(OH)2 materials” J. Nanosci. &

Nanotech. 2, 6, 1533-4880 (2002).

References

5. C.Y. Wang, J. Sun, H.K. Liu and S.X. Dou, “TMHP- BASED POLYMER

ELECTROLYTE FOR NICKEL-METAL HYDRIDE BATTERY”, Proceedings of

the 204 th Electrochemical Society, Orlando, Florida, October 12-17, 2003.

6. C.Y. Wang, J. Sun, H.K. Liu, S.X. Dou, D. MacFarlace and M. Forsyth,

“POTENTIAL APPLICATION OF SOLID ELECTROLYTE P11OH IN NI/MH

BATTERIES”, Two-pages’s abstract has been accepted in “The International

Conference on Synthetic Metals (ICSM) 2004”, Wollongong, Australia, 28 June,

2004.

7. J. Wang, D. Zhou, J. Chen, C.Y. Wang, C.O. Too and G.G Wallace,

“Electrochemical synthesis of polypyrrole film using stainless steel mesh as substrate for battery application”, Two-pages’s abstract has been accepted in “The

International Conference on Synthetic Metals (ICSM) 2004”, Wollongong, Australia,

28 June, 2004.

References

References

1. T.N. Veziroglu, International Journal of Hydrogen Energy, 1, (2) 99-129

(1987).

2. J. Töpler, O. Bernauer, and H. Buchner, J. Less-Common Metals, 74, 385

(1980).

3. C. Folonari, G. Iemmi, F. Manfredi, and A. Rolle, J. Less-Common Met, 74, 371

(1980).

4. K. Nomura and Y. Ishido, Energy Conversion, 19, 49 (1979).

5. J.H.N.V. Vucht, F.A. Kuijpers, and H.C.A.M. Bruning, Philips Res. Repts., 25,

133 (1970).

6. P. Dantzer, Material Science and Engineering, A329-331, 313 (2002).

7. N. Furukawa, J. Power Sources, 54, 45 (1994).

8. P. Ruetschhi, F. Meli, and J. Desilvestro, J. Power Sources, 57, 87 (1995).

9. S.K. Dhar, S.R. Ovshhisky, P.R. Gifford, D.A.Corrigan, M.A. Fetcenko, and S.

Venkatesan, J. Power Sources, 65, 1 (1997).

10. N. Vassal, E. Salmon, and J.-F. Fauvarque, J. Electrochem. Soc., 146, 20 (1999).

11. J.T. Brown and M.G. Klein. in 12th Annual Battery Conference on Applications

and Advances. 1997. New York: IEEE.33

12. R. Barnard, C.F. Randell, and F.L. Tye, J.Appl. Electrochem., 10, 109 (1980).

13. P.V. Kamath, M. Dixit, L. Indira, A.K. Shukla, V.G. Lumar, and N.

Munichandraiah, J. Electrochem. Soc., 141, 2956 (1994).

14. M. Dixit and P.V. Kamath, J. Power Sources, 56, 97 (1995).

15. J.R.V. Beek, H.C. Donkersloot, and J.J.G. Willems, Power Sources 10, L.J.

Pierce, Editor. 1985. p. 317. References

16. G.S. Nagarajan and J.W.V. Zee, J. Power Sources. 70, 173 (1998).

17. S.R. Ovshinsky and M.A. Fetcenko, Russ. J. Science, 260, 176 (1993).

18. P. Ruetschi, F. Meli, and J. Desilvestro, J. Power Sources, 57, 85 (1995).

19. L. Schwalbach and T. Riesterer, Appl. Phys. A, 32, 169 (1983).

20. L.Y. Zhang, Rare earths science, technology and applications III, R.G. Bautista,

Editor. 1997, Warrendale: TMS. p. 225-227.

21. H. Bode, K. Dehmelt, and J. Witte, Electrochem. Acta., 11, 1079 (1966).

22. Z.S. Wronski, International Materials Reviews, 46, 1 (2001).

23. F. Feng, M. Geng, and D.O. Northwood, International Journal of Hydrogen

Energy, 26, 725 (2001).

24. M. Kanda, M. Yamamoto, K. Kanno, Y. Satoh, H. Hayashida, and M. Suzuki, J.

Less-Common Met., 172/174, 1227 (1991).

25. D. Linden, in Handbook of Batteries (2nd Ed.). 1995, McGraw-Hill Inc.: New

York, USA. p. P. 33.1.

