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

Abstract

To provide an electrolytic capacitor in which a decrease in capacitance over time is suppressed, a cathode body included in 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 includes a valve metal and has a dielectric oxide film formed on a foil surface. The cathode body has the anode foil of the valve metal and a carbon layer laminated on the cathode foil. The interface resistance between the cathode foil and the carbon layer is 1.8 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 electrolytic capacitors, not only is a dielectric oxide film intentionally formed on the anode foil, but an oxide film also forms on the surface of the cathode foil due to reactions with air and the chemical action of the electrolyte. Therefore, electrolytic capacitors can be considered series capacitors, with capacitance (also known as Capacitance) on both the anode and cathode sides. In such electrolytic capacitors, it is important to ideally make the capacitance on the cathode side approach infinity in order to efficiently utilize the capacitance on the anode side.

[0006] 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 1). The carbon layer prevents the electrolyte from penetrating the surface of the cathode foil and prevents the new formation and growth of an oxide film on the cathode foil, thereby causing the capacitance developed on the cathode side to 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. 2006-190878 Summary of the Invention [Problem to be solved by the invention]

[0008] However, simply forming a carbon layer on the cathode foil will cause the capacitance of the electrolytic capacitor to deteriorate over a long period of time, until the rate of decrease in capacitance of the electrolytic capacitor falls below a limit, and the electrolytic capacitor reaches the end of its life.

[0009] 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 degradation of capacitance over time is suppressed, a cathode body included in the electrolytic capacitor, and a method for manufacturing the electrolytic capacitor. [Means for solving the problem]

[0010] As a result of extensive research, the present inventors have found the following: When a carbon layer is laminated on a cathode foil, the interface resistance between the cathode foil and the carbon layer is at least 1.8 mΩ cm. 2 When the temperature was below 100°C, the rate of decrease in capacitance of the electrolytic capacitor was suppressed to 30% or less compared to the initial value.

[0011] 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 1.8 mΩ cm 2 It is characterized by the following:

[0012] Furthermore, as a result of intensive research, the present inventors have further discovered the following: When a carbon layer is laminated on a cathode foil, the interface resistance between the cathode foil and the carbon layer is at least 1.6 mΩ cm. 2 The rate of decrease in capacitance due to deterioration over time changed significantly around this boundary. 2 Compared with the super- 2 In the following cases, the deterioration of the capacitance of the electrolytic capacitor over time was effectively suppressed.

[0013] Based on this further finding, the interface resistance between the cathode foil and the carbon layer is 1.6 mΩ cm 2 The following may be true:

[0014] 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 carbon layer is formed, the carbon fills the recesses of the surface-expanding layer, creating an anchor effect that brings the cathode foil and the carbon layer into close contact with each other, further reducing the interfacial resistance between the cathode foil and the carbon layer.

[0015] 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.

[0016] In order to solve the above-mentioned problems, a cathode body of an electrolytic capacitor is also one aspect of the present invention. The cathode body of this electrolytic capacitor 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 1.8 mΩ cm. 2 It is characterized by the following:

[0017] The interface resistance between the cathode foil and the carbon layer was 1.6 mΩ cm 2 The following may be true:

[0018] 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 1.8 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.

[0019] In the cathode body manufacturing process, the interface resistance was 1.6 mΩ cm 2The carbon layer may be pressed against the cathode foil by pressing until the carbon layer reaches the thickness of 0.1 mm or less. [Effects of the Invention]

[0020] According to the present invention, the deterioration of the capacitance of an electrolytic capacitor over time can be suppressed. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a graph showing the relationship between the interface resistance and the rate of change in capacitance (ΔCap). DETAILED DESCRIPTION OF THE INVENTION

[0022] 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.

[0023] (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 having a dielectric oxide film formed on its surface, a cathode body, an electrolyte, and a separator. The anode foil and the cathode body are disposed opposite each other, and the separator and the electrolyte are interposed between the anode foil and the cathode body. The anode foil and the cathode body are arranged in a stacked configuration in which they are alternately stacked with the separator sandwiched between them, or in a wound configuration in which they are wound with the separator sandwiched between them. A solid electrolyte layer may be disposed between the anode foil and the cathode body in addition to the electrolyte.

[0024] (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.

[0025] 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.

[0026] 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.

[0027] (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.

[0028] 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.

[0029] This carbon layer has an interface resistance between the carbon layer and the cathode foil of at least 1.6 mΩ·cm 2 It is preferable that the interface resistance is 1.6 mΩ cm or less. 2 If the interfacial resistance is less than 1.6 mΩ cm, the rate of decrease in capacitance over time of the electrolytic capacitor can be kept low. 2 If this value is exceeded, the rate at which the capacitance of the electrolytic capacitor decreases will become rapidly large.

