Patent Publication Number: US-2022238965-A1

Title: Secondary battery and assembled battery

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of foreign priority to Japanese Patent Application No. 2021-11630, filed on Jan. 28 2021, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a secondary battery provided with an electrode terminal and an assembled battery including a plurality of secondary batteries. 
     2. Description of the Related Art 
     Secondary batteries such as lithium ion secondary batteries and nickel hydrogen batteries are currently used in a wide variety of fields such as vehicles and mobile devices. A secondary battery normally has electrode bodies, which are power generation elements, and a battery case for housing the electrode bodies. Furthermore, a structure in which electrode terminals connected to the electrode bodies in a battery case are led out to the case exterior is adopted in such a secondary battery. Then, the electrode terminals led out to the case exterior are connected to the exterior apparatus, another battery, or the like via the exterior connection member such as a bus bar. 
     A structure disclosed in JP 2011-124024 A is mentioned as one example of a conventional technique relating the electrode terminals. The electrode terminal disclosed in JP 2011-124024 A includes an exterior terminal with a good weld quality to a bus bar, and a base part having one end joined to an exterior terminal and the other end connected to an electrode body. In the technique disclosed in JP 2011-124024 A, joining the base part and the exterior terminal by ultrasonication is recommended. Joining the constitution members of electrode terminals by metal-to-metal joining using ultrasonication or the like enables to provide suitable joint strength and improve the conductivity at the interface between the members. 
     SUMMARY 
     However, the study made by the present inventor revealed that techniques to join the constitution members of electrode terminals by metal-to-metal joining have room for improvement. Specifically, thermal welding such as laser welding is normally used for connecting an electrode terminal led out to the exterior of a battery case and a bus bar. When large heat upon such thermal welding is transmitted to the joining mark (metal junction part) formed by metal-to-metal joining, the metal junction part may be deteriorated by heat, and the joint strength and conductivity may be lower in some cases. 
     The present disclosure has been made to solve such problems and has an object to provide a technique for suppressing thermal deterioration of metal junction parts formed on contact surfaces of constitution members of electrode terminals. 
     To reach the above objective, the technique disclosed herein provides a secondary battery having the constitution described below. 
     The secondary battery disclosed herein includes: an electrode body; a battery case housing the electrode body; and an electrode terminal connected to the electrode body in the battery case and partly exposed to the exterior of the battery case. The electrode terminals of such a secondary battery include a first member having one end positioned in the battery case and another end positioned outside the battery case and a plate-shaped second member connected to the first member at an outside of the battery case. Furthermore, the first member includes a shaft part penetrating the battery case and a plate-shaped connection part that is formed on a top edge of the shaft part and in surface contact with the second member. Then, the secondary battery disclosed herein includes a metal junction part and a heat-insulating part formed on a contact interface between a connection part of the first member and the second member, and the heat-insulating part is formed outward of the metal junction part in a radial direction centered on the metal junction part. 
     In the secondary battery disclosed herein, a metal junction part is formed on the contact interface between the first member and the second member, which are constitution members of electrode terminals. Such a metal junction part is a joining mark by metal-to-metal joining such as ultrasonic joining, laser welding, or resistance welding. Then, in the secondary battery disclosed herein, a heat-insulating part is formed outward of the metal junction part in a radial direction centered on the metal junction part. Due to this constitution, the heat-insulating part can block great heat (for example, welding heat or the like) even when such a great heat is applied to a region outside the heat-insulating part, and therefore, the heat deterioration of the metal junction part can be suppressed. 
     In a suitable embodiment of the secondary battery disclosed herein, the heat-insulating part is a cavity formed by a part of a bottom surface of the second member separating from a top surface of the connection part. According to such a constitution, the thermal deterioration of the metal junction part can be suppressed at a low cost. 
     In a suitable embodiment of the secondary battery disclosed herein, the heat-insulating part is a heat-insulating material filled in a space between a bottom surface of the connection part and a top surface of the second member. According to such a constitution, the thermal deterioration of the metal junction part can be more suitably suppressed. 
     In a suitable embodiment of the secondary battery disclosed herein, the heat-insulating part is a ring-shaped heat-insulating part formed so as to surround the metal junction part in plan view Due to this constitution, the thermal deterioration of the metal junction part can be more suitably suppressed. 
     In a suitable embodiment of the secondary battery disclosed herein, the first member and the second member are constituted of different metal materials. Then, the technique disclosed herein can be particularly suitably used for joining dissimilar metals, in which metal bonding is recommended because the thermal deterioration of the metal junction part can be suppressed. 
