Impact of the F– for O2– substitution in Na3V2(PO4)2F3–yOy on their transport properties and electrochemical performance

The series of polyanionic compounds Na3V 3+ 2-yV 4+ y(PO4)2F3-yOy (0 ≤ y ≤ 2) attracts much attention as positive electrode material for Na-ion batteries, because of its high operating potential and stable cycling performance. A series of nanospherical Na3V 3+ 2-yV 4+ y(PO4)2F3-yOy (NVPFOy) materials with y = 0.8, 1.35, 1.6 and 2 was synthesized using a solvothermal reaction and changes in the vanadium average oxidation state were fully characterized combining analyses by Raman and infrared spectroscopies and X-ray diffraction. Raman spectroscopy, beyond checking for the absence of a carbon coating, was in fact used for its sensitivity to the vanadium environment and turn out to be an efficient characterization technique to estimate the oxygen content within the Na3V 3+ 2-yV 4+ y(PO4)2F3-yOy family. The impact of the oxygen content on the transport properties was evaluated by electrochemical impedance spectroscopy. The material with y = 1.35 demonstrates the smallest electrical resistivity in the series, as well as the best rate capability and cyclability upon long-term cycling, despite no carbon coating and a high mass loading positive electrode