Abstract:
A secondary battery having an upper battery assembly including an upper can housing a cap unit having an electrode terminal adapted to electrically connect to an external device and a gasket inserted between the cap unit and one end portion of the upper can for sealing and insulating the cap unit and the upper can. The secondary battery also has a lower battery assembly including a lower can housing an electrode assembly, the electrode assembly having a first electrode, a second electrode, and a separator interposed between the first electrode for preventing a short circuit between the two electrodes. A first electrode tap connects the first electrode to the cap unit and a second electrode tap connects the second electrode to the lower can.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application claims the priority of Korean Patent Application No. 2005-0019829, filed on Mar. 9, 2005, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference.  
         [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an engagement structure of a cap unit and a can in a cylindrical type secondary battery and a method of fabricating the same.  
         [0004]     2. Description of Related Art  
         [0005]     Since secondary batteries have many advantages such as rechargeability, miniaturization, and high capacity, they are frequently used and have become popular. Secondary batteries can be classified into Ni-MH batteries and Li-ion batteries depending on the electrode active material.  
         [0006]     It is known that liquid electrolyte, solid polymer electrolyte, or gel type electrolyte are used as the electrolyte of Li-ion secondary batteries. Also, the lithium secondary battery can be classified into a can type or a pouch type depending on the container into which an electrode assembly is inserted.  
         [0007]     In a can type lithium secondary battery, the electrode assembly is inserted into a can which is formed by a deep drawing method using metallic materials such as aluminum or an aluminum alloy. Typically, liquid electrolyte is used in a can type secondary battery structure.  
         [0008]     Can type secondary batteries can be classified into hexahedronal can types and cylindrical can types. Since the hexahedronal can type can be formed in a thin hexahedronal shape, it can allow installation flexibility when it is used in electronic devices such as a mobile phone. The cylindrical can type is usually used in high capacity electronic devices, in which a plurality of secondary batteries are combined into a single battery pack.  
         [0009]      FIG. 1  is a cross-sectional view illustrating an exemplary structure of a conventional cylindrical can type secondary battery.  
         [0010]     A method of fabricating a conventional cylindrical can type secondary battery will be described with reference to  FIG. 1 . First, two electrode plates  25  having a generally rectangular shape and separators  21 ,  23  interposed between the electrode plates  25  for preventing short circuits between the plates are stacked and wound in a jelly roll configuration to provide an electrode assembly  20 . Each electrode plate is formed by coating an active material slurry on a charge collector composed of metallic foil.  
         [0011]     The charge collector has uncovered areas absent the active material slurry on either end of the charge collector in a direction in which the electrode plates  21 ,  23  are wound. On the uncovered areas, electrode taps  27 ,  29  are provided for each electrode plate. The electrode tap  27  is electrically connected to a cap unit  80 . The electrode tap  29  is electrically connected to a cylindrical can  10 . The cap unit  80  is insulated from the cylindrical can  10  to provide a path for connecting the electrode assembly to external circuits during charging/discharging of the battery. From the electrode assembly  20 , the electrode tap  27  is extracted toward an opening of the cylindrical can  10  and the other electrode tap  29  is extracted in an opposite direction.  
         [0012]     The electrode assembly  20  with upper and lower insulation plates  13   a ,  13   b  is inserted into the cylindrical can  10  through the can opening. Beads for preventing floating of the electrode assembly in the can are formed, and electrolyte is injected. An insulation gasket  30  is provided around the inside wall of the can opening and a cap unit  80  for covering the can opening is installed inside the gasket  30 .  
         [0013]     In the cap unit  80 , a bent assembly welded to one of the electrode taps  27 , a positive thermal coefficient (PTC) element  60 , and a cap-up having an electrode terminal are included. The bent assembly typically includes a bent  40  and a current interrupt device (CID)  50  which is used in combination with the bent  40  to cut off a current path.  
         [0014]     Subsequently, the side wall of the can opening is pressed toward the center and bottom of the can by using the cap-up  70  inserted into the inside of the gasket  30  as a stopper. In addition, tubing work is performed to form an exterior of the battery.  
         [0015]     The electrode tap  27  is formed to have enough length to allow the electrode tap  27  to be welded to the bent  40 . Also, the electrode tap  27  is bent while the bent assembly is inserted into the cap opening into which the gasket  30  is already installed. In this process, a small space between the bent assembly and the electrode assembly  20  is provided for process convenience.  
         [0016]     After the electrode assembly  20  and the upper insulation plate  13   a  are inserted into the can, bead areas are provided for fixing these components to the can. However, the bead areas also contribute to formation of a space between the electrode assembly  20  and the cap unit  80 . Such spaces are not desired in view of miniaturization and high capacity of the battery.  
