Electrolytic capacitor, cathode, and manufacturing method of electrolytic capacitor

Abstract

This invention provides an electrolytic capacitor that suppresses ESR increases even at high temperatures, a cathode of the electrolytic capacitor, and a method for manufacturing the electrolytic capacitor. The electrolytic capacitor includes an anode foil, a cathode, and an electrolyte. The anode foil is formed of a valve metal and has a dielectric oxide film formed on its surface. The cathode has a valve metal cathode foil and a carbon layer deposited on the cathode foil. The interfacial resistance between the cathode foil and the carbon layer is 1.1 mΩ·cm² or less.

Description

[Technical Field]

[0001] The present invention relates to an electrolytic capacitor, a cathode comprising the electrolytic capacitor, and a method for manufacturing the electrolytic capacitor. [Previous Technology]

[0002] An electrolytic capacitor is configured such that the anode foil and cathode foil face each other, wherein the anode foil has a dielectric oxide film formed on a valve metal such as tantalum or aluminum, and the cathode foil is formed of a foil of the same type of valve metal or another valve metal. An electrolyte separator is present between the anode foil and the cathode foil. The electrolyte separator is present between the anode foil and the cathode foil and is in close contact with the uneven surface of the anode foil, thus functioning as the actual cathode.

[0003] In recent years, electrolytic capacitors, in which a solid electrolyte exists between the anode and cathode foils instead of a liquid electrolyte, have become increasingly common. Electrolytic capacitors incorporating this solid electrolyte are small, have high capacitance, and low equivalent series resistance, making them indispensable for the miniaturization and high functionality of electronic devices. Manganese dioxide or 7,7,8,8-tetracyanoquinodimethane (TCNQ) complexes are known as solid electrolytes. Furthermore, 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). Regarding conductive polymers, high conductivity is exhibited when polyanions such as organic sulfonic acids are used as dopants during chemical oxidative polymerization or electrolytic oxidative polymerization, and they also exhibit excellent adhesion to the dielectric oxide film.

[0004] Among them, electrolytic capacitors containing solid electrolytes lack the repair function of dielectric oxide film defects compared to electrolytic capacitors containing electrolyte. Therefore, so-called hybrid electrolytic capacitors, in which a solid electrolyte is present between the anode foil and the cathode foil and which are impregnated with electrolyte, have also attracted attention.

[0005] Here, in order to reduce the ESR (equivalent series resistance), also known as the equivalent series resistance, an electrolytic capacitor is proposed in which a metal carbide with low specific resistance, such as TiC, WC, or ZrC, is formed on the surface of the cathode foil (for example, see Patent Document 1). According to this proposal, it is reported that the ESR of the electrolytic capacitor can be reduced because the adhesion between the metal carbide and the conductive polymer becomes good.

[0006] However, there is a problem that an oxide film gradually grows on the surface of metal carbides. Therefore, an electrolytic capacitor has been proposed that uses a dry plating method, such as ion plating, to form carbon on the surface of the cathode foil (for example, see Patent Document 2). Furthermore, 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 on the cathode side gradually become almost infinite, and setting the capacitance of the electrolytic capacitor only as the anode capacitance. [Prior Art Documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2005-109272 [Patent Document 2] Japanese Patent Application Publication No. 2006-190878 [Summary of the Invention]

[0008] [Problem to be Solved by the Invention] It has been confirmed that even for electrolytic capacitors containing a cathode body with a carbon layer formed on the surface of the cathode foil, the ESR increases significantly when exposed to a high-temperature environment above 120°C. One of the reasons for the increase in ESR under this high-temperature environment is the following phenomenon.

[0009] That is, sometimes electrolytic capacitors contain water. For example, sometimes an aqueous solvent is used as the electrolyte, or a mixture of water and an organic solvent is used as the electrolyte. In addition, when the electrolyte solvent is an alcohol-based solvent and contains a salt of a carboxylic acid or an ionicly dissociable carboxylic acid as a solute, water is generated due to the esterification process.

[0010] If the moisture comes into contact with the cathode foil, an oxide film is formed on the cathode foil through the hydration reaction between the cathode foil and the moisture, and this oxide film gradually grows. If an oxide film of insulating material is formed and grows on the cathode foil, the ESR of the electrolytic capacitor becomes higher. Even if a carbon layer is formed on the surface of the cathode foil, an oxide film will still form because the electrolytic capacitor is exposed to a high-temperature environment and moisture enters between the cathode foil and the carbon layer and comes into contact with the cathode foil.

[0011] This invention was proposed to solve the aforementioned problems, and its object is to provide an electrolytic capacitor that suppresses the rise of ESR even under high-temperature environments, a cathode included in the electrolytic capacitor, and a method for manufacturing the electrolytic capacitor. [Means for Solving the Problems]

[0012] The inventors conducted diligent research and obtained the following insights. Specifically, when a carbon layer is deposited on the cathode foil, and the interfacial resistance between the cathode foil and the carbon layer exceeds 1.1 mΩ·cm², the ESR of the electrolytic capacitor increases significantly when the electrolytic capacitor is exposed to a high-temperature environment above 120°C. On the other hand, when a carbon layer is deposited on the cathode foil, and the interfacial resistance between the cathode foil and the carbon layer is suppressed to below 1.1 mΩ·cm², the increase in the ESR of the electrolytic capacitor is suppressed even when exposed to a high-temperature environment above 120°C.

