Patent Publication Number: US-11664560-B2

Title: Secondary battery and method of manufacturing the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS: 
     This application is a Continuation of U.S. patent application Ser. No. 16/248,017, filed Jan. 15, 2019, which claims priority to Japanese Patent Application No. 2018-005265 filed in the Japan Patent Office on Jan. 17, 2018, each of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a secondary battery and a method of manufacturing the secondary battery. 
     Description of Related Art 
     Prismatic secondary batteries such as alkali secondary batteries and non-aqueous electrolyte secondary batteries are used in power sources for driving, for example, electric vehicles (EVs) and hybrid electric vehicles (HEVs or PHEVs). 
     Each of the prismatic secondary batteries includes a battery case formed of a prismatic exterior body in the form of a tube having an opening and a bottom and a sealing plate that seals the opening of the exterior body. In the battery case, an electrode body and an electrolyte solution are accommodated, and the electrode body is formed of a positive-electrode sheet, a negative-electrode sheet, and a separator. A positive-electrode terminal and a negative-electrode terminal are secured to the sealing plate. The positive-electrode terminal is electrically connected to the positive-electrode sheet with a positive-electrode current collector interposed therebetween. The negative-electrode terminal is electrically connected to the negative-electrode sheet with a negative-electrode current collector interposed therebetween. 
     In such a prismatic secondary battery, the positive-electrode terminal and the positive-electrode current collector are preferably connected to each other by crimping and welding, and the negative-electrode terminal and the negative-electrode current collector are preferably connected to each other by crimping and welding. 
     For example, Japanese Published Unexamined Patent Application No. 2011-076867 (Patent Document 1) discloses a technique of laser welding by which a crimped portion of a terminal is welded to a current collector. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a secondary battery that enables the reliability of a joint between a terminal and a current collector to be improved, and a method of manufacturing the secondary battery. 
     A method of manufacturing a secondary battery according to an embodiment of the present invention is a method of manufacturing a secondary battery including an electrode body that includes a positive-electrode sheet and a negative-electrode sheet, an exterior body that has an opening and that accommodates the electrode body, a sealing plate that has a terminal insertion hole and that seals the opening, a terminal that extends through the terminal insertion hole, a current collector that has a terminal connection hole and that is electrically connected to the positive-electrode sheet or the negative-electrode sheet, and a tapered portion that is formed around the terminal connection hole and that has an inner diameter that gradually increases toward an end portion thereof before the terminal and the current collector are welded to each other. The method includes an insertion step of inserting the terminal into the terminal connection hole, a crimping step of crimping the terminal on the tapered portion to form a crimped portion of the terminal, and a welding step of welding the terminal and the current collector to each other by radiating energy rays toward at least one of the crimped portion of the terminal and the tapered portion of the current collector. The crimping step includes crimping the terminal such that a gap is formed between the crimped portion and the tapered portion. The welding step incudes melting at least one of the terminal and the current collector due to the energy rays such that a melt of metal of which the at least one of the terminal and the current collector is composed flows into the gap and a solidified portion that is solidified from the melt has a recessed portion. 
     With the method of manufacturing the secondary battery according to the embodiment of the present invention, the terminal and the current collector are more firmly connected to each other, and the secondary battery has high reliability. In the case where the melted metal flows into the gap between the crimped portion of the terminal and the tapered portion of the current collector such that the solidified portion that is solidified from the melted metal has the recessed portion, overlap, which is a welding defect, can be inhibited from occurring. In addition, the solidified portion can be effectively inhibited from having a bulge shape. Accordingly, the solidified portion is unlikely to be damaged, and the reliability of the secondary battery increases. The terminal and the current collector are preferably composed of a metal. The terminal may be a positive-electrode terminal or a negative-electrode terminal, and the current collector may be a positive-electrode current collector or a negative-electrode current collector. 
     A bottom of the recessed portion is preferably located nearer to the sealing plate than a surface of the current collector farther from the sealing plate. 
     In this case, the solidified portion is more unlikely to be damaged, and the reliability of the secondary battery increases. In addition, overlap, which is a welding defect, can be more effectively inhibited from occurring. 
     The crimping step preferably includes crimping the terminal such that the terminal projects in a first direction beyond the surface of the current collector farther from the sealing plate where the first direction is a direction from the sealing plate toward the current collector. The welding step preferably includes making a volume of a melted portion of the terminal due to the energy rays larger than a volume of a melted portion of the current collector due to the energy rays. 
     This enables the melted metal to readily enter the gap between the crimped portion of the terminal and the tapered portion of the current collector, the terminal and the current collector are more firmly connected to each other, and the reliability of the secondary battery increases. In addition, the solidified portion is more unlikely to be damaged, and the reliability of the secondary battery increases. 
     A thickness of the solidified portion at a bottom of the recessed portion in a thickness direction of the current collector is preferably more than a thickness of the solidified portion at a side wall of the recessed portion in a radial direction of the terminal connection hole. 
     In this case, the terminal and the current collector are more firmly connected to each other, and the reliability of the secondary battery increases. In addition, the solidified portion is more unlikely to be damaged, and the reliability of the secondary battery increases. 
     It is preferable that a portion of the terminal that is farthest from the sealing plate in a thickness direction of the sealing plate be not melted due to the energy rays in the welding step. 
     In this case, a manufacturing apparatus, a jig, and other components are unlikely to come into contact with the solidified portion in the secondary battery. In the secondary battery, the solidified portion is unlikely to be damaged. 
     The current collector is preferably composed of copper or copper alloy. The terminal preferably includes a portion composed of aluminum or aluminum alloy and a portion composed of copper or copper alloy. The crimping step preferably includes crimping the portion composed of copper or copper alloy. 
     This enables the negative-electrode terminal and the negative-electrode current collector to be more firmly connected to each other and enables a conductive member composed of aluminum or aluminum alloy to be firmly connected to the negative-electrode terminal. 
