Patent Publication Number: US-2022231356-A1

Title: Secondary cell and method for manufacturing the same

Description:
TECHNICAL FIELD 
     The present invention relates to a secondary cell and a method for manufacturing the same. 
     BACKGROUND ART 
     A typical secondary cell has a structure in which an electrode body is housed in an outer package, e.g., a casing. The electrode body includes a positive electrode layer, an intermediate layer, and a negative electrode layer that are stacked in this order. Moreover, a positive electrode current collector is disposed on the outer surface of the positive electrode layer while a negative electrode current collector is disposed on the outer surface of the negative electrode layer. 
     A secondary cell including a solid electrolyte as an intermediate layer is particularly called an all-solid-state cell that is assumed to be superior in productivity and cell characteristics (including an energy density) to a cell including a liquid electrolyte. 
     In order to obtain an all-solid-state cell with excellent characteristics, a contact resistance between the layers is desirably kept at a sufficiently low resistance. Specifically, it is desirable to improve contact between the layers in the electrode body, contact between the positive electrode layer and the positive electrode current collector, contact between the negative electrode layer and the negative electrode current collector, and, when a plurality of electrode bodies are stacked, contact between the electrode bodies. 
     For example, a conventional all-solid-state cell described in Japanese Patent No. 6123642 includes a pressurizing portion for applying a retaining pressure in the stacking direction (perpendicularly to the stacking surface) of the cell. 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, when a secondary cell (all-solid-state cell) is provided with the pressurizing portion and a pressure control unit for controlling the pressurizing portion as described in Japanese Patent No. 6123642, the secondary cell including the pressurizing portion and the pressure control unit may increase in volume. Since the pressurizing portion and the pressure control unit do not store energy, the provision thereof may deteriorate the performance (particularly an energy density) of the overall secondary cell. 
     In other words, in order to secure contact between layers in the electrode body of the conventional secondary cell, the overall secondary cell is likely to upsize with deteriorated cell performance. An object of the present invention is to provide a secondary cell that can be downsized with improved cell performance, and a method for manufacturing the same. 
     Solution to Problem 
     In order to solve the problem, a secondary cell according to an embodiment of the present invention includes an outer package and an object to housed in the outer package, wherein the object to be housed includes an electrode body, and the outer package has a pressing portion for locally pressing at least one of the obverse side and the reverse side of the object to be housed. 
     The pressing portion is preferably at least one protrusion provided on the inner surface of the outer package. The pressing portion preferably configured to press one of the obverse side and the reverse side of the object to be housed at least at the central portion of the outer package. 
     More preferably, the outer package has a flat outer surface. Moreover, a corner in the outer package preferably includes an acute-angled portion. Furthermore, the electrode body preferably contains a powder material. 
     A method for manufacturing a secondary cell as an embodiment according to the present invention includes: holding an object to be housed having an electrode body, between an obverse-side plate and a reverse-side plate, at least one of the obverse-side plate and the reverse-side plate having a pressing portion for locally pressing at least one of the obverse side and the reverse side of the object to be housed; and forming an outer package of the secondary cell by connecting the obverse-side plate and the reverse-side plate via side plates extending between the obverse-side plate and the reverse-side plate. 
     The manufacturing method preferably further includes connecting a side plate of the side plates extending from the outer edge of one plate of the obverse-side plate and the reverse-side plate toward the other plate of the obverse-side plate and the reverse-side plate, to one of the other plate and a side plate of the side plates extending from the other plate toward the one plate. 
     More preferably, the obverse-side plate and the reverse-side plate are connected to each other via the side plates such that a corner in the outer package of the secondary cell includes an acute-angled portion. Furthermore, the outer package of the secondary cell is preferably formed in a vacuum. 
     Advantageous Effect of Invention 
     According to the secondary cell and the method for manufacturing the same of the present invention, the object to be housed including the electrode body is pressed by the pressing portion of the outer package, thereby keeping a low contact resistance in the electrode body without the external pressurizing portion of the related art. Thus, the cell performance of the secondary cell is expected to improve. Moreover, additional configurations for pressurization, for example, the pressurizing portion and the pressure control unit are not necessary, thereby downsizing the secondary cell. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a secondary cell according to an embodiment of the present invention. 
