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^-1. 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 LiMn₂O₄/Li₄Ti₅O₁₂ full cell with a high energy density of 128 Wh kg^-1 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.