A Template for Enhanced Lithium Ion Battery Electrodes

Technische Universiteit Delft

Sustainable Mobility

A Template for Enhanced

Lithium Ion Battery Electrodes

Deepak Pratap Singh

High performance Li-Ion Batteries

Deepak Pratap Singh

Technische Universiteit Delft

Sustainable Mobility

A Template for Enhanced

Lithium Ion Battery Electrodes

Deepak Pratap Singh

High performance Li-Ion Batteries

Deepak Pratap Singh

Technische Universiteit Delft

Sustainable Mobility

Templated

Li Ion

Battery

Electrodes

Deepak Pratap Singh

Deepak Pratap Singh

Samenstelling promotiecommissie:

Rector Magnificus Voorzitter

Prof. dr. F.M. Mulder Technische Universiteit Delft, promotor Dr. ir. M. Wagemaker Technische Universiteit Delft, copromotor Prof. dr. P.H.L. Notten Technische Universiteit Eindhoven Prof. dr. ir. J.E. ten Elshof Universiteit Twente

Prof. dr. B. Dam Technische Universiteit Delft

Prof. dr. E.H. Brück Technische Universiteit Delft

Dr. H.P.C.E. Kuipers Shell Nederlands B.V.

The Research described in this thesis was carried out in the section Fundamental Aspects of Materials and Energy (FAME), Department of Radiation Science and Technology at Delft University of Technology.

The research described in this thesis was financially supported by the Shell-TUDelft Sustainable Mobility Program.

ISBN: 978-94-6259-107-3

Copyright © 2014 by Deepak Pratap Singh

All Right Reserved by the author encourages the non-commercial communication of scientific contents, provided the proper permission is obtained and citation to the source is given. Parts of the thesis are published in scientific journals and copyright is subject to different terms and conditions. (email: deepak.tnw@gmail.com)

Art direction & lay-out: Esther Beekman (www.estherontwerpt.nl) Printed by: Ipskamp drukkers BV, Enschede

Battery Electrodes

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op maandag 17 maart 2014 om 15:00 uur door

Deepak Pratap Singh

Master of Science in Nano Science, Amity University geboren te Pilibhit, India

Impact of Mobility on Society

Sustainable Energy, Mobility and Storage Battery: Electrochemical Energy Storage Lithium Ion Battery

Scientific questions and scope of the thesis Structure of the thesis

References

Experiments and Techniques

Material preparation Material Characterization Electrode Preparation Soft Lithography

Cell fabrication and Electrochemical Testing References

Dynamic solubility limits in nano-sized olivine LiFePO₄

Introduction Methods

Results and Discussion Conclusion

References

Facile micro templating LiFePO₄ electrodes for high performance Li-ion batteries

Introduction Methods

Results and Discussion Conclusion

References

Templating Li₄Ti₅O₁₂ spinel electrodes for high rate performance and energy density Li-ion batteries

Introduction Methods

Results and Discussion

10 13 14 16 23 24 25 31 32 37 45 49 55 59 61 62 63 63 73 74 79 80 81 82 90 92 97 98 99 101 1.1 1.2 1.3 1.4 1.5 1.6 1.7 2 2.1 2.2 2.3 2.4 2.5 2.6 3 3.1 3.2 3.3 3.4 3.5 4 4.1 4.2 4.3 4.4 4.5 5 5.1 5.2 5.3

Methods

Results and Discussion Conclusion

References

Nanostructured TiO₂ anatase Micro Patterned

3D electrodes for high performance Li-ion batteries

Introduction Methods

Results and Discussion Conclusion

References

Appendix 1: Dynamic solubility limits in nano-sized olivine LiFePO₄ - XRD, Neutron Diffraction and Rietveld Refinement

Appendix 2: Dynamic solubility limits in nano-sized olivine LiFePO₄ - Diffuse Interface Calculations

Appendix 3: Facile micro templating LiFePO₄ electrodes for high performance Li-ion batteries

- Removal of NaHCO₃ Template from LiFePO4 electrodes - k value for templated electrode

Appendix 4: Nanostructured TiO₂ Anatase Micro Patterned 3D electrodes for

high performance Li-ion batteries

- Structural & Electrochemical characterization of TiO₂ electrodes - Cross-section SEM of TiO₂ microelectrodes after (dis)charge cycle

Summery Samenvatting Acknowledgement List of pucliations Curriculum Vitae 119 123 126 127 131 132 133 134 142 143 149 157 161 165 173 181 189 195 199 6.3 6.4 6.5 6.6 7 7.1 7.2 7.3 7.4 7.5

Introduction

“Knowing yourself is the beginning of all wisdom.”

10

Since the origins of life on earth, energy has been an inevitable requirement for its existence, and will remain so for ages to come. For instance, in prehistoric times - the sun and food were the exclusive source of energy for all living entities on the planet – and the mobility of these entities was predominated by muscle power. It was only around the Stone Age that humans marked the most important scientific advances of humanity by taming - stone for tools, fire for energy, land for farming, and inventing the wheel for mobility.

These early technological developments and aspiration had not only advanced the Stone Age civilisation through the Bronze and the Iron Age to the Modern day society by endorsing value to human life, but also transformed energy demands towards more dense energy sources such as the fossil fuels. With an ever-growing population and rapidly depleting fossil energy resources, the current energy, and mobility, powered evolution has become unsustainable - both, economically and environmentally. Hence, to keep the wheel in motion, the energy must be obtained from sustainable sources and should be used and stored efficiently.

This thesis explores one such pathway in which ‘batteries’ are an integral component to achieve sustainability in energy and eventually mobility. Including this chapter, this thesis contains total seven chapters and a summary. This introductory chapter starts with giving short historical overview of mobility and its impact on society, evolution of modern-day energy storage system i.e. the battery. Furthermore, it explores technical aspects of state of the art lithium ion batteries for sustainable mobility application. Finally, the scientific questions and scope of this thesis. The contents of this chapter are as following;

1.1 Impact of mobility on society – Economic and Environmental 1.2 Sustainable Energy, Mobility and Storage – Time Line 1.3 Batteries : Electrochemical Energy Storage

1.4 Lithium ion battery – Power and Energy Density 1.5 Scientific questions and scope of the thesis 1.6 Structure of the thesis

1.1 Impact of Mobility on Society

1.1.1. Economic Impact

In human history, mobility has always been instrumental in shaping economic prosperity. Today’s major trade sea route via the Suez Canal plays the same role as the ancient Silk

Route, which evolved through the 3rd millennium BCE until 1CE by connecting most

prosperous civilizations primarily in the Indus Valley, ancient Egypt and Mesopotamia. It extended further into Europe in the west and China in the east. In these early days sailing ships and animal powered carts were the only modes of mobility, probably and unarguably the most environmentally friendly too. Over time, this allowed civilizations and empires, having the advantage of access to the sea and the ability to build large ships and road networks, to become prosperous and powerful. Fleets of sailing ships became

11

emblems of economic and feudal supremacy. Several empires got the opportunity to expand their local markets, while a privileged few expanded their territories as well. Sailing ships and horses, which were the only components of mobility for thousands of years, became redundant overnight with the advent of steam engines in the early 19th century. The age of steam had dawned upon us and changed the components of mobility and energy, forever. The ability of coal and wood powered steam engines to work on demand had laid down the foundation for the industrial revolution in Europe. The positioning of steam engines on ships, and later as railway locomotives had not only mechanized sea and land transportation, but also became essential for the rapid movement of goods and manpower required for economic growth and industrialization. A few decades later, with the construction of the internal combustion engine, the era of fossil fuel powered mobility and economy had dawned. In the last six to seven decades, this dawn has raised the human population much faster than it has ever done in the entire human history. This has gone hand in hand with our energy demands and dependence on countries with surplus fossil fuels. This critical dependency on fossil fuels has separated the world into two parts (1) the developed and energy rich nations and (2) the developing and energy deprived nations. The energy deprived nations like India, among many others, end up spending heavily (~ $22 billion, 2010)1 _to

provide subsidised imported fuel and energy. This enormous spending further fuels the economic inequality of the country and will continue to do so, unless they migrate to non conventional energy resources.

