Solid electrolytic capacitor and method for manufacturing solid electrolytic capacitor

Abstract

To provide a solid electrolytic capacitor improved in opportunity for defect repair and reduced in leakage current, and a method for manufacturing the solid electrolytic capacitor.SOLUTION: A solid electrolytic capacitor includes a capacitor element, a conductive polymer layer and an electrolyte. The capacitor element comprises an anode foil and a cathode body facing each other. The conductive polymer layer is formed by the impregnation of a dispersion containing a solvent and particles or powder of a conductive polymer. The electrolyte is impregnated in the capacitor element. Herein, the cathode body has a cathode foil and a carbon layer. The cathode foil comprises a valve metal and has a widening layer formed on a surface thereof. The carbon layer is laminated on the widening layer, and is in contact with the conductive polymer layer on a side opposite to the widening layer. The amount of particles or powder of the conductive polymer contained in the widening layer is less than the amount of particles or powder of the conductive polymer contained in the carbon layer on a surface side facing the conductive polymer layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a hybrid-type solid electrolytic capacitor that uses a solid electrolyte and an electrolytic solution in combination, and a method for manufacturing the solid electrolytic capacitor. [Background technology]

[0002] Electrolytic capacitors that use valve metals such as tantalum or aluminum can achieve a small size and large capacitance by enlarging the surface of the valve metal serving as the anode-side counter electrode in the form of a sintered body or etched foil. In particular, solid electrolytic capacitors, in which a dielectric oxide film is covered with a solid electrolyte, are small, have large capacitance, and low equivalent series resistance. They are also easy to fabricate into chips and are suitable for surface mounting, making them essential for the miniaturization, high functionality, and cost reduction of electronic devices.

[0003] Known 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 have rapidly become popular as solid electrolytes. An example of such a conductive polymer is poly(3,4-ethylenedioxythiophene) (PEDOT). Conductive polymers exhibit high conductivity when polyanions such as organic sulfonic acids are used as dopants during chemical oxidative polymerization or electrolytic oxidative polymerization, and they also exhibit excellent adhesion to dielectric oxide films.

[0004] Here, defects in the dielectric oxide film formed on the anode foil and the oxide film formed naturally or intentionally on the cathode foil result in large leakage current. Defects in anode and cathode foils are caused, for example, by the heat generated during reflow soldering when mounting solid electrolytic capacitors. When heat is applied to the foil during the reflow process, the oxide film cannot keep up with the expansion of the valve metal due to the difference in thermal expansion coefficient between the valve metal and the oxide film, resulting in defects in the oxide film.

[0005] In liquid-type electrolytic capacitors that use an electrolyte, the electrolyte repairs defects, thereby suppressing leakage current. However, solid electrolytic capacitors, in which the capacitor element is impregnated with an electrolyte and does not have a conductive polymer layer, are less effective at repairing defects in the dielectric oxide film. Therefore, so-called hybrid-type solid electrolytic capacitors, in which a conductive polymer layer is formed on a capacitor element consisting of an anode foil and a cathode foil facing each other and the voids in the capacitor element are impregnated with an electrolyte, have attracted attention. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-114540 Summary of the Invention [Problem to be solved by the invention]

[0007] However, even when a liquid electrolyte is used in combination with a solid electrolytic capacitor, compared to a liquid electrolytic capacitor that does not have a conductive polymer layer, solid electrolytic capacitors tend to have poor repair properties for defects in the oxide film and have large leakage currents.

[0008] The present invention has been proposed to solve the above-mentioned problems, and its purpose is to provide a solid electrolytic capacitor and a method for manufacturing a solid electrolytic capacitor that improves the chances of defect repair and reduces leakage current. [Means for solving the problem]

[0009] In order to solve the above-mentioned problems, the solid electrolytic capacitor of the present invention comprises a capacitor element formed by placing an anode foil and a cathode body opposite each other, a conductive polymer layer formed by impregnating the capacitor element with a dispersion containing conductive polymer particles or powder and a solvent, and an electrolyte solution impregnated into the capacitor element, wherein the cathode body has a cathode foil made of a valve metal and having a surface-expanding layer formed on its surface, and a carbon layer laminated on the surface-expanding layer and in contact with the conductive polymer layer on the side opposite the surface-expanding layer, and wherein the amount of conductive polymer particles or powder contained in the surface-expanding layer is less than the amount of conductive polymer particles or powder contained in the surface-expanding layer of the carbon layer facing the conductive polymer layer.

