Method of forming a separator for alkaline electrolyte secondary electric cell

An alkaline electrolyte secondary electric cell comprises at least one positive electrode and one negative electrode positioned either side of a separator composed of polyolefin fibers grafted with a vinyl monomer. The cell contains a device for absorbing and retaining nitrogen in a strongly basic medium, with a pH of at least 12. This device is constituted by the separator. A vinyl monomer solution is forced into the pores of the separator by drawing the solution through the separator, for example by using a suction pump.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention concerns an alkaline electrolyte secondary electric 
cell, in particular a nickel-hydridable metal cell (Ni-MH). 
2. Description of the Prior Art 
The main advantage of secondary cells is their ability to store energy. 
However, a completely charged cell which is not used rapidly loses part of 
its charge. Nickel-cadmium (Ni-Cd) storage cells, for example, have long 
been used as an autonomous energy source. They are known to have good 
charge retention, i.e. when stored in the charged state the capacity falls 
slowly. The charge lost by a completely charged Ni-Cd cell is about 20% 
over 7 days at 40.degree. C. 
Since modern portable appliances require even more powerful autonomous 
energy sources, a new cell has recently been developed. This is the 
nickel-hydridable metal storage cell (Ni-M). Such a storage cell has a 
specific energy which is at least equal to that of the Ni-Cd storage cell 
but the self-discharge rate is high and thus the user is greatly 
inconvenienced. The charge lost by a Ni-MH storage cell which is stored in 
its completely charged state is about twice as much as that for a Ni-Cd 
storage cell, i.e. 40% over 7 days at 40.degree. C. This poor result is 
due to the fact that the MH electrode, once charged, has a higher reducing 
character than the Cd electrode. 
Self-discharge is generally attributed in part to nitrogen-containing 
shuttles. Ammonia and nitrites present in the storage cell are oxidized to 
nitrates at the positively charged electrode, discharging it. Also, 
nitrates and nitrites are reduced to ammonia at the negatively charged 
electrode, discharging that as well. Those reactions can occur a number of 
times since the species generated at the positive electrode will react at 
the negative electrode where they are transformed into species which are 
capable of reacting at the positive electrode. This is why they are called 
shuttles. 
Reduction of nitrates and nitrites to ammonia is accelerated at an MH 
electrode. If such a reaction is the rate limiting step in the kinetics of 
the nitrogen-containing shuttle, then over a given period more shuttles 
can be produced in an Ni-MH storage cell. This hypothesis is generally 
accepted as the explanation for the high self-discharge rate in Ni-MH 
cells (Ikoma et al., J. Electrochem. Soc., 143, 6, 1996, 1904-1907). 
In a cell, the positive and negative electrodes are separated by an 
insulative material which assures ionic conduction while preventing 
electrical contact between the two electrodes. In order to maintain 
electrical insulation between the two electrodes, the separator must be 
mechanically and chemically stable under service conditions. It must 
retain its properties during the entire service life of the cell. Further, 
a high ionic conductivity requires that the separator be uniformly wetted 
by the electrolyte. 
In order to limit the influence of the nitrogen-containing species, it has 
been proposed to replace the polyamide separator generally used in Ni-MH 
storage cells by a polyolefin separator with a higher chemical stability 
in that medium. A polyamide separator is a potential source of 
nitrogen-containing impurities due to its deterioration in the highly 
alkaline electrolyte used in Ni-MH storage cells (U.S. Pat. No. 
5,278,001). 
The most frequently used separators at this time are based on polyethylene 
and/or polypropylene. Separators composed of fibers with a polypropylene 
core surrounded by a polyethylene sleeve are known in themselves, for 
example. However, polyethylene separators are difficult to wet in an 
aqueous electrolyte. In order to improve wettability, manufacturers have 
turned towards using separators which are grafted with hydrophilic 
monomers, which are generally vinyl compounds. The grafting method which 
has proved to be the most effective employs ionizing radiation, with the 
irradiation and grafting being carried out in two steps or simultaneously. 
