Micro-porous diaphragm for lithium battery and preparation method of micro-porous diaphragm

Abstract

The invention relates to a microporous diaphragm for a lithium battery and a preparation method of the microporous diaphragm. Specifically, the present invention provides a microporous separator for a lithium battery, which is a porous single-layer film containing a polyolefin resin, and which exhibits excellent elastic recovery performance in both a horizontal direction and a vertical direction of a plane. The invention also provides a preparation method of the high-plane elastic recovery lithium ion battery diaphragm.

Description

A microporous diaphragm for lithium battery and preparation method thereof Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to a microporous diaphragm for lithium batteries and a preparation method thereof. Background Art

[0002] Polyolefin microporous membranes are widely used as separation membranes used in the separation and selective permeation of various substances, as well as insulating materials. For example, polyolefin microporous membranes are used as precision filtration membranes, separators for fuel cells, separators for capacitors, parent materials for functional membranes filled with functional materials in order to express new functions, separators for batteries, etc. Among these uses, polyolefin microporous membranes are particularly suitable for use as separators for lithium-ion batteries widely used in laptop computers, portable phones, digital cameras, etc. As a reason for this, polyolefin microporous membranes have excellent membrane mechanical strength and closed pore properties.

[0003] Furthermore, microporous separators for lithium batteries are also required to have good in-plane elastic recovery properties.

[0004] However, the planar elastic recovery rate of the commonly used polyolefin microporous membranes is low, especially the wet-process lithium battery separators, which are less than 14% in both the transverse and longitudinal directions. This will affect the irreversible deformation of the microporous membrane during the battery assembly process, causing the microporous membrane to deviate from the battery design dimensions, thereby affecting the electrochemical safety performance.

[0005] At present, there is no microporous membrane that has both high mechanical strength and good planar elastic recovery, while being thin, having excellent pore structure and surface. It is especially difficult to obtain a wet-process lithium-ion battery membrane with excellent planar elastic recovery.

[0006] Summary of the Invention

[0007] By controlling the proportion of low-molecular-weight segments in the microporous separator, the present invention achieves a lithium-ion battery separator with excellent planar elastic recovery, using a commonly used polyolefin resin as the primary raw material. Furthermore, the present invention addresses the issue of high membrane surface resistance, reducing the surface resistance of the present lithium-ion battery separator by over 20% compared to prior art. Furthermore, the membrane's pore size, air permeability, and mechanical strength are acceptable, making it suitable for use in lithium-ion batteries.

[0008] The present invention also found that the above-mentioned diaphragm can be obtained by using a low-temperature multi-point distributed stretching process in the longitudinal (MD) stretching, using a low-temperature low-ratio stretching in the transverse (TD1) stretching, and controlling the difference between the MD stretching ratio and the TD1 stretching ratio.

[0009] In some specific embodiments, the present invention provides a microporous membrane for a lithium battery, which is a porous monolayer membrane comprising a polyolefin resin, wherein the intrinsic viscosity index of the microporous membrane is 700-1500 ml / g, preferably 900-1200 ml / g; in the microporous membrane, the amount of the polyolefin segment component with a weight average molecular weight of less than 100,000 accounts for 15-30 mol%, preferably 15-25 mol%, more preferably 15-21 mol%; the longitudinal elastic recovery rate of the microporous membrane measured under the following conditions is greater than or equal to 14%, preferably Greater than or equal to 18%, and the transverse elastic recovery rate is greater than or equal to 14%, preferably greater than or equal to 18%, the conditions include: cutting a diaphragm spline with a width of 15 mm from one of the longitudinal direction (MD direction) or the transverse direction (TD direction), stretching the diaphragm spline from L0 = 100 mm in the direction at a speed of 50 mm / min to 50% elongation, holding it for 60 seconds, and then standing it for natural shrinkage for 3 minutes, and then measuring the length L1, and calculating the elastic recovery rate = (1.5*L0-L1) / (0.5*L0)*100%.

[0010] Preferably, in the microporous separator, the polyolefin segment component with a weight average molecular weight of less than 10,000 accounts for 0.5-2.5 mol%, preferably 0.5-1.5 mol%.

[0011] Preferably, the molecular weight distribution of the polyolefin resin is 3-6, preferably 3.5-5.0.

[0012] Preferably, the microporous membrane satisfies one or more of the following combinations:

[0013] a. Surface resistance is 0.2-0.7Ω, preferably 0.3-0.6Ω;

[0014] b. The average pore size is 25-50 nm, preferably 30-45 nm;

[0015] c. A thickness of 1-30 μm, preferably 4-12 μm;

[0016] d. Air permeability is 10-300 sec / 100 cc, preferably 60-170 sec / 100 cc;

[0017] e. The tensile strength in the MD or TD direction is 2000-4000kgf / cm 2 , preferably 2800-4000kgf / cm 2 .