         [0017]     When the cap unit is installed in the can opening into which the gasket is already inserted and clamping for enveloping the battery is performed, if a high pressure is applied to the side wall of the can opening to improve sealing reliability, the bead areas may be significantly crushed, distorted, or deformed.  
         [0018]     The deformation of the bead areas may be greatly varied. When the deformation is significant, the bead areas may be weakened and finally fractured by even a small external force causing air or humidity to flow into the battery or electrolyte to leak.  
         [0019]     Also, the deformation of the bead areas may cause irregular heights of each battery module so that the battery module may not sufficiently contact probes of a manufacturing device in subsequent forming processes in which the initial charging and discharging for secondary batteries are executed.  
       SUMMARY OF THE INVENTION  
       [0020]     The present invention provides a can type secondary battery and a method of fabricating the same by which problems such as deformation of beads and an unnecessary gap between an electrode assembly and a cap unit are prevented.  
         [0021]     A can type secondary battery and a method of fabricating the same is provided having increased resistance to deformation during a fabricating process, a minimized battery size, and an increased battery capacity.  
         [0022]     A can type secondary battery and a method of fabricating the same is provided, by which clamping can be performed at a sufficient pressure to guarantee reliability of a battery sealing condition, and by which the length of the battery can be efficiently utilized.  
         [0023]     According to an exemplary embodiment of the present invention, a secondary battery is provided having an upper battery assembly including an upper can housing a cap unit having an electrode terminal adapted to electrically connect to an external device and a gasket inserted between the cap unit and one end portion of the upper can for sealing and insulating the cap unit and the upper can. The secondary battery also has a lower battery assembly including a lower can housing an electrode assembly, the electrode assembly having a first electrode, a second electrode, and a separator interposed between the first electrode for preventing a short circuit between the two electrodes. A first electrode tap connects the first electrode to the cap unit and a second electrode tap connects the second electrode to the lower can.  
         [0024]     The upper battery assembly may be formed by inserting the cap unit into the inside wall of the upper can in which the gasket has been installed and robustly combining them by clamping.  
         [0025]     The upper and lower battery assemblies may be combined with each other by a screw engagement using threads formed on both of a lower portion of the upper can and an upper portion of the lower can or by a sleeve engagement using sleeves for inserting one of an upper portion of the lower can and a lower portion of the upper can into the other. Also, they may be combined by welding.  
         [0026]     When the upper and lower battery assemblies are combined by a screw engagement, an electrode tap of the electrode assembly may be electrically connected to a bent portion of the cap unit. In this case, when the upper can is rotated for a screw engagement, means can be provided for preventing rotation of the electrode tap caused by friction with the bent portion. For example, a trench may be provided for storing the electrode tap on a top surface of the upper insulation plate installed on the electrode assembly.  
         [0027]     The can type secondary battery according to the present invention may be used in electric power tools such as power drills or power saws which consume abundant currents in a short time and have a high power and a high resistance to external vibration or impact.  
         [0028]     According to another aspect of the present invention, there is provided a method of fabricating a can type secondary battery, the method including the processes of: forming an upper battery assembly by inserting a gasket in an inside wall of an upper can; installing a cap unit on the gasket in the upper can; and combining the upper can with the cap unit by a clamping work for downwardly and inwardly pressing the inside wall of the upper can; forming a lower battery assembly by inserting an electrode assembly into a lower can; and combining the upper and lower battery assemblies with each other.  
         [0029]     The process of forming a lower battery assembly may include: inserting the electrode assembly into the lower can together with upper and lower insulation plates; and bending an electrode tap extracted from the electrode assembly under the lower insulation plate and welding the electrode tap to a bottom surface of the lower can.  
         [0030]     The process of combining the upper and lower battery assemblies may include: forming threads on corresponding portions of the upper and lower cans; and engaging the upper and lower battery assemblies by screwing the upper and lower cans with respect to each other.  
         [0031]     Otherwise, the process of combining the upper and lower battery assemblies may include: forming sleeves in one of the upper and lower cans; and engaging the upper and lower cans by forcibly inserting one of the upper and lower cans into the other. In this case, an adhesive or gasket having conductivity may be applied to engagement contact portions between the upper and lower battery assemblies.  
         [0032]     In the process of combining the upper and lower battery assemblies, a welding work may be performed to the upper and lower battery assemblies without or after engaging them by using a screw or sleeve engagement. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]      FIG. 1  is a cross-sectional elevation view of a conventional cylindrical can type secondary battery.  
         [0034]      FIG. 2  is a cross-sectional elevation view of a cylindrical can type secondary battery in which upper and lower battery assemblies are engaged by using a screw engagement method according to an embodiment of the present invention.  
