Electrolytic capacitor, cathode body, and method of manufacturing an electrolytic capacitor

Abstract

To provide an electrolytic capacitor with suppressed ESR increase even in a high temperature environment, a cathode body of the electrolytic capacitor, and a manufacturing method for the electrolytic capacitor.SOLUTION: The electrolytic capacitor includes an anode foil, a cathode body, and an electrolytic solution. The anode foil is made of a valve metal and has a dielectric oxide film formed on a foil surface. The cathode body has a valve metal cathode foil and a carbon layer laminated on the cathode foil. The interface resistance between the cathode foil and the carbon layer is 1.1 mΩ cm2 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrolytic capacitor, a cathode body included in the electrolytic capacitor, and a method for manufacturing the electrolytic capacitor. [Background technology]

[0002] An electrolytic capacitor consists of an anode foil made of a valve metal such as tantalum or aluminum with a dielectric oxide film formed on it, and a cathode foil made of the same or another valve metal, facing each other. An electrolyte is interposed between the anode and cathode foils. The electrolyte is in close contact with the irregular surface of the anode foil and functions as a true cathode.

[0003] In recent years, electrolytic capacitors with a solid electrolyte between the anode and cathode foils instead of a liquid electrolyte have become increasingly popular. These solid electrolytes offer compact size, high capacitance, and low equivalent series resistance, making them essential for miniaturizing and enhancing the functionality of electronic devices. Examples of solid electrolytes include manganese dioxide and 7,7,8,8-tetracyanoquinodimethane (TCNQ) complexes. Conductive polymers derived from monomers with π-conjugated double bonds are rapidly gaining popularity as solid electrolytes. Examples of such conductive polymers include poly(3,4-ethylenedioxythiophene) (PEDOT). Conductive polymers exhibit high conductivity when polyanions such as organic sulfonic acids are used as dopants during chemical or electrolytic oxidation polymerization, demonstrating excellent adhesion to dielectric oxide films.

[0004] However, electrolytic capacitors with solid electrolytes are less effective at repairing defects in the dielectric oxide film than electrolytic capacitors with liquid electrolytes. Therefore, so-called hybrid electrolytic capacitors, in which a solid electrolyte is interposed between the anode foil and the cathode foil and the foil is impregnated with liquid electrolyte, have also attracted attention.

[0005] In order to reduce the equivalent series resistance (ESR), electrolytic capacitors have been proposed in which metal carbides with low specific electrical resistance, such as TiC, WC, and ZrC, are formed on the surface of the cathode foil (see, for example, Patent Document 1). According to this proposal, it is reported that the ESR of electrolytic capacitors can be reduced because the adhesion between the metal carbide and the conductive polymer is improved.

[0006] However, there is a problem in that an oxide film gradually grows on the surface of metal carbide. To address this issue, an electrolytic capacitor has been proposed in which carbon is formed on the surface of the cathode foil using a dry plating method such as ion plating (see, for example, Patent Document 2). The purpose of this solid electrolytic capacitor is to prevent the formation of an oxide film on the surface of the cathode foil, thereby making the capacitance developed on the cathode side approach infinity and limiting the capacitance of the electrolytic capacitor to the anode capacitance alone. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-109272 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-190878 Summary of the Invention [Problem to be solved by the invention]

[0008] It has been confirmed that even in electrolytic capacitors equipped with a cathode body in which a carbon layer is formed on the surface of the cathode foil, the ESR rises significantly when exposed to a high-temperature environment of 120°C or higher. The following phenomenon is thought to be one of the causes of the increase in ESR in this high-temperature environment.

[0009] That is, electrolytic capacitors may contain moisture. For example, an aqueous solvent or a mixture of water and an organic solvent may be used as the solvent for the electrolyte. Also, if the solvent for the electrolyte is alcoholic and contains a carboxylic acid or a salt of an ionically dissociable carboxylic acid as a solute, moisture is generated as the esterification proceeds.

[0010] When this moisture comes into contact with the cathode foil, a hydration reaction between the cathode foil and the moisture causes an oxide film to form on the cathode foil, and this oxide film continues to grow. When an oxide film, which is an insulating component, forms and grows on the cathode foil, the ESR of the electrolytic capacitor increases. Even if a carbon layer is formed on the surface of the cathode foil, if the electrolytic capacitor is exposed to a high-temperature environment and moisture gets in between the cathode foil and the carbon layer and comes into contact with the cathode foil, an oxide film will form.

[0011] The present invention has been proposed to solve the above-mentioned problems, and an object of the present invention is to provide an electrolytic capacitor in which an increase in ESR is suppressed even in a high-temperature environment, a cathode body included in the electrolytic capacitor, and a method for manufacturing the electrolytic capacitor. [Means for solving the problem]

[0012] As a result of extensive research, the inventors of the present invention have found the following: By laminating a carbon layer on a cathode foil, the interface resistance between the cathode foil and the carbon layer is 1.1 mΩ cm. 2 When the ESR exceeds 1.1 mΩ cm, the ESR of the electrolytic capacitor increases significantly when exposed to a high-temperature environment of 120°C or higher. On the other hand, when a carbon layer is laminated on the cathode foil and the interface resistance between the cathode foil and the carbon layer is 1.1 mΩ cm, 2 When the temperature was kept below this value, the ESR of the electrolytic capacitor was prevented from increasing even when the electrolytic capacitor was exposed to a high-temperature environment of 120°C or higher.

