JP2024050386A - Solid electrolytic capacitor and manufacturing method - Google Patents

Abstract

[Problem] To provide a solid electrolytic capacitor that achieves both low ESR and high voltage resistance. [Solution] The solid electrolytic capacitor includes a capacitor element having an anode foil, a cathode body, and an electrolyte. The anode foil has a surface that is enlarged by tunnel-shaped etching pits, and has a dielectric film on the surface. The cathode body faces the anode foil. The electrolyte includes a solid electrolyte layer that contains a conductive polymer. This solid electrolyte layer has a weight of 250 mg/cm3 or less per unit volume of the capacitor element. [Selected Figure] Figure 1

Description

The present invention relates to a solid electrolytic capacitor that uses a solid electrolyte layer or a combination of a solid electrolyte layer and a liquid component as the electrolyte, and a manufacturing method thereof.

An electrolytic capacitor has anode and cathode foils made of valve metals such as tantalum or aluminum. The anode foil is enlarged by forming the valve metal into a sintered or etched foil, and the enlarged surface has a dielectric film formed by a process such as anodizing. An electrolyte is interposed between the anode and cathode foils.

This electrolytic capacitor has the advantage that it is easier to obtain a large capacitance compared to other types of capacitors because the specific surface area can be increased by expanding the surface area of the anode foil. Electrolytic capacitors contain electrolyte in the form of an electrolytic solution. The contact area of the electrolyte with the dielectric film of the anode foil increases. This makes it easier to increase the capacitance of the electrolytic capacitor. However, the electrolyte evaporates to the outside over time, and the electrolytic capacitor experiences a decrease in capacitance and an increase in electrostatic dissipation factor over time, leading to drying out.

Among electrolytic capacitors, solid electrolytic capacitors using solid electrolytes have been attracting attention. Manganese dioxide and 7,7,8,8-tetracyanoquinodimethane (TCNQ) complexes are known as solid electrolytes. In recent years, conductive polymers derived from monomers with π-conjugated double bonds, such as poly(3,4-ethylenedioxythiophene) (PEDOT), which has a slow reaction rate and excellent adhesion to dielectric films, have rapidly become popular as solid electrolytes. Conductive polymers use acid compounds such as polyanions as dopants, and have a partial structure in the monomer molecule that acts as a dopant, resulting in high conductivity. Therefore, solid electrolytic capacitors have the advantage of low equivalent series resistance (ESR).

However, compared to liquid-type electrolytic capacitors, in which the capacitor element is impregnated with an electrolyte, solid electrolytic capacitors are poor at repairing defects in the anodized film, which is the dielectric, and there is a risk of increased leakage current. For this reason, so-called hybrid-type solid electrolytic capacitors have also been proposed. Hybrid-type solid electrolytic capacitors have a driving electrolyte that is impregnated into the voids in the capacitor element in which the solid electrolyte layer is formed.

JP 2008-109068 A Patent No. 4536625

Capacitors are used for a variety of purposes. For example, in the field of power electronics, power from an AC power source is converted to DC power by a converter circuit, and this DC power is then converted to the desired AC power by an inverter circuit. In this type of power supply circuit, smoothing capacitors are provided to suppress pulsations in the DC output from the converter circuit and smooth the DC power before inputting it to the inverter circuit. In addition, decoupling capacitors are provided near semiconductor switching elements such as gallium nitride to ensure stable operation of the semiconductor switching elements and to remove noise.

As the power electronics field becomes increasingly powerful, solid electrolytic capacitors are required to have a higher withstand voltage. For example, in some fields such as power electronics, capacitors with a high withstand voltage of at least 200 V are expected. However, it has not been easy to achieve a high withstand voltage while maintaining the low ESR, which is one of the advantages of solid electrolytic capacitors.

The present invention has been proposed to solve the above problems, and its purpose is to provide a solid electrolytic capacitor and a manufacturing method that achieve both low ESR and high voltage resistance.

In order to solve the above problems, the solid electrolytic capacitor of the present embodiment includes a capacitor element having an anode foil, a cathode body, and an electrolyte, the anode foil has a surface enlarged by tunnel-shaped etching pits and has a dielectric coating on the surface, the cathode body faces the anode foil, the electrolyte includes a solid electrolyte layer containing a conductive polymer, and the solid electrolyte layer has a weight of 250 mg/cm3 or ^less per unit volume of the capacitor element.

The solid electrolyte layer may include the conductive polymer, a solvent for a conductive polymer liquid in which the conductive polymer is dispersed or dissolved, and an additive added to the conductive polymer liquid, and the total weight of the conductive polymer, the solvent, and the additive may be 250 mg/ ^cm3 or less per unit volume of the capacitor element.

