Secondary battery with graft-polymerized separator

A secondary battery comprising a positive-electrode active material enabling the battery to discharge and to be charged, an electrolyte solution, a separator, and a negative-electrode active material enabling the battery to discharge and to be charged, wherein the separator is obtained by subjecting a porous polymer substrate to plasma treatment and polymerizing a monomer in the presence of the plasma-treated substrate thereby to graft-polymerize the monomer onto the substrate.

FIELD OF THE INVENTION 
The present invention relates to a secondary battery. 
BACKGROUND OF THE INVENTION 
As separators for secondary batteries including lithium secondary 
batteries, porous polymer films have been used hitherto which have 
excellent permeability to electrolyte solutions. However, due to the 
porous nature of such a porous polymer film separator, use thereof as the 
separator in a secondary battery has inevitably had a practical problem 
that as the battery is repeatedly charged and discharged, dendrites grow 
and penetrate through pores of the porous film to cause a short-circuit or 
the dendrites fall from the electrode to cause a decrease in capacity. 
JP-A-53-84134 discloses a separator for secondary batteries, such as 
lithium secondary batteries, which is obtained by grafting an electrolyte 
solution-permeable polymer onto a porous substrate thereby to fill the 
pores of the substrate with the polymer. (The term "JP-A" as used herein 
means an "unexamined published Japanese patent application".) However, 
this separator film is not suited for practical use because it swells too 
much. Although a measure for controlling the swelling of the film is 
disclosed in JP-B-62-17822, this technique has failed to provide a 
substantial solution to the swelling problem. (The term "JP-B", as used 
herein, means an "examined Japanese patent publication".) 
JP-A-2-82457 discloses a method of producing a separator which comprises 
impregnating a porous resin film with a monomer and then irradiating the 
resulting film with ultraviolet rays to polymerize the monomer, thereby to 
fill the pores of the film with the polymer. However, this method has a 
drawback that the adhesion between the porous resin and the polymer is so 
poor that the separator develops voids at the interface between the porous 
resin and the polymer. Thus, this method has been unable to solve the 
problem of dendritic growth. 
Further, JP-A-3-41107 discloses a separator modification utilizing 
plasma-initiated polymerization. In general, plasma-initiated 
polymerization is a technique of polymerizing a gaseous-phase or 
liquid-phase monomer by irradiating the monomer with a plasma (as 
described in "Plasma Polymerization" written by Yoshihito Osada et al., 
published by Tokyo Kagaku Dohjin, Japan). This method, however, has also 
failed to improve the adhesion between the polymer substrate and the 
polymer formed by plasma polymerization to a desired level. In addition, 
even in the case where the polymerization of a monomer was conducted by 
irradiating a substrate with a plasma to graft-polymerize the monomer onto 
the substrate, the thus-obtained separator was unable to sufficiently 
inhibit dendritic growth. 
SUMMARY OF THE INVENTION 
A first object of the present invention is to prevent dendritic growth. 
A second object of the present invention is to provide a secondary battery 
which, when charged and discharged repeatedly, suffers only a slight 
decrease in capacity. 
A third object of the present invention is to provide a secondary battery 
using a battery separator having excellent dimensional stability. 
A fourth object of the present invention is to provide a secondary battery 
using a battery separator which comprises a porous substrate and a polymer 
which virtually fills all the pores of the substrate, and in which the 
adhesion of the polymer to the substrate is excellent. 
The above and other objects of the present invention are accomplished with 
a secondary battery comprising a positive-electrode active material 
enabling the battery to discharge and to be charged, an electrolyte 
solution, a separator, and a negative-electrode active material enabling 
the battery to discharge and to be charged, wherein the separator is 
obtained by subjecting a porous polymer substrate to plasma treatment and 
polymerizing a monomer in the presence of the plasma-treated substrate 
thereby to graft-polymerize the monomer onto the substrate.

