Electrolyte for electrolytic capacitor and electrolytic capacitor

Abstract

Provided is an electrolyte solution for electrolytic capacitors, which makes it possible to prevent the expansion of an external container during reflow soldering.　An electrolyte solution for electrolytic capacitors, the electrolyte solution being provided with, in an external container, a capacitor element which comprises a positive electrode foil having a dielectric oxide film on a surface thereof, a negative electrode foil, and a solid electrolyte layer and an electrolyte solution with which the capacitor element is impregnated, the electrolyte solution being characterized in that the electrolyte solution comprises an organic carboxylic acid having a hydroxy group, a boron compound and a solvent, the content of the boron compound is 0.6 mole or more relative to 1 mole of the organic carboxylic acid having a hydroxy group, the solvent comprises a protic organic solvent, and the increase in the pressure in the external container can be prevented even when the electrolytic capacitor is exposed to a high temperature of 250℃ or higher.

Description

[Technical field]

[0001] The present invention relates to an electrolytic solution for electrolytic capacitors that can suppress expansion of an outer container even when exposed to a high-temperature atmosphere in a solder reflow process or the like, and to an electrolytic capacitor using this electrolytic solution. [Background technology]

[0002] In recent years, there has been an increasing demand for higher reliability in AV equipment and in-vehicle electrical equipment. The electrolytic capacitors used therein also need to be smaller, have larger capacity, and have improved performance in terms of equivalent series resistance (hereinafter referred to as ESR) in the high-frequency range. In particular, with the trend toward higher frequencies in electronic devices, there is a demand for electrolytic capacitors with superior ESR characteristics in the high-frequency range, and large capacitance electrolytic capacitors.Recently, in order to reduce ESR in such high-frequency ranges, solid electrolytic capacitors that use a solid electrolyte such as a conductive polymer with higher electrical conductivity than conventional electrolytes for driving devices and an electrolyte solution have been studied and commercialized.

[0003] Specifically, Patent Documents 1 and 2 disclose solid electrolytic capacitors using solid electrolytes made of conductive polymers such as polypyrrole, polythiophene, and polyaniline. In these solid electrolytic capacitors, an electrolyte solution is used in which a solute is a complex compound of an organic acid such as phthalic acid, maleic acid, adipic acid, or salicylic acid, and an inorganic acid such as boric acid, phosphoric acid, or hydrofluoric acid, typically an ammonium salt or amine salt of borodisalicylic acid, borodiglycolic acid, or borodisalic acid, and various organic solvents such as γ-butyrolactone, γ-valerolactone, or ethylene glycol. Such solid electrolytic capacitors have a large capacity and a small leakage current, and are therefore attracting attention as electrolytic capacitors with high characteristics. Furthermore, Patent Document 3 discloses an electrolytic capacitor that uses an electrolyte solution containing an amine salt of a boron complex of a dicarboxylic acid or a hydroxycarboxylic acid as an electrolyte, an aprotic organic solvent such as γ-butyrolactone, and an aromatic carboxylic acid having a sulfonic acid group or a salt thereof, and a protic organic solvent such as ethylene glycol as additives.

[0004] On the other hand, these electrolytic capacitors are required to have high heat resistance when they are mounted on a circuit board, in addition to heat resistance when the electronic device they are built in is in use. Specifically, a solder reflow process is used to perform surface mounting of electrolytic capacitors, but in recent years, there has been a demand for lead-free products, particularly for environmental conservation, and the solder reflow process requires high heat resistance of 250°C or more. In such a high-temperature solder reflow process, gas is generated from the electrolyte in the electrolytic capacitor, causing an increase in pressure inside the outer container, which expands the outer container and, for example, increases the height dimension of the electrolytic capacitor. In this case, not only does this cause problems when the electrolytic capacitor mounted on the board is installed in the device, but it may also cause the characteristics of the electrolytic capacitor itself to deteriorate, and there is a risk of losing its reliability (see Patent Documents 4 and 5). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2015-165550 A [Patent Document 2] Patent No. 5052746 [Patent Document 3] Patent No. 4863626 [Patent Document 4] JP 2004-186523 A [Patent Document 5] Patent No. 2007-59611 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention aims to provide an electrolytic solution for electrolytic capacitors that contains a protic organic solvent such as a polyhydric alcohol as the electrolyte and has excellent heat resistance, which is capable of suppressing dimensional changes due to expansion of an outer container even when exposed to high temperatures of 250°C or more, such as those used in a solder reflow process, and an electrolytic capacitor that uses this electrolyte. [Means for solving the problem]

