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

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 Na2 MnFe(CN)6 and NaTi2 (PO4 )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.