Patent Publication Number: US-2012034504-A1

Title: Flat secondary battery electrode group, method for manufacturing same, and flat secondary battery with flat secondary battery electrode group

Description:
TECHNICAL FIELD 
     The present invention relates to secondary batteries represented by lithium ion secondary batteries, particularly to an electrode group for a flat secondary battery (hereinafter referred to as a “flat electrode group”), a method for fabricating the same, and a flat secondary battery including the flat electrode group. 
     BACKGROUND ART 
     In lithium ion secondary batteries which have widely been used as power sources of portable electronic devices, a carbon-based material capable of inserting and extracting lithium is used as a negative electrode active material, and composite oxide of transition metal and lithium (e.g., LiCoO 2 , etc.) is used as a positive electrode active material. This provides the lithium ion secondary battery with high potential and high discharge capacity. 
     The lithium ion secondary battery is fabricated by the following method. First, a positive electrode and a negative electrode are wound into spiral shape with a separator (a porous insulator) interposed therebetween. The obtained electrode group, and a nonaqueous electrolytic solution are placed in a battery case made of stainless steel, nickel-plated iron, aluminum, etc. Then, an opening of the battery case is hermetically sealed with a sealing plate. 
     The electrode (the positive or negative electrode) is fabricated by the following method. A mixture of materials (an active material, a binder, and a conductive agent if necessary) in a slurry state is applied to a current collector, and is dried (fabrication of a base of the electrode). Then, the electrode base is compressed to a predetermined thickness by pressing etc. Increasing an amount of the active material applied to the current collector can increase a density of the active material in the electrode, thereby increasing a capacity of the lithium ion secondary battery. 
     Due to variety of functions and reduced size of the electronic devices and telecommunication devices, the lithium ion secondary batteries of smaller size, and higher capacity have been required. Particularly in the electronic devices and the telecommunication devices which are thinned down, a flat lithium ion secondary battery including power generation components (an electrode group etc.) placed in a battery case has been used to save space for the battery, or to correspond with the shape of the device in which the secondary battery is mounted. 
     For example, Patent Document 1 proposes a method for fabricating a flat electrode group.  FIGS. 6(   a )- 6 ( b ) are schematic cross-sectional views sequentially illustrating steps of a method for fabricating the flat electrode group of Patent Document 1. 
     First, a positive electrode, a negative electrode, and a porous insulator are wound around a cylindrical core (not shown) to fabricate a cylindrical electrode group  91 . Then, as shown in  FIG. 6(   a ), cylindrical jigs  93 ,  94  are inserted in a hollow part  92  of the electrode group  91 , and the jigs  93 ,  94  are moved outward in a radial direction of the electrode group  91 . This changes a lateral cross-section of the electrode group  91  from a substantially round shape to an elliptic shape as shown in  FIG. 6(   b ). Then, the electrode group  91  is pressed to fabricate a flat electrode group (not shown). 
     CITATION LIST 
     Patent Document  
     [Patent Document 1] Japanese Patent Publication No. 2006-278184 
     SUMMARY OF THE INVENTION 
     Technical Problem  
     According to the method disclosed by Patent Document 1, parts of the electrode group  91  shown in  FIG. 6(   b ) in contact with the jigs  93 ,  94  are positioned on a major axis of the electrode group  91 . Thus, when the electrode group  91  is pressed, an electrode mixture layer may be cracked, or separated from a current collector at the parts of the electrode group  91  in contact with the jigs  93 ,  94  (hereinafter merely referred to as “separation of the electrode mixture layer”). This may reduce a capacity of the secondary battery. 
     Due to the cracking or separation of the electrode mixture layer, the electrode mixture layer may drop from the current collector (hereinafter merely referred to as “drop of the electrode mixture layer”). An internal short circuit may occur when the dropped electrode mixture layer penetrates the porous insulator. 
     In view of the foregoing, the present invention has been achieved. The present invention is concerned with providing a flat and highly safe secondary battery which can be fabricated without cracking or separation of the electrode mixture layer. 
     Solution to the Problem  
     A flat electrode group according to the present invention is formed by winding a positive electrode and a negative electrode with a porous insulator interposed therebetween, and flattening the wound product by pressing. Bent portions are provided at ends of the electrode group in a direction of a long axis thereof, respectively, and parts of the bent portions at the innermost of the flat electrode group (hereinafter referred to as “innermost parts of the bent portions”) are positioned opposite each other relative to a center line which passes a midpoint of the electrode group in a direction of a thickness thereof, and extends in the direction of the long axis. The innermost parts of the bent portions may be symmetric about a point on the center line. The “center line” is, e.g., a major axis of the flat electrode group. The “point on the center line” is, e.g., a point of intersection of a major axis and a minor axis of the flat electrode group (the minor axis is a line extending in a direction of a short axis of the flat electrode group), or the center of a lateral cross-section of the flat electrode group. 
     The flat electrode group can be fabricated without cracking or separation of the electrode mixture layer, and can provide the flat secondary battery with high safety. 
     The flat electrode group of the present invention is fabricated by the following method. First, the positive electrode and the negative electrode are wound with the porous insulator interposed therebetween to form an intermediate electrode group having a parallelogram-shaped lateral cross-section. Then, the intermediate electrode group is pressed to form a flat electrode group. Through the pressing, bent portions are formed at ends of the flat electrode group in a direction of a long axis thereof, respectively, and innermost parts of the bent portions are positioned opposite each other relative to the center line. 
     The intermediate electrode group may be pressed with a spacer having curved portions at longitudinal ends thereof inserted in a hollow part of the intermediate electrode group. This can ensure the size of the hollow part. Thus, increase in volume of the electrode group through charge/discharge can easily be absorbed by the hollow part. This can reduce expansion of the battery due to expansion of the electrode, and can prevent the occurrence of an internal short circuit etc. due to the expansion of the battery. 