The electrolyte may include only the solid electrolyte layer, and the solid electrolyte layer may have a weight per unit volume of the capacitor element of 120 mg/cm ³ or more and 150 mg/cm ³ or less.

The electrolyte may include a liquid component that fills the voids in the capacitor element.

The electrolyte may include a liquid component that fills voids in the capacitor element, and the solid electrolyte layer may have a weight per unit volume of the capacitor element of 15 mg/cm ³ or more and 250 mg/cm ³ or less.

The electrolyte may include a liquid component that fills voids in the capacitor element, and the solid electrolyte layer may have a weight per unit volume of the capacitor element of 60 mg/cm ³ or more and 180 mg/cm ³ or less.

The solid electrolyte layer may contain a compound having a hydroxyl group. The compound having a hydroxyl group may be one or more selected from the group consisting of ethylene glycol, butanediol, diethylene glycol, and glycerin.

In order to solve the above-mentioned problems, a method for manufacturing a solid electrolytic capacitor according to the present embodiment is a method for manufacturing a solid electrolytic capacitor having an anode foil, a cathode body, and a solid electrolyte layer, and includes an anode foil forming step of forming a tunnel-shaped etching pit on a surface of the anode foil and further forming a dielectric film on the surface, an element forming step of forming a capacitor element in which the cathode body faces the anode foil, and an electrolyte layer forming step of attaching a conductive polymer solution in which a conductive polymer is dispersed or dissolved between the anode foil and the cathode body to form the solid electrolyte layer containing the conductive polymer, a solvent for the conductive polymer solution in which the conductive polymer is dispersed or dissolved, and an additive added to the conductive polymer solution, and in the electrolyte layer forming step, the conductive polymer, the solvent, and the additive are contained so that the total weight of the conductive polymer, the solvent, and the additive is 250 mg/ ^cm3 or less per unit volume of the capacitor element.

According to the present invention, the solid electrolytic capacitor achieves both low ESR and high voltage resistance.

1 is a graph showing the relationship between the weight of a solid electrolyte layer and the withstand voltage, and the relationship between the weight of a solid electrolyte layer and ESR, in accordance with Examples 1 to 7. 11 is a graph showing the relationship between the weight of the solid electrolyte layer and the withstand voltage, and the relationship between the weight of the solid electrolyte layer and the ESR, relating to Examples 8 to 14.

(Solid electrolytic capacitor)
A solid electrolytic capacitor has a pair of electrode bodies and an electrolyte layer. One electrode body is an anode foil with a dielectric film formed on the foil surface. The other electrode body is a cathode body. The cathode body is positioned opposite the anode foil. This pair of electrode bodies is positioned opposite each other with an electrolyte layer in between. The assembly of this pair of electrode bodies and electrolyte layer is called a capacitor element.

The capacitor element may include a separator. The separator is interposed between the pair of electrode bodies to isolate the anode foil and cathode body to prevent short circuits and to hold the electrolyte layer in place. If the shape of the electrolyte layer can be maintained by itself and the pair of electrode bodies can be isolated by the electrolyte layer, the separator can be omitted from the capacitor element.

An anode lead is connected to the anode foil, and a cathode lead is connected to the cathode body. The solid electrolytic capacitor is electrically connected to the mounting circuit via these anode and cathode leads. By being conductive to the mounting circuit, the solid electrolytic capacitor becomes a passive element that obtains capacitance through the dielectric polarization action of the dielectric film and stores and discharges electric charge.

One example of a method for manufacturing this solid electrolytic capacitor is outlined below. First, the anode foil is formed in an anode foil formation process, which enlarges the surface area of the anode foil and then forms a dielectric film. This anode foil is then placed opposite a cathode body in the element formation process to form a capacitor element. This capacitor element is then subjected to a repair chemical treatment.

Then, an electrolyte layer formation process is performed in which a conductive polymer liquid in which conductive polymer particles or powder are dispersed or dissolved is impregnated between the anode foil and the cathode body to form an electrolyte layer. The open end of the case into which the capacitor element is inserted is then sealed with a sealer, and aging is performed to form a solid electrolytic capacitor.

(Electrode body)
In such solid electrolytic capacitors, the electrode body is a foil body made of a valve metal. In the wound type, a long strip shape obtained by stretching the valve metal is often used, while in the flat type, a flat plate obtained by stretching the valve metal is often used. Valve metals include aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony. The purity is preferably 99.9% or more for the anode foil and 99% or more for the cathode body, but impurities such as silicon, iron, copper, magnesium, and zinc may be contained.