26. L.B. Lave, C.T. Hendrickson, and F.C. McMichael, Science, 268, 993 (1995).

27. S.K. Dhar, S.R. Ovshinsky, P.R. Gifford, D.A. Corrigan, M.A. Fetcenko, and S.

Venkatesan, J. Power Sources, 65, 1 (1997).

28. T. Sakai, A. Takagi, K. Kinoshita, N. Kuriyama, H. Miyamura, and H. Ishikawa,

J. Less-Common Met., 172/174, 1194 (1991).

29. J.J.C. Kopera, Inside the Nickel Metal Hydride Battery. 2002, Texaco Ovonic

Battery System LLC. p. 5.

30. K. Hong, J. Power Sources, 96, 85 (2001).

31. R.P. Thomas and J.R. Gibb, J. Electrochem. Soc., 93, 198 (1948).

32. J.Völkl and G. Alefeld, in Diffusion in Solids, Recent Developments, A.S.

Nowick and J.J. Burton, Editors, Academic Press: New York. p. 232-270. References

33. H. Peisl, Physik in unserer Zeit, 2, 9 (1978).

34. G. Sandrock. in Int. Symp. On Metal-Hydrogen Systems-Fundamentals and

Applications. 1998: Hangzhou, China.PO: 01

35. G. Sandrock, J. Alloys Compd., 293-295, 877 (1999).

36. A. Anani, A. Visintin, K. Petrov, S. Srinivasan, J.J. Reily, J.R. Johnson, R.B.

Schwarz, and P.B. Desch, J. Power Sources, 47, 261 (1994).

37. G.G. Libowitz, H.F. Hayes, and T.R.P. Gibb, J.Phys. Chem., 62, 76 (1958).

38. J.J. Reily and R.H. Wiswall, Inorg. Chem., 13, 218 (1974).

39. J.J. Reily, Hydrogen Storage Materials, Batteries and (The

Electrochemical Society Proceedings Series), D.A. Corrigan and S. Srinivasan,

Editors. 1992, Pennington: New Jersey. p. 24.

40. J.J. Reily and J.R. Johnson. in First World Hydrogen Energy Conf. 1976. Coral

Gables, FL: Int. Assoc, Hydrogen Energy.P.8

41. D. Noreus, Z. Phys. Chem., 163, 575 (1989).

42. E.W. Justi, H.H. Ewe, A.W. Kalberlah, M.N. Saridakis, and M.H. Schaeffer,

Energy Conversion, 10, 183 (1970).

43. D. Shaltiel, I. Jacob, and D. Davidov, J. Less-Common Met., 53, 117 (1977).

44. Y. Gamo, Y. Moriwaki, T. Yamashita, and M. Fukuda, in U.S. Pat. 4, 144, 103.

1979.

45. D.G. Ivey and D.O. Northwood, Z. Phys. Chem., 147, 191 (1986).

46. R.M.V. Essen and K.H.J. Buschow, Mat. Res. Bull., 15, 1149 (1980).

47. F.Pourarian, H.Fuiji, W.E. Wallace, V.K. Sinha, and H.K. Smith, J. Phys.

Chem., 85, 3150 (1981).

48. J.H.N.V. Vucht, F.A. Kuijpers, and H.C.A.M. Bruning, 25 (1970) 133, Philips

Res. Repts. 1970. p. 133. References

49. K.H.J. Buschow, P.C.B. Bouten, and A.R. Miedema, Rep. Prog. Phys, 45, 937

(1982).

50. A. Percheron-Guegan, J.C. Achard, J. Sarradin, and G. Bronoel, in HYDRIDES

FOR ENERGY STORAGE, A.F. Andresen and A.J. Maeland, Editors. 1978,

Pergamon Press: Oxford. p. 485.

51. G.D. Sandrock, in Hydrogen Energy System, T.N. Veziroglu and W. Seifritz,

Editors. 1978, Pergamon Press: Oxford. p. 1625.

52. C. Iwakura and M. Matsuoka, Prog. Batteries Battery Mater., 10, 81 (1991).

53. H. Yukawa and M. Morinaga, in Advances in Quantum Chemistry, P. Löwdin,

J.R. Sabin, M.C. Zerner, E. Brändas, L. Kövér, J. Kawai, and H. Adachi,

Editors, Academic Press: New York. p. 83-108.

54. C. Delmazures, in Secondary Batteries, E.J. Wade, Editor. 1982, The Electrician

Printing and Publishing Co.: London. p. 130.

55. C. Delmas, C. Faure, L. Gautier, L. Guerlou-Demourgues, and A. Rougier, Phil.

Trans. R. Soc. Lond. A, 354, 1545 (1996).

