JP7620827B2 - Electrolytic capacitor and its manufacturing method - Google Patents

Description

The present invention relates to an electrolytic capacitor and a method for manufacturing the same.

Capacitors used in electronic devices are required to have a large capacity and a small equivalent series resistance (ESR) value in the high frequency range. Electrolytic capacitors that use conductive polymers such as polythiophene are promising capacitors with a large capacity and low ESR. The capacitor element of an electrolytic capacitor is obtained, for example, by winding an anode foil and a cathode foil with a separator between them to form an electrode group, and impregnating the electrode group with a dispersion of a conductive polymer.

Patent Document 1 discloses a separator for use in solid electrolytic capacitors, which is constructed by dissolving cellulose in a solvent, applying a DC voltage to a nozzle that sprays the solution, and depositing long fibers with a fiber diameter of 1 μm or less. Patent Document 1 also discloses a separator constructed by dissolving a synthetic resin such as polyethylene terephthalate (PET) or aramid in an organic solvent or water, and applying a DC voltage to a nozzle that sprays the solution, and depositing long non-fibrillated fibers with a fiber diameter of 1 μm or less.

Patent Document 2 discloses a separator for use in an electrochemical element, which contains 20-80% by mass of natural cellulose fiber A, 10-50% by mass of natural cellulose fiber B, and 10-50% by mass of beaten regenerated cellulose fiber. Natural cellulose fiber A has a length-weighted average fiber length of 0.30-1.19 mm and a CSF of 500-50 ml. Natural cellulose fiber B has a length-weighted average fiber length and a maximum distribution fiber length of 1.20-1.99 mm and a CSF of 500-50 ml.

JP 2010-245150 A International Publication No. 2017/047699

Currently, there is a demand for electrolytic capacitors with higher characteristics (e.g., low ESR, high voltage resistance and reliability, etc.).

One aspect of the present invention relates to an electrolytic capacitor comprising an anode foil having a dielectric layer on its surface, a cathode foil, a separator interposed between the anode foil and the cathode foil, a hydroxyl group-containing compound, and a capacitor element including a conductive polymer, the separator including synthetic fibers and cellulosic fibers, the conductive polymer and the hydroxyl group-containing compound adhering to the surface and interior of the separator, the hydroxyl group-containing compound being at least one compound (excluding polymers) selected from the group consisting of sugars and polyhydric alcohols, and the hydroxyl group-containing compound being unevenly distributed in the separator.

Another aspect of the present invention relates to a method for producing an electrolytic capacitor, comprising a first step of obtaining an intermediate comprising an anode foil having a dielectric layer on its surface, a cathode foil, a separator interposed between the anode foil and the cathode foil, and a hydroxyl-containing compound attached to the surface and inside of the separator and unevenly distributed in the separator, and a second step of impregnating the intermediate with a treatment liquid containing a conductive polymer to obtain the capacitor element, the separator comprising synthetic fibers and cellulosic fibers, and the hydroxyl-containing compound being at least one compound (excluding polymers) selected from the group consisting of sugars and polyhydric alcohols.

The present invention provides an electrolytic capacitor with excellent characteristics.

1 is a cross-sectional view illustrating an example of an electrolytic capacitor according to an embodiment of the present invention. FIG. 2 is a perspective view showing a part of the wound body of FIG. 1 in a developed form.

[Electrolytic capacitor]
The electrolytic capacitor according to the embodiment of the present invention includes an anode foil having a dielectric layer on its surface, a cathode foil, a separator interposed between the anode foil and the cathode foil, a hydroxyl-containing compound, and a capacitor element including a conductive polymer. The separator includes synthetic fibers and cellulosic fibers. The conductive polymer and the hydroxyl-containing compound are attached to the surface and interior of the separator. Hereinafter, the anode foil, the cathode foil, and the separator are collectively referred to as an "electrode group."

The hydroxyl group-containing compound is at least one compound selected from the group consisting of sugars and polyhydric alcohols. However, the hydroxyl group-containing compound does not include polymers. For example, it does not include polymers that may be included in the category of polyhydric alcohols. Examples of such polymers include polyvinyl alcohol (PVA) and polyalkylene oxides having alcoholic hydroxyl groups at their terminals. The term "polymer" as used here is not particularly limited, but means, for example, a molecule with a weight average molecular weight of 1000 or more.

Hydroxyl-containing compounds are unevenly distributed in the separator. "Hydroxyl-containing compounds are unevenly distributed in the separator" means that the hydroxyl-containing compounds are distributed in higher concentration inside the separator than in the surface layer of the separator. The surface layer of the separator refers to a very thin region including the outer surface of the separator, and the maximum depth of the surface layer from the outer surface of the separator is, for example, 25% or less of the thickness of the separator. The concentration of hydroxyl-containing compounds in the surface layer of the separator is easily affected by the concentration of hydroxyl-containing compounds outside the separator, and can also be said to represent the concentration of hydroxyl-containing compounds outside the separator.

The separator is a porous sheet made of fibers. The hydroxyl-containing compound and conductive polymer attached to the inside of the separator means the hydroxyl-containing compound and conductive polymer attached to the wall surface inside the pores of the separator. The hydroxyl-containing compound and conductive polymer attached to the surface of the separator that faces the anode foil or cathode foil means the hydroxyl-containing compound and conductive polymer attached to the surface of the separator that faces the anode foil or cathode foil. When the capacitor (electrode group) is disassembled and the cathode foil is removed, the hydroxyl-containing compound and conductive polymer attached to the cathode foil can be considered to be the hydroxyl-containing compound and conductive polymer on the surface of the separator.

