Solid electrolytic capacitor element and solid electrolytic capacitor

Abstract

This solid electrolytic capacitor element to be included in a solid electrolytic capacitor comprises: a positive electrode body; a dielectric layer formed on a surface of the positive electrode body; and a negative electrode part covering at least part of the dielectric layer. The negative electrode part includes a solid electrolyte layer covering at least part of the dielectric layer. A metal-particle-containing layer is included in at least part of the negative electrode part. Metal particles contained in the metal-particle-containing layer include first metal particles that contain silver. The first metal particles each include a core particle that contains silica and a silver-containing coating layer that covers the core particle.

Description

Solid electrolytic capacitor element and solid electrolytic capacitor

[0001] The present disclosure relates to a solid electrolytic capacitor element and a solid electrolytic capacitor.

[0002] The solid electrolytic capacitor includes a solid electrolytic capacitor element, an exterior housing that seals the solid electrolytic capacitor element, and an external electrode electrically connected to the solid electrolytic capacitor element. The solid electrolytic capacitor element includes an anode body, a dielectric layer formed on the surface of the anode body, and a cathode section that covers at least a portion of the dielectric layer. The cathode section includes, for example, a solid electrolyte layer containing a conductive polymer that covers at least a portion of the dielectric layer, and a cathode extraction layer that covers at least a portion of the solid electrolyte layer. The cathode extraction layer includes, for example, a carbon layer that covers at least a portion of the solid electrolyte layer and a metal particle-containing layer that covers at least a portion of the carbon layer. The cathode extraction layer is electrically connected to a cathode-side external electrode via a cathode lead.

[0003] To ensure high conductivity, metal particle-containing layers are often formed using a conductive paste containing silver particles and a resin binder, but this has drawbacks, such as high costs due to the high cost of silver particles.

[0004] It has been proposed to use copper particles instead of silver particles in the metal particle-containing layer. For example, Patent Document 1 proposes a tantalum solid electrolytic capacitor using a sintered body made of fine powder of a valve metal, in which an oxide film layer, a manganese dioxide layer, and a conductive layer made of fine powder of carbon are sequentially formed on the sintered body, and a conductive paste containing copper powder is formed thereon.

[0005] Japanese Patent Application Publication No. 4-85915

[0006] Solid electrolytic capacitors are generally soldered to a substrate through a reflow process that exposes them to high temperatures. Although using copper particles instead of silver particles in the conductive paste used for the cathode can reduce costs, solid electrolytic capacitors with a solid electrolyte layer containing a conductive polymer suffer from a significant increase in leakage current after exposure to high temperatures.

[0007] A first aspect of the present disclosure relates to a solid electrolytic capacitor element including an anode body, a dielectric layer formed on a surface of the anode body, and a cathode section covering at least a portion of the dielectric layer, wherein the cathode section includes a solid electrolyte layer covering at least a portion of the dielectric layer and includes a metal particle-containing layer in at least a portion of the cathode section, the solid electrolyte layer includes a conductive polymer, the metal particles included in the metal particle-containing layer include first metal particles containing silver, and the first metal particles include core particles containing silica and a silver-containing coating layer coating the core particles.

[0008] A second aspect of the present disclosure relates to a solid electrolytic capacitor including at least one of the above solid electrolytic capacitor elements and an exterior body that seals the solid electrolytic capacitor element.

[0009] This reduces the manufacturing cost of the solid electrolytic capacitor and reduces leakage current after exposure to high temperatures.

[0010] FIG. 1 is a cross-sectional schematic view of a solid electrolytic capacitor according to an embodiment of the present disclosure.

[0011] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0012] The metal particles used in the metal particle-containing layer that constitutes part of the cathode of a solid electrolytic capacitor are required to have high electrical conductivity. The content of the metal particles in the metal particle-containing layer is relatively high (e.g., 80 mass% or more). Therefore, using copper particles or the like as highly conductive metal particles instead of silver particles is expected to significantly reduce costs.

[0013] Solid electrolytic capacitors are generally soldered to a substrate through a reflow process in which they are exposed to high temperatures. Depending on the application, solid electrolytic capacitors may also be used in high-temperature environments. When copper particles are used in a metal particle-containing layer of a solid electrolytic capacitor, even if the initial leakage current is small, the leakage current may increase after the solid electrolytic capacitor is exposed to high temperatures. Furthermore, a large leakage current may cause a short circuit, resulting in an increase in the product defect rate (hereinafter sometimes referred to as the short-circuit defect rate).

[0014] The reason why leakage current increases after exposure to high temperatures in solid electrolytic capacitors with a metal particle-containing layer containing copper particles is thought to be due to the following reasons. When a solid electrolytic capacitor is exposed to high temperatures, the copper particles, which are more easily ionized than silver particles, ionize and migrate from the metal particle-containing layer to the solid electrolyte layer due to the effects of heat or gas generated from the polymer film. The copper ions easily interact with the conductive polymer components (e.g., dopants) contained in the solid electrolyte layer, causing the copper ions to migrate into the solid electrolyte layer. The copper ions interact or react with surrounding components (e.g., conductive polymer components) or are reduced. The copper-containing components then reach the dielectric layer. When a solid electrolytic capacitor is exposed to high temperatures, organic components such as the resin binder or its cured product contained in the cathode expand, generating internal stress. This stress is transmitted to the very thin dielectric layer, making it susceptible to damage. If copper components (e.g., copper ions, components resulting from interaction of copper ions with components of a conductive polymer, metallic copper, copper compounds, or other conductive components containing copper) are present in the damaged portion of the dielectric layer, the anode body and the cathode portion become electrically conductive via the copper components, resulting in a relatively large leakage current. If a solid electrolytic capacitor absorbs moisture from the atmosphere during storage, exposure to high temperatures can further increase the stress applied to the dielectric layer, making it more likely to be damaged. Therefore, in this case, the leakage current becomes even more pronounced. In addition, in capacitors without a solid electrolyte layer containing a conductive polymer, there is almost no interaction between copper ions and components of the conductive polymer, and therefore the migration of copper ions toward the dielectric layer itself is suppressed.

[0015] In view of the above, (1) a solid electrolytic capacitor element according to a first aspect of the present disclosure includes an anode body, a dielectric layer formed on the surface of the anode body, and a cathode section covering at least a portion of the dielectric layer. The cathode section includes a solid electrolyte layer covering at least a portion of the dielectric layer, and includes a metal particle-containing layer in at least a portion of the cathode section. The solid electrolyte layer includes a conductive polymer. The metal particles included in the metal particle-containing layer include first metal particles containing silver. The first metal particles include core particles containing silica and a silver-containing coating layer coating the core particles.

[0016] In the solid electrolytic capacitor element of the present disclosure, the first metal particles contain core particles containing silica, thereby reducing the silver content in the metal particle-containing layer. This allows costs to be kept low. Furthermore, both silver and silica are difficult to ionize and barely interact with the components of the conductive polymer. Therefore, the migration of ions of the components of the first metal particles to the solid electrolyte layer, as occurs with copper particles, is suppressed. This reduces leakage current after the solid electrolytic capacitor is exposed to high temperatures. Since the generation of large leakage current is suppressed, the short-circuit defect rate can be reduced. Furthermore, the present disclosure allows leakage current to be kept low even when the solid electrolytic capacitor is exposed to a high-humidity environment (including a high-temperature, high-humidity environment). In other words, the solid electrolytic capacitor exhibits excellent moisture resistance, ensuring high reliability. The present disclosure also ensures high moisture resistance comparable to that of a conventional silver paste layer containing silver particles.

