As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having high energy density and voltage, extended service-life, and low self discharge rate, which has been commercialized and widely used.
Based on the construction of electrodes and an electrolyte, the lithium secondary battery may be classified as a lithium-ion battery, a lithium-ion polymer battery, or a lithium polymer battery. Among them, the lithium-ion polymer battery has been increasingly used because the lithium-ion polymer battery has a low possibility of electrolyte leakage and can be easily manufactured. The lithium-ion polymer battery (LiPB) is constructed in a structure in which an electrode assembly manufactured by thermally welding electrodes (cathodes and anodes) and separators is impregnated with an electrolyte. Mostly, the lithium-ion polymer battery is constructed in a structure in which a stacking type electrode assembly is mounted in a pouch-shaped case made of an aluminum laminate sheet in a sealed state. For this reason, the lithium-ion polymer battery is often referred to as a pouch-shaped battery.
FIGS. 1 and 2 typically illustrate a general structure of a representative lithium-ion polymer battery including a stacking type electrode assembly.
Referring to these drawings, a lithium-ion polymer battery 100 is constructed in a structure in which an electrode assembly 300 including cathodes, anodes, and separators disposed respectively between the cathodes and the anodes is mounted in a pouch-shaped battery case 200, cathode and anode tabs 301 and 302 of the electrode assembly 300 are welded to two electrode leads 400 and 410, respectively, and the electrode assembly 300 is sealed in the battery case 200 while electrode leads 400 and 410 are exposed to the outside of the battery case 200.
The battery case 200 is made of a soft wrapping material, such as an aluminum laminate sheet. The battery case 200 includes a case body 210 having a hollow receiving part 230 for receiving the electrode assembly 300 and a cover 220 connected to the case body 210 at one side thereof.
The electrode assembly 300 of the lithium-ion polymer battery 100 may be constructed in a jelly-roll type structure in addition to the stacking type structure shown in FIG. 1. The stacking type electrode assembly 300 is constructed in a structure in which the cathode tabs 301 and the anode tabs 302 are welded to the electrode leads 400 and 410, respectively, and insulative films 500 are attached to the upper and lower surfaces of the electrode leads 400 and 410 for securing electrical insulation and sealability between the electrode leads 400 and 410 and the battery case 200.
When a lithium secondary battery, such as the lithium-ion polymer battery, is exposed to high temperature, or when a large amount of current flows in a short time due to overdischarge, an external short circuit, a nail penetration, a local crush, or a drop-induced short circuit, the battery is heated due to IR heat generation with the result that the battery may catch fire or explode. As the temperature of the battery is increased, the reaction between the electrolyte and the electrodes is accelerated. As a result, heat of reaction is generated, and therefore, the temperature of the battery is further increased, which accelerates the reaction between the electrolyte and the electrodes. As a result, the temperature of the battery is sharply increased, and therefore, the reaction between the electrolyte and the electrodes is accelerated. This vicious cycle causes a thermal runaway phenomenon in which the temperature of the battery is sharply increased. When the temperature of the battery is increased to a predetermined temperature level, the battery may catch fire. Also, as a result of the reaction between the electrolyte and the electrodes, gas is generated, and therefore, the internal pressure of the battery is increased. When the internal pressure of the battery is increased to a predetermined pressure level, the lithium secondary battery may explode. This possibility that the lithium secondary battery catches fire and explodes is the most fatal disadvantage of the lithium secondary battery.
Especially, the battery case of the lithium-ion polymer battery is made of a soft wrapping material having low strength. As a result, the battery case of the lithium-ion polymer battery is easily deformed when the battery falls or external impacts are applied to the battery. As shown in FIG. 2, a space 230a is provided at the upper end of the electrode assembly 300 in the battery case 200 such that the electrode tabs of the electrode assembly are connected to the electrode leads 400 and 410 by welding. Consequently, when external impacts are applied to the battery at the upper end of the battery due to the falling of the battery, the electrode assembly 300 is moved toward the space 230a formed at the upper end of the electrode assembly 300, and, therefore, the welded parts (mainly, the cathode region) of the electrode leads 400 and 410 are brought into contact with the outermost electrodes (mainly, anode electric collectors) of the electrode assembly 300 or the upper end of the electrode assembly 300 with the result that an internal short circuit may occur. The falling of the battery frequently occurs during the use of the battery. Consequently, the demand of a technology for more efficiently securing the safety of the battery is very high.
Some conventional arts propose a method of attaching adhesive tape to predetermined positions of the electrode assembly and a method of filling the space formed at the upper end of the electrode assembly with a foreign material, in order to prevent the occurrence of the internal short circuit due to the movement of the electrode assembly. However, these methods have a problem in that the adhesive tape and the foreign material chemically react with the electrolyte, and therefore, the performance of the battery is reduced.
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