Process of preparing an electrode for an electrochemical cell with a porous support and an electrode obtained by said process

A process of preparing an electrode for an electrochemical cell comprising a foamlike porous metal support (1) loaded with active material, wherein, on each side of said support there is placed a separator layer (2, 3) made of polyolefin fibers, and wherein the assembly thus obtained is compressed so as to produce an adhesive bond between the layers of separator and the entire surface of said support.

BACKGROUND OF THE INVENTION 
This invention relates to a process of preparing an electrode for an 
electrochemical cell with a porous support and an electrode obtained by 
said process. It more particularly but not exclusively relates to 
electrochemical cells having alkaline electrolytes with helical electrodes 
wherein at least one electrode comprises a porous metal support loaded 
with active material. This support displays a matrix structure similar to 
that of a sponge wherein the cells are interconnected in a 
three-dimensional matrix: It is referred to as "foamlike" material. The 
porosity of the foamlike material, exclusive of active material, is 
greater than 90%. 
After the active material is loaded in the electrode, said electrode is 
compressed so as to adjust its thickness, to facilitate its assembly, 
thereby ensuring proper operation of the cell. 
When two electrodes are helically wound with an interposed separator strip, 
it is necessary to employ a separator displaying a high tensile strength 
(greater than 50 N for a separator having a width of 5 cm for example). 
Such a separator is ordinarily composed of a sheet made of polyamide or 
polypropylene fibers having a diameter on the order of 25 micrometers. 
For electrochemical reasons, it would be advantageous to create a separator 
having much finer fibers, that is, of a diameter on the order of 5 
micrometers or even less. However, the tensile strength of such a 
separator would be much too low to enable it to be helically wound with 
two electrodes using known methods. 
SUMMARY OF THE INVENTION 
The object of this invention is to obviate this disadvantage and to provide 
for the embodiment of separators having very fine fibers, particularly in 
accumulators with helical electrodes. 
The object of this invention is a process of preparing an electrode for an 
electrochemical cell comprising a foamlike porous metal support loaded 
with active material, wherein there is placed on each side of said support 
a layer of a separator made of polyolefin fibers, and wherein the assembly 
obtained in this manner is compressed so as to produce an adhesive bond 
between the separator layers and the entire surface of said support. 
A strong adhesive bond is achieved between the separators and the support 
due to the nature of said support. Indeed, such an adhesive bond does not 
exist in the case of electrodes of the sintered type, the plastic-coated 
type or the felt type. 
By virtue of the aforesaid process, an electrode with a "foamlike" support 
may be equipped with separators of polyolefin fibers with a diameter of 
less than 5 micrometers, wherein these separators display a tensile 
strength which is one-tenth that displayed by conventional separators. 
The gram weight of these separators preferably ranges from 5 to 50 
g/m.sup.2. 
Such electrodes may be helically wound using the usual winding processes, 
with the possible addition of another interposed separator. Such a 
separator, made of polyamide or polypropylene, comprises fibers of a 
diameter on the order of 20 micrometers, with a gram weight ranging from 
60 to 90 g/m.sup.2. 
According to a preferred embodiment, the dimensions of the two layers of 
separator are selected such that these two layers may be joined and thus 
protect the edges of the support. Of course, the elongation of the various 
components during compression should be taken into consideration. 
It is preferable for said support to be equipped with its electrical 
connection prior to adding its two separator layers. 
A further object of this invention is an electrode for an electrochemical 
cell comprising a foamlike porous metal support, which is filled with 
active material and equipped with an electrical connection, wherein on 
both of its sides there are embedded, respectively, two layers of a 
separator of polyolefin fibers having a diameter of less than 5 
micrometers, wherein the gram weight of said separator ranges from 5 to 50 
g/m.sup.2. 
This invention displays numerous advantages. 
First, the fibrous structure of the separator substantially improves the 
flexibility of the electrode. This greatly facilitates winding. 
Furthermore, it is possible to employ foamlike supports that are more 
fragile or have lower gram weights than the usual supports. 
In addition, for a given quantity of separator material, the separator 
consisting of very fine fibers according to the invention displays 
increased permeability to the electrolyte and improved filtering and 
insulating capacities to prevent the passage of undesirable solids and 
electrons through the separator, while retaining the same permeability to 
ions. 
The process according to the invention provides for incorporating an 
additional intermediate separator; should one of the separator layers 
contain a defect which might weaken its filtering and insulating 
properties at a given point, the superimposition of several layers will 
almost completely compensate for the effects of such a defect. 
Because separators having very fine fibers are less expensive than 
conventional separators, it is possible to either lower the total cost of 
separators in a battery or to adopt the option of multiple layers to 
provide improved performance at the same cost. 
Finally, the two separator layers of an electrode according to the 
invention protect the environment from particles of active material which 
the electrode may release. This obviates the need for treating the surface 
and body of these electrodes with particle removal products, a treatment 
known as fibrillation or "post-coating." The amount of particulate matter 
present at the work stations is substantially reduced. This results in 
improved working conditions and reduced risks to the health of the 
operators. 
Although the process according to the invention is particularly 
advantageous for the helical electrodes of cylindrical batteries, it is 
also extremely useful for flat electrodes. Indeed, electrodes with a 
porous support are obtained wherein the coating is bonded to the entire 
surface of the support; many of the aforesaid advantages therefore also 
apply in this instance. 
Other characteristics and advantages of this invention shall be made 
apparent in the following description of embodiments which are provided as 
non-limiting examples for purposes of illustration.

