CA1040259A - Method of preparing a battery plate by coating an aluminum core with lead - Google Patents

Abstract

Abstract of the Disclosure This invention relates to a method of producing battery plates which have reduced internal resistance over present lead-antimony plates and which are smaller than such plates in both size and weight. The method comprises the steps of:
(a) placing an aluminum core to be coated and a supply of lead in a reduced pressure chamber containing an inert gas;
(b) connecting said core as a cathode electrode and the lead as an anode electrode to an electrical power source to produce an electric field, and (c) coating said core with lead particles dislodged from the anode by ionizing the gas to produce ion bombardment of the electrodes, wherein the step of coating also causes the removal of any surface coating of aluminum oxide on the aluminum core by ion bombardment during the coating process.

Description

iO'~59 This invention relates to battery plates, and more particularly, to an improvement in the method of producing said plates. In producing battery plates, it is desirable to produce plates which are capable of carrying high currents for rapid charging and output performance, while at the same time reducing their weight and size. The present invention teaches the production of battery plates which have a reduced internal resistance -~
over present lead-antimony plates and which are smaller in size and weight.
This application is a division of my application 221,286, filed March 4, 1975.
It is a further object of the present invention to provide a method for forming battery plates of higher conductivity and superior charging capability.
This invention may be generally defined as a method of preparing a battery plate having a substantially oxide-free aluminum core and a coating of substantially pure lead on the core, said method comprising the steps of:
Ca) placing an aluminum core to be coated and a supply of lead in a reduced pressure chamber containing an inert gas;
(b) connecting said core as a cathode electrode and the lead as an anode electrode to an electrical power source to produce an electric field;
and (c) coating said core with lead particles dislodged from the anode by ionizing the gas to produce ion bombardment of the electrodes, wherein the step of coating also causes the removal of any surface coating of aluminum oxide on the aluminum core by ion bombardment during the coating process.
In accordance with this inventionf in order to obtain as light a battery as possible, the core of the plate of the battery is made of a lightweight conductive metal, preferably aluminum. While other metals such as copper or silver are considered, aluminum has the highest conductivity per unit of weight, is most economical and has adequate physical strength, and therefore is preferred.

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-The aluminum should preferably be of a commercially pure type.
The alumimAm is formed into a shape suitable for a battery plate. The plate is then cleaned in a well known manner in a solution oA4 a detergent in which a suitable wetting agent has been placed. It is thereafter washed in water and is provided with a barrier layer to the lead.
In accordance with this invention, the barrier layer is provided and the internal resistance of the battery cell is minimized by removing the natural aluminum oxide which coats the surface of the aluminum while simul-taneously replacing it with a protective conductive coating. A protective coating may be applied using metals, such as zinc, nickel, or chrome, but zinc is preferred because it is a better thermal conductor and its thermal expansion and contraction characteristics are more compatible with thoseof aluminum. The protective coating is applied by dipping the core in a solution consisting of from 13 to 70 ounces of zinc oxide per gallon of sodium hydroxide at room temperature. After the first dipping of from 2 to 5 seconds, a second dipping for from 2 to 5 seconds is advisedO To improve the bonding of the coating additives, they are added to the solution into which the core is dipped. These may be a solution of 1 to 5 ounces of either ferric chloride or rochelle salts to a gallon of water. These additives insure that a tightly bonded pure zinc film, without cracks or blisters is left on the aluminum surface.
The solution into which the aluminum plate is dipped is at room temperature. A weight of about 0.02 mg. of zinc per square inch is desirable.
The other indicated metals namely, nickel, or chrome, may be deposited using the same method as described for zinc. The aluminum oxide is removed while simultaneously depositing the zinc coating to prevent reformation of the aluminum oxide, which is an insulator, and makes poor contact with other materials.
If an inspection reveals, after the first zinc dipping that there is still impurities on the surface, the battery plate is dipped into a solution of 50% nitric acid at room temperature for a few seconds. There-

