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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to an electrically conductive material
comprising a polymer of a compound having conjugated double bonds, which
is provided on a specific base material, and to a secondary battery using
this type of electrically conductive material.
It is known that polymers having conjugated double bonds in the main chain,
such as polyacetylene, poly-p-phenylene, polythienylene, polypyrrole,
polyaniline, and poly-p-phenylene-vinylene, are remarkably improved in
electric conductivity when they are treated with a P- or N-type doping
agent such as arsenic pentafluoride, antimony pentafluoride, iodine,
bromine, sulfur trioxide, n-butyllithium, or sodium naphthalene, whereby
they are changed from an insulator to a semiconductor or a conductor.
These electrically conductive materials (so-called "electrically
conductive polymers") are obtained in the form of powder, grain, bulk, or
film, which is used either as such or after molding thereof in accordance
with the purpose of use thereof. They are now under investigation as to
the application thereof to a wide variety of fields involving not only
functional elements such as an antistatic material, an electromagnetic
wave shielding material, a photoelectric conversion element, an optical
memory (holographic memory), and various sensors but also a display
element (electrochromism), a switch, various hybrid materials (transparent
conductive film, and the like), various terminal equipment, and a
secondary battery.
However, this type of electrically conductive polymer is generally poor in
moldability and processability. Particularly in order to obtain a film
form of an electrically conductive polymer, a special process must be
adopted. Today, known films of such electrically conductive polymers
include a polyacetylene film which is generally prepared by blowing an
acetylene gas against a glass wall coated with a polymerization catalyst
to form a film and peeling the film from the glass wall, and polypyrrole
and polythienylene films which are prepared by forming a film on an
electrolysis electrode according to an electrochemical oxidation reaction
(electrolytic oxidation polymerization) and peeling the film from the
electrode.
Among the above-mentioned conventional electrically conductive polymer
films, the polyacetylene film disadvantageously is so unstable in air as
to undergo progressive oxidative deterioration, and has a low mechanical
strenght, while the polypyrrole and polythienylene films and the like
obtained by the above-mentioned electrolytic oxidation polymerization
disadvantageously have their film size restricted by the size of the
electrolysis electrode, and involve complicated steps and a high cost.
Further, Journal of Electronic Materials, Vol. 13, No. 1, pp. 211-230
(1984) revealed an electrically conductive material prepared by immersing
a filter paper in 0.01M aqueous HCl containing FeCl.sub.3.6H.sub.2 O,
bringing the filter paper into contact with pyrrole vapor or immersing the
filter paper in a pyrrole solution to effect gas-phase or solution
polymerization of the pyrrole on the filter paper. It further revealed an
electrically conductive material prepared by bringing a pyrrole vapor into
contact with a filter paper after immersion thereof in a solution of
FeCl.sub.3.6H.sub.2 O--C.sub.2 H.sub.5 OH to effect gas-phase
polymerization of the pyrrole into polypyrrole on the filter paper.
However, the former, namely the electrically conductive material prepared
by the method involving immersion of a filter paper in 0.01M aqueous HCl
containing FeCl.sub.3.6H.sub.2 O, contains water and disadvantageously
undergoes drastic reduction in electric conductivity when dried. Thus,
this electrically conductive material can be used only in a wet state
(hydrous state). This presents a problem such that this material cannot be
used, for example, as the electrode material of a secondary battery of the
non-aqueous electrolytic solution system in reality. Further, in this
electrically conductive material, iron compounds used for the polypyrrole
formation remains as an impurity without being removed. The presence of
this impurity presents problems of providing low performance and limited
use and application of the electrically conductive material when it
remains as it is due to its low electrical conductivity. On the other
hand, the latter, namely the electrically conductive material prepared by
the method involving immersion in a solution of FeCl.sub.3.6H.sub.2
O--C.sub.2 H.sub.5 OH, has an electric conductivity as low as 1/1000 of
that of the above-mentioned material prepared by the method involving
immersion in 0.01M aqueous HCl containing FeCl.sub.3.6H.sub.2 O, thus
presenting a problem of being notably poor in performance as the
electrically conductive material.
