Solid electrolytic capacitor

Abstract

This solid electrolytic capacitor includes a first electrode, a dielectric layer covering at least a part of the first electrode, and a second electrode covering at least a part of the dielectric layer. The second electrode includes a solid electrolyte layer, a conductive carbon layer covering at least a part of the solid electrolyte layer, and a metal particle layer covering at least a part of the carbon layer. The solid electrolyte layer includes a pyrrole electrolytic polymerization layer, the carbon layer includes carbon particles and a binder, and the binder includes rubber particles.

Description

Solid electrolytic capacitor Cross-reference to related applications

[0001] This disclosure claims the benefit of priority for Japanese Patent Application No. 2024-169322, filed on September 27, 2024, with the Japan Patent Office, and the entire content of the said patent application is incorporated herein by reference.

[0002] The present invention relates to a solid electrolytic capacitor.

[0003] A solid electrolytic capacitor generally includes a first electrode, a dielectric layer formed on the first electrode, and a second electrode formed on the dielectric layer. The second electrode generally includes a solid electrolyte layer and an electrode lead-out layer formed on the solid electrolyte layer. The electrode lead-out layer includes, for example, a carbon layer and a metal particle layer formed on the carbon layer. A conductive polymer is used in the solid electrolyte layer. In particular, the development of solid electrolytic capacitors using polypyrrole, which has excellent conductivity, is progressing.

[0004] Patent Document 1 proposes, in a "solid electrolytic capacitor formed by sequentially laminating a solid electrolyte layer made of a conductive polymer, a carbon conductor layer, and a silver conductor layer on the surface of a sintered body formed of valve action metal powder after forming a dielectric oxide film on the surface of the sintered body to form a capacitor anode body, characterized in that the heat treatment temperature for forming the carbon conductor layer is 30 to 120°C", "using at least one of polyester resin, epoxy resin, acrylic resin, phenolic resin, and rubber-based resin as a binder for the carbon conductor layer and the silver conductor layer".

[0005] Japanese Patent Application Laid-Open No. 2006-339182

[0006] Polypyrrole can be synthesized by chemical polymerization using an oxidizing agent, but residues of the oxidizing agent are mixed in as impurities. On the other hand, attempts have also been made to synthesize polypyrrole by electrolytic polymerization. In electrolytic polymerization, impurities are difficult to be doped, and a dense electrolytic polymerization layer can be formed, so high capacitance and high performance can be expected.

[0007] However, with the increasing performance of electronic devices, the demand for improved performance in solid electrolytic capacitors is constantly rising. Recent solid electrolytic capacitors are required to maintain their performance even under severe heat cycle testing. Under severe heat cycle testing, a phenomenon is observed in which the ESR (Equivalent Series Resistance) of solid electrolytic capacitors increases significantly.

[0008] In view of the foregoing, the first aspect of the present disclosure relates to a solid electrolytic capacitor comprising: a first electrode; a dielectric layer covering at least a portion of the first electrode; and a second electrode covering at least a portion of the dielectric layer, wherein the second electrode comprises a solid electrolyte layer; a conductive carbon layer covering at least a portion of the solid electrolyte layer; and a metal particle layer covering at least a portion of the carbon layer, wherein the solid electrolyte layer includes a pyrrole electrolytic polymerization layer; and the carbon layer includes carbon particles and a binder, wherein the binder includes rubber particles.

[0009] According to this disclosure, the rise in ESR can be significantly suppressed in a solid electrolytic capacitor in which the solid electrolyte layer comprises a pyrrole electrolytic polymerization layer. Novel features of the present invention are described in the appended claims, but the present invention, both in terms of structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in reference to the drawings.

[0010] This is a schematic cross-sectional view of a capacitor element of a solid electrolytic capacitor according to one embodiment of the present disclosure.

[0011] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be given as examples, but other numerical values, materials, etc. may be applied as long as the effects of this disclosure are obtained. In addition, components other than those characteristic of this disclosure may be replaced with components of known capacitors. In this specification, when "range of numerical value A to numerical value B" is used, that range includes numerical values ​​A and B. When multiple materials are given as examples, one of them may be selected and used alone, or two or more may be used in combination.

[0012] Here, "solid electrolytic capacitor" is a general term for electrolytic capacitors that have a solid electrolyte layer. Electrolytic capacitors with a solid electrolyte layer (solid electrolytic capacitors) may further contain liquid components such as electrolyte.

[0013] A solid electrolytic capacitor (hereinafter also referred to as "capacitor (C)") according to one embodiment of the present disclosure comprises a first electrode, a dielectric layer covering at least a portion of the first electrode, and a second electrode covering at least a portion of the dielectric layer. Hereinafter, the smallest unit of a solid electrolytic capacitor including the first electrode, the dielectric layer, and the second electrode may be referred to as a "capacitor element." Capacitor (C) is a concept that encompasses both solid electrolytic capacitors and capacitor elements. Note that "capacitor" may be read as "capacitor."

