Quaternary salt solution electrolyte for electrolytic capacitors.

The present invention relates to an electrolyte for an aluminum electrolytic capacitor, and more particularly to an electrolyte which can reduce internal resistance of an electrolytic capacitor and broaden the temperature range over which the capacitor can be used, through the use of an electrolyte containing quaternary phosphonium salts as solute dissolved in aprotic solvents.

Conventional electrolytes for aluminum electrolytic capacitors generally comprise some type of ionizable com­pound dissolved in a solvent.

The chemical or electric properties of the electro­lyte, such as electric conductivity and scintillation voltage become main factors for determining the electric properties of the electrolytic capacitors.

For example, a solution of boric acid or ammonium borate in ethylene glycol, which has low electric conduc­tivity and high scintillation voltage, has been used for aluminum electrolytic capacitors operating of 200 V or higher (U.S. Patent 1,891,207). The maximum operating temperature for such an electrolyte system is less than 100°C and normally 85°C because a large amount of water formed during the rapid esterification of glycol and borate species at higher temperature impairs the electric properties of the electrolytic capacitors. The minimum operating temperature in such a system is above -20°C since glycol freezes at -17°C.

On the other hand, a solution of an amine or an quaternary ammonium salt of carboxylic acid in an aprotic solvent such as N,N-dimethylformamide or γ-butyrolactone is used for low voltage electrolytic capacitors, which have good low temperature characteristics (U.S. Patents 3,812,039 and 4,473,864).

However, electrolytes which have high electric conductivity over the wider temperature range are desirable for imparting lower equivalent series resistance (ESR) and lower impedance at high frequencies to the electrolytic capacitor containing them.

As the result of various investigations on dis­covering electrolyte systems having higher electric conduc­tivity, the inventors have discovered that an electrolyte containing a quaternary phosphonium salt dissoved in an aprotic solvent shows higher electric conductivity than those of conventional electrolyte systems and have succeeded in accomplishing this invention.

As to electrolytes using quaternary phosphonium salts, U.S. Patent 3,325,697 refers to quaternary phos­phonium salts as one of cations having a reducible group such as nitro groups, etc. which are used for sealed capacitors with non-gas producing electrolyte but there are neither practical illustrations nor examples about this case in the aforesaid patent. Also, we have not yet found any other patents using quaternary phosphonium salts for such electrolyte systems.

Thus, it is a feature of this invention to provide an aluminum electrolytic capacitor utilizing an electrolyte containing a quaternary phosphonium salt as solute dissolved in an aprotic solvent. This electrolyte can reduce the internal resistance of the electrolytic capacitor and broaden the temperature range over which the electrolytic capacitor can be used.

The drawing is a graph showing the relation between the electric conductivity of the electrolyte obtained in Example 1 and temperature.

The electrolyte of this invention is a solu­tion of a quaternary phosphonium salt as solute dissolved in an aprotic solvent.

The cation component of the solute preferred in this invention is a quaternary phosphonium ion represented by the following general formula (I): <chemistry id="chem0001" num="0001"><img file="EP0246825A2_D0001.tif" /></chemistry> wherein R₁, R₂, R₃ and R₄ each represents a hydrocarbon group having from 1 to 20 carbon atoms, and preferably an alkyl group having from 1 to 10 carbon atoms or an aryl group, said adjoining alkyl groups may combine with each other to form a ring.

Specific examples of the alkyl group or aromatic group for R₁, R₂, R₃ and R₄ in the general formula (I) described above are a methyl group, an ethyl group, a propyl group, a butyl group, a phenyl group and a benzyl group.

Also, specific examples of the quaternary phosphoni­um ion shown by the general formula (I) described above are tetramethylphosphonium, tetraethylphosphonium, tetrapropyl­phosphonium, tetrabutylphosphonium, methyltriethylphosphoni­um, methyltripropylphosphonium, methyltributylphosphonium, dimethyldiethylphosphonium, dimethyldipropylphosphonium, dimethyldibutylphosphonium, trimethylethylphosphonium, tri­methylpropylphosphonium, trimethylbutylphosphonium, ethyl­tripropylphosphonium, ethyltributylphosphonium, diethyldi­propylphosphonium, diethyldibutylphosphonium, triethylpro­ pylphosphonium, triethylbutylphosphonium, propyltributyl­phosphonium, dipropyldibutylphosphonium, tripropylbutylphos­phonium, trimethyldecylphosphonium, trimethylphenylphos­phonium, triethylphenylphosphonium, tripropylphenylphos­phonium, tributylphenylphosphonium, methyltriphenylphos­phonium, ethyltriphenylphosphonium, propyltriphenylphos­phonium, butyltriphenylphosphonium, tetraphenylphosphonium and trimethylbenzylphosphonium.