     A secondary battery according to an embodiment of the present invention includes an electrode body that includes a positive-electrode sheet and a negative-electrode sheet, an exterior body that has an opening and that accommodates the electrode body, a sealing plate that has a terminal insertion hole and that seals the opening, a terminal that extends through the terminal insertion hole, and a current collector that has a terminal connection hole and that is electrically connected to the positive-electrode sheet or the negative-electrode sheet. The terminal is inserted in the terminal connection hole. The terminal includes a crimped portion that has an outer diameter larger than a minimum inner diameter thereof around the terminal connection hole. The crimped portion and the current collector are joined to each other with a solidified portion that is solidified from a melt of at least one of the crimped portion and the current collector. The solidified portion has a recessed portion. A bottom of the recessed portion is located nearer to the sealing plate than a surface of the current collector facing the electrode body. 
     A tapered portion is preferably formed around the terminal connection hole and preferably has an inner diameter that gradually increases in a direction away from the sealing plate. 
     At least a part of the crimped portion preferably projects toward the electrode body beyond the surface of the current collector facing the electrode body. 
     A thickness of the solidified portion at a bottom of the recessed portion in a thickness direction of the current collector is preferably more than a thickness of the solidified portion at a side wall of the recessed portion in a radial direction of the terminal connection hole. 
     According to the present invention, a secondary battery having high reliability can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG.  1    is a perspective view of a secondary battery according to an embodiment. 
         FIG.  2    is a sectional view of  FIG.  1    taken along line II-II. 
         FIG.  3    is a plan view of a positive-electrode sheet according to the embodiment. 
         FIG.  4    is a plan view of a negative-electrode sheet according to the embodiment. 
         FIG.  5    is a plan view of an electrode element according to the embodiment. 
         FIG.  6    illustrates positive-electrode tab groups that are connected to a positive-electrode current collector and negative-electrode tab groups that are connected to a negative-electrode current collector. 
         FIG.  7    illustrates a surface of a sealing plate facing an electrode body after a first positive-electrode current collector and a first negative-electrode current collector are secured. 
         FIG.  8 A  is a sectional view of the vicinity of a positive-electrode terminal taken along the transverse direction of the sealing plate before the positive-electrode terminal is crimped. 
         FIG.  8 B  is a sectional view of the vicinity of the positive-electrode terminal taken along the transverse direction of the sealing plate after the positive-electrode terminal is crimped. 
         FIG.  8 C  is a sectional view of the vicinity of the positive-electrode terminal taken along the transverse direction of the sealing plate after the positive-electrode terminal and the first positive-electrode current collector are welded. 
         FIG.  9 A  is an enlarged view of the positive-electrode terminal near a crimped portion thereof in  FIG.  8 B . 
         FIG.  9 B  is an enlarged view of the positive-electrode terminal near the crimped portion in  FIG.  8 C . 
         FIG.  10    illustrates the surface of the sealing plate facing the electrode body after a second positive-electrode current collector is secured to the first positive-electrode current collector and a second negative-electrode current collector is secured to the first negative-electrode current collector. 
         FIG.  11    is a sectional view of the vicinity of a negative-electrode terminal taken along the longitudinal direction of the sealing plate. 
         FIG.  12    is an enlarged view of the vicinity of a joint between the first negative-electrode current collector and the second negative-electrode current collector in  FIG.  11   . 
         FIG.  13 A  and  FIG.  13 B  illustrate perspective views of the sealing plate and a cover after components are secured. 
         FIG.  14    is a sectional view of the vicinity of the positive-electrode terminal taken along the transverse direction of the sealing plate. 
         FIG.  15    is a sectional view of the vicinity of the positive-electrode terminal taken along the longitudinal direction of the sealing plate. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The structure of a prismatic secondary battery  20  that corresponds to a secondary battery according to an embodiment will hereinafter be described. The present invention is not limited to the embodiment described below. 
     As illustrated in  FIG.  1    and  FIG.  2   , the prismatic secondary battery  20  includes a battery case  100  formed of a prismatic exterior body  1  in the form of a prism having an opening and a bottom and a sealing plate  2  that seals the opening of the prismatic exterior body  1 . The prismatic exterior body  1  and the sealing plate  2  are preferably composed of a metal and are preferably composed of, for example, aluminum or aluminum alloy. In the prismatic exterior body  1 , an electrode body  3  and an electrolyte are accommodated, and the electrode body  3  is formed of positive-electrode sheets and negative-electrode sheets that are stacked with separators interposed therebetween. 
     Positive-electrode tabs  40  and negative-electrode tabs  50  are disposed on an end portion of the electrode body  3  facing the sealing plate  2 . The positive-electrode tabs  40  are electrically connected to a positive-electrode terminal  7  with a second positive-electrode current collector  6   b  and a first positive-electrode current collector  6   a  interposed therebetween. The negative-electrode tabs  50  are electrically connected to a negative-electrode terminal  9  with a second negative-electrode current collector  8   b  and a first negative-electrode current collector  8   a  interposed therebetween. 
     The first positive-electrode current collector  6   a , the second positive-electrode current collector  6   b , and the positive-electrode terminal  7  are preferably composed of a metal and more preferably composed of aluminum or aluminum alloy. An outer insulating member  10  composed of a resin is disposed between the positive-electrode terminal  7  and the sealing plate  2 . An inner insulating member  11  composed of a resin is disposed between the first positive-electrode current collector  6   a  and the and the sealing plate  2  and between the second positive-electrode current collector  6   b  and the sealing plate  2 . 
     The first negative-electrode current collector  8   a , the second negative-electrode current collector  8   b , and the negative-electrode terminal  9  are preferably composed of a metal and more preferably composed of copper or copper alloy. The negative-electrode terminal  9  preferably includes a portion composed of aluminum or aluminum alloy and a portion composed of copper or copper alloy. In this case, the portion composed of copper or copper alloy is preferably connected to the first negative-electrode current collector  8   a , and the portion composed of aluminum or aluminum alloy preferably projects toward the outside beyond the sealing plate  2 . An outer insulating member  12  composed of a resin is disposed between the negative-electrode terminal  9  and the sealing plate  2 . An inner insulating member  13  composed of a resin is disposed between the first negative-electrode current collector  8   a  and the sealing plate  2  and between the second negative-electrode current collector  8   b  and the sealing plate  2 . 
