U.S. patent application number 10/546736 was filed with the patent office on 2006-07-13 for method of manufacturing electrode for electrochemical device.
Invention is credited to Yoshitsugu Hirokawa.
Application Number | 20060153972 10/546736 |
Document ID | / |
Family ID | 32929637 |
Filed Date | 2006-07-13 |
United States Patent
Application |
20060153972 |
Kind Code |
A1 |
Hirokawa; Yoshitsugu |
July 13, 2006 |
Method of manufacturing electrode for electrochemical device
Abstract
A method of manufacturing an electrode for an electrochemical
device is provided with the steps of: supplying, onto a collector,
a powdered mixture containing a binder and an active material; and
heating the powdered mixture to form an electrode layer on the
collector, that allows continuous mass production of electrodes for
electrochemical devices.
Inventors: |
Hirokawa; Yoshitsugu;
(Tokyo, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
32929637 |
Appl. No.: |
10/546736 |
Filed: |
February 24, 2004 |
PCT Filed: |
February 24, 2004 |
PCT NO: |
PCT/JP04/02102 |
371 Date: |
August 24, 2005 |
Current U.S.
Class: |
427/79 |
Current CPC
Class: |
H01G 11/86 20130101;
H01G 11/38 20130101; H01M 4/04 20130101; H01M 4/02 20130101; H01M
4/0483 20130101; Y02E 60/10 20130101; H01M 4/0404 20130101; H01G
11/42 20130101; Y02E 60/13 20130101; H01G 11/22 20130101; H01M
4/043 20130101; H01G 9/155 20130101; H01M 4/0471 20130101 |
Class at
Publication: |
427/079 |
International
Class: |
B05D 5/12 20060101
B05D005/12 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 25, 2003 |
JP |
2003-047119 |
Jul 3, 2003 |
JP |
2003-190789 |
Claims
1. A method of manufacturing an electrode for an electrochemical
device, comprising the steps of: supplying, onto a collector, a
powdered mixture containing a binder and an active material; and
heating the powdered mixture to form an electrode layer on the
collector.
2. The method according to claim 1, wherein the electrochemical
device is an electric double layer capacitor, and the active
material is a carbonaceous material.
3. The method according to claim 1, wherein the powdered mixture
further contains an electroconductivity-imparting agent.
4. The method according to claim 2, wherein 100 parts by weight of
the powdered mixture contains 0.1 to 50 parts by weight of the
binder and 50 to 99.9 parts by weight of a carbonaceous material
for use as the active material and the
electroconductivity-imparting agent.
5. The method according to claim 1, wherein the binder contains at
least one of particulate rubber and particulate thermoplastic
elastomer.
6. The method according to claim 1, wherein the binder contains
particulate rubber and particulate thermoplastic elastomer.
7. The method according to claim 5, wherein the thermoplastic
elastomer is a graft polymer of an ethylene/acrylic (or
methacrylic) ester copolymer graft polymerized
radical-polymerizable monomer thereon.
8. The method according to claim 1, wherein the binder has a
particle diameter of 0.01 to 10 .mu.m.
9. The method according to claim 1, further comprising the step of
press-forming the powdered mixture under heating or after heating.
Description
TECHNICAL FIELD
[0001] The invention relates to a method of manufacturing an
electrode for an electrochemical device.
BACKGROUND ART
[0002] Recently, with the developments in electronic instruments,
there has been a demand for development of compact, lightweight,
high-energy-density, rechargeable electrochemical devices. In
particular, there has been an expansion in the use of
high-energy-density lithium-ion secondary batteries and long-life,
high-current-producible electric double layer capacitors and redox
capacitors, because of their advantages.
[0003] Recently, there has been a drastic increase in the demand
for electric double layer capacitors serving as memory-backup power
sources, which use an electric double layer formed on an interface
between a polarized electrode and an electrolyte. Attention for
electric double layer capacitors has also been focused on
large-capacitance-demanding applications such as electric vehicle
power sources.
[0004] In the fields of cellular phones, laptop computers and the
like, there has been a sharp increase in the demand for lithium-ion
secondary batteries using LiCoO.sub.2, LiNiO.sub.2,
LiMn.sub.2O.sub.4, or the like as a positive electrode active
material and graphite or the like as a negative electrode active
material, because of their performance. In addition, attention has
been focused on the capacity size of redox capacitors using
oxidation-reduction reaction (pseudo-capacitance of electric double
layer) on the surface of metal oxides or electrically conductive
polymers.
[0005] These electrochemical devices have been practically
developed for the last dozen years or so, and many patent
applications and literatures on these techniques have been
published in recent years. For example, methods of manufacturing
electrodes for electric double layer capacitors are proposed which
include kneading activated carbon powder with a solvent of a liquid
electrolyte such as sulfuric acid to form a slurry and forming the
slurry by pressing (U.S. Pat. No. 3,288,641). However, the
electrode formed by this method has a rigid porous structure and
can easily crack or collapse. Thus, it cannot endure long-term
use.
[0006] In order to produce electrodes having anti-cracking or
anti-collapsing properties and good form retention, it is proposed
that the method of manufacturing electrodes should include
preforming a kneaded product of a carbonaceous material such as
activated carbon, a binder such as polytetrafluoroethylene (PTFE)
and a liquid lubricant and then drawing or rolling the preform into
a sheet-shaped product (Japanese Patent Application Laid-Open
(JP-A) No. S63-107011 and JP-A No. H02-235320).
