Vacancy-induced anion and cation redox chemistry in cation-deficient F-doped anatase TiO2

Abstract

The incorporation of point defects such as cationic vacancies into electrode materials has been considered as an effective strategy to improve the charge-transfer and ion-diffusion kinetics and allow insertion and migration of multivalent ions. However, they suffer from low specific capacity and electrochemical irreversibility. To elucidate the origin of these issues, we investigated the F-doped and cation-deficient anatase TiO₂ through Mg²⁺ insertion for understanding the redox activity of vacancy structure. Our first-principles calculations showed that charge transfer inductively occurs from anion O²⁻/F⁻ to cation Ti⁴⁺ near vacancies, forming oxidized anions F^(1−x)−/O^(2−y)− and reduced cations Ti^(4−z)+. We further found that cooperative cationic and anionic redox reactions, Ti^3.83+ + 0.19e⁻ → Ti^3.64+, O^1.94− + 0.06e⁻ → O²⁻ and F^0.93− + 0.07e⁻ → F⁻, take place during Mg²⁺ insertion. The peculiar anionic redox reaction of oxidized F 2p states is attributed to low-coordination fluorine ions, which was demonstrated by the previous NMR characterization. Our calculations showed that the fluorine redox reaction contributes 26.5% of the total redox capacity. The present results provided chemical clues to use the vacancy structure design to develop efficient cationic and anionic redox materials for improving the energy density and cyclic stability of battery materials.