[0007] As a result of extensive research conducted by the present inventors in order to achieve the above object, the present inventors have obtained the following findings. It has been found that conventional electrolytic capacitors, particularly those disclosed in Patent Documents 1 and 2, which use borodisalicylic acid as a solute, which is a composite compound of an organic carboxylic acid having a hydroxyl group and boric acid, and which use protic organic solvents such as polyhydric alcohols such as ethylene glycol and propylene glycol as a solvent, from which excellent ESR characteristics can be obtained, have insufficient heat resistance at high temperatures during processes such as solder reflow.

[0008] On the other hand, when the relationship between the content of boric acid per mole of organic carboxylic acid in the electrolyte of an electrolytic capacitor and the heat resistance was investigated, the content of boric acid per mole of organic carboxylic acid in the electrolyte of the electrolytic capacitor disclosed in Patent Documents 1, 2, and 3 is at most 0.5 to 0.55 moles. According to the present inventor's research, it was found that when the content of boric acid per mole of organic carboxylic acid is small, the above-mentioned phenomenon of insufficient heat resistance occurs. Furthermore, it has been found that the above-mentioned problem of heat resistance can be solved by using an electrolyte in an electrolytic capacitor, which contains, as solutes, an organic carboxylic acid having a hydroxyl group and a boron compound in a protic organic solvent such as a polyhydric alcohol, and the content of the boron compound per mole of the organic carboxylic acid having a hydroxyl group is as large as 0.6 moles or more, more preferably 0.7 moles or more, and particularly preferably 0.9 moles or more.

[0009] On the other hand, in the case of an electrolytic capacitor using an electrolyte solution mainly composed of an aprotic organic solvent such as γ-butyrolactone as a solvent, there is no problem with heat resistance such as expansion of the outer container. In addition, in the case of an electrolyte solution using an aprotic organic solvent as a solvent, as the content of boric acid relative to the organic carboxylic acid increases, for example, when the content of boric acid relative to 1 mole of organic carboxylic acid exceeds 0.5 moles, the solubility of the solute components, boric acid, organic carboxylic acid, or a composite compound thereof, decreases. However, in the case of a protic organic solvent, the solubility of these solute components is maintained, and it has been found that the resistance of the resulting electrolyte solution can be maintained low.

[0010] The present invention is based on the above findings and has the following aspects. (1) An electrolytic solution for an electrolytic capacitor, the electrolytic solution comprising a capacitor element having an anode foil, a cathode foil, and a solid electrolyte layer, the capacitor element being impregnated with an electrolytic solution, the capacitor element being contained in an outer container, The electrolyte for an electrolytic capacitor comprises an organic carboxylic acid having a hydroxyl group, a boron compound, and a solvent, the content of the boron compound being 0.6 moles or more per mole of the organic carboxylic acid having a hydroxyl group, the solvent comprising a protic organic solvent, and is characterized in that it is possible to suppress an increase in pressure inside an outer container when the electrolytic capacitor is exposed to a high temperature of 250°C or more.

