KR20200083918A - Porous separator for secondary battery - Google Patents

Abstract

The present invention relates to a porous separator for a secondary battery, which comprises: a polyolefin-based porous substrate; and a heat-resistant coating layer which is applied to at least one surface of the porous substrate and includes a composite and a filler in which a natural polymer compound dissociated to at least a portion of the surfaces of the acrylic polymer particles is bound and, more specifically, to a porous separator for a secondary battery coated with a coating composition having excellent heat resistance and adhesion.

Description

Porous separator for secondary batteries {POROUS SEPARATOR FOR SECONDARY BATTERY}

The present invention relates to a porous separator for a secondary battery.

Lithium secondary batteries are widely used as power sources for various electric products that require miniaturization and weight reduction, such as smartphones, laptops, and tablet PCs. Development of a lithium secondary battery having a large, long-life and high stability is required.

In order to improve the stability, a separator is formed in which micropores are formed to separate the positive electrode and the negative electrode to prevent internal shorts and facilitate the movement of lithium ions during the charging and discharging process. As the furnace, polyolefin-based polyethylene or polypropylene is used.

Due to the non-ideal behavior of the battery, when the separator is exposed to a high temperature environment, mechanical shrinkage or damage of the separator may occur. In this case, the positive electrode and the negative electrode may contact and ignite the battery. In order to overcome this problem, there is a need for a technique for securing the stability of the battery by suppressing thermal shrinkage of the separator. In particular, as the market has expanded from small-sized batteries to medium-sized and large-sized batteries, the standard of stability required for the separator has been raised, and in order to meet this, the thermal resistance of the separator is improved by coating inorganic particles with high thermal resistance with an adhesive organic binder and coating them. How to increase is known. However, the separator manufactured by the prior art cannot secure the desired adhesiveness, and it is difficult to apply it to the separator having various shapes and sizes at once. Therefore, it is necessary to develop a separator having high heat resistance and excellent adhesion at the same time.

In addition, lithium contained in the secondary battery may explode due to a rapid reaction upon contact with moisture. In particular, the separator using an aqueous slurry needs to reduce the water content to prevent such an explosion.

Korean Registered Patent No. 10-1801049, Korean Patent Publication No. 10-2013-0114152, etc. use natural polymer compounds as viscosity modifiers for slurry compositions to change the viscosity of the slurry to increase the dispersibility of the particles, or to improve the coatability. Height is composing a configuration. However, the configuration described in the above patent is a simple mixing of a natural polymer compound with a binder and the like, and the storage stability of the prepared slurry is poor, and there is no effect of improving heat resistance or water content characteristics.

Korean Registered Patent No. 10-1801049 Korean Patent Publication No. 10-2013-0114152

The present invention is to solve the problems of the prior art described above, one object of the present invention is to provide a porous separator for a secondary battery comprising a coating layer formed from a coating composition having a low water content, excellent heat resistance and adhesion.

Another object of the present invention is to provide a porous separator for a secondary battery having excellent electrode adhesion or shutdown characteristics.

One aspect of the present invention is a polyolefin-based porous substrate; And it is applied to at least one surface of the porous substrate, and provides a porous separator for a secondary battery comprising a heat-resistant coating layer comprising a composite and a filler bonded to a natural polymer compound dissociated on at least a portion of the surface of the acrylic polymer particles.

In one embodiment, the natural polymer compound may be at least one selected from the group consisting of plant-based polymer compounds, animal polymer compounds and microbial polymer compounds.

In one embodiment, the filler is Al ₂ O ₃ , AlOOH, Al(OH) ₃ , CaCO ₃ , SiO ₂ , SiC, BaTiO ₃ , TiO ₂ , talc, (meth) acrylic copolymer, (meth) acrylic- It may include at least one selected from the group consisting of styrene-based copolymers, polymethyl methacrylate, polystyrene and polysilsesquioxane, and the average particle size of the filler may be 0.1 to 3 μm.

In one embodiment, the thermal shrinkage of the separator 150° in the vertical and horizontal directions may be 10.0% or less, respectively.

In one embodiment, the moisture content of the heat-resistant coating layer may be 1,000ppm or less.

In one embodiment, the heat-resistant coating layer may have an adhesive strength to the electrode of 10 gf/cm or more.

In one embodiment, the heat-resistant coating layer may have a shutdown characteristic at 130 ℃ or less.

In one embodiment, the heat-resistant coating layer is applied to one surface of the porous substrate, and is applied to the other surface of the porous substrate so as to face the heat-resistant coating layer, the adhesive strength to the electrode may include an adhesive coating layer of 10gf / cm or more .

In one embodiment, the heat-resistant coating layer is applied to one surface of the porous substrate, and is applied to the other surface of the porous substrate so as to face the heat-resistant coating layer, may further include a shutdown coating layer having a shutdown characteristic at 130 ℃ or less. .

In one embodiment, the shutdown coating layer may include a polymer having a glass transition temperature (T _g ) of 50 to 100°C.

According to an aspect of the present invention, it is possible to provide a porous separator for a secondary battery coated with a coating composition having a low water content, excellent heat resistance and adhesion.

According to another aspect of the present invention, it is possible to provide a separator having excellent stability by applying the coating composition.

It should be understood that the effects of the present invention are not limited to the above-described effects, and include all effects that can be deduced from the configuration of the invention described in the detailed description or claims of the present invention.

Figure 1 shows the modification of the PAA-casein complex contained in the coating composition according to an embodiment of the present invention, the complex is a case of dissociated casein (Casein) on the surface of polyacrylic acid (PAA) .
2 is a schematic diagram of a reactor for preparing a coating composition according to an embodiment of the present invention.
3 is a result of cold-blending the casein aqueous solution and the polyacrylic acid solution.
FIG. 4(A) shows the casein aggregation as a result of leaving the mixture prepared by cold mixing the aqueous casein solution and the polyacrylic acid solution for 2 days, and FIG. 4(B) shows that casein and polyacrylic acid are separated from each other in the mixture. It is confirmed.
Figure 5 is a comparison of each other after changing the pH and preparing the coating composition.
Figure 6 shows the solubility curve of the aqueous casein solution.
7 is to compare the casein content after changing the casein content to prepare a coating composition.
Figure 8 is a comparison of each other after changing the solid content and preparing the coating composition.
Figure 9 is a scanning electron microscope (Scanning electron microscope) images of the particles of the coating composition observed, (a) Preparation Example 1-5, (b) Comparative Preparation Example 1-5, (c) Comparative Preparation Example 1-6 The particles are taken.
10 is a scanning electron microscope image of the coating layer of the separator prepared according to Example 1-5 of the present invention, each magnification is (a) 1,000 times, (b) 3,000 times, (c) 5,000 times, ( d) 10,000 times.
11 shows the results of thermomechanical analysis of the separator according to an embodiment of the present invention.
12 shows the results of measuring the dispersion stability of the coating composition according to an embodiment of the present invention.

