Solid electrolytic capacitor element and solid electrolytic capacitor

Abstract

This solid electrolytic capacitor element comprises a positive electrode body, a dielectric layer that is formed on the surface of the positive electrode body, a solid electrolyte layer that covers at least a part of the dielectric layer, and a negative electrode lead-out layer that covers at least a part of the solid electrolyte layer. The negative electrode lead-out layer comprises a carbon layer which is in contact with the solid electrolyte layer and covers at least a part of the solid electrolyte layer. The carbon layer includes a carbonaceous material and a transition metal ion component. A solid electrolytic capacitor that keeps initial ESR low is provided by using a solid electrolytic capacitor element in which the content of the transition metal ion component in the carbon layer is less than or equal to 17,000 ppm by mass.

Description

Technical Field

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

Background Art

[0002] A solid electrolytic capacitor includes a solid electrolytic capacitor element, a resin exterior or case for encapsulating 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 portion covering at least a part of the dielectric layer. The cathode portion includes a solid electrolyte layer containing a conductive polymer covering at least a part of the dielectric layer, and a cathode lead-out layer covering at least a part of the solid electrolyte layer. The cathode lead-out layer includes, for example, a carbon layer and a silver paste layer.

[0003] Patent Document 1 proposes a solid electrolytic capacitor including a carbon layer containing carbon particles and silicic acid and / or silicate.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Means for Solving the Problems

[0005] A solid electrolytic capacitor element according to one aspect of the present disclosure includes an anode body, a dielectric layer formed on the surface of the anode body, a solid electrolyte layer covering at least a part of the dielectric layer, and a cathode lead-out layer covering at least a part of the solid electrolyte layer. The cathode lead-out layer includes a carbon layer that contacts the solid electrolyte layer and covers at least a part of the solid electrolyte layer. The carbon layer includes a carbonaceous material and a transition metal ion component, and the content of the transition metal ion component in the carbon layer is 17000 ppm or less on a mass basis.

[0006] Other aspects of the solid electrolytic capacitor of the present disclosure include at least one of the above-described solid electrolytic capacitor elements.

[0007] According to the present disclosure, the initial equivalent series resistance (ESR) in the solid electrolytic capacitor can be kept low.

Brief Description of the Drawings

[0008]

Figure 1

Embodiments for Carrying Out the Invention

[0009] Prior to the description of the embodiments, problems in the prior art are briefly shown below. In a solid electrolytic capacitor, when air enters inside, due to the action of moisture or oxygen contained in the air, the conductive polymer may be oxidized and deteriorated, or the dopant contained in the solid electrolyte layer may be decomposed, resulting in deterioration of the solid electrolyte layer and a decrease in the conductivity of the solid electrolyte layer. When the conductivity of the solid electrolyte layer is low, the initial performance of the solid electrolytic capacitor deteriorates (for example, the ESR increases or the capacitance decreases). Also, during the use of the solid electrolytic capacitor, the conductivity of the solid electrolyte layer decreases, leading to a decrease in the performance of the solid electrolytic capacitor, such as an increase in ESR or a decrease in capacitance. The deterioration of the solid electrolyte layer is particularly significant under a high-temperature environment. The solid electrolytic capacitor may be used under a high-temperature environment depending on the application. Also, the solid electrolytic capacitor is generally soldered to a substrate through a reflow process in which it is exposed to high temperatures. When the solid electrolytic capacitor is exposed to high temperatures, the deterioration of the solid electrolyte layer becomes more significant and the decrease in conductivity becomes significant, so that the decrease in capacitor performance is likely to become apparent.

[0010] In a solid electrolytic capacitor, a carbon layer is provided so as to cover at least a part of the solid electrolyte layer. The carbon layer is formed, for example, by applying a liquid or paste-like dispersion in which a carbonaceous material is dispersed in a liquid medium to the surface of the solid electrolyte layer and drying it. In order to obtain a dispersion in which the carbonaceous material is dispersed with high dispersibility, the dispersion is usually prepared by wet-milling the carbonaceous material using a liquid medium and finely dispersing it in the liquid medium. Wet-milling is generally carried out using a bead mill using stainless steel beads. However, it has been clarified that when wet-milling is carried out using stainless steel beads, a large amount of transition metal ions are mixed into the dispersion and the formed carbon layer. Transition metal ions may act as oxidizing agents. It has been clarified that the presence of transition metal ions contained in the dispersion and the carbon layer contributes to the oxidative degradation of the conductive polymer as described above.

[0011] In view of the above, in a solid electrolytic capacitor element according to one aspect of the present disclosure, in a carbon layer containing a carbonaceous material and a transition metal ion component, the content of the transition metal ion component in the carbon layer is controlled to 17000 ppm or less on a mass basis. Thereby, the initial ESR of the solid electrolytic capacitor can be kept low. Further, even when the solid electrolytic capacitor is used or when the solid electrolytic capacitor is exposed to a high temperature, an increase in ESR can be kept low. Therefore, a solid electrolytic capacitor element and a solid electrolytic capacitor excellent in stability over time and thermal stability can be obtained. By enhancing these stabilities, the reliability of the solid electrolytic capacitor element and the solid electrolytic capacitor can be enhanced. It is considered that the ESR can be kept low because the oxidation reaction of the conductive polymer is reduced, the deterioration of the solid electrolyte layer is reduced, and the high conductivity of the solid electrolyte layer can be ensured and maintained.

[0012] Hereinafter, the solid electrolytic capacitor and the solid electrolytic capacitor element of the present disclosure (hereinafter, may be simply referred to as a capacitor element) will be described more specifically with reference to the drawings as necessary.

[0013] [Solid electrolytic capacitor] The solid electrolytic capacitor includes one or more capacitor elements. At least one of the capacitor elements included in the solid electrolytic capacitor may include a carbon layer in which the content of the transition metal ion component is within the above range. Preferably, 50% or more of the number of capacitor elements included in the solid electrolytic capacitor includes a carbon layer in which the content of the transition metal ion component is within the above range, more preferably 75% or more, and even more preferably all of the capacitor elements include a carbon layer in which the content of the transition metal ion component is within the above range.