         FIG. 2  schematically illustrates an outer package and an object to be housed in the outer package in order to describe the structure of the secondary cell of  FIG. 1 . 
         FIG. 3A  illustrates a modification of the outer package, in which one of an obverse-side plate and a reverse-side plate is flat. 
         FIG. 3B  illustrates a modification of the outer package, in which a central portion protrudes inward along the width direction and the depth direction of the outer package. 
         FIG. 4A  illustrates an enlarged perspective view and a modification of pressing portions, in which a plurality of protrusions are formed in the width direction with a constant height along the depth direction. 
         FIG. 4B  illustrates an enlarged perspective view and a modification of pressing portions, in which protrusions vary in height along the depth direction. 
         FIG. 4C  illustrates an enlarged perspective view and a modification of the pressing portions, in which a plurality of columnar protrusions are formed. 
         FIG. 5  schematically illustrates a reverse-side plate connected to side plates, the housed object placed on the reverse-side plate, and an obverse-side plate before being connected to the side plates, in order to describe a method for manufacturing the secondary cell according to the embodiment of the present invention. 
         FIG. 6  illustrates the obverse-side plate connected to the side plates after the state of  FIG. 5 . 
         FIG. 7A  schematically illustrates a modification of the manufacturing method, in which the side plates are connected in advance to the obverse-side plate. 
         FIG. 7B  schematically illustrates a modification of the manufacturing method, in which a side plate is connected in advance to each of the obverse-side plate and the reverse-side plate. 
         FIG. 7C  schematically illustrates a modification of the manufacturing method, in which side plates on the obverse-side plate and side plates on the reverse-side plate are connected to each other. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     A schematic diagram in  FIG. 1  illustrates a secondary cell  10  according to an embodiment of the present invention. The secondary cell  10  includes an outer package  12  and an object  14  housed in the outer package  12 . The housed object  14  includes an electrode body. 
     The detailed structure of the electrode body is not illustrated. Typically, an electrode body having a laminated structure is used, in which an intermediate layer is interposed between a positive electrode layer and a negative electrode layer. The positive electrode layer, the intermediate layer, and the negative electrode layer that are included in the electrode body may be made of powder materials. Moreover, a positive electrode current collector is disposed on the outer surface of the positive electrode layer of the electrode body while a negative electrode current collector is disposed on the outer surface of the negative electrode layer. The housed object  14  may include a laminated package containing the electrode body in a sealed manner, in addition to the electrode body. The housed object may include a plurality of stacked electrode bodies. If a plurality of electrode bodies are stacked, the electrode bodies may be placed in various forms including a series connection, a parallel connection, and a combination of a series connection and a parallel connection. For example, in the case of a parallel connection of electrode bodies, in a part (internal structure) other than the outer surface of the overall housed object, a positive electrode current collector is preferably disposed between the positive electrode layers of the electrode bodies so as to electrically connect the positive electrode layers, and a negative electrode current collector is preferably disposed between the negative electrode layers of the electrode bodies so as to electrically connect the negative electrode layers. 
     As illustrated in  FIG. 1 , the inner surface of the outer package  12  has a plurality of asperities including inward convex portions (protrusions) that serve as pressing portions  12   a  for pressing the housed object  14 . The pressing portions  12   a  press the stacking surfaces (the extended surfaces of the layers) of the electrode body included in the housed object  14 . Specifically, the pressing portions  12   a  press the obverse side and the reverse side of the housed object  14  in a height direction Z that is the stacking direction (perpendicular to the stacking surfaces) of the electrode body having the laminated structure in  FIG. 1  (the obverse side is an upper side in the height direction Z while the reverse side is a lower side in the height direction Z, the obverse and reverse sides may be switched). 