     An electrode body holder  14  that is formed of a resin sheet is disposed between the electrode body  3  and the prismatic exterior body  1 . The electrode body holder  14  is preferably molded by bending an insulating sheet composed of a resin into a bag shape or a box shape. The sealing plate  2  has an electrolytic solution injection hole  15 . The electrolytic solution injection hole  15  is sealed by a sealing member  16 . A gas exhausting valve  17  is disposed in the sealing plate  2 . The gas exhausting valve  17  is broken when the pressure in the battery case  100  becomes a certain pressure or more, and gas in the battery case  100  is discharged therefrom to the outside of the battery case  100 . An annular projection  2   c  is formed on the surface of the sealing plate  2  on the inner side of the battery around the gas exhausting valve  17 . 
     A method of manufacturing the prismatic secondary battery  20  and the structure thereof will now be described in detail. 
     Positive-Electrode Sheet 
       FIG.  3    is a plan view of a positive-electrode sheet  4 . The positive-electrode sheet  4  includes a main body in which positive electrode active material mixture layers  4   b  including a positive electrode active material are formed on both surfaces of a rectangular positive-electrode core  4   a . The positive-electrode core  4   a  projects from an end side of the main body. The positive-electrode core  4   a  that projects forms the positive-electrode tabs  40 . The positive-electrode tabs  40  may be parts of the positive-electrode core  4   a  as illustrated in  FIG.  3   . Other members that are connected to the positive-electrode core  4   a  may be used as the positive-electrode tabs  40 . A positive-electrode protection layer  4   d  having an electric resistance larger than the electric resistance of the positive electrode active material mixture layers  4   b  are preferably disposed on a portion of the positive-electrode tab  40  adjacent to the positive electrode active material mixture layers  4   b . A metal foil such as an aluminum foil or aluminum alloy foil is preferably used as the positive-electrode core  4   a . For example, a lithium transition metal composite oxide is preferably used as the positive electrode active material. 
     Negative-Electrode Sheet 
       FIG.  4    is a plan view of a negative-electrode sheet  5 . The negative-electrode sheet  5  includes a main body in which negative electrode active material mixture layers  5   b  including a negative electrode active material are formed on both surfaces of a rectangular negative-electrode core  5   a . The negative-electrode core  5   a  projects from an end side of the main body. The negative-electrode core  5   a  that projects forms the negative-electrode tabs  50 . The negative-electrode tabs  50  may be parts of the negative-electrode core  5   a  as illustrated in  FIG.  4   . Other members that are connected to the negative-electrode core  5   a  may be used as the negative-electrode tabs  50 . A metal foil such as a copper foil or copper alloy foil is preferably used as the negative-electrode core  5   a . For example, a carbon material or a silicon material is preferably used as the negative electrode active material. 
     Manufacture of Electrode Body Element 
     The above method is used to manufacture  50  positive-electrode sheets  4  and  51  negative-electrode sheets  5 . These are stacked with polyolefin rectangular separators interposed therebetween to manufacture multilayer electrode body elements (a first electrode body element  3   a  and a second electrode body element  3   b ). As illustrated in  FIG.  5   , the multilayer electrode body elements (the first electrode body element  3   a  and the second electrode body element  3   b ) include positive-electrode tab groups (a first positive-electrode tab group  40   a  and a second positive-electrode tab group  40   b ) and negative-electrode tab groups (a first negative-electrode tab group  50   a  and a second negative-electrode tab group  50   b ). The positive-electrode tabs  40  of the positive-electrode sheets  4  are stacked on end portions of the positive-electrode tab groups. The negative-electrode tabs  50  of the negative-electrode sheets  5  are stacked on end portions of the negative-electrode tab groups. 
     Some of the separators are located on both outer surfaces of each electrode body element. The electrode sheets and the separators that are stacked can be secured with, for example, a tape. Alternatively, adhesive layers may be formed on each separator, the separator and the corresponding positive-electrode sheet  4  may adhere to each other, and the separator and the corresponding negative-electrode sheet  5  may adhere to each other. The separators may be formed in a jig zag pattern to stack the positive-electrode sheets  4  and the negative-electrode sheets  5 . 
     The size of each separator in a plan view is preferably equal to or larger than the size of each negative-electrode sheet  5 . Each positive-electrode sheet  4  or each negative-electrode sheet  5  may be disposed between two separators, and the positive-electrode sheets  4  and the negative-electrode sheets  5  may be stacked after the peripheries of the separators are thermally welded to each other. A belt-like positive-electrode sheet and a belt-like negative-electrode sheet may be wound with a belt-like separator interposed therebetween to form a wound electrode body element. 
     Connection between Current Collector and Tab 
     The two electrode body elements, which are the first electrode body element  3   a  and the second electrode body element  3   b , are manufactured in the above manner. The first electrode body element  3   a  and the second electrode body element  3   b  may have the same structure or may have different structures. The positive-electrode tabs  40  of the first electrode body element  3   a  form the first positive-electrode tab group  40   a . The negative-electrode tabs  50  of the first electrode body element  3   a  form the first negative-electrode tab group  50   a . The positive-electrode tabs  40  of the second electrode body element  3   b  form the second positive-electrode tab group  40   b . The negative-electrode tabs  50  of the second electrode body element  3   b  form the second negative-electrode tab group  50   b.    
       FIG.  6    illustrates the first positive-electrode tab group  40   a  and the second positive-electrode tab group  40   b  that are connected to the second positive-electrode current collector  6   b  and the first negative-electrode tab group  50   a  and the second negative-electrode tab group  50   b  that are connected to the second negative-electrode current collector  8   b . The second positive-electrode current collector  6   b  and the second negative-electrode current collector  8   b  are disposed between the first electrode body element  3   a  and the second electrode body element  3   b . The first positive-electrode tab group  40   a  and the second positive-electrode tab group  40   b  are disposed on the second positive-electrode current collector  6   b . The first negative-electrode tab group  50   a  and the second negative-electrode tab group  50   b  are disposed on the second negative-electrode current collector  8   b . The first positive-electrode tab group  40   a  and the second positive-electrode tab group  40   b  are welded to the second positive-electrode current collector  6   b  to form welds  60 . The first negative-electrode tab group  50   a  and the second negative-electrode tab group  50   b  are welded to the second negative-electrode current collector  8   b  to form welds  61 . A welding method is preferably ultrasonic welding or resistance welding. The laser welding can be used for connection. In the positive-electrode current collectors  6 , a current collector opening  6   e  is formed at a position at which the positive-electrode current collector faces the electrolytic solution injection hole  15 . 
