Porous substrate for separation membrane and separation membrane for electrochemical device including the same

Abstract

The present invention relates to a porous substrate for a separation membrane. The porous substrate according to the present invention has small and uniform pores, and therefore has excellent physical strength and durability, and can ensure a high breakdown voltage, resulting in a low incidence of short circuits.

Description

[Technical field]

[0001] This application claims priority to Korean Patent Application No. 10-2021-0059551, filed on May 7, 2021. The present invention relates to a separator for an electrochemical device, the electrochemical device may be a primary battery or a secondary battery, and the secondary battery may include a lithium ion secondary battery. [Background technology]

[0002] Polyolefin microporous membranes are widely used for battery separators, including lithium batteries, diaphragms for electrolytic capacitors, moisture-permeable waterproof clothing, various filtration membranes, etc. When such polyolefin microporous membranes are used as battery separators, their performance is closely related to the characteristics, productivity, and safety of the battery. Therefore, in addition to excellent mechanical properties and permeability, lithium-ion battery separators in particular are required to have the following characteristics: a shutdown characteristic (whereby the pores are blocked by abnormal heat generation to stop the battery reaction) in order to prevent the battery from overheating, ignition, and explosion caused by short circuits in external circuits, overcharging, etc.; and a heat shrinkage resistance (whereby the shape is maintained even at high temperatures to prevent dangerous situations in which the positive and negative electrode materials directly react with each other).

[0003] Generally, a microporous membrane made of only polyethylene has a low meltdown temperature, and a microporous membrane made of only polypropylene has a high shutdown temperature, so battery separators made of microporous membranes mainly composed of polyethylene and polypropylene have been proposed.

[0004] For example, Japanese Patent No. 3235669 discloses a battery separator having excellent heat shrinkage resistance and shutdown characteristics, the battery separator having at least one first layer formed of a polymer selected from low-density polyethylene, an ethylene-butene copolymer, or an ethylene-hexene copolymer, and at least one second layer formed of a polymer selected from high-density polyethylene, ultra-high molecular weight polyethylene, or polypropylene.

[0005] Japanese Patent No. 3422496 discloses a battery separator having excellent shutdown characteristics, the battery separator having at least one first layer formed of a polymer selected from an ethylene-butene copolymer, an ethylene-hexene copolymer, an ethylene-methacrylate copolymer, or polyethylene, and at least one second layer formed of a polymer selected from polyethylene or polypropylene.

[0006] Japanese Patent No. 2883726 discloses a battery separator with excellent shutdown and meltdown properties, which is formed by simultaneously extruding polypropylene with a melting point of 150°C or higher and polyethylene with a melting point of 100°C to 140°C, uniaxially stretching the resulting laminated film at a temperature of -20°C [melting point of polyethylene (Tm0) - 30]°C, and further stretching in the same direction at a temperature of (Tm0-30)°C to (Tm0-2)°C to make it porous.

[0007] Japanese Patent Application Laid-Open No. 11-329390 proposes a battery separator with excellent shutdown characteristics and strength, which is formed from two microporous strength layers made of polypropylene material and a blocking layer made of a filler-containing polyethylene material interposed between them, with the blocking layer made of a filler-containing polyethylene material being formed from a microporous membrane manufactured by a particle stretching method.

[0008] The separator is bonded to the electrode to manufacture the battery, and the bonding is performed by a lamination process in which the electrode and the separator are laminated and then heat and / or pressure are applied. The higher the heat and pressure conditions applied in the lamination process, the higher the adhesive strength with the electrode. In recent years, in order to improve productivity, the process speed has been increased to shorten the time that heat is applied to the separator, so the adhesive strength is secured by increasing the pressure to secure the adhesive strength, but there is a problem that the separator is deformed by high pressure. In particular, when a porous substrate for the separator that is weak against heat and pressure is used, the thickness is significantly reduced and the pores are significantly damaged, which reduces not only the performance of the battery but also the dielectric breakdown voltage of the separator, which may result in Hi-pot defects and low voltage defects. Therefore, it is necessary to develop a porous polymer film substrate for the separator that is less deformed even under high pressure lamination conditions. Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention provides a porous substrate for a separator having a low thickness deformation rate and a high dielectric breakdown voltage, and a separator including the same. It will be readily understood that the objects and advantages of the present invention can be achieved by the means or methods described in the claims and combinations thereof. [Means for solving the problem]

[0010] The present invention relates to a porous substrate for a separation membrane, the porous substrate comprising polyethylene and / or polypropylene, the substrate having a porous characteristic including a plurality of pores therein, the porosity being 30 vol% to 60 vol%, and the pore distribution being such that the full width at half maximum (FWHM) value of the Gaussian distribution measured through a pore size distribution is 4.0 nm or less.

[0011] In a second aspect of the present invention, in the first aspect, the pore distribution has a half-width value of 3.0 nm or less.

[0012] A third aspect of the present invention is the first or second aspect, wherein the difference between the maximum pore size (Mps) and the average pore size (mps) is 30 nm.

[0013] A fourth aspect of the present invention is any one of the first to third aspects, wherein the difference between the maximum pore size (Mps) and the average pore size (mps) is 20 nm.

[0014] A fifth aspect of the present invention is the third aspect, wherein the average pore size (mps) is 10 nm to 100 nm.

[0015] A sixth aspect of the present invention is the third aspect, wherein the average pore size (mps) is 20 nm to 30 nm.

[0016] A seventh aspect of the present invention is any one of the first to sixth aspects, wherein the porous substrate has a BET of 20m 2 / g~60m 2 / g.

[0017] An eighth aspect of the present invention is any one of the first to seventh aspects, wherein the porous substrate has a thickness of 5 μm to 20 μm.

[0018] A ninth aspect of the present invention is any one of the first to eighth aspects, wherein the polymer resin contains 90 wt % or more of a polyolefin resin with respect to 100 wt % of the polymer resin.

[0019] A tenth aspect of the present invention is any one of the first to ninth aspects, wherein the polyolefin resin has a PDI (poly dispersity index) value of 2.5 to 6.5.

[0020] A tenth aspect of the present invention relates to a separator for an electrochemical element, the separator comprising a porous substrate for a separator according to any one of the first to ninth aspects and a heat-resistant layer formed on one or both surfaces of the porous substrate, the heat-resistant layer comprising a binder resin and inorganic particles.

[0021] An eleventh aspect of the present invention relates to an electrochemical device, comprising a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode, the separator being related to the tenth aspect. Effect of the Invention

[0022] The porous substrate according to the present invention has small pores and uniform pore distribution. When used in a separator, the substrate exhibits little thickness deformation and high breakdown voltage when the separator and an electrode are laminated together and subjected to a lamination process. [Brief description of the drawings]

[0023] The drawings attached to this specification are illustrative of preferred embodiments of the present invention and serve to facilitate a better understanding of the technical concept of the present invention together with the above-mentioned contents of the invention, and therefore the present invention should not be interpreted as being limited to only the matters shown in such drawings. Meanwhile, the shape, size, scale, ratio, etc. of elements in the drawings described in this specification may be exaggerated in order to emphasize a clearer description.

