Description
SEPARATOR HAVING POROUS COATING LAYER AND
ELECTROCHEMICAL DEVICE CONTAINING THE SAME
Technical Field
[I] The present invention relates to a separator used for an electrochemical device such
as a lithium secondary battery and an electrochemical device having the same. More
particularly, the present invention relates to a separator in which a porous coating layer
made of a mixture of a binder polymer and inorganic particles is formed on the surface
of a porous substrate, and an electrochemical device containing the same.
Background Art
[21 Recently, there has been an increasing interest in energy storage technology.
Batteries have been widely used as energy sources in the fields of cellular phones,
camcorders, notebook computers, PCs and electric cars, resulting in intensive research
and development into them. In this regard, electrochemical devices are one of the
subjects of great interest. Particularly, development of rechargeable secondary batteries
has been the focus of attention. Recently, in the development of such batteries, designs
of new electrodes and batteries to improve capacity density and specific energy are
mainly studied.
[31 Among currently used secondary batteries, lithium secondary batteries developed in
early 1990's have a higher drive voltage and a much higher energy density than those
of conventional batteries using a liquid electrolyte solution such as Ni-MH batteries,
Ni-Cd batteries, and H2S04-Pb batteries. For these reasons, the lithium secondary
batteries have been advantageously used. However, such a lithium secondary battery
has disadvantages in that organic electrolytes used therein may cause safety-related
problems such as ignition and explosion of the batteries and that processes for manufacturing
such a battery are complicated. Recently, lithium-ion polymer batteries have
been considered as one of the next-generation batteries since the above disadvantages
of the lithium ion batteries are solved. However, the lithium-ion polymer batteries have
a relatively lower battery capacity in comparison to the lithium ion batteries, and its
discharging capacity is insufficient at low temperature. Thus, it is urgent to solve these
disadvantages of the lithium-ion polymer batteries.
[41 Such electrochemical devices have been produced from many companies, and battery
safety characteristics are different in the electrochemical devices. Accordingly, it is
important to evaluate and ensure the safety of the electrochemical batteries. First of all,
malfunction of the electrochemical device should not cause any damage to users. For
this purpose, the Safety Regulation strictly regulates ignition and explosion in the electrochemical
devices. In the safety characteristics of the electrochemical device,
overheating of the electrochemical device may cause thermal runaway, and explosion
may occur when a separator is pierced. In particular, a polyolefin-based porous
substrate commonly used as a separator of an electrochemical device shows extreme
thermal shrinking behavior at a temperature of 100°C or above due to its inherent characteristics
and its manufacturing processes such as elongation, which may cause an
electric short circuit between positive and negative electrodes.
[51 In order to solve the above safety-related problems of the electrochemical device,
Korean Patent Registration No. 10-0727248 and No. 10-0727247 disclose a separator
10 having a porous coating layer formed by coating at least one surface of a porous
substrate 1 having many pores with a mixture of inorganic particles 3 and a binder
polymer 5 (see FIG. I). In the separator, the inorganic particles 3 in the porous coating
layer formed on the porous substrate 1 serve as a kind of spacer that keeps a physical
shape of the porous coating layer, so the inorganic particles 3 restrain thermal
shrinkage of the porous substrate when the electrochemical device is overheated. In
addition, interstitial volumes exist among the inorganic particles, thereby forming
micro pores.
[61 As mentioned above, the porous coating layer formed on the porous substrate contributes
to the improvement of safety. However, due to the introduction of inorganic
particles, the life span of an electrochemical device, particularly a high temperature
cycle or storage life, may be deteriorated. The above documents disclose various kinds
of binder polymers and their combinations, but they do not specifically disclose any
binder polymer combination solving the above problem.
[71 Meanwhile, the inorganic particles of the porous coating layer may be disintercalated
due to the stress occurring during the assembling process of an electrochemical device
such as a winding process. The disintercalated inorganic particles act as a local defect
of the electrochemical device, thereby giving a bad influence on the safety of the electrochemical
device. Thus, more endeavors for solving this problem are demanded.
