JP7849412B2 - Electrolytic capacitor and method for manufacturing an electrolytic capacitor - Google Patents

Description

This invention relates to an electrolytic capacitor and a method for manufacturing an electrolytic capacitor.

An electrolytic capacitor is a basic energy storage device consisting of a capacitor element in which an electrolyte is impregnated within a dielectric layer formed on an anode foil and a cathode foil, with a separator placed between them. Electrolytic capacitors can be classified into two types: one in which a non-solid electrolyte (liquid electrolyte) is introduced into the capacitor element, and another in which a solid electrolyte, such as a conductive polymer, is introduced. Furthermore, there are also so-called "hybrid" electrolytic capacitors that incorporate both a solid electrolyte and a liquid electrolyte within the capacitor element, aiming to combine the advantages of both solid and non-solid electrolytes.

In conventional electrolytic capacitors, in order to meet various market demands, for example, by increasing the water content in the electrolyte to lower impedance, the amount of hydrogen gas generated by the hydration reaction between the metal elements of the electrode foil and water increases. Alternatively, in other cases, when trying to miniaturize and increase the capacitance of the product, for example, the amount of hydrogen gas generated increases as the capacitance of the foil increases, and this hydrogen gas increases the internal pressure of the reduced outer casing space. This generation of hydrogen gas can lead to problems such as bulging of the exterior, leakage, and activation (opening) of explosion-proof valves.

Therefore, conventionally, nitro compounds containing nitro groups have sometimes been incorporated into the electrolyte (see Patent Document 1: Japanese Patent Publication No. 4-10512). Nitro compounds have the effect of suppressing the generation of hydrogen gas within electrolytic capacitors by reducing the nitro group and absorbing hydrogen ions.

Japanese Unexamined Patent Publication No. 4-10512 Japanese Patent Publication No. 2023-145741

As illustrated in Patent Document 1 (Japanese Unexamined Patent Publication No. 4-10512), the hydrogen gas generation suppression effect of nitro compounds (nitro groups) is well known. Conventionally, a limited number of nitrobenzene compounds have been used in electrolytes for electrolytic capacitors (Patent Document 2: Japanese Unexamined Patent Publication No. 2023-145741, paragraph 0032).

However, the nitro compounds used until now had relatively low solubility in polar solvents such as ethylene glycol, and at relatively high concentrations, they reduced the voltage rating of electrolytic capacitors. Therefore, their use was limited to relatively low concentrations.

Furthermore, the inventors discovered a new problem: conventional nitro compounds exhibit a significant decrease in dielectric strength, particularly after high-temperature loading (being left at high temperatures for a certain period).

Therefore, the present invention aims to provide an electrolytic capacitor and a method for manufacturing an electrolytic capacitor that can suppress the generation of hydrogen gas by using a nitro compound, thereby suppressing the decrease in withstand voltage after high-temperature loading and enabling stable operation in a temperature range that includes high temperatures.

The present invention solves the above-mentioned problem by a solution described below as one embodiment.

The electrolytic capacitor according to the present invention comprises a capacitor element having an anode foil on which a dielectric layer is formed, a cathode foil, and a separator disposed between the anode foil and the cathode foil, and an electrolyte impregnated within the capacitor element, wherein the electrolyte contains a 4-nitropyridine N-oxide compound.

It is preferable that the 4-nitropyridine N-oxide compound is 4-nitropyridine N-oxide.

Furthermore, hydrogen generation within electrolytic capacitors becomes more problematic the higher the water content in the electrolyte. Therefore, the present invention is more preferable in a form where the solvent of the electrolyte contains water.

Furthermore, the method for manufacturing an electrolytic capacitor according to the present invention is characterized by incorporating a 4-nitropyridine N-oxide compound into the electrolyte.

It is preferable that the 4-nitropyridine N-oxide compound is 4-nitropyridine N-oxide.

A solvent containing water can be suitably used as the solvent for the electrolyte.

The electrolytic capacitor according to the present invention can suppress the generation of hydrogen gas within the electrolytic capacitor by using a 4-nitropyridine N-oxide compound, thereby suppressing the decrease in withstand voltage after high-temperature loading and enabling stable operation in a temperature range that includes high temperatures.

Figure 1 is a schematic diagram (front cross-sectional view) showing an example of an electrolytic capacitor according to this embodiment. Figure 2 is a schematic diagram (exploded perspective view) showing an example of a capacitor element in an electrolytic capacitor according to this embodiment. Figure 3 is a schematic diagram (a schematic diagram of the basic configuration) showing an example of a capacitor element in the electrolytic capacitor according to this embodiment. Figure 4 is a flowchart showing an example of a manufacturing method for an electrolytic capacitor according to this embodiment. Figures 5A and 5B are graphs showing the results of Example 1, Comparative Examples 1 and 2, and Reference Example 1 of Test 3. Figures 6A and 6B are graphs showing the results of Example 2, Comparative Example 4, and Reference Example 1 of Test 3. Figure 7 is a graph showing the results of Example 3, Comparative Example 5, and Reference Example 1 of Test 4.

