An electrolyte composition and a metal-ion battery employing the same are provided. The electrolyte composition includes a metal halide, a solvent, and an additive. The solvent is an ionic liquid or organic solvent. The molar ratio of the metal halide to the solvent is from 1:1 to 2.2:1. The amount of additive is from 1 wt % to 25 wt %, based on the total weight of the metal halide and the solvent. The additive is monochloroethane, trichlorethylene, dichloroethane, trichloroethane, phosphorus trichloride, phosphorus pentachloride, methyl pyidine, methyl nicotinate, or a combination thereof.