[0013] Based on this insight, in order to solve the aforementioned problem, the electrolytic capacitor of the present invention includes an anode foil, a cathode body, and an electrolyte. The electrolytic capacitor is characterized in that: the anode foil is formed of a valve metal and has a dielectric oxide film formed on the surface of the foil; the cathode body has a cathode foil of valve metal and a carbon layer deposited on the cathode foil; and the interface resistance between the cathode foil and the carbon layer is less than 1.1 mΩ·cm2.

[0014] Alternatively, the electrolyte may be configured to contain water.

[0015] Alternatively, the electrolyte may contain alcohols as solvents and carboxylic acids, salts of carboxylic acids, or both as solutes.

[0016] Alternatively, the cathode foil may have an extended surface layer on its surface, and the carbon layer may be present on the extended surface layer. If the extended surface layer is formed on the basis of the carbon layer, the cathode foil and the carbon layer are closely connected due to the anchoring effect produced by the unevenness of the carbon material entering the extended surface layer, and the interface resistance between the cathode foil and the carbon layer is more easily reduced.

[0017] Alternatively, the carbon layer can be pressed onto the cathode foil. If the carbon layer is pressed onto the cathode foil on the basis of forming the carbon layer, the interfacial resistance between the cathode foil and the carbon layer is more easily reduced. Furthermore, it is preferable that the pressing of the carbon layer onto the cathode foil and the formation of the expanded surface layer on the cathode foil coexist. If the carbon layer is pressed onto the cathode foil with the expanded surface layer formed, the carbon material of the carbon layer is pressed into the pores of the expanded surface layer, and the carbon layer deforms along the convex and concave surfaces of the expanded surface layer, further improving the adhesion and fixation between the carbon layer and the cathode foil. Therefore, the interfacial resistance between the cathode foil and the carbon layer is more easily reduced.

[0018] It can also be configured as follows: in addition to including the electrolyte, it further includes a solid electrolyte layer formed on the surface of the anode foil and the cathode body.

[0019] In addition, in order to solve the aforementioned problem, the cathode body of the electrolytic capacitor is also an example of the present invention, characterized in that: it includes a cathode foil and a carbon layer formed on the surface of the cathode foil, wherein the interface resistance between the cathode foil and the carbon layer is 1.1 mΩ·cm2 or less.

[0020] Furthermore, in order to solve the aforementioned problem, a method for manufacturing an electrolytic capacitor is also an aspect of the present invention. This method includes an anode foil, a cathode body, and an electrolyte, characterized by comprising: a cathode body fabrication step, wherein after forming a carbon layer on a cathode foil of valve metal, the carbon layer is pressed onto the cathode foil by a pressing process until the interface resistance becomes below 1.1 mΩ·cm², thereby fabricating the cathode body; a capacitor element fabrication step, wherein the anode foil, on which a dielectric oxide film is formed on its surface, faces the cathode body fabricated in the cathode body fabrication step, thereby fabricating a capacitor element; and an impregnation step, wherein the electrolyte is impregnated into the capacitor element. [Effects of the Invention]

[0021] According to the present invention, even when the electrolytic capacitor is exposed to a high temperature environment, the increase in ESR caused by moisture can be suppressed.

Implementation Method

[0023] Hereinafter, an electrolytic capacitor and a manufacturing method according to embodiments of the present invention will be described. Furthermore, the present invention is not limited to the embodiments described below.

[0024] (Overall Structure) An electrolytic capacitor is a passive element that stores and discharges charge by obtaining electrostatic capacitance through the dielectric polarization of a dielectric oxide film. The electrolytic capacitor includes: an anode foil with a dielectric oxide film formed on its surface, a cathode, an electrolyte, and a separator. The anode foil and cathode are arranged facing each other, and the separator and electrolyte exist between the anode foil and the cathode. The anode foil and cathode are arranged in a laminated type, with the separator alternately stacked, or in a wound type, where the separator is wound while being sandwiched.

[0025] The electrolyte is disposed between the anode foil and the cathode body in the form of an electrolyte solution or a mixture of electrolyte and solid electrolyte. The electrolyte is in close contact with the dielectric oxide film on the anode foil, becoming the true cathode for transmitting the electric field of the foil. A separator prevents short circuits between the anode foil and the cathode body and also retains the electrolyte.

[0026] (Electrode Foil) The cathode body includes cathode foil. The cathode foil of both the anode foil and the cathode body is a foil made of a valve metal. Valve metals include aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony, etc. Regarding purity, the ideal purity for the anode foil is above 99.9%, and the ideal purity for the cathode foil is around 99% or above, but it may also contain impurities such as silicon, iron, copper, magnesium, and zinc.

[0027] An expanded surface layer with an expanded surface structure is formed on one or both sides of the anode foil and cathode foil. The expanded surface layer is formed by electrolytic etching, chemical etching, or sandblasting, or by vapor deposition or sintering of metal particles onto the foil. That is, the expanded surface layer includes tunnel-like pits, sponge-like pits, or voids between dense powder particles. Electrolytic etching can be exemplified by applying direct current or alternating current to an acidic aqueous solution containing halide ions, such as hydrochloric acid, for direct current etching or alternating current etching. In chemical etching, the anode foil and cathode foil are immersed in an acidic or alkaline solution. Furthermore, the tunnel-like pits can be formed along the entire length of the foil or a length that does not reach the center of the foil.

