Patent Publication Number: US-2017365420-A1

Title: Electric double-layer capacitor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of International application No. PCT/JP2016/053862, filed Feb. 9, 2016, which claims priority to Japanese Patent Application No. 2015-044112, filed Mar. 5, 2015, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an electric double-layer capacitor. 
     BACKGROUND OF THE INVENTION 
     Conventionally, capacitors have been widely used in various kinds of electronic devices such as cellular phones. Examples of known capacitors include an electric double-layer capacitor (EDLC). The electric double-layer capacitor involves no chemical reaction at charging and discharging unlike a secondary battery, and thus advantageously has a long product lifetime and can complete charging and discharging in a short time with a large current. For this reason, the electric double-layer capacitor has been tested for application in, for example, usage requiring a long product lifetime and usage requiring a large current. 
     Patent Document 1 discloses an exemplary electric double-layer capacitor. In the electric double-layer capacitor disclosed in Patent Document 1, a separator is provided to cover a negative electrode, and accordingly, separate the negative electrode from a positive electrode. 
     Patent Document 1: Japanese Patent Application Laid-open No. 2014-63789 
     SUMMARY OF THE INVENTION 
     It is a continual requirement to reduce the thickness of electric double-layer capacitors. 
     The present invention is mainly intended to provide a thin electric double-layer capacitor. 
     An electric double-layer capacitor according to an embodiment of the present invention includes a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode includes a positive-electrode collector electrode and a positive-electrode polarizable electrode. The positive-electrode polarizable electrode is provided on the positive-electrode collector electrode. The negative electrode includes a negative-electrode collector electrode and a negative-electrode polarizable electrode. The negative-electrode polarizable electrode is provided on the negative-electrode collector electrode. The negative electrode faces to the positive electrode. The electrolyte is interposed between the positive electrode and the negative electrode. The separator is provided between the positive-electrode polarizable electrode and the negative-electrode polarizable electrode which oppose each other. The separator has a flat plate shape. No polarizable electrode is provided on an outer surface of the collector electrode of each one of the positive electrode and the negative electrode, which are positioned outermost in a thickness direction. Since, in the electric double-layer capacitor according to the present embodiment, the separator has a flat plate shape, and no polarizable electrode is provided on the outer surface of the collector electrode of each one of the positive electrode and the negative electrode, which is positioned outermost in the thickness direction, the electric double-layer capacitor can have a reduced thickness. 
     Preferably, the separator is larger than the positive electrode and the negative electrode, and the electric double-layer capacitor further includes a bonding layer that bonds peripheral parts of the separators to at least one of the positive electrode and the negative electrode. 
     It is also preferable that at least one of the positive electrode and the negative electrode includes a facing part which faces the other of the positive electrode and the negative electrode, and an outside part positioned outside the facing part, and the electric double-layer capacitor further includes a bonding layer that bonds the outside part and the peripheral part of the separator. 
     Preferably, the bonding layer is separated from the facing part in which the positive electrode and the negative electrode face to each other and does not extend into the facing part. In this case, blockage of pores in a portion of the separator, which is positioned on the facing part in which the positive electrode and the negative electrode face to each other, is unlikely to occur due to a bonding agent applied for forming the bonding layer. This configuration can prevent reduction of the capacitance of the electric double-layer capacitor. 
     It is preferable that the porosity of at least part of a portion of the separator, which is positioned between the bonding layer and the facing part in plan view, is lower than the porosity of a portion of the separator positioned on the facing part in plan view. In this case, blockage of pores in a portion of the separator, which is positioned on the facing part in which the positive electrode and the negative electrode face each other, is unlikely to occur due to a bonding agent applied to form the bonding layer and having reached at this portion in plan view. This configuration can prevent reduction of the capacitance of the electric double-layer capacitor. 
     The thickness of a portion of the separator, which is positioned between the bonding layer and the facing part in plan view, is preferably smaller than the thickness of a portion of the separator positioned on the facing part in plan view. In this case, blockage of pores in a portion of the separator, which is positioned on the facing part in which the positive electrode and the negative electrode face to each other, is unlikely to occur due to a bonding agent applied to form the bonding layer and having reached at this portion in plan view. This configuration can prevent reduction of the capacitance of the electric double-layer capacitor. 
