HIGH ENERGY DENSITY BATTERY FOR WEARABLE ELECTRONICS AND

SENSORS

Thesis

Submitted to

The School of Engineering of the

UNIVERSITY OF DAYTON

In Partial Fulfillment of the Requirements for

The Degree of

Master of Science in Electrical Engineering

By

Asha Palanisamy

Dayton, Ohio

December, 2016

HIGH ENERGY DENSITY BATTERY FOR WEARABLE ELECTRONICS AND

SENSORS

Name: Palanisamy, Asha

APPROVED BY:

______Guru Subramanyam, Ph.D. Jitendra Kumar, Ph.D. Advisory Committee Chairman Advisory Committee Co-Chairman Department Chair and Professor Senior Research Scientist Department of Electrical and University of Dayton Research Computer Engineering Institute

______Vamsy Chodavarapu, Ph.D. Committee member Associate Professor Department of Electrical and Computer Engineering

______Robert J. Wilkens, Ph.D., P.E. Eddy M. Rojas, Ph.D., M.A., P.E. Associate Dean for Research and Innovation Dean, School of Engineering Professor School of Engineering

ii

© Copyright by

Asha Palanisamy

All rights reserved

2016

iii ABSTRACT

HIGH ENERGY DENSITY BATTERY FOR WEARABLE ELECTRONICS AND

SENSORS

Name: Palanisamy, Asha University of Dayton

Advisors: Dr. Guru Subramanyam & Dr. Jitendra Kumar

Wearable electronics and sensors are being extensively developed for several applications such as health monitors, watches, wristbands, eyeglasses, socks and smart clothing. Energy storage devices such as rechargeable batteries make wearable devices to become more independent from power outlets, or in other words, make device portability.

Battery energy density determines how long a battery powered device will work before it needs a recharge. Longer the time before battery needs recharge, better it is for device applications. Therefore, the goal of battery researchers and engineers is to develop a battery that can provide high energy density and longer device operation. The state-of- the-art battery is the lithium- battery (LIB) technology outperforming any other battery for the aforementioned applications. Even LIB is limited in energy storage

(energy density ~200 Wh/kg) and requires frequent battery charge. Some other major challenges associated with LIBs are high cost, low cycle life (restricted to 500 – 1000 cycles), safety, and negative environmental impacts.

iv Further improvement in LIB is very limited as the technology is reaching the theoretical limit and therefore new battery technology with greater energy density and overall better performance must be developed in order to match the ever increasing power demand in fast growing electronics. Lithium sulfur battery (LSB) (energy density

~2600 Wh/kg) is one of the most promising batteries for next generation energy storage, enabling approximately 10 times more energy storage in LSB than LIB. Furthermore, sulfur is inexpensive, abundant and environmental friendly. Therefore, LSB is expected to be more economical, safe and environmentally sustainable compared to LIB. However, performance (cycle life, thermal stability and safety) of current LSB technology do not meet commercialization standards at current development stage and thus, open for further technological advancements. My thesis focuses on the development of novel materials that when successfully developed will improve the overall performance of LSB and can meet commercialization standards by combining thermal and dendrite-proof solid ion conducting ceramic based (no liquid spillage) along with solid state and flexible S- being developed in the Electrochemical Energy Systems Laboratory at the University of Dayton Research Institute (UDRI) of University of Dayton (UD).

v ACKNOWLEDGEMENTS

I am deeply indebted to my advisor, Dr. Jitendra Kumar, who introduced me to the subject ‘Solid state batteries’ and thrived my interest on this new technology. His guidance and inspiration helped me to complete this work.

I am grateful to my department chair, Dr. Guru Subramanyam, for his incessant support and encouragement.

My heartfelt thanks to Dr. Priyanka Bhattacharya for aiding me in my research work through her brilliant ideas and valuable thoughts.

Special thanks to Dr. Vamsy Chodavarapu for his valuable suggestions on my research work.

I would like to thank the Department of Electrical and Computer Engineering, University of Dayton (UD), for the financial support that allowed me to pursue this research work and the University of Dayton Research Institute (UDRI), for providing me with the opportunity to work in an excellent lab.

Finally, my deepest gratitude goes to my family and friends for their unflagging love and motivation throughout my studies.

vi TABLE OF CONTENTS

ABSTRACT ……………………………………………………………….………….....iv

ACKNOWLEDGEMENTS……………………………………………………….…...... vi

LIST OF FIGURES………………………………………………………………...... x

LIST OF TABLES…………………………………………………………………….....xii

LIST OF ABBREVATIONS AND NOTATIONS…………………………………...... xiii

CHAPTER 1 INTRODUCTION……………………………………………….…………1

1.1 Wearable Electronics and Need for Flexible Energy Storage Systems

(Batteries)……………………………………………………………………...1

1.2 The State-of-the-Art Battery Technology: Lithium-Ion Battery (LIB)...... 7

1.2.1 Working Principle of LIB………………………………………....9

1.2.2 Challenges in LIB…………………………..……………….…...11

1.2.3 Solutions to the Challenges of LIB…………………….....……...11

1.3 Next Generation Li Batteries…….………………………………….…….....12

1.4 Next Generation Battery: Lithium-Sulfur Battery (LSB) ………...……..…..13

1.4.1 Working Principle of LSB….…………………………………....13

1.4.2 Challenges in LSB…………..…………………………….……..16

1.4.3 Solutions to the Challenges of LSB…….....……………….….....17

1.5 Liquid Electrolyte vs. Solid Electrolyte…………………………...……...... 18

1.6 LAGP Solid Electrolyte…………………………………...………………....19

vii 1.7 Comparison of LIB and LSB Technologies……………………..………..….19

1.8 Thesis Objectives………………………………………….………………....22

CHAPTER 2 SOLUTIONS TO THE CHALLENGES OF LSB…………………….….23

2.1 Components and Fabrication of Li-S Cell ….………………………...... ….23

2.1.1 Tools Used to Fabricate Cell Components.……….………………..24

2.2 Li-S Cell Fabrication and Electrochemical Testing…...... …………………...24

2.2.1 Tools Used for Testing the Li-S Cells…...……………….………..26

2.3 Effect of Solid Electrolyte (LAGP) Based S-Cathode on Li-S Performance..26

2.3.1 Preparation of S-Cathode with LAGP …………....……...……...26

2.3.2 Preparation of S-Cathode without LAGP …………...………...... 27

2.3.3 Fabrication of Li-S with and without LAGP Based S-Cathode.....27

2.3.4 Testing of Li-S with and without LAGP Based S-Cathode...……28

2.3.5 Battery Performance Comparison…...…………………………...28

2.4 Effect of Concentration of S-Cathode (PVDF) on Li-S Performance.30

2.4.1 Preparation of S-Cathode with Varying Concentration of PVDF

Binder (5 wt% and 10 wt%) ……...……………..………...…..…31

2.4.2 Fabrication and Testing of Li-S with Variable PVDF

Concentrations………………………………………………...…31

2.4.3 Battery Performance Comparison………...………………...……31

2.5 Effect of Solid Electrolyte (LAGP) Based on Li-S Performance....33

2.5.1 Fabrication of LAGP Based Polyethylene (PE) Separator…....…34

viii 2.5.2 Preparation of S-Cathode of Li-S with LAGP Coated PE

Separator…………………………………………………...…….34

2.5.3 Fabrication and Testing of Li-S with LAGP Coated PE

Separator…..………...……………..…………………………….35

2.5.4 Battery Performance Comparison….…………………………….35

CHAPTER 3 CNT COATED PAPER CURRENT COLLECTOR FOR S-

CATHODES….…..…………………………………………………………...…….…...38

3.1 Flexible CNT Paper-Based vs Traditional Aluminum Metal Current

Collector…………...………………………………..…………………...…….....38

3.1.1 Preparation of the CNT Solution for CNT (P-B) Substrate...... 40

3.1.2 Preparation of S-Cathode with CNT (P-B) Substrate….……...…...40

3.1.3 Fabrication and Testing of Li-S with CNT Coated Flexible Current

Collector………………………….………………..……………………..41

3.1.4 Battery Performance Comparison………………………………. …41

CHAPTER 4 CONCLUSIONS AND FUTURE WORK…………………………...... 44

REFERENCES……………………………………………………………………...…...46

ix LIST OF FIGURES

Figure 1.1 Wearable Devices Requiring Flexible Batteries…………………………..…...2

Figure 1.2 Samsung’s Note 7 Explosion Due to Battery Failure……………………..…...4

Figure 1.3 (a) Battery Folding Technique (Kirigami) for Rigid Unit Battery Cell Block...5

Figure 1.3 (b) Flexible Substrate Based Flexible LIB Battery for Wearable Devices..…..5

