Solid electrolytic capacitor and method for manufacturing same

Abstract

Provided are a solid electrolytic capacitor having good frequency characteristics, and a method for manufacturing the same. An anode foil has an expanded layer formed on a foil surface, a dielectric film formed on the expanded layer, and a pseudo boehmite film formed on the dielectric film. A conductive polymer is contained at 50% or more in the bottom of the expanded layer, with respect to the content in the surface layer of the expanded surface layer. For this solid electrolytic capacitor, included are: a step for forming an expanded surface layer on an anode foil; a step for, after performing the step regarding the expanded surface, forming a pseudo boehmite film on the anode foil; a step for forming an oxide film on the anode foil so that the pseudo boehmite film remains on a dielectric film; and a step for forming a layer of a conductive polymer by causing a conductive polymer dispersion liquid containing the conductive polymer and a high boiling point solvent to adhere to at least the anode foil and to dry the same, so that the conductive polymer is adjusted to be contained at 50% or more in the bottom part of the expanded surface layer, with respect to the content in the surface layer of the expanded surface layer.

Description

Solid electrolytic capacitor and its manufacturing method

[0001] The present invention relates to a solid electrolytic capacitor and a method for manufacturing the same.

[0002] A solid electrolytic capacitor uses a valve metal, such as tantalum or aluminum, as anode and cathode foils. The anode foil is enlarged by shaping the valve metal into a sintered or etched foil, and the enlarged surface is coated with a dielectric film by anodizing or other processes. A solid electrolyte layer, which adheres closely to the anode foil and acts as the true cathode, is interposed between the anode and cathode foils.

[0003] Solid electrolytic capacitors can achieve a large capacitance in a small size by etching the valve metal anode to increase the surface area of ​​the anode. In particular, solid electrolytic capacitors, in which the dielectric film is covered with a solid electrolyte, are small, have large capacitance, and have low equivalent series resistance (low ESR), making them essential for miniaturizing electronic devices, improving their functionality, and reducing their costs.

[0004] Known examples of solid electrolytes include manganese dioxide and 7,7,8,8-tetracyanoquinodimethane (TCNQ) complexes. In recent years, conductive polymers derived from monomers with π-conjugated double bonds, such as poly(3,4-ethylenedioxythiophene) (PEDOT), have rapidly become popular as solid electrolytes, due to their slow reaction rate and excellent adhesion to dielectric films. Examples of conductive polymers include externally doped conductive polymers that exhibit conductivity by using low-molecular-weight anions or polyanions as dopants during chemical oxidative polymerization or electrolytic oxidative polymerization, and self-doped conductive polymers that exhibit conductivity and solubility in solvents by incorporating a substructure within the monomer molecule that acts as a dopant.

[0005] The conductive polymer layer is formed, for example, as follows: A capacitor element consisting of an anode foil, a cathode foil, and a separator is impregnated with a polymerization solution. The polymerization solution contains a monomer, which is a monomer unit of the conductive polymer. The monomer is allowed to penetrate into the pits by impregnation with the polymerization solution, and then a conductive polymer is produced by electrolytic polymerization or chemical polymerization and attached to the anode foil (see, for example, Patent Document 1). This forms a conductive polymer layer.

[0006] However, it has been reported that the average depth of the pits where a conductive polymer film is formed is up to 16 μm (see Patent Document 2). During polymerization, the monomer present at the back of the pit moves to the vicinity of the pit entrance and is consumed, so the monomer concentration at the back of the pit becomes low, and a conductive polymer film is not formed in the deep part. Furthermore, even when a liquid with a high liquid concentration is used, only a thick film is formed near the pit entrance, and the film does not progress in the direction of the pit depth.

[0007] Powder laminated foils are sometimes used as anode foils. Powder laminated foils are anode foils with a core of stretched foil on which a surface-expanded layer is formed by vapor-depositing or sintering valve metal powder, or anode foils without a core, with a surface-expanded layer formed by pressure-molding and sintering valve metal powder. The inventors have confirmed that even when powder laminated foils are used as anode foils, no conductive polymer is formed in the depths toward the center of the foil. In this powder laminated foil, too, the monomer near the center of the foil's thickness migrates to the surface layer of the powder laminated foil during polymerization and is consumed, resulting in a dilution of the monomer concentration in the depths toward the foil's thickness.

[0008] Therefore, there is a method for forming a dielectric film down to the finest details by impregnating aluminum foil with an aqueous solution containing polyaniline, which is obtained by polymerizing a solvent-soluble aniline derivative (see, for example, Patent Document 3). However, in recent years, poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid, known as PEDOT:PSS, has become more popular than polyaniline as a conductive polymer. PEDOT is poorly soluble in water and other substances.

[0009] Solid electrolytic capacitors are used in a variety of applications. For example, in the field of power electronics, a power conversion circuit converts power from an AC power source into DC power using a converter circuit, and then converts this DC power into desired AC power using an inverter circuit. A smoothing capacitor is provided in the power conversion circuit to suppress pulsation in the DC output from the converter circuit and smooth the DC before inputting it to the inverter circuit. Furthermore, a decoupling capacitor is provided near a semiconductor switching element to ensure stable operation of the element and remove noise.

[0010] Because the field of power electronics handles large amounts of power, solid electrolytic capacitors are required to have a high withstand voltage. Therefore, for example, a pseudo-boehmite film may be formed on top of a dielectric film formed on an anode foil. The pseudo-boehmite film has a dense interior and functions as a resistive layer, improving the withstand voltage of the solid electrolytic capacitor. However, the pseudo-boehmite film has the problem of impregnating the anode foil with conductive polymers.

[0011] JP 2021-64737 A JP 2-288217 A JP 2000-340462 A

[0012] In recent years, in the field of power electronics, MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors) using wide bandgap semiconductors such as SiC and GaN have been attracting attention. These devices using wide bandgap semiconductors have advantages over conventional power devices using silicon, such as lower on-resistance, faster switching speeds, and the ability to operate in high-temperature environments.

[0013] Taking advantage of the characteristics of wide bandgap semiconductors, power converters and other devices can achieve high efficiency by suppressing power loss through low on-resistance and high-speed switching. As a result, cooling components such as cooling fans and heat sinks can be simplified, and passive components can be made smaller through higher frequencies, contributing to cost reduction and energy savings.

[0014] In response to recent trends in equipment with power devices such as power converters, the solid electrolytic capacitors incorporated into such equipment must also have frequency characteristics compatible with wide bandgap semiconductors. That is, solid electrolytic capacitors must have high capacitance and low equivalent series resistance (ESR) even in the high frequency range.

[0015] The present invention has been proposed to solve the above problems, and an object of the present invention is to provide a solid electrolytic capacitor having good frequency characteristics and a method for manufacturing the same.

[0016] In order to solve the above problems, the solid electrolytic capacitor of this embodiment is a solid electrolytic capacitor comprising an anode foil and a layer of a conductive polymer, wherein the anode foil has a surface-expanding layer formed on the foil surface, a dielectric film formed on the surface-expanding layer, and a pseudo-boehmite film formed on the dielectric film, and the conductive polymer is contained in a bottom portion of the surface-expanding layer in an amount of 50% or more based on the content in the surface layer of the surface-expanding layer.

[0017] The conductive polymer may be poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid.

[0018] The surface-expanding layer may have a plurality of tunnel-shaped pits.

[0019] The surface-expanding layer may be formed by laminating powder of a valve metal.

[0020] The dielectric film may have a withstand voltage of 250 V or more. The dielectric film may have a thickness that corresponds to a formation voltage of 250 V or more.

[0021] Furthermore, in order to solve the above-mentioned problems, the method for manufacturing a solid electrolytic capacitor of this embodiment is a method for manufacturing a solid electrolytic capacitor including an anode foil and a conductive polymer layer, and includes: a surface-expanding step of forming a surface-expanding layer on the anode foil; a pre-chemical conversion step of forming a pseudo-boehmite film on the anode foil after the surface-expanding step; a chemical conversion step of forming a dielectric film on the anode foil so that the pseudo-boehmite film remains on the dielectric film; and a polymer layer-forming step of forming the conductive polymer layer by applying a conductive polymer dispersion containing the conductive polymer and a high-boiling-point solvent to at least the anode foil and drying it at a temperature lower than the boiling point of the high-boiling-point solvent, so that the conductive polymer is contained in the bottom portion of the surface-expanding layer at 50% or more based on the content in the surface layer of the surface-expanding layer.

