Conductive polymer dispersion and method for producing conductive polymer dispersion, solid electrolytic capacitor and method for producing solid electrolytic capacitor

Abstract

To provide a conductive polymer dispersion that is resistant to a decrease in capacitance while maintaining the effect of suppressing an increase in ESR of a solid electrolytic capacitor, a manufacturing method thereof, a solid electrolytic capacitor made from the conductive polymer dispersion, and a manufacturing method thereof.SOLUTION: A conductive polymer dispersion forms a solid electrolyte layer in a capacitor element. A manufacturing method of the conductive polymer dispersion includes a mixing step of mixing at least a solvent, a conductive polymer, and an alkaline component, and in the mixing step, both a volatile alkaline component and a non-volatile alkaline component are mixed into the conductive polymer dispersion as the alkaline component.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a conductive polymer dispersion in which a conductive polymer is dispersed, the conductive polymer dispersion, a solid electrolytic capacitor in which a solid electrolyte layer is formed using the conductive polymer dispersion, and a method for producing a solid electrolytic capacitor using the conductive polymer dispersion. [Background technology]

[0002] Electrolytic capacitors using valve metals such as tantalum or aluminum can achieve a small size and large capacitance by enlarging the surface area of the dielectric by forming the valve metal anode electrode into a sintered body or etched foil. In particular, solid electrolytic capacitors, in which a dielectric oxide film is covered with a solid electrolyte, are small, have large capacitance, and have low equivalent series resistance, making them essential for miniaturizing electronic devices, improving their functionality, and reducing their costs.

[0003] Known solid electrolytes include manganese dioxide and 7,7,8,8-tetracyanoquinodimethane (TCNQ) complexes. In recent years, conductive polymers derived from monomers with π-conjugated double bonds, such as poly(3,4-ethylenedioxythiophene) (PEDOT), which has a slow reaction rate and excellent adhesion to dielectric oxide films, have rapidly become popular as solid electrolytes. Conductive polymers exhibit high conductivity when acid compounds such as polyanions are used as dopants during chemical oxidative polymerization or electrolytic oxidative polymerization.

[0004] However, compared to liquid-type electrolytic capacitors, in which the capacitor element is impregnated with an electrolyte solution and no solid electrolyte layer is used, solid electrolytic capacitors are less able to repair defects in the dielectric oxide film, which can lead to increased leakage current. Therefore, so-called hybrid-type solid electrolytic capacitors, in which a solid electrolyte layer is formed on a capacitor element made up of a pair of opposing electrode foils and the voids in the capacitor element are impregnated with an electrolyte solution, have attracted attention.

[0005] The solid electrolyte layer is formed by immersing a capacitor element, which has a pair of electrode foils facing each other, in a liquid composition containing a conductive polymer. When the capacitor element is immersed in the liquid composition, the conductive polymer adheres to the capacitor element, forming a solid electrolyte layer. The capacitor element with the solid electrolyte layer formed is further impregnated with an electrolyte solution, and the capacitor element impregnated with the electrolyte solution is housed in an outer case. A sealing member having an elastomer layer such as ethylene propylene rubber or butyl rubber is inserted into the opening of the outer case to seal the interior.

[0006] Liquid compositions containing dopants, which are acid compounds, are highly acidic. Therefore, it has been pointed out that the electrode foil of a solid electrolytic capacitor may corrode. To address this issue, a solid electrolytic capacitor has been proposed in which an alkaline component is incorporated into the solid electrolyte layer (see, for example, Patent Document 1). Specifically, the anode body is immersed in a liquid composition containing a conductive polymer, a polyanion, an alkaline component, and a solvent, or the liquid composition is applied or dropped onto the anode body, and then the solvent from the liquid composition is removed by one or more drying steps. Ammonia is preferably proposed as the alkaline component. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2018 / 123255 Summary of the Invention [Problem to be solved by the invention]

[0008] When solid electrolytic capacitors were made using a liquid composition containing ammonia, the seal cracked and the capacitance decreased in high-temperature environments such as 150°C or 180°C. After investigating this phenomenon, it was determined that when the solvent in the liquid composition was removed by drying, a large amount of ammonia, which has a low boiling point, evaporated, making it impossible to maintain a neutral state within the capacitor element.

[0009] However, ammonia is useful from the viewpoint of improving voltage resistance characteristics and suppressing an increase in ESR in high-temperature environments, and there are also compounds useful for solid electrolytic capacitors among the volatile alkaline components that are easily evaporated during the solvent removal process of the liquid composition, making it difficult to replace it with other compounds.

