PNNL

Abstract

We report on the energetics and dynamics of Li diffusion in bulk anatase and amorphous TiO2 using molecular dynamics (MD) simulations and density functional theory (DFT) calculations. Using MD simulations, for both anatase and amorphous TiO2, diffusion characteristics are first studied for an isolated Li ion, followed by simulations of Li concentrations ranging from 10% to 100% in order to explore the concentration effect on the diffusivity. The isolated Li diffusion mechanism, revealed from the MD simulations, occurs via zigzag hops between the octahedral sites in anatase. The corresponding barrier for this process obtained fromDFT-NEB calculations is 480 meV.MDsimulations also show that isolated Li ion diffusivity is much slower in the amorphous TiO2 than in anatase TiO2. DFT-NEB results for the diffusion in amorphous titania indicate that Li encounters deep energy wells within the amorphous network that are in the electronvolt range, confirming our MD observation of low Li diffusivity. A monotonic decrease in diffusion barriers with increasing Li concentration is observed in the case of amorphous titania whereas a non-monotonic variation is seen in anatase, with the lowest barrier observed at 50% Li concentration. At low Li concentrations (75% for amorphous titania, the Li diffusivity in amorphous is found to be much higher than in anatase. Our MD simulations suggest that the underlying reason for these differences is related to changes in diffusion mechanism. Our simulations therefore indicate a strong correlation between Li ion concentration and the observed transport characteristics, offering new insights into ion conduction mechanisms that are of importance to solid-state devices used for energy storage applications.