This invention relates to an electrolytic capacitor having structural features in the composition of its electrolyte and a method for manufacturing the same. Therefore, this invention is applicable to electrolytic capacitors of the type in which an electrolyte is impregnated into a capacitor element. Hereinafter, as an embodiment for carrying out the present invention, an example of an electrolytic capacitor 1 of the type in which an electrolyte 6 as a non-solid electrolyte is introduced into a capacitor element 2 (a type in which a solid electrolyte is not introduced) will be described. However, due to the nature of the present invention described above, the electrolytic capacitor according to the present invention is not limited to the type of electrolytic capacitor 1 described here. That is, the electrolytic capacitor according to the present invention also includes, for example, a so-called "hybrid type" electrolytic capacitor in which an electrolyte is introduced into a capacitor element together with a solid electrolyte such as a conductive polymer. Furthermore, as an embodiment for carrying out the present invention, an example of a lead-terminal type (lead-wire type) electrolytic capacitor 1 having lead terminals (lead wires) 10, 11 will be described. However, due to the nature of the present invention described above, the electrolytic capacitor according to the present invention is not limited to this terminal configuration. That is, the electrolytic capacitor according to the present invention also includes, for example, a screw-terminal type having screw terminals, a substrate-free type with riveted external terminals erected, and other surface-mount type electrolytic capacitors. Furthermore, as an embodiment of the present invention, an electrolytic capacitor 1 having a wound-type capacitor element 2 will be described as an example. However, due to the nature of the present invention described above, the electrolytic capacitor according to the present invention is not limited to this element form. That is, the electrolytic capacitor according to the present invention also includes electrolytic capacitors of element forms such as multilayer type and coin type.

Hereinafter, this specification will describe in detail embodiments for carrying out the present invention with reference to the drawings. Figure 1 is a schematic diagram (front cross-sectional view) showing an example of an electrolytic capacitor 1 according to this embodiment. However, for ease of viewing, the cross-sectional structure of the capacitor element 2 is not shown. Figures 2 and 3 are schematic diagrams illustrating an example of the capacitor element 2 in the electrolytic capacitor 1 according to this embodiment. Figure 2 is an exploded perspective view showing the formation (winding) process S01 of the capacitor element 2. Figure 3 is an explanatory diagram schematically showing the basic configuration of the capacitor element 2.

As shown in Figure 1, the electrolytic capacitor 1 according to this embodiment houses a capacitor element 2 impregnated with an electrolyte 6 in a bottomed cylindrical outer case 7. The gap between the capacitor element 2 and the inner wall of the outer case 7 may be filled with a filler (not shown) made of an insulating material such as epoxy resin. For the outer case 7, for example, an aluminum case with relatively high airtightness, heat resistance, and corrosion resistance can be used. An explosion-proof valve 8, which is a pressure valve, is provided at the bottom of the outer case 7. The explosion-proof valve 8 is designed so that if a relatively rapid and large amount of gas is generated inside the electrolytic capacitor 1 and the internal pressure rises above a certain level, the thin-walled portion will crack, allowing the gas to escape. Furthermore, for mounting purposes, the outer case 7 is covered with an outer sleeve (not shown) made of an insulating material such as polyvinyl chloride.

A sealing body 9 is fitted into the opening 7a of the outer casing 7. The lead terminals (anode lead terminal 10 and cathode lead terminal 11) connected to the capacitor element 2 pass through through holes in the sealing body 9 and are led out of the electrolytic capacitor 1 through these holes. The sealing body 9 is crimped by the opening 7a of the outer casing 7, forming a first crimped portion 7c perpendicular to the mounting direction of the sealing body 9 (Z direction in the drawing). The sealing body 9 is crimped by the open end 7b of the outer casing 7, forming a second crimped portion 7d parallel to the mounting direction of the sealing body 9 (Z direction in the drawing). Thus, the outer casing 7 is sealed by the sealing body 9 and also sealed by the two crimped portions 7c and 7d.

Next, as shown in Figure 2, the wound capacitor element 2 according to this embodiment is formed by winding the anode foil 3 and cathode foil 4 together with a separator 5 in between, and holding them with a retaining material (not shown), such as tape, to form a cylindrical capacitor element 2.

The basic configuration, as shown in Figure 3, includes an anode foil 3, a cathode foil 4, and a separator 5 disposed between the anode foil 3 and the cathode foil 4. Anode lead terminals 10 are connected to the anode foil 3. Cathode lead terminals 11 are connected to the cathode foil 4.

The electrode foils (anode foil 3 and cathode foil 4) may be made of valve metals such as aluminum or tantalum. Alternatively, a valve metal alloy composed of multiple types of valve metals may be used. Or, a valve metal alloy may be used in which one or more valve metals are mixed with other metallic elements or non-metallic elements to the extent that they can function as electrode foils 3 and 4. It should be noted that the presence of unintended impurities in these valve metals or valve metal alloys, within the limits that allow them to function as electrode foils 3 and 4, is permissible.

The electrode foils 3 and 4 may be formed from multiple layers, consisting of a core substrate and a coating material covering the substrate. In this case, at least the outer coating layer should be made of the valve metal or valve metal alloy described above. That is, the composition of the substrate and the composition of the coating material may be the same or different.

