KR20220128244A - Polyolefin Microporous Membrane - Google Patents

Abstract

The present invention relates to a polyolefin microporous membrane, and more specifically, to a polyolefin microporous membrane having excellent heat resistance. The polyolefin microporous membrane of the present invention has an average pore size of 30 to 65 nm on at least one surface of both surfaces. A coating layer is formed on one side or both sides of the polyolefin microporous membrane.

Description

Polyolefin Microporous Membrane

The present invention relates to a polyolefin microporous membrane, and more particularly, to a water-based coating polyolefin microporous membrane having excellent heat resistance.

Polyolefin-based microporous membranes used as separators for lithium ion secondary batteries are closely related to battery life, output, and safety. Therefore, suitable permeability, mechanical properties, heat resistance, etc. are required for the polyolefin microporous membrane.

Recently, in order to improve the safety of a lithium ion secondary battery, a technique for improving the heat resistance of a microporous membrane through organic or organic/inorganic coating on the surface of a substrate has been applied. In particular, as the demand for water-based coatings in consideration of the environment and cost increases, studies on the composition of binders and water-based coating layers are being actively conducted, but studies on the substrate surface suitable for water-based coatings are incomplete.

An object of the present invention is to provide a polyolefin microporous membrane for battery separators having excellent heat resistance.

The present invention provides a polyolefin microporous membrane having an average pore size of 65 nm or less on at least one of both surfaces.

In addition, the present invention provides a coated porous film in which a water-based coating layer is formed on the above-described polyolefin microporous film.

The polyolefin microporous membrane for battery separator according to the present invention exhibits excellent heat resistance by adjusting the average pore size of the surface of the surface in contact with the coating layer to 65 mm or less. As a result, it is possible to improve the safety of a lithium ion secondary battery including a coating separator for a battery manufactured using the polyolefin microporous membrane according to the present invention. In addition, since the polyolefin microporous film of the present invention exhibits excellent heat resistance properties, it is possible to thin the coating layer, and as a result, a high-capacity secondary battery having high output characteristics can be manufactured, and also the secondary battery manufacturing cost can be reduced. have.

Hereinafter, the present invention will be described in detail. However, it is not limited only by the following content, and each component may be variously modified or selectively mixed as needed. Accordingly, it should be understood to include all modifications, equivalents and substitutes included in the spirit and scope of the present invention.

<Polyolefin microporous membrane>

polyolefin microporous membrane

The polyolefin microporous membrane of the present invention is characterized in that the average pore size of at least one of both surfaces is 65 nm or less. For example, the average pore size of the surface of the polyolefin microporous membrane may be 30 to 65 nm, for example, 40 to 65 nm. When the average pore size of the surface of the polyolefin microporous membrane satisfies the above-mentioned range, the adhesive strength of the contact surface between the polyolefin microporous membrane and the coating layer increases during coating, so that the peel strength of the coated microporous membrane is improved and heat shrinkage is improved. . On the other hand, when the average pore size of the surface of the polyolefin microporous membrane exceeds the above-mentioned range, the adhesive strength is weak compared to the case where the above-mentioned range is satisfied, and the heat shrinkage reduction rate of the coated microporous membrane compared to the substrate decreases. can occur In addition, when the average pore size of the surface of the polyolefin microporous membrane is less than the above-mentioned range, the amount of electrolyte impregnation may decrease and ionic resistance may increase, and as a result, the output characteristics and lifespan characteristics of the battery may be reduced.

Among the pores on the surface of the polyolefin microporous membrane of the present invention, the ratio (number ratio) of pores having a pore size of 100 nm or more may be 15% or less, for example, 3 to 15%, and in another example 5 to 13.5%. When the ratio (number ratio) of pores having a pore size of 100 nm or more satisfies the above-mentioned range, the adhesive strength of the contact surface between the polyolefin microporous membrane and the coating layer during coating further increases, so that the peel strength of the coated microporous membrane is further increased. improved and heat shrinkage is further improved, enabling thinning of the coating layer. When the ratio (number ratio) of pores having a pore size of 100 nm or more exceeds the above-mentioned range, the adhesive strength is weak compared to the case where the above-mentioned range is satisfied, and the reduction rate of heat shrinkage of the coated microporous membrane compared to the substrate is decreased. Problems can arise.

