First-principles study of competing mechanisms of nondilute Li diffusion in spinel LixTiS2

We report on a first-principles study of nondilute Li diffusion in spinel LixTiS2 with the aim of elucidating the role of crystal structure and chemistry on Li mobility in intercalation compounds used as electrodes in Li-ion batteries. In contrast to transition-metal oxide spinels, where Li ions occupy tetrahedral interstitial sites, Li ions in spinel LixTiS2 preferentially occupy octahedral sites. This makes spinel LixTiS2 a useful model system to explore diffusion mechanisms in three-dimensional intercalation compounds with octahedral Li occupancy. Elementary Li hops between neighboring octahedral sites pass through intermediate tetrahedral sites. High coordination of these intermediate tetrahedral sites by octahedral sites causes the migration barrier to be sensitive to the local Li concentration and configuration. Kinetic Monte Carlo simulations predict diffusion mechanisms mediated by triple vacancies and divacancies, which leads to a strong concentration dependence of the chemical diffusion coefficient. Insights from this study combined with those gathered in past first-principles studies of layered intercalation compounds indicate that crystal structures with activated states that are highly coordinated by Li sites will result in diffusion mechanisms mediated by vacancy clusters, producing a chemical diffusion coefficient that decreases with increasing Li composition.