Such separators introduce no or very few nitrogen-containing species, but 
the other electrochemical components present in the cell remain unwanted 
sources of nitrogen-containing compounds. Thus self-discharge of a Ni-MH 
storage cell remains very much higher than that observed under the same 
conditions for a Ni-Cd storage cell. 
The aim of the present invention is to propose a separator which increases 
charge retention in alkaline electrolyte secondary cells, in particular 
nickel-hydridable metal type cells. 
SUMMARY OF THE INVENTION 
The present invention consists in a secondary electric cell comprising at 
least one positive electrode and one negative electrode positioned either 
side of a separator composed of fibers of a polyolefin grafted with a 
vinyl monomer and containing means for absorbing and retaining nitrogen in 
a strongly basic medium, with a pH of at least 12, said means being 
constituted by said separator. 
It has been established that, surprisingly, apart from the fact that it 
does not generate nitrogen-containing species, the separator of the 
present invention efficiently traps nitrogen-containing species 
originating from the other electrochemical components. It can therefore 
substantially reduce the quantity of nitrogen-containing shuttles which 
contribute to self-discharge of Ni-MH storage cells and therefore increase 
their charge retention. 
The separator has an ability to absorb and retain nitrogen in a strongly 
basic medium, with a pH of at least 12, in a proportion of at least 
3.times.10.sup.-4 moles of nitrogen per gram of separator when the fibers 
are constituted by at least two polyolefins. 
The separator may, for example, be constituted by fibers with a 
polypropylene core which is surrounded by a sleeve of polyethylene or a 
mixture of these fibers with fibers which contain only polyethylene. 
The separator can absorb and retain nitrogen in a strongly basic medium, 
with a pH of at least 12, in a proportion of more than 5.times.10.sup.-4 
moles of nitrogen per gram of separator when said fibers are constituted 
by a single polyolefin. 
The polyolefin is preferably selected from polyethylene and polypropylene. 
These polymers have the advantage of having high chemical stability in an 
alkaline medium. 
The vinyl monomer is preferably selected from acrylic acid and methacrylic 
acid. Grafting with hydrophilic groups enhances the wettability of the 
separator. 
The present invention also consists in a process for the production of a 
separator which can absorb and retain nitrogen in a strongly basic medium, 
composed of polyolefin fibers grafted with a vinyl monomer, the process 
comprising the following steps: 
impregnating a porous separator composed of ungrafted polyolefin fibers 
with an aqueous solution containing a vinyl monomer by forcing the 
solution to penetrate into all of the pores of the separator, such that 
the volume of the solution retained by the separator after impregnation is 
at least equal to the pore volume of the separator (.gtoreq.100% of the 
pore volume); 
positioning the impregnated separator between two films of polyolefin with 
no gas being present between the surface of the impregnated separator and 
the surface of the film; 
irradiating the assembly constituted by the separator and the films with 
ultraviolet radiation to graft the monomer over the entire surface of each 
of the fibers; 
rinsing and drying the grafted separator. 
Merely immersing the separator in the solution is insufficient for the 
solution containing the monomer to bathe the entire surface of each fiber 
and to be distributed homogeneously over the entire length of the fibers 
to the core of the separator. The solution must be forced to occupy all of 
the pores of the separator. This can be achieved by drawing the solution 
through the separator, for example using a suction pump. 
The solution preferably also contains a grafting initiator, for example 
benzophenone. 
Further, the separator must be constantly bathed in the monomer solution 
for the entire duration of the grafting operation. Loss of solution which 
may occur during this step, for example by evaporation, must therefore be 
limited. 
The volume of the solution retained by the separator after impregnation and 
irradiation is preferably at least equal to the pore volume of the 
separator (.gtoreq.100% of the pore volume). 
