Solid electrolytic capacitor and manufacturing method

Abstract

Provided are a solid electrolytic capacitor that exhibits low equivalent series resistance even at high frequencies, and a manufacturing method. The solid electrolytic capacitor comprises a positive electrode foil, a negative electrode body, and an electrolyte layer. The positive electrode foil is composed of a valve metal, and a dielectric oxide film is formed on the surface thereof. The negative electrode body comprises: a negative electrode foil composed of a valve metal; and an electrically conductive layer formed on the surface of the negative electrode foil. The electrolyte layer is interposed between the positive electrode foil and the negative electrode foil, and contains an electrolyte solution and an electrically conductive polymer. The electrolyte solution contains a phosphoric acid compound having a C1-10 alkyl group.

Description

Solid electrolytic capacitor and manufacturing method

[0001] The present invention relates to a solid electrolytic capacitor in which an electrolyte layer contains an electrolytic solution and a conductive polymer, and a manufacturing method thereof.

[0002] Electrolytic capacitors use valve metals such as tantalum or aluminum as anode and cathode foils. The anode foil is enlarged by shaping the valve metal into a sintered or etched foil, and the enlarged surface is coated with a dielectric oxide film. Electrolytic capacitors can be considered series capacitors, with capacitances on both the anode and cathode sides. Therefore, the cathode capacitance is also very important for efficient use of the anode capacitance.

[0003] To efficiently utilize the anode capacitance, the cathode foil is also etched to increase its surface area. However, there is a limit to how much the cathode foil can be enlarged due to its thickness. Therefore, electrolytic capacitors have been proposed in which a metal nitride film, such as titanium nitride, is formed on the cathode foil. Titanium is evaporated in a nitrogen gas environment using vacuum arc deposition, a type of ion plating, to deposit titanium nitride on the surface of the cathode foil. Because metal nitrides are inert, a natural oxide film is difficult to form, and the cathode capacitance theoretically approaches infinity. Furthermore, the vapor-deposited film forms fine irregularities, expanding the cathode's surface area.

[0004] An electrolyte is interposed between the anode foil and the cathode foil. The electrolyte is in close contact with the uneven surface of the anode foil and functions as the true cathode. The electrolyte uses, for example, ethylene glycol or γ-butyrolactone as a solvent and contains a carboxylic acid such as 1,6-decanedicarboxylic acid, 1,7-octanedicarboxylic acid, or azelaic acid, or its salt, as a solute. Over time, the electrolyte evaporates and escapes from the electrolytic capacitor. As a result, the electrolytic capacitor's capacitance decreases over time as it dries out, and the tangent of the loss angle (tan δ) increases over time, eventually reaching the end of its life.

[0005] Therefore, electrolytic capacitors in which a conductive polymer is interposed between the anode and cathode foils instead of an electrolyte are widely used. The conductive polymer is derived from a monomer with a π-conjugated double bond and is doped with an external dopant molecule. An example of this conductive polymer is poly(3,4-ethylenedioxythiophene) (PEDOT). An example of the dopant is polystyrene sulfonic acid.

[0006] However, electrolytic capacitors with solid electrolytes are less effective at repairing defects in the dielectric oxide film than electrolytic capacitors with liquid electrolytes. Therefore, so-called hybrid electrolytic capacitors, in which a solid electrolyte is interposed between the anode foil and the cathode foil and the foil is impregnated with liquid electrolyte, have also attracted attention.

[0007] JP 2000-114108 A JP 2008-10657 A

[0008] With the digitalization of electronic devices, there is a growing demand for capacitors that are small, have large capacitance, and have low ESR (equivalent series resistance). Solid electrolytic capacitors have a large capacitance compared to film capacitors and ceramic capacitors, and because conductive polymers have high conductivity, they have good ESR. For this reason, solid electrolytic capacitors are increasingly being used for high-frequency smoothing applications, for example. In recent years, digital devices have begun to operate in the high-frequency range of several tens of kHz, and solid electrolytic capacitors are also required to have low ESR even in the high-frequency range.

[0009] The present invention has been proposed to solve the above problems, and its object is to provide a solid electrolytic capacitor that exhibits low ESR even in the high frequency range, and a method for manufacturing the same.

[0010] In order to solve the above-described problems, the solid electrolytic capacitor of the present embodiment includes an anode foil made of a valve metal and having a dielectric oxide film formed on a surface thereof, a cathode body facing the anode foil, and an electrolyte layer interposed between the anode foil and the cathode body and containing an electrolyte solution and a conductive polymer, wherein the cathode body has a cathode foil made of a valve metal and a conductive layer formed on the surface of the cathode foil, and the electrolyte solution contains a phosphate compound having an alkyl group having 1 to 10 carbon atoms.

[0011] 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.

[0012] The conductive layer may include carbon material, titanium, titanium nitride, titanium carbide, and composites or mixtures thereof.

[0013] The amount of the phosphate compound may be 4 mmol or more per 100 g of the electrolyte solution.

[0014] The amount of the phosphate compound may be 4 mmol or more and 16 mmol or less per 100 g of the electrolyte solution.

[0015] The electrolytic solution may contain one or more selected from the group consisting of ethylene glycol, glycerin, and sulfolane.

[0016] The cathode foil may have a surface enlarging layer on a surface of the cathode foil, and the conductive layer may be formed on the surface enlarging layer.

[0017] Furthermore, in order to solve the above-described problems, a method for manufacturing a solid electrolytic capacitor according to the present embodiment is a method for manufacturing a solid electrolytic capacitor including an anode foil, a cathode body, and an electrolyte layer, and includes the steps of: forming a conductive layer on a surface of a cathode foil containing a valve action metal to fabricate the cathode body; preparing an electrolyte solution containing a phosphate compound having an alkyl group having 1 to 10 carbon atoms; and forming an electrolyte layer by interposing the electrolyte solution and a conductive polymer between the anode foil and the cathode body.

