Patent Publication Number: US-8974549-B2

Title: Manufacturing method for film-covered electrical device

Description:
TECHNICAL FIELD 
     The present invention relates to a manufacturing method and manufacturing apparatus for a film-covered electrical device, which is an electrical device element contained in laminate film, typified by a battery or capacitor. 
     BACKGROUND ART 
     Electrical devices typified by batteries and electrolytic capacitors are manufactured by filling a case which is made of metal or the like and which contains an electrode group with an electrolytic solution and then sealing the case, where the electrode group is an electrical device element. 
     Conventionally, a case placed upright is filled with a predetermined amount of electrolytic solution and left at rest for an extended period of time to allow the electrolytic solution to gradually permeate spaces in the electrode group. However, since an electrode group is generally made of electrode plates stacked densely, it takes time to allow the electrolytic solution to permeate the spaces in the electrode group. The case needs to be left at rest, for example, for a whole day and night in order for the electrolytic solution to permeate the spaces among the electrodes on its own. This means very inefficient production. 
     Also, since the electrolytic solution is absorbed very slowly, if the required amount of electrolytic solution is supplied at once into the case, the electrolytic solution will overflow the case. Methods adopted to deal with this situation include a method in which a watertight cover is placed on an opening of the case and filled with a predetermined amount of electrolytic solution. However, the method involves placing the covers one by one on the cases, making it difficult to increase manufacturing efficiency. 
     Japanese Patent No. 3467135 discloses an electrolytic-solution filling method intended to solve this problem. The method depressurizes an opening of a case, fills the opening with the electrolytic solution to temporarily form a pool, fills the depressurized case with the electrolytic solution, allows the electrolytic solution to permeate spaces in an electrode group, and then increases pressure in the case to make the electrolytic solution in the pool permeate the spaces in the electrode group. 
     By depressurizing the case once, the method expels air from the spaces in the electrode group so that the air will not obstruct permeation of the electrolytic solution. After creating a condition in which the electrode can permeate the spaces easily, the method fills the case with the electrolytic solution. Then, the method further pressurizes the case to cause the electrolytic solution in the pool to permeate. The combination of depressurization and pressurization not only reduces the time required for the electrolytic solution to be absorbed, but also prevents the electrolytic solution from spattering upon pressure release. 
     DISCLOSURE OF THE INVENTION 
     In addition to electrical devices which use a metal case, film-covered electrical devices which use a laminate material for outer covering have been developed, where the laminate material is a thin film created by laminating a metal layer of aluminum or the like and heat-fusible resin layers via adhesive layers. Generally, laminate materials have a structure in which a thin metal layer of aluminum or the like has both sides coated with a thin resin layer. The laminate materials are resistant to acid and alkali, and are lightweight and flexible. 
     However, the laminate film for film-covered electrical devices has flexibility, unlike metal cases. That is, the laminate film, which deforms easily, has a problem not encountered by metal cases which hardly deforms when filled with an electrolytic solution. 
     The electrolytic solution that is poured into an opening of the laminate film flows into between principal surfaces of the electrical device element and the laminate film without forming a pool at the opening. This is because the laminate film is flexible. Thus, the method disclosed in Japanese Patent No. 3467135 cannot be adopted as it is, where the method seals the electrical device element temporarily from the outside by means of the pool and impregnates the electrolytic solution forming the pool into the electrical device element by means of the pressure difference between the electrical device element and the outside. 
     Also, the electrolytic solution is not impregnated into the electrode group at uniform speed, but is impregnated irregularly depending on the area to be impregnated. The irregular impregnation with the electrolytic solution appears as creases in the surfaces of the laminate film due to the flexibility of the laminate film. 
     Also, the irregular impregnation with the electrolytic solution partially produces, on the surfaces, areas with low ionic conductivity between positive and negative layers, resulting in reduced electrical performance characteristics of the battery. Besides, since the laminate film deforms easily, binding forces between electrode layers are weak, and consequently the irregular impregnation can cause separators to be creased. 
     In view of the above circumstances, an object of the present invention is to provide a manufacturing method and manufacturing apparatus for a film-covered electrical device, where the manufacturing method and apparatus are capable of filling the film-covered electrical device with an electrolytic solution in a short time without causing irregular impregnation when impregnating the electrolytic solution or producing creases in separators. 
     To achieve the above object, the present invention provides a manufacturing method for a film-covered electrical device, comprising: holding, in a vacuum container, a bag-shaped laminate film that contains a power generating element and that has an opening, by pinching the bag-shaped laminate film at positions corresponding to two principal surfaces of the power generating element, the power generating element having a positive layer and a negative layer stacked via a separator; reducing pressure in the vacuum container; pouring an electrolytic solution into the bag-shaped laminate film through the opening with the pressure in the vacuum container kept reduced until the entire power generating element is immersed in the electrolytic solution; and increasing the reduced pressure in the vacuum container. 
     The present invention makes it possible to fill the laminate film with the electrolytic solution in a short time without producing creases in the separator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a configuration of a filling apparatus used to fill a film-covered battery according to the present invention with an electrolytic solution; 