[0013] Based on this finding, and in order to solve the above-mentioned problems, the electrolytic capacitor of the present invention is an electrolytic capacitor comprising an anode foil, a cathode body, and an electrolyte, wherein the anode foil is made of a valve metal and has a dielectric oxide film formed on the foil surface, and the cathode body has a cathode foil of valve metal and a carbon layer laminated on the cathode foil, and the interface resistance between the cathode foil and the carbon layer is 1.1 mΩ cm 2 It is characterized by the following:

[0014] The electrolyte may contain water.

[0015] The electrolytic solution may contain an alcohol as a solvent and a carboxylic acid, a carboxylic acid salt, or both as a solute.

[0016] The cathode foil may have a surface-expanding layer on its surface, and the carbon layer may be formed on the surface-expanding layer. When the surface-expanding layer is formed in addition to the carbon layer, the carbon material penetrates into the recesses of the surface-expanding layer, creating an anchor effect that adheres the cathode foil to the carbon layer, making it easier to further reduce the interfacial resistance between the cathode foil and the carbon layer.

[0017] The carbon layer may be pressure-welded to the cathode foil. In addition to forming a carbon layer, pressure-welding the carbon layer to the cathode foil facilitates further reduction of the interface resistance between the cathode foil and the carbon layer. It is more preferable that pressure-welding of the carbon layer to the cathode foil and formation of a surface-expanding layer on the cathode foil coexist. When the carbon layer is pressure-welded to the cathode foil on which the surface-expanding layer is formed, the carbon material of the carbon layer is forced into the pores of the unevenness of the surface-expanding layer, and the carbon layer deforms along the uneven surface of the surface-expanding layer, further improving the adhesion and fixation between the carbon layer and the cathode foil. This facilitates further reduction of the interface resistance between the cathode foil and the carbon layer.

[0018] In addition to the electrolytic solution, the battery may further include a solid electrolyte layer formed on the surfaces of the anode foil and the cathode body.

[0019] In order to solve the above-mentioned problems, a cathode body of an electrolytic capacitor is also one aspect of the present invention. This cathode body includes a cathode foil and a carbon layer formed on the surface of the cathode foil, and the interface resistance between the cathode foil and the carbon layer is 1.1 mΩ cm. 2 It is characterized by the following:

[0020] In order to solve the above-mentioned problems, a method for manufacturing an electrolytic capacitor is also one aspect of the present invention. This method for manufacturing an electrolytic capacitor includes an anode foil, a cathode body, and an electrolyte, and after forming a carbon layer on a cathode foil made of a valve metal, the method achieves an interface resistance of 1.1 mΩ cm. 2 the anode foil having a dielectric oxide film formed on its surface is placed opposite the cathode body fabricated in the cathode body fabrication step to fabricate a capacitor element; and an impregnation step of impregnating the capacitor element with an electrolyte. [Effects of the Invention]

[0021] According to the present invention, even if an electrolytic capacitor is exposed to a high-temperature environment, an increase in ESR caused by moisture can be suppressed. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a graph showing the change over time in the rate of increase in ESR of each example and each comparative example in a temperature environment of 125° C. [Figure 2] 1 is a graph showing the change over time in the rate of increase in ESR for each of Examples and Comparative Examples in a temperature environment of 135° C. [Figure 3] 1 is a graph showing the change over time in the rate of increase in ESR for each example and each comparative example in a temperature environment of 150° C. DETAILED DESCRIPTION OF THE INVENTION

[0023] DETAILED DESCRIPTION OF THE INVENTION An electrolytic capacitor and a manufacturing method thereof according to an embodiment of the present invention will be described below. However, the present invention is not limited to the following embodiment.

[0024] (Overall composition) An electrolytic capacitor is a passive element that obtains capacitance through the dielectric polarization of a dielectric oxide film and stores and discharges electric charge through the capacitance. This electrolytic capacitor includes an anode foil with a dielectric oxide film formed on its surface, a cathode body, an electrolyte, and a separator. The anode foil and the cathode body are arranged opposite each other, with the separator and electrolyte interposed between them. The anode foil and the cathode body are arranged in a stacked configuration, where they are alternately stacked with the separator sandwiched between them, or in a wound configuration, where they are wound with the separator sandwiched between them.

[0025] The electrolyte, either in the form of a liquid electrolyte or a layer of both a liquid electrolyte and a solid electrolyte, is placed between the anode foil and the cathode body. The electrolyte adheres to the dielectric oxide film on the anode foil, forming a true cathode that transmits the foil's electric field. The separator prevents shorting between the anode foil and the cathode body and also retains the electrolyte.

[0026] (electrode foil) The cathode body includes a cathode foil. The anode foil and the cathode foil of the cathode body are foil bodies made of valve metals. Valve metals include aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony. The purity of the anode foil is preferably 99.9% or higher, and that of the cathode foil is preferably about 99% or higher, but impurities such as silicon, iron, copper, magnesium, and zinc may be contained.