[0014] A capacitor (C) is divided into an anode and a cathode. The anode and cathode are insulated by a dielectric layer. The first electrode constitutes the anode, and the second electrode constitutes the cathode.

[0015] [First Electrode] The first electrode includes an anode. The anode comprises, for example, a porous sintered body containing a valve-acting metal, or a foil (metal foil) containing a valve-acting metal.

[0016] Examples of valve metals include titanium, tantalum, aluminum, and niobium. The anode may contain one or more valve metals. The first electrode may contain the valve metal in the form of an alloy or compound.

[0017] The porous sintered body may be a sintered body of metal particles containing a valve-acting metal. In particular, the porous sintered body is preferably a sintered body of metal particles containing Ta. That is, the anode may be a porous sintered body containing tantalum.

[0018] Electrode wires may be embedded in the porous sintered body. The electrode wires are used to connect to the anode lead terminals. That is, the anode portion may include a porous sintered anode body and an anode wire having an embedded portion embedded in the anode body and a protruding portion that extends to the outside of the anode body.

[0019] The thickness of the porous sintered body is not particularly limited, but for example, it is between 15 μm and 5 mm. The thickness of the first electrode made of metal foil is not particularly limited, but for example, it is between 15 μm and 300 μm.

[0020] [Second Electrode] The second electrode comprises a solid electrolyte layer, a conductive carbon layer covering at least a portion of the solid electrolyte layer, and a metal particle layer covering at least a portion of the carbon layer. The solid electrolyte layer is formed to cover at least a portion of the dielectric layer formed on the surface of the first electrode (anode). The solid electrolyte layer includes a pyrrole electropolymer layer. The pyrrole electropolymer layer is dense, has a uniform film thickness and high smoothness, and has high conductivity, so high capacity and high performance can be expected. On the other hand, the pyrrole electropolymer layer tends to be highly elastic and have high hardness and rigidity.

[0021] A detailed examination of solid electrolytic capacitors that showed a significant increase in ESR during a rigorous heat cycle test revealed that delamination had occurred at the interface between the solid electrolyte layer and the carbon layer, a phenomenon not observed after previous tests. Furthermore, it was found that the delamination behavior during the heat cycle test differed depending on the composition of the solid electrolyte layer. Therefore, in solid electrolytic capacitors where the solid electrolyte layer is a pyrrole electrolytic polymerization layer, improvements are necessary to suit the properties of the pyrrole electrolytic polymerization layer. To address this, we attempted to improve the carbon layer to suppress delamination between the solid electrolyte layer and the carbon layer. We discovered that delamination was significantly suppressed when the carbon layer contained carbon particles and a binder, and the binder contained rubber particles.

[0022] Rubber particles used as binders in the field of electrochemical devices are generally dispersed in an aqueous solvent. Therefore, carbon layers containing rubber particles as a binder are formed using a paste in which carbon particles and rubber particles are dispersed in an aqueous solvent. Carbon layers formed using such a paste are considered to have excellent adhesion to the pyrrole electropolymer layer. The pyrrole electropolymer layer is composed of polypyrrole, and unlike other conductive polymers such as polythiophene, it is generally highly hydrophilic. Therefore, rubber particles dispersed in an aqueous solvent are suitable as a binder.

[0023] Conventional carbon layers use water-soluble polymers such as carboxymethylcellulose (CMC) as binders. However, carbon layers containing only water-soluble polymers as binders tend to be highly rigid. When both the pyrrole electropolymer layer and the carbon layer have high rigidity, interfacial delamination is likely to occur when stress is applied to the interface between the solid electrolyte layer and the carbon layer. Furthermore, the surface of a carbon layer containing only water-soluble polymers as binders tends to be smooth. Therefore, the adhesion between the uneven pyrrole electropolymer layer and the carbon layer tends to decrease even further. In addition, because water-soluble polymers cover a large portion of the carbon particle surface, the bulk resistance (volume resistivity) of the carbon layer increases.

[0024] In this embodiment, since rubber particles, which have rubber elasticity and are particulate, are used as a binder for the carbon layer, the rigidity of the carbon layer is low. Therefore, when stress is applied to the interface between the solid electrolyte layer and the carbon layer, the carbon layer is thought to have the effect of relieving the stress. Furthermore, the rubber particles are scattered on the surface of the carbon particles and bond the carbon particles to each other by point adhesion. That is, since the rubber particles do not coat the surface of the carbon particles in a film-like manner, the bulk resistance of the carbon layer can be kept low compared to, for example, when only a water-soluble polymer is used as a binder. Also, when rubber particles are scattered on the surface of the carbon particles, the rough shape of the carbon particles is maintained, so the surface of the carbon layer containing rubber particles is thought to be less likely to become smooth. For these reasons, it is thought that delamination at the interface between the solid electrolyte layer and the carbon layer is significantly suppressed, and the increase in the ESR of the solid electrolytic capacitor is reduced. Note that the binder only needs to include rubber particles, and rubber particles may be used in combination with other binders. For example, rubber particles may be used in combination with a water-soluble polymer such as CMC.

