Patent Publication Number: US-2022223985-A1

Title: Terminal component and secondary battery

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a terminal component and a secondary battery. The present application claims priority based on Japanese Patent Application No. 2021-003789 filed on Jan. 13, 2021, and the entire contents of the application are incorporated herein by reference. 
     2. Description of the Related Art 
     Japanese Patent Application Publication No. 2013-77546 discloses a secondary battery including a fuse in an electrode terminal. The secondary battery disclosed in this publication includes a portion that functions as a fuse in a current collecting member inside a battery case. It is considered that the portion that functions as a fuse can cut off electrical connection by melting a portion of the fuse that is narrower than the surrounding portions when an overcurrent flows through the current collecting member. 
     Japanese Patent Application Publication No. 2014-86177 discloses a pressure-type current cutoff device for a sealed battery provided with an inversion plate, the current cutoff device being arranged outside a battery case. It is considered that with the pressure-type current cutoff device disclosed in this publication, the inversion plate deforms in response to a pressure rise inside the battery case, thereby electrically cutting off the inversion plate from a terminal connected to the inversion plate. 
     SUMMARY OF THE INVENTION 
     Where a member used for a secondary battery includes a part serving as a fuse, the part serving as a fuse may have a higher electrical resistance than other parts. Therefore, when an electric current flows, the heat quantity generated in the part serving as a fuse can be larger than that in the other parts. The present inventor has found that where a part serving as a fuse is provided inside a battery case, heat may be trapped inside the battery case due to heat generation in that part. As a result, the temperature inside the battery case may rise, an electrolytic solution may be decomposed, and battery performance may be deteriorated. 
     The present inventor proposes a novel structure that functions as a fuse for a secondary battery. 
     A terminal component disclosed herein is to be attached to a battery case so that a part of the terminal component is connected to an internal terminal inside the battery case, and a part is exposed to the outside of the battery case, the terminal component including a first metal and a second metal overlapped on the first metal. The first metal has a part to be connected to the internal terminal, and the second metal has a part to be exposed to the outside of the battery case. At an interface where the first metal and the second metal are overlapped, one of the first metal and the second metal has a protrusion having a flat top portion. The other metal is joined to the top portion. A cross section of the protrusion orthogonal to a projection direction of the protrusion is set such that fusing occurs when a current equal to or higher than a predetermined current value flows between the first metal and the second metal. 
     In such a terminal component, the protrusion of one of the first metal and the second metal functions as a fuse. 
     The first metal and the second metal may be configured of different metals. The first metal and the second metal may be joined by metal joining at a distal end of the protrusion. 
     In a secondary battery including a battery case and an electrode terminal attached to the battery case, the electrode terminal may include a part configured of the terminal component described above. The protrusion of the terminal component may be provided on the outside of the battery case. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional view of a lithium ion secondary battery  10 ; 
         FIG. 2  is a cross-sectional view showing a II-II cross section of  FIG. 1 ; 
         FIG. 3  is a sectional view taken along line III-III of  FIG. 2 ; and 
         FIG. 4  is a cross-sectional view schematically showing a terminal component  200 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the terminal component and secondary battery disclosed herein will be described. The embodiment described herein is, of course, not intended to specifically limit the present disclosure. The present disclosure is not limited to the embodiment described herein, unless otherwise specified. Each drawing is schematically drawn and does not necessarily reflect the actual configuration. In addition, members and parts that perform the same action are designated, as appropriate, by the same reference numerals, and duplicate description thereof will be omitted. 
     Secondary Battery 
     In the present description, the “secondary battery” means a device capable of charging and discharging. The secondary battery is inclusive of a battery generally called a lithium ion battery, a lithium secondary battery, or the like, a lithium polymer battery, a lithium ion capacitor, or the like. Here, a lithium ion secondary battery will be illustrated as a form of the secondary battery. 