Hereinafter, the present invention will be described with reference to the accompanying drawings. However, the present invention may be implemented in various different forms, and thus is not limited to the embodiments described herein. In addition, in order to clearly describe the present invention in the drawings, parts irrelevant to the description are omitted, and like reference numerals are assigned to similar parts throughout the specification.

Throughout the specification, when a part is "connected" to another part, this includes not only "directly connected" but also "indirectly connected" with another member in between. . Also, when a part “includes” a certain component, this means that other components may be further provided instead of excluding other components, unless specifically stated otherwise.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Coating composition

The coating composition according to an aspect of the present invention may include a complex in which a natural polymer compound dissociated to at least a part of the surface of the acrylic polymer particles is bound.

1 shows the case where the acrylic polymer is PAA and the natural polymer compound is casein in the composite. Referring to FIG. 1, a dissociated natural polymer compound is bound to a part of the surface of the acrylic polymer particle. The dissociated natural polymer compound may promote crosslinking and/or bonding between the complexes, and these properties may improve the heat resistance and adhesion of the slurry containing the complex.

The coating composition may be water-soluble.

"Non-aqueous coating solution" is a coating solution that is used by dissolving a non-aqueous binder in an organic solvent. It has excellent heat resistance and adhesion, but requires a separate post-treatment because an uneven coating layer may be formed due to gelation during drying. And, there is a disadvantage that it is difficult to shorten the processing speed due to the risk of explosion during drying, and the storage stability is poor due to volatility of the organic solvent and causes environmental pollution.

The "aqueous coating solution" is a coating solution that is used by dissolving a water-soluble binder in water, and is economical and eco-friendly compared to a non-aqueous coating solution. However, a general aqueous coating solution has a disadvantage of poor adhesion, or closing of pores in a separator to decrease air permeability. However, the coating composition of the present invention may have excellent heat resistance and adhesion compared to conventional aqueous coating solutions.

The natural polymer compound may be one selected from the group consisting of a plant polymer compound, an animal polymer compound, a microbial polymer compound and a combination of two or more of them, and preferably, an animal polymer compound.

The animal polymer compound may be one selected from the group consisting of collagen, casein, albumin, gelatin and a combination of two or more of them.

The casein is a protein obtainable from milk and is composed of α-casein, β-casein, κ-casein, and trace amounts of other caseins. The casein used in the present invention may have a ratio of α, β, and κ of 40 to 50: 20 to 40: 10 to 15, respectively.

The solid content of the coating composition may be 5 to 25% by weight, preferably 10 to 20% by weight, more preferably 15 to 20% by weight. If the solid content of the coating composition is less than 5% by weight, the adhesive strength may decrease, and if it exceeds 25% by weight, the viscosity may be excessively high, resulting in poor coating properties.

The coating composition may have a pH of 4 to 5. If the pH of the composition is outside the above range, the complex may not be formed.

Manufacturing method of coating composition

Method for producing a coating composition according to another aspect of the present invention, (a) injecting water and a natural polymer compound in the reactor and adjusting the pH to dissociate the natural polymer compound; And (b) reacting by introducing a water-soluble initiator and an acrylic monomer into the reactor.

In step (a), the pH of the reactor can be adjusted to a value capable of dissociating the natural polymer compound. For example, if the natural polymer compound is casein, the pH of step (a) may be 3 or less or 7 or more.

As the water-soluble initiator, ammonium peroxide (Ammonium persulfate, APS), potassium peroxide (Potassium persulfate, KPS) and similar properties of initiator may be used.

The acrylic monomer may be saponification before being added in step (b). When the acrylic monomer is saponified and then added, mixing property with the dissociated natural polymer compound may be improved, and the complex may be more easily formed.

The concentration of the natural polymer compound may be 1 to 10% by weight. If the concentration is less than 1% by weight, the effect of improving heat resistance and adhesiveness may be reduced, and if it is more than 10% by weight, it may be gelled and cannot be used.

Porous separator for secondary battery

The porous separator according to an aspect of the present invention includes a polyolefin-based porous substrate; And it is applied to at least one surface of the porous substrate, it may include a heat-resistant coating layer comprising a composite and filler is a natural polymer compound dissociated to at least a portion of the surface of the acrylic polymer particles.

The heat-resistant coating layer may have a thickness of 0.5 to 10㎛. If the thickness of the heat-resistant coating layer is less than 0.5 μm, a sufficient heat-resistance improving effect may not be realized, and if it is more than 10 μm, the air permeability of the separator is lowered, and the separation film is thickened, which may be detrimental to assembly and high capacity of the secondary battery. .

The heat-resistant coating layer may be formed by preparing a slurry using the above-described coating composition, and then applying it to at least one surface of the porous substrate and drying it. Therefore, the natural polymer compound may be at least one selected from the group consisting of plant-based polymer compounds, animal-based polymer compounds, and microbial polymer compounds.

The composite may serve to stably fix the filler on the surface of the porous substrate. Based on 100 parts by weight of the filler, if the content of the composite is less than 1 part by weight, the expression of this function may be insufficient, and if it exceeds 150 parts by weight, the pores of the heat-resistant coating layer are insufficiently formed to degrade the performance of the secondary battery. Can. Therefore, the content of the composite is 1 part by weight, 5 parts by weight, 10 parts by weight, 15 parts by weight, 20 parts by weight, 25 parts by weight, 30 parts by weight, 35 parts by weight, 40 parts by weight based on 100 parts by weight of the filler, 45 parts, 50 parts, 55 parts, 60 parts, 65 parts, 70 parts, 75 parts, 80 parts, 85 parts, 90 parts, 95 parts, 100 parts, 105 parts Parts, 110 parts by weight, 115 parts by weight, 120 parts by weight, 125 parts by weight, 130 parts by weight, 135 parts by weight, 140 parts by weight, 145 parts by weight or 150 parts by weight, but is not limited thereto.