Also, examples of the quaternary phosphonium ion shown by the general formula (I) described above wherein the adjoining alkyl groups combine with each other to form a ring are alicyclic phosphonium ions represented by the following general formula (II) or (III): <chemistry id="chem0002" num="0002"><img file="EP0246825A2_D0002.tif" /></chemistry> wherein, R₁ and R₂ each represents an alkyl group of an aryl group as defined above about the general formula (I) and ℓ and m each represents an integer of from 4 to 6.

Examples of the alicyclic phosphonium ions shown by the general formula (II) or (III) described above are 1,1-­dimethylphosphoranium, 1-methyl-1-ethylphosphoranium, 1,1-­diethylphsphoranium, 1,1-dimethylphosphorinanium, 1-methyl-­1-ethylphosphorinanium, 1,1-diethylphosphorinanium, 1,1-­pentamethylenephosphorinanium, etc.

Furthermore, polycations shown by the general for­mula (IV) described below can be also effectively used as the quaternary phosphonium ions: <chemistry id="chem0003" num="0003"><img file="EP0246825A2_D0003.tif" /></chemistry> wherein, R₁, R₂, and R₃ each represents an alkyl group or an aryl group as defined above about the general formula (I) described above and n represents an integer of from 1 to 20.

In order to provide an electrolyte having higher electric conductivity, a quaternary phosphonium ion having a low molecular weight (e.g., having up to 10 carbon atoms in the total alkyl and/or aryl groups), such as, for example, a tetraethylphosphonium ion or a methyltriethyl­phosphonium ion is preferred, while in order to obtain an electrolyte having high scintillation voltage, a quaternary phosphonium ion having a high molecular weight, such as, for example a tetrabutylphosphonium ion or a tetrahexylphosphonium ion,is preferred.

The anion component of the solute for use in this invention is a conjugate base of an organic or inorganic acid and examples of the acid are: <ul id="ul0001" list-style="none"><li>(1) organic carboxylic acids and organic weak acids such as phenols having pKa of at least 1, etc.;</li><li>(2) organic strong acids such as picric acid, sulfonic acid, etc.;</li><li>(3) inorganic weak acids having pKa of at least 1, such as boric acid, phosphoric acid, phosphorous acid, hypophos­phorous acid, carbonic acid, silic acid, etc., and the alkyl or aryl derivatives thereof;</li><li>(4) inorganic strong acids such as nitric acid, sulfuric acid, sulfurous acid, thiocyanic acid, etc., and the deriva­tives thereof; and</li><li>(5) very strong acids having pKa of up to 0 (i.e., 0 or a negative value) containing halogen atom.</li></ul>

The anions selected from the organic or inorganic weak acids are preferred since they are excellent in the anodic oxidation film-forming faculty on aluminum and, in particular, dicarboxylic acids having the total carbon atom number of at least 10 and boric acid are good "film-forming anions", having high scintillation voltage, and hence can be used for medium to high voltage capacitors.

The anions of the strong acids are apt to corrode the oxidation film because of their high acidity and hence are used for capacitors for low voltage of up to 100 volts.

Also, the anions of the acids containing halogen atom liberate a halogen ion unpreferably for aluminum and hence are used for capacitors for low voltage of up to 10 volts only.

Suitable carboxylic acids for the quaternary phos­phonium salts of carboxylic acids, which are used in this invention, are aliphatic and aromatic monovalent or poly­valent carboxylic acids having 1 to 30 total carbon atoms.

