TECHNICAL FIELD
The present disclosure relates to a separator for a lithium secondary battery, a
method for manufacturing the same, and a lithium secondary battery including the same.
The present application claims priority to Korean Patent Application No. 10-2020-
10 0041002 filed on April 3, 2020 in the Republic of Korea, the disclosures of which are
incorporated herein by reference.
BACKGROUND ART
Recently, energy storage technology has been given an increasing attention.
15 Efforts into research and development for electrochemical devices have been actualized
more and more, as the application of energy storage technology has been extended to
energy for cellular phones, camcorders and notebook PC and even to energy for electric
vehicles. In this context, electrochemical devices have been most spotlighted. Among
such electrochemical devices, development of rechargeable secondary batteries has been
20 focused. More recently, active studies have been conducted about designing a novel
electrode and battery in order to improve the capacity density and specific energy in
developing such batteries.
Among the commercially available secondary batteries, lithium secondary
3
batteries developed in the early 1990’s have been spotlighted, since they have a higher
operating voltage and significantly higher energy density as compared to conventional
batteries, such as Ni-MH, Ni-Cd and sulfuric acid-lead batteries using an aqueous
electrolyte.
Although such secondary batteries have been produced from 5 many production
companies, safety characteristics thereof show different signs. Evaluation and
securement of safety of such secondary batteries are very important. The most important
consideration is that secondary batteries should not damage users upon their malfunction.
For this purpose, safety standards strictly control ignition and smoke emission in secondary
10 batteries. With regard to safety characteristics of secondary batteries, there is great
concern about explosion when a secondary battery is overheated to cause thermal runaway
or perforation of a separator.
Such a separator prevents a short-circuit between a cathode and an anode, and
provides a lithium-ion transport channel. Therefore, the separator is an important factor
15 affecting the safety and output characteristics of a battery. However, a polyolefin-based
porous polymer substrate used conventionally as a separator for an electrochemical device
shows a severe heat shrinking behavior at a temperature of 130°C or higher due to its
material property and a characteristic during its manufacturing process, including
orientation, thereby causing a short-circuit between a cathode and an anode.
20 Under these circumstances, there has been an attempt to apply a porous coating
layer including inorganic particles and a thermoplastic binder polymer to the porous
polymer substrate in order to increase the heat resistance. When forming such a porous
coating layer, heat resistance may be improved as compared to the porous polymer
4
substrate used alone. However, when the separator is exposed to a high-temperature
environment higher than the melting point of the binder polymer, improvement of thermal
safety is limited.
Meanwhile, high adhesion is also required between the porous coating layer and
5 an electrode.
DISCLOSURE
Technical Problem
The present disclosure is designed to solve the problems of the related art, and
10 therefore the present disclosure is directed to providing a separator for a lithium secondary
battery which has improved heat resistance and shows high adhesion to an electrode.
The present disclosure is also directed to providing a method for manufacturing
the separator for a lithium secondary battery having the above-mentioned characteristics.
In addition, the present disclosure is directed to providing a lithium secondary
15 battery including the separator having the above-mentioned characteristics.
Technical Solution
In one aspect of the present disclosure, there is provided a separator for a lithium
secondary battery according to any one of the following embodiments.
20 According to the first embodiment, there is provided a separator for a lithium
secondary battery, including:
a porous polymer substrate having a plurality of pores; and
a porous coating layer formed on at least one surface of the porous polymer
5
substrate and including a plurality of inorganic particles and a binder polymer, wherein the
binder polymer includes a thermosetting phenolic resin having at least one hydroxyl group
and aromatic group.
According to the second embodiment, there is provided the separator for a lithium
secondary battery as defined in the first embodiment, wherein the 5 thermosetting phenolic
resin includes at least one of a novolac-type phenol resin represented by the following
Chemical Formula 1 and a resol-type phenol resin represented by the following Chemical
Formula 2:
[Chemical Formula 1]
10
wherein n is an integer of 1 or more, and R represents any one of H and optionally
substituted C1-C100 alkyl, alkoxy and alkenyl groups.
[Chemical Formula 2]
15
6
wherein each of n and m is an integer of 1 or more, and R represents any one of H
and optionally substituted C1-C100 alkyl, alkoxy and alkenyl groups.
According to the third embodiment, there is provided the separator for a lithium
secondary battery as defined in the first or the second embodiment, wherein the
thermosetting phenolic resin is thermally 5 cured at 120-200°C.
According to the fourth embodiment, there is provided the separator for a lithium
secondary battery as defined in any one of the first to the third embodiments, wherein the
thermosetting phenolic resin is a phenol-formaldehyde resin.
According to the fifth embodiment, there is provided the separator for a lithium
10 secondary battery as defined in any one of the first to the fourth embodiments, wherein the
content of the thermosetting phenolic resin is 0.1-20 wt% based on 100 wt% of the content
of the inorganic particles.
According to the sixth embodiment, there is provided the separator for a lithium
secondary battery as defined in any one of the first to the fifth embodiments, wherein the
15 binder polymer further includes a non-phenolic resin in an amount of 0.1-50 wt% based on
100 wt% of the content of the inorganic particles.
According to the seventh embodiment, there is provided the separator for a lithium
secondary battery as defined in any one of the first to the sixth embodiments, wherein the
porous polymer substrate has a thickness of 1-20 μm, and the thickness of the porous
20 coating layer is 0.5-20 μm based on single surface coating.
In another aspect of the present disclosure, there is provided a method for
manufacturing a separator according to any one of the following embodiments.
