JPH11307402A - Electrode for electric double layer capacitor and electric double layer capacitor having the electrode - Google Patents

Abstract

(57) [Problem] To provide an electrode for an electric double layer capacitor having excellent mechanical strength, high capacity and low resistance. An electrode is composed of a carbonaceous material and a binder,
As the binder, polytetrafluoroethylene and 1 to 50% by weight of a fluorine-containing polymer resin (for example, an ethylene / tetrafluoroethylene copolymer resin) that can be melt-molded with respect to the polytetrafluoroethylene are used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electrode for an electric double layer capacitor, and more particularly to an electrode for an electric double layer capacitor having excellent mechanical strength and high capacity.

[0002]

2. Description of the Related Art An electric double layer capacitor is based on the principle that electric charges are accumulated in an electric double layer formed at an interface between an electrode and an electrolytic solution. Activated carbon having a high specific surface area, carbonaceous materials such as carbon black, and fine particles of metal or conductive metal oxide are used. The electrodes are joined to a current collector made of a low-resistance layer or foil such as metal or graphite for efficient charging and discharging. As the current collector, a valve metal such as aluminum having high electrochemical corrosion resistance and a stainless steel such as SUS304 and SUS316L are usually used.

[0003] Electrolyte for an electric double layer capacitor includes an organic electrolyte and an aqueous electrolyte. However, since the operating voltage is high and the energy density in the charged state can be increased, the electric double layer capacitor using the organic electrolyte is used. Is attracting attention. When an organic electrolytic solution is used, if water is present inside the electric double layer capacitor cell, the performance is deteriorated due to the electrolysis of the water, so it is necessary to sufficiently dehydrate the electrodes,
Usually, a drying treatment of heating under reduced pressure is performed.

Activated carbon is generally used as a main component as a polarizable electrode. However, since activated carbon is usually in powder form, it is mixed with a binder such as polytetrafluoroethylene to form a sheet-like electrode. Then, the electrode body is formed by being electrically connected to the current collector through the conductive adhesive layer.

As the binder, polytetrafluoroethylene is used because of its excellent heat resistance and chemical stability.
It is known that a continuous fine porous structure is formed by mixing and kneading with active carbon and then stretching to form polytetrafluoroethylene into fibers to form a continuous microporous structure, which is used as an electrode (Japanese Patent Publication No. 7-105316). . However, in this case, if the polytetrafluoroethylene is not sufficiently fiberized,
There has been a problem that sufficient bonding between the carbonaceous fine powders is not performed and the mechanical strength of the electrode is reduced. In order to supplement the mechanical strength, the amount of polytetrafluoroethylene used can be increased, but in this case, the amount of activated carbon is relatively small, so that the capacity of the electric double layer capacitor per unit volume is reduced. descend.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and has an electrode suitable for a high-capacity electric double layer capacitor having excellent mechanical characteristics. An object is to provide an electric double layer capacitor having electrodes.

[0007]

According to the present invention, there is provided an electrode for an electric double layer capacitor comprising a carbonaceous material and a binder, wherein the binder is one to one of polytetrafluoroethylene and polytetrafluoroethylene. Disclosed are an electrode for an electric double layer capacitor characterized by comprising 50% by weight of a melt-moldable fluoropolymer resin, and an electric double layer capacitor having the electrode.

The present inventors have conducted intensive studies to obtain a high capacity electrode for an electric double layer capacitor having excellent mechanical properties using polytetrafluoroethylene as a binder. It has been found that it is effective to add a small amount of a possible fluoropolymer resin.

The electrode of the present invention is preferably a porous sheet in which polytetrafluoroethylene is fibrillated to form a continuous microporous structure, and the structure holds a carbonaceous material. Polytetrafluoroethylene is three-dimensionally fiberized by application of external stress such as shearing, rolling, or stretching to form a continuous microporous structure. Since the carbonaceous material can be densely contained in the continuous microporous structure, the electrode composed of the structure gives a large capacitor capacity and has a moderate flexibility due to the presence of the fine fibers. High mechanical strength. Then, in this case, it is considered that the fluoropolymer resin thermally stabilizes the porous structure of polytetrafluoroethylene bonded by fibrillation.