DETAILED DESCRIPTION OF THE INVENTION 
The porous substrate to be irradiated with plasma in order to produce the 
separator employed in the battery of the present invention may be made of 
a polymer such as a polyolefin, poly(vinyl halide), polyester, polyamide, 
polysulfone, cellulose, polyurethane, or the like. Preferred examples of 
the substrate polymer include polyethylene, polypropylene, polybutene, 
polyisobutylene, ethylene-propylene copolymers, poly(vinyl chloride), 
poly(1,1-dichloroethylene), poly(chlorotrifluoroethylene), 
poly(1,1-dichloro-2-fluoroethylene), 
poly(1,2-dichloro-1,2-difluoroethylene), poly(1,1-difluoroethylene), 
poly-(1,2-difluoroethylene), polytetrafluoroethylene, 
poly[(pentafluoroethyl)ethylene], polyhexafluoropropylene, 
poly(3,3,3-trifluoropropylene), poly(vinyl fluoride), poly(ethylene 
terephthalate), poly(butylene terephthalate), poly(ethylene adipate), 
poly(oxy-2,2,3,3,4,4-hexafluoropentamethyleneoxyadipoyl), 
poly(oxytetraphthaloyloxy-1,4-phenyleneisopropylidene-1,4-phenylene), 
poly(oxytetramethyleneoxyterephthaloyl), nylon 3, nylon 6,6, nylon 4,6, 
nylon 5,6, nylon 6, nylon 6,2, nylon 11, 
poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene), 
poly(oxy-1,4-phenylenesulfonyl-1,4-phenyleneoxy-1,4-phenylenemethylene 
1,4-phenylene), carboxymethyl cellulose, hydroxyethyl cellulose, 
triacetoxy cellulose, diacetoxy cellulose, and 
poly(oxytetramethyleneoxycarbonyliminohexaethyleneiminocarbonyl). Of 
these, polyethylene, polypropylene, ethylene-propylene copolymers, and 
polytetrafluoroethylene are particularly preferred. 
For use in producing the separator to be employed in the battery of the 
present invention, it is preferable if the porous polymer substrate has a 
porosity of from 30 to 90%, with the more preferred range of the porosity 
thereof being from 40 to 90%. 
Further, the maximum pore diameter of the porous polymer substrate is 
preferably from 0.01 to 50 .mu.m, more preferably from 0.01 to 20 .mu.m, 
and most preferably from 0.01 to 5 .mu.m. 
As the monomer for use in producing the separator to be employed in the 
battery of the present invention, any vinyl compound may be used as long 
as the compound is polymerizable. It is, however, preferred that the 
monomer contain a group capable of dissolving in the electrolyte solution 
to be employed in the battery of the present invention. The group capable 
of dissolving in the electrolyte solution may be anionic, nonionic, or 
cationic. Preferred examples of such a group include ethylene oxide 
groups, propylene oxide groups, ester groups, carbonyl groups, amide 
groups, amino groups, nitrile groups, alkyl groups, phenyl groups, 
sulfonic acid groups, phosphoric acid groups, and carboxylic acid groups. 
Preferred examples of the monomer to be used for producing the separator 
include compounds represented by general formulae (1), (2), and (3), alone 
or in combination thereof, described below. 
##STR1## 
In general formula (1), R.sub.1 represents a hydrogen atom or an alkyl 
group having from 1 to 3 carbon atoms, and preferably is a hydrogen atom 
or a methyl group. R2 represents --C(O)--X.sub.1 --(CH.sub.2 CH.sub.2 
O).sub.n --R.sub.3, --C(O)--Y--(CH.sub.2).sub.x --C.sub.6 H.sub.5, or 
--CN, in which X.sub.1 and Y represent an oxygen atom or --N(R.sub.4)--, 
in which R.sub.4 represents a hydrogen atom or an alkyl group having from 
1 to 3 carbon atoms. X.sub.1 preferably is an oxygen atom, 
--N(CH.sub.3)--, or --N(C.sub.2 H.sub.5)--. n is an integer preferably 
from 0 to 50, more preferably from 0 to 23, and most preferably from 0 to 
9. R.sub.3 represents a hydrogen atom, an alkyl group having from 1 to 3 
carbon atoms, an aralkyl group, or an aryl group having from 6 to 14 
carbon atoms, preferably, a hydrogen atom, an alkyl group having 1 or 2 
carbon atoms, an aralkyl group having from 7 to 10 carbon atoms, or 
--C.sub.6 H.sub.5, and more preferably, a hydrogen atom, a methyl group, 
or C.sub.6 H.sub.5 group. x is an integer preferably from 1 to 5, and more 
preferably, from 1 to 3. 
