Abstract: There are provided an aqueous electrolyte solution having an extended potential window, in particular, an aqueous electrolyte solution whose potential window is further wider than those exhibited by conventional concentrated aqueous electrolyte solutions, and an aqueous electrolyte solution in which the cycle characteristics can be improved. A non-aqueous electrolyte solution capable of achieving a higher energy density is provided, the non-aqueous electrolyte solution containing easily available and inexpensive materials and having further improved characteristics. One aqueous electrolyte solution of the present embodiment contains a salt of at least one selected from the group consisting of sodium, magnesium, potassium and lithium, and a chaotropic additive. One other non-aqueous electrolyte solution of the present embodiment contains a salt of at least one selected from the group consisting of sodium, magnesium, potassium and lithium, and a chaotropic additive.