[0027] Anode and cathode foils have a surface-expanding layer formed on one or both surfaces. The surface-expanding layer is formed by electrolytic etching, chemical etching, sandblasting, or by vapor-depositing or sintering metal particles onto the foil. That is, the surface-expanding layer consists of tunnel-shaped pits, spongy pits, or voids between densely packed powder particles. Examples of electrolytic etching include DC etching and AC etching, which apply DC or AC in an acidic aqueous solution containing halogen ions, such as hydrochloric acid. Chemical etching involves immersing the anode and cathode foils in an acid or alkaline solution. The tunnel-shaped pits may be formed long enough to penetrate the foil or may extend short of the center of the foil.

[0028] The dielectric oxide film of the anode foil is typically an oxide film formed on the surface layer of the anode foil. For example, if the anode foil is an aluminum foil, this dielectric oxide film is aluminum oxide formed by oxidizing the surface layer of the surface-expanding layer. The dielectric oxide film is intentionally formed by a chemical conversion treatment in which a voltage is applied in a solution free of halogen ions, such as an aqueous solution of adipic acid, boric acid, or phosphoric acid. The surface layer of the cathode foil may also be intentionally formed by this chemical conversion treatment, or may be naturally formed. The natural oxide film that naturally forms on the surface layer of the cathode foil is formed by the cathode foil reacting with oxygen in the air.

[0029] (cathode body) The cathode body includes a carbon layer in addition to a cathode foil. The carbon layer is laminated on the cathode foil. The carbon layer is a layer containing a carbon material. The carbon material is fibrous carbon, carbon powder, or a mixture thereof. The fibrous carbon or carbon powder is preferably subjected to a porosity treatment such as an activation treatment or an opening treatment to form pores.

[0030] Examples of carbon powders include natural plant tissues such as coconut husks, synthetic resins such as phenols, activated carbon derived from fossil fuels such as coal, coke, and pitch, carbon blacks such as ketjen black, acetylene black, channel black, and thermal black, carbon nanohorns, amorphous carbon, natural graphite, artificial graphite, graphitized ketjen black, and mesoporous carbon. Examples of fibrous carbon include carbon nanotubes and carbon nanofibers. Carbon nanotubes may be single-walled carbon nanotubes, which have a single graphene sheet, or multi-walled carbon nanotubes (MWCNTs), which have two or more graphene sheets rolled coaxially to form multiple tube walls.

[0031] The carbon layer and cathode foil are adjusted to have low interfacial resistance. Low interfacial resistance correlates with the degree of adhesion between the carbon layer and the cathode foil. Therefore, lowering interfacial resistance reduces the gap between the carbon layer and the cathode foil. A smaller gap between the carbon layer and the cathode foil reduces the likelihood of moisture in the electrolyte coming into contact with the cathode foil surface. Reducing the chance of contact between the cathode foil and moisture suppresses the hydration reaction between the cathode foil and moisture, making it more difficult for an oxide film to form on the cathode foil surface. Even if an oxide film does form, it is less likely to grow. The more difficult it is for an insulating oxide film to form on the cathode foil, the less likely the increase in the ESR of the electrolytic capacitor will be.

[0032] However, considering the correlation between the interface resistance and the degree of adhesion, the effect of this mechanism in suppressing the increase in ESR is thought to be inversely proportional to the low interface resistance. 2 At this point, the change in ESR of electrolytic capacitors in high-temperature environments is significantly different, exceeding the range that can be explained by this mechanism. 2 If the ESR of the electrolytic capacitor is less than 1.1 mΩ cm, the increase in the ESR of the electrolytic capacitor is suppressed even if the electrolytic capacitor is exposed to a high-temperature environment of 120°C or higher. 2If the temperature exceeds 120°C, the ESR of the electrolytic capacitor will increase significantly when exposed to a high temperature environment of 120°C or higher.

[0033] Therefore, the interface resistance between the carbon layer and the cathode foil was set to 1.1 mΩ cm 2 The ESR of the electrolytic capacitor is adjusted as follows. This significantly suppresses an increase in the ESR of the electrolytic capacitor even when the electrolytic capacitor is exposed to a high-temperature environment of 120°C or higher. A suitable method for adjusting the interfacial resistance between the carbon layer and the cathode foil is to press the carbon layer and the cathode foil together. In the press process, the laminate of the carbon layer and the cathode foil is sandwiched between press rollers and a linear press pressure is applied. The linear press pressure is preferably about 0.01 to 100 t / cm. Furthermore, the press temperature, which is the temperature of the press rollers during pressing, is preferably about 0 to 200°C.

[0034] Prior to press processing, a carbon layer is formed on the cathode foil by vacuum deposition, sputtering, ion plating, CVD, coating, electrolytic plating, electroless plating, or the like. In the coating method, a carbon material is dispersed in a dispersion solvent to prepare a slurry, which is then applied to the cathode foil by a slurry casting method, doctor blade method, spray atomization, or the like, and dried. In the vacuum deposition method, the carbon material is evaporated by applying an electric current to heat the carbon material in a vacuum, or by irradiating the carbon material with an electron beam in a vacuum, thereby forming a film of the carbon material on the cathode foil. In the sputtering method, a carbon target and the cathode foil are placed in a vacuum chamber, and an inert gas is introduced into the vacuum chamber and a voltage is applied, causing the plasma of the inert gas to collide with the target, and carbon material particles knocked out from the target are deposited on the cathode foil.

