Silicon-Based Anodes with Long Cycle Life for Lithium-Ion Batteries Achieved by Significant Suppression of Their Volume Expansion in Ionic-Liquid Electrolyte.

Elemental Si has a high theoretical capacity and has attracted attention as an anode material for high energy density lithium-ion batteries. Rapid capacity fading is the main problem with Si-based electrodes; this is mainly because of a massive volume change in Si during lithiation-delithiation. Here, we report that combining an ionic-liquid electrolyte with a charge capacity limit of 1000 mA h g significantly suppresses Si volume expansion, improving the cycle life. Phosphorus-doping of Si also enhances the suppression and increases the Li diffusion coefficient. In contrast, the Si layer expands significantly in an organic electrolyte even with the charge capacity limit and even in an ionic-liquid electrolyte without the limit. We demonstrated that the homogeneously distributed Si lithiation-delithiation, phase-transition control from the Si to Li-rich Li-Si alloy phases, formation of a surface film with structural and/or mechanical stability, and faster Li diffusion contribute to suppressing Si volume expansion.