1.1.2. Environmental Impact

Our dependence on fossil fuels, has not only enshrined inequality into the global economic order, but also made ecology unsustainable. To meet the rising energy and raw material demands of a growing and developing population, the over-exploitation of fossil fuels has compromised the ecosystem adversely and irreversibly. As Figure 1 shows, the global carbon emission is rampant and much faster than in any timescale of the entire human history. More than 80 percent of today’s energy demands relies on fossil fuels. Out of this, around 40 per cent of fossil energy is used for transportation only, making transport the second highest carbon emitter after coal powered installations. This emission has been held responsible for enhancing global warming and triggering increasingly fluctuating weather patterns, including devastating floods, drought and storms.

Yet, economic priorities and easy access to cheap fossil fuels has selectively and overwhelmingly snubbed environmental issues. No significant effort was made to transit towards clean renewable energy source until the 1970’s Arab Oil embargo and the aftermath of 1991 Gulf war, which had literally threatened to stop the perpetual wheel in motion, .i.e. economic growth. In the year following the gulf war, probably for the first time in history, more than 108 heads of government came together to address the need for

renewable energy to replace fossil fuels, to reduce vehicle emissions, and stop climate

change. The ambitious Rio declaration was signed in the UN’s Rio Earth Summit 92, and later the Kyoto protocol in 2012. Nevertheless, considering the level of these treaties,

12

Figure 1: Timeline of global carbon emission. The impact of 19th century’s fossil fuel powered industrialization on environment is visible as the exponential increase in Carbon emission. (CDAIC, ORNL, US Dept. of Energy).

Figure 2: Idea of sustainable mobility. Oil shortages in The Netherlands during Second World War, led people to use cars without gasoline/fuel. The Hague, may 14th 1941 (National Archive)3._.

and the ever-increasing CO₂ emissions, it seems much less has been achieved than anticipated.

13

1.2 Sustainable Energy, Mobility and Storage

Apart from environmental, the limited availability of easy accessible fossil fuels is a driver for policymakers to further emphasize on energy generation using clean and alternate energy sources like hydropower, solar and wind energy.

1.2.1. Sustainable Energy Generation

Among all renewable energy sources, Hydroelectric power, since its inception has been a major contributor to meet global energy demand and is now credited for generating one- fifth of world's total electricity output. During the past five years or so, wind and solar energy utilization has also marked the fastest growth among all renewable sources and blossomed in emerging and energy deprived economies. Contrary to renewables, Nuclear energy, for its ability to produce efficient energy from fairly small amounts of fuel and zero CO₂ emission, has also become popular as clean and alternative energy source. With alternative fuel cycles and reactors, the amount of radioactive waste may be reduced. These arguments have been used to state that nuclear power has the potential for long-term generation of energy. Had it not been prioritized for military and intimidated by geo-politics, Nuclear Power has all the potential to become a successful venture for clean energy by replacing coal-fired power plants for future electricity generation. The best current example would be France, which implemented nuclear energy to generate 75 percent of its electricity demand. Often mentioned draw backs are military nuclear use and geo-politics, In addition, from the Kyshtym, Chernobyl, Three Miles Island to the Fukushima disasters, nuclear power plants also proved to carry large risks upon failure, leading to a nuclear waste ‘legacy’.

Various energy organizations have projected that, by 2060 the alternate energy resources could buyout a large chunk of coal-powered electricity, and will at least reduce CO₂ emission andenergy cost. However, unlike Hydro and Nuclear power, Solar and Wind are intermittent sources of energy as their contribution is strongly dictated by availability of sunlight and wind on any given day and during seasons. Therefore, the need to utilize clean energy from sun and wind, either on grid or as autonomous power system, obliges efficient energy storage systems.

1.2.2. Sustainable Mobility

i. Electric Vehicles

Despite the progress towards green energy production, the CO₂ emission is constantly increasing with ever-growing population and increasing prosperity. The worldwide growth of the automobile market exemplifies the increase in fossil fuel consumption and CO₂ emission. This demands a reconsideration of the 19th _{century invention, the}

electrical vehicle, to enable zero emission mobility for the 21st _{century. The first electric}

vehicles arose in the 1830’s, decades before the invention of the internal combustion engine, and held the record of the fastest car of the century. However, in the early 20th

century, several technical factors, including economic and fossil fuel powered ambitions, restrained the expansion of electric mobility.

14

ii. Portable Electronics - Tele mobility

Thanks to the invention of the telegraph in 19th century and its nearly exponential development; the idea of tele-mobility has emerged as an option for sustainable mobility via the use of portable and mobile devices. Today portable and wireless communication devices have not only leveraged and customized the mobility for 6 billion mobile phone subscribers worldwide, but are also assisting in reducing CO₂ emission. By assuming only one percent of the total phone calls made per day have helped their respective callers to reduce at least one kilometre of work related travelling, one would easily endorse the role mobile phones in reducing fossil fuel consumption hence resulting reduction in CO₂ emissions.

1.2.3. Energy Storage

The very basic ideas of sustainable energy and mobility, from solar power to a smart grid, from electric cars to mobile phones, may not have been realised without the progress in electrochemical energy storage and conversion by means of Fuel Cells and Batteries.

1.3 Battery: Electrochemical Energy Storage

Like hydropower and wind power, the history of electrochemical storage, also dated back to ancient times. In 1936, during excavations in Khujut Rabu near Baghdad (ancient Mesopotamia), German archaeologists found terra cotta jars each containing an iron rod surrounded by rolled-up sheets of copper, later named as the Baghdad Battery. Later it was believed that the ancient galvanic cells were used for electroplating of gold or silver on jewellery, where lemon juice or vinegar was used as an electrolyte. The recent timeline of energy storage devices has embarked from 1745, when Pieter Van Musschenbroek invented the Leyden jar to store static electricity. The basic principle was quite similar to that known by the ancient Greeks; which is that after being rubbed amber could attract light particles. Nevertheless, it was Benjamin Franklin in 1749, who coined the term ‘Battery’ to elucidate the set of capacitors. Later in 1800, Alesandro Volta invented the Volta pile in 1800, which was credited as the first ever electrochemical cell. Volta’s cell consisted of zinc and copper electrodes and sulphuric acid as electrolyte. In the battery chemical free energy is converted into electrical energy as expressed by ¢G = -nFE, where ¢G is the change in Gibbs free energy, n is the electron transfer number, F is Faraday constant, and E is the cell potential. Following the same principle, various types of primary and secondary (rechargeable) batteries have been invented since then, as shown on a time line in Figure 3 and 4. Zinc Carbon or alkaline batteries are well known examples of primary batteries and are intended for single use only. Due to their low power, they are often used for devices with low current drain.

The secondary (rechargeable) batteries work on reversible electro-chemical reactions. During discharge, an exothermic reaction decreasing the Gibbs Free energy in the conversion towards electrical energy leads to spontaneous oxidation and reduction at the negative and positive electrode, respectively. This process is reversible by applying an external potential, reversing the reactions, and increasing the Gibbs Free energy, the

15

latter representing the potentially available chemical energy. All rechargeable batteries such as lead-acid, Nickel metal hydride, nickel-cadmium, and Li-ion follow the same working principle, but with different chemistries. The ability to recharge and reuse has made secondary batteries an important choice for sustainable mobility and energy storage.