     As has been mentioned, the inner surface of the outer package  12  has a plurality of asperities and thus the obverse side and the reverse side of the housed object  14  are pressed by the pressing portions  12   a  at the inward convex portions of the asperities. In other words, the pressing portions  12   a  locally press the obverse side and the reverse side of the housed object  14 . The pressing portions  12   a  are particularly formed into inward convex portions at a central portion provided in a width direction X (orthogonal to the height direction Z that is the stacking direction of the electrode body), thereby securing a pressing force at the central portion where a pressing force is likely to decrease. 
     In this configuration, the outer package  12  desirably includes an elastic body (e.g., metals such as aluminum and stainless steel and synthetic resin) that facilitates the application of a pressing force from the outer package  12  to the housed object  14 . In other words, the housed object  14  is preferably pressed by an elastic force of the outer package  12 . 
       FIG. 2  illustrates the structure of the secondary cell  10  in which the outer package  12  includes an elastic body. As illustrated in  FIG. 2 , before the object  14  is housed in the outer package  12  according to the present embodiment, the upper surface and the lower surface of the outer package  12  internally have a plurality of asperities as the pressing portions  12   a , the upper and lower surfaces extending orthogonally to the height direction Z in  FIG. 2 . Furthermore, the entire outer package  12  is considerably curved inward like large undulations (the overall surfaces are curved to protrude inward). A central portion  18  in the width direction X particularly has a small clearance in the height direction Z, the clearance being smaller in the height direction Z than the dimension of the object  14  to be housed. 
     When the secondary cell  10  in  FIG. 1  is formed by using the outer package  12  and the object  14  to be housed in  FIG. 2 , the object  14  to be housed is inserted into an opening as indicated by an arrow in  FIG. 2 , the opening being formed in a depth direction Y (a direction perpendicular to the height direction Z and the width direction X and also perpendicular to the plane of  FIG. 2 ) of the outer package  12  so as to receive the object  14  to be housed. When the object  14  to be housed is inserted into the outer package  12  that is entirely curved inward like large undulations as illustrated in  FIG. 2 , the outer package  12  is deformed along the outside shape of the housed object  14  into a shape illustrated in  FIG. 1  (a rectangular edge and a substantially flat outer surface). However, the elasticity of the elastic body constituting the outer package  12  allows the outer package  12  to return toward the shape of  FIG. 2  (entirely curved inward like large undulations). Thus, as indicated by arrows in  FIG. 1 , a pressing force derived from an elastic force is applied from the pressing portions  12   a  to the housed object  14 . A large pressing force is applied particularly to the central portion where a clearance is smaller than the housed object  14  before the object  14  to be housed is inserted. In this way, the present embodiment can increase a pressing force at the central portion that is remote from ends in the width direction X and is likely to receive a small pressing force. 
     Furthermore, the pressing portions  12   a  are locally provided in the present embodiment, allowing a pressing force applied by the elasticity of the outer package  12  to the object  14  to be housed to concentrate on the pressing portions  12   a . This can more efficiently apply a pressing force to the object  14  to be housed while concentrating the pressing force at points to be pressed, as compared with a pressure uniformly applied to the overall inner surface of the outer package  12 . 
     Moreover, in the present embodiment, corners  16  at ends in the width direction X in the outer package  12  have acute angles as illustrated in  FIGS. 1 and 2 . In other words, the upper surface and the lower surface in the height direction Z in the outer package  12  are entirely angled inward, ensuring the application of a pressing force to the obverse side and the reverse side of the object  14  to be housed. 
     As has been discussed, a pressing force derived from the shape and material of the outer package  12  is applied to the housed object  14  in the secondary cell  10  according to the present embodiment, eliminating the need for providing pressurizing portion and a pressure control unit in addition to the outer package  12  and the housed object  14  unlike in the related art. Thus, the secondary cell  10  of the present embodiment can improve in cell performance with a contact resistance reduced in the electrode body by a pressing force applied to the housed object  14 , and can be downsized by eliminating the need for a redundant configuration. 
     Furthermore, the outer surface of the outer package  12  is substantially flat while the object  14  is housed as illustrated in  FIG. 1 . Thus, if multiple secondary cells  10  are placed next to one another to obtain high power (series connection) or a large capacity (parallel connection), no clearance is made between the adjacent secondary cells  10 . This does not form any redundant spaces in the overall configuration, thereby keeping a high energy density per volume in the assembly of the secondary cells  10 . 