[0017] The coating layer of the first metal particles contains silver, which makes it easier to ensure high conductivity of the metal particle-containing layer. Because the core particles of the first metal particles are silica, they have a lower specific gravity than particles formed entirely of metal. Therefore, the paste for forming the metal particle-containing layer can cover the solid electrolyte layer with a smaller mass. By using such a paste, it is possible to form a metal particle-containing layer with high conductivity while reducing the cost per unit volume. Therefore, the initial equivalent series resistance (ESR) of the solid electrolytic capacitor can be kept low. In addition, the coating layer of the first metal particles contains silver, which suppresses oxidative degradation of the first metal particles, even when exposed to high-temperature or high-humidity environments, compared to copper particles. Therefore, by using the first metal particles, it is possible to suppress the increase in ESR when exposed to high temperatures or high-temperature, high-humidity environments, compared to copper particles. Therefore, the high reliability of the solid electrolytic capacitor can be ensured.

[0018] (2) In the above (1), the ratio of the first metal particles to the total metal particles may be 10 mass % or more. Cost reduction effects can be achieved depending on the ratio of the first metal particles. Furthermore, compared to the case of copper particles, the leakage current suppression effect can be enhanced depending on the ratio of the first metal particles.

[0019] (3) In the above (1) or (2), the average aspect ratio of the core particles may be from 1 to 10. When the aspect ratio of the core particles is in this range, it is relatively easy to ensure contact between the first metal particles and to highly disperse the first metal particles in the paste.

[0020] (4) In any one of (1) to (3) above, the average proportion of the silver-containing coating layer in the first metal particles may be 0.1% by mass or more and 50% by mass or less. When the proportion of the silver-containing coating layer is in this range, it is easy to balance low cost and high conductivity.

[0021] (5) In any one of (1) to (4) above, the metal particles may include second metal particles containing silver. Here, the second metal particles are at least one type selected from the group consisting of silver particles and silver alloy particles. By including the second metal particles containing silver in the metal particles, higher conductivity of the metal particle-containing layer can be ensured, and the ESR of the solid electrolytic capacitor can be kept low. Furthermore, an increase in the ESR of the solid electrolytic capacitor can be suppressed even after exposure to a high-temperature environment or a high-temperature, high-humidity environment, ensuring high reliability.

[0022] (6) The present disclosure also includes a solid electrolytic capacitor including at least one solid electrolytic capacitor element according to any one of (1) to (5) above and an exterior body that seals the solid electrolytic capacitor element.

[0023] (7) In the above (6), the solid electrolytic capacitor may include a plurality of stacked solid electrolytic capacitor elements.

[0024] In this specification, the metal particle-containing layer containing the first metal particles may be referred to as the “first metal particle-containing layer.” Furthermore, the solid electrolytic capacitor element may be simply referred to as the “capacitor element.”

[0025] The cathode portion includes, for example, a solid electrolyte layer and a cathode extraction layer that covers at least a portion of the solid electrolyte layer. When the cathode extraction layer and the cathode lead are connected with a conductive adhesive, the cathode portion herein also includes a conductive adhesive layer (hereinafter sometimes referred to as a first conductive adhesive layer) interposed between the cathode extraction layer and the cathode lead. In a solid electrolytic capacitor including multiple capacitor elements, when the multiple capacitor elements are fixed with a conductive adhesive, the cathode portion herein also includes a conductive adhesive layer (hereinafter sometimes referred to as a second conductive adhesive layer) that fixes adjacent capacitor elements together (more specifically, the cathode portion of one of the capacitor elements).

[0026] In any one of the above (1) to (7), the cathode section may include a first metal particle-containing layer in at least a portion of at least one selected from the group consisting of the cathode extraction layer, the first conductive adhesive layer, and the second conductive adhesive layer. For example, the cathode extraction layer may include a first layer (also referred to as a carbon layer) that contains conductive carbon and covers at least a portion of the solid electrolyte layer, and the first metal particle-containing layer serving as a second layer that covers at least a portion of the first layer. The cathode section may include a metal particle-containing layer other than the first metal particle-containing layer (hereinafter sometimes referred to as a second metal particle-containing layer or a third metal particle-containing layer). For example, the cathode extraction layer may include a carbon layer serving as the first layer and a second metal particle-containing layer serving as the second layer, and the first metal particle-containing layer may serve as a first conductive adhesive layer interposed between the second metal particle-containing layer and the cathode lead. The solid electrolytic capacitor may also include a laminate in which a plurality of capacitor elements, each including a first layer and a cathode extraction layer including a second metal particle-containing layer as a second layer, are stacked via a first metal particle-containing layer as a second conductive adhesive layer. In such a laminate, the cathode extraction layer and the cathode lead of each capacitor element may be connected via a third metal particle-containing layer or the first metal particle-containing layer as a first conductive adhesive layer.

[0027] The capacitor element and solid electrolytic capacitor of the present disclosure will be described in more detail below, including the above (1) to (7). At least one selected from the components described below can be arbitrarily combined with at least one of the above (1) to (5) for the solid electrolytic capacitor element of the present disclosure and (6) to (7) for the solid electrolytic capacitor, as long as such combination is technically possible.

[0028] [Solid Electrolytic Capacitor] A solid electrolytic capacitor includes one or more capacitor elements.

[0029] (Capacitor Element) (Anode Body) The anode body included in the capacitor element may contain a valve metal, an alloy containing a valve metal, or a compound containing a valve metal. The anode body may contain one of these materials or a combination of two or more of them. Examples of valve metals include aluminum, tantalum, niobium, and titanium.

[0030] The anode body has a porous portion at least in its surface layer. Such a porous portion provides the anode body with fine irregularities at least on its surface. An anode body having a porous portion in its surface layer can be obtained, for example, by roughening the surface of a substrate containing a valve metal (such as a sheet-like (e.g., foil-like, plate-like) substrate). The roughening may be performed, for example, by etching. The anode body may also be a compact or sintered body of particles containing a valve metal. The compact and the sintered body may each constitute a porous portion entirely. The compact and the sintered body may each be in the form of a sheet, a rectangular parallelepiped, a cube, or a similar shape.

[0031] The anode body typically has an anode lead portion and a cathode forming portion. The porous portion may be formed in the cathode forming portion, or may be formed in the cathode forming portion and the anode lead portion. The cathode portion is typically formed in the cathode forming portion of the anode body via a dielectric layer. The anode lead portion is used, for example, for electrical connection with an external electrode on the anode side.

[0032] (Dielectric Layer) The dielectric layer is formed, for example, so as to cover at least a portion of the surface of the anode body. The dielectric layer is an insulating layer that functions as a dielectric. The dielectric layer is formed by anodizing a valve metal on the surface of the anode body using a chemical conversion treatment or the like. Because the dielectric layer is formed on the porous surface of the anode body, the surface of the dielectric layer has a fine uneven shape as described above.

[0033] The dielectric layer contains an oxide of the valve metal. For example, when tantalum is used as the valve metal, the dielectric layer contains Ta. ２ O ５ When aluminum is used as the valve metal, the dielectric layer contains Al ２ O ３The dielectric layer is not limited to these examples, but may be any layer that functions as a dielectric.

[0034] (Cathode part) The cathode part is formed so as to cover at least a part of the dielectric layer formed on the surface of the anode body. Each layer constituting the cathode part can be formed by a known method according to the layer configuration of the cathode part.

