Method for manufacturing a solid electrolytic capacitor

Abstract

To provide a manufacturing method of a solid electrolytic capacitor, capable of achieving both of a withstand voltage characteristic.SOLUTION: A manufacturing method of a solid electrolytic capacitor, contains: a coating layer formation step; a voltage application step; and a conductive polymer formation step. In the coating layer formation step, an anode foil is coated with a conductive polymer. The voltage application step is the later step from the coating layer formation step, and is for applying a voltage to the anode foil to which a conductive polymer is coated. The conductive polymer formation step is the later step from the voltage application step, and is for forming a conductive polymer layer.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] An electrolytic capacitor using a valve metal such as aluminum can obtain a small size and a large capacitance by etching the valve metal as an anode electrode to expand the surface area of the anode electrode. In particular, a solid electrolytic capacitor in which a dielectric oxide film is covered with a solid electrolyte is small in size, large in capacitance, and low in equivalent series resistance, and is essential for miniaturization, high functionality, and cost reduction of electronic devices.

[0003] As the solid electrolyte, manganese dioxide and 7,7,8,8 - tetracyanoquinodimethane (TCNQ) complexes are known. In recent years, conductive polymers derived from monomers having a π - conjugated double bond, such as poly(3,4 - ethylenedioxythiophene) (PEDOT), which have a slow reaction rate and excellent adhesion to the dielectric oxide film, have rapidly spread as solid electrolytes. Conductive polymers include conductive polymers using an external dopant that exhibits conductivity by using a low - molecular - weight anion or polyanion as a dopant during chemical oxidative polymerization or electrolytic oxidative polymerization, or self - doped conductive polymers having a partial structure acting as a dopant within the monomer molecule and having conductivity and solubility in a solvent.

[0004] However, a solid electrolytic capacitor has a poor defect - repairing effect on the dielectric oxide film compared to a liquid - type electrolytic capacitor in which the capacitor element is impregnated with an electrolytic solution and does not have a conductive polymer layer, and there is a risk of an increase in leakage current. Therefore, a so - called hybrid - type solid electrolytic capacitor in which a conductive polymer layer is formed on a capacitor element with a pair of electrode foils facing each other and the voids of the capacitor element are impregnated with an electrolytic solution has also attracted attention.

[0005] Here, when forming a dielectric oxide film on an anode foil for medium and high voltage applications, a hydration treatment may be performed by immersing the aluminum foil in boiling pure water before the formation treatment to form a pseudo-boehmite film layer. When the formation treatment is performed after forming the pseudo-boehmite film layer, the pseudo-boehmite film layer is transformed into a dielectric oxide film layer composed of highly crystalline γ-alumina. The highly crystalline dielectric oxide film has a high breakdown voltage compared to its thickness and can form a solid electrolytic capacitor with a high breakdown voltage and a high capacitance.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] The dielectric oxide film layer grows from the boundary between the pseudo-boehmite film layer and the valve action metal toward both sides of the pseudo-boehmite film layer and the valve action metal. As this growth progresses, the pseudo-boehmite film layer becomes thinner. However, it is rare for all of the pseudo-boehmite film layer to be replaced by the dielectric oxide film layer. Therefore, the structure of the anode foil is such that a pseudo-boehmite film layer remains on the dielectric oxide film layer.

[0008] The inside of the pseudo-boehmite film layer is dense. Therefore, it is difficult for the conductive polymer to penetrate due to the obstruction of the pseudo-boehmite film layer, and the adhesion of the conductive polymer to the dielectric oxide film layer decreases. The decrease in the adhesion of the conductive polymer to the dielectric oxide film layer reduces the conductive path between the conductive polymer layer, which serves as the true cathode, and the dielectric oxide film layer, deteriorating the frequency characteristics. Here, the frequency characteristics refer to the capacitance and equivalent series resistance (ESR) in each frequency region. A general-purpose solid electrolytic capacitor is required to have good capacitance and equivalent series resistance from the low-frequency region to the medium and high-frequency regions.

[0009] It is also conceivable to insert a step of removing the pseudo-boehmite film layer. If the pseudo-boehmite film layer is removed, it becomes easier for the conductive polymer to attach to the dielectric oxide film layer, and the adhesion between the dielectric oxide film and the conductive polymer is improved. However, the presence of the pseudo-boehmite film layer also has the merit of improving the withstand voltage of the solid electrolytic capacitor. If the pseudo-boehmite film layer is removed, the withstand voltage characteristics capable of withstanding medium and high voltage applications will be lost.

[0010] There may be a case where the dielectric oxide film layer is made of amorphous alumina. In this case, after forming a porous film layer having pores along the thickness direction of the anode foil, it is transformed into amorphous alumina so as to fill these pores. Also, for the anode foil in which the inside of this porous film layer is dense and the porous film layer remains on the surface layer side of the dielectric oxide film layer, similar to the pseudo-boehmite film layer, it brings good withstand voltage characteristics to the solid electrolytic capacitor, but deteriorates the frequency characteristics.

[0011] The present invention has been proposed to solve the above problems, and its object is to provide a method for manufacturing a solid electrolytic capacitor that achieves both high withstand voltage characteristics and good frequency characteristics.

Means for Solving the Problems

[0012] In order to solve the above problems, a method for manufacturing a solid electrolytic capacitor according to the present invention is a method for manufacturing an electrolytic capacitor in which a conductive polymer layer is interposed between an anode foil and a cathode foil, including a coating layer forming step of coating the anode foil with a conductive polymer, a voltage application step that is a subsequent step to the coating layer forming step and applies a voltage to the anode foil coated with the conductive polymer, and a conductive polymer layer forming step that is a subsequent step to the voltage application step and forms the conductive polymer layer.

[0013] Further include an anode foil manufacturing process for forming the anode foil. In the anode foil manufacturing process, a dielectric oxide film layer may be located on the valve action metal foil, and an anode foil may be formed such that a pseudo-boehmite film layer or a porous film layer is located on the dielectric oxide film layer.

[0014] Include an element winding process of winding the anode foil and the cathode foil. The coating layer forming process may be performed after the element winding process, and the coating layer forming process and the voltage application process may be continuous.

[0015] Include an element winding process of winding the anode foil and the cathode foil. The coating layer forming process may be interposed after the anode foil manufacturing process and before the element winding process, and the voltage application process and the conductive polymer layer forming process may be performed after the element winding process.

[0016] Include an element winding process of winding the anode foil and the cathode foil. The coating process and the voltage application process may be interposed after the anode foil manufacturing process and before the element winding process, and the conductive polymer layer forming process may be performed after the element winding process.

[0017] Include a surface expansion process of expanding the anode foil. In the surface expansion process, tunnel-shaped etching pits extending in the foil thickness direction may be formed on the anode foil.

[0018] After interposing the conductive polymer layer between the anode foil and the cathode foil, an electrolytic solution impregnation process of impregnating with an electrolytic solution may be further included.

[0019] Without impregnating with an electrolytic solution, only the conductive polymer layer may be interposed as an electrolyte between the anode foil and the cathode foil.