         FIG. 2  is a sectional view showing an example of a holding fixture according to the present invention; 
         FIG. 3  is a perspective view showing another example of a holding fixture according to the present invention; 
         FIG. 4  is a schematic perspective view of battery elements illustrating a principal surface and edge surfaces of the battery elements; 
         FIG. 5  is a graph showing a relationship between the time required for filling and amount of the electrolytic solution absorbed into element, where a pushing force on the principal surface is used as a parameter; 
         FIG. 6A  is a perspective view of a film-covered electrical device which is an application example of the present invention; 
         FIG. 6B  is a front view of a film-covered electrical device which is another application example of the present invention; 
         FIG. 6C  is a schematic diagram of a battery element accommodated in a laminate film as viewed in the direction of a principal surface to illustrate pooling conditions of the electrolytic solution; 
         FIG. 7  is a schematic diagram of a battery element divided into four areas into which the electrolytic solution permeates through respective four edge surfaces; 
         FIG. 8A  is a schematic diagram illustrating how the electrolytic solution is absorbed through the edge surfaces in a final stage of filling; 
         FIG. 8B  is a schematic diagram showing how the electrolytic solution pooled at top and on both sides has been absorbed before the electrolytic solution pooled in the bottom is absorbed; 
         FIG. 8C  is a schematic diagram showing how the electrolytic solution pooled at top and on both sides is being absorbed after the electrolytic solution pooled in the bottom has been absorbed; 
         FIG. 9  is a schematic diagram showing an example of a system for supplying an electrolytic solution to bag-shaped laminate film containing a plurality of battery elements; 
         FIG. 10  is a schematic diagram showing another example of a system for supplying an electrolytic solution to bag-shaped laminate film containing a plurality of battery elements; 
         FIG. 11  is a schematic diagram showing another example of a system for supplying an electrolytic solution to bag-shaped laminate film containing a plurality of battery elements; 
         FIG. 12  is a schematic diagram showing another example of a system for supplying an electrolytic solution to bag-shaped laminate film containing a plurality of battery elements; 
         FIG. 13  is a schematic diagram showing an example of a mechanism for maintaining the laminate film in an open state; 
         FIG. 14A  is a schematic diagram showing another example of a mechanism for maintaining the laminate film in an open state; 
         FIG. 14B  is a schematic diagram illustrating a configuration and travel directions of claws in the mechanism shown in  14 A; 
         FIG. 15A  is a schematic diagram showing another example of a mechanism for maintaining the laminate film in an open state; 
         FIG. 15B  is a schematic diagram illustrating a configuration and travel directions of claws in the mechanism shown in  15 A; 
         FIG. 16  is a schematic diagram showing an example of a mechanism for cleaning that part of laminate film which is to be heat-fused; 
         FIG. 17  is a schematic diagram showing another example of a mechanism for cleaning that part of laminate film which is to be heat-fused; and 
         FIG. 18  is a flowchart illustrating a manufacturing method for a film-covered electrical device according to the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Next, an embodiment of the present invention will be described with reference to the drawings. 
       FIG. 1  is a schematic diagram showing a configuration of a filling apparatus used to fill a film-covered battery according to the present invention with an electrolytic solution. 
     (Film-Covered Battery) 
     First, a configuration of film-covered battery  10  according to the present embodiment will be outlined. 
     Film-covered battery  10  includes battery element  11 , a positive collector and negative collector installed on battery element  11 , an outer cover made of one sheet of laminate film  12  and containing battery element  11  together with electrolytic solution  20 , a positive tab connected to the positive collector, and a negative tab connected to the negative collector. 
     Battery element  11  includes a plurality of positive plates and negative plates stacked alternately via separators. 
     Herein, regarding battery element  11 , surfaces perpendicular to a stacking direction will be referred to as principal surfaces  11   a  and surfaces parallel to the stacking direction will be referred to as edge surfaces  11   b , as shown in  FIG. 4 . 
     Each positive plate is made of aluminum foil coated with a positive electrode while each negative plate is made of copper foil coated with a negative electrode. Extension strips, which extend from a stack area, are not coated with electrode material. The extension strips from the positive plates are joined together by ultrasonic welding, and so are the extension strips from the negative plates, to form, respectively, the positive collector and negative collector which are relays. At the same time, the positive tab is connected to the positive collector and the negative tab is connected to the negative collector also by ultrasonic welding. 
     One sheet of laminate film  12  is folded into two to surround battery element  11  by sandwiching battery element  11  from both sides in the thickness direction of battery element  11 . Laminate film  12  is a laminate of heat-fusible resin layer, a metal layer, and a protective layer. With the heat-fusible resin layers of PP (polypropylene) placed facing the battery, laminate film  12  seals battery element  11  as the heat-fusible resin layers are fused together by heat. 
     Electrolytic solution  20  is prepared using 1 mol/liter of LiPF 6  as a supporting electrolyte and a mixture of propylene carbonate and ethylene carbonate (ratio by mass is 50:50) as a solvent. 
     (Filling Apparatus) 
     Next, a configuration of filling apparatus  1  according to the present embodiment will be described. 
     Filling apparatus  1  includes vacuum container  2 , holding fixture  3 , electrolytic-solution supply line  4 , evacuation line  5 , gas inlet line  6 , and controller  7 . 
     Controller  7  controls the operation of holding fixture  3 , a vacuum pump (not shown) connected to evacuation line  5 , and liquid delivery system  41  connected to electrolytic-solution supply line  4 . Operation of the components controlled by controller  7  will be described in detail below. 
     Vacuum container  2  houses holding fixture  3 . Wall surfaces of vacuum container  2  are connected with electrolytic-solution supply line  4 , evacuation line  5 , and gas inlet line  6 . 
