Abstract:
A secondary battery having a current interrupt device (CID) between a negative current collecting plate and a case of the secondary battery. The secondary battery includes: an electrode assembly including a positive electrode, a separator, and a negative electrode; a case housing the electrode assembly; a cap assembly coupled to the case for sealing the case; a positive current collecting plate connected to the positive electrode and the cap assembly; an insulator in the case adjacent an end plate of the case; and a negative current collecting plate connected to the negative electrode and the end plate of the case, the end plate being curved convexly toward an inner cavity of the case.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2008-0133808 filed in the Korean Intellectual Property Office on Dec. 24, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a secondary battery, and more particularly, to a secondary battery having a current interrupt device (CID) between the bottom of a case and a negative current collecting plate. 
     2. Description of the Related Art 
     As generally known, a secondary battery can be recharged and discharged, unlike a primary battery. Secondary batteries may be classified into low-capacity batteries and high-capacity batteries. 
     For example, a low-capacity secondary battery includes unit batteries, and is mainly used for small portable electronic devices, such as cellular phones, laptop computers, and camcorders, whereas high-capacity batteries are used as the power supply for driving motors in hybrid electric vehicles and the like. A high-capacity secondary battery forms a battery module by connecting a plurality of unit batteries in the form of a pack, and is used as the power supply for driving motors in hybrid electric vehicles and the like. 
     Each of the unit batteries includes an electrode assembly including a positive electrode, a separator, and a negative electrode, a case for accommodating the electrode assembly, and a cap assembly coupled with the case to seal the case and having an electrode terminal electrically connected with the electrode assembly. 
     For example, in a cylindrical secondary battery, the positive and negative electrodes in the electrode assembly respectively include non-coating portions on which an active material is not coated, and the positive electrode non-coating portion and the negative electrode non-coating portion are positioned at opposite sides to each other. 
     A negative current collecting plate is attached to the negative electrode non-coating portion, and a positive current collecting plate is attached to the positive electrode non-coating portion. The negative current collecting plate is connected to the case and the positive current collecting plate is connected to the cap assembly to thus draw current to the outside. 
     When the negative current collecting plate is connected to the case, the case serves as a negative electrode terminal. When the positive current collecting plate is connected to the cap assembly, the cap assembly serves as a positive electrode terminal. The cap assembly and the case are coupled with each other in an insulation structure through a gasket. 
     The cap assembly includes a cap plate, a positive temperature device, a vent plate, an insulator, a middle plate, a sub-plate, and a connecting member that are sequentially provided from the outside. The connecting member electrically connects the positive current collecting plate and the middle plate. The vent plate and the sub-plate are connected to each other by welding, with the insulator and the middle plate interposed therebetween. 
     The vent plate forms a vent which is to be connected to the sub-plate, the vent and the sub-plate form a connection portion, and as the connection portion is formed by welding, they are easily broken and disconnected when the internal pressure of the battery rises, to thus cut off current. That is, the current interrupt device (CID) is formed between the vent plate and sub-plate of the cap assembly. 
     To this end, the vent plate is provided with notches so that the circumference of the vent has a smaller thickness than other portions of the vent plate have. However, such a cap assembly interrupts current while the vent is separated as the notches are ruptured by explosion in the event of an increase of the internal pressure of the battery. 
     In this manner, the current interrupt device cuts off current after explosion, so that the current interrupt device is not able to properly prevent the explosion of the battery. Moreover, the sub-plate forming the connection portion together with the vent is formed in a plate shape, and hence there is a large dispersion of an operating pressure, i.e., separation pressure, at which the connection portion of the sub-plate and the vent is disconnected. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE INVENTION 
     The embodiments of the present invention provide a secondary battery which can effectively serve to prevent explosion since current is interrupted by the disconnection of a connection portion before explosion. 
     The embodiments of the present invention provide a secondary battery which can reduce the dispersion of the operating pressure of a disconnection portion since the connection portion can be designed so as to be disconnected under a predetermined operating pressure. 
     The embodiments of the present invention provide a secondary battery which improves reliability with respect to current interruption because a disconnected state of the connection portion is firmly maintained even when the internal pressure of the case decreases again as the connection portion between a bottom of the case and a negative current collecting plate is disconnected and the bottom of the case is broken outwardly. 