[0010] Although not limited to this, it is believed that if conductive polymer particles or powder adhere to the location where a defect occurs, the conductive polymer particles or powder act as a barrier, preventing the electrolyte from repairing the defect. Based on this assumption, if the amount of conductive polymer particles or powder adhering to the oxide film can be reduced, the opportunity to repair the defect increases, improving the defect repair function and leading to a reduction in leakage current.

[0011] However, because the conductive polymer layer serves as the true cathode in an electrolytic capacitor, it must be adhered to the dielectric oxide film formed on the anode foil. Meanwhile, the total number of defects can also be reduced by repairing defects that occur naturally or intentionally in the oxide film formed on the cathode foil. Therefore, a carbon layer is disposed on the cathode foil as a barrier to prevent the conductive polymer particles or powder from migrating from the conductive polymer layer to the etching layer where the oxide film is located. Furthermore, the amount of conductive polymer particles or powder contained in the surface-expanding layer is set to be less than the amount of conductive polymer particles or powder contained in the surface-side carbon layer facing the conductive polymer layer.

[0012] This improves the chances of repairing defects on the cathode side, thereby reducing the number of defects in the entire solid electrolytic capacitor and reducing leakage current. While this effect cannot be achieved unless an oxide film is formed on the cathode foil's surface-enlarging layer, this oxide film may be naturally formed or intentionally formed. Furthermore, the carbon layer allows the electrolyte to pass through.

[0013] In addition, since it is only necessary to prevent defects on the cathode side from being blocked by the conductive polymer particles or powder, it is not necessary to have the conductive polymer particles or powder completely absent in the surface-expanding layer. Also, the amount of the conductive polymer particles or powder contained in the carbon layer on the surface-expanding layer side facing the surface-expanding layer may be less than the amount of the conductive polymer particles or powder on the surface-expanding layer side of the carbon layer and more than the amount of the conductive polymer particles or powder in the surface-expanding layer, i.e., a gradient in the density of the conductive polymer particles or powder may be formed in the carbon layer.

[0014] The carbon layer may be compressed and pressed against the etching layer. This results in a more irregular arrangement of the carbon material constituting the carbon layer, resulting in the voids extending from the surface of the carbon layer to the surface-expanding layer being interrupted or the voids extending from the surface of the carbon layer to the surface-expanding layer being serpentine, forming a so-called labyrinth structure. Even if conductive polymer particles or powder enter the voids, they cannot reach the surface-expanding layer and are instead captured by the carbon layer. As a result, the penetration of the conductive polymer particles or powder into the surface-expanding layer is suppressed. Furthermore, the compressed carbon layer more densely distributes the carbon material over the entire surface of the cathode foil, suppressing the penetration of the conductive polymer particles or powder into the surface-expanding layer for the entire cathode body.

[0015] Furthermore, by pressing the carbon layer, the size of the openings of the voids in the carbon layer becomes smaller, and the average void size in the carbon layer becomes smaller than the particle size of the conductive polymer particles or powder that would affect the repair of defects in the oxide film. Furthermore, the carbon material is less likely to be liberated from the carbon layer, which prevents the carbon material from adhering to the dielectric oxide film of the anode foil, reducing its insulating properties, or adhering to defects in the dielectric oxide film.

[0016] To solve the above-mentioned problems, another aspect of the present invention is a method for manufacturing a solid electrolytic capacitor, which method includes the steps of: forming a carbon layer on a surface-expanding layer of a cathode foil made of a valve metal; pressing the carbon layer against the cathode foil; forming a capacitor element by arranging a cathode body including the cathode foil and the carbon layer and an anode foil opposite each other to form a capacitor element; impregnating the capacitor element with a dispersion containing conductive polymer particles or powder and a solvent; and impregnating the capacitor element with an electrolyte solution, thereby compressing the carbon layer and bringing it into pressure contact with the etching layer. [Effects of the Invention]

[0017] According to the present invention, the defect repair function of the oxide film on the cathode side can be improved, and leakage current can be reduced. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a graph showing leakage currents in an example, a comparative example, and a reference example. DETAILED DESCRIPTION OF THE INVENTION

[0019] DETAILED DESCRIPTION OF THE INVENTION A solid electrolytic capacitor and a manufacturing method thereof according to an embodiment of the present invention will be described below, but the present invention is not limited to the embodiment described below.