Advantages of the Invention

[0020] According to the present invention, high withstand voltage and good frequency characteristics of the solid electrolytic capacitor are compatible.

Brief Description of the Drawings

[0021]

Figure 1

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Embodiments for Carrying Out the Invention

[0022] (Solid electrolytic capacitor) A solid electrolytic capacitor is a passive element that stores and discharges electric charges according to its capacitance. This solid electrolytic capacitor has, for example, a wound or laminated shape. In the present embodiment, the wound type will be exemplified and described, but the solid electrolytic capacitor of the present invention is not limited thereto, nor is it limited to the other embodiments to be described.

[0023] The wound-type solid electrolytic capacitor has a cylindrical capacitor element in which a pair of electrode foils are wound. The capacitor element includes an anode foil and a cathode foil as a pair of electrode foils, a separator, and a conductive polymer layer. A dielectric oxide film is formed on the surface of the anode foil. The anode foil and the cathode foil are wound with the separator in between. The conductive polymer layer contains a conductive polymer and is formed between the anode foil and the cathode foil so as to cover at least a part of the dielectric oxide film layer on the surface of the anode foil. This conductive polymer layer serves as the true cathode of the solid electrolytic capacitor. In addition to the conductive polymer layer using a conductive polymer as an electrolyte, an electrolytic solution may be used in combination, and the electrolytic solution is filled in the voids of the capacitor element on which the conductive polymer layer is formed.

[0024] (Manufacturing Method) This solid electrolytic capacitor is manufactured through an anode foil manufacturing process for manufacturing the anode foil, a cathode foil manufacturing process for manufacturing the cathode foil, an element winding process for winding the anode foil, the cathode foil, and the separator, a coating layer forming process for forming a coating layer, a voltage application process for applying a voltage to the anode foil coated with the conductive polymer, a conductive polymer layer forming process for forming the conductive polymer layer, an electrolytic solution impregnation process for impregnating with the electrolytic solution, and a final manufacturing process for sealing 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. Also, one process includes a plurality of sub-processes, and some of these sub-processes may be performed during other processes, or incorporated between one sub-process and the next sub-process of other processes.

[0025] (Anode Foil Manufacturing Process) The anode foil is a long foil body made of a valve action metal. The valve action metal is aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, antimony, etc. The purity is desirably 99.9% or more for the anode foil, but impurities such as silicon, iron, copper, magnesium, and zinc may be included.

[0026] This anode foil undergoes a surface area expansion process for the foil obtained by stretching a valve metal. In the surface area expansion process, the surface of the foil is made porous to increase the specific surface area. In this surface area expansion process, the stretched foil is used as a core, and a powder of the valve metal is vapor-deposited or sintered on the surface, or an etching treatment is performed. In the etching treatment, a large number of tunnel-shaped pits are formed by digging from the foil surface in the thickness direction for medium and high voltage applications. In this etching treatment, typically, a direct current is passed through an acidic aqueous solution containing halogen ions such as hydrochloric acid to form pits, and a direct current is passed through an acidic aqueous solution such as nitric acid to expand the diameter of the pits.

[0027] After the anode foil undergoes the surface area expansion process, it undergoes a pre-forming treatment process for forming a pseudo-boehmite film layer, a porous film layer, etc., and a forming treatment process for forming a dielectric oxide film layer. The dielectric oxide film layer is a film formed on the surface layer of the expanded anode foil. If the anode foil is made of aluminum, it is a layer of aluminum oxide obtained by oxidizing the porous region after the surface area expansion process. A pseudo-boehmite film layer or a porous film layer exists on the foil surface layer rather than the dielectric oxide film layer.

[0028] The pseudo-boehmite film layer contains a hydrated oxide of the valve metal. If the valve metal is aluminum, it is Al2O3·xH2O. This pseudo-boehmite film layer has a dense interior, which deteriorates the impregnability of the conductive polymer of the solid electrolyte to the anode foil. On the other hand, it functions as a resistance layer to improve the withstand voltage of the solid electrolytic capacitor. When the pseudo-boehmite film layer exists, the dielectric oxide film layer contains γ-alumina, which is a crystalline oxide.

[0029] The structure in which the pseudo-boehmite film layer exists on the surface layer rather than the dielectric oxide film layer is formed by undergoing the pre-forming treatment process and the forming process in this order. In the pre-forming process, a pseudo-boehmite film layer is formed on the surface of the anode foil. In the forming process, it grows from the interface with the unoxidized valve metal toward both sides of the pseudo-boehmite film layer and the valve metal, and the pseudo-boehmite film layer is transformed into a dielectric oxide film layer.

[0030] When forming a pseudo-boehmite film layer in this pre-forming treatment process, the enlarged anodic foil is immersed in pure water at 80°C or above or boiling water. The immersion time may be determined according to the target thickness of the pseudo-boehmite film layer and the balance between breakdown voltage and capacitance. In the forming process, a voltage is applied to the anodic foil in a forming solution without halogen ions. As the forming solution, a phosphoric acid-based forming solution such as ammonium dihydrogen phosphate, a boric acid-based forming solution such as ammonium borate, or an adipic acid-based forming solution such as ammonium adipate can be used. The applied voltage may be according to the target breakdown voltage.

[0031] The porous film layer is a layer of oxide film in which columnar pores extending in the thickness direction are formed from the foil surface. This porous film layer also has a dense interior, which deteriorates the impregnability of the conductive polymer of the solid electrolyte to the anodic foil. On the other hand, it functions as a resistance layer to improve the breakdown voltage of the solid electrolytic capacitor. When the porous film layer exists, the dielectric oxide film layer contains amorphous alumina. Note that both the pseudo-boehmite film layer and the porous film layer are layers with lower resistance than the dielectric oxide film layer.

[0032] A structure in which a porous film layer exists in the surface layer instead of the pseudo-boehmite layer relative to the dielectric oxide film layer is formed by undergoing a porous forming process. In the porous forming process, a voltage is applied to the anodic foil in an acid solution such as oxalic acid, phosphoric acid, chromic acid, and sulfuric acid. In this porous forming process, a porous film layer with innumerable fine pores appears and grows. Then, by the forming process, it grows from the surface of the valve metal towards both sides of the porous film layer and the valve metal, and the pores are filled, and the dielectric oxide film layer is formed with a thickness corresponding to the applied voltage on the surface of the valve metal.

[0033] (Cathode Foil Manufacturing Process) The cathode foil is also a long foil made of a valve-acting metal as the material, and a purity of 99% or more is desirable. After stretching the valve-acting metal into a foil shape, it may be transferred to a surface area expanding process if necessary. Also, after the surface area expanding process, a thin oxide film layer may be intentionally formed on the cathode foil. This oxide film layer may be provided with a pseudo-boehmite film layer or a porous film layer on top of the γ-alumina layer. That is, also in the cathode foil manufacturing process, a pre-forming process and a forming process, or a forming process in which a porous film layer is also formed may be incorporated.

