Patent Publication Number: US-2022216550-A1

Title: Rechargeable battery

Description:
TECHNICAL FIELD 
     The present invention relates to a rechargeable battery, and more particularly, to a very small rechargeable battery. 
     BACKGROUND ART 
     A rechargeable battery differs from a primary battery in that it can be repeatedly charged and discharged, while the latter is incapable of being recharged. Low-capacity rechargeable batteries are used in portable electronic devices such as mobile phones, laptop computers, and camcorders, and large-capacity batteries are widely used as power sources for driving a motor such as for hybrid vehicles. 
     Representative rechargeable batteries include a nickel-cadmium (NiCd) battery, a nickel metal hydride (NiMH) battery, a lithium (Li) battery, and a lithium ion (Li ion) rechargeable battery. In particular, lithium ion rechargeable batteries are about three times higher in operating voltage than the nickel-cadmium batteries or nickel metal hydride batteries, which are widely used as power sources for portable electronic equipment. Further, it is widely used because of its high energy density per unit weight. 
     In particular, in recent years, as the demand for wearable devices such as headphones, earphones, smartwatches, and body-attached medical devices using Bluetooth increases, the need for ultra-small rechargeable batteries with high energy density is increasing. 
     It is an important task to secure required electrical capacity within a limited size, and to effectively reduce a weight and improve structural stability of such a miniature rechargeable battery. 
     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. 
     DISCLOSURE 
     An embodiment of the present invention has been made in an effort to provide a rechargeable battery capable of effectively reducing weight, improving electrical capacity, and efficiently securing structural stability. 
     An embodiment of the present invention provides a rechargeable battery including: an electrode assembly in which a separator is provided between a first electrode and a second electrode; a case configured to have an opening at a side thereof to accommodate the electrode assembly; a cap assembly coupled to the opening to close and seal the case; a first electrode tab configured to extend from the first electrode and to be coupled to the case; and a second electrode tab configured to extend from the second electrode and to be coupled to the cap assembly, wherein the cap assembly includes a metal layer to which the second electrode tab is coupled and a plastic layer stacked on an outer surface of the metal layer. 
     The cap assembly may include a stack portion in which the plastic layer is stacked on an outer surface of the metal layer, and a terminal portion in which the outer surface of the metal layer is exposed to the outside. 
     The first electrode, the second electrode, and the separator may be wound around a center pin, an upper surface may face the cap assembly, and a lower surface may face the bottom of the case in the electrode assembly; the first electrode tab may extend from the lower surface of the electrode assembly to be coupled to a bottom of the case; and the second electrode tab may extend from the upper surface of the electrode assembly to be coupled to metal layer of the cap assembly. 
     It may further include an insulating member configured to surround the electrode assembly between the electrode assembly and the case and to insulate the electrode assembly from the case. 
     An upper end of the center pin may protrude from the upper surface of the electrode assembly toward the cap assembly, and the second electrode tab may be supported on the upper end of the center pin to be welded to the metal layer of the cap assembly. 
     The terminal portion may be formed at a central side corresponding to the center pin in the cap assembly. 
     A side surface of the metal layer may be surrounded by the plastic layer. 
     The plastic layer may extend outside a circumference of the metal layer to form a first fusion portion, and the first fusion portion may be fused to the case in the cap assembly. 
     The case may include a second fusion portion of a plastic material surrounding an upper end of a sidewall surrounding the opening, and the first fusion portion may be fused to the second fusion portion. 
     The second fusion portion may be positioned on an upper surface and an outer surface of the sidewall except for an inner surface facing the electrode assembly from the upper end thereof. 
     The cap assembly may have an annular stepped groove extending from the first fusion portion, and the second fusion portion may be seated in the stepped groove, and may be fused with the first fusion portion. 
     The rechargeable battery may have a coin shape with a diameter-to-height ratio of 1 or more. 
     According to the embodiment of the present invention, it is possible to provide a rechargeable battery capable of effectively reducing weight, improving electrical capacity, and efficiently securing structural stability. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a perspective view of a rechargeable battery according to an embodiment of the present invention. 
         FIG. 2  illustrates a schematic perspective view of the rechargeable battery of  FIG. 1 . 
         FIG. 3  illustrates a cross-sectional view taken along a line A-A of the rechargeable battery illustrated in  FIG. 1 . 
         FIG. 4  illustrates an enlarged view of an area B of  FIG. 3 . 
     
