U.S. patent application number 14/850590 was filed with the patent office on 2015-12-31 for method for manufacturing separator, separator manufactured by the method and method for manufacturing electrochemical device including the separator.
This patent application is currently assigned to LG CHEM, LTD.. The applicant listed for this patent is LG CHEM, LTD.. Invention is credited to Jang-Hyuk HONG, Jong-Hun KIM, Joo Sung LEE.
Application Number | 20150380702 14/850590 |
Document ID | / |
Family ID | 48136226 |
Filed Date | 2015-12-31 |
United States Patent
Application |
20150380702 |
Kind Code |
A1 |
LEE; Joo Sung ; et
al. |
December 31, 2015 |
METHOD FOR MANUFACTURING SEPARATOR, SEPARATOR MANUFACTURED BY THE
METHOD AND METHOD FOR MANUFACTURING ELECTROCHEMICAL DEVICE
INCLUDING THE SEPARATOR
Abstract
A method for manufacturing a separator includes (S1) preparing a
porous substrate having pores, (S2) coating at least one surface of
the porous substrate with a first solvent, (S3) coating the first
solvent with a slurry containing inorganic particles dispersed
therein and formed by dissolving a binder polymer in a second
solvent, (S4) drying the first and second solvents simultaneously
to form a porous organic-inorganic composite layer on the porous
substrate. Since the phenomenon that the pores of the porous
substrate are closing by the binder polymer is minimized, it is
possible to prevent the resistance of the separator from increasing
due to the formation of the porous organic-inorganic composite
layer.
Inventors: |
LEE; Joo Sung; (Daejeon,
KR) ; HONG; Jang-Hyuk; (Daejeon, KR) ; KIM;
Jong-Hun; (Daejeon, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LG CHEM, LTD. |
Seoul |
|
KR |
|
|
Assignee: |
LG CHEM, LTD.
Seoul
KR
|
Family ID: |
48136226 |
Appl. No.: |
14/850590 |
Filed: |
September 10, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13328607 |
Dec 16, 2011 |
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14850590 |
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PCT/KR2011/007859 |
Oct 20, 2011 |
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13328607 |
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Current U.S.
Class: |
429/144 ;
427/58 |
Current CPC
Class: |
B05D 3/104 20130101;
Y02E 60/10 20130101; H01M 2/1646 20130101; H01M 10/0525 20130101;
B05D 2252/02 20130101; B05D 3/0254 20130101; B05D 1/26 20130101;
H01M 2/1653 20130101; H01M 2/166 20130101; H01M 2/1686 20130101;
H01M 2/145 20130101 |
International
Class: |
H01M 2/14 20060101
H01M002/14; H01M 10/0525 20060101 H01M010/0525; H01M 2/16 20060101
H01M002/16 |
Claims
1. A method for manufacturing a separator, comprising: (S1)
preparing a porous substrate having pores; (S2) coating at least
one surface of the porous substrate with a composition consisting
essentially of a first solvent; (S3) coating the composition
consisting essentially of the first solvent with a slurry
containing inorganic particles dispersed therein and formed by
dissolving a binder polymer in a second solvent; and (S4) drying
the first and second solvents simultaneously to form a porous
organic-inorganic composite layer on the porous substrate, wherein
binder polymer in the second solvent is substantially prevented
from entering the pores of the porous substrate in the manufactured
separator.
2. The method for manufacturing a separator according to claim 1,
wherein the binder polymer of the porous organic-inorganic
composite layer is present on the surfaces of the inorganic
particles in whole or in part as a coating layer, wherein the
inorganic particles fixedly connect to each other by the coating
layer in an adhered state, and wherein vacant spaces present among
the inorganic particles form pores.
3. The method for manufacturing a separator according to claim 1,
wherein the porous substrate is a polyolefin-based porous film.
4. The method for manufacturing a separator according to claim 1,
wherein the porous substrate has a thickness of 1 to 100 .mu.m.
5. The method for manufacturing a separator according to claim 1,
wherein the binder polymer has a solubility parameter of 15 to 45
MPa.sup.1/2.
6. The method for manufacturing a separator according to claim 1,
wherein the binder polymer is selected from the group consisting of
polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene
fluoride-co-trichloroethylene, polymethylmethacrylate,
polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone,
polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene
oxide, polyarylate, cellulose acetate, cellulose acetate butyrate,
cellulose acetate propionate, cyanoethylpullulan,
cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose,
pullulan, carboxyl methyl cellulose, or their mixtures.
