Solid electrolyte capacitor element, solid electrolyte capacitor, and carbon paste for solid electrolyte capacitor element

Abstract

A solid electrolyte capacitor element according to the present invention comprises: a positive electrode body; a dielectric layer that is formed on a surface of the positive electrode body; a solid electrolytic layer that covers at least a portion of the dielectric layer; and a carbon layer that covers at least a portion of the solid electrolytic layer and that contains carbon particles. The ratio D / G of the intensity of D band to the intensity of G band in a Raman spectrum of the carbon layer is not more than 1.5.

Description

Solid electrolytic capacitor element, solid electrolytic capacitor, and carbon paste for solid electrolytic capacitor element

[0001] The present disclosure relates to a solid electrolytic capacitor element, a solid electrolytic capacitor including the same, and a carbon paste used to form a carbon layer of the solid electrolytic capacitor element.

[0002] A solid electrolytic capacitor includes a solid electrolytic capacitor element and a resin outer casing or case that seals the solid electrolytic capacitor element. The solid electrolytic capacitor element includes, for example, an anode body, a dielectric layer formed on the surface of the anode body, a solid electrolyte layer containing a conductive polymer component 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-containing layer that covers at least a portion of the carbon layer. The performance of a solid electrolytic capacitor is greatly affected by the condition of the solid electrolyte layer. The carbon layer can cause degradation of the solid electrolyte layer, resulting in reduced capacitor performance.

[0003] Patent Document 1 proposes a solid electrolytic capacitor in which an anode body, a dielectric film, a solid electrolyte layer, and a conductive paste layer are formed in this order, and a conductive polymer compound is used as the solid electrolyte, and the conductive paste layer contains an acrylic resin and a cellulose-based resin as binders, and the thickness of the conductive paste layer is 20 to 40 μm.

[0004] Japanese Patent Application Publication No. 11-297574

[0005] In Patent Document 1, after a solid electrolytic capacitor is exposed to a high-temperature environment, the resistance of the conductive paste layer increases significantly, resulting in a significant increase in equivalent series resistance (ESR). Because solid electrolytic capacitors are sometimes used in high-temperature environments depending on the application, it is necessary to reduce the increase in ESR even when exposed to a high-temperature environment.

[0006] 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; a solid electrolyte layer covering at least a portion of the dielectric layer; and a carbon layer covering at least a portion of the solid electrolyte layer and containing carbon particles, wherein a ratio of a D band intensity to a G band intensity (D / G) in a Raman spectrum of the carbon layer is 1.5 or less.

[0007] A second aspect of the present disclosure relates to a solid electrolytic capacitor including at least one of the above-described solid electrolytic capacitor elements.

[0008] A third aspect of the present disclosure relates to a carbon paste for a solid electrolytic capacitor element, the carbon paste comprising carbon particles and a dispersion medium, for forming a carbon layer to be included in a solid electrolytic capacitor element, wherein a ratio of D band intensity to G band intensity: D / G in a Raman spectrum of a dried coating of the carbon paste is 1.5 or less.

[0009] In the solid electrolytic capacitor, the ESR can be kept low after exposure to a high-temperature environment.

[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] When a solid electrolytic capacitor is exposed to a high-temperature environment, the oxygen contained in the air that has entered the capacitor tends to cause significant oxidative degradation of the solid electrolyte layer, which is thought to result in a decrease in conductivity and a decrease in ESR. In a high-temperature, high-humidity environment, the effects of moisture, in addition to the effects of oxygen, become more pronounced. In this case, in addition to the oxidative degradation of the solid electrolyte layer, the effect of moisture also tends to promote de-doping, in which dopants are removed from the solid electrolyte layer, which tends to result in a significant decrease in ESR.

[0013] If air easily penetrates into the carbon layer or if the adhesion between the solid electrolyte layer and the carbon layer is poor, air easily penetrates into the solid electrolyte layer, resulting in the above-mentioned decrease in ESR.

[0014] In view of the above, in the solid electrolytic capacitor element according to the present disclosure, the D / G ratio in the Raman spectrum of the carbon layer is 1.5 or less. When the D / G ratio of the carbon layer is within this range, it is believed that the surface condition of the carbon particles is improved, and π-conjugated systems are spread across the surface layers of the carbon particles. It is believed that such carbon particles approach other carbon particles having graphite structures contained in the carbon layer and molecules of components having π-covalent systems (e.g., molecules of compounds having acid groups, binder molecules, or additive molecules, as described below) through π-π interactions. Since the distance between carbon particles is shortened, conductive paths are more likely to spread within the carbon layer. Therefore, high conductivity of the carbon layer is achieved, thereby enabling the initial ESR to be kept low. It is also believed that the high conductivity of the carbon layer is influenced by the low bulk resistance of the carbon particles themselves. (1) A solid electrolytic capacitor element according to a first aspect of the present disclosure includes an anode body, a dielectric layer formed on a surface of the anode body, a solid electrolyte layer covering at least a portion of the dielectric layer, and a carbon layer covering at least a portion of the solid electrolyte layer and containing carbon particles, wherein a ratio of a D band intensity to a G band intensity (D / G) in a Raman spectrum of the carbon layer is 1.5 or less.

[0015] Furthermore, in the solid electrolytic capacitor element according to the present disclosure, the distance between carbon particles and the distance between the carbon particles and molecules of compounds having acid groups, binders, additives, etc. are shortened as described above, which is believed to reduce the air permeability of the carbon layer. This prevents air from entering the solid electrolyte layer, thereby suppressing oxidative degradation and dedoping of the solid electrolyte layer, even when the solid electrolytic capacitor is exposed to a high-temperature or high-humidity environment. As a result, it is believed that the reduction in ESR when the solid electrolytic capacitor is exposed to a high-temperature or high-humidity environment is reduced. It is also believed that the reduction in ESR when the solid electrolytic capacitor is exposed to a high-temperature or high-humidity environment is reduced. Thus, the solid electrolytic capacitor element according to the present disclosure not only has a low initial ESR, but also maintains a low ESR even after exposure to a high-temperature or high-humidity environment, thereby achieving high reliability.

