NON-AQUEOUS LIQUID ELECTROLYTE FOR ELECTROLYTIC CAPACITORS AND ELECTROLYTIC CAPACITORS USING SAME

Abstract: Non-aqueous liquid electrolytes for electrolytic cap­ acitors which comprise diazabicycloalkenes and/or salts thereof as a solute. The electrolytes are effective in improving low temperature and high temperature characteristics when applied as electrolytic capacitors. The capacitors using the el­ ectrolytes are also described.

Description: This invention relates to electrolytic capacitors and more particularly, to electrolytic capacitors having non-aqueous liquid electrolytes. Typical electrolytic capacitors have those electrolytes which comprise ionogens dissolved in ethylene glycol. This type of electrolytic capacitor is disadvantageous in relatively poor electrolytic characteristics at low temperatures. In order to overcome the above disadvantage, there has been proposed in Japanese Laid-open Patent Application No. 54-104559 an electrolyte for electrolytic capacitors in which phthalates are dissolved in a mixed solvent of dimethylformamide and ethylene glycol. Japanese Laid-open Patent Application No. 54-7564 describes an electrolyte which contains amine salts of maleic acid dissolved in a mixed solvent of γ-butyrolactone and ethylene glycol. However, these electrolytes have the respective drawbacks. More particularly, the electrolyte using dimethylformamide is apt to pass through a sealing rubber material and has a short life on drying-up at high temperatures. On the other hand, the solvent mainly composed of γ-butyrolactone is unlikely to pass through a sealing material and is expected to have a long life at high temperatures. However, when there are used solutes which are obtained by neutralizing maleic acid with amines, a high specific conductance cannot be expected. It is accordingly an object of the invention to provide electrolytic capacitors using improved electrolytes that have improved low temperature characteristics with improved high temperature characteristics over a long time. It is another object of the invention to provide electrolytic capacitors having stable characteristic properties over a wide temperature range and a long life. It is a further object of the invention to provide electrolytic capacitors with electrolytes in which diazabicycloalkenes and/or salts thereof are used as a solute by which a high specific conductance is ensured with an improved dissipation or dielectric loss at low to high temperatures. The above objects are achieved, according to the invention, by an electrolytic capacitor comprising at least one valve metal electrode having a dielectic film formed thereon, and a non-aqueous liquid electrolyte which comprises, in a chemically and thermally stable solvent, at least one compound serving as a solute and selected from the group consisting of diazabicycloalkenes of the following formula, and salts thereof, in which m is an integer of from 2 to 11, n is an integer of from 2 to 6, and both rings are independently unsubstituted or substituted with a lower alkyl group, in an amount in the range of from 1 to 50 % by weight of the solvent used. Fig. 1 is a schematic view of a wound electrolytic capacitor section as one applicable embodiment of the present invention; Fig. 2 is a graphical representation of a variation rate of capacitance in relation to time for different electrolytic capacitors of the invention and for comparison; Fig. 3 is a graphical representation of a tangent of the loss angle in relation to time for different electrolytic capacitors of the invention and for comparison; and Fig. 4 is a graphical representation of a leakage current in relation to time for different electrolytic capacitors of the invention and for comparison. The present invention is characterized by using, as a solute in the electrolyte of the electrolytic capacitor, diazabicycloalkenes having the formula defined before and/or salts thereof. In the formula, it is preferred that m is an integer of from 3 to 5, and n is an integer of 2 or 3. The diazabicycloalkenes of the aforeindicated formula may be those described in Japanese Patent Publication No. 46-37503. Preferable examples of the diazabicycloalkenes include 1,8-diazabicyclo[5.4.0]undecene-7, 1,5-diazabicyclo[4.3.0]nonene-5, 1,8-diazabicyclo[5.3.0]decene-7, 1,4-diazabicyclo[3.3.0]octene-4, 10-methyl-1,8-diazabicyclo[5.4.0]undecene-7, 3-methyl-1,5-diazabicyclo[4.3.0]nonene-5, and mixtures thereof. Most preferably, 1,8-diazabicyclo[5.4.0]undecene-7 and 1,5-diazabicyclo[4.3.0]nonene-5 are mentioned. The salts of the diazabicycloalkenes may be those salts obtained from acids including inorganic and organic acids. Examples of the inorganic acids include boric acid, phosphoric acid, and polyphosphoric acid. Examples of the organic acids are: (1) carboxylic acids including (A) polycarboxylic acids (dibasic to tetrabasic polycarboxylic acids) which include (a) aliphatic saturated polycarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, and sebacic acid, (b) aliphatic unsaturated polycarboxylic acids such as maleic acid, fumaric acid, and itaconic acid, and (c) aromatic polycarboxylic acids such as phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, and pyromellitic acid, (B) monocarboxylic acids having from 1 to 30 carbon atoms which include (a) saturated monocarboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, lauric acid, myristic acid, stearic acid, and behenic acid, (b) unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, and oleic acid, (C) oxycarboxylic acids which include (a) aliphatic oxycarboxylic acids such as glycolic acid, lactic acid, and tartaric acid, and (b) aromatic oxycarboxylic acids such as salicyclic acid and mandelic acid, and (D) sulfur-containing polycarboxylic acids such as thiodipropionic acid; (2) phenols including, for example, monohydric phenols, e.g. phenols and naphthols, which include phenol, alkylphenols such as cresol, xylenol, ethylphenol, n and iso-propylphenol, n and iso-amylphenol, iso-nonylphenyl, and iso-dodecylphenol, methoxyphenols such as eugenol, guaiacol and the like, naphthol and cyclohexylphenol, and polyhydric phenols such as catechol, resorcin,pyrogallol, and phloroglucinol; (3) sulfonic acids such as paratoluenesulfonic acid, and dodecylbenzenesulfonic acid sulfosalicylic acid; and (4) mixtures thereof. Of these acids, phthalic acid, maleic acid and mixtures thereof are preferred. The ratio by equivalent of a diazabicycloalkene and an acid is generally in the range of 1:0.1 to 1:5, preferably from 1:0.3 to 1:1.5. The solvents for the diazabicycloalkenes and/or salts thereof may be γ-butyrolactone or mixtures thereof with other solvents. In the mixed solvents, solvents to be mixed with γ-butyrolactone include, for example, monohydric alcohols such as propyl alcohol, butyl alcohol, and amyl alcohol, polyhydric alcohols such as ethylene glycol, propylene glycol, and glycerine, ethers such as ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol dimethyl ether, and diethylene glycol dimethyl ether, amides such as N-methylformamide, N,N-dimethylformamide, N,N-diethylformamide, and N,N-diethylacetoamide, sulfoxide such as dimethylsulfoxide, oxazolidines such as 3-methyl-1,3-oxazolidin-2-one, and 3-ethyl-1,3-oxazolidin-2-one, and mixtures thereof. Of these, 3-methyl-1,3-oxazolidin-2-one, ethylene glycol and mixtures thereof are preferred. The solvent mixed with γ-butyrolactone is used in amounts not greater than 90% by weight, preferably not greater than 50% by weight, of the total amount of a mixed solvent. The amount of the diazabicycloalkene and /or a salt thereof is generally in the range of from 1 to 50% by weight, preferably from 5 to 30% by weight, of the solvent used. In other words, the electrolyte comprises from 1 to 35% by weight, preferably from 5 to 25% by weight, of a diazabicycloalkene and/or a salt thereof and, correspondingly, from 99 to 65% by weight, preferably 95 to 75% by weight, of a solvent for the diazabicycloalkene and/or salt thereof. The solvent may be γ-butyrolactone or a mixture of γ-butyrolactone and up to 90% by weight, preferably up to 50%, by weight of other solvents indicated above. If the amount of the diazabicycloalkenes and/or salts thereof is less than 1% by weight, the electric conductivity is not satisfactory. Over 50% by weight, low temperature characteristics are not improved further. Diazabicycloalkenes have an amidine structure of N - C - N, in which electrons are not localized and which are not expected in ordinary amines. When the alkenes are ionized, the proton is stabilized by resonance and is thus considered to exhibit strong basicity. Presumably, this results in promotion of dissociation of a weak acid and thus, a high specific conductance is obtained. It is also considered that since the alkenes are cyclic in structure, so that they are unlikely to decompose even at high temperatures with a long-term stability. Referring now to the drawings, and in particular to Fig. 1, there is schematically shown a partially unrolled capacitor roll 10 of a conventional construction. The capacitor 10 comprises a pair of electrode foils 11, 12 at least one of which is made of a suitable film-forming valve metal such as aluminium, which has a dielectric oxide film thereon. The oxide film may be formed by anodizing processes known in the art. The electrode foils 11, 12 are separated from each other by means of dielectric spacers 13 which are composed, for example, of paper. These spacers 13 are impregnated with the electrolyte. Terminals 14, 15 are secured to the respective electrode foils and extend in the same or opposite directions. The electrodes foils 11, 12 and the dielectric sheets 13 are convolutely wound into a roll and placed into a casing not shown. The electrolyte may be used in