7. The method for manufacturing a separator according to claim 1,
wherein the first and second solvents are independently selected
from the group consisting of acetone, tetrahydrofuran, methylene
chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone
(NMP), cyclohexane, water, or their mixtures.
8. The method for manufacturing a separator according to claim 7,
wherein the first and second solvents are the same kind of
solvent.
9. The method for manufacturing a separator according to claim 1,
wherein the difference in solubility parameters between the binder
polymer and the first solvent, between the binder polymer and the
second solvent, and between the first solvent and the second
solvent is 5.0 Mpa.sup.0.5 or less, respectively.
10. The method for manufacturing a separator according to claim 1,
wherein the first solvent has a boiling point equal to or higher
than that of the second solvent.
11. The method for manufacturing a separator according to claim 1,
wherein the average particle diameter of the inorganic particles is
0.001 to 10 .mu.m.
12. The method for manufacturing a separator according to claim 1,
wherein the weight ratio of the inorganic particles to the binder
polymers is 50:50 to 99:1.
13. The method for manufacturing a separator according to claim 1,
wherein the first solvent has a coating thickness of 10 to 250
.mu.m.
14. A separator manufactured by the method according to claim
1.
15. An electrochemical device, comprising: a cathode; an anode; and
a separator interposed between the cathode and the anode and formed
according to the method for manufacturing a separator, defined in
claim 1.
16. The electrochemical device to claim 15, wherein the
electrochemical device is a lithium secondary battery.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a Continuation of copending application
Ser. No. 13/328,607, filed on Dec. 16, 2011, which was a
Continuation of PCT International Application No.
PCT/KR2011/007859, filed on Oct. 20, 2011, all of which are hereby
expressly incorporated by reference into the present
application.
TECHNICAL FIELD
[0002] The present invention relates to a method for manufacturing
a separator for an electrochemical device such as a lithium
secondary battery, a separator manufactured by the method, and an
electrochemical device including the separator. More specifically,
the present invention relates to a method for manufacturing a
separator in which an organic-inorganic composite porous layer
composed of a mixture of inorganic particles and binder polymers is
formed on at least one surface of a porous substrate, a separator
manufactured by the method, and an electrochemical device including
the separator.
BACKGROUND ART
[0003] Recently, there has been increasing interest in energy
storage technologies. As the application fields of energy storage
technologies have been extended to mobile phones, camcorders,
notebook computers and even electric cars, efforts have
increasingly been made towards the research and development of
electrochemical devices. Under these circumstances, secondary
batteries capable of repeatedly charging and discharging, in
particular, have attracted considerable attention as the most
promising electrochemical devices. In recent years, extensive
research and development has been conducted to design new
electrodes and batteries for the purpose of improving capacity
density and specific energy of the batteries.
[0004] Many secondary batteries are currently available. Lithium
secondary batteries developed in the early 1990's have drawn
particular attention due to their advantages of higher operating
voltages and much higher energy densities than conventional aqueous
electrolyte-based batteries such as Ni-MH batteries, Ni--Cd
batteries, and H.sub.2SO.sub.4--Pb batteries. However, such lithium
ion batteries suffer from safety problems, such as fire or
explosion, when encountered with the use of organic electrolytes
and have a disadvantage in that they are complicated to
manufacture. In attempts to overcome the disadvantages of lithium
ion batteries, lithium ion polymer batteries have been developed as
next-generation batteries. More research is still urgently needed
to improve the relatively low capacities and insufficient
low-temperature discharge capacities of lithium ion polymer
batteries in comparison with lithium ion batteries.
[0005] Many companies have produced a variety of electrochemical
devices with different safety characteristics. It is very important
to evaluate and ensure the safety of such electrochemical devices.
The most important consideration for safety is that operational
failure or malfunction of electrochemical devices should not cause
injury to users. For this purpose, regulatory guidelines strictly
restrict potential dangers (such as fire and smoke emission) of
electrochemical devices. Overheating of an electrochemical device
may cause thermal runaway or a puncture of a separator may pose an
increased risk of explosion. In particular, porous polyolefin
substrates commonly used as separators for electrochemical devices
undergo severe thermal shrinkage at a temperature of 100.degree. C.
or higher in view of their material characteristics and production
processes including elongation. This thermal shrinkage behavior may
cause short circuits between a cathode and an anode.
