ELECTROLYTIC SOLUTION FOR ELECTROLYTIC CAPACITOR AND ELECTROLYTIC CAPACITOR PREPARED BY USING IT

Abstract: This new electrolytic soln. for driving an electrolytic capacitor contains as a solute a quaternary ammonium salt of a carboxylic acid, selected from the following gp.: phthalic, tetrahydrophthalic, hexahydrophthalic, maleic acids, and alkyl- and nitro-substd. derivs. of these acids. The solute is dissolved in an organic solven. The quaternary ammonium salt has the general formula X1-A-X2 where X1 is (I); X2 is (II) or COOH, R1, R2, R3 and R4 are, independently, alkyl, aryl, cycloalkyl, aralkyl, and/or hydroxyalkyl gps., or by a combination of two of these gps., the combination of two of R1, R2, R3, R4 can be a gp. of valency two that can form a ring contg. the central nitrogen atom. A is a carbonic acid residual gp. The quaternary ammonium salt can be a tetraalkylammonium salt and/or a trialkylhydroxyalkylammonium salt, a phthalic acid monotetraethyl ammonium salt, or a maleic acid monotetraethyl ammonium salt. The amt. of quaternary ammonium salt should be 1-40 wt. % of the electrolytic soln. The electrolytic capacitor is prepared from this electrolytic soln.

Description: This invention relates to an electrolyte for use in electrolytic capacitors and also to an electrolytic capacitor using the electrolyte. A typical electrolyte for electrolytic capacitors which has been hitherto used is an electrolyte of an ionogen dissolved in ethylene glycol. However, this type of electrolyte has a low specific conductance and tends to cause an impedance characteristic to deteriorate. To avoid this, usual practice is to add water to the electrolyte, but such an electrolyte is elevated in vapor pressure at high temperatures and is apt to react with an aluminum electrode, thus being unsuitable for use in a high temperature range. In order to improve the above drawbacks, there have been proposed an electrolyte using γ-butyrolactone as a solvent and a triethylamine salt of phthalic acid as described in Japanese Laid-open Patent Application No. 61-70711 and an electrolyte using an amine salt of maleic acid dissolved in a mixed solvent of γ-butyrolactone and ethylene glycol as described in Japanese Laid-open Patent Application No. 54-7564. Moreover, Japanese Laid-open Patent Application No. 59-78522 describes electrolytes using quaternary ammonium salts of linear dicarboxylic acids, and Japanese Laid-open Patent Application No. 61-93610 teaches electrolytes using tetralkylammonium salts of 1,10-decanedicarboxylic acid or 1,6-decanedicarboxylic acid. However, the known electrolytes comprising triethylamine salts of phthalic acid or maleic acid dissolved in a γ-butyrolactone solvent are disadvantageous in that the acid dissociation is caused by the proton equilibrium of the triethylamine, so that ions are produced only in small amounts. Thus, a sufficiently high specific conductance cannot be obtained. On the other hand, with electrolytes using the quaternary ammonium salts of linear dicarboxylic acids or the tetraalkylammonium salts of 1,10-decanedicarboxylic acid or 1,6-decanedicarboxylic acid, the specific conductance becomes low because of the rather inappropriate combination with the acid. Accordingly, electrolytic capacitors using these electrolytes cannot disadvantageously lower the impedance characteristic. The EP-A-0 227 433 which represents an earlier application being post-published discloses, in the priority relevant documents thereof, an electrolyte solution for an electrolytic capacitor which comprises as a solute quaternary ammonium salts of a carboxylic acid selected from maleic acid and/or citraconic acid, or an aromatic carboxylic acid including unsubstituted or substituted phthalic acid. The present invention provides an electrolyte for electrolytic capacitors comprising a solution of a quaternary ammonium salt of a carboxylic acid, the carboxylic acid being selected from the group consisting of phthalic acid, maleic acid, tetrahydrophthalic acid, hexahydrophthalic acid, or a quaternary ammonium salt of an alkyl or nitro-substituted compound of tetrahydrophthalic acid, hexahydrophthalic acid, phthalic acid and maleic acid, in an alcohol, ether, amide, oxazolidinone, dimethylsulfoxide and/or gamma-butyrolactone, whereby the concentration of the respective quaternary ammonium salt is in the range of 1 wt% to 40 wt%, with the provision that for the quaternary ammonium salts other than tetramethylammonium salts, phthalic acid, maleic acid, nitrophthalic acid and citraconic acid are excluded as the carboxylic acid. The invention also provides an