[0024] [Figure 1] 1 is a schematic diagram showing a state of deformation due to compression in a cross section of a porous substrate of a comparative example. [Diagram 2] 1 is a cross-sectional view of a porous substrate according to an embodiment, showing a schematic diagram of a deformation state due to compression. [Diagram 3] 1 shows an exemplary Gaussian distribution of pores for determining the half-width of the pore size. [Figure 4] 1 is a graph showing an example of a dry sample curve, a wet sample curve, and a 1 / 2 dry sample curve that can be obtained through a porometer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The present invention will be described in detail below. Prior to that, the terms or words used in the present specification and claims should not be interpreted limited to their ordinary or dictionary meanings, but should be interpreted with meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term to best describe his / her invention. Therefore, it should be understood that the embodiments described in this specification and the configurations shown in the drawings are merely the most preferred embodiments of the present invention, and do not fully represent the technical idea of ​​the present invention, and therefore there may be various equivalents and modifications that can replace them at the time of this application.

[0026] Throughout this specification, when a part is said to "comprise" certain elements, this means that it can further include other elements, rather than excluding other elements, unless specifically stated to the contrary.

[0027] In addition, the terms "about," "substantially," and the like, used throughout this specification, when given manufacturing and material tolerances inherent in the referred meaning, are used to mean a numerical value or a approximation thereof, and are used to prevent unconscionable infringers from unfairly taking advantage of the disclosure in which precise or absolute numerical values ​​are mentioned to aid in the understanding of this application.

[0028] Throughout this specification, the term "A and / or B" means "A or B or both."

[0029] The present invention relates to a porous substrate applicable to a separator for an electrochemical device. In the present invention, the electrochemical device is a device that converts chemical energy into electrical energy by an electrochemical reaction, and includes a primary battery and a secondary battery. The secondary battery is capable of being charged and discharged, and includes a lithium ion battery, a nickel-cadmium battery, a nickel-hydrogen battery, etc.

[0030] separation membrane In the present specification, the separator serves as a porous ion-conducting barrier that allows ions to pass while blocking electrical contact between the negative and positive electrodes in an electrochemical device. It is preferable that a plurality of pores are formed therein and the pores are interconnected so that gas or liquid can pass from one side of the separator to the other side.

[0031] The separation membrane according to the present invention includes a porous substrate made of a polymer material having a plurality of pores. In addition, the separation membrane may further include another layer disposed on at least one surface of the porous substrate in terms of material or function, if necessary. In one embodiment of the present invention, the separation membrane may include the porous substrate and a heat-resistant layer, and for example, the heat-resistant layer may be formed on at least one surface of the porous substrate. The heat-resistant layer may include inorganic particles and / or a binder resin.

[0032] In one embodiment of the present invention, the inorganic particles in the heat-resistant layer may have a layered structure bound by a binder resin, and may exhibit a porous structure resulting from spaces (interstitial volumes) formed between the inorganic particles. Such a porous structure has the effect of improving the electrolyte retention of the separator. Meanwhile, in one embodiment of the present invention, when the separator includes a heat-resistant layer, the heat-resistant layer may be 3 vol% to 40 vol% relative to 100 vol% of the total volume of the separator, and simultaneously or independently, the heat-resistant layer may be 5 vol% to 50 vol% relative to 100% of the total thickness of the separator.

[0033] porous substrate In one embodiment of the present invention, the porous substrate may be in the form of a porous sheet containing a polymer resin and having a plurality of pores. The pores include open pores, and the open pores are connected to each other, so that gas or liquid can pass from one side of the porous substrate to the other side. In one embodiment of the present invention, the porous substrate preferably has an air permeability of 2000 sec / 100 ml or less and a porosity of 30 vol% to 60 vol% in terms of battery output and cycle characteristics.

[0034] In the present invention, the air permeability (sec / 100 ml) means the time (seconds) required for 100 ml of air to pass through a sample such as a porous substrate or a separation membrane having a size of 1 square inch under a certain air pressure. In one embodiment of the present invention, the air permeability can be measured according to the standard regulations in the technical field. For example, the air permeability can be measured using a known Gurley Densometer according to ASTM D726-58 or ASTM D726-94, and for example, air at a pressure of 0.304 (kPa) or 1.215 kN / m 2 of water pressure in one square inch (or 6.54 cm 2In another embodiment, the air permeability is measured at room temperature at 4.8 inches H according to the Gurley method of the Japanese Industrial Standard (JIS-P8117). 2 The time taken for 100 ml of air to pass through 1 square inch of sample under a constant pressure of O can be measured and expressed in seconds. In one embodiment of the present invention, the air permeability can be measured, for example, using an Asahi Seiko EG01-55-1MR instrument according to the standard.

[0035] The porosity means the ratio of the volume occupied by pores to the total volume, and is expressed in units of vol%. It can be used interchangeably with terms such as porosity and net density. In the present invention, the measurement of the porosity is not particularly limited, and a method known in the art can be applied. For example, it can be measured by BET (Brunauer-Emmett-Teller) measurement method using nitrogen gas, water intrusion porosimeter method, capillary flow porometer, or mercury penetration method (Hg porosimeter). Alternatively, in one embodiment of the present invention, the true density of the porous substrate is calculated from the density (apparent density) of the obtained porous substrate, the composition ratio of the materials contained in the porous substrate, and the density of each component, and the porosity of the porous substrate can be calculated from the difference between the apparent density and the net density. For example, the porosity can be calculated by the following [Equation 1].

[0036] [Formula 1] Porosity (vol%) = {1-(apparent density / true density)} x 100

[0037] On the other hand, in the above formula, the apparent density can be calculated from the following [Formula 2].

[0038] [Formula 2] Apparent density (g / cm3 ) = {Weight of porous substrate [g] / (Thickness of porous substrate [cm] × Area of ​​porous substrate [cm 2 ])}

[0039] In the present invention, the porous substrate has small and uniform pores, and when used as a separator for an electrochemical device, it can exhibit excellent dimensional stability and voltage resistance.

[0040] In the present invention, the porous substrate preferably has a maximum pore size (Mps) of 80 nm or less, and a difference between the maximum pore size (Mps) and the average pore size (mps) of 30 nm or less, preferably 20 nm or less. The average pore size (mps) is preferably in the range of 10 nm to 100 nm.

[0041] In one embodiment of the present invention, the pore size can be calculated from the pore size distribution measured using a capillary flow porometer. The capillary flow porometer can be measured using a porometer manufactured by Porous Materials Inc. and Galwick solution, and the measurement method described below can be referred to.

[0042] As a specific example, a porometer is used to measure the relationship between air pressure and flow rate for a porous substrate in a dry state (dry sample) and a porous substrate in a wet state (wet sample), and an air permeability curve for the dry sample (dry curve) and a air permeability curve for the wet sample (wet curve) are obtained as shown in Figure 4, from which the size and distribution of the pores can be confirmed.