Figure 1.4 (a) Working Principle of LIB…...……………………………………...…….10

Figure 1.4 (b) Charge-Discharge Voltage Profile of Li-Ion Cell (with LCO Cathode)....10

Figure 1.5 Specific Energies of Potential Battery Technologies Useful for Wearable

Devices and Sensors..…………………………………………………………………....12

Figure 1.6 (a) Working Principle of LSB…………………………………………….….14

Figure 1.6 (b) Charge-Discharge Profile of LSB…………..……………………….…....14

Figure 1.6 (c) Li-S Cell and Dissolution of Polysulfides Issue…..……….……………..14

Figure 1.7 LAGP-coated PE Solid Electrolyte……………………..…………………....19

Figure 1.8 (a) First Charge/Discharge Capacity of Li-Ion and Li-S Cells…………….....21

Figure 1.8 (b) Cyclability of Li-Ion and Li-S Cells…………………………………..….21

Figure 2.1 Assembling of Cell Components in Li-S Cell……………………………..…23

x Figure 2.2 (a) Flexible Lithium ……………………………………………..……25

Figure 2.2 (b) Flexible S-cathode……….…………………………………………….…25

Figure 2.2 (c) Flexible LAGP-Coated PE Separator………………………………….…25

Figure 2.3 (a) First Charge/Discharge Capacity with/without LAGP in the Cathode…...29

Figure 2.3 (b) Cyclability of Li-S Cell with/without LAGP in the Cathode………...... 29

Figure 2.4 (a) First Charge/Discharge Capacity with 5 wt% and 10 wt% of PVDF binder in the S-cathode……………..…………………………………………………………....32

Figure 2.4 (b) Cyclability of Li-S Cells with 5 wt% and 10 wt% of PVDF binder in the

S-cathode……………..…………………………………………………………….…….32

Figure 2.5 (a) First Charge/Discharge Capacity of Li-S Cells with/without LAGP in the

PE Separator……………………………………………………………………………...36

Figure 2.5 (b) Cyclability and Coulombic Efficiency (%) of Li-S Cells with/without

LAGP in the PE Separator………………………………………………………….……36

Figure 3.1 (a) Flexible CNT Paper-Based Current Collector Substrate …………….…..39

Figure 3.1 (b) S-cathode with CNT Paper-Based Current Collector Substrate….……....39

Figure 3.2 (a) First Charge/Discharge Capacity with Aluminum Foil and CNT Paper-

Based Current Collector Substrate in the S-cathode…………………...….………..……42

Figure 3.2 (b) Cyclability of Li-S Cells with Aluminum Foil and CNT Paper-Based

Current Collector Substrate in the S-cathode………………………...…………..……...42

xi LIST OF TABLES

Table 1.1 State-Of-The-Art Cathode Materials for LIBs and their Corresponding

Electrochemical Parameters…………………………………………………………..…...8

Table 1.2 Liquid Electrolyte vs. Solid Electrolyte………………………...…………..…18

Table 1.3 Comparison of LIB and LSB Technology………………………………….....20

xii LIST OF ABBREVIATIONS AND NOTATIONS

LIB Lithium Ion Battery

LSB Lithium Sulfur Battery

CCS Customer Communication Services

CPU Central Processing Unit

GPS Global Positioning System

BMS Battery Management System

Li Lithium

S/S8 Sulfur

EV

LiC6 Lithiated Graphite

C Carbon

LiPF6 Lithium Hexaflurophosphate

LiClO4 Lithium Perchlorate

LiBF4 Lithium Tetrafluroborate

xiii LiCF3SO3 Lithium Triflate

LiAsF6 Lithium Hexafluroarsenate Monohydrate

EC Ethylene Carbonate

PC Propylene Carbonate

EMC Ethyl Methyl Carbonate

VC Vinyl Carbonate

DMC Dimethyl Carbonate

LiCoO2/LCO Lithium Cobalt Oxide

LiMn2O4 Lithium Manganese Oxide

LiNiMnCoO2 Lithium Nickel Manganese Cobalt Oxide

LiFePO4 Lithium Iron Phosphate

LiNiCoAlO2 Lithium Nickel Cobalt Aluminum Oxide

Li4Ti5O12 Lithium Titanate

Li-ion Lithium-ion

Li-O2/Li-air Lithium-air

Pb-acid Lead acid

Ni-Cd Nickel-Cadmium

Ni-MH Nickel Metal Hydride

xiv Zn-air Zinc-air

LiTFSI Lithium Bis(Trifluromethanesulfonyl)imide

LiNO3 Lithium Nitrate

DOL 1,3-Dioxolane

DME Dimethoxyethane v/v Volume/Volume

Li-S Lithium-Sulfur

LiPS Lithium Polysulfides

Li2S Lithium Sulfide

LAGP Lithium Aluminum Germanium Phosphate

PE Polyethylene

PEO Polyethylene Oxide

CNT Carbon Nanotubes

S-cathode Sulfur cathode

PVDF Polyvinylidene Fluoride

NMP N-methyl-2-pyrrolidinone

OCV Open-Circuit Voltage

MWCNT Multi-walled Carbon Nanotubes

xv Al Aluminum

EB PVD Electron Beam Physical Vapor Deposition

SEI Solid Electrolyte Interface

P-B Paper-Based

Oct-SWCNT Octadecylamine functionalized single-walled carbon nanotubes

CC Constant Current

xvi CHAPTER 1

INTRODUCTION

This chapter outlines the need for flexible batteries in wearable devices, describes the state-of-the-art Lithium-ion battery (LIB) technology, limitations of LIB, next generation high energy density battery (e.g. lithium-sulfur battery, LSB), and finally compares LIB and LSB technologies in terms of their performance, energy density, and cyclability.

1.1 Wearable Electronics and Need for Flexible Energy Storage Systems (Batteries)

The demand for wearable electronics such as smart watches, smart goggles, smart shoes, smart clothing, and fitness trackers keeps increasing every day. According to

Customer Communication Services (CCS) insight, 84 million wearable devices were sold in 2015 (US $15 billion) and expected to sell 245 million (US $25 billion) wearables by

2019.[1] Smart watches are the highest sold electronics among the wearables. However, a major problem with smart watches is that they drain the battery quickly due to the consumption of energy by various components such as a display, Central Processing Unit

(CPU), Global Positioning System (GPS) and bluetooth operation. For example,

Samsung’s Gear S2 smart watch uses a LIB with a capacity of 300 mAh and typically the battery lasts only for 2-3 days.[2] Some of the wearable devices that require flexible

1 batteries are shown in Figure 1.1.

Figure 1.1: Wearable Devices Requiring Flexible Batteries[3]

The use of a rigid battery in Samsung’s smartwatch may add up to the total weight (watch weight 51g) of the smart watch and makes it less attractive for wearable devices and hinder the progress of newer and better electronics. However, improving battery performance is not an easy task as compared to improving electronics itself.

Moore’s law states that the number of transistors used in an integrated circuit doubles every two years. This is very true in today’s electronics as can be seen from the speed of current computers (up to 4GHz in 2016) compared to earlier computers (740 KHz to

8MHz in the 1970s).[4] This leads to a better performance, although desirable at lower costs and higher energy efficiency in the electronic market. However, the development of batteries doesn’t follow Moore’s law. This is because the that carry charge in batteries are larger than the electrons in the transistors and hence take up more space in the form of cell components – anode, cathode and electrolyte.[5]

Doubling a battery’s energy storage capability every two years is quite challenging because of the limited battery chemistries available. For example, doubling

2 the energy density of tesla’s LIBs took over a decade. However, the other battery performances and cost were not compromised. New battery chemistries with inherently higher theoretical capacities may bring faster advancement in battery technology if their limitations are addressed in a timely manner.[6] Development of high energy storage batteries with high performance and low cost is important for today’s electronics.

For wearable applications, batteries need to be flexible, thin, lightweight, with high energy (that can last long before the battery needs re-charging), and environmentally compatible. The state-of-the-art LIB does not possess all the aforementioned required wearable battery characteristics and mostly employs flammable liquid . The use of hazardous liquid electrolytes may cause the battery to explode due to internal short-circuiting or over-charging due to malfunctioning of battery management system

(BMS). Therefore, an alternative safer battery with higher energy density must be explored. After 35 legitimate reports of battery explosion in the latest Note 7, Samsung stopped sales & shipping of their new Samsung Galaxy Note 7 across the globe. The problem behind the Samsung’s battery explosion is mainly due to use of highly flammable liquid electrolyte in the LIB (3500 mAh), and possible malfunctioning of

BMS. The separator which was used to separate the anode and cathode might got punctured due to the growth of lithium dendrites on the anode and this may have led the of the battery to touch each other which in turn led to internal short-circuiting of the battery. The rise of battery’s internal temperature due to internal short-circuit can heat up the liquid electrolyte leading internal gas pressure build-up and caused the battery to explode (Thermal runaway) (Figure 1.2).[7]

3

Figure 1.2: Samsung’s Note 7 Explosion Due to Battery Failure[8]

The one possible solution to this problem is to replace flammable liquid electrolyte with non-flammable solid electrolyte and developing better BMS. The focus of present study relates to the use of solid electrolyte and determines battery performance.

