The electrolyte is formed by applying a liquid mixture containing a conjugated polymer, a first dopant (specifically, a first polymer dopant), and a metal ion, to an anode body having a dielectric layer. The electrolyte is formed so as to cover at least part of the dielectric layer. The liquid mixture may be a solution or a liquid dispersion. The application of the liquid mixture to the anode body may be performed once, and may be repeated twice or more times.

The liquid mixture can be prepared through, for example, a sub-step of polymerizing a precursor of the conjugated polymer in the presence of the first polymer dopant and an oxidizing agent, to obtain a first mixture containing the conjugated polymer and the first polymer dopant, and a sub-step of further mixing the metal ion with the first mixture, to prepare the liquid mixture, as a second mixture.

As long as the metal ion is contained in the liquid mixture at the time of application to the anode body having a dielectric layer, excellent impregnation can be obtained. Therefore, the metal ion may not be necessarily contained in the reaction system during polymerization of the precursor. The amount of the metal ion contained in the reaction system is preferably small so that the metal ion gives no influence on the reaction system. In the reaction system, for example, the amount of the metal ion may be 0.01 equivalents or less or 0.001 equivalents or less, relative to 1 equivalent of the anionic group of the first polymer dopant. In particular, the polymerization of the precursor is preferably performed in the absence of the metal ion.

The metal ion may be mixed in the form of a salt, with the first mixture. When the salt is dissolved in the solvent or the like and mixed in this state with the first mixture, the metal ion is dissociated in some cases. Such cases are also encompassed in being mixed in the form of a salt. For example, the metal ion may be mixed in the form of a salt of the first dopant (specifically, the second polymer dopant) and the metal ion, with the first mixture. The second polymer dopant is encompassed in the aforementioned first dopant, for which the description of the first dopant can be referred to. The second polymer dopant may be used singly, or in combination of two or more kinds. The first polymer dopant contained in the first mixture may be the same as at least part of the second polymer dopant, or may be different from all of the second polymer dopant. When the second cation is used, it may be added to the first mixture or may be added to the liquid mixture. The second cation is added in the form of a salt, as necessary. The salt may be a salt with the first dopant, a salt with the second dopant, or a salt with another anion.

The thus-obtained liquid mixture containing a metal ion may be used to form a first portion, and the liquid mixture prepared without using a metal ion (e.g., the first mixture) may be used to form a second portion. The first portion may be formed by attaching a liquid mixture once, or by attaching it twice or more. Likewise, the second portion may be formed by attaching a liquid mixture for the second portion once, or by attaching it twice or more.

When a plurality of liquid mixtures are used for forming the electrolyte, the compositions of the respective liquid mixtures (e.g., the kinds of each monomer, dopant, metal ion, second cation, etc., and the amounts of them) may be the same or different.

Examples of the precursor of a conjugated polymer used in the liquid mixture include: raw material monomers of a conjugated polymer; and oligomers and prepolymers in which a plurality of molecular chains of raw material monomers are connected. The precursor may be used singly, or in combination of two or more kinds.

The oxidizing agent may be added to the liquid mixture, or may be applied to the anode body with the dielectric layer formed at its surface, before or after bringing the liquid mixture into contact with the anode body. Examples of such an oxidizing agent include compounds capable of producing Fe³⁺ (ferric sulfate, etc.), persulfates (sodium persulfate, ammonium persulfate, etc.), and hydrogen peroxide. The oxidizing agent may be used singly, or in combination of two or kinds.

The liquid mixture usually contains a solvent. Examples of the solvent include water, an organic solvent, and a mixed solvent of water and an organic solvent (water-soluble organic solvent, etc.).

In the case of using other conductive materials, additives, and the like, they may be added to the liquid mixture.