In order to obtain the above-mentioned electrode, for example, a mixture comprising a carbonaceous material, polytetrafluoroethylene, the above-mentioned fluoropolymer resin and a liquid lubricant is kneaded, then molded into a sheet, and the molded product is subjected to a rolling treatment. And / or stretching. In this case, the stretching may be uniaxial or multiaxial, and the liquid lubricant is removed before or after the rolling and / or stretching.
In particular, when the mixture is subjected to extrusion such as paste extrusion or screw extrusion, and then subjected to rolling and / or stretching, polytetrafluoroethylene is fiberized vertically and horizontally to form a three-dimensional network structure. This is preferable because an electrode sheet having a small thickness can be manufactured continuously.

The polytetrafluoroethylene in the present invention is a powder obtained through a coagulation step from a dispersion obtained by a dispersion polymerization method carried out in an aqueous medium, which is one of the general polymerization methods for polytetrafluoroethylene. A so-called fine powder in the form is preferably used.

Although polytetrafluoroethylene in the present invention cannot be melt-molded, it may be not only a homopolymer of tetrafluoroethylene but also a so-called trace copolymer in which a comonomer is copolymerized to such an extent that it cannot be melt-molded. As a comonomer to be copolymerized in such a trace copolymer, a fluorine-containing ethylenic monomer can be used, and specifically, hexafluoropropylene, chlorotrifluoroethylene, perfluoro (alkyl vinyl ether), (perfluoroalkyl) ethylene and the like can be used. No. In this case, the polymerization unit based on the comonomer is contained in the polytetrafluoroethylene in an amount of 1.0% by weight or less.

In the electrode of the present invention, it is preferable that polytetrafluoroethylene is contained in an amount of 1 to 50% by weight based on the carbonaceous material. When the amount of polytetrafluoroethylene is large, the mechanical strength of the electrode increases, but the capacity decreases. If the amount of polytetrafluoroethylene is less than 1% by weight, the strength of the electrode will be weak. More preferably, the amount of polytetrafluoroethylene is 3 to 30% by weight.

In the present invention, the melt-moldable fluorine-containing polymer includes a tetrafluoroethylene / perfluoro (alkyl vinyl ether) copolymer, a tetrafluoroethylene / hexafluoropropylene copolymer, and a tetrafluoroethylene / ethylene copolymer. Copolymer, tetrafluoroethylene / propylene copolymer, chlorotrifluoroethylene polymer, chlorotrifluoroethylene / ethylene copolymer, chlorotrifluoroethylene /
Alkyl vinyl ether copolymer, vinylidene fluoride polymer, vinylidene fluoride / hexafluoropropylene copolymer, fluoroalkyl acrylate polymer, fluoroalkyl methacrylate polymer, perfluoro (butenyl vinyl ether) cyclization Examples thereof include polymers.

Of these, a tetrafluoroethylene / ethylene copolymer or a vinylidene fluoride polymer is particularly preferred. However, in the present specification, the term “A / B-based copolymer” refers to a copolymer containing a polymerized unit based on A and a polymerized unit based on B, and a polymerized unit based on a monomer other than A and B. Or a copolymer containing no more than 30% by weight. Further, the C-based polymer is a polymer having a polymerized unit based on C as a main component, and is a polymer based on another monomer other than the homopolymer including the polymerized unit based on C and the polymerized unit based on C. A copolymer containing 30% by weight or less of a unit is indicated. In the above copolymer, since the copolymer containing the polymerized unit based on tetrafluoroethylene is a resin that can be melt-molded, the content of the polymerized unit based on tetrafluoroethylene is 99% by weight.
Is less than.

The content of the fluoropolymer is 1 to 50% by weight based on polytetrafluoroethylene in the electrode. If the amount is less than 1% by weight, the effect of adding the above fluoropolymer is not substantially exhibited. On the other hand, if it exceeds 50% by weight, the electrical resistance of the electrode increases and the charge / discharge cycle durability deteriorates.