A surface-expanding layer is formed on one or both sides of the anode foil. The surface-expanding layer is an etching layer having numerous tunnel-shaped etching pits. The tunnel-shaped etching pits are holes dug in the thickness direction of the foil. The tunnel-shaped pits may penetrate the foil, or may be long enough so that their deepest parts remain within the foil. The tunnel-shaped etching pits are typically formed by passing a direct current in an acidic aqueous solution, such as hydrochloric acid, in which halogen ions are present. The tunnel-shaped etching pits are further enlarged by passing a direct current in an acidic aqueous solution, such as nitric acid.

The cathode body may be, for example, a foil-shaped cathode foil. Alternatively, the cathode body may be a laminate of a metal layer such as silver and a carbon layer. A surface expansion layer may be formed on one or both sides of the cathode foil. A plain foil without a surface expansion layer may be used as the cathode foil. The surface expansion layer of the cathode foil is an etching layer, a sintered layer in which valve metal powder is sintered, or a vapor deposition layer in which valve metal particles are vapor-deposited on the foil. In other words, the surface expansion layer of the cathode foil is made up of tunnel-shaped pits, spongy pits, or gaps between densely packed powder or particles.

The dielectric film is formed on the uneven surface of the surface expansion layer. The dielectric film is typically an oxide film formed on the surface of the anode foil. If the anode foil is an aluminum foil, it is an aluminum oxide layer formed by oxidizing the surface of the surface expansion layer. In the chemical conversion treatment to form the dielectric film, a voltage is applied to the anode foil in a chemical conversion solution to achieve the desired withstand voltage. The chemical conversion solution is a solution that does not contain halogen ions, and examples of such solutions include phosphoric acid-based chemical conversion solutions such as ammonium dihydrogen phosphate, boric acid-based chemical conversion solutions such as ammonium borate, and adipic acid-based chemical conversion solutions such as ammonium adipate.

The cathode body may have a natural oxide film or a thin oxide film (about 1 to 10 V) formed by chemical conversion treatment. The natural oxide film is formed when the cathode body reacts with oxygen in the air.

(Electrolyte layer)
The electrolyte layer is attached to at least a part of the dielectric film of the anode foil, and serves as the true cathode of the solid electrolytic capacitor. Preferably, the electrolyte layer is in close contact with the entire dielectric film and connected to the surface of the cathode foil. The electrolyte layer is a solid electrolyte layer, or is composed of a solid electrolyte layer and a liquid component. The solid electrolyte layer contains a conductive polymer. The liquid component is a driving electrolyte solution or a solvent portion of the electrolyte solution that is impregnated in the voids of the capacitor element on which the solid electrolyte layer is formed.

(Solid electrolyte layer)
Conductive polymers are self-doped conjugated polymers doped with dopant molecules within the molecule or doped with external dopant molecules. Conjugated polymers are obtained by chemical oxidative polymerization or electrolytic oxidative polymerization of monomers or their derivatives having π-conjugated double bonds. Doped conjugated polymers exhibit high electrical conductivity. In other words, electrical conductivity is exhibited by adding a small amount of dopant, such as an acceptor that easily accepts electrons or a donor that easily gives electrons, to a conjugated polymer.

The solid electrolyte layer is formed using a conductive polymer liquid. The conductive polymer liquid is a liquid in which conductive polymer particles or powder are dispersed or dissolved. Additives are added to the conductive polymer liquid as necessary. At least the anode foil, the pair of electrode bodies and the separator, or the capacitor element are immersed in the conductive polymer liquid and then dried. In addition to immersion, the conductive polymer liquid may be applied by dripping or spraying. As a result, some or all of the unvolatilized solvent remains in the conductive polymer liquid, and the conductive polymer and additives adhere to the conductive polymer and additives, forming the solid electrolyte layer.

This solid electrolyte layer has a weight of 250 mg/cm3 or ^less per unit volume of the capacitor element. In other words, in the electrolyte formation process, the conductive polymer liquid is impregnated and dried so that the total weight of the conductive polymer, the remaining solvent in the conductive polymer liquid, and the additives remains in the capacitor element at a ratio of 250 mg/ ^cm3 or ^less per unit volume of the capacitor element. When the weight is 250 mg/cm3 or less per unit volume of the capacitor element, low ESR and high withstand voltage are both achieved. On the other hand, when the weight per unit volume of the capacitor element exceeds 301 mg/ ^cm3 , the ESR deteriorates.

There is no limitation on the weight adjustment of the solid electrolyte layer. For example, the weight of the solid electrolyte layer can be adjusted by changing the type and amount of components that volatilize from the solid electrolyte layer using the drying temperature, drying time, and pressure after immersion in the conductive polymer liquid as variables.