     The positive electrode preferably includes a positive-electrode facing part that faces the negative electrode, a positive-electrode terminal part connected with the positive-electrode facing part and extending outward from the positive-electrode facing part in a first direction, and a positive-electrode outside part connected with the positive-electrode facing part and extending toward a side opposite to the positive-electrode terminal part in the first direction. The negative electrode preferably includes a negative-electrode facing part that faces the positive electrode, a negative-electrode terminal part connected with the negative-electrode facing part and extending in the first direction, and a negative-electrode outside part connected with the negative-electrode terminal part and extending toward a side opposite to the negative-electrode terminal part in the first direction. In this case, it is preferable that the positive-electrode terminal part and the negative-electrode terminal part are disposed without overlapping each other in plan view, the positive-electrode outside part and the negative-electrode outside part are disposed without overlapping each other in plan view, and the positive-electrode terminal part, the negative-electrode terminal part, the positive-electrode outside part, and the negative-electrode outside part are bonded with the bonding layer. This configuration leads to further improvement of the bonding strength of a laminated body including the separator, the positive electrode, and the negative electrode. 
     The present invention provides a thin electric double-layer capacitor. 
    
    
     
       BRIEF EXPLANATION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an electric double-layer capacitor according to an embodiment of the present invention. 
         FIG. 2  is a schematic plan view of the electric double-layer capacitor according to the embodiment of the present invention. 
         FIG. 3  is a schematic plan view of a negative electrode in the embodiment of the present invention. 
         FIG. 4  is a schematic plan view of a positive electrode in the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     A preferred embodiment of the present invention will be described below. However, the embodiment described below is merely exemplary. The present invention is not limited by the embodiment described below. 
     Any drawing to be referred in the embodiments and the like is schematically illustrated. The dimensional ratio and the like of an object illustrated in the drawing are different from the dimensional ratio and the like of the real object in some cases. The dimensional ratio and the like of an object are different between the drawings in some cases. The specific dimensional ratio and the like of an object are to be determined based on the following description. 
       FIG. 1  is a schematic cross-sectional view of an electric double-layer capacitor according to the present embodiment. 
       FIG. 2  is a schematic plan view of the electric double-layer capacitor according to the present embodiment.  FIG. 2  omits illustration of an exterior body  10 . 
     As illustrated in  FIG. 1 , this electric double-layer capacitor  1  includes a negative electrode  11 , a positive electrode  12 , a separator  13 , a bonding layer  14 , and the exterior body  10 . 
     The negative electrode  11  and the positive electrode  12  face to each other with the separator  13  interposed therebetween. Specifically, a plurality of the negative electrodes  11  and a plurality of the positive electrodes  12  are alternately laminated with the separator  13  interposed therebetween. Each negative electrode  11  is electrically connected with a negative-electrode wiring member (not illustrated) and extended out of the exterior body  10 . Each positive electrode  12  is electrically connected with a positive-electrode wiring member (not illustrated) and extended out of the exterior body  10 . 
     Negative Electrode  11   
     Each negative electrode  11  includes a negative-electrode collector electrode  11   a.  The negative-electrode collector electrode  11   a  may be formed of, for example, aluminum foil. The negative-electrode collector electrode  11   a  may have a thickness of, for example, 10 μm to 30 μm approximately. 
     A negative-electrode polarizable electrode  11   b  is provided on each negative-electrode collector electrode  11   a . Specifically, the negative-electrode collector electrode  11   a  positioned outermost in a thickness direction (lamination direction) among the positive electrodes  12  and the negative electrodes  11  is provided with the negative-electrode polarizable electrode  11   b  only on an inner principal surface but not on an outer principal surface. The negative-electrode polarizable electrodes  11   b  of the other negative electrodes  11  are provided on both principal surfaces of the negative-electrode collector electrodes  11   a.  In other words, each negative-electrode polarizable electrode  11   b  is provided only on one of the principal surfaces of the corresponding negative-electrode collector electrode  11   a , which faces to the positive electrode  12 . The negative-electrode polarizable electrode  11   b  may have a thickness of, for example, 10 μm to 30 μm approximately. 
     As illustrated in  FIGS. 1 and 3 , each negative electrode  11  includes a negative-electrode facing part  11 A, a negative-electrode terminal part  11 B, and a negative-electrode outside part  11 C. The negative-electrode facing part  11 A faces to the positive electrode  12 . The negative-electrode terminal part  11 B is connected with the negative-electrode facing part  11 A. Specifically, in the present embodiment, the negative-electrode terminal part  11 B extends toward an x 1  side from a portion of the negative-electrode facing part  11 A on a y 1  side in a y-axis direction orthogonal to an x-axis direction. The negative-electrode outside part  11 C is connected with the negative-electrode facing part  11 A. The negative-electrode outside part  11 C extends from the negative-electrode facing part  11 A toward an x 2  side in the x-axis direction. Specifically, in the present embodiment, the negative-electrode outside part  11 C extends toward the x 2  side from a portion of the negative-electrode facing part  11 A on a y 1  side in the y-axis direction. The negative-electrode polarizable electrode  11   b  is provided only to the negative-electrode facing part  11 A but not to the negative-electrode terminal part  11 B nor the negative-electrode outside part  11 C. The negative-electrode terminal part  11 B and the negative-electrode outside part  11 C are parts of the negative-electrode collector electrode  11   a.    