[0028] The dielectric oxide film of the anode foil is typically an oxide film formed on the surface of the anode foil. Regarding this dielectric oxide film, for example, if the anode foil is aluminum foil, it is aluminum oxide formed by oxidizing the surface layer. The dielectric oxide film is intentionally formed by a formation process in a solution free of halide ions, such as an aqueous solution of adipic acid, boric acid, or phosphoric acid. An oxide film can also be intentionally formed on the surface of the cathode foil by this formation process, or it can form naturally. A naturally occurring oxide film on the surface of the cathode foil is formed by the reaction of the cathode foil with oxygen in the air.

[0029] (Cathode Body) The cathode body includes a cathode foil and a carbon layer. The carbon layer is deposited on the cathode foil. The carbon layer is a layer containing 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 porous treatment such as activation treatment or pore-forming opening treatment.

[0030] Carbon powder includes, for example, natural plant tissues such as coconut shells, synthetic resins such as phenols, activated carbon derived from fossil fuels such as coal, coke, or asphalt, carbon black such as Ketjen black, acetylene black, channel black, or thermal carbon black, carbon nanohorns, amorphous carbon, natural graphite, artificial graphite, graphitized Ketjen black, mesoporous carbon, etc. Fibrous carbon includes, for example, carbon nanotubes and carbon nanofibers. Carbon nanotubes can be single-layer carbon nanotubes with a single layer of graphene sheet, or multi-layer carbon nanotubes (multi-wall carbon nanotubes, MWCNTs) with two or more layers of graphene sheet coiled coaxially and having multiple layers of tube walls.

[0031] Here, the carbon layer and the cathode foil are adjusted by reducing the interfacial resistance. The degree of low interfacial resistance is related to the tightness of the contact between the carbon layer and the cathode foil. Therefore, by reducing the interfacial resistance, the gap between the carbon layer and the cathode foil can be reduced. If the gap between the carbon layer and the cathode foil is small, it is difficult for water in the electrolyte to contact the surface of the cathode foil. If the contact opportunity between the cathode foil and water decreases, the hydration reaction between the cathode foil and water is suppressed, making it difficult to form an oxide film on the surface of the cathode foil. In addition, even if an oxide film is formed, it is difficult for the oxide film, which is an insulator, to form on the cathode foil, and the increase in the ESR of the electrolytic capacitor is suppressed.

[0032] However, regarding the suppression effect of ESR rise based on this mechanism, considering the correlation between interface resistance and the degree of contact, it is considered to be inversely proportional to the degree of interface resistance. However, with the interface resistance between the cathode foil and the carbon layer at 1.1 mΩ·cm², the change in ESR of the electrolytic capacitor under high-temperature conditions is significantly different, exceeding the range that can be explained by this mechanism. If the interface resistance between the carbon layer and the cathode foil is below 1.1 mΩ·cm², the rise in ESR of the electrolytic capacitor is suppressed even when exposed to high-temperature environments above 120°C. On the other hand, if the interface resistance between the carbon layer and the cathode foil exceeds 1.1 mΩ·cm², the ESR of the electrolytic capacitor exposed to high-temperature environments above 120°C will rise significantly.

[0033] Therefore, the interfacial resistance between the carbon layer and the cathode foil is adjusted to below 1.1 mΩ·cm². This significantly suppresses the rise in the ESR of the electrolytic capacitor, even when exposed to high temperatures above 120°C. As a method for adjusting the interfacial resistance between the carbon layer and the cathode foil, a pressing process that involves pressing the carbon layer and the cathode foil together is suitable. In the pressing process, a pressing line is applied by clamping the laminate of the carbon layer and the cathode foil with a pressure roller. Ideally, the pressing line pressure is around 0.01 t / cm to 100 t / cm. Furthermore, the ideal temperature of the pressure roller during pressing, i.e., the pressing temperature, is around 0°C to 200°C.

[0034] Furthermore, as a pre-processing stage, a carbon layer is pre-formed on the cathode foil using methods such as vacuum evaporation, sputtering, ion plating, chemical vapor deposition (CVD), coating, electrolytic plating, and electroless plating. In the case of coating, carbon material is dispersed in a dispersion solvent to prepare a slurry, which is then applied to the cathode foil using methods such as slurry casting, doctor blade application, or spraying and dried. In the case of vacuum evaporation, carbon material is evaporated by electrically heating it in a vacuum, or by irradiating it with an electron beam in a vacuum, thereby forming a carbon film on the cathode foil. In the case of sputtering, a target containing carbon material and a cathode foil are placed in a vacuum container, an inert gas is introduced into the vacuum container, and a voltage is applied, causing the plasma-coated inert gas to collide with the target, resulting in carbon material particles ejected from the target accumulating on the cathode foil.

[0035] 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 0.5 V or more and about 3 V on the cathode foil by a formation process. While the oxide film formed on the cathode foil affects the increase of the interfacial resistance of the electrolytic capacitor, 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 0.5 V or more and about 3 V, the improvement in adhesion is strongly influenced compared to oxide films outside this range, and the interfacial resistance between the carbon layer and the cathode foil easily decreases to below 1.1 mΩ·cm².

