electrolytic capacitors

Abstract

The present invention relates to an electrolytic capacitor including a resin layer, and provides an electrolytic capacitor whose ESR increase over time is suppressed. The electrolytic capacitor comprises: a capacitor element having an anode foil, a cathode foil, and an electrolyte; a housing for housing the capacitor element; a sealing member for sealing the housing; and a resin layer disposed near the sealing member. The resin layer disposed near the sealing member comprises an epoxy resin composition containing an ester bond. In contrast, the electrolyte of the electrolytic capacitor contains 45% by weight or less of a compound having a hydroxyl group in the solvent, including 0% by weight.

Description

Technical Field

[0001] The present invention relates to an electrolytic capacitor including a resin layer. Background Art

[0002] Electrolytic capacitors using valve-action metals such as tantalum and aluminum achieve compactness and high capacitance by forming the valve-action metal, which serves as the anode-side counter electrode, into a sintered body or etched foil, thereby expanding the dielectric surface. This type of electrolytic capacitor is constructed so that the dielectric oxide film of the anode and the counter electrode are in close contact, with the electrolyte filling the gap. Specifically, electrolytic capacitors include liquid electrolytic capacitors containing only an electrolyte, hybrid electrolytic capacitors containing an electrolyte and a solid electrolyte, and bipolar electrolytic capacitors with a dielectric film formed on both electrodes.

[0003] The electrolyte contains ethylene glycol or γ-butyrolactone as a solvent and carboxylic acids such as 1,6-decanedicarboxylic acid, 1,7-octanedicarboxylic acid, and azelaic acid or their salts as solutes. The electrolyte is in direct contact with the dielectric film and acts as a true cathode, and has a repairing effect on the dielectric film. However, the electrolyte will evaporate and dissipate to the outside of the electrolytic capacitor over time. Therefore, the electrolytic capacitor faces drying up, the electrostatic capacitance decreases over time, and the tangent of the loss angle (tanδ) increases over time, eventually reaching the end of its life.

[0004] Therefore, the capacitor element is housed in an outer casing with a bottom, and the opening of the outer casing is sealed with a sealing member, thereby sealing the electrolyte impregnated in the capacitor element to suppress the evaporation of the electrolyte, thereby achieving a long life. However, the sealing member uses an elastomer such as butyl rubber or ethylene propylene diene rubber (ethylene propylene diene monomer, EPDM). Therefore, the electrolyte is not completely enclosed in the casing, but gradually evaporates to the outside of the electrolytic capacitor through the sealing member. Therefore, as in Patent Document 1, in order to suppress the evaporation of the electrolyte caused by the penetration of the sealing member, a scheme of covering the sealing member with a resin layer has been proposed.

[0005] The resin layer generally has an excellent thermal insulation effect. If the thermal insulation of the electrolytic capacitor becomes higher, the heat generated in the shell is difficult to dissipate and is easily stored. Therefore, as in Patent Document 2, it is proposed to use ethylene glycol as a solvent for the electrolyte. The reason is that ethylene glycol has a thermal conductivity about twice that of γ-butyrolactone or cyclopentane sulfone. In Patent Document 2, it is suggested that the greater the proportion of ethylene glycol in the solvent, the more improved the thermal conductivity is, but in order to contain other types of solvents, ethylene glycol is preferably set to 90% by mass or less.

[0006] [Prior art literature]

[0007] [Patent Document]

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 60-245106

[0009] Patent Document 2: WO2018 / 123525 Summary of the Invention

[0010] [Problems to be solved by the invention]

[0011] The resin layer contains a curable resin composition. Examples of curable resin compositions include epoxy resin compositions containing an epoxy resin and an acid anhydride hardener. Epoxy resin compositions are epoxy resin compositions cured with an acid anhydride hardener. Epoxy resin compositions containing an acid anhydride hardener have improved heat resistance, and since the reaction temperature between the epoxy resin and the acid anhydride hardener is above 100°C, the curing reaction can be accelerated using temperature as a trigger. Therefore, epoxy resin compositions containing an epoxy resin and an acid anhydride hardener are considered suitable for the resin layer.

[0012] The inventors used epoxy resin cured with an anhydride hardener to partially coat an electrolytic capacitor. They then exposed the capacitor to a 150°C temperature environment. They found that after 400 hours of exposure, the equivalent series resistance (ESR) of the capacitor increased significantly.

[0013] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an electrolytic capacitor including a resin layer, wherein an increase in ESR over time is suppressed.

[0014] [Technical means to solve the problem]

[0015] The inventors of the present invention have conducted diligent research and have come to the following conclusion: the time-dependent increase in the ESR of electrolytic capacitors is unique to the case where an epoxy resin composition comprising an epoxy resin and an anhydride hardener (hereinafter referred to as an anhydride-hardened epoxy resin) is used to form a resin layer and ethylene glycol is used as a solvent for the electrolyte.

[0016] Furthermore, the inventors and others conducted extensive research based on this understanding and found that anhydride-hardened epoxy resin reacts with water or ethylene glycol and dissolves into the electrolyte. The presence of anhydride-hardened epoxy resin in the electrolyte is the cause of the increased ESR of electrolytic capacitors. Anhydride-hardened epoxy resin contains ester bonds in its chemical structure. On the other hand, even when water is not intentionally added to electrolytic capacitors, a small amount of water is mixed into the electrolytic capacitor during the manufacturing process. Ethylene glycol is a compound with hydroxyl groups, and compounds with hydroxyl groups have structures that are easily accessible to water. Therefore, when the compound with hydroxyl groups permeates the sealing member, water also permeates, reacting with the ester bonds in the anhydride-hardened epoxy resin, and the anhydride-hardened epoxy resin is hydrolyzed. In addition, an esterification reaction occurs between the carbonyl groups generated by hydrolysis and the compound with hydroxyl groups. Thus, it is believed that a portion of the anhydride-hardened epoxy resin component that reacts with water or the compound with hydroxyl groups permeates the sealing member and dissolves into the electrolyte.

