The instant invention provides a solid, polymeric electrolyte in membrane form, constituted by a solid solution of an ionic compound in a crosslinked polyether, characterized in that said polymeric electrolyte is obtained by (1) blending a prepolymer, an ionic compound, a plasticizer, and a free-radical-generating activator; and (2) applying the resulting blend on an inert carrier and exposing said blend to a source of ultraviolet light, thermal radiation or electron beam, until the blend is completely crosslinked. The instant invention also provides an electrolyte separator for use in electrochemical devices, optical and electrochemical displays and sensors, which comprises the polymeric electrolyte of the instant invention.

The present invention relates to a solid, polyether-based, polymeric 
electrolyte, to a process for preparing it, and to its use in 
electrochemical devices. 
In the art solid polymeric electrolytes, also referred to as "ionic 
conductive polymers" are known, which are constituted by a solution of a 
ionic compound totally dissolved in a plastic, solid, macromolecular 
material, which is the polymerization product of monomers containing least 
one heteroatom, in particular oxygen. 
Generally, said macromolecular material is polyethylene oxide (PEO), or 
other polyethers disclosed in U.S. Pat. No. 4,471,037; French patents 
2,523,769 and 2,568,574; EP-13,037 and EP-13,199. 
The problems associated with such solid, polymeric electrolytes generally 
consist in their displaying a satisfactory ionic conductivity only at 
higher-than-room temperatures and in the poor characteristics of 
mechanical strength and dimensional stability of the corresponding 
electrolitic membranes. All the above makes the solid polymeric 
electrolyte known from the prior art not really interesting for a 
practical use. 
Solid, polymeric, polyvinylether-based electrolytes, characterized by good 
mechanical strength and satisfactory ionic conductivity at low 
temperatures, are disclosed in U.S. Pat. No. 4,886,716; and in Italian 
patent No. 22,482 A/90. 
Unfortunately, the preparation of said electrolytes requires complex, 
multistep processes which comprise copolymerizing suitable vinyl ethers at 
temperatures of from -75.degree. C. to -80.degree. C., during a time 
period of 30-60 minutes, dissolving the resulting solid, crosslinked 
polyvinylether in a suitable solvent, mixing the resulting solution with a 
solution containing an ionic compound, and finally evaporating off the 
solvent, in order to obtain a membrane. 
Recently, solid, polyether-based polymeric electrolytes, displaying good 
characteristics of mechanical strength and ionic conductivity have been 
disclosed in Italian patent application No. 1008 A/92. 
The preparation of said solid, polymeric electrolytes, which is based on 
the use of macromers having a controlled molecular weight (which, hence, 
contain a known number of side ethylene oxide based chains) and a 
methacrylic group at each chain terminal end, in the crosslinking step 
requires, however, using a suitable difunctional comonomer in order to 
obtain dimensionally stable crosslinked systems capable of keeping 
embedded, inside their interior, an active plasticizer.

The purpose of the present invention is of overcoming the above reported 
drawbacks which affect the prior art. 
In particular, the Applicant has found now, according to the present 
invention, that the adoption of a particular prepolymer which contains, 
already included in its molecular structure, the active side ethylene 
oxide based chains and a large enough number of methacrylic groups, makes 
it possible the crosslinked polymeric material to be directly obtained by 
exposure to suitable sources, without using a difunctional comonomer. 
The preparation results consequently possible of a solid polymeric 
electrolyte in membrane form, provided with improved mechanical 
characteristics, dimensional stability and ionic conductivity, over the 
corresponding characteristics respectively displayed by the polymeric 
electrolytes known from the prior art. 
In accordance therewith, according to a first aspect thereof, the present 
invention relates to a solid, polymeric electrolyte in membrane form, 
constituted by a solid solution of an ionic compound in a crosslinked 
polyether, characterized in that said polymeric electrolyte is obtained: 
(1) by blending: 
(a) a prepolymer having the formula (I) 
##STR1## 
and having a molecular weight comprised within the range of from 10,000 
to 100,000, and wherein 
R is an ethyl or methyl radical; 
x is an integer comprised within the range of from 2 to 5; 
n is comprised within the range of from 0.7 to 0.95; 
m is comprised within the range of from 0.3 to 0.05 and wherein n+m is 1 in 
an amount comprised within the range of from 20 to 80% by weight, with 
(b) an ionic compound, in an amount comprised within the range of from 1 to 
30% by weight, in the presence, or less, of 
(c) a plasticizer selected from an oligomer or a dipolar aprotic solvent, 
in an amount comprised within the range of from 0 to 80% by weight; and 
(d) a free-radical-generating activator, in an amount comprised within the 
range of from 0 to 10% by weight; and, finally, 
(2) applying the resulting blend the on an inert carrier and exposing said 
blend to a source of ultraviolet light, thermal radiation or electron 
beam, until the system is completely crosslinked. The prepolymer (I) can 
be prepared by means of the cationic polymerization of a vinyl ether of 
formula: 
EQU CH.sub.2 .dbd.CH--O--(CH.sub.2 --CH.sub.2 --O).sub.x --R (II) 
wherein: 
R and x have the above reported meaning, with a vinyloxy ethoxy ethyl 
methacrylate of formula: 
##STR2## 
with a molar ratio of both monomers to each other comprised within the 
range of from 70:30 to 95:5. 