[0022] The surface-expanding step may form the surface-expanding layer by a plurality of tunnel-shaped pits.

[0023] The surface enlarging step may be performed by layering powder of a valve metal.

[0024] The anodizing step may apply an anodizing voltage of 250 V or more to the anode foil.

[0025] According to the present invention, a solid electrolytic capacitor having a high capacitance and a low ESR in a high frequency range can be realized.

[0026] 1 is a cross-sectional schematic diagram of a pit in the surface-expanding layer. It is a graph showing the results of EDX analysis and illustrating the elemental ratio of sulfur in Examples 1 and 2 and Comparative Examples 1 and 2. It is a graph showing the results of EDX analysis and illustrating the sulfur element content for each depth zone of the surface-expanding layer in Examples 1 and 2 and Comparative Examples 1 and 2. It is an SEM image of Example 2, where (a) is the surface layer of the surface-expanding layer and (b) is the bottom of the surface-expanding layer. It is an SEM image of Comparative Example 1, where (a) is the surface layer of the surface-expanding layer and (b) is the bottom of the surface-expanding layer. It is a graph showing the capacitance of the solid electrolytic capacitors of Examples 1 and 2 and Comparative Examples 1 and 2 by frequency. It is a graph showing the ESR of the solid electrolytic capacitors of Examples 1 and 2 and Comparative Examples 1 and 2 by frequency. It is a graph showing the capacitance of the solid electrolytic capacitors of Example 1 and Comparative Examples 2 and 3 by frequency. It is a graph showing the ESR of the solid electrolytic capacitors of Example 1 and Comparative Examples 2 and 3 by frequency. 1 is a graph showing the capacitance of the solid electrolytic capacitors of Example 1, Examples 3 to 6, and Comparative Examples 2 and 4 for each frequency. FIG. 2 is a graph showing the ESR of the solid electrolytic capacitors of Example 1, Examples 3 to 6, and Comparative Examples 2 and 4 for each frequency. FIG. 3 is a graph showing the capacitance of the solid electrolytic capacitors of Reference Examples 1 to 3 for each frequency. FIG. 4 is a graph showing the ESR of the solid electrolytic capacitors of Reference Examples 1 to 3 for each frequency. FIG. 5 is a graph showing the results of EDX analysis, illustrating the elemental ratio of sulfur in Example 7 and Comparative Example 5. FIG. 6 is a graph showing the results of EDX analysis, illustrating the sulfur content of each depth zone of the surface-expanding layer in Example 7 and Comparative Example 5, based on the sulfur content in the surface layer of the surface-expanding layer. FIG. 7 is a graph showing the capacitance of the solid electrolytic capacitors of Example 7 and Comparative Example 5 for each frequency. FIG. 8 is a graph showing the ESR of the solid electrolytic capacitors of Example 7 and Comparative Example 5 for each frequency.

[0027] Hereinafter, a solid electrolytic capacitor and a manufacturing method thereof according to an embodiment of the present invention will be described, but the present invention is not limited to the embodiment described below.

[0028] A solid electrolytic capacitor is a passive device that stores and discharges electric charge through capacitance. The capacitor element of a solid electrolytic capacitor includes an anode foil, a cathode foil, a separator, and a conductive polymer layer (hereinafter referred to as the polymer layer) made of a valve metal. A dielectric film is formed on the surface of the anode foil. The polymer layer contains a conductive polymer and is formed between the anode foil and the cathode foil, covering at least a portion of the dielectric film on the anode foil surface. This polymer layer serves as the true cathode of the solid electrolytic capacitor.

[0029] This solid electrolytic capacitor is manufactured through an anode foil manufacturing process to manufacture an anode foil, a cathode foil manufacturing process to manufacture a cathode foil, a subassembly process to laminate the anode foil, the cathode foil, and a separator, a polymer layer forming process to form a polymer layer within the capacitor element, and a final manufacturing process to seal the capacitor element in an outer case. The order of these processes can be freely rearranged as much as possible, and parallel processing is also possible. Furthermore, one process may include multiple sub-processes, some of which may be performed within other processes, or may be inserted between one sub-process and the next sub-process of another process.

[0030] Here, the anode foil manufacturing process includes a surface enlargement process. In the surface enlargement process, a surface enlargement layer is formed on the surface of the anode foil to increase the specific surface area. In other words, the surface of the anode foil is made porous. Next, in the anode foil manufacturing process, prior to the chemical conversion process in which a dielectric film is formed, the anode foil with the surface enlargement layer formed thereon undergoes a pre-chemical conversion process in which a pseudo-boehmite film is formed. The pseudo-boehmite film contains a hydrated oxide of a valve metal. If the valve metal is aluminum, Al ２ O ３ ・xH ２ It is O.

[0031] The dielectric coating is formed in the chemical conversion process after a pre-chemical conversion process. In the chemical conversion process, the pseudo-boehmite coating is transformed into a dielectric coating from the boundary surface with the unoxidized valve metal toward both sides of the pseudo-boehmite coating and the valve metal, and the dielectric coating grows. The dielectric coating formed by transforming the pseudo-boehmite coating contains γ-alumina, which is a crystalline oxide. In the chemical conversion process, the pseudo-boehmite coating is left on the surface layer. Therefore, the pseudo-boehmite coating is formed on the dielectric coating.

[0032] In the polymer layer forming step, the polymer layer is formed using a conductive polymer dispersion. The conductive polymer dispersion is a dispersion in which conductive polymer particles or powder are dispersed. At least the anode foil, or in addition, the cathode foil and the separator, are immersed in or coated with the conductive polymer dispersion, and then dried, whereby the conductive polymer forms a polymer layer and adheres between the anode foil and the cathode foil.

[0033] In this polymer layer formation process, a polymer layer is formed using a conductive polymer dispersion liquid to which a high-boiling-point solvent has been added. The high-boiling-point solvent has a boiling point higher than the drying temperature at which the conductive polymer dispersion liquid is dried, and remains in the capacitor element at least until the drying process of the conductive polymer dispersion liquid is completed, suppressing the aggregation of the conductive polymer and reducing the size of the conductive polymer particles. In other words, the drying temperature is set to a temperature lower than the boiling point of the high-boiling-point solvent. For example, the high-boiling-point solvent has a boiling point of 150°C or higher, and the drying temperature is set to 110°C. Note that the high-boiling-point solvent may remain in the capacitor element after the drying process is completed.

[0034] Thus, the anode foil manufacturing process includes a pre-chemical conversion process in which a pseudo-boehmite film is formed on an anode foil having a surface-expanding layer, and a polymer layer formation process in which a conductive polymer dispersion liquid containing a high-boiling-point solvent is applied and dried. This manufacturing method achieves a uniform distribution of the conductive polymer in the surface-expanding layer from the surface to the bottom. Specifically, the amount of conductive polymer contained in the bottom of the surface-expanding layer is 50% or more of the content in the surface layer of the surface-expanding layer. When the surface-expanding layer is divided into three equal parts in the depth direction, the region containing the surface is referred to as the surface layer, the region containing the bottom is referred to as the bottom portion, and the region sandwiched between the surface layer and the bottom is referred to as the intermediate portion.

[0035] Here, the pits constituting the surface expansion layer can be represented by an equivalent circuit in which resistances for each depth zone are connected in series in the depth direction, and the capacitors for each depth zone are connected in series to the combined resistance up to that depth zone. In other words, the pits comprise RC circuits of resistance components and capacitance components, each with a high resistance value depending on the depth. In this case, there is a difference in the charging rate between the shallow depth zone and the deep depth zone. Therefore, when solid electrolytic capacitors are used in high frequency bands, the bottom of the pit does not generally have a time constant R × C small enough to be sufficiently charged and discharged. Therefore, typical solid electrolytic capacitors are unable to fully utilize their high capacitance at high frequencies.

[0036] Meanwhile, in this solid electrolytic capacitor, the conductive polymer content in the bottom of the surface-expanding layer is 50% or more, based on the content in the surface layer of the surface-expanding layer. A sufficient amount of conductive polymer contained in the bottom of the pits creates a conductive path that bypasses the resistance component of the RC circuit at the bottom of the pits. Therefore, even when used at high frequencies, the bottom of the pits is sufficiently charged and discharged, allowing the solid electrolytic capacitor to exhibit high capacitance. Furthermore, the large number of conductive paths created by the polymer layer at the bottom of the surface-expanding layer reduces the electrical resistance at the bottom of the surface-expanding layer, improving the ESR of the solid electrolytic capacitor.