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after, the battery plate is rinsed in pure water and dipped in the original zinc oxide-sodium hydroxide mixture again.
Thereafter, the aluminum plate which is now coated with pure zinc has a second mechanical barrier layer of an electrically conductive metal, such as silver, a few angstroms thick, deposited over the zinc coating. This is achieved by placing the battery core for from 2 to 4 seconds in a regular silver plating solution. By way of example, the silver plating solution can be a mixture of 1/2 to 3/4 ounce of silver cyanide to a gallon of water.
This solution is mixed with an equal solution of 10 to 12 ounces of potassium cyanide to a gallon of water. Temperature of the solution is maintained between 70 to 85 degrees. The voltage which is applied to the electrolytic solution is from 4 to 6 volts and the current density is from 15 to 25 amperes per square foot.
The two electrically conductive barrier layers which are deposited on the aluminum core prevent any grain migration between the aluminum and the lead and avoid any galvanic action that otherwise acts to deteriorate the cell and tend to increase its resistance.
The final lead coating which is deposited on the cell can be applied by electroplating, spraying, sintering, or any other well-known tech-niques. With a cell of this construction, purer lead may be employed than otherwise. Antimony is usually present in standard battery plates. It is added to the lead or alloying constituents to increase hardness of the lead uset in the manufacture of the plates. Such plates release a large proportion of the surface antimony which migrates toward the negative plates, thereby contaminating the electrolyte and adversely affecting battery performance, as by increasing the positive plate corrosion, and increasing the self- -tischarge rate. These problems may be avoided when pure lead is used. With ; the battery plate made in accordance with the present invention, there is no neet for a hart lead plate and therefore, pure lead can be used.
Lead normally has up to 0.5% of antimony, which has little effect 1~4(~Z59 in cell operation. Antimony may be removed from the lead which has been deposited on the battery plate by dipping the lead covered battery plate into a solution made of a mixture of 2 to 5 volumes of hydrogen peroxide with one volume of sulfuric acid having a specific gravity of l.Z, for a time on the order of one hour.
In order to obtain a film of pure lead with good adherence qualities directly on an aluminum core, it is necessary to use one or more of the following vacuum techniques: thermal evaporation, plasma deposition (sputtering~ or ion implantation ~thermal evaporation with ionization).

Thermal evaporation of lead onto an aluminum core is accomplished by heating pure lead in a vacuum chamber causing it to vaporize and recondense onto the core, to form a lead film thereon. The steps include placing pure lead into a filament boat which is connected to a high current low voltage power supply. The heat from the filament is transferred to the lead by conduction. When the temperature of the lead reaches the evaporation point, it begins to evaporate. The vaporized molecules move from their source to the other solid surfaces in the vacuum chamber such as the core and the ~~ chamber walls condensing thereon due to the temperature differential. Con-densation produces the lead film formation on the aluminum core. No removal of the aluminum oxide is obtained in this process. However, such removal is a desirable objective in making high performance battery plates, and thus where this technique is used the aluminum oxide is substantially removed by a sodium hydroxide bath or the like prior to placing the aluminum into the vacuum chamber.
EXAMPLE I
r A pure lead film was deposited on an aluminum core having its oxide coating substantially removed, in a vacuum chamber having a pressure of 2 x 10 5 torr. The rate of deposition was 50 angstroms per minute.
EXAMPLE II
In a vacuum chamber having a pressure of 2 x la 7 torr, a film _ 4 _ -.. .... ... . ..
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1~4(~2S9 of pure lead was obtained on an aluminum core with a deposition rate of 15,000 angstroms per minuteO
In both examples a uniform film having good adhesion properties was obtained.
A second vacuum deposition technique known as ion sputtering may be employed. In ion sputtering two electrodes are immersed in an inert gas medium at reduced pressureO The material to be coated, for example, an aluminum core, is utilized as the cathode. The coating material to be deposited on the aluminum, for example, lead, is utilized as the anode. A
plasma is maintained by either a DoCo or R~Fo potential which continually ionizes the inert gas molecules. The electrodes are bombarded by the gaseous ions in the presence of a high electric field. The kinetic energy of the gaseous ions (on the order of 150 to 600 electron volts) exceeds the bonding energy of the surface molecules of the electrodes causing them to be dis-lodget from the surfaceO In the case of the aluminum cathode, this produces a removal of the aluminum oxide surface layer for effectively cleaning the aluminum plate~ In the case of pure lead this 6Ombardment is effective for dislodging molecules from t~e surface. The lead molecules are accelerated by the electric field thereby to cause a lead film to condense and adhere to the cleaned aluminum cathode.
EXAMPLE III
In a vacuum chamber of 1.0 to 15 x 10 3 torr and utilizing 50 to 600 watts of R.F. power, a deposition rate of approximately 1,000 angstroms per minute of lead onto the cleaned aluminum core is obtained.
A third technique combines the first two techniques and produces an increased deposition rate. The third technique is known as ion-plating.
As in the sputtering technique, electrodes are immersed in an inert gas medium such as argon at reduced pressure. The aluminum substrate which is to receive the film is connected as the cathode~ A plasma is maintained either by a D.C. or R.F. potential. An evaporation source is utilized as the ~-~