[0010] The present invention has been proposed to solve the above-mentioned problems, and its object is to provide a conductive polymer dispersion in which conductive polymer particles or powder are dispersed, which maintains the effect of suppressing an increase in ESR and is resistant to a decrease in capacitance, a method for producing this conductive polymer dispersion, a solid electrolytic capacitor made from this conductive polymer dispersion, and a method for producing this solid electrolytic capacitor. [Means for solving the problem]

[0011] In order to solve the above-mentioned problems, the conductive polymer dispersion of the present invention is a conductive polymer dispersion for forming a solid electrolyte layer in a capacitor element having a pair of electrode foils facing each other, and is characterized in that it contains at least a solvent, a conductive polymer, and an alkaline component, and the alkaline component contains both a volatile alkaline component and a non-volatile alkaline component.

[0012] The volatile alkaline component may have a boiling point of −30° C. or lower, and the non-volatile alkaline component may have a boiling point of 1000° C. or higher.

[0013] The volatile alkaline component may be ammonia, and the non-volatile alkaline component may be one or more selected from the group consisting of sodium hydroxide, potassium hydroxide, and calcium hydroxide.

[0014] The molar ratio of the volatile alkali component (A) to the hardly volatile alkali component (B) may be in the range of A:B=2:8 to 8:2.

[0015] In addition, in order to solve the above-mentioned problems, the solid electrolytic capacitor of the present invention comprises a capacitor element having a pair of electrode foils facing each other, and an electrolyte layer formed within the capacitor element and containing a solid electrolyte layer and an electrolytic solution, wherein the solid electrolyte layer contains both a volatile alkaline component and a non-volatile alkaline component.

[0016] The electrolytic solution may contain at least glycols as a solvent.

[0017] Furthermore, in order to solve the above-mentioned problems, the method for manufacturing a solid electrolytic capacitor of the present invention includes a dispersion preparation step of mixing at least a solvent, a conductive polymer, and an alkaline component to prepare a conductive polymer dispersion; an attachment step of using the conductive polymer dispersion to attach a conductive polymer to a capacitor element having a pair of electrode foils facing each other; a drying step of drying the capacitor element that has undergone the attachment step; and an impregnation step of impregnating an electrolytic solution into the capacitor element that has undergone the drying step, wherein in the dispersion preparation step, both a volatile alkaline component and a non-volatile alkaline component are added to the conductive polymer dispersion as the alkaline component.

[0018] In the drying step, the capacitor element is exposed to a temperature environment equal to or higher than the boiling points of the solvent and the volatile alkaline component and lower than the boiling point of the non-volatile alkaline component. [Effects of the Invention]

[0019] According to the present invention, it is possible to suppress the deterioration of the sealing member and the decrease in capacitance caused by the high acidity of the conductive polymer dispersion. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a graph showing the relationship between the molar concentration of ammonia relative to the total of volatile alkaline components and refractory alkaline components and ΔCap. [Figure 2] 1 is a graph showing the relationship between the molar concentration of sodium hydroxide relative to the total of volatile and hardly-volatile alkaline components and ΔESR. DETAILED DESCRIPTION OF THE INVENTION

[0021] (Overall composition) The following describes a conductive polymer dispersion and a solid electrolytic capacitor according to an embodiment of the present invention. A solid electrolytic capacitor is a passive element that stores and discharges electric charge by capacitance, and is a so-called hybrid-type solid electrolytic capacitor that uses both a solid electrolyte layer and an electrolytic solution. Hereinafter, a hybrid-type solid electrolytic capacitor will be simply referred to as a solid electrolytic capacitor. The solid electrolytic capacitor may have, for example, a wound type or a laminated type shape. In this embodiment, a wound type is described as an example, but the solid electrolytic capacitor of the present invention is not limited to this, nor is it limited to the other embodiments described.

[0022] A wound-type solid electrolytic capacitor has a cylindrical capacitor element. The capacitor element is inserted into a cylindrical outer case with a bottom. A sealer having an elastomer layer such as ethylene propylene rubber or butyl rubber is attached to the open end of the outer case, and the open end is sealed by crimping. The capacitor element includes a pair of electrode foils, separated into an anode side and a cathode side, a separator, and an electrolyte layer. The pair of electrode foils are wound with the separator interposed between them, and the electrolyte layer is formed between the pair of electrode foils. A dielectric oxide film is formed on the surface of the electrode foil on the anode side. An anode lead and a cathode lead are connected to each of the pair of electrode foils, and the anode lead and the cathode lead are pulled out from the seal rubber.