     As another aspect of the technique disclosed herein, an assembled battery including a plurality of secondary batteries mutually electrically connected via a flat plate-shaped bus bar is provided. In the assembled battery disclosed herein, at least one of the plurality of secondary batteries is the secondary battery having the above structure. The bus bar is in surface contact with a top surface of the second member, and the bus bar and the second member are connected by a thermally welded part penetrating the bus bar to reach the second member. Then, in the assembled battery disclosed herein, the thermally welded part is formed upward of the heat-insulating part or outward of the heat-insulating part in a radial direction centered on the metal junction part. Due to this constitution, the heat generated upon forming the thermally welded part is blocked by the heat-insulating part, and therefore, thermal deterioration of the metal junction part upon connecting the bus bar can be suitably suppressed. 
     In a suitable embodiment of the assembled battery disclosed herein, the heat-insulating part is a cavity formed by a part of the bottom surface of the second member separating from the top surface of the connection part. According to such a constitution, the thermal deterioration of the metal junction part can be suppressed at a low cost. 
     In an embodiment in which a cavity is disposed as the heat-insulating part, assuming that, in plan view, a distance from a center of the metal junction part to an outer periphery of the metal junction part is taken as L 1 , a distance from a center of the metal junction part to an inner edge of the heat-insulating part is taken as L 2 , and a distance from a center of the metal junction to a center of the thermally welded part is taken as L 3 , it is preferred to satisfy the following formula (1). Due to this constitution, the heat generated upon forming the thermally welded part can be sufficiently blocked by the heat-insulating part, and therefore, the thermal deterioration of the metal junction part can be more suitably suppressed. 
         L 3&gt; L 2&gt; L 2− L 1   (1)
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view schematically illustrating a secondary battery according to one embodiment; 
         FIG. 2  is a sectional view schematically illustrating the vicinity of the electrode terminal of a secondary battery according to one embodiment; 
         FIG. 3  is a plan view explaining the positional relationship between the metal junction part and the heat-insulating part in the secondary battery according to one embodiment: 
         FIG. 4  is a perspective view schematically illustrating an assembled battery according to one embodiment; 
         FIG. 5  is a sectional view schematically illustrating the structure of a connection part between an electrode terminal and a bus bar in the assembled battery according to one embodiment; 
         FIG. 6  is a plan view explaining the positional relationship among the metal junction part, the heat-insulating pan, and the thermally welded part in the assembled battery according to one embodiment; 
         FIG. 7  is a plan view explaining the positional relationship among the metal junction part, the heat-insulating part, and the thermally welded part in an assembled battery according to another embodiment; 
         FIG. 8  is a plan view explaining the positional relationship among the metal junction part, the heat-insulating part, and the thermally welded part in the assembled battery according to another embodiment; 
         FIG. 9  is a plan view explaining the positional relationship among the metal junction part, the heat-insulating part, and the thermally welded part in the assembled battery according to another embodiment; 
         FIG. 10  is a plan view explaining the positional relationship among the metal junction part, the heat-insulating part, and the thermally welded part in an assembled battery according to another embodiment; and 
         FIG. 11  is a plan view explaining the positional relationship among the metal junction part, the heat-insulating part, and the thermally welded part in an assembled battery according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Now, one embodiment of the technique disclosed herein is described below. Note that the following embodiment is not intended to limit the technique disclosed herein. Matters other than those specifically mentioned in the description but necessary for implementing the technique disclosed herein may be recognized as design matters for a person skilled in the art based on conventional techniques in the art. That is, the technique disclosed herein can be implemented based on the content disclosed in the present description and common general technical knowledge in the art. 
     In the drawings referred to in the explanation below, members and sites having the same effect are assigned with the same numerals or symbols. The dimensional relationships (lengths, widths, thicknesses, etc.) in each figure do not reflect actual dimensional relationships. The symbol X represents a “width direction”, the symbol Y represents a “depth direction”, and the symbol Z represents a “height direction” in each figure. However, these directions are defined for explanatory convenience and are not intended to limit the mode of installation of the secondary battery or the assembled battery during use or production. 
     In addition, the term “secondary battery” in the present description refers to a power storage device in general that causes a discharging and charging reaction by the transfer of a charge carrier between a pair of electrodes (positive and negative electrodes) via electrolytes. Such secondary batteries encompass a so-called storage battery such as a secondary lithium battery, a nickel hydrogen battery, and a nickel cadmium battery, and a capacitor such as an electric double layer capacitor. The technique disclosed herein is not limited to a specific type of secondary battery and can be applied to all secondary batteries having electrode terminals and assembled batteries constructed using such secondary batteries without particular limitations. 
     1. Secondary Battery 
     Hereinafter, the structure of the secondary battery according to the present embodiment is described with reference to  FIGS. 1 to 3 .  FIG. 1  is a perspective view schematically illustrating a secondary battery according to the present embodiment.  FIG. 2  is a sectional view schematically illustrating the structure in the vicinity of the electrode terminal of a secondary battery according to the present embodiment.  FIG. 3  is a plan view explaining the positional relationship between the metal junction part and the heat-insulating part in the electrode terminal of the secondary battery according to the present embodiment. 