[0025] Because the surface of the carbon layer containing rubber particles is difficult to smooth, a significant improvement in adhesion strength due to the anchoring effect with the pyrrole electropolymer layer can be expected. In particular, when the surface area ratio (Sdr) of the solid electrolyte layer at the interface between the pyrrole electropolymer layer and the carbon layer is 0.38 or higher, the adhesion strength between the pyrrole electropolymer layer and the carbon layer is significantly improved.

[0026] The interface area ratio (Sdr) is a parameter measured in accordance with ISO 25178. For example, the Sdr of a perfectly flat surface is 0. The correlation between the adhesion strength between the pyrrole electropolymer layer and the carbon layer and the interface area ratio (Sdr) is relatively high, and controlling the surface state of the pyrrole electropolymer layer using Sdr as an indicator makes it easier to improve the adhesion strength between the pyrrole electropolymer layer and the carbon layer. The interface area ratio (Sdr) of the solid electrolyte layer surface at the interface between the pyrrole electropolymer layer and the carbon layer may be 0.82 or higher, or 1.00 or higher. However, if the interface area ratio (Sdr) is too large, the film quality of the pyrrole electropolymer layer deteriorates, so an Sdr of 3.00 or lower is preferable.

[0027] When the adhesion strength between the pyrrole electropolymer layer and the carbon layer is A, and the adhesion strength between the carbon layer and the metal particle layer is B, it is preferable that the relationship A > B is satisfied. In this case, when stress occurs at the interface between the pyrrole electropolymer layer and the carbon layer, the interface between the carbon layer and the metal particle layer is thought to have a stress-relieving effect, making interfacial delamination between the pyrrole electropolymer layer and the carbon layer even less likely to occur. In particular, when the average particle size of the metal particles is 0.5 μm or more and 12 μm or less, the stress-relieving effect that suppresses interfacial delamination between the pyrrole electropolymer layer and the carbon layer becomes larger while ensuring high conductivity.

[0028] The adhesion strength A between the pyrrole electropolymer layer and the carbon layer, and the adhesion strength B between the carbon layer and the metal particle layer, should be measured according to the "Test Method for Adhesive Tapes and Adhesive Sheets" in accordance with JIS Z0237 (2009). Single films of the pyrrole electropolymer layer, carbon layer, and metal particle layer may be used as the measurement samples.

[0029] For measuring adhesion strength A, a pyrrole electropolymerized layer is formed by passing an arbitrary current through a polymerization solution containing pyrrole monomer and dopant, using a strip-shaped metal plate (such as SUS) of approximately 10 mm x 40 mm as the anode, an anode of any size, and a cathode made of the same material. After washing and drying the pyrrole electropolymerized layer, it is dipped into a dispersion of carbon particles and a binder in water (carbon paste) to form a carbon layer on top of the pyrrole electropolymerized layer. The thickness of the pyrrole electropolymerized layer should be approximately 15 μm to 70 μm, and the thickness of the carbon layer should be approximately 1 μm to 10 μm.

[0030] A sample for measuring adhesion strength B can be prepared by applying carbon paste to the surface of a smooth glass substrate, then applying a metal paste containing metal particles (e.g., silver particles), a binder (e.g., epoxy resin), and a solvent, and curing the laminated coating after drying. The thickness of the carbon layer and the metal particle layer should each be approximately 5 μm to 20 μm.

[0031] <Solid Electrolyte Layer> The solid electrolyte layer may include a pyrrole electropolymerization layer. The pyrrole electropolymerization layer is a layer formed of polypyrrole synthesized by electropolymerization. The pyrrole electropolymerization layer may be formed, for example, by immersing an anode body with a conductive pre-coat layer formed on it in a polymerization solution containing pyrrole monomer and a dopant (e.g., a sulfonate having a naphthalene skeleton), and allowing the electropolymerization reaction of the pyrrole monomer to proceed with the pre-coat layer as the base electrode. Unlike polypyrrole layers formed by other methods such as chemical polymerization, the pyrrole electropolymerization layer formed in this way has a uniform film thickness, high smoothness, is dense and strong, and has high conductivity, so high capacity and high performance can be expected.

[0032] The pre-coat layer is not particularly limited, but may be formed by depositing an oxidizing agent onto the surface of the dielectric layer, immersing the anode in an aqueous solution of pyrrole monomer, and then drying it. Alternatively, it may be formed by depositing a self-doped conductive polymer onto the surface of the dielectric layer and then drying the anode.

[0033] The solid electrolyte layer may contain two or more solid electrolyte components. The solid electrolyte layer may be, for example, a laminate of two or more different layered solid electrolyte components. In this case, the carbon layer may be in direct contact with the pyrrole electropolymerization layer.