The filler may serve to improve the thermal stability of the heat-resistant coating layer. The filler is Al ₂ O ₃ , AlOOH, Al(OH) ₃ , CaCO ₃ , SiO ₂ , SiC, BaTiO ₃ , TiO ₂ , talc, (meth)acrylic copolymer, (meth)acrylic-styrene copolymer, poly It may include one or more selected from the group consisting of methyl methacrylate, polystyrene and polysilsesquioxane.

If the content of the filler is less than 1% by weight based on the total weight of the heat-resistant coating layer, the effect of improving thermal stability may be insignificant, and if it exceeds 70% by weight, precipitation occurs in the slurry during coating and dispersion stability decreases, and uniformity of the coating layer This can degrade. Therefore, the content of the filler is 1% by weight, 5% by weight, 10% by weight, 15% by weight, 20% by weight, 25% by weight, 30% by weight, 35% by weight, 40% by weight based on the total weight of the heat-resistant coating layer , 45% by weight, 50% by weight, 55% by weight, 60% by weight, 65% by weight, or 70% by weight, but is not limited thereto.

If the average particle size of the filler is less than 0.1 μm, dispersibility may decrease, and if it is more than 3 μm, formation of a thin film having a thickness of 3 μm or less may be difficult. Therefore, the average particle size of the filler may be 0.1 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, or 3 μm, but is not limited thereto. This average particle size may be the D50 average particle size of the volume average.

The heat-resistant coating layer may further include a dispersant. The dispersant is partially adsorbed on the surface of the filler, and the rest is dispersed in a dispersion solvent, so that all components including the filler before and after coating serve to stably maintain the dispersion state.

As the dispersant, one or more selected from methyl cellulose, carboxymethyl cellulose, and salts thereof may be used as the ionic cellulose semi-synthetic polymer, and one or more selected from associative polyurethane-based or alkali-swellable acrylic resins may be used as the synthetic polymer.

When the content of the dispersant is less than 1 part by weight based on 100 parts by weight of the filler, the content of the dispersant adsorbed on the filler surface may decrease, resulting in insufficient slurry dispersibility during coating, and when it exceeds 15 parts by weight, the viscosity of the slurry is excessive. As it rises, aggregation between fillers may occur, and the smoothness of the coating layer may decrease. Therefore, the content of the dispersant may be 1 part by weight, 3 parts by weight, 5 parts by weight, 7 parts by weight, 9 parts by weight, 11 parts by weight, 13 parts by weight or 15 parts by weight based on 100 parts by weight of the filler, but is limited thereto. It does not work.

The horizontal and vertical heat shrinkage of the separator may be 5.0% or 0.55% or less at 105°C, and 5.0% or 1.2% or less at 120°C, depending on the temperature. In addition, the heat shrinkage of the separator 150°C in the horizontal and vertical directions may be 10.0% or less, 5.0% or less, or 1.0% or less, respectively. The heat-resistant coating layer including the coating composition according to an aspect of the present invention may improve the heat resistance of the separator to reduce the thermal contraction rate of the separator.

The water content of the coating layer may be 1000 ppm or less or 500 ppm or less. The natural polymer compound bound to the surface of the acrylic polymer particle can prevent the acrylic polymer from containing moisture, thereby reducing the water content.

The heat-resistant coating layer may have an adhesive strength to the electrode of 10 gf/cm or more. For example, the adhesive strength of the heat-resistant coating layer to the electrode may be 10gf/cm, 15gf/cm, 20gf/cm, 25gf/cm, 30gf/cm, 35gf/cm or 40gf/cm, but is not limited thereto.

The adhesive strength may be improved by further including an adhesive polymer in which the heat-resistant coating layer has excellent adhesive strength with an electrode, but is not limited thereto. The adhesive polymer is a polymer having a glass transition temperature (T _g ) of 0 to 150° C., an acrylic copolymer, a methacrylic copolymer, (meth)acrylic-styrene copolymer, and (meth)acrylic-acrylic. Nitrile copolymer, silicone-acrylic copolymer, epoxy-acrylic copolymer, polybutadiene, polyisoprene, butadiene-styrene random copolymer, isoprene-styrene random copolymer, acrylonitrile-butadiene copolymer, acrylonitrile-butadiene- One or more of styrene copolymer, butadiene-styrene block copolymer, styrene-butadiene-styrene-block copolymer, ethylene-vinyl acetate copolymer, acrylic-urethane copolymer, and vinyl acetate-based copolymer may be used, but is not limited thereto. It is not. The adhesive polymer may be a polymer exhibiting heat seal adhesion.

If the content of the adhesive polymer is less than 1 part by weight based on 100 parts by weight of the filler, the expression of adhesive performance may be insufficient, and when it exceeds 50 parts by weight, agglomeration occurs in the slurry during formation of the coating layer, resulting in dispersion stability. Deterioration, and the air permeability of the separator may deteriorate. Therefore, the content of the adhesive polymer is 1 part by weight, 5 parts by weight, 10 parts by weight, 15 parts by weight, 20 parts by weight, 25 parts by weight, 30 parts by weight, 35 parts by weight, 40 parts by weight based on 100 parts by weight of the filler Part, 45 parts by weight or 50 parts by weight, but is not limited thereto.

The heat-resistant coating layer may have a shut-down characteristic at 130°C or less. Shutdown characteristics are characteristics that can block the flow of current by closing the micropores inside the separator when the temperature inside the battery is abnormally increased, for example, at least a part of the porous substrate or coating layer is melted to be implemented by closing the pores Can.

Such a shutdown characteristic may be implemented by further including a polymer having a shutdown characteristic of the heat resistant coating layer, but is not limited thereto. A shutdown polymer having a shutdown characteristic may have a weight average molecular weight (M _w ) of 50,000 to 500,000, and an average particle size of 0.01 to 1 μm. The shutdown polymer may have a glass transition temperature (T _g ) of 50 to 100°C or a melting point (T _m ) of 105 to 145°C. When the glass transition temperature or melting point of the shutdown polymer is outside the above range, the pores are closed at an excessively low temperature, or the pores are closed at a temperature higher than the shutdown temperature of the porous substrate, so that the shutdown effect of the coating layer may be insufficient. Although the exact mechanism of action is not known, when the glass transition temperature falls within the above range, a part of the amorphous part of the polymer may be melted to close the pores below 130°C, and if the melting point falls within the above range, the polymer may melt and melt at 130°C It seems that the pores can be closed below.

