Medium‐Entropy Doping in Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ Cathode via Mg/Cu/Cr Triple Doping: Site‐Specific Optimization Unlocks High Conductivity and Ultralong Cyclability

Abstract Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ (NFPP) has emerged as a promising cathode material for sodium‐ion batteries due to its expansive sodium‐ion diffusion channels, cost‐effectiveness, and optimal operating voltage. However, the inherent presence of NaFePO ₄ impurities, coupled with substantial volume fluctuations and poor intrinsic electronic conductivity, significantly hinder its practical application. To address these limitations, a tailored medium‐entropy (ME) doping strategy is employed to synthesize Na ₄ Fe _2.91 Mg _0.03 Cu _0.03 Cr _0.03 (PO ₄ ) ₂ P ₂ O ₇ /C (NFPP‐ME). This material demonstrates remarkable electrochemical performance, including an ultra‐long cycle life with a capacity retention of 75.3 mAh g⁻¹ after 10 000 cycles at 10 C (79.12% capacity retention) and excellent rate capability, achieving 72.4 mAh g⁻¹ at 50 C. Systematic experimental investigations and theoretical calculations reveal that Mg doping enhances the structural stability of the material, while Cu doping suppresses the formation of the inactive NaFePO ₄ impurity phase. Cr doping introduces defect states at the valence band edge, thereby enhancing the electronic conductivity. Furthermore, in situ X‐ray diffraction (XRD) studies validate that the doping strategy minimizes volume fluctuations, contributing to prolonged cycling stability. The insights gained from this study offer important guidance for the design of high‐performance, durable polyanionic materials for advanced sodium‐ion battery applications.