[0020] (Each part configuration) A solid electrolytic capacitor is a passive device that stores and discharges electric charge through capacitance. It is classified as a hybrid type that uses both a conductive polymer layer and an electrolyte. Hereinafter, hybrid-type solid electrolytic capacitors will be referred to simply as solid electrolytic capacitors. This solid electrolytic capacitor has a wound or stacked capacitor element. The capacitor element includes an anode foil, a cathode body, a conductive polymer layer, an electrolyte, and a separator.

[0021] The anode foil and the cathode foil of the cathode body are foil bodies made of valve metals. 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, and that of the cathode foil is preferably approximately 99% or higher. However, impurities such as silicon, iron, copper, magnesium, and zinc may be included. For example, the cathode foil may be made of aluminum material designated by the temper symbol H in accordance with JIS standard H0001, known as H material, or aluminum material designated by the temper symbol O in accordance with JIS standard H0001, known as O material.

[0022] Anode and cathode foils have a surface-expanding layer formed on one or both surfaces of the foil. The surface-expanding layer is formed by electrolytic etching, chemical etching, sandblasting, or by vapor-depositing or sintering metal particles onto the foil. That is, the surface-expanding layer consists of tunnel-shaped pits, spongy pits, or voids between densely packed powder particles. Examples of electrolytic etching include DC etching and AC etching, in which DC or AC is applied in an acidic aqueous solution containing halogen ions, such as hydrochloric acid. Chemical etching involves immersing the metal foil in an acid or alkaline solution. The tunnel-shaped pits may be formed long enough to penetrate the foil or may not reach the center of the foil.

[0023] The dielectric oxide film on the anode foil is typically an oxide film formed on the surface of the anode foil. If the anode foil is made of aluminum, it is aluminum oxide formed by oxidizing the surface of the surface-expanding layer. This dielectric oxide film is intentionally formed by chemical conversion treatment in which a voltage is applied in a solution free of halogen ions, such as an aqueous solution of adipic acid, boric acid, or phosphoric acid. An oxide film is also formed on the surface of the cathode foil either intentionally by this chemical conversion treatment or naturally. The natural oxide film that naturally forms on the surface of the cathode foil is formed by the cathode foil reacting with oxygen in the air.

[0024] The cathode body includes a carbon layer in addition to the cathode foil. The carbon layer is laminated on the surface-expanding layer of the cathode foil. The carbon layer is a layer containing a carbon material. The carbon material is fibrous carbon, carbon powder, or a mixture thereof. The fibrous carbon or carbon powder is preferably subjected to a porosity treatment such as an activation treatment or an opening treatment to form pores.

[0025] Examples of carbon powders include natural plant tissues such as coconut husks, synthetic resins such as phenols, activated carbon derived from fossil fuels such as coal, coke, and pitch, carbon blacks such as ketjen black, acetylene black, channel black, and thermal black, carbon nanohorns, amorphous carbon, natural graphite, artificial graphite, graphitized ketjen black, and mesoporous carbon. Examples of fibrous carbon include carbon nanotubes and carbon nanofibers. Carbon nanotubes may be single-walled carbon nanotubes, which have a single graphene sheet, or multi-walled carbon nanotubes (MWCNTs), which have two or more graphene sheets rolled coaxially to form multiple tube walls.

[0026] Methods for forming a carbon layer on a cathode foil include vacuum deposition, sputtering, ion plating, CVD, coating, electrolytic plating, and electroless plating. In the coating method, a carbon material is dispersed in a dispersion solvent to prepare a slurry, which is then coated on the cathode foil by a slurry casting method, doctor blade method, spray atomization method, or the like, and dried. In the deposition method, the carbon material is evaporated by heating with electrical current in a vacuum, or by irradiating the carbon material with an electron beam in a vacuum, forming a film of the carbon material on the cathode foil. In the sputtering method, a target made of a carbon material and the cathode foil are placed in a vacuum chamber, and an inert gas is introduced into the vacuum chamber and a voltage is applied, causing the plasma-generated inert gas to collide with the target, and carbon material particles knocked out from the target are deposited on the cathode foil.