     Holding fixture  3  holds bag-shaped laminate film  12  by pinching laminate film  12  from both sides (both principal surfaces  11   a ) in the thickness direction of battery element  11  when electrolytic solution  20  is filled into bag-shaped laminate film  12  containing battery element  11 . 
     Preferably a space (V L  in  FIG. 6C ) is provided below battery element  11  to pool electrolytic solution  20  as described later, and thus it is preferable to place battery element  11  above bottom surface  12   b  of laminate film  12 , with a space provided therebetween. As shown in  FIG. 6B , positive tab  104   a  and negative tab  104   b  connected to battery element  11  face away from each other. Furthermore, positive tab  104   a  and negative tab  104   b  are fixed in place beforehand, being sealed in laminate film  12 . Consequently, since battery element  11  is supported by positive tab  104   a  and negative tab  104   b , battery element  11  can be positioned, being separated from bottom surface  12   b  of laminate film  12 . 
     However, laminate film  12  of film-covered battery  10  is flexible. Consequently, if electrolytic solution  20  is poured without laminate film  12  being supported in some way or other, electrolytic solution  20  will flow around to principal surfaces  11   a  of battery element  11  without pooling on the sides of edge surfaces  11   b  of battery element  11 . Also, this may cause the separators to be creased if the battery element is impregnated irregularly with electrolytic solution  20 . 
     Thus, holding fixture  3  pinches laminate film  12  at positions corresponding to principal surfaces  11   a  of battery element  11 . This prevents poured electrolytic solution  20  from flowing around to principal surfaces  11   a  of battery element  11  and allows electrolytic solution  20  to be pooled once, surrounding edge surfaces  11   b  of battery element  11 . Besides, even if the battery element is impregnated irregularly with electrolytic solution  20 , holding fixture  3  holds down battery element  11  via laminate film  12  at positions corresponding to principal surfaces  11   a  of battery element  11 , thereby preventing the separators from being creased. 
     Now, a configuration example of holding fixture  3  and a method for holding laminate film  12  and battery element  11  will be described more concretely. 
       FIG. 2  shows an example of holding fixture  3 . Holding fixture  3 A shown in  FIG. 2  includes a plurality of plate members  3   a  fixedly placed at predetermined intervals. The intervals between plate members  3   a  form sockets  3   b  which receive bag-shaped laminate film  12  containing battery element  11 . 
       FIG. 3  shows another example of holding fixture  3 . Holding fixture  3 B shown in  FIG. 3  has a structure in which fixed end plate  3   d  and a plurality of movable plates  3   e  placed at predetermined intervals are mounted on a base  3   f . Fixed end plate  3   d  is mounted at an end of base  3   f . A plurality of movable plates  3   e  are arranged parallel to fixed end plate  3   d . Fixed end plate  3   d  is fixed to base  3   f . On the other hand, movable plates  3   e  are installed in such a way as to be movable on base  3   f . Piston  3   c  is installed on the side opposite to fixed end plate  3   d . Piston  3   c  pushes movable plate  3   e  mounted at the end opposite to fixed end plate  3   d.    
     As shown in  FIG. 3 , pieces of bag-shaped laminate film  12  containing battery element  11  are inserted one each between fixed end plate  3   d  and movable plate  3   e  or between each pair of adjacent movable plates  3   e . When piston  3   c  pushes movable plates  3   e , pushing force is applied to bag-shaped laminate film  12  containing battery element  11 . That is, holding fixture  3 B can adjust pushing force on film-covered battery  10  during filling with the electrolytic solution. 
     Incidentally, in  FIG. 1 , bag-shaped laminate film  12  containing battery element  11  is pinched by holding fixture  3  in such a way that positive tab  104   a  and negative tab  104   b  described later will face in a direction perpendicular to the plane of the paper. 
     Laminate film  12  containing battery element  11  and being pinched by holding fixture  3  has been shaped like a bag. That is, laminate film  12  is heat-fused on three sides other than the side along which laminate film  12  is folded. Specifically, laminate film  12  has been heat-fused on the two sides from which positive tab  104   a  and negative tab  104   b  extend, but the side opposite the folded side has not yet been heat-fused. Laminate film  12  is bag-shaped to allow electrolytic solution  20  to be poured through the side has not yet been heat-fused with the side serving as opening  12   a . That is, in  FIG. 1 , holding fixture  3  pinches bag-shaped laminate film  12  from both sides in the thickness direction of battery element  11  with the unfused side up in such a way that positive tab  104   a  and negative tab  104   b  will face in the direction perpendicular to the plane of the paper. Incidentally, although one sheet of laminate film  12  folded into two is taken as an example according to the present embodiment, the present invention is not limited to this and two sheets of laminate film may be used alternatively. In that case, the laminate film is shaped like a bag in advance by heat-fusing three sides. 
     One end of electrolytic-solution supply line  4  is connected to a tank (not shown) which stores the electrolytic solution. The other end is installed on an upper wall of vacuum container  2 , being positioned such that electrolytic solution  20  supplied through electrolytic-solution supply line  4  can be poured through opening  12   a  of laminate film  12  which opens upward. 
     Evacuation line  5 , which is used to evacuate vacuum container  2 , has valve  5   a  and vacuum pump  5   b.    
     Gas inlet line  6  is used to introduce dry air or inert gas into vacuum container  2  evacuated by evacuation line  5  and thereby break the vacuum in vacuum container  2 . Gas inlet line  6  includes valve  6   a  and a gas storage tank (not shown). 
     (Pressure Range of Pushing Force) 
     Next, description will be given of a pressure range of the pushing force exerted by holding fixture  3  on film-covered battery  10  which is being filled with the electrolytic solution. 