[0036] Another method for adjusting the interfacial resistance between the carbon layer and the cathode foil is to form an expansion layer on the surface of the cathode foil. By also forming an expansion layer on the surface of the cathode foil, the carbon material of the carbon layer enters the unevenness of the expansion layer, which can reduce the interfacial resistance between the carbon layer and the cathode foil. If the carbon layer and the cathode foil are pressed after forming the expansion layer on the cathode foil, the interfacial resistance can be reduced more easily.

[0037] In addition, as a method for adjusting the interfacial resistance between the carbon layer and the cathode foil, the selection of carbon material contained in the carbon layer can be listed. As the carbon material, carbon black as spherical carbon is preferred. By using spherical carbon black with an average primary particle size of 100 nm or less, the carbon layer becomes dense, and the carbon layer is more likely to adhere closely to the expanded surface layer, thus the interfacial resistance is more likely to decrease.

[0038] Furthermore, the carbon material contained in the carbon layer can be flake-shaped or scaly graphite and carbon black as spherical carbon. The flake-shaped or scaly graphite preferably has an aspect ratio of minor axis to major axis of 1:5 to 1:100. If a carbon layer containing this combination of carbon materials is deposited on a cathode foil, the carbon layer is compressed, and pressed against the expansion layer, the carbon black is easily rubbed into the expansion layer by the graphite. The graphite easily deforms along the uneven surface of the expansion layer and easily accumulates on the uneven surface. Moreover, the graphite acts as a pressure cap, confining the spherical carbon inside the expansion layer. Therefore, the interfacial resistance between the carbon layer and the cathode foil easily decreases.

[0039] Furthermore, the interfacial resistance can be measured in the following manner. That is, the potential of the surface of the carbon layer of the cathode body to which the carbon layer is formed is measured at multiple locations. In other words, the surface of the carbon layer whose potential is measured is the surface located opposite to the surface in close contact with the cathode foil, or the exposed surface of the cathode body. In the potential measurement, an application probe is brought into contact with the surface of the carbon layer, and a measuring probe is also brought into contact with the surface of the carbon layer. A predetermined DC current is applied between the probes, and the resistance calculated at this time is the "interfacial resistance of the cathode". As a suitable device for obtaining the interfacial resistance of the cathode, the electrode resistance measuring system RM2610 manufactured by HIOKI Electric Co., Ltd. can be cited as an example.

[0040] (Electrolyte) The solvent of the electrolyte is water, a protic organic polar solvent, or a non-protic organic polar solvent, and may be a single type or a combination of two or more. The solute contains both anionic and cationic components. Typical solutes are salts of organic acids, salts of inorganic acids, or salts of complex compounds of organic and inorganic acids, which may be used alone or in combination of two or more. Alternatively, acids that become anions and bases that become cations may be added separately as solute components to the electrolyte.

[0041] As proton-based polar organic solvents, examples include: monohydric alcohols, polyhydric alcohols, and oxygen-containing alcohol compounds. Examples of monohydric alcohols include: ethanol, propanol, butanol, pentanol, hexanol, cyclobutanol, cyclopentanol, cyclohexanol, benzyl alcohol, etc. Examples of polyhydric alcohols and oxygen-containing alcohol compounds include: ethylene glycol, propylene glycol, glycerol, methyl cellosolve, ethyl cellosolve, methoxypropylene glycol, dimethoxypropanol, etc.

[0042] Examples of aprotic organic polar solvents include sulfonium-based, amide-based, lactone-based, cyclic amide-based, nitrile-based, and sulfene-based solvents. Examples of sulfonium-based solvents include: dimethylsulfonium, ethylmethylsulfonium, diethylsulfonium, cyclobutanesulfonium, 3-methylcyclobutanesulfonium, 2,4-dimethylcyclobutanesulfonium, etc. Examples of amide-based solvents include: N-methylmethamide, N,N-dimethylmethamide, N-ethylmethamide, N,N-diethylmethamide, N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide, hexamethylphosphatamide, etc. Examples of lactones and cyclic amides include: γ-butyrolactone, γ-valerolactone, δ-valerolactone, N-methyl-2-pyrrolidone, ethyl acetate carbonate, propyl acetate carbonate, butyl acetate carbonate, and isobutyl acetate carbonate. Examples of nitrile compounds include: acetonitrile, 3-methoxypropionitrile, and glutaronitrile. Examples of urethane compounds include: dimethyl urethane.

[0043] As organic acids, examples include: oxalic acid, succinic acid, glutaric acid, pimelic acid, octanoic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, maleic acid, adipic acid, benzoic acid, toluic acid, heptanoic acid, malonic acid, 1,6-decanedicarboxylic acid, 1,7-octanedicarboxylic acid, azelaic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, and other carboxylic acids, phenols, and sulfonic acids. Additionally, as inorganic acids, examples include: boric acid, phosphoric acid, phosphorous acid, hypophosphorous acid, carbonic acid, and silicic acid. As complex compounds of organic and inorganic acids, examples include: borodisalicylic acid, borodioxalic acid, and borodiglycolic acid.