[0034] The solid electrolyte portion other than the pyrrole electropolymerization layer may include polymers with a π-conjugated polymer as the basic framework, such as polyaniline, polythiophene, and polyacetylene, as conductive polymers. Such polymers also include derivatives. For example, polymers of monomers having substituents may also be used. Specifically, polythiophene includes poly(3,4-ethylenedioxythiophene), etc. The polypyrrole contained in the pyrrole electropolymerization layer may also be a derivative of polypyrrole.

[0035] The solid electrolyte portion other than the pyrrole electropolymerization layer can be formed, for example, by chemical polymerization and / or electropolymerization of raw material monomers on the dielectric layer. Alternatively, it can be formed by coating the dielectric layer with a solution containing a conductive polymer or a dispersion containing a conductive polymer. Polypyrrole other than the electropolymerization layer may also be used.

[0036] The weight-average molecular weight of the polypyrrole contained in the pyrrole electropolymerization layer is not particularly limited, but is, for example, between 1,000 and 1,000,000.

[0037] Each solid electrolyte portion may contain a dopant. The dopant may be, for example, a low-molecular-weight compound having an acidic group (or anionic group) (hereinafter also referred to as a "low-molecular-weight dopant") or a high-molecular-weight compound (hereinafter also referred to as a "high-molecular-weight dopant"). A single dopant may be used, or two or more may be used in combination.

[0038] As low-molecular-weight dopants, low-molecular-weight compounds having anionic groups can be used. Examples of such compounds include cyclic compounds in which anionic groups are bonded to aromatic rings such as benzene, naphthalene, and anthracene, or to fused rings of aromatic and aliphatic rings. As high-molecular-weight dopants, high-molecular-weight compounds having anionic groups can be used.

[0039] Examples of the anionic group include a sulfonic acid group, a carboxyl group, a phosphoric acid group (—O—P(═O)(—OH)2), a phosphonic acid group (—P(═O)(—OH)2), etc. Among them, the sulfonic acid group is preferable, and a combination of a sulfonic acid group and an anionic group other than the sulfonic acid group may also be used.

[0040] Specific examples of the low molecular weight dopant include alkylbenzene sulfonic acids such as benzenesulfonic acid and p-toluenesulfonic acid, naphthalenesulfonic acid, anthraquinone sulfonic acid, etc.

[0041] Examples of the polymer dopant include a homopolymer or copolymer of a monomer having a sulfonic acid group (for example, a vinyl monomer having a sulfonic acid group). Examples of the vinyl monomer having a sulfonic acid group include aliphatic vinyl monomers having a sulfonic acid group such as vinyl sulfonic acid, allyl sulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid, and aromatic vinyl monomers having a sulfonic acid group such as styrene sulfonic acid.

[0042] The amount of the dopant contained in the solid electrolyte layer is preferably 10 to 1000 parts by mass, and more preferably 50 to 200 parts by mass, relative to 100 parts by mass of the conductive polymer.

[0043] <Carbon layer> The carbon layer and the metal particle layer constitute the cathode lead-out layer. The carbon layer contains carbon particles and a binder. The binder contains rubber particles.

[0044] Examples of the rubber particles include styrene-butadiene copolymer (SBR), styrene conjugated diene copolymer, acrylonitrile-butadiene copolymer, etc. Among them, SBR having appropriate viscosity and elasticity is preferable. SBR may contain monomer units other than styrene units and butadiene units, but it is preferable to use SBR containing 60 mol% or more in total of styrene units and butadiene units. Also, the ratio of styrene units in the total of styrene units and butadiene units is preferably 30 mol% to 70 mol%.

[0045] As monomer units other than styrene units and butadiene units, acrylonitrile units, acrylic acid units, methacrylic acid units, 2-ethylhexyl acrylate units, butyl acrylate units and the like are preferable.

[0046] The average particle diameter of the rubber particles is, for example, 50 nm to 200 nm, preferably 50 to 150 nm, and more preferably 100 to 120 nm. Rubber particles having such a small average particle diameter can sufficiently exhibit the binding property due to point adhesion between fine carbon particles.

[0047] The average particle diameter of the rubber particles can be measured, for example, by a laser diffraction scattering type particle size distribution measuring device (for example, Microtrack manufactured by Nikkiso Co., Ltd.). Specifically, a cumulative volume distribution is obtained, and the particle diameter D50 at which the cumulative volume becomes 50% is determined as the average particle diameter based on volume.