Disclosure of Invention
Technical Problem
[81 The present invention is designed to solve the problems of the prior art, and therefore
an object of the invention is to provide a separator capable of improving life span characteristics
of electrochemical devices by introducing a porous coating layer with
inorganic particles, and an electrochemical device containing such a separator.
[91 Another object of the present invention is to provide a separator capable of
improving safety of an electrochemical device by solving the problem that inorganic
particles in a porous coating layer formed on a porous substrate are disintercalated
during an assembling process of the electrochemical device, and an electrochemical
device containing such a separator.
Technical Solution
[lo] In order to accomplish the first object, the present invention provides a separator,
which includes a porous substrate having a plurality of pores; and a porous coating
layer formed on at least one surface of the porous substrate and made of a mixture of a
plurality of inorganic particles and a binder polymer, wherein the binder polymer
includes a first polyvinylidene fluoride-based copolymer having solubility of 25
weight% or more with respect to acetone at 35OC; a second polyvinylidene fluoridebased
copolymer having solubility of 10 weight% or less with respect to acetone at
35°C; and a polymer having a cyano group.
[I 11 In the separator of the present invention, the first polyvinylidene fluoride-based
copolymer may be polyvinylidene fluoride-co-hexafluoropropylene, and the second
polyvinylidene fluoride-based copolymer may be polyvinylidene fluorideco-
chlorotrifluoroethylene. Also, the polymer having a cyano group may be cyanoethylpullulan,
cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose,
and so on.
[I21 In the separator of the present invention, it is preferred that, based on the entire
weight of the porous coating layer, the content of the first polyvinylidene fluoridebased
copolymer is 5 to 30 weight%, the content of the second polyvinylidene
fluoride-based copolymer is 1 to 10 weight%, and the content of the polymer having a
cyano group is 0.1 to 5 weight%, respectively.
[I31 The separator of the present invention may be interposed between positive and
negative electrodes and used for electrochemical devices such as lithium secondary
batteries and super capacitors.
Advantageous Effects
[I41 The separator according to the present invention solves deterioration of life span
characteristics of an electrochemical device by adopting a porous coating layer with
inorganic particles, and also solves the problem of disintercalation of inorganic
particles in the porous coating layer formed on a porous substrate during an assembling
process of the electrochemical device, thereby improving safety of the electrochemical
device.
~151 Accordingly, the separator of the present invention is very useful for electrochemical
devices such as lithium secondary batteries and super capacitors, particularly middle or
large batteries for electric vehicles or hybrid vehicles.
Brief Description of Drawings
[I61 Other objects and aspects of the present invention will become apparent from the
following description of embodiments with reference to the accompanying drawing in
which:
~171 FIG. 1 is a sectional view schematically showing a separator;
[ 181 FIG. 2 is a SEM (Scanning Electron Microscope) photograph showing surfaces of
separators prepared according to an embodiment and comparative examples;
~191 FIG. 3 is a graph showing measurement results of high-temperature storage life characteristics
of the batteries according to the embodiment and the comparative examples;
and
Pol FIG. 4 is a graph showing measurement results of high-temperature cycle life characteristics
of the batteries according to the embodiment and the comparative examples.
Best Mode for Carrying out the Invention
[211 Hereinafter, preferred embodiments of the present invention will be described in
detail with reference to the accompanying drawings. 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.
[221 A separator of the present invention includes a porous substrate having a plurality of
pores; and a porous coating layer formed on at least one surface of the porous substrate
and made of a mixture of a plurality of inorganic particles and a binder polymer. The
binder polymer includes a first polyvinylidene fluoride-based copolymer having
solubility of 25 weight% or more with respect to acetone at 35OC; a second
polyvinylidene fluoride-based copolymer having solubility of 10 weight% or less with
respect to acetone at 35OC; and a polymer having a cyano group.
