Several hydrophilic ionic liquids dissolved in water with different molalities are studied as “water-in-ionic liquid” supporting electrolytes. They exhibit wide electrochemical stability window from 3 to 4.4 V, high ionic conductivities and good flowability, making them promising supporting electrolytes for aqueous high-voltage rechargeable batteries. In such supporting electrolytes, enhanced redox activities of lithium insertion/extraction in anodic TiO2 film at low negative potential and Fe3+/Fe2+ couple at moderate positive potential were observed. Cyclability of an aqueous TiO2/Fe2+ hybrid battery is demonstrated with a voltage of 1.7 V.