[0017] However, the present inventors have diligently studied and found that the hydrolysis and elution of the epoxy resin composition having an ester bond into the electrolyte can be solved by re-estimating the amount of the compound having a hydroxyl group relative to the entire solvent.

[0018] The present invention is based on the above findings. The electrolytic capacitor of the present invention is characterized by comprising: a capacitor element having an anode foil, a cathode foil, and an electrolyte; a case for accommodating the capacitor element; a sealing member for sealing the case; and a resin layer disposed near the sealing member, wherein the resin layer comprises an epoxy resin composition containing an ester bond, and the solvent of the electrolyte contains a compound having a hydroxyl group at a concentration of 45 wt % or less, including 0 wt %.

[0019] The epoxy resin composition may include an epoxy resin and an acid anhydride hardener as raw materials, and may have a chemical structure including an epoxy resin and an acid anhydride hardener.

[0020] In the solvent of the electrolyte, the compound having a hydroxyl group may account for 10 wt % or more.

[0021] The compound having a hydroxyl group may be ethylene glycol, diethylene glycol, or polyethylene glycol.

[0022] The electrolyte may contain 35 wt % or more of sulfolane based on the total amount of the solvent of the electrolyte.

[0023] The capacitor element may also have a solid electrolyte.

[0024] The electrolyte solution may contain 76 mmol or less of cationic components per 100 g of the electrolyte solution. Alternatively, the electrolyte solution may contain 25 mmol or less of cationic components per 100 g of the electrolyte solution.

[0025] [Effects of the Invention]

[0026] According to the present invention, it is possible to suppress an increase in the ESR of an electrolytic capacitor including a resin layer over time. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a graph showing the relationship between the ratio of ethylene glycol and ESR.

[0028] Figure 2 These are perspective images captured from multiple directions in Comparative Example 1, Comparative Example 2, and Example 1. DETAILED DESCRIPTION

[0029] Hereinafter, an electrolytic capacitor according to an embodiment of the present invention will be described. However, the present invention is not limited to the embodiment described below.

[0030] (Overall structure)

[0031] An electrolytic capacitor is a passive component that stores and discharges electric charge through electrostatic capacitance. The electrolytic capacitor includes a liquid electrolytic capacitor having only an electrolyte, and a hybrid electrolytic capacitor using a solid electrolyte such as a conductive polymer or gel and an electrolyte. In addition, the electrolytic capacitor includes an electrolytic capacitor having a dielectric oxide film formed only on the anode side, and a bipolar electrolytic capacitor having a dielectric oxide film formed on both sides of the electrode. Furthermore, the electrolytic capacitor includes an electrolytic capacitor with lead terminals, an electrolytic capacitor provided with a base and surface-mounted on a substrate, and an electrolytic capacitor that is entirely coated with resin together with other electronic circuit components. Hereinafter, these various combinations of electrolytic capacitors will be collectively referred to as electrolytic capacitors.

[0032] An electrolytic capacitor comprises a capacitor element, a housing, and a sealing member. The housing houses the capacitor element. The sealing member is fastened to the opening of the housing and seals the opening of the housing. The capacitor element comprises an anode foil, a cathode foil, a separator, and an electrolyte. The anode foil and the cathode foil face each other with a separator interposed therebetween. A dielectric oxide film is formed on the surface of the anode foil. A dielectric oxide film is also formed on the cathode foil as needed. The electrolyte comprises an electrolyte. In addition to the electrolyte, a solid electrolyte may also be included. The solid electrolyte separator is present between the anode foil and the cathode foil and is in close contact with the dielectric oxide film. The electrolyte is filled in the gap portion of the capacitor element. In addition, the electrolytic capacitor may also have a base as needed.

[0033] (Sealing member)

[0034] The sealing member is mounted on the housing by fastening. The housing is made of aluminum, an aluminum alloy containing aluminum or manganese, or stainless steel, and is a cylindrical body with a bottom and an opening at the other end. During the fastening process, the opening of the housing is bent inward and flattened, so that the housing is in close contact with the sealing member. The sealing member with elastic force is a plate body comprising an elastomer, or a plate body formed by laminating a synthetic resin plate or a metal plate relative to the elastomer. The elastomer is in close contact with the housing that has been fastened by elastic force and maintains the airtightness in the housing. However, with respect to the sealing member comprising an elastomer, the transmittance of the volatilized electrolyte is not zero. As the elastomer, butyl rubber, ethylene propylene diene rubber (EPDM), silicone rubber and butyl rubber can be listed.

[0035] (Resin layer)

[0036] The electrolytic capacitor also includes a resin layer. The resin layer suppresses the evaporation of the electrolyte. That is, it can be provided at any part of the electrolytic capacitor as long as it is near a sealing member where there is a risk of the electrolyte passing through. For example, the resin layer covers at least a portion of the surface of the sealing member. The resin layer covers the sealing member by resin molding, thereby reducing the permeability of the electrolyte through the sealing member. That is, the so-called vicinity does not need to be separated, and also includes a contact configuration. In addition, for example, in the case where the electrolytic capacitor is a chip type including a base, the resin layer is, for example, provided at or around the part where the base contacts the shell and is near the sealing member. The electrolyte that passes through the sealing body is suppressed from evaporating by the resin layer. In other words, the resin layer also has the risk of reacting with water or ethylene glycol in the electrolyte that passes through the sealing body and dissolving into the electrolyte through the sealing body.

