Polyolefin Microporous Membrane

Abstract

The present invention relates to a polyolefin microporous membrane, and more specifically, to a polyolefin microporous membrane having excellent strength, resistance characteristics and a low shutdown temperature.

Description

Technology Field

[0001] The present invention relates to a polyolefin microporous membrane (also referred to as a perforated polyolefin film) that is widely used as a separator for material separation, selective permeation, etc., and as an isolator for electrochemical reaction devices such as alkaline batteries, lithium secondary batteries, fuel cells, and capacitors. In particular, the present invention is a polyolefin microporous membrane suitable for use as a separator for non-aqueous electrolyte secondary batteries such as lithium-ion batteries, and is utilized as a separator having higher safety and output characteristics compared to conventional polyolefin microporous membranes. Background Technology

[0002] Polyolefin microporous membranes are used as filters, separators for fuel cells, and separators for capacitors. Polyolefin microporous membranes have excellent mechanical strength, shutdown characteristics, and ion permeability, and are particularly suitable for use as separators for non-aqueous electrolyte secondary batteries, such as lithium-ion batteries, which are widely used in notebook personal computers, mobile phones, and electric vehicles (EVs).

[0003] Recently, the capacity of batteries has been increasing, driven by the miniaturization of electronic devices and expansion into automotive applications; consequently, there is a growing demand for thinner separators. However, thinning separators reduces their strength, making them susceptible to short circuits caused by electrodes or foreign substances, or film rupture due to impact. Consequently, this decrease in resistance to foreign substances and impact leads to reduced battery safety. Therefore, there is a need for polyolefin microporous membranes that possess higher strength than conventional polyolefin microporous membranes.

[0004] In addition, as the capacity and output of the battery increase, even if the electrochemical reaction is stopped by the shutdown function of the separator, the temperature inside the battery continues to rise, causing the separator to rupture and short-circuit. Therefore, a low-temperature shutdown function is required for the separator to increase the safety of the battery. However, if the shutdown temperature is lowered to increase safety, there is a problem that the strength of the porous film decreases.

[0005] Accordingly, there is a need to develop a polyolefin microporous membrane that has excellent strength and a low shutdown temperature. The problem to be solved

[0006] The present invention provides a polyolefin microporous membrane having excellent strength, resistance characteristics and a low shutdown temperature.

[0007] In addition, the present invention provides a secondary battery comprising the polyolefin microporous membrane. means of solving the problem

[0008] The present invention relates to a molecular weight of 3 x 10 at the maximum peak in a differential molecular weight distribution curve according to gel permeation chromatography (GPC). 5 The present invention provides a polyolefin microporous membrane having a g / mol or higher, a maximum peak top melting point on the melting curve obtained during the second heating step by differential scanning calorimetry (DSC) of 133 ℃ or lower, and a difference (△Tm = Tm2- Tm1) between the maximum peak top melting point on the melting curve obtained during the second heating step by DSC and the maximum peak top melting point on the melting curve obtained during the first heating step (Tm1) of 8 ℃ or lower. Effects of the invention

[0009] The present invention provides a polyolefin microporous membrane having excellent strength, resistance characteristics, and a low shutdown temperature. Accordingly, the polyolefin microporous membrane according to the present invention can be applied as a separator in a secondary battery requiring high capacity and high power output. Specific details for implementing the invention

[0010] The present invention will be described in detail below. However, it is not limited to the following description, and each component may be modified in various ways or optionally combined as needed. Accordingly, it should be understood that the invention includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the invention.

[0012] Polyolefin Microporous Membrane

[0013] Separators require high strength, high output, and low-temperature shutdown characteristics. Conventionally, to achieve high strength characteristics, methods such as using ultra-high molecular weight polyethylene (UHPE) or increasing the elongation ratio were employed, and to achieve high output characteristics, methods such as increasing the porosity, large pore size, and improving pore size uniformity were used. Additionally, to achieve low-temperature shutdown, methods such as adding low-melting point components were employed. However, since UHPE has a slow diffusion rate and high viscosity, the addition of low molecular weight components is necessary; however, when low molecular weight components are added, the affinity between UHPE and the low molecular weight components is low due to the difference in molecular weight, making it easy to form a non-uniform structure and degrading ion permeability (output characteristics). Furthermore, as the elongation ratio increases, orientation proceeds and the melting point increases, leading to an increase in shutdown temperature and a decrease in safety. Additionally, when increasing the porosity, the resin content decreases, which lowers strength, while the pore occlusion area increases, which also leads to an increase in shutdown temperature.