According to the eighth embodiment, there is provided a method for
7
manufacturing a separator for a lithium secondary battery, including the steps of:
(S1) preparing an inorganic particle dispersion by dispersing inorganic particles in
a first solvent, and dissolving a thermosetting phenolic resin having at least one hydroxyl
group and aromatic ring therein; and
(S2) coating slurry for forming a porous coating layer 5 including the inorganic
particle dispersion on at least one surface of a porous polymer substrate, followed by
drying, to form a porous coating layer.
According to the ninth embodiment of the present disclosure, there is provided the
method for manufacturing a separator for a lithium secondary battery as defined in the
10 eighth embodiment, which further includes a step of mixing the inorganic particle
dispersion of step (S1) with a polymer solution including a non-phenolic resin dissolved in
a second solvent.
According to the tenth embodiment of the present disclosure, there is provided the
method for manufacturing a separator for a lithium secondary battery as defined in the
15 ninth embodiment, wherein each of the first solvent and the second solvent independently
includes any one compound selected from water, C2-C5 alcohols, acetone, tetrahydrofuran,
methylene chloride, chloroform, dimethyl formamide, N-methyl-2-pyrrolidone, methyl
ethyl ketone and cyclohexane, or a mixture of two or more of them.
According to the eleventh embodiment of the present disclosure, there is provided
20 the method for manufacturing a separator for a lithium secondary battery as defined in any
one of the eighth to the tenth embodiments, wherein the inorganic particle dispersion
further includes a dispersing agent.
According to the twelfth embodiment of the present disclosure, there is provided
8
the method for manufacturing a separator for a lithium secondary battery as defined in any
one of the eighth to the eleventh embodiments, wherein the thermosetting phenolic resin
includes at least one of a novolac-type phenol resin represented by the following Chemical
Formula 1 and a resol-type phenol resin represented by the following Chemical Formula 2:
[5 Chemical Formula 1]
wherein n is an integer of 1 or more, and R represents any one of H and optionally
substituted C1-C100 alkyl, alkoxy and alkenyl groups.
10 [Chemical Formula 2]
wherein each of n and m is an integer of 1 or more, and R represents any one of H
and optionally substituted C1-C100 alkyl, alkoxy and alkenyl groups.
According to the thirteenth embodiment of the present disclosure, there is provided
15 the method for manufacturing a separator for a lithium secondary battery as defined in any
9
one of the eighth to the twelfth embodiments, wherein the thermosetting phenolic resin is
thermally cured at 120-200°C.
According to the fourteenth embodiment of the present disclosure, there is
provided the method for manufacturing a separator for a lithium secondary battery as
defined in any one of the eighth to the thirteenth embodiments, wherein 5 the content of the
thermosetting phenolic resin is 0.1-20 wt% based on 100 wt% of the content of the
inorganic particles.
In still another aspect of the present disclosure, there is provided a lithium
secondary battery including a cathode, an anode and a separator interposed between the
10 cathode and the anode, wherein the separator is the same as defined in any one of the
above-described embodiments.
Advantageous Effects
The separator for a lithium secondary battery according to an embodiment of the
15 present disclosure includes a porous coating layer containing a plurality of inorganic
particles and a binder polymer, wherein the binder polymer includes a thermosetting
phenolic resin having at least one hydroxyl group and aromatic ring. The thermosetting
phenolic resin having such a structure functions as a dispersing agent in slurry for forming
a porous coating layer as well as functions as a binder polymer for fixing the inorganic
20 particles. Therefore, the inorganic particles are mixed well with the binder polymer in the
slurry, and thus the adhesion between an electrode and the porous coating layer is
maintained well, even when using no dispersing agent or using a reduced amount of
dispersing agent. In addition, it is possible to reduce the content of the dispersing agent
10
and to increase the content of the binder polymer, and thus to improve the adhesion to the
electrode.
Meanwhile, when an electrochemical device including the separator is exposed to
high temperature, due to ignition or the like, the thermosetting phenolic resin is thermally
cured to form a network structure. As a result, it is possible to provide 5 a separator with
reinforced heat resistance and improved thermal safety.
BEST MODE
Hereinafter, preferred embodiments of the present disclosure will be described in
10 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 disclosure on the
basis of the principle that the inventor is allowed to define terms appropriately for the best
15 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 disclosure, so it
should be understood that other equivalents and modifications could be made thereto
without departing from the scope of the disclosure.
Throughout the specification, the expression ‘a part includes or comprises an
20 element’ does not preclude the presence of any additional elements but means that the part
may further include the other elements, unless otherwise stated.
In general, a lithium secondary battery uses a porous polymer substrate as a
11
separator. Such a porous polymer substrate may include a non-woven web substrate
including polymer fibers and a film substrate made of a film obtained by melt extrusion of
a polymer. Such polymer substrates, particularly, film substrates show a heat shrinking
behavior undesirably. Therefore, in order to improve the heat resistance of a separator, a
porous coating layer including a plurality of inorganic particles and 5 a binder polymer has
been introduced to at least one surface of the porous polymer substrate. When the porous
coating layer is formed as mentioned above, it is possible to improve the heat resistance of
the separator as compared to the porous polymer substrate used alone.
A thermoplastic fluorinated binder polymer has been used largely as a binder
10 polymer. When the separator is exposed to a high-temperature environment higher than
the melting point of the binder polymer, improvement of thermal safety is limited. In
addition, high adhesion between the porous coating layer and an electrode is required to
realize the performance of the lithium secondary battery sufficiently.
The present inventors have conductive intensive studies to solve the above15
mentioned problems. Therefore, the present disclosure is directed to providing a
separator, which includes a porous coating layer showing improved heat resistance, when
an electrochemical device including the separator is exposed to high temperature, due to
ignition, or the like, and has high adhesion to an electrode.
