An electrolyte composition and an energy storage device employing the same are provided. The electrolyte composition includes a solid and a solution. The solid includes a core and a metal layer encapsulating the core, where the metal layer is selected from a group consisting of Zn, Al, Mg, Li, Na and the metal oxides thereof. In particular, the solid has a first density and the solution has a second density, and the ratio between the first density and the second density is from about 0.97 to 1.03.