[0023] The electrolyte layer comprises a solid electrolyte layer and an electrolyte solution. The solid electrolyte layer is formed so as to cover at least a portion of the dielectric oxide film layer on the surface of the anode foil. The electrolyte solution is filled in the voids of the capacitor element on which the solid electrolyte layer is formed. The solid electrolyte layer contains a conductive polymer and is formed by immersing the capacitor element in a conductive polymer dispersion and drying it. The conductive polymer dispersion is a liquid in which conductive polymer particles or powder are dispersed, and by immersing the capacitor element, the conductive polymer adheres to the capacitor element as a solid electrolyte layer. By drying the capacitor element, the solvent of the conductive polymer dispersion is removed. By immersing the capacitor element on which the solid electrolyte layer has been formed, the capacitor element is impregnated with the electrolyte solution.

[0024] (Conductive polymer dispersion) The conductive polymer in the conductive polymer dispersion is a doped conjugated polymer, which is obtained by chemical oxidative polymerization or electrolytic oxidative polymerization of a monomer or its derivative having a π-conjugated double bond. By performing a doping reaction on a conjugated polymer, the conductive polymer exhibits high conductivity. Specifically, conductivity is exhibited by adding a small amount of a dopant, such as an acceptor that readily accepts electrons or a donor that readily donates electrons, to the conjugated polymer. When an acceptor or donor is added to a conjugated polymer, π electrons are extracted from the conjugated polymer, creating negatively charged atoms (positive holes), while electrons are supplied to the donor, creating negatively charged carriers, thereby exhibiting conductivity.

[0025] 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.

[0026] Among the above conjugated polymers, preferred are conjugated polymers obtained by polymerizing thiophene or its derivatives, 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. Preferred thiophene derivatives are compounds 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. Suitable alkyl and alkoxy groups have 1 to 16 carbon atoms.

[0027] In particular, a polymer of 3,4-ethylenedioxythiophene known as EDOT, i.e., poly(3,4-ethylenedioxythiophene) known as PEDOT, is particularly preferred. Also, alkylated ethylenedioxythiophenes, in which an alkyl group is added to 3,4-ethylenedioxythiophene, may be used, such as methylated ethylenedioxythiophene (i.e., 2-methyl-2,3-dihydro-thieno[3,4-b][1,4]dioxin) and ethylated ethylenedioxythiophene (i.e., 2-ethyl-2,3-dihydro-thieno[3,4-b][1,4]dioxin).

[0028] 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.

[0029] Examples of polyanions include substituted or unsubstituted polyalkylenes, substituted or unsubstituted polyalkenylenes, substituted or unsubstituted polyimides, substituted or unsubstituted polyamides, and substituted or unsubstituted polyesters, including polymers consisting only of structural units having anionic groups, and polymers consisting of structural units having anionic groups and structural units not having anionic groups. Specific examples of polyanions include polyvinyl sulfonic acid, polystyrene sulfonic acid, polyallylsulfonic acid, polyacrylsulfonic acid, polymethacrylic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprenesulfonic acid, polyacrylic acid, polymethacrylic acid, and polymaleic acid. The number-average molecular weight of the polyanion is 1,000 to 2,000,000, preferably 10,000 to 500,000. If the number average molecular weight is less than 1,000, the resulting conductive polymer will have insufficient conductivity and reduced dispersibility, which is undesirable. If the number average molecular weight exceeds 2,000,000, the viscosity of the mixture will increase, which is undesirable.

[0030] This conductive polymer dispersion contains a volatile alkali component and a non-volatile alkali component in addition to a solvent and a conductive polymer. The volatile alkali component and the non-volatile alkali component are neutralizers for the conductive polymer dispersion, the acidity of which increases due to the presence of a dopant, etc. As long as the conductive polymer dispersion contains a volatile alkali component and a non-volatile alkali component, it may contain two or more types of volatile alkali components or two or more types of non-volatile alkali components.

[0031] The solvent for the conductive polymer dispersion may be any solvent that disperses conductive polymer particles or powder, such as water, an organic solvent, or a mixture thereof. Examples of the organic solvent include polar solvents, alcohols, esters, hydrocarbons, carbonate compounds, ether compounds, chain ethers, heterocyclic compounds, and nitrile compounds.

[0032] Examples of polar solvents include N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide. Examples of alcohols include methanol, ethanol, propanol, and butanol. Examples of esters include ethyl acetate, propyl acetate, and butyl acetate. Examples of hydrocarbons include hexane, heptane, benzene, toluene, and xylene. Examples of carbonate compounds include ethylene carbonate and propylene carbonate. Examples of ether compounds include dioxane and diethyl ether. Examples of linear ethers include ethylene glycol dialkyl ether, propylene glycol dialkyl ether, polyethylene glycol dialkyl ether, and polypropylene glycol dialkyl ether. Examples of heterocyclic compounds include 3-methyl-2-oxazolidinone. Examples of nitrile compounds include acetonitrile, glutarodinitrile, methoxyacetonitrile, propionitrile, and benzonitrile.