     Lithium-Ion Secondary Battery  10   
       FIG. 1  is a partial cross-sectional view of a lithium ion secondary battery  10 . In  FIG. 1 , a state in which the inside is exposed is drawn along a wide surface on one side of a substantially rectangular parallelepiped battery case  41 . The lithium ion secondary battery  10  shown in  FIG. 1  is a so-called sealed battery.  FIG. 2  is a cross-sectional view showing a II-II cross section of  FIG. 1 . In  FIG. 2 , a partial cross-sectional view of a substantially rectangular parallelepiped battery case  41  in a state where the inside is exposed along a narrow surface on one side is schematically drawn. 
     As shown in  FIG. 1 , the lithium ion secondary battery  10  includes an electrode body  20 , a battery case  41 , a positive electrode terminal  42 , and a negative electrode terminal  43 . 
     Electrode Body  20   
     The electrode body  20  is accommodated in the battery case  41  in a state of being covered with an insulating film (not shown) or the like. The electrode body  20  includes a positive electrode sheet  21  as a positive electrode element, a negative electrode sheet  22  as a negative electrode element, and separator sheets  31  and  32  as separators. The positive electrode sheet  21 , the first separator sheet  31 , the negative electrode sheet  22 , and the second separator sheet  32  are long strip-shaped members, respectively. 
     In the positive electrode sheet  21 , a positive electrode active material layer  21   b  including a positive electrode active material is formed on both sides of a positive electrode current collecting foil  21   a  (for example, an aluminum foil) having a predetermined width and thickness, except for a non-formation portion  21   a   1  that is set to a constant width at one end in the width direction. For example, in a lithium ion secondary battery, the positive electrode active material is a material capable of releasing lithium ions during charging and absorbing lithium ions during discharging, such as a lithium transition metal composite material. Various positive electrode active materials have been generally proposed in addition to the lithium transition metal composite material, and the type of the positive electrode active material is not particularly limited. 
     In the negative electrode sheet  22 , a negative electrode active material layer  22   b  including a negative electrode active material is formed on both sides of a negative electrode current collecting foil  22   a  (here, a copper foil) having a predetermined width and thickness, except for a non-formation portion  22   a   1  that is set to a constant width at one end in the width direction. For example, in a lithium ion secondary battery, the negative electrode active material is a material capable of occluding lithium ions during charging and releasing the occluded lithium ions during discharging, such as natural graphite. Various negative electrode active materials have been generally proposed in addition to natural graphite, and the type of the negative electrode active material is not particularly limited. 
     For the separator sheets  31  and  32 , for example, a porous resin sheet which has a required heat resistance and through which an electrolyte can pass is used. Various separator sheets have been proposed for the separator sheets  31  and  32 , and the type thereof is not particularly limited. 
     Here, the negative electrode active material layer  22   b  is formed, for example, to be wider than the positive electrode active material layer  21   b . The width of the separator sheets  31  and  32  is larger than that of the negative electrode active material layer  22   b . The non-formation portion  21   a   1  of the positive electrode current collecting foil  21   a  and the non-formation portion  22   a   1  of the negative electrode current collecting foil  22   a  are directed to opposite sides in the width direction. Further, the positive electrode sheet  21 , the first separator sheet  31 , the negative electrode sheet  22 , and the second separator sheet  32  are oriented in the length direction, stacked in this order and wound. The negative electrode active material layer  22   b  covers the positive electrode active material layer  21   b  with the separator sheets  31  and  32  interposed therebetween. The negative electrode active material layer  22   b  is covered with separator sheets  31  and  32 . The non-formation portion  21   a   1  of the positive electrode current collecting foil  21   a  protrudes from one side of the separator sheets  31  and  32  in the width direction. The non-formation portion  22   a   1  of the negative electrode current collecting foil  22   a  protrudes from the separator sheets  31  and  32  on the opposite side in the width direction. 
     As shown in  FIG. 1 , the above-described electrode body  20  is flattened along one plane including the winding axis so as to be accommodated in the case body  41   a  of the battery case  41 . The non-formation portion  21   a   1  of the positive electrode current collecting foil  21   a  is arranged on one side, and the non-formation portion  22   a   1  of the negative electrode current collecting foil  22   a  is arranged on the opposite side along the winding axis of the electrode body  20 . 