[0016] (2) The present disclosure also includes a solid electrolytic capacitor including at least one solid electrolytic capacitor element according to (1) above.

[0017] The present disclosure also includes a carbon paste for a solid electrolytic capacitor element that is useful for forming the above-described carbon layer. The carbon paste of the present disclosure has a D / G ratio of 1.5 or less in the Raman spectrum of a dried film of the carbon paste. It can be said that such a dried film is equivalent to a sample of the carbon layer in a solid electrolytic capacitor element in terms of Raman spectrum measurement. More specifically, (3) the carbon paste for a solid electrolytic capacitor element according to the present disclosure is a carbon paste for a solid electrolytic capacitor element that contains carbon particles and a dispersion medium and is used to form a carbon layer included in a solid electrolytic capacitor element. Here, the ratio of the D band intensity to the G band intensity (D / G) in the Raman spectrum of a dried film of the carbon paste is 1.5 or less.

[0018] The Raman spectrum of the dried carbon layer or carbon paste film shows a peak at 1300 cm －１ ~1400cm －１ Around (more specifically, 1350 cm －１ The D band was observed around 1550 cm －１~1650cm －１ Around (more specifically, 1590 cm －１ A G band is observed near the G band. The G band is a peak derived from the graphite structure of the carbon particles, and the D band is a peak derived from structural defects in the carbon particles. It can be said that the smaller the ratio of the D band intensity to the G band intensity (=D / G ratio) in the Raman spectrum, the better the surface condition of the carbon particles contained in the carbon layer or the dried coating of the carbon paste.

[0019] (4) In any one of the above (1) to (3), the D / G ratio may be 1 or less.

[0020] (5) In any one of the above (1) to (4), the carbon particles may have an average particle diameter D50 of 0.1 μm or more and 10 μm or less.

[0021] (6) In any one of the above (1) to (5), the carbon particles may include scaly particles.

[0022] (7) In any one of the above (1) to (6), the carbon layer may contain one or more compounds having an acid group.

[0023] The solid electrolytic capacitor, solid electrolytic capacitor element, and carbon paste of the present disclosure will be described in more detail below, including the above (1) to (7), with reference to the drawings as necessary. At least one of the above (1) to (7) may be combined with at least one of the elements described below, provided that this is not technically inconsistent.

[0024] [Solid Electrolytic Capacitor] A solid electrolytic capacitor includes one or more solid electrolytic capacitor elements, which may hereinafter be simply referred to as capacitor elements.

[0025] (Capacitor Element) (Anode Body) The anode body included in the capacitor element may contain a valve metal, an alloy containing a valve metal, a compound containing a valve metal, or the like. The anode body may contain one of these materials or a combination of two or more of them. Examples of preferred valve metals include aluminum, tantalum, niobium, and titanium. Anode bodies having a porous surface can be obtained by roughening the surface of a substrate containing a valve metal (e.g., a sheet-like (e.g., foil-like, plate-like) substrate) by etching or the like. The roughening can be performed by etching or the like. The anode body may also be a compact or sintered body of particles containing a valve metal. Note that both the compact and the sintered body have a porous structure. The compact and the sintered body may each be in the form of a sheet, a rectangular parallelepiped, a cube, or similar shapes.

[0026] The anode body typically has an anode lead and a cathode forming portion. The cathode portion is typically formed on the cathode forming portion of the anode body via a dielectric layer. An anode lead is connected to the anode lead.

[0027] (Dielectric Layer) The dielectric layer is an insulating layer that functions as a dielectric and is formed so as to cover at least a portion of the surface of the anode body. The dielectric layer is formed by anodizing the valve metal on the surface of the anode body using a chemical conversion treatment or the like. The dielectric layer may be formed so as to cover at least a portion of the anode body. The dielectric layer is usually formed on the surface of the anode body. 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.

[0028] The dielectric layer contains an oxide of a valve metal. For example, when tantalum is used as the valve metal, the dielectric layer contains Ta2O5, and when aluminum is used as the valve metal, the dielectric layer contains Al2O3. However, the dielectric layer is not limited to this, as long as it functions as a dielectric.

[0029] (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 (e.g., 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.

[0030] The solid electrolyte layer contains a conductive polymer and may further contain at least one of a dopant and an additive, as necessary.

[0031] Conductive polymers that can be used include known polymers used in solid electrolytic capacitors, such as π-conjugated conductive polymers. Examples of conductive 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 also include homopolymers, copolymers of two or more monomers, and derivatives thereof (e.g., substituted products having substituent groups). For example, polythiophenes include poly(3,4-ethylenedioxythiophene).

[0032] The conductive polymers may be used alone or in combination of two or more.

[0033] The solid electrolyte layer may further contain a dopant, for example, at least one selected from the group consisting of anions and polyanions.

[0034] Examples of anions include, but are not limited to, sulfate ions, nitrate ions, phosphate ions, borate ions, organic sulfonate ions, and carboxylate ions. Examples of dopants that generate sulfonate ions include benzenesulfonic acid, p-toluenesulfonic acid, and naphthalenesulfonic acid.

