U.S. patent application number 11/329594 was filed with the patent office on 2006-07-13 for electrode for electrochemical cell, method of manufacturing the same, and electrochemical cell includng the electrode.
Invention is credited to Dong-min Im, Jin-hwan Park, Mi-jeong Song.
Application Number | 20060151318 11/329594 |
Document ID | / |
Family ID | 36652170 |
Filed Date | 2006-07-13 |
United States Patent
Application |
20060151318 |
Kind Code |
A1 |
Park; Jin-hwan ; et
al. |
July 13, 2006 |
Electrode for electrochemical cell, method of manufacturing the
same, and electrochemical cell includng the electrode
Abstract
An electrode for an electrochemical cell is provided. The
electrode comprises an electrode active material coated on a
current collector. The surface of the electrode active material has
a greater porosity than the portion nearest the current collector.
The electrode includes an active material with controlled porosity,
where the porosity of the inner portion is equal to or less than
the porosity of the surface of the electrode after the electrode is
roll-pressed. As a result, the impregnating characteristics of the
electrolytic solution are improved and decreases in capacity upon
charging and discharging at high rates are prevented. Therefore,
excellent charge and discharge characteristics are obtained. In
addition, cells including the inventive electrodes exhibit
excellent charge and discharge characteristics.
Inventors: |
Park; Jin-hwan; (Jung-gu,
KR) ; Song; Mi-jeong; (Suwon-si, KR) ; Im;
Dong-min; (Seocho-gu, KR) |
Correspondence
Address: |
CHRISTIE, PARKER & HALE, LLP
PO BOX 7068
PASADENA
CA
91109-7068
US
|
Family ID: |
36652170 |
Appl. No.: |
11/329594 |
Filed: |
January 10, 2006 |
Current U.S.
Class: |
204/284 |
Current CPC
Class: |
H01M 2004/021 20130101;
H01M 4/0471 20130101; H01M 4/1393 20130101; H01M 4/0435 20130101;
H01M 4/133 20130101; H01M 4/0404 20130101; Y02E 60/10 20130101;
H01M 4/13 20130101; H01M 10/0525 20130101; H01M 4/139 20130101 |
Class at
Publication: |
204/284 |
International
Class: |
C25B 11/02 20060101
C25B011/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 11, 2005 |
KR |
10-2005-0002449 |
Claims
1. An electrode for an electrochemical cell comprising an electrode
active material coated on a current collector, wherein a porosity
of the electrode active material near a surface of the electrode
active material is greater than a porosity of the electrode active
material near the current collector.
2. The electrode of claim 1, wherein the surface of the electrode
active material which contacts an electrolytic solution has the
greatest porosity.
3. The electrode of claim 1, wherein the electrode active material
is manufactured by sintering an active material and a pore forming
material.
4. The electrode of claim 3, wherein the pore forming material is
selected from the group consisting of thermally decomposable
materials, materials capable of dissolving in an electrolytic
solution and mixtures thereof.
5. The electrode of claim 4, wherein the thermally decomposable
material comprises a compound selected from the group consisting of
ammonium carbonate, ammonium bicarbonate, ammonium oxalate and
mixtures thereof.
6. The electrode of claim 4, wherein the material capable of
dissolving in the electrolytic solution comprises a compound
selected from the group consisting of LiClO.sub.4, LiBF.sub.4,
LiPF.sub.6, LiCF.sub.3SO.sub.3, and mixtures thereof.
7. The electrode of claim 3, wherein the pore forming material is
present in the electrode active material in an amount ranging from
about 0.1 to about 10% by weight based on the total weight of the
electrode active material.
8. An electrochemical cell comprising the electrode of any of
claims 1 to 7.
9. The electrochemical cell of claim 8, wherein the electrochemical
cell is selected from the group consisting of lithium ion batteries
and lithium ion polymer batteries.
10. A method of manufacturing an electrode for an electrochemical
cell, the method comprising: coating a current collector with an
electrode active material; coating the electrode active material
with a mixture of a pore forming material and the electrode active
material to form an electrode; roll-pressing the electrode; and
sintering the roll-pressed electrode.
11. A method of manufacturing an electrode for an electrochemical
cell, the method comprising: coating a current collector with an
electrode active material; coating the electrode active material
with a pore forming material to form an electrode; roll-pressing
the electrode; and sintering the roll-pressed electrode.