When the solid electrolytic capacitor is not of the hybrid type but of the non-hybrid type, and the electrolyte layer has only a solid electrolyte layer and does not contain a liquid compound, the solid electrolyte layer preferably has a density of 120 mg/ ^cm3 or more and 150 mg/ ^cm3 or less per unit volume of the capacitor element. When the non-hybrid type solid electrolyte layer falls within this range, the ESR is particularly low and minimized, and the withstand voltage is particularly high and maximized. When the density deviates above or below this range, the ESR increases compared to this range, and the withstand voltage decreases compared to this range, although the absolute values of the ESR and withstand voltage are low.

When the solid electrolytic capacitor is a hybrid type, the voids of the capacitor element are impregnated with a liquid component, and the electrolyte layer is composed of a solid electrolyte layer and a liquid component, the solid electrolyte layer is preferably 15 mg/cm ³ or more and 250 mg/cm ³ or less per unit volume of the capacitor element. In other words, the conductive polymer liquid is preferably impregnated and dried so that the total weight of the conductive polymer, the conductive polymer solvent, and the additives remains in the capacitor element at a ratio of 15 mg/cm ³ or more and 250 mg/cm ³ per unit volume of the capacitor element. When the hybrid type solid electrolyte layer falls within this range, the ESR is particularly low and the withstand voltage is particularly high. When the range is deviated above or below, the ESR increases compared to this range, and the withstand voltage decreases compared to this range, although the absolute values are low ESR and high withstand voltage.

Furthermore, when the solid electrolytic capacitor is a hybrid type, the solid electrolyte layer is particularly preferably 60 mg/ ^cm3 to 180 mg/ ^cm3 per unit volume of the capacitor element. In this range, the ESR is further minimized and the withstand voltage is further maximized.

In such a solid electrolyte layer, any known conjugated polymer can be used without any particular limitation. Examples include polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene, polyphenylenevinylene, polyacene, polythiophenevinylene, etc. These conjugated polymers may be used alone or in combination of two or more kinds, or may be a copolymer of two or more kinds of monomers.

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

In particular, a polymer of 3,4-ethylenedioxythiophene called EDOT, that is, poly(3,4-ethylenedioxythiophene) called PEDOT, is particularly preferred. A substituent may be added to 3,4-ethylenedioxythiophene. For example, an alkylated ethylenedioxythiophene having an alkyl group having 1 to 5 carbon atoms added as a substituent may be used. Examples of alkylated ethylenedioxythiophene include methylated ethylenedioxythiophene (i.e., 2-methyl-2,3-dihydro-thieno[3,4-b][1,4]dioxine), ethylated ethylenedioxythiophene (i.e., 2-ethyl-2,3-dihydro-thieno[3,4-b][1,4]dioxine), butylated ethylenedioxythiophene (i.e., 2-butyl-2,3-dihydro-thieno[3,4-b][1,4]dioxine), and 2-alkyl-3,4-ethylenedioxythiophene.

Any known dopant can be used without any particular limitation. The dopant may be used alone or in combination of two or more kinds. A polymer or monomer may also be used. For example, the dopant may be an inorganic acid such as a polyanion, boric acid, nitric acid, or phosphoric acid, or an organic acid such as acetic acid, oxalic acid, citric acid, tartaric acid, squaric acid, rhodizonic acid, croconic acid, salicylic acid, p-toluenesulfonic acid, 1,2-dihydroxy-3,5-benzenedisulfonic acid, methanesulfonic acid, trifluoromethanesulfonic acid, borodisalicylic acid, bisoxalateborate acid, sulfonylimide acid, dodecylbenzenesulfonic acid, propylnaphthalenesulfonic acid, or butylnaphthalenesulfonic acid.

Examples of polyanions include substituted or unsubstituted polyalkylenes, substituted or unsubstituted polyalkenylenes, substituted or unsubstituted polyimides, substituted or unsubstituted polyamides, and substituted or unsubstituted polyesters, and include polymers consisting only of structural units having an anionic group, and polymers consisting of structural units having an anionic group and structural units not having an anionic group. Specific examples of polyanions include polyvinyl sulfonic acid, polystyrene sulfonic acid, polyallylsulfonic acid, polyacrylsulfonic acid, polymethacrylsulfonic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprenesulfonic acid, polyacrylic acid, polymethacrylic acid, and polymaleic acid.

The solvent for the conductive polymer liquid, i.e., the remaining solvent in the solid electrolyte layer, is sufficient as long as the conductive polymer disperses or dissolves, and is preferably water or a mixture of water and an organic solvent. Examples of organic solvents include polar solvents, alcohols, esters, hydrocarbons, carbonate compounds, ether compounds, chain ethers, heterocyclic compounds, and nitrile compounds.

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 chain 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.