The above-mentioned fluoropolymer resin is preferably contained in the electrode in an amount of 0.1 to 5% by weight, particularly 0.5 to 3% by weight, based on the carbonaceous material. If the amount is less than 0.1% by weight, the effect of adding the above fluoropolymer resin is not substantially exhibited. On the other hand, when the content exceeds 5% by weight, the electric resistance of the electrode increases.

In the present invention, since the strength of the electrode is increased by adding the above-mentioned fluoropolymer resin, sufficient mechanical properties can be obtained even if the amount of the binder in the electrode is small, especially 10% by weight or less. And, as a result, the amount of carbonaceous material in the electrode can be increased, so that a high capacity can be achieved.

In the present invention, the carbonaceous material contained in the continuous microporous structure constituting the electrode has a specific surface area of 700 to 3000 m ² / g, particularly 1000 to 2500.
Those having m ² / g are preferred because of their large capacity. Specific examples include activated carbon, carbon black, and polyacene.
In particular, when carbon black such as acetylene black or Ketjen black is added as a conductive material to the electrode so as to have a specific surface area of 700 to 3000 m ² / g as a main component and a conductive material of 5 to 20% by weight, high capacity and high conductivity are obtained. This is preferable because an electrode sheet having a characteristic property is obtained. As the activated carbon, any of phenol-based, rayon-based, acrylic-based, bitch-based or coconut husk-based activated carbon can be used.

In the present invention, it is preferable to use a powdery carbonaceous material because the carbonaceous material having the same specific surface area has a higher capacity than a fibrous material. When the carbonaceous powder has a particle size of 0.1 to 200 μm, particularly preferably 1 to 50 μm, the strength when molded into a sheet-like electrode is preferably high. However, if necessary, for example, the crushed length of 0.1 to 2
Carbon fibers of 00 μm, especially 1 to 50 μm, can also be used.

The organic electrolyte used for the electric double layer capacitor of the present invention is not particularly limited, and any known organic solvent containing an ion-dissociable salt can be used. Among them, R ¹ R ² R ³ R ⁴ N ⁺ , R ¹ R ² R ³ R ⁴ P ⁺
(Where R ¹ , R ² , R ³ and R ⁴ are alkyl groups, which may be the same or different), and BF ₄ ⁻ , PF ₆ ⁻ , and ClO ₄ ⁻ , CF
₃ SO ₃ ^- preferably a salt consisting of an anion using an organic electrolyte obtained by dissolving in an organic solvent such as.

The organic solvent is preferably a carbonate such as propylene carbonate, butylene carbonate, diethyl carbonate, etc., a lactone such as γ-butyrolactone, sulfolane, or a mixed solvent of two or more selected from these.

As the separator of the electric double layer capacitor of the present invention, cellulose paper, mixed paper of cellulose and glass fiber, glass fiber mat, porous polypropylene sheet, porous polytetrafluoroethylene sheet and the like can be used. Glass fiber mats having high heat resistance and a low moisture content, and thin but high-strength cellulose paper are preferred.

In the electric double layer capacitor of the present invention, for example, a pair of band-like electrode bodies are used as a positive electrode body and a negative electrode body, and a band-like separator is interposed therebetween to be wound to form an element, which is housed in a bottomed cylindrical case. Impregnating the device with an organic electrolyte solution,
It is preferable to obtain by closing with a lid made of a thermosetting insulating resin having a positive electrode terminal and a negative electrode terminal. At this time, the material of the case is preferably aluminum, and a rubber ring is arranged on the peripheral portion of the lid, and it is preferable that the case is curled.

Further, for example, a plurality of rectangular electrode bodies are formed into the same number of positive electrode bodies and negative electrode bodies, and separators are interposed therebetween so as to be alternately laminated to form an element. The lead is taken out and housed in a bottomed square aluminum case, and the element is impregnated with an organic electrolytic solution. Then, a lid having a positive electrode terminal and a negative electrode terminal is attached, and sealed by laser welding or the like. A double layer capacitor may be configured. With such a cylindrical or square structure, an electric double layer capacitor having a high capacity and a large capacity per unit volume can be obtained.

