Fluorine‐Free Water‐in‐Salt Electrolyte for Green and Low‐Cost Aqueous Sodium‐Ion Batteries

Abstract The highly concentrated “water‐in‐salt” electrolyte (WiSE) containing sodium acetate and potassium acetate demonstrates surprising performance (specific capacity of 37 mA h −1 g −1 at the 5th cycle and average discharge voltage of 0.82 V) in aqueous sodium‐ion batteries (SIBs) based on Na 2 MnFe(CN) 6 and NaTi 2 (PO 4 ) 3 . The fluorine‐free electrolyte offers a wide electrochemical stability window and compatibility with Al current collector. The electrolyte, current collector, and the electrode materials based on abundant elements make the proposed battery chemistry safe, low‐cost, and environmentally friendly.