The electrode foils 3 and 4 have an expanded surface structure due to a surface expansion treatment. Typical surface expansion treatments include etching. Alternatively, the expanded surface structure may be formed by other methods such as metal powder deposition or sintering. In this case, for example, the electrode foils 3 and 4 themselves and the expanded surface structure can be formed by coating the core substrate with the valve metal or valve metal alloy powder by deposition or sintering. The expanded surface structure of the electrode foils 3 and 4 increases the specific surface area and thus the capacitance.

On the surface of the enlarged anode foil 3, a dielectric oxide film 3a is formed by chemical conversion treatment. On the other hand, chemical conversion treatment is optional for the enlarged cathode foil 4. That is, in a polarized electrolytic capacitor 1, chemical conversion treatment is not applied, and normally, a natural oxide film (not shown) is formed on the surface of the cathode foil 4 by oxygen in the air. Alternatively, a treatment other than chemical conversion treatment may be applied; for example, valve metal or valve metal alloy powder may be further coated on the surface of the enlarged cathode foil 4. This can increase the dielectric constant and improve the capacitance. Alternatively, the surface of the enlarged cathode foil 4 may be treated with chemical conversion treatment in the same way as the anode foil 3, resulting in a non-polarized electrolytic capacitor 1.

The lead terminals 10 and 11 are connected to the electrode foils 3 and 4 by stitch connections, cold pressure welding, etc., and are led out from the capacitor element 2.

The separator 5 (electrolytic paper) can be made from paper made from natural cellulose fibers such as Manila hemp pulp, or from cloth, sheets, films, etc., made from synthetic fibers such as nylon, or from blends or mixed products of these materials. Of these, synthetic fibers have the advantage of allowing the selection of materials with particularly excellent heat resistance. Examples of such synthetic fibers include nylon, polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), and polyphenylene sulfide (PPS). Figure 3 schematically shows the basic configuration of the capacitor element 2, and while it shows one separator 5, the number is not limited, and multiple separators (for example, three in Figure 2) may be used as needed to achieve the purpose of separating the anode foil 3 and the cathode foil 4.

Furthermore, the capacitor element 2 is impregnated with an electrolyte 6. As schematically shown in Figure 3, in the polarized electrolytic capacitor 1, the electrolyte 6 exists in the gap between the anode foil 3 and the cathode foil 4. The electrolyte 6, by contacting the dielectric layer (oxide film 3a) formed on the anode foil 3 and the cathode foil 4, functions as a true cathode, effectively acting as the counter electrode to the anode foil 3 in place of the cathode foil 4. While the separator 5 allows the components of the electrolyte 6 to pass freely between the anode foil 3 and the cathode foil 4, depending on the configuration and material of the separator 5, the electrolyte 6 may also be impregnated within the separator 5.

The electrolyte solution 6 comprises an electrolyte and a solvent for dissolving or dispersing the electrolyte.

The electrolyte can be an organic acid, an inorganic acid, a compound of an organic acid and an inorganic acid, a derivative thereof, or a salt thereof. One of these may be used alone, or two or more may be used in mixture. For example, both an organic acid and an inorganic acid may be used.

Examples of organic acids and their derivatives include monocarboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, benzoic acid, caprylic acid, and their derivatives. Examples of dicarboxylic acids include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, fumaric acid, maleic acid, phthalic acid, azelaic acid, sebacic acid, 1,6-decanedicarboxylic acid, 5,6-decanedicarboxylic acid, 1,10-decanedicarboxylic acid, and their derivatives. Examples of hydroxycarboxylic acids include citric acid, salicylic acid, and their derivatives. Examples of inorganic acids and their derivatives include boric acid, sulfamic acid, and their derivatives. Furthermore, examples of complex compounds of organic and inorganic acids and their derivatives include boron complexes of dicarboxylic acids or hydroxycarboxylic acids. Examples include borodisuoic acid, borodimalonic acid, borodisuccinic acid, borodiadipic acid, borodimaleic acid, borodiphthalic acid, borodiglycolic acid, borodicitric acid, borosalicylic acid, and their derivatives.

Furthermore, examples of salts of organic acids, inorganic acids, complex compounds of organic and inorganic acids, and their derivatives include ammonium salts, alkylammonium salts, amine salts, amidine salts, sodium salts, potassium salts, etc. Examples of amine salts include salts of dimethylamine, diethylamine, trimethylamine, triethylamine, ethyldimethylamine, diethylmethylamine, methanolamine, ethanolamine, dimethanolamine, diethanolamine, trimethanolamine, triethanolamine, pyrrolidine, piperidine, piperazine, morpholine, methylmorpholine, ethylmorpholine, oxazolidine, thiomorpholine, thiazolidine, etc.

The solvent can consist of one or more organic solvents, one or more organic solvents and water, or water alone.

Examples of organic solvents that can be used as solvent components include, as protic solvents, monohydric alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol, and butyl alcohol; dihydric alcohols such as ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, and propylene glycol; trihydric alcohols such as glycerin; and derivatives thereof. Examples of aprotic solvents include lactone compounds such as γ-butyrolactone; sulfolane, methylsulfolane, dimethylsulfolane, ethylene carbonate, propylene carbonate, pyrrolidine, 2-pyrrolidinone, N-methyl-2-pyrrolidinone, 1,3-dimethyl-2-imidazolidinone, tetrahydrofuran, acetonitrile, N-methylformamide, N,N-dimethylformamide, nitrobenzene; and derivatives thereof. Of these, one may be used alone, or two or more may be used in mixture. For example, both protic and aprotic solvents may be used.