Examples of R.sub.2 include --C(O)O(CH.sub.2 CH.sub.2 O).sub.n CH.sub.3, 
--C(O)O(CH.sub.2 CH.sub.2 O).sub.n --C.sub.6 H.sub.5, --C(O)OCH.sub.2 
C.sub.6 H.sub.5, --C(O)OC.sub.6 H.sub.5, --C(O)CH.sub.3, --C(O)OC.sub.2 
H.sub.5, --C(O)OC.sub.3 H.sub.7, --C(O)NH.sub.2, --C(O)N(CH.sub.3).sub.2, 
and --CN. 
##STR2## 
In general formula (2), R.sub.5 represents a hydrogen atom or an alkyl 
group having from 1 to 3 carbon atoms, and preferably is a hydrogen atom 
or a methyl group. X.sub.2 represents --C(O)--X.sub.3 --C(O)--, in which 
X.sub.3 represents --O--X.sub.4 --O--or --O--(CH.sub.2 CH.sub.2 O).sub.m 
--. X.sub.4 is an alkylene group, preferably one having from 1 to 20 
carbon atoms, more preferably, one having from 1 to 10 carbon atoms, and 
most preferably, one having from 1 to 6 carbon atoms. 
m is an integer of 1 or more, preferably from 1 to 50, more preferably from 
1 to 20, and most preferably from 1 to 10. 
Examples of X.sub.2 include --C(O)--O--(CH.sub.2 CH.sub.2 O).sub.m 
--C(O)--, --C(O)--O--(CH.sub.2).sub.6 --O--C(O)--, 
--C(O)O--CH(CH.sub.3)--CH.sub.2 CH.sub.2 OC(O)--, and --C(O)O--CH.sub.2 
C(CH.sub.3).sub.2 CH.sub.2 OC(O)--. 
##STR3## 
In general formula (3), R.sub.6 represents a hydrogen atom or an alkyl 
group having from 1 to 3 carbon atoms, and preferably is a hydrogen atom 
or a methyl group. R.sub.7 represents an alkyl group or a group having the 
formula --CH.sub.2 O--R.sub.8, in which R.sub.8 represents a hydrogen atom 
or an alkyl group having from 1 to 10, preferably 1 to 5, more preferably 
1 to 3 carbon atoms. Preferred examples of R.sub.7 include alkyl groups 
having from 1 to 10 carbon atoms, --CH.sub.2 OH, --CH.sub.2 OCH.sub.3, and 
--CH.sub.2 OCH.sub.2 CH.sub.3. More preferred examples of R.sub.7 are 
alkyl groups having from 1 to 3 carbon atoms, --CH.sub.2 OH, --CH.sub.2 
OCH.sub.3, and --CH.sub.2 OCH.sub.2 CH.sub.3. 
Specific examples of compounds of general formula (1) are given below, but 
the compound is not, of course, limited to these examples. 
##STR4## 
Specific examples of compounds of general formula (2) are given below, but 
the compounds are not, of course, limited to these examples. 
##STR5## 
Specific examples of compounds of general formula (3) are given below, but 
compounds are not, of course, limited to these examples. 
##STR6## 
In conducting polymerization, any one of the compounds of general formulae 
(1), (2), and (3) may be polymerized alone, or two or more of these 
compounds may be polymerized in combination with one another in an 
arbitrary proportion. 
Water or an organic solvent, preferably water, may be used as a reaction 
medium for the polymerization. 
In the case where water is used as a reaction medium to polymerize a 
hydrophobic monomer, it is preferred to use a surfactant so that the 
polymerization is conducted while the monomer is kept in a dispersed or 
emulsified state. 
The process for the plasma treatment and polymerization for producing a 
separator is explained below. The polymerization of the monomer is 
conducted while a porous polymer substrate, which has undergone plasma 
surface treatment, is immersed in a solution or dispersion of the monomer. 
Although the product of the polymerization of the monomer may contain a 
non-graft-portion in the polymer substrate, it is preferable for the 
polymerization product to contain a grafted polymer. 
Each step of the process is explained below. 
The plasma surface treatment for activating the surfaces of the porous 
polymer substrate may be conducted by using ordinary discharge treatment. 
Details of the discharge plasma treatment are given, for example, in J. R. 
Hollahan and A. T. Bell, Techniques and Applications of Plasma Chemistry, 
Wiley, New York, 1974 and in other books and publications. 