[0027] The conductive polymer layer is a layer containing particles or powder of a conductive polymer. In this specification, the term "conductive polymer" refers to a polymer that is conductive, and also includes a conductive polymer compound composed of a conductive polymer and a dopant. Furthermore, in this specification, the term "particles or powder of a conductive polymer" refers to a particulate or powdery conductive polymer, and also includes aggregates formed by agglomeration of conductive polymer particles or powder.

[0028] The conductive polymer is a conjugated polymer or a doped conjugated polymer. Any known conjugated polymer can be used without any particular limitation. Examples include polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene, polyphenylene vinylene, polyacene, and polythiophene vinylene. A representative conductive polymer is poly(3,4-ethylenedioxythiophene), also known as PEDOT, doped with polystyrene sulfonic acid (PSS). These conjugated polymers may be used alone or in combination of two or more types, or may even be copolymers of two or more types of monomers.

[0029] The solvent for the electrolyte is not particularly limited, and protic organic polar solvents or aprotic organic polar solvents can be used. Representative examples of protic polar solvents include monohydric alcohols, polyhydric alcohols, oxyalcohol compounds, and water. Representative examples of aprotic polar solvents include sulfones, amides, lactones, cyclic amides, nitriles, and oxides including sulfoxides.

[0030] The solute contained in the electrolytic solution includes anionic and cationic components, and is typically an organic acid or its salt, an inorganic acid or its salt, or a composite compound of an organic acid and an inorganic acid or an ionically dissociable salt thereof, and is used alone or in combination of two or more. An acid that becomes an anion and a base that becomes a cation may be added separately to the electrolytic solution as solute components.

[0031] Furthermore, other additives can be added to the electrolyte. Examples of additives include polyethylene glycol, complex compounds of boric acid and polysaccharides (e.g., mannitol, sorbitol), complex compounds of boric acid and polyhydric alcohols, boric acid esters, nitro compounds, phosphate esters, and colloidal silica. These may be used alone or in combination of two or more. Nitro compounds suppress the generation of hydrogen gas in electrolytic capacitors. Examples of nitro compounds include o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, and p-nitrophenol.

[0032] Examples of materials for the separator include cellulose papers such as kraft, Manila hemp, esparto, hemp, and rayon, and mixtures thereof; polyester-based resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and derivatives thereof; polytetrafluoroethylene-based resins, polyvinylidene fluoride-based resins, vinylon-based resins; polyamide-based resins such as aliphatic polyamides, semi-aromatic polyamides, and wholly aromatic polyamides; polyimide-based resins, polyethylene resins, polypropylene resins, trimethylpentene resins, polyphenylene sulfide resins, acrylic resins, and polyvinyl alcohol resins. These resins can be used alone or in combination.

[0033] The separator serves to retain the conductive polymer layer and the electrolyte and to prevent short-circuiting between the anode foil and the cathode body. If the conductive polymer layer can maintain its shape without a separator, each part of the capacitor element including the conductive polymer layer can retain the electrolyte, and the conductive polymer layer has a thickness sufficient to prevent short-circuiting between the anode foil and the cathode body, then a separator may be unnecessary.

[0034] (Overall composition) A cathode body is formed by forming a surface-enlarging layer on the cathode foil and anode foil, forming a dielectric oxide film on the anode foil, and laminating a carbon layer on the cathode foil by coating or the like. The anode foil and cathode body are placed opposite each other with a separator interposed between them to form a capacitor element. A conductive polymer layer is formed within the capacitor element by impregnating this capacitor element with a dispersion containing conductive polymer particles or powder and a solvent. In this specification, a "dispersion containing conductive polymer particles or powder" may also be referred to as a "conductive polymer dispersion."

