Patent Publication Number: US-2015086821-A1

Title: Flat wound secondary battery and method for producing same

Description:
TECHNICAL FIELD 
     The present invention relates to, for example, an on-board flat wound secondary battery having high capacity and a method for producing the same. 
     BACKGROUND ART 
     In recent years, lithium-ion secondary batteries including positive and negative electrodes with separators interposed between the electrodes have been developed with high energy densities as the power sources of electric vehicles and so on. As lithium-ion secondary batteries have been widely used with higher performance, a simple production process and low cost have been required. Under the present circumstances, a technique is disclosed in which a shaft core having a wound electrode is, for example, a stainless shaft core or a seamless cylinder of synthetic resin, and the ring-shaped shaft core is flattened with a wound electrode body after the winding of the electrode (Patent Literature 1). 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Unexamined Patent Application Published No. 2002-280055 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The related art requires insertion of the cylindrical core onto the spindle of a winder before the electrode is wound by the winder. This may suppress productivity improved by automation. Moreover, if separators are welded to the resin core, irregularities on a welded portion may wrinkle the electrodes and form a gap between the electrodes. 
     The present invention has been devised in view of the problem. An object of the present invention is to provide a flat wound secondary battery and a method for producing the same with a simple structure that can simplify a production process and suppress wrinkles formed on an electrode by irregularities on a welded portion. 
     Solution to Problem 
     The present invention includes multiple solutions, for example, a flat wound secondary battery having a wound electrode body including a positive electrode and a negative electrode that are wound flat around a shaft core with a separator interposed between the electrodes, the shaft core including a wound resin sheet having higher flexural rigidity than the positive electrode, the negative electrode, and the separator, the shaft core including an innermost portion that forms the innermost periphery of the shaft core and an extended portion to a winding terminal end of the resin sheet from the innermost portion, and the separator including a bonded portion to the extended portion and a separator winding portion that winds only the separator at least one turn around the shaft core so as to connect the separator to the bonded portion. 
     Advantageous Effects of Invention 
     The present invention can provide a flat wound secondary battery and a method for producing the same with a simple structure that can simplify a production process with high reliability. Other problems, configurations, and effects will be clarified in the following embodiments: 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an external perspective view of a lithium-ion secondary battery according to a first embodiment. 
         FIG. 2  is an exploded perspective view of the lithium-ion secondary battery shown in  FIG. 1 . 
         FIG. 3  is an exploded perspective view of a power generating element assembly shown in  FIG. 2 . 
         FIG. 4A  is a developed perspective view of a wound electrode body shown in  FIG. 3 . 
         FIG. 4B  is an explanatory drawing showing the configuration of a shaft core and a schematic diagram viewed in a direction B of  FIG. 4A . 
         FIG. 4C  shows the flattened shaft core. 
         FIG. 5  shows the positional relationship among a resin sheet, a separator, a negative plate, and a positive plate at the beginning of winding. 
         FIG. 6  is a structural example of the winder. 
         FIG. 7  is a schematic diagram for explaining the resin sheet wound around a winding core. 
         FIG. 8A  is a cross-sectional conceptual diagram showing a bonded structure of the shaft core and the separators according to the first embodiment. 
         FIG. 8B  is an explanatory drawing of a winding method around the shaft core according to the first embodiment. 
         FIG. 9  is a cross-sectional conceptual diagram showing an example of a method of bonding the shaft core and the separators according to the first embodiment. 
         FIG. 10  is a cross-sectional conceptual diagram showing a bonded structure of a shaft core and separators according to a second embodiment. 
         FIG. 11  is a cross-sectional conceptual diagram showing an example of a method of bonding the shaft core and the separators according to the second embodiment. 
         FIG. 12  is a cross-sectional conceptual diagram showing a bonded structure of a shaft core and separators according to a third embodiment. 
         FIG. 13  is a cross-sectional conceptual diagram showing a method of bonding the shaft core and the separators according to the third embodiment. 
         FIG. 14  is a cross-sectional conceptual diagram showing a bonded structure of a shaft core and separators according to a fourth embodiment. 
         FIG. 15A  is an explanatory drawing of a winding method of a shaft core according to a fifth embodiment. 
         FIG. 15B  is a cross-sectional conceptual diagram showing a bonded structure of the shaft core and separators according to the fifth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Referring to  FIGS. 1 to 15B , embodiments of the present invention will be described below. 
     The present invention is a flat wound secondary battery that has a flat wound electrode body around a shaft core with a separator interposed between a positive electrode and a negative electrode. The shaft core includes a resin sheet wound with higher flexural rigidity than the positive electrode, the negative electrode, and the separator. The shaft core includes an innermost portion that forms the innermost periphery of the shaft core and an extended portion to a winding terminal end from the innermost portion. The separator includes a bonded portion to the extended portion and a separator winding portion that winds only the separator at least one turn around the shaft core so as to connect to the bonded portion. 
     First Embodiment 
     In the present embodiment, an example of a lithium-ion secondary battery as a flat wound secondary battery will be described below. 