56. F. Oliva, J. Leonardi, J.F. Laurent, C. Delmas, J.J. Braconnier, M. Figlarz, F.

Fievet, and A.d. Guibert, J. Power Sources, 8, 229 (1982).

57. A. Delahaye-Vidal, B. Beaudoin, N. Sac-Epée, K. Tekaia-Elhsissen, A.

Audemer, and M. Figlarz, Solid State Ionics, 84, 239 (1996).

58. in International Centre for Diffraction Data (JCDD), Newton Square: PA,USA.

59. J.D. Bernal and H.D. Megaw, Proc. R. Soc. London, Ser. A, 151, 384 (1935).

60. M. Figlarz, J. Guenot, and S.L. Bihan, C.R. Acad Sci. Paris, C270, 2131 (1970).

61. L. Neel, Nuovo Cimento, 6-X, Suppl., 3, 942 (1957).

62. P. Genin, A. Delahaye-Vidal, K. Tekaia-Elhsissen, P. Genin, and M. Figlarz,

Eur. J. Solid State Inorg. Chem., 28, 505 (1991). References

63. A. Delahaye-Vidal, K. Tekaa-Ehlsissen, P. Genin, and M.Figlarz, Eur. J. Solid

State Inorg. Chem., 31, 823 (1994).

64. O. Glemser and J. Einerhand, Z. Anorg. Allg. Chem., 261, 26 (1950).

65. A.N. Mansour, C.A. Melendres, M. Pankuc, and R.A. Brizzolara, J.

Electrochem. Soc., 141, L69 (1994).

66. C. Delmas, C. Fouassier, and P. Hagenmuller, Physica, 99 B, 81 (1980).

67. F.P. Kober, J. Electrochem. Soc., 112, 1064 (1965).

68. U. Palmqvist, l. Eriksson, J. García-García, N. Simic, E. Ahlberg, and R.

Sjövall, J. Power Sources, 99, 15 (2001).

69. C. Yang and P. Chen, J. Power Sources (Chinese), 23, 37 (1999).

70. R. Barnard, C.F. Randeli, and F.L. Tye, J. Appl. Electrochem., 10, 127 (1980).

71. B.C. Cornilsen, X.Y. Shan, and P.L. Loyselle, J. Power Sources, 29, 453 (1990).

72. D.A. Corringan and S.L. Knight, J. Electrochem. Soc., 136, 613 (1989).

73. A. Sugimoto, S. Ishida, and K. Hanawa, J. Electrochem. Soc., 146, 1251 (1999).

74. M. Watada, M. Ohnishi, Y. Harada, and M. Oshitani. in 34th Internet Power

Sources Symp. (IEEE). 1990.299

75. Z. Hengbin, L. Hansan, C. Xuejing, L. Shujia, and S. Chiachung, Materials

Chemistry and Physics, 79, 37 (2003).

76. V.G. Kumar, N. Munichandraiah, P.V. Kamath, and A.K. Shukla, J. Power

Sources, 56, 111 (1995).

77. C. Faure and C. Delmas, J. Power Sources, 35, 279 (1991).

78. G.H.A. Therese, P.V. Kamath, and J. Gopalakrishnan, J. Solid State Chem., 128,

38 (1997).

79. R.S. Jayashree and P.V. Kamath, J. Power Sources, 107, 120 (2002).

80. S.L. Bihan and M. Figlarz, J. Cryst. Growth, 13-14, 458 (1972). References

81. W. Feitknecht, Helv. Chim. Acta, 13, 500 (1930).

82. R. Allman, Chimia, 24, 99 (1970).

83. M. Dixit, P.V. Kamath, and J. Gopalakrishnan, J. Electrochem. Soc., 146, 79

(1999).

84. W. Stahlin and H.R. Oswald, Acta. Cryst., B26, 860 (1970).

85. R.D. Armstrong and E.A. Charles, J. Power Sources, 25, 89 (1989).

86. C. Faure, C. Delmas, M. Fouassier, and P. Willmann, J. Power Sources, 35, 249

(1991).

87. L. Demourgues-Guerlou and C. Delmas, J. Power Sources, 45, 281 (1993).

88. L. Demourgues-Guerlou, C. Denage, and C. Delmas, J. Power Sources, 52, 269

(1994).

89. K.T. Ehlisissen, A. Delahaye-Vidal, P. Genin, M. Figlarz, and P. Willman, J.

Mater. Chem., 3, 883 (1993).

90. B. Liu, Y. Zhang, H. Yuan, H. Yang, and E. Yang, International Journal of

Hydrogen Energy, 25, 333 (2000).