[0006] In order to solve the above safety problems of
electrochemical devices, a separator has been suggested in which a
mixture of inorganic particles and a binder polymer is coated on at
least one surface of a highly porous substrate to form a porous
organic-inorganic composite coating layer. For example, Korean
Unexamined Patent Publication No. 2007-0019958 discloses a method
for manufacturing a separator, in which a porous substrate such as
a polyolefin film is coated with a slurry containing inorganic
particles dispersed therein and formed by dissolving a binder
polymer in a solvent and then dried to provide a porous
organic-inorganic composite coating layer on the porous
substrate.
[0007] The inorganic particles present in the porous coating layer
serve as spacers that help to maintain a physical shape of the
porous coating layer to inhibit the porous substrate from thermal
shrinkage when an electrochemical device overheats or to prevent
short circuits between both electrodes of the electrochemical
device when thermal runaway takes place. Vacant spaces present
between the inorganic particles form fine pores.
[0008] As described above, the organic-inorganic composite porous
coating layer contributes to thermal stability of the separator,
but tends to increase the resistance of the separator since the
binder polymer flows into the pores of the porous substrate and
closes a part of the pores when the organic-inorganic composite
porous coating layer is formed.
DISCLOSURE
Technical Problem
[0009] The present invention is designed to solve the problems of
the prior art, and therefore it is an object of the present
invention to provide a method for manufacturing a separator, which
may minimize the phenomenon that pores of a porous substrate are
closed by a binder polymer when an organic-inorganic composite
porous coating layer is formed. It is another object of the
invention to provide a separator manufactured by the method. It is
still another object of the invention to provide an electrochemical
device including the separator.
Technical Solution
[0010] According to an aspect of the present invention, there is
provided a method for manufacturing a separator, which includes
(S1) preparing a porous substrate having pores; (S2) coating at
least one surface of the porous substrate with a first solvent;
(S3) coating the first solvent with a slurry containing inorganic
particles dispersed therein and formed by dissolving a binder
polymer in a second solvent; and (S4) drying the first and second
solvents simultaneously to form a porous organic-inorganic
composite layer on the porous substrate.
[0011] In the method for manufacturing a separator, the binder
polymer of the porous organic-inorganic composite layer is
preferably present on the surfaces of the inorganic particles in
whole or in part as a coating layer, the inorganic particles
preferably fixedly connect to each other by the coating layer in a
closely adhered state, and the pores are preferably formed by
vacant spaces present among the inorganic particles.
[0012] In the method for manufacturing a separator, the porous
substrate is preferably a polyolefin-based porous film, and the
porous substrate preferably has a thickness of 1 to 100 .mu.m.
[0013] In the method for manufacturing a separator, the binder
polymer preferably has a solubility parameter of 15 to 45
MPa.sup.1/2. The binder polymer may be selected from the group
consisting of polyvinylidene fluoride-co-hexafluoropropylene,
polyvinylidene fluoride-co-trichloroethylene,
polymethylmethacrylate, polybutylacrylate, polyacrylonitrile,
polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl
acetate, polyethylene oxide, polyarylate, cellulose acetate,
cellulose acetate butyrate, cellulose acetate propionate,
cyanoethylpullulan, cyanoethylpolyvinylalcohol,
cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl
cellulose, or their mixtures.
[0014] In the method for manufacturing a separator, the first and
second solvents may be independently selected from the group
consisting of acetone, tetrahydrofuran, methylene chloride,
chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP),
cyclohexane, water, or their mixtures.
[0015] In the method for manufacturing a separator, the difference
in solubility parameters between the binder polymer and the first
solvent, between the binder polymer and the second solvent, and
between the first solvent and the second solvent is preferably 5.0
Mpa.sup.0.5 or less, respectively, in consideration of the easiness
of coating and the prevention of gelation of the binder polymer. In
this aspect, it is more preferred that the first and second
solvents employ the same kind of solvent.
[0016] In the method for manufacturing a separator, the average
particle diameter of the inorganic particles is preferably 0.001 to
10 .mu.m, and the weight ratio of the inorganic particles to the
binder polymers is preferably 50:50 to 99:1.