[231 As mentioned above, the introduction of inorganic particles into a porous coating
layer formed on a porous substrate may cause deterioration of life span characteristics
of an electrochemical device. The inventors found that, when a porous coating layer is
formed using the three-component polymers at the same time, inorganic particles of
the formed coating layer are minimally exposed to the separator surface due to phase
separation, which accordingly allows improvement of life span characteristics of an
electrochemical device and also solves the problem of disintercalating of inorganic
particles. The present invention is conceived on the ground of the above.
[241 In the separator of the present invention, the first and second polyvinylidene fluoridebased
copolymers essentially contain a vinylidene fluoride component, and they have
solubility of 25 weight% or more and 10 weight% or less, respectively. If
polyvinylidene fluoride-based components with different solubility to a solvent used in
forming a porous coating layer are used in mixture, polymers are phase-separated,
thereby forming a porous coating layer of which inorganic particles are minimally
exposed to a separator surface. The second polyvinylidene fluoride-based copolymer
has a low solubility to acetone, so it is solidified in advance while the porous coating
layer is formed, so the second polyvinylidene fluoride-based copolymer is generally
positioned in a lower portion of the porous coating layer. On the contrary, the first
polyvinylidene fluoride-based copolymer has a high solubility to acetone, so it is solidified
more slowly and generally located in an upper portion of the porous coating
layer. If the first polyvinylidene fluoride-based copolymer has solubility less than 25
weight% with respect to acetone at 35OC or the second polyvinylidene fluoride-based
copolymer has solubility more than 10 weight% with respect to acetone at 35OC, the
mentioned polymer phase separation may not occur, so the above effects may not be
realized. The first and second polyvinylidene fluoride-based copolymers may be
polyvinylidene fluoride-co-hexafluoropropylene and polyvinylidene fluorideco-
chlorotrifluoroethylene, respectively. Mole ratios of hexafluoropropylene and
chlorotrifluoroethylene may be 10 to 30 mole% and 5 to 30 mol%, respectively, but
not limitedly.
~251 In addition, in the separator of the present invention, the polymer having a cyano
group plays a role of preventing inorganic particles of the porous coating layer from
cohering with each other. The polymer having a cyano group may use cyanoethylpullulan,
cyanoethylpolyvinylalcohol, cyanoethylcellulose and cyanoethylsucrose,
in single or in mixture, but not limitedly.
[261 In the separator of the present invention, based on the entire weight of the porous
coating layer, the content of the first polyvinylidene fluoride-based copolymer is
preferably 5 to 30 weight%, the content of the second polyvinylidene fluoride-based
copolymer is preferably 1 to 10 weight%, and the content of the polymer having a
cyano group is preferably 0.1 to 5 weight%. Also, it is apparent to those having
ordinary skill in the art that any other polymer may be further mixed thereto if the
effects of the present invention are not deteriorated.
~271 In the separator of the present invention, the inorganic particles used for forming a
porous coating layer serve as a kind of spacer that keeps a physical shape of the porous
coating layer, so the inorganic particles restrain thermal shrinkage of the porous
substrate when the electrochemical device is overheated. In addition, interstitial
volumes exist among the inorganic particles, thereby forming micro pores. The
inorganic particles are not specially limited if they are electrically and chemically
stable. In other words, inorganic particles causing no oxidation or reduction reaction in
an operating voltage range (for example, 0 to 5V based on Li/Lic) of an electrochemical
device may be used in the present invention. In particular, in case an
inorganic particle with ion transferring capability is used, it is possible to enhance the
performance of the electrochemical device by increasing ion conductivity.
[281 In addition, in case an inorganic particle with a high dielectric constant is used, it
contributes to the increase of dissociation of electrolyte salt, for example lithium salt,
in the liquid electrolyte, thereby improving ion conductivity of the electrolyte.
~291 Due to the above reasons, it is preferred that the inorganic particles may include
inorganic particles having a dielectric constant of 5 or above, preferably 10 or above,
inorganic particles having lithium-ion transferring capability, or their mixtures. The
inorganic particle having a dielectric constant of 5 or above may be for example BaTiO
3, Pb(Zr,Ti)03( PZT), Pbl.xLaxZr1.yTiy0(P3L ZT), PB(Mg3Nb213)03-PbTi(0P3M N-PT),
hafnia (Hf02), SrTi03, Sn02, Ce02, MgO, NiO, CaO, ZnO, Zr02, SO2, Y203, A1203,
Sic, Ti02, and their mixtures, but not limitedly.
