GB2082378A - Nickel cadmium batteries - Google Patents

Abstract

Nickel cadmium storage batteries have a positive plate strip with a residual charge higher than that of the negative plate strip. The strips are wound into a coil with a separator and inserted into a cell container. The cadmium electrode may be made by immersing a nickel plaque in cadmium nitrate.

Description

SPECIFICATION Process for reducing creep leakage This invention relates to a process for the preparation of nickel cadmium storage batteries and battery cells produced thereby, more particularly, to a process for preparation of nickel cadmium storage batteries with reduced creep leakage and reduced creep leakage nickel cadmium battery cells.

Creep leakage is a phenomenam which occurs in nickel cadmium storage battery cells during periods when the cell is under load at about zero volts. The occurrence of such creep leakage phenomena is evidenced by the formation of a white potassium carbonate (K2 CO3) residue or a film of liquid potassium hydroxide electrolyte (KOH) in the vicinity of the seals at the positive battery terminal. The presence of such white residue or liquid electrolyte on the battery exterior and in the vicinity of the positive terminal is not only undesirable insofar as the battery is concerned but can be harmful and damaging to adjacent positions of the equipment to which the battery cell is connected. In many applications, such as in memory preservation in computers, the battery may be under load of about zero volts per cell for extending periods.

When creep leakage occurs in such computers, the damage which might result can be substantial.

In the instant invention it has been discovered that creep leakage can be reduced and substantially eliminated. It has been discovered that, by building the nickel cadmium battery cell so that the negative plate has a residual charge lower than the residual charge of the positive plate, creep leakage can be eliminated. Such battery cells, having a lower negative plate residual charge than the positive plate residual charge, are particularly suited for uses which involve long overcharge and low rate discharge, such as in microprocessing applications, where creep leakage is particularly harmful.

In the practice of the instant invention, the positive and the negative plate materials may be formed in the conventional manner and the anodization and cathodization of the respective materials modified to attain the required residual charge relationship of the plates. The positive plate material might be formed in conventional manner and, with slight alteration, the negative plate material might be formed in accordance with U. S. patent No. 4,139,423. Preferably for reasons more apparent later herein, the positive plate material might be formed in conventional manner and the negative plate material formed in accordance with U. S. patent 4,1 66,01 0, with modification.With modification and adjustment in the electrolyte added to the cell, forming the negative plate material in accordance with the '010 patent affords the additional advantages of energy savings, water conservation, space saving and reduced impact on work environment, all without creep leakage.

The positive and the negative plate materials are conventionally formed for nickel cadmium storage batteries, by deposit or build-up of the positive and the negative materials on a carrier, such as an expanded or stamped metal plate, woven metal screen or sintered metal frame. The carrier or plaque is then loaded with electrochemically active positive or negative material, as the case may be, is cured or dried, then immersed in electrolyte, charged and discharged, washed, dried and then cut into plate size strips. The plate size strips are wound or coiled, with a separator strip therebetween.The wound or coiled unit, quite often referred to as a jelly-roll, is then loaded into a container with the negative plate electrically connected to the negative terminal, usually the metal container itself, and the positive plate electrically connected to the positive terminal, conventionally a post or terminal electrically insulated from the metal container and, by the separator, from the negative plate. In the instance of nickel cadmium batteries, the electrochemically active positive plate material is nickel hydroxide and the electrochemically active negative plate material is cadmium.

In the conventional or standard processing of positive plate materials for nickel cadmium batteries, after the carrier or plaque has been suitably loaded with nickel hydroxide, the loaded carrier or plaque is immersed in Na OH electrolyte. The immersed loaded carrier is first anodized to 70% of theoretical charge input and is then cathodized to discharge the carrier of 70% of the input during anodization. The anodized and cathodized positive plate material is then removed from the electrolyte, immersed in distilled water to remove the electrolyte and is then dried. The positive plate material is then cut into battery size strips.

The conventional or standard processing of negative plate material for nickel cadmium batteries is carried out by loading the carrier or plaque with cadmium material, such as cadmium hydroxide. This loading is accomplished by immersing a sintered nickel metal plaque in a cadmium nitrate solution, withdrawing and heating the plaque until substantially dry, then immersing the dried plaque in sodium hydroxide and then in water. The plaque is withdrawn from the water and heated in air until substantially dried. The foregoing procedure is repeated, step-by-step, until the desired electrochemical capacity of the plaque is attained.

Once the electrochemical capacity of the negative plaque has been attained, in the conventional or standard process for manufacture of nickel cadmium batteries, the air dried, electrochemically loaded plaque is immersed in Na OH electrolyte. The immersed plaque is first cathodized 100% of the theoretical charge input, then anodized or discharged 80% of the cathodized input and next again cathodized but this time to 10% of theoretical charge input.

The negative plaque is then removed from the electrolyte, immersed in distilled water to remove the electrolyte and then air dried. The negative plate material is then cut into battery size strips.

The battery size negative and positive strips are wound or coiled with the separator therebetween, inserted into the container and the battery is formed.

