JP2003100569A - Electric double layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric double layer capacitor which undergoes no characteristic deterioration and is excellent in long-term reliability. SOLUTION: An electric double layer capacitor utilizes an electric double layer formed at the interface between a polarizable electrode 9 and an electrolyte 6. The polarizable electrode 9 comprises a positive polarizable electrode 1 and a negative polarizable electrode 5 which are disposed so as to oppose each other via a separator 7. The electrode 1 is made smaller than the electrode 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitor, and more particularly to an electric double layer capacitor utilizing an electric double layer formed at an interface between an electrode made of activated carbon or the like and an electrolytic solution.

[0002]

2. Description of the Related Art A conventional electric double layer capacitor having a small size has a separator such as polypropylene nonwoven fabric sandwiched between a pair of polarizable electrodes having a polarizable electrode layer mainly composed of activated carbon on a collector. The device has a coin-type structure in which the device is impregnated with an electrolytic solution, housed in a metal container, and sealed in a metal container with a sealing plate and a gasket. In addition to this, for a relatively large capacity, a laminated sheet with a separator sandwiched between a pair of sheet-shaped polarizable electrodes is wound into an element, which is impregnated with an electrolytic solution and housed in a metal container. A wound-type capacitor having an opening hermetically sealed with a sealing member is manufactured. These electric double layer capacitors are mainly used for backup of IC memories and actuators.

Further, a laminated electric double layer capacitor having an element in which a separator is sandwiched between a large number of sheet-like polarizable electrode layers for large current and large capacity is also disclosed.
154106 and Japanese Patent Laid-Open No. 4-286108.

The polarizable electrodes constituting these electric double layer capacitors are mainly made of activated carbon having a large surface area, and in a wound type capacitor, activated carbon powder or the like is used as a binder such as PTFE (polytetrafluoroethylene). In the coin type, the above-mentioned sheet-like activated carbon or activated carbon fiber cloth is used.

By the way, when the voltage applied to the electric double layer capacitor is gradually increased, a decomposition reaction of the electrolytic solution occurs at a certain potential or higher, oxygen gas is generated on the anode (positive electrode) side, and hydrogen gas is generated on the cathode (negative electrode) side. Occurs. Therefore,
The potential just before the gas generation reaction occurs at each of the anode and cathode is the oxidation potential and the reduction potential of the electrolytic solution,
The maximum voltage (withstand voltage) that can be used as a capacitor is the difference between the oxidation potential and the reduction potential.

The above withstand voltage is about 1 V, which is the decomposition potential of water when an aqueous electrolyte solution is used, and slightly different depending on the electrolyte and solvent used when an organic electrolyte solution is used. However, it is known that the decomposition potential is +2 to 3 V on the positive electrode side and −2 to 3 V on the negative electrode side, and the withstand voltage is about 4 to 6 V.

In this electric double layer capacitor, when a positive voltage is applied to the positive electrode and a negative voltage is applied to the negative electrode, an anion and a cation face each other at the interface between the positive electrode and the negative electrode, forming an electric double layer. Further, there is a diffusion layer on the outside of the diffusion layer, which is generated by the diffusion of a part of the ions due to the force for maintaining the homogeneity of the solution phase, and the ions are distributed according to the Boltzmann distribution law. In the bulk region of the electrolyte, there are almost the same number of cations and anions,
It is electrically neutral. The potential distribution at this time is shown in FIG. There is a potential gradient in the positive and negative electric double layers, but there is almost no potential difference in the bulk region of the electrolytic solution.

At this time, the difference between the positive electrode potential and the negative electrode potential is the voltage applied to the capacitor, and if this is within the withstand voltage of the electrolytic solution, the capacitor operates stably. However, when a voltage is applied to the capacitor, the potentials of the positive electrode and the negative electrode move from the equilibrium potential in opposite directions by the same amount, but generally, the activated carbon electrode has an oxidizing surface functional group, and the equilibrium potential is the positive electrode. Since it is shifted to the side, there is a sufficient margin up to the decomposition potential on the negative electrode side, but the decomposition potential reaches first on the positive electrode side. For this reason, when the organic electrolytic solution was used, the driving voltage of the capacitor was suppressed to about 2 to 2.5 V, although the decomposition voltage of the electrolytic solution was 4 to 6V.

[0009]

As described above, the electric double layer capacitor is a capacitor utilizing the electric storage function of the electric double layer formed between the two electrodes facing each other. However, conventionally, many problems have occurred because the structure has an electrode facing structure.

For example, in the wound type structure mainly used for large-capacity capacitors, a separator is sandwiched between a positive electrode and a negative electrode and wound, but for some reason, a winding deviation occurs in the electrode width direction. , It is easy for the positions of the winding ends of both poles to shift. In a coin-type capacitor, the electrode sheet is usually punched out in a circular shape, but the center is often slightly displaced when the separator sheet is sandwiched and stacked.

