Synergistic Configurational Entropy and Iron Vacancy Engineering in Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 Cathode for High‐Power‐Density and Ultralong‐Life Na‐Ion Full Batteries

Abstract Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 cathode exhibits extensive potential for high‐power applications, owing to its large sodium ion diffusion channels, low cost, and suitable operating voltage. However, it suffers significant capacity degradation due to the inevitable NaFePO 4 impurity. Herein, a synergistic strategy is proposed that integrates high entropy doping with Fe vacancy engineering, which not only preserves the phase purity but also provides additional active sites and further stabilizes its crystal structure. A novel Na 4 Fe 2.61 (Ni, Co, Mn, Cu, Zn, Mg) 0.05 (PO 4 ) 2 P 2 O 7 cathode has been successfully synthesized by a simple sol‐gel method, which exhibits an ultralong cycle life (over 15000 cycles at 5 A g −1 ) and outstanding rate capability (61.1 mAh g⁻¹ at 10A g −1 ). Additionally, a combined solid‐solution and biphasic reaction mechanism in sodium storage process is thoroughly confirmed. Notably, benefiting from the rational design of N/P ratio and well‐matched capacitive contributions, the full cells assembled with hard carbon anodes exhibit superior cycling durability, sustaining over 1000 cycles at a high current density of 1 A g⁻¹ without severe capacity deterioration. Such highly durable full cells with low N/P ratioand common ester‐based electrolytes have never been reported before. The present work offers new perspectives to expedite the commercialization of low‐cost, high‐power‐density sodium‐ion batteries.