Boosting Na ⁺ Storage and Thermal Stability of Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ via High-Entropy Engineering

Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) possesses a stable NASICON-type framework and a suitable redox potential, making it attractive as a sodium-ion battery cathode. Yet, its utility is limited by poor electronic conductivity and sluggish Na⁺ diffusion. Here, we design a high-entropy Na₄Fe_2.75Mn_0.05Mg_0.05Cr_0.05Cu_0.05Al_0.05(PO₄)₂P₂O₇ (HE-NFPP) synthesized through spray drying and sintering. HE-NFPP achieves a compaction density of 2.34 g/cm³ under 300 MPa, rivaling that of LiFePO₄. The high-entropy incorporation of Mn, Mg, Cr, Cu, and Al enables enhanced electron transitions and improved intrinsic conductivity. Simultaneously, the creation of wide, interconnected 3D Na⁺ channels significantly reduces migration barriers, accelerating transport and electrode kinetics. In situ optical fiber thermometry reveals suppressed heat evolution, leading to enhanced thermal stability, uniform reaction processes, and safer operation. Additionally, the redistribution of the local electrostatic field minimizes cation repulsion and mechanical strain during Na⁺ (de)intercalation, ensuring structural robustness. These synergistic effects yield excellent rate capability and cycling durability, underscoring the potential of entropy engineering as a versatile strategy to optimize polyanionic cathodes for next-generation sodium-ion batteries.