Specific examples of these carboxylic acids are aliphatic monocarboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, undecylic acid, lauric acid, tridecylic acid, myristic acid, pentadecylic acid, palmitic acid, heptadecylic acid, stearic acid, nonadecylic acid, arachidinic acid, isobutyric acid, isovaleric acid, isocaproic acid, ethylbutyric acid, methylvaleric acid, isocaprylic acid, propylvaleric acid, ethylcaproic acid, isocaprylic acid, tuberculostearic acid, pivalic acid, 2,2-dimethylbutanoic acid, 2,2-dimethylpenta­noic acid, 2,2-dimethylhexanoic acid, 2,2-dimethylheptanoic acid, 2,2-dimethyloctanoic acid, 2-methyl-2-ethylbutanoic acid, 2-methyl-2-ethylpentanoic acid, 2-methyl-2-ethyl­hexanoic acid, 2-methyl-2-ethyl-heptanoic acid, 2-methyl-2-­propylpentanoic acid, 2-methyl-2-propylhexanoic acid, 2-­methyl-2-propylheptanoic acid, acrylic acid, crotonic acid, isocrotonic acid, 3-butenoic acid, pentenoic acid, hexenoic acid, heptenoic acid, octenoic acid, nonenoic acid, decenoic acid, undecenoic acid, dodecenoic acid, tudzuic acid, physeteric acid, goshuyuic acid, palmitoleic acid, petro­selinic acid, oleic acid, elaidic acid, vaccenic acid, gadoleic acid, methacrylic acid, 3-methylcrotonic acid, tiglic acid, methylpentenoic acid, cyclopentanecarboxylic acid, cyclohexanecarboxylic acid, etc.; aliphatic di­carboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, tridecanedioic acid, tetradecanodioic acid, pentadecanedioic acid, hexadecanedioic acid, hepta­decanedioic acid, octadecanedioic acid, nonadecanedioic acid, eicosanedioic acid, methylmalonic acid, ethylmalonic acid, propylmalonic acid, butylmalonic acid, pentylmalonic acid, hexylmalonic acid, dimethylmalonic acid, methylethyl­malonic acid, diethylmalonic acid, methylpropylmalonic acid, methylbutylmalonic acid, ethylpropylmalonic acid, dipropyl­malonic acid, ethylbutylmalonic acid, propylbutylmalonic acid, dibutylmalonic acid, methylsuccinic acid, ethyl­ succinic acid, 2,2-dimethylsuccinic acid, 2,3-dimethylsuc­cinic acid, 2-methylglutaric acid, 3-methylglutaric acid, 3-­methyl-3-ethylglutaric acid, 3,3-diethylglutaric acid, maleic acid, citranconic acid, 1,5-octanedicarboxylic acid, 5,6-decanedicarboxylic acid, 1,7-decanedicarboxylic acid, 4,6-dimethyl-4-nonene-1,2-dicarboxylic acid, 4,6-dimethyl-­1,2-nonanedicarboxylic acid, 1,7-dodecanedicarboxylic acid, 5-ethyl-1,10-decanedicarboxylic acid, 6-methyl-6-dodecene-­1,12-dicarboxylic acid, 6-methyl-1,12-dodecanedicarboxylic acid, 6-ethylene-1,12-dodecanedicarboxylic acid, 6-ethyl-­1,12-dodecanedicarboxylic acid, 7-methyl-7-tetradecene-1,14-­dicarboxylic acid, 7-methyl-1,14-tetradecanedicarboxylic acid, 3-hexyl-4-decene-1,2-dicarboxylic acid, 3-hexyl-1,2-­decanedicarboxylic acid, 6-ethylene-9-hexadecene-1,16-di­carboxylic acid, 6-ethyl-1,16-dexadecanedicarboxylic acid, 6-phenyl-1,12-dodecanedicarboxylic acid, 7,12-dimethyl-7,11-­octadecadiene-1,18-dicarboxylic acid, 7,12-dimethyl-1,18-­octadecanedicarboxylic acid, 6,8-diphenyl-1,14-tetradecane­dicarboxylic acid, 1,1-cyclopentanedicarboxylic acid, 1,2-­cyclopentanedicarboxylic acid, 1,1-cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, 4-cyclohexene-1,2-­dicarboxylic acid, 5-norbornene-2,3-dicarboxylic acid, etc.; aromatic monocarboxylic acids (including o, m, p-isomers thereof) such as benzoic acid, toluic acid, ethylbenzoic acid, propylbenzoic acid, isopropylbenzoic acid, butylbenzo­ ic acid, isobutylbenzoic acid, sec-butylbenzoic acid, tert-­butylbenzoic acid, hydroxybenzoic acid, anisic acid, ethoxy­benzoic acid, propoxybenzoic acid, isopropoxybenzoic acid, butoxybenzoic acid, isobutoxybenzoic acid, sec-butoxybenzoic acid, tert-butoxybenzoic acid, aminobenzoic acid, N-methyl-­aminobenzoic acid, N-ethylaminobenzoic acid, N-propylamino­benzoic acid, N-isopropylaminobenzoic acid, N-butyl­aminobenzoic acid, N-isobutylaminobenzoic acid, N-sec­butylaminobenzoic acid, N-tert-butylaminobenzoic acid, N,N-dimethylaminobenzoic acid, N,N-diethylaminobenzoic acid, nitrobenzoic acid, resorcyclic acid, etc.; and aromatic polyvalent carboxylic acids such as phthalic acid, isophthalic acid, terephthalic acid, 3-­nitrophthalic acid, 4-nitrophthalic acid, trimellitic acid, hemimellitic acid, trimesic acid, pyromellitic acid, etc.