[0033] The volatile alkaline component is an alkaline component whose boiling point is lower than the temperature environment in which the capacitor element immersed in the conductive polymer dispersion is placed when drying the capacitor element. Examples of this volatile alkaline component include alkaline components with a boiling point of −30° C. or lower, and ammonia is a preferred example. Ammonia suppresses an increase in the equivalent series resistance (ESR) of the solid electrolytic capacitor even when the solid electrolytic capacitor is exposed to a high-temperature environment of 150 to 280° C., such as during reflow soldering during mounting.

[0034] The non-volatile alkaline component is an alkaline component having a boiling point higher than the temperature environment in which the capacitor element immersed in the conductive polymer dispersion is placed when the capacitor element is dried. Examples of the volatile alkaline component include alkaline components with a boiling point of 1000°C or higher, and specifically, one or more types selected from the group consisting of sodium hydroxide, potassium hydroxide, and calcium hydroxide. The volatile alkaline component suppresses deterioration of the sealing member and suppresses a decrease in the capacitance of the solid electrolytic capacitor.

[0035] The molar ratio of the volatile alkali component to the non-volatile alkali component, where A is the volatile alkali component and B is the non-volatile alkali component, is preferably in the range of A:B=2:8 to 8:2. If the volatile alkali component exceeds 80 mol% of the total alkali component, the capacitance of the solid electrolytic capacitor decreases significantly. On the other hand, if the volatile alkali component is less than 20 mol% of the total alkali component, the ESR increases significantly in high-temperature environments.

[0036] Furthermore, by adding a volatile alkali component and a hardly-volatile alkali component, the pH of the conductive polymer dispersion is preferably adjusted to a range from 2 to 7, more preferably from 2 to 5. By adjusting the pH within this range, deterioration of the sealing member is further suppressed, and a decrease in the capacitance of the solid electrolytic capacitor is further suppressed.

[0037] The conductive polymer dispersion may contain a polyhydric alcohol in addition to the solvent, conductive polymer, volatile alkali component, and non-volatile alkali component. Examples of the polyhydric alcohol include sorbitol, ethylene glycol, diethylene glycol, triethylene glycol, polyoxyethylene glycol, glycerin, polyoxyethyleneglycerin, xylitol, erythritol, mannitol, dipentaerythritol, pentaerythritol, and combinations of two or more thereof. Because the polyhydric alcohol has a high boiling point, it can remain in the solid electrolyte layer even after the drying process, resulting in reduced ESR and improved voltage resistance.

[0038] Furthermore, the conductive polymer dispersion may contain other compounds. For example, conventional additives such as organic binders, surfactants, antifoaming agents, coupling agents, antioxidants, and UV absorbers may be added. The ESR can be significantly reduced by adding additives to the conductive polymer dispersion or by increasing the number of times the conductive polymer dispersion is impregnated into the capacitor element.

[0039] In the dispersion preparation process for this conductive polymer dispersion, for example, a monomer constituting the conductive polymer and an acid or its alkali metal salt that releases a dopant constituting the conductive polymer are added, a supporting electrolyte is added, and electrolytic oxidation polymerization is carried out with stirring. Impurities and residual monomers are then removed by purification means such as ultrafiltration, cation exchange, and anion exchange. A conductive polymer dispersion is obtained by this dispersion preparation process. The volatile alkali component and the non-volatile alkali component may be added to the conductive polymer dispersion after the impurities and residual monomers have been removed during the dispersion preparation process.