[0037] The resin layer comprises an anhydride-hardened epoxy resin. The resin layer may contain compounds other than anhydride-hardened epoxy resins. An anhydride-hardened epoxy resin is an epoxy resin composition comprising an epoxy resin and an anhydride hardener as raw materials. The epoxy resin is hardened using an anhydride-based hardener. Furthermore, the anhydride-hardened epoxy resin has an ester bond in its chemical structure.

[0038] Specifically, anhydride-hardened epoxy resins are produced by reacting hydroxyl groups, etc., present in the epoxy resin with the anhydride groups of the anhydride hardener to form a monoester. The resulting carboxyl groups then react with epoxy groups to form a diester, while also forming hydroxyl groups, which then react with the next anhydride group, in a sequential reaction. Therefore, anhydride-hardened epoxy resins have ester bonds in their chemical structure.

[0039] As can be seen from the reaction system, the epoxy resin composition containing an epoxy resin and an acid anhydride hardener in its chemical structure does not mean that the epoxy resin and the acid anhydride hardener are directly incorporated into the chemical structure in the same state as before the reaction. The epoxy resin and the acid anhydride hardener contained in the chemical structure means that the epoxy resin and the acid anhydride hardener are incorporated into the chemical structure after substitution of the respective groups for bonding.

[0040] The epoxy resin contained in anhydride-cured epoxy resins is an epoxy oligomer having two or more reactive epoxy groups at the ends. The epoxy resin undergoes cross-linking between the epoxy resins through the addition reaction of an anhydride hardener, transforming from a liquid to a solid resin. Typical examples of epoxy resins include bisphenol A diglycidyl ether, a condensation product of bisphenol A and epichlorohydrin. Examples of epoxy resins include other glycidyl epoxy resins and alicyclic epoxides such as 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexanecarboxylate.

[0041] Examples of glycidyl epoxy resins include bisphenols obtained by glycidylating bisphenols, such as bisphenol A, bisphenol F, bisphenol AD, bisphenol S, tetramethylbisphenol A, tetramethylbisphenol F, tetramethylbisphenol AD, tetramethylbisphenol S, tetrabromobisphenol A, tetrachlorobisphenol A, and tetrafluorobisphenol A.

[0042] Examples of glycidyl epoxy resins include those obtained by glycidylating dihydric phenols, such as biphenol, dihydroxynaphthalene, and 9,9-bis(4-hydroxyphenyl)fluorene.

[0043] Examples of glycidyl epoxy resins include those obtained by glycidylating trisphenols, such as 1,1,1-tris(4-hydroxyphenyl)methane and 4,4-(1-(4-(1-(4-hydroxyphenyl)-1-methylethyl)phenyl)ethylene)bisphenol.

[0044] Examples of glycidyl epoxy resins include epoxy resins obtained by glycidylating tetraphenols, such as 1,1,2,2-tetrakis(4-hydroxyphenyl)ethane.

[0045] Examples of glycidyl epoxy resins include novolac-type epoxy resins obtained by glycidylating novolacs, such as phenol novolac, cresol novolac, bisphenol A novolac, brominated phenol novolac, and brominated bisphenol A novolac.

[0046] Examples of the glycidyl epoxy resin include epoxy resins obtained by glycidylating polyphenols and aliphatic ether epoxy resins obtained by glycidylating polyols such as glycerin and polyethylene glycol.

[0047] Examples of glycidyl epoxy resins include ether-ester epoxy resins obtained by glycidylating hydroxycarboxylic acids, ester epoxy resins obtained by glycidylating polycarboxylic acids, glycidyl amine compounds, and amine epoxy resins. Examples of hydroxycarboxylic acids include p-hydroxybenzoic acid and β-hydroxynaphthoic acid. Examples of polycarboxylic acids include phthalic acid and terephthalic acid. Examples of amine compounds include 4,4-diaminodiphenylmethane and m-aminophenol. Examples of amine epoxy resins include triglycidyl isocyanurate.

[0048] Examples of the acid anhydride curing agent contained in the acid anhydride curing epoxy resin include phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, methylnadic anhydride, succinic anhydride, dodecenylsuccinic anhydride, chloric anhydride, maleic anhydride, and dichloromaleic anhydride. Furthermore, the acid anhydride curing agent may be an aromatic polycarboxylic acid anhydride or a styrene-maleic anhydride copolymer.

[0049] (Capacitor element)

[0050] (Electrolyte)

[0051] The electrolyte impregnated in the capacitor element is a solution of an ion-decomposable salt that decomposes into anionic and cationic components, or a solvent containing no ion-decomposable salt. The proportion of the compound having a hydroxyl group in the solvent relative to the total solvent is 45% by weight or less, including 0% by weight. If the solvent contains 0% by weight, the compound having a hydroxyl group may not be present. Preferably, the compound having a hydroxyl group contains 10% or more by weight relative to the total solvent.

[0052] Examples of compounds having hydroxyl groups include protic organic polar solvents. Examples of protic organic polar solvents include monohydric alcohols, polyhydric alcohols, and oxygen-containing alcohol compounds. Examples of monohydric alcohols include ethanol, propanol, butanol, pentanol, hexanol, cyclobutanol, cyclopentanol, cyclohexanol, and benzyl alcohol. Examples of polyhydric alcohols and oxygen-containing alcohol compounds include alkylene oxide adducts of polyhydric alcohols such as ethylene glycol, diethylene glycol, propylene glycol, glycerol, methyl cellosolve, ethyl cellosolve, methoxypropylene glycol, dimethoxypropanol, polyethylene glycol, and polyoxyethylene glycerol.

[0053] When the ratio of the compound having a hydroxyl group to the entire solvent is 45 wt % or less, even when an epoxy resin having an ester bond, such as an anhydride-cured epoxy resin, is used for the resin layer, the increase in the ESR of the electrolytic capacitor over time is suppressed.

