Patent Publication Number: US-2016233478-A1

Title: Rectangular electricity storage device and method for producing the same

Description:
TECHNICAL FIELD 
     The present invention relates to a rectangular electricity storage device in which a plurality of electrode groups are housed in an outer package can and to a method for producing the same. 
     BACKGROUND ART 
       FIG. 25  is a longitudinal cross-sectional view that conceptually illustrates an example of an existing rectangular electricity storage device. As illustrated in  FIG. 25 , the existing rectangular electricity storage device includes a plurality of electrode groups  101 , a bottom-closed cylindrical outer package can  102  that houses these electrode groups  101 , a cover plate  103  that seals an opening  102   a  of the outer package can  102 , and a positive electrode external terminal  104  and a negative electrode external terminal (not illustrated) that are provided on the cover plate  103  (refer to, for example, PTL 1). Although not illustrated in the figure, in each of the electrode groups  101 , a plurality of positive electrode plates and a plurality of negative electrode plates are alternately stacked with separators therebetween. Each of the positive electrode plates has a positive electrode tab protruding from an edge facing the opening  102   a  of the outer package can  102 . Each of the negative electrode plates has a negative electrode tab protruding from an edge facing the opening  102   a  of the outer package can  102 . Each of the electrode groups  101  includes a positive electrode terminal portion  105  which is a bundle including a plurality of overlapping positive electrode tabs belonging to the electrode group  101 , and a negative electrode terminal portion (not illustrated) which is a bundle including a plurality of overlapping negative electrode tabs belonging to the electrode group  101 . 
     In the existing rectangular electricity storage device, an end of a positive electrode lead plate  106  is welded to each positive electrode terminal portion  105 . A plurality of positive electrode lead plates  106  provided on the corresponding positive electrode terminal portions  105  are bundled into one, and another end of the bundled positive electrode lead plates  106  are welded on the positive electrode external terminal  104  or fixed to the positive electrode external terminal  104  with a screw. The bundled positive electrode lead plates  106  are folded in a space in the outer package can  102 , the space being formed between the cover plate  103  and the electrode groups  101 . In this manner, each of the positive electrode terminal portions  105  is electrically connected to the positive electrode external terminal  104  with the corresponding positive electrode lead plate  106  therebetween. Similarly, each of the negative electrode terminal portions is electrically connected to the negative electrode external terminal with a corresponding negative electrode lead plate therebetween. 
     In a process for producing the existing rectangular electricity storage device, first, a plurality of electrode groups  101  to be housed in an outer package can  102  are prepared, and, in each of the electrode groups  101 , a positive electrode lead plate  106  and a negative electrode lead plate are welded on a positive electrode terminal portion  105  and a negative electrode terminal portion, respectively. Next, the electrode groups  101  are stacked so that the positive electrode lead plates  106  and the negative electrode lead plates attached to the electrode groups  101  are oriented in the same direction, and the electrode groups  101  are housed in the outer package can  102  so that the positive electrode lead plates  106  and the negative electrode lead plates are led out from an opening of the outer package can  102 . Consequently, the electrode groups  101  are fixed to a predetermined position in the outer package can  102 . Subsequently, the positive electrode lead plates  106  are bundled into one and welded to a positive electrode external terminal  104  in the outside of the outer package can  102 . Similarly, the negative electrode lead plates are bundled into one and welded to a negative electrode external terminal in the outside of the outer package can  102 . Subsequently, the bundled positive electrode lead plates  106  and the bundled negative electrode lead plates are folded and housed in the outer package can  102 . An opening  102   a  of the outer package can  102  is sealed with a cover plate  103 . 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2011-165475 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the existing rectangular electricity storage device, it is necessary that, after the electrode groups  101  are housed in the outer package can  102 , the positive electrode lead plates  106  be bundled into one and welded to the positive electrode external terminal  104  or fixed to the positive electrode external terminal  104  with a screw, and that the negative electrode lead plates be bundled into one and welded to the negative electrode external terminal or fixed to the negative electrode external terminal with a screw. The reason for this is as follows. Even if a plurality of electrode groups  101  can be stacked on one another without misalignment before the electrode groups  101  are housed in the outer package can  102 , misalignment of the electrode groups  101  may occur when the positive electrode lead plates  106  are bundled into one and welded to the positive electrode external terminal  104  or fixed to the positive electrode external terminal  104  with a screw, and the negative electrode lead plates are bundled into one and welded to the negative electrode external terminal or fixed to the negative electrode external terminal with a screw. When misalignment of the electrode groups  101  occurs, it is difficult to house the electrode groups  101  in the outer package can  102 . 
     In view of the circumstances described above, it is necessary to house the electrode groups  101  in the outer package can  102  in advance. Accordingly, as the positive electrode lead plates  106  and the negative electrode lead plates, it is necessary to use plates having such a length that when the electrode groups  101  are housed in the outer package can  102 , the plates can be led to the outside of the outer package can  102 . Therefore, a space for housing the positive electrode lead plates  106  and the negative electrode lead plates is provided in the outer package can  102 . The positive electrode lead plates  106  and the negative electrode lead plates are folded and housed in the space when the opening  102   a  of the outer package can  102  is sealed with the cover plate  103 . 
     In recent years, with an increase in the capacity of electricity storage devices, the current drawn from the electricity storage devices has been increasing. Therefore, the Joule heat generated due to the electrical resistance of lead plates increases, and the lead plates may cause a significant energy loss. In order to reduce the Joule heat generated in lead plates, the cross-sectional area of each of the lead plates may be increased to decrease the electrical resistance of the lead plate. An example of a simple method for increasing the cross-sectional area of a lead plate is to increase the thickness of the lead plate. 
     On the other hand, if the thickness of each lead plate is increased, it is necessary to increase the size of the space in which the lead plates are folded and housed. For example, the inner dimensions of the outer package can  102  may be increased, or a ratio of the space occupied by the electrode groups  101  in the outer package can  102  may be decreased. However, in these methods, an improvement in the volume energy density may be hindered. Furthermore, in the case where the thickness of a lead plate is increased, during the folding of the lead plate, breakage and damage tend to occur in a folded portion of the lead plate. In order to prevent the occurrence of such breakage and damage, the lead plate needs to have a thickness of less than 0.2 mm. 
     Accordingly, an object of the present invention is to provide a rectangular electricity storage device having a high volume energy density and a low energy loss between an external terminal and electrode groups. 
     Solution to Problem 
     An aspect of the present invention relates to a rectangular electricity storage device. The rectangular electricity storage device includes a first electrode group and a second electrode group, a bottom-closed cylindrical outer package can, a cover plate that seals an opening of the outer package can, an external terminal disposed on the cover plate, and a connection member. In each of the first electrode group and the second electrode group, a plurality of first electrode plates and a plurality of second electrode plates having a polarity opposite to the first electrode plates are stacked. In the outer package can, the first electrode group and the second electrode group are housed in a stacked state. The first electrode plates are each provided with an electrode tab that protrudes from an edge toward the opening of the outer package can, the edge facing the opening. The first electrode group is provided with a first terminal portion that extends from an end surface toward the opening of the outer package can, the end surface facing the opening, the electrode tabs provided on the first electrode plates belonging to the first electrode group overlap and form a bundle, and the bundle constitutes the first terminal portion. The second electrode group is provided with a second terminal portion that extends from an end surface toward the opening of the outer package can, the end surface facing the opening, the electrode tabs provided on the first electrode plates belonging to the second electrode group overlap and form a bundle, and the bundle constitutes the second terminal portion. The connection member mechanically and electrically couples the first electrode group and the second electrode group to each other. Specifically, the connection member includes a first connecting portion welded to the first terminal portion, a second connecting portion welded to the second terminal portion, and a coupling portion that mechanically and electrically couples the first connecting portion and the second connecting portion to each other. An inner surface of the cover plate is provided with a projecting portion that extends from the inner surface toward a bottom surface of the outer package can, and the projecting portion is electrically connected to the external terminal and is welded to at least any one of the first terminal portion, the second terminal portion, and the connection member. 
     Another aspect of the present invention relates to a method for producing a rectangular electricity storage device. The production method includes steps (i) to (v). In the step (i), a connection member is prepared. In the step (ii), a first terminal portion provided on a first electrode group is welded to a first connecting portion of the connection member. In the step (iii), a second electrode group is stacked on the first electrode group, and a second terminal portion provided on the second electrode group is welded to a second connecting portion of the connection member. After the steps (i) to (iii), in the step (iv), a projecting portion provided on an inner surface of a cover plate is welded to at least any one of the first terminal portion, the second terminal portion, and the connection member. After the step (iv), in the step (v), the first electrode group and the second electrode group are housed in an outer package can, and an opening of the outer package can is sealed with the cover plate. 
     Advantageous Effects of Invention 
     According to the aspects of the present invention, the volume energy density is increased, and an energy loss between an external terminal and electrode groups is decreased. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view that conceptually illustrates a rectangular electricity storage device according to an embodiment of the present invention. 
         FIG. 2  is an exploded perspective view of the rectangular electricity storage device. 
         FIG. 3  is a side view illustrating an inner structure of the rectangular electricity storage device, viewed from a first sidewall side of an outer package can included in the rectangular electricity storage device. 
         FIG. 4  is a side view illustrating an inner structure of the rectangular electricity storage device, viewed from a second sidewall side of an outer package can included in the rectangular electricity storage device. 
         FIG. 5A  is a longitudinal cross-sectional view that conceptually illustrates a structure of a positive electrode of each of a plurality of electrode groups included in the rectangular electricity storage device. 
         FIG. 5B  is a longitudinal cross-sectional view that conceptually illustrates a structure of a negative electrode of each of a plurality of electrode groups included in the rectangular electricity storage device. 
         FIG. 6A  is a perspective view illustrating a structure of a positive electrode terminal member included in the rectangular electricity storage device. 
         FIG. 6B  is a perspective view illustrating a structure of a negative electrode terminal member included in the rectangular electricity storage device. 
         FIG. 7A  is a perspective view illustrating a structure of a positive electrode connection member included in the rectangular electricity storage device. 
         FIG. 7B  is a perspective view illustrating a structure of a negative electrode connection member included in the rectangular electricity storage device. 
         FIG. 8  is a perspective view illustrating a state in which the positive electrode connection member and the negative electrode connection member are connected to electrode groups. 
         FIG. 9  is a perspective view used for illustrating a step (A) included in a first welding step. 
         FIG. 10  is a perspective view used for illustrating a step (B) included in the first welding step. 
         FIG. 11  is a perspective view used for illustrating a step (C) included in the first welding step. 
         FIG. 12  is a perspective view used for illustrating a step (D) included in the first welding step. 
         FIG. 13  is a perspective view used for illustrating a step (E) included in a second welding step. 
         FIG. 14  is a perspective view used for illustrating a step (F) included in the second welding step. 
         FIG. 15A  is a perspective view illustrating a structure of each of a positive electrode connection member and a negative electrode connection member included in a rectangular electricity storage device according to a first modification. 
         FIG. 15B  is a perspective view illustrating a state in which the positive electrode connection member and the negative electrode connection member included in the rectangular electricity storage device according to the first modification are connected to electrode groups. 
         FIG. 16  is a perspective view used for illustrating a step (A′) included in a first welding step of the first modification. 
         FIG. 17  is a perspective view used for illustrating a step (B′) included in the first welding step of the first modification. 
         FIG. 18  is a perspective view used for illustrating a step (C′) included in the first welding step of the first modification. 
         FIG. 19  is a perspective view used for illustrating a step (D′) included in the first welding step of the first modification. 
         FIG. 20A  is a perspective view illustrating a structure of each of a positive electrode connection member and a negative electrode connection member included in a rectangular electricity storage device according to a second modification. 
         FIG. 20B  is a perspective view illustrating a state in which the positive electrode connection member and the negative electrode connection member included in the rectangular electricity storage device according to the second modification are connected to electrode groups. 
         FIG. 21  is a perspective view used for illustrating a step (A′) included in a first welding step of the second modification. 
         FIG. 22  is a perspective view used for illustrating a step (B′) included in the first welding step of the second modification. 
         FIG. 23  is a perspective view used for illustrating a step (C′) included in the first welding step of the second modification. 
         FIG. 24  is a perspective view used for illustrating a step (D′) included in the first welding step of the second modification. 
         FIG. 25  is a longitudinal cross-sectional view that conceptually illustrates an example of an existing rectangular electricity storage device. 
