3D Interconnected Porous Sodium Super Ionic Conductor-Structured Na₄Fe₃(PO₄)₂(P₂O₇)/C as a Cathode Material for Sodium-Ion Batteries

Na4Fe3(PO4)2P2O7 (NFPP), with a high theoretical specific capacity of 129 mAh g–1, is regarded as a promising cathode material for sodium-ion batteries. However, its practical application in sodium-ion batteries is severely constrained by its intrinsic low electronic conductivity and the presence of NaFePO4 impurities. In this study, a sol–gel method combined with solid-state calcination is successfully employed to synthesize NFPP/C, a sodium super ionic conductor, with a three-dimensional interconnected porous structure. The NFPP/C material exhibits a uniform surface carbon coating and a continuous mesoporous structure. After annealing at 550 °C for 10 h, NFPP/C demonstrates excellent electrochemical performance, including a high discharge capacity of 104.6 mAh g–1 at 0.1 C, superior long-term cycling stability (79.3% capacity retention after 2000 cycles at 10 C), and outstanding rate capability (71.4 mAh g–1 at 20 C). This method provides a universal synthesis strategy for high-performance phosphate cathode materials with three-dimensional interconnected porous architectures for sodium-ion batteries.