[0037] The reason for this is presumed to be as follows, although not limited thereto. First, when the conductive polymer dispersion is dried, the conductive polymer aggregates as the dispersion medium evaporates, increasing the particle size. Furthermore, the pseudo-boehmite film is dense inside. The conductive polymer with an increased particle size is blocked by the pseudo-boehmite film and therefore has difficulty penetrating to the bottom of the surface-expanding layer. When the evaporation of the dispersion medium is suppressed and the aggregation of the conductive polymer is suppressed, the conductive polymer becomes smaller in particle size. Therefore, even in the presence of the pseudo-boehmite film, the conductive polymer easily penetrates to the bottom of the surface-expanding layer. The high-boiling-point solvent remains when the conductive polymer dispersion is dried, maintaining the small particle size of the conductive polymer.

[0038] Therefore, even if the conductive polymer is dispersed in a conductive polymer dispersion, the bottom of the surface-expanding layer contains 50% or more of the conductive polymer, based on the content in the surface layer of the surface-expanding layer. This ensures that the solid electrolytic capacitor has a high capacitance and a low ESR, even when used in high-frequency bands. Moreover, because the polymer layer is formed using a conductive polymer dispersion, the capacitor element is not immersed in a highly acidic polymerization solution, preventing the capacitor element from being exposed to strong acids. Furthermore, even for tunnel-shaped pits with an average depth of 16 μm or more, the bottom of the surface-expanding layer contains 50% or more of the conductive polymer, based on the content in the surface layer of the surface-expanding layer.

[0039] The high-boiling-point solvent added to the conductive polymer dispersion is preferably a glycol-based compound. The glycol-based compound reduces the resistance of the conductive polymer and improves the withstand voltage of the solid electrolytic capacitor. Examples of glycol-based compounds include ethylene glycol, diethylene glycol, glycerin, 1,3-propanediol, and polyethylene glycol.

[0040] The surface-expanding layer may be either a layer of valve metal powder vapor-deposited or sintered on the surface of a core of drawn valve metal foil, or an etched layer obtained by etching. That is, the anode foil may be either a powder-laminated foil with a valve metal powder layer as the surface-expanding layer, or an etched foil obtained by etching the surface of a drawn foil. The etched layer may be composed of tunnel-shaped pits or spongy pits. Tunnel-shaped etching pits are holes dug in the foil thickness direction. A surface-expanding layer composed of spongy pits is a sponge-like layer with a series of fine voids extending in a spatial manner.

[0041] Alternatively, the anode foil may be produced by pressure-molding a valve metal powder into a foil shape and sintering the resulting foil. This anode foil has no core and is entirely composed of a surface-expanded layer. The anode foil without a core is divided into three equal parts in the depth direction from the foil surface to the center of the foil thickness, with the region including the surface being the surface layer, the region including the center of the foil thickness being the bottom layer, and the region sandwiched between the surface layer and the bottom layer being the intermediate region.

[0042] In the surface-enlarging step of forming tunnel-shaped etching pits, a direct current is typically passed through the anode foil in an acidic aqueous solution such as hydrochloric acid, in which halogen ions are present. The tunnel-shaped etching pits may be further enlarged by passing a direct current through an acidic aqueous solution such as nitric acid. In the surface-enlarging step of forming spongy pits, an alternating current is typically passed through the anode foil in an acidic aqueous solution such as hydrochloric acid, in which halogen ions are present.

[0043] The powder layer is produced by attaching valve action metal particles to a foil and then sintering the particles under vacuum or in a reducing atmosphere. Alternatively, the powder layer can be produced by, for example, a resistance heating vapor deposition method or an electron beam heating vapor deposition method. The powder layer produced by this vapor deposition method is formed by heating and evaporating the valve action metal with resistance heat or electron beam energy, and then depositing the vapor of the valve action metal particles on the surface of the foil.

[0044] Preferably, the surface-expanding layer is composed of tunnel-shaped pits. The purpose of the pseudo-boehmite coating is to improve voltage resistance, and the dielectric coating also becomes thick. If the dielectric coating is made thicker, the pits in the surface-expanding layer become more easily filled during the chemical conversion process. Therefore, a tunnel-shaped pit that allows for diameter expansion is preferable. Furthermore, the tunnel-shaped pits are holes dug in the foil thickness direction, and the denser the surface-expanding layer, the more difficult it is for conductive polymer particles to reach the bottom of the pits. Therefore, solid electrolytic capacitors having a surface-expanding layer with tunnel-shaped pits clearly exhibit the effect of achieving both high capacitance and low ESR in the high-frequency range.

[0045] More preferably, the surface-expanding layer is a vapor-deposited layer or a sintered layer. That is, more preferably, the anode foil is a powder laminated foil. The powder laminated foil has a denser surface-expanding layer than an etched foil with tunnel-shaped pits, making it more difficult for conductive polymer particles to reach the bottom of the pits. However, if a conductive polymer dispersion containing a high-boiling point solvent is used in the polymer layer formation process and the conductive polymer dispersion is dried at a drying temperature lower than the boiling point of the high-boiling point solvent, a large polymer layer is formed at the bottom of the powder laminated foil, exceeding the effect of an etched foil with tunnel-shaped pits, and the effect of achieving both high capacitance and low ESR in the high-frequency range of the solid electrolytic capacitor is clearly achieved.

[0046] The pre-chemical conversion step is not particularly limited as long as it can form a pseudo-boehmite coating. For example, in the pre-chemical conversion step, the surface-expanded anode foil is immersed in pure water at 80°C or higher or boiling water. The immersion time can be determined based on the balance between the withstand voltage and the capacitance, depending on the target thickness of the pseudo-boehmite coating. This pre-chemical conversion step forms a pseudo-boehmite layer on the surface of the anode foil, following the irregularities of the surface-expanded layer.

[0047] In the chemical conversion step, the pseudo-boehmite film grows from the interface with the unoxidized valve metal toward both sides of the valve metal, transforming the pseudo-boehmite film into a dielectric film. Therefore, in the chemical conversion step, a voltage is applied to the anode foil in a chemical conversion solution that does not contain halogen ions. The chemical conversion solution may be a phosphoric acid-based chemical conversion solution such as ammonium dihydrogen phosphate, a boric acid-based chemical conversion solution such as ammonium borate, or an adipic acid-based chemical conversion solution such as ammonium adipate. The applied voltage may be determined according to the desired withstand voltage.

[0048] If the anode foil is made of aluminum, a layer of aluminum oxide is formed along the irregularities of the surface-expanding layer during the chemical conversion process. This aluminum oxide layer becomes the dielectric film. However, the anode foil may be made of a valve metal other than aluminum. Valve metals include aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony. The purity of the anode foil is preferably 99.9% or higher, but impurities such as silicon, iron, copper, magnesium, and zinc may be included.

[0049] In the chemical conversion step, it is preferable to apply a voltage of at least 250 V so that the rated withstand voltage is at least 250 V. In other words, if a dielectric film with a thickness of 1.1 to 1.5 nm is required to obtain a withstand voltage of 1 V, a dielectric film with a thickness of 275 nm to 375 nm or more is formed. When a pseudo-boehmite film is required, a solid electrolytic capacitor with a high withstand voltage of at least 250 V is expected.

[0050] Furthermore, application of a high voltage tends to generate voids in the dielectric film. Therefore, the chemical conversion process may include a depolarization treatment. The depolarization treatment may include a heat treatment, a phosphate treatment, or both. The heat treatment involves exposing the anode foil to a temperature environment of 450°C or higher in the atmosphere, for example, to open isolated voids within the dielectric film. The phosphate treatment enlarges cracks and openings that lead to the voids. In this phosphate treatment, the anode foil is immersed in a phosphoric acid solution or an ammonium dihydrogen phosphate solution. This allows the chemical conversion solution to easily penetrate into the voids.

[0051] In addition to the high-boiling-point solvent, a sugar alcohol such as sorbitol may be added to the conductive polymer dispersion. Sorbitol improves the withstand voltage of the solid electrolytic capacitor. Generally, sorbitol solidifies near the pit entrances of the surface-expanding layer, preventing the conductive polymer from penetrating to the bottom of the surface-expanding layer. However, the high-boiling-point solvent allows the sorbitol to penetrate to the bottom of the surface-expanding layer in addition to the conductive polymer. The sorbitol prevents dielectric breakdown that may occur near the bottom of the surface-expanding layer, further improving the withstand voltage of the solid electrolytic capacitor.

