Patent Publication Number: US-2023163424-A1

Title: Energy storage apparatus and method for manufacturing energy storage apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Japanese Patent Application No. 2020-082644, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an energy storage apparatus including an energy storage device having an external terminal and a bus bar, and a method for manufacturing the energy storage apparatus. 
     BACKGROUND ART 
     Conventionally, an assembled battery including a plurality of prismatic secondary batteries including external terminals is known (for example, see Patent Document 1). The assembled battery includes a negative electrode terminal, a positive electrode terminal, the external terminal (a negative electrode external terminal or a positive electrode external terminal), and a bus bar connecting between the negative electrode external terminals or between the positive electrode external terminals. In this assembled battery, the external terminal and the bus bar are connected by laser welding. In this assembled battery, the negative electrode terminal is made of a copper material, and the bus bar is made of an aluminum material. The negative electrode external terminal includes a clad region including a copper material portion and an aluminum material portion. 
     By the way, in order to secure the connection strength between the external terminal and the bus bar, when a welding joining width is secured during the laser welding of the external terminal and the bus bar, a depth of penetration of the external terminal and the bus bar increases. On the other hand, when penetration by welding reaches an interface between different metal layers of the external terminal during the welding of the external terminal having the clad region and the bus bar, bonding strength at this interface (bonding strength between adjacent metal layers) decreases. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: WO 2013/065523 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     An object of an embodiment is to provide an energy storage apparatus including an energy storage device in which an external terminal is formed of a clad material and a bus bar is connected to the external terminal, the energy storage apparatus in which a decrease in bonding strength between metal layers constituting the clad material is prevented, and a method for manufacturing the energy storage apparatus. 
     Means for Solving the Problems 
     An energy storage apparatus of the embodiment includes: an energy storage device including a metal external terminal; and a metal bus bar that is laminated on the external terminal and welded to the external terminal. A welded portion that welds the bus bar and the external terminal is formed on the bus bar, the welded portion is extended from the bus bar to the external terminal, the external terminal includes a clad material including a first metal layer adjacent to the bus bar and a second metal layer adjacent to the first metal layer, the welded portion welds the bus bar and the first metal layer, and an end of the welded portion does not reach an interface between the first metal layer and the second metal layer. 
     In the energy storage apparatus, the welded portion may include a first welded portion and a second welded portion that partially overlap each other. 
     In the energy storage apparatus, in a section of the external terminal and the bus bar in a laminating direction of the external terminal and the bus bar, a dimension of the welded portion at a boundary position between the external terminal and the bus bar in a direction orthogonal to the laminating direction may be larger than a thickness of the first metal layer. 
     In the energy storage apparatus, the welded portion when viewed from the laminating direction of the external terminal and the bus bar may have an annular shape having no corner or an arc shape having no corner. 
     An energy storage apparatus of the embodiment includes: an energy storage device including a metal external terminal; and a bus bar that is formed of a metal containing aluminum, the aluminum being laminated on the external terminal and welded to the external terminal. A welded portion that welds the bus bar and the external terminal is formed on the bus bar, the welded portion is extended from the bus bar to the external terminal, the external terminal includes a positive electrode external terminal and a negative electrode external terminal, the positive electrode external terminal is made of a metal containing aluminum, the negative electrode external terminal includes a clad material including a first metal layer adjacent to the bus bar and a second metal layer adjacent to the first metal layer, the first metal layer is formed of a metal containing aluminum, the second metal layer is formed of a metal containing copper, the welded portion welds the bus bar and the first metal layer, and an end of the welded portion does not reach an interface between the first metal layer and the second metal layer. 
     A method for manufacturing an energy storage apparatus of the embodiment includes: laminating a metal bus bar on a metal external terminal of an energy storage device; applying heat to the bus bar from a side opposite to the external terminal to weld the bus bar and the external terminal; and forming a welded portion that welds the bus bar and the external terminal to each other on the bus bar. The welded portion is extended from the bus bar to the external terminal, the external terminal includes a clad material including a first metal layer adjacent to the bus bar and a second metal layer adjacent to the first metal layer, the welded portion welds the bus bar and the first metal layer, and an end of the welded portion does not reach an interface between the first metal layer and the second metal layer. 
     In the method for manufacturing the energy storage apparatus, the welded portion may be formed in an annular shape having no corner or an arc shape having no corner when viewed from a laminating direction of the external terminal and the bus bar. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view illustrating an energy storage apparatus according to an embodiment. 
