Patent Publication Number: US-10763462-B2

Title: Secondary battery

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is a divisional application of U.S. application Ser. No. 13/885,572 filed May 15, 2013, which is the National Stage Application of International Application No. PCT/JP2011/072985, filed Oct. 5, 2011, which claims priority to Japanese Patent Application Nos. 2010-258365 filed Nov. 18, 2010 and 2011-185051 filed Aug. 26, 2011, the entire contents of all are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a secondary battery. 
     BACKGROUND ART 
     Provided is a high-output and/or high-capacity battery module in which a plurality of flat batteries having electrodes led out of exterior members are stacked and electrically connected in series and/or parallel to each other. 
     One example of flat battery usable in the battery module is a non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery is a battery in which a stacked electrode assembly is accommodated together with a non-aqueous electrolytic solution between exterior members. The stacked electrode assembly has a positive electrode, a negative electrode and a separator for preventing a short circuit between the positive and negative electrodes. For example, aluminum laminate sheets are usable as the exterior members. Outer peripheral portions of the aluminum laminate sheets are sealed by fusion bonding to form a battery package such that the stacked electrode assembly can be accommodated in the battery package. 
     There is known a technique for fusion bonding an outer peripheral portion of the separator with the seal portions of the aluminum laminate sheets in order to prevent a displacement of the stacked electrode assembly in the secondary battery (see Patent Document 1). 
     PRIOR ART DOCUMENTS 
     Patent Document 
     Patent Document 1: Japanese Laid-Open Patent Publication No. H11-250873 
     SUMMARY OF THE INVENTION 
     In order to maintain the performance of the battery, it is important to contain a predetermined amount of electrolytic solution in the stacked electrode assembly. It is thus preferable to refill the stacked electrode assembly with the electrolytic solution in the case where the amount of the electrolytic solution in the stacked electrode assembly becomes insufficient due to some reason. 
     In the non-aqueous electrolyte secondary battery of Patent Document 1 in which the outer peripheral portion of the separator is joined with the exterior members, however, any consideration is not given to the refilling of the stacked electrode assembly with the electrolytic solution. This may result in a deterioration of battery performance during long-term use. 
     For the purpose of prevention of battery performance deterioration in the secondary battery in which the outer peripheral portion of the separator is joined with the exterior member such as aluminum laminate sheets, it is also desirable to take measures to prevent the outer peripheral portion of the separator from being broken from the joint part. Any measures against such breakage are not however taken in the non-aqueous electrolyte secondary battery of Patent Document 1. This makes it difficult to increase the operation life of the battery. 
     The present invention has been made to solve the above prior art problems. It is accordingly an object of the present invention to provide a secondary battery in which a separator of a stacked electrode assembly is joined at an outer peripheral portion thereof between exterior members so as to allow refilling of the stacked electrode assembly with an electrolytic solution, favorably prevent the separator from being broken from its joint part and thereby maintain battery performance during long-term use. 
     In order to achieve the above object, there is provided according to the present invention a secondary battery, comprising: a stacked electrode assembly having a positive electrode, a negative electrode and a separator; an electrolytic solution; and exterior members accommodating therebetween the stacked electrode assembly together with the electrolytic solution, wherein the secondary battery comprises: a plurality of joint parts at which an outer peripheral portion of the separator is joined with the exterior members; and a holding part formed at least between the joint parts so as to hold therein the electrolytic solution; and wherein a sum of perimeters of the joint parts is longer than a perimeter of a rectangle of minimum area enclosing therein all of the joint parts. 
     In the present invention, the outer peripheral portion of the separator is joined with the exterior members by forming a plurality of separate joint parts, rather than by forming a continuous single joint part, while forming the holding part at least between the joint parts to hold therein the electrolyte solution. In this configuration, the stacked electrode assembly can be refilled with the electrolyte solution from the holding part between the joint parts. Further, the sum of the perimeters of the joint parts is made longer than the perimeter of the rectangle of minimum area enclosing therein all of the joint parts so that, even though the separator becomes smaller in thickness at the joint parts, the occurrence of breakage in the joint parts can be prevented by improvement in strength against tensile force in the present invention. It is therefore possible to maintain the performance of the secondary battery during long-term use. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a battery module according to one exemplary embodiment of the present invention. 