[0007] In this method including the kneading step, however, PTFE
can be partially formed into fibers and partially not formed into
fibers. In the process of forming a thin film-shaped electrode
sheet, therefore, uneven surfaces can be easily formed, and thus
performance of the resulting electrochemical device can be
unsatisfactory.
[0008] There is also proposed a method including the steps of
mixing activated carbon powder, PTFE and a solvent to form a paste,
applying the paste to a collector, drying it, then heating it to
the melting point of PTFE or higher, and press-forming it to form a
thin-film electrode and to increase its density (JP-A No.
H09-36005). However, this method has complicated processes.
[0009] There is also proposed a method including the steps of
mixing activated carbon powder, an aqueous dispersion of a
styrene/butadiene polymer and a water-soluble thickening binder to
form a paste, applying the paste to a collector, drying it, and
then press-forming it to form a thin-film electrode and to increase
its density (JP-A No. H11-162794). In this method, however, the
viscosity of the slurry highly depends on the solid content, and
thus a slight increase in slurry concentration can lead to a great
change in slurry viscosity. Therefore, the coating performance can
easily be degraded, and continuous production is not possible by
this method.
[0010] As mentioned above, it is difficult to achieve continuous
production by any of the conventional manufacturing methods, and
for example, a long electrode sheet cannot be efficiently produced.
The conventional methods have a problem with mass production.
[0011] The invention has been made in order to solve the problems
with the above prior art, and it is an object of the invention to
provide a method of manufacturing an electrode for electrochemical
device suitable for industrial-scale mass production.
DISCLOSURE OF INVENTION
[0012] The inventors have made active investigations on processes
for manufacturing electrodes for electrochemical devices such as
lithium-ion secondary batteries and electric double layer
capacitors. As a result, the inventors have found that electrodes
for an electrochemical device can be efficiently manufactured by a
simplified process including the steps of using a powdered mixture
containing a binder and an active material, supplying the mixture
onto a collector by spreading or any other method and heating the
mixture to form an electrode layer, and finally have made the
invention.
[0013] Thus, the invention is directed to a method of manufacturing
an electrode for an electrochemical device, comprising the steps
of: supplying, onto a collector, a powdered mixture containing a
binder and an active material; and heating the powdered mixture to
form an electrode layer on the collector.
[0014] Preferably, the electrochemical device is an electric double
layer capacitor, and the active material is a carbonaceous
material.
[0015] The powdered mixture may further contain an
electroconductivity-imparting agent.
[0016] When the electrochemical device is an electric double layer
capacitor, 100 parts by weight of the powdered mixture preferably
contains 0.1 to 50 parts by weight of the binder and 50 to 99.9
parts by weight of a carbonaceous material for use as the active
material and the electroconductivity-imparting agent. Herein the
term "carbonaceous material" may encompass the "active material"
for use as an electrode material and the
"electroconductivity-imparting agent" as described later.
[0017] The binder preferably contains at least one of particulate
rubber and particulate thermoplastic elastomer.
[0018] The binder also preferably contains particulate rubber and
particulate thermoplastic elastomer.
[0019] The thermoplastic elastomer is preferably a graft polymer of
an ethylene/acrylic (or methacrylic) ester copolymer graft
polymerized radical-polymerizable monomer thereon.
[0020] The binder preferably has a particle diameter of 0.01 to 10
.mu.m.
[0021] The powdered mixture is preferably further press-formed
under heating or after heating.
BRIEF DESCRIPTION OF DRAWINGS
[0022] FIG. 1 is a flow chart showing a manufacturing method
according to the invention; and
[0023] FIG. 2 is a schematic diagram showing a manufacturing
apparatus for use in a manufacturing method according to the
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
<1> Material Components
[0024] According to the invention, there is provided a method of
manufacturing an electrode for an electrochemical device,
comprising the steps of: supplying, onto a collector, a powdered
mixture containing a binder and an active material; and heating the
powdered mixture. In the method of the invention, "the binder" and
"the active material" a reused as essential components of the
powdered mixture, and "the electroconductivity-imparting agent" is
used as an optional component, which are described below.
(1) Binder
[0025] The binder for use in the invention can be melted or
softened by heat to bond the active material to the collector. The
type of the binder may be rubber, thermoplastic elastomer, or the
like. In particular, the binder for use in the invention preferably
contains "particulate rubber" and/or "particulate thermoplastic
elastomer."
<Rubber>
[0026] Any rubber may be used as the binder. Examples of the rubber
include diene rubbers mainly composed of a conjugated diene-derived
monomer unit such as butadiene and isoprene; acrylic rubbers mainly
composed of an acrylic acid ester- and/or methacrylic acid
ester-derived monomer unit; and fluorocarbon rubbers. Diene rubbers
and acrylic rubbers are particularly preferred. Particulate diene
rubbers having a crosslinked structure and particulate acrylic
rubbers having a crosslinked structure are more preferred.
Particulate diene rubbers can be produced by adjusting the
polymerization conditions such as polymerization temperature, the
amount of a polymerization initiator, and the amount of a chain
transfer agent. Particulate acrylic rubbers can be produced by
copolymerizing an acrylic acid ester and/or a methacrylic acid
ester, a multifunctional ethylenically unsaturated monomer and
optionally any other monomer.
[0027] Examples of the acrylic acid ester include ethyl acrylate,
propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl
acrylate, tert-butyl acrylate, n-amyl acrylate, isoamyl acrylate,
n-hexyl acrylate, 2-ethylhexyl acrylate, hexyl acrylate, nonyl
acrylate, lauryl acrylate, stearyl acrylate, hydroxyethyl acrylate,
and hydroxypropyl acrylate.
