Solid electrolytic capacitor and method for manufacturing same

Abstract

Provided are a solid electrolytic capacitor having good frequency characteristics and a method for manufacturing the same. The anode foil has an expanded surface layer formed on the foil surface, a dielectric film formed on the expanded surface layer, and a pseudo-boehmite film formed on the dielectric film. The conductive polymer is contained in an amount of 50% or more at the bottom of the expanded-surface layer based on the content in the surface layer of the expanded-surface layer. The solid electrolytic capacitor includes: a step for forming a surface expansion layer on an anode foil; a step for forming a pseudo-boehmite film on the anode foil after the surface expansion step; a step for forming the oxide film on the anode foil such that a pseudo-boehmite film remains on the dielectric film; and a step in which a conductive polymer dispersion containing a conductive polymer and a high-boiling-point solvent is adhered to at least the anode foil and dried, and the conductive polymer is adjusted so as to contain 50% or more of the conductive polymer in the bottom of the surface-expanded layer based on the content of the conductive polymer in the surface layer of the surface-expanded layer, thereby forming a layer of the conductive polymer.

Description

TECHNICAL FIELD

[0001] The present application relates to a solid electrolytic capacitor and a manufacturing method thereof. BACKGROUND

[0002] The solid electrolytic capacitor includes a valve action metal such as tantalum or aluminum as an anode foil and a cathode foil. The anode foil is surface-extended by making a sintered body or an etching foil of the valve action metal, and has a dielectric coating film on the surface-extended surface by a treatment such as anode oxidation. A solid electrolyte layer which is in close contact with the anode foil and functions as a true cathode is interposed between the anode foil and the cathode foil.

[0003] In the solid electrolytic capacitor, the surface area of the anode electrode is surface-extended by etching the valve action metal as the anode electrode, and thus it is possible to be small-sized and obtain a large capacity. In particular, the solid electrolytic capacitor in which the solid electrolyte covers the dielectric coating film is small-sized, large-capacity, low equivalent series resistance (low ESR), and indispensable in the small-sizing, high-functioning, and low-costing of electronic devices.

[0004] As the solid electrolyte, manganese dioxide, a 7,7,8,8-tetracyanoquinodimethane (TCNQ) complex are known. In recent years, a conductive polymer derived from a monomer having a π conjugated double bond such as poly(3,4-ethylenedioxythiophene) (PEDOT) which is slow in reaction rate and excellent in close contact with the dielectric coating film is rapidly popularized as the solid electrolyte. As the conductive polymer, a conductive polymer using an external dopant which exhibits conductivity by using a low molecular anion, a polyanion or the like as a dopant at the time of chemical oxidation polymerization or electrolytic oxidation polymerization; or a self-doping type conductive polymer which exhibits conductivity by having a partial structure which functions as a dopant within a monomer molecule and is imparted with solubility in a solvent can be listed.

[0005] The layer of the conductive polymer is formed, for example, as follows. That is, a polymerization solution is impregnated in a capacitor element composed of an anode foil, a cathode foil, and a separator. The polymerization solution contains a monomer which is a monomer unit of the conductive polymer. By the impregnation of the polymerization solution, the monomer penetrates into the recesses, and a conductive polymer is generated by electrolytic polymerization or chemical polymerization and attached to the anode foil (for example, refer to Patent Document 1). Thus, the layer of the conductive polymer is formed.

[0006] One report states that the average depth of the pits forming the conductive polymer film is up to 16 μm (see Patent Document 2). During polymerization, monomers present deep within the pits migrate to the vicinity of the pit entrance and are consumed, thus diluting the monomer concentration deep within the pits and preventing the formation of a conductive polymer film at depth. Furthermore, even when using a liquid with a high concentration, only the vicinity of the pit entrance forms a thick film, which does not extend into the pit depth.

[0007] Sometimes, powder-laminated foil is used as the anode foil. Powder-laminated foil is an anode foil with an expanded surface layer formed by evaporating or sintering valve-acting metal powder onto the surface of a stretched foil as the core, or an anode foil with an expanded surface layer and no stretched core formed by pressurizing and sintering valve-acting metal powder. The inventors have confirmed that even when powder-laminated foil is used as the anode foil, conductive polymers do not form deep within the foil towards its thickness center. With this powder-laminated foil, because monomers near the foil's thickness center migrate to the surface of the powder-laminated foil and are consumed during polymerization, the monomer concentration in the deeper layers becomes thinner towards the foil's thickness direction.

[0008] Therefore, there is a method in which an aqueous solution of polyaniline, which is polymerized from a solvent-soluble aniline derivative, is impregnated in an aluminum foil, thereby forming a coating to the fine details of the dielectric coating (see, for example, Patent Document 3). However, in recent years, poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate, known as PEDOT:PSS, has become increasingly popular as a conductive polymer compared to polyaniline. PEDOT is poorly soluble in water and the like.

[0009] Solid electrolytic capacitors are used in a variety of applications. For example, in the field of power electronics, in power conversion circuits that use converter circuits to convert AC power to DC power and inverter circuits to convert the DC power back to the desired AC power, smoothing capacitors are used to suppress and smooth the DC ripple output from the converter circuit before it is input to the inverter circuit. Additionally, decoupling capacitors are placed near semiconductor switching elements to ensure stable operation and reduce noise.

[0010] In the field of power electronics, solid electrolytic capacitors require high voltage withstand capabilities to handle large amounts of power. Therefore, for example, a boehmite coating is sometimes formed on the dielectric coating formed on the anode foil. The dense interior of the boehmite coating functions as a resistive layer, improving the voltage withstand capability of the solid electrolytic capacitor. However, the boehmite coating has the problem of degrading the impregnation of conductive polymers into the anode foil.

[0011] Existing technical documents

[0012] Patent documents

[0013] Patent Document 1: Japanese Patent Application Publication No. 2021-64737

[0014] Patent Document 2: Japanese Patent Application Publication No. 2-288217

[0015] Patent Document 3: Japanese Patent Application Publication No. 2000-340462 Summary of the Invention

[0016] The problem that the invention aims to solve

[0017] In recent years, MOSFETs (Metal-Oxide Semiconductor Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors), which use wide-bandgap semiconductors such as SiC and GaN, have attracted much attention in the field of power electronics. Compared with conventional power devices using silicon, devices using these wide-bandgap semiconductors have advantages such as low on-resistance, high-speed switching, and the ability to operate in high-temperature environments.

[0018] By effectively utilizing the advantages of this wide-bandgap semiconductor, power converters and the like suppress power loss by employing low on-resistance and high-speed switching, thereby moving towards higher efficiency. As a result, it helps to simplify cooling components such as cooling fans and heat sinks, miniaturize passive components due to high frequency, and thus reduce costs and save energy.

[0019] Recent trends in devices incorporating power converters and other power components necessitate that solid electrolytic capacitors integrated into these devices possess frequency characteristics comparable to those of wide-bandgap semiconductors. Specifically, solid electrolytic capacitors require high capacitance in the high-frequency region and low equivalent series resistance (ESR).

[0020] The present invention is proposed to solve the aforementioned problems, and its purpose is to provide a solid electrolytic capacitor with good frequency characteristics and a method for manufacturing the same.

[0021] Methods for solving problems

[0022] To address the aforementioned issues, the solid electrolytic capacitor of this embodiment is a solid electrolytic capacitor comprising an anode foil and a layer of conductive polymer. The anode foil has an extended surface layer formed on the foil surface, a dielectric coating formed on the extended surface layer, and a pseudo-boehmite coating formed on the dielectric coating. Based on the content in the surface layer of the extended surface layer, the bottom of the extended surface layer contains more than 50% of the conductive polymer.

[0023] The conductive polymer may be poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid.

[0024] The expanded surface layer may have multiple tunnel-shaped pits.

[0025] The expanded surface layer can be formed by stacking powders of valve-acting metal.

[0026] It can have a withstand voltage of 250 V or higher. The dielectric coating can have a thickness corresponding to a chemical conversion voltage of 250 V or higher.

[0027] In addition, to solve the aforementioned problem, the manufacturing method of the solid electrolytic capacitor of this embodiment is a method for manufacturing a solid electrolytic capacitor including an anode foil and a layer of conductive polymer, comprising the following steps: an expansion layer forming an expansion layer on the anode foil; a chemical conversion pre-process after the expansion layer forming a pseudo-boehmite coating on the anode foil; a chemical conversion process in which the pseudo-boehmite coating is retained on the dielectric coating on the anode foil; and a polymer layer forming process in which a conductive polymer dispersion containing the conductive polymer and a high-boiling-point solvent is attached to the anode foil at least to a content of the conductive polymer at the bottom of the expansion layer based on the content in the surface layer, and the conductive polymer is dried at a temperature lower than the boiling point of the high-boiling-point solvent to form the layer of conductive polymer.