     In the present description, the “parallelogram” may include a shape which is slightly deformed from a perfect parallelogram. Being “symmetric about a point” may include a positional relationship which is slightly deviated from perfect point symmetry. The “midpoint” may include a position which is slightly misaligned from a perfect midpoint. Unless otherwise deviated from the scope of the advantages of the present invention, modifications may be made to the flat electrode group, the intermediate electrode group, the hollow part of the intermediate electrode group, the shape of the core, the positions of the innermost parts of the bent portions, etc. 
     In the present description, the expression that two parts are symmetric about a particular point designates that the two parts are not positioned on a major axis of the flat electrode group, the intermediate electrode group, or the core, and are positioned symmetric about the particular point. 
     Advantages of the Invention  
     According to the present invention, the flat electrode group can be fabricated without cracking or separation of the electrode mixture layer. Thus, the present invention can provide the flat secondary battery with high safety. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1(   a )- 1 ( c ) are schematic cross-sectional views illustrating a flat electrode group of the present invention. 
         FIGS. 2(   a )- 2 ( d ) are schematic cross-sectional views sequentially illustrating steps of a method for fabricating the flat electrode group of the present invention. 
         FIGS. 3(   a )- 3 ( b ) are schematic cross-sectional views sequentially illustrating steps of another method for fabricating the flat electrode group of the present invention. 
         FIG. 4  is a perspective view, partially cut away, of a flat secondary battery of the present invention. 
         FIGS. 5(   a )- 5 ( c ) are schematic cross-sectional views sequentially illustrating steps of a method for fabricating a flat electrode group according to a comparative example. 
         FIGS. 6(   a )- 6 ( b ) are schematic cross-sectional views sequentially illustrating steps of a method for fabricating a conventional flat electrode group. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be described below with reference to the drawings. The present invention is not limited to the following embodiment. 
       FIGS. 1(   a )- 1 ( c ) are schematic cross-sectional views of a flat electrode group  1  according to an embodiment of the present invention. The flat electrode group  1  of the present embodiment is formed by winding a negative electrode  2  and a positive electrode  3  with a porous insulator  4  interposed therebetween, and flattening the wound product by pressing, and has a hollow part  7 . The negative electrode  2 , the positive electrode  3 , and the porous insulator  4  are bent (bent portions  8 ,  9 ) at longitudinal ends of a lateral cross-section of the flat electrode group  1  (ends in a direction of a long axis). Innermost parts  8 A,  9 A of the bent portions are not positioned on a major axis  6  of the flat electrode group  1 , but are positioned opposite each other relative to the major axis  6 . 
     Specifically, in the electrode group  1  shown in  FIG. 1(   a ), the innermost parts  8 A,  9 A of the bent portions are symmetric about a point of intersection X of a minor axis  5  and a major axis  6  of the flat electrode group  1  (a substantial center of the lateral cross-section of the flat electrode group  1 ). A displacement H 1  of the innermost part of the bent portion from the major axis  6  is approximately the same as a displacement H 2  of the innermost part of the bent portion from the major axis  6 . In the electrode group  1  shown in  FIG. 1(   b ), the displacement H 1  of the innermost part of the bent portion from the major axis  6  is larger than the displacement H 2  of the innermost part of the bent portion from the major axis  6 . 
     In the electrode group  1  shown in  FIG. 1(   c ), the displacement H 1  of the innermost part of the bent portion from the major axis  6  is smaller than the displacement H 2  of the innermost part of the bent portion from the major axis  6 . 
     Each of the electrode groups shown in  FIGS. 1(   a )- 1 ( c ) can provide the advantages of the present embodiment (described below). 
       FIGS. 2(   a )- 2 ( d ) are schematic cross-sectional views sequentially illustrating steps of a method for fabricating the flat electrode group  1  of the present embodiment. 
       FIG. 2(   a ) shows an early stage of a step of winding the negative electrode  2  and the positive electrode  3  sandwiching the porous insulator  4  therebetween (a stack) to form the flat electrode group  1 . A core  33  around which the stack is wound includes an upper core  32  and a lower core  30 . The upper core  32  and the lower core  30  have parallelogram-shaped lateral cross-sections, respectively. The upper core  32  has a corner  36 , and the lower core  30  has a corner  35 . The upper core  32  has an inner axis  34  to sandwich and hold the stack in the beginning of the winding. The core  33  further includes a pressing cylinder  31  for pressing a wound product (the wound stack). When the core  33  is rotated in a direction A shown in  FIG. 2(   a ), the stack is wound to form an intermediate electrode group  1   a  shown in  FIG. 2(   b ). The intermediate electrode group  1   a  has corners  8   a,    9   a  corresponding to the corners  35 ,  36  of the core  33 , and a hollow part  7   a.    FIG. 2(   c ) shows pressing of the intermediate electrode group  1   a  in a direction of the minor axis, and  FIG. 2(   d ) shows a lateral cross-section of the flat electrode group  1  obtained by the pressing. 
     Specifically, with the corners  35 ,  36  of the core  33  positioned opposite each other relative to a major axis L of the core  33 , the corners  8   a ,  9   a  are positioned opposite each other relative to the major axis  6  of the intermediate electrode group  1   a . In the step shown in  FIG. 2(   c ), the intermediate electrode group  1   a  is pressed in the direction of the minor axis. The pressing does not bend the corners  8   a ,  9   a  only, but forms bent portions  8   b ,  9   b  which include the corners  8   a ,  9   a,  respectively, and are bent in a larger area than the corners  8   a ,  9   a.  The bent portions  8   b ,  9   b  become part of bent portions 8, 9 of the flat electrode group  1  after the pressing, and innermost parts  8 A,  9 A of the bent portions are positioned opposite each other relative to the major axis  6 . With the bent portions  8 ,  9  formed in this manner, a bend of the corners  8   a ,  9   a  or a residual stress associated with the bend can be reduced even when additional bending stress is applied to the bent portions  8 ,  9  in pressing the intermediate electrode group. This can reduce a width of the crack generated in the electrode mixture layers of the negative electrode  2  and the positive electrode  3 , and can reduce the separation of the electrode mixture layers. 