     If the intermediate layer of the electrode body provided in the housed object  14  is a solid-electrolyte layer, which is made of a powder material and is formed by a dry process, and the overall secondary cell  10  (all-solid-state cell) is fabricated with a large pressing force, a certain pressure is obtained in the housed object  14 . Thus, the use of powder materials eliminates the need for a large pressure from the outer package of the housed object, so that the asperities on the inner surface of the outer package only require a small depth and the inner surface only requires a small degree of curvature with respect to the outer surface. Hence, in this case, a volume in the outer package  12  is not so small before the insertion of the object  14  to be housed, so that the object  14  to be housed is easily inserted into the outer package  12 . 
     In  FIG. 2 , the upper side (facing the obverse side of the object  14  to be housed) and the lower side (facing the reverse side of the object  14  to be housed), in the height direction Z, of the inner surface of the outer package  12  are entirely curved inward before the insertion of the object  14  to be housed. It is not always necessary to curve both of the obverse side and the reverse side. 
       FIGS. 3A and 3B  illustrate outer packages  32  and  34  according to modifications. In the outer package  32  illustrated in  FIG. 3A , a plate constituting the inner surface of the lower side (reverse side) in the height direction Z is curved inward like the outer package  12  of  FIG. 2  while a plate constituting the inner surface of the upper side (obverse side) in the height direction Z is flattened. In this way, the outer package  32  has a flat inner surface on one of the obverse side and the reverse side. Also in this configuration, when the object  14  to be housed is inserted into the outer package  32 , a pressing force is applied by elasticity from the curved plate to the housed object  14 . If a necessary pressing force is obtained only by a pressing force from one of the obverse side and the reverse side, it is not always necessary to press the housed object  14  from both of the obverse side and the reverse side. Hence, like the outer package  32  of  FIG. 3A , one of the plates on the obverse side and the reverse side may be flattened (the obverse side) or right-angled portions (corners on the obverse side) may be included instead of acute-angled corners. 
     The plate curved to protrude inward on the reverse side in the outer package  32  of  FIG. 3A  is curved along the width direction X (a distance from the flat outer surface with the housed object  14  changes along the width direction X) but is not curved in the depth direction Y (perpendicular to the height direction Z and the width direction X). However, the plate of the outer package  32  may be also curved along the depth direction Y. In the outer package  34  illustrated in  FIG. 3B , plates on the obverse side and the reverse side are curved to protrude inward in both of the width direction X and the depth direction Y. A distance between the plates on the obverse side and the reverse side is minimized particularly at a central portion in the width direction X and the depth direction Y. The object  14  to be housed in the outer package  34  having such a shape is housed with a pressing force received from the obverse side and the reverse side, the pressing force changing along the width direction X and the depth direction Y. Thus, a desired pressing force can be applied to a desired point. For example, corners that are likely to be broken by an excessive force may not receive a pressing force. 
     In order to apply a pressing force changing along the width direction X and the depth direction Y to the housed object  14 , the pressing portions  12   a  in  FIG. 1  may be deformed in the depth direction Y in addition to curvatures on the overall plate in both directions.  FIGS. 4A, 4B, and 4C  illustrate enlarged perspective views of the pressing portions  12   a  and modifications thereof. For the sake of simplicity,  FIGS. 4A, 4B, and 4C  only illustrate the inner surface of the plate on the obverse side of the outer package  12 . The plate is entirely flattened and is not curved to protrude inward. 
     Pressing portions  42   a  of an outer package  42  in  FIG. 4A  are protrusions formed in the width direction X with a constant height along the depth direction Y. This shape is wavy in cross section in an X-Z plane and is rectangular in cross section in a Y-Z plane, which is relatively easily formed. Pressing portions  44   a  of an outer package  44  in  FIG. 4B  are protrusions formed in the width direction X with a height changing along the depth direction Y. This shape is wavy in cross section in the X-Z plane and in cross section in the Y-Z plane, which can be formed by processing for deforming the flat surface into a wavy form in the width direction X and in the depth direction Y. According to another modification, columnar protrusions may be formed as pressing portions  46   a  of an outer package  46  in  FIG. 4C  instead of the wavy form. 
