Abstract:
A secondary-battery electrode group unit with its electrodes and current collecting members reliably fixed to each other and enabling a large current flow is provided, and a method of manufacturing the secondary-battery electrode group unit is also provided. A conductive member is mechanically and electrically connected to a current collecting portion of a current collecting member and a terminal forming portion to form, besides a current path passing through the current collecting portion and the terminal forming portion, another current path allowing a current to flow from a part of the current collecting portion to the terminal forming portion.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to secondary-battery electrode group unit for use in a secondary battery such as a lithium ion secondary battery, and to a method of manufacturing the secondary-battery electrode group unit. 
       BACKGROUND ART 
       [0002]    In some secondary batteries such as a lithium ion secondary battery including a wound electrode group, electrodes (a positive electrode and a negative electrode) formed from a metal foil and current collecting members (a positive current collecting member and a negative current collecting member) are connected to each other utilizing a plurality of tabs integrally formed with the electrodes. If the electrodes and the current collecting members are connected to each other utilizing the plurality of tabs, however, the wound electrode group including the electrodes is not fixed with respect to the current collecting members as opposed to the tabs which are fixed to the current collecting members. Therefore, the wound electrode group in a container may be vibrated to break the tabs if strong vibration is continuously applied to the secondary battery from the outside. Collectively welding the plurality of tabs increases the electrical resistance at the welded portion, increasing the power loss. Thus, there has been developed a secondary battery with a tabless structure in which the current collecting members are directly connected to the electrodes by laser welding without utilizing the tabs to enhance the resistance to vibration and reduce the electrical resistance between the electrodes and the current collecting members [see Japanese Patent No. 3738177 (Patent Document 1), for example]. 
       RELATED-ART DOCUMENT 
     Patent Document 
       [0000]    
       
         Patent Document 1: Japanese Patent No. 3738177 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0004]    In order to directly connect the electrodes and the current collecting members to each other without utilizing the tabs, however, it is necessary to make current collecting portions of the current collecting members thin so as to allow laser welding. If the current collecting members each include a terminal forming portion forming a terminal and a current collecting portion including a plurality of welded portions to be subjected to laser welding, a current collected by the current collecting portion flows in a concentrated manner through a portion at which the terminal forming portion and the current collecting portion are connected to each other. If a large current is collected, a large amount of heat is generated at the portion of connection between the thin current collecting portion and the terminal forming portion, which may cause the current collecting member to be fused and cut off.  FIG. 8  schematically illustrates passages (current passages) through which a current flows and which are formed in a portion of a positive current collecting member according to the related art. 
         [0005]    An object of the present invention is to provide a secondary-battery electrode group unit with its electrodes and current collecting members reliably fixed to each other and enabling a current flow that is large compared to the related art, and to provide a method of manufacturing the secondary-battery electrode group unit. 
       Solution to Problem 
       [0006]    The present invention provides a secondary-battery electrode group unit basically including a wound electrode group, a positive current collecting member, and a negative current collecting member. The wound electrode group is formed by winding a layered member including a positive electrode, a negative electrode, and a separator. The positive electrode has an applied layer formed by applying a positive active material mixture to a first metal foil and an unapplied portion on which the positive active material mixture is not applied along the applied layer of the positive active material mixture. The negative electrode has an applied layer formed by applying a negative active material mixture to a second metal foil and an unapplied portion on which the negative active material mixture is not applied along the applied layer of the negative active material mixture. The positive electrode and the negative electrode are layered via the separator such that the unapplied portion of the positive electrode and the unapplied portion of the negative electrode project in directions opposite to each other. The positive current collecting member is welded to the unapplied portion of the positive electrode. The unapplied portion of the positive electrode projects beyond the separator at one end portion of the wound electrode group. The negative current collecting member is welded to the unapplied portion of the negative electrode. The unapplied portion of the negative electrode projects beyond the separator at the other end portion of the wound electrode group. At least one of the positive current collecting member and the negative current collecting member includes a terminal forming portion configured to form a terminal and a current collecting portion including a plurality of portions to be welded that are provided to face the unapplied portion and welded to a part of the unapplied portion by laser welding. If the current collecting portion is thick enough to be laser-welded to the unapplied portion and there is only one current path from the current collecting portion to the terminal forming portion, and if a large current flows through the current path, all the current concentrates on the portion of connection between the terminal forming portion and the current collecting portion, which may cause heat generation. Thus, in the present invention, a conductive member is mechanically and electrically connected to the current collecting portion and the terminal forming portion to form, besides a current path passing through the current collecting portion and the terminal forming portion, another current path allowing a current to flow from a part of the current collecting portion to the terminal forming portion. Forming such current path enables providing a first current path passing through the current collecting portion and the terminal forming portion and a second current path bypassing the first current path. This allows suppressing heat generation at the current collecting portion even if the thickness of the current collecting portion is reduced in the lithium ion secondary battery which generates a large current or the like. 