     (1) Overall Constitution 
     As illustrated in  FIG. 1 , the secondary battery  1  according to the present embodiment includes an electrode body (not illustrated), a battery case  10 , and an electrode terminal  20 . Each constitution thereof is explained below. 
     (2) Electrode Body 
     The electrode body is a power generation element housed in the battery case  10 . The structure of the electrode body is not particularly limited, and various structures that can be adopted in a general secondary battery can be adopted without particular limitations. For example, a structure wherein a positive electrode and a negative electrode are superimposed via a separator may be adopted for the electrode body. Examples of this type of electrode body structure include a wound electrode in which long belt-shaped positive electrode, negative electrode, and separator are rolled up, a laminated electrode body in which rectangular sheet-shaped positive electrode, negative electrode, and separator are stacked, and the like. Note that, regarding the specific structure and the material of each member (the positive electrode, the negative electrode, and the separator) constituting the electrode body, those that may be adopted in a general secondary battery (for example, lithium ion secondary battery) may be adopted without particular limitations. That is, the structure and the material of each member constituting the electrode body do not limit the technique disclosed herein, and thus, specific explanations therefor are omitted. 
     Although the illustration is omitted, in the secondary battery according to the present embodiment, an electrolyte is also housed in the battery case  10  in addition to the electrode body. As the electrolyte, non-aqueous liquid electrolyte (non-aqueous electrolyte) containing a non-aqueous solvent and a supporting electrolyte, a solid electrolyte obtained by molding a powdery electrolyte into a sheet shape, or the like may be used. Note that the specific component of the electrolyte does not limit the technique disclosed herein, and thus, specific explanations therefor are omitted. 
     (3) Battery Case 
     The battery case  10  is a container that houses the electrode body or the electrolyte mentioned above. As illustrated in  FIG. 1 , the battery case  10  according to the present embodiment is a container having a flat rectangular shape. The rectangular battery case  10  includes a case main body  12  having a flat rectangular shape opened at the top surface and a plate-shaped lid  14  for closing the top surface opening of the case main body  12 . Although details will be described later, a terminal insertion hole  14   a,  which is an opening for inserting electrode terminals  20  (first members  30 ), is disposed at each of both ends of the lid  14  in the width direction X as illustrated in  FIGS. 1 and 2 . Note that the outer shape of the battery case is not limited to the shape as mentioned above and may be appropriately changed according to the standard of exterior apparatuses, the shape of the electrode, or the like. For example, the battery case may be a bottomed cylindrical case. The material of the battery case  10  is not particularly limited as long as the material having the desired strength. Suitable examples of the materials of the battery case  10  include metal materials that are light and have good thermal conductivity (for example, aluminum, stainless steel, nickel-plated steel, and the like). 
     (3) Electrode Terminal 
     As illustrated in  FIG. 1 , the secondary battery  1  according to the present embodiment includes a pair of electrode terminals  20 . These electrode terminals  20  are partly exposed to the exterior of the battery case  10 . Although the illustration is omitted, each electrode terminal  20  is connected to an electrode body (positive electrode or negative electrode) in the battery case  10 . In the present description, an electrode terminal connected to the positive electrode side of the electrode body refers to a “positive electrode terminal”, and an electrode terminal connected to the negative electrode side refers to a “negative electrode terminal”. Then, as illustrated in  FIG. 2 , the electrode terminals  20  in the present embodiment includes a first member  30  and a second member  40 . Hereinafter, the constituent member of the electrode terminals  20  in the present embodiment is specifically described. Note that the structure of the electrode terminal  20  explained below may be applied to either one of the positive electrode terminal and the negative electrode terminal or both of them. 
     (a) First Member 
     The first member  30  is a member having one end positioned in the battery case  10  and another end positioned outside the battery case  10 . As illustrated in  FIG. 2 , the first member  30  includes a shaft part  32  penetrating the battery case  10 . The shaft part  32  is a pillar-shaped member extending in the height direction Z. The upper end of this shaft part  32  is exposed to the exterior of the battery case  10  (above the lid  14 ). The lower end of the shaft part  32  is housed in the battery case  10 . The outer shape of the shaft part  32  is not particularly restricted and may be a columnar shape or a prismatic shape. However, considering the easiness of closing the terminal insertion hole  14   a  of the lid  14  (sealing the battery case  10 ), the outer shape of the shaft part  32  is preferably columnar. A connection part  34  is formed at the upper end of the shaft part  32  in the present embodiment. The connection part  34  is a plate-shaped member coming into surface contact with a second member  40  that will be described below. Note that the shape of the connection part  34  in plan view is not particularly restricted and may be circular or may be rectangular. In the present embodiment, a rivet part  36  is formed at the lower end of the shaft part  32 . The rivet part  36  is formed by crimping for pressedly deforming the lower edge of the tubular-shaped shaft part  32  toward the outside in the radial direction. This rivet part  36  fixes a third member  50 , which will be mentioned below, to the battery case  10  (lid  14 ) and electrically connects the first member  30  and the third member  50 . 