[0011] (2) The electrolyte according to (1), wherein the boron compound is boric acid. (3) The electrolyte solution according to (1) or (2), wherein the organic carboxylic acid having a hydroxy group is an aromatic carboxylic acid having a hydroxy group. (4) The electrolyte solution according to any one of (1) to (3) further containing ammonia or a primary, secondary or quaternary amine. (5) The electrolyte solution according to any one of (1) to (4), wherein the protic organic solvent is ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, benzyl alcohol, or glycerin. (6) The electrolyte solution according to any one of (1) to (5), wherein the content of the boron compound is 0.7 to 2 mol per 1 mol of the organic carboxylic acid having a hydroxy group. (7) The protic organic solvent The medium The electrolyte solution according to any one of (1) to (6), wherein the content of said first metal oxide is 65 mass % or more based on the electrolyte solution. (8) The electrolyte solution according to any one of (1) to (7) above, having a water content of 1% by mass or less. (9) The electrolyte solution according to any one of (1) to (8), having a pH of 2 to 6.

[0012] (10) The electrolyte solution according to any one of (1) to (9) further containing a nitro compound. (11) The electrolyte solution according to any one of (1) to (10), wherein the organic carboxylic acid having a hydroxy group and the boron compound form a composite compound. (12) The electrolytic solution according to any one of (1) to (11), wherein the solid electrolyte layer is a layer of polythiophene, polypyrrole, polyaniline, or a derivative thereof. (13) The electrolytic solution according to any one of (1) to (12), wherein the electrolytic capacitor is exposed to a high temperature of 250° C. or higher during a reflow process of the electrolytic capacitor. (14) An electrolytic capacitor using any one of the electrolyte solutions (1) to (13) above. (15) A reflow soldering method using the electrolytic capacitor according to (14). Effect of the Invention

[0013] According to the present invention, there is provided an electrolyte for electrolytic capacitors that can suppress an increase in pressure inside an outer container and suppress expansion of the outer container even when an electrolytic capacitor using a protic organic solvent such as a polyhydric alcohol as the electrolyte is exposed to high temperatures of 250°C or higher, or even 260°C or higher, during a solder reflow process, etc., and an electrolytic capacitor using this electrolyte. Furthermore, electrolytic capacitors using the electrolytic solution of the present invention are suppressed from undergoing dimensional changes and changes in characteristics due to expansion of the outer container even during the solder reflow process at high temperatures of 250°C or higher, and also have high reliability with little change in characteristics even in high-temperature operating environments.

[0014] The mechanism by which the increase in pressure inside the outer container can be suppressed and the heat resistance can be improved in the electrolytic capacitor using the electrolytic solution of the present invention is believed to be as follows. By increasing the content of the boron compound to 1 mole of organic carboxylic acid having a hydroxyl group contained in the electrolytic solution using a protic organic solvent as a solvent to 0.6 moles or more, or even more, it is possible to suppress the generation of carbon dioxide and the like due to the decomposition of the organic carboxylic acid having a hydroxyl group, its composite compound with the boron compound, or their salts. It is believed that the suppression of the generation of carbon dioxide and the like due to this decomposition can be prevented more reliably as the content of the boron compound to 1 mole of organic carboxylic acid increases to 0.7 moles or more, or even 0.9 moles or more. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described. In the present specification, when a range of numerical values ​​is described, if the units of the lower limit and the upper limit are the same, the unit of the lower limit may be omitted. <Electrolytic capacitor> The electrolyte of the present invention is used in a solid 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 electrolytic capacitor is obtained by a known method. 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.

[0016] The solid electrolyte layer used in the present invention can be obtained by chemically oxidizing or electrolytically oxidizing polymerizing 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 polymerization 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 chemically oxidizing doping.

[0017] 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 dioxyfuran, 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 preferred because they are excellent in terms of low ESR of the electrolytic capacitor.

[0018] 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.

[0019] 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. potassium sulfonate, 8-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-2-yl)octane-1-sulfonic acid, sodium 8-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-2-yl)octane-1-sulfonate, potassium 8-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-2-yl)octane-1-sulfonate, sodium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-methyl-1-propanesulfonate, sodium 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.

[0020] The dispersion medium may be water or an organic solvent, such as alcohols, ketones, esters, ethers, cellosolves, aromatic hydrocarbons, or aliphatic hydrocarbons.

