US2726279A - Anode for primary cells and method for making same - Google Patents

Description

Unite 2,726,279 Patented Dec. 6, 1955 ANODE FOR PRIMARY CELLS AND METHOD FOR MAKING SAME Andr R. Gobat, North Caldwell, N. J., assignor to International Telephone and Telegraph Corporation, a corporation of Maryland No Drawing. Application August 23, 1951, Serial No. 243,357

6 Claims. Cl. 136-120) This invention relates to the deferred type of primary cell which, when supplied with or immersed in electrolyte, is ready for use, and more particularly to an anode for such cells.

Dry primary cells of the deferred type have been proposed heretofore for storage in the dry state and operative by addition of a suitable electrolyte. In such cells, an anode of magnesium can be used. The magnesium anode, however, tends to deteriorate during storage because of its high reactive characteristic. When such cells are stored, the anode, unless kept exceptionally dry such as in a dehumidified atmosphere, tends to corrode in reaction with the moist atmosphere and certain salts ordinarily included in the cell assembly. This corrosion of the anode, of course, reduces the eifective life of the cell and is one of the main difliculties encountered with the deferred type of primary cell. Effort has been made heretofore to overcome this deterioration of the magnesium anode, but with only little success. It has been proposed, for example, to treat the anode with a dilute aqueous solution of chromic acid or alkali metal chromate or dichromate, after which it is dried and included in the cellassembly. Such treatment, however, tends to depress the activity of the anode during service operation, thus reducing the voltage and current output of the cell. Furthermore, such treatment is only partially effective in inhibiting the anode corrosion in the presence of moist atmosphere.

One of the objects of this invention is to provide an improved magnesium anode for the deferred type of primary cell which has a high resistance to moist atmospheres and which resists deterioration during long storage.

Another object of the invention is to provide a magnesium anode for use in a primary cell with a coating having a catalytic eifect on the activity of the anode when supplied with electrolyte in a cell assembly and yet which inhibits activity of the anode while stored in cell assemblies, even though in the presence of moist atmosphere; and a further object is to provide a method of applying such coating.

One of the features of the invention is the particular protective coating, and another is the method by which the coating is formed on the surface of the magnesium anode. A strip of magnesium is selected of desired size and shape depending on the type of cell to be made, whether of the flat electrode type or coiled electrode form, and preferably of thin foil thickness. The magnesium strip is then prepared in accordance with the principles of this invention as follows:

(1) The magnesium strip is cleaned and degreased by any suitable method such as by immersion in a hot bath of trichloroethylene obtainable as a commercial solvent known as Tri-clean.

(2) The magnesium strip, depending upon the type of cell to be formed, may be specially formed at this stage of the preparation. The special shaping, such as making the foil strip corrugated or ridged, relieves the foil of certain strains and renders it uniform in the shape desired.

Corrugations are also sometimes desirable to increase the active surface of the anode for a given area and to enable rapid access of electrolyte to the surfaces of electrodes and for the escape of gases.

(3) Where the magnesium anode has been subjected to a special shaping operation following the cleaning and degreasing step, it is preferable to again clean the magnesium strip by subjecting it to an etching bath which may comprise, for example, a bath of acetic acid and sodium nitrate, after which the strip is washed with water for about 30 seconds.

(4) The magnesium strip is next subjected to a bath of mercuric salts and acetic acid, whereby it becomes coated with magnesium amalgam. Any soluble mercuric salt may be used, convenient salts for this preparation being mercuric acetate and mercuric nitrate. The proportions of the bath are not critical. Where mercuric acetate is used, for example, the proportions may be in the amount between 5 and 100 grams of mercuric acetate per liter of solution and the acetic acid may be in the amount of 2 /2 to 200 milliliters of acetic acid per liter of solution. The only pre-requisite is to provide suflicient mercuric ions in solution to effect a thin coating on the magnesium foil. By way of example a satisfactory ratio of these two ingredients used in practicing the invention is 30 grams mercuric acetate per liter of solution and 15 milliliters acetic acid per liter of solution. The immersion of the anode in this bath may be for a short period of about 10 seconds at room temperature, the period being varied according to the bath concentration and temperature.

(5) The treated magnesium strip is next washed in water for about 30 seconds to remove traces of the soluble salts.

(6) In order to maintain the magnesium amalgam coating on the magnesium strip and protect it against moist atmosphere, the strip is next subjected to a bath of potassium dichromate for about 60 seconds at room temperature using in the neighborhood of 50 grams of potassium dichromate per liter of solution. The strength of this bath may be varied considerably depending upon the period of immersion and temperature. This bath acts as a fixing bath and in eflfect appears to provide a protective coating over the magnesium amalgam which protects it from moist atmosphere, but which when wetted by a suitable electrolyte permits immediate reaction.

(7) After the fixing bath the magnesium strip is again washed with water, this time for about 20 seconds.

(8) The strip is removed from the washing operation and dipped through a bath of acetone to enhance rapid drying and is then dried.

From the foregoing description it will be clear that the treatment of the magnesium anode serves two purposes: one, to provide it with a coating of magnesium amalgam, and second, to protect the magnesium amalgam coating from deterioration. The amalgamated magnesium surface When supplied with an electrolyte appears to have a catalytic action which provides a higher voltage for the cell assembly than obtained heretofore with magnesium anodes.