[0018] According to the present invention, the solid electrolytic capacitor has a low ESR at least in the high frequency range.

[0019] 1 is a graph showing ESR at each elapsed time for Example 1 and Comparative Examples 1, 2, and 3. 2 is a graph showing ESR at each elapsed time for Example 2 and Comparative Examples 1, 2, and 4.

[0020] Hereinafter, a solid electrolytic capacitor according to an embodiment will be described, but the present invention is not limited to the embodiment described below.

[0021] (Solid Electrolytic Capacitor) A solid electrolytic capacitor is a passive element that obtains capacitance through the dielectric polarization of a dielectric oxide film and stores and discharges electric charge. This solid electrolytic capacitor is constructed by housing a capacitor element in a case and sealing the case opening with a sealing body. The capacitor element includes an anode foil, a cathode foil, a separator, and an electrolyte layer. The anode foil and the cathode body face each other via the separator and are wound or laminated. A dielectric oxide film is formed on the surface of the anode foil. The electrolyte layer consists of a solid electrolyte layer containing a conductive polymer and an electrolyte solution. The solid electrolyte layer is interposed between the anode foil and the cathode foil and is in close contact with the dielectric oxide film. The electrolyte solution is impregnated into the voids of the capacitor element on which the solid electrolyte layer is formed.

[0022] (Anode foil) The anode foil is a long foil formed by stretching a valve metal. Valve metals include aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony. The purity of the anode foil is preferably 99.9% or higher, but impurities such as silicon, iron, copper, magnesium, and zinc may be contained.

[0023] The anode foil has an enlarged surface, and can be prepared as a compact formed from valve metal powder, a sintered compact, or an etched foil formed by etching rolled foil. The enlarged surface structure is composed of tunnel-like pits, spongy pits, or voids between densely packed powder particles. The enlarged surface structure is typically formed by direct current etching or alternating current etching in an acidic aqueous solution containing halogen ions, such as hydrochloric acid, or by vapor deposition or sintering of metal particles or the like into the core. The etching pits may be formed so as to penetrate the anode foil.

[0024] The dielectric oxide film is typically an oxide film formed on the surface of an anode foil. For example, if the anode foil is an aluminum foil, the dielectric oxide film is aluminum oxide obtained by oxidizing the surface-expanding structure. The dielectric oxide film is formed by chemical conversion treatment in which a voltage is applied in an aqueous solution of adipic acid, boric acid, phosphoric acid, or the like.

[0025] (Cathode body) The cathode body includes a cathode foil, which is a foil body made of an elongated valve metal. The purity of the cathode foil is preferably 99% or more, but it may contain impurities such as silicon, iron, copper, magnesium, and zinc. The cathode foil is a plain foil with a flat surface, or has a surface-expanding layer formed on its surface by surface-expanding. An oxide film may be formed intentionally or naturally on the surface-expanding layer. A thin dielectric oxide film (approximately 1 to 10 Vfs) may be intentionally formed by chemical conversion treatment. The natural oxide film is formed when the cathode foil reacts with oxygen in the air.

[0026] This cathode body further includes a conductive layer and has a laminated structure of a cathode foil and the conductive layer. The conductive layer contains a conductive material and is a layer with higher conductivity than an oxide film. This conductive layer is laminated on one or both sides of the cathode foil and is located as the outermost layer of the cathode body. Examples of conductive materials include titanium, zirconium, tantalum, niobium, nitrides or carbides of these, aluminum carbide, carbon materials, and composites or mixtures of these. This conductive layer may be a laminate of multiple layers, and each layer may be of a different type.

[0027] The carbon material may be fibrous carbon, carbon powder, or a mixture thereof. It may also be fibrous carbon or carbon powder that has been subjected to a porous treatment, such as an activation treatment or an opening treatment to form pores. Examples of carbon powder include activated carbon derived from natural plant tissues such as coconut husks, synthetic resins such as phenols, and fossil fuels such as coal, coke, and pitch; carbon blacks such as ketjen black, acetylene black, and channel black; carbon nanohorns; amorphous carbon; natural graphite; artificial graphite; graphitized ketjen black; and mesoporous carbon. Examples of fibrous carbon include carbon nanotubes and carbon nanofibers. Carbon nanotubes may be single-walled carbon nanotubes, which have a single graphene sheet, or multi-walled carbon nanotubes (MWCNTs), which have two or more graphene sheets coaxially rolled together to form multiple tube walls.

[0028] These conductive materials are attached to the cathode foil by coating, vapor deposition, heat treatment, or the like. Coating methods are suitable, for example, for forming a conductive layer made of a carbon material. A slurry containing a conductive material, a binder, and a solvent is applied to the cathode body by a slurry casting method, a doctor blade method, a spray atomization method, or the like, and dried, and the cathode foil and the conductive layer are adhered to each other by pressing, if necessary. Vapor deposition methods are suitable, for example, for forming a metallic conductive layer made of titanium, and include vacuum arc deposition, sputter deposition, and electron beam deposition. Heat treatment involves attaching a powder of a conductive material to the surface of the cathode foil and sintering it.