The hydroxyl groups of the hydroxyl-containing compound improve the adhesion of the conductive polymer. By distributing the hydroxyl-containing compound unevenly on the separator, the separator can stably hold a large amount of conductive polymer. In other words, the adhesiveness of the conductive polymer to the separator can be improved, and the conductive polymer can efficiently perform its role as an electrolyte. As a result, an electrolytic capacitor with good characteristics (especially low ESR) is obtained.

When a separator containing synthetic fibers and cellulosic fibers is used, an electrolytic capacitor with good characteristics (particularly low ESR) can be obtained. Synthetic fibers are advantageous in terms of improving heat resistance and voltage resistance. Cellulosic fibers are advantageous in terms of improving the separator's ability to absorb treatment liquid containing hydroxyl-containing compounds. In other words, they are advantageous in terms of unevenly distributing hydroxyl-containing compounds in the separator (low ESR).

If the separator does not contain synthetic fibers and is made of cellulosic fibers, the separator has low strength and heat resistance, is prone to short circuits when exposed to high temperatures, and has low voltage resistance. On the other hand, if the separator does not contain cellulosic fibers and is made of synthetic fibers, it may be difficult for the hydroxyl-containing compound to penetrate into the pores of the separator when the separator is impregnated with a treatment liquid containing the hydroxyl-containing compound. In other words, it may be difficult to distribute the hydroxyl-containing compound unevenly on the separator, which may reduce the adhesion of the conductive polymer in the separator and increase the ESR.

In the capacitor element, the mass ratio M _I of the hydroxyl-containing compound to the conductive polymer inside the separator is preferably larger than the mass ratio M _O of the hydroxyl-containing compound to the conductive polymer in the surface layer of the separator. The ratio of the mass ratio M _O to the mass ratio M _I : M _O /M _I may be 0.5 or more and 0.95 or less, or 0.7 or more and 0.9 or less.

The distribution of the hydroxyl group-containing compound in the capacitor element can be determined by the following method.
(I) Disassemble the electrolytic capacitor, cut the separator to a predetermined size, and obtain a separator sample. Dry the separator sample and measure the mass of the dried separator sample. Extract the hydroxyl group-containing compound from the separator sample with ion-exchanged water.

(II) Separately, prepare a separator sample of the same size, and peel off the surface layer (or any attached matter on the surface) of the separator sample. The peeling method can be a method of thinly slicing the surface layer and removing it, a method of scraping it off, a method of applying adhesive tape to the surface layer and then peeling it off, or the like. For the separator sample after the surface layer has been removed, measure the mass of the separator sample and extract the hydroxyl group-containing compounds attached to the inside of the separator sample with ion-exchanged water in the same manner as above.

(III) The above mass ratios M _I and M _O are calculated from the concentrations of the hydroxyl-containing compounds contained in each extract measured by FT-IR analysis and the mass of each separator sample, and M _O /M _I is calculated. When _{M O} /M _I < 1, it can be considered that the concentration of the hydroxyl-containing compounds is higher in the interior of the separator than in the surface layer.

The separator preferably contains 20 parts by mass or more and 60 parts by mass or less of cellulosic fibers (40 parts by mass or more and 80 parts by mass or less of synthetic fibers) per 100 parts by mass of the total of cellulosic fibers and synthetic fibers, and more preferably contains 20 parts by mass or more and 50 parts by mass or less of cellulosic fibers (50 parts by mass or more and 80 parts by mass or less of synthetic fibers).

(Separator)
The separator can be produced by a papermaking method, and a mixed sheet containing synthetic fibers and cellulosic fibers can be used for the separator. The separator may be a woven fabric or a nonwoven fabric. The thickness of the separator is, for example, 20 μm or more and 200 μm or less. The separator may be formed by overlapping a plurality of fiber sheets. The plurality of fiber sheets may be of the same type or different types.

Cellulosic fibers include natural cellulose fibers. Examples of natural cellulose fibers include Manila hemp pulp, esparto pulp, softwood pulp, hardwood pulp, and sisal pulp. Cellulosic fibers may also be regenerated cellulose fibers, semi-synthetic cellulose fibers, and the like. Regenerated cellulose fibers include fibers spun using a solution in which natural cellulose is dissolved in a solvent (e.g., rayon). Semi-synthetic cellulose fibers include fibers made by adding other materials to natural cellulose to form fibers (e.g., acetate). Cellulosic fibers may be used alone or in combination of two or more types.

From the viewpoint of short circuit resistance, the cellulosic fibers may be fibrillated.

The synthetic fiber (fiber material) preferably contains at least one selected from the group consisting of aramid (aromatic polyamide), polyethylene terephthalate (PET), and vinylon (acetalized PVA). Among these, aramid is more preferable from the viewpoints of improving heat resistance and fiber strength (tensile strength). The synthetic fiber may be used alone or in combination of two or more types.

From the viewpoint of improving short-circuit resistance and absorbency of the treatment solution containing the conductive polymer (reducing ESR), it is preferable that the synthetic fibers are fibrillated. Fibrillation is performed by beating or the like.

In addition to the above examples, the synthetic fibers (fiber materials) may include polybutylene terephthalate, polyphenylene sulfide, nylon, polyimide, polyamideimide, polyetherimide, etc. The separator may include glass fiber.

The separator may be carbonized. The separator may be partially carbonized. The carbonization can appropriately reduce the density of the separator and increase the absorbency of the treatment liquid containing the conductive polymer. In the carbonization, mainly the cellulose-based fibers may be carbonized.

The separator may contain heat-sealable binder fibers. The separator may be heated to heat-seal the binder fibers, thereby bonding the fibers together.