[0054] The rapid change in ESR at 45 wt% is speculation and is not limited to the mechanism described above. It can be considered as follows. First, the anhydride-cured epoxy resin reacts with water and hydrolyzes. In addition, dehydration condensation occurs as the reverse reaction of hydrolysis. Hydrolysis and dehydration condensation are reversible reactions, so they maintain a state of equilibrium. However, in addition to the hydrolysis reaction and dehydration condensation reaction, an esterification reaction also occurs between the carbonyl group generated by hydrolysis and the compound having a hydroxyl group. The esterification reaction disrupts the equilibrium state of hydrolysis and dehydration condensation and promotes the hydrolysis reaction. As a result, these epoxy resins decompose and dissolve into the electrolyte, and the ESR of the electrolytic capacitor increases. However, if the proportion of the compound having a hydroxyl group relative to the entire solvent is less than 45 wt%, it can be considered that the more hydrolysis is promoted, the less rapid the esterification reaction will occur, and the more difficult it is to disrupt the equilibrium state of hydrolysis and dehydration condensation.

[0055] Furthermore, if the proportion of the compound having a hydroxyl group relative to the total solvent is within the range of 10 wt% to 45 wt%, the increase in the ESR of the electrolytic capacitor over time is further suppressed, which is preferable. It is believed that when using an electrolyte solution containing a hydroxyl group compound such as ethylene glycol as a solvent in conjunction with a solid electrolyte, a proportion of 10 wt% or more relative to the total solvent improves the conductivity of the conductive polymer by altering its higher-order structure and reorienting the polymer chain's crystal structure. However, γ-butyrolactone does not achieve this effect.

[0056] In addition to compounds having hydroxyl groups, the solvent may also be composed of an aprotic organic polar solvent. Representative aprotic organic polar solvents include sulfones, amides, lactones, cyclic amides, nitrile systems, and sulfoxides. Examples of sulfones include dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methylsulfolane, and 2,4-dimethylsulfolane. 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 hexamethylphosphoramide. 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 nitrile-based compounds include acetonitrile, 3-methoxypropionitrile, and glutaronitrile, while examples of sulfoxide-based compounds include dimethyl sulfoxide.

[0057] Among them, sulfolane is particularly preferred. Due to its high boiling point, sulfolane is difficult for the solvent to evaporate, which can extend the life of the electrolytic capacitor. In addition, sulfolane does not contain ether bonds. For example, compared with compounds with ether bonds such as γ-butyrolactone, sulfolane can be said to have a structure that is difficult for water to access. Therefore, even compared with compounds with ether bonds, sulfolane does not attract water when passing through the sealing member, and can further suppress the dissolution of part of the anhydride-cured epoxy resin component through the sealing member into the electrolyte. Furthermore, if compounds with ether bonds are compared with compounds with hydroxyl groups, compounds with hydroxyl groups are obviously more accessible to water.

[0058] Regarding sulfolane, when used as a mixed solvent with a compound having a hydroxyl group, such as ethylene glycol, the proportion of sulfolane in the solvent is preferably 55% by weight or more and less than 100% by weight, and more preferably 55% by weight or more and 90% by weight or less. Furthermore, from the perspective of further suppressing the partial elution of the anhydride-cured epoxy resin component into the electrolyte, the proportion of sulfolane in the solvent is preferably 35% by weight or more.

[0059] Examples of organic acids having an anion component as the solute 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, heptanoic acid, malonic acid, 1,6-decanedicarboxylic acid, 1,7-octanedicarboxylic acid, azelaic acid, resorcinolic acid, pyrogallol, gallic acid, gentisic acid, protocatechuic acid, pyrocatechuic acid, trimellitic acid, and pyromellitic acid, as well as phenols and sulfonic acids. Inorganic acids include boric acid, phosphoric acid, phosphorous acid, hypophosphorous acid, carbonic acid, and silicic acid. Examples of composite compounds of organic acids and inorganic acids include boron disalicylic acid, boron dioxalic acid, boron diglycolic acid, boron dimalonic acid, boron disuccinic acid, boron diadipic acid, boron diazelaic acid, boron dibenzoic acid, boron dimaleic acid, boron dilactic acid, boron dimalic acid, boron ditartaric acid, boron dicitric acid, boron diphthalic acid, boron di(2-hydroxy)isobutyric acid, boron diresorcinolic acid, boron dimethylsalicylic acid, boron dinaphthoic acid, boron dimandelic acid, and boron di(3-hydroxy)propionic acid.

[0060] In addition, examples of salts of at least one of an organic acid, an inorganic acid, and a complex compound of an organic acid and an inorganic acid include ammonium salts, quaternary ammonium salts, quaternary amidine salts, amine salts, sodium salts, potassium salts, and the like. Examples of quaternary ammonium ions of quaternary ammonium salts include tetramethylammonium, triethylmethylammonium, and tetraethylammonium. Examples of quaternary amidine salts include ethyldimethylimidazolium and tetramethylimidazolium. Examples of amine salts include salts of 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, tributylamine, ethyldimethylamine, and ethyldiisopropylamine.

[0061] The cationic component in the solute is preferably added in an amount of 76mmol or less per 100g of the electrolyte, more preferably in an amount of 51mmol or less per 100g of the electrolyte, more preferably in an amount of 34mmol or less per 100g of the electrolyte, particularly preferably in an amount of 25mmol or less per 100g of the electrolyte, and most preferably in an amount of 17mmol or less per 100g of the electrolyte. Furthermore, the ratio of the cationic component to the anionic component of the solute can be equimolar, or can be an excess of cationic component or an excess of anionic component.

