Abstract: A lithium ion secondary battery is provided having a cap plate, an insulation plate, and a terminal plate, each shaped to prevent the terminal plate from rotating when assembling a cap assembly. The cap plate has an anti-rotation groove formed on a lower surface thereof and the insulation plate has an insulation plate protrusion formed on an upper surface thereof. When the anti-rotation groove is coupled to the insulation plate protrusion, the terminal plate may be prevented from rotating.

Abstract: A secondary battery including a can for receiving an electrolyte and an electrode assembly including first and second electrode plates and a cap assembly including a cap plate covering an upper portion of the can. An electrode terminal connected to one of the first and second electrode plates is inserted into a center portion of the cap plate. A safety vent is formed at a first side of the cap plate and an electrode lead wire is connected to an upper surface of the electrode terminal to connect the electrode terminal to an external terminal. A washer is installed on an upper surface of the cap assembly in order to insulate the cap plate from the electrode lead wire and has an exposure part for exposing the safety part allowing the location of the safety vent to be noted without disassembly of the secondary battery.