[0035] The cathode section includes, for example, a solid electrolyte layer covering at least a portion of the dielectric layer and a cathode extraction layer covering at least a portion of the solid electrolyte layer. The cathode section may further include a first conductive adhesive layer interposed between the cathode extraction layer and the cathode lead. The cathode section may also include a second conductive adhesive layer that fixes adjacent capacitor elements together.

[0036] As described above, the first metal particle-containing layer may be included in at least a portion of at least one selected from the group consisting of the cathode extraction layer, the first conductive adhesive layer, and the second conductive adhesive layer. The cathode extraction layer, which is closer to the solid electrolyte layer, has a greater effect on leakage current than the first conductive adhesive layer and the second conductive adhesive layer. In the present disclosure, when the cathode portion includes the first metal particle-containing layer at least in the cathode extraction layer, the effect of reducing leakage current after the solid electrolytic capacitor is exposed to high temperatures is more pronounced.

[0037] The components of the cathode section will be described below.

[0038] (Solid Electrolyte Layer) The solid electrolyte layer is formed on the surface of the anode body, with the dielectric layer interposed therebetween, so as to cover the dielectric layer. The solid electrolyte layer does not necessarily have to cover the entire dielectric layer (the entire surface), but may be formed so as to cover at least a portion of the dielectric layer. The solid electrolyte layer constitutes at least a portion of the cathode part of the solid electrolytic capacitor.

[0039] The solid electrolyte layer includes a conductive polymer. The conductive polymer includes, for example, a conjugated polymer and a dopant. The solid electrolyte layer may further include an additive, if necessary.

[0040] Conjugated polymers include known conjugated polymers used in solid electrolytic capacitors, such as π-conjugated polymers. Examples of conjugated polymers include polymers with a basic skeleton of polypyrrole, polythiophene, polyaniline, polyfuran, polyacetylene, polyphenylene, polyphenylene vinylene, polyacene, and polythiophene vinylene. Of these, polymers with a basic skeleton of polypyrrole, polythiophene, or polyaniline are preferred. The above polymers may contain at least one monomer unit constituting the basic skeleton. The monomer unit may also include a monomer unit having a substituent. The above polymers may also include homopolymers and copolymers of two or more monomers. For example, polythiophenes include poly(3,4-ethylenedioxythiophene) (PEDOT).

[0041] The solid electrolyte layer may contain one type of conjugated polymer or a combination of two or more types of conjugated polymers.

[0042] The weight average molecular weight (Mw) of the conjugated polymer is not particularly limited, but is, for example, 1,000 or more and 1,000,000 or less.

[0043] In this specification, the weight average molecular weight (Mw) is a value calculated as polystyrene measured by gel permeation chromatography (GPC), which is typically performed using a polystyrene gel column and a water / methanol (volume ratio 8 / 2) mobile phase.

[0044] The dopant may be, for example, at least one selected from the group consisting of anions and polyanions.

[0045] Examples of anions include, but are not limited to, sulfate ions, nitrate ions, phosphate ions, borate ions, organic sulfonate ions, carboxylate ions, etc. Examples of dopants that generate sulfonate ions include benzenesulfonic acid, p-toluenesulfonic acid, and naphthalenesulfonic acid.

[0046] Examples of the polyanion include a polymer anion, etc. The solid electrolyte layer may include, for example, a conjugated polymer including a monomer unit corresponding to a thiophene compound and a polymer anion.

[0047] Examples of polymer anions include polymers having multiple anionic groups. Such polymers include polymers containing monomer units having anionic groups. Examples of the anionic groups include sulfonic acid groups and carboxyl groups.

[0048] In the solid electrolyte layer, the anionic group of the dopant may be contained in a free form, an anion form, or a salt form, or may be contained in a form bound to or interacting with the conjugated polymer. In this specification, all of these forms may be simply referred to as an "anionic group," "sulfonic acid group," "carboxy group," or the like.

[0049] Examples of polymer anions having a carboxy group include, but are not limited to, polyacrylic acid, polymethacrylic acid, and copolymers using at least one of acrylic acid and methacrylic acid.

[0050] Examples of polymer anions having sulfonic acid groups include polymeric polysulfonic acids. Specific examples of polymeric polysulfonic acids include, but are not limited to, polyvinyl sulfonic acid, polystyrene sulfonic acid (including copolymers and substituted products having substituents), polyallylsulfonic acid, polyacrylic sulfonic acid, polymethacrylic sulfonic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprene sulfonic acid, polyester sulfonic acids (such as aromatic polyester sulfonic acids), and phenolsulfonic acid novolac resins.

[0051] The amount of the dopant contained in the solid electrolyte layer is, for example, 10 to 1000 parts by mass, or may be 20 to 500 parts by mass, or 50 to 200 parts by mass, relative to 100 parts by mass of the conjugated polymer.

[0052] The solid electrolyte layer may further contain at least one selected from the group consisting of known additives and known conductive materials other than conductive polymers, as needed. Examples of conductive materials include at least one selected from the group consisting of conductive inorganic materials such as manganese dioxide and TCNQ complex salts. A layer for enhancing adhesion may be interposed between the dielectric layer and the solid electrolyte layer.

[0053] The solid electrolyte layer may be a single layer or may be composed of multiple layers. For example, the solid electrolyte layer may be composed of a first solid electrolyte layer covering at least a portion of the dielectric layer and a second solid electrolyte layer covering at least a portion of the first solid electrolyte layer. The types, compositions, contents, etc. of the conjugated polymers, dopants, additives, etc. contained in each layer may be different or the same.

[0054] The solid electrolyte layer is formed, for example, by polymerizing a treatment solution containing a conjugated polymer precursor and a dopant on a dielectric layer. The polymerization can be performed by at least one of chemical polymerization and electrolytic polymerization. Examples of the conjugated polymer precursor include a monomer, an oligomer, or a prepolymer. The solid electrolyte layer may be formed by applying a treatment solution (e.g., a dispersion or solution) containing a conductive polymer to a dielectric layer and then drying the applied solution. Examples of the dispersion medium (or solvent) include at least one selected from the group consisting of water and organic solvents. The treatment solution may further contain other components (e.g., at least one selected from the group consisting of a dopant and an additive). For example, the solid electrolyte layer may be formed using a treatment solution containing a conductive polymer (e.g., PEDOT), a dopant (e.g., a polyanion such as polystyrene sulfonate), and, if necessary, an additive.

[0055] When a treatment liquid containing a precursor of a conjugated polymer is used, an oxidizing agent is used to polymerize the precursor. The oxidizing agent may be contained in the treatment liquid as an additive. Alternatively, the oxidizing agent may be applied to the anode body on which the dielectric layer has been formed before or after the treatment liquid is brought into contact with the anode body. Examples of such oxidizing agents include Fe ３＋Examples of the oxidizing agent include compounds capable of generating fluorine-containing nitrite (e.g., ferric sulfate), persulfates (e.g., sodium persulfate, ammonium persulfate), and hydrogen peroxide. The oxidizing agent can be used alone or in combination of two or more.

[0056] The process of forming a solid electrolyte layer by immersion in a treatment solution and polymerization (or drying) may be performed once or may be repeated multiple times, with the same conditions, such as the composition and viscosity of the treatment solution, or at least one of the conditions being changed each time.

[0057] (Cathode Extraction Layer) The cathode extraction layer needs to include at least a first layer that is in contact with the solid electrolyte layer and covers at least a portion of the solid electrolyte layer, and may include the first layer and a second layer that covers at least a portion of the first layer.

