Solid electrolytic capacitors

Abstract

This invention provides a solid electrolytic capacitor with low ESR even in the high-frequency region. The solid electrolytic capacitor includes an anode foil, a cathode foil, and an electrolyte layer. The anode foil contains a valve-acting metal and has a dielectric oxide film formed thereon. The cathode foil contains a valve-acting metal and faces the anode foil. The electrolyte layer contains a conductive polymer and an electrolyte solution, with a dielectric layer existing between the anode foil and the cathode foil. The electrolyte solution contains a butylated phosphate compound and an aliphatic dicarboxylic acid, wherein the phosphate compound content is 32 mmol or less per 100 g of the electrolyte solution.

Description

Solid electrolytic capacitors This invention relates to a solid electrolytic capacitor. Solid electrolytic capacitors comprise a valve-acting metal, such as tantalum or aluminum, as the anode foil and cathode foil. The anode foil is enlarged by forming the valve-acting metal into a sintered body or etching a foil shape, and the enlarged surface is treated with a dielectric oxide film such as anodizing. Between the anode foil and the cathode foil, there exists a solid electrolyte layer that is in close contact with the anode foil and functions as the actual cathode. Manganese dioxide and 7,7,8,8-tetracyanoquinodimethane (TCNQ) complexes are known as solid electrolytes. In recent years, poly(3,4-ethylenedioxythiophene) (PEDOT), with its high conductivity and excellent adhesion to dielectric oxide films, has rapidly gained popularity as a solid electrolyte. PEDOT typically exhibits high conductivity by being doped with polystyrene sulfonic acid (PSS) or by possessing a portion of its structure within the molecule that functions as a dopant. However, compared to liquid-type electrolytic capacitors that impregnate the capacitor element with electrolyte, solid-type electrolytic capacitors are less effective at repairing defects in the dielectric oxide film, potentially leading to increased leakage current. Therefore, hybrid-type solid-type electrolytic capacitors have attracted attention, which form a solid electrolyte layer within the capacitor element and impregnate the gaps in the capacitor element with electrolyte. The electrolyte may contain, for example, γ-butyrolactone or cyclobutane as a solvent, and triethylamine borosilicate, ethyldimethylamine phthalate, mono-1,2,3,4-tetramethylimidazoline phthalate, or mono-1,3-dimethyl-2-ethylimidazoline phthalate as a solute. [Prior Art Documents] [Patent Documents] [Patent Document 1] Japanese Patent Application Publication No. 2008-10657 [The problem that the invention aims to solve] With the digitization of electronic devices, there is a growing demand for small, high-capacity capacitors with low equivalent series resistance (ESR). Electrolytic capacitors offer larger capacitance compared to film capacitors or ceramic capacitors. Furthermore, solid electrolytic capacitors containing conductive polymers such as poly(3,4-epoxythiophene) (PEDOT) exhibit excellent ESR due to the high conductivity of these polymers. Digital devices operate in high-frequency regions exceeding tens of kHz, requiring solid electrolytic capacitors to have low ESR even in these high-frequency areas. This invention addresses this problem by providing a solid electrolytic capacitor with low ESR even in high-frequency regions. [Means for Solving the Problem] To address the aforementioned issues, the solid electrolytic capacitor of this embodiment includes: an anode foil containing a valve-acting metal and having a dielectric oxide film formed thereon; a cathode foil containing a valve-acting metal and facing the anode foil; and an electrolyte layer containing a conductive polymer and an electrolyte, with a dielectric layer existing between the anode foil and the cathode foil, wherein the electrolyte contains a butylated phosphate compound and an aliphatic dicarboxylic acid, and the phosphate compound is present in amounts of 32 mmol or less per 100 g of the electrolyte. The phosphate compound may be less than 8 mmol per 100 g of the electrolyte. This results in a significant decrease in the ESR of the solid electrolytic capacitor and a significant decrease in leakage current. The phosphoric acid compound may be one or a mixture of two or more selected from the group consisting of dibutyl phosphate, tributyl phosphate, dibutyl phosphite, and tributyl phosphite. The aliphatic dicarboxylic acid may be one or a mixture of two or more selected from the group consisting of azelaic acid, succinic acid, glutaric acid, and citralic acid. The aliphatic dicarboxylic acid may be one or a mixture of two or more selected from the group consisting of azelaic acid, succinic acid, and glutaric acid. The electrolyte may contain one or more of the group consisting of ethylene glycol, glycerol, and cyclobutane. The electrolyte may contain ammonia. When the phosphate compound is set to 1 in the molar ratio, the electrolyte may contain 0.25 or more of the aliphatic dicarboxylic acid. [Effects of the Invention] According to the present invention, the solid electrolytic capacitor has low ESR, at least in the high-frequency region. Hereinafter, a solid electrolytic capacitor