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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 LixTiO2 (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.