Microporous separator for lithium batteries and method for preparing the same

Abstract

The present invention relates to a microporous separator for lithium batteries and a method for preparing the same. Specifically, the present invention provides a microporous separator for lithium batteries that is a porous monolayer membrane containing a polyolefin resin, and has excellent elastic recovery performance in both the transverse and longitudinal directions of the plane. The present invention also provides a method for preparing a lithium ion battery separator with high planar elastic recovery rate.

Description

[Technical Field]

[0001] This invention relates to the field of lithium-ion battery technology, and more particularly to a microporous separator for lithium batteries and a method for preparing the same. [Background technology]

[0002] Polyolefin microporous membranes are widely used as separation membranes and separator materials for the separation and selective permeation of various substances. For example, polyolefin microporous membranes are used as microfiltration membranes, fuel cell separators, capacitor separators, base materials for functional membranes in which functional materials are filled into the pores to exhibit new functions, and battery separators. Of these uses, polyolefin microporous membranes are particularly well-suited for use as separators in lithium-ion batteries, which are widely used in laptop computers, mobile phones, and digital cameras. This is because polyolefin microporous membranes have excellent mechanical strength and pore sealing properties.

[0003] Furthermore, microporous separators for lithium batteries are also required to have good planar elasticity recovery properties.

[0004] However, currently used polyolefin microporous membranes, particularly lithium battery separators manufactured by wet processes, have low planar elasticity recovery rates, both in the lateral and longitudinal directions, which are lower than 14%. This causes irreversible deformation of the microporous membrane during battery assembly, resulting in a microporous membrane that deviates from the battery's design size and affecting electrochemical safety performance.

[0005] Currently, a microporous separator that possesses both high mechanical strength and good planar elasticity recovery, is thin, and has excellent pore structure and surface properties does not yet exist. In particular, obtaining a lithium-ion battery separator using a wet manufacturing method that exhibits superior planar elasticity recovery is difficult. [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] This invention makes it possible to obtain a lithium-ion battery separator with excellent planar elasticity recovery rate when using commonly used polyolefin resins as the main raw material, by controlling the proportion of low molecular weight chain segments in the microporous separator. Furthermore, this invention solves the problem of high surface resistance of separators, and the surface resistance of the lithium-ion battery separator according to this invention is reduced by more than 20% compared to conventional technology. Moreover, the pore size, air permeability, and mechanical strength of the separator according to this invention are acceptable and suitable for use in lithium-ion batteries.

[0007] The present invention further discovered that the above-mentioned separator can be obtained by using low-temperature multi-point dispersion stretching in the vertical direction (MD) and low-temperature low-magnification stretching in the horizontal direction (TD1), and by controlling the difference between the MD stretching magnification and the TD1 stretching magnification. [Means for solving the problem]

[0008] In some specific embodiments, the present invention provides a microporous separator for lithium batteries, which is a porous single-layer film containing a polyolefin resin, wherein the characteristic viscosity index of the microporous separator is 700 ml / g to 1500 ml / g, preferably 900 ml / g to 1200 ml / g, the proportion of polyolefin chain segment components with a weight-average molecular weight of 100,000 or less in the microporous separator is 15 mol% to 30 mol%, preferably 15 mol% to 25 mol%, and more preferably 15 mol% to 21 mol%, and the elastic recovery rate in the longitudinal direction of the microporous separator, measured under the following conditions, is 14% or more, preferably 18% or more, and the elastic recovery rate in the transverse direction is 14% or more, preferably 18% or more, wherein the conditions are as follows: a separator sample piece with a width of 15 mm is cut in either the longitudinal (MD direction) or transverse (TD direction), and the separator sample piece is cut from L0 = 100 mm, SelectedAfter stretching the material to 50% elongation at a speed of 50 mm / min in one direction, holding it for 60 seconds, and then allowing it to contract naturally for 3 minutes, the length L1 was measured. This includes calculating the elastic recovery rate using the following formula. Elasticity recovery rate = (1.5 * L0 - L1) / (0.5 * L0) * 100 %

[0009] Preferably, in the microporous separator, the proportion of polyolefin chain segment components with a weight-average molecular weight of 10,000 or less is 0.5 mol% to 2.5 mol%, and more preferably 0.5 mol% to 1.5 mol%.

[0010] Preferably, the molecular weight distribution of the polyolefin resin is between 3 and 6, and more preferably between 3.5 and 5.0.

