Method of producing a polymer based solid electrolyte for an electrochemical cell

A method of producing a solid electrolyte for an electrochemical cell, the cell comprising at least one first complexing polymer presenting in its monomer pattern at least one heteroatom and at least one ionizable alkaline salt complexed in the said polymer, wherein said complexing polymer is mixed with at least one second polymer miscible with the said complexing polymer and having cross-linkable functions; said cross-linkable functions then being physically cross-linked, with the complexing polymer being brought to an essentially amorphous state during said cross-linking operation. The electrolyte obtained can be used in electrochemical cells operating at ambient temperature.

BACKGROUND OF THE INVENTION 
This invention relates to a method for producing an electrolyte, 
comprising, at least one first polymer, called the complexing polymer, 
having at least one hetero atom in its monomer pattern, together with at 
least one complexed ionizable alkaline salt in the said polymer. 
A number of studies have been conducted on such electrolytes. 
French Pat. No. 2 442 513 presents an electrolyte of this type in which the 
polymer carries, partially or totally, the homo and/or copolymers, 
essentially noncross-linked chains, derived from one or several monomers 
holding a heteroatom capable of forming donor-acceptor type links with the 
ionizable salt cation. 
However, the complex formed has a strong tendency to crystallize at ambient 
temperature (reduction of the lability between chains), a temperature 
which is below the melting temperature of the crystallites, which 
considerably reduces ionic mobility within the network. 
The consequence of such a property is a substantial reduction in the ionic 
conductivity between 100.degree. C. and ambient temperature, a reduction 
which makes it necessary to use such solid polymer electrolytes above 
80.degree. C., because it is necessary to reach the melting temperature of 
the crystalline zones to obtain enough ionic conductivity, e.g. about 
10.sup.-4 (ohm.cm).sup.-1. 
Also known, i.e. from European patent application No. 0 037 776, is the use 
of polymers or oligomers, which are cross-linked, in particular 
chemically, to constitute the complexing polymer, with as low a vitreous 
temperature as possible. 
However, such polymers have proved to have little use as solid electrolytes 
in electrochemical cells in ambient temperature applications because of 
their ionic conductivity and because of the instability of the cross-links 
in the presence of the electrodes, in particular the negative electrode; 
this is particularly the case for di, tri or multi isocyanate chemical 
cross-linking in relation to lithium, which reduces them. 
The embodiments of the present invention solve the above problems. 
SUMMARY OF THE INVENTION 
The purpose of the invention is to supply a solid electrolyte which can be 
used in electrochemical cells operating at ambient temperature. The 
invention provides a method of producing a solid electrolyte for 
electrochemical cells, comprising, on the one hand, at least a first 
polymer, called the complexing polymer, presenting in its monomer pattern 
at least one heteroatom, and on the other hand, at least one ionizable 
alkaline salt complexed in the said polymer, characterized by the fact 
that a mixture is made of the said complexing polymer with at least one 
additional polymer, miscible with the said complexing polymer, and having 
cross-linkable functions; then the said cross-linkable functions are 
cross-linked physically, the complexing polymer being brought to an 
essentially amorphous state during this cross-linking operation. 
The physical cross-linking or reticulation operation can be performed, for 
example, by irradiation or by hot calendering. 
The physical cross-linking avoids introducing, into the polymer mixture, 
chemical cross-linking functions which are unstable in relation to the 
materials constituting the electrodes, in particular lithium. 
More specifically, the radiation may be ultraviolet, X or gamma radiation, 
but it may also be particle radiation, e.g. electrons, neutrons or alpha 
particles. 
The hot calendering actuating the cross-linking can be performed from 
50.degree. C. upwards. 
The complexing polymer used includes one or several heteroatoms, such as 
oxygen or nitrogen. 