Beyond LIB technology, LSB technology is most prominent and theoretically can pack up to 10 times more energy than LIB. The use of a bendable/flexible high energy density metal anode(lithium, Li) and an environmentally benign, low cost, high energy density S-cathode in LSB makes it suitable to be employed in wearable electronics. This requires all components of the battery to be bendable/flexible. Moreover, in order to make wearable batteries safe, the electrolyte used in the battery must be solid and non- flammable to avoid any health and safety hazards due to flammable liquid spillage under abuse conditions.

Furthermore, designing a flexible battery for wearable electronics is another key challenge. To a certain extent, flexibility in a non-flexible battery can be achieved through connecting rigid cells with a flexible (spring) string. Researchers from the

4 Arizona State University developed a flexible LIB using a technique called “Kirigami paper folding technique” (Figure 1.3 (a)) that as mentioned above would allow certain degree of flexibility and in near term expected to create greater impact in the future smart watches.

(a)

(b)

Figure 1.3: (a) Battery Folding Technique (Kirigami) for Rigid Unit Battery Cell

Block[9] and (b) Flexible Substrate Based Flexible LIB Battery for Wearable Devices[9]

5 They replaced the original rigid LIB in Samsung’s Gear S2 smart watch with their paper-Kirigami based LIB which was sewed to the elastic band and found to be functional after several bendings and stretchings (Figure 1.3 (b)).[9] The demo LIB was able to provide only 80 mAh which was much less than the original battery capacity (300 mAh). It is expected that much more capacity can be achieved (up to 700 mAh), if battery’s surface area can be increased, for example, making a battery which can fit (if flexible) and replace conventional (non-energy storage, only mechanical in nature) strap of smart watch. Though the state-of-the-art LIB is preferred for most wearables due to its high performance, cycling stability and cell voltage, the energy that it can store is limited to 200 Wh/kg. To make them flexible, energy densities of LIB are significantly compromised. Same problem will persist with any other battery technology if energy storage device is not made inherently flexible. Therefore, developing a new battery with higher energy than LIB and making all the cell components flexible (in order to make the whole battery flexible) can be a viable path to achieve goals of advancing modern and future wearables. With LSB, the energy density of the battery can be as high as 2600

Wh/kg, and at the same time if battery can be made flexible without use of folding techniques as practiced in prior research, presents most promising solution. Making a flexible LSB is easier than making a flexible LIB as lithium is already flexible, and LSB cathode are much thinner and energy denser ((1675 mAh/g)[10] compared to the cathode of LIB and can be easily diffused in a suitable flexible (carbon) matrix. Therefore, full flexibility in LSB can be easily realized if electrolyte can be made flexible and solution free.

6 LSB technology is considered to be the promising near-future battery technology due to its ability to store energy (1675 mAh/g) that is 3-5 times higher than the LIB technology.[10] These batteries are also relatively light due to the low and moderate weight of lithium (Li) and sulfur (S).[11] S is abundant in nature and thus LSB when fully developed will be cheaper and less detrimental to the environment compared to LIB.

However, there are quite a few challenges faced by the LSB before they can be commercialized and they are discussed later in this chapter.

1.2 The State-of-the-Art Battery Technology: Lithium-Ion Battery (LIB)

Today’s state-of-the-art LIBs are widely used in portable electronic devices

(laptops, smartphones, tablets, digital cameras and wearable devices), space vehicles, power grid support (micro-grid), and electric vehicles (EVs) ), providing long cycle life and decent specific energy (150-200 Wh/kg). LIBs comprise of three main cell components- anode (negative ), cathode (positive electrode) and electrolyte.

Typically the LIBs use a graphite or carbon anode (C) whose capacity ranges between 300 and 350 mAh/g.[12] Non-aqueous electrolytes comprising of salts such as lithium hexaflurophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluroborate (LiBF4), lithium triflate (LiCF3SO3) and lithium hexafluroarsenate monohydrate (LiAsF6) along with a combination of organic solvents selected from ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), vinyl carbonate (VC), and dimethyl carbonate (DMC) are also employed.[13]

LiPF6 or LiCF3SO3 dissolved in aforementioned solvents are most commonly used electrolytes among the others. Lithium Cobalt Oxide (LiCoO2) is the majorly used

7 cathode material in LIBs with a specific energy of 150-200 Wh/kg. Some of the other cathode materials used in LIBs depend on the type of applications and they are listed in table 1.1.

Table 1.1: State-Of-The-Art Cathode Materials for LIBs and their Corresponding

Electrochemical Parameters [14]

Cathode Voltages Specific Applications Advantages Drawbacks materials energy

(capacity)

Lithium Nominal – 150-200 Mobile High Limited Cobalt 3.6V Wh/kg phones, specific power, Oxide Typical tablets, energy short cycle (LiCoO2) operating laptops and life, safety range – 3.0 - cameras risks and 4.2V/cell expensive Lithium Nominal – 100-150 Power tools, High power, Low Manganese 3.7V (3.8V) Wh/kg medical long cycle specific Oxide Typical devices, and life, energy (LiMn2O4) operating electric inexpensive, range – 3.0 - powertrains thermal 4.2V/cell stability and safety Lithium Nominal – 150-220 E-bikes, EVs, high power, Low Nickel 3.6V, 3.7V Wh/kg medical and long cycle specific Manganese Typical industrial life, and energy Cobalt operating equipment safety Oxide range – 3.0 - (LiNiMnC 4.2V/cell oO2)

Lithium Nominal – 90- Portable and Long cycle Low Iron 3.2V, 3.3V 120Wh/kg stationary life, thermal specific Phosphate Typical needing high stability, energy (LiFePO4) operating load currents and safety

8 range – 2.5 - and endurance 3.65V/cell Lithium Nominal – 200- Medical High Unsafe and Nickel 3.6V 260Wh/kg; devices, specific costly Cobalt Typical 300Wh/kg electric power energy and Aluminum operating predictable train and good cycle Oxide range – 3.0 - industrial life (LiNiCoAl 4.2/cell O2)

Lithium Nominal – 70- Electric Good cycle Low Titanate 2.4V 80Wh/kg powertrain, life and fast voltage, low (Li4Ti5O12) Typical solar powered charging specific operating street lighting, energy and range – 1.8 - and expensive 2.85V/cell Uninterruptibl e Power Supply

1.2.1 Working Principle of LIB

A cathode (e.g. LiCoO2) and an anode (e.g. graphite) separated by a porous polymer separator soaked in liquid or gel or solid electrolyte constitute a LIB. During charge (energy storage), Li+ de-intercalate from the cathode, transfer to the anode via the electrolyte where they get stored, and electrons enter the anode through the external circuit to maintain an electrical balance (Equation 1). The discharge (energy release/generation) process is the reverse of the charge process in which Li+ de- intercalate from the anode, transfer to the cathode via the electrolyte and get stored in the cathode, and electrons are released to an external circuit that can be fed to a load

(Equation 2). Thus, LIBs work on the principle of intercalation chemistry (depicted in the

Figure 1.4 (a)). Figure 1.4 (b) shows the typical voltage-capacity curve of a LIB.

9 [15] 퐿푖퐶표푂2 + 퐶6 → 퐿푖1−푥 퐶표푂2 + 퐶6퐿 푥 ……………………… (1)

[15] 퐿푖1−푥 퐶표푂2 + 퐶6퐿푥 → 퐿푖퐶표푂2 + 퐶6 ………….…………... (2)

(a)

(b)

Figure 1.4: (a) Working Principle of LIB [16] and (b) Charge-Discharge Voltage Profile of

Li-Ion Cell (with LCO Cathode) [17]

The cell in Figure 1.4 (b) was cycled at a current of 8 mA which corresponds to

C/20 rate.[17] The C-rate of a battery is a measure of the rate at which a battery is discharged/charged relative to its maximum capacity. A 1C rate means that the current will discharge/charge the entire battery in 1 hour. The initial charge and discharge

10 capacities of the cell were 126 mAh/g and 111 mAh/g.[17] The discharge capacity was observed to be lesser than the charge capacity (Figure 1.4 (b)). This might be due to side reactions of the graphite anode with the electrolyte (also known as formation of solid electrolyte interface (SEI)) which consume Li+ and reduce the cell coulombic efficiency

(CE = discharge capacity/charge capacity).