according to an embodiment of the present invention will be described. However, the present invention is not limited to the embodiments described below. (Solid Electrolytic Capacitor) A solid electrolytic capacitor is a passive component that achieves electrostatic capacitance and performs charge storage and discharge through the dielectric polarization of a dielectric oxide film. The solid electrolytic capacitor is formed by housing the capacitor element in a casing and sealing the casing opening with a sealing body. The capacitor element includes an anode foil, a cathode foil, a separator, and an electrolyte layer. The anode foil, cathode foil, and separator face each other and are wound or laminated. A dielectric oxide film is formed on the surface of the anode foil. The electrolyte layer includes a solid electrolyte layer containing conductive polymers and an electrolyte solution. The solid electrolyte layer exists between the anode foil and the cathode foil, and is in close contact with the dielectric oxide film. The electrolyte solution is impregnated in the voids of the capacitor element where the solid electrolyte layer is formed. (Electrode Foil) Anode foil and cathode foil are long strips of foil extending the valve-acting metal. Valve-acting metals include aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony. Regarding purity, the ideal purity for anode foil is above 99.9%, and for cathode foil, it is around 99% or higher, although impurities such as silicon, iron, copper, magnesium, and zinc may be present. Anode foil, whether formed from powder of valve-acting metal, sintered from a formed body, or etched from rolled foil, has an expanded surface. That is, the expanded surface structure includes tunnel-like pits, sponge-like pits, or dense inter-powder voids. Typically, the expanded surface structure is formed by direct current or alternating current etching or alternating current etching in an acidic aqueous solution containing halide ions, such as hydrochloric acid, or by core evaporation or sintering of metal particles. Cathode foil can also have an expanded surface structure through evaporation, sintering, or etching. Dielectric oxide film is typically formed on the surface of the anode foil. For example, if the anode foil is aluminum foil, the dielectric oxide film is aluminum oxide formed by oxidizing the expanded structure. The dielectric oxide film is formed by chemical conversion treatment of the anode foil in an aqueous solution of adipic acid, boric acid, or phosphoric acid, by applying a voltage. A thin oxide film (around 1 V to 10 V) can also be formed on the surface of the cathode foil by chemical conversion treatment as needed. (Electrolyte) The electrolyte is contained within the voids of a capacitor element having a solid electrolyte layer. This electrolyte is a solution in which anionic and cationic components are added to a solvent. Typically, the anionic and cationic components are salts of organic acids, salts of inorganic acids, or salts of complex compounds of organic and inorganic acids, added to the solvent by dissociation into ionic salts of anionic and cationic components. Alternatively, an acid as an anionic component and a base as a cationic component can be added to the solvent separately. Furthermore, the electrolyte may also be in a solvent that does not contain anionic components, or both anionic and cationic components. The electrolyte contains both a butyl-containing phosphoric acid compound and an aliphatic dicarboxylic acid. The phosphoric acid compound has at least one butyl group. Because the electrolyte contains both the butyl-containing phosphoric acid compound and the aliphatic dicarboxylic acid, a low ESR and a low leakage current can be maintained for a long time, even when using solid electrolytic capacitors in high-frequency regions such as 100 kHz. This ESR is lower than that obtained by adding either the butyl-containing phosphoric acid compound or the aliphatic dicarboxylic acid, which has a better ESR, to the electrolyte. However, the concentration of butyl phosphate compound relative to 32 mmol or less per 100 g of electrolyte is preferable, and preferably 8 mmol or less per 100 g of electrolyte. If it is 48 mmol or more, the ESR deteriorates compared to the case where the electrolyte contains only butyl phosphate compound, and the synergistic effect of butyl phosphate compound and aliphatic dicarboxylic acid cannot be obtained. On the other hand, if it is 32 mmol or less, the ESR becomes good compared to the case where the electrolyte contains only butyl phosphate compound. If it is 8 mmol or less, in addition to the low ESR, the LC (leak current) becomes good compared to the case where the electrolyte contains only butyl phosphate compound and aliphatic dicarboxylic acid. There is no particular limitation on the amount of aliphatic dicarboxylic acid added, and a synergistic effect of the ESR of the butylated phosphate compound and the aliphatic dicarboxylic acid can be obtained. Preferably, if the phosphate compound is set to 1 in the molar ratio, the aliphatic dicarboxylic acid is 0.25 or more. At least within this range, when using solid electrolytic capacitors in the high-frequency region such as 100 kHz, low ESR can be maintained for a long time, and the LC is also