         FIG.  2    is a perspective view illustrating an energy storage device in the energy storage apparatus. 
         FIG.  3    is a side view of the energy storage device. 
         FIG.  4    is a sectional view taken along a line IV-IV in  FIG.  2   . 
         FIG.  5    is a sectional view taken along a line V-V in  FIG.  2   . 
         FIG.  6 A  is a schematic plan view illustrating connection between an external terminal of the energy storage device and a bus bar. 
         FIG.  6 B  is a schematic sectional view taken along a line VI-VI in  FIG.  6 A . 
         FIG.  7 A  is a schematic plan view illustrating connection between an external terminal and a bus bar of an energy storage device according to a comparative example. 
         FIG.  7 B  is a schematic sectional view taken along a line VII-VII in  FIG.  7 A . 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     An energy storage apparatus of the embodiment includes: an energy storage device including a metal external terminal; and a metal bus bar that is laminated on the external terminal and welded to the external terminal. A welded portion that welds the bus bar and the external terminal is formed on the bus bar, the welded portion is extended from the bus bar to the external terminal, the external terminal includes a clad material including a first metal layer adjacent to the bus bar and a second metal layer adjacent to the first metal layer, the welded portion welds the bus bar and the first metal layer, and an end of the welded portion does not reach an interface between the first metal layer and the second metal layer. 
     According to the above configuration, the welded portion does not reach the interface between the metal layers constituting the clad material, so that the bonding strength of each metal layer constituting the clad material can be secured. 
     In the energy storage apparatus, the welded portion may include a first welded portion and a second welded portion that partially overlap each other. 
     According to the above configuration, the bonding strength between the bus bar and the external terminal can be ensured by ensuring a wide bonding width of the welded portion. 
     In the energy storage apparatus, in a section of the external terminal and the bus bar in a laminating direction of the external terminal and the bus bar, a dimension in a direction orthogonal to the laminating direction at a boundary position between the external terminal and the bus bar may be larger than a thickness of the first metal layer. 
     According to the above configuration, because the dimension (dimension in the direction orthogonal to the laminating direction) of the welded portion is secured at the boundary position between the external terminal and the bus bar, the bonding strength between the bus bar and the external terminal can be secured, whereby the bonding strength between the bus bar and the external terminal can be secured while the bonding strength between the metal layers of the clad material is secured. 
     In the energy storage apparatus, the welded portion when viewed from the laminating direction of the external terminal and the bus bar may have an annular shape having no corner or an arc shape having no corner. 
     According to the above configuration, since the welded portion has an annular shape or an arc shape without a corner portion, even if a force is applied to the welded portion, the force is less likely to concentrate on a part of the welded portion. 
     An energy storage apparatus of the embodiment includes: an energy storage device including a metal external terminal; and a bus bar that is formed of a metal containing aluminum, the aluminum being laminated on the external terminal and welded to the external terminal. A welded portion that welds the bus bar and the external terminal is formed on the bus bar, the welded portion is extended from the bus bar to the external terminal, the external terminal includes a positive electrode external terminal and a negative electrode external terminal, the positive electrode external terminal is made of a metal containing aluminum, the negative electrode external terminal includes a clad material including a first metal layer adjacent to the bus bar and a second metal layer adjacent to the first metal layer, the first metal layer is formed of a metal containing aluminum, the second metal layer is formed of a metal containing copper, the welded portion welds the bus bar and the first metal layer, and an end of the welded portion does not reach an interface between the first metal layer and the second metal layer. 
     According to the above configuration, the welded portion does not reach the interface between the metal layers constituting the clad material, so that the bonding strength of each metal layer constituting the clad material can be secured. 
     A method for manufacturing an energy storage apparatus of the embodiment includes: laminating a metal bus bar on a metal external terminal of an energy storage device; applying heat to the bus bar from a side opposite to the external terminal to weld the bus bar and the external terminal; and forming a welded portion that welds the bus bar and the external terminal to each other on the bus bar. The welded portion is extended from the bus bar to the external terminal, the external terminal includes a clad material including a first metal layer adjacent to the bus bar and a second metal layer adjacent to the first metal layer, the welded portion welds the bus bar and the first metal layer, and an end of the welded portion does not reach an interface between the first metal layer and the second metal layer. 