(Cathode Body)

For the cathode body, likewise for the anode body, a metal foil may be used. The metal is preferably a valve metal, such as aluminum, tantalum, and niobium, or an alloy containing a valve metal. The metal constituting the cathode body, however, is not limited thereto. A surface of the metal foil may be roughened, as necessary. The metal foil may be provided at its surface with a chemical conversion film, or with a surface film of a metal different from the metal constituting the metal foil (i.e., a dissimilar metal) or of a non-metal. Examples of the dissimilar metal and the non-metal include a metal, such as titanium, and a non-metal, such as carbon.

(Separator)

When a metal foil is used for the cathode body, a separator may be disposed between the metal foil and the anode body. As the separator, without particular limitation, for example, a nonwoven fabric including cellulose, polyethylene terephthalate, vinylon, or polyamide (e.g., aliphatic polyamide, aromatic polyamide such as aramid) fibers may be used. In the present disclosure, since the electrolyte is formed using a liquid mixture containing a metal ion, excellent impregnation of the liquid mixture into the separator can be ensured. Therefore, even when the anode body and the cathode body are stacked with the separator interposed therebetween, and the liquid mixture is applied to the stack to form an electrolyte, the electrolyte can be formed with high coverage.

(Liquid Component)

When the electrolytic capacitor contains a liquid component, this is advantageous in ensuring even higher capacitance. Furthermore, even when the electrolytic capacitor is exposed to high temperatures, the changes in ESR can be suppressed low.

The liquid component contains a solvent. In view of suppressing the volatilization of the solvent, the solvent preferably contains at least a polyhydric alcohol. In addition to the polyhydric alcohol, a sulfone compound, a lactone compound, a carbonate compound, and the like may be used. The solvent may be used singly, or in combination of two or more kinds.

Examples of the sulfone compound include sulfolane, dimethyl sulfoxide, and diethyl sulfoxide. Examples of the lactone compound include γ-butyrolactone and γ-valerolactone. Examples of the carbonate compound include dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, and fluoroethylene carbonate.

The proportion of the polyhydric alcohol in the whole solvent contained in the liquid component is, for example, 50 mass % or more, and may be 75 mass % or more. The proportion of the polyhydric alcohol in the whole solvent contained in the liquid component is 100 mass % or less.

The polyhydric alcohol includes, for example, a glycerin compound, a sugar alcohol compound, and a glycol compound.

Examples of the glycerin compound include glycerin, a polyglycerin (diglycerin, triglycerin, etc.), or derivatives thereof. The number of repeating glycerin units in the polyglycerin is, for example, 2 or more and 20 or less, and may be 2 or more and 10 or less. Examples of the sugar alcohol compound include a sugar alcohol (erythritol, mannitol, pentaerythritol, etc.) or derivatives thereof. The derivatives are exemplified by alkylene oxide adducts (an adduct in which one alkylene oxide is added per one hydroxy group of glycerin, polyglycerin, or sugar alcohol). The alkylene oxide adducts are exemplified by C_2-4alkylene oxide adducts (ethylene oxide adduct, etc.).

Examples of the glycol compound include an alkylene glycol (C_2-4alkylene glycol (e.g., ethylene glycol, propylene glycol), etc.), a polyalkylene glycol (polyC_2-4alkylene glycol (e.g., diethylene glycol, dipropylene glycol, triethylene glycol, polyethylene glycol), etc.), and a polyalkylene oxide adduct (polyC_2-4alkylene oxide adduct (e.g., polyethylene oxide adduct), etc.) of a sugar alcohol (glycerin, erythritol, mannitol, pentaerythritol, etc.).

The solvent preferably contains, among polyhydric alcohols, any one selected from ethylene glycol, glycerin, diethylene glycol, triethylene glycol, and propylene glycol.

The liquid component may contain a solute. Examples of the solute include an acid component and a base component.

