Perspective—Electrolyte Design for Aqueous Batteries: From Ultra-High Concentration to Low Concentration？

High voltage aqueous Li-ion batteries have the potential for sustainable large-scale energy storage due to their intrinsic advantages of safety, low cost, as well as environmental friendliness. “Water-in-salt” electrolytes have significantly enhanced the energy density of aqueous Li-ion batteries by extending the aqueous electrolyte stability window to 3.0 V. However, the cathodic limit and salt concentration need to be reduced, enabling to operate low cost LiMn 2 O 4 /Li 4 Ti 5 O 12 cells. In addition, gravimetric energy density of LiMn 2 O 4 /Li 4 Ti 5 O 12 , self-discharge rate, and operation temperature range remain rarely studied. This perspective aims to briefly summarize the opportunity and challenges of aqueous batteries for practical applications.