Abstract:
A rechargeable battery and a method of forming the rechargeable battery. An electrode assembly includes a first electrode and a second electrode, and a separator disposed between the first electrode and the second electrode to prevent a short circuit between the first electrode and the second electrode. A first electrode tap is formed on an end portion of the first electrode and a second electrode tap is formed on an end portion of a second electrode, each end portion being absent active materials. A can is adapted to receive the electrode assembly and have the electrode assembly welded to an inner side of the can. A cap assembly surrounded by a gasket insulates the cap assembly and seals an opening of the can.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to and the benefit of Korean Patent Application No. 10-2005-0035287, filed on Apr. 27, 2005, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a coupling structure of a tap and a can in a rechargeable battery and a method of forming the rechargeable battery.  
         [0004]     2. Description of Related Art  
         [0005]     Since rechargeable batteries have many advantages such as rechargeability, miniaturization, and high capacity, there has been rapid development of rechargeable batteries as they have become more widely used in recent years. Rechargeable batteries can be classified into nickel-hydrogen (Ni-MH) batteries and lithium ion (Li-ion) rechargeable batteries depending on electrode active materials.  
         [0006]     A liquid electrolyte, solid polymer electrolyte, or gel type electrolyte can be used as the electrolyte of the lithium rechargeable battery. Also, lithium rechargeable batteries can be classified into can types and pouch types depending on the container into which an electrode assembly is inserted.  
         [0007]     In can type lithium rechargeable batteries, the electrode assembly is located in a can formed from metallic materials such as aluminum-containing metal by the use of a deep drawing method and the like. Typically, liquid electrolyte is used in can type rechargeable batteries.  
         [0008]     Can type rechargeable batteries can be further classified into polygonal can types and cylindrical can types. The polygonal can type is formed from a container in a hexahedron shape or in a thin shape in which the edges of the hexahedron are rounded. The cylindrical can type is widely used in high-capacity electronic and electrical devices, in which a plurality of rechargeable batteries are combined into a single battery pack.  
         [0009]      FIGS. 1 and 2  are a front cross-sectional view and an exploded perspective view, respectively, illustrating a structure of a conventional cylinder type rechargeable battery and a method of forming a cylinder type rechargeable battery will be described with reference to  FIGS. 1 and 2 . First, two electrodes  25  having a rectangular plate shape and separators  21 ,  23  interposed between the electrodes  25  to prevent short circuits between the two electrodes  25  are stacked and wound in a jelly roll configuration to provide an electrode assembly  20 . Each electrode is formed by coating active material slurry on a charge collector made of a metallic foil.  
         [0010]     Uncovered areas absent the slurry are located at either end of the charge collector. The uncovered areas are provided with electrode taps  27 ,  29  for each electrode plate. The electrode taps  27 ,  29  are electrically connected to a cylindrical can  10  and a cap assembly  80  insulated from the cylindrical can  10  to form a part of a path for connecting the electrode assembly to external circuits during charging and discharging of the rechargeable battery. From the electrode assembly  20 , one electrode tap is drawn upward to an opening of the cylindrical can  10  and the other electrode tap is drawn downward.  
         [0011]     The electrode assembly is inserted into the cylindrical can  10  through the opening with upper and lower insulating plates  13   a ,  13   b . Then, beads for preventing floating of the electrode assembly in the can are formed in the cylindrical can  10  and electrolyte is injected into the cylindrical can  10 . An insulating gasket  30  is provided on the inner side of the can in the vicinity of the opening and a cap assembly  80  for capping the opening is provided on the inside of the gasket  30 .  
         [0012]     In the cap assembly  80 , a bent assembly, a positive thermal coefficient (PTC) element  60 , and a cap-up  70  having an electrode terminal are included. The bent assembly typically includes a bent  40  at the lower side and a current interrupt device (CID)  50  which would be fractured in combination with the bent  40  to cut off a current path.  
         [0013]     Subsequently, clamping work is performed to seal the cylindrical can  10  by pressing the sidewall of the opening of the cylindrical can  10  toward the center of the can using the cap-up  70  inserted into the gasket  30  as a stopper. In addition, tubing work is performed to form an exterior of the battery.  
         [0014]     In connecting the electrode taps  27 ,  29 , the electrode tap  29  extending downward is welded to a bottom surface of the cylindrical can with the lower insulating plate  13   b  therebetween and the electrode tap extending upward  27  is welded to the bent  40  through a hole in the upper insulating plate  13   a.    
         [0015]     The upward electrode tap  27  has a length sufficient to easily weld the upward electrode tap  27  and a protrusion  42  of the bent  40  to each other. The electrode tap  27  is bent and the bent assembly is inserted into the opening of the cylindrical can in which the gasket  30  is provided. This work requires a space between the bent assembly and the electrode assembly  20 .  