12. The method of claim 10, wherein the pore forming material is
selected from the group consisting of thermally decomposable
materials, materials capable of dissolving in an electrolytic
solution and mixtures thereof.
13. The method of claim 12, wherein the thermally decomposable
material comprises a compound selected from the group consisting of
ammonium carbonate, ammonium bicarbonate, ammonium oxalate and
mixtures thereof.
14. The method of claim 12, wherein the material capable of
dissolving in the electrolytic solution comprises a compound
selected from the group consisting of LiClO.sub.4, LiBF.sub.4,
LiPF.sub.6, LiCF.sub.3SO.sub.3, and mixtures thereof.
15. The method of claim 10, wherein the pore forming material is
present in the electrode in an amount ranging from about 0.1 to
about 10% by weight based on the total amount of the electrode
active material.
16. The method of claim 11, wherein the pore forming material is
selected from the group consisting of thermally decomposable
materials, materials capable of dissolving in an electrolytic
solution and mixtures thereof.
17. The method of claim 16, wherein the thermally decomposable
material comprises a compound selected from the group consisting of
ammonium carbonate, ammonium bicarbonate, ammonium oxalate and
mixtures thereof.
18. The method of claim 16, wherein the material capable of
dissolving in the electrolytic solution comprises a compound
selected from the group consisting of LiClO.sub.4, LiBF.sub.4,
LiPF.sub.6, LiCF.sub.3SO.sub.3, and mixtures thereof.
19. The method of claim 11, wherein the pore forming material is
present in the electrode in an amount ranging from about 0.1 to
about 10% by weight based on the total weight of the electrode
active material.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
[0001] This application claims priority to and the benefit of
Korean Patent Application No. 10-2005-0002449, filed on Jan. 11,
2005, in the Korean Intellectual Property Office, the entire
content of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to an electrode having
controlled porosity for an electrochemical cell, and to an
electrochemical cell having the same. More particularly, the
invention is directed to an electrode in which non-uniform porosity
of the electrode active material, which occurs during
roll-pressing, is prevented, thereby improving the charge and
discharge characteristics of the electrochemical cell including the
electrode.
BACKGROUND OF THE INVENTION
[0003] Electrochemical cells, e.g. secondary batteries, are used in
portable electronic devices, and the demand for these
electrochemical cell has been increasing. As portable devices
become smaller and lighter in weight, they require high performance
and electrochemical cells having high capacity are required.
[0004] In order to obtain an electrochemical cell with high
capacity, an electrode material having high capacity can be used,
or the density of the electrode can be increased using physical
methods.
[0005] When an electrode material having high capacity is used, the
electrode material can be a metal, such as lithium or the like.
However, when lithium or the like is repeatedly charged and
discharged, lithium dendrite grows on the surface of the electrode,
short-circuiting the electrode. As such, use of a metal decreases
stability. On the other hand, use of a carbon material is safe
because no side reactions occur and a variety of carbon material
powders can be made. However, the use of carbon material results in
low electrical capacity. Therefore, to increase the electrical
capacity, the electrode can be roll-pressed or the like to increase
density.
[0006] However, when the electrode is roll-pressed or the like,
electrode density increases, but porosity decreases due to a
decrease in volume, thus decreasing the impregnating
characteristics of the electrolytic solution. If this happens, the
electrolytic solution cannot sufficiently penetrate the inner
portion of the electrode, thereby decreasing the contact area
between the electrode and the electrolytic solution. Accordingly,
ions are not sufficiently transported to the electrode and
sufficient cell capacity cannot be obtained. Further, when the cell
is charged and discharged at a high rate, cell performance
decreases.
[0007] Many techniques have been developed to improve the
impregnating characteristics of the electrolytic solution. For
example, in Japanese Laid-open Patent No. 1994-060877, the
impregnating characteristics are improved by treating the anode
with plasma, or by adsorbing a wetting agent onto the anode. When
plasma-treated, the surface of the anode becomes rough. When the
wetting agent is adsorbed into the anode, interfacial tension
between the electrode and electrolytic solution decreases.
[0008] In Japanese Laid-open Patent No. 1996-162155, the
impregnating characteristics of the electrolytic solution are
improved by adding a non-ionic surfactant to the electrolytic
solution. In this technique, the non-ionic surfactant acts as the
wetting agent and is directly added not into the electrode but into
the electrolytic solution.
