Solid electrolytic capacitor and manufacturing method

Abstract

The present invention provides a solid electrolytic capacitor in which the rise in ESR is suppressed even after thermal stress loading, and a method for manufacturing the same. [Solution] The solid electrolytic capacitor comprises a pair of electrode foils, an electrolyte layer, and a compound having a functional group having pH buffering capacity. The pair of electrodes contain a valve metal, and a dielectric oxide film is formed on the surface of one of the foils. The electrolyte layer is interposed between the pair of electrode foils and contains an electrolyte solution and a conductive polymer. The compound having a functional group having pH buffering capacity adheres to part or all of the electrode foils. The manufacturing method includes an element assembly step of assembling a capacitor element by interposing a pair of electrode foils or further a separator, a compound attachment step of impregnating the capacitor element with a liquid containing a compound having a functional group having pH buffering capacity after the element assembly step, and a conductive polymer attachment step of impregnating the capacitor element with a dispersion of a conductive polymer after the compound attachment step.

Description

Technical Field

[0001] The present invention relates to a solid electrolytic capacitor including an electrolyte solution and a conductive polymer in an electrolyte layer, and a method for manufacturing the same.

Background Art

[0002] An electrolytic capacitor using a valve metal such as tantalum or aluminum is provided with an anode electrode composed of a sintered body obtained by sintering a powder composed of a valve metal, or an anode electrode obtained by etching an extended foil body composed of a valve metal to increase the surface area, whereby a small size and a large capacitance can be obtained. In particular, a solid electrolytic capacitor in which a dielectric oxide film is covered with a solid electrolyte is small in size, large in capacitance, and low in equivalent series resistance, and is essential for miniaturization, high functionality, and cost reduction of electronic devices.

[0003] As the solid electrolyte, manganese dioxide and 7,7,8,8 - tetracyanoquinodimethane (TCNQ) complex are known. In recent years, conductive polymers derived from monomers having a π - conjugated double bond, such as poly(3,4 - ethylenedioxythiophene) (PEDOT), which have a slow reaction rate and excellent adhesion to the dielectric oxide film, have rapidly spread as solid electrolytes. Examples of the conductive polymer include a conductive polymer using an external dopant for expressing conductivity. The conductive polymer using an external dopant is produced by using a low - molecular - weight anion or polyanion as a dopant during chemical oxidative polymerization or electrolytic oxidative polymerization. In addition, examples of the conductive polymer include a self - doped conductive polymer. The self - doped conductive polymer has a molecule acting as a dopant in the monomer molecule, and is provided with conductivity expression and solubility in a solvent.

[0004] However, solid electrolytic capacitors, unlike liquid electrolytic capacitors which have an electrolyte impregnation solution in the capacitor element and no solid electrolyte layer, have poor repair capabilities for defects in the dielectric oxide film, which may 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 with the anode foil and cathode foil facing each other, and electrolyte is impregnated into the voids of the capacitor element, are attracting attention.

[0005] Furthermore, this hybrid type of solid electrolytic capacitor is attracting attention because its equivalent series resistance (ESR) is less affected by the electrical conductivity of the electrolyte. Generally, the ESR of electrolytic capacitors that use only an electrolyte is easily affected by the electrical conductivity of the electrolyte, and the lower the electrical conductivity of the electrolyte, the higher the ESR tends to be. However, the ESR of solid electrolytic capacitors that have both a solid electrolyte layer and an electrolyte is less affected by the conductivity of the electrolyte. In other words, even if the conductivity of the electrolyte is low, the ESR of the product can be kept sufficiently low. This is because the electrical conductivity of the conductive polymer used in solid electrolytic capacitors is orders of magnitude higher than that of the electrolyte, and therefore the electrical conductivity of the conductive polymer has a significant impact on the ESR. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2006-114540 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, in solid electrolytic capacitors that use both a solid electrolyte and an electrolyte solution, conductivity deteriorates due to the dedoping reaction of the dopant, and the ESR of the solid electrolytic capacitor increases. Regarding this increase in ESR associated with the dedoping reaction, Patent Document 1 reports that the dedoping reaction can be suppressed by making the molar ratio of acidic and basic components, which are solute components in the electrolyte solution, acidic. This report hypothesizes that the dedoping reaction is suppressed because the dopant, which is an acidic component, and the acidic component in the electrolyte solution maintain an equilibrium state.

[0008] However, solid electrolytic capacitors can be subjected to thermal stress, such as when used in high-temperature environments (e.g., above 115°C) or during the reflow soldering process when mounting them to circuit boards. This thermal stress promotes the dedoping reaction, even if the solute components in the electrolyte are excessively acidic. As a result, the ESR of solid electrolytic capacitors increases after being subjected to thermal stress.

[0009] The present invention was proposed to solve the above problems, and its objective is to provide a solid electrolytic capacitor having a solid electrolyte and an electrolyte solution as the electrolyte, in which the rise in ESR is suppressed even after thermal stress loading, and a method for manufacturing the same. [Means for solving the problem]

[0010] The inventors fabricated solid electrolytic capacitors with a fixed amount of cationic component and a varied amount of anionic component, and with a varied amount of cationic component and a fixed amount of anionic component. They then subjected these solid electrolytic capacitors to a reflow process to apply thermal stress and measured the ESR before and after the thermal stress load. The leftmost result in Figure 1 (labeled "Cation only" in Figure 1) is the result where only triethylamine, a cationic component, was added, without the addition of azelaic acid, an anionic component. As a result, as shown in Figure 1, the ESR changes before and after thermal stress load were almost the same for each solid electrolytic capacitor with a fixed amount of cationic component, whereas the solid electrolytic capacitors with a fixed amount of anionic component showed large and small ESR changes before and after thermal stress load.