[0011] Preferably, the microporous separator satisfies one or more combinations of the following a to e: a. The surface resistance is 0.2Ω to 0.7Ω, preferably 0.3Ω to 0.6Ω. b. The average pore size is 25 nm to 50 nm, preferably 30 nm to 45 nm. c) The thickness is 1 μm to 30 μm, preferably 4 μm to 12 μm. d. The air permeability is 10 sec / 100cc to 300 sec / 100cc, preferably 60 sec / 100cc to 170 sec / 100cc. e. The tensile strength in the MD or TD direction is 2000 kgf / cm². 2 ~4000 kgf / cm² 2 The pressure is preferably 2800 kgf / cm². 2 ~4000 kgf / cm² 2 That is the case.

[0012] Preferably, the polyolefin resin is selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, and UHDPE), polypropylene, polybutylene, polymethylpentene, copolymers thereof, and mixtures thereof.

[0013] Preferably, in the polyolefin resin, the proportion of the number of polyolefin chain segment components having a weight average molecular weight of 100,000 or less is 10 mol% to 30 mol%, preferably 10 mol% to 25 mol%, more preferably 13 mol% to 25 mol%, and still more preferably 13 mol% to 20 mol%.

[0014] Preferably, in the polyolefin resin, the proportion of the number of polyolefin chain segment components having a weight average molecular weight of 10,000 or less is 0 mol% to 2 mol%, preferably 0.3 mol% to 1.0 mol%.

[0015] Preferably, the microporous separator is a separator prepared by a wet process.

[0016] In some specific embodiments, the present invention provides a method for preparing a microporous separator for a lithium battery, the method comprising: step (a) of melt-kneading a mixture containing a polyolefin resin and a plasticizer to form a melt; step (b) of extruding the melt and curing it as a thick plate; step (c) of stretching the thick plate in the longitudinal direction (MD direction) and the transverse direction (TD direction) to obtain a stretched body; step (d) of removing the plasticizer from the stretched body, drying it, and obtaining the microporous separator for the lithium battery, wherein, in the polyolefin resin according to step (a), the proportion of the number of polyolefin chain segment components having a weight average molecular weight of 100,000 or less is 10 mol% to 30 mol%, preferably 10 mol% to 25 mol%, more preferably 13 mol% to 25 mol%, and still more preferably 13 mol% to 20 mol%, and the intrinsic viscosity index of the polyolefin resin is 800 ml / g to 1600 ml / g, preferably 1000 ml / g to 1300 ml / g.

[0017] Preferably, in the polyolefin resin according to step (a), the proportion of the number of polyolefin chain segment components having a weight average molecular weight of 10,000 or less is 0 mol% to 2 mol%, preferably 0.3 mol% to 1.5 mol%.

[0018] Preferably, the molecular weight distribution of the polyolefin resin is between 3 and 6, preferably between 3.5 and 5.0.

[0019] Preferably, in the microporous separator, the proportion of the number of polyolefin chain segment components having a weight average molecular weight of 100,000 or less is 15 mol% to 30 mol%, preferably 15 mol% to 25 mol%, more preferably 15 mol% to 21 mol%.

[0020] Preferably, in the microporous separator, the proportion of the number of polyolefin chain segment components having a weight average molecular weight of 10,000 or less is 0.5 mol% to 2.5 mol%, preferably 0.5 mol% to 1.5 mol%.

[0021] Preferably, the elastic recovery rate in the longitudinal direction of the microporous separator measured under the following conditions is 14% or more, preferably 18% or more, and the elastic recovery rate in the transverse direction is 14% or more, preferably 18% or more. The conditions are as follows: Cut a separator sample piece with a width of 15 mm in either the longitudinal direction (MD direction) or the transverse direction (TD direction), and from a position where L0 = 100 mm, Selected Stretch the separator sample piece at a speed of 50 mm / min to 50% elongation in that direction, then hold it for 60 s, leave it as it is, and allow it to naturally shrink for 3 min, and then measure the length L1. This includes calculating the elastic recovery rate using the following formula. Elastic recovery rate = (1.5 * L0 - L1) / (0.5 * L0) * 100 %

[0022] Preferably, the weight ratio of the polyolefin resin to the plasticizer is between 15:85 and 45:55, preferably between 20:80 and 30:70, and more preferably 25:75.

[0023] Preferably, an extruder is used for melt kneading in step (a), preferably the extruder temperature is 160°C to 250°C, and the extruder screw speed is 60 r / min to 100 r / min.

[0024] Preferably, in step (b), the mixture is attached to an extrusion casting roller via a die head to form a thick plate by cooling and curing, and the temperature of the casting roller is 20°C to 30°C.

[0025] Preferably, in step (c), the stretching in the vertical direction is performed at a stretching temperature of 80°C to 120°C, with a stretching ratio of 4 to 9 times, preferably 4 to 7 times, and the stretching method is multi-point dispersed stretching, preferably with 3 to 7 points.