[0009] When an electrode operates, its temperature increases and
the electrode active material expands, thereby decreasing the
amount of the electrolytic solution which eventually becomes
insufficient. In Japanese Laid-open Patent No. 1999-086849, an
electrode having a high-temperature electrolytic solution and a
conductive material is used to prevent insufficiency of the
electrolytic solution.
[0010] These conventional techniques, in which the surface of the
electrode is modified or the temperature changed, are effective.
However, when the porosity of the electrode active material is
decreased from roll-pressing or the like, and the contact surface
between the electrode and electrolytic solution decreases, it
becomes very difficult to improve impregnating characteristics.
[0011] In particular, when the electrode is roll-pressed, the
pressure applied is greatest at the surface of the electrode such
that porosity decreases and density increases from the inner
portion to the surface of the electrode. Accordingly, even when the
inner portion of the electrode is sufficiently porous, the surface
of the electrode is less porous and the electrolytic solution
cannot sufficiently penetrate the inner portion of the electrode.
As a result, a method of manufacturing an electrode with sufficient
porosity is required.
SUMMARY OF THE INVENTION
[0012] In one embodiment, the present invention is directed to an
electrode with controlled porosity for an electrochemical cell, and
to an electrochemical cell having the electrode. In another
embodiment, the present invention is directed to a method of
manufacturing the electrode.
[0013] According to an embodiment of the present invention, an
electrode for an electrochemical cell comprises an electrode active
material coated on a current collector, wherein the porosity of the
electrode active material near the exposed surface is greater than
the porosity of the electrode active material nearest the current
collector.
[0014] In one embodiment, the porosity is greatest at the surface
of the electrode active material contacting the electrolytic
solution.
[0015] In another embodiment, the porosity increases as the time of
contact between the electrode active material and the electrolytic
solution increases.
[0016] The electrode active material may be a sinter of an active
material and a pore forming material.
[0017] According to another embodiment of the present invention, an
electrochemical cell includes the electrode.
[0018] According to yet another embodiment of the present
invention, a method of forming an electrode for an electrochemical
cell comprises coating a current collector with an electrode active
material, coating the coated current collector with a mixture of a
pore forming material and the electrode active material to form an
electrode, roll-pressing the electrode, and sintering the
roll-pressed electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The above and other features and advantages of the present
invention will become more apparent by reference to the following
detailed description when considered in conjunction with the
attached drawings in which:
[0020] FIG. 1 is an energy dispersive X-ray spectroscopy (EDS)
image of a cross section of a roll-pressed cobalt oxide electrode
according to the prior art;
[0021] FIG. 2 is a scanning electron microscopy (SEM) image of a
carbon anode according to one embodiment of the present invention
in which the carbon anode is coated with a pore forming material
and roll-pressed; and
[0022] FIG. 3 is an SEM image of the carbon anode of FIG. 2 after
removal of the pore forming material.
DETAILED DESCRIPTION OF THE INVENTION
[0023] An electrode for an electrochemical cell according to one
embodiment of the present invention includes an electrode active
material having constant porosity which can retain that porosity
after roll-pressing. Such an electrode imparts improved charge and
discharge characteristics. In conventional electrodes, porosity
substantially decreases near the surface of the electrode when
roll-pressed, adversely affecting the impregnating characteristics
of the electrolytic solution.
[0024] An electrode is roll-pressed to decrease its volume and to
increase its energy density. Upon roll-pressing, the thickness of
the active material layer coated on the current collector decreases
to about half the original thickness or less. The greatest pressure
is applied to the portion of the active material located farthest
from the current collector, thereby imparting the greatest density
and least porosity to the portion of the electrode material located
farthest from the current collector.
[0025] FIG. 1 is an energy dispersive X-ray spectroscopy (EDS)
image of a cross section of a prior art electrode formed by coating
a current collector with a mixture of cobalt oxide, a conducting
agent and a binder, and roll-pressing the resulting electrode. The
cobalt oxide is used as the cathode active material. In FIG. 1, the
yellow portions represent the cobalt oxide used as the active
material. As can be seen, the density of the active material
gradually increases from the current collector to the surface,
indicating a gradual decrease in porosity. As seen in FIG. 1, the
porosity is greatest nearest the current collector and the porosity
is least nearest the surface, which is common in roll-pressed
electrodes.