     (b) Second Member 
     The second member  40  is a plate-shaped member connected to the first member  30  outside the battery case  10 . As illustrated in  FIG. 1 , the second member  40  in the present embodiment is a substantially rectangular plate-shaped member in plan view. The plate-shaped second member  40  is disposed outside the battery case  10  (above the lid  14 ). Note that the shape of the second member in plan view is not particularly restricted and may be circular. Then, as illustrated in  FIG. 2 , the bottom surface  40   a  of the second member  40  (a surface on the battery case  10  side) is in surface contact with the top surface  34   a  of the connection part  34  of the first member  30 . Although details will be described later, a metal junction part  70  is formed on the contact interface between the top surface  34   a  of this connection part  34  and the bottom surface  40   a  of the second member  40 , and the first member  30  and the second member  40  are connected by the metal junction part  70 . A recessed part  42  is formed on the bottom surface side of the second member  40  in the present embodiment, and the connection part  34  of the first member  30  is fitted into the recessed part  42 . Due to this constitution, the movement of the second member  40  in the width direction X or the depth direction Y when vibration or external force is applied to the electrode terminal  20  is regulated, and the breakage of the metal junction part  70  by the movement of the second member  40  can be prevented. Furthermore, a locking part  44  protruding toward the shaft part  32  of the first member  30  is formed at the lower edge of the sidewall of the recessed part  42  in the present embodiment. Due to this constitution, the movement of the second member  40  in the height direction Z can be regulated, and therefore, the breakage of the metal junction part  70  by the movement of the second member  40  can be more surely prevented. 
     (c) Third Member 
     The electrode terminal  20  in the present embodiment includes a third member  50 . Although the illustration is omitted, the third member  50  has a current collector part extending in the height direction Z. The current collector part of this third member  50  is connected to an electrode body (a positive electrode or a negative electrode) at the lower edge thereof Then, as illustrated in  FIG. 2 , the upper edge  52  of the third member  50  is bent along the inner surface of the battery case  10  (lid  14 ). Then, the upper edge  52  of the third member  50  is fixed to the battery case  10  (lid  14 ) by the rivet part  36  of the first member  30  and electrically connected to the first member  30 . Due to this constitution, the electrode body in the battery case  10  and the second member  40  outside the battery case  10  are electrically connected via the third member  50  and the first member  30 . Note that this third member  50  is not an essential constitution member of the electrode terminal in the technique disclosed herein. For example, an electrode terminal with no third member can be constructed when a part of the first member is extended in the height direction to connect the electrode body. 
     (4) Insulating Member 
     Next, the secondary battery  1  according to the present embodiment includes an insulating member that prevents the conduction between the electrode terminal  20  and the battery case  10 . As illustrated in  FIG. 2 , the insulating member in the present embodiment includes a gasket  62  and an insulator  64 . 
     The gasket  62  is an insulating member disposed on the outer surface (the top surface of the lid body  14 ) of the battery case  10 . The gasket  62  is a box-shaped insulating member having a housing part  62   a  on the top surface side. The connection part  34  of the first member  30  and the second member  40  are housed in this housing part  62   a  of the gasket  62 . Due to this constitution, the lid  14  and the first member  30  are insulated from each other, and the lid  14  and the second member  40  are insulated from each other. Furthermore, a tubular protrusion part  62   b  having a terminal insertion hole  62   c  is formed on the gasket  62  in the present embodiment. This protrusion part  62   b  of the gasket  62  is inserted into the terminal insertion hole  14   a  of the lid  14 . Then, the protrusion part  62   b  of the gasket  62  intervenes between the shaft part  32  of the first member  30  and the lid  14  in the terminal insertion hole  14   a.  Due to this constitution, the first member  30  and the lid  14  can be insulated. The gasket  62  is pressed by crimping when the rivet part  36  of the first member  30  is formed. Therefore, the gasket  62  is compressed between the lid  14  and the second member  40  (or between the lid  14  and the connection part  34 ). Due to this constitution, gaps generated when each member is assembled to the lid  14  are sealed, and therefore, the distribution of liquid (water intrusion and electrolyte leakage) between the inside and the outside of the battery case  10  can be prevented. Note that the gasket  62  is preferably formed by an easily elastically deformable insulating resin. As examples of the material of the gasket  62 , fluorinated resins such as perfluoroalkoxy fluororesins (PFA), polyphenylene sulfide resins (PPS), aliphatic polyamides, and the like may be mentioned. 
