# Patent

## Title
Conductive polymer suspension and method for producing the same, conductive polymer material, electrolytic capacitor, and solid electrolytic capacitor and method for producing the same

## Abstract

The present invention provides a conductive polymer suspension for providing a conductive polymer material having a high conductivity and a method for producing the same, and in particular, a solid electrolytic capacitor having a low ESR and a method for producing the same. The conductive polymer suspension is produced by: synthesizing a conductive polymer by chemical oxidative polymerization of a monomer giving the conductive polymer by using an oxidant in a solvent containing a dopant consisting of a low-molecular organic acid or a salt thereof; purifying the conductive polymer; and mixing the purified conductive polymer and an oxidant in an aqueous solvent containing a polyacid component.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a conductive polymer suspension and a method for producing the same, a conductive polymer material, an electrolytic capacitor, and a solid electrolytic capacitor and a method for producing the same.
2. Description of the Related Art
Conductive organic materials are used for electrodes of capacitors, electrodes of dye-sensitized solar cells and electrodes of electroluminescence displays and the like. As such conductive organic materials, those conductive polymers obtained by polymerizing pyrrole, thiophene, aniline and the like are known.
Such conductive polymers are generally supplied as a dispersion (suspension) or a solution in an aqueous solvent or as a solution in an organic solvent, which is used as a conductive polymer material by removing the solvent at the time of use. However, even when the types of the conductive polymers are the same, the physical properties of the obtained conductive polymer materials are different from each other depending on the conditions of the dispersions; and hence the methods for producing such dispersions have hitherto been studied in various manners.
Japanese Patent Application Laid-Open No. 07-90060 discloses a solution (dispersion) of polythiophene and a method for producing the solution, and a technique of application of the solution (dispersion) to an antistatic treatment of a plastic molded article. The dispersion of polythiophene includes, as the dispersion medium, water or a mixture of a water-miscible organic solvent with water, a polythiophene formed of a structural unit of 3,4-dialkoxythiophene and a polyanion derived from polystyrenesulfonic acid having a molecular weight within a range from 2,000 to 500,000. The polythiophene is obtained by chemical oxidative polymerization, in the presence of the polyanion, of polystyrenesulfonic acid having a molecular weight within a range from 2,000 to 500,000. It is described that a transparent antistatic film can be formed in this manner.
Japanese Patent Application Laid-Open No. 2004-59666 discloses a water dispersion of a complex between poly(3,4-dialkoxythiophene) and a polyanion and a method for producing the water dispersion, and a technique related to a coating composition containing the water dispersion and a coated substrate having a transparent conductive film formed by applying the composition. The water dispersion is obtained by polymerizing 3,4-dialkoxythiophene in an aqueous solvent by using peroxodisulfuric acid as an oxidant in the presence of a polyanion. Alternatively, the water dispersion is obtained by chemical oxidative polymerization of 3,4-dialkoxythiophene in an aqueous solvent by using an oxidant in the presence of a polyanion under the pH of the reaction solution lowered by adding an acid selected from the group consisting of water-soluble inorganic acids and organic acids. It is described that a conductive thin film excellent in transparency can be formed in this manner.
However, by the methods described in Japanese Patent Application Laid-Open Nos. 07-90060 and 2004-59666, specifically by a method which conducts the chemical oxidative polymerization of 3,4-dialkoxythiophene in one stage in the presence of the polyanion acting as a dopant, it is difficult to control the doping rate. In other words, the undoped polyanion, namely, the polyanion making no contribution to the conductivity comes to be present in an excessive amount, and thus, the methods are far from being satisfactory methods as the production method for obtaining a conductive polymer material having a higher conductivity.
The conductive polymer film obtained by the method described in Japanese Patent Application Laid-Open No. 7-90060 has a sufficient conductivity as an antistatic material, but hardly attains a conductivity sufficiently satisfying the requirement for lowering the ESR, for example, when used as a solid electrolyte in a capacitor. In other words, the surface resistivity of an antistatic film is generally classified to fall within a range from 105 to 1014Ω/□; when the conductivity is too high, there is a possibility of occurrence of an intense electrostatic discharge, and hence it is understood that the antistatic film does not have a conductivity so high as to rapidly dissipate the static charge of an charged object. Therefore, a material usable as an antistatic film is high in resistance as the solid electrolyte for use in capacitors and hence cannot satisfy the requirement of low resistance. Additionally, a capacitor including a solid electrolyte that contains excessive polyanions has a drawback of poor reliability, in particular, poor properties in a high humidity atmosphere.
SUMMARY OF THE INVENTION
An object of the present invention is to solve the above-described problems, specifically, to provide a conductive polymer suspension for providing a conductive polymer material having a high conductivity and a method for producing the same, and in particular to provide a solid electrolytic capacitor having a low ESR and a method for producing the same.
A method for producing a conductive polymer suspension of the present invention comprises:
(a) synthesizing a conductive polymer (P1) by chemical oxidative polymerization of a monomer (M1) giving the conductive polymer by using an oxidant (O1) in a solvent containing a dopant consisting of a low-molecular organic acid or a salt thereof;
(b) purifying the conductive polymer (P1); and
(c) mixing the purified conductive polymer (P1) and an oxidant (O2) in an aqueous solvent containing a polyacid component to obtain the conductive polymer suspension.
A conductive polymer suspension of the present invention is obtained by the above-described method. A conductive polymer material of the present invention is obtained by removing the solvent from the above-described conductive polymer suspension.
An electrolytic capacitor of the present invention comprises, as an electrolyte, the above-described conductive polymer suspension. A solid electrolytic capacitor of the present invention comprises a solid electrolyte layer including the above-described conductive polymer material.
A first method for producing a solid electrolytic capacitor of the present invention comprises:
forming a dielectric layer on the surface of an anode conductor consisting of a valve action metal; and
forming a solid electrolyte layer comprising the conductive polymer material by applying or impregnating the above-described conductive polymer suspension onto the dielectric layer and by removing the solvent from the conductive polymer suspension.
A second method for producing a solid electrolytic capacitor of the present invention, comprises:
forming a dielectric layer on the surface of an anode conductor consisting of a valve action metal;
forming a first solid electrolyte layer comprising a conductive polymer (P2) by chemical oxidative polymerization or electrolytic polymerization of a monomer (M2) giving the conductive polymer on the dielectric layer; and
forming a second solid electrolyte layer by applying or impregnating the above-described conductive polymer suspension onto the first solid electrolyte layer and by removing the solvent from the conductive polymer suspension.
The present invention can provide a conductive polymer suspension for proving a conductive polymer material having a high conductivity and a method for producing the same, and in particular, a solid electrolytic capacitor having a low ESR and a method for producing the same.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic sectional view illustrating the structure of the solid electrolytic capacitor according to an embodiment of the present invention; and
FIG. 2 is a chart showing the X-ray diffractions of the conductive polymer films formed in Example 1 and Comparative Example 2.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method for producing a conductive polymer suspension of the present invention is described.
First, a conductive polymer (P1) is synthesized by chemical oxidative polymerization of a monomer (M1) giving the conductive polymer by using an oxidant (O1) in a solvent containing a dopant consisting of a low-molecular organic acid or a salt thereof (step (a)). The step (a) enables to obtain the conductive polymer (P1) having a high degree of polymerization and a high degree of crystallization.
The solvent in which the reaction is conducted is preferably selected as a solvent satisfactory in the compatibility with the monomer (M1). The solvent may be water, an organic solvent or a water-mixed organic solvent. Specific examples of the organic solvent include: alcohol solvents such as methanol, ethanol and propanol; aromatic hydrocarbon solvents such as benzene, toluene and xylene; and aliphatic hydrocarbon solvents such as hexane. The organic solvent may be used alone or in combinations of two or more thereof. Among these, ethanol or a mixed solvent composed of ethanol and water is preferable.
The monomer (M1) can be appropriately selected from the monomers giving conductive polymers. Specific examples of the monomer (M1) include pyrrole, thiophene, aniline and derivatives of these. Specific examples of the pyrrole derivatives include: 3-alkylpyrroles such as 3-hexylpyrrole; 3,4-dialkylpyrroles such as 3,4-dihexylpyrrole; 3-alkoxypyrroles such as 3-methoxypyrrole; and 3,4-dialkoxypyrroles such as 3,4-dimethoxypyrrole. Specific examples of the thiophene derivatives include: 3,4-ethylenedioxythiophene and derivatives thereof; 3-alkylthiophene such as 3-hexylthiophene; and 3-alkoxythiophenes such as 3-methoxythiophene. Specific examples of the aniline derivatives include: 2-alkylanilines such as 2-methylaniline; and 2-alkoxyanilines such as 2-methoxyaniline. Among these, 3,4-ethylenedioxythiophene represented by the following formula (1) or derivatives thereof are preferable. Specific examples of the derivatives of 3,4-ethylenedioxythiophene include 3,4-(1-alkyl)ethylenedioxythiophene such as 3,4-(1-hexyl)ethylenedioxythiophene. The monomer (M1) may be used alone or in combinations of two or more thereof.




The concentration of the monomer (M1) in the solvent is not particularly limited because the monomer (M1) can be removed in the step (b) even when the concentration is excessive. However, for the purpose of obtaining the conductive polymer (P1) having a high conductivity with a satisfactory yield, the concentration of the monomer (M1) is preferably 0.5 to 70% by weight and more preferably 1 to 50% by weight.
As the dopant, a low-molecular organic acid or a salt thereof is used. Specific examples of the low-molecular organic acid and a salt thereof include alkylsulfonic acids, benzenesulfonic acid, naphthalenesulfonic acid, anthraquinonesulfonic acid, camphorsulfonic acid and derivatives of these, and the iron(III) salts of these. The low-molecular organic acid may be a monosulfonic acid, a disulfonic acid or a trisulfonic acid. Specific examples of the derivatives of the alkylsulfonic acids include 2-acrylamide-2-methylpropanesulfonic acid. Specific examples of the derivatives of benzenesulfonic acid include phenolsulfonic acid, styrenesulfonic acid, toluenesulfonic acid and dodecylbenzenesulfonic acid. Specific examples of the derivatives of naphthalenesulfonic acid include 1-naphthalenesulfonic acid, 2-naphthalenesulfonic acid, 1,3-naphthalenedisulfonic acid, 1,3,6-naphthalenetrisulfonic acid and 6-ethyl-1-naphthalenesulfonic acid. Specific examples of the derivatives of anthraquinonesulfonic acid include anthraquinone-1-sulfonic acid, anthraquinone-2-sulfonic acid, anthraquinone-2,6-disulfonic acid and 2-methylanthraquinone-6-sulfonic acid. Among these, 1-naphthalenesulfonic acid, 2-naphthalenesulfonic acid, 1,3,6-naphthalenetrisulfonic acid, anthraquinonedisulfonic acid, p-toluenesulfonic acid, camphorsulfonic acid and the iron(III) salts of these are preferable. Because of a marked effect on the high crystallization of the polymer, camphorsulfonic acid is more preferable. Camphorsulfonic acid may also be an optically active substance. Additionally, iron(III) p-toluenesulfonate is also preferable because it also functions as the oxidant (O1). The dopant may be used alone or in combinations of two or more thereof.
The amount of the dopant used is not particularly limited because the dopant can be removed in the step (b) even when it is added in an excessive amount. However, for the purpose of obtaining the conductive polymer (P1) having a high conductivity, the amount of the dopant used is preferably 1 to 100 parts by weight and more preferably 1 to 20 parts by weight, in relation to 1 part by weight of the monomer (M1).
The oxidant (O1) is not particularly limited. Examples of the usable oxidant (O1) include: iron(III) salts of inorganic acids such as iron(III) chloride hexahydrate, anhydrous iron(III) chloride, iron(III) nitrate nonahydrate, anhydrous ferric nitrate, iron(III) sulfate n-hydrate (n=3 to 12), ammonium iron(III) sulfate dodecahydrate, iron(III) perchlorate n-hydrate (n=1,6) and iron(III) tetrafluoroborate; copper(II) salts of inorganic acids such as copper(II) chloride, copper(II) sulfate and copper(II) tetrafluoroborate; nitrosonium tetrafluoroborate; persulfates such as ammonium persulfate, sodium persulfate and potassium persulfate; periodates such as potassium periodate; hydrogen peroxide, ozone, potassium hexacyanoferrate(III), tetraammonium cerium(IV) sulfate dihydrate, bromine and iodine; and iron(III) salts of organic acids such as iron(III) p-toluenesulfonate. Among these, iron(III) salts of inorganic acids or organic acids, or persulfates are preferable, and ammonium persulfate or iron(III) p-toluenesulfonate is more preferable. In particular, iron(III) p-toluenesulfonate is more preferable because it also functions as the organic acid dopant. The oxidant (O1) may be used alone or in combinations of two or more thereof.
The amount of the oxidant (O1) used is not particularly limited because the oxidant (O1) can be removed in the step (b) even when it is added in an excessive amount. However, for the purpose of obtaining the polymer having a high conductivity by allowing the reaction to occur in a more moderate oxidative atmosphere, the amount of the oxidant (O1) used is preferably 0.5 to 100 parts by weight and more preferably 1 to 40 parts by weight, in relation to 1 part by weight of the monomer (M1).
The step (a) can also be conducted in the presence of a surfactant. The solubility of the monomer (M1) in water is low. Accordingly, when water is used as the solvent, the use of a surfactant enables to improve the dispersibility of the monomer (M1). The surfactant may be an anionic surfactant, a cationic surfactant, an amphoteric surfactant or a nonionic surfactant. Dodecylbenzenesulfonic acid or polyethylene glycol is preferable. The surfactant may be used alone or in combinations of two or more thereof.
The amount of the surfactant used is not particularly limited because the surfactant can be removed in the step (b) even when it is added in an excessive amount. However, the amount of the surfactant used is preferably 0.01 to 10 parts by weight and more preferably 0.1 to 5 parts by weight, in relation to 1 part by weight of the monomer (M1).
The conductive polymer (P1) obtained by chemical oxidative polymerization of the monomer (M1) has the structural unit derived from the monomer (M1). For example, when 3,4-ethylenedioxythiophene represented by the formula (1) is used as the monomer (M1), the obtained conductive polymer (P1) has the structural unit represented by the following formula (2).




It is preferable to conduct the chemical oxidative polymerization under stirring. The reaction temperature of the chemical oxidative polymerization is not particularly limited; however, the chemical oxidative polymerization is conducted with the upper limit of the reaction temperature set at the reflux temperature of the solvent used, and the reaction temperature is preferably 0 to 100° C. and more preferably 10 to 50° C. When the reaction temperature is inappropriate, the conductivity of the obtained conductive polymer (P1) is sometimes degraded. The reaction time of the chemical oxidative polymerization depends on the type and the amount of the oxidant (O1) used, the reaction temperature, the stirring conditions and the like; the reaction time is preferably about 5 to 100 hours. It is to be noted that when the conductive polymer (P1) is produced, the reaction liquid turns deep blue.
Next, the conductive polymer (P1) is purified (step (b)). Specifically, from the reaction solution containing the conductive polymer (P1) having been obtained by chemical oxidative polymerization, the conductive polymer (P1) is separated and washed to remove the dopant, the monomer (M1), the oxidant (O1) and the oxidant after having undergone the reaction. The step (b) enables to obtain the conductive polymer (P1) having a high purity.
Examples of the method for separating the conductive polymer (P1) from the reaction solution include a filtration method and a centrifugal separation method.
The washing with a solvent is preferably conducted by using a solvent capable of dissolving the monomer (M1) and/or the oxidant (O1) without dissolving the conductive polymer (P1). Specific examples of the washing solvent include water and alcohol solvents such as methanol, ethanol and propanol. The washing solvent may be used alone or in combinations of two or more thereof. The extent of the washing can be checked by pH measurement or calorimetric observation of the washing solvent having been used for washing.
Further, it is preferable that the conductive polymer (P1) is washed with a hot water and/or heat treated because the metal component derived from the oxidant (O1) can be removed to a higher extent. The heat treatment temperature is not particularly limited as long as it is equal to or lower than the decomposition temperature of the conductive polymer (P1); the heat treatment is preferably conducted at a temperature lower than 300° C. An ion exchange treatment conducted with an ion exchange resin is also effective as a method for removing the metal component derived from the oxidant.
The impurities contained in the conductive polymer (P1) can be quantitatively determined by ICP luminescence analysis, ion chromatography or the like.
Next, the purified conductive polymer (P1) and an oxidant (O2) are mixed in an aqueous solvent containing a polyacid component to obtain the conductive polymer suspension (step (c)). In the step (c), the polyacid component also acts as a dispersant, and hence a conductive polymer suspension satisfactory in dispersion can be obtained. The dispersion mechanism probably involves at least the doping effect of the polyanion derived from the polyacid component.
As the aqueous solvent, water is preferable, and a mixed solvent composed of water and a water-soluble organic solvent may also be adopted. Specific examples of the water-soluble organic solvent include: protic polar solvents such as methanol, ethanol, propanol and acetic acid; and aprotic polar solvents such as N,N-dimethylformamide, dimethyl sulfoxide, acetonitrile and acetone.
The concentration of the conductive polymer (P1) in the aqueous solvent is preferably 0.1 to 20% by weight and more preferably 0.5 to 10% by weight.
As the polyacid component, a polyacid or a salt thereof can be used. Specific examples of the polyacid include: polycarboxylic acids such as polyacrylic acid, polymethacrylic acid and polymaleic acid; polysulfonic acids such as polyvinylsulfonic acid, poly(2-acrylamido-2-methylpropanesulfonic acid) and polystyrenesulfonic acid; and copolymers having these structural units. Specific examples of the salts of the polyacids include lithium salts, sodium salts, potassium salts and ammonium salts of polyacids. Among these, polystyrenesulfonic acid having the structural unit represented by the following formula (3) is preferable. The polyacid component may be used alone or in combinations of two or more thereof.




For the purpose of obtaining the conductive polymer (P1) having a high conductivity, the weight average molecular weight of the polyacid component is preferably 2,000 to 500,000 and more preferably 10,000 to 200,000.
For the purpose of obtaining the conductive polymer (P1) having a high conductivity, the amount of the polyacid component used is preferably 20 to 3,000 parts by weight and more preferably 30 to 1,000 parts by weight, in relation to 100 parts by weight of the conductive polymer (P1).
As the oxidant (O2), the same oxidant as the oxidant (O1) can be used. Among them, ammonium persulfate, hydrogen peroxide or the like is preferable. For the purpose of obtaining the conductive polymer (P1) having a high conductivity, the amount of the oxidant (O2) used is preferably 0.5 to 50 parts by weight and more preferably 1 to 30 parts by weight, in relation to 1 part by weight of the conductive polymer (P1).
The temperature of the step (c) is not particularly limited; however, it is preferably within a range from 0° C. to 100° C. and more preferably within a range from 10° C. to 50° C. The time of the step (c) is not particularly limited; however, it is about 5 to 100 hours.
It is preferable to apply the above-described ion exchange treatment after the step (c).
It is preferable to further include a step (step (d)) of mixing at least one selected from erythritol and pentaerythritol during or after the step (c). The step (d) allows erythritol or pentaerythritol to interact with the polyacid component (undoped dopant anion (resistive component)) which is located in the vicinity of the conductive polymer (P1) in the conductive polymer suspension. The interaction reduces the resistance between the conductive polymer (P1) particles and at the same time increases the density of the conductive polymer (P1), and hence the conductive polymer (P1) is enabled to have a further higher conductivity.
Erythritol is higher in crystallinity than the polyhydric alcohols such as sorbitol and maltitol and accordingly low in hygroscopicity and hence is preferable from the viewpoint of easiness in handling. Erythritol is also known as a food additive used as a sweetener, is excellent in safety and stability, and is several-fold higher in solubility in water than in nonaqueous solvents such as ethylene glycol and glycerin, and hence erythritol provides an advantage in the degree of freedom in designing the addition amount thereof.
Pentaerythritol is characterized by being slowly sublimed when heated, and by undergoing dehydration and polymerization when heated at a temperature equal to or higher than the melting point thereof. Thus, pentaerythritol has an advantage that the physical properties of an organic material are accordingly altered to improve the density and the strength thereof. Such reactivity originates from the chemical structure of pentaerythritol, and hardly results from such chemical structure as that of erythritol or sorbitol.
The mixing of erythritol or pentaerythritol in a concentration equal to or higher than the concentration of the conductive polymer (P1) in the conductive polymer suspension attains a more marked effect. It is to be noted that the upper limit of the mixing amount is not particularly limited as long as it is dissolved completely in the conductive polymer suspension.
A resin that functions as binding action may be added to the obtained conductive polymer suspension. Specific examples of the resin include polyester resin, polyamide resin, polyimide resin, polyether resin and polystyrene resin. From the viewpoint of not impairing the conductivity, the amount of the resin added is preferably 0.01 to 20 parts by weight in relation to 100 parts by weight of the conductive polymer suspension.
The conductive polymer suspension of the present invention usually shows deep blue color.
By removing the solvent from the conductive polymer suspension of the present invention, a conductive polymer material can be obtained. The conductive polymer material has a high conductivity. It is to be noted that the conductive polymer (P1) is high in the degree of crystallization and disperses light and hence the conductive polymer material has no transparency and show a color close to black.
The removal of the solvent can be conducted by drying the conductive polymer. The drying temperature is not particularly limited as long as the drying temperature is equal to or lower than the decomposition temperature of the conductive polymer; however the drying temperature is preferably 300° C. or lower.
The conductive polymer suspension of the present invention can be used as an electrolyte of an electrolytic capacitor. The conductive polymer material obtained by removing the solvent from the conductive polymer suspension of the present invention can be used as the solid electrolyte layer of a solid electrolytic capacitor. The high conductivity of the conductive polymer (P1) contained in the conductive polymer suspension or of the conductive polymer material obtained by removing the solvent from the conductive polymer suspension enables to obtain a capacitor having a low ESR. Further, the degree of crystallization of the conductive polymer (P1) is high and hence the oxygen barrier property is also correlatively high, and thus the improvement of the reliability of the capacitor is also sufficiently promising.
FIG. 1 shows a schematic sectional view illustrating the structure of the solid electrolytic capacitor according to an embodiment of the present invention. The solid electrolytic capacitor has a structure in which dielectric layer 2, solid electrolyte layer 3 and cathode conductor 4 are formed in this order on anode conductor 1.
Anode conductor 1 is formed of: a plate, a foil or a wire of a valve action metal; a sintered body made of fine particles of a valve action metal; a porous metal subjected to a surface area enlargement treatment by etching; or the like. Specific examples of the valve action metal include tantalum, aluminum, titanium, niobium and zirconium, and alloys of these. Among these, at least one valve action metal selected from aluminum, tantalum and niobium is preferable.
Dielectric layer 2 is a layer that can be formed by the electrolytic oxidation of the surface of anode conductor 1, and is also formed in the vacancy portions of a sintered body or a porous body. The thickness of dielectric layer 2 can be appropriately adjusted by the voltage of the electrolytic oxidation.
Solid electrolyte layer 3 includes at least the conductive polymer material obtained by removing the solvent from the above-described conductive polymer suspension. Examples of the method for forming solid electrolyte layer 3 include a method in which the above-described conductive polymer suspension is applied or impregnated onto dielectric layer 2, and the solvent is removed from the conductive polymer suspension.
The application or impregnation method is not particularly limited. However, for the purpose of sufficiently filling the conductive polymer suspension in the interior of the pores of the porous material, dielectric layer 2 is preferably allowed to stand for a few minutes to a few tens minutes after application or impregnation. A repeated immersion method, a pressure reduction method or a pressurization method is preferable.
The removal of the solvent from the conductive polymer suspension can be conducted by drying the conductive polymer. The drying temperature is not particularly limited as long as it is within a temperature range in which the solvent removal is possible; however, from the viewpoint of the prevention of element degradation by heat, the upper limit of the drying temperature is preferably lower than 300° C. It is necessary to appropriately optimize the drying time according to the drying temperature; the drying time is not particularly limited as long as it is within a range in which the conductivity is not impaired.
Further, solid electrolyte layer 3 may also include: conductive polymers formed of pyrrole, thiophene, aniline and derivatives of these; oxide derivatives such as manganese dioxide and ruthenium oxide; and organic semiconductors such as TCNQ (7,7,8,8-tetracyanoquinodimethane complex salt).
For example, solid electrolyte layer 3 may also be designed to have a two-layer structure formed of first solid electrolyte layer 3a and second solid electrolyte layer 3b. First solid electrolyte layer 3a including a conductive polymer (P2) may be formed on dielectric layer 2 by chemical oxidative polymerization or electrolytic polymerization of a monomer (M2) giving the conductive polymer. Second solid electrolyte layer 3b is formed by applying or impregnating the above-described conductive polymer suspension onto first solid electrolyte layer 3a and by removing the solvent from the conductive polymer suspension.
As the monomer (M3), at least one selected from pyrrole, thiophene, aniline and derivatives of these can be used. As the dopant used when the conductive polymer (P2) is obtained by chemical oxidative polymerization or electrolytic polymerization of the monomer (M2), sulfonic acid compounds such as alkylsulfonic acid, benzenesulfonic acid, naphthalenesulfonic acid, anthraquinonesulfonic acid, camphorsulfonic acid and derivatives of these are preferable. With respect to the molecular weight of the dopant, dopants ranging from low molecular weight compounds to polymers can be used in an appropriately selected manner. The solvent may be only water or a mixed solvent composed of water and a water-soluble organic solvent.
The conductive polymer (P1) included in first solid electrolyte layer 3a and the conductive polymer (P2) included in second solid electrolyte layer 3b are preferably the same polymer.
Cathode conductor 4 is not particularly limited as long as cathode conductor 4 is a conductor; however, cathode conductor 4 may also be designed to have a two-layer structure formed of carbon layer 4a made of graphite or the like and silver conductive resin layer 4b. 
EXAMPLES
Hereinafter, the present invention is described more specifically with reference to Examples; however, the present invention is not limited only to these Examples.
Example 1
<Step (a)>
In 30 ml of ethanol as the solvent, 1 g of 3,4-ethylenedioxythiophene as the monomer (M1), 1 g of camphorsulfonic acid as the dopant and 9 g of iron (III) p-toluenesulfonate functioning as the oxidant (O1) and the dopant were dissolved. The obtained solution was stirred at room temperature for 24 hours to conduct the chemical oxidative polymerization, and thus poly(3,4-ethylenedioxythiophene) was synthesized. In this case, the solution turned from yellow to deep blue.
<Step (b)>
The obtained solution was filtered by using a reduced pressure filtration apparatus to collect a powder. The powder was washed with purified water to remove the excessive oxidant (O1) and dopant. The washing with purified water was repeated until the acidity of the filtrate came to be pH 6 to 7. Thereafter, the powder was washed with ethanol to remove the monomer (M1), the oxidant (O1) and the oxidant (iron(II) p-toluenesulfonate) after having undergone the reaction were removed. The washing with ethanol was conducted until the filtrate came to be colorless and transparent.
<Step (c)>
In 50 ml of water, 0.5 g of the purified powder was dispersed, and thereafter 3.3 g of an aqueous solution containing 20% by weight of polythiophenesulfonic acid (weight average molecular weight: 50,000) as the polyacid component was added. To the mixed solution, 1.5 g of ammonium persulfate as the oxidant (O2) was added and stirred at room temperature for 24 hours. The polythiophene suspension thus obtained was deep navy blue.
The obtained polythiophene suspension was dropped onto a glass substrate in an amount of 100 μl, the substrate was dried in a thermostatic chamber at 125° C. to form a conductive polymer film, and the surface resistance (Ω/□) and the film thickness of the conductive polymer film were measured by the four-terminal method, and the conductivity (S/cm) of the conductive polymer film was derived. The result thus obtained is shown in Table 1. Additionally, for the purpose of evaluating the crystallinity of the formed conductive polymer film, the X-ray diffraction of the conductive polymer film was measured, wherein the measurement was conducted by scanning the 2θ range from 5 to 40°. The measurement result thus obtained is shown in FIG. 2. Further, a portion of the formed conductive polymer film was sampled, and the glass transition temperature of the conductive polymer film was measured by using a differential scanning calorimeter (DSC). The result thus obtained is shown in Table 2.
Example 2
A polythiophene suspension was produced by operating in the same manner as in Example 1 except that a polystyrenesulfonic acid having a weight average molecular weight of 2,000 was used as the polyacid component. Then, a conductive polymer film was formed in the same manner as in Example 1, and thereafter the conductivity of the conductive polymer film was derived. The result thus obtained is shown in Table 1.
Example 3
A polythiophene suspension was produced by operating in the same manner as in Example 1 except that a polystyrenesulfonic acid having a weight average molecular weight of 14,000 was used as the polyacid component. Then, a conductive polymer film was formed in the same manner as in Example 1, and thereafter the conductivity of the conductive polymer film was derived. The result thus obtained is shown in Table 1.
Example 4
A polythiophene suspension was produced by operating in the same manner as in Example 1 except that a polystyrenesulfonic acid having a weight average molecular weight of 500,000 was used as the polyacid component. Then, a conductive polymer film was formed in the same manner as in Example 1, and thereafter the conductivity of the conductive polymer film was derived. The result thus obtained is shown in Table 1.
Example 5
A polythiophene suspension was produced by operating in the same manner as in Example 1 except that in the step (b) the obtained powder was washed with purified water and ethanol, and then further washed with boiling hot purified water. Then, a conductive polymer film was formed in the same manner as in Example 1, and thereafter the conductivity of the conductive polymer film was derived. The result thus obtained is shown in Table 1.
Example 6
A polythiophene suspension was produced by operating in the same manner as in Example 1 except that in the step (b) the obtained powder was washed with purified water and ethanol, and then further heat treated in a thermostatic chamber set at 125° C. Then, a conductive polymer film was formed in the same manner as in Example 1, and thereafter the conductivity of the conductive polymer film was derived. The result thus obtained is shown in Table 1.
Example 7
<Step (a)>
In 100 ml of water as the solvent, 1 g of 3,4-ethylenedioxythiophene as the monomer (M1) was dispersed by using 2.3 g of dodecylbenzenesulfonic acid functioning as a dopant and a surfactant, and stirred at room temperature for 1 hour to be sufficiently dispersed. Thereafter, 2.4 g of ammonium persulfate was added as the oxidant (O1) to the dispersion. The obtained dispersion was stirred at room temperature for 100 hours to conduct the chemical oxidative polymerization. In this case, the dispersion turned from yellow to deep blue.
<Step (b)>
From the obtained dispersion, a powder was collected by using a centrifugal separator (5,000 rpm). The powder was washed by decantation in the centrifugal separator by using purified water to remove the excessive oxidant (O1) and dopant. The washing with purified water was repeated until the acidity of the supernatant liquid came to be pH 6 to 7.
A polythiophene suspension was produced by performing the step (c) in the same manner as in Example 1. Then, a conductive polymer film was formed in the same manner as in Example 1, and thereafter the conductivity of the conductive polymer film was derived. The result thus obtained is shown in Table 1.
Example 8
<Step (a)>
In 100 ml of water as the solvent, 1 g of 3,4-ethylenedioxythiophene as the monomer (M1) and 1 g of camphorsulfonic acid as the dopant were dispersed by using 2 g of polyethylene glycol (weight average molecular weight: 4,000) functioning as a surfactant. The mixture thus obtained was stirred to be sufficiently dispersed at room temperature for 1 hour, and then 2.4 g of ammonium persulfate as the oxidant (O1) was added to the obtained dispersion. The dispersion thus obtained was stirred at room temperature for 100 hours to conduct the chemical oxidative polymerization. In this case, the dispersion turned from yellow to deep blue.
A polythiophene suspension was produced by performing the step (b) and the step (c) in the same manner as in Example 7. Then, a conductive polymer film was formed in the same manner as in Example 1, and thereafter the conductivity of the conductive polymer film was derived. The result thus obtained is shown in Table 1.
Example 9
In 10 g of the polythiophene suspension obtained in Example 1, 1 g of erythritol was further dissolved at room temperature to produce a polythiophene suspension. Then, a conductive polymer film was formed in the same manner as in Example 1, and thereafter the conductivity of the conductive polymer film was derived. The result thus obtained is shown in Table 1.
Example 10
In 10 g of the polythiophene suspension obtained in Example 1, 1 g of pentaerythritol was further dissolved at room temperature to produce a polythiophene suspension. Then, a conductive polymer film was formed in the manner described in Example 1, and thereafter the conductivity of the conductive polymer film was derived. The result thus obtained is shown in Table 1.
Example 11
A porous aluminum was used as the anode conductor consisting of a valve action metal and an oxide coating film to be a dielectric layer was formed on the surface of the aluminum by anodic oxidation. Next, the anode conductor with the dielectric layer formed thereon was immersed in and taken out from the polythiophene suspension produced in Example 1, and then dried and solidified at 125° C. to form a solid electrolyte layer. On the solid electrolyte layer, a graphite layer and a silver-containing resin layer were formed in this order to obtain a solid electrolytic capacitor.
The ESR (equivalent series resistance) of the obtained solid electrolytic capacitor was measured by using an LCR meter at a frequency of 100 kHz. The ESR value was normalized from the value corresponding to the total cathode area to the value corresponding to the unit area (1 cm2). The result thus obtained is shown in Table 3.
Example 12
A porous aluminum was used as the anode conductor consisting of a valve action metal and an oxide coating film was formed on the surface of the aluminum metal by anodic oxidation. Next, the anode conductor with the dielectric layer formed thereon was immersed in and taken out from a monomer solution prepared by dissolving 10 g of pyrrole as the monomer (M2) in 200 ml of purified water and immersed in and taken out from an oxidant solution prepared by dissolving 20 g of p-toluenesulfonic acid as the dopant and 10 g of ammonium persulfate as the oxidant in 200 ml of purified water, in this order and repeatedly 10 times, to conduct the chemical oxidative polymerization, and thus the first solid electrolyte layer was formed.
The polythiophene suspension produced in Example 1 was dropped onto the first solid electrolyte layer, and dried and solidified at 125° C. to form the second solid electrolyte layer. On the second solid electrolyte layer, a graphite layer and a silver-containing resin layer were formed in this order to obtain a solid electrolytic capacitor.
The ESR (equivalent series resistance) of the obtained solid electrolytic capacitor was measured in the same manner as in Example 11. The result thus obtained is shown in Table 3.
Example 13
A solid electrolytic capacitor was produced in the same manner as in Example 12 except that the polythiophene suspension produced in Example 9 was used, and the ESR (equivalent series resistance) of the solid electrolytic capacitor was measured. The result thus obtained is shown in Table 3.
Example 14
A solid electrolytic capacitor was produced by operating in the same manner as in Example 11 except that a porous tantalum was used as the anode conductor consisting of a valve action metal, and the ESR (equivalent series resistance) of the solid electrolytic capacitor was measured. The result thus obtained is shown in Table 3.
Comparative Example 1
In 20 ml of water, 2 g of polystyrenesulfonic acid (weight average molecular weight: 4,000), 0.5 g of 3,4-ethylenedioxythiophene and 0.05 g of iron(III) sulfate were dissolved, and air was introduced into the obtained solution over a period of 24 hours to produce a polythiophene solution. Then, a conductive polymer film was formed in the same manner as in Example 1, and thereafter the conductivity of the conductive polymer film was derived. The result thus obtained is shown in Table 1.
Comparative Example 2
A polythiophene solution was produced by operating in the same manner as in Comparative Example 1 except that a polystyrenesulfonic acid having a weight average molecular weight of 50,000 was used. Then, a conductive polymer film was formed in the same manner as in Example 1, and thereafter the derivation of the conductivity, the measurement of the X-ray diffraction and the measurement of the glass transition temperature of the conductive polymer film were conducted. The results thus obtained are shown in Table 1, FIG. 2 and Table 2, respectively.
Comparative Example 3
A solid electrolytic capacitor was produced by operating in the same manner as in Example 12 except that the polythiophene solution produced in Comparative Example 2 was used, and the ESR (equivalent series resistance) of the solid electrolytic capacitor was measured. The result thus obtained is shown in Table 3.









TABLE 1







Conductivity (S/cm)



















Example 1
240



Example 2
140



Example 3
210



Example 4
165



Example 5
255



Example 6
260



Example 7
190



Example 8
215



Example 9
370



Example 10
315



Comparative Example 1
105



Comparative Example 2
110



















TABLE 2







Glass transition



temperature (° C.)



















Example 1
271.5



Comparative Example 2
251.0



















TABLE 3







ESR (mΩ · cm2)



















Example 11
2.0



Example 12
1.8



Example 13
1.4



Example 14
2.1



Comparative Example 3
3.2










As shown in Table 1, the conductive polymer films formed in Examples 1 to 10 were all higher in conductivity than the conductive polymer films formed in Comparative Examples 1 and 2. In this way, by passing through the steps (a) to (c), (1) the choice of the dopant is made wider, (2) a dopant capable of enhancing the degree of crystallization can be selected, (3) a solvent composition having a high compatibility with the monomer can be selected to lead to a high degree of polymerization, and (4) washing is easily conducted and hence a high purity can be attained.
As shown in Table 2, it was found that the conductive polymer film formed in Example 1 was higher in glass transition temperature and also higher in the degree of polymerization than the conductive polymer film formed in Comparative Example 2.
Additionally, in each of Examples 5 and 6 where poly(3,4-ethylenedioxythiophene) was washed with a hot water and/or heat treated in the step (b), the conductivity was found to be made higher. This is probably because the hot water used enabled the unnecessary components to be higher in solubility or the applied heating enabled the volatile components to be removed, and hence a further higher purity level was able to be attained.
Additionally, in each of Examples 9 and 10 in which the step (d) was conducted after the step (c), the conductivity was found to be made higher. This is probably because the interaction with the polystyrenesulfonic acid which is located in the vicinity of the particles of the poly(3,4-ethylenedioxythiophene) in the polythiophene suspension reduced the resistance between the particles of the poly(3,4-ethylenedioxythiophene) and at the same time increased the density of the poly(3,4-ethylenedioxythiophene).
From the measurement results of the X-ray diffraction shown in FIG. 1, it was able to be verified that the crystallinity of the conductive polymer film formed in Example 1 was higher than that of the conductive polymer film formed in Comparative Example 2. Consequently, the poly(3,4-ethylenedioxythiophene) in the obtained polythiophene solution is satisfactory in the electron conduction between the polymer chains and hence comes to be highly conductive. It is to be noted that the conductive polymer film formed in Example 1 was high in the degree of crystallization so as to disperse light, and hence had no transparency to show a color close to black.
Additionally, as shown in Table 3, the solid electrolytic capacitors produced in Examples 11 to 14 were allowed to reduce the resistance of the solid electrolyte due to the high conductivity of the poly(3,4-ethylenedioxythiophene) and hence allowed to reduce the resistance (ESR) of the solid electrolytic capacitor.

## Claims

1. A method for producing a conductive polymer suspension, comprising:
(a) synthesizing a conductive polymer (P1) by chemical oxidative polymerization of a monomer (M1) giving the conductive polymer by using an oxidant (O1) in a solvent containing a dopant consisting of a low-molecular organic acid or a salt thereof;
(b) purifying the conductive polymer (P1); and
(c) mixing the purified conductive polymer (P1) and an oxidant (O2) in an aqueous solvent containing a polyacid component to obtain the conductive polymer suspension.

2. The method for producing a conductive polymer suspension according to claim 1, wherein the monomer (M1) is at least one selected from pyrrole, thiophene, aniline and derivatives of these.

3. The method for producing a conductive polymer suspension according to claim 2, wherein the monomer (M1) is 3,4-ethylenedioxythiophene.

4. The method for producing a conductive polymer suspension according to claim 1, wherein the dopant is at least one selected from benzenesulfonic acid, naphthalenesulfonic acid, camphorsulfonic acid and derivatives of these, and salts of these.

5. The method for producing a conductive polymer suspension according to claim 1, wherein the step (a) is conducted in the presence of a surfactant.

6. The method for producing a conductive polymer suspension according to claim 1, wherein in the step (b) the conductive polymer (P1) is washed by using a solvent capable of dissolving the monomer (M1) and/or the oxidant (O1).

7. The method for producing a conductive polymer suspension according to claim 6, wherein in the step (b) the conductive polymer (P1) is further washed by a hot water and/or heat treated.

8. The method for producing a conductive polymer suspension according to claim 1, wherein the polyacid component is a polystyrenesulfonic acid.

9. The method for producing a conductive polymer suspension according to claim 8, wherein a weight average molecular weight of the polystyrenesulfonic acid is 2,000 to 500,000.

10. The method for producing a conductive polymer suspension according to claim 1, further comprising:
(d) mixing at least one selected from erythritol and pentaerythritol.

11. A conductive polymer suspension obtained by the method according to claim 1.

12. A conductive polymer material obtained by removing the solvent from the conductive polymer suspension according to claim 11.

13. A solid electrolytic capacitor comprising a solid electrolyte layer including the conductive polymer material according to claim 12.

14. The solid electrolytic capacitor according to claim 13, comprising an anode conductor consisting of a valve action metal and a dielectric layer formed on the surface of the anode conductor, wherein the solid electrolyte layer is formed on the dielectric layer.

15. The solid electrolytic capacitor according to claim 14, wherein the valve action metal is at least one selected from aluminum, tantalum and niobium.

16. An electrolytic capacitor comprising, as an electrolyte, the conductive polymer suspension according to claim 11.

17. A method for producing a solid electrolytic capacitor, comprising:
forming a dielectric layer on the surface of an anode conductor consisting of a valve action metal; and
forming a solid electrolyte layer comprising the conductive polymer material by applying or impregnating the conductive polymer suspension according to claim 11 onto the dielectric layer and by removing the solvent from the conductive polymer suspension.

18. The method for producing a solid electrolytic capacitor according to claim 17, wherein the valve action metal is at least one selected from aluminum, tantalum and niobium.

19. A method for producing a solid electrolytic capacitor, comprising:
forming a dielectric layer on the surface of an anode conductor consisting of a valve action metal;
forming a first solid electrolyte layer comprising a conductive polymer (P2) by chemical oxidative polymerization or electrolytic polymerization of a monomer (M2) giving the conductive polymer on the dielectric layer; and
forming a second solid electrolyte layer by applying or impregnating the conductive polymer suspension according to claim 11 onto the first solid electrolyte layer and by removing the solvent from the conductive polymer suspension.

20. The method for producing a solid electrolytic capacitor according to claim 19, wherein the conductive polymer (P2) is a polymer obtained by chemical oxidative polymerization or electrolytic polymerization of at least one selected as the monomer (M2) from pyrrole, thiophene, aniline and derivatives of these.

21. The method for producing a solid electrolytic capacitor according to claim 19, wherein the valve action metal is at least one selected from aluminum, tantalum and niobium.