     Battery Case  41   
     As shown in  FIG. 1 , the electrode body  20  is accommodated in the battery case  41 . The battery case  41  has a case body  41   a  having a substantially rectangular parallelepiped angular shape with one side open, and a lid  41   b  mounted on the opening. In this embodiment, the case body  41   a  and the lid  41   b  are formed of aluminum or an aluminum alloy mainly composed of aluminum, from the viewpoint of weight reduction and ensuring the required rigidity. 
     Case Body  41   a    
     As shown in  FIGS. 1 and 2 , the case body  41   a  has a substantially rectangular parallelepiped angular shape with one side open. The case body  41   a  has a substantially rectangular bottom surface portion  61 , a pair of wide surface portions  62  and  63 , and a pair of narrow surface portions  64  and  65 . Each of the pair of wide surface portions  62  and  63  rises from the long side of the bottom surface portion  61 . Each of the pair of narrow surface portions  64  and  65  rises from the short side of the bottom surface portion  61 . An opening  41   a   1  surrounded by a pair of wide surface portions  62  and  63  and a pair of narrow surface portions  64  and  65  is formed on one side surface of the case body  41   a.    
     Lid  41   b    
     The lid  41   b  is mounted on the opening  41   a   1  of the case body  41   a  surrounded by the long sides of the pair of wide surface portions  62  and  63  and the short sides of the pair of narrow surface portions  64  and  65 . The peripheral edge of the lid  41   b  is joined to the edge of the opening  41   a   1  of the case body  41   a . Such joining may be performed by, for example, continuous welding with no gaps. Such welding can be achieved, for example, by laser welding. 
     In this embodiment, a positive electrode terminal  42  and a negative electrode terminal  43  are attached to the lid  41   b . The positive electrode terminal  42  includes an internal terminal  42   a  and an external terminal  42   b . The negative electrode terminal  43  includes an internal terminal  43   a  and an external terminal  43   b . The internal terminals  42   a  and  43   a  are attached to the inside of the lid  41   b  with an insulator  72  interposed therebetween. The external terminals  42   b  and  43   b  are attached to the outside of the lid  41   b  with a gasket  71  interposed therebetween. The internal terminals  42   a  and  43   a  extend inside the case body  41   a . The internal terminal  42   a  of the positive electrode is connected to the non-formation portion  21   a   1  of the positive electrode current collecting foil  21   a . The internal terminal  43   a  of the negative electrode is connected to the non-formation portion  22   a   1  of the negative electrode current collecting foil  22   a.    
     As shown in  FIG. 1 , the non-formation portion  21   a   1  of the positive electrode current collecting foil  21   a  of the electrode body  20  and the non-formation portion  22   a   1  of the negative electrode current collecting foil  22   a  are attached to the internal terminals  42   a  and  43   a  that are attached to both sides of the lid  41   b  in the longitudinal direction. The electrode body  20  is accommodated in the battery case  41  in a state of being attached to the internal terminals  42   a  and  43   a  attached to the lid  41   b . Here, the wound electrode body  20  is illustrated by way of example. The structure of the electrode body  20  is not limited to such a form. The structure of the electrode body  20  may be, for example, a laminated structure in which a positive electrode sheet and a negative electrode sheet are alternately laminated with a separator sheet interposed therebetween. Further, a plurality of electrode bodies  20  may be accommodated in the battery case  41 . 
       FIG. 3  is a sectional view taken along line III-III of  FIG. 2 .  FIG. 3  shows a cross section of a part where the negative electrode terminal  43  is attached to the lid  41   b . In this embodiment, a member in which dissimilar metals are joined is used for the external terminal  43   b  of the negative electrode. In  FIG. 3 , the cross-sectional shape of the external terminal  43   b  is schematically shown without showing the structure of the metals constituting the external terminal  43   b , the interface between the dissimilar metals, the gap between the dissimilar metals, or the like. 
