Aqueous Li-ion batteries attract increasing attention as an ultimately safe rechargeable battery based on non-flammable and non-toxic aqueous electrolytes. However, their low operation voltage, limited by water’s electrochemical window (1.23 V), is a prime issue that has hindered their widespread applications. Here, by exploring optimized eutectic systems of several Li salts, we discovered a room-temperature hydrate melt of Li salts, in which all water molecules participate in Li⁺ hydration shells with retaining fluidity. As a new class of aqueous liquid electrolyte, a surprisingly wide 3.2 V stability window was achieved to unlock the water’s voltage limitation. Using the hydrate melt electrolyte, we observed, for the first time, the reversible reactions of Li₄Ti₅O₁₂ and LiNi_0.5Mn_1.5O₄ electrodes in an aqueous system (Fig. 1), both of which are far beyond the water’s stability window. As a proof of concept, we demonstrated charge-discharge cycling of a 3.1 V-class LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂ battery, as well as a 2.5 V-class LiCoO₂/Li₄Ti₅O₁₂ battery. We rationalized the unusual stability from the viewpoint of the unique liquid structure exclusively achieved in the hydrate melt state, on the basis of both experimental and theoretical approaches. We believe that our discovery of hydrate melt electrolytes will open an expanded research avenue for safe and high-voltage aqueous energy storage, breaking away from flammable, expensive, and toxic organic solvents.