         FIG. 2  is a perspective view of a cell unit in the battery module of  FIG. 1 . 
         FIG. 3  is a perspective exploded view of the cell unit of  FIG. 2  as viewed from the back side. 
         FIG. 4  is a perspective view of a flat battery according to one exemplary embodiment of the present invention. 
         FIG. 5  is a plan view of substantial part of the flat battery, showing joint parts at which the outer peripheral portion of a separator is joined with exterior members, according to one embodiment of the present invention. 
         FIG. 6A  is a section view taken along line  6 A- 6 A of  FIG. 5 ; and  FIG. 6B  is a section view taken along line  6 B- 6 B of  FIG. 5 . 
         FIG. 7A  is an enlarged view of an area enclosed by broken line  7 A in  FIG. 5 ; and  FIG. 7B  is an enlarged view of an area enclosed by broken line  7 B in  FIG. 5 . 
         FIGS. 8A and 8B  are schematic views showing how a tensile force acts on the flat battery in a direction of outline arrow. 
         FIGS. 9A to 9E  are schematic view showing examples of the shape and arrangement of a plurality of joint parts and the relationship of the sum (La) of the perimeters of the joint parts and the perimeter (Lb) of a rectangle of minimum area encompassing all of the joint parts. 
         FIGS. 10A to 10C  are schematic view showing examples of the shape and arrangement of a plurality of joint parts and the relationship of the sum (La) of the perimeters of the joint parts and the perimeter (Lb) of a rectangle of minimum area encompassing all of the joint parts. 
         FIG. 11  is a plan view of substantial part of a flat battery, showing joint parts, according to another exemplary embodiment of the present invention. 
         FIG. 12  is schematic views of joint parts formed in Examples and Comparative Example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, exemplary embodiments of the present invention will be described below with reference to the drawings. It is noted that: in the drawings, like parts and portions are designated by like reference numerals to omit repeated explanations thereof; and the dimensions of the respective parts and portions may be exaggerated for purposes of illustration and may be different from the actual dimensions. Further, x-axis and y-axis indicate the lateral and longitudinal of flat battery  10 , respectively, in the drawings. 
     As shown in  FIGS. 1 and 2 , battery module  100  includes cell unit  130  formed with a plurality of flat batteries  10  (as secondary batteries), insulating cover  140  having electrical insulating properties and case  120  accommodating therein cell unit  130  and insulating cover  140 . Although battery module  100  can be used solely, it is feasible to provide an assembled battery with desired current, voltage and capacity characteristics by series and/or parallel connection of a plurality of battery modules  100 . 
     Case  120  has rectangular box-shaped lower case member  122  and lid-shaped upper case member  124 . An edge portion of upper case member  124  is wound around and fixed by crimping to an edge portion of a peripheral wall of lower case member  122 . Each of lower case member  122  and upper case member  124  is formed from a relatively thin steel plate or aluminum plate. Lower case member  122  and upper case member  124  have through holes  126  formed in respective four corner portions thereof such that stacked battery modules  100  can be maintained as the assembled battery by insertion of bolts (not shown) in through holes  126 . Herein, reference numerals  131  and  132  designate output terminals arranged to protrude from front opening portions of lower case member  122 . 
     As shown in  FIG. 2 , cell unit  130  has stacked body  132  in which a plurality of flat batteries  10  are electrically connected together and a plurality of spacers  160  and  161  supporting the batteries. Each of spacers  160  and  161  has electrical insulating properties. Spacers  160  are arranged on a front side of stacked body  132 , whereas spacers  161  (as a supporting member) are arranged on a back side of stacked body  132 . 
     For example, spacers  161  on the back side of stacked body  132  are positioned in such a manner to hold outer peripheral portions  32  of exterior members  30  of flat batteries  10  as shown in  FIG. 3 . Spacers  161  have through holes  162  formed in longitudinally opposite end portions thereof such that through holes  162  can be aligned with through holes  126  on the back side of lower case member  122  and upper case member  124 . 
     As shown in  FIGS. 4 to 7 , flat battery  10  is configured as e.g. a lithium-ion secondary battery and has a structure that stacked electrode assembly  20  is accommodated together with an electrolytic solution between exterior members  30 . Flat battery  10  includes electrodes  41  and  42  (referred to as “tabs”) let to the outside from exterior members  30 . In  FIG. 5 , reference numeral  21  designates a positive electrode or a negative electrode. For purposes of clarity, only separators  22  are illustrated in  FIG. 6 . 
     Stacked electrode assembly  20  includes a positive electrode or electrodes, a negative electrode or electrodes and a separator or separators  22  stacked alternately together. The positive electrode has a positive electrode active material layer formed of e.g. a lithium-transition metal composite oxide such as LiMn 2 O 4 , whereas the negative electrode has a negative electrode active material layer formed of e.g. a carbon material and a lithium-transition metal composite oxide. Separator  22  is formed of e.g. porous PE (polyethylene) having permeability to allow permeation of an electrolyte material. 