         [0007]    If the terminal forming portion is integrally provided at a central portion of the current collecting portion, the conductive member preferably has a through hole formed at its center portion for the terminal forming portion to pass therethrough. The same applies if the terminal forming portion and the current collecting portion are discrete members, the current collecting portion has a through hole formed at its center portion for the terminal forming portion to pass therethrough, and the current collecting portion and the terminal forming portion are welded to each other, with the terminal forming portion fitted in the through hole of the current collecting portion and the terminal forming portion contacting the current collecting portion. The terminal forming portion and the conductive member are welded to each other and the current collecting portion and the conductive member are welded to each other at a portion other than the plurality of welded portions, with the terminal forming portion fitted in the through hole and with the conductive member contacting the current collecting portion. The configuration allows the conductive member to be fixed with respect to the terminal forming portion and the current collecting portion, ensuring formation of a plurality of current paths and preventing concentration of a current. 
         [0008]    Preferably, the plurality of welded portions of the current collecting member are each formed by deforming a part of the current collecting portion to be convex toward the unapplied portion, and are disposed to extend radially about the terminal forming portion. Thus forming the plurality of welded portions to perform laser welding increases the area of contact between the current collecting member and the unapplied portion, reducing the electrical resistance. In addition, the welding location is clearly defined, facilitating the welding work. 
         [0009]    If the current collecting portion and the conductive member are both circular in outline shape, a radius of the outline of the conductive member is preferably smaller than a radius of the outline of the current collecting portion. The configuration ensures formation of another current path originating from various portions of the current collecting member and passing through the conductive member. In addition, preferably, the radius of the outline of the conductive member is equal to or more than half the radius of the outline of the current collecting portion. If the radius of the outline of the conductive member is less than half the radius of the outline of the current collecting portion, the current path passing through the current collecting member and the current path passing through the conductive member may be too close to each other to obtain a sufficient effect of suppressing heat generation even if a current path is formed using the conductive member. 
         [0010]    In order to manufacture the secondary-battery electrode group unit discussed above, the plurality of welded portions of the current collecting portion may be welded to the unapplied portion by laser welding, and thereafter a conductive member may be welded to the current collecting portion and the terminal forming portion to form, besides a current path passing through the current collecting portion and the terminal forming portion, another current path allowing a current to flow from a part of the current collecting portion to the terminal forming portion. Use of the conductive member as a discrete member facilitates welding of the current collecting member, and allows formation of a plurality of current paths to the terminal forming portion, preventing heat generation. Use of a discrete member advantageously facilitates changing the electrical resistance by changing the thickness and the material of the conductive member in accordance with the amount of a flowing current. 
         [0011]    The secondary-battery electrode group unit manufactured as described above can be applied to a secondary battery. The secondary battery may use a wound electrode group, and may be a lithium ion secondary battery, for example. Use of the secondary-battery electrode group unit according to the present invention in the secondary battery enables the electrodes and the current collecting members to be reliably fixed to each other. Since the current paths have a low electrical resistance, heat generation can be suppressed even in a secondary battery that generates a large current such as a lithium ion secondary battery. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1A  is a plan view of a lithium ion secondary battery according to an embodiment of the present invention,  FIG. 1B  is a cross-sectional view taken along the line IB-IB of  FIG. 1A , and  FIG. 1C  is a bottom view of the lithium ion secondary battery according to the embodiment of the present invention. 
           [0013]      FIG. 2A  is a plan view of a current collecting member according to the embodiment of the present invention, and  FIG. 2B  is a front view of the current collecting member according to the embodiment of the present invention. 