[0048] The content rate of the rubber particles in the carbon layer is, for example, 0.5% by mass or more and 2.0% by mass or less, and may be 1.0% by mass to 1.5% by mass. By setting the content rate of the rubber particles to 0.5% by mass or more, the adhesion between the solid electrolyte layer and the carbon layer can be sufficiently increased, and the effect of suppressing the interfacial peeling between the solid electrolyte layer and the carbon layer becomes more remarkable. Further, by setting the content rate of the rubber particles to 2.0% by mass or less, a large proportion of the carbon particles in the carbon layer can be ensured, so that the conductivity of the carbon layer can be maintained high. When the carbon layer contains a binder other than the rubber particles (for example, a water-soluble polymer), the content rate of the binder other than the rubber particles in the carbon layer is, for example, 0.5% by mass or more and 2.0% by mass or less, and may be 1.0% by mass to 1.5% by mass. The content rate of the binder other than the rubber particles in the carbon layer may be not more than the content rate of the rubber particles.

[0049] The average particle diameter of the carbon particles is, for example, 0.5 μm or more and 5 μm or less, and may be 1.0 μm to 3.0 μm. Carbon particles having such an average particle diameter are excellent in contact with the solid electrolyte layer (pyrrole electrolytic polymerization layer) and exhibit excellent conductivity.

[0050] The average particle size of carbon particles can be determined using a laser diffraction scattering particle size distribution analyzer, similar to how the average particle size of rubber particles is determined.

[0051] The type of carbon material constituting the carbon particles is not particularly limited. Examples of carbon materials include graphite, graphene, carbon black, soft carbon, and hard carbon. As graphite, a carbon material having a graphite-type crystalline structure is used, and either artificial graphite or natural graphite may be used. Carbon nanotubes and carbon fibers may also be used as carbon materials. Fibrous carbon materials such as carbon nanotubes and carbon fibers may be cut to appropriate lengths. These carbon materials may be used individually or in combination of two or more. Among these, graphite particles, which have high crystallinity and excellent conductivity, are preferred as carbon particles.

[0052] The aspect ratio of the carbon particles is preferably around 8.0 (for example, in the range of 5 to 15, preferably in the range of 6 to 10). The aspect ratio of the carbon particles can be determined from a cross-sectional image of the capacitor element (carbon layer). For carbon particles within the observation field, the maximum diameter D1 and the maximum diameter D2 in the direction perpendicular to this maximum diameter D1 are measured, and the aspect ratio of each particle is calculated by dividing D1 by D2. The aspect ratio is calculated for at least 500 carbon particles, and the average value is determined.

[0053] The thickness of the carbon layer is not particularly limited. The average thickness of the carbon layer may be, for example, 0.5 μm to 10 μm, or 1 μm to 3 μm. The average thickness can be determined, for example, by measuring the thickness at multiple locations (for example, 10 locations) of the carbon layer in an electron microscope image of a cross-section in the thickness direction of the carbon layer and averaging the results.

[0054] <Metal Particle Layer> The metal particle layer contains metal particles. The type of metal constituting the metal particles is not particularly limited, but from the viewpoint of conductivity, silver or silver alloy is preferred. Therefore, the metal particle layer may be a silver particle-containing layer containing silver particles or silver alloy particles.

[0055] The shape of the metal particles is not particularly limited. The shape of the metal particles may be, for example, spherical or flaky. The average aspect ratio of spherical metal particles is, for example, less than 1.5. The average aspect ratio of flaky metal particles is 1.5 or more, for example, 2 or more. From the viewpoint of densely arranging the metal particles in the metal particle layer, it is preferable that the metal particles include at least spherical particles. The aspect ratio of metal particles can be determined in the same way as the aspect ratio of carbon particles. That is, the aspect ratio of metal particles can be determined in the same way as the aspect ratio of carbon particles described above, but by replacing "carbon particles" with "metal particles".

[0056] The average particle size of the metal particles is, for example, 0.5 μm to 12 μm, and may be between 1 μm and 5 μm. Metal particles with such an average particle size have excellent contact with the carbon layer and exhibit excellent conductivity. In particular, the metal particle layer preferably contains silver particles between 1 μm and 5 μm.

[0057] The average particle size of metal particles can be determined using a laser diffraction scattering particle size distribution analyzer, similar to the average particle size of carbon particles. Alternatively, the average particle size of metal particles can be determined from a cross-sectional image of a capacitor element (metal particle layer). In this case, the cross-sectional area of ​​at least 500 metal particles within the observation field is calculated, and the average value of the diameter of their corresponding circles is used to determine the average particle size.

[0058] The metal particle layer may further contain a binder. The binder is not particularly limited, but may be the same as the binder in the carbon layer, or a curable resin such as epoxy resin may be used. From the viewpoint of ensuring sufficient conductivity, the content of metal particles in the metal particle layer may be 50% by mass or more, and may be 70% by mass or more.

[0059] The thickness of the metal particle layer is not particularly limited. The average thickness of the metal particle layer may be, for example, 0.1 μm to 50 μm, or 1 μm to 20 μm. The average thickness can be determined, for example, by measuring the thickness at multiple locations (for example, 10 locations) of the metal particle layer in an electron microscope image of the cross-section in the thickness direction of the metal particle layer and averaging the results.

