Impact of the F^– for O^2– Substitution in Na₃V₂(PO₄)₂F_3–yO_y on Their Transport Properties and Electrochemical Performance

The series of polyanionic compounds Na₃V³⁺_2–yV⁴⁺_y(PO₄)₂F_3–yO_y (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 Na₃V³⁺_2–yV⁴⁺_y(PO₄)₂F_3–yO_y (NVPFO_y) materials with y = 0.8, 1.35, 1.6, and 2 were synthesized using a solvothermal reaction, and changes in the vanadium average oxidation state were fully characterized by combining analyses of 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 turned out to be an efficient characterization technique to estimate the oxygen content within the Na₃V³⁺_2–yV⁴⁺_y(PO₄)₂F_3–yO_y 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.