The shutdown polymer is (meth)acrylic copolymer, (meth)acryl-styrene copolymer, (meth)acryl-acrylonitrile copolymer, silicone-(meth)acrylic copolymer prepared by emulsion polymerization or suspension polymerization. , Epoxy- (meth) acrylic copolymer, ethylene copolymer and propylene copolymer may be used at least one, but is not limited thereto. For example, the glass transition temperature of the shutdown polymer may be 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃ or 100℃, but is limited thereto. It does not work.

Based on 100 parts by weight of the filler, if the content of the shutdown polymer is less than 1 part by weight, the expression of shutdown performance may be insufficient, and when it exceeds 50 parts by weight, flexibility of the coating layer is lowered and battery assembly property is reduced or fine cracks at low temperature (crack) may occur. Therefore, the content of the shutdown polymer is 1 part by weight, 5 parts by weight, 10 parts by weight, 15 parts by weight, 20 parts by weight, 25 parts by weight, 30 parts by weight, 35 parts by weight, 40 parts by weight based on 100 parts by weight of the filler , 45 parts by weight or 50 parts by weight, but is not limited thereto.

The heat-resistant coating layer may be applied to one surface of the porous substrate, and applied to the other surface of the porous substrate so as to face the heat-resistant coating layer, and may include an adhesive coating layer having an adhesive strength to an electrode of 10 gf/cm or more. The adhesive force may be 10gf/cm, 15gf/cm, 20gf/cm, 25gf/cm, 30gf/cm, 35gf/cm or 40gf/cm, but is not limited thereto.

When the battery is assembled, the adhesive coating layer may have excellent adhesion to an electrode contacting a separator.

The adhesive coating layer may include an adhesive polymer having excellent adhesion to an electrode. The adhesive polymer may have a glass transition temperature of 0 to 150°C, and the types thereof are as described above. The adhesive coating layer is coated and dried with 1 to 30 parts by weight of the adhesive polymer, 1 to 30 parts by weight of a binder resin, 0.1 to 10 parts by weight of a dispersant, 1 to 70 parts by weight of a filler, and 20 to 70 parts by weight of a dispersion solvent It may be formed by. When the content of the adhesive polymer is out of the above range, adhesiveness with an electrode may be poor, or agglomeration may occur in the slurry to deteriorate the air permeability of the separator. Thus, the adhesive polymer may be included in 1 part by weight, 5 parts by weight, 10 parts by weight, 15 parts by weight, 20 parts by weight, 25 parts by weight or 30 parts by weight in the coating slurry, but is not limited thereto.

The binder resin is polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-trichloroethylene (PVDF-CTFE), acrylic copolymer, methacryl-based Copolymer, (meth)acrylic-styrene copolymer, (meth)acryl-acrylonitrile copolymer, silicone-(meth)acrylic copolymer, epoxy-(meth)acrylic copolymer, polybutadiene, polyisoprene, butadiene One or more selected from -styrene copolymer, isoprene-styrene copolymer, acrylonitrile-butadiene copolymer, acrylonitrile-butadiene-styrene copolymer, butadiene-styrene copolymer, and styrene-butadiene-styrene copolymer may be used. . The binder resin not only serves to stably fix the adhesive polymer to the surface of the porous substrate, but also can impart flexibility to the adhesive coating layer, and if the content is less than 1 part by weight, the adhesive coating layer is applied to the porous substrate It may not adhere properly, and if it exceeds 30 parts by weight, the air permeability may increase excessively.

The properties of the dispersant and filler are as described above.

The dispersion solvent is water, cyclopentane, cyclohexane, toluene, xylene, methyl ethyl ketone, cyclohexanone, ethyl acetate, butyl acetate, acetonitrile, propionitrile, tetrahydrofuran, ethylene glycol diethyl ether, methanol, One or more selected from ethanol, isopropanol, ethylene glycol, ethylene glycol monomethyl ether, N-methylpyrrolidone, and N,N-dimethylformamide can be used. The dispersion solvent is all volatilized in the drying process after applying the slurry, so that only a small portion may remain in the shutdown coating layer. When the content of the dispersion solvent is less than 20 parts by weight, the viscosity of the slurry may rise excessively, resulting in uneven coating layer thickness, and if it exceeds 70 parts by weight, the viscosity may drop excessively, resulting in uneven distribution of the components of the coating layer.

The thickness of the adhesive coating layer may be 0.5 ~ 10㎛. If the thickness is less than 0.5 µm, adhesiveness may be insufficient, and if it is more than 10 µm, air permeability may decrease, and battery assembly may be poor due to thickening of the separator or disadvantageous in increasing the capacity of the battery. Other adhesive forces are as described above.

The heat-resistant coating layer is applied to one surface of the porous substrate, and is applied to the other surface of the porous substrate so as to face the heat-resistant coating layer, and may further include a shutdown coating layer having a shutdown characteristic at 130°C or less. The shutdown characteristic may be implemented at, for example, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, 125°C or 130°C, but is not limited thereto.

The shutdown coating layer may have a function of blocking the movement of ionic materials by dissolving when the temperature inside the battery increases above a certain temperature and closing the pores of the separator.

The shutdown coating layer may include a shutdown polymer having a glass transition temperature (T _g ) of 50 to 100°C. The shutdown coating layer may be formed by applying a coating slurry comprising 0.1 to 50 parts by weight of the shutdown polymer, 1 to 40 parts by weight of a binder resin, and 10 to 100 parts by weight of a dispersion solvent, followed by drying. If the content of the shutdown polymer is outside the above range, the shutdown characteristics may be poor, or the flexibility of the coating layer may be insufficient, resulting in poor battery assembly or micro cracks at low temperatures. Therefore, the shutdown polymer is 0.1 parts by weight, 1 part by weight, 5 parts by weight, 10 parts by weight, 15 parts by weight, 20 parts by weight, 25 parts by weight, 30 parts by weight, 35 parts by weight, 40 parts by weight, 45 parts by weight or 50 parts by weight may be included in the coating slurry, but is not limited thereto.

