Abstract:
Disclosed is a secondary battery that enhances the safety and reliability thereof by preventing the danger of ignition and explosion when it is compressed or damaged due to an external impact. The secondary battery includes an electrode assembly including a first electrode plate, a second electrode plate, and a separator; a can accommodating the electrode assembly; and a cap assembly covering a top opening of the can. The cap assembly includes a cap plate having a first hole at a central portion thereof and electrically connected to a first electrode tab protruding from the first electrode plate, an insulation plate located under the cap plate, a terminal plate located under the insulation plate, an electrode terminal penetrating the cap plate, the insulation plate, and the terminal plate and electrically connected to a second electrode tab protruding from the second electrode plate, and a short-circuit plate located under the cap plate. The short-circuit plate is disposed in a manner that the short-circuit plate contacts the terminal plate or the second electrode tab whenever a distorting pressure is applied to the secondary battery.

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application earlier filed in the Korean Intellectual Property Office on 5 Dec. 2008 and there duly assigned Serial No. 10-2008-0123205. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a battery, and more particularly, to a secondary battery including a short-circuit plate installed on the bottom surface of a cap plate. 
     2. Description of the Related Art 
     A lithium ion secondary battery includes a positive electrode plate, a negative electrode plate, an electrolyte, and a separator, and repeats charge and discharge operations during use thereof. 
     In order to test the safety of a lithium ion secondary battery, many safety tests such as a penetration test, a compression test, and an oven test are carried out to eliminate the danger caused by damage to the lithium ion secondary battery during use thereof. 
     When a lithium ion secondary battery is compressed and damaged by an external impact, it loses its function as a battery and may ignite or explode. This is caused by a short circuit generated between a negative electrode collector and a positive electrode coating portion in the battery when the battery is compressed. Therefore, there have been many studies for eliminating the danger of ignition and explosion when a lithium ion secondary battery is compressed and damaged due to an external impact. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in effort to solve the above problems, and the present invention provides a secondary battery that enhances the safety and reliability thereof by preventing the danger of ignition and explosion when it is compressed or damaged by an external impact. 
     The present invention is not limited to the above-mentioned object, and those skilled in the art can clearly understand other unmentioned objects of the present invention. 
     In accordance with an aspect of the present invention, there is provided a secondary battery includes an electrode assembly having a first electrode plate, a second electrode plate, and a separator disposed between the first and the second plates, a can accommodating the electrode assembly and having a top opening on a top surface of the can, and a cap assembly covering the opening of the can. The cap assembly includes a cap plate electrically connected to a first electrode tab protruding from the first electrode plate, an insulation plate located under the cap plate, a terminal plate located under the insulation plate, an electrode terminal penetrating the cap plate, the insulation plate, and the terminal plate and electrically connected to a second electrode tab protruding from the second electrode plate, and a short-circuit plate located under the cap plate. The short-circuit plate is disposed in a manner that the short-circuit plate contacts the terminal plate or the second electrode tab whenever a distorting pressure is applied to the secondary battery. 
     The short-circuit plate may include a first plate contacting a bottom surface of the cap plate, a second plate connected to the first plate, a third plate connected to the second plate. The third plate may be disposed under the terminal plate, and the third plate may contact the terminal plate or the second electrode tab whenever a distorting pressure is applied to the secondary battery. 
     A top surface of the third plate may have a polygonal shape. A corner of the top surface of the third plate may have an acute angle. 
     The second plate may maintain a first distance from the terminal plate. The first distance is about 1 mm to 3 mm. The third plate may maintain a second distance from the terminal plate. The second distance is about 0.5 mm to 1.0 mm. 
     The short-circuit plate may be welded to the cap plate and be electrically connected to the cap plate. The short-circuit plate may contact the cap plate, and the terminal plate may contact the electrode terminal. 
     The short-circuit plate may be made of nickel or a nickel alloy. 
     The cap assembly may further include a gasket disposed inside a first hole of the cap plate. The electrode terminal penetrates the cap plate through the first hole and the gasket prevents the electrode terminal from contacting the cap plate. 