1.2.2 Challenges in LIB

Even though LIBs are widely used and most advanced battery today, they are faced with several challenges including

 Highly flammable organic liquid electrolyte: responsible for reduced battery

safety.

 Limited energy density (200 Wh/Kg): responsible for limited device operation

time.

 Expensive cathode material (LCO): responsible for high battery cost.

 Dendrite formation on anode: responsible for battery electrical short and battery

fire hazards. Also, irreversible chemical reaction between anode and electrolyte

and cathode/electrolyte responsible for limited battery cycle and overall battery

life.

 Adverse ecological impacts from battery material components.

1.2.3 Solutions to the Challenges of LIB

The above challenges may be overcome by developing cost-effective battery chemistries with higher energy densities than the state-of-the-art Li-ion technologies, and replacing the flammable organic liquid electrolytes with inflammable and highly

11 conducting solid electrolytes to enhance battery safety. These recommendations form the basis of this thesis and will be discussed in detail below.

1.3 Next Generation Li Batteries

As discussed in section 1.2, the current state-of-the-art LIBs are practically limited to a capacity of ∼200mAh/g [18] and an energy density of ∼200 Wh/kg which is not sufficient to meet future requirements of wearable electronics that desire prolonged working duration on single charge.

LSBs and lithium-air (Li-O2) battery technologies are the most promising candidates for meeting future demands of the wearable devices market in terms of energy density and cost. The specific energies of some of the current and future battery technologies which can potentially be used for wearable devices are shown in Figure 1.5.

Figure 1.5: Specific Energies of Potential Battery Technologies Useful for Wearable

Devices and Sensors[19] [20]

12 The lighter shades in the bar indicate the theoretical specific energies and darker shades indicate the practical specific energies of the battery. As can be seen from the above figure, the lithium-air battery technology has a higher theoretical energy density of

11,680 Wh/kg than all other battery chemistries known today.[21] However, the practical energy density achieved by Li-air batteries is very low and research is still in early stage and will take many years to overcome the challenges of Li-air batteries. The focus of this thesis is on the next generation LSB technology which is currently in advanced stage of development and on the verse of commercialization in very near future.

1.4 Next-Generation Battery: Lithium-Sulfur Battery (LSB)

The LSB is a with high theoretical specific capacity of 1675 mAh/g and energy density of 2600 Wh/kg.[10] They are also low weight systems compared to LIBs because of the low atomic weight of Li and moderate weight of S.[11]

Furthermore, S is a promising cathode due to its high energy density, low-cost, abundance, and non-toxicity.

1.4.1 Working Principle of LSB

The working principle of LSB involves chemical reactions as depicted in Figure

1.6 (a). The LSB employs a metallic Li anode (negative electrode), a S-based cathode

(positive electrode) and an organic liquid electrolyte usually consisting of 1M LiTFSI/0.1

M LiNO3 (lithium bis(trifluromethanesulfonyl)imide/lithium nitrate) dissolved in DOL

(1,3-dioxolane) and DME (1,2-dimethoxyethane) (1:1=v/v). A polymer separator separates the anode and cathode.

13 (a)

(b)

(c)

Figure 1.6: (a) Working Principle of LSB

(b) Charge-Discharge Profile of LSB[10] and (c) Li-S Cell and Dissolution of Polysulfides

Issue[22]

14 During discharge, Li+ from the Li anode move towards the S cathode through the electrolyte where it reduces elemental sulfur (S8) to form lithium polysulfides (LiPS).

Electrons liberated at anode passes through external load and enter cathode to complete and continue electrochemical reaction. The following reaction takes place during discharge.

[11] 푆8 → 퐿푖2푆8 → 퐿푖2푆7 + 퐿푖2푆6 → 퐿푖2푆5 → 퐿푖2푆4 → 퐿푖2푆3 → 퐿푖2푆2 → 퐿푖2푆 … (3)

During charge, Li+ from the S cathode move towards the Li anode through electrolyte and LiPS are oxidized to S8 at the cathode. Electrons from outer power source enters anode to reduce Li+ coming from cathode to Li metal that is plated on anode. This process is necessary to complete and continue electrochemical reaction.

[11] 퐿푖2푆 → 퐿푖2푆2 → 퐿푖2푆3 → 퐿푖2푆4 → 퐿푖2푆5 → 퐿푖2푆6 → 퐿푖2푆7 → 퐿푖2푆8 → 푆8 … (4)

The charge and discharge profile of LSB can be seen in the Figure 1.6 (b). The first step shows the discharge profile and the second step shows the charge profile of the

LSB. The discharge profile consists of a high voltage plateau (~2.3 V) and a low voltage plateau (~2.1 V). The high voltage plateau is due to the formation of higher order polysulfides from Li2S8 to Li2S3 and the low voltage plateau is due to the formation of lower order polysulfides from Li2S2 to Li2S.

Region I: Elemental sulfur, S8, is reduced to Li2S8 represented by the high voltage plateau at 2.2-2.3V. These higher order LiPS dissolve into the liquid electrolyte due to their high polarity and the highly polar nature of the electrolyte.

[10] 푆8 + 2퐿푖 → 퐿푖2푆8 ………………. (5)

15 Region II: The dissolved Li2S8 is further reduced to Li2S7, Li2S6, Li2S5, Li2S4, and Li2S3.

The voltage of the cell decreases during this reaction.

[10] 퐿푖2푆8 + 2퐿푖 → 퐿푖2푆8−푛 + 퐿푖2푆푛 . (6)

Region III: The lower order polysulfides are further reduced to Li2S2 and Li2S (insoluble polysulfides) which are represented by the lower voltage plateau at 1.9-2.1V. This region typically contributes to the major capacity of the Li-S cell.

[10] 2퐿푖2푆푛 + (2푛 − 4)퐿푖 → 푛퐿푖2푆2 .. (7)

[10] 퐿푖2푆푛 + (2푛 − 2)퐿푖 → 푛퐿푖2푆 .... (8)

Region IV: The Li2S2 is completely reduced to Li2S (lithium sulfide). This region has lower voltage due to the insoluble and non-conductive nature of Li2S.

[10] 퐿푖2푆2 + 2퐿푖 → 퐿푖2푆 ……….... (9)

1.4.2 Challenges in LSB

Although LSB has high theoretical energy density and capacity, the commercial use of LSB is limited due to rapid capacity fading (polysulfide dissolution) and low coulombic efficiency.

Below are some of the challenges that prevent the commercialization of LSB.

1. Sulfur is highly insulating in nature, which has a poor electrical conductivity of

5x10-30 S cm-1 at 25°C.[23] To make sulfur conducting, high percentage of

conducting carbon is used which will negatively affect both volumetric and

gravimetric density of S-cathode.

16 2. During cycling, LiPS dissolves in the liquid electrolyte (Figure 1.6 (c)). These

dissolved higher order polysulfides migrate towards the lithium metal anode and

get reduced to Li2S2/ Li2S (insoluble polysulfides) and coated on the surface of

the lithium metal which not only leads to the loss of active material (sulfur) but

also hinder Li+ diffusion from and to Li which is required to continue battery

charge/discharge at high C rates. These lower order polysulfides are oxidized to

higher order LiPS which diffuse back to the cathode and are oxidized to form S8.

This mechanism keeps recurring resulting in the well-known “shuttle effect”.

3. The insoluble and insulating lower order polysulfides (Li2S2 to Li2S) corrodes the

lithium metal anode and hinders the transport of Li+ resulting in lower coulombic

efficiency, short cycle life and an increase in the charge transfer resistance of the

cell.[24]

4. The lithium metal undergoes undesirable electrochemical reaction when it comes

in contact with the electrolyte. The resultant micro fibrous deposits of the lithium

metal which adopt needle-like structure (dendrites) eventually puncture the

separator causing internal short circuit.

1.4.3 Solutions to the Challenges of LSB

1. A sulfur-carbon composite can be used in the cathode to improve the

conductivity of sulfur. The electronic conductivity of carbon is 102 S cm-1 at

25°C [25] and sulfur is 5×10−30 S cm-1 at 25 °C.[23]

2. Since liquid electrolyte (flammable) is not safe for wearable devices, a solid

electrolyte (non-flammable) must be used to improve the safety. Liquid

electrolytes have ionic conductivities in the range of 11.93 mS cm-1 at 25°C.[26]

17 LAGP (lithium aluminum germanium phosphate), a type of solid electrolyte used

in this project, has a superionic conductivity of 10-2 S cm-1 at 25°C.[27]

3. LAGP can possibly hold polysulfides in its micro/nano pores to prevent capacity

fading.

