Structure of Nickel Hydroxide-Based Electrode Doped With Cobalt for an Electrochemical Generator

The present invention relates to the structure of a positive electrode base on nickel hydroxide and possessing an improved current-carrying capacity. A positive electrode with this type of structure can be used for the assembly of electrochemical generators using liquid electrolyte, for example those of the nickel-cadmium type.

The invention relates more especially to an improved electrode structure derived from the structure described in U.S. Pat. No. 4,356,242.

Even then this patent was more especially concerned by the description of the structure of a positive electrode comprising, on the one hand, a porous fibrous support of open porosity forming and/or supporting a metallic current collector, made of nickel in particular and inert with respect to the electrochemical reactions necessarily occurring in the cycles of charge-discharge of the generator and, on the other, a nickel hydroxide-based active material partially filling the pores of the collector.

The earlier patent also described a process for manufacturing such an electrode structure from a thin, non-conducting substrate of porous structure, in particular from a sheet or felt of non-woven, interlocking fibres or a porous tissue. This felt may, for example, be based on polyvinyl chloride, polypropylene or the polyacrylamide known under the trademark NYLON.

The first essential step of the procedure described thus consists in first constructing the current collector by electroplating, preferably with nickel, this collector then forming a surface which is itself porous, closely moulding the extended surface of the substrate and penetrating into the three-dimensional space of the porous substrate. If required, the initial, porous substrate made of organic fibres can be removed by thermal oxidation. This first phase is then followed by a second phase which entails the deposition of nickel hydroxide-based active material in the three-dimensional bulk of the collector.

More particularly, a manufacturing process for a positive electrode structure of this type comprises the immersion of the highly porous collector in an electrolyte solution, of nickel nitrate in particular, and electrochemical impregnation in the interior of the porous collector in order to bring about in the pores of the latter the deposition of nickel hydroxide which will constitute the active material of the electrode, this electrochemical operation being regulated in a manner so as to permit only partial filling of the pores of the collector with the active material.

It will be profitable to refer to the French patent mentioned above or to the corresponding U.S. Pat. No. 4,356,242 for further details relating to the construction and performance of such electrode structures, as well as to French patent No. 84,0110.

Moreover, it is a known practice designed to improve certain properties of the nickel electrodes manufactured from collectors of the sintered type (improvement of the stability of the electrode during the cycling operations, limitation of the variations of volume during cycling, improved propensity to charging at high temperature or at low charging loads), to introduce into the impregnating solution of active material relatively low amounts of cobalt nitrate (for example, 5% of cobalt with respect to the nickel content). In other words, the nickel hydroxide-based active material deposited thus contains a low amount of cobalt hydroxide distributed in a homogeneous manner throughout its bulk. Although the presence of nickel hydroxide really does confer on nickel electrodes of the sintered type improved properties mentioned above, up to the present it has never been observed that the effect of cobalt is to increase the bulk capacity and, in a general manner, the energetic yield of the electrode of the type under consideration.

The invention results from the discovery that the incorporation of cobalt into the active material under appropriate conditions can, nonetheless, lead to a preceptible increase, which may be as high as 20% to 25%, of the bulk capacity of the nickel hydroxide-based electrodes constructed from collectors such as those described in the earlier patents cited above, and more especially of the non-woven fibrous support type and of the isotropic, cross-liked type with open porosity.

In order to attain the appropriate conditions mentioned above, the invention proposes the subdivision of the operation of electrochemical impregnation with nickel hydroxide mentioned above of the porous collector which had first been constructed into at least two distinct, successive steps separated by a non-electrochemical impregnation of the electrode structure partially impregnated with active material interposed at the end of the first of the two steps mentioned and which entails the use of a concentrated solution of cobalt nitrate ("doping" bath) containing notably 100 to 1,000 g/l but preferably 200 to 800 g/l, of cobalt nitrate or a salt equivalent in the sense of its being capable of being reduced to cobalt hydroxide at the second step of the electrochemical impregnation.

The process according to the invention consequently implies that the partially impregnated electrode is withdrawn from the electrochemical impregnation bath containing nickel hydroxide and dried at least roughly before being placed in the non-electrochemical impregnation bath (doping) containing the cobalt salt, the electrode, partially impregnated with nickel hydroxide and modified by chemical impregnation being then reintroduced into the electrolytic bath in order to effect the second stage of the electrochemical impregnation by means of nickel hydroxide which is essential to the manufacture of the final products.

Advantageously, the first step of the electrochemical impregnation is regulated so that the quantity of nickel hydroxide deposited in the pores in the interior of the collector amounts to about 70% to about 90% of the final or desired quantity of nickel hydroxide in the structure of the electrode, the remaining 30% to 10% of nickel hydroxide being then deposited during the second stage of the electrochemical impregnation.

The non-electrochemical impregnation of the partly finished electrode by means of a solution of a cobalt salt which can subsequently be transformed into cobalt hydroxide, particularly during the second step mentioned above, may be carried out in any appropriate manner: immersion of the partly finished electrode in a concentrated solution of the salt, cobalt nitride in particular, spraying of the partially impregnated electrode with a concentrated solution of cobalt salt, application by means of a buffer, etc. . . . The non-electrochemical impregnation operation (doping), in particular its duration in relation to the concentration of salt used, is conducted in such a manner as to incorporate into the electrode a final quantity of cobalt in the form of cobalt hydroxide, for example, which varies between about 2% and about 10%, and is more advantageously of the order of 4% to 6%, for example of final cobalt hydroxyde with respect of a concentration of nickel hydroxide in the final electrode.