Battery Performance: Power and Energy Density

Battery performance can be measured by evaluating the energy density and the power density along with its life cycle. Depending upon the requirement dictated by the application, batteries can be optimised for high energy or high power output. Gravimetric energy density is the amount of energy stored per weight unit and Power density is the maximum amount of power that can be supplied per weight unit. Figure 5, illustrating energy and powder density of various type of rechargeable batteries, suggests that lithium ion batteries have both, high energy and high power density. In combination with the high volumetric energy density this makes lithium ion the best suited system for electric mobility and portable electronics.

Figure 3 : Time line of primary Batteries.4

16

1.4 Lithium Ion Battery

A typical Lithium ion battery consists of two insertion material electrodes having different lithium chemical potential. The lithium-salt electrolyte filled separator allows Lithium ion transfer but prevents electrical contact as illustrated in Figure 6. During discharge the lithium ions at the negative electrode, having a high chemical potential, are driven via the electrolyte towards the positive electrode having a lower chemical potential. The charge-compensating electron is forced to travel via the external circuit providing the useable electrical energy. This change in lithium concentrations in the electrode materials results in structural changes and phase transformations. During charging the application of an external potential reverses, the process and the lithium ions migrate back via the electrolyte to the negative electrode where they combine with the electrons. This charge/discharge process involves electronic and ionic charge transport in the solid-state electrodes and ionic transport in the electrolyte.

1.4.1. Energy and Power density

High energy applications require Lithium ion batteries with high energy and power density. The overall energy density of any lithium ion battery is determined by the amount of lithium ions that can be stored per unit mass of electrode material and the potential difference between the positive and negative terminal. The power density, determining the (dis)charge time, is limited by the internal resistance of the battery, which mainly originates from the resistance of the charge transport in the liquid electrolyte (ionic) and in the electrodes (electronic and ionic).

17

1.4.2. Lithium Ion batteries so far

In 1991, Sony introduced the LiCoO₂/ Graphitic Carbon (GC) chemistry based batteries which have dominated the market ever since. The large potential difference between electrodes (graphite 0.1 V, LiCoO₂ 4.0 V) and facile charge transport in the solid state provides high energy and power density which has leveraged portable electronic industries. However, issues such as, (1) degradation of carbonate electrolytes at the low graphite voltages and, (2) the susceptibly of layered Li_0.5CoO₂ to structural changes at overcharging conditions refrains the use of LiCoO₂/GC based batteries for high power application. In order to employ lithium ion battery for electric mobility and onboard power applications for portable & mobile devices, battery research now focuses on improving the energy and power density, along with improving safety and cycle life. Among the large group of Lithium insertion/host materials, lithium Iron phosphates (LiFePO₄)5 _and

titanium oxides (TiO₂, Li₄Ti₅O₁₂)3,6 _{have become low cost and safer alternates for positive}

and negative electrodes, respectively. However, the benefit of a flat charge (dis)charge profile, thermal stability and minimal environmental impact, goes at the expense of battery performance. For example, batteries using LiFePO₄ (operating at 3.4 V vs Li/ Li+_{) as cathode and TiO}

2 (1.7 V vs Li/Li+) or Li4Ti5O12 (1.5 V vs Li/Li+) as anode, operate

within the stability window of most organic lithium electrolytes which minimizes the risks and reduced cycle life issues related to Solid Electrolyte Interface growth and lithium metal plating. However, the low operating voltage results in a significantly smaller energy and power density compared to LiCoO₂/GC based batteries.

18

1.4.3. Power and Energy Density: role of particles size and electrode morphology

i. Power density : Nanosizing of Electrode Material

To realize the full potential of insertion electrode materials, such as LiFePO₄ and Titanium based oxide materials, various strategies have been implemented that address the relatively poor charge transport properties, including nano structuring the crystallites

7-14_{, doping with high valence metal ions}15-18_{, and mixing active material particles with}

conducting phases19-23_{. Among all, the combination of nano sizing of electrode material}

and surface modification by carbon coating 24,25_{, has turned out as the most effective}

strategy to increase the charge transport in the solid state. Reduction in particle size shortens the ionic diffusion path d through solid-state crystallite, resulting in reduction of the lithium ion diffusion time t~d2_{=D (where D is the diffusion constant), thereby}

increasing the rate of lithium insertion/removal. Simultaneously, the surface modification by electronically conducting carbon coating, improves electronic transport through neighboring crystallites towards the current collector as shown in figure 7.

- Size dependent Effect: Altered structure and Voltage profile

The consequence of reducing the particle size towards nano sizes is that strain, surface and interface energies are completely altered as compared to bulk properties of insertion electrode materials.1,26-28 _{These size dependent effect includes the modification}

of solubility limits resulting in higher storage capacities of insertion materials and reduction of the miscibility gap29_{. The reduced miscibility gap in combination with the}

particle size distribution and surface storage phenomena30,31 _{alters the voltage profile}26

for nano materials, as shown in Figure 8.

Certainly, the nano sizing of insertion material and tuning of the electrode morphology in three dimensions will improve the ionic and electronic power density. However,

19

this advantage is often accompanied by disadvantages such as lower tap density, which lowers the volumetric energy density. Another side effect of nanosizing is the high surface reactivity between electrode and electrolyte, which could lead to enhancement of undesired side reactions such as degradation of the electrolyte at the electrode surface2,32,33 _{potentially limiting the overall performance of the battery.}

ii. Energy Density

The energy density of lithium insertion materials, determined by the amount of lithium ions that can be stored per unit mass, is exceptionally high among all the existing battery technologies, shown in Figure 5. However, for the high power applications, such as electric mobility, the overall energy density of lithium ion batteries is severely limited by the presence of inactive material in a battery pack. These inactive parts are the metallic current collectors, the separator soaked with liquid electrolyte and other packaging components. Table 1 shows that the relative amount of the active material in a power rated cell is limited to 26 percent of the total battery weight, and reduces the overall energy density.34 _{While one can assume that the energy density can be improves by}

simply increasing the relative amount (thickness) of active material with respect to inactive components present in the battery, shown in Table 1. However, the thicker electrodes have a higher internal resistance due to charge transport limitations.

Figure 8: Gibbs free energy (a) and voltage profile & solubility limits (b) in Lithium insertion material for micron-sized particles. Impact of nanosizing on the Gibbs free energy (c) and solubility limits and (d) voltage profile.

20

Table 1: The relative amount of active and inactive component in the cells, customized for energy and power specific applications.34

High Energy Cell

(100A-Hr)

High Power Cell

(10A-Hr) Material & Components Quantity (g) Weight percent Quantity (g) Weight Percent Negative Electrode Anode Material (Graphite ) 563.6 16.4 14.1 4.3 Current Collector (Copper) 151.9 4.4 41.6 12.8 Binder (PVDF) 69.7 2.0 Positive Electrode Cathode Material 1408.6 41 74.4 22.9 Current Collector (Aluminum ) 63 1.8 19.4 6 Binder (PVDF) 92.9 2.7 Rest of Cell Packaging 358.1 36.3 175.5 54 Separator 60.5 1.8 16.4 5 Electrolyte 618 18 44.0 13.5 Other (including carbon black) 46.6 1.4 12.6 3.9 Total 3432.7 100.0 325 100.0

21

The observation that thicker electrodes (high loading density) provide relatively less capacity at the same (dis)charge rates, shown in Figure 9, indicates that the rate limiting charge transport phenomena is either the electron or the ionic transport through the electrode, scaling with the electrode thickness. Therefore, increasing the energy density of batteries requires lowering the internal resistance of electrodes by improving the charge transport. Better ionic and electronic wiring, resulting in a lower internal resistance, will allow thicker electrodes, thereby increasing the relative amount of active material, and hence the energy density of the battery.