[0035] The solvent for conductive polymer dispersions is generally water, as long as it disperses conductive polymer particles or powder. However, ethylene glycol may also be used as the dispersion solvent if necessary. It has been shown that using ethylene glycol as the dispersion solvent can reduce the electrical properties of the product, particularly the ESR characteristics. To improve the impregnation and conductivity of conductive polymer dispersions, various additives may be added to the conductive polymer dispersion or neutralization may be performed by adding cations. Conductive polymer dispersions can be prepared by preparing particles or powder of polyethylenedioxythiophene (PEDOT), a conductive polymer doped with polystyrene sulfonate (PSS), and dispersing them in a solvent. Alternatively, polystyrene sulfonate (PSS) and ethylenedioxythiophene (EDOT) particles or powder can be mixed in water and polymerized in the water to form a conductive polymer dispersion.

[0036] The impregnation method of the conductive polymer dispersion may be immersing the capacitor element in the conductive polymer dispersion, or by dripping or spraying. Furthermore, the dispersion may be impregnated not only into the entire capacitor element, but also into the anode foil or cathode body, and then the capacitor element may be assembled. To promote the impregnation of the conductive polymer dispersion into the capacitor element, a decompression treatment or a pressurization treatment may be performed as necessary. This deposition process may be repeated multiple times.

[0037] As a result, in the solid electrolytic capacitor, the carbon layer is formed on the surface-expanding layer of the cathode foil, and the conductive polymer layer is in close contact with the dielectric oxide film on the anode side and on the carbon layer, i.e., the side opposite the surface-expanding layer, on the cathode side. The carbon material constituting the carbon layer is arranged more irregularly, resulting in the voids extending from the surface of the carbon layer to the surface-expanding layer being interrupted or meandering, creating a so-called labyrinth structure. Therefore, conductive polymer particles or powder are trapped in the carbon layer rather than passing through the surface-expanding layer from the surface of the carbon layer, thereby preventing the conductive polymer particles or powder from penetrating the surface-expanding layer.

[0038] Furthermore, when the carbon layer is divided in the depth direction from the conductive polymer layer side to the surface expansion layer side, the surface side of the carbon layer is the area of ​​the carbon layer that faces the conductive polymer layer, and the surface expansion layer side of the carbon layer is the area of ​​the carbon layer that faces the surface expansion layer.

[0039] Furthermore, the average size of the voids in the carbon layer that connect the surface-expanding layer and the conductive polymer layer may be equal to or smaller than the median diameter of the conductive polymer particles or powder. For example, the voids in the carbon layer may average several hundred nanometers, while the median diameter per particle of the conductive polymer particles or powder may be approximately 450 nm. Therefore, when the conductive polymer layer is formed, fewer conductive polymer particles or powder in the dispersion pass through the carbon layer and are present on the surface-expanding layer side.

[0040] The voids extending from the surface side of the carbon layer to the surface-expanding layer may be cut off midway, or the voids extending from the surface side of the carbon layer to the surface-expanding layer may be made to snake to form a so-called labyrinth structure, or the size of the voids extending from the surface side of the carbon layer to the surface-expanding layer may be made smaller than the size of the conductive polymer particles or powder, or in order to use these in combination, it is preferable to compress the carbon layer and press it against the surface-expanding layer.

[0041] To compress the carbon layer and press it onto the surface-expanding layer, for example, a pressing process is performed in which the carbon layer is pressed against the cathode foil. In the pressing process, the cathode body consisting of the carbon layer and the cathode foil is sandwiched between press rollers and a linear pressure is applied. The linear pressure is 0.01 to 100 t / cm. 2 Furthermore, compressing the carbon layer and pressing it against the surface-expanding layer reduces the risk of carbon material being released from the carbon layer and reaching the anode foil. This reduces the amount of carbon material that reaches the anode foil and adheres to the dielectric oxide film, reducing its insulating properties, or that adheres to defects in the dielectric oxide film, hindering the repair of those defects.

[0042] The carbon material is preferably carbon black, which is spherical carbon. By using spherical carbon black with an average primary particle size of 100 nm or less, the carbon layer becomes dense and is easily attached to the surface-expanding layer, thereby reducing the gap connecting the conductive polymer layer and the surface-expanding layer.

