With its theoretical capacity of 4200 mAh silicon received significant attention as a viable alternative to graphitic carbon in lithium ion batteries.[1] However, silicon has substantial limitations and suffers from poor passivating behavior in organic electrolytes and top-of-charge electrolyte side reactions, which results in large irreversible capacity loss and gradual electrolyte consumption with a lithium inventory shift, due to poor SEI formation. Furthermore, large crystallographic volume changes (~320%) are observed during cycling, which translates to particle cracking, particle isolation, and electrode delamination. This in turn affects not only the SEI stability but also results in the loss of electrode integrity.

To overcome the loss of electronic connectivity and mechanical integrity of the bulk electrode during extended charge-discharge cycles, it is important to identify an effective polymer binder. Consequently it is essential to understand the effect of polymer binder on the electrochemical performance for Si/C anodes.

On the one hand this study focuses on the determination of the mechanical and adhesion binder properties before and with cycling. Stress-strain curves, indentation measurements and adhesion test give information about properties needed to withstand the large volume changes during cycling. Furthermore the evolution of binder deterioration is monitored and gives information about effect of the polymer binder on adhesion, crack initiation, growth, and particle delamination.

On the other hand, the determination of chemical changes in polymer binders with cycling is carried out. Films of the binder polymers are spin coated onto doped Si wafers using thicknesses of same order of magnitude as those calculated for composite electrodes. Electrochemical studies give information about Li⁺ doping, reduction and protection layer formation. The film surface exposed to the electrolyte as well as the substrate/binder interface are analyzed in terms of conductivity and chemical changes. By understanding these changes and their significance regarding cell life, effective new binder candidates can be developed, which can lead to more efficient LIB systems.

Acknowledgements

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, under the Applied Battery Research for Transportation (ABR) Program and Award Number DE-EE0006443.

[1] T. D. Hatchard, J. R. Dahn, Journal of the Electrochemical Society 2004, 151, A838.