WO2008039197A1

WO2008039197A1 - Solid electrolytic capacitor and method of manufacturing the same

Info

Publication number: WO2008039197A1
Application number: PCT/US2006/038025
Authority: WO
Inventors: Kaoru Ueno; Larry Dominey
Original assignee: Nippon Chemi-Con Corporation
Priority date: 2006-09-29
Filing date: 2006-09-29
Publication date: 2008-04-03
Also published as: WO2008039214A1; JP5170707B2; JP2010505271A

Abstract

To provide a solid electrolytic capacitor with a high voltage resistance property and a method of manufacturing the same, a solid electrolytic capacitor is formed including an electrolyte layer on an anode electrode formed of aluminum by a polymerization reaction in which a polymerizable monomer or a monomer solution is mixed with oxidant wherein a plane symmetric dimethylpyridine is included in the electrolyte layer.

Description

Specification Title of the Invention

Solid electrolytic capacitor and method of manufacturing the same Background of the Invention

1. Field of the Invention

The present invention relates to a solid electrolytic capacitor and a method of manufacturing the same and, more particularly to a solid electrolytic capacitor with a high voltage resistance property and a method of manufacturing the same.

2. Description of the Related Art Conventionally, an electrolytic capacitor utilizing metal with valve action, such as aluminum, has been commonly used since when metal with valve action as an anode electrode is made into the form of etching foil and the like to obtain a surface-roughened dielectric, a downsized electrolytic capacitor with a large capacitance is provided. Particularly, a solid electrolytic capacitor employing a solid electrolyte has good properties such as small size, a large capacitance, and low equivalent series resistance. In addition to these properties, ease of making into chips and suitability for surface mounting is important. As a result, the solid electrolytic capacitor is now indispensable for making electronic equipment smaller and more powerful. A conductive polymer with high electrical conductance, which is excellent in adhesion to an oxide film layer of an anode electrode, has been used as a solid electrolyte employed in a solid electrolytic capacitor. There have been known, for example, polyaniline, polythiophene, polyethylenedioxythiophene, and the like as the conductive polymers .

Polyethylenedioxythiophene (hereinafter referred to as PEDT) has received attention as a conductive polymer capable of achieving high voltage resistance because of high voltage resistance against the thickness of an oxide film. A capacitor employing PEDT uses chemical oxidative polymerization and is made as follows. A capacitor element, formed by winding anode electrode foils and cathode electrode foils via separators, is impregnated with EDT (ethylenedioxythiophene) and an oxidant solution. It is then heated to form a PEDT polymer layer between both electrodes to result in the formation of a solid electrolytic capacitor (Japanese Unexamined Patent Publication No. 9 (1997) -293639) . Although such a solid electrolytic capacitor as described above can be used for in-car application and inverter application, there is disclosed that to resolve the problem that working voltage increases from 20 WV to 35 WV, withstand voltage is increased by forming a link composed of a compound having a vinyl group and a boric acid compound in a capacitor element (Japanese Unexamined Patent Publication No. 2003-100560) . Summary of the Invention

However, even with such an art, the achievement of high voltage resistance is not good enough. Accordingly, one object of the invention of the application which is proposed in order to resolve such problems of the prior art as described above, is to provide a solid electrolytic capacitor with a high voltage resistance property and a method of manufacturing of the same.

Object of the Invention

To achieve said object, the solid electrolytic capacitor in accordance with the present invention is characterized in that a solid electrolytic capacitor is produced by forming an electrolyte layer on an anode electrode formed of aluminum by a polymerization reaction in which a polymerizable monomer or a monomer solution is mixed with an oxidant including plane symmetric dimethylpyridine in the electrolyte layer. The aluminum employed in the present invention includes aluminum alloy.

The polymerizable monomer employed in the present invention includes, for example, ethylenedioxythiophene (EDT) , chemical oxidative polymerization of which forms polyethylenedioxythiophene (PEDT) . Here, the use of 3, 4-ethylenedioxythiophene as a polymerizable monomer forms poly (3, 4-ethylenedioxythiophene) as the electrolyte layer. Furthermore, iron (III) p-toluenesulfonate dissolved in a 1-butanol solution is preferably used as the oxidant.

To the oxidant, plane symmetric dimethylpyridine, a Lewis base, is added. Preferably, to the oxidant, the above dimethylpyridine is added to achieve a molar ratio of 0.9.