[0028] In the surface enlargement process, the surface enlargement layer can be formed through multiple tunnel-shaped pits.

[0029] In the expansion process, powder of valve action metal can be stacked.

[0030] In the chemical conversion process, a chemical conversion voltage of 250 V or higher can be applied to the anode foil.

[0031] The effects of the invention

[0032] According to the present invention, it is possible to realize a solid electrolytic capacitor with high capacitance and low ESR in the high frequency region. Attached Figure Description

[0033] Figure 1 This is a cross-sectional schematic diagram of the pits in the expanded surface layer.

[0034] Figure 2 The graph shows the results of EDX analysis and the elemental ratio of sulfur in Examples 1 and 2, and Comparative Examples 1 and 2.

[0035] Figure 3The graph shows the results of EDX analysis and the sulfur content of each depth zone of the expanded layer in Examples 1 and 2, and Comparative Examples 1 and 2.

[0036] Figure 4 These are SEM images of Example 2, (a) is the surface of the expansion layer, and (b) is the bottom of the expansion layer.

[0037] Figure 5 The images are SEM images of Comparative Example 1, (a) is the surface of the expansion layer, and (b) is the bottom of the expansion layer.

[0038] Figure 6 The graph shows the capacitance of the solid electrolytic capacitors of Examples 1 and 2, and Comparative Examples 1 and 2, at each frequency.

[0039] Figure 7 The graphs show the ESR of the solid electrolytic capacitors of Examples 1 and 2, and Comparative Examples 1 and 2, at each frequency.

[0040] Figure 8 The graph shows the capacitance of the solid electrolytic capacitors of Example 1, Comparative Example 2, and Comparative Example 3 at each frequency.

[0041] Figure 9 The graphs show the ESR of the solid electrolytic capacitors of Example 1, Comparative Example 2, and Comparative Example 3 at each frequency.

[0042] Figure 10 The graph shows the capacitance of the solid electrolytic capacitors of Examples 1, 3 to 6, and Comparative Examples 2 and 4 at each frequency.

[0043] Figure 11 The graphs show the ESR of the solid electrolytic capacitors of Examples 1, 3 to 6, and Comparative Examples 2 and 4 at each frequency.

[0044] Figure 12 The graph shows the capacitance of the solid electrolytic capacitors of Reference Examples 1 to 3 at each frequency.

[0045] Figure 13 The graph shows the ESR of the solid electrolytic capacitors of Reference Examples 1 to 3 at each frequency.

[0046] Figure 14 The graph shows the results of EDX analysis and the elemental ratio of sulfur in Example 7 and Comparative Example 5.

[0047] Figure 15The results of EDX analysis are shown, along with graphs illustrating the sulfur content of each depth zone of the expanded surface layer in Example 7 and Comparative Example 5, based on the sulfur content of the surface layer.

[0048] Figure 16 The graph shows the capacitance of the solid electrolytic capacitors of Example 7 and Comparative Example 5 at each frequency.

[0049] Figure 17 The graph shows the ESR of the solid electrolytic capacitors of Example 7 and Comparative Example 5 at each frequency. Detailed Implementation

[0050] The following describes a solid electrolytic capacitor and its manufacturing method according to embodiments of the present invention. It should be noted that the present invention is not limited to the embodiments described below.

[0051] A solid electrolytic capacitor is a passive component that stores and discharges charge through capacitance. The capacitor element of a solid electrolytic capacitor includes: an anode foil formed of a valve-acting metal, a cathode foil, a separator, and a layer of conductive polymer (hereinafter referred to as the polymer layer). A dielectric coating is formed on the surface of the anode foil. The polymer layer, containing conductive polymers, is formed between the anode and cathode foils and is formed as part of the dielectric coating covering at least the surface of the anode foil. This polymer layer becomes the true cathode of the solid electrolytic capacitor.

[0052] This solid electrolytic capacitor is manufactured through the following processes: an anode foil manufacturing process; a cathode foil manufacturing process; an assembly process of stacking the anode foil, cathode foil, and separator; a polymer layer forming process to form a polymer layer within the capacitor element; and a final manufacturing process of sealing the capacitor element in an outer casing. The order of these processes can be rearranged as flexibly as possible and can be processed in parallel. Furthermore, a process may include multiple detailed processes, some of which can be performed in other processes or incorporated between a detailed process of another process and the next detailed process.

[0053] Here, in the anode foil manufacturing process, an expansion process is performed. In this expansion process, an expansion layer is formed on the surface of the anode foil, increasing its specific surface area. In other words, the surface of the anode foil is made porous. Next, in the anode foil manufacturing process, before the chemical conversion process for forming the dielectric coating, a pre-chemical conversion process is performed to form a pseudo-boehmite coating on the anode foil with the expansion layer. The pseudo-boehmite coating contains a hydrated oxide of the valve-acting metal; if the valve-acting metal is aluminum, then it is Al₂O₃·xH₂O.

[0054] The dielectric coating is formed during the chemical conversion process after the pre-chemical conversion process. In the chemical conversion process, the boehmite coating is transformed into a dielectric coating from the boundary surface with the unoxidized valve metal towards both sides of the boehmite coating and the valve metal, causing the dielectric coating to grow. The dielectric coating after the boehmite coating transformation contains γ-alumina as a crystalline oxide. During the chemical conversion process, the boehmite coating remains on the surface. Therefore, a boehmite coating is formed on the dielectric coating.

[0055] In addition, in the polymer layer formation process, a conductive polymer dispersion is used to form the polymer layer. The conductive polymer dispersion is a dispersion of conductive polymer particles or powders. By immersing at least the anode foil, or at least the cathode foil and a separator, in the conductive polymer dispersion, or by coating at least the anode foil, or at least the cathode foil and a separator, and then drying, the conductive polymer forms a polymer layer that adheres between the anode foil and the cathode foil.

[0056] In this polymer layer formation process, a conductive polymer dispersion containing a high-boiling-point solvent is used to form the polymer layer. The high-boiling-point solvent has a boiling point higher than the drying temperature used to dry the conductive polymer dispersion, and remains within the capacitor element at least until the drying process is complete. This inhibits the aggregation of the conductive polymer and reduces the particle size of the conductive polymer. In other words, the drying temperature is set below the boiling point of the high-boiling-point solvent. For example, if the high-boiling-point solvent has a boiling point of 150°C or higher, the drying temperature is set to 110°C. It should be noted that the high-boiling-point solvent may also remain within the capacitor element after the drying process is complete.

[0057] Thus, the anode foil manufacturing process includes a pre-chemical conversion step of forming a pseudo-boehmite coating on the anode foil with the expanded surface layer. Additionally, in the polymer layer formation step, a conductive polymer dispersion containing a high-boiling-point solvent is coated and dried. Through this manufacturing method, the distribution of conductive polymers within the expanded surface layer is nearly uniform from the surface to the bottom. Specifically, the amount of conductive polymers at the bottom of the expanded surface layer is at least 50% of the amount present at the surface of the expanded surface layer. It should be noted that when the expanded surface layer is divided into three equal parts in the depth direction, the area belonging to the surface is called the surface layer, the area belonging to the bottom is called the bottom, and the area between the surface layer and the bottom is called the middle portion.

[0058] Here, the pit constituting the extended layer can be represented by an equivalent circuit consisting of resistors connected in series in each depth band, capacitors in each depth band, and a combined resistor connected in series up to that depth band. That is, the pit includes an RC circuit group with resistive and capacitive components having high resistance values ​​depending on the depth. In this case, a difference arises between the charging rate of the shallow depth band and the charging rate of the deep depth band. Therefore, typically, when using solid electrolytic capacitors in the high-frequency band, the bottom of the pit does not have a time constant R×C small enough to allow for sufficient charging and discharging. Consequently, ordinary solid electrolytic capacitors cannot fully utilize their high capacitance at high frequencies.