On the other hand, there has recently been proposed a secondary battery
prepared by using an electrically conductive polymer as mentioned above as
the electrode material.
Although such an electrically conductive polymer usually has a slight
electric conductivity as described above, the electric conductivity
thereof can be dramatically increased by doping since it can be doped with
a dopant such as any one of various anions and cations, or can be undoped.
In constituting a secondary battery with such an electrically conductive
polymer as the electrode material, an electrically conductive polymer
capable of being doped with anions is used as the anode material, and/or
an electrically conductive material capable of being doped with cations is
used as the cathode material, while a solution containing a dopant as
mentioned above is used as the electrolytic solution. Thus, there can be
produced a secondary battery capable of charging and discharging via
electrochemically reversible doping and undoping.
Known electrically conductive polymers of the kind as described above
include the aforementioned polymers having conjugated double bonds in the
main chain, such as polyacetylene, poly-p-phenylene, polypyrrole,
polythienylene, polyaniline, and poly-p-phenylene-vinylene. In an instance
of polyacetylene, it is used as the electrode material for at least one of
the anode and the cathode, while anions such as BF.sub.4.sup.-,
ClO.sub.4.sup.-, SbF.sub.6.sup.- or PF.sub.6.sup.-, or cations such as
Li.sup.+, Na.sup.+ or R.sub.4 N.sup.+ (wherein R represents an alkyl
group) are employed to constitute an electrochemically reversible system
capable of doping and undoping.
These electrically conductive polymers are obtained in the form of powder,
grain, bulk, or film. In the case of using a powdery, grainy, or bulky
form of an electrically conductive polymer as the electrode material in
constituting a secondary battery with a non-aqueous electrolytic solution
or a solid electrolyte, there is needed a step of press-molding the
polymer into an electrode either as such or after addition of an adequate
electrically conductive material for improving the electric conductivity
and/or a thermoplastic resin for improving the mechanical strength of the
resulting electrode. In this respect, the use of a film form of an
electrically conductive polymer provides, for example, such a
characteristic feature that the film can be only punched with a
predetermined size into an electrode to considerably facilitate the
electrode production.
Known examples of such an electrically conductive polymer film include not
only polyacetylene, polypyrrole, and polythienylene films as described
before, but also composite electrically conductive films obtained by
coating a base material such as a PET film with a solution containing an
oxidizing agent and a polymer binder and bringing the resulting base
material into contact with a vapor of pyrrole, aniline, or the like to
form a layer of an electrically conductive polymer film on the base
material.
However, in the case of using a conventional electrically conductive
polymer film as mentioned above as a battery electrode material
constituting a secondary battery, a polyacetylene film quite
disadvantageously undergoes polymer deterioration due to slight amounts of
oxygen and water present in the battery, leading to a poor performance of
the electrode, and causes, for example, a rapid increase in charging
voltage and a decrease in charging and discharging efficiency during
cycles, leading to a shortened cycling life span. Further, there have
arisen problems such that the film is liable to be oxidized with oxygen
contained in a working atmosphere, leading to a difficult and complicated
production of electrodes, and that the preservability of electrodes is
poor due to grave deterioration of the materail by oxidation.
In the case of using a polythienylene or polypyrrole film prepared by the
electrochemical oxidation polymerization reaction, not only is the size of
the film restricted by the size of the electrolysis electrode, but also a
complicated production process and a need for a special production
apparatus are involved, thus leading to a high battery production cost.
Further, since a difficulty is encountered in obtaining a thick and
uniform film, combined use of this film as a battery electrode with a
collector involves such problems that the contact of the film with the
collector may become poor during charging and discharging cycles, and that
the battery reaction may occur concentrately in a portion of the
electrode, thus causing deterioration in battery performance.