     In the flat electrode group  1  fabricated in this manner, drop of the electrode mixture layer due to the cracking or separation of the electrode mixture layer can be prevented, thereby preventing the occurrence of an internal short circuit due to the drop of the electrode mixture layer. This can provide the flat secondary battery with high safety. 
     A method for fabricating the flat electrode group  1  will be described in detail below. The winding step shown in  FIG. 2(   a ) includes a main winding step, and a finishing step. In the main winding step, the stack is sandwiched between the upper core  32  and the lower core  30 , and is held by the inner axis  34  and the lower core  30 . With a predetermined tension applied to each of the negative electrode  2 , the positive electrode  3 , and the porous insulator  4 , the core  33  is rotated a predetermined number of times in the direction A shown in  FIG. 2(   a ). Thus, the stack is wound. 
     In the finishing step, the rest of the stack is wound. Under facility-based constraints, it is difficult to wind the rest of the stack at the same tension applied in the main winding step. Thus, in winding a longitudinal end of the stack, the tension applied thereto may become zero, thereby loosening the stack. To reduce the loosening, the stack is sandwiched between the pressing cylinder  31  and the core  33 , and the core  33  is rotated at least one time to wind the stack being pressed. Further, while the wound product is pressed by the pressing cylinder  31 , an adhesive tape made of polypropylene (this is adhered to a last wound end of the stack) is adhered to an outer peripheral surface of the wound product. When the wound product fabricated in this manner is removed from the core  33 , the intermediate electrode group  1   a  having a parallelogram-shaped lateral cross-section as shown in  FIG. 2(   b ) is obtained. Then, in the pressing step shown in  FIG. 2(   c ), the intermediate electrode group  1   a  is pressed in the direction of the minor axis to form the flat electrode group  1  shown in  FIG. 2(   d ). 
       FIG. 3(   a ) schematically shows pressing of the intermediate electrode group  1   a  in the direction of the minor axis with a spacer  37  inserted in the hollow part  7   a  of the intermediate electrode group  1   a . The spacer  37  includes curved portions  37 A at longitudinal ends, respectively. The spacer  37  is inserted in the hollow part  7   a  in such a manner that the curved portions  37 A are positioned at longitudinal ends of the hollow part  7   a  of the intermediate electrode group  1   a.  Thus, as compared with the case where the intermediate electrode group  1   a  is pressed without the spacer  37  inserted in the hollow part  7   a , the hollow part  7  is formed larger (see  FIG. 3(   b )). Therefore, expansion of the negative electrode  2  and the positive electrode  3  due to charge/discharge (hereinafter merely referred to as “expansion of the negative electrode  2  and the positive electrode  3 ”) can easily be absorbed by the hollow part  7   a , and warpage of the negative electrode  2  and the positive electrode  3  due to charge/discharge can be reduced. 
     In the present embodiment, the shape of the lateral cross-section of the intermediate electrode group  1   a  is not limited to the shape shown in  FIGS. 1(   a )- 1 ( c ) as long as the two diagonal corners of the intermediate electrode group  1   a  are not positioned on the same line perpendicular to the direction in which the intermediate electrode group  1   a  is pressed. The advantages similar to those of the present embodiment can be obtained in this case. However, when the two corners are positioned on the same line perpendicular to the pressing direction of the intermediate electrode group  1   a , the electrode mixture layer may be cracked or separated in pressing the intermediate electrode group  1   a,  and the internal short circuit may occur. 
     In other words, the pressing direction of the intermediate electrode group  1   a  is not limited to the direction of the minor axis of the intermediate electrode group  1   a,  and the intermediate electrode group  1   a  can be pressed in any direction except for a direction perpendicular to a line connecting the diagonal corners. The advantages similar to those of the present embodiment can be obtained in this case. However, when the intermediate electrode group  1   a  is pressed in the direction perpendicular to the line connecting the two diagonal corners, the electrode mixture layer may be cracked or separated in pressing the intermediate electrode group  1   a,  and the internal short circuit may occur. 
     In view of easy designing of the core  33 , or easy winding, the core  33  having the corners  35 ,  36  which are symmetric about a center of the core  33  is preferably used in fabricating the electrode group  1 . Thus, the electrode group  1  shown in  FIG. 1(   a ) is preferable. However, forming the electrode group  1  of  FIG. 1(   a ) with high yield is not easy. Therefore, even when the core (the core  33  having the corners  35 ,  36  which are symmetric about the center of the core  33 ) is used, the electrode group  1  shown in  FIG. 1(   b ) or  FIG. 1(   c ) may be formed. However, the electrode group  1  shown in  FIG. 1(   b ) or  FIG. 1(   c ) can provide substantially the same advantages as those of the electrode group  1  shown in  FIG. 1(   a ) because the innermost parts  8 A,  9 A of the bend portions are positioned opposite each other relative to the major axis  6 . 
     When the two diagonal corners of the intermediate electrode group  1   a  are positioned opposite each other relative to the major axis  6  of the intermediate electrode group  1   a,  the advantages described above can be obtained. Thus, the lateral cross-section of the intermediate electrode group  1   a  is not necessarily parallelogram-shaped as long as the hollow part  7   a  of the intermediate electrode group  1   a  is parallelogram-shaped when viewed in lateral cross-section. 
     Materials of the flat secondary battery will be described below. 
     The positive electrode  3  is formed by applying positive electrode mixture paste to one or both of surfaces of a positive electrode current collector, drying the paste, and rolling the obtained product to a predetermined thickness. The positive electrode current collector is made of, for example, foil or nonwoven fabric of aluminum or an aluminum alloy, and has a thickness of 5-30 μm. The positive electrode mixture paste is prepared by mixing and dispersing a positive electrode active material, a conductive agent, and a binder in a dispersion medium using a distributor such as a planetary mixer etc. 