[0043] The carbon material contained in the carbon layer may be a combination of flake-like or scaly graphite and spherical carbon, i.e., carbon black. The flake-like or scaly graphite preferably has an aspect ratio of its minor axis to its major axis in the range of 1:5 to 1:100. When a carbon layer containing this combination of carbon materials is laminated on a cathode foil, compressed, and pressed against the surface-expanding layer, the carbon black is easily rubbed into the surface-expanding layer by the graphite. The graphite easily deforms along the uneven surface of the surface-expanding layer and easily accumulates on the uneven surface. The graphite then acts as a pressure lid, holding the spherical carbon within the surface-expanding layer. This further enhances the adhesion and fixation between the carbon layer and the cathode foil, thereby reducing voids. Furthermore, using flake-like or scaly graphite as the carbon material extends the path through which conductive polymer particles or powder pass from the surface layer side to the surface-expanding layer side within the carbon layer, thereby further inhibiting the conductive polymer particles or powder from passing through the carbon layer.

[0044] The electrolyte can pass through the carbon layer, and by impregnating the capacitor element with the electrolyte, the electrolyte is filled in the conductive polymer layer, the carbon layer, and the surface-expanding layer.

[0045] In such solid electrolytic capacitors, the presence of the carbon layer prevents many of the conductive polymer particles or powder from migrating to the surface-expanding layer. Furthermore, the average size of the voids in the carbon layer that connect the surface-expanding layer and the conductive polymer layer is smaller than the particle size of the conductive polymer particles or powder that would affect the repair of defects in the oxide film, so many of the conductive polymer particles or powder in the dispersion are unable to migrate to the surface-expanding layer. In other words, the amount of conductive polymer particles or powder present in the surface-expanding layer is smaller than that present on the surface of the carbon layer.

[0046] In addition, the amount of conductive polymer particles or powder on the surface-expanding layer side of the carbon layer is less than the amount of conductive polymer particles or powder on the surface-expanding layer side of the carbon layer, and is greater than the amount of conductive polymer particles or powder within the surface-expanding layer. In other words, the amount of conductive polymer particles or powder gradually decreases from the surface side of the carbon layer toward the surface-expanding layer.

[0047] In this way, even if a defect occurs in the oxide film in the surface-expanding layer, the defect is blocked by the conductive polymer particles or powder, reducing the possibility that the electrolyte will be unable to reach the defect, increasing the chance of repairing the defect, and thus suppressing the leakage current of the solid electrolytic capacitor.

[0048] The amount of conductive polymer particles or powder can be confirmed by a known method. For example, the amount of conductive polymer particles or powder in the cross section of the cathode body can be calculated by combining elemental analysis such as SEM-EDX (scanning electron microscope-energy dispersive X-ray analysis). [Example]

[0049] The solid electrolytic capacitor and the manufacturing method of the present invention will be described in more detail below based on examples, but the present invention is not limited to the following examples.

[0050] Aluminum foil was selected as the anode and cathode foils. The anode and cathode foils were subjected to AC etching, forming a surface-enlarging layer consisting of spongy etching pits on both sides of the foil. In the AC etching process, the cathode foil was immersed in an acidic aqueous solution containing approximately 8% by weight of hydrochloric acid as the main electrolyte at a liquid temperature of 25°C, and an AC current of 10 Hz and a current density of 0.14 A / cm was applied. 2 A current of 1000kJ / cm2 was applied to the substrate for approximately 5 minutes. Furthermore, a chemical conversion treatment was applied to the anode and cathode foils, forming a dielectric oxide film on the surface of the surface-expanding layer of the anode foil and an oxide film on the surface of the surface-expanding layer of the cathode foil. For the chemical conversion treatment, after removing the chlorine that had adhered during the AC etching process with a phosphoric acid solution, a voltage was applied in an ammonium dihydrogen phosphate solution.

[0051] A carbon layer was laminated on the surface-expanding layer of the cathode foil, completing a cathode body comprising the cathode foil and carbon layer. Carbon black was selected as the carbon material for the carbon layer. Carbon black powder, styrene butadiene rubber (SBR) as a binder, and a carboxymethyl cellulose sodium (CMC-Na) aqueous solution as a dispersant-containing solution were mixed and kneaded to prepare a slurry, which was then uniformly applied to the cathode foil. The slurry was then heated and dried to volatilize the solvent.