According to the first embodiment, there is provided a separator for a lithium
20 secondary battery, including:
a porous polymer substrate having a plurality of pores; and
a porous coating layer formed on at least one surface of the porous polymer
substrate and including a plurality of inorganic particles and a binder polymer, wherein the
12
binder polymer includes a thermosetting phenolic resin having at least one hydroxyl group
and aromatic group.
In the separator according to an embodiment of the present disclosure, the
thermosetting phenolic resin is thermally cured to form a network structure, when the
separator is exposed to high temperature in an abnormal environment. 5 Herein, the
network structure refers to a structure formed by the thermal curing of the thermosetting
phenolic resin, wherein the network structure may be a three-dimensional network
structure.
When the thermosetting phenolic resin in the porous coating layer undergoes a
10 thermal curing reaction and forms a network structure at high temperature in an abnormal
environment as mentioned above, it is possible to improve the heat resistance and
dimensional stability, as compared to a separator using a thermoplastic polymer. In other
words, when the network structure is formed by the thermal curing reaction, the inorganic
particles may be retained firmly in the network structure up to a significantly high
15 temperature.
[Thermosetting Phenolic Resin]
The thermosetting phenolic resin has at least one hydroxyl group and aromatic ring.
The thermosetting phenolic resin is thermally cured at high temperature to form a
20 network structure. Particularly, the thermosetting phenolic resin may be thermally cured
at a high temperature of 120°C or higher, particularly at a high temperature of 120-200°C.
Meanwhile, the thermosetting phenolic resin having at least one hydroxyl group
and aromatic ring may function as a dispersing agent in the slurry for forming a porous
13
coating layer. In this manner, it is possible to disperse inorganic particles more easily in
the slurry for forming a porous coating layer, and thus another dispersing agent may not be
used or use thereof may be reduced.
The thermosetting phenolic resin may include at least one of a novolac-type
phenol resin represented by the following Chemical Formula 1 and 5 a resol-type phenol
resin represented by the following Chemical Formula 2, and may further include a
modified phenol resin. Particularly, any thermosetting phenolic resin obtained from
aldehyde condensation of phenol and derivatives thereof may be used.
Herein, Chemical Formula 1 is the same as represented hereinafter.
10 [Chemical Formula 1]
wherein n is an integer of 1 or more, and R represents any one of H and optionally
substituted C1-C100 alkyl, alkoxy and alkenyl groups. Herein, the number of carbon
15 atoms may be 1-100 or less, 1-50 or less, 1-30 or less, or 1-10 or less.
As used herein, ‘novolac-type phenol resin’ may be obtained by the reaction of a
phenol compound with an aldehyde compound in the presence of an acidic catalyst.
Since the novolac-type phenol resin has a phenolic hydroxyl group, it is soluble in alkali.
However, when the novolac-type phenol resin has an excessively high molecular weight,
14
its solubility is reduced.
Since the novolac-type resin has little methylol group reactive to curing, it is
possible to form a cured product having a three-dimensional structure, when it is heated
after adding a curing agent, such as hexamethylene tetraamine ((CH2)6N4), thereto.
The novolac-type phenol resin may be present as an ortho- 5 or para-binding.
According to an embodiment of the present disclosure, the novolac-type phenol resin may
have a ratio of ortho-binding/para-binding of less than 0.2, particularly less than 0.1.
According to an embodiment of the present disclosure, the novolac-type phenol
resin may have at least two kinds of novolac-type phenol resins having a different ratio of
10 ortho-binding/para-binding. Herein, when using a high-para novolac-type phenol resin
having a larger amount of para-binding as compared to ortho-binding, curing occurs more
rapidly at high temperature so that the separator may be provided with more improved
safety.
The novolac-type phenol resin preferably has a weight average molecular weight
15 (Mw) of 1,000-4,000 and a polydispersity (Mw/Mn, wherein Mn is a number average
molecular weight) of 2.0 or less. A novolac-type phenol resin having a weight average
molecular weight within the above-defined range is advantageous in terms of ensuring heat
resistance and flowability during the preparation of slurry and forming a thin film-type
porous coating layer. Meanwhile, a novolac-type phenol resin having a polydispersity of
20 2.0 or less is more advantageous in terms of improvement of dispersibility of inorganic
particles. The novolac-type phenol resin may have an equivalent of hydroxyl groups of
100-200 g/eq.
Chemical Formula 2 is the same as represented hereinafter.
15
[Chemical Formula 2]
wherein each of n and m is an integer of 1 or more, and R represents any one of H
and optionally substituted C1-C100 alkyl, alkoxy and alkenyl groups.
As used herein, ‘resol-type phenol resin’ may be obtained by 5 reacting a phenol
compound with an aldehyde compound in the presence of a metal catalyst. The resoltype
phenol resin has a hydroxyl group and reacts with the novolac-type phenol resin to
form a crosslinked structure.
Herein, the metal catalyst may include manganese, magnesium, zinc, or the like.
10 Particularly, the metal catalyst may be a metal acetate. For example, the metal catalyst
may include manganese acetate, magnesium acetate and zinc acetate. Such metal
catalysts may be used alone or in combination.
The resol-type phenol resin preferably has a weight average molecular weight of
420-1,500 and has a polydispersity (Mw/Mn, wherein Mn is a number average molecular
15 weight) of 2.0 or less. A resol-type phenol resin having a weight average molecular
weight within the above-defined range is advantageous in terms of ensuring heat resistance
and flowability during the preparation of slurry and forming a thin film-type porous
coating layer. Meanwhile, a resol-type phenol resin having a polydispersity of 2.0 or less
is more advantageous in terms of improvement of dispersity of inorganic particles. The
16
resol-type phenol resin may have an equivalent of hydroxyl groups of 100-200 g/eq.
