The Hydrotropic Effect of Ionic Liquids in Water-in-Salt Electrolytes*

Water-in-salt electrolytes have successfully expanded the electrochemical stability window of aqueous electrolytes beyond 2 V. Further improvements in stability can be achieved by partially substituting water with either classical organic solvents or ionic liquids. Here, we study ternary electrolytes composed of LiTFSI, water, and imidazolium ionic liquids. We find that the LiTFSI solubility strongly increases from 21 mol kg^-1 in water to up to 60 mol kg^-1 in the presence of ionic liquid. The solution structure is investigated with Raman and NMR spectroscopy and the enhanced LiTFSI solubility is found to originate from a hydrotropic effect of the ionic liquids. The increased reductive stability of the ternary electrolytes enables stable cycling of an aqueous lithium-ion battery with an energy density of 150 Wh kg^-1 on the active material level based on commercially relevant Li₄ Ti₅ O₁₂ and LiNi_0.8 Mn_0.1 Co_0.1 O₂ electrode materials.