Electrolyte for electrolytic capacitor and electrolytic capacitor

Abstract

The present invention provides an electrolyte solution for an electrolytic capacitor which is capable of maintaining high performance over a long period of time even in severe environments, and an electrolytic capacitor using this electrolyte solution.　This electrolyte solution is for an electrolytic capacitor that comprises a capacitor element, the capacitor having an anode foil that has a dielectric oxide film layer on a surface thereof, a cathode foil, and a solid electrolyte layer, and an electrolyte solution, the electrolyte solution being impregnated into the capacitor element. The electrolyte solution comprises an organic solvent that contains the following compound (a) and / or compound (b).　(a) An aromatic carboxylic acid having a sulfo group and a hydroxy group, and a boron compound; (b) A composite compound of an aromatic carboxylic acid having a sulfo group and a hydroxy group, and a boron compound

Description

[Technical field]

[0001] The present invention relates to an electrolyte for an electrolytic capacitor that can maintain high performance such as low ESR for a long period of time even in a severe environment, and to an electrolytic capacitor using this electrolyte. [Background technology]

[0002] In recent years, there has been an increasing demand for higher reliability in AV equipment and in-vehicle electrical equipment. For the electrolytic capacitors used therein, there is also a need for improvements in performance, such as small size, large capacity, and equivalent series resistance (hereinafter, also referred to as ESR) in the high-frequency range.

[0003] In particular, with the trend toward higher frequencies in electronic devices, there is a demand for electrolytic capacitors with superior equivalent series resistance (ESR) characteristics in the high frequency range. Recently, in order to reduce the ESR in such high frequency ranges, electrolytic capacitors that use solid electrolytes such as conductive polymers with higher electrical conductivity than conventional electrolytes for driving, or that use both a solid electrolyte and an electrolyte, have been studied and commercialized.

[0004] Specifically, electrolytic capacitors are used that contain a solid electrolyte made of conductive polymers such as polypyrrole, polythiophene, and polyaniline, as well as ammonium salts and amine salts of phthalic acid, maleic acid, adipic acid, salicylic acid, and the like, and ammonium and amine salts of borodisalicylic acid, which is a composite compound of salicylic acid and boric acid, and an electrolytic solution made of an organic solvent such as γ-butyrolactone, γ-valerolactone, and ethylene glycol. Such electrolytic capacitors have a large capacity and a small leakage current, and are therefore known as electrolytic capacitors with high performance (see Patent Documents 1 to 3).

[0005] Patent Document 4 discloses an electrolytic capacitor having a solid electrolyte layer and an electrolyte solution, the solid electrolyte layer containing a π-conjugated conductive polymer and a first sulfonic acid, and the electrolyte solution containing a solvent and an acid component containing a second sulfonic acid. It is disclosed that the electrolyte solution of this electrolytic capacitor may contain a third component such as a carboxylic acid, boric acid, or phosphonic acid together with the second sulfonic acid in order to stabilize the ESR while suppressing corrosion of the electrodes, and it is described that the third component is preferably a carboxylic acid. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 5052746 [Patent Document 2] JP 2015-165550 A [Patent Document 3] Patent No. 5305569 [Patent Document 4] International Publication (WO) 2017 / 159243 Summary of the Invention [Problem to be solved by the invention]

[0007] In recent years, electrolytic capacitors used in in-vehicle electrical equipment and the like are required to have high performance characteristics, such as low ESR, that can be maintained for a long period of time in harsh high-temperature environments, such as maximum operating temperatures of 85 to 150°C. An object of the present invention is to provide an electrolytic solution for electrolytic capacitors capable of maintaining a low ESR for a long period of time even in harsh high-temperature environments such as those in which the maximum operating temperature is 85 to 150°C, particularly in in-vehicle AV equipment and electrical equipment, and an electrolytic capacitor using this electrolytic solution. [Means for solving the problem]

[0008] As a result of extensive research, the present inventors have found that the above-mentioned object of maintaining a low ESR even in a severe high-temperature environment can be achieved by an electrolytic capacitor having a solid electrolyte layer and an electrolyte solution, when the electrolyte solution contains the following compound (a) and / or (b): (a) An aromatic carboxylic acid having a sulfo group and a hydroxy group, and a boron compound. (b) A composite compound of an aromatic carboxylic acid having a sulfo group and a hydroxy group and a boron compound. On the other hand, even when an aromatic carboxylic acid having a sulfo group and a hydroxy group is contained, an electrolyte solution containing a carboxylic acid compound instead of a boron compound, or Even in the case where a boron compound is contained, the above-mentioned object is hardly achieved in an electrolyte solution containing, for example, an aromatic carboxylic acid having only a sulfo group and no hydroxy group, instead of an aromatic carboxylic acid having a sulfo group and a hydroxy group, and therefore the above-mentioned result was obtained unexpectedly.