Additives to the conductive polymer liquid include polyhydric alcohols, organic binders, surfactants, dispersants, defoamers, coupling agents, antioxidants, UV absorbers, etc. Examples of polyhydric alcohols include sorbitol, ethylene glycol, propanediol, butanediol, pentanediol, hexanediol, heptanediol, octanediol, diethylene glycol, triethylene glycol, polyoxyalkylene glycol, glycerin, polyglycerin, polyoxyalkylene glycerin, xylitol, erythritol, mannitol, dipentaerythritol, pentaerythritol, sulfolane, methylsulfolane, or a combination of two or more of these.

As the solvent, additive, or both that constitute such a conductive polymer liquid, a compound having a hydrophilic group such as a hydroxyl group or a hydrophilic molecule is preferred. Examples of compounds having a hydroxyl group include polyhydric alcohols such as ethylene glycol, butanediol, diethylene glycol, and glycerin. Compounds having a hydroxyl group cause a change in the higher-order structure of the conductive polymer, which reduces the ESR of the solid electrolytic capacitor and improves the voltage resistance. In addition, polyhydric alcohols have a high boiling point and tend to remain in the electrolyte layer and form a solid electrolyte layer.

In addition, butanediol, diethylene glycol, polyoxyethylene glycol, glycerin, and sulfolane are particularly preferred as the solvent, additive, or both that make up such a conductive polymer liquid. Although it is speculated, the boiling points of butanediol, diethylene glycol, polyoxyethylene glycol, glycerin, and sulfolane are high, for example, at 200°C or higher. Therefore, it is thought that the flexibility of the conductive polymer is increased when it is dried after being immersed in the conductive polymer liquid. A conductive polymer with increased flexibility is more likely to adhere to the electrode body. When the adhesion between the conductive polymer and the electrode body is improved, a low ESR can be maintained even with a small amount of solid electrolyte layer.

(Liquid component)
The liquid component is a driving electrolyte or a solvent portion of the driving electrolyte. Examples of the solvent for the driving electrolyte include protic organic polar solvents and aprotic organic polar solvents, which may be used alone or in combination of two or more kinds.

Examples of protic organic solvents that serve as 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 alkylene oxide adducts of polyhydric alcohols such as ethylene glycol, diethylene glycol, propylene glycol, glycerin, methyl cellosolve, ethyl cellosolve, methoxypropylene glycol, dimethoxypropanol, polyglycerin, polyethylene glycol, polyoxyethylene glycerin, and polypropylene glycol.

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

When the solute of the driving electrolyte is added to the liquid component, the solute is an anion component and a cation component. The solute is typically a salt of an organic acid, a salt of an inorganic acid, or a salt of a complex compound of an organic acid and an inorganic acid, and is used alone or in combination of two or more kinds. An acid that becomes an anion and a base that becomes a cation may be added separately to the solvent.

Organic acids that act as anionic solutes include carboxylic acids such as oxalic acid, succinic acid, glutaric acid, pimelic acid, suberic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, maleic acid, adipic acid, benzoic acid, toluic acid, enanthic acid, malonic acid, 1,6-decanedicarboxylic acid, 1,7-octanedioic acid, azelaic acid, undecanedioic acid, dodecanedioic acid, tridecanedioic acid, t-butyl adipic acid, 11-vinyl-8-octadecenedioic acid, resorcylic acid, phloroglucinic acid, gallic acid, gentisic acid, protocatechuic acid, pyrocatechuic acid, trimellitic acid, and pyromellitic acid, as well as phenols and sulfonic acids.

Inorganic acids include boric acid, phosphoric acid, phosphorous acid, hypophosphorous acid, carbonic acid, silicic acid, etc. Composite compounds of organic acids and inorganic acids include borodisalicylic acid, borodioxalic acid, borodiglycolic acid, borodimalonic acid, borodisuccinic acid, borodiadipic acid, borodiazelaic acid, borodibenzoic acid, borodimaleic acid, borodilactic acid, borodimalic acid, boroditartaric acid, borodicitric acid, borodiphthalic acid, borodi(2-hydroxy)isobutyric acid, borodiresorcylic acid, borodimethylsalicylic acid, borodinaphthoic acid, borodimandelic acid, and borodi(3-hydroxy)propionic acid, etc.

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

Furthermore, other additives can be added to the electrolyte. Examples of additives include alkylene oxide adducts of polyhydric alcohols such as polyethylene glycol and polyoxyethylene glycerin, complex compounds of boric acid and polysaccharides (mannitol, sorbitol, etc.), complex compounds of boric acid and polyhydric alcohols, boric acid esters, nitro compounds (o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, p-nitrophenol, p-nitrobenzyl alcohol, etc.), and phosphate esters. These may be used alone or in combination of two or more.

(Separator)
Examples of the separator 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 fully 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.