[0026] However, according to one embodiment of the present
invention, an electrode for an electrochemical cell has a porosity
near its surface equal to or greater than the porosity nearest the
current collector. According to this embodiment, the electrode has
a different porosity pattern that conventional electrodes, in which
porosity near the surface is less than the porosity near the
current collector. When the porosity near the surface is greater
than the porosity near the current collector, the impregnating
characteristics of the surface (where the electrolytic solution
first contacts the electrode) are improved, enabling the
electrolytic solution to easily penetrate the inner portion of the
electrode.
[0027] The electrode can be used in any type of electrochemical
cell. For example, it can be used as a cathode or anode of a
lithium battery. In particular, it can be used as a carbon anode.
Metal material can easily be impregnated with the electrolytic
solution, but the carbon material has relatively bad impregnating
characteristics. Specifically, when the density of the carbon
material increases during roll-pressing or the like, the
impregnating characteristics become very poor. A nonlimiting
example of a suitable carbon material is graphite, or the like.
[0028] The porosity of the electrode active material may be
greatest at the surface contacting the electrolytic solution. In
conventional roll-pressed electrodes, the porosity of the surface
contacting the electrolytic solution is least, thus preventing
electrons or ions from moving to the electrode active material near
the current collector via the electrolytic solution. This
substantially decreases the contact area between the electrode and
the electrolytic solution. As a result, cell performance decreases.
When the porosity near the surface contacting the electrolytic
solution is increased, this problem is overcome and the
impregnating characteristics of the electrode are improved.
[0029] The porosity of the electrode active material may increase
as contact time between the electrode active material and the
electrolytic solution increases. The electrode active material may
further include a pore forming material that can be dissolved in
the electrolytic solution. After the cell is formed and the
electrode contacts the electrolytic solution, the pore forming
material begins to dissolve in the electrolytic solution and
generates pores, thereby increasing the porosity at the surface of
the electrode.
[0030] The electrode active material may be formed by sintering the
active material with the pore forming material. The pore forming
material included in the roll-pressed electrode active material
thermally decomposes by sintering such that pores are produced and
non-uniform density resulting from the roll-pressing is prevented.
The size of the pores formed by sintering, and the porosity, can be
controlled by selecting the particle size, distribution and the
like of the pore forming material.
[0031] Any material capable of producing pores can be used as the
pore forming material. Nonlimiting examples of suitable materials
for the pore forming material include thermally decomposable
materials, materials capable of dissolving in the electrolytic
solution and mixtures thereof.
[0032] FIG. 2 is a scanning electron microscopy (SEM) image of an
electrode according to one embodiment of the present invention in
which the electrode is further coated with the pore forming
material and then roll-pressed. When the electrode is sintered at a
predetermined temperature, the pore forming material decomposes and
porosity increases. FIG. 3 is an SEM image of the surface of the
electrode after sintering. As can be seen, the porosity of the
electrode in FIG. 3 is greater than that of the electrode in FIG.
2.
[0033] when the pore forming material is a thermally decomposable
material, it is vaporized by heat, leaving pores. When the pore
forming material is not decomposable by heat, it generates pores
after contacting the electrolytic solution. When the pore forming
material is a mixture of thermally decomposable materials and
materials capable of dissolving in the electrolytic solution, some
of the pore forming material thermally decomposes by heat to form
pores which the electrolytic solution penetrates, and the remaining
pore forming material dissolves in the penetrated electrolytic
solution, thereby producing pores.
[0034] Nonlimiting examples of suitable thermally decomposable
materials include ammonium carbonate, ammonium bicarbonate,
ammonium oxalate, and the like.
[0035] Nonlimiting examples of suitable materials capable of
dissolving in the electrolytic solution include salts that can
easily dissolve in non-aqueous electrolytes, such as lithium salt
or the like. Nonlimiting examples of suitable lithium salts include
LiClO.sub.4, LiBF.sub.4, LiPF.sub.6, LiCF.sub.3SO.sub.3, and the
like.
[0036] The amount of the pore forming material in the electrode
ranges from about 0.1 to 10% by weight based on the total weight of
the electrode active material. When the amount of the pore forming
material is greater than about 10% by weight, the density of the
electrode decreases. When the amount of the pore forming material
is less than about 0.1% by weight, controlled porosity is difficult
to achieve.
[0037] An electrochemical cell according to one embodiment of the
present invention includes the above-described electrode for an
electrochemical cell. The electrochemical cell may be a lithium
battery, but is not limited thereto.