electrolytic capacitor as specified in claim 5. The resultant electrolytic capacitor has improved low temperature characteristics and improved characteristics with time at high temperatures, thus ensuring a prolonged life. Fig. 1 is a schematic perspective view illustrating the respective members of an electrolytic capacitor; and Figs. 2-4 are, respectively, graphs of characteristics, in relation to time, of aluminum electrolytic capacitors having ratings 10 V and 2200 µF ( 16 x 32) and using known electrolytes and electrolytes of the invention, in which Fig. 2 is a characteristic graph showing a variation in electrostatic capacity by application of a rated voltage, Fig. 3 is a characteristic graph showing a variation in tangent of the loss angle by application of a rated voltage, and Fig. 4 is a characteristic graph of a variation in leakage current in the absence of an applied voltage. The quaternary ammonium salts are those compounds of the following general formula in which X₁ represents X₂ represents or COOH in which each R₁, each R₂, each R₃ and each R₄ independently represent an alkyl group, an aryl group, a cycloalkyl group, an aralkyl group and/or a hydroxyalkyl group, or the two of these substituents may combine to form a divalent group which forms a ring along with the nitrogen atom, and A represents a carboxylic acid residue of a carboxylic acid selected from the group consisting of phthalic acid, maleic acid, tetrahydrophthalic acid, hexahydrophthalic acid, an alkyl or nitro-substituted compound of tetrahydrophthalic acid, hexahydrophthalic acid, maleic acid and phthalic acid, whereby the concentration of the respective quaternary ammonium salt is in the range of 1 wt% to 40 wt%, with the provision that for the quaternary ammonium salts other than tetramethylammonium salts, phthalic acid, maleic acid, nitrophthalic acid and citraconic acid are excluded as the carboxylic acid. In the above general formula, R₁, R₂, R₃ and R₄ are, respectively, a C₁₋₁₂ alkyl group (such as methyl, ethyl, propyl, butyl, octyl, dodecyl and the like), an aryl group (such as phenyl and the like), a cycloalkyl group (such as cyclohexyl and the like), an aralkyl group (such as benzyl and the like), and a C₂₋₄ hydroxyalkyl group (such as hydroxyethyl and the like). Alternatively, these two substituents may combine to give a divalent group such as, for example, -(CH₂)n- wherein n is 5 or 6, or - CH₂CH₂OCH₂CH₂-, and form a ring along with the nitrogen atom. In the practice of the invention, the tetrahydrophthalic acid includes cyclohexene-1,2-dicarboxylic acid and cyclohexene-4,5-dicarboxylic acid. The alkyl-substituted compound of the carboxylic acid is one whose alkyl moiety has 1 - 3 carbon atoms, e.g. a methyl-substituted compound. An example of the alkyl-substituted compound is dimethylmaleic acid. Examples of the nitro-substituted compounds include 3- or 4-nitrophthalic acid. These carboxylic acids and their alkyl or nitro-substituted compounds may be used in combination. Examples of the quaternary ammonium salts include tetraalkylammonium salts (whose alkyl moiety has generally 1-12 carbon atoms, e.g. a tetramethylammonium salt, a tetraethylammonium salt, a tetrapropylammonium salt, a tetrabutylammonium salt, a methyltriethylammonium salt, an ethyltrimethylammonium salt and the like), aryltrialkylammonium salts (such as a phenyltrimethylammonium salt and the like), cyclohexyltrialkylammonium salts (such as a cyclohexyltrimethylammonium salt and the like), arylalkyltrialkylammonium salts (such as a benzyltrimethylammonium salt and the like), trialkylhydroxyalkylammonium salts (such as a trimethylhydroxyethylammonium salt and the like), N,N-dialkylpiperidinium salts (such as an N,N-dimethylpiperidinium salt and the like), and mixtures thereof. Of these quaternary ammonium salts, the tetraalkylammonium salts and the trialkylhydroxyalkylammonium salts are preferred. Most preferably, a tetraethylammonium salt is used. The equivalent ratio between the carboxyl groups of a carboxylic acid and the quaternary ammonium base is generally in the range of 1.2:1.0 - 2.8:1.0, preferably 1.5:1.0 - 2.5:1.0, and most preferably 2.0:1.0. In other words, a mono-salt having a molar ratio between the acid and the base of 1:1 is most preferably used. The electrolyte for an electrolytic capacitor according to the invention comprises the above-defined salt and alcohols, e.g. monohydric alcohols such as butyl alcohol, diacetone alcohol, benzyl alcohol, amino alcohols and the like, dihydric alcohols such as ethylene glycol, propylene glycol, diethylene glycol, hexylene glycol, phenyl