Deformation mechanisms of silicon electrodes in rechargeable Li-ion batteries

Abstract

Silicon has received a significant attention as a promising anode material for rechargeable Li-ion batteries owing to its high theoretical capacity (4200 mAhg-1) and relatively low operating voltage (0.4V vs Li/Li+). However, cycling performance of Si electrodes remains very limited due to large volume changes upon alloying and de-alloying with lithium, resulting in poor mechanical stability and early capacity fade. To address this issue, sound understanding of mechanical behavior of silicon electrodes during electrochemical lithium insertion is required. In this study, molecular dynamic simulation is employed to simulate the deformation behavior of lithiated amorphous silicon (a-Si) under nanoindentation at different stages of lithiation. The results are used to characterize the effect of Li concentration on hardness and modulus of the electrode, as well as the lithiation-induced plastic deformation. The effect of Li concentration on mechanical properties and deformation mechanisms are thoroughly explored during the nanoindentation process. The results indicate the transition of the properties of a-Si electrode with lithiation; from a hard, covalent solid into a soft, ductile alloy which can accommodate large plastic deformation. Obvious elastic and plastic deformations occur under the nanoindentation of a-Si and LixSi. Clearly different deformation mechanisms of Li-poor structures and Li-rich structures under indentation are revealed. Plastic deformation of a-Si is governed by the increasing fivefold coordinates of silicon network structure. In contrast, plastic flow of Li-rich structures is found to be governed by migration of shear transformation zones. Increased disorder of Li-Si alloys with electrochemical Li insertion might play a vital role on the formation of shear transformation zones in Li-rich LixSi alloys