Patent Publication Number: US-2022238961-A1

Title: Battery and method of manufacturing same

Description:
This nonprovisional application is based on Japanese Patent Application No. 2021-011115 filed on Jan. 27, 2021, with the Japan Patent Office, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present technology relates to a battery and a method of manufacturing the battery. 
     Description of the Background Art 
     There has been conventionally known a battery including an electrode assembly including a positive electrode plate, a negative electrode plate, and a separator disposed between the positive electrode plate and the negative electrode plate, wherein an adhesive layer is provided between each electrode plate (the positive electrode plate and the negative electrode plate) and the separator. 
     When adhesive force between the electrode plate (particularly, the positive electrode plate) and the separator is too strong in the electrode assembly, a clearance may not be sufficiently provided between the electrode plates in a cell drying step, with the result that a path for drying out moisture from a cell may be unable to be appropriately secured. In this case, remaining moisture reacts with an electrode active material to generate a gas, with the result that the cell is likely to be expanded. Therefore, it is necessary to secure a path for appropriately drying out moisture during the drying step. In view of the above, the conventional battery does not necessarily have a sufficient configuration. 
     SUMMARY OF THE INVENTION 
     An object of the present technology is to provide a battery and a method of manufacturing the battery so as to suppress expansion of a cell. 
     A battery according to the present technology includes an electrode assembly including a positive electrode plate, a negative electrode plate, and a separator disposed between the positive electrode plate and the negative electrode plate. A first adhesive layer is provided between the positive electrode plate and the separator, and a second adhesive layer is provided between the negative electrode plate and the separator. A coating weight of the first adhesive layer is smaller than a coating weight of the second adhesive layer, and a total of the coating weight of the first adhesive layer and the coating weight of the second adhesive layer is more than or equal to 0.03 g/m 2  and less than or equal to 0.15 g/m 2 . 
     A method of manufacturing a battery according to the present technology includes forming an electrode assembly including a positive electrode plate, a negative electrode plate, and a separator disposed between the positive electrode plate and the negative electrode plate, the separator having a first surface facing the positive electrode plate and a second surface facing the negative electrode plate. The forming of the electrode assembly includes: forming a first adhesive layer on the first surface of the separator and forming a second adhesive layer on the second surface of the separator; adhering the separator and the positive electrode plate to each other with the first adhesive layer being interposed between the separator and the positive electrode plate; and adhering the separator and the negative electrode plate to each other with the second adhesive layer being interposed between the separator and the negative electrode plate. A coating weight of the first adhesive layer is smaller than a coating weight of the second adhesive layer, and a total of the coating weight of the first adhesive layer and the coating weight of the second adhesive layer is more than or equal to 0.03 g/m 2  and less than or equal to 0.15 g/m 2 . 
     It should be noted that in the present specification, the “coating weight” of the adhesive layer refers to the mass of adhesive particles included in the adhesive layer per unit area of an adhesion surface. A theoretical value of the “coating weight” is found by (the area of the adhesive layer per unit area)×(the thickness of the adhesive layer)×(the density of the adhesive layer); however, the theoretical value of the “coating weight” can be uniquely found also by measuring the number of adhesive particles included per unit area. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a prismatic secondary battery. 
         FIG. 2  is a cross sectional view showing a structure of an electrode assembly according to one embodiment. 
         FIG. 3  is a cross sectional view showing a separator and adhesive layers in the electrode assembly shown in  FIG. 2 . 
         FIG. 4  is a cross sectional view showing a structure of an electrode assembly according to a modification. 
         FIG. 5  is a cross sectional view showing a separator and adhesive layers in the electrode assembly shown in  FIG. 4 . 
         FIG. 6  is a plan view showing an exemplary arrangement of an adhesive layer on the separator. 
         FIG. 7  is a plan view showing another exemplary arrangement of the adhesive layer on the separator. 
         FIG. 8  is a plan view showing still another exemplary arrangement of the adhesive layer on the separator. 
         FIG. 9  is a plan view showing yet another exemplary arrangement of the adhesive layer on the separator. 