[0052] After the carbon layer was formed on the surface-expanding layer of the cathode foil, a pressing process was carried out to press the carbon layer onto the surface-expanding layer. In the pressing process, the cathode body was sandwiched between press rollers and pressed with a pressure of 5.38 kNcm. -1 The pressure was applied using a press machine manufactured by Takumi Giken Co., Ltd. The diameter of the press roller was 180 mm, the width of the press treatment was 130 mm, and the cathode body was transported once at 3 m / min.

[0053] An aluminum tab-shaped lead terminal was stitched to each of the anode foil and cathode body. A separator was sandwiched between the anode foil and cathode body, and the two were then wound together to create a capacitor element comprising the anode foil, cathode body, and separator. A manila separator was used as the separator. After winding, the capacitor element was immersed in an ammonium dihydrogen phosphate aqueous solution and an electric current was passed through it to repair any defects that had occurred during winding. The capacitor element was then removed from the ammonium dihydrogen phosphate aqueous solution and dried in a temperature environment of 105°C for 30 minutes.

[0054] Next, a conductive polymer dispersion was prepared. This dispersion consisted of a powder of polyethylenedioxythiophene (PEDOT), a conductive polymer doped with polystyrene sulfonic acid (PSS), dispersed in water. The capacitor element was immersed in this dispersion. While immersed, it was exposed to a pressure environment of 30 kPa for 120 seconds. After this, the capacitor element was removed and dried at 150°C for 30 minutes. This immersion and drying process was repeated twice. This resulted in a conductive polymer layer containing polyethylenedioxythiophene (PEDOT) doped with polystyrene sulfonic acid (PSS) as the conductive polymer, which was adhered to the dielectric oxide film of the anode foil and also laminated on the carbon layer of the cathode body.

[0055] Next, an electrolyte solution was prepared, and the capacitor element with the conductive polymer layer formed thereon was impregnated with the electrolyte. The electrolyte solution was prepared by adding ammonium azelaate as a solute to ethylene glycol as a solvent. The capacitor element was inserted into a cylindrical outer case with a bottom, and a rubber seal was attached to the open end and sealed by crimping.

[0056] The solid electrolytic capacitor was subjected to an aging treatment. The solid electrolytic capacitor of the example thus fabricated had a rated withstand voltage of 25 WV, a rated capacitance of 270 μF, and a size of 10 mm in diameter and 8 mm in height.

[0057] For comparison with the solid electrolytic capacitors of the Examples, comparative solid electrolytic capacitors were fabricated as follows. The cathode body of the comparative solid electrolytic capacitor includes only a cathode foil. No carbon layer is laminated on the surface-expanding layer of the cathode foil. Except for this, the comparative solid electrolytic capacitor was fabricated with the same configuration, composition, manufacturing method, and conditions as the solid electrolytic capacitors of the Examples.

[0058] In addition, a reference example solid electrolytic capacitor was fabricated as follows for comparison with the example solid electrolytic capacitor. The cathode body of the reference example solid electrolytic capacitor had a carbon layer made of carbon nanotubes on the surface expansion layer of the cathode foil. However, to reduce adhesion with the carbon layer, the surface expansion layer was not formed on the cathode foil, and the pressing step of pressing the carbon layer against the cathode foil was also omitted. Except for this, the reference example solid electrolytic capacitor was fabricated with the same configuration, composition, manufacturing method, and conditions as the example solid electrolytic capacitor.

[0059] Thirty solid electrolytic capacitors each of the Example, Comparative Example, and Reference Example were fabricated, and the leakage current (LC) of each was measured. To measure the leakage current, the solid electrolytic capacitors were placed in a 20°C environment, a constant voltage of 25V was applied, and the leakage current was measured after a predetermined period of time. The average, maximum, and minimum leakage current values ​​for the Example, Comparative Example, and Reference Example obtained as a result of the measurements are shown in Table 1 below. Based on Table 1, Figure 1 was also created, with the Example, Comparative Example, and Reference Example series arranged on the horizontal axis and the leakage current on the vertical axis.

