Defect Chemistry, Surface Structures, and Lithium Insertion in Anatase TiO2

Abstract

Atomistic simulation techniques are used to investigate the defect properties of anatase TiO₂ and Li_xTiO₂ both in the bulk and at the surfaces. Interatomic potential parameters are derived that reproduce the lattice constants of anatase, and the energies of bulk defects and surface structures are calculated. Reduction of anatase involving interstitial Ti is found to be the most favorable defect reaction in the bulk, with a lower energy than either Frenkel or Schottky reactions. The binding energies of selected defect clusters are also presented: for the Ti³⁺−Li⁺ defect cluster, the binding energy is found to be approximately 0.5 eV, suggesting that intercalated Li ions stabilize conduction band electrons. The Li ion migration path is found to run between octahedral sites, with an activation energy of 0.45−0.65 eV for mole fractions of lithium in Li_xTiO₂ of x ≤ 0.1. The calculated surface energies are used to predict the crystal morphology, which is found to be a truncated bipyramid in which only the (101) and (001) surfaces are expressed, in accord with the available microscopy data. Calculations of defect energies at the (101) surface suggest that single Ti³⁺ defects and neutral Ti³⁺−Li⁺ pairs tend to segregate to the surface.