[0038] One method of manufacturing the lithium battery will now be
described. However, it is understood that other methods may be used
to manufacture the lithium battery.
[0039] First, a cathode active material composition is formed by
mixing a cathode active material, a conducting agent, a binder, and
a solvent. The cathode active material composition is coated
directly on a metal current collector and dried, thereby forming a
cathode plate. Alternatively, the cathode active material
composition is cast onto a separate support, peeled, and then
laminated on the metal current collector, thereby forming the
cathode plate.
[0040] The cathode active material may be any lithium-containing
metal oxide commonly used in the art. Nonlimiting examples of
suitable cathode active materials include LiCoO.sub.2,
LiMn.sub.xO.sub.2x, LiNi.sub.1-xMn.sub.xO.sub.2x(x=1, 2),
Ni.sub.1-x-yCo.sub.xMn.sub.yO.sub.2 where 0.ltoreq.x.ltoreq.0.5,
0.ltoreq.y.ltoreq.0.5, and the like. Specifically, the cathode
active material may be a compound in which lithium can be oxidized
and reduced, such as LiMn.sub.2O.sub.4, LiCoO.sub.2, LiNiO.sub.2,
LiFeO.sub.2, V.sub.2O.sub.5, TiS, MoS, or the like.
[0041] One nonlimiting example of a suitable conducting agent is
carbon black.
[0042] Nonlimiting examples of suitable binders include vinylidene
fluoride/hexafluoropropylene copolymers, polyvinylidenefluoride,
polyacrylonitrile, polymethylmethacrylate, polytetrafluoroethylene,
styrene butadiene rubber polymers, and mixtures thereof.
[0043] Nonlimiting examples of suitable solvents include
N-methylpyrrolidone, acetone, water, and the like.
[0044] The amounts of cathode active material, conducting agent,
binder, and solvent are the same as in conventional lithium
batteries.
[0045] The lithium battery further comprises a separator, which can
be any separator commonly used in lithium batteries. One exemplary
separator has low resistance to the movement of the ions of the
electrolytic solution and good electrolytic solution binding
capacity. Nonlimiting examples of suitable materials for the
separator include glass fiber, polyester, Teflon, polyethylene,
polypropylene, polytetrafluoroethylene (PTFE), and mixtures thereof
in the form of woven or non-woven fabrics. In lithium ion
batteries, polyethylene, polypropylene or the like is used as the
separator, and is capable of being wound. In lithium ion polymer
batteries, separators with good impregnating capacity for organic
electrolytic solutions are used.
[0046] One exemplary method of manufacturing the separator will now
be described. However, it is understood that other methods can also
be used.
[0047] First, a separator composition is produced by mixing a
polymer resin, a filling agent, and a solvent. The separator
composition is directly coated on a surface of an electrode,
thereby forming a separator film. Alternatively, the separator
composition is cast onto a support, dried, peeled, and laminated on
a surface of the electrode.
[0048] The polymer resin can be any material capable of bonding to
an electrode plate. Nonlimiting examples of suitable polymer resins
include vinylidenefluoride/hexafluoropropylene copolymers,
polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate,
and mixtures thereof.
[0049] The electrolytic solution may contain a compound selected
from the group consisting of LiPF.sub.6, LiBF.sub.4, LiSbF.sub.6,
LiAsF.sub.6, LiClO.sub.4, LiCF.sub.3SO.sub.3,
Li(CF.sub.3SO.sub.2).sub.2N, LiC.sub.4F.sub.9SO.sub.3, LiSbF.sub.6,
LiAlO.sub.4, LiAlCl.sub.4,
LiN(C.sub.xF.sub.2x+1SO.sub.2)(C.sub.yF.sub.2y+1SO.sub.2) where
each of x and y is a natural number, LiCl, Lil, and mixtures
thereof. This compound is dissolved in a solvent selected from the
group consisting of propylene carbonate, ethylene carbonate,
diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate,
butylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran,
2-methyltetrahydrofuran, .gamma.-butyrolactone, dioxolane,
4-methyldioxorane, N,N-dimethylformamide, dimethylacetamide,
dimethylsulfoxide, dioxane, 1,2-dimethoxyethane, sulforane,
dichloroethane, chlorobenzene, nitrobenzene, dimethylcarbonate,
methylethylcarbonate, diethylcarbonate, methylpropylcarbonate,
methylisopropylcarbonate, ethylpropylcarbonate, dipropylcarbonate,
dibutylcarbonate, diethyleneglycol, and mixtures thereof.