4. An LAGP coated PE (polyethylene) separator can be used for single ion

conduction and to mitigate polysulfide shuttling. LAGP coated PE separator can

act as a barrier that physically prevents the polysulfide dissolution entering the

anode region.

1.5 Liquid Electrolyte vs. Solid Electrolyte

Table 1.2. shows a comparison between commonly used liquid and solid electrolytes.

Table 1.2: Liquid Electrolyte vs Solid Electrolyte[28]

Liquid electrolyte Solid electrolyte It will not prevent dendrites It will act as a physical from creating short circuits barrier to prevent dendrites unless a mechanically strong from creating short circuits. separator is used. Thermal runaway (above It is safe up to 500 °C 85°C)

High room temperature ionic Low room temperature ionic conductivity conductivity

18 1.6 LAGP Solid Electrolyte

LAGP is a glass-ceramic electrolyte that behaves as a single ion conductor. It has an electronic conductivity of 10-2 S cm-1 at 25°C.[27] LAGP is brittle and hard to fabricate in thin film form as desired in many battery applications. However, LAGP when mixed with ion conducting polymer or coated on polymer can be flexible, as shown in Figure

1.7. Figure 1.7 is a flexible solid state electrolyte comprised of LAGP and PEO:Li salt suitable for wearable applications.

Figure 1.7: LAGP-coated PE Solid Electrolyte

Furthermore, LAGP when coated on non-conducting separator can improve separator functionality including ionic conduction, dendrite prevention, and improvement in thermal stability. Moreover, LAGP coated separator of LAGP-polymer composite can limit the shuttling of dissolved polysulfides and hence improve battery cycle life.

1.7 Comparison of LIB and LSB Technologies

Table 1.3 shows the comparison of LIB and LSB technologies.

19 Table 1.3: Comparison of LIB and LSB Technology

LIB LSB Low theoretical specific energy High theoretical specific energy density (200 Wh/kg) density (2600 Wh/Kg)

It works on intercalation chemistry Chemical reaction occurs

Heavyweight compared to lithium Lightweight sulfur

Low theoretical capacity (250 High theoretical capacity (1675 mAh/g). It can accommodate only mAh/g). Each sulfur atom can 0.5-0.7 lithium ions per atom.[11] accommodate 2 lithium ions.[11]

High cost Expected to be low cost since sulfur is cheap and abundant

High environmental impacts Less environmental impact (pollution and toxic to humans)

A Li-ion cell was fabricated with a graphite anode, lithium hexaflurophosphate

(1M LiPF6 dissolved in ethylene carbonate (EC)/ dimethyl carbonate (DMC) (1:1, v/v)) electrolyte, and a lithium cobalt oxide (LCO) cathode. An aluminum metal current collector was used as a substrate for the LCO cathode. The weight of the active material

(LCO) was calculated to be 32.9 mg. The cell was charged and discharged using arbin battery cycler.

A Li-S cell was fabricated to compare its performance with that of the LIB cell.

The fabricated Li-S cell used a lithium metal anode, a liquid electrolyte of 1M LiTFSI/0.1

M LiNO3 (lithium Bis(trifluromethanesulfonyl)imide/ lithium nitrate) dissolved in DOL

(1,3 dioxolane) and DME (1,2 dimethoxyethane) (1:1, v/v) and a sulfur-based cathode. A

20 CNT-paper based current collector was used as a substrate for the S-cathode to provide good electronic conduction. The preparation process of sulfur composite and the CNT- paper based current collector is explained in detail in chapter 3. The weight of the active material (sulfur) used was 1.8mg.

(a) 4.3 LIB- 159 mAh/g 3.9 Cycle 1- Lithium ion cell 3.5

3.1 LIB- 158 mAh/g LSB- 1332 mAh/g

Voltage (v) Voltage 2.7 Cycle 1- Lithium sulfur cell

2.3

1.9 LSB- 1402 mAh/g

1.5 0 200 400 600 800 1000 1200 1400 1600 Capacity (mAh/g)

(b) 1600

1400

) 1200

1000

800 Lithium sulfur cell

600

Discharge capacity capacity (mAh/g Discharge 400 Lithium ion cell 200

0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 Number of cycles

Figure 1.8: (a) First Charge/Discharge Capacity

and (b) Cyclability of Li-Ion and Li-S Cells

21 As seen in Figure 1.8 (a), the first charge/discharge capacity of the Li-S cell was approximately 9 times higher than that of the Li-ion cell. Also, it was observed in Figure

1.8 (b) that the discharge capacities of Li-S cell were not as stable as LIB, which indicates the degradation of the sulfur cathode. Noticeably, the nominal voltage of the Li-

S cell is 2.1 V whereas the nominal voltage of the Li-ion cell is 3.7 V. For applications where high voltage is required, multiple Li-S cells can be connected in series.

1.8 Thesis Objectives

The objective of this research is to develop a LSB better than the state-of-the-art

LIB in terms of performance, energy density, and cyclability. In Chapter 1, a Li-S cell better than Li-ion cell was developed. The goal of Chapter 2 is to overcome the challenges of LSB and to improve the safety of LSB by incorporating LAGP solid electrolyte in the sulfur-cathode and PE (polyethylene) separator. The effect of binder concentration on the cell performance was also studied. Chapter 3 aims to develop a flexible LSB by incorporating flexible CNT (carbon nanotubes) paper-based current collector in the sulfur cathode to demonstrate high energy density and wearability.

Chapter 4 will conclude this research work and discuss future work aimed at making LSB more suitable for wearables.

22 CHAPTER 2

SOLUTIONS TO THE CHALLENGES OF LSB

The goal of this chapter is to overcome the challenges of LSB by incorporating

LAGP (Lithium Aluminum Germanium Phosphate) solid electrolyte in the S-cathode and

PE (Polyethylene) separator of LSB. Moreover, the effect of binder concentration was studied for the improved performance of LSB.

2.1 Components and Fabrication of Li-S Cell

Figure 2.1: Assembling of Cell Components in Li-S Cell

23 As shown in figure 2.1, a Li-S cell consists of a lithium metal anode and a sulfur cathode isolated by a separator. The liquid electrolyte consists of 1M LiTFSI/0.1 M

LiNO3 (Lithium Bis(trifluromethanesulfonyl)imide/ lithium nitrate) dissolved in DOL

(1,3 dioxolane) and DME (1,2 Dimethoxyethane) (1:1, v/v). The electrolyte acts as a medium for the passage of Li+ ions from the anode to cathode and vice versa. The diameter of the lithium metal anode, separator, and sulfur cathode used were 15mm,

19mm and 14mm respectively.

2.1.1 Tools Used to Fabricate Cell Components

Below are some of the tools used to fabricate the cell components.

 Agar mortar and pestle

 Thinky mixer (enables mixing and dispersion of materials)

 Dr. Blade

 Pipette

 Vacuum oven

 Balance

 Punches (14mm (cathode), 15mm (anode) and 19mm (separator))

 Glove box

 Insulated tweezers

 Cell crimper

2.2 Li-S Cell Fabrication and Electrochemical Testing

The flexible components used in the Li-S cells for testing can be seen in figure

2.2. Lithium metal being soft has the tendency to deform easily (figure 2.2 (a)) and has a

24 very high theoretical capacity of 3860 mAh/g.[29] It was used as anode due to its high theoretical capacity, lowest anode potential (-3.04V) and its light weight (0.53g/cm3).[29]

(a) (b)

(c)

Figure 2.2: (a) Flexible Lithium Anode (b) Flexible S-cathode (c) Flexible LAGP-Coated

PE Separator

Conductive agents such as carbon, CNT (carbon nanotubes) and LAGP were added to the S-cathode to improve the conductivity of the sulfur and to prevent the polysulfide dissolution into the electrolyte. The flexibility of the S-cathode can be seen in figure 2.2 (b). The PE separator was coated with LAGP material to improve the ionic conductivity and to prevent the polysulfides entering the anode region. The flexibility of

LAGP coated PE separator can be seen in the figure 2.2 (c). The components of the cell were assembled in an argon-filled glove box.

25 2.2.1 Tools Used for Testing the Li-S Cells

After fabrication, the cells were tested for impedance; open circuit voltage (OCV) and galvanostatic charge/discharge at different currents using Arbin battery test equipment. The battery tester was connected to the computer and was used through battery test software called MITS pro (by Arbin). MITS pro software can control the cell’s current, measure the cell’s voltage and can stop the measurement of cell when a preset voltage or current is achieved. The parameters used to test the Li-S cells were a cut-off voltage of 3.0 V for charge and 1.7 V for discharge. The cells were charged and discharged using constant current (galvanostatic mode) at a C rate of C/20 (0.05C) for the first two cycles and C/5 (0.2C) for the rest of the cycles at room temperature.

