Investigation of si anode for LI ion batteries

Figure 5.1: Cycling performance of nanoporous Si with different residual film thickness.. 97 Figure 5.2: SEM image of nanoporous Si with ~ 100 nm residual film thickness a before and b a

INVESTIGATION OF SILICON ANODE FOR

LITHIUM ION BATTERIES

OMAMPULIYUR SWAMINATHAN RAJAMOULY

(M Eng., MIT; B Eng (Hons.), NUS)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILIOSOPHY

NANO- SYSTEMS (AMM&NS)

NATIONAL UNIVERSITY OF SINGAPORE

2013

Declaration

I hereby declare that this thesis is my original work and it has been written by me in its entirety I have duly acknowledged all the sources of

information which have been used in the thesis

This thesis has also not been submitted for any degree in any university

previously

Omampuliyur Swaminathan Rajamouly

17 January 2013

Acknowledgements

This project would not have been feasible without the guidance, support and encouragement of many people I express my heartfelt gratitude to Prof Choi (SMA, NUS) for his invaluable guidance, unflinching encouragement and constant belief in me The weekly meetings with Prof Choi were crucial for me to be constantly cognizant of the project objectives and it helped me through many a lean patch Prof Fitzgerald (SMA, MIT) has been a great inspirational figure and was the main reason for me to embark upon the anode investigation project, for which I thank him earnestly Prof Thompson (SMA, MIT) has helped me greatly by guiding me through several critical twists and turns of the project and I thank him sincerely Prof Lu (NUS) allowed me to use his laboratory facilities for half-cell assembly and I

am greatly indebted to him Prof Thong’s (NUS) lab was of indispensable value for my SEM and EDX analyses and I am very thankful to him Prof Shao-Horn (MIT) helped me gain important electrochemical insights with respect to the project and I genuinely thank her

I would like to thank Koo Chee Keong for his help with numerous SEM and EDX analyses This project wouldn’t have become a reality without the help of Xia Hui and Zhu Jing in assembling several half-cells, almost every week Maruf has been of great help in this project and aided me with many an explorative discussion, and I am immensely thankful to him I am very much indebted to Mohammed Khalid and Zheng Han for all their help in the Glancing Angle Deposition of gold I am sincerely thankful to Zheng Fei,

Tze Haw and Yun Jia; my senior laboratory mates for bringing me up to speed with respect to all the operating procedures of laboratory equipment I am also very thankful to Lim Walter (lab officer), Hong Yanling (SMA) and Juliana (SMA) for all the help during this project Over the course of this project my laboratory mates: Yudi, Ria, Zongbin, Cheng He, Changquan, Bihan, Zhu Mei, Thi and Jia Xin have helped one way or the other and I thank them all I would also like to thank Lin Thu and Wang Kai for all their help and wish them the very best in their projects

I am greatly indebted to all my friends for their support and encouragement over the course of this project I thank my friend Krishandan for helping me with the thankless and frustrating task of proof-reading my thesis

The ever growing fountain of scientific knowledge has only been made possible by the ardent works of earlier intellectual stalwarts and was

succinctly expressed by Isaac Newton in : “If I have seen further, it is by

standing on the shoulders of giants” I would like to take this opportunity to

express my boundless gratitude to all the researchers whose works have been the basis and inspiration for my work

“Parents, Teachers and only then god” has been a timeless Indian

adage, which underscores the importance of parents in our lives I owe my existence and all my opportunities to my kind and loving parents My parents have given me infinite support and immense encouragement over the course of

my life and this research project Though words cannot express the gratitude I feel for them, I try and sincerely thank them for all the help, support and love