[0044] Examples of salts that are at least one of the salts of organic acids, inorganic acids, or complex compounds of organic and inorganic acids include: ammonium salts, quaternary ammonium salts, quaternary amidine salts, amine salts, sodium salts, potassium salts, etc. Examples of quaternary ammonium ions that are quaternary ammonium salts include: tetramethylammonium, triethylmethylammonium, tetraethylammonium, etc. Examples of quaternary amidine salts include: ethyldimethylimidazolinetonium, tetramethylimidazolinetonium, etc. Examples of amine salts that are amine salts include: primary amines, secondary amines, tertiary amines. Examples of primary amines include: methylamine, ethylamine, propylamine, etc. Examples of secondary amines include: dimethylamine, diethylamine, ethylmethylamine, dibutylamine, etc. Examples of tertiary amines include: trimethylamine, triethylamine, tripropylamine, tributylamine, ethyldimethylamine, ethyldiisopropylamine, etc. The electrolyte can be prepared by adding salts with ionic dissociation properties, which consist of organic acids, inorganic acids, or complex compounds of organic and inorganic acids that become anions, and bases that become cations.

[0045] The solvent and solute used as the electrolyte are a combination of reaction sources containing water or esterification. That is, as a first type of electrolyte, the solvent contains water, and there are no other particular limitations. As a second type of electrolyte, a monohydric alcohol, polyhydric alcohol, oxygen-containing alcohol compound, or a mixture thereof is used as the solvent, and a carboxylic acid or a salt of a carboxylic acid is used as the solute, and there are no other particular limitations. Water is generated by the esterification reaction of the alcohol and the carboxylic acid, which then undergoes a hydration reaction with the cathode foil. As a third type of electrolyte, it is a mixture of the first and second types of electrolytes.

[0046] Furthermore, other additives may be added to the electrolyte. Examples of additives include: polyethylene glycol, compounds of boric acid and polysaccharides (mannitol, sorbitol, etc.), compounds of boric acid and polyols, borate esters, nitro compounds, phosphate esters, colloidal silicon dioxide, etc. These can be used alone or in combination of two or more. Nitro compounds suppress the amount of hydrogen produced in the electrolytic capacitor. Examples of nitro compounds include: o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, p-nitrophenol, etc.

[0047] The electrolyte is impregnated in a capacitor element after preparation. The capacitor element is formed by facing an anode foil with a dielectric oxide film and a cathode separator with a carbon layer deposited on a cathode foil. When impregnating the capacitor element with the electrolyte, pressure reduction or pressure treatment may be performed as needed to promote impregnation. The impregnation step may also be repeated multiple times. Furthermore, when a solid electrolyte layer is used, the electrolyte is impregnated in a capacitor element in which a 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 monomers or their derivatives having π-conjugated double bonds. By doping the conjugated polymer, the conductive polymer exhibits high conductivity.

[0049] As conjugated polymers, known substances may be used without particular limitation. Examples include: polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene oxide, polyacene, polythiophene oxide, etc. As conductive polymers, poly(3,4-phenylene sulfonic acid, PSS) doped with polystyrene sulfonic acid (PSS) and called poly(3,4-phenylene sulfonyl dioxythiophene) may be representatively cited. These conjugated polymers may be used alone, or in combination of two or more monomers, or further as copolymers of two or more monomers.

[0050] Dopants may be any known substances without particular limitation. Examples include: inorganic acids such as boric acid, nitric acid, and phosphoric acid; acetic acid, oxalic acid, citric acid, ascorbic acid, tartaric acid, squaric acid, rosinic acid, ketone acid, salicylic acid, p-toluenesulfonic acid, 1,2-dihydroxy-3,5-benzene disulfonic acid, methanesulfonic acid, trifluoromethanesulfonic acid, borosilicate, bis(oxalate-boronic acid), sulfonyliminic acid, dodecylbenzenesulfonic acid, propylnaphthalenesulfonic acid, butylnaphthalenesulfonic acid, and other organic acids. Additionally, polyanionic dopants may be used, including: polyvinylsulfonic acid, polystyrene sulfonic acid, polyallyl sulfonic acid, polyacrylic acid sulfonic acid, polymethacrylic acid sulfonic acid, poly(2-acrylamide-2-methylpropanesulfonic acid), polyisoprene sulfonic acid, polyacrylic acid, polymethacrylic acid, polymaleic acid, etc. Dopants may be used alone or in combination of two or more. Alternatively, polymers or monomers can also be used.

[0051] This solid electrolyte layer is formed by impregnating a dispersion containing conductive polymers into a capacitor element. The solvent for the dispersion can be any medium that disperses conductive polymer particles or powders; water is primarily used. Ethylene glycol can also be used as the dispersion solvent if necessary. However, using ethylene glycol as the dispersion solvent can reduce the electrical properties of the product, especially its ESR characteristics. To improve the impregnation and conductivity of the dispersion, various additives can be used in the dispersion, or neutralization can be achieved by adding cations.

[0052] As an impregnation method for a dispersion of conductive polymers, capacitor elements can also be impregnated in the dispersion, or drop coating or spray coating can be performed. Furthermore, not limited to the entire electrode pair, capacitor elements can also be assembled after the dispersion is impregnated in the anode foil or cathode body. To promote the impregnation of the dispersion into the electrode pair, depressurization or pressurization treatment can be performed as needed. This adhesion step can also be repeated multiple times.