     According to the present invention, when an external impact or a compressive force is applied to a secondary battery, a short-circuit plate located under a cap plate makes contact with a terminal plate or an electrode tab having the opposite polarity to that of the short-circuit plate, inducing an electrical short circuit. Therefore, the energy of the secondary battery may be consumed in advance to prevent the danger of ignition and explosion, enhancing the safety and reliability of the secondary battery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
         FIG. 1  is a perspective view of a secondary battery according to an embodiment of the present invention; 
         FIG. 2  is an exploded perspective view of the secondary battery according to the embodiment of the present invention; 
         FIG. 3  is a sectional view of the secondary battery according to the embodiment of the present invention; 
         FIG. 4  is a perspective view of a short-circuit plate according to the embodiment of the present invention; 
         FIGS. 5 and 6  are views illustrating the function of the short-circuit plate according to the embodiment of the present invention; and 
         FIG. 7  is a perspective view of a short-circuit plate according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Details of the following embodiments of the present invention are contained herein and in the accompanying drawings. The advantages and characteristics of the present invention and methods for achieving them will be apparent with reference to the embodiments of the present invention that will be described in detail with reference to the drawings. The same reference numerals are used throughout the specification to refer to the same or like elements. 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Here, the mentioned secondary battery may refer to a lithium ion battery, but the present invention is not limited thereto. In the embodiments of the present invention, it should be understood that a bare cell and a secondary battery have the same meaning. 
       FIGS. 1 to 3  are a perspective view, an exploded perspective view, and a sectional view of a secondary battery according to an embodiment of the present invention respectively, and  FIG. 4  is a perspective view of a short-circuit plate  400  according to the embodiment of the present invention. 
     Referring to  FIGS. 1 to 3 , the secondary battery according to the embodiment of the present invention includes a can  100 , an electrode assembly  200  accommodated inside the can  100 , and a cap assembly  300  covering an top opening  100   a  of the can  100 . The element including the can  100 , the electrode assembly  200 , and the cap assembly  300  may be referred to as a bare cell  10 . 
     The can  100  has a substantially rectangular parallelepiped shape, and has a top opening  100   a  at a top surface of the can  100 . The can  100  may be made of a metal, and may function as a terminal by itself. The electrode assembly  200  is inserted into the can  100  through the top opening  100   a.    
     The electrode assembly  200  includes a first electrode plate  210 , a second electrode plate  220 , and a separator  230 . In the electrode assembly  200 , the separator  230  is located between the first electrode plate  210  and the second electrode plate  220 . The first electrode plate  210 , the separator  230 , and the second electrode plate  220  are wound so as to have a jelly-roll configuration. 
     The first electrode plate  210  includes a first electrode collector (not shown) and a first electrode coating portion (not shown). 
     When the first electrode plate  210  is the positive electrode, the first electrode collector is made of aluminum foil of high conductivity. 
     The first electrode coating portion is located on the first electrode collector, and includes a first electrode active material, a conductive material, and a binder. Here, the first electrode active material may be lithium cobalt oxide (LiCoO 2 ), lithium manganese oxide (LiMn 2 O 4 ), or lithium nickel oxide (LiNIO 2 ). Carbon black may be used as the conductive material. The binder may be formed by dissolving and dispersing PVDF, SBR, or PTFE with a volatile solvent such as NMP, an organic solvent, or water. 
     The first electrode collector includes first electrode non-coating portions at the ends of the first electrode collector, in which the first electrode coating portion is not formed. A first electrode tab  215  is attached to the first electrode non-coating portion such that it protrudes toward the top opening  100   a  of the can  100 . The first electrode tab  215  may be made of aluminum. A first insulation tape (not shown) is formed at a portion of the electrode assembly  200 , around which the first electrode tab  215  is drawn, to prevent a short circuit with elements other than the can  100 . 
     The second electrode plate  220  includes a second electrode collector (not shown) and a second electrode coating portion (not shown). 
     When the second electrode plate  220  is the negative electrode, the second electrode collector is made of copper foil of high conductivity. 
     The second electrode coating portion is located on the second electrode collector, and may include a second electrode active material, a conductive material, and a binder. Here, the second electrode active material may be a carbon material, Si, Sn, a tin oxide, a composite tin alloy, a transition metal oxide, a lithium metal nitride, or a lithium metal oxide. A carbon material is mainly used as the second electrode active material. Carbon black is used as the conductive material. The binder may be formed by dissolving and dispersing PVDF, SBR, or PTFE with a volatile solvent such as NMP, an organic solvent, or water. The second electrode plate  220  may not need to use a conductive material because the conductivity of the second electrode coating material itself is high. 
