Nondilute diffusion from first principles: Li diffusion in LixTiS2

We investigate Li diffusion in the favorable intercalation compound LixTiS2 as a function of Li concentration, x, from first principles. We find that Li ions hop between neighboring octahedral interstitial sites of the TiS2 host by passing through an adjacent tetrahedral site. The migration barriers for these hops are significantly reduced when the end points belong to a divacancy. We use a cluster expansion within kinetic Monte Carlo simulations to describe the configuration dependence of the migration barriers and predict a diffusion coefficient that varies by several orders of magnitude with Li concentration, exhibiting a maximum close to x=0.5. The kinetic Monte Carlo simulations predict that diffusion is mediated predominantly by divacancies. We also find that the migration barriers depend strongly on the c-lattice parameter, which decreases as Li ions are removed from LixTiS2.