A novel bimetallic-polyanion Na4Fe2.82Ni0.18(PO4)2P2O7 cathode with superior rate performance and long cycle-life for sodium-ion batteries

Na4Fe3(PO4)2P2O7, with the advantages of strong structural stability, suitable voltage window, low cost and green pollution-free, is considered as a highly researchable cathode material for sodium-ion batteries. However, the low conductivity of Na4Fe3(PO4)2P2O7 and the inevitable electrochemical inertia from maricite-NaFePO4 impurity during the preparation process lead to the low reversible capacity and poor cycling stability. In this work, a new bimetallic-polyanion cathode material Na4Fe2.82Ni0.18(PO4)2P2O7 capped with an in-situ nitrogen-doped carbon layer is proposed. Ni-substituting inhibits the generation of electrochemical inert maricite-NaFePO4 impurity and enhances the Na+ ion conductivity. Meanwhile, a lower volume-change during the charging/discharging process can be achieved in Na4Fe2.82Ni0.18(PO4)2P2O7 electrode compared with conventional Na4Fe3(PO4)2P2O7 electrode. With the synergistic effect of in-situ N-doping and Ni-substituting, an ultra-high initial discharge capacity (128.4 mAh g−1 at 0.2C), superior rate performance (62.5 mAh g−1 at 50C) and significant long-term cycle life (capacity retention rate of about 83 % after 3000 cycles at 10C) can be achieved in the optimal Na4Fe2.82Ni0.18(PO4)2P2O7@C-N (Ni6-NFPP@C-N) cathode. This study suggests a practical strategy to produce Na4Fe3(PO4)2P2O7-based electrode material with no impurities and superior electrochemical properties for sodium-ion batteries.