KR102093063B1 - Polyolefin Microporous Membrane - Google Patents

Abstract

The present invention relates to a polyolefin microporous membrane which has excellent basic properties (mechanical properties, thermal stability, etc.) and coating properties required as a separator for a secondary battery, and at the same time, exhibits a good rate of increase in air permeation resistance even after a coating layer is formed, thereby improving the output characteristics of the battery.

Description

Polyolefin Microporous Membrane

The present invention relates to a polyolefin microporous membrane, and more particularly, it has excellent basic properties (mechanical properties, thermal stability, etc.) and coating properties required as a separator for a secondary battery, and at the same time shows a good rate of increase in air permeation resistance even after the coating layer is formed. It relates to a polyolefin microporous membrane capable of improving the output characteristics.

With the rapid development of portable information and communication equipment and electric vehicles, lithium-ion secondary batteries having a relatively high energy density are attracting much attention as power sources, and studies to improve battery performance by making lithium-ion secondary batteries have a higher energy density Development continues.

When a lithium ion secondary battery having a high energy density is used as a power source for a portable information communication device and an electric vehicle and is exposed to constant charging and discharging and high external temperatures, ignition may occur due to a membrane rupture. As a method for solving this problem, an attempt has been made to prevent the separation of the membrane by forming an additional coating layer on the microporous membrane having excellent heat resistance and mechanical properties.

However, in the case of the multi-layer porous membrane, the surface shape is more complicated than that of the single-layer porous membrane, making it difficult to uniformly form the coating layer, and as a result, the performance of the porous membrane is deteriorated. Therefore, there is a continuing need for a fine multi-layer porous film having a constant coating layer.

The present invention provides a polyolefin microporous membrane having excellent coating properties and having a low rate of increase in air permeation resistance even after introduction of the coating layer.

In addition, the present invention provides a lithium ion secondary battery comprising the polyolefin microporous membrane.

The present invention provides a polyolefin microporous membrane that satisfies Expression 1 below.

[Equation 1]

(R _{SCI, Front} -R _{SCE, Front} ) / (R _{SCI, Back} -R _{SCE, Back} ) × 100 ≤ 70

In the above formula 1,

R _{SCI, Front} is the specular component included-reflectance ( _SCI ) for the front side,

R _{SCE, Front} is the specular component excluded-reflectance (R _SCE ) for the front surface,

R _{SCI, Back} is the total light reflectance on the back side,

R _{SCE, Back} is the diffuse light reflectance on the _back side,

The light reflectance was measured at 340 to 740 nm with a D65 light source.

The polyolefin microporous membrane according to the present invention has a surface with a regular reflectance control, and exhibits excellent coating properties even when a multilayered film is formed. In addition, the separator for a battery manufactured using the polyolefin microporous membrane according to the present invention can improve the output characteristics of the battery by maintaining excellent strength and air permeability.

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

<Polyolefin microporous membrane>

As the polyolefin resin constituting the polyolefin microporous membrane of the present invention, conventional polyolefin resins known in the art can be used without limitation. Examples 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 60% by mass or more, with respect to the total mass of the polyolefin resin. The polyethylene resin may have a weight average molecular weight (Mw) in the range of 2.0 × 10 ⁵ to 4.0 × 10 ⁶ , for example, in the range of 2.5 × 10 ⁵ to 3.5 × 10 ⁶ . 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 polyethylenes having different weight average molecular weights (Mw) or densities may be mixed.

The content of the polypropylene resin may be, for example, 50% by mass or less, and 40% by mass or less, relative to the total mass of the polyolefin resin. The polypropylene resin may have a weight average molecular weight (Mw) of, for example, 7.0 × 10 ⁴ to 3.2 × 10 ⁶ , and another example of 9.5 × 10 ⁴ to 3.0 × 10 ⁶ . As the polypropylene, high density polypropylene (HDPP), medium density polypropylene (MDPP), or low density polypropylene (LDPP) may be used without limitation. Alternatively, two or more polypropylenes having different weight average molecular weights (Mw) or densities may be mixed.

