1 In-Situ Characterization of Li-Ion Battery Electrodes Using Atomic

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In-situ characterization of Li-ion battery electrodes using Atomic Force Microscopy

Thesis

Presented in Partial Fulfillment of the Requirements for Master of Science in the

Graduate School of The Ohio State University

Rahul Kumar Raghava Reddi

Graduate Program in Mechanical Engineering

The Ohio State University

2018

Thesis Committee

Dr. Hanna Cho, Advisor

Dr. Jung Hyun Kim, Advisor

Copyrighted by

Rahul Kumar Raghava Reddi

2018

Abstract

The wetting of the electrodes due to liquid electrolyte used in Li-ion batteries can significantly impact the mechanochemical properties of polymer binders. To observe and characterize this, we employed in-situ atomic force microscopy (AFM) based force spectroscopy. Two different polymer binders for Si anodes, sodium (Na) alginate and polyvinylidene fluoride (PVdF), are immersed in a solution of di-methyl carbonate (DMC) and analyzed using a micro-cantilever with a Si tip, to mimic the actual Si – binder interface in Si anodes. Na-alginate is found to have orders of magnitude higher adhesive forces than

PVdF after immersion in liquid electrolyte, which can be attributed to a functional carboxyl group in Na-alginate, possibly leading to a formation of hydrogen bonds and/or ion-dipole interaction with Si. In comparison PVdF demonstrates considerably weaker adhesive forces owing to Van der Waals’ interaction. It is also found that Na-alginate retains its mechanical strength better than PVdF in liquid electrolyte, by retaining more than 90% of its Youngs modulus, while the Young’s modulus of PVdF decreases to lower than 80% after immersion in electrolyte for 4 hours. Our nano-scale AFM analysis data agrees well with literature that are mostly based on macro-scale characterization. Furthermore, this study demonstrates the benefits of using force spectroscopy, AM-FM, and in-situ AFM techniques to characterize the evolution of interfaces between electrode components at a nano-scale under battery operating conditions.

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Acknowledgments

I would like to acknowledge my advisors, Prof. Cho and Prof. Kim, for their patience, support and guidance through the course of my study. This work would never have been possible without their profound help. I would also like to acknowledge the faculty and staff of the Mechnical Engineering Department at The Ohio State University, for offering their knowledge and resources, helping me progress towards completing my degree.

A special thanks to Michael Lee, for working with me throughout the course of my study and being a wonderful research partner. I would also like to thank all my lab mates and friends for sharing, supporting and encouraging me through good and bad times.

Vita

Personal Information

Name: Rahul Reddi

Education: Bachelor of Technology in Mechanical Engineering,

Indian Institute of Technology Roorkee, India [2011-2015]

Work Experience: Research Assistant,

National Center for Aerospace Innovation and Research,

Indian Institute of Technology Bombay, India [2015-2016]

Contact Information: Phone- 614-632-2275

Email- [email protected]

Fields of Study

Major Field: Mechanical Engineering

Table of Contents

Abstract ...... iii Acknowledgments...... iv Vita ...... v List of Figures ...... vii Chapter 1. Introduction ...... 1 Chapter 2. Background ...... 4 2.1. Lithium ion batteries ...... 4 2.1.1. Components of the Li-ion battery ...... 5 2.1.2. Significance of binder in Li-ion battery electrodes...... 7 2.2. Atomic force microscopy (AFM) ...... 10 2.2.1. Modes of imaging ...... 11 2.2.2. Force Spectroscopy ...... 13 2.2.3. AMFM- Viscoelastic Force Mapping ...... 15 Chapter 3. Experimentation and Methodology ...... 17 3.1. Sample Preparation ...... 19 3.2. Experiments ...... 20 3.2.1. AC/Tapping Mode in Air ...... 20 3.2.2. AC/Tapping Mode in liquid ...... 21 3.2.3. Force Spectroscopy ...... 21 3.2.4. AM-FM Mode ...... 23 Chapter 4. Results and Discussion ...... 25 4.1. Characterization of PVdF ...... 25 4.2. Sodium Alginate ...... 34 4.3. PVdF with other electrode components ...... 36 4.4. AC Mode in liquid and Force Spectroscopy ...... 45 Chapter 5. Conclusion and future work ...... 52 Bibliography ...... 54 Appendix A. Detailed procedure for sample fabrication ...... 58 vi

List of Figures

Figure 1. Comparison of various types of batteries in terms of volumetric and gravimetric energy densities [3] ...... 4 Figure 2. Schematic of a typical Li-ion half cell[4] ...... 5 Figure 3.Schematic of a Li-ion battery electrode [5] ...... 6 Figure 4.Schematic of a typical AFM system[41] ...... 10 Figure 5. Schematic representation of imaging using contact mode[50]...... 11 Figure 6. Schematic representation of imaging using tapping mode[50] ...... 12 Figure 7. Depiction of cantilever deflection vs. tip-substrate distance[41] ...... 13 Figure 8. Schematic of operation in AM-FM Mode[50] ...... 16 Figure 9. (a) Schematic of the Fluid-lite Cell (b) Schematic of the EC Cell [50](c) SEM image of the AC-160 cantilever. Scale bar is 30 µm...... 17 Figure 10. (a) Typical force vs. indentation curve from a PVdF sample measurement (b) Curve fitting using the JKR force model for a sample force curve (c) Parameters available to fit the mathematical models to the force curves ...... 22 Figure 11. (a) SEM image of PVdF. Scale bar is 30 µm...... 26 Figure 12. (a) Topography of PVdF-A using AC Mode (b) Topography of PVdF-C using AC Mode. All scale bars are 10 µm...... 26 Figure 13. Height, amplitude and phase data of PVdF-A, using AC mode. Scale bar is 1 µm...... 27 Figure 14. Height, amplitude and phase data of PVdF-C, using AC mode. Scale bar is 1µm...... 27 Figure 15. Height, amplitude and Young’s modulus data of PVdF-A , using AM-FM mode, along with a histogram of Young’s modulus measurements. Scale bar is 1 µm. Figure 16. Height, amplitude and Young’s modulus data of PVdF-A, using AM-FM mode, along with a histogram of Young’s modulus data values. Scale bar is 1 µm...... 28 Figure 17. Height, amplitude and Young’s modulus data of PVdF-A, using AM-FM mode, along with a histogram of Young’s modulus data values. Scale bar is 1 µm...... 29 Figure 18. Height, amplitude and Young’s modulus data of PVdF-C, using AM-FM mode, along with a histogram of Young’s modulus data values. Scale bar is 5 µm...... 29 Figure 19. Height, amplitude and Young’s modulus data of PVdF-C, using AM-FM mode, along with a histogram of Young’s modulus data values. Scale bar is 1 µm...... 31 Figure 20. Height, amplitude and Young’s modulus data of PVdF-C, using AM-FM mode, along with a histogram of Young’s modulus data values. Scale bar is 1 µm...... 31 Figure 21. Height, amplitude and Young’s modulus data of PVdF-C, using AM-FM mode, along with a histogram of Young’s modulus data values. Scale bar is 1 µm...... 32 Figure 22. Height, amplitude and Young’s modulus data of PVdF-B, using AM-FM mode, along with a histogram of Young’s modulus data values. Scale bar is 5 µm...... 32

