KR20230136139A - Polyolefin microporous membrane - Google Patents

Abstract

The present invention relates to a polyolefin microporous film, and more specifically, to a polyolefin microporous film having excellent heat resistance.

Description

Polyolefin microporous membrane

The present invention relates to a polyolefin microporous membrane, and more specifically, to a water-based coated polyolefin microporous membrane with excellent heat resistance.

The polyolefin-based microporous membrane used as a separator for lithium-ion secondary batteries is closely related to the lifespan, output, and safety of the battery. Therefore, the polyolefin microporous film is required to have appropriate permeability, mechanical properties, heat resistance, etc.

Recently, in order to improve the safety of lithium-ion secondary batteries, technology has been applied to improve the heat resistance of microporous films through organic or organic/inorganic coatings on the surface of the substrate. In particular, as demand for water-based coatings in consideration of environment and cost increases, research on the composition of binders and water-based coating layers is actively underway, but research on substrate surfaces suitable for water-based coatings is insufficient.

The technical task of the present invention is to provide a polyolefin microporous membrane for battery separator having excellent heat resistance.

The present invention provides a polyolefin microporous film in which the average pore size of at least one of both surfaces is 65 nm or less.

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

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

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

<Polyolefin microporous membrane>

Polyolefin microporous membrane

The polyolefin microporous film 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 film 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, thereby improving the peel strength of the coated microporous membrane and improving heat shrinkage. . On the other hand, when the average pore size of the surface of the polyolefin microporous membrane exceeds the above-mentioned range, problems such as weaker adhesive strength and lower heat shrinkage reduction rate of the coated microporous membrane compared to the substrate compared to the case that satisfies the above-mentioned range. It can happen. In addition, when the average pore size of the surface of the polyolefin microporous film is less than the above-mentioned range, the amount of electrolyte impregnated is reduced, which may increase ionic resistance, and as a result, the output characteristics and life characteristics of the battery may deteriorate.

Among the pores on the surface of the polyolefin microporous film of the present invention, the proportion (number ratio) of pores with 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 with a pore size of 100 nm or more satisfies the above-mentioned range, the adhesive strength of the contact surface of the polyolefin microporous film and the coating layer increases further during coating, thereby further increasing the peeling strength of the coated microporous film. This improves heat shrinkage further, making it possible to thin the coating layer. If the ratio (number ratio) of pores with a pore size of 100 nm or more exceeds the above-mentioned range, the adhesive strength becomes weaker and the heat shrinkage reduction rate of the coated microporous film compared to the substrate becomes smaller compared to the case that satisfies the above-mentioned range, etc. Problems may arise.

The polyolefin microporous film of the present invention may have a surface porosity of 10 to 25%, for example, 15 to 25%. When the surface porosity of the polyolefin microporous membrane satisfies the range, the adhesive strength of the contact surface between the polyolefin microporous membrane and the coating layer increases further during coating, thereby further improving the peel strength of the coated microporous membrane and further improving heat shrinkage. . On the other hand, when the surface porosity of the polyolefin microporous membrane exceeds the above-mentioned range, problems such as weaker adhesive strength and lower heat shrinkage reduction rate of the coated microporous membrane compared to the substrate compared to the case satisfying the above-mentioned range may occur. You can. In addition, when the surface porosity of the polyolefin microporous film is less than the above-mentioned range, the amount of electrolyte impregnated is reduced, which may increase ionic resistance, and as a result, the output characteristics and life characteristics of the battery may deteriorate.

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

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

The content of the polyethylene resin may be, for example, 50% by mass or more, and in other examples, 60% by 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) can be used without limitation. Alternatively, two or more types of polyethylene with different viscosity average molecular weight (Mv) or density may be mixed.

The content of the polypropylene resin may be, for example, 50% by mass or less, and in another example, 40% by 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. can be used without limitation. Alternatively, two or more types of polypropylene with different viscosity average molecular weight (Mv) or density may be mixed.

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

If necessary, conventional additives known in the art, such as antioxidants and pore formers, may be included as long as they do not impair the effect of the present invention.