[0009] The present invention is based on the above findings and has the following aspects. (1) An electrolyte for an electrolytic capacitor, comprising: an anode foil, a cathode foil, and a capacitor element having a solid electrolyte layer, the anode foil and the cathode foil each having a dielectric oxide film layer on the surface thereof; and an electrolyte impregnated in the capacitor element, 1. An electrolytic solution for an electrolytic capacitor, comprising an organic solvent containing the following compound (a) and / or compound (b): (a) an aromatic carboxylic acid having a sulfo group and a hydroxy group, and a boron compound (b) A composite compound of an aromatic carboxylic acid having a sulfo group and a hydroxy group and a boron compound.

[0010] (2) The electrolyte for an electrolytic capacitor according to the above (1), wherein the aromatic carboxylic acid having a sulfo group and a hydroxy group is 5-sulfosalicylic acid. (3) The electrolyte for an electrolytic capacitor according to (1) or (2) above, wherein the boron compound is boric acid. (4) The electrolyte for an electrolytic capacitor according to any one of the above (1) to (3), further comprising ammonia or a primary, secondary or quaternary amine. (5) The electrolyte for an electrolytic capacitor according to any one of the above (1) to (4), wherein the organic solvent is polyalkylene glycol, γ-butyrolactone, γ-valerolactone, ethylene glycol, diethylene glycol, triethylene glycol or benzyl alcohol.

[0011] (6) The electrolyte solution for an electrolytic capacitor according to any one of (1) to (5) above, wherein the content of the aromatic carboxylic acid having a sulfo group and a hydroxy group is 0.1 parts by mass or more. (7) The electrolyte for an electrolytic capacitor according to any one of (1) to (6) above, wherein the content of the boron compound is 0.1 mol or more per 1 mol of the aromatic carboxylic acid having a sulfo group and a hydroxy group. (8) The electrolyte for an electrolytic capacitor according to any one of (1) to (7) above, wherein the content of the composite compound of the aromatic carboxylic acid having a sulfo group and a hydroxy group and the boron compound is 0.05 parts by mass or more. (9) The electrolyte for an electrolytic capacitor according to any one of (1) to (8) above, which has a water content of 0.5% by mass or less. (10) The electrolyte for an electrolytic capacitor according to any one of (1) to (9) above, which has a pH of 2 to 6. (11) The electrolyte for an electrolytic capacitor according to any one of (1) to (10) above, further comprising an additive comprising a nitro compound.

[0012] (12) The electrolyte for an electrolytic capacitor according to any one of the above (1) to (11), wherein the solid electrolyte layer is a layer of polythiophene, polypyrrole, polyaniline, or a derivative thereof. (13) An electrolytic capacitor using the electrolytic solution according to any one of (1) to (12) above. Effect of the Invention

[0013] The electrolyte of the present invention provides an electrolyte for electrolytic capacitors that can maintain a large capacity and a low ESR for a long period of time even in a severe high-temperature environment, such as a maximum operating temperature of 85 to 150°C. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] <Electrolytic capacitor> The electrolyte of the present invention is used in an electrolytic capacitor having an anode and a cathode having a dielectric oxide film layer on the surface, a capacitor element having a solid electrolyte, and an electrolyte impregnated in the capacitor element. The solid electrolyte used in the present invention is a conductive polymer doped with a dopant component. The conductive polymer may be polypyrrole, polythiophene, polyaniline, or a derivative thereof.

[0015] The solid electrolyte layer used in the present invention can be obtained by chemically oxidizing or electrolytically oxidizing a monomer of a conductive polymer in the presence of the dopant component. Alternatively, the solid electrolyte layer can be obtained by contacting a dispersion or a solution in which a conductive polymer formed in the form of fine particles by chemically oxidizing polymerization is dispersed in a solvent such as water. In addition, in the chemically oxidizing or electrolytically oxidizing polymerization, a part or all of the dopant component and the monomer of the conductive polymer may be replaced with a monomer having a functional group capable of undergoing chemical oxidizing doping.