The so-called polymer chain can be constituted in the following pattern: 
##STR1## 
R and R' can be the hydrogen CH.sub.3, --CH.sub.2 --.sub.n CH.sub.3, a 
polyether chain, for example, a polyoxyethylene chain or a 
polyoxypropylene chain, or a polysequenced chain of polyethers, or an 
elastomer chain. 
The presence of such chains causes a structural disorder which is favorable 
to the lowering of the melting temperature of the crystallites which are 
likely to form within the complexing polymer. 
As to the ionizable salt, formula MX, this is not limiting at all, and is 
the type in which: 
M.sup.+ =Li.sup.+, Na.sup.+, K.sup.+, Ca.sup.2+, NH.sub.4.sup.+. 
X.sup.- =1.sup.-, ClO.sub.4.sup.-, BF.sub.4.sup.-, AsF.sub.6.sup.-, 
CF.sub.3 SO.sub.3.sup.-, CF.sub.3 CO.sub.3.sup.-, B.sub.12 
H.sub.12.sup.2-, B.sub.10 Cl.sup.2-.sub.10, B.phi..sub.4.sup.-, 
.phi. designating C.sub.6 H.sub.5, or an alkyl or an aryl chain. 
It is possible to use several ionizable salts together. 
Furthermore, it is advantageous that at least one part of the ionizable 
salt includes an anion so that it will be cross-linked, after radiation, 
on the second polymer. This anion may be selected from within the group 
formed by: 
EQU CF.sub.3 CF.sub.2 COO.sup.-, CF.sub.3 (CF.sub.2).sub.6 CF.sub.2 
SO.sub.3.sup.-, CH.sub.3 (CH.sub.2)5 CH.sub.2 SO.sub.3.sup.-, CH.sub.2 
.dbd.CH--(CH.sub.2).sub.3 --SO.sub.3.sup.-, 
the sulfonate polystyrenes, the polymetacrylates. 
These anions create individual barriers to the mobility of the anions which 
show the crystalization of the complexed polymer, which cannot attain 
electrical neutrality within the crystalline unit. 
The second polymer is advantageously selected from within the group formed 
by the type acrylic polybutadienenitrile elastomers, methyl 
polyethylene-acrylate, and the elastomer polyesters. 
The second polymer is at a low proportion by weight in relation to the 
complexing polymer; this proportion could, for example, be from 1 to 25%. 
As an illustration one among many, Perbunan could be used as the second 
polymer at 3.5% by weight, the complexing polymer being preferably formed 
by a polyoxythylene (PEO) with a molecular weight in the range 5000 to 
7,000,000. 
In accordance with the invention, the cross-linking can also be done after 
having introduced a monomer and/or a macromer which could be used as an 
agent which could be grafted onto the chain/or an agent bridging between 
chains during the cross-linking. So it is that the following could be used 
for this purpose: butadiene, light linear ethers, glycol polyethylene, and 
also, components holding acrylates in their chain, for example methyl 
acrylate, acrylonitriles, styrenes, for example butadiene-styrene, styrene 
grafted onto a polydimethyl ether or polyether chain; the chains may be 
linear or branching. Each of the above-mentioned agents can also be 
grafted onto an inorganic matrix such as silica, alumina, zircon, glass, 
etc . . . , in order to modify the structure of any crystallites formed in 
the complexing polymer. For example, silica powder may be added into the 
polymer mixture, with a glycol polyethylene already grafted onto the 
silica powder by known methods. 
The second polymer with its cross-linkable functions is provided for 
several reasons: 
1. It creates a structural disorder which controls the miscibility of the 
polymers in mixture and, therefore, contributes to all the crystallization 
faults to be encountered later by the stoichiometric complex regulation 
processes. 
2. It can play a surface active role in relation to any crystallites 
formed, and, therefore, contribute to their dispersal as well as 
contributing to limiting their size. This would be the case during 
copolymerizing under radiation of acrylic components (lyophiles) and of 
macromers containing a complexing oligomer chain. 
