Phase Transition Behavior of Acetate Electrolyte and Its Application in Aqueous Sodium-Ion Batteries

Acetate salts, owing to their low cost and high safety, show promise as supporting electrolytes for aqueous batteries. Building on the "water-in-salt" (WiS) strategy, recent studies propose using mixed cations to increase total salt concentration, thereby optimizing the electrochemical stability window (ESW) and battery performance. In this study, various Na-K binary acetate aqueous solutions were investigated, and their phase transition behaviors at room temperature were first reported, analyzed through thermal and electrochemical characterizations. When applied in a full-cell system (sodium manganese hexacyanoferrate/sodium titanium phosphate), the 16 mol kg^-1 potassium acetate +8 mol kg^-1 sodium acetate exhibited a matching ESW and better interface features than higher-concentration alternatives, making it more suitable for aqueous batteries. It maintained a specific capacity of 32 mAh g^-1 after 500 cycles. These findings highlight the importance of multidimensional considerations for WiS electrolytes to meet application requirements.