The thickness of the polyolefin microporous film is not particularly limited, and may be, for example, 2 to 50 um, and in another example 3 to 30 um.

As the polyolefin resin constituting the polyolefin microporous membrane of the present invention, conventional polyolefin resins known in the art may be used without limitation. For example, it may include a polyethylene resin, a polypropylene resin, or a mixture thereof.

The content of the polyethylene resin may be, for example, 50 mass% or more, for example, 60 mass% or more, based on the total mass of the polyolefin resin. The polyethylene resin may have a viscosity average molecular weight (Mv) of, for example, 2.0×10 ⁵ to 4.0×10 ⁶ g/mol, and in another example, 2.5×10 ⁵ to 3.5×10 ⁶ g/mol. As the polyethylene, high-density polyethylene (HDPE), medium-density polyethylene (MDPE), or low-density polyethylene (LDPE) may be used without limitation. Alternatively, two or more types of polyethylene having different viscosity average molecular weights (Mv) or densities may be mixed.

The content of the polypropylene resin may be, for example, 50 mass% or less, for example, 40 mass% or less, based on the total mass of the polyolefin resin. The polypropylene resin may have a viscosity average molecular weight (Mv) of, for example, 7.0×10 ⁴ to 3.2×10 ⁶ g/mol, and in another example, 9.5×10 ⁴ to 3.0×10 ⁶ g/mol. As the polypropylene, atactic polypropylene (Atactic PP), syndiotactic polypropylene (Syndiotactic PP), isotactic polypropylene (isotactic PP), etc. may be used without limitation. Alternatively, two or more polypropylenes having different viscosity average molecular weights (Mv) or densities may be mixed.

In order to improve the properties as a battery separator, the polyolefin resin may further include a polyolefin that imparts a shutdown function. Examples of polyolefins that impart a shutdown function include low-density polyethylene (LDPE) or polyethylene wax. As the low-density polyethylene (LDPE), at least one selected from the group consisting of branched LDPE, linear LDPE, and ethylene/α-olefin copolymer prepared by a single site catalyst may be used, and the amount of added sugar It may be appropriately adjusted within a range known in the art.

If necessary, conventional additives known in the art, for example, antioxidants, pore formers, etc. may be included in a range that does not impair the effects of the present invention.

The polyolefin microporous membrane according to the present invention may have a multilayer structure in which several layers of the microporous membrane are stacked.

Manufacturing method of polyolefin microporous membrane

The method for producing the polyolefin microporous membrane of the present invention is not particularly limited, and for example, (1) melt-kneading polyolefin and a plasticizer to prepare a polyolefin solution, (2) extruding the polyolefin solution and cooling it to form a sheet (3) a first stretching step of stretching the sheet-like material to form a film, (4) removing the plasticizer from the film, (5) drying the film from which the plasticizer is removed, ( 6) a second stretching step of re-stretching the dried film and (7) a heat treatment step. Hereinafter, each manufacturing process will be described in detail.

(1) Preparation step of polyolefin solution

A polyolefin solution is prepared by melt-kneading a polyolefin resin and a plasticizer. As a method of melt-kneading a composition comprising a polyolefin-based resin and a plasticizer, a conventional method known in the art may be used without limitation. For example, a method of melt-kneading a polyolefin-based resin and a plasticizer at a temperature of 100 to 250° C. and using a twin-screw extruder may be used.

The content of the polyolefin-based resin is not particularly limited, and may be included, for example, in an amount of 20 to 50% by weight based on the total weight of the composition including the polyolefin-based resin and a plasticizer, and in another example, it may be included in an amount of 20 to 40% by weight.

In the present invention, as the plasticizer, conventional components known in the art may be used without limitation, and for example, any organic compound forming a single phase with the polyolefin-based resin at an extrusion temperature may be used.