         [0016]     Since extra length of the upward electrode tap  27  is difficult to dispose after welding, the length of the upward electrode tap is determined in a workable range. The portion of the upward electrode tap  27  remaining after welding may be positioned in a space between the cap assembly and the electrode assembly. The upper insulating plate  13   a  may serve to prevent a short circuit between the upward electrode tap  27  and the other electrode  25  of the electrode assembly.  
         [0017]     A center pin  18  shown in  FIG. 2  may be provided in a hollow for winding the electrode assembly  20 .  
         [0018]     In welding the downward electrode tap  29 , in the partial section shown in  FIG. 3 , the electrode assembly  20  is inserted into the cylindrical can  10 . Resistance welding is then performed by inserting a welding rod  115  into a hollow space at the center of the electrode assembly  20  and allowing current to flow in the welding rod  115  such that the electrode tap  29  comes in close contact with the bottom surface of the can  10 . A welding point  291  is formed at a position between the electrode tap  29  and the can  10  with which the narrow end of the welding rod  115  comes in contact. Since the welding rod  115  is inserted into a small space at the center of the electrode assembly for the welding, it is difficult to perform the welding while moving the welding rod. Thus, a one-point welding  291  is performed.  
         [0019]     A strong weld is typically needed between the electrode taps of the electrode assembly and the safety bent or the can. If the weld is not strong, the electrical connection between the electrode tap and the safety bent or the can is often made through a surface contact. The electrical connection with the surface contact generates contact resistance which is greater than the welding resistance. Accordingly, the internal resistance of the battery increases and the charging and discharging efficiency decreases.  
         [0020]     Specifically, in case of the one-point welding on the bottom surface of the can, the strength against tension easily satisfies a predetermined criterion. However, when the electrode assembly rotates inside the can, the strength against the rotation is very weak because there is only one welding point formed at the center of the electrode assembly. Accordingly, the weld may be destroyed, rapidly increasing the internal resistance. Thus, there is a need for a rechargeable battery in which the reliability of the electrical connection between the bottom surface of a can and a downward electrode tap is improved.  
       SUMMARY OF THE INVENTION  
       [0021]     A rechargeable battery and a method of forming the rechargeable battery are provided in which the strength of the weld between the bottom surface of a can and a downward electrode tap is increased and low inner resistance of a connection portion can be maintained to enhance a charging and discharging efficiency and to increase an amount of discharge.  
         [0022]     According to an exemplary embodiment of the present invention, a rechargeable battery includes an electrode assembly having two electrodes in which electrode taps are formed and a short circuit prevention separator disposed between the two electrodes. The rechargeable battery also includes a can which receives the electrode assembly and a cap assembly which caps the can, the cap assembly having a gasket at an opening of the can. One electrode tap is bent inside the can and is welded to the inner side at one or more points and another electrode tap is welded to the bottom surface of the can at one or more points.  
         [0023]     The electrode tap to be welded to the inner side of the can may be folded inside the rechargeable battery two or more times. The electrode tap to be welded to the bottom surface of the can protrudes from the lower end of the electrode assembly, is bent upward, is bent downward again at the upper portion of the can, is bent again at the lower end of the electrode assembly, and then is welded to the bottom surface of the can at two or more points. The upper portion of the can where the electrode tap is bent indicates the portion than the center of the height of the sidewall. The electrode tap may be bent at the top end of the can.  
         [0024]     If the length of a portion of the electrode tap other than the portion in contact with the bottom surface of the can or the portion parallel to the bottom surface is equal to the length of the portion of the electrode tap between the sidewall of the can and the sidewall of the electrode assembly, the length of the portion of the electrode tap may be greater than the height of the inner side of the can and less than double the height of the inner side of the can. When the electrode tap is welded to the sidewall of the can without the portion parallel to the bottom surface of the can, the length of the electrode tap extending downward from the electrode assembly is greater than the height of the can and smaller than double the height of the can.  
         [0025]     According to another embodiment of the present invention, a method of forming a rechargeable battery is provided, the method including inserting an electrode tap of an electrode assembly into a can, locating a body of the electrode assembly outside the can, and then welding the inserted electrode tap to the inner side of the can at two or more points; and bending the welded electrode tap and inserting the electrode assembly including all the electrode tap into the can.  
         [0026]     In the method, after welding the electrode tap, a lower insulating plate is disposed on the bottom surface and then the electrode assembly is inserted into the can. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  is a front cross-sectional view illustrating a structure of a conventional cylinder type rechargeable battery.  