[0021] 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° C. or higher. Specific examples of the high-boiling organic solvent include N-methyl-2-pyrrolidone, dimethyl sulfoxide, γ-butyrolactone, sulfolane, dimethyl sulfone, ethylene glycol, diethylene glycol, triethylene glycol, and polyethylene glycol. Among these, ethylene glycol or γ-butyrolactone is more preferred in that it can form a solid electrolyte layer containing a conductive polymer with a uniform surface.

[0022] 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 %, 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 drawback that the drying process requires 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.

[0023] The anode and cathode used in the electrolytic capacitor are preferably valve metals, specifically, one selected from the group consisting of aluminum, tantalum, niobium, and titanium, and aluminum is particularly preferred. The valve metals are usually used in the form of sintered bodies or foils. The electrolytic capacitor can be a chip type or a wound type depending on the shape of the anode and cathode used.

[0024] 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.

[0025] <Electrolyte> The electrolytic solution in the present invention contains an electrolyte and an organic solvent, and the electrolyte contains an organic carboxylic acid having a hydroxy group and a boron compound. The organic carboxylic acid having a hydroxy group is preferably one having 1 to 4, more preferably 1 or 2, and particularly preferably 1 ring structure, and preferably having 1 to 4, more preferably 1 or 2, and particularly preferably 1 hydroxy group.

[0026] Preferred specific examples of organic carboxylic acids having a hydroxy group include glycolic acid, lactic acid, 3-hydroxypropionic acid, mandelic acid, salicylic acid, 3-methylsalicylic acid, 4-methylsalicylic acid, 5-methylsalicylic acid, 6-methylsalicylic acid, 3-nitrosalicylic acid, 4-nitrosalicylic acid, 5-nitrosalicylic acid, 3,5-dinitrosalicylic acid, 3-sulfosalicylic acid, 4-sulfosalicylic acid, 5-sulfosalicylic acid, and 5-sulfosalicylic acid. Examples include licylic acid, 4-hydroxysalicylic acid, 5-hydroxysalicylic acid, 3-methoxysalicylic acid, 4-methoxysalicylic acid, 5-methoxysalicylic acid, methylenedisalicylic acid, 1-hydroxy-2-naphthoic acid, 2-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid, 3-hydroxy-7-methyl-2-naphthoic acid, 4,4'-methylenebis(3-hydroxy-2-naphthoic acid), etc. Among the above, aromatic carboxylic acids are preferred, and salicylic acid is particularly preferred.

[0027] The boron compound contained in the electrolytic solution in the present invention includes boric acid or boric acid esters (trimethyl borate, triethyl borate, tripropyl borate, tributyl borate, triphenyl borate, etc.), etc. In particular, boric acid, which has excellent characteristics, is preferable.

[0028] In the electrolyte of the present invention, the ratio of the boron compound to the organic carboxylic acid having a hydroxyl group is important, and the boron compound is contained in an amount of 0.6 mol or more, preferably 0.65 mol or more, more preferably 0.7 mol or more, and even more preferably 0.9 mol or more, per mol of the organic carboxylic acid having a hydroxyl group. When the content ratio of the boron compound is within the above range, a sufficient effect of suppressing the expansion of the outer container can be obtained when exposed to a high temperature of 250° C. or more. On the other hand, if the content of the boron compound is large, the ESR characteristics of the capacitor may deteriorate under high temperature conditions such as 100°C or higher. Therefore, the content of the boron compound is preferably 5 mol or less, more preferably 3.5 mol or less, even more preferably 2.0 mol or less, and particularly preferably 1.5 mol or less, per 1 mol of the organic carboxylic acid having a hydroxyl group. The boron compound contained in the electrolytic solution of the present invention may exist separately from an organic carboxylic acid having a hydroxyl group in the electrolytic solution, or a part or all of both may exist as a composite compound of an organic carboxylic acid having a hydroxyl group and a boron compound.