     A secondary battery according to one exemplary embodiment of the present invention includes: an electrode assembly including a positive electrode, a separator, and a negative electrode; a case including an end plate and at least one wall extending therefrom defining an inner cavity, the case housing the electrode assembly in the inner cavity; a cap assembly coupled to the case for sealing the case; a positive current collecting plate including a first side connected to the positive electrode in the case and a second side connected to the cap assembly; an insulator in the case adjacent the end plate; and a negative current collecting plate including a first side connected to the negative electrode and a second side adjacent the insulator and connected to the end plate at a connection portion of the end plate, wherein the end plate is curved convexly toward the inner cavity of the case. 
     The insulator may include: an external circumferential surface portion corresponding to an internal circumferential surface of the at least one wall of the case; a planar portion adjacent the negative current collecting plate; and a recessed portion curved concavely toward the inner cavity of the case and contacting the end plate, wherein an aperture extends between the planar portion and the recessed portion near a center of the insulator, and wherein the negative current collecting plate is connected to the end plate through the aperture. 
     The negative current collecting plate may further include a protruding portion extending toward and connected to the end plate through the aperture of the insulator at the connection portion. 
     A portion of the insulator may face the end plate and be curved. The portion of the insulator facing the end plate may contact the end plate and have a curvature corresponding to a curvature of the end plate. 
     The end plate may be invertible convexly away from the inner cavity of the case to provide a separating space between the end plate and the negative current collecting plate. The end plate may be invertible convexly away from the inner cavity of the case to disconnect the negative current collecting plate from the end plate when an internal pressure in the case is greater than a reference operating pressure. 
     The end plate may include at least one notch and be collapsible away from the inner cavity of the case at the at least one notch upon an increase of an internal pressure in the case. 
     The at least one notch may include at least one outer circumferential notch along at least a portion of an outer circumference of the end plate. 
     The at least one notch may include central notches arranged in a cross pattern at the connection portion of the end plate. The end plate may be configured to rupture away from the inner cavity at the central notches when the internal pressure in the case is greater than a rupturing pressure. 
     The at least one notch may further include at least one inner circumferential notch connecting the central notches in a circumferential direction. 
     A portion of the insulator may face the end plate and have a polyhedron shape. A portion of the end plate may face and contact the insulator and have a polyhedron shape. 
     According to another exemplary embodiment of the present invention, a secondary battery having a current interrupt device between a negative current collecting plate of the secondary battery and a case of the secondary battery includes: an electrode assembly including a positive electrode, a separator, and a negative electrode; a case including an end plate and at least one wall extending therefrom defining an inner cavity, the case housing the electrode assembly in the inner cavity; a cap assembly coupled to the case for sealing the case; a positive current collecting plate connecting the positive electrode to the cap assembly; an insulator adjacent the end plate; and a negative current collecting plate connected to the negative electrode and connectable to the end plate at a connection portion of the end plate, wherein the end plate is curved toward the inner cavity of the case and is connected to the negative current collecting plate when an internal pressure in the case is less than a reference operating pressure, and wherein the end plate is curved away from the inner cavity of the case and is configured to be disconnected from the negative current collecting plate when the internal pressure in the case is greater than the reference operating pressure. 
     In one embodiment, the end plate includes at least one notch and is collapsible away from the inner cavity of the case at the at least one notch when the internal pressure in the case is greater than the reference operating pressure. 
     As set forth above, according to one exemplary embodiment of the present invention, the bottom of the case is protruded inward and connected to the negative current collecting plate at the connection portion through an aperture of an insulator, and is configured such that the connection portion is disconnectable to interrupt current before an explosion of the case, thereby preventing explosion. 
     Since the bottom of the case is curved convexly toward the inside of the case and connected to the negative current collecting plate at the connection portion, the connection portion can be designed so as to be disconnected under a predetermined operating pressure, thus reducing the dispersion of the operating pressure of the connection portion. 
     Even when the internal pressure of the case decreases after the bottom of the case and the negative current collecting plate are disconnected and the bottom of the case is broken outwardly, the bottom of the case remains inverted concavely away from the inner cavity of the case, thus firmly maintaining a disconnected state of the connection portion. As a result, reliability with respect to current interruption is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional perspective view of a secondary battery according to a first exemplary embodiment of the present invention. 
         FIG. 2  is an exploded sectional view of a current interrupt device of the secondary battery of  FIG. 1 . 
         FIG. 3  is a bottom view of a case of the secondary battery of  FIG. 2 . 
         FIG. 4  is a sectional view showing a connected state when a connection portion of a current interrupt device is connected. 