         [0028]      FIG. 2  is an exploded perspective view illustrating the structure of the rechargeable battery of  FIG. 1 .  
         [0029]      FIG. 3  is a partial cross-sectional view illustrating an electrode tap being welded to the bottom surface of a can in the conventional cylinder type rechargeable battery of  FIG. 1 .  
         [0030]      FIG. 4  is a cross-sectional view illustrating welding an electrode tap of an electrode assembly to a can according to an embodiment of the present invention.  
         [0031]      FIG. 5  is a cross-sectional view illustrating bending the electrode tap of  FIG. 4 .  
         [0032]      FIG. 6  is a cross-sectional view of the electrode assembly of  FIG. 4  inserted into a can.  
         [0033]      FIG. 7  is a front cross-sectional view of a rechargeable battery according to an exemplary embodiment of the present invention.  
         [0034]      FIG. 8  is a front cross-sectional view of a rechargeable battery according to another exemplary embodiment of the present invention.  
         [0035]      FIGS. 9 and 10  are cross-sectional views illustrating alternative embodiments of the electrode assembly coupled to the can according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     First, a method of forming a rechargeable battery according to the present invention will be described. Similarly to the conventional method, two electrodes having a rectangular plate shape are stacked and wound in a jelly-roll configuration to form an electrode assembly. Since separators are disposed between and above or below the two electrodes, the separator is located such that it prevents a short circuit between the two electrodes.  
         [0037]     The electrode plate is formed by coating a charge collector of a metallic foil or aluminum or copper mesh with an active material slurry. The slurry is typically made by agitating a granular active material, a subsidiary conductor, a binder, and a plasticizer in a solvent added thereto. The solvent is removed in the subsequent process of forming electrodes.  
         [0038]     Uncovered areas absent the slurry exist are located at either end of the charge collector in a direction in which the electrode plates are wound. The uncovered areas are provided with electrode taps for each electrode plate. As seen in the exemplary embodiment shown in  FIG. 4 , one electrode tap  127  is drawn upward and the other electrode tap  129  is drawn downward. The electrode tap  129  is electrically connected to a cylindrical can  10 . The electrode tap  127  is electrically connected to a cap assembly (not shown) insulated from the cylindrical can  10  to form a part of a path for connecting the electrode assembly to external circuits during charging and discharging of the rechargeable battery.  
         [0039]     The can is formed out of iron, aluminum alloy, or the like by the use of a deep drawing method. Subsequently, as shown in FIGS.  4  to  6 , a process of coupling the electrode assembly  120  and the can  10  including a process of welding the electrode tap  129  is performed.  
         [0040]     First, as shown in  FIG. 4 , the electrode assembly  120  from which the electrodes taps  127 ,  129  are drawn upward and downward, respectively. One end of the electrode tap  129  is connected to an electrode at the outermost part of the electrode assembly  120  and the other end is to be securely welded to an upper end portion of the sidewall of the can at two or more welding points  149  in parallel.  
         [0041]     As shown in  FIG. 4 , a lower insulating plate  113   b  may be first provided on the bottom surface of the can  10 . Then, as shown in  FIG. 5 , the electrode assembly  120  is inserted into the can  10  such that a portion of the electrode tap  129  is folded onto itself. Finally, as shown in  FIG. 6 , the electrode tap  129  is located in the space between the electrode assembly  120  and the sidewall of the can  10  such that the electrode tap  129  is folded in part. In a rechargeable battery having a space between the electrode assembly and the can, the folded electrode tap may serve to prevent the electrode assembly from moving within the can.  
         [0042]      FIG. 7  depicts a fully assembled rechargeable battery according to an exemplary embodiment of the present invention.  
         [0043]     The electrode assembly  120  is inserted into the can  10  and the upper insulating plate  113   a  is located on the electrode assembly. The upward electrode tap  127  is protruded through a hole of the upper insulating plate to prevent the electrode assembly from floating. Subsequently, clamping the sidewall of the can by pressing a part of the sidewall of the can toward the center of the can is carried out.  
         [0044]     Electrolyte solution is injected into the can  10  in which the electrode assembly  120  has been installed and the gasket  130  is inserted into the upper opening of the can to contact the sidewall of the can.  
         [0045]     The cap assembly  180  as described above is provided inside the gasket  130 . A bent having a connection portion protruding downward from the central portion is positioned at the lowermost side. A current interrupt device (CID)  150  which would be deformed broken down with the upward deformation of the electrical connection portion  142  of the vent  140  due to the internal pressure is provided above the vent  140 . A positive thermal coefficient (PTC) element  160  connected to the CID  150 . The PTC element  160  cuts off a current path in the battery when the battery is overheated. A cap-up  170  having an electrode terminal protruded outward to provide an electrical connection to external devices is provided above the PTC element  160 . Instead of the PTC element  160  or above the PTC element, an additional protection circuit board may be further provided.  
