JP7827766B2 - Electrolytic capacitor and method for manufacturing the same - Google Patents

Description

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

An electrolytic capacitor is an electricity storage device based on a capacitor element consisting of an anode foil with an oxide film formed as a dielectric layer, a cathode foil, and a separator disposed between them, into which an electrolyte is introduced (impregnated). There are two types of electrolytic capacitors: one in which an electrolytic solution is introduced as a non-solid electrolyte into the capacitor element, and one in which a solid electrolyte such as a conductive polymer is introduced. There are also so-called "hybrid types" of electrolytic capacitors, in which an electrolytic solution is introduced together with a solid electrolyte into the capacitor element, thereby taking advantage of the benefits of both solid and non-solid electrolytes.

The electrolyte in an electrolytic capacitor contains at least a solute (electrolyte) and a solvent that dissolves or disperses the solute. Regarding the composition of the electrolyte in an electrolytic capacitor, the water content in the solvent is sometimes increased to lower the resistance of the electrolyte, with the aim of lowering the ESR and impedance of the electrolytic capacitor. However, when the water content in the electrolyte, which is prone to chemical reactions, increases, especially at high temperatures, the dissolved metal elements (e.g., Al ³⁺ ) of the electrode foil undergo a hydration reaction with water (i.e., OH ⁻ ). The resulting hydroxide (e.g., Al(OH) ₃ ) precipitates on the surface of the electrode foil, degrading the foil and increasing leakage current. Furthermore, hydrogen gas generated by the hydration reaction and water electrolysis increases internal pressure, causing the explosion-proof valve to open in a relatively short time, thereby shortening the lifespan.

Consequently, conventional electrolyte solutions containing aqueous solvents containing a certain amount of water have been formulated with phosphoric acid to prevent hydration degradation of electrode foils (Patent Document 1: JP 2004-186239 A).

Japanese Patent Application Laid-Open No. 2004-186239

Although the inhibitory effect of phosphoric acid on electrode foil hydration degradation is well known, there has been a problem in that the amount of phosphoric acid added to the electrolyte solution is limited, resulting in an insufficient inhibitory effect on electrode foil hydration degradation. Specifically, phosphate ions released into the electrolyte react with eluted metal elements (e.g., Al ³⁺ ) in the electrode foil, resulting in the formation of a water-resistant film on the surface of the electrode foil (e.g., AlPO ₄ ). This inhibits hydration degradation of the electrode foil, but phosphoric acid (phosphate ions) gradually disappears from the electrolyte. Therefore, a relatively small amount of phosphoric acid does not sufficiently inhibit hydration degradation of the electrode foil, and the inhibitory effect is difficult to sustain. Therefore, adding a relatively large amount of phosphoric acid to enhance the inhibitory effect on electrode foil hydration degradation has the disadvantage that, due to its acidic nature, phosphoric acid itself has the property of degrading electrode foil, which actually accelerates electrode foil degradation, increases leakage current, accelerates hydration reactions, and shortens the electrode foil's lifespan.

The present invention was made in consideration of the above circumstances, and aims to provide an electrolytic capacitor and a method for manufacturing an electrolytic capacitor that can suppress leakage current and achieve a long life despite containing a relatively large amount of phosphate ion source in an electrolyte composition in which a predetermined amount of phosphate ion source is blended with an aqueous electrolyte containing a predetermined amount of water.

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

An electrolytic capacitor according to the present invention is a low-voltage electrolytic capacitor comprising: 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 solution impregnated in the capacitor element, wherein the electrolyte solution contains an aqueous solvent containing 55 wt % or more of water, a phosphate ion source, and a phosphonic acid chelating agent which is a compound having a phosphonic acid group and having chelating ability (however, excluding ethylenediaminetetra(methylenephosphonic acid) (EDTMP), ethylenediaminetetrakis(methylenephosphonic acid) (EDTPO), and ethylenediamine-N,N'-bis(methylenephosphonic acid), and substances having the same chemical structure as these).

According to the present invention, the phosphonic acid-based chelating agent adsorbs to the surface of the electrode foil and acts as a protective film, protecting the electrode foil from the effects of excess phosphate ions. Furthermore, the protective film moderately slows down the excessive reaction between the metal elements of the electrode foil, such as Al ³⁺ , and the phosphate ions, thereby suppressing the loss of phosphate ions. As a result, the present invention can suppress leakage current and achieve a long life, even when the electrolyte contains a relatively large amount of a phosphate ion source, for example, 120.0 to 500.0 mmol/kg.

The electrolytic solution is characterized by containing an organic acid ammonium salt , which is preferably composed mainly of ammonium adipate with ammonium formate added thereto.

The phosphate ion source can be one or more substances selected from the group consisting of phosphoric acid, phosphorous acid, hypophosphorous acid, salts of each of these three acids, esters of each of the three acids and salts of these esters, and condensates of each of the three acids and salts of these condensates. Furthermore, the phosphate ion source is preferably one or more substances selected from the group consisting of phosphoric acid, phosphate salts, phosphate esters, and salts of phosphate esters.

The phosphonic acid chelating agent can be one or more materials selected from the group including 1-hydroxyethylidene-1,1-diphosphonic acid, nitrilotris(methylenephosphonic acid), and 2-phosphonobutane-1,2,4-tricarboxylic acid .

The method for producing an electrolytic capacitor according to the present invention is characterized in that an electrolytic solution is prepared by blending an aqueous solvent containing at least 55 wt % or more of water, a phosphate ion source, and a phosphonic acid chelating agent which is a compound having a phosphonic acid group and chelating ability (however, excluding ethylenediaminetetra( methylenephosphonic acid) (EDTMP), ethylenediaminetetrakis(methylenephosphonic acid) ( EDTPO), ethylenediamine-N,N'-bis(methylenephosphonic acid), and substances having the same chemical structure as these ).

According to the present invention, the phosphonic acid-based chelating agent adsorbs to the surface of the electrode foil and acts as a protective film, protecting the electrode foil from the effects of excess phosphate ions. Furthermore, the protective film moderately slows down the excessive reaction between the metal elements of the electrode foil, such as Al ³⁺ , and the phosphate ions, thereby suppressing the loss of phosphate ions. As a result, the present invention can suppress leakage current and achieve a long life, even when the electrolyte contains a relatively large amount of a phosphate ion source, for example, 120.0 to 500.0 mmol/kg.

The electrolytic solution is characterized in that it contains an organic acid ammonium salt. The organic acid ammonium salt is preferably composed mainly of ammonium adipate with ammonium formate added.

The phosphate ion source can be one or more substances selected from the group consisting of phosphoric acid, phosphorous acid, hypophosphorous acid, salts of each of these three acids, esters of each of the three acids and salts of these esters, and condensates of each of the three acids and salts of these condensates. Furthermore, the phosphate ion source is preferably one or more substances selected from the group consisting of phosphoric acid, phosphate salts, phosphate esters, and salts of phosphate esters.