good. Phosphoric acid compounds containing a butyl group are, for example, dibutyl phosphate, tributyl phosphate, dibutyl phosphite, or tributyl phosphite. It is sufficient that one or more phosphoric acid compounds are included in the electrolyte. Aliphatic dicarboxylic acids are, for example, azelaic acid, succinic acid, glutaric acid, or citrate. It is sufficient that one or more aliphatic dicarboxylic acids are included in the electrolyte. Preferably, the aliphatic dicarboxylic acid is selected from one or more of the group consisting of azelaic acid, succinic acid, or glutaric acid. Compared to phosphoric acid compounds containing a butyl group, azelaic acid, succinic acid, and glutaric acid deteriorate the ESR of solid electrolytic capacitors. However, if a phosphoric acid compound containing a butyl group is also included in the electrolyte, the ESR of the solid electrolytic capacitor is better, and the ESR reduction effect is higher, compared to the case where only a phosphoric acid compound containing a butyl group is included. Furthermore, even if citral acid, which has a high ESR reduction effect, is used as an aliphatic dicarboxylic acid, it will result in a better ESR for solid electrolytic capacitors compared to electrolytes containing only butyl phosphate compounds, and electrolytes containing only citral acid. If the electrolyte contains both butylated phosphoric acid compounds and aliphatic dicarboxylic acids, it may also contain other types of anionic components as solutes. Among other types of anionic components, examples of organic acids include: oxalic acid, succinic acid, glutaric acid, pimelic acid, octanoic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, maleic acid, adipic acid, benzoic acid, toluic acid, enanthic acid, malonic acid, 1,6-decanedicarboxylic acid, 1,7-octanedicarboxylic acid, azelaic acid, undecanediic acid, dodecanediic acid, tridecanediic acid, tributyl adipic acid, 11-vinyl-8-octadecenoic acid, resorcinolic acid, 2,4,6-trihydroxybenzoic acid, gallic acid, gentisic acid, protocatechuic acid, pyrocatechuic acid, trimellitic acid, pyromellitic acid, etc., as well as carboxylic acids, phenols, and sulfonic acids. In addition, examples of inorganic acids include: boric acid, phosphoric acid, phosphorous acid, hypophosphorous acid, carbonic acid, and silicic acid. Examples of complex compounds of organic and inorganic acids include: borosilicate, borosilicate oxalic acid, borosilicate diglycolic acid, borosilicate dimalonic acid, borosilicate disuccinic acid, borosilicate diadipic acid, borosilicate diazelic acid, borosilicate dibenzoic acid, borosilicate dimaleic acid, borosilicate dilactic acid, borosilicate dimalic acid, borosilicate ditartaric acid, borosilicate dicitric acid, borosilicate diphthalic acid, borosilicate di(2-hydroxy)isobutyric acid, borosilicate diresorcinic acid, borosilicate dimethylsalicylic acid, borosilicate dinaphtholic acid, borosilicate dimandelic acid, and borosilicate di(3-hydroxy)propionic acid. Salts of at least one of organic acids, inorganic acids, and complex compounds of organic and inorganic acids include, for example, ammonium salts, quaternary ammonium salts, quaternary amidine salts, amine salts, sodium salts, potassium salts, etc. Quaternary ammonium ions, as quaternary ammonium salts, include tetramethylammonium, triethylmethylammonium, tetraethylammonium, etc. Quaternary amidine salts include ethyldimethylimidazolium, tetramethylimidazolium, etc. Amine salts include salts of primary, secondary, and tertiary amines. Primary amines include methylamine, ethylamine, propylamine, etc.; secondary amines include dimethylamine, diethylamine, ethylmethylamine, dibutylamine, etc.; and tertiary amines include trimethylamine, triethylamine, tributylamine, ethyldimethylamine, ethyldiisopropylamine, etc. The preferred cation species for the solute are ammonia, ammonium salts, or quaternary ammonium salts. If ammonium is present in the electrolyte, a lower ESR can be maintained for a longer period when using solid electrolytic capacitors in the high-frequency region, such as 100 kHz, compared to the case where no other cation species are present. There are no particular limitations on the solvent for the electrolyte; either protic or non-protic polar organic solvents can be used. Examples of protic organic 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 ethylene glycol, diethylene glycol, propylene glycol, glycerol, methyl cellosolve, ethyl cellosolve, methoxypropylene glycol, dimethoxypropanol, polyglycerol, polyethylene glycol or polyoxyethylene glycerol, polypropylene glycol, and other epoxide adducts of polyhydric alcohols. As a nonprotic organic polar solvent, sulfonium-based, amide-based, lactone-based, cyclic amide-based, nitrile-based, and sulfonium-based solvents can be used. Examples of sulfonium-based solvents include: dimethylsulfonium, ethylmethylsulfonium, diethylsulfonium, cyclobutanesulfonium, 3-methylcyclobutanesulfonium, and 2,4-dimethylcyclobutanesulfonium. Examples of amide-based solvents include: N-methylmethamide, N,N-dimethylmethamide, N-ethylmethamide, N,N-diethylmethamide, N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide, and hexamethylphosphatamide. Examples of lactones and cyclic amides include: γ-butyrolactone, γ-valerolactone, δ-valerolactone, N-methyl-2-pyrrolidone, ethyl acetate carbonate, propyl acetate carbonate, butyl acetate carbonate, and isobutyl acetate carbonate. Examples of nitrile compounds include: acetonitrile, 3-methoxypropionitrile, and glutaronitrile. Examples of urethane compounds include: dimethyl urethane. Preferably, the electrolyte solvent contains polyols, monools, or oxygen-containing alcohol compounds such as ethylene glycol or glycerol, or other types of solvents. These solvents readily undergo esterification reactions with aliphatic dicarboxylic acids. If the aliphatic dicarboxylic acid decomposes through esterification, the acidity of the electrolyte decreases, making it easier for dedoping to occur from the conductive polymer. However, adding a butylated phosphate compound to the electrolyte can suppress dedoping of the conductive polymer and maintain a low ESR. Additionally, the electrolyte solvent may also contain cyclobutane or other types of solvents. Other additives may also be added to the electrolyte. Examples of additives include: compounds of boric acid with polysaccharides (mannitol, sorbitol, etc.), compounds of boric acid with polyols, borate esters, and nitro compounds (o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, p-nitrophenol, p-nitrobenzyl alcohol, etc.). These can be used alone or in combination of two or more. (Solid Electrolyte Layer) In the solid electrolyte layer, the conductive polymer is either a self-doped conjugated polymer doped with intramolecular dopants or a conjugated polymer doped with external dopants. Conjugated polymers are obtained through chemical oxidative polymerization or electrolytic oxidative polymerization of monomers or their derivatives possessing π-conjugated double bonds. By doping conjugated polymers, conductive polymers exhibit high conductivity. That is, conductivity is achieved by adding small amounts of dopants, such as electron acceptors or electron donors, to the conjugated polymer. As conjugated polymers, known polymers can be used without particular limitation. Examples include: polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene vinylene, polyacene, and polythiophene vinylene. These conjugated polymers can be used alone, or in combination of two or more, and further, they can be copolymers of two or more monomers. Among the conjugated polymers, those polymerized from thiophene or its derivatives are preferred, particularly those polymerized from 3,4-epethyldioxothiophene (i.e., 2,3-dihydrothiophene[3,4-b][1,4]dioxin), 3-alkylthiophene, 3-alkoxythiophene, 3-alkyl-4-alkoxythiophene, 3,4-alkylthiophene, 3,4-alkoxythiophene, or their derivatives. As thiophene derivatives, compounds selected from thiophenes having substituents at the 3 and 4 positions are preferred, wherein the substituents at the 3 and 4 positions of the thiophene ring can form a ring together with the carbons at the 3 and 4 positions. The number of carbons in the alkyl or alkoxy group is preferably 1 to 16. Especially preferred are polymers of 3,4-epoxythiophene known as EDOT, i.e., poly(3,4-epoxythiophene) known as PEDOT. Alternatively, alkylated epoxythiophene with an alkyl group added to it can also be used, such as methylated epoxythiophene (i.e., 2-methyl-2,3-dihydro-thieno[3,4-b][1,4]dioxin) and ethylated epoxythiophene (i.e., 2-ethyl-2,3-dihydro-thieno[3,4-b][1,4]dioxin), etc. Dopant may be any known dopant without particular limitation. Dopant may be used alone or in combination of two or more. In addition, polymers or monomers may be used. Examples of dopant include: inorganic acids such as polyanionic acid, boric acid, nitric acid, and phosphoric acid; and organic acids such as acetic acid, oxalic acid, citric acid, tartaric acid, squaric acid, rhodizonic acid, croconic acid, salicylic acid, p-toluenesulfonic acid, 1,2-dihydroxy-3,5-benzenedisulfonic acid, methanesulfonic acid, trifluoromethanesulfonic acid, borodisalicylic acid, bis(oxalato)boronic acid, sulfonylidinylimine, dodecylbenzenesulfonic acid, propylnaphthalenesulfonic acid, and butylnaphthalenesulfonic acid. Polyanionic compounds include, for example, substituted or unsubstituted polyalkylene, substituted or unsubstituted polyolefin, substituted or unsubstituted polyimide, substituted or unsubstituted polyamide, and substituted or unsubstituted polyester. Examples include polymers containing only constituent units with anionic groups, polymers containing constituent units with anionic groups and those without anionic groups. Specifically, examples of polyanionic compounds include: polyvinyl sulfonic acid, polystyrene sulfonic acid, polyallyl sulfonic acid, polyacrylic acid sulfonic acid, polymethacrylic acid sulfonic acid, poly(2-acrylamide-2-methylpropanesulfonic acid), polyisoprene sulfonic acid, polyacrylic acid, polymethacrylic acid, and polymaleic acid. In addition to conductive polymers, the solid electrolyte layer may also contain various additives such as polyols. Examples of polyols include: sorbitol, ethylene glycol, diethylene glycol, triethylene glycol, polyoxyethylene glycol, glycerol, polyglycerol, polyoxyethylene glycerol, xylitol, erythritol, mannitol, dipentaerythritol, pentaerythritol, or combinations of two or more of these. Because polyols have high boiling points, they may remain in the solid