The present invention will be described in more detail below with reference to examples. Note that the present invention is not limited to the following examples.

(Examples 1 to 7)
The solid electrolytic capacitors of Examples 1 to 7 and Comparative Example 1 were fabricated as follows. The solid electrolytic capacitors of Examples 1 to 7 and Comparative Example 1 were non-hybrid types, and did not contain any liquid component in the electrolyte layer.

First, both electrodes were made of aluminum foil in the shape of a long strip. Tunnel-shaped pits were formed on both sides of the anode foil by DC etching. A dielectric film was also formed on the anode foil by chemical conversion. In the chemical conversion, the applied voltage reached 650V. Pits were formed on both sides of the cathode foil by AC etching, and an oxide film was formed by chemical conversion at a chemical conversion voltage of 3Vfs. Lead wires were connected to both electrodes, and the two electrodes were wound facing each other with a separator made of Manila hemp in between. Then, a repair chemical conversion was performed using an aqueous solution of ammonium borate.

Next, the capacitor element was immersed in the conductive polymer liquid, and after the conductive polymer liquid was impregnated into the capacitor element, the capacitor element was dried. Polyethylenedioxythiophene doped with polystyrene sulfonic acid (PEDOT/PSS) was dispersed in the conductive polymer liquid as a conductive polymer. The solvent for the conductive polymer liquid was a mixture of water and ethylene glycol. Sorbitol was added to the conductive polymer liquid. The conductive polymer liquid contained 48.5 wt% water, 48.5 wt% ethylene glycol, 1 wt% PEDOT/PSS, and 2 wt% sorbitol.

As a result, a solid electrolyte layer was formed in the capacitor element. Here, the weight of the solid electrolyte layer differs between Examples 1 to 7 and Comparative Example 1. The weight of the solid electrolyte layer is the weight per unit volume of the capacitor element, and is hereinafter simply referred to as the solid electrolyte layer weight. The weight of the solid electrolyte layer was adjusted by drying the capacitor element after impregnation with the conductive polymer liquid at 110°C and increasing or decreasing the drying time.

In this way, the solid electrolytic capacitors of Examples 1 to 7 and Comparative Example 1 were manufactured with the same structure, composition, manufacturing method, and manufacturing conditions, except for the weight of the solid electrolyte layer.

(Dielectric strength and ESR)
The withstand voltage of the solid electrolytic capacitors of Examples 1 to 7 and Comparative Example 1 was measured. The method for measuring the withstand voltage was as follows. That is, a voltage was applied to the solid electrolytic capacitor at 105° C. The initial voltage was 200 V, and the applied voltage was increased by 1 V every 10 seconds. The voltage when the current flowing through the solid electrolytic capacitor reached 1 mA was defined as the withstand voltage.

The ESR of the solid electrolytic capacitors of Examples 1 to 7 and Comparative Example 1 was also measured. The method for measuring the withstand voltage is as follows. The ESR was measured at room temperature using an LCR meter manufactured by NF Corporation. The measurement frequency was 100 kHz, and the AC amplitude was a sine wave of 0.5 Vms.

The following Table 1 shows the measurement results of the withstand voltage and ESR together with the weight of the solid electrolyte layer of Examples 1 to 7 and Comparative Example 1. In Table 1, the weight of the solid electrolyte layer is expressed as the amount of solid electrolyte per unit volume of the element.
(Table 1)

As shown in Table 1, the weight of the solid electrolyte layer is the weight (mg/ ^cm3 ) converted to a unit volume of the capacitor element, and was varied from 18 mg/ ^cm3 to 301 mg/ ^cm3 so that Examples 1 to 7 and Comparative Example 1 had unique values. Note that the weight of each solid electrolyte layer was calculated by subtracting the weight of the capacitor element before immersion in the conductive polymer liquid from the weight of the capacitor element after immersion in the conductive polymer liquid and drying.

Based on Table 1, the relationship between the weight of the solid electrolyte layer and the withstand voltage, and the relationship between the weight of the solid electrolyte layer and the ESR are shown in the graph in Figure 1. In Figure 1, the square marks plot the withstand voltage, and the circle marks plot the ESR. Note that the ESR is expressed in logarithm.

As shown in Table 1 and FIG. 1, when the weight of the solid electrolyte layer is 18 mg/ ^cm3 , the withstand voltage exceeds 200 V, and increases significantly as the weight of the solid electrolyte layer increases from 18 mg/ ^cm3 to 120 mg/ ^cm3 . The withstand voltage is maximized from 120 mg/ ^cm3 to 150 mg/ ^cm3 . At 180 mg/ ^cm3 , the withstand voltage is significantly higher than 200 V, but drops sharply compared to 150 mg/ ^cm3 . There is little change in the withstand voltage beyond 180 mg/ ^cm3 .