[0028] Examples of the methacrylic acid ester include ethyl
methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl
methacrylate, isobutyl methacrylate, tert-butyl methacrylate,
n-amyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate,
2-ethylhexyl methacrylate, octyl methacrylate, isodecyl
methacrylate, lauryl methacrylate, tridecyl methacrylate, stearyl
methacrylate, hydroxyethyl methacrylate, and hydroxypropyl
methacrylate.
[0029] Examples of the monomer copolymerizable with the acrylic
acid ester and/or the methacrylic acid ester include aromatic vinyl
compounds such as styrene, a-methylstyrene, 2-methylstyrene,
3-methylstyrene, and 4-methylstyrene; .alpha.,.beta.-unsaturated
nitrile compounds such as acrylonitrile and methacrylonitrile;
ethylenically unsaturated carboxylic acids such as acrylic acid,
methacrylic acid, itaconic acid, fumaric acid, and maleic acid;
acrylamide compounds such as acrylamide and methacrylamide; and
sulfonic acid group-containing monomers such as
acrylamidemethylpropanesulfonic acid and styrenesulfonic acid, and
salts thereof.
[0030] Examples of the multifunctional ethylenically unsaturated
monomer include ethylene glycol dimethacrylate, diethylene glycol
dimethacrylate, polyethylene glycol polymethacrylate,
trimethylolpropane trimethacrylate, and divinylbenzene.
[0031] Examples of the diene rubbers include polybutadiene,
optionally carboxy-modified styrene/butadiene copolymer, and
acrylonitrile/butadiene copolymer and hydrogenated product
thereof.
[0032] Examples of the acrylic rubbers include 2-ethylhexyl
acrylate/methacrylic acid/acrylonitrile/ethylene glycol
dimethacrylate copolymer, 2-ethylhexyl acrylate/methacrylic
acid/methacrylonitrile/diethylene glycol dimethacrylate copolymer,
butyl acrylate/acrylonitrile/diethylene glycol dimethacrylate
copolymer, and butyl acrylate/acrylic acid/trimethylolpropane
trimethacrylate copolymer.
[0033] The particulate rubber may be kept in the form of particles
when used in the preparation of the powdered mixture. Preferably,
the particulate rubber is dispersed in a liquid medium and used in
the form of a liquid dispersion. The liquid dispersion may be an
aqueous dispersion or an organic solvent dispersion. In view of
environmental considerations, the aqueous dispersion is preferred.
In the liquid dispersion, the particulate rubber generally has a
particle diameter of 0.01 to 10 .mu.m, preferably of 0.05 to 1
.mu.m. Using such particulate rubber, adhesion of the active
material to the collector can be enhanced, and the total
consumption of the binder can be reduced.
<Thermoplastic Elastomer>
[0034] In the method of the invention, a thermoplastic elastomer
that is not adhesive at or around room temperature can keep the
powdered mixture in a powder state and then can function as an
active binder component similarly to the above rubber after heat
treatment. Such a thermoplastic elastomer is preferably used in the
form of particles.
[0035] The thermoplastic elastomer for use is generally a copolymer
having partly a crystal structure in its molecule. Specific
examples of the thermoplastic elastomer for use in the invention
include acrylic (or methacrylic) ester copolymers such as
ethylene/methyl acrylate copolymer, ethylene/methyl methacrylate
copolymer, ethylene/ethyl acrylate copolymer, and ethylene/ethyl
methacrylate copolymer; acrylic (or methacrylic) acid copolymer
such as ethylene/acrylic acid copolymer and ethylene/methacrylic
acid copolymer; graft polymers such as any of the above acrylic (or
methacrylic) ester copolymers graft polymerized by
radical-polymerizable monomer thereon; and styrene-containing block
copolymers such as styrene/butadiene/styrene block copolymer,
styrene/isoprene/styrene block copolymer,
styrene/ethylene.butylene/styrene block copolymer, and
styrene/ethylene.propylene/styrene block copolymer.
[0036] Among the above thermoplastic elastomers, the graft polymer
of an ethylene/acrylic (or methacrylic) ester copolymer graft
polymerized radical-polymerizable monomer thereon is preferably
used. For example, such a graft polymer comprises an ethylene/ethyl
acrylate copolymer graft polymerized by methyl methacrylate,
acrylonitrile, methacrylic acid or the like.
[0037] Any method may be used to form particulate thermoplastic
elastomer. For example, a method for use in producing particulate
thermoplastic elastomer includes dissolving the thermoplastic
elastomer in a liquid medium at a temperature of at its melting
point or higher and then cooling the solution to precipitate
particles in the liquid medium. In such a particle preparation
method, a dispersing agent, an emulsifier or the like may be used
as needed. Alternatively, a liquid dispersion of the thermoplastic
elastomer may be spray-dried to form a powder of particles before
use. The particle diameter of the thermoplastic elastomer is
generally from 0.01 to 10 .mu.m, preferably from 0.02 to 5.0
.mu.m.
[0038] The particulate thermoplastic elastomer is preferably used
in combination with the particulate rubber. The blend ratio of
particulate rubber/particulate thermoplastic elastomer is generally
from 1/10 to 10/1, preferably from 2/5 to 5/2. At a particulate
rubber/particulate thermoplastic elastomer blend ratio of at least
1/10, a reduction in adhesion of the active material to the
collector can be prevented. If the blend ratio is 10/1 or less,
coalescence between the active material and the particle of
electroconductivity-imparting agent as described later can be
prevented, so that a reduction in performance of electrochemical
devices can be prevented.
