Solid electrolytic capacitor and method for manufacturing a solid electrolytic capacitor

Abstract

The present invention provides a high-voltage solid electrolytic capacitor and a method for manufacturing a solid electrolytic capacitor. [Solution] The solid electrolytic capacitor comprises an anode, a cathode, and a solid electrolyte layer interposed between the anode and the cathode. The anode has a valve-acting metal substrate and a dielectric oxide film on the valve-acting metal substrate. The dielectric oxide film comprises a void layer and a void repair layer. The void layer is located on the surface side of the dielectric oxide film and has a plurality of microvoids formed thereon. The void repair layer is located on the boundary side of the dielectric oxide film with the valve-acting metal substrate. The solid electrolyte layer contains particles of a conductive polymer of poly(2-ethyl-3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid. These conductive polymer particles have a larger diameter than the hole diameter of the microvoids in the void layer.

Description

Technical Field

[0001] The present invention relates to a solid electrolytic capacitor and a method for manufacturing a solid electrolytic capacitor.

Background Art

[0002] Capacitors are used in various applications. For example, in the field of power electronics, a power supply circuit that converts power of an AC power supply into DC power by a converter circuit and converts the DC power into desired AC power by an inverter circuit is provided with a smoothing capacitor for suppressing and smoothing DC pulsation output from the converter circuit before inputting the DC power to the inverter circuit. Further, for stable operation of semiconductor switching elements such as gallium nitride and noise removal, a decoupling capacitor is provided in the vicinity of the semiconductor switching element.

[0003] Along with the increase in power in recent years, the demand for higher capacitance of capacitors has become stronger. Electrolytic capacitors can achieve higher capacitance more easily than film capacitors, and can readily meet this demand for higher capacitance. An electrolytic capacitor is provided with a valve metal such as tantalum or aluminum as an anode foil and a cathode foil. The anode foil is surface-expanded by forming the valve metal into a shape such as a sintered body or an etched foil, and has a dielectric oxide film formed on the surface-expanded surface by a treatment such as anodic oxidation. An electrolyte is interposed between the anode foil and the cathode foil.

[0004] Electrolytic capacitors can increase the specific surface area of the anode foil through a surface expansion process, so they have large capacitance and can meet the demand for higher capacitance. In addition, electrolytic capacitors include an electrolyte in the form of an electrolytic solution. Since the electrolytic solution can penetrate into deep portions of the surface-expanded surface of the anode foil, the contact area between the electrolyte and the dielectric oxide film can be increased. Therefore, the capacitance of the electrolytic capacitor can be further increased, which is suitable for the demand for higher capacitance accompanying the increase in power in recent years. However, the electrolytic solution evaporates and diffuses to the outside over time, which causes a decrease in capacitance and an increase in dissipation factor of the electrolytic capacitor over time, eventually leading to dry-up.

[0005] Therefore, among electrolytic capacitors, solid electrolytic capacitors, which use a solid electrolyte, are attracting attention. Solid electrolytic capacitors are free from or have suppressed effects from electrolyte drying. In addition, solid electrolytic capacitors have the advantage of having a low equivalent series resistance (ESR).

[0006] Known solid electrolytes include manganese dioxide and 7,7,8,8-tetracyanoquinodimethane (TCNQ) complexes. In recent years, conductive polymers derived from monomers with π-conjugated double bonds, such as poly(3,4-ethylenedioxythiophene) (PEDOT), have rapidly become popular as solid electrolytes due to their slow reaction rates and excellent adhesion to dielectric oxide films. Conductive polymers utilize acid compounds such as polyanions as dopants, and also possess substructures within the monomer molecule that act as dopants, resulting in high conductivity. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2003-160647 [Patent Document 2] Japanese Patent Publication No. 2008-258224 [Overview of the project] [Problems that the invention aims to solve]

[0008] On the other hand, in fields such as power electronics, high-voltage capacitors are in demand. For example, inverters used to drive motors in electric vehicles utilize smoothing capacitors with a voltage rating of 470V, but even higher voltage-rated smoothing capacitors are needed to achieve even faster charging. Electrolytic capacitors using electrolytes have high voltage ratings due to the electrolyte's defect repair effect on the dielectric oxide film, but even with such electrolytic capacitors, meeting the requirement for high voltage ratings is not easy.

[0009] Furthermore, solid electrolytic capacitors have difficulty achieving high voltage ratings compared to electrolytic capacitors using electrolytes, due to considerations of adhesion to the anode foil and the defect repair function of the dielectric oxide film. Generally, increasing the dielectric oxide film thickness is considered to increase the voltage rating of a capacitor. However, in the case of solid electrolytic capacitors, even if the dielectric oxide film is thickened, it is difficult to achieve a voltage rating exceeding 400V, and moreover, even the capacitance, which is the advantage of solid electrolytic capacitors, is greatly reduced.

[0010] Figure 6 is a graph showing the relationship between the formation voltage and the breakdown voltage for forming the dielectric oxide film in a solid electrolytic capacitor. As shown in Figure 6, the relationship between the breakdown voltage and the formation voltage is well proportional up to the breakdown voltage of the solid electrolytic capacitor in the 300V region. However, when applying a breakdown voltage of 350V or more to a solid electrolytic capacitor, a formation voltage far exceeding the target breakdown voltage is required. To apply a breakdown voltage of 470V to a solid electrolytic capacitor, a large formation voltage exceeding 1000V is required. It appears difficult to achieve a breakdown voltage of 400V by increasing the formation voltage.

[0011] In other words, achieving high voltage resistance in solid electrolytic capacitors to cope with the increasing power demands of recent years has not been easy. Consequently, current solid electrolytic capacitors are predominantly limited to a voltage resistance of around 100V, and in fields requiring voltage resistance exceeding 400V, for example, even with their long lifespan and low equivalent series resistance, solid electrolytic capacitors have not been a viable option due to the excessive power required for production.

[0012] This invention was proposed to solve the above problems, and its objective is to provide a high-voltage solid electrolytic capacitor and a method for manufacturing a solid electrolytic capacitor. [Means for solving the problem]

[0013] To solve the above problems, the solid electrolytic capacitor of this embodiment comprises an anode, a cathode, and a solid electrolyte layer interposed between the anode and the cathode. The anode has a valve-acting metal substrate and a dielectric oxide film on the valve-acting metal substrate. The dielectric oxide film has a void layer located on the surface side of the dielectric oxide film where a plurality of microvoids are formed, and a void repair layer on the boundary side of the dielectric oxide film with the valve-acting metal substrate. The solid electrolyte layer contains particles of a conductive polymer of poly(2-ethyl-3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid, and the particles of the conductive polymer are larger in diameter than the hole diameter of the microvoids in the void layer.

[0014] Furthermore, in order to solve the above problems, the method for manufacturing a solid electrolytic capacitor of this embodiment is a method for manufacturing a solid electrolytic capacitor comprising an anode body, a cathode body and a solid electrolyte layer based on a valve metal, comprising: an anode body processing step for processing the anode body; a conductive polymer liquid preparation step for preparing a dispersion of a conductive polymer of poly(2-ethyl-3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid; an element formation step for forming a capacitor element by facing the anode body and the cathode body; and applying the dispersion liquid to the surface of the anode body or inside the capacitor element. The anode body processing step includes a solid electrolyte layer formation step, the anode body processing step includes a first chemical treatment in which a predetermined chemical treatment voltage is applied to a valve-acting metal substrate, a void introduction treatment in which, after the first chemical treatment, the anode body is immersed in one or more of an acidic solution, an alkaline solution, and pure water to form a void layer containing a plurality of microvoids, and a second chemical treatment in which, after the void introduction treatment, a chemical treatment voltage lower than that of the first chemical treatment is applied, the conductive polymer liquid preparation step is in which the conductive polymer having a diameter larger than the hole diameter of the microvoids is dispersed in the dispersion.