Figure 9: The rate test for LiFePO₄ at two different loading.

iii. Electrode Morphology

Recently, various three dimensional35-37 _{(3D) electrode architectures and nano}

morphologies have been put forward to achieve optimal electronic and ionic wiring around the active material in electrode matrix.36,38 _{Nanosizing of active material and assembling}

them in a 3D morphology helps to improve the facile electronic/ionic transport through the electrodes to maximize the power output, Figure 10. The suggested 3D architecture mainly includes interdigitated, trench and concentric morphologies.36,38,39 _However,

the 3D electrodes are often fabricated in the tubular morphology by direct synthesis of active material using polycarbonate40 _{and alumina as a template.}41,42 _{The another}

fabrication method is to deposit active materials on 3D current collectors43,44 _using

electro deposition39_{, Atomic layer deposition(ALD)}45,46_{, chemical vapour deposition}47,48

(CVD), RF. It should be noted that these 3D electrode processing techniques are preliminary dictated by their ability; 1) to produce the electrode material in the desired crystallographic phase and, 2) to control the electrode morphologies with sufficient free

22

volume in the structure to be filled by the electrolyte. In the case of micro batteries this free volume is also required to be interdigitated with the counter electrode.35

However, the limited amount of active material loading and excess of inactive components, such as the 3D metallic support, will compromise the overall volumetric and gravimetric energy density of the battery.

Figure 10: The charge transport in lithium-ion battery electrode, and the impact of electrode morphology. (a) The compact solid electrode maintains the high energy density but the limited ionic transport results in a poor power density, (b) Increasing the electrode thickness of the conventional porous electrode the energy density will increase at the expense of power density. (c) The 3D electrodes morphology has the advantage of dedicated ionic and electronic wiring around the active material resulting in high power densities. However, the limited amount of active material loading and the excess of inactive material (current collector) cause poor volumetric and gravimetric energy density.

23

1.5 Scientific questions and scope of the thesis

As already mention in the previous sections of this chapter, the transition towards sustainable and electric mobility requires lithium ion batteries with high energy and power densities. Despite having an enviable reputation compared to other battery technologies, at present, the lithium ion batteries are not able to provide an efficient solution (fast charging, longer run time between the charges) for complete electric vehicles. The main reason is the tradeoff between the energy and power density, which often have the contradictory demands. The energy density simply requires the large amount of active material loading. Whereas, the high power density needs the facile charge transport through the active material (ionic and electronic) and electrode matrix (ionic), requiring smaller crystallites size of the active material to reduce charge transport distances in the crystallites and on the other hand a porous electrode morphology for liquid enabling fast ionic transport through liquid electrolyte. The advantage of nanosizing and porous morphologies has inevitable drawback reinforced with it. Apart from the high surface relativity and altered voltage profile (shown in figure 8), the combination of nanosized active material and porous electrode reduces the overall energy density of the battery. The first question is then, what is the impact of nanosizing on the active material on the Li insertion characteristics? Secondly, what is the role of electrode morphology, and how this influences the high rate electrochemical performance and energy density of the battery? Furthermore, explore the strategies to tune the electrode morphology to achieve optimal power density in high energy density electrodes.

The research described in this thesis ranges from the fundamental studies on electrode material at nano scale, to developing the novel templating strategies and electrode processing techniques to tuning the electrode morphology for high performance batteries. This is achieved by using two distinct electrode processing and templating techniques.

Chapter 3 presents a systematic study revealing the impact of particle size and composition on the thermodynamics of LiFePO₄. This was accomplished by combining X-ray and neutron diffraction, the latter allowing the direct determination of Li occupancies in the coexisting heterosite Li_xαFePO₄ and triphylite Li_xβFePO₄phases. The results were compared with thermodynamic calculations based on the diffuse interface giving a rationale for the observations. Chapter 4 and Chapter 5 are focused on the

development of a novel and low cost templating technique. “Kitchen salts” (bicarbonate salts) were used to tune the electrode morphology to optimise the charge transport through high tap density electrodes. The high tap density electrodes, templated by sodium and ammonium bicarbonate salts, significantly improved the capacity during high rate electrochemical testing. Apart from this coarse and rugged templating method, state of the art soft lithography techniques were used to fabricate 3D microelectrodes. Chapter 6 and Chapter 7 are based on this soft lithography approach to fabricate 3D microelectrode which have the prospect to integrate complete 3D batteries.

24

1.6 Structure of the thesis

This thesis contains seven chapters, including this introduction (Chapter 1), and explores various facets of lithium-ion battery technology, starting from the fundamental property of electrode material at nano scale, to tuning the macro-porous electrode morphology for developing high performance batteries.

• Chapter 2 contains a short description of the experimental techniques, synthesis and electrode fabrication methods used at various stages of the research. This includes neutron, and x-ray diffraction, carbonate templating, soft lithography templating and electrochemical testing.

The impact of Nano sizing electrode materials:

• Chapter 3 explores the effect of nanosizing on Li1-xFePO4. The structural impact is

revealed by determining lithium occupancies in Li_1-xFePO₄ using the combination of neutron and x-ray diffraction.

Tailoring electrode morphologies for high Power and Energy Density:

• Chapter 4 is focused on improving and optimising the power output of high energy density C/LiFePO₄ electrodes by developing a novel and low cost “Kitchen Salt” templating technique. Sodium Carbonate salt is used to template high tap density electrodes and sustained significant capacity during high rate electrochemical testing. • Chapter 5 is the extension of “Kitchen salt” templating methods. To generalise the templating strategy, ammonium bicarbonate salt was used to template C/Li₄Ti₅O₁₂ electrode without compromising the electrode tap density.

3D Microelectrode morphologies:

• Chapter 6 explores the low cost soft lithography technique to fabricate 3D microelectrode and complete micro battery. The method was demonstrated by patterning TiO₂ and LiFePO₄ microelectrodes. Electrodes were tested at variable C rates. In addition to this, an attempt was made to integrate solid electrolyte LiPON over 3D TiO₂ microelectrode.

• Chapter 7 is focused on TiO2 anatase microelectrodes prepared by soft lithography,

and discusses the synthesis conditions, electrochemical performance, and impact of charge discharge cycle on the electrode morphology.

25

References

1 National Geographic - India: Drumbeat of Demand, 2012

2 A CITIZENS’ GUIDE TO ENERGY SUBSIDIES IN INDIA International institute for sustainable development global subsidies initiative March (2012).

3 The Nationaal Archief of Netherlands The Dutch Ministry of Education, Culture & Science http://www.flickr.com/photos/nationaalarchief/2948560477/in/ photostream/

4 Michael Root. An In-Depth Guide to Construction, Design,and Use- The TAB™

Battery Book. (McGraw Hill).

5 Padhi, A. K., Nanjundaswamy, K. S. & Goodenough, J. B. Phospho-olivines as positive-electrode materials for rechargeable lithium batteries. J Electrochem Soc 144, 1188-1194 (1997).

6 Ferg, E., Gummow, R. J., Dekock, A. & Thackeray, M. M. Spinal Anodes for Lithium-Ion Batteries. J. Electrochem. Soc. 141, L147-L150 (1994).

7 Wagemaker, M., Borghols, W. J. H. & Mulder, F. M. Large impact of particle size on insertion reactions, A case of anatase LixTiO2 J. Am. Chem. Soc. 129, 4323 (2007).