         FIG. 10  is a diagram showing a relation between a ratio of coating weights of the adhesive layers formed on both surfaces of the separator and a cell expansion ratio (with respect to a cell thickness in a standard state). 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present technology will be described. It should be noted that the same or corresponding portions are denoted by the same reference characters, and may not be described repeatedly. 
     It should be noted that in the embodiments described below, when reference is made to number, amount, and the like, the scope of the present technology is not necessarily limited to the number, amount, and the like unless otherwise stated particularly. Further, in the embodiments described below, each component is not necessarily essential to the present technology unless otherwise stated particularly. 
     It should be noted that in the present specification, the terms “comprise”, “include”, and “have” are open-end terms. That is, when a certain configuration is included, a configuration other than the foregoing configuration may or may not be included. Further, the present technology is not limited to one that necessarily exhibits all the functions and effects stated in the present embodiment. 
     In the present specification, the term “battery” is not limited to a lithium ion battery, and may include another battery such as a nickel-metal hydride battery. In the present specification, the term “electrode” may collectively represent a positive electrode and a negative electrode. Further, the term “electrode plate” may collectively represent a positive electrode plate and a negative electrode plate. 
       FIG. 1  is a perspective view of a prismatic secondary battery  1 . As shown in  FIG. 1 , prismatic secondary battery  1  includes a battery case  100 , an electrode assembly  200 , a positive electrode terminal  300 , a negative electrode terminal  400 , and an insulating member  500 . 
     Battery case  100  is constituted of: a prismatic exterior body  110  that is provided with an opening and that has a prismatic tubular shape having a bottom; and a sealing plate  120  that seals the opening of prismatic exterior body  110 . Each of prismatic exterior body  110  and sealing plate  120  is preferably composed of a metal, and is preferably composed of aluminum or an aluminum alloy. 
     Electrode assembly  200  is accommodated in battery case  100  together with an electrolyte solution. Each of positive electrode terminal  300  and negative electrode terminal  400  is fixed to sealing plate  120  with an insulating member  500  being interposed therebetween, insulating member  500  being composed of a resin. 
     The battery of the present technology is not limited to the prismatic shape. The shapes of battery case  100  and electrode assembly  200  are not particularly limited. For example, electrode assembly  200  may be of a stack type, flat type, or cylindrical type. Preferably, electrode assembly  200  is a stack type electrode assembly. 
     When manufacturing prismatic secondary battery  1 , electrode assembly  200  is accommodated in prismatic exterior body  110 , and then prismatic exterior body  110  is sealed by sealing plate  120 . In this state, a drying step is performed to dry inside of battery case  100 . By the drying step, moisture included in an adhesive material used in the manufacturing of electrode assembly  200  is dried out. Thereafter, the electrolyte solution is injected into battery case  100  through an injection hole provided in sealing plate  120 . 
       FIG. 2  is a cross sectional view showing a structure of electrode assembly  200  including positive electrode plates  210 , negative electrode plates  220 , separators  230 , and adhesive layers  240 .  FIG. 3  is a cross sectional view showing separator  230  and adhesive layers  240  in electrode assembly  200 . 
     Electrode assembly  200  has a structure in which the plurality of positive electrode plates  210 , the plurality of negative electrode plates  220 , and the plurality of separators  230  are stacked in the order of positive electrode plate  210 , separator  230 , negative electrode plate  220 , and separator  230 . Generally, negative electrode plates  220  are disposed as electrode plates located at both ends of electrode assembly  200  in the stacking direction. 
     Electrode assembly  200  may have a structure in which one separator  230  is folded in a meandering manner to be interposed between positive electrode plates  210  and negative electrode plates  220 ; however, in the example of  FIG. 2 , separators  230  in the form of individual sheets are used. 
