Solid electrolyte, solid electrolytic capacitor, conductive polymer fluid dispersion, manufacturing method for solid electrolyte, and manufacturing method for conductive polymer fluid dispersion

Abstract

To provide a solid electrolyte establishing both of low ESR and high voltage resistance of a solid electrolytic capacitor, the solid electrolytic capacitor, conductive polymer fluid dispersion, and manufacturing methods therefor.SOLUTION: A solid electrolyte is formed from conductive polymer fluid dispersion containing a conductive polymer. The conductive polymer is Bu-PEDOT doped with PSS. A mole ratio of the Bu-PEDOT(A) to the doped PSS(B) is A:B=1:2.8 to 1:4.7. The solid electrolyte is manufactured by a method including a production step of producing the conductive polymer fluid dispersion and a dehydration step of evaporating at least part of a liquid component from the conductive polymer fluid dispersion. The conductive polymer fluid dispersion is manufactured by a method including an addition step of adding the PSS and Bu-EDOT to a solvent of the fluid dispersion. In the addition step, the Bu-EDOT(C) and PSS(D) are added with a feed composition ratio in terms of a molar ratio of C:D=1:4 to 1:12.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a solid electrolyte provided in a solid electrolytic capacitor, a solid electrolytic capacitor, a conductive polymer dispersion for forming the solid electrolyte, and methods for producing these. [Background technology]

[0002] Capacitors are used in a variety of applications. For example, in the field of power electronics, power from an AC power source is converted into DC power by a converter circuit, and this DC power is then converted into desired AC power by an inverter circuit. A smoothing capacitor is provided in a power supply circuit to suppress pulsation in the DC output from the converter circuit and smooth the DC before inputting it to the inverter circuit. Furthermore, a decoupling capacitor is provided near a semiconductor switching element such as gallium nitride to ensure stable operation of the semiconductor switching element and remove noise.

[0003] With the recent trend toward higher power consumption, there is a growing demand for higher capacitance in capacitors. Electrolytic capacitors are easier to achieve higher capacitance than film capacitors, and are therefore more likely to meet this demand. 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 a dielectric film is formed on the enlarged surface by anodizing or other processes. An electrolyte is interposed between the anode and cathode foils.

[0004] This electrolytic capacitor can increase the specific surface area by enlarging the anode foil, resulting in a large capacitance and meeting the demand for higher capacity. Furthermore, the electrolytic capacitor contains an electrolyte in the form of an electrolytic solution. The contact area of ​​the electrolytic solution with the dielectric film on the anode foil increases. This allows the electrolytic capacitor to have a larger capacitance, making it suitable for meeting the demand for higher capacity associated with the recent trend toward higher power. However, the electrolytic solution evaporates and dissipates over time, causing the electrolytic capacitor to experience a decrease in capacitance and an increase in dielectric loss tangent, resulting in drying out.

[0005] Therefore, solid electrolytic capacitors using conductive polymers as electrolytes have also attracted attention. Conductive polymers are polymerized monomers with π-conjugated double bonds, such as poly(3,4-ethylenedioxythiophene) (PEDOT). During chemical oxidation polymerization or electrolytic oxidation polymerization of the conductive polymer, an acid compound such as a polyanion is used as a dopant, resulting in high conductivity. Therefore, solid electrolytic capacitors have the advantage of low equivalent series resistance (ESR). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-160647 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-258224 Summary of the Invention [Problem to be solved by the invention]

[0007] Thus, compared to electrolytic capacitors using electrolytes, solid electrolytic capacitors are free from the risk of electrolyte drying up and are easier to achieve low equivalent series resistance. Meanwhile, in some fields, such as power electronics, capacitors with high withstand voltages are expected, with at least 400 V becoming the standard in electric vehicles (Battery Electric Vehicles). Electrolytic capacitors using electrolytes have a high withstand voltage due to the ability of the electrolyte to repair defects in the dielectric film, but even these electrolytic capacitors do not easily meet the high withstand voltage requirement.

[0008] Furthermore, solid electrolytic capacitors have difficulty meeting the high voltage requirements compared to electrolytic capacitors that use liquid electrolytes due to the need for adhesion to the anode foil and the ability to repair defects in the dielectric film. In particular, achieving high voltage while maintaining the low ESR, which is an advantage of solid electrolytic capacitors, has not been easy.

[0009] The present invention has been proposed to solve the above-mentioned problems, and its object is to provide a solid electrolyte that achieves both low ESR and high withstand voltage in a solid electrolytic capacitor, a solid electrolytic capacitor using this solid electrolyte, a conductive polymer dispersion for forming this solid electrolytic capacitor, and methods for producing these. [Means for solving the problem]

[0010] In order to solve the above problems, the solid electrolyte of the present invention is a solid electrolyte formed from a conductive polymer dispersion liquid containing a conductive polymer, wherein the conductive polymer is poly(2-butyl-3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid, and the molar ratio of the poly(2-butyl-3,4-ethylenedioxythiophene) (A) to the doped polystyrene sulfonic acid (B) is A:B=1:2.8 to 1:4.7.

[0011] Another aspect of the present invention is a solid electrolytic capacitor comprising this solid electrolyte, an anode having a dielectric film, and a cathode facing the anode across the solid electrolyte.