The binder resin is polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-trichloroethylene (PVDF-CTFE), acrylic copolymer, methacryl-based Copolymer, (meth)acrylic-styrene copolymer, (meth)acryl-acrylonitrile copolymer, silicone-(meth)acrylic copolymer, epoxy-(meth)acrylic copolymer, polybutadiene, polyisoprene, butadiene One or more selected from -styrene copolymer, isoprene-styrene copolymer, acrylonitrile-butadiene copolymer, acrylonitrile-butadiene-styrene copolymer, butadiene-styrene copolymer, and styrene-butadiene-styrene copolymer may be used. . The binder resin not only serves to stably fix the shutdown polymer on the surface of the porous substrate, but also can impart flexibility to the shutdown coating layer, and if the content is less than 1 part by weight, the shutdown coating layer is properly adhered to the base film It may not, and if it exceeds 40 parts by weight, the air permeability may be excessively increased.

The dispersion solvent is water, cyclopentane, cyclohexane, toluene, xylene, methyl ethyl ketone, cyclohexanone, ethyl acetate, butyl acetate, acetonitrile, propionitrile, tetrahydrofuran, ethylene glycol diethyl ether, methanol, One or more selected from ethanol, isopropanol, ethylene glycol, ethylene glycol monomethyl ether, N-methylpyrrolidone, and N,N-dimethylformamide can be used. The dispersion solvent is all volatilized in the drying process after applying the slurry, so that only a small portion may remain in the shutdown coating layer. If the content of the dispersing solvent is less than 10 parts by weight, the viscosity of the slurry may rise excessively, resulting in uneven coating layer thickness, and if it exceeds 100 parts by weight, the viscosity may drop excessively, resulting in uneven distribution of the components of the coating layer.

The thickness of the shutdown coating layer may be 0.5 to 10㎛. If the thickness is less than 0.5 µm, the shutdown function may be insufficient, and if it is more than 10 µm, air permeability may be reduced, and battery assembly may be poor due to thickening of the separator or disadvantageous in increasing the capacity of the battery. Other shutdown characteristics are as described above.

According to one aspect of the present invention, a heat-resistant coating layer comprising a composite and a filler in which a natural polymer compound is bonded to at least a part of the surface of an acrylic polymer particle can provide a porous separator for a secondary battery coated on one or both surfaces of a porous substrate, At least one of the heat-resistant coating layers may have shutdown characteristics and adhesion characteristics to electrodes. Alternatively, a porous separator for a secondary battery having a heat-resistant coating layer coated on one surface of the porous substrate and a shutdown coating layer having shutdown characteristics on the other surface or an adhesive coating layer having adhesion characteristics to electrodes may be provided.

Hereinafter, embodiments of the present invention will be described in more detail. However, the following experimental results are only representative of the experimental results among the above examples, and the scope and contents of the present invention may be reduced or limited by examples and the like. Each effect of the various embodiments of the present invention, which are not explicitly set forth below, will be described in detail in the corresponding section.

The following experimental results are to prepare a coating composition and a separator with different composition contents according to an embodiment of the present invention, and to compare and analyze each property of each other.

1. Coating composition

Production Example 1 and Comparative Production Example 1

A casein aqueous solution was added to the reactor and stirred. Ammonium (NH ₄ ⁺ ) was added so that the pH of the reactor was 8, and the casein was dissociated. Dropping the acrylic acid (Acrylic acid, AA) monomer saponification with NH ₄ ⁺ in the first feeder and the top of the reactor, the reaction was carried out by dropwise addition of peroxide, ammonium persulfate with 2 feeders. After the reaction was completed, a coating composition was obtained.

FIG. 1 shows a modified form of the PAA-casein complex in which casein dissociated to a part of the surface of the polyacrylic acid particles included in the coating composition, and FIG. 2 shows a method of manufacturing the coating composition.

Comparative Production Example 1-7

According to Patent Documents 1 and 2, natural polymer compounds and particulate polymers were mixed. The coating composition was prepared by cold-blending the 10% by weight aqueous casein solution and the 20% by weight polyacrylic acid solution in a weight ratio of 1:9, respectively, but excessive sediment was generated, and thus it could not be used for coating the separator.

Referring to FIG. 3 immediately after mixing of the composition, when simple mixing of casein and polyacrylic acid, dispersibility was poor to form a lump, and the lump prevented stirring of the reactor impeller. 4 is 2 days after mixing the composition of Comparative Preparation Example 1-7, referring to (A) and (B) of FIG. 4, only casein is separately aggregated to separate and the composition has poor storage stability. Can be confirmed.

The composition and properties of the coating composition prepared according to Preparation Example 1 and Comparative Preparation Example 1 are shown in Tables 1 and 2 below.

Referring to Tables 1 and 2, it can be seen that the viscosity of the coating composition increases as casein and solids content increase.

(1) pH

FIG. 5 compares the coating compositions of Preparation Examples 1-5 and Comparative Preparation Examples 1-2 and 1-3 with different pH, and FIG. 6 shows the solubility curve of casein according to pH.

5 and 6, the coating composition of Preparation Example 1-5 has a milky white formulation by forming particles, but the coating composition of Comparative Preparation Examples 1-2 and 1-3 is due to an increase in casein solubility according to pH. A solution was formed and desired properties could not be obtained.

(2) Casein content

7 is a state of the coating composition of Preparation Example 1 having a casein content of 1 to 10% by weight, it can be confirmed visually that the opacity of the composition also increases according to the content of casein. The coating composition of Comparative Production Example 1-4 having a casein content of 15% could not be used for coating a separator due to gelation.

(3) Solid content

8 is a comparison of the coating composition of Comparative Preparation Examples 1-5 and 1-6 having a solid content of 15% by weight with the coating compositions of Preparation Examples 1-3 and 1-5. The coating composition of Comparative Preparation Example 1-5 had poor storage stability, resulting in a layer separation phenomenon, and thus, the viscosity was measured low. In the coating composition of Comparative Production Example 1-6, unreacted grit formed 333 mg per 100 g, resulting in poor reactivity.

2. Separator

Examples 1-1 to 1-5 and Comparative Examples 1-1 to 1-2

Polyethylene having a thickness of 14 μm and an air permeability of 170 sec/100 mL with an aqueous coating slurry comprising 30 parts by weight of solids and 70 parts by weight of water consisting of 2% by weight of the coating composition, 3% by weight of carboxymethyl cellulose (CMC), and 95% by weight of alumina. (Polyethylene) A porous membrane was coated with a thickness of 4 μm on a cross-section basis using a bar coating method to prepare a separator.