[0017] In the method for manufacturing a separator, the first
solvent preferably has a coating thickness of 10 to 250 .mu.m, and
the coating thickness of the slurry is preferably adjusted so that
a finally produced porous organic-inorganic composite layer has a
thickness of 0.1 to 20 .mu.m.
[0018] The separator of the present invention manufactured by the
above method may be interposed between a cathode and an anode to
manufacture an electrochemical device such as lithium secondary
batteries and supercapacitor devices.
Advantageous Effects
[0019] The separator manufactured by the method of the present
invention exhibits the following advantageous effects.
[0020] First, the porous organic-inorganic composite layer
restrains thermal shrinkage of a porous substrate when an
electrochemical device overheats, and also prevents a short circuit
between both electrodes when thermal runaway takes place.
[0021] Second, since the problem of the binder polymer in the
slurry flowing into the pores of the porous substrate when the
organic-inorganic composite porous coating layer is formed has been
solved, it is possible to minimize the phenomenon of the pores of
the porous substrate closing. Accordingly, the resistance increase
of the separator caused by the formation of the porous
organic-inorganic composite layer decreases.
DESCRIPTION OF DRAWINGS
[0022] FIG. 1 is a view schematically illustrating the process of a
method for manufacturing a separator according to an embodiment of
the present invention.
BEST MODE
[0023] Hereinafter, preferred embodiments of the present invention
will be described in detail with reference to the accompanying
drawing. Prior to the description, it should be understood that the
terms used in the specification and the appended claims should not
be construed as limited to general and dictionary meanings, but
interpreted based on the meanings and concepts corresponding to
technical aspects of the present invention on the basis of the
principle that the inventor is allowed to define terms
appropriately for the best explanation. Therefore, the description
proposed herein is just a preferable example for the purpose of
illustrations only, not intended to limit the scope of the
invention, so it should be understood that other equivalents and
modifications could be made thereto without departing from the
spirit and scope of the invention.
[0024] The present invention provides a method for manufacturing a
separator. The method of the present invention will now be
described in detail.
[0025] First, a porous substrate having pores is prepared (S1).
[0026] The porous substrate may be any porous substrate commonly
used in electrochemical devices. Examples of such porous substrates
include various porous polymer membranes and non-woven fabrics. As
the porous polymer membranes, for example, porous polyolefin
membranes used in separators for electrochemical devices,
particularly, lithium secondary batteries may be used. The
non-woven fabrics may be, for example, those composed of
polyethylene phthalate fibers. The material or shape of the porous
substrate may vary according to intended purposes. Examples of
materials suitable for the porous polyolefin membranes include
polyethylene polymers, such as high density polyethylene, linear
low density polyethylene, low density polyethylene and ultrahigh
molecular weight polyethylene, polypropylene, polybutylene and
polypentene. These polyolefins may be used alone or as a mixture
thereof. Examples of materials suitable for the non-woven fabrics
include polyolefins and polymers having higher heat resistance than
polyolefins. The thickness of the porous substrate is preferably
from 1 to 100 .mu.m, more preferably from 5 to 50 .mu.m, but is not
particularly limited to this range. The pore size and porosity of
the porous substrate are preferably from 0.001 to 50 .mu.m and 10
to 95%, respectively, but are not particularly limited to these
ranges.
[0027] Subsequently, at least one surface of the porous substrate
is coated with a first solvent (S2).
[0028] In the present invention, the first solvent means a solvent
which may dissolve a binder polymer, described later. The solvent
capable of dissolving the binder polymer (namely, the first
solvent) preferably has a solubility parameter similar to that of
the binder polymer to be used and has a boiling point equal to or
higher than that of a second solvent. It is because the second
solvent is desirably dried together with or faster than the first
solvent during a drying process, described later.
[0029] Non-limiting examples available as the first solvent include
acetone, tetrahydrofuran, methylene chloride, chloroform,
dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane,
water, or their mixtures.
[0030] The first solvent preferably has a coating thickness of 10
to 250 .mu.m in consideration of the effect caused by the coating
of the first solvent and the effectiveness according to a drying
rate.
[0031] After that, the formed first solvent is coated with a slurry
containing inorganic particles dispersed therein and formed by
dissolving a binder polymer in the second solvent (S3).