In the case of using a composite electrically conductive film as mentioned
above, since the polymer binder is used in order to keep the oxidizing
agent on the base material, an electrically conductive polymer obtained by
the polymerization reaction is in the form of a compositie electric
conductive made of a mixture of a polymer of pyrrole or aniline with the
polymer binder. This decreases the concentration of the polymer of pyrrole
or aniline having an electric conductivity in the electrically conductive
polymer. Thus, when it is used as an electrode material, a problem of poor
performance arises due to the disadvantageous reduction in the effective
polymer concentration since the same performance as that in the case of
using, for example, a conventional electrically conductive polymer film as
mentioned above cannot be attained even is desired.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an electrically conductive
material comprising a specific base material and, polymerized thereon, a
compound having conjugated double bonds, which does not involve the
above-mentioned problems, which is stable in air, can be readily produced,
and has a high electric conductivity, and which can be rendered
electrically conductive, for example, in an arbitrary direction in an
arbitrary portion.
Another object of the present invention is to provide an electrically
conductive material comprising a specific base material and, polymerized
thereon, a compound having conjugated double bonds, which can be readily
rendered electrically conductive, for example, in one surface thereof.
A still further object of the present invention is to provide a secondary
battery prepared by using an electrically conductive material of the kind
as described above, which greatly facilitates the control of an electrode
preparation atmosphere since the control must not be so severe as compared
with that in the case where a polyacetylene film is used as the electrode
material, and which uses an electrode not only improved in itself in
preservability but also causing neither denaturing nor decomposition even
when exposed to oxygen and water present inside the battery or excessive
charging to avoid a rapid increase in voltage in the course of charging,
leading to improvements in charging efficiency and cycling life span.
A further object of the present invention is to provide a secondary battery
of the kind as described above, the cycling life span of which is improved
by providing better contact of an electrode with a collector.
A still further object of the present invention is to provide a secondary
battery of the kind as described above, the charging and discharging
characteristics of which is improved by improving the liquid containing
capacity of an electrode itself.
The above-mentioned objects have been attained by the electrically
conductive material of the present invention containing no substantial
amount of water which is prepared by polymerizing in the presence of an
oxidizing agent a compound having conjugated double bonds on a base
material having spaces capable of retaining the oxidizing agent in a gas
phase to form a polymer of the above-mentioned compound on the base
material.
The above-mentioned oxidizing agent is a compound having an activity of
polymerizing a monomer compound having conjugated double bonds. The
oxidizing agent may be used in the form of a single compound as described
above or in combination of two or more kinds of such compounds. Usually
used in a metallic salt containing a residue group of a strong acid, a
halogen or a cyano group, a peroxide, a nitrogen oxide, or the like.
Specific examples of such compounds include Fe(ClO.sub.4).sub.3,
Fe(BF.sub.4).sub.3, Fe.sub.2 (SiF.sub.6).sub.3, Cu(ClO.sub.4).sub.2,
Cu(BF.sub.4).sub.2, CuSiF.sub.6, FeCl.sub.3, CuCl.sub.2, K.sub.3
[Fe(CN).sub.6 ], RuCl.sub.3, MoCl.sub.5, WCl.sub.6. Those compounds can
also be used as they have water of crystallization or as they are obtained
in the form of a solution. In addition, other compounds such as
(NH.sub.4).sub.2 S.sub.2 O.sub.8, K.sub.2 S.sub.2 O.sub.8, Na.sub.2
S.sub.2 O.sub.8, NaBO.sub.3, H.sub.2 O.sub.2, NOBF.sub.4, NO.sub.2
BF.sub.4, NO.sub.2 PF.sub.6, NOClO.sub.4, NOAsF.sub.6, and NOPF.sub.6 can
be used.
A material having spaces capable of retaining the above-mentioned oxidizing
agent is used as the base material. Such spaces are satisfactory if only
their size is enough to retain the oxidizing agent at least in the form of
molecules or aggregates. It is not preferred that the spaces be too small
to retain the oxidizing agent in the form of molecules or too large to
retain the oxidizing agent in the form of aggregates. These spaces are
distributed in the form of micropores or voids having any of various
shapes on or inside the base material. In the case of micropores, the
average size thereof is specifically 0.001 to 100 .mu.m, preferably 0.005
to 50 .mu.m. It has been known that the depth of the micropores is 0.001
.mu.m or deeper, preferably 0.005 .mu.m or deeper.