On the other hand, the ESR rapidly decreases as the weight of the solid electrolyte layer increases from 18 mg/ ^cm3 to 120 mg/ ^cm3 . The ESR then reaches a minimum between 120 mg/ ^cm3 and 180 mg/ ^cm3 . The ESR starts to increase from 250 mg/ ^cm3 . The ESR at 301 mg/ ^cm3 and above reaches about 28 times the ESR at 250 mg/ ^cm3 .

Thus, a solid electrolytic capacitor with a solid electrolyte weight of 250 mg/ ^cm3 or less achieves both low ESR and high withstand voltage. A non-hybrid solid electrolytic capacitor with a solid electrolyte weight of 120 mg/ ^cm3 or more and 150 mg/ ^cm3 or less achieves both maximized withstand voltage and minimized ESR.

(Examples 8 to 14)
Furthermore, solid electrolytic capacitors of Examples 8 to 14 and Comparative Example 2 were produced. The solid electrolytic capacitors of Examples 8 to 14 and Comparative Example 2 were hybrid types, and the electrolyte layer was composed of a solid electrolyte layer and a liquid component. The liquid component was composed of 59.4 wt% ethylene glycol, 39.6 wt% polyethylene glycol, and 1 wt% ammonium azelaate. The average molecular weight of the polyethylene glycol was 1000. The liquid component was impregnated into the capacitor element. The other configurations, compositions, manufacturing methods, and manufacturing conditions were the same as those of Examples 1 to 7 and Comparative Example 1, except for the weight of the solid electrolyte layer.

(Dielectric strength and ESR)
The withstand voltage was measured for the solid electrolytic capacitors of Examples 8 to 14 and Comparative Example 2. The measuring method and conditions for the withstand voltage and ESR were the same as those of Examples 1 to 7 and Comparative Example 1. The method for confirming the weight of each solid electrolyte layer was also the same as those of Examples 1 to 7 and Comparative Example 1.

The measurement results of the withstand voltage and ESR are shown in Table 2 below, together with the weights of the solid electrolyte layers of Examples 8 to 14 and Comparative Example 2. In Table 2, the weights of the solid electrolyte layers are expressed as the amount of solid electrolyte per unit volume of the element.
(Table 2)

As shown in Table 2, the weight of the solid electrolyte layer is the weight (mg/ ^cm3 ) converted to a unit volume of the capacitor element, and was changed from 18 mg/ ^cm3 to 301 mg/ ^cm3 so that Examples 8 to 14 and Comparative Example 2 had unique values. Based on this Table 2, the relationship between the weight of the solid electrolyte layer and the withstand voltage, and the relationship between the weight of the solid electrolyte layer and the ESR are shown in the graph of Figure 2. In Figure 2, the plot of square marks indicates the withstand voltage, and the plot of circle marks indicates the ESR. Note that the ESR is expressed in logarithm.

As shown in Table 2 and Fig. 2, when the weight of the solid electrolyte layer is 18 mg/ ^cm3 , the withstand voltage greatly exceeds 200 V, and increases significantly as the weight of the solid electrolyte layer increases from 18 mg/ ^cm3 to 120 mg/ ^cm3 . The withstand voltage is maximized from 60 mg/ ^cm3 to 180 mg/ ^cm3 . After 180 mg/ ^cm3 , the withstand voltage decreases as the weight of the solid electrolyte layer increases. However, even when the weight of the solid electrolyte layer is 301 mg/ ^cm3 , the withstand voltage greatly exceeds 200 V.

On the other hand, the ESR is maintained low regardless of the weight of the solid electrolyte layer from 18 mg/ ^cm3 to 250 mg/ ^cm3 . However, the ESR significantly deteriorates from 301 mg/ ^cm3 onwards, reaching about 11 times the ESR at 250 mg/ ^cm3 .

Thus, regardless of whether the solid electrolytic capacitor is hybrid or non-hybrid, a solid electrolyte layer weight of 250 mg/ ^cm3 or less achieves both low ESR and high withstand voltage. In particular, a hybrid solid electrolyte capacitor with a solid electrolyte layer weight of 60 mg/ ^cm3 or more and 150 mg/ ^cm3 achieves both maximized withstand voltage and particularly low ESR.

(Examples 15 to 18)
Furthermore, solid electrolytic capacitors of Examples 15 to 18 were produced. The solid electrolytic capacitors of Examples 15 to 18 were of the same non-hybrid type as Example 4, but the solvent of the conductive polymer liquid was different from that of Example 4. Except for the difference in the solvent of the conductive polymer liquid, the solid electrolytic capacitors of Examples 15 to 18 were produced with the same configuration, composition, manufacturing method, and manufacturing conditions as Example 4.