[0026] Preferably, a preheating step is provided before stretching in the vertical direction, the preheating temperature is 60°C to 100°C, and preferably, the preheating is gradient preheating, with 2 to 4 stages set for the gradient preheating, and the temperature difference between two adjacent gradients is 7°C to 25°C.

[0027] Preferably, in step (c), the stretching temperature in the transverse direction (TD1) is set to 90°C to 125°C, preferably 90°C to 115°C, the stretching ratio is set to 4 to 8 times, preferably 4 to 6 times, and the absolute value of the difference between the stretching ratio in the vertical direction and the stretching ratio in the transverse direction is set to 1 or less.

[0028] Preferably, step (d) further includes a second transverse stretching (TD2 stretching) step after the drying step, wherein the stretching temperature for the second transverse stretching is 125°C to 140°C, preferably 130°C to 140°C, and the stretching ratio is 1.4 to 1.8 times.

[0029] Preferably, the polyolefin resin is selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, and UHDPE), polypropylene, polybutylene, polymethylpentene, copolymers thereof, and mixtures thereof. [Modes for carrying out the invention]

[0030] Before further describing the present invention, the following section compiles the specific terms used in this specification, the examples, and the appended claims. The definitions set forth herein should be read and understood by those skilled in the art in light of the remainder of the invention. All technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art unless otherwise defined.

[0031] As used herein, the terms “one” and “other” are used solely for illustrative purposes and are not intended to express or imply relative importance or to implicitly specify the number of technical features being described.

[0032] As used herein, the term "about" means, when referring to a specific value, to include its variation, such as ±10%, ±5%, ±1%, or ±0.1%.

[0033] As used herein, the term “essentially the same” means that, when referring to two values, the difference between the two values ​​is less than 10%, 5%, or 1% of the mean of the two values.

[0034] As used herein, the term "polyolefin" may refer to a polyolefin monomer (i.e., a single type of polyolefin), a polyolefin copolymer, or a polyolefin comixture.

[0035] As used herein, the term “comixture” means two or more homopolymers, copolymers, or physical mixtures of homopolymers and copolymers having different molecular structures. Specifically, a comixture may include different polymers, i.e., at least two polymers having different chemical properties (e.g., polyethylene, polypropylene, and / or ethylene-propylene copolymers having different chemical properties), and / or polymers having the same chemical properties but different characteristics (e.g., two different polyethylenes having different characteristics (density, molecular weight, molecular weight distribution, rheology, additives (composition and / or percentage), etc.)).

[0036] As used herein, the term "longitudinal direction" is also called the MD direction and refers to the direction of operation of the machine.

[0037] As used in this specification, the term "lateral direction," also known as the TD direction, refers to the direction perpendicular to the machine's operating direction.

[0038] In a first aspect, the present invention provides a microporous separator for lithium batteries, which is a porous single-layer film containing a polyolefin resin, wherein the characteristic viscosity index of the microporous separator is 700 ml / g to 1500 ml / g, preferably 900 ml / g to 1200 ml / g, the proportion of polyolefin chain segment components with a weight-average molecular weight of 100,000 or less in the microporous separator is 15 mol% to 30 mol%, preferably 15 mol% to 25 mol%, more preferably 15 mol% to 21 mol%, the elastic recovery rate in the longitudinal direction of the microporous separator measured under the following conditions is 14% or more, preferably 18% or more, and the elastic recovery rate in the transverse direction is 14% or more, preferably 18% or more, the above conditions being: a separator sample piece with a width of 15 mm is cut in either the longitudinal direction (MD direction) or the transverse direction (TD direction), and the separator sample piece is cut from L0 = 100 mm, Selected After stretching the material to 50% elongation at a speed of 50 mm / min in one direction, holding it for 60 seconds, and then allowing it to contract naturally for 3 minutes, the length L1 was measured. This includes calculating the elastic recovery rate using the following formula. Elasticity recovery rate = (1.5 * L0 - L1) / (0.5 * L0) * 100 %

[0039] Preferably, the upper limit of the elastic recovery rate of the microporous separator measured under the above conditions is 50% or less, preferably 30% or less, and more preferably 25% or less. By providing the above elastic recovery rate, the processing stability of the separator can be ensured, and it is possible to avoid the separator's rebound being too large during the battery or separator preparation process, which would affect the manufacturing of the product.

[0040] According to some preferred embodiments of the present invention, in the microporous separator, the proportion of polyolefin chain segment components with a weight-average molecular weight of 10,000 or less is 0.5 mol% to 2.5 mol%, preferably 0.5 mol% to 1.5 mol%.