[0014] (Capacitor element) (Anode body) The anode body can include a valve metal, an alloy containing a valve metal, and a compound containing a valve metal, etc. These materials can be used alone or in combination of two or more. As the valve metal, for example, aluminum, tantalum, niobium, and titanium are preferably used. An anode body with a porous surface can be obtained, for example, by roughening the surface of a base material (such as a foil-shaped or plate-shaped base material) containing a valve metal by etching or the like. The roughening can be performed, for example, by an etching treatment. Also, the anode body may be a molded body or a sintered body of particles containing a valve metal. Note that the molded body and the sintered body have a porous structure as a whole.

[0015] (Dielectric layer) The dielectric layer is an insulating layer that functions as a dielectric formed so as to cover at least a part of the surface of the anode body. The dielectric layer is formed by anodizing the valve metal on the surface of the anode body by a forming treatment or the like. The dielectric layer only needs to be formed so as to cover at least a part of the anode body. The dielectric layer is usually formed on the surface of the anode body. Since the dielectric layer is formed on the porous surface of the anode body, it is formed along the inner wall surfaces of the pores and pits on the surface of the anode body.

[0016] The dielectric layer contains an oxide of the valve-acting metal. For example, when tantalum is used as the valve-acting metal, the dielectric layer contains Ta2O5, and when aluminum is used as the valve-acting metal, the dielectric layer contains Al2O3. Note that the dielectric layer is not limited to this, and any material that functions as a dielectric may be used.

[0017] (Cathode portion) The cathode portion includes a solid electrolyte layer that covers at least a part of the dielectric layer and a cathode lead-out layer that covers at least a part of the solid electrolyte layer. The cathode portion is usually formed on at least a part of the surface of the anode body via the dielectric layer. Hereinafter, the solid electrolyte layer and the cathode lead-out layer will be described.

[0018] (Solid electrolyte layer) The solid electrolyte layer is formed on the surface of the anode body via the dielectric layer so as to cover the dielectric layer. The solid electrolyte layer does not necessarily need to cover the entire dielectric layer (the entire surface), and it may be formed so as to cover at least a part of the dielectric layer. The solid electrolyte layer constitutes at least a part of the cathode portion in the solid electrolytic capacitor.

[0019] The solid electrolyte layer contains a conductive polymer. The solid electrolyte layer may further contain at least one of a dopant and an additive, if necessary.

[0020] As the conductive polymer, known ones used in solid electrolytic capacitors, such as π-conjugated conductive polymers, can be used. Examples of the conductive polymer include polymers having a basic skeleton of polypyrrole, polythiophene, polyaniline, polyfuran, polyacetylene, polyphenylene, polyphenylene vinylene, polyacene, and polythiophene vinylene. Among these, polymers having a basic skeleton of polypyrrole, polythiophene, or polyaniline are preferred. The above polymers also include homopolymers, copolymers of two or more monomers, and derivatives thereof (substituted products having substituents, etc.). For example, polythiophene includes poly(3,4-ethylenedioxythiophene).

[0021] The conductive polymer may be used alone or in combination of two or more kinds.

[0022] The weight average molecular weight (Mw) of the conductive polymer is not particularly limited, but is, for example, 1,000 or more and 1,000,000 or less.

[0023] In this specification, the weight average molecular weight (Mw) is a value in terms of polystyrene measured by gel permeation chromatography (GPC). Note that GPC is usually measured using a polystyrene gel column and water / methanol (volume ratio 8 / 2) as a mobile phase.

[0024] The solid electrolyte layer can further contain a dopant. As the dopant, for example, at least one selected from the group consisting of anions and polyanions is used.

[0025] Examples of the anion include, but are not particularly limited to, sulfate ion, nitrate ion, phosphate ion, borate ion, organic sulfonate ion, carboxylate ion, etc. Examples of the dopant that generates sulfonate ion include benzenesulfonic acid, p-toluenesulfonic acid, and naphthalenesulfonic acid, etc.

[0026] Examples of the polyanion include polymer-type polysulfonic acid and polymer-type polycarboxylic acid, etc. Examples of the polymer-type polysulfonic acid include polyvinylsulfonic acid, polystyrenesulfonic acid, polyallylsulfonic acid, polyacrylicsulfonic acid, and polymethacrylicsulfonic acid, etc. Examples of the polymer-type polycarboxylic acid include polyacrylic acid, polymethacrylic acid, etc. The polyanion also includes polyester sulfonic acid, phenol sulfonic acid novolak resin, etc. However, the polyanion is not limited thereto.

[0027] The dopant may be contained in the solid electrolyte layer in a free form, an anion form, or a salt form, or may be contained in a form bound or interacting with the conductive polymer.

[0028] The amount of the dopant contained in the solid electrolyte layer is, for example, 10 to 1000 parts by mass, 20 to 500 parts by mass, or 50 to 200 parts by mass with respect to 100 parts by mass of the conductive polymer.

[0029] The solid electrolyte layer may be a single layer or may be composed of a plurality of layers. When the solid electrolyte layer is composed of a plurality of layers, the conductive polymers contained in each layer may be the same or different. Also, the dopants contained in each layer may be the same or different.

[0030] The solid electrolyte layer may further contain, if necessary, known additives and known conductive materials other than the conductive polymer. Examples of such conductive materials include at least one selected from the group consisting of conductive inorganic materials such as manganese dioxide and TCNQ complex salts. Note that a layer for enhancing adhesion or the like may be interposed between the dielectric layer and the solid electrolyte layer.

[0031] The solid electrolyte layer is formed, for example, by polymerizing a precursor on the dielectric layer using a treatment liquid containing a precursor of the conductive polymer. The polymerization can be carried out by at least one of chemical polymerization and electrolytic polymerization. Examples of the precursor of the conductive polymer include monomers, oligomers, or prepolymers. The solid electrolyte layer may be formed by attaching a treatment liquid (for example, a dispersion or a solution) containing the conductive polymer to the dielectric layer and then drying it. Examples of the dispersion medium (or solvent) include water, organic solvents, or mixtures thereof. The treatment liquid may further contain at least one selected from the group consisting of other components (such as dopants and additives).