The form of the base material having the above-mentioned characteristics is
specifically a porous material [powder, molding (plate molding or the
like), sheet, film, filament], woven fabric, non-woven fabric, a fibrous
material by more than two filaments, or the like.
The base material to be used may be either organic or inorganic. Usable
organic base materials include materials of polyolefin, polyvinyl halide,
polyfluorocarbon, polyester, polyamide, polyimide, polyacrylic,
polycarbonate, as well as their copolymer and mixture types. Usable
inorganic base materials include materials of carbon, metal, alloy, metal
oxide, metal carbide, metal nitride, and their mixture types. A base
material made of a mixture of organic and inorganic base materials may
also be used.
Specific examples of such organic base materials include resins which
contain no hydroxyl group, which is referred to as "hydrophobic resins"
hereinafter, such as polyethylene, polypropylene, ethylene-propylene
copolymers, polyvinyl chloride, polyvinylidene chloride, polyvinyl
fluoride, polytetrafluoroethylene, polyethylene terephthalate,
polybutylene terephthalate, polystyrene, polyamides, polyimides,
polyamide-imides, ethylene-vinyl acetate copolymers, polyacrylonitrile,
polymethacrylonitrile, polymethyl methacrylate, polybutyl methacrylate,
polystyrene-acrylonitrile, and polycarbonate. Specific examples of such
inorganic base materials include such materials as active carbon, carbon
black, graphite, chromium, titanium, nickel, gold, platinum, tantalum,
copper, silver, iron, stainless steel, alumina, silica, silica-alumina,
zirconia, beryllium oxide, potassium titanate, silicon carbide, boron
carbide, titanium carbide, molybdenum carbide, tantalum carbide, boron
nitride, silicon nitride, and niobium nitride.
Pyrrole and thiophene compounds can be used as the compound having
conjugated double bonds to be use in the present invention. They may be
used alone or in mixture. Preferred examples are pyrrole and thiophene
compounds having no substituents in the 2- and 5-positions of the skeletal
structure of a pyrrole or thiphene ring. Specific examples of pyrrole
compounds include pyrrole, N-methylpyrrole, N-ethylpyrrole,
N-n-propylpyrrole, N-n-butylpyrrole, N-phenylpyrrole, N-tolylpyrrole,
N-naphthylpyrrole, 3-methylpyrrole, 3,5-dimethylpyrrole, 3-ethylpyrrole,
3-n-propylpyrrole, 3-n-butylpyrrole, 3-phenylpyrrole, 3-tolylpyrrole,
3-naphthylpyrrole, 3-methoxypyrrole, 3,5-dimethoxypyrrole,
3-ethoxypyrrole, 3-n-propyxypyrrole, 3-phenoxypyrrole,
3-methyl-N-methylpyrrole, 3-methoxy-N-methylpyrrole, 3-chloropyrrole,
3-bromopyrrole, 3-methylthiopyrrole, and 3-methylthio-N-methylpyrrole.
Specific examples of thiophene compounds include 2,2'-bithiophene,
3-methyl-2,2'-bithiophene, 3,3'-dimethyl-2,2'-bithiophene,
3,4-dimethyl-2,2'-bithiophene,
3,4-dimethyl-3',4'-dimethyl-2,2'-bithiophene, 3-methoxy-2,2'-bithiophene,
3,3'-dimethoxy-2,2'-bithiophene, 2,2',5',2"-terthiophene,
3-methyl-2,2',5',2"-thiophene, and 3,3'-dimethyl-2,2',5',2"-terthiophene.