Also, specific examples of phenols in the quaternary phosphonium salts of phenols for use in this invention are phenol, catechol, resorcinol, hydroquinone, phloroglucinol, pyrogallol, 1,2,4-trihydroxybenzene, o-nitrophenol, m-nitro­phenol, p-nitrophenol, 2,4-dinitrophenol, 2,5-dinitrophenol, 2,6-dinitrophenol, 3,4-dinitrophenol, 4-nitrocatechol, 2-­nitroresorcinol, etc.

Picric acid which is an organic strong acid is a strong acid having pKa of -0.2. Sulfonic acid which is also an organic strong acid is an aliphatic or aromatic mono­ valent or polyvalent sulfonic acid. The total carbon atom number of the sulfonic acid for use in this invention is in the range of from 1 to 30.

Specific examples of the sulfonic acid are methane­sulfonic acid, ethanesulfonic acid, propanesulfonic acid, butanesulfonic acid, pentanesulfonic acid, hexanesulfonic acid, heptanesulfonic acid, octanesulfonic acid, nonanesulfonic acid, decanesulfonic acid, vinylsulfon­ic acid, allylsulfonic acid, 1,2-ethanedisulfonic acid, 1,4-­butanedisulfonic acid, benzenesulfonic acid, p-toluenesul­fonic acid, p-ethylbenzenesulfonic acid, xylenesulfonic acid, naphthalenesulfonic acid, phenolsolfonic acid, nitro­benzenesulfonic acid, 2,4-dinitrobenzenesulfonic acid, picrylsulfonic acid, pyridine-3-sulfonic acid, m-benzenedi­sulfonic acid, toluene-3,4-disulfonic acid, etc.

The inorganic weak acids and the derivatives thereof for use in this invention are, for example, boric acid, boric acid derivatives represented by the general formula (V) described below, phosphoric acid, phosphoric acid esters represented by the general formula (VI) described below, phosphorous acid, phosphorous acid derivatives represented by the general formula (VII), hypophosphorous acid, hypo­phosphorous acid derivatives represented by the general formula (VIII), carbonic acid, carbonic acid monoesters represented by the general formula (IX), silic acid, etc. <chemistry id="chem0004" num="0004"><img file="EP0246825A2_D0004.tif" /></chemistry> wherein, R₅, R₆, R₇ and R₈ each represents an alkyl group having from 1 to 10 carbon atoms or an aryl group and also R₆ or R₇ may be a hydrogen atom.

Specific examples of the boric acid derivatives shown by the general formula (V) described above are methane boric acid, ethane boric acid, phenyl boric acid, etc.

Specific examples of the phosphoric acid derivatives shown by the general formula (VI) described above are phosphoric acid monomethyl ester, phosphoric acid dimethyl ester, phosphoric acid monophenyl ester, etc.

Specific examples of the phosphorous acid deriva­tives shown by the general formula (VII) described above are phosphorous acid monomethyl ester, methylphosphonic acid, methylphosphonic acid methyl ester, etc.

Specific examples of the hypophosphorous acid derivatives shown by the general formula (VIII) described above are methylphosphinic acid, dimethylphosphinic acid, phenylphosphinic acid, etc.

Also, specific examples of the carbonic acid mono­esters shown by the general formula (IX) described above are carbonic acid monomethyl ester, carbonic acid monophenyl ester, etc.