       FIG. 1  is an external perspective view of the lithium-ion secondary battery according to the present embodiment.  FIG. 2  is an exploded perspective view of the lithium-ion secondary battery shown in  FIG. 1 . 
     As shown in  FIGS. 1 and 2 , a lithium-ion secondary battery  1  includes a battery container  2  that contains a wound electrode body  3 . The battery container  2  includes a battery case  11  having an opening  11   a  and a battery lid  21  that closes the opening  11   a  of the battery case  11 . As shown in  FIG. 4A , the wound electrode body  3  includes a positive plate  34  and a negative plate  32  that are stacked with separators  33  and  34  interposed between the positive and negative plates  34  and  32 . In this state, the wound electrode body  3  is flat wound around a resin sheet  81  wrapped around a winding core  110  of a winder  100 . The wound electrode body  3  with a sheet insulating protective film  41  disposed around the wound electrode body  3  is stored in the battery container  2 . 
     The battery container  2  includes the battery case  11  and the battery lid  21 . The battery case  11  and the battery lid  21  are both made of an aluminum alloy. The battery lid  21  is welded to the battery case  11  by laser welding. The battery container  2  is a flat square container shaped like a rectangular parallelepiped includes a pair of wide sides PW, a pair of narrow sides PN, a bottom PB, and the battery lid  21 . A positive terminal  51  and a negative terminal  61  (a pair of electrode terminals) are disposed on the battery lid  21  with an insulating member interposed between the terminals and the battery lid  21 . The positive and negative terminals  51  and  61  constitute a lid assembly  4 . In addition to the positive terminal  51  and the negative terminal  61 , the battery lid  21  includes a gas release vent  71  that is opened to discharge gas in the battery container  2  when a pressure in the battery container  2  exceeds a predetermined value, and an electrolyte inlet  72  that fills the battery container  2  with an electrolyte. 
     The positive terminal  51  and the negative terminal  61  are longitudinally separated from each other on one side and the other side of the battery lid  21 . The positive terminal  51  and the negative terminal  61  have external terminals  52  and  62  that are disposed outside the battery lid  21  and connection terminals  53  and  63  that are disposed inside the battery lid  21  so as to be electrically connected to the external terminals  52  and  62 . The external terminal  52  and the connection terminal  53  on the positive side are made of an aluminum alloy while the external terminal  62  and the connection terminal  63  on the negative side are made of a copper alloy. 
     The connection terminals  53  and  63  and the external terminals  52  and  62  are disposed with an insulting member (not shown) interposed between the terminals and the battery lid  21 , electrically insulating the terminals from the battery lid  21 . The connection terminals  53  and  63  include current collecting terminals  54  and  64  that are extended from the inside of the battery lid  21  to the bottom of the battery case  11  so as to be electrically connected to the wound electrode body  3 . The wound electrode body  3  is disposed so as to be supported between the current collecting terminal  54  of the positive terminal  51  and the current collecting terminal  64  of the negative terminal  61 . The lid assembly  4  and the wound electrode body  3  constitute a power generating element assembly  5 . 
     Subsequently, in order to obtain insulation between the power generating element assembly  5  and the battery case  11 , the wound electrode body  3  is inserted from the opening  11   a  of the battery case  11  so as to locate the insulating protective film  41  between the power generating element assembly  5  and the battery case  11 , and then the battery lid  21  and the battery case  11  are welded by laser welding. After that, an electrolyte is poured into the battery container  2  from the electrolyte inlet  72  of the battery lid  21 , and then the electrolyte inlet  72  is closed by an electrolyte stopper  73 . The electrolyte stopper  73  is welded to the battery lid  21  by laser welding. 
     The electrolyte contains, for example, 1 mol/L of LiPF 6  (lithium hexafluorophosphate) in a mixed solution of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) with a volume ratio of 1:1:1. 
     In this example, the electrolyte is LiPF 6 . The electrolyte is not limited to LiPF 6  and may be, for example, LiClO 4 , LiAsF 6 , LiBF 4 , LiB(C 6 H 5 ) 4 , CH 3 SO 3 Li, CF 3 SOLi, or a mixture of these substances. Moreover, a solvent for a non-aqueous electrolyte is a mixed solvent of EC and DMC in the example of the present embodiment. Alternatively, the mixed solvent may contain at least one of propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, and propionitrile. The mixing ratio is not limited. Power is supplied from the wound electrode body  3  to an external load through the external terminals  52  and  62 , or external generated power is charged to the wound electrode body  3  through the external terminals  52  and  62 . 
       FIG. 3  is an exploded perspective view of the detail of the power generating element assembly shown in  FIG. 2 . 
     The power generating element assembly  5  is fabricated as follows: the positive terminal  51  and the negative terminal  61  are attached to the battery lid  21  via the insulating member to fabricate the lid assembly  4 , and then a positive uncoated portion  34   b  and a negative uncoated portion  32   b  of the wound electrode body  3  are electrically connected to the positive terminal  51  and the negative terminal  61  of the lid assembly  4  by ultrasonic bonding. 
