Microporous separator for lithium battery and method for preparing 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] The present invention relates to the field of lithium ion battery technology, and in particular 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 whose pores are filled with functional materials to exhibit new functions, battery separators, etc. Among these uses, polyolefin microporous membranes are particularly suitable for use as separators for lithium-ion batteries, which are widely used in laptop computers, mobile phones, digital cameras, etc. This is because polyolefin microporous membranes have excellent mechanical strength and pore sealing properties.

[0003] Microporous separators for lithium batteries are also required to have good planar elasticity recovery.

[0004] However, currently commonly used polyolefin microporous membranes, especially those used in wet-processed lithium battery separators, have low planar elastic recovery rates, less than 14% in both the transverse and longitudinal directions, which can lead to irreversible deformation of the microporous membrane during battery assembly, resulting in a microporous membrane that deviates from the designed battery size and affecting electrochemical safety performance.

[0005] Currently, there is no microporous separator that combines high mechanical strength and good planar elasticity recovery, has a thin thickness, and has an excellent pore structure and surface. In particular, it is difficult to obtain a lithium-ion battery separator by a wet process that has excellent planar elasticity recovery. Summary of the Invention [Problem to be solved by the invention]

[0006] By controlling the proportion of low molecular weight chain segments in the microporous separator, the present invention can obtain a lithium ion battery separator with excellent planar elastic recovery when using a commonly used polyolefin resin as the main raw material. Furthermore, the present invention also solves the problem of high separator surface resistance, reducing the surface resistance of the lithium ion battery separator of the present invention by more than 20% compared to conventional techniques. Furthermore, the pore size, air permeability, and mechanical strength of the separator of the present invention are acceptable, making it suitable for use in lithium ion batteries.

[0007] The present inventors have further discovered that the above separator can be obtained by using low-temperature multi-point dispersed stretching in the machine direction (MD) stretching and low-temperature low-magnification stretching in the transverse direction (TD1) stretching, 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 a lithium battery, which is a porous monolayer film containing a polyolefin resin, wherein the microporous separator has a characteristic viscosity index of 700 ml / g to 1500 ml / g, preferably 900 ml / g to 1200 ml / g, and the ratio 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%, and more preferably 15 mol% to 21 mol%, and the microporous separator has an elastic recovery rate in the machine direction of 14% or more, preferably 18% or more, and an elastic recovery rate in the transverse direction of 14% or more, preferably 18% or more, measured under the following conditions: a separator sample piece having a width of 15 mm is cut in either the machine direction (MD) or the transverse direction (TD), and the separator sample piece is cut from a position L0=100 mm. SelectedThe specimen was stretched in the direction at a speed of 50 mm / min to 50% elongation, then held for 60 seconds, and then left to shrink naturally for 3 minutes, after which the length L1 was measured. 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 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 %, and more preferably 0.5 mol % to 1.5 mol %.

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

[0011] Preferably, the microporous separator satisfies one or a combination 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 / 100 cc to 300 sec / 100 cc, preferably 60 sec / 100 cc to 170 sec / 100 cc; e. Tensile strength in MD or TD direction is 2000 kgf / cm 2 ~4000kgf / cm 2 and preferably 2800 kgf / cm 2 ~4000kgf / cm 2 is.

[0012] Preferably, the polyolefin resin is selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE), polypropylene, polybutylene, polymethylpentene, copolymers thereof, and co-blends 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 even 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 %, and more 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; (b) extruding the melt and hardening it into a slab; (c) stretching the thick plate in the machine direction (MD) and the transverse direction (TD) to obtain an elongated body; and (d) removing the plasticizer from the elongated body and drying it to obtain the microporous separator for lithium batteries; Here, in the polyolefin resin in 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 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.

[0017] Preferably, in the polyolefin resin in 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 %, and more preferably 0.3 mol % to 1.5 mol %.

[0018] Preferably, the molecular weight distribution of the polyolefin resin is in the range of 3 to 6, and more preferably in the range of 3.5 to 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 %, and 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 %, and more preferably 0.5 mol % to 1.5 mol %.

[0021] Preferably, the elastic recovery rate of the microporous separator in the machine direction 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, measured under the following conditions: cutting a separator sample piece 15 mm wide in either the machine direction (MD) or the transverse direction (TD), and cutting the separator sample piece from L0=100 mm. Selected The specimen was stretched in the direction at a speed of 50 mm / min to 50% elongation, then held for 60 seconds, and then left to shrink naturally for 3 minutes, after which the length L1 was measured. Calculating the elastic recovery rate using the following formula: Elasticity 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), and the temperature of the extruder is preferably 160°C to 250°C, and the screw speed of the extruder is preferably 60 r / min to 100 r / min.