[0032] The inorganic particles are not specifically limited so long
as they are electrochemically stable. In other words, the inorganic
particles can be used without particular limitation in the present
invention if they do not undergo oxidation and/or reduction in an
operating voltage range applied to an electrochemical device (for
example, 0-5 V for Li/Li.sup.+). In particular, a high dielectric
constant of the inorganic particles can contribute to an increase
in the degree of dissociation of a salt (e.g., a lithium salt) in a
liquid electrolyte to improve the ionic conductivity of the
electrolyte.
[0033] For these reasons, it is preferred that the inorganic
particles have a high dielectric constant of at least 5, preferably
at least 10. Non-limiting examples of inorganic particles having a
dielectric constant of at least 5 include BaTiO.sub.3, Pb (Zr,
Ti)O.sub.3 (PZT), Pb.sub.1-xLa.sub.xZr.sub.1-yTi.sub.yO.sub.3
(PLZT, here 0<x<1 and 0<y<1),
Pb(Mg.sub.1/3Nb.sub.2/3)O.sub.3-PbTiO.sub.3 (PMN-PT), hafnia
(HfO.sub.2), SrTiO.sub.3, SnO.sub.2, CeO.sub.2, MgO, NiO, CaO, ZnO,
ZrO.sub.2, Y.sub.2O.sub.3, Al.sub.2O.sub.3, TiO.sub.2 and SiC
particles. These inorganic particles may be used alone or as a
mixture of two or more kinds thereof.
[0034] The inorganic particles may be those having the ability to
transport lithium ions, that is, those containing lithium atoms and
having the ability to transfer lithium ions without storing the
lithium. Non-limiting examples of inorganic particles having the
ability to transport lithium ions include lithium phosphate
(Li.sub.3PO.sub.4) particles, lithium titanium phosphate
(Li.sub.xTi.sub.y(PO.sub.4).sub.3, 0<x<2, 0<y<3)
particles, lithium aluminum titanium phosphate
(Li.sub.xAl.sub.yTi.sub.z(PO.sub.4).sub.3, 0<x<2,
0<y<1, 0<z<3) particles, (LiAlTiP).sub.xO.sub.y type
glass (0<x<4, 0<y<13) particles such as
14Li.sub.2O-9Al.sub.2O.sub.3-38TiO.sub.2-39P.sub.2O.sub.5
particles, lithium lanthanum titanate (Li.sub.xLa.sub.yTiO.sub.3,
0<x<2, 0<y<3) particles, lithium germanium
thiophosphate (Li.sub.xGe.sub.yP.sub.zS.sub.w, 0<x<4,
0<y<1, 021 z<1, 0<w<5) particles such as
Li.sub.3.25Ge.sub.0.25P.sub.0.75S.sub.4 particles, lithium nitride
(Li.sub.xN.sub.y, 0<x<4, 0<y<2) particles such as
Li.sub.3N particles, SiS.sub.2 type glass (Li.sub.xSi.sub.yS.sub.z,
0<x<3, 0<y<2, 0<z<4) particles such as
Li.sub.3PO.sub.4--Li.sub.2S--SiS.sub.2 particles, and
P.sub.2S.sub.5 type glass (Li.sub.xP.sub.yS.sub.x, 0<x<3,
0<y<3, 0<z<7) particles such as
LiI--Li.sub.2S--P.sub.2S.sub.5 particles. These inorganic particles
may be used alone or as a mixture of two or more kinds thereof.
[0035] There is no particular restriction on the average particle
diameter of the inorganic particles. The average particle diameter
of the inorganic particles is preferably limited to the range of
0.001 to 10 .mu.m. Within this range, a uniform thickness and an
optimal porosity of the coating layer can be obtained. An average
particle diameter of less than 0.001 .mu.m may cause deterioration
of dispersibility. Meanwhile, an average particle diameter
exceeding 10 .mu.m may increase the thickness of the coating
layer.
[0036] The binder polymer preferably employs a polymer having a
glass transition temperature, T.sub.g) of -200 to 200.degree. C.,
since such a polymer may improve mechanical properties such as
flexibility and elasticity of a finally produced coating layer.
[0037] In addition, the binder polymer does not have to possess ion
conductivity, but the performance of an electrochemical device may
be further improved if a polymer having ion conductivity is used.
Therefore, it is preferable for the binder polymer to have the
highest dielectric constant possible. In fact, since the degree of
dissociation of a salt depends on the dielectric constant of an
electrolyte solvent, the higher the dielectric constant of the
binder polymer, the more the degree of dissociation of a salt may
be enhanced in the electrolyte. The dielectric constant of the
binder polymer is available in the range of 1.0 to 100 (a
measurement frequency=1 kHz), particularly preferably 10 or
above.
