U.S. patent application number 12/369634 was filed with the patent office on 2009-06-18 for electrode material for a polarized electrode.
Invention is credited to Junji Ito.
Application Number | 20090152510 12/369634 |
Document ID | / |
Family ID | 27654355 |
Filed Date | 2009-06-18 |
United States Patent
Application |
20090152510 |
Kind Code |
A1 |
Ito; Junji |
June 18, 2009 |
ELECTRODE MATERIAL FOR A POLARIZED ELECTRODE
Abstract
The present invention provides polarized electrodes that ensure
a satisfactory packing ratio of activated carbon particles and
conductive particulates and favorable contact between these and an
electrolyte, and demonstrate high capacitance. The present
invention relates to the following electrode materials for
polarized electrodes: (1) electrode materials for polarized
electrodes containing a rubber-based emulsion, activated carbon
particles, conductive particulates, and a surfactant; and (2)
electrode materials for polarized electrodes containing a
dispersion of polytetrafluoroethylene, activated carbon particles,
conductive particulates, and a surfactant, wherein the solid
content is 25% by weight or more.
Inventors: |
Ito; Junji; (Komaki-Shi,
JP) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET, FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
27654355 |
Appl. No.: |
12/369634 |
Filed: |
February 11, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10502905 |
Dec 13, 2004 |
7491352 |
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PCT/JP03/00781 |
Jan 28, 2003 |
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12369634 |
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Current U.S.
Class: |
252/511 |
Current CPC
Class: |
H01G 11/38 20130101;
H01G 11/42 20130101; Y02E 60/13 20130101 |
Class at
Publication: |
252/511 |
International
Class: |
H01B 1/20 20060101
H01B001/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 29, 2002 |
JP |
2002-20454 |
Claims
1. An electrode material for a polarized electrode, the electrode
material comprising: a material prepared by: a first step of
dispersing conductive particles and a surfactant in water; a
subsequent second step of dispersing activated carbon particles,
different from the conductive particles, in the dispersion obtained
in the first step; and a third step of mixing a binder comprising
rubber containing emulsion or a dispersion of
polytetrafluoroethylene in the dispersion obtained in the second
step.
2. A polarized electrode obtained by forming an electrode layer on
a conductive substrate using the electrode material for a polarized
electrode of claim 1.
3. An electrode material for a polarized electrode according to
claim 1, containing a dispersion of polytetrafluoroethylene,
activated carbon particles, conductive particulates, and a
surfactant, wherein the solid content is 25% by weight or more.
4. An electrode material for a polarized electrode according to
claim 1, wherein a rubber component of the rubber-based emulsion is
at least one member selected from the group consisting of butyl
rubber, natural rubber, butadiene rubber, isoprene rubber, nitrile
rubber, acrylic rubber, chloroprene rubber, ethylene-propylenediene
rubber, styrene-butadiene rubber, and carboxy styrene-butadiene
rubber.
Description
TECHNICAL FIELD
[0001] The present invention relates to electrode materials for
preparing polarized electrodes (hereinafter, simply referred to as
"electrode materials"), methods for preparing the same, and
polarized electrodes using the electrode materials.
BACKGROUND OF THE INVENTION
[0002] Conventionally, carbon electrodes containing activated
carbon particles are used as polarized electrodes for electric
double layer capacitors. The carbon electrodes are generally
manufactured by applying electrode materials containing activated
carbon particles onto a conductive substrate, and drying the
laminate. A currently known method for manufacturing polarized
electrodes includes applying onto a conductive substrate electrode
materials obtained by dispersing activated carbon particles,
conductive particulates, etc. in a dispersion of
polytetrafluoroethylene (referred to as "PTFE", hereinafter) as a
binder; and drying the laminate.
[0003] The capacitance of an electric double layer capacitor varies
depending on the area (effective area) where the activated carbon
particles and the conductive particulates contained in the
polarized electrodes contact with the electrolyte. More
specifically, the capacitance is closely related to the packing
ratio of the activated carbon particles and conductive particulates
in the polarized electrodes. Therefore, to increase the
capacitance, the packing ratio of the activated carbon particles
and conductive particulates needs to be improved.
[0004] In a slurry method which includes dispersing activated
carbon particles, conductive particulates, a binder, etc. in a
dispersion medium to prepare slurry-like electrode materials; and
forming an electrode layer of the polarized electrode using the
same, the capacitance of a polarized electrode is generally
proportional to the solid content of the electrode materials.
Therefore, to increase the capacitance of the polarized electrode,
electrode materials with high solid content need to be prepared in
the slurry method.
[0005] In general, however, the electrode materials with high solid
content cannot exhibit favorable application properties in many
cases since such electrode materials are thickened due to a high
degree of interaction between the binder and the solid content,
such as activated carbon particles, conductive particulates, etc.
For example, according to the conventional slurry method, when the
solid content is 25% by weight or more, the viscosity of the
electrode materials is about 6,000 cP (the viscosity being measured
under atmospheric conditions at 25.degree. C. with a Brookfield
viscometer). Thus, a high viscosity makes it difficult to
demonstrate good application properties.