Due to this shift, in the portions where the electrodes are not evenly opposed to each other, the voltage balance in the electrodes may be disturbed, and the voltage applied to the capacitor may be concentrated, resulting in a large load. In particular, on the positive electrode side, as described above, the margin until reaching the decomposition potential is small, so that the electrolyte is decomposed even if the load is increased even a little, and the performance of the capacitor is deteriorated.

In particular, when a paper made of rayon or hemp is used as the separator, the decomposition product of the electrolytic solution causes deterioration, which may cause a short circuit.

An object of the present invention is to solve the above-mentioned problems of the prior art and to provide an electric double layer capacitor which is free from characteristic deterioration and has excellent long-term reliability.

[0014]

As a result of earnest efforts for the purpose of developing an electric double layer capacitor having a high withstand voltage and excellent long-term reliability, the present inventors have found that the polarizable electrode of the positive electrode side of the polarizable electrodes is It has been found that by making the size smaller than the polarizable electrode on the negative electrode side, an electric double layer capacitor having high withstand voltage and less deterioration can be manufactured.

That is, the present invention relates to an electric double layer capacitor comprising an electric double layer formed at an interface between a polarizable electrode and an electrolytic solution, wherein the polarizable electrodes are opposed to each other with a separator interposed therebetween. In the polarizable electrode, the polarizable electrode on the positive electrode side is smaller than the polarizable electrode on the negative electrode side, which is an electric double layer capacitor.

Further, the present invention is the above electric double layer capacitor, wherein the polarizable electrode is made of carbon powder or activated carbon fiber.

Further, the present invention is the above electric double layer capacitor, wherein the carbon powder is made of conductive carbon.

In the present invention, the conductive carbon is
The electric double layer capacitor is characterized in that it is activated carbon powder or carbon black.

Further, the present invention is the above electric double layer capacitor, wherein the separator is a rayon paper.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION For example, in the case of a wound type electric double layer capacitor, it is manufactured by stacking a reel-shaped electrode sheet cut into a predetermined width with a reel-shaped separator and winding the same. However, at this time, the width of the electrode sheet on the positive electrode side is made slightly smaller (1 to 2 mm) than that of the negative electrode so that the positive electrode can be prevented from protruding even when winding deviation occurs. By making the positive electrode smaller than the negative electrode in this way, the electrolytic solution can be prevented from reaching the decomposition potential, so that the withstand voltage is improved and the long-term reliability is improved.

As the activated carbon used for the polarizable electrode,
Phenolic resin-based activated carbon, coconut husk-based activated carbon, petroleum coke-based activated carbon and the like are available. Phenolic resin-based activated carbon is preferably used because a large capacity electric double layer capacitor can be obtained and the purity is high. Further, as activated carbon, in order to obtain an electric double layer capacitor having a large capacity and a low internal resistance, powder has an average particle diameter of 20 μm or less, and fibrous has a fiber diameter of 10 to 20 μm and both have a specific surface area. It is preferable to use 1000 to 3000 m ² / g of activated carbon.

As the activated carbon activation treatment method, there are a steam activation treatment method and a molten KOH activation treatment method.
It is preferable to use activated carbon obtained by the steam activation treatment method because an electric double layer capacitor with less deterioration of characteristics and high reliability can be obtained.

As the binder for the polarizable electrode, a polytetrafluoroethylene-based binder is preferably used because it has high chemical resistance when an organic solvent-based electrolyte is used.

The electrolytic solution used in the present invention is not particularly limited, and those usually used for electric double layer capacitors, that is, those obtained by dissolving an electrochemically stable electrolyte in a polar organic solvent are used. Used as appropriate.

As the above electrolyte, (C_TwoH₅)_FourN
⁺Or (C_FourH₉)_FourN⁺, (C_TwoH ₅)_FourP⁺Such as
Quaternary onium cation and BF_Four ⁻And PF₆ ⁻, ClO
_Four ⁻A salt composed of anions such as
It

As the above organic solvent, carbonates such as propylene carbonate, butylene carbonate and diethyl carbonate, lactones such as γ-butyrolactone and sulfolane can be preferably used. These may be used alone or in combination of two or more.

As the separator used in the present invention,
The separator is not particularly limited, and usual separators for electric double layer capacitors, such as rayon paper, glass fiber mixed paper, polypropylene nonwoven fabric, and the like can be used.

[0028]

EXAMPLES The present invention will be specifically described below with reference to examples and conventional examples.

Example 1 Phenolic resin-based steam activated activated carbon powder (average particle size 5 μm) as a polarizable electrode
Ethanol was added to a mixture of 80% by weight, 10% by weight of carbon black (made by Nippon Graphite Co., Ltd.) and 10% by weight of polytetrafluoroethylene-based binder, and the mixture was kneaded and rolled to obtain a width of 8 cm, a length of 10 cm and a thickness of 0. .
A 6 cm sheet was formed and dried at 250 ° C. for 2 hours.