2.3 Effect of Solid Electrolyte (LAGP) Based S-Cathode on Li-S Performance

LAGP is a glass ceramic material used in the cathode matrix for fast lithium ion conduction and to provide improved cell capacity or reduced voltage polarization. LAGP has already been proven for Li-O2 (Lithium-Air Battery) that the capacity of the battery increases with the concentration of LAGP in the cathode.[30] Similar results were obtained when LAGP solid electrolyte was incorporated in the S-cathode of Li-S cells. LAGP is used to improve the ionic conductivity of sulfur cathode and it can possibly hold lithium polysulfides in its micro/nano pores but this needs further characterization to prove it.

2.3.1 Preparation of S-Cathode with LAGP

A sulfur-carbon-LAGP composite was prepared using melt diffusion method. A

60 wt% of sulfur (ALDRICH), 20 wt% of carbon (SuperP) and 20 wt% of LAGP powder was hand milled together using agar mortar and pestle for about 10 minutes. Then the

26 composite was taken for melt diffusion process for about 12 hours at 155C. The S- cathode slurry was prepared using 90 wt% of 60S-20C-20LAGP composite, 5 wt% of multi-walled carbon nanotubes (NANO AMER) and 5wt% of PVDF (Polyvinylidene fluoride) binder in an NMP (N-methyl-2-pyrolidinone) solvent. All these materials were finely mixed using a thinky mixture for about 20 minutes. The slurry was cast on aluminum foil substrate using Dr. Blade (300µm thickness) and allowed to dry at 75C for few hours until it was completely dry. Later, the electrodes (cathode) were punched out using 14 mm punch and further dried in a vacuum oven at 55C overnight. The dried sulfur were taken into an argon-filled dry box where Li-S cell was fabricated.

2.3.2 Preparation of S-Cathode without LAGP

A sulfur-carbon composite was prepared using melt diffusion method. An 80 wt% of sulfur (ALDRICH) and 20 wt% of carbon (SuperP) was hand milled together using agar mortar and pestle for about 10 minutes. Then the composite was taken for melt diffusion process for about 12 hours at 155C. The S-cathode slurry was prepared by using 90 wt% of 80S-20C composite, 5 wt% of multi-walled carbon nanotubes (NANO

AMER) and 5 wt% of PVDF binder in an NMP solvent. The S-cathode preparation process were similar to the Li-S cell with LAGP in the S-cathode (refer section 2.3.1).

2.3.3 Fabrication of Li-S with and without LAGP Based S-Cathode

Two Li-S cells were fabricated with a lithium metal anode, PE separator, a liquid electrolyte of 1M LiTFSI/0.1 M LiNO3 (lithium Bis(trifluromethanesulfonyl)imide/ lithium nitrate) dissolved in DOL (1,3 dioxolane) and DME (1,2 dimethoxyethane) (1:1, v/v)and a sulfur based cathode with/without LAGP. The quantity of the liquid electrolyte

27 used in the Li-S cell with LAGP in the S-cathode was 45 µl and without LAGP in the S-

Cathode was 80 µl.

2.3.4 Testing of Li-S with and without LAGP Based S-Cathode

The Li-S cells were charged and discharged using constant current (galvanostatic mode) at a C rate of C/20 for the first two cycles and C/5 for the rest of the cycles. The performances of the cells were analyzed at room temperature.

2.3.5 Battery Performance Comparison

The initial charge/discharge capacity of the Li-S cell with/without LAGP in the S- cathode is shown in the figure 2.3 (a). Both of the Li-S cells show two discharge plateaus during discharge corresponding to the expected charge/discharge profile of an LSB. The first high voltage discharge plateau (2.3V) in both of the cells indicates that there was higher order polysulfide formation and dissolution (Li2S8 down to Li2S3) in the electrolyte/cathode. The second low voltage discharge plateau (2V/2.1V) indicates that the major capacity was contributed by the formation of insoluble polysulfides Li2S2/Li2S

(insulating by nature) as they were dominant species. It was observed that the first charge/discharge capacity of the Li-S cell with LAGP in the S-cathode (826/779 mAh/g) was less than the charge/discharge capacity of the Li-S cell without LAGP in the S- cathode (831/795 mAh/g) when cycled at a rate of C/20. The slight lowering in charge/discharge capacity in the first cycle of Li-S cell with LAGP in the S-cathode may be due to the improper wetting of the electrodes by the electrolyte which in turn reduces the conductivity of the ions (electrode wetting problem will improve over time).

28 (a) 3.1 2.9 831 mAh/g Cathode with LAGP 2.7 Cathode without LAGP 2.5

2.3 826 mAh/g

Voltage (V) Voltage 2.1 779 mAh/g

1.9

1.7 795 mAh/g

1.5 0 200 400 600 800 Charge/discharge capacity (mAh/g)

(b) 900

800 Cathode with LAGP 700 Cathode without LAGP 600

500

400

300

200

100 Discharge capacity (mAh/g)

0 0 10 20 30 40 50 60 70 80 90 100 Number of cycles

Figure 2.3: (a) First Charge/Discharge Capacity and (b) Cyclability of Li-S Cell

with/without LAGP in the Cathode

However, the cycle life of the Li-S cell with LAGP in the S-cathode shows significant higher capacities than the Li-S cell without LAGP in the S-cathode (figure 2.3

(b)). The Li-S cell without LAGP in the S-cathode indicates lower discharge capacities

29 due to high polysulfide dissolution in the electrolyte which leads to loss of active material

(sulfur) in the cathode.

The sulfur loading of Li-S cell with LAGP in the sulfur cathode was 1.375mg cm-2 less than the Li-S cell without LAGP in the sulfur cathode (2.193 mg cm-2). Despite low sulfur loading, Li-S cell with LAGP in the S-cathode was able to provide higher discharge capacities. Moreover, the quantity of the liquid electrolyte (45µl) used in the

Li-S cell with LAGP in the S-cathode was less than the Li-S cell without LAGP in the sulfur cathode (80 µl). This proves that using LAGP in the S-cathode definitely has an effect on the cell’s capacity and it minimizes the polysulfide dissolution thereby improving the conduction of Li+ ions between the electrode and electrolyte.

2.4 Effect of Concentration of S-Cathode Binder (PVDF) on Li-S Performance

PVDF (Polyvinylidene fluoride) is one of the commonly used binders for the LSB due to its good electrochemical stability and good adhesion property. It dissolves in an organic solvent like NMP with high boiling point. [31] Typically for Li-S cells, the S- cathode slurry is prepared by using the active material (sulfur), electron conducting carbon, and a PVDF binder. The slurry was casted onto aluminum (Al foil) current collector. Then the casted mixture was made to dry at a certain temperature under vacuum. A binder is used to provide good adhesion and mechanical strength for the electrodes casted on the current collector without peel-off or crack.

30 2.4.1 Preparation of S-Cathode with Varying Concentration of PVDF Binder (5 wt% and 10 wt%)

The Li-S cell with LAGP in the S-cathode (refer section 2.3.1) was used to show the effect of PVDF 5 wt% (binder) in the S-cathode. To show the effect of PVDF 10 wt% in the S-cathode, the cathode slurry was prepared with 85 wt% of 60S-20C-20LAGP composite, 5 wt% of multi-walled carbon nanotubes (NANO AMER) and 10 wt% of

PVDF binder in a NMP solvent. The preparation of S-cathode was similar to the section

2.3.1. The sulfur loading density of the cathode was 2.53 mg cm-2 and the thickness of the

S-cathode was 300µm.

2.4.2 Fabrication and Testing of Li-S with Variable PVDF Concentrations

Two Li-S cells were fabricated with a lithium metal anode, PE separator, a liquid electrolyte of 1M LiTFSI/0.1 M LiNO3 (lithium Bis(trifluromethanesulfonyl)imide/ lithium nitrate) dissolved in DOL (1,3 dioxolane) and DME (1,2 dimethoxyethane) (1:1, v/v) and a sulfur based cathode with a varying concentration of PVDF binder (5 wt% and

10 wt%). The Li-S cells were charged and discharged using constant current

(galvanostatic mode) at a C rate of C/20 for the first two cycles and C/5 for the rest of the cycles. The performance of the cells was analyzed at room temperature.

2.4.3 Battery Performance Comparison

Compared with Li-S cell using 10 wt% of PVDF binder in the S-cathode, the Li-S cell using 5 wt% of PVDF binder in the S-cathode showed a better charge/discharge capacity (figure 2.4 (a)). The voltage polarization of the Li-S cell with 5 wt% of PVDF

31 in the S-cathode was found to be less than the Li-S cell with 10 wt% of PVDF in the S- cathode.