The inorganic strong acids and the derivatives thereof for use in this invention are, for example, nitric acid, sulfuric acid, sulfuric acid monoesters shown by the general formula (X) described below, sulfurous acid, sul­furous acid monoesters shown by the general formula (XI) described below, and thiocyanic acid. <chemistry id="chem0005" num="0005"><img file="EP0246825A2_D0005.tif" /></chemistry> wherein, R₉ and R₁₀ each represents an alkyl group having from 1 to 10 carbon atoms or an aryl group.

Also, the acid containing halogen atom is an acid showing a very strong acidicity owing to the halogen atom having high electronegativity contained therein. Specific examples of such an acid are HBF₄ (pKa = -5), HPF₆ (pKa = -20), HAsF₆, HSbF₆, CF₃SO₃H (pKA = -13), CF₃COOH (pKa = 0), C₄F₉SO₃H, C₄F₉CO₂H, HCℓO₄ (pKa = -5), etc.

As the aprotic solvent for dissolving the quaternary phos­phonium salts in this invention, there are amide solvents such as N-methylformamide, N-ethylformamide, N,N-dimethyl­formamide, N,N-diethylformamide, N-methylacetamide, N-­ethylacetamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methylpyrrolidinone, N-methyloxazolidinone, N,Nʹ-dimethyl­imidazolidinone, etc.; lactone solvents such as γ-butyro­lactone, γ-valerolactone, δ-valerolactone, etc.; carbonate solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, etc.; nitrile solvents such as 3-meth­ oxypropionitrile, etc.; and phosphoric acid ester solvents such as trimethyl phosphate, etc. These solvents can be used singly or as a mixture thereof.

Among these, solvents composed mainly of aprotic solvents such as amides and lactones are preferred, and particularly solvents composed mainly of γ-butyrolactone are preferred by the following reasons.

γ-butyrolactone has a broad freezing point-boiling point range and is less toxic. γ-butyrolactone also does not attack the sealing gaskets compared with amide solvents, and electrolytic capacitor using this solvent is hardly suffered serious damage from halogen ion generated from the washing agent for solder flux which will be transmitted through a closure.

The amount of the quaternary phosphonium salt to be dissolved in the aforesaid solvent depends upon the desired electric conductivity and scintillation voltage but is generally less than the saturated concentration, preferably from 0.1 to 40% by weight, and more preferably from 5 to 30% by weight for low voltage capacitors and from 1 to 20% by weight for medium to high voltage capacitors, based on the weight of the electrolyte.

The electrolyte of this invention is substantially composed of the quaternary phosphonium salt and the aprotic solvent but may further contain a small amount of an auxiliary solute for various purposes such as the prevention of electrolytic corrosion, the reduction of leakage current, etc.

Now the invention will be explained in greater detail with reference to the following examples and com­parison examples, but the invention is not limited thereto.

EXAMPLE 1

An electrolyte was obtained by dissolving monotetra­ethylphosphonium maleate in an amount of 20% by weight based on a weight of an electrolyte in γ-butyrolactone as the solvent. The electric conductivity of the electrolyte at 25°C was 12.1 mS/cm and the scintillation voltage was 93 V when a constant electric current of 4 mA/cm² was applied to one set of plus and minus smooth aluminum foils in the elec­trolyte. In addition, the result of measuring the change of the electric conductivity of the electrolyte by the change of temperature is shown in the accompanying drawing.

EXAMPLE 2

An electrolyte was obtained by the same way as in Example 1 except for using monotetramethylphosphonium maleate in place of monotetraethylphosphonium maleate. The electric conductivity at 25°C and the scintillation voltage of the electrolyte were 10.7 mS/cm and 88 V, respectively.

EXAMPLE 3

An electrolyte was obtained by the same way as in Example 1 except for using monotetrabtylphosphonium maleate in place of monotetraethylphosphonium maleate. The electric conductivity at 25°C and scintillation voltage of the elec­trolyte were 6.6 mS/cm and 103 V, respectively.

COMPARISON EXAMPLE 1

An electrolyte was obtained by dissolving monotri­ethylammonium maleate in an amount of 20% by weight based on a weight of an electrolyte in γ-butyrolactone as the solvent. The electric conductivity of the electrolyte at 25°C was 4.2 mS/cm.