Herein, weight average molecular weight may be determined by using gel
permeation chromatography (GPC, PL GPC220, Agilent Technologies).
Particularly, determination of weight average molecular weight may be carried out
under the following 5 analysis conditions:
- Column: PL MiniMixed B x 2
- Solvent: THF or DMF
- Flow rate: 0.3 mL/min
- Sample concentration: 2.0 mg/mL
10 - Injection amount: 10 μL
- Column temperature: 40°C
- Detector: Agilent RI detector
- Standard: Polystyrene (corrected with tertiary function)
- Data processing: ChemStation
15 According to an embodiment of the present disclosure, the thermosetting phenolic
resin may include 2,3,4-trihydroxybenzophenone, 2,4,6-trihydroxybenzophenone, 2,2’-
tetrahydroxybenzophenone, 4,4’-tetrahydroxybenzophenone, 2,3,4,3’-
tetrahydroxybenzophenone, 2,3,4,4’-tetrahydroxybenzophenone, 2,3,4,2’-tetrahydroxy 4’-
methylbenzophenone, 2,3,4,4’-tetrahydroxy 3’-methoxybenzophenen, 2,3,4,2’-
20 pentahydroxybenzophenone, 2,3,4,6’-pentahydroxybenzophenone, 2,4,6,3’-
hexahydroxybenzophenone, 2,4,6,4’-hexahydroxybenzophenone, 2,4,6,5’-
hexahydroxybenzophenone, 3,4,5,3’-hexahydroxybenzophenone, 3,4,5,4’-
hexahydroxybenzophenone, 3,4,5,5’-hexahydroxybenzophenone, bis(2,4-
17
dihydroxyphenyl)methane, bis(p-hydroxyphenyl)methane, tri(p-hydroxyphenyl)methane,
1,1,1-tri(p-hydroxyphenyl)ethane, bis(2,3,4-trihydroxyphenyl)methane, 2,2-bis(2,3,4-
trihydroxyphenyl)propane, 1,1,3-tris(2,5-dimethyl 4-hydroxyphenyl)-3-phenyl propane,
4,4’-[1-[4-[1-[4-hydroxyphenyl]-1-methylethyl]phenyl]ethylidene]bisphenol, bis(2,5-
dimethyl 4-hydroxyphenyl)-2-hydroxyphenylmethane, or the like. Such 5 compounds may
be used alone, or two or more of them may be used in combination to prepare a resin. For
example, the thermosetting phenolic resin may be a phenol-formaldehyde resin.
According to an embodiment of the present disclosure, when the thermosetting
phenolic resin is thermally cured under an abnormal high-temperature environment, it may
10 form a network structure having a structure represented hereinafter.
For example, the network structure may be represented by the following Chemical
Formula 3:
According to an embodiment of the present disclosure, once the thermosetting
18
phenolic resin is thermally cured to form a network structure, the inorganic particles in the
porous coating layer may be disposed in the network structure.
Herein, the content of the thermosetting phenolic resin may be particularly 0.1-20
wt%, and more particularly 0.5-15 wt%, based on 100 wt% of the content of the inorganic
particles. Within the above defined range, it is possible to improve 5 the heat resistance,
while not causing a significant increase in the interfacial resistance between the porous
polymer substrate and the porous coating layer.
[Inorganic Particles]
10 According to the present disclosure, there is no particular limitation in the
inorganic particles, as long as they are electrochemically stable. In other words, there is
no particular limitation in the inorganic particles that may be used herein, as long as they
cause no oxidation and/or reduction in the range (e.g. 0-5 V based on Li/Li+) of operating
voltage of an applicable electrochemical device. Particularly, when using inorganic
15 particles having a high dielectric constant as the inorganic particles, it is possible to
improve the ion conductivity of an electrolyte by increasing the dissociation degree of an
electrolyte salt, such as a lithium salt, in a liquid electrolyte.
For the above-mentioned reasons, the inorganic particles may be inorganic
particles having a dielectric constant of 5 or more, inorganic particles having lithium-ion
20 transportability or a combination thereof.
The inorganic particles having a dielectric constant of 5 or more may include any
one selected from the group consisting of Al2O3, SiO2, ZrO2, AlOOH, TiO2, BaTiO3,
Pb(Zr,Ti)O3 (PZT, wherein 0 < x < 1), Pb1-xLaxZr1-yTiyO3 (PLZT, wherein 0 < x < 1, 0 < y
19
< 1), (1-x)Pb(Mg1/2Nb2/3)O3-xPbTiO3 (PMN-PT, wherein 0 < x < 1), hafnia (HfO2), SrTiO3,
SnO2, CeO2, MgO, NiO, CaO, ZnO, ZO3 and SiC, or a mixture of two or more of them.
The inorganic particles having lithium-ion transportability may be any one
selected from the group consisting of include lithium phosphate (Li3PO4), lithium titanium
phosphate (LixTiy(PO4)3, 0 < x < 2, 0 < y < 3), lithium aluminum 5 titanium phosphate
(LixAlyTiz(PO4)3, 0 < x < 2, 0 < y < 1, 0 < z < 3), (LiAlTiP)xOy-based glass (1 < x < 4, 0 <
y < 13), lithium lanthanum titanate (LixLayTiO3, 0 < x < 2, 0 < y < 3), lithium germanium
thiophosphate (LixGeyPzSw, 0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5), lithium nitride
(LixNy, 0 < x < 4, 0 < y < 2), SiS2-based glass (LixSiySz, 0 < x < 3, 0 < y < 2, 0 < z < 4) and
10 P2S5-based glass (LixPySz, 0 < x < 3, 0 < y < 3, 0 < z < 7), or a mixture of two or more of
them.