[0012] Another embodiment of the present invention is a conductive polymer dispersion liquid containing poly(2-butyl-3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid, in which the content ratio of the poly(2-butyl-3,4-ethylenedioxythiophene) (A) to the polystyrene sulfonic acid (B) is A:B=1:2.8 to 1:4.7 in terms of molar ratio.

[0013] In order to solve the above-mentioned problems, the method for producing a solid electrolyte of the present invention includes a producing step of producing a conductive polymer dispersion in which poly(2-butyl-3,4-ethylenedioxythiophene) (A) and polystyrene sulfonic acid (B) are doped into the poly(2-butyl-3,4-ethylenedioxythiophene) so that the molar ratio of the poly(2-butyl-3,4-ethylenedioxythiophene) (A) to the polystyrene sulfonic acid (B) is A:B=1:2.8 to 1:4.7, and a drying step of evaporating at least a part of a liquid component from the conductive polymer dispersion.

[0014] Furthermore, in order to solve the above-mentioned problems, the method for producing a conductive polymer dispersion of the present invention produces a conductive polymer dispersion in which poly(2-butyl-3,4-ethylenedioxythiophene) (A) and polystyrene sulfonic acid (B) are doped into the poly(2-butyl-3,4-ethylenedioxythiophene) so that the molar ratio of the poly(2-butyl-3,4-ethylenedioxythiophene) (A) to the polystyrene sulfonic acid (B) is A:B=1:2.8 to 1:4.7.

[0015] The method may include an addition step of adding the polystyrene sulfonic acid and 2-butyl-3,4-ethylenedioxythiophene to a solvent, and in the addition step, the 2-butyl-3,4-ethylenedioxythiophene (C) and the polystyrene sulfonic acid (D) may be added at a molar ratio of C:D=1:4 to 1:12. [Effects of the Invention]

[0016] According to the present invention, a solid electrolytic capacitor that combines low ESR and high withstand voltage can be realized. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the embodiments described below.

[0018] (solid electrolyte) A solid electrolyte is interposed between the anode foil and cathode foil of a solid electrolytic capacitor. A solid electrolytic capacitor is a passive device that stores and discharges electric charge through capacitance. The anode foil and cathode foil are arranged facing each other with a separator interposed between them, and the solid electrolyte is interposed between the anode foil and cathode foil. Within the solid electrolytic capacitor, the solid electrolyte is in contact with the dielectric film formed on the anode foil and functions as the true cathode of the solid electrolytic capacitor.

[0019] The solid electrolyte is formed using a conductive polymer dispersion. The conductive polymer dispersion is a liquid in which conductive polymer particles or powder are dispersed, and the conductive polymer adheres to the anode foil, cathode foil, and separator as a solid electrolyte. That is, the solid electrolyte contains a conductive polymer.

[0020] The conductive polymer is attached as a solid electrolyte by immersing each of the anode foil, cathode foil, and separator in a conductive polymer dispersion and drying them. Alternatively, a capacitor element may be fabricated by winding or stacking the anode foil and cathode foil with the separator interposed therebetween, and then the capacitor element may be immersed in the conductive polymer dispersion and dried to form a solid electrolyte. In addition to immersion, the conductive polymer dispersion may be applied by dropwise or spray coating. The drying process may be repeated multiple times, or drying may be performed under reduced pressure.

[0021] The conductive polymer is poly(2-butyl-3,4-ethylenedioxythiophene) (hereinafter also referred to as Bu-PEDOT) represented by the following chemical formula (1), which is formed by adding a butyl group as a substituent to the 2-position of 3,4-ethylenedioxythiophene. The butyl group may be any of n-butyl, sec-butyl, isobutyl, and tert-butyl. [ka]

[0022] This conductive polymer exhibits conductivity through doping. The dopant is polystyrene sulfonate, also known as PSS. If Bu-PEDOT is A and the doped PSS is B, PSS is doped with Bu-PEDOT at a molar ratio of A:B = 1:2.8 to 1:4.7. This conductive polymer gives the solid electrolytic capacitor high voltage resistance and low ESR.

[0023] The number average molecular weight of the PSS is 1,000 to 2,000,000, and 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, while if the number average molecular weight exceeds 2,000,000, the viscosity of the mixture will increase, which is undesirable.

[0024] In addition to the conductive polymer, the solid electrolyte may contain various additives such as polyhydric alcohols. The polyhydric alcohols change the higher-order structure of the conductive polymer and enhance the reorientation of the crystalline structure of the polymer chains. This reduces the ESR of the electrolytic capacitor and improves its withstand voltage. Examples of polyhydric alcohols include sorbitol, ethylene glycol, diethylene glycol, triethylene glycol, polyoxyethylene glycol, glycerin, polyoxyethyleneglycerin, xylitol, erythritol, mannitol, dipentaerythritol, pentaerythritol, and combinations of two or more of these.

[0025] Polyhydric alcohols remain in the solid electrolyte layer due to their high boiling point. The polyhydric alcohols change the higher-order structure of the conductive polymer and reorient the crystalline structure of the polymer chains. As a result, the polyhydric alcohols improve carrier mobility and the electrical conductivity of the conductive polymer, further reducing the ESR of the solid electrolytic capacitor and further increasing the withstand voltage.

