JPH11340093A - Electric double layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric double layer capacitor which is thin and by which dry-up of electrolytic solution is prevented and ESR(equivalent series resistance) is largely reduced. SOLUTION: An electric double layer capacitor uses an aqueous solution base electrolytic solution and a conductive rubber sheet or a conductive plastic sheet 10a as collector 10. In this case, a swollen graphite layer 10b is formed on one side or both sides of the conductive rubber sheet or the conductive plastic sheet 10a. Since the electric double layer capacitor uses a collector comprised of a swollen graphite layer 10b formed on the surface of the conductive rubber sheet or the conductive plastic sheet 10a, its thickness can be reduced, dry-up of the electrolytic solution can be prevented and the ESR can be largely reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric double layer capacitor, and more particularly to an improvement in a current collector material.

[0002]

2. Description of the Related Art FIG. 7 is a schematic sectional view showing an example of the structure of a basic cell of a conventional electric double layer capacitor. The basic cell of this electric double layer capacitor is a separator 1 made of an electrically insulating and ion-permeable porous film.
A polarizable electrode 2 formed by sandwiching the separator 1 and made of powdered activated carbon, activated carbon fiber, or a solidified form of these activated carbons with a binder such as Teflon, into which an electrolyte is impregnated; The gasket 3 holds the separator 1 and the polarizable electrode 2 from the side, and a conductive rubber sheet or a conductive plastic sheet formed on the outer surface of the polarizable electrode 2 and provided with conductivity by carbon powder or the like. It comprises a pair of current collectors 4 and a pair of terminal plates 5 arranged on the current collectors 4 for taking out terminals.

Further, since the surface of the current collector 4 is not smooth, contact resistance is generated between the surface of the current collector 4 and the polarizable electrode 2 and between the surface of the current collector 4 and the terminal plate 5. To reduce this contact resistance, dozens of kg /
A mechanism (not shown) for applying a pressure of ² cm ² and maintaining this pressure is provided. The withstand voltage of such a basic cell depends on the electrolytic solution, and is 1.0 V in the case of an aqueous solution system, and is about 2.0 to 3.0 V in the case of an organic solvent system, though it differs depending on the constituent compounds. The electric double layer capacitor has a structure in which a required number of basic cells are stacked in series according to a required withstand voltage in order to achieve a predetermined withstand voltage.

[0004]

Such an electric double layer capacitor has been mainly used for backing up a memory or the like, but in recent years, it has required a large current for automobiles and electronic devices. Use in applications is increasing. However, the conventional electric double-layer capacitor has a contact resistance between the surface of the current collector 4 and the polarizable electrode 2 and between the surface of the current collector 4 and the terminal plate 5, so that an equivalent series resistance (hereinafter, referred to as “equivalent series resistance”). ESR) is large. Therefore, when a large current flows from the electric double layer capacitor by discharging, the ESR
Has the disadvantage that a voltage drop occurs.

In order to reduce such a voltage drop and allow a large current to flow from the electric double layer capacitor, the shapes of the polarizable electrode 2 and the current collector 4 must be reduced to reduce the ESR. In the field of electronic devices, the size of the devices has been reduced, and accordingly, thinner electric double layer capacitors have been required. However, the conventional electric double layer capacitor has a mechanism (not shown) for applying a pressure of several tens of kg / cm ² from the terminal plates 5 on both sides in order to reduce the contact resistance. . Therefore, it has been difficult to reduce the thickness of the electric double layer capacitor.

In order to solve this problem, an electric double layer capacitor which does not require the above-mentioned pressure holding mechanism and has reduced contact resistance and is stabilized has a large oil absorption between the current collector and the polarizable electrode. One in which an intermediate layer mainly composed of carbon is formed is disclosed in Japanese Patent Application Laid-Open No. 1-136326. Further, Japanese Patent Application Laid-Open No. 02-281607 discloses an electric double layer capacitor in which a graphite sheet is provided between a current collector and a polarizable electrode. However,
Even when carbon with large oil absorption is provided in the intermediate layer, contact resistance is generated between carbon particles, between carbon particles and current collector, and between carbon particles and polarizable electrode, so contact resistance is greatly reduced. I can't. Further, when a graphite sheet is provided, it is difficult to reduce the thickness because the thickness of the sheet itself is large.