     If excessive pressure is applied by holding fixture  3  to principal surfaces  11   a  of battery element  11 , battery element  11  will be squeezed excessively, preventing electrolytic solution  20  from infiltrating the battery element. Consequently, pores in the separators and electrodes cannot be impregnated with electrolytic solution  20 . This is because electrolytic solution  20  enters the battery element mainly by seeping into contact surfaces of the separators and electrodes by capillary action, and the application of too much pressure makes it difficult for the solution to infiltrate the contact surfaces. On the other hand, if the applied pressure is too low, spaces may be formed among positive plates, separators, and negative plates, electrolytic solution  20  may infiltrate into the spaces, causing the separators to lose firmness or become creased, and the creases may remain after the battery is completed. 
     A relationship between the time required for filling and amount of the electrolytic solution absorbed into element is shown in  FIG. 5 , where a pushing force on the principal surface is used as a parameter. 
       FIG. 5  shows measurement results obtained by a filling method which uses the pressure difference and measurement results obtained by a filling method which does not use a pressure difference. The filling method which uses a pressure difference is a method in which electrolytic solution  20  pooled at locations facing edge surfaces  11   b  is sucked by the negative pressure of battery element  11  kept under vacuum. This method is a feature of the present invention and will be described in detail later. On the other hand, the method which does not use a pressure difference is a method which causes battery element  11  to be impregnated with electrolytic solution  20  by gravity, by the capillary action of the contact surfaces between the electrodes and separators, and by the capillary action of the pores in the separators without creating any pressure difference between the inside and outside of battery element  11 . 
     Regarding the filling method which uses a pressure difference, an experiment was conducted under two conditions: the pushing force of holding fixture  3  was set to 0.25 kPa and 0.5 kPa. Regarding the filling method which does not use a pressure difference, the experiment was conducted only under one condition: the pushing force of holding fixture  3  was set to 0.5 kPa. 
     It can be seen from  FIG. 5 , that the time required for filling with a prescribed amount of electrolytic solution is shorter when the pressure of 0.25 kPa is applied than when the pressure of 0.5 kPa is applied. At first, battery element  11  is impregnated with the electrolytic solution at approximately the same rate under 0.25 kPa and 0.5 kPa, but toward the end of filling, i.e., when the prescribed amount of electrolytic solution has been almost filled, the impregnation rate slows down and thus a longer time is required for filling when the higher pressure of 0.5 kPa is used. In short, if the pushing force exerted by holding fixture  3  is too high, a longer time will be required for filling. 
     However, even if the same pressure of 0.5 kPa is used, the filling method which uses a pressure difference can finish filling in a far shorter time than the filling method which does not use a pressure difference. 
     In the above experiment, to prevent the impregnation rate from slowing down toward the end of filling, the pushing force of the holding fixture may be reduced gradually as the filling proceeds from the initial stage to the end of impregnation. Also, the pushing force of the holding fixture may be reduced to almost zero near the end of impregnation. 
     (Electrolytic-Solution Filling Method) 
     Next, a method of filling with electrolytic solution  20  using filling apparatus  1  according to the present embodiment will be described with reference to a flowchart of  FIG. 18 . 
     The method of filling with electrolytic solution  20  according to the present embodiment fills battery element  11  with electrolytic solution  20  using a pressure difference. The following procedures are used for filling with electrolytic solution  20 . 
     First, in vacuum container  2 , filling apparatus  1  pinches laminate film  12  containing battery element  11  using holding fixture  3  and thereby holds the entire principal surfaces of battery element  11  (Step S 1 ). When laminate film  12  is held by holding fixture  3 , the top side of laminate film  12  containing battery element  11  forms opening  12   a  which remains to be heat-fused. 
     Next, with valve  5   a  open, filling apparatus  1  operates vacuum pump  5   b  of evacuation line  5  to depressurize vacuum container  2  (Step S 2 ). When a predetermined degree of vacuum is reached, filling apparatus  1  closes valve  5   a . In this state, the pressures in vacuum container  2  and in battery element  11  contained therein have equally been reduced to a predetermined level. 
     Next, using a mechanism for maintaining an open state (described later), filling apparatus  1  maintains opening  12   a  of laminate film  12  in an open state (Step S 3 ). 
     Next, filling apparatus  1  supplies electrolytic solution  20  through electrolytic-solution supply line  4  (Step S 4 ) and pours supplied electrolytic solution  20  through opening  12   a  at the top of laminate film  12  (Step S 5 ). Incidentally, the supplying of electrolytic solution  20  through electrolytic-solution supply line  4  will be described in detail later. Since battery element  11  has its entire principal surfaces held in the thickness direction by holding fixture  3 , there is no space for electrolytic solution  20  to flow in on the sides of principal surfaces  11   a . Also, since battery element  11  is held by holding fixture  3 , there is little space among the positive plates, separators, and negative plates for electrolytic solution  20  to flow in. Furthermore, the pressures in vacuum container  2  and in battery element  11  contained therein have equally been reduced to a predetermined level. Consequently, electrolytic solution  20  is not sucked into battery element  11  by negative pressure in battery element  11 . Electrolytic solution  20  is poured until entire battery element  11  is immersed in electrolytic solution  20 . Consequently, electrolytic solution  20  is pooled on the sides of edge surfaces  11   b  of battery element  11 . That is, of the six surfaces of battery element  11 , four surfaces—top, bottom, and two lateral edge surfaces—excluding principal surfaces  11   a  are surrounded by electrolytic solution  20 . 
     Next, filling apparatus  1  opens valve  6   a  of gas inlet line  6  to introduce gas into vacuum container  2 , and thereby increases the pressure in vacuum container  2  (Step S 6 ). Although the introduction of gas raises the pressure in vacuum container  2 , the inside of battery element  11  surrounded by electrolytic solution  20  remains depressurized as a result of evacuation. Consequently, there is a pressure difference between the inside of battery element  11  surrounded by electrolytic solution  20  and the inside of vacuum container  2 . That is, since there is a vacuum in battery element  11 , electrolytic solution  20  is sucked into battery element  11  by the negative pressure, thereby filling battery element  11  with electrolytic solution  20  rapidly. 