     As shown in  FIG. 3 , the lid  41   b  has an attachment hole  41   b   1  for attaching the external terminal  43   b  of the negative electrode. The attachment hole  41   b   1  penetrates the lid  41   b  at a predetermined position of the lid  41   b . The internal terminal  43   a  and the external terminal  43   b  of the negative electrode are attached to the attachment hole  41   b   1  of the lid  41   b  with the gasket  71  and the insulator  72  interposed therebetween. On the outside of the attachment hole  41   b   1 , a step  41   b   2  on which the gasket  71  is mounted is provided around the attachment hole  41   b   1 . The step  41   b   2  is provided with a seat surface  41   b   3  on which the gasket  71  is arranged. The seat surface  41   b   3  is provided with a projection  41   b   4  for positioning the gasket  71 . 
     Here, as shown in  FIG. 3 , the external terminal  43   b  of the negative electrode includes a head  43   b   1 , a shaft  43   b   2 , and a caulking piece  43   b   3 . The head  43   b   1  is a part arranged outside the lid  41   b . The head  43   b   1  is a part that is larger than the attachment hole  41   b   1  and arranged at the gasket  71 . The shaft  43   b   2  is a part mounted in the attachment hole  41   b   1  with the gasket  71  interposed therebetween. The shaft  43   b   2  protrudes downward from a substantially central portion of the head  43   b   1 . As shown in  FIG. 3 , the caulking piece  43   b   3  is a part caulked to the internal terminal  43   a  of the negative electrode inside the lid  41   b . The caulking piece  43   b   3  extends from the shaft  43   b   2  and is bent and caulked to the internal terminal  43   a  of the negative electrode after being inserted into the lid  41   b.    
     Gasket  71   
     As shown in  FIG. 3 , the gasket  71  is a member attached to the attachment hole  41   b   1  and the seat surface  41   b   3  of the lid  41   b . In this embodiment, the gasket  71  includes a seat  71   a , a boss  71   b , and a side wall  71   c . The seat  71   a  is apart mounted on the seat surface  41   b   3  provided on the outer surface around the attachment hole  41   b   1  of the lid  41   b . The seat  71   a  has a substantially flat surface corresponding to the seat surface  41   b   3 . The seat  71   a  is provided with a depression corresponding to the projection  41   b   4  of the seat surface  41   b   3 . The boss  71   b  projects from the bottom surface of the seat  71   a . The boss  71   b  has an outer shape along the inner side surface of the attachment hole  41   b   1  so as to be mounted on the attachment hole  41   b   1  of the lid  41   b . The inner surface of the boss  71   b  serves as a mounting hole for mounting the shaft  43   b   2  of the external terminal  43   b . The side wall  71   c  rises upward from the peripheral edge of the seat  71   a . The head  43   b   1  of the external terminal  43   b  is mounted on a part surrounded by the side wall  71   c  of the gasket  71 . 
     The gasket  71  is arranged between the lid  41   b  and the external terminal  43   b  to ensure insulation between the lid  41   b  and the external terminal  43   b . Further, the gasket  71  ensures the airtightness of the attachment hole  41   b   1  of the lid  41   b . From this point of view, it is preferable to use a material having excellent chemical resistance and weather resistance. In this embodiment, PFA is used for the gasket  71 . PFA is a copolymer of tetrafluoroethylene and perfluoroalkoxyethylene (Tetrafluoroethylene Perfluoroalkylvinylether Copolymer). The material used for the gasket  71  is not limited to PFA. 
     Insulator  72   
     The insulator  72  is a member mounted inside the lid  41   b  around the attachment hole  41   b   1  of the lid  41   b . The insulator  72  includes a base  72   a , a hole  72   b , and a side wall  72   c . The base  72   a  is a part arranged along the inner surface of the lid  41   b . In this embodiment, the base  72   a  is a substantially flat plate-shaped part. The base  72   a  is arranged along the inner side surface of the lid  41   b , and has a size such that the base does not protrude from the lid  41   b  so that it can be housed in the case body  41   a . The hole  72   b  is provided correspondingly to the attachment hole  41   b   1 . In this embodiment, the hole  72   b  is provided in a substantially central portion of the base  72   a . On the side surface of the lid  41   b  facing the inner side surface, a recessed step  72   b   1  is provided around the hole  72   b . The step  72   b   1  accommodates the distal end of the boss  71   b  of the gasket  71  mounted in the attachment hole  41   b   1 . The side wall  72   c  rises downward from the peripheral edge of the base  72   a . A proximal portion  43   a   1  provided at one end of the internal terminal  43   a  of the negative electrode is accommodated in the base  72   a . Since the insulator  72  is arranged inside the battery case  41 , it is preferable that the insulator  72  have a required chemical resistance. In this embodiment, PPS is used for the insulator  72 . PPS is a polyphenylene sulfide resin. The material used for the insulator  72  is not limited to PPS. 