[301 In particular, the inorganic particles such as of BaTi03, Pb(Zr,Ti)03 (PZT), Pbl.,LaX
Zr1.yTiy03(P LZT), PB(Mg3Nb213)03-PbTi(0P3M N-PT) and hafnia (Hf02)s how a high
dielectric constant of 100 or above and have piezoelectricity since charges are
generated to make a potential difference between both surfaces when a certain pressure
is applied thereto to extend or shrink them, so the above inorganic particles may
prevent generation of an internal short circuit of both electrodes caused by an external
impact and thus further improve the safety of the electrochemical device. In addition,
in case the inorganic particles having a high dielectric constant are mixed with the
inorganic particles having lithium ion transferring capability, their synergistic effect
may be doubled.
[311 In the present invention, the inorganic particle having lithium ion transferring capability
means an inorganic particle containing lithium atom and having a function of
moving a lithium ion without storing the lithium. The inorganic particle having lithium
ion transferring capability may transfer and move lithium ions due to a kind of defect
existing in the particle structure, so it is possible to improve lithium ion conductivity in
the battery and also improve the performance of the battery. The inorganic particle
having lithium ion transferring capability may be lithium phosphate (Li3P04),l ithium
titanium phosphate (LixTiy(P04)30, < x < 2,0 < y < 3), lithium aluminum titanium
phosphate (LixA1yTiz(P04)03 ,< x < 2,0 < y < 1,O < z < 3), (LiA1TiP),OYt ype glass (0
< x < 4,0 < y < 13) such as 14Li20-9A1203-38Ti02-39P20l5it,h ium lanthanum titanate
(LixLayTi030, < x < 2,0 < y < 3), lithium germanium thiophosphate (LixGeyPzS,,0 < x
< 4,0 < y < 1,O < z < 1,O < w < 5) such as Li3.25Ge0.25P0.7li5thSi4u,m nitrides (LixNy0,
< x < 4,0 < y < 2) such as Li3N, SiS2t ype glass (LixSiySz0, < x < 3,0 < y < 2,0 < z <
4) such as Li3P04-Li2S-SiS2P, 2S5t ype glass (Li,P,S,, 0 < x < 3,0 < y < 3,0 < z < 7)
such as LiI-Li2S-P2S5o,r their mixtures, but not limitedly.
[321 In the separator according to the present invention, the size of inorganic particles in
the porous coating layer is not specially limited, but the particle size is preferably
0.001 to 1Opm in order to form a coating layer with a uniform thickness and ensure
suitable porosity. If the particle size is less than 0.001pm, a dispersing property of
inorganic particles may be deteriorated, so it is not easy to control properties of the
separator. If the particle size exceeds IOpm, the thickness of the porous coating layer is
increased, which may deteriorate mechanical properties. Also, an excessively great
pore size may increase the possibility of internal short circuit while a battery is charged
or discharged.
[331 A ratio of the inorganic particles to the binder polymer in the porous coating layer
formed in the separator according to the present invention is preferably 50:50 to 99: 1,
more preferably from 70:30 to 95:5. If the ratio of the inorganic particles to the binder
polymer is less than 50:50, the content of polymer is so great that the thermal stability
of the separator may be not much improved. In addition, pore size and porosity may be
decreased due to the reduction of interstitial volume formed among the inorganic
particles, thereby deteriorating the performance of a final battery. If the content of
inorganic particles exceeds 99 parts by weight, the peeling resistance of the porous
coating layer may be weakened since the content of binder polymer is so small. The
thickness of the porous coating layer composed of the inorganic particles and the
binder polymer is not specially limited but is preferably 0.01 to 20pm. Also, pore size
and porosity are not specially limited, but the pore size is preferably 0.001 to 1Opm and
the porosity is preferably 10 to 90%. The pore size and porosity are mainly dependent
on the size of inorganic particles. For example, in case inorganic particles have a
diameter of lpm or less, the formed pores are also approximately lpm or less. The pores
as mentioned above are filled with electrolyte injected later, and the filled electrolyte
plays a role of transferring ions. In case the pore size and porosity are respectively less
than 0.001pm and lo%, the porous coating layer may act as a resistance layer. In case
the pore size and porosity are respectively greater than 1Opm and 90%, mechanical
properties may be deteriorated.
