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

To find alternatives to lithium-ion batteries, much effortis beingdevoted to finding electrode materials that allow reversible Mg2+ disinsertion/insertion. Due to the strong Coulomb forcebetween Mg2+ and electrode materials, certain modificationmethods are often needed to reduce the energy barrier of Mg jumpingin materials. The bronze-phase TiO2(B) has attracted considerableattention as a promising anode for lithium-ion batteries, but it hasproven to be difficult for Mg to insert therein. The PBE+U calculationsshow that the low capacity of perfect TiO2 as the anodeof magnesium-ion batteries (MIB) is mainly due to the high diffusionenergy barrier of the Mg ion (at least 1.27 eV). Both the defectsof oxygen vacancies and F-doping can significantly reduce the bandgap value and the Mg-ion migration barriers. The results of CM5 chargesand charge density differences prove that the local lattice distortioncaused by oxygen vacancies can cause the distribution of Ti3+ away from Mg, resulting in a significant decrease in the Mg-Obinding energy, which is beneficial to the reduction of the migrationbarrier. F-doping can make the binding energy of Mg at each site tendto be the same, so that the energy barrier of Mg jumping along theway is lower. In addition, the coexistence of oxygen vacancy and Fdoping in TiO2(B) has synergistic effects in improving Mg-ion diffusion.