[0043] The capillary flow porosity measurement method is to measure the pore size by the pressure required to expel the wetting solution filling the pores of the porous substrate by wetting the porous substrate with a wetting solution having low surface tension and then pressurizing the substrate with gas. For example, the porous substrate to be measured is wetted with a wetting solution such as Galwick solution, and the air pressure on one side of the porous substrate is gradually increased. At this time, when the applied air pressure becomes greater than the capillary attraction of the wetting solution present in the pores, the wetting solution filling the pores is expelled, and the size and distribution of the pores can be measured by the pressure and flow rate at the moment of expulsion. In one embodiment of the present invention, a non-reactive gas can be used instead of the air.

[0044] In one specific embodiment of the present invention, the measurement can be performed at a measurement pressure in the range of 0 to 3500 MPa, and within the range, the minimum air pressure can be 30 psi or more and the maximum air pressure can be 500 psi or less.

[0045] The minimum air pressure may refer to the bubble point pressure. The bubble point refers to the pressure at which a pressure curve begins to be drawn in a capillary flow porometer, and may reflect the maximum pore size among the pore diameters of a porous substrate. That is, when the air pressure is gradually increased, the wetting liquid filled in the pores of the separation membrane substrate is expelled and moves in the order of pores with larger diameters, and the air flow rate gradually increases accordingly, and the sample is finally dried. Here, the pressure at which the wetting liquid begins to move is called the bubble point pressure.

[0046] Meanwhile, the maximum air pressure may be the pressure when the sample finally becomes dry, and may reflect the minimum pore size. Alternatively, the maximum air pressure may refer to the pressure at the point where the pressure curve (wet curve) of a capillary flow porometer measured using the wetting liquid intersects with the air permeability curve of a dry sample. The "dry curve" refers to the pressure distribution required to expel the existing gas filling the pores by pressurizing a dry separation membrane substrate not infiltrated with a wetting liquid using air or a non-reactive gas.

[0047] FIG. 4 shows the pressure curves of a wet sample and a dry sample measured using a capillary flow porometer. The point where the pressure curves of the wet sample and the dry sample intersect can be regarded as the maximum air pressure.

[0048] On the other hand, in the present invention, the condition for air at pressure P to enter a pore of diameter D can be expressed by the Washburn equation shown below in [Equation 3], where the surface tension of the wetting liquid is γ and the contact angle of the wetting liquid is θ. P = (4γcosθ) / D……(Equation 3)

[0049] Therefore, by measuring the pressure at which the wetting liquid is squeezed out of the pores, the diameter of the pores can be calculated.

[0050] On the other hand, when the air flow rate of the wet sample at the pressure Pj is Fw,j and the air flow rate of the dry sample is Fd,j, the cumulative filter flow rate (CFF, unit: %) and the pore size distribution (PSF, unit: %) are calculated by the following Equation 4 and Equation 5, respectively. CFF=[(Fw, j / Fd, j)×100]……(Formula 4) PSF=(CFF)j+1-(CFF)j……(Formula 5)

[0051] By combining the above formulas 3 to 5, a pore size distribution curve showing the relationship between the pore diameter D and the pore size distribution PSF can be obtained based on the pressure change of the air flow rate in the dry and wet states. An example of such a pore size distribution curve is shown in Figure 3. Various physical property values ​​related to the pores can be obtained from the pore size distribution curve shown in Figure 3.

[0052] In one embodiment of the present invention, the bubble point may represent the maximum diameter of the pores, and the intersection point of the wet sample curve with the dry sample curve may represent the minimum diameter of the pores, and the intersection point of the wet sample curve with the 1 / 2 dry sample curve, which is half the value of the dry sample air permeability curve, may represent the average pore diameter (see FIG. 4).

[0053] In addition, in the present invention, the porous substrate has a full width at half maximum peak height (FWHM) of 4.0 nm or less, 3.0 nm or less, 2.0 nm or less, or 1.5 nm or less in a pore diameter distribution according to a normal distribution (Gaussian distribution) measured through the pore size distribution (see FIG. 3). It may be preferably 3.0 nm or less. The full width at half maximum may be defined as the difference in size between two points on the x-axis that are half the maximum value on the y-axis (the most frequent value among the pore sizes) in a normal distribution for a pore size distribution shown by classifying pores formed inside a porous substrate according to their size. In the distribution, the x-axis represents the size (diameter) of the pores, and the y-axis represents the frequency of the number of pores corresponding to the pore size on the x-axis (e.g., the number of pores or the percentage of the number of pores). In the present invention, the unit of the x-axis may be expressed in nm or μm. Referring to FIG. 3, the full width at half maximum (FWHM) is the two points on the x-axis that correspond to 1 / 2 (1 / 2T) of the mode (T) on the y-axis. 2 and X 1 The difference can be expressed as an absolute value. Meanwhile, the normal distribution can represent symmetric and asymmetric distributions based on the maximum value, and any distribution other than normal.

[0054] In a preferred embodiment of the present invention, the half-width can be determined from the distribution of the remaining pores excluding the pores having the maximum diameter. Meanwhile, in the present invention, the shape of the pores can be circular, elliptical or amorphous, and the cross section can be a closed curve. The diameter of the pores means the longest distance between any two points in the closed curve. When the above range is satisfied, the porous substrate has a small pore size and a high uniformity of the pore size. A porous substrate having such characteristics can exhibit high dimensional stability and high dielectric breakdown voltage.

[0055] On the other hand, the porous substrate has a BET surface area of ​​20 m 2 / g~60m 2 The larger the BET surface area, the higher the porosity and the smaller the pore size. Also, for the same porosity, the smaller the pore size, the larger the BET surface area. In the present invention, the BET surface area can be measured using the adsorption equation of the BET (Brunauer, Emmett and Teller) model, and when the adsorption isotherm is measured at the boiling point of nitrogen at -196°C up to 1 bar (1 atmosphere), the measured N 2 The amount of adsorption can be measured and calculated from the adsorption isotherm.

[0056] In the present invention, the porous substrate may have a thickness of 5 μm to 20 μm in terms of thinning the electrochemical device and increasing the energy density. If the thickness of the porous substrate is less than this range, the conductive barrier function is insufficient, whereas if the thickness is too much greater than this range (i.e., too thick), the resistance of the separator may increase excessively.

[0057] Meanwhile, in one embodiment of the present invention, the polymer resin may contain a thermoplastic resin having a melting point of less than 200° C. from the viewpoint of imparting a shutdown function, and may preferably contain one or more polyolefin-based resins. The shutdown function refers to a function in which, when the battery temperature becomes high, the polymer resin melts and blocks the pores of the porous substrate, thereby blocking the movement of ions between the positive electrode and the negative electrode, thereby preventing thermal runaway of the battery.

[0058] Examples of the polyolefin resin include polyethylene, polypropylene, polybutene, polypentene, etc., and may include one or a mixture of two or more of them. As a specific example, the polyolefin resin may include two or more selected from polyethylene, polypropylene, and polypentene. As another example, the polyolefin resin may be polyethylene and / or polypropylene.