[0006] Therefore, in the slurry method, the solid content of the
electrode materials in view of ease of preparation, ease of
applying, etc. is preferably adjusted to be about 20% by
weight.
[0007] In recent years, lightweight and miniaturized polarized
electrodes have been demanded, and thus the development of high
capacity polarized electrodes for lightweight and miniaturized
designs is called for. Thus, it is desired to produce practical
polarized electrodes using electrode materials with a solid content
of about 30% by weight. The development of electrode materials with
low viscosity and excellent application properties (e.g., slurry
flowability) even when the solid content thereof is 30% by weight
or more needs to be realized.
DISCLOSURE OF THE INVENTION
[0008] A main object of the present invention is to provide
polarized electrodes that ensure a favorable packing ratio of the
activated carbon particles and conductive particulates to be
contained in the polarized electrodes and satisfactory contact
between these and the electrolyte, and demonstrate excellent
capacitance.
[0009] The inventor carried out intensive research in view of the
above-described problems of the prior art and as a result found
that the above object can be achieved by preparing
specifically-structured electrode materials using particular
materials in a specific procedure to accomplish the present
invention.
[0010] The present invention relates to the following electrode
materials, methods for preparing the same, and polarized electrodes
using the electrode materials.
[0011] 1. An electrode material for a polarized electrode
containing a rubber-based emulsion, activated carbon particles,
conductive particulates, and a surfactant.
[0012] 2. An electrode material for a polarized electrode
containing a dispersion of polytetrafluoroethylene, activated
carbon particles, conductive particulates, and a surfactant,
wherein the solid content is 25% by weight or more.
[0013] 3. An electrode material for a polarized electrode according
to Item 1, wherein a rubber component of the rubber-based emulsion
is at least one member selected from the group consisting of butyl
rubber, natural rubber, butadiene rubber, isoprene rubber, nitrile
rubber, acrylic rubber, chloroprene rubber, ethylene-propylenediene
rubber, styrene-butadiene rubber, and carboxy styrene-butadiene
rubber.
[0014] 4. A method for preparing an electrode material for a
polarized electrode, the method comprising:
[0015] a first step of dispersing conductive particles and a
surfactant in water;
[0016] a second step of dispersing activated carbon particles in
the dispersion obtained in the first step; and
[0017] a third step of mixing a rubber-based emulsion or a
dispersion of polytetrafluoroethylene in the dispersion obtained in
the second step.
[0018] 5. A preparation method according to Item 4, wherein at
least two types of activated carbon particles with different mean
particle diameters are used.
[0019] 6. A preparation method according to Item 5, wherein the at
least two types of activated carbon particles are dispersed in the
dispersion in the second step in the order such that the type of
activated carbon particles with the smallest mean particle diameter
is dispersed first, and then a) the other type of activated carbon
particles is dispersed or b) the other types of activated carbon
particles are dispersed in order of their increasing mean particle
diameter.
[0020] 7. An electrode material for a polarized electrode prepared
by a method according to Item 4.
[0021] 8. A polarized electrode obtained by forming an electrode
layer on a conductive substrate using an electrode material for a
polarized electrode according to any one of Items 1 to 3 and 7.
[0022] Hereinafter, electrode materials, preparation methods
thereof, and polarized electrodes of the present invention will be
described in detail.
Electrode Materials of the Invention
[0023] Electrode materials of the invention include the following
two types of electrode material:
[0024] (1) electrode materials of the first aspect of the invention
containing a rubber-based emulsion, activated carbon particles,
conductive particulates, and a surfactant; and
[0025] (2) electrode materials of the second aspect of the
invention containing a dispersion of PTFE, activated carbon
particles, conductive particulates, and a surfactant, wherein the
solid content is 25% by weight or more.
[0026] The rubber-based emulsion contained in the electrode
materials of the first aspect of the invention and the PTFE
dispersion contained in the electrode materials of the second
aspect of the invention serve as binders.
[0027] Hereinafter, ingredients constituting the electrode material
of the invention will be described.
Binder
[0028] A rubber-based emulsion (the first aspect of the invention)
or a PTFE dispersion (the second aspect of the invention) is used
as a binder for the electrode materials of the present
invention.
[0029] Any rubber-based emulsion can be used without limitation as
long as it functions as a binder. Preferably, for example, a
rubber-based emulsion is obtained by dispersing water in a known
rubber solution and then emulsifying the dispersion.
[0030] Rubber components contained in the rubber-based emulsion are
not limited, and can be selected from known rubbers according to
the desired characteristics of the electrode materials. Examples of
rubbers include, for example, butyl rubber, natural rubber,
butadiene rubber, isoprene rubber, nitrile rubber, acrylic rubber,
chloroprene rubber, ethylene-propylenediene rubber,
styrene-butadiene rubber, carboxy styrene-butadiene rubber, etc.