The polymerization reaction is carried out in an inert solvent, at a 
temperature comprised within the approximate range of from -60.degree. C. 
to -30.degree. C. and in the presence of a Friedel-Crafts catalyst, used 
in an amount comprised Within the range of from 0.5 to 3.0 mol per 100 mol 
of monomers (II) and (III). 
Examples of suitable catalysts for the desired purpose are etherated boron 
trifluoride, aluminum trichloride, alkyl aluminum halides and tin 
tetrachloride. 
Examples of suitable solvents for the desired purpose are chlorinated 
solvents, as dichloromethane; and hydrocarbons, as toluene. 
At the end of the polymerization, the catalyst is deactivated by adding an 
aliphatic alcohol, as methanol. The prepolymer is then recovered by means 
of usual separation techniques, as a colourless, thick liquid having a 
weight average molecular weight of the order of 10,000-100,000 as a 
function of the polymerization temperature. 
In the preferred embodiment, in formula (I) R represents the methyl 
radical. 
The resulting, prepolymer can be characterized by means of such analytical 
techniques, as NMR and FT-IR. The results from such analyses confirm the 
absence of the vinylic double bond, and the presence of the methacrylic 
double bond, which is present at the same level (5-30% by mol), at which 
it was initially introduced. 
The vinyl ether (II) can be prepared by reacting ethyl vinyl ether with a 
polyoxyethylene glycol mono-methyl or -ethyl ether. The monomer (III) can 
be obtained by reacting diethylene glycol monovinyl ether with 
methacryloyl chloride in ethyl ether and in the presence of pyridine. The 
monomers (II) and (III) are obtained with a higher than 99% purity, by 
using usual separation techniques. 
The prepolymer (I) is then blended with an ionic compound, in the presence, 
or less, of a plasticizer and a free radical generating activator, until a 
homogeneous solution is obtained; the prepolymer is used in amounts 
comprised within the range of from 20 to 80% by weight, preferably of from 
30 to 70% by weight. 
Suitable ionic compounds for the purposes of the present invention are 
salts, preferably perchlorates, borates, fluoroborates, thiocyanates, 
hexafluoroarsenates, trifluoroacetates and trifluoromethanesulfonates of 
(monovalent or polyvalent) metals, and, in particular, lithium, sodium, 
potassium, calcium, copper, zinc, magnesium, lead, tin and aluminum. 
Preferred for the purpose according to the invention are lithium salts and, 
in particular, lithium perchlorate, and in this case it is used in amounts 
comprised within the range of from 1 to 30% by weight, preferably in 
amounts of the order of 3-10% by weight. 
According a preferred embodiment, a plasticizer is present in the blend, in 
an amount comprised within the range of from 20 to 80% by weight, 
preferably of from 30 to 70%. 
Suitable plasticizers for the purposes of the present invention can be 
selected from oligomers containing ethylene oxide chains or aprotic 
dipolar solvents with high dielectric constant, low volatility and 
dissociative properties for lithium salts. 
Examples of such oligomers are oligoethylene glycol dialkylethers (Diglyme, 
Tetraglyme) and low-molecular-weight polyethylene glycol dialkylethers 
(PEGME). 
Examples of such solvents are propylene carbonate (PC), ethylene carbonate 
(EC), dimethoxyethane (DME), and mixtures thereof. 
The free radical generating activator (d), which performs the function of 
speeding up the crosslinking reaction, thus lowering the exposure times, 
is present in an amount comprised within the range of from 0.5 to 10% by 
weight, preferably of from 0.1 to 5% by weight. 
Suitable free-radical generating activators for the purposes of the present 
invention are, e.g., benzophenone or benzoin methyl ether. 