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

Method for manufacturing polyolefin microporous membrane

The method for producing the polyolefin microporous film of the present invention is not particularly limited, and includes, 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-like product. forming a step, (3) a first stretching step of stretching the sheet-like material to form a film, (4) removing a plasticizer from the film, (5) drying the film from which the plasticizer has been 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 process of polyolefin solution

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

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

In the present invention, the plasticizer may be any conventional component known in the art without limitation, and may be, for example, any organic compound that forms a single phase with the polyolefin resin at the extrusion temperature.

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 such as paraffin wax. 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; There are fatty acid alcohols with 10 to 20 carbon atoms, such as palmitic alcohol, stearic acid alcohol, and oleic acid alcohol. The above plasticizers can 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, making it suitable for use as a plasticizer in a wet method.

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

In addition to the polyolefin resin described above, other resins, inorganic particles, additives, etc. commonly used in the art may be included.

(2) Forming process of sheet shape

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

When a polyolefin solution is extruded from a die and cooled to form a gel-like sheet, surface pores can be controlled, for example, by controlling the cooling rate. For example, it may be cooled at a rate of 40°C/min or more, for example, 100°C/min or more, in another example, 100 to 650°C/min, and in another example, 300 to 650°C/min. If the cooling rate is below the above-mentioned range, phase separation between the resin and the plasticizer and between polyethylene and polypropylene in the resin may proceed excessively, resulting in uneven initial micropore formation, resulting in surface pore size and surface porosity within a certain range. It may become difficult to control the pore size and ensure uniform pore size.

The cooling method is not particularly limited, and methods known in the art, such as cooling by contact with a cooling roll (Chill), cooling by cold air, or cooling by impregnation in cold water, can be used.

(3) First stretching 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 film are primarily controlled through stretching and heating in the first stretching process.

In the present invention, the first stretching process can be performed at a predetermined magnification by a conventional method in the relevant technical field, such as 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 simultaneous biaxial stretching or sequential stretching may be performed.

In the first stretching process, the stretching ratio varies depending on the thickness of the sheet-shaped material, but for example, in biaxial stretching, it is appropriate to carry out at least 2 times x 2 times or more in any direction, for example, in the range of 3 to 10 times. can do.

During the first stretching process, the structure and pore size of the fibrils can be adjusted by adjusting the stretching temperature and 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°C, and in another example, 105 to 125°C. When stretching within the above range, the sheet can be stretched to have appropriate air permeability and mechanical strength without blocking pores in the sheet. If the first stretching temperature exceeds the above-mentioned temperature range, the resin in the sheet may melt, the molecular chains may not be oriented due to stretching, and the pore size may become non-uniform. On the other hand, if the first stretching temperature is below the above-described temperature range, the sheet may be damaged by stretching due to insufficient softening, and high-magnification stretching may not be achieved.

(4) Plasticizer removal process

Using a cleaning solvent, the plasticizer is extracted from the stretched film. Since the polyolefin phase is phase-separated from the plasticizer, when the plasticizer is removed, a porous film with a plurality of pore structures is obtained. As a method of removing the plasticizer, conventional methods known in the relevant technical field can be used without limitation.

In the present invention, any organic solvent that can extract a plasticizer can be used as a cleaning solvent without particular restrictions. For example, halogenated hydrocarbons such as methylene chloride, 1,1,1-trichloroethane, and fluorocarbons, which have high extraction efficiency and are easy to dry; Hydrocarbons such as n-hexane and cyclohexane; Alcohols such as ethanol and isopropanol; Ketones such as acetone and 2-butanone 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 plasticizers are highly volatile and toxic, water can be used to suppress volatilization of the organic solvent, if necessary.

(5) Drying process

The polyolefin microporous film obtained by removing the plasticizer can be dried using a conventional drying method known in the art. For example, heat drying method, air drying method, etc. can be used.

(6) Second stretching process

Subsequently, the dried film is re-stretched in at least one axis direction. The size of the surface pores of the porous film is secondarily adjusted through stretching and heating in the second stretching process to manufacture the final form of the fine porous film.