[0015] The process may include an aging step in which a voltage is applied to the capacitor element after the element formation step and the solid electrolyte layer formation step, and the second chemical formation step may be the aging step.

[0016] Said step of preparing said conductive polymer solution may include a polymerization step of producing said conductive polymer having a larger diameter than the pore diameter of said microvoids.

Effects of the Invention

[0017] According to the present invention, the withstand voltage of a solid electrolytic capacitor can be increased.

Brief Description of Drawings

[0018] [Figure 1] It is a schematic diagram showing an anode body of a solid electrolytic capacitor. [Figure 2] It is a flowchart showing a method for manufacturing an anode body of a solid electrolytic capacitor. [Figure 3] It is a flowchart showing a method for manufacturing a solid electrolytic capacitor. [Figure 4] It is a graph showing the relationship between applied voltage and leakage current in Example 1, Comparative Example 1, and Comparative Example 2. [Figure 5] It is a graph showing the withstand voltage of Example 1, Comparative Example 1, and Comparative Example 2. [Figure 6] It is a graph showing the relationship between formation voltage and withstand voltage in a solid electrolytic capacitor.

Mode for Carrying Out the Invention

[0019] (Solid Electrolytic Capacitor) A solid electrolytic capacitor includes an anode body, a cathode body, and a solid electrolyte layer made of a conductive polymer. The anode body and the cathode body are disposed opposite to each other. A separator is interposed between the anode body and the cathode body. An assembly of the anode body, the cathode body, and the separator is referred to as a capacitor element. This capacitor element is classified into, for example, a stacked type, a wound type, and a flat plate type depending on its shape. In the stacked type, anode bodies and cathode bodies are alternately stacked with separators interposed therebetween. In the wound type, an anode body and a cathode body are wound with a separator interposed therebetween. In the flat plate type, a conductive polymer and a cathode body are stacked on the anode body.

[0020] The separator holds the conductive polymer and maintains the shape of the solid electrolyte layer. The separator also prevents short circuits between the anode and cathode. If the solid electrolyte layer maintains its shape on its own and isolates the anode and cathode, the separator can be omitted from the solid electrolytic capacitor configuration.

[0021] The conductive polymer is formed using a conductive polymer liquid. The conductive polymer liquid is a dispersion in which conductive polymers are dispersed. A solid electrolyte layer is formed by impregnating a separator with this conductive polymer liquid and drying it. Alternatively, a solid electrolyte layer is formed by coating the anode with this conductive polymer and drying it. The solid electrolyte layer adheres closely to the anode and functions as a true cathode.

[0022] An anode lead is connected to the anode body, and a cathode lead is connected to the cathode body. The solid electrolytic capacitor is electrically connected to the mounted circuit via these anode and cathode leads. By conducting electricity with the mounted circuit, the solid electrolytic capacitor becomes a passive element that stores and discharges electric charge by obtaining capacitance through the dielectric polarization effect of the dielectric oxide film.

[0023] (Anode) Figure 1 is a schematic diagram showing the anode of a solid electrolytic capacitor. As shown in Figure 1, the anode is a foil with a thickness on the order of micrometers. This anode comprises a valve-acting metal substrate 1, a dielectric oxide film 2, and a hydrated oxide film 3, extending from the center in the thickness direction toward both foil surfaces. That is, the valve-acting metal substrate 1 is located at the center in the thickness direction of the anode. The hydrated oxide film 3 is located at the outermost layer of the anode. The dielectric oxide film 2 is located between the valve-acting metal substrate 1 and the hydrated oxide film 3.

[0024] Furthermore, the dielectric oxide film 2 is further divided into a void repair layer 21 and a void layer 22. The void repair layer 21 is the region on the valve-acting metal substrate 1 side and is in contact with the valve-acting metal substrate 1. The void layer 22 is a region away from the valve-acting metal substrate 1, in other words, it is on the surface side of the dielectric oxide film 2. An unrepaired layer 23 may be interposed between the void repair layer 21 and the void layer 22. Alternatively, the entire region of the unrepaired layer 23 may be replaced by the void repair layer 21, and the void repair layer 21 and the void layer 22 may be in contact.

[0025] The valve metal substrate 1 is a core layer in which the base metal remains as unoxidized valve metal. In the wound type, this valve metal substrate 1 is a long strip formed by stretching the valve metal, and in the laminated type, this valve metal substrate 1 is a flat plate or a sintered body formed by molding and sintering powder into a flat plate shape. The valve metals include aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony. The purity of the anode is preferably 99.9% or higher, but impurities such as silicon, iron, copper, magnesium, and zinc may be present.

[0026] The hydrated oxide film 3 is a layer of needle-shaped structures containing hydrated aluminum oxide. The hydrated oxide is AlOOH·xH2O or Al2O3·xH2O. The hydrated oxide film 3 is dense internally and functions as a resistive layer to improve the voltage withstand capability of the solid electrolytic capacitor. However, the hydrated oxide film 3 may be completely replaced and disappear with the dielectric oxide film 2 during the manufacturing process of the solid electrolytic capacitor, or the step of forming the hydrated oxide film 3 may be eliminated altogether.

[0027] This hydrated oxide film 3 is formed by hot water treatment of the valve metal substrate 1. In the hot water treatment, the valve metal substrate 1 is immersed in pure water at 80°C or higher or boiling. Through this hot water treatment, the valve metal substrate 1 is replaced by the hydrated oxide film 3 from the surface to the depths. The immersion time should be determined by balancing the dielectric strength and capacitance, depending on the desired thickness of the hydrated oxide film 3.

[0028] The dielectric oxide film 2 is a layer of aluminum oxide containing γ-alumina, which is a crystalline oxide. Through the first chemical conversion treatment, the dielectric oxide film 2 replaces the hydrated oxide film 3 with the dielectric oxide film 2, starting from the interface between the valve metal substrate 1 and the hydrated oxide film 3 and moving towards the surface of the hydrated oxide film 3. That is, the hydrated oxide is converted to γ-alumina from the interface between the valve metal substrate 1 and the hydrated oxide film 3. The entire surface of the hydrated oxide film 3 may also be replaced with the dielectric oxide film 2.

[0029] In the first chemical treatment, a voltage corresponding to the desired withstand voltage is applied to the anode body, which has formed a hydrated oxide film 3 in a halogen-ion-free chemical treatment solution. As the chemical treatment solution, a phosphoric acid-based solution such as ammonium dihydrogen phosphate, a boric acid-based solution such as ammonium borate, or an adipic acid-based aqueous solution such as ammonium adipate can be used.

[0030] The void layer 22 contains numerous minute voids within the layer, the voids being too small for conductive polymers to penetrate. In other words, the void layer 22 is a region of the dielectric oxide film 2 containing numerous minute voids whose void diameter is smaller than the particle diameter of the conductive polymers in terms of their equivalent circular diameter. To put it another way, the solid electrolyte layer contains conductive polymers with a diameter larger than the void diameter of the minute voids. The void repair layer 21 extends from the interface with the valve-acting metal substrate 1 and is a layer in which the minute voids have disappeared or become fewer than those in the void layer 22.