Such rubbers can be used singly or in combination.
[0031] Due to the combination of the electrolyte with which a
polarized electrode contacts, certain types of rubber may expand on
contact with the electrolyte, resulting in reduced rubber porosity.
Therefore, it is preferable to select rubbers with a swelling
resistance against the electrolyte (i.e., solubility resistance
against the electrolyte) considering the type of electrolyte to be
used. For example, butyl rubber, ethylene propylene diene rubber,
etc., which have excellent solubility resistance against propylene
carbonate, are preferably selected when an electrolyte containing
propylene carbonate is used.
[0032] In general, the rubber-based emulsion can be prepared by
dissolving rubber in a suitable solvent, dispersing water in the
obtained rubber solution, and emulsifying the dispersion. Solvents
can be appropriately selected according to the type of rubber
without limitation as long as rubber components can be dissolved.
For example, toluene is suitably used for butyl rubber, ethylene
propylene diene rubber, etc. Cyclohexane is suitable for natural
rubber, nitrile rubber, etc.
[0033] The amount of solvent can be suitably adjusted according to
the rubber type within the range that the rubber components can be
sufficiently dissolved. The amount of water to be dispersed in the
rubber solution is not limited, and can be appropriately adjusted
depending on the desired characteristics of the rubber-based
emulsion.
[0034] For emulsification, a kneader, mixer, homogenizer, etc.
(hereinafter, referred to as "a known stirrer") can be used. The
solid-content (amount of rubber) of the rubber-based emulsion is
not limited, and is generally about 30 to about 80% by weight, and
preferably about 40 to about 60% by weight.
[0035] Any PTFE dispersion can be used without limitation as long
as it functions as a binder, and known PTFE dispersions can be
used. The present invention preferably employs a dispersion
obtained by mixing about 150 to about 250 parts by weight of water,
and preferably about 180 to about 220 parts by weight of water,
with 100 parts by weight of PTFE, and stirring the mixture to form
a dispersion. The mean particle diameter of PTFE is not limited,
but is preferably about 0.2 to about 0.3 .mu.m. A known stirrer can
be used for stirring PTFE and water.
[0036] The content of the binder (rubber-based emulsion or PTFE
dispersion) in the electrode materials is not limited, and is
generally adjusted so that the rubber or PTFE amount
(solid-content) is about 4 to about 10% by weight, and preferably
about 5 to about 7% by weight, per 100% by weight of the electrode
material.
Activated Carbon Particles
[0037] Any activated carbon particles can be used without
limitation, and known activated carbon particles for use in carbon
electrodes, etc. can be used. Activated carbon particles obtained
by activating coconut husks, wood flour, petroleum pitch, phenol
resin, etc. using vapors, various agents, alkalis, etc. can be
mentioned, for example. Such kinds of activated carbon particles
can be used singly or in combination.
[0038] The mean particle diameter of the activated carbon particles
is not limited, but is usually about 0.5 to about 50 .mu.m, and
preferably about 2 to about 30 .mu.m.
[0039] The content of activated carbon particles in the electrode
material is not limited and is usually about 70 to about 90% by
weight, and preferably about 85 to about 90% by weight, per 100% by
weight of the electrode material.
Conductive Particulates
[0040] Any conductive particulates can be used without limitation,
and usable is at least one member selected from the group, used as
conductive particulates in carbon electrodes, etc., consisting of
known carbon-based particles, conductive polymer particles, and
metal particles.
[0041] Examples of conductive particulates include, for example,
carbon-based particles, such as carbon black (e.g., acetylene
black, acetylene black containing boron in a molecule, ketjenblack,
etc.), graphite, mesophase carbon, carbon nanotubes, etc.;
conductive polymer particles, such as polyacetylene,
polyparaphenylene, polyphenylene vinylene, polypyrrole,
polyaniline, etc.; metal particles, such as aluminium, tantalum,
titanium, nickel, vanadium oxide, ruthenium oxide, titanium
nitride, potassium manganate, cobalt acid lithium, etc. Such kinds
of conductive particulates can be used singly or in
combination.
[0042] The mean particle diameter of the conductive particulates
can be appropriately determined without limitation within the range
of 0.01 to 20 .mu.m according to the type of conductive
particulates as follows: the carbon-based particles are preferably
adjusted to about 0.5 to about 20 .mu.m, and more preferably about
1 to about 15 .mu.m; the conductive polymer particles are
preferably adjusted to about 0.05 to about 20 .mu.m, and more
preferably about 0.1 to about 3 .mu.m; and the metal particles are
preferably adjusted to about 0.01 to about 10 .mu.m, and more
preferably about 0.05 to about 1 .mu.m.
[0043] The content of conductive particulates in the electrode
materials is not limited and is generally about 5 to about 10% by
weight, and preferably about 6 to about 8% by weight, per 100% by
weight of the electrode material.
Surfactant
[0044] The surfactant used may be selected from known surfactants,
including anionic surfactants, cationic surfactants, amphoteric
surfactants, and nonionic surfactants.