[0016] Specifically, the monomer of the conductive polymer is 3,4-ethylenedioxythiophene, methyl-3,4-ethylenedioxythiophene, ethyl-3,4-ethylenedioxythiophene, propyl-3,4-ethylenedioxythiophene, 3,4-propylenedioxythiophene, methyl-3,4-propylenedioxythiophene, ethyl-3,4-propylenedioxythiophene, propyl-3,4-propylenedioxythiophene, 3,4-ethylenedioxyfuran, methyl-3,4-ethylenedioxyfuran, ethyl-3,4-ethylenedioxyfuran, propyl-3,4-ethylenedioxyfuran, Examples of such thiophenes include 3,4-propylenedioxyfuran, 3,4-propylenedioxyfuran, methyl-3,4-propylenedioxyfuran, ethyl-3,4-propylenedioxyfuran, propyl-3,4-propylenedioxyfuran, 3,4-ethylenedithiathiophene, methyl-3,4-ethylenedithiathiophene, ethyl-3,4-ethylenedithiathiophene, propyl-3,4-ethylenedithiathiophene, 3,4-propylenedithiathiophene, methyl-3,4-propylenedithiathiophene, ethyl-3,4-propylenedithiathiophene, and propyl-3,4-propylenedithiathiophene. Among the above, 3,4-ethylenedioxythiophene, methyl-3,4-ethylenedioxythiophene, and ethyl-3,4-ethylenedioxythiophene are particularly preferable because they provide an electrolytic capacitor with a low ESR.

[0017] The dopant component may have a functional group that can cause chemical oxidation doping of the polymer, and is preferably a sulfate ester group, a phosphate ester group, a phosphate group, a carboxyl group, a sulfo group, etc. Among these, from the viewpoint of doping effect, the sulfate ester group, the carboxyl group, and the sulfo group are more preferable, and the sulfo group is particularly preferable. Specific examples of the dopant component include polyvinyl sulfonic acid, polystyrene sulfonic acid, polyallyl sulfonic acid, polyethyl acrylate sulfonic acid, polybutyl acrylate sulfonic acid, polyacryl sulfonic acid, polymethacryl sulfonic acid, poly(2-acrylamido-2-methyl sulfonic acid), polyisoprene sulfonic acid, polyvinyl carboxylic acid, polystyrene carboxylic acid, polyallyl carboxylic acid, polyacryl sulfonic acid, polymethacryl carboxylic acid, poly(2-acrylamido-2-methyl carboxylic acid), polyisoprene carboxylic acid, paratoluene sulfonic acid, xylene sulfonic acid, methylnaphthalene sulfonic acid, butylnaphthalene sulfonic acid, or metal salts thereof. These may be homopolymers or copolymers of two or more kinds. Among these, polystyrene sulfonic acid is particularly preferred.

[0018] Examples of monomers having functional groups that can undergo the above-mentioned chemical oxidation doping include 6-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-2-yl)hexane-1-sulfonic acid, sodium 6-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-2-yl)hexane-1-sulfonate, lithium 6-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-2-yl)hexane-1-sulfonate, and 6-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-2-yl)hexane-1-sulfonate. 8-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-2-yl)octane-1-sulfonic acid, 8-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-2-yl)octane-1-sulfonic acid sodium salt, 8-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-2-yl)octane-1-sulfonic acid potassium salt, 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-methyl-1-propanesulfonic acid sodium salt, 3-[ Sodium (2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-ethyl-1-propanesulfonate, sodium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-propyl-1-propanesulfonate, sodium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-methyl-1-propanesulfonate, sodium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-methyl-1-propanesulfonate, sodium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl) methoxy]-1-methyl-1-propanesulfonic acid potassium, 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-methyl-1-propanesulfonic acid, 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-methyl-1-propanesulfonic acid ammonium, 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-methyl-1-propanesulfonic acid triethylammonium, and the like.

[0019] The dispersion medium may be water or an organic solvent, such as alcohols, ketones, esters, ethers, cellosolves, aromatic hydrocarbons, or aliphatic hydrocarbons.

[0020] The conductive polymer dispersion or solution of the present invention may contain a high-boiling organic solvent. Among high-boiling organic solvents, particularly preferred are those having a boiling point of 150 to 250° C. Specific examples of the high-boiling organic solvent include N-methyl-2-pyrrolidone, dimethyl sulfoxide, γ-butyrolactone, sulfolane, dimethyl sulfone, ethylene glycol, diethylene glycol, and triethylene glycol. Among these, ethylene glycol or γ-butyrolactone is particularly preferred in that a solid electrolyte layer containing a conductive polymer with a uniform surface can be formed.