EXAMPLES 4 TO 13

Electrolytes were obtained by the same way as in Example 1 except for using tetraethylphosphonium formate (Example 4), tetraethylphosphonium stearate (Example 5), monotetraethylphosphonium adipate (Example 6), monotetra­ethylphosphonium sebacate (Example 7), monotetraethylphos­phonium eicosanedioate (Example 8), monotetraethylphosphonium dimethylmalonate (Example 9), monotetraethylphoshponium 6-­ethyl-1,16-hexadecanedicarboxylate (Example 10), monotetra­ethylphosphonium cyclopentylmalonate (Example 11), tetra­ethylphosphonium benzoate (Example 12), and monotetraethyl­phosphonium phthalate, respectively, in place of monotetra­ethylphosphonium maleate. The electric conductivities and scintillation voltages of these electrolytes are shown in Table 1 below.

EXAMPLES 14 TO 23

Electrolytes were obtained by the same ways as in Examples 1, 4 and 6 to 13 except for using N,N-dimethyl­formamide as a solvent in place of γ-butyrolactone. The electric conductivities and scintillation voltages of the electrolytes are shown in Table 2 below.

EXAMPLES 24 TO 34

Electrolytes were obtained by the same ways as in Examples 1 and 4 to 13 except for using propylene carbonate as a solvent in place of γ-butyrolactone. The electric conductivities and scintillation voltages of the electro­lytes are shown in Table 3 below.

EXAMPLES 35 to 43

Electrolytes were obtained by the same ways as in Examples 1, 4, 6, 7 and 9 to 13 except for using trimethyl phosphate as a solvent in place of γ-butyrolactone. The electric conductivities and scintillation voltages of the electrolytes are shown in Table 4 below.

In addition, the figures in the parentheses in the following tables are the highest voltages attained since spark was not generated.

Also, in the following tables, the following abbreviations were used. GBL: γ-Butyrolactone DMF: N,N-Dimethylformamide PC: Propylene Carbonate TMP: Trimethyl Phosphate <tables id="tabl0001" num="0001"><img file="EP0246825A2_D0006.tif" /></tables><tables id="tabl0002" num="0002"><img file="EP0246825A2_D0007.tif" /></tables><tables id="tabl0003" num="0003"><img file="EP0246825A2_D0008.tif" /></tables><tables id="tabl0004" num="0004"><img file="EP0246825A2_D0009.tif" /></tables>

EXAMPLE 44

An electrolyte was obtained by dissolving tetra­butylphosphonium phenolate in an amount of 20% by weight based on a weight of an electrolyte in γ-butyrolactone as the solvent. The electric conductivity of the electrolyte at 25°C was 5.0 mS/cm and the scintillation voltage of the electrolyte was 105 V when a constant electric current of 4 mA/cm² was applied to one set of plus and minus smooth aluminum foils in the electrolyte.

COMPARISON EXAMPLE 2

An electrolyte was obtained by dissolving triethyl­ammonium phenolate in an amount of 20% by weight based on a weight of an electrolyte in γ-butyrolactone as the solvent. The electric conductivity of the electrolyte at 25°C was 0.4 mS/cm.

EXAMPLE 45

An electrolyte was obtained by dissolving tetra­butylphosphonium methanesulfonate in an amount of 20% by weight based on a weight of an electrolyte in γ-butyro­lactone as the solvent. The electric conductivity of the electrolyte at 25°C was 5.8 mS/cm and the highest voltage attained was 17 V.

COMPARISON EXAMPLE 3

An electrolyte was obtained by dissolving triethyl­ammonium methanesulfonate in an amount of 20% by weight based on a weight of an electrolyte in γ-butyrolactone as the solvent. The electric conductivity of the electrolyte at 25°C was 3.1 mS/cm.

EXAMPLE 46

An electrolyte was obtained by dissolving a tetra­ethylphosphonium salt of boric acid in an amount of 20% by weight based on a weight of an electrolyte in γ-butyro­lactone as the solvent. The electric conductivity of the electrolyte at 25°C was 2.8 mS/cm and the scintillation voltage was 420 V when a constant electric current of 4 mA/cm² was applied to one set of plus and minus smooth aluminum foils in the electrolyte.

EXAMPLE 47

An electrolyte was obtained by the same way as in Example 46 except for using a tetrabutylphosphonium salt of boric acid in place of the tetraethylphosphonium salt of boric acid. The electric conductivity at 25°C and the scintilltion voltage of the electrolyte were 0.9 mS/cm and 520 V, respectively.

