AN ELECTROLYTE FOR ELECTROLYTIC CAPACITOR

Abstract: An electrolyte for electrolytic capacitor containing within an aprotic solvent alkyl quarternary ammonium salts of phenol or nitrophenols as solute.

Description: This invention relates to an electrolyte for electrolytic capacitor containing, within an aprotic solvent, alkyl quarternary ammonium salts of phenol or nitrophenols as solute. An electrolytic capacitor is formed by using an anode electrode of the so-called valve metal such as aluminium or tantalum, said metal being caused to form an insulating oxide layer on its surface, said insulating oxide layer acting as a dielectric layer, contacting said insulating oxide layer with an electrolyte layer, arranging a current collector electrode generally referred to as cathode. The electrolyte for the electrolytic capacitor, as above mentioned, contacts directly with the dielectric layer and acts as a true cathode, that is, the electrolyte lies between the dielectric layer of the electrolytic capacitor and the current collector electrode, and hence its resistance is inserted in series for the electrolytic capacitor. Therefore, the characteristic of the electrolyte results in an important factor controlling the characteristic of the electrolytic capacitor. For example, if the conductivity of an electrolyte is lower, an equivalent series resistance within the electrolytic capacitor is increased and hence the high-frequency characteristic and the loss characteristic may be lowered. That is the reason for the demand for high conductivity electrolyte and as such a high conductivity electrolyte, a solution of an organic acid or its salt, such as adipic acid in glycols, such as ethylene glycol or alcohols has been used. The latest expansion of demands for electrolytic capacitors having higher electric characteristics has not been satisfied with the conductivity of the present electrolyte. Especially in the case of the present electrolyte, when a desired conductivity is not obtained or a low solubility solute is used, the conductivity has been improved by intentionally adding water. However, in the latest utilizing condition of the electrolytic capacitor required to be used at above 100°C and for many hours, the presence of water causes the dielectric layer to worsen, the internal vapor pressure of the electrolytic capacitor to increase and hence the life of the electrolytic capacitor is worsened because of a breakage of the seal part or the volatilization of an electrolyte. Therefore, the present electrolytic capacitor has had a drawback of being unable to hold a stable characteristic over a long term. To solve the problem, it is known that an ammonium or a primary, secondary or tertiary amine salt of an organic acid can be used as a solute in an aprotic solvent, as disclosed in the Japanese patent publication Nos. 55-3989, 57-56763, 58-32769 and the U.S Patent No. 4,117,531. Hitherto, it has been known that ammonium or primary, secondary or tertiary amine salts of phenol or nitrophenols are difficult to form in a polar solvent such as water owing to their weak acidities though the acidities of nitrophenols may be increased by the increase of nitro-group. However, phenol or nitrophenols can form the salts of ammonium or primary, secondary or tertiary amine in an aprotic solvent. Now, it has been discovered that alkyl quaternary ammonium salts of phenol or nitrophenols have a considerable solubility in an aprotic solvent and the solution has an extremely high conductivity. Thus, a principal object of the invention is to provide an electrolyte of substantially non water series high conductivity with use of an aprotic solvent. An electrolyte for electrolytic capacitor according to the invention is characterized by containing alkyl quaternary ammonium salts of phenol or nitrophenols as the solute in the aprotic solvent. The alkyl quarternary ammonium salts of phenol or nitrophenols according to the invention are represented by the following general formula, in which n represents an integer of 0 to 3, and A is alkyl quarternary ammonium containing 4 alkyl groups having 1 to 6 carbon atoms. The nitrophenols to be preferably used in the invention may be selected from the group of o-nitrophenol, m-nitrophenol, p-nitrophenol, 2,3-dinitrophenol, 2,4-dinitrophenol, 2,5-dinitrophenol, 2,6-dinitrophenol, 3,4-dinitrophenol, 3,5-dinitrophenol or 2,4,6-trinitrophenol. The alkyl quarternary ammonium to be preferably used in the invention may be selected from the following but not limited thereto: tetramethylammonium, tetraethylammonium, tetrabutylammonium. The aprotic solvent to be used in the invention may be selected from the following but not limited thereto : (1) Amide system solvent: N-methylformamide, N,N-dimethylformamide, N-ethylformamide, N,N-diethylformamide, N-methylacetoamide, N,N-dimethylacetoamide, N-ethylacetoamide, N,N-diethylacetoamide, hexamethylphosphorinamide (2) Oxide compounds: di-methylsulfoxide (3) Nitrile compounds : acetonitrile (4) Cyclic esters, cyclic amides :γ-butyrolactone, N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate. An electrolyte for an electrolytic capacitor according to the invention may conveniently be prepared by adding one equivalent amount of phenol or nitrophenols to a methylene chloride solution of alkyl ammonium hydrogen-sulfate for reaction with subsequent addition of two equivalent amounts of alkali hydroxide. A deposit is removed from the reaction product with subsequent vacuum drying to obtain an anhydrous salt and then said anhydrous salt is dissolved in an aprotic solvent until a desired conductivity is obtained, for example, as an approximately 10% to 20% by weight solution. (Examples 1 to 11) An electrolyte for an electrolytic capacitor according to the invention will be exemplified in the following examples with 20% by weight solution for alkyl quarternary ammonium salts of phenol and various nitrophenols with respect to the conductivity as shown in Table 1. Further, as a comparative example the conventional standard electrolytes composed of 78% by weight of ethylene glycol, 10% by weight of water and 12% by weight of ammonium adipate are shown also in Table 1. As is understandable from the result above, the electrolyte of the present invention has higher conductivty than that of the conventional electrolyte. Further, a series of electrolytic capacitors for comparative test are prepared with the electrolytes of the examples and each of the capacitor characteristics is compared The electrolytic capacitors for the tests are prepared by rolling three superposed layers, a cathode and an anode of aluminium foil and a separator paper between them, to form a cylindrical capacitor element. The capacitor element is impregnated with one of the electrolytes of the examples and is then introduced into an external case and sealed respectively. Each electrolytic capacitor consists of the same dimentional capacitor element having 16V rated voltage and 180 µF rated capacity. The actual high-temperature load test obtained at 110 °C, 1000 Hours for electrolytes exemplified in examples 1 to 11 and the comparative example are shown at the mean value of 10 load tests in Table 2. As understandable from the results above, the electrolytic capacitors using the electrolytes of the present invention have lower capacitor losses or tan δ when compared with those of the conventional electrolyte because of the higher conductivity of the electrolyte of the present invention. Furthermore, as a result of the essentially non-aqueous system, the electrolytic capacitor using the electrolyte of the present invention shows no abnormal exterior or decrease of the electrostatic capacity due to an increase of the internal pressure and the extremely small change of the characteristic values in comparison between the initial value and the value after the load test. The electrolyte according to the invention may provide an electrolytic capacitor for a power device such as a switching regulator used at high frequency and requiring high efficiency and for various electrical machines and apparatuses used at high temperature for a long period because of the maintenance of lower capacitor losses and the stable characteristics when used at high temperature for a long period.