     The internal terminal  43   a  of the negative electrode includes the proximal portion  43   a   1  and a connection piece  43   a   2  (see  FIGS. 1 and 2 ). The proximal portion  43   a   1  is a part mounted on the base  72   a  of the insulator  72 . In this embodiment, the proximal portion  43   a   1  has a shape corresponding to the inside of the side wall  72   c  around the base  72   a  of the insulator  72 . As shown in  FIGS. 1 and 2 , the connection piece  43   a   2  extends from one end of the proximal portion  43   a   1  and extends into the case body  41   a  to be connected to the non-formation portion  22   a   1  of the negative electrode of the electrode body  20 . 
     In this embodiment, the gasket  71  is attached to the outside of the lid  41   b  while the boss  71   b  is being mounted on the attachment hole  41   b   1 . The external terminal  43   b  is mounted on the gasket  71 . At this time, the shaft  43   b   2  of the external terminal  43   b  is inserted into the boss  71   b  of the gasket  71 , and the head  43   b   1  of the external terminal  43   b  is arranged on the seat  71   a  of the gasket  71 . The insulator  72  and the internal terminal  43   a  are attached to the inside of the lid  41   b . As shown in  FIG. 3 , the caulking piece  43   b   3  of the external terminal  43   b  is bent and caulked to the proximal portion  43   a   1  of the internal terminal  43   a . The caulking piece  43   b   3  of the external terminal  43   b  and the proximal portion  43   a   1  of the negative electrode terminal  43  may be partially metal-joined in order to improve conductivity. 
     For the internal terminal  42   a  of the positive electrode of the lithium ion secondary battery  10 , the required level of oxidation-reduction resistance is not higher than that of the negative electrode. From the viewpoint of required oxidation-reduction resistance and weight reduction, aluminum can be used for the internal terminal  42   a  of the positive electrode. By contrast, for the internal terminal  43   a  of the negative electrode, the required level of oxidation-reduction resistance is higher than that of the positive electrode. From this point of view, copper may be used for the internal terminal  43   a  of the negative electrode. Meanwhile, as the bus bar to which the external terminal  43   b  is connected, aluminum or an aluminum alloy may be used from the viewpoint of weight reduction and cost reduction. 
     The present inventor has studied the use of copper or copper alloy for a part of the external terminal  43   b  that is joined to the internal terminal  43   a , and the use of aluminum or an aluminum alloy for a part of the external terminal  43   b  that is connected to the bus bar. In order to realize such a structure, in this embodiment, a member obtained by dissimilar metal joining of copper and aluminum is used as the external terminal  43   b . The structure of the terminal component  200  used as the external terminal  43   b  will be described hereinbelow. 
     Terminal Component  200   
       FIG. 4  is a cross-sectional view schematically showing the terminal component  200 . As shown in  FIGS. 1 and 2 , the terminal component  200  is attached to the battery case  41  so that a part of the terminal component is connected to the internal terminal  43   a  inside the battery case  41 , and a part is partially exposed to the outside of the battery case  41 . 
     The terminal component  200  includes a first metal  201  and a second metal  202  overlapped on the first metal  201 . A part of the first metal  201  is connected to the internal terminal  43   a  inside the battery case  41 . The second metal  202  is exposed to the outside of the battery case  41 . The first metal  201  and the second metal  202  are configured of different metals. 