[0011] From these results, the inventors have found that the change in ESR before and after thermal stress loading is determined more by the amount of cationic components contained in the electrolyte layer than by the effects of pH or acid-base ratio. However, cationic components in the electrolyte contribute to improving the ionic conductivity of the electrolyte layer and also have the effect of improving the corrosion resistance of the electrode foil by preventing an increase in the relative ratio of anionic components.

[0012] Therefore, in order to solve the above problems, the solid electrolytic capacitor of this embodiment comprises a pair of electrode foils, an electrolyte layer interposed between the pair of electrode foils and containing an electrolyte solution and a conductive polymer, and a compound attached to the electrode foils and having a functional group having pH buffering capacity.

[0013] The device includes a separator interposed between the pair of electrode foils, wherein the compound may adhere to the separator in addition to the electrode foils, or to the separator instead of the electrode foils.

[0014] The functional group having pH buffering capacity may be an acid anhydride or a phosphonic acid group.

[0015] The compound may be attached in a film-like manner.

[0016] The compound may have a silanol group or a phosphonic acid group as an adsorption group.

[0017] Also, in order to solve the above problems, a method for manufacturing a solid electrolytic capacitor according to the present embodiment includes an element assembling step of assembling a capacitor element by interposing a separator between a pair of electrode foils, and after the element assembling step, a compound adhesion step of impregnating the capacitor element with a liquid agent containing a compound having a functional group having a pH buffering ability, and after the compound adhesion step, a conductive polymer adhesion step of impregnating the capacitor element with a conductive polymer dispersion liquid in which a conductive polymer is dispersed.

[0018] A drying step of drying the capacitor element may be included between the compound adhesion step and the conductive polymer adhesion step.

[0019] The functional group having the pH buffering ability may be an acid anhydride or a phosphonic acid group.

[0020] The compound may have a silanol group or a phosphonic acid group as an adsorption group.

Advantages of the Invention

[0021] According to the present invention, in a solid electrolytic capacitor having a solid electrolyte and a liquid, an increase in ESR can be suppressed even after a thermal stress load.

Brief Description of the Drawings

[0022] [Figure 1] It is a graph showing the relationship between the ratio of the anion (acid) component and the cation (base) component contained in the liquid and the change in ESR before and after reflow. [Figure 2] It is a graph showing the change over time of the ESR of the solid electrolytic capacitors of Examples 1 and 3 to 6 and Comparative Examples 1 and 6. [Figure 3] It is a graph showing the change over time of the ESR of the solid electrolytic capacitors of Example 2 and Comparative Examples 2, 3, 5, and 7. [Figure 4] It is a graph showing the change over time of the capacitance of the solid electrolytic capacitors of Example 1 and Examples 3 to 6 and Comparative Examples 1 and 6. [Figure 5] It is a graph showing the change over time of the capacitance of the solid electrolytic capacitors of Example 2 and Comparative Examples 2, 3, 5 and 7. [Figure 6] It is a graph showing the change over time of the ESR of the solid electrolytic capacitors of Example 2 and Comparative Examples 2 and 7 to 9. [Figure 7] It is a graph showing the change over time of the capacitance of the solid electrolytic capacitors of Example 2 and Comparative Examples 2 and 7 to 9.

Mode for Carrying Out the Invention

[0023] Hereinafter, the solid electrolytic capacitor according to the embodiment will be described. Note that the present invention is not limited to the embodiments described below.

[0024] (Solid Electrolytic Capacitor) A solid electrolytic capacitor is a passive element that obtains capacitance by the dielectric polarization action of a dielectric oxide film and stores and discharges electric charges. This solid electrolytic capacitor is formed by housing a capacitor element in a case and sealing the case opening with a sealing body. The capacitor element includes a pair of electrode foils, a separator, and an electrolyte layer.

[0025] The pair of electrode foils are an anode foil and a cathode foil. The anode foil and the cathode foil face each other through a separator and are wound or laminated. The dielectric oxide film is formed on the surface of the anode foil. The electrolyte layer is composed of a solid electrolyte layer containing a conductive polymer and an electrolytic solution. The solid electrolyte layer is interposed between the anode foil and the cathode foil and adheres to the dielectric oxide film. The electrolytic solution impregnates the voids of the capacitor element in which the solid electrolyte layer is formed. This electrolyte layer functions as a true cathode.

[0026] (Electrode Foil) The anode foil and cathode foil are long foil bodies made of valve metal. Valve metals include aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony. The purity is preferably 99.9% or higher for the anode foil and approximately 99% or higher for the cathode foil, but impurities such as silicon, iron, copper, magnesium, and zinc may be present.

[0027] The anode foil is made by sintering a sintered body of valve metal powder, or by etching a foil made by stretching valve metal, thereby expanding its surface. The expanded surface structure consists of tunnel-shaped pits, spongy pits, or voids between densely packed powder particles. The tunnel-shaped pits may be excavated while leaving the depth of the anode foil intact, or they may be formed to penetrate the anode foil.

[0028] This enlarged surface structure is formed by electrolytic etching, chemical etching, or sandblasting, or by depositing or sintering metal particles onto a foil body. Electrolytic etching includes DC etching or AC etching, which involves applying DC or AC current in an acidic aqueous solution containing halogen ions such as hydrochloric acid. Chemical etching involves immersing the metal foil in an acidic or alkaline solution.

[0029] A dielectric oxide film is typically an oxide film formed on the surface of an anode foil, and if the anode foil is made of aluminum, it is aluminum oxide obtained by oxidizing the expanded surface structure region. This dielectric oxide film is formed by a chemical conversion treatment in which a voltage is applied in a halogen-free solution such as adipic acid, boric acid, or phosphoric acid. Furthermore, the dielectric oxide film may be produced by forming a layer of metal nitride, metal carbide, or metal carbonitride by vapor deposition, or by using a material that contains carbon on its surface.