[0021] The content of the organic solvent in the conductive polymer dispersion or solution is preferably 1 to 20 mass %, particularly preferably 5 to 15 mass %. If the content of the organic solvent is less than 1 mass %, there is a problem that the effect of forming a solid electrolyte layer containing a conductive polymer with a uniform surface is slightly inferior, and if it exceeds 20 mass %, there is a problem that the drying process takes a long time. The conductive polymer dispersion or solution may contain a binder resin, a surfactant, or an alkaline compound in order to adjust the film-forming property and film strength. The conductive polymer dispersion is a dispersion in which the conductive polymer is dispersed in a dispersion medium, and a part of the conductive polymer may be dissolved in the dispersion medium.

[0022] The anode and cathode used in the electrolytic capacitor are preferably made of a valve metal, specifically, one selected from the group consisting of aluminum, tantalum, niobium and titanium, of which aluminum is preferred. The valve metal is usually used in the form of a sintered body or foil. The electrolytic capacitor can be of the chip type or the wound type, depending on the shape of the anode and cathode used.

[0023] In the electrolytic capacitor of the present invention, the solid electrolyte may be formed by contacting the capacitor element with a dispersion or solution of the conductive polymer by means of immersion or the like and then drying the solvent, or by immersing the capacitor element in a monomer solution of the conductive polymer and then chemically polymerizing or electrolytically polymerizing the solid electrolyte.

[0024] (electrolyte) The electrolytic solution in the present invention contains an electrolyte and an organic solvent, and the electrolyte contains the following (a) and / or (b): (a) an aromatic carboxylic acid having a sulfo group and a hydroxy group, and a boron compound (b) A composite compound of an aromatic carboxylic acid having a sulfo group and a hydroxy group and a boron compound. Examples of aromatic carboxylic acids having a sulfo group and a hydroxy group include compounds having a sulfo group and a hydroxy group on an aromatic ring of an aromatic carboxylic acid preferably having 1 to 4, more preferably 1 or 2, and particularly preferably 1 ring structure. The number of sulfo groups and hydroxy groups that the aromatic ring has is each independently preferably 1 to 4, more preferably 1 or 2, and particularly preferably 1.

[0025] Preferred specific examples of aromatic carboxylic acids having a sulfo group and a hydroxy group include 5-sulfosalicylic acid, 4-sulfosalicylic acid, 3-sulfosalicylic acid, 6-sulfosalicylic acid, 3-hydroxy-4-sulfobenzoic acid, 3-hydroxy-5-sulfobenzoic acid, 2,4-dihydroxy-5-sulfobenzoic acid, 2-hydroxy-4-methyl-5-sulfobenzoic acid, 2-hydroxy-4-methoxy-5-sulfobenzoic acid, 3-hydroxy-7-sulfo-2-naphthalenecarboxylic acid, 3-hydroxy-6-sulfo-2-naphthalenecarboxylic acid, and 3-hydroxy-6-sulfo-2-naphthalenecarboxylic acid. Examples of the sulfonylsalicylic acid include 5-sulfosalicylic acid, 3-hydroxy-5-sulfo-2-naphthalenecarboxylic acid, 3-hydroxy-8-sulfo-2-naphthalenecarboxylic acid, 1-hydroxy-4-sulfo-2-naphthalenecarboxylic acid, 1-hydroxy-6-sulfo-2-naphthalenecarboxylic acid, 1-hydroxy-7-sulfo-2-naphthalenecarboxylic acid, 2-hydroxy-4-sulfo-1-naphthalenecarboxylic acid, 2-hydroxy-6-sulfo-1-naphthalenecarboxylic acid, and 2-hydroxy-7-sulfo-1-naphthalenecarboxylic acid. Among these, 5-sulfosalicylic acid is preferred.

[0026] Examples of the boron compound contained in the electrolytic solution of the present invention include boric acid, boronic acids (methylboronic acid, ethylboronic acid, butylboronic acid, isobutylboronic acid, cyclopentylboronic acid, cyclohexylboronic acid, phenylboronic acid, 4-tert-butylphenylboronic acid, 4-ethoxyphenylboronic acid, 2-naphthaleneboronic acid, 2-anthraceneboronic acid, etc.), boric acid esters (trimethyl borate, triethyl borate, tripropyl borate, tributyl borate, triphenyl borate, etc.), and boric acid esters (allyl pinacol borate, etc.). Among these, boric acid, which has excellent characteristics, is preferred.