[0052] The conductive polymer dispersed in the conductive polymer dispersion is PEDOT:PSS. However, any type of conductive polymer may be used as long as it is in the form of a solid in the conductive polymer dispersion. The conductive polymer is a self-doped type doped with an internal dopant molecule or a conjugated polymer doped with an external dopant molecule.

[0053] Conjugated polymers can be obtained by chemical oxidative polymerization or electrolytic oxidative polymerization of a monomer having a π-conjugated double bond or a derivative thereof. Examples of conjugated polymers include polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene, polyphenylene vinylene, polyacene, and polythiophene vinylene. These conjugated polymers can be used alone or in combination of two or more types, or can be a copolymer of two or more types of monomers.

[0054] The conjugated polymers include those obtained by polymerizing thiophene or its derivatives, and specific examples include 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, and derivatives thereof. Examples of thiophene derivatives include compounds selected from thiophenes having substituents at the 3rd and 4th positions, and the substituents at the 3rd and 4th positions of the thiophene ring may form a ring together with the carbon atoms at the 3rd and 4th positions. The alkyl group or alkoxy group preferably has 1 to 16 carbon atoms.

[0055] Any known dopant can be used without any particular limitation. A single dopant may be used, or two or more dopants may be used in combination. Furthermore, a polymer or a monomer may be used. Examples of dopants include inorganic acids such as polyanions, boric acid, nitric acid, and phosphoric acid, and organic acids such as acetic acid, oxalic acid, citric acid, tartaric acid, squaric acid, rhodizonic acid, croconic acid, salicylic acid, p-toluenesulfonic acid, 1,2-dihydroxy-3,5-benzenedisulfonic acid, methanesulfonic acid, trifluoromethanesulfonic acid, borodisalicylic acid, bisoxalateborate acid, sulfonylimide acid, dodecylbenzenesulfonic acid, propylnaphthalenesulfonic acid, and butylnaphthalenesulfonic acid.

[0056] Examples of the polyanion include a substituted or unsubstituted polyalkylene, a substituted or unsubstituted polyalkenylene, a substituted or unsubstituted polyimide, a substituted or unsubstituted polyamide, and a substituted or unsubstituted polyester, and include a polymer consisting only of a structural unit having an anionic group, and a polymer consisting of a structural unit having an anionic group and a structural unit not having an anionic group. Specific examples of the polyanion include polyvinyl sulfonic acid, polystyrene sulfonic acid, polyallyl sulfonic acid, polyacryl sulfonic acid, polymethacrylic acid, poly(2-acrylamido-2-methylpropane sulfonic acid), polyisoprene sulfonic acid, polyacrylic acid, polymethacrylic acid, and polymaleic acid.

[0057] The dispersion medium for the conductive polymer dispersion may be any medium capable of dispersing conductive polymer particles or powder, such as water, an organic solvent, or a mixture thereof. Examples of the organic solvent include polar solvents, alcohols, esters, hydrocarbons, carbonate compounds, ether compounds, chain ethers, heterocyclic compounds, and nitrile compounds.

[0058] Examples of polar solvents include N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide. Examples of alcohols include methanol, ethanol, propanol, and butanol. Examples of esters include ethyl acetate, propyl acetate, and butyl acetate. Examples of hydrocarbons include hexane, heptane, benzene, toluene, and xylene. Examples of carbonate compounds include ethylene carbonate and propylene carbonate. Examples of ether compounds include dioxane and diethyl ether. Examples of linear ethers include ethylene glycol dialkyl ether, propylene glycol dialkyl ether, polyethylene glycol dialkyl ether, and polypropylene glycol dialkyl ether. Examples of heterocyclic compounds include 3-methyl-2-oxazolidinone. Examples of nitrile compounds include acetonitrile, glutarodinitrile, methoxyacetonitrile, propionitrile, and benzonitrile.

[0059] The cathode foil, which serves as the cathode body, is a long foil made of a valve metal, preferably with a purity of 99% or higher. In the cathode foil manufacturing process for wound solid electrolytic capacitors, the valve metal is stretched into a foil, and the cathode foil is then subjected to a surface-expanding process as needed. After the surface-expanding process, a thin oxide layer of approximately 1 to 10 Vfs may be intentionally formed on the cathode foil. The surface-expanding layer may also be naturally formed with an oxide layer, which is formed by the cathode foil reacting with oxygen in the air. Furthermore, the cathode foil may be formed with a layer of metal nitride, metal carbide, or metal carbonitride by vapor deposition, or may have a carbon-containing layer on its surface.

[0060] In a flat-plate solid electrolytic capacitor, a laminate of a metal layer and a carbon layer may be used as the cathode body instead of a cathode foil. In this case, instead of the cathode foil manufacturing process for a wound-type solid electrolytic capacitor, a flat-plate capacitor element is produced as the cathode body, in which a carbon layer and a metal layer are formed on a polymer layer. The carbon layer is formed by coating a slurry or paste on the polymer layer after the polymer layer is formed on the anode foil, and then curing by heating. The metal layer is, for example, a silver layer, and is formed by coating a slurry or paste on the carbon layer and then curing by heating. Alternatively, a laminated type may be used in which multiple flat-plate capacitor elements are stacked.

[0061] In the assembly process, if the solid electrolytic capacitor is a wound type, the anode foil and cathode foil are wound with a separator interposed therebetween to form a cylindrical wound body. The separator is overlapped so that one end protrudes beyond the other ends of the anode foil and cathode foil. The protruding separator is wound first to form a winding core, and the winding is then continued by using the winding core as a winding shaft. Before winding, aluminum lead terminals are connected to the anode foil and cathode foil by, for example, stitching, cold welding, ultrasonic welding, or laser welding. If the solid electrolytic capacitor is a laminated type, the assembly process includes a polymer layer formation process and a cathode body manufacturing process for a cathode body consisting of a carbon layer and a metal layer.

[0062] The separator between the anode and cathode foils prevents shorting between them and maintains the polymer layer between them. If the polymer layer can maintain its shape and separate the anode and cathode foils, the separator can be eliminated from the solid electrolytic capacitor. Examples of separator materials include cellulose paper (e.g., kraft, Manila, esparto, hemp, rayon) and their blends; polyester resins (e.g., polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and their derivatives); polytetrafluoroethylene resins, polyvinylidene fluoride resins, vinylon resins; polyamide resins (e.g., 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 alone or in combination.

[0063] In the final assembly process, the capacitor element is inserted into a cylindrical outer case with a bottom. The outer case is a metal container with one bottom and the other open, and is made of aluminum, an aluminum alloy containing aluminum and manganese, or stainless steel. After the capacitor element is housed in the outer case, the open end of the outer case is sealed with a sealant. The sealant is made of rubber or a laminate of rubber and a hard substrate, and examples of rubber include ethylene propylene rubber and butyl rubber. This sealant is fitted into the end of the outer case, and the open end of the outer case is crimped to seal it. Lead terminals connected to the anode foil and cathode foil are extended from the sealant.

[0064] After the capacitor element is sealed in the outer case, the final manufacturing process for a solid electrolytic capacitor is an aging process. In the aging process, a DC voltage is applied to the solid electrolytic capacitor, and the electrolyte repairs defects in the dielectric film and other areas. In solid electrolytic capacitors that do not contain electrolyte, the DC voltage applied in the aging process insulates the conductive polymer present at or near defects in the dielectric film.

[0065] The capacitor element does not have to be inserted into a cylindrical outer case with a bottom. For example, the capacitor element may be covered with a laminate film. The capacitor element may also be sealed by molding it with a resin such as a heat-resistant resin or an insulating resin, or by forming the resin into a thin film on the capacitor element using a method such as dip coating or printing. In the case of a flat-plate solid electrolytic capacitor, the outer case may be omitted.

[0066] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0067] Example 1 The solid electrolytic capacitor of Example 1 was manufactured as follows. First, aluminum foil was prepared as an anode foil. This aluminum foil was then transferred to a surface-expanding step in the anode foil manufacturing process, where tunnel-shaped etching pits were formed on both sides of the aluminum foil. In the surface-expanding step, a direct current was passed through the aluminum foil in an aqueous solution containing hydrochloric acid to form pits, and then a direct current was passed through the aluminum foil in an aqueous solution containing nitric acid to expand the diameter of the pits.