    
    
     MODE FOR INVENTION 
     In the following detailed description, only certain embodiments of the present invention have been shown and described, simply by way of illustration. 
     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. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
     In this specification, redundant descriptions for the same constituent elements will be omitted. 
     In this specification, it is to be understood that when one component is referred to as being “connected” or “coupled” to another component, it may be connected or coupled directly to the other component or may be connected or coupled to the other component with a further component intervening therebetween. In this specification, it is to be understood that when one component is referred to as being “connected” or “coupled directly” to another component, it may be connected to or coupled to the other component without being connected to a component intervening therebetween. 
     It is also to be understood that the terminology used herein is only for the purpose of describing particular embodiments, and is not intended to be limiting of the invention. 
     Singular forms are to include plural forms unless the context clearly indicates otherwise. 
     It will be further understood that terms “comprises” or “have” used in the present specification specifies the presence of stated features, numerals, steps, operations, components, parts, or a combination thereof, but do not preclude the presence or addition of one or more other features, numerals, steps, operations, components, parts, or a combination thereof. 
     In addition, as used herein, the term “and/or” includes any plurality of combinations of items or any of a plurality of listed items. In this specification, “A or B” may include “A”, “B”, or “A and B”. 
     An embodiment of the present invention relates to a secondary battery, and although a size thereof may vary, it may be an ultra-small rechargeable battery with a very small size such that it can be used in headphones, earphones, smartwatches, wearable devices, etc. 
     In addition, the embodiment of the present invention may be applied to various forms and types of rechargeable batteries such as a prismatic, a cylindrical, and a fin type, but may preferably be a coin type or button type of battery. The coin type or button type of battery has a same shape as a thin coin or button, may be used as an ultra-small battery, and may be defined as a battery in which a ratio of a height H to a diameter L of the rechargeable battery (height/diameter) is 1 or less. 
     The coin-type or button-type of battery may have a cylindrical shape having a circular cross-sectional shape, but the present invention is not limited thereto, and may have an oval or polygonal cross-sectional shape. 
     Herein, as illustrated in  FIG. 2 , the diameter L indicates a maximum distance between sides of a case  180  in which an electrode assembly  200  is built, the height H indicates a minimum distance from a flat bottom surface of a cell to a flat top surface of the cell, and it may be understood as from a lower surface of the case  180  to an upper surface of a cap assembly  100 . 
     The rechargeable battery according to an embodiment of the present invention may have various sizes, types, and shapes, but hereinafter, for convenience of description of an embodiment of the present invention, a coin-type battery having a circular cross-sectional shape will be described as an example. 
       FIG. 1  illustrates a perspective view of a rechargeable battery according to an embodiment of the present invention, and  FIG. 2  illustrates a schematic perspective view of the rechargeable battery of  FIG. 1 . Meanwhile,  FIG. 3  illustrates a cross-sectional view taken along a line A-A of the rechargeable battery of  FIG. 1 . 
     The rechargeable battery according to an embodiment of the present invention includes: the electrode assembly  200  in which a separator  13  is positioned between a first electrode  11  and a second electrode  12 ; the case  180  having an opening at a side thereof, the cap assembly  100  coupled to the opening to seal the case  180 ; a first electrode tab  232  extending from the first electrode  11  and coupled to the case  180 ; and a second electrode tab  234  extending from the second electrode  12  and coupled to the cap assembly  100 , and the cap assembly  100  includes a metal layer  120  to which the second electrode tab  234  is coupled and a plastic layer  140  stacked on an outer surface of the metal layer  120 . 
     The electrode assembly  200  includes a first electrode  11  and a second electrode  12  each of which may have a coated region and an uncoated region. The first electrode  11  may serve as a positive electrode, and the second electrode  12  may serve as a negative electrode, or vice versa. The separator  13  for electrical insulation may be positioned between the first electrode  11  and the second electrode  12 . 
     In an embodiment of the present invention, the electrode assembly  200  may be provided as a stacked type, but as illustrated in  FIG. 2  and  FIG. 3 , may have a jelly-roll form in which the first electrode  11 , the second electrode  12 , and the separator  13  are wound together around a center pin  210 . 