[341 The separator of the present invention may further include other additives in addition
to the inorganic particles and the binder polymer explained above as components of the
porous coating layer.
[351 In addition, in the separator of the present invention, the porous substrate having a
plurality of pores may use any porous substrate, such as a polyolefin-based porous
substrate, commonly used for electrochemical devices. The polyolefin-based porous
substrate may adopt any polyolefin-based porous substrate commonly used as a
separator of electrochemical devices, particularly lithium secondary batteries. For
example, the polyolefin-based porous substrate may be a membrane or a non-woven
fabric formed using any polyolefin-based polymer such as polyethylene like HDPE
(high density polyethylene), LDPE (low density polyethylene), LLDPE (linear low
density polyethylene and UHMWPE (ultra high molecular weight polyethylene),
polypropylene, polybutylene and polypentene, or their mixtures. Thickness of the
porous substrate is not specially limited, but preferably 5 to 50pm. Pore size and
porosity of the porous substrate are also not specially limited, but preferably 0.01 to
50pm and 10 to 95%, respectively.
[361 The separator having a porous coating layer with electrode active particles according
to the present invention may be manufactured in a common way, and a preferable
example is explained below, but the present invention is not limited thereto.
[371 First, the above-mentioned three-component polymer is dissolved in a solvent to
make a binder polymer solution.
[381 Subsequently, inorganic particles are added to the binder polymer solution and
dispersed therein. The solvent preferably has a low boiling point. It will help uniform
mixture and easy removal of the solvent afterward. Non-limiting examples of usable
solvents include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide,
N-methyl-2-pyrrolidone (NMP), cyclohexane, and water, or their mixtures.
Among them, acetone is most preferred. The inorganic particles are preferably
pulverized after being added to the binder polymer solution. At this time, the time
required for pulverization is suitably 1 to 20 hours, and the particle size of the
pulverized particles is preferably 0.001 and IOpm, as mentioned above. Conventional
pulverization methods may be used, and ball milling is particularly preferred.
[391 After that, a porous substrate is coated with the binder polymer solution in which the
inorganic particles are dispersed, under the humidity condition of 10 to 80%, and then
dried.
[401 In order to coat the porous substrate with the binder polymer solution in which the
inorganic particles are dispersed, common coating methods well known in the art may
be used. For example, various methods such as dip coating, die coating, roll coating,
comma coating or their combinations may be used. In addition, the porous coating
layer may be formed on both surfaces of the porous substrate or on any one surface
thereof selectively.
[411 The separator prepared as mentioned above according to the present invention may
be used for an electrochemical device. In other words, the separator of the present
invention may be useful as a separator interposed between positive and negative
electrodes. At this time, in case a polymer that is gellable at swelling in liquid
electrolyte is used as a binder polymer component, after a battery is assembled using
the separator, the injected electrolyte and the binder polymer may be reacted and then
gelated to form a gel-type organicfinorganic composite electrolyte.
[421 The electrochemical device may be any device in which electrochemical reactions
may occur, and specific examples of the electrochemical devices include all kinds of
primary batteries, secondary batteries, fuel cells, solar cells or capacitors such as a
super capacitor. In particular, among the secondary batteries, lithium secondary
batteries such as a lithium metal secondary battery, a lithium ion secondary battery, a
lithium polymer secondary battery or a lithium ion polymer secondary battery are
preferred.
[431 The electrochemical device may be made according to common methods well known
in the art. For example, the electrochemical device may be made by interposing the
above separator between positive and negative electrodes, and then injecting an
electrolyte therein.