electrolyte layer after the drying process, resulting in reduced ESR or improved voltage withstand capability. (Separator) Separators can be made from materials such as kraft paper, Manila hemp, esparto, hemp, rayon, and other cellulose and blended paper; polyester resins such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate, and their derivatives; polytetrafluoroethylene resins; polyvinylidene fluoride resins; vinylon resins; aliphatic polyamines; semi-aromatic polyamines; fully aromatic polyamines; polyimide resins; polyethylene resins; polypropylene resins; trimethylpentene resins; polyphenylene sulfide resins; acrylic resins; and polyvinyl alcohol resins. These resins can be used alone or in combination. [Example] The solid electrolytic capacitors of the embodiments are described in further detail below. However, the present invention is not limited to the embodiments described below. (Example 1) The solid electrolytic capacitor of Example 1 was fabricated. The anode foil was aluminum foil, which was enlarged by etching and formed with a dielectric oxide film by chemical conversion treatment at a chemical conversion voltage of 63.6 Vfs. The cathode foil was enlarged by etching and formed with an oxide film by chemical conversion treatment at a chemical conversion voltage of 3 Vfs. Leads were connected to each of the anode and cathode foils, and the anode and cathode foils were wound facing each other through a Manila steel separator. The wound body underwent a repair chemical conversion by immersion in an aqueous solution of ammonium dihydrogen phosphate for 15 minutes. Then, it was dried at 105°C. Next, the wound body is immersed in a dispersion of poly(ethylene thiophene / PSS) doped with polystyrene sulfonate as a conductive polymer. Ethylene glycol is added to the dispersion. After immersing the wound body in the dispersion, it is dried at 110°C for 30 minutes. This allows the conductive polymer to adhere to the wound body, and the ethylene glycol to be impregnated in a solid electrolyte layer containing the conductive polymer. Next, an electrolyte is prepared by immersing the electrolyte in a wound containing a solid electrolyte. Ethylene glycol is used as the solvent for the electrolyte. Azelaic acid and dibutyl phosphate are added to the electrolyte to a concentration of 16 mmol each per 100 g of electrolyte. Additionally, 16 mmol of ammonia is included as a cation per 100 g of electrolyte. The capacitor element, impregnated with electrolyte, is inserted into a bottomed cylindrical outer casing. A sealing rubber seal is installed at the open end of the outer casing, and a tight seal is achieved. Each solid electrolytic capacitor undergoes an aging process by applying voltage. The manufactured solid electrolytic capacitors have a rated withstand voltage of 35 WV and a rated capacitance of 270 μF. Corresponding to Example 1, solid electrolytic capacitors of Comparative Example 1 and Comparative Example 2 were fabricated. The solid electrolytic capacitor of Comparative Example 1 differs from that of Example 1 in that only dibutyl phosphate is added to the electrolyte instead of azelaic acid. Dibutyl phosphate is added so that its content is 32 mmol per 100 g of electrolyte. The solid electrolytic capacitor of Comparative Example 2 differs from that of Example 1 in that only azelaic acid is added to the electrolyte instead of dibutyl phosphate. Azelaic acid is added so that its content is 32 mmol per 100 g of electrolyte. The solid electrolytic capacitors of Comparative Example 1 and Comparative Example 2 are identical to those of Example 1 in all other respects, including the use of ammonia as a cation, other structures, manufacturing methods, and manufacturing conditions. (ESR 1) The ESR of the solid electrolytic capacitors of Example 1, Comparative Examples 1 and 2 was measured. The ESR measurement frequency was 100 kHz. To measure the ESR, the solid electrolytic capacitors were exposed to a temperature environment of 160°C. Then, the ESR was measured at a time when the time elapsed before exposure to this high temperature environment was zero hours, and at a time when the exposure to this high temperature environment had lasted for 1200 hours. The ratio of the ESR at the time of zero hours to the time after 1200 hours, i.e., ΔESR, was also calculated. ΔESR is the percentage of the ESR at the time after 1200 hours to the ESR at the time of zero hours. The results are shown in Table 1 below. (Table 1) As shown in Table 1, the ESR of the solid electrolytic capacitor of Example 1 showed little change even after being exposed to a high-temperature environment at 160°C for 1200 hours, and the value was the smallest. This confirms that adding azelaic acid, which has a relatively poor ESR, to dibutyl phosphate, which has a relatively good ESR, results in a better ESR compared to dibutyl phosphate. (Examples 2 to 4) Solid electrolytic capacitors of Examples 2 to 4 were fabricated. The solid electrolytic capacitors of Examples 2 to 4 differ from the solid electrolytic capacitor of Example 1 in the type of butyl-containing phosphate compound. Tributyl phosphate was used in Example 2, dibutyl phosphite was used in Example 3, and tributyl phosphite was used in Example 4. Compared to Example 1, the solid electrolytic capacitors of