LiVPO4F1-yOy Tavorite-Type Compositions: Influence of the Concentration of Vanadyl-Type Defects on the Structure and Electrochemical Performance

Mixed-valence LiVPO4F1-yOy materials were obtained for the first time over a large composition range (here 0.35 <= y <= 0.75) through a single-step solid-state synthesis. Interestingly, the competition between the ionic character of the V3+-F bond and the strong covalency of the V4+=O vanadyl bond originates complex crystal chemistry at the local scale, which allows stabilization of a solid solution between LiVPO4F and LiVPO4O despite a significant deviation from Vegard's law for the cell parameters. A combined study using IR, Raman, and X-ray absorption spectroscopies highlights the effect of the vanadyl environment on the electronic structure of the vanadium orbitals and a fortiori the electrochemical behavior. Our results underline that the electrochemical performance of LiVPO4F1-yOy-type materials can be controlled by tuning the concentration of vanadyl-type defects, i.e., by playing on the competition between the ionic V3+-F bond and the covalent V4+=O bond.