The electrolyte 6 according to this embodiment further contains a 4-nitropyridine N-oxide compound (NpNO compound) _, which is one of the nitro compounds. The 4-nitropyridine N-oxide compound referred to here is a compound containing 4- _{nitropyridine} N-oxide ( _C5H4N2O3 ) or a derivative thereof, as shown in the following chemical formula ( ₁ ).

In chemical formula (1), ^X1 , ^X2 , ^X3 , and ^X4 each independently represent a hydrogen atom or an atom or group other than hydrogen, and they may be the same or different.

As a basic embodiment, 4-nitropyridine N-oxide is used, but as shown in the chemical formula (1) above, one or more hydrogen atoms ^X1 - ^X4 of 4-nitropyridine N-oxide may be substituted with other atoms or groups. As such derivatives, for example, a compound in which one or more hydrogen atoms of 4-nitropyridine N-oxide are substituted with an alkyl group (expressed in chemical formula (1), ^X1 , ^X2 , ^X3 , and ^X4 each independently represent a hydrogen atom or an alkyl group, and may be the same or different NpNO compound) can be used. Specifically, for example, 3-methyl-4- _{nitropyridine N-oxide (C6H6N2O3 )} (expressed in chemical formula ( ₁ ), an NpNO compound in _which ^X1 , ^X3 , and ^X4 are hydrogen atoms (H) and ^X2 is a methyl group ( _-CH3 )) can be used.

NpNO compounds, as nitro compounds containing a nitro group, have the effect of suppressing the generation of hydrogen gas within the electrolytic capacitor 1, thereby suppressing the rise in internal pressure of the electrolytic capacitor 1. As a result, bulging of the exterior, leakage of electrolyte, and even the operation (opening) of the explosion-proof valve 8, which occur as the internal pressure rises, can be prevented. Since hydrogen generation within the electrolytic capacitor 1 becomes more problematic the higher the water content in the electrolyte 6, incorporating NpNO compounds into the electrolyte 6, which uses a water-containing solvent, effectively suppresses the generation of hydrogen gas and the rise in internal pressure.

Furthermore, NpNO compounds exhibit superior solubility in polar solvents such as ethylene glycol compared to conventional substances like nitroacetophenone and nitrobenzoic acid. This is presumed to be because NpNO compounds possess a pyridine ring and an oxygen coordination bond, resulting in relatively high polarity and excellent solubility in polar solvents. In fact, when the inventors examined the solubility in ethylene glycol, one of the main solvents, by evaluating the presence or absence of crystals after heating and dissolving the substance in a solvent and leaving it at room temperature for one day, they found that 4-nitropyridine N-oxide could dissolve at more than twice the concentration (mass%) of both 3'-nitroacetophenone and nitrobenzoic acid.

Furthermore, the NpNO compound hardly causes the voltage drop of the electrolytic capacitor 1 that occurs when conventional nitro compounds are blended at relatively high concentrations or after high-temperature loading. In other words, the NpNO compound exhibits a clear voltage drop suppression effect compared to these conventional substances. As shown in the examples described later, in the case where an electrolyte 6 containing 4-nitropyridine N-oxide was used, the electrolytic capacitor 1 operated normally even after a constant voltage was applied at a high temperature of 125°C for 500 hours without short-circuiting due to dielectric breakdown or other issues that may occur when the voltage drops. In fact, the voltage drop (spark voltage) measured using an oxide film 3a-forming foil for the electrolyte 6 after high-temperature loading at 125°C hardly decreased compared to before high-temperature loading, at both the conventional normal amount range of 0.03 mol/L and the higher 0.12 mol/L. Furthermore, in another example, it has been confirmed that the dielectric strength does not decrease even when 4-nitropyridine N-oxide is added at an even higher concentration (0.18 mol/L). Thus, stable and favorable results are obtained regardless of the amount (concentration) of the NpNO compound. Therefore, the amount (concentration) of the NpNO compound in the electrolyte 6 can be set to, for example, 0.03-0.30 mol/L, 0.06-0.30 mol/L, 0.12-0.30 mol/L, 0.18-0.30 mol/L, etc., increasing up to approximately 0.3 mol/L, the upper limit of solubility in ethylene glycol. Moreover, the NpNO compound can be added at even higher concentrations in solvents such as water, which are more easily soluble.

In addition, the electrolyte 6 may contain known functional substances that can be incorporated into the electrolyte 6 of the electrolytic capacitor 1, in addition to the NpNO compound. These functional substances refer to substances that have some useful function in the electrolytic capacitor 1, such as oxide film 3a repair function, pressure resistance function, high temperature resistance function, low temperature resistance function, and hydrogen gas absorption function. Some functional substances can function as electrolytes (solutes), but when such substances are incorporated as functional substances, they are primarily incorporated for their effects due to useful functions other than electrolyte (solute) function. Examples of such functional substances include phosphoric acid compounds, chelate compounds, specific polymers, and carbohydrates.

