Lithium ion secondary battery

The present disclosure relates to a lithium ion secondary battery, which can prevent a cylindrical can from being cracked due to an external shock applied during an assembling process while controlling a rupture pressure of the cylindrical can. The cylindrical lithium ion secondary battery includes a cylindrical can, an electrode assembly accommodated in the cylindrical can with an electrolyte, and a cap assembly sealing the cylindrical can, wherein the cylindrical can has a bottom portion and a side portion extending from the bottom portion to the cap assembly, and a safety vent having a thickness gradient is formed in the bottom portion.