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.