<Dispersing Agent>
[0039] In the invention, water- or organic solvent-soluble polymers
may be used as dispersing agents for the
electroconductivity-imparting agent as described later. Any of the
above particulate rubbers or particulate thermoplastic elastomers
may be used as the dispersing agent as long as they are water- or
organic solvent-soluble.
[0040] Examples of the water-soluble polymer include celluloses
such as carboxymethylcellulose (CMC), methylcellulose and
ethylcellulose, polyvinyl alcohol, polyvinyl methyl ether,
polyacrylic acid or its salt, oxidized starch, phosphorylated
starch, casein, and various types of modified starch.
[0041] The organic solvent-soluble polymer should satisfy the
requirements: (1) it should be soluble in a solvent; (2) it should
be insoluble in a liquid electrolyte for use in capacitors; and (3)
it should be electrochemically stable in the liquid electrolyte for
use in capacitors. Examples complying with the requirements include
acrylonitrile polymers such as polyacrylonitrile,
acrylonitrile/acrylic acid ester copolymers, and
acrylonitrile/methacrylic acid ester copolymers; and fluoropolymers
such as polyvinylidene fluoride.
[0042] The water- or organic solvent-soluble polymer is preferably
used in the kneading step (see Step S4 in FIG. 1) for producing
electroconductivity-imparting agent dispersion B as described
later. It can provide fluidity and viscosity to the
electroconductivity-imparting agent when used in the kneading step.
It is preferably added in an amount of 1 to 5% by weight, more
preferably of 1 to 3% by weight, relatively to the
electroconductivity-imparting agent. If the amount of the
dispersing agent is too small, the viscosity of the
electroconductivity-imparting agent can be insufficient in the
kneading step, and kneading can be insufficient. If the amount of
the dispersing agent is too large, the internal resistance can
easily be increased.
<Amount of Blended Binder>
[0043] In the manufacturing method of the invention, the binder is
generally used in a total amount of 0.1 to 50 parts by weight,
preferably of 2 to 30 parts by weight, in order to form 100 parts
by weight of the powdered mixture.
[0044] If the amount of the binder is too small, it can be
difficult to form the powdered mixture into the shape of a sheet by
heating, pressing or the like. If the amount of the binder is too
large, the internal resistance of the electrochemical device can be
high.
(2) Active Material
[0045] The active material on which electrolyte ions will be
adsorbed in electric double layer capacitors may comprise activated
carbon, polyacene, or the like and is preferably a powder with a
specific surface area of at least 30 m.sup.2/g, preferably of 200
to 3500 m.sup.2/g. Fibers or powders such as carbon fibers, carbon
whiskers, and graphite, having specific surface areas within the
above range, may also be used as far as they does not degrade the
extrusion formability. The activated carbon may be phenol-, rayon-,
acryl-, pitch-, or coconut husk-derived activated carbon. The
materials disclosed in JP-A Nos. 11-317333 and 2002-25867 such as
non-porous carbon comprising graphite-like microcrystalline carbon
having an expanded distance between the microcrystalline carbon
phases may also be used as the electrode active material. The
active material preferably has a particle diameter of 0.1 to 100
.mu.m, more preferably of 1 to 20 .mu.m, because in such a case,
electrodes for use in capacitors can easily be produced in the form
of thin films, and high capacitance densities can be obtained.
[0046] Examples of positive electrode active materials for use in
lithium-ion secondary batteries include lithium-containing
composite metal oxides such as LiCoO.sub.2, LiNiO.sub.2,
LiMnO.sub.2, LiMn.sub.2O.sub.4, LiFePO.sub.4, and LiFeVO.sub.4;
transition metal sulfides such as TiS.sub.2, TiS.sub.3 and
amorphous MoS.sub.3; and transition metal oxides such as
Cu.sub.2V.sub.2O.sub.3, amorphous V.sub.2O-P.sub.2O.sub.5,
MoO.sub.3, V.sub.2O.sub.5, and V.sub.6O.sub.13.
Electrically-conductive polymers such as polyacetylene and
poly(p-phenylene) may also be used. Examples of negative electrode
active materials include carbonaceous materials such as amorphous
carbon, graphite, natural graphite, meso carbon micro beads (MCMB),
and pitch-derived carbon fibers; and electrically conductive
polymers such as polyacene.
[0047] The active material for use in redox capacitors may be a
metal oxide such as ruthenium oxide (RuO.sub.2).
(3) Electroconductivity-Imparting Agent
[0048] In the invention, a carbonaceous material such as acetylene
black, ketjen black and carbon black is optionally used as the
electroconductivity-imparting agent in mixture with the active
material. Preferably, any of these electroconductivity-imparting
agents is dispersed in the form of fine particles with the above
dispersing agent and then mixed with the above active material. The
electroconductivity-imparting agent used in combination can
significantly improve the electrical contact between the active
material matters so that the electrochemical devices can have low
internal resistance and high capacitance density.
[0049] The carbonaceous material (including the active material and
the electroconductivity-imparting agent) for forming an electric
double layer capacitor is generally used in an amount of 50 to 99.9
parts by weight, preferably of 70 to 98 parts by weight, more
preferably of 80 to 96 parts by weight, to form 100 parts by weight
of the powdered mixture. Concerning the blend ratio of the active
material with the electroconductivity-imparting agent, generally
0.1 to 20 parts by weight, preferably 2 to 10 parts by weight of
the electroconductivity-imparting agent is blended with 100 parts
by weight of the active material.
