JP3488260B2 - Method for manufacturing solid electrolytic capacitor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a solid electrolytic capacitor, and more particularly to a solid electrolytic capacitor using a solid electrolyte made of an organic conductive compound such as polypyrrole. 2. Description of the Related Art In recent years, there has been a demand for miniaturization of electronic devices and more efficient mounting on a printed circuit board. Along with this, there is an increasing demand for electrolytic capacitors having a chip and a low profile. Also, with the diversification of electronic devices, chip-type electrolytic capacitors are required to have various characteristics. In a solid electrolytic capacitor, in addition to a solid electrolyte composed of a metal oxide semiconductor such as manganese dioxide, a solid electrolyte composed of an organic conductive compound such as tetracyanoquinodimethane (TCNQ), polypyrrole, or polyaniline is also used. It has been proposed that it is applied to. Solid electrolytic capacitors using these organic conductive compounds have higher conductivity than manganese dioxide, etc.
Significant improvement in electrical characteristics, particularly ESR characteristics, can be expected. [0004] In the formation of an electrolyte layer made of an organic conductive compound such as polypyrrole, for example, the anode body is immersed in a pyrrole solution containing an oxidizing agent to form a pyrrole thin film on the surface of the anode body (chemical polymerization). ) And then applying a voltage (electrolytic polymerization) while immersing in a solution in which pyrrole is dissolved. [0005] Such a proposal has a background that a strong electrolyte layer cannot be obtained with an electrolyte layer formed only by chemical polymerization. As a result, it is very difficult to provide a means for extracting an electrode such as a conductive paste formed on the surface of the electrolyte layer, and the application of the conductive paste may damage the electrolyte layer, thereby causing a short circuit accident or a function of extracting an electrode of the electrolyte layer. Is not sufficient, a desired capacitance may not be obtained. In addition, since a strong electrolyte layer cannot be obtained, the moisture resistance is poor, so that it is very difficult to maintain the initial electrical characteristics for a long period of time. On the other hand, according to electrolytic polymerization, although a strong electrolyte layer can be obtained, it is necessary to apply a voltage to the anode body. However, a dielectric (oxide film layer), which is an insulator, is formed on the surface of the anode body, and it is difficult to directly form an electrolyte layer on the surface of the anode body by electrolytic polymerization. Therefore, as a pretreatment, an electrolyte layer is first formed by chemical polymerization, and electrolytic polymerization is performed using the electrolyte layer as an electrode, thereby obtaining a strong electrolyte layer. However, in the electropolymerization, since a voltage is applied in a pyrrole solution containing an oxidizing agent, there is a disadvantage that it is difficult to reuse the pyrrole solution. Coupled with the necessity of applying a voltage to each capacitor element and strictly managing the liquid level to prevent the pyrrole solution from creeping up on a non-selected part of the anode body, for example, the terminal part, ,
There were aspects that were not always optimal for mass production. Therefore, it is conceivable to form an electrolyte layer made of polypyrrole only by chemical polymerization. In that case, as described above, it is necessary to consider the electrode extraction means on the cathode side, and not only the adhesion between the electrolyte layer made of polypyrrole and the anode body, but also if the electrical adhesion with the cathode body must be ensured. No. That is, if a pyrrole solution and an oxidizing agent solution are applied and dropped on the surface of the anode body, an electrolyte layer made of polypyrrole is generated, and its adhesion is ensured, but the cathode body is further arranged on this electrolyte layer. Also,
Almost no adhesion between the electrolyte layer and the cathode body can be obtained. In particular, when a foil element is used as the bipolar electrode body and wound together with the separator to form a capacitor element, forming an electrolyte layer on the surface of the foil-shaped anode body and winding this is because of the fragility of the electrolyte layer. Is also extremely difficult. Therefore, when a foil-shaped anode body and a cathode body are used, in order to sufficiently adhere to the anode body (anode foil) and the cathode body (cathode foil), it is necessary to use a wound capacitor element. In order to form an electrolyte layer having a sufficient thickness even in the central portion, it is necessary to immerse the capacitor element several times in a pyrrole solution containing an oxidizing agent. Alternatively, it is conceivable to separately impregnate the pyrrole solution and the oxidizing agent solution to form an electrolyte layer by chemical polymerization inside the capacitor element, and to repeat this plural times. However, it has been found that when immersion or impregnation is performed several times, the electrical characteristics of the capacitor element, particularly the leakage current characteristics, are extremely deteriorated. In the case of a solid electrolytic capacitor using, for example, manganese dioxide as the electrolyte, it is necessary to perform a heat treatment in the process of producing manganese dioxide, and this heat treatment has an adverse effect on the oxide film layer. In the case of producing polypyrrole, such a heat treatment is not required, and there is no inconvenience such as applying a reverse voltage to the anode foil as compared with the production step by the electrolytic polymerization reaction.