[0029] In vacuum arc evaporation, a voltage is applied to a material source in a vacuum chamber to melt and vaporize it, then the vaporized material source reacts with a reactive gas, and the material source reacted with the reactive gas forms a film on a cathode foil. In sputter evaporation, a plasma is generated in an environment filled with a reactive gas and a target is placed, and the material source is ejected from the target while reacting with the reactive gas, and the material source reacted with the reactive gas forms a film on a cathode foil. In electron beam evaporation, an electron beam is irradiated onto a material source in a vacuum chamber to melt and vaporize it, then the evaporated material source reacts with the reactive gas, and the material source reacted with the reactive gas forms a film on a cathode foil.

[0030] After laminating the conductive layer and the cathode foil, they are preferably pressed together by press working. In press working, for example, the cathode body consisting of the conductive layer and the cathode foil is sandwiched between press rollers and a linear press pressure is applied. The press pressure is preferably about 0.01 to 100 t / cm. This press working creates a pressed structure in which the conductive material is forced into the pores of the surface-expanding layer, and also creates a pressed structure in which the conductive material is deformed along the uneven surface of the surface-expanding layer. This pressed structure improves the adhesion and fixation between the conductive layer and the cathode foil, and reduces the ESR of the solid electrolytic capacitor.

[0031] (Solid electrolyte layer) The conductive polymer of the solid electrolyte layer is a self-doped type doped with intramolecular dopant molecules or a conjugated polymer doped with external dopant molecules. Conjugated polymers are obtained by chemical oxidative polymerization or electrolytic oxidative polymerization of monomers or their derivatives having π-conjugated double bonds. By performing a doping reaction on a conjugated polymer, the conductive polymer exhibits high conductivity. In other words, conductivity is exhibited by adding a small amount of a dopant, such as an acceptor that easily accepts electrons or a donor that easily donates electrons, to a conjugated polymer.

[0032] As the conjugated polymer, any known polymer can be used without any particular limitation. Examples include polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene, polyphenylene vinylene, polyacene, polythiophene vinylene, etc. These conjugated polymers may be used alone or in combination of two or more types, or may even be a copolymer of two or more types of monomers.

[0033] Among the above conjugated polymers, preferred are conjugated polymers obtained by polymerizing thiophene or a derivative thereof, and preferred are conjugated polymers obtained by polymerizing 3,4-ethylenedioxythiophene (i.e., 2,3-dihydrothieno[3,4-b][1,4]dioxin), 3-alkylthiophene, 3-alkoxythiophene, 3-alkyl-4-alkoxythiophene, 3,4-alkylthiophene, 3,4-alkoxythiophene, or derivatives thereof. The thiophene derivative is preferably a compound selected from thiophenes having substituents at the 3rd and 4th positions, and the substituents at the 3rd and 4th positions of the thiophene ring may form a ring together with the carbon atoms at the 3rd and 4th positions. The alkyl group or alkoxy group preferably has 1 to 16 carbon atoms.

[0034] In particular, a polymer of 3,4-ethylenedioxythiophene known as EDOT, i.e., poly(3,4-ethylenedioxythiophene) known as PEDOT, is particularly preferred. Furthermore, alkylated ethylenedioxythiophenes in which an alkyl group is added to 3,4-ethylenedioxythiophene may also be used, such as methylated ethylenedioxythiophene (i.e., 2-methyl-2,3-dihydro-thieno[3,4-b][1,4]dioxin) and ethylated ethylenedioxythiophene (i.e., 2-ethyl-2,3-dihydro-thieno[3,4-b][1,4]dioxin).

[0035] Any known dopant can be used without any particular limitation. A single dopant may be used, or two or more dopants may be used in combination. Furthermore, a polymer or a monomer may be used. Examples of dopants include inorganic acids such as polyanions, 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, bisoxalateborate acid, sulfonylimide acid, dodecylbenzenesulfonic acid, propylnaphthalenesulfonic acid, and butylnaphthalenesulfonic acid.

[0036] Examples of the polyanion include a substituted or unsubstituted polyalkylene, a substituted or unsubstituted polyalkenylene, a substituted or unsubstituted polyimide, a substituted or unsubstituted polyamide, and a substituted or unsubstituted polyester, and include a polymer consisting only of a structural unit having an anionic group, and a polymer consisting of a structural unit having an anionic group and a structural unit not having an anionic group. Specific examples of the polyanion include polyvinyl sulfonic acid, polystyrene sulfonic acid, polyallyl sulfonic acid, polyacryl sulfonic acid, polymethacrylic acid, poly(2-acrylamido-2-methylpropane sulfonic acid), polyisoprene sulfonic acid, polyacrylic acid, polymethacrylic acid, and polymaleic acid.

[0037] In addition to the conductive polymer, the solid electrolyte layer may contain various additives such as polyhydric alcohols. Examples of polyhydric alcohols include sorbitol, ethylene glycol, diethylene glycol, triethylene glycol, polyoxyethylene glycol, glycerin, polyglycerin, polyoxyethyleneglycerin, xylitol, erythritol, mannitol, dipentaerythritol, pentaerythritol, and combinations of two or more thereof. Polyhydric alcohols have high boiling points and can remain in the solid electrolyte layer even after the drying process, resulting in reduced ESR and improved voltage resistance.

[0038] (Electrolyte) The electrolyte is a solution in which an anionic component and a cationic component are added to a solvent. The anionic component and the cationic component are typically salts of organic acids, salts of inorganic acids, or salts of complex compounds of organic acids and inorganic acids, and are added to the solvent in the form of an ion-dissociating salt that dissociates into the anionic component and the cationic component. An acid that becomes the anionic component and a base that becomes the cationic component may be added separately to the solvent. Furthermore, the electrolyte does not necessarily need to contain either the anionic component or the cationic component, or both the anionic component and the cationic component, in the solvent.