     
    
    
     REFERENCE SIGNS LIST 
     
         
           1 A,  1 B,  1 C,  1 D electrode group 
           11 A,  11 B,  11 C,  11 D positive electrode terminal portion 
           111 A,  111 B,  111 C,  111 D first surface 
           112 A,  112 B,  112 C,  112 D second surface 
           12 A,  12 B,  12 C,  12 D negative electrode terminal portion 
           121 A,  121 B,  121 C,  121 D first surface 
           122 A,  122 B,  122 C,  122 D second surface 
           13  end surface 
           14  positive electrode plate 
           141  edge 
           142  positive electrode tab 
           15  negative electrode plate 
           151  edge 
           152  negative electrode tab 
           16  separator 
           17 A,  17 B,  17 C,  17 D first surface 
           18 A,  18 B,  18 C,  18 D second surface 
           2  outer package can 
           21  opening 
           22  bottom surface 
           23  first sidewall 
           24  second sidewall 
           3  cover plate 
           31  inner surface 
           32 ,  33  nut 
           4  positive electrode terminal member 
           41  positive electrode base portion 
           411  main surface 
           412  edge 
           42  positive electrode external terminal 
           43  positive electrode projecting portion 
           5  negative electrode terminal member 
           51  negative electrode base portion 
           52  negative electrode external terminal 
           53  negative electrode projecting portion 
           6  positive electrode connection member 
           61 A,  61 B,  61 C,  61 D positive electrode connecting portion 
           611 A,  611 B,  611 C,  611 D first edge 
           612 A,  612 B,  612 C,  612 D second edge 
           613 A,  613 B,  613 C facing part 
           614 A,  614 B,  614 C non-facing part 
           615 A,  615 B,  615 C,  615 D first side edge 
           616 A,  616 B,  616 C,  616 D second side edge 
           62   a ,  62   b ,  62   c  positive electrode coupling portion 
           62   d ,  62   e ,  62   f  positive electrode coupling portion 
           7  negative electrode connection member 
           71 A,  71 B,  71 C,  71 D negative electrode connecting portion 
           713 B,  713 C,  713 D facing part 
           714 B,  714 C,  714 D non-facing par 
           72   a ,  72   b ,  72   c  negative electrode coupling portion 
           72   d ,  72   e ,  72   f  negative electrode coupling portion 
           8  electric insulation sheet 
           81 ,  82  window 
           9 ,  9 A,  9 B ultrasonic welder 
           91 ,  91 A horn 
           92 ,  92 A anvil 
           911  first welding end 
           912  second welding end 
         L distance 
         L 1   a , L 1   b , L 1   c  distance 
         L 2   a , L 2   b , L 2   c  distance 
         RA 1 , RB 1 , RC 1 , RD 1 , RP 1  predetermined region 
         RA 2 , RB 2 , RC 2 , RD 2 , RP 2  predetermined region 
           101  electrode group 
           102  outer package can 
           102   a  opening 
           103  cover plate 
           104  positive electrode external terminal 
           105  positive electrode terminal portion 
           106  positive electrode lead plate 
       
    
     DESCRIPTION OF EMBODIMENTS 
     A rectangular electricity storage device according to an embodiment of the present invention includes a first electrode group and a second electrode group, a bottom-closed cylindrical outer package can, a cover plate that seals an opening of the outer package can, an external terminal disposed on the cover plate, and a connection member. In each of the first electrode group and the second electrode group, a plurality of first electrode plates and a plurality of second electrode plates having a polarity opposite to the first electrode plates are stacked. The first electrode group and the second electrode group are housed in the outer package can in a stacked state. The first electrode plates are each provided with an electrode tab that protrudes from an edge toward the opening of the outer package can, the edge facing the opening. The first electrode group is provided with a first terminal portion that extends from an end surface toward the opening of the outer package can, the end surface facing the opening, the electrode tabs provided on the first electrode plates belonging to the first electrode group overlap and form a bundle, and the bundle constitutes the first terminal portion. The second electrode group is provided with a second terminal portion that extends from an end surface toward the opening, the end surface facing the opening of the outer package can, the electrode tabs provided on the first electrode plates belonging to the second electrode group overlap and form a bundle, and the bundle constitutes the second terminal portion. The connection member mechanically and electrically couples the first electrode group and the second electrode group to each other. Specifically, the connection member includes a first connecting portion welded to the first terminal portion, a second connecting portion welded to the second terminal portion, and a coupling portion that mechanically and electrically couples the first connecting portion and the second connecting portion to each other. An inner surface of the cover plate is provided with a projecting portion that extends from the inner surface toward a bottom surface of the outer package can, and the projecting portion is electrically connected to the external terminal and is welded to at least any one of the first terminal portion, the second terminal portion, and the connection member. 
     The term “rectangular electricity storage device” covers devices having a rounded corner or a rounded edge. The first electrode plates are positive electrode plates, and the second electrode plates are negative electrode plates. Alternatively, the first electrode plates are negative electrode plates, and the second electrode plates are positive electrode plates. The electricity storage device is not particularly limited as long as the electricity storage device is a device that can perform charging and discharging. Typical examples of the electricity storage device include batteries and capacitors (condensers). Examples of the batteries include a lead storage battery, a lithium-ion battery, and a molten-salt battery. Examples of the capacitors include an electric double-layer capacitor and a lithium-ion capacitor. 
     According to the rectangular electricity storage device described above, in a process for producing the rectangular electricity storage device, before the first electrode group and the second electrode group are housed in the outer package can, the first electrode group and the second electrode group can be integrated by being fixed to the connection member in a state in which these electrode groups are stacked. Misalignment is unlikely to occur in the first electrode group and the second electrode group which are fixed to the connection member. Accordingly, even before the first electrode group and the second electrode group are housed in the outer package can, the projecting portion provided on the cover plate can be welded to at least any one of the first terminal portion, the second terminal portion, and the connection member without causing misalignment of the first electrode group and the second electrode group. Thus, the cover plate is fixed to the first electrode group and the second electrode group, and the first electrode group and the second electrode group are electrically connected to the external terminal through the connection member. Even after welding of the cover plate, misalignment does not substantially occur in the first electrode group and the second electrode group, and thus the first electrode group and the second electrode group can be housed in the outer package can  2 . 
     Therefore, the rectangular electricity storage device does not require the space for housing lead plates that are folded, the space being necessary for the existing rectangular electricity storage device. Consequently, the ratio of the total volume of the electrode groups to the volume of the rectangular electricity storage device increases, resulting in an improvement in the volume energy density. From the viewpoint of improving the volume energy, in a direction from the bottom surface of the outer package can to the opening of the outer package can, a ratio of a height of the first terminal portion from the end surface, the first terminal portion being provided on the first electrode group, to a distance from the end surface of the first electrode group to the inner surface of the cover plate, the end surface facing the opening of the outer package can, is preferably 0.9 or less. 
     In the existing rectangular electricity storage device, the thickness of each lead plate must be small in order to prevent breakage and damage from occurring in a folded portion of the lead plate. In contrast, in the rectangular electricity storage device described above, since such breakage and damage do not occur in the connection member, the connection member can have a large thickness. Even when the connection member has a large thickness, the volume energy density does not significantly decrease. 
     Therefore, according to the rectangular electricity storage device, the electrical resistance of the connection member is low, and an energy loss between the external terminal and the electrode groups is decreased. From the viewpoint of decreasing the energy loss, the connection member is preferably formed of at least one metal selected from the group consisting of aluminum, copper, and nickel. The thickness of the connection member is preferably 0.1 mm or more and 2.0 mm or less, and more preferably 0.5 mm or more and 1.5 mm or less. 
     In a specific preferred structure of the rectangular electricity storage device, the first terminal portion and the second terminal portion each have a first surface oriented in a first direction that is the same as a direction in which the first electrode group and the second electrode group are stacked and a second surface oriented in a direction opposite to the first direction, and the first connecting portion and the second connecting portion are welded to the first surface of the first terminal portion and the second surface of the second terminal portion, respectively. The first connecting portion and the second connecting portion each have a first edge facing the opening of the outer package can and a second edge on the side opposite to the opening, and the first edges or the second edges are mechanically and electrically coupled to each other by the coupling portion. 
     The connection member is formed by, for example, bending a single metal flat plate, and thus has high mechanical strength. Therefore, the occurrence of misalignment is prevented in the first electrode group and the second electrode group that are fixed to the connection member. 
     More specifically, the rectangular electricity storage device has the following structures (1) and (2). In the structure (1), the first surface of the first terminal portion and the second surface of the second terminal portion are surfaces that face each other, and the second edges are mechanically and electrically coupled to each other by the coupling portion. In the structure (2), the first surface of the first terminal portion and the second surface of the second terminal portion are respectively a back surface of the second surface of the first terminal portion and a back surface of the first surface of the second terminal portion, the second surface of the first terminal portion and the first surface of the second terminal portion facing each other, and the first edges are mechanically and electrically coupled to each other by the coupling portion. 
     By combining these structures (1) and (2), a connection member that has a recess and a protrusion and that can be formed from a single metal flat plate is obtained. This connection member is used in a rectangular electricity storage device in which three or more electrode groups are housed in an outer package can. In this case, the volume energy density can be improved in the rectangular electricity storage device. 
     In a specific more preferred structure of the structure (2), the first edge of the first connecting portion and the first edge of the second connecting portion each have a coupled region to which the coupling portion is coupled and an exposed region to which the coupling portion is not coupled, and the first connecting portion and the second connecting portion are respectively welded to the first terminal portion and the second terminal portion in a part close to the exposed region. 
     For example, in the case where ultrasonic welding or resistance welding is used as welding means, it is necessary to sandwich a welding position with a first welding tool and a second welding tool. In the process for producing the rectangular electricity storage device, when the welding position is close to the coupled region, it is difficult to sandwich the welding position with the first welding tool and the second welding tool. In contrast, when the welding position is close to the exposed region, it is easy to sandwich the welding position with the first welding tool and the second welding tool. Consequently, welding of the first and second terminal portions and the connection member is easily performed in the production process. 
     In another specific preferred structure of the rectangular electricity storage device, the first connecting portion includes a facing part that faces the second connecting portion, and a non-facing part that does not face the second connecting portion, and the first connecting portion is welded to the first terminal portion or the projecting portion in the non-facing part. 
     For example, in the case where ultrasonic welding or resistance welding is used as welding means, it is necessary to sandwich a welding position with a first welding tool and a second welding tool. In the process for producing the rectangular electricity storage device, when the welding position is the facing part of the first connecting portion, a first welding tool and a second welding tool that have special shapes are necessary in order to sandwich the first connecting portion and the first terminal portion at the welding position. In contrast, when the welding position is the non-facing part of the first connecting portion, the welding position can be sandwiched with a first welding tool and a second welding tool that have been hitherto used. 
     In another specific preferred structure of the rectangular electricity storage device, the first connecting portion and the second connecting portion each have a side edge, and the side edges are mechanically and electrically connected to each other by the coupling portion. 
     For example, in the case where ultrasonic welding or resistance welding is used as welding means, it is necessary to sandwich a welding position with a first welding tool and a second welding tool. According to the rectangular electricity storage device, the welding position is easily sandwiched with the first welding tool and the second welding tool. Consequently, welding of the first and second terminal portions and the connection member is easily performed in the production process. 
     A production method according to an embodiment of the present invention is a method for producing the rectangular electricity storage device described above and includes steps (i) to (v). In the step (i), the connection member is prepared. In the step (ii), the first terminal portion provided on the first electrode group is welded to the first connecting portion of the connection member. In the step (iii), the second electrode group is stacked on the first electrode group, and the second terminal portion provided on the second electrode group is welded to the second connecting portion of the connection member. After the steps (i) to (iii), in the step (iv), the projecting portion provided on the inner surface of the cover plate is welded to at least any one of the first terminal portion, the second terminal portion, and the connection member. After the step (iv), in the step (v), the first electrode group and the second electrode group are housed in the outer package can, and the opening of the outer package can is sealed with the cover plate. 
     According to the above production method, before the first electrode group and the second electrode group are housed in the outer package can, in the steps (ii) and (iii), the first electrode group and the second electrode group are integrated by being fixed to the connection member in a state in which these electrode groups are stacked. Misalignment is unlikely to occur in the first electrode group and the second electrode group which are fixed to the connection member. Accordingly, even before the first electrode group and the second electrode group are housed in the outer package can, in the step (iv), the projecting portion provided on the cover plate can be welded to at least any one of the first terminal portion, the second terminal portion, and the connection member without causing misalignment of the first electrode group and the second electrode group. Thus, the cover plate is fixed to the first electrode group and the second electrode group, and the first electrode group and the second electrode group are electrically connected to the external terminal through the connection member. Even after the step (iv), misalignment does not substantially occur in the first electrode group and the second electrode group, and thus the first electrode group and the second electrode group can be housed in the outer package can  2 . 
     Therefore, according to the above production method, the space for housing lead plates that are folded, the space being necessary for the existing rectangular electricity storage device, is unnecessary. Consequently, in a rectangular electricity storage device to be produced, the ratio of the total volume of the electrode groups to the volume of the rectangular electricity storage device increases, resulting in an improvement in the volume energy density. 
     In the existing rectangular electricity storage device, the thickness of each lead plate must be small in order to prevent breakage and damage from occurring in a folded portion of the lead plate. In contrast, in the above production method, since such breakage and damage do not occur in the connection member, the connection member can have a large thickness. Even when the connection member has a large thickness, the volume energy density does not significantly decrease. Therefore, according to the above production method, the electrical resistance of the connection member is low, and an energy loss between the external terminal and the electrode groups is decreased in a rectangular electricity storage device to be produced. 
     From the viewpoint of increasing the strength of the connection member, in the step (i), the first connecting portion, the second connecting portion, and the coupling portion are preferably formed by bending a single metal flat plate. From the viewpoint of decreasing the energy loss, the metal flat plate is preferably formed of at least one metal selected from the group consisting of aluminum, copper, and nickel. Furthermore, the metal flat plate preferably has a thickness of 0.1 mm or more and 2.0 mm or less, and more preferably 0.5 mm or more and 1.5 mm or less. 
     Next, details of a rectangular electricity storage device according to an embodiment and a method for producing the rectangular electricity storage device will be specifically described with reference to the drawings. 
     [1] Structure of Rectangular Electricity Storage Device 
       FIG. 1  is a perspective view that conceptually illustrates a rectangular electricity storage device of the present embodiment.  FIG. 2  is an exploded perspective view of the rectangular electricity storage device. The rectangular electricity storage device of the present embodiment includes four electrode groups  1 A to  1 D, a bottom-closed cylindrical outer package can  2 , a cover plate  3 , a positive electrode terminal member  4 , a negative electrode terminal member  5 , a positive electrode connection member  6 , a negative electrode connection member  7 , and an electric insulation sheet  8 . Hereinafter, in the position in which an opening  21  of the outer package can  2  is directed upward (refer to  FIG. 2 ), a width direction of the outer package can  2  is defined as an X-direction, a thickness direction of the outer package can  2  is defined as a Y-direction, and a height direction of the outer package can  2  is defined as a Z-direction. The Z-direction coincides with a direction from a bottom surface  22  of the outer package can  2  to the opening  21 . 
     [1-1] Electrode Group 
       FIG. 3  is a side view illustrating an inner structure (specifically, a connection structure of a positive electrode) of the rectangular electricity storage device, viewed from a first sidewall  23  (refer to  FIG. 2 ) side of the outer package can  2 .  FIG. 4  is a side view illustrating an inner structure (specifically, a connection structure of a negative electrode) of the rectangular electricity storage device, viewed from a second sidewall  24  (refer to  FIG. 2 ) side of the outer package can  2 . As illustrated in  FIGS. 2 to 4 , the electrode groups  1 A to  1 D are housed in the outer package can  2  in a state in which the electrode groups  1 A to  1 D are stacked in the Y-direction. An electrolyte is housed in the outer package can  2  together with the electrode groups  1 A to  1 D. The electrode groups  1 A to  1 D each have an end surface  13  facing the opening  21  of the outer package can  2  in the assembled state of the rectangular electricity storage device. The electrode groups  1 A to  1 D are respectively provided with positive electrode terminal portions  11 A to  11 D and negative electrode terminal portions  12 A to  12 D that extend from the end surfaces  13  thereof to the opening  21  (in the Z-direction). In this embodiment, the electrode groups  1 A to  1 D have the same structure, the same shape, and the same dimensions. The end surfaces  13  of the electrode groups  1 A to  1 D are flush with each other. Furthermore, a height T 1  (refer to  FIGS. 2 and 3 ) of each of the positive electrode terminal portions  11 A to  11 D from the end surfaces  13 , which are flush with each other, with respect to the Z-direction, and a height T 2  (refer to  FIGS. 2 and 4 ) of each of the negative electrode terminal portions  12 A to  12 D from the end surfaces  13  with respect to the Z-direction are all the same. 