     In terms of weight reduction and thermal conduction, exterior members  30  are formed from sheets of e.g. polymer-metal laminate film in which a metal (or alloy) such as aluminum, stainless steel, nickel or cupper is coated with an insulating material such as propylene film. Each of exterior members  30  includes body portion  31  covering stacked electrode assembly  20  and outer peripheral portion  32  extending along peripheral edge of body portion  31 . Outer peripheral portions  32  are joined at part or the whole thereof by fusion bonding. 
     Tabs  41  and  42  are adapted to take an electric current out of stacked electrode assembly  20  and arranged to extend on a front side of flat battery  10 . 
     Flat battery  10  has a plurality of joint parts  40  at which outer peripheral portions of separators  22  are joined with exterior members  30  in order to prevent a displacement of stacked electrode assembly  20 . Flat battery  10  further includes holding part  50  formed at least between joint parts  40  to hold therein the electrolytic solution in such a manner that stacked electrode assembly  20  can be refilled with the electrolytic solution. The joining can be done by thermal fusion bonding, ultrasonic fusion bonding, welding etc. In  FIGS. 5, 7 and 8 , joint parts  40  are indicated by hatching for ease of understanding. 
     More specifically, the outer peripheral portions of separators  22  are joined with the exterior members  30  by forming a plurality of separate joint parts  40  as shown in  FIGS. 5, 6 and 7  rather than by forming a continuous single joint part. The continuous single joint part is hereinafter sometimes simply referred to as “continuous joint part” for the sake of simplicity. 
     The dimensions of joint parts  40  can be set, for example, as follows: W 1  (width)=2 mm, W 2  (distance)=1 mm, W 3  (length)=2 mm and W 4  (distance from the fused joint (joint part  40 ) between the outer peripheral portions of separators  22  and exterior members  30  to the fused joint between outer peripheral portions  32  of exterior members  30 )=5 mm as shown in  FIGS. 7(A)  and (B). There is no particular limitation on the ratio of the sum of the areas of joint parts  40  to the total area of separator  22 . For example, the ratio of the sum of the areas of joint parts  40  to the total area of separator  22  can be set to 0.1% to 1%. 
     In the case of a battery in which separators are joined between exterior members by a continuous joint part, the battery has a so-called bag structure that an electrolytic solution is accumulated and sealed in a space between the continuous joint part and the joint between outer peripheral portions of the exterior members. In such a structure, it is not possible to make effective use of the sealed electrolytic solution. 
     On the other hand, separators  22  are joined with the exterior member  30  by a plurality of joint parts  40  in the present embodiment. As separators  22  are not fused together in at least the region between the joint parts  40 , holding parts  50  are formed in a small space between separators  22  in this region so as to hold therein electrolytic solution  51  (see  FIG. 6 ). Stacked electrode assembly  20  can be thus refilled with electrolytic solution  51  from holding part  50  under the action of capillarity when the amount of electrolytic solution  51  in stacked electrode assembly  20  becomes insufficient due to some reason. By such refilling of stacked electrode assembly  20  with electrolytic solution  51 , it is possible to maintain a state in which stacked electrode assembly  20  contains a predetermined amount of electrolytic solution  51  over a long period of time so that flat battery  10  can secure battery performance during long-term use. 
     Even when gas is accumulated between the stacked electrodes during battery manufacturing process, such accumulated gas can be easily released from the power generating portion of the battery to outer peripheral portions  32  through region S 1  between joint parts  40 . This makes it possible to prevent a deterioration in power generating efficiency caused by the accumulated gas. 
     As shown in  FIGS. 5 and 7 , not only region S 1  between joint parts  40  but also clearance region S 2  from the uppermost one of the plurality of joint parts  40  in the drawing to exterior members  30 , clearance region S 3  from the lowermost one of the plurality of joint parts  40  in the drawing to the exterior members  30  and clearance region S 4  between the plurality of joint parts  40  and the back-side ends of exterior members  30  are adapted to serve as holding part  50 . This makes it possible to hold a larger amount of electrolytic solution  51  for refilling of stacked electrode assembly  20  so that flat battery  10  can secure battery performance during longer-term use. 
     As shown in  FIGS. 3 to 5 , outer peripheral portion  32  of exterior member  30  includes a plurality of supporting parts  33  coupled by an adhesive to battery-supporting spacers  161  and extension part  34  located between supporting parts  33  and extending radially outwardly of the battery. The outer peripheral portion of separator  22  has tongue part  23  extending toward extension parts  34  of exterior members  30  such that joint parts  40  are formed on tongue part  23 . This makes it possible to make effective use of the dead space that is supported by spacers  161  and does not substantially contribute to power generation, increase the hold amount of electrolytic solution  51  and improve the operation lifetime of the battery. 