[0050] In the case of a positive electrode for lithium-ion
batteries, the electroconductivity-imparting agent is generally
used in an amount of 1 to 20 parts by weight, preferably of 2 to 10
parts by weight, to form 100 parts by weight of the powdered
mixture. In the case of a negative electrode for lithium-ion
batteries, the carbonaceous material (including the active material
and the electroconductivity-imparting agent) is generally used in
an amount of 50 to 99.9 parts by weight, preferably of 70 to 98
parts by weight, to form 100 parts by weight of the powdered
mixture. The blend ratio of active
material/electroconductivity-imparting agent is generally from
200/1 to 5/1, preferably from 100/1 to 10/1.
<2> Manufacturing Flow and Manufacturing Apparatus
[0051] An example of the method of manufacturing electrodes for
electrochemical devices according to the invention is described
with reference to FIG. 1 showing a manufacturing method flow and
FIG. 2 schematically showing a manufacturing apparatus. While the
manufacturing method flow and the manufacturing apparatus are
designed to be applied to the production of electrodes for electric
double layer capacitors, the method of manufacturing electrodes for
electrochemical devices according to the invention may also be
applied to make electrodes for any other electrochemical devices
such as lithium-ion secondary batteries and redox capacitors.
[0052] In this embodiment, the manufacturing method include first
mixing and dissolving the thermoplastic elastomer in a liquid
medium (step S1), then heating the liquid mixture to a temperature
of at least the melting point of the thermoplastic elastomer (step
S2), and performing precipitation by cooling (step S3) to form
particulate thermoplastic elastomer A, which may be used in the
form of a dispersion in a liquid medium. If necessary, the
precipitate of thermoplastic elastomer A may be recovered and dried
by any known method such as spraying (step S3') before use.
[0053] Concurrently, the electroconductivity-imparting agent (e.g.
acetylene black), one of the carbonaceous materials for forming an
electrode layer, and the dispersing agent (e.g. an aqueous solution
of carboxymethylcellulose) are kneaded (step S4) and then diluted
with any properly selected solvent (step S5) to form an
electroconductivity-imparting agent dispersion B.
[0054] The prepared particulate thermoplastic elastomer A,
electroconductivity-imparting agent dispersion B, a carbonaceous
active material other than the electroconductivity-imparting agent
(e.g. activated carbon), and the particulate rubber are mixed in a
mixer 6 (see FIG. 2) such as a kneader (step S6). The mixture is
dried under reduced pressure (step S7) to give a powdered mixture D
for forming an electrode layer.
[0055] If necessary, the manufacturing method of the invention may
include the steps of kneading the electroconductivity-imparting
agent, the particulate rubber, the dispersing agent, and a solvent
(step S8), and diluting them with any properly selected solvent
(step S9) to form an electrically conductive adhesive C, which may
be used to bond an electrode layer to the collector. Specifically,
the electrically conductive adhesive C is applied to a collector
sheet S using a coating apparatus 2 (step S10). A specific amount
of the electrically conductive adhesive C is stored in an adhesive
tank 21 such that the adhesive C can easily be applied (see FIG.
2). The adhesive tank 21 is equipped with a set of rolls 23a and
23b, and the lower portion of the lower roll 23b is dipped in the
adhesive. When the rolls are rotated, the electrically conductive
adhesive C deposited on the surface of the roll 23b is lifted up,
scraped and adjusted with a blade 22 to have a specific amount, and
then transferred and applied to the lower surface of the collector
sheet S.
[0056] The collector sheet E coated with the electrically
conductive adhesive C is turned around by means of drive rolls 3
and 4 and allowed to pass through an optionally provided drying
oven 5 to be dried (step S11).
[0057] A feeder 7 loaded with the powdered mixture D is placed
above the collector sheet E passing through the drying oven 5. A
specific amount of the powdered mixture D is fed (spread) onto the
upper surface of the collector sheet E from the lower port of the
feeder 7 (step S12). The powdered mixture D deposited on the upper
surface of the collector sheet E is then leveled off with a blade 8
(step S13).
[0058] The collector sheet E is subsequently introduced into a
heating apparatus 9, in which the temperature is kept at the
melting point of the thermoplastic elastomer A or higher, for
example, at 200.degree. C. In the process of heating the powdered
mixture D, the particulate thermoplastic elastomer in the mixture
is melted to form an electrode layer comprising the carbonaceous
materials (the electroconductivity-imparting agent and the active
material) bonded with the elastomer (step S14). In this process,
the particulate rubber, which is an additive in the powdered
mixture, also plays a role of a binder together with the
thermoplastic elastomer.
[0059] The collector sheet E having the formed electrode layer is
rolled through a pair of press rolls 10 under heating or after
heating (step S15). The press-forming step is preferably used,
because the resulting electrode can have improved adhesion and a
uniform surface.
[0060] The collector sheet E having the formed electrode layer is
wound up on a take-up roll 11 to give an electrode sheet F for use
in electrochemical devices. The sheet is then cut or formed into
the desired shape for use as an electrochemical device electrode.
After the step of cutting or the like, vacuum drying is preferably
performed to remove residual water, solvent or the like from the
electrode sheet. For example, vacuum drying is performed under the
conditions of a temperature of 200.degree. C., an atmospheric
pressure of 6.7.times.10.sup.4 Pa and a holding time of 7
hours.
[0061] Using the above process, a long high-capacitance-density
electrode sheet for use in electrochemical devices can continuously
be manufactured in a stable manner. It should be noted that the
steps of preparing, applying and drying the electrically conductive
adhesive C (steps S8 to S11) are not essential. The powdered
mixture D may be heated through a pre-heated collector without
using the heating apparatus 9.