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

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

[0026] Preferably, a preheating step is performed before the stretching in the machine direction, and the preheating temperature is 60°C to 100°C. Preferably, the preheating is a gradient preheating temperature increase, and the gradient preheating temperature increase is set to 2 to 4 stages, 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 90°C to 125°C, preferably 90°C to 115°C, the stretching ratio is 4 to 8 times, preferably 4 to 6 times, and the absolute value of the difference between the stretching ratio in the longitudinal direction and the stretching ratio in the transverse direction is 1 or less.

[0028] Preferably, the step (d) further includes a second transverse stretching (TD2 stretching) step after the drying step, in which the stretching temperature in 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, UHDPE), polypropylene, polybutylene, polymethylpentene, copolymers thereof, and co-blends thereof. DETAILED DESCRIPTION OF THE INVENTION

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

[0031] The terms "one" and "another" as used herein are used for descriptive purposes only and are not to be construed as expressing or implying relative importance or as implicitly designating the number of technical features shown.

[0032] As used herein, the term "about," when referring to a particular value, is meant to include variations therein, such as ±10%, ±5%, ±1%, or ±0.1% of the particular value.

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

[0034] As used herein, the term "polyolefin" may refer to a polyolefin monomer (ie, a single type of polyolefin), a polyolefin copolymer, or a polyolefin co-blend.

[0035] As used herein, the term "co-mixture" refers to a physical mixture of two or more homopolymers, copolymers, or homopolymers and copolymers having different molecular architectures. Specifically, a co-mixture can include different polymers, i.e., at least two polymers with different chemistries (e.g., polyethylene, polypropylene, and / or ethylene-propylene copolymers with different chemistries), and / or polymers with the same chemistry but different properties (e.g., two different polyethylenes with different properties (density, molecular weight, molecular weight distribution, rheology, additive (composition and / or percentage), etc.)).

[0036] As used herein, the term "machine direction," also known as MD direction, refers to the direction of machine movement.

[0037] As used herein, the term "cross direction", also referred to as TD direction, refers to the direction perpendicular to the direction of machine motion.

[0038] In a first aspect, the present invention provides a microporous separator for a lithium battery, which is a porous monolayer film containing a polyolefin resin, wherein the microporous separator has a characteristic viscosity index of 700 ml / g to 1500 ml / g, and preferably 900 ml / g to 1200 ml / g; the microporous separator has a ratio of the number of polyolefin chain segment components having a weight-average molecular weight of 100,000 or less of 15 mol% to 30 mol%, and preferably 15 mol% to 25 mol%, and more preferably 15 mol% to 21 mol%; and the microporous separator has an elastic recovery rate in the machine direction of 14% or more, and preferably 18% or more, and an elastic recovery rate in the transverse direction of 14% or more, and preferably 18% or more, measured under the following conditions: cutting a separator sample piece having a width of 15 mm in either the machine direction (MD) or the transverse direction (TD), and cutting the separator sample piece from a position L0=100 mm: Selected The specimen was stretched in the direction at a speed of 50 mm / min to 50% elongation, then held for 60 seconds, and then left to shrink naturally for 3 minutes, after which the length L1 was measured. 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 ensuring the above elastic recovery rate, processing stability of the separator can be ensured, and it is possible to avoid the separator's excessive repulsion during the battery or separator preparation process from affecting product manufacturing.

[0040] According to some preferred embodiments of the present invention, 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 %.

[0041] In the present invention, the polyolefin resin can be any polyolefin resin commonly used in the art for preparing microporous separators for lithium batteries, such as polyethylene (e.g., LDPE). (Low-Density Polyethylene) , LLDPE (Linear Low-Density Polyethylene) , HDPE (High-Density Polyethylene) , UHDPE (Ultra High-Density Polyethylene) The polymer may be selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, copolymers thereof, and co-mixtures thereof, among which polyethylene and / or polypropylene are preferred.