91. S. Akiko, I. Shintaro, and H. Kenzo, J. Electrochem. Soc., 146, 1251 (1999).

92. H. Chen, J.M. wang, T.Pan, H.M. Xiao, J.Q. Zhang, and C.N. Cao, International

Journal of Hydrogen Energy, 27, 489 (2002).

93. C. Tessier, C. Faure, L. Guerlou-Demourgues, C. Denage, G. Nabias, and C.

Delmas, J. Electrochem. Soc., 149, A1136 (2002).

94. J.-H. Choy, Y.-M. Kwon, K.-S. Han, S.-W. Song, and S.H. Chang, Materials

Letters, 34, 356 (1998).

95. D.-Y. Shin, in US Patent 5,498,403. 1996.

96. X. Wang, H. Luo, P.V. Parkhutik, A.-C. Millan, and E. Matveenva, J. Power

Sources, 115, 153 (2003). References

97. H. Gleiter, Prog. Mater. Sci., 33, 223 (1989).

98. M. Akinc, N. Jongen, J. Lemaitre, and H. Hofmann, Journal of the European

Ceramic Society, 18, 1559 (1998).

99. X. Xia, L.L. Shen, Z.P. Guo, H.K. Liu, and G. Walter, Journal of Nanoscience

and Nanotechnology, 2, 45 (2002).

100. C. Zhaorong, L. Gongan, Z. Yujuan, C. Jianguo, and D. Yunchang, J. Power

Sources, 74, 252 (1998).

101. S.R. Ovshinsky, B. Aladjov, R.T. Young, S. Venkatesan, and S.K. Dhar, in US

Patent 6, 086, 843. 2000.

102. P. Kelson, A.D. Sperrin, and F.L. Tye, in Power Sources 4, D.H. Collins, Editor.

1973, Academic Press: London.

103. D. Tuomi and G.J.B. Crawford, J. Electrochem. Soc., 115, 450 (1968).

104. Z. Takehara, M. Kato, and S. Yoshizawa, Electrochim, Acta, 16, 833 (1971).

105. J.P. Harivel, B. Morignat, J. Labat, and J.F. Laurent, in Power Sources 1966,

D.H. Collins, Editor. 1966, Academic Press, London. p. 239.

106. D.E. Fenton, J.M. Parker, and P.V. Wright, Polymer, 14, 589 (1973).

107. V. Wright, Br. Polymer J., 7, 319 (1975).

108. M.B. Armand, J.M. Chabagno, and M. Duclot. in Extended Abstracts of 2nd

International Meeting on Solid Electrolytes, St Andrews. 1978: Scotland

109. M. Kono, E. Hayashi, and M. Watanabe, J. Electrochem. Soc., 146, 1626 (1999).

110. M. Mohri, Y. Tajima, H. Tanaka, T. Yoneda, and M. Kasahara, Sharp Tech. J.,

34, 97 (1986).

111. T. Yoneda, S. Satoh, and M. Mohri, Sharp Tech. J., 38, 55 (1987).

112. N. Kuriyama, T. Sakai, H. Miyamura, A. Kato, and H. Ishikawa, J. Electrochem.

Soc., 137, 355 (1990). References

113. N. Kuriyama, T. Sakai, H. Miyamura, A. Kato, and H. Ishikawa, Solid State

Ionics, 40/41, 906 (1990).

114. N. Vassal, E. Salmon, and J.-F. Fauvarque, Electrochim. Acta, 45, 1527 (2000).

115. C. Iwakura, S. Nohara, N. Furukawa, and H. Inoue, Solid State Ionics, 148, 487

(2002).

116. C.-C. Yang, J. Power Sources, 109, 22 (2002).

117. E. Lifshin, X-ray Characterization of Materials. 1999, Weinheim, Germany:

Wiley-Vch Verlag. 37-39.

118. A.K. Shukla, V.G. Kumar, and N. Munichandraiah, J. Electrochem. Soc., 141,

2956 (1994).

119. L. Indira, M. Dixit, and P.V. Kamath, J. Power Sources, 52, 93 (1994).

120. J. Dai, S.F.Y. Li, T.D. Xiao, D.M. Wang, and D.E. Reisner, J. Power Sources,

89, 40 (2000).

121. K.S. Han, L. Guerlou-Demourgues, and C. Delmas, Solid State Ionics, 84, 227

(1996).