[Manufacturing method for electrolytic capacitors]
Next, this specification will describe a method for manufacturing the electrolytic capacitor 1 according to this embodiment. Figure 4 is a flowchart showing an example of a method for manufacturing the electrolytic capacitor 1 according to this embodiment.

As shown in Figure 4, the electrolytic capacitor 1 according to this embodiment is manufactured, for example, by performing a capacitor element formation step S01, an electrolyte introduction step S02, a sealing step S03, and an aging step S04. Of these, the operation of preparing an electrolyte 6 of a specific composition in the electrolyte introduction step S02 is a novel and advantageous feature common to all electrolytic capacitors according to the present invention. On the other hand, for the other operations, known manufacturing processes and operations for this embodiment and other types, such as hybrid types, terminal configurations other than leaded types, and element configurations other than wound types, can be used in addition to the methods exemplified herein.

In the capacitor element formation process S01, the anode foil 3 is subjected to a surface widening treatment and a chemical conversion treatment beforehand. For example, the etching treatment involves applying a DC or AC voltage to the raw material metal foil, such as aluminum foil, in an etching solution (a strongly acidic aqueous solution such as hydrochloric acid). This creates an uneven surface on the metal foil, increasing its specific surface area. Subsequently, the chemical conversion treatment involves applying a DC voltage to the etched foil in a chemical conversion solution (a weakly acidic aqueous solution such as ammonium borate). This forms an oxide film 3a as a dielectric layer on the surface of the etched foil. The cathode foil 4 is also subjected to a surface widening treatment and, if necessary, a chemical conversion treatment or other treatments.

Furthermore, the anode lead terminals 10 and cathode lead terminals 11 are connected to the anode foil 3 and cathode foil 4 by stitch connections, cold pressure welding, or the like. The connection of the lead terminals 10 and 11 can be performed before or after the chemical conversion treatment.

Then, as shown in Figure 2, the anode foil 3 and cathode foil 4 are overlapped with a separator 5 in between and wound to form a cylindrical shape, and a retaining material such as tape (not shown) is attached to hold the wound state. This forms the capacitor element 2.

In the hybrid type, after the formation of the capacitor element 2, the chemical conversion treatment may be performed again at any time to repair any damaged oxide film 3a. This re-chemical conversion treatment is optional; for example, it is not prohibited to perform the re-chemical conversion treatment on this embodiment, nor is it prohibited to refrain from performing it on the hybrid type.

Next, in the electrolyte introduction step S02, the electrolyte 6 is prepared and introduced into the capacitor element 2 to impregnate it. The preparation of the electrolyte 6 can be performed at any time before introducing the electrolyte 6. The composition of the electrolyte 6 is as already described for the electrolyte 6 according to this embodiment. That is, a 4-nitropyridine N-oxide compound (NpNO compound) is further added to the basic configuration in which the electrolyte is dissolved or dispersed in a solvent. Specifically, for example, 0.03 to 0.30 mol/L of the NpNO compound can be added to the electrolyte 6. Also, for example, 4-nitropyridine N-oxide or its derivatives can be added as the NpNO compound. Furthermore, any known functional substances may be optionally added to the electrolyte 6.

The electrolyte 6 is introduced by immersing the capacitor element 2 in the electrolyte 6 for a predetermined time. As mentioned above, NpNO compounds and other functional substances may be added during the preparation of the electrolyte 6. Alternatively, functional substances may be introduced into the capacitor element 2 by immersing it in a liquid containing NpNO compounds and other functional substances separately from the electrolyte 6, either before or after introducing the electrolyte 6 into the capacitor element 2. The introduction of the electrolyte 6 and functional substances may be carried out under controlled pressure as needed. Furthermore, after introduction, excess liquid may be removed using a centrifuge as needed.

Next, in the sealing process S03, the capacitor element 2 is housed in a bottomed cylindrical outer case 7, and the sealing body 9 is attached to the opening 7a of the outer case 7. The lead terminals 10 and 11 drawn out from the capacitor element 2 are passed through the through-holes of the sealing body 9, thereby leading them out of the electrolytic capacitor 1 through the through-holes. The opening 7a of the outer case 7 is crimped in a direction perpendicular to the mounting direction of the sealing body 9 (Z direction in the drawing). This forms the first crimped portion 7c shown in Figure 1. The open end 7b of the outer case 7 is also crimped in a direction parallel to the mounting direction of the sealing body 9 (Z direction in the drawing). This forms the second crimped portion 7d shown in Figure 1. As a result, the outer case 7 is sealed by the sealing body 9 and sealed by the two crimped portions 7c and 7d. In this way, the electrolytic capacitor 1 is formed. For mounting purposes, the outer case 7 is further covered with an outer sleeve made of an insulating material such as polyvinyl chloride. This coating can be applied before or after the aging process S04 described below.

Finally, in the aging process S04, a DC voltage is applied to the formed electrolytic capacitor 1 under high-temperature conditions. This activates the oxide film 3a repair function of predetermined components in the electrolyte 6, repairing areas where the oxide film 3a was peeled off during the manufacturing process, as well as areas where the oxide film 3a is relatively thin, thereby suppressing leakage current and stabilizing performance. Furthermore, the aging process also provides a debugging effect, such as eliminating unexpected initial defects.