     Each of positive electrode plates  210  includes a positive electrode core body; and a positive electrode composite layer formed on the positive electrode core body. For the positive electrode core body, there can be used: a foil of a metal, such as aluminum, stable in a potential range of positive electrode plate  210 ; a film having the metal disposed on a surface layer thereof; or the like, for example. The positive electrode composite layer can include a positive electrode active material, a conductive material, and a binder. The positive electrode composite layer is generally formed on each of both surfaces of the positive electrode core body. Positive electrode plate  210  can be produced by forming the positive electrode composite layer on each of the both surfaces of the positive electrode core body. The positive electrode composite layer can be formed on each of the both surfaces of the positive electrode core body by applying a positive electrode composite slurry including the positive electrode active material, the conductive material, the binder, and the like, drying the applied film, and then performing rolling. 
     Examples of the positive electrode active material for forming the positive electrode composite layer include a lithium-containing transition metal oxide. Examples of metal element(s) included in the lithium-containing transition metal oxide include at least one selected from magnesium (Mg), aluminum (Al), calcium (Ca), scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), gallium (Ga), germanium (Ge), yttrium (Y), zirconium (Zr), tin (Sn), antimony (Sb), tungsten (W), lead (Pb), and bismuth (Bi). Among these, the positive electrode active material preferably include at least one selected from cobalt, nickel, and manganese. 
     Examples of the conductive material for forming the positive electrode composite layer include carbon materials such as carbon black (CB), acetylene black (AB), Ketjen Black, and graphite. Examples of materials usable as the binder include an organic-resin-based material, an acrylic-resin-based material, an epoxy-resin-based material, a styrene-butadiene-rubber-based material, a silicone-rubber-based material, a polyvinylidene-difluoride (PVdF)-based material, and the like. Each of them may be solely used or two or more of them may be used in combination. 
     Each of negative electrode plates  220  includes: a negative electrode core body; and a negative electrode composite layer formed on the negative electrode core body. For the negative electrode core body, there can be used: a foil of a metal, such as copper, stable in a potential range of negative electrode plate  220 ; a film having the metal disposed on a surface layer thereof; or the like, for example. The negative electrode composite layer can include not only a negative electrode active material but also a binder. The negative electrode composite layer is generally formed on each of both surfaces of the negative electrode core body. Negative electrode plate  220  can be produced by forming the negative electrode composite layer on each of the both surfaces of the negative electrode core body. The negative electrode composite layer can be formed on each of the both surfaces of the negative electrode core body by applying a negative electrode composite slurry including the negative electrode active material, the conductive material, the binder, and the like, drying the applied film, and then performing rolling. 
     The negative electrode active material for forming the negative electrode composite layer is not particularly limited as long as lithium ions can be reversibly occluded and released, and examples of materials usable as the negative electrode active material include: a carbon material such as natural graphite or artificial graphite; a metal that can be alloyed with lithium, such as silicon (Si) or tin (Sn); an alloy including a metal element such as silicon or tin; a composite oxide; and the like. Each of the above-listed negative electrode active materials may be solely used or two or more of the above-listed negative electrode active materials may be used in combination. The negative electrode active material preferably includes the carbon material, the silicon material, or the lithium metal. More preferably, the negative electrode active material is mainly composed of the carbon material. 
     As the binder included in the negative electrode composite layer, the same material as that in the positive electrode composite layer can be used. When the negative electrode composite slurry is prepared using a water-based solvent, a styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like may be used. 
     As positive electrode plate  210 , for example, a comparatively large-sized positive electrode plate can be used which has a size with a long side of more than or equal to 100 mm (as one example, about 140 mm) and a short side of more than or equal to 50 mm (as one example, a height of about 70 mm). For example, 50 or more (as one example, about 60) positive electrode plate  210  and 50 or more (as one example, about 60) negative electrode plate  220  can be stacked. 
     Each of separators  230  is constituted of: a porous substrate  231 ; and a porous heat-resistant layer  232  formed on one surface of substrate  231 . By providing heat-resistant layer  232 , separator  230  is less likely to be raptured due to, for example, introduction of a foreign matter, penetration of a nail, or the like, or can be suppressed from being contracted upon an increase in temperature. In order to increase cost effectiveness while suppressing an increase in thickness of electrode assembly  200 , it is preferable to form heat-resistant layer  232  only on one surface of substrate  231 . 