[0031] Generally, in electrolytic capacitors using an electrolyte, voids within the dielectric oxide film 2 reduce the breakdown voltage. Therefore, in electrolytic capacitors using an electrolyte, voids are reduced by depolarization treatment. On the other hand, in solid electrolytic capacitors, even if voids generated by the first chemical conversion treatment are reduced by depolarization treatment, the effect of improving the breakdown voltage is small. Rather, in solid electrolytic capacitors, actively introducing a void layer 22 containing many minute voids, even though these are minute voids without conductive polymers, into the dielectric oxide film 2 improves the breakdown voltage of the solid electrolytic capacitor.

[0032] This is a hypothesis, and not limited to this, but in solid electrolytic capacitors, the conductive component is a conductive polymer. Since the conductive polymer has a larger diameter than the hole diameter of the minute voids, it is difficult for it to penetrate the void layer 22. At the very least, the concentration of conductive polymer in the void layer 22 is lower than the concentration of conductive polymer in the solid electrolyte layer.

[0033] However, the liquid component of the conductive polymer solution penetrates into the minute voids, and some of it remains even after the drying process. The liquid component of the conductive polymer solution exhibits a repair effect on defects in the dielectric oxide film 2. That is, the void layer 22 has minute voids, and while the conductive polymer is excluded, the liquid component of the conductive polymer solution, which has a repair effect on defects in the dielectric oxide film 2, is filled in. As a result, the risk of dielectric breakdown of the solid electrolytic capacitor is reduced, and the withstand voltage is improved.

[0034] Furthermore, charges from the conductive polymer may probabilistically be distributed on the surface of the void repair layer 21. When these charges distributed on the surface of the void repair layer 21 are injected into a weak area of ​​the void repair layer 21, the resistance of that weak area decreases. When the resistance of the weak area into which the charges have been injected decreases, the charges accumulated on the surface of the void repair layer 21 concentrate at that weak area, causing dielectric breakdown of the void repair layer 21.

[0035] However, if the conductive polymer has a larger diameter than the diameter of the minute voids in the void layer 22, the void layer 22 acts as a barrier, increasing the separation distance between the conductive polymer and the void repair layer 21. This increased separation distance reduces the probability of the conductive polymer's charge being distributed on the surface of the void repair layer 21. As a result, charge is less likely to accumulate on the surface of the void repair layer 21, and the resistance of the weak parts of the void repair layer 21 is less likely to decrease. Consequently, dielectric breakdown of the void repair layer 21 becomes less likely. Therefore, by having conductive polymer particles with a larger diameter than the diameter of the minute voids in the void layer 22, the dielectric strength of the solid electrolytic capacitor is improved.

[0036] Furthermore, the hydrated oxide film 3, being dense internally, prevents the penetration of conductive polymers. The presence of the hydrated oxide film 3 creates a conductive polymer exclusion layer with a thickness equal to the sum of the void layer 22 and the hydrated oxide film 3. Therefore, the voltage withstand capability of the solid electrolytic capacitor is further improved.

[0037] Such a void layer 22 is formed by introducing a void introduction treatment after the first chemical conversion treatment. In the void introduction treatment, the number of minute voids in the dielectric oxide film 2 is increased. That is, the void introduction treatment forms a void layer 22 with an increased number of minute voids in the dielectric oxide film 2.

[0038] In the void introduction process, the anode body on which the dielectric oxide film 2 is formed is immersed in a void treatment solution consisting of an acidic solution, an alkaline solution, or pure water, or a combination thereof. Examples of acidic solutions, alkaline solutions, or pure water include phosphoric acid solution, sulfuric acid solution, nitric acid solution, sodium hydroxide solution, potassium hydroxide solution, ammonia solution, or pure water at 60°C or higher. Immersion in an acidic solution, alkaline solution, or pure water causes the dielectric oxide film 2 to stretch, the gas within the dielectric oxide film 2 to expand, or the layers of the dielectric oxide film 2 to chemically dissolve, thereby generating new minute voids or expanding large voids.

[0039] In the first chemical conversion treatment aimed at achieving high dielectric strength, shrinkage occurs due to a decrease in specific volume when the hydrated oxide film 3 is converted to a dielectric oxide film 2, resulting in the formation of coarse voids within the dielectric oxide film 2. These coarse voids are distributed in a range of approximately 50 nm to 100 nm in equivalent circular diameter. In the void introduction treatment, numerous microvoids are formed, creating open pathways of interconnected microvoids that lead to the coarse voids. These open pathways provide clues for repairing the coarse voids.

[0040] In the void introduction process, a void layer 22 is formed from the interface with the void processing liquid toward the deeper parts of the anode body. The void introduction process transforms the surface region of the dielectric oxide film 2 into a void layer 22. The thickness of the void layer 22 is determined by the immersion time in the processing liquid and the temperature of the processing liquid.

[0041] The void repair layer 21 is a region of the dielectric oxide film 2 in which, after generating open pathways of interconnected minute voids through a void introduction process, the voids are repaired and eliminated by a second chemical conversion process. The void repair layer 21 repairs the dielectric oxide film 2 sequentially from the interface between the valve-acting metal substrate 1 and the dielectric oxide film 2 toward the surface of the dielectric oxide film 2 by the second chemical conversion process, filling in both large and minute voids.

[0042] In the second chemical conversion treatment, a predetermined voltage is applied to the anode body, which has formed a void layer 22 in a conversion solution that is free of halogen ions. As the conversion solution, phosphoric acid-based conversion solutions such as ammonium dihydrogen phosphate, boric acid-based conversion solutions such as ammonium borate, adipic acid-based conversion solutions such as ammonium adipate, or conductive polymer liquid dispersion media and additives can be used. In the second chemical conversion treatment, the conversion solution reaches the coarse voids through open pathways made up of minute voids. Therefore, in the second chemical conversion treatment, the coarse voids and minute voids are repaired and disappear.

[0043] The applied voltage for the second chemical treatment is lower than the applied voltage for the first chemical treatment. In the second chemical treatment, a void repair layer 21 is generated that is free of large voids and free of or contains few small voids, while leaving the void layer 22 intact. In other words, the applied voltage for the second chemical treatment is set to a maximum such that the void repair layer 21 reaches the void layer 22, or does not completely eliminate the void layer 22.

[0044] From the viewpoint of further improving the withstand voltage of solid electrolytic capacitors, it is preferable to change the applied voltage of the second chemical treatment for forming the void repair layer 21 depending on the type of conductive polymer. When polyethylene dioxythiophene (PEDOT:PSS) doped with polystyrene sulfonic acid is used as the conductive polymer, it is preferable that the applied voltage of the second chemical treatment be lower than the first chemical treatment voltage and 400V or less, and that the withstand voltage of the void repair layer 21 be set to 400V or less. Assuming that a void repair layer 21 with a thickness of 0.9 to 1.5 nm is required to obtain a withstand voltage of 1 V, it is preferable to form a void repair layer 21 with a thickness of 360 nm or more or 600 nm or less.