In the step (2) of the present invention, the resulting homogeneous 
solution is applied onto a solid carrier which can be a plastic material, 
glass, or a metal sheet; furthermore, the solution can be directly applied 
onto the surface of a lithium anode or of a composite cathode generally 
constituted by an oxide, or a sulfide, of a transition metal (V, Mn, Co, 
Ni, W, Ti), in mixture with an ionically conductive material and an 
electronic conductor such as carbon black or acetylene black. 
The exposure of the resulting blend to a source of U.V. light, thermal 
radiation, or electron beam, until the complete crosslinking of the system 
is obtained, which usually requires a few tens of seconds, leads to 
obtaining the solid polymeric electrolyte in membrane form with a 
thickness of the order of 50-200 microns. 
The so obtained polymeric electrolyte displays a transition temperature 
comprised within the range of from -100.degree. C. to -60.degree. C., as a 
function of the composition of the blend. 
The polymeric electrolyte of the present invention is mechanically strong, 
dimensionally stable, and highly conductive, even at low temperatures. The 
solid polymeric electrolyte can be used as an electrolytic separator in 
electrochemical devices, optical displays and sensors. 
The following examples are illustrative and non-limitative of the purview 
of the present invention. 
EXAMPLE 1 
Preparation of vinyl ether: 
EQU CH.sub.2 .dbd.CH--O--(CH.sub.2 --CH.sub.2 --O).sub.3 --CH.sub.3 
To a three-necked flask of 500 ml of capacity, equipped with reflux 
condenser and kept under a flowing nitrogen stream, ethyl vinyl ether (1.8 
mol), triethylene glycol monomethyl ether (0.6 mol) and mercury acetate 
(5.7.10.sup.-3 mol) are charged. 
The reaction mixture is heated up to its reflux temperature and is kept 
under refluxing conditions for 10 hours. 
During this time period, the temperature increases from its initial value 
of 39.degree. C., up to its end value of 42.degree. C. The reaction is 
then quenched by means of the addition of solid potassium carbonate, and 
the reaction mixture is submitted to distillation, firstly under 
atmospheric pressure, in order to remove any ethyl vinyl ether excess and 
the ethyl alcohol obtained as a reaction byproduct, and then under reduced 
pressure (20 torr) in order to separate the title vinyl ether from 
unreacted glycol monomethyl ether. 
The resulting vinyl ether shows a purity higher than 99% and is obtained in 
a yield, based on the starting triethylene glycol monomethyl ether, of 
approximately 80%. Its structure is confirmed by NMR and IR spectroscopy 
and mass spectrometry. 
EXAMPLE 2 
Preparation of vinyloxy ethoxy ethyl methacrylate 
##STR3## 
To a three-necked flask of 250 ml of capacity, equipped with reflux 
condenser, 85 g (0.8 mol) of diethylene glycol, 80 ml (0.8 mol) of ethyl 
vinyl ether and 4.8 g (15.2 mmol) of mercury acetate Hg(CH.sub.3 
--COO).sub.2 are charged in the sequence stated. 
The reaction mixture is heated up to its refluxing temperature 
(70.degree.-80.degree. C.) and is kept under refluxing conditions for 10 
hours. 
The reaction product is recovered by extraction with methylene chloride and 
is separated from the corresponding divinyl ether by distillation. In that 
way, 40 g (0.3 mol) of diethylene glycol monovinyl ether (yield 40%) is 
obtained, which is then added to a solution containing 53 ml of pyridine 
and 40 ml of ethyl ether. To the resulting mixture, kept at room 
temperature and under a flowing nitrogen stream, 38.3 g (0.32 mol) is 
slowly added dropwise of methacryloyl chloride, which causes the immediate 
precipitation of pyridinium hydrochloride. The reaction is carried out 
until the monovinyl ether is totally disappeared. 
The difunctional product is recovered from the reaction mixture by HPLC, 
using silica as the stationary phase and, as eluent, a mixture of hexane 
and ethyl acetate with a volumetric ratio of 8:1 hexane:ethyl acetate. The 
resulting product displays a higher purity than 99% and its yield, based 
on the diethylene glycol monovinyl ether used as the starting product, is 
of approximately 70%. 
EXAMPLE 3 
Prepolymer preparation 
##STR4## 
An amount of 4.92 g (25.9 mmol) of monovinyl ether from Example 1 and 0.58 
g (2.9 mmol) of vinyloxy ethoxy ethyl methacrylate from Example 2, 
dissolved in 10 ml of anhydrous methylene chloride are charged to a glass 
reactor of 100 ml of capacity, equipped with mechanical stirring means of 
impeller type and inlet fittings for nitrogen and reactant feed. The 
resulting mixture is cooled down -60.degree. C., then, with vigorous 
stirring, 0.32 mol of ether-BF.sub.3 complex dissolved in 2 ml of 
methylene chloride is rapidly added. 