COMPARISION EXAMPLE 4

When the conventional ammonium borate was added in an amount of 10% by weight based on a weight of an elec­trolyte to γ-butyrolactone as the solvent and the mixture was heated, the ammonium salt was not dissolved and hence the mixture was unsuitable for electrolyte.

EXAMPLE 48

An electrolyte was obtained by the same way an in Example 46 except for using a tetraethylphosphonium salt of metaneboric acid in place of the tetraethylphosphonium borate. The electric conductivity at 25°C and the scintil­lation voltage of the electrolyte were 4.6 mS/cm and 210 V, respectively.

EXAMPLE 49

An electrolyte was obtained by dissolving tetra­butylphosphonium salt of phosphoric acid in an amount of 20% by weight based on a weight of an electrolyte in γ-butyro­lactone as the solvent. The electric conductivity of the electrolyte at 25°C was 6.5 mS/cm.

EXAMPLE 50

An electrolyte was prepared by the same way as in Example 49 except for using N,N-dimethylformamide as the solvent in place of γ-butyrolactone. The electric conduc­tivity of the electrolyte at 25°C was 7.6 mS/cm.

EXAMPLE 51

An electrolyte was prepared by the same way as in Example 49 except for using propylene carbonate as the solvent in place of γ-butyrolactone. The electric conduc­tivity of the electrolyte at 25°C was 6.5 mS/cm.

EXAMPLE 52

An electrolyte was obtained by dissolving tetra­ ethylphosphonium salt of phosphorous acid in an amount of 20% by weight based on a weight of an electrolyte in γ-­butyrolactone as the solvent. The electric conductivity of the electrolyte at 25°C was 8.2 mS/cm and the highest voltage attained was 170 V when a constant electric current of 5 mA/cm² was applied to one set of plus and minus smooth aluminum foils in the electrolyte.

EXAMPLE 53

An electrolyte was obtained by dissolving tetra­ethylphosphonium salt of carbonic acid in an amount of 20% by weight based on a weight of an electrolyte in γ-butyro­lactone as the solvent. The electric conductivity of the electrolyte at 25°C was 8.3 mS/cm and the scintillation voltage thereof was 208 V when a constant electric current of 4 mA/cm² was applied to one set of plus and minus smooth aluminum foils in the electrolyte.

EXAMPLE 54

An electrolyte was prepared by the same way as in Example 53 except for using N,N-dimethylformamide as the solvent in place of γ-butyrolactone.

EXAMPLE 55

An electrolyte was prepared by the same way as in Example 53 except for using a tetrabutylphosphonium salt of carbonic acid in place of the tetraethylphosphonium salt of carbonic acid.

EXAMPLE 56

An electrolyte was prepared by the same way as in Example 55 except for using propylene carbonate as the solvent in place of γ-butyrolactone.

The electric conductivities and scintilation voltages of these electrolytes are shown in Table 5 below. <tables id="tabl0005" num="0005"><img file="EP0246825A2_D0010.tif" /></tables>

EXAMPLE 57

An electrolyte was obtained by dissolving methyltri­butylphosphonium methyl carbonate in an amount of 20% by weight based on a weight of an electrolyte in γ-butyro­lactone as the solvent. The electric conductivity of the electrolyte at 25°C was 3.6 mS/cm and the scintillation voltage of the electrolyte was 180 V when a constant elec­tric current of 5 mA/cm² was applied to one set of plus and minus smooth aluminum foils in the electrolyte.

EXAMPLE 58

An electrolyte was obtained by dissolving a tetra­butylphosphonium salt of silic acid in an amount of 20% by weight based on a weight of an electrolyte in γ-butyro­lactone as the solvent. The electric conductivity of the electrolyte at 25°C was 4.5 mS/cm and the scintillation voltage of the electrolyte was 410 V when a constant elec­tric current of 4 mA/cm² was applied to one set of plus and minus smooth aluminum foils in the electrolyte.

EXAMPLE 59

An electrolyte was obtained by the same way as in Example 58 except for using N,N-dimethylformamide as the solvent in place of γ-butyrolactone. The electric conduc­tivity at 25°C and the scintillation voltage of the elec­trolyte were 6.1 mS/cm and 500 V, respectively.