[0035] Furthermore, to reduce the interfacial resistance between the carbon layer and the cathode foil, it is preferable to intentionally form an oxide film on the cathode foil at 0.5V to 3V by chemical conversion treatment. The oxide film formed on the cathode foil increases the interfacial resistance of the electrolytic capacitor, but it also improves the adhesion between the carbon layer and the cathode foil. When a carbon layer is formed on a cathode foil with an oxide film at 0.5V to 3V, the adhesion improvement is more pronounced than when an oxide film outside this range is used, and the interfacial resistance between the carbon layer and the cathode foil is 1.1 mΩ·cm. 2 It easily drops below this.

[0036] Another method for adjusting the interfacial resistance between the carbon layer and the cathode foil is to form a surface-expanding layer on the surface of the cathode foil. By forming a surface-expanding layer on the surface of the cathode foil, the carbon material of the carbon layer penetrates into the irregularities of the surface-expanding layer, thereby reducing the interfacial resistance between the carbon layer and the cathode foil. If a surface-expanding layer is formed on the cathode foil and then the carbon layer and the cathode foil are pressed together, the interfacial resistance can be reduced even more easily.

[0037] The interface resistance between the carbon layer and the cathode foil can be adjusted by selecting the carbon material contained in the carbon layer. Carbon black, which is spherical carbon, is preferred as the carbon material. By using spherical carbon black with an average primary particle size of 100 nm or less, the carbon layer becomes dense and is more likely to adhere to the surface-expanding layer, which reduces the interface resistance.

[0038] The carbon material contained in the carbon layer may also be a mixture of flake or scale-like graphite and spherical carbon, i.e., carbon black. The flake or scale-like graphite preferably has an aspect ratio of the minor axis to the major axis in the range of 1:5 to 1:100. When a carbon layer containing this combination of carbon materials is laminated on a cathode foil, compressed, and pressed against the surface-expanding layer, the carbon black is easily rubbed into the surface-expanding layer by the graphite. The graphite easily deforms along the uneven surface of the surface-expanding layer and easily accumulates on the uneven surface. The graphite then acts as a pressing lid, holding the spherical carbon within the surface-expanding layer. This facilitates a reduction in the interfacial resistance between the carbon layer and the cathode foil.

[0039] The interfacial resistance can be measured as follows. That is, the potential of the surface of the carbon layer of a cathode body on which the carbon layer is formed is measured at multiple positions. The surface of the carbon layer on which the potential is measured is, in other words, the surface opposite to the surface in close contact with the cathode foil, or the exposed surface of the cathode body. In measuring the potential, a test probe for application is brought into contact with the surface of the carbon layer, and a test probe for measurement is also brought into contact with the surface of the carbon layer. A predetermined DC current is applied between the test probes, and the calculated resistance is the "interfacial resistance at the cathode." An example of a suitable device for measuring the interfacial resistance at the cathode is the RM2610 electrode resistance measurement system manufactured by Hioki E.E. Corporation.

[0040] (electrolyte) The solvent of the electrolyte is water, a protic organic polar solvent, or an aprotic organic polar solvent, and may be used alone or in combination of two or more. The solute includes anionic and cationic components. The solute is typically a salt of an organic acid, a salt of an inorganic acid, or a salt of a complex compound of an organic acid and an inorganic acid, and may be used alone or in combination of two or more. An acid that becomes an anion and a base that becomes a cation may be added separately to the electrolyte as solute components.

[0041] Examples of protic organic polar solvents include monohydric alcohols, polyhydric alcohols, and oxyalcohol compounds. Examples of monohydric alcohols include ethanol, propanol, butanol, pentanol, hexanol, cyclobutanol, cyclopentanol, cyclohexanol, and benzyl alcohol. Examples of polyhydric alcohols and oxyalcohol compounds include ethylene glycol, propylene glycol, glycerin, methyl cellosolve, ethyl cellosolve, methoxypropylene glycol, and dimethoxypropanol.

[0042] Representative examples of aprotic organic polar solvents include sulfones, amides, lactones, cyclic amides, nitriles, and sulfoxides. Examples of sulfones include dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methyl sulfolane, and 2,4-dimethyl sulfolane. Examples of amides include N-methylformamide, N,N-dimethylformamide, N-ethylformamide, N,N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide, and hexamethylphosphoric amide. Examples of lactones and cyclic amides include γ-butyrolactone, γ-valerolactone, δ-valerolactone, N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate, butylene carbonate, and isobutylene carbonate. Examples of nitriles include acetonitrile, 3-methoxypropionitrile, glutaronitrile, etc. Examples of sulfoxides include dimethyl sulfoxide, etc.