[0058] Examples of the first layer include a layer containing conductive particles and metal foil. Examples of the conductive particles include at least one selected from conductive carbon and metal powder. For example, the cathode extraction layer may be formed of a first layer containing conductive carbon (carbon layer) and a second layer containing metal powder or metal foil. When metal foil is used as the first layer, the cathode extraction layer may be formed of this metal foil.

[0059] Examples of conductive carbon include graphite (artificial graphite, natural graphite, etc.).

[0060] The second layer containing metal powder can be formed, for example, by laminating a composition containing metal powder on the surface of the first layer. Examples of such second layers include metal particle-containing layers formed using a paste containing metal powder and a resin binder. While thermoplastic resins can be used as the resin binder, thermosetting resins such as imide resins and epoxy resins are preferred. To facilitate high conductivity of the second layer, silver-containing particles may be used as the metal powder. Examples of silver-containing particles include first metal particles and second metal particles (specifically, silver particles and silver alloy particles). The second layer may contain one type of silver-containing particle or a combination of two or more types. To ensure high conductivity of the second layer, silver particles and first metal particles are preferred as the silver-containing particles. The silver particles may contain a small amount of impurities. The second layer containing silver-containing particles may be a first metal particle-containing layer or a second metal particle-containing layer. The second layer may include, for example, silver particles and silver alloy particles, may include first metal particles, or may include first metal particles and at least one of silver particles and silver alloy particles.

[0061] When a metal foil is used as the first layer, the type of metal is not particularly limited. It is preferable to use a valve metal (aluminum, tantalum, niobium, etc.) or an alloy containing a valve metal for the metal foil. If necessary, the surface of the metal foil may be roughened. The surface of the metal foil may be provided with a chemical conversion coating, or may be provided with a coating of a metal (dissimilar metal) or a nonmetal different from the metal constituting the metal foil. Examples of dissimilar metals and nonmetals include metals such as titanium and nonmetals such as carbon (e.g., conductive carbon).

[0062] The coating of the dissimilar metal or non-metal (for example, conductive carbon) may be the first layer, and the metal foil may be the second layer.

[0063] When the cathode extraction layer includes the first metal particle-containing layer, the cathode extraction layer may be entirely composed of the first metal particle-containing layer, the first layer may be composed of the first metal particle-containing layer, or the second layer may be composed of the first metal particle-containing layer. For example, the cathode extraction layer may include a first layer (carbon layer) containing conductive carbon and a second layer including the first metal particle-containing layer that covers at least a portion of the first layer.

[0064] The cathode extraction layer is formed by a known method depending on its layer structure. For example, when the cathode extraction layer includes a metal foil as the first or second layer, the first or second layer is formed by laminating the metal foil so as to cover at least a portion of the solid electrolyte layer or the first layer. The first layer containing conductive particles is formed, for example, by applying a conductive paste or liquid dispersion containing conductive particles and, if necessary, a resin binder (e.g., a water-soluble resin, a curable resin, etc.) to the surface of the solid electrolyte layer. The second layer containing metal powder is formed, for example, by applying a paste containing metal powder and a resin binder to the surface of the first layer. During the formation of the cathode extraction layer, drying treatment, heating treatment, etc. may be performed as necessary.

[0065] From the viewpoint of suppressing an increase in leakage current due to the migration of copper ions in a high-temperature environment, it is preferable that the cathode extraction layer (e.g., metal particle-containing layer) does not contain copper particles or copper alloy particles. From the same viewpoint, even when the cathode extraction layer contains at least one of copper particles and copper alloy particles, it is preferable that the total proportion of these particles is small. For example, the total proportion of copper particles and copper alloy particles relative to all metal particles contained in the metal particle-containing layer is, for example, less than 10% by mass, more preferably 5% by mass or less, or 1% by mass or less. Even when the cathode extraction layer contains a metal foil, it is preferable that the metal foil does not contain copper, or even if the metal foil contains copper, the copper content of the metal foil is low. The copper content of the metal foil is, for example, less than 10% by mass, more preferably 5% by mass or less, or 1% by mass or less. Furthermore, even when the metal-containing layers (e.g., the first layer and the second layer) constituting the cathode extraction layer do not contain copper, or even if they contain copper, the proportion of copper relative to all metals contained in the metal-containing layer may be less than 10% by mass, 5% by mass or less, or 1% by mass or less.

[0066] (First conductive adhesive layer) The solid electrolytic capacitor may include a cathode lead. In the solid electrolytic capacitor, the cathode lead is connected to the cathode extraction layer via the first conductive adhesive layer. When the solid electrolytic capacitor includes multiple capacitor elements, the cathode extraction layer and cathode lead of some of the capacitor elements may be connected via the first conductive adhesive layer. The first conductive adhesive layer electrically connects the cathode extraction layer and cathode lead of the capacitor element.

[0067] The first conductive adhesive layer may be formed using a known conductive adhesive. Examples of known conductive adhesives include a paste containing conductive particles and a resin binder (such as a curable resin). The first conductive adhesive layer formed using a known conductive adhesive may be a second metal particle-containing layer formed using a known silver-containing adhesive (such as a silver-containing paste). Such a first conductive adhesive layer may be formed, for example, by arranging the above-described paste (including a silver-containing paste) so as to be sandwiched between the cathode extraction layer and the cathode lead. For example, the above-described paste may be applied or transferred to a portion of the surface of the cathode extraction layer, and then a portion of one end of the cathode lead may be overlapped on the formed paste coating. In the process of forming the first conductive adhesive layer, drying treatment, heating treatment, and the like may be performed as necessary.

[0068] The first conductive adhesive layer may be a first metal particle-containing layer. In this case, the cathode part includes the first metal particle-containing layer interposed between the cathode extraction layer and the cathode lead.

[0069] (Second Conductive Adhesive Layer) When the solid electrolytic capacitor includes multiple capacitor elements, the multiple capacitor elements may be fixed via a second conductive adhesive layer. For example, when the solid electrolytic capacitor includes a stack of multiple capacitor elements, the multiple capacitor elements may be stacked via a second conductive adhesive layer. The second conductive adhesive layer may be in contact with the cathode lead layer of each capacitor element. The second conductive adhesive layer electrically connects the multiple capacitor elements.

[0070] The second conductive adhesive layer may be formed using a known conductive adhesive. Examples of known conductive adhesives include pastes containing conductive particles and a resin binder (such as a curable resin). The second conductive adhesive layer formed using a known conductive adhesive may be a third metal particle-containing layer formed using a known silver-containing adhesive (e.g., a silver-containing paste). Such a second conductive adhesive layer may be formed, for example, by sandwiching the above-described paste (including a silver-containing paste) between adjacent capacitor elements. For example, the above-described paste may be applied or transferred to a portion of the surface of the cathode extraction layer of a capacitor element, and another capacitor element may be placed on the resulting paste coating. During the formation of the second conductive adhesive layer, drying treatments, heating treatments, and the like may be performed as necessary.

[0071] The second conductive adhesive layer may be a first metal particle-containing layer, in which case adjacent solid electrolytic capacitor elements are fixed via the first metal particle-containing layer.

[0072] The first metal particle-containing layer included in the cathode part will be described in more detail below.

[0073] (First Metal Particle-Containing Layer) The first metal particle-containing layer contains metal particles and typically contains a resin binder or a cured product thereof.

[0074] The metal particles include first metal particles. The metal particles may further include second metal particles, and the second metal particles are specifically at least one type selected from the group consisting of silver particles and silver alloy particles. In addition to the first metal particles or the first metal particles and the second metal particles, the metal particles may further include third metal particles other than the first metal particles and the second metal particles.