     The positive electrode active material may be lithium cobaltate or modified lithium cobaltate (e.g., lithium cobaltate containing aluminum or magnesium as a solid solution), lithium nickelate or modified lithium nickelate (e.g., nickel partially substituted with cobalt etc.), lithium manganate or modified lithium manganate, etc. 
     The conductive agent may be carbon black, such as acetylene black, Ketchen black, channel black, furnace black, lamp black, thermal black, etc., or various type of graphites used alone or in combination. 
     The binder for the positive electrode may be, for example, poly(vinylidene fluoride) (PVdF), modified PVdF, polytetrafluoroethylene (PTFE), or a rubber particle binder containing an acrylate unit. 
     The negative electrode  2  is formed by applying negative electrode mixture paste to one or both of surfaces of a negative electrode current collector, drying the paste, and rolling the obtained product to a predetermined thickness. The negative electrode current collector is made of, for example, rolled copper foil, electrolytic copper foil, or nonwoven fabric of copper fiber, and has a thickness of 5-25 μm. The negative electrode mixture paste is prepared by mixing and dispersing a negative electrode active material and a binder (together with a conductive agent and a thickener as needed) in a dispersion medium using a distributor such as a planetary mixer etc. 
     The negative electrode active material may be, for example, various types of natural graphite, artificial graphite, silicon-based composite materials such as silicide etc., or various types of alloy compositions. 
     Various types of binders can be used as the binder for the negative electrode, e.g., poly(vinylidene fluoride) (PVdF), and modified PVdF. In view of easy insertion of lithium ions, styrene-butadiene-rubber (SBR) particles or modified SBR particles etc. may preferably be used as the binder for the negative electrode. 
     The thickener may be a viscous solution, such as poly(ethylene oxide) (PEO), poly(vinyl alcohol) (PVA), etc. In view of easy dispersion of the mixture, and thickening effect, a cellulose-based resin, such as carboxymethylcellulose (CMC), or modified CMC, may preferably be used as the thickener. 
     The porous insulator  4  is not limited as long as the porous insulator can be durable for use in the flat secondary battery. In particular, the porous insulator is preferably a single layer or multiple layers of a microporous film made of a polyolefin-based resin, such as polyethylene, polypropylene, etc. A microporous insulating layer may be formed on a film, and a thickness of the porous insulator  4  is preferably 10-25 μm. 
     The flat secondary battery of the present embodiment will be described below.  FIG. 4  is a perspective view, partially cut away, illustrating a flat secondary battery  25  including the flat electrode group  1  of the present embodiment. The flat secondary battery  25  is fabricated by the following method. The flat electrode group  1 , and an insulating frame  27  are placed in a flat battery case  21  having a close end. A negative electrode lead  23  drawn from an upper part of the flat electrode group  1  is connected to a terminal  20  (an insulating gasket  29  is attached to a periphery of the terminal  20 ), and a positive electrode lead  22  drawn from the upper part of the flat electrode group  1  is connected to a sealing plate  26 . The sealing plate  26  is inserted in an opening of the battery case  21 , and the sealing plate  26  and the battery case  21  are welded along a rim of the opening of the battery case  21 . Thus, the battery case  21  is sealed. A predetermined amount of a nonaqueous electrolytic solution (not shown) is fed into the battery case  21  through a plug hole in the sealing plate  26 , and the plug hole is stopped with a plug  24 . Thus, the flat secondary battery 25 is fabricated. The above-described fabrication method is merely an example, and the method for fabricating the flat secondary battery  25  is not limited thereto. 
     Various types of lithium compounds, such as LiPF 6 , LiBF 4 , etc., may be used as electrolyte salt of the nonaqueous electrolytic solution. As a solvent of the nonaqueous electrolytic solution, ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), or ethyl methyl carbonate (MEC) may be used alone or in combination. To form a good coating on the positive electrode, or to ensure stability in overcharge, vinylene carbonate (VC), cyclohexylbenzene (CHB), or modified cyclohexylbenzene may preferably be used as the solvent of the nonaqueous electrolytic solution. 
     EXAMPLES  
     In the following examples, safety of the flat secondary battery including an electrode group having a lateral cross-section of  FIG. 1(   a ) was evaluated. 
     1. Method for Fabricating Flat Secondary Battery 
     Example 1  
     (a) Fabrication of Positive Electrode  3   
     In a dual arm kneader, 100 parts by weight (pbw) of lithium cobaltate (a positive electrode active material), 2 pbw of acetylene black (a conductive agent), 2 pbw of poly(vinylidene fluoride) (a binder), and an appropriate amount of N-methyl-2-pyrrolidone were stirred to obtain positive electrode mixture paste. 
     Then, the positive electrode material paste was applied to each surface of 15 μm thick aluminum foil (a positive electrode current collector), and dried to form a positive electrode base having a 100 μm thick positive electrode mixture layer on each surface of the aluminum foil. 
     Then, the positive electrode base was pressed to a total thickness of 165 μm. The pressing reduced the thickness of each of the positive electrode mixture layers to 75 μm. The pressed positive electrode base was cut into a predetermined width to obtain a positive electrode  3 . 
     (b) Fabrication of Negative Electrode  2   
     In a dual arm kneader, 100 pbw of artificial graphite (a negative electrode active material), 2.5 pbw of a dispersion of styrene-butadiene rubber particles (a binder, containing 40 wt. % of a solid content) (1 pbw in terms of a solid content of the binder), 1 pbw of carboxymethyl cellulose (a thickener), and an appropriate amount of water were stirred to obtain negative electrode material paste. 
     The negative electrode material paste was applied to each surface of 10 μm thick copper foil (a negative electrode current collector), and dried to obtain a negative electrode base having a 100 μm thick negative electrode mixture layer on each surface of the copper foil. 
     Then, the negative electrode base was pressed to a total thickness of 170 μm. The pressing reduced the thickness of each of the negative electrode mixture layers to 80 μm. The pressed negative electrode base was cut into a predetermined width to obtain a negative electrode  2 . 