[0043] Examples of organic acids include carboxylic acids such as oxalic acid, succinic acid, glutaric acid, pimelic acid, suberic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, maleic acid, adipic acid, benzoic acid, toluic acid, enanthic acid, malonic acid, 1,6-decanedicarboxylic acid, 1,7-octanedicarboxylic acid, azelaic acid, undecanedioic acid, dodecanedioic acid, and tridecanedioic acid, as well as phenols and sulfonic acids. Examples of inorganic acids include boric acid, phosphoric acid, phosphorous acid, hypophosphorous acid, carbonic acid, and silicic acid. Examples of composite compounds of organic and inorganic acids include borodisalicylic acid, borodioxalic acid, and borodiglycolic acid.

[0044] Examples of at least one salt of these organic acid salts, inorganic acid salts, and organic and inorganic acid complex compounds include ammonium salts, quaternary ammonium salts, quaternized amidinium salts, amine salts, sodium salts, and potassium salts. Examples of quaternary ammonium ions in quaternary ammonium salts include tetramethylammonium, triethylmethylammonium, and tetraethylammonium. Examples of quaternized amidinium salts include ethyldimethylimidazolinium and tetramethylimidazolinium. Examples of amines in amine salts include primary amines, secondary amines, and tertiary amines. Examples of primary amines include methylamine, ethylamine, and propylamine. Examples of secondary amines include dimethylamine, diethylamine, ethylmethylamine, and dibutylamine. Examples of tertiary amines include trimethylamine, triethylamine, tripropylamine, tributylamine, ethyldimethylamine, and ethyldiisopropylamine. An ionically dissociable salt containing an anion component, which is an organic acid, inorganic acid, or complex compound of an organic acid and an inorganic acid, and a cation component, which is a base, may be added to the electrolytic solution.

[0045] The solvent and solute of the electrolyte solution are a combination containing water or an esterification source. That is, in the first type of electrolyte solution, the solvent contains water, and other components are not particularly limited. In the second type of electrolyte solution, the solvent is a monohydric alcohol, a polyhydric alcohol, an oxyalcohol compound, or a mixture thereof, and the solute is a carboxylic acid or a carboxylic acid salt, and other components are not particularly limited. The esterification reaction between the alcohol and the carboxylic acid generates water, which reacts with the cathode foil as a hydration reaction. In the third type of electrolyte solution, the first and second types of electrolyte solutions are mixed.

[0046] Furthermore, other additives can be added to the electrolyte. Examples of additives include polyethylene glycol, complex compounds of boric acid and polysaccharides (e.g., mannitol, sorbitol), complex compounds of boric acid and polyhydric alcohols, boric acid esters, nitro compounds, phosphate esters, and colloidal silica. These may be used alone or in combination of two or more. Nitro compounds suppress the generation of hydrogen gas in electrolytic capacitors. Examples of nitro compounds include o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, and p-nitrophenol.

[0047] After preparation, the electrolyte is impregnated into a capacitor element. The capacitor element is composed of an anode foil on which a dielectric oxide film is formed and a cathode body in which a carbon layer is laminated on a cathode foil, with a separator interposed between them. When the capacitor element is impregnated with the electrolyte, a depressurization treatment or a pressurization treatment may be performed as necessary to promote the impregnation. The impregnation process may be repeated multiple times. When a solid electrolyte layer is used in combination, the electrolyte is impregnated into the capacitor element on which the solid electrolyte layer is formed.

[0048] (solid electrolyte) The solid electrolyte layer contains a conductive polymer. The conductive polymer is a conjugated polymer or a doped conjugated polymer. The conjugated polymer is obtained by chemical oxidative polymerization or electrolytic oxidative polymerization of a monomer having a π-conjugated double bond or a derivative thereof. By performing a doping reaction on the conjugated polymer, the conductive polymer exhibits high conductivity.

[0049] Any known conjugated polymer can be used without any particular limitation. Examples include polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene, polyphenylene vinylene, polyacene, and polythiophene vinylene. A representative conductive polymer is poly(3,4-ethylenedioxythiophene), also known as PEDOT, doped with polystyrene sulfonic acid (PSS). These conjugated polymers may be used alone, in combination with two or more types, or as a copolymer of two or more types of monomers.

[0050] Any known dopant can be used without any particular limitation. Examples of the dopant include inorganic acids such as boric acid, nitric acid, and phosphoric acid, and organic acids such as acetic acid, oxalic acid, citric acid, ascot acid, tartaric acid, squaric acid, rhodizonic acid, croconic acid, salicylic acid, p-toluenesulfonic acid, 1,2-dihydroxy-3,5-benzenedisulfonic acid, methanesulfonic acid, trifluoromethanesulfonic acid, borodisalicylic acid, bisoxalateborate acid, sulfonylimide acid, dodecylbenzenesulfonic acid, propylnaphthalenesulfonic acid, and butylnaphthalenesulfonic acid. Polyanions can also be used as dopants, including polyvinylsulfonic acid, polystyrenesulfonic acid, polyallylsulfonic acid, polyacrylicsulfonic acid, polymethacrylic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprenesulfonic acid, polyacrylic acid, polymethacrylic acid, and polymaleic acid. The dopant may be used alone or in combination of two or more kinds. Also, a polymer or a monomer may be used.