       FIG. 5A  is a longitudinal cross-sectional view that conceptually illustrates a structure of a positive electrode of each of the electrode groups  1 A to  1 D.  FIG. 5B  is a longitudinal cross-sectional view that conceptually illustrates a structure of a negative electrode of each of the electrode groups  1 A to  1 D. As illustrated in  FIGS. 5A and 5B , in each of the electrode groups  1 A to  1 D, a plurality of positive electrode plates  14  and a plurality of negative electrode plates  15  are alternately stacked with separators  16  therebetween. In this embodiment, the number of the negative electrode plates  15  is larger than the number of the positive electrode plates  14  by  1 , and two outer layers are each constituted by the negative electrode plate  15 . In an example, the number of the positive electrode plates  14  is 30, and the number of the negative electrode plates  15  is 31. The number of the positive electrode plates  14  may be the same as the number of the negative electrode plates  15 , one outer layer may be constituted by the positive electrode plate  14 , and another outer layer may be constituted by the negative electrode plate  15 . Alternatively, the number of the positive electrode plates  14  may be larger than the number of the negative electrode plates  15  by  1 , and two outer layers may each be constituted by the positive electrode plate  14 . 
     As illustrated in  FIG. 5A , the positive electrode plates  14  are each provided with a positive electrode tab  142  that protrudes from an edge  141  to the opening  21  (in the Z-direction), the edge  141  facing the opening  21  (refer to  FIG. 3 ) of the outer package can  2  in the assembled state of the rectangular electricity storage device. The positive electrode tabs  142  that are provided on the positive electrode plates  14  belonging to each of the electrode groups  1 A to  1 D project from the same position on the edges  141 , overlap, and form a bundle. The bundle formed in this manner constitutes each of positive electrode terminal portions  11 A to  11 D. Accordingly, as illustrated in  FIG. 2 , the positive electrode terminal portions  11 A to  11 D respectively have first surfaces  111 A to  111 D directed in the Y-direction and second surfaces  112 A to  112 D directed in a direction opposite to the Y-direction. 
     In this embodiment, in all the positive electrode plates  14  included in the electrode groups  1 A to  1 D, the positive electrode tabs  142  are provided at the same position on the edges  141 . Accordingly, two positive electrode terminal portions selected from the four positive electrode terminal portions  11 A to  11 D face each other in any combination thereof. Alternatively, in each of the electrode groups  1 A to  1 D, the positive electrode tabs  142  may be provided at different positions on the edges  141 . 
     As illustrated in  FIG. 5B , the negative electrode plates  15  are each provided with a negative electrode tab  152  that protrudes from an edge  151  to the opening  21  (in the Z-direction), the edge  151  facing the opening  21  (refer to  FIG. 3 ) of the outer package can  2  in the assembled state of the rectangular electricity storage device. The negative electrode tabs  152  that are provided on the negative electrode plates  15  belonging to each of the electrode groups  1 A to  1 D project from the same position on the edges  151 , overlap, and form a bundle. The bundle formed in this manner constitutes each of negative electrode terminal portions  12 A to  12 D. Accordingly, as illustrated in  FIG. 2 , the negative electrode terminal portions  12 A to  12 D respectively have first surfaces  121 A to  121 D directed in the Y-direction and second surfaces  122 A to  122 D directed in a direction opposite to the Y-direction. 
     In this embodiment, in all the negative electrode plates  15  included in the electrode groups  1 A to  1 D, the negative electrode tabs  152  are provided at the same position on the edges  151 . Accordingly, two negative electrode terminal portions selected from the four negative electrode terminal portions  12 A to  12 D face each other in any combination thereof. Alternatively, in each of the electrode groups  1 A to  1 D, the negative electrode tabs  152  may be provided at different positions on the edges  151 . 
     [1-2] Electric Insulation Sheet 
     The electric insulation sheet  8  is a sheet that prevents a positive electrode and a negative electrode from being electrically short-circuited in the outer package can  2 . Specifically, the electric insulation sheet  8  has an outer peripheral shape that is the same as or slightly smaller than the shape of the opening  21  of the outer package can  2 . As illustrated in  FIG. 2 , two windows  81  and  82  are formed in the electric insulation sheet  8 . All the positive electrode terminal portions  11 A to  11 D are allowed to pass through the window  81 , and all the negative electrode terminal portions  12 A to  12 D are allowed to pass through the window  82 . In this state, the electric insulation sheet  8  covers the end surfaces  13  of the electrode groups  1 A to  1 D. 
     [1-3] Cover Plate 
     As illustrated in  FIG. 3 , the opening  21  of the outer package can  2  is sealed by the cover plate  3 . Specifically, the cover plate  3  has an outer peripheral shape slightly larger than the shape of the opening  21  of the outer package can  2  and is fixed to an opening end surface of the outer package can  2  by welding means such as laser welding. The opening  21  of the outer package can  2  is preferably hermetically sealed by the cover plate  3  in this manner. This structure prevents a liquid from leaking from the rectangular electricity storage device and prevents foreign materials from entering the rectangular electricity storage device. 
     [1-4] Positive Electrode Terminal Member and Negative Electrode Terminal Member 
       FIGS. 6A and 6B  are perspective views that illustrate structures of a positive electrode terminal member  4  and a negative electrode terminal member  5 , viewed from directions opposite to each other. As illustrated in  FIGS. 6A and 6B , the positive electrode terminal member  4  includes a positive electrode base portion  41 , a positive electrode external terminal  42 , and a positive electrode projecting portion  43 . The positive electrode base portion  41  is a rectangular flat plate. The positive electrode external terminal  42  is a bolt with a thread groove (not illustrated) and is provided to protrude from a main surface  411  of the positive electrode base portion  41 . The positive electrode projecting portion  43  is formed on an edge  412  of the positive electrode base portion  41  and electrically connected to the positive electrode external terminal  42  through the positive electrode base portion  41 . In this embodiment, the positive electrode projecting portion  43  is provided to be perpendicular to the main surface  411  of the positive electrode base portion  41  and evenly extends along the edge  412  of the positive electrode base portion  41 . The positive electrode projecting portion  43  may be slightly inclined with respect to the perpendicular of the main surface  411  of the positive electrode base portion  41 . The shape of the positive electrode projecting portion  43  is not limited to a flat shape and may be another shape such as a rod-like shape. 
     The negative electrode terminal member  5  includes a negative electrode base portion  51 , a negative electrode external terminal  52 , and a negative electrode projecting portion  53 . In this embodiment, the negative electrode terminal member  5  has the same shape and dimensions as the positive electrode terminal member  4 . The negative electrode terminal member  5  may have a shape and dimensions that are different from those of the positive electrode terminal member  4 . 
     As illustrated in  FIG. 2 , the positive electrode terminal member  4  and the negative electrode terminal member  5  are provided on the cover plate  3 . Specifically, the positive electrode terminal member  4  is fixed to the cover plate  3  as illustrated in  FIG. 3 . That is, the positive electrode external terminal  42  passes through the cover plate  3  from an inner surface  31  side of the cover plate  3 , and a nut  32  is fitted to the positive electrode external terminal  42  in this state. The nut  32  is tightened toward a base of the positive electrode external terminal  42 . With this structure, the positive electrode base portion  41  is fixed to the cover plate  3  while being pressed against the inner surface  31  of the cover plate  3 . As a result, the positive electrode terminal member  4  is fixed to the cover plate  3 . In this fixed state, the positive electrode projecting portion  43  extends from the inner surface  31  of the cover plate  3  toward the bottom surface  22  of the outer package can  2  (in a direction opposite to the Z-direction). In this embodiment, the positive electrode projecting portion  43  is directed in a direction opposite to the Y-direction with respect to the positive electrode external terminal  42 . 
     As illustrated in  FIG. 4 , the negative electrode terminal member  5  is fixed to the cover plate  3  similarly to the positive electrode terminal member  4 . That is, the negative electrode external terminal  52  passes through the cover plate  3  from the inner surface  31  side of the cover plate  3 , and a nut  33  is fitted to the negative electrode external terminal  52  in this state. However, in this embodiment, the negative electrode projecting portion  53  is directed, with respect to the negative electrode external terminal  52 , in a direction (Y-direction) opposite to the direction in which the positive electrode projecting portion  43  is directed. 
     Herein, the positive electrode base portion  41  and the positive electrode projecting portion  43  have such shapes and dimensions that the positive electrode projecting portion  43  can face a non-facing part  614 A of a positive electrode connecting portion  61 A included in a positive electrode connection member  6  described below (refer to  FIG. 3 ) in the assembled state of the rectangular electricity storage device. The negative electrode base portion  51  and the negative electrode projecting portion  53  have such shapes and dimensions that the negative electrode projecting portion  53  can face a non-facing part  714 D of a negative electrode connecting portion  71 D included in a negative electrode connection member  7  described below (refer to  FIG. 4 ) in the assembled state of the rectangular electricity storage device. 
     [1-5] Positive Electrode Connection Member and Negative Electrode Connection Member 
       FIGS. 7A and 7B  are perspective views that illustrate structures of a positive electrode connection member  6  and a negative electrode connection member  7 , viewed from directions opposite to each other.  FIG. 8  is a perspective view illustrating a state in which the positive electrode connection member  6  and the negative electrode connection member  7  are connected to the electrode groups  1 A to  1 D. As illustrated in  FIG. 8 , the positive electrode connection member  6  and the negative electrode connection member  7  are each a member that mechanically and electrically couples the four electrode groups  1 A to  1 D to each other, the electrode groups  1 A to  1 D being housed in the outer package can  2 . Each of the positive electrode connection member  6  and the negative electrode connection member  7  is preferably formed of at least one metal selected from the group consisting of aluminum, copper, and nickel. 
     &lt;Positive Electrode Connection Member&gt; 
     As illustrated in  FIGS. 7A and 7B , the positive electrode connection member  6  includes four positive electrode connecting portions  61 A to  61 D and three positive electrode coupling portions  62   a  to  62   c . The positive electrode connection member  6  is formed by bending a single metal flat plate having a particular shape. Accordingly, the positive electrode connecting portions  61 A to  61 D and the positive electrode coupling portions  62   a  to  62   c  each have a flat shape. Specifically, the positive electrode connecting portions  61 A to  61 D have a flat shape parallel to the X-Z plane and sequentially arranged in the Y-direction. The positive electrode coupling portions  62   a  to  62   c  have a flat shape parallel to the X-Y plane. The metal flat plate used for forming the positive electrode connection member  6  preferably has a thickness of 0.5 mm or more and 1.5 mm or less. 
     The positive electrode connecting portion  61 A includes a facing part  613 A that faces a part of the positive electrode connecting portion  61 B, and a non-facing part  614 A that extends from the facing part  613 A to a lateral side (in a direction opposite to the X-direction) and that does not face the positive electrode connecting portion  61 B. The positive electrode connecting portion  61 B includes a facing part  613 B that faces the positive electrode connecting portion  61 A and a non-facing part  614 B that does not face the positive electrode connecting portion  61 A. The whole of the positive electrode connecting portion  61 C faces the positive electrode connecting portion  61 B. The positive electrode connecting portion  61 D faces a part of the positive electrode connecting portion  61 C. The positive electrode connecting portion  61 C includes a facing part  613 C that faces the positive electrode connecting portion  61 D and a non-facing part  614 C that does not face the positive electrode connecting portion  61 D. In this embodiment, the widths of the positive electrode connecting portions  61 B and  61 C with respect to the X-direction are substantially the same as the widths of the positive electrode terminal portions  11 B and  11 C with respect to the X-direction, respectively. The widths of the positive electrode connecting portions  61 A to  61 D with respect to the Z-direction are respectively smaller than the height T 1  of the positive electrode terminal portions  11 A to  11 D from the end surfaces  13  of the electrode groups  1 A to  1 D (refer to  FIG. 3 ). 
     The positive electrode connecting portions  61 A to  61 D respectively have first edges  611 A to  611 D that face the opening  21  (refer to  FIG. 3 ) of the outer package can  2 , and second edges  612 A to  612 D disposed on the opposite side of the opening  21  in the assembled state of the rectangular electricity storage device. The second edge  612 A of the positive electrode connecting portion  61 A is mechanically and electrically coupled to the second edge  612 B of the positive electrode connecting portion  61 B with the positive electrode coupling portion  62   a  therebetween. The second edge  612 C of the positive electrode connecting portion  61 C is mechanically and electrically coupled to the second edge  612 D of the positive electrode connecting portion  61 D with the positive electrode coupling portion  62   c  therebetween. 
     Furthermore, the first edge  611 B of the positive electrode connecting portion  61 B is mechanically and electrically coupled to the first edge  611 C of the positive electrode connecting portion  61 C with the positive electrode coupling portion  62   b  therebetween. Specifically, the facing part  613 B of the positive electrode connecting portion  61 B is mechanically and electrically coupled to the facing part  613 C of the positive electrode connecting portion  61 C with the positive electrode coupling portion  62   b  therebetween. Accordingly, each of the first edges  611 B and  611 C has a coupled region to which the positive electrode coupling portion  62   b  is directly coupled and an exposed region to which the positive electrode coupling portion  62   b  is not coupled. 
     As illustrated in  FIG. 8  (also refer to  FIG. 2 ), the positive electrode connection member  6  is arranged in the following positional relationship with respect to the positive electrode terminal portions  11 A to  11 D. Specifically, the whole of the positive electrode connecting portion  61 B faces the second surface  112 B of the positive electrode terminal portion  11 B. The whole of the positive electrode connecting portion  61 C faces the first surface  111 C of the positive electrode terminal portion  11 C. The facing part  613 A of the positive electrode connecting portion  61 A faces a part of the first surface  111 A of the positive electrode terminal portion  11 A. The whole of the positive electrode connecting portion  61 D faces a part of the second surface  112 D of the positive electrode terminal portion  11 D. Herein, as illustrated in  FIG. 3 , the positive electrode coupling portions  62   a  to  62   c  each have such dimensions that the positive electrode connection member  6  can be arranged with respect to the positive electrode terminal portions  11 A to  11 D without deforming the positive electrode terminal portions  11 A to  11 D or with a small amount of deformation of the positive electrode terminal portions  11 A to  11 D. 
     In this arrangement relationship, the positive electrode connection member  6  is welded to the positive electrode terminal portions  11 A to  11 D as follows (refer to  FIGS. 2 and 8 ). Specifically, the positive electrode connecting portion  61 A is welded to the first surface  111 A of the positive electrode terminal portion  11 A in the facing part  613 A thereof. The positive electrode connecting portion  61 B is welded to the second surface  112 B of the positive electrode terminal portion  11 B in the non-facing part  614 B thereof (in this embodiment, the non-facing part  614 B is a part close to the exposed region of the first edge  611 B). In other words, the first surface  111 A of the positive electrode terminal portion  11 A and the second surface  112 B of the positive electrode terminal portion  11 B are surfaces that face each other, and the positive electrode connecting portions  61 A and  61 B are respectively welded to the first surface  111 A and the second surface  112 B. 