     Positive Electrode  12   
     Each positive electrode  12  includes a positive-electrode collector electrode  12   a.  The positive-electrode collector electrode  12   a  may be formed of, for example, aluminum foil. The positive-electrode collector electrode  12   a  may have a thickness of, for example, 10 μm to 30 μm approximately. 
     A positive-electrode polarizable electrode  12   b  is provided on each positive-electrode collector electrode  12   a . Specifically, the positive-electrode collector electrode  12   a  positioned outermost in the thickness direction (lamination direction) among the positive electrodes  12  and the negative electrodes  11  is provided with the positive-electrode polarizable electrode  12   b  only on an inner principal surface but not on an outer principal surface. The positive-electrode polarizable electrodes  12   b  of the other positive electrodes  12  are provided on both principal surfaces of the positive-electrode collector electrodes  12   a.  In other words, each positive-electrode polarizable electrode  12   b  is provided only on one of the principal surface of the corresponding positive-electrode collector electrode  12   a , which faces to the negative electrode  11 . The positive-electrode polarizable electrode  12   b  may have a thickness of, for example, 10 μm to 30 μm approximately. 
     As illustrated in  FIGS. 1 and 4 , the positive electrode  12  includes a positive-electrode facing part  12 A, a positive-electrode terminal part  12 B, and a positive-electrode outside part  12 C. The positive-electrode facing part  12 A faces to the negative electrode  11 . The positive-electrode terminal part  12 B is connected with the positive-electrode facing part  12 A. Specifically, in the present embodiment, the positive-electrode terminal part  12 B extends toward the x 1  side from a portion of the positive-electrode facing part  12 A on the y 2  side in the y-axis direction. The positive-electrode outside part  12 C is connected with the positive-electrode facing part  12 A. The positive-electrode outside part  12 C extends from the positive-electrode facing part  12 A toward the x 2  side in the x-axis direction. Specifically, in the present embodiment, the positive-electrode outside part  12 C extends toward the x 2  side from a portion of the positive-electrode facing part  12 A on the y 2  side in the y-axis direction. The positive-electrode polarizable electrode  12   b  is provided only to the positive-electrode facing part  12 A but not to the positive-electrode terminal part  12 B nor the positive-electrode outside part  12 C. The positive-electrode terminal part  12 B and the positive-electrode outside part  12 C are parts of the positive-electrode collector electrode  12   a.    
     Separator  13   
     Each separator  13  is provided between the corresponding negative and positive electrodes  11  and  12  adjacent to each other. The separator  13  has a flat plate shape larger than the negative electrode  11  and the positive electrode  12 . The separator  13  separates the negative electrode  11  and the positive electrode  12  from each other. Specifically, the separator  13  is interposed between the negative-electrode polarizable electrodes  11   b  of the negative electrodes  11  and the positive-electrode polarizable electrodes  12   b  of the positive electrodes  12 . The separator  13  may be formed of, for example, a porous sheet including a plurality of open cells. 
     Exterior Body  10   
     The negative electrode  11 , the positive electrode  12 , and the separator  13  are housed in the exterior body  10 . The negative electrode  11  and the positive electrode  12  are connected with a negative-electrode extended terminal (not illustrated) and a positive-electrode extended terminal (not illustrated), respectively, provided outside the exterior body  10 . The exterior body  10  may be formed of, for example, an aluminum laminate, both surfaces of which are covered by resin layers. 
     Electrolyte 
     The exterior body  10  is filled with an electrolyte. The electrolyte is interposed between the negative electrodes  11  and the positive electrodes  12 . Specifically, the electrolyte is interposed between the negative-electrode polarizable electrodes  11   b  of the negative electrodes  11  and the positive-electrode polarizable electrodes  12   b  of the positive electrodes  12  via the separator  13 . 
