Nondilute diffusion from first principles: Li diffusion in Li_xTiS₂

We investigate Li diffusion in the favorable intercalation compound Li_xTiS₂ 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 Li_xTiS₂.