[441 There is no special limitation in electrodes that may be used together with the
separator of the present invention, and the electrode may be manufactured in a form
that electrode active materials are united to electrode current collectors according to
one of common methods well known in the art. Among the electrode active materials,
positive electrode active material may adopt common positive electrode active material
available for a positive electrode of conventional electrochemical devices. Particularly,
the positive electrode active material preferably uses lithium manganese oxides,
lithium cobalt oxides, lithium nickel oxides, lithium iron oxides or lithium composite
oxides thereof, not limitedly. Also, non-limiting examples of negative electrode active
materials are lithium intercalation materials such as lithium metal, lithium alloy,
carbon, petroleum coke, activated carbon, graphite or other carbonaceous materials.
Non-limiting examples of the positive electrode current collector include a foil made of
aluminum, nickel or combinations thereof, and non-limiting examples of the negative
electrode current collector include a foil made of copper, gold, nickel, copper alloys or
combinations thereof.
[451 The electrolyte solution useable in the present invention includes a salt represented
by the formula of A+B-, wherein A+ represents an alkali metal cation such as LiC, NaC,
K+ or their combinations, and B- represents an salt containing an anion such as PF;, BF
4-, C1-, Br., I-, C104-, A sF6-,C H3C02-C, F3S03-N, (CF3S02)2-C, (CF2S023)- or their combinations.
The salt may be dissolved or dissociated in an organic solvent composed of
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),
ethylmethyl carbonate (EMC), gamma-butyrolactone (y-butyrolactone), or their
mixtures. However, the electrolyte solution useable in the present invention is not
limited to the above examples.
The electrolyte solution may be injected in a suitable step during the manufacturing
process of a battery, according to the manufacturing process and desired properties of a
final product. In other words, the electrolyte solution may be injected before a battery
is assembled, during a final step of the assembly process of a battery, or the like.
To apply the separator of the present invention to a battery, a folding process and a
laminating or stacking process of the separator and the electrode may be used in
addition to a general winding process. The separator of the present invention has
excellent peeling resistance, so inorganic particles are not easily disintercalated during
the battery assembling processes.
Mode for the Invention
Hereinafter, various preferred examples of the present invention will be described in
detail for better understandings. However, the examples of the present invention may
be modified in various ways, and they should not be interpreted as limiting the scope
of the invention. The examples of the present invention are just for better understandings
of the invention to persons having ordinary skill in the art.
Embodiment 1
Polyvinylidene fluoride-co-hexafluoropropylene (containing 1 8 mol% of hexafluoropropylene)
having solubility of 40 weight% or more with respect to acetone at 35OC,
polyvinylidene fluoride-co-chlorotrifluoroethylene (containing 20 mol% of chlorotrifluoroethylene)
having solubility of 5 weight% or less with respect to acetone at 35OC,
and cyanoethylpullulan were respectively added at a weight ratio of 8:2:2 to acetone
and dissolved at 50°C for about 12 hours to make a binder polymer solution.
Al2O3 powder and BaTi03 powder mixed at a weight ratio of 9: 1 were added to the
prepared binder polymer solution such that a weight ratio of binder polymer I inorganic
particles = 10190, and then it was pulverized and dispersed for 12 hours or more by ball
milling to make a slurry. The prepared slurry was applied to a polyethylene1
polypropylenelpolyethylene porous membrane and then dried. The coating thickness
was controlled to be about 5pm in its section.
Com~arativeE xam~le1
The comparative example 1 was identical to the embodiment 1, except that inorganic
particle was not added, and only the polyvinylidene fluoride-co-chlorotrifluoroethylene
binder of the embodiment 1 was used as the binder polymer for coating with a
thickness of about 1 to 2pm in section. The separator coated with only the
polyvinylidene fluoride-co-chlorotrifluoroethylene binder exhibits excellent high temperature
characteristics in a cell, so it is useful as a comparative example.