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Figure 23. Height, amplitude and Young’s modulus data of PVdF-B, using AM-FM mode, along with a histogram of Young’s modulus data values. Scale bar is 5 µm...... 33 Figure 24. Height, amplitude and Young’s modulus data of PVdF-B, using AM-FM mode, along with a histogram of Young’s modulus data values. Scale bar is 1 µm...... 33 Figure 25. (a) SEM image of PVdF sample. Scale bar is 100 µm (b) Topography of Na- Alginate using AC mode. Scale bar is 10 µm...... 34 Figure 26. Height, Amplitude, Phase of Na-Alginate, using AC Mode. Scale bar is 1 µm...... 35 Figure 27. (a) SEM image of PVdF + Super P (10:90 wt. ratio) sample. Scale bar is 20 µm. (b) Height, Amplitude, Phase of PVdF + Super P (10:90 wt. ratio), using AC Mode. Scale bar is 1 µm...... 36 Figure 28. (a) SEM image of PVdF + LNMO (20:80 wt. ratio) sample. Scale bar is 20 µm. (b) Height, Amplitude, Phase of PVDF + LNMO, using AC Mode. Scale bar is 5 µm...... 38 Figure 29. Height, Amplitude, Young’s modulus of LNMO + PVdF (80:20 wt. ratio) using AM-FM, with a histogram of Young’s modulus data values. Scale bar is 5µm ..... 39 Figure 30. Height, Amplitude, Young’s modulus of LNMO + PVdF (80:20 wt. ratio) using AM-FM, with a histogram of Young’s modulus data values. Scale bar is 1 µm. ... 39 Figure 31. Height, Amplitude, Young’s modulus of LNMO + PVdF (80:20 wt. ratio) using AM-FM, with a histogram of Young’s modulus data values. Scale bar is 1 µm. ... 40 Figure 32. Height, Amplitude, Young’s modulus of LNMO + PVdF (80:20 wt. ratio) using AM-FM, with a histogram of Young’s modulus data values. Scale bar is 1 µm. ... 40 Figure 33. Force curves on PVdF + LNMO (20:80 wt. ratio) sample ...... 42 Figure 34. (a) SEM image of PVdF + Super P + LNMO (10:10:80 wt. ratio) sample. Scale bar is 20 µm ...... 43 Figure 35. Height, Amplitude, Phase of LNMO + PVdF + Super P (80:10:10 wt. ratio), using AC Mode. Scale bar is 1 µm...... 44 Figure 36. Height, Amplitude, and Young’s modulus of LNMO + PVdF + Super P (80:10:10 wt. ratio) using AM-FM, with a histogram of Young’s modulus data values. Scale bar is 1µm...... 44 Figure 37. AFM nano-indentation/force spectroscopy on PVdF, for each hour immersed in electrolyte: (a) AFM topography, (b) modulus map, (c) histograms of modulus. The scale bar is 1 µm...... 46 Figure 38. AFM nano-indentation/force spectroscopy on Na-alginate, for each hour immersed in electrolyte: (a) AFM topography, (b) modulus map, (c) histograms of modulus. The scale bar is 1 µm...... 47 Figure 39. Force Spectroscopy Results (a) Force-distance curves on PVdF, for each hour immersed in electrolyte, (b) modulus of PVdF as a function of immersion time, (c) force- distance curve on Na-alginate, for each hour immersed in electrolyte, (d) modulus of Na- alginate as a function of immersion time...... 48 Figure 40. Adhesion trend of Na-alginate with time of immersion ...... 49

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Chapter 1. Introduction

Energy is the driving force for all forms of activities that we perform and observe in our daily lives. It is used in all sectors of industry, transportation, communications, agriculture, and a variety of household services. Due to the importance of energy as a resource, there has been a tremendous focus on its production, storage, sustainability, and conservation.

The last few decades particularly have seen electrical energy develop as the main resource for all kinds of industrial and household applications. From electric trains to television remotes, everything utilizes electricity supplied directly or from storage devices like batteries. The advancement of electronics and more recently, electric vehicles (EVs) has proportionally increased the demand for compact rechargeable batteries with high energy densities. Lithium ion (Li-ion) batteries appeared as a welcome solution to this demand, because of their superior energy density, low self-discharge and high voltages.

However, there are certain critical limitations in the application of Li-ion batteries, especially in areas like EVs. The ever-increasing consumer demand for better performing batteries with high capacities has led to a lot of research in developing new battery materials. The battery electrode composite has evolved with the introduction of silicon (Si) based anodes to replace conventional graphite anodes, delivering capacities many fold higher. New cathode materials were also introduced for high-voltage applications. Though this indicated better performance, the Si anodes demonstrated a volume expansion of more than 300% [1] and there were parasitic reactions occurring at the electrolyte-cathode

1 interfaces in the case of the high-voltage cathodes [2]. Polyvinylidene fluoride (PVdF) is a standard binder material in the electrodes, adopted due to its chemical and electrochemical inertness with lithium or electrolyte, under battery operating conditions. PVdF binds to the other electrode components by means of weak Van der Waals’ forces, with its Youngs modulus degrading drastically when wetted by electrolyte. In case of these new battery materials, this led to mechanical failure of the electrodes, causing cracking and delamination, leading to battery failure. This led to the introduction of polymers like sodium alginate, with stronger bonding types like hydrogen bonding for better adhesion to the electrode components, also demonstrating better cycle life.

A strong affinity between binders and other electrode components, and high elastic modulus of binders are thus found to be critical mechanical properties in determining the long-term mechanical integrity of battery. It is because the stronger bonding forces and higher stiffness of binders helps the endurance limit of the binder exceed the internal stresses in the electrode caused by the volume expansion. The favorable mechanical properties are considered to improve the electrochemical stability and cycle lifetime of Li- ion batteries. Even though we can correlate the binding strength to the nature of chemical bonding based on the knowledge of electrochemistry, the actual impact of the chemical bonding in the particle-to-particle interconnectivity and the resulting long-term electrochemical stability are still in the gray area.

To understand the mechano-chemical behavior of these polymers and other electrode components, it is essential that their properties, especially critical ones like adhesive forces and Youngs modulus, are monitored and measured at a micro/nano scale,

2 in an environment which emulates battery operating conditions. Atomic Force Microscopy

(AFM) is a great tool for this purpose because of the ease of operation under in-situ conditions., offering techniques like force spectroscopy.The current research aims to characterize two popular binder materials for Li-ion batteries, PVdF and sodium alginate, using AFM, with the objective of developing a technique to understand the fundamental mechano-chemical characteristics of polymer binders, which is the main limiting factor for improving binder functionality.