[0045] Examples of anionic surfactants include fatty acid sodium
salts, fatty acid potassium salts, sodium alkylbenzenesulfonates,
sodium alkylsulfonates, carboxymethylcellulose (CMC), CMC ammonium,
triethanolamine, ammonium alkylbenzenesulfonates, sodium
alkylsulfonates, etc.
[0046] Examples of cationic surfactants include alkyltrimethyl
ammonium chlorides, alkyldimethylbenzyl ammonium chlorides,
etc.
[0047] Examples of amphoteric surfactants include alkyl betaines,
amide betaine, etc.
[0048] Examples of nonionic surfactants include sucrose fatty acid
esters, polyoxyethylene alkyl ethers, polyvinyl alcohol, polyvinyl
pyrrolidone, etc.
[0049] Among the above-mentioned surfactants, polyvinyl alcohol,
polyvinyl pyrrolidone, and carboxymethylcellulose may be preferably
used in the present invention.
[0050] In general, the surfactant is appropriately selected from
the above depending on the type of charge on the surface of the
conductive particulates contained in the electrode materials as
follows: anionic surfactants are preferable when the charge of the
surface of conductive particulates is acidic (negative); and
cationic surfactants are preferred when the surface charge is
alkaline (positive).
[0051] The content of the surfactant in the electrode materials is
not limited, and is generally about 0.1 to about 5% by weight, and
more preferably about 0.5 to about 2% by weight per 100% by weight
of the electrode material.
Other Ingredients (Additives, Etc.)
[0052] The electrode material of the invention may contain water in
addition to the above-mentioned essential ingredients. Various
additives may be contained within the range that does not adversely
affect the effects of the electrode materials of the invention.
[0053] Distilled water is preferably used as water. Water contained
in the invention may be distinguished as water as a binder
component, and water which can be added as needed so as to adjust
the solid-content (i.e., viscosity) of the electrode materials,
etc.
[0054] Additives can be used without limitation, and, for example,
fillers, adhesion enhancement agents, rubber antidegradants, rubber
crosslinking agents, rubber crosslinking accelerators, etc. can be
mentioned.
[0055] Examples of fillers include silica, zinc oxide, magnesium
oxide, magnesium carbonate, wollastonite, diatomaceous earth, mica,
calcium carbonate, aluminium hydroxide, alumina, magnesium
hydroxide, clay, talc, wood flour, pulp, etc. Fillers are added
generally in an amount of about 0.5 to about 5 parts by weight per
100 parts by weight of the rubber (solid content) contained in the
electrode materials.
[0056] Examples of adhesion enhancement agents include stearic
acid, polyacrylic acid, vinyl acetate, polyvinyl alcohol, etc. The
adhesion enhancement agents are usually added in an amount of about
1 part by weight per 100 parts by weight of the rubber
(solid-content) contained in the electrode materials.
[0057] Examples of rubber antidegradants include
2,6-di-t-butyl-4-methylphenol (BHT),
2,2'-methylenebis-(4-methyl-6-t-butylphenol) (MBMBP), etc. The
rubber antidegradants are usually added in an amount of about 0.5
parts by weight per 100 parts by weight of the rubber
(solid-content) contained in the electrode materials.
[0058] Examples of rubber crosslinking agents include resin
crosslinking agents, sulfur crosslinking agents, oxime crosslinking
agents, metal crosslinking agents, etc. The rubber crosslinking
agents are usually added in an amount of about 1 to about 2-parts
by weight per 100 parts by weight of the rubber (solid-content)
contained in the electrode materials.
[0059] Examples of rubber crosslinking accelerators include
N,N-diphenylguanidine (DPG), tetrabutylthiuram disulfide (TBTD),
tetramethylthiuramdisulfide (TMTD),
tetramethylthioperoxydicarboxylic acid diamide (TMTM),
dipentamethylenethiuramtetrasulfide (DPTT), zinc
phenyldithiocarbamate (ZnPDC), zinc dimethyldithiocarbamate
(ZnMDC), zinc dibutyldithiocarbamate (ZnBDC),
2-mercaptobenzothiazole (MBT), dibenzothiazyldisulfide (MBTS),
N-cyclohexylbenzothiazyl-2-sulfenic amide (CBS),
N-t-butylbenzothiazyl-2-sulfenic amide (BBS),
N-cyclohexylthiophthalimide (CPT), N-butyraldehyde aniline (BAA),
zinc white, etc. The rubber crosslinking accelerators are usually
added in an amount of about 1 to about 2 parts by weight per 100
parts by weight of the rubber (solid-content) contained in the
electrode materials.
Preparation Method of the Electrode Materials of the Invention
[0060] Methods for preparing the electrode materials of the first
aspect of the invention are not limited as long as a rubber-based
emulsion, activated carbon particles, conductive particulates, and
a surfactant are contained in the electrode materials. For example,
the electrode materials can be prepared by mixing these ingredients
(to which the above-mentioned additives may be added) and stirring
the mixture to make it uniform.
