Unlocking Fast-Charging and Durable Sodium-Ion Full Batteries via Entropy-Driven Coordination Regulation of Na 4 Fe 3 (PO 4 ) 2 P 2 O 7

Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) (NFPP) is a low-cost, mixed-polyanion cathode that offers an appropriate operating voltage and large theoretical capacity, yet its intrinsically low electronic conductivity and sluggish ion-diffusion kinetics hamper practical performance. To overcome these limitations, we apply a high-entropy doping strategy that tailors the local coordination environment of the NFPP framework, enhancing both electronic and ionic transports without compromising structural integrity. A series of Na 4 Fe 3–5 x (MgMnTiCrCu) x (PO 4 ) 2 P 2 O 7 ( x = 0, 0.02, 0.03, 0.04, denoted as HE-NFPP- x ) were synthesized via ball milling. The incorporation of multiple metal ions at Fe sites promotes electron transfer by shortening the Fe–O and Fe–Fe bond lengths and reducing the band gap. The lattice expansion and elongated Na–O bonds enhance the sodium ion diffusion kinetics in HE-NFPP. The optimized HE-0.03 exhibits remarkable cycling stability and exceptional fast-charging performance. This work provides valuable insights into the performance enhancement of sodium-ion battery cathodes through entropy-mediated coordination tuning.