     Through holes  35  are formed in extension parts  34  such that pins in spacers  161  can be inserted in through holes  35  so as to restrict the supporting position of flat battery  10  relative to spacers  161 . 
     In  FIG. 8(A) , schematically indicated by a two-dot chain line is breakage line  61  that could be developed at or around joint parts  40  when a tensile force acts on separator  22  in the direction of outline arrow  60 . Under such a tensile force, a tensile stress is exerted on each of joint parts  40  in the directions of solid-line arrows  62  as schematically indicated in  FIG. 8(B) . 
     When the tensile stress is excessively exerted on each joint part  40  in the directions of solid-line arrows  62  in  FIG. 8(B) , there occurs breakage in separator  22  etc. along breakage line  61  in  FIG. 8(A) . As breakage line  61  extends along the periphery (three sides) of each of joint parts  40 , the battery is made able to withstand a larger tensile stress by increasing the sum of the perimeters of joint parts  40 . 
     It is thus preferable to increase the number of joint parts  40  and to increase the sum of the perimeters of joint parts  40  as shown in  FIG. 5  in the case where a larger tensile stress acts on each joint part  40 . 
     In particular, the sum (La) of the perimeters of joint parts  40  is preferably made longer than the perimeter (Lb) of rectangle  63  of minimum area enclosing therein all of joint parts  40 . The configuration of rectangle  63  can be defined as indicated by a broken line in  FIGS. 9 and 10 . By satisfaction of such a condition, separator  22  can attain an improved strength against tensile force during use of flat battery  10  in a vehicle such as automotive vehicle where vibrations occur during running. This makes it possible to prevent the occurrence of breakage in joint parts  40  at which separator  22  is made thinner. 
     Herein, examples of the shape and arrangement of a plurality of joint parts  40  and the relationship of La and Lb are schematically shown in  FIGS. 9(A)  to (E) and  FIGS. 10(A)  to (C). 
     In  FIG. 5  and  FIGS. 9(A)  and (C), joint parts  40  are all made the same in size and aligned in a line in the x-axis direction. The relationship of La and Lb is either La&lt;Lb ( FIG. 9(C) ) or La&gt;Lb ( FIG. 5  and  FIG. 9(A) ). The present invention is not limited to this example. 
     It is feasible to align joint parts  40  of different sizes in a line in the x-axis direction as shown in  FIG. 9(B) . The condition of La&gt;Lb is satisfied in this example. 
     As shown in  FIGS. 9(D)  and (E), it is feasible to displace any one or ones of joint parts  40  relative to the other joint parts  40  in the y-axis direction rather than to arrange joint parts  40  in a line in the x-axis direction. The relationship of La and Lb is La&gt;Lb ( FIG. 9(D) ) or La&lt;Lb ( FIG. 9(E) ). 
     Further, joint parts  40  may be formed along the x-axis direction and arranged in two columns in the y-axis direction as shown in  FIG. 10(A) . The condition of La&gt;Lb is satisfied in this example. Even in such an arrangement, holding part  50  can be formed so as to hold therein electrolytic solution  51  and refill stacked electrode assembly  20  with electrolytic solution  51  as long as there are some spaces left at least between joint parts  40  in the vertical direction of the drawing. 
     As shown in  FIG. 10(B) , joint parts  40  may be arranged in two rows in the x-axis direction and in two columns in the y-axis direction. In this example, the condition of La&gt;Lb is satisfied. 
     As shown in  FIG. 10(C) , joint parts  40  may be arranged in two columns in the y-axis direction with five joint parts  40  aligned in the x-axis direction in the upper column. The condition of La&gt;Lb is satisfied in this example. The position of space between joint parts  40  in the upper column are displaced in the x-axis direction from the position of space between joint parts  40  in the lower column. Even in such an arrangement, holding part  50  can be formed so as to hold there in electrolytic solution  51  and refill stacked electrode assembly  20  with electrolytic solution  51 . 
     The arrangement of joint parts  40  is not limited to the direction parallel to the x-axis or y-axis direction. Although not shown in the drawings, it is feasible to arrange joint parts  40  in any direction inclined relative to the x-axis or y-axis direction. 
     As described above, flat battery  10  according to the present embodiment has joint parts  40  at which the outer periphery of separator  22  is joined with exterior members  30  and holding part  50  formed at least between joint parts  40  so as to hold therein electrolytic solution  51  and refill stacked electrode assembly  20  with electrolytic solution  51 . It is therefore possible to maintain the state in which stacked electrode assembly  20  contains a predetermined amount of electrolytic solution  51  over a long period of time by refilling stacked electrode assembly  20  with electrolytic solution  51  from holding part  50  between joint parts  40 , whereby the secondary battery can secure battery performance during long-term use. Further, it is easier to release the gas accumulated between the stacked electrodes to outer peripheral portions  32  through the region between joint parts  40  and is possible to prevent a deterioration in power generating efficiency caused by the accumulated gas. It is furthermore possible to improve the strength of separator  22  against tensile strength and prevent the occurrence of breakage in joint parts  40  in which separator  22  is made smaller in thickness by controlling the sum (La) of the perimeters of joint parts  40  to be longer than the perimeter of rectangle  63  of minimum area enclosing therein all of joint parts  40 . 