[0041] In the present invention, any polyolefin resin commonly used in the art for the preparation of microporous separators for lithium batteries can be used as the polyolefin resin, for example, polyethylene (e.g., LDPE). (Low-density polyethylene) LLDPE (Linear low-density polyethylene) HDPE (High-density polyethylene) UHDPE (Ultra-high-density polyethylene) Polyethylene and / or polypropylene can be selected from polypropylene, polybutylene, polymethylpentene, copolymers thereof, and comixtures thereof (including), with polyethylene and / or polypropylene being preferred.

[0042] According to the present invention, the polyolefin resin used can be any resin capable of producing the microporous separator for lithium batteries required by the present invention. To provide a microporous separator with a high planar elasticity recovery rate, preferably, in the polyolefin resin, the proportion of polyolefin chain segment components with a weight-average molecular weight of 100,000 or less is 10 mol% to 30 mol%, preferably 10 mol% to 25 mol%, more preferably 13 mol% to 25 mol%, and even more preferably 13 mol% to 20 mol%. Preferably, in the polyolefin resin, the proportion of polyolefin chain segment components with a weight-average molecular weight of 10,000 or less is 0 mol% to 2 mol%, preferably 0.3 mol% to 1.0 mol%. Even more preferably, the molecular weight distribution of the polyolefin resin is 3 to 6, preferably 3.5 to 5.0.

[0043] In some specific embodiments of the present invention, the microporous separator satisfies one or more combinations of the following a to e: a. The surface resistance is 0.2Ω to 0.7Ω, preferably 0.3Ω to 0.6Ω. b. The average pore size is 25 nm to 50 nm, preferably 30 nm to 45 nm. c) The thickness is 1 μm to 30 μm, preferably 4 μm to 12 μm. d. The air permeability is 10 sec / 100cc to 300 sec / 100cc, preferably 60 sec / 100cc to 170 sec / 100cc. e. The tensile strength in the MD or TD direction is 2000 kgf / cm². 2 ~4000 kgf / cm² 2 The pressure is preferably 2800 kgf / cm². 2 ~4000 kgf / cm² 2 That is the case.

[0044] To obtain a microporous separator with a high planar elasticity recovery rate, the microporous separator according to the present invention is preferably a separator prepared by a wet manufacturing method.

[0045] In a second aspect, the present invention provides a method for preparing a microporous separator for lithium batteries, the preparation method being: (a) A step of melting and kneading a mixture containing polyolefin resin and a plasticizer to form a molten product, (b) a step of extruding the molten material and hardening it into a thick plate, (c) A step of stretching the aforementioned thick plate in the longitudinal direction (MD direction) and the transverse direction (TD direction) to obtain an elongated body, The process includes (d) removing the plasticizer from the stretched material and drying it to obtain a microporous separator for the lithium battery, Here, in the polyolefin resin related to step (a), the proportion of polyolefin chain segment components with a weight-average molecular weight of 100,000 or less is 10 mol% to 30 mol%, preferably 10 mol% to 25 mol%, more preferably 13 mol% to 25 mol%, and even more preferably 13 mol% to 20 mol%, and the characteristic viscosity index of the polyolefin resin is 800 ml / g to 1600 ml / g, preferably 1000 ml / g to 1300 ml / g.

[0046] According to the present invention, a microporous separator with a high planar elasticity recovery rate, suitable for use in lithium batteries, can be obtained by treating a mixture containing a suitable polyolefin resin and a plasticizer according to the preparation method of the present invention.

[0047] In the present invention, any polyolefin resin commonly used in the art for the preparation of microporous separators for lithium batteries can be used as the polyolefin resin. For example, it can be selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, and UHDPE), polypropylene, polybutylene, polymethylpentene, copolymers thereof, and mixtures thereof, of which polyethylene and / or polypropylene are preferred.

[0048] In the present invention, the plasticizer is a low molecular weight solvent capable of dissolving the polyolefin resin, and may be, for example, liquid paraffin, diethyl phthalate, palm oil, etc., and the kinematic viscosity at 40 °C is 35 mm 2 / s to 120 mm 2 / s of liquid paraffin is preferred, and the kinematic viscosity at 40 °C is 40 mm 2 / s to 55 mm 2 / s of liquid paraffin is more preferred. The test method for the kinematic viscosity uses GB / T 265.

[0049] According to the present invention, in step (a), the weight ratio of the polyolefin resin to the plasticizer is preferably between 15:85 and 45:55, and more preferably between 20:80 and 30:70. The weight ratio of the polyolefin resin to the plasticizer may specifically be 20:80, 21:79, 22:78, 23:77, 24:76, 25:75, 26:74, 27:73, 28:72, 29:71, or 30:70, etc. By applying the above-mentioned weight ratio of the polyolefin resin to the plasticizer, the repulsive performance in the planar direction of the microporous separator can be improved.

