Electrochemical performance of mixed valence Na3V2O2x(PO4)2F3−2x/C as cathode for sodium-ion batteries

A composite made of a mixed-valence sodium-vanadium fluorophosphate and 6.4% wt. carbon, Na3V2O2x(PO4)2F3−2x/C (0 < x < 1), has been prepared. Structural and magnetic characterization confirmed the +3/+4 oxidation state of vanadium in the phase. Morphological and texture analyses showed that carbon forms a network surrounding the particles, leading to a mesoporous composite with a high specific area of 67 m2 g−1. Electrochemical characterization conducted in Swagelok cells by cyclic voltammetry and galvanostatic cycling indicated that sodium extraction/insertion proceeds through a complex mechanism in two voltage pseudo-plateaux at 3.6 and 4.1 V vs. Na/Na+. Rate capability of the material ranges from specific capacities of 100 mAh g−1 at C/20 to 75 mAh g−1 at 5C. Cycling stability at 1C showed coulombic efficiency higher than 99% and capacity retention of 95% after 200 cycles.