     The second electrode collector includes second electrode non-coating portions (not shown) at the ends of the second electrode collector, in which the second electrode coating portion is not formed. A second electrode tab  225  is attached to the second electrode non-coating portion and protrudes toward the top opening  100   a  of the can  100 . The second electrode tab  225  may be made of copper (Cu) and nickel (Ni). A second insulation tape (not shown) is formed at a portion of the electrode assembly  200 , around which the second electrode tab  225  is drawn, to prevent a short circuit with elements other than the electrode terminal  350 . 
     Although the first electrode plate  210  is assumed to be the positive electrode and the second electrode plate  220  the negative electrode in the above description, the first electrode plate  210  may be the negative electrode and the second electrode plate  220  may be the positive electrode. In this case, the materials of the collectors and active materials would be replaced each other. 
     In general, in a square type secondary battery, the can  100  performing the function of a terminal is the positive electrode. In this case, when the first electrode plate  210  is the positive electrode, the outermost electrode plate of the jelly-roll type electrode assembly  200  may be the first electrode plate  210 . On the other hand, when the first electrode plate  210  is the negative electrode, the outermost electrode plate of the jelly-roll type electrode assembly  200  may be the second electrode plate  220 , i.e. the positive electrode. 
     In the following description of the embodiment of the present invention, it is assumed that the first electrode plate  210  is the positive electrode and the second electrode plate  220  is the negative electrode. 
     The separator  230  is formed of a porous membrane using polyethylene (PE), polypropylene (PP), or their composite film. The separator  230  interrupts electrical conduction between the first electrode plate  210  and the second electrode plate  220  of the electrode assembly  200 , and enables smooth movement of lithium ions. The separator  230  prevents the first electrode plate  210  and the second electrode plate  220  from making contact with each other and also prevents the temperature of the secondary battery from increasing further through a shut-down of the secondary battery when the temperature of the secondary battery starts to increase due to an external short circuit or the like. 
     Ceramic layers made by mixing a ceramic material with a binder may be formed on surfaces of the first electrode plate  210  and the second electrode plate  220  to prevent a short circuit between the first electrode plate  210  and the second electrode plate  220  in addition to the separator  230 . However, this is not within the spirit of the present invention, and a detailed description thereof will be omitted. 
     In the electrode assembly  200 , lithium ions are moved from the first electrode plate  210  to the second electrode plate  220  to be intercalated thereby during a charge operation of the battery. On the other hand, lithium ions are deintercalated from the second electrode plate  220  to the first electrode plate  210  to apply a voltage to an external power source during a discharge operation of the battery. 
     The cap assembly  300  includes a cap plate  310 , an insulation plate  320 , a terminal plate  330 , a gasket  340 , an electrode terminal  350 , and a short-circuit plate  400 . The cap assembly  300  is coupled to the electrode assembly  200  at around the top opening  100   a  of the can  100  together with a separate insulation case  360  so as to seal the can  100 . 
     The cap plate  310  is a metal plate having a size corresponding to the size of the top opening  100   a  of the can  100 . A first hole  310   a  is formed at a central portion of the cap plate  310 . An electrolyte injection hole is formed on one side of the cap plate  310 , and a safety vent (not shown) is formed on one surface of the cap plate  310  on the opposite side to the electrolyte injection hole with respect to the first hole  310   a . After the electrolyte is poured into the can  100 , the electrolyte injection hole is sealed by a cover such as a ball. The electrolyte injection hole and the safety vent have general shapes. The cap plate  310  makes contact with the first electrode tab  215  and is electrically connected to it. Hence, the cap plate  310  has the same polarity as that of the first electrode plate  210 . 
     The insulation plate  320  is located under the cap plate  310 , and has a second hole  320   a  at a position corresponding to the first hole  310   a . The insulation plate  320  may be made of the same insulation material as that of the gasket  340 . 
     The terminal plate  330  is located under the insulation plate  320 , and has a third hole  330   a  at a position corresponding to the second hole  320   a . The terminal plate  330  is made of nickel or a nickel alloy. 
     The electrode terminal  350  penetrates the cap plate  310 , the insulation plate  320 , and the terminal plate  330  through the first hole  310   a , the second hole  320   a , and the third hole  330   a . The electrode terminal  350  makes contact with the second electrode tab  225  and is electrically connected to it. Hence, the electrode terminal  350  has the same polarity as that of the second electrode plate  220 . Since the electrode terminal  350  is inserted into the third hole  330   a  of the terminal plate  330  so as to make contact with the second electrode tab  225 , the terminal plate  330  is also electrically connected to the second electrode tab  225  and has the same polarity as that of the second electrode plate  220 . 