[0030] The cathode foil may be a plain foil without a surface expansion structure, or it may have a surface expansion structure by vapor deposition, sintering, or etching, similar to the anode foil. An oxide film may be intentionally or naturally formed on the surface expansion layer. Intentionally, a thin dielectric oxide film (approximately 1-10 Vfs) may be formed by chemical conversion treatment. The naturally occurring oxide film is formed by the reaction of the cathode foil with oxygen in the air.

[0031] Lead terminals are connected to the anode foil and cathode foil. These lead terminals are connected to the anode foil and cathode foil respectively by stitching, ultrasonic welding, or other methods. These lead terminals are responsible for the electrical connection between the anode foil, cathode foil, connection point, and the outside.

[0032] (Separator) Examples of separators include cellulose and mixed papers such as kraft, Manila hemp, esparto, hemp, and rayon; polyester resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and their derivatives; polyamide resins such as polytetrafluoroethylene resins, polyvinylidene fluoride resins, vinylon resins; polyamide resins such as aliphatic polyamides, semi-aromatic polyamides, and fully aromatic polyamides; polyimide resins; polyethylene resins; polypropylene resins; trimethylpentene resins; polyphenylene sulfide resins; acrylic resins; and polyvinyl alcohol resins. These resins can be used individually or in combination.

[0033] (Compounds having functional groups with pH buffering capacity) A compound having a pH-buffering functional group (hereinafter referred to as "specific compound") is attached to the entire surface or part of the surface of a pair of electrode foils, separators, or multiple or all of them. This specific compound has a carboxyl group, a phosphonic acid group, etc., as the pH-buffering functional group. Due to the pH-buffering functional group, the specific compound acts as a polyvalent acid. The specific compound undergoes a neutralization reaction with cations contained in the electrolyte layer. However, the specific compound acting as a polyvalent acid has multiple acid dissociation constants. Therefore, the neutralization reaction proceeds stepwise in a specific pH range corresponding to the acid dissociation constant until all the acid components of the specific compound are completely neutralized by the cations. As a result, the pH change due to the neutralization of the specific compound is gradual. In other words, the pH-buffering functional group is a functional group that gives the specific compound multiple acid dissociation constants. By buffering the cations contained in the electrolyte layer with this specific compound, rapid pH fluctuations in the electrolyte layer are suppressed.

[0034] While this is a hypothesis and not limited to this mechanism, it is hypothesized that this specific compound suppresses the increase in ESR of solid electrolytic capacitors after thermal stress loading through the following mechanism: Cationic cations in the electrolyte act on dopants under thermal stress, making dopant dedoping reactions more likely. When dedoping reactions are accelerated, the conductivity of the solid electrolyte layer decreases, leading to an increase in the ESR of the solid electrolytic capacitor after thermal stress loading. Therefore, by buffering the cations in the electrolyte, the dedoping reaction of dopants by cations is suppressed, thereby suppressing the decrease in conductivity of the solid electrolyte layer and thus suppressing the increase in ESR of the solid electrolytic capacitor even after thermal stress loading.

[0035] Functional groups with pH buffering capacity include acid anhydrides that acquire pH buffering capacity through chemical reactions occurring within electrolytic capacitors. Examples of acid anhydrides include succinic acid anhydride. Acid anhydrides themselves do not have pH buffering properties, but succinic acid anhydride, for example, readily undergoes ring opening by hydrolysis as shown in formula (1) below, becoming succinic acid, a divalent acid, which acquires pH buffering properties and buffers cations in the electrolyte. [ka]

[0036] As functional groups with pH buffering capacity, carboxyl groups, epoxy groups, acrylic acid groups, and amino groups, which act as monovalent acids, are unsuitable. Amino groups act to promote the dedoping reaction of conductive polymers. Carboxyl groups are unsuitable because it is difficult to obtain pH buffering capacity when they are monovalent acids. Furthermore, epoxy groups and acrylic acid groups, presumably due to their compatibility with the electrolyte solvent, act to worsen the adhesion of conductive polymers to the electrode foil, thereby degrading the ESR of solid electrolytic capacitors.

[0037] Preferably, the specific compound further has an adsorption group and forms a self-assembled film that is deposited on the entire surface or a portion of the surface of a pair of electrode foils, separators, or multiple or all of them. Examples of adsorption groups include silanol groups and phosphonic acid groups. Examples of silanol groups include triethoxysilanol groups and trimethoxysilanol groups.

[0038] Examples of functional groups having pH buffering capacity include carboxyl groups, phosphate groups, and phosphonic acid groups. Examples of adsorbent groups include carboxyl groups, phosphate groups, phosphonic acid groups, alkoxysilanol groups, hydroxyl groups, and amino groups. Furthermore, examples of groups that act as both functional groups having pH buffering capacity and adsorbent groups include carboxyl groups, phosphate groups, and phosphonic acid groups. In addition, a specific compound may have multiple functional groups having pH buffering capacity. The multiple functional groups having pH buffering capacity that a specific compound possesses may be two or more types. Examples of such specific compounds include [3-(triethoxysilyl)propyl]succinic anhydride, [3-(trimethoxysilyl)propyl]succinic anhydride, nitrilotris(methylenephosphonic acid), glycine-N,N-bis(methylenephosphonic acid), etidronic acid, and pyrophosphate.

