Solid polymer electrolyte and preparation method therefor

A solid polymer electrolyte having a high ionic conductivity and a high mechanical strength, and a preparation method therefor. The solid polymer electrolyte comprises a metal salt and a polymer blend of a fluoropolymer and a polyether comprising either or both of an ethylene oxide unit and a propylene oxide unit as a monomeric unit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a solid polymer electrolyte and a 
preparation method therefor. More particularly, the present invention 
relates to a solid polymer electrolyte which is superior in ionic 
conductivity and mechanical strength, and a preparation method therefor. 
The solid polymer electrolyte according to the present invention is 
suitable for electrochemical devices such as batteries, particularly for 
secondary batteries of high energy density. 
2. Description of the Prior Art 
As electronic and information systems increasingly feature size reduction 
and portability, research and development is now being made on 
light-weight and high-voltage secondary batteries, among which metal 
lithium secondary batteries are promising power sources for these systems 
because of their light weight and high energy density. In general, such a 
lithium battery employs metal lithium as its negative electrode and a 
nonaqueous electrolytic solution containing a lithium salt as its 
electrolyte. 
It is, however, known that dendrites (branching tree-like crystals) are 
generated on the metal lithium during repeated charge and discharge cycles 
when it uses metal lithium as negative electrode for lithium secondary 
battery, resulting in a short circuit within the battery and deterioration 
of the cycle characteristic of the battery. 
Attention is now focused on lithium ion secondary batteries, which have 
already been put to practical use. Such a lithium ion secondary battery 
employs, instead of the metal lithium negative electrode, a negative 
electrode which comprises a host such as of a carbon material and lithium 
ions and utilizes an intercalation and deintercalation reaction of the 
lithium ions in the host. The lithium ion secondary battery generally has 
a lower theoretical negative electrode capacity than the metal lithium 
secondary battery, but is superior in the cycle characteristic and 
reliability. 
In general, the lithium secondary batteries (including the metal lithium 
secondary batteries and the lithium ion secondary batteries) employ 
organic electrolytic solutions as their electrolytes. However, the use of 
such a liquid electrolyte imposes problems associated with the reliability 
of the battery, e.g., deterioration of the battery which may result from 
leakage of the electrolytic solution out of the battery, vaporization of a 
solvent of the electrolytic solution and dissolution of an electrode 
material in the electrolytic solution. Further, the organic electrolytic 
solution contains a flammable organic solvent and, hence, the leakage of 
the solvent may result in ignition. 
There is a demand for a battery which employs a solid electrolyte composed 
of an inorganic material or a polymeric material and is free from the 
solution leakage out of the battery. Particularly, a solid electrolyte 
composed of a polymeric material (hereinafter referred to as "solid 
polymer electrolyte") has the advantages of relatively easy preparation, 
low costs and light weight. The solid polymer electrolyte is attractive 
because totally solid batteries featuring a smaller thickness and a shape 
variation can be provided by employing the solid polymer electrolyte. 
The solid polymer electrolyte is highly safe, but has a lower ionic 
conductivity than the conventional organic electrolytic solutions. 
Exemplary polymers currently used for the solid polymer electrolyte 
include polyethers such as polyethylene oxide and polypropylene oxide. 
Since linear polymers such as polyethylene oxide and polypropylene oxide 
are crystalline polymers, a solid polymer electrolyte composed of such a 
polymer and an electrolytic salt has a satisfactory ionic conductivity at 
a high temperature, but a low ionic conductivity at a temperature not 
higher than ordinary temperature. 
A known approach to the problem of the reduction in the ionic conductivity 
is to use an amorphous polymer obtained by cross-linking polyethylene 
oxide and/or polypropylene oxide into a graft structure or a network 
structure. 
However, a solid polymer electrolyte composed of such an amorphous polymer 
still has a lower ionic conductivity than the organic electrolytic 
solution, and does not exhibit a satisfactory ionic conductivity at a 
temperature not higher than ordinary temperature. 
To further improve the ionic conductivity, an organic solvent is added to 
the aforesaid ion conductive polymer to such an extent that exudation of 
the organic solvent does not occur, or the ion conductive polymer is 
formed into a thin film for reduction of the resistance of the entire ion 
conductor. A totally solid battery employing a solid polymer electrolyte 
thus obtained has a reduced internal resistance. 
However, the solid polymer electrolyte composed of the polymer cross-linked 
into a network structure is superior in the ionic conductivity, but has a 
very low mechanical strength. Therefore, if the solid polymer electrolyte 
is employed for a battery, the solid polymer electrolyte may be damaged by 
pressure applied thereto during fabrication of the battery or during the 
charge and discharge process of the battery. 
To solve this problem, various solid polymer electrolytes are proposed. 