     According to the above configuration, the welded portion does not reach the interface between the metal layers constituting the clad material, the energy storage apparatus in which the bonding strength of each metal layer constituting the clad material is ensured can be manufactured. 
     In the method for manufacturing the energy storage apparatus, the welded portion may be formed in an annular shape having no corner or an arc shape having no corner when viewed from a laminating direction of the external terminal and the bus bar. 
     According to the above configuration, when the welding is performed with a general welding width, an annular or arc-shaped welded portion having no corner where force is hardly concentrated on one portion can be formed. 
     From the above, the energy storage apparatus including the energy storage device in which the external terminal is formed of the clad material and the bus bar is connected to the external terminal, the energy storage apparatus in which the decrease in bonding strength between metal layers constituting the clad material is prevented, and a method for manufacturing the energy storage apparatus can be provided. 
     Hereinafter, an embodiment of the present invention will be described with reference to  FIGS.  1  to  7 B . In the embodiment, a chargeable and dischargeable secondary battery will be described as an example of the energy storage device. Names of the constituent members (constituent elements) of the embodiment are those in the embodiment, and are sometimes different from the names of the constituent members (constituent elements) in the background art. 
     As illustrated in  FIG.  1   , an energy storage apparatus  11  includes an energy storage device  1  including a metal external terminal  4  and a metal bus bar  6  that is laminated on the external terminal  4  and welded to the external terminal  4 . As illustrated in  FIG.  6 B , in the bus bar  6  of the energy storage apparatus  11 , a welded portion  7  that welds the external terminal  4  and the bus bar  6  is formed at a joining position between the external terminal  4  and the bus bar  6 . The energy storage apparatus  11  of the embodiment includes a plurality of energy storage devices  1  (see  FIG.  1   ). 
     The plurality of energy storage devices  1  are arranged in a predetermined direction (first direction). For example, the first direction is a direction orthogonal to a laminating direction of the external terminal  4  and the bus bar  6 . Each of the plurality of energy storage devices  1  is a primary battery, a secondary battery, a capacitor, or the like. The energy storage device  1  of the embodiment is a nonaqueous electrolyte secondary battery that can be charged and discharged. More specifically, the energy storage device  1  is a lithium ion secondary battery utilizing electron transfer caused by transfer of the lithium ion. The energy storage device  1  is what is called a prismatic lithium ion secondary battery. 
     As illustrated in  FIGS.  2  to  5   , the energy storage device  1  includes an electrode assembly  2  including a positive electrode and a negative electrode, a case  3  accommodating the electrode assembly  2 , and the external terminal  4  that is disposed outside the case  3  and electrically connected to the electrode assembly  2 . In addition to the electrode assembly  2 , the case  3  and the external terminal  4 , the energy storage device  1  includes a current collector  5  that makes the electrode assembly  2  and the external terminal  4  conductive with each other. 
     The electrode assembly  2  includes a winding core  21  and a layered product  22  in which a positive electrode and a negative electrode are insulated from each other and laminated, the layered product  22  being wound around a winding core  21  (see  FIG.  5   ). When the lithium ion moves between the positive electrode and the negative electrode in the electrode assembly  2 , the energy storage device  1  is charged and discharged. 
     For example, the positive electrode includes a strip-shaped metal foil and a positive active material layer formed on the metal foil. For example, the negative electrode includes a strip-shaped metal foil and a negative active material layer formed on the metal foil. 
     In the electrode assembly  2  of the embodiment, the positive electrode and the negative electrode are wound while being insulated from each other by a separator. That is, in the electrode assembly  2  of the embodiment, the layered product of the positive electrode, the negative electrode, and the separator is wound. The separator is a member having an insulating property. The separator is disposed between the positive electrode and the negative electrode. Thus, in the electrode assembly  2  (particularly; the layered product), the positive electrode and the negative electrode are insulated from each other. The separator holds the electrolyte solution in the case  3 . Thus, during charge-discharge of the energy storage device  1 , the lithium ion moves between the positive electrode and the negative electrode that are alternately layered with the separator interposed therebetween. 
     The case  3  includes a case body  31  including an opening, and a lid plate  32  that closes the opening of the case body  31 . The case  3  accommodates the electrolyte solution in an internal space  33  together with the electrode assembly  2 , the current collector  5 , and the like. The case  3  is made of metal having resistance to the electrolyte solution. For example, the case  3  of the embodiment is formed of aluminum or a metal material containing aluminum such as an aluminum alloy. The case  3  may be formed of a metal material such as stainless steel and nickel, a composite material obtained by bonding resin such as nylon to aluminum, or the like. 