Examples of the acid component include a carboxylic acid (aliphatic carboxylic acid, aromatic carboxylic acid (including polycarboxylic acid, such as phthalic acid and pyromellitic acid), etc.), a sulfur-containing acid (sulfuric acid, sulfonic acid (e.g., aliphatic sulfonic acid, aromatic sulfonic acid), etc.), a boron-containing acid (boric acid, halogenated boric acid (e.g., tetrafluoroboric acid), or partial esters thereof, etc.), a phosphorus-containing acid (phosphoric acid, halogenated phosphoric acid (e.g., hexafluorophosphoric acid), etc.), phosphonic acid, phosphinic acid, or partial esters thereof, etc.), nitric acid, and nitrous acid. As the acid component, a condensate (borodisalicylic acid, borodiglycolic acid, borodioxalic acid, etc.) of a carboxylic acid and an inorganic acid (boric acid, phosphoric acid, etc.) may be used. The aromatic sulfonic acid includes an aromatic sulfonic acid having a hydroxyl or carboxyl group in addition to a sulfo group (oxyaromatic sulfonic acid (e.g., phenol-2-sulfonic acid), and a sulfoaromatic carboxylic acid (e.g., p-sulfobenzoic acid, 3-sulfophthalic acid, 5-sulfosalicylic acid), etc.). A polymeric acid component may also be used. The polymeric acid component can include, for example, at least one selected from the group consisting of polyacrylic acid, polymethacrylic acid, polyvinylsulfonic acid, polystyrenesulfonic acid, polyallylsulfonic acid, polyacrylsulfonic acid, polymethacrylsulfonic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), and polyisoprene sulfonic acid. The liquid component may contain one kind of acid component, or two or more kinds of acid components.

Examples of the base component include ammonia, an amine (specifically, primary amine, secondary amine, and tertiary amine), a quaternary ammonium compound, and an amidinium compound. The amine may be aliphatic, aromatic, or heterocyclic. Examples of the amine include trimethylamine, diethylamine, triethylamine, ethylenediamine, aniline, pyrrolidine, imidazole, and 4-dimethylaminopyridine. Examples of the quaternary ammonium compound include an amidine compound (including an imidazole compound). The liquid component may contain one kind of base component, or two or more kinds of base components.

The liquid component may contain the acid component and the base component, each in a free state, or in the form of an ion, or in the form of a salt. The liquid component may contain an organic salt. Examples of the organic salt include those in which at least one of the acid component and the base component is organic.

The concentration of the solute in the liquid component is, for example, 0.1 mass % or more and 25 mass % or less, and may be 0.5 mass % or more and 15 mass % or less. When the concentration of the solute is in the range as above, dedoping of the dopant is likely to be suppressed.

(Others)

The electrolytic capacitor may be of a wound type, and may be of a chip type or a laminate type. The electrolytic capacitor has at least one capacitor element, and may have a plurality of capacitor elements. For example, a solid electrolytic capacitor may include a laminate of two or more capacitor elements, or may include two or more wound capacitor elements. The configuration or number of capacitor elements may be selected depending on the type or application of the electrolytic capacitor.

In the following, the electrolytic capacitor of the present disclosure will be described more specifically, based on embodiments. However, the electrolytic capacitor of the present disclosure is not limited to the following embodiments.

FIG. 1 is a schematic cross-sectional view of an electrolytic capacitor according to the present embodiment, and FIG. 2 is a partially-exploded schematic view of a capacitor element according to the electrolytic capacitor.

The electrolytic capacitor illustrated in FIG. 1 includes a capacitor element 10, a bottomed case 11 housing the capacitor element 10, an encapsulation member 12 closing an opening of the bottomed case 11, a seat plate 13 covering the encapsulation member 12, lead wires 14A and 14B each extended from the encapsulation member 12 and passing through the seat plate 13, lead tabs 15A and 15B each connecting the lead wire with an electrode of the capacitor element 10, and a liquid component (not shown). The open end of the bottomed case 11 is curled so as to be crimped onto the encapsulation member 12.