[0039] In particular, [3-(triethoxysilyl)propyl]succinate anhydride or [3-(trimethoxysilyl)propyl]succinate anhydride are preferred as specific compounds. When [3-(triethoxysilyl)propyl]succinate anhydride or [3-(trimethoxysilyl)propyl]succinate anhydride is selected as a specific compound, the capacitance of the solid electrolytic capacitor can be maintained particularly well, which is why it is preferred. This is a hypothesis and not limited to this, but it is thought that [3-(triethoxysilyl)propyl]succinate anhydride and [3-(trimethoxysilyl)propyl]succinate anhydride have low solubility in the dielectric oxide film of the anode foil and the oxide film of the cathode foil, so the capacitance of the solid electrolytic capacitor can be maintained particularly well.

[0040] The amount of specific compound to be deposited can be appropriately selected according to the cation concentration in the electrolyte. Preferably, the amount of specific compound deposited is such that the molar ratio of pH-buffering functional groups present inside the capacitor element is equal to or greater than the molar ratio of cation components in the electrolyte. Even if the molar ratio of pH-buffering functional groups is lower than the molar ratio of cation components in the electrolyte, the effect of suppressing the rise in ESR after thermal stress loading of the solid electrolytic capacitor can still be obtained, but a particularly good suppression effect can be obtained when the amounts are equal or greater. However, if the adsorbent groups exhibit strong acidity, it is preferable to reduce the amount of specific compound deposited or to appropriately select the processing conditions of the compound deposition process so as not to cause dissolution of the dielectric oxide film during the compound deposition process in which the liquid agent containing the specific compound is impregnated into the capacitor element.

[0041] (Solid electrolyte layer) Conductive polymers are either self-doped polymers, doped with dopant molecules within the molecule, or conjugated polymers, doped with external dopant molecules. Conjugated polymers are obtained by chemical oxidation polymerization or electrolytic oxidation polymerization of monomers or derivatives thereof that have π-conjugated double bonds. Conductive polymers exhibit high conductivity by undergoing a doping reaction. That is, conductivity is exhibited by chemically or electrochemically adding small amounts of dopants, such as acceptors that readily accept electrons or donors that readily donate electrons, to conjugated polymers.

[0042] Any known conjugated polymer can be used without particular limitation. Examples include polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene, polyphenylenevinylene, polyacene, and polythiophenevinylene. These conjugated polymers may be used individually, in combination of two or more types, or as copolymers of two or more monomers.

[0043] Among the above-mentioned conjugated polymers, conjugated polymers formed by polymerizing thiophene or its derivatives are preferred, and conjugated polymers formed 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 are preferred. As the thiophene derivative, compounds selected from thiophenes having substituents at the 3rd and 4th positions are preferred, and the substituents at the 3rd and 4th positions of the thiophene ring may form a ring together with the carbon atoms at the 3rd and 4th positions. The number of carbon atoms in the alkyl or alkoxy group is suitable to be 1 to 16.

[0044] In particular, a polymer of 3,4-ethylenedioxythiophene called EDOT, i.e., poly(3,4-ethylenedioxythiophene) called PEDOT, is especially preferred. Monomers to which substituents have been added to 3,4-ethylenedioxythiophene may also be used. For example, alkylated ethylenedioxythiophene to which alkyl groups have been added as substituents may be used, such as methylated ethylenedioxythiophene (i.e., 2-methyl-2,3-dihydrothieno[3,4-b][1,4]dioxin), ethylated ethylenedioxythiophene (i.e., 2-ethyl-2,3-dihydrothieno[3,4-b][1,4]dioxin), and butylated ethylenedioxythiophene (i.e., 2-butyl-2,3-dihydrothieno[3,4-b][1,4]dioxin).

[0045] Dopants can be any known substance without particular limitation. Dopants may be used alone or in combination of two or more. Polymers or monomers may also be used. Examples of dopants include polyanions, inorganic acids such as boric acid, nitric acid, and phosphoric acid, and organic acids such as acetic acid, oxalic acid, citric acid, tartaric acid, squalaneic 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.

[0046] Polyanions include, for example, substituted or unsubstituted polyalkylenes, substituted or unsubstituted polyalkenes, substituted or unsubstituted polyimides, substituted or unsubstituted polyamides, and substituted or unsubstituted polyesters, which are polymers consisting only of structural units having anionic groups, or polymers consisting of structural units having anionic groups and structural units not having anionic groups. Specifically, examples of polyanions include polyvinyl sulfonic acid, polystyrene sulfonic acid, polyallyl sulfonic acid, polyacryl sulfonic acid, polymethacrylate sulfonic acid, poly(2-acrylamido-2-methylpropane sulfonic acid), polyisoprene sulfonic acid, polyacrylic acid, polymethacrylate, and polymaleic acid.

[0047] The solid electrolyte layer may contain various additives in addition to the conductive polymer, such as polyhydric alcohols. Examples of polyhydric alcohols include sorbitol, ethylene glycol, diethylene glycol, triethylene glycol, polyoxyethylene glycol, glycerin, polyglycerin, polyoxyethylene glycerin, xylitol, erythritol, mannitol, dipentaerythritol, pentaerythritol, or combinations of two or more of these. Because polyhydric alcohols have high boiling points, they can remain in the solid electrolyte layer even after the drying process, resulting in reduced ESR and improved dielectric strength.

[0048] (electrolyte) An electrolyte is a solution in which an anionic component and a cationic component are added to a solvent. The anionic and cationic components are typically salts of organic acids, inorganic acids, or complex compounds of organic and inorganic acids, and are added to the solvent by ionic dissociable salts that dissociate into anionic and cationic components. The acid that becomes the anionic component and the base that becomes the cationic component may be added to the solvent separately. Furthermore, the electrolyte may not contain only the anionic component, only the cationic component, or both the anionic and cationic components in the solvent.