For example, Japanese Unexamined Patent Publication No. SHO 63(1988)-102104 
proposes a composite solid polymer electrolyte obtained by impregnating an 
electrolytic polymer such as polyethylene oxide in a polymeric porous film 
such as of a polycarbonate or polyvinyl chloride. Japanese Unexamined 
Patent Publication No. HEI 8(1996)-148163 proposes a composite solid 
polymer electrolyte containing a powdery insulative material such as glass 
or a ceramic or a powdery ion conductive material dispersed therein. 
Further, Japanese Unexamined Patent Publications No. HEI6(1994)-140051 and 
No. HEI6(1994)-150941 propose solid polymer electrolytes composed of a 
polymer blend of polyvinyl alcohol and polyethylene oxide or a copolymer 
of vinyl alcohol and ethylene oxide. 
However, these solid polymer electrolytes suffer the following drawbacks. 
The composite solid polymer electrolyte (ion conductor) comprising the 
polymeric porous film and the polymeric electrolyte has a greater 
resistance because the content of the electrolyte component serving for 
ion conduction is reduced. Therefore, if the composite solid polymer 
electrolyte is employed for a battery, a reduction in the battery capacity 
and an increase in the internal resistance may result. 
Further, the composite solid polymer electrolyte containing the powdery 
glass or ceramic dispersed therein has a high mechanical strength, but the 
preparation thereof is costly because an additional step for particle size 
classification of the particles is required for formation of a homogenous 
solid polymer electrolyte film. 
The solid polymer electrolyte composed of the polymer blend of polyvinyl 
alcohol and polyethylene oxide or the copolymer of vinyl alcohol and 
ethylene oxide is excellent in the ionic conductivity and the mechanical 
strength. However, hydroxyl groups in polyvinyl alcohol are reactive with 
lithium. Therefore, if the solid polymer electrolyte is employed for a 
metal lithium battery or a lithium ion battery, the hydroxyl groups react 
with lithium, so that it is difficult to maintain polyvinyl alcohol stable 
in the battery. Therefore, an electrode containing the solid polymer 
electrolyte has problems associated with the stability and cycle 
characteristic. 
SUMMARY OF THE INVENTION 
As the result of intensive studies, inventors of the present invention have 
found that the problems described above can be solved by the following 
means, and attained the present invention. 
In accordance with one aspect of the present invention, there is provided a 
solid polymer electrolyte comprising a metal salt and a polymer blend of a 
fluoropolymer and a polyether having either or both of an ethylene oxide 
unit and a propylene oxide unit as a monomeric unit. 
In accordance with another aspect of the present invention, there is 
provided a method for preparing a solid polymer electrolyte, comprising 
the steps of: dissolving a metal salt and a fluoropolymer in an organic 
solvent; mixing a polyether having either or both of an ethylene oxide 
unit and a propylene oxide unit as a monomeric unit with the resulting 
solution at an optimum temperature; irradiating the resulting mixture with 
ionizing radiation while maintaining the mixture at the optimum 
temperature; and removing the organic solvent as required.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A solid polymer electrolyte according to the present invention contains a 
metal salt, a fluoropolymer, and a polyether having either or both of an 
ethylene oxide unit and a propylene oxide unit as a monomeric unit and, as 
required, a nonaqueous solvent. 
The polyether to be used in the present invention is not particularly 
limited, as long as the polymer has an ethylene oxide unit and/or a 
propylene oxide unit as a monomeric unit. Examples of specific polyethers 
include polyethylene oxide (e.g. polyethylene oxide diacrylate), 
polypropylene oxide (e.g. polypropylene oxide diacrylate), a polymer blend 
of polyethylene oxide and polypropylene oxide, a copolymer (block 
copolymer and/or random copolymer) of ethylene oxide and propylene oxide, 
a graft copolymer having a backbone of polysiloxane or polyphosphazene and 
side chains of polyethylene oxide and/or polypropylene oxide having a 
relatively low molecular weight on the order of about 100 to 10,000, 
preferably about 100 to 1,000, and polymers obtained by cross-linking 
polymers having ethylene oxide units and/or propylene oxide units into a 
network structure (hereinafter sometimes referred to as "cross-linked 
network polymers"). 
It is particularly preferred to use a cross-linked network polymer 
comprising polyethylene oxide and/or polypropylene oxide as the polyether 
in the present invention. It is known that, when polyethylene oxide or 
polypropylene oxide having a weight-average molecular weight of greater 
than 200,000 is heated in a nonaqueous solvent (e.g., carbonate solvent) 
more than 50.degree. C., the viscosity thereof steeply rises. On the other 
hand, a certain quantity of heat should be applied to dissolve the 
fluoropolymer in the carbonate solvent. Therefore, it is difficult to 
homogeneously blend the fluoropolymer and the aforesaid high molecular 
weight polymer to obtain a polymer blend. For this reason, it is preferred 
to use a polyether which does not undergo the aforesaid drastic change nor 
decomposition even if a required quantity of heat is applied thereto. More 
specifically, the polyether preferably has a low molecular weight, more 
preferably a weight-average molecular weight of 1,000 to 50,000, at a 
stage where the polyether is blended with the fluoropolymer in the 
carbonate solvent. 