8 Borghols, W. J. H., Wagemaker, M., Lafont, U., Kelder, E. M. & Mulder, F. M. Impact of nanosizing on lithiated rutile TiO2. Chem Mater 20, 2949-2955, doi:10.1021/ cm703376e (2008).

9 Kavan, L. et al. Li insertion into Li4Ti5O12 p(Spinel) - Charge capability vs. particle size in thin-film electrodes. J. Electrochem. Soc. 150, A1000-A1007 (2003). 10 Delacourt, C., Poizot, P., Levasseur, S. & Masquelier, C. Size effects on

carbon-free LiFePO4 powders. Electrochem Solid St 9, A352-A355 (2006).

11 Sudant, G., Baudrin, E., Larcher, D. & Tarascon, J. M. Electrochemical lithium reactivity with nanotextured anatase-type TiO2. J. Mater. Chem. 15, 1263-1269 (2005).

12 Hu, Y. S., Kienle, L., Guo, Y. G. & Maier, J. High lithium electroactivity of nanometer-sized rutile TiO2. Adv. Mater. 18, 1421-+ (2006).

13 Wagemaker, M., van Eck, E. R. H., Kentgens, A. P. M. & Mulder, F. M. Li-Ion Diffusion in the Equilibrium Nanomorphology of Spinel Li4+xTi5O12. Journal of

26

14 Borghols, W. J. H. et al. The electronic structure and ionic diffusion of nanoscale LiTiO2 anatase. Physical Chemistry Chemical Physics 11, 5742-5748, doi:10.1039/ b823142g (2009).

15 Chen, C. H. et al. Studies of Mg-substituted Li4-xMgxTi5O12 spinel electrodes (0 <= x <= 1) for lithium batteries. J Electrochem Soc 148, A102-A104 (2001). 16 Jung, H.-G., Yoon, C. S., Prakash, J. & Sun, Y.-K. Mesoporous Anatase TiO2

with High Surface Area and Controllable Pore Size by F−-Ion Doping: Applications for High-Power Li-Ion Battery Anode. The Journal of Physical Chemistry C 113, 21258-21263, doi:10.1021/jp908719k (2009).

17 Ali, Z. et al. Design and evaluation of novel Zn doped mesoporous TiO2 based anode material for advanced lithium ion batteries. J Mater Chem 22, 17625-17629 (2012).

18 Wagemaker, M., Ellis, B. L., Luetzenkirchen-Hecht, D., Mulder, F. M. & Nazar, L. F. Proof of Supervalent Doping in Olivine LiFePO4. Chem Mater 20, 6313-6315, doi:10.1021/cm801781k (2008).

19 Wang, Y. Q. et al. Rutile-TiO2 Nanocoating for a High-Rate Li4Ti5O12 Anode of a Lithium-Ion Battery. J Am Chem Soc 134, 7874-7879, doi:Doi 10.1021/Ja301266w (2012).

20 Park, K. S., Benayad, A., Kang, D. J. & Doo, S. G. Nitridation-Driven Conductive Li4Ti5O12 for Lithium Ion Batteries. J Am Chem Soc 130, 14930-+, doi:Doi 10.1021/Ja806104n (2008).

21 Herle, P. S., Ellis, B., Coombs, N. & Nazar, L. F. Nano-network electronic conduction in iron and nickel olivine phosphates. Nat Mater 3, 147-152, (2004).

22 Jung, H. G. et al. Microscale spherical carbon-coated Li4Ti5O12 as ultra high power anode material for lithium batteries. Energ Environ Sci 4, 1345-1351, doi:Doi 10.1039/C0ee00620c (2011).

23 Chung, S.-Y., Bloking, J. T. & Chiang, Y.-M. Electronically conductive phospho-olivines as lithium storage electrodes. Nat Mater 1, 123-128 (2002).

24 Gaberscek, M., Dominko, R. & Jamnik, J. Is small particle size more important than carbon coating? An example study on LiFePO4 cathodes. Electrochem

Commun 9, 2778-2783, doi:10.1016/j.elecom.2007.09.020 (2007).

25 Dominko, R., Gaberscek, M., Bele, A., Mihailovic, D. & Jamnik, J. Carbon nanocoatings on active materials for Li-ion batteries. J Eur Ceram Soc 27, 909-913, doi:DOI 10.1016/j.jeurceramsoc.2006.04.133 (2007).

27

26 Van der Ven, A. & Wagemaker, M. Effect of surface energies and nano-particle size distribution on open circuit voltage of Li-electrodes. Electrochem Commun 11, 881-884, doi:10.1016/j.elecom.2009.02.015 (2009).

27 Wagemaker, M., Mulder, F. M. & van der Ven, A. The role of surface and interface energy on phase stability of nanosized insertion compounds. Adv. Mater. 21, 1-7 (2009).

28 Van der Ven, A., Garikipati, K., Kim, S. & Wagemaker, M. The Role of Coherency Strains on Phase Stability in LixFePO4: Needle Crystallites Minimize Coherency Strain and Overpotential. J Electrochem Soc 156, A949-A957, doi:10.1149/1.3222746 (2009).

29 Meethong, N., Huang, H. Y. S., Carter, W. C. & Chiang, Y. M. Size-dependent lithium miscibility gap in nanoscale Li1-xFePO4. Electrochem Solid St 10, A134-A138 (2007).

30 Chen, J. S. et al. Constructing Hierarchical Spheres from Large Ultrathin Anatase TiO2 Nanosheets with Nearly 100% Exposed (001) Facets for Fast Reversible Lithium Storage. J Am Chem Soc 132, 6124-6130, doi:10.1021/ja100102y. 31 Doyle, M., Fuller, T. F. & Newman, J. Modeling of Galvanostatic Charge and

Discharge of the Lithium/Polymer/Insertion Cell. J Electrochem Soc 140, 1526-1533, doi:10.1149/1.2221597 (1993).

32 Wagemaker, M., van de Krol, R. & van Well, A. A. Nano-morphology of lithiated thin film TiO2 anatase probed with in situ neutron reflectometry. Physica B-Condensed

Matter 336, 124-129 (2003).

33 Singh, D. P., Mulder, F. M., Abdelkader, A. M. & Wagemaker, M. Facile Micro Templating LiFePO4 Electrodes for High Performance Li-Ion Batteries. Adv

Energy Mater 3, 572-578, doi:DOI 10.1002/aenm.201200704 (2013).

34 Linda, G. R., Cuenca;. Costs of Lithium-Ion Batteries for Vehicles. Center for

Transportation Research, Argonne National Laboratory (2000).

35 Arthur, T. S. et al. Three-dimensional electrodes and battery architectures. MRS bulletin 36, 523-531 (2011).

36 Long, J. W., Dunn, B., Rolison, D. R. & White, H. S. Three-Dimensional Battery Architectures. Chem Rev 104, 4463-4492, doi:10.1021/cr020740l (2004).

37 Rolison, D. R. et al. Multifunctional 3D nanoarchitectures for energy storage and conversion. Chem Soc Rev 38, 226-252, doi:Doi 10.1039/B801151f (2009).

28

38 Roberts, M. et al. 3D lithium ion batteries-from fundamentals to fabrication. J Mater Chem 21, 9876-9890 (2011).

39 Edström, K., Brandell, D., Gustafsson, T. & Nyholm, L. Electrodeposition as a Tool for 3D Microbattery Fabrication. (2011).

40 Sides, C. R., Croce, F., Young, V. Y., Martin, C. R. & Scrosati, B. A High-Rate, Nanocomposite LiFePO4 ∕ Carbon Cathode. Electrochemical and Solid-State

Letters 8, A484-A487, doi:10.1149/1.1999916 (2005).