[0049] Examples of organic acids that act as anionic 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, resorcinic acid, phloroglucic 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 complex compounds of organic and inorganic acids include borodisalicylic acid, borodisuoic 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, borodiresorcinic acid, borodimethylsalicylic acid, borodinafthoic acid, borodimandelic acid, and borodi(3-hydroxy)propionic acid.

[0050] Furthermore, examples of salts of organic acids, inorganic acids, and composite compounds of organic and inorganic acids include ammonium salts, quaternary ammonium salts, amidinium quaternary salts, amine salts, sodium salts, potassium salts, etc. Examples of quaternary ammonium ions in quaternary ammonium salts include tetramethylammonium, triethylmethylammonium, and tetraethylammonium. Examples of amidinium quaternary 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; and examples of tertiary amines include trimethylamine, triethylamine, tributylamine, ethyldimethylamine, and ethyldiisopropylamine.

[0051] The solvent for the electrolyte is not particularly limited, but a protic organic polar solvent or an aprotic organic polar solvent can be used. Examples of protic organic solvents include monohydric alcohols, polyhydric alcohols, and oxyalcohol compounds. Examples of monohydric alcohols include ethanol, propanol, butanol, pentanol, hexanol, cyclobutanol, cyclopentanol, cyclohexanol, and benzyl alcohol. Examples of polyhydric alcohols and oxyalcohol compounds include ethylene glycol, diethylene glycol, propylene glycol, glycerin, methyl cellosolve, ethyl cellosolve, methoxypropylene glycol, dimethoxypropanol, and alkylene oxide adducts of polyhydric alcohols such as polyethylene glycol and polyoxyethylene glycerin.

[0052] As aprotic organic polar solvents, sulfones, amides, lactones, cyclic amides, nitriles, sulfoxides, etc., may be used. Examples of sulfones include dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methylsulfolane, and 2,4-dimethylsulfolane. Examples of amides include N-methylformamide, N,N-dimethylformamide, N-ethylformamide, N,N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide, and hexamethylphosphoric amide. Examples of lactones and cyclic amides include γ-butyrolactone, γ-valerolactone, δ-valerolactone, N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate, butylene carbonate, and isobutylene carbonate. Examples of nitrile-based compounds include acetonitrile, 3-methoxypropionitrile, and glutaronitrile. Examples of sulfoxide-based compounds include dimethyl sulfoxide.

[0053] Furthermore, other additives can be added to the electrolyte. Examples of additives include complex compounds of boric acid and polysaccharides (such as mannitol and sorbitol), complex compounds of boric acid and polyhydric alcohols, boric acid esters, and nitro compounds (such as o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, p-nitrophenol, and p-nitrobenzyl alcohol). These may be used individually or in combination of two or more.

[0054] (Manufacturing method) In a method for manufacturing such a solid electrolytic capacitor, it is preferable that the compound deposition step, in which a specific compound is deposited, is performed after the element assembly step of the capacitor element, and before the conductive polymer deposition step, in which the solid electrolyte layer is formed. In other words, it is preferable that the conductive polymer deposition step is performed after the compound deposition step. If the compound deposition step is performed first, and the conductive polymer deposition step is performed after the compound deposition step, the ESR and capacitance of the solid electrolytic capacitor will be maintained well without significant deterioration, even if the solid electrolytic capacitor is exposed to a high-temperature environment for a long time.

[0055] The reason why ESR and capacitance do not deteriorate significantly with this sequence of processes is as follows: First, the conductive polymer is dispersed in a conductive polymer dispersion in the form of particles or powder, and the solid electrolyte layer is formed by impregnating the capacitor element with the conductive polymer dispersion. Furthermore, by immersing the capacitor element in a solution to which the specific compound has been added, the specific compound having adsorbent groups forms a self-assembled film that is deposited on the entire surface or a portion of the surface of the pair of electrode foils, separators, or multiple or all of them. If the compound attachment process is performed first, it is possible to prevent the additives in the conductive polymer dispersion from dissolving into the solution to which the specific compound has been added, thereby suppressing a rapid deterioration of ESR and capacitance.

[0056] In particular, when ethylene glycol or sorbitol is added to a conductive polymer dispersion, the effect of preventing a rapid deterioration of ESR and capacitance is significantly enhanced by performing the compound deposition process first, followed by the conductive polymer deposition process.

[0057] (Component assembly process) In the device assembly process, anode foil and cathode foil, each having a dielectric oxide film formed on them, are wound together with a separator in between to create a cylindrical winding body. The separator is overlapped so that one end protrudes beyond the other end of the anode foil and cathode foil. The protruding separator is wound first to create the core, and then the winding is continued using this core as the winding shaft. Before winding, lead terminals, such as aluminum ones, are connected to the anode foil and cathode foil by stitching, cold welding, ultrasonic welding, or laser welding.

[0058] After winding, a device formation process may be provided to repair the exposed base metal portion of the valve metal when the anode foil, on which the dielectric oxide film layer has been formed, is cut to a desired width, as well as defects in the anode foil and cathode foil caused by physical stress such as winding. In the device formation process, the wound body is immersed in a formation solution and a voltage is applied. As the formation solution, a phosphoric acid-based solution such as ammonium dihydrogen phosphate, a boric acid-based solution such as ammonium borate, an adipic acid-based solution such as ammonium adipate, or a solution made by mixing boric acid and a dicarboxylic acid such as citric acid can be used. Preferably, the voltage applied during device formation is, for example, 0.1 to 1.2 times the formation voltage. Furthermore, as a method of applying the voltage during device formation, a method of applying a constant voltage from the start of device formation, or a method of gradually increasing the applied voltage at regular intervals can be appropriately selected.