Non-limiting examples of plasticizers that can be used include liquid paraffin (or paraffin oil) such as nonan, decane, decalin, liquid paraffin (LP), aliphatic or cyclic paraffin wax such as hydrocarbon; phthalic acid esters such as dibutyl phthalate and dioctyl phthalate; fatty acids having 10 to 20 carbon atoms, such as palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid; and fatty acid alcohols having 10 to 20 carbon atoms, such as palmitic alcohol, stearic alcohol, and oleic alcohol. The above plasticizers may be used alone or in combination of two or more.

Among the plasticizers, liquid paraffin is harmless to the human body and has a high boiling point and low volatile components, so it is suitable for use as a plasticizer in a wet method.

The content of the plasticizer is not particularly limited, and may be included, for example, in an amount of 50 to 90% by weight, for example, in an amount of 60 to 80% by weight, based on the total weight of the composition including the polyolefin-based resin and the plasticizer. .

In addition to the above-described polyolefin-based resin, other resins, inorganic particles, additives, and the like conventional in the art may be included.

(2) Formation process of sheet-like article

The polyolefin solution prepared in step (1) is extruded in a die having an extruder and cooled to form a sheet-like article.

When the polyolefin solution is extruded from a die and cooled to form a gel-like sheet, for example, the surface pores can be controlled by controlling the cooling rate. In one example, the cooling may be performed at a rate of 40 °C/min or more, for example 100 °C/min or more, 100 to 650 °C/min in another example, and 300 to 650 °C/min in another example. When the cooling rate is less than the above range, the phase separation between the resin and the plasticizer and the phase separation between the polyethylene and the polypropylene in the resin proceed excessively, and the initial micropore formation may become non-uniform. It may be difficult to secure one pore size.

The cooling method is not particularly limited, and methods known in the art, for example, cooling in contact with a chill roll, cooling with cold air, or cooling by immersion in cold water may be used.

(3) 1st extending process

The sheet-like material obtained in step (2) is biaxially stretched one or more times to form a film. The size of the surface and internal pores of the porous membrane is primarily controlled through stretching and heating in the first stretching process.

In the present invention, the first stretching step may be performed at a predetermined magnification by a conventional method in the art, for example, a tenter method, a roll method, an inflation method, a rolling method, or a combination of these methods. In the case of biaxial stretching, either biaxial stretching may be performed simultaneously or sequential stretching may be performed.

In the first stretching step, the draw ratio is different depending on the thickness of the sheet-like material, but for example, in biaxial stretching, it is appropriate to perform at least 2 times x 2 times or more in any direction, for example, in the range of 3 to 10 times. can do.

In the first stretching process, the structure and pore size of the fibril may be adjusted by controlling the stretching temperature and the air volume.

In the first stretching process of the present invention, the stretching temperature may be, for example, in the range of 100 to 130 ℃, as another example, it may be in the range of 105 to 125 ℃. When stretching within the above range, the stretching may be performed to have adequate air permeability and mechanical strength without blocking pores (pores) in the sheet. When the first stretching temperature exceeds the above-mentioned temperature range, the resin in the sheet is melted so that the molecular chains are not oriented by stretching and the pore size may become non-uniform. On the other hand, when the first stretching temperature is less than the above-mentioned temperature range, softening of the sheet may become insufficient, which may cause breakage by stretching, and stretching of a high magnification may not be achieved.

(4) plasticizer removal process

Using a washing solvent, a plasticizer is extracted from the stretched film. Since the polyolefin phase is phase-separated from the plasticizer, a porous membrane in which a plurality of pore structures are formed is obtained when the plasticizer is removed. As a method for removing the plasticizer, a conventional method known in the art may be used without limitation.