[0034] (Element winding process) The anode foil and the cathode foil produced through the anode foil manufacturing process and the cathode foil manufacturing process are wound with a separator interposed therebetween to produce a cylindrical wound body. One end of the separator is overlapped so as to protrude from one end of the anode foil and the cathode foil, and the protruding separator is wound first to produce a core part, and then the core part is used as a winding shaft to continue winding. Before winding, for example, aluminum lead terminals are connected to the anode foil and the cathode foil by stitching, cold welding, ultrasonic welding, laser welding, etc.

[0035] Examples of the separator include cellulose such as kraft, manila hemp, esparto, hemp, rayon, and mixed papers thereof, polyester resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and their derivatives, polytetrafluoroethylene resins, polyvinylidene fluoride resins, vinylon resins, polyamide resins such as aliphatic polyamide, semi-aromatic polyamide, and wholly aromatic polyamide, polyimide resins, polyethylene resins, polypropylene resins, trimethylpentene resins, polyphenylene sulfide resins, acrylic resins, polyvinyl alcohol resins, etc. These resins can be used alone or in combination.

[0036] The separator separates the anode foil and the cathode foil to prevent short circuits therebetween, and holds the conductive polymer layer between the cathode foil and the anode foil. When an electrolytic solution is used in combination, the separator also holds the electrolytic solution. When the shape of the conductive polymer layer is self-retained and the anode foil and the cathode foil can be isolated by the conductive polymer layer, the separator can be excluded from the solid electrolytic capacitor.

[0037] In addition, an element forming process may be provided to repair the exposed base metal portion of the valve action metal and the defects of the anode foil and the cathode foil caused by physical stress such as winding when the anode foil with a dielectric oxide film layer formed thereon is cut to a desired width after the element winding process. In the element forming process, the wound body is immersed in a forming solution and a voltage is applied. As the forming solution, a phosphoric acid-based forming solution such as ammonium dihydrogen phosphate, a boric acid-based forming solution such as ammonium borate, an adipic acid-based forming solution such as ammonium adipate, or a forming solution in which a dicarboxylic acid such as boric acid and citric acid is mixed can be used. The voltage is preferably, for example, a value 0.1 to 1.2 times the forming voltage as the applied voltage during element formation. Also, as the voltage application method during element formation, a method of applying a constant voltage from the start of element formation or a method of gradually increasing the applied voltage step by step at regular intervals is appropriately selected.

[0038] A coating layer containing a conductive polymer and a conductive polymer layer are formed on each of the anode foil after the anode foil manufacturing process, the cathode foil after the cathode foil manufacturing process, and the separator, or for the capacitor element after the element winding process, the coating layer forming process, the voltage application process, and the conductive polymer layer forming process are performed in this order. That is, the voltage application process is sandwiched between the coating layer forming process and the conductive polymer layer forming process.

[0039] In the coating layer formation step and the conductive polymer layer formation step, both sides or one side of the anode foil is covered with a conductive polymer. In this coating layer formation step and conductive polymer layer formation step, the anode foil, the cathode foil, the separator, or the capacitor element is immersed in a dispersion liquid containing a conductive polymer, and the anode foil, the cathode foil, the separator, or the capacitor element is impregnated with the conductive polymer. In this coating layer formation step and conductive polymer layer formation step, a pressure reduction treatment or a pressure application treatment may be performed as necessary to promote the impregnation of the conductive polymer. In addition to immersion, the dispersion liquid of the conductive polymer may be drop-coated or spray-coated.

[0040] After immersion, the solvent of the dispersion liquid is removed by a drying process. In the drying process, the anode foil, the cathode foil, the separator, or the capacitor element is exposed in a temperature environment of, for example, 40°C or higher and 200°C or lower for a range of 3 minutes or longer and 180 minutes or shorter. This drying process may be repeated a plurality of times. Drying may be performed in a reduced pressure environment, for example, reducing the pressure to a pressure of 5 kPa or higher and 100 kPa or lower.

[0041] In addition, in the coating layer formation step and the conductive polymer layer formation step, the conductive polymers to be impregnated may be different. Also, in the coating layer formation step and the conductive polymer layer formation step, the amounts of the conductive polymers to be impregnated may be different. However, the conductive polymer forming the coating layer is required to be a material having permeability to water and various solvents. In the voltage application step after the coating layer formation step, the anode foil or the capacitor element is immersed in a formation solution to apply a voltage. This is because water and various solvents used in the formation solution need to permeate through the coating layer and come into contact with the dielectric oxide film layer, the pseudo-boehmite film layer, or the porous film layer.

[0042] In the voltage application process, the anode foil, both the anode foil and the cathode foil, or the capacitor element is immersed in the formation solution, and a voltage is applied. As the formation solution, a phosphoric acid-based formation solution such as ammonium dihydrogen phosphate, a boric acid-based formation solution such as ammonium borate, an adipic acid-based formation solution such as ammonium adipate, or a formation solution in which a dicarboxylic acid such as boric acid and citric acid is mixed can be used. The voltage is preferably, for example, a value 0.1 to 1.2 times the formation voltage as the applied voltage during voltage application. Also, as the voltage application method during voltage application, a method of applying a constant voltage from the start of voltage application or a method of gradually increasing the applied voltage at regular intervals is appropriately selected.

[0043] In this way, by passing through the coating layer formation process, the voltage application process, and the conductive polymer layer formation process in this order, good withstand voltage characteristics and frequency characteristics of the solid electrolytic capacitor are compatible. That is, a high withstand voltage of the solid electrolytic capacitor and a high capacitance and a low equivalent series resistance (ESR) over a wide frequency band are compatible.

[0044] This reason is speculative and not limited to this mechanism, but it can be considered as follows. First, due to the coating layer formation process, the conductive polymer penetrates through the pseudo-boehmite film layer or the porous film layer with a dense interior and adheres to the dielectric oxide film layer. As a result, the number of conductive paths between the conductive polymer layer and the dielectric oxide film layer increases, and the frequency characteristics are improved. On the other hand, although the withstand voltage should decrease due to the generation of this conductive path, the voltage application process insulates the conductive polymer in contact with the dielectric oxide film layer, improving the withstand voltage. Thereby, good withstand voltage characteristics and frequency characteristics of the solid electrolytic capacitor are compatible. However, if the conductive polymer layer formation process is omitted or the coating layer formation process and the conductive polymer layer formation process are continuous without intervening the voltage application process, it is impossible to make the good withstand voltage characteristics and frequency characteristics of the solid electrolytic capacitor compatible.

[0045] Here, the conductive polymer is a self-doped type doped by dopant molecules within the molecule or a conjugated polymer doped by external dopant molecules. The conjugated polymer is obtained by chemically oxidatively polymerizing or electrochemically oxidatively polymerizing a monomer having a π-conjugated double bond or its derivative. By performing a doping reaction on the conjugated polymer, the conductive polymer exhibits high conductivity. That is, conductivity is exhibited by adding a small amount of a dopant such as an acceptor that easily accepts electrons or a donor that easily donates electrons to the conjugated polymer.

[0046] As the conjugated polymer, known ones can be used without particular limitation. For example, polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene, polyphenylene vinylene, polyacene, polythiophene vinylene, etc. can be mentioned. These conjugated polymers may be used alone, may be combined of two or more kinds, or may be a copolymer of two or more kinds of monomers.