     The electrode terminal  350  is inserted into the first hole  310   a  of the cap plate  310  through the gasket  340 . The gasket  340 , in the first hole  310   a , is located between the electrode terminal  350  and cap plate  310 , and therefore the electrode terminal  350  is insulated from the cap plate  310 . 
     The short-circuit plate  400  is located under the cap plate  310 , and when the bare cell  10  is compressed by an external pressure, the short-circuit plate  400  is short-circuited with the terminal plate  330  of the cap assembly  300  or the second electrode tab  225  to consume almost all charge energy of the secondary battery. The short-circuit plate  400  will be described in detail hereinafter. 
     Referring to  FIG. 4  together with  FIG. 3 , the secondary battery according to the embodiment of the present invention includes the short-circuit plate  400  located under the cap plate  310  on one side of the cap plate  310 . 
     The short-circuit plate  400  includes a first plate  410 , a third plate  430 , and a second plate  420  that connects the first plate  410  to the third plate  430 . The first plate  410  is located under the cap plate  310  on one side of the cap plate  310 , and a third plate  430  is located under the terminal plate  330 . The upper surface of the third plate  430  is spaced apart from the terminal plate  330  so as to face the terminal plate  330 . The first plate  410 , the second plate  420 , and the third plate  430  have flat rectangular shapes, but are not limited thereto. In other words, they may have polygonal or circular shapes. The short-circuit plate  400  is made of a metal such as nickel (Ni) or a nickel alloy, copper, or aluminum. 
     The first plate  410  is welded to the cap plate  310  and is electrically connected to it. Hence, the first plate  410  may have the same polarity as that of the first electrode plate  210 , and the second and third plates  420  and  430  electrically connected to the first plate  410  may have the same polarity as that of the first electrode plate  210 . 
       FIGS. 5 and 6  are views illustrating the function of the short-circuit plate  400  according to the embodiment of the present invention. 
     Referring to  FIG. 5  together with  FIG. 3 , when the secondary battery is compressed or damaged due to a longitudinal compression test or an external impact caused by a carelessness of the user, the battery loses its function and may ignite or explode. This is caused by a short circuit between the first electrode coating layer and the second electrode collector. 
     As illustrated in  FIG. 5 , in the secondary battery according to the embodiment of the present invention, when the secondary battery is distorted by a pressure applied in the longitudinal direction of the secondary battery, the short-circuit plate  400  may contact the terminal plate  330 . More particularly, the third plate  430  of the short-circuit plate  400  may contact the terminal plate  330 , causing an electrical short circuit. However, the degree of compression generated in the secondary battery becomes different depending on a portion of the secondary battery to which a pressure is applied, and the degree of compression or distortion determines which portion of the short-circuit plate  400  contacts the terminal plate  330  to cause an electrical short circuit. Since the short-circuit plate  400  has the same polarity as that of the first electrode plate  210  and the terminal plate  330  has the same polarity as that of the second electrode plate  220 , an electrical short circuit may be caused by them. 
     As mentioned above, when the secondary battery is compressed or distorted by a pressure, the terminal plate  330  and the short-circuit plate  400  make contact with each other so as to be short-circuited, consuming the power of the secondary battery. In other words, since the short-circuit plate  400  and the terminal plate  330  are made of metals, they are mutually short-circuited to emit the energy of the secondary battery in advance and thus minimizing emission of heat. 
     Hence, before a short circuit is caused between coating portions having the opposite polarities or between an electrode collector and coating portions, the terminal plate  330  and the short-circuit plate  400  make contact with each other to consume the power of the secondary battery, thereby remarkably reducing the possibility of ignition or explosion of the secondary battery. 
     In addition, even when a short circuit is caused between a collector and a coating portion having the opposite polarities in the electrode assembly  200 , the terminal plate  330  and the short-circuit plate  400  may be short-circuited together. Hence, the power of the electrode assembly  200  is also consumed at a contact portion between the terminal plate  330  and the short-circuit plate  400 , reducing the possibility of explosion of the secondary battery. 