     The electrolyte includes a cation, an anion, and a solvent. Examples of preferable cations include tetraethylammonium salt. Examples of preferable anions include tetrafluoroborate ion (BF 4− ) and bis (trifluoromethylsulfonyl) imide ((CF 3 SO 2 ) 2 N − ). Examples of preferable solvents include an aqueous solvent of carbonate compounds such as propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, nitrile compound, and water. 
     The electrolyte may be, for example, a crosslinkable gel electrolyte or ionic liquid made of an imidazole compound. 
     Bonding Layer  14   
     As illustrated in  FIGS. 1 and 2 , each bonding layer  14  is provided to bond peripheral parts of the separators  13  adjacent to each other in the thickness direction with one of the negative and positive electrodes  11  and  12  interposed therebetween. In the present invention, the bonding layer may bond any separator with the outside part of one of the negative and positive electrodes. 
     The bonding layers  14  are provided outside the facing parts  11 A and  12 A in the x-axis direction. The bonding layers  14  are separated from the facing parts  11 A and  12 A in the x-axis direction and do not extend into the facing parts  11 A and  12 A. With this configuration, a bonding agent for forming the bonding layers  14  can be effectively prevented from entering into the separator  13  and filling voids in a portion of the separator  13 , which is positioned between the facing parts  11 A and  12 A. Thus, the capacitance of the electric double-layer capacitor  1  can be prevented from decreasing. 
     To effectively prevent the bonding agent that forms the bonding layer  14  from entering into the separator  13  and filling voids in the portion of the separator  13  which is positioned between the facing parts  11 A and  12 A, it is preferable that the porosity of at least part of a portion of the separator  13 , which is positioned between the bonding layer  14  and each of the facing parts  11 A and  12 A in plan view, is lower than the porosity of a portion of the separator  13  positioned on each of the facing parts  11 A and  12 A in plan view. It is preferable that the porosity of at least part of the portion of the separator  13 , which is positioned between the bonding layer  14  and each of the facing parts  11 A and  12 A in plan view is substantially zero. From the same viewpoint, it is preferable that the thickness of the portion of the separator  13 , which is positioned between the bonding layer  14  and each of the facing parts  11 A and  12 A is smaller than the thickness of the portion of the separator  13 , which is positioned on each of the facing parts  11 A and  12 A. In these cases, the bonding agent applied on the separator  13  is unlikely to reach at the portion of the separator  13 , which is positioned on each of the facing parts  11 A and  12 A through the portion of the separator  13 , which is positioned between the bonding layer  14  and each of the facing parts  11 A and  12 A. For this reason, in the present embodiment, the terminal parts  11 B and  12 B and the outside parts  11 C and  12 C are provided in regions in which the bonding layers  14  are provided. The terminal parts  11 B and  12 B and the outside parts  11 C and  12 C, which are not porous bodies, each have a high strength of bonding with the bonding layer  14 . Thus, each component can be effectively prevented from peeling off. 
     To more effectively prevent each component from peeling off, it is preferable that the positive-electrode terminal part  12 B and the negative-electrode terminal part  11 B are disposed without overlapping in plan view, the positive-electrode outside part  12 C and the negative-electrode outside part  11 C are disposed without overlapping in plan view, and the positive-electrode terminal part  12 B, the negative-electrode terminal part  11 B, the positive-electrode outside part  12 C, and the negative-electrode outside part  11 C are bonded with the bonding layer  14 . 
     When a bent-type separator is employed as in the electric double-layer capacitor disclosed in Patent Document 1, the electric double-layer capacitor is likely to have a large thickness due to the elasticity of the separator. In the electric double-layer capacitor  1  according to the present embodiment, however, the separator  13  having a flat plate shape is employed. In addition, no polarizable electrodes  11   b  and  12   b  are provided on the outer surfaces of the collector electrodes  11   a  and  12   a  of electrodes positioned outermost in the thickness direction among the positive electrodes  12  and the negative electrodes  11 . This configuration leads to the electric double-layer capacitor  1  having a small thickness. 
     DESCRIPTION OF REFERENCE SYMBOLS 
     
         
           1 : electric double-layer capacitor 
           10 : exterior body 
           11 : negative electrode 
           11   a : negative-electrode collector electrode 
           11   b : negative-electrode polarizable electrode 
           11 A: negative-electrode facing part 
           11 B: negative-electrode terminal part 
           11 C: negative-electrode outside part 
           12 : positive electrode 
           12   a : positive-electrode collector electrode 
           12   b : positive-electrode polarizable electrode 
           12 A: positive-electrode facing part 
           12 B: positive-electrode terminal part 
           12 C: positive-electrode outside part 
           13 : separator 
           14 : bonding layer