         [0046]     To install the cap assembly  180 , the gasket  130  is first located on the cap assembly  180  and the upward electrode tap  27  is then welded to the connection portion  142  protruding downward in the safety vent  140 . The welding is carried out at two or more points apart from each other to enhance the welding strength which provides greater resistance to rotation of the electrode assembly  120 .  
         [0047]     When the elements of the cap assembly  180  are inserted into the gasket, a clamping work is carried out. Subsequently, a tubing work including coating the outside with a sheathing material is carried out.  
         [0048]     According to another exemplary embodiment of the present invention as shown in  FIG. 8 , the downward electrode tap  129  is bent at one portion and may be divided into a portion parallel to the sidewall of the can and an end portion parallel to the bottom surface of the can. The portion parallel to the sidewall of the can has a length corresponding to about one and half times the height of the sidewall of the can and the end portion parallel to the bottom surface of the can has a length substantially corresponding to the radius of the bottom surface of the cylindrical can.  
         [0049]     The end portion of the electrode tap parallel to the bottom surface of the can is first inserted into the can and is brought into contact with the bottom surface of the can. The end portion of the electrode tap is welded to the bottom surface of the can at two or more separate points. In the past, only the central portion of the bottom surface of the can was welded. However, in the present embodiment, the welding work is performed symmetrical to both sides of the center of the bottom surface of the can or to a plurality of points with a constant interval in between the points. The welding work may be carried out by laser welding, ultrasonic welding, or resistance welding, depending upon the material of the electrode tap.  
         [0050]     When the downward electrode tap  129  and the can  10  are welded to each other, the lower insulating plate  113   b  is inserted into the can before the electrode assembly is inserted. The lower insulating plate  113   b  serves to prevent short circuits between the two electrodes  125  due to the electrical connection between the electrode assembly  120  and the electrode tap  129  or between the electrode assembly  120  and the bottom surface of the can.  
         [0051]     The electrode assembly  120  is then inserted into the can  10 . Since the electrode tap  129  protruding from the lower end of the electrode assembly is longer than the sidewall of the can  129 , the electrode tap may be folded such that the electrode assembly  120  is completely insertable into the can  10 . When a portion of the electrode tap  129  protruding downward is folded onto itself, the end portion of the electrode tap is positioned below. The electrode tap  129  is then folded onto itself again. The whole electrode tap  129  is received in the can  10  in this folded state. The folded portion is positioned above the center of the can when the electrode assembly  120  is inserted into the can  10 .  
         [0052]     When the length of the electrode tap  129  other than the end portion is less than the height of the sidewall of the can, a part of the electrode assembly be located on the inside of the can to hinder the welding work between the end portion of the electrode tap and the bottom surface of the can. Accordingly, the electrode tap  129  should have a length greater than the height of the sidewall of the can.  
         [0053]     On the other hand, in a further exemplary embodiment as shown in  FIG. 9 , the end portion of the electrode tap  129  can be welded to the sidewall of the can at two or more welding points  149 ′ which are positioned at the central portion of the sidewall of the can. In this case, the length of the electrode tap  129  is not necessarily greater than the height of the sidewall of the can  10 . That is, the electrode tap  129  may be welded to the central portion of the sidewall of the can  10  and has a length sufficient to prevent the electrode assembly  120  from entering the can  10  during the welding work. Additionally, the length of the electrode tap  129  may be greater than the length between the welding point and the lower end of the electrode assembly at the time of receiving the electrode assembly in the can.  
         [0054]      FIG. 10  is a cross-sectional view illustrating a case wherein a center pin  150  is provided in the central space of the electrode assembly according to yet another embodiment of the present invention. As shown in  FIG. 10 , since the welding does not need to be carried out using the central space of the electrode assembly, the lower insulating plate  113   b  does not contain a hole.  
         [0055]     In the rechargeable battery according to the present invention, the welding strength between the bottom surface of the can and the downward electrode tap is enhanced, the resistance of the electrical connection portion is low to reduce the external internal resistance of the rechargeable battery and the charging and discharging efficiency and the amount of discharge is enhanced.  
         [0056]     Specifically, the present invention reduces the likelihood that the electrode assembly will rotate due to external forces and thus, the welding between the bottom surface of the can and the downward electrode tap, is more likely to be maintained. Thus, the reliability of the rechargeable battery is enhanced.  
         [0057]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Therefore, the scope of the invention is defined by the appended claims, and all variations within the scope of the claims will be understood to be included in the present invention.