Atomistic Diffusion Pathways of Lithium Ions in Crystalline Lithium Silicides from ab Initio Molecular Dynamics Simulations

The LixSiy class of compounds exhibits a broad variety of crystal structures with high experimentally observed lithium diffusivities. We explore lithium diffusion in a series of LixSiy by means of ab initio molecular dynamics simulations and find a strong variability of diffusion coefficients in the defect-free crystal structures. We explain the microscopic origin of these variations in order to characterize the mobility of lithium ions, both from a local and from a long-range perspective. Our study reveals the existence of important interstitial sites. We identify different types of diffusion pathways in our simulation trajectories and report their energy profiles. It turns out that the diffusive behavior of lithium in these compounds is governed by only a few diffusion paths. We show the connection between diffusion mechanisms and energy barriers and especially highlight the relevance of point defects. We observe considerable structural relaxation within a radius of about 3.5 angstrom around the diffusion path.