In the present invention, the washing solvent is not particularly limited as long as it is an organic solvent capable of extracting the plasticizer and can be used. For example, halogenated hydrocarbons such as methylene chloride, 1,1,1-trichloroethane, and fluorocarbons with high extraction efficiency and easy drying; hydrocarbons such as n-hexane and cyclohexane; alcohols such as ethanol and isopropanol; Ketones, such as acetone and 2-butanone, etc. can be used. For example, when liquid paraffin is used as a plasticizer, methylene chloride can be used as an organic solvent.

Since most organic solvents used in the process of extracting the plasticizer are highly volatile and toxic, water may be used to suppress volatilization of the organic solvent if necessary.

(5) drying process

The polyolefin microporous membrane obtained by removing the plasticizer may be dried using a conventional drying method known in the art. As an example, a heat-drying method, an air-drying method, etc. may be used.

(6) 2nd drawing process

Then, the dried film is re-stretched again at least in the uniaxial direction. The size of the surface pores of the porous membrane is secondarily adjusted through stretching and heating in the second stretching process, thereby preparing a final microporous membrane.

In the present invention, the second stretching process may be performed by a tenter method or the like in the same manner as the first stretching process while heating the film. The stretching may be uniaxial stretching or biaxial stretching.

The temperature of the second stretching process may be, for example, the crystal dispersion temperature of the polyolefin resin constituting the microporous film or more, the crystal dispersion temperature + 40 ° C. or less, and as another example, the crystal dispersion temperature + 10 ° C. or more, the crystal The dispersion temperature may be in the range of up to +40 °C. The crystal dispersion temperature refers to a value obtained by measuring the temperature characteristics of dynamic viscoelasticity based on ASTM D4065. When the polyolefin resin is polyethylene, the crystal dispersion temperature thereof is generally 90 to 100°C.

By adjusting the second stretching temperature to the above-mentioned range, it is possible to prevent a decrease in air permeability and a deviation in physical properties in the width direction of the sheet when stretching in the transverse direction (width direction: TD direction). By setting the second stretching temperature within the above range, it is possible to suppress the occurrence of variations in air permeation resistance in the stretched sheet width direction and to control the pore size of the membrane surface within the scope of the present invention. At a temperature exceeding the above range, it may become difficult to control the pore size within the range of the present invention due to excessive melting of the polyolefin resin, and at a temperature below the above range, the film may be damaged by stretching due to insufficient softening of the film. It may be difficult to obtain a uniform surface pore size because easy, uniform stretching cannot be achieved.

In addition, by adjusting the temperature of the second stretching process to the above-mentioned range, the polyolefin resin can be sufficiently softened, and the polyolefin resin can be stretched uniformly by preventing film breakage. As an example, the second stretching temperature may be in the range of 90 to 140 °C, as another example, it may be in the range of 120 to 140 °C.

The magnification in the uniaxial direction of the second stretching may be, for example, 1.0 to 1.8 times, and in another example, 1.2 to 1.6 times. For example, in the case of uniaxial stretching, the length is 1.0 to 1.8 times in the longitudinal direction (machine direction: MD direction) or in the TD direction. In the case of biaxial stretching, it is adjusted by 1.0 to 1.8 times in the MD and TD directions, respectively. In the case of biaxial stretching, the respective draw ratios in the MD direction and the TD direction may be the same or different. By adjusting the draw ratio in the above range, it is possible to prevent deterioration of permeability, electrolyte solution absorption and compression resistance, and excessively thin fibrils and deterioration of heat shrinkage resistance can be prevented.

(7) heat treatment process

Then, the re-stretched film is fixed and heat-treated. As the heat treatment method, a conventional method known in the art may be performed without limitation, and as an example, a heat setting treatment and/or a heat relaxation treatment may be used.

In particular, by performing the heat setting treatment, the crystal structure of the fibrils formed by the second stretching process is stabilized to have the surface pore size of the present invention, and a microporous membrane having a uniform pore size distribution can be manufactured.

In the present invention, the heat setting treatment is performed within a temperature range above the crystal dispersion temperature and below the melting point of the polyolefin resin constituting the microporous film. In this case, the heat setting treatment may be performed by a tenter method, a roll method, or a rolling method.