In an advantageous variation, the solution is circulated between the 
separator and the films during irradiation. Thus the separator is 
surrounded by solution and loss through evaporation is prevented. A 
reserve of Fresh solution containing the monomer to be grafted is always 
in contact with the separator and depleted solution is evacuated. 
Rinsing is preferably carried out using deionized water Solvents which are 
difficult to eliminate and/or liable to react chemically with the grafted 
separator must be avoided. 
In a separator produced by the process described above the monomer is 
homogeneously grafted over the entire surface of each of the fibers. 
The invention further consists in a nickel-hydridable metal secondary 
electric cell comprising at least one positive electrode and one negative 
electrode positioned either side of a separator produced by the process 
described above. 
In a first embodiment of the invention, the polyolefin fibers are 
constituted by a mixture of polyethylene and polypropylene and the monomer 
is acrylic acid, the separator being capable of absorbing and retaining 
nitrogen in a proportion which is at least 3.times.10.sup.-4 moles of 
nitrogen per gram of separator. 
In a second embodiment of the invention, the polyolefin fibers are 
constituted by a single polyolefin selected from polyethylene and 
polypropylene and the monomer is acrylic acid, the separator being capable 
of absorbing and retaining nitrogen in a proportion which is at least 
5.times.10.sup.-4 moles of nitrogen per gram of separator. 
Further features and advantages of the present invention will become 
apparent from the following examples which, of course, are given by way of 
non-limiting illustration and with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Example 1 
A non-woven material grafted separator A comprising fibers constituted by a 
mixture of 25% by weight of polyethylene and 75% of polypropylene was 
produced by the process of the present invention shown in FIG. 1. 
During a first step, the ungrafted separator was impregnated with an 
aqueous solution 2 containing acrylic acid and a photoinitiator. A 
photoinitiator is an agent which increases the sensitivity of the polymer 
to ultraviolet radiation. The solution was drawn through the separator by 
means of a suction pump 3 to allow the solution to completely penetrate to 
the core of the separator. Thus all of the fibers constituting the 
separator could be reached for grafting. The separator weight gain 
(difference between the initial weight of the separator and its weight 
after impregnation, with respect to the initial weight) was 348%, 
representing 129% of the pore volume of the separator. 
The second step comprised simultaneous irradiation and grafting. Two 50 
.mu.m thick films 4 of co-extruded polypropylene which was transparent to 
UV were positioned in intimate contact with each face of the separator so 
that there was no gas between the separator and the films. The separator 
between the two films 4 underwent ultraviolet irradiation at a power of 2 
kW to effect grafting. A portion of the solution impregnating the 
separator escaped during this operation (evaporation . . . ) and the 
weight gain was only 312% after irradiation, i.e. 116% of the pore volume. 
The grafted separator 6 was then rinsed with deionized water at 7 then 
dried at 8 for 12 hours at 70.degree. C. 
By way of comparison, a separator A' grafted under analogous conditions to 
those applied to separator A but without using suction during impregnation 
or polypropylene films during irradiation had a weight gain of only 245% 
after impregnation, i.e. 91% of the pore volume, which dropped to 107% 
after irradiation (40% of the pore volume). Under these conditions the 
solution had clearly not penetrated to the core of the separator. 
The ability of the grafted separator A to absorb and retain nitrogen was 
measured as follows. 
The receptacle used was a 250 cm.sup.3 conical flask, the ground-in stopper 
of which had been lubricated with plenty of silicone grease. The flask 
contained 125 cm.sup.3 of an aqueous 8N KOH solution which initially 
contained 15.times.10.sup.-4 moles of NH.sub.3, into which 2 grams of 
separator A was introduced. After stirring, the flask was left for at 
least three days at 40.degree. C. then for 2 hours at SOC to prevent the 
ammonia from evaporating when the stopper was opened. A 100 ml sample of 
the solution was removed and the remaining ammonia was measured using the 
KJELDAHL method. NH.sub.3 was distilled and recovered in 10 cm.sup.3 of a 
0.1N HCl solution. Back titration of the HCl was carried out using an 
aqueous 0.1N potassium hydroxide KOH solution in the presence of a color 
indicator, in this case a 1% by weight alcoholic methyl red solution. 