[0047] Among the above conjugated polymers, a conjugated polymer formed by polymerizing thiophene or its derivative is preferred, and a conjugated polymer polymerized from 3,4-ethylenedioxythiophene (that is, 2,3-dihydrothieno[3,4-b][1,4]dioxin), 3-alkylthiophene, 3-alkoxythiophene, 3-alkyl-4-alkoxythiophene, 3,4-alkylthiophene, 3,4-alkoxythiophene or their derivatives is preferred. As the thiophene derivative, a compound selected from thiophenes having substituents at the 3-position and 4-position is preferred, and the substituents at the 3-position and 4-position of the thiophene ring may form a ring together with the carbons at the 3-position and 4-position. The number of carbon atoms of the alkyl group or alkoxy group is suitably 1 to 16.

[0048] In particular, a polymer of 3,4-ethylenedioxythiophene called EDOT, that is, poly(3,4-ethylenedioxythiophene) called PEDOT is particularly preferred. Further, an alkylated ethylenedioxythiophene in which an alkyl group is added to 3,4-ethylenedioxythiophene may also be used. For example, methylated ethylenedioxythiophene (that is, 2-methyl-2,3-dihydro-thieno[3,4-b][1,4]dioxin), ethylated ethylenedioxythiophene (that is, 2-ethyl-2,3-dihydro-thieno[3,4-b][1,4]dioxin), etc. may be mentioned.

[0049] As the dopant, known ones can be used without particular limitation. The dopant may be used alone or in combination of two or more. Further, a polymer or a monomer may be used. For example, as the dopant, polyanion, boric acid, inorganic acids such as nitric acid and phosphoric acid, 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, borodisalicylate acid, bisoxalate borate acid, sulfonylimide acid, dodecylbenzenesulfonic acid, propylnaphthalenesulfonic acid, butylnaphthalenesulfonic acid, etc. may be mentioned.

[0050] The polyanion is, for example, a substituted or unsubstituted polyalkylene, a substituted or unsubstituted polyalkenylene, a substituted or unsubstituted polyimide, a substituted or unsubstituted polyamide, a substituted or unsubstituted polyester, a polymer composed only of constitutional units having an anion group, and a polymer composed of constitutional units having an anion group and constitutional units not having an anion group. Specifically, examples of the polyanion include polyvinylsulfonic acid, polystyrenesulfonic acid, polyallylsulfonic acid, polyacrylicsulfonic acid, polymethacrylicsulfonic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprenesulfonic acid, polyacrylic acid, polymethacrylic acid, polymaleic acid, etc.

[0051] In the case where this conductive polymer is added and used as a solvent for the dispersion liquid impregnated into an anode foil or the like in the coating layer formation step and the conductive polymer layer formation step, any solvent in which the particles or powder of the conductive polymer can be dispersed may be used. For example, water, an organic solvent, or a mixture thereof is used. Examples of the organic solvent include polar solvents, alcohols, esters, hydrocarbons, carbonate compounds, ether compounds, chain ethers, heterocyclic compounds, nitrile compounds, and the like.

[0052] Examples of the polar solvent include N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and the like. Examples of the alcohols include methanol, ethanol, propanol, butanol, and the like. Examples of the esters include ethyl acetate, propyl acetate, butyl acetate, and the like. Examples of the hydrocarbons include hexane, heptane, benzene, toluene, xylene, and the like. Examples of the carbonate compounds include ethylene carbonate, propylene carbonate, and the like. Examples of the ether compounds include dioxane, diethyl ether, and the like. Examples of the chain ethers include ethylene glycol dialkyl ether, propylene glycol dialkyl ether, polyethylene glycol dialkyl ether, polypropylene glycol dialkyl ether, and the like. Examples of the heterocyclic compounds include 3-methyl-2-oxazolidinone, and the like. Examples of the nitrile compounds include acetonitrile, glutaronitrile, methoxyacetonitrile, propionitrile, benzonitrile, and the like.

[0053] The dispersion may contain a polyhydric alcohol in addition to a solvent and a conductive polymer. Examples of the polyhydric alcohol include sorbitol, ethylene glycol, diethylene glycol, triethylene glycol, polyoxyethylene glycol, glycerin, polyoxyethylene glycerin, xylitol, erythritol, mannitol, dipentaerythritol, pentaerythritol, or a combination of two or more thereof. Since the polyhydric alcohol has a high boiling point, it can remain in the conductive polymer layer even after the drying process, and effects such as ESR reduction and breakdown voltage improvement can be obtained.

[0054] Other compounds may be contained in the dispersion of the conductive polymer. For example, conventional additives such as an organic binder, a surfactant, a dispersant, an antifoaming agent, a coupling agent, an antioxidant, and an ultraviolet absorber may be added. It is also possible to significantly reduce the ESR by adding an additive to the dispersion of the conductive polymer or increasing the number of times the dispersion of the conductive polymer is impregnated into the capacitor element.

[0055] (Electrolyte impregnation step) The solid electrolytic capacitor produced through this conductive polymer layer formation step does not use an electrolyte in combination, has good frequency characteristics only by the conductive polymer layer, and achieves both good breakdown voltage and good frequency characteristics. When not using the electrolyte in combination or reducing the amount of the electrolyte, deterioration of the solid electrolytic capacitor due to increased internal pressure or dry-up can be suppressed.

[0056] However, when using the electrolyte in combination, the ESR can be further reduced in the frequency range around 120 Hz, and the electrolyte may be used in combination from the viewpoint of ESR reduction. When using the electrolyte in combination, the electrolyte is filled in the voids of the capacitor element on which the conductive polymer layer is formed. When impregnating the capacitor element with the electrolyte, a pressure reduction treatment or a pressure increase treatment may be performed as necessary to promote the impregnation. The impregnation step may be repeated a plurality of times.

[0057] The solvent of the electrolytic solution is not particularly limited, but a protic organic polar solvent or an aprotic organic polar solvent can be used. As the protic polar solvent, monohydric alcohols, polyhydric alcohols, oxyalcohol compounds, water, etc. are typically cited, for example, ethylene glycol or propylene glycol. As the aprotic polar solvent, sulfone-based, amide-based, lactones, cyclic amide-based, nitrile-based, sulfoxide-based, etc. are typically cited, for example, sulfolane, γ-butyrolactone, ethylene carbonate or propylene carbonate.

[0058] The solute contained in the electrolytic solution contains anionic and cationic components. Typically, it is an organic acid such as adipic acid or benzoic acid or its salt, an inorganic acid such as boric acid or phosphoric acid or its salt, or a composite compound of an organic acid and an inorganic acid such as borodisalicylate or its ion-dissociable salt, and is used alone or in combination of two or more. As at least one kind of salt of these organic acid salts, inorganic acid salts, and composite compounds of organic acids and inorganic acids, ammonium salts, quaternary ammonium salts, quaternized amidinium salts, amine salts, sodium salts, potassium salts, etc. are cited. The acid serving as the anion and the base serving as the cation may be separately added to the electrolytic solution as solute components.