[0062] Cationic components tend to promote the dedoping reaction of the conductive polymer in the solid electrolyte under a high-temperature environment such as the reflow step when installing the electrolytic capacitor. If the dedoping reaction is promoted by the cationic component, the conductivity of the solid electrolyte decreases and the ESR of the electrolytic capacitor increases. On the other hand, when a resin layer is arranged near the sealing member, the resin layer will hinder the evaporation of the cationic component through the sealing member, thereby suppressing the reduction of the cationic component in the electrolytic capacitor. Therefore, if a resin layer is arranged near the sealing member, many dedoping reactions caused by the cationic component will occur, and the ESR of the electrolytic capacitor will increase.

[0063] However, as long as the cationic component of the solute in the electrolyte is an amount of 76 mmol or less per 100 g of the electrolyte, the increase in ESR is suppressed compared to the case of exceeding 76 mmol. Furthermore, as long as the cationic component of the solute in the electrolyte is an amount of 51 mmol or less per 100 g of the electrolyte, for example, compared to the case of 76 mmol per 100 g of the electrolyte, the increase in ESR is drastically suppressed. As long as the cationic component of the solute in the electrolyte is an amount of 25 mmol or less per 100 g of the electrolyte, for example, compared to the case of 51 mmol per 100 g of the electrolyte, the increase in ESR is further drastically suppressed.

[0064] Furthermore, other additives may be added to the liquid. Examples of additives include complex compounds of boric acid and polysaccharides (such as mannitol and sorbitol), complex compounds of boric acid and polyols, boric acid esters, nitro compounds (such as o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, p-nitrophenol, and p-nitrobenzyl alcohol), and phosphoric acid esters. These may be used alone or in combination of two or more.

[0065] (Electrode Foil)

[0066] Anode and cathode foils are long strips of valve metal. Examples of valve metals include aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony. The purity of anode foil is ideally 99.9% or higher, while that of cathode foil is around 99% or higher. Impurities such as silicon, iron, copper, magnesium, and zinc may also be present.

[0067] The anode foil is a sintered body formed by sintering valve metal powder, or an etched foil formed by etching a stretched foil, and its surface is expanded. The expanded surface structure includes tunnel-shaped pits, sponge-like pits, or spaces between dense powders. Typically, the expanded surface structure is formed by applying direct current or alternating current (DC) etching or alternating current (AC) etching in an acidic aqueous solution containing halogen ions, such as hydrochloric acid, or by vapor-depositing or sintering metal particles on the core. The cathode foil can also have an expanded surface structure through vapor deposition, sintering, or etching.

[0068] The dielectric oxide film is typically formed on the surface of the anode foil. If the anode foil is made of aluminum, it is aluminum oxide formed by oxidizing the porous structure region. The dielectric oxide film is formed by a chemical conversion treatment in an aqueous solution of adipic acid, boric acid, or phosphoric acid, for example, by applying a voltage. Alternatively, a thin dielectric oxide film (approximately 1V to 10V) can be formed on the surface of the cathode foil through a chemical conversion treatment as needed. Furthermore, the dielectric oxide film can be produced by using a layer formed by vapor deposition containing metal nitrides, metal carbides, or metal carbonitrides, or by using a layer containing carbon on the surface.

[0069] (Partition)

[0070] Examples of the separator include: cellulose such as kraft, manila hemp, esparto, hemp, rayon, and mixed papers thereof; polyester resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and derivatives thereof; polytetrafluoroethylene resins; polyvinylidene fluoride resins; vinylon resins; polyamide resins such as aliphatic polyamides, semi-aromatic polyamides, and wholly aromatic polyamides; polyimide resins; polyethylene resins; polypropylene resins; trimethylpentene resins; polyphenylene sulfide resins; acrylic resins; polyvinyl alcohol resins, etc. These resins can be used alone or in combination.

[0071] (Solid Electrolyte)

[0072] When a solid electrolyte is formed in a capacitor element, the solid electrolyte 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. Conductivity is exhibited by adding a small amount of an acceptor that easily accepts electrons or a donor that easily donates electrons to the conjugated polymer. If an acceptor or a donor is added to the conjugated polymer, in the case of the acceptor, π electrons are extracted from the conjugated polymer to generate negative charge carriers (holes); in the case of the donor, electrons are donated to generate negative charge carriers, thereby exhibiting conductivity.

[0073] As the conjugated polymer, known conjugated polymers can be used without particular limitation. Examples include polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene, polyphenylene vinylene, polyacene, and polythiophene vinylene. These conjugated polymers can be used alone or in combination of two or more, or as copolymers of two or more monomers.

[0074] Among the conjugated polymers, preferred are conjugated polymers obtained by polymerizing thiophene or its derivatives, and preferably are conjugated polymers obtained by polymerizing 3,4-ethylenedioxythiophene (i.e., 2,3-dihydrothieno[3,4-b][1,4]dioxin), 3-alkylthiophene, 3-alkoxythiophene, 3-alkyl-4-alkoxythiophene, 3,4-alkylthiophene, 3,4-alkoxythiophene, or derivatives thereof. Thiophene derivatives are preferably compounds selected from thiophenes having substituents at the 3- and 4-positions, wherein the substituents at the 3- and 4-positions of the thiophene ring may form a ring together with the carbon atoms at the 3- and 4-positions. The number of carbon atoms in the alkyl or alkoxy group is preferably 1 to 16, and particularly preferred are polymers of 3,4-ethylenedioxythiophene known as EDOT (3,4-ethylenedioxythiophene), namely, poly(3,4-ethylenedioxythiophene) known as PEDOT (Poly(3,4-polyethylenedioxythiophene)). Alternatively, alkylated ethylenedioxythiophene having an alkyl group added to 3,4-ethylenedioxythiophene may be used, such as methylated ethylenedioxythiophene (i.e., 2-methyl-2,3-dihydro-thieno[3,4-b][1,4]dioxin) and ethylated ethylenedioxythiophene (i.e., 2-ethyl-2,3-dihydro-thieno[3,4-b][1,4]dioxin).

