1. Field of the Invention
The present invention relates to a center pin and a lithium ion secondary battery, and more particularly, to a center pin that has a hollow body and a solid extinguishant inside the hollow body, and a lithium ion secondary battery having the same. The solid extinguishant has a certain shape and can be made by compressing a powder extinguishing substance. Top and bottom of the center pin are initially closed to reduce a dead volume in a battery. The top and bottom of the center pin open at high temperature so that the solid extinguishant is liquefied and flows out of the center pin. The liquefied extinguishant at the high temperature prevent the battery from igniting.
2. Description of the Related Art
As weights of portable apparatuses, such as video cameras, mobile telephones, and portable computers, are reduced and performance of the portable apparatuses is improved, researches have been performed on the secondary batteries that are used for the driving power sources of the portable apparatuses. The secondary batteries include a nickel cadmium (Ni—Cd) battery, a nickel metal hybrid (Ni-MH) battery, a nickel zinc (Ni—Zn) battery, and a lithium ion (Li-ion) secondary battery. The Li-ion secondary battery is rechargeable, can be made in a small size, and has large capacity. The operation voltage and the energy density per unit weight of the Li-ion secondary battery are high. Therefore, the Li-ion secondary battery is widely used for the portable apparatuses and for other types of electronic devices.
A lithium ion secondary battery includes a cylindrical can, an electrode assembly that is disposed inside the cylindrical can, an electrolyte filled in the cylindrical can, a cap assembly installed at an upper part of the cylindrical can, and a center pin that is inserted into the core of the electrode assembly. The electrode assembly has a positive electrode plate, a negative electrode plate, and a separator that is disposed between the positive electrode plate and the negative electrode plate. The electrode assembly is disposed inside the cylindrical can, and an electrolyte is filled in the cylindrical can. The electrode assembly produces electricity through chemical reaction with the electrolyte.
The cap assembly covers the open top of the cylindrical can, and has terminals to supply electricity generated in the can to an external device that is electrically connected to the can. The cap assembly includes a safety vent and a current interception unit. The safety vent is distorted or broken when gas is generated in the can increasing pressure. If the safety vent is distorted or broken, the current interception unit is also damaged, and current is intercepted.
The center pin is inserted in order to prevent the electrode assembly from being distorted or floating. The center pin is commonly formed of a metal such as stainless steel. The center pin functions as a path for supplying gas generated in the can to the cap assembly.
On the other hand, a void volume or a dead volume, which is an empty space, exists in a battery. The dead volume is formed between the cap assembly and the electrode assembly, and is also formed inside the center pin. The dead volume causes a safety problem in a battery. If a battery is overcharged, voltage of the battery and temperature of the battery increase. In this case, gas is generated inside the battery. The pressure of the gas distorts the safety vent installed in the cap assembly to intercept current and to prevent a dangerous situation. The dead volume, however, prevents the pressure of the gas from quickly increasing, and as a result, prevents the gas from quickly distorting the safety vent. Therefore, the temperature of the battery further rises to an ignition point, which causes an explosion of the battery.
Therefore, it is necessary to provide a secondary battery that effectively reduces the dead volume and that efficiently prevents the explosion of the battery during an abnormal operation of the battery.