1t~4l~'~59 anode and may be a boat or filament together with the evaporant lead. Posi-tive ions in the plasma are accelerated towards the cathode (the aluminum core) by the electrical field gradient thereby bombarding and continuously cleaning the plate's core prior to the film deposition.
While the surface of the aluminum is cleaned by the plasma bombardment, the pure lead is evaporated by heatO The evaporated lead becomes ionized on passing through the plasma. The ionized lead atoms are accelerated towards the cathode (the aluminum core) simultaneously with the sputtering action which is cleaning the surface. In this manner the aluminum surface is sputtered, removing the aluminum oxide, both before and during plating.
This maintains a clea~ surface until the first monolayer of the lead film is deposited on the surface, and at the same time roughens the surface to insure strong adhesion of the first monolayer. The higher the velocity with which ~ ~ -the lead particles are accelerated, the deeper the penetration of the lead onto the surface of the aluminum.
The plasma bombarding of the core is essential for it is effec-tive to remove the naturally occurring aluminum oxide from the surface of the core. Further, it cleans and etches the surface to reduce surface deectsO
The removal of the aluminum oxide and the cleaning of the surface produce greatly improved electrical characteristics. Removal of the oxide also -~ facilitates ths proper deposition of lead onto the aluminum, a result which has not heretofore been obtainable without the utilization of intermediate te~ositions of metals such as zinc between the lead and aluminum surfaces.
It is desirable to provide shutters between the cathode and anode to collect the removed aluminum oxide and other particles which are freed from the aluminum substrate by the spu~tering process to thereby reduce contamination.
EXAMPLE IV
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Ion plating in a vacuum chamber having a pressure of from 5 x 10 5 to 8 x 10 4 torr, a film of 1,000 to 15,000 angstroms per minute of .: . .

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1~4~%59 high purity lead was deposited on the aluminum core. The dielectric was kept at about 10 to 13 mm at 20 to 25 millitorr with a current density of approxi-mately 003 to 0.8 m amps per cm .
Film deposition rates are nearly linear functions of the deposi-tion period. By varying the process parameters of pressure, power and electrode distance the film deposition rate can be changed as desired. The lead may be vaporized by resistance heating in a dielectric crucible heated by eddy currents induced by an external R.F. diode coilO Use of this technique permits deposition rates up to about 200,000 angstroms per minuteO
Although the foregoing description discloses a technique for forming battery plates wherein lead is directly coated onto aluminum as the aluminum oxide layer is removed, it is also possible to utilize the present invention where a mechanical barrier coating is first placed over the aluminum.
- For example, as disclosed in the first embodiment of the invention, an aluminum core can have its oxide removed and replaced with pure zinc followed by a thin layer of silver and thereafter a lead film. Additional lead is then deposited over the lead film by the present method. A similar example is an oxide free aluminum plate ~hich has been silver plated and then has a lead film formed thereon.
- 20 A third variation is to first deposit a lead film on the aluminum by the present method and to then add a lead coating by the electrochemical plating technique of the first embodiment. A sufficiently porous coating formed in this manner will eliminate the need for lead paste usually required for negative plates.
A final variation is to deposit directly by the vacuum techniques of the present embodiment lead oxide on the aluminum core.
Utilization of plates formed according to this invention will not be subject to the usual hydrogen embrittlement. This performance is superior ' in comparison to lead-antimony type plates. Further, the use of such plates will produce a battery cell having higher efficiency and discharge rate at ., .
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- - -. ' low temperatures. The plates are thinner than the standard plates and thus a - larger number of plates can be placed in a given cell. A partial improvement in the performance of a battery having plates according to the present invention is due to the absence of the antimony usually present in lead.
The present process by use of pure lead eliminates the electrical couple form-ed between lead and antimony which tends to cause self-discharging and create internal resistance in the battery. An additional advantage is the improved circulation of electrolyte between the thinner plates which carries away small bubbles usually present in a standard cell further reducing internal resistance.
While I have described embodiments of this invention in some detail, it will be understood that this description and illustrations are offered merely by way of example, and that the invention is to be limited in scope only by the appended claims.

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Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of preparing a battery plate having a substantially oxide-free aluminum core and a coating of substantially pure lead over the core, said method comprising the steps of: placing an aluminum core to be coated and a supply of lead in a reduced pressure chamber containing an inert gas; connecting said core as a cathode electrode and the lead as an anode electrode to an electrical power source to produce an electric field; and coating said core with lead particles dislodged from the anode by ionizing the gas to produce ion bombardment of the electrodes, wherein the step of coating also causes the removal of any surface coating of aluminum oxide on the aluminum core by ion bombardment during the coating process.

2. A method according to claim 1, wherein the step of coating the core includes the sub-steps of: heating the lead supply in said chamber to its vaporization point; and condensing lead on said plate by maintaining said plate at a temperature less than the vaporization point of lead.