[0050] In order to further improve the repulsive performance in the planar direction of the microporous separator, in the polyolefin resin, the proportion of the number of polyolefin chain segment components with a weight average molecular weight of 10,000 or less is set to 0 mol% to 2 mol%, preferably 0.3 mol% to 1.5 mol%. Preferably, the molecular weight distribution of the polyolefin resin is set to 3 to 6, preferably 3.5 to 5.0.

[0051] According to the present invention, in step (a), the mixture can be processed using any conventional method capable of forming it as a molten product, for example, an extruder can be used for melt kneading. In some preferred embodiments, the parameters of the extruder include the extruder temperature and the extruder screw speed, wherein the extruder temperature is 150°C to 250°C, preferably 180°C to 240°C, and the extruder screw speed is 60 r / min to 100 r / min, preferably 70 r / min to 90 r / min.

[0052] According to the present invention, in step (b), the method for extruding the molten material and hardening it into a thick plate can be any method that can form a thick plate of a desired thickness. For example, the mixture may be attached to an extrusion casting roller via a die head and a thick plate may be formed by cooling and hardening. Preferably, the temperature of the casting roller is 20°C to 30°C. The roll speed of the casting roller may be 3 m / min to 8 m / min.

[0053] According to the present invention, in step (c), an elongated body is obtained by performing stretching in the longitudinal direction (i.e., longitudinal stretching or MD stretching) and stretching in the transverse direction (i.e., transverse stretching or TD1 stretching), respectively. Specifically, the transverse stretching may be performed after the longitudinal stretching, or the longitudinal stretching may be performed after the transverse stretching, but the former is preferred.

[0054] To further improve the rebound performance of the microporous separator in the planar direction, in step (c), stretching in the longitudinal direction is performed at a stretching temperature of 80°C to 120°C, with a stretching ratio of 4 to 9 times, preferably 4 to 7 times, and the stretching method is multi-point dispersed stretching, preferably with 3 to 7 points. In multi-point dispersed stretching, the stretching points are the speed ratio between two adjacent stretching rollers with different linear velocities, i.e., the ratio of the rear roller linear speed to the front roller linear speed, and the stretching ratio for each stretching point is 1.1 to 4. In the case of multi-point dispersed stretching, the stretching ratio refers to the sum of the stretching ratios of each point, preferably each stretching ratio is independently 1.1 to 3, preferably 1.1 to 2.4. Preferably, there is at least one set of three adjacent enlargement points, there is an increasing relationship between the enlargement ratios, and the absolute value of the difference in enlargement ratios between adjacent enlargement points is > 0.1. For example, in the case of three-point enlargement, the distribution of enlargement ratios may be 1.3 / 1.6 / 1.9 or 1.5 / 1.9 / 2.0, and in the case of five-point enlargement, the distribution of enlargement ratios may be 1.1 / 1.4 / 2.0 / The ratios may also be 2.4 / 1.1, or 1.1 / 1.6 / 2.0 / 2.1 / 1.1, or 1.3 / 1.3 / 1.6 / 2.1 / 1.3, or 1.1 / 1.1 / 1.4 / 1.6 / 2.0. In the case of seven-point enlargement, the distribution of enlargement ratios may also be 1.1 / 1.1 / 1.2 / 1.3 / 1.4 / 1.55 / 1.6, or 1.1 / 1.2 / 1.4 / 1.7 / 1.2 / 1.1 / 1.1.

[0055] Generally, three adjacent points of increasing magnification are not only present at the last three magnification points of the magnification section, but preferably also in the preceding and intermediate sections of the magnification. at least one of The three adjacent stretching points should have a magnification-increasing relationship.

[0056] The increasing magnification of longitudinal stretching is advantageous due to sufficient stretching and polymer orientation, resulting in the formation of filaments with superior mechanical properties and a uniform size distribution, and better elastic recovery of the microporous membrane.

[0057] According to some more preferred embodiments of the present invention, a preheating step is provided before stretching in the longitudinal direction, and the preheating temperature is 60°C to 100°C. Preferably, the preheating is gradient preheating, and the gradient preheating can be set in 2 to 4 stages, preferably the temperature difference between two adjacent gradients is 7°C to 25°C. The preheating time may be 2s to 100s, preferably 4s to 60s. In the case of gradient preheating, the preheating times of two adjacent gradients may be the same or different. for example, Each is independent of the others, ranging from 1s to 25s.

[0058] To further improve the rebound performance of the microporous separator in the planar direction, in step (c), the stretching temperature in the transverse direction is 90°C to 125°C, preferably 90°C to 115°C, and the stretching ratio is 4 to 8 times, preferably 4 to 6 times. Preferably, the absolute value of the difference between the stretching ratio in the vertical direction and the stretching ratio in the transverse direction is 1 or less, more preferably 0.2 to 0.9, and even more preferably 0.4 to 0.8.