glycol and the like, trihydric alcohols such as glycerine, 3-methylpentane-1,3,5-triol and the like, and hexitol; ethers, e.g. monoethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, ethylene glycol phenyl ether and the like, diethers such as ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether and the like; amides, e.g. formamides such as N-methylformamide, N,N-dimethylformamide, N-ethylformamide, N,N-diethylformamide and the like, acetamides such as N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide and the like, propionamides such as N,N-dimethylpropionamide and the like, and hexamethylphosphoryl amide; oxazolidinones such as N-methyl-2-oxazolidinone, N-ethyl-2-oxazolidinone, 3,5-dimethyl-2-oxazolidinone and the like; dimethylsulfoxide, γ-butyrolactone and the like; and mixtures thereof. Of these, amides, oxazlidinones and γ-butyrolactone are preferred. More preferably, γ-butyrolactone alone or a mixed solvent comprising a major proportion of γ-butyrolactone is used. In the latter case, the content of the γ-butyrolactone is preferably in the range of 50 - 100% based on the total weight of the solvent. The electrolyte of the invention may further comprise water, if necessary. The content of water is generally not larger than 10% based on the weight of the electrolyte. The amount of the quaternary ammonium salt in the electrolyte is in the range of 1 wt% - 40 wt%, preferably 5 wt% - 30 wt%. The amount of the organic solvent described above is in the range of 60 wt% - 99 wt%, preferably 70 wt% - 95 wt%. The electrolyte of the invention obtained as described above has a high specific conductance and good stability at high temperatures. The use of the electrolytes results in an electrolytic capacitor which has a low impedance and is stable at high temperatures. More particularly, when a quaternary ammonium salt as described above is used in combination with a solvent comprised mainly of γ-butyrolactone, very high conductance is obtained. In γ-butyrolactone, quaternary ammonium salts as described above are very stable at high temperatures. In addition, the reaction between the quaternary ammonium salts as described above and the solvent and the reaction between carboxylic acids described above and tetraethylammonium do rarely take place. Presumably, this is why the stability at high temperatures is excellent. In the practice of the invention, the concentration of the quaternary ammonium salt is in the range of 1 wt% - 40 wt%. If the amount is less than 1 wt%, the specific conductance does not become satisfactorily high. Over 40 wt%, precipitation will take place. (Examples) The present invention is illustrated by way of examples. In Table 1, there are shown electrolytes according to the invention and electrolytes given for comparison along with their specific conductance at normal temperatures. As will be clear from Table 1, higher specific conductances can be obtained than the conductances of the known electrolytes. In Table 2, there are shown initial characteristics of capacitors using the electrolytes of References 1, 2 and 3 and Examples, 1, 2 and 3 indicated in Table 1. The capacitor used was an aluminum electrolytic capacitor having ratings of 10 V and 220 µF ( 16 x 32). Fig. 1 schematically shows an arrangement of the respective parts or members of this type of aluminum electrolytic capacitor. In Fig. 1, an anode foil 1 used as an anode of aluminum and a cathode foil 2 used as a cathode of aluminum are convolutely wound so that they are in face-to-face relation through a separator 3, thereby providing a capacitor element. The anode foil 1 and the cathode foil 2 of the element have leading tabs 4, respectively. The thus arranged element is impregnated with each electrolyte and is sealingly encased in a case to obtain an electrolytic capacitor. As will be apparent from Table 2, the electrolytes of Examples of the invention have significantly lower tan δ (120Hz) and impedance (100 KHz) characteristics than the known electrolytes. As described hereinbefore, the electrolytes for electrolytic capacitors and electrolytic capacitors using such electrolytes according to the invention have high specific conductance, and the loss of the capacitor can be improved over a wide range of normal to low temperatures. Because the electrolytes do rarely deteriorate at high temperatures, electrolytic capacitors whose characteristics do not vary over a long term can be obtained. Thus, the present invention provides an electrolytic capacitor which exhibits stable characteristics over a wide temperature range of from low to high temperatures.