The phosphonic acid chelating agent can be one or more materials selected from the group including 1-hydroxyethylidene-1,1-diphosphonic acid, nitrilotris(methylenephosphonic acid), and 2-phosphonobutane-1,2,4-tricarboxylic acid .

According to the present invention, an electrolyte composition in which a predetermined amount of phosphate ion source is blended with an aqueous electrolyte containing a predetermined amount of water can suppress leakage current and achieve a long life, even while containing a relatively large amount of the phosphate ion source.

FIG. 1 is a schematic view (front cross-sectional view) showing an example of an electrolytic capacitor according to this embodiment. FIG. 2 is an explanatory diagram illustrating an example of a capacitor element in the electrolytic capacitor according to this embodiment. FIG. 3 is a flowchart showing an example of a method for manufacturing an electrolytic capacitor according to this embodiment. FIG. 4 is an explanatory diagram illustrating an example of a capacitor element forming step in the method shown in FIG. 5A and 5B are graphs showing the results of Test 1. FIG. 6 is a graph showing the results of Test 3. FIG. 7 is a graph showing the results of Test 4.

The present invention relates to an electrolytic capacitor having structural characteristics in the composition of the electrolyte solution, and to a method for manufacturing the same. Below, as an embodiment of the present invention, an electrolytic capacitor of the type in which an electrolyte solution is introduced as a non-solid electrolyte into the capacitor element (a type in which a solid electrolyte is not introduced) will be described as an example. However, due to the nature of the present invention, the electrolytic capacitor according to the present invention is not limited to the type of electrolytic capacitor described here, and also includes, for example, so-called "hybrid type" electrolytic capacitors in which an electrolyte solution is introduced into the capacitor element together with a solid electrolyte such as a conductive polymer.

This specification will now be described in detail with reference to the drawings. Figure 1 is a schematic diagram (front cross-sectional view) illustrating an example of an electrolytic capacitor 1 according to this embodiment. However, for ease of understanding, the cross-sectional structure of the capacitor element 2 is not shown. Figure 2 is an explanatory diagram illustrating an example of the capacitor element 2 in the electrolytic capacitor 1 according to this embodiment, and schematically illustrates an example of the basic configuration of the capacitor element 2. This specification will now be described using an electrolytic capacitor 1 having a wound-type capacitor element 2 as an example of one embodiment. However, this element configuration is not limited to this, and other configurations, such as stacked and coin-type, are also possible. This specification will now be described using an example of a lead-type electrolytic capacitor 1 having lead terminals 4 as an example of one embodiment. However, this terminal configuration is not limited to this, and other configurations, such as screw terminals and board-mounted types, are also possible. This specification will now be described using an example of an aluminum electrolytic capacitor 1 having electrode foils 8 and 9 primarily made of aluminum or an aluminum alloy. However, this foil configuration is not limited to this, and other configurations, such as electrode foils primarily made of a valve metal other than aluminum or a valve metal alloy, are also possible.

As shown in FIG. 1 , the electrolytic capacitor 1 according to this embodiment includes a capacitor element 2 impregnated with an electrolyte 3 housed in a cylindrical case 6 with a bottom. The opening of the case 6 is sealed with a sealing material 5. Lead terminals 4 (anode terminal 4a and cathode terminal 4b) joined to the capacitor element 2 fit into through holes provided in the sealing material 5 and extend from the through holes to the outside of the electrolytic capacitor 1. The opening edge 6a of the case 6 crimps the sealing material 5, sealing the inside of the case 6. The case 6 is provided with an explosion-proof valve 7, which serves as a pressure valve. When the internal pressure of the electrolytic capacitor 1 reaches a certain level, the valve opens to release gas within the electrolytic capacitor 1, thereby preventing explosion. The number and location of the explosion-proof valves 7 are not limited; for example, they may be provided in the sealing material 5, or in both the case 6 and the sealing material 5. Furthermore, in a packaged product, the case 6 is typically packaged in an exterior packaging material (not shown).

Next, as shown in FIG. 2, the capacitor element 2 according to this embodiment includes an anode foil 8, a cathode foil 9, and a separator 10 disposed between the anode foil 8 and the cathode foil 9. An anode terminal 4a is bonded to the anode foil 8. A cathode terminal 4b is bonded to the cathode foil 9.

The electrode foils (anode foil 8 and cathode foil 9) are primarily composed of aluminum or an aluminum alloy. "Main material" here means that trace amounts of unintended other elements, such as less than 1.0% by mass (e.g., less than 0.5%, 0.1%, 0.05%, or 0.01%, by mass), are permitted. Meanwhile, other elements intentionally included in the aluminum alloy can be one or more of the following elements: Ta, Nb, Ti, Cr, Hf, Zr, Zn, W, Ni, V, Fe, Cu, Mn, Mg, Ga, Si, and B. The other metals constituting the aluminum alloy are not limited to valve metals; they may also include metallic elements other than valve metals or non-metallic elements. These other elements may be included in any amount within the range that allows the electrode foils 8 and 9 in the aluminum electrolytic capacitor 1 to function. If the content (by mass) of the intended additives is relatively small, the composition may contain more than the content (by mass) of unintended impurities. Note that foil forms other than those of this embodiment that can be applied to the present invention include foil forms in which the aluminum described above is replaced with other valve metals.

The electrode foils 8 and 9 may be formed of two layers: a core substrate and a coating material that covers the substrate. In this case, it is sufficient that at least the outer layer of coating material is primarily made of aluminum or an aluminum alloy. In other words, the composition of the substrate and the composition of the coating material may be the same or different.

The electrode foils 8, 9 have an expanded surface structure. The expanded surface structure is typically formed by etching. Alternatively, the expanded surface structure may be formed by vapor deposition, sintering, or the like of a primary metal powder. In this case, for example, the electrode foils 8, 9 themselves can be formed and the expanded surface structure can be formed by coating the core substrate with aluminum or aluminum alloy powder by vapor deposition or sintering. The expanded surface structure of the electrode foils 8, 9 can increase the specific surface area and thus the capacitance.

An oxide film 8a serving as a dielectric layer is formed on the surface of the enlarged anode foil 8 by chemical conversion treatment. Specifically, chemical conversion treatment is an anodizing process in which the target metal (here, the anode foil 8) is used as the anode and a voltage is applied between it and a chemical conversion solution to form the oxide film 8a.