     When the electrode assembly  200  corresponds to a winding type, as illustrated in  FIG. 2  and  FIG. 3 , the electrode assembly  200  may have a flat upper surface and lower surface, and may have a curved side surface, and a winding axis may be parallel to a height direction of the rechargeable battery. That is, the electrode assembly  200  may have an upper surface facing the cap assembly  100  and a lower surface facing a bottom of the case  180 . 
     The first electrode  11  is formed to have a long band shape, and includes a negative coating region, which is a region where a negative active material layer is coated to a current collector of a metal foil (e.g., a Cu foil), and a negative uncoated region, which is a region where no active material is coated. The negative uncoated region may be positioned at a first end portion of the negative electrode in a longitudinal direction thereof. 
     The second electrode  12  is formed to have a long band shape, and includes a positive coating region, which is a region where a positive active material layer is coated to a current collector of a metal foil (e.g., an Al foil), and a positive uncoated region, which is a region where no active material is coated. The positive uncoated region may be positioned at a first end portion of the positive electrode in a longitudinal direction thereof. 
     The case  180  has an accommodation space in which the electrode assembly  200  is accommodated, and an opening may be formed at a first side thereof. In an embodiment of the present invention, the case  180  has an opening formed on an upper surface thereof as illustrated in  FIG. 1  to  FIG. 3 , and the electrode assembly  200  may be accommodated therein. The opening may be formed on the side or the bottom unlike  FIG. 1  to  FIG. 3 , but for the purpose of description of the present invention, the following will be described based on the opening formed on the upper surface of the case  180 . 
     The case  180  may be made of an electrically conductive metal material such as aluminum or stainless steel, and a shape of the case  180  may be various as needed, such as a cylindrical shape or a square shape, but as an embodiment of the present invention, a cylindrical case  180  in which the wound electrode assembly  200  is accommodated is illustrated in  FIG. 1  and  FIG. 2 . 
     The cap assembly  100  may be coupled to the opening of the case  180 , and may seal the accommodation space of the case  180 . A coupling method with the case  180  may be various methods such as metal-to-metal welding, plastic welding, coupling using tape or an adhesive, and the like. 
     The first electrode tab  232  may extend from the first electrode  11  of the electrode assembly  200  to be coupled to the case  180 . The first electrode tab  232  may be integrally provided with the first electrode  11  or may be separately manufactured and coupled to the first electrode  11  by welding or the like. 
     The first electrode tab  232  may be made of a metal material with electrical conductivity, and is electrically connected to the first electrode  11  to have the same polarity. In addition, the case  180  coupled to the first electrode tab  232  to be electrically connected thereto may have the same polarity as that of the first electrode  11 . 
     The first electrode tab  232  and the case  180  may have a mutual coupling relationship using a separate medium, or may be directly coupled to each other by welding or the like as illustrated in  FIG. 2 . 
     The second electrode tab  234  may extend from the second electrode  12  of the electrode assembly  200  to be coupled to the cap assembly  100 . The second electrode tab  234  may be integrally provided with the second electrode  12  or may be separately manufactured and coupled to the second electrode  12  by welding or the like. 
     The second electrode tab  234  may be made of a metal material with electrical conductivity, and is electrically connected to the second electrode  12  to have the same polarity. In addition, the cap assembly  100  coupled to the second electrode tab  234  to be electrically connected thereto may have the same polarity as that of the second electrode  12 . The second electrode tab  234  and the cap assembly  100  may have a mutual coupling relationship using a separate medium, or may be directly coupled to each other by welding or the like as illustrated in  FIG. 2 . 
     Meanwhile, referring to  FIG. 3 , the first electrode tab  232  and the second electrode tab  234  may extend from the outside of the insulating member  220  to contact the case  180 , and may be used in a form in which an outer surface is coated with an electrically insulating material for electrical insulation from the case  180 . 
     Meanwhile, the cap assembly  100  includes a metal layer  120  and a plastic layer  140 . The metal layer  120  may be made of a plate made of a metal material, and the second electrode tab  234  may have the same polarity as that of the second electrode  12  by being coupled thereto by welding or the like. 
     The plastic layer  140  may have a stacked structure on an outer surface of the metal layer  120  by injection molding or a bonding method using an adhesive, and has electrical insulation. 
     According to an embodiment of the present invention, the cap assembly  100  may be manufactured to include the plastic layer  140 , thereby reducing a weight thereof compared to a case where the cap assembly  100  is entirely made of a metal material, and furthermore, as the plastic layer  140  and the metal layer  120  are provided together, it is possible to secure structural stability such as improving rigidity. 
