Phase-manipulated hierarchically core-shell Na3(VO1-xPO4)2F1+2x (0≤x≤1)@Na3V2(PO4)3 and its synergistic effect with conformally wrapped reduced graphene oxide framework towards high-performance cathode for sodium-ion batteries

Sodium-ion batteries (SIBs) are regarded as promising candidates for large-scale stationary energy storage systems because of the high abundance of Na resources. Na superionic conductor polyanionic cathode (NASICON), such as Na 3 (VO 1-x PO 4 ) 2 F 1+2x (0 ≤ x ≤ 1, NVPF 1+2x ) and Na 3 V 2 (PO 4 ) 3 (NVP), shows intriguing electrochemical performances due to their robust open structure and superior cycle stability. Inspired by phase manipulation and conductive reduced graphene oxide (rGO) incorporation, a hierarchical NVPF 1+2x @NVP core-shell structure is elaborately synthesized and conformally wrapped by rGO via a facial microwave-assisted hydrothermal approach. F-free surface of NVPF 1+2x @NVP particle possesses intimate contact with rGO framework without strong mutual repulsive force while conductive rGO endows NVPF 1+2x @NVP with high electronic conductivity to ensure fast electron transport . Combining the synergistic effect between delicate design of core-shell structure and conductive rGO framework, the NVPF 1+2x @NVP/rGO-600 °C electrode achieves a high discharge capacity of 105.9 mAh g −1 at 0.5 C with an impressive capacity retention of 99.1% after 200 cycles, delivering an excellent rate capability of 90.6 mAh g −1 at 10 C. Moreover, the NVPF 1+2x @NVP/rGO-600 °C electrode possesses an alleviated polarization, small impedance and fast electron/Na  +   diffusion properties due to this synergistic effect. The full cell using NVPF 1+2x @NVP/rGO-600 °C cathode and hard carbon (HC) anode shows an initial discharge capacity of 80.0 mAh g −1 at 0.2 C and rate capability of 45.2 mAh g −1 at 10 C, which could light an LED lamp and manifest its prospective application. Our findings provide new insights into a broadly applicable material design combining morphological phase manipulation with conductive rGO framework incorporation, which synergistically boosts the electrochemical performance of NASICON cathode towards advanced SIBs with a higher energy density , faster charging rate and more practical applications.