       FIG. 4A  is an external perspective view specifically showing a developed part of the wound electrode body in  FIG. 3 .  FIG. 4B  is an explanatory drawing showing the configuration of a shaft core  80  and a schematic diagram viewed in a direction B of  FIG. 4A .  FIG. 4C  shows the flattened shaft core.  FIG. 5  is a developed view of the positional relationship among the resin sheet, the separator, the negative plate, and the positive plate at the beginning of winding. 
     The wound electrode body  3  includes the negative plate (negative electrode)  32  and the positive plate (positive electrode)  34  that are wound flat around the shaft core  80  with the separators  33  and  35  interposed between the plates. As shown in  FIG. 4A , the outermost electrode of the wound electrode body  3  is the negative plate  32  on which the separator  35  is wound. The separators  33  and  35  insulate the positive plate  34  from the negative plate  32 . 
     As shown in  FIG. 5 , a negative coated portion  32   a  of the negative plate  32  is larger in width than a positive coated portion  34   a  of the positive plate  34 . Thus, the positive uncoated portion  34   a  is always held by the negative coated portion  32   a . The positive uncoated portion  34   b  and the negative uncoated portion  32   b  are connected to the positive and negative current collecting terminals  54  and  64  that are bundled on a flat portion and are connected to the external terminals  52  and  62  by welding or the like. The separators  33  and  35  are larger in width than the negative coated portion  32   a  but are wound at positions where metal foil surfaces are exposed on the ends of the positive uncoated portion  34   b  and the negative uncoated portion  32   b . This does not hamper welding of the bundled terminals. 
     The positive plate  34  has the positive coated portion  34   a  formed by applying a positive active material mixture to both surfaces of positive electrode foil serving as a positive current collector, and the positive uncoated portion (foil exposed portion)  34   b  not coated with a positive active material mixture on one end in the width direction of the positive electrode foil. 
     The negative plate  32  has the negative coated portion  32   a  formed by applying a negative active material mixture to both surfaces of negative electrode foil serving as a negative current collector, and the negative uncoated portion (foil exposed portion)  32   b  not coated with a negative active material mixture on one end in the width direction of the positive electrode foil. The positive uncoated portion  34   b  and the negative uncoated portion  32   b  are regions where the metal surfaces of electrode foil are exposed. The positive and negative uncoated portions  34   b  and  32   b  are wound so as to be located on one end and the other end in a winding axial direction (X direction in  FIG. 4 ). 
     For the negative plate  32 , 10 parts by weight of polyvinylidene fluoride (hereinafter, will be called PVDF) were added as a binding agent to 100 parts by weight of amorphous carbon powder that is a negative electrode active material. Moreover, N-methylpyrrolidone (hereinafter, will be called NMP) was added as a dispersing solvent to the powder and was mixed to prepare a negative material mixture. The negative material mixture was applied to both surfaces of copper foil (negative electrode foil) having a thickness of 10 μm, except for a current collecting portion (negative uncoated portion). After that, the foil was dried, pressed, and cut to obtain a negative plate that had a portion coated with the negative active material without containing copper foil with a thickness of 70 μm. 
     In the present embodiment, the negative active material was amorphous carbon. The negative active material is not limited to amorphous carbon and thus may be natural graphite allowing insertion and desorption of lithium ions, various artificial graphite materials, carbonaceous materials such as coke, a compound of materials such as Si and Sn (e.g., SiO or TiSi 2 ), or a composite material thereof. The forms of particles include scaly, spherical, fibrous, and massive forms and are not particularly limited. 
     For the positive plate  34 , 10 parts by weight of scaly graphite as a conductive material and 10 parts by weight of PVDF as a binding agent were added to 100 parts by weight of lithium manganate (chemical formula: LiMn 2 O 4 ) that is a positive active material. Moreover, NMP was added as a dispersing solvent to the material and then mixed to prepare a positive material mixture. The positive material mixture was applied to both surfaces of aluminum foil (positive electrode foil) having a thickness of 20 μm, except for an uncoated current collecting portion (positive uncoated portion). After that, the foil was dried, pressed, and cut to obtain a positive plate that has a portion coated with the positive active material without containing aluminum foil with a thickness of 90 μm. 
     In the present embodiment, the positive active material was lithium manganate. The positive active material may be another lithium manganate having a spinel crystal structure, a lithium manganese complex oxide partially substituted by or doped with a metallic element, lithium cobaltate having a laminar crystal structure, lithium titanate, or a lithium-metal composite oxide obtained by substitution or doping of some of these substances with metallic elements. 
     In the present embodiment, the bonding material of a coated portion on the positive plate and the negative plate is PVDF. The bonding material may be polytetrafluoroethylene (PTFE), polyethylene, polystyrene, polybutadiene, butyl rubber, nitrile rubber, styrene-butadiene rubber, polysulfide rubber, nitrocellulose, cyanoethyl cellulose, various latexes, acrylonitrile, vinyl fluoride, vinylidene fluoride, propylene fluoride, chloroprene fluoride, a polymer containing an acrylic resin, and a mixture of these substances. 