     A method for manufacturing a secondary cell will be described below according to an embodiment of the present invention.  FIGS. 5 and 6  schematically illustrate the method for manufacturing a secondary cell  50  according to the present embodiment. In  FIG. 2 , the object  14  to be housed is inserted into the opening provided in the depth direction Y of the outer package  12 , whereas in  FIGS. 5 and 6 , an outer package  51  is divided into a reverse-side plate  52  (a lower plate in the height direction Z) and an obverse-side plate  56  (the other upper plate in the height direction Z), and the object  14  to be housed is held between the obverse-side plate  56  and the reverse-side plate  52 . 
     In  FIG. 5 , a plurality of pressing portions  52   a  are provided to locally press the reverse side (the lower surface of the housed object  14  in the height direction Z) of the object  14  to be housed, and the reverse side of the object  14  to be housed is disposed on the reverse-side plate  52  that is entirely curved to protrude upward. Furthermore, side plates  54  extending in the height direction Z are connected to the outer edges (the left and right ends in the width direction X) of the reverse-side plate  52 . In this configuration, the side plate  54  is connected such that an acute angle is formed at a corner  53  between the reverse-side plate  52  and the side plate  54 . Thus, the side plates  54  extend to tilt slightly inward in the width direction X with respect to the height direction Z. The side plates  54  are preferably connected to the reverse-side plate  52  such that the outer edges of the reverse-side plate  52  are bent or drawn to form the side plates  54 . In this case, as indicated at the corner  53 , the reverse-side plate  52  and the side plate  54  are formed by a combined material. 
     Moreover, pressing portions  56   a  are provided to locally press the obverse side (the upper surface of the housed object  14  in the height direction Z) of the housed object  14  from above in the height direction Z, and the obverse-side plate  56  entirely curved to protrude downward is pressed (pressurized) to the obverse side of the housed object  14  and the distal ends of the side plates  54 . In this configuration, the reverse-side plate  56  curved to protrude upward and the obverse-side plate  54  curved to protrude downward are elastic bodies that are pressed (pressurized) to be deformed along the shape of the housed object  14 . Hence, when the outer surface (undersurface) of the reverse-side plate  52  and the outer surface (top surface) of the obverse-side plate  56  are deformed flat as illustrated in  FIG. 6 , the side plates  54  are placed along the height direction Z. In this state, the distal ends of the side plates  54  in the height direction Z are connected to the outer edges of the obverse-side plate  56  by means such as welding. 
     The secondary cell  50  including the outer package  51  illustrated in  FIG. 6  is manufactured thus. Specifically, the obtained outer package  51  has a flat outer surface and an inner surface having the pressing portions  56   a  that press the obverse side of the housed object  14  and the pressing portions  52   a  that press the reverse side of the housed object  14 . In the outer package  51 , the housed object  14  is held between the obverse-side plate  56  having the pressing portions  56   a  and the reverse-side plate  52  having the pressing portions  52   a , and the obverse-side plate  56  and the reverse-side plate  52  are connected to each other via the side plates  54  extending between the obverse-side plate  56  and the reverse-side plate  52 . 
     In this manufacturing method, the side plates  54  are attached in advance (bent in advance) to the outer edges of the reverse-side plate  52  with an acute angle at the corner  53 , thereby placing the side plates  54  in the height direction Z when the curved reverse-side plate  52  and obverse-side plate  56  are elastically deformed to flatten the outer surface. As illustrated in  FIG. 6 , although the side plates  54  are placed in the height direction Z, the pressing portions  52   a  of the reverse-side plate  52  and the pressing portions  56   a  of the obverse-side plate  56 , particularly the pressing portions  52   a  and  56   a  near the side plates  54  are protruded inward, so that acute angles are formed at the corner  53  between the reverse-side plate  52  and the side plate  54  and at the corner  55  between the obverse-side plate  56  and the side plate  54  in the outer package  51 . 