[0039] The electrolyte solution contains a phosphoric acid compound having an alkyl group. The number of carbon atoms in the alkyl group is preferably 1 or more and 10 or less. As the linear chain of the alkyl group becomes longer, it becomes less soluble in the solvent of the electrolyte solution, but it becomes less susceptible to hydrolysis, and chemical stability is improved. Within this range, the balance between the solubility of the phosphoric acid compound having an alkyl group in the solvent of the electrolyte solution and chemical stability is good, and if the alkyl group is a butyl group, the balance between the solubility in the solvent of the electrolyte solution and chemical stability is particularly good. It is sufficient for the phosphoric acid compound to have at least one such alkyl group.

[0040] Such phosphate compounds include dibutyl phosphate, tributyl phosphate, dibutyl and tributyl phosphite, triethyl phosphite, trimethyl phosphite, triisopropyl phosphate, and diisopropyl phosphite.

[0041] The electrolyte may contain one or more of these phosphate compounds. When the cathode body has a conductive layer and the electrolyte contains such a phosphate compound, the solid electrolytic capacitor exhibits good ESR in the high frequency range of 100 kHz, for example, even when exposed to a high temperature environment of 160°C, for example. Furthermore, when the cathode body has a conductive layer and the electrolyte contains such a phosphate compound, the leakage current (LC) is well maintained even when exposed to a high temperature environment for a long period of time.

[0042] Preferably, the amount of the phosphate compound is 4 mmol or more per 100 g of electrolyte, and more preferably 4 mmol or more and 16 mmol or less per 100 g of electrolyte. When the amount is 4 mmol or more, the solid electrolytic capacitor exhibits good ESR in the high frequency range such as 100 kHz even when exposed to a high temperature environment. Furthermore, when the amount is 16 mmol or more per 100 g of electrolyte, the change in ESR in the high temperature environment and in the high frequency range such as 100 kHz is small. Therefore, in terms of other capacitor properties and cost, it is more preferable that the amount of the phosphate compound is 16 mmol or less per 100 g of electrolyte.

[0043] As long as the phosphate compound is contained, the electrolyte solution may contain, as an anion component, an organic acid, an inorganic acid, or a composite compound of an organic acid and an inorganic acid. Examples of the organic acid include carboxylic acids such as oxalic acid, succinic acid, glutaric acid, pimelic acid, suberic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, maleic acid, adipic acid, benzoic acid, toluic acid, enanthic acid, malonic acid, 1,6-decanedicarboxylic acid, 1,7-octanedioic acid, azelaic acid, undecanedioic acid, dodecanedioic acid, tridecanedioic acid, t-butyl adipic acid, 11-vinyl-8-octadecenedioic acid, resorcylic acid, phloroglucinic acid, gallic acid, gentisic acid, protocatechuic acid, pyrocatechuic acid, trimellitic acid, and pyromellitic acid, as well as phenols and sulfonic acids. Furthermore, examples of inorganic acids include boric acid, phosphoric acid, phosphorous acid, hypophosphorous acid, carbonic acid, silicic acid, etc. Examples of composite compounds of organic acids and inorganic acids include borodisalicylic acid, borodioxalic acid, borodiglycolic acid, borodimalonic acid, borodisuccinic acid, borodiadipic acid, borodiazelaic acid, borodibenzoic acid, borodimaleic acid, borodilactic acid, borodimalic acid, boroditartaric acid, borodicitric acid, borodiphthalic acid, borodi(2-hydroxy)isobutyric acid, borodiresorcylic acid, borodimethylsalicylic acid, borodinaphthoic acid, borodimandelic acid, and borodi(3-hydroxy)propionic acid, etc.

[0044] Furthermore, examples of at least one salt of an organic acid, an inorganic acid, or a complex compound of an organic acid and an inorganic acid include ammonium salts, quaternary ammonium salts, quaternized amidinium salts, amine salts, sodium salts, and potassium salts. Examples of quaternary ammonium ions of quaternary ammonium salts include tetramethylammonium, triethylmethylammonium, and tetraethylammonium. Examples of quaternized amidinium salts include ethyldimethylimidazolinium and tetramethylimidazolinium. Examples of amine salts include salts of primary amines, secondary amines, and tertiary amines. Examples of primary amines include methylamine, ethylamine, and propylamine. Examples of secondary amines include dimethylamine, diethylamine, ethylmethylamine, and dibutylamine. Examples of tertiary amines include trimethylamine, triethylamine, tributylamine, ethyldimethylamine, and ethyldiisopropylamine.

[0045] The solvent for the electrolyte is not particularly limited, and protic organic polar solvents or aprotic organic polar solvents can be used. Examples of protic organic solvents include monohydric alcohols, polyhydric alcohols, and oxyalcohol compounds. Examples of monohydric alcohols include ethanol, propanol, butanol, pentanol, hexanol, cyclobutanol, cyclopentanol, cyclohexanol, and benzyl alcohol. Examples of polyhydric alcohols and oxyalcohol compounds include ethylene glycol, diethylene glycol, propylene glycol, glycerin, methyl cellosolve, ethyl cellosolve, methoxypropylene glycol, dimethoxypropanol, polyglycerin, and alkylene oxide adducts of polyhydric alcohols such as polyethylene glycol and polyoxyethylene glycerin.

[0046] Examples of aprotic organic polar solvents that may be used include sulfones, amides, lactones, cyclic amides, nitriles, and sulfoxides. Examples of sulfones include dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methyl sulfolane, and 2,4-dimethyl sulfolane. Examples of amides include N-methylformamide, N,N-dimethylformamide, N-ethylformamide, N,N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide, and hexamethylphosphoric amide. Examples of lactones and cyclic amides include γ-butyrolactone, γ-valerolactone, δ-valerolactone, N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate, butylene carbonate, and isobutylene carbonate. Examples of nitriles include acetonitrile, 3-methoxypropionitrile, glutaronitrile, etc. Examples of sulfoxides include dimethyl sulfoxide, etc.

