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

Water-in-salt electrolytes have successfully expanded the electrochemical stability window of aqueous beyond 2 V. Further improvements in can be achieved by partially substituting water with either classical organic solvents or ionic liquids. Here, we study ternary composed LiTFSI, water, and imidazolium We find that LiTFSI solubility strongly increases from 21 mol kg?1 to up 60 presence liquid. The solution structure is investigated Raman NMR spectroscopy enhanced found originate a hydrotropic effect increased reductive enables stable cycling an lithium-ion battery energy density 150 Wh on active material level based commercially relevant Li4Ti5O12 LiNi0.8Mn0.1Co0.1O2 electrode materials.