[0060] (Table 1) TIFF2025147196000002.tif56161

[0061] The working example is a solid electrolytic capacitor in which, first, a surface-expanding layer is formed on the cathode foil, second, a carbon layer is formed on the surface-expanding layer, and third, a pressing process is performed to press the carbon layer against the surface-expanding layer, thereby compressing the carbon layer and pressing it against the surface-expanding layer. On the other hand, the comparative example does not have any carbon layer to block the surface-expanding layer and prevent communication between the conductive polymer layer and the surface-expanding layer. Furthermore, the reference example has a carbon layer that blocks the surface-expanding layer, but no surface-expanding layer is formed on the cathode foil, and the pressing process to press the carbon layer against the cathode foil has not been performed.

[0062] 1, the solid electrolytic capacitors of the Comparative Example and Reference Example did not exhibit suppressed leakage current, while the solid electrolytic capacitors of the Example exhibited lower leakage current than the solid electrolytic capacitors of the Comparative Example and Reference Example. That is, in the solid electrolytic capacitors of the Example, by pressing the carbon, it was possible to break the voids formed by the carbon black (carbon material) from the surface side of the carbon layer toward the surface-expanding layer midway, or to create a so-called labyrinth structure in which the voids from the surface side of the carbon layer toward the surface-expanding layer snake. This confirmed that the conductive polymer could not migrate through the carbon layer to the oxide film of the surface-expanding layer and adhere thereto, preventing defects from being blocked by the conductive polymer and increasing the opportunity for the defects to be repaired by the electrolyte.

[0063] In the solid electrolytic capacitor of the Reference Example, it was confirmed that simply attaching a carbon layer to the cathode foil without forming a surface-expanding layer on the cathode foil and without controlling the voids in the carbon layer to the size as in the Examples would actually worsen the leakage current. It is believed that the carbon material liberated from the carbon layer adheres to the dielectric oxide film of the anode foil, reducing the insulating properties of the dielectric oxide film and sealing defects in the dielectric oxide film.

Claims

1. a capacitor element formed by opposing an anode foil and a cathode body; a conductive polymer layer formed by impregnation with a dispersion containing conductive polymer particles or powder and a solvent; an electrolyte impregnated in the capacitor element; Equipped with The cathode body is a cathode foil made of a valve metal and having a surface-expanding layer formed on its surface; a carbon layer laminated on the surface-expanding layer and in contact with the conductive polymer layer on the surface opposite to the surface-expanding layer; and the amount of the conductive polymer particles or powder contained in the surface-expanding layer is smaller than the amount of the conductive polymer particles or powder contained in the surface layer side of the carbon layer facing the conductive polymer layer; A solid electrolytic capacitor characterized by:

2. the amount of the conductive polymer particles or powder contained in the carbon layer on the surface-expanding layer side facing the surface-expanding layer is less than the amount of the conductive polymer particles or powder on the surface layer side of the carbon layer and is greater than the amount of the conductive polymer particles or powder in the surface-expanding layer; 2. The solid electrolytic capacitor according to claim 1,

3. the electrolyte can pass through the carbon layer; 3. The solid electrolytic capacitor according to claim 1, wherein:

4. the carbon layer is compressed and pressed against the surface-expanding layer; 4. The solid electrolytic capacitor according to claim 1, wherein:

5. the carbon layer includes voids formed by the plurality of carbon materials, the average size of the voids is equal to or smaller than the median diameter of the conductive polymer particles or powder; 5. The solid electrolytic capacitor according to claim 1, wherein:

6. an oxide film is formed on the surface-expanding layer; 6. The solid electrolytic capacitor according to claim 1, wherein:

7. a carbon layer forming step of forming a carbon layer on the surface-expanding layer of the valve metal cathode foil; a pressing step of pressing the carbon layer against the cathode foil; an element forming step of forming a capacitor element by placing an anode foil and a cathode body including the cathode foil and the carbon layer opposite each other; a dispersion impregnation step of impregnating the capacitor element with a dispersion containing conductive polymer particles or powder and a solvent; an electrolyte impregnation step of impregnating the capacitor element with an electrolyte; containing, A method for manufacturing a solid electrolytic capacitor, comprising:

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