[0040] The supporting electrolyte can be any compound that releases dopants contained in conventional conductive polymers, and examples thereof include inorganic acids such as boric acid, nitric acid, phosphoric acid, tungstophosphoric acid, and molybdophosphoric acid; organic acids such as acetic acid, oxalic acid, citric acid, tartaric acid, squaric acid, rhodizonic acid, croconic acid, and salicylic acid; and sulfonic acids such as methanesulfonic acid, dodecylsulfonic acid, trifluoromethanesulfonic acid, p-toluenesulfonic acid, dodecylbenzenesulfonic acid, 1,2-dihydroxy-3,5-benzenedisulfonic acid, naphthalenesulfonic acid, naphthalenedisulfonic acid, propylnaphthalenesulfonic acid, and butylnaphthalenesulfonic acid, as well as salts thereof. Polycarboxylic acids such as polyacrylic acid, polymethacrylic acid, and polymaleic acid; polysulfonic acids such as polystyrenesulfonic acid and polyvinylsulfonic acid, as well as salts thereof, can also be used as supporting electrolytes. Furthermore, boron complexes such as borodisalicylic acid, borodioxalic acid, borodimalonic acid, borodisuccinic acid, borodiadipic acid, borodimaleic acid, borodiglycolic acid, borodilactic acid, borodihydroxyisobutyric acid, borodimalic acid, boroditartaric acid, borodicitric acid, borodiphthalic acid, borodihydroxybenzoic acid, borodimandelic acid, and borodibenzlic acid, sulfonylimide acids, and salts thereof can also be used as supporting electrolytes.

[0041] Examples of salts include alkali metal salts such as lithium salt, sodium salt, and potassium salt; alkylammonium salts such as ammonium salt, ethylammonium salt, and butylammonium salt; dialkylammonium salts such as diethylammonium salt and dibutylammonium salt; trialkylammonium salts such as triethylammonium salt and tributylammonium salt; and tetraalkylammonium salts such as tetraethylammonium salt and tetrabutylammonium salt.

[0042] The electrolytic oxidation polymerization is carried out by any of the methods of the constant potential method, the constant current method, and the potential sweep method. In the case of the constant potential method, a potential of 1.0 to 1.5 V with respect to a saturated calomel electrode is suitable, and in the case of the constant current method, a potential of 1 to 10,000 μA / cm 2In the case of potential sweeping, it is preferable to sweep the potential in the range of 0 to 1.5 V against a saturated calomel electrode at a rate of 5 to 200 mV / sec. There is no strict limit to the polymerization temperature, but it is generally in the range of 10 to 60°C. The polymerization time is generally in the range of 10 minutes to 30 hours.

[0043] Furthermore, in the dispersion preparation process for this conductive polymer dispersion, for example, a monomer constituting the conductive polymer and an acid or its alkali metal salt that releases a dopant constituting the conductive polymer are added, an oxidizing agent is added, and the mixture is stirred until chemical oxidative polymerization is completed. Subsequently, residual monomers and impurities are removed by a purification method such as ultrafiltration, cation exchange, or anion exchange. A conductive polymer dispersion is obtained by this dispersion preparation process. The volatile alkali component and the non-volatile alkali component may be added to the conductive polymer dispersion after the residual monomers and impurities have been removed during the dispersion preparation process.

[0044] Examples of oxidizing agents that can be used include trivalent iron salts such as iron(III) p-toluenesulfonate, iron(III) naphthalenesulfonate, and iron(III) anthraquinonesulfonate, and peroxodisulfates such as peroxodisulfate, ammonium peroxodisulfate, and sodium peroxodisulfate. A single compound or two or more compounds may be used. In chemical oxidative polymerization, the polymerization temperature is not strictly limited, but is generally in the range of 10 to 60°C. The polymerization time is generally in the range of 10 minutes to 30 hours.

[0045] In the electrolytic oxidation polymerization and chemical oxidation polymerization, any solvent can be used without limitation as long as it can dissolve the desired amount of monomer and supporting electrolyte and does not adversely affect the electrolytic polymerization. Examples of suitable solvents include water, methanol, ethanol, isopropanol, butanol, ethylene glycol, acetonitrile, butyronitrile, acetone, methyl ethyl ketone, tetrahydrofuran, 1,4-dioxane, γ-butyrolactone, methyl acetate, ethyl acetate, methyl benzoate, ethyl benzoate, ethylene carbonate, propylene carbonate, nitromethane, nitrobenzene, sulfolane, and dimethyl sulfolane. These solvents may be used alone or in combination.

[0046] (Solid electrolytic capacitor) The anode and cathode electrode foils are long foils made of valve metals such as aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony. The purity of the anode electrode foil is preferably 99.9% or higher, and that of the cathode electrode foil is preferably 99% or higher, although impurities such as silicon, iron, copper, magnesium, and zinc may be included.

[0047] The anode electrode foil has a porous structure on its surface, either as a sintered body made by sintering valve metal powder or as an etched foil made by etching a stretched foil. The porous structure consists of tunnel-like pits, spongy pits, or voids between densely packed powder particles. The porous structure is typically formed by direct current etching or alternating current etching, in which direct current or alternating current is applied in an acidic aqueous solution containing halogen ions such as hydrochloric acid, or by vapor deposition or sintering metal particles or the like into the core. The cathode electrode foil may also have a porous surface if necessary.