     Meanwhile, according to an embodiment of the present invention, as shown in  FIG. 1  to  FIG. 3 , the cap assembly  100  may have a stack portion in which the plastic layer  140  is stacked on an outer surface of the metal layer  120 , and a terminal portion in which the outer surface of the metal layer  120  is exposed to the outside. 
     In  FIG. 1 , a portion on which the plastic layer  140  is stacked corresponds to the stack portion, a portion where the metal layer  120  is exposed at a central side corresponds to the terminal portion, and in  FIG. 2 , the stack portion and the terminal portion are illustrated in a same manner as well. 
     In an embodiment of the present invention, the metal layer  120  of the cap assembly  100  is connected to the second electrode tab  234  to have same polarity as that of the second electrode  12 , and the plastic layer  140  forms a terminal portion exposing some of the metal layer  120  to the outside so that it can be connected to an external electronic device or the like while limiting unnecessary electrical connection by forming a stack portion covering the outer surface of the metal layer  120 . 
     In  FIG. 1  to  FIG. 3 , it is illustrated that a stack portion in which the plastic layer  140  is stacked is formed on the outer surface of the metal layer  120  in an area of the cap assembly  100  except for a central side, and a terminal portion in which the outer surface of the metal layer  120  is directly exposed to the outside is formed at the central side without the plastic layer  140  according to an embodiment of the present invention. However, a position or shape of the terminal portion may be set in various ways as needed. 
     According to an embodiment of the present invention, it is possible to form a stack portion including the plastic layer  140  in the cap assembly  100 , to effectively achieve overall weight reduction, and at the same time to eliminate unnecessary electrical connection in the stack portion. 
     Further, in an embodiment of the present invention, it is possible to effectively implement the terminal portion while omitting a separate configuration by simply and effectively implementing the terminal portion by removing only the plastic layer  140  from the cap assembly  100 . 
     In the meantime, in an embodiment of the present invention, in the electrode assembly  200 , the first electrode  11 , the second electrode  12 , and the separator  13  may be wound around a center pin  210 , an upper surface thereof may face the cap assembly  100 , and a lower surface thereof may face bottom of the case  180 ; the first electrode tab  232  may extend from a lower surface of the electrode assembly  200  to be coupled to the bottom of the case  180 ; and the second electrode tab  234  may extend from an upper surface of the electrode assembly  200  to be coupled to the metal layer  120  of the cap assembly  100 . 
     In  FIG. 2  and  FIG. 3 , according to an embodiment of the present invention, the wound electrode assembly  200  in which the first electrode  11 , the second electrode  12 , and the separator  13  are wound around the center pin  210  corresponding to a winding axis is illustrated. Referring to  FIG. 2 , the upper surface of the electrode assembly  200  may face the cap assembly  100 , and the lower surface of the electrode assembly  200  may face the bottom of the case  180 . 
     As such, it is possible to effectively utilize an accommodation space of the case  180  to improve energy density by using the wound electrode assembly  200  in a state in which the center pin  210  vertically extends, and furthermore, it may be effective in realizing a flat rechargeable battery such as a coin-type battery. 
     Meanwhile, the first electrode tab  232  may extend from the lower surface of the electrode assembly  200  as illustrated in  FIG. 3 , and thus may extend along a simple and efficient path to be coupled to the bottom of the case  180 . 
     In addition, the second electrode tab  234  may extend from the upper surface of the electrode assembly  200  as illustrated in  FIG. 3 , and thus may be coupled to the cap assembly  100  by reducing a distance from the cap assembly  100  and extending along a simple and efficient path. 
     Meanwhile, as illustrated in  FIG. 3 , according to an embodiment of the present invention, it may further include an insulating member  220  that wraps the electrode assembly  200  between the electrode assembly  200  and the case  180 , and insulates the electrode assembly  200  from the case  180 . 
     The insulating member  220  may surround the electrode assembly  200 , and may electrically insulate the electrode assembly  200  in relation to the case  180  and the cap assembly  100 . The insulating member  220  may be made of an electrically insulating material such as a polymer, and may be provided in the form of a film to surround the electrode assembly  200 , or may be provided as a liquid and applied to the outer surface of the electrode assembly  200  and then undergo a curing process etc. 
     Meanwhile, according to an embodiment of the present invention, an upper end of the center pin  210  may protrude from the upper surface of the electrode assembly  200  toward the cap assembly  100 , and the second electrode tab  234  may be supported on the upper end of the center pin  210  to be welded to the metal layer  120  of the cap assembly  100 . 