     A part of the first metal  201  is connected to the internal terminal  43   a  inside the battery case  41  when the terminal component  200  is used as the external terminal  43   b . In this embodiment, the first metal is configured of copper. The first metal  201  has a shaft  201   a  and a flange  201   b . The shaft  201   a  is a part serving as the shaft  43   b   2  to be inserted into the attachment hole  41   b   1  of the lid  41   b . The flange  201   b  is a substantially rectangular flat plate-shaped part that is provided at one end of the shaft  201   a  and is wider than the shaft  201   a . The shaft  201   a  is provided with a part  201   c  that serves as the caulking piece  43   b   3  that is to be further caulked to the internal terminal  43   a  on the side opposite to the side where the flange  201   b  is provided. 
     The second metal  202  is a part exposed to the outside of the battery case  41  when the terminal component  200  is used as the external terminal  43   b . In this embodiment, the second metal is configured of aluminum. 
     In this embodiment, the first metal  201  includes a protrusion  201   d  having a flat top portion  201   d   1 . The protrusion  201   d  is provided at the center of a facing surface  201   b   1  of the flange  201   b . The protrusion  201   d  is a substantially disk-shaped part. 
     The second metal  202  is a flat plate-shaped metal member overlapped on the first metal  201 . The second metal  202  has a substantially rectangular shape in which the facing surface  202   a  facing the first metal  201  corresponds to the facing surface  201   b   1  of the first metal  201 . 
     The second metal  202  is joined to the top portion  201   d   1  of the protrusion  201   d  of the first metal  201 . In this embodiment, the first metal  201  and the second metal  202  are metal-joined at the top portion  201   d   1  of the protrusion  201   d  of the first metal  201 . The metal-joined joint portion  203  is formed at the top portion  201   d   1  and the central portion of the facing surface  202   a . The method of joining the top portion  201   d   1  of the protrusion  201   d  and the second metal  202  is not particularly limited, and for example, the joining can be performed by a method such as ultrasonic pressure welding, friction welding, resistance welding, and the like. The joint portion  203  joined in this way is formed by solid-phase joining without using an adhesive layer of an adhesive or a solder. 
     In this embodiment, a region other than the protrusion  201   d  is a gap between the facing surface  201   b   1  of the first metal  201  and the facing surface  202   a  of the second metal  202 . However, the region between the facing surface  201   b   1  and the facing surface  202   a  is not limited to such a form. A member that does not electrically connect the first metal  201  and the second metal  202  may be arranged in the region. For example, when a vibration is applied to the lithium ion secondary battery  10 , the vibration may be transmitted to the terminal component  200  via a bus bar. In order to alleviate the concentration of such an external load on the protrusion  201   d , for example, a gasket  71  or the like may be arranged in the region. Such a member that does not electrically connect the first metal  201  and the second metal  202  may be partially or entirely arranged in the region. 
     When the lithium ion secondary battery  10  is charged or discharged, a current flows through the terminal component  200  used as the external terminal  43   b . At this time, a current also flows through the first metal  201  and the second metal  202 . As shown in  FIG. 4 , the protrusion  201   d  of the first metal  201  has a narrower cross-sectional area through which an electric current passes than surrounding portions. As a result, when a current flows through the terminal component  200  due to charging or discharging of the lithium ion secondary battery  10 , the current is concentrated in the protrusion  201   d . In this way, the amount of Joule heat generated in the portion where the current is concentrated is larger than in the other portions. Further, the joint portion  203  is formed with a dissimilar metal joint in which the first metal  201  and the second metal  202  are joined. Therefore, when a current flows through the terminal component  200 , the amount of Joule heat generated in in the protrusion  201   d  and the joint portion  203  is larger than in the surrounding portions. 
     Here, the cross section of the protrusion  201   d  that is orthogonal to the projection direction of the protrusion  201   d  is set such that fusing occurs when a current equal to or higher than a predetermined current value flows between the first metal  201  and the second metal  202 . Here, the projection direction is a direction perpendicular to the facing surface  201   b   1  provided with the protrusion  201   d.    
     Here, the predetermined current value is set based on, for example, a peak current value in the normal usage mode of the battery. Although not limited to this, the predetermined current value can be set to twice or more the above-mentioned peak current value. 