         FIG. 5  is a sectional view showing a disconnected state when the connection portion of the current interrupt device of  FIG. 4  is disconnected. 
         FIG. 6  is a sectional view showing a cut away state after the current interrupt device of  FIG. 4  is disconnected. 
         FIG. 7  is an exploded sectional view of a current interrupt device of a secondary battery according to a second exemplary embodiment of the present invention. 
         FIG. 8  is a bottom view of a case of the secondary battery of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. The drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
       FIG. 1  is a sectional perspective view of a secondary battery according to a first exemplary embodiment of the present invention. Referring to  FIG. 1 , the secondary battery according to the first exemplary embodiment includes an electrode assembly  10 , a case  20 , a cap assembly  30 , a positive current collecting plate  40 , and a negative current collecting plate  50 . The case  20  accommodates the electrode assembly  10  and an electrolyte. 
     The electrode assembly  10  includes a positive electrode  11 , a separator  12 , and a negative electrode  13 . The electrode assembly  10  is formed by winding the positive electrode  11 , the negative electrode  13 , and the separator  12  of an insulating material disposed therebetween. 
     In one example, the electrode assembly  10  may be formed in a cylindrical shape. A sector pin  14  is disposed at the center of the cylindrical electrode assembly  10 . The sector pin  14  maintains the cylindrical shape of the spirally wound electrode assembly  10 . 
     The positive electrode  11  and the negative electrode  13  are made of a thin metal foil and form a current collecting body, and include respective coating portions  11   a  and  13   a  and non-coating portions  11   b  and  13   b  which are differentiable according to the application or absence of an active material. That is, the coating portions  11   a  and  13   a  have an active material applied thereto, and the non-coating portions  11   b  and  13   b  do not have an active material applied thereto. 
     The case  20  has a space in which the electrode assembly  10  is inserted, and may be formed in a cylindrical or rectangular shape which is open at one end. The case  20  is connected to the negative current collecting plate  50 , and serves as a negative electrode terminal in the secondary battery. The case  20  is made of a conductive metal, such as aluminum, an aluminum alloy, or nickel-plated steel. 
     A cap assembly  30  is coupled with the open end of the case  20  through a gasket  31  configured to seal the case  20  accommodating the electrode assembly  10  and the electrolyte. The cap assembly  30  of the present exemplary embodiment may be provided with a current interrupt device (not shown) or not, as illustrated in  FIG. 1 . 
     Before a detailed description of the cap assembly  30  is provided, the positive current collecting plate  40  will be discussed. The positive current collecting plate  40  is connected to the non-coating portion  11   b  of the positive electrode  11  at the cap assembly  30  side to connect the positive electrode  11  to the cap assembly  30 . 
     The cap assembly  30  includes a cap plate  32  and a sub-plate  33 . The cap plate  32  is connected to the positive current collecting plate  40 , and serves as a positive electrode terminal in the secondary battery. The cap plate  32  has a terminal  32   a  protruding to the outside and a vent hole  32   b.    
     A positive temperature coefficient element  34  is installed between the cap plate  32  and the sub-plate  33 . The positive temperature coefficient element  34  forms or interrupts current flow between the cap plate  32  and the sub-plate  33 . That is, when a preset temperature is exceeded, the electrical resistance of the positive temperature coefficient element  34  increases to a virtually infinite level, thereby stopping the flow of charging or discharging current. 
     The sub-plate  33  is installed inside the cap plate  32  and connected to the electrode assembly  10 . That is, the sub-plate  33  is electrically connected to the positive current collecting plate  40  through a connection member  35 . 
     An insulating member  36  is provided on the positive current collecting plate  40 . The insulating member  36  covers the periphery of the positive current collecting plate  40  below a beading portion  21 . Hence, the positive current collecting plate  40  is electrically connected to the cap plate  32  through the connection member  35  and the sub-plate  33 . 
     After the cap assembly  30  is inserted into the case  20 , the cap assembly  30  is clamped and fixed to the case  20 . Hereupon, the beading portion  21  and a clamping portion  22  are formed, and the gasket  31  provides an airtight seal between the case  20  and the cap assembly  30 . 
     The negative current collecting plate  50  is connected to a non-coating portion  13   b  of the negative electrode  13  on one side and connected to the case  20  on the other side. In order to construct a current interrupt device (CID) between the negative current collecting plate  50  and the case  20 , an insulator  60  is interposed between the negative current collecting plate  50  and a bottom  23  of the case  20  (i.e., an endplate  23  of the case  20 ) to thus electrically insulate the negative current collecting plate  50  and the bottom  23  of the case  20 . 
