An ab initio study of reduction of V₂O₅ through the formation of oxygen vacancies and Li intercalation

Two methods of reduction of V₂O₅ have been investigated: oxygen vacancy formation and lithium intercalation. The electronic structure, geometry, and energetics of these reduced systems are examined. Oxygen vacancies in bulk α-V₂O₅ have been investigated by using gradient-corrected density functional theory (GGA) and density functional theory corrected for on-site Coulomb interactions in strongly correlated systems (GGA+U). The GGA calculation predicts a delocalized defect electronic state. This disagrees with experimental evidence, which indicates that oxygen vacancies produce a localized reduced vanadium state in the band gap. The DFT+U results for U = 4.0 eV are consistent with available UPS and XPS data, indicating strong localization on the vanadium atoms nearest the vacancy, and showing reduced V(IV) species. Intercalation of Li in V₂O ₅, which has important potential applications in energy storage devices, is also reported at the GGA+ U level, using the value of U obtained from the oxygen-deficient calculation, and localized reduction is demonstrated. These results are again in agreement with available UPS data and crystallographic data, indicating good transferability of this value of U among the systems of interest. Calculated lithium intercalation energies for both the α- and γ-V₂O₅ phases are reported, and the structure and relative stability of the deintercalated γ-V ₂O₅ phase are also examined.