Nondilute diffusion from first principles: Li diffusion inLixTiS2

We investigate Li diffusion in the favorable intercalation compound ${\text{Li}}_{x}{\text{TiS}}_{2}$ as a function of Li concentration, $x$, from first principles. We find that Li ions hop between neighboring octahedral interstitial sites of the ${\text{TiS}}_{2}$ 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 ${\text{Li}}_{x}{\text{TiS}}_{2}$.