     Porous substrate  231  can solely function as a separator. Substrate  231  is composed of a resin layer or a nonwoven fabric. As substrate  231 , a porous film having ion permeability and electric insulation property can be used. The thickness of substrate  231  is, for example, approximately more than or equal to 1 μm and less than or equal to 20 μm. Examples of the material of substrate  231  include olefin resins such as polyethylene, polypropylene, ethylene-propylene copolymer, ethylene, propylene, and copolymers with other α-olefins. Substrate  231  is preferably composed of a polyolefin layer. The melting point of substrate  231  is generally approximately less than or equal to 200° C. 
     The porous film included in substrate  231  is provided with a multiplicity of pores for permeation of lithium ions; however, the unevenness of the surface of the porous film is smaller than the unevenness of the surface of heat-resistant layer  232 , and the surface of the porous film is flatter than the surface of heat-resistant layer  232 . The size (maximum length) of each hole or recess in the surface of substrate  231  is, for example, less than 0.5 μm, and is preferably less than 0.3 μm. 
     Heat-resistant layer  232  can be composed of a resin having a higher melting point or softening point than that of the resin of substrate  231 , such as an aramid resin, polyimide, or polyamideimide. Preferably, heat-resistant layer  232  is mainly composed of an inorganic compound. Heat-resistant layer  232  is preferably composed of: insulative inorganic particles; and a binder that binds the inorganic particles and that binds the inorganic particles and substrate  231 . As with substrate  231 , heat-resistant layer  232  has ion permeability and electric insulation property. The thickness of heat-resistant layer  232  is, for example, approximately more than or equal to 1 μm and less than or equal to 10 μm, and is preferably approximately more than or equal to 1 μm and less than or equal to 6 μm. 
     As the inorganic particles to be the main component of heat-resistant layer  232 , at least one selected from alumina, boehmite, aluminum hydroxide, silica, titania, and zirconia can be used, for example. Preferably, alumina is used as the inorganic particles. The content of the inorganic particles is preferably approximately more than or equal to 85 mass % and less than or equal to 99.9 mass %, and is more preferably approximately more than or equal to 90 mass % and less than or equal to 99.5 mass %, with respect to the mass of heat-resistant layer  232 . 
     The shape of each of the inorganic particles is not particularly limited, and particles each having a spherical shape, a quadrangular prism shape, or the like can be used, for example. The average particle diameter of the particles each having a spherical shape or the average length of sides of the particles each having a quadrangular prism shape is preferably approximately more than or equal to 0.1 μm and less than or equal to 1.5 μm, and is more preferably approximately more than or equal to 0.5 μm and less than or equal to 1.2 μm. When the particle diameters of the inorganic particles fall within the above range, heat-resistant layer  232  can be formed to have excellent ion permeability and excellent durability. The average particle diameter of the inorganic particles and the average length of the sides of the inorganic particles are measured by observing the surface of heat-resistant layer  232  using a scanning electron microscope (SEM). Specifically, the average particle diameter of the inorganic particles and the average length of the sides of the inorganic particles can be calculated by randomly selecting  100  inorganic particles in an SEM image of heat-resistant layer  232 , measuring the diameters of circles circumscribed on the particles or measuring the lengths of the sides of the particles, and performing averaging. 
     As the binder included in heat-resistant layer  232 , the same binder as the binder included in each of the positive electrode composite layer and the negative electrode composite layer can be used. The content of the binder is preferably approximately more than or equal to 0.1 mass % and less than or equal to 15 mass %, and is more preferably approximately more than or equal to 0.5 mass % and less than or equal to 10 mass %, with respect to the mass of heat-resistant layer  232 . Heat-resistant layer  232  is formed by, for example, applying a slurry containing the inorganic particles and the binder onto one surface of the porous film of substrate  231  and drying the applied film. 
     Heat-resistant layer  232  thus formed has a higher permeability for moisture and ions than that of substrate  231 . 