[0053] Alternatively, the solid electrolyte layer can also be formed by known electrolytic polymerization or chemical polymerization. In chemical polymerization, the solid electrolyte layer is formed by immersing a capacitor element in a liquid formed by dissolving a monomer and an oxidant in a solvent and then drying it; or by alternately immersing the electrode pair in a liquid formed by dissolving a monomer in a solvent and a liquid formed by dissolving an oxidant in a solvent and then drying it. For example, using 3,4-einyldioxythiophene as a polymerizable monomer and an alcoholic solution (ethanol, etc.) of ferric p-toluenesulfonate as an oxidant, the capacitor element is immersed in the mixture of the polymerizable monomer and the oxidant, and a polymerization reaction of a conductive polymer is generated by heating, thereby forming a solid electrolyte. In addition, a water washing treatment can be performed before and after the heat treatment to remove unreacted monomers or residual monomers.

[0054] In electrolytic polymerization, the solid electrolyte layer is formed by introducing a capacitor element into an electrolytic polymerization solution containing at least a monomer, a supporting electrolyte, and a solvent, and applying a voltage between the anode and cathode. The electrolytic polymerization solution can be a monomer that becomes conductive through electrolytic polymerization. Thiophene monomers or pyrrole monomers are suitable as monomers. When using these monomers, the capacitor element is immersed in an aqueous solution for electrolytic polymerization containing the monomer and sodium 1-naphthalenesulfonate as a supporting electrolyte in a stainless steel container, and a predetermined voltage is applied. In this way, a solid electrolyte layer can be uniformly formed by electrolytic polymerization using water-soluble monomers (e.g., thiophene or pyrrole).

[0055] (Separator) Separators may include kraft paper, Manila hemp, esparto, hemp, rayon and other cellulose and mixed paper, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate and its derivatives, polyester resins such as polytetrafluoroethylene resins, polyvinylidene fluoride resins, vinylon resins, aliphatic polyamines, semi-aromatic polyamines, fully aromatic polyamines and other polyamine resins, polyimide resins, polyethylene resins, polypropylene resins, trimethylpentene resins, polyphenylene sulfide resins, acrylic resins, polyvinyl alcohol resins, etc., which may be used alone or in combination.

[0056] Furthermore, the separator is responsible for maintaining the solid electrolyte layer and the electrolyte, as well as preventing short circuits between the anode foil and the cathode. In cases where the solid electrolyte layer can maintain its shape even without the separator, each part of the capacitor element containing the solid electrolyte layer can retain the electrolyte, and the solid electrolyte layer has a thickness sufficient to prevent short circuits between the anode foil and the cathode, the separator may not be necessary.

[0057] (Manufacturing method) This electrolytic capacitor can be manufactured by the following steps: an anode foil manufacturing step, manufacturing an anode foil; a cathode body manufacturing step, manufacturing a cathode body; an element manufacturing step, manufacturing a capacitor element with the anode foil and cathode body facing each other; a solid electrolyte layer forming step, forming a solid electrolyte layer on the capacitor element; and an electrolyte impregnation step, impregnating the capacitor element in which the solid electrolyte layer has been formed with electrolyte.

[0058] In the anode foil fabrication step, valve metal is extended to form an anode foil, and an expanded surface layer is formed on the anode foil. A dielectric oxide film is then formed on the surface of the expanded surface layer. In the cathode body fabrication step, valve metal is extended to form a cathode foil, and an expanded surface layer is formed on the cathode foil. Furthermore, in the cathode body fabrication step, a carbon layer is formed on the cathode foil, and the carbon layer is pressed onto the cathode foil by a pressing process, setting the interface resistance between the carbon layer and the cathode foil to 1.1 mΩ·cm² or less.

[0059] In the element fabrication step, the anode foil with a dielectric oxide film is overlapped with the cathode body separator. In the case of a multilayer type, the anode foil, cathode body, and separator are alternately multilayered. In the case of a winding type, the anode foil and cathode body with overlapped separators are wound. In the solid electrolyte layer formation step, a dispersion containing conductive polymers is impregnated in the capacitor element. In the electrolyte impregnation step, an electrolyte is impregnated in the capacitor element having a solid electrolyte layer formed through the solid electrolyte layer formation step.

[0060] Accordingly, an electrolytic capacitor with an interfacial resistance of 1.1 mΩ·cm² or less between the cathode foil and the carbon layer is manufactured. This electrolytic capacitor suppresses the rise in ESR even when exposed to high temperatures above 120°C. It is particularly useful when the electrolyte solvent is partially or entirely aqueous, or contains alcohols as a solvent, and contains carboxylic acids, salts of carboxylic acids, or both as solutes. [Example]

[0061] Hereinafter, the electrolytic capacitor and manufacturing method of the present invention will be described in further detail based on embodiments. Furthermore, the present invention is not limited to the following embodiments.

