A lithium ion secondary battery includes: an electrode assembly having first electrodes, second electrodes, and separators interposed between each of the first electrodes and each of the second electrodes; a can containing the electrode assembly; a cap assembly having a cap plate and a first electrode terminal having a first polarity insulated from the cap plate, the cap assembly covering a top opening of the can; and a battery accessory mounted on the cap assembly. At least one protrusion is provided on an upper surface of the cap plate, and the protrusion is combined in a complementary manner with a corresponding member of the battery accessory. As a result, it is possible to easily engage the battery accessories with the bare cell, and to easily perform subsequent processes using the complementary engagement.