JP2004048055A - Electric double layer capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide electric double layer capacitor with less performance degradation caused by a charge and discharge cycle, and excellent operation reliability for a long term. <P>SOLUTION: The electric double layer capacitor includes a polarizable electrode and organic electolyte forming an electric double layer on the surface of the polarizable electrode. The polarizable electrode with a main component of active carbon formed sheet-like is bonded to a collector via a carbon based conductive adhesive layer containing polymide resin or polyamideimide resin as a binder component. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to an electric double layer capacitor (hereinafter referred to as EDLC), and more particularly to an EDLC excellent in operation reliability.

EDLC is based on the principle of accumulating electric charge in the electric double layer formed in the electrolyte solution on the surface of the polarizable electrode. In order to improve the capacity density of EDLC, the polarizable electrode has a high specific surface area activated carbon, Carbon materials such as carbon black, fine particles of metal or conductive metal oxide, and the like are used. In order to efficiently charge and discharge these polarizable electrodes having a high specific surface area, these polarizable electrodes are joined to a layer called a current collector, such as a metal or graphite, having a low electron conductivity resistance or a foil. For these current collectors, valve metals such as aluminum having high electrochemical resistance and stainless steel such as SUS304 and SUS316L are usually used.

There are two types of EDLC, one using an organic electrolyte and the other using an aqueous electrolyte. The EDLC uses an organic electrolyte because the operating voltage is high and the energy density of the charged state can be increased. Is attracting attention. When using an organic electrolyte, the presence of water inside the EDLC causes the water to electrolyze and degrade performance, so the polarizable electrode must be highly dehydrated and is usually dried by heating under reduced pressure. Processing is performed.

Activated carbon is mainly used for the polarizable electrode, and the activated carbon is usually in the form of powder. For example, the current collected is formed into a sheet using a binder containing a fluorine-containing resin such as polytetrafluoroethylene (PTFE). Used as an electrode electrically connected to the body. At this time, many of them are bonded via a conductive adhesive layer so that both are brought into close contact with each other and the electrical contact resistance is reduced. However, the fluorine-containing resin has a property that it is difficult to bond, and it is difficult to obtain a large bonding strength.

Since the conductive adhesive layer requires electrochemical corrosion resistance, a carbon material such as carbon black or graphite is preferably used as a filler that gives electronic conductivity to the conductive adhesive layer. Further, various binder components are used for the conductive adhesive layer in order to ensure the bonding strength. As binder components used for this purpose, resins such as cellulose and polyvinyl alcohol (Patent Documents 1 and 2) and inorganic binder components such as water glass (Patent Document 3) are known.

However, when a conductive adhesive containing these resin binder components is used, the polarizable electrode and the current collector may be peeled off due to insufficient resistance to the organic electrolyte. Moreover, even if the binder has good heat resistance, the heat resistance is around 150 ° C., so it cannot be dried at high temperature, and the performance of EDLC is deteriorated due to the electrolysis of residual moisture adsorbed on the activated carbon. There were problems such as. In addition, inorganic binders such as water glass have high heat resistance but insufficient adhesive strength with a metal current collector, and there is a problem in that EDLC performance deteriorates due to elution of alkali components and residual moisture.

JP 59-3915 (Claims) JP-A-62-200715 (page 2, upper left column, last line to upper right column, 3 lines) JP-A-2-82608 (Claims)

The object of the present invention is to provide an EDLC that can reduce the problem in the prior art, that is, EDLC, in particular, moisture in the polarizable electrode as much as possible, has strong electrical connection between the electrode and the current collector, and does not cause performance deterioration. And

The EDLC of the present invention is a polarizable electrode mainly composed of activated carbon formed into a sheet shape in an electric double layer capacitor having a polarizable electrode and an organic electrolyte that forms an electric double layer on the surface of the polarizable electrode. Is bonded to the current collector through a carbon-based conductive adhesive layer containing polyimide resin or polyamideimide resin as a binder component.

The EDLC according to the present invention is remarkably small in capacity deterioration and increase in internal resistance when 3000 charge / discharge cycles are repeated under test conditions that cause high-temperature accelerated deterioration, and has excellent operational reliability when used for a long period of time. It turns out that it is EDLC.