The polyether may be solidified, for example, by irradiation with ionizing 
radiation. The solidification may be achieved by cross-linking the 
polyether to increase the molecular weight of the polymer. The 
solidification is preferably performed, for example, after the polyether 
is applied on a base, so that the solid polymer electrolyte can be formed 
into a desired configuration. The solid polymer electrolyte composed of 
the cross-linked network polyether is superior in the mechanical strength. 
Further, even if a considerable amount of a nonaqueous solvent is 
contained in the solid polymer electrolyte, the solid polymer electrolyte 
is free from exudation of the nonaqueous solvent. Thus, the solid polymer 
electrolyte has a high ionic conductivity even at ordinary temperature. As 
described above, the cross-linked network polymer of polyethylene oxide 
and/or polypropylene oxide is particularly preferred as the polyether to 
be used in the present invention. 
The fluoropolymer to be used in the present invention is not particularly 
limited, as long as it is soluble in a particular organic solvent within 
such a temperature range that no drastic chemical change occurs in the 
polyether due to decomposition or polymerization of the polyether. 
Examples of specific fluoropolymers include homopolymers such as 
polyvinylidene fluoride, polyvinyl fluoride, polytetrafluoroethylene and 
polyhexafluoropropylene, and copolymers and terpolymers derived from 
plural kinds of monomers selected from the group consisting of vinylidene 
fluoride, vinyl fluoride, tetrafluoroethylene and hexafluoropropylene. In 
the present invention, polyvinylidene fluoride is particularly preferred 
because it is highly soluble in the organic solvent and highly compatible 
with the metal salt. Further, the polyvinylidene fluoride preferably has a 
weight-average molecular weight of 10,000 to 1,000,000, more preferably 
100,000 to 500,000, for a high mechanical strength and a high solubility 
in the solvent. 
The weight ratio of the fluoropolymer to the polyether is preferably 10 to 
100 parts by weight relative to 100 parts by weight of the polyether. If 
the proportion of the fluoropolymer is smaller than 10 parts by weight, 
the mechanical strength of the resulting solid polymer electrolyte is not 
satisfactory. If the proportion is greater than 100 parts by weight, the 
ionic conductivity of the resulting solid polymer electrolyte is 
undesirably reduced. 
The metal salt to be used in the present invention is not particularly 
limited, as long as it serves as an electrolyte for a battery. Examples of 
specific metal salts include lithium perchlorate (LiClO.sub.4), lithium 
hexafluorophosphate (LiPF.sub.6), lithiumborofluoride (LiBF.sub.4), 
lithium trifluoromethanesulfonate (LiCF.sub.3 SO.sub.3), lithium 
bis(trifluoromethylsulfonyl)imide (Li(CF.sub.3 SO.sub.2).sub.2 N), a 
lithium tris(trifluoromethylsulfonyl)methide (Li(CF.sub.3 SO.sub.2).sub.3 
C), sodium perchlorate (NaClO.sub.4), sodium borof luoride (NaBF.sub.4), 
magnesium perchlorate (Mg(ClO.sub.4).sub.2) and magnesium borofluoride 
(Mg(BF.sub.4).sub.2). These metal salts may be used either alone or as a 
mixture. 
The weight ratio of the metal salt to the polymer blend is preferably 5 to 
100 parts by weight relative to 100 parts by weight of the polymer blend. 
If the proportion of the metal salt is smaller than 5 parts by weight, the 
ionic conductivity of the resulting solid polymer electrolyte is 
undesirably reduced because a smaller number of carrier ions are present 
in the solid polymer electrolyte. If the proportion is greater than 100 
parts by weight, the metal salt is not sufficiently dissociated in the 
resulting solid polymer electrolyte, and the crystallinity of the polymer 
blend at ordinary temperature is undesirably increased because of a rise 
in the glass transition temperature thereof. 
A nonaqueous solvent is preferably added to the polymer blend to impart the 
solid polymer electrolyte with an ionic conductivity high enough for 
practical use. 
Examples of specific nonaqueous solvents include solvents having high 
dielectric constants, such as ethylene carbonate (EC), propylene carbonate 
(PC), butylene carbonate (BC), .gamma.-butyrolactone (BL) and 
.gamma.-valerolactone (VL), and solvents having low viscosities, such as 
1,2-dimethoxyethane (DME), ethoxymethoxyethane (EME), 1,2-diethoxyethane 
(DEE), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl 
carbonate (DEC), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2MeTHF) 
and dioxane (DO). These solvents may be used either alone or as a mixture. 
To increase the ionic conductivity of the solid polymer electrolyte, it is 
preferred to use a solvent mixture containing a cyclic carbonate solvent 
having a high dielectric constant and a low viscosity solvent in a desired 
mixing ratio. As the high dielectric constant solvent, propylene carbonate 
and ethylene carbonate are preferably used either alone or as a mixture 
for minimization of environmental influences. 