Table of Contents

Acknowledgements ii

Summary x

List of Tables xii

List of Figures xiii

List of Acronyms xx

Chapter 1 Introduction 1

1.1 Background 1

1.2 Motivation 2

1.3 Organization of Thesis 3

Chapter 2 Literature Review 5

2.1 Introduction 5

2.2 Battery 5

2.2.1 Primary Batteries 8

2.2.2 Secondary Batteries 9

2.2.3 Lithium-ion Battery 10

2.3 Electrolytes for LIB 15

2.4 Cathodes for LIB 17

2.5 Anodes for LIB 18

2.5.1 Intercalation and de-intercalation 19

2.5.2 Conversion Reaction 20

2.5.3 Alloying and de-alloying 20

2.6 Silicon Anode 25

2.6.1 Si powder anodes 27

2.6.2 Si dispersed in inactive or active matrix 28

2.6.3 Si thin films 30

2.6.4 Si nanostructures 33

2.7 Solid Electrolyte Interface (SEI) layer 38

2.8 Nanofabrication 40

2.8.1 Metal Assisted Chemical Etching 41

2.8.2 Glancing Angle Deposition 43

2.8.3 Laser Interference Lithography 44

2.9 Summary 47

Chapter 3 Experimental Details 49

3.1 Introduction 49

3.2 Nanoporous and nanopillar Si 49

3.3 Stainless Steel substrate preparation 52

3.4 Silicon Sputtering 55

3.5 BHF Cleaning 59

3.6 Glancing Angle Deposition of Gold 60

3.7 MACE for nanoporous Si 62

3.8 Laser Interference Lithography for Nanopillars 64

3.8.1 Photo Resist Coating 64

3.8.2 Llyod’s mirror setup, exposure and development 65

3.8.3 O2 Plasma 68

3.9 Thermal Deposition of Gold 69

3.10 MACE for nanopillar Si 70

3.11 Thin-film thickness measurement 71

3.12 Weight measurement 72

3.13 Scanning Electron Microscopy 73

3.14 Energy Dispersive X-ray Spectroscopy 75

3.15 Half-cell assembly and testing 75

Chapter 4 Silicon - Thin film 81

4.1 Introduction 81

4.2 Effect of Si thin film thickness 81

4.3 Effect of RF sputtering power 84

4.3.1 EDX analysis 86

4.3.2 Voltage profile and differential capacity 87

4.3.3 Thermodynamic calculations 90

4.4 Effect of oxygen incorporation on cycling performance 92

4.5 Summary 94

Chapter 5 Nanoporous Si 96

5.1 Introduction 96

5.2 Residual film 96

5.3 Native Oxide Layer 99

5.4 Charging rate vs capacity 102

5.5 Effect of nanoporous Si thickness 104

5.6 750 nm nanoporous Si – 50 cycles 110

5.7 Summary 111

Chapter 6 Nanopillar Si 113

6.1 Introduction 113

6.2 200 nm nanopillar Si 113

6.3 450 nm nanopillar Si 115

6.4 750 nm nanopillar Si 116

6.5 Charging rate vs capacity 120

6.6 Areal specific capacity comparison 122

6.7 Summary 123

Chapter 7 Conclusions 124

7.1 Summary 124

7.2 Recommendations 127

Bibliography 130 Appendix - List of Publications / Conferences 144

Summary

In this work, we investigate the performance of Silicon as rechargeable Li-ion battery (LIB) anode The objective of this study was to evaluate the effect of sputtering parameters on the anode performance of Si thin film, and

to investigate nanostructuring via Metal Assisted Chemical Etching (MACE)

as a possible methodology to mitigate the debilitating problem of volume expansion of Si on lithiation Though Si has a much higher theoretical specific capacity (4200mAhg-1) than commercially used carbon (370mAhg-1) based anodes, Si undergoes large volume expansion (400%) which often leads to pulverization and limits the use of Si as an anode material

We have observed that Si thin film anode’s cycling performance greatly depends on the thickness of the thin film, in that for thicknesses greater than 200 nm the cycling performance is significantly degraded Therefore, if one wants to increase the areal capacity any further while maintaining good cycling performance, nanostructuring is the direction to pursue Also, increasing the RF sputtering power during Si film deposition resulted in higher gravimetric capacity and first cycle Coulombic Efficiency (CE), due to decreased oxygen incorporation Oxygen incorporation helped in enhancing cycling performance with the tradeoff of reduced specific capacity

Nanoporous Si were fabricated via Glancing Angle Deposition (GLAD) of gold followed by Metal Assisted Catalytic Etching (MACE) We have found that the residual layer thickness after MACE is critical in ensuring good cycling performance, as removing it completely would mean peel off of

the nanoporous layer from the substrate but too thick a residual film resulted

in degraded cycling performance To ensure good cycling performance the residual layer thickness should be limited to a maximum of 100 nm Native surface oxide played a role in controlling the CE of the anode during cycling and it is paramount that any native surface oxide is removed before half-cell assembly to ensure high CE Nanoporous Si layer has one-dimensional nanostructures and consequently had good rate performance We also observed that thinner nanoporous Si layer does not cycle as well as thicker nanoporous Si layer as thinner nanoporous Si layer was not sufficiently mechanically robust during cycling Though 750 nm nanoporous Si was found

to cycle very well initially, their cycling performance eventually degraded due

to pulverization and peel off in certain areas of the substrate

Ordered Si nanopillars were fabricated via Laser Interference Lithography (LIL) and gold deposition followed by MACE Good cycling performance was observed for Si nanopillars of different heights: 200, 450 and