In the EDLC of the present invention, the binder component contained in the carbon-based conductive adhesive layer is a polyimide resin or a polyamideimide resin, and the heat resistance temperature of these resins is usually in the range of 200 to 400 ° C. and has high heat resistance. Polyimide resin is a general term for those having an imide structure (—CO—NR—CO—) in its main chain, and is excellent in chemical resistance, mechanical properties, dimensional stability, and electrical characteristics. Polyimide resins can be broadly classified into linear polyimide resins and curable polyimide resins. Linear polyimide resins include thermoplastic resins and non-thermoplastic resins, and curable resins include thermosetting resins and photocurable resins. Regardless of which type of polyimide resin is selected, the resin must be a solvent. It is preferable to use a varnish dissolved in the adhesive.

There are two types of polyimide resin varnishes: one in which a polyimide resin soluble in a solvent is dissolved in the solvent, and one in which a polyimide resin precursor such as polyamic acid is dissolved in a solvent, which becomes an imide resin by high-temperature heat treatment. Can be used in the same manner. For the varnish of polyimide resin, Ube Industries' “U-varnish” (polyamic acid dissolved in a solvent), Shin Nippon Rika Co., Ltd.'s “Rika Coat” (solvent-soluble polyimide resin dissolved in a solvent) DuPont's "Pier ML", Hitachi Chemical's "PIQ", Toray's "Trenice" and Asahi Kasei's "Pimel". Further, varnishes obtained by dissolving a polyamideimide resin soluble in a solvent in a solvent include “N7525” and “NA-11” manufactured by Toyobo.

Thus, in the EDLC of the present invention, since the heat resistance of the polyimide resin or polyamideimide resin that is the binder component of the carbon-based conductive adhesive layer is high, it is in the activated carbon by heating at high temperature or heat treatment under reduced pressure. Moisture can be removed by high drying. In addition, this binder component is resistant to an organic electrolyte, and the adhesion strength of a polarizable electrode sheet having a fluorine-containing resin as a binder to a current collector such as metal is extremely excellent. For this reason, even if a charge / discharge cycle is repeated at a large current density or a voltage is applied for a long period of time, the operation performance is stable, and at the same time, the increase in internal resistance of the electrode can be reduced.

Since the resin used for the binder component can be obtained as a resin powder or varnish, carbon black and graphite fine particles are highly used as conductive fillers in those resins dissolved in a solvent such as N-methyl-2-pyrrolidone (NMP). Dispersed to form a suspension adhesive, and this adhesive is applied to the surface of the current collector by dropping, brushing, spraying, etc., and then a separately prepared sheet of polarizable electrode is pressure-bonded to the surface, preferably Is firmly bonded by heating and drying at a high temperature of 250 ° C. or higher, and further under reduced pressure.

A preferred EDLC of the present invention is a carbon-based conductive adhesive layer in which a binder component is a heat-cured varnish in which a solvent-soluble polyimide resin, polyamide-imide resin or a precursor of these resins is dissolved in an organic solvent. It contains 10 to 70% by weight of the polyimide resin or polyamideimide resin in the total amount of the polyimide resin or polyamideimide resin and the conductive filler. When the polyimide resin or polyamideimide resin is contained in the carbon-based conductive adhesive layer in an amount of 10% by weight or more, a practical bonding strength can be obtained. The following is preferable.

The organic electrolyte used in the EDLC of the present invention is not particularly limited, and an organic electrolyte containing ion-dissociable salts in a known organic solvent can be used. Among them, a salt comprising a quaternary onium cation such as R ₄ N ⁺ or R ₄ P ⁺ (R is an alkyl group) and an anion such as BF ₄ ⁻ , PF ₆ ⁻ , ClO ₄ ⁻ or CF ₃ SO ₃ ^−. It is preferable to use an organic electrolytic solution in which is dissolved in an organic solvent.

As the organic solvent, carbonates such as propylene carbonate, butylene carbonate and diethyl carbonate, lactones such as γ-butyllactone, sulfolane or a mixed solvent thereof can be preferably used.