41 Che, G., Lakshmi, B. B., Fisher, E. R. & Martin, C. R. Carbon nanotubule membranes for electrochemical energy storage and production. Nature 393, 346-349 (1998). 42 Nishizawa, M., Mukai, K., Kuwabata, S., Martin, C. R. & Yoneyama, H. Template

Synthesis of Polypyrrole-Coated Spinel LiMn2 O 4 Nanotubules and Their Properties as Cathode Active Materials for Lithium Batteries. J Electrochem Soc 144, 1923-1927, doi:10.1149/1.1837722 (1997).

43 Gowda, S. R., Reddy, A. L. M., Zhan, X. B., Jafry, H. R. & Ajayan, P. M. 3D Nanoporous Nanowire Current Collectors for Thin Film Microbatteries. Nano Lett 12, 1198-1202, doi:Doi 10.1021/Nl2034464 (2012).

44 Taberna, P. L., Mitra, S., Poizot, P., Simon, P. & Tarascon, J. M. High rate capabilities Fe3O4-based Cu nano-architectured electrodes for lithium-ion battery applications. Nat Mater 5, 567-573 (2006).

45 Wang, W. et al. Three-Dimensional Ni/TiO2 Nanowire Network for High Areal Capacity Lithium Ion Microbattery Applications. Nano Lett 12, 655-660, doi:Doi 10.1021/Nl203434g (2012).

46 Kim, S. W. et al. Fabrication and Electrochemical Characterization of TiO2 Three-Dimensional Nanonetwork Based on Peptide Assembly. Acs Nano 3, 1085-1090, doi:Doi 10.1021/Nn900062q (2009).

47 Che, G., Jirage, K. B., Fisher, E. R., Martin, C. R. & Yoneyama, H. Chemical-‐Vapor Deposition-Based Template Synthesis of Microtubular TiS2 Battery Electrodes. J Electrochem Soc 144, 4296-4302, doi:10.1149/1.1838181 (1997).

48 Chan, C. K. et al. High-performance lithium battery anodes using silicon nanowires.

Nat Nano 3, 31-35, (2008).

49 Hong, J., Wang, C., Dudney, N. J. & Lance, M. J. Characterization and Performance of LiFePO4 Thin-Film Cathodes Prepared with Radio-Frequency Magnetron-Sputter Deposition. J Electrochem Soc 154, A805-A809, doi:10.1149/1.2746804 (2007).

29

50 Iriyama, Y. et al. Preparation of LiFePO4 Thin Films by Pulsed Laser Deposition and Their Electrochemical Properties. Electrochemical and Solid-State Letters 7, A340-A342,

Experiments and Techniques

“The language of experiment is more authoritative

than any reasoning: facts can destroy

our ratiocination—not vice versa.”

32

The performance of a battery depends on a number of material properties, including the ionic and electronic mobility in the electrodes, which is strongly influenced by crystallite size and electrode morphology. The general aim of this thesis which is improving the electrode morphology, requires preparing, characterizing, and testing of electrodes. This chapter describes various techniques that were applied during the course of this thesis research to prepare, characterize, and test electrode materials, electrode morphologies, and batteries.

2.1 Material preparation

2.1.1. Nano Sizing LiFePO₄

2.1.2. Synthesis of sub-stoichiometric Li(x)FePO₄

2.2 Material Characterization

2.2.1. Electron Microscopy:

- Transmission Electron Microscopy - Scanning Electron Microscopy 2.2.2. X-Ray Powder Diffraction 2.2.3. Neutron Diffraction

2.3. Electrode preparation

2.3.1. Standard Electrode Fabrication

2.3.2. Electrode Templating using carbonates salt

2.4 Soft Lithography

2.4.1. Types of Soft lithography technique 2.4.2. Fabrication of Microelectrodes:

2.4.3. Preparation of Sols and slurry for soft lithography

2. 5 Cell fabrication and electrochemical measurement

2.5.1. Battery Assembly& Testing: 2.5.2. Electrochemical Testing: 2.5.3. Lifecycle test and C rate: 2.5.4. Battery Voltage:

2.5.5. Capacity, Energy density, and Power density:

2.1 Material preparation

2.1.1. Nano sizing of LiFePO₄:

Nano sized LiFePO₄ was prepared by high-energy ball milling using a Fritsch Pulversette 6 planetary monomill1_{. In general, high-energy planetary ball milling involves crushing and}

disintegration of powders by high-energy collisions with the grinding balls in the grinding bowl of a planetary ball mill. The grinding bowl rotates around its own axis on a main disk whereas the supporting disk is rotating in the opposite direction at a different speed.

33

Due to the difference in speeds, the grinding ball cross the bowl with high speed and hit the material on the opposite wall of the bowl. Along with frictional forces, this high-energy impact grinds the sample material, on the opposite wall of bowl, into a smaller grain size.

The average crystallite size resulting from milling depends on various factors such as amount of sample material, number and material of balls, the size of the balls, the rotational speed, and the time of grinding. To grind 3.0 g of LiFePO₄ an 80 ml stainless steel jar and 32 stainless steel balls were used with a diameter of 10 mm. The rotation speed and time were optimized for synthesizing LiFePO₄ in various sizes as listed in Table 1.

Figure 1: Schematic diagram showing working principle of planetary ball milling.

Table 1: High energy Ball Milling sequence, optimized to produce LiFePO₄ in different crystallite size.

Sr. number Ball milling Sequence Crystallite Size

1 No ball milling 140 nm

2 15min at 300rpm 75 nm

3 60min at 300rpm 39 nm

4 90min at 300rpm - 30min at 450rpm 27 nm

34

2.1.2. Synthesis of sub-stoichiometric Li_xFePO₄

To study the effect of nano sizing on the structure of sub stoichiometric nano sized Li_xFePO₄, materials were prepared with various lithium fractions, x, by chemicaldelithiation and subsequent lithiation.

Though LiFePO₄ is stable in air, the presence of water and oxygen on particle surface can lead to undesired side reactions during (de)lithiation. This holds true especially for nano-sized particles, intrinsically having a larger specific surface area compared to micron size particles. The presence of oxygenand moisture on the surface of nano-sized particles could escalate surface related side reactions during lithiation. To avoid the reaction with oxygen and moisture, all chemicals were thoroughly dried before delithiation and lithiation, and the chemical delithiation and lithiation of LiFePO₄ is carried out in an argon-filled glove box with H₂O and O₂ levels of less than 1ppm.

i. Lithium Extraction

Chemical extraction of lithium from LiFePO₄ (Phostech) was carried out by oxidization using NO₂BF₄ (Aldrich) under an argon atmosphere. NO₂BF₄ is a strong oxidizing agent with a high redox potential of NO₂+_/NO

2 at 5.1 V against Li+/Li (0.0V) and Fe+3/Fe+2 (3.4

volt). The oxidation process can be expressed as

(2.1)

The LiFePO₄ powder is then dispersed in an excess of acetonitrile. For 1 g of LiFePO_4, around 1.8 g of NO₂BF₄ was added. To guarantee complete de-lithiation, the mixture was stirred for 48 to 72 hours at room temperature. Once the supernatant liquid containing impurity (unreacted LiBF₄ etc.) was filtered out, the samples were extensively washed using acetonitrile or ethanol and dried in a vacuum oven at around 100-120°C. After drying, the material was lithiated in an argon-filled glove box.

ii. Lithium Insertion

Chemical lithiation of Li_xFePO₄ for various lithium fractions (x) was carried out by reduction using n-butyllithium under an argon atmosphere. The lithiation of FePO₄ is driven by the low redox potential of n-butyllithium (0.9 V) against Fe+3_/Fe+2 _{(3.4 volt).}

De-lithiated Li₀FePO₄ powder was dispersed in an excess of n-hexane (anhydrous 95%, Aldrich) and a predetermined amount of n-butyl lithium was added while stirring. The materials were left to react for a few days to guarantee complete reaction with the n-butyllithium. Once the supernatant liquid containing impurity and unreacted n-butyllithium were filtered out, the lithiated LixFePO4 powder was extensively washed

using n-hexane and dried in a vacuum oven at around 100-120°C. iii. Controlling Lithium fraction (x) in Li_xFePO₄

The procedure to add lithium into host crystallites is well established and provides decent control of the lithium fraction (x) in Li_xFePO_4, and is shown in the following equation:

35

(2.2)

Where, x is Li fraction in Li_xFePO₄ and is acquired by tuning the molar ratio of FePO₄ and n-butyllithium.

iv. Amount of n-butyllithium required for x= 0.3 in Li_xFePO₄ The molar weight of a 1.6 mol/L solution of n-butyllithium is given by :

(2.3) Where molar weight of n-butyllihtium is 64.06 g/mol and density of n-butyllithium in hexane (is 680g/L).