[0032] When using a treatment liquid containing a precursor of a conductive polymer, an oxidizing agent is used to polymerize the precursor. The oxidizing agent may be contained in the treatment liquid as an additive. Further, the oxidizing agent may be applied to the anode body before or after bringing the treatment liquid into contact with the anode body on which the dielectric layer is formed. Examples of such an oxidizing agent include sulfates, sulfonic acids or salts thereof. The oxidizing agent can be used alone or in combination of two or more. Examples of sulfates include salts of sulfuric acid or persulfuric acid such as ferric sulfate and sodium persulfate with metals. Examples of the metal constituting the salt include alkali metals (such as sodium and potassium), iron, copper, chromium, zinc, etc. The sulfonic acid or its salt has a function as a dopant in addition to the function as an oxidizing agent. As the sulfonic acid or its salt, the low-molecular-weight sulfonic acid or its salt exemplified for the dopant is used.

[0033] The step of forming the solid electrolyte layer by immersion in the treatment liquid and polymerization (or drying) may be performed once, or may be repeated a plurality of times. In each time, conditions such as the composition and viscosity of the treatment liquid may be the same, or at least one condition may be changed.

[0034] (Cathode lead-out layer) The cathode lead-out layer only needs to include at least a carbon layer that contacts the solid electrolyte layer and covers at least a part of the solid electrolyte layer, and may include a carbon layer and a metal-containing layer that covers the carbon layer. Examples of the metal-containing layer include at least one selected from the group consisting of a layer containing metal powder and a metal foil. By providing the metal-containing layer, it becomes easier to draw out electric charges from the solid electrolytic capacitor element.

[0035] (Carbon layer) The carbon layer contains a carbonaceous material and a transition metal ion component. The carbon layer may contain conductive particles other than the carbonaceous material (e.g., metal powder). However, the content of the transition metal ion component in the carbon layer is 17,000 ppm or less on a mass basis. By controlling the content of the transition metal ion component to be this low, high conductivity of the solid electrolyte layer can be ensured, and the initial ESR of the solid electrolytic capacitor can be suppressed low. The carbon layer may contain at least one selected from the group consisting of, for example, a polymer component and an additive, if necessary.

[0036] The carbon layer is formed, for example, by applying a dispersion containing a constituent component of the carbon layer and a liquid medium so as to cover at least a part of the surface of the solid electrolyte layer and drying it. The dispersion is generally prepared by wet grinding a constituent component of the carbon layer (specifically, a carbonaceous material, one selected from the group consisting of a binder and an additive as necessary, etc.) and a liquid medium with a bead mill. During this wet grinding, it is considered that metal components (mainly transition metal components) contained in the constituent members of the bead mill (e.g., container, disk, beads) are mixed into the dispersion in the form of ions. The transition metal ion component contained in the carbon layer is mainly mixed during this wet grinding. In the dispersion used for forming the carbon layer, for example, by using ceramic beads containing zirconia beads etc. as the beads used for wet grinding, or by removing the transition metal ion component from the dispersion obtained by wet grinding, the content of the transition metal ion component can be reduced. These methods may be combined. The removal of the transition metal ion component may be performed, for example, by bringing an ion exchanger into contact with the dispersion.

[0037] Examples of the transition metal ion component include ions of metals belonging to Groups 3 to 11 of the periodic table. Specific examples of the transition metal ions contained in the transition metal ion component include, for example, Group 4 metal ions of the periodic table (such as titanium ions and zirconium ions), Group 5 metal ions (such as vanadium ions and niobium ions), Group 6 metal ions (such as chromium ions and molybdenum ions), Group 7 metal ions (such as manganese ions), Group 8 metal ions (such as iron ions), Group 9 metal ions (such as cobalt ions), Group 10 metal ions (such as nickel ions), Group 11 metal ions (such as copper ions), and the like. The transition metal ion component may contain metal ions of the 4th to 6th periods of the periodic table, may contain metal ions of the 4th and 5th periods, or may contain metal ions of the 4th period. The transition metal ion component may contain one kind of transition metal ion, but often contains two or more kinds.

[0038] In the carbon layer, the valence of each ion of the contained transition metal ion component is not particularly limited. The valence of each transition metal ion may be monovalent or divalent or higher.

[0039] The transition metal ion component may contain at least one selected from the group consisting of iron ions, nickel ions, and copper ions. These ions tend to act as oxidizing agents and are likely to cause oxidative degradation of the conductive polymer. Therefore, when the transition metal ion component contains such ions, the effect by controlling the content of the transition metal ion component is more significantly exhibited.

[0040] The content of the transition metal ion component in the carbon layer may be 17,000 ppm or less, preferably 15,000 ppm or less, may be 10,000 ppm or less, and may be 5,000 ppm or less on a mass basis. When the content of the transition metal ion component is within such a range, the initial ESR of the solid electrolytic capacitor can be suppressed low. Also, even when the solid electrolytic capacitor is used for a long period or the solid electrolytic capacitor is exposed to high temperatures, an increase in ESR can be significantly reduced. The lower the content of the transition metal ion component in the carbon layer, the more preferable, but it is difficult to make it 0 ppm. Therefore, the content of the transition metal ion component in the carbon layer is usually more than 0 ppm on a mass basis.

[0041] From the viewpoint of suppressing the initial ESR and the increase in ESR low, the content of iron ions in the carbon layer is more preferably 5,000 ppm or less, and even more preferably 4,000 ppm or less on a mass basis. Also, when the content (mass basis) of iron ions in the carbon layer is 1,500 ppm or less or 1,000 ppm or less (preferably 800 ppm or less), the initial ESR can be suppressed even lower, and even when the solid electrolytic capacitor is used for a long period or the solid electrolytic capacitor is exposed to high temperatures, an increase in ESR can be further reduced.