The reaction is allowed to proceed during 3 hours, with the reaction 
temperature being kept at -60.degree. C. Then, a methanol excess is added 
to the reaction mixture, in order to stop the polymerization and remove 
any hydroxy groups possibly formed during the polymerization reaction. The 
prepolymer is recovered as a colourless, thick liquid, by extraction with 
a chlorinated solvent; the yield is quantitative. 
The prepolymer is characterized by NMR and FT-IR spectroscopy, which 
confirm the absence of double bonds of vinylic character and the presence 
of double bonds of methacrylic character, at the same level (10% by mol) 
as initially charged. 
EXAMPLES 4-8 
Preparation of electrolytic membranes 
The electrolytic polyether-based membranes are prepared according to the 
following procedure, carried out inside a dry-box, under an argon 
atmosphere and with a humidity level lower than 5 ppm, using: 
the prepolymer from Example 3; 
LiClO.sub.4 as the ionic compound; 
propylene carbonate (PC) or tertaethylene glycol dimethyl ether (TGME) as 
plasticizer, in such amounts as reported in following Table: 
TABLE 1 
______________________________________ 
Prepolymer Plasticizer LiClO.sub.4 
%- %- %- 
Formulae 
mg weight Type mg weight 
mg weight 
______________________________________ 
PVE- 510 46.1 PC 520 47.6 70 6.3 
MMA1 
PVE- 510 48.2 PC 503 47.3 47 4.4 
MMA2 
PVE- 430 41.4 PC 490 47.3 118 11.3 
MMA3 
PVE- 410 46.2 TGME 420 47.4 56 6.3 
MMA4 
PVE- 406 47.2 TGME 416 48.4 37 4.3 
MMA5 
______________________________________ 
The resulting blends are cast into constant-thickness films on glass 
supports and are submitted to U.V. light exposure for 20 seconds. Thus, 5 
homogeneous, clear membranes of approximately 100 microns of thickness are 
obtained. The glass transition temperatures (Tg), as determined by DSC 
(differential scanning calorimetry), are reported in following Table 2. 
TABLE 2 
__________________________________________________________________________ 
Glass transition temperature (Tg .degree.C.) 
PVE-MMA1 
PVE-MMA2 
PVE-MMA3 
PVE-MMA4 
PVE-MMA5 
__________________________________________________________________________ 
-90.degree. C. 
-95.degree. C. 
-82.degree. C. 
-94.degree. C. 
-96.degree. C. 
__________________________________________________________________________ 
The membrane conductivity measurement is carried out in a cell provided 
with two symmetrical electrodes of carbon steel, between which said 
membrane is compressed, by applying a sinusoidal alternating voltage of 
100 mV of amplitude. 
In Table 3 and in FIG. 1, the values and trends of ionic conductivity are 
respectively reported, which were obtained by means of impedance 
spectroscopy measurements at the temperatures of 60.degree. C., 40.degree. 
C., 20.degree. C., 0.degree. C. and -20.degree. C., within the frequency 
range of from 0.5 to 65,000 Hertz. 
TABLE 3 
______________________________________ 
Conductivity (S/cm) .multidot. 10.sup.3 
Formulations 
60.degree. C. 
40.degree. C. 
20.degree. C. 
0.degree. C. 
-20.degree. C. 
______________________________________ 
PVE-MMA1 4.30 3.10 1.80 0.94 0.27 
PVE-MMA2 4.20 3.10 1.90 0.64 0.21 
PVE-MMA3 3.80 2.70 1.70 0.93 0.28 
PVE-MMA4 0.67 0.61 0.45 0.028 0.01 
PVE-MMA5 0.50 0.42 0.27 0.12 0.042 
______________________________________ 
In particular, in FIG. 1, in which on the ordinate the values of 
conductivity expressed as S/cm, and on the abscissa the values of 
temperature, expressed as degrees Kelvin, are reported, the lines in the 
chart have the following meaning: 
-- -- PVE-MMA1; --.vertline.-- PVE-MMA2; --*-- PVE-MMA3; --.quadrature.-- 
PVE-MMA4; --x-- PVE-MMA5. 
In the chart displayed in FIG. 2, the line (-- --) relates to the values of 
conductivity displayed by a membrane (PVE-MMA1) prepared according to the 
present invention, whereas the line (-- --) relates to a membrane prepared 
according to the prior art (Italian patent application No. 1008 A/92).