The properties of the aqueous coating slurry used in Example 1 and Comparative Example 1 are shown in Table 3 below. The coating composition of Comparative Preparation Example 1-5 was used after sufficiently stirring before removing the layer separation phenomenon.

Experimental Example 1

Further, in order to confirm the particle size of the formed particles, an aqueous coating slurry prepared from the coating compositions of Preparation Examples 1-5, Comparative Preparation Examples 1-5, and 1-6 was photographed with a scanning electron microscope (SEM). The results are shown in FIG. 9.

In Table 3, D mean, D10, D50, and D90 are values measured by the particle size analyzer for the distribution of particles in the slurry. D mean is an average value of particle size, and Dx is a particle size value corresponding to x%.

Referring to Table 3, regardless of the casein content, D10 and D50 are constant, but in the case of D90, it can be seen that Comparative Example 1-1 without casein is about 10% smaller than other slurries. In addition, Comparative Example 1-1 has a pH of about 0.6 and a viscosity of about 20 cps lower than that of other slurries.

Referring to (a) of FIG. 9, it can be confirmed that in Preparation Examples 1-5, casein on the surface of the PAA-casein complex promoted the binding between the complexes to form particles of 8.67 to 34.9 μm . 9(b) and (c) of the particles of Comparative Preparation Examples 1-5 and 1-6 enlarged by 50,000 to 100,000 times, Comparative Example 1-5 having a casein content of 15% is PAA- The particle size of the casein complex is 248 to 280 nm and the casein content of 20% is Comparative Production Example 1-6, the particle size of the PAA-casein complex is 376 to 382 nm, and the higher the casein content, the more the particle size of the PAA-casein complex also increases. Can be confirmed.

From the above results, it can be confirmed that casein in the slurry increases the size by forming a complex with the polyacrylic acid particles and induces bonding between the complexes to increase the viscosity and pH.

In addition, in order to confirm the stability of the slurry, the sedimentation rate of the slurry according to the casein content is shown in Table 3, and the dispersion stability index (TSI; Turbiscan Stability Index) is measured and shown in FIG. 12.

Referring to Table 3, as the content of casein increased, the sedimentation rate of the slurry tended to decrease. The higher the casein content, the better the dispersion and the smaller the particles, or the shape of the particles may be irregular.

Referring to Figure 12, it was confirmed that the stability of the slurry is improved as the casein content increases.

The physical properties of the separators prepared according to Example 1 and Comparative Example 1 were measured and are shown in Table 4 below.

Additionally, the slurry coating layer of the separator prepared according to Examples 1-5 was photographed by SEM, and the results are shown in FIG. 10.

When the thermal contraction rate of the separator is high, the possibility of a short circuit is increased due to deformation of the separator during heat generation of the battery. Lithium contained in the lithium secondary battery may explode upon contact with moisture, and thus, if the moisture content of the separator is high, stability of the battery is impaired.

Referring to Table 4, when the coating composition of the present invention is used, it can be seen that 150°C heat shrinkage and water content are significantly improved compared to the separation membrane of Comparative Example 1-1 that does not contain casein, thereby improving stability. In addition, it can be seen that as the content of casein increased, the tensile strength of MD (Mechanical Direction) and TD (Transverse Direction) improved.

Referring to FIG. 10, when the slurry containing the coating composition of the present invention was coated on a porous fabric, the coating layer was well formed in a form in which air permeability could be secured.

Samples having a length of 10 mm were taken from the separation membranes prepared according to Examples 1-5 and Comparative Examples 1-1 and 1-2, followed by thermomechanical analysis to shut down and melt down temperature and 105°C, 120°C and 130 The heat shrinkage at ℃ was measured and the results are shown in Table 5 and FIG. 11 below. It was measured while heating at a rate of 10°C/min in a temperature range of 20 to 200°C.

Referring to Table 5 and FIG. 11, the heat shrinkage of all three separators was improved compared to the porous fabric, and among them, the thermal shrinkage of 105°C and 120°C was the best in the separator of Example 1-5. The shutdown temperature and the meltdown temperature were about 4° C. lower than that of the porous fabric.

3. Introduction of additional functional layers

Preparation Example 2-1

100 parts by weight of high density polyethylene having a weight average molecular weight (M _w ) of 380,000; 90 parts by weight of paraffin oil having a kinematic viscosity of 70 cSt (based on 40° C.), and 60 parts by weight of solid paraffin wax; And 5 parts by weight of a phosphite ester antioxidant; after kneading in a twin screw extruder, after forming a base sheet having a thickness of 2,100 μm in a twin screw extruder (200°C, 400 rpm) with a T-die of 350 mm width. A cooling roll having a surface temperature of 40°C was passed between the nip roll. The particle size of the cooling roll and the nip roll is in a ratio of 1.5: 1.0, and the nip roll has an inverse gradient of 1,900,000 mm in radius.

After performing the sequential stretching to stretch the porous sheet 10 times in the traveling direction (MD) at a temperature of 110 to 130° C., and to stretch 10 times in the vertical direction (TD) of the traveling direction, both the MD and TD directions are fixed to methylene. The paraffin contained in the sheet was extracted and removed by immersion in chloride. Subsequently, it was heat-set in a hot air oven at 130°C for 4 minutes to prepare a porous fabric.

Preparation Example 2-2

100 parts by weight of high density polyethylene having a weight average molecular weight (M _w ) of 380,000; 10 parts by weight of polypropylene having a weight average molecular weight (M _w ) of 350,000; 90 parts by weight of paraffin oil having a kinematic viscosity of 70 cSt (based on 40° C.), and 110 parts by weight of solid paraffin wax having a weight average molecular weight (M _w ) of 3,800 at 90° C. for 1 hour; And 7 parts by weight of a phosphite ester antioxidant; after kneading in a twin screw extruder, after forming a base sheet having a thickness of 2,100 μm in a twin screw extruder (200°C, 400 rpm) with a T-die having a width of 350 mm. A cooling roll having a surface temperature of 40°C was passed between the nip roll. The particle size of the cooling roll and the nip roll is in a ratio of 1.5: 1.0, and the nip roll has an inverse gradient of 1,900,000 mm in radius.