750 nm, due to their mechanical robustness over many cycles This is important as it ensures that to increase areal capacity further, higher nanopillars can be used Nanopillars being one-dimensional nanostructures had good rate performance Nanopillar Si performed significantly better with respect to areal specific capacity compared to nanoporous and thin film Si Further increase in areal capacity by increasing the height of the nanopillars could not be investigated in this work due to the slow deposition rates during the initial thin film sputtering

List of Tables

Table 2.1: Primary Batteries [46] 9

Table 2.2: Secondary Batteries [46] 10

Table 2.3: Li-Si system summary at 415 oC [85], [86] 25

Table 3.1: Summary of sputtering power 57

Table 4.1: Thin film deposition rate vs RF power 87

Table 7.1: Si anode summary 126

Table 7.2: Nano-structured Si anode comparison 127

List of Figures

Figure 2.1: Energy Storage Domains [43] 6

Figure 2.2: Zn | Cu cell 7

Figure 2.3: Portable Secondary Battery market [47], [48] 11

Figure 2.4: Total LIB market revenue and growth forecasts, 2006-2016 [48] 11

Figure 2.5: Comparison of the different battery technologies in terms of volumetric and gravimetric energy density [3] 12

Figure 2.6: First C/LiCoO2 battery [1] 14

Figure 2.7: Charge Discharge in 18650-cell [55] 15

Figure 2.8: Electrolytes' Li Ionic conduction [1] 17

Figure 2.9: Generic Voltage vs Li Concentration for a single phase system [78] 22

Figure 2.10: Generic Voltage vs Li Concentration for a two phase system [79] 22

Figure 2.11: Theoretical Capacity and Voltage vs Li for alloying anodes 24

Figure 2.12: Li-Si phase diagram [88] 26

Figure 2.13: Galvanostatic profile for micro-Si (10 um) anode (for different cycle numbers) [5] 26

Figure 2.14: Cycling behavior of SiOx with different oxygen content and particle [17] 29

Figure 2.15: Cycling performance profiles of silicon film electrodes with

different thicknesses [24] 32

Figure 2.16: Cycle life of 50nm a-Si thin film anode vs designated charge capacity [27] 33

Figure 2.17: Capacity vs cycle number (a) for Si nanowires at C/20 [92], (b) for Si nanowires at C/5 rate [30] 35

Figure 2.18: Cycling of Si nanowires made by MACE [34] 36

Figure 2.19: Nanosphere cycling [39] 37

Figure 2.20: Si nano -pillar and -well cycling [40] 38

Figure 2.21: GLAD deposited Si cycling [93] 38

Figure 2.22: SEM image of (a) Si nanowires etched with Au catalyst and H2O2/HF solution [114] (b) Si nanowires etched in an Ag(NO)3/HF solution [120] 42

Figure 2.23: Schematic diagram of GLAD without substrate rotation, (a) Initial ad-atoms condensation and shadowing (b) Columnar film development from ad-atom nuclei, and (c) directed growth towards the flux [124] 44

Figure 2.24: GLAD nanostructures with constant α (a) no substrate rotation, (b) substrate rotation by 180o, (c) slow rotation of substrate, and (d) faster rate of rotation of substrate [124] 44

Figure 2.25: Schematic diagram illustrating the formation of standing wave from a two-beam-interference [127] 45

Figure 2.26: Total dose distribution on PR due to the superposition of two perpendicular exposures [129] 46

Figure 2.27: SEM image of PR pattern with (a) 90o rotation in between exposures and (b) 30o rotation in between exposures [130] 46

Figure 3.1: Schematic for fabrication of nanoporous Si 50

Figure 3.2: Schematic for fabrication of nanopillar Si 51

Figure 3.3: SS surface after polishing and acetone cleaning 54

Figure 3.4: SS surface after polishing and HF cleaning 54

Figure 3.5: Sputtering [134] 56

Figure 3.6: SS Substrate held in position during sputtering with the help of clean room tape 59

Figure 3.7: Schematic diagram of the GLAD set up [137] 61

Figure 3.8: Au clusters after GLAD [41] 62

Figure 3.9: Nanoporous Si 63

Figure 3.10: Lloyd’s Mirror Interference Lithography setup [139] 65

Figure 3.11: Double exposure Interference Lithography [139] 66

Figure 3.12: Tilt view of patterned PR 67

Figure 3.13: Top view of patterned PR 68

Figure 3.14: O2 Plasma to remove residual PR 68

Figure 3.15: Schematic of a thermal evaporator [135] 69

Figure 3.16: Ordered Si nanopillars 71

Figure 3.17: Thickness measurement using step profiler 72

Figure 3.18: Microbalance 73

Figure 3.19: Schematic diagram of a SEM 73

Figure 3.20: Schematic of half-cell assembly 76

Figure 3.21: Discharge / Charge cycle of a Si anode with Li/Li+ counter

electrode 78

Figure 4.1: Cycling performance of n-Si thin films with different thickness sputtered at 85W 82