Among the aforesaid low viscosity solvents, linear carbonate solvents such 
as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and diethyl 
carbonate (DEC) are preferred. The linear carbonate solvents are highly 
stable with respect to metal lithium or a carbon material containing 
lithium ions intercalated thereto, when the resulting solid polymer 
electrolyte is employed as an electrolyte of a totally solid battery. 
The nonaqueous solvent is preferably contained in the polymer blend in such 
a proportion that the concentration of the metal salt can be adjusted to 
0.001 to 5.0 moles/liter, more preferably 0.05 to 2.0 moles/liter. If the 
metal salt concentration is lower than 0.001 mole/liter, the ionic 
conductivity of the resulting solid polymer electrolyte is undesirably 
reduced due to shortage of carrier ions. If the metal salt concentration 
is higher than 5.0 moles/liter, the ionic conductivity of the resulting 
polymeric electrolyte is undesirably reduced because the metal salt is not 
sufficiently dissociated. 
The weight ratio of the polymer blend to the nonaqueous solvent is 
preferably 30 to 100 parts by weight relative to 100 parts by weight of 
the nonaqueous solvent. If the proportion of the polymer blend is smaller 
than 30 parts by weight, the exudation of the nonaqueous solvent may 
result, making it difficult to shape the resulting solid polymer 
electrolyte as desired. Even if the shaping of the solid polymer 
electrolyte is possible, the electrolyte may have an insufficient 
mechanical strength. If the proportion of the polymer blend is greater 
than 100 parts by weight, the effect of addition of the nonaqueous solvent 
cannot be expected for ensuring a high ionic conductivity. 
A method for preparing the solid polymer electrolyte according to the 
present invention will hereinafter be described. 
First, the fluoropolymer and the metal salt are dissolved in an organic 
solvent in which the fluoropolymer and the metal salt are soluble. If the 
fluoropolymer are less soluble in the solvent at ordinary temperature, the 
solvent is preferably heated so that the fluoropolymer can completely be 
dissolved in the solvent. 
The fluoropolymer dissolvable organic solvent is preferably capable of 
dissolving a required amount of the metal salt therein and less volatile 
at an elevated temperature at which the fluoropolymer is soluble therein. 
It is preferred that the organic solvent can readily be removed after the 
resulting solid polymer electrolyte is formed into a film. Examples of 
specific organic solvents include N-methylpyrrolidone, acetonitrile, 
acetone, dimethylformamide, dimethylacetamide, acetylacetone, 
cyclohexanone, ethyl methyl ketone, dimethylaminopropylamine, 
hexamethylphosphoramide and diethylenetriamine in addition to the 
nonaqueous solvents described above. Where the solid polymer electrolyte 
containing the nonaqueous solvent is to be prepared, the organic solvent 
capable of dissolving the fluoropolymer and the metal salt therein is 
preferably selected from the nonaqueous solvents described above. Among 
the aforesaid organic solvents, a solvent stable with respect to an 
electrode is particularly preferred. 
For subsequent solidification of the polymer blend by ionizing radiation, 
aphoto-polymerization initiator may be added to the organic solvent. 
Examples of specific photo-polymerization initiators include carbonyl 
compounds such as benzoyl compounds, sulfur compounds and onium salts. The 
photo-polymerization initiator is preferably used in a proportion of 0.1 
to 5.0 parts by weight relative to 100 parts by weight of the polyether. 
Thereafter, the polyether is mixed with the resulting fluoropolymer 
solution at a temperature optimum for the fluoropolymer and the polyether. 
The mixing is preferably carried out speedily. 
For shaping of the solid polymer electrolyte, the resulting mixture is 
applied on a substrate, spread on a template, or injected into a sealed 
vessel, and then irradiated with ionizing radiation at the optimum 
temperature for solidification thereof. The solidification of the mixture 
occurs because the polymerization degree of the polyether is increased by 
the cross-linking of the polyether. The application of the mixture can be 
achieved by spreading the mixture on a substrate such as of glass, 
aluminum or a stainless steel with a doctor blade, a roller coater, a bar 
coater or the like. The ionizing radiation may be active light radiation 
such as ultraviolet radiation (UV). 
The temperature optimum for the fluoropolymer and the polyether ranges from 
a lower limit temperature at which the fluoropolymer solution is not 
solidified, to an upper limit temperature at which the polyether is not 
decomposed or thermally polymerized. It is preferred that the substrate, 
the template or the sealed vessel to be used for formation of the film of 
the solid polymer electrolyte is preliminarily heated to the optimum 
temperature. As required, the resulting solid polymer electrolyte may be 
freed of the organic solvent by desiccation in vacuo or at ordinary 
temperature. 