[0027] The electrolyte of the present invention preferably contains ammonia, primary to quaternary amines, or an amidine compound in addition to an aromatic carboxylic acid having a sulfo group and a hydroxy group and a boron compound. As such primary to quaternary amines, tertiary amines are preferred. Examples of tertiary amines include trialkylamines (trimethylamine, dimethylethylamine, methyldiethylamine, triethylamine, dimethyl n-propylamine, dimethylisopropylamine, methylethyl n-propylamine, methylethylisopropylamine, diethyl n-propylamine, diethylisopropylamine, tri-n-propylamine, triisopropylamine, tri-n-butylamine, tri-tert-butylamine, etc.), and phenyl group-containing amines (dimethylphenylamine, methylethylphenylamine, diethylphenylamine, etc.).

[0028] Among them, trialkylamines are preferred, and more preferred are those containing one or more selected from the group consisting of trimethylamine, dimethylethylamine, methyldiethylamine and triethylamine.

[0029] The organic solvent used in the electrolytic solution may be a protic polar solvent or an aprotic polar solvent, and may be used alone or in combination of two or more kinds. Examples of protic polar solvents include monohydric alcohols (methanol, ethanol, propanol, butanol, pentanol, hexanol, cyclobutanol, cyclopentanol, cyclohexanol, benzyl alcohol, etc.), polyhydric alcohols and oxyalcohol compounds (ethylene glycol, propylene glycol, glycerin, methyl cellosolve, ethyl cellosolve, methoxypropylene glycol, dimethoxypropanol, etc.), polyalkylene glycols (polyethylene glycol, polypropylene glycol, etc.), and the like.

[0030] Aprotic polar solvents include γ-butyrolactone, γ-valerolactone, amides (N-methylformamide, N,N-dimethylformamide, N-ethylformamide, N,N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide, hexamethylphosphoric amide, etc.), sulfolanes (sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, etc.), chain sulfones (dimethylsulfone, ethylmethylsulfone, ethylisopropylsulfone), and cyclic amides. (N-methyl-2-pyrrolidone, etc.), carbonates (ethylene carbonate, propylene carbonate, isobutylene carbonate, etc.), nitriles (acetonitrile, etc.), sulfoxides (dimethyl sulfoxide, etc.), 2-imidazolidinones [1,3-dialkyl-2-imidazolidinone (1,3-dimethyl-2-imidazolidinone, 1,3-diethyl-2-imidazolidinone, 1,3-di(n-propyl)-2-imidazolidinone, etc.), 1,3,4-trialkyl-2-imidazolidinone (1,3,4-trimethyl-2-imidazolidinone, etc.)], etc.

[0031] Among the above, the organic solvent used in the electrolytic solution is preferably at least one selected from the group consisting of sulfolane, γ-butyrolactone, propylene carbonate, ethylene carbonate, ethylene glycol, diethylene glycol, triethylene glycol, benzyl alcohol, and glycerin, because it provides a high capacitance to the capacitor. In particular, γ-butyrolactone, ethylene glycol, or polyethylene glycol is preferred.

[0032] The content of the aromatic carboxylic acid having a sulfo group and a hydroxy group in the electrolytic solution of the present invention is preferably 0.1 to 40 mass%, more preferably 1 to 20 mass%, and particularly preferably 3 to 10 mass%. If the content is less than 0.1 mass%, it is difficult to obtain sufficient electrical properties, while if it exceeds 40 mass%, the maintenance rate of low ESR in a high-temperature environment becomes poor. The content of the boron compound is preferably 0.1 mole or more, more preferably 0.25 to 1 mole, and particularly preferably 0.4 to 0.6 moles, per mole of the aromatic carboxylic acid having a sulfo group and a hydroxy group. If the content is less than 0.1 mole per mole of the carboxylic acid, sufficient electrical properties are difficult to obtain, whereas if it exceeds 1 mole, it is difficult to maintain a low ESR in a high temperature environment.