[0038] In addition to the above functions, the binder polymer may
have a high degree of electrolyte swelling as it is gelled when
swelling in a liquid electrolyte. Accordingly, the binder polymer
preferably has a solubility parameter of 15 to 45 MPa.sup.1/2, more
preferably 15 to 25 MPa.sup.1/2 and 30 to 45 MPa.sup.1/2.
Therefore, hydrophilic polymers having many polarity groups are
preferred to hydrophobic polymers such as polyolefin-based
materials. If the solubility parameter is lower than 15 MPa.sup.1/2
or greater than 45 MPa.sup.1/2, the binder polymer may not easily
swell by a common liquid electrolyte for a battery.
[0039] Non-limiting examples of the binder polymer includes
polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene
fluoride-co-trichloroethylene, polymethylmethacrylate,
polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone,
polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene
oxide, polyarylate, cellulose acetate, cellulose acetate butyrate,
cellulose acetate propionate, cyanoethylpullulan,
cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose,
pullulan, carboxyl methyl cellulose, or their mixtures.
[0040] The weight ratio of the inorganic particles to the binder
polymers is preferably in the range of 50:50 to 99:1 and more
preferably 70:30 to 95:5. If the weight ratio of the inorganic
particles to the binder polymers is less than 50:50, the pore size
and porosity of the coating layer may be reduced since the content
of the binder polymer increases. Meanwhile, if the inorganic
particles are present in an amount exceeding 99 parts by weight,
the peeling resistance of the formed coating layer may deteriorate
since the content of the binder polymer is small.
[0041] In the present invention, the second solvent means a solvent
capable of dissolving the binder polymer. The solvent of the binder
polymer (namely, the second solvent) preferably has a solubility
parameter similar to that of the binder polymer to be used and a
boiling point lower than that of the binder polymer. It is for
uniform mixing and easy removal of the solvent afterward.
Non-limiting examples available as the second solvent includes
acetone, tetrahydrofuran, methylene chloride, chloroform,
dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane,
water, or their mixtures. The second solvent may be different from
the first solvent.
[0042] In the method for manufacturing a separator according to the
present invention, the difference in solubility parameters between
the binder polymer and the first solvent, between the binder
polymer and the second solvent, and between the first solvent and
the second solvent is preferably 5.0 Mpa.sup.1/2 or less,
respectively, in consideration of the easiness of coating and the
prevention of gelation of the binder polymer. In this aspect, it is
more preferred that the first and second solvents employ the same
kind of solvent.
[0043] The slurry containing inorganic particles dispersed therein
and formed by dissolving the binder polymer in the second solvent
may be produced by dissolving the binder polymer in the second
solvent and then adding and dispersing inorganic particles, without
being limited thereto. The inorganic particles may be added after
being crushed to a suitable size, but it is preferred to add
inorganic particles to the binder polymer solution and then perform
ball milling or the like so that the inorganic particles are
crushed and dispersed simultaneously.
[0044] The coating thickness of the slurry applied to the porous
substrate is preferably controlled so that the porous
organic-inorganic composite layer finally formed after drying has a
thickness of 0.1 to 20 .mu.m, in consideration of the safety
improvement of the battery and the resistance of the separator.
[0045] The coating of the first solvent according to the step S2
and the coating of the slurry according to the step S3 may be
carried out by various techniques such as slot die coating, slide
coating, curtain coating, or the like in a sequential or
non-sequential way. In particular, the coating processes in the
steps S2 and S3 are preferably carried out sequentially or
simultaneously in aspect of productivity. A preferred embodiment of
the sequential coating is illustrated in FIG. 1.
[0046] Referring to FIG. 1, a die 1 having two slots 3a and 3b is
used to carry out the coating of the first solvent according to the
step S2 the coating of the slurry according to the step S3. The
first solvent 7 is supplied through the first slot 3a. In addition,
the inorganic particles are dispersed through the second slot 3b,
and the slurry 5 is supplied to the second solvent in which the
binder polymer is dissolved. If the porous substrate 9 is supplied
onto a rotating roller, the porous substrate 9 is coated with the
first solvent 7, and sequentially the first solvent 7 is coated
with the slurry 5.