     Fusing occurring when a current equal to or higher than the predetermined current value flows is, for example, a process in which one of the first metal  201  and the second metal  202  is melted, thereby electrically separating the first metal and the second metal from each other in the protrusion. 
     The fusing referred to herein may occur when the protrusion  201   d  reaches the melting point and melts, or when the protrusion  201   d  does not melt and the other second metal  202  melts at the joint portion  203 . 
     Since the cross section of the protrusion  201   d  orthogonal to the projection direction is set to such a cross section, the protrusion  201   d  functions as a fuse that cuts off the electrical connection of the first metal  201  and the second metal  202  when an overcurrent occurs. 
     In the terminal component  200  disclosed herein, the dimensions of the protrusion  201   d  that functions as a fuse can be set, as appropriate, according to the assumed overcurrent, metal types of the first metal  201  and the second metal  202 , and the like. An example will be described below. 
     Described hereinbelow is the setting of the dimensions of the protrusion  201   d  when a cut-off current is 940 A and an energizing time is 100 sec. The first metal  201  having the disk-shaped protrusion  201   d  is configured of copper, and the second metal  202  is configured of aluminum. A case will be considered in which the second metal  202 , which has a melting point lower than that of the first metal  201 , melts when the above-mentioned current flows through the protrusion  201   d  of the first metal  201 . 
     First, the conditions under which the second metal  202 , which has a melting point lower than that of the first metal  201 , melts will be considered. The heat quantity Q m  required for the second metal  202  to melt is represented by Q m =m×c×ΔT by using the mass m, the specific heat c, and the temperature difference ΔT. 
     The melting point of aluminum constituting the second metal  202  is 660.3° C. When the room temperature is 25° C., the temperature difference ΔT required to melt the second metal  202  is 635.3° C. The specific gravity ρ of aluminum is 2.7 g/cm 3 . The specific heat c of aluminum is 0.9 J/(g·° C.). Let S be the area of the joint portion  203 . In this embodiment, in the joint portion  203 , the second metal of 0.1 mm is melted, and the first metal  201  and the second metal  202  are disconnected. 
     The heat quantity Q m  for melting the second metal  202  is expressed by the following formula. 
         Q   m   =S× 0.1 (mm)×2.7 (g/cm 3 )×0.9 (J/(g·° C.))×635.3 (° C.)
 
     Next, the Joule heat generated at the protrusion  201   d  of the first metal  201  will be considered. The Joule heat Q h  generated at the protrusion  201   d  of the first metal  201  is represented by Q h =R×I 2 ×t by using the resistance value R, the current value I, and the energization time t. 
     As described above, the assumed current value I is 940 A. The energization time t is 100 sec. The resistance value R is represented by R=ρ v ×L/S by using the volume resistivity ρ v , the length L through which the current flows, and the cross-sectional area S through which the current flows. For example, at 20° C., the volume resistivity ρ v  of copper constituting the first metal  201  is 1.69 μΩ·cm. The length L through which the current flows is the height of the protrusion  201   d.    
     The Joule heat Q h  generated at the protrusion  201   d  of the first metal  201  is expressed by the following formula. 
         Q   h =1.69 (μΩ·cm)× L/S ×(940 (Å)) 2 ×100 (sec)
 
     When the Joule heat Q h  generated at the protrusion  201   d  of the first metal  201  is higher than the heat quantity Q m  required for melting the second metal  202 , fusing occurs at the protrusion  201   d . That is, the cross-sectional area and height of the protrusion  201   d  of the first metal  201  can be set so as to satisfy Q m &lt;Q h . 
     For example, the height of the protrusion  201   d  of the first metal  201  can be 0.1 mm and the diameter can be 6 mm. At this time, when the above current flows, a Joule heat of 5.3 J is generated at the protrusion  201   d , and the second metal  202  in contact with the protrusion  201   d  is melted to cause fusing. 
     Contact resistance is generated in the joint portion  203  because the joining interface is obtained by joining dissimilar metals. Further, as the temperature of the first metal  201  rises, the volume resistivity of the protrusion  201   d  rises. That is, more Joule heat can be generated at the joint portion  203  than in the above calculation. For example, the dimensions of the protrusion  201   d  may be adjusted, as appropriate, by using computer simulation or by performing a preliminary test using a sample simulating the structure of the terminal component  200 . 