The chemical conversion treatment can be performed, for example, by the following procedure. Specifically, the chemical conversion treatment involves immersing the capacitor element 2 in a chemical conversion solution bath, and applying a predetermined voltage between the anode terminal 4a as the anode and the chemical conversion solution for a predetermined period of time (e.g., applying a voltage of 100 V for 5 minutes). The capacitor element 2 is then removed from the chemical conversion solution bath and dried. Examples of chemical conversion solutions include aqueous solutions of ammonium adipate, ammonium borate, ammonium phosphate, ammonium glutarate, ammonium azelaate, ammonium tartrate, ammonium sebacate, ammonium pimelate, and ammonium suberate.

On the other hand, the surface-enlarged cathode foil 9 is not further processed, and a natural oxide film (not shown) is formed on the surface of the cathode foil 9 by oxygen in the air. However, this configuration is not limited to this, and, for example, a valve metal or valve metal alloy powder such as titanium may be vapor-deposited on the surface of the surface-enlarged cathode foil 9. This increases the dielectric constant and improves the capacitance. Alternatively, the surface of the surface-enlarged cathode foil 9 may be subjected to chemical conversion treatment in the same way as the anode foil 8, resulting in a non-polar electrolytic capacitor 1.

The separator 10 separating the anode foil 8 and the cathode foil 9 is made of paper made from natural cellulose fibers such as Manila hemp pulp, or cloth, sheet, or film made from synthetic fibers such as nylon, or a blend or spun product thereof. Synthetic fibers have the advantage of being able to be selected from materials with particularly excellent heat resistance. Examples of such synthetic fibers include nylon, polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), and polyphenylene sulfide (PPS). Note that Figure 2 is a schematic diagram illustrating an example of the basic configuration of a capacitor element 2, and while one separator 10 is shown, the number is not limited; for example, three separators may be used, as shown in Figure 4.

The capacitor element 2 is also impregnated with an electrolyte solution 3. As shown schematically in Figure 2, the electrolyte solution 3 is present in the gap between the anode foil 8 and the cathode foil 9. By contacting the dielectric layer (oxide film 8a) formed on the anode foil 8 and the cathode foil 9, the electrolyte solution 3 functions in place of the cathode foil 9 as a true cathode, essentially serving as the counter electrode to the anode foil 8. However, as long as the electrolyte solution 3 can perform its function, it does not have to completely fill the space between the electrode foils 8 and 9; there may be areas between the electrode foils 8 and 9 that are not filled with the electrolyte solution 3. The separator 10 allows the components of the electrolyte solution 3 to pass freely between the anode foil 8 and the cathode foil 9. However, depending on the configuration and material of the separator 10, the electrolyte solution 3 may also be impregnated within the separator 10.

The electrolyte solution 3 contains a solute (electrolyte) and a solvent that dissolves or disperses the solute. The solvent in this embodiment is an aqueous solvent containing a predetermined amount of water. The water content is not limited, but the electrolyte solution 3 may contain 40% or more by weight of water, and may further contain 50% or more, 60% or more by weight, 70% or more by weight, 80% or more by weight, or 85% or more by weight. The high water content in the electrolyte solution 3 is achieved by the inclusion of a phosphate ion source and a phosphonic acid chelating agent, as described below. The aqueous solvent may further contain components other than water (e.g., an organic solvent) or may be composed solely of water. Such an aqueous solvent increases the solubility of the electrolyte and the mobility of ions through the action of water, thereby lowering the resistivity of the electrolyte solution 3. The reduced resistance of the electrolyte solution 3 results in a lower ESR and lower impedance of the electrolytic capacitor 1.

Examples of organic solvents that can be used as solvent components include protic solvents such as monohydric alcohols like methyl alcohol, ethyl alcohol, propyl alcohol, and butyl alcohol; dihydric alcohols like ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, and propylene glycol; and trihydric alcohols like glycerin, as well as derivatives thereof. Aprotic solvents include lactone compounds like gamma-butyrolactone; sulfolane, methyl sulfolane, dimethyl sulfolane, 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. One of these solvents may be used alone, or two or more may be mixed and used. For example, both protic and aprotic solvents may be used.

The solute, which is an electrolyte, can be an organic acid, an inorganic acid, a composite compound of an organic acid and an inorganic acid, or a derivative or salt thereof. One of these may be used alone, or two or more may be mixed and used. For example, an organic acid and an inorganic acid may be used together.

Examples of organic acids and their derivatives include monocarboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, benzoic acid, and caprylic acid, as well as 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, and 1,10-decanedicarboxylic acid, as well as their derivatives. Examples of hydroxycarboxylic acids include citric acid and salicylic acid, as well as their derivatives. Examples of inorganic acids and their derivatives include boric acid and sulfamic acid, as well as their derivatives. Furthermore, examples of composite compounds of organic acids and inorganic acids and derivatives thereof include boron complexes of dicarboxylic acids or hydroxycarboxylic acids, such as borodisalic acid, borodimalonic acid, borodisuccinic acid, borodiadipic acid, borodimaleic acid, borodiphthalic acid, borodiglycolic acid, borodicitric acid, borodisalicylic acid, and derivatives thereof.

In addition, examples of salts of organic acids, inorganic acids, hybrid compounds of organic acids and inorganic acids, and derivatives thereof include ammonium salts, alkylammonium salts, amine salts, amidine salts, sodium salts, and potassium salts. 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, and thiazolidine.

In the examples described below, a mixture of two electrolytes, ammonium adipate and ammonium formate, is used as the solute.

Furthermore, the electrolyte solution 3 according to this embodiment contains a phosphate ion source. The phosphate ion source is a general term for compounds that generate phosphate ions in the electrolyte solution 3. The phosphate ions (PO ₄ ³⁻ ) generated from the phosphate ion source suppress hydration deterioration of the electrode foils 8 and 9 caused by the hydration reaction product (Al(OH) ₃ ) of water (OH ⁻ ) and aluminum ions (Al ³⁺ ) in the aqueous solvent. The phosphate ions (PO ₄ ³⁻ ) have significantly stronger adsorptivity or adhesiveness to the surfaces of the electrode foils 8 and 9 than hydroxide ions (OH ⁻ ), suppressing contact between the electrode foils 8 and 9 and the hydroxide ions (OH ⁻ ), thereby improving water resistance. The phosphate ions (PO ₄ ³⁻ ) also react with aluminum ions (Al ³⁺ ) eluted from the electrode foils 8 and 9, producing aluminum phosphate (AlPO ₄ ) that forms a water-resistant film that protects the surfaces of the electrode foils 8 and 9. Furthermore, the reaction between phosphate ions (PO ₄ ^3- ) and aluminum ions (Al ³⁺ ) competes with the hydration reaction, thereby suppressing the hydration reaction. Thus, electrolytic capacitor 1 having electrolyte solution 3 containing a phosphate ion source can suppress hydration deterioration of electrode foils 8, 9, thereby suppressing leakage current. Furthermore, electrolytic capacitor 1 can suppress the generation of hydrogen gas due to hydration reactions and electrolysis of water, thereby suppressing an increase in internal pressure of electrolytic capacitor 1 and extending the life of electrolytic capacitor 1.