     In an embodiment of the present invention, the electrode assembly  200  may include the center pin  210  in a center thereof, and the center pin  210  may be made of various materials having electrical insulation. 
     The center pin  210  may protrude toward the cap assembly  100  with an upper end thereof facing the cap assembly  100  rather than the upper surface of the electrode assembly  200 . The second electrode tab  234  may be in contact with the metal layer  120  of the cap assembly  100  by having a coupling portion thereof with the cap assembly supported on the upper end of the center pin  210 . 
     Accordingly, the second electrode tab  234  to be welded with the metal layer  120  of the cap assembly  100  may be stably maintained in contact with the metal layer  120 , and thus the second electrode tab  234  may be effectively coupled to the cap assembly  100  through external laser welding, etc. before and after coupling between the case  180  and the cap assembly  100 . 
     Meanwhile, in an embodiment of the present invention, the cap assembly  100  may be formed with the terminal portion on a central side corresponding to the center pin  210 .  FIG. 1  to  FIG. 3  illustrate the cap assembly  100  in which the terminal portion is formed at the central side thereof according to an embodiment of the present invention. 
     In the cap assembly  100 , the terminal portion corresponds to an area where the plastic layer  140  is not stacked, and even when the cap assembly  100  in which the plastic layer  140  is coupled to the metal layer  120  by injection molding or the like is directly coupled to the case  180 , the second electrode tab  234  may be positioned under the terminal portion where the metal layer  120  is exposed, and thus even when the plastic layer  140  uses the pre-stacked cap structure and precedes the coupling between the case  180  and the cap assembly  100 , it is advantageous because the second electrode tab  234  and the metal layer  120  may be welded to each other by a laser method or the like through the exposed outer surface of the terminal portion. 
     Meanwhile, in an embodiment of the present invention, a side surface of the metal layer  120  may be surrounded by the plastic layer  140 . That is, in the cap assembly  100 , a plate made of a metal material constituting the metal layer  120  may be coupled to the plastic layer  140  on a side surface thereof as well as an upper surface thereof. 
       FIG. 3  illustrates the plastic layer  140  surrounding the side surface of the metal layer  120  according to an embodiment of the present invention, and  FIG. 4 , which illustrates an enlarged area B of  FIG. 3 , shows the plastic layer  140  at a side of the metal layer  120 . 
     A surface of the metal layer  120  in contact with the plastic layer  140  may be etched by a laser or chemical method to improve bonding strength with the plastic layer  140 , and etching may be performed on the upper surface as well as the side surface of the metal layer  120  according to an embodiment of the present invention. 
     That is, according to an embodiment of the present invention, the plate constituting the metal layer  120  is coupled to the plastic layer  140  on the upper surface as well as on the side, thereby effectively improving a mutual bonding force. 
     Meanwhile, in the cap assembly  100 , the plastic layer  140  may extend outside a circumference of the metal layer  120  to form a first fusion portion  145 , and the first fusion portion  145  may be fused to the case  180 . 
     Referring to  FIG. 3  and  FIG. 4 , in the cap structure, an outer diameter of the plastic layer  140  is set to be larger than that of the metal layer  120 , and a portion of the plastic layer  140  that extends outside the circumference of the metal layer  120  corresponds to the first fusion portion  145 . 
     The first fusion portion  145  may be thermally fused to the case  180  while being melted by using a laser or the like. In the case  180 , the bonding force with the first fusion portion  145  may be improved by etching a surface of the case  180  to be bonded to plastic using a laser or a chemical agent. 
     According to an embodiment of the present invention, mutual coupling is effectively achieved while having an efficient electrical insulation structure between the cap structure and the case  180 , by forming the first fusion portion  145  made of only the plastic layer  140  by removing the metal layer  120  in an edge region of the cap structure. 
     Meanwhile, in an embodiment of the present invention, the case  180  may include a second fusion portion  190  of a plastic material surrounding an upper end of a side wall surrounding the opening, and the first fusion part  145  may be fused to the second fusion portion  190 . 
     In  FIG. 3  and  FIG. 4 , the case  180  having the second fusion portion  190  is illustrated, and an interface between the first fusion portion  145  and the second fusion portion  190  is represented by a dotted line to help understanding. 
     Referring to  FIG. 3  and  FIG. 4 , the upper surface of the case  180  may correspond to the opening, and the second fusion portion  190  made of plastic may be provided at at least an upper end of a sidewall surrounding the opening. The second fusion portion  190  may be coupled to an upper end of the sidewall of the case  180  by injection molding or the like. 
