Lithium-salt monohydrate melt As the market of an energy storage system grows, the demand for the batteries with thehigh reliability, superior safety, and high energy density has been aggrandized daily. For commercialized Li-ion batteries, thenon-aqueous electrolytes have been universally adopted owing to its wide potential window (>3 V), which is much essential for achieving high energy density. However, their high volatility, the flammability, and the toxicity pose a serious safety issue for the use in the wide fields. Indeed, several well-publicized incidents related to theLi-ion batteries have amplified the concerns about their overall safety (Schreiner et al., 2010). Aqueous electrolytes have been considered as an alternative since Dahn group suggested in 1995. Aqueous electrolytes are quite non-flammable and low-toxicity, and can also reduce facility cost for the humidity control in the manufacturing line. However, their intrinsically narrow potential windows (~1.25 V for pure water) have limited the voltage and the energy density ofthe aqueous Li-ion batteries, thus impeding their specific commercialization