[0042] From the viewpoint of suppressing the initial ESR and the increase in ESR low, the content of nickel ions in the carbon layer is more preferably 5,000 ppm or less, and even more preferably 4,500 ppm or less on a mass basis. When the content (mass basis) of nickel ions in the carbon layer is 2,500 ppm or less or 2,000 ppm or less, the initial ESR can be suppressed even lower, and even when the solid electrolytic capacitor is used for a long period or the solid electrolytic capacitor is exposed to high temperatures, an increase in ESR can be further reduced.

[0043] From the viewpoint of suppressing the initial ESR and the increase in ESR to a low level, the content of copper ions in the carbon layer is more preferably 150 ppm or less, and even more preferably 100 ppm or less, based on mass. Further, when the content of copper ions in the carbon layer (based on mass) is 30 ppm or less or 15 ppm or less (preferably 10 ppm or less), the initial ESR can be suppressed to an even lower level, and even when the solid electrolytic capacitor is used for a long period or exposed to a high temperature, the increase in ESR can be further reduced.

[0044] Typical metal ions may be mixed into the carbon layer. Among typical metal ions, there are those that promote the progress of side reactions in the carbon layer and its vicinity (such as the solid electrolyte layer). Therefore, from the viewpoint of maintaining high conductivity of the solid electrolyte layer, carbon layer, etc., it is preferable that the content of typical metal ions in the carbon layer is also low. Examples of such typical metal ions include ions of Group 12 metals in the periodic table (such as zinc ions). The carbon layer may contain one kind of such metal ions or two or more kinds.

[0045] The total content of typical metal ions such as zinc ions (such as Group 12 metal ions in the periodic table) in the carbon layer is preferably less than 15 ppm, more preferably 14 ppm or less, and may be 12 ppm or less, based on mass. When the content of typical metal ions is within such a range, it is easier to maintain higher conductivity of the solid electrolyte layer, carbon layer, etc. The content of such typical metal ions is usually more than 0 ppm, based on mass.

[0046] When determining the content of metal ions in the carbon layer from the carbon layer of a solid electrolytic capacitor element, it can be determined by the following procedure.

[0047] A sample is prepared by embedding a solid electrolytic capacitor in a curable resin and curing the curable resin. The carbon layer on the capacitor element is exposed by subjecting the sample to polishing, milling, or the like. The transition metal ions contained in the exposed carbon layer surface are confirmed by qualitative analysis using energy dispersive X-ray spectroscopy (EDX). When it is confirmed that the carbon layer contains transition metal ions, the carbon layer is scraped off and a predetermined amount of sample (sample A) is collected, and its mass (m0) is measured. Sample A is mixed with an aqueous nitric acid solution having a concentration of 1.0% by mass and left at room temperature (20 °C or higher and 35 °C or lower) for 1 day. The resulting mixture (sample B) is separated into a solid (sample C) and a liquid (sample D) by centrifugation. Using the separated liquid sample D, the concentration of metal ions contained in sample B is determined by inductively coupled plasma (ICP) emission spectroscopy. From this concentration and the mass m0, the content of each metal ion in the carbon layer is determined. For ICP emission spectroscopy, for example, Optima 5300DV manufactured by PerkinElmer is used.

[0048] Note that sample A is collected from the carbon layer in an exposed state by removing the metal-containing layer. However, for the purpose of excluding the influence of the metal contained in the metal-containing layer, when determining the content of metal ions in the carbon layer, the concentration of metal ions other than the metal (for example, silver) contained in the metal-containing layer may be determined.

[0049] As the carbonaceous material, usually, a conductive carbonaceous material is used. Examples of the carbonaceous material include graphite (artificial graphite, natural graphite, vapor-grown carbon, etc.), carbon black, and amorphous carbon. The carbon layer may contain one kind of carbonaceous material or two or more kinds. The carbonaceous material may be in the form of particles or fibers, but it preferably contains at least particles.

[0050] The dispersion for forming the carbon layer is prepared by wet grinding using a bead mill. Therefore, the carbon layer contains the ground carbonaceous material. Such carbonaceous materials include, for example, particles with a relatively small average particle size after grinding. The average particle size of such particles is, for example, 10 μm or less, may be 5 μm or less, and may be 1.5 μm or less or 1 μm or less. When the carbonaceous material of the carbon layer contains particles having such an average particle size, a conductive path between the particles is easily formed, and since high conductivity of the carbon layer can be obtained, the initial ESR can be further suppressed to a lower level. Also, the high conductivity of the carbon layer is maintained even when the solid electrolytic capacitor is used for a long period of time or when the solid electrolytic capacitor is exposed to high temperatures, so it becomes easier to ensure high reliability of the solid electrolytic capacitor element and the solid electrolytic capacitor. The lower limit of the average particle size of the above particles is not particularly limited, but it may be determined so that the volume resistivity of the carbon layer, for example, becomes 1.0 Ω·cm or less.

[0051] Note that the above average particle size refers to the cumulative 50% particle size (median diameter) in the volume-based particle size distribution measured using a particle size distribution measuring device of the dynamic light scattering method or the laser diffraction / scattering method. For example, when the average particle size is 10 μm or less, a particle size distribution measuring device of the dynamic light scattering method is used, and when the average particle size exceeds 10 μm, a particle size distribution measuring device of the laser diffraction / scattering method is used. As the particle size distribution measuring device by the dynamic light scattering method, for example, the light scattering photometer DLS-8000 manufactured by Otsuka Electronics Co., Ltd. is used. As the particle size distribution measuring device of the laser diffraction / scattering method, for example, the MT3200II manufactured by Microtrac is used.

[0052] When determining the above average particle diameter for a carbonaceous material collected from the carbon layer of a solid electrolytic capacitor element, as a sample for measuring the average particle diameter, for example, a dispersion liquid containing a sample E obtained by the following procedure is used. First, the above-mentioned solid sample C is washed with water, washed with an organic solvent, and dried to obtain a carbonaceous material (sample E). As the organic solvent, for example, among those exemplified as the organic liquid medium for wet grinding described later, one that can dissolve the polymer component that cannot be removed by washing with water may be selected. The sample E is dispersed in a liquid dispersion medium using a surfactant to prepare a dispersion liquid for measurement. As the dispersion medium, for example, pure water or an organic medium that is liquid at room temperature (for example, 20°C to 35°C) is used. The type and concentration of the surfactant, the type of the dispersion medium, and the concentration of the sample E in the dispersion liquid may each be selected within a range that can prepare a dispersion liquid suitable for measuring the average particle diameter.