[0045] When using poly(2-ethyl-3,4-ethylenedioxythiophene) (Et-PEDOT:PSS) doped with polystyrene sulfonic acid as the conductive polymer, the applied voltage for the second chemical treatment to form the void repair layer 21 is lower than the applied voltage for the first chemical treatment, but is set to exceed 400V, such as 490V, and it is preferable to set the withstand voltage of the void repair layer 21 to exceed 400V. If a void repair layer 21 with a thickness of 0.9 to 1.5 nm is required to obtain a withstand voltage of 1 V, then, for example, an applied voltage of 490 V will form a void repair layer 21 with a thickness of 440 nm or more or 740 nm or less.

[0046] Poly(2-ethyl-3,4-ethylenedioxythiophene) is a polymer with ethylated ethylenedioxythiophene as its monomer unit. Ethylated ethylenedioxythiophene is 2-ethyl-2,3-dihydrothieno[3,4-b][1,4]dioxin, which is formed by adding an ethyl group to 3,4-ethylenedioxythiophene. Alternatively, poly(2-butyl-3,4-ethylenedioxythiophene) or poly(2-propyl-3,4-ethylenedioxythiophene), which are doped with polystyrene sulfonic acid, may be used as the conductive polymer.

[0047] The mechanism for improved dielectric strength related to the void repair layer 21 is speculative and not limited to this, but is speculated to be as follows. First, when the applied voltage of the second chemical treatment is low, the resistance of the void repair layer 21 becomes uniformly low. Although the charge of the conductive polymer may probabilistically be distributed on the surface of the void repair layer 21, if the resistance of the void repair layer 21 is uniformly low, the current will be dispersed and flow uniformly within the void repair layer 21. Therefore, charge does not accumulate on the surface of the void repair layer 21. Consequently, dielectric breakdown due to charge concentration at the weak points of the void repair layer 21 does not occur.

[0048] Furthermore, in relation to polyethylene dioxythiophene (PEDOT:PSS) doped with polystyrene sulfonic acid, the withstand voltage of the void repair layer 21 is set to 400V or less. This prevents charge from accumulating on the surface of the void repair layer 21, making it less susceptible to dielectric breakdown, and further improving the withstand voltage of the solid electrolytic capacitor.

[0049] Here, the characteristics of the interface of the void repair layer 21 differ depending on the type of conductive polymer, and since the probability and amount of charge transfer from the conductive polymer differ, the voltage at which the void repair layer 21 undergoes dielectric breakdown also differs.

[0050] Therefore, in relation to poly(2-ethyl-3,4-ethylenedioxythiophene) (Et-PEDOT:PSS) doped with polystyrene sulfonic acid, the withstand voltage of the void repair layer 21 may be set to more than 400V. Even if the withstand voltage of the void repair layer 21 exceeds 400V, no charge accumulates on the surface of the void repair layer 21. Consequently, dielectric breakdown of the void repair layer 21 does not occur, and the withstand voltage of the solid electrolytic capacitor is further improved.

[0051] Furthermore, the second chemical treatment does not necessarily require the void repair layer 21 to reach the void layer 22. Therefore, the predetermined applied voltage of the second chemical treatment may be set even lower to create an unrepaired layer 23 between the void repair layer 21 and the void layer 22. In other words, the unrepaired layer 23 is a layer in which large voids and small voids have not been repaired by the second chemical treatment, and it contains more voids than the void repair layer 21.

[0052] The liquid component of the conductive polymer solution has permeated the unrepaired layer 23 via the void layer 22, and together with the void layer 22, it forms a layer that stores the liquid component which has a repairing effect on defects in the dielectric oxide film 2. Moreover, due to the void layer 22 located closer to the surface, or both the hydrated oxide film 3 and the void layer 22, the conductive polymer is absent or present in small amounts in this unrepaired layer 23. Therefore, the unrepaired layer 23, together with the void layer 22, also improves the dielectric strength of the solid electrolytic capacitor.

[0053] The remaining hydrated oxide film 3 after replacement with dielectric oxide film 2 may be adjusted in thickness or completely removed by acid treatment. Specifically, a series of steps are repeated in which the anode foil is briefly immersed in phosphoric acid or the like to acid-treat it, dissolving the hydrated oxide film 3 from the surface, and then a repair chemical conversion treatment is performed. In the repair chemical conversion treatment, the anode is immersed in the chemical conversion solution and a voltage is applied. As the chemical conversion solution, a phosphoric acid-based solution such as ammonium dihydrogen phosphate, a boric acid-based solution such as ammonium borate, an adipic acid-based solution such as ammonium adipate, or a solution made by mixing boric acid and dicarboxylic acid such as citric acid can be used.

[0054] The hydrated oxide film 3 and the dielectric oxide film 2 may be formed on the surface expansion layer of one or both sides of the anode. The surface expansion layer is a surface layer that has been treated to increase its surface area compared to its projected area, and is an etched layer formed by etching the foil body, a sintered layer formed by adhering and sintering valve metal powder to the foil body, or a vapor-deposited layer formed by depositing valve metal particles onto the foil body. That is, the surface expansion layer has a porous structure and consists of tunnel-shaped pits, sponge-like pits, or densely packed powder or voids between particles. The dielectric oxide film 2 and the hydrated oxide film 3 are formed along the irregularities of this surface expansion layer.

[0055] Tunnel-shaped etching pits are holes carved in the thickness direction of the anode and may penetrate the anode. These tunnel-shaped etching pits are typically formed by passing a direct current through an acidic aqueous solution containing halogen ions, such as hydrochloric acid. The tunnel-shaped etching pits are further expanded by passing a direct current through an acidic aqueous solution containing halogen ions, such as nitric acid. Sponge-like etching pits form a spongy expanded layer with a series of fine voids in a spatial manner. These sponge-like etching pits are formed by passing an alternating current through an acidic aqueous solution containing halogen ions, such as hydrochloric acid.

[0056] The sintered layer is produced by attaching powder of the same or different valve metal as the foil to the foil and sintering it. The powder is obtained by methods such as grinding, atomization, melt spinning, rotating disk method, and rotating electrode method. The powder is pasteified with a binder or solvent, applied to the foil, and dried. Then, it is sintered by heating in a vacuum or reducing atmosphere. The atomization method can be water atomization, gas atomization, or water-gas atomization. The vapor-deposited layer is produced, for example, by resistance heating vapor deposition or electron beam heating vapor deposition. This vapor-deposited layer is formed by heating the same or different valve metal as the foil using resistance heat or electron beam energy to evaporate it, and depositing the vapor of valve metal particles onto the surface of the foil.

[0057] Figure 2 is a flowchart showing the anode processing steps for such an anode. First, a surface expansion step is performed to form a porous surface expansion layer on one or both sides of the foil (step S01). To increase the voltage resistance of the solid electrolytic capacitor, it is preferable to form tunnel-shaped etching pits in the surface expansion step.

[0058] After the surface expansion step, a hot water treatment step is performed to form a hydrated oxide film 3 on the surface of the valve-acting metal substrate 1 (step S02). The hot water treatment step for forming the hydrated oxide film 3 may be omitted.

[0059] After the hot water treatment process, a first chemical conversion treatment is performed (step S03) in which the hydrated oxide film 3 is transformed into a dielectric oxide film 2, starting from the interface between the valve-acting metal substrate 1 and the hydrated oxide film 3 and moving toward the outer surface of the hydrated oxide film 3. In the first chemical conversion treatment, the hydrated oxide film 3 may be completely removed.