[0059] (Compound adhesion process) In the compound deposition process, in which a specific compound is attached, the solvent of the solution to which the specific compound is added is not particularly limited, but various solvents can be used, such as water, alcohols such as ethanol, aprotic polar solvents such as tetrahydrofuran, and low polar solvents such as hexane. There are no particular limitations on the immersion conditions, but the capacitor element is immersed in the solution at 10°C to 80°C for 1 minute to 720 minutes.

[0060] After immersing the capacitor element in a solution containing a specific compound, it is preferable to dry the capacitor element before the conductive polymer deposition step. There are no particular limitations on the conditions for this drying step, but for example, the capacitor element is left standing for 1 minute to 720 minutes in a temperature environment of 40°C to 200°C. This removes the solvent while leaving behind the specific compound that has formed a self-assembled film. The drying step may be repeated multiple times. Drying may also be performed under reduced pressure, for example, by reducing the pressure to 5 kPa or more and 100 kPa or less.

[0061] (Conductive polymer adhesion process) In the conductive polymer deposition process, the solvent used for the conductive polymer dispersion is, for example, water, an organic solvent, or a mixture thereof. Examples of organic solvents include polar solvents, alcohols, esters, hydrocarbons, carbonate compounds, ether compounds, linear ethers, heterocyclic compounds, and nitrile compounds.

[0062] 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, glutalodinitrile, methoxyacetonitrile, propionitrile, and benzonitrile.

[0063] The conductive polymer dispersion may contain additives such as polyhydric alcohols in addition to the solvent and conductive polymer. Examples of polyhydric alcohols include sorbitol, ethylene glycol, diethylene glycol, triethylene glycol, polyoxyethylene glycol, glycerin, polyoxyethylene glycerin, xylitol, erythritol, mannitol, dipentaerythritol, pentaerythritol, or combinations of two or more of these. Because polyhydric alcohols have high boiling points, they can remain in the solid electrolyte layer even after the drying process following the conductive polymer deposition process, resulting in ESR reduction and improved voltage resistance.

[0064] Other additives that may be added include, for example, conventional additives such as organic binders, surfactants, dispersants, defoamers, coupling agents, antioxidants, and UV absorbers. It is also possible to significantly reduce the ESR 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.

[0065] To promote the impregnation of the conductive polymer dispersion into the capacitor element, vacuum or pressure treatment may be applied as needed. The impregnation process may be repeated multiple times. After impregnating the capacitor element with the conductive polymer dispersion, the solvent in the dispersion is removed by a drying process. In the drying process, the anode foil, cathode foil, separator, or the capacitor element itself is exposed to a temperature environment of, for example, 40°C to 200°C for 3 minutes to 180 minutes. This drying process may be repeated multiple times. Drying may also be performed under reduced pressure, for example, by reducing the pressure to 5 kPa to 100 kPa. Alternatively, the drying process may be divided into a pre-drying process and a main drying process.

[0066] The order of the manufacturing processes for other solid electrolytic capacitors can be rearranged, omitted, and processed in parallel as much as possible. Furthermore, one process may include multiple sub-processes, some of which may be performed during other processes or incorporated between one sub-process of one process and the next. For example, after the conductive polymer deposition process, a sealing body is combined with the capacitor element, the capacitor element is impregnated with electrolyte, the capacitor element is housed in an outer case with a closed bottom at one end and an open end at the other, and the capacitor element is sealed in the outer case with the sealing body. After sealing the capacitor element in the outer case, the solid electrolytic capacitor is completed after undergoing an aging process. In the aging process, a DC voltage is applied to the solid electrolytic capacitor, and the electrolyte repairs defects such as the dielectric oxide film layer. [Examples]

[0067] The solid electrolytic capacitors and their manufacturing methods described below will be explained in more detail. However, the present invention is not limited to the embodiments described below.

[0068] (Examples) Solid electrolytic capacitors of Examples 1 to 6 and Comparative Examples 1 to 7 were fabricated. First, anode foil and cathode foil were fabricated using aluminum foil. The anode foil and cathode foil were enlarged by etching. A dielectric oxide film was formed on the anode foil by chemical conversion treatment. Then, in the element assembly process, lead wires were connected to each of the anode foil and cathode foil, and the anode foil and cathode foil were wound facing each other with a Manila paper separator in between. This capacitor element underwent a restoration chemical conversion by applying current at an ammonium dihydrogen phosphate aqueous solution at a liquid temperature of 40°C for 20 minutes under the conditions of an applied voltage of 56.5V and a current density of 10mA. After that, it was dried at 105°C.

[0069] Next, the compound deposition process was carried out, and the capacitor elements of Examples 1 to 6 and Comparative Examples 1 to 5 were immersed in a solution containing the compounds listed in Table 1 below. This compound deposition process was omitted for the capacitor elements of Comparative Examples 6 and 7. (Table 1) TIFF2026083435000003.tif141161

[0070] As shown in Table 1, the solutions in which the capacitor elements of Examples 1 to 6 were immersed contained a specific compound. On the other hand, the solutions in which the capacitor elements of Comparative Examples 1 to 5 were immersed did not contain a compound with a functional group having pH buffering capacity, i.e., the specific compound. As the solvent for the solutions in which the capacitor elements of Examples 1 to 6 and Comparative Examples 1 to 5 were immersed, a mixed solvent of water and ethanol was used for Examples 1 and 2 and Comparative Examples 2 to 5, while water was used for Examples 3 to 6 and Comparative Example 1. The solution temperature was room temperature, and the capacitor elements were immersed for 30 minutes. After immersion, the capacitor elements of Examples 1 to 6 and Comparative Examples 1 to 5 were dried by exposing them to a temperature environment of 130°C for 15 minutes.