The capacity of the separator to absorb and retain nitrogen is defined as 
the difference between the number of moles of NH.sub.3 initially 
introduced into the flask and the number of moles of NH.sub.3 present in 
the solution for 1 gram of separator. 
For separator A, the titration showed that 8.6.times.10.sup.-4 moles of 
NH.sub.3 remained in the solution. The trapping capacity of separator A 
was thus 3.2.times.10.sup.-4 moles of nitrogen per gram of separator. 
The distribution of the monomer on the surface of the fibers was studied 
using scanning electron microscopy (SEM). A sample of grafted separator 
was coated with resin and polished to enable the fibers to be observed in 
the transverse direction. The sample surface was then brought into contact 
with a solution of a cesium salt then rinsed with distilled water and 
dried. Microscopic observation of several samples of separator A showed 
that all of the fibers 20 had been grafted with monomer 21 (see FIG. 3) 
and the monomer was uniformly distributed over the entire surface all 
along the fibers. 
An Ni-MH storage cell I was produced with a positive electrode the active 
material of which was nickel hydroxide and a negative electrode the active 
material of which was a metal alloy capable of absorbing hydrogen. These 
two electrodes were separated by 0.5 g of grafted separator. The assembly 
was wound and positioned in an AA format casing filled with an aqueous 
electrolyte composed of a mixture of potassium hydroxide KOH, sodium 
hydroxide NaOH and lithium hydroxide LiOH. The quantity of 
nitrogen-containing species contained in the storage cell which could 
contribute to the nitrogen-containing shuttles described above 
corresponded to 1.4.times.10.sup.-4 moles of nitrogen. 
Storage cell I containing separator A thus had a trapping capacity of 
1.6.times.10.sup.-4 moles of nitrogen, which was higher than the quantity 
of nitrogen present in the cell. 
Storage cell I was electrochemically evaluated when stored under the 
following conditions. Storage cell I was charged then discharged twice 
then the capacity C.sub.2 discharged in the second cycle at a rate of C/5 
(a rate which will discharge the nominal capacity in 5 hours) was 
measured. It was charged a third time over 16 hours at a rate of C/10 (a 
rate which will discharge the nominal capacity in 10 hours). The storage 
cell was then stored for 7 days at 40.degree. C. on open circuit. After 
returning to ambient temperature, the cell was completely discharged at a 
rate of C/5 (a rate which will discharge the nominal capacity in 5 hours), 
to determine the remaining capacity C.sub.3. 
The loss of capacity P is defined as the difference between the discharged 
capacity C.sub.2 obtained during the second discharge and the discharged 
capacity C.sub.3 obtained after being left for 7 days at 40.degree. C., 
divided by the discharged capacity C.sub.2 obtained during the second 
discharge: 
##EQU1## 
For storage cell I containing separator A, the loss of capacity P was 21%. 
Example 2 
A grafted separator B in accordance with the present invention analogous to 
separator A was produced using the process described in Example 1 with the 
exception that the irradiation step was carried out as follows, as 
illustrated in FIG. 2. 
Two 50 .mu.m thick co-extruded polypropylene films 4 which were transparent 
to UV were positioned either side of the separator so that a small space 
(at least 0.5 mm thick) remained between the surface 11 of the separator 
and each of the films 4. This space 12 was filled with the impregnating 
solution described in Example 1 so as to force out the gas. The solution 
between the separator and the films was forced to circulate, for example 
under gravity, so that the solution bathing the surface 11 of the 
separator was continuously renewed. The separator then underwent 
ultraviolet irradiation at 5 at a power of 2 kW to effect grafting. The 
separator was thus bathed in the solution circulating between the 
polypropylene walls and simultaneously exposed to the UV radiation. 