[0047] Preferably, the electrolyte contains ethylene glycol, glycerin, or sulfolane as a solvent or other species in the solvent. Ethylene glycol, glycerin, and sulfolane cause a change in the higher-order structure of the conductive polymer. This improves the initial ESR of the solid electrolytic capacitor and also suppresses deterioration of the ESR under high-temperature conditions.

[0048] Furthermore, other additives can also be added to the electrolyte solution. Examples of additives include complex compounds of boric acid and polysaccharides (mannite, sorbite, etc.), complex compounds of boric acid and polyhydric alcohols, boric acid esters, and nitro compounds (o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, p-nitrophenol, p-nitrobenzyl alcohol, etc.). These may be used alone or in combination of two or more.

[0049] (Separator) Examples of separators include cellulose papers such as kraft, Manila hemp, esparto, hemp, and rayon, and mixtures thereof; polyester-based resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and derivatives thereof; polytetrafluoroethylene-based resins, polyvinylidene fluoride-based resins, vinylon-based resins; polyamide-based resins such as aliphatic polyamides, semi-aromatic polyamides, and wholly aromatic polyamides; polyimide-based 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.

[0050] The solid electrolytic capacitors of the examples will be described in more detail below, but the present invention is not limited to the examples described below.

[0051] (Examples 1 and 2) Solid electrolytic capacitors of Comparative Examples 1 to 4 and Examples 1 and 2 were fabricated. First, anode and cathode foils were fabricated using aluminum foil. The anode foils were etched to enlarge their surface area, and then a dielectric oxide film was formed by chemical conversion using an adipic acid aqueous solution at a chemical conversion voltage of 61.7 Vfs. The cathode foils were etched to enlarge their surface area, and then a dielectric oxide film was formed by chemical conversion using an adipic acid aqueous solution at a chemical conversion voltage of 3 Vfs. A conductive layer was laminated on the cathode foils of Comparative Example 2, Example 1, and Example 2. The conductive layer was a 100 nm thick titanium carbide layer formed on the surface of the cathode foil by vacuum deposition. No conductive layer was formed on Comparative Examples 1, 3, and 4.

[0052] Lead wires were connected to the anode foil and cathode foil or cathode body, and the anode foil and cathode foil or cathode body were wound facing each other with a cellulose separator interposed between them. The wound body was repaired by immersing it in an ammonium dihydrogen phosphate aqueous solution for 20 minutes, and then dried at 105°C.

[0053] This wound body was immersed in a conductive polymer dispersion to adhere a conductive polymer to the dielectric oxide film of the anode foil, the cathode foil, and the separator. In the conductive polymer dispersion, particles of poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid were dispersed as a conductive polymer, and ethylene glycol was added. After the first immersion of the wound body in the conductive polymer dispersion, the wound body was dried at 125°C for 30 minutes. The wound body was further immersed in the conductive polymer dispersion, and after the second immersion, the wound body was dried at 150°C for 30 minutes.

[0054] The wound body with the conductive polymer attached was then impregnated with an electrolyte. The electrolytes of each Example and Comparative Example contained ethylene glycol as a solvent. The electrolytes of Comparative Examples 1 and 2 contained only 16 mmol of azelaic acid per 100 g of electrolyte, the electrolytes of Comparative Example 3 and Example 1 contained 16 mmol each of azelaic acid and dibutyl phosphate per 100 g of electrolyte, and the electrolytes of Comparative Example 4 and Example 2 contained 16 mmol each of azelaic acid and tributyl phosphate per 100 g of electrolyte. Furthermore, the electrolytes of each Example and Comparative Example contained 16 mmol of ammonia as a solute cation component.

[0055] The capacitor elements impregnated with the electrolyte were inserted into a cylindrical outer case with a bottom. A rubber seal was attached to the open end of the outer case and sealed by crimping. Each solid electrolytic capacitor was subjected to an aging treatment by applying a voltage. Each solid electrolytic capacitor fabricated had a diameter of 10.0 mm and a height of 10.0 mm, a rated withstand voltage of 35 WV, a rated capacitance of 330 μF for Comparative Examples 1, 3, and 4, and a rated capacitance of 390 μF for Comparative Example 2 and Examples 1 and 2.

[0056] (High-Frequency ESR Part 1) The ESR of the solid electrolytic capacitors of Examples 1 and 2 and Comparative Examples 1 to 4 was measured. Each solid electrolytic capacitor was exposed to a temperature environment of 160°C, and the ESR was measured at each elapsed time. The measurement frequency was 100 kHz, which is in the high-frequency range. The ESR immediately before exposure to the temperature environment of 160°C, i.e., at zero hours elapsed, and the ESR after 800 hours have elapsed are shown in Table 1 below. The ESR of Comparative Examples 1, 2, and 3 and Example 1 is shown in the graph of FIG. 1, and the ESR of Comparative Examples 1, 2, 4, and Example 2 is shown in the graph of FIG. 2. The horizontal axis of the graphs in FIGS. 1 and 2 represents elapsed time, and the vertical axis represents ESR.

[0057] (Table 1)

[0058] As shown in Table 1 and Figures 1 and 2, Comparative Examples 3 and 4, in which dibutyl phosphate or tributyl phosphate was added to the electrolyte, achieved a lower ESR than Comparative Example 1 at both zero and 800 hours. The difference between Comparative Example 3 and Comparative Example 1 was 0.0015 Ω after 800 hours, and the difference between Comparative Example 4 and Comparative Example 1 was 0.0016 Ω after 800 hours. Comparative Example 2, which had a conductive layer on the cathode foil, had an ESR that was almost the same as Comparative Example 1 at both zero and 800 hours.