     Separator  230  is adhered to the positive electrode composite layer of positive electrode plate  210  and the negative electrode composite layer of negative electrode plate  220  by adhesive layer  240  including adhesive particles. In the present embodiment, heat-resistant layer  232  of separator  230  and the positive electrode composite layer of positive electrode plate  210  are adhered to each other by an adhesive layer  241  (first adhesive layer), and substrate  231  of separator  230  and the negative electrode composite layer of negative electrode plate  220  are adhered to each other by an adhesive layer  242  (second adhesive layer) as shown in  FIG. 2 . 
     The thickness of adhesive layer  240  is approximately more than or equal to 0.1 μm and less than or equal to 1 μm, or is approximately more than or equal to 0.2 μm and less than or equal to 0.9 μm, for example. The thickness of adhesive layer  240  is determined by amount, particle diameters, or the like of the adhesive particles. Adhesive layer  240  is formed by, for example, applying a slurry containing the adhesive particles onto the surface of separator  230  and drying the slurry. For the slurry of the adhesive particles, a so-called emulsion can be used in which minute adhesive particles are dispersed in water. In this case, separator  230  can be obtained to have both surfaces on which adhesive layers  240  each composed of the adhesive particles are formed. 
     Electrode assembly  200  is obtained by: stacking negative electrode plate  220 , separator  230  having adhesive layers  240  formed thereon, positive electrode plate  210 , and separator  230  having adhesive layers  240  formed thereon in this order; and performing thermal pressing. Adhesive force between separator  230  and each electrode plate (each of positive electrode plate  210  and negative electrode plate  220 ) can be adjusted by changing thermal pressing conditions such as temperature, pressure, and pressure application time. The thermal pressing conditions cannot be made different between an interface between separator  230  and positive electrode plate  210  and an interface between separator  230  and negative electrode plate  220 . 
     As the adhesive particles included in adhesive layer  240 , a plurality of different types of adhesive particles may be used; however, in consideration of productivity and the like, the same type of adhesive particles are preferably used. That is, the same type of adhesive particles are preferably present between substrate  231  of separator  230  and positive electrode plate  210 , and between heat-resistant layer  232  of separator  230  and negative electrode plate  220 . Here, for example, the same type of adhesive particles refer to adhesive particles provided as the same product, and a production lot may be different. 
     The average particle diameter of the adhesive particles is, for example, approximately more than or equal to 0.1 μm and less than or equal to 1 μm, and is preferably approximately more than or equal to 0.5 μm and less than or equal to 0.7 μm. The average particle diameter of the adhesive particles is measured by observing the surface of separator  230  using a SEM as with the average particle diameter of the inorganic particles included in heat-resistant layer  232 . 
     The adhesive particles of adhesive layer  240  can be melted or softened in the thermal pressing step. Due to the adhesive particles being melted or softened, adhesive layer  240  is strongly adhered to the surfaces of separator  230  and the electrode plate (each of positive electrode plate  210  and negative electrode plate  220 ), thereby obtaining excellent adhesion. Adhesive layer  240  is composed of, for example, a resin having a glass transition temperature of less than or equal to 80° C. Adhesive layer  240  preferably includes an acrylic-resin-based adhesive material, an epoxy-resin-based adhesive material, a styrene-butadiene-rubber-based adhesive material, a silicone-rubber-based adhesive material, and a PVdF-based adhesive material. Among them, the acrylic-resin-based adhesive material is more preferable. 
     As described above, in electrode assembly  200 , the mass of the adhesive particles per unit area at the interface between the negative electrode composite layer of negative electrode plate  220  and heat-resistant layer  232  is larger than the mass of the adhesive particles per unit area at the interface between the positive electrode composite layer of positive electrode plate  210  and substrate  231 . 
     In other words, the coating weight of adhesive layer  241  (first adhesive layer) provided between positive electrode plate  210  and separator  230  is smaller than the coating weight of adhesive layer  242  (second adhesive layer) provided between negative electrode plate  220  and separator  230 . That is, as shown in  FIG. 3 , the coating weight of adhesive layer  241  (first adhesive layer) formed on heat-resistant layer  232  (first surface) is smaller than the coating weight of adhesive layer  242  (second adhesive layer) formed on substrate  231  (second surface). 
     Further, in electrode assembly  200 , the total of the coating weight of adhesive layer  241  (first adhesive layer) provided between positive electrode plate  210  and separator  230  and the coating weight of adhesive layer  242  (second adhesive layer) provided between negative electrode plate  220  and separator  230  is approximately more than or equal to 0.03 g/m 2  and less than or equal to 0.15 g/m 2 . Here, the coating weight of adhesive layer  241  (first adhesive layer) provided between positive electrode plate  210  and separator  230  is preferably approximately more than or equal to 0.01 g/m 2  and less than or equal to 0.05 g/m 2 . 