[0060] After the first chemical conversion treatment step, a void introduction treatment is added to increase the number of voids in the dielectric oxide film 2 (step S04). The void introduction treatment introduces minute voids into the dielectric oxide film 2 to form a void layer 22, and also releases the coarse voids generated in the first chemical conversion treatment.

[0061] After the void introduction treatment, a second chemical conversion treatment is performed (step S05) to repair coarse and minute voids from the interface with the valve-acting metal substrate 1 and replace them with a void repair layer 21. In the second chemical conversion treatment, the applied voltage is set lower than that of the first chemical conversion treatment, so that the void layer 22 remains. After this, if necessary, a series of steps of dissolving the hydrated oxide film 3 and performing a repair chemical conversion treatment may be repeated to adjust the thickness of the hydrated oxide film 3, including zero.

[0062] Here, the void repair layer 21 can also be formed by performing a second chemical conversion treatment during the aging treatment of the solid electrolytic capacitor. Figure 3 is a flowchart showing the manufacturing method when the void repair layer 21 is formed by the aging treatment.

[0063] As shown in Figure 3, after the surface expansion process (step S01), the hot water treatment process (step S02), the first chemical conversion treatment process (step S03), and the void introduction process (step S04), an element formation process (step S05) for assembling the capacitor element and a solid electrolyte layer formation process (step S06) for forming a solid electrolyte layer inside the capacitor element are performed.

[0064] In the element formation process, the anode and cathode, which have undergone void introduction treatment, or a separator is added to them. In the solid electrolyte layer formation process, the assembly of the anode, cathode, and separator is impregnated and dried with a conductive polymer liquid. Alternatively, in the solid electrolyte layer formation process, the conductive polymer liquid is applied to the anode and dried, and the cathode is laminated onto the solid electrolyte layer.

[0065] To promote the impregnation of the conductive polymer liquid, vacuum treatment or pressurization treatment may be applied as needed during the solid electrolyte layer formation process. The impregnation process may be repeated multiple times. After impregnating the capacitor element with the conductive polymer liquid, the liquid component of the conductive polymer liquid is removed by a drying process to remove any penetration into the void layer 22.

[0066] A second chemical conversion treatment is performed after this solid electrolyte layer formation process (step S07). This second chemical conversion treatment is an aging treatment in which a lower voltage than that of the first chemical conversion treatment is applied to the solid electrolytic capacitor. During the aging treatment, the void repair layer 21 spreads from the interface between the valve-acting metal substrate 1 and the dielectric oxide film 2. In this aging treatment as well, the applied voltage is lower than that of the first chemical conversion treatment and is set to a level that does not cause the void layer 22 to disappear.

[0067] Preferably, the void repair layer 21 is formed by a second chemical conversion treatment before the element formation process. Forming the void repair layer 21 by a second chemical conversion treatment before the element formation process results in a higher capacitance of the solid electrolytic capacitor compared to when the void repair layer 21 is formed by an aging treatment.

[0068] (Solid electrolyte layer) The conductive polymer in the solid electrolyte layer is a conjugated polymer that is either self-doped by an intramolecular dopant molecule or externally doped by an external dopant molecule, such as polyethylene dioxythiophene (PEDOT:PSS) doped with polystyrene sulfonic acid and poly(2-ethyl-3,4-ethylenedioxythiophene) (Et-PEDOT:PSS) doped with polystyrene sulfonic acid.

[0069] Conjugated polymers are obtained by chemical oxidation polymerization or electrolytic oxidation polymerization of monomers or derivatives thereof that have π-conjugated double bonds. Doped conjugated polymers exhibit high conductivity. That is, conductivity is achieved by adding a small amount of dopant, such as an acceptor that readily accepts electrons or a donor that readily donates electrons, to the conjugated polymer.

[0070] In addition to polyethylenedioxythiophene and poly(2-ethyl-3,4-ethylenedioxythiophene), other known conjugated polymers can be used without particular limitation. Examples include polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene, polyphenylenevinylene, polyacene, and polythiophenevinylene. These conjugated polymers may be used individually, in combination of two or more types, or as copolymers of two or more monomers.

[0071] Among the above-mentioned conjugated polymers, conjugated polymers formed by polymerizing thiophene or its derivatives are preferred, and conjugated polymers formed by polymerizing 3,4-ethylenedioxythiophene (i.e., 2,3-dihydrothieno[3,4-b][1,4]dioxin), 3-alkylthiophene, 3-alkoxythiophene, 3-alkyl-4-alkoxythiophene, 3,4-alkylthiophene, 3,4-alkoxythiophene, or derivatives thereof are preferred. As the thiophene derivative, compounds selected from thiophenes having substituents at the 3rd and 4th positions are preferred, and the substituents at the 3rd and 4th positions of the thiophene ring may form a ring together with the carbons at the 3rd and 4th positions. The number of carbon atoms in the alkyl or alkoxy group is suitable to be 1 to 16. In particular, polymers of 3,4-ethylenedioxythiophene called EDOT, i.e., poly(3,4-ethylenedioxythiophene) called PEDOT are preferred.

[0072] Furthermore, substituents may be added to 3,4-ethylenedioxythiophene. For example, alkylated ethylenedioxythiophene, in which an alkyl group having 1 to 5 carbon atoms is added as a substituent, may be used. Examples of alkylated ethylenedioxythiophene include methylated ethylenedioxythiophene (i.e., 2-methyl-2,3-dihydrothieno[3,4-b][1,4]dioxin), ethylated ethylenedioxythiophene (i.e., 2-ethyl-2,3-dihydrothieno[3,4-b][1,4]dioxin), butylated ethylenedioxythiophene (i.e., 2-butyl-2,3-dihydrothieno[3,4-b][1,4]dioxin), and 2-alkyl-3,4-ethylenedioxythiophene. In particular, polymers of 2-ethyl-3,4-ethylenedioxythiophene, referred to as Et-EDOT, i.e., poly(2-ethyl-3,4-ethylenedioxythiophene), are preferred.

[0073] Dopants can be any known substance without particular limitation. Dopants may be used alone or in combination of two or more. Polymers or monomers may also be used. Examples of dopants include polyanions, inorganic acids such as boric acid, nitric acid, and phosphoric acid, and organic acids such as acetic acid, oxalic acid, citric acid, tartaric acid, squalaneic acid, rhodizonic acid, croconic acid, salicylic acid, p-toluenesulfonic acid, 1,2-dihydroxy-3,5-benzenedisulfonic acid, methanesulfonic acid, trifluoromethanesulfonic acid, borodisalicylic acid, bisoxalateborate acid, sulfonylimide acid, dodecylbenzenesulfonic acid, propylnaphthalenesulfonic acid, and butylnaphthalenesulfonic acid.

[0074] Polyanions include, for example, substituted or unsubstituted polyalkylenes, substituted or unsubstituted polyalkenes, substituted or unsubstituted polyimides, substituted or unsubstituted polyamides, and substituted or unsubstituted polyesters, which are polymers consisting only of structural units having anionic groups, or polymers consisting of structural units having anionic groups and structural units not having anionic groups. Specifically, examples of polyanions include polyvinyl sulfonic acid, polystyrene sulfonic acid, polyallyl sulfonic acid, polyacryl sulfonic acid, polymethacrylate sulfonic acid, poly(2-acrylamido-2-methylpropane sulfonic acid), polyisoprene sulfonic acid, polyacrylic acid, polymethacrylate, and polymaleic acid.