Claims: An electrolyte for an electrolytic capacitor consisting of an aprotic solvent and alkyl quarternary ammonium salts of phenol or nitrophenols having the general formula, in which n represents an integer of 0 to 3, and A is alkyl ammonium containing 4 alkyl groups having 1 to 6 carbon atoms. An electrolyte for an electrolytic capacitor according to claim 1, wherein the aprotic solvent is selected from the group of N-methylformamide, N,N-dimethylformamide, N-ethylformamide, N,N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide, γ-butyrolactone, N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate, dimethyl sulfoxide, acetonitrile or a mixture thereof. An electrolyte for an electrolytic capacitor according to claim 1, wherein the nitrophenol is selected from the group of o-nitrophenol, m-nitrophenol, p-nitrophenol, 2,3-dinitrophenol, 2,4-dinitrophenol, 2,5-dinitrophenol, 2,6-dinitrophenol, 3,4-dinitrophenol, 3,5-dinitrophenol or 2,4,6-trinitrophenol. An electrolyte for an electrolytic capacitor according to claim 1, wherein the alkyl quarternary ammonium is selected from the group of tetramethylammonium, tetraethylammonium, tetrabutylammonium.

Inventors: SHINOZAKI, FUMIHIKO, TSUJI, TATSUNORI, YOKOYAMA, YUTAKA
Applicants: NIPPON CHEMI-CON CORPORATION
Application year: 1987
Main classification: H01G9
IPC codes: H01G9