[0035] Examples of polyanions include polymeric polysulfonic acids and polymeric polycarboxylic acids. Polymeric polysulfonic acids include polyvinyl sulfonic acid, polystyrene sulfonic acid, polyallyl sulfonic acid, polyacrylic sulfonic acid, and polymethacrylic sulfonic acid. Polymeric polycarboxylic acids include polyacrylic acid and polymethacrylic acid. Polyanions also include polyester sulfonic acid and phenol sulfonic acid novolac resin. However, polyanions are not limited to these.

[0036] 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 bonded to or interacting with the conductive polymer.

[0037] The amount of the dopant contained in the solid electrolyte layer may be, for example, 10 parts by mass or more and 1,000 parts by mass or less, 20 parts by mass or more and 500 parts by mass or less, or 50 parts by mass or more and 200 parts by mass or less, relative to 100 parts by mass of the conductive polymer.

[0038] The solid electrolyte layer may be a single layer or may be composed of multiple layers. When the solid electrolyte layer is composed of multiple layers, the conductive polymer contained in each layer may be the same or different. Furthermore, the dopant contained in each layer may be the same or different.

[0039] The solid electrolyte layer may further contain known additives and known conductive materials other than the conductive polymer component, as needed. 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. A layer for enhancing adhesion may be interposed between the dielectric layer and the solid electrolyte layer.

[0040] The solid electrolyte layer is formed, for example, by using a treatment liquid containing a precursor of a conductive polymer and polymerizing the precursor on the dielectric layer. The polymerization can be performed by at least one of chemical polymerization and electrolytic polymerization. Examples of the precursor of the conductive polymer include a monomer, an oligomer, and a prepolymer. The solid electrolyte layer may be formed by applying a treatment liquid (e.g., a dispersion or solution) containing the conductive polymer to the dielectric layer and then drying it. Examples of the dispersion medium (or solvent) include water, an organic solvent, or a mixture thereof. The treatment liquid may further contain other components (e.g., at least one selected from the group consisting of a dopant and an additive).

[0041] When a treatment liquid containing a precursor of a conductive 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 oxidizing agent (such as ferric sulfate), persulfates (such as sodium persulfate and ammonium persulfate), and hydrogen peroxide. The oxidizing agent can be used alone or in combination of two or more.

[0042] 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 conditions, such as the composition and viscosity of the treatment solution, remaining the same for each time, or at least one of the conditions may be changed.

[0043] (Cathode extraction layer) A capacitor element typically includes a cathode extraction layer that covers at least a portion of the solid electrolyte layer. The solid electrolyte layer and the cathode extraction layer may form a cathode portion of the capacitor element. In the capacitor element of the present disclosure, the cathode extraction layer includes a carbon layer that covers at least a portion of the solid electrolyte layer. The cathode extraction layer may include a carbon layer and a metal-containing layer that covers a portion of the carbon layer.

[0044] (Carbon Layer) The carbon layer contains carbon particles (more specifically, conductive carbon particles). The carbon layer may further contain a compound having an acid group. The carbon layer may further contain a binder. The carbon layer may also contain an additive.

[0045] (D / G Ratio) In the present disclosure, the D / G ratio in the Raman spectrum of the carbon layer is 1.5 or less. Such a small D / G ratio can reduce the initial ESR, as described above, and can also keep the ESR of the solid electrolytic capacitor low even when exposed to a high-temperature environment or a high-temperature, high-humidity environment. The increase in ESR when exposed to a high-temperature environment or a high-temperature, high-humidity environment can be particularly significantly suppressed when the D / G ratio is 1 or less (preferably 0.8 or less).

[0046] The D / G ratio of the carbon layer and the dried coating of the carbon paste is affected by the D / G ratio of the carbon particles (raw material particles) used to prepare the carbon paste for forming the carbon layer, but is different from the D / G ratio of the raw material particles. More specifically, the D / G ratio of the carbon layer and the dried coating of the carbon paste tends to be smaller than the D / G ratio measured on the surface of the raw material particles. This indicates that the surface condition of the carbon particles in the carbon layer or the dried coating is improved compared to the surface condition of the raw material carbon particles. The D / G ratio of the carbon layer and the dried coating can be adjusted, for example, by using carbon particles (raw material particles) with a relatively small D / G ratio, using carbon particles with a relatively small average particle diameter D50, selecting the shape of the carbon particles, selecting the type of compound having an acid group, binder, or additive, or adjusting the amounts of these.

[0047] (Carbon Particles) The average particle diameter D50 of the carbon particles is, for example, 0.1 μm or more and 15 μm or less. When the D / G ratio is within the above range, as the D50 of the carbon particles decreases, the initial ESR and the fluctuations in ESR after exposure to a high-temperature environment tend to decrease. As the D50 of the carbon particles decreases, the contact points between the solid electrolyte layer and the carbon particles increase, which is effective in reducing the initial ESR. From the viewpoint of further reducing the fluctuations in the initial ESR and ESR after exposure to a high-temperature environment, the D50 of the carbon particles is preferably 0.1 μm or more and 10 μm or less, more preferably 0.1 μm or more and 5 μm or less, and may be 0.1 μm or more and 2 μm or less (or 1 μm or less).

[0048] In this specification, the average particle diameter D50 refers to the cumulative 50% particle diameter (median diameter) in a volume-based particle size distribution measured using a particle size distribution measuring device by dynamic light scattering (DLS).

[0049] The shape of the carbon particles is not particularly limited. From the viewpoint of easily obtaining π-π interactions between the carbon particles in the carbon layer and easily increasing the packing rate of the carbon particles, it is preferable that the carbon layer contains at least flaky carbon particles. Examples of the flaky carbon particles include flaky graphite particles and flaky graphene particles.

[0050] The average aspect ratio of the scaly carbon particles is, for example, from 1 to 20,000, and may be from 1 to 7,000. When the average aspect ratio is in this range, the conductive paths in the carbon layer are more easily widened, which is advantageous in terms of increasing the conductivity.