[0059] On the other hand, in this solid electrolytic capacitor, based on the content in the surface layer of the expansion layer, more than 50% conductive polymer is contained in the bottom of the expansion layer. The presence of a sufficient amount of conductive polymer in the bottom of the pit creates a conductive path that bypasses the resistive component of the RC circuit. Therefore, even at high frequencies, sufficient charging and discharging can be achieved at the bottom of the pit, resulting in high capacitance for the solid electrolytic capacitor. Furthermore, due to the large number of conductive paths based on the polymer layer generated at the bottom of the expansion layer, the resistance at the bottom of the expansion layer decreases, leading to good ESR for the solid electrolytic capacitor.

[0060] The reasons are not limited to this, but the following speculations exist. First, during the drying of the conductive polymer dispersion, the conductive polymers aggregate and their particle size increases as the dispersion medium evaporates. Additionally, the pseudoboehmite coating is internally dense. The larger conductive polymer particles are blocked by the pseudoboehmite coating, making it difficult for them to penetrate to the bottom of the diffusion layer. If the evaporation of the dispersion medium and the aggregation of the conductive polymers are suppressed, the conductive polymers become smaller particles. Therefore, even in the presence of the pseudoboehmite coating, they can easily penetrate to the bottom of the diffusion layer. High-boiling-point solvents remain during the drying of the conductive polymer dispersion, maintaining the small particle size of the conductive polymers.

[0061] Therefore, even when the conductive polymer is dispersed in a conductive polymer dispersion, the conductive polymer content at the bottom of the expansion layer is more than 50%, based on the content in the surface layer. This results in high capacitance and low ESR in solid electrolytic capacitors, even when used in high-frequency bands. Furthermore, because a conductive polymer dispersion is used to form the polymer layer, the capacitor element is not immersed in a highly acidic polymer solution, preventing exposure to strong acids. Additionally, even for tunnel-like pits with an average depth of 16 μm or more, the conductive polymer content at the bottom of the expansion layer is more than 50%, based on the content in the surface layer.

[0062] It should be noted that the high-boiling-point solvent added to the conductive polymer dispersion is preferably a diol-based compound. Diol-based compounds reduce the resistance of the conductive polymer and improve the voltage withstand capability of the solid electrolytic capacitor. Examples of diol-based compounds include: ethylene glycol, diethylene glycol, glycerol, 1,3-propanediol, and polyethylene glycol.

[0063] The expansion layer can be any of the following: a foil obtained by stretching the valve-acting metal as the core; a layer of valve-acting metal powder deposited or sintered on the surface; or an etched layer that has undergone etching treatment. That is, the anode foil can be any of the following: a powder layer stacked foil with a layer of valve-acting metal powder as the expansion layer; or an etched foil whose surface has been etched after stretching. The etched layer can be composed of tunnel-like pits or sponge-like pits. Tunnel-like etched pits are holes drilled in the foil thickness direction. An expansion layer composed of sponge-like pits is a sponge-like layer that expands by connecting small gaps into a spatial structure.

[0064] Alternatively, the anode foil can be manufactured by pressing and molding valve-acting metal powder into a foil shape and then sintering it. This anode foil has no core and is composed entirely of an expanded layer. For the coreless anode foil, the area from the foil surface to the center of the foil in the thickness direction is divided into three equal parts in the depth direction. The area belonging to the surface is called the surface layer, the area belonging to the center of the foil in the thickness direction is called the bottom layer, and the area sandwiched between the surface layer and the bottom layer is called the middle layer.

[0065] In the process of widening the surface area of ​​the etched pits to form tunnel-like patterns, a direct current is typically passed through the anode foil in an acidic aqueous solution containing halide ions, such as hydrochloric acid. The tunnel-like etched pits can then be widened by passing a direct current through an acidic aqueous solution containing nitric acid. In the process of widening the surface area of ​​the etched pits to form sponge-like patterns, an alternating current is typically passed through the anode foil in an acidic aqueous solution containing halide ions, such as hydrochloric acid.

[0066] Regarding the powder layer, it is produced by attaching valve-acting metal particles to a foil and then sintering it under heat in a vacuum or reducing atmosphere. Alternatively, the powder layer can be produced, for example, by resistance heating vapor deposition or electron beam heating vapor deposition. In this vapor deposition method, the powder layer is formed by evaporating the valve-acting metal using resistance heating or electron beam energy, causing the vapor of the valve-acting metal particles to deposit on the surface of the foil to form a film.

[0067] Preferably, the expanded surface layer is composed of tunnel-shaped pits. The pseudo-boehmite coating aims to improve voltage withstand capability, and the dielectric coating is also thickened. If the dielectric coating is thickened, the pits in the expanded surface layer are easily filled during the chemical conversion process. Therefore, a tunnel-shaped structure that allows for diameter expansion is preferred. Furthermore, since the tunnel-shaped pits are holes drilled in the foil thickness direction, the denser the expanded surface layer, the more difficult it is for conductive polymer particles to reach the bottom of the pits. Therefore, in solid electrolytic capacitors with expanded surface layers having tunnel-shaped pits, a clear effect of combining high capacitance and low ESR in the high-frequency region is exhibited.

[0068] More preferably, the expansion layer is a vapor-deposited layer or a sintered layer. That is, more preferably, the anode foil is a powder-laminated foil. Compared with an etched foil that forms tunnel-like pits, the expansion layer of the powder-laminated foil is denser, and it is more difficult for the conductive polymer particles to reach the bottom of the pits. However, in the polymer layer formation process, if a conductive polymer dispersion containing a high-boiling-point solvent is used, and the conductive polymer dispersion is dried at a drying temperature lower than the boiling point of the high-boiling-point solvent, a large amount of polymer layer is formed at the bottom of the powder-laminated foil, exceeding the effect of an etched foil that forms tunnel-like pits. This clearly exhibits the effect of combining high capacitance and low ESR in the high-frequency region of a solid electrolytic capacitor.

[0069] There are no particular limitations on the pre-chemical conversion process as long as it can form a pseudo-boehmite coating. For example, in the pre-chemical conversion process, the expanded anode foil is immersed in pure water at or above 80°C or boiling. The immersion time can be determined based on the target thickness of the pseudo-boehmite coating and by balancing voltage withstand and capacitance. Through this pre-chemical conversion process, a pseudo-boehmite layer is formed on the surface of the anode foil along the unevenness of the expanded layer.

[0070] In the chemical conversion process, growth is achieved by extending the pseudoboehmite coating from the boundary with the unoxidized valve-acting metal towards both sides of the valve-acting metal, thus transforming the pseudoboehmite coating into a dielectric coating. Therefore, in the chemical conversion process, a voltage is applied to the anode foil in a chemical conversion solution free of halide ions. As the chemical conversion solution, phosphoric acid-based solutions such as ammonium dihydrogen phosphate, boric acid-based solutions such as ammonium borate, and adipic acid-based solutions such as ammonium adipate can be used. The applied voltage simply needs to correspond to the target withstand voltage.

[0071] If the anode foil is made of aluminum, an aluminum oxide layer is formed along the unevenness of the expansion layer through a chemical conversion process. This aluminum oxide layer becomes the dielectric coating. However, the anode foil can also be made of a valve-acting metal other than aluminum. Valve-acting metals include aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony. Ideally, the purity of the anode foil should be above 99.9%, but it may contain impurities such as silicon, iron, copper, magnesium, and zinc.

[0072] In the chemical conversion process, it is preferable to apply a voltage of at least 250 V to achieve, for example, a rated withstand voltage of 250 V or higher. In other words, if a dielectric coating with a thickness of 1.1 nm to 1.5 nm is required to obtain a withstand voltage of 1 V, then a dielectric coating with a thickness of 275 nm to 375 nm or higher is formed. In cases where a pseudo-thin boehmite coating is required, a solid electrolytic capacitor with a high withstand voltage of at least 250 V is desirable.

[0073] It should be noted that applying a high voltage can easily create pores within the dielectric coating. Therefore, depolarization treatment may also be included in the chemical conversion process. Depolarization treatment includes heat treatment, phosphoric acid treatment, or both. In heat treatment, for example, at temperatures above 450°C exposed to the atmosphere, isolated pores within the dielectric coating are opened. In phosphoric acid treatment, cracks and openings communicating with the pores are enlarged. In this phosphoric acid treatment, the anode foil is immersed in a phosphoric acid solution or an ammonium dihydrogen phosphate solution. This allows the chemical conversion solution to easily penetrate into the pores.

