Oxidation under Air of Tavorite LiVPO4F: Influence of Vanadyl-Type Defects on Its Electrochemical Properties

Tavorite-type compositions offer a very rich crystal chemistry, among which (LiVPO4F)-P-III has the highest theoretical energy density (i.e., 655 Wh/kg). In this article, an in-depth study of vanadyl-type defects generated by temperature-controlled oxidation of (LiVPO4F)-P-III under air is proposed, and the influence of the defects on the electrochemical properties is demonstrated. A combination of high resolution synchrotron diffraction, infrared spectroscopy, and magic angle spinning nuclear magnetic resonance was used to fully characterize the materials thus generated, from their average long-range structure to their local structure with the presence of defects. The increase of the annealing temperature tends to substitute oxygen for fluorine with the formation of a series of LiVPO4F1-xOx compositions. The miscibility domains appear to be narrow at the two ends of the solid solution (i.e., in the composition ranges LiVPO4F[1.0.9]O[0.0.1] and LiVPO4F[0-0.1]O[1-0.9]). The presence of vanadyl-type defects obtained as localized or more extended ones, depending on the annealing conditions, affects drastically the electrochemical properties of these Tavorite LiVPO4F-type materials.