     (c) Fabrication of Flat Electrode Group  1   
     A flat electrode group  1  was fabricated by the method shown in  FIGS. 2(   a )- 2 ( d ). 
     Specifically, as shown in  FIG. 2(   a ), a stack formed by sandwiching the porous insulator  4  between the negative electrode  2  and the positive electrode  3  was sandwiched between an upper core  32  and a lower core  30 , and was held between an inner axis  34  and a lower core  30 . 
     Then, a tension of 1000 gf was applied to the negative electrode  2  and the positive electrode  3 , and a tension of 500 gf was applied to the porous insulator  4  to rotate a core  33  in a direction A shown in  FIG. 2(   a ). The stack was wound 7 times in a main winding step, and was wound 3 times in a finishing step while the wound product was pressed by a pressing cylinder  31  at a pressure of 0.06 MPa. Then, a polypropylene adhesive tape was adhered to an outer peripheral surface of the wound product to fix a longitudinal end of the stack to the outer peripheral surface, and then the wound product was removed from the core  33 . Thus, an intermediate electrode group  1   a  shown in  FIG. 2(   b ) was obtained. 
     Then, as shown in  FIG. 2(   c ), the intermediate electrode group  1   a  was pressed in a direction of a minor axis thereof. Thus, a flat electrode group  1  shown in  FIG. 2(   d ) was obtained. In the obtained flat electrode group  1 , innermost parts  8 A,  9 A of bent portions were symmetric about a point of intersection X. 
     (d) Fabrication of Flat Secondary Battery  25   
     The obtained flat electrode group  1  and an insulating frame  27  were placed in a flat battery case  21  having a closed end. A negative electrode lead  23  was connected to a terminal  20 , and a positive electrode lead  22  was connected to a sealing plate  26 . The sealing plate  26  was inserted in an opening of the battery case  21 , and the sealing plate  26  and the battery case  21  were welded along a rim of the opening of the battery case  21 . Then, a predetermined amount of a nonaqueous electrolytic solution was fed into the battery case  21  through a plug hole, and the plug hole was stopped with a plug  24 . Thus, a flat secondary battery  25  was obtained. 
     Example 2  
     A flat secondary battery of Example 2 was fabricated in the same manner as Example 1 except for the tension applied in winding the stack. 
     Specifically, with the stack of Example 1 held by the core  33 , a tension of 800 gf was applied to the negative electrode  2  and the positive electrode  3 , and a tension of 200 gf was applied to the porous insulator  4  to rotate the core  33  in the direction A shown in  FIG. 2(   a ). In the flat electrode group  1  obtained in this manner, the innermost parts  8 A,  9 A of the bent portions were symmetric about the point of intersection X. 
     Example 3  
     A flat secondary battery of Example 3 was fabricated in the same manner as Example 1 except that the intermediate electrode group  1   a  was pressed by a method shown in  FIGS. 3(   a )- 3 ( b ). 
     Specifically, as shown in  FIG. 3(   a ), a 0.5 mm thick spacer  37  was inserted in a hollow part  7   a  of the intermediate electrode group  1   a.  Curved portions  37 A were formed at longitudinal ends of the spacer  37 , respectively, and the spacer  37  was inserted in the hollow part  7   a  of the intermediate electrode group  1   a  in such a manner that the curved portions  37 A are positioned at ends in a direction of a major axis of the hollow part  7   a  of the intermediate electrode group  1   a . Then, with the spacer  37  inserted in the hollow part  7   a  of the intermediate electrode group  1   a,  the intermediate electrode group  1   a  was pressed in the direction of the minor axis thereof. Thus, a flat electrode group  1  shown in  FIG. 3(   b ) was obtained. In the flat electrode group  1 , the innermost parts  8 A,  9 A of the bent portions were symmetric about the point of intersection X. 
     Comparative Example 1  
     A flat secondary battery of Comparative Example 1 was fabricated in the same manner as Example 1 except that the stack of Example 1 was wound by a method shown in  FIGS. 5(   a )- 5 ( c ). 
       FIG. 5(   a ) is a schematic cross-sectional view illustrating an early stage of a step of winding the stack of Example 1. A core  47  for winding the stack includes a left core  43  and a right core  45 . The left core  43  and the right core  45  have rhomboid-shaped lateral cross-sections, respectively. The left core  43  has a corner  44 , and the right core  45  has a corner  48 . The right core  45  includes an inner axis  46  to sandwich and hold stack in the beginning of the winding. The core  47  further includes a pressing cylinder  31  for pressing a wound product. When the core  47  is rotated in a direction A shown in  FIG. 5(   a ), the stack is wound to form an intermediate electrode group  49   a  having corners  58   a ,  59   a  corresponding to the corners  44 ,  48  as shown in  FIG. 5(   b ). Specifically, the intermediate electrode group  49   a  has a rhomboid-shaped lateral cross-section.  FIG. 5(   c ) schematically shows a lateral cross-section of a flat electrode group  49  formed by flattening the intermediate electrode group  49   a  of  FIG. 5(   b ) by pressing. Innermost parts  58 A,  59 A of bent portions of the flat electrode group  49  are positioned on a major axis  56  of the flat electrode group  49 . 
     (c) Fabrication of Flat Electrode Group  49   
     As shown in  FIG. 5(   a ), the stack of Example 1 was sandwiched between the left core  43  and the right core  45 , and was held by the inner axis  46  and the right core  45 . 
     A tension of 1000 gf was applied to the negative electrode  2  and the positive electrode  3 , and a tension of 500 gf was applied to the porous insulator  4  to rotate the core  47  in the direction A shown in  FIG. 5(   a ). The stack was wound 7 times in the main winding step, and was wound 3 times in the finishing step while the wound product was pressed by the pressing cylinder  31  at a pressure of 0.06 MPa. Then, a polypropylene adhesive tape was adhered to an outer peripheral surface of the wound product to fix a longitudinal end of the stack to the outer peripheral surface, and then the wound product was removed from the core  47 . Thus, an intermediate electrode group  49   a  shown in  FIG. 5(   b ) was obtained. 
     Then, as shown in  FIG. 5(   c ), the intermediate electrode group  49   a  was pressed in a direction of a minor axis thereof to obtain a flat electrode group  49 . The obtained flat electrode group  49  had bent portions  58 ,  59  at ends in a direction of the major axis, and innermost parts  58 A,  59 A of the bent portions were positioned on the major axis  56 . 
     Table 1 shows the details of Examples 1-3 and Comparative Example 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Positions of innermost parts 
                 Winding 
               