On the other hand, in the conventional composition of electrolyte 3, when the amount of phosphate ions is relatively small, hydration deterioration of electrode foils 8 and 9 is not sufficiently suppressed, and the suppression effect is difficult to sustain. Furthermore, if the phosphate ion content is increased, the phosphate ions themselves cause a side reaction that accelerates the deterioration of electrode foils 8 and 9, thereby increasing leakage current and shortening the lifespan of electrolytic capacitor 1. Therefore, it was not possible to increase the phosphate ion content in electrolyte 3.

In contrast, the electrolyte solution 3 according to this embodiment contains a phosphonic acid chelating agent in addition to a phosphate ion source. The phosphonic acid chelating agent adsorbs to the surfaces of the electrode foils 8 and 9 and acts as a protective film, protecting the electrode foils 8 and 9 from the effects of excess phosphate ions. Therefore, even if the electrode foils 8 and 9 contain a relatively large amount of phosphate ions, the electrode foils 8 and 9 can fully exhibit the expected hydration degradation suppression effect without experiencing adverse side reactions. Furthermore, the protective film formed by the phosphonic acid chelating agent moderately slows the excessive reaction between Al ³⁺ and phosphate ions, thereby suppressing the loss of phosphate ions. Therefore, the hydration degradation suppression effect of the electrode foils 8 and 9, which is expected from the presence of a relatively large amount of phosphate ions, can be exhibited for a long period of time. As a result, the electrolytic capacitor 1 according to this embodiment contains a relatively large amount of water and a relatively large amount of a phosphate ion source in the electrolytic solution 3, which reduces the resistance of the electrolytic solution 3 due to the water, thereby achieving a low ESR and low impedance for the electrolytic capacitor 1, and also sufficiently suppresses leakage current due to the phosphate ion source, thereby extending the life of the electrolytic capacitor 1.

Specifically, the electrolyte solution 3 according to this embodiment contains a relatively large amount of the phosphate ion source, for example, 120.0 to 500.0 mmol/kg, 120.0 to 400.0 mmol/kg, 120.0 to 300.0 mmol/kg, 120.0 to 200.0 mmol/kg, 120.0 to 170.0 mmol/kg, 120.0 to 160.0 mmol/kg, or 125.0 to 155.0 mmol/kg, yet can suppress leakage current and achieve a long life. This content (120.0 to 500.0 mmol/kg) corresponds to a blending amount of about 1.2 to 5.0 mass% in the electrolyte solution 3 when the phosphate ion source is phosphoric acid (H ₃ PO ₄ ), for example, and is a higher concentration than the standard blending amount (appropriate blending amount) of about 1.0 mass% to about 100.0 mmol/kg to 105.0 mmol/kg of phosphoric acid in the composition of the electrolyte solution 3 to which no phosphonic acid chelating agent is added. However, from another perspective, the phosphonic acid chelating agent enables the blended phosphate ions to be utilized more efficiently, and therefore the amount of the phosphate ion source does not necessarily need to be extremely large (e.g., 500.0 mmol/kg), but can be appropriately large (e.g., 120.0 mmol/kg), thereby sufficiently suppressing leakage current and sufficiently achieving a long life.

Phosphate ion sources that can be used include phosphoric acid (orthophosphoric acid), phosphorous acid, hypophosphorous acid, salts of each of these three acids, esters of each of the three acids and salts of these esters, and condensates of each of the three acids and salts of these condensates. One of these may be used alone, or two or more may be used in combination.

Examples of salts of phosphoric acid, phosphorous acid, and hypophosphorous acid include ammonium salts, aluminum salts, sodium salts, calcium salts, and potassium salts. Examples of esters of phosphoric acid, phosphorous acid, and hypophosphorous acid include alkyl phosphate esters such as ethyl phosphate, diethyl phosphate, butyl phosphate, and dibutyl phosphate. Examples of salts of these esters include ammonium salts, aluminum salts, sodium salts, potassium salts, and calcium salts.

Examples of condensates of phosphoric acid, phosphorous acid, and hypophosphorous acid include condensed phosphoric acid, which is a condensate of phosphoric acid. Examples of condensed phosphoric acid include linear condensed phosphoric acids such as pyrophosphoric acid, tripolyphosphoric acid, and tetrapolyphosphoric acid, cyclic condensed phosphoric acids such as metaphosphoric acid and hexametaphosphoric acid, and compounds in which such linear condensed phosphoric acids are combined with cyclic condensed phosphoric acids. Examples of salts of condensates of phosphoric acid, phosphorous acid, and hypophosphorous acid include ammonium salts, aluminum salts, sodium salts, potassium salts, and calcium salts.

Phosphoric acid and phosphoric acid compounds decompose in electrolyte 3 to produce phosphate ions. Phosphorous acid and phosphorous acid compounds decompose in electrolyte 3 to produce phosphite ions, which are then oxidized to produce phosphate ions. Hypophosphorous acid and hypophosphite compounds decompose in electrolyte 3 to produce hypophosphorous acid, which is then oxidized to produce phosphate ions. Condensates are relatively stable in electrolyte 3 at room temperature in the neutral range, but can gradually decompose depending on temperature or pH conditions or over time to produce phosphate ions, phosphite ions, hypophosphite ions, etc. Therefore, phosphoric acid, phosphate salts, phosphate esters, and salts of phosphate esters, which can directly produce phosphate ions, are suitable phosphate ion sources, with phosphoric acid and phosphate salts being even more suitable.

The phosphonic acid chelating agent is a compound having phosphonic acid (H ₃ PO ₃ ), i.e., a phosphonic acid group (—P(═O)(OH) ₂ ), and chelating ability (the ability to form a complex with a metal ion). Examples of phosphonic acid chelating agents include 1-hydroxyethylidene-1,1-diphosphonic acid (abbreviation: HEDP), nitrilotris(methylenephosphonic acid) (abbreviation: NTMP), 2-phosphonobutane-1,2,4-tricarboxylic acid (abbreviation: PBTC), and ethylenediaminetetra(methylenephosphonic acid) (abbreviation: EDTMP). Of these, one type may be used alone, or two or more types may be used in combination. An appropriate type of phosphonic acid chelating agent may be selected depending on the specifications of the electrolytic capacitor 1 (e.g., heat resistance temperature, etc.). Taking into account the results of the tests (examples) described below, it can be seen that, overall, of the four phosphonic acid chelating agents listed above, EDTMP, HEDP, and NTMP are more preferable, and HEDP and NTMP are even more preferable.