     The insulator  64  is a plate-shaped insulating member having an opening  64   a  and disposed between the battery case  10  (lid  14 ) and the third member  50 . Due to this constitution, the lid  14  and the third member  50  are insulated. Then, the opening  64   a  of the insulator  64  is arranged so as to overlap with the terminal insertion hole  62   c  of the gasket  62  in plan view, and the shaft part  32  of the first member  30  is inserted thereinto. Due to this constitution, the contact between the shaft part  32  of the first member  30  and the lid  14  can be surely prevented. In the crimping when the rivet part  36  is formed at the lower edge of the first member  30 , the lower surface of the protrusion part  62   b  of the gasket  62  is pressed against the periphery of the opening  64   a  of the insulator  64 . Due to this constitution, the gasket  62  and the insulator  64  are crimped, and therefore, the distribution of liquid between the inside and the outside of the battery case  10  can be more surely prevented. Note that the insulator  64  is preferably formed by an easily elastically deformable insulating resin, as with the gasket  62 . 
     (5) Connection between First Member and Second Member 
     As described above, in the secondary battery  1  according to the present embodiment, a metal junction part  70  is formed on the contact interface between the connection part  34  of the first member  30  and the second member  40 . Such a metal junction part  70  is a joining mark formed when the first member  30  and the second member  40  are joined by metal-to-metal joining such as ultrasonic joining, laser welding, or resistance welding. As illustrated in  FIG. 2 , the metal junction part  70  in the present embodiment is formed on the shaft center SC of the shaft part  32  of the first member  30 . As one example, when the metal junction part  70  is formed using ultrasonic joining, the ultrasonic joining may be performed while holding the first member  30  and the second member  40  such that the horn and the anvil are placed on the shaft center SC of the shaft part  32 . Note that, although it is not intended to limit the technique disclosed herein, the metal junction part  70  can be substantially circular in plan view, as illustrated in  FIG. 3 . However, the shape of the metal junction part  70  in plan view can be substantially rectangular (substantially square, substantially oblong) depending on the joining means or the joining condition. Note that, in view of suitably connecting the first member  30  and the second member  40 , it is preferred to appropriately adjust the joining condition such that the diameter of the metal junction part  70  will be 0.3 mm or larger. Alternatively, in view of more suitably connecting the first member  30  and the second member  40 , the diameter of the metal junction part  70  is more preferably 0.5 mm or larger and particularly preferably 1.0 mm or larger. Meanwhile, the upper limit of the diameter of the metal junction part  70  is not particularly limited, and may be 10.0 mm or smaller, may be 5.0 mm or smaller, and may be 2.0 mm or smaller. 
     As described above, the first member  30  and the second member  40  can be connected with low resistance and high strength by forming the metal junction part  70  on the contact interface between the connection part  34  of the first member  30  and the second member  40 . However, if the metal junction part  70  is thermally deteriorated by application of great heat when a bus bar is connected or the like, the conductivity and the joining strength may greatly decrease. For suppressing such thermal deterioration of the metal junction part  70 , a heat-insulating part  80  is formed on the contact interface between the connection part  34  and the second member  40  in the secondary battery  1  according to the present embodiment. This heat-insulating part  80  is formed outward of the metal junction part  70  in the radial direction centered on the metal junction part  70  as illustrated in  FIG. 3 . This enables to block great heat by the heat-insulating part  80  even when such a great heat is applied to a region above the heat-insulating part  80  or outside the heat-insulating part  80  and therefore, to suppress the heat deterioration of the metal junction part  70  by welding a bus bar or the like. 
     Note that, in the present embodiment, the heat-insulating part  80  is a cavity formed between the connection part  34  and the second member  40 . Specifically, the second member  40  in the present embodiment is provided with a recessed groove  46  separating from the top surface  34   a  of the connection part  34  on a part of the bottom surface  40   a  of the second member  40 . Due to this constitution, the heat-insulating part  80 , constituted of a cavity, can be formed on the connection interface between the connection part  34  and the second member  40 . According to such a constitution, the thermal deterioration of the metal junction part  70  can be suppressed at a low cost. Note that the recessed groove for forming the heat-insulating part may be formed on the top surface of the connection part of the first member instead of on the bottom surface of the second member. In addition, the mode of the heat-insulating part is not particularly restricted as long as the heat conduction to the metal junction part can be blocked. For example, the heat-insulating part may be formed by filling a heat-insulating material in a space on the contact interface between the connection part of the first member and the second member. As the heat-insulating material, inorganic heat-insulating materials excellent in heat resistance (such as grass wool, a coating layer containing alumina particles, and a coating layer containing silica particles) are suitable. The thermal deterioration of the metal junction part  70  can be more suitably suppressed by using such a heat insulating material. 