The solid polymer electrolyte according to the present invention is 
applicable to any of various batteries such as primary batteries and 
secondary batteries, and suitable for totally solid primary and secondary 
batteries, particularly for totally solid secondary batteries. For 
example, the solid polymer electrolyte can be used for a totally solid 
secondary battery having a construction as described below. 
Used as a positive electrode for the totally solid secondary battery is an 
electrode including a positive electrode collector and a composite 
positive electrode material applied on the positive electrode collector 
and composed of a positive electrode active material, a conductor, a 
binder and an ion conductive solid polymer electrolyte containing a metal 
salt. 
The positive electrode active material is not particularly limited, but may 
be composed of an oxide of at least one metal selected from the group 
consisting of cobalt, nickel, vanadium, manganese, niobium and the like. 
LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2 O.sub.4 are particularly preferred. 
Other exemplary positive electrode active materials include chalcogen 
compounds such as titanium disulfide and molybdenum disulfide, and 
electron conductive polymeric compounds such as polypyrrole and 
polyaniline. 
The conductor for the positive electrode is not particularly limited, but 
examples thereof include carbon materials such as carbon black, acetylene 
black and Ketchen black, powdery graphite materials (natural graphite and 
artificial graphite), powdery metals and fibrous metals. 
The binder for the positive electrode is not particularly limited, but 
examples thereof include fluoropolymers such as polytetrafluoroethylene 
and polyvinylidene fluoride, polyolefine polymers such as polyethylene, 
polypropylene and ethylene-propylene-diene terpolymer, and 
styrene-butadiene rubber. 
The solid polymer electrolyte according to the present invention can be 
used as the metal-salt-containing ion conducting solid polymer electrolyte 
for the positive electrode. 
The positive electrode collector is not particularly limited, but exemplary 
materials for the collector include metals, metal alloys and carbon 
materials, more specifically, titanium, aluminum and stainless steels. 
Used as a negative electrode for the totally solid secondary battery is an 
electrode including a negative electrode collector and a composite 
negative electrode material applied on the negative electrode collector 
and composed of a negative electrode active material, a binder and an ion 
conductive polymeric electrolyte containing a metal salt. 
The negative electrode active material is not particularly limited, but 
lithium, lithium alloys and/or substances capable of absorbing and 
desorbing lithium are preferably used as the negative electrode active 
material. Examples of specific negative electrode active materials include 
(1) metal lithium and lithium alloys such as lithium aluminum alloys, 
lithium tin alloys, lithium lead alloys and Wood's alloys; (2) substances 
which can electrochemically be doped and dedoped with lithium ions, such 
as conductive polymers (e.g., polyacetylene, polythiophene, 
polyparaphenylene and the like), pyrolytic carbon materials, carbon 
materials pyrolyzed in a gas phase in the presence of a catalyst, carbon 
materials obtained by baking pitch, coke, tar and the like, and carbon 
materials obtained by baking polymers such as cellulose, a phenolic resin 
and the like; (3) graphite materials (natural graphite, artificial 
graphite, expansive graphite and the like) which can be intercalated and 
deintercalated with lithium ions; and (4) inorganic compounds (WO.sub.2, 
MoO.sub.2 and the like) which can be doped and dedoped with lithium ions. 
These materials may be used either alone or as a composite thereof. 
The binder for the negative electrode is not particularly limited, but 
examples thereof include fluoropolymers such as polytetrafluoroethylene 
and polyvinylidene fluoride, polyolefine polymers such as polyethylene, 
polypropylene and ethylene-propylene-diene terpolymer, and 
styrene-butadiene rubber. 
The solid polymer electrolyte according to the present invention can be 
used as the metal-salt-containing ion conductive polymeric electrolyte for 
the negative electrode. 
The negative electrode collector is not particularly limited, but exemplary 
materials for the collector include metals and metal alloys, more 
specifically, copper, nickel and stainless steels. 
The shape of the battery is not particularly limited, but the battery may 
be of a coin type, a button type, a sheet type, a cylindrical type, a 
square type or the like. 
In accordance with the present invention, a solid polymer electrolyte is 
provided which is stable with respect to metal lithium or a lithium 
absorbable or insertable carbon material and has a high ionic conductivity 
and stability within a wide temperature range. 
Since the solid polymer electrolyte exhibits a high mechanical strength, 
the electrolyte is insusceptible to damage which may otherwise result from 
pressure applied thereto during fabrication of a battery or during the 
charge and discharge process of the battery. Therefore, the thickness of 
the ion conductive solid polymer electrolyte can be reduced. Further, the 
solid polymer electrolyte also functions as a separator and, hence, there 
is no need to provide a separator such as of a polyethylene nonwoven 
fabric between electrodes in the totally solid battery, unlike in a 
battery utilizing a liquid electrolyte. The solid polymer electrolyte, if 
having a reduced thickness, has a correspondingly reduced resistance. 