[0075] (Conductive polymer liquid) This solid electrolyte layer is formed in the solid electrolyte layer formation process by applying and drying a conductive polymer liquid to the anode body, or by impregnating and drying it into the capacitor element. The conductive polymer liquid is a dispersion in which conductive polymers are dispersed.

[0076] The dispersion medium or solvent of the conductive polymer liquid is a type of liquid component that remains in the solid electrolyte layer and is not particularly limited as long as it has some degree of vaporization that allows some to remain even after drying. The dispersion medium or solvent is typically water, or an organic solvent, or a mixture of an organic solvent and water. Examples of organic solvents include polar solvents, alcohols, esters, hydrocarbons, carbonate compounds, ether compounds, chain ethers, heterocyclic compounds, and nitrile compounds.

[0077] 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 linear ethers include ethylene glycol dialkyl ether, propylene glycol dialkyl ether, polyethylene glycol dialkyl ether, and polypropylene glycol dialkyl ether. Examples of heterocyclic compounds include 3-methyl-2-oxazolidinone. Examples of nitrile compounds include acetonitrile, glutalodinitrile, methoxyacetonitrile, propionitrile, and benzonitrile.

[0078] The conductive polymer liquid may contain various additives that remain in the solid electrolyte layer. For example, the conductive polymer liquid may contain additives such as ethylene glycol and polyhydric alcohols such as glycerin. Polyhydric alcohols have high boiling points and tend to remain in the solid electrolyte layer when the conductive polymer liquid is dried. Polyhydric alcohols cause changes in the higher-order structure of the conductive polymer and reorientation of the crystalline structure of the polymer chains, resulting in reduced ESR and further improved voltage withstand capability of the solid electrolytic capacitor. In addition, the conductive polymer liquid may contain sugar alcohols such as sorbitol. Sugar alcohols improve the chemical formation properties of the dielectric oxide film and further increase the voltage withstand capability of the solid electrolytic capacitor.

[0079] Conventional additives such as organic binders, surfactants, dispersants, defoamers, coupling agents, antioxidants, and UV absorbers may be added to the conductive polymer solution. The pH of the conductive polymer solution is preferably between 2 and 9. Dissolution of the separator, anode, and cathode is suppressed when the pH is within the range of 2 to 9.

[0080] Here, the conductive polymer is produced by a polymerization process. In the polymerization process, monomers, dopants, and an oxidizing agent are mixed and stirred until chemical oxidative polymerization is complete. After the polymerization process, residual monomers and impurities are removed by purification methods such as ultrafiltration, cation exchange, and anion exchange. This yields a conductive polymer liquid. The conductive polymer is dispersed using, for example, ultrasound. In the polymerization process, instead of dopants, an acid that releases dopants or an alkali metal salt thereof may be mixed in.

[0081] When the dopant is polystyrene sulfonic acid (PSS), the number-average molecular weight of PSS is 1,000 to 2,000,000, preferably 10,000 to 500,000. A number-average molecular weight of less than 1,000 is undesirable because the resulting conductive polymer will have insufficient conductivity and reduced dispersibility, while a number-average molecular weight exceeding 2,000,000 is undesirable because the viscosity of the mixture will increase.

[0082] Preferred oxidizing agents include iron salts and persulfates of inorganic and organic acids. Examples include ferric chloride hexahydrate, anhydrous ferric chloride, ferric nitrate nonahydrate, ferric nitrate, ferrous sulfate, ferric sulfate, ferric sulfate n-hydrate, ferric ammonium sulfate dodecahydrate, ferric perchlorate n-hydrate, ferric tetrafluoroborate, cupric chloride, cupric sulfate, cupric tetrafluoroborate, nitrosonium tetrafluoroborate, ammonium persulfate, sodium persulfate, potassium persulfate, potassium periodate, hydrogen peroxide, ozone, potassium hexacyanoferric, tetraammonium cerium(IV) sulfate dihydrate, bromine, iodine, iron dodecylbenzenesulfonate, ferric p-toluenesulfonate, ferric naphthalenesulfonate, ferric anthraquinonesulfonate, periodic acid, iodic acid, etc. The oxidizing agent may be a single compound or two or more compounds may be used.

[0083] In particular, it is preferable to use a combination of an iron salt such as ferrous sulfate and peroxodisulfate as the oxidizing agent. The iron salt acts as an oxidation initiator and catalyst, while the peroxodisulfate acts as an oxidation accelerator. That is, the divalent iron ions of the oxidizing agent react with persulfate ions to form radical sulfate ions, and the oxidation reaction of the monomer is promoted by these radical sulfate ions. The divalent iron ions that react with persulfate ions become trivalent iron ions, oxidize the monomer, return to divalent iron ions, and then react with persulfate ions again, repeating this cycle. It is preferable to adjust the pH of the polymerization solution to the acidic side by adding sulfuric acid or the like. For example, adjusting the polymerization solution to the acidic side, such as pH=1.9, promotes the polymerization reaction.

[0084] The solvent to which the monomer, dopant, and oxidizing agent are added can be any solvent that can dissolve the desired amount of monomer, dopant, and oxidizing agent without adversely affecting chemical oxidation polymerization. Examples of solvents include water, methanol, ethanol, isopropanol, butanol, ethylene glycol, acetonitrile, butyronitrile, acetone, methyl ethyl ketone, tetrahydrofuran, 1,4-dioxane, γ-butyrolactone, methyl acetate, ethyl acetate, methyl benzoate, ethyl benzoate, ethylene carbonate, propylene carbonate, nitromethane, nitrobenzene, sulfolane, and dimethylsulfolane. These solvents may be used individually or in mixtures of two or more.

[0085] PSS also functions as a dispersant for the monomer 2-ethyl-3,4-ethylenedioxythiophene (Et-EDOT). Increased dispersibility of the monomer in the solvent leads to smaller particle sizes of the conductive polymer; conversely, low dispersibility of the monomer in the solvent leads to larger particle sizes. To produce conductive polymers with a median diameter larger than the minute voids in the void layer 22, the amount of PSS added is reduced to decrease monomer dispersibility.

[0086] When the maximum diameter of the microvoids is 20 nm, a 1:1 molar ratio of monomer to PSS yields a conductive polymer with an exceptionally large median diameter. Furthermore, the conductive polymer is prevented from approaching the void repair layer 21 by the void layer 22, increasing the separation distance between the conductive polymer and the void repair layer 21. This improves the voltage withstand capability of the solid electrolytic capacitor.

[0087] (Cathole body) The cathode body is a cathode foil stretched from a valve metal. A purity of 99% or higher is desirable for the cathode foil. Like the anode body, a widening layer is formed on the cathode foil. A plain foil without a widening layer may also be used as the cathode foil. The cathode foil may have a naturally occurring oxide film or a thin oxide film (approximately 1-10V) formed by chemical conversion treatment. The naturally occurring oxide film is formed by the reaction of the cathode foil with oxygen in the air.