The plane symmetric dimethylpyridine employed in the present invention includes 2, 6-dimethylpyridine and 3, 5-dimethylpyridine, but 2, 6-dimethylpyridine is more preferable. The chemical structural formulas of pyridine, 2, 6-dimethylpyridine, and 3, 5-dimethylpyridine Lewis bases, are shown below. Formula 1

Formula 2

Formula 3

In order to manufacture such a solid electrolytic capacitor as described above, plane symmetric dimethylpyridine is incorporated in the polymerization reaction liquid and the resultant mixture is polymerized to form the electrolyte layer on the anode electrode formed of aluminum by the polymerization reaction in which the polymerizable monomer is mixed with the oxidant . The following three methods are used in the polymerization reaction.

In the first method the polymerizable monomer or the monomer solution is adhered to the anode electrode, then an oxidant solution is adhered thereto, and subsequent heating causes the polymerization reaction to proceed.

In the second method after adhering the oxidant solution to the anode electrode, the polymerizable monomer or the monomer solution is adhered thereto, and subsequent heating causes the polymerization reaction to proceed. In the third method after mixing the polymerizable monomer or the monomer solution with the oxidant solution, the resultant mixture is adhered to the anode electrode, and subsequent heating causes the polymerization reaction to proceed.

In the first and second methods, the plane symmetric dimethylpyridine is added to the monomer or the monomer solution and the oxidant solution. In the third method, the plane symmetric dimethylpyridine is added to the mixture . With this feature, the acidity of the oxidant is relieved, an attack to the anode electrode due to the oxidant is suppressed, an electrode voltage resistance property is improved, and the voltage resistance of the electrolytic capacitor is improved.

Moreover, the plane symmetric dimethylpyridine has high vapor pressure and remains in a conductive polymer layer even after heating and oxidative polymerization.

Brief Description of the Drawings

Fig. 1 is a graph showing voltage-current properties of an Al/PEDT capacitor with a conventionally formed voltage oxide of 40 V; Fig. 2 is a graph showing voltage-current properties for the case of adding pyridine into the oxidant in PEDT polymerization of the capacitor of Fig. 1;

Detailed Description of the Invention

One embodiment of the present invention will be described with the use of the drawings. A solid electrolytic capacitor employed in the present invention incorporates plane symmetric dimethylpyridine in an electrolyte layer in the formation of an electrolyte layer formed of polyethylene-dioxythiophene (PEDT) on an anode electrode formed of aluminum by a polymerization reaction in which a polymerizable monomer or a monomer solution (EDT) is mixed with an oxidant.

The Al/PEDT capacitor which will be used has a very low equivalence series resistance (ESR) and high heat resistance that an Al electrolytic capacitor does not have, while significantly lower breakdown voltage than voltage necessary for oxide formation and high leakage currently occurs. It is suspected that low voltage resistance in the capacitor is caused by the dissolution or deterioration of an aluminum oxide film due to protons released from monomers during polymerization of Al oxide and the PEDT.

Hence, when a thicker voltage oxide film than that of an Al electrolytic capacitor to the rated voltage of an Al/PEDT capacitor is employed, in other words, when the ratio of working voltage to formation voltage (V_w/V_f) for the Al/PEDT capacitor is much smaller than that of the Al electrolytic capacitor is employed, the V_w/V_f ratio for the Al electrolytic capacitor is not more than 0.8, whereas the ratio for the Al/PEDT capacitor is less than 0.3. Accordingly, if the thickness of the oxide film can be reduced almost as much as the Al electrolytic capacitor, and the V_w/V_f ratio can be increased, a preferably high voltage product by which electricity can be saved for oxide formation can be obtained. Moreover, the capacitance of the Al/PEDT can be increased dramatically so that the capacitance is doubled by simply replacing 100 V oxide with 50 V oxide.

On the other hand, according to the voltage-current property of a Ta/PEDT capacitor, upon constantly charging current flows at a low electric field to a middle electric field, current increases exponentially near oxide film formation voltage, and finally breakdown voltage occurs. The breakdown voltage is close to the oxide film formation voltage, and its value is proportional to the oxide film formation voltage. Ta oxide is known as a very stable oxide which is not dissolved or degraded by general acids, other than hydrofluoric acid.

