Cylindrical secondary battery with cap assembly configured for deformation during gas discharge

A cylindrical secondary battery comprising:

• a cylindrical can having one open end to form an opening;
• an electrode assembly accommodated the cylindrical can;
• and
• a cap assembly including a vent plate on an outermost side of the cylindrical can in a first direction away from the opening, the vent plate including at least one notch, a cap down spaced apart from the vent plate in a second direction of the opening and electrically connected to the electrode assembly, a support plate between the vent plate and the cap down coupled to the vent plate, and an insulating member between the support plate and the cap down to insulate the support plate and the cap down from each other,
• wherein, the vent plate is configured to rupture and separate from the cap assembly based on pas inside the cylindrical can being discharged toward the opening;
• wherein, the support plate includes a first support part and a second support part, wherein the second support part is configured to transition from a first state to a second state based on the gas inside the cylindrical can being discharged toward the opening, wherein in the first state the second support part is in contact with a lower surface of the vent plate and in the second state the second support part is inverted upward into an inverted position.