Secondary battery

A secondary battery including a bare cell. The bare cell includes an exposed electrode tab (124, 125) and a lead (300c, 300b) line welded to the electrode tab and configured to electrically connect the bare cell to a device electrically connected to the lead line. The lead line includes a welding region (320) and a lead region (330) excluding the welding region. The lead line is welded to the electrode tab in the welding region. The welding region includes a first layer (312) and a second layer (314). The second layer has a higher electrical conductivity than the first layer.