     The electrolyte solution is a non-aqueous solution electrolytic solution. The electrolyte solution is obtained by dissolving an electrolyte salt in an organic solvent. Examples of the organic solvent include cyclic carbonate esters such as propylene carbonate and ethylene carbonate, and chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Examples of the electrolyte salt include LiClO 4 , LiBF 4 , and LiPF 6 . 
     The case  3  is formed by overlapping and joining an opening peripheral edge of the case body  31  and a peripheral edge of the lid plate  32  (see  FIG.  4   ). The case  3  includes an internal space  33  defined by the case body  31  and the lid plate  32 . In the embodiment, the opening peripheral edge of the case body  31  and the peripheral edge of the lid plate  32  are joined by welding. 
     The case body  31  includes a closing portion  311  that has a plate shape and includes an inner surface facing the inside of the case  3  and an outer surface facing the outside of the case  3 , and a cylindrical body portion  312  that is connected to a peripheral edge of the closing portion  311 , extends to the inner surface side of the closing portion  311 , and surrounds the inner surface of the closing portion  311  (see  FIG.  2   ). 
     The closing portion  311  is a member positioned at a lower end of the case body  31  (that is, a bottom wall of the case body  31  is formed when the opening looks upward) when the case body  31  is disposed such that the opening looks upward. The closing portion  311  has a rectangular shape as viewed in a normal direction of the closing portion  311 . The four corners of the closing portion  311  have an arc shape. 
     Hereinafter, as illustrated in  FIG.  2   , a long side direction of the closing portion  311  is defined as an X-axis direction, a short side direction of the closing portion  311  is defined as a Y-axis direction, and the normal direction of the closing portion  311  is defined as a Z-axis direction. 
     The lid plate  32  is a plate-like member that closes the opening of the case body  31 . Specifically, the lid plate  32  abuts on the case body  31  so as to close the opening of the case body  31 . More specifically, the peripheral edge of the lid plate  32  overlaps with the opening peripheral edge of the case body  31  such that the lid plate  32  closes the opening. A boundary portion between the lid plate  32  and the case body  31  laminated in this manner is welded. Thus, the case  3  is configured. 
     The lid plate  32  includes a gas release valve  321  capable of discharging gas in the case  3  to the outside (see  FIG.  3   ). The gas release valve  321  discharges the gas from the inside of the case  3  to the outside when internal pressure of the case  3  increases to a predetermined pressure. 
     The external terminal  4  is a member that is electrically connected to an external terminal of another energy storage device, an external device, or the like. The external terminal  4  of the embodiment has an outer surface  40  that is a surface to which the bus bar or the like can be welded. The outer surface  40  of the embodiment is a flat surface. The external terminal  4  has a plate shape expanding along the lid plate  32 . Particularly, a flange of the external terminal  4  has a rectangular plate shape as viewed in the Z-axis direction. 
     The external terminal  4  has a flange that extends along the case  3  outside the case  3 , and a shaft that extends from the flange, penetrates the case  3 , and is electrically connected to the electrode assembly  2  (see  FIG.  4   ). Furthermore, the external terminal  4  includes an external terminal (positive electrode external terminal)  4   a  connected to the positive electrode and an external terminal (negative electrode external terminal)  4   b  connected to the negative electrode. 
     In the external terminal  4   a , a flange  401   a  and a shaft  402   a  are integrated. The external terminal  4   a  is formed of aluminum or a metal material containing aluminum such as an aluminum alloy. 
     In the external terminal  4   b , a flange  401   b  and a shaft  402   b  are separate bodies (separate members). The flange  401   b  and the shaft  402   b  are connected by swaging the shaft  402   b.    
     The external terminal  4   b  has a clad material including a plurality of (two layers in the example of the embodiment) metal layers laminated in the laminating direction (Z-axis direction) of the bus bar  6  and the external terminal  4 . Specifically, in the external terminal  4   b , the flange  401   b  is made of a clad material. The metal layers adjacent to each other in the plurality of metal layers are made of different kinds of metals. 