[0061] Methods for preparing the electrode materials of the second
aspect of the invention are not limited as long as the electrode
materials contain a PTFE dispersion, activated carbon particles,
conductive particulates and a surfactant, and the solid content is
25% by weight or more. For example, the electrode materials can be
prepared by mixing these ingredients (to which the above-mentioned
additives may also be added) and stirring the mixture to make it
uniform. Water may be added thereto or it may be warmed as needed
so as to have a desired solid content.
[0062] The above-mentioned ingredients can be used in either
preparation method. The mixing ratio of each ingredient may be
determined considering the above-described suitable content range
of each ingredient per 100% by weight of the electrode material. A
known stirrer can be used for stirring each ingredient.
[0063] As is clear from the above description, there is no
limitation to the methods for preparing electrode materials of the
invention. In particular, an electrode material fully capable of
improving the capacitance of a polarized electrode can be prepared
according to a specific preparation method comprising the following
three steps:
(1) a first step of dispersing conductive particulates and a
surfactant in water; (2) a second step of dispersing activated
carbon particles in the dispersion obtained in the first step; and
(3) a third step of mixing a rubber-based emulsion or a PTFE
dispersion to the dispersion obtained in the second step.
[0064] Hereinafter, the preparation method comprising the
above-described three steps is referred to as "the preparation
method of the invention".
[0065] The preparation method of the invention is characterized by
mixing each ingredient constituting the electrode material in a
specific order. Each step is now described.
[0066] In the first step, conductive particulates and a surfactant
are dispersed in water.
[0067] The mixing rate of the conductive particulates and a
surfactant to water can be appropriately determined without
limitation according to the type of each ingredient, desired
characteristics of electrode materials, etc. In general, in 100
parts by weight of water, the conductive particulates may be
dispersed in an amount of about 20 to about 30 parts by weight, and
preferably about 25 to about 30 parts by weight. The surfactant is
dispersed in the same amount of water in an amount of about 0.1 to
about 0.5 parts by weight, and preferably about 0.2 parts by
weight.
[0068] The order of dispersing conductive particulates and a
surfactant is not limited, and either may be added first. A known
stirrer can be used for dispersion.
[0069] In the second step, activated carbon particles are dispersed
in the dispersion obtained in the first step.
[0070] Any activated carbon particles can be used without
limitation. In particular, at least two types of activated carbon
particles with different mean particle diameters are preferably
used in the preparation method of the invention. The viscosity of
the electrode materials obtained using at least two types of
activated carbon particles with different mean particle diameters
can be reduced more compared to the case where activated carbon
particles with a single mean particle diameter are used.
[0071] The two or more types of activated carbon particles with
different mean particle diameters are preferably classified as
follows: assuming the smallest mean particle diameter to be "A",
the second smallest mean particle diameter should be about "3A";
and the third smallest mean particle diameter should be about "9A".
For example, when the kind of activated carbon particles of the
smallest mean particle diameter are of 3 .mu.m size, the kind of
activated carbon particles of the second smallest mean particle
diameter are of about 9 .mu.M size.
[0072] The mixing ratio of the activated carbon particles with
different mean particle diameters is not limited. In general, these
are mixed in about equal proportions, or the kind of activated
carbon particles with the smallest mean particle diameter may be
included in excess by about 10 to 50% by weight as compared to the
other activated carbon particles.
[0073] In the second step, it is preferable to disperse the two or
more types of activated carbon particles with different mean
particle diameters in the dispersion obtained in the first step in
such a manner that the type of the activated carbon particles with
the smallest mean particle diameter is dispersed first, and then a)
the other type of activated carbon particles is dispersed or b) the
other types of activated carbon particles are dispersed in order of
their increasing mean particle diameter.
[0074] The activated carbon particles may be dispersed in an amount
of about 200 to about 300 parts by weight, and preferably about 200
to about 250 parts by weight, per 100 parts by weight of the
dispersion obtained in the first step.
[0075] In the third step, a rubber-based emulsion or PTFE
dispersion (binder) is mixed in the dispersion obtained in the
second step.
[0076] The binder is usually mixed in an amount of about 5 to about
10 parts by weight, and preferably about 5 to about 6 parts by
weight, per 100 parts by weight of the dispersion obtained in the
second step. The mixing ratio of the binder can be suitably
adjusted depending on the desired characteristics of the electrode
material, and is not necessarily limited to the above-mentioned
range.
[0077] When the preparation method of the invention is adopted as a
method for preparing the electrode materials of the second aspect
of the invention, the mixing ratio of the binder, water, etc. can
be appropriately adjusted so that the solid content of the
electrode material becomes 25% by weight or more.
[0078] In the preparation method of the invention, conductive
particulates and a surfactant are dispersed in water prior to the
activated carbon particles. Thus, the viscosity of the electrode
materials can be reduced by 1,000 cP (viscosity measured under
atmospheric conditions at 25.degree. C. with a Brookfield
viscometer) as compared with electrode materials prepared by the
conventional method in which the same amounts of all ingredients
are dispersed at once.