[0014] The present invention aims to solve the problems of the aforementioned prior art by controlling the molecular weight of the maximum peak in the differential molecular weight distribution curve according to GPC of the polyolefin microporous membrane, the melting point of the maximum peak on the melting curve obtained at the second heating step by DSC, and △Tm (Tm2-Tm1) to the aforementioned ranges, thereby providing a polyolefin microporous membrane having uniform pores, excellent strength and ion permeability, and low shutdown temperature and resistance.

[0015] The polyolefin microporous membrane of the present invention has a molecular weight of 3 x 10 at the maximum peak in the differential molecular weight distribution curve according to gel permeation chromatography (GPC). 5 It is g / mol or more, and the maximum peak top melting point on the melting curve obtained during the second heating step by differential scanning calorimetry (DSC) is 133 ℃ or less, and the difference between the maximum peak top melting point on the melting curve obtained during the second heating step by DSC (Tm2) and the maximum peak top melting point on the melting curve obtained during the first heating step (Tm1) (△Tm = Tm2- Tm1) is 8 ℃ or less.

[0016] In the differential molecular weight distribution curve according to GPC of the above polyolefin microporous membrane, the molecular weight of the maximum peak is 3 x 10 5 g / mol or more, e.g., 3 x 10⁻⁶ 5 Up to 6 x 10 5It may be g / mol. If the molecular weight of the maximum peak in the GPC differential molecular weight distribution curve is below the aforementioned range, the piercing strength is low, which may cause difficulties in winding during thin film formation, or the shutdown characteristics at low temperatures may be poor, leading to reduced stability. If the molecular weight of the maximum peak satisfies the aforementioned range, the entanglement density in the amorphous region increases, allowing for excellent tensile strength and tensile elongation to be secured, and excellent mechanical strength, shutdown characteristics, cell resistance, and dielectric strength can be provided through the homogenization of pore diameter and promotion of orientation.

[0017] The above polyolefin microporous membrane may have a maximum peak top melting point on the melting curve obtained during the second heating step using Differential Scanning Calorimetry (DSC) of 133°C or lower, for example, between 128°C and 133°C. By controlling the maximum peak top melting point on the melting curve obtained during the second heating step using DSC to the aforementioned range, a polyolefin microporous membrane with excellent permeability and shutdown characteristics can be obtained. To impart low-temperature shutdown characteristics, a low-molecular-weight polyolefin with a low melting point may be mixed with an ultra-high molecular weight polyolefin; however, in this case, it is easy to form a non-uniform structure, which may lead to a decrease in strength. The present invention allows for maintaining excellent strength by improving affinity with plasticizers or low molecular weight components and homogenizing the structure by controlling the maximum peak top melting point on the melting curve obtained during the second heating step by DSC to the aforementioned range.

[0018] The above polyolefin microporous membrane may have a difference (△Tm = Tm2 - Tm1) between the maximum peak top melting point (Tm2) on the melting curve obtained during the second heating step by DSC and the maximum peak top melting point (Tm1) on the melting curve obtained during the first heating step, which is 8°C or less, for example, 4 to 8°C. If △Tm(Tm2 - Tm1) is less than the aforementioned range, the strength may decrease, and if it exceeds the aforementioned range, the pore size may increase, which may lead to a decrease in battery performance.

[0019] The above polyolefin microporous membrane may have an average pore size of 30 nm or less, for example, 20 to 30 nm, as measured using a pore analyzer (Porometer). By controlling the pore size to the aforementioned range, the present invention controls swelling properties and crystal acceleration, thereby forming a uniform structure at the time of casting and, as a result, providing excellent stability. When the average pore size is 20 nm or more, ion permeability is superior, and when it is 30 nm or less, the battery life can be extended.

[0020] The bubble point (BP) pore size of the above polyolefin microporous membrane may be 40 nm or less, for example, 30 to 40 nm. If the BP pore size is less than the aforementioned range, the protrusion strength may be low and stability may be reduced, and if it exceeds the aforementioned range, the difference in molecular weight between the low molecular weight component and the BP may be large, resulting in reduced affinity and a non-uniform structure, and the ion permeability (output characteristics) may deteriorate, increasing the shutdown (SD) temperature and reducing safety.

[0021] The difference between the maximum pore size and the average pore size of the above-mentioned polyolefin microporous membrane may be 13 nm or less. If the pore size of the microporous membrane is small, the resistance increases, and if the pore size is large, the dielectric breakdown voltage (BDV) decreases, which may lead to a decrease in dielectric strength. By controlling the swelling and crystal acceleration by adjusting the pore uniformity to the aforementioned range, the present invention can form a uniform structure at the time of casting, and as a result, provide excellent stability.

[0022] The above polyolefin microporous membrane may have a protrusion strength of 90 gf / (g / m²) per unit mass. If the protrusion strength per unit mass is less than the aforementioned range, metal ions may be leached out during charge-discharge cycles, and the leached metal ions may grow in the microporous membrane to form dendrites, causing internal short circuits, and metal ions that pass through the microporous membrane may be deposited on the negative electrode, thereby degrading the performance of the battery.

