Hybrid Electrolytes Enabling in‐situ Interphase Protection and Suppressed Electrode Dissolution for Aqueous Sodium‐Ion Batteries

Abstract Aqueous sodium‐ion batteries are promising candidates for grid‐scale energy storage due to their high safety and low cost. However, the constant dissolution of vanadium‐based electrodes and protective solid‐electrolyte interphase (SEI) in aqueous electrolytes severely limits the cycle life. Herein, using adiponitrile as a functional co‐solvent, we designed a super‐concentrated aqueous sodium electrolyte (Na + /H 2 O=1, molar ratio) to confine almost all water molecules within the primary solvation shell of Na + with decreased activity. Such a unique solvation structure not only expands the electrochemical stability window of the electrolyte to 2.75 V, but also greatly alleviates the dissolution of vanadium‐based electrodes and NaF‐rich SEI. The assembled Na 3 V 2 (PO 4 ) 3 /NaTi 2 (PO 4 ) 3 battery delivers an average Coulombic efficiency of 99.6 % with 71 % capacity retention after 1000 cycles at 5 C. In addition, the freezing point of the electrolyte could be reduced −79 °C while retaining appreciable low‐temperature conductivity due to the disrupted hydrogen bond among water molecules.