(a) 3.1 676 mAh/g 2.9 PVDF 5wt%

2.7 PVDF 10wt%

2.5

2.3 826 mAh/g

Voltage (V) Voltage 2.1 779 mAh/g

1.9

1.7 604 mAh/g 1.5 0 200 400 600 800 Charge/discharge capacity (mAh/g)

(b) 900

800 PVDF 10wt% 700 PVDF 5wt% 600

500

400

300

200

Discharge capacity (mAh/g) 100

0 0 10 20 30 40 50 60 70 80 90 100 Number of cycles

Figure 2.4: (a) First Charge/Discharge Capacity and (b) Cyclability of Li-S cells with 5

wt % and 10 wt% of PVDF Binder in the S-cathode

This proves that the kinetic characteristics of the S-cathode with 5 wt% of PVDF in the S-cathode were better. Moreover, the cyclability of Li-S cell with 5 wt% of PVDF

32 in the S-cathode showed greater capacities compared to that of the Li-S cell with 10 wt% of PVDF in the S-cathode (figure 2.4 (b)). This indicates that higher the concentration of

PVDF, poorer the cell performance. Since PVDF is used to bind well the electrode material onto the current collector, higher concentration of PVDF will make the surface of the S-cathode much dense, block the cathode pores and make it difficult for the liquid electrolyte to get absorbed in cathode. This, in turn, creates difficulty for the Li+ ions to move from the anode to cathode. More concentration of PVDF will also block the pores of the carbon while not allowing the carbon to hold the sulfur material. Hence, optimizing binder concentration is crucial in achieving higher cell capacity and overall electrochemical performance.

2.5 Effect of Solid Electrolyte (LAGP) Based Separator on Li-S Performance

A porous separator is used as a physical barrier to separate the two electrodes of a battery cell. Separator is filled with liquid electrolyte that allows the transport of ions from anode to cathode and vice versa. A traditional PE separator would not prevent the polysulfide dissolution in the electrolyte and lithium metal dendrite formation due to large pores in it.

A solid electrolyte (like LAGP) can be used to modify the pores of PE separator and improve the cell performance by mitigating deficiency of PE separator. An LAGP- coated PE separator could enhance the transport of lithium ions from lithium metal anode to S-cathode and vice versa.

33 2.5.1 Fabrication of LAGP Based Polyethylene (PE) Separator

LAGP material with a thickness of ~ 130 nm was coated on both the sides of the

PE separator by using an Electron Beam Physical Vapor Deposition (EB PVD) technique to enhance single ion conduction and prevent lithium metal dendrites.[32]

LAGP coated PE separator was preferred over traditional PE separator to address two major problems in LSB and they are listed below.

1. During repeated cell cycling, the lithium metal anode electrochemically reacts with organic liquid electrolyte and enhances the growth of lithium dendrites on the anode.

When a traditional PE separator is used, the volume change of the lithium dendrites can easily break the SEI (Solid Electrolyte Interface) layer and dendrites can lead to short circuiting of the cell when they reach the cathode.[33]

2. During discharge, the higher order polysulfides (Li2Sx, 4 ≤ 8) dissolve in the liquid electrolyte and move towards the lithium metal anode where they are further reduced to

Li2S2/ Li2S which are insoluble and they accumulate on the lithium metal surface preventing the Li+ diffusion. Later the insoluble polysulfides diffuse back to the sulfur cathode and are re-oxidised to higher order polysulfides. [34] This ‘shuttle mechanism’ can result in loss of active material (sulfur) in the cathode and low coulombic efficiency of the cell.

2.5.2 Preparation of S-Cathode of Li-S with LAGP Coated PE Separator

The S-cathode slurry was prepared by using 90 wt% of 60S-20C-20LAGP composite, 5 wt% of single-walled carbon nanotubes (ALDRICH) and 5 wt% of PVDF binder in an NMP solvent. The slurry making process were similar to the section 2.3.1.

34 The sulfur loading density of the cathode was 0.9365 mg cm-2 and the thickness of the S- cathode was 200 µm. For the Li-S cell with traditional PE separator, the same Li-S cell with LAGP in the S-cathode was used (refer section 2.3.1).

2.5.3 Fabrication and Testing of Li-S with LAGP Coated PE Separator

Two Li-S cells were fabricated with a lithium metal anode, a liquid electrolyte of

1M LiTFSI/0.1M LiNO3 (Lithium Bis(trifluromethanesulfonyl)imide/lithium nitrate) dissolved in a mixture of DOL (1,3 dioxolane) and DME (1,2 Dimethoxyethane) (1:1, v/v), LAGP incorporated S-cathode (refer section 2.3.1) and PE separator (with/without

LAGP). The Li-S cells were charged and discharged using constant current (galvanostatic mode) at a C rate of C/20 for the first two cycles and C/5 for the rest of the cycles. The performance of the cells was analyzed at room temperature.

2.5.4 Battery Performance Comparison

A uniform and dense coating of LAGP on both the sides of the PE separator will improve the Li+ conduction and suppress the growth of lithium dendrites on the anode.

Also, it will minimize the shuttle effect by acting as a physical barrier that prevents the dissolved polysulfides entering the lithium metal region. It can be seen from figure 2.5

(a) that the Li-S cell with LAGP-coated PE separator shows a high initial discharge capacity (838 mAh/g) and a low charge capacity (647 mAh/g) compared to the Li-S cell with traditional PE (Celgard) separator (779/826 mAh/g). The low initial charge capacity of the Li-S cell with LAGP-coated PE separator might be due to the blockage of polysulfides in the pores of the separator that hinders the diffusion of Li+ ions.

35 (a) 3.1 PE seperator 2.9 647 mAh/g LAGP-coated PE separator 2.7

2.5

2.3 826 mAh/g

Voltage (V) Voltage 2.1 779 mAh/g

1.9 838 mAh/g 1.7

1.5 0 200 400 600 800 Charge/discharge capacity (mAh/g)

(b) 900 120 800 110 700 100 90 600 80 500 70 400 60 300 50 200 40

100 30 Coulombic efficiency (%) Discharge capacity (mAh/g) 0 20 0 10 20 30 40 50 60 70 80 90 100 Number of cycles PE separator LAGP-coated PE separator PE Coulombic eff (%) LAGP coulombic eff(%)

Figure 2.5: (a) First Charge/Discharge Capacity and (b) Cyclability and Coulombic

Efficiency (%) of Li-S Cells with/without LAGP in the PE Separator

It was also noted that the second plateau of the Li-S cell with LAGP-coated PE separator was long and quite stable (2V) compared to the Li-S with traditional PE separator. The second plateau contributes to the major capacity of the cell due to the presence of dominant species (Li2S2 and Li2S). This proves that the polysulfide

36 dissolution was controlled in the first cycle and the discharge capacity was improved for the Li-S cell with LAGP coated PE separator. However, the cyclability of Li-S cell with

LAGP-coated PE separator (figure 2.5 (b)) exhibits significant capacity fading whereas the cell with traditional PE separator shows a high cycling stability, but the results need to be verified with repeated experimentation.

The low cycling stability of Li-S cell with LAGP-coated PE separator might be due to the blockage of insoluble lithium polysulfides (Li2S2/ Li2S) in the pores of the separator which impede the diffusion of Li+ ions from the anode to cathode. It was

퐶ℎ푎푟𝑔푒 푐푎푝푎푐𝑖푡푦 observed that the coulombic efficiency ( ∗ 100) of the Li-S cell with 퐷𝑖푠푐ℎ푎푟𝑔푒 푐푎푝푎푐𝑖푡푦

LAGP-coated PE separator was less than 100% in the first cycle and a drop in capacity implies that the pores of the separator are being blocked by the polysulfides. The consecutive cycles 100% coulombic efficiency implies that the separator blocks the diffusion of polysulfides to the lithium metal anode and minimizes the shuttle effect

(figure 2.5(b)). The columbic efficiency of Li-S cell with PE separator was found to be over 100% and this indicates high shuttling.

37 CHAPTER 3

CNT COATED PAPER CURRENT COLLECTOR SUBSTRATE FOR

S- CATHODES

This chapter primarily focuses on flexible CNT paper-based substrate as a current collector for improved cycle life performance in S-cathodes. A study was made between the Li-S cells using the traditional metal current collector (Aluminum foil) and flexible

CNT paper-based (PB) current collector in terms of electrical conductivity, adhesion of active electrode materials and cyclability.

3.1 Flexible CNT Paper-Based vs Traditional Aluminum Metal Current Collector

A current collector is typically used to improve the electronic conduction of the electrodes. It will provide a conducting path through the mixture (sulfur & carbon) and also offer a physical support for the active material of the electrode. A current collector chosen for the electrodes must be electrochemically stable with the electrolyte and electrode material.