     The positive electrode connecting portion  61 C is welded to the first surface  111 C of the positive electrode terminal portion  11 C in the non-facing part  614 C thereof (in this embodiment, the non-facing part  614 C is a part close to the exposed region of the first edge  611 C). From the viewpoint of the relationship with the positive electrode connecting portion  61 B, this structure is understood as follows. Specifically, the second surface  112 B of the positive electrode terminal portion  11 B and the first surface  111 C of the positive electrode terminal portion  11 C are respectively back surfaces of the first surface  111 B of the positive electrode terminal portion  11 B and the second surface  112 C of the positive electrode terminal portion  11 C, the first surface  111 B and the second surface  112 C facing each other. The positive electrode connecting portions  61 B and  61 C are respectively welded to the second surface  112 B and the first surface  111 C. 
     The positive electrode connecting portion  61 D is welded to the second surface  112 D of the positive electrode terminal portion  11 D. From the viewpoint of the relationship with the positive electrode connecting portion  61 C, this structure is understood as follows. Specifically, the first surface  111 C of the positive electrode terminal portion  11 C and the second surface  112 D of the positive electrode terminal portion  11 D are surfaces that face each other, and the positive electrode connecting portions  61 C and  61 D are respectively welded to the first surface  111 C and the second surface  112 D. 
     Furthermore, as illustrated in  FIG. 3 , the positive electrode connecting portion  61 A is welded to the positive electrode projecting portion  43  in the non-facing part  614 A thereof. In this manner, the positive electrode plates  14  included in each of the electrode groups  1 A to  1 D are electrically connected to the positive electrode external terminal  42  through the positive electrode connection member  6 . 
     &lt;Negative Electrode Connection Member&gt; 
     As illustrated in  FIGS. 7A and 7B , the negative electrode connection member  7  includes four negative electrode connecting portions  71 A to  71 D and three negative electrode coupling portions  72   a  to  72   c . Similarly to the positive electrode connection member  6 , the negative electrode connection member  7  is formed by bending a single metal flat plate having a particular shape. Accordingly, the negative electrode connecting portions  71 A to  71 D and the negative electrode coupling portions  72   a  to  72   c  each have a flat shape. Specifically, the negative electrode connecting portions  71 A to  71 D have a flat shape parallel to the X-Z plane and sequentially arranged in the Y-direction. The negative electrode coupling portions  72   a  to  72   c  have a flat shape parallel to the X-Y plane. The metal flat plate used for forming the negative electrode connection member  7  preferably has a thickness of 0.5 mm or more and 1.5 mm or less. 
     In this embodiment, the negative electrode connection member  7  has the same shape and the same dimensions as the positive electrode connection member  6 . The negative electrode connecting portion  71 A corresponds to the positive electrode connecting portion  61 D. The negative electrode connecting portion  71 B corresponds to the positive electrode connecting portion  61 C. The negative electrode connecting portion  71 C corresponds to the positive electrode connecting portion  61 B. The negative electrode connecting portion  71 D corresponds to the positive electrode connecting portion  61 A. The negative electrode coupling portion  72   a  corresponds to the positive electrode coupling portion  62   c . The negative electrode coupling portion  72   b  corresponds to the positive electrode coupling portion  62   b . The negative electrode coupling portion  72   c  corresponds to the positive electrode coupling portion  62   a . The negative electrode connection member  7  may have a shape and dimensions different from those of the positive electrode connection member  6 . Modifications relating to the shape of the negative electrode connection member  7  and the positive electrode connection member  6  will be described below. 
     As illustrated in  FIG. 8  (also refer to  FIG. 2 ), the negative electrode connection member  7  is arranged in the following positional relationship with respect to the negative electrode terminal portions  12 A to  12 D. Specifically, the whole of the negative electrode connecting portion  71 B faces the second surface  122 B of the negative electrode terminal portion  12 B. The whole of the negative electrode connecting portion  71 C faces the first surface  121 C of the negative electrode terminal portion  12 C. The whole of the negative electrode connecting portion  71 A faces a part of the first surface  121 A of the negative electrode terminal portion  12 A. A facing part  713 D of the negative electrode connecting portion  71 D faces a part of the second surface  122 D of the negative electrode terminal portion  12 D. Herein, as illustrated in  FIG. 4 , the negative electrode coupling portions  72   a  to  72   c  each have such dimensions that the negative electrode connection member  7  can be arranged with respect to the negative electrode terminal portions  12 A to  12 D without deforming the negative electrode terminal portions  12 A to  12 D or with a small amount of deformation of the negative electrode terminal portions  12 A to  12 D. 
     In this arrangement relationship, the negative electrode connection member  7  is welded to the negative electrode terminal portions  12 A to  12 D as follows (refer to  FIGS. 2 and 8 ). Specifically, the negative electrode connecting portion  71 A is welded to the first surface  121 A of the negative electrode terminal portion  12 A. The negative electrode connecting portion  71 B is welded to the second surface  122 B of the negative electrode terminal portion  12 B in a non-facing part  714 B thereof. In other words, the first surface  121 A of the negative electrode terminal portion  12 A and the second surface  122 B of the negative electrode terminal portion  12 B are surfaces that face each other, and the negative electrode connecting portions  71 A and  71 B are respectively welded to the first surface  121 A and the second surface  122 B. 
     The negative electrode connecting portion  71 C is welded to the first surface  121 C of the negative electrode terminal portion  12 C in a non-facing part  714 C thereof. From the viewpoint of the relationship with the negative electrode connecting portion  71 B, this structure is understood as follows. Specifically, the second surface  122 B of the negative electrode terminal portion  12 B and the first surface  121 C of the negative electrode terminal portion  12 C are respectively back surfaces of the first surface  121 B of the negative electrode terminal portion  12 B and the second surface  122 C of the negative electrode terminal portion  12 C, the first surface  121 B and the second surface  122 C facing each other. The negative electrode connecting portions  71 B and  71 C are respectively welded to the second surface  122 B and the first surface  121 C. 
     The negative electrode connecting portion  71 D is welded to the second surface  122 D of the negative electrode terminal portion  12 D in the facing part  713 D thereof. From the viewpoint of the relationship with the negative electrode connecting portion  71 C, this structure is understood as follows. Specifically, the first surface  121 C of the negative electrode terminal portion  12 C and the second surface  122 D of the negative electrode terminal portion  12 D are surfaces that face each other, and the negative electrode connecting portions  71 C and  71 D are respectively welded to the first surface  121 C and the second surface  122 D. 
     Furthermore, as illustrated in  FIG. 4 , the negative electrode connecting portion  71 D is welded to the negative electrode projecting portion  53  in the non-facing part  714 D thereof. In this manner, the negative electrode plates  15  included in each of the electrode groups  1 A to  1 D are electrically connected to the negative electrode external terminal  52  through the negative electrode connection member  7 . 
     According to the rectangular electricity storage device of the present embodiment, in a process for producing the rectangular electricity storage device as described below, before the electrode groups  1 A to  1 D are housed in the outer package can  2 , the electrode groups  1 A to  1 D can be integrated by being fixed to the positive electrode connection member  6  and the negative electrode connection member  7  in a state in which the electrode groups  1 A to  1 D are stacked. The positive electrode connection member  6  and the negative electrode connection member  7  are each formed by bending a single metal flat plate and thus have high mechanical strength. Therefore, misalignment is unlikely to occur in the electrode groups  1 A to  1 D which are fixed to the positive electrode connection member  6  and the negative electrode connection member  7  having high mechanical strength. 
     Accordingly, even before the electrode groups  1 A to  1 D are housed in the outer package can  2 , the positive electrode projecting portion  43  and the negative electrode projecting portion  53  that are fixed to the cover plate  3  can be welded to the positive electrode connection member  6  and the negative electrode connection member  7 , respectively, without causing misalignment of the electrode groups  1 A to  1 D. Thus, the cover plate  3  is fixed to the electrode groups  1 A to  1 D. The positive electrode plates  14  included in each of the electrode groups  1 A to  1 D are electrically connected to the positive electrode external terminal  42  through the positive electrode connection member  6 . The negative electrode plates  15  included in each of the electrode groups  1 A to  1 D are electrically connected to the negative electrode external terminal  52  through the negative electrode connection member  7 . Even after welding of the cover plate  3 , misalignment does not substantially occur in the electrode groups  1 A to  1 D, and thus the electrode groups  1 A to  1 D can be housed in the outer package can  2 . 
     Therefore, the rectangular electricity storage device of the present embodiment does not require the space for housing lead plates that are folded, the space being necessary for the existing rectangular electricity storage device (refer to  FIG. 25 ). Consequently, the ratio of the total volume of the electrode groups  1 A to  1 D to the volume of the rectangular electricity storage device increases, resulting in an improvement in the volume energy density. From the viewpoint of improving the volume energy, a ratio of the height T 1  of the positive electrode terminal portions  11 A to  11 D or the height T 2  of the negative electrode terminal portions  12 A to  12 D (T 2 =T 1  in the present embodiment) to a distance L from the end surfaces  13  of the electrode groups  1 A to  1 D, the end surfaces  13  being flush with each other, to the inner surface  31  of the cover plate  3  (refer to  FIG. 3 or 4 ) is preferably 0.9 or less. 
     In the existing rectangular electricity storage device, the thickness of each lead plate must be small in order to prevent breakage and damage from occurring in a folded portion of the lead plate. In contrast, in the rectangular electricity storage device of the present embodiment, since such breakage and damage do not occur in the positive electrode connection member  6  and the negative electrode connection member  7 , each of the positive electrode connection member  6  and the negative electrode connection member  7  can have a large thickness. Even when the positive electrode connection member  6  and the negative electrode connection member  7  each have a large thickness, the volume energy density does not significantly decrease. 
     Therefore, according to the rectangular electricity storage device of the present embodiment, the electrical resistance of the positive electrode connection member  6  is low, and an energy loss between the positive electrode external terminal  42  and the electrode groups  1 A to  1 D is decreased. Similarly, the electrical resistance of the negative electrode connection member  7  is low, and an energy loss between the negative electrode external terminal  52  and the electrode groups  1 A to  1 D is decreased. From the viewpoint of decreasing the energy loss, each of the positive electrode connection member  6  and the negative electrode connection member  7  is preferably formed of at least one metal selected from the group consisting of aluminum, copper, and nickel. The thickness of each of the positive electrode connection member  6  and the negative electrode connection member  7  is preferably 0.5 mm or more and 1.5 mm or less. 
     [2] Method for Producing Rectangular Electricity Storage Device 
     In a method for producing the rectangular electricity storage device of the present embodiment, a preparation step, a first welding step, a second welding step, and a sealing step are sequentially performed. In the first welding step, steps (A) to (D) are sequentially performed, and in the second welding step, steps (E) and (F) are sequentially performed. Hereinafter, in the assembled state of the rectangular electricity storage device illustrated in  FIG. 2 , the surfaces of the electrode groups  1 A to  1 D that are to be directed in the Y-direction are respectively referred to as first surfaces  17 A to  17 D, and the surfaces of the electrode groups  1 A to  1 D that are to be directed in a direction opposite to the Y-direction are respectively referred to as second surfaces  18 A to  18 D. 
     [2-1] Preparation Step 
     First, in the preparation step, a positive electrode connection member  6  and a negative electrode connection member  7  are prepared (refer to  FIGS. 7A and 7B ). Specifically, the positive electrode connection member  6  is formed by preparing a metal flat plate punched to have a particular shape, and bending the metal flat plate. The negative electrode connection member  7  having the same shape and the same dimensions as the positive electrode connection member  6  is formed by the same method. The negative electrode connection member  7  may have a shape and dimensions different from the positive electrode connection member  6 . The metal flat plate used for forming the positive electrode connection member  6  and the negative electrode connection member  7  is preferably formed of at least one metal selected from the group consisting of aluminum, copper, and nickel. The metal flat plate preferably has a thickness of 0.5 mm or more and 1.5 mm or less. 
     In the preparation step, besides the positive electrode connection member  6  and the negative electrode connection member  7 , electrode groups  1 A to  1 D that respectively include positive electrode terminal portions  11 A to  11 D and negative electrode terminal portions  12 A to  12 D, an outer package can  2  in which the electrode groups  1 A to  1 D are to be housed, a cover plate  3  for sealing an opening  21  of the outer package can  2 , and an electric insulation sheet  8  are prepared. A positive electrode terminal member  4  and a negative electrode terminal member  5  are fixed to the cover plate  3  using nuts  32  and  33 , respectively. 
     [2-2] First Welding Step 
     &lt;Step (A)&gt; 
       FIG. 9  is a perspective view used for illustrating the step (A) included in the first welding step. As illustrated in  FIG. 9 , in the step (A), first, the electrode group  1 C is arranged so that the positive electrode terminal portion  11 C and the negative electrode terminal portion  12 C provided on the electrode group  1 C are directed in the horizontal direction, and the first surface  17 C of the electrode group  1 C is directed upward. The electric insulation sheet  8  is arranged in a state in which the positive electrode terminal portion  11 C and the negative electrode terminal portion  12 C are respectively passed through windows  81  and  82  formed in the electric insulation sheet  8 . 
     Next, the positive electrode connection member  6  is arranged so that the whole of the positive electrode connecting portion  61 C faces an upper surface (the first surface  111 C illustrated in  FIG. 2 ) of the positive electrode terminal portion  11 C. Subsequently, a non-facing part  614 C of the positive electrode connecting portion  61 C and the positive electrode terminal portion  11 C are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the non-facing part  614 C and the positive electrode terminal portion  11 C to each other by welding. Specifically, an ultrasonic welder  9  including a horn  91  that generates ultrasonic waves and an anvil  92  that serves as a pedestal is prepared. The anvil  92  is inserted between a non-facing part  614 B of a positive electrode connecting portion  61 B and the non-facing part  614 C of the positive electrode connecting portion  61 C, and the horn  91  is arranged above the non-facing part  614 C of the positive electrode connecting portion  61 C. Subsequently, the horn  91  is lowered to bring a leading end surface of the horn  91  into contact with a predetermined region RC 1  of the non-facing part  614 C and to sandwich the non-facing part  614 C and the positive electrode terminal portion  11 C from the top and the bottom with the horn  91  and the anvil  92 . In this state, ultrasonic waves are generated from the horn  91 , thereby joining the non-facing part  614 C and the positive electrode terminal portion  11 C to each other by welding. 