[0026] (Conductive polymer dispersion) The solid electrolyte is formed using a conductive polymer dispersion. The conductive polymer dispersion is a liquid in which conductive polymer particles or powder are dispersed. At least the anode foil is immersed in the conductive polymer dispersion as the target object. In addition to the anode foil, the cathode foil, the separator, or both may also be immersed in the conductive polymer dispersion as the target object. A capacitor element consisting of an anode foil, a cathode foil, and a separator stacked together may also be immersed in the conductive polymer dispersion as the target object. This causes the conductive polymer to adhere to the target object, forming a solid electrolyte layer on and within the target object.

[0027] The conductive polymer dispersion may be applied by dropwise application or spray application, in addition to immersion. The conductive polymer dispersion may be applied by immersion or application once or multiple times. The conductive polymer dispersion may be impregnated into the object to which the conductive polymer is to be attached while reducing the pressure inside the object, or while further pressurizing the conductive polymer dispersion. After immersion or application, a drying step is performed to evaporate liquid components such as the dispersion medium in the conductive polymer dispersion. The liquid components are evaporated partially or completely. When a polyhydric alcohol is contained in the conductive polymer dispersion and the polyhydric alcohol is left in the solid electrolyte layer, the temperature in the drying step is set to a temperature lower than the boiling point of the polyhydric alcohol.

[0028] The conductive polymer is preferably added to the conductive polymer dispersion at a concentration of 0.1 wt% to 7.0 wt%. This range makes the conductive polymer less likely to aggregate. This range also improves heat resistance, preventing the degradation of the solid electrolytic capacitor's properties even when the solid electrolyte is exposed to high-temperature environments.

[0029] The dispersion medium for the conductive polymer liquid is water or a mixture of water and an organic solvent. Suitable examples of the organic solvent include polar solvents, alcohols, esters, hydrocarbons, carbonate compounds, ether compounds, chain ethers, heterocyclic compounds, nitrile compounds, and sulfone-based solvents.

[0030] 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. Examples of sulfones include dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methyl sulfolane, and 2,4-dimethyl sulfolane.

[0031] The conductive polymer dispersion may contain a pH adjuster, a polyhydric alcohol to be contained in the solid electrolyte, and various additives, as needed. The pH adjuster neutralizes the dopant to improve the acidity of the conductive polymer dispersion, preventing dissolution of the anode foil, cathode foil, and separator. Examples of pH adjusters include ammonia, water-soluble alkylamines such as ethylamine and diethylamine, water-soluble arylamines such as aniline and benzylamine, and water-soluble heterocyclic amines such as pyridine and imidazole. Examples of pH adjusters include alkali metal or alkaline earth metal hydroxides such as sodium hydroxide and calcium hydroxide, alkali metal or alkaline earth metal carbonates such as sodium carbonate and calcium carbonate, and alkali metal or alkaline earth metal alkoxides such as sodium methoxide and calcium methoxide.

[0032] Examples of additives include organic binders, surfactants, dispersants, antifoaming agents, coupling agents, antioxidants, and ultraviolet absorbers. Examples of organic binders include polyvinyl alcohol, methyl methacrylate, and hydroxypropyl methylcellulose. Examples of surfactants include sorbitan oleate, polyoxyethylene sorbitan oleate, and alkylphenol polyoxyethylene ether. Examples of dispersants include polyvinylpyrrolidone, carboxymethylcellulose, and polyoxyethylene. Examples of antifoaming agents include silicone oil, polyglycerin laurate, and polyglycerin polyphosphate. Examples of coupling agents include silane coupling agents, polyimide coupling agents, and titanate esters. Examples of antioxidants include hydroquinone, methylhydroquinone, and 1,4-dihydroxybenzene. Examples of ultraviolet absorbers include benzophenone, octocrylene, and avobenzone.

[0033] In the process of producing this conductive polymer dispersion, the conductive polymer is produced by a polymerization process. In the polymerization process, the monomer 2-butyl-3,4-ethylenedioxythiophene (hereinafter also referred to as Bu-EDOT), the dopant PSS, and the oxidizing agent are added to the solvent in the addition process, and the mixture is stirred until chemical oxidative polymerization is complete. In chemical oxidative polymerization, there are no strict limitations on the polymerization temperature, but it is generally in the range of 0 to 60°C. The polymerization time is generally in the range of 10 minutes to 30 hours.

[0034] In the addition step, any solvent can be used without particular limitation as long as it can dissolve the desired amount of monomer and oxidizing agent and does not adversely affect the chemical oxidative polymerization. Examples of 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.

[0035] In the addition step, Bu-EDOT (C) and PSS (D) are added in a molar ratio of C:D = 1:4 to 1:12. As a result, if Bu-PEDOT is A and doped PSS is B, PSS is doped with Bu-PEDOT in a molar ratio of A:B = 1:2.8 to 1:4.7.