[0029] The electrolytic solution of the present invention preferably contains ammonia, primary to quaternary amines, or an amidine compound in addition to an organic carboxylic acid having a hydroxyl 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.).

[0030] Of these, the tertiary amine is preferably a trialkylamine, and more preferably includes one or more selected from the group consisting of trimethylamine, dimethylethylamine, methyldiethylamine, and triethylamine. When the electrolyte solution of the present invention contains ammonia, or a primary to quaternary amine, or an amidine compound, it is believed that some or all of these form some kind of complex compound with an organic carboxylic acid having a hydroxyl group and / or a boron compound or complex compound, although the structure is not necessarily clear.

[0031] The organic solvent used in the electrolytic solution of the present invention is a protic organic solvent, and one or more types of solvents may be used alone. In the present invention, the use of a protic organic solvent is necessary, and when an aprotic organic solvent such as γ-butyrolactone is used as the solvent, the expansion of the outer container during reflow soldering is small even without the addition amount of the boron compound of the present invention, which does not meet the object of the present invention. In addition, the electrolytic solution of the present invention is characterized in that the content ratio of the boron compound per mole of an organic carboxylic acid having a hydroxyl group is 0.6 mol or more. However, when an aprotic organic solvent such as γ-butyrolactone is used, as shown in Comparative Example 6, when the content ratio of boric acid per mole of salicylic acid, which is an organic carboxylic acid having a hydroxyl group, is 1 mole, the solute salicylic acid and / or boric acid is not sufficiently dissolved. The protic organic solvent in the electrolytic solution of the present invention is preferably 65% ​​by mass or more, more preferably 70% by mass or more, particularly preferably 80% by mass or more, and most preferably 85% by mass or more.

[0032] Examples of the protic organic solvent include monohydric alcohols (methanol, ethanol, propanol, butanol, pentanol, hexanol, cyclohexanol, benzyl alcohol, etc.), polyhydric alcohols and alkoxy alcohol compounds (ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, glycerin, methyl cellosolve, ethyl cellosolve, methoxypropylene glycol, dimethoxypropanol, etc.), polyalkylene glycols (polyethylene glycol, polypropylene glycol, etc.), etc. Among these, those having a boiling point of 150°C or higher are preferable, and examples of these include monohydric alcohols (hexanol, cyclohexanol, benzyl alcohol, etc.), polyhydric alcohols and alkoxy alcohol compounds (ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, glycerin, methyl cellosolve, ethyl cellosolve, methoxypropylene glycol, dimethoxypropanol, etc.), polyalkylene glycols (polyethylene glycol, polypropylene glycol, etc.), etc.

[0033] Among the above, the organic solvent used in the electrolytic solution is preferably at least one selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, benzyl alcohol, and glycerin, because it provides a high capacitance to the capacitor. Ethylene glycol, diethylene glycol, or polyethylene glycol is particularly preferred.

[0034] The electrolyte of the present invention may contain an aprotic solvent. However, as shown in the comparative example described later, when an aprotic solvent such as γ-butyrolactone is used, the solubility of the organic carboxylic acid and / or boron compound having a hydroxyl group, which is the solute of the electrolyte of the present invention, in γ-butyrolactone is small, and the contribution to preventing the expansion of the outer container during reflow soldering is small. Therefore, even if the electrolyte of the present invention contains an aprotic solvent, it is preferably 80 mass % or less, preferably 40 mass % or less, more preferably 20 mass % or less, and particularly preferably 10 mass % or less of the total solvent.

[0035] Aprotic 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), cyclic amides (N-methylformamide, N,N-dimethylformamide, N-ethylformamide, N,N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide, hexamethylphosphoric amide, etc.), ethyl-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.

[0036] The content of the organic carboxylic acid having a hydroxyl 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 decreases. The content of the boron compound is 0.6 moles or more relative to 1 mole of the organic carboxylic acid having a hydroxy group, and is 0.65 moles or more. M The content is preferably at least 1 mole, more preferably 0.7 to 2.0 moles, and particularly preferably 0.9 to 1.5 moles. If the content is less than 0.6 moles per mole of the organic carboxylic acid, it is difficult to obtain a sufficient effect of suppressing blistering during reflow.