Further, if the current collector is made thin in order to make the shape of the electric double layer capacitor thin, the gas permeability of the current collector becomes large. Therefore, there arises a problem that the reliability of the electric double layer capacitor is reduced due to the dry-up of the electrolytic solution. As shown in FIG. 8, as a current collector plate having reduced gas permeability, conductive layers 7 and 8 are formed on both surfaces of an insulating layer 6 having a sufficiently low gas permeability, and through holes 9 are formed in the insulating layer 6. One formed and made conductive between the conductive layers 7 and 8 is disclosed in JP-A-59-208714. However, the method using the current collector plate has a disadvantage that the cross-sectional area of the through-hole 9 must be reduced to some extent in order to suppress gas permeability, and therefore the internal resistance increases.

Therefore, an object of the present invention is to reduce the thickness
An object of the present invention is to provide an electric double layer capacitor in which dry-up of an electrolytic solution is prevented and ESR is greatly reduced.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide an electric double layer capacitor using an aqueous electrolytic solution and a conductive rubber sheet or a conductive plastic sheet as a current collector. The problem is solved by an electric double layer capacitor characterized in that an expanded graphite layer is formed on one or both surfaces of a conductive plastic sheet.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described with reference to the drawings, but the electric double layer capacitor of the present invention is not limited to the following embodiments. FIG. 1 is a schematic sectional view showing an example of a basic cell of the electric double layer capacitor of the present invention. The basic cell of the electric double layer capacitor includes a separator 1, a pair of polarizable electrodes 2 formed with the separator 1 interposed therebetween, a gasket 3 for holding the separator 1 and the polarizable electrode 2 from the side, and a polarizable electrode. The current collector 10 includes a pair of current collectors 10 formed on the outer surface thereof, and a pair of terminal plates 5 arranged on the current collector 10 for taking out terminals.

As the separator 1, a porous film having electronic insulation and ion permeability is used. An example of such a porous film is a nonwoven fabric made of a synthetic resin or the like. The above-mentioned polarizable electrode 2 is preferably electrically conductive, stable with respect to an electrolytic solution, and has a large surface area. For example, powdered activated carbon or activated carbon fiber, or these activated carbons such as Teflon, phenolic resin, etc. The one solidified with a binder and impregnated with an electrolytic solution is used. As the electrolytic solution, a solution in which sulfuric acid, potassium hydroxide, or the like is dissolved in water is used. The gasket 3 maintains the shape of the basic cell, prevents leakage of the electrolyte, and prevents short-circuiting due to contact between the upper and lower current collectors 18. The material of the gasket 3 is, for example, AB
A resin material such as S is used.

The current collector 10 includes a conductive rubber sheet or a conductive plastic sheet 10a provided with conductivity by carbon powder or the like, and an expansion formed on the surface of the conductive rubber sheet or the conductive plastic sheet 10a. And a graphite layer 10b. Examples of the material of the conductive rubber sheet or the conductive plastic sheet 10a include conductive butyl rubber, conductive rubber such as polyethylene, polypropylene, nitrile, styrene, and butadiene, conductive elastomer, and conductive plastic. And the like.

The expanded graphite forming the expanded graphite layer 10b is a conductivity-imparting material obtained by rapidly heating a graphite intercalation compound to about 800 to 1000 ° C. The shape of the expanded graphite is powdery or particulate. is there. Since this expanded graphite has expanded 100 times or more as compared with normal graphite, it tends to change its shape as compared with normal graphite. The thickness of the expanded graphite layer 10b is preferably set to 10 μm or less. If the film thickness exceeds 10 μm, the expanded graphite layer 10
b may peel off. Even if the film thickness is larger than 10 μm, the contact resistance between the surface of the current collector 10 and the polarizable electrode 2 and between the surface of the current collector 10 and the terminal plate 5 and the electric double layer capacitor ESR The reduction effect is hardly improved.

