Li+ intercalation in isostructural Li2VO3 and Li2VO2F with O2− and mixed O2−/F− anions

Abstract

Mixed-anion materials for Li-ion batteries have been attracting attention in view of their tunable electrochemical properties. Herein, we compare two isostructural (Fmm) model intercalation materials Li₂VO₃ and Li₂VO₂F with O²⁻ and mixed O²⁻/F⁻ anions, respectively. Synchrotron X-ray diffraction and pair distribution function data confirm large structural similarity over long-range and at the atomic scale for these materials. However, they show distinct electrochemical properties and kinetic behaviour arising from the different anion environments and the consequent difference in cationic electrostatic repulsion. In comparison with Li₂VO₃ with an active V^4+/5+ redox reaction, the material Li₂VO₂F with oxofluoro anions and the partial activity of V^3+/5+ redox reaction favor higher theoretical capacity (460 mA h g⁻¹vs. 230 mA h g⁻¹), higher voltage (2.5 V vs. 2.2 V), lower polarization (0.1 V vs. 0.3 V) and faster Li⁺ chemical diffusion (∼10⁻⁹ cm² s⁻¹vs. ∼10⁻¹¹ cm² s⁻¹). This work not only provides insights into the understanding of anion chemistry, but also suggests the rational design of new mixed-anion battery materials.