Tuning the Electrolyte Solvation Structure via a Nonaqueous Co-Solvent to Enable High-Voltage Aqueous Lithium-Ion Batteries.

"Water-in-salt" electrolytes have significantly expanded the electrochemical stability window of the aqueous electrolytes from 1.23 to 3 V, making highly safe 3.0 V aqueous Li-ion batteries possible. However, the awkward cathodic limit located at 1.9 V (versus Li/Li) and the high cost of the expensive salts hinder the practical applications. In this work, an ideal "bisolvent-in-salt" electrolyte is reported to tune the electrolyte solvation structure via introducing sulfolane as the co-solvent, which significantly enhances the cathodic limit of water to 1.0 V (versus Li/Li) at a significantly reduced salt concentration of 5.7 mol kg. Due to the competitive coordination of sulfolane, water molecules that should be in the primary solvation sheath of Li are partly substituted by the electrochemically stable sulfolane, significantly decreasing the hydrogen evolution. Meanwhile, the unique electrolyte structures enable the formation and stabilization of a robust solid electrolyte interphase. As a result, a 2.4 V LiMnO/LiTiO full cell with a high energy density of 128 Wh kg is realized. The hybrid water/sulfolane electrolytes provide a brand new strategy for designing aqueous electrolytes with an expanded electrochemical stability window at a low salt concentration.