[0075] (First Metal Particle) The first metal particle includes a core particle and a silver-containing coating layer that coats the core particle. The core particle includes, for example, silica. The silica may be crystalline or amorphous. Furthermore, the silica may be porous or non-porous. The core particle may be fused silica.

[0076] The average aspect ratio of the core particles is, for example, from 1 to 100, and may be from 1 to 20. From the viewpoint of facilitating the formation of a relatively uniform silver-containing coating layer, the average aspect ratio is preferably from 1 to 10, and more preferably from 1 to 5. Furthermore, when the average aspect ratio is within this range, the core particles are easily dispersed in a paste for forming the first metal particle-containing layer, and the first metal particles are easily arranged with high packing in the first metal particle-containing layer.

[0077] The shape of the core particle is not particularly limited, and may be spherical (including oval spheres), flake-like, irregular, etc. From the viewpoints of facilitating the formation of a relatively uniform silver-containing coating layer, facilitating dispersion in a paste, and facilitating high loading in the first metal particle-containing layer, the shape of the core particle is preferably spherical (including oval spheres).

[0078] In this specification, spherical particles refer to particles having a sphericity of 0.6 or more and 1 or less. Flake-like particles refer to particles having a flat or thin flake shape.

[0079] The silver-containing coating layer may be made of silver or a silver alloy. From the viewpoint of obtaining high conductivity, the silver-containing coating layer is preferably made of silver. In this case, the silver may contain a small amount of impurities.

[0080] The average ratio of the silver-containing coating layer in the first metal particles may be, for example, 0.1% by mass or more and 50% by mass or less, 1% by mass or more and 40% by mass or less, 5% by mass or more and 30% by mass or less, or 10% by mass or more and 30% by mass or less. When the ratio of the silver-containing coating layer is within such a range, most of the surface of the core particle is covered with the silver-containing coating layer, making it easy to ensure high conductivity of the first metal particles. In addition, it is easy to adjust the specific gravity of the first metal particles to an appropriate range, making it easy to highly disperse the first metal particles in the paste. Therefore, it is easy to ensure a good balance between cost reduction effects and high conductivity of the first metal particle-containing layer.

[0081] The first metal particles may include one type of particle, or may include a combination of two or more types of particles in which at least one of the core particle and the silver-containing coating layer has a different composition.

[0082] The shape of the first metal particles is not particularly limited and may be spherical (including oval spheres), flake-like, irregular, or the like. The first metal particles may include particles of one type of shape, or may include a combination of particles of two or more types of shapes. The first metal particles preferably include at least spherical particles. In this case, the first metal particles are easily dispersed in the paste, and the first metal particles are easily arranged in the first metal particle-containing layer with high packing. Furthermore, in the first metal particle-containing layer, many contact points between particles can be ensured. Therefore, higher conductivity of the first metal particle-containing layer can be ensured. This tends to enhance the effect of suppressing the initial ESR low. The first metal particles may include, for example, spherical particles and particles of other shapes.

[0083] In this specification, the sphericity of a particle can be estimated by acquiring a cross-sectional image containing multiple particles (e.g., 10 or more particles) and analyzing the contours of the particles contained in the image. The ratio of the diameter of a circle (hereinafter referred to as the "equivalent circle") equal to the area within the closed curve formed by the contours to the diameter of the smallest circle circumscribing the contours is calculated. The average value of this ratio for multiple particles is taken as the particle sphericity. For example, when a particle contains spherical particles and particles of other shapes, multiple particles are selected from the spherical particles and the sphericity is calculated using the above procedure. The cross-sectional image may be an image obtained using a scanning electron microscope (SEM).

[0084] The above cross-sectional image can be obtained, for example, by the following procedure. First, the solid electrolytic capacitor is embedded in a curable resin, and the curable resin is cured. The cured product is wet- or dry-polished to expose a cross-section parallel to the thickness direction of the cathode portion (a cross-section on which the stacking state of each layer of the cathode portion can be confirmed). The exposed cross-section is smoothed by ion milling to obtain a sample for photographing. If necessary, the cross-sectional image can be analyzed using image analysis-based particle size distribution measurement software (e.g., MAC-View (Mountec Co., Ltd.)) to identify the outline of each particle.

[0085] The average aspect ratio of the core particles can also be determined from the cross-sectional image. More specifically, a plurality of first metal particles (e.g., 10 or more) from which core particles can be observed are arbitrarily selected in the cross-sectional image, and the maximum length a of each core particle is determined. For each core particle, the maximum length b in a direction perpendicular to the maximum length a is determined, and the ratio a / b is defined as the aspect ratio of each core particle. The ratios a / b for the plurality of core particles are averaged to determine the average aspect ratio of the core particles.

[0086] The average particle size of the first metal particles may be, for example, 1 μm to 20 μm, or 1 μm to 10 μm. When the average particle size is in this range, contact between the first metal particles is easily ensured, and higher conductivity of the first metal particle-containing layer is easily obtained.

[0087] In this specification, the average particle size of particles can be estimated by acquiring a cross-sectional image containing multiple particles (e.g., 10 or more particles) and analyzing the contour lines of the particles contained in the image. The average particle size can be estimated by determining the diameter of an equivalent circle equal to the area within the closed curve formed by the contour lines and averaging the diameters. Preparation of a sample for the cross-sectional image and analysis of the image are performed, for example, using the same procedures as for determining sphericity. If necessary, the cross-sectional image can be analyzed using the above-mentioned software to identify the contour of each particle and determine the diameter of the equivalent circle or the smallest circumscribing circle having the same area as the area enclosed by the contour.

[0088] The ratio of the first metal particles to all the metal particles contained in the first metal particle-containing layer is, for example, 10% by mass or more, or may be 30% by mass or more, 50% by mass or more, or 60% by mass or more. As the ratio of the first metal particles increases, the average specific gravity of the metal particles decreases, which has the effect of lowering the cost per unit volume. From this perspective, the ratio of the first metal particles to all the metal particles contained in the first metal particle-containing layer may be 80% by mass or more, or even more than 90% by mass. The ratio of the first metal particles to all the metal particles contained in the first metal particle-containing layer is 100% by mass or less.

[0089] The first metal particles can be obtained by a known method or a method similar thereto. Alternatively, commercially available products may be used as the first metal particles. The core particles may be coated with the silver-containing coating layer by plating, vapor deposition (vapor deposition, sputtering, etc.), or the like.

[0090] In the first metal particle-containing layer, the mass ratio of Si to the metal such as Ag (=Si / metal (Ag, etc.)) may be 0.1 or more and 10 or less, 0.2 or more and 5.0 or less, or 0.2 or more and 3.0 or less. The mass ratio of Si to the metal is determined by measuring a cross section of the first metal particle-containing layer using an electron probe microanalyzer (EPMA).

[0091] In the cross section of the first metal particle-containing layer, the ratio of the area occupied by the first metal particles to the total area occupied by the metal particles (= area of ​​first metal particles / area of ​​all metal particles) may be 0.20 or more and 1.00 or less (e.g., 0.50 or more and 1.00 or less) or 0.40 or more and 1.00 or less (e.g., 0.40 or more and 0.95 or less). This area ratio is determined by energy dispersive X-ray spectroscopy (EDX) using the cross-sectional image.