For this reason, the Al/PEDT capacitor has also breakdown voltage close to the oxide film formation voltage, if the oxide film is not dissolved or degraded.

Furthermore, the Al oxide is dissolved or degraded in a 6O⁰C oxidant solution, and further the Al oxide is dissolved in molten p-toluenesulfonic acid of a byproduct at 150⁰C.

So, the Al oxide dissolution or degradation would be important factors for low breakdown voltage or high leakage current. Accordingly, if the pH in a monomer/oxidant/PEDT mixture during polymerization reaction is kept in the range that the Al oxide is stable (pH=3 to 10) , it is possible to obtain a good voltage-current property, it being possible to obtain breakdown voltage close to the oxide film formation voltage and low leakage current.

Hence, in accordance with the present invention, the effect of some Lewis base additives to the Al/PEDT capacitor on its voltage-current property was investigated in order to enhance the conductivity. (Experiment) (Specimen and pretreatment)

An Al plate with a thickness of 0.5 mm was cut into a 0.64 cm diameter circle . The specimen was immersed in a 85 vol% H3PO₄/I5 vol% HNO₃ mixture at 85 to 9O⁰C for 3 minutes, rinsed with water, rinsed with methanol, dried, and then stored in a desiccator. Just before anodic oxide formation, the specimen was immersed in 1 mol dm^"3 NaOH at room temperature for 3 minutes, immersed in 10 vol% HNO₃ for 1 minute, rinsed with water, rinsed with methanol, and then dried. (Oxide film formation) Anodic oxide was formed by applying a 0.83 mol dm^"3 ammonium adipate solution with a current density of up to 1 mAcm^"2 until the desired formation voltage was reached, followed by maintaining the voltage for 10 minutes. The formed specimen was then rinsed with water, rinsed with methanol to remove water, dried and then stored in the desiccator. (Masking and reformation)

The specimens were masked with a polyimide tape with a thickness not more than 0.05 mm to define the sample area (diameter of 6 mm) . The reformation of oxide was carried out in the same electrolyte solution as was used for the oxide film formation by keeping at prescribed formation voltage for 1 minute in order to repair any damage that might be caused during masking. (Monomer and oxidant)

The monomer and oxidant employed in the present invention were 3, 4-ethylenedioxythiophene (Baytron ® M V2) and a 54 wt% iron (III) p-toluenesulfonate 1-butanol solution (Baytron ® C-B54) , respectively. In order to investigate the effect of Lewis base additives, oxidant solutions containing pyridine with a molar ratio of 0.9 2, 6-dimethylpyridine, and 3, 5-dimethylpyridine were used. (Polymer coating on Al oxide and electrical contact)

After mixing said monomer and said oxidant solution thoroughly at a molar ratio of 1.0/0.3, the mixture was deposited on the Al oxide film to form a PEDT film. For the oxidant without a Lewis base, the specimen was spun at 600 rpm for 20 seconds to obtain a uniform film. For the oxidant with a Lewis base, spin coating was not employed since most of the mixture flew off a spinning disc due to its lower viscosity- resulting from the fact that the addition of the Lewis base causes polymerization rate to slow down. So the mixture was spread by tilting the specimen to cover the whole sample area. The polymer synthesis was carried out at 60° C for 30 minutes, followed by at 90° C or 15O⁰C for 60 continuous minutes. Electrical contact between a Cu wire and the PEDT film was made with an Ag paste. (Voltage-current curve measurement)

The specimens were set in a stainless chamber with a drying agent (P₂O₅) . After leaving for 1 to 2 days, a voltage-current (V-i) curve was measured by applying ramp voltage with 100 mVs^"1. Current was determined from voltage on resistance with 100 or 10 kΩ. (Conventional example) There is shown below the effect of polymerization temperature with no Lewis base additives during oxidative polymerization.

PEDT was synthesized at different temperatures to investigate the effect of the polymerization temperature on voltage-current (V-i) curve. The melting point of p-toluenesulfonic acid is about 105° C, so that the oxide film will not be damaged by molten p-toluenesulfonic acid if polymerization reaction is carried out below the melting point as diffusion of solid p-toluenesulfonic acid is likely to be limited.

