GB2044801A

GB2044801A - Electrodepositing nickel over alkaline cell electrodes

Info

Publication number: GB2044801A
Application number: GB7837928A
Authority: GB
Original assignee: TAMIL NADU ALKALINE BATTERIES
Current assignee: TAMIL NADU ALKALINE BATTERIES
Priority date: 1978-09-25
Filing date: 1978-09-25
Publication date: 1980-10-22

Abstract

Manufacture or treatment of alkaline cell electrodes or plates (especially negative electrodes) in which the electrodes whether in the charged or discharged state are subject to an electro-deposition of nickel over the exterior surface area of the electrodes. The electrodes are preferably subject to a high current density for short duration nickel plating ("flash nickel" plating). The treatment has particular practical application to sintered nickel electrodes impregnated with cadmium hydroxide, i.e. for sealed nickel cadmium cells as well as to sintered zinc electrodes. The invention includes electrodes when so manufactured or treated and also cells or batteries containing them.

Description

SPECIFICATION Alkaline cell electrodes and their manufacture The object of this invention is to provide an improvement in the manufacture of electrodes for alkaline cells or batteries more particularly nickel-cadmium cells e.g. of the sealed type and also in the resulting electrodes (especially negative electrodes) and cells incorporating them.

The plates or electrodes of sealed nickelcadmium cells are usually made of sintered nickel with their pores or porous matrix filled with active materials such as hydroxides of nickel in the case of the positive electrode and cadmium hydroxide and cadmium in the negative electrode. The cells are operated with a limited amount of electrolyte in order to facilitate internal oxygen recombination during overcharge.The serviceable life of such cells is frequently limited by failure of the negative electrode and common failures are due to an increase in the mean particle size of the active materials of the negative electrode due to charge-discharge cycling or ageing (e.g. due to storage without use) which can lead to passivation of the electrode, blockage of the pores or clogging of the separator by cadmium hydroxide and cadmium dendrites causing inter-electrode bridging and resulting electrical shorting of the cell.

Particularly during use of the cell, precipitation of cadmium hydroxide occurs partly in the pores of the separator and which may be also caused by electrophoretic migration of the cadmium hydroxide particles between the negative and positive electrodes. Those particles of cadmium hydroxide in the pores of the separator which made contact with the negative electrode will be reduced to metallic cadmium during charge thus constituting a "growth" of cadmium into the separator. A continued growth of such cadmium crystallites by reduction of additional particles of cadmium hydroxide in the separator will eventually reach the positive electrode causing an internal short at that point.

Such nucleation and growth of cadmium from the exterior surface of the negative electrode takes place at random and the frequency of occurrence depends on the presence or absence of any cadmium hydroxide film on the exterior surface of the electrode when in the discharge state.

The incorporation of cadmium hydroxide into the pore space of sintered nickel plates by the vacuum impregnation process inevitably leaves behind a thin film of cadmium hydroxide on the exterior surface of the electrode plate and such film formation is accentuated by the more efficient electrochemical process due to the fact that metallic cadmium is itself electro deposited on the surface and becomes firmly bonded to the nickel substrate of the plate.

On ageing or cycling of the cell this film of cadmium hydroxide already present on the external surface of the electrode during manufacture is likely to provide a large number of nucleation sites for the growth of cadmium hydroxide contributing to failure of the cell.

A particular object of the invention is to minimise formation of cadmium crystallites on the exterior surface of the negative electrode and thus minimise the growth of cadmium hydroxide crystals or cadmium dendrites on the surface.

In order to achieve this and in accordance with this invention, the method of manufacture of alkaline cell electrodes (especially negative electrodes) is characterised by subjecting the electrodes whether in the charged or discharged state, to an electro-deposition of nickel over the exterior surface of the electrodes.

The invention further includes the provision of electrodes, especially negative electrodes, when provided with an electro-deposition of nickel and also alkaline cells or batteries incorporating such electrodes.

In practice the procedure may be as follows, i.e. as applied to sintered negative electrodes for sealed nickel cadmium cells.

After impregnation of the sintered nickel plates or electrodes with cadmium hydroxide they are formed and washed while in the charged state and then placed in a nickel plating bath such as a Watts bath where they are preferably subject to a high current density for short duration nickel plating ("flash nickel" plating), and the current density being of the order of 0.01 to 0.5 amp/cm2 snd for a duration calculated to give a deposit of 1 to 10 micro-metres (,um) on the exterior or apparent area of the electrodes. These values are given by way of practical example only and may be varied according to requirements.