[0050] The separator is positioned between the cathode plate and
the anode plate, thus forming a cell structure. The cell structure
is wound or folded and then sealed in a cylindrical or rectangular
battery case. The organic electrolytic solution is then injected
into the battery case to complete the lithium ion battery.
[0051] The cell structures can be stacked to form a bi-cell
structure, which is impregnated with the organic electrolytic
solution. The resulting cell is sealed in a pouch to form a lithium
ion polymer battery.
[0052] A method of manufacturing an electrode for an
electrochemical cell according to one embodiment of the present
invention will now be described. However, it is understood that
other methods can be used.
[0053] First, an electrode active material is coated on a current
collector. Then, a mixture of a pore forming material and an
electrode active material is coated on the resulting current
collector, thereby forming an electrode. The electrode is then
roll-pressed and sintered to form an electrode for an
electrochemical cell.
[0054] Alternatively, an electrode active material is coated on a
current collector and a pore forming material is coated on the
active material. The dual coated electrode is then roll-pressed and
sintered, thereby forming an electrode for an electrochemical
cell.
[0055] The inventive electrodes can be used in any type of
electrochemical cell, for example, they can be cathodes or anodes
of lithium batteries. In one embodiment, the electrode is a carbon
anode. Metal material can be easily impregnated with the
electrolytic solution, while carbon material has relatively bad
impregnating characteristics. Specifically, when the density of the
carbon material increases during roll-pressing or the like, the
impregnating characteristics become very poor. One nonlimiting
example of a suitable carbon material is graphite or the like.
[0056] Any material capable of generating pores can be used as the
pore forming material. The pore forming material may be a thermally
decomposable material, a material capable of dissolving in the
electrolytic solution, or a mixture thereof. The thermally
decomposable material may be ammonium carbonate, ammonium
bicarbonate, ammonium oxalate or the like. the material capable of
dissolving in the electrolytic solution may be a salt that is
easily dissolved in a non-aqueous electrolyte, such as a lithium
salt or the like. For example, the material capable of easily
dissolving in the non-aqueous electrolyte can be LiClO.sub.4,
LiBF.sub.4, LiPF.sub.6, LiCF.sub.3SO.sub.3, or the like.
[0057] The amount of the pore forming material in the electrode
ranges from about 0.1 to about 10% by weight based on the total
weight of the electrode active material. When the amount of the
pore forming material is greater than about 10% by weight, the
electrode density decreases. When the amount of the pore forming
material is less than about 0.1% by weight, controlled porosity is
difficult to achieve.
[0058] the present invention will now be described in detail with
reference to the following examples. These examples are provided
for illustrative purposes only, and do not limit the scope of the
present invention.
Manufacturing Anode Electrodes
EXAMPLE 1
[0059] 150 ml of distilled water was added to a mixture of 97 g of
graphite powder, 1.5 g of styrene butadiene rubber (SBR), and 1.5 g
of carboxy methyl cellulose (CMC). The mixture was stirred for 30
minutes using a mechanical mixing device, thus forming a first
slurry.
[0060] The first slurry was coated on a 10 .mu.m thick Cu current
collector to a thickness of about 100 .mu.m and a loading lever of
8 mg/cm.sup.2 using a doctor blade. The first slurry was then
dried.
[0061] 150 ml of distilled water was then added to a mixture of 5 g
of ammonium bicarbonate, 97 g of graphite powder, 1.5 g of SBR, and
1.5 g of CMC and stirred using a mechanical mixing device for 30
minutes to produce a second slurry. The second slurry was coated on
the first slurry to a loading level of about 2 mg/cm.sup.2.
[0062] The resulting coated Cu current collector was roll-pressed
to a density of 1.7 mg/cm.sup.3 and dried in a vacuum at
145.degree. C. for 3 hours, thereby forming an anode plate.
EXAMPLE 2
[0063] 150 ml of distilled water was added to a mixture of 97 g of
graphite powder, 1.5 g of SBR, and 1.5 g of CMC and stirred for 30
minutes using a mechanical mixing device, thereby forming a
slurry.