FIG. 1 shows the voltage-current curve of an Al/oxide₄ov/PEDT capacitor with polymer prepared at 90°C and 15O⁰C in heat treatment twice. In Fig. 1, the voltage-current curve of a Ta/PEDT capacitor is also shown as a comparative. In the case of the Ta/PEDT capacitor, the charging current, which was almost independent of the applied voltage corresponding to its capacitance, flew at low voltage (low electric field) , then the current exponentially increased, and finally a current jump occurred near oxide formation voltage. When ramp voltage was applied from 0 V again on the sample after the current jump occurred, the current increased linearly with the applied voltage. That is, the current increased according to Ohm's law. This indicates that the sample was short circuited, i.e. , breakdown occurred at the time of the current jump.

Additionally, in the case of the Al/PEDT in the absence of sterically hindered Lewis base additives in which the heat treatment was carried out at both the temperatures described above, much higher (a few orders of magnitude) current flowed as compared with the Ta/PEDT capacitor even at low voltage. The Al/PEDT capacitor prepared at 90° C showed lower leakage current than that prepared at 150° C, but the current was still much higher than that of the Ta/PEDT. The breakdown (current jump) voltage was close to the oxide film formation voltage, i.e., similar to the Ta/PEDT capacitor, for both the heat treatment temperatures, although the leakage current was much higher than the Ta/PEDT. (Comparative example)

There is shown an example in which, pyridine is added as a Lewis base during oxidative polymerization to the conventional example, and the effect of a Lewis base on a voltage-current property is shown below.

Fig. 2 shows voltage-current curves of an Al/PEDT capacitor, where the PEDT was synthesized with and without pyridine in oxidant. As a comparative example, a voltage-current curve of a Ta/PEDT capacitor with a 40 V oxide film is also shown. It seems that the addition of pyridine somewhat reduces the current density. However, the value was much higher than that of the Ta/PEDT capacitor. This indicates that the addition of pyridine did not work for protecting an Al oxide film from a chemical attack by protons. (Example 1)

The Example 1 examines the effect on a voltage-current property in which 2, 6-dimethylpyridine is substituted for the pyridine in the Comparative example described above.

It is examined about the voltage-current curve of an Al/PEDT capacitor with different oxide formation voltage, for example 40, 70 and 100V with PEDT synthesized with an oxidant containing 2, 6-dimethylpyridine. Although spikes of current are observed at very low voltage (a few volts) and middle voltage (0.6 V_f or lower) , the general phenomena is very similar to that of a Ta/PEDT capacitor. In other words, constant charging current flows first, then increases exponentially near formation voltage (near 40, 70 and 100 V) , and finally breakdowns. As a result, the oxide was protected from the proton attack, and a good voltage-current property close to that of the Ta/PEDT capacitor was obtained. (Example 2) The Example 2 examines the effect on a voltage-current property in which 3, 5-dimethylpyridine is substituted for the 2, 6-dimethylpyridine in Example 1 described above.

The voltage-current curve of an Al/PEDT capacitor with different oxide formation voltage, for example 40, 70 and 100V, was examined with PEDT synthesized with an oxidant containing 3, 5-dimethylpyridine in the same manner as the above Example 1. Methyl substitutions at 3- and 5-position are away from the ring nitrogen, so that interaction with the iron (III) oxidant is expected. At low voltage (less than 0.5 to 0.6 V_f) , constant charging current flowed similarly to the Al/PEDT capacitor with the 2, 6-dimethylpyridine in Example 1. However at voltage above 0.5 to 0.6 V_f, leakage current increased, and then breakdown occurred near formation voltage (near 40, 70 and 100 V). With this feature, the addition of 3, 5-dimethylpyridine provided better results than the addition of pyridine, but worse results than the addition of 2, 6-dimethylpyridine . For this reason, the highest additive effect on the voltage-current property of the Al/PEDT capacitor was 2, 6-dimethylpyridine, followed by 3, 5-dimethylpyridine . The addition of pyridine brought about no changes .