     For example, as illustrated in  FIG.  6 B , the plurality of metal layers included in the flange  401   b  (external terminal  4   b ) include a first metal layer  41  located closest to the bus bar  6  in the Z-axis direction (that is, adjacent to the bus bar  6 ), and a second metal layer  42  adjacent to the first metal layer  41 . The flange  401   b  of the embodiment is configured of the first metal layer  41  and the second metal layer  42 . For example, the thickness of the first metal layer  41  of the embodiment ranges from 0.2 mm to 1.0 mm, and the thickness of the second metal layer  42  ranges from 0.2 mm to 3.0 mm. The thickness of the first metal layer  41  and the thickness of the second metal layer  42  are not limited to these ranges. 
     For example, the first metal layer  41  is formed of aluminum or a metal material containing aluminum such as an aluminum alloy. For example, the second metal layer  42  is formed of a metal material containing copper such as copper or a copper alloy. 
     The current collector  5  is disposed in the case  3  and directly or indirectly connected to the electrode assembly  2  in a conductive manner (see  FIGS.  4  and  5   ). The current collector  5  is formed of a member having conductivity. 
     The bus bar  6  connects the different energy storage devices  1  to each other or connects the energy storage device  1  to an external input and output terminal or the like in a conductive manner (see  FIG.  1   ). The bus bar  6  of the embodiment is overlapped on the outer surface  40  of the external terminal  4  and welded to the external terminal  4  (see  FIG.  6 B ). In addition, the bus bar  6  of the embodiment is made of the same material as the external terminal  4   a . Furthermore, the bus bar  6  of the embodiment is made of the same material as the first metal layer  41  of the external terminal  4   b . Specifically, the bus bar  6  is formed of aluminum or a metal material containing aluminum such as an aluminum alloy. 
     The welded portion  7  is a member formed by welding the external terminal  4   b  (particularly, the flange  401   b ) and the bus bar  6  (see  FIGS.  6 A and  6 B ). The welded portion  7  is extended from the bus bar  6  to the external terminal  4   b , namely, formed between the bus bar  6  and the external terminal  4   b . In other words, the welded portion  7  is formed continuously from the bus bar  6  to the external terminal  4   b . The welded portion  7  of the embodiment is formed by laser welding between the external terminal  4   b  and the bus bar  6 . The welded portion  7  of the embodiment includes welding tracks  710 ,  720  that are a mark of a laser irradiation position (specifically, a mark due to a center of the laser irradiation position). The width of the laser beam with which the bus bar  6  is irradiated in the laser welding of the embodiment is less than or equal to 1.0 mm. 
     The method for manufacturing the energy storage apparatus  11  includes a process of forming the welded portion  7  when the external terminal  4   b  and the bus bar  6  are welded to each other. Specifically, the method for manufacturing the energy storage apparatus  11  includes: laminating the metal bus bar  6  on the metal external terminal  4   b  of the energy storage device  1 ; applying heat to the bus bar  6  from the side opposite to the external terminal  4   b  to weld the bus bar  6  and the external terminal  4   b  to each other; and forming the welded portion  7  where the bus bar  6  and the external terminal  4   b  are welded to each other at the welding position between the external terminal  4   b  and the bus bar  6 , namely, forming the welded portion  7  between the external terminal  4   b  and the bus bar  6 . In other words, the welded portion  7  is formed continuously from the bus bar  6  to the external terminal  4   b.    
     In the Z-axis direction, an end  70  of the welded portion  7  does not reach an interface  43  between the first metal layer  41  and the second metal layer  42 . Specifically, the end  70  of the welded portion  7  exists in the first metal layer  41 . More specifically, the end  70  of the welded portion  7  is located closer onto the side of the bus bar  6  than the interface  43  between the first metal layer  41  and the second metal layer  42  (see  FIG.  6 B ). That is, in the Z-axis direction, the end  70  is located in front of the interface  43  between the first metal layer  41  and the second metal layer  42 . 
     The end  70  of the welded portion  7  is an end on the side of the external terminal  4   b  out of both ends of the welded portion  7  in the Z-axis direction. That is, the end  70  of the welded portion  7  becomes a lower end of the welded portion  7  when the case body  31  is disposed such that the opening looks upward (that is, when the bus bar  6  is disposed above the external terminal  4   b ). 
     The welded portion  7  of the embodiment includes a first welded portion  71  and a second welded portion  72  partially overlapping each other. Specifically, the first welded portion  71  and the second welded portion  72  overlap each other at least at a boundary position between the external terminal  4   b  and the bus bar  6  (the outer surface  40  of the external terminal  4   b ). The first welded portion  71  includes a first welding track  710 , and the second welded portion  72  includes a second welding track  720  (see  FIG.  6 A ). 