In addition, there is no particular limitation in the average particle diameter of the
inorganic particles. However, the inorganic particles preferably have an average particle
diameter of 0.001-10 μm in order to form a coating layer with a uniform thickness and to
15 provide suitable porosity. Particularly, the inorganic particles may have an average
particle diameter of 100 nm or more, 150 nm or more, or 200 nm or more, and 1000 nm or
less, 900 nm or less, 800 nm or less, or 700 nm or less.
[Non-Phenolic resin]
20 According to an embodiment of the present disclosure, the porous coating layer
may further include a non-phenolic resin as a binder polymer, besides the above-described
thermosetting phenolic resin.
The non-phenolic resin may be one used currently for forming a porous coating
20
layer in the art. Particularly, a polymer having a glass transition temperature (Tg) of -200
to 200°C may be used. This is because such a polymer can improve the mechanical
properties, such as flexibility and elasticity, of the finally formed porous coating layer.
Such a non-phenolic resin functions as a binder which connects and stably fixes the
inorganic particles with one another, and thus contributes to prevention 5 of degradation of
mechanical properties of a separator having a porous coating layer.
In addition, it is not essentially required for the non-phenolic resin to have ion
conductivity. However, when using a polymer having ion conductivity, it is possible to
further improve the performance of an electrochemical device. Therefore, a non-phenolic
10 resin having a dielectric constant as high as possible may be used. In fact, since the
dissociation degree of a salt in an electrolyte depends on the dielectric constant of the
solvent for the electrolyte, a non-phenolic resin having a higher dielectric constant can
improve the salt dissociation degree in an electrolyte. The non-phenolic resin may have a
dielectric constant ranging from 1.0-100 (measured at a frequency of 1 kHz), particularly
15 10 or more.
In addition to the above-mentioned function, the non-phenolic resin may be
characterized in that it is gelled upon the impregnation with a liquid electrolyte and thus
shows a high degree of swelling. Thus, the non-phenolic resin has a solubility parameter
(i.e., Hildebrand solubility parameter) of 15-45 MPa1/2 or 15-25 MPa1/2 and 30-45 MPa1/2.
20 Therefore, hydrophilic polymer resins having many polar groups may be used more
frequently as compared to hydrophobic polymer resins, such as polyolefins. When the
solubility parameter is less than 15 MPa1/2 and more than 45 MPa1/2, it is difficult for the
non-phenolic resin to be swelled with a conventional liquid electrolyte for a battery.
21
Non-limiting examples of the non-phenolic resin include polyvinylidene fluoride,
polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-cotrichloroethylene,
polyvinylidene fluoride-co-trifluoroethylene, polyvinylidene fluoride-cochlorotrifluoroethylene,
polymethyl methacrylate, polyethylhexyl acrylate, polybutyl
acrylate, polyacrylonitrile, polyvinyl pyrro1idone, polyvinyl acetate, 5 polyethylene-co-vinyl
acetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate,
cellulose acetate propionate, cyanoethylpullulan, cyanoethyl polyvinylalchol, cyanoethyl
cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, etc.
10 The separator according to an embodiment of the present disclosure may further
include other additives, such as a dispersing agent, as ingredients of the porous coating
layer, besides the inorganic particles and a binder polymer.
Although there is no particular limitation in the thickness of the porous coating
layer, the porous coating layer may have a thickness of 0.5-20 μm, particularly 1.5-10 μm,
15 based on single-side coating. In addition, the porosity of the porous coating layer is not
particularly limited, but the porous coating layer preferably has a porosity of 35-85%.
[Porous Polymer Substrate]
In the separator according to an embodiment of the present disclosure, the porous
20 polymer substrate may be a porous polymer film substrate or a porous polymer nonwoven
web substrate.
The porous polymer film substrate may be a porous polymer film including
polyolefin, such as polyethylene or polypropylene. For example, such a polyolefin
22
porous polymer film substrate may realize a shut-down function at a temperature of 80-
150°C.
Herein, the polyolefin porous polymer film may be formed of polymers including
polyolefin polymers, such as polyethylene, including high-density polyethylene, linear
low-density polyethylene, low-density polyethylene or ultrahigh-5 molecular weight
polyethylene, polypropylene, polybutylene, or polypentene, alone or in combination of two
or more of them.
In addition, the porous polymer film substrate may be obtained by molding various
polymers, such as polyesters, other than polyolefins, into a film shape. Further, the
10 porous polymer film substrate may have a stacked structure of two or more film layers,
wherein each film layer may be formed of polymers including the above-mentioned
polymers, such as polyolefins or polyesters, alone or in combination of two or more of
them.
In addition, the porous polymer film substrate and porous polymer nonwoven web
15 substrate may be formed of polymers including polyethylene terephthalate, polybutylene
terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetherether
ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, or polyethylene
naphthalene, alone or in combination, besides the above-mentioned polyolefins.
In addition, although there is no particular limitation in the thickness of the porous
20 polymer substrate, the porous polymer substrate has a thickness of 1-20 μm, particularly 5-
15 μm. Although there is no particular limitation in the size of the pores present in the
porous polymer substrate and porosity, the pore size and porosity may be 0.01-50 μm and
20-75%, respectively.