In U. S. patents 4,139,423 and 4,166,010, the positive plate material is formed in the conventional manner and the formation of the negative plate material is modified. Taking, first, the procedure of the '423 patent, the loading of the carrier or plaque with electrochemically active cadmium material is carried out in the same manner as in the conventional or standard processing described above.Once the desired electrochemical capacity of the plaque is attained however, and before the loaded plaque is immersed in Na OH electrolyte and cathodized and anodized, the dried plaque is heated to a temperature between 220 -300 C to convert the Cd (OH)2 in the plaque to CdO and the temperature is then raised to a temperature between 400"-550"C to convert the Ni (OH)2 remaining in the plaque to NiO and convert nitrates to nitrogen oxide gas and to drive such gas off of the plaque. This reduces the nitrate ion level in the negative plate material and improves t'le charge retention of the battery cell formed therewith.After the temperature has been raised, the Cd (OH)2 has been converted to CdO and the nickel oxide has been formed and nitrogen oxide gas is driven off, the negative plate plaque material of the '423 patent is immersed in Na OH electrolyte and cathodized and anodized in conventional manner.

The process for the formation of the negative plate material in the '010 U.S. patent is a modification of the '423 patent procedure. Rather than immersing the plaque in Na OH electrolyte after the nitrogen oxide is driven off and cathodizing and anodizing the negative plate plaque material, in the process of the '010 patent, after the nitrogen oxide is driven off, the plaque is immersed in Na OH electrolyte and cathodized until about 20-40% of the CdO is converted to Cd. The plaque in the process of the '010 patent is not anodized.

In the conventional or standard process and in the processes of the '423 and '010 U.S.

patents, the residual charge in the negative plate strip is higher than the residual charge in the positive plate strip. It is this higher negative plate strip residual charge which, in the present invention, it has been discovered causes creep leakage and which the instant invention eliminates. It has been discovered that, if the residual charge in the positive plate is maintained at a level of from about 5% to about 25% of total positive plate capacity, preferably about 10% to about 20% of plate capacity, at the time the cell is assembled and, at such time, the residual charge in the positive plate is higher than in the negative plate, creep leakage is avoided.

Higher residual charge in the negative plate material or strip than in the positive plate material or strip can be avoided or eliminated by increasing the residual charge in the positive plate material or strip or by lowering the residual charge in the negative plate material or strip. Thus, in the formation of the positive plate material, the cathodization to discharge the anodized positive plate material might be decreased or the anodization might be increased without change in cathodization. Such increase in the residual charge in the positive plate material would, of course, require increase in energy input into the positive plate material during charging, would increase the time and space requirements for charging and would increase costs.Thus, in the practice of the instant invention, to reduce cost it is preferred to reduce the residual charge on the negative plate material.

There are three charging steps in the standard or conventional process for the forming of negative plate materials which might be modified or changed to reduce residual charge and to insure, when the negative plate material is cut into plate strips and combined with a positive plate strip and a separator, that the residual change of the negative plate will be less than the positive plate. Thus, initial cathodization or cathodization after anodization might be decreased or the amount of anodization might be increased. In any event, in the practice of the instant invention when the cell is assembled the residual charge on the positive plate strip is greater than the residual charge on the negative plate strip.To assure that such positive plate residual charge at assembly will be higher than the residual charge of the negative plate, a charge of not substantially less than 5% preferably not substantially less than 10%, of the total charge capacity of the positive plate has been found to be suitable. If such residual charge of the positive plate is substantially higher than 25%, preferably higher than 20%, the capacity of the cell might be adversely affected.

The residual charge on the negative plate of the nickel cadmium battery cell produced in accordance with the '423 U.S. patent might be reduced to a charge less than the residual charge on the positive plate by decreasing the cathodizing, increasing the anodizing or by a combination of cathodizing decrease and anodizing increase.

Negative plate materials processed in accordance with the dehydration process of the 010 U.S. patent are most suitable to reduced residual charge of the instant invention. In carrying out the process of the instant invention, the cathodizing of the plaque after the plaque is dehydrated and the nitrogen oxide is driven off is eliminated. Thus, the negative plate strip cut from such plaque is assembled into the battery cell with the positive plate strip and separator without the negative plate strip being electrically formed. This is accomplished without compromise in plate purity, eliminates creep leakage and, at the same time, results in additional savings. The negative plate should be brushed before assembly.Such brushing can take place after the plaque or plate has been dehydrated and the nitrogen oxide is driven off in the presence of electrolyte to re-hydrate the plate or the dehydrated plate may be brushed and additional electrolyte might be added to the cell at assembly to re-hydrate the negative plate.

Three groups of 1 /3-AA nickel cadmium battery cells, the size most commonly used in microprocewssor applications, where creep leakage is most commonly found, were built and identified as Group C, Group D, and Group NR. Each Group contained ten battery cells.

The battery cells of Group C were of standard construction. The battery cells of Group D were built in accordance with the process of U.S. patent No. 4,139,423 with the negative plate dehydrated then cathodized and anodized. The battery cells of Group NR were built with negative plates which were dehydrated but which were neither cathodized nor anodized. Thus, the cells of Groups C and D all contained negative plates with a residual charge higher than the residual charge of the positive plate. The cells of Group NR were built with negative plates without residual charge and positive plates with residual charge in accordance with the instant invention.