In the present invention, the second stretching process can be performed using 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, in the range of more than the crystal dispersion temperature of the polyolefin resin constituting the microporous membrane, and less than or equal to the crystal dispersion temperature + 40° C., and as another example, more than the crystal dispersion temperature + 10° C., more than the crystal dispersion temperature + 10° C. It may be in the range of dispersion temperature + 40°C or less. 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, its crystal dispersion temperature 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 the occurrence of deviation in physical properties in the sheet width direction 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 variation in air permeability resistance in the width direction of the stretched sheet and at the same time adjust the pore size of the membrane surface within the range of the present invention. At temperatures exceeding the above-mentioned range, it may become difficult to control the pore size and surface opening ratio within the range of the present invention due to excessive melting of the polyolefin resin, and at temperatures below the above-mentioned range, softening of the film becomes insufficient, resulting in insufficient stretching. It can be easily damaged and cannot achieve uniform elongation, making it difficult to secure a uniform surface pore size.

In addition, by adjusting the temperature of the second stretching process to the above-mentioned range, the polyolefin resin can be sufficiently softened, and film rupture can be prevented and stretched uniformly. In one example, the second stretching temperature may range from 90 to 140°C, and in another example, it may range from 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 stretching is 1.0 to 1.8 times in the longitudinal direction (machine direction: MD direction) or TD direction. In the case of biaxial stretching, it is adjusted to 1.0 to 1.8 times in the MD and TD directions, respectively. In the case of biaxial stretching, each stretching ratio in the MD direction and TD direction may be the same or different. By adjusting the draw ratio to the above-mentioned range, it is possible to prevent a decrease in permeability, electrolyte absorption, and compression resistance, and to prevent excessive thinning of fibrils and a decrease in heat shrinkage resistance.

(7) Heat treatment process

Then, the re-stretched film is fixed and heat treated. The heat treatment method may be any conventional method known in the art without limitation, and for example, heat setting treatment and/or heat relaxation treatment may be used.

In particular, by performing heat setting treatment, the crystal structure of the fibrils formed by the second stretching process is stabilized, making it possible to manufacture a microporous film having the surface pore size and surface porosity of the present invention and a uniform pore size distribution.

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 membrane. At this time, the heat setting treatment can be performed by a tenter method, a roll method, or a rolling method.

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

The polyolefin microporous film according to the present invention has a peeling strength of the coating layer of 45gf/15mm or more, and a maximum melt heat shrinkage (%) within 10% in the MD and TD directions. In addition, the polyolefin microporous film according to the present invention has a low shrinkage rate after coating, and the shrinkage rate reduction before and after coating may be 65% or more, for example, 75 to 95%.

coating layer

The polyolefin microporous film according to the present invention may have a coating layer formed on one or both sides. The coating layer may be formed on both surfaces of the polyolefin microporous film with an average pore size of 65 nm or less.

The coating liquid forming the coating layer includes inorganic particles and a polymer binder. The coating liquid may further include a solvent as needed.

Non-limiting examples of usable inorganic particles include Alumina, Barium Sulfate, Aluminum Hydroxide, Barium Titanium Oxide, Magnesium Oxide, and Magnesium Hydroxide. ), Clay, Titanium Oxide, Glass Powder, Boehmite, etc. The above inorganic particles can be used alone or in a mixture of two or more types.

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

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

Non-limiting examples of polymer 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, and polyvinyl sulfo. nate, chlorosulfonated polyethylene, perfluorosulfonated ionomer, sulfonated polystyrene, styrene-acrylic acid copolymer, sulfonated butyl rubber, polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoride Propylene (PVdF-HFP), polyvinylpyrrolidone, polyacrylonitrile, polyvinylidene fluoride-trichloroethylene, polyvinylidene fluoride-chlorotrifluoroethylene (PVdF-TFE), polymethylmethacryl Latex, polyvinyl acetate, ethylene-co-vinylacetate copolymer, polyethylene oxide, cellulose acetate, cellulose acetate butylate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, Cyanoethyl sucrose, pullulan, carboxymethyl cellulose, acrylonitrile styrene butadiene copolymer, polyimide, etc. The polymer binder can be used alone or in a mixture of two or more types.

