A secondary battery includes a Z stack electrode assembly including a separator bent in a Z shape and including a plurality of bent areas, a first electrode plate on a lower portion of each of the bent areas, and a second electrode plate on an upper portion of each of the bent areas, and an exterior portion configured to accommodate the Z stack electrode assembly, and an outermost end area of the separator is bent to be thermally fused to a bent area of the plurality of bent areas of the separator.