Reactions of high-energy active particles, such as radicals and ions, 
generated in a discharge plasma take place within an extremely thin 
surface layer of the polymer substrate and, as a result, C--C bonds or 
C--H bonds in the molecules of the polymer are cleaved and polymeric 
radicals are formed. These reactions are followed by reactions between 
radicals and elimination reactions, thereby to form crosslinks and 
unsaturated bonds. In some cases, polar groups are newly introduced 
through reactions with an active gas such as oxygen. 
On the other hand, polymeric radicals which have not undergone the above 
reactions, which are secondary reactions, can take part in graft 
polymerization when brought into contact with the monomer. In other words, 
the polymeric radicals can be utilized as graft polymerization-initiating 
sites. 
In producing a discharge plasma, audio-frequency waves having a frequency 
on the order of from several kilohertz (kHz) to several tens of kHz, 
radio-frequency waves having a frequency of 13.56 MHz, or microwaves 
having a frequency on the order of GHz may be utilized. 
For producing a separator to be employed in the present invention, 
high-voltage discharge for the plasma treatment is generally conducted 
under a reduced pressure, preferably at any of various gas pressures 
higher than 0.001 Torr and lower than 20 Torr. For example, it is more 
preferred to conduct the plasma surface treatment by irradiating the 
polymer substrate with a plasma for a period of from 5 seconds to 10 
minutes under conditions of a gas pressure of from 0.01 to 1.0 Torr and an 
output of from 5 to 500 W. 
As a plasma gas source, use may be made of a gas which does not deteriorate 
the substrate polymer, such as a noble gas (e.g., helium or argon), 
nitrogen, or a residual inorganic gas. 
The step of grafting a monomer onto the plasma-treated polymer substrate is 
now explained. This graft polymerization can be carried out in various 
ways, which are generally divided into two methods: (1) a method in which 
the polymer substrate already activated with plasma is directly treated 
with a degassed monomer solution to allow the polymer to react with the 
monomer; and, (2) a method in which the activated polymer substrate is 
first brought into contact with air or oxygen gas to form peroxides, the 
peroxides are subsequently pyrolyzed to oxy-radicals, and the resulting 
polymer substrate is then treated with a degassed monomer solution to 
allow the polymer to react with the monomer. Both of these two methods are 
usable for producing a separator to be employed in the battery of the 
present invention. 
It is generally desirable for the graft polymerization reaction to be 
carried out in vacuo (e.g., under a high vacuum). However, it is possible 
to conduct the polymerization at a higher pressure (e.g., ordinary 
pressure) in the absence of air or oxygen, for example, in a nitrogen or 
argon gas atmosphere. 
The temperature for the graft polymerization varies depending on the 
polymerization activity of the monomer employed. In general, however, the 
graft polymerization may be conducted at a temperature of from 0.degree. 
to 100.degree. C., preferably, from room temperature to 90.degree. C. 
The reaction time for the graft polymerization is not particularly limited, 
but a practically preferred range thereof is from 10 minutes to 10 hours. 
When the film thus obtained by the above-described process is used as the 
separator for the battery in this invention, the thus-obtained separator 
may be impregnated with an electrolyte solution containing a supporting 
electrolyte dissolved therein. 
As the supporting electrolyte, a salt of a Group Ia or IIa element of the 
periodic table is used. Examples of these salts include LiBF.sub.4, 
LiClO.sub.4, LiCF.sub.3 SO.sub.3, LiPF.sub.6, lithium toluenesulfonate, 
NaClO.sub.4, NaBF.sub.4, NaCl, LiCl, LiOH, and NaOH. These may be used 
alone or in combination of two or more thereof. 
The solvent for the supporting electrolyte is not particularly limited as 
long as it dissolves the electrolyte therein. Examples of the solvent 
include propylene carbonate, butylene carbonate, ethylene carbonate, 
.gamma.-butyrolactone, dimethoxyethane, tetrahydrofuran, 
methyltetrahydrofuran, acetonitrile, 1,3-dioxolane, nitromethane, 
dimethylformamide, dimethyl sulfoxide, and water. These may be used alone 
or in combination of two or more thereof. 
There is no particular limitation on the concentration of the supporting 
electrolyte in the solution in which the separator is impregnated. 
However, the electrolyte solution generally has a supporting electrolyte 
concentration such that the separator film after impregnation has an ionic 
conductance of preferably 10.sup.-9 S/cm or higher, more preferably 
10.sup.-6 S/cm or higher, most preferably 10.sup.-4 S/cm or higher. 