Figure 4.2: CE of Si thin films with different thickness 83

Figure 4.3: Surface of (a) Si thin film before cycling, (b) 200 nm Si thin film after 50 cycles, (c) 550 nm Si thin film after 50 cycles and (d) 1.1 um n-Si thin film after 50 cycles 84

Figure 4.4: Discharge and Charge Capacity vs cycle of 140 nm Si thin film samples sputtered at 30W, 65W and 85W 85

Figure 4.5: Coulombic efficiency vs cycle of 140 nm Si thin film samples sputtered at 30 W, 65 W and 85W 85

Figure 4.6: EDX of Si sputtered at (a) 85 W, (b) 60 W, (c) 30 W and (d) 30 W sputtered in O2 86

Figure 4.7: Voltage profile for samples sputtered at (a) 30W, (b) 60W and (c)

Figure 5.1: Cycling performance of nanoporous Si with different residual film

thickness 97

Figure 5.2: SEM image of nanoporous Si with ~ 100 nm residual film thickness (a) before and (b) after cycling 97

Figure 5.3: SEM image of nanoporous Si with ~ 300 nm residual film thickness (a) before and (b) after cycling 97

Figure 5.4: SEM image of nanoporous Si with ~ 500 nm residual film thickness (a) before and (b) after cycling 98

Figure 5.5: 450 nm nanoporous Si (a) with BHF dip and (b) without BHF dip 100

Figure 5.6: Cycling Performance of samples with and without BHF dip 100

Figure 5.7: Effect of BHF dip on CE 101

Figure 5.8: Capacities of nanowire samples with different surface oxide thickness [159] 101

Figure 5.9: Nanoporous Si Capacity vs Current density 103

Figure 5.10: Rate performance of VLS nanowires [86] 104

Figure 5.11: Cycling of nanoporous Si with different thickness 105

Figure 5.12: (a) Voltage Profile & (b) Differential Capacity of 450 nm nanoporous Si; (c) Voltage Profile & (d) Differential Capacity of 750 nm nanoporous Si 106

Figure 5.13: SEM images of 450 nm nanoporous Si (a) before cycling, (b) after cycling acetone wash, and (c)&(d) after cycling DIW rinse at different magnifications 107

Figure 5.14: SEM images of 750 nm nanoporous Si (a) before cycling, (b) & (c) after cycling and acetone wash at different magnifications, and (d) after

cycling DIW rinse 107

Figure 5.15: Top view of 750 nm nanoporous Si after cycling and acetone clean 109

Figure 5.16: 750 nm nanoporous Si cycling performance – 50 cycles 110

Figure 5.17: 750 nm nanoporous Si after cycling (a) Big islands, (b) Big and small islands, and (c) complete peel off 111

Figure 6.1: SEM image of 200 nm nanopillar Si (a) before cycling, (b) after cycling & DIW rinse 114

Figure 6.2: 200 nm nanopillar Si cycling 114

Figure 6.3: SEM image of 450 nm nanopillar Si (a) before cycling, (b) after cycling & acetone rinse, and (c) after cycling & DIW rinse 115

Figure 6.4: 450 nm nanopillar Si cycling 116

Figure 6.5: 750 nm nanopillar Si cycling 118

Figure 6.6: SEM image of 750 nm nanopillar Si (a) before cycling, (b) after cycling & acetone rinse, (c) & (d) after cycling & DIW rinse at different magnifications 119

Figure 6.7: (a) Voltage Profile & (b) Differential Capacity of 750 nm nanopillar Si 119

Figure 6.8: Nanopillar Si: Capacity vs Current density 121

Figure 6.9: Rate performance of nanopillar array (SPA) [40] 121

Figure 6.10: Areal specific capacity comparison 122

Figure 7.1: Total battery specific capacity as a function of anode specific capacity (Cc is the cathode specific capacity) [89] 129