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

If necessary, conventional additives known in the art, for example, antioxidants, pore formers, and the like, 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 satisfies Equation 1 below:

[Equation 1]

(R _{SCI, Front} -R _{SCE, Front} ) / (R _{SCI, Back} -R _{SCE, Back} ) × 100 ≤ 70

In the above formula 1,

R _{SCI, Front} is the specular component included-reflectance ( _SCI ) for the front side,

R _{SCE, Front} is the specular component excluded-reflectance (R _SCE ) for the front surface,

R _{SCI, Back} is the total light reflectance on the back side,

R _{SCE, Back} is the diffuse light reflectance on the _back side,

The light reflectance was measured at 340 to 740 nm with a D65 light source.

In the present invention, the value of (R _{SCI, Front} -R _{SCE, Front} ) / (R _{SCI, Back} -R _{SCE, Back} ) × 100 is 70 or less, for example 20 to 70, another example 40 to 70, another Examples may be 40 to 60. When the value of (R _{SCI, Front} -R _{SCE, Front} ) / (R _{SCI, Back} -R _{SCE, Back} ) × 100 has a value in the above range, excellent coating properties can be secured, and the above range If it deviates, the speculative resistance also increases, and battery power and cycle characteristics may deteriorate.

The polyolefin microporous membrane according to the present invention may satisfy Equation 2 below:

[Equation 2]

(R _{SCI, Back} -R _{SCE, Back} )-(R _{SCI, Front} -R _{SCE, Front} ) ≥ 0.5

In the formula 2,

R _{SCI, Front} is the total light reflectance to the front,

R _{SCE, Front} is the diffuse light reflectance to the front,

R _{SCI, Back} is the total light reflectance on the _back side,

R _{SCE, Back} is the diffuse light reflectance on the _back side,

The light reflectance was measured at 340 to 740 nm with a D65 light source.

In the present invention, the value of (R _{SCI, Back} -R _{SCE, Back} )-(R _{SCI, Front} -R _{SCE, Front} ) may be 0.5 or more, for example, 0.5 to 3, and another example, 0.5 to 2. If the values of (R _{SCI, Back} -R _{SCE, Back} )-(R _{SCI, Front} -R _{SCE, Front} ) are out of the above-mentioned range, the increase in speculation resistance also increases, leading to a decrease in battery output and cycle characteristics. Can be.

In addition, the polyolefin microporous membrane according to the present invention may satisfy the following equations 3 and 4:

[Equation 3]

0.95 ≤ (R _{SCI, Front} -R _{SCE, Front} ) ≤ 1.65

[Equation 4]

2.15 ≤ (R _{SCI, Back} -R _{SCE, Back} ) ≤ 3.00

In the above formulas 3 and 4,

R _{SCI, Front} is the total light reflectance to the front,

R _{SCE, Front} is the diffuse light reflectance to the front,

R _{SCI, Back} is the total light reflectance on the _back side,

R _{SCE, Back} is the diffuse light reflectance on the _back side,

The light reflectance was measured at 340 to 740 nm with a D65 light source.

For example, the front surface of the polyolefin microporous membrane may be an air side surface, and a back surface may be a cooling roll surface.

The shutdown temperature (SDT) of the polyolefin microporous membrane according to the present invention may be less than 135.8 ° C, for example, 118 ° C or more and less than 135.8 ° C, in another example 120 ° C to 135.2 ° C, and in another example 125 ° C to 134.5 ° C. In addition, the melt-down temperature (MDT) of the polyolefin microporous membrane according to the present invention may be 160 ° C or higher, for example, 160 ° C to 210 ° C, and in another example, 160 ° C to 180 ° C.

The polyolefin microporous membrane according to the present invention may have a multi-layer structure in which the microporous membrane is stacked in multiple layers.

In the polyolefin microporous membrane according to the present invention, a coating layer may be formed on one side or both sides. The coating layer may include inorganic particles or crosslinked polymer particles.

Non-limiting examples of usable inorganic particles include alumina, aluminum hydroxide, barium titanium oxide, magnesium oxide, magnesium hydroxide, and the like. Clay, titanium oxide, glass powder, boehmite, or mixtures of two or more thereof.

The size of the inorganic particles is not limited, but may be in the range of 0.001 to 10 μm for uniform film formation and proper porosity.

The content of the inorganic particles may be, for example, 50 to 90% by weight based on the total weight of the coating layer, and another example may be 60 to 85% by weight. When the content of the inorganic particles falls within the above-described range, a heat resistance effect due to the use of the inorganic particles can be achieved.