     The shaft core  80  includes the wound resin sheet  81  having higher flexural rigidity than the positive plate  34 , the negative plate  32 , and the separators  33  and  35 . As shown in  FIG. 4E , the shaft core  80  includes an innermost portion  82  forming the innermost periphery of the shaft core  80  and an extended portion  83  from the innermost portion  82  to the winding terminal end. 
     The resin sheet  81  is larger in thickness than the negative plate  32 , the positive plate  34 , and the separators  33  and  35  and is made of an insulating resin material having rigidity. The width of the resin sheet  81  is desirably not smaller than that of the negative coated portion  32   a  in the winding axial direction (X direction) so as to wind the negative coated portion  32   a  in contact with the overall outermost surface of the shaft core  80 . Moreover, when the positive uncoated portion  34   b  and the negative uncoated portion  32   b  are collectively welded in the thickness direction (Z direction), the resin sheet  81  preferably has a width that does not cause insulation between pieces of metallic foil. In the present embodiment, the width of the resin sheet  81  is set at the same width as the separators  33  and  35 . 
     The shaft core  80  includes the wound resin sheet  81  having higher flexural rigidity than the negative plate  32 , the positive plate  34 , and the separators  33  and  35 . Thus, the elastic force of the shaft core  80  can tightly place the separators  33  and  35  and the negative plate  32  along the outer surface of the shaft core  80 , and also place the positive plate  34 , which is located outside the shaft core  80 , along the shaft core  80 . This can prevent looseness of the winding start ends of the separators  33  and  35 , the negative plate  32 , and the positive plate  34  toward the center of winding. 
     In the present embodiment, the shaft core  80  includes the resin sheet  81  that is a PP sheet having a thickness of 150 μm. The resin sheet  81  used in a battery does not cause problems such as deterioration, has higher flexural rigidity than the negative plate  32 , and can tightly place the negative plate  32  along the outer periphery of the shaft core  80 . The material, dimensions, and so on of the resin sheet  81  are not limited as long as the resin sheet  81  has insulation. 
       FIG. 6  shows a structural example of the winder  100 . 
     The winder  100  includes a spindle  101  that is rotatably supported at the center of the winder and is rotated clockwise by a rotating unit (not shown). There is provided, on one side of the spindle  101 , a feeder for feeding the positive electrode  34 , the separator  33  (first separator), the negative electrode  32 , the separator  35  (second separator), and the resin sheet  81  to the spindle  101 . 
     The feeder holds the rolls of the positive electrode  34 , the separator  33 , the negative electrode  32 , the separator  35 , and the resin sheet  81  in this order from the upper right of the feeder. The rolls are fed to the spindle  101  from the outer end of the winder. The winder  100  further includes feed rollers  160   a  to  160   e  that feed the electrodes  34  and  32 , the separators  33  and  35 , and the resin sheet  81  for a predetermined length and cutters  161   a  to  161   e  that cut the electrodes  34  and  32 , the separators  33  and  35 , and the resin sheet  81  at a predetermined length. 
     The spindle  101  includes a flat winding core  102  that has a holding portion  103  for holding the winding start end of the resin sheet  81 . Bonding means  167  is provided near the winding core  102 . The bonding means  167  rotates the winding core  102  to form the wound electrode body  3  and then bonds adhesive tape  163  to prevent unwinding of the wound electrode body  3 . A predetermined length of the adhesive tape  163  is fed by a feed mechanism  164 , is cut by a cutter  165  at a predetermined length, and is bonded to the wound electrode body  3 . 
     Furthermore, a heater head  170  and a heater lifting mechanism  171  are provided near the spindle  101 . The heater head  170  that thermally welds the separators  33  and  35  to the resin sheet  81  wound around the winding core  102 . The heater lifting mechanism  171  lifts the heater head  170  to a predetermined position and then presses the heater head  170 . 
     Moreover, a temporarily pressing mechanism  178  is provided to hold the resin sheet  81  to be cut without being unwound. In another embodiment, the separators may be bonded with adhesive tape instead of thermal welding. Thus, in this case, the heater head  107  and the heater lifting mechanism  171  are replaced with a mechanism (not shown) similar to the bonding means  167  for bonding the tape. 
       FIG. 7  is an explanatory drawing of a method of winding the resin sheet around the winding core. 
     The winding core  102  is provided to wind the resin sheet  81  so as to form the shaft core  80 . The winding core  102  is shaped like a flat plate that is larger in width than the resin sheet  81 . The winding core  102  is rotatably fixed to the spindle  101  so as to align the winding axis with the center of rotation of the spindle  101 . 
     The winding core  102  includes the holding portion  103  for holding the winding start end of the resin sheet  81 . The holding portion  103  is configured to increase or reduce the width of an insertion groove  103   a  formed along the winding axial direction. The end of the resin sheet  81  is inserted into the insertion groove  103   a , and then the groove width is reduced so as to hold the winding start end of the resin sheet  81 . 
     The winding start end of the resin sheet  81  is inserted into the insertion groove  103   a  and is held by the holding portion  103 . The winding core  102  is then rotated to cut the resin sheet  81  with the cutter  161   e  at a length of at least one turn of the resin sheet  81  around the winding core  102 . The resin sheet  81  is pressed to the winding core  102  with the temporarily pressing roller of the temporarily pressing mechanism  178  and thus is held without being unwound. 