     In the positive electrode connection member  6  of the present embodiment, the positive electrode connecting portion  61 D does not face the non-facing part  614 C of the positive electrode connecting portion  61 C. Therefore, while it is difficult to bring the horn  91  into contact with the facing part  613 C of the positive electrode connecting portion  61 C from above the facing part  613 C because of the presence of the positive electrode connecting portion  61 D, the horn  91  can be easily brought into contact with the non-facing part  614 C of the positive electrode connecting portion  61 C by lowering the horn  91  from above the non-facing part  614 C. Accordingly, a horn having a shape the same as an existing horn can be used as the horn  91 . Furthermore, in the positive electrode connection member  6  of the present embodiment, a region (exposed region) serving as an edge of the non-facing part  614 B of the first edge  611 B and a region (exposed region) serving as an edge of the non-facing part  614 C of the first edge  611 C are not coupled with a positive electrode coupling portion  62   b  but are exposed (refer to  FIGS. 7A and 7B ). Therefore, the anvil  92  is easily inserted between the non-facing parts  614 B and  614 C. Accordingly, in the step (A), welding of the positive electrode connecting portion  61 C and the positive electrode terminal portion  11 C is easily performed. 
     Furthermore, in the step (A), the negative electrode connection member  7  is arranged so that the whole of the negative electrode connecting portion  71 C faces an upper surface (the first surface  121 C illustrated in  FIG. 2 ) of the negative electrode terminal portion  12 C. Subsequently, a non-facing part  714 C of the negative electrode connecting portion  71 C and the negative electrode terminal portion  12 C are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the non-facing part  714 C and the negative electrode terminal portion  12 C to each other by welding. Specifically, the anvil  92  is inserted between a non-facing part  714 B of a negative electrode connecting portion  71 B and the non-facing part  714 C of the negative electrode connecting portion  71 C, and the horn  91  is arranged above the non-facing part  714 C of the negative electrode connecting portion  71 C. Subsequently, the horn  91  is lowered to bring the leading end surface of the horn  91  into contact with a predetermined region RC 2  of the non-facing part  714 C and to sandwich the non-facing part  714 C and the negative electrode terminal portion  12 C from the top and the bottom with the horn  91  and the anvil  92 . In this state, ultrasonic waves are generated from the horn  91 , thereby joining the non-facing part  714 C and the negative electrode terminal portion  12 C to each other by welding. 
     In the negative electrode connection member  7  of the present embodiment, the negative electrode connecting portion  71 D does not face the non-facing part  714 C of the negative electrode connecting portion  71 C. Therefore, while it is difficult to bring the horn  91  into contact with the facing part  713 C of the negative electrode connecting portion  71 C from above the facing part  713 C because of the presence of the negative electrode connecting portion  71 D, the horn  91  can be easily brought into contact with the non-facing part  714 C of the negative electrode connecting portion  71 C by lowering the horn  91  from above the non-facing part  714 C. Furthermore, in the negative electrode connection member  7  of the present embodiment, the anvil  92  is easily inserted between the non-facing parts  714 B and  714 C as in the positive electrode connection member  6 . Accordingly, in the step (A), welding of the negative electrode connecting portion  71 C and the negative electrode terminal portion  12 C is easily performed. 
     &lt;Step (B)&gt; 
       FIG. 10  is a perspective view used for illustrating the step (B) included in the first welding step. As illustrated in  FIG. 10 , in the step (B), first, the top and the bottom of the electrode group  1 C subjected to the step (A) are reversed to direct the first surface  17 C of the electrode group  1 C downward and to direct the second surface  18 C of the electrode group  1 C upward. Subsequently, the electrode group  1 B is superposed on the second surface  18 C of the electrode group  1 C so that a second surface  18 B of the electrode group  1 B is directed upward. At this time, a positive electrode terminal portion  11 B and a negative electrode terminal portion  12 B provided on the electrode group  1 B are respectively passed through the windows  81  and  82  formed in the electric insulation sheet  8 . In addition, the upper surface (the second surface  112 B illustrated in  FIG. 2 ) of the positive electrode terminal portion  11 B is allowed to face the whole of the positive electrode connecting portion  61 B. Furthermore, the upper surface (the second surface  122 B illustrated in  FIG. 2 ) of the negative electrode terminal portion  12 B is allowed to face the whole of the negative electrode connecting portion  71 B. 
     Subsequently, a non-facing part  614 B of the positive electrode connecting portion  61 B and the positive electrode terminal portion  11 B are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the non-facing part  614 B and the positive electrode terminal portion  11 B to each other by welding. Specifically, the anvil  92  is inserted between the non-facing part  614 B of the positive electrode connecting portion  61 B and the non-facing part  614 C (in  FIG. 10 , the non-facing part  614 C is hidden by the anvil  92 ) of the positive electrode connecting portion  61 C, and the horn  91  is arranged above the non-facing part  614 B of the positive electrode connecting portion  61 B. Subsequently, the horn  91  is lowered to bring the leading end surface of the horn  91  into contact with a predetermined region RB 1  of the non-facing part  614 B and to sandwich the non-facing part  614 B and the positive electrode terminal portion  11 B from the top and the bottom with the horn  91  and the anvil  92 . In this state, ultrasonic waves are generated from the horn  91 , thereby joining the non-facing part  614 B and the positive electrode terminal portion  11 B to each other by welding. 
     In the positive electrode connection member  6  of the present embodiment, the positive electrode connecting portion  61 A does not face the non-facing part  614 B of the positive electrode connecting portion  61 B. Therefore, while it is difficult to bring the horn  91  into contact with the facing part  613 B of the positive electrode connecting portion  61 B from above the facing part  613 B because of the presence of the positive electrode connecting portion  61 A, the horn  91  can be easily brought into contact with the non-facing part  614 B of the positive electrode connecting portion  61 B by lowering the horn  91  from above the non-facing part  614 B. Furthermore, in the positive electrode connection member  6  of the present embodiment, the exposed region of the first edge  611 B and the exposed region of the first edge  611 C are not coupled with the positive electrode coupling portion  62   b  but are exposed as described above. Therefore, the anvil  92  is easily inserted between the non-facing parts  614 B and  614 C. Accordingly, in the step (B), welding of the positive electrode connecting portion  61 B and the positive electrode terminal portion  11 B is easily performed. 
     Furthermore, in the step (B), a non-facing part  714 B of the negative electrode connecting portion  71 B and the negative electrode terminal portion  12 B are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the non-facing part  714 B and the negative electrode terminal portion  12 B to each other by welding. Specifically, the anvil  92  is inserted between the non-facing part  714 B of the negative electrode connecting portion  71 B and the non-facing part  714 C of the negative electrode connecting portion  71 C, and the horn  91  is arranged above the non-facing part  714 B of the negative electrode connecting portion  71 B. Subsequently, the horn  91  is lowered to bring the leading end surface of the horn  91  into contact with a predetermined region RB 2  of the non-facing part  714 B and to sandwich the non-facing part  714 B and the negative electrode terminal portion  12 B from the top and the bottom with the horn  91  and the anvil  92 . In this state, ultrasonic waves are generated from the horn  91 , thereby joining the non-facing part  714 B and the negative electrode terminal portion  12 B to each other by welding. 
     In the negative electrode connection member  7  of the present embodiment, the negative electrode connecting portion  71 A does not face the non-facing part  714 B of the negative electrode connecting portion  71 B. Therefore, while it is difficult to bring the horn  91  into contact with the facing part  713 B of the negative electrode connecting portion  71 B from above the facing part  713 B because of the presence of the negative electrode connecting portion  71 A, the horn  91  can be easily brought into contact with the non-facing part  714 B of the negative electrode connecting portion  71 B by lowering the horn  91  from above the non-facing part  714 B. Furthermore, in the negative electrode connection member  7  of the present embodiment, the anvil  92  is easily inserted between the non-facing parts  714 B and  714 C as in the positive electrode connection member  6 . Accordingly, in the step (B), welding of the negative electrode connecting portion  71 B and the negative electrode terminal portion  12 B is easily performed. 
     &lt;Step (C)&gt; 
       FIG. 11  is a perspective view used for illustrating the step (C) included in the first welding step. As illustrated in  FIG. 11 , in the step (C), first, the top and the bottom of the electrode groups  1 B and  1 C subjected to the step (B) are reversed to direct the second surface  18 B of the electrode group  1 B downward and to direct the first surface  17 C of the electrode group  1 C upward. Subsequently, the electrode group  1 D is superposed on the first surface  17 C of the electrode group  1 C so that a first surface  17 D of the electrode group  1 D is directed upward. At this time, a positive electrode terminal portion  11 D and a negative electrode terminal portion  12 D provided on the electrode group  1 D are respectively passed through the windows  81  and  82  formed in the electric insulation sheet  8 . In addition, a part of the lower surface (the second surface  112 D illustrated in  FIG. 2 ) of the positive electrode terminal portion  11 D is allowed to face the whole of the positive electrode connecting portion  61 D. Furthermore, a part of the lower surface (the second surface  122 D illustrated in  FIG. 2 ) of the negative electrode terminal portion  12 D is allowed to face the facing part  713 D of the negative electrode connecting portion  71 D. 
     Subsequently, the positive electrode connecting portion  61 D and positive electrode terminal portion  11 D are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the positive electrode connecting portion  61 D and the positive electrode terminal portion  11 D to each other by welding. Specifically, the anvil  92  is inserted between the positive electrode connecting portions  61 C and  61 D, and the horn  91  is arranged above the positive electrode connecting portion  61 D. Subsequently, the horn  91  is lowered to bring the leading end surface of the horn  91  into contact with a predetermined region RD 1  of the positive electrode terminal portion  11 D at a position above the positive electrode connecting portion  61 D and to sandwich the positive electrode connecting portion  61 D and the positive electrode terminal portion  11 D from the top and the bottom with the horn  91  and the anvil  92 . In this state, ultrasonic waves are generated from the horn  91 , thereby joining the positive electrode connecting portion  61 D and the positive electrode terminal portion  11 D to each other by welding. 
     Furthermore, in the step (C), the facing part  713 D of the negative electrode connecting portion  71 D and the negative electrode terminal portion  12 D are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the facing part  713 D and the negative electrode terminal portion  12 D to each other by welding. Specifically, the anvil  92  is inserted between the negative electrode connecting portions  71 C and  71 D, and the horn  91  is arranged above the facing part  713 D of the negative electrode connecting portion  71 D. Subsequently, the horn  91  is lowered to bring the leading end surface of the horn  91  into contact with a predetermined region RD 2  of the negative electrode terminal portion  12 D at a position above the facing part  713 D and to sandwich the facing part  713 D and the negative electrode terminal portion  12 D from the top and the bottom with the horn  91  and the anvil  92 . In this state, ultrasonic waves are generated from the horn  91 , thereby joining the facing part  713 D and the negative electrode terminal portion  12 D to each other by welding. 
     &lt;Step (D)&gt; 
       FIG. 12  is a perspective view used for illustrating the step (D) included in the first welding step. As illustrated in  FIG. 12 , in the step (D), first, the top and the bottom of the electrode groups  1 B to  1 D subjected to the step (C) are reversed to direct the first surface  17 D of the electrode group  1 D downward and to direct the second surface  18 B of the electrode group  1 B upward. Subsequently, the electrode group  1 A is superposed on the second surface  18 B of the electrode group  1 B so that a second surface  18 A of the electrode group  1 A is directed upward. At this time, a positive electrode terminal portion  11 A and a negative electrode terminal portion  12 A provided on the electrode group  1 A are respectively passed through the windows  81  and  82  formed in the electric insulation sheet  8 . In addition, a part of the lower surface (the first surface  111 A illustrated in  FIG. 2 ) of the positive electrode terminal portion  11 A is allowed to face a facing part  613 A of the positive electrode connecting portion  61 A. Furthermore, a part of the lower surface (the first surface  121 A illustrated in  FIG. 2 ) of the negative electrode terminal portion  12 A is allowed to face the whole of the negative electrode connecting portion  71 A. 
     Subsequently, the facing part  613 A of the positive electrode connecting portion  61 A and the positive electrode terminal portion  11 A are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the facing part  613 A and the positive electrode terminal portion  11 A to each other by welding. Specifically, the anvil  92  is inserted between the positive electrode connecting portions  61 A and  61 B, and the horn  91  is arranged above the facing part  613 A of the positive electrode connecting portion  61 A. Subsequently, the horn  91  is lowered to bring the leading end surface of the horn  91  into contact with a predetermined region RA 1  of the positive electrode terminal portion  11 A at a position above the facing part  613 A and to sandwich the facing part  613 A and the positive electrode terminal portion  11 A from the top and the bottom with the horn  91  and the anvil  92 . In this state, ultrasonic waves are generated from the horn  91 , thereby joining the facing part  613 A and the positive electrode terminal portion  11 A to each other by welding. 
     Furthermore, in the step (D), the negative electrode connecting portion  71 A and the negative electrode terminal portion  12 A are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the negative electrode connecting portion  71 A and the negative electrode terminal portion  12 A to each other by welding. Specifically, the anvil  92  is inserted between the negative electrode connecting portions  71 A and  71 B, and the horn  91  is arranged above the negative electrode connecting portion  71 A. Subsequently, the horn  91  is lowered to bring the leading end surface of the horn  91  into contact with a predetermined region RA 2  of the negative electrode terminal portion  12 A at a position above the negative electrode connecting portion  71 A and to sandwich the negative electrode connecting portion  71 A and the negative electrode terminal portion  12 A from the top and the bottom with the horn  91  and the anvil  92 . In this state, ultrasonic waves are generated from the horn  91 , thereby joining the negative electrode connecting portion  71 A and the negative electrode terminal portion  12 A to each other by welding. 
     By performing the first welding step (steps (A) to (D)), the four electrode groups  1 A to  1 D are mechanically and electrically coupled to each other through the positive electrode connection member  6  and the negative electrode connection member  7 . 
     [2-3] Second Welding Step 
     &lt;Step (E)&gt; 
       FIG. 13  is a perspective view used for illustrating the step (E) included in the second welding step. As illustrated in  FIG. 13 , in the step (E), first, a cover plate  3  to which a positive electrode terminal member  4  and a negative electrode terminal member  5  are fixed is arranged as follows with respect to the electrode groups  1 A to  1 D subjected to the step (D). Specifically, in a state in which a positive electrode external terminal  42  and a negative electrode external terminal  52  are located on the side opposite to an end surface  13  of the electrode groups  1 A to  1 D, a positive electrode projecting portion  43  is superposed on a non-facing part  614 A of the positive electrode connecting portion  61 A (refer to  FIG. 13 ), and a negative electrode projecting portion  53  is superposed on a non-facing part  714 D of the negative electrode connecting portion  71 D (refer to  FIG. 14 ). At this time, a part of the positive electrode projecting portion  43  may overlap with the positive electrode terminal portion  11 A. A part of the negative electrode projecting portion  53  may overlap with the negative electrode terminal portion  12 D. 