           [0014]      FIG. 3A  is a plan view illustrating a state in which a conductive member is attached to the current collecting member according to the embodiment of the present invention, and  FIG. 3B  is a front view illustrating a state in which the conductive member is attached to the current collecting member according to the embodiment of the present invention. 
           [0015]      FIG. 4  schematically illustrates current paths formed in a positive current collecting member according to the present invention. 
           [0016]      FIG. 5  is a front view of a terminal forming portion of a current collecting member according to a second embodiment. 
           [0017]      FIG. 6  illustrates a current collecting portion of the current collecting member according to the second embodiment, in which  FIG. 6A  is a plan view of the current collecting portion according to the embodiment, and  FIG. 6B  is a front view of the current collecting portion according to the embodiment. 
           [0018]      FIG. 7  illustrates the current collecting member (the current collecting portion and the terminal forming portion) according to the second embodiment. 
           [0019]      FIG. 8  schematically illustrates current paths formed in a positive current collecting member according to the related art. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0020]    An embodiment in which the present invention is applied to a cylindrical lithium ion secondary battery will be described below with reference to the drawings.  FIG. 1A  is a plan view of a lithium ion secondary battery according to an embodiment of the present invention.  FIG. 1B  is a cross-sectional view taken along the line IB-IB of  FIG. 1A .  FIG. 1C  is a bottom view of the lithium ion secondary battery.  FIGS. 2A and 2B  illustrate a current collecting member according to the embodiment of the present invention.  FIGS. 3A and 3B  illustrate a state in which a conductive member is attached to the current collecting member according to the embodiment of the present invention. In  FIG. 1B , the cross section of a wound electrode group  9  is not illustrated. 
       &lt;Overall Configuration&gt; 
       [0021]    A cylindrical lithium ion secondary battery  1  according to the embodiment includes a battery container body  3 , a positive-electrode battery lid  5 , a negative-electrode battery lid  7 , an axial core  8 , a wound electrode group  9  infiltrated with an electrolyte, a positive current collecting member  11 , and a negative current collecting member  13 . The battery container body  3 , the positive-electrode battery lid  5 , and the negative-electrode battery lid  7  form a battery container  2 . The battery container body  3  is made of a nickel-plated steel material, and has the shape of a cylinder that is open at both ends. The opening portions of the battery container body  3  at both ends are blocked by the positive-electrode battery lid  5  and the negative-electrode battery lid  7 , respectively. Terminal through holes  5   a  and  7   a  are formed in the center portion of the positive-electrode battery lid  5  and the negative-electrode battery lid  7 , respectively. A terminal forming portion  15  of the positive current collecting member  11  and a terminal forming portion  17  of the negative current collecting member  13  are inserted into the terminal through holes  5   a  and  7   a , respectively, via an insulating ring  19 , an O-ring  21 , and a back-up ring  22 . A threaded portion is formed on the outer peripheral portion of the terminal forming portions  15  and  17 . A nut member  25  is screwed on each threaded portion. An insulating washer  23  is disposed between the nut member  25  and the insulating ring  19 . 
         [0022]    The positive-electrode battery lid  5  is provided with an electrolyte injection port  27  configured to allow injection of the electrolyte. The electrolyte injection port  27  is sealed with a bolt  28 . In order to prevent a rise in pressure inside the battery, the positive-electrode battery lid  5  is provided with a gas discharge port  45  including a safety valve  43  configured to discharge a generated gas when the internal pressure is raised by the generated gas. 
       &lt;Wound Electrode Group&gt; 
       [0023]    As illustrated in  FIG. 1 , the wound electrode group  9  is formed by winding a belt-like positive electrode  29  and a belt-like negative electrode  31  about the axial core  8  via a belt-like separator  33 . The cross section of the wound electrode group  9  taken in the direction orthogonal to the axial core  8  has a swirling shape. In the positive electrode  29  according to the embodiment, a positive mixture containing a lithium manganese oxide, which is a lithium transition metal oxide, is generally uniformly applied to both surfaces of an aluminum foil serving as a positive current collector. An unapplied portion  30 , to which no positive mixture is applied, is formed on one side of the aluminum foil in the longitudinal direction. The positive current collecting member  11 , which is made of aluminum, is welded to the unapplied portion  30 . In the negative electrode  31 , a negative mixture containing carbon powder capable of occluding and releasing lithium ions as a negative active material is generally uniformly applied to both surfaces of a rolled copper foil serving as a negative current collector. An unapplied portion  32 , to which no negative mixture is applied, is formed on one side of the copper foil in the longitudinal direction. The negative current collecting member  13  made of copper is welded to the unapplied portion  32 . The wound electrode group  9 , the positive current collecting member  11 , and the negative current collecting member  13  form a secondary-battery electrode group unit  14 . 