After performing the sequential stretching to stretch the porous sheet 10 times in the traveling direction (MD) at a temperature of 110 to 130°C and 9 times in the vertical direction (TD) of the traveling direction, both the MD and TD directions are fixed to methylene The paraffin contained in the sheet was extracted and removed by immersion in chloride. Subsequently, it was heat-set in a hot air oven at 130°C for 4 minutes to prepare a porous fabric.

Example 2-1

After preparing a water-based coating slurry by mixing 30 parts by weight of solids composed of 2% by weight of the coating composition of Preparation Example 1-5, 3% by weight of carboxymethyl cellulose (CMC) and 95% by weight of alumina and 70 parts by weight of water by a ball mill method , After coating the gravure (Gravure) on the porous fabric of Preparation Example 2-1 in a hot air oven at 50 ~ 80 ℃ for 2-3 minutes to prepare a separator having a coating layer having a thickness of 3 μm on the cross-section.

Example 2-2

30 parts by weight of solids composed of 2% by weight of the coating composition of Preparation Example 1-5, 3% by weight of carboxymethyl cellulose and 95% by weight of polyalkylsilsesquioxane having an average particle size of 190 nm, 0.1 parts by weight of polyamineamide carboxylate, and After mixing 69.9 parts by weight of water with a ball mill method to prepare a water-based coating slurry, gravure coating on both sides of the porous fabric of Preparation Example 2-2, followed by drying in a hot air oven at 50 to 80° C. for 2-3 minutes to coat the coating layer. Eggplant separator was prepared.

Example 2-3

2 parts by weight of the coating composition of Preparation Example 1-5, 30 parts by weight of solids composed of 3% by weight of ethyl cellulose and 95% by weight of hydrophilic silica, 0.1 parts by weight of polyamineamide carboxylate and 69.9 parts by weight of water were mixed by a ball mill method After preparing the coating slurry, gravure was coated on both sides of the porous fabric of Preparation Example 2-2, and then dried in a hot air oven at 50 to 80°C for 2-3 minutes to prepare a separator having a coating layer.

Example 2-4

10 parts by weight of ethyl acrylate, 15 parts by weight of methyl methacrylate, 20 parts by weight of n-methyl methacrylate, 1 part by weight of acrylic acid and 5 parts by weight of lauryl methacrylate, 2 parts by weight of monoester sulfosuccinate, sodium dialkyl A reaction mixture was prepared by mixing 2 parts by weight of sulfosuccinate (70%) and 25 parts by weight of ion-exchanged water.

After heating 19.5 parts by weight of ion-exchanged water to 78°C under a nitrogen atmosphere, 0.1 parts by weight of 3% potassium persulfate was added. The reaction mixture and 0.2% by weight of 3% potassium persulfate were added dropwise over 4 hours to polymerize. After maintaining at 80°C for 30 minutes, and adjusting to 60°C for 30 minutes, 0.1 parts by weight of t-butyl hydroperoxide and 0.1 parts by weight of Longgarite-C were added to react for 30 minutes, neutralized with ammonia water, and the glass transition temperature (T _g ) To obtain a polymer aqueous dispersion of about 20°C.

After preparing a water-based coating slurry by mixing 30 parts by weight of solids composed of 2% by weight of the coating composition of Preparation Example 1-5, 3% by weight of carboxymethyl cellulose (CMC) and 95% by weight of alumina and 70 parts by weight of water by a ball mill method , After gravure coating on one side of the porous fabric of Preparation Example 2-1, dried in a hot air oven at 50 to 80° C. for 2 to 3 minutes to form a first coating layer having a thickness of 3 μm on a cross-section basis.

On the other side of the fabric, 30 parts by weight of solids composed of 3% by weight of carboxymethyl cellulose, 12% by weight of the polymer aqueous dispersion, and 85% by weight of alumina and 70 parts by weight of water were mixed to prepare a heat seal coating slurry, followed by gravure coating, followed by 50 A separator was prepared by drying in a hot air oven at ˜80° C. for 2-3 minutes to form a second coating layer having a thickness of 3 μm on a cross-section basis.

The separator was laminated on a graphite-based negative electrode, and then laminated by passing through a heating roll press at about 70° C. to measure adhesive strength. The second coating layer had excellent adhesion properties because the adhesion to the cathode was measured at 25 gf/cm.

Example 2-5

10 parts by weight of ethyl acrylate, 70 parts by weight of methyl methacrylate, 10 parts by weight of n-methyl methacrylate, 2 parts by weight of acrylic acid and 2 parts by weight of hydroxyethyl methacrylate, 2 parts by weight of sodium lauryl ether sulfate, 70% A reaction mixture was prepared by mixing 1 part by weight of sodium dialkyl sulfosuccinate, 3 parts by weight of disodium alkylethoxysulfosuccinate and 50 parts by weight of ion-exchanged water.

After heating 49 parts by weight of ion-exchanged water to 80°C under a nitrogen atmosphere, 0.1% by weight of 3% potassium persulfate was added. The reaction mixture and 0.35 parts by weight of 3% potassium persulfate were added dropwise over 4 hours to polymerize. After maintaining at 80°C for 30 minutes, adjusting to 60°C for 30 minutes, reacting for 30 minutes by adding 0.1 parts by weight of t-butyl hydroperoxide and 0.12 parts by weight of Long Gart-C, neutralizing with ammonia water, and then transferring the glass transition temperature (T _g ) To obtain an aqueous dispersion of a polymer having a temperature of 72°C.

After mixing 2 parts by weight of the coating composition of Preparation Example 1-5, and 40 parts by weight of solids composed of 6% by weight of the aqueous dispersion of the polymer and 60 parts by weight of water with a bead mill method to prepare an aqueous coating slurry, the Preparation Example 2- After coating gravure on both sides of the porous fabric of 1, dried in a hot air oven at 50 to 80°C for 2-3 minutes, a coating layer having a thickness of 4 μm based on a cross section was formed to prepare a separator.

As a result of measuring a shutdown temperature by thermomechanical analysis after collecting a specimen of 10 mm in length from the separator, it was confirmed that a portion of the coating layer was dissolved at 122 to 123° C. to have a shutdown characteristic to close pores. It seems that some of the amorphous regions of the polymer can be dissolved to close the pores.