[0047] Finally, the first solvent applied onto the porous substrate
and the second solvent present in the slurry are dried
simultaneously so that a porous organic-inorganic composite layer
is formed on the porous substrate (S4).
[0048] The porous organic-inorganic composite layer formed
according to the method of the present invention will be described
below.
[0049] The resultant product of the step S3 is configured so that
the porous substrate is coated with the first solvent thereon,
which is also coated with the slurry thereon. If the resultant
product passes through a drier or the like, the slurry applied onto
the first solvent firstly receives heat or hot wind. Therefore, the
second solvent in the slurry applied to an outer region is dried
earlier than the first solvent. Accordingly, before the binder
polymer in the slurry is entirely diffused to the layer composed of
the first solvent, a coating layer of the binder polymer is formed
on the surface of the inorganic particles in whole or in part, from
the inorganic particles present in the outermost region of the
slurry coating layer. At this time, the inorganic particles are
present substantially in a closely adhered state, and as the
inorganic particles fixedly connect to each other by the coating
layer of the binder polymer, vacant spaces are formed among the
inorganic particles to form pores. Therefore, the dispersion of the
binder polymer in the slurry into the pores of the porous substrate
due to the first solvent layer is minimized. Accordingly, the
phenomenon that the pores of the porous substrate are closing by
the binder polymer in the slurry is minimized, and so it is
possible to prevent the resistance of the separator from increasing
due to the formation of the porous organic-inorganic composite
layer.
[0050] The separator may be interposed between a cathode and an
anode, followed by winding or lamination to manufacture an
electrochemical device. An electrochemical device manufactured by
the method may be any device in which electrochemical reactions
occur, and specific examples thereof include all kinds of primary
batteries, secondary batteries, fuel cells, solar cells, and
capacitors such as supercapacitor devices. Particularly preferred
are lithium secondary batteries, including lithium metal secondary
batteries, lithium ion secondary batteries, lithium polymer
secondary batteries and lithium ion polymer secondary
batteries.
[0051] There is no particular restriction on the production method
of a cathode and an anode to be applied together with the separator
of the present invention. Each of the electrodes can be produced by
binding an electrode active material to an electrode current
collector by suitable methods known in the art. The cathode active
material may be any of those that are commonly used in cathodes of
conventional electrochemical devices. Non-limiting examples of
particularly preferred cathode active materials include lithium
manganese oxides, lithium cobalt oxides, lithium nickel oxides,
lithium iron oxides and lithium composite oxides thereof. The anode
active material may be any of those that are commonly used in
anodes of conventional electrochemical devices. Non-limiting
examples of particularly preferred anode active materials include
lithium, lithium alloys, and lithium intercalation materials such
as carbon, petroleum coke, activated carbon, graphite and other
carbonaceous materials. Non-limiting examples of cathode current
collectors suitable for use in the electrochemical device of the
present invention include aluminum foils, nickel foils and
combinations thereof. Non-limiting examples of anode current
collectors suitable for use in the electrochemical device of the
present invention include copper foils, gold foils, nickel foils,
copper alloy foils and combinations thereof.
[0052] The electrochemical device of the present invention can use
an electrolyte consisting of a salt and an organic solvent capable
of dissolving or dissociating the salt. The salt has a structure
represented by A.sup.+B.sup.- wherein A.sup.+ is an alkali metal
cation such as Li.sup.+, Na.sup.+, K.sup.+ or a combination thereof
and B.sup.- is an anion such as PF.sub.6.sup.-, BF.sub.4.sup.-,
Cl.sup.-, Br.sup.-, I.sup.-, ClO.sub.4.sup.-, AsF.sub.6.sup.-,
CH.sub.3CO.sub.2.sup.-, CF.sub.3SO.sub.3.sup.-,
N(CF.sub.3SO.sub.2).sub.2.sup.-, C(CF.sub.2SO.sub.2).sub.3.sup.-or
a combination thereof. Examples of organic solvents suitable for
dissolving or dissociating the salt include, but are not limited
to, propylene carbonate (PC), ethylene carbonate (EC), diethyl
carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate
(DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane,
diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP),
ethyl methyl carbonate (EMC) and .gamma.-butyrolactone. These
organic solvents may be used alone or as a mixture thereof.