Claims: An electrolyte for an electrolytic capacitor comprising a solution of a quaternary ammonium salt of a carboxylic acid, the carboxylic acid being selected from the group consisting of phthalic acid, maleic acid, tetrahydrophthalic acid, hexahydrophthalic acid, or a quaternary ammonium salt of an alkyl or nitro-substituted compound of tetrahydrophthalic acid, hexahydrophthalic acid, maleic acid and phthalic acid, in an alcohol, ether, amide, oxazolidinone, dimethylsulfoxide and/or gamma-butyrolactone, whereby the concentration of the respective quaternary ammonium salt is in the range of 1 wt% to 40 wt%, with the provision that for the quaternary ammonium salts other than tetramethylammonium salts, phthalic acid, maleic acid, nitrophthalic acid and citraconic acid are excluded as the carboxylic acid. An electrolyte according to Claim 1, wherein said quaternary ammonium salt has the following general formula in which X₁ represents X₂ represents or COOH in which each R₁, each R₂, each R₃ and each R₄ independently represent an alkyl group, an aryl group, a cycloalkyl group, an aralkyl group and/or a hydroxyalkyl group, or the two of these substituents combine to form a divalent group which forms a ring along with the nitrogen atom, and A represents a carboxylic acid residue of a carboxylic acid being selected from the group consisting of phthalic acid, maleic acid, tetrahydrophthalic acid, hexahydrophthalic acid, an alkyl or nitro-substituted compound of tetrahydrophthalic acid, hexahydrophthalic acid, maleic acid and phthalic acid, whereby the concentration of the respective quaternary ammonium salt is in the range of 1 wt% to 40 wt%, with the provision that for the quaternary ammonium salts other than tetramethylammonium salts, phthalic acid, maleic acid, nitrophthalic acid and citraconic acid are excluded as the carboxylic acid. An electrolyte according to Claim 1, wherein said quaternary ammonium salt is a tetraalkylammonium salt and/or a trialkylhydroxyalkylammonium salt. An electrolyte according to Claim 3, wherein said tetraalkylammonium salt is a tetraethylammonium salt. An electrolytic capacitor of the type in which a capacitor element is arranged by providing an anode (1) and a cathode (2) in face-to-face relation through a separator (3), wherein the electrolyte for the capacitor consists of a solution of a quaternary ammonium salt of a carboxylic acid, the carboxylic acid being selected from the group consisting of phthalic acid, maleic acid, tetrahydrophthalic acid, hexahydrophthalic acid, or a quaternary ammonium salt of an alkyl or nitro-substituted compound of tetrahydrophthalic acid, hexahydrophthalic acid, maleic acid and phthalic acid, in an alcohol, ether, amide, oxazolidinone, dimethylsulfoxide and/or gamma-butyrolactone, whereby the concentration of the respective quaternary ammonium salt is in the range of 1 wt% to 40 wt%, with the provision that for the quaternary ammonium salts other than tetramethylammonium salts, phthalic acid, maleic acid, nitrophthalic acid and citraconic acid are excluded as the carboxylic acid. An electrolytic capacitor according to Claim 5, wherein said quaternary ammonium salt has the general formula in which X₁ represents X₂ represents or COOH in which each R₁, each R₂, each R₃ and each R₄ independently represent an alkyl group, an aryl group, a cycloalkyl group, an aralkyl group and/or a hydroxyalkyl group or the two of these substituents combine to form a divalent group which forms a ring along with a nitrogen atom, and A represents a carboxylic acid residue of a carboxylic acid being selected from the group consisting of phthalic acid, maleic acid, tetrahydrophthalic acid, hexahydrophthalic acid, an alkyl or nitro-substituted compound of tetrahydrophthalic acid, hexahydrophthalic acid, maleic acid and phthalic acid, whereby the concentration of the respective quaternary ammonium salt is in the range of 1 wt% to 40 wt%, with the provision that for the quaternary ammonium salts other than tetramethylammonium salts, phthalic acid, maleic acid, nitrophthalic acid and citraconic acid are excluded as the carboxylic acid. An electrolytic capacitor according to Claim 6, wherein said quaternary ammonium salt is a tetraalkylammonium salt and/or a trialkylhydroxyalkylammonium salt. An electrolytic capacitor according to Claim 7, wherein said tetraalkylammonium salt is a tetraethylammonium salt.

Inventors: KISHI TAKAAKI - GOKASYOU KITAN, KUWAE YOSHITERU - NISHIYAMA WA, MORI KEIJI - ITINOBE ASHIHARA, NAGARA HISAO - GINZA-CHO -CHOM, SHIMAMOTO HIDEKI - - MIIMINAMI, SHIONO KATSUJI - SENNYUJI GOYO, YOSHIDA SHINGO - TONOMINAMISHI, KISHI, TAKAAKI 22-13, GOKASYOU KITANOSYOU, KUWAE, YOSHITERU 5-6, NISHIYAMA WAKI, MORI, KEIJI 114-5, ITINOBE ASHIHARA, NAGARA, HISAO 333-1, GINZA-CHO 2-CHOME, SHIMAMOTO, HIDEKI 3-1-502, MIIMINAMI-MACHI, SHIONO, KATSUJI 7-3, SENNYUJI GOYOUNOTSUJI-MACHI, YOSHIDA, SHINGO 8-108, TONOMINAMISHIMIZU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, SANYO CHEMICAL IND LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD., SANYO CHEMICAL INDUSTRIES LTD.
Application year: 1987
Main classification: H01G9
IPC codes: H01G9, G02F1, H01M10