[0051] From the viewpoint of easily adjusting the D / G ratio in the carbon layer or dried coating of the carbon paste to the above range, graphite is preferred as the carbon particles. Graphite refers to a material having a graphite-type crystal structure. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. The carbon particles may contain one type of these graphites or a combination of two or more types.

[0052] The carbon layer may contain a carbonaceous material (second carbonaceous material) other than graphite (first carbonaceous material). Examples of the second carbonaceous material include carbon black, carbon nanotubes, and graphene. However, it is preferable that the proportion of the first carbonaceous material in the entire carbonaceous material contained in the carbon layer is high so that the D / G ratio of the Raman spectrum of the carbon layer or the dried coating of the carbon paste falls within the above range. The proportion of the first carbonaceous material (graphite particles) in the entire carbonaceous material contained in the carbon layer is, for example, 90 mass% or more, and may even be 95 mass% or more. The proportion of the first carbonaceous material (graphite particles) in the entire carbonaceous material contained in the carbon layer is 100 mass% or less. The carbonaceous material contained in the carbon layer may be composed of graphite particles alone.

[0053] The content of carbon particles (first carbonaceous material) in the carbon layer is, for example, 50% by mass to 95% by mass, or may be 50% by mass to 75% by mass. When the carbon particle content is in this range, it is easy to achieve a balance between high electrical conductivity of the carbon layer and high adhesion between the carbon layer and the solid electrolyte layer.

[0054] (Compound Having an Acid Group) The carbon layer may contain a compound having an acid group. The acid group may be contained in the carbon layer in a free form, an anion form, or a salt form, or may be contained in a form bonded to or interacting with other components contained in the carbon layer or the solid electrolyte layer. The conductive polymer contained in the solid electrolyte layer has a functional group that is easily positively charged. Therefore, when the carbon layer contains a compound having an acid group, the acid group bonds with the functional group of the conductive polymer in the solid electrolyte layer, thereby enhancing the adhesion between the solid electrolyte layer and the carbon layer. The compound having an acid group may have an aromatic ring or may be a polymer. When the compound having an acid group has an aromatic ring, the π-π interaction with the carbon particles enhances the adhesion between the compound having an acid group and the carbon particles. When the compound having an acid group is a polymer, the polymer chain becomes entangled with the carbon particles, thereby further enhancing the adhesion between the compound having an acid group and the carbon particles. These measures further reduce the permeation of air through the carbon layer, and also improve the adhesion between the carbon layer and the solid electrolyte layer, thereby further reducing the intrusion of air into the solid electrolyte layer.

[0055] Examples of such compounds include aromatic compounds having acid groups and resins having acid groups. Examples of acid groups include sulfonic acid groups, carboxy groups, and phenolic hydroxy groups. The compound having an acid group may have one type of acid group or two or more types. The acid group of the aromatic compound is preferably a sulfonic acid group. The carbon layer may contain one type of compound having an acid group or two or more types. The carbon layer may contain either an aromatic compound having an acid group or a resin having an acid group, or both.

[0056] Examples of aromatic compounds having a sulfonic acid group include aromatic compounds having a carbon number of 6 to 14 and having a sulfonic acid group. Specific examples of such aromatic compounds include, but are not limited to, benzenesulfonic acid, toluenesulfonic acid, and naphthalenesulfonic acid. The aromatic compound has, for example, one to three sulfonic acid groups, and preferably one or two.

[0057] The aromatic compound having a sulfonic acid group also includes an oligomer containing a polymerizable monomer having an aromatic ring with a sulfonic acid group as a monomer unit. The oligomer may contain only such a monomer unit, or may contain other monomer units in addition to such a monomer. The degree of polymerization of the monomer unit in the oligomer may be, for example, 2 or more and 100 or less, or 2 or more and 50 or less.

[0058] The weight average molecular weight (Mw) of the aromatic compound having a sulfonic acid group is, for example, 20,000 or less, or may be 10,000 or less, or 5,000 or less, or may be 1,000 or less.

[0059] The carbon layer may contain one type of aromatic compound having a sulfonic acid group, or may contain two or more types in combination.

[0060] 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.

[0061] The resin having an acid group may have one type of acid group or two or more types of acid groups.

[0062] Among resins having an acid group, resins having a sulfonic acid group include, for example, polymers (more specifically, homopolymers or copolymers) containing a polymerizable monomer having a sulfonic acid group as a monomer unit, such as polystyrene sulfonic acid, polyvinyl sulfonic acid, polyallylsulfonic acid, phenolsulfonic acid novolac resin, and poly-2-acrylamido-2-methylpropanesulfonic acid.

[0063] Among resins having an acid group, resins having a carboxy group include, for example, polymers (more specifically, homopolymers or copolymers) containing a polymerizable monomer having a carboxy group as a monomer unit. Examples of such polymers include polyacrylic acid, polymethacrylic acid, acrylic acid-methacrylic acid copolymer, ethylene-acrylic acid copolymer, (meth)acrylic acid-(meth)acrylic acid ester copolymer (acrylic acid-methyl methacrylate, etc.). Acrylic acid and methacrylic acid are sometimes collectively referred to as (meth)acrylic acid. Acrylic acid esters and methacrylic acid esters are sometimes collectively referred to as (meth)acrylic acid esters.

[0064] As the resin having a carboxy group, a cellulose-based resin (such as cellulose ether) having a carboxy group is also preferred. Examples of such a cellulose-based resin include carboxymethyl cellulose (CMC) or its salts (ammonium salt, sodium salt, potassium salt, etc.). CMC or its salts also function as a binder.