[0053] The polymer component contained in the carbon layer may be hydrophilic (for example, water-soluble, water-dispersible) or hydrophobic. The polymer component may contain one type of polymer or two or more types of polymers.

[0054] Examples of the hydrophilic polymer component include a polymer containing at least one hydrophilic group selected from the group consisting of an acid group and a hydroxy group (hereinafter, may be referred to as the first polymer). Among them, the first polymer having an anionic group such as an acid group and a phenolic hydroxy group is preferable. Examples of the acid group include a sulfone group and a carboxy group. When forming a carbon layer using an aqueous dispersion, from the viewpoint of easily ensuring high dispersibility, the first polymer preferably has a plurality of hydrophilic groups (particularly, a plurality of anionic groups). As the hydrophilic polymer component, it is preferable to use the first polymer having at least an acid group, and the first polymer having an acid group and a hydroxy group may also be used.

[0055] In the carbon layer, the acid groups of the polymer may be contained in a free form, an anionic form, a salt form, or may be contained in a state of interacting or bonding with the components contained in the carbon layer or the solid electrolyte layer. In this specification, all these forms of acid groups may be simply referred to as "acid groups". Also, in the carbon layer, the hydroxy groups of the polymer may be contained in a free form, an anionic form, or may be contained in a state of interacting or bonding with the components contained in the carbon layer or the solid electrolyte layer. In this specification, all these forms of hydroxy groups may be simply referred to as "hydroxy groups". Similarly, for anionic groups, all forms including the free form, anionic form, salt form, and the state of interacting or bonding with the components contained in the carbon layer or the solid electrolyte layer may be simply referred to as "anionic groups".

[0056] The carbon layer may contain one kind of the first polymer or two or more kinds of the first polymer.

[0057] When a polymer anion containing a monomer unit having an anionic group is used as the first polymer, the effect of suppressing dedoping from the solid electrolyte layer is enhanced. Therefore, even when the solid electrolytic capacitor is used for a long period of time or the solid electrolytic capacitor is exposed to high temperatures, the high conductivity of the solid electrolyte layer can be maintained and an increase in ESR can be suppressed to a low level. Such a polymer anion may be referred to as polymer 1A. Examples of polymer 1A include a homopolymer having a monomer unit having an anionic group, a copolymer containing two or more kinds of monomer units having an anionic group, and a copolymer of a monomer unit having an anionic group and another copolymerizable monomer. The monomer unit having an anionic group may be aliphatic or may have at least one ring selected from the group consisting of an aliphatic ring, an aromatic ring, and a heterocyclic ring. The polymer component may contain one kind of polymer 1A or two or more kinds of polymer 1A.

[0058] Examples of the polymeric anions having a sulfone group include polyvinylsulfonic acid, polystyrenesulfonic acid, polyallylsulfonic acid, polyacrylicsulfonic acid, polymethacrylicsulfonic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprenesulfonic acid, polyester sulfonic acid, and phenolsulfonic acid novolak resin. Examples of the polymeric anions having a carboxy group include polyacrylic acid, polymethacrylic acid, and copolymers using at least one of acrylic acid and methacrylic acid. The copolymers include copolymers of at least one of acrylic acid and methacrylic acid and at least one of acrylic acid esters and methacrylic acid esters (such as methyl acrylate and methyl methacrylate). However, the polymeric anions are not limited thereto. These polymeric anions are usually water-soluble.

[0059] As the first polymer, water-soluble cellulose derivatives, saponified products of polyvinyl acetate (partial saponified products, polyvinyl alcohol, etc.) are also preferable. Examples of the water-soluble cellulose derivatives include cellulose ether compounds. Examples of the cellulose ether compounds include carboxymethylcellulose Salt(Alkali metal salts such as sodium salts and potassium salts, ammonium salts, etc.), hydroxyethyl cellulose, hydroxypropyl methyl cellulose, etc. may be mentioned. These first polymers may be referred to as polymer 1B. Polymer 1B is likely to form a hydrogen bond with an organic molecule (specifically, a polymer) contained in the carbon layer, and thus is likely to adopt a dense structure. Therefore, there is little space for gas such as oxygen to diffuse within the polymer component, and it is difficult for gas to permeate. Thus, when the polymer component contains polymer 1B, the oxygen barrier property of the carbon layer can be enhanced, and the adhesion between the solid electrolyte layer and the carbon layer and the adhesion between carbonaceous materials can be enhanced. Therefore, oxidative degradation of the conductive polymer can be reduced, and the conductivity of the solid electrolyte layer can be easily maintained. Further, when polymer 1B is used, an appropriate thickening effect can be obtained, and thus it is easy to enhance the dispersibility of the constituent components in the dispersion. The polymer component may contain one kind of polymer 1B or two or more kinds thereof. The polymer component may contain polymer 1A and polymer 1B.

[0060] The weight average molecular weight Mw of the first polymer is, for example, 2000 or more and 1000000 or less.

[0061] Examples of polymers other than the first polymer include fluororesins, acrylic resins (such as polyacrylic acid esters), polyester resins, polyurethane resins, vinyl resins (such as polyvinyl acetate), polyolefin resins, rubber-like materials (e.g., styrene-butadiene copolymer rubber (SBR)), and epoxy resins. Examples of fluororesins include vinylidene fluoride resins (such as polyvinylidene fluoride and vinylidene fluoride copolymers), and fluorinated olefin resins (such as polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymers, and tetrafluoroethylene-ethylene copolymers). Such polymers may be referred to as the second polymer. The polymer component may contain one kind of the second polymer or two or more kinds thereof. The polymer component may contain the first polymer and the second polymer as needed. The second polymer acts as a binder. When a fluororesin is used among the second polymers, the adhesion to the solid electrolyte layer of the dispersion can be improved, so that the adhesion between the solid electrolyte layer and the carbon layer can be enhanced. Also, when a fluororesin is used, the dispersibility of the components in the dispersion is easily increased.