     The negative current collecting plate  50  is connected to the bottom  23  of the case  20  at a connection portion CP passing through the interposed insulator  60 . For example, the negative current collecting plate  50  may contact the bottom  23  of the case  20  at the connection portion CP. Upon an increase of the internal pressure of the case  20 , the negative current collecting plate  50  is separated from the bottom  23  of the case  20  at the connection portion CP to thus interrupt current. 
       FIG. 2  is an exploded sectional view of the current interrupt device of  FIG. 1 . Referring to  FIG. 2 , that is, the current interrupt device CID may be formed by the negative current collecting plate  50  and the bottom  23  of the case  20  that are mostly insulated through the insulator  60  and partly connected through the connection portion CP. 
     To form the current interrupt device CID, the bottom  23  of the case  20  is curved convexly toward the inside of the case  20 . Further, the bottom  23  of the case  20  is inverted convexly toward the outside of the case  20  and forms a separating space SS during a time from immediately after the bottom  23  of the case  20  is separated from the negative current collecting plate  50  at the connection portion CP due to an increase of the pressure in the case  20  until immediately before the bottom  23  of the case  20  is cut away (see  FIGS. 4 and 5 ). 
     Before further describing the bottom  23  of the case  20 , the insulator  60  will be described in more detail. The insulator  60  includes an external circumferential surface portion  61  corresponding to the internal circumferential surface of the case  20 , a planar portion  62  supporting the negative current collecting plate  50 ; a recessed portion  63  formed concavely and tightly contacting the bottom  23  of the case  20 , and a through hole  64  penetrated at the center of the insulator  60  from the planar portion  62  to the recessed portion  63 . 
     The recessed portion  63  of the insulator  60  may be curved facing the bottom  23  of the case  20  on a cut surface directed toward the negative current collecting plate  50  from the positive current collecting plate  40 , i.e., on a vertical section of  FIGS. 1 and 2 . Further, the recessed portion  63  is curved along the circumferential direction and forms an overall three-dimensional curved surface. 
     The negative current collecting plate  50  further includes a protruding portion  51  connected at the connection portion CP to the bottom  23  of the case  20 . The protruding portion  51  is connected to the bottom  23  of the case  20  through the through hole  64  of the insulator  60 . Therefore, the protruding portion  51  has a diameter D for insertion into the through hole  64  and a height H for reaching the bottom  23  of the case  20  through the through hole  64 . 
     Again, a portion of the bottom  23  of the case  20  that faces and contacts the recessed portion  63  of the insulator  60  is curved on a cut surface directed toward the negative current collecting plate  50  from the positive current collecting plate  40 , i.e., on a vertical section of  FIGS. 1 and 2 . 
     Further, the bottom  23  of the case  20  is curved along the circumferential direction and forms an overall three-dimensional curved surface. Accordingly, the bottom  23  of the case  20  and the recessed portion  63  of the insulator  60  maintain a stable contact and coupling structure because they have curved surfaces in contact with each other. 
     The bottom  23  of the case  20  curved convexly toward the inside of the case  20  and the protruding portion  51  of the negative current collecting plate  50  are connected at the connection portion CP, so that current is interrupted between the negative current collecting plate  50  and the case  20  as the bottom  23  of the case  20  is separated from the connection portion CP upon an increase of the internal pressure of the case  20 . 
     The bottom  23  of the case  20  having an inward convex shape makes clear the connection to and disconnection from the protruding portion  51 , thereby reducing the dispersion of an operating pressure of the connection portion CP, which disconnects the connection portion CP at an internal pressure greater than a predetermined operating pressure. Also, the bottom  23  of the case  20  effectively serves to prevent explosion because it forms the separating space SS without explosion as it is disconnected from the connection portion CP. 
       FIG. 3  is a bottom view of the case  20  of  FIG. 2 . Referring to  FIGS. 2 and 3 , the bottom  23  of the case  20  is provided with at least one notch  24  having a thickness less than the circumferential thickness of the bottom  23  of the case  20 . The notch  24  prevents explosion more effectively since it is cut away upon an additional increase of the internal pressure of the case  20  in a state where current is interrupted by the disconnection of the connection portion CP. 