[0060] [Dielectric layer] The dielectric layer is formed, for example, by anodizing the surface of the first electrode through chemical conversion treatment. Therefore, the dielectric layer may contain an oxide of the valve metal. For example, when a sintered body of tantalum particles is anodized, the dielectric layer contains Ta ２ O ５ It includes. Furthermore, if aluminum is used as the valve metal, the dielectric layer contains aluminum oxide. Note that the dielectric layer is not limited to this, and any material that functions as a dielectric is acceptable. The dielectric layer may also be formed using a vapor phase method such as atomic layer volume deposition (ALD).

[0061] A solid electrolytic capacitor according to an example of this disclosure comprises a porous anode body, an anode wire having an embedded portion embedded in the anode body and a protruding portion protruding to the outside of the anode body, a dielectric layer formed on the surface of the anode body and the protruding portion, and a solid electrolyte layer covering at least a portion of the dielectric layer. The solid electrolytic capacitor may also comprise an anode lead terminal electrically connected to the protruding portion, a cathode lead terminal electrically connected to the solid electrolyte layer, and an outer casing covering the anode body, anode wire, dielectric layer and solid electrolyte layer with a portion of the anode lead terminal and a portion of the cathode lead terminal exposed. The cathode lead terminal is joined to the cathode lead layer, for example, via a conductive adhesive.

[0062] The configuration of an example of an electrolytic capacitor related to this disclosure will be described in more detail below with reference to the drawings.

[0063] Figure 1 is a schematic cross-sectional view of a capacitor element 110 of a solid electrolytic capacitor 100 according to one embodiment of the present disclosure. Figure 2 is a schematic cross-sectional view showing an example of a solid electrolytic capacitor 100.

[0064] The solid electrolytic capacitor 100 comprises a capacitor element 110. The capacitor element 110 includes an anode portion 111, a dielectric layer 118 (not shown in Figure 2), and a cathode portion 115. The solid electrolytic capacitor 100 includes the capacitor element 110, an anode lead terminal 120, a cathode lead terminal 130, a conductive adhesive 141, and an outer casing 101.

[0065] The anode lead terminal 120 includes an anode terminal portion 121 and an anode connection portion 122. The anode terminal portion 121 is exposed on the bottom surface 100b of the solid electrolytic capacitor 100. The anode connection portion 122 is connected to the anode wire 112. The cathode lead terminal 130 includes a cathode terminal portion 131 and a cathode connection portion 132. The cathode terminal portion 131 is exposed on the bottom surface 100b of the solid electrolytic capacitor 100. The cathode connection portion 132 is electrically connected to the cathode lead layer by a conductive adhesive 141.

[0066] The anode section 111 includes an anode body 113 and an anode wire 112. The anode body 113 is a sintered body of valve-acting metal particles and has a rectangular parallelepiped shape. The surface of the anode body 113 is covered with a dielectric layer 118 (not shown in Figure 2), and the surface of the protruding portion 112A of the anode wire 112 is also covered with the dielectric layer 118 (not shown in Figure 2). The protruding portion 112A of the anode wire 112 protrudes from the front surface 110f of the anode body 113 toward the front surface 100f of the solid electrolytic capacitor 100. The other portion of the anode wire 112 is an embedded portion 112B embedded in the anode body 113 and extends toward the back surface 110b of the anode body 113.

[0067] The cathode 115 includes a solid electrolyte layer 114 arranged to cover at least a portion of the dielectric layer 118, a carbon layer 116 formed on the solid electrolyte layer 114, and a metal particle layer 117 formed on the carbon layer 116. The carbon layer 116 is formed using a paste containing carbon particles and rubber particles. The metal particle layer 117 is formed using a paste containing, for example, silver particles and a binder. The carbon layer 116 and the metal particle layer 117 function as a cathode extraction layer.

[0068] (Note) The following technologies are disclosed by the above description. (Technology 1) A solid electrolytic capacitor comprising: a first electrode; a dielectric layer covering at least a part of the first electrode; and a second electrode covering at least a part of the dielectric layer, wherein the second electrode comprises: a solid electrolyte layer; a conductive carbon layer covering at least a part of the solid electrolyte layer; and a metal particle layer covering at least a part of the carbon layer, wherein the solid electrolyte layer includes a pyrrole electrolytic polymerization layer; the carbon layer includes carbon particles and a binder; and the binder includes rubber particles. (Technology 2) The solid electrolytic capacitor according to Technology 1, wherein the developed area ratio (Sdr) of the surface of the solid electrolyte layer at the interface between the pyrrole electrolytic polymerization layer and the carbon layer is 0.38 or more. (Technology 3) The solid electrolytic capacitor according to Technology 1 or 2, wherein the rubber particles include a styrene-butadiene copolymer. (Technology 4) The solid electrolytic capacitor according to any one of Technology 1 to 3, wherein the content of the rubber particles in the carbon layer is 0.5% by mass or more and 2.0% by mass or less. (Technology 5) A solid electrolytic capacitor according to any one of Technologies 1 to 4, wherein the carbon particles include graphite particles with an average particle size of 0.5 μm or more and 5 μm or less. (Technology 6) A solid electrolytic capacitor according to any one of Technologies 1 to 5, wherein the metal particle layer includes silver particles with an average particle size of 0.5 μm or more and 12 μm or less. (Technology 7) A solid electrolytic capacitor according to any one of Technologies 1 to 6, where A is the adhesion strength between the pyrrole electrolytic polymerization layer and the carbon layer, and B is the adhesion strength between the carbon layer and the metal particle layer, and the relationship A > B is satisfied.