(Dielectric strength and ESR)
The withstand voltage was measured for the solid electrolytic capacitors of Examples 15 to 18. The measuring method and conditions for the withstand voltage and ESR were the same as those in Example 4. The method for confirming the weight of each solid electrolyte layer was also the same as that in Example 4.

The measurement results of the withstand voltage and ESR are shown in Table 3 below, together with the weights of the solid electrolyte layers of Examples 15 to 18 and Example 4. In Table 3, the weights of the solid electrolyte layers are expressed as the amount of solid electrolyte per unit volume of the element.
(Table 3)

As shown in Table 3, the solvent of the conductive polymer liquid in Example 4 is ethylene glycol, whereas in Example 15 it is butanediol, in Example 16 it is diethylene glycol, in Example 17 it is glycerin, and in Example 18 it is sulfolane. That is, the solid electrolyte layer in Example 4 contains ethylene glycol as a compound having a hydroxyl group. The solid electrolyte layer in Example 15 contains butanediol as a compound having a hydroxyl group. The solid electrolyte layer in Example 16 contains diethylene glycol as a compound having a hydroxyl group. The solid electrolyte layer in Example 17 contains glycerin as a compound having a hydroxyl group. The solid electrolyte layer in Example 18 contains sulfolane as a high boiling point solvent.

As shown in Table 3, it was confirmed that, regardless of the type of solvent in the solid electrolyte layer, as long as the weight of the solid electrolyte layer per unit volume of the capacitor element is 250 mg/cm3 or ^less , both a high withstand voltage and a low ESR can be obtained. In addition, it was confirmed that when the solvent in the solid electrolyte layer is a compound having a hydroxyl group, such as ethylene glycol, butanediol, diethylene glycol, or glycerin, the withstand voltage can be particularly high and the ESR can be particularly low.

Claims (9)

A capacitor element having an anode foil, a cathode body, and an electrolyte,
the anode foil has a surface enlarged by tunnel-shaped etching pits and has a dielectric coating on the surface;
The cathode body faces the anode foil,
the electrolyte includes a solid electrolyte layer including a conductive polymer;
the solid electrolyte layer has a weight per unit volume of the capacitor element of 250 mg/ ^cm3 or less;
A solid electrolytic capacitor characterized by: the solid electrolyte layer includes the conductive polymer, a solvent for a conductive polymer liquid in which the conductive polymer is dispersed or dissolved, and an additive added to the conductive polymer liquid,
the total weight of the conductive polymer, the solvent, and the additive is 250 mg/ ^cm3 or less per unit volume of the capacitor element;
2. The solid electrolytic capacitor according to claim 1, the electrolyte has only the solid electrolyte layer,
the solid electrolyte layer has a weight per unit volume of the capacitor element of 120 mg/ ^cm3 or more and 150 mg/ ^cm3 or less;
3. The solid electrolytic capacitor according to claim 1 or 2. the electrolyte includes a liquid component that fills voids in the capacitor element;
3. The solid electrolytic capacitor according to claim 1 or 2. the solid electrolyte layer has a weight per unit volume of the capacitor element of 15 mg/ ^cm3 or more and 250 mg/ ^cm3 or less;
5. The solid electrolytic capacitor according to claim 4, the solid electrolyte layer has a weight per unit volume of the capacitor element of 60 mg/ ^cm3 or more and 180 mg/ ^cm3 or less;
6. The solid electrolytic capacitor according to claim 5, the solid electrolyte layer contains a compound having a hydroxy group;
3. The solid electrolytic capacitor according to claim 1 or 2. the compound having a hydroxy group is at least one selected from the group consisting of ethylene glycol, butanediol, diethylene glycol, and glycerin;
8. The solid electrolytic capacitor according to claim 7, A method for manufacturing a solid electrolytic capacitor having an anode foil, a cathode body, and a solid electrolyte layer, comprising the steps of:
an anode foil forming step of forming a tunnel-shaped etching pit on the surface of the anode foil and further forming a dielectric film on the surface;
an element forming step of forming a capacitor element by opposing the anode foil to the cathode body;
an electrolyte layer forming step of applying a conductive polymer solution having a conductive polymer dispersed or dissolved therein between the anode foil and the cathode body to form the solid electrolyte layer containing the conductive polymer, a solvent for the conductive polymer solution in which the conductive polymer is dispersed or dissolved, and an additive added to the conductive polymer solution;
Including,
In the electrolyte layer forming step, the conductive polymer, the solvent, and the additive are contained in an amount of 250 mg/ ^cm3 or less in total per unit volume of the capacitor element;
A method for producing a solid electrolytic capacitor comprising the steps of:

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