Atomistic insights into Li-ion diffusion in amorphous silicon

Silicon has been critically examined for its potential use as an electrode material for Li-ion batteries. Diffusive transport of Li-ions in the crystalline silicon anode is one of the key mechanisms that controls the deformation during lithiation, the rate of the charge-discharge cycle, and eventual mechanical failure. The use of amorphous silicon, instead of its crystalline counterpart, is considered to offer several advantages. The atomistic mechanisms underpinning diffusive transport of Li-ions in amorphous silicon are, however, poorly understood. Conventional molecular dynamics, if used to obtain atomistic insights into the Li-ion transport mechanism, suffers from several disadvantages: the relaxation times of Li ion diffusion in many of the diffusion pathways in amorphous Si are well beyond the short time scales of conventional molecular dynamics. In this work we utilize a sequence of approaches that involve the employment of a novel and recently developed potential energy surface sampling method, kinetic Monte Carlo, and the transition state theory to obtain a realistic evaluation of Li-ion diffusion pathways in amorphous Si. Diffusive pathways are not a priori set but rather emerge naturally as part of our computation. We elucidate the comparative differences between Li-ion diffusion in amorphous and crystalline Si as well as compare our results with past studies based on other methods.