[0062] The second polymer is hardly soluble in water, and usually, a dispersion is prepared in combination with an organic liquid medium. However, not limited to this case, if necessary, water or a mixed medium of water and an organic liquid medium may be used for preparing a dispersion containing the second polymer. The second polymer may be a thermoplastic resin or a curable resin (such as a thermosetting resin or a photocurable resin). In the case of a curable resin, a monomer component may be used as the liquid medium. The polymer component may contain one kind of the second polymer or two or more kinds thereof.

[0063] The content of the polymer component in the carbon layer is Based on 100 parts by mass of the carbonaceous material, For example, it can be selected from a wide range of 0.1 part by mass or more and 5000 parts by mass or less, and may be 0.5 part by mass or more and 1000 parts by mass or less.

[0064] The amount of the first polymer is, for example, 1 part by mass or more and 5000 parts by mass or less, may be 2 parts by mass or more and 1000 parts by mass or less, or may be 10 parts by mass or more and 100 parts by mass or less, with respect to 100 parts by mass of the carbonaceous material. From the viewpoint of ensuring easy workability, the amount of the polymer 1B is preferably 10 parts by mass or less with respect to 100 parts by mass of the carbonaceous material. Further, when the polymer component includes the polymer 1A and the polymer 1B, the amount of the polymer 1B is, for example, 5 parts by mass or more and 50 parts by mass or less, and may be 5 parts by mass or more and 35 parts by mass or less, with respect to 100 parts by mass of the polymer 1A.

[0065] The content of the second polymer in the carbon layer is Based on 100 parts by mass of the carbonaceous material, for example, 10 parts by mass or less, may be 0.1 part by mass or more and 10 parts by mass or less, or may be 0.5 part by mass or more and 5 parts by mass or less.

[0066] Examples of the additive contained in the carbon layer include, but are not limited to, at least one selected from the group consisting of an aromatic compound having a sulfonic acid group, a thickener, a surface conditioner, and a surfactant. When the carbon layer contains an aromatic compound having a sulfonic acid group, the adhesion between the solid electrolyte layer and the carbon layer can be further enhanced, and the dedoping in the solid electrolyte layer can be reduced.

[0067] Examples of the aromatic compound having a sulfonic acid group include aromatic sulfonic acids. The aromatic sulfonic acid may have at least one selected from the group consisting of a hydroxy group and a carboxy group in addition to the sulfonic acid group. In the carbon layer, the forms of the sulfonic acid group, carboxy group, and hydroxy group of the aromatic compound may be included in any of the forms described for the polymer component.

[0068] Examples of the aromatic sulfonic acid include benzenesulfonic acid, p-toluenesulfonic acid, naphthalenesulfonic acid, butylnaphthalenesulfonic acid, phenolsulfonic acid, sulfosalicylic acid, hydroquinonesulfonic acid, hydroquinonedisulfonic acid, catecholsulfonic acid, catechol disulfonic acid, pyrogallolsulfonic acid, pyrogalloldisulfonic acid, or salts thereof (e.g., alkali metal salts (sodium salt, potassium salt, etc.)). The aromatic compounds having a sulfonic acid group include condensates of aromatic sulfonic acids with aldehyde compounds (such as formaldehyde or its polymers (e.g., trioxane)). Specific examples of the condensates include formaldehyde condensates of phenolsulfonic acid, formaldehyde condensates of naphthalenesulfonic acid, formaldehyde condensates of arylphenolsulfonic acid, formaldehyde condensates of anthraquinonesulfonic acid, or salts thereof (e.g., alkali metal salts (sodium salt, potassium salt, etc.)). However, the aromatic compounds having a sulfonic acid group are not limited thereto.

[0069] The carbon layer may contain one kind of the aromatic compound having a sulfonic acid group, or may contain two or more kinds.

[0070] In the carbon layer, the amount of the aromatic compound having a sulfonic acid group is, for example, 4 parts by mass or more and 300 parts by mass or less with respect to 100 parts by mass of the carbonaceous material.

[0071] The thickness of the carbon layer is, for example, 0.1 μm or more and 100 μm or less, may be 0.5 μm or more and 50 μm or less, or may be 1 μm or more and 20 μm or less.

[0072] When preparing the dispersion, examples of the ceramic beads used for wet grinding include zirconia beads, zirconia-silica-based ceramic beads, titania beads, alumina beads, sialon beads, silicon nitride beads, and the like. The ceramic beads have a smooth surface, are less likely to damage the container or disk of the mill, and are advantageous for reducing the incorporation of metal ion components into the dispersion. Although the ceramic beads may contain metal components such as transition metals, even if the constituent components of the ceramic beads are incorporated into the dispersion, the content of the metal ion components is much less compared to the case of using stainless steel beads.

[0073] As the ion exchanger for removing ions from the dispersion, for example, an inorganic exchanger may be used, but the content of the transition metal ion component can be easily reduced by using an ion exchange resin (such as a cation exchange resin). If necessary, an inorganic ion exchanger and an ion exchange resin may be used in combination.

[0074] For wet grinding, a liquid medium is used. Examples of the liquid medium include water, an organic liquid medium, and a mixture of water and an organic liquid medium (such as a water-soluble organic liquid medium). The liquid medium only needs to have fluidity at room temperature (a temperature of 20°C or higher and 35°C or lower). Examples of the organic liquid medium include alcohols (such as ethanol, 2-propanol), ethers (such as diethyl ether, tetrahydrofuran), ketones (such as acetone), nitriles (such as acetonitrile), sulfoxides (such as dimethyl sulfoxide), and N-methyl-2-pyrrolidone. The organic liquid medium may be used alone or in combination of two or more.

[0075] The carbon layer can be formed by immersing an anode body having a dielectric layer with a solid electrolyte layer formed thereon in a liquid dispersion, applying a paste-like dispersion on the surface of the solid electrolyte layer, and then drying.