Chapter 2. Background

2.1. Lithium ion batteries

First developed in the 1970s, Lithium ion (Li-ion) batteries have since undergone a lot of research and development. The first commercial Li-ion battery was released by Sony in

1991. They have become a popular choice since then, compared to other types of batteries like lead-acid and nickel metal hydrides, because of their superior energy density, low self- discharge and high voltages.

Figure 1. Comparison of various types of batteries in terms of volumetric and gravimetric

energy densities [3]

2.1.1. Components of the Li-ion battery

The primary functional components of lithium-ion batteries are electrodes, an electrolyte, and a separator. Conventional positive electrodes have been made with following three materials: a layered oxide (such as lithium cobalt oxide), a polyanion (such as lithium iron phosphate) or a spinel (such as lithium manganese oxide). The most commercially popular negative electrodes have been made with graphite. The electrolyte has been made with a lithium salt (e.g., LiPF6) dissolved in organic solvents, typically a mixture of carbonates such as ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC).

The separator is thin plastic films made with polypropylene (PP) or polyethylene (PE). It has 20 - 25 µm thickness and controlled porosity that absorbs electrolytes in it. It functions as Li-ion flow channel while preventing a short-circuit of cells.

Figure 2. Schematic of a typical Li-ion half cell[4]

Though Li-ion battery has been successfully applied in a variety of electronics and electric vehicles (EVs), there is a constant demand from consumers for more compact, better performing batteries, which has generated a lot of research activity in this area.

Tarascon et.al. [3] point out the limitations in the development of Li-ion batteries regarding the synthesis, characterization and electrochemical performance, and also explain the importance of developing the energy density and cycle life of batteries while ensuring safety in operation. From the discussion about limitations in Li-ion batteries that Tarascon et.al. elaborate, we can conclude that among the components of the battery system, (i.e., electrodes, electrolytes, and separator), the electrodes play a pivotal role in determining the performance of the battery. A Li-ion battery electrode is a composite of active material, polymeric binder, carbon conductors, and metal current collector. The active material stores lithium ions in the electrode; the binder holds the electrode components together; and carbon conductors increase the electronic conductivity.

Figure 3.Schematic of a Li-ion battery electrode [5]

There have been extensive efforts to develop advanced anode and cathode materials for high rate applications. Kang et.al. [5] developed a layered cathode, Li(Ni0.5Mn0.5)O2, which showed a considerable retention in charge and discharge capacity at high rates, and

6 at the same time replaced the conventional and more expensive LiCoO2. Thackeray et.al.

[6] observed that Li2MnO3-stabilized LiMO2 (M = Mn, Ni, Co) electrodes with highly complex ‘‘composite’’ layered-type structures. This composition can be formulated as x

Li2MnO3·(1-x) LiMO2 (M=Mn, Ni, Co) and capable of delivering high specific capacity of ~ 260 mAh g-1 that is close to its theoretical capacity. In contrast, commercial layered

LiMO2 (M= Mn, Ni, Co) electrodes such as LiCoO2 offer only 50–60% (e.g., 140 – 170 mAh g-1) of their theoretical capacity. In addition, there has been extensive research efforts to find high-energy anode materials that can replace the conventional graphite. Chan et.al.

[7] made use of Ge nanowire electrodes fabricated by using vapor-liquid-solid growth on metallic current collector substrates. They reported an initial discharge capacity of 1141 mAh g-1. Courtel et al. [8] and Poizot et al. [9] developed anodes from spinel and nano- sized transition metal oxides, respectively, and reported considerable increase in capacity compared with graphite anodes. Out of all these materials, Silicon would seem to be the best possible alternative for the graphite or carbon anode because its high specific capacity

(> 3000 mAh g-1) [10], [11]. However, Si suffers from 400 % volumetric expansion during lithiation, and Kasavajjula et al. [12] discussed its detrimental roles in terms of mechanical stability and cycling stability.

2.1.2. Significance of binder in Li-ion battery electrodes

In light of these developments, the significance of binders’ function has been emphasized in determining the mechanical integrity of the composite electrode and, thereby, its long- term stability. There have been studies to demonstrate how the selection of binder critically

7 affects the electrochemical performance of electrodes. Chou et.al. [13] reviewed how even though the binder is just a small component of the electrode, it can play a significant role contribution to the battery performance in terms of specific capacity, rate capability, cycle life, etc. Choi et.al. [14] reported how the binder content affects specific capacity and capacity retention, also stating that cyclability is enhanced when a binder is more compatible with liquid electrolyte.

In conventional Li-ion batteries, polyvinylidene fluoride (PVdF) has been adopted as a standard binder because it is chemically and electrochemically inert at battery operating conditions without unwanted reaction with lithium and electrolyte [15]–[17].

PVdF binder, however, intrinsically retains potential environmental and cost issues because it employs N-methylpyrrolidone (NMP) solvent during its manufacturing process

[18]. There were discoveries that aqueous binders performed better with batteries, with the added benefit that they are greener, and cheaper to use with battery electrodes [19]–[21].

More importantly, PVdF binds to the electrode particles via weak van der Waals force and its elastic modulus is degraded significantly in a wet state [1], [22], which unwantedly leads to the mechanical failure of electrodes (e.g., crack or delamination) after cycling [23]. This problem becomes prominent for the case of Si-based anodes due to its large volume expansion [1]. To counter this problem, alternative polymer materials with stronger bonding types (e.g., hydrogen, ion-dipole, or covalent bonding) were actively proposed to enhance the adhesion to electrode particles. Bridel et.al. [24] demonstrated the benefit of using carboxymethyl cellulose (CMC) by showing improved cycle life and low cost compared with those of PVdF. Kovalenko et al. [25] and Magasinski et al. [26]

8 demonstrated the benefit of using styrene-butadiene rubber (SBR), polyacrylic acid (PAA), and alginate by showing improved cycle life, possibly attributed to their stronger chemical bonding through functional groups, and favorable mechanical properties in contact with electrolyte.

Thus, even though the significance of the binders function was established, current practices of binder selection rely upon the physical properties of polymers and macro-scale adhesion testing, mostly in their dry state [27]. In battery cells, however, binders are wetted in liquid electrolytes as a form of nano-scale connectors between heterogeneous electrode particles and metallic current collectors. It is reasonable to assume that a nanometer-scale thin layer of binder in battery electrodes exhibits different mechanical, chemical, and electrochemical behaviors compared with bulk and dry polymers, because of its interaction with electrolyte under variable electric potential and temperature. The material affinity and other mechanochemical properties between the binder and the other components of the electrode can have a significant impact on the mechanical integrity and electrochemical stabilities of the electrode. From the literature available [16], [19], [27]–[40], we can identify that the adhesive forces of the binder with the remaining components of the electrode and its retention of Young’s modulus in electrolyte are important factors that may determine the suitability of a binder. This is because the stronger bonding forces and higher stiffness of binders helps the endurance limit of the binder exceed the internal stresses in the electrode caused by high volume expansions. The favorable mechanical properties are considered to improve the electrochemical stability and cycle life of Li-ion batteries. It is

9 therefore important to characterize various binders at nano-scales, systematically, by identifying the critical properties of the binders and monitoring them in electrolyte.