[0030] 1.6mΩ·cm 2 The phenomenon in which the rate of decrease in capacity over time changes significantly before and after the interface resistance reaches 1.6 mΩ cm is speculated, but is not limited to this, as follows: the interface resistance between the carbon layer and the cathode foil correlates with the diameter and volume of the pores between the carbon layer and the cathode foil. 2 An interface resistance of 1.6 mΩ cm or less corresponds to a gap of a diameter and volume where the electrolyte cannot easily penetrate between the carbon layer and the cathode foil, and the electrolyte cannot come into contact with the surface of the cathode foil. 2 Below this level, the growth of the oxide film on the cathode foil is suppressed, and the cathode side maintains a large capacity. If the cathode side maintains a large capacity, the capacity of the anode side can continue to be extracted efficiently, and the rate of decrease in the capacitance of the electrolytic capacitor over time is kept low.

[0031] However, the interface resistance between the carbon layer and the cathode foil is 1.63 mΩ cm 2 Over 1.8mΩ·cm 2 Although the rate of capacitance loss of the electrolytic capacitor increases rapidly up to this point, the rate of capacitance loss is kept to 30% or less compared to the initial state. The initial state refers to before the electrolytic capacitor is exposed to a temperature environment of 125°C and a load test in a high-temperature environment in which a DC voltage of 2.4V is applied. Therefore, the interface resistance between the carbon layer and the cathode foil is at least 1.8mΩ·cm. 2 The following applies.

[0032] A suitable method for adjusting the interfacial resistance between the carbon layer and the cathode foil is press-bonding the carbon layer and the cathode foil. In press-bonding, 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. The press temperature, which is the temperature of the press rollers during pressing, is preferably about 0 to 200°C.

[0033] 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 coated on 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, 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-generated inert gas to collide with the target, resulting in particles of the carbon material being knocked out of the target and deposited on the cathode foil.

[0034] Furthermore, in order to reduce the interfacial resistance between the carbon layer and the cathode foil, it is preferable to intentionally form an oxide film of 0.5V to 3V on the cathode foil by chemical conversion treatment. The oxide film formed on the cathode foil has the effect of reducing the capacitance of the electrolytic capacitor, but it also has the effect of improving the adhesion between the carbon layer and the cathode foil. When a carbon layer is formed on a cathode foil with an oxide film of 0.5V to 3V, the improvement in adhesion has a stronger effect than when an oxide film outside this range is formed, and the interfacial resistance between the carbon layer and the cathode foil is 1.6mΩ·cm. 2 Therefore, if an oxide film of 0.5V to 3V is intentionally formed on the cathode foil, the effect of the oxide film on capacitance reduction can be overcome and the reduction in capacitance of the electrolytic capacitor can be suppressed.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] (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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] Such an electrolyte solution is prepared and then 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 solution, 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 solution is impregnated into the capacitor element on which the solid electrolyte layer is formed.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] The 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.

[0050] 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.

[0051] The solid electrolyte layer may also be formed by known electrolytic polymerization or chemical polymerization methods. 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.

[0052] 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.).

[0053] (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.

[0054] 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.

[0055] (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.

[0056] 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 1.8 mΩ·cm. 2 Less than or equal to 1.6 mΩ cm, preferably 2 Do the following:

[0057] 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.

[0058] This resulted in an interface resistance between the cathode foil and the carbon layer of 1.8 mΩ cm 2 Less than or equal to 1.6 mΩ·cm 2 The electrolytic capacitor is fabricated as follows: This electrolytic capacitor is suppressed from decreasing in capacitance over time. [Example]

[0059] 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.

[0060] An electrolytic capacitor was fabricated as follows. First, an aluminum foil was used as the cathode foil. The aluminum foil was 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 this magnitude was applied to the substrate for about 5 minutes to expand the surface area of ​​both sides of the aluminum foil.

[0061] The aluminum foil was then subjected to a chemical conversion treatment to form an oxide film on the surface of the surface-expanding layer. In the chemical conversion treatment, chlorine that had adhered during the AC etching process was removed using a phosphoric acid solution, and then a voltage was applied in an ammonium dihydrogen phosphate solution.

[0062] Carbon black was selected as the carbon material for the carbon layer of the cathode body. A slurry was prepared by mixing and kneading carbon black powder, styrene butadiene rubber (SBR) as a binder, and an aqueous solution of carboxymethyl cellulose sodium (CMC-Na) as a dispersant-containing solution.

[0063] This slurry was uniformly applied to a cathode foil. The slurry was then heated and dried to volatilize the solvent, after which the cathode body was pressed. In the pressing process, the cathode body was sandwiched between press rollers and linear pressure was applied to fix the carbon layer onto the cathode foil.

