Cylindrical secondary battery

1 . A cylindrical secondary battery comprising:

a cylindrical can having a circular bottom portion, a side portion extending from the bottom portion, a beading part concavely formed inwardly at one end of the side portion, and a crimping part formed by bending a distal end of the side portion;

an electrode assembly in the can; and

a cap assembly sealing the can, the cap assembly comprising:

a gasket for insulation from the can, an upper portion of the gasket in a direction away from the electrode assembly having a smaller thickness than a lower portion of the gasket;

a cap-up between the crimping part and the beading part; and

a safety vent below the cap-up,

wherein the gasket has an inclined surface that is chamfered on an inner circumferential surface thereof, the inclined surface beginning at an edge of the cap-up, and

wherein a lower surface of an edge of the safety vent is in contact with an upper surface of the cap-up.

2 . The cylindrical secondary battery of claim 1 , wherein the thickness of the upper portion of the gasket decreases toward its end portion.

3 . The cylindrical secondary battery of claim 2 , wherein the gasket has a ring-shaped body, a connection portion extending from the body, and an extension portion extending downwardly from the connection portion, and

wherein a thickness of the body decreases toward its end portion.

4 . The cylindrical secondary battery of claim 3 , wherein the cap assembly further comprises

a cap-down below the safety vent and electrically connected to the electrode assembly, and

wherein the gasket insulates the safety vent and the cap-up from the can.

5 . The cylindrical secondary battery of claim 4 ,

wherein the gasket is bent to surround the edge of the safety vent.

6 . The cylindrical secondary battery of claim 5 , wherein the inclined surface of the gasket is in contact with an upper surface of the edge of the safety vent.

7 . The cylindrical secondary battery of claim 6 , wherein the safety vent is bent to extend over the upper surface of the cap-up, and

wherein a bending position at where the safety vent is bent is at where the inclined surface of the gasket begins.