NOVELTY - A lithium ion secondary battery comprises an electrode assembly (110); a center pin positioned within the electrode assembly and having an open upper end and an open lower end, where the center pin is adapted to increase in length when an interior temperature of the center pin reaches a critical temperature; a cap (155) coupled open upper and lower ends; a can (140) containing the electrode assembly, the center pin, and the caps; and a cap assembly coupled to the can. The cap assembly has safety element that fractures in response to center pin being increased in length.    USE - As lithium ion secondary battery used for laptop computers, digital cameras, and camcorders.    ADVANTAGE - The functional center pin undergoes a chemical reaction and mechanical reaction when the temperature rises above a critical level due to overcharging of the battery, in order to further advance the safety vents operation time and current interruption time. The functional center pin is adapted to reduce any unnecessary void volume inside the battery so that the safety vent's deformation time and current interruption time are advanced in the case of overcharging and the possibility of fire is minimized. The functional center pin can be manufactured at a low cost. The variable-length member can include various functional materials, such as a gasification member and/or a non-flammable member, to operate the safety vent more quickly and accurately and avoid fire in a more active manner. When the battery temperature reaches a critical level or higher due to continuous overcharging, the cap fractures and the center pin acts as a channel to guarantee deformation or fracture of the safety vent. This prevents the battery from exploding due to overcharging. The battery can be manufactured with little cost while maintaining its stability.    DETAILED DESCRIPTION - A lithium ion secondary battery comprises an electrode assembly (110); a center pin positioned within the electrode assembly and having an open upper end and an open lower end, where the center pin is adapted to increase in length when an interior temperature of the center pin reaches a critical temperature; a cap (155) coupled to each of the open upper end and the open lower end; a can (140) containing the electrode assembly, the center pin, and the caps; and a cap assembly coupled to the can. The cap assembly has safety element that fractures in response to the center pin being increased in length. The cap comprises a coupling unit fitted to the center pin and a taper unit integral with the coupling unit positioned on an exterior of the center pin. In the lithium ion secondary battery, a film is positioned at an interface between the coupling unit and the taper unit to seal at least one of the open upper end and the open lower end. The center pin has a gasification for generating gas at a predetermined voltage of 4-4.5V. The center pin has a non-flammable member. The center pin has at least one hole formed along surface of pin. The safety element comprises a safety vent (152) positioned to have a surface contactable by the center pin; and a circuit board (153) positioned on a surface of the safety vent opposite the surface contactable by the center pin.    DESCRIPTION OF DRAWING(S) - The figure shows an exploded perspective view of the lithium ion secondary battery.    Electrode assembly (110)    Can (140)    Safety vent (152)    Circuit board (153)    Cap (155)