[0059] In a specific embodiment of the present invention, the porous substrate contains polyethylene and may contain polypropylene as necessary. In this case, the content of polypropylene is 0 wt% to 5 wt% relative to 100 wt% of the substrate. For example, the content of polypropylene may be less than 5 wt%. Meanwhile, in one embodiment of the present invention, the polyethylene may have a weight average molecular weight (Mw) of 300,000 g / mol to 1,800,000 g / mol, preferably 300,000 g / mol to 1,500,000 g / mol, or 300,000 mol to 1,000,000 mol, or 300,000 mol to 500,000 mol, in order to realize the compressibility range described later.

[0060] Meanwhile, in the present invention, the polyolefin resin preferably accounts for 90 wt % or more, or 95 wt % or more of 100 wt % of the polymer material.

[0061] The polyolefin resin may include, for example, one or a mixture of two or more selected from the group consisting of polyethylene, polypropylene, polybutene, and polypentene. In particular, the polyolefin resin may be polyethylene and / or polypropylene.

[0062] Meanwhile, in a specific embodiment of the present invention, the polymer resin preferably has a polydispersity index (PDI) value in the range of 2.5 to 6.5. In one embodiment, when two or more polymer resins are mixed, the mixed polymer resin can satisfy the PDI. In a more preferred embodiment, the polymer resin preferably contains a content of any one type of polymer resin as a single component in a range satisfying the PDI, based on 100 wt% of the polymer material, of more than 50 wt%, 70 wt% or more, or 90 wt% or more. For example, the polymer resin may be composed of only a single component. The single component means that the chemical structure (particularly, the repeating unit) of the polymer resin is the same, and the PDI satisfies the range of 2.5 to 6.5.

[0063] In one specific embodiment of the present invention, the polymer material may contain 90 wt % or more of polyethylene having a PDI of 2.5 to 6.5 relative to 100 wt % of the polymer material, or may consist solely of such polyethylene.

[0064] When the PDI value is satisfied or the polymer resin content range is satisfied together with or independently of the PDI value, the uniformity of the pore size and distribution in the polymer substrate is improved. Weight average molecular weight (Mw) / Number average molecular weight (Mn) It can be calculated from the ratio of

[0065] Meanwhile, in the present invention, the weight average molecular weight (Mw) and number average molecular weight (Mn) can be measured by gel permeation chromatography (GPC, PL GPC220, Agilent Technologies), and the measurement conditions can be set as follows. - Column: PL Olexis (Polymer Laboratories) -Solvent: TCB (Trichlorobenzene) -Flow rate: 1.0ml / min -Sample concentration: 1.0mg / ml -Injection volume: 200μl -Column temperature: 160°C -Detector: Agilent High Temperature RI detector -Standard: Polystyrene (corrected by a cubic function)

[0066] In another embodiment, the polymeric material may have a melting index (MI) in the range of 0.02 g / 10 min to 1.0 g / 10 min, the MI being based on conditions where a load of 21.6 kg is applied at 190°C.

[0067] In one embodiment of the present invention, the porous substrate may contain polypropylene, and the polypropylene content in the porous substrate is preferably controlled to 5 wt% or less, for example, less than 5 wt%. The higher the polypropylene content, the lower the polymer crystallinity, and thus the compression ratio and permanent deformation rate do not decrease even if the porosity is high and the penetration strength is low, and the failure rate of Hi-Pot, which is a voltage resistance characteristic, can be maintained low. However, if the polypropylene content exceeds the above range, it is chemically unstable and pores are not formed well when manufacturing a porous substrate by a wet method, which is disadvantageous for the development of porous characteristics, so the content is preferably appropriately controlled within the above range.

[0068] In addition, when the polypropylene content is high, it is easier to produce a porous substrate with well-developed pores by a dry method rather than a wet method, but it is more difficult to control the thickness of the porous substrate to be thinner when a dry method is applied than when a wet method is applied.

[0069] Meanwhile, in a specific embodiment of the present invention, the porous substrate may further include at least one polymer resin, such as polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene naphthalene, as necessary.

[0070] In one embodiment of the present invention, the polymeric porous substrate satisfying the above thickness range can be manufactured by a wet manufacturing method using polyethylene.

[0071] Meanwhile, in a specific embodiment of the present invention, the porous substrate may be a porous polymer film prepared by the method described below, and may be a single monolayer film or a multilayer film formed by laminating two or more films.

[0072] In the present invention, a separator satisfying the above values ​​improves the withstand voltage characteristics of a battery, increasing the dielectric breakdown voltage and reducing the rate of short circuit occurrence (Hi-Pot failure rate) even under high voltage conditions.

[0073] Meanwhile, in the present invention, the dielectric breakdown voltage is the maximum voltage that an insulator can withstand, and dielectric breakdown means that when a voltage is applied to an insulator, if the voltage exceeds a certain value, the insulator breaks down and loses its insulating properties.

[0074] In one embodiment of the present invention, the withstand voltage characteristics can be confirmed by measuring the dielectric breakdown voltage of the separator, which can be confirmed by disposing a separator, which is an insulator, between two conductors, applying a voltage, and measuring the voltage at which dielectric breakdown occurs.

[0075] Such dielectric breakdown voltage can be measured, for example, with an AC / DC / IR Hi-Pot tester. For example, a stainless steel mesh and a porous substrate are hot-pressed together at 90°C, 4 MPa, and 1 sec, and then a DC current of 0.5 mA and a voltage rise of 100 V / s (voltage 3 kV, ramp up time 3 s) are set. When the experiment is started, the measurement is completed when the voltage rises and a short circuit occurs, and the voltage at that time is defined as the dielectric breakdown voltage.

[0076] In one embodiment of the present invention, the short circuit occurrence rate (Hi-Pot failure rate) can be evaluated by determining the voltage exhibited by the bottom 1% of test pieces exhibiting low breakdown voltage through a Weibull distribution analysis among the total number of test pieces tested.

[0077] Method for producing porous substrate In one embodiment of the present invention, the porous substrate can be manufactured by a method for manufacturing a polymer film, preferably a wet manufacturing method, for example, the wet manufacturing method includes the steps of (S1) preparing a mixture, (S2) extruding the mixture and forming an extruded sheet, (S3) stretching the extruded sheet, (S4) removing the pore-forming agent, and (S5) heat fixing the extruded sheet.

[0078] In the step (S1), the type of polymer resin is appropriately selected according to the final properties of the separator, and the selected polymer resin is mixed with a pore-forming agent. The polymer resin may be the polymer resin of the porous substrate described above. For example, the polymer resin may be a polyolefin-based polymer resin. Examples of the polyolefin-based polymer resin include one selected from polyethylene, such as high density polyethylene, linear low density polyethylene, low density polyethylene or ultra-high molecular weight polyethylene, polypropylene, polybutylene, polypentene, etc., or a combination of two or more of them.