     In addition, as illustrated in  FIG. 3 , the heat-insulating part  80  in the present embodiment is a ring-shaped heat-insulating part  80  formed so as to surround the metal junction part  70  in plan view. This ring-shaped heat-insulating part  80  enables to prevent great heat from reaching the metal junction part  70  around the heat-insulating part  80 , and therefore, more suitably to suppress the thermal deterioration of the metal junction part  70 . Note that the dimension L 4  (see  FIG. 3 ) from the inner edge  82  to the outer edge  84  of the ring-shaped heat-insulating part  80  is preferably  3  mm or larger, more preferably 5 mm or larger, and particularly preferably 10 mm or larger. This enables to prevent great heat from reaching the metal junction part  70  more suitably. Note that taking the strength of the member (for example, the second member) on which the recessed groove for forming the heat-insulating part is disposed into consideration, the dimension L 4  in  FIG. 3  is preferably 50 mm or smaller, more preferably 30 mm or smaller, and particularly preferably 20 mm or smaller. 
     Note that the technique disclosed herein can be particularly suitably used when the first member and the second member are constituted of dissimilar metal materials. Furthermore, ultrasonic junction is recommended among the above-mentioned metal-to-metal joining methods for connecting this type of dissimilar metals in view of forming a junction part with high strength and low resistance. However, a metal junction part formed by ultrasonic joining tends to show large strength reduction and conductivity reduction after thermal deterioration. On the contrary, the technique disclosed herein can suppress the thermal deterioration of the metal junction part, and therefore, can be particularly suitably used for constructing an electrode terminal having a dissimilar metal bonding recommended to be achieved by ultrasonic joining. 
     As one example of the dissimilar metal bonding recommended to be achieved by ultrasonic joining, an electrode terminal  20  in which the first member  30  is constituted of a copper material and the second member  40  is constituted of an aluminum material can be mentioned. Since an aluminum material and a copper material tend to show high electrical resistance when joined by thermal welding, the use of ultrasonic joining is recommended. For example, when a negative electrode terminal of a lithium ion secondary battery is connected with a negative electrode of an electrode body (negative electrode current collector), the connection between copper materials is recommended, and when the negative electrode terminal is connected with a bus bar, the connection between aluminum materials is recommended. In such a case, a dissimilar metal-to-metal joining of copper materials and aluminum materials is required in the components of a negative electrode terminal. For this issue, it is preferred to form a metal junction part  70  using ultrasonic joining between the first member  30  made of copper and the second member  40  made of aluminum. Due to this constitution, the first member  30  and the second member  40  can be joined with relatively low resistance. Then, as described above, thermal deterioration of the metal junction part  70  can be suppressed by the heat-insulating part  80  in the technique disclosed herein, and therefore, the joint interface between the first member  30  and the second member  40 , which are made of dissimilar metals, can be maintained in a low resistance state. As described above, the technique disclosed herein can be particularly suitably used for constructing a negative electrode terminal of a lithium ion secondary battery for which dissimilar metal-to-metal joining between copper materials and aluminum materials is required. Note that the term “aluminum material(s)” in the present description is a concept encompassing aluminum and alloys mainly composed of aluminum. The term “alloy mainly composed of aluminum” used herein refers to an alloy containing at least 70% or more aluminum. Other elements that may be contained in the aluminum material are not particularly restricted, and examples thereof include silicon, iron, copper, manganese, magnesium, zinc, chromium, titanium, lead, zirconium, and the like. Meanwhile, the term “copper material(s)” is a concept encompassing copper and alloys mainly composed of copper. Note that the term “alloy mainly composed of copper” used herein refers to an alloy containing at least 50% or more copper. Other elements that may be contained in the material are not particularly restricted, and examples thereof include silicon, iron, manganese, magnesium, zinc, chromium, titanium, lead, tin, phosphorus, aluminum, nickel, cobalt, beryllium, zirconium, and the like. 
     2. Assembled Battery 
     Next, one embodiment of an assembled battery constructed by a plurality of secondary batteries is described as another aspect of the technique disclosed herein. In the following description relating to the assembled battery according to the present embodiment,  FIGS. 4 to 6  are mainly referred to.  FIG. 4  is a perspective view schematically illustrating an assembled battery according to the present embodiment.  FIG. 5  is a sectional view schematically illustrating the structure of a connection part between an electrode terminal and a bus bar in the assembled battery according to the present embodiment.  FIG. 6  is a plan view explaining the positional relationship among the metal junction part, the heat-insulating part, and the thermally welded part in the assembled battery according to the present embodiment. Note that, in  FIG. 6 , the indication of a bus bar  110  is omitted for explanatory convenience, and the formed positions of the metal junction part  70  and the heat-insulating part  80  are indicated as dotted lines. The same applies to  FIGS. 7 and 11 , which will be described later. 