(Hydroxy group-containing compound)
The hydroxyl group-containing compound is at least one compound selected from the group consisting of sugars and polyhydric alcohols, and does not contain a polymer. Since the hydroxyl group-containing compound does not contain a polymer, the pores of the separator are The treatment liquid containing the hydroxyl group-containing compound does not contain any polymer, so it has a low viscosity and is easy to impregnate into the separator.

Examples of sugars include glucose. Examples of polyhydric alcohols include mannitol, sorbitol, xylitol, pentaerythritol, and trimethylolpropane. Note that mannitol, sorbitol, xylitol, pentaerythritol, and the like are sometimes called sugar alcohols. The hydroxyl group-containing compounds may be used alone or in combination of two or more.

The hydroxyl-containing compound may be an organic compound containing multiple hydroxyl groups bonded to carbon atoms. The number of hydroxyl groups contained in the hydroxyl-containing compound may be in the range of 2 to 12 (e.g., in the range of 3 to 6). Typically, the hydroxyl-containing compound is a water-soluble compound. Note that PVA typically has 13 or more hydroxyl groups in its molecule.

The melting point of the hydroxyl-containing compound is preferably higher than the temperature at which the capacitor is used. The melting point of the hydroxyl-containing compound may be 50°C or higher, and may be in the range of 80°C to 300°C (e.g., 120°C to 300°C).

The hydroxyl group-containing compound may be at least one selected from the group consisting of glucose, mannitol, sorbitol, xylitol, pentaerythritol, and trimethylolpropane. The melting point of glucose is about 146 to 150°C, that of mannitol is about 165 to 169°C, that of sorbitol is about 93 to 95°C, that of xylitol is about 92 to 97°C, that of pentaerythritol is about 257 to 260°C, and that of trimethylolpropane is about 56 to 58°C. Note that the melting points of these substances may vary depending on the structure (stereoisomer). Glucose, mannitol, and pentaerythritol are preferred because they have high melting points.

(Conductive polymer)
Examples of conductive polymers include polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, and derivatives thereof. The derivatives include polymers having polypyrrole, polythiophene, polyfuran, polyaniline, and polyacetylene as the basic skeleton. For example, derivatives of polythiophene include poly(3,4-ethylenedioxythiophene). These conductive polymers may be used alone or in combination. The conductive polymer may also be a copolymer of two or more monomers. The weight-average molecular weight of the conductive polymer is not particularly limited and may be in the range of 1,000 to 100,000, for example. A preferred example of the conductive polymer is poly(3,4-ethylenedioxythiophene) (PEDOT).

A dopant may be added to the conductive polymer. From the viewpoint of suppressing dedoping from the conductive polymer, it is preferable to use a polymer dopant. Examples of polymer dopants include polyvinyl sulfonic acid, polystyrene sulfonic acid, polyallyl sulfonic acid, polyacryl sulfonic acid, polymethacryl sulfonic acid, poly(2-acrylamido-2-methylpropane sulfonic acid), polyisoprene sulfonic acid, polyacrylic acid, etc. These may be used alone or in combination of two or more. These may be added in the form of a salt. The polymer dopant may exist in the electrolyte in the form of an anion in which a cation (e.g., a proton) is dissociated from at least a part of the acidic group. A preferred example of the dopant is polystyrene sulfonic acid (PSS).

The weight-average molecular weight of the dopant is not particularly limited. From the viewpoint of facilitating the formation of a homogeneous electrolyte layer, the weight-average molecular weight of the dopant may be in the range of 1,000 to 100,000.

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

(Liquid component)
The electrolytic capacitor may contain a non-aqueous solvent or an electrolytic solution as a liquid component. The liquid component may be a substance that is liquid at room temperature (25°C) or a substance that is liquid at the temperature when the electrolytic capacitor is used. A preferred example of the liquid component is a liquid in which the hydroxyl group-containing compound is substantially insoluble. The liquid component has a role of protecting the conductive polymer. In addition, the inclusion of the liquid component can increase the contact between the conductive polymer and the dielectric layer, and can also increase the repairability of defects in the dielectric layer. The electrolytic solution can function as an electrolyte together with the conductive polymer.

The non-aqueous solvent may be an organic solvent or an ionic liquid. Examples of non-aqueous solvents include polyhydric alcohols such as ethylene glycol and propylene glycol, cyclic sulfones such as sulfolane (SL), lactones such as γ-butyrolactone (GBL), amides such as N-methylacetamide, N,N-dimethylformamide, and N-methyl-2-pyrrolidone, esters such as methyl acetate, carbonate compounds such as propylene carbonate, ethers such as 1,4-dioxane, ketones such as methyl ethyl ketone, and formaldehyde.

In addition, a polymer solvent may be used as the non-aqueous solvent. Examples of polymer solvents include polyalkylene glycol, derivatives of polyalkylene glycol, and compounds in which at least one hydroxyl group in a polyhydric alcohol is substituted with polyalkylene glycol (including derivatives). Specifically, examples of polymer solvents include polyethylene glycol (PEG), polyethylene glycol glyceryl ether, polyethylene glycol diglyceryl ether, polyethylene glycol sorbitol ether, polypropylene glycol, polypropylene glycol glyceryl ether, polypropylene glycol diglyceryl ether, polypropylene glycol sorbitol ether, and polybutylene glycol. Examples of polymer solvents further include ethylene glycol-propylene glycol copolymers, ethylene glycol-butylene glycol copolymers, and propylene glycol-butylene glycol copolymers. The non-aqueous solvent may be used alone or in a mixture of two or more.