Phosphonic acid-based chelating agents are distinguished from chelating agents that do not contain phosphonic acid groups because the phosphonic acid groups in the chelating agent are thought to be effective in adsorbing to the surfaces of the electrode foils 8 and 9 and forming protective films. In fact, according to the examples described below, the inclusion of a phosphonic acid-based chelating agent in electrolyte 3 resulted in particularly significant suppression of leakage current and particularly significant longevity of the electrolytic capacitor 1 under high-temperature conditions. The content of the phosphonic acid-based chelating agent in electrolyte 3 can be set within ranges such as 0.1 to 5.0% by mass, 0.1 to 4.0% by mass, 0.1 to 3.5% by mass, 0.1 to 3.0% by mass, 0.1 to 2.5% by mass, 0.1 to 2.0% by mass, and 0.1 to 1.6% by mass.

Furthermore, the electrolyte solution 3 may contain known functional substances that can be blended into the electrolyte solution 3 of the electrolytic capacitor 1. The term "functional substance" as used here refers to a substance that has some useful function in the electrolytic capacitor 1, such as repairing the oxide film 8a, resisting high temperatures, resisting low temperatures, or absorbing hydrogen gas. Some functional substances can also function as electrolytes (solutes), but when such substances are blended as functional substances, they are blended primarily for the purpose of achieving the desired effect through a useful function other than their function as an electrolyte (solute).

[Method of manufacturing electrolytic capacitor]
Next, this specification will describe a method for manufacturing the electrolytic capacitor 1 according to this embodiment. Fig. 3 is a flowchart showing an example of the method for manufacturing the electrolytic capacitor 1 according to this embodiment. Fig. 4 is an explanatory view (schematic perspective view) illustrating an example of the capacitor element formation step S01 in the method shown in Fig. 3.

As shown in FIG. 3, the electrolytic capacitor 1 according to this embodiment is manufactured by, for example, performing a capacitor element formation process S01, an electrolyte introduction process S02, a sealing process S03, and an aging process S04. Of these, the electrolyte introduction process S02, which introduces an electrolyte 3 of a specific composition, includes a novel and advantageous feature common to all electrolytic capacitors according to this embodiment. Meanwhile, processes S01, S03, and S04 include operations that are unnecessary or different for types other than this embodiment (e.g., hybrid types, element configurations other than wound types, terminal configurations other than lead wire types, etc.). These operations may be omitted or modified as appropriate for types other than this embodiment.

In the capacitor element formation process S01, for example, the anode foil 8 is subjected to the aforementioned surface-expanding treatment, such as etching, and the aforementioned chemical conversion treatment, before being joined to the anode terminal 4a. The cathode foil 9 is subjected to the aforementioned surface-expanding treatment, such as etching, before being joined to the cathode terminal 4b. Then, as shown in FIG. 4, a separator 10 is sandwiched between the anode foil 8 and the cathode foil 9 to isolate them, and the anode foil 8, cathode foil 9, and separator 10 are wound to form a cylindrical shape. A holding material (not shown), such as tape or film, is then attached to a predetermined portion of the outer periphery of the cylindrical shape to maintain the wound state. While not typically performed in the electrolytic capacitor 1 according to this embodiment, in the hybrid type, a chemical conversion treatment is typically performed on the cylindrically formed capacitor element 2 to repair any missing oxide film 8a. The chemical conversion treatment here is a re-chemical conversion treatment that repeats the anodizing treatment already performed on the anode foil 8 to repair the damaged oxide film 8a. This re-chemical conversion treatment is optional; for example, it is not prohibited to perform a re-chemical conversion treatment on the electrolytic capacitor 1 of this embodiment type, and conversely, it is not prohibited to not perform a re-chemical conversion treatment on a hybrid type.

Next, in the electrolyte solution introduction step S02, an electrolyte solution 3 of a specific composition is introduced into the capacitor element 2. The composition of the electrolyte solution 3 is as already described for the electrolyte solution 3 according to this embodiment. That is, the solvent is an aqueous solvent containing at least water. The water content is not limited, but may be 40% by mass or more in the electrolyte solution 3, and may even be 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 85% by mass or more. The aqueous solvent may further contain components other than water (e.g., organic solvents), but may also be composed solely of water. The solvent components other than water and the solute components may be the same as those described above, as appropriate.

In addition to the solvent and solute, a phosphate ion source and a phosphonic acid chelating agent are also blended into the electrolyte solution 3. The phosphate ion source is blended in such an amount that the concentration in the electrolyte solution 3 is 120.0 to 500.0 mmol/kg, 120.0 to 400.0 mmol/kg, 120.0 to 300.0 mmol/kg, 120.0 to 200.0 mmol/kg, 120.0 to 170.0 mmol/kg, 120.0 to 160.0 mmol/kg, or 125.0 to 155.0 mmol/kg, etc. Additionally, a phosphonic acid-based chelating agent is blended into the electrolyte solution 3 at a concentration of 0.1 to 5.0% by mass, 0.1 to 4.0% by mass, 0.1 to 3.5% by mass, 0.1 to 3.0% by mass, 0.1 to 2.5% by mass, 0.1 to 2.0% by mass, or 0.1 to 1.6% by mass. Furthermore, the electrolyte solution 3 may be blended with known functional substances that can be blended into the electrolyte solution 3 of the electrolytic capacitor 1.

As an example of introducing the electrolyte solution 3, the capacitor element 2 is immersed in a tank of prepared electrolyte solution 3. After a predetermined time has elapsed, the capacitor element 2 is removed from the tank of electrolyte solution 3. As described above, a known functional substance may be added during the preparation of the electrolyte solution 3. However, the functional substance may also be introduced into the capacitor element 2 by immersing the capacitor element 2 in a tank of a liquid containing the functional substance before or after introducing the electrolyte solution 3 into the capacitor element 2. The introduction of the electrolyte solution 3 and the functional substance may be carried out under controlled pressure as necessary.

Next, the procedure for the sealing step S03 will be described with reference to Figure 1. As an example, the capacitor element 2 is housed in a cylindrical case 6 with a bottom. The lead terminals 4 (anode terminal 4a and cathode terminal 4b) joined to the capacitor element 2 are fitted into through holes in the sealing material 5, and the opening of the case 6 is closed with the sealing material 5. As a result, the lead terminals 4 are extended outside the electrolytic capacitor 1. Next, the opening edge 6a of the case 6 is crimped to seal the inside of the case 6. Note that an explosion-proof valve 7 may be provided in the case 6 and/or the sealing material 5. In this way, the electrolytic capacitor 1 shown in Figure 1 is manufactured. Furthermore, the case 6 may be packaged in an exterior material (not shown). This packaging may be performed before or after the aging step S04 described below.

Finally, in the aging step S04, the manufactured electrolytic capacitor 1 is subjected to an aging treatment in which a predetermined voltage is applied for a predetermined time under high-temperature conditions. This activates the oxide film 8a repair function of predetermined components in the electrolyte 3, repairing areas where the oxide film 8a peeled off during the manufacturing process or relatively thin areas of the oxide film 8a, thereby suppressing leakage current and stabilizing performance. The aging treatment also has a debugging effect, such as eliminating unexpected initial defects.