[0074] In addition to high-boiling-point solvents, sugar alcohols such as sorbitol can be further added to conductive polymer dispersions. Sorbitol improves the voltage withstand capability of solid electrolytic capacitors. Here, sorbitol typically solidifies near the inlet of the pit in the expansion layer, thus hindering the penetration of conductive polymers to the bottom of the expansion layer. However, with high-boiling-point solvents, not only do conductive polymers penetrate to the bottom of the expansion layer, but sorbitol also penetrates to the bottom of the expansion layer. Moreover, sorbitol prevents insulation breakdown that may occur near the bottom of the expansion layer, further improving the voltage withstand capability of the solid electrolytic capacitor.

[0075] The conductive polymer dispersed in the conductive polymer dispersion is PEDOT:PSS. However, the conductive polymer can be of any type as long as it is a solid component in the conductive polymer dispersion. The conductive polymer is a self-doped type, doped with intramolecular dopant molecules, or a conjugated polymer system, doped with external dopant molecules.

[0076] Conjugated polymers are obtained by chemical oxidative polymerization or electrolytic oxidative polymerization of monomers or their derivatives having π-conjugated double bonds. Examples of conjugated polymers include: polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene vinylene, polyacene, and polythiophene vinylene. These conjugated polymers can be used alone or in combination of two or more monomers, and can also be copolymers of two or more monomers.

[0077] Examples of conjugated polymers include those polymerized from thiophene or its derivatives, specifically including 3,4-ethylenedioxythiophene (i.e., 2,3-dihydrothiophene[3,4-b][1,4]dioxan), 3-alkylthiophene, 3-alkoxythiophene, 3-alkyl-4-alkoxythiophene, 3,4-alkylthiophene, 3,4-alkoxythiophene, or derivatives thereof. Examples of thiophene derivatives include compounds selected from thiophenes having substituents at the 3 and 4 positions, wherein the substituents at the 3 and 4 positions of the thiophene ring can form a ring together with the carbons at the 3 and 4 positions. The number of carbons in the alkyl and alkoxy groups is preferably 1 to 16.

[0078] Dopant can be any known dopant without particular limitation. Dopant can be used alone or in combination of two or more. In addition, polymers or monomers can also be used. Examples of dopant include: inorganic acids such as polyanionic acid, boric acid, nitric acid, and phosphoric acid; and organic acids such as acetic acid, oxalic acid, citric acid, tartaric acid, squaric acid, rhodizonica acid, croconic acid, salicylic acid, p-toluenesulfonic acid, 1,2-dihydroxy-3,5-benzenedisulfonic acid, methanesulfonic acid, trifluoromethanesulfonic acid, borodisalicylic acid, bis(oxalate) borate acid, sulfonyl imide acid, dodecylbenzenesulfonic acid, propylnaphthalenesulfonic acid, and butylnaphthalenesulfonic acid.

[0079] Polyanionic polymers include, for example, substituted or unsubstituted polyalkylene compounds, substituted or unsubstituted polyolefin compounds, substituted or unsubstituted polyimides, substituted or unsubstituted polyamides, and substituted or unsubstituted polyesters. Examples include polymers containing only structural units with anionic groups, and polymers containing both structural units with and without anionic groups. Specifically, examples of polyanionic polymers include: polyvinyl sulfonic acid, polystyrene sulfonic acid, polyallyl sulfonic acid, polyacryloyl sulfonic acid, polymethacryloyl sulfonic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprene sulfonic acid, polyacrylic acid, polymethacrylic acid, and polymaleic acid.

[0080] As a dispersion medium for conductive polymer dispersions, any dispersion medium that disperses conductive polymer particles or powders is acceptable, such as water, organic solvents, or mixtures thereof. Examples of organic solvents include: polar solvents, alcohols, esters, hydrocarbons, carbonate compounds, ether compounds, chain ethers, heterocyclic compounds, nitrile compounds, etc.

[0081] Examples of polar solvents include: N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide. Examples of alcohols include: methanol, ethanol, propanol, and butanol. Examples of esters include: ethyl acetate, propyl acetate, and butyl acetate. Examples of hydrocarbons include: hexane, heptane, benzene, toluene, and xylene. Examples of carbonate compounds include: ethylene carbonate and propylene carbonate. Examples of ether compounds include: dioxane and diethyl ether. Examples of chain ethers include: ethylene glycol dialkyl ethers, propylene glycol dialkyl ethers, polyethylene glycol dialkyl ethers, and polypropylene glycol dialkyl ethers. Examples of heterocyclic compounds include: 3-methyl-2-oxazolidinone. Examples of nitrile compounds include: acetonitrile, glutaronitrile, methoxyacetonitrile, propionitrile, and benzonitrile.

[0082] The cathode foil, serving as the cathode body, is a long foil made of a valve-acting metal with an ideal purity of 99% or higher. In the manufacturing process of the cathode foil for a wound solid electrolytic capacitor, after the valve-acting metal is stretched into a foil shape, the cathode foil is transferred to an expansion process as needed. Furthermore, after the expansion process, a thin oxide coating layer of approximately 1–10 Vfs can be intentionally formed on the cathode foil. Alternatively, the oxide coating can be formed naturally in the expansion layer, formed by the reaction of the cathode foil with oxygen in the air. Furthermore, layers containing metal nitrides, metal carbides, or metal carbonitrides can be formed on the cathode foil using vapor deposition, or a carbon-containing layer can be formed on the surface.

[0083] In planar solid electrolytic capacitors, a laminate of a metal layer and a carbon layer can be used as the cathode, replacing the cathode foil. In this case, instead of the cathode foil manufacturing process of a wound solid electrolytic capacitor, a planar capacitor element with a carbon layer and a metal layer formed on a polymer layer as the cathode is manufactured. The carbon layer is formed by preparing a paste or slurry, coating it onto the polymer layer after forming the anode foil, and then curing it by heating. The metal layer, for example, is a silver layer, which is formed by preparing a paste or slurry, coating it onto the carbon layer, and then curing it by heating. Alternatively, a laminate of multiple planar capacitor elements can also be manufactured.

[0084] In the component assembly process, when the solid electrolytic capacitor is of the wound type, a separator is wound around the anode foil and cathode foil to form a cylindrical wound body. The separator is overlapped with one end protruding from one end of the anode foil and the other end of the cathode foil. The protruding separator is wound first to create a core, and then the core is used as a spool for winding. It should be noted that before winding, aluminum leads are connected to the anode foil and cathode foil, for example, by stitching, cold welding, ultrasonic welding, or laser welding. When the solid electrolytic capacitor is of the laminated type, the component assembly process includes a polymer layer formation process and a cathode body manufacturing process consisting of a carbon layer and a metal layer.

[0085] A separator between the anode and cathode foils prevents short circuits between them and maintains the polymer layer between them. Since the polymer layer retains its shape and can isolate the anode and cathode foils, the separator can be removed from the solid electrolytic capacitor. Examples of separators include cellulose materials such as kraft paper, Manila hemp, esparto, hemp, and rayon, as well as blends thereof; polyester resins such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate, as well as their derivatives; polytetrafluoroethylene resins, polyvinylidene fluoride resins, vinylon resins; polyamide resins such as aliphatic polyamides, semi-aromatic polyamides, and fully aromatic polyamides; polyimide resins; polyethylene resins; polypropylene resins; trimethylpentene resins; polyphenylene sulfide resins; acrylic resins; and polyvinyl alcohol resins. These resins can be used alone or in combination.

[0086] In the final assembly process, the capacitor element is inserted into a bottomed cylindrical outer casing. The outer casing is a metal container with a bottom at one end and an open end at the other; materials include aluminum, aluminum alloys containing aluminum and manganese, or stainless steel. After the capacitor element is housed in the outer casing, the open end of the outer casing is sealed using a sealing body. The sealing body is made of rubber or a laminate of rubber and a rigid substrate; examples of rubber include ethylene propylene rubber and butyl rubber. The sealing body is embedded into the end of the outer casing, and the open end of the outer casing is riveted to achieve a seal. Lead terminals for connection to the anode and cathode foils are then led out from the sealing body.

[0087] After the capacitor element is sealed in its outer casing, the solid electrolytic capacitor undergoes an aging process in the final manufacturing step. During the aging process, a DC voltage is applied to the solid electrolytic capacitor, and the electrolyte repairs defects in the dielectric coating and other components. Additionally, in solid electrolytic capacitors without electrolyte, a DC voltage is applied during the aging process to insulate the conductive polymers present in or near defects in the dielectric coating.