The method of retaining the oxidizing agent on the base material may
comprise the step of contacting the base material with the oxidizing agent
itself or a dispersion or solution of the oxidizing agent in an adequate
solvent to make the base material retain the oxidizing agent thereon. In
order to facilitate retention of the oxidizing agent on the base material,
the base material may be preliminarily subjected to an arbitrary treatment
such as washing, degassing, rendering hydrophilic, or rendering
hydrophobic. The oxidizing agent may be retained on all or a predetermined
portion of the base material according to the need. For example, when the
oxidizing agent is retained only on one surface of the base material, the
polymer of the compound having conjugated double bonds is formed only on
one surface portion to obtain an electrically conductive material having
only one surface thereof made electrically conductive. As another example,
when the oxidizing agent is retained in the form of a continuous line in a
given direction on the base material, the polymer of the compound having
conjugated double bonds is formed in the form of a continuous line only on
that portion of the base material to obtain an anisotropic electrically
conductive material having an electric conductivity only in a given
direction. In such a way, electrical conductivity can be imparted to the
base material only on an arbitrary portion only in an arbitrary direction.
Thus, the electrically conductive material of the present invention can
also be utilized as a material for forming an electrically conductive
circuit.
Although the molar ratio of the oxidizing agent to the compound having
conjugated double bonds is associated with the amount of the polymer to be
formed, it is usually 0.001 to 10,000/1, preferably 0.005 to 5,000/1.
The polymer of the compound having conjugated double bonds on the base
material is formed in a gas phase. Specifically, gas phase polymer
formation is effected in the sole presence of a vapor of the compound
having conjugated double bonds or in the conjoint presence of such a vapor
with nitrogen, argon, air, other gas, or a mixture thereof. Although the
whole system may be under high, ordinary, or reduced pressure, the
ordinary pressure is preferred from the viewpoint of process control or
the like.
The reaction temperature is not particularly limited, provided that the
compound having conjugated double bonds can be polymerized at that
temperature. It is usually -20.degree. to 100.degree. C., preferably
0.degree. to 80.degree. C. Although the reaction time is dependent on the
reaction temperature, the amount of the oxidizing agent, the amount of the
compound having conjugated double bonds, etc., it is usually 0.01 to 200
hours, preferably 0.02 to 100 hours. After polymerization, a homogeneous,
dark brown to black polymer appears on a portion of the base material
where the oxidizing agent is retained.
An oxidizing agent may be further retained on the formed polymer as
mentioned above, and the polymerization reaction may be continued while
contacting the oxidizing agent with the same or another kind of compound
having conjugated double bonds, whereby an increase in the amount of the
polymer formed, or formation of two or more kinds of polymers can be
attained.
After completion of the polymerization reaction, the compound having
conjugated double bonds and the oxidizing agent which remains on the base
material are removed. They can be usually removed by immersing the base
material in an alcohol or other organic solvent to effect washing. The
washing in the present invention which is important for completely
removing the oxidizing agent to improve the electrical conductivity is
carried out, depending upon the kinds of the oxidizing agent, as set forth
below: when a metallic salt containing residual group of strong acids or
halogen or cyano group is used as an oxidizing agent, washing is carried
out with organic solvent, and then with alcohol repeatedly; and when a
peroxide or a nitrogen oxide is used as an oxidizing agent, washing is
carried out with water at first to resolve the oxidizing agent and then
with organic solvent and alcohol, repeatedly. By washing, a remaining
amount of the oxidizing agent relative to the amount of the formed polymer
must be held 1% or less, and preferably less than 0.5%, otherwise an
electrically conductive material having high electrical conductivity for a
practical application is not obtained. Thereafter, the base material may
be dried by a conventional drying method to obtain an electrically
conductive material.
In the present invention, a water content of the obtained electrically
conductive material is less than 1.0% and, preferably, less than 0.5%.