[0062] (Examples 1 to 5) Solid electrolytic capacitors of Examples 1 to 5 and Comparative Examples 1 to 3 were manufactured as follows. In these solid electrolytic capacitors, aluminum foil was used for both the anode foil and the cathode foil. An AC etching process was performed on the anode foil and the cathode foil to form an extended layer containing sponge-like etching pits on both sides of the foil. In the AC etching process, the cathode foil was immersed in an acidic aqueous solution at a liquid temperature of 25°C and with approximately 8% by weight hydrochloric acid as the main electrolyte, and an AC current of 10 Hz and a current density of 0.14 A / cm² was applied to the substrate for approximately 5 minutes. Then, a formation process was performed on the anode foil to form a dielectric oxide film on the surface of the extended layer of the anode foil. In the formation process, after removing the chlorine adhering during the AC etching process using an aqueous phosphoric acid solution, a voltage was applied in an aqueous solution of ammonium dihydrogen phosphate.

[0063] A carbon layer is deposited on the extended layer of the cathode foil to complete a cathode body comprising the cathode foil and the carbon layer. Carbon black is selected as the carbon material for the carbon layer. Carbon black powder, styrene butadiene rubber (SBR) as a binder, and an aqueous solution of carboxymethyl cellulose sodium (CMC-Na) containing a dispersant are mixed and kneaded to prepare a slurry, which is then uniformly coated onto the cathode foil. The slurry is then heated and dried to evaporate the solvent.

[0064] After forming a carbon layer on the expanded surface layer of the cathode foil, a pressing process is performed to push the carbon layer onto the expanded surface layer. The cathode body is clamped by a pressure roller, and a pressing line pressure is applied.

[0065] Here, by performing pressing processing under different pressing conditions for each embodiment and comparative example, the interface resistance between the cathode foil and the carbon layer differs in each embodiment and comparative example. The pressing line voltage, pressing temperature, and interface resistance of each embodiment and comparative example are shown in Table 1 below. Furthermore, the variation of the interface resistance with respect to the pressing line voltage or pressing temperature varies, for example, depending on the thickness of the cathode foil or the thickness of the carbon layer.

[0066] (Table 1) Suppression line pressure (kN / cm) Pressing temperature (°C) Interface resistance (mΩ･cm) 2 ) Example 1 7 150 0.51 Example 2 7 70 0.66 Example 3 4 70 0.72 Example 4 2 150 0.73 Example 5 4 70 1.03 Comparative Example 1 3.5 70 1.28 Comparative Example 2 1 150 1.83 Comparative Example 3 2 70 2.2

[0067] Aluminum tag-shaped lead terminals are sewn onto the anode foil and cathode body respectively. A separator is sandwiched between the anode foil and the cathode body and the capacitor element is wound to manufacture a capacitor element including the anode foil, cathode body and separator. A Manila-based separator is used as the separator. After winding, a formation process is performed to repair defects caused by winding.

[0068] Next, a dispersion of the conductive polymer is prepared. The dispersion is prepared by dispersing a powder of poly(ethylene ethyl dioxythiophene) (PEDOT), a conductive polymer doped with polystyrene sulfonate (PSS), in water. The capacitor element is immersed in the dispersion. During immersion, it is exposed to a pressure of 30 kPa for 120 seconds. Afterward, the capacitor element is lifted and dried at 150°C for 30 minutes. The immersion and drying are repeated twice. In this way, the solid electrolyte layer containing poly(ethylene ethyl dioxythiophene) doped with polystyrene sulfonate (PSS) is tightly bonded to the dielectric oxide film of the anode foil, and it is also deposited on the carbon layer of the cathode.

[0069] Next, an electrolyte is prepared and immersed in a capacitor element having a solid electrolyte layer formed thereon. The electrolyte is prepared by using ethylene glycol as a solvent and adding ammonium azelate as a solute. Ethylene glycol is a diol, and azelate is a saturated dicarboxylic acid. Water is generated over time through an esterification reaction between ethylene glycol and azelate. Furthermore, the esterification reaction is promoted by exposure to a high-temperature environment.

[0070] The capacitor element was inserted into a bottomed cylindrical outer casing, and a sealing rubber was installed at the open end, which was then sealed by riveting. The solid electrolytic capacitor was then exposed to a temperature of 115°C for 45 minutes for aging treatment. The solid electrolytic capacitors of the embodiments and comparative examples prepared by the above method have 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] (Evaluation of ESR) The solid electrolytic capacitors of Examples 1 to 5 and Comparative Examples 1 to 3 were placed in three different temperature environments, and their ESR was measured after each elapsed time. A 100 kHz AC signal was applied to the solid electrolytic capacitors during ESR measurement. The percentage increase in ESR after each elapsed time relative to the initial ESR before placement in the three temperature environments was then calculated. The calculation results are shown in Figures 1 to 3. Figure 1 is a graph showing the change in the rate of increase in ESR over time for each solid electrolytic capacitor after placement at 125°C. Figure 2 is a graph showing the change in the rate of increase in ESR over time for each solid electrolytic capacitor after placement at 135°C. Figure 3 is a graph showing the change in the rate of increase in ESR over time for each solid electrolytic capacitor after placement at 150°C.