       &lt;Current Collecting Member&gt; 
       [0024]      FIGS. 2A and 2B  illustrate an example of the current collecting member (the positive current collecting member  11  or the negative current collecting member  13 ) for use in the embodiment. The positive current collecting member  11  and the negative current collecting member  13  are the same as each other except for the material. Therefore, reference numerals for the negative current collecting member  13  are given in parentheses. The positive current collecting member  11  is described as an example. The positive current collecting member  11  integrally includes the terminal forming portion  15  and a current collecting portion  16 . The positive current collecting member  11  is brought closer to the wound electrode group  9  from above the unapplied portion  30  of the positive electrode of the wound electrode group  9  to place the positive current collecting member  11  on the unapplied portion  30 . The unapplied portion  30  and the positive current collecting member  11  are welded to each other by laser welding. For laser welding, the current collecting portion  16  of the positive current collecting member  11  is provided with eight grooves  33  for welding that are convex so as to be in contact with the wound electrode group  9  and that are open in the direction away from the wound electrode group  9 . The grooves  33  are formed in the current collecting portion  16  by pressing, and extend straight radially about the imaginary center point of the positive current collecting member  11 . 
         [0025]    The negative current collecting member  13  is the same as the positive current collecting member  11  except for being welded to the unapplied portion  32  of the negative electrode of the wound electrode group  9  and except for the material. Therefore, the negative current collecting member  13  is not described. 
       &lt;Conductive Member&gt; 
       [0026]    In the embodiment, a conductive member  35  made of aluminum and formed in the shape of a ring in which a circular through hole  37  is formed in the center portion is welded to the positive current collecting member  11 . Meanwhile, a conductive member  36  made of copper and formed in the shape of a ring in which a circular through hole  38  is formed in the center portion is welded to the negative current collecting member  13 . In  FIGS. 3A and 3B , only the current collecting member and the conductive member welded thereto are illustrated, and other components are not illustrated. The positive-side components and the negative-side components are the same as each other except for the material. Therefore, reference numerals for the negative-side components are given in parentheses. The positive-side components are described as an example. The conductive member  35  is welded after the positive current collecting member  11  is welded to the wound electrode group  9 . The terminal forming portion  15  is passed through the through hole  37  formed in the conductive member  35 . The conductive member  35  and the terminal forming portion  15  and the current collecting portion  16  are welded to each other with the conductive member  35  contacting the terminal forming portion  15  and the current collecting portion  16 . The welding is performed for the purpose of mechanically and electrically connecting the terminal forming portion  15  and the conductive member  35  to each other and the current collecting portion  16  and the conductive member  35  to each other. To this end, the welding is performed between a portion of the conductive member  35  around the through hole  37  (a welded portion  39 ) and the terminal forming portion  15 , and between the outer peripheral portion of the conductive member  35  (a welded portion  41 ) and the current collecting portion  16 . The welding is performed through semiconductor laser welding, for example. The welding is performed in the same manner for the negative-side components. 
       &lt;Current Path&gt; 
       [0027]      FIG. 4  schematically illustrates current paths formed by attaching the conductive member  35  to the positive current collecting member  11 .  FIG. 4  is a simplified cross-sectional view in which members other than the wound electrode group  9 , the positive current collecting member  11 , and the conductive member  35  are not illustrated in order to clarify the image of the current paths. Besides a current path in the positive current collecting member  11  from the current collecting portion  16  to the terminal forming portion  15 , a current path passing from the current collecting portion  16  to the terminal forming portion  15  through the welded portion  41 , the conductive member  35 , and the welded portion  39  is formed by attaching the conductive member  35 . Forming such current paths enables providing a first current path passing through the current collecting portion  16  and the terminal forming portion  15  and a second current path bypassing the first current path. This allows suppressing heat generation at the portion of connection between the current collecting portion  16  and the terminal forming portion  15  while reducing the thickness of the current collecting portion  16  in the lithium ion secondary battery which generates a large current. In order to enhance the bypass effect provided by the conductive member  35 , the radius of the outline of the conductive member  35  is decided to be smaller than the radius of the outline of the current collecting portion  16 , and to be equal to or more than half the radius of the outline of the current collecting portion  16 . Current paths are formed in the same manner for the negative-side components, suppressing heat generation at the current collecting portion  18 . 