List of Acronyms

BHF: Buffered Hydrogen Fluoride

CE: Coulombic Efficiency

CVD: Chemical Vapor Deposition

DIW: De-Ionized Water

EDX: Energy-dispersive X-ray

GLAD: Glancing Angle Deposition

LIB: Lithium-Ion Battery

LIL: Laser Interference Lithography

MACE: Metal Assisted Chemical Etching

PECVD: Plasma Enhanced Chemical Vapor Deposition

PR: Photo Resist

RF: Radio Frequency

RIE: Reactive-Ion Etching

SEI: Solid Electrolyte Interface

SEM: Scanning Electron Microscopy

SS: Stainless Steel

TEM: Transmission Electron Microscopy

VLS: Vapor-Liquid-Solid

XRD: X-Ray Diffraction

Chapter 1 Introduction

1.1 Background

The interest in battery technologies has sky rocketed since the introduction of a plethora of handheld devices in the consumer market and also due to the increased need for low emission vehicles Batteries have come

a long way from the primitive galvanic cell with liquid electrolyte to the commercially available solid-state electrolyte Li Ion batteries (LIBs) that are commonly used in laptops, hand phones etc LIBs are rechargeable batteries, which make them environmentally friendly and satisfy the increasing energy storage requirements of hand-held electronic devices The anode of choice for LIBs is carbon which was made commercially available in early 1990s [1] The commonly used carbon based anodes will not be a suitable candidate for rechargeable batteries in the future due to their low specific capacity (372mAhg-1) [2], [3] A lot of research has been devoted to silicon (Si) as a possible LIB anode due to its very high theoretical specific capacity of 4200 mAhg-1 [4] Although Si has a very high specific capacity, a large increase in volume (approximately 400% with the formation of Li22Si5 alloy) during lithiation prevents good cycling performance of Si anodes

To mitigate the problem associated with volume expansion of the Si anode on lithiation, many approaches have been researched such as: Si powder anodes [5]-[8], Si dispersed in an inactive or active matrix [9]-[22], Si thin films [23]-[27] and Si nanostructures [28]-[40]

In this work we investigated sputtered Si thin films and nanostructures etched down from sputtered Si films as LIB anodes

1.2 Motivation

Si nanostructures fabricated on metal current collectors without the aid

of any other composites or host/binder material will allow for maximum capacity extraction from the Si anodes Nanostructured anodes have the following advantages:

1 High surface-area-to-volume ratio ensures that the strain caused by volume expansion during lithiation can be effectively handled

2 The vacant space in between the nanostructures provides enough room for the material to expand without pulverization

3 Without any composites or binders, one-dimensional nanostructures like nanowires or nanopillars are directly connected to the current collector ensuring that each individual nanostructure takes part in lithiation and delithiation This results in higher gravimetric capacity compared to anodes with binders or composites

4 Shorter Li diffusion lengths and increased surface area allow for higher power density and results in good anode rate performance

Thin film anodes cannot cycle well if the thickness is above 200 nm (as we will see in Sections 2.6.3 and 4.2) and to increase areal specific

capacity of Si anodes, nano-structuring is an effective direction The need to increase areal specific capacity and in view of the advantages enumerated above, Si nanowires and nanopillars were investigated in this work The nanowires were fabricated via Glancing Angle Deposition (GLAD) of Au followed by Metal Assisted Catalytic Etching (MACE) [41] The nanopillars were fabricated via Laser Interference Lithography (LIL) and Au deposition followed by MACE [42]

Sputtered Si thin film formed the basis for our nanostructure fabrication, and therefore we performed a thorough investigation of the effect

of sputtering parameters such as RF sputtering power and oxygen incorporation on anode performance Nanowire fabrication used in this work was based on a lithography-free bottom-up technique, making it an easy and relatively cheap technique Nanowire fabrication used in this work is also relatively cheap and easy, as the lithography technique is mask-less

In Chapter 3, various experimental procedures employed in the preparation, fabrication and electrochemical characterization of the various samples prepared in this study will be explained

In Chapter 4, the battery performance of Si thin film anodes will be discussed The effect of thickness and RF sputtering power on Si thin film anode performance, along with the effect of oxygen incorporation in the thin film on cycling performance will be presented

In Chapter 5, the battery performance of nanoporous Si anodes will be discussed The effect of un-etched residual film, native surface oxide and nanowire length on anode performance, along with the rate performance nanowire anode will be presented

In Chapter 6, the battery performance of ordered Si nanopillar anodes will be discussed Cycling performance of Si nanopillars of different heights will be presented along with their rate performance This chapter will conclude with a discussion on areal specific capacity of all the anodes investigated and rationale for the enhanced cycling performance of nanopillars

The thesis will conclude with Chapter 7, which provides a summary of the performance of all the different Si anodes The last section of this chapter will enumerate the scope for future research on nanostructured Si

Chapter 2 Literature Review

by a broad survey on LIB anodes Given the scope and motivation of our research, works done with respect to Si anodes have been carefully reviewed The chapter will end with a review of the literature and theory behind the nano-fabrication techniques used in this research work

appreciate that among the devices that store their energy within (no external fuel tank); batteries have the highest energy density