               
                   
                 of bent portions 
                 tension 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Example 1 
                 Symmetric about point of intersection X 
                 High 
               
               
                 Example 2 
                 Symmetric about point of intersection X 
                 Low 
               
               
                 Example 3 
                 Symmetric about point of intersection X 
                 High 
               
               
                 Comparative 
                 On major axis 
                 High 
               
               
                 Example 1 
               
               
                   
               
            
           
         
       
     
     2. First Evaluation 
     Flat electrode groups of Examples 1-3 and Comparative Example 1, 100 each, were fabricated, and 60 of the 100 flat electrode groups were used to fabricate flat secondary batteries (60 flat secondary batteries were fabricated), and the remaining 40 flat electrode groups were merely placed in battery cases, respectively. Then, these samples were evaluated in the following manner. 
     (a) Whether Battery was Thickened or Not 
     Thicknesses of the flat secondary batteries were measured immediately after the fabrication, and after 500 charge/discharge cycles, and averages of the two measurements were calculated. Batteries which experienced increase in thickness after the 500 cycles by 20% or higher of the thickness immediately after the fabrication were regarded as thickened batteries. 
     (b) Whether Electrode was Warped or Not 
     Lateral cross-sectional images of the flat secondary batteries were taken at the vertical center thereof immediately after the fabrication, and after the 500 charge/discharge cycles by X-ray computerized tomography (hereinafter abbreviated as CT). The images were visually checked to see whether the electrode was warped or not. 
     (c) Whether Electrode Mixture Layer was Cracked, Separated or Not 
     The flat electrode group placed in the battery case was fixed using a thermosetting resin. Then, the flat electrode group was cut in a direction perpendicular to an axis thereof. The cross-section (a lateral cross-section of the flat electrode group) was observed by a measuring microscope to measure a width of the crack in the electrode mixture layer. The electrode mixture layer in which the width of the crack was smaller than 0.1 mm was regarded as an electrode mixture layer which was not cracked, while the electrode mixture layer in which the width of the crack was 0.1 mm or larger was regarded as a cracked electrode mixture layer. The cross-section was observed by a microscope to see whether the electrode mixture layer was separated or not. 
     Table 2 shows the results of (a)-(c). 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                 Whether the 
                 Whether the 
               
               
                   
                 Whether the 
                 Whether the 
                 electrode 
                 electrode 
               
               
                   
                 electrode 
                 battery was 
                 mixture layer 
                 mixture layer 
               
               
                   
                 was warped 
                 thickened 
                 was cracked 
                 was sepa- 
               
               
                   
                 or not 
                 or not 
                 or not 
                 rated or not 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 1 
                 Not warped 
                 Not thickened 
                 Not cracked 
                 Not separated 
               
               
                 Example 2 
                 Not warped 
                 Not thickened 
                 Not cracked 
                 Not separated 
               
               
                 Example 3 
                 Not warped 
                 Not thickened 
                 Not cracked 
                 Not separated 
               
               
                 Compar- 
                 Warped 
                 Thickened 
                 Cracked 
                 Separated 
               
               
                 ative 
               
               
                 Example 1 
               
               
                   
               
            
           
         
       
     