[0068] The aluminum foil that had undergone the surface enlargement process was transferred to a pre-chemical conversion process. In the pre-chemical conversion process, the aluminum foil was immersed in boiled pure water for 15 minutes to form a pseudo-boehmite film on the surface of the aluminum foil. Next, a chemical conversion process was carried out. In the chemical conversion treatment process, the aluminum foil was immersed in an aqueous ammonium borate solution at 90°C, and a current density of 25 mAcm was applied to the aluminum foil. －２ After the voltage reached 600 V, the voltage was maintained for 20 minutes, thereby transforming the pseudo-boehmite coating into a dielectric coating, leaving only the outermost layer. When confirmed by SEM imaging, the thickness of the dielectric coating was 652 nm.

[0069] One side of the anode foil was masked with imide tape to expose four 1 cm diameter foil surfaces. The polymer layer formation process then began. 63 μL of conductive polymer dispersion was dropped onto the four exposed areas and allowed to dry. Polyethylenedioxythiophene (PEDOT) doped with polystyrene sulfonic acid (PSS) (PEDOT:PSS) was dispersed in the conductive polymer dispersion. The dispersion medium was a mixture of 10 wt% ethylene glycol and 90 wt% water. The amount of PEDOT:PSS added was adjusted to 1 wt% of the total conductive polymer dispersion.

[0070] The conductive polymer dispersion was dried by leaving it in a temperature environment of 60°C for 10 minutes and then leaving it in a temperature environment of 110°C for 30 minutes. The boiling point of ethylene glycol is 197°C, which is higher than the drying temperature of 110°C. In other words, the drying temperature was 110°C, which is lower than the boiling point of ethylene glycol, 197°C.

[0071] After the polymer layer formation process was completed, carbon paste was applied to the four exposed regions and allowed to harden by leaving it in a 110°C environment for 30 minutes. Furthermore, silver paste was applied to the four exposed regions on top of the carbon layer, and copper foil was simultaneously adhered to the carbon layer as a lead terminal. The silver paste was applied to one side of the anode foil, and then allowed to harden by leaving it in a 110°C environment for 30 minutes with copper foil adhered to the unhardened silver paste portion. This connected the silver layer and copper foil. The carbon layer, silver layer, and copper foil layer correspond to the cathode body of the solid electrolytic capacitor.

[0072] Example 2 A solid electrolytic capacitor of Example 2 was manufactured. Example 2 differs from Example 1 in the conductive polymer dispersion liquid used in the polymer layer formation process. The dispersion medium of the conductive polymer dispersion liquid of Example 2 is a mixture of 10 wt % ethylene glycol and 90 wt % water. Sorbitol was added to this dispersion medium as a solid content in addition to the conductive polymer. The amount of PEDOT:PSS added was 1 wt % of the total amount of the conductive polymer dispersion liquid. The amount of sorbitol added was 85 wt % of the weight of the combined solid content of the conductive polymer and sorbitol.

[0073] Comparative Examples Solid electrolytic capacitors of Comparative Examples 1 to 3 were manufactured. In Comparative Examples 1 to 3, the conductive polymer dispersion liquid used in the polymer layer formation step differs from that of Example 1. The conductive polymer dispersion liquids of Comparative Examples 1 to 3 do not contain a high-boiling point solvent, but contain only water as a dispersion medium. Furthermore, the conductive polymer dispersion liquid of Comparative Example 1 contains the same amount of sorbitol as in Example 2. The solid electrolytic capacitors of Comparative Examples 1 and 2 were manufactured using the same manufacturing method and under the same manufacturing conditions, except for the composition of the conductive polymer dispersion liquid.

[0074] For the solid electrolytic capacitor of Comparative Example 3, a high-boiling-point solvent impregnation process was added after the polymer layer formation process. In the high-boiling-point solvent impregnation process, after the polymer layer formation process using a conductive polymer dispersion liquid that did not contain a high-boiling-point solvent, the capacitor element with the polymer layer already formed was immersed in ethylene glycol. After removing the capacitor element from the ethylene glycol, it was dried for 5 minutes in a temperature environment of 150°C. This high-boiling-point solvent impregnation process allowed the capacitor element to be impregnated with ethylene glycol. Thus, in Comparative Example 3, the high-boiling-point solvent was not added to the conductive polymer dispersion liquid, but rather the high-boiling-point solvent was added to the capacitor element after the polymer formation process.

[0075] (Observation of Polymer Layer) The cross sections of the anode foils of Examples 1 and 2 and Comparative Examples 1 and 2 were subjected to energy dispersive X-ray analysis to measure the element ratios. The cross section of the anode foil is a surface cut along the thickness direction of the foil. For the EDX analysis, an energy dispersive X-ray spectrometer (XFlash5060FQ, manufactured by Bruker) was used, with an acceleration voltage of 4 kV, an observation magnification of 80 kx, and a working distance (WD) of 10.5 to 11.0 mm.

[0076] 1 is a cross-sectional schematic diagram of a pit in a surface-expanded layer. A dielectric film 2 is formed closer to the pit center than a non-oxidized valve metal region 1, and a pseudo-boehmite film 3 is formed closer to the pit center than the dielectric film 2. In the mapping analysis, the analysis area is adjusted so that the dielectric film 2 does not enter the analysis area, as shown by the range enclosed by rectangle 4 in FIG. 1, by aligning the edge of the analysis area with the boundary between the dielectric film 2 and the pseudo-boehmite film 3.

[0077] The analysis areas were set at the surface, middle, and bottom of the surface-expanding layer. Mapping analysis was then performed for each depth zone. The mapping analysis measured the elemental ratio of sulfur to the total of carbon, aluminum, and sulfur. Sulfur is contained in polyethylenedioxythiophene doped with polystyrene sulfonic acid. Therefore, the atomic ratio of sulfur at each depth indicates the relative content of the conductive polymer at each depth.

[0078] 2 is a graph showing the results of EDX analysis and illustrating the elemental ratio of sulfur in Examples 1 and 2 and Comparative Examples 1 and 2. In the graph of FIG. 2, the horizontal axis represents the top, middle, and bottom of the surface-expanding layer, and the vertical axis represents the elemental ratio of sulfur. In the graph, the circle plots are Example 1, the diamond plots are Example 2, the triangle plots are Comparative Example 1, and the x plots are Comparative Example 2.

[0079] FIG. 3 shows the results of the EDX analysis and is a graph showing the sulfur content for each depth zone of the surface-expanding layer in Examples 1 and 2 and Comparative Examples 1 and 2. In the graph of FIG. 3, the horizontal axis represents the surface (top), middle, and bottom (bottom) of the surface-expanding layer, and the vertical axis represents the sulfur content. The sulfur content indicates the rate of decrease in the sulfur content in the middle and bottom based on the sulfur content in the surface layer of the surface-expanding layer. In the graph, the circle plots are Example 1, the diamond plots are Example 2, the triangle plots are Comparative Example 1, and the x plots are Comparative Example 2.

[0080] As shown in Figures 2 and 3, in Comparative Example 1, the amount of sulfur contained in the bottom portion of the surface-expanding layer is reduced by approximately 65% ​​based on the amount of sulfur contained in the surface layer of the surface-expanding layer. In other words, the conductive polymer contained in the bottom portion of Comparative Example 1 is approximately 35% of the surface layer. In Comparative Example 2, the amount of sulfur contained in the bottom portion of the surface-expanding layer is reduced by approximately 85% based on the amount of sulfur contained in the surface layer of the surface-expanding layer. In other words, the conductive polymer contained in the bottom portion of Comparative Example 2 is approximately 15% of the surface layer.

[0081] In contrast, as shown in Figures 2 and 3, in Example 1, the amount of sulfur contained in the bottom portion of the surface-expanding layer is reduced by approximately 30% based on the amount of sulfur contained in the surface layer of the surface-expanding layer. In other words, the conductive polymer contained in the bottom portion of Example 1 accounts for approximately 70% of the surface layer. In Example 2, the amount of sulfur contained in the bottom portion of the surface-expanding layer is reduced by approximately 45% based on the amount of sulfur contained in the surface layer of the surface-expanding layer. In other words, the conductive polymer contained in the bottom portion of Example 2 accounts for approximately 55% of the surface layer.