[0036] The oxidizing agent used in the polymerization step is preferably an iron salt of an inorganic acid or an organic acid, or a persulfate. Examples include ferric chloride hexahydrate, anhydrous ferric chloride, ferric nitrate nonahydrate, ferric nitrate, ferrous sulfate, ferric sulfate, ferric sulfate n-hydrate, ammonium ferric sulfate dodecahydrate, ferric perchlorate n-hydrate, ferric tetrafluoroborate, cupric chloride, cupric sulfate, cupric tetrafluoroborate, nitrosonium tetrafluoroborate, peroxodisulfate, ammonium peroxodisulfate, sodium peroxodisulfate, potassium persulfate, potassium periodate, hydrogen peroxide, ozone, potassium hexacyanoferric, tetraammonium cerium (IV) sulfate dihydrate, bromine, iodine, iron dodecylbenzenesulfonate, ferric paratoluenesulfonate, ferric naphthalenesulfonate, ferric anthraquinonesulfonate, periodic acid, and iodic acid. The oxidizing agent may be a single compound or two or more compounds.

[0037] In particular, it is preferable to use an iron salt such as ferrous sulfate in combination with a peroxodisulfate as the oxidizing agent. The iron salt acts as an oxidation initiator and catalyst, while the peroxodisulfate acts as an oxidation promoter. Specifically, the divalent iron ions of the oxidizing agent react with persulfate ions to form radical sulfate ions, which then accelerate the oxidation reaction of the monomer. The divalent iron ions reacting with persulfate ions become trivalent iron ions, oxidize the monomer, then return to divalent iron ions, and react again with persulfate ions, repeating this cycle. It is preferable to adjust the pH of the polymerization solution to an acidic side by adding sulfuric acid or the like. Adjusting the polymerization solution to an acidic side, such as pH 1.9, accelerates the polymerization reaction.

[0038] After the polymerization process, residual monomers and impurities are removed by purification methods such as ultrafiltration, cation exchange, and anion exchange. This results in a conductive polymer dispersion. The conductive polymer is dispersed using, for example, ultrasound. In the polymerization process, an acid or its alkali metal salt that releases PSS may be mixed in place of PSS. Alternatively, undoped PSS may remain in the conductive polymer dispersion.

[0039] (Solid electrolytic capacitor) The anode foil and the cathode foil are alternately stacked with a separator sandwiched between them in a laminated arrangement. The laminated type may be a flat type without an exterior packaging, or may be sealed by, for example, covering the capacitor element with a laminate film, or by molding, dip-coating, or printing a resin such as a heat-resistant resin or an insulating resin. Alternatively, the anode foil and the cathode foil may be alternately stacked with a separator sandwiched between them and wound in a wound arrangement. In the wound type, for example, the capacitor element is inserted into a cylindrical exterior case with a bottom, and the open end of the exterior case is sealed with a sealing body by crimping. The sealing body is, for example, rubber such as ethylene propylene rubber or butyl rubber, or a laminate of rubber and a hard substrate.

[0040] The anode foil and cathode foil are made of valve metals, such as 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, and that of the cathode foil is preferably 99% or higher, but impurities such as silicon, iron, copper, magnesium, and zinc may be present.

[0041] When a solid electrolytic capacitor is a flat plate, the anode is a flat plate made of a valve metal. The anode plate is immersed in a conductive polymer dispersion to form a solid electrolyte covering the dielectric film. Next, a carbon paste is printed on the solid electrolyte layer using a screen printer or similar and allowed to dry. A metal paste such as silver paste is then printed on the carbon layer and allowed to dry. These carbon and silver layers correspond to the cathode of the solid electrolytic capacitor.

[0042] The anode 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 foil may also have a porous surface if necessary.

[0043] The dielectric film is the dielectric layer of a solid electrolytic capacitor and is typically an oxide film formed on the surface of an anode foil. If the anode foil is an aluminum foil, the dielectric film is an aluminum oxide layer formed by oxidizing the porous structure region. This dielectric film 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 foil may also be formed with an oxide film as needed, and may further include a layer of metal nitride, metal carbide, or metal carbonitride formed by vapor deposition, or one containing carbon on the surface.

[0044] 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.

[0045] The separator separates the anode and cathode foils to prevent them from shorting out, and also holds the solid electrolyte between them. If the shape of the solid electrolyte can be maintained by itself and the cathode foils can be isolated by the solid electrolyte, the separator can be eliminated from the solid electrolytic capacitor.

[0046] Solid electrolytic capacitors can use a liquid electrolyte in addition to the solid electrolyte. The liquid electrolyte is impregnated into the capacitor element after the conductive polymer deposition and drying processes. This liquid electrolyte at least repairs the dielectric film and also provides ionic conductivity, whereas the conductive polymer solid electrolyte is an electronic conduction medium.

[0047] The solvent for the electrolyte is not particularly limited, and a protic organic polar solvent or an aprotic organic polar solvent can be used. Typical examples of protic polar solvents include monohydric alcohols, polyhydric alcohols, and oxyalcohol compounds, such as ethylene glycol, propylene glycol, and glycerin. Typical examples of aprotic polar solvents include sulfones, amides, lactones, cyclic amides, nitriles, and sulfoxides, such as sulfolane, γ-butyrolactone, ethylene carbonate, and propylene carbonate.

[0048] The solute contained in the electrolyte solution includes anionic and cationic components, typically organic acids such as adipic acid and benzoic acid or their salts, inorganic acids such as boric acid and phosphoric acid or their salts, or composite compounds of organic acids and inorganic acids such as borodisalicylic acid or their ionically dissociable salts, which may be used alone or in combination of two or more. Examples of these salts of organic acids, salts of inorganic acids, and at least one salt of a composite compound of an organic acid and an inorganic acid include ammonium salts, quaternary ammonium salts, quaternized amidinium salts, amine salts, sodium salts, potassium salts, etc. Anionic acids and cationic bases may be added separately to the electrolyte solution as solute components. [Example]

[0049] 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.