[0042] According to the present invention, the polyolefin resin used may be any polyolefin resin capable of producing the microporous separator for lithium batteries required by the present invention. To provide a microporous separator with a high planar elastic recovery rate, the proportion of polyolefin chain segment components having a weight-average molecular weight of 100,000 or less in the polyolefin resin is preferably 10 mol% to 30 mol%, more preferably 10 mol% to 25 mol%, more preferably 13 mol% to 25 mol%, and even more preferably 13 mol% to 20 mol%. Preferably, the proportion of polyolefin chain segment components having a weight-average molecular weight of 10,000 or less in the polyolefin resin is 0 mol% to 2 mol%, preferably 0.3 mol% to 1.0 mol%. 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 / 100 cc to 300 sec / 100 cc, preferably 60 sec / 100 cc to 170 sec / 100 cc; e. Tensile strength in MD or TD direction is 2000 kgf / cm 2 ~4000kgf / cm 2 and preferably 2800 kgf / cm 2 ~4000kgf / cm 2 is.

[0044] In order 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 process.

[0045] In a second aspect, 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; (b) extruding the melt and hardening it into a slab; (c) stretching the thick plate in the machine direction (MD) and the transverse direction (TD) to obtain an elongated body; and (d) removing the plasticizer from the elongated body and drying it to obtain the microporous separator for lithium batteries; Here, in the polyolefin resin in 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 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 having a high planar elastic recovery rate suitable for use in a lithium battery can be obtained by processing a mixture containing a suitable polyolefin resin and a plasticizer according to the preparation method of the present invention.

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

[0048] In the present invention, the plasticizer is a low molecular weight solvent capable of dissolving polyolefin resin, and may be, for example, liquid paraffin, diethyl phthalate, palm oil, etc., and has a kinematic viscosity of 35 mm at 40°C. 2 / s~120mm 2 Preferably, the liquid paraffin has a kinematic viscosity of 40 mm / s at 40°C. 2 / s~55mm 2 It is more preferable that the kinematic viscosity is liquid paraffin having a viscosity of 1 / s. GB / T 265 is used as the test method for the kinematic viscosity.

[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, more preferably between 20:80 and 30:70. Specifically, the weight ratio of the polyolefin resin to the plasticizer may be 20:80, 21:79, 22:78, 23:77, 24:76, 25:75, 26:74, 27:73, 28:72, 29:71, or 30:70. By applying the above weight ratio of the polyolefin resin to the plasticizer, the resilience performance of the microporous separator in the planar direction can be improved.

[0050] To further improve the resilience of the microporous separator in the planar direction, the proportion of polyolefin chain segment components having a weight-average molecular weight of 10,000 or less in the polyolefin resin is 0 mol % to 2 mol %, preferably 0.3 mol % to 1.5 mol %, and the molecular weight distribution of the polyolefin resin is preferably 3 to 6, more 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 a melt, for example, an extruder can be used for melt-kneading. In some preferred embodiments, the extruder parameters include the extruder temperature and extruder screw speed, and 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 melt is extruded and hardened into a thick plate by any method that can form a thick plate of the desired thickness, and for example, the mixture may be extruded through a die head and attached to a casting roller, which is cooled and hardened to form a thick plate, and the temperature of the casting roller is preferably 20° C. to 30° C. The rolling speed of the casting roller may be 3 m / min to 8 m / min.

[0053] According to the present invention, in step (c), stretching in the machine direction (i.e., machine direction stretching or MD stretching) and stretching in the transverse direction (i.e., transverse direction stretching or TD1 stretching) are carried out to obtain an elongated body. Specifically, machine direction stretching may be followed by transverse direction stretching, or transverse direction stretching may be followed by machine direction stretching, but the former is preferred.

[0054] To further improve the resilience of the microporous separator in the planar direction, in step (c), stretching in the machine direction is carried out at a stretching temperature of 80°C to 120°C, with a stretching ratio of 4 to 9, preferably 4 to 7, and the stretching method is multi-point distributed stretching, preferably with 3 to 7 points. In multi-point distributed stretching, a stretching point is the speed ratio between two adjacent stretching rollers with different linear speeds, i.e., the ratio between the rear roller linear speed and the front roller linear speed, and the stretching ratio for each stretching point is 1.1 to 4. In the case of multi-point distributed stretching, the stretching ratio refers to the sum of the stretching ratios at each point, and each stretching ratio is preferably independently 1.1 to 3, preferably 1.1 to 2.4. Preferably, there is at least one set of three adjacent enlargement points, and there is an increasing relationship between the magnifications, and the absolute value of the difference between the enlargement magnifications of the adjacent enlargement points is >0.1. For example, in the case of three-point enlargement, the distribution of the enlargement magnifications may be 1.3 / 1.6 / 1.9 or 1.5 / 1.9 / 2.0; in the case of five-point enlargement, the distribution of the enlargement magnifications may be 1.1 / 1.4 / 2.0 / 2.0. It may 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, and in the case of a 7-point enlargement the distribution of enlargement ratios may 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, the three adjacent points of the increasing enlargement ratio are not only present at the last three enlargement points of the enlargement section, but preferably also at the front, middle and end sections of the enlargement. At least one of means that three adjacent enlargement points should have an increasing magnification relationship.