     In the dimension (width W 1 ) of the welded portion  7  in the direction along the boundary at the boundary position (particularly, the outer surface  40  of the flange  401   b  of the external terminal  4   b ) between the external terminal  4   b  and the bus bar  6  in the Z-axis direction, namely, in the section (see  FIG.  6 B ) in the direction crossing the welding tracks  710  and  720 , the dimension W 1  of the welded portion  7  in the direction orthogonal to the Z-axis direction at the boundary position between the external terminal  4   b  and the bus bar  6  is larger than the thickness T 1  of the first metal layer  41  (see  FIG.  6 B ). That is, the width W 1  of the welded portion  7  at this boundary position is larger than the thickness T 1  of the first metal layer  41 . 
     Furthermore, the shape of the welded portion  7  viewed from the Z-axis direction is an annular shape without a corner (for example, an elliptic shape or a circular shape). That is, the welded portion  7  is formed in an annular shape having no corner when viewed from the Z-axis direction. In the welded portion  7  of the embodiment, the shape of the first welding track  710  and the shape of the second welding track  720  are both annular without corner (in the example of  FIG.  6 A , whet is called a racetrack type). In the welded portion  7  of the embodiment, the welding track is formed in a plurality of concentric circles (for example, a double circle shape). 
     The first welding track  710  is separated from the second welding track  720 . Specifically, the first welding track  710  and the second welding track  720  are separated from each other in the range where the first welded portion  71  and the second welded portion  72  overlap each other at the boundary position between the bus bar  6  and the external terminal  4   b . The first welding track  710  is disposed inside the second welding track  720 . 
     In the conventional energy storage apparatus, when the external terminal  4   b  and the bus bar  6  are welded by the laser welding such that the welding track is formed around the bus bar  6 , as illustrated in  FIGS.  7 A and  7 B , the depth of the penetration between the external terminal  4   b  and the bus bar  6  is increased in order to increase the width of the welded portion  7  at the boundary position between the external terminal  4   b  and the bus bar  6  (the upper surface  40  of the external terminal  4   b ) for the purpose of securing the bonding strength between the external terminal  4   b  and the bus bar  6 . Consequently, in the conventional energy storage apparatus, the end  70  of the welded portion  7  reaches the interface  43  between the first metal layer  41  and the second metal layer  42  that constitute the clad material, and sometimes the bonding strength between the metal layers  41  and  42  is not secured. 
     On the other hand, in the energy storage apparatus  11  of the embodiment, because the end  70  of the welded portion  7  is positioned in front of the interface  43  between the first metal layer  41  and the second metal layer  42  (see  FIG.  6 B ), namely, because the welded portion  7  does not reach the interface  43  between the first metal layer  41  and the second metal layer  42  that constitute the clad material of the external terminal  4   b , the bonding strength between the metal layers  41  and  42  that constitute the clad material is secured. The details are as follows. 
     In the case where the laser beam having a small spot diameter (less than or equal to 1.0 mm) is used for welding the bus bar  6  and the external terminal  4   b , when the width of the welded portion  7  is small, the welding strength between the external terminal  4   b  and the bus bar  6  cannot be secured. On the other hand, when the penetration depth in the welded portion  7  is increased in order to secure the welding strength, the end  70  of the welded portion  7  reaches the interface between the first metal layer  41  and the second metal layer  42  of the external terminal (clad material)  4   b  to generate an alloy of copper and aluminum, so that the strength of the external terminal  4   b  (particularly, the flange  401   b  formed of the clad material) decreases. Therefore, like the method for manufacturing the energy storage apparatus  11  of the embodiment, the end  70  of the welded portion  7  does not reach the interface  43  between the first metal layer  41  and the second metal layer  42  of the clad material (external terminal  4   b ) as a method for welding the external terminal  4   b  (flange  401   b ) and the bus bar  6  with the laser beam having the small spot diameter so as not to lower the interface strength of the clad material. At this point, the spot diameter of the laser welding in the embodiment is the width of the laser beam focused by the laser beam being narrowed at the focal position. That is, the width of the laser that focuses on the upper surface of the bus bar  6  is the spot diameter. 
