CA1119395A - Electrolyte system for electrolytic capacitors - Google Patents

Abstract

48.261 ELECTROLYTE SYSTEM FOR ELECTROLYTIC CAPACITORS

ABSTRACT OF THE DISCLOSURE:
An essentially non-aqueous electrolyte composition for aluminum-anode capacitors is disclosed comprising ethylene glycol and a salt such as the ammonium or amine salt of an unsaturated aliphatic dicarboxylic acid the amine containing up to 12 carbon atoms with or without a small amount of an additive such as boric acid, phos-phoric acid, ammonium dihydrogen phosphate, ammonium pentaborate, ammonium phosphate and mixtures thereof.
The electrolyte offers high electrical conductivity, and greatly increased stability and shelf life, under condi-tions of no voltage, for low voltage capacitors having high capacitance ratings. In one embodiment, the combi-nation of ethylene glycol, as the solvent, with between about 0.001 to 0.1 mole of unsaturated dicarboxylic acid per mole of ethylene glycol and about 0.001 to 0.1 mole of the salt per mole of ethylene glycol is used. Other embodiments are disclosed.

Description

B~C~GRO~D O~ T~i_ IMV~NTION:
The shelf lifc of electrolyte compositions ~or capa-citors has been a problem for over 40 years. Most of ~he re-search has been directed to the formation of an adequate seal for the oxide film on the anode electrode which is covered by a skin or film of an oxide produced by anodic oxidation which functions as a dielectric. The purpose of the seal is to make the anode less susceptible to degradation by hydration of the oxide.
B. Chesnot, in U.S. Patents 3,546,119 and 3,638,077, dlscloses an electrolyte for single and multiple aluminum elec-trode capacitors comprising about 0.5 to 10 moles of boric acid per 11 moles of a solvent such as gamma-butyrolactone, gamma-valerolactone and N-methylpyrrolidone. Chesnot's compositions can contain between 0.1 to 2 moles of a trialkylamine per mole of the solvent. The anion generating agent may be such acids as acetic, acrylic, butyric and citric acids. ThesP compositions are substantially non-aqueous but are dependent on the presence of a small amount of water necessary to reform the anode (oxide) layer produced in situ by the dehydration of boric acid into metaboric acid and water. Without this water balance the elec-trolyte is subject to pre~ature destruction through self-sustained ~. .

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elev~tion of the leakage current or an unwanted change in the electrical characteristics of the capacitor. Such composi~ions also require the presence of the afore-mentioned anode-generat-ing acids in order to attain lowered resistivity. Ethylene glycol is incorporated in the compositions of the '077 p~tent as a solubilizer.
U.S. Patent 3,83~,055, by B. Chesnot, discloses an aqueous electrolyte for aluminum-anode capacitors which is com-posed of maleic acid, a maleate of N-methylbutylamine, triethyl-amine or tributylamine and phosphoric acid dissolved in dimethyl-for~amide (DMF) wherein the maleic acid and the maleate salt are present in an amount between 10-3 moles up to their up limits of solubility per mole of solvent ~DMF) and the phosphoric acid is present in an amount between 10-3 to 10 1 moles per mole of solvent (D~).
These compositions are described as being very slightly aqueous and function where low temperatures, high frequencies and possibly high temperatures in the order of at least 85C are encountered. Stability over a temperature range of -55C to ~125C is disclosed. High faradic efficiency of the anodic oxidation and a limited service voltage, i.e. with maximum build-up voltage,are disclosed by Chesnot. Tests show that the com-bin~tion of the maleic acid and its salt produces an electrical conductivity (mmho/cm at 35C)which is more than additive and that the addition of phosphoric acid raised the build-up voltage without affecting the electrical conductivity by more than 10%.

~119395 Also, a small amount, i.e, 0.01% to 3% by weight of water and various lactones and D~-ethylene glycol solvent combinations can be used with the phosphoric acid being an essential ingre-dient.

