Secondary battery having a sealing gasket with concave and convex portions

A secondary battery, comprising:

• a container;
• a cap assembly fixed to the container to seal the container;
• and
• a gasket between the container and the cap assembly, the gasket having a bent end portion configured to contact the container and/or the cap assembly;
• wherein a plurality of concavo-convex portions are on an inner circumferential surface of the bent end portion and an outer circumferential surface of the bent end portion along an axial direction of the gasket;
• wherein the plurality of concavo-convex portions are continuously along a circumference of the gasket;
• and
• wherein the secondary battery further comprises at least one O-ring in at least one of the plurality of concavo-convex portions.