[0079] The pore former is a material dispersed in a polymer, which shows the heterogeneity of the substrate produced by extrusion, stretching, etc., and is subsequently removed from the substrate. Thus, the portion of the substrate where the pore former was located remains in the form of pores in the substrate. The pore former is preferably a material that is liquid during the extrusion process, but a material that maintains a solid state can also be used. The pore former can be an aliphatic hydrocarbon solvent such as liquid paraffin, paraffin oil, mineral oil, or paraffin wax; a vegetable oil such as soybean oil, sunflower oil, rapeseed oil, palm oil, coconut oil, corn oil, grapeseed oil, cottonseed oil, etc.; or a plasticizer such as dialkyl phthalate. In particular, the plasticizer may be di-2-ethylhexyl phthalate (DOP), di-butyl-phthalate (DBP), di-isononyl phthalate (DINP), di-isodecyl phthalate (DIDP), butyl benzyl phthalate (BBP), etc. Among these, liquid paraffin (LP, also called "liquid paraffin") is particularly preferred.

[0080] In addition, the content of the pore-forming agent in the preparation of the separator can be appropriately adjusted to realize a desired level of porosity. In consideration of improving the air permeability, it is preferable that the content of the pore-forming agent is high, but if it is contained in an excessive amount, it may have a negative effect on the strength of the final substrate. Therefore, the content of the pore-forming agent may be 1 wt% to 80 wt% based on 100 wt% of the total of the polymer resin and the pore-forming agent, and may be adjusted to 70 wt% or less, 60 wt% or less, or 50 wt% or less within the above range as necessary, or may be adjusted to 1 wt% or more, 20 wt% or more, or 40 wt% or more. Meanwhile, in a specific embodiment of the present invention, in order to realize a suitable porosity of the porous substrate, for example, to realize a porosity of about 45% or less, the pore-forming agent may be included in a range of 1 wt% to 60 wt% based on the total amount of the polymer resin and the pore-forming agent.

[0081] Next, the mixture prepared in the above step is extruded through an extruder to obtain an extruded sheet. The extruder is not particularly limited and may be an extruder commonly used in the art, for example, an extruder equipped with a T-die or a tubular die. The extrusion process may be carried out at a normal extrusion temperature, preferably at a temperature 10°C to 100°C higher than the melting point of the polymer resin used. If the extrusion process is carried out excessively beyond the above range, the polymer resin may be thermally degraded, making film formation difficult and deteriorating the physical properties of the substrate produced, which is undesirable. An extruded sheet may be obtained by such an extrusion process.

[0082] The extruded sheet is then put into a stretching process. This stretching process is carried out using a stretching machine commonly used in the art. The stretching machine may be, but is not limited to, a sequential biaxial stretching machine. In this way, the mechanical strength of the porous substrate can be increased by stretching the extruded sheet. The stretching process is carried out in a machine direction (MD, machine direction, longitudinal direction) and / or a transverse direction (TD, vertical direction). The stretching process in all or one of these directions increases the tensile strength in the corresponding stretching direction. If necessary, the separation membrane of the present invention may be stretched in the machine direction (MD) and / or transverse direction (TD) alone (e.g., uniaxial stretching), simultaneously or sequentially (e.g., biaxial stretching) in the stretching process. Meanwhile, in one embodiment of the present invention, the temperature of the membrane may be controlled to 100°C to 130°C, preferably 110°C to 125°C. For example, the temperature of the membrane during the stretching can be controlled in the range of 115° C. to 121° C. When the stretching step is carried out within the above temperature range, a uniform membrane with small pores can be obtained.

[0083] Next, the pore-forming agent is removed from the extruded sheet obtained above. The pore-forming agent is removed by extracting it using a solvent and drying it. Also, as a result of this removal, the spaces occupied by the pore-forming agent are formed as pores. Any solvent capable of extracting the pore-forming agent can be used as the solvent for extracting the pore-forming agent, but preferably, methyl ethyl ketone, methylene chloride, hexane, etc., which have high extraction efficiency and quick drying, are suitable. Preferably, the solvent is methylene chloride, for example, methylene dichloride (MC). As the extraction method, any common solvent extraction method such as an immersion method, a solvent spray method, an ultrasonic method, etc., can be used individually or in combination.

[0084] After the extraction of the pore-forming agent, the substrate is heat-set, thereby finally obtaining a separation membrane having the desired physical properties, porosity, and air permeability. The heat-setting step can be performed using a heating device, such as an oven, capable of applying an appropriate temperature required for heat-setting. In particular, the first dried membrane undergoes heat-setting in order to reduce the shrinkage rate of the final membrane by removing the residual stress. Heat-setting is a method of fixing a membrane and forcibly holding the membrane that is about to shrink by applying heat to remove the residual stress. A higher heat-setting temperature is advantageous for reducing the shrinkage rate, but if the temperature is too high, the membrane will partially melt, causing the formed pores to be blocked and reducing the permeability. The preferred heat-setting temperature is selected within a temperature range in which about 10 to 30 wt% of the crystalline portion of the membrane melts. If the heat setting temperature is selected to be lower than the temperature at which about 10 wt % of the crystalline portion of the film melts, the reorientation of the polyethylene molecules in the film is insufficient, and therefore there is no effect of removing residual stress in the film. If the heat setting temperature is selected to be higher than the temperature at which about 30 wt % of the crystalline portion of the film melts, the pores are blocked due to partial melting, and the permeability decreases.

[0085] In one embodiment of the present invention, the porous substrate may be a single layer, or may be a laminated film in which two or more films are laminated together, at least one of the films included in the laminated film may be formed by the above-mentioned method.

[0086] heat resistant layer In one embodiment of the present invention, the separation membrane may include a heat-resistant layer formed on at least one surface of the porous substrate. The heat-resistant layer may include an adhesive binder resin and inorganic particles, and may have a structure in which a number of micropores are formed inside and these micropores are connected, and may have a structural feature of a porous layer in which gas or liquid can pass from one surface to the other surface. In one embodiment of the present invention, the binder resin and the inorganic particles in the heat-resistant layer may be contained in a weight ratio of 1:99 to 30:70. The ratio may be appropriately adjusted within the above range, and for example, the total of the binder resin and the inorganic particles may be 100 wt%, the binder resin may be 1 wt% or more, 5 wt% or more, or 10 wt% or more, and the inorganic particles may be 80 wt% or more, 85 wt% or more, 90 wt% or more, or 95 wt% or more. In the present invention, it is preferable that the heat-resistant layer has a porous structure in terms of ion permeability.

[0087] The heat-resistant layer may be formed by binding inorganic particles with a binder resin as a medium, and pores may be formed due to interstitial volumes between the inorganic particles. The interstitial volumes are spaces defined by inorganic particles that are in substantial surface contact with each other in a packed structure (closed packed or densely packed) of inorganic particles.