For an example of a cell assembly incorporating a magnesium anode treated according to the principles of this invention, reference may be had to the copending application of A. 1. Warner and A. R. Gobat issued as U. S. Patent No. 2,661,388, December 1, 1953. In such cell the cathode is provided with a silver chloride compound and is disposed in close, spaced relation with respect to the amalgamated magnesium anode with a suitable porous electrical insulating material therebetween. For eight-cell batteries of the silver chloride-magnesium deferred action type, heretofore provided, the best voltage obtainable was about 11.3 volts for a discharge current 3 of approximately 1.5. amperes per square decirneter of electrode surface. With eight-cell batteries made in accordance with the present invention, as more particularly set forth in said copending application, a V0 tage of 12.3

volts was consistently obtained for discharge currents of nearly 2 amperes per square decimeter of electrode surface. The initial voltage rise, after immersion in water, was much quicker for batteries made according to the present invention than for the older type, and the time integral of the voltage, for voltages in excess of 1 volt per cell, was much improved. An individual silver chloride-magnesium cell, for instance, incorporating a magnesium anode treated in accordance with this invention showed the following discharge characteristic. When tested against a fixed discharge conductance of 0.14 mhos per square decimeter electrode surface, the voltage across the discharge conductance rose to one volt within less than 10 seconds. At the end of one minute the voltage exceeded 1.5 volts and the current was greater than 2 ampercs per square decimeter electrode surface. The duration of the cells continuous operating service at a voltage exceeding 1 volt was well over nine minutes.

Besides using the specially treated magnesium anode with a silver chloride cathode, cathodes of many other materials may be used. Any depolarizing material, such as lead dioxide, manganese dioxide, nickel dioxide, copper chloride, silver oxide, and silver peroxide, may be used in a cell assembly with the magnesium anode treated in accordance with the present invention.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made by way of example only and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

I claim:

1. In a deferred-action primary cell activated by being wetted by electrolyte, an anode comprising magnesium having a layer of magnesium amalgam and a chromated coating thereon protecting said layer from deterioration prior to activation of said cell.

2. The. method of improving the resistance to corrosion in a moist atmosphere and the electrode activity when supplied with electrolyte of a magnesium anode for a deferred-action primary cell comprising cleaning a strip of magnesium, subjecting the clean magnesium strip to a bath containing a soluble mercuric salt to provide a layer of magnesium amalgam on the surface of said strip and subjecting said layer to a chromating treatment, thereby rendering it inactive in the presence of moist atmosphere.

3. A method according to claim 2, wherein the mercuric bath includes mercuric acetate and acetic acid.

4. A method according to claim 2, wherein the mercuric bath includes mercuric nitrate and acetic acid.

5. A method according to claim 2, wherein the treatment of the magnesium strip includes washing the strip after forming the layer of magnesium amalgam and wherein said layer is chromated by subjecting the strip to a bath of potassium dichromate.

6. The method of improving the resistance to corrosion in a moist atmosphere and the electrode activity when supplied with electrolyte of a magnesium anode for a deferred-action primary cell comprising etching a strip of magnesium, subjecting the etched strip to a bath including a soluble mercuric salt and acetic acid to form a layer of magnesium amalgam on the surface of the strip, subjecting the strip to a bath of potassium dichromate, washing said strip and drying the strip.

References Cited in the file of this patent UNITED STATES PATENTS 741 Elkington May 17, 1838 1,696,873 Wood Dec. 25, 1928 1,771,190 Polcich July 22, 1930 1,783,770 Beck et al. Dec. 2, 1930 2,143,959 Schumpelt Jan. 17, 1939 2,454,799 Hart et al. Nov. 30, 1948 2,553,449 Freud May 15, 1951 FOREIGN PATENTS 1,789 Great Britain of 1853

Claims (2)

1. IN A DEFERRED-ACTION PRIMARY CELL ACTIVATED BY BEING WETTED BY ELECTROLYTE, AN ANODE COMPRISING MAGNESIUM HAVING A LAYER OF MAGNESIUM AMALGAM AND A CHROMATED COATING THEREON PROTECTING SAID LAYER FROM DETERIORATION PRIOR TO ACTIVATION OF SAID CELL.

2. THE METHOD OF IMPROVING THE RESISTANCE OF CORROSION IN A MOIST ATMOSPHERE AND THE ELECTRODE ACTIVITY WHEN SUPPLIED WITH ELECTROLYTE OF A MAGNESIUM ANODE FOR A DEFERRED-ACTION PRIMARY CELL COMPRISING CLEANING A STRIP OF MAGNESIUM, SUBJECTING THE CLEAN MAGNESIUM STRIP TO A BATH CONTAINING A SOLUBLE MERCURIC SALT TO PROVIDE A LAYER OF MAGNESIUM AMALGAM ON THE SURFACE OF SAID STRIP AND SUBJECTING SAID LAYER TO A CHROMATING TREATMENT, THEREBY RENDERING IT INACTIVE IN THE PRESENCE OF MOIST ATMOSPHERE.