     As each of outer peripheral portions  32  of exterior members  30  has a plurality of supporting parts  33  coupled to battery-supporting spacers  161  and extension part  34  located between supporting parts  33  and extending radially outwardly of the battery; and as the outer peripheral portion of separator  22  has tongue part  23  extending to between extension parts  34  of exterior members  30  such that joint parts  40  are formed on tongue part  23 , it possible to make effective use of the dead space that is supported by spacers  161  and does not substantially contribute to power generation, increase the hold amount of electrolytic solution  51  and improve the operation lifetime of the battery. 
     Another Embodiment 
     Next, a flat battery according to another embodiment of the present invention will be explained below. 
     In general, flat battery  10  according to the present another embodiment includes a plurality of joint parts  40  at which outer peripheral portions of separators  22  are each joined with exterior members  30  and holding part  50  formed between joint parts  40  as shown in  FIG. 11 . Joint parts  40  are arranged in joint regions  45 ,  46  and  47 . In each of joint regions  45 ,  46  and  47 , joint parts  40  are arranged adjacent to and at distance W 2  away from each other. One joint region  44  is located at a distance that is longer than distance W 2  away from the other joint region  46 ,  47 . In each of joint regions  45 ,  46  and  47 , the sum of the perimeters of joint parts  40  is made longer than the perimeter of rectangle  65  of minimum area enclosing therein all of adjacent joint parts  40 . 
     In the above-mentioned embodiment, attention is focused on the relationship between all of joint parts  40  of flat battery  10  and rectangle  63  of minimum area enclosing all these joint parts  40  so as to prevent breakage in joint parts  40  by satisfaction of La&gt;Lb. By contrast, attention is focused on each joint region  45 ,  46 ,  47  constituted by adjacent joint parts  40  for improvement in joint strength in the present embodiment. 
     As shown in  FIG. 11 , the plurality of joint regions  45 ,  46  and  47  are provided on flat battery  10 . Each of joint regions  45 ,  46  and  47  is defined as the region in which the plurality of joint parts  40  are arranged adjacent to each other. These joint regions  45 ,  46  and  47  are located at distance W 5  from each other. Distance W 5  is longer than distance W 2  between adjacent joint parts  40 . Holding portion  50  is formed between joint parts  40  and between joint regions  45 ,  46  and  47 . 
     In one joint region  45 , joint parts  40  are formed so as to satisfy the condition of La (the sum of the perimeters of adjacent joint parts  40 )&gt;Lc (the perimeter of rectangle  65  of minimum area enclosing therein adjacent joint parts  40 ). This allows improvement in the tensile strength in joint region  45  as compared to the case forming a continuous joint in the region corresponding to joint region  45 . As in the case of joint region  45 , joint parts  40  are formed so as to satisfy the condition of La&gt;Lc in each of the other joint regions  46  and  47 . This also allows improvement in the tensile strength in each of joint regions  46  and  47 . The tensile strength of separator  22  can be improved throughout flat battery  10  by improving the tensile strength in each joint region  45 ,  46 ,  47 . 
     As described above, it is possible according to the present another embodiment to refill the stacked electrode assembly with electrolytic solution  51  from holding part  50  and prevent breakage in joint parts  40 , whereby the secondary battery can secure battery performance during long-term use. 
     Although one joint region is constituted by four adjacent joint parts in the present another embodiment, the number of joint parts in one joint region is not particularly limited and can be adjusted as appropriate. The number of joint regions in one flat battery is not also particularly limited and can be adjusted as appropriate although three joint regions are provided in one flat battery in the present another embodiment. 
     Modifications 
     The present invention is not limited to the above embodiments. Various changes and modifications are possible within the scope of the present invention. Although both of positive and negative tabs  41  and  42  are provided on one side of exterior member  30  in flat battery  20  in the above embodiment, the present invention is applicable to the secondary battery in which positive and negative tabs are provided on different sides. It is alternatively feasible to provide holding part  50  and tabs on the same side. 
     Examples 
     Examples of flat battery  10  with a plurality of joint parts  40  will be next described below. It is herein noted that the secondary battery according to the present invention is not limited to the following examples. 
     The operation conditions of Examples and Comparative Example are indicated in TABLE 1. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Dis- 
                 Number 
                 Fused 
                   