     Moreover, although electrolytic solution  20  enters through four edge surfaces  11   b , since the entire principal surfaces of battery element  11  are held by holding fixture  3 , electrolytic solution  20  does not enter through the principal surfaces. This prevents the separators from being creased. In the above example, holding fixture  3  holds the entire areas of principal surfaces  11   a  of battery element  11 , and this is most desirable. However, the operation and effect of the present invention can be achieved even if there are areas which are not held by holding fixture  3  in part of principal surfaces  11   a . For example, most parts of principal surfaces  11   a  including the centers may be held, leaving peripheral parts. 
     Incidentally, if the amount of electrolytic solution  20  which can be pooled around edge surfaces  11   b  is less than the amount required for film-covered battery  10 , the above processes may be repeated beginning with the process of pouring electrolytic solution  20  until the required amount is reached. Also, the process of pouring the electrolytic solution and the process of raising the pressure in vacuum container  2  may be performed simultaneously. One of the processes may be performed continuously while the other process is performed intermittently. 
     As described above, although flexible laminate film is used as covering material for the battery, the method of filling with electrolytic solution  20  according to the present invention can keep laminated surfaces taut and flat, prevent the separators from being creased during filling with electrolytic solution  20 , and carry out filling rapidly. 
     (Pooling Conditions of Electrolytic Solution) 
     Next, pooling conditions of electrolytic solution  20  according to the present embodiment will be described with reference to  FIGS. 6A to 6C . 
     Battery element  11  is impregnated with electrolytic solution  20  through edge surfaces  11   b  of battery element  11 . Battery element  11 , which is rectangular in shape, has four edge surfaces  11   b . To reduce the filling time and prevent creasing of laminate film  12 , it is important to fill battery element  11  with electrolytic solution  20  using all four edge surfaces  11   b  effectively. 
       FIG. 6A  is a perspective view of a film-covered electrical device which is an application example of the present invention. Extension strip  43  made of metal foil and used to draw electric current extends from the electrode in each layer of battery element  11 . Extension strips  43  of the positive layers are connected to positive tab  104   a  in positive collector  103   a . Similarly, extension strips  43  of the negative layers are connected to negative tab  104   b  in negative collector  103   b . The film-covered electrical device shown in  FIG. 6A  has recess  12   e  formed in laminate film  12  to house battery element  11 . The film-covered electrical device shown in  FIG. 6A  is a type in which two sheets of laminate film are stacked face to face and the four sides are sealed. 
     However, the film-covered electrical device according to the present invention may use flat laminate film without a recess. Alternatively, the present invention may be applied to a film-covered electrical device of a type in which three sides are sealed by folding a single sheet of laminate film. A front view of such an example is shown in  FIG. 6B . In  FIG. 6B , bottom surface  12   b  is folded to form a side. 
       FIG. 6C , which is a schematic diagram of a battery element accommodated in a laminate film as viewed in the direction of a principal surface, illustrates pooling conditions of the electrolytic solution.  FIG. 6C  shows a part surrounded by sides  12   a ,  12   b ,  12   c , and  12   d  (sides  12   c  and  12   d  correspond to inner borders of sealing) in  FIG. 6B . Incidentally, positive tab  104   a , negative tab  104   b , extension strips  43 , positive collector  103   a , and negative collector  103   b  are omitted in  FIG. 6C . 
     Regarding the size of battery element  11 , the length is denoted by L and the width is denoted by W. Inner space of laminate film  12  is larger in size than battery element  11 . Electrolytic solution  20  is pooled temporarily in spaces produced by the size difference. Each space is formed by one of edge surfaces  11   b  of battery element  11  and sealed end  12   f  closest to edge surface  11   b  or the bend of the laminate film. Hereinafter, the volumes of the spaces will be referred to as pooling volumes. 
     In  FIG. 6C , the pooling volumes on both sides of film-covered battery  10  placed in a vertical position are denoted by V W  and the pooling volume at the bottom is denoted by V L . 
     Pooling volumes V W  of two spaces are shown in  FIG. 6C : the space formed by left edge surface  11   b  and left sealed end  12   f , and the space formed by right edge surface  11   b  and right sealed end  12   f . That is, pooling volumes V W  are volumes of the spaces formed between the edges of laminate film  12  and two edge surfaces  11   b  which join the other two edge surfaces  11   b  of battery element  11 —edge surface  11   b  located at opening  12   a  and edge surface  11   b  located at the bottom (opposite to edge surface  11   b  located at opening  12   a ). 
     On the other hand, pooling volume V L  is the volume of space formed by edge surface  11   b  at the bottom and the bend of laminate film  12  (the base of laminate film  12 ). That is, pooling volume V L  is the volume of space formed between an edge of laminate film  12  and one of edge surfaces  11   b  of battery element  11 , i.e., bottom edge surface  11   b  opposite to edge surface  11   b  located at opening  12   a.    
     The upper border of V W  is flush with the upper end of battery element  11 . Also, pooling volumes V W  and pooling volume V L  are bounded by lines each of which joins a corner of battery element  11  and a corner of laminate film  12 . If there are some other objects in the spaces, the volumes occupied by the objects are deducted. Examples of the other objects as mentioned here include extension strips  43 , positive collector  103   a , and negative collector  103   b  in  FIG. 6B  as well as insulating coating members covering the collectors. 
     A necessary condition for pooling electrolytic solution  20  on a side of edge surface  11   b  located at opening  12   a , i.e., in the upper part, is given by:
 