EXAMPLE 60

An electrolyte was obtained by the same way as in Example 58 except for using propylene carbonate as the solvent in place of γ-butyrolactone. The electric conduc­tivity at 25°C and the scintillation voltage of the elec­trolyte were 4.7 mS/cm and 500 V, respectively.

EXAMPLE 61

An electrolyte was obtained by dissolving tetra­ethylphosphonium nitrate in an amount of 20% by weight based on a weight of an electrolyte in γ-butyrolactone as the solvent. The electric conductivity of the electrolyte at 25°C was 15.3 mS/cm.

EXAMPLE 62

An electrolyte was prepared by the same way as in Example 61 except for using N,N-dimethylformamide as the solvent in place of γ-butyrolactone. The electric conduc­tivity of the electrolyte at 25°C was 20.6 mS/cm.

EXAMPLE 63

An electrolyte was obtained by the same way as in Example 61 except for using monotetraethylphosphonium salt of sulfurous acid in place of tetraethylphosphonium nitrate. The electric conductivity at 25°C and the highest voltage attained of the electrolyte were 11.7 mS/cm and 100 V, respectively.

EXAMPLE 64

An electrolyte was obtained by dissolving a tetra­ethylphosphonium thiocyanate in an amount of 20% by weight based on a weight of an electrolyte in γ-butyrolactone as the solvent. The electric conductivity of the electrolyte at 25°C was 15.8 mS/cm.

EXAMPLE 65

An electrolyte was obtained by the same way as in Example 64 except for using N,N-dimethylformamide as the solvent in place of γ-butyrolactone. The electric conduc­ tivity of the electrolyte at 25°C was 21.0 mS/cm.

EXAMPLE 66

An electrolyte was obtained by dissolving tetra­ethylphosphonium tetrafluoroborate in an amount of 20% by weight based on a weight of an electrolyte in γ-butyro­lactone as the solvent. The electric conductivity of the electrolyte at 25°C was 17.7 mS/cm. Also, by immersing one set of plus and minus smooth aluminum foils in the elec­trolyte and applying thereto a constant electric current of 4 mA/cm², an oxide film was formed on the aluminum anode. The highest voltage attained in this case was 85 V.

EXAMPLES 67 AND 68

Electrolytes were prepared by the same way as in Example 66 except for using tetramethylphosphonium tetrafluoroborate (Example 67) and tetrabutylphosphonium tetrafluoroborate (Example 68) in place of tetraethylphos­phonium tetrafluoroborate. The electric conductivities of the electrolytes are shown in Table 6 below.

EXAMPLES 69 and 70

Electrolytes were prepared by the same way as in Example 66 except for using N,N-dimethylformamide (Example 69) and propylene carbonate (Example 70) as the solvent in place of γ-butyrolactone. The electric conductivities of the electrolytes obtained are shown in Table 6 below. <tables id="tabl0006" num="0006"><img file="EP0246825A2_D0011.tif" /></tables>

EXAMPLE 71

An electrolyte was obtained by dissolving tetra­ethylphosphonium hexafluoroantimonate in an amount of 20% by weight based on a weight of an electrolyte in γ-butyro­lactone as the solvent. The electric conductivity of the electrolyte at 25°C was 12.0 mS/cm. Also, by immersing one set of plus and minus smooth aluminum foils in the elec­trolyte and applying a constant electric current of 4 mA/cm² to the aluminum foils, an oxide film was formed on the aluminum anode. The highest voltage attained in this case was 270 V.

EXAMPLES 72 TO 74

Electrolytes were prepared by the same way as in Example 71 except for using tetraethylphosphonium hexaflu­orophosphate (Example 72), tetraethylphosphonium perchlorate (Example 73), and tetraethylphosphonium trifluoromethanesul­fonate (Example 74) in place of tetraethylphosphonium hexa­ fluoroantimonate. The electric conductivities of the electrolytes are shown in Table 7 below.

EXAMPLES 75 and 76

Electrolytes were prepared in the same way as in Example 74 except for using N,N-dimethylformamide (Example 75) and propylene carbonate (Example 76) as the solvent in place of γ-butyrolactone, the electric conductivities of the electrolytes are shown in Table 7 below. <tables id="tabl0007" num="0007"><img file="EP0246825A2_D0012.tif" /></tables>