     Since the total of the coating weights of adhesive layers  240  on both surfaces of separator  230  is approximately more than or equal to 0.03 g/m 2  and less than or equal to 0.15 g/m 2  in electrode assembly  200 , the amount of adhesive layers  240  can be appropriate. More specifically, adhesive force between the electrode plate (each of positive electrode plate  210  and negative electrode plate  220 ) and separator  230  can be suppressed from being too strong. As a result, in the drying step in the process of manufacturing of prismatic secondary battery  1 , a clearance to serve as a path for drying out moisture can be readily secured between positive electrode plate  210  and negative electrode plate  220 , thereby suppressing generation of a gas due to a reaction between remaining moisture and the electrode active material as well as expansion of battery case  100  due to the gas. 
     Particularly, with the relatively small coating weight of adhesive layer  241  provided on the positive electrode plate  210  side, the clearance to serve as the path for drying out moisture can be readily secured on the positive electrode plate  210  side. Thus, expansion of battery case  100  can be more effectively suppressed. Further, since heat-resistant layer  232  having a higher moisture permeability than that of substrate  231  is disposed on the positive electrode plate  210  side, the path for drying out moisture can be secured more readily on the positive electrode plate  210  side. 
     It should be noted that the mass of the adhesive particles per unit area at the interface between the positive electrode composite layer and heat-resistant layer  232  can be calculated by measuring the number of the adhesive particles adhered on the surface of the positive electrode composite layer and the surface of heat-resistant layer  232  and multiplying the total volume of the particles by the specific gravity of the particles. The mass of the adhesive particles per unit area at the interface between the negative electrode composite layer and substrate  231  can also be calculated in the same manner. 
       FIG. 4  is a cross sectional view showing a structure of an electrode assembly  200 A according to a modification. Electrode assembly  200 A includes positive electrode plates  210 A, negative electrode plates  220 A, separators  230 A, and adhesive layers  240 A.  FIG. 5  is a cross sectional view showing separator  230 A and adhesive layers  240 A in electrode assembly  200 A. 
     As shown in  FIG. 4 , in electrode assembly  200 A, the coating weight of adhesive layer  241 A provided between positive electrode plate  210 A and separator  230 A is smaller than the coating weight of adhesive layer  242 A provided between negative electrode plate  220 A and separator  230 A. That is, as shown in  FIG. 5 , the coating weight of adhesive layer  241 A formed on substrate  231 A is smaller than the coating weight of adhesive layer  242 A formed on heat-resistant layer  232 A. 
     Also in the modification shown in  FIGS. 4 and 5 , the total of the coating weights of adhesive layers  240 A on the both surfaces of separator  230 A is approximately more than or equal to 0.03 g/m 2  and less than or equal to 0.15 g/m 2 . Thus, the amount of adhesive layers  240 A is appropriate, thereby readily securing a clearance to serve as a path for drying out moisture in the drying step. 
     Each of  FIGS. 6 to 9  is a plan view showing an exemplary arrangement of adhesive layer  240  on separator  230 . Adhesive layer  240  may be arranged in the form of dots as shown in  FIG. 6 , may be arranged in the form of stripes as shown in  FIG. 7 , may be arranged on the entire surface as shown in  FIG. 8 , or may be arranged in the form of islands as shown in  FIG. 9 . Adhesive layers  240  may be arranged in the same manner on the both surfaces of separator  230  when viewed in plan or may be arranged in different manners on the both surfaces of separator  230  when viewed in plan. Adhesive layers  240  may have the same thickness or different thicknesses on the both surfaces of separator  230 . In order to secure the path for drying out moisture, adhesive layer  240  is preferably partially disposed on separator  230  (the examples of  FIGS. 6, 7, and 9 ). 
       FIG. 10  is a diagram showing a relation between a ratio (positive electrode side/both surfaces) of the coating weights of adhesive layers  240  formed on the both surfaces of separator  230  in electrode assembly  200  and a cell expansion ratio (with respect to a cell thickness in a standard state). Table 1 shows numerical values of samples shown in  FIG. 10 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Coating Weight of Adhesive Layer (g/m 2 ) 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Positive Electrode 
                 Negative 
                 Coating Weight 
                   