The coating composition of the present invention is prepared by mixing an inorganic particle slurry dispersed in a solvent and a binder dissolved in the 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 no organic solvent is used. Therefore, environmental problems caused by using organic solvents do not occur.

<Lithium ion secondary battery>

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

The lithium ion secondary battery of the present invention can be manufactured according to conventional methods known in the art, except that the polyolefin microporous film according to the present invention is used as a separator. For example, it can be manufactured by interposing a separator between the anode and the cathode and adding 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. Here, the components of the positive electrode, negative electrode, electrolyte, and other additives, if necessary, excluding the above-described separator, are conventionally known in the art. Complies with the elements of lithium-ion secondary batteries.

For example, the positive electrode can be manufactured using a typical positive electrode active material for lithium ion secondary batteries known in the art, non-limiting examples of which include lithium transition metal composites such as LiMxOy (M = Co, Ni, Mn, CoaNibMnc). Oxides (for example, lithium manganese complex oxides such as LiMn ₂ O ₄ , lithium nickel oxides such as LiNiO ₂ , lithium cobalt oxides such as LiCoO ₂ , and some of the manganese, nickel, and cobalt of these oxides are mixed with other common transition metals or Substituted with aluminum, etc. or vanadium oxide containing lithium, etc.), LiNPO ₄ with olivine structure (N = Fe, Mn, Co, Ni), or chalcogen compounds (for example, manganese dioxide, titanium disulfide, molybdenum disulfide, etc. ), etc.

For example, the negative electrode can be manufactured using common negative electrode active materials for lithium ion secondary batteries known in the art, non-limiting examples of which include materials that can intercalate/deintercalate lithium ions, For example, there are lithium metal, lithium alloy, coke, artificial graphite, natural graphite, organic polymer compound combustion body, carbon fiber, silicon-based, tin-based, etc.

Non-aqueous electrolytes include electrolyte components commonly known in the art, such as electrolyte salts and electrolyte solvents.

The electrolyte salt contains (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 It may be composed of a combination of anions selected from the group consisting of ₂ -, CF ₃ SO ₃ -, N(CF ₃ SO ₂ ) ₂ -, C(CF ₂ SO ₂ ) ₃ -, of which lithium salt is preferable. Specific examples of lithium salts 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 combination of two 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), and fluoroethylene carbonate (FEC), and examples of the linear carbonate include diethyl carbonate (DEC) and dimethyl carbonate. (DMC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), and methyl propyl carbonate (MPC). Examples of the lactone include gammabutyrolactone (GBL), and examples of the ether include dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, etc. 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, etc. In addition, the lactam includes N-methyl-2-pyrrolidone (NMP), and the ketone includes polymethylvinyl ketone. Additionally, halogen derivatives of the above organic solvents can also be used, but are not limited thereto. In addition, the organic solvent can also be used as glyme, diglyme, triglyme, or tetraglyme. These organic solvents can be used individually or in combination of two or more.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are only intended to aid understanding of the present invention and do not limit the scope of the present invention to the examples in any way.

[Example 1]

A 12 um thick polyolefin microporous film with an average pore size of 57.3 nm on one surface, a ratio (number ratio) of pores with a pore size of 100 nm or more among pores on the surface of 9%, and a surface porosity of 16.8%. On the corresponding surface, a water-based slurry (Al ₂ O ₃ : dispersant (3M, FC4430) : binder (Toyochem, CSB-130) = weight ratio of 100:0.3:3) was gravure coated to a thickness of 4 um, followed by hot air drying. Thus, the water-based coating porous film of Example 1 was prepared.

[Example 2-7]

Examples, except that the average surface pore diameter of the microporous membrane, the ratio (number ratio) of pores with a pore size of 100 nm or more, the surface porosity, the thickness of the microporous membrane, or the thickness of the coating layer were changed according to Table 1 below. The water-based coating porous film of each example was manufactured in the same manner as in 1.