[0064] The slurry was coated on a 10 .mu.m thick Cu current
collector to a loading level of 10 mg/cm.sup.2 using a doctor
blade. The slurry was then dried.
[0065] The coated Cu current collector was further coated by
spraying it with ammonium bicarbonate dissolved in ethanol. The
ammonium bicarbonate was coated to a loading level of about 0.1
mg/cm.sup.2.
[0066] The coated Cu current collector was roll-pressed to a
density of 1.7 g/cm.sup.3 and then dried in a vacuum at 145.degree.
C. for 3 hours, thereby forming an anode plate.
EXAMPLE 3
[0067] An anode plate was manufactured as in Example 1 except that
ammonium oxalate was used instead of ammonium bicarbonate.
EXAMPLE 4
[0068] An anode plate was manufactured as in Example 2 except that
ammonium oxalate was used instead of ammonium bicarbonate.
EXAMPLE 5
[0069] An anode plate was manufactured as in Example 1 except that
LiClO.sub.4 was used instead of ammonium bicarbonate.
EXAMPLE 6
[0070] An anode plate was manufactured as in Example 2 except that
LiClO.sub.4 was used instead of ammonium bicarbonate.
EXAMPLE 7
[0071] An anode plate was manufactured as in Example 1 except that
a mixture of ammonium oxalate and LiClO.sub.4 was used instead of
ammonium bicarbonate.
EXAMPLE 8
[0072] An anode plate was manufactured as in Example 2 except that
a mixture of ammonium oxalate and LiClO.sub.4 was used instead of
ammonium bicarbonate.
EXAMPLE 9
[0073] An anode plate was manufactured as in Example 1 except that
10 g of ammonium bicarbonate was used.
EXAMPLE 10
[0074] An anode plate was manufactured as in Example 1 except that
20 g of ammonium bicarbonate was used.
EXAMPLE 11
[0075] An anode plate was manufactured as in Example 2 except that
the ammonium bicarbonate was coated to a loading level of 0.2
mg/cm.sup.2.
EXAMPLE 12
[0076] An anode plate was manufactured as in Example 2 except that
the ammonium bicarbonate was coated to a loading level of 0.4
mg/cm.sup.2.
COMPARATIVE EXAMPLE 1
[0077] An anode plate was manufactured as in Example 1 except that
the pore forming material was not used.
COMPARATIVE EXAMPLE 2
[0078] An anode plate was manufactured as in Example 2 except that
the pore forming material was not used.
Manufacturing of Half Cells
[0079] Each of the anode plates of Examples 1 to 12, and
Comparative Examples 1 and 2 was cut to a size of 2.times.3
cm.sup.2. A half cell was manufactured using each anode plate, a
lithium metal as a counter electrode, and an electrolyte formed by
adding 2.3 wt % of vinylene carbonate (VC) to a solution of
ethylene carbonate (EC), diethylcarbonate (DEC), fluorobenzene
(FB), and dimethyl carbonate (DMC) in a weight ratio of
3:5:1:1.
Charge and Discharge Test
[0080] Each half cell was discharged a constant current of 35 mA
per 1 g of active material until 0.001 V was obtained with respect
to the Li electrode. Thereafter, each half cell was discharged a
constant voltage of 0.001 V until the current decreased to 3.5 mA
per 1 g of the active material.
[0081] Each discharged cell was allowed to sit for about 30 minutes
and then charged at a constant current of 35 mA per 1 g of the
active material until the voltage reached 1.5 V.