     Here, a first distance L 1  between the second plate  420  and the terminal plate  330  is 1 to 3 mm, and a second distance L 2  between the third plate  430  and the terminal plate  330  is 0.5 to 1 mm. When the first distance between the second plate  420  and the terminal plate  330  is smaller than 1 mm or the second distance between the third plate  430  and the terminal plate  330  is smaller than 0.5 mm, the short-circuit plate  400  may touch the terminal plate  330  even in the case of a minor impact, making the secondary battery useless. On the other hand, when the first distance between the second plate  420  and the terminal plate  330  is larger than 3 mm or the second distance between the third plate  430  and the terminal plate  330  is larger than 1 mm, the short-circuit plate  400  may not touch the terminal plate  330  even in the case of a large external impact. In this case, since the short-circuit plate  400  and the terminal plate  330  are not short-circuited even when the first electrode coating portion and the second electrode collector are short-circuited, the internal energy of the secondary battery cannot be consumed by the short-circuit plate  400  and the terminal plate  330 , making it difficult to eliminate the danger of ignition or explosion of the secondary battery. 
     Referring to  FIG. 6 , when the secondary battery is compressed by pressures applied to the upper portion in the transverse direction of the secondary battery as indicated by arrows in  FIG. 6 , the short-circuit plate  400  can make contact with the second electrode tab  225 . More particularly, the third plate  430  of the short-circuit plate  400  may make contact with the second electrode tab  225 , causing an electrical short circuit. However, the degree of compression generated in the secondary battery becomes different depending on a portion of the secondary battery to which a pressure is applied, and the degree of compression determines which portion of the short-circuit plate  400  makes contact with the second electrode tab  225  to cause an electrical short circuit. Since the short-circuit plate  400  has the same polarity as that of the first electrode plate  210  and the second electrode tab  225  has the same polarity as that of the second electrode plate  220 , an electrical short circuit may be caused by them. 
     As mentioned above, when the secondary battery is compressed by a pressure, the second electrode tab  225  and the short-circuit plate  400  make contact with each other so as to be short-circuited, consuming the power of the secondary battery. 
     When the secondary battery is compressed, it is difficult to know whether the short-circuit plate  400  will make contact with the terminal plate  330  or the second electrode plate  225 . The reason is that the degree of compression of the secondary battery becomes different depending on the direction and strength of a pressure and the material of the can  100 . However, in general, when the secondary battery is compressed lightly, the short-circuit plate  400  is short-circuited with the terminal plate  330  close to the short-circuit plate  400 , but when the secondary battery is compressed strongly, the short-circuit plate  400  may be short-circuited with at least one of the terminal plate  330  and the second electrode tab  225 . 
       FIG. 7  is a perspective view of a short-circuit plate  500  according to another embodiment of the present invention. 
     Referring to  FIG. 7 , the short-circuit plate  500  according to the embodiment of the present invention has a different structure from the short-circuit plate  400  of  FIG. 4 , but has the same position, material, and function as those of the short-circuit plate  400 . Therefore, the structure of the short-circuit plate  500  will be described in detail hereinafter. 
     The short-circuit plate  500  includes a first plate  510 , a third plate  530 , and a second plate  520  that connects the first plate  510  to the third plate  530 . The first plate  510  is located under the cap plate  310  on one side of the cap plate  310 . The third plate  530  is located under the terminal plate  330 . The upper surface of the third plate  530  is spaced apart from the terminal plate  330  so as to face the terminal plate  330 . 
     As shown in  FIG. 7 , a corner of the third plate  520  of the short-circuit plate  500  has an acute angle. 
     When a pressure is applied in the longitudinal direction of the secondary battery so as to compress the secondary battery, the short-circuit plate  500  may make contact with at least one of the terminal plate  330  and the second electrode tab  225 . Then, since one side of the third plate  530  has an acute corner, the acute corner portion of the third plate  530  may be driven into the terminal plate  330  or the second electrode tab  225  to be fixed to it or penetrate the terminal plate  330  or the second electrode tab  225  when the third plate  530  makes contact with the terminal plate  330  or the second electrode tab  225 . Hence, when the short-circuit plate  500  makes contact with the terminal plate  330  or the second electrode tab  225 , it is prevented from being separated from the terminal plate  330  or the second electrode tab  225 , thereby consuming the power of the secondary battery further. 
     Although the embodiments of the present invention have been described in detail hereinabove, it should be understood that many variations and modifications of the basic inventive concept herein described will still fall within the spirit and scope of the present invention as defined in the appended claims.