[0051] This solid electrolyte layer is formed by impregnating the capacitor element with a dispersion liquid in which a conductive polymer is dispersed. The solvent for the dispersion liquid can be any solvent that can disperse conductive polymer particles or powder, and water is typically used. If necessary, ethylene glycol may be used as the solvent for the dispersion liquid. It has been found that using ethylene glycol as the solvent for the dispersion liquid can reduce the electrical properties of the product, particularly the ESR characteristics. To improve the impregnation and electrical conductivity of the dispersion liquid, various additives may be used in the dispersion liquid, or neutralization may be performed by adding cations.

[0052] The method of impregnating the conductive polymer dispersion liquid may involve immersing the capacitor element in the dispersion liquid, or applying it by dripping or spraying. Furthermore, the dispersion liquid may be impregnated not only into the entire electrode pair, but also into the anode foil or cathode body, and then the capacitor element may be assembled. To promote the impregnation of the dispersion liquid into the electrode pair, a decompression treatment or a pressurization treatment may be performed as necessary. This application process may be repeated multiple times.

[0053] The solid electrolyte layer may also be formed by a known electrolytic polymerization or chemical polymerization method. In chemical polymerization, the solid electrolyte layer can be formed by impregnating a capacitor element with a solution of a monomer and an oxidizing agent dissolved in a solvent, followed by drying, or by alternately impregnating an electrode pair with a solution of a monomer dissolved in a solvent and a solution of an oxidizing agent dissolved in a solvent, followed by drying. For example, a solid electrolyte can be formed by immersing a capacitor element in a mixed solution of 3,4-ethylenedioxythiophene as the polymerizable monomer and an alcohol solution (e.g., ethanol) of ferric paratoluenesulfonate as the oxidizing agent, and then heating the mixed solution to initiate a polymerization reaction of the conductive polymer. Before or after this heating process, a water washing process may be performed to remove unreacted or excess monomer.

[0054] In electropolymerization, a solid electrolyte layer is formed by placing a capacitor element in an electropolymerization solution containing at least a monomer, a supporting electrolyte, and a solvent, and applying a voltage between the anode and cathode. This electropolymerization solution can be made of a monomer that becomes conductive through electropolymerization. Thiophene monomers and pyrrole monomers are suitable as the monomer. When using these monomers, the capacitor element is immersed in an electropolymerization aqueous solution containing the monomer and the supporting electrolyte, sodium 1-naphthalenesulfonate, in a stainless steel container, and a predetermined voltage is applied. This allows a solid electrolyte layer to be uniformly formed by electropolymerization of a water-soluble monomer (e.g., thiophene, pyrrole, etc.).

[0055] (separator) Examples of materials for the separator include cellulose papers such as kraft, Manila hemp, esparto, hemp, and rayon, and mixtures thereof; polyester-based resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and derivatives thereof; polytetrafluoroethylene-based resins, polyvinylidene fluoride-based resins, vinylon-based resins; polyamide-based resins such as aliphatic polyamides, semi-aromatic polyamides, and wholly aromatic polyamides; polyimide-based resins, polyethylene resins, polypropylene resins, trimethylpentene resins, polyphenylene sulfide resins, acrylic resins, and polyvinyl alcohol resins. These resins can be used alone or in combination.

[0056] The separator serves to retain the solid electrolyte layer and the electrolytic solution and to prevent short-circuiting between the anode foil and the cathode body. If the solid electrolyte layer can maintain its shape without a separator, each part of the capacitor element including the solid electrolyte layer can retain the electrolytic solution, and the solid electrolyte layer has a thickness sufficient to prevent short-circuiting between the anode foil and the cathode body, then the separator may be unnecessary.

[0057] (Manufacturing method) Such an electrolytic capacitor is produced through an anode foil production step of producing an anode foil, a cathode body production step of producing a cathode body, an element production step of producing a capacitor element in which the anode foil and the cathode body are arranged opposite each other, a solid electrolyte layer formation step of forming a solid electrolyte layer on the capacitor element, and an electrolyte solution impregnation step of impregnating the capacitor element with the solid electrolyte layer formed thereon with an electrolyte solution.

[0058] In the anode foil manufacturing process, valve metal is stretched to form the anode foil, a surface-expanding layer is formed on the anode foil, and then a dielectric oxide film is formed on the surface of this surface-expanding layer. In the cathode body manufacturing process, valve metal is stretched to form the cathode foil, and then a surface-expanding layer is formed on the cathode foil. Furthermore, in the cathode body manufacturing process, a carbon layer is formed on the cathode foil, and the carbon layer is pressed against the cathode foil by pressing, reducing the interface resistance between the carbon layer and the cathode foil to 1.1 mΩ·cm. 2 Do the following:

[0059] In the element fabrication process, an anode foil with a dielectric oxide film formed thereon and a cathode body are stacked together with a separator interposed between them. In the case of a laminated type, the anode foil, cathode body, and separator are alternately stacked in multiple layers. In the case of a wound type, the anode foil and cathode body stacked together with the separator interposed between them are wound together. In the solid electrolyte layer formation process, a dispersion liquid in which a conductive polymer is dispersed is impregnated into the capacitor element. In the electrolyte impregnation process, the capacitor element, which has a solid electrolyte layer formed through the solid electrolyte layer formation process, is impregnated with an electrolyte.