     As illustrated in  FIG. 4 , an assembled battery  100  according to the present embodiment includes a plurality of secondary batteries  1 . Specifically, in this assembled battery  100 , a plurality of secondary batteries  1  having a flat rectangular battery case  10  are arranged along the depth direction Y such that the flat surfaces face each other. Then, a pair of end plates  120  are arranged on both outsides in the arrangement direction (depth direction Y) of the assembled battery  100 . The plurality of secondary batteries  1  are each restrained along the arrangement direction by crosslinking this pair of end plates  120  by a restraining beam material  130 . Note that, in the present embodiment, a spacer  140  is disposed between the adjacent secondary batteries  1  in order to equalize the restraint pressure in a flat surface. Note that the number of the secondary batteries  1  constituting the assembled battery  100  is not particularly limited and can be appropriately changed according to the purpose (required electric power of the exterior apparatus, regulation, and the like) of the assembled battery  100 . In addition, it is not necessary that all of the plurality of secondary batteries constituting the assembled battery have the constitution described above in the technique disclosed herein. That is, the assembled battery  100  according to the present embodiment may partly include a secondary battery having a different structure from the secondary battery  1  according to the above embodiment. 
     In the present embodiment, the electrode terminals  20  are connected to each other via the bus bar  110  between adjacent two secondary batteries  1 . A metal material excellent in conductivity and strength is used for this bus bar  110 . As illustrated in  FIG. 5 , the bus bar  110  is thermally welded in surface contact with the top surface  40   b  of the second member  40  of the electrode terminal  20 . Then, the thermally welded part  90 , which is a joining mark of this thermal welding, is formed so as to penetrate the bus bar  110  to reach the second member  40 . Suitable examples of the material of the bus bar  110  in the present embodiment include aluminum materials and the like. Furthermore, in view of preventing the conductivity reduction by thermal welding of dissimilar metal members, the same type of metal material as the second member  40  of the electrode terminal  20  is preferably used for a bus bar  110 . 
     Here, as illustrated in  FIGS. 5 and 6 , the bus bar  110  and the electrode terminal  20  are thermally welded in the present embodiment such that the thermally welded part  90  is formed above the heat-insulating part  80  formed on the contact interface between the connection part  34  of the first member  30  and the second member  40 . Specifically, the laser light is applied to the above part of the heat-insulating part  80  such that a pair of linear thermally welded parts  90  facing each other with the metal junction part  70  interposed therebetween will be formed. Due to this constitution, a thermally welded part  90  can be formed while insulating the heat upon thermal welding by the heat-insulating part  80 , and therefore, thermal deterioration of the metal junction part  70  can be suppressed. 
     Note that, in the technique disclosed herein, the formed position of the thermally welded part  90  (laser applied position in thermal welding) is not limited to the above position of the heat-insulating part  80 . For example, as illustrated in  FIG. 7 , the thermal welding may be performed such that the thermally welded part  90  will be formed outward of the heat-insulating part  80  in the radial direction centered on the metal junction part  70 . Also in this case, the heat upon thermal welding can be insulated by the heat-insulating part  80 , and therefore, thermal deterioration of the metal junction part  70  can be suppressed. 
     Note that the formed position of the thermally welded part  90  is preferably adjusted in view of suitably suppressing the thermal deterioration of the metal junction part  70 . Specifically, assuming that the distance from the center C of the metal junction part  70  to the outer periphery  72  is taken as L 1 , the distance from the center C of the metal junction part  70  to the inner edge  82  of the heat-insulating part  80  is taken as L 2 , and the distance from the center C of the metal junction part  70  to the center of the thermally welded part  90  is taken as L 3 , the formed position of the thermally welded part  90  is preferably adjusted such that the following formula (1) is satisfied. This enables to prevent great heat upon thermal welding from reaching the metal junction part  70  more suitably. In addition, in view of more surely preventing thermal deterioration of the metal junction part  70 , the formed position of the thermally welded part  90  is preferably adjusted so as to satisfy the following formula (2). Note that the following formulas (1) and (2) are conditions that may be applied when the heat-insulating part  80  is a cavity. For example, when the heat-insulating part  80  is filled with a heat-insulating material, it is preferred to adjust the formed position of the thermally welded part  90 , taking the heat-insulating performance (heat conductivity) of the heat-insulating material into consideration. 
