Atomistic Insights into Lithium Storage Mechanisms in Anatase, Rutile, and Amorphous TiO Electrodes.

Density functional theory calculations were used to investigate the phase transformations of LiTiO (at 0 ≤ ≤ 1), solid-state Li diffusion, and interfacial charge-transfer reactions in both crystalline and amorphous forms of TiO. It is shown that in contrast to crystalline TiO polymorphs, the energy barrier to Li diffusion in amorphous TiO decreases with increasing mole fraction of Li due to the changes of chemical species pair interactions following the progressive filling of low-energy Li trapping sites. Sites with longer Li-Ti and Li-O interactions exhibit lower Li insertion energies and higher migration energy barriers. Due to its disordered atomic arrangement and increasing Li diffusivity at higher mole fractions, amorphous TiO exhibits both surface and bulk storage mechanisms. The results suggest that nanostructuring of crystalline TiO can increase both the rate and capacity because the capacity dependence on the bulk storage mechanism is minimized and replaced with the surface storage mechanism. These insights into Li storage mechanisms in different forms of TiO can guide the fabrication of TiO electrodes to maximize the capacity and rate performance in the future.