Examples of the positive-electrode active material for use with the 
separator in the battery of the present invention include oxides, 
sulfides, and selenides of manganese, molybdenum, vanadium, titanium, 
chromium, niobium, cobalt, and nickel, active carbon (described in 
JP-A-60-167280), carbon fibers (described in JP-A-61-10882), polyaniline, 
polymers of amino group-substituted aromatic compounds, polymers of 
heterocyclic compounds, polyacene, and polyyne compounds. Particularly 
useful are active carbon, .gamma.-MnO.sub.2 (described in JP-A-62-108455 
and JP-A-62-108457), a mixture of .gamma.-.beta.-MnO.sub.2 and Li.sub.2 
MnO.sub.3 (U.S. Pat. No. 4,758,484), amorphous V.sub.2 O.sub.5 
(JP-A-61-200667), V.sub.6 O.sub.13, Li.sub.x Ni.sub.y CO.sub.(1-y) O.sub.2 
(0.05.ltoreq.x.ltoreq.1.10, 0.ltoreq.y.ltoreq.1) (JP-A-1-294372), 
MoS.sub.2 (JP-A-61-64083), TiS.sub.2 (JP-A-62-222578), polyaniline 
(JP-A-60-65031, JP-A-60-149628, JP-A-61-281128, JP-A-61-258831, 
JP-A-62-90878, JP-A-62-93868, JP-A-62-119231, JP-A-62-181334, and 
JP-A-63-46223), polyacetylene (JP-A-57-121168, JP-A-57-123659, 
JP-A-58-40781, JP-A-60-124370, JP-A-60-127669, and JP-A-61-285678), and 
polyphenylene. 
In the electrode active materials, an electrically conductive material such 
as carbon, silver (JP-A-63-148554), or a poly(phenylene derivative) 
(JP-A-59-20971) and a binder such as Teflon may generally be incorporated. 
Examples of the negative-electrode active material for use in the battery 
of the present invention include lithium metal, polyacene, polyacetylene, 
polyphenylene, carbon (JP-A-1-204361), calcium metal, sodium metal, 
aluminum, magnesium, zinc, niobium, and alloys of these metals, 
particularly lithium alloys. Examples of such lithium alloys include 
aluminum or magnesium alloys (JP-A-57-65670 and JP-A-57-98977), mercury 
alloys (JP-A-58-111265), Pt alloys (JP-A 60-79670), Sn-Ni alloys 
(JP-A-60-86759), Wood's metal alloys (JP-A-60-167279), electrically 
conductive polymer alloys (JP-A-60-262351), Pd-Cd-Bi alloys 
(JP-A-61-29069), Ga-In alloys (JP-A-61-66368), Pb-Mg alloys 
(JP-A-61-66370), Zn alloys (JP-A-61-68864), Al-Ag alloys (JP-A-61-74258), 
Cd-Sn alloys (JP-A-61-91864), Al-Ni alloys (JP-A-62-119865 and 
JP-A-62-119866), and Al-Mn alloys (U.S. Pat. No. 4,820,599). Of these, 
lithium metal, its alloys with Al, and carbon are useful. 
In the separator employed in the battery of the present invention, the 
amount of the polymer grafted onto the polymer substrate is preferably 
0.001 to 20 mg/cm.sup.2, more preferably 0.01 to 10 mg/cm.sup.2, and most 
preferably 0.1 to 5 mg/cm.sup.2. 
As described above, in producing the separator to be employed in the 
battery of the present invention, a polymer layer can be efficiently 
formed on the surfaces of a polymer substrate. The separator thus formed 
has advantages in that the polymer layer formed on the substrate never 
peels-off the substrate or dissolves away over a prolonged time period (or 
even when the separator is subjected to various treatments), and the 
separator retains its initial performance over a prolonged time period. 
Moreover, in producing the separator, the thickness of the polymer layer 
to be formed on a substrate can be controlled freely, from an extremely 
small to a large thickness, by suitably selecting polymerization 
conditions (temperature, time, monomer temperature, surfactant amount, 
etc.). A further advantage of the separator is that in the polymer 
layer-forming step, the substrate is less damaged as compared to the case 
of polymer film formation utilizing electron beams or the like and, hence, 
the properties originally possessed by the substrate are not substantially 
impaired. 
