Mechanism investigation of high performance Na3V2(PO4)2O2F/reduced graphene oxide cathode for sodium-ion batteries

Abstract Na3V2(PO4)2O2F (NVPOF) is a promising cathode material for sodium-ion batteries (SIBs) due to its high working plateaus, small volume change and large theoretical capacity. However, the electrochemical performance is strongly hampered by its intrinsic inferior kinetics. Herein, NVPOF nanorods are uniformly anchored onto reduced-graphene oxide (rGO) nanosheets by a simple electrostatic assembly method followed by thermal-annealing treatment. The synthesized NVPOF@rGO nanocomposite is applied as cathode material of SIBs. With the assistance of rGO, NVPOF@rGO has improved surface kinetics and reduced polarization, leading to an enhanced rate capability and long cycle stability. The in-situ XRD results indicate that the desodiation process of NVPOF@rGO is related to two-Na/two electrons full extraction process and the crystal phase is changed from Na3V2(PO4)2O2F into Na2V2(PO4)2O2F (1st charge plateau) and then to NaV2(PO4)2O2F (2nd charge plateau), and the sodiation process is the reverse process. The diffusion coefficient of sodium ions is increased greatly with the assistance of rGO nanosheets for NVPOF@rGO nanocomposites. This superior electrochemical performance is attributed to the synergetic effect between rGO nanosheets and the NVPOF nanorod by enhanced electronic conductivity and the improved electrochemical kinetics. Our results may provide a simple route to improve the electrochemical performance for the cathode materials of SIBs.