[0069] [Examples] The present invention will be described in detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0070] 《Example 1》 (1) Fabrication of Capacitor Element (i) Preparation of Anode Body Ta particles (Ta powder) were used as valve-acting metal particles. Ta powder was formed into a rectangular parallelepiped so that one end of an anode wire made of Ta was embedded in the Ta powder, and then the molded body was sintered in a vacuum. This yielded an anode section (first electrode) including an anode body which is a porous sintered body of Ta particles, and an anode wire having an embedded portion embedded in the anode body and a protruding portion that protrudes from the front of the anode body. The anode section, consisting of the anode body and anode wire, was washed using isopropanol (IPA) as a cleaning solution, and then dried at 150°C for 30 minutes.

[0071] (ii) Formation of dielectric layer Fifty anode portions, after cleaning, were arranged in a line at regular intervals, and the anode wires were welded to the elongated plate-shaped first external electrode. The anode body and a portion of the anode wire were immersed in a chemical solution in a chemical conversion tank made entirely of glass, and with the second external electrode made of Ta immersed in the chemical solution, a DC voltage was applied between the first external electrode and the second external electrode to oxidize the surface of the anode body and form a dielectric layer.

[0072] An aqueous solution containing phosphoric acid was used as the chemical conversion solution. The temperature of the chemical conversion solution was 60°C. A DC voltage of 7.3V was applied for 7 hours. After conversion, it was dried at 100°C for 10 minutes. In this way, tantalum oxide (Ta) was applied as a dielectric layer to the surface of the anode body and the surface of the anode wire. ２ O ５ A uniform dielectric layer (approximately 20 nm thick) was formed.

[0073] (iii) Formation of solid electrolyte layer (pre-coat treatment) An aqueous solution of pyrrole monomer (first treatment solution) was prepared. The concentration of monomer in the first treatment solution was 4% by mass. After applying an oxidizing agent to the surface of the dielectric layer, the anode body on which the dielectric layer was formed was immersed in the first treatment solution. After that, it was dried to form a pre-coat layer.

[0074] (Formation of electrolytic polymerization layer) An aqueous dispersion (second treatment solution) containing pyrrole, a dopant (sulfonate having a naphthalene skeleton), and p-nitrophenol was prepared.

[0075] The concentration of pyrrole in the second treatment solution was set to 2% by mass, the concentration of the dopant in the second treatment solution was set to 10% by mass, and the concentration of p-nitrophenol in the second treatment solution was set to 0.5% by mass.

[0076] An anode body with a pre-coated layer was immersed in a second processing solution, and electrolytic polymerization of pyrrole was carried out using the pre-coated layer as the base electrode to form an electrolytic polymerization layer containing polypyrrole. At this time, the relationship between current density and electrolytic polymerization time (profile) was appropriately controlled so that the surface area ratio (Sdr) of the solid electrolyte layer at the interface between the pyrrole electrolytic polymerization layer and the carbon layer was 0.82.

[0077] (iv) Formation of the carbon layer A dispersion (carbon paste) was prepared by dispersing 100 mass of carbon particles (graphite powder with an average particle size of 1.0 μm and an aspect ratio of 8.0) and 1 mass of SBR (average particle size of 150 nm) in water. After applying the carbon paste to the solid electrolyte layer, it was heated at 200°C to form a carbon layer on the surface of the solid electrolyte layer.

[0078] (v) Formation of the metal particle layer A silver paste containing silver particles (average particle size 3.0 μm, aspect ratio 10), an epoxy resin binder, and a solvent was applied to the surface of the carbon layer. Then, it was heated at 200°C to form a silver particle-containing layer and obtain a capacitor element.

[0079] (2) Fabrication of Solid Electrolytic Capacitors Conductive adhesive was applied to the silver particle-containing layer of the capacitor element, and the cathode lead terminals were joined to the silver particle-containing layer. The protruding portion of the anode wire and the anode lead terminals were joined by resistance welding. Next, the capacitor element with each lead terminal joined was placed in a mold and sealed with an outer casing by transfer molding. In this way, a solid electrolytic capacitor (capacitor A1) with a rated voltage Rv of 2.5V was fabricated.