[0059] According to the present invention, in step (d), the plasticizer can be removed from the stretched body by any method, and it is preferable to remove the plasticizer from the stretched body using an extractant. In some embodiments, The aforementioned extractant may be an alkane extractant. or With halogenated hydrocarbon extractants It's fine to have it. , for example It is dichloromethane.

[0060] One method for removing plasticizers from the extensor using an extractant is to remove the plasticizers from the extensor by circulating the extractant. Preferably, the circulating volume of the extractant is 1 m³. 3 / h~5m 3 The value is / h. After extraction, the stretched body is heated and dried by selecting one or more of the following: a heat roller, a heating plate, and hot air, and the drying temperature is preferably 20°C to 150°C.

[0061] According to a preferred embodiment of the present invention, step (d) further includes a second transverse stretching (TD2 stretching) step after the drying step, wherein the stretching temperature for the second transverse stretching is 125°C to 140°C, preferably 130°C to 140°C, and the stretching ratio is 1.4 to 1.8 times.

[0062] As a result of preparation using the above method, the proportion of polyolefin chain segment components with a weight-average molecular weight of 100,000 or less in the obtained microporous separator is 15 mol% to 30 mol%, preferably 15 mol% to 25 mol%, and more preferably 15 mol% to 21 mol%. Preferably, in the microporous separator, the proportion of polyolefin chain segment components with a weight-average molecular weight of 10,000 or less is 0.5 mol% to 2.5 mol%, preferably 0.5 mol% to 1.5 mol%. Furthermore, the elastic recovery rate in the longitudinal direction of the microporous separator measured under the following conditions is 14% or more, preferably 18% or more, and the elastic recovery rate in the transverse direction is 14% or more, preferably 18% or more. The above conditions are met by cutting a separator sample piece with a width of 15 mm in either the longitudinal direction (MD direction) or the transverse direction (TD direction), and cutting the separator sample piece from L0 = 100 mm. Selected After stretching the material to 50% elongation at a speed of 50 mm / min in one direction, holding it for 60 seconds, and then allowing it to contract naturally for 3 minutes, the length L1 was measured. This includes calculating the elastic recovery rate using the following formula. Elasticity recovery rate = (1.5 * L0 - L1) / (0.5 * L0) * 100 %

[0063] In a third aspect, the present invention provides a microporous separator for lithium batteries obtained by the preparation method according to the second aspect of the present invention.

[0064] In the following examples and comparative examples, the test methods for each parameter are as follows:

[0065] (1) The detailed test method for the elastic recovery rate is as follows: Cut a 15mm wide test specimen piece along the MD or TD direction, measure 100mm along the length of the test specimen piece, draw lines at the corresponding ends, and define this length as L0. When a test sample is placed in the enlarger and a line is drawn with the enlargement collet (i.e., Colette Clamp it to a distance of 100mm, and set the extension speed to 50mm / min. The test specimen is stretched to 50% of its length and held for 60 seconds. After removal, it is left at room temperature for 3 minutes to naturally shrink, and the distance between the two ends of the drawn line is measured and labeled L1.

[0066] (2) The test method for surface resistance is as follows: Four separator samples, each 45 mm in diameter, are cut from a flat surface. The samples are immersed in an electrolyte solution (1.0 M LiPF6 in a 1:1:1 EC / EMC / DMC solvent) for 30 minutes in a sealed environment. Approximately 15 ml of 1 mol / L fresh electrolyte solution (1.0 M LiPF6 in a 1:1:1 EC / EMC / DMC solvent) is placed in a surface resistance test fixture. One, two, three, or four separators are then placed in the fixture for testing. Linear fitting is performed with the number of separator layers as the x-coordinate and the separator resistance value as the y-coordinate. The slope of the line and the degree of fit are determined. If the degree of fit is greater than 0.999, the slope is defined as the surface resistance of the separator.

[0067] (3) The average pore size was obtained by measuring at 25°C using a PMI instrument (Chia Yun Instrument Inc., CFP-1500AE model) and galwick permeate (surface tension of 15.9 dynes / cm at 25°C), and the pore size is expressed in nm.

[0068] (4) The thickness shall be measured in accordance with the provisions of GB-T 36363-2018.

[0069] (5) The air permeability was measured in accordance with the provisions of GB / T 36363-2018, with 100 ml of air per 6.45 cm² area under a pressurization of 1.21 kPa.2 The time required to pass through the separator is measured.

[0070] (6) The tensile strength shall be measured in accordance with the provisions of GB / T1040.3-2006.