Non-limiting examples of crosslinked polymer particles 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, polyvinyl Sulfonate, chlorosulfonated polyethylene, perfluorinated sulfonated ionomer, sulfonated polystyrene, styrene-acrylic acid copolymer, sulfonated butyl rubber, polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexa Fluoropropylene (PVdF-HFP), polyvinylpyrrolidone, polyacrylonitrile, polyvinylidene fluoride-trichloroethylene, polyvinylidene fluoride-chlorotrifluoroethylene (PVdF-TFE), polymethyl meta Acrylate, polyvinyl acetate, ethylene-co-vinyl acetate copolymer, polyethylene oxide, cellulose acetate, cellulose acetate Tilate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, acrylonitrile styrene-butadiene copolymer, polyimide Or mixtures of two or more of these.

The thickness of the polyolefin microporous membrane according to the present invention is not particularly limited, and may be 1 to 16 μm, for example, 3 to 14 μm, and another example, 4 to 10 μm.

The method for producing the microporous polyolefin membrane of the present invention is not particularly limited, for example, (1) preparing a polyolefin solution by melt-kneading a polyolefin and a plasticizer, and (2) extruding the polyolefin solution and cooling it to form a sheet. Forming, (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 is removed, ( 6) a second stretching step for re-stretching the dried film and (7) a heat treatment step.

Hereinafter, each manufacturing process will be described in detail.

(1) Process for preparing polyolefin solution

The polyolefin resin and the plasticizer are melt-kneaded to prepare a polyolefin solution. Melting and kneading a composition comprising a polyolefin-based resin and a plasticizer may be performed without limitation using conventional methods known in the art. As an 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 can be used.

The content of the polyolefin-based resin is not particularly limited, and for example, may be included in an amount of 20 to 50% by weight relative to the total weight of the composition comprising the polyolefin-based resin and a plasticizer, and may be included in another example 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 constituting a single phase with the polyolefin-based resin at an extrusion temperature.

Non-limiting examples of plasticizers that can be used include non-, non-, decane, decalin, liquid paraffin (Liquid paraffin, LP), such as liquid paraffin (or paraffin oil), 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; And fatty acid alcohols having 10 to 20 carbon atoms, such as palmitic acid alcohol, stearic acid alcohol, and oleic acid alcohol, or mixtures of two or more thereof. As an example, the liquid paraffin among the plasticizers is harmless to the human body and has a high boiling point and low volatile components, and is suitable for use as a plasticizer in a wet method.

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

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

(2) Step of forming sheet

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

When the polyolefin solution is extruded from the die and cooled to form a gel-like sheet, the surface roughness can be controlled, for example, by controlling the cooling rate. For example, it may be cooled at a rate of 100 ° C./min or more, for example, 100 to 650 ° C./min, and another example, 300 to 650 ° C./min. When the cooling rate is less than 100 ° C / min, the fibril size of the gel sheet increases due to an increase in crystallinity of the polyolefin, and thus surface roughness may increase, and an unsuitable gel due to an increase in stiffness due to high crystallinity A sheet of top may be formed.

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

When cooling using a cooling roll, the temperature of the cooling roll may be 20 ° C or higher, for example, 20 to 35 ° C, and another example, 25 to 30 ° C. In this case, the temperature opposite to the surface of the sheet in contact with the cooling roll, that is, the temperature of the indoor air may be 25 ° C or higher, for example, 25 to 50 ° C, and another example may be 30 to 40 ° C.

The difference between the indoor air temperature and the cooling roll temperature may be, for example, 3 to 15 ° C, and another example may be 3 to 10 ° C. When the difference between the temperature of the indoor air and the temperature of the cooling roll satisfies the above-described range, it is possible to secure excellent coating properties by adjusting the specular reflectance of the surface of the porous membrane within the range according to the present invention.

(3) First stretching process

The sheet-like article obtained in step (2) is biaxially stretched one or more times to form a film.

In the present invention, the first stretching process may be performed at a predetermined magnification by conventional methods in the art, 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, biaxial stretching may be performed at the same time, or any of sequential stretching may be performed.

In the first stretching step, the stretching ratio differs 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. You can.