[0088] A conductive layer may be added to the outermost surface of the cathode foil. The conductive layer is formed, for example, by vapor deposition and contains metal nitrides, metal carbides, or metal carbonitrides, or it is a carbon layer applied by slurry casting, doctor blade method, or spray atomization method. The carbon layer contains carbon material and is made of fibrous carbon, carbon powder, or a mixture thereof. Fibrous carbon includes carbon nanotubes and carbon nanofibers. Carbon powder includes activated carbon made from natural plant tissues such as coconut shells, synthetic resins such as phenol, and fossil fuels such as coal, coke, and pitch, as well as carbon black such as Ketjenblack, acetylene black, and channel black, carbon nanohorns, amorphous carbon, natural graphite, artificial graphite, graphitized Ketjenblack, mesoporous carbon, etc.

[0089] Alternatively, the cathode body is a laminate of a metal layer and a carbon layer, with the carbon layer facing the anode body. The carbon layer is formed by coating it with a paste onto a solid electrolyte layer formed on the anode body, and then curing it by heating. The metal layer is, for example, a silver layer, and the metal layer is formed by coating it with a paste on top of the carbon layer, and then curing it by heating.

[0090] (Separator) Examples of separators include cellulose and mixed papers such as kraft, Manila hemp, esparto, hemp, and rayon; polyester resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and their derivatives; polyamide resins such as 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 individually or in mixtures. [Examples]

[0091] The present invention will be described in more detail below based on the following examples. However, the present invention is not limited to the following examples.

[0092] Solid electrolytic capacitors were fabricated in Example 1, Comparative Example 1, and Comparative Example 2. The solid electrolytic capacitors of Example 1, Comparative Example 1, and Comparative Example 2 differ in the particle size of the conductive polymer contained in the solid electrolyte layer. Aside from the particle size of the conductive polymer in the solid electrolyte layer, the other components were manufactured using the same manufacturing method and conditions in Example 1, Comparative Example 1, and Comparative Example 2, and are identical in all of them. That is, the anodes of Example 1, Comparative Example 1, and Comparative Example 2 were also manufactured using the same manufacturing method and conditions, and the configuration of the anodes of Example 1, Comparative Example 1, and Comparative Example 2 is also identical.

[0093] (Anode) First, an aluminum foil with a thickness of 120 μm and a purity of 99.99% was used as the valve-acting metal substrate 1 for the anode. The anode was then fabricated from this aluminum foil through an anode processing step.

[0094] First, the aluminum foil was immersed in 95°C distilled water for 10 minutes in a hot water treatment, which created a hydrated oxide film 3 on the surface of the aluminum foil. Furthermore, in the first chemical conversion treatment, the hydrated oxide film 3 was dehydrated, leaving the surface layer intact, and converted into a dielectric oxide film 2, which is highly crystalline aluminum oxide. In the first chemical conversion treatment, after creating the hydrated oxide film 3, it was immersed in a 0.5 mol / L boric acid (H3BO3) aqueous solution at a liquid temperature of 85°C, and a 1 mA / cm² test was performed. 2 A constant current was passed through while a voltage was applied. The voltage was applied until the chemical voltage reached 700V.

[0095] Next, a void layer 22 was formed by a void introduction treatment. In the void introduction treatment, the anode body with the dielectric oxide film 2 formed on it was immersed in pure water at a liquid temperature of 95°C for 600 seconds. Subsequently, the process moved to a second chemical conversion treatment to form a void repair layer 21. In the second chemical conversion treatment, the anode body was immersed in a 0.5 mol / L boric acid (H3BO3) aqueous solution at a liquid temperature of 85°C, and a 1 mA / cm² solution was applied. 2 A constant current was passed while a voltage was applied. The voltage was applied until the chemical treatment voltage reached 490V, which is lower than the 700V of the first chemical treatment.

[0096] (Diameter of minute voids) Here, the hole diameter of the minute voids present in the void layer 22 of the anode was measured. For the measurement, a cross-sectional SEM image was obtained by observing the cross-section of the anode at 50k magnification using a scanning transmission electron microscope. This cross-sectional SEM image was binarized, and the hole diameter of all minute voids within an arbitrarily selected rectangular area of ​​0.35 μm (thickness of void layer 22) × 2.5 μm was measured as the equivalent diameter of a circle using image processing. The maximum value among the measurement results was then adopted as the hole diameter of the minute void. As a result, the maximum value of the minute voids present in the void layer 22 was found to be 20 nm.

[0097] (Conductive polymer liquid) Conductive polymer solutions were prepared for use in Example 1, Comparative Example 1, and Comparative Example 2. First, the dopant PSS was added to pure water. In Example 1, 36.073 g of the dopant PSS was added to 917.344 mL of pure water. In Comparative Example 1, 72.147 g of the dopant PSS was added to 881.270 mL of pure water. In Comparative Example 2, 108.220 g of the dopant PSS was added to 845.197 mL of pure water.

[0098] To an aqueous solution of PSS, 18.775 g of ferrous sulfate, whose chemical formula is FeSO4, was added as an oxidizing agent, and 122.061 g of sulfuric acid, whose chemical formula is H2SO4, was added to adjust the pH. The mixture was stirred with a stirrer, and dissolved oxygen was removed by nitrogen bubbling. Subsequently, the mixture was stirred with a homogenizer for 2 hours. To this mixture, 5.747 g of the monomer Et-EDOT was added, and homogenization and stirring with a stirrer were performed for 30 minutes.

[0099] In this case, Example 1 is prepared so that the amount of PSS and Et-EDOT mixed is equal to the amount of Et-EDOT in the initial molar ratio. Comparative Example 1 is prepared so that the amount of PSS and Et-EDOT mixed is twice the amount of Et-EDOT in the initial molar ratio. Comparative Example 2 is prepared so that the amount of PSS and Et-EDOT mixed is three times the amount of Et-EDOT in the initial molar ratio.

[0100] Next, an aqueous solution of ammonium peroxodisulfate, an oxidizing agent with the molecular formula (NH4)S2O8, prepared by dissolving 15.408 g in 84.592 mL of pure water, was added to the mixture to initiate polymerization. The polymerization time was set to 24 hours, during which homogenization and stirring with a stirrer were continued.

[0101] After 24 hours, ion exchange resin was added to the mixture and stirred for another 24 hours. The ion exchange resin was then removed from the mixture and subjected to ultrafiltration to remove any remaining monomers, dopants, and oxidizing agents. After ion exchange and ultrafiltration, the particle size of the conductive polymer was refined using a high-pressure homogenizer. This resulted in the production of the Et-PEDOT:PSS conductive polymer.

[0102] The conductive polymer Et-PEDOT:PSS was dispersed in a conductive polymer solution with water as the dispersion medium. To the conductive polymer solution, 10 vol% ethylene glycol was added relative to the total volume of the conductive polymer solution, and 85 wt% sorbitol was added relative to the weight of the conductive polymer. This prepared a conductive polymer solution containing 1 wt% conductive polymer relative to the total volume of the conductive polymer solution.

[0103] (Median diameter of conductive polymer) The conductive polymer solution before the addition of ethylene glycol and sorbitol was diluted 50 times with water, and the particle size distribution of the conductive polymer of Et-PEDOT:PSS was measured using dynamic light scattering (DLS). As a result, the conductive polymer of Et-PEDOT:PSS in Example 1 had a particle size of 2593 nm at the median diameter. Since the maximum hole diameter of the microvoids in void layer 22 was 20 nm, the particles of the conductive polymer in Example 1 were larger in diameter than the hole diameter of the microvoids. The conductive polymer of Et-PEDOT:PSS in Comparative Example 1 had a particle size of 8.2 nm at the median diameter, and the conductive polymer of Et-PEDOT:PSS in Comparative Example 2 had a particle size of 2.6 nm at the median diameter. Therefore, the particles of the conductive polymers in Comparative Example 1 and Comparative Example 2 were smaller in diameter than the hole diameter of the microvoids.