[0075] The dopant may be any known dopant without particular limitation. Examples include inorganic acids such as boric acid, nitric acid, and phosphoric acid; and organic acids such as acetic acid, oxalic acid, citric acid, ascorbic 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, boron disalicylic acid, bisoxalatoborate, sulfonylimide acid, dodecylbenzenesulfonic acid, propylnaphthalenesulfonic acid, and butylnaphthalenesulfonic acid. Examples of polyanions include polyethylenesulfonic acid, polystyrenesulfonic acid, polyallylsulfonic acid, polyacrylic acidsulfonic acid, polymethacrylic acidsulfonic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprenesulfonic acid, polyacrylic acid, polymethacrylic acid, and polymaleic acid.

[0076] These dopants may be used alone or in combination of two or more. In addition, these dopants may be polymers of a single monomer or copolymers of two or more monomers. In addition, the dopants may be macromolecules or monomers.

[0077] [Example]

[0078] (Example 1 to Example 11)

[0079] Hereinafter, the electrolytic capacitor of the present invention will be further described in detail based on the examples. Furthermore, the present invention is not limited to the following examples. Various electrolytic capacitors are produced by coating a sealing member with a resin layer containing an anhydride hardener epoxy resin. The electrolytic capacitors of Comparative Examples 1 to 3 and Examples 1 to 11 have an electrolyte that uses both an electrolyte solution and a solid electrolyte, and the ratio of ethylene glycol in the solvent of the electrolyte solution is different. The anhydride hardener epoxy resin is an example of an epoxy resin having an ester bond, and ethylene glycol is an example of a compound having a hydroxyl group.

[0080] The electrolytic capacitors of Comparative Examples 1 to 3 and Examples 1 to 11 share the following features: The anode foil is aluminum foil, expanded by etching, and then chemically converted to form a dielectric oxide film. The cathode foil is a flat, unetched aluminum foil. The same anode and cathode foils are used in full electrolytic capacitors, each with leads connected and wound with a Manila separator interposed between them, facing each other. The capacitor elements are then chemically converted by immersion in an aqueous solution of ammonium dihydrogen phosphate at the same concentration for 10 minutes.

[0081] Next, a dispersion of polyethylene dioxythiophene (PEDOT / polystyrene sulfonic acid (PSS)) doped with polystyrene sulfonic acid of the same product, as a conductive polymer, was prepared at the same concentration, and the capacitor element was immersed in it. The capacitor element was then lifted up and dried at 150°C for 30 minutes. The immersion and drying were repeated the same number of times. In this way, a solid electrolyte was formed on the capacitor element. Next, different electrolytes were prepared for each electrolytic capacitor, and the capacitor element with the solid electrolyte was immersed in the electrolyte. The capacitor element was inserted into a bottomed cylindrical outer casing of the same metal type, the same size, and the same shape. A sealing member was installed at the open end and sealed by tightening using the same pressure. Each electrolytic capacitor used a sealing member made of butadiene rubber.

[0082] Each electrolytic capacitor was housed in an aluminum container of the same size and completely embedded in the same type of epoxy resin, forming a resin layer. However, the leads from the electrolytic capacitors were pre-extracted from the epoxy resin to allow for power to the capacitors. The epoxy resin introduced into the container was primarily a bisphenol A liquid epoxy resin, and the hardener was an acid anhydride-based tetrahydromethylphthalic anhydride. This resulted in the outer surface of each electrolytic capacitor's sealing member being completely covered with the anhydride-hardened epoxy resin.

[0083] The electrolytes prepared in the electrolytic capacitors of Comparative Examples 1 to 3 and Examples 1 to 11 are shown in Table 1 below.

[0084] (Table 1)

[0085]

[0086] In Table 1 above, the solvent composition ratio represents the weight ratio relative to the total solvent amount. As shown in Table 1, the composition and composition ratio of the electrolyte solution differed between the electrolytic capacitors. The electrolytic solutions of the electrolytic capacitors of Comparative Examples 1 to 3 contained ethylene glycol at a concentration of 50 wt% or greater relative to the total solvent amount. The electrolytic solutions of the electrolytic capacitors of Examples 1 to 10 contained ethylene glycol at a concentration of 0 wt% to 45 wt% relative to the total solvent amount, with the amount of ethylene glycol varied in increments of 5 wt%.

[0087] In the electrolytic solutions of the electrolytic capacitors of Comparative Examples 2 and 3 and Examples 1 to 10, the remainder of the solvent other than ethylene glycol was sulfolane. The electrolytic solution of the electrolytic capacitor of Example 11 did not use ethylene glycol, and γ-butyrolactone was used instead of the total sulfolane solvent in Example 10. Furthermore, in the electrolytic capacitors of Comparative Examples 1 to 3 and Examples 1 to 11, no solute was added, in order to clarify the influence of the solvent in the electrolyte.

[0088] Each electrolytic capacitor was embedded in an anhydride-cured epoxy resin and then left at 150°C for 400 hours while applying a constant voltage of 35V. The ESR was measured before and after the thermal stress. The ESR was measured at 100kHz. Furthermore, the interior of each electrolytic capacitor after the thermal stress was applied was photographed using an X-ray machine, and the internal state was observed using fluoroscopic images.

[0089] Table 2 below shows the ESR of each electrolytic capacitor before and after thermal stress loading and the results of internal conditions obtained using fluoroscopic images.

[0090] (Table 2)

[0091]

[0092] Based on the ESR after thermal stress loading in Table 2, a graph showing the relationship between the ratio of ethylene glycol and ESR is shown in Figure 1 Furthermore, since the ESR of the groups of Examples 1 to 11 and Comparative Examples 1 to 3 are significantly different, Figure 1 Only the data of the electrolytic capacitors of Examples 1 to 10 are plotted. Figure 1As shown in Table 2, there is a significant difference in ESR between the groups of Examples 1 to 11 and Comparative Examples 1 to 3. The ESR of the groups of Examples 1 to 11 is suppressed to a minimum of 35 times lower and a maximum of 94 times lower than that of the groups of Comparative Examples 1 to 3.

