A lithium ion battery is disclosed. The lithium ion battery includes a can, an electrode assembly, and a cap assembly. A relatively thin emergency rupture member is formed on (or at) a bottom (or bottom surface) of the can. The emergency rupture member may have a line shape which is spaced apart from and parallel to a long side of the bottom of the can. Alternatively, the emergency rupture member may be formed on (and/or along) the long side of the bottom. Compressive forces applied to opposite sides of the can in directions parallel to the emergency rupture member cause the can to rupture along the emergency rupture member.