     In this configuration, the acute angles at the corners  53  and  55  do not mean geometrically strict intersections of straight lines. Even if the intersection of the outer end of one of the reverse-side plate  52  and the obverse-side plate  56  and the side plate  54  is not the intersection of straight lines in cross section, any acute angle may be formed between the plane of the outer end of one of the reverse-side plate  52  and the obverse-side plate  56  and the extending direction of the side plate  54 . For example, if the side plate  54  is formed by bending the reverse-side plate  52 , the corner  53  may be formed into a round corner (including a round portion) instead of an intersection between straight lines depending upon the shape of a tool (a tool with a rounded tip). Also in this case, it is assumed that the corner  53  includes an acute-angled portion as long as an acute angle is formed between the extending direction of the side plate  54  and the extending surface (tangent plane) of the reverse-side plate  52  near the corner  53 . Also in this case, the housed object  14  held between the reverse-side plate  52  and the obverse-side plate  56  securely receives pressing forces from both of the obverse side and the reverse side. In this way, one of the corner  53  and the corner  55  of the outer package  51  may include an acute-angled portion as long as the outer end of one of the reverse-side plate  52  and the obverse-side plate  56  and the side plate  54  are connected to each other so as to securely apply a pressing force to the obverse side and the reverse side of the housed object  14 . 
     In  FIG. 5 , the corner  53  has an acute angle, and a clearance between the side plates  54  and the housed object  14  is illustrated to clarify the curves of the reverse-side plate  52  and the obverse-side plate  56 . However, the clearance between the side plates  54  and the housed object  14  can be reduced depending upon the angle of the corner  53  and the degree of curvatures of the reverse-side plate  52  and the obverse-side plate  56 . The presence of a clearance does not affect the performance of the secondary cell  50 . If the outer package  51  is particularly formed in a vacuum, the performance of the secondary cell  50  is not reduced by the influence of atmospheric components in a clearance or an atmospheric pressure. 
     In  FIGS. 5 and 6 , the two side plates  54  are connected to the outer edges of the reverse-side plate  52  (one plate), and the obverse-side plate  56  (the other plate) is connected to the side plates  54 . The reverse-side plate  52 , the side plates  54 , and the obverse-side plate  56  can be connected in varying combinations. The method for attaching the side plates  54  to the reverse-side plate  52  and the obverse-side plate  56  is not limited to bending. For example, the side plates  54  made of a material separate from the reverse-side plate  52  and the obverse-side plate  56  may be connected to the outer edges of the reverse-side plate  52  and the obverse-side plate  56  by means such as welding.  FIGS. 7A, 7B, and 7C  schematically illustrate modifications of the manufacturing method, in which the relation of connection and the connecting method are changed. In these modifications, the side plates are made of a material separate from the reverse-side plate and the obverse-side plate. In  FIGS. 7A, 7B, and 7C , the illustration of the pressing portions and the housed object is omitted, and only the relation of connection of the reverse-side plate, the side plates, and the obverse-side plate is illustrated. 
     In  FIG. 7A , side plates  78  are connected in advance to the outer edges of an obverse-side plate  76  by methods such as welding but are not connected to a reverse-side plate  72  (actually, the side plates  78  are preferably connected with acute-angled corners). The lower ends of the side plates  78  are then connected to the reverse-side plate  72 , forming an outer package  70 . 
     In  FIG. 7B , a reverse-side side plate  84  is connected in advance to one side (the right side in  FIG. 7B ) of the outer edge of a reverse-side plate  82  while an obverse-side side plate  88  is connected in advance to one side (the left side in  FIG. 7B ) of the outer edge of an obverse-side plate  86 . The upper end of the reverse-side side plate  84  is then connected to the obverse-side plate  86 , and the lower end of the obverse-side side plate  88  is connected to the reverse-side plate  82 , which forms an outer package  80 . 
