Novel Quaternary Ammonium Ionic Liquid Electrolyte for Stable, High Capacity Si Nanowire Based Li-Ion Battery

In recent years, there has been interest in silicon nanowires as an anode for Li-ion battery because of its large gravimetric capacity of 4200 mAh/g. 1D nanostructure are essential because they permit radial strain relaxation of mechanical stress which occurs due to the volumetric expansion (>300%) as result of Li insertion. However, commercialization of silicon nanowire-based material has been impeded by number of key issues such as severe capacity fade, limited cycle life and mechanical degradation as result of structural changes. Overcoming these challenges requires optimization of anode structure and choice of better electrolyte than those commonly used. The electrolyte should enable the formation of a stable solid-electrolyte-interface (SEI). Furthermore, the use of organic electrolyte in lithium batteries poses a flammability issue and limits its operating temperature range.

In this work we show a novel quaternary ammonium based ionic liquid as an electrolyte for high capacity and high stability silicon nanowire based anode. Battery testing in these electrolytes showed steady reversible capacity of 2000 mAh/g and columbic efficiency ~97% even after 50 cycles. The efficiency for the first cycle was 91% with very little loss in the discharge capacity indicating good SEI formation in these ionic liquids. Results are discussed in the terms of the nature of this SEI layer and the role of additives that promotes its formation. New strategies were developed to minimize the capacity fade and obtain battery life > 600 cycles.