2.2. Atomic force microscopy (AFM)

AFM is a very high-resolution form of surface probe microscopy, with demonstrated resolutions on the order of fractions of nanometer. The information is gathered by interactions of a mechanical probe with the surface under observation, as opposed to other forms of microscopy. AFM thus has the capability to not only image the surface at high resolutions, but also measure the mechanical properties through nano-indentation of the surface. The movement of the sample and probe are facilitated by piezoelectric elements controlled electronically, which ensures accurate movements.

Figure 4.Schematic of a typical AFM system[41] 10

2.2.1. Modes of imaging

Contact Mode

Contact Mode AFM, also known as Constant Force Mode, is one of the more commonly used imaging modes in AFM. It is often used in imaging hard materials, in some electrical techniques, and in imaging biological materials, such as cells. In contact mode, the tip is moved across the surface of the sample while it is in contact with the surface, and the contours of the surface are measured either using the deflection of the cantilever directly or, more commonly, using the feedback signal required to keep the cantilever at a constant position. Because the measurement of a static signal is prone to noise and drift, low stiffness cantilevers (i.e. cantilevers with a low spring constant, k) are used to achieve a large enough deflection signal while keeping the interaction force low. Close to the surface of the sample, attractive forces can be quite strong, causing the tip to snap-in to the surface.

Thus, contact mode AFM is almost always done at a depth where the overall force is repulsive, that is, in firm "contact" with the solid surface.

Figure 5. Schematic representation of imaging using contact mode[50]

AC Mode

When the probe tip is close enough to the sample for short-range forces to become detectable, there are often liquid meniscus layers formed due to atmospheric conditions.

Preventing the tip from sticking to the surface because of these menisci presents a major problem for contact mode in ambient conditions. AC Mode, or tapping mode, was developed to bypass this problem. In tapping mode, the cantilever is driven to oscillate up and down at its resonant frequency. This oscillation is achieved with a small piezo element in the cantilever holder. The frequency and amplitude of the driving signal are kept constant, leading to a constant amplitude of the cantilever oscillation as long as there is no drift or interaction with the surface. The interaction of forces acting on the cantilever when the tip comes close to the surface, including Van der Waals forces, dipole-dipole interactions, electrostatic forces, etc. causes the amplitude of the cantilever's oscillation to change (usually decrease) as the tip gets closer to the sample. This amplitude is used as the parameter that goes into the electronic servo that controls the height of the cantilever above the sample. The servo adjusts the height to maintain a set cantilever oscillation amplitude as the cantilever is scanned over the sample.

Figure 6. Schematic representation of imaging using tapping mode[50]

When operating in tapping mode, the phase of the cantilever's oscillation with respect to the driving signal can be recorded as well. This signal channel contains information about the energy dissipated by the cantilever in each oscillation cycle. Samples that contain regions of varying stiffness or with different adhesion properties can give a contrast in this channel that is not visible in the topographic image. This feature is extremely useful for characterizing a polymer having varied mechanical properties, or a mixture of materials present (like in a Li-ion battery).

2.2.2. Force Spectroscopy

Force spectroscopy is a set of techniques for the study of the interactions and the binding forces between individual molecules. These methods can be used to measure the mechanical properties of single polymer molecules or proteins, or individual chemical bonds.

(a) (b)

(c)

Figure 7. Depiction of cantilever deflection vs. tip-substrate distance[41]

Figure 7 shows a typical force curve obtained by indenting a sample using an AFM probe. The slope of the curve depends on the stiffness of the cantilever, and the Young’s modulus of the substrate. By employing a proper contact model (e.g., Hertzian contact model, Johnson-Kendall-Roberts model), the Young’s modulus of the surface can be estimated by fitting the measured force-distance curve. The dip in the curve during retraction, depicted by region (iv) in the figure, indicates the adhesion that the tip has with the surface. For the same probe, the larger the adhesion, the larger the deflection will be.

In a dry state, the adhesive force from the meniscus formation (due to atmospheric moisture conditions) at the tip contact area is quite significant and, therefore, it is not easy to characterize the bonding force between two materials. Whereas when the measurements are performed in a liquid environment, the effect of the meniscus force is eliminated. The analysis of the surface using these methods is known as Force Spectroscopy.

Force spectroscopy, using AFM, is a very versatile tool and has been used to characterize a variety of materials. This technique was also employed previously to research Li-ion batteries. Aurbach et.al. [42] demonstrated the surface phenomena on graphite anodes, during lithium insertion, using in-situ AFM. Jeong et.al. [43] studied the formation of solid electrolyte interphase (SEI) on graphite anodes in Li-ion batteries, and the effect of various organic co-solvents on the SEI. Balke et.al. [44] even succeeded in mapping the ion diffusion in Li-ion battery cathodes using in-situ AFM. Even though the use of in-situ AFM is a well-known technique, it has not been actively employed to study the evolution of physical properties of binder materials in electrolyte, which, as we discussed in the previous section, may be a critical factor for selection of binders. This

14 research study attempts to perform a systematic analysis on the evolution of binder properties of PVdF and Na-alginate, while wetting in liquid electrolyte solvent by using

AFM.

2.2.3. AMFM- Viscoelastic Force Mapping

The roots of AM-FM Mode lie in tapping mode and bimodal AFM. In bimodal AFM, two resonant modes are excited. The first (lower) mode is operated in regular tapping or AM mode, yielding the usual topography and phase data. The second (higher) resonance can also be operated in AM mode or with another feedback approach. Bimodal images can provide qualitative contrast that cannot be obtained with conventional tapping mode. AM-

FM Mode is one such approach for improved quantitative interpretation. As a bimodal technique, it operates simultaneously at two different cantilever frequencies. The cantilever’s first and second flexural resonances are typically used, although operation with higher order resonances is not uncommon.

Figure 8. Schematic of operation in AM-FM Mode[50]

The lower mode operates in standard tapping (AM) mode, and the higher mode operates in frequency modulation (FM) mode with frequency feedback. The AM mode provides topography and loss tangent information. The FM mode, which operates at a much smaller amplitude, acts as a ride-along signal that sensitively probes the tip-sample interaction. By using the same imaging feedback as normal tapping mode, data acquisition in AM-FM Mode is reliable and rapid.