For fabrication of a battery, all components of the battery should be 
accommodated within a battery case of a predetermined volume. By employing 
the solid polymer electrolyte having a reduced thickness, a space to be 
occupied by the electrolyte in the battery can be reduced. Therefore, 
corresponding larger volumes of electrode active materials can be used so 
that the energy density of the battery can be increased. Since the solid 
polymer electrolyte according to the present invention has a high 
mechanical strength, the battery performance can be enhanced. 
By thus employing the solid polymer electrolyte according to the present 
invention, a totally solid battery can be provided which has a high energy 
density, superior battery characteristics in a wide temperature range, and 
a long charge and discharge cycle lifetime. 
EXAMPLES 
The present invention will hereinafter be described more specifically by 
way of examples and comparative examples thereof. In the examples and 
comparative examples, the ionic conductivity and the mechanical tensile 
strength were determined in the following manner: 
Ionic conductivity 
The ionic conductivity of a sample was determined at 20.degree. C. by the 
AC impedance method with the sample held between blocking electrodes of 
nickel. 
Mechanical Tensile Strength 
A sample having a known size was stretched at a low stretching rate of 5 
cm/min with opposite ends thereof held by a tensile strength test machine, 
and a load applied to the sample when the sample was torn was measured for 
determination of the mechanical tensile strength of the sample. 
Examples 1 to 3 
At a temperature of about 120.degree. C., 6.7 parts by weight of 
polyvinylidene fluoride having a weight-average molecular weight of about 
300,000 was completely dissolved in 100 parts by weight of a propylene 
carbonate solution containing lithium perchlorate in a concentration of 1 
mole/liter. The resulting solution was cooled to about 105.degree. C., and 
then 26.7 parts by weight of a polyether (polyethylene oxide diacrylate) 
having a weight-average molecular weight of about 5,000 was added to the 
solution and mixed by stirring. The resulting solution was spread on a 
template, and solidified by cross-linking the polyether by UV radiation. 
Thus, a solid polymer electrolyte of Example 1 was prepared. 
Solid polymer electrolytes of Examples 2 and 3 were prepared in 
substantially the same manner as in Example 1, except that 13.3 parts by 
weight of polyvinylidene fluoride and 20.0 parts by weight of the 
polyether were used in Example 2 and 16.7 parts by weight of 
polyvinylidene fluoride and 16.7 parts by weight of the polyether were 
used in Example 3. 
Relationships of the amount of polyvinylidene fluoride contained in the 
solid polymer electrolyte versus the ionic conductivity and mechanical 
tensile strength of the solid polymer electrolyte according to Examples 1 
to 3 are shown in FIG. 1. 
The solid polymer electrolytes of Examples 1 to 3 were pressed to such an 
extent that the electrolytes were not destroyed. At this time, exudation 
of propylene carbonate (nonaqueous solvent) was not observed. 
Further, the solid polymer electrolytes of Examples 1 to 3 were kept in 
contact with metal lithium for 30 days. As a result, any chemical change 
in the solid polymer electrolytes and metal lithium was not observed. This 
indicates that the solid polymer electrolytes had a high chemical 
stability with respect to metal lithium. 
Comparative Example 1 
First, 33 parts by weight of a polyether (polyethylene oxide diacrylate) 
having a weight-average molecular weight of about 5,000 was added to 100 
parts by weight of a propylene carbonate solution containing lithium 
perchlorate in a concentration of 1 mole/liter, and mixed by stirring. 
Then, the resulting solution was spread on a template, and solidified by 
cross-linking the polyether by UV radiation. Thus, a solid polymer 
electrolyte was prepared. The ionic conductivity and mechanical tensile 
strength of the solid polymer electrolyte are shown in FIG. 1. 
FIG. 1 indicates that the mechanical tensile strength increases as the 
amount of polyvinylidene fluoride contained in the electrolyte is 
increased. FIG. 1 also indicates that, even if the amount of 
polyvinylidene fluoride is increased, the ionic conductivity is kept 
virtually unchanged. This indicates that the addition of polyvinylidene 
fluoride enhances the mechanical tensile strength without changing the 
ionic conductivity. 
Comparative Example 2 
At a temperature of 120.degree. C. to 130.degree. C., 33 parts by weight of 
polyvinylidene fluoride having a weight-average molecular weight of about 
300,000 was completely dissolved in 100 parts by weight of a propylene 
carbonate solution containing lithium perchlorate in a concentration of 1 
mole/liter. The resulting solution was spread on a template, and naturally 
cooled. Thus, a solid polymer electrolyte was prepared. 
When the solid polymer electrolyte was slightly pressed, the solution 
exuded from the electrolyte. Therefore, the solid polymer electrolyte 
cannot be measured the ionic conductivity and the mechanical tensile 
strength. 
Comparative Example 3 
A mixture containing 100 parts by weight of polyvinyl alcohol having a 
weight-average molecular weight of 22,000 and 100 parts by weight of 
polyethylene oxide having a weight-average molecular weight of 500,000 was 
pressed into a film having a thickness of 0.5 mm. The film was immersed in 
a propylene carbonate solution containing lithium perchlorate in a 
concentration of 1 mole/liter at 50.degree. C. for 24 hours, and then the 
resulting film was dried. Thus, a solid polymer electrolyte was prepared. 
