Anion Effects on Crystal Water Reactivity and Cathode‐Electrolyte Interphase of Prussian Blue in Sodium‐Ion Batteries

Abstract Prussian blue (PB) is a promising low‐cost cathode material for sodium‐ion batteries (SIBs), but the impact of crystal water on performance degradation remains unclear. This study explores how PB's crystal water interacts with different electrolyte salts—NaClO 4 and NaTFSI—affecting solvation structure and interfacial stability. Based on the Hofmeister series, it is demonstrated that the strong hydration of ClO 4 – sustains water reactivity, promoting Fe oxidation and solvent decomposition at high voltages. In contrast, the weakly hydrated TFSI – suppresses water‐induced side reactions and facilitates the formation of stable interphases on both cathode and anode. Electrochemical analysis at 4.0 V and 4.2 V revealed that NaTFSI consistently improves reversibility, particularly at 4.2 V, achieving 77.1% capacity retention over 500 cycles—56.8% for NaClO 4 . The results highlight the crucial role of electrolyte‐dependent water coordination in determining PB electrode stability, offering valuable insights for designing electrolytes and interphases for long‐life PB‐based SIBs.