[0048] The dielectric oxide film layer is the dielectric layer of a solid electrolytic capacitor and is typically an oxide film formed on the surface of the anode electrode foil. If the anode electrode foil is made of aluminum, the dielectric oxide film layer is an aluminum oxide layer formed by oxidizing the porous structure region. This dielectric oxide film layer is formed by applying a voltage in a solution free of halogen ions, such as an aqueous solution of adipic acid or boric acid. The cathode electrode foil may also have a dielectric oxide film layer formed thereon as needed, or may have a layer of metal nitride, metal carbide, or metal carbonitride formed thereon by vapor deposition, or may have a surface containing carbon. The dimensions of the anode and cathode foils can be freely set according to the specifications of the solid electrolytic capacitor to be manufactured.

[0049] Examples of separators include cellulose papers such as kraft, Manila hemp, esparto, hemp, and rayon, and mixed papers 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 separator separates the anode and cathode electrode foils to prevent short-circuiting between them, and also maintains the solid electrolyte layer between the electrode foils. If the shape of the solid electrolyte layer can be maintained by itself and the cathode and anode electrode foils can be separated by the solid electrolyte layer, the separator can be eliminated from the solid electrolytic capacitor.

[0051] A capacitor element is fabricated by winding a pair of electrode foils with a separator interposed therebetween. The separator is overlapped so that one end protrudes beyond the other end of the pair of electrode foils. The layer of the pair of electrode foils, cathode foil, and separator is wound by starting with the protruding separator to form a winding core, and then winding the winding core around the winding shaft. A solid electrolyte layer is then formed on the capacitor element, and the capacitor element is impregnated with an electrolyte solution.

[0052] The solid electrolyte layer contains the conductive polymer contained in the conductive polymer dispersion, a volatile alkaline component, a non-volatile alkaline component, or additives such as polyhydric alcohols, and adheres closely to the dielectric oxide film of the anode-side electrode foil to form the true cathode. This solid electrolyte layer is formed by immersing a capacitor element in the conductive polymer dispersion and adhering the conductive polymer to the dielectric oxide film layer. In the conductive polymer attachment step, a reduced pressure or pressurized treatment may be performed as needed to promote impregnation of the conductive polymer into the capacitor element. This attachment step may be repeated multiple times.

[0053] In addition to immersion, the conductive polymer dispersion may be impregnated into the capacitor element by dropwise application, spray application, etc. Furthermore, instead of the entire capacitor element, the anode side electrode foil and the cathode side electrode foil may be immersed in the conductive polymer dispersion, or the anode side electrode foil and the cathode side electrode foil may be dropwise or spray-applied with the conductive polymer dispersion before the capacitor element is assembled.

[0054] After the impregnation step, the capacitor element is dried to remove the solvent from the conductive polymer dispersion. In the drying step, the capacitor element is exposed to a temperature environment of 80°C to 200°C for 3 minutes to 180 minutes. This drying step may be repeated multiple times. The capacitor element may be dried in a reduced pressure environment, for example, at a pressure of 5 kPa to 100 kPa.

[0055] The capacitor element is impregnated with the electrolyte after the conductive polymer deposition step and drying step. This electrolyte is responsible for at least repairing the dielectric oxide film. The solvent in the electrolyte is preferably a glycol-based solvent. When the electrolyte solvent is a glycol-based solvent and the conductive polymer dispersion contains both a volatile alkaline component and a non-volatile alkaline component, deterioration of the sealing member in a high-temperature environment can be effectively suppressed, and a decrease in capacitance can also be effectively suppressed.

[0056] A glycol-based solvent is a dihydric alcohol in which two different carbon atoms in a chain aliphatic hydrocarbon or a cyclic aliphatic hydrocarbon are substituted with an OH group. Examples of glycol-based solvents include ethylene glycol, propylene glycol, diethylene glycol, glycerin, polyethylene glycol, and polyoxyethyleneglycerin. The electrolyte may contain one or more glycol-based solvents as solvents. Furthermore, as long as the electrolyte contains a glycol-based solvent, the electrolyte may contain a solvent other than a glycol-based solvent.

[0057] The electrolyte includes an anion component and a cation component. The electrolyte can be used without any particular limitation and is typically a salt of an organic acid, a salt of an inorganic acid, or a salt of a complex compound of an organic acid and an inorganic acid, and is used alone or in combination of two or more. An acid that becomes an anion and a base that becomes a cation may be added separately to the liquid.