[0088] It should be noted that the capacitor element may not need to be inserted into the bottomed cylindrical outer casing. For example, this can be achieved by coating the capacitor element with a laminated film. Alternatively, the capacitor element can be sealed by molding with a resin such as a heat-resistant resin or an insulating resin, or by forming the resin into a thin film using methods such as dip coating or printing. Furthermore, in the case of a flat-plate solid electrolytic capacitor, the outer casing may be omitted.

[0089] Example

[0090] The present invention will now be described in more detail based on embodiments. It should be noted that the present invention is not limited to the embodiments described below.

[0091] (Example 1)

[0092] The solid electrolytic capacitor of Example 1 is manufactured as follows. First, aluminum foil is prepared as the anode foil. The aluminum foil is transferred to the expansion process of the anode foil manufacturing process to form tunnel-shaped etched pits on both sides of the aluminum foil. In the expansion process, a direct current is passed through the aluminum foil in an aqueous solution containing hydrochloric acid to form pits, and then a direct current is passed through the aluminum foil in an aqueous solution containing nitric acid to expand the diameter of the pits.

[0093] The aluminum foil, after the surface enlargement process, is transferred to the pre-chemical conversion process. In the pre-chemical conversion process, the aluminum foil is immersed in boiling pure water for 15 minutes, forming a pseudo-boehmite coating on the surface of the aluminum foil. Next, the chemical conversion process is carried out. In the chemical conversion process, the aluminum foil is immersed in an ammonium borate aqueous solution at 90°C, while a current density of 25 mAcm⁻¹ is applied. -2 An electric current was applied to the aluminum foil to achieve a chemical conversion voltage of 600 V, and this voltage was maintained for 20 minutes. This resulted in the outermost layer of the pseudo-boehmite coating being retained, which then deteriorated into a dielectric coating. SEM images confirmed that the thickness of the dielectric coating was 652 nm.

[0094] One side of the anode foil is shielded using an imide tape, leaving four 1 cm diameter areas exposed. The process is then transferred to a polymer layer formation step, where 63 μL of a conductive polymer dispersion is added dropwise to each of the four exposed areas, followed by drying. The conductive polymer dispersion contains poly(ethylene dioxythiophene) (PEDOT:PSS) doped with polystyrene sulfonate (PSS). The dispersion medium is a mixture of 10 wt% ethylene glycol and 90 wt% water. The amount of PEDOT:PSS added is adjusted to be 1 wt% relative to the total amount of the conductive polymer dispersion.

[0095] The conductive polymer dispersion was dried by placing it at 60°C for 10 minutes, followed by placing it at 110°C for 30 minutes. Ethylene glycol has a boiling point of 197°C, which is higher than the drying temperature of 110°C. In other words, the drying temperature was 110°C, which is lower than the boiling point of ethylene glycol, 197°C.

[0096] After the polymer layer formation process, carbon paste is applied to the four exposed areas and cured at 110°C for 30 minutes. Next, silver paste is applied to the four exposed areas from the carbon layer, simultaneously bonding copper foil as leads. After the silver paste is applied to one side of the anode foil, with the copper foil bonded to the silver paste before curing, it is cured at 110°C for 30 minutes. This connects the silver layer and the copper foil. The carbon layer, silver layer, and copper foil layer constitute the cathode of a solid electrolytic capacitor.

[0097] (Example 2)

[0098] Manufacturing the solid electrolytic capacitor of Example 2. The conductive polymer dispersion used in the polymer layer formation process of Example 2 differs from that of Example 1. The dispersion medium of the conductive polymer dispersion in Example 2 is a mixture of 10 wt% ethylene glycol and 90 wt% water. For this dispersion medium, sorbitol is added as a solid component in addition to the conductive polymer. The amount of PEDOT:PSS added is 1 wt% relative to the total amount of the conductive polymer dispersion. The amount of sorbitol added is 85 wt% relative to the weight of the solid component after adding the conductive polymer and sorbitol.

[0099] (Comparative example)

[0100] Solid electrolytic capacitors of Comparative Examples 1 to 3 were manufactured. The conductive polymer dispersions used in the polymer layer formation process of Comparative Examples 1 to 3 differed from those of Example 1. The conductive polymer dispersions of Comparative Examples 1 to 3 did not contain high-boiling-point solvents, but only water as the dispersion medium. Furthermore, the conductive polymer dispersion of Comparative Example 1 contained sorbitol in the same amount as in Example 2. The solid electrolytic capacitors of Comparative Examples 1 and 2 were manufactured using the same manufacturing method and under the same manufacturing conditions, except for the composition of the conductive polymer dispersion.

[0101] In Comparative Example 3, the solid electrolytic capacitor underwent a high-boiling-point solvent impregnation process after the polymer layer formation process. In this high-boiling-point solvent impregnation process, after the polymer layer formation process using a conductive polymer dispersion without a high-boiling-point solvent, the capacitor element with the formed polymer layer was immersed in ethylene glycol. After the capacitor element was removed from the ethylene glycol, it was dried at 150°C for 5 minutes. This high-boiling-point solvent impregnation process impregnates the capacitor element with ethylene glycol. Thus, in Comparative Example 3, the high-boiling-point solvent was not added to the conductive polymer dispersion, but rather added to the capacitor element after the polymer formation process.

[0102] (Observation of the polymer layer)

[0103] Energy-dispersive X-ray diffraction (EDXD) analysis was performed on the cross-sections of the anode foils of Examples 1 and 2, as well as Comparative Examples 1 and 2, to determine the elemental ratios. The cross-section of the anode foil is defined as a surface cut along the thickness direction of the foil. In the EDX analysis, an energy-dispersive X-ray spectrometer (Bruker XFlash5060FQ) was used, with the accelerating voltage set to 4 kV, the magnification set to 80 kx, and the working distance (WD) set to 10.5–11.0 mm.

[0104] Figure 1 This is a cross-sectional schematic diagram of the pits in the expanded surface layer. A dielectric coating 2 is formed closer to the center of the pit in the less oxidized valve metal region 1, and a pseudo-boehmite coating 3 is formed closer to the center of the pit than the dielectric coating 2. In mapping analysis, such as... Figure 1 As shown in the area enclosed by the rectangle 4, align the end of the analysis object area with the boundary between the dielectric coating 2 and the pseudo-thin boehm coating 3, and adjust the analysis object area so that the dielectric coating 2 does not enter.

[0105] The analysis targets were defined in the surface, middle, and bottom layers of the expansion layer. Mapping analysis was then performed on each depth band. In this analysis, the elemental ratio of sulfur to the total of carbon, aluminum, and sulfur was determined. Sulfur is contained in poly(ethylene thiothiophene) doped with polystyrene sulfonate. Therefore, the atomic ratio of sulfur at each depth represents the relative content of the conductive polymer at that depth.

[0106] Figure 2 This is a graph showing the results of EDX analysis and illustrating the elemental ratios of sulfur in Examples 1 and 2, and Comparative Examples 1 and 2. Figure 2In the graph, the horizontal axis represents the top, middle, and bottom of the expansion layer, respectively, and the vertical axis represents the elemental ratio of sulfur. In the graph, the circular marker represents Example 1, the diamond marker represents Example 2, the triangle marker represents Comparative Example 1, and the x marker represents Comparative Example 2.

[0107] in addition, Figure 3 This is a graph showing the results of EDX analysis and illustrating the sulfur content of each depth band in the expanded surface layer of Examples 1 and 2, and Comparative Examples 1 and 2. Figure 3 In the graph, the horizontal axis represents the top, middle, and bottom of the expansion layer, respectively, and the vertical axis represents the sulfur content. The sulfur content is based on the sulfur content of the top layer of the expansion layer, representing the rate of reduction of sulfur content in the middle and bottom layers. In the graph, the circular marker represents Example 1, the diamond marker represents Example 2, the triangle marker represents Comparative Example 1, and the x marker represents Comparative Example 2.

[0108] like Figure 2 and Figure 3 As shown, in Comparative Example 1, based on the amount of sulfur contained in the surface layer, the amount of sulfur contained in the bottom of the surface layer is reduced by approximately 65%. In other words, the conductive polymer contained in the bottom of Comparative Example 1 is approximately 35% of that in the surface layer. In Comparative Example 2, based on the amount of sulfur contained in the surface layer, the amount of sulfur contained in the bottom of the surface layer is reduced by approximately 85%. In other words, the conductive polymer contained in the bottom of Comparative Example 2 is approximately 15% of that in the surface layer.