The thermal relaxation treatment may be performed by a tenter method, a roll method, a compression method, or may be performed using a belt conveyor or a floating roll. The thermal relaxation treatment may, for example, be performed in at least one direction in a range of 20% or less of a relaxation rate, and in another example, may be performed in a range of 10% or less of a relaxation rate.

The polyolefin microporous membrane according to the present invention has a peel strength of 45 gf/15 mm or more of the coating layer, a maximum heat of fusion shrinkage (%) within 10% in the MD and TD directions, and a reduction in shrinkage during coating of 65% or more, for example 75 to 95%.

coating layer

A coating layer may be formed on one or both surfaces of the polyolefin microporous membrane according to the present invention. The coating layer may be formed on a surface having an average pore size of 65 nm or less among both surfaces of the polyolefin microporous membrane.

The coating solution forming the coating layer includes inorganic particles and a polymer binder. The coating solution may further include a solvent if necessary.

Non-limiting examples of the inorganic particles that can be used include alumina, barium sulfate, aluminum hydroxide, barium titanate (Barium Titanium Oxide), magnesium oxide (Magnesium Oxide), magnesium hydroxide (Magnesium Hydroxide) ), clay, titanium oxide, glass powder, and boehmite. The inorganic particles may be used alone or in combination of two or more.

The size of the inorganic particles is not limited, but may be in the range of 0.001 to 10 μm. When the size of the inorganic particles satisfies the above-mentioned range, a film having a uniform thickness may be formed and an appropriate porosity may be secured.

The inorganic particles may be used in an amount of 80 to 99% by weight based on the total weight of the solid content of the aqueous coating composition. When the content of the inorganic particles falls within the above-described range, the heat resistance effect according to the use of the inorganic particles may be achieved.

Non-limiting examples of polymeric binders that can be used include polyacrylic acid, polymethacrylic acid, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, butadiene-acrylic acid copolymer, butadiene-methacrylic acid copolymer, polyvinylsulfo. Nate, chlorosulfonated polyethylene, perfluorosulfonated ionomer, sulfonated polystyrene, styrene-acrylic acid copolymer, sulfonated butyl rubber, polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoro Lopropylene (PVdF-HFP), polyvinylpyrrolidone, polyacrylonitrile, polyvinylidene fluoride-trichloroethylene, polyvinylidene fluoride-chlorotrifluoroethylene (PVdF-TFE), polymethylmethacrylic Late, polyvinyl acetate, ethylene-co-vinyl acetate copolymer, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, acrylonitrile-styrene-butadiene copolymer, polyimide, and the like. The polymer binder may be used alone or in combination of two or more.

The coating composition of the present invention is prepared by mixing an inorganic particle slurry dispersed in a solvent and a binder dissolved in a solvent. The coating composition of the present invention may be a water-based coating composition, in which case water is used as a solvent, and an organic solvent is not used. Accordingly, there is no environmental problem caused by using the organic solvent.

<Lithium-ion secondary battery>

The present invention provides a secondary battery, for example, a lithium ion secondary battery including the polyolefin microporous membrane described above.

The lithium ion secondary battery of the present invention may be manufactured according to a conventional method known in the art, except for using the polyolefin microporous membrane according to the present invention as a separator. For example, it can be manufactured by interposing a separator between the positive electrode and the negative electrode and introducing a non-aqueous electrolyte.

The lithium ion secondary battery of the present invention includes a negative electrode, a positive electrode, a separator, and an electrolyte as battery components. It conforms to the elements of phosphorus lithium ion secondary batteries.

For example, the positive electrode may be manufactured using a typical positive electrode active material for a lithium ion secondary battery known in the art, and a non-limiting example thereof is a lithium transition metal composite such as LiMxOy (M = Co, Ni, Mn, CoaNibMnc). Oxides (for example, lithium manganese composite oxides such as LiMn ₂ O ₄ , lithium nickel oxides such as LiNiO ₂ , lithium cobalt oxides such as LiCoO ₂ , and manganese, nickel, and a part of cobalt of these oxides with other conventional transition metals or vanadium oxide containing lithium or the like) or a chalcogen compound (eg, manganese dioxide, titanium disulfide, molybdenum disulfide, etc.).