A basic composition was prepared by dissolving a predetermined organic acid as the electrolyte and a predetermined functional substance as an additive in ethylene glycol and water as the solvents. Nitro compounds shown in Table 1 were then added to the electrolyte solution to the concentrations shown in Table 1, thereby preparing the electrolyte solutions for the examples and comparative examples. Each test was conducted using these example electrolyte solutions. However, for Comparative Example 3, it was not possible to dissolve the entire amount of the added nitro compound, 3'-nitroacetophenone, in the solvent, and therefore the electrolyte solution could not be prepared. Consequently, all tests described later could not be performed for Comparative Example 3. The composition ratio of the basic composition without added nitro compounds was approximately 65% by mass of ethylene glycol, approximately 5% by mass of water, approximately 10% by mass of the electrolyte, and 20% by mass of other additives. This basic composition electrolyte solution was used as Reference Example 1, and Test 3 described later was performed.

[Test 1]
An aluminum electrolytic capacitor with a rated voltage of 450V was manufactured by a conventional method in accordance with the embodiment described above. Etched aluminum foil was used for the anode and cathode foils, and an oxide film was formed on the anode foil by chemical conversion treatment. The anode foil and cathode foil, to which lead terminals were connected, were wound together with a separator in between to form a capacitor element. Next, the electrolyte for each example was introduced into this capacitor element, and it was housed in an outer case and sealed to manufacture an electrolytic capacitor. After that, a predetermined aging treatment was performed.

For each example, an electrolytic capacitor was subjected to a DC voltage of 450V at a temperature of 105°C, and the time until the explosion-proof valve activated (opened) was measured. Ten units were used in each example. The results are shown in Table 2.

[Test 2]
Furthermore, each electrolytic capacitor in each example was subjected to a DC voltage of 450V for 500 hours at a temperature of 125°C to check for the occurrence of a short circuit. Ten capacitors were used in each example. The results are shown in Table 2.

[Test 3]
In addition, anode foil pieces (10 x 20 mm) were prepared by etching and chemical conversion treatment to form an oxide film on aluminum foil pieces, and cathode foil pieces were prepared as plain aluminum foil pieces of the same size. The anode foil pieces and cathode foil pieces were immersed in 2 ^cm² of the electrolyte solution for each example, and a load was applied by passing a constant current while the electrolyte solution was kept at 85°C, and the voltage behavior (initial voltage behavior) was measured.

Furthermore, the medium bottles containing the electrolytes for each example were kept at a constant temperature of 125°C for 500 hours using a forced-convection constant-temperature incubator (PH-202, manufactured by ESPEC). Afterward, the voltage behavior (voltage behavior after high-temperature storage) was measured using the electrolytes from each example after high-temperature storage, while a constant current was applied at 85°C, as described above. The number of samples for each example was one. Figure 5A shows the initial voltage behavior of Example 1, Comparative Examples 1 and 2, and Reference Example 1. Figure 5B shows the voltage behavior after high-temperature storage of Example 1, Comparative Examples 1 and 2, and Reference Example 1. Figure 6A shows the initial voltage behavior of Example 2, Comparative Example 4, and Reference Example 1. Figure 6B shows the voltage behavior after high-temperature storage of Example 2, Comparative Example 4, and Reference Example 1. Reference Example 1 shown in Figures 6A and 6B is a re-explanation of Reference Example 1 shown in Figures 5A and 5B. In the graphs of Figures 5A and 5B, the time range on the horizontal axis (from minimum to maximum value) and the voltage range on the vertical axis (from minimum to maximum value) are matched. Similarly, in the graphs of Figures 6A and 6B, the time range on the horizontal axis (from minimum to maximum value) and the voltage range on the vertical axis (from minimum to maximum value) are matched. Of these, the voltage range on the vertical axis is consistent across all figures: 5A, 5B, 6A, and 6B. In all figures, the same voltage value α, used as a reference value for comparison, is shown as a dashed line. Furthermore, in all figures, except for Reference Example 1, the plots are shifted by adding a constant to the time value to prevent overlap of the voltage behavior of each example. In the voltage behavior of each example shown in each figure, the spark voltage is defined as the voltage at the point when the disturbance in the rising curve due to sparking or scintillation was first observed. A higher spark voltage indicates a higher withstand voltage.

As shown in Table 2, in Example 1 and Comparative Examples 1 and 2, where the nitro compound reagent was added at a conventional amount of 0.03 mol/L (within the normal range), all reagents could be dissolved in the solvent to prepare the electrolyte. In the test (Test 1) where the rated voltage was applied at a high temperature of 105°C, the explosion-proof valves of all tested equipment activated within 2000 to 2500 hours in all cases where the reagent was added. When electrolytic capacitors with the same specifications as the tested equipment, using an electrolyte with the same basic composition as the equipment without the nitro compound, were subjected to the same voltage conditions as in Test 1, the explosion-proof valves activated in almost all cases within 500 to 1000 hours. In comparison, in Test 1, the hydrogen gas generation suppression effect within the electrolytic capacitor was observed in all cases where the reagent was added, resulting in an extended explosion-proof valve activation time and thus an extended lifespan for the electrolytic capacitor. There was no difference in the explosion-proof valve activation times across the categories in Table 2, indicating that the hydrogen gas absorption performance of each reagent was comparable.