[0059] Furthermore, other additives can also be added to the electrolytic solution. Examples of the additives include polyethylene glycol, complex compounds of boric acid and polysaccharides (mannitol, sorbitol, etc.), complex compounds of boric acid and polyhydric alcohols, boric acid esters, nitro compounds, phosphate esters, colloidal silica, etc. These may be used alone or in combination of two or more. The nitro compound suppresses the generation amount of hydrogen gas in the electrolytic capacitor. Examples of the nitro compound include o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, p-nitrophenol, etc.

[0060] (Final manufacturing process) The capacitor element that has undergone the conductive polymer layer formation process or the electrolytic solution impregnation process is inserted into a bottomed cylindrical exterior case. The exterior case is a metal container with a bottom at one end and an opening at the other end, and examples of the material include aluminum, an aluminum alloy containing aluminum and manganese, or stainless steel. After accommodating the capacitor element in the exterior case, the opening end of the exterior case is sealed with a sealing body. The sealing body is a rubber or a laminate of rubber and a rigid substrate, and examples of the rubber include ethylene propylene rubber and butyl rubber. By fitting this sealing body into the end of the exterior case and caulking the opening end of the exterior case, it is sealed. From the sealing body, lead terminals connected to the anode foil and the cathode foil are drawn out.

[0061] After sealing the capacitor element in the exterior case, the solid electrolytic capacitor is completed through an aging process within the final manufacturing process. In the aging process, a DC voltage is applied to the solid electrolytic capacitor, and the electrolytic solution repairs the defective portions such as the dielectric oxide film layer. Also, in a solid electrolytic capacitor that does not contain an electrolytic solution, by applying a DC voltage in the aging process, the conductive polymer existing at or near the defective portion of the dielectric oxide film layer is insulated.

[0062] (All steps of the manufacturing method) According to the manufacturing method of the solid electrolytic capacitor as described above, by sandwiching a voltage application step between the coating layer formation step and the conductive polymer layer formation step, the solid electrolytic capacitor can have both high withstand voltage and good frequency characteristics.

[0063] Here, the coating layer formation step, the voltage application step, and the conductive polymer layer formation step may be performed continuously between other steps, or may be dispersed and performed between a plurality of other steps.

[0064] FIG. 1 is a flowchart showing the entire first manufacturing method of a solid electrolytic capacitor. As shown in FIG. 1, for example, by a cathode foil manufacturing process and an anode foil manufacturing process, a cathode foil and an anode foil with a dielectric oxide film layer having a pseudo-boehmite film layer or a porous film layer remaining on the surface layer are manufactured. These anode foil, cathode foil, and separator are wound by an element winding process. At the end of this element winding process, an element formation application process is incorporated as necessary.

[0065] After the element winding process is completed, a coating layer formation process is performed. After the coating layer formation process, a voltage application process is performed. After the voltage application process, a conductive polymer layer formation process is performed. A dispersion of the conductive polymer is prepared in advance. After the conductive polymer layer formation process is completed, it proceeds to the electrolytic solution impregnation process, and finally, through the final manufacturing process, the manufacturing of the solid electrolytic capacitor is completed. Note that when manufacturing a solid electrolytic capacitor without an electrolytic solution, the electrolytic solution impregnation process is omitted.

[0066] FIG. 2 is a flowchart showing the entire second manufacturing method of a solid electrolytic capacitor. As shown in FIG. 2, for example, by a cathode foil manufacturing process and an anode foil manufacturing process, a cathode foil and an anode foil with a dielectric oxide film layer having a pseudo-boehmite film layer or a porous film layer remaining on the surface layer are manufactured.

[0067] Then, for the anode foil that has undergone the anode foil manufacturing process, before moving on to the element winding process, a coating layer formation process is performed. After going through the coating layer formation process, these anode foil, cathode foil, and separator are wound by the element winding process. If a thin oxide film layer is also formed on the cathode foil, after the cathode foil manufacturing process, a coating layer formation process may be performed on the cathode foil.

[0068] After the element winding process is completed, a voltage application process is performed on the element with the coating layer formed, and a conductive polymer layer formation process is performed after the voltage application process. In this case, since the element formation process in the element winding process overlaps with the voltage application process after the coating layer formation process, it can be omitted. After the conductive polymer layer formation process is completed, it proceeds to the electrolyte impregnation process, and finally, through the final manufacturing process, the manufacturing of the solid electrolytic capacitor is completed. When manufacturing a solid electrolytic capacitor that does not contain an electrolyte, the electrolyte impregnation process is omitted.

[0069] Figure 3 is a flowchart showing the entire third manufacturing method of the solid electrolytic capacitor. As shown in Figure 3, for example, by the cathode foil manufacturing process and the anode foil manufacturing process, a cathode foil and an anode foil with a dielectric oxide film layer having a pseudo-boehmite film layer or a porous film layer remaining on the surface layer are manufactured.

[0070] Then, before moving on to the element winding process for the anode foil that has undergone the anode foil manufacturing process, a coating layer formation process and a voltage application process are performed. After the coating layer formation process and the voltage application process, these anode foil, cathode foil, and separator are wound by the element winding process. If a thin oxide film layer is also formed on the cathode foil, after the cathode foil manufacturing process, a coating layer formation process and a voltage application process may be performed on the cathode foil.

[0071] After the element winding process is completed, a conductive polymer layer formation process is performed. In this case, an element formation process may be performed in the element winding process. After the conductive polymer layer formation process is completed, it proceeds to the electrolyte impregnation process, and finally, through the final manufacturing process, the manufacturing of the solid electrolytic capacitor is completed. When manufacturing a solid electrolytic capacitor that does not contain an electrolyte, the electrolyte impregnation process is omitted.

Example

[0072] Hereinafter, the present invention will be described in more detail based on examples. Note that the present invention is not limited to the following examples.

[0073] (Example 1) The solid electrolytic capacitor of Example 1 was manufactured as follows. First, an aluminum foil was prepared as the anode foil. This aluminum foil was transferred to the surface expansion process of the anode foil manufacturing process, and tunnel-shaped etching pits were formed on both sides of the aluminum foil. In the surface expansion process, 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.

[0074] The aluminum foil that had undergone the surface expansion process was transferred to the pre-formation treatment process. In the pre-formation treatment process, the aluminum foil was immersed in boiled pure water for 15 minutes to form a pseudo-boehmite film layer on the surface of the aluminum foil. Next, the formation treatment process was performed. In the formation treatment process, while the aluminum foil was immersed in an ammonium borate aqueous solution at 90 °C, a current density of 25 mA / cm -2 was applied to the aluminum foil until a predetermined formation voltage was reached, and then the voltage was held for 20 minutes to transform the outermost layer of the pseudo-boehmite film layer into a dielectric oxide film layer while leaving the outermost layer. The formation voltage of the aluminum foil was set to 650 V.