0&lt;2 V   W   +V   L   &lt;V   TOTAL   (1)
 
where V TOTAL  is the total volume of electrolytic solution  20  poured into laminate film  12  during filling.
 
     On the other hand, a condition for filling electrolytic solution  20  into the separators not only through edge surface  11   b  at opening  12   a , but also through edge surfaces  11   b  on both sides and at the bottom is given by:
 
0 &lt;V   W′ 0 &lt;V   L   (2)
 
To pour electrolytic solution  20  through all four edge surfaces  11   b , it is necessary to satisfy condition equations (1) and (2).
 
     Next, the amounts of electrolytic solution  20  absorbed through four edge surfaces  11   b —edge surface  11   b  at opening  12   a , edge surfaces  11   b  on both sides, and edge surface  11   b  at the bottom—will be estimated.  FIG. 7  shows a schematic diagram of battery element  11  divided into four areas into which electrolytic solution  20  permeates through respective four edge surfaces  11   b.    
     Area S W  is a surface area of the area into which electrolytic solution  20  infiltrates through each of edge surfaces  11   b  on both sides. Area S L  is a surface area of the area into which electrolytic solution  20  infiltrates through edge surface  11   b  at opening  12   a  or edge surface  11   b  at the bottom. Area S W  and area S L  are given as follows using length L and width W:
 
 S   W =½ ×W/ 2 ×W=W   2 /4  (3)
 
 S   L   =WL/ 2 −W   2 /4  (4)
 
     The ratio between the amounts of electrolytic solution  20  absorbed through two groups of edge surfaces  11   b  per unit time is given by:
 
 S   W   :S   L   =W   2 /4:( W/ 2)·( L−W/ 2)= W/ 2 :L−W/   (5)
 
     When electrolytic solution  20  is absorbed through all four edge surfaces  11   b , ideally absorption through all four edge surfaces  11   b  will be finished simultaneously. 
     It is not desirable that absorption of electrolytic solution  20  through edge surfaces  11   b  on both sides be finished earlier than absorption through edge surface  11   b  at the bottom. Reasons for this will be described with reference to  FIGS. 8A and 8B . 
     At a stage shown in  FIG. 8A , filling with electrolytic solution  20  is carried out through all four edge surfaces  11   b . Then, it is assumed that the filling proceeds to its final stage shown in  FIG. 8B . In  FIG. 8B , electrolytic solution  20  pooled above edge surface  11   b  at opening  12   a  and in the spaces of pooling volume V W  on both sides has been absorbed. On the other hand, electrolytic solution  20  pooled in the space of pooling volume V L  on a side of edge surface  11   b  at the bottom has not yet been absorbed. Electrolytic solution  20  remaining at this time has to be absorbed only through edge surface  11   b  at the bottom, thus extending the time until the completion of filling. 
     Even if absorption does not finish all at once, if absorption through edge surface  11   b  at the bottom finishes earlier than absorption through edge surfaces  11   b  on both sides, there is no problem. Reasons for this will be described with reference to  FIG. 8C . 
     It is assumed that the filling proceeds to its final stage shown in  FIG. 8C . In  FIG. 8C , electrolytic solution  20  pooled above edge surface  11   b  at opening  12   a  and electrolytic solution  20  pooled in the space of pooling volume V L  on the side of edge surface  11   b  at the bottom have been absorbed. On the other hand, electrolytic solution  20  pooled in the spaces of pooling volume V W  on both sides has not yet been absorbed. In this case, electrolytic solution  20  remaining in the spaces of pooling volume V W  on both sides flows to edge surface  11   b  at the bottom by gravity. Thus, even after all electrolytic solution  20  in the space of pooling volume V L  has been absorbed through edge surface  11   b  at the bottom, edge surface  11   b  at the bottom can absorb electrolytic solution  20  which flows in without being absorbed through edge surfaces  11   b  on both sides. This makes it possible to reduce the time required for filling the battery with electrolytic solution  20 . 
     Conditions for at least absorption through edge surface  11   b  at the bottom to finish earlier than absorption through edge surfaces  11   b  on both sides are given by:
 
 V   L   /S   L   ≦V   W   /S   W  
 
 V   L ≦( S   L   /S   W )· V   W  
 
 V   L ≦(2 /W )·( L−W/ 2)· V   W   (6)
 
