Vanadyl-type defects in Tavorite-like NaVPO4F: from the average long range structure to local environments

Abstract

Tavorite-type compositions offer rich crystal chemistry for positive electrodes in rechargeable batteries, among which LiV^IIIPO₄F has the highest theoretical energy density (i.e. 655 Wh kg⁻¹). In this article, we report for the first time the synthesis of the related Na-based phase crystallizing in the Tavorite-like structure. Its in-depth structural and electronic characterization was conducted by a combination of several techniques, spanning electron and X-ray powder diffraction as well as infrared and X-ray absorption spectroscopy. The magnetic susceptibility measurement reveals an average oxidation state for vanadium slightly higher than V³⁺. This slight oxidation is supported by infrared and X-ray absorption spectroscopies which highlight the presence of V⁴⁺O vanadyl-type defects leading to an approximated NaV^III_0.85(V^IVO)_0.15(PO₄)F_0.85 composition. In this material, the profile of the diffraction lines is governed by a strong strain anisotropic broadening arising from the competitive formation between the ionic V³⁺–F and the covalent V⁴⁺O bonds. This material shows a limited extraction of sodium, close to 15% of the theoretical capacity. Indeed, its electrochemical properties are strongly inhibited by the intrinsic low sodium mobility in the Tavorite framework.