     In  FIG. 7C , reverse-side side plates  94  and obverse-side side plates  98  are connected in advance to the outer edge of a reverse-side plate  92  and the outer edge of an obverse-side plate  96 , respectively. The upper ends of the reverse-side side plates  94  and the lower ends of the obverse-side side plates  98  are connected to each other, which forms an outer package  90 . 
     The secondary cell  10  illustrated in, for example,  FIG. 1  according to the foregoing embodiment is particularly assumed to be an all-solid-state cell that includes a solid electrolyte as the intermediate layer of an electrode body and is manufactured by a dry process. However, the secondary cell  10  may be a liquid-based cell containing liquid electrolyte in the intermediate layer. The housed object  14  may include a plurality of stacked electrode bodies, each having the laminated structure. 
     Moreover, the pressing portions  12   a  in, for example,  FIG. 1  are not always to be provided on both of the obverse side and the reverse side. At least one of the obverse side and the reverse side of the housed object  14  is to be locally pressed. In other words, the pressing portions  12   a  may be provided on only one of the obverse side and the reverse side. 
     The pressing portions  12   a  may have any shapes that can locally press the housed object  14 . For example,  FIG. 4C  illustrates the modification in which the columnar protrusions (pressing portions  46   a ) are cylindrical with dome-shaped tops. The pressing portions  12   a  may be, for example, prisms or dome-shaped portions may be directly attached to a flat plate. Alternatively, the pressing portions  12   a  may be carved from a flat plate or protrusions may be attached to a flat plate. Moreover, the pressing portions  12   a  are not always to be integrated with plates such as the reverse-side plate and the obverse-side plate. Members separate from the plate may be attached to the outer package  12  so as to form the pressing portions  12   a . For example, the outer package may include a sheet disposed inside the plate, and protrusions or a wavy pattern may be formed as the pressing portions  12   a  on the sheet. 
       FIG. 1  illustrates the three pressing portions  12   a  placed along the width direction X. The number of pressing portions  12   a  is not limited to three and thus may be larger than or smaller than three. A pressing portion  12   a  applies a pressure to the housed object  14  at the central portion in the width direction X (the pressing portion  12   a  protrudes inward to the obverse side or the reverse side). A pressure is not to be always applied to the central portion. The pressing portions  12   a  may be provided in various layouts and shapes according to points for pressing. Conversely, it is desirable to avoid pressing structurally fragile portions such as corners (these portions are concave inward). 
     For example, in  FIG. 6 , the outer package  51  of the secondary cell  50  is surrounded by the reverse-side plate  52 , the side plates  54 , and the obverse-side plate  56 . Additionally, lids may be attached to openings near and remote from a viewer of  FIG. 6  in the depth direction Y so as to completely seal the housed object  14 . In this case, the outer package  51  is sealed and evacuated, thereby keeping a vacuum therein and allowing the housed object  14  to be pressed by an atmospheric pressure. Thus, a contact resistance is expected to decrease in the electrode body. However, if the secondary cell  50  is covered with a case or the like in addition to the outer package  51 , sealing is not necessary. In any configuration, an electrode terminal serving as an electrical connecting portion of the electrode body is to be drawn out of the outer package  51 . The inner surface of the outer package  51  is desirably insulated. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10  Secondary cell 
               12  Outer package 
               12   a  Pressing portion 
               14  Object to be housed 
               16  Corner 
               18  Central portion 
               32  Outer package 
               34  Outer package 
               42  Outer package 
               42   a  Pressing portion 
               44  Outer package 
               44   a  Pressing portion 
               46  Outer package 
               46   a  Pressing portion 
               50  Secondary cell 
               51  Outer package 
               52  Reverse-side plate 
               53  Corner 
               54  Side plate 
               55  Corner 
               56  Obverse-side plate 
               70  Outer package 
               72  Reverse-side plate 
               76  Obverse-side plate 
               78  Side plate 
               80  Outer package 
               82  Reverse-side plate 
               84  Reverse-side side plate 
               86  Obverse-side plate 
               88  Obverse-side side plate 
               90  Outer package 
               92  Reverse-side plate 
               94  Reverse-side side plate 
               96  Obverse-side plate 
               98  Obverse-side side plate