For a flexible battery, all of the battery components needs to be flexible and with the traditional current collectors (Aluminum and Nickel), it’s very difficult to achieve the flexibility without cracking the electrodes and losing the capacity.

38 Hence a current collector of light weight, highly conducting combined with flexibility is required for flexible batteries. CNT’s (Carbon Nanotubes) are allotropes of carbon that provide high electrical conductivity, thermal stability and good mechanical strength.[35] The CNT paper-based (P-B) substrate was used as a current collector for S- cathode to provide enhanced electrical conductivity and to provide improved capacity.

There are two major reasons to use CNT (P-B) substrate as a current collector in

Li-S battery and they are listed below.

(1) The metal current collector (Aluminum) wouldn’t be a good choice for the flexible battery as it electrochemically corrodes.

(2) The S-cathode material can be easily peeled off from the metal current collector when bent or folded.[36] This occurs because the sulfur material doesn’t stick well onto the aluminum.

(a) (b)

Figure 3.1: (a) Flexible CNT Paper-Based Current Collector Substrate and (b) S-

Cathode with CNT Paper-Based Current Collector Substrate

39 The flexible CNT (P-B) substrate and the S-cathode materials casted on the CNT

(P-B) substrate (after drying) were shown in the figure 3.1 (a) and figure 3.1 (b) respectively. The CNT (P-B) substrate exhibited a good mechanical strength compared to that of the aluminum metal current collector.

3.1.1 Preparation of the CNT Solution for CNT (P-B) Substrate

The materials used for making CNT solution are Octadecylamine functionalized single-walled carbon nanotubes (Oct-SWCNT), dichlorobenzene and chloroform. A mixture of 5mg Oct-SWCNT, 1g of dichlorobenzene and 0.5ml of chloroform were sonicated in a bath sonicator for 1 hour. A few drops of the well-dispersed Oct-SWCNT solution were placed directly on a printer paper and dried at 85°C for 2 hours. The other side of the printer paper was coated in a similar way. Since one side of the printer paper was not thoroughly coated, the gold coating was done on that side (sputtering technique) to provide a uniform coating for improved conductivity. Later the CNT-coated paper was cut into 14mm discs to be used as substrates for S-cathodes.

3.1.2 Preparation of S-Cathode with CNT (P-B) Substrate

The S-cathode slurry was prepared by using 90wt% of 80S-20C composite, 5wt% of multi-walled carbon nanotubes (NANO AMER) and 5wt% of PVDF binder in an

NMP solvent. The procedure for the preparation of sulfur-carbon composite can be referred in chapter 2 (section 2.3.2). The cathode materials were finely mixed using a thinky mixture for about 20 minutes and the slurry was cast onto a CNT (P-B) substrate

(14mm). It was dried at 75°C under vacuum for 3 hours.

40 3.1.3 Fabrication and Testing of Li-S with CNT Coated Flexible Current Collector

A Li-S cell was fabricated with a lithium metal anode, a traditional PE separator with a liquid electrolyte of 1M LiTFSI/0.1M LiNO3 (80µl) dissolved in a mixture of

DOL (1,3 dioxolane) and DME (1,2 Dimethoxyethane) (1:1, v/v) and a sulfur-based cathode with CNT (P-B) current collector and its performance was evaluated with that of the Li-S cell with aluminum (Al) metal current collector in the S-cathode (refer chapter 2, section 2.3.1 for Al based Li-S cell). The Li-S cell using LAGP material in the cathode was used to show the effect of aluminum current collector in the S-cathode (refer chapter

2, section 2.3.1). The Li-S cells were charged and discharged using constant current

(galvanostatic mode) at a C rate of C/20 for the first two cycles and C/5 for the rest of the cycles. The performance of the cells was analyzed at room temperature.

3.1.4 Battery Performance Comparison

It was observed from figure 3.2 (a) that the Li-S cell with CNT (P-B) current collector in the S-cathode exhibits a high initial discharge capacity of 1402 mAh/g which is approximately 84% of the theoretical capacity of sulfur (1675 mAh/g). The initial charge and discharge capacity of Li-S cell with CNT (P-B) current collector in the S- cathode was found to be high when compared to that of the Li-S cell with Al current collector in the S-cathode. This behavior was due to the high electrical conductivity of the CNT (P-B) current collector, strong adhesion of sulfur-carbon composite onto the

CNT substrate minimizing the polysulfide dissolution and CNT functionalization with nitrogen groups that have shown to interact better with polysulfides. The low initial charge/discharge capacity observed in Li-S cell with Al current collector in the S-cathode was due to the dissolution of polysulfides in the electrolyte leading to capacity loss.

41 (a) 3.1 Cathode with CNT paper 1332 mAh/g 2.9 substrate 826 mAh/g 2.7 Cathode with Al foil substrate 2.5

2.3

Voltage (v) Voltage 2.1 1402 mAh/g 1.9

1.7 779 mAh/g

1.5 0 200 400 600 800 1000 1200 1400 Charge/discharge capacity (mAh/g)

(b) 1600

1400 Cathode with CNT paper substrate 1200 Cathode with Al foil substrate 1000

800

600

400

Discharge Capacity DischargeCapacity (mAh/g) 200

0 0 20 40 60 80 100 Number of cycles

Figure 3.2: (a) First Charge/Discharge Capacity and (b) Cyclability of Li-S Cells with

Aluminum Foil and CNT Paper-Based Current Collector Substrate in the S-Cathode

The cyclability of S-cathode with CNT (P-B) current collector was found to be high when compared to the S-cathode with Al current collector (figure 3.2 (b)).

퐶ℎ푎푟𝑔푒 푐푎푝푎푐𝑖푡푦 Moreover, the mean columbic efficiency ( ∗ 100) of the Li-S cell with 퐷𝑖푠푐ℎ푎푟𝑔푒 푐푎푝푎푐𝑖푡푦

CNT (P-B) current collector in the S-cathode was 97% (high) whereas the mean

42 columbic efficiency of Li-S cell with the aluminum current collector in S-cathode was

111% which indicates high shuttling due to polysulfide dissolution in the electrolyte.

It was noted that the Li-S cell with CNT (P-B) current collector in the S-cathode was able to achieve higher capacities with low sulfur loading (1.125mg cm-2) whereas the sulfur loading of the Li-S cell with Al current collector in the S-cathode was 1.375 mg cm-2. The S-cathode with CNT (P-B) substrate did not have any LAGP material in it and still was able to provide high electrical conductivity improving the capacity. The open circuit voltage (OCV) of the Li-S cell with CNT (P-B) current collector in S-cathode was

2.58V which was higher than the Li-S cell with Al current collector in S-cathode (2.39V).

With the above results, it can be concluded that the electrodes with CNT (P-B) current collector provide better cycling stability and possess good adhesion property over those with traditional Al metal current collectors. A sudden drop of capacity was observed after few cycles in Li-S cell with CNT (P-B) current collector (figure 3.2 (b)) in the S-cathode and this was due to the abrupt malfunctioning of the cell which was then restarted.

43

CHAPTER 4

CONCLUSIONS AND FUTURE WORK

This chapter summarizes the research performed during this thesis, and outlines possible future work.

In chapter 1, the need for flexible batteries in wearable devices, LIB and its limitations, LSB and its challenges were discussed. A battery (LSB) better than state-of- the-art (LIB) was developed.

In chapter 2, a novel approach was proposed to overcome the challenges of LSB and thereby improving the practical energy density of LSB. i) Use of LAGP in the sulfur cathode reduced the polysulfide dissolution in the electrolyte and improved the electronic conduction of the sulfur material. ii) The study of right concentration of the PVDF binder in the sulfur cathode helped to address the challenges of LSB (capacity fading). High amount of binder clogs the pores of carbon and causes cell polarization. iii) The use of

LAGP-coated PE separator minimized the ‘shuttle effect’ in LSB to an extent thereby acting as a physical barrier between the anode and cathode. Also, it minimized the use of liquid electrolyte in LSB to improve the safety of wearables.

However, the use of LAGP-coated PE separator did not provide the expected cell capacity and it might be due to the clogging of polysulfides in the separator pores that hinders the transport of Li+ ions.

44 In chapter 3, the use of CNT paper-based substrate as current collector greatly improved the performance of LSB by providing higher sulfur adhesion on the cathode and reduction of polysulfide dissolution because of nitrogen functionalization.

Since the current research work has provided the feasibility of the flexible cell components to be used in LSB, the future work is to do further research in flexible CNT paper-based current collector and understanding the role of LAGP coated PE separator.

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