Use of choline as the cation in capacitor for electrolytes

Improved electrolytes for electrolytic capacitors contain a choline salt of aromatic dicarboxylic acids or tetracarboxylic acids, cis-aliphatic and cis-alicylic dicarboxylic acids and tetracarboxylic acids in a mole ratio of choline to carboxylic groups of 1 to 2 and an aprotic solvent.

BACKGROUND OF THE INVENTION 
The invention in this case relates to a new and novel electrolyte 
particularly adapted for use in electrolytic capacitors and to capacitors 
containing this electrolyte. 
It is desirable that electrolytes employed for capacitors be as safe as 
possible. Additionally it is desirable that the electrolyte not contain an 
alkali metal as the alkali metal tends to be reduced during operation of 
the capacitor and during this reduction tends to cause pitting of aluminum 
cathodes. Further, the electrolyte should be free of halides as the 
presence of halides tends to deteriorate the capacitors. 
Additionally, for those capacitors to be used under operating conditions of 
high temperatures for example at about 125.degree. C. it is desirable that 
the vapor pressure of the electrolyte be as low as possible at these 
temperatures. This is particularly desirable when the capacitors employed 
are tantalum capacitors as these capacitors are frequently operated for 
long periods of time at high temperatures. 
Ross, U.S. Pat. No. 2,759,132, shows tantalum and aluminum electrolyte 
capacitors containing as an electrolyte a quaternary amine salt of an 
oxidizing anion such as one having a nitro, nitrosa or other oxidizing 
substituent. Other Examples of amines disclosed are the salts of 
tetraethanol ammonium and tri-isopropanol ammonium cations. 
Examples of the anions are the picrates, nitrophenolates, nitrophenyl and 
the nitro-beta-naphtholate. These electrolytes suffer from the 
disadvantages of being relatively unsafe to those employed in the 
production process as the anion are toxic and potentially explosive. 
Ross, U.S. Pat. No. 2,866,139, shows an electrolytic capacitor in which a 
tantalum anode is employed and in which the electrolyte contains a salt of 
an organic amine cation and an aromatic anion possessing a quinoid 
structure. Examples of the anion disclosed are alizarin, 
tetrahyroxyquinone and alizalin sapherole B. Among the cations disclosed 
are the quaternary amines such as the tetraethylammonium and other 
quaternary amines. While the electrolyte of this patent is useful at high 
temperatures as the vapor pressure is low at high temperatures, it is not 
free of halides particularly the chloride. Additionally, the anions 
employed are so expensive so as to prohibit the large scale use of these 
electrolytes. 
Ross, U.S. Pat. No. 3,300,693, also shows electrolytes for use in an 
electrolytic capacitor comprising in a tantalum anode and an electrolyte 
containing a salt of phosphoric acid and a quaternary ammonium cation such 
as the tetraethylammonium cation. While these electrolytes are useful at 
high temperatures as they exhibit low vapor pressures at high temperatures 
they are not free of halides particularly chlorides. 
SUMMARY OF THE INVENTION 
A principle object of this invention is to provide an electrolyte 
particularly adapted for use in an electrolytic capacitors such as 
aluminum electrolytic capacitors or tantalum electrolytic capacitors which 
are free of halides, have a vapor pressure at temperatures such in the 
neighborhood of 125.degree. C. allow efficient and long term operation of 
capacitors containing these electrolytes at these temperatures and which 
present less hazardous conditions during the manufacturing process. 
According to the invention the applicant has developed a new and novel 
electrolyte. The electrolyte of the invention in its broadest aspect 
comprises a solution in an aprotic solvent of a salt of choline and an 
acid selected from the group consisting of the aromatic dicarboxylic 
acids, the aromatic tetracarboxylic acids, the cis-aliphatic dicarboxylic 
acids, the cis-aliphatic tetracarboxylic acids, the cis-alicyclic 
dicarboxylic acids and the cis-alicyclic tetracarboxylic acids, the 
choline and carboxylic groups being present in the mole ratio of about 1 
to 2. 
Since the choline may be manufactured without the use of halides, the 
electrolytes are free of chlorine, unlike most of the quaternary salt 
ammonium compounds formerly employed. Additionally, choline is a safe 
non-toxic material, in fact being a vitamin. 
Further, the electrolyte of the invention has a relatively low vapor 
pressure at high temperatures making it particularly adapted for use in 
tantalum electrolytes designed to be used for long periods of time at high 
operating temperatures. 
It should be noted that German Pat. No. 563,994 shows an electrolyte, for a 
capacitor, containing licithin as an emulsifying agent. 
Licithin is a mixture of the diglcerides of stearic, palmetic, oleric or 
other fatty acid linked to the choline ester of phosphoric acid.