[0050] Example 1 The conductive polymer dispersion of Example 1a was prepared as follows. First, 181.853 g of a PSS aqueous solution, which served as a dopant, was added to 1008.735 mL of pure water. The concentration of the PSS aqueous solution was 17.1 wt%. To this PSS aqueous solution, 23.466 g of ferrous sulfate, which served as a catalyst and had the composition formula FeSO4, was added. Furthermore, 152.576 g of sulfuric acid, which had the chemical formula H2SO4, was added to adjust the pH.

[0051] The mixture was stirred with a stirrer and dissolved oxygen was removed by nitrogen bubbling. The mixture was then stirred with a homogenizer for 2 hours. 8.368 g of the monomer Bu-EDOT was added to the mixture, and the mixture was homogenized and stirred with a stirrer for 30 minutes. The molar ratio of the mixture (181.853 g of PSS aqueous solution, 17.1 wt%) to 8.368 g of Bu-EDOT was [Bu-EDOT]:[PSS] = 1:4. In other words, PSS was added in a molar ratio of 4 times that of Bu-EDOT.

[0052] Next, the mixture was mixed with an aqueous solution of 19.262 g of ammonium peroxodisulfate (NH4)2S2O8 (an oxidizing agent) dissolved in 105.740 mL of pure water to initiate polymerization. The polymerization temperature was 20°C, and the polymerization time was 24 hours. During the polymerization, the mixture was continuously homogenized and stirred with a stirrer.

[0053] After 24 hours, ion exchange resin was added to the mixture and stirred for another 24 hours. The ion exchange resin was then removed from the mixture and subjected to ultrafiltration to remove the remaining monomer, dopant, and oxidant. After ion exchange and ultrafiltration, the conductive polymer particle size was reduced using a high-pressure homogenizer.

[0054] The conductive polymer dispersion of Example 1a was prepared by dispersing the conductive polymer in ethylene glycol as a dispersion medium and then adding sorbitol. The conductive polymer concentration was 1 wt% and the ethylene glycol concentration was 10 wt%. Sorbitol was added so that the total solid content of the conductive polymer and sorbitol was 85 wt%. The pH of the conductive polymer dispersion was adjusted to 4 by adding ammonia. The viscosity of this conductive polymer dispersion was 15 mPa·s or less.

[0055] A solid electrolytic capacitor having the solid electrolyte of Example 1b was fabricated using the conductive polymer dispersion of Example 1a. This solid electrolytic capacitor was fabricated as follows.

[0056] An aluminum foil was prepared as the anode foil. The anode body was an aluminum foil with a purity of 99.99%. This aluminum foil was subjected to a DC etching process to enlarge the surface. By enlarging the surface, tunnel-shaped pits were formed on both sides of the aluminum foil. In this DC etching process, a DC current was passed through the aluminum foil in an aqueous solution containing hydrochloric acid to form pits, and then a DC current was passed through the aluminum foil in an aqueous solution containing nitric acid to enlarge the pits.

[0057] A dielectric film of aluminum oxide was formed on the surface-enlarged aluminum foil by chemical conversion treatment. In the chemical conversion treatment process, the aluminum foil was immersed in a boric acid aqueous solution with a concentration of 20 g / L at a liquid temperature of 90°C, and a current density of 25 mAcm was applied to the aluminum foil. -2 The voltage was increased to 946 V, and the voltage was maintained for 20 minutes.

[0058] The aluminum foil was punched into the desired electrode shape. The exposed end surface was then subjected to a chemical conversion treatment again. In the chemical conversion treatment, the aluminum foil was immersed in a boric acid solution with a concentration of 20 g / L at a liquid temperature of 85°C, and a current density of 1 mAcm was applied to the aluminum foil. -2 After applying a voltage of 851 V, the voltage was maintained for 10 minutes.

[0059] Both sides of the aluminum foil were masked with imide tape, leaving four exposed areas with a diameter of 13 mm so that one side of the aluminum foil was exposed in four places. The conductive polymer dispersion of Example 1a was dripped onto the four exposed areas and dried. The amount of conductive polymer dispersion dripped was 106 μL per exposed area. After dripping the conductive polymer dispersion, the sample was left in a temperature environment of 60°C for 10 minutes, and then left at 110°C for 30 minutes, thereby evaporating the liquid components from the conductive polymer dispersion. As a result, a layer of the solid electrolyte of Example 1b was formed on the aluminum foil.

[0060] Subsequently, a solid electrolyte layer was formed, and then carbon paste was applied to the solid electrolyte and allowed to harden by leaving it in a 110°C environment for 30 minutes. Furthermore, silver paste was applied onto the carbon layer, and at the same time, copper foil measuring 5 mm in width, 4 cm in length, and 20 μm in thickness was adhered as a lead terminal. The silver paste was allowed to harden by leaving it in a 110°C environment for 30 minutes with the copper foil adhered. The carbon layer and silver layer correspond to the cathode foil of the solid electrolytic capacitor. In this way, a solid electrolytic capacitor having the solid electrolyte of Example 1b was produced.