               
               
                   
                 Side 
                 Electrode Side 
                 Ratio 
                 Cell Thickness 
               
               
                   
                 (Heat-Resistant 
                 (Substrate 231 
                 (Positive Electrode 
                 (with respect to 
               
               
                   
                 Layer 232 Side) 
                 Side) 
                 Side/Both Surfaces) 
                 Standard State) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Comparative 
                 0.07 
                 0.07 
                 0.500 
                 103.4% 
               
               
                 Example 1 
               
               
                 Comparative 
                 0.05 
                 0.05 
                 0.500 
                 103.3% 
               
               
                 Example 2 
               
               
                 Example 1 
                 0.06 
                 0.07 
                 0.462 
                 101.7% 
               
               
                 Example 2 
                 0.06 
                 0.08 
                 0.429 
                 101.6% 
               
               
                 Example 3 
                 0.05 
                 0.06 
                 0.455 
                 101.6% 
               
               
                 Example 4 
                 0.05 
                 0.08 
                 0.385 
                 101.5% 
               
               
                 Example 5 
                 0.04 
                 0.05 
                 0.444 
                 101.5% 
               
               
                 Example 6 
                 0.05 
                 0.07 
                 0.417 
                 101.4% 
               
               
                 Example 7 
                 0.04 
                 0.06 
                 0.400 
                 101.2% 
               
               
                 Example 8 
                 0.03 
                 0.06 
                 0.333 
                 101.2% 
               
               
                 Example 9 
                 0.05 
                 0.09 
                 0.357 
                 101.0% 
               
               
                 Example 10 
                 0.04 
                 0.08 
                 0.333 
                 101.0% 
               
               
                 Example 11 
                 0.04 
                 0.09 
                 0.308 
                 100.7% 
               
               
                   
               
            
           
         
       
     
     As shown in  FIG. 10  and Table 1, the expansion ratio of battery case  100  (“Cell Thickness” in  FIG. 10  and Table 1) can be suppressed by setting the ratio of the coating weight on the positive electrode side to be smaller than 0.5, i.e., by setting the coating weight on the positive electrode side to be smaller than the coating weight on the negative electrode side. 
     It should be noted that the present inventors have confirmed that, in each of Comparative Examples A to E shown in Table 2, the expansion ratio of battery case  100  is significantly increased as compared with the results shown in Table 1. It should be noted that in each of Comparative Examples A to E, substrate  231 A of separator  230 A is disposed on the positive electrode side, and heat-resistant layer  232 A is arranged on the negative electrode side as shown in  FIGS. 4 and 5 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Coating Weight of Adhesive Layer (g/m 2 ) 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Positive Electrode 
                 Negative Electrode Side 
                 Coating Weight Ratio 
               
               
                   
                 Side 
                 (Heat-Resistant Layer 
                 (Heat-Resistant Layer 232A 
               
               
                   
                 (Substrate 231A Side) 
                 232A Side) 
                 Side/Both Surfaces) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Comparative 
                 0.30 
                 0.35 
                 0.538 
               
               
                 Example A 
               
               
                 Comparative 
                 0.33 
                 0.50 
                 0.602 
               
               
                 Example B 
               
               
                 Comparative 
                 0.22 
                 0.50 
                 0.694 
               
               
                 Example C 
               
               
                 Comparative 
                 0.35 
                 0.80 
                 0.696 
               
               
                 Example D 
               
               
                 Comparative 
                 0.18 
                 0.35 
                 0.660 
               
               
                 Example E 
               
               
                   
               
            
           
         
       
     
     It is considered that the expansion ratio of battery case  100  was significantly increased in each of Comparative Examples A to E shown in Table 2 mainly due to the following reasons: the total of the coating weights of adhesive layers  240 A on the both surfaces of separator  230 A is large, i.e., is more than or equal to 0.5 g/m 2 ; and substrate  231 A, with which it is more difficult to secure the path for drying out moisture as compared with the case of heat-resistant layer  232 A, is disposed on the positive electrode side. 
     On the other hand, in each of Examples 1 to 11 of the present technology shown in Table 1, it is understandable that the total of the coating weights of adhesive layers  240  on the both surfaces of separator  230  is small, i.e., is less than or equal to 0.15 g/m 2 , the coating weight on the positive electrode side is particularly small, and heat-resistant layer  232 , with which it is easier to secure the path for drying out moisture as compared with the case of substrate  231 , is disposed on the positive electrode side, with the result that the expansion ratio of battery case  100  can be effectively suppressed. 
     Although the embodiments of the present invention have been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being interpreted by the terms of the appended claims.