[0104] (Solid electrolytic capacitor) After the anode body processing and conductive polymer solution preparation processes, the process moved on to the solid electrolytic capacitor element formation and solid electrolyte layer formation processes. In the element formation and solid electrolyte layer formation processes, both sides of the anode body were masked with imide tape, leaving four circular exposed areas with a diameter of 10 mm on one side of the foil. Conductive polymer solution was dropped onto the four exposed areas and dried. 5 μL of conductive polymer solution was dropped onto each exposed area. The conductive polymer solution was dried by standing it for 10 minutes at a temperature of 60°C, and then standing it for 30 minutes at a temperature of 110°C.

[0105] After forming a solid electrolyte layer on the anode body by drying a conductive polymer liquid, the device formation process continued. Carbon paste was applied to four exposed areas and cured by leaving it at 110°C for 30 minutes. Furthermore, silver paste was applied to the four exposed areas on top of the carbon layer, and copper foil was bonded to serve as the cathode lead terminal. The silver paste was cured by leaving it at 110°C for 30 minutes after the copper foil was bonded before curing. The carbon layer, silver layer, and copper foil layer correspond to the cathode body of a solid electrolytic capacitor.

[0106] (Measurement of withstand voltage) The withstand voltage of the solid electrolytic capacitors in Example 1 and Comparative Examples 1 and 2 was measured. The method for measuring the withstand voltage was as follows: In measuring the withstand voltage, a voltage was applied to the solid electrolytic capacitor. The starting voltage was 0V, and the applied voltage was increased by 1V every second. While measuring the leakage current flowing through the solid electrolytic capacitor, the voltage at which the leakage current first reached 10mA on the voltage rise curve was defined as the withstand voltage. Four solid electrolytic capacitors were manufactured, and the average withstand voltage of the four was calculated.

[0107] The withstand voltage results are shown in Figures 4 and 5 and Table 1 below. Figure 4 is a graph showing the relationship between applied voltage and leakage current for Example 1, Comparative Example 1, and Comparative Example 2. Table 1 shows the correspondence between each aspect of Example 1, Comparative Example 1, and Comparative Example 2 and the withstand voltage. Figure 5 is a graph that combines the withstand voltages of Example 1, Comparative Example 1, and Comparative Example 2 into a single graph.

[0108] (Table 1) TIFF2026144441000002.tif41161

[0109] As shown in Figure 4, the solid electrolytic capacitor of Example 1 exhibited a spike exceeding 10mA when the applied voltage was 579V. That is, the withstand voltage of the solid electrolytic capacitor of Example 1 was 579V. Similarly, the withstand voltage of the solid electrolytic capacitor of Comparative Example 1 was 489V. The withstand voltage of the solid electrolytic capacitor of Comparative Example 2 was 506V.

[0110] Here, the solid electrolytic capacitors of Comparative Example 1 and Comparative Example 2 have a dielectric strength that is close to the applied voltage of the second chemical conversion treatment. This is because the defects in the dielectric oxide film 2 are repaired by the liquid component that has penetrated into the void layer 22. Furthermore, the poly(2-ethyl-3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid prevents charge from accumulating on the surface of the void repair layer 21, making the void repair layer 21 less susceptible to dielectric breakdown. In addition, poly(2-ethyl-3,4-ethylenedioxythiophene) has a high dielectric strength.

[0111] Therefore, the solid electrolytic capacitors of Comparative Example 1 and Comparative Example 2 also achieve high withstand voltage without applying a conversion voltage exceeding, for example, 1000V, due to the presence of the void layer 22 and the void repair layer 22.

[0112] Furthermore, the solid electrolytic capacitor of Example 1 achieved a withstand voltage significantly exceeding the applied voltage of the second chemical treatment. As shown in Table 1 and Figure 5, the withstand voltage of Example 1 is even higher than that of Comparative Examples 1 and 2, being 18% higher than that of Comparative Example 1 and 14% higher than that of Comparative Example 2.

[0113] In this Example 1 solid electrolytic capacitor, the particle size of the conductive polymer is larger than the diameter of the microvoid holes in the void layer 22. Therefore, the conductive polymer cannot penetrate the void layer 22, and the separation distance between the conductive polymer and the void repair layer 22 is greater than in Comparative Examples 1 and 2.

[0114] Therefore, the probability of the conductive polymer charge being distributed on the surface of the void repair layer 21 is reduced, making it difficult for charge to accumulate on the surface of the void repair layer 21, and also making it difficult for the resistance of the weak parts of the void repair layer 21 to decrease. As a result, the solid electrolytic capacitor of Example 1 has an even higher voltage rating. [Explanation of symbols]

[0115] 1 Valve-acting metal base material 2 Dielectric oxide film 21 Void Repair Layer 22 Void Layer 23 Unrestored layer 3. Hydrated oxide film

Claims

1. The device comprises an anode, a cathode, and a solid electrolyte layer interposed between the anode and the cathode. The anode body has a valve-acting metal substrate and a dielectric oxide film on the valve-acting metal substrate. The dielectric oxide film is A void layer located on the surface side of the dielectric oxide film, in which multiple microvoids are formed, The void repair layer on the boundary side of the dielectric oxide film with the valve-acting metal substrate, It has, The solid electrolyte layer contains particles of a conductive polymer of poly(2-ethyl-3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid. The conductive polymer particles have a diameter larger than the hole diameter of the microvoids in the void layer. A solid electrolytic capacitor characterized by the following features.

2. A method for manufacturing a solid electrolytic capacitor comprising an anode, a cathode, and a solid electrolyte layer, with a valve-acting metal as the base material, A process for processing the anode body, A conductive polymer solution preparation step for preparing a dispersion of a conductive polymer of poly(2-ethyl-3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid, A device forming step of forming a capacitor element by facing the anode and the cathode, A solid electrolyte layer formation step involves applying the dispersion to the surface of the anode or impregnating the capacitor element with the dispersion, Includes, The aforementioned anode processing step is: A first chemical treatment involves applying a predetermined chemical treatment voltage to a valve-acting metal substrate, After the first chemical treatment, a void introduction treatment is performed in which the anode is immersed in one or more of an acidic solution, an alkaline solution, and pure water to form a void layer containing multiple microvoids. A second chemical treatment is performed after the void introduction treatment, in which a lower chemical treatment voltage than that of the first chemical treatment is applied. Includes, In the conductive polymer liquid preparation step, the conductive polymer having a diameter larger than the hole diameter of the microvoid is dispersed in the dispersion. A method for manufacturing solid electrolytic capacitors characterized by the following.

3. The process includes an aging step in which a voltage is applied to the capacitor element, following the element formation step and the solid electrolyte layer formation step. The second chemical conversion step is the aging step. A method for manufacturing a solid electrolytic capacitor according to claim 2, characterized by the above.

4. The conductive polymer liquid preparation step includes a polymerization step that produces the conductive polymer having a diameter larger than the hole diameter of the microvoid. A method for manufacturing a solid electrolytic capacitor according to claim 2, characterized by the above.

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