[0023] The above polyolefin microporous membrane may have a shutdown (SD) temperature of 143°C or lower, for example, between 137°C and 143°C. When the shutdown temperature is 143°C or lower, it exhibits excellent stability and can be applied as a separator for secondary batteries (e.g., secondary batteries for electric vehicles) that require high energy density, high capacity, and high output. When the shutdown temperature is 137°C or higher, it is possible to prevent the phenomenon in which the pores become closed and the output characteristics deteriorate during normal usage environments or battery manufacturing processes.

[0024] The above polyolefin microporous membrane may have a porosity of 35% or more, for example, 40 to 60%. If the porosity is less than the aforementioned range, the glove may not be maintained after the coating process, and if it exceeds the aforementioned range, the glove may not be maintained after the pressing process.

[0025] The air permeability of the above polyolefin microporous membrane may be 90 sec / 100cc or less. If the air permeability satisfies the aforementioned range, good ion conductivity can be provided when applied to a battery.

[0026] The curvature of the above polyolefin microporous membrane may be 1 or more, for example, 1.1 or more, or 1.2 or more. When the curvature satisfies the aforementioned range, it is effective in preventing short circuits when applied to a battery. In addition, the curvature may be 3 or less, for example, 2 or less, or 1.6 or less. When the curvature satisfies the aforementioned range, excellent permeability and insulation can be provided.

[0027] The above polyolefin has a weight-average molecular weight of 1 x 10 6 Up to 1.5 x 10 6 A first polyolefin having g / mol and a weight-average molecular weight of 1 x 10 5 It may include a second polyolefin with a g / mol or less.

[0028] The above polyolefin may be any known in the art without limitation and may include, for example, polyethylene, polypropylene, or a mixture thereof. For example, the polyolefin may be polyethylene.

[0029] The above polyolefins may include ultra-high density polyethylene (UHPE), high density polyethylene (HDPE), medium density polyethylene (MDPE), or low density polyethylene (LDPE), and linear or branched polyethylene may be used, and two or more types of polyethylene with different weight-average molecular weight, density and / or structure (linear / branched) may be mixed.

[0030] The first polyolefin has a weight-average molecular weight of 1 x 10 6 g / mol or more, e.g., 1 x 10⁻⁶ 6 Up to 2.0 x 10 6It may be g / mol. If the weight-average molecular weight of the first polyolefin is below the aforementioned range, the protrusion strength may be low and stability may be reduced. If it exceeds the aforementioned range, the difference in molecular weight between the low molecular weight component and the first polyolefin is large, so the affinity may be reduced and a non-uniform structure may be formed, and the ion permeability (output characteristics) may deteriorate, increasing the shutdown (SD) temperature and reducing safety.

[0031] The first polyolefin above may have a melting point of 125 to 133°C. If the melting point is 125°C or higher, it is advantageous for preventing melting such as heat fixation and maintaining porosity, and if it is 133°C or lower, the affinity with plasticizers and low molecular weight components is improved, so even when low molecular weight components are added, a uniform pore size can be obtained and it is advantageous for maintaining a good shutdown temperature.

[0032] The second polyolefin above has a weight-average molecular weight of 1 x 10 5 g / mol or less, e.g., 5 x 10⁻⁶ 4 Up to 1 x 10 5 It may be g / mol. When a high molecular weight first polyolefin is used alone, the pore size may become non-uniform and the strength may decrease. When a low molecular weight second polyolefin having a weight-average molecular weight within the aforementioned range is mixed, a polyolefin microporous membrane can be provided that has excellent miscibility with the first polyolefin, has uniform pore size, excellent strength and resistance characteristics, and a low shutdown temperature.

[0033] For example, based on the total weight of the polyolefin, the first polyolefin may comprise 50% or more by weight, for example, 50 to 90% by weight, or 50 to 70% by weight, and the second polyolefin may comprise 50% or less by weight, for example, 10 to 50% by weight, or 30 to 50% by weight. When the content of the first polyolefin satisfies the aforementioned range, a microporous membrane having uniform pore size and excellent strength can be provided, and when the content of the second polyolefin satisfies the aforementioned range, a polyolefin microporous membrane having excellent miscibility with a plasticizer and the first polyolefin and a low shutdown temperature can be provided.

[0034] 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 for low-density polyethylene (LDPE), one or more selected from the group consisting of branched LDPE, linear LDPE, and ethylene / α-olefin copolymers produced by a single-site catalyst may be used, and the amount added thereof can be appropriately controlled within a range known in the relevant art.