[0018] Preferably, the polyolefin resin is selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE), polypropylene, polybutene, polymethylpentene, copolymers thereof, and blends thereof.

[0019] Preferably, in the polyolefin resin, the polyolefin segment component with a weight average molecular weight of less than 100,000 accounts for 10-30 mol%, preferably 10-25 mol%, more preferably 13-25 mol%, and further preferably 13-20 mol%.

[0020] Preferably, in the polyolefin resin, the polyolefin segment component with a weight average molecular weight of less than 10,000 accounts for 0-2 mol%, preferably 0.3-1.0 mol%.

[0021] Preferably, the microporous membrane is a membrane prepared by a wet process.

[0022] In some specific embodiments, the present invention provides a method for preparing a microporous separator for a lithium battery, comprising the following steps:

[0023] (a) melt-kneading a mixture comprising a polyolefin resin and a plasticizer to form a melt;

[0024] (b) extruding and solidifying the melt into a thick sheet;

[0025] (c) stretching the thick sheet in the longitudinal direction (MD direction) and the transverse direction (TD direction) to obtain a stretched body;

[0026] (d) removing the plasticizer from the stretched body and drying the stretched body to obtain the microporous separator for lithium battery;

[0027] Among them, in the polyolefin resin of step (a), the number of polyolefin segment components with a weight-average molecular weight of less than 100,000 accounts for 10-30 mol%, preferably 10-25 mol%, more preferably 13-25 mol%, and further preferably 13-20 mol%; the intrinsic viscosity index of the polyolefin resin is 800-1600 ml / g, preferably 1000-1300 ml / g.

[0028] Preferably, in the polyolefin resin of step (a), the amount of the polyolefin segment component with a weight average molecular weight of less than 10,000 accounts for 0-2 mol%, preferably 0.3-1.5 mol%.

[0029] Preferably, the molecular weight distribution of the polyolefin resin is 3-6, preferably 3.5-5.0.

[0030] Preferably, in the microporous separator, the polyolefin segment component having a weight average molecular weight of 100,000 or less accounts for 15-30 mol %, preferably 15-25 mol %, and more preferably 15-21 mol %.

[0031] Preferably, in the microporous separator, the polyolefin segment component with a weight average molecular weight of less than 10,000 accounts for 0.5-2.5 mol%, preferably 0.5-1.5 mol%.

[0032] Preferably, the longitudinal elastic recovery rate of the microporous diaphragm measured under the following conditions is greater than or equal to 14%, preferably greater than or equal to 18%, and the transverse elastic recovery rate is greater than or equal to 14%, preferably greater than or equal to 18%, and the conditions include: cutting a diaphragm spline with a width of 15 mm in one of the longitudinal direction (MD direction) or the transverse direction (TD direction), stretching the diaphragm spline from L0 = 100 mm in the direction at a speed of 50 mm / min to 50% elongation, holding it for 60 seconds, and then letting it stand and naturally shrink for 3 minutes before measuring the length L1, and calculating the elastic recovery rate as = (1.5*L0-L1) / (0.5*L0)*100%.

[0033] 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.

[0034] Preferably, in step (a), an extruder is used for melt mixing, preferably with an extruder temperature of 160-250° C. and an extruder screw speed of 60-100 r / min.

[0035] Preferably, in step (b), the mixture is extruded through a die and attached to a casting roller to be cooled and solidified to form a thick sheet, and the temperature of the casting roller is 20-30°C.

[0036] Preferably, in step (c), the stretching in the longitudinal direction adopts a stretching temperature of 80-120° C., a stretching ratio of 4-9 times, preferably a stretching ratio of 4-7 times, and a stretching method of multi-point distributed stretching, preferably 3-7 points.

[0037] Preferably, a preheating step is provided before stretching in the longitudinal direction, and the preheating temperature is 60-100°C; preferably, the preheating is a gradient preheating temperature increase, and the gradient preheating temperature increase is set in 2-4 stages, and the temperature difference between two adjacent gradients is 7-25°C.

[0038] Preferably, in step (c), the stretching temperature in the transverse direction (TD1) is 90-125°C, preferably 90-115°C, the stretching ratio is 4-8 times, preferably 4-6 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 less than 1.

[0039] Preferably, in the step (d), a second transverse stretching (TD2 stretching) step may be included after the drying process, the stretching temperature of the second transverse stretching is 125-140° C., preferably 130-140° C., and the stretching ratio is 1.4-1.8 times.