The current collector 10 can be manufactured, for example, by the following manufacturing method. FIG. 2 shows the current collector 10.
It is a schematic diagram which shows an example of the manufacturing process of. First, an expanded graphite / solvent mixture 11 in which expanded graphite and a solvent are mixed at a predetermined ratio is applied thickly on a conductive rubber sheet or a conductive plastic sheet 10a using an applicator 13 and a blade 14 is used to expand the expanded graphite / solvent mixture. The coating surface of the solvent mixture 11 is smoothed. Then, the applied expanded graphite / solvent mixture 11 is applied to a conductive rubber sheet or a conductive plastic sheet 10a.
Is pressed using a reverse roll 12 so that the expanded graphite adheres to the conductive rubber sheet or the conductive plastic sheet 10a. Next, the compressed expanded graphite / solvent mixture 11 is dried with a drier or the like to remove the solvent.

In such an electric double layer capacitor, the current collector 1 in which an expanded graphite layer 10b is formed on the surface of a conductive rubber sheet or a conductive plastic sheet 10a is provided.
Since 0 is used, the contact resistance between the surface of the current collector 10 and the polarizable electrode 2 and the contact resistance between the surface of the current collector 10 and the terminal plate 5 can be significantly reduced. This eliminates the need for a pressure holding mechanism, so that the thickness of the electric double layer capacitor can be reduced, and the ESR of the electric double layer capacitor can be reduced. Further, since the expanded graphite layer 10b is formed on the surface of the conductive rubber sheet or the conductive plastic sheet 10a, the gas permeability of the current collector 10 is reduced, and the dry-up of the electrolytic solution is prevented.

When the expanded graphite layer 10b is pressure-bonded to the surface of the conductive rubber sheet or the conductive plastic sheet 10a using the reverse roll 12 or the like, the conductive rubber sheet or the conductive plastic sheet 10a and the expanded graphite layer 10b , The ESR of the electric double layer capacitor is further reduced, and the electrolyte is prevented from drying up.

By forming an expanded graphite layer 10b on the surface of a conductive rubber sheet or a conductive plastic sheet 10a, the surface between the current collector 10 and the polarizable electrode 2 and between the surface of the current collector 10 and the terminal plate 5 are formed. The reasons why the contact resistance between the electrolyte solution and the electrolyte solution is greatly reduced and the dry-up of the electrolyte solution is prevented are as follows. Expanded graphite layer 10b
Is more likely to cause a shape change than ordinary graphite particles. Therefore, as shown in FIG. 3, when the expanded graphite 15 causes friction with the surface of the conductive rubber sheet or the conductive plastic sheet 10a, the expanded graphite 15 is deformed according to the unevenness of the surface and adheres to the surface. It has conductivity, reduces contact resistance, and reduces gas permeability. On the other hand, as shown in FIG.
Is not deformed in accordance with the unevenness of the surface of the conductive rubber sheet or the conductive plastic sheet 10a. Low.

[0018]

Embodiments are described below. (Embodiment 1) FIG. 4 is a schematic view showing a sectional structure of a test electric double layer capacitor in this embodiment. The weight ratio of expanded graphite powder having a fixed carbon content of 97% or more to water is 1:
3 to prepare an expanded graphite powder / water mixture. An electrically conductive butyl rubber sheet 24a having a thickness of 50 μm was prepared, and an expanded graphite powder / water mixture was uniformly applied and pressed on both surfaces thereof, so that the expanded graphite powder was brought into close contact with the surface of the electrically conductive butyl rubber sheet 24a. This is dried for 120
At 1 ° C. for 1 hour to remove the remaining solvent, and the thickness of one side of the conductive butyl rubber sheet 24a is 10 μm.
The current collector 24 on which the m expanded graphite layer 24b was formed was obtained.