<3> Specific Mode of the Manufacturing Method
(1) Mixing Method
[0062] The powdered mixture for use in the invention may be
produced by uniformly mixing the binder and the active material and
optionally the electroconductivity-imparting agent in a mixer 6
(see FIG. 1, step S6, and FIG. 2). The mixer may be a ribbon mixer,
a co-kneader, an internal mixer (Banbury mixer), a two-arm kneader,
a paddle mixer, a self-cleaning type kneader, a screw extruder, a
screw kneader, a Henschel mixer, a planetary mixer, a Muller mill,
an Eirich mill, a chaser mill, or the like.
[0063] The binder may be used in a dried state but is preferably
subjected to mixing in the form of a dispersion in a liquid medium
so that the binder-containing liquid can be applied in the form of
a coating film around the electroconductivity-imparting agent and
around the active material and that the binder can be uniformly
dispersed together with the electroconductivity-imparting agent and
the active material. In this case, a Henschel mixer having a
high-speed-rotation blade in a cylindrical container, a two-arm
kneader having a Z type rotation blade, a paddle mixer with a
fan-shaped paddle rotating around its axis, or the like is
preferably used to generate stronger stirring force. For uniform
dispersion, the particulate rubber is also preferably added in
portions.
[0064] When the particulate thermoplastic elastomer is used, mixing
is preferably performed at or lower than a temperature of the
melting point of the particulate thermoplastic elastomer, more
preferably at a temperature at least 20.degree. C. lower than the
melting point of the particulate thermoplastic elastomer. At a
temperature higher than the melting point of the thermoplastic
elastomer, the active material and the
electroconductivity-imparting agent can tend to coalesce. The
resulting mixture is dried using a vacuum dryer, a flash dryer or
the like and optionally pulverized to give the powdered mixture
D.
[0065] The powdered mixture generally has an average particle
diameter of 10 to 200 .mu.m, preferably of 15 to 70 .mu.m, more
preferably of 20 to 50 .mu.m.
(2) Feeding, Leveling, and Heat-Melting of Powdered Mixture
[0066] In the manufacturing method of the invention, after the
mixing process, the powdered mixture D may be metered and supplied
onto the collector E using the feeder 7. Alternatively, the
powdered mixture D may be supplied onto the collector E by a
spraying method or electrostatically attached by an electrostatic
spraying method, an electrostatic-fluidization dip coating method
or the like. The feeder 7 may be a circle feeder, a screw feeder, a
rotary feeder, an air blow feeder, a vibrating feeder, or the like.
The surface of the fed powder is preferably leveled off with a
blade, a roll, or the like such that the resulting electrode can
have a uniform thickness.
[0067] In the invention, the powdered mixture D from the feeder 7
is fed onto the collector E, allowed to have a smooth surface, and
then formed into an electrode layer on the collector by heating.
The heating temperature may be a temperature equal to or higher
than the melting point (Tm) of the binder or the glass transition
temperature (Tg) of the binder, preferably a temperature at least
10.degree. C. higher than Tm or Tg. If necessary, the electrode
layer formed by the heat treatment may be roll-pressed under
heating or after heating.
(3) Application of Electrically Conductive Adhesive to Collector
and Drying of it
[0068] In the invention, a metal foil is preferably used as the
collector. Specifically, an aluminum foil is preferably used for
electric double layer capacitors or positive electrodes of
lithium-ion secondary batteries, and a copper foil is preferably
used for negative electrodes of lithium-ion secondary batteries.
When used for electric double layer capacitors, the collector is
preferably precoated with the electrically conductive adhesive C
for the purpose of reducing the internal resistance between the
electrode and the collector (see FIG. 1, step S10). The
electrically conductive adhesive C to be applied to the collector
in advance may be produced by kneading the
electroconductivity-imparting agent such as acetylene black, ketjen
black and carbon black with the above particulate rubber and the
dispersing agent in water or an organic solvent. Specifically, the
electrically conductive adhesive C may be produced by mixing 5 to
40 parts by weight of the particulate rubber with 100 parts by
weight of the electroconductivity-imparting agent such as acetylene
black, ketjen black and carbon black in a kneader having a shearing
function (see FIG. 1, step S8).
[0069] If the amount of the particulate rubber used in the
production of the electrically conductive adhesive C is too small,
adhesion between the resulting electrode and the collector can be
insufficient. If the amount of the particulate rubber is too large,
the electroconductivity-imparting agent can be insufficiently
dispersed so that the internal resistance can be high.
[0070] The kneader for use in the production of the electrically
conductive adhesive C may be a ball mill, a sand mill, a
pigment-dispersing machine, a chaser mill, an ultrasonic dispersing
machine, a homogenizer, a planetary mixer, or the like.
[0071] The electrically conductive adhesive C may be applied to the
collector by any method including a doctor blade method, a dip
method, a reverse roll method, a direct roll method, a gravure
method, an extrusion method, a brush painting method, and the like.
While the adhesive may be applied in any amount, the amount of
application may be adjusted such that the resulting electrically
conductive layer after drying can generally have a thickness of 0.5
to 10 .mu.m, preferably of 2 to 7 .mu.m.
[0072] The electrically conductive adhesive C applied as described
above may be dried by any drying method in a heat-drying apparatus
5 (see FIG. 2). Examples of the drying method include warm air
drying, hot air drying, low humidity air drying, and drying by
application of (far) infrared rays, electron beams, or the like.
The drying conditions may be adjusted such that the liquid medium
can be removed as soon as possible, as far as the electrically
conductive adhesive layer does not crack by stress concentration or
does not come off from the collector.