~561
~581 The comparative example 2 was identical to the embodiment 1, except that
polyvinylidene fluoride-co-chlorotrifluoroethylene and cyanoethylpullulan of the embodiment
1 were used at a weight ratio of 10:2, respectively.
[591
[611 The comparative example 3 was identical to the embodiment 1, except that
polyvinylidene fluoride-co-hexafluoropropylene and cyanoethylpullulan of the embodiment
1 were used at a weight ratio of 10:2, respectively.
[621
[631 Surfaces of the separators prepared according to the embodiment 1 and the comparative
examples 2 and 3 were photographed using SEM (Scanning Electron Microscope),
as depicted in FIG. 2.
[641 Also, in order to evaluate peeling resistance of the porous coating layers of the
separators prepared according to the embodiment and the comparative examples, the
following evaluation was conducted. The term 'peeling force of a porous coating layer',
used herein, means a peeling force measured according to the following test. The
peeling force test was executed as follows. A separator sample was fixed on a glass
plate using a double-sided adhesive tape, and then a tape (a 3M transparent tape) was
firmly attached to the exposed porous coating layer. Subsequently, a tensile force
measuring equipment was used to measure a force required for separating the tape,
thereby evaluating a peeling force of the porous coating layer.
~651 Meanwhile, surface adhesive strength was evaluated as follows. For measuring
surface adhesive strength, two sheets of separators were put between PET films and
then laminated. After that, a tensile strength measuring equipment was use to measure
a force required for separating two laminated separators, thereby evaluating the surface
adhesive strength of the porous coating layer. Measurement results for peeling force
and surface adhesive strength of the embodiment 1 and the comparative examples 2
and 3 are listed in Table 1.
[661
~671 Table 1
[Table I]
[Table ]
Meanwhile, the surface adhesive strength of the comparative example 2 was
measured as being slightly over 10 since separation occurred between the porous
substrate and the porous coating layer at measurement, but it is expected that an actual
surface adhesive strength is much greater than 1Ogf.
D
Embodiment 1
Comparative example 2
Comparative example 3
Preparation of Neg- ative Electrode
96 weight% of carbon powder as a negative electrode active material, 3 weight% of
polyvinylidene fluoride (PVdF) as a coupling agent and 1 weight% of carbon black as
a conductive material were added to N-methyl-2 pyrrolidone (NMP) as a solvent to
make a negative electrode mixture slurry. The negative electrode mixture slurry was
applied to a copper (Cu) film that is a negative current collector with a thickness of
IOpm, and then dried to make a negative electrode, and then roll pressing was
conducted thereto.
Preparation of Positive Electrode
90 weight% of lithium manganese composite oxide as a positive electrode active
material, 6 weight% of carbon black as a conductive material and 4 weight% of PVdF
as a coupling agent were added to N-methyl-2 pyrrolidone (NMP) as a solvent to make
a positive electrode active material slurry. The positive electrode active material slurry
was applied to an aluminum (Al) film that is a positive current collector with a
thickness of 20pm, and then dried to make a positive electrode, and then roll pressing
was conducted thereto.
Peeling force of Porous
coating layer (gf)
100
12
25
The above electrodes and separators were used to make cells. After that, high temperature
(45°C) cycle life and high temperature (60°C) storage life of the cells were
measured in the following way.
Surface adhesive
strength (gf)
100
lo* D
-0
Hig- h Temperature (60°C) Stora-g e Life
[go] The cells prepared according to the embodiment 1 and the comparative examples 1
and 2 were initially chargedldischarged, and then the cells in SOC 50% state were
stored in a 60°C chamber and taken out to measure power variation at every two weeks
at 25"C, SOC 50%. The measurement results are shown in FIG. 3.
[811
[821 Hig- h Temperature Cvcle Life Characteristics
[831 The cells prepared according to the embodiment 1 and the comparative examples 1
and 2 (each three cells) were initially chargedldischarged, and basic chargingldischarging
at high temperature (45°C) was conducted 1000 times at IC. Power variation
was measured at every 200 turns at 25"C, SOC 50%. The measurement results are
shown in FIG. 4.