[0065] Examples of resins having a phenolic hydroxyl group include phenolic resins (such as phenolic resins having a phenolic hydroxyl group).Phenol resins also function as binders.

[0066] The use of resins that function as binders, such as CMC or its salts and phenolic resins, facilitates high adhesion between carbon particles and is also advantageous in further enhancing adhesion between the carbon layer and the solid electrolyte layer. When using such resins, the binders described below may be omitted, or they may be combined with one or more of the binders described below as needed. Furthermore, from the viewpoint of facilitating even higher adhesion between the resin having acid groups and the carbon particles due to the π-π interaction between the carbon particles and the aromatic rings and the entanglement of the polymer chains with the carbon particles, it is also preferable to use phenolic resins, phenolsulfonic acid novolac resins, and the like, as the resin having acid groups. In particular, the use of at least a resin having sulfonic acid groups and an aromatic ring (such as phenolsulfonic acid novolac resin) is more preferable because it can further enhance adhesion to the carbon particles.

[0067] The carbon layer may contain one or more resins having an acid group. For example, a resin having a sulfonic acid group (such as a phenolsulfonic acid novolac resin) may be combined with at least one resin selected from the group consisting of a resin having a carboxy group (such as CMC or a salt thereof) and a resin having a phenolic hydroxy group (such as a phenolic resin). In addition to these resins, the carbon layer may also contain an aromatic compound having a sulfonic acid group.

[0068] The weight average molecular weight (Mw) of the resin having an acid group is larger than that of an aromatic compound (such as an oligomer) having an acid group, although this depends on the type of resin. The Mw of the resin having an acid group may be, for example, larger than 1,000, larger than 10,000, larger than 50,000, or larger than 100,000. The Mw of the resin having an acid group is, for example, 3,000,000 or less.

[0069] The total amount of the compound having an acid group contained in the carbon layer is, for example, 10 parts by mass or more and 100 parts by mass or less, or may be 10 parts by mass or more and 80 parts by mass or less, or 10 parts by mass or more and 50 parts by mass or less, or may be 20 parts by mass or more and 40 parts by mass or less, relative to 100 parts by mass of the carbon particles.

[0070] (Binder) Examples of binders include organic polymers. The organic polymers may be either curable resins or thermoplastic resins. The curable resins may be either thermosetting resins or photocurable resins. The binder may be a curable resin composition containing a curable resin and at least one selected from the group consisting of a polymerization initiator, a curing agent, a curing accelerator, and a curing catalyst. A component different from the resin having an acid group is usually used as the binder. A resin without an acid group is usually used as the binder.

[0071] Examples of organic polymers include cellulose-based resins (such as cellulose ethers and cellulose esters), epoxy resins, acrylic resins (such as polymethyl acrylate and polymethyl methacrylate), polyimide resins, polyamide resins, polyamideimide resins, polyester resins (such as aromatic polyesters, such as polyethylene terephthalate, and aliphatic polyesters), fluororesins, polyurethane resins, vinyl resins, polyolefin resins, and rubber-like materials. Vinyl resins also include polyvinyl acetate and its saponified products (such as partially saponified polyvinyl acetate and polyvinyl alcohol). These organic polymers may be either curable (such as thermosetting or photocurable) or thermoplastic. One type of organic polymer may be used, or two or more types may be used in combination. Either hydrophilic or hydrophobic organic polymers may be used as the organic polymer, or these organic polymers may be used in combination. Of these, epoxy resins, polyvinyl alcohol, and the like are preferred.

[0072] The amount of binder contained in the carbon layer is, for example, 10 parts by mass or more and 50 parts by mass or less, or may be 15 parts by mass or more and 40 parts by mass or less, or may be 20 parts by mass or more and 40 parts by mass or less, relative to 100 parts by mass of carbon particles.

[0073] (Additives) The additives are not particularly limited and include, for example, dispersants, surfactants, antioxidants, preservatives, bases, and acids. The carbon layer may contain one type of additive or a combination of two or more types of additives.

[0074] (Others) The thickness of the carbon layer may be, for example, 0.1 μm or more and 100 μm or less, or 0.5 μm or more and 50 μm or less.

[0075] The carbon layer is formed, for example, using a carbon paste containing the constituent components of the carbon layer and a dispersion medium. More specifically, the carbon layer can be formed by immersing an anode body having a solid electrolyte layer formed thereon in the carbon paste, or by applying the carbon paste to the surface of the solid electrolyte layer. For example, the carbon layer is formed by drying a coating of the carbon paste formed so as to cover the solid electrolyte layer. Drying may be performed under heating, if necessary.

[0076] (Carbon Paste) The present disclosure also encompasses a carbon paste for forming a carbon layer. As described above, the carbon paste has a D / G ratio in the Raman spectrum of a dried film of 1.5 or less, preferably 1 or less (or 0.8 or less).

[0077] The carbon paste is prepared by mixing the components of the carbon layer with a dispersion medium, for example. The dispersion medium is, for example, a medium that is liquid at room temperature (a temperature of 20° C. or higher and 35° C. or lower).

[0078] The dispersion medium is selected depending on the type of compound having an acid group or binder, and may be either water or an organic liquid medium. The organic liquid medium is not particularly limited, and examples thereof include alcohols (including terpene alcohols), ketones, esters, ethers, amides, nitriles, sulfoxides, and hydrocarbons (including terpenes). The carbon paste may contain one type of dispersion medium or a combination of two or more types of dispersion mediums. The carbon paste may contain one type of organic liquid medium or two or more types of organic liquid mediums.

[0079] The carbon paste can be obtained by mixing the constituent components. The mixing can be performed by a known method, for example, a method using a mixer, a kneader, a homogenizer, etc. In preparing the carbon paste, a defoaming treatment may be performed as necessary.