The liquid component may contain an acid component and a base component. Examples of the acid component include maleic acid, phthalic acid, benzoic acid, pyromellitic acid, and resorcylic acid. Examples of the base component include 1,8-diazabicyclo[5,4,0]undecene-7, 1,5-diazabicyclo[4,3,0]nonene-5, 1,2-dimethylimidazolinium, 1,2,4-trimethylimidazoline, 1-methyl-2-ethyl-imidazoline, 1,4-dimethyl-2-ethylimidazoline, 1-methyl-2-heptyl imidazoline, 1-methyl-2-(3'heptyl)imidazoline, 1-methyl-2-dodecyl imidazoline, 1,2-dimethyl-1,4,5,6-tetrahydropyrimidine, 1-methylimidazole, and 1-methylbenzimidazole.

The electrolyte contains a non-aqueous solvent and a solute (e.g., an organic salt) dissolved therein. Examples of the non-aqueous solvent that constitutes the electrolyte include the examples of the non-aqueous solvents described above. Examples of the solute include inorganic salts and organic salts. An organic salt is a salt in which at least one of the anion and the cation contains an organic substance. Examples of the organic salt include trimethylamine maleate, triethylamine borodisalicylate, ethyldimethylamine phthalate, mono 1,2,3,4-tetramethylimidazolinium phthalate, mono 1,3-dimethyl-2-ethylimidazolinium phthalate, etc.

To prevent dopant dedoping, the pH of the liquid component may be less than 7, or may be 5 or less.

(anode foil)
The anode foil may be a metal foil having a dielectric layer formed on its surface. The type of metal constituting the metal foil is not particularly limited. In terms of ease of forming the dielectric layer, examples of metals constituting the metal foil include valve metals such as aluminum, tantalum, niobium, and titanium, and alloys of valve metals. A preferred example is aluminum and an aluminum alloy. Usually, the surface of the anode foil is roughened, and the dielectric layer is formed on the roughened surface. The dielectric layer of the anode foil is in contact with a conductive polymer (electrolyte).

Anode foil is obtained, for example, by roughening the surface of a metal foil by etching or other treatment, and then forming a dielectric layer (oxide film) on the roughened surface of the metal foil by chemical conversion treatment or other treatment.

(cathode foil)
A metal foil can be used for the cathode foil. The type of metal constituting the metal foil is not particularly limited. Examples of metals constituting the metal foil include valve metals such as aluminum, tantalum, niobium, and titanium, and alloys of valve metals. A preferred example is aluminum and an aluminum alloy. In addition, a chemical conversion film may be provided on the surface of the cathode foil, and a coating of a metal (heterogeneous metal) or a nonmetal different from the metal constituting the cathode foil may be provided. Examples of heterogeneous metals and nonmetals include metals such as titanium and nonmetals such as carbon.

The electrolytic capacitor may be a wound electrolytic capacitor having a wound body formed by winding an anode foil and a cathode foil with a separator interposed between the anode foil and the cathode foil. The electrolytic capacitor may also be a stacked electrolytic capacitor having a stack formed by folding an anode foil and a cathode foil in a zigzag shape with a separator interposed between the anode foil and the cathode foil.

Here, FIG. 1 is a cross-sectional view that shows a schematic example of an electrolytic capacitor according to an embodiment of the present invention. FIG. 2 is a perspective view that shows a schematic configuration of the wound body. FIG. 2 is a view in which a part of the wound body in FIG. 1 is developed.

The electrolytic capacitor 200 includes a wound body 100. The wound body 100 is formed by winding an anode foil 10 and a cathode foil 20 with a separator 30 interposed therebetween.

The wound body 100 contains a conductive polymer (not shown) and a hydroxyl-containing compound (not shown). That is, the conductive polymer and the hydroxyl-containing compound are interposed between the anode foil 10 (dielectric layer) and the cathode foil 20, and are attached to the surface and inside of the separator 30. The hydroxyl-containing compound is unevenly distributed in the separator 30.

One end of the lead tabs 50A and 50B is connected to the anode foil 10 and the cathode foil 20, respectively, and the wound body 100 is formed by winding the lead tabs 50A and 50B. The other ends of the lead tabs 50A and 50B are connected to lead wires 60A and 60B, respectively.

A stop tape 40 is placed on the outer surface of the cathode foil 20 located in the outermost layer of the wound body 100, and the end of the cathode foil 20 is fixed by the stop tape 40. When preparing the anode foil 10 by cutting it from a large foil, the wound body 100 may be further subjected to a chemical conversion treatment in order to provide a dielectric layer on the cut surface.

The wound body 100 is housed in the bottomed case 211 so that the lead wires 60A and 60B are located on the open side of the bottomed case 211. The material of the bottomed case 211 can be a metal such as aluminum, stainless steel, copper, iron, brass, or an alloy of these metals.

The wound body 100 is sealed in the bottomed case 211 by placing a sealing member 212 at the opening of the bottomed case 211 in which the wound body 100 is stored, crimping the open end of the bottomed case 211 to the sealing member 212 and curling it, and placing a seat plate 213 on the curled portion.

The sealing member 212 is formed so that the lead wires 60A and 60B can pass through it. The sealing member 212 may be made of any insulating material, and is preferably made of an elastic material. Among them, silicone rubber, fluororubber, ethylene propylene rubber, hypalon rubber, butyl rubber, isoprene rubber, etc., which have high heat resistance, are preferred.

[Method of manufacturing electrolytic capacitor]
The method for producing an electrolytic capacitor according to an embodiment of the present invention includes a first step of obtaining a capacitor element intermediate (hereinafter also simply referred to as intermediate), and a second step of impregnating the intermediate with a treatment liquid containing a conductive polymer to obtain a capacitor element. The intermediate includes an anode foil having a dielectric layer on its surface, a cathode foil, a separator interposed between the anode foil and the cathode foil, and a hydroxyl-containing compound attached to the surface and inside of the separator and unevenly distributed in the separator. It can also be said that the intermediate includes an electrode group and a hydroxyl-containing compound. The separator includes synthetic fibers and cellulosic fibers. The hydroxyl-containing compound is at least one compound (excluding polymers) selected from the group consisting of sugars and polyhydric alcohols.