For this reason, it is unclear why the inconvenience of the rapid increase of the leakage current as described above occurs. Alternatively, it is possible that polypyrrole, which is a conductor, is also generated in the damaged portion of the oxide film layer on the surface of the anode foil, and polypyrrole penetrates into the oxide film layer itself, thereby increasing the leakage current. Since polypyrrole itself has almost no function of restoring an oxide film, even if an aging treatment is performed after forming an electrolyte layer on a capacitor element, an improvement in leakage current characteristics cannot be expected, and a desired electric current cannot be obtained. It becomes difficult to obtain characteristics. As a result, the yield in the manufacturing process becomes extremely poor, and the reliability of the product is significantly impaired. An object of the present invention is to provide a highly reliable solid electrolytic capacitor having stable electrical characteristics in a solid electrolytic capacitor using a capacitor element formed by winding an electrode foil. It is to realize. The present invention provides a method for manufacturing a solid electrolytic capacitor, comprising:
A step of winding a cathode electrode foil to form a capacitor element, and a step of separately impregnating the capacitor element with an oxidant solution and a pyrrole solution, and separately impregnating the capacitor element with an oxidant solution and a pyrrole solution A plurality of times, and in at least the first impregnation step, ammonium persulfate having a concentration of 0.01 to 10% by weight.
Is characterized in that the concentration of ammonium persulfate in the oxidant solution for the second and subsequent times is made higher than that of the first time using an oxidant solution using as an oxidant. In the present invention, the electrolyte layer is formed by a chemical polymerization reaction between an oxidant solution impregnated in a capacitor element and a pyrrole solution impregnated subsequently. The reason why the oxidizing agent solution and the pyrrole solution are separately impregnated is that polypyrrole is generated in a direction in which the pyrrole solution permeates the oxidizing agent solution in the chemical polymerization reaction. This is because if the solution is impregnated, polypyrrole is generated even in the center of the capacitor element. According to the present invention, in the step of separately impregnating the oxidizing agent solution and the pyrrole solution, an increase in leakage current can be suppressed by reducing the concentration of the oxidizing agent solution used in at least the first impregnation step. As a result of the experiment, the use of the oxidizing agent solution having the above-described concentration makes it possible to suppress the deterioration of the leakage current characteristics of the capacitor element on which the electrolyte layer is formed. Although there are many unclear points about the mechanism, in the chemical polymerization reaction between the oxidizing agent solution and the pyrrole solution, the concentration of the oxidizing agent solution affects the amount of polypyrrole to be produced. The extremely thin polypyrrole film produced by the first chemical polymerization reaction, which has little effect on the oxide film layer, becomes a protective film, so to say, that the deterioration of the oxide film layer due to the subsequent chemical polymerization reaction is suppressed. Inferred. Embodiments of the present invention will be described below. The anode foil is made of a valve metal such as aluminum or tantalum. The surface of the anode foil is subjected to an etching treatment in advance to increase its surface area, and a chemical conversion treatment is applied to form an oxide film layer. This oxide film layer is made of aluminum oxide in which the surface of the anode foil is oxidized, and serves as a dielectric of the capacitor. The cathode foil, like the anode foil, is made of foil-like aluminum. Although no oxide film layer is formed on the surface of the cathode foil, the cathode foil is etched to increase the surface area. The capacitor element is formed by winding such an anode foil and a cathode foil via a separator. As the separator, glass paper having a thickness of 80 μm was used in this example. Here, the reason why the glass paper is used as the separator is to suppress the oxidation reaction between the oxidizing agent solution impregnating the capacitor element and the separator in a later step as much as possible and maintain the oxidizing ability of the oxidizing agent solution. Next, the capacitor element is immersed in an electrolytic solution containing ammonium phosphate as a main component and a voltage is applied to the capacitor element to perform a so-called re-chemical treatment. By this re-chemical conversion treatment, it becomes possible to further suppress the leakage current due to the generation of the electrolyte layer made of polypyrrole which is generated later. The reformed capacitor element is washed and dried, and separately impregnated with an oxidizing agent solution and a pyrrole solution. The oxidizing agent solution was composed of 2.5% by weight of ammonium persulfate and water, and the impregnation was performed for 5 to 20 minutes with the capacitor element kept under reduced pressure. This impregnation method may be another method, for example, a method of quantitatively dropping a predetermined amount. The capacitor element impregnated with the oxidizing agent solution is impregnated with a pyrrole solution obtained by dissolving pyrrole in ethanol and water for 10 to 20 minutes under reduced pressure.