[0082] Each cell was subjected to one discharge/charge cycle at 0.1
C, two discharge/charge cycles at 0.2 C, one discharge/charge cycle
at 0.5 C, one discharge/charge cycle at 1 C, and one
discharge/charge cycle at 2 C. High-rate charge/discharge
characteristics were measured using a ratio of high rate charge
capacity to the second 0.2 C cycle charge capacity. The results are
shown in Table 1. TABLE-US-00001 TABLE 1 0.2 C 0.2 C 0.5 C 1 C 2 C
Discharge/ Discharge/ Discharge/ Discharge/ Discharge/ Charge
Charge Charge Charge Charge Capacity Capacity Capacity Capacity
Capacity (mAh) (mAh) (mAh) (mAh) (mAh) Example 1 23.98/20.86
20.83/20.44 20.47/20.29 20.00/19.93 19.63/18.53 High-rate -- 100%
99.3% 97.5% 90.7% charge/discharge characteristics Example 2
24.17/20.96 20.92/20.50 20.57/20.39 20.18/20.07 20.03/18.71
High-rate -- 100% 99.5% 97.9% 91.3% charge/discharge
characteristics Example 3 23.64/20.87 20.73/20.38 20.38/20.24
20.00/19.82 19.54/18.46 High-rate -- 100% 99.3% 97.3% 90.6%
charge/discharge characteristics Example 4 24.02/20.87 20.84/20.40
20.49/20.24 20.03/19.89 19.86/18.53 High-rate -- 100% 99.2% 97.5%
90.8% charge/discharge characteristics Example 5 23.64/21.08
20.91/20.52 20.38/20.31 20.00/19.86 19.51/18.33 High-rate -- 100%
99.0% 96.8% 89.3% charge/discharge characteristics Example 6
23.55/21.12 20.77/20.40 20.24/20.10 19.96/19.65 19.37/18.14
High-rate -- 100% 98.5% 96.3% 88.9% charge/discharge
characteristics Example 7 23.98/20.98 20.83/20.56 20.47/20.35
20.07/20.03 19.63/18.84 High-rate -- 100% 99.0% 97.4% 91.6%
charge/discharge characteristics Example 8 23.92/20.94 20.92/20.63
20.57/20.39 20.28/20.10 20.07/19.00 High-rate -- 100% 98.8% 97.4%
92.1% charge/discharge characteristics Example 9 23.97/20.77
20.80/20.38 20.38/20.24 19.96/19.80 19.47/18.35 High-rate -- 100%
99.3% 97.2% 90.0% charge/discharge characteristics Example 10
24.23/20.87 20.70/20.35 20.32/20.20 20.18/19.96 19.82/18.33
High-rate -- 100% 99.3% 98.1% 90.2% charge/discharge
characteristics Example 11 24.30/20.52 20.70/20.52 20.38/20.17
19.96/19.12 19.40/17.54 High-rate -- 100% 98.3% 93.2% 85.5%
charge/discharge characteristics Example 12 24.06/20.45 20.56/20.40
20.31/20.17 19.96/19.68 19.26/17.75 High-rate -- 100% 98.9% 96.5%
87.0% charge/discharge characteristics Comparative 22.91/19.76
20.30/20.03 20.12/19.67 18.00/17.96 16.57/13.91 Example 1 High-rate
-- 100% 98.2% 89.7% 69.4% charge/discharge characteristics
Comparative 22.42/19.25 20.42/20.16 20.21/20.04 18.13/17.93
16.70/14.23 Example 2 High-rate -- 100% 99.4% 88.9% 70.6%
charge/discharge characteristics
[0083] As shown in Table 1, the charge capacities of the half cells
including the anode plates according to Examples 1 through 12 did
not decrease substantially. Rather, they maintained 85% or more of
their charge/discharge capabilities when charged and discharged at
high rates. This is 20% or greater than the half cells including
the anode plates according to Comparative Examples 1 and 2. These
excellent charge/discharge characteristics can be explained as
follows. The porosity of the electrode active material was
controlled using a pore forming material in Examples 1 through 12
such that the contact area between the electrode active material
and the electrolytic solution was larger than that in Comparative
Examples 1 and 2. As a result, the impregnating characteristics of
the electrolytic solution were improved, the electrolytic solution
could smoothly penetrate into the inner portion of the electrode,
and the area of the electrode in contact with the electrolytic
solution was increased, enabling ions to move more smoothly. Due to
these excellent high-rate charge/discharge characteristics, the
cells have large capacities, thus preventing decreases in
performance.
[0084] An electrode for an electrochemical cell according to the
present invention includes an electrode active material with
controlled porosity. In particular, after the electrode is
roll-pressed, the porosity of the surface of the electrode is equal
to or greater than the porosity of the inner portion of the
electrode. Therefore, the impregnating characteristics of the
electrolytic solution are improved and the capacitance of the
electrode, which occurs when charged and discharged at high rates
does not significantly decrease, thereby improving the charge and
discharge characteristics. Cells including the electrodes exhibit
excellent charge and discharge characteristics.
[0085] While the present invention has been described with
reference to certain exemplary embodiments, it will be understood
by those of ordinary skill in the art that various changes in form
and details may be made without departing from the spirit and scope
of the present invention as defined by the following claims.
* * * * *