By performing the first and second steps, a capacitor element can be obtained in which the separator contains a large amount of conductive polymer.

(First step)
(Step (1a))
An example of the first step may include a step (1a) of obtaining an electrode group by disposing a separator between an anode foil and a cathode foil, and impregnating the electrode group with a treatment liquid containing a hydroxyl group-containing compound. Step (1a) may be performed repeatedly.

In step (1a), for example, the electrode group is immersed in a treatment liquid containing a hydroxyl group-containing compound. The immersion time is, for example, 1 minute or more and less than 20 minutes. Step (1a) may be performed at room temperature or at a temperature other than room temperature (for example, a temperature higher than room temperature). Step (1a) may also be performed under atmospheric pressure or under reduced pressure.

The treatment liquid containing the hydroxyl group-containing compound used in step (1a) usually contains water. Hereinafter, the treatment liquid containing the hydroxyl group-containing compound is also referred to as the "aqueous treatment liquid". The amount of water contained in the liquid (solvent) constituting the aqueous treatment liquid is, for example, in the range of 50 to 100% by mass. Usually, the hydroxyl group-containing compound is dissolved in the aqueous treatment liquid, and the aqueous treatment liquid is an aqueous solution of the hydroxyl group-containing compound. The content (concentration) of the hydroxyl group-containing compound in the aqueous treatment liquid may be in the range of 3 to 50% by mass (for example, in the range of 5 to 15% by mass). The aqueous treatment liquid may contain components other than the hydroxyl group-containing compound as necessary.

The aqueous treatment liquid does not contain a polymer (a polymer with a weight-average molecular weight of 1000 or more). For example, the aqueous treatment liquid does not contain a conductive polymer. The aqueous treatment liquid that does not contain a polymer has a low viscosity, which makes it easier to impregnate the electrode group and to attach the hydroxyl group-containing compound to the inside of the separator.

(Step (2a))
An example of the first step may include a step (2a) of heating and drying the electrode group impregnated with the treatment liquid containing the hydroxyl group-containing compound after the step (1a). An intermediate containing the electrode group and the hydroxyl group-containing compound may be obtained by the step (2a). The step (2a) may be performed under atmospheric pressure or under reduced pressure. The heating temperature in the step (2a) is, for example, 100° C. or higher, 120° C. or higher, or 125° C. or higher. From the viewpoint that the hydroxyl group-containing compound is easily permeated into the inside of the separator (capacitor element), the heating and drying in the step (2a) may be performed at a temperature equal to or higher than the melting point of the hydroxyl group-containing compound.

The first step may include a step of heating and carbonizing the separator. Usually, the separator is partially carbonized. The carbonization can appropriately reduce the density of the separator and increase the absorbency of the treatment liquid containing the hydroxyl group-containing compound and the treatment liquid containing the conductive polymer. In the carbonization, mainly the cellulosic fibers can be carbonized.

The carbonization of the separator may be carried out, for example, by heating the intermediate after step (2a). Step (2a) may also serve as a step of carbonizing the separator. The heating temperature for the carbonization is preferably 130°C or higher and 280°C or lower, and more preferably 160°C or higher and 230°C or lower. The heating time for the carbonization is, for example, 80 minutes or higher and 100 minutes or lower.

In addition, if the separator contains binder fibers, the first step may include a step of heating the separator to heat-seal the binder fibers. This may bond the fibers in the separator together and increase the strength of the separator. The heat-sealing process of the binder fibers is performed, for example, before step (1a) (construction of the electrode group).

(Step (1b))
Another example of the first step may include a step (1b) of obtaining a separator having a hydroxyl-containing compound attached to the surface and inside thereof.

Step (1b) may include step (1b-1) of impregnating the separator with a treatment liquid containing a hydroxyl group-containing compound. Step (1b-1) may be repeated. In step (1b-1), for example, the treatment liquid containing a hydroxyl group-containing compound is applied or sprayed onto the separator. The treatment liquid containing a hydroxyl group-containing compound in step (1b) may be the same as the treatment liquid containing a hydroxyl group-containing compound used in step (1a).

Step (1b) may include a step (1b-2) of heating and drying the separator impregnated with the treatment liquid containing the hydroxyl-containing compound after step (1b-1). Step (1b-2) may be performed under atmospheric pressure or under an environment other than atmospheric pressure (e.g., reduced pressure). The heating temperature in step (1b-2) is, for example, 100°C or higher, 120°C or higher, or 125°C or higher. The heating and drying in step (1b-2) may be performed at a temperature equal to or higher than the melting point of the hydroxyl-containing compound. When the separator contains binder fibers, step (1b-2) may also serve as a step of heat-sealing the binder fibers.

In addition, step (1b) includes a blending step in which synthetic fibers and cellulosic fibers are blended to obtain a separator, and a hydroxyl group-containing compound may be added in the blending step.

Another example of the first step may include a step (2b) of disposing a separator between the anode foil and the cathode foil after step (1b). Step (2b) may produce an intermediate in which the hydroxyl-containing compound is unevenly distributed in the separator. Also, after step (2b), a step of carbonizing the separator may be included. The carbonization of the separator can be performed by heating the intermediate.

In the case of a wound type capacitor, the anode foil and cathode foil may be wound with a separator between them to form an electrode group (wound body). In the case of a laminated type capacitor, the anode foil and cathode foil may be folded in a zigzag pattern with a separator between them to form an electrode group (laminate).