3. It creates a local micro network capable of imprisoning the amorphous 
and conductive form of the complexing chain and of keeping it without 
recrystallizing. 
4. We also found that in certain cases the second polymer could present 
complexing properties in relation to the ionizable salt, for example, when 
it held polydimethyl ethers (polyglymes). 
The irradiation can be performed on a membrane, in the solid state or on a 
gel holding the polymer mixture and the electrolytic ionizable salt. 
Less than 10% by weight phthalocyanines of lithium or of lutetium and also 
porphins, porphyrins, or generally speaking, sands may also be added into 
the irradiated mixture in order to improve the conductivity of the 
electrolyte and, under certain circumstances, to modify its color. 
Cross-linking under radiation may be performed on the heated mixture until 
melting temperature in the crystalline ranges, with the complexing polymer 
brought to an essentially amorphous state characterized by its melting 
within most of the above-mentioned ranges. It is then cooled to ambient 
temperature. 
In this case, the amorphous form is fixed at ambient temperature, thus 
keeping all the texture properties which are desired to obtain a good 
conductivity at ambient temperature. It should be noted that the bringing 
to the proper temperature, which then only requires simply exceeding the 
vitreous transition or melting temperature of the majority of the 
crystallites, can be done under infrared or ultraviolet radiation. 
The cross-linking under radiation can be performed on the polymer mixture, 
as defined above, with an added active mass, the weight ratio between the 
polymer masses and a salt on the one hand, and the active mass on the 
other hand, being from 0.001 to 10. 
In this way a composite solid electrode-electrolyte unit may be produced. 
Among a number of possibilities, in the case of a positive electrode, the 
active mass can include TiS.sub.2, with Li.sub.x FeS.sub.2 with 
0&lt;x.ltoreq.2, NiPS.sub.3, V.sub.6 O.sub.13, WO.sub.3, MoO.sub.3, V.sub.2 
O.sub.5, MnO.sub.2, mixtures of PbO.sub.2 and Bi.sub.2 O.sub.3, as well as 
fluorocarbon polymers, polyacetylenes, or polypyrroles. 
In the case of a negative electrode, the active mass can include metal 
alloys, type LiAl, LiB, LiMg and non stoichiometric compounds derived 
therefrom. 
The polymer mixture may be impregnated with a solid inorganic electrolyte 
such as beta aluminium or Li.sub.3 N, for example.

DETAILED DESCRIPTION OF THE INVENTION 
The following paragraphs, give some examples of making solid electrolytes 
and solid batteries in accordance with the invention. 
EXAMPLE 1 
(Prior Art) 
700 mg of polyoxyethylene (POE) whose molar weight is 4.times.10.sup.6, 300 
mg of LiClO.sub.4 and 50 mg of phthalocyanine of lithium are inserted into 
40 cm.sup.3 of acetonitrile. 
This mixture is then poured into a mold and evaporated in a glove box for 
15 days to remove the acetonitrile. 
The electrolyte membrane obtained in this way is heated to 100.degree. C. 
for one hour, then irradiated (one hour under X-rays). The irradiated 
membrane is installed in an electrochemical cell whose negative electrode 
holds lithium and whose positive electrode holds NiPS.sub.3. The positive 
active mass NiPS.sub.3 is supplemented with 50% (by weight) of the 
preceeding solid electrolyte, nonirradiated, in order to ensure its ionic 
conductivity. 
Five hours after the installation, this cell cannot be discharged at 
ambient temperature. On the other hand, it can be discharged beginning at 
80.degree. C.; we obtain an efficiency of 100% at 30 MA at 80.degree. C. 
The return to ambient temperature preserves the discharge possibilities 
obtained at 80.degree. C. for five hours at most. 
FIG. 1 gives the initial voltage characteristics (E) in volts, current I in 
milliamperes, after this five hour storage time, of such a generator for 
various temperatures. The resistivity of the electrolyte at ambient 
temperature is higher than 10.sup.7 ohm.cm. 