[0092] (Second metal particles) Of the above-mentioned second metal particles, silver particles are preferred. The silver particles may contain a small amount of impurities. The second metal particles may contain silver particles and silver alloy particles. The content of silver particles in the second metal particles is, for example, 80% by mass or more, and may even be 90% by mass or more. The content of silver particles in the second metal particles is 100% by mass or less. The second metal particles may be composed of silver particles only.

[0093] The shape of the second metal particles is not particularly limited and may be spherical (including oval spheres), flake-shaped, irregular, or the like. The second metal particles may contain particles of one type of shape or a combination of particles of two or more types of shapes. For example, the second metal particles may contain at least one type selected from the group consisting of spherical particles and flake-shaped particles. The second metal particles preferably contain at least spherical particles. In this case, they are easily dispersed in a paste, and the second metal particles are easily highly packed in the second metal particle-containing layer. Furthermore, the second metal particle-containing layer can ensure many contact points between particles. Therefore, the second metal particle-containing layer can ensure higher conductivity. This tends to enhance the effect of suppressing the initial ESR low. The second metal particles may, for example, contain spherical particles and particles of other shapes.

[0094] The average particle size of the second metal particles is, for example, 0.01 μm or more and 50 μm or less, and may be 0.1 μm or more and 20 μm or less.

[0095] The aspect ratio and sphericity of the second metal particles may be selected from the ranges described for the first metal particles. The aspect ratio, sphericity, and average particle size of the second metal particles are determined in the same manner as for the first metal particles.

[0096] (Third Metal Particles) Examples of third metal particles other than the first metal particles and the second metal particles include metal particles that are substantially free of precious metals such as silver or gold. Examples of such third metal particles include copper particles, copper alloy particles, nickel particles, and nickel alloy particles. Note that metal particles containing precious metals as impurities (excluding the first metal particles and the second metal particles) are included in the category of third metal particles.

[0097] The first metal particle-containing layer containing third metal particles is advantageous in terms of reducing costs. However, as mentioned above, a lower total ratio of copper particles and copper alloy particles is preferable, and it is also preferable that the first metal particle-containing layer does not contain copper particles or copper alloy particles. Furthermore, from the viewpoint of easily ensuring higher conductivity, a lower content of third metal particles relative to the total metal particles contained in the first metal particle-containing layer is preferable. The total content of the first metal particles and the second metal particles relative to the total metal particles is, for example, 90% by mass or more, and may even be 95% by mass or more. The total content of the first metal particles and the second metal particles relative to the total metal particles is 100% by mass or less. The metal particles may be composed of only first metal particles, or only first metal particles and second metal particles.

[0098] (Resin Binder) Examples of the resin binder include a thermoplastic resin material, a curable resin material, etc. From the viewpoint of relatively small deformation when exposed to high temperatures, it is preferable that the first metal particle-containing layer contains a cured product of the resin binder (specifically, a cured product of a curable resin material).

[0099] The first metal particle-containing layer is formed, for example, using a conductive paste containing metal particles and a resin binder. For example, a coating of the conductive paste is heated to harden the resin binder, thereby forming the first metal particle-containing layer.

[0100] Examples of curable resin materials include resin compositions containing a curable resin (e.g., a thermosetting resin), a component involved in the curing of the curable resin, and, optionally, at least one selected from the group consisting of additives and liquid media. Depending on the type of curable resin, examples of components involved in the curing of the curable resin include polymerization initiators, curing agents, curing accelerators, crosslinking agents, and curing catalysts. These components may be used alone or in combination. Examples of additives include known additives used in conductive pastes for solid electrolytic capacitors.

[0101] The curable resin is preferably an epoxy resin, a polyamide-imide resin, a polyimide resin, a phenolic resin, etc. The resin binder may contain one type of curable resin or a combination of two or more types.

[0102] In the first metal particle-containing layer, the amount of the resin binder or its cured product relative to 100 parts by mass of the metal particles may be, for example, 2 parts by mass to 25 parts by mass, 5 parts by mass to 20 parts by mass, or 10 parts by mass to 20 parts by mass, but is not limited to these ranges.

[0103] (Other) The content of metal particles in the first metal particle-containing layer is determined, for example, taking into consideration the balance between conductivity and adhesion. The content of metal particles may be, for example, 80% by mass or more and 98% by mass or less, or 85% by mass or more and 96% by mass or less. However, the proportion of metal particles is not limited to these ranges. The content of metal particles in the first metal particle-containing layer corresponds to the proportion (mass %) of metal particles in the total dry solid content (i.e., the total amount of components other than the liquid medium (i.e., solvent)) contained in the paste for forming the first metal particle-containing layer. The liquid medium does not include raw materials (such as monomers) for the cured resin binder.

[0104] The thickness of the first metal particle-containing layer is, for example, 0.5 μm or more and 100 μm or less, or may be 1 μm or more and 50 μm or less, or may be 1 μm or more and 20 μm or less.

[0105] The thickness of the first metal particle-containing layer is determined by measuring the thickness of the first metal particle-containing layer at multiple locations (for example, 10 locations) in a cross-sectional image taken by SEM and averaging the measurements.

[0106] The thickness of the first metal particle-containing layer is measured, for example, by taking a cross-sectional image of the portion of the capacitor element that includes the first metal particle-containing layer by SEM. The cross-sectional image is prepared, for example, by the same procedure as that used to determine the sphericity.

[0107] The first metal particle-containing layer can be formed by applying a conductive paste containing at least first metal particles, second metal particles, and a resin binder to at least a portion of at least one component (also referred to as a component) constituting the capacitor element (more specifically, the cathode portion), followed by heat treatment. The component to which the conductive paste is applied includes layers in contact with the first metal particle-containing layer in the cathode portion, such as the solid electrolyte layer, the cathode extraction layer, the first or second layer constituting the cathode extraction layer, and the cathode lead.

[0108] The conductive paste can be obtained by mixing the components. A known method can be used for mixing. The liquid medium used to prepare the conductive paste may be a medium that is liquid at the temperature at which the conductive paste is prepared or applied, and may be a medium that is liquid at room temperature (e.g., 20°C to 35°C). For example, an organic solvent is used as the liquid medium. A combination of an organic solvent and water may also be used as the liquid medium. The liquid medium is selected depending on the type of curable resin, components involved in curing, and additives.

[0109] (Other) The solid electrolytic capacitor may be a wound type, and may be either a chip type or a stacked type. When the solid electrolytic capacitor includes multiple capacitor elements, each capacitor element may be, for example, a wound type or a stacked type. For example, a stacked type solid electrolytic capacitor includes multiple stacked capacitor elements. The configuration of the capacitor element may be selected depending on the type of solid electrolytic capacitor.

[0110] In the capacitor element, for example, one end of a cathode lead is electrically connected to the cathode extraction layer. For example, one end of an anode lead is electrically connected to the anode body (specifically, the anode extraction portion). The other end of the anode lead and the other end of the cathode lead are each extracted from the exterior housing. The other end of each lead exposed from the exterior housing is used for soldering to a substrate on which the solid electrolytic capacitor is to be mounted, and is electrically connected to an external electrode. At least a portion of the external electrode constitutes an external terminal of the solid electrolytic capacitor. Each lead may be a lead wire or a lead frame. Furthermore, without being limited to the use of leads, an end face of the anode extraction portion may be exposed from the exterior housing and connected to an external electrode. A cathode foil may be connected to the cathode extraction layer, and an end face of the cathode foil may be exposed from the exterior housing and connected to an external electrode. The other end face of the lead connected to the cathode extraction layer may be exposed from the exterior housing and connected to an external electrode.