other types of electrolytic capacitors known in the art. For instance, the electrolyte may be applied to a wire capacitor, in which the anode is a film-forming wire having a dielectric oxide formed thereon. The present invention is described in more detail by way of examples. Examples 1 - 6 and Comparative Examples 1 - 3 Electrolytic capacitors of the formulations indicated in Table 1 below were prepared and subjected to measurement of a specific conductance at normal temperatures. The results are shown in Table 1 below. The electrolytes indicated in Table 1 were each used in an aluminium electrolytic capacitor having, respectively, a rated voltage and a capacity of 10V and 47 µF with a size of 5 mm in diameter and 11 mm in length. The respective capacitors were subjected to measurements of a capacitance and an impedance at normal and low temperatures of 20 and - 55 o C. The results are shown in Table 2 below, in which low temperature characteristics are represented by ratios to the normal temperature characteristics and tan δ is a tangent of the loss angle. The values in the figure are each an average of ten measurements. As will be apparent from the above results of Tables 1 and 2, the electrolytes of the capacitors of the invention have higher specific conductances than the electrolytes conventionally used for capacitors. This results in a reduced loss of the capacitors at normal temperatures. In addition, the capacitors of the invention have less reduced characteristics at the low temperatures. Moreover, the electrolytic capacitors using the electrolytes of Examples 1, 2 and 4 and Comparative Examples 1 and 2 were subjected to a life test at a high temperature of 105 o C over 3000 hours. In the test, each capacitor was measured with respect to the capacitance, tangent of the loss angle and leakage current. The capacitance and the tangent were determined by application of a rated voltage to the respective capacitors and the leakage current was determined without application of a voltage. The results are shown in Figures 2 through 4, which reveal that the life characteristics of the capacitors of the invention are equal or superior to those characteristics of the prior art capacitors. Especially, the phthalates of 1,8-diazabicyclo[5.4.0]undecene-7 and 1,5-diazabicyclo[4.3.0]nonene-5 are more effective in prolongation of the life at high temperatures. As will be seen from the above results, the diazabicycloalkenes and salts thereof ensure high specific conductances on use as a solute of electrolyte and can improve the tangent of the loss angle of capacitors. Since the alkenes have a cyclic structure, they are rather unlikely to decompose at high temperatures. Thus, the capacitors of the invention are stable over a wide temperature range.

Claims: An electrolytic capacitor comprising at least one valve metal electrode having a dielectric film formed thereon, and a non-aqueous liquid electrolyte which comprises, in an inert solvent, at least one compound serving as a solute and selected from the group consisting of diazabicycloalkenes of the following formula and salts thereof, in which m is an integer of from 2 to 11, n is an integer of from 2 to 6, and both rings are independently unsubstituted or substituted with a lower alkyl group, in an amount in the range of from 1 to 50 % by weight of the solvent used. The electrolytic capacitor according to Claim 1, wherein, in the formula, m is from 3 to 5 and n is 2 or 3. The electrolytic capacitor according to Claim 1, wherein said at least one compound is 1,8-diazabicyclo[5.4.0]undecene-7. The electrolytic capacitor according to Claim 1, wherein said at least one compound is 1,5-diazabicyclo[4.3.0]nonene-5. The electrolytic capacitor according to Claim 1, wherein said at least one compound is a salt of the diazabicycloalkene with phthalic acid, maleic acid or a mixture thereof. The electrolytic capacitor according to Claim 1, wherein said solvent is γ-butyrolactone. The electrolytic capacitor according to Claim 1, wherein said solvent is a mixed solvent comprising γ-butyrolactone. The electrolyitc capacitor according to Claim 7, wherein said mixed solvent is a mixture of γ-butyrolactone and a compound selected from the group consisting of 3-methyl-1,3-oxazolidin-2-one, ethylene glycol and mixtures thereof. The electolytic capacitor according to Claim 1, wherein said solute is contained in an amount of from 1 to 35 wt % in the electrolyte. The electrolytic capacitor according to Claim 9, wherein said solute is contained in an amount of from 5 to 25 wt% in the electrolyte.

Inventors: AOSHIMA, YOICHI, KUWAE, YOSHITERU, MIYAZAKI, YOSHIO, NAGARA, HISAO, SHIMAMOTO, HIDEKI CITY HAITSU KOURIGAOKA 3-1-502, SHIONO, KAZUSHI, TSUNETSUGU, TSUYOSHI
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD., SANYO CHEMICAL INDUSTRIES, LTD.
Application year: 1986
Main classification: H01G9
IPC codes: H01G9