     The second positive-electrode current collector  6   b  has an opening  6   c . The opening  6   c  is formed within a thin portion  6   d . The second negative-electrode current collector  8   b  has an opening  8   c . The opening  8   c  is formed within a thin portion  8   d.    
     Securing Components to Sealing Plate 
     The outer insulating member  10  is disposed on the sealing plate  2  on the outer surface side of the battery around a positive-electrode terminal insertion hole  2   a . The inner insulating member  11  and the first positive-electrode current collector  6   a  are disposed on the sealing plate  2  on the inner surface side of the battery around the positive-electrode terminal insertion hole  2   a . The positive-electrode terminal  7  is inserted into a through-hole of the outer insulating member  10 , the positive-electrode terminal insertion hole  2   a  of the sealing plate  2 , a through-hole of the inner insulating member  11 , and a terminal connection hole of the first positive-electrode current collector  6   a  from the outside of the battery. An end portion of the positive-electrode terminal  7  is crimped on the first positive-electrode current collector  6   a . Consequently, the positive-electrode terminal  7  and the first positive-electrode current collector  6   a  are secured to the sealing plate  2 . A crimped portion of the positive-electrode terminal  7  and the first positive-electrode current collector  6   a  are preferably welded to each other. 
     The outer insulating member  12  is disposed on the sealing plate  2  on the outer surface side of the battery around a negative-electrode terminal insertion hole  2   b . The inner insulating member  13  and the first negative-electrode current collector  8   a  are disposed on the sealing plate  2  on the inner surface side of the battery around the negative-electrode terminal insertion hole  2   b . The negative-electrode terminal  9  is inserted into a through-hole of the outer insulating member  12 , the negative-electrode terminal insertion hole  2   b  of the sealing plate  2 , a through-hole of the inner insulating member  13 , and a terminal connection hole of the first negative-electrode current collector  8   a  from the outside of the battery. An end portion of the negative-electrode terminal  9  is crimped on the first negative-electrode current collector  8   a . Consequently, the negative-electrode terminal  9  and the first negative-electrode current collector  8   a  are secured to the sealing plate  2 . A crimped portion of the negative-electrode terminal  9  and the first negative-electrode current collector  8   a  are preferably welded to each other. 
       FIG.  7    illustrates the surface of the sealing plate  2  on the inner surface side of the battery after the positive-electrode terminal  7 , the outer insulating member  10 , the inner insulating member  11 , the first positive-electrode current collector  6   a , the negative-electrode terminal  9 , the outer insulating member  12 , the inner insulating member  13 , and the first negative-electrode current collector  8   a  are secured. The inner insulating member  11  on the positive electrode side includes a base  11   a  that is disposed along the sealing plate  2 . A pair of second walls  11   b  that project from the base  11   a  toward the electrode body  3  are disposed on both ends of the base  11   a  in the transverse direction of the sealing plate  2 . A pair of first walls  11   c  that project from the base  11   a  toward the electrode body  3  are disposed on both ends of the base  11   a  in the transverse direction of the sealing plate  2 . An outer circumferential rib  11   d  is disposed along an outer circumference of the base  11   a  of the inner insulating member  11  at a position at which the second walls  11   b  and the first walls  11   c  are not disposed. As illustrated in  FIG.  7   , the first positive-electrode current collector  6   a  and the positive-electrode terminal  7  are connected to each other between the pair of the first walls  11   c.    
     A current-collector projection  6   x  is formed on the surface of the first positive-electrode current collector  6   a  facing the electrode body  3 . The shape of the current-collector projection  6   x  in a plan view is preferably a shape having the longitudinal direction and the transverse direction such as a rectangle, an ellipse, or a track shape. 
     The inner insulating member  13  on the negative electrode side includes a base  13   a  that is disposed along the sealing plate  2 . A pair of third walls  13   b  that project from the base  13   a  toward the electrode body  3  are disposed on both ends of the base  13   a  in the transverse direction of the sealing plate  2 . An outer circumferential rib  13   c  is disposed along an outer circumference of the base  13   a  of the inner insulating member  13  at a position at which the third walls  13   b  are not disposed. 
     A current-collector projection  8   x  is formed on the surface of the first negative-electrode current collector  8   a  facing the electrode body  3 . The shape of the current-collector projection  8   x  in a plan view is preferably a shape having the longitudinal direction and the transverse direction such as a rectangle, an ellipse, or a track shape. 
     Connection between Terminal and Current Collector 
     A method of connecting the negative-electrode terminal  9  and the first negative-electrode current collector  8   a  to each other is taken as an example to describe a method of connecting the positive-electrode terminal  7  and the first positive-electrode current collector  6   a  to each other and a method of connecting the negative-electrode terminal  9  and the first negative-electrode current collector  8   a  to each other. The positive-electrode terminal  7  and the first positive-electrode current collector  6   a  can be connected to each other in the same manner as the negative-electrode terminal  9  and the first negative-electrode current collector  8   a  are connected to each other. 
     As illustrated in  FIG.  8 A , an insertion portion  9   b  that is disposed on a flange  9   a  of the negative-electrode terminal  9  is inserted into a terminal connection hole  8   y  that is formed in the first negative-electrode current collector  8   a . A tapered portion  8   z  is formed around the terminal connection hole  8   y  and has an inner diameter that gradually increases toward the end portion thereof farther from the sealing plate  2 . The insertion portion  9   b  is inserted into the terminal connection hole  8   y  from the sealing plate  2 . The negative-electrode terminal  9  preferably includes a first metal portion  9   x  composed of a first metal and a second metal portion  9   y  composed of a second metal that differs from the first metal. The first metal is preferably aluminum or aluminum alloy. The second metal is preferably copper or copper alloy. A layer composed of a third metal may be formed between the first metal portion  9   x  and the second metal portion  9   y . The third metal is preferably nickel, or another metal. 