[0071] The conductive polymer deposition process was then carried out, in which the capacitor elements were immersed in a conductive polymer dispersion to deposit the conductive polymer onto the dielectric oxide film of the anode foil, the cathode foil, and the separator. The capacitor elements were immersed in the conductive polymer dispersion for 2 minutes under a pressurized environment of 20 kPa. After immersion, the capacitor elements were pre-dried at room temperature for 10 minutes, and then further dried at 150°C for 30 minutes. In the conductive polymer dispersions into which the capacitor elements of Examples 1 and 3-6 and Comparative Examples 1 and 6 were immersed contained dispersions of poly(3,4-ethylenedioxythiophene) fine particles and polystyrene sulfonic acid. In the conductive polymer dispersions into which the capacitor elements of Example 2 and Comparative Examples 2-5 and 7 were immersed were prepared by diluting a dispersion of poly(3,4-ethylenedioxythiophene) fine particles and polystyrene sulfonic acid with sorbitol and ethylene glycol.

[0072] After the conductive polymer deposition process, a sealing body was attached to the capacitor element, and the capacitor element was impregnated with an electrolyte. The capacitor elements of Examples 1 and 3-6 and Comparative Examples 1 and 6 were impregnated with an ethylene glycol-based electrolyte, while the capacitor elements of Example 2 and Comparative Examples 2-5 and 7 were impregnated with a sulfolane-based electrolyte. Subsequently, the capacitor elements were inserted into a bottomed cylindrical outer case, and the opening of the outer case was sealed with a sealing body. Then, each solid electrolytic capacitor was subjected to aging treatment by applying voltage.

[0073] (ESR and capacitance) The changes in ESR and capacitance over time were measured for the solid electrolytic capacitors of Examples 1 to 6 and Comparative Examples 1 to 7. Each solid electrolytic capacitor was subjected to a continuous DC load of 35V while being exposed to a temperature environment of 150°C. The measurement frequency for ESR was set to 100kHz, which is in the high-frequency range. The measurement frequency for capacitance was set to 120Hz.

[0074] Furthermore, in the case of Comparative Example 4, the ESR of the solid electrolytic capacitor was more than two orders of magnitude higher than that of the other solid electrolytic capacitors in the initial stages, i.e., after the completion of the manufacturing of the solid electrolytic capacitor and before applying voltage in a high-temperature environment of 150°C. Therefore, the measurement of ESR and capacitance was discontinued.

[0075] Figure 2 shows the changes in ESR over time for the solid electrolytic capacitors of Examples 1 and 3-6 and Comparative Examples 1 and 6. In the graph, the filled-in circles represent Example 1, the filled-in diamonds represent Example 3, the open diamonds represent Example 4, the squares represent Example 5, the open-circle plots represent Example 6, the triangles represent Comparative Example 1, and the x-marks represent Comparative Example 6.

[0076] Furthermore, the changes in ESR over time for the solid electrolytic capacitors of Example 2 and Comparative Examples 2, 3, 5, and 7 are shown in the graph in Figure 3. In the graph, the filled-in circles represent Example 2, the diamond-shaped plots represent Comparative Example 2, the open-circle plots represent Comparative Example 3, the square plots represent Example 5, and the x-marked plots represent Comparative Example 7.

[0077] As shown in Figure 2, the ESR of the solid electrolytic capacitor in Comparative Example 1 deteriorated compared to Comparative Example 6, which omitted the compound deposition process. On the other hand, the solid electrolytic capacitors in Examples 1, 3-5, which had a specific compound deposited during the compound deposition process, showed better ESR results than Comparative Example 6. Similarly, as shown in Figure 3, the ESR of the solid electrolytic capacitors in Comparative Examples 2 and 3 deteriorated compared to Comparative Example 7, which omitted the compound deposition process. The solid electrolytic capacitor in Comparative Example 5 also deteriorated to the same level of ESR as Comparative Example 7 after 3000 hours. On the other hand, the solid electrolytic capacitor in Example 2, which had a specific compound deposited during the compound deposition process, showed better ESR results than Comparative Example 7.

[0078] This confirmed that by attaching a specific compound having a pH buffering functional group to the entire surface or part of the surface of a pair of electrode foils, separators, or multiple or all of them, the increase in ESR of a solid electrolytic capacitor is suppressed even after thermal stress, regardless of the type of electrolyte or solid electrolyte layer.

[0079] Next, Figure 4 shows the changes in capacitance over time for the solid electrolytic capacitors of Examples 1 and 3-6 and Comparative Examples 1 and 6. In the graph, the filled-in circles represent Example 1, the filled-in diamonds represent Example 3, the open diamonds represent Example 4, the squares represent Example 5, the open-circles represent Example 6, the triangles represent Comparative Example 1, and the x marks represent Comparative Example 6.

[0080] Furthermore, the changes in capacitance over time of the solid electrolytic capacitors of Example 2 and Comparative Examples 2, 3, 5, and 7 are shown in the graph in Figure 5. In the graph, the filled-in circles represent Example 2, the diamond-shaped plots represent Comparative Example 2, the open-circle plots represent Comparative Example 3, the square plots represent Comparative Example 5, and the x-shaped plots represent Comparative Example 7.

[0081] As shown in Figure 4, the capacitance of the solid electrolytic capacitor in Example 1 was equivalent to that of Comparative Example 6, which did not undergo the compound deposition process, even after being subjected to thermal stress. Furthermore, as shown in Figure 5, the capacitance of the solid electrolytic capacitor in Example 2 was equivalent to that of Comparative Example 7, which did not undergo the compound deposition process, even after being subjected to thermal stress.

[0082] This confirmed that when the functional group with pH buffering capacity of a specific compound is an acid anhydride, the capacitance of the solid electrolytic capacitor is maintained even after thermal stress has been applied.