[0037] The electrolytic solution of the present invention is produced by simultaneously or successively adding an organic carboxylic acid having a hydroxyl 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.

[0038] The electrolyte solution of the present invention can be produced by reacting an organic carboxylic acid having a hydroxyl group with a boron compound in advance and adding the reaction product to the above-mentioned organic solvent, or by reacting an organic carboxylic acid having a hydroxyl 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.

[0039] In the electrolytic solution of the present invention, when the amount of water contained therein is preferably 1% by mass or less, excellent properties can be obtained in that the internal pressure increase can be suppressed even when heat of 250°C or more is applied, such as in the case of a solder reflow process. The amount of water contained is more preferably 1% by mass or less, and particularly preferably 0.5% by mass or less. The amount of water contained can be adjusted by removing the water contained in the raw materials of the electrolytic solution, such as the organic solvent used, or in the prepared electrolytic solution. The removal of the water contained can be performed, for example, by heating the electrolytic solution, distilling it off under reduced pressure, treating it with a dehydrating agent, or a combination of these. 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.

[0040] 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.). In addition, an antioxidant can be added to the electrolyte, and examples of the antioxidant include phenol compounds (catechol, hydroquinone, pyrogallol, etc.), amine compounds, azo compounds, silane compounds, quinone compounds, and carboxylic acid compounds.

[0041] <Heat resistance of solid electrolytic capacitors> The solid electrolytic capacitor of the present invention is manufactured according to a known method by immersing a capacitor element having a solid electrolyte layer formed thereon in the electrolytic solution of the present invention to fill voids in the capacitor element with the electrolytic solution of the present invention, inserting the capacitor element into an outer container, attaching a sealing material or the like to the open end, and performing a tightening process. The material of the outer container used in the manufacture of the solid electrolytic capacitor of the present invention can be aluminum, aluminum alloy, other metals, etc. The thickness of the outer container is preferably 0.2 to 0.7 mm, more preferably 0.2 to 0.4 mm. The sealing material of the outer container can be preferably butyl rubber, butadiene rubber, styrene butadiene rubber, EPM, EPDM, or a mixture thereof.

[0042] The solid electrolytic capacitor of the present invention thus obtained has excellent heat resistance, and since dimensional changes and changes in characteristics due to the expansion of the outer container are suppressed even in a lead-free solder reflow process at high temperatures of 250° C. or more, and even at high temperatures of 260° C. or more, dimensional changes during the surface mounting process can be suppressed. Furthermore, in recent years, the capacitor has high reliability with only small changes in characteristics even in high-temperature usage environments such as in-vehicle applications. In addition, the solid electrolytic capacitor of the present invention has excellent properties such as low ESR characteristics in the high frequency range and high capacitance characteristics, and because of the high heat resistance described above, these properties are not deteriorated by heat. EXAMPLES

[0043] 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 various modifications are possible 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.

[0044] 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. Thus, a conductive polymer dispersion containing poly(3,4-ethylenedioxythiophene) doped with 1.3 mass % polystyrene sulfonic acid was prepared.

[0045] (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.

[0046] 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 of salicylic acid, boric acid, dimethylethylamine, and ethylene glycol in the amounts shown in Table 1 were mixed under stirring at 90 to 95°C to produce an electrolyte solution, which was then impregnated into the capacitor element. The capacitor element was then inserted into a cylindrical metal aluminum case with a bottom (case material thickness: 0.3 mm), and a rubber seal was attached to the open end, followed by curling 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 of the sealed electrolytic capacitor were a cylinder having a diameter of 10 mm and a height of 10 mm.