       Second Embodiment 
       [0028]      FIGS. 5 to 7  illustrate a current collecting member for use in a lithium ion secondary battery according to a second embodiment. In  FIGS. 5 to 7 , component parts that are the same as those in the embodiment illustrated in  FIGS. 1 to 4  are denoted by reference numerals obtained by adding 100 to the reference numerals affixed to their counterparts in  FIGS. 1 to 4  to omit their descriptions. 
         [0029]    A positive current collecting member  111  is described as an example. In the second embodiment, the positive current collecting member  111  is formed from a terminal forming portion  115  ( FIG. 5 ) and a current collecting portion  116  ( FIGS. 6A and 6B ) that are discrete members. 
         [0030]    The terminal forming portion  115  is formed from a cylindrical fitting portion  149  to be passed through a through hole  147  of the current collecting portion  116 , and a cylindrical terminal portion  151 . The diameter of the through hole  147  formed in the current collecting portion  116  is just enough to allow passage of the fitting portion  149 . The diameter of the terminal portion  151  is defined to be larger than the diameter of the fitting portion  149 . Therefore, as illustrated in  FIG. 7 , when the fitting portion  149  of the terminal forming portion  115  is passed through the through hole  147 , an abutting portion  153  of the terminal portion  151  contacts an abutted portion  155  at the periphery of the through hole  147 . In the embodiment, a welded portion is formed between the fitting portion  149  of the terminal forming portion  115  and a portion of the current collecting portion  116  around the through hole  147  from the side of the current collecting portion  116  facing the unapplied portion (from the lower side of  FIG. 7  on the paper), thus obtaining the positive current collecting member  111 . The negative current collecting member  113  may be formed in the same manner as the positive current collector  111  according to the embodiment.  FIGS. 5 to 7  also indicate reference numerals for the negative current collector  113 . It is a matter of course that the conductive member  35  illustrated in  FIG. 4  may be attached by welding to the positive current collector  111  and the negative current collector  113  according to the embodiment. In this case, the terminal portion  151  of the terminal forming portion  115  and the conductive member  35  are welded to each other with the fitting portion  149  of the terminal forming portion  115  fitted in the through hole  37  of the conductive member  35  and the conductive member  35  contacting the current collecting portion  116 . Then, the current collecting portion  116  and the conductive member  35  are welded to each other at a portion other than the plurality of welded portions. 
       Other Embodiments 
       [0031]    The material of the conductive member is not limited to that described above, and any conductive material that can be welded to the current collecting member may be selected as appropriate. 
       INDUSTRIAL APPLICABILITY 
       [0032]    According to the present invention, the conductive member allows formation of a current path, besides a current path passing through the current collecting portion and the terminal forming portion, even if the thickness of the current collecting portion of the current collecting member is reduced by adopting the tabless structure. This prevents a current from flowing locally, preventing heat generation. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1  cylindrical lithium-ion secondary battery 
               2  battery container 
               3  battery container body 
               5  positive-electrode battery lid 
               5   a  terminal through hole 
               7  negative-electrode battery lid 
               7   a  terminal through hole 
               8  axial core 
               9  wound electrode group 
               11  positive current collecting member 
               13  negative current collecting member 
               14  secondary-battery electrode group unit 
               15  terminal forming portion 
               16  current collecting portion 
               17  terminal forming portion 
               18  current collecting portion 
               19  insulating ring 
               21  O-ring 
               22  back-up ring 
               23  insulating washer 
               25  nut member 
               27  electrolyte injection port 
               28  bolt 
               29  positive electrode 
               30  unapplied portion 
               31  negative electrode 
               32  unapplied portion 
               33  ( 34 ) groove 
               35  ( 36 ) conductive member 
               37  ( 38 ) through hole 
               39  ( 40 ) welded portion 
               41  ( 42 ) welded portion 
               43  safety valve 
               45  gas discharge port