[0083] (Manufacturing method testing) Solid electrolytic capacitors of Comparative Examples 8 and 9 were fabricated. Comparative Example 8 had the same configuration, composition, and composition ratio as Example 2, and Comparative Example 9 had the same configuration, composition, and composition ratio as Comparative Example 2, but the manufacturing method was different. Unlike Example 2 and Comparative Example 2, the solid electrolytic capacitors of Comparative Examples 8 and 9 underwent a conductive polymer deposition process first to form a solid electrolyte layer after the element assembly process, and then a compound deposition process was performed after the conductive polymer deposition process.

[0084] Then, the changes in ESR and capacitance over time were measured for the solid electrolytic capacitors of Example 2, Comparative Examples 2 and 7 to 9. Each solid electrolytic capacitor was continuously subjected to a 35V DC load while being exposed to a temperature environment of 150°C. The measurement frequency for ESR was set to 100kHz, which is in the high-frequency range. The measurement frequency for capacitance was set to 120Hz.

[0085] Furthermore, the solid electrolyte layer of the solid electrolytic capacitors in Examples 2, Comparative Examples 2, 8, and 9 was formed by immersing the capacitor element in a conductive polymer dispersion prepared by diluting a dispersion of poly(3,4-ethylenedioxythiophene) fine particles and polystyrene sulfonic acid with sorbitol and ethylene glycol. In Examples 2 and Comparative Example 8, a specific compound having a functional group with pH buffering capacity is attached, while in Comparative Examples 2 and 9, a compound different from the specific compound is attached. In Comparative Example 7, the compound attachment step is omitted.

[0086] Figure 6 shows the changes in ESR over time for solid electrolytic capacitors in Example 2, Comparative Examples 2 and 7 to 9. In the graph, the filled-in circles represent Example 2, the filled-in diamonds represent Comparative Example 2, the open-circle plots represent Comparative Example 8, the open-circle plots represent Comparative Example 9, and the x-marked plots represent Comparative Example 7.

[0087] As shown in Figure 6, the ESR of the solid electrolytic capacitors in Comparative Examples 7 to 9 showed a sharp increase after 2000 hours compared to Example 2 and Comparative Example 2. On the other hand, the ESR of the solid electrolytic capacitors in Example 2 and Comparative Example 2 did not show a sharp increase even after time had passed. This confirms that by performing the compound deposition process first, followed by the conductive polymer deposition process, after the device assembly process, a sharp increase in ESR can be prevented even under thermal stress.

[0088] Furthermore, the changes in capacitance over time for the solid electrolytic capacitors of Example 2, Comparative Examples 2 and 7 to 9 are shown in the graph in Figure 7. In the graph, the filled-in circles represent Example 2, the filled-in diamonds represent Comparative Example 2, the open-circle plots represent Comparative Example 8, the open-circle plots represent Comparative Example 9, and the x-marked plots represent Comparative Example 7.

[0089] As shown in Figure 7, the capacitance of the solid electrolytic capacitors in Comparative Examples 8 and 9 showed a sharp drop after 2000 hours compared to Example 2 and Comparative Example 2. In addition, for Comparative Example 9, the capacitance could not be extracted after 2500 hours, and the measurement was stopped. On the other hand, the capacitance of the solid electrolytic capacitors in Example 2 and Comparative Example 2 did not drop sharply even as time passed. This confirmed that by performing the compound deposition process first, followed by the conductive polymer deposition process, after the element assembly process, a sharp drop in capacitance can be prevented even under thermal stress. The reason for this is speculative and not limited to this mechanism, but it is thought to be as follows: By performing the conductive polymer deposition process first and then the compound deposition process, the ethylene glycol and sorbitol added to the conductive polymer solution may dissolve into the solution immersed in during the compound deposition process, worsening the time-dependent changes in ESR and capacitance.

Claims

1. A pair of electrode foils, An electrolyte layer containing an electrolyte solution and a conductive polymer is interposed between the pair of electrode foils, A compound adhering to the electrode foil and having a functional group having pH buffering capacity, To be equipped, A solid electrolytic capacitor characterized by the following features.

2. A separator is interposed between the pair of electrode foils, The compound adheres to the separator in addition to the electrode foil, or adheres to the separator instead of the electrode foil. A solid electrolytic capacitor according to claim 1, characterized by the above.

3. The functional group having pH buffering capacity is an acid anhydride or a phosphonic acid group. A solid electrolytic capacitor according to claim 1 or 2, characterized by the above.

4. The compound is attached in a film-like manner. A solid electrolytic capacitor according to any one of claims 1 to 3, characterized by the above.

5. The aforementioned compound has a silanol group or a phosphonic acid group as an adsorbent group. A solid electrolytic capacitor as described in claim 4, characterized by the above.

6. The capacitor element assembly process involves interposing a separator between a pair of electrode foils, After the element assembly step, a compound adhesion step is performed in which a liquid agent containing a compound having a functional group having pH buffering capacity is impregnated into the capacitor element. After the compound adhesion step, a conductive polymer adhesion step is performed in which a conductive polymer dispersion containing a conductive polymer is impregnated into the capacitor element. Including, A method for manufacturing solid electrolytic capacitors characterized by the following.

7. The compound deposition step and the conductive polymer deposition step include a drying step for drying the capacitor element. A method for manufacturing a solid electrolytic capacitor according to claim 6, characterized by the above.

8. The functional group having pH buffering capacity is an acid anhydride or a phosphonic acid group. A method for manufacturing a solid electrolytic capacitor according to claim 6 or 7, characterized by the above.

9. The aforementioned compound has a silanol group or a phosphonic acid group as an adsorbent group. A method for manufacturing a solid electrolytic capacitor as described in any one of claims 6 to 8, characterized by the above.

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