[0060] This resulted in an interface resistance between the cathode foil and the carbon layer of 1.1 mΩ cm 2 The following electrolytic capacitor is fabricated. This electrolytic capacitor suppresses an increase in ESR even when exposed to a high-temperature environment of 120°C or higher. This is particularly effective when the electrolyte solvent is partially or entirely aqueous, or when the electrolyte contains an alcohol as the solvent and a carboxylic acid, a carboxylic acid salt, or both as the solute. [Example]

[0061] The electrolytic capacitor and the manufacturing method of the present invention will be described in more detail below based on examples, but the present invention is not limited to the following examples.

[0062] (Examples 1 to 5) Solid electrolytic capacitors of Examples 1 to 5 and Comparative Examples 1 to 3 were fabricated as follows. In these solid electrolytic capacitors, aluminum foil was used for the anode and cathode foils. The anode and cathode foils were subjected to an AC etching process to form a surface-enlarging layer consisting of spongy etching pits on both sides of the foil. In the AC etching process, the cathode foil was immersed in an acidic aqueous solution containing approximately 8% by weight of hydrochloric acid as the main electrolyte at a liquid temperature of 25°C, and an AC current of 10 Hz and a current density of 0.14 A / cm was applied. 2 A current of 1000kJ / cm2 was applied to the substrate for approximately 5 minutes. Furthermore, the anode foil was subjected to a chemical conversion treatment to form a dielectric oxide film on the surface of the anode foil's surface-expanding layer. For the chemical conversion treatment, after removing chlorine that had adhered during the AC etching process with a phosphoric acid solution, a voltage was applied in an ammonium dihydrogen phosphate solution.

[0063] A carbon layer was laminated on the surface-expanding layer of the cathode foil, completing a cathode body comprising the cathode foil and carbon layer. Carbon black was selected as the carbon material for the carbon layer. Carbon black powder, styrene butadiene rubber (SBR) as a binder, and a carboxymethyl cellulose sodium (CMC-Na) aqueous solution as a dispersant-containing solution were mixed and kneaded to prepare a slurry, which was then uniformly applied to the cathode foil. The slurry was then heated and dried to volatilize the solvent.

[0064] After the carbon layer was formed on the surface-expanding layer of the cathode foil, a press process was performed to press the carbon layer onto the surface-expanding layer. The cathode body was sandwiched between press rollers and linear pressure was applied.

[0065] Here, the interfacial resistance between the cathode foil and the carbon layer was varied for each example and comparative example by performing press processing under different press conditions. The press line pressure, press temperature, and interfacial resistance for each example and comparative example are shown in Table 1 below. Note that the change in interfacial resistance with respect to the press line pressure and press temperature also differs depending on, for example, the thickness of the cathode foil and the thickness of the carbon layer.

[0066] (Table 1) TIFF0007797789000001.tif81161

[0067] The anode foil and cathode body were each connected to an aluminum tab-shaped lead terminal by stitching. A separator was sandwiched between the anode foil and cathode body, and the resulting capacitor element was fabricated. A Manila separator was used as the separator. After winding, repair and chemical conversion were performed to remove any defects that occurred during winding.

[0068] Next, a conductive polymer dispersion was prepared. The dispersion consisted of a powder of polyethylenedioxythiophene (PEDOT), a conductive polymer doped with polystyrene sulfonate (PSS), dispersed in water. The capacitor element was immersed in the dispersion. While immersed, it was exposed to a pressure environment of 30 kPa for 120 seconds. After this, the capacitor element was removed and dried at 150°C for 30 minutes. The immersion and drying process was repeated twice. This resulted in a solid electrolyte layer containing polyethylenedioxythiophene (PEDOT) doped with polystyrene sulfonate (PSS) as a conductive polymer, which was adhered to the dielectric oxide film on the anode foil and also laminated on the carbon layer of the cathode body.

[0069] Next, an electrolyte solution was prepared and impregnated into the capacitor element with the solid electrolyte layer. The electrolyte solution was prepared by adding ammonium azelaate as a solute to ethylene glycol as a solvent. Ethylene glycol is a dihydric alcohol, and azelaic acid is a saturated dicarboxylic acid. The esterification reaction between ethylene glycol and azelaic acid produces water over time. This esterification reaction is accelerated by exposure to a high-temperature environment.

[0070] The capacitor element was inserted into a cylindrical outer case with a bottom, and a rubber seal was attached to the open end and sealed by crimping. The solid electrolytic capacitor was then subjected to aging treatment by being exposed to a temperature environment of 115°C for 45 minutes. The solid electrolytic capacitors of the examples and comparative examples thus fabricated had a rated withstand voltage of 25 WV, a rated capacitance of 270 μF, and dimensions of 10 mm in diameter and 8 mm in height.