23
Hereinafter, the method for manufacturing a separator according to an
embodiment of the present disclosure will be explained in detail. However, the scope of
the present disclosure is not limited to the following embodiment.
Particularly, inorganic particles are dispersed in a first solvent, 5 and a thermosetting
phenolic resin having at least one hydroxyl group and aromatic ring is dissolved therein to
prepare an inorganic particle dispersion (S1).
Herein, the method may further include a step of mixing a polymer solution
containing a non-phenolic resin dissolved in a second solvent with the inorganic particle
10 dispersion of step (S1). In addition, the inorganic particle dispersion may further include
a dispersing agent.
Herein, the first solvent and the second solvent may be the same or different.
Herein, each of the first solvent and the second solvent preferably has a solubility
parameter similar to the solubility parameter of the binder polymers, i.e., the thermosetting
15 phenolic resin and the non-phenolic resin, respectively, and has a low boiling point. This
is because such a solvent facilitates homogeneous mixing and the subsequent solvent
removal. Non-limiting examples of the solvent that may be used include any one selected
from water, C2-C5 alcohols, acetone, tetrahydrofuran, methylene chloride, chloroform,
dimethyl formamide, N-methyl-2-pyrrolidone, methyl ethyl ketone and cyclohexane, or a
20 mixture of two or more of them.
Then, slurry for forming a porous coating layer including the inorganic particle
dispersion is coated on at least one surface of a porous polymer substrate, followed by
drying, to form a porous coating layer (S2).
24
Although there is no particular limitation in the process for coating the slurry for
forming a porous coating layer onto the porous polymer substrate, it is preferred to use a
slot coating or dip coating process. A slot coating process includes coating a composition
supplied through a slot die onto the whole surface of a substrate and is capable of
controlling the thickness of a coating layer depending on the flux supplied 5 from a metering
pump. In addition, a dip coating process includes dipping a substrate into a tank
containing a composition to carry out coating and is capable of controlling the thickness of
a coating layer depending on the concentration of the composition and the rate of removing
the substrate from the composition tank. Further, in order to control the coating thickness
10 more precisely, it is possible to carry out post-metering through a Mayer bar or the like,
after dipping.
Then, the porous polymer substrate coated with the slurry for forming a porous
coating layer is dried by using a dryer, such as an oven, thereby forming a porous coating
layer on at least one surface of the porous polymer substrate.
15 In the porous coating layer, the inorganic particles may be dispersed in the matrix
of the binder polymer. The binder polymer may be a thermosetting phenolic resin or a
non-phenolic resin, or both.
In addition, the inorganic particles are bound with one another by the binder
polymer, while they are packed and are in contact with each other. Thus, interstitial
20 volumes are formed among the inorganic particles, and the interstitial volumes among the
inorganic particles become vacant spaces to form pores. In other words, the binder
polymer attaches the inorganic particles to one another so that they may retain their
binding states. For example, the binder polymer connects and fixes the inorganic
25
particles with one another. In addition, the pores of the porous coating layer are those
formed by the interstitial volumes among the inorganic particles, which become vacant
spaces. The spaces are defined by the inorganic particles facing one another substantially
in a closely packed or densely packed structure of the inorganic particles.
The inorganic particles, the thermosetting phenolic resin, the 5 non-phenolic resin
and the porous polymer substrate are the same as described above, and detailed description
thereof will be omitted.
In another aspect of the present disclosure, there is provided a lithium secondary
10 battery including a cathode, an anode and a separator interposed between the cathode and
the anode, wherein the separator is the above-described separator according to an
embodiment of the present disclosure.
Particularly, the lithium secondary battery may include a lithium metal secondary
battery, a lithium-ion secondary battery, a lithium polymer secondary battery or a lithium15
ion polymer secondary battery.
The two electrodes, cathode and anode, used in combination with the separator
according to the present disclosure are not particularly limited, and may be obtained by
allowing electrode active materials to be bound to an electrode current collector through a
method generally known in the art. Among the electrode active materials, non-limiting
20 examples of a cathode active material include conventional cathode active materials that
may be used for the cathodes for conventional electrochemical devices. Particularly,
lithium manganese oxides, lithium cobalt oxides, lithium nickel oxides, lithium iron oxides
or lithium composite oxides containing a combination thereof are used preferably. Non26
limiting examples of an anode active material include conventional anode active materials
that may be used for the anodes for conventional electrochemical devices. Particularly,
lithium-intercalating materials, such as lithium metal or lithium alloys, carbon, petroleum
coke, activated carbon, graphite or other carbonaceous materials are used preferably.
Non-limiting examples of a cathode current collector include foil 5 made of aluminum,
nickel or a combination thereof. Non-limiting examples of an anode current collector
include foil made of copper, gold, nickel, copper alloys or a combination thereof.
The electrolyte that may be used in the electrochemical device according to the
present disclosure is a salt having a structure of A+B-, wherein A+ includes an alkali metal
cation such as Li+, Na+, K+ or a combination thereof and B- includes an anion such as PF6
10 -,
BF4
-, Cl-, Br-, I-, ClO4
-, AsF6
-, CH3CO2
-, CF3SO3
-, N(CF3SO2)2
-, C(CF2SO2)3
- or a
combination thereof, the salt being dissolved or dissociated in an organic solvent including
propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl
carbonate (DMC), dipropyl carbonate(DPC), dimethyl sulfoxide, acetonitrile,
15 dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl
methyl carbonate (EMC), gamma-butyrolactone (γ-butyrolactone) or a combination thereof.
However, the present disclosure is not limited thereto.
