Atomistic Insights into Lithium Storage Mechanisms in Anatase, Rutile, and Amorphous TiO₂ Electrodes

Density functional theory calculations were used to investigate the phase transformations of LiTiO₂ (at 0 ≤ x ≤ 1), solid-state Li⁺ diffusion, and interfacial charge-transfer reactions in both crystalline and amorphous forms of TiO₂. It is shown that in contrast to crystalline TiO₂ polymorphs, the energy barrier to Li⁺ diffusion in amorphous TiO₂ 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 TiO₂ exhibits both surface and bulk storage mechanisms. The results suggest that nanostructuring of crystalline TiO₂ 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 TiO₂ can guide the fabrication of TiO₂ electrodes to maximize the capacity and rate performance in the future.