As the above-mentioned base material, there may be used a material having
spaces capable of retaining the oxidizing agent in the above-mentioned way
and showing hydrophobicity at least on one surface thereof. When the
oxidizing agent is to be retained, for example, only on one surface of a
base material having no such hydrophobicity for forming an electrically
conductive polymer layer only on one surface of the base material, the
oxidizing agent may permeate up to the other surface of the base material
because the above-mentioned base material is liable to allow the oxidizing
agent to permeate into all the spaces and be retained there. Thus, an
electrically conductive polymer may be formed in every portion of the base
material by the gas-phase polymerization. In view of this, great care must
be taken in forming an electrically conductive polymer only on one surface
of the base material. This may entail a very complicated procedure in
manufacturing.
When the above-mentioned hydrophobic base material is used, an electrically
conductive polymer layer can be easily provided on either one of the
surfaces of an electrically conductive polymer film. This provides an
advantage in industrial manufacturing, whereby the industrial application
of the electrically conductive polymer film can be expected to further
spread in the fields of a planar heating element, a laminated functional
material for photoelectric conversion, a collector, a separator-integrated
electrode material, a collector-integrated electrode material, and the
like.
The above-mentioned hydrophobicity is 90.degree. C. or more in terms of
contact angle with water. In this case, the oxidizing agent is used in the
form of an aqueous solution, so that retention of the oxidizing agent on
either one of the surfaces of the base material can be readily
materialized. This can be easily understood from the fact that, when a
solvent capable of permeating into the base material, such as methanol,
ethanol, acetonitrile, or tetrahydrofuran, is used as the solvent of the
oxidizing agent, a difficulty is encountered in retaining the oxidizing
agent only on either one of the surfaces of the base material because the
oxidizing agent readily permeates into or up to the hydrophobic surface
even if one surface of the base material is hydrophobic. Where the sheet
base material has, for example, one hydrophobic surface and the other
hydrophilic surface, the oxidizing agent can be easily retained on the
hydrophilic surface by immersing the base material in an aqueous solution
of the oxidizing agent, or coating the hydrophilic surface with an aqueous
solution of the oxidizing agent. Where the sheet base material is
hydrophobic on both of the surfaces, retention of the oxidizing agent on
one surface of the base material can be materialized by repeatedly coating
the one surface with an aqueous solution of the oxidizing agent, or by
treating the one surface with a hydrophilicity-imparting agent such as
polyethylene oxide or polyvinyl alcohol and subsequently immersing the
base material in an aqueous solution of the oxidizing agent or coating the
hydrophilicity-imparted surface with the oxidizing agent. Where the sheet
base material is hydrophilic on both of the surfaces, one surface is
treated with, for example, a silicone or fluorocarbon water repellant to
make the surface hydrophobic, followed by immersion in an aqueous solution
of the oxidizing agent or coating of the hydrophilic surface with an
aqueous solution of the oxidizing agent, whereby retention of the
oxidizing agent on the hydrophilic surface can be materialized.
The secondary battery of the present invention, which can attain the
aforementioned objects, is prepared by using as at least one electrode of
the anode and cathode thereof a film of an electrically conductive
material of the kind as discussed above, which is prepared by polymerizing
in the presence of an oxidizing agent a compound having conjugated double
bonds on a base film material having spaces capable of retaining the
oxidizing agent in a gas phase to form a polymer of the compound having
conjugated double bonds in the above-mentioned spaces. Such an
electrically conductive film material preferably contains no substantial
water just like the above-mentioned electrically conductive material.
Where such an electrically conductive film material is used as an electrode
of a secondary battery, an electrode which can be easily produced at a
relatively low cost and which has a uniform thickness even when the
thickness thereof is large can be materialized.
When a highly conductive inorganic base material (in the form of plate,
gauze, or the like) made of a metal such as gold, platinum, stainless
steel or steel, or a carbonaceous material such as active carbon, carbon
black or graphite is used as the base material which is designed also to
serve as the collector, the contact of the polymer of the compound having
conjugated double bonds as the electrode material with the base material
as the collector can be remarkably increased. This leads to an improvement
in the cycling life span of the battery. When a porous film like a
polyethylene film is used as the base material, the liquid containing
capacity (electrolyte containing capacity) of the electrode itself can be
remarkably improved, thus providing advantages such as an improvement in
the charging and discharging efficiency.