[0059] In contrast, Examples 1 and 2, in which dibutyl phosphate or tributyl phosphate was added to the electrolyte and a conductive layer was provided on the cathode foil, had even lower ESRs of 0.0012 Ω and 0.0007 Ω after 800 hours than Comparative Examples 3 and 4. Although the ESR of Comparative Example 2, which had a conductive layer on the cathode foil, was almost the same as that of Comparative Example 1 at both time zero and 800 hours, Examples 1 and 2 achieved such good ESRs.

[0060] This confirmed that the ESR of a solid electrolytic capacitor can be further reduced by providing a cathode body with a conductive layer formed on the surface of a cathode foil and by adding to the electrolyte a phosphate compound having an alkyl group with 1 to 10 carbon atoms. In particular, the ESR of a solid electrolytic capacitor can be further reduced by adding to the electrolyte a phosphate compound having a butyl group.

[0061] (Examples 3-7) Next, solid electrolytic capacitors of Examples 3 to 7 were fabricated. The solid electrolytic capacitors of Examples 3 to 7 had titanium carbide in the cathode body as the conductive layer, as in Examples 1 and 2, but had a different type of phosphate compound than those of Examples 1 and 2. However, the phosphate compounds of Examples 3 to 7 had an alkyl group having 1 to 10 carbon atoms. In addition, a solid electrolytic capacitor of Comparative Example 5 was fabricated, which had titanium carbide in the cathode body and phosphoric acid in the electrolyte solution. The solid electrolytic capacitors of Examples 3 to 7 and Comparative Example 5 were fabricated using the same manufacturing method and under the same conditions as Example 1, except for the type of phosphate compound, and had the same configuration, composition, and composition ratio.

[0062] (High-Frequency ESR and LC) The ESR of the solid electrolytic capacitors of Example 1, Examples 3 to 7, and Comparative Example 5 was measured. Each solid electrolytic capacitor was exposed to a temperature environment of 150°C, and the ESR was measured at each elapsed time. The measurement frequency was 100 kHz, which is in the high-frequency range. The ESR immediately before exposure to the temperature environment of 150°C, i.e., at zero hours elapsed, and the ESR after 260 hours have elapsed are shown in Table 2 below.

[0063] The LC (leakage current) of the solid electrolytic capacitors of Example 1 and Comparative Example 5 was also measured. Each solid electrolytic capacitor was left in a temperature environment of 150°C for 2,700 hours, and the leakage current after leaving was measured. The leakage current was measured by applying a rated withstand voltage of 35 WV to each solid electrolytic capacitor and maintaining this voltage for 2 minutes. The leakage current results are shown in Table 2 below.

[0064] (Table 2)

[0065] As shown in Table 2, the phosphate compound in Example 1 is dibutyl phosphate. In Example 3, equimolar amounts of dibutyl phosphate and triisopropyl phosphate were mixed into the electrolyte. The phosphate compound in Example 4 is dibutyl phosphite. The phosphate compound in Example 5 is triethyl phosphite. The phosphate compound in Example 6 is trimethyl phosphite. The phosphate compound in Example 7 is triisopropyl phosphate.

[0066] As shown in Table 2, Examples 3 and 4 are solid electrolytic capacitors in which a phosphate compound having a butyl group is contained in the electrolyte solution, similar to Examples 1 and 2. For Examples 3 and 4, the ESR at zero hours and the ESR after 260 hours are almost the same as those of Example 1 in Table 2.

[0067] Furthermore, Examples 4 to 7 are solid electrolytic capacitors in which a phosphate compound having an alkyl group other than a butyl group is contained in the electrolyte. These solid electrolytic capacitors in Examples 4 to 7 have good ESR characteristics similar to those of Examples 1, 3, and 4. Thus, when a cathode body is provided with a conductive layer formed on the surface of a cathode foil and the electrolyte contains a phosphate compound having an alkyl group with 1 to 10 carbon atoms, the ESR of the solid electrolytic capacitor is reduced. Furthermore, Examples 1, 3, and 4, which contain a phosphate compound having a butyl group, have particularly good ESR characteristics, and a butyl group is preferred as the alkyl group.

[0068] When phosphoric acid was added to the electrolyte solution instead of a phosphate compound having an alkyl group with 1 to 10 carbon atoms, as in Comparative Example 5, the leakage current (LC) was significantly worse than in Example 1. By using a phosphate compound having an alkyl group with 1 to 10 carbon atoms, the deterioration of LC as in Comparative Example 5 can be prevented.

[0069] (Examples 8-10) Solid electrolytic capacitors of Examples 8 to 10 were fabricated. The solid electrolytic capacitors of Examples 8 to 10 contained a different type of phosphate compound as the phosphate compound than those of Examples 1 and 2, but the type of conductive layer was different from that of Examples 1 and 2. Furthermore, corresponding to Examples 8 to 10, solid electrolytic capacitors of Comparative Examples 6 to 8 were fabricated, which had the same type of conductive layer but did not contain a phosphate compound in the electrolyte solution.