Therefore, due to the use of the separator, the secondary battery of the 
present invention can show excellent charge-discharge cycle 
characteristics and is substantially free from internal short-circuiting 
and dendritic growth. 
The present invention will be explained below in more detail with reference 
to the following examples, but the invention is not to be construed as 
being limited thereto. Unless otherwise indicated, all parts, percents, 
ratios and the like are by weight. 
EXAMPLE 1 
Plasma irradiation was conducted using plasma irradiator Model BP-1 
manufactured by Samco International Kenkyusho as follows. 
A porous polypropylene film (trade name, Julagard 2500; manufactured by 
Polyplastics Co., Ltd., Japan) having dimensions of 6 cm by 6 cm was 
placed on an electrode plate, and the pressure in the plasma irradiator 
was reduced to 0.675 Torr while nitrogen gas was continuously introduced 
therein at a rate of 30 ml/min. The polypropylene film was then irradiated 
with a plasma at an output of 50 W for 60 seconds. Subsequently, the 
pressure inside the irradiator was raised to ordinary pressure by 
introducing nitrogen gas, and the polypropylene film was then taken out 
from the irradiator. This plasma-treated polypropylene film was immersed 
in 200 g of an aqueous monomer solution which contained, as a monomer, 30 
g of compound 1-2 as specified hereinabove and had been heated beforehand 
to 65.degree. C. The monomer was then allowed to react at 65.degree. C. 
for 3 hours. The polypropylene film was then taken out from the resulting 
monomer solution, subsequently washed with 1 liter of water for one day to 
remove the monomer which has been unreacted, and then dried. It was 
ascertained that a graft polymer had been formed on the resulting film in 
an amount of 2.2 mg per cm.sup.2 of the film. Further, examination with an 
electron microscope revealed that the pores of the porous polypropylene 
film had been clogged. 
This film was then immersed in a 1-mol/liter LiBF.sub.4 solution in 
propylene carbonate/dimethoxyethane=1 (by volume). As a result, although 
the film swelled in the direction of thickness (dry film thickness 31 
.mu.m, after-immersion film thickness 58 .mu.m), no swelling was observed 
in either of the length and width directions. The film after immersion had 
an ionic conductance of 1.times.10.sup.-3 S/cm. 
Using this film as a separator, 100 batteries having a construction as 
illustrated in FIG. 1 were fabricated. In FIG. 1, numeral 1 denotes a 
positive-electrode active material, numeral 2 denotes a nonwoven fabric 
impregnated with an electrolyte solution, numeral 3 denotes a 
negative-electrode active material (lithium), numeral 4 denotes a 
stainless-steel case, numeral 5 denotes an insulating synthetic rubber, 
and numeral 6 denotes the separator. These batteries were subjected to a 
charge-discharge test in which charge and discharge were conducted between 
1.5 V and 3.5 V at a constant current of 2 mA. No internal 
short-circuiting was observed. 
Further, some of the batteries that had undergone 100 cycles of charge and 
discharge were charged and then disassembled, and the lithium surfaces 
were examined with an electron microscope. As a result, no dendritic 
growth was observed. 
The positive electrode employed in each of the above-fabricated batteries 
had been prepared by mixing 90 parts by weight of V.sub.6 O.sub.13 as a 
positive-electrode active material, 5 parts by Weight of Teflon as a 
binder, and 5 parts by weight of carbon black as an electrically 
conductive material to prepare a positive-electrode active material 
mixture, and then pressing the mixture into a pellet having a diameter of 
1.3 cm. As the negative electrode, a lithium metal disk having a diameter 
of 1.3 cm was used which had been obtained by die-cutting. 
EXAMPLES 2 TO 10 
Polymer-grafted films were prepared in the same manner as in Example 1 
except that the monomer, the output and time for plasma irradiation, and 
the temperature for polymerization were changed as shown in Table 1, and 
in the case of using a hydrophobic monomer, the following surfactant A was 
used for dispersing the monomer in an amount shown in Table 1. 
##STR7## 
The grafted polymer amount and ionic conductance for each polymer-grafted 
film are shown in Table 1. In the same manner as in Example 1, batteries 
were fabricated using each of the films and were subjected to a 
charge-discharge test. No dendritic growth was observed in any of the 
batteries. 
TABLE 1 
__________________________________________________________________________ 
Plasma 
Polymeri- 
Grafted 
Monomer irradiation 
zation polymer 
Ionic 
Example 
compound No. 