[0058] Examples of organic acids that can serve as anion components 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-octanedicarboxylic acid, azelaic 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. Examples of inorganic acids include boric acid, phosphoric acid, phosphorous acid, hypophosphorous acid, carbonic acid, and silicic acid. Examples of composite compounds of organic acids and inorganic acids include 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.

[0059] Furthermore, examples of at least one salt of an organic acid, an inorganic acid, or a composite 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.

[0060] Furthermore, other additives can be added to the electrolyte solution. Examples of additives include complex compounds of boric acid and polysaccharides (mannitol, sorbitol, etc.), complex compounds of boric acid and polyhydric alcohols, boric acid esters, nitro compounds (o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, p-nitrophenol, p-nitrobenzyl alcohol, etc.), and phosphate esters. These may be used alone or in combination of two or more.

[0061] The amount of additive to be added is not particularly limited, but it is preferable to add it to an amount that does not deteriorate the characteristics of the solid electrolytic capacitor, for example, 60 wt% or less in the liquid. Among the above additives, it is preferable to add a complex compound of boric acid and polyhydric alcohol to improve the withstand voltage, or to add a nitro compound to absorb gas in the capacitor.

[0062] The electrolyte solution is filled into the voids of the capacitor element with the solid electrolyte layer in an impregnation step after the solid electrolyte layer is formed. The liquid may be impregnated to the extent that the solid electrolyte layer swells. In the impregnation step, a reduced pressure treatment or a pressurized treatment may be performed as necessary. [Example]

[0063] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0064] Conductive polymer dispersions of Examples 1 to 4 and Comparative Examples 1 and 2 were prepared by varying the molar ratio of the volatile alkaline component to the non-volatile alkaline component, and solid electrolytic capacitors were fabricated using each conductive polymer dispersion.

[0065] First, water was used as the solvent to prepare the conductive polymer dispersion. Polystyrene sulfonic acid (PSS)-doped polyethylenedioxythiophene (PEDOT) (PEDOT / PSS) powder was added to the water, and both volatile and non-volatile alkaline components were added and dissolved in water. Ammonia (NH3) was used as the volatile alkaline component, and sodium hydroxide (NaOH) was used as the non-volatile alkaline component.

[0066] In Examples 1 to 4 and Comparative Examples 1 and 2, the ratio of polymer to water and the ratio of alkaline component to water were the same, and the stirring method and conditions were also the same. However, Examples 1 to 4 and Comparative Examples 1 and 2 differ in the molar ratio of the volatile alkaline component to the non-volatile alkaline component added to the conductive polymer dispersion. Table 1 below shows the molar concentrations of ammonia and sodium hydroxide relative to the total alkaline components, which are the combined volatile alkaline component and non-volatile alkaline component added to the conductive polymer dispersion. (Table 1) TIFF0007718571000001.tif50161

[0067] Capacitor elements with solid electrolyte layers formed from this conductive polymer dispersion were fabricated in each example and comparative example as follows. The anode and cathode electrode foils were aluminum foils, which were etched to enlarge their surfaces, and the anode foil was subjected to a chemical conversion treatment to form a dielectric oxide film. Lead wires were connected to each of the anode and cathode electrode foils, and the anode and cathode electrode foils were wound facing each other with a manila separator interposed between them. This resulted in a capacitor element. The capacitor element was then immersed in an ammonium dihydrogen phosphate aqueous solution for 10 minutes for chemical conversion repair.

[0068] This capacitor element was immersed in the conductive polymer dispersion of each Example and Comparative Example. After the capacitor element was removed from the conductive polymer dispersion, it was dried at 150°C for 30 minutes. The immersion and drying were repeated twice. As a result, a solid electrolyte layer was formed on the capacitor element.

[0069] An electrolyte solution containing ethylene glycol as a solvent and ammonium azelaate as a solute was prepared for each example and comparative example. The electrolyte solution was then impregnated into the capacitor element with the solid electrolyte layer formed. The capacitor element was then inserted into an aluminum cylindrical outer case with a bottom, and a butyl rubber sealing rubber was attached to the open end and sealed by crimping.

[0070] Each solid electrolytic capacitor was subjected to an aging treatment by applying a voltage. The rated voltage of each solid electrolytic capacitor was 35 V and the rated capacitance was 47 μF. The solid electrolytic capacitors of Examples 1 to 4 and Comparative Examples 1 and 2 were exposed to a high-temperature environment at 180°C. The ESR before exposure to the high-temperature environment, i.e., before thermal stress loading, and the ESR after 150 hours of exposure in the high-temperature environment were measured, and the rate of change in ESR, ΔESR, was calculated. The capacitance before thermal stress loading and the capacitance after 323 hours of exposure in the high-temperature environment were also measured, and the rate of change in capacitance, ΔCap, was measured. The ESR was measured by passing a 100 kHz AC current through the solid electrolytic capacitor, and the capacitance was measured by passing a 120 Hz AC current through the solid electrolytic capacitor.