Thereafter, washing and drying are performed. By the impregnation of the pyrrole solution, a chemical polymerization reaction occurs inside the capacitor element, and a polypyrrole film, which can be called a first electrolyte layer, is formed on the surface of the anode foil. In addition, each impregnation time of an oxidizing agent solution and a pyrrole solution can be changed as needed. Next, the capacitor element is immersed in an electrolytic solution containing ammonium adipate as a main component, and a voltage is applied to perform a re-chemical conversion treatment again. Then, a cleaning and drying treatment is performed in the same manner as the previous re-chemical conversion treatment. . Further, the capacitor element is impregnated again with the oxidizing agent solution and the pyrrole solution. Here, the capacitor element is first impregnated with the oxidizing agent solution under reduced pressure for about 1 minute. The oxidizing agent solution was an aqueous solution containing ammonium persulfate as a main component, and the amount of ammonium persulfate was 30% by weight, similarly to the previously impregnated solution. Then, the pyrrole solution was impregnated under reduced pressure for about 2 minutes to generate polypyrrole by a chemical polymerization reaction inside the capacitor element. As a result, a second electrolyte layer is sequentially formed on the first electrolyte layer on the surface of the anode foil of the capacitor element. The capacitor element thus formed is housed in, for example, an outer frame having a housing space for housing the capacitor element, and its opening is sealed with a sealing resin such as epoxy resin. And Alternatively, it may be housed in a bottomed cylindrical outer case made of aluminum or the like, and its opening may be sealed with a sealing body, or the outer surface of the capacitor element may be covered with a mold resin. In this embodiment, when the step of separately impregnating the capacitor element with the oxidant solution and the pyrrole solution is performed a plurality of times, at least in the first impregnation step, the capacitor element is impregnated with a low-concentration oxidant solution. ,
Thereafter, a first electrolyte layer made of extremely thin polypyrrole is generated by impregnation with a pyrrole solution. Therefore, even if polypyrrole is generated by the subsequent chemical polymerization reaction,
Damage to the oxide film layer on the anode foil is suppressed, and an increase in leakage current can be suppressed. In the chemical polymerization treatment, the capacitor element is first impregnated with an oxidizing agent solution and then impregnated with a pyrrole solution. Therefore, a chemical polymerization reaction does not occur so that the pyrrole solution permeates the oxidizing agent solution, and the electrolyte layer is not generated only at the end face portion of the capacitor element. Next, the electrical characteristics of the solid electrolytic capacitor manufactured according to the embodiment of the present invention were measured. As a comparative example, a capacitor element similar to that of the example was prepared, and this capacitor element was impregnated with oxidant solutions having different concentrations and a pyrrole solution having the same concentration.