[0111] The capacitor element is sealed with, for example, an exterior housing. For example, the capacitor element and the resin material of the exterior housing (e.g., an uncured thermosetting resin and a filler) may be placed in a mold, and the capacitor element may be sealed with the resin exterior housing by transfer molding, compression molding, or the like. At this time, the other end of the anode lead and the other end of the cathode lead drawn out from the capacitor element are exposed from the mold. Alternatively, the capacitor element may be housed in a bottomed case such that the other end of the anode lead and the other end of the cathode lead are positioned on the opening side of the bottomed case, and the opening of the bottomed case may be sealed with a sealant to form a solid electrolytic capacitor.

[0112] Fig. 1 is a cross-sectional view schematically illustrating the structure of a solid electrolytic capacitor according to an embodiment of the present disclosure. As shown in Fig. 1, the solid electrolytic capacitor 1 includes a capacitor element 2, a resin outer casing 3 that seals the capacitor element 2, and an anode terminal 4 and a cathode terminal 5, at least a portion of which is exposed to the outside of the resin outer casing 3. The anode terminal 4 and the cathode terminal 5 may be made of a metal such as copper or a copper alloy. The resin outer casing 3 has a substantially rectangular parallelepiped outer shape, and the solid electrolytic capacitor 1 also has a substantially rectangular parallelepiped outer shape.

[0113] Capacitor element 2 includes an anode body 6, a dielectric layer 7 covering anode body 6, and a cathode portion 8 covering dielectric layer 7. Cathode portion 8 includes a solid electrolyte layer 9 covering dielectric layer 7, and a cathode extraction layer 10 covering solid electrolyte layer 9. Cathode extraction layer 10 includes a first layer 11 covering solid electrolyte layer 9 and a second layer 12 covering the first layer.

[0114] The anode body 6 includes a region facing the cathode portion 8 and a region not facing the cathode portion 8. Of the region of the anode body 6 not facing the cathode portion 8, an insulating separator 13 is formed in a strip-like shape on the surface of the anode body 6 in a portion adjacent to the cathode portion 8, thereby restricting contact between the cathode portion 8 and the anode body 6. Of the region of the anode body 6 not facing the cathode portion 8, another part is electrically connected to the anode terminal 4 by welding. The cathode terminal 5 is electrically connected to the cathode portion 8 via a first conductive adhesive layer 14.

[0115] In the illustrated example, at least one of the second layer 12 and the first conductive adhesive layer 14 (preferably, at least the second layer 12) may be a first metal particle-containing layer containing first metal particles. By including the first metal particle-containing layer in the cathode portion, it is possible to reduce costs and to reduce leakage current after the solid electrolytic capacitor is exposed to high temperatures. Furthermore, the first metal particles ensure high conductivity of the first metal particle-containing layer, thereby reducing the initial ESR.

[0116] The present invention will be specifically described below based on examples and reference examples, but the present invention is not limited to the following examples.

[0117] Examples 1 to 3 and Comparative Example 1 Capacitor elements or solid electrolytic capacitors were fabricated and evaluated in the following manner.

[0118] (1) Preparation of Anode Body An anode body was prepared by roughening both surfaces of an aluminum foil (thickness: 100 μm) used as a substrate by etching.

[0119] (2) Formation of Dielectric Layer The other end of the anode body was immersed in a chemical conversion solution, and a DC voltage of 2.5 V was applied for 20 minutes to form a dielectric layer containing aluminum oxide.

[0120] (3) Formation of Solid Electrolyte Layer An aqueous solution containing pyrrole monomer and p-toluenesulfonic acid was prepared. The monomer concentration in this aqueous solution was 0.5 mol / L, and the p-toluenesulfonic acid concentration was 0.3 mol / L.

[0121] The anode element on which the dielectric layer had been formed in (2) above and a counter electrode were immersed in the obtained aqueous solution, and electrolytic polymerization was carried out at 25°C and a polymerization voltage of 3 V (polymerization potential relative to the silver reference electrode), thereby forming a solid electrolyte layer.

[0122] (4) Formation of Cathode Portion The anode body obtained in (3) above was immersed in a dispersion liquid in which graphite particles were dispersed in water, removed from the dispersion liquid, and then dried to form a first layer (carbon layer) at least on the surface of the solid electrolyte layer. The drying was performed at 150° C. for 30 minutes.

[0123] Next, a conductive paste containing the metal particles shown in the table was applied to the surface of the first layer, followed by heat treatment at 210°C for 10 minutes to form a second layer, which was a metal particle-containing layer. In this way, a cathode extraction layer composed of the first layer and the second layer was formed. The thickness of the second layer was approximately 10 μm. In this way, a capacitor element was produced.

[0124] The conductive paste used to form the second layer was prepared by mixing the metal particles, resin binder, and liquid medium (or a dispersion or solution containing the resin binder) shown in the table. An epoxy resin composition was used as the resin binder. The ratio of metal particles to the total amount of components other than the liquid medium in the conductive paste (total dry solid content) was 87.5 mass%. The ratio of resin binder to 100 mass parts of the total amount of metal particles was 14 mass parts. The following metal particles were used for each metal particle in the table. For each example, the density of the conductive paste was determined from the composition of the conductive paste. (a) First metal particles: silver-coated particles comprising a core particle made of fused silica and a silver coating layer covering the core particle (silver coverage of approximately 20% by mass, average particle diameter of 4.1 μm, spherical shape (sphericity: 0.9), aspect ratio of core particle: approximately 1, sphericity of core particle: 0.9) (b) Second metal particles: silver particles (aspect ratio: approximately 3, spherical shape (sphericity: 0.6), average particle diameter of 2.0 μm) (c) Third metal particles: copper particles (average particle diameter: approximately 2 μm, sphericity: 0.4) The sphericity of each particle corresponds to the sphericity determined from the cross-sectional image of the metal particle-containing layer using the procedure described above.

[0125] (5) Assembly of Solid Electrolytic Capacitors For Example 1 and Comparative Example 1, solid electrolytic capacitors were further assembled using the capacitor elements obtained in (4) above, according to the following procedure. The cathode extraction layer of the capacitor element was bonded to one end of the cathode lead using a conductive adhesive. One end of the anode body, a portion not covered by the solid electrolyte layer and the cathode extraction layer, was bonded to one end of the anode lead by laser welding. A resin outer casing made of insulating resin was then formed around the capacitor element by molding. At this time, the other end of the anode lead and the other end of the cathode lead were extended from the resin outer casing. In this manner, a solid electrolytic capacitor was completed.

[0126] For the capacitor elements of Examples 1 to 3, the mass ratio of silica to silver (metal) in the cross section of the first metal particle-containing layer was determined according to the procedure described above and was found to be in the range of approximately 0.2 to 3.0. Also, the area ratio of the first metal particles to all metal particles in the cross section of the first metal particle-containing layer was in the range of approximately 0.50 to 1.00.

[0127] [Evaluation] The solid electrolytic capacitors or capacitor elements were used to carry out the following evaluations.

[0128] (a) Leakage Current (LC) of Solid Electrolytic Capacitor The leakage current (LC) of the solid electrolytic capacitors of Example 1 and Comparative Example 1 was evaluated by the following procedure. A 1 kΩ resistor was connected in series to the solid electrolytic capacitor at 25° C., and a rated voltage of 2 V was applied from a DC power supply for 1 minute, after which the leakage current (μA) was measured, and the average value (initial leakage current (initial LC)) of 30 solid electrolytic capacitors was calculated.