[0082] Examples 1 and 2 differ from Comparative Examples 1 and 2 in that a conductive polymer dispersion containing a high-boiling point solvent was used in the polymer layer formation step. It was confirmed that, by undergoing the polymer layer formation step using a conductive polymer dispersion containing a high-boiling point solvent, the anode foil having a surface-expanding layer, a dielectric coating, and a pseudo-boehmite coating had a conductive polymer content of 50% or more in the bottom of the surface-expanding layer, based on the content in the surface layer of the surface-expanding layer.

[0083] Next, SEM images of the cross sections of the surface and bottom of the surface-expanding layer were taken using a scanning cross-section microscope for the anode foils of Example 2 and Comparative Example 1. The acceleration voltage for taking the SEM images was 1.0 kV, the working distance (WD) was 2.1 to 2.3 mm, and the observation magnification was 80 kx. The results of the SEM images are shown in Figures 4 and 5. Figure 4 is an SEM image of Example 2, where (a) is the surface of the surface-expanding layer and (b) is the bottom of the surface-expanding layer. Figure 5 is an SEM image of Comparative Example 1, where (a) is the surface of the surface-expanding layer.

[0084] As shown in Figure 4, in Example 2, a polymer layer containing sorbitol was formed on the surface and bottom of the surface-expanding layer. The sorbitol was transported to the bottom of the surface-expanding layer, and a thick polymer layer due to sorbitol was also formed at the bottom of the surface-expanding layer. On the other hand, as shown in Figure 5, in Comparative Example 1, a large amount of sorbitol, which is a solid component, was deposited on the surface of the surface-expanding layer. Therefore, only a small amount of sorbitol reached the bottom of the surface-expanding layer in Comparative Example 1, and the polymer layer was thinner and more uneven than in Example 2.

[0085] Thus, by undergoing the polymer layer formation process using a conductive polymer dispersion containing a high-boiling point solvent and sorbitol, it was confirmed that in an anode foil having a surface-expanding layer, a dielectric film, and a pseudo-boehmite film, sorbitol was transported to the bottom of the surface-expanding layer in large amounts, and a thick polymer layer was formed at the bottom of the surface-expanding layer. Therefore, the solid electrolytic capacitor of Example 2 is less susceptible to dielectric breakdown even at the bottom of the surface-expanding layer, improving its voltage resistance. Furthermore, the solid electrolytic capacitor of Example 2 has high conductivity at the bottom of the surface-expanding layer, and an increased capacitance extraction rate, which can be expected to result in high capacitance and low ESR.

[0086] (Frequency Characteristic Measurement Test) The capacitance and ESR were measured for each frequency of the solid electrolytic capacitors of Examples 1 and 2 and Comparative Examples 1 and 2. The capacitance and ESR were measured using an LCR meter (ZM2376, manufactured by NF Corporation). The measurements were performed at a temperature of 20°C, and the measurement frequencies were in the range of 1 Hz to 1000 kHz.

[0087] The measurement results of the capacitance are shown in the graph of Fig. 6, and the measurement results of the ESR are shown in the graph of Fig. 7. In each graph, the horizontal axis represents frequency, and the vertical axis represents capacitance or ESR. In the graph, the circle plots represent Example 1, the diamond plots represent Example 2, the triangle plots represent Comparative Example 1, and the x plots represent Comparative Example 2. Table 1 below lists the capacitance (Cap) at 120 Hz, the capacitance at 10 kHz, the ESR at 10 kHz, and the ESR at 100 kHz.

[0088] (Table 1)

[0089] 6 and Table 1, the solid electrolytic capacitors of Examples 1 and 2 have higher capacitances than Comparative Examples 1 and 2. In particular, when used in the high frequency range of 10 kHz, the solid electrolytic capacitors of Examples 1 and 2 have capacitances at least 1.5 times higher than Comparative Examples 1 and 2.

[0090] 7 and Table 1, the solid electrolytic capacitors of Examples 1 and 2 have lower ESR than those of Comparative Examples 1 and 2. When used in the high frequency range of 10 kHz and 100 kHz, the solid electrolytic capacitors of Examples 1 and 2 have ESRs that are one-fourth or less of those of Comparative Examples 1 and 2.

[0091] In this way, by undergoing the polymer layer formation process using a conductive polymer dispersion liquid containing a high-boiling point solvent, an anode foil having a surface-expanding layer, a dielectric film, and a pseudo-boehmite film has a conductive polymer content of 50% or more in the bottom portion of the surface-expanding layer, based on the content in the surface layer of the surface-expanding layer.It has been confirmed that when an anode foil having a surface-expanding layer, a dielectric film, and a pseudo-boehmite film has a conductive polymer content of 50% or more in the bottom portion of the surface-expanding layer, based on the content in the surface layer of the surface-expanding layer, the capacitance of the solid electrolytic capacitor can be well extracted and the ESR can be reduced, even when used in high-frequency bands.

[0092] Furthermore, the capacitance and ESR of the solid electrolytic capacitor of Comparative Example 3 were measured. The measurement methods and conditions for capacitance and ESR were the same as those of Example 1 and Comparative Example 2. The measurement results for capacitance are shown in the graph of FIG. 8 together with those of Example 1 and Comparative Example 2, and the measurement results for ESR are shown in the graph of FIG. 9 together with those of Example 1 and Comparative Example 2. In each graph, the horizontal axis represents frequency, and the vertical axis represents capacitance or ESR. In the graph, the plots marked with circles represent Example 1, the plots marked with xes represent Comparative Example 2, and the plots marked with diamonds represent Comparative Example 3. Table 2 below lists the capacitance at 120 Hz (Cap), the capacitance at 10 kHz, the ESR at 10 kHz, and the ESR at 100 kHz.

[0093] (Table 2)

[0094] In Comparative Example 3, a high-boiling point solvent was not added to the conductive polymer dispersion, and the capacitor element was impregnated with a high-boiling point solvent after the polymer layer was formed. As shown in Figure 8 and Table 2, the solid electrolytic capacitor of Comparative Example 3 had a higher capacitance and a lower ESR than Comparative Example 2. However, the solid electrolytic capacitor of Comparative Example 3 was unable to achieve the high capacitance in the high frequency range as in Example 1, nor was it able to achieve the low ESR in the high frequency range as in Example 1.

[0095] This confirmed that by undergoing the polymer layer formation process using a conductive polymer dispersion liquid containing a high-boiling point solvent, the solid electrolytic capacitor exhibits high capacitance in the high frequency band and low ESR. It is presumed that the reason why Comparative Example 3 has better characteristics than Comparative Example 2 is because ethylene glycol causes a change in the higher-order structure of the conductive polymer.

[0096] (Examples 3 to 6 and Comparative Example 4) Solid electrolytic capacitors of Examples 3 to 6 and Comparative Example 4 were manufactured. Compared to Example 1, Examples 3 to 6 differ in the type of high-boiling-point solvent added to the conductive polymer dispersion. Other configurations of Examples 3 to 6 were the same as Example 1, including the anode foil having a surface-expanding layer, a dielectric coating, and a pseudo-boehmite coating. The solid electrolytic capacitors of Examples 3 to 6 were manufactured using the same manufacturing method and conditions, except for the type of high-boiling-point solvent. However, the drying conditions for the conductive polymer dispersion in the polymer layer formation step were adjusted according to the boiling points of the various high-boiling-point solvents in order to achieve uniform solid content after drying. The drying temperature was adjusted to be below the boiling points of the various high-boiling-point solvents.

[0097] In Example 3, diethylene glycol was added as a high-boiling point solvent to the conductive polymer dispersion liquid instead of ethylene glycol in Example 1. The drying conditions were the same as those in Example 1. In Example 4, glycerin was added as a high-boiling point solvent to the conductive polymer dispersion liquid instead of ethylene glycol in Example 1. The drying conditions were as follows: leaving the conductive polymer dispersion liquid for 10 minutes in a 60°C temperature environment, then leaving it for 30 minutes in a 110°C temperature environment, and then leaving it for 90 minutes in a 150°C temperature environment. In Example 5, 1,3-propanediol was added as a high-boiling point solvent to the conductive polymer dispersion liquid instead of ethylene glycol in Example 1. The drying conditions were the same as those in Example 1. In Example 6, polyethylene glycol was added as a high-boiling point solvent to the conductive polymer dispersion liquid instead of ethylene glycol in Example 1. In Comparative Example 4, tetrahydrofuran was added as a low-boiling point solvent to the conductive polymer dispersion liquid instead of ethylene glycol in Example 1. The drying conditions were as follows: leaving the substrate in a 60° C. environment for 10 minutes, then leaving it in a 110° C. environment for 30 minutes, and then leaving it in a 150° C. environment for 150 minutes.