[0088] In one embodiment of the present invention, the porosity of the heat-resistant layer is 30vol% to 70vol%, and within the range, the porosity may be 35vol% or more, or 40vol% or more, and simultaneously or independently, 65vol% or less, or 60vol%. For example, the porosity may be 40vol% to 60vol%. When the porosity is 70vol% or less, it is possible to ensure mechanical properties that can withstand the pressing process for bonding with the electrode, and the surface opening ratio is not too high, so it is suitable for ensuring adhesive strength. On the other hand, when the porosity is 30vol% or more, it is advantageous in terms of ion permeability.

[0089] Meanwhile, in the present invention, the porosity can be measured using an adsorbent gas such as nitrogen with a BELSORP (BET device) manufactured by BEL JAPAN Co., Ltd., or a method such as mercury intrusion porosimetry. Alternatively, in one embodiment of the present invention, the true density of the electrode active material layer can be calculated from the density (apparent density) of the obtained electrode (electrode active material layer), the composition ratio of the materials contained in the electrode (electrode active material layer), and the density of each component, and the porosity of the electrode active material layer can be calculated from the difference between the apparent density and the true density (net density).

[0090] The thickness of the heat-resistant layer may be 1 μm to 6 μm on one side of the porous substrate. Within the above range, the thickness of the heat-resistant layer may be 2 μm or more, or 3 μm or more, as necessary. Within the above numerical range, the adhesive strength with the electrode is excellent, and as a result, the cell strength of the battery is increased. On the other hand, when the thickness is 6 μm or less, it is advantageous in terms of the cycle characteristics and resistance characteristics of the battery. From this viewpoint, the thickness is preferably 4 μm or less, and more preferably 3 μm or less.

[0091] Non-limiting examples of binder resins that can be used in the heat-resistant layer of the present invention include polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, ethylene-vinyl acetate copolymer, polyethylene oxide, and the like. Examples of the polymer resin include any one selected from the group consisting of polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, and carboxyl methyl cellulose, or a mixture of two or more of these polymer resins, but are not limited thereto.

[0092] In a specific embodiment of the present invention, the inorganic particles usable in the heat-resistant layer are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles usable in the present invention are within the range of the operating voltage of the applied electrochemical element (for example, Li / Li + There are no particular limitations as long as the oxidation and / or reduction reaction does not occur at a potential of 0 to 5 V relative to the reference potential.

[0093] Non-limiting examples of the inorganic particles include BaTiO 3, Pb(Zr,Ti)O 3 (PZT), Pb 1-x La x Zr 1-y Ti y O 3 (PLZT, 0 <x<1、0<y<1)、Pb(Mg 1 / 3 Nb 2 / 3 )O 3 -PbTiO 3 (PMN-PT), hafnia (HfO 2 ), SrTiO 3 , SnO 2 , CEO 2 , MgO, Mg(OH) 2 , NiO, CaO, ZnO, ZrO 2 , SiO 2 , Y 2 O 3 , Al 2 O 3 , SiC, Al(OH) 3 , TiO 2 , aluminum peroxide, zinc tin hydroxide (ZnSn(OH) 6 ), tin-zinc oxide (Zn 2 SnO 4 , ZnSnO 3 ), antimony trioxide (Sb 2 O 3 ), antimony tetroxide (Sb 2 O 4 ), antimony pentoxide (Sb 2 O 5 ) and the like, and may include one or more of these.

[0094] Independently or in combination with the above-listed components, the inorganic particles may include inorganic particles having lithium ion transport capability. Non-limiting examples of inorganic particles having lithium ion transport capability include lithium phosphate (Li 3 PO 4 ), lithium titanium phosphate (Li x Ti y (PO 4 ) 3 , 0 <x<2、0<y<3)、リチウムアルミニウムチタンホスフェート(Lix Al y Ti z (PO 4 ) 3 、 0 < x < 2, 0 < y < 1, 0 < z < 3), 14Li 2 O - 9Al 2 O 3 - 38TiO 2 - 39P 2 O 5 such as (LiAlTiP) x O y -type glass (0 < x < 4, 0 < y < 13), lithium lanthanum titanate (Li x La y TiO 3 , 0 < x < 2, 0 < y < 3), Li 3.25 Ge 0.25 P 0.75 S 4 such as lithium germanium thiophosphate (Li x Ge y P z S w , 0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5), Li 3 N such as lithium nitride (Li x N y , 0 < x < 4, 0 < y < 2), Li 3 PO 4 -Li 2 S - SiS 2 such as SiS 2 -type glass (Li x Si y S z , 0 < x < 3, 0 < y < 2, 0 < z < 4), LiI - Li 2 S - P 2 S 5 such as P 2 S 5 -type glass (Li x P y S z , 0 < x < 3, 0 < y < 3, 0 < z < 7) or mixtures thereof, etc.

[0095] Also, the average diameter (D 50) is not particularly limited, but is preferably in the range of 0.3 μm to 1 μm in order to form a coating layer of uniform thickness and to have a suitable porosity. If it is less than 0.3 μm, the dispersibility of inorganic particles in the slurry prepared for manufacturing the heat-resistant layer may decrease, and if it exceeds 1 μm, the thickness of the coating layer formed may increase.

[0096] In one embodiment of the present invention, the method for forming the heat-resistant layer is, for example, as follows. First, a binder resin is dissolved in a suitable organic solvent to prepare a polymer solution. The solvent preferably has a solubility index similar to that of the binder polymer to be used and a low boiling point. This is to facilitate uniform mixing and subsequent solvent removal. Non-limiting examples of usable solvents include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane, water, or a mixture thereof.

[0097] Next, inorganic particles are added and dispersed in the prepared polymer solution. In the present invention, the content ratio of the inorganic particles to the binder is as described above, and is appropriately adjusted in consideration of the thickness, pore size, and porosity of the heat-resistant layer of the present invention to be finally prepared.

[0098] Next, the inorganic particle slurry prepared above is applied to at least one side of a separator and dried. The method of coating the slurry on a porous substrate is not limited to any particular method, and any common coating method known in the art can be used. For example, various methods such as dip coating, die coating, roll coating, comma coating, or a combination thereof can be used.

[0099] The drying process is performed by appropriately setting temperature and time conditions to minimize the occurrence of surface defects on the surface of the composite porous layer. The drying process can be performed using auxiliary drying devices such as a drying oven or hot air within an appropriate range.

[0100] The separation membrane of the present invention can also be produced by a method in which the heat-resistant layer and the porous substrate are separately prepared, these sheets are superimposed, and composited by thermocompression bonding or an adhesive, etc. As a method for obtaining the heat-resistant layer as an independent sheet, there can be mentioned a method in which the slurry is applied onto a release sheet, the heat-resistant layer is formed by the method described in detail above, and only the heat-resistant layer is peeled off, etc.

[0101] Meanwhile, the present invention provides a secondary battery including the separator, the battery including a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode, the separator being a low resistance separator having the above-mentioned characteristics.