[0079] In the second step of the preparation method of the
invention, the viscosity of the electrode material can be reduced
more efficiently when the two or more types of activated carbon
particles with different mean particle diameters are dispersed in
such a manner that the type of the activated carbon particles with
the smallest mean particle diameter is dispersed first and then a)
the other type of activated carbon particles is dispersed orb) the
other types of activated carbon particles are dispersed in order of
their increasing mean particle diameter. For example, the viscosity
of the electrode material to be obtained can be reduced by about
800 cP (viscosity measured under atmospheric conditions at
25.degree. C. with a Brookfield viscometer) as compared to the case
where an equivalent amount of activated carbon particles of a
single mean particle diameter are used.
[0080] The preparation method of the invention allows activated
carbon particles to be dispersed at high densities. Thus activated
carbon particles in the same amount as the prior art can be
dispersed in a dispersion medium of 80 to about 95% by weight of
the minimum amount required to maintain a fluid state in the
electrode materials prepared by known methods, without gelling
(critical amount of electrode materials to maintain flowability).
As a result, an electrode material with low viscosity compared to
that obtained by the prior art can be prepared even if the solid
content is the same.
[0081] As is clear from the above, due to the attempt to reduce the
viscosity of electrode materials, electrode materials with
excellent application properties (e.g., slurry flowability) can be
prepared compared to conventional electrode materials even with the
same solid content, thereby readily preparing polarized electrodes
with high capacitance.
Polarized Electrode of the Invention
[0082] The polarized electrodes of the invention can be produced by
forming an electrode layer on a conductive substrate using the
electrode materials of the invention.
[0083] Known materials usable as collectors in carbon electrodes,
etc. are preferably used as conductive substrates. For example,
aluminium, nickel, copper, stainless steel, titanium, conductive
rubber, etc. can be mentioned. The conductive substrate can be in
any shape, such as a rod-like, sheet-like, etc.
[0084] The surface of the conductive substrate may be roughened
beforehand by acid treatment, alkali treatment, blast treatment,
etc. A roughened surface can ensure increased adhesion to the
electrode layer. The surface area is consequently increased,
thereby improving the current collection effect. Effective ways to
improve output by increasing adhesion to the electrode layer, in
addition to surface roughening, include forming a colloidal carbon
thin layer on the conductive substrate surface using water glass as
a binder, and a thin layer formed of silane coupling, titanium
coupling, etc.
[0085] The electrode layer can be formed on the conductive
substrate by any method without limitation, such as application
methods (brush coating, spraying, roller, etc.), the doctor blade
method, dipping, etc. The thickness of the electrode layer can be
appropriately determined depending on the use of the finished
product, the type of conductive substrate, etc. and is generally
about 50 to about 250 .mu.m, and preferably about 100 to about 200
.mu.m.
[0086] After forming an electrode layer, the electrode layer may be
pressurized as long as the contact between the electrolyte, and the
activated carbon particles and conductive particulates is not
impaired. Pressurization treatment can be easily carried out by a
press roller, hydraulic press, etc. The pressurization treatment
can further ensure an increase in the packing ratio of the
activated carbon particles and conductive particulates.
[0087] The electrode layer can be dried by any method without
limitation, such as natural drying, drying by heating, etc. In
order to ensure an adequate porosity of the electrode layer, a
short drying period is preferable (e.g., for one to two minutes at
200.degree. C.).
[0088] When a rubber crosslinking agent is contained in the
electrode materials, it is preferable to perform the heat-treatment
for crosslinking after drying (preferably, vacuum drying or drying
under an inert gas). The heat-treatment temperature for
crosslinking is not limited, and is in general preferably about 150
to about 200.degree. C. The heating period can be suitably adjusted
depending on the temperature.
Function
[0089] According to the preparation method of the invention,
conductive particulates and a surfactant are dispersed in water in
the first step, and activated carbon particles are dispersed in the
second step. In this order of dispersion, electric interaction
occurs preferentially between the surfactant and conductive
particulates and further electric repulsion occurs between the
conductive particulates. As a result, the autoagglutination of the
conductive particulates can be efficiently suppressed. However, the
detailed mechanism of this phenomenon is not yet known. In the
electrode materials obtained by the preparation method of the
invention, aggregation and sedimentation of conductive particulates
are suppressed at least one month, and thus it can be expected that
a certain electric repulsion interaction occurs between the
conductive particulates.
[0090] The detailed mechanism of this interaction is not yet known.
It is conceivable that aggregation and sedimentation of conductive
particulates are suppressed since hydrophobic groups of the
surfactant adsorb on the conductive particulate surfaces and the
hydrophilic groups oriented to the water repel one another (FIG.
2).