Figure 2.1: Energy Storage Domains [43]

This places batteries in a special category of electrochemical storage devices The interest in battery technologies has sky rocketed with the introduction of a plethora of handheld devices in the consumer market and also due to increased need for low emission vehicles In 2009, President of the USA, Barack Obama announced US $ 2.4 billion in funding for advanced battery and electric drive vehicles research of which US $ 1.5 billion was for new battery technologies [44] This underscores the commitment and interest

in the research and development of new battery technologies

Though the word battery is used interchangeably with cell, it should be noted that the basic electrochemical unit is referred to as a cell A battery could consist of many cells connected in parallel or series In a cell, two electrodes are connected in such a way that there are two independent paths for electronic and ionic conduction The electrodes are often physically

separated by an ionic conductor which is called the electrolyte The electrode that is more easily oxidized resulting in free electrons is called an anode (negative electrode) The electrode that accepts the electrons from the anode and undergoes reduction is called a cathode (positive electrode) To illustrate this, let us consider a simple Zn | Cu cell as shown in Figure 2.2

Figure 2.2: Zn | Cu cell

The aqueous solution in which the metal electrodes are submerged along with the porous membrane acts as the electrolyte The standard reaction potentials for the reduction of Zn and Cu are [45]:

This illustrates how the stored chemical energy is converted in to electrical energy to drive a load Eventually, Zn metal will get spent completely or all Cu2+ (aq) will be deposited on the cathode as Cu metal

Batteries fall into two general classifications: primary rechargeable) and secondary (rechargeable) batteries Secondary batteries can

(non-be recharged by forcing a current in the opposite direction thereby reversing the redox reactions or any other structural changes

2.2.1 Primary Batteries

A primary battery is usually a lightweight and packaged source of power which is inexpensive and suitable for portable electronic and electric devices, flash lights, toys, etc Primary batteries are used where there is a need

to provide portable power without depending on electric supply The advantages of primary batteries are good shelf life, high energy density at low

to moderate discharge rates, little if any maintenance, and ease of use [46] High capacity primary batteries are also used in military applications and high energy battery packs but majority of primary batteries are cylindrical (used in flash lights and TV remotes) and flat button cells (used in watches) which have now been popular for decades There are a variety of anode-electrolyte-cathode systems for primary batteries Alkaline Zn - MnO2 (1.5 V) and Li - MnO2 (3.0 V) are the most popular types, although they come in many different types and geometries [46] Table 2.1 summarizes some of the common types of primary batteries

Table 2.1: Primary Batteries [46]

Zn - C 1.5 Lowest cost, low energy density and

poor low-temperature performance

Zn

-Chloride

1.5 Low cost, better energy performance

compared to Zn - C but lower than alkaline systems

Zn -

MnO2

1.5 High capacity compared to

Zn - C and good low – temperature performance at a moderate cost

Li - MnO 2 3.0 High energy density, good

low-temperature and high-rate performance

2.2.2 Secondary Batteries

Secondary batteries can be recharged by forcing a current in the opposite direction thereby reversing the redox reactions or any structural changes The applications for this kind of batteries are increasing at a rapid pace owing to the need for rechargeable energy storage systems for hand-held electronics and growing concern of using eco-friendly alternatives (rechargeable batteries can be charged and used many times unlike primary batteries) Typically, secondary batteries are used in hand phones, uninterrupted power systems, hybrid electric vehicles etc., where the batteries are not removed from the devices during charging There are also secondary batteries that are removed and charged separately such as rechargeable AA or AAA cells and these are common in power tools and digital cameras Secondary batteries are also referred to as storage batteries or accumulators in common parlance Table 2.2 gives a short summary of some popular

secondary batteries Note that in each category there are many variations and the table shows a synopsis of only the popular variations

Table 2.2: Secondary Batteries [46]

Cd (Anode), NiOOH (Cathode), KOH (aq.) (electrolyte)

Metal Hydride (Anode), NiOOH (Cathode), KOH (aq.) (electrolyte)

C (Anode), LiCoO2

(Cathode) Non-aq Organic solvent

Good energy density and high rate performance

at a reasonable cost and sealed unlike Pb - acid

High energy density and good cycle life with cost in between

Ni - Cd and Li - ion

High energy density, low self-discharge with long

cycle life

Applications Starting

Lighting Ignition (SLI)

in vehicles

Old consumer electronics like pagers

Aerospace applications and electric vehicles

New generation

of hand-held electronic devices like hand phones and tablets

2.2.3 Lithium-ion Battery

Though Ni-Cd and Ni-MH are still in use in certain niches, Lithium Ion Battery (LIB) has a phenomenal market share as seen in Figure 2.3, which shows the market and growth forecasts for LIB and other portable secondary batteries In 2010, LIBs had close to five times the revenue of Ni-based batteries Growth rate for LIB market is predicted to be above 10% whereas for Ni - Cd it is predicted to be negative 3.1% The market numbers show that