     Subsequently, the positive electrode projecting portion  43  and the non-facing part  614 A of the positive electrode connecting portion  61 A are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the positive electrode projecting portion  43  and the non-facing part  614 A to each other by welding. Specifically, the anvil  92  is arranged below the non-facing part  614 A of the positive electrode connecting portion  61 A, and the horn  91  is arranged above the positive electrode projecting portion  43 . Subsequently, the horn  91  is lowered to bring the leading end surface of the horn  91  into contact with a predetermined region RP 1  of the positive electrode projecting portion  43  at a position above the non-facing part  614 A and to sandwich the positive electrode projecting portion  43  and the non-facing part  614 A from the top and the bottom with the horn  91  and the anvil  92 . In this state, ultrasonic waves are generated from the horn  91 , thereby joining the positive electrode projecting portion  43  and the non-facing part  614 A to each other by welding. 
     &lt;Step (F)&gt; 
       FIG. 14  is a perspective view used for illustrating the step (F) included in the second welding step. As illustrated in  FIG. 14 , in the step (F), first, the top and the bottom of the electrode groups  1 A to  1 D subjected to the step (E) are reversed to direct the second surface  18 A of the electrode group  1 A downward and to direct the first surface  17 D of the electrode group  1 D upward. 
     Subsequently, the negative electrode projecting portion  53  and the non-facing part  714 D of the negative electrode connecting portion  71 D are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the negative electrode projecting portion  53  and the non-facing part  714 D to each other by welding. Specifically, the anvil  92  is arranged below the non-facing part  714 D of the negative electrode connecting portion  71 D, and the horn  91  is arranged above the negative electrode projecting portion  53 . Subsequently, the horn  91  is lowered to bring the leading end surface of the horn  91  into contact with a predetermined region RP 2  of the negative electrode projecting portion  53  at a position above the non-facing part  714 D and to sandwich the negative electrode projecting portion  53  and the non-facing part  714 D from the top and the bottom with the horn  91  and the anvil  92 . In this state, ultrasonic waves are generated from the horn  91 , thereby joining the negative electrode projecting portion  53  and the non-facing part  714 D to each other by welding. 
     By performing the second welding step (steps (E) and (F)), the positive electrode plates  14  included in each of the electrode groups  1 A to  1 D are electrically connected to the positive electrode external terminal  42  through the positive electrode connection member  6 , and the negative electrode plates  15  included in each of the electrode groups  1 A to  1 D are electrically connected to the negative electrode external terminal  52  through the negative electrode connection member  7 . 
     [2-4] Sealing Step 
     After the second welding step is performed, in the sealing step, the electrode groups  1 A to  1 D and an electrolyte are housed in an outer package can  2 , and the cover plate  3  is brought into contact with an opening end surface of the outer package can  2 . In this state, welding is performed on the contact surface between the outer package can  2  and the cover plate  3  by welding means such as laser welding to seal an opening  21  of the outer package can  2  with the cover plate  3 . 
     According to the production method of the present embodiment, as described above, a space for housing lead plates that are folded, the space being necessary for the existing rectangular electricity storage device (refer to  FIG. 25 ), is not necessary. Therefore, in a rectangular electricity storage device to be produced, the ratio of the total volume of the electrode groups  1 A to  1 D to the volume of the device increases, resulting in an improvement in the volume energy density. Furthermore, according to the production method of the present embodiment, the thickness of each of the positive electrode connection member  6  and the negative electrode connection member  7  can be increased. Consequently, the electrical resistance of the positive electrode connection member  6  is low, and an energy loss between the positive electrode external terminal  42  and the electrode groups  1 A to  1 D is decreased in the rectangular electricity storage device produced. Similarly, the electrical resistance of the negative electrode connection member  7  is low, and an energy loss between the negative electrode external terminal  52  and the electrode groups  1 A to  1 D is decreased in the rectangular electricity storage device produced. 
     [3] Modifications 
     [3-1] First Modification 
       FIG. 15A  is a perspective view illustrating a structure of each of a positive electrode connection member  6  and a negative electrode connection member  7  included in a rectangular electricity storage device according to a first modification.  FIG. 15B  is a perspective view illustrating a state in which the positive electrode connection member  6  and the negative electrode connection member  7  are connected to electrode groups  1 A to  1 D. The difference from the structure of the rectangular electricity storage device of the above embodiment will be mainly described in detail below. 
     &lt;Positive Electrode Connection Member&gt; 
     In the first modification, as illustrated in  FIG. 15A , a facing part  613 A of a positive electrode connecting portion  61 A faces the whole of a positive electrode connecting portion  61 B. Regarding a positive electrode connecting portion  61 D, the whole of the positive electrode connecting portion  61 D faces a positive electrode connecting portion  61 C. Furthermore, a positive electrode coupling portion  62   b  is coupled to the whole of first edges  611 B and  611 C, and thus each of the first edges  611 B and  611 C has no exposed region. Other structures are the same as those of the positive electrode connection member  6  illustrated in  FIGS. 7A and 7B , and thus a description thereof is omitted. 
     As illustrated in  FIG. 15B  (also refer to  FIG. 2 ), the positive electrode connection member  6  is arranged in the following positional relationship with respect to the positive electrode terminal portions  11 A to  11 D. Specifically, the facing part  613 A of the positive electrode connecting portion  61 A faces the first surface  111 A of the positive electrode terminal portion  11 A. The whole of the positive electrode connecting portion  61 B faces the second surface  112 B of the positive electrode terminal portion  11 B. The whole of the positive electrode connecting portion  61 C faces the first surface  111 C of the positive electrode terminal portion  11 C. The whole of the positive electrode connecting portion  61 D faces the second surface  112 D of the positive electrode terminal portion  11 D. Herein, the positive electrode coupling portions  62   a  to  62   c  (refer to  FIG. 15A ) each have such dimensions that the positive electrode connection member  6  can be arranged with respect to the positive electrode terminal portions  11 A to  11 D without deforming the positive electrode terminal portions  11 A to  11 D or with a small amount of deformation of the positive electrode terminal portions  11 A to  11 D. 
     In this arrangement relationship, the positive electrode connection member  6  is welded to the positive electrode terminal portions  11 A to  11 D as follows. Specifically, the positive electrode connecting portion  61 A is welded to the first surface  111 A of the positive electrode terminal portion  11 A in the facing part  613 A thereof. The positive electrode connecting portion  61 B is welded to the second surface  112 B of the positive electrode terminal portion  11 B. The positive electrode connecting portion  61 C is welded to the first surface  111 C of the positive electrode terminal portion  11 C. The positive electrode connecting portion  61 D is welded to the second surface  112 D of the positive electrode terminal portion  11 D. 
     Furthermore, the positive electrode connecting portion  61 A is welded to the positive electrode projecting portion  43  in a non-facing part  614 A thereof (refer to  FIG. 3 ). In this manner, the positive electrode plates  14  included in each of the electrode groups  1 A to  1 D are electrically connected to the positive electrode external terminal  42  through the positive electrode connection member  6 . 
     &lt;Negative Electrode Connection Member&gt; 
     In the first modification, the negative electrode connection member  7  has the same shape and the same dimensions as the positive electrode connection member  6 . A negative electrode connecting portion  71 A corresponds to the positive electrode connecting portion  61 D, a negative electrode connecting portion  71 B corresponds to the positive electrode connecting portion  61 C, a negative electrode connecting portion  71 C corresponds to the positive electrode connecting portion  61 B, and a negative electrode connecting portion  71 D corresponds to the positive electrode connecting portion  61 A (refer to  FIG. 15A ). A negative electrode coupling portion  72   a  corresponds to the positive electrode coupling portion  62   c . A negative electrode coupling portion  72   b  corresponds to the positive electrode coupling portion  62   b . A negative electrode coupling portion  72   c  corresponds to the positive electrode coupling portion  62   a . The negative electrode connection member  7  may have a shape and dimensions different from those of the positive electrode connection member  6 . 
     As illustrated in  FIG. 15B  (also refer to  FIG. 2 ), the negative electrode connection member  7  is arranged in the following positional relationship with respect to the negative electrode terminal portions  12 A to  12 D. Specifically, the whole of the negative electrode connecting portion  71 A faces the first surface  121 A of the negative electrode terminal portion  12 A. The whole of the negative electrode connecting portion  71 B faces the second surface  122 B of the negative electrode terminal portion  12 B. The whole of the negative electrode connecting portion  71 C faces the first surface  121 C of the negative electrode terminal portion  12 C. A facing part  713 D of the negative electrode connecting portion  71 D faces the second surface  122 D of the negative electrode terminal portion  12 D. Herein, the negative electrode coupling portions  72   a  to  72   c  ( FIG. 15A ) each have such dimensions that the negative electrode connection member  7  can be arranged with respect to the negative electrode terminal portions  12 A to  12 D without deforming the negative electrode terminal portions  12 A to  12 D or with a small amount of deformation of the negative electrode terminal portions  12 A to  12 D. 
     In this arrangement relationship, the negative electrode connection member  7  is welded to the negative electrode terminal portions  12 A to  12 D as follows. Specifically, the negative electrode connecting portion  71 A is welded to the first surface  121 A of the negative electrode terminal portion  12 A. The negative electrode connecting portion  71 B is welded to the second surface  122 B of the negative electrode terminal portion  12 B. The negative electrode connecting portion  71 C is welded to the first surface  121 C of the negative electrode terminal portion  12 C. The negative electrode connecting portion  71 D is welded to the second surface  122 D of the negative electrode terminal portion  12 D in the facing part  713 D thereof 
     Furthermore, the negative electrode connecting portion  71 D is welded to the negative electrode projecting portion  53  in a non-facing part  714 D thereof (refer to  FIG. 4 ). In this manner, the negative electrode plates  15  included in each of the electrode groups  1 A to  1 D are electrically connected to the negative electrode external terminal  52  through the negative electrode connection member  7 . 
     According to the rectangular electricity storage device of the first modification, as in the rectangular electricity storage device of the embodiment described above, even before the electrode groups  1 A to  1 D are housed in the outer package can  2 , the positive electrode projecting portion  43  and the negative electrode projecting portion  53  that are fixed to the cover plate  3  can be welded to the positive electrode connection member  6  and the negative electrode connection member  7 , respectively, without causing misalignment of the electrode groups  1 A to  1 D. Accordingly, the rectangular electricity storage device of the first modification does not require the space for housing lead plates that are folded, the space being necessary for the existing rectangular electricity storage device (refer to  FIG. 25 ). Therefore, the ratio of the total volume of the electrode groups  1 A to  1 D to the volume of the rectangular electricity storage device increases, resulting in an improvement in the volume energy density. 
     Furthermore, according to the rectangular electricity storage device of the first modification, the thickness of each of the positive electrode connection member  6  and the negative electrode connection member  7  can be increased as in the rectangular electricity storage device of the embodiment described above. Accordingly, the electrical resistance of the positive electrode connection member  6  is low, and an energy loss between the positive electrode external terminal  42  and the electrode groups  1 A to  1 D is decreased. Similarly, the electrical resistance of the negative electrode connection member  7  is low, and an energy loss between the negative electrode external terminal  52  and the electrode groups  1 A to  1 D is decreased. 
     &lt;Method for Producing Rectangular Electricity Storage Device&gt; 
     In a method for producing the rectangular electricity storage device of the first modification, a preparation step, a first welding step, a second welding step, and a sealing step are sequentially performed. In the first welding step, steps (A′) to (D′) are sequentially performed. Since the preparation step, the second welding step, and the sealing step are the same as those in the above embodiment, a description thereof is omitted. The first welding step will now be described with reference to the drawings. 
       FIG. 16  is a perspective view used for illustrating the step (A′) included in the first welding step. As illustrated in  FIG. 16 , in the step (A′), first, electrode groups  1 A to  1 D are stacked so that all positive electrode terminal portions  11 A to  11 D and negative electrode terminal portions  12 A to  12 D provided on the electrode groups  1 A to  1 D are oriented in the same direction. At this time, the electrode groups  1 A to  1 D are stacked so that the positive electrode terminal portions  11 A to  11 D face each other and the negative electrode terminal portions  12 A to  12 D face each other. The electrode groups  1 A to  1 D are arranged so that the positive electrode terminal portions  11 A to  11 D and the negative electrode terminal portions  12 A to  12 D are oriented in the horizontal direction, and a first surface  17 D (surface oriented in the Y-direction in the assembled state of the rectangular electricity storage device (refer to  FIG. 2 )) of the electrode group  1 D is oriented upward. Furthermore, an electric insulation sheet  8  is arranged in a state in which the positive electrode terminal portions  11 A to  11 D and the negative electrode terminal portions  12 A to  12 D are respectively passed through windows  81  and  82  formed in the electric insulation sheet  8 . 
     Next, the positive electrode connection member  6  is arranged so that the positional relationship becomes the same as the positional relationship illustrated in  FIG. 15B  with respect to the positive electrode terminal portions  11 A to  11 D. Subsequently, as illustrated in  FIG. 16 , the positive electrode connecting portion  61 D and the positive electrode terminal portion  11 D are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the positive electrode connecting portion  61 D and the positive electrode terminal portion  11 D to each other by welding. For the ultrasonic welding, an ultrasonic welder  9 A including a horn  91 A that generates ultrasonic waves and an anvil  92 A that serves as a pedestal is used. In this modification, the shape of the horn  91 A is different from the shape of the horn  91  (refer to, for example,  FIG. 9 ) used in the embodiment described above. The horn  91 A includes two upper and lower welding ends that can be inserted between adjacent two positive electrode terminal portions and adjacent two positive electrode connecting portions. Hereinafter, an upper welding end  911  and a lower welding end  912  are referred to as “first welding end  911 ” and “second welding end  912 ”, respectively. 
     Specifically, the anvil  92 A is inserted between the positive electrode connecting portions  61 C and  61 D from the lateral side, and the second welding end  912  of the horn  91 A is arranged above the positive electrode connecting portion  61 D. Subsequently, the horn  91 A is lowered to bring a leading end surface of the second welding end  912  into contact with a predetermined region RD 1  of the positive electrode terminal portion  11 D at a position above the positive electrode connecting portion  61 D and to sandwich the positive electrode connecting portion  61 D and the positive electrode terminal portion  11 D from the top and the bottom with the horn  91 A and the anvil  92 A. In this state, ultrasonic waves are generated from the second welding end  912  of the horn  91 A, thereby joining the positive electrode connecting portion  61 D and the positive electrode terminal portion  11 D to each other by welding. 