In the first stretching process, the structure and pore size of the fibril can be controlled by adjusting 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 ° C, and in another example, in the range of 110 to 125 ° C. In the case of stretching in the above range, it can be stretched to have appropriate air permeability and mechanical strength without blocking pores (pores) in the sheet. When the first stretching temperature exceeds the above-described temperature range, the fibril is melted, the thickness becomes thicker, and the air permeation resistance after the coating layer is formed is rapidly increased, thereby deteriorating the performance of the battery. On the other hand, when the first stretching temperature is lower than the above-described temperature range, the polyolefin microporous membrane is not only produced in a state that does not have the necessary properties (for example, porosity and air permeability), but also after the coating process is carried out, the air permeability is increased It can cause.

(4) Plasticizer removal process

Using a cleaning solvent, a plasticizer is extracted from the stretched film. Since the polyolefin phase is phase-separated from the plasticizer, removing the plasticizer provides a porous membrane having a large number of pore structures. As a method for removing the plasticizer, conventional methods known in the art can be used without limitation.

In the present invention, as the cleaning solvent, any organic solvent capable of extracting a plasticizer may be used without particular limitation. For example, halogenated hydrocarbons such as methylene chloride, 1,1,1-trichloroethane, and fluorocarbon, 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 using liquid paraffin as a plasticizer, methylene chloride can be used as an organic solvent.

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

(5) Drying process

The polyolefin microporous membrane obtained by plasticizer removal can be dried using conventional drying methods known in the art. For example, a heat drying method, an air drying method, or the like can be used.

(6) Second stretching process

Subsequently, the dried film is stretched again at least in the uniaxial direction or more. 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. At this time, it may be uniaxial stretching or biaxial stretching.

The temperature of the second stretching process may be, for example, a crystal dispersion temperature of the polyolefin resin constituting the microporous membrane, a crystal dispersion temperature of + 40 ° C. or less, and another example, a crystal dispersion temperature + 10 ° C. or more, crystal Dispersion temperature + 40 ℃ or less. By adjusting the second stretching temperature in the above-described range, it is possible to prevent a decrease in air permeability and the occurrence of physical property variations in the sheet width direction when stretching in the transverse direction (width direction: TD direction). By making the 2nd stretching temperature within the said range, the generation | occurrence | production of the deviation in the width direction of the extending | stretching sheet of air permeability resistance can be suppressed especially. Here, 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 is generally 90 to 100 ° C.

In addition, by adjusting the temperature of the second stretching process in the above-described range, the polyolefin resin can be sufficiently softened, and it is possible to uniformly stretch by preventing rupture. In one example, the second stretching temperature may range from 90 to 140 ° C, and another example may range from 120 to 140 ° C.

The magnification of the second stretching in the uniaxial direction may be, for example, 1.0 to 1.8 times, and another example may be 1.2 to 1.6 times. For example, in the case of uniaxial stretching, 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, the MD direction and the TD direction are respectively adjusted to 1.0 to 1.8 times. In the case of biaxial stretching, each stretching magnification in the MD direction and the TD direction may be the same or different. By adjusting the stretching magnification to the above-described range, it is possible to prevent deterioration of permeability, electrolytic solution absorption and compressibility, and to prevent fibril from becoming too thin and heat shrinkage resistance.

(7) Heat treatment process

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

In particular, by performing the heat setting treatment, a network structure composed of fibrils formed by the second stretching process is maintained, and the crystals of the porous membrane are stabilized, whereby the microporous diameter is properly adjusted and a fine porous membrane having excellent strength can be produced.

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

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

<Lithium ion secondary battery>

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

The lithium ion secondary battery of the present invention can be manufactured according to a conventional method known in the art, except that the polyolefin microporous membrane according to the present invention is used as a separator. For example, a separation membrane may be interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte may be introduced to produce it.

The lithium ion secondary battery of the present invention includes a negative electrode, a positive electrode, a separator, and an electrolyte as a battery component, wherein the components of the positive electrode, the negative electrode, the electrolyte, and other additives, if necessary, other than the above-described separator are conventionally known in the art. It conforms to the elements of the lithium ion secondary battery.