[0056] Increasing the longitudinal stretching ratio is advantageous due to sufficient stretching and sufficient polymer orientation, which results in the formation of fibrils with better mechanical properties and uniform size distribution, and better elastic recovery of the microporous membrane.

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

[0058] To further improve the resilience 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, preferably 4 to 6. Preferably, the absolute value of the difference between the stretching ratio in the longitudinal 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 invention, in step (d), the plasticizer can be removed from the elongated body by any method, preferably by using an extractant to remove the plasticizer from the elongated body. In some embodiments, The extractant may be an alkane extractant; or With halogenated hydrocarbon extractants It's okay to have , for example Dichloromethane.

[0060] As a method for removing the plasticizer from the elongated body using an extractant, there is a method for removing the plasticizer in the elongated body by circulating the extractant. Preferably, the amount of the circulating extractant is 1 m 3 / h~5m 3 After extraction, the elongated body is dried by heating using one or more of 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, the step (d) further includes a second transverse stretching (TD2 stretching) step after the drying step, in which the stretching temperature in 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] In the microporous separator obtained by the above preparation method, 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%. 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%. Furthermore, the elastic recovery rate in the machine 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, and the conditions are as follows: a separator sample piece having a width of 15 mm is cut in either the machine direction (MD) or the transverse direction (TD), and the separator sample piece is cut from L0=100 mm. Selected The specimen was stretched in the direction at a speed of 50 mm / min to 50% elongation, then held for 60 seconds, and then left to shrink naturally for 3 minutes, after which the length L1 was measured. 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 a lithium battery obtained by the preparation method according to the above 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 elastic recovery rate is as follows. Cut a test sample piece 15 mm wide along the MD or TD direction, measure 100 mm in the length direction of the test sample piece, draw lines on both ends corresponding to the length, and call this length L0. The test specimen was placed in the stretching apparatus and the stretching collet was aligned with the line (i.e., Colette The distance between the clamps is 100 mm, and the stretching speed is 50 mm / min. The test specimen is stretched to 50% of its length and held for 60 seconds, then removed and placed at room temperature for 3 minutes to allow natural shrinkage. The distance between the two ends of the line is measured and recorded as L1.

[0066] (2) The test method for surface resistance is as follows. Cut a total of four separator samples with a diameter of 45 mm in a flat position, immerse the samples in an electrolyte (1.0 M LiPF6 in an EC / EMC / DMC solvent with a volume ratio of 1:1:1) and seal for 30 minutes. Then, place approximately 15 ml of 1 mol / L fresh electrolyte (1.0 M LiPF6 in an EC / EMC / DMC solvent with a volume ratio of 1:1:1) into a sheet resistance test jig, and place one, two, three, or four separators in the jig for testing. Perform linear fitting with the number of separator layers as the abscissa and the separator resistance value as the ordinate, and determine the slope of the straight line and the goodness of fit. If the goodness of fit is greater than 0.999, then the slope is taken as the separator's sheet resistance.

[0067] (3) The average pore size was measured at 25°C using a PMI device (Chia Yun Instrument Inc., Model CFP-1500AE) with a galwick penetrant (surface tension at 25°C: 15.9 dynes / cm). The pore size is expressed in nm.

[0068] (4) Thickness is measured in accordance with the provisions of GB-T 36363-2018.

[0069] (5) Air permeability is measured according to the GB / T 36363-2018 standard, and is measured when 100 ml of air is compressed to an area of ​​6.45 cm under a pressure of 1.21 kPa.2 The time required for the particles to pass through the separator is measured.

[0070] (6) Tensile strength is measured in accordance with the provisions of GB / T1040.3-2006.

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

[0072] (8) Molecular weight distribution is measured in accordance with the provisions of GB / T 36214.4-2018.