[0035] If necessary, conventional additives known in the relevant technical field, such as antioxidants and pore-forming agents, may be included to the extent that they do not impair the effects of the present invention.

[0036] The thickness of the polyolefin microporous membrane according to the present invention is not particularly limited and may be, for example, 10 μm or less.

[0037] The polyolefin microporous membrane according to the present invention may have a single-layer structure or a multi-layer structure in which the microporous membrane is stacked in multiple layers.

[0038] A polyolefin microporous membrane according to the present invention may have a coating layer formed on one or both sides. The coating solution forming the coating layer comprises inorganic particles and a polymer binder. The coating solution may further comprise a solvent as needed.

[0039] Non-limiting examples of usable inorganic particles include alumina, aluminum hydroxide, barium titanium oxide, magnesium oxide, magnesium hydroxide, clay, titanium oxide, glass powder, and boehmite, and these may be used alone or in combination of two or more types.

[0040] The size of the above inorganic particles is not limited, but may be in the range of 0.001 to 10 μm for forming a film of uniform thickness and appropriate porosity.

[0041] The content of the inorganic particles may be, for example, in the range of 50 to 90 weight percent based on the total weight of the coating layer, or, as another example, in the range of 60 to 85 weight percent. When the content of the inorganic particles falls within the aforementioned range, a heat resistance effect can be achieved through the use of inorganic particles.

[0042] Non-limiting examples of available polymer binders include polyacrylic acid, polymethacrylic acid, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, butadiene-acrylic acid copolymer, butadiene-methacrylic acid copolymer, polyvinylsulfonate, chlorosulfonated polyethylene, perfluorosulfonated ionomer, sulfonated polystyrene, styrene-acrylic acid copolymer, sulfonated butyl rubber, polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP), polyvinylpyrrolidone, polyacrylonitrile, polyvinylidene fluoride-trichloroethylene, polyvinylidene fluoride-chlorotrifluoroethylene (PVdF-TFE), polymethyl methacrylate, polyvinyl acetate, and ethylene-co-vinyl acetate. Copolymers, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethylcellulose, acrylonitrile styrene butadiene copolymer, polyimide, etc., may be used alone or in combination of two or more types.

[0043] Both aqueous solvents and organic solvents may be used as the above solvents, and non-limiting examples of usable organic solvents include acetone, tetrahydrofuran, methylene chloride, dimethylformamide, dimethylacetamide, ethanol, methanol, etc., and these may be used alone or in combination of two or more.

[0045] Method for manufacturing a polyolefin microporous membrane

[0046] The method for manufacturing the polyolefin microporous membrane of the present invention is not particularly limited and may include, for example, (1) a step of preparing a polyolefin solution by melt-kneading a polyolefin and a plasticizer; (2) a step of forming a sheet-shaped body by extruding the polyolefin solution and then cooling it; (3) a first stretching step of forming a film by stretching the sheet-shaped body; (4) a step of removing the plasticizer from the film; (5) a step of 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. Each manufacturing process is described in detail below.

[0048] (1) Preparation process of polyolefin solution

[0049] A polyolefin solution is prepared by melt-kneading a polyolefin resin and a plasticizer. Any conventional method known in the art can be used without limitation as a method for melt-kneading a composition containing a polyolefin resin and a plasticizer. For example, a method can 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.

[0050] The content of the polyolefin resin is not particularly limited and, for example, may be included in an amount of 20 to 50 weight% based on the total weight of the composition including the polyolefin resin and the plasticizer, or, as another example, in an amount of 20 to 40 weight%.

[0051] In the present invention, conventional components known in the art can be used as plasticizers without limitation, and for example, any organic compound that forms a single phase with the polyolefin resin at the extrusion temperature can be used.

[0052] Non-limiting examples of usable plasticizers include aliphatic or cyclic hydrocarbons such as liquid paraffin (or paraffin oil) such as nonane, decane, decalin, and liquid paraffin (LP), and paraffin wax; 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; fatty acid alcohols having 10 to 20 carbon atoms such as palmitic acid alcohol, stearic acid alcohol, and oleic acid alcohol, and these can be used alone or in a mixture of two or more types. Among the above plasticizers, liquid paraffin is harmless to the human body, has a high boiling point, and has low volatile components, making it suitable for use as a plasticizer in the wet method.

[0053] The content of the plasticizer is not particularly limited, and for example, it may be included in an amount of 50 to 90 weight% with respect to the total weight of the composition containing the polyolefin resin and the plasticizer, and for another example, in an amount of 60 to 80 weight%.

[0054] In addition to the aforementioned polyolefin-based resin, other resins, inorganic particles, additives, etc., conventional in the relevant technical field may be included.