On the other hand, in an accelerated test (Test 2) where a constant voltage was applied at an even higher temperature of 125°C for 500 hours, in Comparative Examples 1 and 2, where 3'-nitroacetophenone or 4-nitrobenzyl alcohol was added, short circuits occurred in multiple devices due to dielectric breakdown caused by a decrease in withstand voltage. In contrast, in Example 1, where 4-nitropyridine N-oxide was added, no short circuits occurred in any of the devices, and the electrolytic capacitors operated without problems.

In Test 3, where the dielectric strength of the electrolytes in each example was measured under the same high-temperature load of 125°C as in Test 2, as shown in Figures 5A and 5B, Comparative Examples 1 and 2, which had 3'-nitroacetophenone or 4-nitrobenzyl alcohol added, showed a decrease in dielectric strength after high-temperature loading compared to the initial state. In contrast, Example 1, which had 4-nitropyridine N-oxide added, showed no decrease in dielectric strength even after high-temperature loading; in fact, it showed an upward trend compared to the initial state. As shown in Figure 5A, the initial dielectric strength of each example, including Reference Example 1 which had no nitro compounds added, was similar. Therefore, comparing the graphs of each example after high-temperature storage in Figure 5B, it is clear that after high-temperature loading, the dielectric strength of Reference Example 1 hardly changed, the dielectric strength of Comparative Examples 1 and 2 decreased, and the dielectric strength of Example 1 did not decrease at all.

Next, in Example 2 and Comparative Examples 3 and 4, where the nitro compound reagent was added at a concentration exceeding the conventional normal range of 0.12 mol/L, as mentioned above, in Comparative Example 3, the 3'-nitroacetophenone could not be fully dissolved in the solvent, and the electrolyte could not be prepared. On the other hand, with 4-nitrobenzyl alcohol (Comparative Example 4) and 4-nitropyridine N-oxide (Example 2), the entire amount of the reagent added could be dissolved in the solvent, and the electrolyte could be prepared. Furthermore, in the test (Test 1) where the rated voltage was applied at a high temperature of 105°C, the explosion-proof valves of all tested equipment activated within 6000 to 6500 hours in all cases where the reagents were added. In other words, in these examples, the effect of suppressing hydrogen gas generation in the electrolytic capacitor improved with increasing reagent levels, and as a result, the explosion-proof valve activation time was further extended, resulting in a further extension of the electrolytic capacitor's lifespan.

On the other hand, in an accelerated test (Test 2) where a constant voltage was applied at an even higher temperature of 125°C for 500 hours, Comparative Example 4, which had 4-nitrobenzyl alcohol added, showed a twofold increase in the short-circuit rate after high-temperature storage compared to Comparative Example 2, which had the usual amount of the reagent added. While the initial withstand voltage was almost the same for Comparative Example 2 (shown in Figure 5A) and Comparative Example 4 (shown in Figure 6A), a comparison of the withstand voltage after high-temperature storage for each example (shown in Figures 5B and 6B) reveals a significant decrease in withstand voltage in Comparative Example 4, indicating that the decrease in withstand voltage increased with increasing reagent levels.

In Example 2, where 4-nitropyridine N-oxide was added, as shown in Table 2, compared to Example 1, no short circuits occurred in any of the instruments even with increased reagent levels, and the electrolytic capacitors operated without problems. Looking at the actual withstand voltage, as shown in Figures 6A and 6B, there was almost no change even after being left at high temperatures compared to the initial state, and there was almost no decrease in withstand voltage even after high-temperature loading. Therefore, it can be seen that 4-nitropyridine N-oxide does not significantly reduce the withstand voltage even when the amount (concentration) added is increased, and virtually no malfunctions occur.

[Test 4]
Furthermore, for the electrolyte solution containing 4-nitropyridine N-oxide as Example 3, or 4-nitrophenol as Comparative Example 5, added to the basic composition without nitro compounds (Reference Example 1) so that the concentration after addition reached 0.18 mol/L, the voltage behavior (initial voltage behavior) was measured by applying a load with a constant current while maintaining a temperature of 85°C, using the same method as in Test 3. The results are shown in Figure 7. Reference Example 1 shown in Figure 7 is a reproduction of Reference Example 1 shown in Figure 5A. The voltage range on the vertical axis of Figure 7 (from minimum to maximum value) is the same as that in Figures 5A-6B. The voltage value α shown in Figures 5A-6B is similarly shown as a dashed line in Figure 7. Also, in Figure 7, for each example other than Reference Example 1, a constant was added to the time value to shift the plot so that the voltage behavior of each example does not overlap.

As shown in Figure 7, Comparative Example 5, to which 4-nitrophenol was added, had a lower dielectric strength compared to Reference Example 1, which did not contain 4-nitrophenol. However, Example 3, to which 4-nitropyridine N-oxide was added, showed almost the same dielectric strength as Reference Example 1.