[0075] The foil capacitance of this anode foil at 120 Hz was 0.686 μF / cm -2 . The foil capacitance of the anode foil was measured by immersing the anode foil defined with a sample area of 1 cm 2 in an ammonium pentaborate aqueous solution at a temperature of 30 °C, setting the DC bias voltage to 1.5 V and the AC amplitude to 1 V, and using a potentiostat SI1287 and a frequency response analyzer 1252A manufactured by Solartron analytical. Note that for measuring the foil capacitance, a general-purpose electrochemical impedance analyzer, an LCR meter, etc. can also be used.

[0076] Both sides of the anode foil were masked with imide tape so that four 1 cm square foil surfaces were exposed per foil side. Then, the process moved on to the coating layer formation step, where a dispersion of a conductive polymer was dropped onto the eight exposed regions and dried. The conductive polymer was polyethylenedioxythiophene (PEDOT) doped with polystyrene sulfonic acid (PSS) (PEDOT / PSS). Water was used as the solvent for the dispersion. PEDOT / PSS was dispersed at a ratio of 2 wt% with respect to the entire dispersion. 5 μL of this PEDOT / PSS dispersion was dropped onto each exposed region, left standing at room temperature for 5 minutes, and then dried at 110 °C for 30 minutes.

[0077] After going through the coating layer formation step, the process moved on to the voltage application step. In the voltage application step, the anode foil that had gone through the coating layer formation step was immersed in a 60 °C boric acid aqueous solution in which 70 g of boric acid was dissolved in 1 L of water. Then, a formation voltage was applied to this anode foil. The application of the formation voltage was repeated by increasing the voltage at regular intervals from 100 V to 550 V and holding the voltage for a certain time, and the voltage was held for 10 minutes at 550 V, which was the final treatment voltage. When applying the formation voltage, current limiting was performed so that a current of 100 mA or less flowed through the anode foil.

[0078] After going through the voltage application step, the process moved on to the conductive polymer layer formation step. In the conductive polymer layer formation step, a dispersion of a conductive polymer was further dropped onto the eight exposed regions and dried. The conductive polymer was the same as in the coating layer formation step, i.e., PEDOT / PSS, and was dispersed at a ratio of 2 wt% in the dispersion. Sorbitol at 83 wt% with respect to the solid content weight after drying of the dispersion was further added to the dispersion used in the conductive polymer layer formation step. 35 μL of this dispersion was dropped onto each exposed region and dried. Specifically, after dropping the dispersion onto one side of the anode foil, it was left standing at room temperature for 5 minutes, and then left standing at 115 °C for 15 minutes. Next, after dropping the dispersion onto the other side of the anode foil, it was left standing at room temperature for 5 minutes, and then left standing at 115 °C for 15 minutes.

[0079] After the formation process of the conductive polymer layer was completed, carbon paste was applied to eight exposed areas and cured by leaving it in a temperature environment of 130°C for 10 minutes. Further, while applying silver paste onto the carbon layer on the eight exposed areas, a copper foil was adhered as a lead-out terminal. The silver paste was applied onto one side of the anode foil and then pre-cured by leaving it in a temperature environment of 130°C for 5 minutes in a state where a copper foil was adhered to the uncured silver paste portion. Thereby, the silver layer and the copper foil were connected. By the same procedure, after applying silver paste onto the other side of the anode foil and folding back and adhering the copper foil connected to the silver layer on one side, it was left at 130°C for 25 minutes. In this way, for all eight exposed areas, the exposed area formed on one side and the exposed area formed on the other side on the anode foil were connected via the copper foil connected with the silver paste. The above carbon layer, silver layer, and copper foil layer correspond to the cathode foil of the solid electrolytic capacitor.

[0080] In this way, the solid electrolytic capacitor of Example 1 was manufactured. The solid electrolytic capacitor of this Example 1 includes an anode foil on which a pseudo-boehmite film layer is formed on a dielectric oxide film layer, a cathode foil composed of a carbon layer, a silver layer, and a copper foil, and a conductive polymer layer containing PEDOT / PSS interposed between these anode foil and cathode foil. And this conductive polymer layer is formed through a coating layer formation process, a voltage application process, and a conductive polymer layer formation process in this order.

[0081] (Comparative Example) A solid electrolytic capacitor of Comparative Example 1 was manufactured. The anode foil of Example 1 was used for the solid electrolytic capacitor of Comparative Example 1. This anode foil was masked so that 1 cm square exposed areas were formed at eight locations on both sides, the same as in Example 1, and the same cathode foil as in Example 1 was installed. However, the voltage application process was omitted, and after the coating layer formation process, the process moved to the conductive polymer layer formation process. The contents of the coating layer formation process and the conductive polymer layer formation process are the same as those of Example 1.

[0082] The solid electrolytic capacitor of Comparative Example 2 was manufactured. The anode foil of Example 1 was used for the solid electrolytic capacitor of Comparative Example 2. This anode foil was masked in the same manner as in Example 1 so that exposed regions of 1 cm square were formed at eight locations on both sides, and the same cathode foil as in Example 1 was installed. However, the coating layer formation step was omitted, and after going through the conductive polymer layer formation step of Example 1, the same voltage application step as in Example 1 was carried out.

[0083] The solid electrolytic capacitor of Comparative Example 3 was manufactured. The anode foil of Example 1 was used for the solid electrolytic capacitor of Comparative Example 3. This anode foil was masked in the same manner as in Example 1 so that exposed regions of 1 cm square were formed at eight locations on both sides, and the same cathode foil as in Example 1 was installed. In Comparative Example 3, the coating layer formation step, the conductive polymer layer formation step, and the voltage application step were carried out, but the order was different from that of Example 1. That is, in Comparative Example 3, the coating layer formation step, the conductive polymer layer formation step, and the voltage application step were carried out in this order. The contents of the coating layer formation step, the conductive polymer layer formation step, and the voltage application step were the same as those of Example 1.

[0084] The solid electrolytic capacitor of Comparative Example 4 was manufactured. The anode foil of Example 1 was used for the solid electrolytic capacitor of Comparative Example 4. This anode foil was masked in the same manner as in Example 1 so that exposed regions of 1 cm square were formed at eight locations on both sides, and the same cathode foil as in Example 1 was installed. In Comparative Example 4, the voltage application step, the coating layer formation step, and the conductive polymer layer formation step were carried out, but the order was different from that of Example 1. That is, in Comparative Example 4, the voltage application step, the coating layer formation step, and the conductive polymer layer formation step were carried out in this order. The contents of the coating layer formation step, the conductive polymer layer formation step, and the voltage application step were the same as those of Example 1.

[0085] (Characteristic Measurement Test) The capacitance, equivalent series resistance (ESR) for each frequency, and breakdown voltage of the solid electrolytic capacitors of Example 1 and Comparative Examples 1 to 4 above were measured. A plurality of solid electrolytic capacitors of Example 1 and Comparative Examples 1 to 4 were fabricated and used as measurement targets. However, for the solid electrolytic capacitors of Comparative Examples 2 and 3, the conductive polymer layer peeled off after the voltage application step and measurement could not be performed. In Comparative Examples 2 and 3, the conductive polymer layer with a thickness formed by dropping 35 μL adhered to the anode foil, and then the voltage application step was carried out with the anode foil immersed in the liquid, resulting in the peeling off of the conductive polymer layer.