LIB is currently the most popular portable secondary battery and the growth rate for LIB market is predicted to be remarkable in the coming years as seen

in Figure 2.4

Figure 2.3: Portable Secondary Battery market [47], [48]

Figure 2.4: Total LIB market revenue and growth forecasts, 2006-2016 [48]

The reasons for the initial investigation and continued popularity of LIB are as follows [2], [3]:

 High operation voltage due to Li being the most electropositive metal (-3.04 V vs standard hydrogen electrode)

-40 -30 -20 -10 0 10 20

2010 Market Revenue and Growth

 Lightest metal (atomic weight M = 6.94 g mol–1, and specific gravity ρ = 0.53 g cm–3) resulting in high energy density

 Low self-discharge rate

 Wide range of operation temperature

Figure 2.5: Comparison of the different battery technologies in terms of volumetric and

gravimetric energy density [3]

A comparison of volumetric and gravimetric energy densities is shown

in Figure 2.5 and it can be seen that LIB has the highest volumetric and gravimetric energy densities, allowing them to occupy lesser space and weigh considerably less The high electro-positivity of Li along with its low atomic mass and specific gravity results in high energy density for LIB as energy density is the product of specific capacity and average potential at which the electrochemical reactions occur

The basis of LIB operation is that, the transfer of Li+ ions through the electrolyte is balanced by the transfer of electrons through an external circuit

The first rechargeable LIB was made possible by Whittingham’s work on TiS2

[49], which showed that titanium disulfide could reversibly interact with Li to form an intercalation compound, LixTiS2 Li metal was used as the anode and TiS2 was used as the cathode in the first Li / TiS2 cell Li metal anode oxidizes releasing Li+ which travels through the electrolyte to interact with Ti3+ formed

by the reduction of Ti4+ (Ti4+ was reduced by the electrons which were transferred through the external circuit), to form LiTiS2 When all the available Ti4+ is reduced, the cell is fully discharged and could be recharged

by forcing electrons from the cathode to anode, thereby oxidizing Ti4+ to Ti3+and depositing Li+ as Li metal on the anode Even though this set up had a good operating voltage and capacity it proved unviable due to severe shape changes and Li metallic dendrite formation [50] The erratic dendrite formation of Li metal pierced through the electrolyte to form an electrical short with cathode Large current caused due to the shorting of electrodes in conjunction with the inflammable electrolyte solution meant potential fire and explosion hazards

Graphite was found to electrochemically interact with Li+ to form intercalated LixC6 [51] and this meant graphite could be used as an anode instead of the hazardous Li-metal Graphite’s layered structure enabled the insertion of Li+ ions between the carbon planes and theoretically one Li+ could

be inserted for every 6 C atoms, consequently having a theoretical capacity of

372 mAhg-1 The lithiation of the graphite occurs at ~0.1 V vs Li/Li+ and this meant a high cell voltage and therefore good energy density could be achieved Mizushima et al found LiCoO2 to be a potential candidate for

cathode by having higher potential vs Li/Li+ than TiSi2, allowing for higher cell voltages [52] The class of materials, LixMO2 (where M is Co, Mn) was found to be ideal for cathodes in LIBs [53] and they continue to be the cathode material class of choice [54] All this culminated in the first commercialized LIB production by Sony with LiCoO2 as the cathode and graphite as the anode called the 18650 cylindrical cells in the 1990s [1] and the salient operational parameters for this LIB is shown in Figure 2.6 A schematic illustration of the cell operation is shown in Figure 2.7 During the discharge cycle a load is driven by the electrons flowing from the anode and Li+ travels through the electrolyte from the graphite anode to the LiCoO2 cathode When the anode is devoid of Li+ (complete discharge), the cell can be recharged by forcing a current into the cathode

Figure 2.6: First C/LiCoO 2 battery [1]

Figure 2.7: Charge Discharge in 18650-cell [55]

2.3 Electrolytes for LIB

Electrolytes act as ionic conductors and electronic insulators while staying in physical contact with both the electrodes The following are the criteria in choosing a good electrolyte for LIB [56], [57]:

 In order to achieve low cell resistance and high power performance, the electrolyte should have very low ionic resistivity

 The electrolyte should be stable enough to avoid decomposition

of the solvent compounds at the electrolyte-electrode interface (reductive and oxidative environment present near the anode and cathode, respectively) It should also be able to withstand the cell operation voltage range