     3. Consideration of First Evaluation 
     The results shown in Table 2 indicate that the negative electrode  2  and the positive electrode  3  of each of Examples 1-3 were not warped, and the increase in thickness of the battery after the 500 charge/discharge cycles was very small. A product (a device in which the flat secondary battery is mounted) was hardly affected. 
     A presumable reason for the results is as follows. The corners  8   a ,  9   a  of the intermediate electrode group  1   a  are symmetric about the point of intersection of the minor axis  5  and the major axis  6  of the intermediate electrode group  1   a . When the intermediate electrode group  1   a  is pressed in the direction of the minor axis, the generated stress is distributed to form gently curved bent portions  8 ,  9  in the flat electrode group  1 . Thus, when placed in the battery case  21 , the flat electrode group  1  is deformed to return to the shape before the pressing, thereby approaching the inner side surface of the battery case  21 . As a result, the hollow part  7  is formed in the flat electrode group  1 . Even when the negative electrode  2  and the positive electrode  3  expand through repetitive charge/discharge, the expansion of the negative electrode  2  and the positive electrode  3  is absorbed by the sufficiently large hollow part  7  formed in the flat electrode group  1 . This can reduce the occurrence of the warpage of the negative electrode  2  and the positive electrode  3 , and can reduce the increase in thickness of the battery. 
     Although not shown in Table 2, the increase in thickness of the battery was smaller in Examples 2 and 3 than in Example 1. A presumable reason why the increase in battery thickness was smaller in Example 2 is that the tension in winding the stack was small. When the tension in winding the stack is small, the stress generated during the winding is reduced, thereby a reducing residual stress in the electrodes at the bent portions  8 ,  9 . Thus, when a volume of the flat electrode group  1  is increased by the expansion of the negative electrode  2  and the positive electrode  3  through the charge/discharge, the current collectors extend in response to the increase in volume of the flat electrode group  1 . This can reduce the occurrence of the warpage of the negative electrode  2  and the positive electrode  3 , and can reduce the increase in thickness of the battery. 
     A presumable reason why the increase in battery thickness was smaller in Example 3 is that the spacer  37  was used in pressing the intermediate electrode group  1   a.  When the intermediate electrode group  1   a  is pressed with the spacer  37  inserted in the hollow part  7   a , the bent portions  8 ,  9  formed in the flat electrode group  1  are gently curved as compared with the case where the intermediate electrode group  1   a  is pressed without using the spacer  37 . Thus, the flat electrode group  1  inserted in the battery case  21  significantly returns to the original shape, and the hollow part  7  becomes large. The large hollow part  7  can easily absorb the expansion of the negative electrode  2  and the positive electrode  3 . This can further reduce the occurrence of the warpage of the negative electrode  2  and the positive electrode  3 , and can reduce the increase in thickness of the battery. 
     As shown in Table 2, the width of the crack in the electrode mixture layer at the innermost parts  8 A,  9 A of the bent portions was very small in each of Examples 1-3. The separation of the electrode mixture layer at the innermost parts  8 A,  9 A of the bent portions was hardly observed, and the product was hardly affected. 
     A presumable reason for the results is as follows. Since the corners  35 ,  36  of the core  33  are symmetric about a center of the lateral cross-section of the core  33 , the corners  8   a ,  9   a  of the intermediate electrode group  1   a  are symmetric about the point of intersection of the minor axis  5  and the major axis  6  of the intermediate electrode group  1   a . Pressing the intermediate electrode group  1   a  in the direction of the minor axis does not bend the corners  8   a ,  9   a  only, but forms bent portions  8   b ,  9   b  which include the corners  8   a ,  9   a,  respectively, and are bent in a larger area than the corners  8   a ,  9   a.  Thus, as shown in  FIG. 2(   d ), the innermost parts  8 A,  9 A of the bent portions are symmetric about the point of intersection X. 
     With the innermost parts  8 A,  9 A of the bent portions formed in this way, a bend of the corners  8   a ,  9   a  or a residual stress associated with the bend can be reduced even when additional bending stress is applied to the bent portions  8 ,  9  in pressing the intermediate electrode group. This can presumably reduce the width of the crack in the electrode mixture layer, and can reduce the separation of the electrode mixture layer. 
     The negative electrode  2  and the positive electrode  3  of Comparative Example 1 were warped, and the battery was thickened. Specifically, the battery was thickened by 0.6 mm. It is presumed that the increase in thickness significantly affects the product, e.g., the flat secondary battery may be detached from the product. 
     A presumable reason for the results is as follows. In the step shown in  FIG. 5(   a ), the stack was wound without partially reducing the tension. Thus, the innermost parts  58 A,  59 A of the bent portions of the flat electrode group  49  were not easily deformed. Therefore, the flat electrode group  49  was hardly returned to the original shape after inserted in the battery case  21 , and the hollow part  57  was smaller than those of Examples 1-3. It was difficult to absorb the expansion of the negative electrode  2  and the positive electrode  3  by the hollow part  57 , and the negative electrode  2  and the positive electrode  3  were warped. Due to the warpage, the flat electrode group  1  significantly expanded radially outward, and the battery was significantly thickened. 
     In the innermost parts  58 A,  59 A of the bent portions, the electrode mixture layer was cracked, and the crack had a width of 1.1 mm. Microscopic foreign matters may easily enter the crack having such a width. Thus, in Comparative Example 1, the internal short circuit is more likely to occur than in Examples 1-3, and overheat easily occurs. The separation of the electrode mixture layer not only reduces quality due to reduction in capacity, but also exposes the current collector when the separated mixture layer is dropped. Thus, the internal short circuit is likely to occur. 
     A presumable reason for the results is as follows. In Comparative Example 1, the corners  58   a ,  59   a  of the intermediate electrode group  49   a  are formed based on the corners  44 ,  48  of the core  47 . Thus, significant residual stress or distortion in the winding step remains near the corners  58   a ,  59   a.  It is presumed that the cracking or separation of the electrode mixture layer was caused by pressing this intermediate electrode group  49   a.    
     4. Second Evaluation 
     Among the flat secondary batteries which experienced the 500 charge/discharge cycles, 30 batteries were used. Ten of the 30 batteries were used to perform a drop test, another 10 batteries were used to perform a crush test with a round rod, and the remaining 10 batteries were used to perform a heat test at 150° C. 
     (d) Drop Test 
     The flat secondary batteries were charged at a current of 2 A to an upper limit voltage of 4.2 V for 2 hours. Then, the batteries were dropped from a height of 1.5 m on a concrete floor. The drop test was performed 10 times on each of 6 surfaces of each flat secondary battery. Temperatures of heat generated by the batteries were measured at a room temperature of 25° C. to obtain an average of the temperatures. 
     (e) Crush Test with Round Rod 
     The flat secondary batteries were charged at a current of 2 A to an upper limit voltage of 4.2 V for 2 hours. Then, each of the batteries was laid down, and a round rod (10 mm in diameter), which was set perpendicular to the length of the battery, was dropped from a predetermined height to crush the battery. Temperatures of heat generated by the batteries were measured at a room temperature of 25° C. to obtain an average of the temperatures. 
     (f) Heat Test at 150° C. 
     The flat secondary batteries were charged at a current of 2 A to an upper limit voltage of 4.2 V for 2 hours. Then, the batteries were placed in a thermostat, and a temperature in the thermostat was raised from a room temperature to 150° C. at a rate of 5° C./minute. Temperatures of heat generated by the batteries were measured to obtain an average of the temperatures. 
                                     TABLE 3                           Crush test with   Heat test at           Drop test   round rod   150° C.           Temperature of   Temperature of   Temperature of           generated   generated   generated           heat (° C.)   heat (° C.)   heat (° C.)                                                    Example 1   25° C.   25° C.   150° C.           (no heat   (no heat   (no heat           generation)   generation)   generation)       Example 2   25° C.   25° C.   150° C.           (no heat   (no heat   (no heat           generation)   generation)   generation)       Example 3   25° C.   25° C.   150° C.           (no heat   (no heat   (no heat           generation)   generation)   generation)       Comparative   50° C.   120° C.    170° C.       Example 1   (heat generated)   (heat generated)   (heat generated)                    
5. Consideration of Second Evaluation The result shown in Table 3 indicate that Examples 1-3 did not show any defects in the drop test, the crush test with the round rod, and the heat test at 150° C. A presumable reason for the results is that the warpage of the positive electrode  3  and the negative electrode 2 was reduced, and the occurrence of the internal short circuit due to the warpage of the electrode was reduced.
 
