Study of the Binder Influence on Expansion/Contraction Behavior of Si Alloy-Based Anodes for Lithium-Ion Batteries (LIBs)
Authors: Yoon Dong-hwan , Marinaro Mario , Axmann Peter , Wohlfahrt-Mehrens Margret 
Published: February 2020

The potential of silicon-based materials as an anode in lithium-ion batteries has attracted considerable attention due to high specific capacity (mAh/g) and energy (Wh/kg). However, practical implementation of anode with high Si content in Li-ion cells remains still challenging because of a remarkable volume change and subsequent disintegration of the electrodes during cycling. An appropriate polymer binder is believed to be a viable solution to maintain the mechanical stability of silicon-based anodes [1-3].
To understand the mechanical issues in commercially more relevant Si alloy-based electrodes (> 3 mAh cm-2), the real-time changes in volume during cycling need to be monitored. Herein, we study the expansion and contraction behavior of Si alloy-containing anodes with three binders, such as lithium poly(acrylic acid) (LiPAA), poly(vinylidene fluoride) (PVdF), and poly(vinyl alcohol) (PVA), which was investigated via in situ electrochemical dilatometry, cross sectional SEM, and optical microscopy.
Our results demonstrate that, of those tested, electrodes made of LiPAA expands the greatest during lithiation, but also exhibits the highest cyclability and reversibility. We tentatively attribute this result to the strength of the interaction between binder and silicon alloy particles (via hydrogen bonding) dominating the electrode properties, rather than the stiffness of binders (normally determined by Young’s modulus). It was observed that in the 1st cycle Si particle expansion and contraction resulted in an increase in electrode porosity, which buffered against subsequent (de)lithiation volume changes. This was also verified semi-quantitatively via cross sectional SEM analysis using the AvisoTM. Development of an in situ optical microscopy (OM) technique was also undertaken, and the best performing LiPAA system (with the largest volume changes) was selected for validation experiments. Our approach not only enabled measurement of electrode thickness changes during cycling, but also a direct, real-time visualization of the changes particles underwent.
It should be noted that dilatometry is an important method for the facile evaluation of silicon alloy anodes with a range of binders, and will be a useful tool during the development of new binder systems with improved properties. Moreover, in situ optical microscopy is a novel technique, enabling real-time visualization of changes in the anode, cathode, and separator. While some challenges, such as a smooth cross-sectioning, have yet to be addressed, these proof-of-concept experiments provide confirmation of the validity of this technique.
Acknowledgment
This work has been financially supported by the European Union within the H2020 Framework Programme for research, technological development and demonstration (H2020-GV-2014) under grant agreement n° 653531 (FIVEVB) and by the German Federal Ministry of Education and Research (BMBF) and the Israeli Ministry of Science, Technology and Space (MOST) within the project LiMetalFreeSSiBat. The authors acknowledge 3M company for providing the silicon alloy materials.
References
[1] M. N. Obrovac, V. L. Chevrier Chem. Rev. 114, 11444−11502 (2014)
[2] M. Marinaro, D. Yoon, G. Gabrielli, P. Stegmaier, E. Figgemeier, P. C. Spurk, D. Nelis, G. Schmidt, J. Chauveau, P. Axmann, M. Wohlfahrt-Mehrens J. Power Soc. 357, 188 (2017)
[3] R. Kasinathan, M. Marinaro, P. Axmann, M. Wohlfahrt-Mehrens, Energy Technol. 6, 1–9 (2018)