For example, the positive electrode may be manufactured using a positive electrode active material for a lithium ion secondary battery known in the art, and non-limiting examples thereof include lithium transition metal composites 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 other common transition metals or parts of manganese, nickel, and cobalt of these oxides, or Aluminum or lithium-containing vanadium oxide) or chalcogen compounds (eg, manganese dioxide, titanium disulfide, molybdenum disulfide, etc.).

For 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 non-limiting examples thereof include materials capable of intercalating / deintercalating lithium ions, Examples include lithium metal or lithium alloys, coke, artificial graphite, natural graphite, organic polymer compound burners, carbon fibers, silicon-based, tin-based, and the like.

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

The electrolyte salt is ^{(i) Li +, Na +} , K + cations and (ii) selected from the group consisting of _{^{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 - can be made of a combination of an anion selected from the group consisting of, a double lithium salt is preferred . 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 may be used alone or in combination of two or more.

As the electrolyte solvent, cyclic carbonate, linear carbonate, lactone, ether, ester, acetonitrile, lactam, and ketone may be used.

Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), 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 are gamma-butyrolactone (GBL), and examples of the ether include dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxy ethane. 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 is N-methyl-2-pyrrolidone (NMP) and the like, and the ketone is 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 also be used for glyme, diglyme, triglyme, and tetraglyme. These organic solvents may be used alone 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 to aid the understanding of the present invention, and the scope of the present invention is not limited to the examples in any sense.

[Example 1-5]

High-density polyethylene (HDPE) having a weight average molecular weight of 600,000 g / mol is 40% by weight based on the total weight of the composition comprising a polyolefin-based resin and a plasticizer, and fluid paraffin is a polyolefin-based resin and a plasticizer as a plasticizer. A polyolefin composition was prepared at 60% by weight based on the total weight of the composition. The polyethylene resin has a melting point of 135 ° C and a crystal dispersion temperature of 100 ° C.

28 parts by weight of the resulting polyolefin composition was added into a steel-blended twin screw extruder having an inner diameter of 58 mm and an L / D of 42, and 72 parts by weight of liquid paraffin (50 cst at 40 ° C) was fed to the twin screw extruder through a side feeder. Did. Melt-blending was performed at 210 ° C. and 200 rpm to prepare a polyethylene solution. This polyethylene solution was extruded from the T-die mounted on the twin screw extruder. The extrudate was cooled while passing through a cooling roll to form a cooled extrudate, that is, a gel-like sheet.

Using a tenter-stretcher, the gel-like sheet material was simultaneously biaxially stretched at 114 ° C in both longitudinal and transverse directions 6 times. The stretched film was repeatedly immersed in a methylene chloride section adjusted to 25 ° C to remove liquid paraffin, and dried by air flow at room temperature. The dried membrane was re-stretched at 129.8 ° C in a transverse direction at a magnification of 1.4 times by a batch stretching machine. A microporous membrane according to Example 1-5 was prepared by fixing the re-stretched membrane to a batch stretching machine and heat-setting at 125 ° C for 10 minutes.

The indoor air temperature and cooling roll temperature of Example 1-5 and the structure, thickness, and light reflectance of each of the prepared porous membranes are shown in Table 1 below. At this time, the light reflectance was measured at 340 to 740 nm with a D65 light source.

[Comparative Example 1-5]

A microporous membrane according to Comparative Example 1-5 was prepared in the same manner as in Example 1-5, except that the indoor air temperature, the cooling roll temperature, the structure, thickness, and light reflectance of the porous membrane were adjusted as shown in Table 2 below Did.

[Experimental Example-Property evaluation]

The physical property test of the microporous membrane was performed as follows, and the results are shown in Table 3 below.

<Method for measuring physical properties>

Shutdown temperature (SDT)

While heating the polyolefin microporous membranes prepared in Examples 1-5 and Comparative Examples 1-5 at a heating rate of 5 ° C./min, an Oken type air permeability meter [Asahi Seiko Co., Ltd. Ltd.,) product, EGO-1T], and the temperature at which the air resistance reached the detection limit of 1Х10 ⁵ sec / 100cc was called the shutdown temperature (℃).