[0093] The results showed that if the proportion of ethylene glycol in the solvent is kept below 45 wt%, ESR will be suppressed. Figure 1 As shown in the results, it was confirmed that when the proportion of ethylene glycol in the solvent is less than 10 wt%, while the ESR does not increase dramatically within the range of 45 wt% to 100 wt%, the ESR tends to increase, and 10 wt% or more of ethylene glycol is preferably used. In particular, when a solid electrolyte is used in combination with ethylene glycol in the solvent, the conductivity of the conductive polymer is improved by changes in the higher-order structure of the conductive polymer and reorientation of the crystalline structure of the polymer chains. However, γ-butyrolactone does not achieve this effect, so the proportion of ethylene glycol is preferably 10 wt% or more.

[0094] Furthermore, as shown in Table 2, the fluoroscopic images obtained using an X-ray imaging device showed that cracks occurred in the resin layer of the anhydride-cured epoxy resin in which the electrolytic capacitors of Comparative Examples 1 to 3 were embedded. In contrast, no cracks were observed in the resin layer of the anhydride-cured epoxy resin in which the electrolytic capacitors of Examples 1 to 11 were embedded. Here, fluoroscopic images taken from multiple directions of Comparative Examples 1, 2, and Example 1 are shown. Figure 2 .like Figure 2 As shown, cracks were observed within the area enclosed by the dotted lines in Comparative Examples 1 to 3. On the other hand, no cracks were observed in the fluoroscopic images of Example 1. The cracks were observed between the butadiene rubber sealing member and the anhydride-cured epoxy resin layer.

[0095] The results confirmed that if the proportion of ethylene glycol in the solvent exceeds 45wt%, the resin in the resin layer will decompose. Here, the electrolytic capacitors of Comparative Example 1 and Example 11 were placed in an aluminum container and filled with epoxy resin, and then placed in a temperature environment of 150°C for 400 hours. The main agent of the epoxy resin is bisphenol A type liquid epoxy resin, and the hardener is tetrahydromethylphthalic anhydride of the acid anhydride system. After the above-mentioned heat placement, the electrolyte was extracted from the electrolytic capacitors of Comparative Example 1 and Example 11 and filtered. Using high performance liquid chromatography, a test was conducted to determine whether substances from the resin layer were detected in the electrolyte. The results are shown in Table 3 below.

[0096] (Table 3)

[0097]

[0098] In Table 3, EG stands for ethylene glycol, and GBL stands for γ-butyrolactone. An electrolyte solution was prepared and a reference chromatogram obtained using only the electrolyte solution was obtained by high-performance liquid chromatography. If the chromatograms obtained from Comparative Example 1 and Example 11 contained peaks not observed in the reference chromatograms, components derived from the resin layer were considered to have been detected. As shown in Table 3, substances derived from the resin layer were detected in Comparative Example 1. On the other hand, no substances derived from the resin layer were detected in Example 11.

[0099] Based on the above results, it can be understood that since the resin in the resin layer is an anhydride-hardened epoxy resin with an ester bond and ethylene glycol is a compound with hydroxyl groups, the anhydride-hardened epoxy resin decomposes by hydrolysis, and the decomposition products react with the compound with hydroxyl groups, namely ethylene glycol. These reaction products dissolve into the electrolyte, and the components from the resin layer in the electrolyte increase the ESR.

[0100] Furthermore, when an epoxy resin with an ester bond, or at least an anhydride-cured epoxy resin, is used as the resin layer, the content of compounds with hydroxyl groups is suppressed to less than 45 wt% relative to the total solvent content of the electrolyte. This prevents components from the resin layer from eluting into the electrolyte, thus suppressing increases in ESR.

[0101] (Example 12 to Example 19)

[0102] Next, electrolytic capacitors of Examples 12 to 19 were produced. In Examples 12 to 19, azelaic acid was added to the electrolyte as an anionic component of the solute, and ammonia was added to the electrolyte as a cationic component of the solute. The electrolytic capacitors of Examples 12 to 19 varied in the amount of cationic components in the solute in the electrolyte. The electrolytic capacitors of Examples 12 to 19 were housed in aluminum containers of the same size as those of Examples 1 to 11 and, similarly to Examples 1 to 11, were completely encased in an anhydride-cured epoxy resin. Furthermore, the electrolytic capacitors of Examples 12 to 19 were produced using the same method and conditions as Examples 1 to 11.

[0103] The electrolytic capacitors of Examples 12 to 19 were embedded in an anhydride-cured epoxy resin. A constant voltage of 35 V was then applied to each electrolytic capacitor in a 150°C environment for 800 hours. The ESR was measured before and after the thermal stress, and the rate of change in ESR after the thermal stress was calculated. ESR was measured at 100 kHz.

[0104] The composition and addition amount of the electrolyte solutions for Examples 12 to 19, as well as the ESR before thermal stress loading (initial ESR), the ESR after thermal stress loading (post-test ESR), and the rate of change of ESR after thermal stress loading are shown in Table 4 below. Furthermore, the amount of solvent in Table 4 represents the content relative to the total solvent, and the amount of solute in Table 4 is converted per 100g of electrolyte. The electrolytic capacitors for Examples 12 to 19 were wound-type with a diameter of 6mm and a total length of 6mm, a rated voltage of 35V, and a rated capacitance of 47μF.