To attain the desired lithium fraction (x) in host material, the molar weight of n-butyl lithium solution has to balance with the molar weight of the host material.

(2.4) As an example, for 5 g of Li_0.3FePO_4, the amount of n-butyllithium required can be calculated using equation 2.4,

The crystallite size, phase fraction (x) and lithium occupancy in sub-stoichiometric Li_xFePO₄ was probed by a combination of X-ray diffraction and neutron diffraction techniques, shown in Figure 2 and Figure 3.

36

Figure 2: X-Ray Diffraction patterns of various overall compositions of nanosized Li_xFePO₄ (where

x

Figure 3: Rietveld refinement of neutron diffraction data, probing phase fraction and Lithium occupancy in sub-stoichiometric Li_xFePO₄.

37

2.2 Material Characterization

2.2.1. Electron Microscopy

Since 1931 when the first electron microscope was built, electron microscopy has become a valuable characterization tool contributing significantly to all the fields in science. In the present thesis, Transmission Electron Microscopy (TEM) was used to measure the crystallite size of various electrode materials and Scanning Electron Microscopy (SEM) was used to study the particle and electrode morphology.

i. Transmission Electron Microscopy

In transmission electron microscopy (TEM), a high voltage electron beam is produced that interacts with sample. The part of the electron beam that is elastically scatters from the atoms in the sample, carries structural information. The transmitted beam can be directly viewed by magnifying the electron image on photographic film or on phosphor coated fluorescent screen and indirectly by using a CCD camera. In high resolution TEM, the microscope is capable of visualizing planar and line defects, grain boundaries, and many other structural details with atomic scale resolution. Under intermediate magnification, electron diffraction and bright field/dark field imaging modes of the microscope provide formation about morphology, crystal phases, and defects in the sample material.

Example: Specimen preparation and Imaging

Ball milled LiFePO₄ particles were suspended in ethanol using mild ultra-sonication and dispersed on a holy carbon foil (Cu grid) using a pipette. The Tecnai F20ST/STEM (200kV) was used for imaging. For bright field imaging, the undiffracted beam was designated to make a diffraction contrast image. For high-resolution electron microscopy, the central beam was used together with diffracted beams to form an image of the nanosized LiFePO₄ crystals, as shown in Figure 4.

Figure 4: Bright Field image of C-LiFePO₄ (Left)_, High Resolution of TEM image of nanosized Li_0.5FePO₄ (Right).

38

ii. Scanning Electron Microscopy

In scanning electron microscopy (SEM), a focused electron beam is scanned over the sample. The electron beam is generated by a field emission gun using a high electrostatic field. When the electron beam, accelerating the electrons between 1 keV and 30 keV, interacts with the sample secondary electrons and backscattered electrons are produced due to the dissipation of kinetic energy of the electron beam. These signals are detected and contain information about the sample’s surface morphology and composition.

In this thesis the sample morphology, electrode thickness and microstructure, were frequently studied using a scanning electron microscope JEOL 7500F with an inbuilt EDX detector which was applied for elemental analysis and mapping. Figure 5 shows the particle size and morphology of carbon coated LiFePO₄ particles.

2.2.2. X-Ray Powder Diffraction

Since 1895, when X Rays were discovered by Röntgen and later in 1912 when the first X ray diffraction experiment was performed by Friederich, Knipping and von Laue, the X-ray diffraction technique has become indispensable as a non-destructive tool to study the crystal structure and the chemical and phase composition of matter2_.

When X-ray waves interact with an equidistant three dimensional array of atomic planes with spacing d, the elastic interaction induce oscillating dipole moments in the materials electrons (scatterer) producing secondary scattered X-ray waves. The scattered waves, originating from different crystal planes interfere with each other either constructively or destructively. If scattered waves from neighbouring atomic plane will interfere

39

constructively, they produce a Bragg reflection. The interference is constructive when the path difference is an integral multiple of the incident wavelength λ, the condition expressed by Bragg’s law.

(2.5) Where n is an integer, λ is wavelength, d inter planar spacing, µ the incident angle. From Figure 6, the path difference can be expressed as,

(2.6)

For characterization material using X-ray diffraction technique, the PANalytical’s X’Pert PRO X-ray diffractometer applying Cu K® radiation with a monochromator to suppress fluorescence was used.

2.2.3. Neutron Diffraction

In 1945-46, Ernest O. Wollan and Clifford Shull performed pioneering neutron diffraction3

experiments and successfully applied them to many different materials. Unlike the X-rays, neutrons interact directly with the nucleus of the atom and consequently the diffracted intensity of neutrons depends very differently on the type of atom. The relatively high coherent cross section of neutrons makes it possible to determine the position and occupancy of hydrogen and lithium even in the presence of heavy elements i.e. Iron (Fe) (Z= 26).

40

i. Scattering cross section

Considering a highly collimated and mono energetic neutron beam with energy E₀, wave vector k₀ and wavelength λ0=2π/k0 falling on a sample. The number of neutrons that

will be detected in a solid angle dΩ , having energy between E1 and E1+ dE1 and wave

vector k₁ ,is defined as the double differential cross section,

(2.7) Where ©(E₀) is incoming neutron flux and I is number of neutron per second counted at detector. The moment transfer to the neutron is:

(2.8) The transferred energy is:

(2.9) Where m_n is mass of neutron, ħ is Planck’s constant divided by 2π . For elastic scattering,

the incoming k₀ and outgoing k₁ wave vectors have same length and the momentum transfer due to scattering over an angle 2θ is given by:

(2.10) The total number of elastically scattered neutron in a solid angle dΩ , without distinguishing on neutron energy, can be given by integrating Equation 2.7 over the energy which is the differential cross section:

(2.11) The total scatting cross section, i.e. total number of neutrons scattered by the sample is given by:

(2.12)

ii. Coherent and Incoherent Scattering :

In case of elastic scattering, the differential cross-section can be calculated given the transition probability per unit time for the transition from state |i> towards state <f |, as expressed by Fermi’s golden rule:

41

(2.13)

Where V is the interaction potential and

_½

Now equation 2.11 can be expressed as:

(2.14) The neutron-nucleus scattering is isotropic in nature, and defined by the scattering length ‘b’. The scattering length measures the interaction strength of neutron with the nucleus, and interactions strongly depend on the nuclear structure and vary strongly from one nuclear isotope to the other. In contrast to with X-ray scattering, the scattering length for neutrons does not increase linearly with the number of electrons of an atom. Thus, the lighter elements such as the lithium (atomic number 3) can be more visible in the surroundings of heavier elements like Iron (Fe), Phosphorous (P), or Oxygen (O). The scattering length can be complex in nature and the sign (positive or negative) of the real part is subject to the nature of the nucleus and the energy of the incident neutron during scattering. The imaginary part of the scattering length characterizes the absorption (capture of thermal neutron).