The capacitor element 10 is produced from a wound body as illustrated in FIG. 2. The wound body is a semi-finished product of the capacitor element 10, in which a conductive polymer is not yet disposed between an anode body 21 having a dielectric layer at its surface and a cathode body 22. The wound body is formed by winding the anode body 21 connected to the lead tab 15A and the cathode body 22 connected to the lead tab 15B, with a separator 23 interposed therebetween. The outermost layer of the wound body is secured with a winding stop tape 24. FIG. 2 shows a partially exploded state before the outermost layer of the wound body is secured.

The anode body 21 has a metal foil with a roughened surface, and a dielectric layer is formed at the roughened surface. The capacitor element 10 is completed by attaching a conductive polymer compound on at least part of the surface of the dielectric layer. The capacitor element 10 is housed, together with the liquid component (not shown), in a package case.

An exemplary method for producing an electrolytic capacitor will be described below.

(i) Step of Preparing Anode Body 21 Having Dielectric Layer and Cathode Body 22

As a raw material of the anode body 21 and the cathode body 22, a metal foil containing a valve metal is used. In the anode body 21, by etching treatment or the like, a surface of the metal foil is roughened and a plurality of irregularities are formed at the surface of the metal foil. This is followed by forming a dielectric layer on the roughened surface of the metal foil by chemical conversion treatment or the like. A surface of the cathode body 22 may be roughened, as necessary.

(ii) Step of Fabricating Wound Body

The anode body 21 and the cathode body 22 are wound with the separator 23 interposed therebetween, to form a wound body. For the separator 23, a nonwoven fabric mainly composed of a synthetic cellulose or the like can be used. The winding stop tape 24 is placed on the outer surface of the cathode body 22 positioned at the outermost layer of the wound body, to secure the end of the cathode body 22. The wound body may be further subjected to chemical conversion treatment, as necessary.

(iii) Step of Forming Capacitor Element 10

For example, a liquid mixture prepared by the above procedures containing a conjugated polymer, a first polymer dopant, and a metal ion is impregnated into the dielectric layer, to form a film of a conductive polymer (electrolyte) that covers at least part of the dielectric layer. In this way, the capacitor element 10 in which a conductive polymer (electrolyte) is disposed between the anode body 21 and the cathode body 22 is obtained. The step of applying a liquid mixture onto the surface of the dielectric layer may be repeated twice or more. In the case of using a liquid component, after the electrolyte is formed, the liquid component is impregnated into the capacitor element 10.

(iv) Step of Encapsulating Capacitor Element

The capacitor element 10 is housed, together with the liquid component, in the bottomed case 11, so that the lead wires 14A and 14B are positioned on the opening side of the bottomed case 11. Next, the opening of the bottomed case 11 is closed with the encapsulation member 12 having the lead wires 14A and 14B passing therethrough, and the open end is curled so as to be crimped onto the encapsulation member 12, and then, the seat plate 13 is placed on the curled portion. Thus, an electrolytic capacitor as illustrated in FIG. 1 is completed.

In the above embodiments, a wound electrolytic capacitor is described, but the scope of application of the present invention is not limited to the above, and can also be applied to other capacitors, for example, a chip-type electrolytic capacitor including a metal sintered body as the anode body, and a laminate-type electrolytic capacitor including a metal plate as the anode body.

All of the features described in the present specification may be combined in any combination.

EXAMPLES

The present invention will be more specifically described below with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

<<Fabrication of electrolytic capacitors A1 to A14 and B1 to B3>>

A wound electrolytic capacitor (diameter 10 mm×L (length) 10 mm) having a rated voltage of 25 V and a rated capacitance of 330 μF was produced. The specific production method of the electrolytic capacitor will be described below.

(Preparation of Anode Body)

A 100-μm-thick aluminum foil was subjected to etching treatment, to roughen the surface of the aluminum foil. Then, a dielectric layer was formed at the surface of the aluminum foil by chemical conversion treatment. The chemical conversion treatment was performed by immersing the aluminum foil in an ammonium adipate solution, and applying a voltage of 45 V thereto. Then, the aluminum foil was cut, to prepare an anode body.

(Preparation of Cathode Body)

A 50-μm-thick aluminum foil was subjected to etching treatment, to roughen the surface of the aluminum foil. Then, the aluminum foil was cut, to prepare a cathode body.

(Fabrication of Wound Body)

An anode lead tab and a cathode lead tab were connected to the anode body and the cathode body, respectively, and the anode body and the cathode body were wound, together with the lead tabs, with a separator interposed between the anode body and the cathode body. An anode lead wire and a cathode lead wire were respectively connected to the ends of the lead tabs extended from the wound body. The obtained wound body was subjected to chemical conversion treatment again, to form a dielectric layer on the cut end of the anode body. Next, the end of the outer surface of the wound body was secured with a winding stop tape, to complete the wound body. The separator used here was a nonwoven fabric made of cellulose.

(Preparation of liquid mixture)

A mixed solution was prepared by dissolving 3,4-ethylenedioxythiophene (EDOT) and polystyrenesulfonic acid (PSS, weight average molecular weight 100,000) serving as a first polymer dopant, in ion-exchanged water. To the mixed solution, under stirring, iron(III) sulfate (oxidizing agent) dissolved in ion-exchanged water was added, to allow a polymerization reaction to proceed. Upon reaction, the resultant reaction solution was dialyzed, to remove unreacted monomers and excess oxidizing agent, so that a polymer dispersion (first mixture) including polyethylene dioxythiophene doped with PSS (PEDOT/PSS) was obtained. At this time, the doping amount was adjusted so that EDOT was present in an amount of 0.5 equivalents, relative to 1 equivalent of the anionic group of PSS. In B1, instead of PSS, a lithium salt of PSS was used.

Polystyrenesulfonic acid (PSS, weight average molecular weight 100,000) serving as a second polymer dopant, and an aqueous solution of a hydroxide of a cation shown in Tables or an aqueous ammonia were added and mixed. The hydroxide or ammonia was added in such a proportion that the cation shown in Tables was present in an equivalent shown in Tables, relative to 1 equivalent of the anionic group of the polystyrenesulfonic acid, thereby to adjust the pH to about 2.0 to 3.5. In this way, a liquid mixture (second mixture) A was prepared. In B1, as the liquid component, the first mixture was used.

Polystyrenesulfonic acid (PSS, weight average molecular weight 100,000) and aqueous ammonia were added. The aqueous ammonia was added in such a proportion that ammonium ion was present in an amount of 0.2 equivalents, relative to 1 equivalent of the anionic group of the polystyrenesulfonic acid, thereby to adjust the pH to about 2.2. In this way, a liquid mixture B was prepared.

(Formation of Electrolyte Layer)

In a reduced pressure atmosphere (40 kPa), the wound body was immersed in the liquid mixture placed in a predetermined container for 5 minutes, and then, the wound body was pulled out of the liquid mixture. Next, the wound body impregnated with the liquid mixture was dried for 20 minutes in a drying oven at 150° C., to form a conductive polymer layer covering at least part of the dielectric layer. In this way, a capacitor element was formed. In A12 and A13, the wound body was immersed in the liquid mixture A for 2 minutes, and then, the wound body was pulled out of the liquid mixture and dried for 20 minutes in a drying oven at 150° C. In A12, the wound body after drying was immersed in the liquid mixture B for 3 minutes, and in A13, immersed in the first mixture for 3 minutes. In A14, the wound body was immersed in the first mixture for 2 minutes, and then, the wound body was pulled out of the first mixture and dried for 20 minutes in a drying oven at 150° C. In A14, the wound body after drying was immersed in the liquid mixture A for 3 minutes. Thereafter, the capacitor element of each Example was pulled out of the liquid mixture and dried for 20 minutes in a drying oven at 150° C. In this way, a capacitor element was formed.

(Preparation of Liquid Component)

To ethylene glycol serving as a solvent, phthalic acid serving as an acid component and triethylamine serving as a base component were added such that the respective concentrations in the liquid component were 5 mass % and 5 mass %, and mixed. In this way, a liquid component was prepared.