[0080] <Comparative Example 1> A solid electrolytic capacitor (capacitor B1) was fabricated in the same manner as in Example 1, except that CMC was used instead of SBR in the preparation of the carbon paste. The surface area ratio (Sdr) of the solid electrolyte layer at the interface between the pyrrole electrolytic polymerization layer and the carbon layer was controlled to 0.82, the same as in Example 1.

[0081] <Comparative Example 2> A solid electrolytic capacitor (capacitor B2) was fabricated in the same manner as in Comparative Example 1, except that the relationship between current density and electrolytic polymerization time (profile) was appropriately controlled so that the surface area ratio (Sdr) of the solid electrolyte layer at the interface between the pyrrole electrolytic polymerization layer and the carbon layer was 0.37.

[0082] 《Evaluation》 Solid electrolytic capacitors were dried at 170°C for 3 hours, and then cooled to room temperature in a drying chamber. The following evaluation was performed using electrolytic capacitors in this state.

[0083] (Initial ESR) Thirty solid electrolytic capacitors were prepared for each example, and the ESR (mΩ) of each electrolytic capacitor at a frequency of 100 kHz was measured using a four-terminal LCR meter in an environment of 20°C, and the average value was calculated.

[0084] (Heat Cycle Test) Thirty solid electrolytic capacitors were prepared for each example and placed in a -55°C environment for 30 minutes, followed by a 30-minute period in a 105°C environment (-55°C / 105°C heat cycle). This cycle was repeated 20 times. After that, the ESR at 20°C was measured at 100 kHz, and the average value of the 30 ESRs was calculated. The rate of increase of the ESR after the test relative to the initial ESR was then calculated, and the relative value of the rate of increase for each example was determined, with the rate of increase in Example 1 set to 1. A larger relative value indicates a larger rate of change in ESR. In addition, the presence or absence of delamination at the interface between the pyrrole electrolytic polymerization layer and the carbon layer was confirmed using cross-sectional SEM images of the capacitor elements.

[0085] Table 1 shows the main characteristics of each example of solid electrolytic capacitor, and Table 2 shows the evaluation results.

[0086]

[0087]

[0088] Table 2 shows that when a pyrrole electropolymerization layer is used as the solid electrolyte layer, the increase in ESR is significantly suppressed by using SBR as a binder. It can also be seen that the larger the Sdr, the smaller the rate of change in ESR tends to be. At 5 heat cycles, the rate of change in ESR of capacitor B1 was 1.0, indicating that capacitor B1 also provided sufficient performance. From this, it can be seen that the effect of using rubber particles as a binder to suppress the increase in ESR becomes particularly apparent in severe heat cycle tests.

[0089] This disclosure is applicable to solid electrolytic capacitors in which the solid electrolyte layer comprises a pyrrole electrolytic polymerization layer. Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be construed as restrictive. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be construed as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0090] 100: Electrolytic capacitor 110: Capacitor element 111: Anode part 112: Anode wire 113: Anode body 114: Solid electrolyte layer 115: Cathode part 116: Carbon layer 117: Metal particle layer 118: Dielectric layer 120: Anode lead terminal 121: Anode terminal part 122: Anode connection part 130: Cathode lead terminal 131: Cathode terminal part 132: Cathode connection part 141: Conductive adhesive

Claims

1. A solid electrolytic capacitor comprising: a first electrode; a dielectric layer covering at least a portion of the first electrode; and a second electrode covering at least a portion of the dielectric layer, wherein the second electrode comprises a solid electrolyte layer; a conductive carbon layer covering at least a portion of the solid electrolyte layer; and a metal particle layer covering at least a portion of the carbon layer, wherein the solid electrolyte layer includes a pyrrole electrolytic polymerization layer; and the carbon layer includes carbon particles and a binder, wherein the binder includes rubber particles.

2. The solid electrolytic capacitor according to claim 1, wherein the unfolded surface area ratio (Sdr) of the solid electrolyte layer at the interface between the pyrrole electrolytic polymerization layer and the carbon layer is 0.38 or more.

3. The solid electrolytic capacitor according to claim 1, wherein the rubber particles include a styrene-butadiene copolymer.

4. The solid electrolytic capacitor according to claim 1, wherein the content of the rubber particles in the carbon layer is 0.5% by mass or more and 2.0% by mass or less.

5. The solid electrolytic capacitor according to claim 1, wherein the carbon particles include graphite particles with an average particle size of 0.5 μm or more and 5 μm or less.

6. The solid electrolytic capacitor according to claim 1, wherein the metal particle layer contains silver particles with an average particle size of 0.5 μm or more and 12 μm or less.

7. The solid electrolytic capacitor according to claim 1, wherein the adhesion strength between the pyrrole electrolytic polymerization layer and the carbon layer is A, and the adhesion strength between the carbon layer and the metal particle layer is B, and the relationship A > B is satisfied.

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