[0071] The measurement results for each of the examples and comparative examples are shown in Table 2 below. (Table 2) TIFF0007718571000002.tif59164

[0072] FIG. 1 is a graph showing the relationship between the ammonia concentration and the rate of change in capacitance ΔCap relative to the total amount of volatile alkali components and non-volatile alkali components, based on Table 2 above. FIG. 2 is a graph showing the relationship between the sodium hydroxide concentration and the rate of change in ESR ΔESR relative to the total amount of volatile alkali components and non-volatile alkali components, based on Table 2 above.

[0073] As shown in Table 2 and Figure 1, the solid electrolytic capacitor of Comparative Example 1, in which the solid electrolyte layer was formed using a conductive polymer dispersion liquid containing only a volatile alkaline component, ammonia, showed a significant decrease in capacitance when exposed to a high-temperature environment. Also, as shown in Table 2 and Figure 2, the solid electrolytic capacitor of Comparative Example 2, in which the solid electrolyte layer was formed using a conductive polymer dispersion liquid containing only a non-volatile alkaline component, sodium hydroxide, showed a significant increase in ESR when exposed to a high-temperature environment.

[0074] On the other hand, as shown in Table 2 and FIGS. 1 and 2, it was confirmed that the solid electrolytic capacitors of Examples 1 to 4, in which the solid electrolyte layer was formed using a conductive polymer dispersion liquid to which both a volatile alkaline component, i.e., ammonia, and a non-volatile alkaline component, i.e., sodium hydroxide, were added, maintained their capacitance and ESR well even when exposed to a high-temperature environment.

Claims

1. A method for producing a conductive polymer dispersion for forming a solid electrolyte layer of an electrolytic capacitor, comprising: a mixing step of mixing at least a solvent, a conductive polymer, an alkali component, and a polyhydric alcohol; In the mixing step, both a volatile alkali component and a non-volatile alkali component are mixed as the alkali component; A method for producing a conductive polymer dispersion liquid, comprising:

2. The volatile alkali component has a boiling point of −30° C. or lower, The low-volatility alkali component has a boiling point of 1000°C or higher; 2. The method for producing the conductive polymer dispersion according to claim 1,

3. the volatile alkaline component is ammonia, the low-volatile alkaline component is at least one selected from the group consisting of sodium hydroxide, potassium hydroxide, and calcium hydroxide; 3. The method for producing the conductive polymer dispersion according to claim 1 or 2,

4. the molar ratio of the volatile alkali component (A) to the hardly-volatile alkali component (B) is in the range of A:B=2:8 to 8:2; 4. The method for producing the conductive polymer dispersion according to claim 1, wherein the conductive polymer dispersion is a liquid containing a carboxylic acid.

5. an electrolyte layer formed in the capacitor element and including a solid electrolyte layer and an electrolyte solution; the solid electrolyte layer contains both a volatile alkaline component and a hardly-volatile alkaline component, and a polyhydric alcohol; A solid electrolytic capacitor characterized by:

6. the electrolytic solution contains at least a glycol as a solvent; 6. The solid electrolytic capacitor according to claim 5,

7. a dispersion preparation step of preparing a conductive polymer dispersion by mixing at least a solvent, a conductive polymer, an alkali component, and a polyhydric alcohol; an attachment step of attaching a conductive polymer to a capacitor element using the conductive polymer dispersion; a drying step of drying the capacitor element that has been subjected to the adhering step; an impregnation step of impregnating the capacitor element that has undergone the drying step with an electrolyte; Including, In the dispersion preparation step, both a volatile alkali component and a hardly volatile alkali component are added to the conductive polymer dispersion as the alkali component; A method for manufacturing a solid electrolytic capacitor, comprising:

8. In the drying step, the capacitor element is exposed to a temperature environment that is equal to or higher than the boiling points of the solvent and the volatile alkaline component and lower than the boiling point of the non-volatile alkaline component; 8. The method for producing a solid electrolytic capacitor according to claim 7,

9. A conductive polymer dispersion for forming a solid electrolyte layer on a capacitor element of an electrolytic capacitor, The composition contains at least a solvent, a conductive polymer, a polyhydric alcohol, and an alkaline component, The alkaline component contains both a volatile alkaline component and a non-volatile alkaline component; A conductive polymer dispersion characterized by:

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