Phase stability and nondilute Li diffusion in spinel Li1+xTi2O4

We report on a comprehensive first-principles investigation of Li diffusion in spinel Li1+xTi2O4, a viable anode material for Li-ion batteries. Concentration dependent diffusion coefficients are calculated by applying kinetic Monte Carlo simulations to a first-principles parametrized cluster expansion of the configurational energy and migration barriers. Diffusion mechanisms in spinel transition metal oxide intercalation compounds are sensitive to the Li concentration. Below x=0 in Li1+xTi2O4, Li ions occupy tetrahedral sites and individual Li hops between neighboring tetrahedral sites pass through intermediate octahedral sites, which we find to be activated states as opposed to local minima. The migration barriers are sensitive to the overall Li concentration, decreasing as the Li concentration over the tetrahedral sites is increased. A miscibility gap exists between LiTi2O4 and Li2Ti2O4. Li-diffusion mechanisms in Li2Ti2O4 are qualitatively different from those in Li1+xTi2O4 when x < 0, with Li hops between neighboring octahedral sites passing through intermediate tetrahedral sites that are locally stable. The present study provides insight to the origin of the high Li mobility in spinel crystal structures compared to the layered crystal structures of common intercalation compounds used as electrodes in Li-ion batteries.