       FIG. 8A  is a cross-sectional conceptual diagram showing a bonded structure of the shaft core and the separators according to the present embodiment.  FIG. 8B  is an explanatory drawing of a winding method around the shaft core according to the present embodiment. 
     As shown in  FIG. 8A , the shaft core  80  is formed by rotating the winding core  102  while the winding start end of the resin sheet  81  is held by the holding portion  103 . The shaft core  80  includes the innermost portion  82  forming the innermost periphery of the shaft core  80  and the extended portion  83  that is opposed as an overlap margin to the outer periphery of the innermost portion  82 . The extended portion  83  may be so long as to be wound at least one turn around the innermost portion  82 . 
     Subsequently, the winding start end of the separator  33  and the winding start end of the separator  35  are fed between the extended portion  83  and the heater head  170 , the heater head  170  is lifted by the heater lifting mechanism  171 , the stacked winding start ends of the separators  33  and  35  are thermally welded to the outer surface of the extended portion  83  by the heater head  170 , and then the winding start ends of the separators  33  and  35  are integrally bonded to the extended portion  83  of the shaft core  80 . 
     In the present embodiment, the resin sheet  81  is wound around the winding core  102  at least one turn (the total length of the innermost portion  82  and the extended portion  83 ), and then the separators  33  and  35  are thermally welded to be integrally bonded to the outer surface of the extended portion  83  of the shaft core  80 . 
     After that, as shown in  FIG. 8B , the winding core  102  is rotated to wind only the separators  33  and  35  around the shaft core  80  at least one turn, forming a separator winding portion. Moreover, the winding start ends of the negative plate  32  and the positive plate  34  are bonded between the separators  33  and  35  and then are further wound to fabricate the wound electrode body  3  having a predetermined thickness. 
     The wound electrode body  3  is removed along the rotation axis from the extended insertion groove  103   a  of the holding portion  103  so as to be removed from the winding core  102 . Furthermore, the wound electrode body  3  is compressed in a winding thickness direction (Z direction), flattening the shaft core  80  of the wound electrode body  3  in the winding thickness direction as shown in  FIG. 4C  which only illustrates the shaft core  80 . 
     When the separators  33  and  35  are thermally welded to the extended portion  83  of the shaft core  80 , the bonded portion has irregularities caused by the molten resin sheet  81  and separators  33  and  35 . The negative plate  32  and the positive plate  34  that are wound around the bonded portion with such irregularities are not uniformly wound. This may cause wrinkles or uneven step heights so as to form a gap between the electrodes, leading to a reduction in service life. 
     In the present embodiment, the separator winding portion and the shaft core  80  co-operate so as to absorb and reduce irregularities on the bonded portion. 
     In the separator winding portion, the separators  33  and  35  are thermally welded to the shaft core  80 , and then only the separators  33  and  35  are wound at least one turn so as to connect to the bonded portion, thereby absorbing and reducing irregularities on the bonded portion. 
     The shaft core  80  including the resin sheet  81  has a certain level of elasticity. Thus, the formation of the separator winding portion can deform the shaft core  80  so as to dent the overall irregularities on the bonded portion toward the shaft center, achieving a smooth surface. Thus, the negative plate  32  and the positive plate  34  can be uniformly wound around the bonded portion so as to prevent wrinkles and uneven step heights. This can prevent the formation of a gap between the electrodes and a reduction in service life. 
       FIG. 9  is a cross-sectional conceptual diagram showing an example of a method of bonding the shaft core and the separators according to the present embodiment. 
     In the bonding method, the resin sheet  81  having a length of at least one turn (the total length of the innermost portion  82  and the extended portion  83 ) is wound a half turn around the winding core  102 , and then the extended portion  83  is kept protruding in a direction that separates from the innermost portion  82 . Subsequently, the stacked winding start ends of the separators  33  and  35  are fed between the extended portion  83  and the heater head  170 , the heater head  170  is lifted by the heater lifting mechanism  171 , and then the winding start ends of the separators  33  and  35  are thermally welded to the outer surface of the extended portion  83  by the heater head  170 , integrally bonding the winding start ends of the separators  33  and  35  to the extended portion  83  of the shaft core  80 . At this point, a pressing mechanism  268 , which is not shown in the winder  100  of  FIG. 6 , is opposed to the heater head  170  via the resin sheet  81  and the separators  33  and  35  and is used as a rear retainer of the heater head  170 . 
     In the present embodiment, the resin sheet  81  is wound around the winding core  102  and then the separators  33  and  35  are thermally welded so as to be integrated with the outer surface of the extended portion  83  protruding from the winding core  102 . After that, the winding core  102  is rotated so as to fabricate the wound electrode body  3  as in  FIG. 8 . Thus, even if the winding core  102  has a small thickness with low rigidity, the wound electrode body  3  can be fabricated. The same effect can be obtained by bonding of tape (not shown) instead of thermal welding. 
     Second Embodiment 
       FIG. 10  is a cross-sectional conceptual diagram showing a bonded structure of a shaft core and separators according to the present embodiment. 
