Medium-Entropy Doping Strategy for Enhanced Sodium-Ion Battery Cathodes: Optimizing Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ Electrochemical Performance

Despite being recognized as a promising cathode material for sodium-ion batteries due to its low cost and environmental friendliness, Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) faces persistent challenges regarding its rate capability and long-term cycling stability. Herein, we propose a medium-entropy doping strategy that synergistically modulates the crystal structure via multielement cooperation, significantly optimizing the electrochemical properties of the material. The medium-entropy doped cathode material Na 4 Fe 2.8 (NiCoMnV) 0.05 (PO 4 ) 2 P 2 O 7 (NFPP-ME-5) synthesized in this study can deliver an exceptional rate performance of 79.20 mAh g –1 at 20 C and 73.48 mAh g –1 at 50 C. Even more remarkably, the material exhibits extraordinary cycling longevity, maintaining a high capacity retention of 94.26% after 10,000 cycles at 20 C and retaining 95.59% of its initial capacity after 20,000 cycles at an ultrahigh rate of 50 C, with negligible degradation. When assembled into a full cell with a hard carbon anode, it achieves a specific capacity of 69.8 mAh g –1 at 10 C. Structural analysis revealed that medium-entropy doping induces the formation of a porous scaffold structure, which facilitates sodium-ion diffusion while enhancing structural stability. Electrochemical kinetic tests further confirmed that the material possesses optimized sodium-ion diffusion kinetics and enhanced electronic conductivity. This study presents a novel strategy for enhancing the rate capability and cycling stability of NFPP.