[0061] Example 2 A conductive polymer dispersion of Example 2a was prepared. The conductive polymer dispersion of Example 2a differs from that of Example 1a in the charge composition ratio of Bu-EDOT and PSS in the addition step. In Example 2a, 272.779 g of a PSS aqueous solution (concentration 17.1 wt%) as a dopant and 8.368 g of Bu-EDOT as a monomer were added to 917.809 mL of pure water. The charge composition ratio of 272.779 g of a PSS aqueous solution (concentration 17.1 wt%) and 8.368 g of Bu-EDOT was [Bu-EDOT]:[PSS] = 1:6 in molar ratio. In other words, PSS was added in an amount six times the molar amount of Bu-EDOT. Other than the difference in charge composition ratio, Example 2a was prepared using the same manufacturing method and under the same conditions as the conductive polymer dispersion of Example 1a.

[0062] The conductive polymer dispersion of Example 2a was used to prepare the solid electrolyte of Example 2b and a solid electrolytic capacitor having this solid electrolyte. The solid electrolyte of Example 2b and the solid electrolytic capacitor having this solid electrolyte were produced by the same production method and under the same conditions as the solid electrolyte and solid electrolytic capacitor of Example 1b, except that they were produced using the conductive polymer dispersion of Example 2a.

[0063] Example 3 A conductive polymer dispersion of Example 3a was prepared. The conductive polymer dispersion of Example 3a differs from that of Example 1a in the charge composition ratio of Bu-EDOT and PSS in the addition step. In Example 3a, 363.706 g of a PSS aqueous solution (concentration 17.1 wt%) as a dopant and 8.368 g of Bu-EDOT as a monomer were added to 826.882 mL of pure water. The charge composition ratio of 363.706 g of a PSS aqueous solution (concentration 17.1 wt%) and 8.368 g of Bu-EDOT was [Bu-EDOT]:[PSS] = 1:8 in molar ratio. In other words, PSS was added in an amount 8 times the molar ratio of Bu-EDOT. Other than the difference in charge composition ratio, Example 3a was prepared using the same manufacturing method and under the same conditions as the conductive polymer dispersion of Example 1a.

[0064] The conductive polymer dispersion of Example 3a was used to prepare the solid electrolyte of Example 3b and a solid electrolytic capacitor having this solid electrolyte. The solid electrolyte of Example 3b and the solid electrolytic capacitor having this solid electrolyte were produced by the same production method and under the same conditions as the solid electrolyte and solid electrolytic capacitor of Example 1b, except that they were produced using the conductive polymer dispersion of Example 3a.

[0065] Example 4 A conductive polymer dispersion of Example 4a was prepared. The conductive polymer dispersion of Example 4a differs from that of Example 1a in the charge composition ratio of Bu-EDOT and PSS in the addition step. In Example 4a, 545.559 g of a PSS aqueous solution (concentration 17.1 wt%) as a dopant was added to 645.029 mL of pure water, and 8.368 g of Bu-EDOT as a monomer was added. The charge composition ratio of 545.559 g of a PSS aqueous solution (concentration 17.1 wt%) to 8.368 g of Bu-EDOT was [Bu-EDOT]:[PSS] = 1:12 in molar ratio. In other words, PSS was added in an amount 12 times the molar amount of Bu-EDOT. Other than the difference in charge composition ratio, Example 4a was prepared using the same manufacturing method and under the same conditions as the conductive polymer dispersion of Example 1a.

[0066] The conductive polymer dispersion of Example 4a was used to prepare the solid electrolyte of Example 4b and a solid electrolytic capacitor having this solid electrolyte. The solid electrolyte of Example 4b and the solid electrolytic capacitor having this solid electrolyte were produced by the same production method and under the same conditions as the solid electrolyte and solid electrolytic capacitor of Example 1b, except that they were produced using the conductive polymer dispersion of Example 4a.

[0067] (Comparative Example 1) A conductive polymer dispersion liquid of Comparative Example 1a was prepared. The conductive polymer dispersion liquid of Comparative Example 1a differs from Example 1a in the charged composition ratio of Bu-EDOT and PSS in the addition step. In Comparative Example 1a, 90.926 g of a PSS aqueous solution (concentration 17.1 wt%) as a dopant and 8.368 g of Bu-EDOT as a monomer were added to 1099.662 mL of pure water. The charged composition ratio of 90.926 g of a PSS aqueous solution (concentration 17.1 wt%) to 8.368 g of Bu-EDOT was [Bu-EDOT]:[PSS] = 1:2 in molar ratio. In other words, PSS was added so that the molar ratio was twice that of Bu-EDOT. Other than the difference in charged composition ratio, Comparative Example 1a was prepared using the same manufacturing method and under the same conditions as the conductive polymer dispersion liquid of Example 1a.

[0068] The conductive polymer dispersion of Comparative Example 1a was used to prepare the solid electrolyte of Comparative Example 1b and a solid electrolytic capacitor having this solid electrolyte. The solid electrolyte of Comparative Example 1b and the solid electrolytic capacitor having this solid electrolyte were produced by the same production method and under the same conditions as the solid electrolyte and solid electrolytic capacitor of Example 1b, except that they were produced using the conductive polymer dispersion of Comparative Example 1a.