SUMMARY OF THE INVENTIO~:
The instant invention provides electrolytes for capa-citors, using ethylene glycol as the solvent, that are charac-terlzed by exhibiting only moderate changes in capacitance and diss~pation factor and leakage currents that are not excessive when maintained at elevated temperatures, either with or without an applied voltage. Also, the compositions of this invention exhibit a leakage current factor K defined by the formula:

K ~ IL (~ a) ~ MF x V

which i9 below that of compositions using other solvents and well below a standard ammonium pentaborate electrolyte at 7.5 VDC, 300,000 ~, and well below this value at 100 hours and up t~ 500 hours or more shelf time (85C), The compositions of this invention have been tested at 60 VDC and lO,OO0 MF
and found to exhibit long shelf life with a low K value. In the above formula, IL ~ leakage current in microamps; MF ~
capacitance in microfarads; and V ~ applied DC voltage in volts, These and other advantages of using the combination of ethylene glycol and a our carbon atom unsaturated dicarboxylic acid in the form of an ammonium or Cl to C4 primary, secondary ~L119395 or tertiary aliphatic amine salt will be described. Particular advantages are obtained using an electrolyte containing between about 0.001 to 0.1 mole of the acid, i.e. fumaric acid, per mole of ethylene glycol and between about 0.001 to 0.2 mole of the amine, i.e. triethylamine, per mole of ethylene glycol. From this it is seen that the molar concentration of the salts herein disclosed is maintained at no more than about 0.3 per mole of ethylene glycol. The amount of the unsaturated aliphatic acid is determined by the desired conductivity, and the amount of base required is sufficient to attain a final pH of about 4.9 to 7.2. Also, the electrolytes of this invention exhibit low voltage shelf lives that exceed the industry imposed standards of 300,000 MFD and 7.5 VDC, and exhibit very little change in 2000 hours.
According to a broad aspect of the invention there is provided an electrolyte for electrolytic capacitors having aluminium anodes, the electrolyte consisting essentially of the salt formed by the reaction of:
a dicarboxylic acid having the formula R' R
HOOC - C = C - COOH
wherein R and R' are hydrogen or a Cl to C4 alkyl group; and a base con-sisting of ammonia and/or an amine of the formula R-NH2, RR'NH or RR'R"N, wherein R, R' and R" are hydrogen or a Cl to C4 alkyl or alkylene group;
said salt being dissolved in ethylene glycol, the total molar concentration of said acid and base being not more than about 0.3 per mole of said ethylene glycol;
and wherein the pH of said electrolyte is substantially between 4.9 and 7.2.
DESCRIPTION OF THE DRAWINGS:
The accompanying drawing is a graph of low voltage shelf life wherein the abscissa is the number of hours of storage at 85C and the ~9395 ordinate is the K factor or leakage current factor.
DESCRIPTION OF PREFERRED EMBODrMENTS:
In order to demonstrate the invention, a number of electrolytes were prepared and tested for low voltage shelf life as measured by the K
factor (leakage current factor). The compositions are set forth in the following TABLE 1.

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T A B L E

ELECT~OLYTE COMPOSITIONS

NO. IDENTIFICATION INGREDIENTS AMOUNTS
.
1. (Standard glycol) ethylene glycol 16,13 mols, 300, 000 M}~, 7 . 5 VDC boric acid 4, 71 mols .
ammonia O. 95 mols .

~, (M-50) ethylene glycol ~ mols~
12 300, 000 MF, 7 . 5 Vl)C fumaric acid ~.~Z; mols .
lJ triethylamine 0.11 mols.

Composition No. l was prepared by mixing ~6.13 mols. of ethylene glycol with 4.71 moL~ of boric acid and slowly adding 0.95 mols. of ammonia. The mixture is then heated until the boiling point reaches 125C, the final pH is sbout 5..1 and the composition exhibits 2 resistivity o~ 940 okm-centimeter~ at 30C.
Composition No. 2 was prepared by mixing 1.32 mol~ . of ~OSS' ethylene glycol with 0,54 mols. of f~maric acid and slowly 0.1~
adding 0.4 mols. of triethylamine, while keeping the tempera-ture below about 35C until the final p~ is about 6.5 and the resistivity is 420 ohm-centimeters at 30 C.
A number of capac~tors with service voltages of 7.5 V.
with etched aluminum anodes were impregnated with each of the fore~oing electrolytes after aging for four hours, and subjected to low voltage shelf life te~s. The re~ults are sho~ in the ~ 395 48.261 accompanying graph wherein the capacitors bear the same numhers as the aforesaid composition numbers.
Each point on the graph represents an average of ten (lO) capacitance determinations at the indicated hours exposure to a temperature of 85C. Capacitors Nos. l and