[0073] In the manufacturing method of the invention, the powdered
mixture D comprising the binder and the active material and
optionally the electroconductivity-imparting agent is fed onto the
collector and heated and optionally press-formed so that electrodes
can be produced with no influence of changes in electrode paste
viscosity in contrast to the conventional methods and thus uniform
electrodes can be produced.
[0074] According to the invention, the powdered mixture D
comprising the binder and the active material and optionally the
electroconductivity-imparting agent is fed onto the collector,
allowed to have an even surface, and then heated and optionally
pressed to form an electrode. According to the invention,
therefore, the electrode production can be continuously performed,
and thus manufacturing methods suitable for mass production can be
provided.
EXAMPLES
[0075] The invention is further described by means of the examples
below, which are not intended to limit the scope of the invention.
In the examples, "parts or part" and "%" are by weight, unless
otherwise stated. The particle diameter of each of the particulate
rubber and the thermoplastic elastomer for use as the binder is
determined as a number-average particle diameter by measuring the
diameters of randomly selected 100 polymer particles with
transmission electron micrographs and calculating their
arithmetical mean.
<Measurement Methods>
(1) Electrode Density
[0076] An electrode sheet for capacitor of 5 cm.times.5 cm was
taken by cutting and measured for weight and thickness. The weight
and thickness of the collector were subtracted from the measured
weight and thickness, respectively, when the density of the
electrode layer (g/cm.sup.3) was calculated.
(2) Peel Strength of Electrode
[0077] A 100 mm-long 25 mm-wide rectangle test piece of an
electrode sheet for capacitor was taken by cutting and fixed with
its electrode layer surface facing upward. After a cellophane tape
was stuck on the electrode layer surface of the test piece, one end
of the cellophane tape was pulled and peeled off at a pulling rate
of 50 mm/minute in a vertical direction, while stress was measured.
The measurement was performed three times, and the average value
was calculated as the peel strength. Greater peel strength means
greater adhesion of the electrode layer to the collector.
(3) Capacitance and Internal Resistance of Electric Double Layer
Capacitor
[0078] An electric double layer capacitor was charged at 25.degree.
C. for 10 minutes to 2.7 Vat a constant current of 10 mA and then
discharged to 0 V at a constant current of 1 mA. Its capacitance
was determined from the resulting charge/discharge curve, and the
capacitance per unit weight of the electrode layer was calculated
by subtracting the weight of the collector from the weight of the
electrode and dividing the capacitance by the calculated weight of
the electrode layer. Its internal resistance was determined from
the charge/discharge curve by the calculation method according to
Japan Electronics and Information Technology industries Association
Standard No. RC-2377.
<Preparation of Particulate Thermoplastic Elastomer A>
[0079] To a stainless-steel autoclave was added 2500 parts of
purified water, and 25 parts of a suspending agent of polyvinyl
alcohol was dissolved therein. Thereto was added 700 parts of a
backbone polymer of an ethylene/ethyl acrylate copolymer (15% by
mole of the ethyl acrylate unit) and dispersed by stirring.
Separately, 1.5 parts of benzoyl peroxide and 6 parts of
tert-butylperoxymethacryloyloxyethyl carbonate were dissolved in a
graft monomer mixture of 220 parts of methyl methacrylate and 80
parts of acrylonitrile, and the resulting monomer mixture was added
to the autoclave and stirred. The autoclave was then heated to
60.degree. C., and stirring was performed for 2 hours. Thereafter,
the temperature was raised to 80.degree. C., and stirring was
continued for 7 hours so that polymerization was completed and a
mixture of the backbone polymer and the branch polymer was
obtained. The resulting polymer mixture was washed with water and
dried, and then kneaded at 180.degree. C. for 10 minutes at a
rotation speed of 50 min.sup.-1 (50 rpm) in a laboplast mill (B-75
manufactured by Toyo Seiki Seisaku-sho, Ltd.) to give a graft
polymer.
[0080] Ten parts of the resulting graft polymer and 90 parts of
N-methylpyrrolidone were mixed and heated at 140.degree. C. to form
a polymer solution. The solution was then cooled to room
temperature under stirring so that particulate polymer was
precipitated to form a 10% polymer dispersion. The polymer
dispersion was spray-dried to give particulate thermoplastic
elastomer A (0.05 .mu.m in particle diameter).
<Preparation of Electroconductivity-Imparting Agent Dispersion
B>
[0081] In a kneader, 100 parts of acetylene black and 40 parts of
an aqueous solution of 10% carboxymethylcellulose (Cellogen 7H
manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) were kneaded, and
the resulting mixture was diluted with water to give
electroconductivity-imparting agent dispersion B.
<Preparation of Electrically Conductive Adhesive C>
[0082] One hundred parts of acetylene black, 20 parts of an aqueous
solution of 10% carboxymethylcellulose (Cellogen 7H manufactured by
Dai-ichi Kogyo Seiyaku Co., Ltd.), 50 parts of carboxy-modified
styrene/butadiene copolymer latex with a solids content of 40%
(BM-400B with a particle diameter of 0.12 .mu.m manufactured by
ZEON Corporation), and 10.2 parts of soft water were kneaded in a
kneader and then diluted with soft water to give electrically
conductive adhesive C with a solids content of 30% in which
acetylene black had an average particle diameter of 0.5 .mu.m when
measured by light-scattering method.