It is extremely difficult to classify the different components of an electrode because of the extremely thin layer of binder which is mixed along with carbon, covering the active material. AMFM is particularly useful in this regard, because it can be used to detect the presence of binder.

Chapter 3. Experimentation and Methodology

The MFP 3D AFM (Asylum Research) was used in this study. The Fluid Lite Cell (Asylum

Research) and Electrochemical(EC) Cell (Asylum Research) were used to provide a closed, liquid environment for the sample. In order to emulate the actual Si-binder interactions, a cantilever with a silicon tip, AC160TS-R3 (Asylum Research), was used to gather scan and force data. This cantilever's tip has nominal radius of 7 nm and a measured stiffness of 36.1

N/m. The cantilever stiffness is sufficient to provide nanoindentation data for the materials of interest.

Figure 9. (a) Schematic of the Fluid-lite Cell (b) Schematic of the EC Cell [50](c) SEM

image of the AC-160 cantilever. Scale bar is 30 µm.

Figure 9. contd.

Clamping assembly

FKM Membrane/Bellows Cantilever Holder Membrane Threaded Clamp

EC Cell PEEK dish

Bottom sealing assembly

3.1. Sample Preparation

Three kinds of PVdF binder samples were used. Two of the samples were made using a simple solution cast process. For the first sample, 10 g Kynar HSV900 PVdF was added to 50 ml of N-methylpyrrolidone (NMP) solvent, and vigorously stirred to dissolve the

PVdF. This was then left to dry in air till the sample appeared to be dry. This sample type will be referred to as PVDF-A in this document. Additionally, a sample was fabricated using a solution of similar concentrationand a spin coater. This sample type will be referred to as PVDF-B in this document.

From initial experiments, it was evident that the process of sample fabrication has a major effect on the mechanical experiments. Therefore, the samples since were fabricated in an extremely controlled fashion. PVdF binder samples were made by adding

5 g of Kynar HSV900 PVdF to 50 ml of N-methylpyrrolidone (NMP) solvent, and vigorously stirred using an ultrasonic shaker to dissolve the PVdF. A few drops of this solution were then cast on a stainless-steel specimen disc. The sample was then left to dry in a vacuum oven, at 120 oC for 12-14 hours. This sample type will be referred to as PVDF-

C in this document.

A solution of Na-alginate was prepared in a similar way, by dissolving 5 g of Na- alginate in 100 ml of deionized water. A few drops of this solution were then cast on a stainless-steel specimen disc. The sample was then left to dry in a vacuum oven, at 80 degrees Celsius for 8-10 hours. The casting process resulted in a sample of each binder approximately 2 μm thick.

For characterizing electrode surfaces, a few combinations of binder, active material and carbon nano powder were also fabricated using a similar solution cast process. First,

PVdF and Lithium Nickel Manganese Oxide (LNMO) were mixed in a 20:80 ratio and made into a slurry using NMP solvent. The slurry was then cast on a stainless-steel specimen disc and dried at 120 degrees Clesius in a vacuum oven. Next, another sample was made by mixing PVdF and Super P (carbon nanopowder) in a 10:90 ratio and cast on the specimen disc in a similar way. An electrode sample was also fabricated with PVdF,

Super P and LNMO in a 10:10:80 ratio. The slurry was spread on an aluminium current collector with a controlled thickness of 200 µm.

3.2. Experiments

3.2.1. AC/Tapping Mode in Air

The AC 160TS-R3 cantilever was tuned to its resonant frequency (~300 kHz) with a free amplitude of 1.0 V, and its optical laser sensitivity was calibrated against a sapphire sample. The prepared samples were first analyzed in a dry state. The samples were placed on a sample holder and held in place with magnetic clamps. The surface of each sample was scanned using the AC160TS-R3 cantilever in tapping mode, over areas ranging from

50µm x 50µm to 5µm x 5µm. The height, amplitude and phase data of the samples was recorded. The topography of the various samples was studied in order to develop an understanding of the composition, which in turn can help characterize the surfaces of different electrodes.

3.2.2. AC/Tapping Mode in liquid

The AC 160TS-R3 cantilever was tuned to its resonant frequency (~170 kHz) with a free amplitude of 1.0 V. It is to be noted that the resonant frequency of the cantilever in liquid decreases to about half of its resonant frequency in air. Only the binder samples, PVdF and

Na alginate, were studied in a liquid environment. The samples were placed in the Fluid

Lite cell and held in place with magnetic clamp. Dimethyl carbonate (DMC), a common electrolyte solvent used in Li-ion batteries, was then added into the cell using a pipette.

This liquid environment was designed to simulate thewet condition of a binder inside of a battery cell. The samples were again scanned using the same process. The height, amplitude and phase data of the samples were recorded at the end of each hour.

3.2.3. Force Spectroscopy

The stiffness of the AC160TS-R3 cantilever being used was calculated using the Sader method, and was found to be 36.1 N/m. The force data was collected in the same scan area following the tapping mode scans, initially in dry state and at the end of each hour once immersed in DMC. The deflection of the cantilever was recorded as it was pressed into and retracted from the sample of the surface. The force curves were taken regularly across the scan area, dividing it into a 16 x 16 grid. The force curves obtained were then used to calculate the adhesive force via the maximum adhesive deflection. Finally, these force curves were fit to a Hertzian or Johnson-Kendall-Roberts nanoindentation model depending on the prominence of the attractive region. This model was used to calculate the

Young’s modulus of each sample at each point and time. Thus, a large sample size of data for each material property at each time was obtained.

Contact regime

Approximate curve fitted to the force curve

Figure 10. (a) Typical force vs. indentation curve from a PVdF sample measurement (b)

Curve fitting using the JKR force model for a sample force curve (c) Parameters available

to fit the mathematical models to the force curves 22

Figure 10 contd.

3.2.4. AM-FM Mode

Along with characterizing the binder adhesive forces and mechanical properties, it is also essential to observe the changes it experiences when it is being cycled under battery operating conditions. With this idea, a few experiments were performed to attempt to characterize the binder when present in the electrode along with active material and carbon nano-particles. Due to the restriction that the AM-FM can be performed only in air, these

23 experiments were performed only in the dry condition. The AC 160TS-R3 cantilever was tuned to its resonant frequency (~300 kHz) with a free amplitude of 1.0 V. The cantilever was also tuned to a second mode frequency (~1 MHz), with an amplitude of 0.1 V. The prepared samples were analyzed in a dry state. The samples were placed on a sample holder and held in place with magnetic clamps. The surface of each sample was scanned using the

AC160TS-R3 cantilever in AMFM mode, over areas ranging from 50µm x 50µm to 5µm x 5µm. The height, amplitude, phase, frequency, dissipation and Youngs modulus data of the samples were recorded.