     For example, the at least one notch  24  includes one or more outer circumferential notches  241  which are formed along the outer circumference on at least part of the outer circumference of the bottom  23  of the case  20 . The outer circumferential notches  241  may be formed around the entire circumference of the bottom  23  of the case  20  (not shown), or may be divided by a predetermined length into two (e.g., one on both sides, as shown in  FIG. 3 ) or more. 
       FIGS. 4 to 6  are sectional views showing operating states.  FIG. 4  shows a state when the connection portion CP of the current interrupt device CID is connected, and  FIG. 5  shows a state when the connection portion CP is disconnected. The outer circumferential notches  241  induce the inversion of the bottom  23  of the case  20 , which is convex toward the inside of the case  20  in a connected operating state, upon an increase of the internal pressure of the case  20  above a predetermined reference pressure, thereby facilitating inversion of the bottom  23  of the case  20  to an outwardly concave position in which the CID is in a disconnected state. 
       FIG. 6  shows a cut away state after the current interrupt device has disconnected current flow. Referring to  FIG. 6 , even though the internal pressure is reduced due to the outer circumferential notches  241  inducing inversion of the bottom  23  of the case  20 , the separating space SS firmly maintains a disconnected state of the bottom  23  of the case  20  and the negative current collecting plate  50 , thereby improving reliability with respect to current interruption. 
     Referring again to  FIG. 3 , the at least one notch  24  includes one or more central notches  242  which are formed in a cross pattern at the connection portion CP of the bottom  23  of the case  20 . The central notches  242  may be formed over the entire diameter of the bottom  23  of the case  20  (not shown), or may be formed in a cross pattern at a predetermined length at the center part of the bottom  23  of the case  20  as shown in  FIG. 3 . 
     The central notches  242  are able to cut away the center part of the bottom  23  of the case  20  even if they are not inverted from the outer circumferential notches  241 . The central notches  242  firmly maintain a disconnected state of the bottom  23  of the case  20  and the negative current collecting plate  50  because they are directed toward the outside of the case  20  as they are cut away around the connection portion CP, i.e., at the center part of the bottom  23  of the case  20 . 
     The bottom  23  of the case  20  operated as described above may be provided with either the outer circumferential notches  241  or the central notches  242  at the bottom  23  of the case  20 , or may be provided with both the outer circumferential notches  241  and the central notches  242 , as shown in  FIG. 3 . 
     When comparing the following second exemplary embodiment with the first exemplary embodiment, descriptions of identical or similar components will be omitted, and different components will be described. 
       FIG. 7  is an exploded sectional view of a current interrupt device in a secondary battery according to a second exemplary embodiment of the present invention.  FIG. 8  is a bottom view of a case of the secondary battery of  FIG. 7 . 
     Referring to  FIGS. 7 and 8 , a current interrupt device CID 2  according to the second exemplary embodiment is provided within a through hole  64  of an insulator  260 , and where a recessed portion  263  of the insulator  260  and a bottom  223  of a case  220  face and contact each other is formed in a polyhedron. A polygonal line of the recessed portion  263  may be shown on a cut surface directed toward the negative current collecting plate  50  from the positive current collecting plate  40 , i.e., on a vertical section of  FIG. 7 . 
     Further, the recessed portion  263  is formed in a curved line along the circumferential direction of the bottom  223  of the case  220  to form a plurality of curved surfaces divided at angulated parts of the polygonal line. Accordingly, the bottom  223  of the case  220  and the recessed portion  263  of the insulator  260  are in linear contact with each other at the angulated parts, and are in surface contact with each other at the other parts, thereby maintaining a stable contact and coupling structure. 
     The bottom  223  of the case  220  includes at least one notch  224  which further includes inner circumferential notches  243  which are formed in a circumferential direction between the central notches  242  and the outer circumferential notches  241 . The inner circumferential notches  243  may be formed in a singular number (not shown), or may be formed in a plural number having a pattern of concentric circles as shown in  FIG. 8 . 
     Therefore, the inner circumferential notches  243  are formed by connecting the central notches  242  in the circumferential direction. The inner circumferential notches  243  enable the bottom  223  of the case  220  to be cut away between the center part and outer circumferential part of the bottom  223  of the case  220  even if the outer circumferential notches  241  are not inverted. The bottom  223  of the case  220  forms angulated parts at the at least one notch  224 , and comes into contact corresponding to the angulated parts formed on the recessed portion  263 . 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.