Further, as other examples, as the oxidizing agent solution to be impregnated for the first time, 0.01% by weight, 0.05% by weight,
An oxidizing agent solution consisting of 2.5% by weight and 10% by weight of ammonium persulfate was prepared. A solid electrolytic capacitor having the same exterior structure as that of the above-described embodiment was obtained using each of the capacitor elements. The results of measuring the capacitance (CAP), the loss angle tangent (tan δ), and the leakage current (LC) of the solid electrolytic capacitors according to the examples and comparative examples are shown below. In each of the examples and comparative examples, a capacitor element having a rated voltage of 10 V and a rated capacitance of 10 μF is used. Ammonium persulfate weight% CAP (μF) tan δ LC (μA) Comparative Example 1 0.005 11.33 0.080 7.10 Example 1 0.01 10.50 0.075 0.32 Example 2 0.05 11.15 0.074 0.28 Example 3 2.50 10.42 0.086 0.30 Example 4 10.00 11.32 0.070 0.59 Comparative Example 2 15.00 11. 30 0.065 1.04 Comparative Example 3 20.00 10.86 0.056 2.90 Comparative Example 4 30.00 9.89 0.068 2.56 As is clear from these results, If the amount of ammonium persulfate in the oxidizing agent solution is less than 0.01% by weight in the first chemical polymerization reaction, the formation of polypyrrole is not sufficient, so that a protective film cannot be formed. The damage of the oxide film layer due to the formation of polypyrrole in the subsequent impregnation step can hardly be suppressed, and the leakage current increases. On the other hand, if it exceeds 10% by weight, the effect on the oxide film layer on the surface of the anode foil increases, and in this case also, the leakage current increases. When ammonium persulfate is used in an amount of 0.01% by weight to 10% by weight as in the example of the present invention, it is understood that the leakage current characteristics are excellent in comparison with each comparative example. In the embodiment, the capacitor element is impregnated with the oxidizing agent solution and the pyrrole solution twice each, but may be impregnated three times or more as necessary. In that case,
Due to the overlapping of the chemical polymerization reactions, the resulting polypyrrole has a more sufficient thickness, and improvement in reliability can be expected. Also,
In the case of impregnation three or more times, for example, in the case of impregnation four times, it is also possible to use a thin oxidizing agent solution in the first and second times, and the same effect as in the above-described embodiment is exhibited. As described above, the present invention relates to a method for manufacturing a solid electrolytic capacitor, wherein a step of winding an anode and a cathode electrode foil together with a separator to form a capacitor element;
And a step of separately impregnating the capacitor element with an oxidant solution and a pyrrole solution, and performing the step of separately impregnating the capacitor element with an oxidant solution and a pyrrole solution a plurality of times, at least in the first impregnation step,
Ammonium persulfate at a concentration of 0.01 to 10% by weight
Since the concentration of ammonium persulfate in the oxidizing solution after the second time is higher than that in the first time, the oxidizing agent solution used as the oxidizing agent is used. A polypyrrole film with little effect of
The polypyrrole film serves as a protective film, which can suppress damage to the oxide film layer of the anode foil due to the second and subsequent chemical polymerization reactions, can improve electrical characteristics such as leakage current, and can be used for a plurality of times. This makes it possible to easily and densely form the electrolyte layer inside the capacitor element and improve the moisture resistance performance. Further, since no electrolytic polymerization step is required unlike the prior art, the production process is simplified, and special production equipment is not required. Also, there is no damage to the oxide film layer due to voltage application in the electrolytic polymerization step, and in combination with the use of the wound capacitor element, even if the electrolyte layer is made of polypyrrole having a weak mechanical strength, the damage is prevented. be able to. Therefore, particularly, the leakage current characteristics can be stably maintained for a long time.

Claims (1)

(57) [Claim 1] A step of winding an anode and a cathode electrode foil together with a separator to form a capacitor element, and a step of separately impregnating the capacitor element with an oxidant solution and a pyrrole solution. Including, performing the step of separately impregnating the capacitor element with the oxidizing agent solution and the pyrrole solution a plurality of times,
At least in the first impregnation step, ammonium persulfate having a concentration of 0.01 to 10% by weight is used as an oxidizing agent.
The oxidizing agent solution for the second and subsequent times using the oxidizing agent solution
A method for producing a solid electrolytic capacitor, characterized in that the concentration of ammonium persulfate therein is higher than in the first time.