EXAMPLE 2 
Polyoxyethylene (PEO), molecular weight 4.times.10.sup.6 is used as a 
complexing polymer. 
The ionizable salt is LiClO.sub.4. 
The proportion of ionizable salt to the complexing polymer is such that one 
atom of Li corresponds to eight atoms of oxygen coming from the POE. 
The second polymer is made up of acrylic polybutadiene-nitrile, more 
specifically Perbunan, reference 3807 NS 71 24 77/10 supplied by SAFIC 
ALCAN. 
The weight proportion of the second polymer in relation to the complexing 
polymer is close to 3.5%. 
The mixture of the polymers and of the ionizable salt is placed in solution 
in the acetonitrile. 
The solution is then poured on a plane surface and the solvent is 
evaporated using argon. 
In this way a membrane of about 120 cm.sup.2 is made with a thickness close 
to 0.2 mm. 
After drying, the membrane is heated to about 100.degree. C. and then 
irradiated with X-rays emitted by a chromium anti-cathode, the irradiation 
takes place over the complete spectrum for 15 minutes. 
Next, the electrolytic membrane is installed in a button cell, 20 mm in 
diameter and 2 mm thick, having a lithium negative electrode and a 
positive electrode based on a mixture including 50% NiPS.sub.3 and 50% POE 
and LiClO.sub.4, not irradiated. 
All these components are placed into contact under a tension of 2.5 Kg 
using a spiral spring. 
At various current densities, weak pulse discharges are regularly actuated. 
The discharge periods are scheduled between one hour and several weeks and 
the conservation of the performance of the cell in storage is examined. 
Any recrystallizing of the complexing polymer causes lower pulsed discharge 
performance. 
The current I, in mA is plotted along the y axis, for an overvoltage of 1.5 
V, as a function of storage time t, in hours, plotted along the x axis. 
The cell discharge depth is weak in every case. Curve (A) in FIG. 2 is 
obtained. 
EXAMPLE 3 
Same operating conditions as in Example 2--but without irradiation. Curve 
(B) in FIG. 2 is obtained. 
EXAMPLE 4 
Same operating conditions as in Example 2--but the ionizable salt LiClO is 
replaced by LiCF.sub.3 SO.sub.3. Curve (C) in FIG. 2 is obtained. 
EXAMPLE 5 
Same operating conditions as in Example 4, but without irradiation. Curve 
(D) in FIG. 2 is obtained. 
These curves (A), (B), (C), (D) show: 
in the case of exemple 3, recrystallization occurs after about 80 hours of 
storage at ambient temperature. 
in the case of example 5, recrystallization occurs in less than one hour, 
in the case of examples 2 and 5, corresponding to the use of the invention, 
performance evolves very slowly without recrystallization before 1000 
hours. 
EXAMPLE 6 
The starting mixture comprises: 
(POE), molecular weight 4 000 000 
LiClO.sub.4 in the same proportions as in example 2 
a macromer, that is, styrene grafted onto a polyglymous chain having 12 
chain links --CH.sub.2 --CH.sub.2 --O--, designated by SOE.sub.550. 
The proportion of SOE.sub.550 in weight in relation to (POE) is 20%. 
The conditions for building the membrane and placing it in the generator 
are the same as in example 2. Curve (E) in FIG. 3 is obtained. 
And so we note that the presence of a single macromer is not enough to 
substantially delay recrystallization. 
EXAMPLE 7 
Same operating conditions as in example 6, but without irradiation. Curve 
(F) in FIG. 3 is obtained. The results are considerably worse than in the 
preceding example. 
EXAMPLE 8 
Same operating conditions as in example 6, 3.6% by weight Perbunan is added 
to the mixture. Curve (G) in FIG. 3 is obtained. 
EXAMPLE 8' 
Same operating conditions as in example 8, but the proportion of SOE550 is 
5% and not 20%. The recrystallizing time is then about a hundred hours. 