     Thus, filling conditions which need to be satisfied in order to fill electrolytic solution  20  into battery element  11  in a short time are given by above condition equations (1), (2), and (6). 
     (System for Supplying Electrolytic Solution) 
     Next, a system for supplying an electrolytic solution with the filling apparatus according to the present embodiment will be described. 
     In  FIG. 1  intended to illustrate a basic configuration of the present invention, a single film-covered battery  10  is housed in vacuum container  2  and a single electrolytic-solution supply line  4  is provided. 
       FIG. 9  shows an example of a system for supplying an electrolytic solution to a plurality of film-covered batteries. 
     A filling apparatus shown in  FIG. 9  includes a plurality of holding fixtures  3 , relay containers  30  installed for respective holding fixtures  3 , and needle  4   a  connected to electrolytic-solution supply line  4 , all of which are housed in vacuum container  2 . 
     Needle  4   a  is installed in such a way as to be able to move above holding fixtures  3 . 
     Each relay container  30  has opening  30   a , main body  30   b , and supply port  30   c . Opening  30   a  receives electrolytic solution  20  supplied from needle  4   a . Main body  30   b  temporarily pools electrolytic solution  20 . Supply port  30   c  supplies electrolytic solution  20  temporarily pooled in main body  30   b  into bag-shaped laminate film  12  containing battery element  11 . Relay containers  30  are installed above openings  12   a  of respective pieces of laminate film  12  and below needle  4   a.    
     With the present configuration, electrolytic solution  20  is not supplied directly into bag-shaped laminate film  12  containing battery element  11 . That is, with the present configuration, electrolytic solution  20  is supplied into bag-shaped laminate film  12  via relay containers  30  which temporarily pool electrolytic solution  20  supplied from needle  4   a.    
     According to the present embodiment, electrolytic solution  20  is supplied as follows. 
     First, needle  4   a  supplies electrolytic solution  20  into one relay container  30 . Electrolytic solution  20  is pooled in relay container  30  and then supplied into bag-shaped laminate film  12 . In the meantime, needle  4   a  supplies electrolytic solution  20  into adjacent another relay container  30 . Electrolytic solution  20  is pooled in given relay container  30  and then supplied into bag-shaped laminate film  12 . In this way, according to the present embodiment, electrolytic solution  20  is pooled in relay containers  30  once before being supplied into bag-shaped laminate film  12 . 
     As described above, according to the present embodiment, electrolytic solution  20  is supplied into different pieces of bag-shaped laminate film  12  by moving needle  4   a . However, the amount of electrolytic solution  20  absorbed by battery element  11  can exceed the amount of electrolytic solution  20  supplied from traveling needle  4   a . To deal with this, in supplying electrolytic solution  20  to multiple pieces of bag-shaped laminate film  12  by moving single needle  4   a , the system pools electrolytic solution  20  temporarily in relay containers  30  while supplying electrolytic solution  20  in the meantime to different pieces of bag-shaped laminate film  12  one after another by moving needle  4   a.    
       FIG. 10  shows another example of a system for supplying an electrolytic solution to bag-shaped laminate film  12  containing a plurality of battery elements  11 . 
     Configuration shown in  FIG. 10  is the same as the configuration in  FIG. 9  except that valve  30   d  is installed at supply port  30   c  of each relay container  30 . According to the present configuration, electrolytic solution  20  is temporarily pooled in relay containers  30  with valves  30   d  closed and when electrolytic solution  20  is pooled in all relay containers  30 , electrolytic solution  20  can be supplied into different pieces of bag-shaped laminate film  12  all together by opening all valves  30   d  at once. This makes it possible to supply electrolytic solution  20  into different pieces of bag-shaped laminate film  12  uniformly without being affected by the traveling speed of needle  4   a , the supply rate of electrolytic solution  20  that is supplied by needle  4   a , the supply rate of electrolytic solution  20  from relay containers  30 , or the like. 
       FIG. 11  shows another example of a system for supplying an electrolytic solution to bag-shaped laminate film containing a plurality of battery elements. 
     Configuration shown in  FIG. 11  is the same as the configuration in  FIG. 10  except that relay containers  30  are installed externally to vacuum container  2 . More specifically, opening  30   a , main body  30   b , and valve  30   d  of each relay container  30  are installed outside the vacuum container  2  while the tip of supply port  30   c  is installed in vacuum container  2 . 
     According to the configurations in  FIGS. 9 and 10 , since relay containers  30  are installed in vacuum container  2 , electrolytic solution  20  is discharged from relay container  30  by gravity. On the other hand, the present configuration, which can use a pressure difference in addition to the force of gravity to supply an electrolytic solution, can supply electrolytic solution  20  into bag-shaped laminate film  12  rapidly. That is, with the configuration in  FIG. 11 , since relay containers  30  are exposed to atmospheric pressure while the tip of supply ports  30   c  are exposed to vacuum, electrolytic solution  20  can be sucked into vacuum container  2  under negative pressure. 
       FIG. 12  shows another example of a system for supplying an electrolytic solution to a plurality of bag-shaped laminate film containing battery elements. 
     In the filling apparatus shown in  FIG. 12 , electrolytic-solution supply line  4  includes electrolytic solution tank  42 , liquid delivery system  41 , a plurality of supply pipes  4   b , and changeover valve  40 . Electrolytic solution tank  42  pools electrolytic solution  20 . Liquid delivery system  41  delivers electrolytic solution  20  from electrolytic solution tank  42  to vacuum container  2 . Changeover valve  40  is installed between liquid delivery system  41  and the plurality of supply pipes  4   b.    
     In the filling apparatus shown in  FIG. 1  and  FIGS. 9 to 11 , electrolytic-solution supply line  4  has a single supply pipe and a single needle connected to the supply pipe. According to the configurations shown in  FIGS. 9 to 11 , electrolytic solution  20  is supplied into multiple pieces of bag-shaped laminate film accommodating a battery element as the single needle is moved. On the other hand, according to the configuration shown in  FIG. 12 , supply pipes  4   b  and needles  4   a  are stationary, and changeover valve  40  is used to switch among supply pipes  4   b  and thus among needles  4   a , and thereby supply electrolytic solution  20  in turns. In this way, eliminating the need for a mechanism to move supply pipes  4   b  and needles  4   a  simplifies equipment configuration as shown in  FIG. 12 . 