[0040] Preferably, the polyolefin resin is selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE), polypropylene, polybutene, polymethylpentene, copolymers thereof, and blends thereof. DETAILED DESCRIPTION

[0041] Before further describing the present invention, the following sections collect certain terms used in the specification, examples, and appended claims. The definitions listed herein should be read and understood by those skilled in the art in light of the remainder of the present invention. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which this invention belongs.

[0042] The terms “one” and “another” used herein are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features.

[0043] As used herein, the term "about" when referring to a value is meant to encompass variations therefrom, such as ±10%, ±5%, ±1%, or ±0.1% of the particular value.

[0044] As used herein, the term "substantially the same" when referring to two values ​​means that the two values ​​differ by less than 10%, 5%, or 1% from the average of the two values.

[0045] As used herein, the term "polyolefin" may be a polyolefin monomer (ie, a single type of polyolefin), a polyolefin copolymer, or a polyolefin blend.

[0046] As used herein, the term "blend" refers to a physical mixture of two or more homopolymers, copolymers, or homopolymers and copolymers having different molecular structures. Specifically, a blend 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 (e.g., density, molecular weight, molecular weight distribution, rheology, additives (components and / or percentages)), etc.).

[0047] As used herein, the term "machine direction" is also referred to as the MD direction and refers to the direction of machine travel.

[0048] The term "transverse direction" as used herein is also referred to as TD direction, and refers to a direction perpendicular to the running direction of the device.

[0049] A first aspect of the present invention provides a microporous membrane for a lithium battery, which is a porous monolayer membrane comprising a polyolefin resin, wherein the intrinsic viscosity index of the microporous membrane is 700-1500 ml / g, preferably 900-1200 ml / g; in the microporous membrane, the amount of the polyolefin segment component with a weight average molecular weight of less than 100,000 accounts for 15-30 mol%, preferably 15-25 mol%, more preferably 15-21 mol%; the longitudinal elastic recovery rate of the microporous membrane measured under the following conditions is greater than or equal to 14%, preferably greater than or equal to 18%, and the transverse elastic recovery rate is greater than or equal to 14%, preferably greater than or equal to 18%, and the conditions include: cutting a diaphragm spline with a width of 15 mm from one of the longitudinal direction (MD direction) or the transverse direction (TD direction), stretching the diaphragm spline from L0 = 100 mm in the direction at a speed of 50 mm / min to 50% elongation, holding it for 60 seconds, and then letting it stand and naturally shrink for 3 minutes, and then measuring the length L1, and calculating the elastic recovery rate = (1.5*L0-L1) / (0.5*L0)*100%.

[0050] Preferably, the upper limit of the elastic recovery rate of the microporous separator measured under the above conditions does not exceed 50%, preferably does not exceed 30%, and more preferably does not exceed 25%. This elastic recovery rate ensures the processing stability of the separator and prevents excessive rebound of the separator during the battery or separator production process, which may affect product manufacturing.

[0051] According to some preferred embodiments of the present invention, in the microporous membrane, the polyolefin segment component with a weight average molecular weight of less than 10,000 accounts for 0.5-2.5 mol%, preferably 0.5-1.5 mol%.

[0052] In the present invention, the polyolefin resin can use any polyolefin resin commonly used in the art for preparing microporous separators for lithium batteries, for example, it can be selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE), polypropylene, polybutene, polymethylpentene, copolymers thereof, and blends thereof, wherein polyethylene and / or polypropylene are preferred.

[0053] According to the present invention, the polyolefin resin used can be any polyolefin resin capable of producing the microporous lithium battery separator required by the present invention. In order to provide a microporous separator with high planar elastic recovery, preferably, in the polyolefin resin, the number of polyolefin segment components with a weight-average molecular weight of less than 100,000 accounts for 10-30 mol%, preferably 10-25 mol%, more preferably 13-25 mol%, and further preferably 13-20 mol%. Preferably, in the polyolefin resin, the number of polyolefin segment components with a weight-average molecular weight of less than 10,000 accounts for 0-2 mol%, preferably 0.3-1.0 mol%. Further preferably, the molecular weight distribution of the polyolefin resin is 3-6, preferably 3.5-5.0.

[0054] In some embodiments of the present invention, the microporous membrane satisfies one or more of the following combinations:

[0055] a. Surface resistance is 0.2-0.7Ω, preferably 0.3-0.6Ω;

[0056] b. The average pore size is 25-50 nm, preferably 30-45 nm;

[0057] c. A thickness of 1-30 μm, preferably 4-12 μm;

[0058] d. Air permeability is 10-300 sec / 100 cc, preferably 60-170 sec / 100 cc;

[0059] e. The tensile strength in the MD or TD direction is 2000-4000kgf / cm 2 , preferably 2800-4000kgf / cm 2 .