[0033] The electrolytic solution of the present invention is produced by simultaneously or successively adding an aromatic carboxylic acid having a sulfo group and a hydroxy group, a boron compound, and preferably further ammonia or a primary to quaternary amine to the above-mentioned organic solvent so that each of them has the above-mentioned content. When adding these, they can be added while stirring as necessary, and while heating as necessary. These stirring and heating can be performed under appropriate conditions, and for example, heating can be performed at 35 to 120°C.

[0034] The electrolyte solution of the present invention can also be produced by reacting an aromatic carboxylic acid having a sulfo group and a hydroxy group with a boron compound in advance and adding the reaction product to the above-mentioned organic solvent, or by reacting an aromatic carboxylic acid having a sulfo group and a hydroxy group with a boron compound and ammonia or a primary to quaternary amine in advance and adding the reaction product to the above-mentioned organic solvent.

[0035] In either case where the aromatic carboxylic acid having a sulfo group and a hydroxy group and the boron compound are added without reacting them in advance or where they are reacted in advance and then added, the aromatic carboxylic acid and the boron compound are mutually reactive, and therefore the aromatic carboxylic acid having a sulfo group and a hydroxy group reacts with the boron compound, and a composite compound in which the aromatic carboxylic acid and the boron compound have reacted can be formed in the electrolyte. For example, when the aromatic carboxylic acid is an aromatic carboxylic acid having a hydroxyl group and a carboxyl group at adjacent positions on the aromatic ring, and the boron compound is boric acid, the two react with each other to at least temporarily or partially form a composite compound having an anion structure represented by the following formula in the obtained electrolyte solution.

[0036] [ka] In the above formula, n is 1 to 4. When n is 1, R represents a sulfo group. When n is 2 to 4, at least one R represents a sulfo group, and the others may be the same or different and represent an alkyl group, a hydroxyl group, an alkoxy group, or a sulfo group.

[0037] When the electrolytic solution of the present invention contains a complex compound of an aromatic carboxylic acid having a sulfo group and a hydroxy group and a boron compound, the content of the complex compound is preferably 0.05 to 40 mass%, more preferably 0.5 to 20 mass%, and particularly preferably 1.5 to 10 mass%. If the content is less than 0.05 mass%, it is difficult to obtain sufficient electrical properties, while if it exceeds 40 mass%, the maintenance rate of low ESR in a high-temperature environment becomes poor.

[0038] In the electrolytic solution of the present invention, when the water content is preferably 1% by mass or less, excellent properties can be obtained in that an increase in internal pressure can be suppressed even when heat of, for example, 200° C. or more is applied in the case of reflow soldering, etc. The water content is more preferably 0.5% by mass or less, and particularly preferably 0.3% by mass or less. Furthermore, it has been found that the electrolyte solution of the present invention can provide excellent properties in terms of maintaining a low ESR when its pH is preferably 2 to 6. Although the reason for this is unclear, the pH is more preferably 2.5 to 5.5, and particularly preferably 3 to 5.

[0039] <Other substances contained> In order to improve the characteristics of the electrolytic capacitor, such as its life performance and resistance performance, other substances than those mentioned above can be added to the electrolytic solution of the present invention. Such additive substances are not particularly limited. For example, the following substances can be mentioned. Examples of such compounds include phosphorus compounds (such as phosphate esters), polysaccharides (such as mannite, sorbitol), complex compounds of boric acid and polysaccharides (such as mannite, sorbitol), complex compounds of boric acid and polyhydric alcohols (such as ethylene glycol and glycerin), and nitro compounds (o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, p-nitrophenol, p-nitrobenzyl alcohol, etc.).

[0040] An antioxidant may be added to the electrolyte, and examples of such antioxidants include phenol compounds, amine compounds, azo compounds, silane compounds, quinone compounds, and carboxylic acid compounds. EXAMPLES

[0041] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples and can be modified within the scope of the present invention. (Preparation of Conductive Polymer Dispersion) 12.2 g of a 20% by weight aqueous solution of polystyrene sulfonic acid (weight average molecular weight: 50,000), which is a dopant component, was mixed with 187.5 g of water and stirred for 10 minutes. Next, 2.04 g of 3,4-ethylenedioxythiophene was added as a monomer and stirred for another 15 minutes to prepare a monomer solution. The resulting monomer solution was pale yellow in color.