     For example, specifically, like the energy storage apparatus  11  of the embodiment, the first welded portion  71  and the second welded portion  72  partially overlap with each other to secure the wide bonding width of the welded portion  7 , so that the bonding strength between the bus bar  6  and the external terminal  4   b  can be secured. That is, as compared with the case where the first welded portion  71  and the second welded portion  72  are welded while being separated from each other (in the state where the first welded portion  71  and the second welded portion  72  do not overlap each other), when the first welded portion  71  and the second welded portion  72  partially overlap each other like the energy storage apparatus  11  of the embodiment, an area (the size of the width W 1 , see  FIG.  6 ( b ) ) of the welded portion  7  is secured, and the sufficient connection strength can be obtained at the connection (welding) member between the bus bar  6  and the external terminal  4   b.    
     In the energy storage apparatus  11  of the embodiment, the width. W 1  at the boundary position between the external terminal  4   b  of the welded portion  7  and the bus bar  6  is larger than the thickness T 1  of the first metal layer  41 , so that the dimension of the welded portion  7  is sufficiently secured at the boundary position. Consequently, the bonding strength between the bus bar  6  and the external terminal  4   b  can be secured. Thus, the bonding strength between the bus bar  6  and the external terminal  4   b  can be secured while the bonding strength between the metal layers  41  and  42  constituting the clad material is secured. 
     Like the energy storage apparatus  11  of this embodiment, when the clad material including the first metal layer  41  made of aluminum and the second metal layer  42  made of the copper is made to constitute the external terminal  4   b  (particularly, the flange  401   b ), the electric resistance of the external terminal  4   b  can be restrained by making the width W 1  of the welded portion  7  at the boundary position between the bus bar  6  and the external terminal  4   b  larger than the thickness T 1  of the first metal layer  41 . That is, when the width W 1  at the boundary position of the welded portion  7  is made larger than the thickness T 1  of the first metal layer  41  while the end  70  of the welded portion  7  does not reach the interface  43  between the first metal layer  41  and the second metal layer  42 , the layer of aluminum (first metal layer  41 ) having electrical conductivity higher than that of copper is thinned while the welding strength between the external terminal  4   b  and the bus bar  6  is sufficiently secured, so that the electric resistance of the entire external terminal  4   b  can be restrained. 
     In the energy storage apparatus  11  of the embodiment, the bus bar  6  and the external terminal  4   b  are cut at the position where the bus bar  6  and the external terminal  4   b  cross the welded portion  7  (welding tracks  710 ,  720 ), and the section (the section along the laminating direction) of the bus bar  6  and the external terminal  4   b  is etched, so that the penetration depth (the position of the end  70  in the Z-axis direction), the shape, the width W 1 , and the like of the welded portion  7  can be checked because a corrosion speed is different between the welded portion  7  and other portions. 
     Furthermore, in the energy storage apparatus  11  of the embodiment, because the welded portion  7  has the annular shape without the corner, force is hardly concentrated on a portion of the welded portion  7  even when the force is applied to the welded portion  7 . In other words, the welded portion  7  has the annular shape without the corner, so that stress concentration in the welded portion  7  can be restrained. 
     In the method for manufacturing the energy storage apparatus  11  described above, the welded portion  7  does not reach the interface between the metal layers  41 ,  42  that constitute the clad material of the external terminal  4   b , so that the energy storage apparatus  11  in which the bonding strength between the metal layers  41 ,  42  constituting the clad material is secured can be manufactured. 
     In the method for manufacturing the energy storage apparatus  11  of the embodiment, when the welding is performed with a general welding width, the annular welded portion  7  having no corner where the force is hardly concentrated on one portion can be formed. 
     In the energy storage apparatus  11  of the embodiment, the energy storage devices  1  are arranged in the direction orthogonal to the laminating direction of the external terminals  4   b  and the bus bars  6 . Thus, the laminated portion between the external terminal  4   b  and the bus bar  6  in each energy storage device  1  is not hidden by another energy storage device, so that the welding between the external terminal  4   b  and the bus bar  6  can be easily performed. In addition, the laminated portion of the external terminal  4   b  and the bus bar  6  is not hidden by another energy storage device, so that alignment of the bus bar  6  and the external terminal  4   b  before the welding is easy to perform. 
     It is needless to say that the energy storage apparatus and the method for manufacturing the energy storage apparatus of the present invention are not limited to the above-mentioned embodiment, and various modifications can be made without departing from the gist of the present invention. For example, the configuration of another embodiment can be added to the configuration of one embodiment, and a part of the configuration of one embodiment can be replaced with the configuration of another embodiment. Furthermore, a part of the configuration of an embodiment can be deleted. 