[0071] (7) The proportion of polyolefin chain segments with a weight-average molecular weight of 100,000 or less and the proportion of polyolefin chain segments with a weight-average molecular weight of 10,000 or less shall be measured in accordance with the provisions of GB / T 36214.4-2018.

[0072] (8) The molecular weight distribution shall be measured in accordance with the provisions of GB / T 36214.4-2018.

[0073] (9) Characteristic viscosity shall be measured in accordance with the provisions of ISO 1628-3.

[0074] Example 1 A polyethylene resin with a characteristic viscosity of 1150 ml / g and a molecular weight distribution of 5.0 is used as a plasticizer (kinematic viscosity of 45 mm²). 2 The polyethylene resin was mixed with paraffin oil (at a weight ratio of 25:75) and melt-kneaded in an extruder to form a molten product. In this product, the proportion of polyethylene chain segments with a weight-average molecular weight of 100,000 or less in the polyethylene resin was 17 mol%, and the proportion of polyethylene chain segments with a weight-average molecular weight of 10,000 or less was 0.8 mol%. The extruder temperature was 220°C, and the extruder screw rotation speed was 80 r / min. The above molten material was cooled and hardened using a casting roller to form a thick plate. The casting roller temperature was set to 25°C, and the resulting thick plate was subjected to a three-stage preheating process at 60°C / 80°C / 100°C, with a total preheating time of 15 seconds. After this, a stretching process was performed. For stretching in the MD direction, a three-point dispersion stretching method was employed, with a constant stretching temperature of 95°C, a total stretching ratio of 6.29 times, and a distribution of stretching ratios at each point of 1.7 / 1.85 / 2.0. Lateral stretching (TD1 stretching) was performed at a temperature of 110°C, with a lateral stretching ratio of 6 times. After removing the plasticizer (paraffin oil), the material was dried with hot air at 55°C, followed by a second lateral stretching (TD2 stretching) at a stretching temperature of 133°C and a stretching ratio of 1.4 times. After this, a heat setting process was performed, and the heat setting processing temperature was set to 135°C.

[0075] Examples 2-9, Comparative Examples 1-3 The differences between the preparation methods for microporous separators for batteries described in Examples 2-9 and Comparative Examples 1-3 and the method described in Example 1 are shown in detail in Table 1, and the parts not shown in Table 1 are the same as those described in Example 1.

[0076] Example 10 This differs from Example 1 in that five-point dispersion stretching was employed in the MD direction, the stretching temperature was set to a constant 95°C, the total stretching ratio was 9.3 times, and the distribution of stretching ratios at each point was 1.1 / 1.6 / 2.0 / 2.4 / 1.1, while the lateral stretching (TD1 stretching) temperature was 110°C and the lateral stretching ratio was 7.5 times.

[0077] Examples 2-1 to 2-4 The differences from Example 2 are shown in detail in Table 2, and the parts not shown in Table 2 are the same as those in Example 2.

[0078] [Table 1]

[0079] [Table 2]

[0080] As is clear from Examples 1-11 and Comparative Examples 1-2 in Table 1, when the proportion of polyolefin chain segment components with a weight-average molecular weight of 100,000 or less in the microporous separator is 15 mol% to 30 mol%, the elastic recovery rate (>14%), surface resistance, and mechanical strength (in MD and TD) are improved. stretch Strength≧2000kgf / cm 2) is excellent, and when the proportion of polyolefin chain segment components with a weight-average molecular weight of 100,000 or less in the microporous separator exceeds the above range, the elastic recovery rate and mechanical strength decrease significantly. As is clear from the comparison of Example 1 and Example 10, when the stretch ratio in MD and TD is increased, only the MD stretch strength improves slightly, while the TD stretch strength decreases slightly, the elastic recovery rate decreases, and the surface resistance increases.

[0081] As is clear from Examples 2-1 to 2-4 in Table 2, the combination of the extrusion method and raw materials makes it possible to adjust and control the ratio of polyolefin chain segments with a weight-average molecular weight of 100,000 or less in the microporous separator.

[0082] In this specification and the appended claims, the singular forms “a,” “an,” and “the” include the plural form unless otherwise explicitly stated in the context. All technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art, unless otherwise defined. The methods described herein may be carried out in any logically possible order, except for the specific order disclosed herein.

[0083] Representative examples are intended to be helpful in illustrating the present invention and are not intended to limit the scope of the invention, nor should they be construed as such. In fact, it will be apparent to those skilled in the art that, in addition to those shown and described herein, several modifications of the invention and many other examples are included in the examples and the scientific and patent literature cited herein. The examples contain important additional information, examples, and instructions that may be adopted in the implementation of many examples and equivalents of the invention.