[0056] (2) Formation process of sheet-shaped product

[0057] The polyolefin solution prepared in step (1) above is extruded from a die equipped with an extruder and cooled to form a sheet-shaped product. The cooling method is not particularly limited, and methods known in the art, such as cooling by contacting a chill roll, cooling with cold air, or cooling by impregnating with cold water, may be used.

[0059] (3) First stretching process

[0060] The sheet shape obtained in step (2) above is biaxially stretched one or more times to form a film.

[0061] In the present invention, the first stretching process may be performed at a predetermined ratio by conventional methods in the relevant technical field, such as the tenter method, the roll method, the inflation method, the rolling method, or a combination thereof. In the case of biaxial stretching, it is acceptable to perform biaxial stretching simultaneously or sequential stretching. For example, wet stretching may be performed as the first stretching. In this case, the sheet can be made thin without tearing, which is advantageous for completely removing the plasticizer in subsequent processes, and can improve the crystallinity of the polymer.

[0062] In the first stretching process above, the stretching ratio varies depending on the thickness of the sheet-shaped workpiece, but for example, in biaxial stretching, it is appropriate to perform at least 1.5 x 1.5 times in either direction, and for example, it can be performed in a range of 1.5 to 10 times. For example, stretching in the MD direction can be performed at 1.5 to 2 times, and stretching in the TD direction can be performed at 1.5 to 2 times. When the MD stretching ratio falls within the aforementioned range, MD orientation can be suppressed to reduce MD tensile strength. When the TD stretching ratio falls within the aforementioned range, a reduction in protrusion strength can be prevented to ensure excellent compressive strength.

[0063] In the first stretching process of the present invention, the stretching temperature may be, for example, in the range of 100 to 130 ℃, and as another example, in the range of 110 to 125 ℃. When stretching within the above range, the sheet can be stretched to have appropriate air permeability and mechanical strength without blocking the pores within the sheet. If the first stretching temperature exceeds the aforementioned temperature range, the size of the pores within the sheet may increase excessively due to increased fluidity of the sheet-shaped material, and non-uniformity in the pore size distribution may occur, which may result in a decrease in the performance and safety of the battery. On the other hand, if the first stretching temperature is below the aforementioned range, the stiffness of the sheet-shaped material increases, which may cause film breakage and non-uniformity in the distribution of physical properties during the process, and non-uniformity in air permeability may occur after coating.

[0065] (4) Plasticizer removal process

[0066] The plasticizer is extracted from the stretched film using a cleaning solvent. Since the polyolefin phase is phase-separated from the plasticizer, removing the plasticizer yields a porous membrane with a plurality of pore structures. Any conventional method known in the art can be used without limitation to remove the plasticizer.

[0067] In the present invention, any organic solvent capable of extracting a plasticizer can be used as a cleaning solvent without particular limitations. 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; and 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.

[0068] Since most organic solvents used in the process of extracting plasticizers are highly volatile and toxic, water can be used to suppress the volatilization of the organic solvents if necessary.

[0070] (5) Drying process

[0071] The polyolefin microporous membrane obtained by plasticizer removal can be dried using conventional drying methods known in the art. For example, heating drying and air drying methods may be used.

[0073] (6) Second stretching process

[0074] Next, the dried film is re-stretched again in at least one uniaxial direction. In the present invention, the second stretching process can be performed using a tenter method or the like, in the same way as the first stretching process, while heating the film. The stretching may be uniaxial stretching or biaxial stretching. For example, dry stretching may be performed as the second stretching. In this case, the physical entanglement of the polymer can be improved.

[0075] The temperature of the second stretching process may, for example, be in the range of crystal dispersion temperature of the polyolefin resin constituting the microporous membrane and crystal dispersion temperature + 40°C or lower, and as another example, may be in the range of crystal dispersion temperature + 10°C or higher and crystal dispersion temperature + 40°C or lower. By controlling the second stretching temperature to the aforementioned range, a decrease in air permeability and the occurrence of deviations in physical properties in the sheet width direction when stretched in the transverse direction (width direction: TD direction) can be prevented. By keeping the second stretching temperature within the above range, deviations in air permeability resistance in the width direction of the stretched sheet can be suppressed, and at the same time, the pore size of the outer layer of the membrane can be controlled to be larger than the pore size of the inner layer.

[0076] At temperatures exceeding the above range, it may be difficult to control the pore size due to excessive melt migration of the polyolefin resin, and at temperatures below the above range, uneven cracks may occur in the polyolefin fibrils, resulting in unevenness in the outer layer pores.

[0077] 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 ℃.

[0078] In addition, by controlling the temperature of the second stretching process to the aforementioned range, the polyolefin resin can be sufficiently softened and stretched uniformly by preventing film breakage. For example, the second stretching temperature may be in the range of 90 to 140 ℃, and as another example, it may be in the range of 120 to 140 ℃.

