U.S. patent application number 09/296404 was filed with the patent office on 2002-03-14 for process for producing an electrode for a battery.
Invention is credited to IIJIMA, TADAYOSHI, SATOH, JUNICHI.
Application Number | 20020029464 09/296404 |
Document ID | / |
Family ID | 17626152 |
Filed Date | 2002-03-14 |
United States Patent
Application |
20020029464 |
Kind Code |
A1 |
IIJIMA, TADAYOSHI ; et
al. |
March 14, 2002 |
PROCESS FOR PRODUCING AN ELECTRODE FOR A BATTERY
Abstract
A process for producing an electrode for a battery, wherein
electrode active material layers are firmly formed on both surfaces
of a current collector for the electrode. An electrode coating
containing an electrode active material, a binder, a solvent and an
acid is applied to one surface of an electrode current collector,
then dried, and the other surface of the current collector is
cleaned with water, and the electrode active material layer is
formed on the other surface of the current collector.
Inventors: |
IIJIMA, TADAYOSHI; (TOKYO,
JP) ; SATOH, JUNICHI; (TOKYO, JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Family ID: |
17626152 |
Appl. No.: |
09/296404 |
Filed: |
April 23, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09296404 |
Apr 23, 1999 |
|
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PCT/JP97/03845 |
Oct 23, 1997 |
|
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Current U.S.
Class: |
29/623.5 ;
29/623.3; 429/217; 429/218.1; 429/231.4; 429/231.8; 429/231.95;
429/232 |
Current CPC
Class: |
H01M 4/0409 20130101;
H01M 4/133 20130101; Y10T 29/49108 20150115; H01M 4/622 20130101;
H01M 4/0416 20130101; H01M 4/587 20130101; H01M 4/0435 20130101;
Y10T 29/49112 20150115; H01M 4/0411 20130101; H01M 4/04 20130101;
Y10T 29/49115 20150115; H01M 4/0414 20130101; H01M 4/0404 20130101;
H01M 4/64 20130101; H01M 4/0419 20130101; Y02E 60/10 20130101 |
Class at
Publication: |
29/623.5 ;
29/623.3; 429/218.1; 429/231.8; 429/231.95; 429/232; 429/217;
429/231.4 |
International
Class: |
H01M 004/04; H01M
004/48; H01M 004/58; H01M 004/62 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 23, 1996 |
JP |
8-280513/1996 |
Claims
What is claimed is:
1. A process for producing an electrode for a battery which
comprises successively applying an electrode coating containing an
electrode active material, a binder, a solvent and an acid to both
surfaces of an electrode current collector, wherein after applying
the electrode coating to one surface of the electrode current
collector and drying it, the other surface thereof is cleaned with
water prior to the application of the electrode coating
thereto.
2. The process of claim 1, wherein the cleaning is conducted by
using water or a mixture of water and an organic solvent.
3. The process of claim 2, wherein the cleaning is conducted by
using pure water.
4. The process of claim 2, wherein the cleaning is conducted by
using the a mixture of water and an organic solvent.
5. The process of claim 4, wherein the organic solvent is methanol,
ethanol or acetone.
6. The process of claim 4, wherein the organic solvent is used in
an amount of 0 to 60% bar weight based on the mixture of water and
the organic solvent.
7. The process of claim 1, wherein the electrode active material is
a cathode active material or an anode active material.
8. The process of claim 7, wherein the cathode active material is a
carbonaceous material.
9. The process of claim 8, wherein the carbonaceous material is
selected from the group consisting of amorphous carbon, acetylene
black, petroleum coke, coal coke, artificial graphite, natural
graphite, graphite carbon fibers and difficultly graphitizable
carbon.
10. The process of claim 1, wherein the acid is selected from the
group consisting of oxalic acid, formic acid, maleic acid, and
hydrates thereof.
11. The process of claim 10, wherein the acid is used in an amount
of 0.001 to 5 parts by weight per 100 parts by weight of the
electrode active material.
12. The process of claim 1, wherein the electrode current collector
is made of metal foil.
13. The process of claim 11, wherein the acid is used in an amount
of 0.01 to 3 parts by weight per 100 parts by weight of the
electrode active material.
14. The process of claim 7, wherein said cathode active material is
selected from the group consisting of lithium cobaltate and lithium
manganate.