(Second step)
In the second step, the intermediate is impregnated with a treatment liquid containing a conductive polymer to obtain a capacitor element. In the second step, for example, the intermediate may be immersed in the treatment liquid containing a conductive polymer. The immersion time is, for example, 30 seconds or more and 30 minutes or less. The second step may be performed at room temperature or at a temperature other than room temperature (for example, a temperature higher than room temperature). In addition, the second step may be performed under atmospheric pressure or under reduced pressure.

The treatment liquid containing the conductive polymer usually contains water. Hereinafter, the treatment liquid containing the conductive polymer is also referred to as an "aqueous dispersion". The amount of water contained in the aqueous liquid (dispersion medium) constituting the aqueous dispersion is, for example, in the range of 2 to 100 mass %. The aqueous liquid may be water. The conductive polymer is dispersed in the aqueous liquid. In other words, the aqueous dispersion is a suspension in which the conductive polymer is dispersed in the aqueous liquid. The content (concentration) of the conductive polymer in the aqueous dispersion may be in the range of 0.1 to 20 mass % (for example, in the range of 0.5 to 3 mass %).

The viscosity of the aqueous dispersion may be in the range of 1 mPa·s to 100 mPa·s, or in the range of 1 mPa·s to 40 mPa·s (e.g., in the range of 1 mPa·s to 25 mPa·s). The lower the viscosity of the aqueous dispersion, the easier it is to impregnate the intermediate.

The aqueous dispersion does not need to contain a hydroxyl group-containing compound. By not adding a hydroxyl group-containing compound, the viscosity of the aqueous dispersion can be reduced, the aqueous dispersion can be easily impregnated, and the conductive polymer can be easily impregnated into the intermediate (inside the separator). When the electrode group is impregnated with a mixed treatment liquid in which a hydroxyl group-containing compound is added to the aqueous dispersion, it is difficult to attach the conductive polymer to the inside of the separator, and the ESR increases.

A dopant may be added to the conductive polymer. The aqueous dispersion may contain components other than the conductive polymer and the dopant, as necessary.

The second step may further include a step of drying the aqueous dispersion impregnated in the intermediate. The aqueous dispersion may be dried under atmospheric pressure or under reduced pressure. The aqueous dispersion may be dried by heating the aqueous dispersion impregnated in the intermediate. The heating temperature is, for example, 100°C or higher and 120°C or lower.

(Third process)
The method for producing an electrolytic capacitor may include a third step of impregnating the capacitor element with a liquid component (a non-aqueous solvent or an electrolytic solution). The third step may be performed, for example, by injecting the liquid component into a bottomed case for an electrolytic capacitor in which the capacitor element is housed. The non-aqueous solvent or the electrolytic solution may be one of those exemplified above. The method for producing an electrolytic capacitor may include a step of sealing the bottomed case in which the capacitor element is housed.

[Example]
The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

Examples 1 to 4
A wound-type electrolytic capacitor (diameter 10 mm×length 10 mm) having a rated voltage of 35 V and a rated capacitance of 270 μF was fabricated by the following procedure.

(Preparation of cathode foil)
The cathode foil used was an aluminum foil having a thickness of 50 μm.

(Preparation of anode foil)
An Al foil having a thickness of 120 μm was prepared. This Al foil was subjected to a direct current etching process to roughen the surface. Next, the Al foil was subjected to a chemical conversion process to form a dielectric layer (thickness: about 70 nm) to obtain an anode foil. The dielectric layer was formed by immersing the Al foil in an ammonium adipate solution and performing a chemical conversion process at 70° C. for 5 hours while applying a voltage of 50 V to the Al foil. Then, the anode foil was cut to a predetermined size to prepare an anode foil.

(Preparation of separator)
A nonwoven fabric (separator) was obtained by mixing aramid fibers, natural cellulose fibers, and PET fibers in the mass ratio shown in Table 1. In Table 1, a1 to a4 respectively represent the separators produced in Examples 1 to 4. The thickness of separators a1 to a4 was 40 μm.

(Preparation of wound body)
An anode lead tab and a cathode lead tab connected to a lead wire were connected to the prepared anode foil and cathode foil, respectively. Then, the anode foil and the cathode foil were wound with a separator sandwiched therebetween, and the outer surface was fixed with a winding stop tape. In this way, a wound body was produced. Next, the wound body was heated at 200°C for 90 minutes in the atmosphere to carbonize the separator. Next, the wound body was immersed in an ammonium adipate solution, and a voltage of 50V was applied to the anode foil, and a chemical conversion treatment was performed again at 70°C for 60 minutes to form a dielectric layer mainly on the end surface of the anode foil.

(Preparation of treatment solution containing hydroxyl group-containing compound)
Mannitol was dissolved in ion-exchanged water to prepare an aqueous mannitol solution (concentration: 10% by mass) as a treatment liquid containing a hydroxyl group-containing compound. The viscosity of the resulting aqueous mannitol solution was 5 mPa·s or less.

(Preparation of treatment solution containing conductive polymer)
3,4-ethylenedioxythiophene and polystyrenesulfonic acid as a dopant were dissolved in ion-exchanged water to prepare a mixed solution. While stirring the obtained mixed solution, iron sulfate (III) dissolved in ion-exchanged water (oxidizing agent) was added to carry out a polymerization reaction. After the reaction, the obtained reaction solution was dialyzed to remove unreacted monomers and excess oxidizing agent, and a dispersion (aqueous dispersion) containing poly(3,4-ethylenedioxythiophene) doped with about 5% by mass of polystyrenesulfonic acid (PSS) was obtained. Hereinafter, poly(3,4-ethylenedioxythiophene) doped with about 5% by mass of polystyrenesulfonic acid (PSS) may be referred to as "PEDOT:PSS". In addition, the dispersion in which PEDOT:PSS is dispersed may be referred to as "PEDOT:PSS dispersion". Using this PEDOT:PSS dispersion, an aqueous dispersion with a PEDOT:PSS concentration of 2% by mass was prepared as a treatment liquid containing a conductive polymer. The viscosity of the resulting aqueous dispersion was 25 mPa·s.