[0079] In the second stretching process described above, the stretching ratio varies depending on the thickness of the sheet-shaped workpiece, but for example, in biaxial stretching, it is appropriate to perform at least 1.5 x 1.5 times in either direction, and for example, it can be performed in a range of 1.5 to 10 times. For example, stretching in the MD direction can be performed at 1.5 to 2 times, and stretching in the TD direction can be performed at 1.5 to 2 times. When the MD stretching ratio falls within the aforementioned range, MD orientation can be suppressed to reduce MD tensile strength. When the TD stretching ratio falls within the aforementioned range, a reduction in protrusion strength can be prevented to ensure excellent compressive strength.

[0081] (7) Heat treatment process

[0082] Next, the re-stretched film is fixed and heat-treated. The heat treatment method may be any conventional method known in the relevant technical field without limitation, and, for example, heat-setting treatment and / or heat-relaxation treatment may be used.

[0083] In particular, by performing a heat-setting treatment, the network structure composed of fibrils formed by the second stretching process is maintained and the crystals of the porous membrane are stabilized, thereby allowing for the production of a microporous membrane with appropriately controlled micropore diameters and excellent strength. 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.

[0084] Thermal relaxation treatment may be performed using a tenter method, a roll method, or a compression method, or by using a belt conveyor or a floating roll. Thermal relaxation treatment may be performed, for example, in a relaxation rate range of 20% or less in at least one direction, and in another example, in a relaxation rate range of 10% or less.

[0086] Secondary Battery

[0087] The present invention provides a secondary battery, for example, a lithium-ion secondary battery, comprising the aforementioned polyolefin microporous membrane.

[0088] The secondary battery of the present invention can be manufactured according to conventional methods known in the art, except that the polyolefin microporous membrane according to the present invention is used as a separator. For example, it can be manufactured by interposing a separator between a positive electrode and a negative electrode and introducing a non-aqueous electrolyte. Herein, regarding the components of the positive electrode, negative electrode, electrolyte, and other additives if necessary, excluding the aforementioned separator, they conform to the components of conventional secondary batteries known in the art.

[0089] For example, the above-mentioned cathode can be manufactured using a conventional cathode active material for secondary batteries known in the art, and non-limiting examples thereof include lithium transition metal composite oxides such as LiMxOy (M = Co, Ni, Mn, CoaNibMnc) (e.g., lithium manganese composite oxide such as LiMn2O4, lithium nickel oxide such as LiNiO2, lithium cobalt oxide such as LiCoO2, and those in which some of the manganese, nickel, and cobalt of these oxides are substituted with other conventional transition metals or aluminum, or vanadium oxide containing lithium, etc.) or chalcogen compounds (e.g., manganese dioxide, titanium disulfide, molybdenum disulfide, etc.).

[0090] For example, the above-mentioned cathode can be manufactured using a conventional cathode active material for secondary batteries known in the art, and non-limiting examples thereof include materials capable of intercalating / deintercalating lithium ions, such as lithium metal or lithium alloy, coke, artificial graphite, natural graphite, combustion of organic polymer compounds, carbon fiber, silicon-based, tin-based, etc.

[0091] Non-aqueous electrolytes include electrolyte components commonly known in the art, such as electrolyte salts and electrolyte solvents.

[0092] The above electrolyte salt may be composed of a combination of (i) a cation selected from the group consisting of Li+, Na+, and K+ and (ii) anion selected from the group consisting of PF6-, BF4-, Cl-, Br-, I-, ClO4-, AsF6-, CH3CO2-, CF3SO3-, N(CF3SO2)2-, and C(CF2SO2)3-, among which a lithium salt is preferred. Specific examples of lithium salts include LiClO4, LiCF3SO3, LiPF6, LiBF4, LiAsF6, and LiN(CF3SO2)2. These electrolyte salts may be used alone or in a mixture of two or more types.

[0093] Cyclic carbonates, linear carbonates, lactones, ethers, esters, acetonitrile, lactams, and ketones may be used as the above electrolyte solvents.

[0094] Examples of the above-mentioned cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), etc. Examples of the above-mentioned linear carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), etc. Examples of the above-mentioned lactone include gamma-butyrolactone (GBL), and examples of the above-mentioned ethers include dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, etc. Examples of the above esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, butyl propionate, methyl pivalate, etc. Additionally, the above lactam includes N-methyl-2-pyrrolidone (NMP), etc., and the above ketone includes polymethylvinyl ketone. Furthermore, halogen derivatives of the above organic solvent may be used, but are not limited thereto. In addition, glyme, deglyme, triglyme, and tetraglyme may be used as the above organic solvent. These organic solvents may be used alone or in a mixture of two or more types.