Injection of the electrolyte may be carried out in an adequate step during the
process for manufacturing a battery depending on the manufacturing process of a final
20 product and properties required for a final product. In other words, injection of the
electrolyte may be carried out before the assemblage of a battery or in the final step of the
assemblage of a battery.
27
Examples will be described more fully hereinafter so that the present disclosure can
be understood with ease. The following examples may, however, be embodied in many
different forms and should not be construed as limited to the exemplary embodiments set
forth therein. Rather, these exemplary embodiments are provided so that the present
disclosure will be thorough and complete, and will fully convey the 5 scope of the present
disclosure to those skilled in the art.
Example 1
First, aluminum hydroxide (Al(OH)3) particles (particle size: 800 nm) as inorganic
10 particles were introduced to acetone as a first solvent to a solid content of 30%. Herein,
‘solid content’ refers to a content of powder obtained after drying the solvent, as expressed
by a percentage based on the total content of the solvent and composition of ingredients.
Next, isopropyl trioleyl titanate as a dispersing agent and a thermosetting phenolic resin
(Kangnam Chemical, resol-type phenol resin, thermosetting temperature: about 150°C,
15 soluble in acetone, solid content 50%, viscosity: 250 mPas at 25°C) were introduced to the
composition as shown in the following Table 1, and agitation was carried out for 2 hours.
In this manner, an inorganic particle dispersion including the inorganic particles dispersed
in the first solvent and the thermosetting phenolic resin dissolved therein was prepared.
Then, polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) as a non20
phenolic resin was introduced to acetone as a second solvent as shown in Table 1, and
dissolved therein at 50°C for about 4 hours to prepare a polymer solution.
The inorganic particle dispersion and the polymer solution prepared as described
above were mixed with each other in such a manner that the weight ratio of the inorganic
28
particles to the binder polymer might be 75:25, and the inorganic particles were pulverized
and dispersed by using a ball milling process for 3 hours to obtain slurry for forming a
porous coating layer having a solid content of 18%.
After that, the slurry for forming a porous coating layer was coated on both
surfaces of a polyethylene porous film having a thickness of 9 μm 5 (porosity: 43%, air
permeation time: 110 sec/100 mL, resistance: 0.45 ohm) through a coating process at 23°C
under a relative humidity of 45% to a total slurry loading amount of 9.5 g/m2, followed by
drying, to obtain a separator having a porous coating layer.
10 Examples 2-7
The separators were obtained in the same manner as Example 1, except that the
types and contents of the ingredients contained in slurry were controlled as shown in Table
1.
15 Comparative Examples 1-4
The separators were obtained in the same manner as Example 1, except that the
types and contents of the ingredients contained in slurry were controlled as shown in Table
1.
[Table 1]
Inorganic particle dispersion
(parts by weight)
Binder polymer
solution (parts
by weight)
Physical
properties of
slurry
Physical properties of
separator
29
Inorga
nic
partic
les
Polym
er
dispers
ing
agent
(Cyan
o-ethyl
PVA)
Dispersi
ng agent
(Tibased
couplin
g agent)
Themo
-
setting
phenol
ic
resin
Fluorina
ted
binder
polymer
(nonphenolic
resin)
Polyvi
nyl
acetate
binder
polym
er
(nonphenol
ic
resin)
Sediment
ation rate
(um/sec
)
D50
(um)
Thi
ck
nes
s
Load
ing
amou
nt
Air
perm
eabili
ty
Adhesi
on
betwee
n
electro
de and
separat
or
(Lami,
gf/
25mm
)
(Top/
Botto
m)
Heat
shrink
age
(MD/T
D)
C
o
m
p.
E
x.
1
75
0.75 0.75 0 23.5 0 10.8 1.8
18.
0
9.5 284 77/75 38/38
2 1.5 0 0 23.5 0 22.5 3.1
17.
4
9.4 302 54/69 35/34
3 0 1.5 0 23.5 0 12.5 1.8
17.
8
9.4 269 57/61 41/40
4 0.75 0.75 0 5 18.5 8.9 1.5 16. 9.2 278 100/98 47/46
30
8
E
x.
1
75
0 0.75 0.75 23.5 0 10.5 2.0
18.
1
9.6 252 75/79 27/26
2 0 0 1.5 23.5 0 20.1 2.6
17.
9
9.4 249 78/72 23/23
3 0 0 3 22 0 17.2 2.3
17.
5
9.5 234 66/69 20/19
4 0 0 5.25 19.75 0 16.1 2.1
17.
8
9.3 219 62/65 17/16
5 0 0 7.5 17.5 0 12.5 1.9
17.
4
9.2 203 58/56 12/12
6 70 0 0 7 23 0 11.3 2.3
17.
6
9.5 254 81/84 16/16
7 75 0 0 1.5 5 18.5 13.4 1.8
16.
7
9.4 267 91/104 22/21
[Test Methods]
1) Method for Measuring Thickness
The thickness of a separator was measured by using a thickness gauge (Mitutoyo,
5 VL-50S-B).
2) Method for Measuring Thermosetting Temperature of Thermosetting
Phenolic Resin
31
The heat emission peak temperature of a thermosetting phenolic resin measured by
using differential scanning calorimeter (DSC) was determined as the thermosetting
temperature of the resin.
3) Method for Determining 5 Air Permeability
The air permeability of a separator was determined by using a Gurley type air
permeability analyzer according to JIS P-8117. Herein, the time required for 100 mL of
air to pass through a diameter of 28.6 mm and an area of 645 mm2 was measured.