[0069] (Comparative Example 2) A conductive polymer dispersion liquid of Comparative Example 2a was prepared. The conductive polymer dispersion liquid of Comparative Example 2a differs from that of Example 1a in the charge composition ratio of Bu-EDOT and PSS in the addition step. In Comparative Example 2a, 727.412 g of a PSS aqueous solution (concentration 17.1 wt%) as a dopant and 8.368 g of Bu-EDOT as a monomer were added to 463.176 mL of pure water. The charge composition ratio of 727.412 g of a PSS aqueous solution (concentration 17.1 wt%) and 8.368 g of Bu-EDOT was [Bu-EDOT]:[PSS] = 1:16 in molar ratio. In other words, PSS was added so that the molar ratio was 16 times that of Bu-EDOT. Other than the difference in charge composition ratio, Comparative Example 2a was prepared using the same manufacturing method and under the same conditions as the conductive polymer dispersion liquid of Example 1a.

[0070] The conductive polymer dispersion of Comparative Example 2a was used to prepare the solid electrolyte of Comparative Example 2b and a solid electrolytic capacitor having this solid electrolyte. The solid electrolyte of Comparative Example 2b and the solid electrolytic capacitor having this solid electrolyte were produced by the same production method and under the same conditions as the solid electrolyte and solid electrolytic capacitor of Example 1b, except that they were produced using the conductive polymer dispersion of Comparative Example 2a.

[0071] (Comparative Example 3) A conductive polymer dispersion liquid of Comparative Example 3a was prepared. The conductive polymer dispersion liquid of Comparative Example 3a differs from that of Example 1a in the type of monomer that forms the conductive polymer. The monomer used when preparing the conductive polymer dispersion liquid of Comparative Example 3a was 2-ethyl-3,4-ethylenedioxythiophene (hereinafter also referred to as Et-EDOT), in which an ethyl group is added as a substituent to the 2-position of 3,4-ethylenedioxythiophene.

[0072] In Comparative Example 3a, 90.926 g of the dopant PSS aqueous solution (concentration 17.1 wt%) and 7.184 g of the monomer Et-EDOT were added to 1100.846 mL of pure water. The molar ratio of the 90.926 g PSS aqueous solution (concentration 17.1 wt%) and 7.184 g Et-EDOT was [Et-EDOT]:[PSS] = 1:2. In other words, PSS was added in a molar ratio twice that of Et-EDOT. Other than the differences in the type of monomer and the charged composition, Comparative Example 3a was prepared using the same manufacturing method and conditions as the conductive polymer dispersion of Example 1a.

[0073] The conductive polymer dispersion of Comparative Example 3a was used to prepare the solid electrolyte of Comparative Example 3b and a solid electrolytic capacitor having this solid electrolyte. The solid electrolyte of Comparative Example 3b and the solid electrolytic capacitor having this solid electrolyte were produced by the same production method and under the same conditions as the solid electrolyte and solid electrolytic capacitor of Example 1b, except that they were produced using the conductive polymer dispersion of Comparative Example 3a.

[0074] (Measurement of composition ratio) The composition ratios of Bu-PEDOT and PSS doped in the Bu-PEDOT contained in the conductive polymer dispersions of Examples 1a to 4a and Comparative Examples 1a to 3a were analyzed. The conductive polymer dispersion of Example 1a was dropped onto a silicon substrate and dried at 110°C for 30 minutes to form a conductive polymer film on the silicon substrate. This conductive polymer film was analyzed by X-ray photoelectron spectroscopy.

[0075] For X-ray photoelectron spectroscopy, an X-ray photoelectron spectrometer (Kratos Analytical, model ESCA-3400, control software Vision Instrument Manager) was used. The conductive polymer film was placed on a sample stage, and spectra were measured using narrow scan measurement. Magnesium Kα radiation (Mg-Kα) was used as the X-ray source, and the S2p spectrum was obtained by irradiating the conductive polymer film with X-rays. Other narrow scan conditions were an acceleration voltage of 10 kV, an emission current of 15 mA, a pass energy of 75 eV, a step voltage of 0.05 eV, and 35 accumulations.

[0076] Vision Processing was used as the waveform analysis software for the S2p spectrum. The Shirley method was used for background subtraction to separate the S2p spectrum into background and signal. Charge correction was performed by correcting the peak-top binding energy of the C1s spectrum (280–300 eV) obtained under the same measurement conditions as the S2p spectrum to 284.8 eV, and charge correction was also performed for the O1s spectrum. Peak fitting was performed using a Gaussian-Lorentzian mixed function (70% Gaussian function ratio) to approximate individual peaks. For peak fitting, the full width half maximum (FWHM) of the C1s spectrum was set to 1.5–1.9 eV, and the FWHM of the O1s spectrum was set to 2.0–2.5 eV.

[0077] The area ratio of the peak at 164-165 eV due to the S-C bond of Bu-PEDOT to the peak at 168-170 eV due to the S-O bond of PSS was calculated by waveform resolution analysis of the binding energy of Sp2. The ratio of these areas was taken as the composition ratio of Bu-PEDOT to doped PSS.

[0078] (Measurement of breakdown voltage and ESR) The withstand voltage and ESR (equivalent series resistance) of the solid electrolytic capacitors including the solid electrolytes of Examples 1b to 4b and Comparative Examples 1b to 3b were measured. The applied voltage was increased from 0 V to 1 V per second at room temperature, and the voltage when a current of 20 mA flowed through the solid electrolytic capacitor was measured as the withstand voltage using a source meter (Tektronix, model 2410). The ESR was measured at a temperature of 20°C, an AC current level of 1.0 Vrms, and a measurement frequency of 100 kHz using an LCR meter (NF Corporation, ZM2376).