15. The process of claim 12, wherein said metal foil is selected
from the group consisting of foils of copper, aluminum, stainless
steel, nickel and iron.
16. The process of claim 1, wherein said cleaning with water is
effected at a temperature of 0 to 50.degree. C.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a process for producing an
electrode for a battery. In particular, the present invention
relates to a process for producing an electrode for a battery,
wherein an electrode active material layer can be firmly formed on
both surfaces of a current collector for the electrode.
[0003] 2. Description of the Background
[0004] Electrodes used in lithium ion secondary batteries are
presently formed by applying a coating containing an electrode
active material to both surfaces of a current collector of the
electrode and drying the coating. In particular, a coating for the
cathode contains a cathode active material and a binder, wherein
the cathode active material is suitably dispersed therein in such a
manner that it is not broken. The cathode coating is first applied
to one surface of a metal foil as the electrode current collector
and, after drying, it is also applied to the other surface thereof
in the same manner as above, and then dried. Thus, the electrode
active material layers are formed on both surfaces of the current
collector for the electrode. Finally, the current collector for the
electrode having the electrode active material layers formed on
both surfaces thereof is cut into pieces, which is then used as the
electrodes.
[0005] In the case where the coating film is formed on such a metal
foil in the prior art, the adhesion between the metal foil and the
electrode active material layer is insufficient to causing this
problem, it has the electrode active material layer to peel off. In
addressing this problem, it has been proposed, for example, to
increase a resin content of an electrode coating, and to add an
acid. Also, Japanese Patent Unexamined Published Application
(hereinafter referred to as "J. P. KOKAI") No. Hei 2-68855
discloses and specifically describes that the adhesion is improved
by using an acid.
[0006] However, when such an acid is used another problem occurs.
Specifically, when the electrode active material layer is formed on
one surface (hereinafter referred to as "surface A") of the
electrode current collector and then the same layer is formed on
the other surface (hereinafter referred to as "surface B") thereof,
the adhesion of the current collector to surface B is much
decreased as compared with the adhesion of that to surface A.
Therefore, the electrode thus prepared exhibits the peeling off of
the electrode active material layer, particularly from the back
surface (surface B) of the electrode current collector. When such
peeling occurs, the capacity of the battery produced therefrom is
lowered, or the electrode active material layer thus peeled off is
interposed between a separator and, for example, a cathode
electrode, and it breaks the separator to cause the short circuit
of the cathode electrode and the anode electrode,
disadvantageously. Such a product is, therefore, not practically
usable as a battery element.
SUMMARY OF THE INVENTION
[0007] Accordingly, it is an object of the present invention to
provide a process for producing an electrode for a battery by
successively applying an electrode coating containing an electrode
active material, a binder, a solvent and an acid to both surfaces
of an electrode current collector to obtain excellent adhesion
between the electrode active material layer and the electrode
current collector and to avoid peeling-off of the electrode active
material layers from both surfaces of the electrode current
collector.
[0008] In particular, the above object and others are provided by a
process for producing an electrode for a battery by successively
applying an electrode coating containing an electrode active
material, a binder, a solvent and an acid to both surfaces of an
electrode current collector, wherein after applying the electrode
coating to one surface of the electrode current collector and
drying it, the other surface thereof is cleaned with water prior to
the application of the electrode coating thereto.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0009] The present invention is predicated upon the surprising
discovery that the above-described object can be effectively
attained by a process for applying an electrode coating containing
an electrode active material, a binder, a solvent and an acid to
both surfaces of an electrode current collector successively,
wherein after applying the electrode coating to one surface of the
electrode current collector and drying it, the other surface
thereof is cleaned with water prior to the application of the
electrode coating thereto. The present invention has been completed
on the basis of this finding.
[0010] In more detail, the electrode coating used in the present
invention contains an electrode active material, a binder, a
solvent and an acid.
[0011] The electrode active materials usable herein are not
particularly limited and those used hitherto as the electrode
active materials are usable.
[0012] The materials used as the electrode active materials are
various, and they are selected suitably depending on the use for
the cathode or anode. Carbonaceous materials are those usually used
as the cathode active materials. The carbonaceous materials are
those used hitherto and not particularly limited. They are, for
example, amorphous carbon, acetylene black, petroleum coke, coal
coke, artificial graphite, natural graphite, graphite carbon fibers
and difficultly graphitizable carbon.