     In the embodiment, the welded portion  7  viewed from the Z-axis direction has the shape without the corner, but may be a shape with the corner (for example, a polygonal shape having the corner, a linear shape, and a polygonal line shape). 
     The welded portion  7  viewed from the Z-axis direction preferably has the shape without the corner, and may be an arc shape without the corner. Even in this case, because the welded portion  7  has the shape without the corner, the force does not concentrate on a part of the welded portion  7  even when the force is applied to the welded portion  7 . When the welded portion  7  has the arc shape, the arc shape having a length greater than or equal to a semicircle is more preferable from the viewpoint of ensuring the bonding strength between the external terminal  4   b  constituting the clad material and the bus bar  6 . 
     Furthermore, the shape of the welded portion  7  viewed from the Z-axis direction may be a spiral shape without a corner or the like. The shape of the welded portion  7  as viewed from the Z-axis direction may be a curved shape or the like. 
     Although the welded portion  7  includes the first welding track  710  and the second welding track  720  as two welding tracks, the welded portion  7  may include one welding track or at least three welding tracks. When the welded portion  7  includes the plurality of welding tracks, the plurality of welding tracks may be separated from each other or partially overlap each other. When the plurality of welding tracks overlap each other in the welded portion  7 , the output of the welding may be limited (adjusted) such that the end  70  of the welded portion  7  does not reach the interface  43  between the metal layers  41 ,  42 . 
     The dimension (dimension of the width W 1 ) of the welded portion  7  at the boundary position between the external terminal  4   b  and the bus bar  6  in the Z-axis direction (the outer surface  40  of the external terminal  4 ) in the direction along this boundary is larger than the thickness T 1  of the first metal layer  41 , but may be the same as the thickness T 1  of the first metal layer  41  or be smaller than the thickness T 1  of the first metal layer  41 . 
     In the embodiment, the flange (clad material)  401   b  of the external terminal  4   b  is constituted of two metal layers, but may be constituted of a plurality of metal layers of at least three layers. Also in this case, the end  70  of the welded portion  7  is positioned closer to the bus bar  6  than the interface between the first metal layer  41  adjacent to the bus bar  6  and the second metal layer  42  adjacent to the first metal layer  41  in the flange (clad material)  401   b  of the external terminal  4   b , so that the configuration in which the welded portion  7  does not reach the interface  43  between the metal layers  41 ,  42  constituting the clad material can be implemented. 
     Furthermore, in the embodiment, the energy storage apparatus  11  includes the plurality of energy storage devices  1 . However, the energy storage apparatus  11  may include one energy storage device  1 . 
     In the embodiment, the case where the energy storage device is used as the nonaqueous electrolyte secondary battery (for example, the lithium ion secondary battery) that can be charged and discharged has been described. However, the type and the size (capacity) of the energy storage device are arbitrary. In the embodiment, the lithium ion secondary battery has been described as an example of the energy storage device, but the present invention is not limited thereto. For example, the present invention is also applicable to various secondary batteries, primary batteries, and energy storage devices of capacitors such as an electric double layer capacitor. 
     In order to express the present invention, the present invention has been appropriately and sufficiently described above through the embodiment with reference to the drawings, but it should be recognized that a person skilled in the art can easily modify and/or improve the embodiment. Accordingly, as long as the modification or improvement implemented by those skilled in the art are not at a level departing from the scope of the claims described in the claims, the modification or improvement is interpreted to be included in the scope of the claims. 
     DESCRIPTION OF REFERENCE SIGNS 
       1 : energy storage device 
       2 : electrode assembly 
       3 : case 
       4 ,  4   a ,  4   b : external terminal 
       5 : current collector 
       6 : bus bar 
       7 : welded portion 
       11 : energy storage apparatus 
       21 : winding core 
       22 : layered product 
       31 : case body 
       32 : lid plate 
       33 : internal space 
       40 : outer surface (upper surface) 
       41 : first metal layer (metal layer) 
       42 : second metal layer (metal layer) 
       43 : interface 
       70 : end 
       71 : first welded portion 
       72 : second welded portion 
       311 : closing portion 
       312 : body portion 
       321 : gas release valve 
       401   a ,  401   b : flange 
       402   a ,  402   b : shaft 
       710 : first welding track 
       720 : second welding track 
     W 1 : width