(Preparation of Capacitor Element Intermediate)
The wound body was immersed in the aqueous mannitol solution in a container at room temperature under atmospheric pressure for 5 minutes, and then dried at 200°C for 20 minutes in a drying oven at 1 atm (first step). In this way, mannitol was attached to the wound body (surface and interior of the separator) to obtain an intermediate. The melting point of mannitol is about 165 to 169°C, and the drying temperature for the wound body is higher than the melting point of mannitol.

(Fabrication of Capacitor Element)
The intermediate was immersed in the PEDOT:PSS dispersion in a container at room temperature in a reduced pressure atmosphere (40 kPa) for 15 minutes, and then dried at 150° C. for 30 minutes in a drying oven at 1 atm (second step). In this way, a conductive polymer was attached to the intermediate (surface layer and inside of the separator), and a capacitor element was obtained.

When the distribution of mannitol in the capacitor element was examined by the method described above, it was confirmed that mannitol was unevenly distributed in the separator. The M _O /M _I ratio determined by the method described above was 0.78 to 0.85.

(Impregnation of liquid component)
The capacitor element was placed in a bottomed case, and the capacitor element was impregnated with a liquid component at room temperature under atmospheric pressure. The liquid component was a mixed solvent of GBL and SL (mass ratio 50:50).

(Capacitor sealing)
A sealing member and a seat plate were placed in the opening of the bottomed case to seal the capacitor element. In this way, an electrolytic capacitor was completed. After that, an aging treatment was performed at 130°C for 2 hours while applying the rated voltage. Note that A1 to A4 in Table 1 represent the electrolytic capacitors of Examples 1 to 4, respectively.

Comparative Example 1
Mannitol was dissolved in the PEDOT:PSS dispersion to prepare a mixed treatment liquid of PEDOT:PSS and mannitol. In the mixed treatment liquid, the concentration of PEDOT:PSS was 2% by mass, and the concentration of mannitol was 10% by mass. The viscosity of the mixed treatment liquid was 58 mPa·s.

A wound body was produced in the same manner as in Example 1, except that separator a2 was used instead of separator a1.
Without carrying out the first and second steps, the wound body was immersed in the mixed treatment solution in a container at room temperature in a reduced pressure atmosphere (40 kPa) for 15 minutes, and then dried in a drying oven at 1 atmosphere pressure for 30 minutes at 150° C. In this manner, the conductive polymer and mannitol were attached to the wound body (surface layer and inside of the separator), and a capacitor element was obtained.

When the distribution of mannitol in the capacitor element was examined by the method described above, it was found that mannitol was not unevenly distributed in the separator, and the M _O /M _I ratio obtained by the method described above was 1.0.

The capacitor element obtained above was used to fabricate electrolytic capacitor X1 in the same manner as in Example 1.

Comparative Example 2
An electrolytic capacitor X2 was produced in the same manner as in Example 1, except that a separator x1 made of aramid fiber was used instead of the separator a1.

Comparative Example 3
The wound body used was the same as that in Comparative Example 2, including the separator x1. Without carrying out the first and second steps, the wound body was immersed in the same mixed treatment liquid as that in Comparative Example 1 for 15 minutes in a reduced pressure atmosphere (40 kPa) at room temperature, and then dried at 150° C. for 30 minutes in a drying oven with a pressure of 1 atmosphere. In this way, the conductive polymer and mannitol were attached to the wound body (surface layer and inside of the separator), and a capacitor element was obtained.
An electrolytic capacitor X3 was produced in the same manner as in Example 1 using the capacitor element obtained above.

Comparative Example 4
An electrolytic capacitor X4 was produced in the same manner as in Example 1, except that a separator x2 made of natural cellulose fiber was used instead of the separator a1.

The electrolytic capacitors thus fabricated were evaluated as follows.
[Breakdown voltage]
A voltage was applied while increasing at a rate of 1.0 V/sec, and the breakdown voltage (BDV) (V) at which an overcurrent of 0.5 A flows was measured.

[Initial Capacitance and ESR]
In an environment of 20° C., the initial capacitance (C0) (μF) and ESR (R0) (mΩ) were measured using an LCR meter for four-terminal measurement.

[ΔESR]
A load test was performed by leaving the capacitor for 500 hours in a state where a rated voltage of 35 V was applied in an environment of 150° C. After 500 hours, the ESR (R1) was measured in the same manner as above. The ratio of R1 to R0: R1/R0 was calculated and used as ΔESR.

[Short circuit occurrence rate]
Thirty pieces of each of the eight types of capacitors were produced and subjected to the same load test as described above. After 500 hours, each capacitor was checked for the presence or absence of short circuits. The number of short-circuited capacitors among the 30 capacitors was then counted, and the proportion of short-circuited capacitors was calculated as the short circuit occurrence rate.

The evaluation results are shown in Table 1.

Electrolytic capacitors A1 to A4 achieved high capacitance, high BDV, and low ESR. In addition, electrolytic capacitors A1 to A4 showed a small increase in ΔESR and a low incidence of short circuits.

The ESR increased in electrolytic capacitors X1 and X3. This is thought to be because the wound body was immersed in a mixed treatment solution of PEDOT:PSS and mannitol, and it was not possible to incorporate a large amount of PEDOT:PSS into the separator.

