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 By 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 1 Copyrighted by Rahul Kumar Raghava Reddi 2018 2 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. iii 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. iv 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 v 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