Examples 2 to 4 are identical in structure, manufacturing method, and manufacturing conditions, except for the type of butyl-containing phosphate compound. (ESR II) The ESR of the solid electrolytic capacitors of Examples 2 to 4 were measured under the same conditions as in Example 1. The results are shown in Table 2 below. (Table 2) As shown in Table 2, the ESR of the solid electrolytic capacitors in Examples 2 to 4 is similar to that in Example 1, showing little change and small values ​​even after exposure to a high-temperature environment at 160°C for 1200 hours. This confirms that, not limited to dibutyl phosphate, the ESR of solid electrolytic capacitors becomes good even when other types of butyl-containing phosphoric acid compounds and azelaic acid are added to the electrolyte. (Examples 5 to 7) Solid electrolytic capacitors of Examples 5 to 7 were fabricated. The solid electrolytic capacitors of Examples 5 to 7 differ from the solid electrolytic capacitor of Example 1 in the type of aliphatic dicarboxylic acid used. Succinic acid was used in Example 5, glutaric acid in Example 6, and citralic acid in Example 7. Compared to Example 1, the solid electrolytic capacitors of Examples 5 to 7 are identical in structure, manufacturing method, and manufacturing conditions, except for the different type of aliphatic dicarboxylic acid. Corresponding to Examples 5 to 7, solid electrolytic capacitors of Comparative Examples 3 to 5 were manufactured. The difference between the solid electrolytic capacitors of Comparative Examples 3 to 5 and those of Examples 5 to 7 is that a butyl-containing phosphoric acid compound was not added to the electrolyte. Compared with Examples 5 to 7, the solid electrolytic capacitors of Comparative Examples 3 to 5 are identical in structure, manufacturing method, and manufacturing conditions, except for the presence or absence of a butyl-containing phosphoric acid compound. (ESR III) The ESR of the solid electrolytic capacitors of Examples 5 to 7 and Comparative Examples 3 to 5 were measured under the same conditions as in Example 1. The results are shown in Table 3 below. (Table 3) As shown in Table 3, the ESR of the solid electrolytic capacitors in Examples 5 to 7 showed little change and smaller values ​​compared to the corresponding Comparative Examples 3 to 5, even after exposure to a high-temperature environment at 160°C for 1200 hours. This confirms that the ESR of the solid electrolytic capacitors is good, not limited to azelaic acid, even when other types of aliphatic dicarboxylic acids and dibutyl phosphate are added to the electrolyte. In summary, based on the results of Examples 1 to 7, it is confirmed that the solid electrolytic capacitors exhibit good ESR due to the presence of various butylated phosphate compounds and various aliphatic dicarboxylic acids in the electrolyte. Furthermore, as shown in Table 3, the ESR of Comparative Examples 3 and 4 deteriorated compared to Comparative Example 1. However, the ESR of Examples 5 and 6, corresponding to Comparative Examples 3 and 4, improved compared to Comparative Example 1. Therefore, considering the results of Examples 1 and Comparative Example 1 as well, it was confirmed that when selecting aliphatic dicarboxylic acids from the group consisting of azelaic acid, succinic acid, or glutaric acid, the addition of an aliphatic dicarboxylic acid with relatively poor ESR resulted in a better ESR compared to a butylated phosphoric acid compound with relatively good ESR. Moreover, the ESR of Examples 1, 5, and 6 was significantly reduced compared to Comparative Examples 2, 3, and 4, and also improved compared to Example 7. Furthermore, in Example 7, the ESR was improved compared to Comparative Examples 1 and 5. Therefore, when citralic acid was chosen as the aliphatic dicarboxylic acid, the addition of a butyl-containing phosphoric acid compound with a relatively poor ESR resulted in a better ESR compared to citralic acid with a relatively good ESR. That is, it was confirmed that when an aliphatic dicarboxylic acid and a butyl-containing phosphoric acid compound were added to the electrolyte, the ESR of the solid electrolytic capacitor improved compared to a relatively good ESR. (Leakage Current Measurement) The leakage current (LC) of the solid electrolytic capacitors of Comparative Example 1, Examples 1, and Examples 5 to 7 was measured. Each solid electrolytic capacitor was placed at a temperature of 160°C for 2000 hours, and the leakage current after placement was measured. The leakage current was defined as the current flowing through each solid electrolytic capacitor when its rated withstand voltage of 35 WV was applied and maintained for 2 minutes. The LC (leakage current) measurement results of the solid electrolytic capacitors of Comparative Example 1, Examples 1, and Examples 5 to 7 are shown in Table 4 below. (Table 4) As shown in Table 4, the solid electrolytic capacitors of Examples 1 and 5 to 7 exhibited significantly reduced leakage current compared to the solid electrolytic capacitor of Comparative Example 1. This confirms that adding a butylated phosphoric acid compound and an aliphatic dicarboxylic acid to the electrolyte reduces both the ESR and the leakage current. (Examples 8 and 9) As shown in Table 5 below, the ESR and leakage current