[0071] (ESR evaluation) The solid electrolytic capacitors of Examples 1 to 5 and Comparative Examples 1 to 3 were placed under three different temperature environments, and the ESR was measured at each elapsed time. A 100 kHz AC signal was applied to the solid electrolytic capacitor during ESR measurement. The percentage increase in ESR over time relative to the initial ESR before placement under the three different temperature environments was calculated. The calculation results are shown in Figures 1 to 3. Figure 1 is a graph showing the change over time in the ESR increase rate of each solid electrolytic capacitor placed under a 125°C temperature environment. Figure 2 is a graph showing the change over time in the ESR increase rate of each solid electrolytic capacitor placed under a 135°C temperature environment. Figure 3 is a graph showing the change over time in the ESR increase rate of each solid electrolytic capacitor placed under a 150°C temperature environment.

[0072] Table 2 below shows the increase in ESR after 2088 hours of standing in three different temperature environments. (Table 2) TIFF0007797789000002.tif81161

[0073] 1 to 3, the solid electrolytic capacitors of Comparative Examples 1 to 3 continue to show a large increase in ESR over time. On the other hand, the solid electrolytic capacitors of Examples 1 to 5 show a small increase in ESR over time, and the increase in ESR is suppressed. Therefore, after 2088 hours, there is a large difference in ESR between the groups of Comparative Examples 1 to 3 and the groups of Examples 1 to 5.

[0074] Specifically, as shown in Table 2, in a temperature environment of 125°C, the maximum value for Examples 1 to 5 was 131.13, while the minimum value for Comparative Examples 1 to 3 was 148.93, a difference of 17.8. In a temperature environment of 135°C, the maximum value for Examples 1 to 5 was 146.74, while the minimum value for Comparative Examples 1 to 3 was 187.79, a difference of 41.05. In a temperature environment of 150°C, the maximum value for Examples 1 to 5 was 210.22, while the minimum value for Comparative Examples 1 to 3 was 284.28, a difference of 74.06. As the temperature increases, the difference between Examples 1 to 5 and Comparative Examples 1 to 3 becomes more pronounced.

[0075] In the groups of Examples 1 to 5, the interface resistance between the cathode foil and the carbon layer was 1.1 mΩ·cm 2 On the other hand, in the groups of Comparative Examples 1 to 3, the interface resistance between the cathode foil and the carbon layer was 1.28 mΩ cm 2 In this way, in the electrolytic capacitor, the cathode body has a valve metal cathode foil and a carbon layer laminated on the cathode foil, and the interface resistance between the cathode foil and the carbon layer is 1.1 mΩ cm 2 It was confirmed that an increase in ESR in a high-temperature environment can be suppressed by setting the ESR at or below 1.1 mΩ cm. In particular, when the adhesion between the cathode foil and the carbon layer is improved by forming an oxide film of about 0.5 to 3 V on the cathode foil, there is a concern that the oxide film will increase the interface resistance. 2 By adjusting the ratio as follows, it is possible to suppress a decrease in capacitance even if an oxide film is formed on the surface of the cathode foil.

[0076] It was also confirmed that long-term stability could be achieved by limiting the increase in ESR after 288 hours to less than 115% in an environment of 125°C, less than 130% in an environment of 135°C, and less than 150% in an environment of 150°C. This phenomenon becomes more pronounced at higher temperatures.

Claims

1. An electrolytic capacitor comprising an anode foil, a cathode body, and an electrolyte, the anode foil is made of a valve metal and has a dielectric oxide film formed on the foil surface; the cathode body includes a valve metal cathode foil, an oxide film of 0.5 V to 3 V on the cathode foil, and a carbon layer laminated on the oxide film; The interface resistance between the cathode foil and the carbon layer was 1.1 mΩ cm ２ That is: An electrolytic capacitor characterized by:

2. the electrolyte solution contains water; 2. The electrolytic capacitor according to claim 1,

3. the electrolytic solution contains an alcohol as a solvent and a carboxylic acid, a carboxylic acid salt, or both as a solute; 3. The electrolytic capacitor according to claim 1 or 2,

4. the cathode foil has a surface-expanding layer on its surface, and the carbon layer on the surface-expanding layer; 4. The electrolytic capacitor according to claim 1, wherein:

5. the carbon layer is pressed against the cathode foil; 5. The electrolytic capacitor according to claim 1, wherein:

6. a solid electrolyte layer formed on the surfaces of the anode foil and the cathode body in addition to the electrolytic solution; 6. The electrolytic capacitor according to claim 1, wherein:

7. A cathode body of an electrolytic capacitor, a cathode foil; an oxide film of 0.5 V to 3 V on the cathode foil; and a carbon layer formed on a surface of the oxide film; The interface resistance between the cathode foil and the carbon layer is 1.1 mΩ cm ２ That is: A cathode body characterized by:

8. A method for manufacturing an electrolytic capacitor including an anode foil, a cathode body, and an electrolyte, comprising: An oxide film of 0.5V to 3V was formed on a valve metal cathode foil, and a carbon layer was further formed on the oxide film. After that, the interface resistance was 1.1 mΩ cm ２ a cathode body fabrication step of fabricating the cathode body by pressing the carbon layer against the cathode foil by press working until the carbon layer reaches a thickness of: a capacitor element fabrication step of fabricating a capacitor element by placing the anode foil having a dielectric oxide film formed on a surface thereof opposite the cathode body fabricated in the cathode body fabrication step; an impregnation step of impregnating the capacitor element with an electrolyte; containing, A method for manufacturing an electrolytic capacitor, comprising:

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