122. D.A. Corrigan and R.M. Bandert, J. Electrochem. Soc., 136, 723 (1989).

123. P.H.L. Notten and P. Hokkeling, J. Electrochem. Soc., 138, 1877 (1991).

124. S.I.C.d. Torresi, K. Provazi, M. Malta, and R.M. Torresi, J. Electrochem. Soc.,

148, A1179 (2001).

125. A. Yuan, S. Cheng, J. Zhang, and C. Cao, J. Power Sources, 77, 178 (1999).

126. K. Watanabe, M. Koseki, and N. Kumagai, J. Power Sources, 58, 23 (1996).

127. M. Oshitani, H. Yufu, K. Takashima, S. Tsuji, and Y. Matsumaru, J.

Electrochem. Soc., 136, 1590 (1989).

128. R.D. Armstrong, G.W.D. Briggs, and E.A. Charles, J. Appl. Electrochem., 18,

215 (1988). References

129. M. Oshitani, T. Takayama, K. Takashima, and S. Tshuji, J. Appl. Electrochem.,

16, 403 (1986).

130. H. Chen, J.M. Wang, T. Pan, Y.L. Zhao, J.Q. Zhang, and C.N. Cao, J.

Electrochem. Soc., 150, A1399 (2003).

131. C.Y. Wang, S. Zhong, D.H. Bradhurst, H.K. Liu, and S.X. Dou, J. Alloys

Compd., 330-332, 802 (2002).

132. R.J. Doran, Batteries: Proc. 3rd Int. Symp., D.H. Collins, Editor. 1963,

Pergamon Press: London.

133. K. Watanabe and N. Kumagai, J. Power Sources, 76, 167 (1998).

134. A.K. Sood. in Symposium on Nickel Hydroxide Electrodes. 1990:

Electrochemical Society.163

135. C.R. Davidson and S. Srinivasan, J. Electrochem. Soc., 127, (5) 1060 (1980).

136. H.G. Meier, J.R. Vilchie, and A.J. Arvia, J. Appl. Electrochem., 10, 611 (1980).

137. M. Oshitani, Y. Sasaki, and K. Takshima, J. Power Sources, 12, (1984).

138. D. Yunchang, L. Hui, Y. Jiongliang, and C. Zhaorong, J. Power Sources, 56,

115 (1995).

139. D.E. Reisner, A.J. Salkind, P.R. Strutt, and T.D. Xiao, J. Power Sources, 65,

231 (1997).

140. X. Wang, J. Yan, H. Yuan, Z. Zhou, D. Song, Y. Zhang, and L. Zhu, J. Power

Sources, 72, 221 (1998).

141. W. Shinjiro, in US Patent 5,861,131. 1997.

142. R. Young, S.R. Ovshinsky, and L. Xu, in U.S. Patent 5, 905,003. 1998.

143. D.T. Xiao, P.R. Strutt, B.H. Kear, H. Chen, and D.M. Wang, in US Patent

6,162,530. 2000. References

144. N. Kazuhiko, A. Kazunobu, K. Shigefumi, F. Kyioshi, H. Tsutomu, O. Shinji, N.

Hiroshi, Y. Minoru, and M. Hideto, in U.S. Patent 6,197,273. 2000.

145. S. Hiroyuki, I. Hidekatsu, K. Hirokazu, I. Yoichi, and M. Isao, in U.S. Patent

6,129,902. 2000.

146. J. Chen, D.H. Bradhurst, S.X. Dou, and H.K. Liu, J. Mater. Res., 14, 1916

(1999).

147. J. Chen, D.H. Bradhurst, S.X. Dou, and H.K. Liu, J. Electrochem. Soc., 146,

3606 (1999).

148. G. Gille, S. Albrecht, J. Meese-Marktscheffel, A. Olbrich, and F. Schrump, Solid

State Ionics, 148, 269 (2002).

149. A.F. Wells, Structural inorganic chemistry, 4th Ed. 1975, Oxford: Clarendon

Press.

150. L. Glasser, Chem. Rev., 75, 21 (1975).

151. A. Clearfield, Chem. Rev., 88, 21 (1975).

152. J. Sun, D.R. MacFarlane, and M. Forsyth, Electrochem. Acta., 48, (14-16) 1971

(2003).

153. W.H. Zhu, G.-D. Zhang, D.-J. Zhang, and J.-J. Ke, J. Chem. Tech. Biotechnol.,

69, 121 (1997).

154. S. Sato, R. Ikeda, and D. Nakamura, J. Chem. Soc., Faraday Trans., 82, 2053

(1986).

155. D. Mootz and R. Seidel, J. Inclusion. Phenom., 8, 139 (1990).