[0013] The anode active materials are those used hitherto. Various
cathode active materially, are usable without particular
limitation. Various cathode active materials, such as lithium
cobaltate and lithium manganate are usable.
[0014] The electrode coating in the present invention contains
usually about 10 to 75 % by weight, based on the solid content of
the electrode coating, and preferably about 25 to 55 % by weight,
of the electrode active material.
[0015] Any binder may be useable so long as it is conventionally
used in this technical field, without particular limitation. The
binder includes, for example, polyacrylonitrile (PAN), polyethylene
terephthalate, polyvinylidene fluoride (PVDF) and polyvinyl
fluoride.
[0016] The binder is used in an amount of about 1 to 40 parts by
weight, preferably about 2 to 25 parts by weight, and particularly
about 5 to 15 parts by weight, per 100 parts by weight of the
electrode active material.
[0017] The solvent is not particularly limited and any of those
used hitherto for the preparation of electrode coatings can be
used. The solvent includes, for example, N-methylpyrrolidone (NMP),
pyrrolidone, N-methylthiopyrrolidone, dimethylformamide (DMF),
dimethylacetamide and hexamethylphosphamide. They are used either
alone or in the form of a mixture of them.
[0018] The solvent is used in such an amount that the solid content
(nonvolatile matter content) of the electrode coating is about 10
to 80% by weight, preferably about 30 to 60% by weight and
particularly preferably about 35 to 45% by weight.
[0019] The acid may be either an organic acid or an inorganic acid.
As the acids, weak acids are preferred, and weak organic acids are
particularly preferred. Preferred examples of the weak organic
acids include oxalic acid, formic acid and maleic acid, and
hydrates of these acids.
[0020] The acid is used in an amount of usually about 0.001 to 5
parts by weight, preferably about 0.01 to 3 parts by weight, per
100 parts by weight of the electrode active material.
[0021] When the electrode active maters has a low electric
conductivity, an electric conductor can be used, if necessary. As
the electric conductors, the above-described carbonaceous materials
are usable. In this cases the electric conductor is used in an
amount of usually about 1 to 25 parts by weight, preferably about 3
to 15 parts by weight and particularly preferably about 5 to 10
parts by weight, per 100 parts by weight of the active
material.
[0022] Metal foils are preferably used as the electrode current
collectors in the present invention. The metal materials for the
electrode current collectors are not particularly limited, and
various metal materials used hitherto for this purpose are usable.
Such metal materials are, for example, copper, aluminum, stainless
steel, nickel and iron.
[0023] The electrode coating used in the present invention is
prepared by mimed the above-described ingredients together, and it
is in the form of a slurry. The electrode active material must be
suitably dispersed in the electrode coating to such an extent that
it is not broken. The mixing and dispersion are conducted by means
of a planetary mixer, ball mill or the like.
[0024] The electrode coating is applied to both surfaces of the
electrode current collector and dried to form the electrode active
material layers.
[0025] The electrode coating can be applied to the electrode
current collector by a well-known method such as extrusion coating,
gravure coating, reverse roll coating, dip coating, kiss coating,
doctor coating, knife coating, curt coated or screen printing.
[0026] The electrode coating thus applied is dried by a method
which varies depending on the kind of the solvent. For example, it
can be dried with hot air of a temperature of 80 to 300.degree.
C.
[0027] In the present invention, the electrode active material
layer is formed by applying the electrode coating to one surface of
the electrode current collector and then dried under the
above-described conditions. Before the electrode coating is applied
to the back surface (the other surface, surface B) of the electrode
current collector and dried, this surface to be coated must be
cleaned with water. By this cleaning operation, the adhesion of the
electrode active material layer to the back surface (surface B) of
the electrode current collector can be remarkably improved Water
used for the cleaning may be any of pure water (distilled water),
ion-exchanged water, tap water, industrial water, well water, etc.
Among them pure water and ion-exchanged water having a low impurity
content are preferred.