Claims
A separator, comprising:
a porous substrate having a plurality of pores; and
a porous coating layer formed on at least one surface of the porous substrate and
made of a mixture of a plurality of inorganic particles and a binder polymer,
wherein the binder polymer includes:
a first polyvinylidene fluoride-based copolymer having solubility of 25 weight%
or more with respect to acetone at 35OC;
a second polyvinylidene fluoride-based copolymer having solubility of 10
weight% or less with respect to acetone at 35OC; and
a polymer having a cyano group.
The separator according to claim 1,
wherein the first polyvinylidene fluoride-based copolymer is polyvinylidene
fluoride-co-hexafluoropropylene.
The separator according to claim 1,
wherein the second polyvinylidene fluoride-based copolymer is polyvinylidene
fluoride-co-chlorotrifluoroethylene.
The separator according to claim 1,
wherein, based on the entire weight of the porous coating layer, the content of
the first polyvinylidene fluoride-based copolymer is 5 to 30 weight%, the content
of the second polyvinylidene fluoride-based copolymer is 1 to 10 weight%, and
the content of the polymer having a cyano group is 0.1 to 5 weight%, respectively.
The separator according to claim 1,
wherein the polymer having a cyano group is at least one selected from the group
consisting of cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose
and cyanoethylsucrose, or their mixtures.
The separator according to claim 1,
wherein the inorganic particles have a size of 0.001 to 10 ,urn.
The separator according to claim 1,
wherein the inorganic particles are selected from the group consisting of
inorganic particles having a dielectric constant of 5 or above and inorganic
particles having lithium-ion transferring capability, or their mixtures.
The separator according to claim 7,
wherein the inorganic particle having a dielectric constant of 5 or above is an
inorganic particle selected from the group consisting of BaTi03, Pb(Zr,Ti)03
(PZT), Pbl.xLaxZrl,Tiy03( PLZT), PB(Mg3Nb2,3)03-PbTi0(3P MN-PT), hafnia
(Hf02), SrTi03, Sn02, Ce02, MgO, NiO, CaO, ZnO, Zr02, SO2, Y2O3, Al2O3,
Sic and Ti02, or their mixtures.
The separator according to claim 7,
wherein the inorganic particle having lithium ion transferring capability is an
inorganic particle selected from the group consisting of lithium phosphate (Li3P0
4), lithium titanium phosphate (LiXTiy(PO4)0,, < x < 2, 0 < y < 3), lithium
aluminum titanium phosphate (LixA1yTiz(P04)03 ,< x < 2,0 < y < 1,O < z < 3),
(LiAlTiP),O, type glass (0 < x < 4,0 < y < 13), lithium lanthanum titanate (LixLa
,Ti03, 0 < x < 2,0 < y < 3), lithium germanium thiophosphate (LixGeyPzS,, 0 < x
< 4,0 < y < 1,O < z < 1,O < w < 5), lithium nitrides (LixNy0, < x < 4,0 < y < 2),
SiS2 (LixSiySz0, < x < 3,0 < y < 2,0 < z < 4) type glass, and P2S5( LixPySz0, < x
< 3,0 < y < 3,0 < z < 7) type glass, or their mixtures.
The separator according to claim 1,
wherein a weight ratio of the inorganic particles to the binder polymer is 50:50 to
99:l.
The separator according to claim 1,
wherein the porous substrate is a polyolefin-based porous substrate.
The separator according to claim 1,
wherein the polyolefin-based porous substrate is made of any one polymer
selected from the group consisting of polyethylene, polypropylene, polybutylene
and polypentene, or their mixtures.
An electrochemical device, comprising a positive electrode, a negative electrode,
and a separator interposed between the positive electrode and the negative
electrode,
wherein the separator is a separator defined in any one of the claims 1 to 12.
The electrochemical device according to claim 13,
wherein the electrochemical device is a lithium secondary battery.
[15] A separator substantially as herein described with reference to the
foregoing description, tables, examples and the accompanying drawings.