[0109] And such Figure 2 and Figure 3 As shown, in Example 1, based on the amount of sulfur contained in the surface layer, the amount of sulfur contained in the bottom of the surface layer is reduced by approximately 30%. In other words, in Example 1, the amount of conductive polymer contained in the bottom is approximately 70% of that in the surface layer. In Example 2, based on the amount of sulfur contained in the surface layer, the amount of sulfur contained in the bottom of the surface layer is reduced by approximately 45%. In other words, in Example 2, the amount of conductive polymer contained in the bottom is approximately 55% of that in the surface layer.

[0110] The difference between Examples 1 and 2 and Comparative Examples 1 and 2 is that a conductive polymer dispersion containing a high-boiling-point solvent was used in the polymer layer formation process. Furthermore, through the polymer layer formation process using a conductive polymer dispersion containing a high-boiling-point solvent, it was confirmed that in the anode foil having a surface layer, a dielectric coating, and a pseudo-boehmite coating, based on the content in the surface layer, the bottom of the surface layer contained more than 50% conductive polymer.

[0111] Next, for the anode foils of Example 2 and Comparative Example 1, SEM images were captured using a scanning spectroscopy (SEM) of the surface and bottom sections of the expanded layer. The accelerating voltage for SEM image capture was 1.0 kV, the working distance (WD) was 2.1–2.3 mm, and the magnification was 80 kx. The results of the SEM image capture are shown below. Figure 4 and Figure 5 middle. Figure 4 These are SEM images of Example 2, (a) is the surface of the expansion layer, and (b) is the bottom of the expansion layer. Figure 5 The image is a SEM image of Comparative Example 1, (a) is the surface layer of the expansion layer.

[0112] like Figure 4 As shown, in Example 2, it can be confirmed that a polymer layer containing sorbitol is formed on the surface and bottom of the expansion layer. Sorbitol is also transported to the bottom of the expansion layer because a thick polymer layer of sorbitol is also formed at the bottom of the expansion layer. On the other hand, as Figure 5 As shown, in Comparative Example 1, it can be confirmed that sorbitol, as a solid component, is abundantly deposited on the surface of the expansion layer. Therefore, very little sorbitol reaches the bottom of the expansion layer in Comparative Example 1, and the polymer layer is thinner and less uniform compared to Example 2.

[0113] Thus, through a polymer layer formation process using a conductive polymer dispersion containing a high-boiling-point solvent and sorbitol, it was confirmed that in the anode foil having an expansion layer, a dielectric coating, and a pseudo-boehmite coating, sorbitol was also transported in large quantities to the bottom of the expansion layer, and a thick polymer layer was formed at the bottom of the expansion layer. Therefore, in the solid electrolytic capacitor of Example 2, insulation breakdown is less likely to occur even at the bottom of the expansion layer, and the withstand voltage is improved. In addition, in the solid electrolytic capacitor of Example 2, the conductivity at the bottom of the expansion layer is high, and the capacitance utilization rate is improved, so high capacitance and low ESR can be expected.

[0114] (Frequency response measurement test)

[0115] The capacitance and ESR of the solid electrolytic capacitors of Examples 1 and 2, and Comparative Examples 1 and 2, were measured at each frequency. The capacitance and ESR were measured using an LCR meter (manufactured by NF Circuit Design Block Co., Ltd., ZM2376). Measurements were performed at 20°C, and the measurement frequency was set to the range of 1 Hz to 1000 kHz.

[0116] The results of the capacitance measurement are shown in Figure 6 The ESR measurement results are shown in the graph. Figure 7The graphs are shown below. The horizontal axis of each graph represents frequency, and the vertical axis represents capacitance or ESR. In the graphs, circles represent Example 1, diamonds represent Example 2, triangles represent Comparative Example 1, and x-marks represent Comparative Example 2. Table 1 below shows the capacitance (Cap) at 120 Hz, the capacitance at 10 kHz, the ESR at 10 kHz, and the ESR at 100 kHz.

[0117] (Table 1)

[0118]

[0119] like Figure 6 As shown in Table 1, the capacitance of the solid electrolytic capacitors of Examples 1 and 2 is higher than that of Comparative Examples 1 and 2. In particular, when used in the high-frequency region of 10 kHz, the capacitance of the solid electrolytic capacitors of Examples 1 and 2 is at least 1.5 times that of Comparative Examples 1 and 2.

[0120] In addition, such as Figure 7 As shown in Table 1, the ESR of the solid electrolytic capacitors of Examples 1 and 2 is lower than that of Comparative Examples 1 and 2. When used in the high-frequency region of 10 kHz and 100 kHz, the ESR of the solid electrolytic capacitors of Examples 1 and 2 is less than one-quarter of that of Comparative Examples 1 and 2.

[0121] Thus, through a polymer layer formation process using a conductive polymer dispersion containing a high-boiling-point solvent, the anode foil having an expansion layer, a dielectric coating, and a pseudo-boehmite coating contains at least 50% conductive polymer at the bottom of the expansion layer, based on the content in the surface layer. Furthermore, if the anode foil having an expansion layer, a dielectric coating, and a pseudo-boehmite coating contains at least 50% conductive polymer at the bottom of the expansion layer, based on the content in the surface layer, it has been confirmed that even when used at high frequencies, the solid electrolytic capacitor exhibits good capacitance performance and reduced ESR.

[0122] Furthermore, the capacitance and ESR of the solid electrolytic capacitor in Comparative Example 3 were measured. The methods and conditions for measuring capacitance and ESR were the same as those for Example 1 and Comparative Example 2. The results of the capacitance measurement are shown together with those of Example 1 and Comparative Example 2. Figure 8 The ESR measurement results are shown together with those of Example 1 and Comparative Example 2 in the graph. Figure 9The graphs are shown below. The horizontal axis of each graph represents frequency, and the vertical axis represents capacitance or ESR. In the graphs, the circular markers represent Example 1, the x-marks represent Comparative Example 2, and the diamond markers represent Comparative Example 3. Table 2 below shows the capacitance (Cap) at 120 Hz, the capacitance at 10 kHz, the ESR at 10 kHz, and the ESR at 100 kHz.

[0123] (Table 2)

[0124]

[0125] In Comparative Example 3, no high-boiling-point solvent was added to the conductive polymer dispersion; instead, the high-boiling-point solvent was impregnated into the capacitor element after the polymer layer was formed. Figure 8 As shown in Table 2, the solid electrolytic capacitor of Comparative Example 3 has a higher capacitance and a lower ESR than Comparative Example 2. However, the solid electrolytic capacitor of Comparative Example 3 cannot achieve a high capacitance in the high-frequency region as in Example 1, nor can it reduce the ESR in the high-frequency region as in Example 1.

[0126] Therefore, it was confirmed that the solid electrolytic capacitor exhibited high capacitance and reduced ESR in the high-frequency band through the polymer layer formation process using a conductive polymer dispersion containing a high-boiling-point solvent. It should be noted that the superior properties of Comparative Example 3 compared to Comparative Example 2 are presumably due to the change in the higher-order structure of the conductive polymer caused by ethylene glycol.

[0127] (Examples 3 to 6 and Comparative Example 4)

[0128] Solid electrolytic capacitors of Examples 3 to 6 and Comparative Example 4 were manufactured. Compared to Example 1, Examples 3 to 6 differed in the types of high-boiling-point solvents added to the conductive polymer dispersion. Other structures of Examples 3 to 6, including the presence of an expanded surface layer, a dielectric coating, and a pseudo-boehmite coating on the anode foil, were identical to those of Example 1. The solid electrolytic capacitors of Examples 3 to 6 were manufactured using the same manufacturing method and under the same manufacturing conditions, except for the type of high-boiling-point solvent. Regarding the drying conditions of the conductive polymer dispersion in the polymer layer formation process, adjustments were made based on the boiling points of various high-boiling-point solvents to ensure consistent solid content after drying. The drying temperature was adjusted within a range lower than the boiling points of the various high-boiling-point solvents.

[0129] In Example 3, diethylene glycol was added to the conductive polymer dispersion as a high-boiling-point solvent instead of ethylene glycol in Example 1. The drying conditions were the same as in Example 1. In Example 4, glycerol was added to the conductive polymer dispersion as a high-boiling-point solvent instead of ethylene glycol in Example 1. The drying conditions were: 10 minutes at 60°C, 30 minutes at 110°C, and 90 minutes at 150°C. In Example 5, 1,3-propanediol was added to the conductive polymer dispersion as a high-boiling-point solvent instead of ethylene glycol in Example 1. The drying conditions were the same as in Example 1. In Example 6, polyethylene glycol was added to the conductive polymer dispersion as a high-boiling-point solvent instead of ethylene glycol in Example 1. In Comparative Example 4, tetrahydrofuran, a low-boiling-point solvent, was added to the conductive polymer dispersion instead of ethylene glycol in Example 1. The drying conditions were set to be placed at 60°C for 10 minutes, then at 110°C for 30 minutes, and finally at 150°C for 150 minutes.