Chapter 4. Results and Discussion

4.1. Characterization of PVdF

PVdF is one of the most extensively used binders in Li-ion batteries, and therefore extensive study was performed on various samples of PVdF. As we discussed in the sample preparation section, 3 types of PVdF samples were used in the experiments. PVdF-A was solution cast and air dried, PVdF-B was spin coated and PVdF-C was solution cast and dried in vacuum. Figure 11 shows an SEM image of the PVdF -C sample. PVdF is typically found to demonstrate a porous and rugged topography, which is observed from this image.

Figure 12 shows the height images of the PVdF-A and PVdF-C samples obtained from AC mode scans. We can correlate the similarity in both these images, about the topography displayed by the PVdF binder sample. The reduction in concentration, and drying in the vacuum chamber seem to have an effect on the sample, as evident from Figure 12. PVdF-

C appears to be more sparsely distributed and more porous than PVdF-A. This becomes more clear by comparing Figures 13 and 14, which show the topography of the samples at a higher resolution. PVdF-A seems to form more clusters than PVdF-C.

Figures 15-17 show the height, amplitude and Young’s modulus data obtained from

AM-FM scans of PVdF-A. The histograms show that the Young’s modulus value is around

4 GPa. However, in the method of sample fabrication, there was a high chance that there was residual traces of NMP solvent in the sample. This might be affecting the properties of the PVdF and therefore for more reliable data, we can observe PVdF-C

Figure 11. (a) SEM image of PVdF. Scale bar is 30 µm.

Figure 12. (a) Topography of PVdF-A using AC Mode (b) Topography of PVdF-C using

AC Mode. All scale bars are 10 µm.

Figure 13. Height, amplitude and phase data of PVdF-A, using AC mode. Scale bar is 1

µm.

Figure 14. Height, amplitude and phase data of PVdF-C, using AC mode. Scale bar is

1µm.

Figure 15. Height, amplitude and Young’s modulus data of PVdF-A , using AM-FM mode, along with a histogram of Young’s modulus measurements. Scale bar is 1 µm.

Figure 16. Height, amplitude and Young’s modulus data of PVdF-A, using AM-FM

mode, along with a histogram of Young’s modulus data values. Scale bar is 1 µm. 28

Figure 17. Height, amplitude and Young’s modulus data of PVdF-A, using AM-FM

mode, along with a histogram of Young’s modulus data values. Scale bar is 1 µm.

Figure 18. Height, amplitude and Young’s modulus data of PVdF-C, using AM-FM

mode, along with a histogram of Young’s modulus data values. Scale bar is 5 µm. 29

Figures 18-21 show the height, amplitude and Young’s modulus data obtained from

PVdF-C that was fabricated in a more controlled way, and was treated in the vacuum oven for a sufficient amount of time to completely eliminate the residual NMP. The resulting modulus data shows that for most areas, the mean Young’s modulus lies in between 6 and

8 GPa. This value correlates very well to the literature values [45], though those values are based off of macro-scale testing. Although histograms provide a good analysis of the scan data, Young’s modulus mapping can be tricky due to the drastic changes in topography, causing frequency shift. This might cause some areas to appear to have a very low Young’s modulus when in reality they are not. Therefore some of the areas exhibiting such an issue are masked, which lead to the abrupt cut of data in some of the histograms.

Figures 22-24 show the AM-FM data obtained from a sample of PVdF-C. This spin coated sample appears to have a much more uniform texture compared to the solution cast samples. The Young’s modulus data for this sample, however, differs for different areas observed. The histogram from Figure 22 shows that the Young’s modulus of PVdF-C is around 7 GPa, but on the contrary Figures 23 and 24 show that the Young’s modulus for the same material is approximately 2 GPa. It is not clear why there is such a discrepancy in value. These experiments give an estimate of the range of Young’s modulus of PVdF.

This data can hence be used to differentiate between PVdF and other components in a composite mixture(i.e., electrodes). In particular, the AM- can be an effective tool that analyzes data the topography and the mechanical properties simultaneously.

Figure 19. Height, amplitude and Young’s modulus data of PVdF-C, using AM-FM

mode, along with a histogram of Young’s modulus data values. Scale bar is 1 µm.

Figure 20. Height, amplitude and Young’s modulus data of PVdF-C, using AM-FM mode, along with a histogram of Young’s modulus data values. Scale bar is 1 µm.

Figure 21. Height, amplitude and Young’s modulus data of PVdF-C, using AM-FM mode, along with a histogram of Young’s modulus data values. Scale bar is 1 µm.

Figure 22. Height, amplitude and Young’s modulus data of PVdF-B, using AM-FM

mode, along with a histogram of Young’s modulus data values. Scale bar is 5 µm.

Figure 23. Height, amplitude and Young’s modulus data of PVdF-B, using AM-FM

mode, along with a histogram of Young’s modulus data values. Scale bar is 5 µm.

Figure 24. Height, amplitude and Young’s modulus data of PVdF-B, using AM-FM

mode, along with a histogram of Young’s modulus data values. Scale bar is 1 µm. 33

4.2. Sodium Alginate

Figure 25 shows a comparison of the SEM image of Na-alginate and the topographical image obtained using AC mode. The images closely relate to each other in demonstrating the surface texture of the sample. In comparison to the PVdF, the Na-alginate sample showed smoother texture, without large concentrated chunks. Visually, the Na-alginate sample appears to be a thin shiny film on the surface of the specimen disc(i.e., substrate) being used. Figure 26 shows topography data obtained using AC mode, at a higher resolution. The smooth texture reduces the height difference, making scanning easier on the alginate.

Figure 25. (a) SEM image of PVdF sample. Scale bar is 100 µm (b) Topography of Na- Alginate using AC mode. Scale bar is 10 µm.

Figure 26. Height, Amplitude, Phase of Na-Alginate, using AC Mode. Scale bar is 1 µm.

4.3. PVdF with other electrode components

Figure 27. (a) SEM image of PVdF + Super P (10:90 wt. ratio) sample. Scale bar is 20

µm. (b) Height, Amplitude, Phase of PVdF + Super P (10:90 wt. ratio), using AC Mode.

Scale bar is 1 µm.

In this study, we focused on the characterization of composite electrode by using AFM.

First, we examined PVdF and Super P (conductive carbon) composite. Figure 27 shows show the topography and phase data obtained from the composite with 10:90 wt of PVdF

36 and Super P. The texture and topography of this PVdF + Super P sample is observed to be considerably different from the PVdF sample. The carbon nano-particles (50 nm in diameter) are well dispersed throughout the sample. However, it cannot be discerned from the phase data whether the PVdF covered the top surface of the sample, or if there was no covering at all. The skewed ratio of 10:90 (typical electrodes use 1:1 wt. ratio for binder and carbon) could be a reason to not observe a clear cluster of binder particles or patches of the binder covering the carbon particles. The sample was observed to have a crumbly texture, and therefore Force Spectroscopy could have resulted in false data. AM-FM mode scans on the sample were not successful, possibly due to the nature of force between the Si and carbon(i.e., Super P). The net phase while using AM-FM mode was in the attractive region, in which the AM-FM mode cannot operate.