[0070] The solid electrolytic capacitor of Example 8 and the solid electrolytic capacitor of Comparative Example 6 corresponding to Example 8 were fabricated as follows. That is, in Example 8, the conductive layer laminated on the cathode foil was a 100 nm-thick carbon nanotube layer formed on the surface of the cathode foil by vacuum deposition. The cathode foil of Comparative Example 6 had no conductive layer laminated thereon. A cellulose-based separator was sandwiched between the anode foil and the cathode body or cathode foil. Each of the fabricated solid electrolytic capacitors had a diameter of 10.0 mm, a height of 7.7 mm, a rated withstand voltage of 25 WV, and a rated capacity of 270 μF. The other manufacturing methods, manufacturing conditions, capacitor structures, compositions, and composition ratios of Example 8 and Comparative Example 6 were the same as those of Example 1.

[0071] The solid electrolytic capacitor of Example 9 and the solid electrolytic capacitor of Comparative Example 7 corresponding to Example 9 were fabricated as follows. That is, in Example 9, the conductive layer laminated on the cathode foil was a 100 nm-thick carbon black layer formed on the surface of the cathode foil by vacuum deposition. The cathode body was sandwiched between press rollers, and linear pressure was applied. The cathode foil of Comparative Example 7 did not have a conductive layer laminated thereon. A cellulose-based separator was sandwiched between the anode foil and the cathode body or cathode foil. Each of the fabricated solid electrolytic capacitors had a diameter of 10.0 mm, a height of 10.0 mm, a rated withstand voltage of 25 WV, and a rated capacity of 580 μF. The other manufacturing methods, manufacturing conditions, capacitor structures, compositions, and composition ratios of Example 9 and Comparative Example 7 were the same as those of Example 1.

[0072] The solid electrolytic capacitor of Example 10 and the solid electrolytic capacitor of Comparative Example 8 corresponding to Example 10 were fabricated as follows. That is, in Example 10, the conductive layer laminated on the cathode foil was a 100 nm-thick titanium nitride layer formed on the surface of the cathode foil by vacuum deposition. The cathode foil of Comparative Example 8 had no conductive layer laminated thereon. A cellulose-based separator was sandwiched between the anode foil and the cathode body or cathode foil. Each of the fabricated solid electrolytic capacitors had a diameter of 10.0 mm, a height of 10.0 mm, a rated withstand voltage of 25 WV, and a rated capacity of 470 μF. The other manufacturing methods, manufacturing conditions, capacitor structures, compositions, and composition ratios of Example 10 and Comparative Example 8 were the same as those of Example 1.

[0073] (High-Frequency ESR) The ESR of the solid electrolytic capacitors of Example 1 and Comparative Example 2, which are in a corresponding relationship, Example 8 and Comparative Example 6, which are in a corresponding relationship, Example 9 and Comparative Example 7, which are in a corresponding relationship, and Example 10 and Comparative Example 8, which are in a corresponding relationship, was measured. Each solid electrolytic capacitor was exposed to a temperature environment of 150°C, and the ESR was measured at each elapsed time. The measurement frequency was 100 kHz, which is in the high-frequency range. The ESR immediately before exposure to the temperature environment of 150°C, i.e., at zero hours elapsed, and the ESR after 260 hours have elapsed are shown in Table 3 below.

[0074] (Table 3)

[0075] As shown in Table 3, Example 8, in which the conductive layer was a carbon nanotube layer, had a better ESR when used at high frequencies than Comparative Example 6, and when exposed to a high-temperature environment, the difference was even greater compared to Comparative Example 6. Example 9, in which the conductive layer was a carbon black layer, had a better ESR when used at high frequencies than Comparative Example 7, and its superiority remained unchanged even when exposed to a high-temperature environment. Example 10, in which the conductive layer was a titanium nitride layer, had a better ESR when used at high frequencies than Comparative Example 8. Moreover, while Comparative Example 6, which was exposed to a high-temperature environment, experienced a significant deterioration in ESR, Example 10 maintained a low ESR even when exposed to a high-temperature environment.

[0076] Thus, the ESR at high frequencies can be improved by forming a conductive layer containing, for example, a carbon material, titanium, titanium nitride, or a composite or mixture of these, without being limited to a specific type of conductive layer. In particular, the combination of a titanium nitride conductive layer and an electrolyte containing a phosphate compound having an alkyl group having 1 to 10 carbon atoms improves the ESR of the solid electrolytic capacitor compared to a case where a titanium nitride conductive layer is simply laminated on a cathode foil.

[0077] Next, solid electrolytic capacitors of Examples 11 to 16 were fabricated. The solid electrolytic capacitors of Examples 11 to 16 differed from Example 1 only in the amount of phosphate compound added, and were fabricated using the same manufacturing method and under the same conditions as Example 1, with the exception of the amount added, and had the same configuration, composition, and composition ratio.

[0078] (High-Frequency ESR) The ESR of the solid electrolytic capacitors of Comparative Example 2, Example 1, and Examples 11 to 16 was measured. Each solid electrolytic capacitor was exposed to a temperature environment of 150°C, and the ESR was measured at each elapsed time. The measurement frequency was 100 kHz, which is in the high-frequency range. The ESR immediately before exposure to the temperature environment of 150°C, i.e., at zero hours elapsed, and the ESR after 260 hours have elapsed are shown in Table 4 below.

[0079] (Table 4)

[0080] As shown in Table 4, no phosphate compound was added in Comparative Example 2. Examples 1 and 11 to 16 differed in the amount of dibutyl phosphate added per 100 g of electrolyte solution, ranging from 2 mmol to 33 mmol.

[0081] As shown in Table 4, when the amount of the phosphate compound added is 4 mmol or more per 100 g of electrolyte, the ESR after 260 hours is particularly good. Furthermore, when the amount of the phosphate compound added is 16 mmol or 33 mmol per 100 g of electrolyte, there is no change in the ESR after 260 hours.