of various solid electrolytic capacitors with different amounts of dibutyl phosphate were measured. Furthermore, in Table 5, Examples 8, 9, and Comparative Example 6 are identical to Example 1 in all aspects except for the amount of dibutyl phosphate added; all other aspects include the same structure, manufacturing method, and manufacturing conditions. Additionally, as shown in Table 5, the ESR was measured at zero hours before exposure to a temperature of 160°C and after 1200 hours of exposure to that high temperature. The leakage current was measured after 2000 hours of exposure to that high temperature. (Table 5) As shown in Comparative Examples 6 and 2 in Table 5, it can be seen that when 48 mmol of dibutyl phosphate with a relatively good ESR is added relative to 100 g of electrolyte, the ESR deteriorates after 1200 hours compared to azelaic acid with a relatively poor ESR. Furthermore, as shown in Comparative Examples 6 and 1, it can be seen that when 48 mmol of dibutyl phosphate is added relative to 100 g of electrolyte, the leakage current also deteriorates significantly compared to the case where only dibutyl phosphate is present in the electrolyte. In contrast, as shown in Example 9 and Comparative Example 1, it was found that even with the presence of azelaic acid, which has a relatively deteriorating ESR, if the amount of dibutyl phosphate is 32 mmol or less relative to 100 g of electrolyte, the ESR becomes good after 1200 hours compared to the case where only dibutyl phosphate is contained in the electrolyte. That is, it was confirmed that if the electrolyte contains an aliphatic dicarboxylic acid and a butyl-containing phosphoric acid compound, and the amount of the butyl-containing phosphoric acid compound added is 32 mmol or less relative to 100 g of electrolyte, the ESR becomes good. Furthermore, as shown in Table 5, it was confirmed that reducing the amount of butylated phosphate compounds with relatively good ESR and increasing the ratio of aliphatic dicarboxylic acids with relatively poor ESR actually lowers the ESR of the solid electrolytic capacitor. Additionally, as shown in Example 8, it was confirmed that mixing butylated phosphate compounds with relatively poor leakage current at a ratio of 8 mmol or less relative to 100 g of electrolyte resulted in better leakage current compared to the case where only aliphatic dicarboxylic acids were present in the electrolyte. (Examples 10 to 12) Solid electrolytic capacitors of Examples 10 to 12 were manufactured. Furthermore, corresponding to Examples 10 to 12, solid electrolytic capacitors of Comparative Examples 7 and 8 were also manufactured. The amount of aliphatic dicarboxylic acid added differs between the solid electrolytic capacitors of Examples 10 to 12, Comparative Examples 7 and 8 and the solid electrolytic capacitor of Example 1. All other structures, manufacturing methods, and manufacturing conditions of the solid electrolytic capacitors of Examples 10 to 12, Comparative Examples 7 and 8 are the same as those of Example 1. Then, the ESR and leakage current of the solid electrolytic capacitors of Examples 1, 10 to 12, and Comparative Examples 7 and 8 were measured. As shown in Table 6 below, the ESR was measured at zero hours before exposure to a temperature environment of 150°C and at 2700 hours after exposure to that high temperature environment. The leakage current was measured at 2700 hours after exposure to that high temperature environment. (Table 6) The butyl-containing phosphoric acid compound is dibutyl phosphate, and the aliphatic dicarboxylic acid is azelaic acid. As shown in Table 6, in Example 1, the electrolyte contained equal molar amounts of the phosphoric acid compound and azelaic acid. In Example 10, if the phosphoric acid compound was set to 1 in the molar ratio, the electrolyte contained azelaic acid at a ratio of 0.75. In Example 11, if the phosphoric acid compound was set to 1 in the molar ratio, the electrolyte contained azelaic acid at a ratio of 0.5. In Example 12, if the phosphoric acid compound was set to 1 in the molar ratio, the electrolyte contained azelaic acid at a ratio of 0.25. In Comparative Example 7, the electrolyte did not contain phosphoric acid compounds. In Comparative Example 8, the electrolyte did not contain aliphatic dicarboxylic acids. If the phosphoric acid compound was set to 1 in the molar ratio, then the azelaic acid content in the electrolyte was 0. As shown in Table 6, it was confirmed that the solid electrolytic capacitors of Examples 1, 10, 11, and 12 exhibited good ESR regardless of the amount of aliphatic dicarboxylic acid added. In particular, Table 6 confirmed that if the phosphate compound was set to 1 in the molar ratio, the presence of 0.25 or more aliphatic dicarboxylic acid in the electrolyte could suppress ESR degradation and reduce leakage current (LC) even when exposed to high temperatures. (Example 13) A solid electrolytic capacitor of Example 13 was fabricated. The solid electrolytic capacitor of Example 13 differs from the solid electrolytic capacitor of Example 1 in that triethylamine was added as a cationic component. All other structures, manufacturing methods, and manufacturing conditions of the solid electrolytic capacitor of Example 13 are the same as those of Example 1. The ESR of the