Comparative Example 2-1

After mixing the acrylic-acrylonitrile copolymer emulsion latex 2% by weight, 3% by weight of carboxymethylcellulose (CMC) and 95% by weight of alumina by mixing 30 parts by weight of solid content and 70 parts by weight of water by a ball mill method to prepare a water-based coating slurry, After the gravure coating on the porous fabric of Preparation Example 2-1 and dried in a hot air oven at 50 to 80°C for 2-3 minutes, a separator having a coating layer having a thickness of 2.5 μm on a cross-section basis was prepared.

Comparative Example 2-2

After mixing the acrylic-acrylonitrile copolymer emulsion latex 2% by weight, 3% by weight of carboxymethylcellulose (CMC) and 95% by weight of alumina by mixing 30 parts by weight of solid content and 70 parts by weight of water by a ball mill method to prepare a water-based coating slurry, After the gravure coating on the porous fabric of Preparation Example 2-1, drying was performed in a hot air oven at 50 to 80° C. for 2 to 3 minutes to form a first coating layer.

After coating 2 parts by weight of wax and 98 parts by weight of water on the coating layer to form a second coating layer and drying for 1 hour in a hot air oven at 80° C., the thickness of the first coating layer and the second coating layer is 4 μm based on the cross-section. Adjusted as much as possible.

The physical properties of the separators prepared in Example 2 and Comparative Example 2 were measured and are shown in Table 6 below.

Referring to Table 6, using the coating composition of Preparation Example 1, a heat-resistant layer is coated on one or both sides, a heat-resistant layer is coated on one side, and an adhesive layer or a shutdown layer is coated on the other side. , Even though an aqueous slurry was used, it had a low water content and was suitable for a separator for secondary batteries.

Test methods for each of the physical properties measured in the present invention are as follows. The temperature was measured at room temperature (25° C.) unless otherwise stated.

-Thickness ( μm ): The thickness of the membrane specimen was measured using a fine thickness meter.

-Aeration (Gurley, sec/100mL): Using a Gurometer (Densometer) EGO2-5 model of Asahi Seiko Co., Ltd., the time at which 100 mL of air was passed through a membrane specimen having a particle size of 29.8 mm at a measurement pressure of 0.025 MPa was measured.

-Coating amount (g/m ² ): The coating amount was calculated by dividing the weight of the coated slurry by the area of the porous fabric.

-Tensile strength (kgf/cm ² ): A sample of a membrane having a size of 20 x 200 mm in the MD and TD directions was taken, and the force applied until the sample was broken by an Instron tensile strength tester was measured.

-150℃ Heat Shrinkage(%): In a 150℃ oven for 1 hour, a separator specimen with a size of 200×200mm is placed between A4 paper and left to cool, then cooled to room temperature to vertically (Mechanical direction, MD) and transverse direction of the specimen. The contracted length of the transverse direction (TD) was measured, and the heat shrinkage ratio was calculated using the following equation. Table 4 above means that the value of the heat shrinkage of the membrane at 150° C. is less than or equal to the stated value.

(In the above formula, l ₁ is the length in the longitudinal or transverse direction of the specimen before shrinking, and l ₂ is the length in the longitudinal or transverse direction of the specimen after shrinking.

-120℃ Heat Shrinkage(%): After leaving the separator specimen of 200×200mm in size in an oven at 120℃ for 1 hour, put it in between A4 paper and cool it at room temperature to shrink the length of the specimen in the longitudinal and transverse directions. It was measured and the heat shrinkage ratio was calculated using the above formula. Table 6 above means that the value of the thermal contraction rate of 120°C of the separation membrane is equal to or less than the stated value.

-Water content (ppm): 2 g of the sample was taken from the separation membrane, and the water content was measured according to the Karl Fischer method.

The above description of the present invention is for illustration only, and a person having ordinary knowledge in the technical field to which the present invention pertains can understand that it can be easily modified into other specific forms without changing the technical spirit or essential features of the present invention. will be. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive. For example, each component described as a single type may be implemented in a distributed manner, and similarly, components described as distributed may be implemented in a combined form.

The scope of the present invention is indicated by the following claims, and all modifications or variations derived from the meaning and scope of the claims and their equivalent concepts should be construed as being included in the scope of the present invention.

Claims (10)

Polyolefin-based porous substrates; And
A porous separator for a secondary battery, comprising a heat-resistant coating layer applied to at least one surface of the porous substrate and including a composite and a filler in which a natural polymer compound dissociated to at least a portion of the surface of acrylic polymer particles is bonded. According to claim 1,
The natural polymer compound is at least one selected from the group consisting of plant-based polymer compounds, animal polymer compounds and microbial polymer compounds, porous separator for secondary batteries. According to claim 1,
The filler is Al ₂ O ₃ , AlOOH, Al(OH) ₃ , CaCO ₃ , SiO ₂ , SiC, BaTiO ₃ , TiO ₂ , talc, (meth)acrylic copolymer, (meth)acrylic-styrene copolymer, poly It includes at least one selected from the group consisting of methyl methacrylate, polystyrene and polysilsesquioxane,
The average particle size of the filler is 0.1 ~ 3㎛, porous separator for secondary batteries. According to claim 1,
The thermal shrinkage of the separator in the vertical and horizontal directions at 150°C is 10.0% or less, respectively. According to claim 1,
The moisture content of the heat-resistant coating layer is less than 1,000ppm, porous separator for secondary batteries. According to claim 1,
The heat-resistant coating layer, the adhesive force to the electrode is 10gf / cm or more, porous separator for secondary batteries. According to claim 1,
The heat-resistant coating layer has a shutdown characteristic at 130 ℃ or less, a porous separator for a secondary battery. According to claim 1,
The heat-resistant coating layer is applied to one surface of the porous substrate,
A porous separator for a secondary battery, which is applied to the other surface of the porous substrate so as to face the heat resistant coating layer, and includes an adhesive coating layer having an adhesive strength to an electrode of 10 gf/cm or more. According to claim 1,
The heat-resistant coating layer is applied to one surface of the porous substrate,
A porous separator for a secondary battery, which is applied to the other surface of the porous substrate so as to face the heat-resistant coating layer, and further includes a shutdown coating layer having a shutdown characteristic at 130° C. or less. The method of claim 9,
The shutdown coating layer includes a polymer having a glass transition temperature (T _g ) of 50 to 100°C, a porous separator for secondary batteries.

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