[0026]

EXAMPLES Example 1 (Example) The electrode has a specific surface area of 180.
0 m ² / g, high-purity activated carbon powder 80 having an average particle size of 10 μm
Parts by weight, 10 parts by weight of Ketjen Black, 10 parts by weight of polytetrafluoroethylene fine powder (manufactured by Asahi Glass Co., Ltd., trade name: Fluon CD1), and a tetrafluoroethylene / ethylene copolymer resin which is a resin capable of being melt-molded. Powder (polymerized units based on tetrafluoroethylene / polymerized units based on ethylene / polymerized units based on (perfluorobutyl) ethylene in a molar ratio of 56/43.
5 / 0.5. Made by Asahi Glass Co., Ltd., product name: Aflon COP Z
-8820. 2) After mixing 2 parts by weight, the mixture was kneaded while dropping ethanol, and roll-rolled to prepare an electrode sheet having a thickness of 120 µm. This was dried at 200 ° C. for 30 minutes to remove ethanol. When the tensile strength of this sheet was measured, it was 0.3 kg / cm ² .

The above-mentioned electrode sheet was bonded to a 50-μm-thick aluminum foil via a conductive adhesive, and heated to harden the adhesive to form an electrode body. Twenty-four electrode bodies having an effective electrode area of 6.5 cm × 12 cm were obtained from this electrode body, 22 of which were used as a positive electrode body and the remaining 22 were used as a negative electrode body.
The positive electrode body and the negative electrode body were alternately laminated via a glass fiber mat separator having a thickness of 160 μm to obtain a device.
This element was housed in a bottomed rectangular aluminum case having a height of 13 cm, a width of 7 cm, and a thickness of 2.2 cm, and was sealed by laser welding using an aluminum top lid having a positive electrode terminal and a negative electrode terminal, and a liquid inlet was opened. Vacuum drying was performed at 200 ° C. for 5 hours to remove impurities. Next, a 1.5 mol / l propylene carbonate solution of (C ₂ H ₅ ) ₃ (CH ₃ ) NPF ₆ was used as an electrolytic solution to impregnate the device under vacuum, and then a safety valve was arranged at the injection port to set a width of 7 cm and a height of 7 cm. A rectangular electric double layer capacitor having a thickness of 15 cm and a thickness of 2.2 cm was obtained.

The initial electric discharge capacity of the obtained electric double layer capacitor was 1400 F, and the internal resistance was 2.2 mΩ.
The leakage current after charging at 2.5V for 100 hours is 0.2m
A. After charging at 2.5V for 100 hours, 25 ℃
After holding for 30 days, the holding voltage is 2.
The voltage was 30 V, and the voltage holding property was good.

Next, in a 45 ° C. constant temperature bath, the temperature is 0 to 2.5 V.
Repeat the charge / discharge cycle with a constant current of 50A between 300,000 times, measure the discharge capacity and internal resistance after 300,000 cycles, and accelerate the long-term operation reliability of the electric double layer capacitor compared with the initial characteristics Was evaluated. Capacity retention rate is 9
The rate of increase of the internal resistance was 0%, and the charge / discharge reliability at a large current was high.

Example 2 (Example) As a fluorine-containing polymer resin capable of being melt-molded, vinylidene fluoride-based polymer resin powder (vinylidene fluoride) was used instead of 2 parts by weight of tetrafluoroethylene / ethylene copolymer resin powder. Of polymerized units based on styrene / polymerized units based on hexafluoroethylene in a molar ratio of 96/4, manufactured by Atochem, trade name: Kainer 285
One. An electrode was produced in the same manner as in Example 1 except that 2 parts by weight was used. The tensile strength of this sheet was measured.
It was 32 kg / cm ² .