[0091] Subsequently, the conductive particulates, to which the
surfactant is uniformly adsorbed, uniformly cover the surfaces of
the activated carbon particles to be dispersed in the second step,
and the binder interlinks through the surrounding medium. In the
above-mentioned structure, water is present between the binder and
the activated carbon particles, and favorable voids can be formed
after drying, thereby increasing the contact of the electrolyte,
with the activated carbon particles and conductive
particulates.
Effects
[0092] According to the preparation method of the invention, the
viscosity of the electrode material can be sharply reduced compared
to the conventional electrode materials, even if the solid-content
is the same.
[0093] The electrode materials of the invention can produce
polarized electrodes that can ensure a favorable packing ratio of
the activated carbon particles and conductive particulates
contained therein and an excellent contact between these and the
electrolyte, and show superior capacitance as compared to
conventional polarized electrodes.
[0094] The polarized electrode of the invention obtained by forming
the electrode layer on a conductive substrate using the electrode
materials of the present invention has a capacitance as much as
about 2 to 3 times that of conventional polarized electrodes.
[0095] The polarized electrode of the invention having such
characteristics is particularly useful for a polarized electrode of
an electric double layer capacitor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0096] FIG. 1 is a view schematically illustrating the structure of
a prior-art electrode material.
[0097] FIG. 2 is a view schematically illustrating the structure of
an electrode material of the invention.
DESCRIPTION OF REFERENCE NUMERALS
[0098] 1. activated carbon particle [0099] 2. activated carbon
particle [0100] 3. conductive particulate [0101] 4. binder [0102]
5. surfactant
BEST MODE FOR CARRYING OUT THE INVENTION
[0103] The present invention will be described below in further
detail with reference to Examples and Comparative Examples.
However, the scope of the invention is not limited to these
Examples.
Preparatory Example
[0104] Ingredients were mixed in the amounts shown in Table 1 to
prepare five types of electrode materials, referred to as samples 1
to 5. Subsequently, the viscosity and state of the electrode
materials of the samples 1 to 5 were examined. The viscosity of the
electrode materials was measured under atmospheric conditions at
25.degree. C. with a Brookfield viscometer (the same applies to
Table 2). The state of the electrode materials was observed
visually.
[0105] "BP-20" (trade name, product of Kuraray Chemical, Inc.) was
used as activated carbon particles. "DENKA BLACK" (trade name,
product of DENKI KAGAKU KOGYO K.K.) was used as the conductive
particulates.
[0106] In Table 1, PTFE denotes "PTFE dispersion"
TABLE-US-00001 TABLE 1 Activated Activated Conductive Mixing
Viscosity Binder carbon 1 carbon 2 particulates Surfactant Water
method (cP) State Mixing 10 35 35 20 0.2 250 ratio (solid content)
Sample 1 PTFE particle particle DENKA -- distilled 1 6,100
semisolid diameter diameter BLACK water 30 .mu.m 30 .mu.m Sample 2
PTFE particle particle DENKA CMC distilled 1 5,512 viscous diameter
diameter BLACK water liquid 30 .mu.m 30 .mu.m Sample 3 PTFE
particle particle DENKA CMC distilled 2 4,034 viscous diameter
diameter BLACK water liquid 30 .mu.m 30 .mu.m Sample 4 PTFE
particle particle DENKA CMC distilled 2 3,130 viscous diameter
diameter BLACK water liquid 15 .mu.m 30 .mu.m Sample 5 PTFE
particle particle DENKA CMC distilled 3 2830 viscous diameter
diameter BLACK water liquid 15 .mu.m 30 .mu.m
Description of Mixing Methods
[0107] Mixing method 1 is the method in which all ingredients are
dispersed at once.
[0108] Mixing method 2 includes dispersing conductive particulates
and a surfactant beforehand in water, subsequently dispersing
activated carbon particles in the resultant dispersion, and then
dispersing a binder therein. Either conductive particulates or a
surfactant may be dispersed first.
[0109] Mixing method 3 is applied to the case where two or more
types of activated carbon particles with different mean particle
diameters are used as activated carbon particles according to
mixing method 2. In mixing method 3, the activated carbon particles
are dispersed in the dispersion in the order such that the type of
activated carbon particles with the smallest mean particle diameter
is dispersed first and then a) the other type of activated carbon
particles is dispersed or b) the other types of activated carbon
particles are dispersed in order of their increasing mean particle
diameter.
Results of Table 1
[0110] Sample 2 containing a surfactant showed further reduced
viscosity as compared with the conventional electrode material
(sample 1) containing no surfactant and obtained by dispersing the
ingredients all at once.
[0111] Sample 3 showed further reduced viscosity as compared with
the sample 2. Sample 3 contained the same ingredients as in the
sample 2 but was obtained in a different dispersion order such that
conductive particulates and a surfactant were dispersed beforehand
in water, and then activated carbon particles were dispersed in the
resultant dispersion.
[0112] Sample 4, obtained by using the same dispersion order as in
sample 3, showed further reduced viscosity as compared with the
sample 3 due to the use of two or more types of activated carbon
particles with different mean particle diameters.