DETAILED DESCRIPTION 
Example 1 
FIG. 1 shows a support 1 made of porous nickel, with a gram weight of 5 
g/dm.sup.2 and a thickness of 1.1 mm, displaying an initial porosity 
greater than 90%. This support is loaded with 17 g/dm.sup.2 of active 
material made of nickel hydroxide, cobalt and cadmium. This support may be 
equipped with an electrical connection (not shown). 
Two layers of separators 2 and 3 are placed on either side of the support 
1. Each has a thickness on the order of 0.1 mm and is made of 
polypropylene fibers with a diameter ranging from 0.5 to 5 micrometers 
inclusive. Its gram weight is 50 g/m.sup.2. 
After compression, an electrode 10 with a thickness of 0.7 mm (see FIG. 2) 
and three components 1', 2', 3' is obtained. The thickness of each layer 
2', 3' is on the order of 0.015 mm. FIG. 3 shows, on an enlarged scale, 
the nickel walls 5 of the support 1' forming cavities for the active 
material 4; the fibers of the separator 3' are embedded in the support 1', 
which ensures a very strong adhesive bond over the entire surface of this 
support. 
The electrode 10 is associated to a conventional cadmium negative electrode 
to produce a nickel-cadmium battery with helical electrodes. During the 
winding operation, a piece of a conventional polyamide separator with a 
gram weight ranging from 70 to 90 g/m.sup.2 is employed to initiate the 
operation and to better protect the core of the spiral which always 
constitutes the area with the highest exposure to insulation defects. 
Example II 
The same procedure is followed as in Example I using a support 1 of porous 
nickel with a gram weight of 3.5 g/dm.sup.2 and a thickness of 1.1 mm, 
loaded with 17 g/dm.sup.2 of active material composed of cobalt, cadmium 
and nickel hydroxide. The gram weight of the separator layers is 16 
g/m.sup.2 and the diameter of its constituent polypropylene fibers ranges 
from 0.5 to 5 micrometers. 
The electrode 10 with its two separator layers 2' and 3' embedded in the 
support 1' displays a thickness of 0.7 mm after compression. It is 
associated to a conventional negative electrode as in Example I, with an 
additional intermediate separator having fibers with a diameter of 25 
micrometers, displaying a gram weight of 60 g/m.sup.2 that is less than 
that of conventional separators. 
Example III 
The same support is employed as that of Example II loaded with the same 
active material at 18 g/dm.sup.2. 
The gram weight of the separator layers is 25 g/m.sup.2 and the diameter of 
the polypropylene fibers ranges from 0.5 to 5 micrometers. The assembly is 
compressed such that the positive electrode 10 is 0.7 mm thick. 
A negative electrode is made according to the same process, wherein the 
gram weight of the porous support is 5 g/dm.sup.2 and wherein the active 
material is an alloy of lanthanum and nickel. The active material load is 
7 g/dm.sup.2. To this support there are associated two separator layers 
made of polypropylene fibers with a diameter ranging from 0.5 to 5 
micrometers inclusive, and having a gram weight of 25 g/m.sup.2. The 
assembly is compressed such that the negative electrode has a thickness of 
0.4 mm. 
These two electrodes are placed face to face and helically wound to form a 
low pressure nickel-hydrogen cell. During the winding operation, a piece 
of conventional separator is employed as in the first example. 
Of course, the invention is not limited to the examples which have just 
been described. Any equivalent means may be substituted for the means 
described without departing from the scope of the invention.