         [0035]      FIG. 3  is a cross-sectional elevation view of a cylindrical can type secondary battery in which upper and lower battery assemblies are engaged by using a sleeve engagement method according to another embodiment of the present invention.  
         [0036]      FIG. 4  is a cross-sectional elevation view of cylindrical can type secondary batteries according to  FIG. 3  in which an adhesive is applied to engagement contact portions between upper and lower battery assemblies.  
         [0037]      FIG. 5  is a cross-sectional elevation view of cylindrical can type secondary batteries according to another embodiment of the present invention in which an adhesive is applied to engagement contact portions between upper and lower battery assemblies.  
         [0038]      FIG. 6  is a partial perspective view showing an exemplary use of a secondary battery of the present invention in a power tool. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0039]     A method of fabricating a cylindrical can type secondary battery according to an embodiment of the present invention will be described with reference to  FIG. 2 .  
         [0040]     First, a method of fabricating a lower battery assembly will be described. Similarly to a conventional method, two electrode plates  25  having a generally rectangular sheet shape are stacked and wound into a jelly roll configuration to provide an electrode assembly. Separators  21 ,  23  are disposed between the two electrode plates  25  and on any surface where the wound electrode plates make contact with the other when they are wound in order to prevent a short circuit.  
         [0041]     The electrode plate may be formed by coating an active material slurry on a charge collector which may be a metallic foil or mesh made of aluminum or copper. The slurry is typically made by agitating a granular active material, a subsidiary conductor, a binder, and a plasticizer with a solvent being added. The solvent is removed in an electrode forming process.  
         [0042]     There are areas of the charge collector absent slurry at either end of the charge collector. On the uncovered areas, one electrode tap is provided for each electrode plate. One of the electrode taps is upwardly extracted through an opening of a cylindrical can, whereas the other electrode tap is downwardly extracted in an opposite direction.  
         [0043]     In a cylindrical can  10   a , a lower insulation plate  13   b , an electrode assembly  20 , and an upper insulation plate  13   a  are sequentially installed. The electrode tap  29  downwardly extracted is bent such that the bent portion is parallel to a lower surface of the electrode assembly  20 , and welded to the bottom surface of the cylindrical can. In this case, the lower insulation plate  13   b  is disposed between the bent portion of the electrode tap  29  and the bottom surface of the electrode assembly  20  so that the electrode tap  29  is not short-circuited.  
         [0044]     In order to weld the electrode tap to the bottom surface of the cylindrical can, a welding rod is inserted through a center hole of the electrode assembly and through a center hole of the lower insulation plate. Then, the electrode tap  29  and the bottom surface of the cylindrical can are welded to each other.  
         [0045]     The electrode tap  27  is upwardly extracted through a thru-hole of the upper insulation plate  13   a  and bent to make contact with a top surface of the upper insulation plate. The upper insulation plate may have a trench for storing the electrode tap on its top surface. The trench may be formed with a depth sufficient to allow the electrode tap  27  to make contact with an electrical connection portion  42  of bent  40  which in turn is connected to the CID  50 , without a separate connection structure being provided and to retain the connection state between the electrode tap  27  and the electrical connector  42 .  
         [0046]     If the upper insulation plate  13   a  does not contain a trench, the bent electrode tap  27  may be fixed on the upper insulation plate  13   a  by using an adhesive or a gluing agent. In addition, the bent portion of the electrode tap  27  does not necessarily have a flat surface but may have a structure such as a plate spring.  
         [0047]     Electrolyte may then be inserted into the electrode assembly of the lower cylindrical can.  
         [0048]     The lower can  10   a  has threads  100  on its outer rim. The upper end of the electrode assembly  20 , the upper insulation plate  13   a , and the level of the electrolyte may be approximately level with the top end of the cylindrical can.  
         [0049]     Now, a method of fabricating an upper battery assembly will be described. First, an upper cylindrical can  10   b  is prepared. The upper cylindrical can also have mating threads  101  around its inner rim, which will be combined with the threads  100  around the outer rim of the lower cylindrical can  10   a.    
         [0050]     Around the inside wall of the upper cylindrical can, an insulation gasket  30  is installed. Around the lower end of the insulation gasket  30 , an inner rim portion is inwardly protruded so that the cap unit can be appropriately positioned in the upper can.  
         [0051]     The cap unit  80  for covering the can opening is installed on the inside of the gasket  30 . The cap unit  80  includes: a bent  40  having a connection portion which protrudes in its center and a CID  50  is used in combination with the bent  40  to cut off a current path. The cap unit  80  also includes a PTC element  60  connected to CID  50  for cutting off a current path in the battery when the battery is overheated and a cap-up  70  having an electrode terminal outwardly protruded to provide an electrical connection to external devices. A protection circuit board (not shown) may be further provided on the PTC element  60 .  