[Comparative Example 1-2]

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

The pore size of the surface of the microporous membrane was calculated by the following method. The microporous films prepared in each Example and Comparative Example were sputtered using an ion sputter (Hitachi MC1000) at 15 mA, 180 sec. After preprocessing, measurements were made using a scanning electron microscope (SEM, Hitachi SU-70) under the following conditions: size 1,280 An SEM image was obtained.

Image using image processing software Halcon (ver.13.0 MVtec) to remove noise from 1,280 A binarization process was performed. Regarding binarization, binarization processing is performed on the image that has undergone the noise removal process above (dyn_threshold, input image: image from which the noise has been removed/threshold image: image processing (mean_image, LowPass mask width: 21, height: 21) ), the pixels in the image area were selected from the input image, and all the pixels that satisfied the threshold conditions for binarization were gathered and the connected values were output.

Each of the output values is an area corresponding to each pore in the surface SEM image. The ratio of the area corresponding to the pores to the total area was expressed as surface porosity (%). In addition, each output value assumes that each pore is a circle in the surface SEM image, calculates the pore size (B) from the area of the pore (A), and calculates the average pore size (B _ave. ) from these values. was calculated.

After calculating the size of each pore using the above method, the proportion of pores with an average pore size of 100 nm or more among all pores was calculated according to the formula below.

Proportion of pores with an average pore size of 100 nm or more (%) = m/n x 100

n: total number of pores

m: Number of pores with a pore size of 100 nm or more among B _a to B _n

[Experimental example - physical property evaluation]

Physical property tests were conducted on the microporous membranes of each Example and Comparative Example using the following method, and the results are shown in Table 2 below.

Peel strength

Attach the coated side of the sample to be measured to a steel plate using double-sided tape (3M), then use a tensile strength meter (Instron) to pull the steel plate and sample to 180° at a speed of 300 mm/min to peel off the coating layer. The intensity was measured.

Heat shrinkage rate (%)

Using TMA (SII Nano Technology Inc., SS6100), the maximum TMA thermal contraction rate of the microporous membrane (width 3 mm, length 10 mm) of each example and comparative example was measured (measurement mode: stretching mode, measurement temperature: 30-200 ℃, temperature increase rate: 5 ℃/min, load: 2gf).

As shown in Table 2, it can be confirmed that the polyolefin microporous film of Examples 1-7, which has an average surface pore size according to the present invention, has a low shrinkage rate after coating and an excellent shrinkage reduction rate due to coating. In particular, in the case of Examples 4 and 5, where the ratio (number ratio) of pores with a pore size of 100 nm or more is low, an excellent heat shrinkage reduction effect after coating can be confirmed even though the coating layer was manufactured thin. On the other hand, in the case of the microporous films 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 coating is not large.

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

Claims (10)

A polyolefin microporous film having an average pore size of 30 to 65 nm on at least one of both surfaces. The method of claim 1, wherein, among surface pores with an average pore size of 30 to 65 nm, the ratio (number ratio) of pores with a pore size of 100 nm or more is 15% or less, and the polyolefin micropolymer has a surface openness ratio of 10 to 25%. Porous membrane. The polyolefin according to claim 1 or 2, wherein a coating layer is formed on one or both sides 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 film according to claim 3, wherein the coating layer is a water-based coating layer. The polyolefin microporous film of claim 3, wherein the coating layer includes inorganic particles and a polymer binder. The method of claim 3, wherein the inorganic particles are alumina, barium sulfate, aluminum hydroxide, barium titanium oxide, magnesium oxide, and magnesium hydroxide. , a polyolefin microporous film containing at least one member selected from the group consisting of clay, titanium oxide, glass powder, and boehmite. The polyolefin microporous film according to claim 3, wherein the peeling strength of the coating layer (measured with a tensile strength meter manufactured by Instron (speed 300 mm/min)) is 45 gf/15 mm or more. The polyolefin microporous film according to claim 3, wherein the maximum melt heat shrinkage (as measured by SII TMA) is within 10% in the MD and TD directions. The polyolefin microporous film according to claim 3, wherein the shrinkage reduction rate before and after coating is 65% or more. A lithium ion secondary battery comprising the polyolefin microporous film according to claim 1 or 2.