     Subsequently, as illustrated in  FIG.  8 B , an end of the insertion portion  9   b  of the negative-electrode terminal  9  is deformed such that the diameter of the end is increased to form a crimped portion  9   c . Consequently, a region of the insertion portion  9   b  on the end side is crimped on the first negative-electrode current collector  8   a . The crimped portion  9   c  is crimped on the tapered portion  8   z . A gap  95  is formed between the tapered portion  8   z  and the insertion portion  9   b . The crimped portion  9   c  has an outer diameter larger than the minimum inner diameter thereof around the terminal connection hole  8   y . A part of the crimped portion  9   c  projects away from the sealing plate  2  beyond a surface  8   a   1  of the first negative-electrode current collector  8   a  farther from the sealing plate  2 . A recessed end portion  9   d  is preferably formed on an end surface of the insertion portion  9   b . The recessed end portion  9   d  that is formed on the end surface of the insertion portion  9   b  enables the crimped portion  9   c  to be more stably formed. Energy rays such as laser rays are radiated toward the gap  95  to melt the crimped portion  9   c  and the tapered portion  8   z  of the first negative-electrode current collector  8   a . The melted portions are solidified, and, as illustrated in  FIG.  8 C , a solidified portion  70  is formed. The negative-electrode terminal  9  and the first negative-electrode current collector  8   a  are joined to each other with the solidified portion  70 . The metal of which the negative-electrode terminal  9  or the first negative-electrode current collector  8   a  that is melted is composed flows into the gap  95  and solidifies. The solidified portion  70  has a recessed portion  70   a.    
     In the case where the negative-electrode terminal  9  and the first negative-electrode current collector  8   a  are connected to each other in the above manner, the negative-electrode terminal  9  and the first negative-electrode current collector  8   a  are more firmly connected to each other, and the prismatic secondary battery  20  has high reliability. In the case where the melted metal flows into the gap  95  between the crimped portion  9   c  of the negative-electrode terminal  9  and the tapered portion  8   z  of the first negative-electrode current collector  8   a  such that the solidified portion  70  that is solidified from the melted metal has the recessed portion  70   a , overlap, which is a welding defect, can be inhibited from occurring. In addition, the solidified portion  70  can be effectively inhibited from having a bulge shape. Accordingly, the solidified portion  70  is unlikely to be damaged, and the reliability of the prismatic secondary battery  20  increases. 
     The bottom of the recessed portion  70   a  is located nearer to the sealing plate  2  (lower side in  FIG.  9 B ) than the surface  8   a   1  (the upper surface of the first negative-electrode current collector  8   a  in  FIG.  9 B ) of the first negative-electrode current collector  8   a  farther from the sealing plate  2 . This makes overlap, which is a welding defect, unlikely to occur during welding. Accordingly, a part of the solidified portion  70  can be effectively prevented from chipping and falling, and a short circuit is effectively prevented from occurring. In addition, the solidified portion  70  is more unlikely to be damaged. 
     The negative-electrode terminal  9  is preferably crimped such that the crimped portion  9   c  of the negative-electrode terminal  9  projects away from the sealing plate  2  (upward in  FIG.  8 B ) beyond the surface  8   a   1  of the first negative-electrode current collector  8   a  farther from the sealing plate  2 . The amount of the melt (the volume of the melted portion) of the negative-electrode terminal  9  due to the energy rays is preferably larger than the amount of the melt (the volume of the melted portion) of the first negative-electrode current collector  8   a  due to the energy rays. This enables the melted metal to readily enter the gap  95 , and the negative-electrode terminal  9  and the first negative-electrode current collector  8   a  are more firmly connected to each other. 
     A bottom and side walls that define the recessed portion  70   a  correspond to the solidified portion  70  that is solidified from the melted metal due to the energy rays. The thickness of a portion  70   x  of the solidified portion  70  that is located at the bottom of the recessed portion  70   a  in the thickness direction (vertical direction in  FIG.  9 B ) of the first negative-electrode current collector  8   a  is preferably more than the thickness of a portion  70   y  of the solidified portion  70  that is located at one of the side walls of the recessed portion  70   a  in the radial direction (left-right direction in  FIG.  9 B ) of the terminal connection hole  8   y . With this structure, the negative-electrode terminal  9  and the first negative-electrode current collector  8   a  are connected in a more preferable state. In addition, the solidified portion  70  is more unlikely to be damaged. 
     It is preferable that a portion of the negative-electrode terminal  9  that is farthest from the sealing plate  2  in the thickness direction of the sealing plate  2  be not melted due to the energy rays. In this case, a manufacturing apparatus, a jig, and other components are unlikely to come into contact with the solidified portion  70 . 
     The energy rays may be continuously radiated or may be radiated in a pulsed manner. The recessed portion  70   a  is preferably annular when viewed from the thickness direction (direction vertical to the sealing plate  2 ) of the sealing plate  2 . 
     Connection between First Current Collector and Second Current Collector 
       FIG.  10    illustrates the surface of the sealing plate  2  facing the electrode body  3  after the second positive-electrode current collector  6   b  is secured to the first positive-electrode current collector  6   a  and the second negative-electrode current collector  8   b  is secured to the first negative-electrode current collector  8   a . The second positive-electrode current collector  6   b  that is connected to the first positive-electrode tab group  40   a  and the second positive-electrode tab group  40   b  is disposed on the base  11   a  of the inner insulating member  11 . A part of the second positive-electrode current collector  6   b  is disposed on the first positive-electrode current collector  6   a . The thin portion  6   d  of the second positive-electrode current collector  6   b  is welded to the first positive-electrode current collector  6   a , and a weld  62  is formed. The weld  62  is formed so as to be away from the opening  6   c . The weld  62  is preferably formed by energy rays such as laser rays. 
     The second negative-electrode current collector  8   b  that is connected to the first negative-electrode tab group  50   a  and the second negative-electrode tab group  50   b  is disposed on the base  13   a  of the inner insulating member  13 . A part of the second negative-electrode current collector  8   b  is disposed on the first negative-electrode current collector  8   a . The thin portion  8   d  of the second negative-electrode current collector  8   b  is welded to the first negative-electrode current collector  8   a , and a weld  63  is formed. The weld  63  is formed so as to be away from the opening  8   c . The weld  63  is preferably formed by energy rays such as laser rays. 