As an example, the negative electrode may be manufactured using a conventional negative electrode active material for a lithium ion secondary battery known in the art, and a non-limiting example thereof is a material capable of intercalating/deintercalating lithium ions is used, For example, lithium metal, lithium alloy, coke, artificial graphite, natural graphite, organic high molecular compound combustor, carbon fiber, silicon type, tin type, etc. exist.

The non-aqueous electrolyte includes electrolyte components commonly known in the art, such as an electrolyte salt and an electrolyte solvent.

The electrolyte salt includes (i) a cation selected from the group consisting of Li+, Na+, K+ and (ii) PF ₆ -, BF ₄ -, Cl-, Br-, I-, ClO ₄ -, AsF ₆ -, CH ₃ CO ₂ -, CF ₃ SO ₃ -, N(CF ₃ SO ₂ ) ₂ -, C(CF ₂ SO ₂ ) ₃ - may be composed of a combination of anions selected from the group consisting of, of which lithium salt is preferable. Specific examples of the lithium salt include LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , and LiN(CF ₃ SO ₂ ) ₂ . These electrolyte salts can be used individually or in mixture of 2 or more types.

The electrolyte solvent may be cyclic carbonate, linear carbonate, lactone, ether, ester, acetonitrile, lactam, or ketone.

Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), and the like, and examples of the linear carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), and methyl propyl carbonate (MPC). Examples of the lactone include gamma butyrolactone (GBL), and examples of the ether include dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane and the like. Examples of the ester include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, butyl propionate, methyl pivalate and the like. In addition, the lactam includes N-methyl-2-pyrrolidone (NMP) and the like, and the ketone includes polymethylvinyl ketone. In addition, a halogen derivative of the organic solvent may also be used, but is not limited thereto. In addition, the organic solvent may be glyme, diglyme, triglyme, or tetraglyme. These organic solvents can be used individually or in mixture of 2 or more types.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are provided to help the understanding of the present invention, and the scope of the present invention is not limited to the examples in any sense.

[Example 1]

On the corresponding side of a 12 um thick polyolefin microporous membrane having an average pore size of 57.3 nm on one surface and a ratio (number ratio) of pores having a pore size of 100 nm or more among the pores on the surface of 9%, an aqueous slurry (Al ₂ O ₃ : Dispersant (3M, FC4430): Binder (Toyochem, CSB-130) = 100: 0.3: 3 by weight ratio of 3) was gravure-coated to a thickness of 4 μm, dried with hot air, and the water-based coating porosity of Example 1 The membrane was prepared.

[Example 2-6]

The same method as in Example 1, except that the average surface pore diameter of the microporous membrane, the ratio (number ratio) of pores having a pore size of 100 nm or more, the thickness of the microporous membrane or the thickness of the coating layer were changed according to Table 1 below to prepare a water-based coating porous membrane of each example.

[Comparative Example 1-2]

The same method as in Example 1, except that the average surface pore diameter of the microporous membrane, the ratio (number ratio) of pores having a pore size of 100 nm or more, the thickness of the microporous membrane or the thickness of the coating layer were changed according to Table 1 below to prepare a water-based coating porous membrane of each comparative example.

The pore size of the surface of the microporous membrane was calculated by the following method. The microporous membranes prepared in Examples and Comparative Examples were subjected to ion sputtering (Ion Sputter, Hitachi MC1000) at 15 mA, 180 sec. After pretreatment with the condition, using a scanning electron microscope (SEM, Hitachi SU-70), size 1,280 X 960, magnification 10K, acceleration voltage 5 kv, emission current 31 uA, Brightness 5, Contrast 1, ABC conditions were measured. Thus, an SEM image was obtained.