[Test 1]
Aluminum electrolytic capacitors with a rated voltage of 16 V were fabricated using a conventional method in accordance with the present embodiment. Etched aluminum foil was used for the anode and cathode foils, and the anode foil was subjected to a chemical conversion treatment to form an oxide film. Capacitor elements were assembled, and then an electrolyte solution with the composition shown in Table 1 was introduced into the capacitor elements. The functional materials in Table 1 contained multiple substances, with the same composition ratio for each example (the same applies to the functional materials tested below). The electrolytic capacitors were then assembled and subjected to the specified aging treatment. Each electrolytic capacitor was exposed to a 115°C temperature environment with no applied voltage, and leakage current was measured at predetermined intervals after applying 16 V for one minute. Five capacitors were tested for each example. The results are shown in Figures 5A and 5B.

The following commercially available phosphoric acid and phosphonic acid chelating agents were used (the same applies to all tests below).
Phosphoric acid (Rasa Industries)
HEDP: 1-hydroxyethylidene-1,1-diphosphonic acid (manufactured by Chelest Corporation, Chelest PH-210; "Chelest" is a registered trademark; the same applies hereinafter)
NTMP: nitrilotris(methylenephosphonic acid) (manufactured by Chelest Corporation, Chelest PH-320)
PBTC: 2-phosphonobutane-1,2,4-tricarboxylic acid (Chelest PH-430, manufactured by Chelest Corporation)
EDTMP: ethylenediaminetetra(methylenephosphonic acid) (manufactured by Chelest Corporation, Chelest PH-540)

Figure 5A is a graph showing the average value for each example, as well as the error ranges representing the maximum and minimum values. However, there are some areas where the error range is too small to be seen. Figure 5B is a graph in which the vertical axis range of the graph in Figure 5A has been reduced to make the results of each example easier to see. However, for ease of viewing, the error range has been omitted. Note that Comparative Example 2 does not appear in Figure 5B.

As shown in Figures 5A and 5B, compared to Reference Example 1, which uses a conventional standard amount of phosphoric acid in the electrolyte (104.9 mmol/kg), Comparative Example 1, which uses a higher amount of phosphoric acid (130.7 mmol/kg), exhibited significantly higher leakage current. Furthermore, Comparative Example 2, which uses an even higher amount of phosphoric acid (156.4 mmol/kg) than Comparative Example 1, exhibited an even higher leakage current. In contrast, Examples 1 to 4, which use approximately the same amount of phosphoric acid as Comparative Example 1 (128.4 to 129.6 mmol/kg) in combination with various phosphonic acid chelating agents, exhibited lower leakage current than Reference Example 1. Furthermore, Examples 5 to 8, which use approximately the same amount of phosphoric acid as Comparative Example 2 (153.6 to 155.0 mmol/kg) in combination with various phosphonic acid chelating agents, also exhibited lower leakage current than Reference Example 1.

Specifically, in Reference Example 1, the leakage current began to increase noticeably approximately 3,000 hours after the start of testing, reaching a maximum value of 115.98 μA after 9,469 hours. In Comparative Example 1, the leakage current began to increase sharply approximately 500 hours after the start of testing, reaching a maximum value of 369.56 μA after 4,060 hours. In Comparative Example 2, the leakage current began to increase sharply approximately 500 hours after the start of testing, exceeding 500 μA and dropping out of range before 1,000 hours had elapsed.

In contrast, the leakage current in Examples 1 to 8 was 16.15 μA or less (Example 6: after 508 hours) throughout the entire test period, which was significantly lower than that of Comparative Examples 1 and 2 and Reference Example 1. Furthermore, the leakage current in Examples 1 to 5, 7, and 8 was 12.62 μA or less (Example 8: after 1084 hours) throughout the entire test period.

Furthermore, among all Examples 1-8, Examples 1-4, which contained relatively low amounts of phosphoric acid, tended to have lower leakage current compared to Examples 5-8, which contained relatively high amounts of phosphoric acid. It is believed that the phosphonic acid chelating agent enabled more efficient use of phosphate ions, allowing for a moderate increase in the phosphate ion source to adequately suppress leakage current. Among the phosphonic acid chelating agents, Examples 5-8, which contained relatively high amounts of phosphoric acid, HEDP (Example 5) and PBTC (Example 7) tended to have lower leakage current than NTMP (Example 6) and EDTMP (Example 8), demonstrating superiority. Furthermore, regardless of the amount of phosphoric acid, PBTC (Examples 3 and 7) particularly tended to have lower leakage current, demonstrating superiority. Considering the test conditions, for electrolytic capacitors expected to be used at temperatures up to approximately 115°C (for example, electrolytic capacitors with a heat resistance temperature set at 110-120°C), among the phosphonic acid chelating agents, NTMP and EDTMP are suitable, HEDP is more suitable, and PBTC is even more suitable.

[Test 2]
Aluminum electrolytic capacitors with a rated voltage of 16 V were manufactured using a conventional method in accordance with the present embodiment. Etched aluminum foil was used for the anode and cathode foils, and the anode foil was subjected to a chemical conversion treatment to form an oxide film. A capacitor element was assembled, and then an electrolyte solution with the composition shown in Table 2 was introduced into the capacitor element. The electrolytic capacitor was then assembled and subjected to a predetermined aging treatment. Each manufactured electrolytic capacitor was exposed to the electrical conditions and temperature environment shown in Table 3, and the presence or absence of failure (open explosion-proof valve or inoperable state due to inability to pass current) was investigated every 500 hours. Five capacitors were tested for each example. The results are shown in Table 3.

As shown in Table 3, overall, the lifespan was shorter in Comparative Example 3 (130.7 mmol/kg), which had a higher amount of phosphoric acid than in Reference Example 2, which used a conventional standard amount of phosphoric acid in the electrolyte (104.9 mmol/kg). Furthermore, in Comparative Example 4 (156.4 mmol/kg), which had an even higher amount of phosphoric acid than Comparative Example 3, the lifespan was either shorter than Comparative Example 3 or similar to that of Comparative Example 3. In contrast, Examples 9 to 12, which used approximately the same amount of phosphoric acid as Comparative Example 3 (128.4 to 129.6 mmol/kg) in combination with various phosphonic acid chelating agents, showed longer lifespans than Reference Example 2. Furthermore, Examples 13 to 16, which used approximately the same amount of phosphoric acid as Comparative Example 4 (153.6 to 155.0 mmol/kg) in combination with various phosphonic acid chelating agents, also showed longer lifespans than Reference Example 2.