FIG. 1 shows a vertical sectional view of the coin type electric double layer capacitor of the first embodiment. A polarizable electrode 9 composed of a positive electrode side polarizable electrode 1 and a negative electrode side polarizable electrode 5 obtained by punching out the electrode sheet obtained by the above method into a positive electrode diameter of 9.5 mm and a negative electrode diameter of 10 mm was attached to a graphite-based conductive adhesive. The agent 2 was adhered to the stainless steel container 10. The container 10 and the polarizable electrode 9 were dried at 300 ° C. under reduced pressure for 2 hours, and then the polarizable electrode 9 was impregnated with the electrolytic solution 6 in a glove box in a dry nitrogen atmosphere. The electrolyte solution 6 contains 0.7 mol of tetraethylammonium tetrafluoroborate.
It was prepared by dissolving in propylene carbonate at a concentration of 1 / l. Polarizable electrode 9 impregnated with this electrolytic solution 6
Were made to face each other through a separator 7 made of rayon paper, and a polypropylene gasket 8 was used for caulking and sealing.

Example 2 Phenolic resin-based steam activated activated carbon powder (average particle size 5 μm) as a polarizable electrode
By coating a mixture of 80% by weight, 10% by weight of carbon black (manufactured by Nippon Graphite Co., Ltd.) and 10% by weight of polyvinylidene fluoride-based binder on an Al foil having a thickness of 30 μm, width 10 cm, length 10 m, thickness The sheet was 150 μm. After winding this into a reel, the positive electrode has a width of 2
7 mm, the negative electrode was cut into 28 mm, and these were dried at 250 ° C. for 2 hours.

FIG. 2 is a perspective view showing the outline of the wound type electric double layer capacitor of the second embodiment. Sheet-like positive electrode side polarizable electrode 12 and negative electrode side polarizable electrode 14 obtained by the above method
A lead tab 15 made of Al was attached to the sheet, and the separator 13 made of rayon-based paper was sandwiched to wind the element into a device having a diameter of 16 mm, and the same electrolytic solution as in Example 1 was impregnated. Thereafter, the winding type electric double layer capacitor was assembled by housing in a bottomed cylindrical outer case 11 made of Al, inserting a rubber cap 16 into the opening of the outer case, and caulking and curling.

(Prior Art Example 1) Both the positive electrode and the negative electrode of the electrode sheet obtained in the same manner as in Example 1 were punched to have a diameter of 10 mm. A coin type electric double layer capacitor was assembled in the same manner as in Example 1 except for the above.

(Conventional Example 2) A reel electrode obtained in the same manner as in Example 2 was cut into a width of 28 mm for both the positive electrode and the negative electrode.
A wound electric double layer capacitor was assembled in the same manner as in Example 2 except for the above.

Example 1, Example 2, Conventional Example 1, Conventional Example 2
The electric double layer capacitor of is charged with an applied voltage of 2.7V,
The initial capacitance (F) when discharged in Example 1 and Conventional Example 1 was about 0.5 mA, and in Example 2 and Conventional Example 2 was about 25 mA.
And ESR (Ω) were successively stored in a constant temperature bath at 70 ° C. for 1000 hours while applying a voltage of 2.7 V to the cell, and the capacity and ESR were measured. Table 1 shows the measurement results
Shown in.

[0036]

[Table 1]

As is apparent from Table 1, the electric double layer capacitors of Examples 1 and 2 in which the positive electrode of the present invention is smaller than the negative electrode are compared with the electric double layer capacitors of Conventional Example 1 and Conventional Example 2. Therefore, capacity deterioration and internal resistance increase are small,
It can be seen that the long-term reliability is high.

[0038]

As described above, according to the present invention,
It is possible to provide an electric double layer capacitor having no deterioration in characteristics and excellent in long-term reliability.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of a coin-type electric double layer capacitor of Example 1 of the present invention.

FIG. 2 is a perspective view showing an outline of a wound electric double layer capacitor according to a second embodiment of the present invention.

FIG. 3 is a diagram showing a state in which a voltage is applied to the electric double layer capacitor and a potential distribution at that time.

[Explanation of symbols]

1 Positive electrode side polarizable electrode 2 Conductive adhesive 3 container cases 4 container lid 5 Negative electrode side polarizable electrode 6 Electrolyte 7 separator 8 gasket 9-minute polar electrode 10 containers 11 exterior case 12 Positive electrode side polarizable electrode 13 separator 14 Negative polarizable electrode 15 Lead tab 16 caps

Claims (5)

[Claims] 1. An electric double layer capacitor comprising an electric double layer formed at an interface between a polarizable electrode and an electrolytic solution, wherein the polarizable electrodes are opposed to each other with a separator interposed therebetween. An electric double layer capacitor in which the polarizable electrode on the positive electrode side is smaller than the polarizable electrode on the negative electrode side. 2. The electric double layer capacitor according to claim 1, wherein the polarizable electrode is made of carbon powder or activated carbon fiber. 3. The electric double layer capacitor according to claim 2, wherein the carbon powder is made of conductive carbon. 4. The electric double layer capacitor according to claim 3, wherein the conductive carbon is activated carbon powder or carbon black. 5. The electric double layer capacitor according to claim 1, wherein the separator is rayon paper.