Example 1
[0083] In a kneader, 75 parts of a high-purity activated carbon
powder with a specific surface area of 1500 m.sup.2/g and an
average particle diameter of 10 .mu.m, 16.7 parts of
electroconductivity-imparting agent dispersion B, 12.5 parts of
particulate rubber of 40% carboxy-modified styrene/butadiene
copolymer latex (BM-400B), and 2 parts of particulate thermoplastic
elastomer A were kneaded, and then dried under reduced pressure to
be powdered. The resulting powdered mixture had an average particle
diameter of 15 .mu.m. After drying, electrically conductive
adhesive C was applied with a thickness of 5 .mu.m to the
collector. The powdered mixture was spread onto the adhesive-coated
collector using a screw feeder and allowed to pass through a roll
to have an even surface (at a collector sheet feeding speed of 4
m/minute). The collector with the spread powdered mixture was then
heated to 200.degree. C. so that the thermoplastic elastomer was
melted. Thereafter, the collector with the mixture was roll-pressed
to give a capacitor electrode sheet with an electrode thickness of
300 .mu.m.
[0084] The forming of the electrode sheet was continuously
performed for 20 minutes, and it was demonstrated that a long
capacitor electrode sheet was stably formed.
[0085] Two 4 cm.times.height 6 cm pieces each with a lead terminal
portion were cut from the capacitor electrode sheet. The electrode
faces of the two pieces of the capacitor electrode sheet were
opposed, and a 25 .mu.m-thick polyethylene separator was sandwiched
between them. The assembly was held between two pieces of 2
mm-thick, 5 cm-wide, 7 cm-high polypropylene sheet to form a
device.
[0086] A solution of 1.5 mol/L triethylmonomethylammonium
tetrafluoroborate in propylene carbonate was used as a liquid
electrolyte. The device was heated under vacuum at 200.degree. C.
for 3 hours so that impurities such as water were removed from the
device. Thereafter, the device was vacuum-impregnated with the
liquid electrolyte, and held in the polypropylene container to form
an electric double layer capacitor. The DC resistance and
capacitance of the capacitor were measured at a current density of
20 mA/cm.sup.2, and the capacitance per unit weight of the
electrode layer (capacitance density) and the volume resistance
were calculated, which showed good capacitor performance. Table 1
shows the results of evaluation of the respective characteristics
of the resulting electrode sheet and electric double layer
capacitor.
Example 2
[0087] A capacitor electrode sheet and an electric double layer
capacitor were prepared and subjected to measurement using the
process of Example 1 except that 12.5 parts of particulate rubber
of 40% acrylate latex (composition: 2-ethylhexyl
acrylate/acrylonitrile/methacrylic acid/ethylene glycol
dimethacrylate=81/15/3/1 with a particle diameter of 0.15 .mu.m)
and 5 parts of particulate thermoplastic elastomer A were
alternatively used. It was demonstrated that a long capacitor
electrode sheet was stably formed and that the capacitor had good
performance. The results are shown in Table 1.
Example 3
[0088] A capacitor electrode sheet and an electric double layer
capacitor were prepared and subjected to measurement using the
process of Example 2 except that 16.7 parts of a polymer dispersion
(with a particle diameter of 0.20 .mu.m and a solids content of
30%) was used as the particulate rubber, wherein the polymer
dispersion was produced by emulsion polymerization of a monomer
mixture of 74 parts of butyl acrylate, 25 parts of methyl
methacrylate and 1 part of ethylene glycol dimethacrylate in the
presence of 5 parts of polyvinyl alcohol. It was demonstrated that
a long capacitor electrode sheet was stably formed and that the
capacitor had good performance. The results are shown in Table
1.
Example 4
[0089] A capacitor electrode sheet and an electric double layer
capacitor were prepared and subjected to measurement using the
process of Example 1 except that particulate thermoplastic
elastomer A was not used. It was demonstrated that a long capacitor
electrode sheet was stably formed and that the capacitor had good
performance. The results are shown in Table 1.
Comparative Example 1
[0090] Similarly to Example 1, 75 parts of the high-purity
activated carbon powder, 16.7 parts of
electroconductivity-imparting agent dispersion B, 12.5 parts of
particulate rubber of 40% carboxy-modified styrene/butadiene
copolymer latex (BM-400B), and 2 parts of particulate thermoplastic
elastomer A were used, but in place of the powdered mixture of
Example 1 produced by heat-drying the materials, an
electrode-forming slurry with a solids content of 40% was prepared
by kneading the materials with a planetary mixer and adding water
thereto. The electrode-forming slurry was applied onto the
collector treated with electrically conductive adhesive C and dried
at 200.degree. C. Thereafter, an electric double layer capacitor
was prepared and subjected to evaluation using the process of
Example 1.
[0091] The electrode-forming slurry prepared by the above method
was placed in a glass beaker and stirred for 1 hour so that it lost
fluidity. At that time, it had a solids content of 41%. Thus, it
has been demonstrated that when such a carbonaceous material with a
large specific surface area is used, particularly when an activated
carbon powder is used as an active material, the electrode-forming
slurry can lose fluidity by a slight change in concentration to
make continuous operation difficult. TABLE-US-00001 TABLE 1
Comparative Example 1 Example 2 Example 3 Example 4 Example 1
Electrode Density (g/cm.sup.3) 0.55 0.56 0.55 0.54 0.53 Peel
Strength (N/cm) 0.06 0.08 0.11 0.06 0.09 Internal Resistance
(.OMEGA.F) 4.2 3.7 3.6 4.0 4.5 Capacitance (F/gr) 55.6 57.3 56.5
56.2 55.2 Continuous Sheet Good Good Good Good Poor Formability
INDUSTRIAL APPLICABILITY
[0092] As described above, according to the invention, there is
provided a manufacturing method that allows continuous mass
production of electrodes for electrochemical devices.
* * * * *