[0105] (Table 4)

[0106]

[0107] As shown in Table 4, the ESR change rate of the electrolytic capacitor of Example 18, which contained 76 mmol of ammonia as a cationic component, was suppressed to approximately 55%, compared to the electrolytic capacitor of Example 19, which contained 86 mmol of ammonia as a cationic component. Furthermore, the ESR change rate of the electrolytic capacitor of Example 17, which contained 51 mmol of ammonia, was suppressed to approximately 24%, compared to the electrolytic capacitor of Example 18, which contained 76 mmol of ammonia. Furthermore, the ESR change rate of the electrolytic capacitor of Example 14, which contained 25 mmol of ammonia, was suppressed to approximately 18%, compared to the electrolytic capacitor of Example 17, which contained 51 mmol of ammonia.

[0108] Thus, if an electrolytic capacitor has a resin layer, the cationic component tends to promote the dedoping reaction, increasing the ESR. However, it has been confirmed that by adjusting the cationic component to 76 mmol or less per 100 g of the electrolyte, the increase in ESR can be suppressed. By adjusting the cationic component to 51 mmol or less per 100 g of the electrolyte, the increase in ESR can be further suppressed. By adjusting the cationic component to 25 mmol or less per 100 g of the electrolyte, the ESR after thermal stress loading can be further suppressed.

[0109] (Example 20 to Example 23)

[0110] Electrolytic capacitors of Examples 20 to 23 and Comparative Example 4 were produced. The electrolyte solutions of Examples 20 to 23 and Comparative Example 4 differed in the ratio of sulfolane in the solvent. Sulfolane is an example of a compound that lacks hydroxyl groups and, consequently, ether bonds. In order to clarify the effects of the electrolyte solvent, no solute was added to the electrolytic capacitors of Examples 20 to 23 and Comparative Example 4. Furthermore, the electrolytic capacitors of Examples 20 to 23 and Comparative Example 4 were produced using the same method and conditions as Examples 1 to 11, including being housed in aluminum containers of the same size and completely embedded in an anhydride-cured epoxy resin.

[0111] After filling with anhydride-cured epoxy resin, each electrolytic capacitor was left for 5500 hours in a temperature environment of 150° C. Tan δ was measured before and after the thermal stress loading, and the rate of change in tan δ after the thermal stress loading was calculated.

[0112] Table 5 shows the electrolyte compositions and amounts of each solution added for Examples 20 to 23 and Comparative Example 4, as well as the tan δ before thermal stress loading (initial tan δ), the tan δ after thermal stress loading (post-test tan δ), and the rate of change of tan δ after thermal stress loading. The electrolytic capacitors used in each of the Examples and Comparative Examples were wound-type capacitors with a diameter of 10 mm and a total length of 10 mm, a rated voltage of 35 V, and a rated capacitance of 270 μF.

[0113] (Table 5)

[0114]

[0115] As shown in Table 5, the electrolyte solution of Comparative Example 4 consists solely of ethylene glycol, which has hydroxyl groups. In the electrolyte solutions of Examples 22 and 23, the ethylene glycol ratio is reduced, and instead of reducing the ethylene glycol ratio, sulfolane is increased. The electrolyte solution of Example 23 consists solely of sulfolane, which has neither hydroxyl groups nor ether bonds. Comparing Comparative Example 4, Examples 22, and 23, it can be confirmed that reducing the ethylene glycol ratio and increasing the sulfolane ratio suppresses the increase in tan δ before and after thermal stress loading.

[0116] Furthermore, in Examples 20 and 21, the amount of ethylene glycol was kept constant, while the amount of γ-butyrolactone was varied. In Example 21, the amount of γ-butyrolactone was reduced compared to Example 20, but sulfolane was added. Comparison of Examples 20 and 21 confirmed that the addition of sulfolane at a ratio of 35 wt% or greater relative to the total amount of solvent in the electrolyte suppressed the increase in tan δ before and after thermal stress loading.

[0117] It can be understood that by reducing the amount of compounds having hydroxyl groups and compounds having ether bonds added to the electrolyte and instead adding 35 wt % or more of cyclopentane sulfone containing neither hydroxyl groups nor ether bonds relative to the total amount of the solvent in the electrolyte, the components from the resin layer will not be further dissolved into the electrolyte, and the deterioration of the characteristics of the electrolytic capacitor will be further suppressed.

Claims

1. An electrolytic capacitor, characterized in that include: A capacitor element comprising an anode foil, a cathode foil, and an electrolyte; a housing made of aluminum, an aluminum alloy containing aluminum or manganese, or stainless steel, and housing the capacitor element; A sealing member for sealing the shell; as well as a resin layer disposed near the sealing member, The resin layer comprises an epoxy resin composition containing an ester bond, In the solvent of the electrolyte solution, the compound having a hydroxyl group accounts for 10 wt % or more and 45 wt % or less.

2. The electrolytic capacitor according to claim 1, wherein The epoxy resin composition uses epoxy resin and acid anhydride hardener as raw materials.

3. The electrolytic capacitor according to claim 1, wherein The epoxy resin composition has a chemical structure including an epoxy resin and an acid anhydride hardener.

4. The electrolytic capacitor according to any one of claims 1 to 3, characterized in that The compound having a hydroxyl group is ethylene glycol, diethylene glycol, or polyethylene glycol.

5. The electrolytic capacitor according to any one of claims 1 to 3, characterized in that The electrolyte contains 35 wt % or more of sulfolane based on the total amount of the solvent in the electrolyte.

6. The electrolytic capacitor according to any one of claims 1 to 3, characterized in that The capacitor element further comprises a solid electrolyte.

7. The electrolytic capacitor according to claim 6, wherein: The cationic component in the electrolyte solution is contained in an amount of 76 mmol or less per 100 g of the electrolyte solution.

8. The electrolytic capacitor according to claim 6, wherein: The electrolyte solution contains 25 mmol or less of cationic components per 100 g of the electrolyte solution.

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