Graphene modified sodium vanadium fluorophosphate as a high voltage cathode material for sodium ion batteries

Substantial interests have been paid to Na-based electrode materials due to the abundance and low cost of sodium resources on earth. In this study, we improved the electrochemical performance of NaVPO4F cathode by coating graphene sheets. The structure, composition and morphology of NaVPO4F/graphene composite were investigated by XRD, FT-IR, FE-SEM and TEM. FE-SEM and TEM analysis indicate that the graphene sheets were successfully coated on the surface of NaVPO4F. The NaVPO4F/graphene composite could be operated at a high working potential (3.95 V for average voltage), and displayed the highest capacity of 120.9 mAh g−1. After 50 charge/discharge cycles at 0.05 C, the capacity retention of the cathode could reach up to 97.7%. Although the discharge capacity of NaVPO4F/graphene decreased to 70.1 mAh g−1 under a high rate of 0.5 C, it returned back to 113.2 mAh g−1 once the rate was reduced to 0.05 C. EIS measurement further reveals that the NaVPO4F/graphene electrode exhibited a faster sodium-ion diffusivity and less resistance compared with the pure NaVPO4F. Our results demonstrate that NaVPO4F/graphene is a promising cathode material for sodium ion batteries.