Figure 28 (a) shows the SEM image a sample of PVdF and LNMO mixed in a wt. ratio of 20:80, and Figure 28 (b) shows the results obtained from using tapping mode. The overall topography observed is very similar to the SEM image. However the phase data obtained indicates a couple of big phase shifts compared to the rest of the scan area. This could be a result of a difference in the covering of these areas by PVdF. This shows that just the phase data is insufficient for characterizing the surface of an electrode with multiple components.

Figure 28. (a) SEM image of PVdF + LNMO (20:80 wt. ratio) sample. Scale bar is 20

µm. (b) Height, Amplitude, Phase of PVDF + LNMO, using AC Mode. Scale bar is 5

µm.

Figure 29. Height, Amplitude, Young’s modulus of LNMO + PVdF (80:20 wt. ratio) using AM-FM, with a histogram of Young’s modulus data values. Scale bar is 5µm

Figure 30. Height, Amplitude, Young’s modulus of LNMO + PVdF (80:20 wt. ratio) using AM-FM, with a histogram of Young’s modulus data values. Scale bar is 1 µm.

Figure 31. Height, Amplitude, Young’s modulus of LNMO + PVdF (80:20 wt. ratio) using AM-FM, with a histogram of Young’s modulus data values. Scale bar is 1 µm.

Figure 32. Height, Amplitude, Young’s modulus of LNMO + PVdF (80:20 wt. ratio) using AM-FM, with a histogram of Young’s modulus data values. Scale bar is 1 µm. 40

Figures 29-31 show AM-FM data from the LNMO + PVdF (80:20 wt. ratio) sample. This is a much higher ratio than what is conventionally used in electrode preparation, and therefore we expect that a lot of the surface could be covered by the PVdF.

This is evident from the modulus maps, where we observe that the range of the values is similar to the values from pure PVdF sample in earlier section 4.1. However, we observe from Figure 29 that there are areas on the LNMO surface which might not be covered by

PVdF. Figure 30 shows one of these areas at a higher resolution, and its modulus mapping data suggests that the exposed material (dark color) would be LNMO, surrounded by PVdF

(bright color with ~ 7 GPa of modulus value). The drastic difference in the modulus between PVdF and LNMO may cause a large shift in phase and make it difficult to properly determine the Young’s modulus of the LNMO using AM-FM mode. This is also apparent from Figure 33, where force curves were taken from an area similar to the one in Figure

30. The cantilever being used was not stiff enough to indent the surface of the LNMO, but was sufficiently stiff to indent the surface of the PVdF. In contrast, Figures 31 and 32 show areas of the same sample, where the LNMO is completely covered by the PVdF binder.

This result cleary demonstrates the unique capability of AFM in characterizing the distribution of materials components (e.g., binder) in composite electrode, which cannot be detected by SEM.

Figure 33. Force curves on PVdF + LNMO (20:80 wt. ratio) sample

This is also verified by performing some force curves. It can be observed that the area which appears to be PVdF can be indented with the cantilever, as shown in Figure 33.

However, the cantilever cannot indent the LNMO because it is not sufficiently stiff. Also, the drastic change in frequency because of the high difference in moduli between the two materials causes problems in AM-FM mode operation. The mode is designed to operate when the net force between the tip and the substrate is in the repulsive mode [50], and the drastic change in modulus causes the phase to transition to the attractive region. However, unlike in AC Mode, the surface constituents can still be differentiated with certainty, because of the Modulus maps. These results demonstrate how AMFM can be a promising technique for characterizing Li-ion battery electrodes, by monitoring the changes in morphology and mechanical properties.

Figure 34 shows the SEM image and the AC Topography data of the sample comprised of PVdF + LNMO + Super P. From the SEM image, it can be observed that the binder and carbon mixture is well spread out, and it is unclear what areas are being covered by this mixture. The phase diagram also does not display any major shifts, which might be due to a uniform covering on the surface. Figure 35 shows the AMFM data for the same sample, and it is observed that most of the area appears to be covered by PVdF. The method of fabrication of the sample could be a reason for the uniform binder covering observed here.

Figure 34. (a) SEM image of PVdF + Super P + LNMO (10:10:80 wt. ratio) sample.

Scale bar is 20 µm

Figure 35. Height, Amplitude, Phase of LNMO + PVdF + Super P (80:10:10 wt. ratio),

using AC Mode. Scale bar is 1 µm.

Figure 36. Height, Amplitude, and Young’s modulus of LNMO + PVdF + Super P

(80:10:10 wt. ratio) using AM-FM, with a histogram of Young’s modulus data values.

Scale bar is 1µm. 44

4.4. AC Mode in liquid and Force Spectroscopy

After obtaining the topography data and Young’s modulus data in dry state, experiments were performed to study these properties in electrolyte. As discussed earlier in section 2.1., it is important to observe the change in binder properties in electrolyte, which can have a huge impact on the mechanical integrity of the electrode. From the data obtained for each hour of immersion in DMC, a conventional solvent used in electrolytes, the topography of neither sample changed considerably, as can be observed from Figure 37(a) and Figure 38

(a). This shows that both of the binders are fairly stable in electrolyte. Adhesive force data was also collected in an environment of DMC. This also ensures that the effect of meniscus is not a part of the adhesive forces observed.

(a) (b) (c)

Figure 37. AFM nano-indentation/force spectroscopy on PVdF, for each hour immersed

in electrolyte: (a) AFM topography, (b) modulus map, (c) histograms of modulus. The

scale bar is 1 µm.

(a) (b) (c)

Figure 38. AFM nano-indentation/force spectroscopy on Na-alginate, for each hour

immersed in electrolyte: (a) AFM topography, (b) modulus map, (c) histograms of

modulus. The scale bar is 1 µm.

Figures 37 and 38 show the force maps obtained from the PVdF and Na Alginate samples with each hour of immersion in DMC. The properties of the binder samples in the dry state and wet state were analyzed using the gathered force data. The adhesion of polymer substrates is known to decrease upon wetting in liquid electrolyte. Figures 39(a) and 39(c) show the force-indentation curves of the PVdF and Na-alginate binder samples

47 respectively. From these curves, we can clearly observe the drastic difference in the adhesive force from the dry state to the wet state. However, it should be noted that surface moisture interactions between AFM cantilever and polymer surfaces under ambient air are quite significant, and therefore make dry state analysis of these components incapable of representing the true mechanical interaction between them.