[0102] In the present invention, the positive electrode comprises a positive electrode current collector and a positive electrode active material layer on at least one surface of the current collector, the positive electrode active material layer including a positive electrode active material, a conductive material, and a binder resin. The positive electrode active material is a lithium manganese composite oxide (LiMn 2 O4, LiMnO 2 etc.), lithium cobalt oxide (LiCoO 2 ), lithium nickel oxide (LiNiO 2 ) and compounds substituted with one or more transition metals; chemical formula Li 1+x Mn 2-x O 4 (where x is 0 to 0.33), LiMnO 3 , LiMn 2 O 3 , LiMnO 2 Lithium manganese oxides such as lithium copper oxide (Li 2 CuO 2 );LiV 3 O 8 , LiV 3 O 4 , V 2 O 5, Cu 2 V 2 O 7 and other vanadium oxides; chemical formula LiNi 1-x M x O 2 (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x = 0.01 - 0.3) nickel-site type lithium nickel oxide represented by the formula; chemical formula LiMn 1-x M x O 2 (where M = Co, Ni, Fe, Cr, Zn or Ta, and x = 0.01 - 0.1) or Li 2 Mn 3 MO 8 (where M = Fe, Co, Ni, Cu or Zn) lithium manganese composite oxide represented by the formula; a part of Li in the chemical formula is replaced by an alkaline earth metal ion in LiMn 2 O 4 ; disulfide compound; Fe 2 (MoO 4 ) 3 can contain one or a mixture of two or more of the above.

[0103] In the present invention, the negative electrode includes a negative electrode current collector and a negative electrode active material layer containing a negative electrode active material, a conductive material, and a binder resin on at least one surface of the current collector. The negative electrode uses, as the negative electrode active material, carbon such as lithium metal oxide, graphitizable carbon, graphite-based carbon; LixFe 2 O 3 (0 ≤ x ≤ 1), LixWO 2 (0 ≤ x ≤ 1), Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8) and other metal composite oxides; lithium metal; lithium alloy; silicon-based alloy; tin-based alloy; SnO, SnO 2 、PbO、PbO 2 、Pb 2 O 3 、Pb 3 O 4 、Sb2 O 3 , Sb 2 O 4 , Sb 2 O 5 , GeO, GeO 2 , Bi 2 O 3 , Bi 2 O 4 , and Bi 2 O 5 The conductive material may include one or a mixture of two or more selected from the group consisting of metal oxides such as those mentioned above, conductive polymers such as polyacetylene, Li-Co-Ni based materials, and titanium oxides.

[0104] In a specific embodiment of the present invention, the conductive material may be, for example, any one selected from the group consisting of graphite, carbon black, carbon or metal fiber, metal powder, conductive whisker, conductive metal oxide, activated carbon, and polyphenylene derivatives, or a mixture of two or more of these conductive materials. More specifically, the conductive material may be any one selected from the group consisting of natural graphite, artificial graphite, super-p, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, denka black, aluminum powder, nickel powder, zinc oxide, potassium titanate, and titanium oxide, or a mixture of two or more of these conductive materials.

[0105] The current collector is not particularly limited as long as it has high conductivity without inducing a chemical change in the battery. For example, stainless steel, copper, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like can be used.

[0106] The binder resin may be a polymer commonly used in electrodes in the art. Non-limiting examples of such binder resins include polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polyethylhexyl acrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, ethylene-vinyl acetate copolymer, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, and cellulose acetate propionate. Examples of suitable cellulose acetate copolymers include, but are not limited to, cyanoethyl acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, and carboxyl methyl cellulose.

[0107] The electrode assembly prepared as above can be placed in a suitable case and an electrolyte can be injected to manufacture a battery.

[0108] In the present invention, the electrolyte is A+ B - A salt having the structure: + Li + , Na + , K + or a combination thereof, such as B - PF 6 - , B.F. 4 - , Cl - , Br - , I - , ClO 4 - , AsF 6 - , C.H. 3 CO 2 - , C.F. 3 SO 3 - , N(CF 3 SO 2 ) 2 - , C(CF 2 SO 2 ) 3 - or a combination thereof, dissolved or dissociated in an organic solvent such as, but not limited to, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), gamma butyrolactone (g-butyrolactone), or a mixture thereof.

[0109] The present invention also provides a battery module including a battery having the electrode assembly as a unit cell, a battery pack including the battery module, and a device including the battery pack as a power source. Specific examples of the device include, but are not limited to, a power tool powered by a battery-type motor, electric vehicles including electric vehicles (EV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), and the like, electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters (E-scooters), electric golf carts, and power storage systems.

[0110] Hereinafter, the present invention will be described in detail with reference to examples in order to specifically explain the present invention. However, the examples of the present invention can be modified into various different forms, and the scope of the present invention should not be construed as being limited to the examples detailed below. The examples of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0111] [Example] 1. Porous Substrate Preparation [Example 1] 30 parts by weight of polyethylene resin (Mw 475,500 g / mol, PDI 3.6) and 70 parts by weight of liquid paraffin oil (dynamic viscosity at 40°C: 40 cSt) were charged into a biaxial extruder, kneaded, and then extruded. After extrusion, the mixture was passed through a T-die and a cooling casting roll to form a sheet, which was then biaxially stretched using a tenter-type simultaneous stretching machine with MD stretching followed by TD stretching. The liquid paraffin oil, which was the diluent, was extracted from the stretched sheet with methylene chloride, and the sheet was heat-set at approximately 128°C to obtain a porous substrate for separation membranes.

[0112] [Example 2] 30 parts by weight of polyethylene resin (Mw 930,800 g / mol, PDI 3.3) and 70 parts by weight of liquid paraffin oil (dynamic viscosity at 40°C: 40 cSt) were charged into a biaxial extruder, kneaded, and then extruded. After extrusion, the mixture was passed through a T-die and a cooling casting roll to form a sheet, which was then biaxially stretched using a tenter-type sequential stretching machine with MD stretching followed by TD stretching. The liquid paraffin oil, which was the diluent, was extracted from the stretched sheet with methylene chloride, and the sheet was heat-set at approximately 128°C to obtain a porous substrate for separation membranes.

[0113] [Comparative Example 1] A polyethylene resin composition was prepared by mixing a first polyethylene resin (Mw 250,000 g / mol) and a second polyethylene resin (Mw 800,000 g / mol). The composition had a molecular weight (Mw) of 497,000 g / mol and a PDI of 12.8. A porous substrate for a separation membrane was then obtained in the same manner as in the previous example.

[0114] [Comparative Example 2] A polyethylene resin composition was prepared by mixing a first polyethylene resin (Mw 250,000 g / mol) and a second polyethylene resin (Mw 800,000 g / mol). The composition had a molecular weight (Mw) of 381,000 g / mol and a PDI of 10.5. A porous substrate for a separation membrane was then obtained in the same manner as in the previous example.

[0115] The porous substrates obtained in the respective Examples and Comparative Examples are summarized in the following [Table 1].