     The terminal component  200  proposed herein is provided with a protrusion  201   d  at the joining interface between the first metal  201  and the second metal  202 . The first metal  201  and the second metal  202  are joined at the top portion  201   d   1  of the protrusion  201   d . As described above, the cross section of the protrusion  201   d  that is orthogonal to the projection direction of the protrusion  201   d  is set in at least a part thereof such that fusing occurs when a current equal to or higher than a predetermined current value flows. In this case, the terminal component  200  is fused when a current equal to or higher than a predetermined current value flows. Therefore, the terminal component  200  can have a function as a fuse. 
     The protrusion  201   d  may be configured to be provided outside the battery case  41 . With this configuration, even when the current is concentrated in the protrusion  201   d  and a large Joule heat is generated as compared with the surroundings, the temperature inside the battery case  41  is not affected. Therefore, as compared with the case where a fuse is provided inside the battery case  41 , it is possible to suppress the decomposition of an electrolytic solution inside the battery case  41  due to the heat generated by the protrusion  201   d . In other words, it is possible to suppress deterioration of battery performance due to heat generation in a part that functions as a fuse. 
     In the terminal component  200  disclosed herein, the first metal  201  and the second metal  202  may be configured of different metals. For example, by configuring the first metal  201  of the same metal type as the metal type of the bus bar, the joining strength between the first metal  201  and the joining interface of the bus bar can be increased. By using the same metal type as the internal terminal  43   a  for the second metal  202 , the joining strength between the second metal  202  and the internal terminal  43   a  can be increased. In this way, by using dissimilar metals for the first metal  201  having a part connected to the internal terminal and the second metal  202  having a part exposed to the outside of the battery case, it is possible to exclude the possibility of a dissimilar metal joint location being provided at the bus bar joining interface outside the battery case. 
     A metal joint made of dissimilar metals is formed at the joint portion  203  in which different metals are joined. Such a joint portion has a higher electrical resistance than a joint portion composed of the same type of metal or a portion formed by thinning a part of one type of metal. Greater Joule heat is generated at the joint portion  203  where different metals are joined. When the terminal component  200  configured of different metals and a terminal component configured of the same metal are set to the same cut-off current, the protrusion  201   d  of the terminal component  200  configured of different metals can have a thicker and shorter shape. In other words, it is possible to realize stronger mechanical strength while imparting a fuse function to the protrusion  201   d.    
     In the terminal component  200  disclosed herein, the first metal  201  and the second metal  202  are metal-joined at the top portion  201   d   1  of the protrusion. The first metal  201  and the second metal  202  are joined by so-called solid-phase joining without interposing an intermediate layer such as a solder or a brazing material. By joining the first metal  201  and the second metal  202  without interposing an intermediate layer in this way, good conduction between the first metal  201  and the second metal  202  is ensured during normal use of the battery as well. 
     In the above-described embodiment, the first metal  201  includes the protrusion  201   d , but this embodiment is not limiting. The second metal may have a protrusion having a flat top portion, and the first metal may be joined to the protrusion. At this time, the surface of the first metal facing the second metal is preferably a flat surface. 
     In the above-described embodiment, the protrusion has a disk shape, but this embodiment is not limiting. The protrusion may be a polygonal flat plate-shaped part such as a quadrangular prism. Further, the cross-sectional shape and area of the protrusion do not have to be constant. The cross section of the protrusion may be tapered from the proximal end of the protrusion toward the top portion. The cross section of the protrusion may be thickened from the proximal end of the protrusion toward the top portion. 
     The terminal component and secondary battery disclosed herein have been described in various ways. Unless otherwise specified, the embodiments of the terminal component and battery mentioned herein do not limit the present disclosure. Further, the secondary battery disclosed herein can be variously modified, and constituent elements thereof and processes referred to herein can be omitted, as appropriate, or combined, as appropriate, unless a specific problem occurs.