The ability of separator B to absorb and retain nitrogen was measured as 
described in Example 1. Titration showed that 8.6.times.10.sup.-4 moles of 
NH.sub.3 remained in the solution. The trapping capacity of separator B 
was thus 3.2.times.10.sup.-4 moles of nitrogen per gram of separator. 
Example 3 
By way of comparison, a commercially available grafted separator C, 
reference number 700/30, grafted by SCIMAT, was studied. Separator C was 
non-woven material comprising fibers constituted by a mixture of 25% by 
weight of polyethylene and 75% by weight of polypropylene. 
The ability of separator C to absorb and retain nitrogen was measured as 
described in Example 1. Titration showed that 9.6.times.10.sup.-4 moles of 
NH.sub.3 remained in the solution. The trapping capacity of separator C 
was thus 2.7.times.10.sup.-4 moles of nitrogen per gram of separator. 
The monomer distribution on the fiber surface was studied by scanning 
electron microscopy (SEM) as described in Example 1. Microscopic 
observation of several samples of separator C showed that certain fibers 
30 had not been grafted (see FIG. 4) and that the monomer 32 had not been 
homogeneously distributed over the entire surface of the fibers 31. 
A Ni-MH storage cell II analogous to that described in Example 1, with the 
exception that it comprised separator C, was produced. Storage cell II 
thus had a trapping capacity of 1.times.10.sup.-4 moles of nitrogen, which 
was less than the quantity of nitrogen present in the storage cell. 
In the case of the storage cell II containing separator C, a loss of 
capacity P on storage of 53% was observed. 
Example 4 
A grafted separator D was produced in accordance with the present invention 
as described in Example 1. Separator D was a non-woven material comprising 
fibers constituted exclusively by polypropylene. 
The ability of separator D to absorb and retain nitrogen was measured as 
described in Example 1. Titration showed that 3.6.times.10.sup.-4 moles of 
NH.sub.3 remained in the solution. The trapping capacity of separator C 
was thus 5.7.times.10.sup.-4 moles of nitrogen per gram of separator. 
A Ni-MH storage cell III was produced which was analogous to that described 
in Example 1 except that the quantity of nitrogen-containing species 
present in the storage cell corresponded to 2.4.times.10.sup.-4 moles of 
nitrogen and that it comprised separator D. Storage cell III thus had a 
trapping capacity of 2.85.times.10.sup.-4 moles of nitrogen, which was 
higher than the quantity of nitrogen present in the cell. 
In the case of the storage cell III containing separator D, a loss of 
capacity P on storage of 21% was observed. 
Example 5 
By way of comparison, a commercially available grafted separator E, 
reference number 700/9, grafted by SCIMAT, was studied. Separator E was a 
non-woven material comprising fibers exclusively constituted by 
polypropylene. 
The ability of separator E to absorb and retain nitrogen was measured as 
described in Example 1. Titration showed that 5.6.times.10.sup.-4 moles of 
NH.sub.3 remained in the solution. The trapping capacity of separator C 
was thus 4.7.times.10.sup.-4 moles of nitrogen per gram of separator. 
A Ni-MH storage cell IV was produced which was analogous to that described 
in Example 4 except that it comprised separator E. Storage cell IV thus 
had a trapping capacity of 2.35.times.10.sup.-4 moles of nitrogen, which 
was lower than the quantity of nitrogen present in the cell. 
In the case of the storage cell IV containing separator E, a loss of 
capacity P on storage of 48% was observed. 
The results obtained in Examples 1 to 5 are summarized in the table below. 
TABLE 1 
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Trapping 
Self 
Cell Separator moles discharge 
ref ref fibers N/g % 
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I A PP + PE 3.2 .times. 10.sup.-4 
21 
-- B PP + PE 3.2 .times. 10.sup.-4 -- 
II C PP + PE 2.7 .times. 10.sup.-4 53 
III D PP 5.7 .times. 10.sup.-4 21 
IV E PP 4.7 .times. 10.sup.-4 48 
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