Na1.51Fe[Fe(CN)6]0.87·1.83H2O Hollow Nanospheres via Non‐Aqueous Ball‐Milling Route to Achieve High Initial Coulombic Efficiency and High Rate Capability in Sodium‐Ion Batteries

Prussian blue analogues (PBAs) have attracted extensive attention as cathode materials in sodium‐ion batteries (SIBs) due to their low cost, high theoretical capacity, and facile synthesis process. However, it is of great challenge to control the crystal vacancies and interstitial water formed during the aqueous co‐precipitation method, which are also the key factors in determining the electrochemical performance. Herein, an antioxidant and chelating agent co‐assisted non‐aqueous ball‐milling method to generate highly‐crystallized Na2‐xFe[Fe(CN)6]y with hollow structure is proposed by suppressing the speed and space of crystal growth. The as‐prepared Na2‐xFe[Fe(CN)6]y hollow nanospheres show low vacancies and interstitial water content, leading to a high sodium content. As a result, the Na‐rich Na1.51Fe[Fe(CN)6]0.87·1.83H2O hollow nanospheres exhibit a high initial Coulombic efficiency, excellent cycling stability, and rate performance via a highly reversible two‐phase transition reaction confirmed by in situ X‐ray diffraction. It delivers a specific capacity of 124.2 mAh g−1 at 17 mA g−1, presenting ultra‐high rate capability (84.1 mAh g−1 at 3400 mA g−1) and cycling stability (65.3% capacity retention after 1000 cycles at 170 mA g−1). Furthermore, the as‐reported non‐aqueous ball‐milling method could be regarded as a promising method for the scalable production of PBAs as cathode materials for high‐performance SIBs. With the cooperation of antioxidant and chelating agent, the Na1.51Fe[Fe(CN)6]0.87·1.83H2O hollow nanospheres synthesized by non‐aqueous ball‐milling method possess a high initial Coulombic efficiency (up to 98.8%), excellent cycling stability, and rate performance, making them promising candidates for high‐performance cathode materials in large‐scale energy storage systems of sodium‐ion batteries.