     A feature of the present embodiment is that the winding start end of a separator  33  is thermally welded to the inner surface of an extended portion  83  of a shaft core  80  while the winding start end of a separator  35  is thermally welded to the outer surface of the extended portion  83  of the shaft core  80 , bonding the shaft core  80  to the separators  33  and  35 . 
     The separators  33  and  35  are fed between a winding core  102  and a heater head  170  so as to hold the winding terminal end of a resin sheet  81  wound around the winding core  102 . The winding start end of the separator  33  is opposed to the inner surface of the extended portion  83  while the winding start end of the separator  35  is opposed to the outer surface of the extended portion  83 . 
     Subsequently, the heater head  170  is lifted by a heater lifting mechanism  171 , and then the extended portion  83  held between the winding start ends of the separators  33  and  35  is thermally welded by the heater head  170 , integrally bonding the separators  33  and  35  to the extended portion  83  of the shaft core  80 . 
     After that, the winding core  102  is rotated to wind only the separators  33  and  35  around the shaft core  80  at least one turn, forming a separator winding portion. Moreover, the winding start ends of a negative plate  32  and a positive plate  34  are bonded between the separators  33  and  35  and then are further wound to fabricate a wound electrode body  3  having a predetermined thickness. As in the first embodiment, the wound electrode body  3  is removed from the winding core  102  and then is compressed in a winding thickness direction (Z direction), flattening the shaft core  80  in a winding thickness direction. 
     For example, if a material having high heat resistance is applied to the surfaces of the separators  33  and  35  so as to be opposed to the positive plate, it may be difficult to thermally weld the separators  33  and  35  to the resin sheet  81  in the first embodiment. In the first embodiment, however, the surfaces of the separators  33  and  35  opposed to the resin sheet  81  can be thermally welded and thus can be easily bonded with reliability, achieving particularly high effectiveness. 
       FIG. 11  is a cross-sectional conceptual diagram showing an example of a method of bonding the shaft core and the separators according to the present embodiment. 
     In the bonding method, the resin sheet  81  having a length of at least one turn (the total length of the innermost portion  82  and the extended portion  83 ) is wound a half turn around the winding core  102 , and then the extended portion  83  is kept protruding in a direction that separates from the innermost portion  82 . Subsequently, the winding start end of the separator  33  is opposed to the inner surface of the extended portion  83  while the winding start end of the separator  35  is opposed to the outer surface of the extended portion  83 . 
     The heater head  170  is then lifted by the heater lifting mechanism  171  and the winding start ends of the separators  33  and  35  are thermally welded to the inner and outer surfaces of the extended portion  83  by the heater head  170 , integrally bonding the winding start ends of the separators  33  and  35  to the extended portion  83  of the shaft core  80 . 
     At this point, a pressing mechanism  268 , which is not shown in a winder  100  of  FIG. 6 , is opposed to the heater head  170  via the resin sheet  81  and the separators  33  and  35  and is used as a rear retainer of the heater head  170 . 
     Alternatively, a pair of heater heads  170  may be prepared to thermally weld the separators  33  and  35  while holding the inner and outer surfaces of the separators  33  and  35 . With this configuration, for example, even if a material having high heat resistance and poor heat transfer is applied to one surface of each of the separators  33  and  35  at the position of a positive electrode, the surfaces of the separators  33  and  35  opposed to the resin sheet  81  can be thermally welded and thus can be easily bonded with reliability. 
     Third Embodiment 
       FIG. 12  is a cross-sectional conceptual diagram showing a bonded structure of a shaft core and separators according to the present embodiment. 
     A feature of the present embodiment is that the winding start ends of separators  33  and  35  are thermally welded to the inner surface of an extended portion  83  of a shaft core  80  so as to bond the shaft core  80  to the separators  33  and  35 . 
     As shown in  FIG. 12 , the shaft core  80  is formed by rotating the winding core  102  one turn while the winding start end of a resin sheet  81  is held by a holding portion  103 . The shaft core  80  includes an innermost portion  82  and an extended portion  83  serving as an overlap margin on the outer periphery of the innermost portion  82 . The extended portion  83  is opposed to the outer periphery of the innermost portion  82 . 
     Subsequently, the winding start end of the separator  35  is fed between the winding terminal end of the resin sheet  81  and the outer surface of the resin sheet  81  opposed to the inner surface of the winding terminal end of the extended portion  83  (in the present embodiment, the outer surface of the innermost portion  82 ). 
     Subsequently, a heater head  170  is lifted by a heater lifting mechanism  171 , and then the stacked winding start ends of the separators  33  and  35  are thermally welded to the inner surface of the extended portion  83  by the heater head  170 , integrally bonding the separators  33  and  35  to the extended portion  83  of the shaft core  80 . 