During plating the electrodes are preferably located in a vertical position e.g. along a groove in a rectangular plating tank such that the edges of the groove just shield the coined portions of the electrodes in order to prevent a build-up of nickel in that area.

If the electrodes are in their discharge state before plating, a somewhat higher current density and longer duration of plating is usually necessary to achieve the same result.

After plating, the finished electrodes are washed free of sulphate ions, dried and charge-discharge cycled in an alkali solution (e.g. 20 to 30% potassium hydroxide solution) before installation in sealed nickel cadmium cells.

The flash nickel plated negative electrodes have a smooth lustrous surface similar to that of matt nickel plating electro deposited on a plane substrate. This is in sharp contrast to the relatively coarse texture and bluish/grey appearance of conventional negative electrodes. The difference may be attributed to the presence of a thin film of cadmium hydroxide/cadmium crystallites on the sintered surface of conventional negative electrodes.

It may be mentioned here that flash nickel plating of sintered nickel positive electrodes is unnecessary owing to the nature of the reaction at such positive electrodes.

In practical tests carried out on sealed nickel cadmium cells incorporating the flash nickel plated negative electrodes, the cells had four positive and five negative electrodes together with a non-woven nylon separator and were positive limited with a negative/positive capacity ratio of 1.5.

It was found that after some sixty cycles of continuous charge-discharge at a constant current and at a one hour rate, that the condition of the flash nickel plated electrodes remained substantially unchanged with negligible smudging of the adjacent side of the separator as compared with that which occurred in cells subject to the same tests and having conventional negative electrodes. The cells were also tested for any gas leakage.

A further advantageous result achieved is that a significantly higher oxygen re-combination rate appears to be obtained for sealed cell operation with no residual hydrogen.

Such increased rate of oxygen recombination is believed to be due to a faster local-cell action with more rapid oxygen reduction on a pure nickel substrate rather than on a cadmium substrate, the nickel acting as a catalyst in the action. The absence of any pressure build up due to hydrogen may be explained by the positive limited nature of the cells as well as by recombination of any traces of hydrogen with oxygen on the catalytically active surface of the nickel plating.

Apart from slight loss of capacity other desired characteristics of the cells containing the plate negative electrodes remain substantially the same.

Whereas the invention has particular practical application to sintered plate negative electrodes for nickel cadmium cells, it is also applicable to sintered plate zinc electrodes for alkaline batteries, although the advantages in this case are partially offset by the high concentration of zincate ions in solution and by some galvanic corrosion of zinc in contact with nickel.

Claims

1. Method of manufacture or treatment of alkaline cell electrodes or plates (especially negative electrodes) characterised by subjecting the electrodes whether in the charged or discharged state, to an electrodeposition of nickel over the exterior surface area of the electrodes.

2. Method of manufacture or treatment of alkaline cell electrodes or plates according to claim 1 wherein the electrodes are subject to a high current density for short duration nickel plating ("flash nickel" plating) in a nickel plating bath such as a Watts bath.

3. Method of manufacture or treatment of alkaline cell electrodes or plates according to claim 2 wherein the high current density is of the order of 0.01 to 0.5 amp/cm2 for a duration calculated to give a deposit of 1 to 10 micro-metres (yam) on the exterior or apparent area of the electrodes.

4. Method of manufacture or treatment of alkaline cell electrodes or plates according to any of the preceding claims wherein the electrodes are supported in a vertical position during their nickel plating, e.g. by groove location in a plating bath.

5. Method of manufacture or treatment of alkaline cell electrodes or plates according to claim 4 wherein location such as groove location of the electrodes in a plating bath is arranged to shield coined portions of the electrodes in order to prevent or minimise build up of nickel deposition on such portions.

6. Method of manufacture or treatment of alkaline cell electrodes or plates according to any of the preceding claims wherein after their nickel plating the electrodes are washed free of sulphate ions, dried and charge-discharge cycled in an alkali solution (e.g. 20 to 30% potassium hydroxide solution ) prior to installation in alkaline cells such as sealed nickel cadmium cells.

7. Method of manufacture or treatment of alkaline cell electrodes or plates substantially as herein described.

8. Alkaline cell electrodes or plates when subject to electro deposition of nickel thereon by the method of manufacture or treatment according to any of the preceding claims.

9. Alkaline cell electrodes or plates according to claim 8 wherein the electrodes or plates are of sintered nickel impregnated with cadmium hydroxide, e.g. for sealed nickel cadmium cells.

10. Alkaline cell electrodes or plates according to claim 8 wherein the electrodes or plates are of sintered zinc.

11. Alkaline cells or batteries having electrodes or plates according to any of claims 8 to 10.