     (Mechanism for Maintaining Laminate Film in an Open State) 
       FIG. 13  shows an example of a mechanism for maintaining laminate film  12  in an open state, according to the present embodiment. 
     A pair of suction apparatuses  50  can maintain laminate film  12  in an open state by pulling opening  12   a  of laminate film  12  from outside using vacuum suction. 
     Next, a method for maintaining an open state on a filling apparatus equipped with suction apparatus  50  will be described. 
     First, laminate film  12  is sucked while under vacuum in the atmosphere. Next, when laminate film  12  is opened, the needle of electrolytic-solution supply line  4  is inserted in opening  12   a . Alternatively, a frame member may be inserted in opening  12   a . After the needle or frame member is inserted, the suction operation of suction apparatus  50  may be stopped. Subsequently, electrolytic solution  20  is poured in an evacuated environment. 
       FIGS. 14A and 14B  show another example of the mechanism for maintaining laminate film  12  in an open state, according to the present embodiment. 
     As shown in  FIG. 14A , the mechanism for maintaining an open state has a plurality of claws  60  each equipped with hook-shaped tip  61 . As shown in  FIGS. 14A and 14B , claws  60  include two types arranged alternately: claws movable in the direction of arrow a and claws movable in the direction of arrow b opposite the direction of arrow a. 
     Next, a method for maintaining an open state on a filling apparatus equipped with claws  60  will be described. 
     First, tips  61  of claws  60  are inserted in opening  12   a  of laminate film  12 . Next, claws  60  are moved in direction a and direction b, thereby maintaining laminate film  12  in an open state. 
       FIGS. 15A and 15B  show another example of the mechanism for maintaining laminate film  12  in an open state, according to the present embodiment. A configuration shown in  FIG. 15A  maintains an open state using claws  70  equipped with tips  71 . However, unlike claws  60  in  FIGS. 14A and 14B , claws movable in the direction of arrow a and claws movable in the direction of arrow b are not arranged alternately. That is, as shown in  FIG. 15B , claws  70  are placed one each on left and right sides: claw  70  movable in the direction of arrow a and claw  70  movable in the direction of arrow b. Tips  71  are shaped such as to entirely cover that part of laminate film  12  which is to be heat-fused. Thus, even if electrolytic solution  20  discharged from needle  4   a  spatters, the present configuration can prevent spatters  20   a  from attaching to that part of laminate film  12  which is to be heat-fused. In this way, claws  70  configured as shown in  FIGS. 15A and 15B  not only maintains laminate film  12  in an open state, but also prevents spatters  20   a  of electrolytic solution  20  from attaching to laminate film  12 , enabling reliable heat fusion. 
     (Mechanism for Cleaning the Part of Laminate Film that is to be Heat Fused) 
     The present invention includes the process of temporarily pooling electrolytic solution  20  discharged from needle  4   a  on the sides of edge surfaces  11   b  of laminate film  12 . Discharged electrolytic solution  20  may spatter and attach to that part of laminate film  12  which is to be heat-fused. Attached electrolytic solution  20  may obstruct reliable heat fusion. Thus, it is preferable to wipe off any electrolytic solution  20  that is attached to the area to be heat-fused. 
       FIG. 16  shows an example of a mechanism for cleaning that part of laminate film which is to be heat-fused. 
     Cleaning mechanism  85  shown in  FIG. 16  has wiper  81  at the tip of shaft  80 . Preferably, wiper  81  is made of material such as nonwoven fabric or sponge which can be impregnated with electrolytic solution  20 . After electrolytic solution  20  is poured, but before heat fusion, cleaning mechanism  85  is inserted in opening  12   a  of laminate film  12  and moved in the direction perpendicular to the plane of the paper in  FIG. 16  with wiper  81  placed in contact with the area to be heat-fused in laminate film  12 . This allows wiper  81  to wipe any electrolytic solution  20  off that area of laminate film  12  which is to be heat-fused. 
       FIG. 17  shows another example of a mechanism for cleaning that part of laminate film which is to be heat-fused. 
     Cleaning mechanism  95  shown in  FIG. 17  includes two pulleys  90  and wiping belt  91  installed over pulleys  90 . Pulleys  90  can be rotated by a driver (not shown), driving, in turn, wiping belt  91 . 
     A cleaning method using cleaning mechanism  95  will be outlined below. 
     After electrolytic solution  20  is poured, but before heat fusion, one of pulleys  90  of cleaning mechanism  95  is inserted in opening  12   a  of laminate film  12 . 
     With wiping belt  91  placed in contact with that area of laminate film  12  which is to be heat-fused, cleaning mechanism  95  is moved in the direction perpendicular to the plane of the paper in  FIG. 17 . Consequently, wiping belt  91  wipes any electrolytic solution  20  off that area of laminate film  12  which is to be heat-fused. Subsequently, before cleaning any other piece of laminate film  12 , pulleys  90  are rotated by a predetermined amount. That is, pulleys  90  are rotated so as to retract that part of wiping belt  91  which has been contaminated as a result of cleaning and pulley  90  bring a clean part of wiping belt  91  into contact with that area of laminate film  12  which is to be heat-fused. 
     In this way, cleaning mechanism  95  can clean that area of laminate film  12  which is to be heat-fused always using an uncontaminated cleaning surface of wiping belt  91 . This ensures more reliable cleaning and thereby enables more reliable heat fusion. 
     (Method for Heat-Fusing Laminate Film) 
     According to the present invention, filling with electrolytic solution  20  is done in vacuum container  2 . Thus, if laminate film  12  is heat-fused with vacuum container  2  evacuated, an additional process of evacuation can be omitted. 
     The present invention has been described with reference to an embodiment, but the present invention is not limited to the embodiment described above. It is to be understood that various changes and modifications which may occur to those skilled in the art may be made to the configuration and details of the present invention without departing from the scope of the present invention. 
     This application claims priority from Japanese Patent Application No. 2008-20951 filed Jan. 31, 2008, which is incorporated herein by reference in its entirety.