[0064] Aluminum foil was used as the anode foil. A surface-expanding layer consisting of spongy etching pits was formed on both sides of the aluminum foil. For the AC etching treatment, 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, a chemical conversion treatment was performed on the anode foil to form a dielectric oxide film on the surface of the surface-expanding layer of the anode foil. For the chemical conversion treatment, after removing the chlorine that had adhered during the AC etching process with a phosphoric acid solution, a voltage was applied in an adipic acid solution.

[0065] Aluminum tab-shaped lead terminals were ultrasonically connected to the anode foil and cathode body, respectively. A zigzag-folded separator was prepared, and the cathode body and anode foil were alternately sandwiched between the folds to form a laminate of the cathode body, anode foil, and separator, with the cathode body and anode foil facing each other via the separator. Regenerated cellulose fiber was used as the separator. The laminate was secured with imide tape to prevent it from opening.

[0066] The laminate was impregnated with an electrolyte solution. The electrolyte solution contained gamma-butyrolactone as a solvent and tetramethylimidazolinium phthalate as a solute. After impregnation with the electrolyte solution, the laminate was sealed in a laminate material. This resulted in the production of a laminate cell electrolytic capacitor. The laminate material was made of aluminum with a thickness of 110 μm. After the laminate cell was produced, it was subjected to an aging treatment. The aging treatment involved applying a voltage of 3.35 V for 60 minutes in a temperature environment of 105°C.

[0067] Several electrolytic capacitors were fabricated in this way. The cathode body of each electrolytic capacitor had different interface resistance between the carbon layer and the cathode foil due to the different pressing conditions used to press the carbon layer onto the cathode foil. The pressing line pressure, pressing temperature, and interface resistance of the cathode body for each electrolytic capacitor are shown in Table 1 below. (Table 1) TIFF0007797790000001.tif89161

[0068] (Capacity test) The capacitance of each electrolytic capacitor having various interfacial resistances shown in Table 1 was measured over time. To measure the capacitance, the electrolytic capacitor was exposed to a temperature environment of 125°C and a DC voltage of 2.4 V was applied. The initial capacitance before the application of the DC voltage of 2.4 V in the 125°C temperature environment and the capacitance after 750 hours had passed were measured. The capacitance was measured by applying a 10 kHz AC signal to the electrolytic capacitor. The rate of change in capacitance after 750 hours relative to the initial capacitance (ΔCap) was then calculated.

[0069] The results of the capacitance calculations are shown in Table 2 below. The results of Table 2 are also shown in the graph in Figure 1. Figure 1 is a graph showing the relationship between interfacial resistance and the rate of change in capacitance (ΔCap). The graph in Figure 1 has interfacial resistance on the horizontal axis and the rate of change in capacitance (ΔCap) on the vertical axis.

[0070] (Table 2) TIFF0007797790000002.tif89161

[0071] As shown in Table 2 and Figure 1, the interface resistance between the carbon layer and the cathode foil was 1.63 mΩ cm 2 In the following cases, it can be confirmed that the capacitance of the electrolytic capacitor is kept to less than 26% of the initial value even after 750 hours of load in a high-temperature environment. On the other hand, when the interface resistance between the carbon layer and the cathode foil is 1.80 mΩ cm 2 However, when the interfacial resistance between the carbon layer and the cathode foil exceeds 1.80 mΩ cm, the capacitance of the electrolytic capacitor decreases rapidly. 2 It can be confirmed that the rate of capacitance reduction of the electrolytic capacitor can be kept to 30% or less compared to the initial value.

[0072] As a result, the cathode body of the electrolytic capacitor 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 at least 1.6 mΩ cm, even taking into account tolerances. 2 It was confirmed that the decrease in capacity over time is suppressed if the oxide film is formed on the cathode foil at a voltage of about 0.5 to 3 V to improve the adhesion between the cathode foil and the carbon layer. In particular, although there is a concern that the oxide film will increase the rate of capacity decrease, the interface resistance of the present invention is 1.6 mΩ cm 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.

[0073] Furthermore, the cathode body of the electrolytic capacitor 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 at least 1.8 mΩ cm, even taking into account tolerances.2 It was confirmed that if the temperature is below this range, the decrease in capacity over time can be suppressed to 30% or less compared to the initial value.

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 is 1.6 mΩ cm ２ That is: An electrolytic capacitor characterized by:

2. the cathode foil has a surface-expanding layer on its surface, and the carbon layer on the surface-expanding layer; 2. The electrolytic capacitor according to claim 1,

3. the carbon layer is pressed against the cathode foil; 3. The electrolytic capacitor according to claim 1 or 2,

4. 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 1.6 mΩ cm ２ That is: A cathode body characterized by:

5. 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 1.6 mΩ cm ２ a cathode body fabricating 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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