[0072] In addition, the increase rate of ESR after being left to stand for 2088 hours at three different temperatures is shown in Table 2 below. (Table 2) Interface resistance (mΩ･cm) 2 ) 125℃ (%) 135℃ (%) 150℃ (%) Example 1 0.51 117.27 119.58 148.71 Example 2 0.66 115.59 123.34 161.22 Example 3 0.72 125.10 134.72 191.17 Example 4 0.73 131.13 143.24 210.22 Example 5 1.03 125.95 146.74 200.15 Comparative Example 1 1.28 148.93 187.88 284.28 Comparative Example 2 1.83 163.14 203.79 301.03 Comparative Example 3 2.2 186.08 227.36 297.60

[0073] As shown in Figures 1 to 3, in the solid electrolytic capacitors of Comparative Examples 1 to 3, the ESR continues to increase significantly over time. On the other hand, in the solid electrolytic capacitors of Examples 1 to 5, the increase in ESR relative to the passage of time is small, and the increase in ESR is suppressed. Therefore, after 2088 hours, there is a large difference in ESR between the group of Comparative Examples 1 to 3 and the group of Examples 1 to 5.

[0074] Specifically, as shown in Table 2, at a temperature of 125°C, the maximum value in the group of Examples 1 to 5 was 131.13, while the minimum value in Comparative Examples 1 to 3 was 148.93, a difference of 17.8. At a temperature of 135°C, the maximum value in the group of Examples 1 to 5 was 146.74, while the minimum value in Comparative Examples 1 to 3 was 187.79, a difference of 41.05. At a temperature of 150°C, the maximum value in the group of Examples 1 to 5 was 210.22, while the minimum value in Comparative Examples 1 to 3 was 284.28, a difference of 74.06. Thus, the higher the temperature, the more significantly the difference between the group of Examples 1 to 5 and the group of Comparative Examples 1 to 3 widens.

[0075] Examples 1 to 5 belong to the group where the interfacial resistance between the cathode foil and the carbon layer is 1.1 mΩ·cm² or less. On the other hand, Comparative Examples 1 to 3 belong to the group where the interfacial resistance between the cathode foil and the carbon layer is 1.28 mΩ·cm² or more. Thus, it was confirmed that in an electrolytic capacitor, by using a cathode foil having a valve metal as the cathode body and a carbon layer deposited on the cathode foil, and setting the interfacial resistance between the cathode foil and the carbon layer to 1.1 mΩ·cm² or less, the increase in ESR under high-temperature conditions can be suppressed. In particular, when the adhesion between the cathode foil and the carbon layer is improved by forming an oxide film of about 0.5 V to 3 V on the cathode foil, although there is a concern that the interfacial resistance will increase due to the oxide film, by adjusting the interfacial resistance to 1.1 mΩ·cm² or less as in the present invention, the reduction in electrostatic capacitance can be suppressed even when an oxide film is formed on the surface of the cathode foil.

[0076] Furthermore, it was confirmed that long-term stability was achieved by suppressing the increase rate of ESR after 288 hours to less than 115% at 125°C, less than 130% at 135°C, and less than 150% at 150°C. This phenomenon is particularly pronounced at higher temperatures. [Simplified Explanation of the Diagram]

[0022] Figure 1 is a graph showing the change over time in the ESR increase rate of each embodiment and each comparative example at a temperature of 125°C. Figure 2 is a graph showing the change over time in the ESR increase rate of each embodiment and each comparative example at a temperature of 135°C. Figure 3 is a graph showing the change over time in the ESR increase rate of each embodiment and each comparative example at a temperature of 150°C.

Claims

1. An electrolytic capacitor comprising an anode foil, a cathode body, and an electrolyte, characterized in that: the anode foil is formed of a valve metal and has a dielectric oxide film formed on the surface of the foil; the cathode body comprises a cathode foil of valve metal, an oxide film of 0.5 V to 3 V on the cathode foil, and a carbon layer deposited on the oxide film; the interface resistance between the cathode foil and the carbon layer is less than 1.1 mΩ·cm².

2. The electrolytic capacitor as claimed in claim 1, wherein the electrolyte comprises water.

3. The electrolytic capacitor as claimed in claim 1 or claim 2, wherein the electrolyte comprises an alcohol as a solvent and comprises a carboxylic acid, a salt of a carboxylic acid, or both as a solute.

4. The electrolytic capacitor as claimed in claim 1 or claim 2, wherein the cathode foil has an expanded surface layer on its surface and the carbon layer is present on the expanded surface layer.

5. The electrolytic capacitor as claimed in claim 1 or claim 2, wherein the carbon layer is pressed onto the cathode foil.

6. The electrolytic capacitor as claimed in claim 1 or claim 2, wherein in addition to the electrolyte, it further includes a solid electrolyte layer formed on the surfaces of the anode foil and the cathode body.

7. A cathode body, which is the cathode body of an electrolytic capacitor, characterized in that: it comprises a cathode foil, an oxide film of 0.5 V to 3 V on the cathode foil, and a carbon layer formed on the surface of the oxide film, wherein the interface resistance between the cathode foil and the carbon layer is less than 1.1 mΩ·cm2.

8. A method for manufacturing an electrolytic capacitor, comprising an anode foil, a cathode body, and an electrolyte, characterized in that it includes: The cathode fabrication step involves forming an oxide film of 0.5 V to 3 V on a cathode foil of valve metal and then forming a carbon layer on the oxide film. The carbon layer is then pressed onto the cathode foil by a pressing process until the interface resistance becomes below 1.1 mΩ·cm2, thereby fabricating the cathode. The capacitor element fabrication step involves placing the anode foil with a dielectric oxide film on its surface facing the cathode fabrication step, thereby fabricating a capacitor element. And an impregnation step, in which the electrolyte is impregnated in the capacitor element.

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