Achieving Fast and Stable Sodium Storage in Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) via Entropy Engineering.

Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) (NFPP) has been considered a promising cathode material for sodium-ion batteries (SIBs) owing to its environmental friendliness and economic viability. However, its electrochemical performance is constrained by connatural low electronic conductivity and inadequate sodium ion diffusion. Herein, a high-entropy substitution strategy is employed in NFPP to address these limitations. Ex situ X-ray diffraction analysis reveals a single-phase electrochemical reaction during the sodiation/desodiation processes and the increased configurational entropy in HE-NFPP endows an enhanced structure, which results in a minimal volume variation of only 1.83%. Kinetic analysis and density functional theory calculation further confirm that the orbital hybrid synergy of high-entropy transition metals offers a favorable electronic structure, which efficaciously boosts the charge transfer kinetics and optimizes the sodium ion diffusion channel. Based on this versatile strategy, the as-prepared high-entropy Na 4 Fe 2.5 Mn 0.1 Mg 0.1 Co 0.1 Ni 0.1 Cu 0.1 (PO 4 ) 2 (P 2 O 7 ) (HE-NFPP) cathode can deliver a prominent rate performance of 55 mAh g -1 at 10 A g -1 and an ultra-long cycling lifespan of over 18 000 cycles at 5 A g -1 . When paired with a hard carbon (HC) anode, HE-NFPP//HC full cell exhibits a favorable cycling durability of 1000 cycles. This high-entropy engineering offers a feasible route to improve the electrochemical performance of NFPP and provides a blueprint for exploring high-performance SIBs.