Claims

1. A microporous separator for lithium batteries, which is a porous single-layer film containing a polyolefin resin, wherein the characteristic viscosity index of the microporous separator is 700 ml / g to 1500 ml / g, the proportion of polyolefin chain segment components with a weight-average molecular weight of 100,000 or less is 15 mol% to 30 mol%, and the elastic recovery rate of the microporous separator in the longitudinal direction, measured under the following conditions, is 14% or more, and the elastic recovery rate in the transverse direction is 14% or more, wherein the above conditions are met by cutting a separator sample piece with a width of 15 mm in either the longitudinal direction (MD direction) or the transverse direction (TD direction), and the separator sample piece is L ０ = Starting from 100 mm, stretch it to 50% extension at a speed of 50 mm / min in the selected direction, hold it for 60 seconds, leave it to contract naturally for 3 minutes, and then the length L １ A microporous separator for lithium batteries, characterized by measuring and calculating the elastic recovery rate using the following formula. Elasticity recovery rate = (1.5 * L) ０ -L １ ) / (0.5*L ０ ) *100%

2. The microporous separator for lithium batteries according to claim 1, wherein the proportion of polyolefin chain segment components with a weight-average molecular weight of 10,000 or less is 0.5 mol% to 2.5 mol%.

3. The microporous separator for lithium batteries according to claim 1, wherein the molecular weight distribution of the polyolefin resin is between 3 and 6.

4. The microporous separator satisfies one or more combinations of the following a to e: a. The surface resistance is 0.2Ω to 0.7Ω. b. The average pore size is 25 nm to 50 nm. c. The thickness is 1 μm to 30 μm. d. The air permeability is 10 sec / 100cc to 300 sec / 100cc. e. The tensile strength in the MD or TD direction is 2000 kgf / cm². ２ ~4000kgf / cm ２ The microporous separator for lithium batteries according to claim 1.

5. The microporous separator for a lithium battery according to claim 1, wherein the polyolefin resin is selected from polyethylene, polypropylene, polybutylene, polymethylpentene, copolymers thereof, and comixtures thereof.

6. The microporous separator for lithium batteries according to claim 1, wherein the polyolefin resin has a weight-average molecular weight of 100,000 or less, and the proportion of polyolefin chain segment components is 10 mol% to 30 mol%.

7. The microporous separator for lithium batteries according to claim 1, wherein the polyolefin resin has a weight-average molecular weight of 10,000 or less, and the proportion of polyolefin chain segment components is 0 mol% to 2 mol%.

8. A method for preparing a microporous separator for a lithium battery according to any one of claims 1 to 7, (a) A step of melting and kneading a mixture containing polyolefin resin and a plasticizer to form a molten product, (b) a step of extruding the molten material and hardening it into a thick plate, (c) A step of stretching the aforementioned thick plate in the longitudinal direction (MD direction) and the transverse direction (TD direction) to obtain an elongated body, The process includes (d) removing the plasticizer from the stretched material and drying it to obtain a microporous separator for the lithium battery, The preparation method is characterized in that the characteristic viscosity index of the polyolefin resin is 800 ml / g to 1600 ml / g.

9. The preparation method according to claim 8, wherein the weight ratio of the polyolefin resin to the plasticizer is between 15:85 and 45:

55.

10. In process (a), an extruder is used for melting and kneading, with the extruder temperature being 160°C to 250°C and the extruder screw speed being 60 r / min to 100 r / min. Furthermore, the preparation method according to claim 8, wherein in step (b), the mixture is attached to an extrusion casting roller via a die head to form a thick plate by cooling and hardening, and the casting roller temperature is 20°C to 30°C.

11. The preparation method according to claim 8, wherein in step (c), stretching in the vertical direction is performed at a stretching temperature of 80°C to 120°C, the stretching ratio is 4 to 9 times, and the stretching method is multi-point dispersed stretching.

12. The preparation method according to claim 8, wherein a preheating step is provided before stretching in the vertical direction, the temperature of the preheating is 60°C to 100°C, the preheating is gradient preheating, the gradient preheating has 2 to 4 stages, and the temperature difference between two adjacent gradients is 7°C to 25°C.

13. The preparation method according to claim 8, wherein in step (c), the stretching temperature in the transverse direction is 90°C to 125°C, the stretching ratio is 4 to 8 times, and the absolute value of the difference between the stretching ratio in the vertical direction and the stretching ratio in the transverse direction is 1 or less.

14. The preparation method according to claim 8, wherein step (d) further includes a second transverse stretching step after the drying step, the stretching temperature for the second transverse stretching being 125°C to 140°C and the stretching ratio being 1.4 to 1.8 times.

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