High‐Energy Aqueous Sodium‐Ion Batteries

Abstract Water‐in‐salt electrolytes (WISE) have largely widened the electrochemical stability window (ESW) of aqueous electrolytes by formation of passivating solid electrolyte interphase (SEI) on anode and also absorption of the hydrophobic anion‐rich double layer on cathode. However, the cathodic limiting potential of WISE is still too high for most high‐capacity anodes in aqueous sodium‐ion batteries (ASIBs), and the cost of WISE is also too high for practical application. Herein, a low‐cost 19 m (m: mol kg −1 ) bi‐salts WISE with a wide ESW of 2.8 V was designed, where the low‐cost 17 m NaClO 4 extends the anodic limiting potential to 4.4 V, while the fluorine‐containing salt (2 m NaOTF) extends the cathodic limiting potential to 1.6 V by forming the NaF–Na 2 O–NaOH SEI on anode. The 19 m NaClO 4 –NaOTF–H 2 O electrolyte enables a 1.75 V Na 3 V 2 (PO 4 ) 3 ∥Na 3 V 2 (PO 4 ) 3 full cell to deliver an appreciable energy density of 70 Wh kg −1 at 1 C with a capacity retention of 87.5 % after 100 cycles.