[0076] (Metal-containing layer) Among the metal-containing layers, the layer containing metal powder can be formed, for example, by laminating a composition containing metal powder on the surface of the carbon layer. As such a metal-containing layer, for example, a metal paste layer formed using a composition containing metal powder such as silver particles and a resin (binder resin) can be used. As the resin, a thermoplastic resin can be used, but it is preferable to use a thermosetting resin such as an imide-based resin or an epoxy resin.

[0077] Among the metal-containing layers, the type of metal constituting the metal foil is not particularly limited, but it is preferable to use a valve metal such as aluminum, tantalum, niobium or an alloy containing a valve metal. If necessary, the surface of the metal foil may be roughened by an etching treatment or the like. A chemical conversion film may be provided on the surface of the metal foil, or a film of a metal (dissimilar metal) or a non-metal different from the metal constituting the metal foil may be provided. Examples of the dissimilar metal include metals such as titanium. Examples of the non-metal material include carbon (such as a conductive carbonaceous material).

[0078] The thickness of the metal-containing layer is, for example, 0.1 μm or more and 100 μm or less, and may be 0.5 μm or more and 50 μm or less, or may be 1 μm or more and 20 μm or less.

[0079] (Others) The solid electrolytic capacitor may be of a wound type, or may be either a chip type or a multilayer type. For example, the solid electrolytic capacitor may include a laminate of two or more capacitor elements. The configuration of the capacitor element may be selected according to the type of the solid electrolytic capacitor.

[0080] In a capacitor element, one end of a cathode terminal is electrically connected to a cathode lead-out layer. The cathode terminal is joined to the cathode layer, for example, by applying a conductive adhesive to the cathode layer. One end of an anode terminal is electrically connected to the anode body. The other ends of the anode terminal and the cathode terminal are each drawn out from a resin exterior or a case. The other ends of the terminals exposed from the resin exterior or the case are used for soldering to a substrate on which the solid electrolytic capacitor is to be mounted, etc.

[0081] The capacitor element is encapsulated using a resin exterior or a case. For example, the capacitor element and the resin material for the exterior (e.g., an uncured thermosetting resin and a filler) may be placed in a mold, and the capacitor element may be encapsulated with the resin exterior by a transfer molding method, a compression molding method, or the like. At this time, the portions on the other end sides of the anode terminal and the cathode terminal connected to the anode lead drawn out from the capacitor element are each exposed from the mold. Also, the capacitor element may be housed in a bottomed case such that the portions on the other end sides of the anode terminal and the cathode terminal are positioned on the opening side of the bottomed case, and the solid electrolytic capacitor may be formed by sealing the opening of the bottomed case with a sealing body.

[0082] FIG. 1 is a cross-sectional view schematically showing 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 exterior 3 that encapsulates the capacitor element 2, and an anode terminal 4 and a cathode terminal 5, at least a part of each of which is exposed outside the resin exterior 3. The anode terminal 4 and the cathode terminal 5 can be made of a metal such as copper or a copper alloy. The resin exterior 3 has a substantially rectangular parallelepiped outer shape, and the solid electrolytic capacitor 1 also has a substantially rectangular parallelepiped outer shape.

[0083] The capacitor element 2 includes an anode body 6, a dielectric layer 7 covering the anode body 6, and a cathode body 8 covering the dielectric layer 7. The cathode body 8 includes a solid electrolyte layer 9 covering the dielectric layer 7 and a cathode lead-out layer 10 covering the solid electrolyte layer 9, and constitutes the above-described cathode portion. In the illustrated example, the cathode lead-out layer 10 has a carbon layer 11 and a metal paste layer 12 as a metal-containing layer. According to the present disclosure, the content of the transition metal ion component in the carbon layer 11 is low. Therefore, oxidative degradation of the conductive polymer is suppressed, so that a decrease in the conductivity of the solid electrolyte layer is suppressed, and thus the initial ESR of the solid electrolytic capacitor can be kept low.

[0084] The anode body 6 includes a region facing the cathode body 8 and a region not facing the cathode body 8. In the region of the anode body 6 that does not face the cathode body 8, an insulating separation layer 13 is formed so as to cover the surface of the anode body 6 in a strip shape at a portion adjacent to the cathode body 8, and contact between the cathode body 8 and the anode body 6 is restricted. A part of the region of the anode body 6 that does not face the cathode body 8 is electrically connected to the anode terminal 4 by welding. The cathode terminal 5 is electrically connected to the cathode body 8 via an adhesive layer 14 formed of a conductive adhesive.

[0085] [Examples] Hereinafter, the present invention will be specifically described based on examples and comparative examples, but the present invention is not limited to the following examples.

[0086] 《Solid electrolytic capacitor E1》 The solid electrolytic capacitor 1 (solid electrolytic capacitor E1) shown in FIG. 1 was manufactured and its characteristics were evaluated in the following manner.

[0087] (1) Preparation of anode body 6 The anode body 6 was manufactured by roughening both surfaces of an aluminum foil (thickness: 100 μm) as a base material by etching.

[0088] (2) Formation of dielectric layer 7 A portion on the other end side of the anode body 6 was immersed in a forming solution, and a DC voltage of 70 V was applied for 20 minutes to form a dielectric layer 7 containing aluminum oxide.

[0089] (3) Formation of the solid electrolyte layer 9 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 concentration of p-toluenesulfonic acid was 0.3 mol / L.

[0090] The obtained aqueous solution was immersed with the anode body 6 on which the dielectric layer 7 was formed in the above (2) and a counter electrode, and electrolytic polymerization was carried out at 25 °C with a polymerization voltage of 3 V (polymerization potential with respect to the silver reference electrode), thereby forming the solid electrolyte layer 9.

[0091] (4) Formation of the cathode body 8 Graphite particles as a carbonaceous material and a dispersant (such as a cellulose derivative) were wet-milled together with water using a bead mill (SC mill manufactured by Nippon Coke Co., Ltd.) to prepare a liquid dispersion. Zirconia beads were used as the beads. The mass ratio of the graphite particles to the dispersant was 100:50.