2 were rated at 300,000 MFD.
The electrolytes formed in accordance with this invention employ ethylene glycol as the solvent contain-ing the ammonium or amine salt of an unsaturated aliphatic dicarboxylic acid represented by the cis and trans isomers, maleic and fumaric acid, maleic anhydride, and the methyl-substituted members of the series including citraconic acid, mesaconic acid, dimethyl maleic acid and dimethyl fumaric acid. In one embodiment the acids used contain two central, double-bonded carbon atoms irrespective of the length of the side chains. Also, mixtures of the dicarboxylic acids can be used, as well as their anhydrides.
The nitrogen-containing base having up to 12 carbon atoms, used to obtain a final pH of between about 4.9 to 7.2 can be ammonia or a Cl to C4 alkyl or alkylene-substi-tuted primary, secondary or tertiary amine. Examples of such amines may be represented by the formulas:
RNH2; RR'NH and RR'R"N
whereln R, R' and R" are hydrogen or Cl to C4 alkyl or alkylene groups and mixed alkyl or alkylene groups.

~19395 Specific example~ o~ amine~ ~re methyl am~ne, dimcthyl-a~lne, ethylamine, diethylamine, trimethylamine, n-propylamine, di-n-propylamine, triethylamine and n-butylamine. Other examples are the di- and tri-butylamines and the mixed ~lkyl amines N-methyl-n-butylamine, N-methylethyla~ine, the preferred amine being triethylamine. Mlxtures of amines or an amine and ammonia can be used.
The following TABLE II lists additional examples of electrolyte compositions coming within the invention wherein tbe molar concentration of each ingredient is shown in paren-theses:

T A B L E II

1. Ethylene glycol ( 1.61 M) Maleic acid ( 0.03 M) Ammonia ( O.03 M) 2. Ethylene glycol ( 1.61 M) M~leic acid ( 0.02 ~) Ammonia (0.064 M) Boric acid (0.042 M)

3. Ethylene glycol ( 1.6L M) Maleic acid ( 0.06 ~) TriethyLamine ( 0.06 M) ` 4, E.hylene slycol (1.61 ~
Maleic a cid (0 . 06 M ) Triethylamine (0.16 M) Phosphoric acid (0.01 M~

~. Ethylene glycol (~
,~ Maleic acid (; ~
Trlethylamine ~ ~ M) Ammoni~n dihydrogen phosphate (0, 0 087M ) 6, Ethylene glycol (1.32 M) Fumaric a cid (0 . 0 54 M ) Triethylamine (0,11 M) 7. Ethylene glycol (4,08 M) Fumaric acid (0.052 M) Triethylamine (0.13 M) Ammonium p~ntaborate (0,008 M) It is noted that such additives as boric acid, phosphoric acid, ammonium dihydrogen phosphate and ammonium pentaborate have been included in some of the cornpositions of Table II.
Such additives are included in small amounts fo~ stability of the equivalent series resistance ~ESR) as required for certain capàcitors without substantially affecting the shelf life.
Contrary to expecta-ion, it W25 found that the additives listed above had to be limited to less than 0. 045 molar concen-tration to achieve the desired capaci.or properties, The pH of the compositions is adjusted to the indicated levels so that the corrosivity of the compositions to alurninum $s eliminated.

48.261 ~19395 In one embodiment of this invention an electrolyte for electrolytic capacitors having aluminum anodes is provided consisting essentially of the salt formed by the reaction of a dicarboxylic acid having the formula R' R
HOOC C = C - COOH

wherein R and R' are substituents of group consisting of hydrogen and Cl to C4 alkyl; and a base which may be ammonia or an amine of the formulas R-NH2; RR'NH and RR'R"N

wherein R, R' and R" are hydrogen and Cl to C4 alkyl and alkylene groups dissolved in ethylene glycol, with the total molar concentration of the salt being about 0.3 per mole of ethylene glycol.

:

Claims (4)

1. An electrolyte for electrolytic capacitors having aluminium anodes, the electrolyte consisting essentially of the salt formed by the reaction of:
a dicarboxylic acid having the formula wherein R and R' are hydrogen or a C1 to C4 alkyl group;
and a base consisting of ammonia and/or an amine of the formula R-NH2, RR'NH or RR'R"N, wherein R, R' and R" are hydrogen or a C1 to C4 alkyl or alkylene group;
said salt being dissolved in ethylene glycol, the total molar concentration of said acid and base being not more than about 0.3 per mole of said ethylene glycol;
and wherein the pH of said electrolyte is substantially between 4.9 and 7.2.

2. The electrolyte of claim 1, wherein said acid is maleic acid.

3. The electrolyte of claim 1, wherein the base to acid ratio is substantially two.

4. The electrolyte of claim 1 or claim 3, wherein said acid is fumaric acid.