Effect of Oxygen Vacancies and F-Doping on TiO₂(B) as Anode for Mg-Ion Batteries

Abstract

To find alternatives to lithium-ion batteries, much effort is being devoted to finding electrode materials that allow reversible Mg 2+ disinsertion/insertion. Due to the strong Coulomb force between Mg 2+ and electrode materials, certain modification methods are often needed to reduce the energy barrier of Mg jumping in materials. The bronze-phase TiO 2 (B) has attracted considerable attention as a promising anode for lithium-ion batteries, but it has proven to be difficult for Mg to insert therein. The PBE+U calculations show that the low capacity of perfect TiO 2 as the anode of magnesium-ion batteries (MIB) is mainly due to the high diffusion energy barrier of the Mg ion (at least 1.27 eV). Both the defects of oxygen vacancies and F-doping can significantly reduce the band gap value and the Mg-ion migration barriers. The results of CM5 charges and charge density differences prove that the local lattice distortion caused by oxygen vacancies can cause the distribution of Ti 3+ away from Mg, resulting in a significant decrease in the Mg–O binding energy, which is beneficial to the reduction of the migration barrier. F-doping can make the binding energy of Mg at each site tend to be the same, so that the energy barrier of Mg jumping along the way is lower. In addition, the coexistence of oxygen vacancy and F doping in TiO 2 (B) has synergistic effects in improving Mg-ion diffusion.