solid electrolytic capacitors of Examples 1 and 13 was measured. As shown in Table 7 below, the ESR was measured at zero hours before exposure to a temperature environment of 150°C and at 260 hours after exposure to the high temperature environment. (Table 7) As shown in Table 7, the ESR of solid electrolytic capacitors is good regardless of the type of cation in the electrolyte. In particular, if the cation is ammonia, it is confirmed that the ESR degradation at high frequencies can be suppressed even when exposed to high temperature environments. (Examples 14 to 20) Solid electrolytic capacitors of Examples 14 to 20 were manufactured. Additionally, a solid electrolytic capacitor of Comparative Example 9 was manufactured. The solid electrolytic capacitors of Examples 14 to 20 and Comparative Example 9 differ from the solid electrolytic capacitor of Example 13 in the type of solvent used in the electrolyte. All other structures, manufacturing methods, and manufacturing conditions of the solid electrolytic capacitors of Examples 14 to 20 and Comparative Example 9 are the same as those of Example 13. Then, the ESR of the solid electrolytic capacitors of Examples 13 to 20 and Comparative Example 9 was measured. As shown in Table 8 below, the ESR was measured at zero hours before exposure to a temperature environment of 150°C and at 260 hours after exposure to that high temperature environment. (Table 8) As shown in Table 8, the solvent in Example 13 was ethylene glycol. The solvent in Example 14 was a mixture of ethylene glycol and glycerol. The solvent in Example 15 was the same as that in Example 14, a mixture of ethylene glycol and glycerol, but the mixing ratios in Examples 14 and 15 were different. In Example 14, ethylene glycol accounted for 90 wt% and glycerol accounted for 10 wt% of the solvent. In Example 15, ethylene glycol accounted for 40 wt% and glycerol accounted for 60 wt% of the solvent. The solvent in Example 16 was glycerol. The solvent in Example 17 was cyclobutane. The solvent in Example 18 was a mixture of glycerol and polyethylene glycol with an average molecular weight of approximately 300. The solvent in Example 19 was the same as in Example 18, a mixture of glycerol and polyethylene glycol with an average molecular weight of approximately 300. In Example 18, glycerol accounted for 70 wt% and polyethylene glycol accounted for 30 wt% of the solvent. In Example 18, glycerol and polyethylene glycol were in equal weight proportions in the solvent. The solvent in Example 20 was γ-butyrolactone. The solvent in Comparative Example 9 was γ-butyrolactone, but it did not contain a phosphate compound having a butyl group. As shown in Table 8, Examples 16 to 19 showed better ESR after exposure to high temperatures compared to Example 20. Examples 13 to 15 showed better ESR at a measurement frequency of 100 kHz compared to Examples 16 to 19, and their ESR was even better after exposure to high temperatures. Examples 13 to 15 are solid electrolytic capacitors in which ethylene glycol is contained in the solvent. Examples 16 to 19 are solid electrolytic capacitors in which glycerol or cyclobutane is contained in the solvent. It was thus confirmed that a low ESR can be maintained when glycerol or cyclobutane is contained in the electrolyte. Furthermore, it was confirmed that an even lower ESR can be maintained when ethylene glycol is contained in the electrolyte. none none

Claims

1. A solid electrolytic capacitor, characterized in that it comprises: An anode foil comprising a valve-acting metal and having a dielectric oxide film formed thereon; a cathode foil comprising a valve-acting metal and facing the anode foil; The electrolyte layer comprises a conductive polymer and an electrolyte, and is separated between the anode foil and the cathode foil. The electrolyte comprises a butylated phosphoric acid compound and an aliphatic dicarboxylic acid, wherein the phosphoric acid compound is 32 mmol or less per 100 g of the electrolyte, and when the phosphoric acid compound is set to 1 in the molality, the electrolyte contains 0.25 to 0.75 of the aliphatic dicarboxylic acid.

2. The solid electrolytic capacitor as claimed in claim 1, wherein, The phosphoric acid compound is less than 8 mmol per 100 g of the electrolyte.

3. The solid electrolytic capacitor as described in claim 1 or claim 2, wherein, The phosphoric acid compound is one or a mixture of two or more selected from the group consisting of dibutyl phosphate, tributyl phosphate, dibutyl phosphite, and tributyl phosphite.

4. The solid electrolytic capacitor as described in claim 1 or claim 2, wherein, The aliphatic dicarboxylic acid is selected from one or more of the group consisting of azelaic acid, succinic acid, glutaric acid, and citralic acid.

5. The solid electrolytic capacitor as claimed in claim 1 or claim 2, wherein, The aliphatic dicarboxylic acid is one or a mixture of two or more selected from the group consisting of azelaic acid, succinic acid, and glutaric acid.

6. The solid electrolytic capacitor as claimed in claim 1 or claim 2, wherein, The electrolyte contains one or more of the group consisting of ethylene glycol, glycerol, and cyclobutane.

7. The solid electrolytic capacitor as claimed in claim 1 or claim 2, wherein, The electrolyte contains ammonia.