[0042] The amount of polystyrene sulfonic acid contained in the monomer solution was 119 parts by mass relative to 100 parts by mass of 3,4-ethylenedioxythiophene contained in the monomer solution. While stirring the monomer solution, 0.012 g of iron sulfate (III) as an oxidizing agent and 4.46 g of ammonium persulfate were dropped and stirred at room temperature for 15 hours to carry out chemical oxidation polymerization. At this time, the monomer solution changed from light yellow to dark blue. Next, 50.1 g of amphoteric ion exchange resin (Organo Corporation product name: MB-1, ion exchange type: -H, -OH) was added to the obtained reaction solution and stirred for 2 hours. As a result, the pH of the reaction solution changed from 1.15 to 1.83. As a result, a conductive polymer dispersion containing poly(3,4-ethylenedioxythiophene) doped with 1.3 mass% polystyrene sulfonic acid was prepared.

[0043] (Fabrication of capacitor elements) The surface was etched and then chemically treated to form an oxide film layer. An aluminum anode foil with a lead terminal attached and an aluminum cathode foil with a surface etched and a lead terminal attached were wound together with a separator (thickness 0.05 mm) made of cellulose fiber between them to produce a capacitor element.

[0044] Example 1 The capacitor element obtained above was immersed in the conductive polymer dispersion liquid, and after the capacitor element was pulled out, the solvent was evaporated to form a solid electrolyte layer made of a conductive polymer. Next, the components in the amounts shown in Table 1 were mixed at 50°C while stirring to produce an electrolyte solution, which was then impregnated into the capacitor element. The capacitor element was then inserted into a cylindrical aluminum case with a bottom, and a rubber seal was attached to the open end, followed by a curling process to seal. After that, an aging treatment was performed at 105°C to produce an aluminum electrolytic capacitor. The rated voltage of this electrolytic capacitor was 35 V, and the outer dimensions after sealing were a cylinder having a diameter of 10 mm and a height of 10 mm.

[0045] (Examples 2 to 7, Comparative Examples 1 to 4) The electrolytic capacitors of Examples 2 to 7 and Comparative Examples 1 to 6 were produced in the same manner as in Example 1, except that an electrolytic solution obtained by mixing the components in the amounts shown in Table 1 with stirring at 95°C was used. Table 1 shows the pH and water content of the electrolytic solution used in each of the electrolytic capacitors of Examples 1 to 7 and Comparative Examples 1 to 6.

[0046] (Capacitor test) The electrolytic capacitors thus fabricated were subjected to a load test at rated voltage in an atmosphere at 150°C, and the ESR (100kHz) was measured. The results are shown in Table 2.

[0047] [Table 1]

[0048] [Table 2]

[0049] When the electrolyte solutions of Examples 1 to 7 and Comparative Examples 1 to 4 were analyzed by liquid chromatography mass spectrometry (LC-MS), the compounds detected in each electrolyte solution were as follows: In the following, 5-sulfosalicylic acid is represented as 5-Ssa, 4-sulfosalicylic acid as 4-Ssa, sulfophthalic acid as Spa, salicylic acid as Sa, phthalic acid as Pa, and 4-nitrophenol as Np. <lc-ms> Column: Silica C18M 4E, Showa Denko Mobile phase: 5 mM ammonium formate aqueous solution / acetonitrile = 80 / 20 (V / V) Detector: ACQUITY QDa detector, manufactured by Japan Waters Ionization method: ESI Negative

[0050] Electrolyte of Example 1 m / z: 217(5‐Ssa - H) - ,221(2(5-Ssa) + B(OH)3- 3H2O - 2H) 2- ,443(2(5-Ssa) + B(OH)3- 3H2O - H) - Electrolyte of Example 2 m / z: 217(5‐Ssa - H) - ,221(2(5-Ssa) + B(OH)3- 3H2O - 2H) 2- ,443(2(5-Ssa) + B(OH)3- 3H2O - H) - Electrolyte of Example 3 m / z: 217(5‐Ssa - H) - ,221(2(5-Ssa) + B(OH)3- 3H2O - 2H) 2- ,443(2(5-Ssa) + B(OH)3- 3H2O - H) - Electrolyte of Example 4 m / z: 217(5‐Ssa - H) - ,221(2(5-Ssa) + B(OH)3- 3H2O - 2H) 2- ,443(2(5-Ssa) + B(OH)3- 3H2O - H) - Electrolyte of Example 5 m / z: 138 (Np - H) - ,217(5‐Ssa - H) - ,221(2(5-Ssa) + B(OH)3- 3H2O - 2H) 2- ,443(2(5-Ssa) + B(OH)3- 3H2O - H) - Electrolyte of Example 6 m / z: 217(4‐Ssa - H) - ,221(2(4-Ssa) + B(OH)3- 3H2O - 2H) 2- ,443(2(4-Ssa) + B(OH)3- 3H2O - H) -