[0098] The boiling point of diethylene glycol is 245°C, which is higher than the drying temperature of 110°C in the polymer layer forming step. The boiling point of glycerin is 290°C, which is higher than the drying temperature of 110°C in the polymer layer forming step. The boiling point of 1,3-propanediol is 213°C, which is higher than the drying temperature of 110°C in the polymer layer forming step. The boiling point of polyethylene glycol is higher than the drying temperature of 110°C in the polymer layer forming step, and although it depends on the molecular weight, it is 200°C or higher for molecular weights of 300 or more. The boiling point of tetrahydrofuran is 66°C, which is lower than the drying temperature of 110°C in the polymer layer forming step.

[0099] (Frequency Characteristic Measurement Test) The capacitance and ESR of the solid electrolytic capacitors of Examples 3 to 6 and Comparative Example 4 were measured at each frequency. The capacitance and ESR were measured using the same method and under the same conditions as in Example 1. The capacitance measurement results are shown in the graph of FIG. 10, and the ESR measurement results are shown in the graph of FIG. 11. The horizontal axis of each graph represents frequency, and the vertical axis represents capacitance or ESR. In the graphs, the plots with solid circles represent Example 1, the plots with diamonds represent Example 3, the plots with squares represent Example 4, the plots with solid triangles represent Example 5, the plots with open circles represent Example 6, the plots with x's represent Comparative Example 2, and the plots with open triangles represent Comparative Example 4. Table 3 below lists the capacitance (Cap) at 120 Hz, the capacitance at 10 kHz, the ESR at 10 kHz, and the ESR at 100 kHz.

[0100] (Table 3)

[0101] As shown in Fig. 10 and Table 3, the solid electrolytic capacitors of Examples 3 to 6 exhibit high capacitance in the high frequency range, similar to Example 1. Furthermore, as shown in Fig. 11 and Table 3, the solid electrolytic capacitors of Examples 3 to 6 maintain low ESR even in the high frequency range, similar to Example 1. Thus, it was confirmed that by adding a high-boiling point solvent other than ethylene glycol to the conductive polymer dispersion, the solid electrolytic capacitor exhibits high capacitance in the high frequency range and maintains low ESR even in the high frequency range.

[0102] (Reference Examples 1 to 3) Solid electrolytic capacitors of Reference Examples 1 to 3 were manufactured. In Reference Examples 1 to 3, the pseudo-boehmite film was removed. To remove the pseudo-boehmite film, a removal process and a repair chemical conversion process were repeated multiple times. In the removal process, after the pre-chemical conversion process and the chemical conversion process were completed, the anode foil was immersed in a phosphoric acid aqueous solution to dissolve the pseudo-boehmite film. In the repair chemical conversion process, a constant current was passed through the anode foil while immersing it in a boric acid aqueous solution to repair defects in the dielectric film caused by the removal process.

[0103] The conductive polymer dispersion of Reference Example 1 contains ethylene glycol, making it the same composition as Example 1. The conductive polymer dispersion of Reference Example 2 does not contain a high-boiling-point solvent and contains only water as a dispersion medium, making it the same composition as Comparative Example 1. The conductive polymer dispersion of Reference Example 3 is the same as Reference Example 2, except that after a polymer layer formation process using a conductive polymer dispersion that does not contain a high-boiling-point solvent, the capacitor element on which the polymer layer had already been formed was immersed in ethylene glycol. In Reference Example 3, as in Comparative Example 3, the ethylene capacitor element was removed from the ethylene glycol and then dried for 5 minutes in a temperature environment of 150°C. In Reference Example 3, the capacitor element was impregnated with ethylene glycol through this high-boiling-point solvent impregnation process. Thus, in Reference Example 3, the high-boiling-point solvent was not added to the conductive polymer dispersion, but rather was added to the capacitor element after the polymer formation process.

[0104] (Frequency Characteristic Measurement Test) The capacitance and ESR of the solid electrolytic capacitors of Reference Examples 1 to 3 were measured for each frequency. The capacitance and ESR were measured using the same method and under the same conditions as in Example 1. The capacitance measurement results are shown in the graph of FIG. 12, and the ESR measurement results are shown in the graph of FIG. 13. In each graph, the horizontal axis represents frequency, and the vertical axis represents capacitance or ESR. In the graph, the circle plot represents Reference Example 1, the x plot represents Reference Example 2, and the triangle plot represents Reference Example 3. Table 4 below lists the capacitance (Cap) at 120 Hz, the capacitance at 10 kHz, the ESR at 10 kHz, and the ESR at 100 kHz.

[0105] (Table 4)

[0106] 12 and Table 4, the solid electrolytic capacitors of Reference Examples 1 to 3 exhibit high capacitance in the high frequency range, and as shown in FIG. 13 and Table 4, the solid electrolytic capacitors of Reference Examples 1 to 3 maintain low ESR even in the high frequency range.

[0107] Thus, when the pseudo-boehmite coating is removed, the solid electrolytic capacitor has good capacitance and ESR in the high frequency range, regardless of whether a high-boiling point solvent is added to the conductive polymer dispersion or whether the capacitor element is immersed in a high-boiling point solvent after the polymer layer is formed. In other words, when the pseudo-boehmite coating is not present, there is no problem of uneven distribution of the conductive polymer in the surface-expanding layer.

[0108] In other words, it was confirmed that the problem of uneven distribution of the conductive polymer within the surface-expanding layer is a problem unique to anode foils on which a pseudo-boehmite film is formed, and the effect of adding a high-boiling point solvent to the conductive polymer dispersion is an effect unique to anode foils on which a pseudo-boehmite film is formed.

[0109] (Voltage Withstanding Test) Next, the voltage withstanding tests were performed on Examples 1 to 6, Comparative Examples 1 to 3, and Reference Examples 1 to 3. The voltage withstanding tests were performed at 25°C by increasing the voltage from 0 V at intervals of 1 V per second and examining the current-time rising curve. The voltage (V) at which a current of 20 mA was observed due to a spike or scintillation in the current rising curve was defined as the voltage withstanding test. The results are shown in Table 5 below.

[0110] (Table 5)

[0111] As shown in Table 5, the solid electrolytic capacitors of Reference Examples 1 to 3 had good capacitance and ESR in the high frequency range, but the withstand voltage did not meet the 250 V required in the power electronics field. On the other hand, the solid electrolytic capacitors of Examples 1 to 6 had a withstand voltage that fully met the 250 V required in the power electronics field, and had good capacitance and ESR in the high frequency range.

[0112] Specifically, by undergoing a polymer layer formation process using a conductive polymer dispersion containing a high-boiling point solvent and sorbitol, an anode foil having a surface-expanding layer, a dielectric film, and a pseudo-boehmite film is obtained in which the conductive polymer content in the bottom portion of the surface-expanding layer is 50% or more, based on the content in the surface layer of the surface-expanding layer. This results in a withstand voltage of 250 V or more, good capacitance and ESR in the high-frequency range, and is particularly useful in the field of power electronics when incorporated into devices using wide-bandgap semiconductors such as SiC and GaN.

[0113] (Example 7 and Comparative Example 5) Solid electrolytic capacitors of Example 7 and Comparative Example 5 were manufactured. The anode foils of Example 7 and Comparative Example 5 differ from those of Example 1 and Comparative Example 2 in that they are powder laminate foils. The solid electrolytic capacitor of Example 7 is the same as Example 1 except that the anode foil is a powder laminate foil, and was manufactured using the same manufacturing method and the same manufacturing conditions. Furthermore, the solid electrolytic capacitor of Comparative Example 5 is the same as Comparative Example 2 except that the anode foil is a powder laminate foil, and was manufactured using the same manufacturing method and the same manufacturing conditions.

[0114] The powder laminated foil for the anode was fabricated as follows: In the surface-expanding step, aluminum powder was laminated on an aluminum substrate and sintered. Next, in the pre-chemical conversion step, a pseudo-boehmite coating was formed, and in the chemical conversion treatment step, the pseudo-boehmite coating was transformed into a dielectric coating, leaving only the outermost layer.