The film was kept in contact with metal lithium for 10 days. As a result, 
the metal lithium was chemically changed by a chemical reaction. 
As can be understood from the results in Examples 1 to 3 and Comparative 
Examples 1 to 3, the solid polymer electrolytes according to the present 
invention each had a high ionic conductivity and a high mechanical 
strength and were superior in the chemical or electrochemical stability 
and the long-term reliability. The use of the cross-linked network 
polyether facilitated the preparation of the solid polymer electrolyte, 
and made it possible for the solid polymer electrolyte to maintain a 
satisfactory mechanical strength. Further, even where the solid polymer 
electrolyte contained the organic solvent in such a great amount that a 
weight ratio of the solvent to the polymer is 100:30 (parts by weight), 
the exudation of the solvent did not occur. Therefore, if the solid 
polymer electrolyte according to the present invention is employed for an 
electrochemical device, particularly for a battery, the electrochemical 
device is expected to have a reduced internal resistance and an improved 
reliability. 
Example 4 
At a temperature of about 120.degree. C., 13.3 parts by weight of 
polyvinylidene fluoride having a weight-average molecular weight of about 
100,000 was completely dissolved in 100 parts by weight of a mixture 
consisting of ethylene carbonate and dimethyl carbonate in a volume ratio 
of 1:1 dissolving lithium hexafluorophosphate in a concentration of 0.5 
mole/liter. The resulting solution was cooled to about 105.degree. C., and 
then 20.0 parts by weight of a polyether (polyethylene oxide diacrylate) 
having a weight-average molecular weight of about 20,000 was added to the 
solution and mixed by stirring. The resulting solution was spread on a 
template, and solidified by cross-linking the polyether by UV radiation. 
Thus, a solid polymer electrolyte of Example 4 was prepared. 
An ionic conductivity of the solid polymer electrolyte at 20.degree. C. was 
1.4.times.10.sup.-3 S/cm. 
Example 5 
6.7 parts by weight of polyvinylidene fluoride having a weight-average 
molecular weight of about 500,000 and 26.7 parts by weight of polyether 
(polyethylene oxide diacrylate) having a weight-average molecular weight 
of about 2,000 were mixed and dissolved in N-methylpyrrolidone. The 
resulting solution was spread on a template, solidified by cross-linking 
the polyether by UV radiation, dried under the reduced pressure at 
100.degree. C. for 3 hours to remove the solvent completely, and immersed 
in a mixture consisting of ethylene carbonate and diethyl carbonate in a 
volume ratio of 1:1 containing lithium perchlorate in a concentration of 
2.0 mole/liter for 24 hours. After the obtained solid polymer electrolyte 
was taken out and then exposed in a dry box for about 5 hours, an ionic 
conductivity of the solid polymer electrolyte was measured. The ionic 
conductivity of the solid polymer electrolyte at 20.degree. C. was 
1.2.times.10.sup.-3 S/cm. 
Examples 6 to 9 
A mixture of 75 wt % of a polymer electrolyte mixture consisting of lithium 
perchlorate, polyvinylidene fluoride having a weight-average molecular 
weight of about 300,000 and a polyether (polyethylene oxide diacrylate) 
having a weight-average molecular weight of about 5,000 in a weight ratio 
of 4:6:9 with 25 wt % of a nonaqueous mixed solvent of ethylene carbonate 
and diethyl carbonate in a volume ratio of 1:1 was spread on a template, 
and solidified by cross-linking the polyether by UV radiation. Thus, a 
solid polymer electrolyte of Example 6 was prepared in substantially the 
same manner as in Examples 1 to 3. 
Solid polymer electrolytes of Examples 7, 8 and 9 were prepared in 
substantially the same manner as in Example 6, except that a mixture of 50 
wt % of a polymer electrolyte mixture with 50 wt % of a nonaqueous mixed 
solvent was used in Example 7, a mixture of 75 wt % of a polymer 
electrolyte mixture with 25 wt % of a nonaqueous mixed solvent was used in 
Example 8, and amixture of 15 wt % of a polymer electrolyte mixture with 
85 wt % of a nonaqueous mixed solvent was used in Example 9. 
Relationships of the proportion of the nonaqueous mixed solvent in the 
solid polymer electrolyte versus the ionic conductivity of the solid 
polymer electrolyte according to Examples 6 to 9 are shown in FIG. 2. 
As is apparent from FIG. 2, it can be seen that the more the proportion of 
the nonaqueous mixed solvent increases, the more the ionic conductivity of 
the solid polymer electrolyte increases. However, it was observed that the 
solvent was exuded from the solid polymer electrolyte when the proportion 
of the nonaqueous mixed solvent became 85 wt %. 
