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

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-NaClO₄ and NaTFSI-affecting solvation structure and interfacial stability. Based on the Hofmeister series, it is demonstrated that the strong hydration of ClO₄ ^- 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 NaClO₄. 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.