Patent ID: 7517607

Claim:
A lithium ion secondary battery including an electrode assembly, which is formed of a positive electrode plate, a negative electrode plate and an isolation film, which are wound together, the lithium ion secondary battery further including a can, the can comprising: a pair of first walls having a plate shape, which are separated from each other by a predetermined distance; a pair of second walls having a plate shape, which have an area less than that of the pair of first walls and connect two longitudinal sides of the pair of first walls; a third wall having a plate shape, which has an area less than that of the pair of second walls and which is connected to both of the pair of first walls and the pair of second walls; and an opening, which is opposed to the third wall, to allow the electrode assembly to be coupled thereto; a cap assembly, welded to the pair of first walls and the pair of second walls, which closes the opening of the can and prevents the electrode assembly from being detached from the can, the cap assembly comprising a cap plate; and an electrolyte injected into the can through the cap plate, wherein a safety vent is obtained by forming a v-shaped notch and an auxiliary v-shaped notch on respective inner surfaces and outer surfaces of at least one of the first walls, the pair of second walls, the third wall and the cap plate, respectively, wherein the auxiliary v-shaped notch is formed at a surface opposite to a surface at which the v-shaped notch is formed, and a position at which the v-shaped notch is formed corresponds to a position at which the auxiliary v-shaped notch is formed such that a vertex of the v-shaped notch is aligned with a vertex of the auxiliary v-shaped notch.