In electrolytic capacitors X2 and X3 using separator x1 (synthetic fibers only), the capacitance dropped significantly and the ESR increased. This is thought to be because the influence of the synthetic fibers containing PVA was greater in separator x1, making it impossible to contain a large amount of mannitol in the separator. In electrolytic capacitor X4 using separator x2 (natural fibers only), the BDV was low and the incidence of short circuits increased.

The present invention can be used in electrolytic capacitors and their manufacturing methods.

10: anode foil, 20: cathode foil, 30: separator, 40: stop tape, 50A, 50B: lead tabs, 60A, 60B: lead wires, 100: wound body, 200: electrolytic capacitor, 211: bottomed case, 212: sealing member, 213: seat plate

Claims (22)

a capacitor element including an anode foil having a dielectric layer on a surface thereof, a cathode foil, a separator interposed between the anode foil and the cathode foil, a hydroxyl group-containing compound, and a conductive polymer;
The separator includes synthetic fibers and cellulosic fibers,
the conductive polymer and the hydroxyl group-containing compound are attached to a surface layer and an inside of the separator,
the hydroxyl group-containing compound is at least one compound (excluding polymers) selected from the group consisting of sugars and polyhydric alcohols,
The electrolytic capacitor, wherein the hydroxyl group-containing compound is unevenly distributed in the separator. 2. The electrolytic capacitor according to claim 1, wherein in the capacitor element, a mass ratio M _I of the hydroxyl-containing compound to the conductive polymer inside the separator is greater than a mass ratio M _O of the hydroxyl-containing compound to the conductive polymer in a surface layer of the separator. 3. The electrolytic capacitor according to claim 2, wherein a ratio of the mass ratio M _{O to the mass ratio M I :} M _O /M _I is 0.7 or more and 0.9 or less. The electrolytic capacitor according to any one of claims 1 to 3, wherein the synthetic fibers include at least one selected from the group consisting of aramid, polyethylene terephthalate, and vinylon. The electrolytic capacitor according to any one of claims 1 to 4, wherein the synthetic fibers are fibrillated. The electrolytic capacitor according to any one of claims 1 to 5, wherein the hydroxyl group-containing compound includes at least one selected from the group consisting of glucose, mannitol, sorbitol, xylitol, pentaerythritol, and trimethylolpropane. The electrolytic capacitor according to any one of claims 1 to 6, wherein the conductive polymer comprises poly(3,4-ethylenedioxythiophene) doped with polystyrenesulfonic acid. The electrolytic capacitor according to any one of claims 1 to 7, wherein the capacitor element contains a non-aqueous solvent or an electrolyte. a first step of obtaining an intermediate including an anode foil having a dielectric layer on a surface thereof, a cathode foil, a separator interposed between the anode foil and the cathode foil, and a hydroxyl group-containing compound attached to a surface layer and an interior of the separator and unevenly distributed in the separator;
a second step of impregnating the intermediate with a treatment liquid containing a conductive polymer to obtain a capacitor element;
The separator includes synthetic fibers and cellulosic fibers,
The method for producing an electrolytic capacitor, wherein the hydroxyl group-containing compound is at least one compound (excluding polymers) selected from the group consisting of sugars and polyhydric alcohols. The method for producing an electrolytic capacitor according to claim 9, wherein the first step includes a step (1a) of obtaining an electrode group by disposing the separator between the anode foil and the cathode foil, and impregnating the electrode group with a treatment liquid containing the hydroxyl group-containing compound. The method for producing an electrolytic capacitor according to claim 10, wherein the first step includes a step (2a) of heating and drying the electrode group impregnated with the treatment liquid containing the hydroxyl group-containing compound at a temperature equal to or higher than the melting point of the hydroxyl group-containing compound after the step (1a). The method for manufacturing an electrolytic capacitor according to any one of claims 9 to 11, wherein the first step includes a step of carbonizing the separator. The first step comprises:
(1b) obtaining the separator having the hydroxyl group-containing compound attached to the surface and the inside of the separator;
After the step (1b), a step (2b) of disposing the separator between the anode foil and the cathode foil;
The method for producing the electrolytic capacitor according to claim 9 , comprising: The method for manufacturing an electrolytic capacitor according to claim 13, wherein the step (1b) includes a step (1b-1) of impregnating the separator with a treatment liquid containing the hydroxyl group-containing compound. The method for producing an electrolytic capacitor according to claim 14, wherein the step (1b) includes a step (1b-2) of heating and drying the separator impregnated with the treatment liquid containing the hydroxyl group-containing compound at a temperature equal to or higher than the melting point of the hydroxyl group-containing compound after the step (1b-1). The method for producing an electrolytic capacitor according to claim 15, further comprising a step of carbonizing the separator after step (2b). The step (1b) includes a blending step of blending the synthetic fibers and the cellulosic fibers to obtain the separator,
The method for producing an electrolytic capacitor according to any one of claims 13 to 16, wherein the hydroxyl group-containing compound is added in the mixing step. The method for manufacturing the electrolytic capacitor according to any one of claims 9 to 17, further comprising a third step of impregnating the capacitor element with a non-aqueous solvent or an electrolytic solution. The method for manufacturing an electrolytic capacitor according to any one of claims 9 to 18, wherein the synthetic fibers include at least one selected from the group consisting of aramid, polyethylene terephthalate, and vinylon. The method for manufacturing an electrolytic capacitor according to any one of claims 9 to 19, wherein the synthetic fibers are fibrillated. The method for producing an electrolytic capacitor according to any one of claims 9 to 20, wherein the hydroxyl group-containing compound includes at least one selected from the group consisting of glucose, mannitol, sorbitol, xylitol, pentaerythritol, and trimethylolpropane. The method for producing an electrolytic capacitor according to any one of claims 9 to 21, wherein the conductive polymer comprises poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid.