Formation of a Solid Electrolyte Interphase in Hydrate-Melt Electrolytes

Aqueous Li-ion batteries using nonflammable aqueous electrolytes have been continuously studied to achieve the ultimate safety of the battery system. However, they have a major drawback of low operation voltage resulting from the narrow potential window of aqueous electrolytes. Recently, a room-temperature hydrate melt of Li salts has been discovered as a new class of stable aqueous electrolyte with a widened potential window (over 3 V) that enables the reversible operation of high-voltage (3 V-class) aqueous Li-ion batteries. An important factor contributing to the wide potential window is the formation of a solid electrolyte interphase (SEI) on negative electrodes, but its detailed mechanism has not been fully understood yet. Here, we study the SEI formation in the hydrate-melt electrolyte in relation with the composition and morphology of the electrodes investigated via X-ray photoelectron spectroscopy and scanning electron microscopy. We demonstrate that the formation of a stable SEI depends on the type of the electrodes used as well as the electrolyte salt concentrations.