Multication Entropy Engineering Enables Fast Sodium Storage in Long-Life Na4Fe3(PO4)2(P2O7)-Based Cathodes

Na4Fe3(PO4)2(P2O7) (NFPP) is a promising iron-based mixed-polyanion cathode material for sodium-ion batteries, but its practical application remains limited by low electronic conductivity, sluggish Na+ transport, and insufficient cycling stability. Herein, a multication entropy engineering strategy was adopted to synthesize a multication-substituted NFPP cathode, Na4Fe2.5Mg0.1Al0.1V0.1Co0.1Cu0.1(PO4)2(P2O7). The introduction of Mg, Al, V, Co, and Cu preserves the parent NFPP framework while inducing slight lattice contraction, reducing particle agglomeration, and promoting the graphitization of the surface carbon coating. These structural and interfacial modifications facilitate electron transport and Na+ diffusion, leading to improved electrochemical kinetics. The NFPP-IE cathode delivers discharge capacities of 119.0, 106.8, and 92.4 mAh g–1 at 0.1, 0.5, and 5 C, respectively, and maintains 97.6% capacity retention after 500 cycles at 0.5 C and 99.9% retention after 1000 cycles at 5 C. Kinetic analyses based on CV, GITT, and in situ EIS/DRT show that the enhanced performance is associated with a higher capacitive contribution, increased Na+ diffusion coefficients, reduced charge-transfer resistance, and mitigated interfacial polarization. Moreover, the electrode morphology remains well preserved after cycling, supporting improved electrode integrity. This work demonstrates that multication compositional regulation can couple structural, microstructural, and interfacial modifications to promote fast and durable Na+ storage in NFPP-based cathodes.