An electrolyte composition and a metal-ion battery employing the same are provided. The electrolyte composition includes a metal chloride, a chlorine-containing ionic liquid, and an additive, wherein the additive has a structure represented by Formula (I) [M]i[(A(SO2CxF2x+1)y)b−]j  Formula (I), wherein M can be imidazolium cation, ammonium cation, azaannulenium cation, . . . etc., wherein M has a valence of a; a can be 1, 2, or 3; A can be N, O, Si, or C; x can be 1, 2, 3, 4, 5, or 6; y can be 1, 2, or 3; b can be 1, 2, or 3; i can be 1, 2, or 3; j can be 1, 2, or 3; a/b=j/i; and when y is 2 or 3, the (SO2CxF2x+1) moieties are the same or different.