Figure 39. Force Spectroscopy Results (a) Force-distance curves on PVdF, for each hour immersed in electrolyte, (b) modulus of PVdF as a function of immersion time, (c) force- distance curve on Na-alginate, for each hour immersed in electrolyte, (d) modulus of Na-

alginate as a function of immersion time. 48

Figure 40. Adhesion trend of Na-alginate with time of immersion

It is known that the PVdF binds through weak Van der Waal’s forces,which are in the pN scale, in the wetted state [27]. The low adhesion force of PVdF, which is below 1nN, cannot be measured accurately with the Si cantilever (AC160TS-R3) used in this study because of its high stiffness and therefore limited sensitivity. However, in the case of the

Na- alginate, we can see a slight adhesion, as shown in Figure 39(c). The adhesion force between Si – Na-alginate also decreases with immersion time as shown in Fig. 40, but still exhibits adhesion forces that are orders of magnitude higher than PVdF. The adhesive force after 1 hour of immersion is 11.96 nN, and decreases to 4.59 nN at the end of 3 hours of immersion. The stronger adhesion force of Na-alginate compared with the PVdF has been explained by its carboxyl moieties that can offer (i) hydrogen bonding with the hydroxyl moieties on Si surfaces and/or (ii) dipole-ion interaction with Si [20]. 49

A softer cantilever could be more sensitive to observe this adhesion force, because for the same force there would be greater deflection, making it more sensitive. Figure 40 shows the trend of adhesion of Na-alginate with the Si tip, with time of immersion in DMC.

A few trials with softer cantilevers having similar tip radius and shape, however, demonstrated even lesser adhesion than what was observed with the AC160. This shows that there are other factors affecting the adhesion between the cantilever tip and measured sample. It is based on the contact mechanics between the tip and the sample. The porosity of the sample, hardness, the tip shape and contact area can be some of the few among many factors which can affect the adhesion forces. Due to the lack of literature and experimental data, it will remain as a future study.

The Young’s modulus of PVdF was found to be 7.8 GPa in the dry state. This modulus agrees closely with the elastic modulus of PVdF measured by Kim et. al.[29] using nanoindentation. Despite using a different nanoindentation model, the Oliver-Pharr method, this group found PVdF to have a Young’s modulus of 7.5 GPa. The Young’s modulus of Na-alginate was found to be 4.31 GPa in the dry state. In addition, as discussed earlier, Si – binder interactions need to be measured in liquid electrolyte to mimic actual battery operating conditions. Figure 37 shows that on immersion in DMC, the Young’s modulus of PVdF drastically decreased to 378.8 MPa after the first hour of immersion and continued to marginally decrease with time. In comparison, Figure 38 shows that the

Young’s modulus of Na-alginate decreased drastically to 121.2 MPa during the first 1 h period of immersion but recovered with time after 1 h and reached to 3.82 GPa at 3 h.

Repeated experiments showed the same trend. This recovery in Young’s modulus

50 demonstrates that the Na-alginate is able to retain its mechanical strength in the liquid electrolyte to a greater extent than PVdF. A direct comparison of the elastic moduli of the two materials over time can be noted in Figure 39. A greater elastic modulus is indicative of the binder’s capacity to resist the internal tensile and compressive forces that result from the volumetric changes of the battery electrodes, thereby maintaining the mechanical integrity of the electrode. Since the study did not take place for more than 4 hours, it is also uncertain how the behavior might change over extended periods of time like days or weeks, which can also be a future study.

The results clearly demonstrate the physical property changes of PVdF and Na- alginate at their nano-scales. Na-alginate’s retention of mechanical strength and higher adhesive forces can help withstand the large volume expansions during battery cycling.

This supports earlier reports, which are mostly based on macro-scale characterization, very well. These experiments also demonstrate AFM’s capability of characterizing the material properties of binder samples immersed in liquid electrolyte, which can be further applied to compare the fundamental interfacial properties of other battery materials.

Chapter 5. Conclusion and future work

In this study to observe mechanical properties of polymer binders for Li-ion batteries, experiements were performed using AFM techniques. The experiments were performed while immersing PVdF and Na-alginate thin-films in DMC solvent to mimic the actual Li- ion battery conditions. The interaction forces between Si-cantilever and the binder samples simulate their mechanochemical interactions in actual Si anodes. The changes in the elastic modulus for the PVdF in liquid electrolyte were drastic, decreasing it by more than 80%.

The Na-alginate, in comparison, regained more than 90% of its Young’s modulus after 3 hours of immersion in liquid electrolyte. The Na-alginate also displays an adhesive force of 11.96 nN after 1 hour of immersion, which gradually falls to 4.59 nN at the end of 3 hours. The PVdF displayed no adhesion region in the force curves, implying that its adhesion forces are lower than the minimum force that can be measured by the micro- cantilever (AC160TS-R3) being used. It can be concluded that Na-alginate is better suited than PVdF for application in high capacity Si anodes, attributed to its superior mechanical properties and adhesive forces. The results correlate well with previous findings and provide a more fundamental understanding of significant factors in choosing binder materials for high capacity Li-ion battery electrodes. This study was performed using only

DMC, which is only one part of the electrolyte solvent. Future work would be to characterize the adhesive forces and elastic modulus properties of these binders in actual electrolyte compositions including ethylene carbonate (EC) and lithium hexaflurophosphate (LiPF6) under cyclic voltage. The Electrochemical(EC) cell, shown in

Figure 9 can be used for these experiments. Controlling the temperature of the environment while performing these experiments is also an essential area to be explored.

The AM-FM mode results show that it is a promising technique for characterizing the surface properties of Li-ion battery electrodes, which is not possible with conventional microscopy techniques such as SEM and TEM due to the extremely thin and fragile nature of the binder film. Future work in this area would be to analyze different combinations of electrode composites, while cross checking with force spectroscopy results, and map the changes in electrode composition that take place during electrode cycling using the

Young’s modulus maps of AM-FM.

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Appendix A. Detailed procedure for sample fabrication

The following are the steps to fabricate the PVdF-C sample used in the study.

Step 1: 5 g of Kynar HSV900 PVdF was measured using a measuring scale and added to a mixing beaker. 50 ml of N-methylpyrrolidone (NMP) solvent was measured in a measuring beaker and then added to the beaker.

Step 2: The beaker was then sealed using Parafilm and placed in a QXULTRASONIC industrial ultrasonic cleanser. The temperature was set at 50 degrees Celsius and the machine was turned on.

The beaker was left ultra-sonic vibration chamber for about half an hour. The vigorous stirring dissolved the PVdF, resulting in the solution.

Step 3: A stainless-steel specimen disc, obtained from Asylum Research, of diameter 10 mm, was used as a substrate to cast the solution. A few drops (2-5) of the solution were cast on to the disc using a pipette, ensuring that there are no air bubbles in the droplet.

Step 4: The sample was then put in a vacuum oven chamber, VWR Symphony “414004-580”. The oven was set to a temperature of 120 degrees Celsius and timed to run for 12-14 hours.

Step 5: The sample was removed from the oven and stored in a container with some silica gel, and stored in the vacuum dessicator

A similar process is followed for the fabrication of sodium alginate, and all other mixtures, where only Step 1 involves different components.