     A method of connecting the first negative-electrode current collector  8   a  and the second negative-electrode current collector  8   b  to each other is taken as an example to describe a method of connecting the first positive-electrode current collector  6   a  and the second positive-electrode current collector  6   b  to each other and a method of connecting the first negative-electrode current collector  8   a  and the second negative-electrode current collector  8   b  to each other. 
       FIG.  11    is a sectional view of the vicinity of the negative-electrode terminal  9  taken along the longitudinal direction of the sealing plate  2 . The second negative-electrode current collector  8   b  includes a tab joint  8   b   1  that is connected to the negative-electrode tabs  50  (the first negative-electrode tab group  50   a  and the second negative-electrode tab group  50   b ) and a current-collector joint  8   b   2  that is connected to the first negative-electrode current collector  8   a . A step portion  8   b   3  is disposed between the tab joint  8   b   1  and the current-collector joint  8   b   2 . The tab joint  8   b   1  is disposed on the base  13   a  of the inner insulating member  13 . The current-collector joint  8   b   2  is disposed on the first negative-electrode current collector  8   a . The thin portion  8   d  of the current-collector joint  8   b   2  is welded to the first negative-electrode current collector  8   a . When the first negative-electrode current collector  8   a  and the second negative-electrode current collector  8   b  are welded to each other, the opening  8   c  is used to check whether there is no gap between the first negative-electrode current collector  8   a  and the thin portion  8   d  of the second negative-electrode current collector  8   b . Alternatively, the opening  8   c  is used to check whether the size of a gap between the first negative-electrode current collector  8   a  and the thin portion  8   d  of the second negative-electrode current collector  8   b  is equal to or less than a predetermined size. This enables the first negative-electrode current collector  8   a  and the second negative-electrode current collector  8   b  to be stably welded to each other. The presence or absence of the gap or the size of the gap is preferably checked by using reflection of light. 
     The energy rays are preferably radiated toward a position away from the opening  8   c  to form the weld  63  at the position away from the opening  8   c . The weld  63  can be stably formed so as to be firmer than in the case where the weld  63  is formed along an edge of the opening  8   c.    
     An insulating member recessed portion  13   x  is formed on the base  13   a  of the inner insulating member  13 . The insulating member recessed portion  13   x  faces the back surface of the first negative-electrode current collector  8   a  opposite the surface to which the second negative-electrode current collector  8   b  is welded. This inhibits the inner insulating member  13  from being damaged due to heat that is generated when the first negative-electrode current collector  8   a  and the second negative-electrode current collector  8   b  are welded to each other. 
     The current-collector projection  8   x  is formed on a portion of the first negative-electrode current collector  8   a  that is not covered by the second negative-electrode current collector  8   b . This prevents the first negative-electrode current collector  8   a  from being oriented in an incorrect direction with certainty when the first negative-electrode current collector  8   a  is connected to the negative-electrode terminal  9  and secured to the sealing plate  2 . The shape of the current-collector projection  8   x  in a plan view is preferably asymmetric. The shape of the current-collector projection  8   x  in a plan view is preferably a shape having the longitudinal direction and the transverse direction. A plurality of the current-collector projections  8   x  may be formed. For example, the current-collector projections  8   x  each of which has a perfect circle shape or a square shape in a plan view may be formed. A part of the second negative-electrode current collector  8   b  is disposed on a flat surface of the first negative-electrode current collector  8   a.    
     Cover 
       FIG.  13 A  and  FIG.  13 B  illustrate perspective views of the vicinity of the positive-electrode terminal  7  after the second positive-electrode current collector  6   b  is connected to the first positive-electrode current collector  6   a . In  FIG.  13 A  and  FIG.  13 B , the first positive-electrode tab group  40   a  and the second positive-electrode tab group  40   b  that are connected to the second positive-electrode current collector  6   b  are not illustrated.  FIG.  14    is a sectional view of the vicinity of the positive-electrode terminal  7  taken along the transverse direction of the sealing plate  2  with a cover  80  connected to the inner insulating member  11 . 
     The cover  80  composed of a resin is connected to the inner insulating member  11 . The cover  80  is disposed between the first positive-electrode current collector  6   a  and the electrode body  3 . This prevents the electrode body  3  from coming into contact with the first positive-electrode current collector  6   a  and the sealing plate  2  although the electrode body  3  moves toward the sealing plate  2  in some cases. In the case where the cover  80  and the inner insulating member  11  are separated components, the secondary battery is more readily manufactured. 
     The positive-electrode tabs  40  and the positive-electrode terminal  7  can be connected by the first positive-electrode current collector  6   a  and the second positive-electrode current collector  6   b . In this case, a portion of the cover  80  that is nearest to the electrode body  3  in the thickness direction of the sealing plate  2  is preferably located nearer to the electrode body  3  than a portion of the positive-electrode terminal  7 , a portion of the first positive-electrode current collector  6   a , and a portion of the second positive-electrode current collector  6   b  that are nearest to the electrode body  3 . 
     The positive-electrode tabs  40  and the positive-electrode terminal  7  can be connected only by the first positive-electrode current collector without using the second positive-electrode current collector. In this case, the portion of the cover  80  that is nearest to the electrode body  3  in the thickness direction of the sealing plate  2  is preferably located nearer to the electrode body  3  than the portion of the positive-electrode terminal  7  and the portion of the first positive-electrode current collector that are nearest to the electrode body  3 . 
     The cover  80  composed of a resin is connected to the inner insulating member  11  after the second positive-electrode current collector  6   b  is connected to the first positive-electrode current collector  6   a . The cover  80  is preferably connected to the inner insulating member  11  after the second positive-electrode current collector  6   b  that is connected to the first positive-electrode tab group  40   a  and the second positive-electrode tab group  40   b  is connected to the first positive-electrode current collector  6   a  and before the first electrode body element  3   a  and the second electrode body element  3   b  are integrated into one piece. 