[0028] The electrode current collector can be cleaned by, for
example, a method which comprises winding a cloth around a roller,
impregnating the cloth with water and rotating it to bring it into
contact with the current collector; a method which comprises
winding a cloth around a rod, impregnating the cloth with water and
reciprocating the rod widthwise in contact with the electrode
current collector; or a method wherein water or water vapor is
directly jetted to the electrode current collector. Other
embodiments of the cleaning method are obvious to those skilled in
the art. However, in immersion methods such as a dipping method,
bad effects may be exerted, such as falling-off of the dried
coating layer and. therefore, the layer must be carefully protected
by, for example, masking.
[0029] For facilitating the drying operation after the cleaning, a
mixture of water and an organic solvent may be used for the
cleaning so far as the cleaning effect is not reduced. The organic
solvents usable herein are those highly soluble in water such as
methanol, ethanol and acetone.
[0030] The amount of the organic solvent is usually 0 to about 60%
by weight, preferably 0 to about 50% by weight, based on the
mixture.
[0031] The cleaning temperature is usually about 5 to 50.degree.
C., preferably about 10 to 40.degree. C. When this temperature is
excessively low, the defining eject is reduced and, on the
contrary, when it is excessively high the foil is possibly
denatured after the cleanup. When the temperature is kept in this
range, water vapor can be used in place of water. The term "water"
thus indicates herein not only water but also water vapor.
[0032] When the electrode coating is applied to the back surface of
the electrode current collector, the surface is preferably dry.
Therefore, the water is wiped off with a dry cloth or air is blown
against the electrode current collector to dry it.
[0033] The thickness of the electrode having the electrode active
material layers on both sides of the current collector can be
controlled, if necessary, by roller pressing or the like.
[0034] The electrode material thus obtained is then cut into pieces
having predetermined width and length. Preferably, a part of the
electrode current collector is left free from the electrode active
material layer so as to provide electric contact with the outside.
The material partially lacking the electrode active material layer
can be formed by, for example, a method wherein an uncoated pant is
formed in the coating step or a method wherein the electrode active
material layer is once formed and then a part thereof is
removed.
[0035] The adhesion of the electrode active material layer, formed
at first on one surface (surface A) of the electrode current
collector to this surface A is higher that of the electrode active
material layer, formed on the other surface (surface B) thereof to
this surface B. The reasons why the adhesion to the surface B is
inferior to that of the surface A have not been elucidated yet.
Although it has not been theoretically proved yet, the mechanism is
supposed to be as follows: The acid contained in the coating is
evaporated during the drying and adheres to the surface B of the
electrode current collector to form some compound with a metal of
the electrode current collector. Although it was considered to
solve such a problem of the poor adhesion, by differentiating the
ratio of the electrode active material to the binder on the surface
A from the ratio on the surface B, such a method causes a problem
that the variety of the electrode coatings to be prepared is
increased in number, and the steps in the production process age
increased to make the process complicated. The present inventors
have surprisingly discovered that the cleaning of the surface B
with water after the coating of the surface A and before the
coating of the surface B of the current collector is very effective
in efficiently improving the adhesion without necessitating such
complicated steps.
[0036] The present invention will now be further illustrated by
reference to certain examples, which are provided solely for
purposes of illustration and are not intended to be limitative.
EXAMPLE 1
[0037] 1. Composition for Cathode Coating Material
[0038] The coating composition for the cathode is as follows:
1 parts by weight Graphite (cathode active material) 100 Acetylene
black (electric conductor) 5 PVDF (hinder) 10 NMP (solvent) 115
Oxalic acid dihydrate (acid) 1
[0039] The cathode coating was prepared as described below.
[0040] 10 parts by weight of the binder were dissolved in 50 parts
by weight of the solvent to obtain 60 parts bar weight of a
lacquer. 10 parts by weight of the lacquer was added to 5 parts by
weight of acetylene black and the resultants mixture was kneaded.
The balance (50 parts by weight) of the lacquer and 65 parts by
weight of the solvent were added to the kneaded mixture, and they
were thoroughly mixed together. 100 parts by weight of the cathode
active material was mixed therein and then 1 part by weight of
oxalic acid dihydrate was added thereto to obtain the cathode
coating.
