Electrolytic Capacitor

The present invention relates to electrolyte systems for electrolytic capacitors and specifically to novel solutes for such electrolytes.

Conventional electrolytes for electrolytic capacitors generally contain a solvent and a conducting species or ionogen. For use below 100.degree. C., the solvent frequently is ethylene glycol and contains a boric acid or borate solute.

A major drawback of this system is that glycol and borate species, e.g. boric acid or ammonium pentaborate, interact to form glycol polyborates and water. This esterification reaction results in an increase in the viscosity and resistivity of the system. The water formed, if contained, results in the build-up of internal pressures at elevated temperatures and can be the cause of capacity loss due to hydration of the anode or cathode foil. If, as is more common, the water vapor formed escapes through the capacitor case, the esterification equilibrium is continually being shifted in the direction for formation of more polymer and more water. The reaction becomes a continuous source of water, and operation of the capacitor near or above 100.degree. becomes impractical.

A simpler solute, not subject to esterification and/or polymer formation is desirable. Ammonia and amine salts of carboxylic acid such as formic acid, adipic acid, and succinic acid have been used, but these tend to form amides when heated at or above 100.degree., as indicated for the ammonium salt shown below:

this reaction is accompanied by an undesirable increase in the resistivity of the electrolyte.

It has been discovered that an ammonium or amine salt of an amic acid or an amate as solute provides an electrolyte with the desired resistivity and stability at operating temperatures. Specifically useful are the amate or salt derived from maleic, succinic, phthalic, or hexahydrophthalic acid which may be obtained by reacting the corresponding anhydride with ammonia or amine of the formula R.sub.1 R.sub.2 NH in which R.sub.1 and R.sub.2 are selected from hydrogen or alkyl, cycloalkyl, or aryl group or together with N form a heterocyclic group. When one mol of the anhydride is reacted with one mol of ammonia or a primary or secondary amine, the corresponding amic acid is produced. When 2 mols of ammonia or primary or secondary amine are used per mol of anhydride, the corresponding amate is formed. Alternately, the amate may be produced by reacting an amic acid with ammonia or the same or different amine. Schematically ##STR1##

The amate was shown above as dissociated. Actually

In the equations, R.sub.1, R.sub.2, R.sub.1 ", R.sub.2 ", and R.sub.3 "are selected from hydrogen or alkyl, cycloalkyl, or aryl group or together with N form a heterocyclic ring. Suitable compounds are ammonia, piperidine, aniline, benzylamine, morpholine, methylamine, dimethylamine, n-propylamine, diisopropylamine, t-butylamine, or triethylamine. R' is the hydrocarbon group of the anhydride which is maleic, succinic, phthalic, or hexahydrophthalic anhydride.

A typical preparation of the amic acids and amates or salts of this invention involves treating the anhydride in ether with ammonia or with an appropriate amine in ether. In the table below, the anhydride, amine used, product, yield, melting point range, and % N or neutralization equivalent, calcd. and found, are given.

The amic acid salt or amate may be preformed and added to the desired solvent or produced in-situ. In addition, at least one inorganic acid may be present, specifically phosphoric or boric acid to improve leakage current, forming properties, and conductivity as taught by the prior art. Small amounts of water may be added to improve specific resistance at room temperature without adversely affecting performance at operating temperatures.

The drawing shows a capacitance section 10 in partly unrolled condition. Anode 11 is of a valve metal having on its surface an insulating oxide layer. Cathode 12 may also be a valve metal or of another metal, such as silver. Electrolyte absorbent films 13 and 14, e.g. of Kraft paper, are positioned between the anode and the cathode. Tabs 15 and 16, e.g. of aluminum, are connected to electrodes 11 and 12 respectively to function as terminals for the capacitance section. The electrolyte of the present invention is not shown in the drawing but in use completely impregnates the capacitance section.

In the following examples N-methylformamide is abbreviated as NMF and dimethylformamide as DMF. Table II shows formulations based on maleamic acid compounds and present specific resistivity at 25.degree. C. in ohm-cm (.OMEGA.cm), maximum formation voltage at 25.degree. C. for Al foil, and maximum formation voltage at 85.degree. C. for Al foil.

Table III presents data on succinamic acid compounds and hexahydrophthalamic compounds. Specific resistivity in ohm-cm (.OMEGA.-cm), maximum formation voltage at 25.degree. C., 85.degree. C., and 105.degree. C. (except as noted) are presented as are comparative data on stability after heating for specified times and at specified temperatures.

The Tables II and III show that these solutes show reasonable resistivity stability vs. time at operating temperatures, e.g. 85.degree. C. or 105.degree. C. The data also show that formation capabilities do not deteriorate with heating.

When low temperatrue performance is not critical, glycol is the preferred solvent for operation up to 200V. For low voltage electrolytes (up to 100V), particularly for low temperature operation, a glycol-methylcellosolve system is preferred.

The electrolyte system of the present invention as described in the samples is merely illustrative and not exhaustive is scope. Modifications and variations, as well as substitution of equivalents, may be made without departing from the spirit of the invention as defined in the appended claims.