                 Fused 
               
               
                   
                 Width 
                 Length 
                 tance 
                 of 
                 joint 
                 Perim- 
                 joint 
               
               
                   
                 W1 
                 W3 
                 W2 
                 fused 
                 area 
                 eter 
                 strength 
               
               
                   
                 (mm) 
                 (mm) 
                 (mm) 
                 joints 
                 (mm 2 ) 
                 (mm) 
                 (N) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Exam- 
                 2 
                 2 
                 1 
                 6 
                 24 
                 48 
                 210 
               
               
                 ple 1 
               
               
                 Exam- 
                 1 
                 1 
                 1 
                 12 
                 12 
                 48 
                 212 
               
               
                 ple 2 
               
               
                 Compa- 
                 12 
                 2 
                 0 
                 1 
                 24 
                 28 
                 128 
               
               
                 rative 
               
               
                 Exam- 
               
               
                 ple 
               
               
                   
               
            
           
         
       
     
     In each of Examples 1 and 2, flat battery  10  was provided to satisfy the condition: the sum of the perimeters of the fused joints (joint parts  40 ) (La)&gt;the perimeter of rectangle  63  of minimum area enclosing all of fused joints  40 . The fused joints of Example 1 and the fused joints of Example 2 are respectively schematically shown in  FIGS. 12(A) and 12(B) . 
     Examples 1 and 2 were different from each other in terms of the size of fused joints  40 , the distance between fused joints  40  and the number of fused joints  40 . The total area of fused joints  40  was made larger in Example 1 than in Example 2. In each of Examples 1 and 2, holding part  50  was formed between the plurality of fused joints  40 . 
     In Comparative Example, a conventional flat battery with continuous fused joint  140  was provided (the number of fused joints was 1). In  FIG. 12(C) , fused joint  140  of Comparative Example is shown. The total area of fused joint  140  was the same as that of Example 2. 
     Flat batteries  10  of Examples 1 and 2 had a higher level of fused joint strength than that of the conventional flat battery of Comparative Example as is seen from TABLE 1. As is seen from comparison of Example 1 and Comparative Example, flat battery  10  of Example 1 had a higher fused joint strength than that of Comparative Example even though there was no difference in the total area of the fused joints. It has thus been shown by the above results that flat battery  10  according to the present invention can ensure improved strength against tensile force applied to fused joints  40  as compared to the conventional flat battery.