[0129] Next, the solid electrolytic capacitors were left standing at 185°C for 4 hours, and then left standing in a humidified environment of 85°C and 85% RH for 12 hours. Next, the solid electrolytic capacitors were left standing again at 185°C for 4 hours, and then left standing in a humidified environment of 85°C and 85% RH for 12 hours. The solid electrolytic capacitors were then heated at 295°C for 6 minutes, simulating a reflow process. The leakage current after this heating (reflow) was measured in the same manner as for the initial leakage current, and the average value (leakage current after reflow (post-reflow LC)) of 30 solid electrolytic capacitors was determined.

[0130] (b) Short-circuit defect rate of solid electrolytic capacitors In the measurement of leakage current after reflow in (a) above, the percentage of solid electrolytic capacitors measured to have a leakage current exceeding 1 mA was calculated out of 30. This percentage was defined as the short-circuit defect rate.

[0131] (c) ESR For the capacitor elements obtained in Examples 1 to 3 and Comparative Example 1, the initial ESR (mΩ) of the capacitor elements at a frequency of 100 kHz was measured using a four-terminal LCR meter in an environment of 25° C. Then, the average initial ESR of 40 capacitor elements was calculated.

[0132] The capacitor elements were randomly divided into two groups of 40 elements each. A heat resistance test was performed on the capacitor elements in one group by leaving them at 145°C for 450 hours. The ESR of the capacitor elements after the heat resistance test was measured in the same manner as the initial ESR, and the average value of the 40 capacitor elements (ESR after the heat resistance test) was calculated.

[0133] The capacitor elements of the other group were subjected to a humidity test in which they were left standing in a humidified environment of 85°C and 85% RH for 450 hours. After the humidity test, the ESR of the capacitor elements was measured in the same manner as for the initial ESR, and the average value (ESR after humidity test) of the 40 capacitor elements was calculated.

[0134] The evaluation results of the solid electrolytic capacitor are shown in Table 1, and the evaluation results of the capacitor element are shown in Table 2. In the table, E1 to E3 are Examples 1 to 3, and C1 is Comparative Example 1. Note that the ESR value after the heat resistance test and the humidity resistance test for C1 in Table 2, as well as the density of the conductive paste, are estimated values ​​obtained through simulation.

[0135]

[0136] As shown in Table 1, the initial LC of the solid electrolytic capacitor E1, which uses first metal particles in the cathode portion, and the solid electrolytic capacitor C1, which uses copper particles, are not significantly different. For E1, even after the solid electrolytic capacitor is exposed to a high temperature and humidity environment and then heated to simulate a reflow process, the leakage current (post-reflow LC) remains low, almost unchanged from the initial LC. Furthermore, for E1, the percentage of solid electrolytic capacitors exhibiting a large leakage current exceeding 1 mA (LC failure rate) is 0%. In contrast to the results for E1, for C1, the post-reflow LC was approximately 40 times that of E1, and the LC failure rate was also very high at 36.7%.

[0137] The reason why LC increased after reflow in C1 is thought to be as follows. In C1, it is thought that some of the copper particles ionized and migrated to the solid electrolyte layer during heating in a high-temperature environment or during the reflow process, and the copper components reached the insulating dielectric layer. The copper components that migrated to the dielectric layer then caused a current to flow between the anode body and the cathode, resulting in an increase in leakage current.

[0138] Furthermore, in the first metal particles used in E1, the core particles are coated with a silver-containing coating layer, thereby suppressing the migration of constituent ions of the first metal particles in high-temperature environments or during the above-mentioned heating. Furthermore, when using the first metal particles, the silver coating layer ensures high conductivity comparable to that of silver particles or silver alloy particles. Additionally, since the core particles of the first metal particles are silica, they have a lower specific gravity than silver particles, silver alloy particles, copper particles, etc. This reduces the cost per unit volume and allows the solid electrolyte layer to be coated with a small mass of paste. Therefore, using the first metal particles not only reduces costs but also reduces leakage current after exposure to high temperatures, resulting in high reliability.

[0139]

[0140] As shown in Table 2, the initial ESR of the capacitor element E1 is significantly lower than that of C1. The first metal particles used in E1 ensure high conductivity due to the silver coating layer, ensuring high conductivity of the metal particle-containing layer. Therefore, the initial ESR of the capacitor element can be kept low. Furthermore, when the first metal particles and second metal particles (e.g., silver particles) are used in combination, the initial ESR can be further reduced due to the high conductivity of the second metal particles while keeping the specific gravity of the metal particles relatively low (comparison of E1 with E2 and E3). Furthermore, in E1 to E3, which use first metal particles (and second metal particles), oxidation degradation of the metal particles is suppressed even during heat resistance tests and humidity resistance tests, so the ESR of the capacitor element after the heat resistance test or humidity resistance test can be kept low, resulting in high reliability.

[0141] In addition, E2 and E3 use silver particles as the first metal particles in combination with the second metal particles, which suppresses the migration of the metal components constituting the metal particles as in C1, which uses copper particles. Therefore, E2 and E3 also achieve leakage current suppression effects equivalent to or superior to E1 in Table 1.

[0142] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0143] The solid electrolytic capacitor of the present disclosure can reduce leakage current after exposure to high temperatures while keeping costs low. It can also reduce leakage current after reflow processing. Furthermore, the solid electrolytic capacitor of the present disclosure has a low initial ESR, and can reduce fluctuations in ESR even after exposure to high-temperature or high-temperature, high-humidity environments. Therefore, the present disclosure can provide a highly reliable solid electrolytic capacitor at a low cost. Therefore, the solid electrolytic capacitor can be applied to a variety of applications, including those requiring high reliability. However, these are merely examples, and the applications of the solid electrolytic capacitor are not limited to these examples.

[0144] 1: Solid electrolytic capacitor 2: Capacitor element 3: Outer package (resin outer package) 4: Anode lead (anode terminal) 5: Cathode lead (cathode terminal) 6: Anode body 7: Dielectric layer 8: Cathode portion 9: Solid electrolyte layer 10: Cathode extraction layer 11: First layer 12: Second layer 13: Separation portion 14: First conductive adhesive layer

Claims

1. an anode body, a dielectric layer formed on a surface of the anode body, and a cathode portion covering at least a portion of the dielectric layer, the cathode section includes a solid electrolyte layer covering at least a portion of the dielectric layer, and at least a portion of the cathode section includes a metal particle-containing layer, The solid electrolyte layer includes a conductive polymer, the metal particles contained in the metal particle-containing layer include first metal particles containing silver; The first metal particle includes a core particle containing silica and a silver-containing coating layer coating the core particle.

2. The solid electrolytic capacitor element according to claim 1 , wherein the ratio of the first metal particles to the total amount of the metal particles is 10 mass % or more.

3. 3. The solid electrolytic capacitor element according to claim 1, wherein the average aspect ratio of the core particles is 1 or more and 10 or less.

4. 3 . The solid electrolytic capacitor element according to claim 1 , wherein an average ratio of the silver-containing coating layer in the first metal particles is 0.1% by mass or more and 50% by mass or less.

5. the metal particles include second metal particles containing silver; 3. The solid electrolytic capacitor element according to claim 1, wherein the second metal particles are at least one type selected from the group consisting of silver particles and silver alloy particles.

6. 3. A solid electrolytic capacitor comprising: at least one solid electrolytic capacitor element according to claim 1; and an exterior body that seals the solid electrolytic capacitor element.

7. The solid electrolytic capacitor according to claim 6 , comprising a plurality of said solid electrolytic capacitor elements stacked together.