(◎: Less than 134.5 ° C, ○: 134.5 ° C or more and 135.2 ° C or less, △: 135.2 ° C or more and 135.8 ° C or less, ×: more than 135.8 ° C)

Meltdown temperature (MDT)

After reaching the shutdown temperature (SDT), heating was continued at a heating rate of 5 ° C / min, and the temperature at which the air permeation resistance reached 1Х10 ⁵ sec / 100cc was referred to as the meltdown temperature (MDT) (° C).

(◎: 170 ° C or more, ○: 166 ° C or more and less than 170 ° C, △: 162 ° C or more and less than 166 ° C, ×: less than 162 ° C)

Speculation resistance

It was measured according to JIS P8117 using an Oken type speculative resistance meter (Asahi Seiko Co., Ltd., EGO-1T). The speculative resistance means the time (Sec) for 100 cc of air to pass through a membrane of a certain area under a constant pressure.

As shown in Table 3, it was confirmed that the polyolefin microporous membrane according to Example 1-5 having the light reflectance according to the present invention exhibits excellent effects in all measured physical properties. On the other hand, the microporous membrane according to Comparative Example 1-5 showed poor heat resistance (shutdown temperature and meltdown temperature) compared to the microporous membrane according to Example 1-5. In particular, it can be confirmed that the multi-layer film of Comparative Examples 1-3 has a very high rate of increase in air permeability after the introduction of the coating layer, and in the case of the battery using this, it is predicted that a rise in internal resistance causes side reactions and cycle performance to deteriorate.

Claims (9)

Polyolefin microporous membrane satisfying the following formula 1:
[Equation 1]
(R _{SCI, Front} -R _{SCE, Front} ) / (R _{SCI, Back} -R _{SCE, Back} ) × 100 ≤ 70
In the above formula 1,
R _{SCI, Front} is the specular component included-reflectance ( _SCI ) for the front side,
R _{SCE, Front} is the specular component excluded-reflectance (R _SCE ) for the front surface,
R _{SCI, Back} is the total light reflectance on the back side,
R _{SCE, Back} is the diffuse light reflectance on the _back side,
The light reflectance was measured at 340 to 740 nm with a D65 light source. The polyolefin microporous membrane according to claim 1, wherein the polyolefin microporous membrane satisfies the following Equation 2:
[Equation 2]
(R _{SCI, Back} -R _{SCE, Back} )-(R _{SCI, Front} -R _{SCE, Front} ) ≥ 0.5
In the formula 2,
R _{SCI, Front} is the total light reflectance to the front,
R _{SCE, Front} is the diffuse light reflectance to the front,
R _{SCI, Back} is the total light reflectance on the _back side,
R _{SCE, Back} is the diffuse light reflectance on the _back side,
The light reflectance was measured at 340 to 740 nm with a D65 light source. The polyolefin microporous membrane according to claim 1, wherein the polyolefin microporous membrane satisfies the following formulas 3 and 4:
[Equation 3]
0.95 ≤ (R _{SCI, Front} -R _{SCE, Front} ) ≤ 1.65
[Equation 4]
2.15 ≤ (R _{SCI, Back} -R _{SCE, Back} ) ≤ 3.00
In equations 3 and 4 above
R _{SCI, Front} is the total light reflectance to the front,
R _{SCE, Front} is the diffuse light reflectance to the front,
R _{SCI, Back} is the total light reflectance on the _back side,
R _{SCE, Back} is the diffuse light reflectance on the _back side,
The light reflectance was measured at 340 to 740 nm with a D65 light source. The polyolefin microporous membrane according to claim 1, wherein the front surface of the polyolefin microporous membrane is an air side and the back side is a cooling roll side. The polyolefin microporous membrane according to claim 1, wherein the polyolefin comprises at least one of polyethylene and polypropylene. The polyolefin microporous membrane according to claim 1, wherein the polyolefin microporous membrane has a shutdown temperature (SDT) of 118 ° C or more and less than 135.8 ° C, and a meltdown temperature (MDT) of 160 ° C to 210 ° C. According to claim 1, The polyolefin microporous membrane is a coating layer is formed on one or both sides, the coating layer is a polyolefin microporous membrane comprising inorganic particles or crosslinked polymer particles. The polyolefin microporous membrane according to claim 1, which has a multi-layer structure. A lithium ion secondary battery comprising the polyolefin microporous membrane according to any one of claims 1 to 8.