The polarizable electrode material of the EDLC of the present invention can be used as long as it is an electrochemically inert material with a high specific surface area, but is preferably a polarizable electrode material mainly composed of activated carbon powder having a large specific surface area. . In addition to the activated carbon powder, materials having a large specific surface area such as carbon black, metal fine particles, and conductive metal oxide fine particles can be preferably used. In many cases, polarizable electrodes mainly composed of these polarizable electrode materials are used for both positive and negative electrodes to form EDLC, but only one of the positive and negative electrodes is used as the polarizable electrode, and the other is used as the other one. A nonpolarizable electrode material that can be charged / discharged, that is, a nonpolarizable electrode mainly composed of an active material for a secondary battery may be used.

The current collector for electrically connecting the polarizable electrodes may be any material that is excellent in conductivity and electrochemically durable, such as valve metals such as aluminum, titanium, and tantalum, and stainless steel. Carbon materials such as conductive rubber containing noble metals such as gold and platinum, graphite, glassy carbon, and carbon black can be preferably used.

The production method of the EDLC of the present invention is to form a polarizable electrode by mixing a mixture of activated carbon powder and carbon black with a fluorine-containing resin into a sheet shape. It is preferable to use a carbon-based conductive adhesive made of a suspension containing any one of the bodies as a binder component, join the current collector, and heat dry at a temperature of 200 ° C. or higher. In order to perform drying highly and quickly, it is preferable to perform heat drying at 250 ° C. or higher.

Hereinafter, the present invention will be further described with reference to Examples (Examples 1 to 3) and Comparative Examples (Examples 4 and 5), but the present invention is not limited thereto.

The palm activated carbon powder (average particle size 10 μm, specific surface area 1800 m ² / g) obtained by the steam activation method is kneaded by adding ethanol to 80% by weight, PTFE 10% by weight and carbon black 10% by weight, and the kneaded product is made into a sheet. Then, after rolling and rolling to a thickness of 0.3 mm, a 40 mm square sheet was cut out, and this was applied to the surface of an aluminum foil (thickness: 0.1 mm) current collector subjected to an etching treatment. It was bonded and fixed via a conductive adhesive layer comprising 20% by weight of the binder component shown in FIG.

That is, a carbon-based conductive adhesive which is a suspension obtained by mixing 20% by weight of the binder component and 80% by weight of the graphite fine particles with NMP was obtained. The carbon-based conductive adhesive is applied to the surface of the aluminum foil of the current collector, and a sheet-like polarizable electrode is pressure-bonded to the surface, and then the binder contained in each suspension shown in Table 1 The components were heated at a heat-resistant allowable temperature for 3 hours under reduced pressure and dried to remove moisture in each electrode.

The polarizable electrode joined to the dried current collector is transferred to a glove box filled with low-humidity argon gas. The electrode was sufficiently impregnated, and a separator made of a nonwoven fabric of polypropylene fibers was sandwiched between the polarizable electrodes so as to face each other, and an EDLC was assembled.

After measuring the initial discharge capacity and internal resistance of the obtained EDLC, the battery was charged and discharged with a constant current of 1 A between 0 and 2.8 V in a constant temperature bath at 40 ° C. for 3000 cycles, and the discharge capacity after 3000 cycles. In addition, by measuring the internal resistance and observing the change in performance before and after, the long-term operational reliability of the EDLC was evaluated in an accelerated manner.

The EDLC according to the present invention is an EDLC excellent in operational reliability during long-term use.

Claims (3)

In an electric double layer capacitor having a polarizable electrode and an organic electrolyte that forms an electric double layer on the surface of the polarizable electrode, the polarizable electrode mainly composed of activated carbon formed into a sheet is made of polyimide resin or polyamide An electric double layer capacitor characterized by being bonded to a current collector through a carbon-based conductive adhesive layer containing an imide resin as a binder component. 2. The electric double layer capacitor according to claim 1, wherein the polyimide resin or polyamideimide resin is contained in an amount of 10 to 70% by weight in the carbon-based conductive adhesive layer. The electric double layer capacitor according to claim 1 or 2, wherein the organic electrolytic solution is an organic electrolytic solution containing a quaternary ammonium salt or a quaternary phosphonium salt as an electrolyte.