[0116] [Table 1]

[0117] 2. Evaluation of the physical properties of porous substrates (1) Pore size distribution A sample was obtained from the center of the width direction of the porous substrate obtained in each Example and Comparative Example, with a size of 5 cm in the TD direction and 5 cm in the MD direction. A drying curve was obtained for each obtained sample using a Perm-Porometer (CFP-1500A) from Porous Materials Inc. (PMI). In addition, a wet curve was obtained after filling the porous substrate with Galwick solution. The measurement pressure was in the range of 0 to 3500 MPa. From this, the maximum pore size (Mps), the average pore size (mps), the half-width value, and the difference between the maximum pore size (Mps) and the average pore size (mps) were calculated. In addition, the calculated pore size distribution results are shown in FIG. 3.

[0118] In the obtained curve, the bubble point may represent the maximum diameter of the pores, the intersection point of the wet sample curve with the dry sample curve may represent the minimum diameter of the pores, and the intersection point of the wet sample curve with the 1 / 2 dry sample curve, which is half the value of the dry sample air permeability curve, may represent the average pore diameter (see FIG. 4).

[0119] (2) Measurement of BET surface area The adsorption isotherm of the porous substrate obtained in each Example and Comparative Example was measured up to 1 bar using a BET-specific surface area analyzer (BEL, Microtrac) at -196°C. The measured N 2 The Brunauer-Ennett-Teller model (BET) was used to calculate the BET surface area from the adsorption isotherms.

[0120] (3) Porosity measurement The porosity was calculated using the following [Formula 1] and [Formula 2]. [Formula 1] Porosity (vol%) = {1-(apparent density / true density)} x 100 [Formula 2] Apparent density (g / cm 3 ) = {Weight of porous substrate [g] / (Thickness of porous substrate [cm] × Area of ​​porous substrate [cm 2])}

[0121] Three samples each having a size of 5 cm in the MD / TD directions were obtained from the porous substrate of each Example and Comparative Example. The thickness of the porous substrate was determined by using an average value after measuring five points for each sample, and the weight of each sample was measured using a balance. The average value of the three samples was calculated and introduced into Equation 1 and Equation 2 to calculate the porosity. Meanwhile, the true density of each sample was calculated based on theoretically confirmed values ​​such as the molecular weight of each component applied.

[0122] (4) Measurement of air permeability and rate of change in air permeability The air permeability of the porous substrates obtained in each of the Examples and Comparative Examples was confirmed. The air permeability was measured according to the Fraser test method based on the ASTM D737 standard, and an Asahi Seiko EG01-55-1MR air permeability tester was used.

[0123] [Air permeability tester equipment setting conditions] Measurement pressure: 0.5kg / cm 2 , Cylinder pressure: 2.5kg / cm 2 , set time: 10 seconds

[0124] Ten or more test pieces were extracted from the porous substrate obtained in each Example and Comparative Example, and the average of the data (DATA) was recorded.

[0125] On the other hand, each porous substrate was hot pressed at 90° C., 4 MPa, and 1 sec, and then its air permeability was measured, and the rate of change in air permeability (%) of each porous substrate was calculated according to the following [Formula 1]. [Formula 1] [(Initial air permeability - Air permeability after pressing) / Initial air permeability] x 100

[0126] (5) Thickness deformation rate The initial thickness and the thickness after pressing of the porous substrate were measured using a contact type thickness meter. The measurements were performed at 5 mm intervals over a distance of 30 cm along the TD direction of the porous substrate. The measurements along the TD direction were performed five times at different MD positions, and the arithmetic average was taken as the thickness of the porous substrate. Meanwhile, the thickness change rate (%) of each porous substrate was calculated using the following [Equation 4]. [Formula 4] [(initial thickness - thickness after pressing) / initial thickness] x 100

[0127] It was confirmed that the porous substrate of Example 1 had a lower thickness change rate than those of Comparative Examples 1 and 2.

[0128] (6) Breakdown voltage Thirty test pieces were prepared for each example and comparative example, and their withstand voltage characteristics were evaluated. A stainless steel mesh and a porous substrate were hot-pressed at 90°C, 4 MPa, and 1 sec, and then a DC current of 0.5 mA and a voltage rise of 100 V / s (voltage 3 kV, ramp up time 3 s) were set. When the experiment started, the measurement was completed when the voltage rose to cause a short circuit in each test piece, and the voltage at that time was measured as the breakdown voltage. The voltages of the bottom 1% of test pieces showing low breakdown voltages were measured through a Weibull distribution analysis of the total number of test pieces tested, and are summarized in Table 1.

[0129] As confirmed by this experiment, the porous substrate according to the embodiment has small and uniform pores, and therefore has superior voltage resistance and thickness deformation rate compared to the porous substrate according to the comparative example.

[0130] (7) Measurement of polydispersity index The polydispersity index was calculated according to the following formula 5. [Formula 5] polydispersity index= Weight average molecular weight (Mw) / Number average molecular weight (Mn)

[0131] The weight average molecular weight (Mw) and number average molecular weight (Mn) were measured by gel permeation chromatography (GPC, PL GPC220, Agilent Technologies) under the following measurement conditions. - Column: PL Olexis (Polymer Laboratories) -Solvent: TCB (1,2,4-Trichlorobenzene, Wako Pure Chemical Industries, Ltd.) -Flow rate: 1.0ml / min -Sample concentration: 1.0mg / ml -Injection volume: 200μl -Column temperature: 160°C -Detector: Agilent High Temperature RI detector -Standard: Polystyrene (corrected by a cubic function)

Claims

1. A porous substrate for a separation membrane, The porous substrate comprises a polyolefin-based polymer resin, the polymer resin comprises polyethylene and / or polypropylene, and the substrate has a porous characteristic including a plurality of pores therein; Its porosity is 30 vol % to 60 vol %, The pore distribution has a Gaussian distribution measured through a pore size distribution with a full width at half maximum (FWHM) value of 1.5 nm or less; The polymer resin contains 90 wt % or more of a polyolefin resin relative to 100 wt % of the polymer resin, The polyolefin resin has a poly dispersity index (PDI) value of 2.5 to 6.

5.

2. 2. The porous substrate for separation membranes according to claim 1, wherein the difference between the maximum pore size (Mps) and the average pore size (mps) is 30 nm or less.

3. 2. The porous substrate for a separation membrane according to claim 1, wherein the difference between the maximum pore size (Mps) and the average pore size (mps) is 20 nm or less.

4. The porous substrate for a separation membrane according to claim 2, wherein the average pore size (mps) is 10 nm to 100 nm.

5. The porous substrate for a separation membrane according to claim 2, wherein the average pore size (mps) is 20 nm to 30 nm.

6. The porous substrate has a BET of 20 m ２ / g to 60m ２ The porous substrate for a separation membrane according to claim 1 , wherein the molecular weight of the porous substrate is 1000 or more.

7. The porous substrate for separation membranes according to claim 1, having a thickness of 5 μm to 20 μm.

8. The porous substrate for a separation membrane according to any one of claims 1 to 7 and a heat-resistant layer formed on one or both sides of the surface of the porous substrate, The heat-resistant layer comprises a binder resin and inorganic particles.

9. An electrochemical device comprising a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode, the separator being as defined in claim 8 .

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