[0130] Diethylene glycol has a boiling point of 245°C, which is higher than the drying temperature of 110°C in the polymer layer formation process. Glycerin has a boiling point of 290°C, also higher than the drying temperature of 110°C in the polymer layer formation process. 1,3-Propane glycol has a boiling point of 213°C, also higher than the drying temperature of 110°C in the polymer layer formation process. Polyethylene glycol has a boiling point higher than the drying temperature of 110°C in the polymer layer formation process, but this varies depending on the molecular weight; for molecular weights above 300, it is above 200°C. Tetrahydrofuran has a boiling point of 66°C, lower than the drying temperature of 110°C in the polymer layer formation process.

[0131] (Frequency response measurement test)

[0132] The capacitance and ESR of the solid electrolytic capacitors of Examples 3 to 6 and Comparative Example 4 were measured at each frequency. The capacitance and ESR were measured using the same methods and conditions as in Example 1. The capacitance measurement results are shown below. Figure 10 The ESR measurement results are shown in the graph. Figure 11 The graphs are shown below. The horizontal axis of each graph represents frequency, and the vertical axis represents capacitance or ESR. In the graphs, solid circles represent Example 1, rhombuses represent Example 3, quadrilaterals represent Example 4, solid triangles represent Example 5, hollow circles represent Example 6, x-marks represent Comparative Example 2, and hollow triangles represent Comparative Example 4. Furthermore, Table 3 below shows the capacitance (Cap) at 120 Hz, capacitance at 10 kHz, ESR at 10 kHz, and ESR at 100 kHz.

[0133] (Table 3)

[0134]

[0135] like Figure 10 As shown in Table 3, the solid electrolytic capacitors of Examples 3 to 6, like those of Example 1, exhibit high capacitance in the high-frequency region. Furthermore, as... Figure 11 As shown in Table 3, the solid electrolytic capacitors of Examples 3 to 6, like those of Example 1, also maintain low ESR in the high-frequency region. Thus, it is confirmed that even when a high-boiling-point solvent other than ethylene glycol is added to the conductive polymer dispersion, the solid electrolytic capacitor exhibits high capacitance and maintains low ESR in the high-frequency region.

[0136] (Refer to Examples 1 to 3)

[0137] Solid electrolytic capacitors of Reference Examples 1 to 3 were manufactured. In Reference Examples 1 to 3, the pseudo-boehmite coating was removed. During the removal of the pseudo-boehmite coating, multiple removal and repair chemical conversion processes were performed. In the removal process, after the pre-chemical conversion process and the chemical conversion process, the anode foil was immersed in an aqueous phosphoric acid solution to dissolve the pseudo-boehmite coating. In the repair chemical conversion process, to repair defects in the dielectric coating caused by the removal process, the anode foil was immersed in an aqueous boric acid solution while a constant current was applied to the anode foil.

[0138] The conductive polymer dispersion of Reference Example 1 contains ethylene glycol, thus having the same composition as Example 1. The conductive polymer dispersion of Reference Example 2 does not contain a high-boiling-point solvent and only contains water as a dispersion medium, thus having the same composition as Comparative Example 1. The conductive polymer dispersion of Reference Example 3 is the same as that of Reference Example 2, but after the polymer layer formation process using a conductive polymer dispersion without a high-boiling-point solvent, the capacitor element with the polymer layer formed is immersed in ethylene glycol. Reference Example 3 is the same as Comparative Example 3, after the ethylene capacitor element is lifted from the ethylene glycol, it is dried at a temperature of 150°C for 5 minutes. In Reference Example 3, ethylene glycol is impregnated in the capacitor element through this high-boiling-point solvent impregnation process. Thus, in Reference Example 3, the high-boiling-point solvent is not added to the conductive polymer dispersion, but rather added to the capacitor element after the polymer formation process.

[0139] (Frequency response measurement test)

[0140] The capacitance and ESR of the solid electrolytic capacitors of Reference Examples 1 to 3 were measured at each frequency. The capacitance and ESR were measured using the same methods and conditions as in Example 1. The capacitance measurement results are shown below. Figure 12The ESR measurement results are shown in the graph. Figure 13 The graphs are shown below. The horizontal axis of each graph represents frequency, and the vertical axis represents capacitance or ESR. In the graphs, the circle markings are from Reference Example 1, the x markings are from Reference Example 2, and the triangle markings are from Reference Example 3. Furthermore, Table 4 below shows the capacitance (Cap) at 120 Hz, the capacitance at 10 kHz, the ESR at 10 kHz, and the ESR at 100 kHz.

[0141] (Table 4)

[0142]

[0143] like Figure 12 As shown in Table 4, the solid electrolytic capacitors of Reference Examples 1 to 3 exhibit high capacitance in the high-frequency region. Furthermore, as... Figure 13 As shown in Table 4, the solid electrolytic capacitors of Reference Examples 1 to 3 also maintain low ESR in the high-frequency region.

[0144] Thus, even when the boehmite coating is removed, the solid electrolytic capacitor maintains good capacitance and ESR in the high-frequency region regardless of whether a high-boiling-point solvent is added to the conductive polymer dispersion or whether the capacitor element after polymer layer formation is impregnated with a high-boiling-point solvent. In other words, without the boehmite coating, there is no issue of uneven distribution of conductive polymers within the expansion layer.

[0145] In other words, the problem of uneven distribution of conductive polymers within the expansion layer is a unique problem in the formation of anode foils with pseudo-boehmite coatings. It has been confirmed that the effect of adding a high-boiling-point solvent to the conductive polymer dispersion is a unique effect in the formation of anode foils with pseudo-boehmite coatings.

[0146] (Withstand voltage test)

[0147] Next, the withstand voltage of Examples 1 to 6, Comparative Examples 1 to 3, and Reference Examples 1 to 3 was measured. When measuring the withstand voltage, the voltage was increased from 0 V at 1 V intervals per second at 25°C, and the current-time rise curve was observed. The withstand voltage was defined as the voltage (V) at which a 20 mA current was applied, observed due to spikes or scintillation, in the current rise curve. The results are shown in Table 5 below.

[0148] (Table 5)

[0149]

[0150] As shown in Table 5, the solid electrolytic capacitors of Reference Examples 1 to 3 exhibit good capacitance and ESR in the high-frequency region, but their withstand voltage does not meet the 250 V requirement in the field of power electronics. On the other hand, the solid electrolytic capacitors of Examples 1 to 6 have a withstand voltage that fully meets the 250 V requirement in the field of power electronics, and exhibit good capacitance and ESR in the high-frequency region.

[0151] Specifically, through a polymer layer formation process using a conductive polymer dispersion containing a high-boiling-point solvent and sorbitol, the anode foil having an extended surface layer, a dielectric coating, and a pseudo-boehmite coating contains at least 50% conductive polymer at the bottom of the extended surface layer, based on the content in the surface layer. This results in a voltage withstand capability of 250 V or higher, good capacitance and ESR in the high-frequency region, and is particularly useful in power electronics, especially when assembled into devices using wide-bandgap semiconductors such as SiC and GaN.

[0152] (Example 7 and Comparative Example 5)

[0153] Solid electrolytic capacitors of Example 7 and Comparative Example 5 were manufactured. The difference between Example 7 and Comparative Example 5 and Example 1 and Comparative Example 2 is that the anode foil in Example 7 and Comparative Example 5 is a powder-laminated foil. The solid electrolytic capacitor of Example 7 is identical to that of Example 1, except that the anode foil is a powder-laminated foil, and was manufactured using the same manufacturing method and the same manufacturing conditions. Similarly, the solid electrolytic capacitor of Comparative Example 5 is identical to that of Comparative Example 2, except that the anode foil is a powder-laminated foil, and was manufactured using the same manufacturing method and the same manufacturing conditions.

[0154] The powder-coated foil of the anode is fabricated as follows. That is, in the expansion process, aluminum powder is laminated onto an aluminum substrate and sintered. Next, a pseudo-boehmite coating is formed in the pre-chemical conversion process. In the chemical conversion treatment process, the outermost layer of the residual pseudo-boehmite coating is transformed into a dielectric coating.