     Furthermore, in the step (A′), the negative electrode connection member  7  is arranged so that the positional relationship becomes the same as the positional relationship illustrated in  FIG. 15B  with respect to the negative electrode terminal portions  12 A to  12 D. Subsequently, as illustrated in  FIG. 16 , the facing part  713 D of the negative electrode connecting portion  71 D and the negative electrode terminal portion  12 D are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the facing part  713 D and the negative electrode terminal portion  12 D to each other by welding. Specifically, the anvil  92 A is inserted between the negative electrode connecting portions  71 C and  71 D from the lateral side, and the second welding end  912  of the horn  91 A is arranged above the facing part  713 D of the negative electrode connecting portion  71 D. Subsequently, the horn  91 A is lowered to bring the leading end surface of the second welding end  912  into contact with a predetermined region RD 2  of the negative electrode terminal portion  12 D at a position above the facing part  713 D and to sandwich the facing part  713 D and the negative electrode terminal portion  12 D from the top and the bottom with the horn  91 A and the anvil  92 A. In this state, ultrasonic waves are generated from the second welding end  912  of the horn  91 A, thereby joining the facing part  713 D and the negative electrode terminal portion  12 D to each other by welding. 
       FIG. 17  is a perspective view used for illustrating the step (B′) included in the first welding step. In the step (B′), the positive electrode connecting portion  61 C and the positive electrode terminal portion  11 C are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the positive electrode connecting portion  61 C and the positive electrode terminal portion  11 C to each other by welding. Specifically, the anvil  92 A is inserted between the positive electrode connecting portions  61 B and  61 C from the lateral side, and the second welding end  912  of the horn  91 A is inserted between the positive electrode connecting portions  61 C and  61 D from the front. Subsequently, the horn  91 A is lowered to bring the leading end surface of the second welding end  912  into contact with a predetermined region RC 1  of the positive electrode connecting portion  61 C and to sandwich the positive electrode connecting portion  61 C and the positive electrode terminal portion  11 C from the top and the bottom with the horn  91 A and the anvil  92 A. In this state, ultrasonic waves are generated from the second welding end  912  of the horn  91 A, thereby joining the positive electrode connecting portion  61 C and the positive electrode terminal portion  11 C to each other by welding. 
     Furthermore, in the step (B′), as illustrated in  FIG. 17 , the negative electrode connecting portion  71 C and the negative electrode terminal portion  12 C are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the negative electrode connecting portion  71 C and the negative electrode terminal portion  12 C to each other by welding. Specifically, the anvil  92 A is inserted between the negative electrode connecting portions  71 B and  71 C from the lateral side, and the second welding end  912  of the horn  91 A is inserted between the negative electrode connecting portions  71 C and  71 D from the front. Subsequently, the horn  91 A is lowered to bring the leading end surface of the second welding end  912  into contact with a predetermined region RC 2  of the negative electrode connecting portion  71 C (in  FIG. 17 , the predetermined region RC 2  overlaps with the leading end surface (lower surface) of the second welding end  912  and thus is hidden by the second welding end  912 ). In addition, the negative electrode connecting portion  71 C and the negative electrode terminal portion  12 C are sandwiched from the top and the bottom with the horn  91 A and the anvil  92 A. In this state, ultrasonic waves are generated from the second welding end  912  of the horn  91 A, thereby joining the negative electrode connecting portion  71 C and the negative electrode terminal portion  12 C to each other by welding. 
       FIG. 18  is a perspective view used for illustrating the step (C′) included in the first welding step. In the step (C′), the positive electrode connecting portion  61 B and the positive electrode terminal portion  11 B are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the positive electrode connecting portion  61 B and the positive electrode terminal portion  11 B to each other by welding. Specifically, the anvil  92 A is inserted between the positive electrode connecting portions  61 B and  61 C from the lateral side, and the first welding end  911  of the horn  91 A is inserted between the positive electrode connecting portions  61 A and  61 B from the front. Subsequently, the horn  91 A is raised to bring a leading end surface of the first welding end  911  into contact with a predetermined region RB 1  of the positive electrode connecting portion  61 B and to sandwich the positive electrode connecting portion  61 B and the positive electrode terminal portion  11 B from the top and the bottom with the horn  91 A and the anvil  92 A. In this state, ultrasonic waves are generated from the first welding end  911  of the horn  91 A, thereby joining the positive electrode connecting portion  61 B and the positive electrode terminal portion  11 B to each other by welding. 
     Furthermore, in the step (C′), as illustrated in  FIG. 18 , the negative electrode connecting portion  71 B and the negative electrode terminal portion  12 B are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the negative electrode connecting portion  71 B and the negative electrode terminal portion  12 B to each other by welding. Specifically, the anvil  92 A is inserted between the negative electrode connecting portions  71 B and  71 C from the lateral side, and the first welding end  911  of the horn  91 A is inserted between the negative electrode connecting portions  71 A and  71 B from the front. Subsequently, the horn  91 A is raised to bring the leading end surface of the first welding end  911  into contact with a predetermined region RB 2  of the negative electrode connecting portion  71 B and to sandwich the negative electrode connecting portion  71 B and the negative electrode terminal portion  12 B from the top and the bottom with the horn  91 A and the anvil  92 A. In this state, ultrasonic waves are generated from the first welding end  911  of the horn  91 A, thereby joining the negative electrode connecting portion  71 B and the negative electrode terminal portion  12 B to each other by welding. 
       FIG. 19  is a perspective view used for illustrating the step (D′) included in the first welding step. In the step (D′), a facing part  613 A of the positive electrode connecting portion  61 A and the positive electrode terminal portion  11 A are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the facing part  613 A and the positive electrode terminal portion  11 A to each other by welding. Specifically, the anvil  92 A is inserted between the positive electrode connecting portions  61 A and  61 B from the lateral side, and the first welding end  911  of the horn  91 A is arranged below the facing part  613 A of the positive electrode connecting portion  61 A. Subsequently, the horn  91 A is raised to bring the leading end surface of the first welding end  911  into contact with a predetermined region RA 1  of the positive electrode terminal portion  11 A at a position below the facing part  613 A and to sandwich the facing part  613 A and the positive electrode terminal portion  11 A from the top and the bottom with the horn  91 A and the anvil  92 A. In this state, ultrasonic waves are generated from the first welding end  911  of the horn  91 A, thereby joining the facing part  613 A and the positive electrode terminal portion  11 A to each other by welding. 
     Furthermore, in the step (D′), as illustrated in  FIG. 19 , the negative electrode connecting portion  71 A and the negative electrode terminal portion  12 A are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the negative electrode connecting portion  71 A and the negative electrode terminal portion  12 A to each other by welding. Specifically, the anvil  92 A is inserted between the negative electrode connecting portions  71 A and  71 B from the lateral side, and the first welding end  911  of the horn  91 A is arranged below the negative electrode connecting portion  71 A. Subsequently, the horn  91 A is raised to bring the leading end surface of the first welding end  911  into contact with a predetermined region RA 2  of the negative electrode terminal portion  12 A at a position below the negative electrode connecting portion  71 A and to sandwich the negative electrode connecting portion  71 A and the negative electrode terminal portion  12 A from the top and the bottom with the horn  91 A and the anvil  92 A. In this state, ultrasonic waves are generated from the first welding end  911  of the horn  91 A, thereby joining the negative electrode connecting portion  71 A and the negative electrode terminal portion  12 A to each other by welding. 
     In the production method of the first modification, since all the electrode groups  1 A to  1 D are stacked in the step (A′), the work necessary in the production method of the above embodiment, that is, the work in which the electrode groups  1 A to  1 D are sequentially stacked in the steps (A) to (D) is unnecessary. Accordingly, it is sufficient that the work necessary in the steps (A′) to (D′) is a simple operation in which the horn  91 A and the anvil  92 A are moved upward or downward relative to the stacked electrode groups  1 A to  1 D. Consequently, according to the production method of the first modification, the production of the rectangular electricity storage device is simplified. 
     [3-2] Second Modification 
       FIG. 20A  is a perspective view illustrating a structure of each of a positive electrode connection member  6  and a negative electrode connection member  7  included in a rectangular electricity storage device according to a second modification.  FIG. 20B  is a perspective view illustrating a state in which the positive electrode connection member  6  and the negative electrode connection member  7  are connected to electrode groups  1 A to  1 D. The difference from the structure of the rectangular electricity storage device of the above embodiment will be mainly described in detail below. 
     &lt;Positive Electrode Connection Member&gt; 
     In the second modification, as illustrated in  FIG. 20A , a facing part  613 A of a positive electrode connecting portion  61 A faces the whole of a positive electrode connecting portion  61 B. Regarding a positive electrode connecting portion  61 D, the whole of the positive electrode connecting portion  61 D faces a positive electrode connecting portion  61 C. The positive electrode connecting portions  61 A to  61 D respectively include first side edges  615 A to  615 D directed in the X-direction and second side edges  616 A to  616 D directed in a direction opposite to the X-direction. The first side edge  615 A of the positive electrode connecting portion  61 A and the first side edge  615 B of the positive electrode connecting portion  61 B are mechanically and electrically coupled to each other with a positive electrode coupling portion  62   d  therebetween. The second side edge  616 B of the positive electrode connecting portion  61 B and the second side edge  616 C of the positive electrode connecting portion  61 C are mechanically and electrically coupled to each other with a positive electrode coupling portion  62   e  therebetween. Furthermore, the first side edge  615 C of the positive electrode connecting portion  61 C and the first side edge  615 D of the positive electrode connecting portions  61 D are mechanically and electrically coupled to each other with a positive electrode coupling portion  62   f  therebetween. Accordingly, in the second modification, the whole of first edges  611 A to  611 D are exposed without being coupled to positive electrode coupling portions. Other structures are the same as those of the positive electrode connection member  6  illustrated in  FIGS. 7A and 7B , and thus a description thereof is omitted. 
     As illustrated in  FIG. 20B  (also refer to  FIG. 2 ), the positive electrode connection member  6  is arranged in the following positional relationship with respect to the positive electrode terminal portions  11 A to  11 D. Specifically, the facing part  613 A of the positive electrode connecting portion  61 A faces the first surface  111 A of the positive electrode terminal portion  11 A. The whole of the positive electrode connecting portion  61 B faces the second surface  112 B of the positive electrode terminal portion  11 B. The whole of the positive electrode connecting portion  61 C faces the first surface  111 C of the positive electrode terminal portion  11 C. The whole of the positive electrode connecting portion  61 D faces the second surface  112 D of the positive electrode terminal portion  11 D. Herein, the positive electrode coupling portions  62   d  to  62   f  (refer to  FIG. 20A ) each have such dimensions that the positive electrode connection member  6  can be arranged with respect to the positive electrode terminal portions  11 A to  11 D without deforming the positive electrode terminal portions  11 A to  11 D or with a small amount of deformation of the positive electrode terminal portions  11 A to  11 D. 
     In this arrangement relationship, the positive electrode connection member  6  is welded to the positive electrode terminal portions  11 A to  11 D as follows. Specifically, the positive electrode connecting portion  61 A is welded to the first surface  111 A of the positive electrode terminal portion  11 A in the facing part  613 A thereof. The positive electrode connecting portion  61 B is welded to the second surface  112 B of the positive electrode terminal portion  11 B. The positive electrode connecting portion  61 C is welded to the first surface  111 C of the positive electrode terminal portion  11 C. The positive electrode connecting portion  61 D is welded to the second surface  112 D of the positive electrode terminal portion  11 D. 
     Furthermore, the positive electrode connecting portion  61 A is welded to the positive electrode projecting portion  43  in a non-facing part  614 A thereof (refer to  FIG. 3 ). In this manner, the positive electrode plates  14  included in each of the electrode groups  1 A to  1 D are electrically connected to the positive electrode external terminal  42  through the positive electrode connection member  6 . 
     &lt;Negative Electrode Connection Member&gt; 
     In the second modification, the negative electrode connection member  7  has the same shape and the same dimensions as the positive electrode connection member  6 . A negative electrode connecting portion  71 A corresponds to the positive electrode connecting portion  61 D, a negative electrode connecting portion  71 B corresponds to the positive electrode connecting portion  61 C, a negative electrode connecting portion  71 C corresponds to the positive electrode connecting portion  61 B, and a negative electrode connecting portion  71 D corresponds to the positive electrode connecting portion  61 A (refer to  FIG. 20A ). A negative electrode coupling portion  72   d  corresponds to the positive electrode coupling portion  62   f . A negative electrode coupling portion  72   e  corresponds to the positive electrode coupling portion  62   e . A negative electrode coupling portion  72   f  corresponds to the positive electrode coupling portion  62   d . The negative electrode connection member  7  may have a shape and dimensions different from those of the positive electrode connection member  6 . 
     As illustrated in  FIG. 20B  (also refer to  FIG. 2 ), the negative electrode connection member  7  is arranged in the following positional relationship with respect to the negative electrode terminal portions  12 A to  12 D. Specifically, the whole of the negative electrode connecting portion  71 A faces the first surface  121 A of the negative electrode terminal portion  12 A. The whole of the negative electrode connecting portion  71 B faces the second surface  122 B of the negative electrode terminal portion  12 B. The whole of the negative electrode connecting portion  71 C faces the first surface  121 C of the negative electrode terminal portion  12 C. A facing part  713 D of the negative electrode connecting portion  71 D faces the second surface  122 D of the negative electrode terminal portion  12 D. Herein, the negative electrode coupling portions  72   d  to  72   f  ( FIG. 20A ) each have such dimensions that the negative electrode connection member  7  can be arranged with respect to the negative electrode terminal portions  12 A to  12 D without deforming the negative electrode terminal portions  12 A to  12 D or with a small amount of deformation of the negative electrode terminal portions  12 A to  12 D. 
     In this arrangement relationship, the negative electrode connection member  7  is welded to the negative electrode terminal portions  12 A to  12 D as follows. Specifically, the negative electrode connecting portion  71 A is welded to the first surface  121 A of the negative electrode terminal portion  12 A. The negative electrode connecting portion  71 B is welded to the second surface  122 B of the negative electrode terminal portion  12 B. The negative electrode connecting portion  71 C is welded to the first surface  121 C of the negative electrode terminal portion  12 C. The negative electrode connecting portion  71 D is welded to the second surface  122 D of the negative electrode terminal portion  12 D in the facing part  713 D thereof 
     Furthermore, the negative electrode connecting portion  71 D is welded to the negative electrode projecting portion  53  in a non-facing part  714 D thereof (refer to  FIG. 4 ). In this manner, the negative electrode plates  15  included in each of the electrode groups  1 A to  1 D are electrically connected to the negative electrode external terminal  52  through the negative electrode connection member  7 . 