[0082] (Examples 17-25) Next, solid electrolytic capacitors of Examples 17 to 25 were fabricated. The solid electrolytic capacitor of Example 17 differs from Example 1 only in the cation species contained in the electrolyte solution. Except for the solvent species, it was fabricated using the same manufacturing method and conditions as Example 1, and had the same configuration, composition, and composition ratio. The solid electrolytic capacitors of Examples 18 to 25 differ from Example 17 only in the solvent species contained in the electrolyte solution. Except for the cation species and solvent species, it was fabricated using the same manufacturing method and conditions as Example 1, and had the same configuration, composition, and composition ratio. In addition, as Comparative Example 9, a solid electrolytic capacitor identical to Example 25 was fabricated, except that the electrolyte solution did not contain a phosphate compound.

[0083] (High-Frequency ESR) The ESR of the solid electrolytic capacitors of Comparative Example 9, Example 1, and Examples 17 to 25 was measured. Each solid electrolytic capacitor was exposed to a temperature environment of 150°C, and the ESR was measured at each elapsed time. The measurement frequency was 100 kHz, which is in the high-frequency range. The ESR immediately before exposure to the temperature environment of 150°C, i.e., at zero hours elapsed, and the ESR after 260 hours have elapsed are shown in Table 5 below.

[0084] (Table 5)

[0085] As shown in Table 5, the solid electrolytic capacitor of Example 17 differs from Example 1 in that triethylamine is added to the electrolyte solution instead of ammonia. The solid electrolytic capacitors of Examples 18 to 25 and Comparative Example 9 also have triethylamine added to the electrolyte solution as a cation species. In Example 18, the solvent species of the electrolyte solution are ethylene glycol and glycerin, with ethylene glycol accounting for 90 wt % of the solvent and glycerin accounting for 10 wt % of the solvent. In Example 19, the solvent species of the electrolyte solution are ethylene glycol and glycerin, with ethylene glycol accounting for 40 wt % of the solvent and glycerin accounting for 60 wt % of the solvent.

[0086] In Example 20, the solvent type of the electrolyte solution is glycerin. In Example 21, the solvent type of the electrolyte solution is sulfolane. In Example 22, the solvent type of the electrolyte solution is equal amounts by weight of sulfolane and polyethylene glycol having an average molecular weight of 300. In Example 23, the solvent type of the electrolyte solution is glycerin and polyethylene glycol having an average molecular weight of 300, with glycerin accounting for 70 wt % of the solvent and polyethylene glycol having an average molecular weight of 300 accounting for 30 wt % of the solvent. In Example 24, the solvent type of the electrolyte solution is γ-butyrolactone. In Example 25, the solvent type of the electrolyte solution is glycerin and polyethylene glycol having an average molecular weight of 300, with the glycerin and polyethylene glycol having an average molecular weight of 300 being equal amounts by weight. In Comparative Example 9, dibutyl phosphate is not contained in the electrolyte solution, and the solvent type of the electrolyte solution is γ-butyrolactone.

[0087] As shown in Table 5, the ESR of Example 1 was equivalent to that of Example 17 before exposure to a high-temperature environment, but after exposure to a high-temperature environment, it was lower than that of Example 18. That is, although any type of cation species added to the electrolyte does not prevent the reduction of ESR, ammonia is particularly preferable.

[0088] Furthermore, as shown in Table 5, in Examples 17 to 23, the ESR after exposure to a high-temperature environment was 0.0260Ω or less. In Example 24, the ESR after exposure to a high-temperature environment was 0.0330Ω or more. As described above, any type of solvent added to the electrolyte does not prevent the reduction of ESR, but one or a mixture of two or more selected from the group consisting of ethylene glycol, glycerin, and sulfolane is particularly preferred.

Claims

1. an anode foil containing a valve metal and having a dielectric oxide film formed on its surface; a cathode body facing the anode foil; an electrolyte layer interposed between the anode foil and the cathode body, the electrolyte layer containing an electrolytic solution and a conductive polymer; Equipped with the cathode body has a cathode foil containing a valve metal and a conductive layer formed on a surface of the cathode foil, the electrolyte solution contains a phosphate compound having an alkyl group having 1 to 10 carbon atoms; A solid electrolytic capacitor characterized by:

2. 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; 2. The solid electrolytic capacitor according to claim 1,

3. the conductive layer comprises a carbon material, titanium, titanium nitride, titanium carbide, or a composite or mixture thereof; 3. The solid electrolytic capacitor according to claim 1, wherein:

4. the amount of the phosphate compound is 4 mmol or more per 100 g of the electrolyte solution; 3. The solid electrolytic capacitor according to claim 1, wherein:

5. the amount of the phosphate compound is 4 mmol or more and 16 mmol or less per 100 g of the electrolyte solution; 3. The solid electrolytic capacitor according to claim 1, wherein:

6. the electrolyte solution contains one or more selected from the group consisting of ethylene glycol, glycerin, and sulfolane; 3. The solid electrolytic capacitor according to claim 1, wherein:

7. the cathode foil has a surface enlarging layer on a surface of the cathode foil, the conductive layer is formed on the surface-expanding layer; 3. The solid electrolytic capacitor according to claim 1, wherein:

8. A method for manufacturing a solid electrolytic capacitor including an anode foil, a cathode body, and an electrolyte layer, comprising: forming a conductive layer on a surface of a cathode foil containing a valve metal to fabricate the cathode body; an electrolyte solution preparation step of preparing an electrolyte solution containing a phosphoric acid compound having an alkyl group having 1 to 10 carbon atoms; an electrolyte layer forming step of forming the electrolyte layer by interposing the electrolytic solution and a conductive polymer between the anode foil and the cathode body; containing, A method for manufacturing a solid electrolytic capacitor, comprising: