JP2021163862A - Solid electrolytic capacitor - Google Patents

Abstract

To provide a solid electrolyte capacitor with improved characteristics of high capacitance (Cap) and low ESR.SOLUTION: A solid electrolytic capacitor includes a capacitor element that faces an anode foil and a cathode foil via a separator, and a solid electrolyte layer containing a conductive polymer formed in the capacitor element. The ratio of the loading amount of the solid electrolyte layer per unit area of an electrode foil including an anode foil and a cathode foil to the loading amount of the solid electrolyte layer per unit area of the separator is 1.5:1 to 0.5:1.SELECTED DRAWING: None

Description

The present invention relates to a solid electrolytic capacitor.

An electrolytic capacitor using a metal having a valve action such as tantalum or aluminum can expand the dielectric surface by forming the valve action metal as the anode side counter electrode into a sintered body or an etching foil or the like. .. As a result, it is widely used because it can obtain a small size and a large capacity. In particular, a solid electrolytic capacitor using a solid electrolyte as an electrolyte has a small size, a large capacity, and a low equivalent series resistance (ESR). Since such solid electrolytic capacitors are easy to chip and have characteristics such as being suitable for surface mounting, they are indispensable for miniaturization, high functionality, and cost reduction of electronic devices. ..

Manganese dioxide and 7,7,8,8-tetracyanoquinodimethane (TCNQ) complexes are known as solid electrolytes used in solid electrolytic capacitors. Further, in recent years, a technique focusing on a conductive polymer such as polyethylene dioxythiophene (hereinafter referred to as PEDOT) having a slow reaction rate and excellent adhesion to an oxide film layer of an anode electrode has been developed (Patent Document 1). Existing.

Japanese Unexamined Patent Publication No. 2-15511

In recent years, solid electrolytic capacitors used for in-vehicle applications have been demanded to have high capacitance (Cap) and low ESR, especially in the frequency range of 10 to 50 kHz. For conductive polymers such as PEDOT, the characteristics of solid electrolytic capacitors improve as the amount added to the device increases up to a certain amount. However, when a certain amount or more of the conductive polymer is added, the characteristics are not significantly improved, and the product cost increases because a large amount of the conductive polymer is used. Therefore, since there are advantages in both performance and cost of the solid electrolytic capacitor, it has been desired to improve the characteristics of Cap and ESR by using a smaller amount of the conductive polymer.

The present invention has been proposed to solve the above problems. An object of the present invention is to provide a solid electrolytic capacitor having improved Cap and ESR characteristics.

The inventors of the present invention have obtained the finding that controlling the solid content distribution of the solid electrolyte layer in each member of the capacitor element within a specific range leads to improvement of the electrochemical characteristics of the solid electrolytic capacitor. The present invention has been completed.

That is, the solid electrolytic capacitor of the present invention includes a capacitor element formed so as to face the anode foil and the cathode foil via a separator, and a solid electrolyte layer containing a conductive polymer formed in the capacitor element. The ratio of the loading amount of the solid electrolyte layer per unit area of the anode foil and the electrode foil including the cathode foil to the loading amount of the solid electrolyte layer per unit area of the separator is 1.5: 1 to 0. .5: 1.

The ratio of the loading amount of the solid electrolyte layer per unit area of the anode foil to the loading amount of the solid electrolyte layer per unit area of the cathode foil may be 1: 1 to 1: 1.5.

According to the present invention, it is possible to provide a solid electrolytic capacitor having improved Cap and ESR characteristics.

It is an SEM image and an EDX image of an anode foil imaged by a scanning cross-section microscope. It is an SEM image and an EDX image of a cathode foil imaged by a scanning cross-section microscope.

Hereinafter, a typical manufacturing procedure for manufacturing the solid electrolytic capacitor according to the present invention will be disclosed. Further, in the description of each step, the configuration of the solid electrolytic capacitor will be specifically described.

(Manufacturing method of solid electrolytic capacitor)
An example of a method for manufacturing a solid electrolytic capacitor according to the present invention is as follows.
First step: An anode foil and a cathode foil having an oxide film layer formed on the surface are wound around a separator to form a capacitor element, and the capacitor element is repaired and formed.
Second step: The condenser element is impregnated with a conductive polymer dispersion in which conductive polymer particles are dispersed in a solvent and dried to form a solid electrolyte layer.
Third step: The capacitor element is immersed in a predetermined electrolytic solution, and the void portion in the capacitor element on which the solid electrolyte layer is formed is filled with the electrolytic solution.
Fourth step: The capacitor element is inserted into the outer case, a sealing rubber is attached to the end of the opening, and after sealing by crimping, aging is performed to form a solid electrolytic capacitor.

(1) First Step The following members are used for forming the capacitor element in the first step.
(Electrode foil)
The anode foil is made of a valve acting metal such as aluminum. Alternatively, tantalum, niobium, titanium or the like may be used as the anode material. The surface of the anode foil is roughened by an etching process to form a large number of etching pits. Further, on the surface of the anode foil, an oxide film layer which becomes a dielectric by applying a voltage in an aqueous solution of ammonium borate, ammonium phosphate, ammonium adipate or the like is formed.

As the cathode foil, one made of the same metal as the anode foil and whose surface is etched is used. If necessary, the cathode foil may be subjected to chemical conversion treatment. In addition, a layer made of metal nitride, metal carbide, and metal carbonitride can be formed on the cathode foil by a vapor deposition method. Further, carbon may be contained in the surface of the cathode foil.

(Separator)
As the separator, a separator made of a non-woven fabric mainly composed of synthetic fibers or a separator made of glass fibers can be used. As the synthetic fiber, polyester fiber, nylon fiber, rayon fiber and the like are suitable. Moreover, you may use the separator made of natural fiber.

(Chemical solution of restoration chemicals)
Examples of the chemical conversion solution for repairing and chemicalizing the condenser element include a phosphoric acid-based chemical conversion solution such as ammonium dihydrogen phosphate and diammonium hydrogen phosphate, a boric acid-based chemical conversion solution such as ammonium borate, and adipic acid such as ammonium adipate. A chemical conversion solution of the system can be used. The immersion time is preferably 5 to 120 minutes.

(2) Second step (conductive polymer dispersion)
As the conductive polymer, known ones can be used without particular limitation. For example, polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene, polyphenylene vinylene, polyacene, polythiophene vinylene and the like can be mentioned. These conductive polymers may be used alone, in combination of two or more, and may be a copolymer of two or more monomers.

Examples of the dopant include inorganic acids such as boric acid, nitrate and phosphoric acid, acetic acid, oxalic acid, citric acid, ascot acid, tartrate acid, squaric acid, logizonic acid, croconic acid, salicylic acid, p-toluenesulfonic acid, 1, Organic acids such as 2-dihydroxy-3,5-benzenedisulfonic acid, methanesulfonic acid, trifluoromethanesulfonic acid, bisoxalate borate acid, sulfonylimide acid, dodecylbenzenesulfonic acid, propylnaphthalenesulfonic acid, butylnaphthalenesulfonic acid Can be mentioned. Examples of the polyanion include polyvinyl sulfonic acid, polystyrene sulfonic acid (PSS), polyallyl sulfonic acid, polyacrylic sulfonic acid, polymethacrylic sulfonic acid, poly (2-acrylamide-2-methylpropanesulfonic acid), and polyisoprene sulfonic acid. , Polyacrylic acid, polymethacrylic acid, polymaleic acid and the like.

Among these, PSD-doped PEDOT (PEDOT / PSS) is preferable. Water is mainly used as the solvent for the conductive polymer dispersion. As the stock solution containing PEDOT powder and polystyrene sulfonic acid, it is preferable to use a solution containing 1 to 5% of solid content.

Further, in order to improve the impregnation property and the conductivity of the conductive polymer dispersion, various additives may be added to the conductive polymer dispersion or neutralization may be performed by adding a cation. In particular, when sorbitol or sorbitol and a polyhydric alcohol are used as additives, ESR can be reduced and deterioration of withstand voltage characteristics due to lead-free reflow or the like can be prevented.

(Impression of conductive polymer dispersion)
The time for impregnating the conductive polymer dispersion with the capacitor element is determined by the size of the capacitor element. For example, it is desirable that a capacitor element having a diameter of about 5 mm and a length of about 3 mm has a diameter of 5 mm or more, and a capacitor element having a diameter of about 9 mm and a length of about 5 mm has a length of 10 seconds or more, and impregnation is required for at least 5 seconds. Further, after impregnation in this way, it is preferable to hold the product in a reduced pressure state. The impregnation of the conductive polymer dispersion may be performed a plurality of times, if necessary.

(Drying of conductive polymer dispersion)
As a result of diligent studies, the inventors of the present invention can make the distribution of the amount of the PEDOT / PSS solid electrolyte layer mounted on each member of the capacitor element suitable for improving the electrochemical characteristics of the solid electrolytic capacitor. I got the finding.

Specifically, the ratio of the loading amount of the solid electrolyte layer per unit area of the electrode foil including the anode foil and the cathode foil to the loading amount of the solid electrolyte layer per unit area of the separator is 1.5: 1 to 0. It becomes .5: 1. That is, the solid electrolyte layer is mounted on both the separator and the electrode foil side. Until now, a larger amount of solid electrolyte layer was mounted on the separator than on the electrode foil side. However, the ESR characteristics and the Cap characteristics are improved by mounting more solid electrolyte layers on the electrode foil side as well.

Further, the ratio of the loading amount of the solid electrolyte layer per unit area of the anode foil to the loading amount of the solid electrolyte layer per unit area of the cathode foil is preferably 1: 1 to 1: 1.5. That is, the loading amount on the cathode foil side is equal to or greater than the loading amount on the anode foil side. By mounting in this way, the conductive path is surely formed, the resistance on the surface of the cathode foil is reduced, and the characteristics of Cap and ESR are improved.

Further, since the solid electrolyte layer is uniformly distributed in the electrode foil, the conductive path of the solid electrolyte layer formed in the electrode foil is satisfactorily formed, and the resistance on the surface of the electrode foil can be reduced. It leads to the improvement of the characteristics of.

(3) Third step (electrolyte solution)
If necessary, an electrolytic solution may be further used. As the solvent that can be used in the electrolytic solution, it is preferable to use a solvent having a boiling point of 120 ° C. or higher, which is the life test temperature. Examples of the solvent include lactone-based solvents such as γ-butyrolactone, polyhydric alcohols such as ethylene glycol, sulfone-based solvents such as sulfolane, and amide-based solvents such as N, N-dimethylformamide. Examples of the lactone system include γ-butyrolactone, γ-valerolactone, and δ-valerolactone. Polyhydric alcohols include ethylene glycol, diethylene glycol, dipropylene glycol, 1,2-propanediol, glycerin, 1,3-propanediol, 1,3-butanediol, 2-methyl-2,4-pentanediol and the like. Low molecular weight polyhydric alcohol is preferable.

Examples of the sulfone system include dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methyl sulfolane, 2,4-dimethyl sulfolane and the like. As the amide system, N-methylformamide, N, N-dimethylformamide, N-ethylformamide, N, N-diethylformamide, N-methylacetamide, N, N-dimethylacetamide, N-ethylacetamide, N, N- Examples thereof include diethylacetamide and hexamethylphosphoricamide. In particular, when a mixed solvent composed of a low molecular weight polyhydric alcohol such as ethylene glycol and γ-butyrolactone is used, the initial ESR characteristics are improved, and the high temperature characteristics are also improved.

That is, when a mixed solvent composed of ethylene glycol and γ-butyrolactone was used, the initial ESR was lowered and the capacitance was changed after a long period of use as compared with the case where a solvent containing no ethylene glycol was used. It has been found that the rate (ΔCap) is small. The reason is considered to be that ethylene glycol has an effect of promoting the elongation of the polymer chain of the conductive polymer, so that the conductivity is improved and the ESR is lowered. In addition, since a protonic solvent having a hydroxyl group such as ethylene glycol has a higher affinity with a separator, an electrode foil, and a conductive polymer than γ-butyrolactone or sulfolane, the electrolytic solution evaporates when an electrolytic capacitor is used. In the process of doing so, it is considered that the electric charge is easily transferred between the separator, the electrode foil, the conductive polymer and the electrolytic solution, and ΔCap becomes small. The amount of ethylene glycol added in the mixed solvent is preferably 5 wt% or more, more preferably 40 wt% or more, and most preferably 60 wt% or more.

Further, by adding a predetermined amount of γ-butyrolactone as the solvent of the electrolytic solution, the impregnation property of the electrolytic solution into the capacitor element can be improved. By using ethylene glycol with relatively high viscosity and γ-butyrolactone with low viscosity, the impregnation property of the capacitor element is improved, the initial characteristics and good characteristics after long-term use are maintained, and the charge / discharge characteristics at low temperature are maintained. Becomes good. The amount of γ-butyrolactone added in the mixed solvent is preferably 40 wt% or less.

Further, at least one solvent selected from sulfolane, 3-methylsulfolane, and 2,4-dimethylsulfolane may be used. Since these sulfolane-based solvents have a high boiling point, the evaporation of the electrolytic solution is suppressed and the high temperature characteristics are improved.

As the solute of the electrolytic solution, a salt of a composite compound of an organic acid and an inorganic acid is used. Examples of the salt include at least one ammonium salt, a quaternary ammonium salt, a quaternary amidinium salt, an amine salt and the like. Examples of the organic acid include phthalic acid, isophthalic acid, terephthalic acid, maleic acid, adipic acid, benzoic acid, toluic acid, enanthic acid, malonic acid, 1,6-decandicarboxylic acid, 1,7-octanedicarboxylic acid and azeline. Examples thereof include carboxylic acids such as acids, salicylic acids, oxalic acids and glycolic acids, and phenols. Examples of the inorganic acid include boric acid, phosphoric acid, phosphite, hypophosphoric acid, phosphoric acid ester, carbonic acid, silicic acid and the like. Examples of the composite compound of an organic acid and an inorganic acid include borodisalicylic acid, borodioxalic acid, and borodiglycolic acid.

Further, examples of at least one salt of the composite compound of the organic acid and the inorganic acid include an ammonium salt, a quaternary ammonium salt, a quaternized amidinium salt, an amine salt and the like. Examples of the quaternary ammonium ion of the quaternary ammonium salt include tetramethylammonium, triethylmethylammonium, and tetraethylammonium. Examples of the quaternized amidinium include ethyldimethylimidazolinium and tetramethylimidazolinium. Examples of amines in amine salts include primary amines, secondary amines, and tertiary amines. Primary amines include methylamine, ethylamine and propylamine, secondary amines include dimethylamine, diethylamine, ethylmethylamine and dibutylamine, and tertiary amines include trimethylamine, triethylamine, tributylamine and ethyldiisopropylamine. And so on.

Furthermore, as additives for the electrolytic solution, polyoxyethylene glycol, a complex compound of boric acid and polysaccharides (mannit, sorbit, etc.), a complex compound of boric acid and polyhydric alcohol, and a nitro compound (o-nitrobenzoic acid) , M-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, p-nitrophenol, etc.), phosphate esters and the like.

(Filling conditions for electrolytic solution)
When the above electrolytic solution is filled in the capacitor element, the filling amount is arbitrary as long as it can fill the gap in the capacitor element, but it is preferably 3 to 100% of the gap in the capacitor element.

(Action / effect)
As described above, the anode foil and the cathode foil include a condenser element formed in the condenser element so as to face the anode foil and the cathode foil via a separator, and a solid electrolyte layer containing a conductive polymer formed in the condenser element. The ESR characteristics are improved by the ratio of the loading amount of the solid electrolyte layer per unit area of the electrode foil to be loaded and the loading amount of the solid electrolyte layer per unit area of the separator being 1: 1 to 1: 1.5. do. Similarly, the ratio of the loading amount of the solid electrolyte layer per unit area of the anode foil to the loading amount of the solid electrolyte layer per unit area of the cathode foil is 1: 1 to 1: 1.5. And the characteristics of ESR are improved.

As described above, the load distribution of the solid electrolyte layer in each member of the capacitor element is controlled within a suitable range. Until now, more solid electrolyte layers were formed on the separator than on the electrode foil, and the characteristics of Cap and ESR were not satisfactory. However, by controlling the loading distribution of the solid electrolyte layer as described above, the characteristics of Cap and ESR are improved. Further, by mounting a larger amount of solid content on the cathode foil than on the anode foil, a conductive path is surely formed, and the characteristics of Cap and ESR are improved.

The present invention will be described in more detail based on examples. The present invention is not limited to the following examples.

(Example 1)
In Example 1, a roughened aluminum foil was used as the anode foil, and an oxide film layer was formed on the surface. As the cathode foil, a foil having an oxide film layer equivalent to 3 Vfs formed on the surface of a roughened aluminum foil was used. As the separator, a separator made of a Manila-based material was used. The anode foil and the cathode foil were wound around the separator to form a capacitor element having an element shape of 10φ × 16.5L. This capacitor element was impregnated with ethylene glycol mixed with a conductive polymer dispersion at a ratio of 60:40.

This capacitor element was inserted into a bottomed cylindrical outer case, a sealing rubber was attached to the end of the opening, and the capacitor element was sealed by crimping. After that, aging was performed by applying a voltage to form a solid electrolytic capacitor. The rated voltage of this solid electrolytic capacitor was 63 WV, and the rated capacity was 150 μF. The loading ratio of the solid electrolyte layer was 43.8% for the separator, 27.5% for the anode foil, and 28.7% for the cathode foil.

(Examples 2 and 3)
The method for manufacturing the solid electrolytic capacitor of Examples 2 and 3 is the same as that of the above-mentioned Example 1. However, the loading ratio of the solid electrolyte layer in Example 2 was 53.4% for the separator, 20.6% for the anode foil, and 26% for the cathode foil. The loading ratio of the solid electrolyte layer in Example 3 was 63.5% for the separator, 17.8% for the anode foil, and 18.6% for the cathode foil.

(Comparative Examples 1 to 3)
The method for manufacturing the solid electrolytic capacitor of Comparative Examples 1 to 3 is the same as that of Example 1 above. However, the loading ratio of the solid electrolyte layer of Comparative Example 1 was 27.7% for the separator, 48% for the anode foil, and 24.3% for the cathode foil. The loading ratio of the solid electrolyte layer of Comparative Example 2 was 84.1% for the separator, 12.1% for the anode foil, and 3.8% for the cathode foil. The loading ratio of the solid electrolyte layer of Comparative Example 3 was 85.1% for the separator, 10.3% for the anode foil, and 4.6% for the cathode foil.

<Examination of solid content loading distribution>
First, the cross sections of the electrode foils of the solid electrolytic capacitors of Example 2 and Comparative Examples 1 and 3 were observed. 1 (a) to 1 (c) are photographs of SEM images of the cross section of the anode foil taken by a scanning cross-section microscope. FIGS. 1 (d) to 1 (f) are EDX images of the results of energy dispersive X-ray analysis of the cross section of the anode foil by a scanning cross-section microscope. Similarly, FIGS. 2A to 2C are photographs of SEM images of a cross section of the cathode foil. 2 (d) to 2 (f) are EDX images of the results of energy dispersive X-ray analysis of the cross section of the cathode foil. The acceleration voltage at the time of imaging was 4 kV, and the observation magnification was 3000 times.

Examining FIGS. 1 (b) and 2 (b), it was found that the solid electrolyte layer was uniformly mounted on the etching pits in both the anode foil and the cathode foil of Example 2. On the other hand, when FIG. 1 (a) and FIG. 2 (a) are examined, it is observed that a space is generated in the etching pits in both the anode foil and the cathode foil of Comparative Example 3, and the solid electrolyte layer is unevenly mounted. rice field. Further, when FIGS. 1 (c) and 2 (c) were examined, in Comparative Example 1, it was observed that a space was formed in the etching pits, especially in the cathode foil, and the solid electrolyte layer was unevenly mounted.

Therefore, the distribution of the carbon loading amount was confirmed using the EDX image. In FIGS. 1 (d) to (f) and FIGS. 2 (d) to 2 (f), the white portion indicates the distribution of carbon.

From the EDX images of Comparative Example 3 of FIGS. 1 (d) and 2 (d), it was found that a large amount of carbon was concentrated on the surface layer side of both the anode foil and the cathode foil. Further, in Example 2 of FIGS. 1 (e) and 2 (e), although carbon was collected on the surface layer side in both the anode foil and the cathode foil, a uniform distribution was confirmed also on the residual core side. .. In the anode foil of Comparative Example 1 of FIG. 1 (f), carbon was distributed without being concentrated on the surface layer side, but in the cathode foil of Comparative Example 1 of FIG. 2 (f), a large amount of carbon was concentrated on the surface layer side. It was found that the particles were attached to the surface. From this result, it was confirmed that carbon was uniformly distributed in the etching pits in the anode foil and the cathode foil of Example 2. This indicates that the solid electrolyte layer is uniformly distributed in the electrode foil.

<Calculation of solid electrolyte layer loading ratio in device>
Table 1 shows the ratio of the loading amount of the solid electrolyte layer (also referred to as solid content) contained in each of the anode foil, the cathode foil, and the separator for the capacitor elements of Examples 1 to 3 and Comparative Examples 1 to 3. The amount of solid content loaded was measured by drying the produced capacitor element at 150 ° C. for 18 hours and measuring the amount of solid content in each member.

From Table 1, in the capacitor elements of Examples 1 to 3, the ratio of the amount of the solid electrolyte layer mounted per unit area of the electrode foil including the anode foil and the cathode foil to the amount of the solid electrolyte layer mounted per unit area of the separator. However, it was found to be 1.5: 1 to 0.5: 1. That is, in the capacitor elements of Examples 1 to 3, there is no large difference in the amount of solid content contained in the separator and the electrode foil, and the solid content is relatively uniformly mounted in each member of the capacitor element.

On the other hand, in Comparative Example 1, the solid content is concentrated on the electrode foil side. Further, in Comparative Examples 2 and 3, a large amount of solid content is mounted on the separator. From the above, it is shown that in the capacitor elements of Comparative Examples 1 to 3, the solid content is concentratedly mounted on some members of the capacitor element, and the solid content is not uniformly mounted on each member of the capacitor element.

Similarly, the amount of solids loaded in the electrode foil was also examined. As a result, in the capacitor elements of Examples 1 to 3, the ratio of the amount of the solid electrolyte layer mounted per unit area of the anode foil to the amount of the solid electrolyte layer mounted per unit area of the cathode foil is 1: 1 to 1: 1. It turned out to be 1.5. That is, it is clear that in the capacitor elements of Examples 1 to 3, there is no large difference in the amount of solid content contained in the anode foil and the cathode foil, and the solid content is relatively uniformly mounted in the electrode foil. It became. Further, it was clarified that the loading amount on the cathode foil side is equal to or larger than the loading amount on the anode foil side.

In Comparative Examples 1 to 3, it was clarified that the solid content of the anode foil was overwhelmingly large in all the capacitor elements, and the solid content was not uniformly mounted on the electrode foil. That is, in the capacitor elements of Comparative Examples 1 to 3, there was no capacitor element in which the amount of the cathode foil mounted was equal to or greater than the amount mounted on the anode foil side.

<Equivalent series resistance equivalent to the capacitance of solid electrolytic capacitors>
Capacitance (Cap) and equivalent series resistance (ESR) at 50 kHz were measured for the solid electrolytic capacitors of Examples 1-3 and Comparative Example 1-3. The results are shown in Table 2.

From Table 2, it was found that Examples 1 to 3 had a higher capacitance than Comparative Examples 1 to 3. It was considered that the improvement in capacity was due to the fact that the solid electrolyte layer was mounted on both the separator and the electrode foil side. In addition, Examples 1 to 3 had better ESR characteristics than Comparative Examples 1 to 3. It was considered that by mounting more solid electrolyte layers on the cathode foil side, a conductive path was surely formed, resistance on the surface of the cathode foil was reduced, and ESR characteristics and capacitance were improved.

Claims (2)

A capacitor element that faces the anode foil and the cathode foil via a separator,
A solid electrolyte layer containing a conductive polymer formed in the capacitor element, and the like.
The ratio of the loading amount of the solid electrolyte layer per unit area of the anode foil and the electrode foil including the cathode foil to the loading amount of the solid electrolyte layer per unit area of the separator is 1.5: 1 to 0. .5: 1 solid electrolytic capacitor. The first aspect of claim 1, wherein the ratio of the loading amount of the solid electrolyte layer per unit area of the anode foil to the loading amount of the solid electrolyte layer per unit area of the cathode foil is 1: 1 to 1: 1.5. Solid electrolytic capacitor.

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