     In the present embodiment, the resin sheet  81  is wound around a winding core  102  at least one turn (the total length of an innermost portion and an extended portion), and then the separators  33  and  35  are thermally welded to the inner surface of the extended portion  83  of the shaft core  80 , integrally bonding the separators  33  and  35  to the inner surface of the extended portion  83  of the shaft core  80 . Subsequently, the winding core  102  is rotated to wind the separators  33  and  35  around the shaft core  80  at least one turn, the winding start ends of a negative plate  32  and a positive plate  34  are bonded between the separators  33  and  35  and then are further wound to fabricate a wound electrode body  3  having a predetermined thickness. The wound electrode body  3  is removed along the rotation axis from an extended insertion groove  103   a  of the holding portion  103  so as to be removed from the winding core  102 . Furthermore, the wound electrode body  3  is compressed in a winding thickness direction (Z direction), flattening the shaft core  80  of the wound electrode body  3  in the winding thickness direction. 
     According to the present embodiment, the winding start ends of the separators  33  and  35  are held between the inner surface of the extended portion  83  and the outer surface of the resin sheet  81  opposed to the inner surface (the outer surface of the innermost portion  82  in the present embodiment) and thus the winding start ends of the separators  33  and  35  are bonded by the friction of holding on the resin sheet  81  as well as bonding by welding. This can more firmly bond the separators  33  and  35  to the shaft core  80 . 
       FIG. 13  is a cross-sectional conceptual diagram showing a method of bonding the shaft core and the separators according to the present embodiment. 
     In the bonding method, the resin sheet  81  having a length of at least one turn (the total length of the innermost portion  82  and the extended portion  83 ) is wound around the winding core  102  a half turn, and then the extended portion  83  is kept protruding in a direction that separates from the innermost portion  82 . Subsequently, the winding start ends of the separators  33  and  35  are fed at a position opposed to the inner surface of the extended portion  83 . The heater head  170  is then lifted by the heater lifting mechanism  171  and the stacked winding start ends of the separators  33  and  35  are thermally welded to the inner surface of the extended portion  83  by the heater head  170 , integrally bonding the winding start ends of the separators  33  and  35  to the extended portion  83  of the shaft core  80 . At this point, a pressing mechanism  268 , which is not shown in a winder  100  of  FIG. 6 , is opposed to the heater head  170  via the resin sheet  81  and the separators  33  and  35  and is used as a rear retainer of the heater head  170 . 
     In the present embodiment, the resin sheet  81  is wound around the winding core  102 , and then the separators  33  and  35  are thermally welded to the protruding portion of the resin sheet  81  from the winding core  102 , that is, the inner surface of the extended portion  83  of the shaft core  80 , integrally bonding the separators  33  and  35  to the inner surface of the extended portion  83  of the shaft core  80 . The winding core  102  is then rotated so as to fabricate the wound electrode body  3  as in  FIG. 8 . Thus, even if the winding core  102  has a small thickness and low rigidity, the wound electrode body  3  can be fabricated. The same effect can be obtained by bonding of tape (not shown) instead of thermal welding. The positional relationship between the pressing mechanism  268  and the heater head  170  in the winder  100  of  FIG. 6  may be vertically reversed. 
     Fourth Embodiment 
       FIG. 14  is a cross-sectional conceptual diagram showing a bonded structure of a shaft core and separators according to the present embodiment. 
     A feature of the present embodiment is that separators  33  and  35  are bonded to a shaft core  80  while the winding start ends of the separators  33  and  35  are held between the inner surface of an extended portion  83  of the shaft core  80  and the outer surface of a resin sheet  81  opposed to the inner surface (the outer surface of an innermost portion  82  in the present embodiment). 
     As shown in  FIG. 14 , the shaft core  80  is formed by rotating a winding core  102  one turn while the winding start end of the resin sheet  81  is held by a holding portion  103 . The shaft core  80  includes an innermost portion  82  and an extended portion  83  serving as an overlap margin on the outer periphery of the innermost portion  82 . The extended portion  83  is opposed to the outer periphery of the innermost portion  82 . 
     Subsequently, the winding start ends of the separators  33  and  35  are fed at a position opposed to the inner surface of the winding terminal end of the resin sheet  81 . A touch roller  179  for preventing unwinding is lifted to hold the separators  33  and  35  between the extended portion  83  and the outer surface of the resin sheet  81  opposed to the inner surface of the extended portion  83 . This prevents removal of the separators  33  and  35  with a friction force so as to integrally bond the separators  33  and  35  to the shaft core  80 . 
     In the present embodiment, the resin sheet  81  is wound around the winding core  102  at least one turn, and then the first separator  33  and the second separator  35  are stacked on the inner surface of the extended portion  83  and are fixed by the touch roller for preventing unwinding. After that, the winding core  102  is rotated one turn to wind the separators  33  and  35  around the shaft core  80  at least one turn. The touch roller  171  is then retracted and the winding core  102  is further rotated to wind the separators  33  and  35 . 
     The shaft core  80  and the separators  33  and  35  are bonded using friction forces and thus the resin sheet  81  preferably has a large friction coefficient. As the separators  33  and  35  are held with a longer length between the inner surface of the extended portion  83  and the outer surface of the resin sheet  81 , a larger friction force can be obtained. For example, the separators  33  and  35  are preferably wound around the innermost portion  82  at least a half turn, preferably at least one turn. 