EXAMPLE 9 
Same operating conditions as in example 7, but 3.6% of Perbunan is added. 
Curve (H) in FIG. 3 is obtained. 
EXAMPLE 10 
Same operating conditions as in example 8, but the ionizable salt is made 
up of LiCF.sub.3 SO.sub.3. Curve (I) in FIG. 3 is obtained. 
Observe that the curves corresponding to the mixtures which are not 
irradiated or do not contain the second polymer tend to quickly 
recrystallize. 
EXAMPLE 10' 
Same operating conditions as in example 10, (POE, LiCF.sub.3 SO.sub.3, 
SOE.sub.550 -20% in weight in relation to POE, Perbunan -3.5% in weight in 
relation to POE), but the cross-linking is obtained by hot calendering 
(0.1 mm at 60.degree. C.) without irradiation. The same results are 
obtained as in the case in example 10. 
EXAMPLE 11 
Same operating conditions as in example 10, but the Perbunan content goes 
from 3.5% to 10%. Curve (J) in FIG. 3 is obtained. The performance is 
weaker than in curve (I) but the recrystallization is blocked in the same 
way as in example 10. 
EXAMPLE 12 
Same operating conditions as in example 8, but only 5% of SEO.sub.550 is 
used. Recrystallization takes place after about one hundred hours. 
EXAMPLE 13 
Same conditions as in example 10 but the SEO.sub.550 is replaced by 
SEO.sub.5000 --113 chain links --O--CH.sub.2 --CH.sub.2 -- on a styrene 
link. The results are identical to those in example 10. 
EXAMPLE 14 
Same operating conditions as in example 2, a bridging agent made up of a bi 
styrenic linear polyether at the ends, known under the reference (V.sub.2 
PE).sub.6800 is also incorporated in the polymer mixture. 
The weight proportion of this agent in relation to the PEO is 20%. Curve 
(K) in FIG. 4 is obtained. 
EXAMPLE 15 
Same operating conditions as in example 14, but without irradiation. Curve 
(L) in FIG. 4 is obtained. 
EXAMPLE 16 
Same operating conditions as in example 14, but the LiClO.sub.4 is replaced 
by LiCF.sub.3 SO.sub.3. Curve (M) in FIG. 4 is obtained. 
EXAMPLE 17 
Same conditions as in example 16, but without irradiation. Curve (N) in 
FIG. 4 is obtained. 
EXAMPLE 18 
Same operating conditions as in example 2, but lutetium phthalocyanine is 
also incorporated in the polymer mixture. Also, the membrane is installed 
between a lithium electrode and a counter electrode constituted by a tin 
oxide type electronic conductor. The polymer then has the property of 
changing color under electric voltage. 
EXAMPLE 19 
20% by weight sodium second beta alumina is incorporated in the mixture 
described in example 2. The average performance obtained is 100 .mu.A on 
the cells and no recrystallizing is observed after 1000 hours. 
EXAMPLE 20 
20% by weight of a silica is added to the mixture described in example 2, 
chemically grafted, according to a known method. Polydimethyl ether 
complexing chain links made up of oligomers of 10 to 200 chain links 
--O.sub.-- CH.sub.2 --CH.sub.2 --, are chemically grafted by a known 
method to the silica before it is added to the mixture. The results 
obtained are identical with the results obtained in example 8. 
EXAMPLE 21 
Same operating conditions as in example 2, but the ionizable salt includes, 
by weight: 
90% LiClO.sub.4 
10% CF.sub.3 CF.sub.2 COOLi 
The same results are obtained as in example 4. 
EXAMPLE 22 
Same operating conditions as in example 21, but CF.sub.3 CF.sub.2 COOLi is 
replaced by CH.sub.2 .dbd.CH--(CH.sub.2).sub.3 --SO.sub.3 Li. The same 
results are obtained as in example 10.