[0115] The conductive polymer dispersion used in Example 1 was composed of PEDOT:PSS, ethylene glycol, and water. 10 wt % of the dispersion medium was ethylene glycol, and the remaining 90 wt % was water. PEDOT:PSS was added so that it accounted for 1 wt % of the weight of the conductive polymer dispersion. The drying temperature in the polymer layer formation step was 110°C, which is lower than the boiling point of ethylene glycol, 197°C. The conductive polymer dispersion used in Comparative Example 5 was composed of PEDOT:PSS and water. The dispersion medium of the conductive polymer dispersion used in Comparative Example 5 was entirely water, and ethylene glycol was not added to the conductive polymer dispersion in Comparative Example 5.

[0116] (Observation of Polymer Layer) The element ratios were measured by energy dispersive X-ray analysis of the cross sections of the anode foils of Example 7 and Comparative Example 5. The surface of the surface-expanding layer was divided into three equal sections from the surface of the core to the surface of the surface-expanding layer, and the analysis areas were set at the surface, middle, and bottom of the surface-expanding layer.

[0117] Fig. 14 shows the results of EDX analysis and is a graph showing the elemental ratio of sulfur in Example 7 and Comparative Example 5. Fig. 15 shows the results of EDX analysis and is a graph showing the sulfur content of each depth zone of the surface-expanding layer in Example 7 and Comparative Example 5, based on the sulfur content of the surface layer of the surface-expanding layer. In both graphs, the plots marked with circles correspond to Example 7, and the plots marked with crosses correspond to Comparative Example 5.

[0118] As shown in Figures 14 and 15, in Comparative Example 5, in which powder laminated foil was used as the anode, the amount of sulfur contained in the bottom portion of the surface-expanding layer was reduced by more than 95% based on the amount of sulfur contained in the surface layer of the surface-expanding layer. In other words, the conductive polymer contained in the bottom portion of Comparative Example 5 was less than 5% of the surface layer. In Comparative Example 2, in which etched foil was used as the anode, the reduction was approximately 85%, or approximately 15% of the surface layer. In other words, when powder laminated foil was used as the anode, it was more difficult to form a polymer layer at the bottom portion of the surface-expanding layer.

[0119] In contrast, as shown in Figures 14 and 15, in Example 7, in which powder laminated foil was used as the anode, the amount of sulfur contained in the bottom portion of the surface-expanding layer was reduced by approximately 20% based on the amount of sulfur contained in the surface layer of the surface-expanding layer. In other words, the conductive polymer contained in the bottom portion of Example 7 was approximately 80% of the surface layer. In Example 1, in which etched foil was used as the anode, the amount of sulfur was reduced by approximately 45%, or approximately 55% of the surface layer. In other words, even when powder laminated foil was used as the anode, when ethylene glycol was added to the dispersion, a polymer layer was actually more likely to form in the bottom portion of the surface-expanding layer.

[0120] Thus, when a powder laminated foil is used as the anode, the polymer layer formed at the bottom of the surface-expanding layer is more difficult to form than when an etched foil is used as the anode. However, when a powder laminated foil is used as the anode, a high-boiling point solvent is contained in the dispersion medium of the conductive polymer dispersion, and the anode is dried at a temperature lower than that of the high-boiling point solvent, it has been confirmed that the polymer layer is more easily formed than when an etched foil is used as the anode.

[0121] (Frequency Characteristic Measurement Test) The capacitance and ESR of the solid electrolytic capacitors of Example 7 and Comparative Example 5 were measured for each frequency. The measurement methods and conditions for capacitance and ESR were the same as those of Example 1 and Comparative Example 2. The capacitance measurement results are shown in the graph of FIG. 16, and the ESR measurement results are shown in the graph of FIG. 17. The horizontal axis of each graph represents frequency, and the vertical axis represents capacitance or ESR. In the graphs, the plots marked with circles represent Example 7, the plots marked with black circles represent Example 7, and the plots marked with xes represent Comparative Example 5. Table 6 below lists the capacitance at 120 Hz (Cap), the capacitance at 10 kHz, the ESR at 10 kHz, and the ESR at 100 kHz, along with those of Example 1 and Comparative Example 2.

[0122] (Table 6)

[0123] 16 and Table 6, the solid electrolytic capacitor of Example 7 has a higher capacitance than the same powder laminated foil of Comparative Example 5. In particular, when used in the high frequency range of 10 kHz, the capacitance of the solid electrolytic capacitor of Comparative Example 5 drops significantly, whereas the capacitance of the solid electrolytic capacitor of Example 7 is nearly twice that of Comparative Example 5.

[0124] 17 and Table 6, the solid electrolytic capacitor of Example 7 has a lower ESR than that of Comparative Example 5. In the high frequency range of 10 kHz and 100 kHz, the solid electrolytic capacitor of Example 7 has an ESR that is about 1 / 20 of that of Comparative Example 5.

[0125] Thus, even if the anode foil is a powder laminate foil, by undergoing a polymer layer formation process using a conductive polymer dispersion containing a high-boiling point solvent, the anode foil having a surface-expanding layer, a dielectric film, and a pseudo-boehmite film has a conductive polymer content of 50% or more in the bottom of the surface-expanding layer, based on the content in the surface layer of the surface-expanding layer. It has been confirmed that, in an anode foil having a surface-expanding layer, a dielectric film, and a pseudo-boehmite film, when the bottom of the surface-expanding layer contains 50% or more of the conductive polymer, based on the content in the surface layer of the surface-expanding layer, the capacitance of the solid electrolytic capacitor can be well extracted and the ESR can be reduced, even when used in high-frequency bands.

[0126] Furthermore, when the anode foil is a powder laminate foil, if a conductive polymer dispersion without a high-boiling point solvent is used and the conductive polymer content at the bottom of the surface-expanding layer is less than 50% of the content in the surface-expanding layer, the ESR is significantly worse than in the case of an etched foil. However, as can be seen from a comparison between Examples 1 and 7, when the anode foil is a powder laminate foil, a conductive polymer dispersion without a high-boiling point solvent is used and the conductive polymer content at the bottom of the surface-expanding layer is 50% or more of the content in the surface-expanding layer, the ESR is actually significantly lower than in the case of an etched foil.

Claims

1. A solid electrolytic capacitor comprising an anode foil and a layer of a conductive polymer, the anode foil having a surface expansion layer formed on the foil surface, a dielectric film formed on the surface expansion layer, and a pseudo-boehmite film formed on the dielectric film, the conductive polymer being contained in a bottom portion of the surface expansion layer at 50% or more based on the content in a surface layer of the surface expansion layer.

2. The solid electrolytic capacitor according to claim 1, wherein the conductive polymer is poly(3,4-ethylenedioxythiophene) doped with polystyrenesulfonic acid.

3. The solid electrolytic capacitor according to claim 1 or 2, wherein the surface-expanding layer has a plurality of tunnel-shaped pits.

4. The solid electrolytic capacitor according to claim 1 or 2, wherein the surface-expanding layer is formed by laminating powder of a valve metal.

5. The solid electrolytic capacitor according to claim 1 or 2, characterized in that it has a withstand voltage of 250 V or more.

6. The solid electrolytic capacitor according to claim 1 or 2, wherein the dielectric film has a thickness corresponding to a formation voltage of 250 V or more.

7. A method for manufacturing a solid electrolytic capacitor comprising an anode foil and a layer of a conductive polymer, comprising: a surface-expanding step of forming a surface-expanding layer on the anode foil; a pre-chemical step of forming a pseudo-boehmite film on the anode foil after the surface-expanding step; a chemical step of forming a dielectric film on the anode foil such that the pseudo-boehmite film remains on the dielectric film; and a polymer layer forming step of forming a layer of the conductive polymer by applying a conductive polymer dispersion liquid containing the conductive polymer and a high-boiling point solvent to at least the anode foil and drying it at a temperature lower than the boiling point of the high-boiling point solvent, so that the conductive polymer is contained in the bottom part of the surface-expanding layer at 50% or more based on the content in the surface layer of the surface-expanding layer.

8. The method for producing a solid electrolytic capacitor according to claim 7, wherein the surface-expanding step forms the surface-expanding layer by forming a plurality of tunnel-shaped pits.

9. The method for producing a solid electrolytic capacitor according to claim 7, wherein the surface enlarging step comprises laminating powder of a valve metal.

10. The method for producing a solid electrolytic capacitor according to claim 7, wherein the chemical formation step applies a chemical formation voltage of 250 V or more to the anode foil.

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