     The cover  80  includes a cover portion  80   a  that faces the first positive-electrode current collector  6   a . The cover portion  80   a  is disposed between the first positive-electrode current collector  6   a  and the electrode body  3 . The cover portion  80   a  preferably faces a joint between the first positive-electrode current collector  6   a  and the second positive-electrode current collector  6   b . The cover  80  includes a pair of cover joints  80   b  that extend from the cover portion  80   a  toward the sealing plate  2 . The cover joints  80   b  are connected to the inner insulating member  11 . The first walls  11   c  of the inner insulating member  11  preferably extend toward the electrode body  3  beyond the surface of the first positive-electrode current collector  6   a  facing the electrode body  3 . The cover joints  80   b  are preferably connected to portions of the first walls  11   c  that are located nearer to the electrode body  3  than the surface of the first positive-electrode current collector  6   a  facing the electrode body  3 . The first walls  11   c  of the inner insulating member  11  can have connection openings  11   e . Each cover joint  80   b  includes a wall  80   b   1  that extends from the cover portion  80   a  toward the sealing plate  2  and a connection projection  80   b   2  that is disposed on a side surface of the wall  80   b   1 . The cover  80  can be connected to the inner insulating member  11  in a manner in which the connection projections  80   b   2  of the cover joints  80   b  are fitted into the connection openings  11   e  of the first walls  11   c . The cover joints  80   b  may have connection openings, and connection projections that are disposed on the first walls  11   c  of the inner insulating member  11  may be fitted into the connection openings. 
     The first walls  11   c  are preferably disposed on both ends of the inner insulating member  11  in the transverse direction of the sealing plate  2 . The cover joints  80   b  are preferably disposed on both ends of the cover  80  in the transverse direction of the sealing plate  2 . The cover joints  80   b  and the corresponding first walls  11   c  are preferably connected to each other. Consequently, the inner insulating member  11  and the cover  80  are more stably connected to each other. 
     A gap is preferably formed between the first positive-electrode current collector  6   a  and the cover portion  80   a  in the thickness direction of the sealing plate  2 . With this structure, the cover portion  80   a  can deform so as to bend when the electrode body  3  comes into contact with the cover portion  80   a , and an impact can be alleviated. The distance between the first positive-electrode current collector  6   a  and the cover  80  in the thickness direction of the sealing plate  2  is preferably 1 mm or more, more preferably 3 mm or more. Root openings  80   c  are more preferably formed in the cover portion  80   a  at the roots of the cover joints  80   b . This enables an impact to be more effectively alleviated. 
     A gap is preferably formed between the cover portion  80   a  and the positive-electrode terminal  7  in the thickness direction of the sealing plate  2 , and it is preferable that the cover portion  80   a  and the positive-electrode terminal  7  be not in contact with each other. This effectively prevents a load from being applied to a joint between the positive-electrode terminal  7  and the first positive-electrode current collector  6   a.    
     As illustrated in  FIG.  15   , the second positive-electrode current collector  6   b  includes a tab joint  6   b   1  that is connected to the positive-electrode tabs  40  and a current-collector joint  6   b   2  that is connected to the first positive-electrode current collector  6   a . A step portion  6   b   3  is formed between the tab joint  6   b   1  and the current-collector joint  6   b   2 . The distance between the sealing plate  2  and the current-collector joint  6   b   2  in the thickness direction of the sealing plate  2  is longer than the distance between the sealing plate  2  and the tab joint  6   b   1 . The cover portion  80   a  is preferably located nearer to the electrode body  3  than the current-collector joint  6   b   2  in the thickness direction of the sealing plate  2 . This enables the secondary battery to have an increased volume energy density and enables the secondary battery to be readily manufactured. The positive-electrode terminal  7  includes a flange  7   a , an insertion portion  7   b , and a crimped portion  7   c . The inner insulating member  11  has an insulating member recessed portion  11   x.    
     At least one support portion  80   d  that projects from the cover portion  80   a  toward the first positive-electrode current collector  6   a  is disposed on the cover portion  80   a . The support portion  80   d  is preferably disposed between the pair of the cover joints  80   b . A plurality of the support portions  80   d  may be disposed on the cover portion  80   a . An end of each support portion  80   d  is preferably in contact with the first positive-electrode current collector  6   a . Alternatively, a small gap may be formed between the end of the support portion  80   d  and the first positive-electrode current collector  6   a . For example, the size of the gap between the end of each support portion  80   d  and the first positive-electrode current collector  6   a  (the distance between the end of the support portion  80   d  and the first positive-electrode current collector  6   a  in the thickness direction of the sealing plate  2 ) is preferably 3 mm or less, more preferably 1 mm or less, further preferably 0.5 mm or less. The end of each support portion  80   d  can come into contact with the first positive-electrode current collector  6   a  when the cover portion  80   a  bends. The support portion  80   d  enables the cover  80  to be inhibited from being damaged. 
     A wall portion that extends in the longitudinal direction of the sealing plate  2 , a wall portion that extends in the transverse direction of the sealing plate  2 , or a columnar portion can be disposed on the cover portion  80   a  and used as the support portion  80   d.    
     The surface of the cover portion  80   a  facing the electrode body  3  in the thickness direction of the sealing plate  2  is preferably nearer to the electrode body  3  than the portion of the first positive-electrode current collector  6   a  and the portion of the second positive-electrode current collector  6   b  that are nearest to the electrode body  3 . This causes the electrode body  3  to come into contact with the cover portion  80   a  first even when the electrode body  3  moves toward the sealing plate  2 , and accordingly, the electrode body  3  can be inhibited from coming into contact with the first positive-electrode current collector  6   a  and the second positive-electrode current collector  6   b.    
     The first positive-electrode current collector  6   a  preferably includes the current-collector projection  6   x  on the surface facing the electrode body  3 . The end portion of the current-collector projection  6   x  facing the electrode body  3  is preferably located nearer to the electrode body  3  than the end portion of the positive-electrode terminal  7  facing the electrode body  3 , and the current-collector projection  6   x  preferably faces the cover portion  80   a . This enables a load can be effectively inhibited from being applied to the joint between the positive-electrode terminal  7  and the first positive-electrode current collector  6   a  even when the electrode body  3  comes into contact with the cover portion  80   a.    
     It is only necessary for the cover  80  to be disposed near the first positive-electrode current collector  6   a , or near the first negative-electrode current collector  8   a , or both. The cover  80  may be disposed between the first positive-electrode current collector  6   a  and the electrode body  3 , and the cover may not be disposed between the first negative-electrode current collector  8   a  and the electrode body  3 . 