 The boiling point of the electrolyte should be sufficiently high

to prevent evaporation and pressure buildup within the cell

 It is preferred for the electrolyte to be non-toxic and inexpensive

Liquid electrolytes consist of a liquid non-aqueous organic solvent and

a dissolved Li salt Gel electrolytes are organic liquid electrolytes (Solvent and

Li Salt) impregnated into host polymers such as poly(vinylidene fluoride) [58] and their ionic conductivity increases with increasing incorporation of the liquid electrolyte Dry polymer electrolytes are prepared by dissolution of lithium salts into polyether [59] and Li ionic conductivity steeply drops with temperature Generally, solid amorphous oxides have poor Li ion conductivity, but LiPON has been shown to have an ionic conductivity of 3.3x10-6 S cm-1[60] and it can be sputtered from Li3PO4 in nitrogen ambient making it an electrolyte of choice for thin film solid-state batteries Amorphous (Li3PO4-63Li2S-36SiS2 [61]) and crystalline (Li3.25Ge0.25P0.75S4 [62]) sulfide solid electrolytes have also been reported to have good Li ionic conductivities, but are difficult to sputter But amongst all these electrolytes, liquid electrolyte (with LiPF6 salt) has the best Li ion conductivity next only to aqueous solutions as seen in Figure 2.8 Co-solvent, ethylene carbonate mixed with diethyl carbonate (1:1 vol.) has been found to be a good solvent combination for a variety of electrodes [63] Liquid electrolytes are ideal when experimenting with nanostructures with high aspect ratio as they have no problem in coating the entire electrode LiPF6 (1.0M) (salt) dissolved in 1:1 ethylene carbonate: diethyl carbonate (organic solvent) was chosen as the electrolyte in this work due to its high Li ion conductivity and conformity of the electrolyte when used in conjunction with nanostructured electrodes

Figure 2.8: Electrolytes' Li Ionic conduction [1]

2.4 Cathodes for LIB

Cathodes for LIB should act as a source of Li as the anodes (such as graphite) are not lithiated in the beginning Therefore, the cathodes must be stable Li-based compounds so that the anode can be lithiated before use This

is unlike Li-metal batteries where Li metal is the anode LiCoO2 is widely used as the cathode in commercial LIBs because Li intercalation/de-intercalation in LiCoO2 happens around 4V vs Li/Li+, and LiCoO2 has exhibited good cycling performance [54], [64] Though LiNiO2 was considered for LIB cathode as it had higher specific capacity than LiCoO2

[65], it proved untenable for safety reasons due to exothermic reactions with the electrolyte and occasional collapse of the lithiated structure Though LiCoO2 has a high theoretical specific capacity (274 mAhg-1) only 50% of it can be utilized, because a high concentration of Co4+ due to Li+ removal leads

to decomposition of the cathode material caused by the interaction of Co4+

with the electrolyte [66] LiMnO2 has good specific capacity (270 mAhg-1) [67] but the layered phase suffers from structural instabilities [55] Olivine LiFePO4 has good chemical stability and specific capacity (110 mAhg-1) [68], but suffers from poor electronic and Li ion conductivity [54] Li(Ni1/3Mn1/3Co1/3)O2 is isostructural to LiCoO2 and has a good specific capacity (160 mAhg-1) in the high voltage range coupled with good high power performance [54] For these reasons it is touted to be the second generation cathode for commercial LIBs In our work, LIB cathodes do not play a significant role as we use Li as the counter electrode to Si electrode to make our half-cells

2.5 Anodes for LIB

Anodes in LIB act as the electron providers while driving of an external load The materials used as anodes in LIB can be broadly classified into the following:

1 Materials where Li storage and removal happens via intercalation and de-intercalation of Li in the anode

2 Materials where Li storage and removal happens via chemical conversion reactions

3 Materials where Li storage and removal happens via alloying and de-alloying of the anode material with Li

2.5.1 Intercalation and de-intercalation

Carbon is an ideal example to illustrate the storage of Li via intercalation The insertion of Li into carbon proceeds according to the reaction shown in Eq (2.3),

by Li intercalation between graphene sheets with an in-plane LiC2 structure [56], [57] and possible Li storage in nanocavities [72]

Spinel-Li4Ti5O12 is another material where Li storage happens via intercalation and de-intercalation This material has very small volume change upon lithiation resulting in reduced strain, thereby making it a good candidate for LIB anode Spinel-Li4Ti5O12 has a theoretical capacity is 175 mAhg-1 and hollow spherical structures of this material has shown a specific capacity of

140 mAhg-1 [73] The low specific capacity of this material makes it untenable