[0079] (Each measurement result) The analysis results of the composition ratio of Bu-PEDOT and PSS doped into the Bu-PEDOT, as well as the measurement results of the withstand voltage and ESR are shown in Table 1 below. Table 1 also lists the monomers used in the polymerization. In Table 1, the measured composition ratio is the actual composition ratio analyzed, and is expressed as the ratio of the doped PSS to the conjugated polymer.

[0080] (Table 1) TIFF2025175627000002.tif86168

[0081] As shown in Table 1 above, the solid electrolytic capacitor with the solid electrolyte of Comparative Example 1b has a high withstand voltage but a significantly deteriorated ESR. Comparative Example 1b is a solid electrolyte containing a conductive polymer in which Bu-PEDOT is doped with 2.12 times the molar amount of PSS. The solid electrolytic capacitor with the solid electrolyte of Comparative Example 2b has a good withstand voltage but a high ESR, although not as high as Comparative Example 1b. Comparative Example 2b is a solid electrolyte containing a conductive polymer in which Bu-PEDOT is doped with 6.01 times the molar amount of PSS.

[0082] In contrast, the solid electrolytic capacitors including the solid electrolytes of Examples 1b to 4b have good withstand voltage and low ESR. Examples 1b to 4b are solid electrolytes including a conductive polymer in which PSS is doped into Bu-PEDOT at a molar ratio of 2.8 to 4.7.

[0083] In Comparative Example 3b, the molar ratio of doped PSS to the conjugated polymer is in the range of 2.8 to 4.7, but Et-PEDOT is used as the conjugated polymer. Therefore, unlike Examples 1b to 4b, the solid electrolytic capacitor including the solid electrolyte of Comparative Example 3b has a low ESR but a low withstand voltage.

[0084] Thus, it was confirmed that the conductive polymer is poly(2-butyl-3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid, and when the molar ratio of poly(2-butyl-3,4-ethylenedioxythiophene) (A) to the doped polystyrene sulfonic acid (B) is A:B = 1:2.8 to 1:4.7, the solid electrolytic capacitor achieves both high withstand voltage and low ESR.

[0085] As shown in Table 1 above, when a conductive polymer dispersion is prepared by adding Bu-EDOT (C) and PSS (D) in a molar ratio of C:D = 1:4 to 1:12, it was confirmed that the molar ratio of Bu-PEDOT (A) to doped PSS (B) falls within the range of A:B = 1:2.8 to 1:4.7. Therefore, it was confirmed that solid electrolytic capacitors can achieve both high withstand voltage and low ESR by using a conductive polymer dispersion prepared by adding Bu-EDOT (C) and PSS (D) in a molar ratio of C:D = 1:4 to 1:12.

Claims

1. A solid electrolyte formed from a conductive polymer dispersion liquid containing a conductive polymer, the conductive polymer is poly(2-butyl-3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid; the molar ratio of the poly(2-butyl-3,4-ethylenedioxythiophene) (A) to the doped polystyrene sulfonic acid (B) is A:B=1:2.8 to 1:4.7; A solid electrolyte characterized by:

2. The solid electrolyte according to claim 1; an anode having a dielectric coating; a cathode facing the anode with the solid electrolyte interposed therebetween; To have A solid electrolytic capacitor characterized by:

3. It comprises poly(2-butyl-3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid, the molar ratio of the poly(2-butyl-3,4-ethylenedioxythiophene) (A) to the polystyrene sulfonic acid (B) is A:B=1:2.8 to 1:4.7; A conductive polymer dispersion characterized by:

4. a generating step of generating a conductive polymer dispersion liquid in which poly(2-butyl-3,4-ethylenedioxythiophene) (A) and polystyrene sulfonic acid (B) are doped into the poly(2-butyl-3,4-ethylenedioxythiophene) so that the molar ratio of the poly(2-butyl-3,4-ethylenedioxythiophene) (A) to the polystyrene sulfonic acid (B) is 1:2.8 to 1:4.7; a drying step of evaporating at least a part of the liquid component from the conductive polymer dispersion; containing, A method for producing a solid electrolyte, characterized by:

5. producing a conductive polymer dispersion liquid in which poly(2-butyl-3,4-ethylenedioxythiophene) (A) and polystyrene sulfonic acid (B) are doped into the poly(2-butyl-3,4-ethylenedioxythiophene) so that the molar ratio of the poly(2-butyl-3,4-ethylenedioxythiophene) (A) to the polystyrene sulfonic acid (B) is A:B=1:2.8 to 1:4.7; A method for producing a conductive polymer dispersion liquid, comprising:

6. an adding step of adding the polystyrene sulfonic acid and 2-butyl-3,4-ethylenedioxythiophene to a solvent, In the adding step, the 2-butyl-3,4-ethylenedioxythiophene (C) and the polystyrene sulfonic acid (D) are added at a charge composition ratio of C:D=1:4 to 1:12 in terms of molar ratio; 6. The method for producing the conductive polymer dispersion according to claim 5,

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