     Etching may be formed on a metal surface of the case  180  on which the second fusion portion  190  is formed using a laser or a chemical agent, and the second fusion portion  190  may be coupled to the case  180  by injection molding or the like. 
     Meanwhile, the first fusion portion  145  of the cap assembly  100  may be thermally fused to the second fusion portion  190  of the case  180  to be coupled to each other. When the first fusion portion  145  and the second fusion portion  190  are fused, an interface therebetween may disappear, but in  FIG. 3  and FIG.  4 , the interface between the first fusion portion  145  and the second fusion portion  190  is indicated by a dotted line for better understanding. 
     According to an embodiment of the present invention, plastic welding between the cap assembly  100  and the case  180  may be effectively performed by forming the second fusion portion  190  in a portion of the case  180  coupled to the plastic layer  140  of the cap assembly  100 . 
     Meanwhile, as illustrated in  FIG. 3  and  FIG. 4 , the second fusion portion  190  may be positioned on an upper surface and an outer surface of the sidewall except for an inner surface facing the electrode assembly  200  from an upper side thereof. 
     Forming the second fusion portion  190  on the inner surface of the sidewall of the case  180  is the same as reducing a diameter of the opening of the case  180  in the end, and movement of the electrode assembly  200  embedded through the opening may be prevented or a size of the electrode assembly  200  may be limited. 
     Therefore, according to an embodiment of the present invention, the second fusion portion  190  is formed on the upper surface and the outer surface of the case  180  except for the inner surface from the side surface thereof, but the present invention is not limited thereto, and the second fusion portion  190  may be formed in various areas as needed. 
     The cap assembly  100  may have an annular stepped groove  147  extending on the first fusion portion  145 , and the second fusion portion  190  may be seated in the stepped groove  147  and fused with the first fusion portion  145 . 
     Referring to  FIG. 4 , in the cap assembly  100 , the stepped groove  147  indicated by a dotted line may be formed in the first fusion portion  145 . The stepped groove  147  may be provided to correspond to a shape of the second fusion portion  190  so that the second fusion part  190  may stably contact the first fusion portion  145  when the cap assembly  100  is seated on the case  180 . 
       FIG. 4  illustrates the annular first fusion portion  145  extending along an edge of the cap assembly  100  according to an embodiment of the present invention, and referring to  FIG. 3 , the stepped groove  147  may be provided to extend along the first fusion portion  145  and to have an annular shape that is recessed inward from a lower portion of the first fusion portion  145 . 
     Meanwhile, the rechargeable battery according to an embodiment of the present invention may have a coin shape with a ratio of a diameter L to a height H of 1 or more. 
     That is, according to the present invention, the coin-type battery may be understood as a flat and thin battery having the ratio of the diameter L to the height H of 1 or more, and may be advantageous in realizing an ultra-small size. 
     That is, an embodiment of the present invention may be applied to a very small coin-type battery. Accordingly, it is possible to reduce the weight by using the plastic layer  140  in a space that is limited to a small size, while at the same time, it is possible to effectively secure an accommodation space for the case  180 , and it is possible to form the terminal portion while effectively reducing the components. 
     While the present invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the following claims. 
     
       
         
           
               
             
               
                   
               
               
                 &lt;Description of Symbols&gt; 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                   
                 100: cap assembly 
               
               
                   
                   
                 120: metal layer 
               
               
                   
                   
                 140: plastic layer 
               
               
                   
                   
                 145: first fusion portion 
               
               
                   
                   
                 147: step groove 
               
               
                   
                   
                 180: case 
               
               
                   
                   
                 190: second fusion portion 
               
               
                   
                   
                 200: electrode assembly 
               
               
                   
                   
                 210: center pin 
               
               
                   
                   
                 220: insulating member 
               
               
                   
                   
                 232: first electrode tab 
               
               
                   
                   
                 234: second electrode tab