         L 3− L 2&gt; L 2− L 1   (1)
 
         L 3− L 2&gt;( L 2− L 1)×1.5   (2)
 
     In view of more surely preventing thermal deterioration of the metal junction part  70 , the condition of the thermal welding (such as welding temperature, scanning speed, or the like) is preferably adjusted such that the width w of the linear thermally welded part  90  satisfies the following formula (3). 
         L 3− L 2&gt; w    (3)
 
     3. Other Embodiments 
     As stated above, one embodiment of the technique disclosed herein is described. Note that the embodiment mentioned above indicates an example of a secondary battery (or assembled battery) to which the technique disclosed herein is applied and is not intended to limit the technique disclosed herein. 
     For example, in the embodiment mentioned above, a ring-shaped heat-insulating part  80  that surrounds the metal junction part  70  in plan view is formed. However, the specific planar shape of the heat-insulating part is not particularly restricted as long as the heat-insulating part is formed outward of the metal junction part in the radial direction centered on the metal junction part. For example, as illustrated in  FIG. 8 , the heat-insulating part may be a rectangular ring-shaped heat-insulating part  80 A surrounding the metal junction part  70  in plan view. Also, in a case where such a rectangular ring-shaped heat-insulating part  80 A is formed, the heat upon forming (thermally welding) the thermally welded part  90  is appropriately insulated, and the thermal deterioration of the metal junction part  70  can be suppressed. Furthermore, the planar shape of the heat-insulating part may not be a ring shape as illustrated in  FIGS. 6 to 8 . For example, as illustrated in  FIG. 9 , a pair of planar rectangular heat-insulating parts  80 B may be formed so as to face each other with the metal junction part  70  interposed therebetween. Note that, when such a pair of heat-insulating part  80 B is formed, the thermally welded part  90  is formed above each heat-insulating part  80 B (or at a position outward of each heat-insulating part  80 B in the radial direction). Due to this constitution, the heat upon thermal welding is appropriately insulated at the heat-insulating part  80 B, and the thermal deterioration of the metal junction part  70  can be suppressed. 
     In the technique disclosed herein, a region for performing thermal welding (that is, the thermally welded part  90 ) is not particularly restricted as long as it is above the heat-insulating part (or outward of the heat-insulating part in the radial direction) and may be appropriately changed. For example, as illustrated in  FIG. 6 or 10 , thermal welding may be performed such that a part of the thermally welded part  90  will be stuck out from the heat-insulating part  80  or  80 C. Even in such a case, thermal deterioration of the metal junction part  70  can be sufficiently suppressed as long as the thermally welded part  90  stuck out from the heat-insulating part  80  or  80 C is formed outward of the heat-insulating part  80  or  80 C in the radial direction centered on the metal junction part  70 . For example, thermal welding is performed inn  FIG. 10  so that the upper edge  92  of the thermally welded part  90  will be stuck out from the rectangular heat-insulating part  80 C. However, the upper edge  92  of the thermally welded part  90  is formed outward of the heat-insulating part  80 C when seen on the straight line A connecting the upper edge  92  of the thermally welded part  90  and the metal junction part  70 . In this case, the heat upon forming the upper edge  92  of the thermally welded part  90  can be insulated in the heat-insulating part  80 C, and therefore, thermal deterioration of the metal junction part  70  can be sufficiently suppressed. However, taking the possibility that the heat going around the heat-insulating part is transmitted to the metal junction part into consideration, it is preferred to form the ring-shaped heat-insulating part  80  or  80 A as illustrated in  FIGS. 6 to 8 . 
     Furthermore, in the mode described above, a pair of linear thermally welded parts  90  facing each other with the metal junction part  70  interposed therebetween is formed. However, the planar shape of the thermally welded part is not a linear shape, and various shapes may be adopted. For example, as illustrated in  FIG. 11 , a ring-shaped thermally welded part  90 A surrounding the heat-insulating part  80  may be formed. Also in this case, the heat upon thermal welding can be appropriately insulated by the heat-insulating part  80 , and therefore, thermal deterioration of the metal junction part  70  can be suppressed. Note that, in  FIG. 11 , a ring-shaped thermally welded part  90 A having a larger radius than the outer edge  84  of the heat-insulating part  80  is formed, and the thermally welded part  90 A is formed outward of the heat-insulating part  80  in the radial direction centered on the metal junction part  70 . However, the radius of the thermally welded part  90 A may be smaller than the outer edge  84  of the heat-insulating part  80  and larger than the inner edge  82 , and the ring-shaped thermally welded part  90 A may be formed above the heat-insulating part  80 . Even in this case, the heat upon thermal welding is appropriately insulated in the heat-insulating part  80 , and thermal deterioration of the metal junction part  70  can be suppressed. 
     As described above, specific embodiments of the present disclosure are explained in detail, but these are mere examples and do not limit the scope of claims. The invention disclosed in claims encompasses variations and modifications of the above specific embodiments in various ways.