     According to the rectangular electricity storage device of the second modification, as in the rectangular electricity storage device of the embodiment described above, even before the electrode groups  1 A to  1 D are housed in the outer package can  2 , the positive electrode projecting portion  43  and the negative electrode projecting portion  53  that are fixed to the cover plate  3  can be welded to the positive electrode connection member  6  and the negative electrode connection member  7 , respectively, without causing misalignment of the electrode groups  1 A to  1 D. Accordingly, the rectangular electricity storage device of the second modification does not require the space for housing lead plates that are folded, the space being necessary for the existing rectangular electricity storage device (refer to  FIG. 25 ). Therefore, the ratio of the total volume of the electrode groups  1 A to  1 D to the volume of the rectangular electricity storage device increases, resulting in an improvement in the volume energy density. 
     Furthermore, according to the rectangular electricity storage device of the second modification, the thickness of each of the positive electrode connection member  6  and the negative electrode connection member  7  can be increased as in the rectangular electricity storage device of the embodiment described above. Accordingly, the electrical resistance of the positive electrode connection member  6  is low, and an energy loss between the positive electrode external terminal  42  and the electrode groups  1 A to  1 D is decreased. Similarly, the electrical resistance of the negative electrode connection member  7  is low, and an energy loss between the negative electrode external terminal  52  and the electrode groups  1 A to  1 D is decreased. 
     &lt;Method for Producing Rectangular Electricity Storage Device&gt; 
     In a method for producing the rectangular electricity storage device of the second modification, a preparation step, a first welding step, a second welding step, and a sealing step are sequentially performed. In the first welding step, steps (A′) to (D′) are sequentially performed. Since the preparation step, the second welding step, and the sealing step are the same as those in the above embodiment, a description thereof is omitted. The first welding step will now be described with reference to the drawings. 
       FIG. 21  is a perspective view used for illustrating the step (A′) included in the first welding step. As illustrated in  FIG. 21 , in the step (A′), first, electrode groups  1 A to  1 D are stacked so that all positive electrode terminal portions  11 A to  11 D and negative electrode terminal portions  12 A to  12 D provided on the electrode groups  1 A to  1 D are oriented in the same direction. At this time, the electrode groups  1 A to  1 D are stacked so that the positive electrode terminal portions  11 A to  11 D face each other and the negative electrode terminal portions  12 A to  12 D face each other. The electrode groups  1 A to  1 D are arranged so that the positive electrode terminal portions  11 A to  11 D and the negative electrode terminal portions  12 A to  12 D are oriented in the horizontal direction, and a first surface  17 D (surface oriented in the Y-direction in the assembled state of the rectangular electricity storage device (refer to  FIG. 2 )) of the electrode group  1 D is oriented upward. Furthermore, an electric insulation sheet  8  is arranged in a state in which the positive electrode terminal portions  11 A to  11 D and the negative electrode terminal portions  12 A to  12 D are respectively passed through windows  81  and  82  formed in the electric insulation sheet  8 . 
     Next, the positive electrode connection member  6  is arranged so that the positional relationship becomes the same as the positional relationship illustrated in  FIG. 20B  with respect to the positive electrode terminal portions  11 A to  11 D. Subsequently, as illustrated in  FIG. 21 , the positive electrode connecting portion  61 D and the positive electrode terminal portion  11 D are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the positive electrode connecting portion  61 D and the positive electrode terminal portion  11 D to each other by welding. For the ultrasonic welding, an ultrasonic welder  9 B including the horn  91 A used in the first modification and the anvil  92  used in the embodiment is used. Specifically, the anvil  92  is inserted between the positive electrode connecting portions  61 C and  61 D from the front, and the second welding end  912  of the horn  91 A is arranged above the positive electrode connecting portion  61 D. Subsequently, the horn  91 A is lowered to bring the leading end surface of the second welding end  912  into contact with a predetermined region RD 1  of the positive electrode terminal portion  11 D at a position above the positive electrode connecting portion  61 D and to sandwich the positive electrode connecting portion  61 D and the positive electrode terminal portion  11 D from the top and the bottom with the horn  91 A and the anvil  92 . In this state, ultrasonic waves are generated from the second welding end  912  of the horn  91 A, thereby joining the positive electrode connecting portion  61 D and the positive electrode terminal portion  11 D to each other by welding. 
     Furthermore, in the step (A′), the negative electrode connection member  7  is arranged so that the positional relationship becomes the same as the positional relationship illustrated in  FIG. 20B  with respect to the negative electrode terminal portions  12 A to  12 D. Subsequently, the facing part  713 D of the negative electrode connecting portion  71 D and the negative electrode terminal portion  12 D are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the facing part  713 D and the negative electrode terminal portion  12 D to each other by welding. Specifically, the anvil  92  is inserted between the negative electrode connecting portions  71 C and  71 D from the front, and the second welding end  912  of the horn  91 A is arranged above the facing part  713 D of the negative electrode connecting portion  71 D. Subsequently, the horn  91 A is lowered to bring the leading end surface of the second welding end  912  into contact with a predetermined region RD 2  of the negative electrode terminal portion  12 D at a position above the facing part  713 D and to sandwich the facing part  713 D and the negative electrode terminal portion  12 D from the top and the bottom with the horn  91 A and the anvil  92 . In this state, ultrasonic waves are generated from the second welding end  912  of the horn  91 A, thereby joining the facing part  713 D and the negative electrode terminal portion  12 D to each other by welding. 
       FIG. 22  is a perspective view used for illustrating the step (B′) included in the first welding step. As illustrated in  FIG. 22 , in the step (B′), the positive electrode connecting portion  61 C and the positive electrode terminal portion  11 C are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the positive electrode connecting portion  61 C and the positive electrode terminal portion  11 C to each other by welding. Specifically, the anvil  92  is inserted between the positive electrode connecting portions  61 B and  61 C from the front, and the second welding end  912  of the horn  91 A is inserted between the positive electrode connecting portions  61 C and  61 D from the front. Subsequently, the horn  91 A is lowered to bring the leading end surface of the second welding end  912  into contact with a predetermined region RC 1  of the positive electrode connecting portion  61 C (in  FIG. 22 , the predetermined region RC 1  overlaps with the leading end surface (lower surface) of the second welding end  912  and thus is hidden by the second welding end  912 ). In addition, the positive electrode connecting portion  61 C and the positive electrode terminal portion  11 C are sandwiched from the top and the bottom with the horn  91 A and the anvil  92 . In this state, ultrasonic waves are generated from the second welding end  912  of the horn  91 A, thereby joining the positive electrode connecting portion  61 C and the positive electrode terminal portion  11 C to each other by welding. 
     Furthermore, in the step (B′), the negative electrode connecting portion  71 C and the negative electrode terminal portion  12 C are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the negative electrode connecting portion  71 C and the negative electrode terminal portion  12 C to each other by welding. Specifically, the anvil  92  is inserted between the negative electrode connecting portions  71 B and  71 C from the front, and the second welding end  912  of the horn  91 A is inserted between the negative electrode connecting portions  71 C and  71 D from the front. Subsequently, the horn  91 A is lowered to bring the leading end surface of the second welding end  912  into contact with a predetermined region RC 2  of the negative electrode connecting portion  71 C and to sandwich the negative electrode connecting portion  71 C and the negative electrode terminal portion  12 C from the top and the bottom with the horn  91 A and the anvil  92 . In this state, ultrasonic waves are generated from the second welding end  912  of the horn  91 A, thereby joining the negative electrode connecting portion  71 C and the negative electrode terminal portion  12 C to each other by welding. 
       FIG. 23  is a perspective view used for illustrating the step (C′) included in the first welding step. As illustrated in  FIG. 23 , in the step (C′), the positive electrode connecting portion  61 B and the positive electrode terminal portion  11 B are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the positive electrode connecting portion  61 B and the positive electrode terminal portion  11 B to each other by welding. Specifically, the anvil  92  is inserted between the positive electrode connecting portions  61 A and  61 B from the front, and the second welding end  912  of the horn  91 A is inserted between the positive electrode connecting portions  61 B and  61 C from the front. Subsequently, the horn  91 A is lowered to bring the leading end surface of the second welding end  912  into contact with a predetermined region RB 1  of the positive electrode terminal portion  11 B (in  FIG. 23 , the predetermined region RB 1  overlaps with the leading end surface (lower surface) of the second welding end  912  and thus is hidden by the second welding end  912 ) at a position above the positive electrode connecting portion  61 B. In addition, the positive electrode connecting portion  61 B and the positive electrode terminal portion  11 B are sandwiched from the top and the bottom with the horn  91 A and the anvil  92 . In this state, ultrasonic waves are generated from the second welding end  912  of the horn  91 A, thereby joining the positive electrode connecting portion  61 B and the positive electrode terminal portion  11 B to each other by welding. 
     Furthermore, in the step (C′), the negative electrode connecting portion  71 B and the negative electrode terminal portion  12 B are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the negative electrode connecting portion  71 B and the negative electrode terminal portion  12 B to each other by welding. Specifically, the anvil  92  is inserted between the negative electrode connecting portions  71 A and  71 B from the front, and the second welding end  912  of the horn  91 A is inserted between the negative electrode connecting portions  71 B and  71 C from the front. Subsequently, the horn  91 A is lowered to bring the leading end surface of the second welding end  912  into contact with a predetermined region RB 2  of the negative electrode terminal portion  12 B at a position above the negative electrode connecting portion  71 B and to sandwich the negative electrode connecting portion  71 B and the negative electrode terminal portion  12 B from the top and the bottom with the horn  91 A and the anvil  92 . In this state, ultrasonic waves are generated from the second welding end  912  of the horn  91 A, thereby joining the negative electrode connecting portion  71 B and the negative electrode terminal portion  12 B to each other by welding. 
       FIG. 24  is a perspective view used for illustrating the step (D′) included in the first welding step. As illustrated in  FIG. 24 , in the step (D′), a facing part  613 A of the positive electrode connecting portion  61 A and the positive electrode terminal portion  11 A are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the facing part  613 A and the positive electrode terminal portion  11 A to each other by welding. Specifically, the anvil  92  is arranged at a position below the facing part  613 A of the positive electrode connecting portion  61 A. At this time, the anvil  92  is brought into contact with the lower surface (the second surface  112 A illustrated in  FIG. 2 ) of the positive electrode terminal portion  11 A. The second welding end  912  of the horn  91 A is inserted between the positive electrode connecting portions  61 A and  61 B from the front. Subsequently, the horn  91 A is lowered to bring the leading end surface of the second welding end  912  into contact with a predetermined region RA 1  of the facing part  613 A (in  FIG. 24 , the predetermined region RA 1  overlaps with the leading end surface (lower surface) of the second welding end  912  and thus is hidden by the second welding end  912 ). In addition, the facing part  613 A and the positive electrode terminal portion  11 A are sandwiched from the top and the bottom with the horn  91 A and the anvil  92 . In this state, ultrasonic waves are generated from the second welding end  912  of the horn  91 A, thereby joining the facing part  613 A and the positive electrode terminal portion  11 A to each other by welding. 
     Furthermore, in the step (D′), the negative electrode connecting portion  71 A and the negative electrode terminal portion  12 A are brought into contact with each other, and ultrasonic welding is performed on the contact surface, thereby joining the negative electrode connecting portion  71 A and the negative electrode terminal portion  12 A to each other by welding. Specifically, the anvil  92  is arranged at a position below the negative electrode connecting portion  71 A. At this time, the anvil  92  is brought into contact with the lower surface (the second surface  122 A illustrated in  FIG. 2 ) of the negative electrode terminal portion  12 A. The second welding end  912  of the horn  91 A is inserted between the negative electrode connecting portions  71 A and  71 B from the front. Subsequently, the horn  91 A is lowered to bring the leading end surface of the second welding end  912  into contact with a predetermined region RA 2  of the negative electrode connecting portion  71 A and to sandwich the negative electrode connecting portion  71 A and the negative electrode terminal portion  12 A from the top and the bottom with the horn  91 A and the anvil  92 . In this state, ultrasonic waves are generated from the second welding end  912  of the horn  91 A, thereby joining the negative electrode connecting portion  71 A and the negative electrode terminal portion  12 A to each other by welding. 
     In the production method of the second modification, since all the electrode groups  1 A to  1 D are stacked in the step (A′), the work necessary in the production method of the above embodiment, that is, the work in which the electrode groups  1 A to  1 D are sequentially stacked in the steps (A) to (D) is unnecessary. Accordingly, it is sufficient that the work necessary in the steps (A′) to (D′) is a simple operation in which the horn  91 A and the anvil  92  are moved upward or downward relative to the stacked electrode groups  1 A to  1 D. Consequently, according to the production method of the second modification, the production of the rectangular electricity storage device is simplified. 
     The structures of respective portions of the present invention are not limited to the embodiments described above, and various modifications can be made within the technical scope described in claims. For example, in the rectangular electricity storage device, instead of the welding to the positive electrode connection member  6  or in addition to the welding, the positive electrode projecting portion  43  may be welded to at least any one of the positive electrode terminal portions  11 A to  11 D. Instead of the welding to the negative electrode connection member  7  or in addition to the welding, the negative electrode projecting portion  53  may be welded to at least any one of the negative electrode terminal portions  12 A to  12 D. 
     In the rectangular electricity storage device, the positive electrode terminal member  4  and the positive electrode connection member  6  may not be provided, and the positive electrode plates  14  included in each of the electrode groups  1 A to  1 D may be electrically connected to the inner surface of the outer package can  2  having electrical conductivity. In this case, at least a part of the outer peripheral surface of the outer package can  2  is used as a positive electrode external terminal. In the rectangular electricity storage device, the negative electrode terminal member  5  and the negative electrode connection member  7  may not be provided, and the negative electrode plates  15  included in each of the electrode groups  1 A to  1 D may be electrically connected to the inner surface of the outer package can  2  having electrical conductivity. In this case, at least a part of the outer peripheral surface of the outer package can  2  is used as a negative electrode external terminal. 
     Furthermore, the structures of respective portions of the rectangular electricity storage device can be applied to various secondary batteries and capacitors, such as a lead storage battery, a lithium-ion battery, a sodium-ion battery, a molten-salt battery, a lithium-ion capacitor, and an electric double-layer capacitor as long as they are rectangular electricity storage devices in which a plurality of electrode groups are housed in the outer package can  2  in a state where the electrode groups are stacked. The structures of respective portions of the rectangular electricity storage device may be applied to primary batteries. 
     The rectangular electricity storage device and the method for producing the rectangular electricity storage device according to the present invention are useful as, for example, large-scale power storage devices for household or industrial use and power supplies installed in an electric vehicle or a hybrid vehicle.