[0096] The present invention will be explained in more detail below through examples. However, the following examples are intended only to aid in understanding the invention and do not limit the scope of the invention in any way.

[0098] [Examples 1-3]

[0099] Microporous membranes of each example were prepared using the polyolefins of Table 1 below. During the preparation of the microporous membranes, two stretching processes were performed, and the first and second stretching processes were carried out at the same magnification ratio, as shown in Table 1. According to the following method, the molecular weight of the maximum peak in the differential molecular weight distribution curve by Gel Permeation Chromatography (GPC) of the microporous membranes according to each example, the melting point of the maximum peak on the melting curve obtained at the second heating step by Differential Scanning Calorimetry (DSC), and △Tm (Tm2-Tm1) were measured, and the results are shown in Table 1 below.

[0101] [Comparative Example 1-4]

[0102] Microporous membranes of each comparative example were prepared in the same manner as the examples, except for those specified in Table 1 below. In the same manner as the examples, the molecular weight of the maximum peak in the differential molecular weight distribution curve according to GPC, the melting point of the maximum peak on the melting curve obtained at the second heating step using Differential Scanning Calorimetry (DSC), and △Tm (Tm2-Tm1) were measured, and the results are shown in Table 1 below.

[0104] GPC maximum peak molecular weight

[0105] Under the following conditions, the derivative of the curve was obtained from the logarithm of each molecular weight according to the gel permeation chromatography (GPC) method, and the logarithm of the molecular weight log(M) was plotted on the horizontal axis and the value obtained by differentiating the concentration fraction by the logarithm of the molecular weight dw / d log(M) was plotted on the vertical axis to obtain a differential molecular weight (weight-average molecular weight) distribution curve. From the obtained differential molecular weight distribution curve, the molecular weight at which the value obtained by differentiating the concentration fraction by the logarithm of the molecular weight dw / d log(M) is maximum was calculated as the maximum peak molecular weight.

[0106] · Measuring device: GPC-150C (WATERS CORPORATION)

[0107] · Column: SHODEX UT806M (Showa Denko Co., Ltd.)

[0108] · Column temperature: 135 ℃

[0109] · Solvent (Mobile Phase): O-Dichlorobenzene

[0110] · Solvent flow rate: 1.0 mL / min

[0111] · Sample concentration: 0.1 wt% (Dissolution conditions: 135 ℃ / 1H)

[0112] · Injection volume: 500 μL

[0113] · Detector: Differential Refractometer (RI Detector, WATERS CORPORATION)

[0114] · Calibration curve: Obtained using monodisperse polystyrene standard samples

[0116] DSC 2nd heat Tm

[0117] The maximum peak top melting point on the melting curve obtained during the second heating step was measured using Differential Scanning Calorimetry (DSC).

[0119] △Tm

[0120] After measuring the maximum peak top melting point (Tm2) on the melting curve obtained during the second heating step by DSC and the maximum peak top melting point (Tm1) on the melting curve obtained during the first heating step, △Tm(Tm2 - Tm1) was calculated.

[0122]

[0123] PO 1: Ultra-high molecular weight polyethylene (UHMWPE), weight-average molecular weight (Mw) 1.5 x 10⁻⁶ 6 g / mol, Tm 132.4 ℃

[0124] PO 2: UHMWPE, Mw 1.5 x 106 g / mol, Tm 131.6 ℃

[0125] PO 3: UHMWPE, Mw 1.5 x 10 6 g / mol, Tm 134 ℃

[0126] PO 4: UHMWPE, Mw 2.4 x 10 6 g / mol, Tm 136 ℃

[0127] PO 5: Polyethylene (PE), Mw 7 x 10 4 g / mol, Tm 133 ℃

[0128] PO 6: Polypropylene (PP), Mw 3.8 x 10 5 g / mol, Tm 137 ℃

[0130] [Physical Property Evaluation]

[0131] The physical properties of the microporous membranes of each example and comparative example were measured using the following method, and the results are shown in Table 2 below.

[0133] MD strength

[0134] Tensile strength was measured at a speed of 100 mm / min using an Instron tensile strength tester.

[0136] Public ratio

[0137] The public ratio was calculated using the following formula.

[0138] Public share rate (%) = [(w2 - w1) / w2] × 100

[0139] w1: Mass of the polyolefin microporous membrane of each example and comparative example

[0140] w2: Mass of a pore-free membrane of the same size made of the polyolefin composition used in each example and comparative example

[0142] Protrusion strength

[0143] The maximum load was measured when each microporous membrane with a thickness T1 (㎛) was pierced at a speed of 2 mm / sec using a needle with a diameter of 1 mm and a spherical tip (radius of curvature R: 0.5 mm). The measured maximum load L1 (gf) was converted into the maximum load L2 per unit volume using the formula: L2 = L1 / M1 and was used as the protrusion strength per unit mass.