Specifically, under conditions of 115°C temperature with no applied voltage, all five devices in Comparative Example 4 failed after 5,500 hours, all five devices in Comparative Example 3 failed after 6,500 hours, and all five devices in Reference Example 2 failed after 9,500 hours. In contrast, in Examples 9 to 16, none of the five devices failed even after 9,500 hours, demonstrating a longer lifespan than Comparative Examples 3 and 4 and Reference Example 2.

At a temperature of 115°C with a rated voltage of 16V applied, all five devices in Comparative Example 4 failed after 5,500 hours, all five devices in Comparative Example 3 failed after 6,500 hours, and all five devices in Reference Example 2 failed after 9,500 hours. In contrast, in Examples 9 to 16, none of the five devices failed even after 9,500 hours, demonstrating a longer lifespan than Comparative Examples 3 and 4 and Reference Example 2.

Under a temperature condition of 125°C with no applied voltage, all five devices failed after 3,000 hours in Comparative Examples 4 and 3, and all five devices failed after 3,500 hours in Reference Example 2. In contrast, all five devices failed after 5,000 hours in Examples 15 and 16, all five devices failed after 7,000 hours in Examples 11 and 14, and some devices failed after 8,000 hours in Examples 9, 10, 12, and 13. Although there were differences between the Examples, all Examples 9 to 16 had a longer lifespan than Comparative Examples 3 and 4 and Reference Example 2.

While there was no difference between the Examples at 115°C, at 125°C, Examples 9-12, which contained relatively less phosphoric acid, tended to have a longer lifespan than Examples 13-16, which contained relatively more phosphoric acid. It is believed that the phosphonic acid chelating agent enabled more efficient use of phosphate ions, allowing for a moderate increase in the phosphate ion source to adequately suppress leakage current. As noted above, at 125°C, while Comparative Examples 3 and 4 and Reference Example 2 failed very quickly, all Examples 9-16 maintained a certain lifespan, and in particular, Examples 9-14 had a lifespan more than twice that of Comparative Examples 3 and 4 and Reference Example 2. Thus, the combined use of phosphoric acid and a phosphonic acid chelating agent produced a particularly remarkable effect, achieving a very stable and long lifespan, even under relatively harsh high-temperature conditions.

Among the phosphonic acid chelating agents, at 125°C, HEDP (Example 13) and NTMP (Example 14) tended to have longer life than PBTC (Example 15) and EDTMP (Example 16) in Examples 13-16, which contain relatively high amounts of phosphoric acid. Considering the test conditions, for electrolytic capacitors expected to be used at temperatures up to approximately 125°C (for example, electrolytic capacitors with a heat resistance temperature set to 120-130°C), among the phosphonic acid chelating agents, PBTC and EDTMP are preferred, with HEDP and NTMP being more preferred.

[Test 3]
Furthermore, in order to investigate the blending amount (content) of the phosphate ion source, electrolyte solutions of each example with the compositions shown in Table 4 were prepared. An aluminum electrode made of a 2 cm x 5 cm plain aluminum foil and a platinum electrode were connected to an ammeter (Yokogawa Measurement Corporation, Digital Multimeter TY710). Both electrodes were placed in 70 g of electrolyte at room temperature in a glass beaker to form a test apparatus. In this test apparatus, the current value was measured 5 minutes after placing the electrodes in the electrolyte. Table 4 shows the electrolyte composition and the measurement results of the current value. The measurement results are also shown in a graph in Figure 6.

The electrode reaction in this test device is as follows:
Aluminum electrode reaction: Al ⇒ Al ³⁺ + 3e ^- (Al + 3OH ^- ⇒ Al(OH) ₃ + 3e ^- )
Platinum electrode reaction 3H ₂ O + 3e ^- ⇒ 3OH ^- + 3/2H ₂

In other words, the magnitude of the current value measured here represents the degree of hydration deterioration of the aluminum foil. At the same time, in each example, the lower the current value, the better the hydration deterioration of the foil is suppressed by the phosphoric acid in the electrolyte.

As shown in Table 4 and Figure 6, compared to Comparative Example 5, which had an electrolyte composition that did not contain a phosphonic acid chelating agent in addition to phosphoric acid, Examples 17-20, which contained various phosphonic acid chelating agents in addition to phosphoric acid, exhibited low current values and suppressed foil hydration degradation at all phosphoric acid levels. More specifically, at a conventional standard amount of phosphoric acid in the electrolyte (approximately 100 mmol/kg), the difference in current values between Examples 17-20 and Comparative Example 5 was relatively small. However, when the amount of phosphoric acid was doubled (approximately 200 mmol/kg), tripled (approximately 300 mmol/kg), quadrupled (approximately 400 mmol/kg), or five times (approximately 500 mmol/kg), Comparative Example 5 exhibited a relatively steep increase in current value, while Examples 17-20 exhibited a relatively gradual increase in current value, and Examples 17, 18, and 20 exhibited a particularly gradual increase in current value. As a result, the difference in current value between Examples 17-20 and Comparative Example 5 became relatively large. Therefore, by using a phosphonic acid chelating agent in combination with phosphoric acid, it is possible to incorporate an extremely large amount of phosphoric acid (phosphate ion source) into the electrolyte, up to about five times the standard amount, and sufficiently suppress deterioration of the electrode foil, thereby reducing leakage current and extending the lifespan of the electrolytic capacitor, as confirmed in Tests 1 and 2.

Based on the test results to date, in the present invention, by using a phosphonic acid chelating agent in combination, the amount (content) of the phosphate ion source in the electrolyte can be set within the following ranges: 120.0 to 500.0 mmol/kg, 120.0 to 400.0 mmol/kg, 120.0 to 300.0 mmol/kg, 120.0 to 200.0 mmol/kg, 120.0 to 170.0 mmol/kg, 120.0 to 160.0 mmol/kg, 125.0 to 155.0 mmol/kg, 130.0 to 155.0 mmol/kg, 130.0 to 150.0 mmol/kg, etc.

Among the phosphonic acid chelating agents, HEDP (Example 17), NTMP (Example 18), and EDTMP (Example 20) tended to produce lower current values compared to PBTC (Example 19). Even with an extremely high amount of phosphoric acid in the electrolyte of approximately 500 mmol/kg, PBTC (Example 19) suppressed the current to a low value of 54.09 μA, HEDP (Example 17) suppressed it to an even lower value of 20.65 μA, NTMP (Example 18) suppressed it to 14.87 μA, and EDTMP (Example 20) suppressed it to an even lower value of 15.53 μA. Considering the test conditions, for electrolytic capacitors and the like expected to be used at room temperature, among the phosphonic acid chelating agents, PBTC is preferred, HEDP is more preferred, and NTMP and EDTMP are even more preferred.