If the wavelength of the incoming neutron is large with respect to the nucleus, the approximate interaction potential V (r) in equation 2.13, due to an array of nuclei at the position R_i, is the Fermi pseudo potential and can be expressed as,

_(2.15)

Where <b_i> is the scattering length density of ith nucleus.

Using equation 2.11 and equation 2.13, the differential cross section can be expressed as,

(2.16)

(2.17) In equation 2.17, the coherent part is the interference between the scattered wave from the system in which all nuclei have an average scattering length <b_i>, and shows strong constructive and destructive interference as a function of the wave vector Q, and resulting in the Bragg peaks. The incoherent part is the independent of the wave

42

vector Q and only based on the deviation of the average scattering length. Therefore, the incoherent scattering does not show the constructive or destructive interference, and adds up to the coherent part in order to increase the total scattering length density. iii. Coherent Nuclear Scattering by a Crystal:

A crystal is an array of nuclei with spatial periodicity, whose structural information is defined by a unit cell. For an ideal crystal, the unit cell can be defined as a set of three non-planer translation vectors, a₁, a₂, a₃ which has a volume va, expressed as,

(2.18) The origin of the lth _{unit cell can be written as a lattice vector L, where u,v and w are}

integers, known as cell indices.

(2.19)

Assuming the unit cell consists of n atoms, if r_j is the position of jth atom in the unit cell

with respect to the origin of the unit cell, then the position of jth atom with respect to lth

unit cell can be given by,

(2.20)

The reciprocal lattice for a crystal with the lattice vector L given in Equation 2.20 can be defined in such a way that it will satisfy,

(2.21)

The arbitrary reciprocal vector G can be written as,

(2.22)

Here b_¹ (where ¹ =1, 2, 3) is the basic lattice vector of the reciprocal lattice, defined by,

(2.23)

From equation 2.18 and 2.21, one can drive,

(2.24) (Where, ¹,v = 1,2,3…)

43

The coherent scattering from a crystal having only one atom per unit cell at position r_j= 0, using equation 2.16, can be given by,

(2.25)

Equations 2.19, 2.22 and 2.24 suggest that there will be always constructive interference for Q = 2¼G shown in equation 2.25. The coherent scattering for a crystal containing N unit cells can be given as:

(2.26)

Similarly, the coherent scattering for a crystal containing n atoms per unit cells given by:

(2.27)

Where, F(Q) is the nuclear structure factor of the unit cell and can be denoted by,

(2.28)

2.2.4. Diffraction Pattern and Interpretation:

The coherent scattering by a crystal can be depicted by the Ewald’s circle (sphere) in reciprocal space. Ewald’s construction in reciprocal space represents all-possible points where a plane wave could satisfy the Bragg equation. Figure 7 shows the Ewald circle with a diffraction angle for which the Bragg condition is satisfied. Where, the origin O is the (0 0 0) reciprocal lattice point and P is the origin of the incoming wave vector k₀(=1=λ). The sphere having centre P, radius k0 and going through the origin O is an

Ewald sphere.

For elastic scattering, the diffracted wave vector from origin P has its end at H general point (h k l) on the sphere surface (where distance OH is 1=d_hkl = |h|_hkl), to ensure k₀ = k₁ and satisfies k₁= k₀ + h_hkl. The magnitude of the reciprocal vector h_hkl between the points, H, which rises with increasing 2µ, is given by,

(2.29)

44 i. Rietveld Refinement

The Rietveld refinement model is the most commonly applied technique to extract information from the Neutron and X-ray diffraction patterns, based on the algorithm developed by Hugo Rietveld in the 1960’s.2 _{Rietveld refinement is an optimization tool,}

which requires a standard crystal structure to measure the differences between the measured diffraction data and model structures (calculated). The quality of refinement is primarily based on (1) the quality of diffraction data, and (2) on the set of sequences followed to refine the parameters. During Rietveld refinement the following group of equations are solved by a nonlinear least square method, calculated by;

(2.31)

The equation 2.31 can be formulated as,

Figure 7: Ewald’s Construction in reciprocal space of a diffraction experiment. where the incident wave vector is k₀, the diffracted wave vector is k₁ and where k₀= k₁ (Elastic scattering) and k₁= k₀ + h.

45

Where I _i is the measured data, I _i is a calculated value at point i of the diffraction pattern,k is the pattern (phase) scale factor, n is the total number of measured data points, Â2 is the minimized function (fit error) and w

i is the weight factor for the

ith _{data point and equal to the inverse of variance of measured point i and given by}

wi=[¾ 2_(I

i )]

-1_{. The least square fitting continue until the parameter of the}

calculated model is adjusted to the measured data i.e. until Â2 reaches to the minimum

possible value.

ii. Diffraction Peak Broadening

In an ideal case, the infinitely thick perfect sample and a perfect diffractometer would produce extremely sharp diffraction peaks. However in reality, the Braggs peaks even in good powder diffraction patterns are broad. This broadening arises due to (a) instrumental and (b) sample contribution. The instrumental contribution is primarily caused by the divergence of a beam (spectral distribution, mosaic spread of monochromator) whereas the broadening due to the powder sample is triggered by small crystallite sizes and strain caused by imperfections in the lattice

For small crystallite, according to the Williomson Hall method,4 _{the width of a diffraction}

peak B_L at the Bragg position µ due to a crystallite of finite size D, given by

(2.32)

The average particle and domain sizes were extracted using the Scherrer formula. Using the GSAS algorithm,5 _{equation 2.32 can be further reduced to,}

(2.33)

Where, D_av is the average crystallite size in Angstrom (Å), λ is the wavelength (Å) of incident X-rays, K is the Scherer constant and X is the profile coefficient (either Gaussian or Lorentzian).

2.3 Electrode Preparation

2.3.1. Standard Lithium Ion Battery Electrode Fabrication

The standard lithium-ion battery electrodes were prepared by mixing active material with PVDF binder and Super P carbon black in NMP (Aldrich) solvent. First, the active material and carbon black were mixed in a given ratio using a mortar-pestle. The grinding continued with a frequent scrabbling of the mortar wall until the both the powers were homogenously mixed. Later, this mixture was added to NMP containing the PVDF binder and stirred for a few hours. The overall energy density of the battery is defined by amount of active material loaded on the electrodes. To maximize the active

obs calc

46

material loading in a single coating, the ratio of these electrode components (active material, carbon black, binder) were optimized and given in Table 2.The resulting slurry was casted with different thickness using a doctor blade onto carbon coated aluminium foil. After drying, the electrode layers were mechanically compressed and the resultant electrode thickness was measured by using a Timos Vectra-Touch precision instrument and SEM.

Table 2: The ratio of electrode components to achieve maximum active material loading in single coating (for doctor blade thickness ~ 300µm)

Crystallite Size Active material (LiFePO₄) Carbon Black TIMCAL Binder (PVDF)

Solvent Solid to Liquid Ratio (w/w)

> 150 nm 90% 5% 5% NMP 1:3

< 100 nm 80% 10% 10% NMP 1:3

< 20 nm 60% 20% 20% NMP 1:3

Figure 8: Cross-sectional scanning electron micrograph of the C-LiFePO₄ electrode.

2.3.2. Electrode Templating using carbonate salts

The working of a Lithium ion battery essentially requires a facile charge (ionic electronic) transport in the solid state and ionic transport through the electrolyte and through the porous electrode morphology. However, during the calendaring (mechanical