[0053] The electrolyte may be injected in any suitable step during
manufacturing of the electrochemical device depending on the
manufacturing processes and desired physical properties of a final
product. Specifically, the electrolyte may be injected before
battery assembly or in the final step of battery assembly.
MODE FOR INVENTION
[0054] Hereinafter, the present invention will be explained in
detail with reference to embodiments. The embodiments of the
present invention, however, may take several other forms, and the
scope of the invention should not be construed as being limited to
the following examples. The embodiments of the present invention
are provided to more fully explain the present invention to those
having ordinary knowledge in the art to which the present invention
pertains.
EXAMPLE 1
[0055] Polyvinylidene fluoride-co-chlorotrifluoroethylene copolymer
(PVdF-CTFE) and cyanoethylpullulan were added to acetone in a
weight ratio of 10:2 and dissolved therein at 50.degree. C. for at
least about 12 hours to prepare a polymer solution. Inorganic
particles in which Al.sub.2O.sub.3 powder and BaTiO.sub.3 powder
were mixed in a weight ratio of 9:1 were added to the prepared
polymer solution until the weight ratio of the binder polymer to
the inorganic particles reached 10:90. The inorganic particles were
crushed and dispersed using a ball mill for at least 12 hours to
prepare a slurry. The inorganic particles of the slurry had an
average particle diameter of 600 nm.
[0056] The prepared slurry and a separately prepared acetone were
sequentially supplied through the slot die illustrated in FIG. 1 to
coat one surface of a 12 .mu.m thick porous polyethylene membrane
(porosity 45%). The coating thickness of the acetone was 20 .mu.m,
and the coating amount of the slurry was set to be 60 .mu.m so that
a finally formed porous organic-inorganic composite layer has a
thickness of 4 .mu.m.
[0057] Subsequently, the coated substrate was passed through a
dryer adjusted to have a temperature of 50.degree. C. to dry the
solvents, completing the manufacture of a separator.
[0058] The separator was found to have a Gurley value of 190
sec/100 mL in a good level. In addition, the separator had a
resistance of 0.6.OMEGA. in a good level.
Comparative Example 1
[0059] A separator was manufactured in the same manner as in
Example 1, except that acetone was not applied and only the slurry
was applied through the slot die shown in FIG. 1. The coating
amount of the slurry was adjusted so that a finally formed porous
organic-inorganic composite layer had a thickness of 4 .mu.m.
[0060] The Gurley value of the separator was 230 sec/100 mL, and
the resistance of the separator increased to 1.0.OMEGA..
Comparative Example 2
[0061] A separator was manufactured in the same manner as in
Comparative Example 1, except that both surfaces of the
polyethylene porous membrane were coated with the slurry in a dip
coating manner, instead of the slot die coating. The coating amount
of the slurry applied to both surfaces was adjusted so that finally
formed porous organic-inorganic composite layers respectively had a
thickness of 2 .mu.m (the sum of thicknesses of both surfaces was 4
.mu.m).
[0062] The Gurley value of the separator was 290 sec/100 mL, and
the resistance of the separator increased to 1.4.OMEGA..
[0063] The separators manufactured according to the Example and
Comparative Examples above were respectively interposed between a
cathode and an anode and wound to assemble an electrode structure.
The anode was prepared by forming an anode active material layer
containing anode active material particles made of graphite on a
copper foil, and the cathode was prepared by forming a cathode
active material layer containing lithium cobalt oxide on an
aluminum foil. A non-aqueous electrolyte prepared by dissolving 1
mol of lithium hexafluorophosphate in an organic solvent in which
ethylene carbonate and ethyl methyl carbonate were mixed in a
volume ratio of 1:2 was injected into the assembled electrode
structure to manufacture a lithium secondary battery.
[0064] C-rate of each lithium secondary battery prepared as above
was measured. The measurement results are shown in Table 1
below.
TABLE-US-00001 TABLE 1 Discharge Battery of Battery of rate Battery
of Comparative Comparative condition Example 1 Example 1 Example 2
Ratio of 0.2 C 99.6% 99.4% 99.2% capacity to 1.0 C 99.8% 95.8%
95.5% design 2.0 C 93.9% 91.4% 90.7% capacity
[0065] As shown in Table 1, there was no great difference at the
low rate discharge, but the discharge capacity of a battery
adopting the separator of Example with a low resistance was high
under a high rate discharge condition in comparison to batteries
adopting the separators of the comparative examples.
* * * * *