The entire procedure which has just been outlined is advantageously carried out starting from a collector of high porosity, for example, of a non-woven fibrous type or of an isotropic, cross-linked type of open porosity, the pores of which have diameters of from 3 to 100, and in particular from 15 to 100 microns, the two steps of the electrochemical impregnation being so regulated that the final amount of active material deposited at the end of the second electrochemical impregnation step occupies about 45% to 65% and in particular 55% to 60% of the initial volume of the pores of the porous collector initially employed.

The different steps of the process described in the foregoing can be carried out by known procedures. For the details relating to them reference may again be made to the earlier patents. In addition and as an example, mention will be made of the procedure described by Kanigene for carrying out the electroplating of the initial, felt-based or fibre-based porous substrate and the procedure of Kandler for carrying out the steps of electrochemical impregnation, for example such as that described in the published German patent application 1,133,422 or in the Swiss patent 389,699.

Preferred experimental conditions will be outlined below which are applicable for the preparation of the improved electrodes according to the invention:

Nature of the doping bath:

Solutions presently used contains 50 g/l of cobalt nitrate. Depending on the Co content desired, this concentration may be varied between 200 and 800 g/l.

Temperature of the doping bath:

May vary between ambient temperature and 45.degree. C.

Time: Very short--less than 1 mn.

Impregnation by the doping solution may be carried out either by immersion or by spraying or by means of a buffer.

The operation is carried out between the two electrochemical impregnations in nickel hydroxide, on electrodes which have been rinsed and dried.

The electrode impregnated by the solution of cobalt nitrate is immersed in an unstirred electrochemical impregnation bath and the voltage is applied as soon as possible (ideally, immersion takes place with the voltage applied).

The amount of nickel hydroxide deposited in the electrode at the time of doping amounts to between 70% and 90% of the final quantity.

At least the initial phase of the second electrochemical impregnation (following doping) is carried out in the absence of agitation in order to prevent a too large dilution in the bath of the cobalt salts which must be fixed in the electrode.

It will be observed that the most favourable results are obtained when a collector of high porosity of the felt type is used for the manufacture of the positive electrodes according to the invention. Nonetheless, very much higher mass yields are also obtained with electrodes constructed from collectors of high porosity of the isotropic, cross-liked type of open porosity, even though the increase of mass capacity does not attain the 20% to 25% supplementary capacity which can be obtained with the electrodes manufactured from porous felts.

It is also striking that the increase in mass capacity appears to be due to the heterogeneity of the distribution of cobalt within the active material deposited in the interior of the fibrous support of high porosity which constitutes the current collector. In fact, the increase in mass capacity which the invention makes possible is not observed on completion of the standard process of electrochemical impregnation carried out in a single step, and employing from the beginning a nickel nitrate-based electrolyte solution containing a proportion of cobalt nitrate corresponding to a final concentration of cobalt hydroxide which is about 5% of the final concentration of nickel hydroxide.

This micro-heterogeneity can be detected by any appropriate means, for example with the aid of a microprobe of Castaing.

In this respect, the invention thus relates to the new products which are constituted by the structures of the positive electrode, the active material of which is constituted by nickel hydroxide within which the active cobalt, particularly in the form of cobalt hydroxide, is dispersed in a heterogeneous manner, the collector being of high porosity, in particular of the non-woven fibrous type or of the isotropic, cross-linked type of open porosity.

In particular, the invention relates to the structures of the type mentioned above in which the innermost part in direct contact with the collector, the thickness of the plating which is formed by the active material and which binds directly to the porosities of the current collector, is essentially free of cobalt. In particular, it can be observed that the major portion of the cobalt thus incorporated into the plating is dispersed in a region occupying approximately the mid-position in the thickness of the plating.

In particular, the major portion of the cobalt present, for example the cobalt present as the hydroxide in the order of 2% to 10%, and particularly from 4% to 6%, of the nickel hydroxide present is concentrated in approximately the median position of the plating.

In order words, the electrode according to the invention may be characterized as possessing, within the plating of active material itself and more particularly in their median zones, zones of microvolume (for example, of the order of 1-10.mu..sup.3) in which cobalt hydroxide predominates over nickel hydroxide.

Preferred structures of the invention may be characterized as comprising a nickel-based metallic collector, the pores of which were initially 3 to 150, and in particular 15 to 100 microns in diameter, and in that the active material occupies 45% to 65%, and in particular 55% to 60%, of the initial total volume of these pores, and in that the thickness of the final structure of the electrode varies between about 0.1 and 4 millimeters. In particular, structure of appropriate electrodes will have thicknesses which vary between about 0.2 and 3 millimeters.

As will be obvious and as follows from the preceding discussion, the invention is in no way limited to those of its modes of application and construction which have been given more special consideration; on the contrary, it encompasses all possible variants; in particular, the invention relates to modified electrodes of this type in which the principal metal of the collector is a metal other than nickel, for example, copper.

Among the variants of the invention which constitute obvious equivalents of the structure and of the process claimed below, mention will be made of the structures obtained by procedures in which the operation of electrochemical impregnation is subdivided into more than two steps and into which are introduced several non-electrochemical impregnation steps using a cobalt salt under the conditions which have been described above. It will be obvious that the number of these steps must be limited, if only on account of the thinness of the electrode available and of the limited volume of the pores of the collector which is available for partial filling.