         [0052]     The components of the cap unit  80  may be sequentially installed on the inside of the gasket. However, a cap unit  80  in which the above components have already been assembled may be inserted into the inside of the gasket  30 . Clamping is performed to provide an upper battery assembly. Specifically, the side wall of the upper can  10   b  is pressed toward the center of the can while the lower end of the upper can  10   b  or an outer portion of the bent is supported by a support so as to maintain their original shape. As a result, it is possible to apply a pressure to sufficiently seal a battery during clamping.  
         [0053]     Then, the upper and lower battery assemblies are combined with each other. The outer diameter of the upper end of the lower battery assembly is engaged with the inner diameter of the lower end of the upper battery assembly using the threads  100 ,  101  formed on their corresponding wall sides. That is, the upper end of the lower battery assembly is screwed into the lower end of the upper battery assembly until they are fully engaged.  
         [0054]     When the lower battery assembly is fully inserted into the upper battery assembly, the upper end of the lower battery assembly  10   a  is in contact with the lower end of the gasket  30 , and the electrical connection portion  42  downwardly protruded in the bent  40  is electrically connected to the electrode tap  27  provided on the top surface of the upper insulation plate  13   a . Conductive paste may be applied to the connection portion to reduce contact resistance.  
         [0055]     When the lower battery is combined with the upper battery assembly, the friction on the electrical connection portion  42  subjects the electrode tap  27  to rotational forces. However, the rotation is suppressed because the electrode tap  29  disposed in the lower surface of the electrode assembly  20  is welded to the bottom of the cylindrical lower can  10 .  
         [0056]     According to this exemplary embodiment, a gap between the electrode assembly and the bent can be significantly reduced as compared to a conventional battery in which beads are provided and the electrode tap is welded to the bent.  
         [0057]     Another embodiment is shown in  FIG. 3  wherein the upper and lower battery assemblies are structurally similar, but combined differently.  
         [0058]     Referring to  FIG. 3 , the upper end of the lower can is forcibly inserted into the inside of the lower end of the upper can in an interference fit fashion. In order to facilitate insertion, the lower end portion  103  of the upper can  10   d  has a sleeve having an inner diameter fitted to the outer diameter of the upper end portion of the lower can  10   c.    
         [0059]     Similar to the embodiment shown in  FIG. 2 , the upper end of the lower can  10   c  makes contact with the lower end of the upper can  10   d  when they are combined, and the connection portion  42  downwardly protruded under the bent is electrically connected to the electrode tap fixed on the top surface of the upper insulation plate  13   a . According to the structure shown in  FIG. 3 , it is not necessary to screw the upper and lower battery assemblies with respect to each other, and thus, it is possible to neglect synchronous rotation of the electrode tap  27  caused by the friction with the electrode connection portion  42 .  
         [0060]     Similarly, tape or other adhesive material may be applied to engagement contact portions between the upper and lower battery assemblies to function as a gasket. In one exemplary embodiment, the tape or the adhesive material may have conductivity. After the upper and lower battery assemblies are combined, spot welding or line welding can be applied to some portions around the side wall.  
         [0061]     Tubing work may then be performed to envelope an exterior of the battery by using a method as is known in the art.  
         [0062]     Tape or other adhesive materials  107  may be applied to engagement contact portions between the upper and lower battery assemblies as shown in  FIG. 4  to function as a gasket. In one exemplary embodiment, the inlayed tape or adhesive material may have conductivity.  
         [0063]     A thread  101   a  on the inner surface of the sleeve  103  may be adapted to combine the upper can  10   f  and the lower can  10   e  as shown in  FIG. 5  Additionally as shown in  FIG. 5 , after the upper and lower battery assemblies are combined in a screw engagement, spot welding  105  or line welding can be applied to some portions around the side wall. A battery  300  of the present invention has a high resistance to external vibration and impact, allowing it to be used to drive the motor of a power tool  200  as shown in  FIG. 6 .  
         [0064]     According to the present invention, there is no need to form beads in a cylindrical battery. Therefore, clamping or pressing is performed without beads, and thus, deformation of beads does not affect the subsequent battery fabricating processes.  
         [0065]     Also, it is possible to prevent insufficient sealing of the battery caused when clamping is performed at an insufficient pressure.  
         [0066]     Since there is almost no gap between the connection portion of the bent of the cap unit and the electrode tap of the electrode assembly when the upper and lower battery assemblies are combined, the size of the electrode assembly can be relatively increased compared to a comparably sized conventional secondary battery. Therefore, it is possible to increase battery capacity and minimize battery size.  
         [0067]     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 differences within the scope will be construed as being included in the present invention.