Secondary battery

A secondary battery comprising:

• An electrode assembly;
• A battery can configured to accommodate the electrode assembly and having an open upper portion;
• and
• A top cap bonded to the upper portion of the battery can, a terminal region protruding upward from the top cap having a wall thickness (t2),
• Wherein an end of the upper portion of the battery can includes a bent part that is bent inward toward the top cap to fix the top cap,
• Wherein the secondary battery is configured such that, when an internal pressure of the battery can exceeds a predetermined value, a region (R) in which the top cap and the bent part overlap each other when viewed from above is reduced by deformation of the top cap or the bent part so as to allow the top cap to be displaced with respect to the battery can in a direction away from the electrode assembly,
• Wherein the battery can has a width (W) defined orthogonally to a longitudinal axis of the battery can, the bent part having a bent length (A) extending towards the longitudinal axis, and a ratio A/W of the bent length (A) of the bent part to the width (W) of the battery can ranging from 2.27×10−2 to 7.5×10−2, and
• wherein a ratio A/t2 of the bent length (A) of the bent part to the wall thickness (t2) of the terminal region of the top cap ranges from 1 to 5.