DETAILED DESCRIPTION OF THE INVENTION 
While the electrolyte of the invention may comprise a salt of choline and 
an acid selected from the group consisting of the aromatic dicarboxylic 
acids, the tetracarboxylic acids, the cis-aliphatic dicarboxylic and 
tetracarboxylic acids and the cis-alicyclic dicarboxylic and 
tetracarboxylic acids it has been found that superior results are obtained 
with the choline salts of an acid of the group consisting of 
ortho-phthalic acid, maleic acid, hexahydrophthalic acid and pyromellitic 
acid and that particularly useful particular excellent results have been 
obtained with the choline half salt of O-phthalic acid. 
While any aprotic solvent may be employed as the solvent for the 
electrolytes of the invention, particularly useful are: dimethylformamide, 
dimethylacetamide, butyrolactone and dimethylsulfoxide. These solvents may 
be used individually or in mixtures. Preferably a mixture of 
dimethylformamide in a major amount and another of the aprotic solvents in 
a minor amount is employed. In addition, the electrolyte may contain a 
protic cosolvent such as water or a glycol for example ethylene glycol in 
an amount of up to 25% by weight. 
Further, a small amount of phosphoric acid, an amount of up to 0.1% by 
weight may be employed in order to ensure cathode passivation. 
Electrolytes consisting essentially of a solution of about 3.0-25.0 weight 
% of the choline salt, about 23.0-96.8 weight % of a aprotic solvent, up 
to 25% by weight of the aprotic cosolvent with the addition of up to 0.1% 
by weight of phosphoric acid have been found to be highly useful and 
especially preferred are electrolytes consisting essentially of about 
70.8% of dimethylformamide, 12 weight % of butrolactone, about 13.0-15.0 
weight % of the choline half ester of phthalic acid, about 0.005-0.02% by 
weight of phosphoric acid and the remainder water and electrolytes 
consisting essentially of about 70 weight % of dimethylformamide, 12 
weight % of dimethylacetamide, about 13.0-15.0 weight % of the choline 
half salt of phthalic acid, about 0.005-0.02 weight % of phosphoric acid 
and the remainder water. The novel electrolyte of the invention may be 
employed with an electrolytic capacitor in which the anode and cathode may 
be formed of metals known to be employed in such capacitors for example 
aluminum, tantalum, titanium and niobium. 
Preferred embodiments of the invention will now be described with reference 
to the figures of the drawing and the following examples. 
EXAMPLE 1 
732K Electrolyte 
This electrolyte was made by adding the following chemicals in descending 
order: 
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CHEMICAL WT % 
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Dimethylformamide 70.00 
85% Phosphoric Acid 0.01 
Phthalic Acid 8.00 
add slowly with vigorous stirring 
70% Choline bicarbonate 
10.00 
Butyrolactone 12.00 
Heat to 130 C. and allow to cool 
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EXAMPLE 2 
Electrolyte B 
This electrolyte was made by adding the following chemicals in descending 
order: 
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CHEMICAL WT % 
______________________________________ 
Dimethylformamide 70.00 
Dimethylacetamide 12.00 
85% Phosphoric Acid 0.01 
Phthalic Acid 8.00 
add slowly with vigorous stirring 
70% Choline bicarbonate 
10.00 
Heat to 130 C. and allow to cool 
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The initial properties of these electrolytes and a standard glycol 
electrolyte are shown in the following table: 
TABLE I 
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PROPERTY 732K 743P GLYCOL 
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10% pH 4.22 4.26 6.00 
100% pH 7.46 7.55 7.50 
Scintillation V 
&gt;72 &gt;70 &gt;100 
Res. ohmcm, 
30 C. 101.4 100.9 145. 
-20 256 264 5,230 
-40 -- 502 31,500 
-55 899 1085 -- 
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As it will be noted the properties of the electrolytes of the invention 
improved results particularly in regard to resistivity at low 
temperatures. 
In order to evaluate the electrolytes for use in capacitors aluminum 
electrolyte capacitors comprising aluminum foil anodes and aluminum foil 
cathodes separated by a Manila paper impregnated by the electrolytes of 
the invention and the standard glycol electrolyte were manufactured and 
tested as follows. 
The capacitors where subjected to a DC operating life test which was 
carried out in the 105.degree. C. oven. The capacitors were removed every 
500 hours from the oven and cooled to room, temperature readings were then 
made on each of the capacitors which were then returned to high 
temperature conditions. The performance of the various electrolytes are 
compared in the following graphs. 
In FIG. 1 the change in the ESR at 120 Hz is plotted against time, and FIG. 
2 the change of the ESR at 20,000 Hz is similarly plotted against time. In 
both cases it is shown that the capacitors which contained the 743P 
composition were very stable over a considerable period of time and more 
stable than the capacitors employing the standard glycol electrolyte. In 
FIG. 3 the change in capacitance with time in capacitors at 105.degree. is 
shown. As shown in this figure capacitance of the capacitors containing 
the electrolyte of the invention (743P) is significantly more stable. The 
shelf life of the capacitors employing 743P electrolytes of the invention 
as compared to the capacitors containing the standard glycol fill 
electrolyte are shown in the following table: 
TABLE II 
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Time I1 (5) I1 (10) SHELF Cap ESR ESR 
Hr. ma ma FACTOR uF 120 Hz 
20 KHz 
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743P ELECTROLYTE 
0.0 0.060 0.036 -- 16,735 
10.1 6.59 
100 0.046 0.029 1.01 16,132 
9.6 6.45 
250 0.042 0.027 0.94 16,051 
9.6 6.81 
GLYCOL ELECTROLYTE 
0.0 0.061 0.039 -- 17,319 
12.3 9.02 
100 0.055 0.034 1.00 16,890 
12.6 11.09 
250 0.049 0.030 1.00 16,858 
12.2 8.08 
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These data show that the DMF composition which uses the Choline as the 
cation (743P) is no more aggressive than is the standard glycol 
composition. 
Finally, the effect of low temperatures on the resistivity of the 734P 
electrolyte and the glycol fill electrolyte as shown in the FIG. 4. As 
shown therein the 734P electrolyte exhibits a significantly lower 
resistivity particularly at low temperatures than that of the glycol fill 
electrolyte. 
While the instant tests have shown the use of the electrolytes of the 
invention in aluminum electrolyte capacitors the electrolytes of the 
invention should be equally useful in electrolytes containing tantalum 
anode. 
While the present invention has been described with reference to 
particularly embodiments thereof it will be understood that numerous 
modifications can be made by those skilled in the art without actually 
departing from the scope of the invention.