Using this electrode, an electric double layer capacitor was prepared in the same manner as in Example 1, and evaluated in the same manner as in Example 1. The initial discharge capacity was 1390 F, the internal resistance was 2.3 mΩ, and the leakage was The current was 0.2 mA and the holding voltage was 2.29V. The capacity retention rate after 300,000 charge / discharge cycles was 91%, and the increase rate of the internal resistance was 9%.

Example 3 (Comparative Example) An electrode sheet was prepared in the same manner as in Example 1 except that the tetrafluoroethylene / ethylene copolymer resin powder was not used. When the tensile strength of this sheet was measured, it was 0.15 kg / cm ² . Using this electrode, an electric double layer capacitor was produced in the same manner as in Example 1 and evaluated in the same manner as in Example 1. The initial discharge capacity was 1350 F and the internal resistance was 2.5 m.
Ω, the leakage current was 0.5 mA, and the holding voltage was 1.85 V. The capacity retention rate after 300,000 charge / discharge cycles was 80%, and the increase rate of the internal resistance was 30%.

Example 4 (Comparative Example) An electrode sheet was prepared in the same manner as in Example 1 except that the amount of the tetrafluoroethylene / ethylene copolymer resin powder was changed to 8 parts by weight. When the tensile strength of this sheet was measured, it was 0.31 kg / cm
^{Was 2} . Using this electrode, an electric double layer capacitor was produced in the same manner as in Example 1 and evaluated in the same manner as in Example 1. As a result, the initial discharge capacity was 1190 F, the internal resistance was 4.5 mΩ, and the leakage current was 0. 0.5 mA, and the holding voltage was 1.78 V. The capacity retention rate after 300,000 charge / discharge cycles is 58%, and the internal resistance rise rate is 25%.
Met.

Example 5 An electrode sheet was prepared in the same manner as in Example 2, except that 7 parts by weight of polytetrafluoroethylene fine powder and 1 part by weight of vinylidene fluoride polymer resin powder were used. . When the tensile strength of this sheet was measured, it was 0.5 kg / cm ² . Using this electrode, an electric double layer capacitor was produced in the same manner as in Example 1 and evaluated in the same manner as in Example 1. As a result, the initial discharge capacity was 1410 F, the internal resistance was 2.2 mΩ, and the leakage current was 0. 0.2 mA, and the holding voltage was 2.29 V. The capacity retention rate after 300,000 charge / discharge cycles is 89
%, And the increase rate of the internal resistance was 10%.

[0035]

The electrode for an electric double layer capacitor of the present invention can provide an electric double layer capacitor excellent in mechanical strength, low in internal resistance and excellent in voltage retention. In addition, because of its excellent mechanical strength, the amount of polytetrafluoroethylene fine powder added as a binder can be reduced, and the electric double layer capacitor can have high capacity and low resistance.

Claims (5)

[Claims] An electrode for an electric double layer capacitor comprising a carbonaceous material and a binder, wherein the binder can be melt-molded in an amount of 1 to 50% by weight based on polytetrafluoroethylene and polytetrafluoroethylene. An electrode for an electric double layer capacitor characterized by comprising a fluoropolymer resin. 2. The carbonaceous material has a specific surface area of 700 to 3000.
^{2. The} electrode for an electric double layer capacitor according to claim 1, comprising m ² / g of activated carbon and carbon black, wherein the carbon black is contained in the electrode in an amount of 5 to 20% by weight. 3. The electrode according to claim 1, wherein the carbonaceous material contains 1 to 50% by weight of polytetrafluoroethylene and 0.1 to 5% by weight of the fluoropolymer resin.
The electrode for an electric double layer capacitor according to the above. 4. The electrode for an electric double layer capacitor according to claim 1, wherein the fluorine-containing polymer is an ethylene / tetrafluoroethylene-based copolymer or a vinylidene fluoride-based polymer. 5. An electric double layer capacitor comprising a positive electrode and a negative electrode comprising an electrode containing a carbonaceous material and a binder, and an organic electrolytic solution, wherein the electrodes are formed.
Or an electric double-layer capacitor comprising the electrode according to 4.