10 4) Method for Determining Adhesion between Electrode and Separator (Lami
Strength)
Each of the separators according to Examples and Comparative Examples was cut
into a size of 25 mm X 100 mm. The prepared separator was stacked with an anode, and
the resultant stack was inserted between PET films having a thickness 100 μm, and then
15 adhesion was carried out by using a flat press. Herein, the flat press was used under
heating and pressurization at 70°C and a pressure of 600 kgf for 1 second. The adhered
separator and anode (artificial graphite, carbon black, carboxymethyl cellulose (CMC) and
styrene-butadiene rubber (SBR) at a weight ratio of 96:1:2:2) was attached to slide glass by
using a double-sided tape. The end (10 mm or less from the end of the adhesive surface)
20 of the separator adhesive surface was peeled and attached to a 25 mm x 100 mm PET film
by using a single-sided adhesive tape so that they might be connected with each other in
the longitudinal direction. Then, the slide glass was mounted to the lower holder of a
UTM instrument (LLOYD Instrument LF Plus), the PET film adhered to the separator was
32
mounted to the upper holder of the UTM instrument, and the force required for separating
the anode from the porous coating layer facing the anode was determined by applying
force at an angle of 180° at a rate of 300 mm/min.
5) Determination 5 of Heat Shrinkage
The heat shrinkage was calculated according to the formula of (Initial length -
Length after heat shrinking at 150°C for 30 minutes) / (Initial length) x 100.

WHAT IS CLAIMED IS:
1. A separator for a lithium secondary battery, comprising:
a porous polymer substrate having a plurality of pores; and
a porous coating layer formed on at least one surface of 5 the porous polymer
substrate and comprising a plurality of inorganic particles and a binder polymer, wherein
the binder polymer comprises a thermosetting phenolic resin having at least one hydroxyl
group and aromatic group.
10 2. The separator for a lithium secondary battery according to claim 1, wherein
the thermosetting phenolic resin comprises at least one of a novolac-type phenol resin
represented by the following Chemical Formula 1 and a resol-type phenol resin represented
by the following Chemical Formula 2:
[Chemical Formula 1]
15
wherein n is an integer of 1 or more, and R represents any one of H and optionally
substituted C1-C100 alkyl, alkoxy and alkenyl groups.
[Chemical Formula 2]
34
wherein each of n and m is an integer of 1 or more, and R represents any one of H
and optionally substituted C1-C100 alkyl, alkoxy and alkenyl groups.
3. The separator for a lithium secondary battery according 5 to claim 1, wherein
the thermosetting phenolic resin is thermally cured at 120-200°C.
4. The separator for a lithium secondary battery according to claim 1, wherein
the thermosetting phenolic resin is a phenol-formaldehyde resin.
10
5. The separator for a lithium secondary battery according to claim 1, wherein
the content of the thermosetting phenolic resin is 0.1-20 wt% based on 100 wt% of the
content of the inorganic particles.
15 6. The separator for a lithium secondary battery according to claim 1, wherein
the binder polymer further comprises a non-phenolic resin in an amount of 0.1-50 wt%
based on 100 wt% of the content of the inorganic particles.
7. The separator for a lithium secondary battery according to claim 1, wherein
35
the porous polymer substrate has a thickness of 1-20 μm, and the thickness of the porous
coating layer is 0.5-20 μm based on single surface coating.
8. A method for manufacturing a separator for a lithium secondary battery,
comprising 5 the steps of:
(S1) preparing an inorganic particle dispersion by dispersing inorganic particles in
a first solvent, and dissolving a thermosetting phenolic resin having at least one hydroxyl
group and aromatic ring therein; and
(S2) coating slurry for forming a porous coating layer comprising the inorganic
10 particle dispersion on at least one surface of a porous polymer substrate, followed by
drying, to form a porous coating layer.
9. The method for manufacturing a separator for a lithium secondary battery
according to claim 8, which further comprises a step of mixing the inorganic particle
15 dispersion of step (S1) with a polymer solution comprising a non-phenolic resin dissolved
in a second solvent.
10. The method for manufacturing a separator for a lithium secondary battery
according to claim 9, wherein each of the first solvent and the second solvent
20 independently comprises any one compound selected from water, C2-C5 alcohols, acetone,
tetrahydrofuran, methylene chloride, chloroform, dimethyl formamide, N-methyl-2-
pyrrolidone, methyl ethyl ketone and cyclohexane, or a mixture of two or more of them.
36
11. The method for manufacturing a separator for a lithium secondary battery
according to claim 8, wherein the inorganic particle dispersion further comprises a
dispersing agent.
12. The method for manufacturing a separator for a lithium 5 secondary battery
according to claim 8, wherein the thermosetting phenolic resin comprises at least one of a
novolac-type phenol resin represented by the following Chemical Formula 1 and a resoltype
phenol resin represented by the following Chemical Formula 2:
[Chemical Formula 1]
10
wherein n is an integer of 1 or more, and R represents any one of H and optionally
substituted C1-C100 alkyl, alkoxy and alkenyl groups.
[Chemical Formula 2]
15 wherein each of n and m is an integer of 1 or more, and R represents any one of H
37
and optionally substituted C1-C100 alkyl, alkoxy and alkenyl groups.
13. The method for manufacturing a separator for a lithium secondary battery
according to claim 8, wherein the thermosetting phenolic resin is thermally cured at 120-
5 200°C.
14. The method for manufacturing a separator for a lithium secondary battery
according to claim 8, wherein the content of the thermosetting phenolic resin is 0.1-20 wt%
based on 100 wt% of the content of the inorganic particles.
10
15. A lithium secondary battery comprising a cathode, an anode and a separator
interposed between the cathode and the anode, wherein the separator is the same as defined
in any one of claims 1 to 7.