[0041] 2. Preparation of Cathode
[0042] The cathode coating prepared as described above, was applied
to a roiled copper foil having a thickness of 18 .mu.m by the
nozzle coating method and then dried in a drying furnace at
110.degree. C. (coating of surface A). The back surface (surface B)
of the foil having the coated surface A was wiped with a non-woven
fabric imprecated with water and then dried. The foil was rolled
and the back surface (surface B) was coated in the some manner as
that for surface A. The electrode current collector having the
electrode active material layers on both surfaces was
compression-molded with a roller press and cut into pieces to be
used as cathodes.
EXAMPLE 2
[0043] The same procedure as that of Example 1 was repeated except
that the pure water was replaced by tap water.
EXAMPLE 3
[0044] The some procedure as that of Example 1 was repeated except
that the pure water was replaced by water/ethanol (weight ratio:
1/3).
EXAMPLE 4
[0045] The same procedure as that of Example 1 was repeated except
that the pure water was replaced by water/ethanol (weight ratio:
1/3).
Comparative Example 1
[0046] The same procedure as that of Example 1 was repeated except
that surface B was coated directly after the coating of surface A
without any treatment, to obtain a sample of Comparative Example
1.
Comparative Example 2
[0047] The same procedure as that of Example 1 was repeated except
that the pure water was replaced by ethanol, to obtain a sample of
Comparative Example 2.
Comparative Example 3
[0048] The same procedure as that of Example 1 was repeated except
that the pure water was replaced by methyl ethyl ketone, to obtain
a sample of Comparative Example 3.
[0049] 3. Evaluation Method
[0050] Determination of Contact Angle
[0051] The contact angles at three points, close to one another, of
the surface of each sample (copper foil having uncoated surface B)
were determined with a contact angle meter (CA-D; a product of
Kyowa Kaimen Kagaku K. K.) by using pure water as the liquid for
the determination, and the average of them was taken as the contact
angle of the sample. The larger the contact angle, the higher the
hydrophobicity.
[0052] Adhesion
[0053] The electrode current collector was cut into pieces of 1
cm.times.10 cm. A pressure-sensitive adhesive double coated tape
was applied to a supporting plate. An end part (3 cm) of the
surface, to be tested, of the electrode current collector (surface
with electrode active material layer) was stuck on the tape. The
electrode current collector was bent at such a curvature that it
would not be broken off, while the other unstuck edge was pulled at
a constant rate of 2 cm/sec with a tensile testing machine, to
conduct the peeling test. The degree of exposure of the copper foil
was determined according to the following criteria:
[0054] A: Less than 20% of the copper foil was exposed.
[0055] B: 20 to 80% of the copper foil was exposed.
[0056] C: More than 80% of the copper foil was exposed.
[0057] Solvent Resistance
[0058] The electrode active material laborer remaining after the
above-described tests was rubbed five times with a cotton swab
impregnated with methyl ethyl ketone, and the solvent resistance
thereof was evaluated according to the following criteria:
[0059] A: The electrode acquire material layer was not peeled off
at all.
[0060] B: The electrode active material layer was slightly peeled
off.
[0061] C: The electrode active material layer was wholly peeled off
after rubbing five times or less.
[0062] D: The electrode active material layer was wholly peeled off
after rubbing twice or less.
[0063] The test results are shown in following Table 1.
2 TABLE 1 Contact Angle Peeling Resistance Solvent Resistance
Surface A 81.7.degree. A A (before coating) Example 1 75.0.degree.
A B Example 2 70.3.degree. A B Example 3 62.3.degree. A B Example 4
45.5.degree. A C Comp. Ex. 1 41.1.degree. B D Comp. Ex. 2
40.0.degree. A D Comp. Ex. 3 23.2.degree. B D
[0064] It is thus clear that the adhesion of the electrode active
material layer to the back surface is surprisingly improved by
cleaning the back surface with water after forming the electrode
active material layer on the front surface of the electrode current
collector and before the application of the electrode coating to
the back surface. Thus, according to the process of the present
invention, crack formation in the electrode active material layer
and peeling-off of the tile layer from the electrode current
collector can be effectively prevented. Further, since the amount
of the binder in the electrode coating is reduced, the relative
amount of the electrode active material can be increased.
[0065] Having described the present invention, it will now be
apparent that many changes and modifications may be made to the
above-described embodiments without departing from the spirit and
the scope of the present invention.
* * * * *