Image processing software Halcon (ver.13.0 MVtec) was used to remove noise from the 1,280 X 870 part of the SEM image (smoothed by averaging, mean_image, LowPass mask width: 3, height: 3) was subjected to binarization processing. Binarization processing (dyn_threshold, input image: image from which the noise has been removed/threshold image: image processing (mean_image, LowPass mask) width: 21, height: 21 In the threshold image/offset:30/extraction region:dark) obtained in ), pixels in the image region were selected from the input image, and pixels satisfying the threshold condition for binarization were gathered and all connected values were output.

Each of the output values is a region corresponding to each pore in the surface SEM image. Each pore was assumed to be a circle, the pore size (B) was calculated from the area (A) of the pore, and the average pore size (B _ave. ) was calculated from these values.

B _ave. =(B _a +B _b +…+B _n )/n

After obtaining the size of each pore by the above method, the proportion of pores having an average pore size of 100 nm or more among all pores was calculated according to the following formula.

Proportion of pores with average pore size greater than 100 nm (%) = m/n x 100

n: the total number of pores

m: the number of pores having a pore size of 100 nm or more among B _a to B _n

[Experimental Example - Evaluation of Physical Properties]

The microporous membrane of each Example and Comparative Example was tested for physical properties by the following method, and the results are shown in Table 2 below.

peel strength

It was measured at a speed of 300 mm/min using a tensile strength meter (Instron).

Heat shrinkage (%)

TMA (SII Nano Technology Inc., SS6100) was used to measure the TMA maximum thermal contraction rate of the microporous membranes (width 3 mm, length 10 mm) of each Example and Comparative Example (measuring mode: stretching mode, measuring temperature: 30-200° C., temperature increase rate: 5° C./min, load: 2 gf).

As shown in Table 2, it can be seen that the polyolefin microporous membrane of Examples 1-6 having an average surface pore size according to the present invention has a low shrinkage after coating and an excellent shrinkage reduction ratio according to coating. In particular, in the case of Examples 4 and 5, in which the ratio (number ratio) of pores having a pore size of 100 nm or more is low, excellent heat shrinkage reduction effect after coating can be confirmed even though the thickness of the coating layer is made thin. On the other hand, in the case of the microporous membranes of Comparative Examples 1 and 2 having an average pore diameter larger than the average surface pore diameter according to the present invention, it can be confirmed that the reduction rate of heat shrinkage due to the coating is not large.

Claims (10)

A polyolefin microporous membrane having an average pore size of 30 to 65 nm on at least one of both surfaces. The polyolefin microporous membrane according to claim 1, wherein the ratio (number ratio) of pores having a pore size of 100 nm or more among the pores on the surface having an average pore size of 30 to 65 nm is 15% or less. The polyolefin according to claim 1 or 2, wherein a coating layer is formed on one or both surfaces of the polyolefin microporous membrane, and the coating layer is formed on the surface of the polyolefin microporous membrane having an average pore size of 30 to 65 nm. microporous membrane. The polyolefin microporous membrane according to claim 3, wherein the coating layer is an aqueous coating layer. The polyolefin microporous membrane according to claim 3, wherein the coating layer includes inorganic particles and a polymer binder. According to claim 5, wherein the inorganic particles are alumina, barium sulfate, aluminum hydroxide (Aluminum Hydroxide), barium titanate (Barium Titanium Oxide), magnesium oxide (Magnesium Oxide), magnesium hydroxide (Magnesium Hydroxide) A polyolefin microporous film comprising at least one selected from the group consisting of , clay, titanium oxide, glass powder, and boehmite. The polyolefin microporous membrane according to claim 3, wherein the peel strength of the coating layer (measured with a tensile strength meter (speed 300 mm/min) of Instron Co., Ltd.) is 45 gf/15 mm or more. The polyolefin microporous film according to claim 3, wherein the maximum heat of fusion shrinkage (measured by TMA manufactured by SII) is within 10% in the MD direction and the TD direction. The polyolefin microporous membrane according to claim 3, wherein the reduction in shrinkage during coating is 65% or more. A lithium ion secondary battery comprising the polyolefin microporous membrane according to claim 1 or 2.