[Test 4]
Furthermore, in order to investigate the blending amount (content) of the phosphonic acid chelating agent, electrolyte solutions of each example with the compositions shown in Table 5 were prepared. Using a test device with the same configuration as in Test 3, the current value was measured 5 minutes after placing the electrodes in the electrolyte solution. Table 5 shows the electrolyte solution composition and the measurement results of the current value. The measurement results are also shown in a graph in Figure 7.

As shown in Table 5 and Figure 7, compared to Comparative Example 6, which had an electrolyte composition that did not incorporate a phosphonic acid chelating agent with phosphoric acid, Examples 21 to 24, which incorporated various phosphonic acid chelating agents with phosphoric acid, exhibited low current values and suppressed foil hydration degradation regardless of the amount of phosphonic acid chelating agent. That is, compared to Comparative Example 2 (7.45 μA), the current value was already significantly reduced to less than 13.5 μA when 0.2% by weight of phosphonic acid chelating agent was incorporated (Examples 21-1, 22-1, 23-1, and 24-1), and was even more significantly reduced to less than 9.0 μA when 0.6% by weight or more of phosphonic acid chelating agent was incorporated (Examples 21-2 to 21-4, 22-2 to 22-4, 23-2 to 23-4, and 24-2 to 24-4). Thus, a significant effect was already achieved compared to the comparative example at a blending rate of 0.2% by mass of phosphonic acid chelating agent, and a stable and significant effect was maintained up to a blending rate of 1.5% by mass of phosphonic acid chelating agent. This suggests that blending at least 0.1 to 0.2% by mass of phosphonic acid chelating agent together with phosphoric acid can sufficiently suppress electrode foil deterioration, thereby reducing leakage current and extending the lifespan of electrolytic capacitors, as confirmed in Tests 1 and 2.

Based on the test results to date, in the present invention, the amount (content) of phosphonic acid chelating agent in the electrolyte can be set to 0.1 to 0.2% by mass or more. For example, by replacing part of the functional substance in this example with a phosphonic acid chelating agent and setting the upper limit, the amount can be set to a range of 0.1 to 5.0% by mass, 0.1 to 4.0% by mass, 0.1 to 3.5% by mass, 0.1 to 3.0% by mass, 0.1 to 2.5% by mass, 0.1 to 2.0% by mass, 0.1 to 1.6% by mass, 0.2 to 5.0% by mass, 0.2 to 4.0% by mass, 0.2 to 3.5% by mass, 0.2 to 3.0% by mass, 0.2 to 2.5% by mass, 0.2 to 2.0% by mass, 0.2 to 1.6% by mass, 0.2 to 1.5% by mass, etc.

REFERENCE SIGNS LIST 1 Electrolytic capacitor 2 Capacitor element 3 Electrolyte 4 Lead terminal 4a Anode terminal 4b Cathode terminal 5 Sealing material 6 Case 6a Opening edge 7 Explosion-proof valve 8 Anode foil 8a Oxide film 9 Cathode foil 10 Separator S01 Capacitor element forming process S02 Electrolyte introduction process S03 Sealing process S04 Aging process

Claims (14)

A low-voltage electrolytic capacitor,
a capacitor element including 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 solution impregnated in the capacitor element,
The electrolytic capacitor is characterized in that the electrolytic solution contains an aqueous solvent containing 55 wt % or more of water, a phosphate ion source, and a phosphonic acid-based chelating agent which is a compound having a phosphonic acid group and having chelating ability (however, excluding ethylenediaminetetra(methylenephosphonic acid) (EDTMP), ethylenediaminetetrakis(methylenephosphonic acid) (EDTPO), ethylenediamine-N,N'-bis(methylenephosphonic acid), and substances having the same chemical structure as these). 2. The electrolytic capacitor according to claim 1, wherein the electrolytic solution contains an organic acid ammonium salt. 3. The electrolytic capacitor according to claim 2 , wherein the organic ammonium salt is composed mainly of ammonium adipate with ammonium formate added thereto. 2. The electrolytic capacitor according to claim 1, wherein the phosphate ion source is one or more substances selected from the group consisting of phosphoric acid, phosphorous acid, hypophosphorous acid, salts of each of these three acids, esters of each of the three acids and salts of the esters, and condensates of each of the three acids and salts of the condensates. 5. The electrolytic capacitor according to claim 4, wherein the phosphate ion source is one or more substances selected from the group consisting of phosphoric acid, phosphate salts, phosphate esters, and salts of phosphate esters. 2. The electrolytic capacitor according to claim 1, wherein the phosphonic acid chelating agent is one or more substances selected from the group consisting of 1-hydroxyethylidene-1,1-diphosphonic acid, nitrilotris(methylenephosphonic acid), and 2-phosphonobutane-1,2,4-tricarboxylic acid. 2. The electrolytic capacitor according to claim 1, wherein the phosphate ion source is contained in the electrolyte in an amount of 120.0 to 500.0 mmol/kg (excluding 0.75 wt % or less in the electrolyte). A low-voltage electrolytic capacitor,
A method for manufacturing an electrolytic capacitor, comprising blending, into an electrolytic solution, an aqueous solvent containing at least 55 wt % or more of water, a phosphate ion source, and a phosphonic acid chelating agent which is a compound having a phosphonic acid group and having chelating ability (excluding ethylenediaminetetra(methylenephosphonic acid) (EDTMP), ethylenediaminetetrakis(methylenephosphonic acid) (EDTPO), ethylenediamine-N,N'-bis(methylenephosphonic acid), and substances having the same chemical structure as these). 9. The method for manufacturing an electrolytic capacitor according to claim 8, wherein the electrolytic solution contains an organic acid ammonium salt. 10. The method for manufacturing an electrolytic capacitor according to claim 9 , wherein the organic ammonium salt is composed mainly of ammonium adipate with ammonium formate added thereto. 9. The method for producing an electrolytic capacitor according to claim 8, wherein the phosphate ion source is one or more substances selected from the group consisting of phosphoric acid, phosphorous acid, hypophosphorous acid, salts of each of these three acids, esters of each of the three acids and salts of the esters, and condensates of each of the three acids and salts of the condensates. 12. The method for manufacturing an electrolytic capacitor according to claim 11, wherein the phosphate ion source is one or more substances selected from the group consisting of phosphoric acid, phosphate salts, phosphate esters, and salts of phosphate esters. 9. The method for producing an electrolytic capacitor according to claim 8, wherein the phosphonic acid chelating agent is one or more substances selected from the group consisting of 1-hydroxyethylidene-1,1-diphosphonic acid, nitrilotris(methylenephosphonic acid), and 2-phosphonobutane-1,2,4-tricarboxylic acid. 9. The method for producing an electrolytic capacitor according to claim 8, wherein the phosphate ion source is blended in the electrolyte solution in an amount of 120.0 to 500.0 mmol/kg (excluding an amount of 0.75 wt % or less in the electrolyte solution).