Example 10 
Evaluation of Battery 
A thin battery as shown in FIG. 3 was fabricated. Used for a positive 
electrode were aluminum for a positive electrode collector, LiCoO.sub.2 
for a positive electrode active material, and acetylene black for a 
conductor. Used for a negative electrode were copper for a negative 
electrode collector and carbon powder for a negative electrode active 
material. 
The thin battery thus fabricated was evaluated through a 
charging/discharging cycle test in which the battery was charged up to a 
limit charged voltage of 4.2 V and discharged to a limit discharged 
voltage of 2.7 V at a constant current of 0.1 mA/cm.sup.2. 
Hereafter, a procedure for fabricating the thin battery will be described. 
Preparation of Positive Electrode 
100 parts by weight of LiCoO.sub.2 was mixed with 10 parts by weight of 
acetylene black and 10 parts by weight of polyvinylidene fluoride followed 
by adding N-methylpyrrolidone to form a slurry. The slurry casted on 
aluminum by a screen coating method was dried under the reduced pressure 
to remove the solvent completely. Accordingly, a positive electrode sheet 
was fabricated which was 40 cm.sup.2 in area and weighted 170 mg. 
Preparation of Negative Electrode 
100 parts by weight of carbon powder was mixed with 10 parts by weight of 
polyvinylidene fluoride followed by adding N-methylpyrrolidone to form a 
slurry. The slurry casted on copper by a screen coating method was dried 
under the reduced pressure to remove the solvent completely. Accordingly, 
a negative electrode sheet was fabricated which was 40 cm.sup.2 in area 
and weighted 110 mg. 
Preparation of Electrolyte 
In the same manner as in Examples 1 to 3, a mixture of 13.3 parts by weight 
of polyvinylidene fluoride having a weight-average molecular weight of 
about 300,000 dissolved in 100 parts by weight of a mixture consisting of 
ethylene carbonate and diethyl carbonate in a volume ratio of 1:1 
dissolving lithium perchlorate in a concentration of 1.0 mole/liter with 
20.0 parts by weight of a polyether (polyethylene oxide diacrylate) having 
a weight-average molecular weight of about 5,000 was casted on each 
electrode by the screen coating method, and solidified by cross-linking 
the polyether by UV radiation. 
Preparation of Battery 
By contacting two electrodes thus fabricated each other, sealing an edge of 
them with a sealer made of modified polypropylene and covering them with a 
aluminum laminated film, the thin battery was fabricated. 
10 cells of the thin battery thus fabricated were evaluated through a 
charging/discharging cycle test. As a result, the discharge capacity of 
the battery was an average of 20.2 mAh per cell in the first 
charging/discharging cycle and an average of 19.5 mAh per cell in the 50th 
cycle. 
Comparative Example 4 
10 cells of a thin battery were prepared in substantially the same manner 
as in Example 10, except that a solid polymer electrolyte was prepared by 
mixing only 33.0 parts by weight of a polyether (polyethylene oxide 
diacrylate) having a weight-average molecular weight of about 5,000 with 
100 parts by weight of a mixture consisting of ethylene carbonate and 
diethyl carbonate in a volume ratio of 1:1 containing lithium perchlorate 
in a concentration of 1.0 mole/liter. These batteries thus fabricated were 
evaluated through a charging/discharging cycle test. 
As a result, the tests of 3 of 10 cells were canceled because the 
performance of these cells had remarkably fallen off during the test. As 
for the rest of 7 cells, the discharge capacity of the battery was an 
average of 19.8 mAh per cell in the first charging/discharging cycle and 
an average of 19.0 mAh per cell in the 50th cycle. 
From these results, it can be imagined that because of a high mechanical 
strength, the solid polymer electrolyte including polyvinylidene fluoride 
could stand the effect of the change of the electrodes occurred during the 
charging and discharging process, therefore the performance of the battery 
had not fallen at all during the test. 
As described above, the solid polymer electrolyte according to the present 
invention contains a polymer blend of a fluoropolymer and a polyether 
comprising an ethylene oxide unit and/or a propylene oxide unit as a 
monomeric unit. Therefore, the solid polymer electrolyte exhibits an 
enhanced mechanical strength as well as a high ionic conductivity. 
Further, a battery employing the solid polymer electrolyte according to 
the present invention has a smaller thickness and a lower internal 
resistance, and is superior in the chemical or electrochemical stability 
and the long-term reliability. 
Where a nonaqueous solvent is contained in the polymer blend, the solid 
polymer electrolyte has an ionic conductivity high enough for practical 
use, and maintains a satisfactory mechanical strength. 
Further, the solid polymer electrolyte can readily be prepared through the 
preparation method according to the present invention, which includes the 
steps of mixing a polyether and a fluoropolymer and irradiating the 
resulting mixture with ionizing radiation. The preparation method is thus 
simplified without the need for copolymerization of the polyether and the 
fluoropolymer.