Activated carbon powder and powdered phenolic resin were mixed at a weight ratio of 6: 4, molded into 70 × 50 × 0.5 mm, and heat-treated at 900 ° C. in a nitrogen atmosphere. (Electrolytic solution) for 24 hours to obtain a polarizable electrode 22. This polarizable electrode 22 has a thickness of 20 μm.
m, and a separator 21 made of a polypropylene nonwoven fabric was interposed therebetween. The current collector 24 formed as described above is pressed from both sides thereof, and then the current collectors 2 on both sides are pressed.
Gasket 23 made of ABS so as not to contact each other
Was arranged around the polarizable electrode 22. Next, a pair of AB
S support members 26 were arranged at both ends of the surface of the current collector 24, and the support member 26 and the gasket 23 were formed so as to sandwich both ends of the current collector 24. Furthermore, an epoxy resin adhesive is applied around the gasket 23 and the support 26,
The electrolyte impregnated in the polarizable electrode 22 was sealed.

Next, a terminal plate 25 for taking out a terminal is provided.
With a pressure of 5 kg / cm ² from both sides and a current collector 24
, And the four corners of the terminal plate 25 were fixed with bolts 27 and nuts 28. To prevent a short circuit between the two poles,
A non-conductive butyl rubber packing 29 was mounted between the bolt 5 and the bolt 27. A 40% by weight aqueous sulfuric acid solution as an electrolytic solution is injected into the thus assembled basic cell through an injection hole (not shown) formed in the side surface of the gasket 23, and after the electrolyte solution is injected, the injection hole is thermally fused. To form a basic cell of an electric double layer capacitor. Similarly, the pressure applied from both sides of the terminal plate 25 is 2
A basic cell of 0 kg / cm ² was prepared.

(Comparative Example 1) Example 1 was repeated except that a conductive butyl rubber sheet coated with graphite powder having a purity of 98% or more was used as the current collector 24 instead of the conductive butyl rubber sheet pressed with expanded graphite powder. The terminal board 25
2 of pressure applied from both sides of the 5 and 20 kg / cm ²
Various types of basic cells were produced.

Comparative Example 2 Example 1 was repeated except that a conductive butyl rubber sheet obtained by pressing a 125 μm-thick graphite sheet was used as the current collector 24 in place of the conductive butyl rubber sheet obtained by pressing expanded graphite powder. The pressure applied from both sides of the terminal plate 25 is 5 and 20 k.
Two types of basic cells of g / cm ² were prepared.

(Comparative Example 3) A current collector 24 having a thickness of 25
10% through-holes were formed in an aluminum foil of
5 kg / c on 0 μm conductive butyl rubber sheet
After press-bonding with m ² , expanded graphite was applied to the aluminum foil side in the same manner as in Example 1, and a thickness of 50 μm was again applied thereon.
Except that the conductive rubber sheet was used and pressed at 5 kg / cm ^2, as in Example 1 and the terminal plate 2
Two types of basic cells were prepared in which the pressure applied from both sides of 5 was 5 and 20 kg / cm ² .

Using the basic cells produced in Example 1 and Comparative Examples 1 to 3, electric double layer capacitors were obtained. The ESR of the electric double layer capacitor, the water vapor permeability of the current collector, and the thickness of the basic cell were measured using the following measurement methods. Table 1 shows the results.

(Measurement of ESR) An ESR was obtained by applying an AC voltage of 1 kHz and 10 mV _{rms to} an electric double layer capacitor and measuring a current and a phase difference. In the sample bottle (water vapor permeability) .phi.30 mm, placed 40 wt% aqueous sulfuric acid solution, placing the current collector in a sample bottle onto an epoxy into a sample bottle a current collector by applying a pressure of 10 kg / cm ² It was crimped with a system adhesive. This sample bottle was left in a drier set at 70 ° C., and determined from the change in weight of the sample bottle. (Thickness of basic cell) The central part of the basic cell was measured using a digital caliper, and the value obtained by dividing the thickness by the thickness of the terminal plates 25 at both ends was defined as the thickness of the basic cell. The thickness of the basic cell was measured for a basic cell in which the pressure from both sides of the terminal plate 25 was 5 kg / cm ² .