     The present embodiment eliminates the need for bonding the shaft core  80  and the separators  33  and  35  by thermal welding and thus does not have irregularities caused by thermal welding on a bonded portion. Thus, even if a negative plate  32  and a positive plate  34  are wound on the bonded portion, wrinkles or uneven step heights do not occur. Moreover, the step of thermal welding can be eliminated and thus an improved production tact can be expected. 
     Fifth Embodiment 
       FIG. 15A  is an explanatory drawing of a winding method of a shaft core according to the present embodiment.  FIG. 15B  is a cross-sectional conceptual diagram showing a bonded structure of the shaft core and separators according to the present embodiment. 
     Unlike in the foregoing embodiments, a feature of the present embodiment is that the winding start ends of separators  33  and  35  are partially held by a holding portion  103  of a winding core  102 , the winding core  102  is rotated one turn with a resin sheet  81  disposed on the separator  33 , and thus the winding start ends of the separators  33  and  35  are bonded to a shaft core  80  while being held between the inner surface of an extended portion  83  of the shaft core  80  and the outer surface of the resin sheet  81  opposed to the inner surface (the outer surface of an innermost portion  82  in the present embodiment). Furthermore, the winding start ends of the separators  33  and  35  are located inside the shaft core  80 . 
     First, the stacked separators  33  and  35  are inserted into an insertion groove  103   a  of the holding portion  103 , and then the groove width of the insertion groove  103  is reduced while the winding start ends of the separators  33  and  35  are protruded by a predetermined length. Thus, the separators  33  and  35  are held by the holding portion  103  of the winding core  102 . 
     As shown in  FIG. 15A , the resin sheet  80  is then disposed on the separator  33  held by the holding portion  103 . Subsequently, a touch roller  179  for preventing unwinding is lifted and then the winding core  102  is rotated one turn, forming the shaft core  80  around the winding core  102  as shown in  FIG. 15 . The shaft core  80  includes the innermost portion  82  and the extended portion  83  serving as an overlap margin on the outer periphery of the innermost portion  82 . The extended portion  83  is opposed to the outer periphery of the innermost portion  82 . The winding start side of the separator  33  and the winding start side of the separator  35  are partially disposed at a position opposed to the inner surface of the winding terminal end of the resin sheet  81 . The separators  33  and  35  extending along the shaft core  80  are held by the shaft core  80  so as to be integrally bonded to the shaft core  80 . The winding start ends of the separators  33  and  35  are protruded from a point between the innermost portion  82  and the extended portion  83  to the center of the shaft core  80  and are disposed inside the shaft core  80 . 
     In the present embodiment, as an example of the winding start ends, the winding start sides of the separators  33  and  34  are partially held by the holding portion  103  of the winding core  102 . The present invention is not limited to this configuration. The winding start ends of the separators  33  and  34  may be held instead. Moreover, in the present embodiment, the touch roller  179  for preventing unwinding was used to extend the shaft core  80  and the separators  33  and  35  along the winding core  102  but the present invention is not limited to the use of the touch roller  179 . The wound electrode body  3  can be fabricated without using the touch roller  179 . Furthermore, in the present embodiment, only the parts of the winding start sides of the separators  33  and  34  are held by the holding portion  103  and the insertion groove  103   a  of the winding core  102 . The present invention is not limited to this configuration. The end or a part of the shaft core  80  may be held concurrently with or separately from the separators  33  and  35 . 
     The present embodiment eliminates the need for bonding the shaft core  80  and the separators  33  and  35  by thermal welding. Unlike in the fourth embodiment, the touch roller  179  for preventing unwinding is used to extend the shaft core  80  and the separators  33  and  35  along the winding core  102  but is not used to fix the separators  33  and  35 . This can achieve stable production and increase the rotation speed of the winding core  102 . Thus, an improved production tact can be expected. 
     The embodiments of the present invention were described in detail. The present invention is not limited to the embodiments and thus various design changes may be made within the spirit of the invention as described in the appended claims. For example, the configurations of the embodiments specifically described to illustrate the present invention are not intended to limit the scope of the present invention. The configuration of one of the embodiments may be partially replaced with the configuration of another one of the embodiments or the configuration of one of the embodiments may be added to the configuration of another one of the embodiments. The addition, deletion, and replacement of configurations are possible partially in the configurations of the embodiments. 
     LIST OF REFERENCE SIGNS 
     
         
           1  lithium-ion secondary battery 
           2  battery container 
           3  wound electrode body 
           4  lid assembly 
           5  power generating element assembly 
           11  battery case 
           21  battery lid 
           32  negative plate (negative electrode) 
           33  separator (first separator) 
           34  positive plate (positive electrode) 
           35  separator (second separator) 
           41  insulating protective film 
           51  positive terminal (electrode terminal) 
           52 , 62  external terminal 
           53 , 63  connection terminal 
           54 , 64  current collecting terminal 
           61  negative terminal (electrode terminal) 
           71  gas release vent 
           72  electrolyte inlet 
           73  electrolyte stopper 
           80  shaft core 
           81  resin sheet 
           82  innermost portion 
           83  extended portion 
           100  winder 
           101  spindle 
           170  heater head