[0080] The content of carbon particles in the dry solid content in the carbon paste, the amount of the compound having an acid group (such as the amount of the aromatic compound having a sulfonic acid group and the amount of the resin having an acid group) relative to 100 parts by mass of the carbon particles, and the amount of the binder can each be selected from the range of content or amount for the carbon layer.

[0081] The viscosity of the carbon paste at 25° C. is, for example, 3000 mPa·s or less, and may be 1000 mPa·s or less. Carbon pastes with low viscosities of 300 mPa·s or less or 100 mPa·s or less at 25° C. can also be prepared. The viscosity of the carbon paste can be measured using a Brookfield viscometer at a rotation speed of 10 rpm.

[0082] <Analysis of Carbon Layer, Carbon Paste, or Constituent Component> (1) Raman Spectrum In this specification, the Raman spectrum of a dried carbon layer or carbon paste coating refers to a Raman spectrum measured on a cross section of a dried carbon layer or carbon paste coating under the following conditions: Raman spectrometer: NanoPhoton RamanFORCE PAV; Irradiated laser light wavelength: 532 nm; Laser power density: 870 W / cm ２ Diffraction grating: 300 / cm Exposure time: 10 s Measurement wave number range: 0 cm －１ More than 4700cm －１ Below temperature: 25℃

[0083] For Raman spectroscopy, samples prepared using the following procedure can be used. First, a solid electrolytic capacitor is embedded in a curable resin, and the curable resin is cured. The cured product is polished or cross-section polished to remove the carbon layer from the surface, exposing a cross section parallel to the surface of the carbon layer. In this way, a measurement sample (Sample A) is obtained. For the dried carbon paste coating, the carbon paste is applied to the surface of a polyethylene terephthalate (PET) sheet to a thickness of 10 μm after drying, and then dried at 100°C for 1 hour to form a dry coating. This dried coating is then embedded in a curable resin together with the PET sheet, and the curable resin is cured. A cross section parallel to the surface of the dried coating is exposed to obtain a measurement sample (Sample B) in the same manner as above, except that the cured product is obtained by exposing the cross section. Raman spectra are measured for the exposed cross sections of Samples A and B. The Raman spectrum of the carbon particles (raw material particles) used in preparing the carbon paste can be obtained by thoroughly drying the carbon particles and then directly measuring the surface of the carbon particles under the same conditions as above, without embedding them in a curable resin.

[0084] (2) Average Particle Diameter D50 As a particle size distribution measuring device using the dynamic light scattering method, for example, a light scattering photometer DLS-8000 manufactured by Otsuka Electronics Co., Ltd. is used.

[0085] When D50 is determined for a carbonaceous material extracted from a carbon paste that forms a carbon layer, a dispersion containing sample F obtained by the following procedure is used as the sample for measuring D50. A predetermined amount of carbon paste is extracted and dried under reduced pressure to obtain a dried product (sample C). An appropriate amount of water is added to the dried product (sample D). The resulting mixture (sample D) is centrifuged to separate it into a solid (sample E) and a liquid (liquid I). The solid sample E is washed with water, washed with an organic solvent, and dried to obtain a carbonaceous material (sample F). The liquid (liquid II) obtained by washing with water and an organic solvent is collected separately. A dispersion for measurement is prepared by dispersing sample F in a liquid dispersion medium using a surfactant.

[0086] Furthermore, when determining D50 for a carbonaceous material extracted from the carbon layer of a solid electrolytic capacitor element, a dispersion containing sample K obtained by the following procedure is used as the sample for measuring D50. First, a solid electrolytic capacitor is embedded in a curable resin and the curable resin is cured to prepare a sample. The sample is subjected to polishing, milling, or other processes to expose the carbon layer on the capacitor element. The exposed carbon particles are scraped off to obtain a predetermined amount of sample (sample G). Sample G is mixed with a 1.0% by mass aqueous nitric acid solution and left at room temperature (20°C to 35°C) for one day. The resulting mixture (sample H) is centrifuged to separate it into a solid (sample J) and a liquid (liquid III). The solid sample J is washed with water, an organic solvent, and dried to obtain a carbonaceous material (sample K). The liquid (liquid IV) obtained by washing with water and an organic solvent is collected separately. A dispersion liquid for measurement is prepared by dispersing sample K in a liquid dispersion medium using a surfactant.

[0087] As the organic solvent for washing the solid sample E or sample J, for example, one that can dissolve the polymer components that cannot be removed by washing with water may be selected from the organic liquid media exemplified for the carbon paste. As the dispersion medium for preparing the dispersion, for example, pure water or an organic medium that is liquid at room temperature (e.g., 20°C or higher and 35°C or lower) may be used. The type and concentration of the surfactant, the type of dispersion medium, and the concentration of sample F or sample K in the dispersion may each be selected within a range that allows the preparation of a dispersion suitable for measuring D50.

[0088] (3) Ratios of First Carbonaceous Material, Compound Having Acid Group, Binder, and Additive When the ratio of carbon particles (first carbonaceous material) is determined from a carbon paste, first, the mass of the above-mentioned sample F is measured, and the type of carbonaceous material is identified from sample F using a known analytical method. When two or more types of carbonaceous materials are contained, they are separated using a known separation method, and the ratio of each carbonaceous material is determined. The mass of the first carbonaceous material is determined from this ratio and the mass of sample F. The compound having an acid group, binder, and additive are separated from liquid I and liquid II recovered when obtaining the above-mentioned sample E and sample F using a known separation method, and the masses of the compound having an acid group, binder, and additive are measured. The types of the compound having an acid group, binder, and additive are identified using a known analytical method. The ratio (mass %) of each component is determined by dividing the mass of each component by the sum of the mass of the separated compound having an acid group, binder, and additive and the mass of the carbonaceous material, and converting the result into a percentage.