     When the batteries of Comparative Example 1 were disassembled and checked after the 500 charge/discharge cycles, defects such as deposition of lithium, break of the electrode, buckling of the electrode, and drop of the electrode mixture layer, etc. were observed. In each of the drop test, the crush test with the round rod, and the heat test at 150° C., the temperature of generated heat was high. A presumable reason for the heat generation is that the internal short circuit was caused due to the drop of the electrode mixture layer, the break of the electrode, or the buckling of the electrode in the winding step. 
     The above results clarified that forming the intermediate electrode group  1   a  having the parallelogram-shaped lateral cross-section by winding the stack can reduce the cracking or separation of the electrode mixture layer at the innermost parts  8 A,  9 A of the bent portions in pressing the intermediate electrode group  1   a.    
     In the winding step shown in  FIG. 2(   a ), the corners  35 ,  36  of the core  33  are symmetric about the center of the lateral cross-section of the core  33 . Thus, the corners  8   a ,  9   a  of the intermediate electrode group  1   a  shown in  FIG. 2(   b ) are symmetric about the point of intersection X of the minor axis  5  and the major axis  6  of the intermediate electrode group  1   a.    
     In the pressing step shown in  FIG. 2(   c ), the intermediate electrode group  1   a  is pressed in the direction of the minor axis. The pressing does not bend the corners  8   a ,  9   a  only, but forms the bent portions  8   b ,  9   b  which include the corners  8   a ,  9   a,  respectively, and are bent in a larger area than the corners  8   a ,  9   a.  Thus, the innermost parts  8 A,  9 A of the bent portions of the flat electrode group  1  are symmetric about the point of intersection X. 
     With the innermost parts  8 A,  9 A of the bent portions are formed in this manner, a bend of the corners  8   a ,  9   a  or a residual stress associated with the bend can be reduced even when additional bending stress is applied to the bent portion  8 ,  9  in pressing the intermediate electrode group. This can presumably reduce the cracking of the electrode mixture layers of the negative electrode  2  and the positive electrode  3 , and can reduce the separation of the electrode mixture layers. 
     When the core  47  having the rhomboid-shaped lateral cross-section which is laterally symmetric with respect to the minor axis  55 , and is longitudinally symmetric with respect to the major axis  6  as shown in  FIGS. 5(   a )- 5 ( c ) is used, the innermost parts  58 A,  59 A of the bent portions are formed based on the corners  44 ,  48  of the core  47 . Thus, the innermost parts  58 A,  59 A of the bent portions presumably have a residual stress or distortion derived from the winding. It is presumed that the cracking or separation of the negative electrode  2  and the positive electrode  3  is more likely to occur when the parts are pressed. 
     In Examples 1-3, the innermost parts  8 A,  9 A of the bent portions which are symmetric about the point of intersection X have been described. However, the positional relationship between the innermost parts  8 A,  9 A of the bent portions is not limited to those of Examples 1-3. For example, as shown in  FIG. 1(   b ) or  FIG. 1(   c ), the innermost parts  8 A,  9 A of the bent portions which are positioned opposite each other relative to the major axis  6  can provide the advantages similar to those of Examples 1-3. 
     In  FIGS. 1(   a )- 1 ( c ),  FIGS. 2(   b )- 2 ( d ),  FIGS. 3(   a )- 3 ( b ), and  FIGS. 5(   b )- 5 ( c ), the lateral cross-section of the flat electrode group is schematically illustrated to avoid complication of the drawings. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, innermost parts of bent portions are positioned opposite each other relative to a major axis of a flat electrode group, and separation or drop of an electrode mixture layer during pressing can be reduced. This can provide can provide a highly safe flat secondary battery. Thus, the battery of the present invention is useful as a battery mounted in devices which requires safety (e.g., portable devices or vehicles). 
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
           1  Electrode group 
           1   a  Intermediate electrode group 
           2  Negative electrode 
           3  Positive electrode 
           4  Porous insulator 
           5  Minor axis 
           6  Major axis 
           7  Hollow part 
           8 ,  9  Bent portion 
           8 A,  9 A Innermost part of bent portion 
           8   a ,  9   a  Corner 
           8   b ,  9   b  Bent portion 
           25  Flat secondary battery 
           30  Lower core 
           31  Cylinder 
           32  Upper core 
           33  Core 
           34  Inner axis 
           35  Corner 
           36  Corner 
           37  Spacer