[0092] The anode body 6 on which the solid electrolyte layer 9 obtained in the above (3) was formed was immersed in the liquid dispersion, taken out from the dispersion liquid, and then dried to form a carbon layer 11 on at least the surface of the solid electrolyte layer 9. The drying was carried out at 150 to 200 °C for 10 to 30 minutes.

[0093] Next, a silver paste containing silver particles and a binder resin (epoxy resin) was applied to the surface of the carbon layer 11 and heated at 150 to 200 °C for 10 to 60 minutes to cure the binder resin, thereby forming a metal paste layer 12. Thus, the cathode body 8 composed of the carbon layer 11 and the metal paste layer 12 was formed. The capacitor element 2 was fabricated as described above.

[0094] (5) Assembly of the solid electrolytic capacitor 1 The cathode body 8 of the capacitor element 2 obtained in the above (4) and one end of the cathode terminal 5 were joined by an adhesive layer 14 of a conductive adhesive. One end of the anode body 6 protruding from the capacitor element 2 and one end of the anode terminal 4 were joined by laser welding. Next, a resin exterior body 3 formed of an insulating resin was formed around the capacitor element 2 by mold molding. At this time, the other end of the anode terminal 4 and the other end of the cathode terminal 5 were in a state of being drawn out from the resin exterior body 3. In this way, a solid electrolytic capacitor was completed. A total of 20 solid electrolytic capacitors were produced in the same manner as above.

[0095] (6) Evaluation The following evaluations were performed using the solid electrolytic capacitor.

[0096] (a) Measurement of the metal ion content in the carbon layer The content of metal ions in the carbon layer was measured by the procedure described above.

[0097] (b) Measurement of ESR The ESR of the solid electrolytic capacitor was measured by the following procedure. In an environment of 20 °C, using an LCR meter for four-terminal measurement, the initial ESR (mΩ) at a frequency of 100 kHz of each solid electrolytic capacitor was measured. Then, the average value (initial ESR) of the 20 solid electrolytic capacitors was obtained.

[0098] Next, the solid electrolytic capacitor was subjected to an acceleration test by applying the rated voltage to the solid electrolytic capacitor for 500 hours in an environment of 145 °C. Then, in the same procedure as in the case of the initial ESR, the ESR was measured in an environment of 20 °C, and the average value (ESR after the acceleration test) of the 20 solid electrolytic capacitors was obtained.

[0099] 《Solid electrolytic capacitor E2》 For the formation (4) of the cathode body 8 of the solid electrolytic capacitor E1 andNext, strip-shaped cation exchange resin was added to the obtained liquid dispersion and mixed for 1 hour, after which the cation exchange resin was taken out. The liquid dispersion was prepared in this way. A solid electrolytic capacitor 1 (solid electrolytic capacitor E2) was fabricated and evaluated in the same manner as in the case of the solid electrolytic capacitor E1, except that the obtained liquid dispersion was used.

[0100] 《Solid electrolytic capacitor C1》 In the formation (4) of the cathode body 8 of the solid electrolytic capacitor E1 and Next, stainless steel beads were used as the beads. A solid electrolytic capacitor C1 was fabricated and evaluated in the same manner as in the case of the solid electrolytic capacitor E1, except for this.

[0101] Table 1 shows the results of the initial ESR of the fabricated solid electrolytic capacitor and the ESR after a 500-hour accelerated test. The ESR is shown as a relative value when the initial ESR of the solid electrolytic capacitor E1 is set to 1.

[0102]

Table 1

Industrial Applicability

[0103] According to the present disclosure, the initial ESR of the solid electrolytic capacitor can be suppressed low. In addition, an increase in ESR when the solid electrolytic capacitor is used for a long time or when the solid electrolytic capacitor is exposed to high temperatures can be suppressed low. Therefore, the solid electrolytic capacitor element and the solid electrolytic capacitor can be used in various applications that require high reliability.

Explanation of Signs

[0104] 1: Solid electrolytic capacitor, 2: Capacitor element, 3: Resin exterior, 4: Anode terminal, 5: Cathode terminal, 6: Anode body, 7: Dielectric layer, 8: Cathode body, 9: Solid electrolyte layer, 10: Cathode lead-out layer, 11: Carbon layer, 12: Metal paste layer, 13: Separation layer, 14: Adhesive layer

Claims

1. An anode body, a dielectric layer formed on the surface of the anode body, a solid electrolyte layer covering at least a part of the dielectric layer, and a cathode lead-out layer covering at least a part of the solid electrolyte layer, wherein the cathode lead-out layer includes a carbon layer that contacts the solid electrolyte layer and covers at least a part of the solid electrolyte layer, the carbon layer contains a carbonaceous material and a transition metal ion component, and the content of the transition metal ion component in the carbon layer is 17,000 ppm or less on a mass basis. A solid electrolytic capacitor element.

2. The transition metal ion component includes iron ions, and the content of the iron ions in the carbon layer is 5,000 ppm or less on a mass basis. The solid electrolytic capacitor element according to Claim 1.

3. The transition metal ion component includes nickel ions, and the content of the nickel ions in the carbon layer is 5,000 ppm or less on a mass basis. The solid electrolytic capacitor element according to Claim 1 or 2.

4. The transition metal ion component includes copper ions, and the content of the copper ions in the carbon layer is 150 ppm or less on a mass basis. The solid electrolytic capacitor element according to any one of Claims 1 to 3.

5. The carbonaceous material includes particles having an average particle diameter of 10 μm or less. The solid electrolytic capacitor element according to any one of Claims 1 to 4.

6. The carbon layer includes a polymer having at least one selected from the group consisting of acid groups and hydroxy groups. The solid electrolytic capacitor element according to any one of Claims 1 to 5.

7. The carbon layer includes a fluororesin. The solid electrolytic capacitor element according to any one of Claims 1 to 5.

8. A solid electrolytic capacitor including at least one solid electrolytic capacitor element according to any one of Claims 1 to 7.

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