[0113] Sample 5 showed further reduced viscosity as compared to
Sample 4, which was produced by gradually dispersing the various
activated carbon particles in the dispersion in order of increasing
mean particle diameter from the smallest mean particle diameter,
even though the ingredients in the electrode material were the same
as in the sample 4, except that two or more types of activated
carbon particles with different mean particle diameters were
used.
Examples 1 to 7 and Comparative Examples 1 and 2
[0114] As shown in Table 2, 9 types of electrode materials were
prepared using a PTFE dispersion and various rubber-based emulsions
as a binder. Water was used in the smallest amount required for the
electrode materials to maintain flowability (critical amount of
electrode material to flow).
[0115] In Table 2, PTFE designates "PTFE dispersion" and IIR, NBR,
SBR, and Acryl each denote rubber emulsions. Mixing methods are as
described above.
[0116] Table 2 shows the solid content (% by weight) of the
obtained electrode materials.
TABLE-US-00002 TABLE 2 Activated Activated Conductive Mixing Solid
content Binder carbon 1 carbon 2 particulates Surfactant method (%
by weight) Mixing ratio 10 35 35 20 0.2 (solid content) Example 1
PTFE particle particle DENKA CMC 2 45 diameter diameter BLACK 15
.mu.m 30 .mu.m Example 2 PTFE particle particle DENKA CMC 2 40
diameter diameter BLACK 30 .mu.m 30 .mu.m Example 3 PTFE particle
particle DENKA CMC 3 40 diameter diameter BLACK 15 .mu.m 30 .mu.m
Example 4 IIR particle particle DENKA CMC 2 45 diameter diameter
BLACK 15 .mu.m 30 .mu.m Example 5 NBR particle particle DENKA CMC 2
45 diameter diameter BLACK 15 .mu.m 30 .mu.m Example 6 SBR particle
particle DENKA CMC 2 40 diameter diameter BLACK 15 .mu.m 30 .mu.m
Example 7 Acryl particle particle DENKA CMC 2 45 diameter diameter
BLACK 15 .mu.m 30 .mu.m Comparative PTFE particle particle DENKA
CMC 1 20 Example 1 diameter diameter BLACK 15 .mu.m 30 .mu.m
Comparative PTFE particle particle DENKA -- 1 20 Example 2 diameter
diameter BLACK 15 .mu.m 30 .mu.m
[0117] Ingredients were mixed and dispersed at high speed for a
predetermined period with a homogenizer manufactured by NIPPON
SEIKI. The electrode materials and the binder were mixed at a low
speed using a universal mixer manufactured by ODAIRA SEISAKUSHO,
LTD. so that the binder might be fibrillated to a desired
extent.
[0118] Subsequently, the obtained electrode materials were spread
over a 50 .mu.m aluminium etched foil with a roll coater, and dried
at 180.degree. C., giving a polarized electrode 150 .mu.m thick
after drying. A sufficient coating film was obtained in every
case.
[0119] The obtained polarized electrodes were punched out into disk
shapes of a diameter of 20 mm. Two disks were brought into contact
with a separator sandwiched, and were immersed in a 1 molar
propylene-carbonate solution of a tetraammonium tetrafluoroborate,
and the resultant was filled into a coin type cell produced by
HOUSEN SEISKUSHO, LTD., giving examination cells.
[0120] The obtained examination cells were charged at 2.5 V and 1
mA, DC for 15,000 seconds, and constant-current discharge at 1 mA
was carried out. The capacitance of the polarized electrodes
calculated from the charge period is shown in Table 3.
TABLE-US-00003 TABLE 3 Capacitance (F) Solid content (g/cm.sup.3)
Example 1 8.6 0.69 Example 2 6.6 0.65 Example 3 8.5 0.69 Example 4
8.1 0.67 Example 5 8.2 0.68 Example 6 8.3 0.67 Example 7 8.0 0.67
Comparative Example 1 3.4 0.41 Comparative Example 2 3.1 0.38
[0121] The polarized electrodes obtained in Examples 1 to 7 had a
capacitance about 2 to 3 times larger than the polarized electrodes
obtained in comparative examples 1 and 2.
INDUSTRIAL APPLICABILITY
[0122] According to the preparation method of the invention,
electrode materials of reduced viscosity compared to the
conventional electrode materials can be prepared even when the
amount of solid-content is the same.
[0123] The electrode materials of the invention can produce, as
compared to conventional polarized electrodes, polarized electrodes
that ensure a favorable packing ratio of the activated carbon
particles and conductive particulates contained in the polarized
electrodes and a satisfactory contact between these and the
electrolyte, and demonstrate excellent capacitance.
[0124] Polarized electrodes of the present invention obtained by
forming an electrode layer on a conductive substrate using the
electrode material of the invention have capacitance about 2 to 3
times greater than conventional polarized electrodes.
[0125] The polarized electrodes of the invention having such
characteristics are particularly useful for polarized electrodes of
electric double layer capacitors.
* * * * *