[0145] airway

[0146] Air permeability was measured according to JISP8117 using an Oken-type air resistance meter (Asahi Seiko Co. Ltd., EGO-1T).

[0148] Average Attack and Bubble Point (BP) Attack

[0149] The average pore size and bubble point (BP) pore size were measured using a capillary flow porometer (Porous Materials, Inc., CFP-1500A). Galwick (15.9 dynes / com) was used as the reagent for pore size measurement, and after installing damage-prevention filter paper (ADVANTEC 5A) in the sample holder, each microporous membrane sample (size: Φ25) was inserted. The procedure was carried out in the order of measuring in dry pressurization mode followed by wet pressurization mode (pressure: 0–2,000 KPa, pressurization rate: 20 cc / min).

[0151] Shutdown temperature

[0152] While heating the microporous membranes of each example and comparative example at a heating rate of 5 °C / min, the air resistance was measured using an air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T), and the temperature at which the air resistance reached the detection limit (1.0 × 10⁵ sec / 100 cm³ Air) was determined and set as the shutdown temperature (°C) by the heating air resistance method. The measurement cell was constructed of an aluminum block and had a structure with a thermocouple located directly below the microporous membrane. The sample was cut into a square with sides of 50 mm × 50 mm, and the temperature was measured while fixing the periphery with an O-ring.

[0154] Curvature rate

[0155] The curvature rate was calculated using the following formula.

[0156] Curvature τ = (d × (ε / 100) × 500 / (3 × L × 10¹³⁰ × Rgas)) 0.5

[0157] d: Average pore size measured by a phorometer (㎛)

[0158] ε: Public rate (%)

[0159] L: Film thickness (㎛)

[0160] Gas Permeability Constant (Rgas) (㎥ / (㎡·sec·Pa)): 0.0001 / (G×(6.424×10⁻⁶) -4 )×(0.01276×101325))

[0161] G: Gulli (Tugido, sec / 100㎤)

[0163]

[0164] As shown in Table 2 above, the polyolefin microporous membrane of Examples 1-3 according to the present invention exhibited excellent physical properties across all measured parameters. On the other hand, the polyolefin microporous membrane of Comparative Example 1-4, in which at least one of the following falls outside the scope of the present invention—namely, the molecular weight of the maximum peak in the differential molecular weight distribution curve by gel permeation chromatography (GPC), the melting point of the maximum peak on the melting curve obtained during the second heating step by differential scanning calorimetry (DSC), and the difference between the melting point of the maximum peak on the melting curve obtained during the second heating step by DSC (Tm2) and the melting point of the maximum peak on the melting curve obtained during the first heating step (Tm1) (△Tm = Tm2 - Tm1)—exhibited inferior physical properties across all measured parameters compared to Examples 1-3.

Claims

Claim 1 The molecular weight of the maximum peak in the differential molecular weight distribution curve according to Gel Permeation Chromatography (GPC) is 3 x 10 5 A polyolefin microporous membrane having a g / mol or higher, a maximum peak top melting point on the melting curve obtained during the second heating step by differential scanning calorimetry (DSC) of 133 °C or lower, a difference (△Tm = Tm2 - Tm1) between the maximum peak top melting point on the melting curve obtained during the second heating step by DSC and the maximum peak top melting point on the melting curve obtained during the first heating step (Tm1) of 8 °C or lower, and a protrusion strength per unit mass of 90 gf / (g / ㎡) or higher. Claim 2 A polyolefin microporous membrane according to claim 1, wherein the average pore size measured using a pore analyzer (Porometer) is 30 nm or less, the bubble point (BP) pore size is 40 nm or less, and the difference between the maximum pore size and the average pore size is 13 nm or less. Claim 3 delete Claim 4 A polyolefin microporous membrane according to claim 1, wherein the shutdown (SD) temperature is 143 ℃ or lower, the porosity is 35% or higher, the air permeability is 90 sec / 100cc or lower, and the curvature is 1 to 3. Claim 5 In claim 1, the polyolefin has a weight-average molecular weight of 1 x 10 6 Up to 1.5 x 10 6 A first polyolefin having g / mol and a weight-average molecular weight of 1 x 10 5 Polyolefin microporous membrane comprising a second polyolefin with a g / mol or less. Claim 6 In claim 5, a polyolefin microporous membrane having a melting point of the first polyolefin of 125 to 133 ℃. Claim 7 In claim 5, a polyolefin microporous membrane comprising 50 to 90 weight% of the first polyolefin and 10 to 50 weight% of the second polyolefin, based on the total weight of the polyolefin. Claim 8 A secondary battery comprising a polyolefin microporous membrane according to any one of claims 1, 2 and 4 through 7.

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