[0089] When the content of carbon particles (first carbonaceous material) is determined from the carbon layer, the content (mass%) of the first carbonaceous material can be determined in the same manner as when determining it from the carbon paste, except that Sample K is used instead of Sample F, and Liquids III and IV are used instead of Liquids I and II. In the case of a compound having an acid group, a binder, or an additive, the content can also be determined from the carbon layer in the same manner as in the case of the carbon paste.

[0090] When a compound having an acid group is contained in the carbon layer or the carbon paste, the presence of the acid group can be confirmed using the above-mentioned Liquids I and II, or Liquids III and IV, with a Fourier transform infrared spectrometer (FTIR) or the like.

[0091] (Metal-Containing Layer) Examples of the metal-containing layer include a metal particle-containing layer and a metal foil. Examples of the metal particles include silver particles and silver alloy particles. The metal particle-containing layer can be formed, for example, by laminating a paste containing metal particles on the surface of a carbon layer. The paste is prepared, for example, by mixing metal particles, a resin (also referred to as a binder resin), and, if necessary, a liquid medium. Although a thermoplastic resin can be used as the resin, it is preferable to use a thermosetting resin such as an imide resin or an epoxy resin.

[0092] The type of metal constituting the metal foil 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 different from the metal constituting the metal foil (also referred to as a dissimilar metal) or a non-metal. Examples of dissimilar metals include metals such as titanium. Examples of non-metals include carbon (e.g., conductive carbon).

[0093] (Other) The solid electrolytic capacitor may be a wound type, and may be either a chip type or a laminated type. The solid electrolytic capacitor may include at least one capacitor element, or may include multiple capacitor elements. For example, the solid electrolytic capacitor may include a laminate of two or more capacitor elements. When the solid electrolytic capacitor includes multiple capacitor elements, each capacitor element may be, for example, a wound type or a laminated type. The configuration of the capacitor element may be selected depending on the type of solid electrolytic capacitor.

[0094] In the capacitor element, one end of the cathode lead is electrically connected to the cathode extraction layer. One end of the anode lead is electrically connected to the anode body. The other end of the anode lead and the other end of the cathode lead are each drawn out from the resin exterior body or the case. The other end of each lead exposed from the resin exterior body or the case is used for soldering to a substrate on which the solid electrolytic capacitor is to be mounted. Each lead may be a lead wire or a lead frame.

[0095] The capacitor element is sealed using a resin outer casing or case. For example, the capacitor element and the resin material of the outer casing (e.g., an uncured thermosetting resin and a filler) may be placed in a mold, and the capacitor element may be sealed in the resin outer casing 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.

[0096] 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.

[0097] 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 carbon layer 11 covering solid electrolyte layer 9, and a metal-containing layer 12 covering carbon layer 11.

[0098] 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 an adhesive layer 14 formed of a conductive adhesive.

[0099] EXAMPLES The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0100] <Solid Electrolytic Capacitors E1 to E4 and C1 to C2> The solid electrolytic capacitors shown in FIG. 1 were fabricated in the following manner, and their characteristics were evaluated.

[0101] (1) Preparation of Anode Body 6 Both surfaces of an aluminum foil (thickness: 100 μm) serving as a substrate were roughened by etching to prepare an anode body 6 .

[0102] (2) Formation of Dielectric Layer 7 The cathode forming portion of the anode body 6 was immersed in a chemical conversion solution, and a DC voltage of 70 V was applied for 20 minutes to form a dielectric layer 7 containing aluminum oxide.

[0103] (3) Formation of Solid Electrolyte Layer 9 Separation portion 13 was formed by attaching insulating resist tape between a region of anode body 6 on which dielectric layer 7 was formed, where a solid electrolyte layer was to be formed, and a region of anode body 6 on which a solid electrolyte layer was not to be formed.

[0104] A polymerization solution containing pyrrole (a monomer of a conductive polymer), naphthalenesulfonic acid (a dopant), and water was prepared. A portion of the anode body 6 and a Ti electrode serving as a counter electrode were immersed in the polymerization solution, and a voltage was applied to the anode body so that the potential of the anode body 6 relative to a silver / silver chloride reference electrode was 2.8 V to perform electrolytic polymerization. A solid electrolyte layer 9 was formed on the portion of the anode body 6.

[0105] (4) Formation of Cathode Extraction Layer 10 (4-1) Formation of Carbon Layer 11 Carbon paste was applied to the surface of the anode body 6 obtained in (3) above and dried to form a carbon layer 11 (thickness: 10 μm) at least on the surface of the solid electrolyte layer 9. Drying was performed at 100 to 250° C. for 5 to 30 minutes.

[0106] The carbon paste was prepared by adding flake graphite particles and, if necessary, at least one of the acid group-containing compound and binder shown in Table 1 to a dispersion medium and mixing them using a bead mill. The D / G ratio and D50 of the graphite particles used were adjusted so that the D / G ratio of the dried carbon paste coating would be the value shown in the table. A liquid medium was used as the dispersion medium, depending on the type of acid group-containing compound and binder, so as to dissolve these components and obtain a uniform paste. The viscosity of the carbon paste measured using the above-described procedure was 20 mPa·s at 25°C. The graphite particle content in the carbon paste was 10% by mass (graphite particle content in the dry solids content of the carbon paste: 66.7% by mass). Per 100 parts by mass of graphite particles, the amounts of the CMC salt and phenolic resin among the acid group-containing compounds were 20 parts by mass, and the amount of phenolsulfonic acid novolac resin was 30 parts by mass. In E3, 20 parts by mass of epoxy resin was used as a binder with respect to 100 parts by mass of graphite particles.

