Vacancy‐Free Prussian Blue Nanocrystals with High Capacity and Superior Cyclability for Aqueous Sodium‐Ion Batteries

Abstract Aqueous Na‐ion batteries have the competitive advantages of being low costing and possessing inherent safety, and they are particularly suitable for large‐scale energy‐storage applications. However, the development of this new battery system is hindered by the lack of suitable Na hosts with sufficiently high capacity and cycle life. Herein, we report vacancy‐free Na 2 CoFe(CN) 6 nanocubes synthesized by a controlled crystallization reaction and reveal their use as a new aqueous cathode with reversible Na‐storage behavior. Owing to its perfect lattice framework with two redox‐active sites, this material exhibits a high reversible capacity of 130 mA h g −1 , a strong rate capability at 20 C, and superior cyclability with 90 % capacity retention over 800 cycles, all of which indicate that this material may possibly serve as a high‐performance and long‐life cathode for aqueous Na‐ion batteries. Also, the synthetic strategy described in this work may be extended to a wide range of Prussian blue compounds for the fabrication of well‐shaped nanocubes with few defects for energy storage, catalysis, and other applications.