Electronic and Ionic Coupled Engineering Strategy of Na₄Fe₃(PO₄)₂(P₂O₇) for High-Rate and Long-Cycling Sodium-Ion Batteries

Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) has emerged as a promising cathode material for sodium-ion batteries (SIBs) due to its robust structural stability, extensive sodium-ion diffusion pathways, and high safety. However, its practical implementation is constrained by inherent limitations such as poor electronic conductivity and reduced capacity under high-rate conditions. In this study, we engineered a dual electronic-ionic coupling strategy to synergistically enhance the electrochemical dynamic behavior of the NFPP material. The proposed NFPP was synthesized via a sol–gel method, realized strategic Mg-substitution at Fe sites within the NFPP lattice and reduced graphene oxide (rGO) coating to establish a three-dimensional conductive framework. The optimized composite (NFPP/rGO-0.15Mg) demonstrates a reversible capacity of 110.1 mAh·g^–1at 1C with 99% capacity retention over 500 cycles. Remarkably, it maintains 97.0 mAh·g^–1at 20C and retains 94.82% of its initial capacity after 6000 cycles, demonstrating exceptional cycling stability. In situ XRD analysis confirms the minimal volumetric expansion (1.3%) during charge/discharge processes. Theoretical calculation results show that Mg doping reduces the material’s bandgap and sodium-ion migration energy barrier. Furthermore, NFPP/rGO-0.15Mg demonstrates robust electrochemical performance under low-temperature conditions (−15 °C) and full-cell configurations. These findings offer crucial implications for the rational design of advanced polyanionic cathode materials to address the evolving demands of advanced SIBs.