The cells of Groups C and D each contained 0.46cc of 31% potassium hydroxide. The cells of the Group NR in addition to 0.46cc of 31% potassium hydroxide had 0.12cc of distilled water added to replace the water calculated for the rehydration of the CdO.

For the purposes of test, five battery cells from each. Group were charged and discharged through eight cycles, as follows: CHARGE DISCHARGE Current Time Current Cut-Off Cycle # mA Hrs. Temp. mA Volts 1 10 27 Room 200 1.0 2 10 21 ,, 200 1.0 3 10 21 ,, 200 1.0 4 10 96 ,, 200 1.0 5 10 48 ,, 100 1.0 6 10 24 ,, 50 1.0 7 10 20 ,, 20 1.0 8 10 72 ,, 50 0 The remaining five battery cells from each Group were charged and discharged through two cycles, as follows: CHARGE DISCHARGE Current Time Current Cut-Off Cycle # mA Hrs. Temp. mA Volts 1 10 24 Room 200 1.0 2 10 96 ,, 200 1.0 Three cells from each test Group were shorted by welding a metal strap to the cell with one end of the strap welded to the positive terminal and the other end to the can or container making up the negative terminal of the cell. All cells, both those shorted and the remaining unshorted cells were stored at room ambient temperature.

None of the unshorted stored cells showed any evidence of white fuzzy coating.

All of the shorted test cells of Groups C and D, both of which Groups were made up of cells with a higher negative plate residual charge than the positive plate residual charge, developed a white fuzzy coating in the vicinity of the positive terminal.

None of the shorted Group NR test cells, made up of cells with a negative plate residual charge lower than the residual charge of the positive plate showed any white fuzzy coating when tested for the same period as the Groups C and D cells.

Claims (14)

1. A process for the manufacture of nickel cadmium storage cells having reduced creep leakage, which process comprises preparing a positive plate strip, charging said positive plate strip, preparing a negative plate strip, with said positive plate strip charged and at a residual charge higher than said negative plate, winding said positive and said negative plate strips, with a separator therebetween, into a coil and inserting said wound coil into a cell container.

2. A process as claimed in claim 1 wherein the positive plate strip is charged by first anodizing said positive plate strip and then cathodizing said anodized strip to partially discharge said strip.

3. A process as claimed in claim 1 or claim 2, wherein the residual charge in said positive plate strip at the time said positive plate strip is wound into a coil with said negative plate strip and said separator is higher than the charge in said negative plate strip.

4. A process as claimed in any one of the preceding claims wherein the negative plate strip, before said strips are wound with said separator, is first cathodized, then anodized and then cathodized and, at the time of winding with said positive plate strip and said separator, has a residual charge less than the residual char je of said positive plate strip.

5. A process as claimed in any one of claims 1 to 3 wherein the negative plate strip is heated and dehydrated before said negative plate strip and said positive plate strip with a higher residual charge are wound into a coil with said separator therebetween.

6. A process as claimed in any one of claims 1 to 3 wherein the negative plate strip is prepared by immersing a porous nickel plaque in a cadmium nitrate solution, said plaque is removed from said solution and heated in air until substantially dried, immersed in a hydroxide solution, rinsed with water, reheated in air until substantially dried, and retreated until a desired electrochemical loading of of said plaque is attained and when said desired loading is attained is heated to a temperature not substantially less than 200 C and not substantially more than 300 C until substantially all of the Cd (OH)2 in said plaque is converted to CdO and nitrate impurities in said plaque are converted to nitrogen oxide gases and volatize said gases.

7. A process as claimed in claim 6, wherein the plaque is heated to a temperature not substantially less than 400 C and not substantially more than 500 C to convert said nitrate impurities in said plaque to nitrogen oxide gases and volatize said gases.

8. A process as claimed in claim 6 or claim 7 wherein the negative plate strip, after said nitrate impurities are converted to said gases and are volatized, is cooled and wound into a coil with said positive plate strip at a higher residual charge and said separator.

9. A process as claimed in claim 6 or claim 7 wherein the negative plate strip, after said nitrate impurities are converted to said gases and are volatized, is cooled and wound into a coil with said positive plate strip at a higher residual charge and said separator.

10. A nickel cadmium storage cell having reduced creep leakage wherein the residual charge in the positive plate of said cell is higher than the residual charge in the negative plate.

11. A nickel cadmium storage cell, as claimed in claim 10 wherein the residual charge in the positive plate is not substantially less than 5% and not substantially more than 25% of the total charge capacity of the positive plate.

12. A nickel cadmium storage cell, as claimed in claim 10, wherein the residual charge in the positive plate is not substantially less than 10% and not substantially more than 20% of the total charge capacity of the positive plate.

13. A process for the manufacture of nickel cadmium storage cells as claimed in claim 1 substantially as hereinbefore described.

14. A nickel cadmium storage cell when produced by a process as claimed in any one of claims 1 to 9.