[0086] First, the frequency characteristics of the solid electrolytic capacitors of Example 1 and Comparative Examples 1 to 4 were measured. The frequency characteristics were measured using an LCR meter ZM2376 manufactured by NF Circuit Design Block Co., Ltd. The measurement was carried out at a temperature of 20°C, and the measurement frequency range was from 10 Hz to 1 MHz. Next, the breakdown voltage of each solid electrolytic capacitor was measured. A source meter 2410 manufactured by Tektronix was used for the breakdown voltage measurement. The measurement was carried out at room temperature with the applied voltage increased by 1 V per second from 0 V. Also, the voltage when a current of 20 mA flowed through the solid electrolytic capacitor to be measured was defined as the breakdown voltage of the solid electrolytic capacitor.

[0087] The measurement results of the capacitance and equivalent series resistance (ESR) are shown in FIGS. 4 to 7. The horizontal axis of each figure is the frequency, the vertical axis is the breakdown voltage, capacitance, or equivalent series resistance. The solid line graph represents the group of Example 1, and the dotted line graph represents the group of Comparative Examples. Also, the measurement results of the breakdown voltage are shown in FIG. 8. The horizontal axis is the breakdown voltage, the vertical axis is the current value, the solid line graph represents the group of Example 1, and the dotted line graph represents the group of Comparative Examples.

[0088] Figure 4 is a graph showing the relationship between frequency and capacitance in Example 1 and Comparative Example 1. As shown in Figure 4, in the entire frequency range from the low frequency region such as 120 Hz to the medium and high frequency regions of 10 kHz and 100 kHz or more, the capacitance of the group in Example 1 exceeded that of the group in Comparative Example 1. The difference between Example 1 and Comparative Example 1 is that Example 1 went through the coating layer formation process, voltage application process, and conductive polymer layer formation process in this order, while in Comparative Example 1, the voltage application process after the coating process layer formation process was omitted, and it went through the coating layer formation process and conductive polymer layer formation process in this order.

[0089] Figure 5 is a graph showing the relationship between frequency and capacitance in Example 1 and Comparative Example 4. As shown in Figure 5, in the entire frequency range from the low frequency region such as 120 Hz to the medium and high frequency regions of 10 kHz and 100 kHz or more, the capacitance of the group in Example 1 exceeded that of the group in Comparative Example 4. The difference between Example 1 and Comparative Example 4 is that Example 1 went through the coating layer formation process, voltage application process, and conductive polymer layer formation process in this order, while Comparative Example 4 went through the voltage application process, coating layer formation process, and conductive polymer layer formation process in this order.

[0090] Furthermore, Figure 6 is a graph showing the relationship between frequency and ESR in Example 1 and Comparative Example 1. As shown in Figure 6, in the entire frequency range from the low frequency region to the medium and high frequency regions, the ESR of the group in Example 1 was lower than that of the group in Comparative Example 1. Figure 7 is a graph showing the relationship between frequency and ESR in Example 1 and Comparative Example 4. As shown in Figure 7, in the entire frequency range from the low frequency region to the medium and high frequency regions, the ESR of the group in Example 1 was lower than that of the group in Comparative Example 4.

[0091] As described above, as shown in Figures 4 to 7, by going through the coating layer formation process, voltage application process, and conductive polymer layer formation process in this order, the capacitance and ESR of the solid electrolytic capacitor are improved in the entire frequency range, and it can be confirmed that the frequency characteristics of the solid electrolytic capacitor are improved.

[0092] Furthermore, the average capacitance of the solid electrolytic capacitor of Example 1 at 120 Hz was 0.621 μF, the average capacitance of the solid electrolytic capacitor of Comparative Example 1 at 120 Hz was 0.438 μF, and the average capacitance of the solid electrolytic capacitor of Comparative Example 4 at 120 Hz was 0.426 μF. Also, the average capacitances of the solid electrolytic capacitor of Example 1 at 10 kHz and 100 kHz were 0.313 μF and 0.161 μF, respectively, the average capacitances of the solid electrolytic capacitor of Comparative Example 1 at 10 kHz and 100 kHz were 0.099 μF and 0.062 μF, respectively, and the average capacitances of the solid electrolytic capacitor of Comparative Example 4 at 10 kHz and 100 kHz were 0.079 μF and 0.040 μF, respectively.

[0093] Furthermore, the average ESR of the solid electrolytic capacitor of Example 1 at 120 Hz was 150 Ω, the average ESR of the solid electrolytic capacitor of Comparative Example 1 at 120 Hz was 1367 Ω, and the average ESR of the solid electrolytic capacitor of Comparative Example 4 at 120 Hz was 1681 Ω. In addition, the average ESRs of the solid electrolytic capacitor of Example 1 at 10 kHz and 100 kHz were 24.7 Ω and 4.3 Ω, respectively, the average ESRs of the solid electrolytic capacitor of Comparative Example 1 at 10 kHz and 100 kHz were 65.5 Ω and 7.9 Ω, respectively, and the average ESRs of the solid electrolytic capacitor of Comparative Example 4 at 10 kHz and 100 kHz were 118 Ω and 18.7 Ω, respectively.

[0094] Next, FIG. 8 is a graph showing the breakdown voltages in Example 1, Comparative Example 1, and Comparative Example 4, where (a) shows Example 1, (b) shows Comparative Example 1, and (c) shows Comparative Example 4. As shown in FIG. 8, the breakdown voltage of Example 1 was 390 V, the breakdown voltage of Comparative Example 1 was 330 V, and the breakdown voltage of Comparative Example 4 was 451 V. That is, although inferior to Comparative Example 4, Example 1 had a better breakdown voltage than Comparative Example 1. However, Comparative Example 4 was significantly inferior to Example 1 in terms of frequency characteristics.

[0095] Generally, as shown in FIGS. 4 to 8, by going through the coating layer formation step, the voltage application step, and the conductive polymer layer formation step in this order, the capacitance and ESR of the solid electrolytic capacitor are improved in the entire frequency range, it can be confirmed that the frequency characteristics of the solid electrolytic capacitor are improved, and it can be confirmed that the withstand voltage of the solid electrolytic capacitor is improved. That is, it was confirmed that by going through the coating layer formation step, the voltage application step, and the conductive polymer layer formation step in this order, both good withstand voltage and good frequency characteristics of the solid electrolytic capacitor can be achieved. Furthermore, it was also confirmed that good frequency characteristics can be achieved without using an electrolytic solution in combination.

[0096] (Example 2) The solid electrolytic capacitor of Example 2 was manufactured. The solid electrolytic capacitor of Example 2 is the same as that of Example 1 in terms of manufacturing method and manufacturing conditions, except that after going through the coating layer formation step, the voltage application step, and the conductive polymer layer formation step in this order, it proceeds to the electrolytic solution impregnation step and the electrolytic solution is impregnated. The solvent of the electrolytic solution is ethylene glycol, and ammonium borate and azelaic acid are added to this electrolytic solution. In the impregnation step, the anode foil was immersed in the electrolytic solution for 10 minutes under a reduced pressure environment of room temperature and 100 kPa. For the cathode foil, an aluminum foil with a thickness of 50 μm was used. A Manila-based separator with a thickness of 50 μm was interposed between the anode foil and the cathode foil.