From the above, it was confirmed that nitro compounds have an effect of suppressing hydrogen gas generation in electrolytic capacitors, and that this effect improves with increasing concentration. On the other hand, conventional substances, exemplified by nitrobenzene compounds, particularly reduced the withstand voltage of electrolytic capacitors after high-temperature loading (Comparative Examples 1 and 2), and it was revealed that the decrease in withstand voltage increased with increasing concentration (Comparative Example 4). Furthermore, some conventional substances, at high concentrations, already reduced the withstand voltage even without high-temperature loading (Comparative Example 5), or had insufficient solubility in polar solvents such as ethylene glycol, making it difficult to increase the concentration (Comparative Example 3). In contrast, the 4-nitropyridine N-oxide compound according to the present invention has excellent solubility in polar solvents such as ethylene glycol, allowing for sufficient concentration increases, and does not reduce the withstand voltage even at high concentrations. Moreover, the 4-nitropyridine N-oxide compound does not reduce the withstand voltage even after high-temperature loading, and can maintain almost the same withstand voltage reduction suppression effect regardless of the amount added and temperature.

Therefore, by incorporating a high concentration of 4-nitropyridine N-oxide compound into the electrolyte, it is possible to stably operate the electrolytic capacitor in a temperature range including high temperatures while greatly suppressing hydrogen gas generation and hardly reducing the withstand voltage. As for the amount (concentration) of 4-nitropyridine N-oxide compound in the electrolyte, for example, the concentration ranges of 0.03 to 0.12 mol/L, 0.03 to 0.18 mol/L, and 0.03 to 0.20 mol/L, corresponding to the concentration range in the examples, are naturally applicable. Furthermore, as other examples, concentrations higher than the conventional normal range of 0.03 to 0.06 mol/L, such as 0.06 to 0.12 mol/L, 0.06 to 0.18 mol/L, 0.06 to 0.20 mol/L, 0.12 to 0.18 mol/L, and 0.12 to 0.20 mol/L, can also be set. Furthermore, the concentration can be increased to approximately 0.3 mol/L, the upper limit of solubility in ethylene glycol, allowing for concentrations of 0.03–0.30 mol/L, 0.06–0.30 mol/L, 0.12–0.30 mol/L, 0.18–0.30 mol/L, etc. It can also be formulated at even higher concentrations in solvents that dissolve more easily, such as water.

Furthermore, after the electrolyte is impregnated into the capacitor element, some of the NpNO compound is adsorbed onto the electrode foil, etc., or consumed during the aging process. Therefore, the content (concentration) of NpNO in the electrolyte of an electrolytic capacitor is somewhat lower than the blending amount (concentration). In addition, some NpNO compound may be lost due to centrifugation of the sample, which is a common analytical method in this field. It is difficult to uniformly define the content (concentration) corresponding to the blending amount (concentration) as it also depends on the product specifications. However, for relatively high-concentration blends such as 0.12 mol/L or 0.18 mol/L, the content (concentration) in the electrolyte of an electrolytic capacitor is usually 0.03 mol/L or higher. On the other hand, for relatively low-concentration blends such as 0.03 mol/L, the content (concentration) can be as low as 0.01 mol/L, or even as low as 0.001 mol/L. Therefore, the content (concentration) of the NpNO compound according to the present invention in the electrolyte of an electrolytic capacitor can be, for example, 0.001 to 0.12 mol/L, 0.001 to 0.18 mol/L, 0.001 to 0.20 mol/L, 0.001 to 0.30 mol/L, 0.01 to 0.12 mol/L, 0.01 to 0.18 mol/L, 0.01 to 0.20 mol/L, 0.01 to 0.30 mol/L, 0. 03-0.12mol/L, 0.03-0.18mol/L, 0.03-0.20mol/L, 0.03-0.30mol/L, 0.06-0.12mol/L, 0.06-0.18mol/L, 0.06 ~0.20 mol/L, 0.06~0.30 mol/L, 0.12~0.18 mol/L, 0.12~0.20 mol/L, 0.12~0.30 mol/L, 0.18~0.30 mol/L, etc.

1 Electrolytic capacitor 2 Capacitor element 3 Anode foil 3a Oxide film 4 Cathode foil 5 Separator 6 Electrolyte 7 Outer case 7a Opening 7b Open end 7c First crimping part 7d Second crimping part 8 Explosion-proof valve 9 Sealing body 10 Anode lead terminal 11 Cathode lead terminal S01 Capacitor element formation process S02 Electrolyte introduction process S03 Sealing process S04 Aging process

Claims (6)

A capacitor element having an anode foil on which a dielectric layer is formed, a cathode foil, and a separator disposed between the anode foil and the cathode foil,
The capacitor element comprises an electrolyte impregnated within it,
The electrolytic capacitor is characterized in that the electrolyte contains a 4-nitropyridine N-oxide compound. The electrolytic capacitor according to claim 1, characterized in that the 4-nitropyridine N-oxide compound is 4-nitropyridine N-oxide. The electrolytic capacitor according to claim 1 or 2, characterized in that the solvent of the electrolyte is a solvent containing water. A method for manufacturing an electrolytic capacitor, characterized by incorporating a 4-nitropyridine N-oxide compound into the electrolyte. The method for manufacturing an electrolytic capacitor according to claim 4, characterized in that the 4-nitropyridine N-oxide compound is 4-nitropyridine N-oxide. The method for manufacturing an electrolytic capacitor according to claim 4 or 5, characterized in that a solvent containing water is used as the solvent for the electrolyte.