The use of the base material also as the separator or the collector as
described above allows the battery assembling process to be greatly
simplified, since the steps of battery assembling including those of
separately preparing a separator or a collector in the battery assembling,
and disposing it between two electrodes in close contact therebetween or
between an electrode and a battery case in close contact therebetween can
be dispensed with.
In the secondary battery of the present invention, there is an embodiment
wherein electrodes made of the above-mentioned electrically conductive
material are used as the anode and cathode, and an embodiment wherein an
electrode made of the above-mentioned electrically conductive material is
used as one of the two electrodes while the other electrode uses an
electrode material selected from among metals, metallic oxides, other
inorganic compounds, known electrically conductive polymers and organic
compounds other than the reaction product used in the present invention,
and organometallic compounds. As an example, in the embodiment wherein the
above-mentioned electrically conductive material is used only in the anode
while a metal is used as the electrode material of the cathode, the metal
constituting the cathode has preferably an electronegativity of 1.6 or
less. Examples of metals having such an electronegativity include Li, Na,
K, Mg, Al, and alloys thereof. Particularly preferred are Li and its
alloys.
Where the present invention is applied in a secondary battery of a
non-aqueous electrolyte type, a solution of an electrolyte in an organic
solvent is used as the electrolytic solution. Examples of such an
electrolyte include cations of metals having an electronegativity of 1.6
or less, organic cations, and salts thereof with anions. Examples of onium
ions include quaternary ammonium ions, carbonium ions, and oxonium ions.
Examples of anions include BF.sub.4.sup.-, ClO.sub.4.sup.-,
PF.sub.6.sup.-, AsF.sub.6.sup.-, CF.sub.3 SO.sub.3.sup.-, I.sup.-,
Br.sup.-, Cl.sup.-, and F.sup.-. Specific examples of the electrolyte
include lithium tetrafluoroborate (LiBF.sub.4), lithium perchlorate
(LiClO.sub.4), lithium hexafluorophosphate (LiPF.sub.6), lithium
tetrachloroaluminate (LiAlCl.sub.4), tetraethylammonium tetrafluoroborate
[(C.sub.2 H.sub.5).sub.4 NBF.sub.4 ]tetraethylammonium perchlorate
[(C.sub.2 H.sub.5).sub.4 NClO.sub.4 ], lithium trifluoromethanesulfonate
(LiCF.sub.3 SO.sub.3), lithium iodide (LiI), and lithium bromide (LiBr),
to which the electrolyte is, however, not limited. When, for example, a
battery, wherein the electrically conductive material according to the
present invention is used in the anode and the cathode while an
electrolytic solution of LiBF.sub.4 as the electrolyte disolved therein is
used, is in the process of being charged, the electrically conductive
material in the anode is doped with BF.sub.4.sup.- in the electrolytic
solution, while that in the cathode is doped with Li.sup.+ in the
electrolytic solution. In contrast, when the battery is in the process of
being discharged, BF.sub.4.sup.- and Li.sup.+ doped in the anode and the
cathode, respectively, are released into the electrolytic solution.
An organic aprotic solvent having a high dielectric constant is preferably
used as the solvent for dissolving therein the electrolyte. Such an
organic solvent includes nitriles, carbonates, ethers, nitro compounds,
amides, sulfur containing compounds, chlorinated hydrocarbons, ketones,
esters, and so on. They may be used alone or in mixture. Representative
examples of such an organic solvent include acetonitrile, propionitrile,
butyronitrile, benzonitrile, propylene carbonate, ethylene carbonate,
tetrahydrofuran, dioxolane, 1,4-dioxane, nitromethane,
N-N-dimethylformamide, dimethyl sulfoxide, sulfolane, 1,2-dichloroethane,
.gamma.-butyrolactone, 1,2-dimethoxyethane, methyl phosphate, and ethyl
phosphate, to which the solvent is, however, not limited.