     The inner insulating member  11  has a liquid inlet  11   f  that faces the electrolytic solution injection hole  15  that is formed in the sealing plate  2 . A tubular portion  11   g  that extends toward the electrode body  3  is disposed around the liquid inlet  11   f . The cover portion  80   a  is preferably located nearer to the electrode body  3  than the end portion of the tubular portion  11   g  facing the electrode body  3  in the thickness direction of the sealing plate  2 . This causes the electrode body  3  to come into contact with the surface of the cover portion  80   a  facing the electrode body  3  earlier than the tubular portion  11   g  when the electrode body  3  moves toward the sealing plate  2 , and accordingly, a load can be inhibited from being applied locally to the electrode body  3 . 
     Manufacture of Electrode Body 
     The first positive-electrode tab group  40   a , the second positive-electrode tab group  40   b , the first negative-electrode tab group  50   a , and the second negative-electrode tab group  50   b  are bent such that the upper surface of the first electrode body element  3   a  and the upper surface of the second electrode body element  3   b  in  FIG.  10    are in contact with each other directly or with another member interposed therebetween. Consequently, the first electrode body element  3   a  and the second electrode body element  3   b  are integrated into the electrode body  3 . The first electrode body element  3   a  and the second electrode body element  3   b  are preferably integrated with a tape. Alternatively, the first electrode body element  3   a  and the second electrode body element  3   b  are preferably integrated by being disposed in the electrode body holder  14  in the form of a box or a bag. 
     The outer surface of the first positive-electrode tab group  40   a  that is bent preferably faces the inner surface of one of the pair of the second walls  11   b . The outer surface of the second positive-electrode tab group  40   b  that is bent preferably faces the inner surface of the other of the pair of the second walls  11   b . The outer surface of the first negative-electrode tab group  50   a  that is bent preferably faces the inner surface of one of the pair of the third walls  13   b . The outer surface of the second negative-electrode tab group  50   b  that is bent preferably faces the inner surface of the other of the pair of the third walls  13   b.    
     The electrode body  3  that is covered by the electrode body holder  14  that is molded out of a resin sheet into a box shape or a bag shape is inserted into the prismatic exterior body  1 . The sealing plate  2  and the prismatic exterior body  1  are welded to each other. The opening of the prismatic exterior body  1  is sealed by the sealing plate  2 . An electrolyte solution is poured into the prismatic exterior body  1  via the electrolytic solution injection hole  15  that is formed in the sealing plate  2 . Subsequently, the electrolytic solution injection hole  15  is sealed by the sealing member such as a blind rivet. 
     The electrode body holder  14  and the second walls  11   b  preferably overlap, and the electrode body holder  14  and the third walls  13   b  preferably overlap when viewed from the direction perpendicular to the side walls of the prismatic exterior body  1  that has a larger area. This prevents the first positive-electrode tab group  40   a , the second positive-electrode tab group  40   b , the first negative-electrode tab group  50   a , and the second negative-electrode tab group  50   b  from coming into contact with the prismatic exterior body  1  with more certainty. 
     In the prismatic secondary battery  20  according to the above embodiment, the positive-electrode tab groups (the first positive-electrode tab group  40   a  and the second positive-electrode tab group  40   b ) and the negative-electrode tab groups (the first negative-electrode tab group  50   a  and the second negative-electrode tab group  50   b ) are disposed on the end portion of the electrode body  3  facing the sealing plate  2 . The positive-electrode tab groups are bent and connected to the surface of the second positive-electrode current collector  6   b  facing the electrode body  3 , and the second positive-electrode current collector  6   b  is disposed along the sealing plate  2 . The negative-electrode tab groups are bent and connected to the surface of the second negative-electrode current collector  8   b  facing the electrode body  3 , and the second negative-electrode current collector  8   b  is disposed along the sealing plate  2 . With this structure, the secondary battery has an increased volume energy density. 
     Others 
     According to the above embodiment described by way of example, the electrode body  3  is formed of the two electrode body elements. The present invention, however, is not limited thereto. The electrode body  3  may be formed of three or more electrode body elements. The electrode body elements are not limited to multilayer electrode bodies and may be wound electrode bodies each of which is obtained by winding a belt-like positive-electrode sheet and a belt-like negative-electrode sheet with a belt-like separator interposed therebetween. The electrode body  3  may has a single multilayer electrode body. The electrode body  3  may has a single wound electrode body obtained by winding the belt-like positive-electrode sheet and the belt-like negative-electrode sheet with the belt-like separator interposed therebetween. 
     The cover  80  is preferably composed of a resin. For example, the cover  80  is preferably composed of polypropylene (PP), polyethylene (PE), or polyphenylene sulfide (PPS). 
     Examples of the energy rays used for welding include laser rays and electron beams. 
     In the prismatic secondary battery  20  according to the above embodiment described by way of example, the positive-electrode current collector that electrically connects the positive-electrode terminal and the positive-electrode tabs to each other is formed of two components. However, the positive-electrode current collector may be a single component. In the prismatic secondary battery  20  according to the above embodiment described by way of example, the negative-electrode current collector that electrically connects the negative-electrode terminal and the negative-electrode tabs to each other is formed of two components. However, the negative-electrode current collector may be a single component. A current interrupt mechanism may be disposed on a conductive path between the positive-electrode terminal and the positive-electrode sheets or on the conductive path between the negative-electrode terminal and the negative-electrode sheets. 
     The materials of the positive-electrode sheets, the negative-electrode sheets, the separators, the electrolyte, and other components can be known materials. According to the present invention, a battery system of the secondary battery is not limited. For example, the secondary battery can be a non-aqueous electrolyte secondary battery such as a lithium-ion battery. According to the present invention, the shape of the secondary battery is not limited to a specific shape. 
     While detailed embodiments have been used to illustrate the present invention, to those skilled in the art, however, it will be apparent from the foregoing disclosure that various changes and modifications can be made therein without departing from the spirit and scope of the invention. Furthermore, the foregoing description of the embodiments according to the present invention is provided for illustration only, and is not intended to limit the invention.