[0097] (Comparative Example) A solid electrolytic capacitor of Comparative Example 5 using an electrolytic solution in combination was manufactured corresponding to the solid electrolytic capacitor of Example 2. The solid electrolytic capacitor of Comparative Example 5 is different from Example 2 in that only the conductive polymer layer formation step is performed and the coating layer formation step and the voltage application step are omitted. In the conductive polymer layer formation step in Comparative Example 5, it was performed under the same procedure and the same conditions as the conductive polymer layer formation step, except that a total of 40 μL of the dispersion liquid applied in the coating layer formation step and the conductive polymer layer formation step in Example 2 was dropped.

[0098] (Characteristic Measurement Test) The capacitance, equivalent series resistance (ESR), and withstand voltage of the solid electrolytic capacitors of Example 2 and Comparative Example 5 above were measured for each frequency. A plurality of solid electrolytic capacitors of Example 2 and Comparative Example 5 were fabricated, each serving as a measurement target, and the average value of the measurement results was calculated. The measurement procedure and conditions were the same as those of Example 1. The average values of the measurement results of capacitance and equivalent series resistance (ESR) are shown in FIGS. 9 to 10. The horizontal axis of each figure is the frequency, the vertical axis is the withstand voltage, capacitance, or equivalent series resistance. The solid line graph represents Example 2, and the dotted line graph represents Comparative Example 5. Also, the measurement results of the withstand voltage are shown in FIG. 11. The horizontal axis is the withstand voltage, and the vertical axis is the current value. The solid line graph represents the group of Example 1, and the dotted line graph represents the group of the comparative examples.

[0099] FIG. 9 is a graph showing the relationship between frequency and capacitance in Example 2 and Comparative Example 5, and FIG. 10 is a graph showing the relationship between frequency and ESR in Example 2 and Comparative Example 5. The solid electrolytic capacitors of Example 2 and Comparative Example 5 use a conductive polymer layer and an electrolytic solution in combination. As can be seen by comparing with FIGS. 9 and 10 and Comparative Examples 1 to 4, in such solid electrolytic capacitors of Example 2 and Comparative Example 5, it can be confirmed that the capacitance and ESR become equivalent over the entire frequency range due to the improvement of the capacitance and the reduction of the ESR of the solid electrolytic capacitor of Comparative Example 5.

[0100] Here, the capacitance of the solid electrolytic capacitor of Example 2 at 120 Hz was 0.654 μF, and the capacitance of the solid electrolytic capacitor of Comparative Example 5 at 120 Hz was 0.665 μF. Also, the ESR of the solid electrolytic capacitor of Example 2 at 120 Hz was 62 Ω, and the ESR of the solid electrolytic capacitor of Comparative Example 5 at 120 Hz was 101 Ω. Thus, it can be confirmed that, compared with Example 1, the solid electrolytic capacitor of Example 2 using the electrolytic solution in combination has a further reduced ESR.

[0101] On the one hand, FIG. 11 is a graph showing the breakdown voltages in Example 2 and Comparative Example 5. As shown in FIG. 11, the breakdown voltage of the solid electrolytic capacitor in Example 2 was 465 V, while that of Comparative Example 5 was 376 V. Thus, although the solid electrolytic capacitor of Comparative Example 5 has good frequency characteristics by using a conductive polymer layer and an electrolytic solution in combination, its breakdown voltage is significantly inferior, and good frequency characteristics and good breakdown voltage cannot be achieved simultaneously. On the other hand, it can be confirmed that the solid electrolytic capacitor of Example 2 has both good frequency characteristics and good breakdown voltage.

[0102] As described above, as shown in FIGS. 9 to 11, by passing through the coating layer formation step, the voltage application step, and the conductive polymer layer formation step in this order, even when an electrolytic solution is used in combination, the capacitance and ESR of the solid electrolytic capacitor are improved in the entire frequency range, and it can be confirmed that the frequency characteristics of the solid electrolytic capacitor are improved. In addition, it can be confirmed that the ESR of the solid electrolytic capacitor is further reduced.

[0103] Furthermore, as shown in FIGS. 4 to 11, when the electrolytic solution is not used in combination and the coating layer formation step, the voltage application step, and the conductive polymer layer formation step are not passed through in this order, the frequency characteristics of the solid electrolytic capacitor deteriorate with only the conductive polymer layer. However, if the coating layer step, the voltage application step, and the conductive polymer layer formation step are passed through in this order without using the electrolytic solution in combination, it was confirmed that even with only the conductive polymer layer, the frequency characteristics of the solid electrolytic capacitor are improved, and good frequency characteristics and good breakdown voltage can be achieved simultaneously.

Claims

1. A method for manufacturing an electrolytic capacitor having a conductive polymer layer interposed between an anode foil and a cathode foil, an anode foil manufacturing step of forming the anode foil in which a dielectric oxide film layer is located on a valve-acting metal foil and a pseudo-boehmite film layer or a porous film layer is located on the dielectric oxide film layer, a coating layer forming step of coating the anode foil with a conductive polymer, a voltage application step which is a subsequent step to the coating layer forming step and applies a voltage to the anode foil coated with the conductive polymer, a conductive polymer layer forming step which is a subsequent step to the voltage application step and forms the conductive polymer layer, comprising, characterized by a method for manufacturing a solid electrolytic capacitor.

2. including an element winding step of winding the anode foil and the cathode foil, the coating layer forming step is performed after the element winding step, and the coating layer forming step and the voltage application step are continuous, characterized by the method for manufacturing a solid electrolytic capacitor according to Claim 1.

3. including an element winding step of winding the anode foil and the cathode foil, the coating layer forming step is interposed after the anode foil manufacturing step and before the element winding step, the voltage application step and the conductive polymer layer forming step are performed after the element winding step, characterized by the method for manufacturing a solid electrolytic capacitor according to Claim 1.

4. including an element winding step of winding the anode foil and the cathode foil, the coating layer forming step and the voltage application step are interposed after the anode foil manufacturing step and before the element winding step, the conductive polymer layer forming step is performed after the element winding step, characterized by the method for manufacturing a solid electrolytic capacitor according to Claim 1.

5. including a surface expansion step of expanding the anode foil, in the surface expansion step, forming tunnel-shaped etching pits extending in the foil thickness direction on the anode foil, characterized by the method for manufacturing a solid electrolytic capacitor according to any one of Claims 1 to 4.

6. further including an electrolytic solution impregnation step of impregnating an electrolytic solution after interposing the conductive polymer layer between the anode foil and the cathode foil, characterized by the method for manufacturing a solid electrolytic capacitor according to any one of Claims 1 to 5.

7. Without impregnating the electrolytic solution, only the conductive polymer layer is interposed as an electrolyte between the anode foil and the cathode foil, The method for manufacturing a solid electrolytic capacitor according to any one of claims 1 to 5, characterized by

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