Strong Impact of the Oxygen Content in Na₃V₂(PO₄)₂F_3–<i>y</i>O_<i>y</i> (0 ≤ <i>y</i> ≤ 0.5) on Its Structural and Electrochemical Properties

Abstract

Polyanionic materials such as Na3V2(PO4)2F3–yOy (0 ≤ y ≤ 2) are of high interest as positive electrode for Na-ion batteries since they offer competitive electrochemical performances compared to sodiated transition metal oxides. The composition Na3V2(PO4)2F3 (y = 0) has the highest theoretical energy density among the series, but surprisingly a lot of discrepancies are reported throughout the literature considering its structure and its electrochemical properties. We will show that most of the compounds reported as being Na3VIII2(PO4)2F3 are in fact slightly oxidized due to synthesis conditions resulting in a partial oxygen substitution for fluorine. To get an in-depth understanding of this system, a series of compositions Na3V2(PO4)2F3–yOy (0 ≤ y ≤ 0.5, i.e., near the fluorine-rich composition) was synthesized and characterized combining synchrotron X-ray diffraction, X-ray absorption spectroscopy, solid state nuclear magnetic resonance spectroscopy, and galvanostatic electrochemical tests. The structural features and electrochemical signatures of these oxidized compounds will be carefully compared to those recently obtained for Na3VIII2(PO4)2F3 by Bianchini et al.