[0073] (9) Characteristic viscosity is 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 45 mm 2 The polyethylene resin is mixed with paraffin oil (of which the weight average molecular weight is 100,000 or less) at a weight ratio of 25:75 and melt-kneaded in an extruder to form a melt, where the proportion of the number of polyethylene chain segments with a weight average molecular weight of 10,000 or less in the polyethylene resin is 17 mol %, and the proportion of the number of polyethylene chain segments with a weight average molecular weight of 10,000 or less is 0.8 mol %, the extruder temperature is 220°C, and the extruder screw rotation speed is 80 r / min. The above melt was cooled and hardened using a casting roller to form a thick plate. The casting roller temperature was set to 25°C. The resulting thick plate was subjected to a three-stage preheating treatment at 60°C / 80°C / 100°C, with a total preheating time of 15s. After this, it was stretched. The stretching in the MD direction was performed using three-point distributed stretching, with a constant stretching temperature of 95°C, a total stretching ratio of 6.29 times, and a distribution of the stretching ratios at each point of 1.7 / 1.85 / 2.0. The transverse stretching (TD1 stretching) temperature was 110°C, and the transverse stretching ratio was 6 times. After removing the plasticizer (paraffin oil), the sheet was dried with hot air at 55°C, and then stretched a second time in the transverse direction (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 at a heat setting process temperature of 135°C.

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

[0076] Example 10 Stretching in the MD direction was performed using five-point distributed stretching, with a constant stretching temperature of 95°C, a total stretching ratio of 9.3 times, and a distribution of stretching ratios at each point of 1.1 / 1.6 / 2.0 / 2.4 / 1.1. This differs from Example 1 in that the transverse stretching (TD1 stretching) temperature was 110°C and the transverse stretching ratio was 7.5 times.

[0077] Examples 2-1 to 2-4 Differences from Example 2 are shown in detail in Table 2, and 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 to 11 and Comparative Examples 1 and 2 in Table 1, when the ratio of the number of polyolefin chain segment components having 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 Enlarge Strength≧2000kgf / cm 2), and when the proportion of the number 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 a comparison between Examples 1 and 10, when the stretch ratio in MD and TD is increased, only the MD tensile strength improves slightly, while the TD tensile 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 ratio of the number of polyolefin chain segments with a weight-average molecular weight of 100,000 or less in the microporous separator can be adjusted and controlled by combining the extrusion method and raw materials.

[0082] In this specification and the appended claims, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. All technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art, unless otherwise defined. The methods described herein can be carried out in any order that is logically possible, excluding the specific order disclosed.

[0083] The representative examples are intended to be illustrative of the invention and are not intended to, and should not be construed as, limiting the scope of the invention. Indeed, multiple modifications of the invention and many other embodiments in addition to those shown and described herein will be apparent to those skilled in the art, including the examples and the scientific and patent literature cited herein. The examples contain important additional information, examples, and guidance that can be employed by the practice of many embodiments of the invention and equivalents thereof.

Claims

1. A microporous separator for a lithium battery which is a porous monolayer film containing a polyolefin resin, the microporous separator having a characteristic viscosity index of 700 ml / g to 1500 ml / g, preferably 900 ml / g to 1200 ml / g, and a ratio of the number of polyolefin chain segment components having a weight-average molecular weight of 100,000 or less in the microporous separator of 15 mol% to 30 mol%, preferably 15 mol% to 25 mol%, more preferably 15 mol% to 21 mol%, and an elastic recovery rate in the machine direction of the microporous separator of 14% or more, preferably 18% or more, and an elastic recovery rate in the transverse direction of the microporous separator of 14% or more, preferably 18% or more, measured under the following conditions: a separator sample piece having a width of 15 mm is cut in either the machine direction (MD) or the transverse direction (TD), and the separator sample piece is subjected to L ０ The sample was stretched from the point of length L = 100 mm in the above direction at a speed of 50 mm / min to 50% elongation, and then held for 60 seconds. After that, the sample was left as it was for 3 minutes to allow natural shrinkage, and the length L １ The elastic recovery rate is calculated as follows: ０ -L １ ) / (0.5 * L ０ ) * 100%.

2. 2. The microporous separator for lithium batteries according to claim 1, wherein the proportion of the number of polyolefin chain segment components having a weight average molecular weight of 10,000 or less in the microporous separator is 0.5 mol% to 2.5 mol%, preferably 0.5 mol% to 1.5 mol%.