[0107] (4-2) Formation of Metal-Containing Layer 12 A silver paste containing silver particles and a binder resin (epoxy resin) was applied to the surface of carbon layer 11, and then heated at a temperature of 100 to 250°C for 5 to 30 minutes to harden the binder resin, thereby forming metal-containing layer 12, which is a silver paste layer. In this way, cathode extraction layer 10 composed of carbon layer 11 and metal-containing layer 12 was formed. Capacitor element 2 was produced in the above manner.

[0108] (5) Assembly of a Solid Electrolytic Capacitor The cathode portion 8 of the capacitor element 2 obtained in (4) above was joined to one end of the cathode terminal 5 with an adhesive layer 14 of a conductive adhesive. One end of the anode body 6 protruding from the capacitor element 2 was joined to one end of the anode terminal 4 by laser welding. Next, a resin outer casing 3 made of an insulating resin was formed around the capacitor element 2 by molding. At this time, the other end of the anode terminal 4 and the other end of the cathode terminal 5 were pulled out from the resin outer casing 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.

[0109] [Evaluation] The solid electrolytic capacitors of the examples and comparative examples prepared above were evaluated as follows.

[0110] (1) D / G Ratio of Dried Film of Carbon Paste Using the carbon paste used to form the carbon layer, the Raman spectrum of the dried film was measured according to the procedure described above, and the D / G ratio was determined.

[0111] (2) D50 of Graphite Particles Using the carbon paste used to form the carbon layer, the D50 of the carbon particles (graphite particles) contained in the carbon paste was determined according to the procedure described above. The D50 was roughly consistent with the D50 of the graphite particles used.

[0112] (3) Initial ESR In an environment of 20°C, the ESR value (mΩ) of the solid electrolytic capacitor at a frequency of 100 kHz was measured using an LCR meter for four-terminal measurement. ０ The initial ESR of each solid electrolytic capacitor was expressed as a relative value when the initial ESR value of the solid electrolytic capacitor C1 was set to 1.

[0113] (4) ESR Change After High-Temperature Test The solid electrolytic capacitors whose initial ESR had been measured were subjected to a high-temperature test by being placed in a 125°C environment for 1000 hours. The ESR value (X １ ) (mΩ) to X ０ The change in ESR was measured in the same manner as in Example 1. The change in ESR was calculated using the following formula: Change in ESR = X １ / X ０

[0114] (5) ESR Change After High-Temperature, High-Dampness Test The solid electrolytic capacitors whose initial ESR had been measured were subjected to a high-temperature, high-humidity test by being placed in an environment of 125°C and 80% RH for 1000 hours. The ESR value (X ２ ) (mΩ) to X ０ The change in ESR was measured in the same manner as in Example 1. The change in ESR was calculated using the following formula: Change in ESR = X ２ / X ０

[0115] The evaluation results are shown in Table 1. Solid electrolytic capacitors E1 to E4 are examples, and C1 and C2 are comparative examples.

[0116]

[0117] As shown in Table 1, when the D / G ratio of the dried coating (or carbon layer) of the carbon paste was 1.5 or less, the ESR change after the high-temperature test was kept low, and the initial ESR was also low (E1 to E4). Furthermore, for E1 to E4, the ESR change after the high-temperature, high-humidity test was also lower than in the comparative examples. When the D / G ratio was 1.5 or less, the ESR change after the high-temperature test or high-temperature, high-humidity test tended to decrease as the D50 of the carbon particles contained in the carbon layer decreased. When the D / G ratio was 1 or less (preferably 0.8 or less), the ESR change after the high-temperature test or high-temperature, high-humidity test became significantly smaller.

[0118] 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.

[0119] According to the present disclosure, the solid electrolytic capacitor can maintain a low ESR even when exposed to a high-temperature environment or a high-temperature, high-humidity environment, and therefore can be used in a variety of applications requiring high reliability.

[0120] 1: Solid electrolytic capacitor 2: Capacitor element 3: Resin exterior body 4: Anode terminal 5: Cathode terminal 6: Anode body 7: Dielectric layer 8: Cathode portion 9: Solid electrolyte layer 10: Cathode lead layer 11: Carbon layer 12: Metal-containing layer 13: Separation portion 14: Adhesive layer

Claims

1. an anode body; a dielectric layer formed on a surface of the anode body; a solid electrolyte layer covering at least a portion of the dielectric layer; and a carbon layer covering at least a portion of the solid electrolyte layer and containing carbon particles; A solid electrolytic capacitor element, wherein the ratio of the intensity of a D band to the intensity of a G band in a Raman spectrum of the carbon layer: D / G is 1.5 or less.

2. 2. The solid electrolytic capacitor element according to claim 1, wherein the D / G ratio is 1 or less.

3. 3. The solid electrolytic capacitor element according to claim 1, wherein the carbon particles have an average particle diameter D50 of 0.1 μm or more and 10 μm or less.

4. The solid electrolytic capacitor element according to claim 1 , wherein the carbon particles include scaly particles.

5. 3. The solid electrolytic capacitor element according to claim 1, wherein the carbon layer contains one or more compounds having an acid group.

6. A solid electrolytic capacitor comprising at least one solid electrolytic capacitor element according to claim 1 or 2.

7. A carbon paste for a solid electrolytic capacitor element, comprising carbon particles and a dispersion medium, for forming a carbon layer included in a solid electrolytic capacitor element, A carbon paste for a solid electrolytic capacitor element, wherein the ratio of D band intensity to G band intensity in a Raman spectrum of a dried film of the carbon paste: D / G is 1.5 or less.