Figs. 1, 2 and Examples 1 and 2 show that in Conventional example (Fig. 1) and Comparative example (Fig. 2), current flows together with voltage application, and that in Examples 1 and 2, voltage increases even when no current flows, with a voltage resistance property enhanced. Furthermore, Example

1 shows better properties than Example 2. Moreover, Tables 1 to 3 show voltage-current properties of an Al/PEDT capacitor with film formation voltage different from PEDT synthesized with oxidant with a hindered Lewis base and without a Lewis base, to which verification experiments were carried out. Table 1 shows a film formation voltage of 40 V, Table

2 shows a film formation voltage of 70 V, and Table 3 shows a capacitor with a film formation voltage of 100 V. It should be noted that Example (1) shows the addition of 2, 6-dimethylpyridine to the oxidant, that Example (2) shows the addition of 3, 5-dimethylpyridine to the oxidant, that Comparative example shows the addition of pyridine to the oxidant, and that the Conventional example shows no additives being added to the oxidant.

Example (1) represents first charges where formation is occasionally imperfect, as shown by spikes in leakage current

(LC) . The 2nd and later sweeps are perfectly flat with very low LC as reformation. Example (2) represents complete during the first charge, (not shown)

(Table 1)

(Table 2 )

(Table 3]

This reveals that the highest additive effect on the voltage-current properties of the Al/PEDT capacitor was 2, 6-dimethylpyridine, followed by 3, 5-dimethyIpyridine, even in the case of using the capacitor with different formation voltage. The addition of pyridine brought about no changes as compared with no additives. Effects of the Invention

As has been described above, in accordance with the present invention, concerning the solid electrolytic capacitor, forming an electrolyte layer on the anode electrode formed of aluminum by a polymerization reaction in which the polymerizable monomer or the monomer solution is mixed with an oxidant, the electrical resistance of the solid electrolytic capacitor can be increased exponentially by incorporating plane symmetric dimethylpyridine, particularly 2,6- dimethylpyridine and 3, 5-dimethylpyridine in the electrolyte layer .

Claims

Claim 1. A solid electrolytic capacitor comprising an electrolyte layer on an anode electrode formed of aluminum by a polymerization reaction in which a polymerizable monomer or a monomer solution is mixed with an oxidant, wherein plane symmetric dimethylpyridine is added to the oxidant.

Claim 2. The solid electrolytic capacitor according to Claim 1, wherein said plane symmetric dimethylpyridine comprises 2,6- dimethylpyridine.

Claim 3. The solid electrolytic capacitor according to claim 1, wherein said plane symmetric dimethylpyridine comprises 3, 5-dimethylpyridine .

Claim 4. The solid electrolytic capacitor according to Claim 1, wherein the polymerizable monomer comprises 3, 4-ethylenedioxythiophene .

Claim 5. The solid electrroytic capacitor of Claim 1, wherein the oxidant comprises iron (III) p-toluenesulfonate dissolved in a solvent.

Claim 6. A method of manufacturing of a solid electrolytic capacitor comprising forming an electrolyte layer on an anode electrode formed of aluminum by a polymerization reaction in which a polymerizable monomer or a monomer solution is mixed with an oxidant, wherein the method includes mixing a plane symmetric dimethylpyridine in a polymerization reaction liquid with the oxidant and polymerizing the resultant mixture.

Claim 7. The method according to Claim 6, further comprising adhering the polymerizable monomer or the monomer solution to said anode electrode, then adhering the oxidant containing said plane symmetric dimethylpyridine thereto, and then performing the polymerization reaction.

Claim 8. The method according to Claim 6, further comprising adhering the oxidant containing said plane symmetric dimethylpyridine to said anode electrode, and then adhering the polymerizable monomer or the monomer solution thereto, and subsequently performing the polymerization reaction.

Claim 9. The method according to Claim 6, wherein after mixing said polymerizable monomer or monomer solution with an oxidant solution, the mixture is adhered to the anode electrode, and the subsequent polymerization reaction then proceeds.

Claim 10. The method according Claim 6, wherein the polymerizable monomer or the monomer solution comprises 3, 4-ethylenedioxythiophene .

Claim 11. The method according to Claim 7, wherein the polymerizable monomer comprises 3, 4-ethylenedioxythiophene .

Claim 12. The method according to Claim 8, wherein the polymerizable monomer comprises 3, 4-ethylenedioxythiophene .

Claim 13. The method according to Claim 9, wherein the polymerizable monomer comprises 3 , 4-ethylenedioxythiophene .

Claim 14. The method according to Claim 6, wherein the oxidant comprises iron (III) p-toluenesulfonate dissolved in a solution.