3. 3. The microporous separator for lithium batteries according to claim 1, wherein the polyolefin resin has a molecular weight distribution of 3 to 6, preferably 3.5 to 5.

0.

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, 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 / 100 cc to 300 sec / 100 cc, preferably 60 sec / 100 cc to 170 sec / 100 cc; e. Stretch strength in MD or TD direction is 2000 kgf / cm ２ ~4000kgf / cm ２ , preferably 2800 kgf / cm ２ ~4000kgf / cm ２ The microporous separator for a lithium battery according to any one of claims 1 to 3,

5. 5. The microporous separator for lithium batteries according to claim 1, wherein the polyolefin resin is selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, and UHDPE), polypropylene, polybutylene, polymethylpentene, copolymers thereof, and co-mixtures thereof.

6. 6. The microporous separator for lithium batteries according to claim 1, wherein 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 even more preferably 13 mol% to 20 mol%.

7. The microporous separator for lithium batteries according to any one of claims 1 to 6, wherein 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%.

8. The microporous separator for a lithium battery according to any one of claims 1 to 7, wherein the microporous separator is prepared by a wet process.

9. 1. A method for preparing a microporous separator for a lithium battery, comprising: Step (a) of melt-kneading a mixture containing a polyolefin resin and a plasticizer to form a melt; (b) extruding the melt and hardening it into a slab; (c) stretching the thick board in the machine direction (MD) and the transverse direction (TD) to obtain an elongated body; and (d) removing the plasticizer from the elongated body and drying it to obtain the microporous separator for lithium batteries; Here, in the polyolefin resin in 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 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.

10. In the polyolefin resin in step (a), the ratio 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 %, 10. The preparation method according to claim 9, wherein the molecular weight distribution of the polyolefin resin is 3 to 6, preferably 3.5 to 5.

0.

11. In the microporous separator, the ratio 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%, Furthermore, in the microporous separator, the ratio 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%, Furthermore, the elastic recovery rate in the machine direction of the microporous separator 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, measured under the following conditions: a separator sample piece 15 mm wide is cut in either the machine direction (MD) or the transverse direction (TD), and the separator sample piece is cut into L ０ The sample was stretched from the point of length L = 100 mm in the above direction at a speed of 50 mm / min to 50% elongation, and then held for 60 seconds. After that, the sample was left as it was for 3 minutes to allow natural shrinkage, and the length L １ The elastic recovery rate is calculated as follows: ０ -L １ ) / (0.5 * L ０ 11. The method of claim 9 or 10, comprising calculating as follows:

12. The preparation method according to any one of claims 9 to 11, wherein 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.

13. In step (a), an extruder is used for melt-kneading, and preferably, the temperature of the extruder is 160°C to 250°C and the screw speed of the extruder is 60 r / min to 100 r / min; 13. The method according to any one of claims 9 to 12, further comprising extruding the mixture through a die head and attaching it to a casting roller to form a slab by cooling and hardening in step (b), wherein the casting roller temperature is between 20°C and 30°C.

14. In step (c), the stretching in the longitudinal direction is carried out 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 distributed stretching, preferably 3 to 7 points; The method according to any one of claims 9 to 13, further comprising a preheating step before the stretching in the machine direction, wherein the preheating temperature is 60°C to 100°C, and preferably the preheating is a gradient preheating temperature increase, the gradient preheating temperature increase is set to 2 to 4 stages, and the temperature difference between two adjacent gradients is 7°C to 25°C.

15. The method according to any one of claims 9 to 14, wherein in step (c), the stretching temperature in the transverse direction is 90°C to 125°C, preferably 90°C to 115°C, the stretching ratio is 4 to 8 times, preferably 4 to 6 times, and the absolute value of the difference between the stretching ratio in the longitudinal direction and the stretching ratio in the transverse direction is 1 or less.

16. The method according to any one of claims 9 to 15, wherein in step (d), after the drying step, the method further comprises a second transverse stretching step, wherein the stretching temperature in the second transverse stretching step is 125°C to 140°C, preferably 130°C to 140°C, and the stretching ratio is 1.4 to 1.8 times.

17. 17. The method according to any one of claims 9 to 16, wherein the polyolefin resin is selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE), polypropylene, polybutylene, polymethylpentene, copolymers thereof, and co-mixtures thereof.

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