Water 
Surfactant* 
Output 
time temperature 
amount 
conductance 
No. (amount (g)) 
(g) (wt %) 
(W) (sec) (.degree.C.) 
(mg/cm.sup.2) 
(S/cm) 
__________________________________________________________________________ 
1 1-2(30) 170 -- 50 60 65 2.2 1.0 .times. 10.sup.-3 
2 1-2(30) 170 -- 40 60 65 2.5 1.1 .times. 10.sup.-3 
3 1-6(25) 175 -- 40 60 65 3.1 1.2 .times. 10.sup.-3 
4 1-9(30) 170 8 50 90 65 2.7 0.9 .times. 10.sup.-3 
5 1-11(30) 
170 10 40 60 65 2.6 1.0 .times. 10.sup.-3 
6 1-2(36), 2-3(4) 
160 10 50 90 60 2.1 1.0 .times. 10.sup.-3 
7 1-2(36), 2-7(4) 
160 10 50 60 65 2.0 1.0 .times. 10.sup.-3 
8 1-9(36), 2-8(4) 
160 10 50 60 65 2.3 0.9 .times. 10.sup.-3 
9 1-11(36), 2-6(4) 
160 10 50 60 65 3.0 1.0 .times. 10.sup.-3 
10 1-2(36), 2-3(2), 
160 10 50 60 65 2.7 1.0 .times. 10.sup.-3 
3-1(2) 
__________________________________________________________________________ 
*Based on the amount of monomer 
COMATIVE EXAMPLE 1 
One hundred batteries were fabricated in the same manner as in Example 1 
except that untreated Julagard 2500 was used in place of the 
polymer-grafted film. 
The batteries thus obtained were subjected to the same charge-discharge 
test as that in Example 1. As a result, two of the batteries suffered 
internal short-circuiting. 
Further, some of the batteries that had undergone 100 cycles of charge and 
discharge were charged and then disassembled, and the lithium surfaces 
were examined with an electron microscope. Dendritic growth was observed. 
COMATIVE EXAMPLE 2 
A porous polypropylene film (Julagard 2500) having dimensions of 6 cm by 6 
cm was irradiated with electron beams at a dose of 20 Mrad. This film was 
immersed in the same monomer solution as that used in Example 1, and 
polymerization and washing were then conducted in the same manner as in 
Example 1. As a result, a film onto which a polymer had been grafted in an 
amount of 5 mg/cm.sup.2 was obtained. After being dried, this film was 
immersed in the same electrolyte solution as that used in Example 1. As a 
result, the film not only swelled in the direction of thickness (dry film 
thickness 32 .mu.m, swelled film thickness 61 .mu.m), but also swelled in 
the length and width directions by 10%. 
COMATIVE EXAMPLE 3 
30 parts by weight of compound 1-7 as specified hereinabove was mixed with 
70 parts by weight of a 1-mol/liter LiBF.sub.4 solution in propylene 
carbonate/di-methoxyethane=1/1 (by volume). Thereto was added 
2,2-methoxy-2-phenylacetophenone as a photosehsitizer in an amount of 0.2% 
by weight based on the amount of the monomer. A polypropylene nonwoven 
fabric was immersed in the thus-prepared solution and then irradiated with 
ultraviolet rays for 10 minutes using a 200-W UV lamp, thereby 
polymerizing the monomer. Batteries were fabricated using the resulting 
film and subjected to a charge-discharge test in the same manner as in 
Example 1. The batteries that had undergone 100 cycles of charge and 
discharge were charged and then disassembled in an argon atmosphere, and 
the lithium surfaces were examined with an electron microscope. Dendritic 
growth was observed. 
COMATIVE EXAMPLE 4 
Graft polymerization was conducted in the same manner as in Example 1 
except that 200 g of an aqueous solution containing 30 g of compound B 
(specified below) and 29 g of surfactant A used above was used as an 
aqueous monomer solution. 
##STR8## 
As a result, a film onto which a polymer had been grafted in an amount of 
1.8 mg/cm.sup.2 was obtained. This film had an ionic conductance of 
3.0.times.10.sup.-5 S/cm. Batteries were fabricated using this film and 
subjected to a charge-discharge test in the same manner as in Example 1 
except that a constant current of 0.1 mA was used. After the test, the 
lithium surfaces were examined and, as a result, dendritic growth was 
observed. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.