[0051] Electrolyte of Comparative Example 1 m / z: 217(5‐Ssa - H) - Electrolyte of Comparative Example 2 No detection Electrolyte of Comparative Example 3 m / z: 217(5‐Ssa - H) - Electrolyte of Comparative Example 4 m / z: 137(Sa-H) - ,283(2Sa + B(OH)3- 3H2O - H) - Electrolyte of Comparative Example 5 m / z: 245 (Spa - H) - Electrolyte of Comparative Example 6 m / z: 165(Pa - H) - ,217(5‐Ssa - H) -

[0052] As can be seen from Tables 1 and 2, the electrolytic capacitors of Examples 1 to 7 are superior to the electrolytic capacitors of Comparative Examples 1 to 3 in that they can maintain a low ESR even at a high temperature of 150° C. On the other hand, it can be seen that none of Comparative Example 4, in which an aromatic carboxylic acid having no sulfo group was used instead of an aromatic carboxylic acid having a sulfo group and a hydroxy group, Comparative Example 5, in which 4-sulfophthalic acid having no hydroxy group was used, and Comparative Example 6, in which phthalic acid was used instead of a boron compound, could maintain a low ESR. In addition, the above LC-MS results suggest that in the electrolyte solutions of Examples 1 to 7, at least a portion of the aromatic carboxylic acid having a sulfo group and a hydroxy group and the boron compound contained in the electrolyte solution form a complex compound. [Industrial Applicability]

[0053] The electrolyte of the present invention is widely used as an electrolyte in electrolytic capacitors which are widely used in power sources for various consumer devices such as AV equipment, mobile phones, and notebook computers, in-vehicle electrical equipment, industrial equipment, and the like. The entire contents of the specification, claims, drawings, and abstract of Japanese Patent Application No. 2019-109812, filed on June 12, 2019, are hereby incorporated by reference as the disclosure of the specification of the present invention.

Claims

1. An electrolytic solution for an electrolytic capacitor, comprising: an anode foil, a cathode foil, and a capacitor element having a solid electrolyte layer, the anode foil and the cathode foil each having a dielectric oxide film layer on a surface thereof; and an electrolytic solution impregnated in the capacitor element, The electrolyte for an electrolytic capacitor is characterized in that the electrolyte comprises an organic solvent containing the following compound (a) and / or (b), and the content of the following 5-sulfosalicylic acid or 4-sulfosalicylic acid is 3 to 40 parts by mass. (a) 5-sulfosalicylic acid or 4-sulfosalicylic acid, and boric acid (b) A complex compound of 5-sulfosalicylic acid or 4-sulfosalicylic acid and boric acid

2. 2. The electrolytic solution for an electrolytic capacitor according to claim 1, further comprising ammonia or a primary to quaternary amine.

3. 3. The electrolyte for an electrolytic capacitor according to claim 1, wherein the organic solvent is polyalkylene glycol, γ-butyrolactone, γ-valerolactone, ethylene glycol, diethylene glycol, triethylene glycol or benzyl alcohol.

4. 4. The electrolyte for an electrolytic capacitor according to claim 1, wherein the content of the boric acid is 0.1 mol or more per 1 mol of the 5-sulfosalicylic acid or 4-sulfosalicylic acid.

5. 5. The electrolyte for an electrolytic capacitor according to claim 1, wherein the content of the 5-sulfosalicylic acid or the composite compound of 4-sulfosalicylic acid and boric acid is 0.05 parts by mass or more.

6. 6. The electrolytic solution for electrolytic capacitors according to claim 1, wherein the water content is 0.5% by mass or less.

7. 7. The electrolytic solution for an electrolytic capacitor according to claim 1, which has a pH of 2 to 6.

8. 8. The electrolyte for an electrolytic capacitor according to claim 1, further comprising an additive comprising a nitro compound.

9. 9. The electrolyte for an electrolytic capacitor according to claim 1, wherein the solid electrolyte layer is a layer of polythiophene, polypyrrole, polyaniline, or a derivative thereof.

10. An electrolytic capacitor using the electrolytic solution according to any one of claims 1 to 9.

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