[0026]

[Table 1]

The ESR of the first embodiment was smaller than that of the first comparative example regardless of the magnitude of the pressure applied to the terminal plate 25. This indicates that forming an expanded graphite layer on the surface of the conductive butyl rubber sheet is effective in reducing ESR. In Example 1 and Comparative Example 1, the water vapor transmission rate of Comparative Example 1 was higher. This is considered to be because the general graphite powder used in Comparative Example 1 hardly changes the shape of the particles, and the adhesion between the conductive butyl rubber sheet and the graphite particles is not sufficient.

In Example 1 and Comparative Example 2, there was no significant change in ESR, and there was almost no change in the water vapor transmission rate. However, comparing the cell thickness, Comparative Example 2 was 40% or more thicker. Example 1 and Comparative Example 3
In Comparative Example 3, although the water vapor transmission rate of Comparative Example 3 was fairly good, the ESR value of Comparative Example 3 was at least three times that of Example 1.

(Example 2) An electric double layer capacitor was prepared using a current collector 24 in which the thickness of the expanded graphite layer 24b was 1, 5, 10, 20, and 30 µm per side, and ESR was performed.
Was measured. FIG. 6 shows the results. Here, the pressure applied from both sides of the terminal plate 25 was 5 kg / cm ² .

From the results of Example 2, it was found that the ESR value was not significantly deteriorated even if the film thickness per one side was extremely small. Conversely, when the film thickness exceeds 10 μm,
Even if it is applied thicker, the effect is not so much seen, and the expanded graphite layer 24b may peel off. From this,
It was found that the thickness of the expanded graphite layer 24b applied to the surface of the conductive butyl rubber sheet 24a should be 10 μm or less. In this embodiment, the conductive butyl rubber is used as a material constituting the current collector, but instead of the conductive butyl rubber, a polyethylene type, a polypropylene type,
Similar results can be obtained by using any of nitrile, styrene and butadiene conductive rubbers, conductive elastomers and conductive plastics.

[0031]

As described above, in the electric double layer capacitor of the present invention, since the current collector having the expanded graphite layer formed on the surface of the conductive rubber sheet or the conductive plastic sheet is used. It is possible to reduce the thickness, prevent the electrolyte solution from drying up, and significantly reduce the ESR.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an example of a basic cell of an electric double layer capacitor of the present invention.

FIG. 2 is a schematic view showing an example of a manufacturing process of a current collector used for the electric double layer capacitor of the present invention.

FIG. 3 is a schematic sectional view showing an example of a current collector used in the electric double layer capacitor of the present invention.

FIG. 4 is a schematic sectional view showing an example of a current collector used in a conventional electric double layer capacitor.

FIG. 5 is a schematic sectional view showing the structure of a basic cell of the electric double layer capacitor used in the example.

FIG. 6 is a graph showing a relationship between the film thickness per one surface of the expanded graphite layer on the surface of the conductive rubber sheet and E in the electric double layer capacitor of the present invention.
It is the graph which showed the relationship of SR.

FIG. 7 is a schematic sectional view showing an example of a basic cell of a conventional electric double layer capacitor.

FIG. 8 is a schematic sectional view showing an example of a current collector used in a conventional electric double layer capacitor.

[Explanation of symbols]

Reference Signs List 10 current collector 10a conductive rubber sheet or conductive plastic sheet 10b expanded graphite layer

Claims (3)

[Claims] 1. An electric double-layer capacitor using an aqueous electrolytic solution and a conductive rubber sheet or a conductive plastic sheet as a current collector, wherein one or both sides of the conductive rubber sheet or the conductive plastic sheet are provided. An electric double layer capacitor characterized by having an expanded graphite layer formed thereon. 2. The electric double layer capacitor according to claim 1, wherein the thickness of the expanded graphite layer is 10 μm or less. 3. The expanded graphite layer according to claim 1, wherein the expanded graphite in the form of powder or particles is uniformly applied to the surface of the conductive rubber sheet or the conductive plastic sheet and pressed. The electric double layer capacitor according to claim 2.