The concentration of the electrolytic solution used in the present
invention is usually 0.001 to 10 mol/1, preferably 0.1 to 3 mol/1.
The electrolytic solution may be used either by pouring it or by
incorporating it into an electrode using the electrically conductive
material according to the present invention.
In the present invention, a solid electrolyte may be used instead of the
above-mentioned electrolytic solution of the electrolyte. Examples of such
a solid electrolyte include electrically conductive solid electrolytes
based on lithium, such as LiI, LiI-Al.sub.2 O.sub.3, Li.sub.3 N, and
LiSICON; glasses of a lithium ion conduction type, such as Li.sub.2
S-P.sub.2 S.sub.5 -LiI; electric conductors of a lithium ion conduction
type having a structure of a .gamma..sub.II -Li.sub.3 PO.sub.4 type, such
as Li.sub.4 SiO.sub.4 -Li.sub.3 PO.sub.4 ; polyelectrolytes of a lithium
ion conduction type, such as polyethylene oxide-LiClO.sub.4, and their
mixtures with an additive.
Although the foregoing description has been given for the method of forming
an electrode without any doping treatment of an electrically conductive
material, the electrically conductive material may be preliminarily doped
with a dopant before use thereof as an electrode.
In the present invention, electrodes may be covered with drainboard-like or
porous glass, Teflon, polyethylene, plate, or the like in order to fix the
electrodes in an electrolyte.
In the battery of the present invention, a filter paper of a glass fiber,
or a porous film of Teflon, polyethylene, polypropylene, or nylon may be
used as the separator.
Further, as at least one of the anode and the cathode, there may be used an
electrically conductive material having an electric conductivity on one
surface thereof according to the present invention, which is prepared by
treating with an oxidizing agent a base material having spaces capable of
retaining the oxidizing agent and having only one hydrophobic surface to
allow the oxidizing agent to be retained only on the one surface, and
polymerizing a compound having conjugated double bonds on the base
material in a gas phase to form a polymer of the above-mentioned compound
only on the one surface of the base material.
Where an electrically conductive material comprising an inorganic base
material is used with the base material also serving as the collector, a
metallic foam having a porosity of 70 to 98% and containing an
electrically conductive polymer formed in the cell spaces of the foam may
be used as the base material.
The use of an electrode made of the metallic foam containing the
electrically conductive polymer formed in the cell spaces of the foam by
gas-phase polymerization as metnioned above is advantageous in that the
electrode has a uniform thickness easily attained due to large polymer
retentivity in cell spaces of the foam even when the amount of the formed
polymer is increased in order to increase the capacity. Furthermore, since
the polymer is formed and retained up to the inside of the micropores of
the foam, the polymer in the electrode never peels or scales off from the
metallic foam as the base material by mechanical shock or the like. Thus,
the mechanical strength of the electrode is remarkably improved as
compared with that of an electrode prepared by press-bonding to a
collector a polymer film formed by the conventional electrolytic
polymerization and peeled off from an electrolytic electrode.
Usable materials of the above-mentioned metallic foam include gold,
platinum, silver, copper, nickel, stainless steel, nickel-aluminum alloys,
nickel-chromium alloys, copper-nickel alloys, and nickel-chromium-aluminum
alloys. When the above-mentioned electrically conductive material
comprising the metallic foam with a porosity of 70 to 90% as the base
material is used as a collector-integrated type electrode, the bond of the
electrically conductive polymer to the collector is improved, leading to a
prolonged cycling life span of the resulting battery. Moreover, since the
metallic foam as the base material of the electrode is porous, the liquid
containing capacity of the electrode itself is improved, contributing to
betterment in the charging and discharging efficiency of the electrode.
The reasons why the porosity of the metallic foam is set within a range of
70 to 98% as mentioned above are as follows. When the porosity is less
than 70%, the specific area of the metallic foam (the ratio of the surface
area to the volume in the metallic foam) is too smal | | |
