Dually Decorated Na₃V₂(PO₄)₂F₃ by Carbon and 3D Graphene as Cathode Material for Sodium-Ion Batteries with High Energy and Power Densities

The dually decorated Na₃V₂(PO₄)₂F₃ by in-situ formed carbon and three-dimensional graphene (NVPF/C@3DG) is prepared via two hydrothermal steps and a high-temperature calcination. Benefiting from the high electronic conductivity of carbon and 3DG, high surface area and the resulted high diffusion coefficients of Na⁺, NVPF/C@3DG displays superior electrochemical performance to the control NVPF/C. Between 2.5 and 4.5 V (vs. Na⁺/Na), NVPF/C@3DG presents the initial discharge capacity of 123.6 mAh g⁻¹ at 0.2 C (25.6 mA g⁻¹) and capacity retention of 92.1 % after 60 cycles, while NVPF/C shows initial discharge capacity of 119 mAh g⁻¹ and capacity retention of 84.1 % under the identical test conditions. Even at 15 C, the initial discharge capacity and capacity retention after 1000 cycles of NVPF/C@3DG are as high as 91.2 mAh g⁻¹ and 82.9 %, respectively, much better than the initial discharge capacity of 71.2 mAh g⁻¹ and the corresponding capacity retention of 76.4 % for NVPF/C, respectively.