[0047] (Examples 2 to 10, Comparative Examples 1 to 5) The electrolytic capacitors of Examples 2 to 10 and Comparative Examples 1 to 5 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 90 to 95°C was used. Comparative Example 6 The components in the amounts shown in Table 1 were mixed and dissolved at 90 to 95° C. with stirring, but when the mixture was cooled to 25° C., a white solid precipitated, making it impossible to prepare an electrolyte solution.

[0048] Table 1 shows the moisture content (mass%) and pH of the electrolyte used in each of the electrolytic capacitors of Examples 1 to 10 and Comparative Examples 1 to 5. The moisture content was measured by Karl Fischer coulometric titration using a trace moisture analyzer CA-200 (manufactured by Mitsubishi Chemical Analytech Co., Ltd.). The pH was measured using a pH meter HM-30R manufactured by DKK-TOA Corporation.

[0049] (Capacitor heating test) The electrolytic capacitor prepared above was heated in a tabletop reflow furnace (STR-2000, manufactured by Synapex) so that the peak temperature was 260° C. In the tabletop reflow furnace, the capacitor was first heated to 180° C. at an average heating rate of 2.1° C. / sec, held at that temperature for 60 seconds, then raised to 260° C. at an average heating rate of 2.1° C. / sec, held at that temperature for 1 second, and then returned to room temperature at an average cooling rate of 4.2° C. / sec. Through this heating test, the dimensional expansion ΔL (mm) in the height direction of the cylindrical electrolytic capacitor was calculated using the following formula, which is shown in Table 1. In the formula, the "height dimension" is the distance between the centers of both bottom surfaces of the cylindrical capacitor, and was measured using a vernier caliper. ΔL = (height dimension after reflow test) - (height dimension before reflow test)

[0050] [Table 1]

[0051] As can be seen from Table 1, the electrolytic capacitors of Examples 1 to 10, which used an electrolyte solution containing 0.6 moles or more of boric acid per mole of an organic carboxylic acid having a hydroxyl group, had a smaller ΔL (mm) than the electrolytic capacitors of Comparative Examples 1 to 5, and suppressed the expansion of the outer container even at a high temperature of 260°C. [Industrial Applicability]

[0052] The present invention is widely used as an electrolyte in electrolytic capacitors that are widely used in power sources for various consumer devices such as AV devices, 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. 2019-207008, filed on November 15, 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, the electrolytic solution comprising a capacitor element having an anode foil, a cathode foil, and a solid electrolyte layer, the capacitor element being impregnated with an electrolytic solution, the capacitor element being contained in an outer container, the electrolytic solution contains an aromatic carboxylic acid having a hydroxy group, a boron compound, a solvent, and further ammonia or a primary to quaternary amine; the content of the boron compound is 0.6 moles or more and 2.0 moles or less relative to 1 mole of the aromatic carboxylic acid having a hydroxy group, the boron compound is boric acid or a borate ester; the solvent comprises a protic organic solvent, An electrolyte for an electrolytic capacitor, characterized in that an increase in pressure inside an outer container can be suppressed even when the electrolytic capacitor is exposed to a high temperature of 250° C. or more.

2. 2. The electrolyte of claim 1, wherein the boron compound is boric acid.

3. 3. The electrolyte solution according to claim 1, wherein the protic organic solvent is at least one selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, benzyl alcohol, and glycerin.

4. The electrolyte according to any one of claims 1 to 3, wherein the protic organic solvent is contained in an amount of 65 mass % or more based on the electrolyte.

5. The electrolyte according to any one of claims 1 to 4, having a water content of 1% by mass or less.

6. 6. The electrolyte according to claim 1, having a pH of 2 to 6.

7. The electrolyte according to any one of claims 1 to 6, further comprising a nitro compound.

8. 8. The electrolyte solution according to claim 1, wherein the aromatic carboxylic acid having a hydroxy group and the boron compound form a composite compound.

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

10. The electrolytic solution according to any one of claims 1 to 9, wherein the electrolytic capacitor is exposed to a high temperature of 250°C or more during a reflow process of the electrolytic capacitor.

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

12. A reflow soldering method using the electrolytic capacitor according to claim 11.

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