[0155] The conductive polymer dispersion used in Example 1 consisted of PEDOT:PSS, ethylene glycol, and water. 10 wt% of the dispersion medium was ethylene glycol, and the remaining 90 wt% was water. PEDOT:PSS was added to make its weight relative to the conductive polymer dispersion 1 wt%. The drying temperature in the polymer layer formation process was 110°C, lower than the boiling point of ethylene glycol, 197°C. The conductive polymer dispersion used in Comparative Example 5 consisted of PEDOT:PSS and water. The dispersion medium in the conductive polymer dispersion used in Comparative Example 5 was entirely water, and no ethylene glycol was added to the conductive polymer dispersion in Comparative Example 5.

[0156] (Observation of the polymer layer)

[0157] For the anode foils of Example 7 and Comparative Example 5, energy-dispersive X-ray analysis of their cross-sections was performed to determine the elemental ratios. Then, the surface of the expansion layer was divided into three equal regions from the surface of the expansion layer to the core surface, and the analysis target regions were set at the surface, middle and bottom of the expansion layer.

[0158] Figure 14 This is a graph showing the results of EDX analysis and the elemental ratio of sulfur in Example 7 and Comparative Example 5. Additionally, Figure 15 This is a graph showing the results of EDX analysis and the sulfur content of each depth zone of the expanded surface layer in Example 7 and Comparative Example 5, based on the sulfur content of the surface layer. In both graphs, the circular markers represent Example 7, and the x-marks represent Comparative Example 5.

[0159] like Figure 14 and Figure 15 As shown, in Comparative Example 5 where a powder-laminated foil is used as the anode, the amount of sulfur contained in the surface layer of the expanded layer is reduced by more than 95%, based on the amount of sulfur contained in the surface layer. In other words, the conductive polymer contained in the bottom of Comparative Example 5 is less than about 5% of that in the surface layer. It should be noted that in Comparative Example 2 where an etched foil is used as the anode, the amount of sulfur is reduced by about 85%, or about 15% of that in the surface layer. That is, if a powder-laminated foil is used as the anode, it is more difficult to form a polymer layer at the bottom of the expanded layer.

[0160] In contrast, such as Figure 14 and Figure 15 As shown, in Example 7 where a powder-laminated foil is used as the anode, the amount of sulfur contained in the surface layer of the expanded layer is reduced by approximately 20%, based on the amount of sulfur contained in the surface layer. In other words, the conductive polymer contained in the bottom of Example 7 is approximately 80% of that in the surface layer. In Example 1 where an etched foil is used as the anode, the reduction is approximately 45%, or approximately 55% of that in the surface layer. That is, even when a powder-laminated foil is used as the anode, the addition of ethylene glycol to the dispersion makes it easier for a polymer layer to form at the bottom of the expanded layer.

[0161] Thus, when using a powder-laminated foil at the anode, it is more difficult to form a polymer layer at the bottom of the expanded layer compared to using an etched foil at the anode. However, it has been found that if a powder-laminated foil is used at the anode, and the dispersion medium of the conductive polymer dispersion contains a high-boiling-point solvent, and drying is performed at a temperature lower than that of the high-boiling-point solvent, then a polymer layer is more easily formed compared to using an etched foil at the anode.

[0162] (Frequency response measurement test)

[0163] The capacitance and ESR of the solid electrolytic capacitors of Example 7 and Comparative Example 5 were measured at each frequency. The methods and conditions for measuring capacitance and ESR were the same as those for Example 1 and Comparative Example 2. The capacitance measurement results are shown below. Figure 16 The ESR measurement results are shown in the graph. Figure 17 The graphs are shown below. The horizontal axis of each graph represents frequency, and the vertical axis represents capacitance or ESR. In the graphs, circles represent Example 7, black circles represent Example 7, and x-marks represent Comparative Example 5. Additionally, Table 6 below lists the capacitance (Cap) at 120 Hz, capacitance at 10 kHz, ESR at 10 kHz, and ESR at 100 kHz, along with Examples 1 and Comparative Example 2.

[0164] (Table 6)

[0165]

[0166] like Figure 16 As shown in Table 6, the solid electrolytic capacitor of Example 7 exhibits a higher capacitance compared to Comparative Example 5, which also uses powder laminated foil. In particular, when used in the high-frequency region of 10 kHz, the capacitance of the solid electrolytic capacitor of Comparative Example 5 decreases significantly, while the capacitance of the solid electrolytic capacitor of Example 7 is nearly twice that of Comparative Example 5.

[0167] In addition, such as Figure 17 As shown in Table 6, the ESR of the solid electrolytic capacitor of Example 7 is lower than that of Comparative Example 5. In the high-frequency region of 10 kHz and 100 kHz, the ESR of the solid electrolytic capacitor of Example 7 is about one-twentieth of that of Comparative Example 5.

[0168] Thus, even if the anode foil is a powder-laminated foil, through a polymer layer formation process using a conductive polymer dispersion containing a high-boiling-point solvent, the anode foil having an expansion layer, a dielectric coating, and a pseudo-boehmite coating contains at least 50% conductive polymer at the bottom of the expansion layer, based on the content in the surface layer. Furthermore, it has been confirmed that if the anode foil having an expansion layer, a dielectric coating, and a pseudo-boehmite coating contains at least 50% conductive polymer at the bottom of the expansion layer, based on the content in the surface layer, then even when used in high-frequency bands, the solid electrolytic capacitor exhibits good capacitance performance and reduced ESR.

[0169] Furthermore, if the anode foil is a powder-laminated foil, and a conductive polymer dispersion without the addition of high-boiling-point solvents is used, and the content of the conductive polymer at the bottom of the expansion layer is less than 50% of the content in the surface layer of the expansion layer, the ESR deteriorates significantly compared to the case of etched foil. However, comparing Example 1 and Example 7, it can be seen that when the anode foil is a powder-laminated foil, a conductive polymer dispersion without the addition of high-boiling-point solvents is used, and the content of the conductive polymer at the bottom of the expansion layer is more than 50% of the content in the surface layer of the expansion layer, the ESR is confirmed to decrease significantly compared to the case of etched foil.

Claims

1. A solid electrolytic capacitor comprising an anode foil and a layer of conductive polymer, characterized in that, The anode foil has The extended layer formed on the foil surface The dielectric coating formed in the expanded surface layer, and A pseudo-thin boehmite coating is formed on the dielectric coating. Based on the content in the surface layer of the expanded layer, the bottom of the expanded layer contains more than 50% of the conductive polymer.

2. The solid electrolytic capacitor as described in claim 1, characterized in that, The conductive polymer is poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid.

3. The solid electrolytic capacitor as described in claim 1 or 2, characterized in that, The expanded surface layer has multiple tunnel-shaped pits.

4. The solid electrolytic capacitor as described in claim 1 or 2, characterized in that, The expanded surface layer is formed by stacking powders of valve-acting metal.

5. The solid electrolytic capacitor as described in claim 1 or 2, characterized in that, It has a withstand voltage of over 250 V.

6. The solid electrolytic capacitor as described in claim 1 or 2, characterized in that, The dielectric coating has a thickness corresponding to a chemical conversion voltage of 250 V or higher.

7. A method for manufacturing a solid electrolytic capacitor, comprising an anode foil and a layer of conductive polymer, characterized in that... It includes the following processes: The expansion process of forming an expansion layer on the anode foil; The process preceding the chemical conversion of the anode foil after the expansion process; A chemical conversion process in which the dielectric coating is formed on the anode foil in such a way that the pseudo-boehmite coating remains on the dielectric coating; as well as In order to form a polymer layer process, the conductive polymer is formed by having a conductive polymer dispersion containing at least 50% of the conductive polymer at the bottom of the expanded layer, based on the content in the surface layer of the expanded layer, by having the conductive polymer dispersion containing the conductive polymer and a high-boiling-point solvent adhere at least to the anode foil and by drying it at a temperature lower than the boiling point of the high-boiling-point solvent.

8. The method for manufacturing a solid electrolytic capacitor as described in claim 7, characterized in that, In the surface-expanding process, the surface-expanding layer is formed through multiple tunnel-shaped pits.

9. The method for manufacturing a solid electrolytic capacitor as described in claim 7, characterized in that, In the expansion process, powder of valve action metal is stacked.

10. The method for manufacturing a solid electrolytic capacitor as described in claim 7, characterized in that, In the chemical conversion process, a chemical conversion voltage of 250 V or higher is applied to the anode foil.

Images