[0060] In order to obtain a microporous separator with high in-plane elastic recovery, the microporous separator of the present invention is preferably a separator prepared by a wet process.

[0061] A second aspect of the present invention provides a method for preparing a microporous separator for a lithium battery, the method comprising the following steps:

[0062] (a) melt-kneading a mixture comprising a polyolefin resin and a plasticizer to form a melt;

[0063] (b) extruding and solidifying the melt into a thick sheet;

[0064] (c) stretching the thick sheet in the longitudinal direction (MD direction) and the transverse direction (TD direction) to obtain a stretched body;

[0065] (d) removing the plasticizer from the stretched body and drying the stretched body to obtain the microporous separator for lithium battery;

[0066] Among them, in the polyolefin resin of step (a), the number of polyolefin segment components with a weight-average molecular weight of less than 100,000 accounts for 10-30 mol%, preferably 10-25 mol%, more preferably 13-25 mol%, and further preferably 13-20 mol%; the intrinsic viscosity index of the polyolefin resin is 800-1600 ml / g, preferably 1000-1300 ml / g.

[0067] According to the present invention, a microporous separator with high planar elastic recovery can be obtained by treating a mixture containing an appropriate polyolefin resin and a plasticizer according to the preparation method of the present invention, which is suitable for use in lithium batteries.

[0068] In the present invention, the polyolefin resin can use any polyolefin resin commonly used in the art for preparing microporous separators for lithium batteries, for example, it can be selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE), polypropylene, polybutene, polymethylpentene, copolymers thereof, and blends thereof, wherein polyethylene and / or polypropylene are preferred.

[0069] In the present invention, the plasticizer is a small molecule solvent that can dissolve polyolefin resin, such as liquid paraffin, diethyl phthalate, palm oil, etc., preferably with a kinematic viscosity of 35-120 mm at 40°C. 2 / s liquid paraffin, further preferably with a kinematic viscosity of 40-55 mm at 40 ° C 2 The kinematic viscosity test method is GB / T 265.

[0070] 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. Specifically, the weight ratio of the polyolefin resin to the plasticizer can 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 using such a weight ratio of polyolefin resin to plasticizer, the in-plane resilience of the microporous separator can be improved.

[0071] To further improve the rebound resilience of the microporous separator in the planar direction, the polyolefin segment component having a weight-average molecular weight of less than 10,000 in the polyolefin resin accounts for 0-2 mol%, preferably 0.3-1.5 mol%. Preferably, the molecular weight distribution of the polyolefin resin is 3-6, preferably 3.5-5.0.

[0072] According to the present invention, in step (a), any existing method capable of forming a melt of the mixture can be used for processing, for example, an extruder can be used for melt mixing. In some preferred embodiments, the parameters of the extruder include: an extruder temperature of 150-250°C, preferably 180-240°C, and an extruder screw speed of 60-100 rpm, preferably 70-90 rpm.

[0073] According to the present invention, in step (b), the method for extruding and solidifying the melt into a slab can be any method that can form a slab of desired thickness. For example, the mixture can be extruded through a die and attached to a casting roll for cooling and solidification to form a slab. The casting roll is preferably at a temperature of 20-30° C. The casting roll can have a rolling speed of 3-8 m / min.

[0074] According to the present invention, in step (c), stretching in the longitudinal direction (i.e., longitudinal stretching, MD stretching) and in the transverse direction (i.e., transverse stretching, TD1 stretching) are performed separately to obtain a stretched body. Specifically, longitudinal stretching can be performed first and then transverse stretching, or transverse stretching can be performed first and then longitudinal stretching, with the former being preferred.

[0075] In order to further improve the rebound performance of the microporous diaphragm in the plane direction, in step (c), the stretching in the longitudinal direction adopts a stretching temperature of 80-120°C, a stretching ratio of 4-9 times, preferably 4-7 times, and a stretching method of multi-point distributed stretching, preferably 3-7 points. In multi-point distributed stretching, the stretching point is the speed ratio between two adjacent stretching rollers with different linear speeds, that is, the linear speed of the rear roller / the linear speed of the front roller, and the stretching ratio of each stretching point is 1.1-4. When multi-point distributed stretching is adopted, the stretching ratio refers to the total stretching ratio of each point, and preferably the stretching ratio of each stretching point is independently 1.1-3, preferably 1.1-2.4. Preferably, there is at least one group of three adjacent stretching points, the ratios are in an increasing relationship, and the absolute value of the difference in stretching ratios between adjacent stretching points is greater than 0.1. For example, during 3-point stretching, the stretching ratio distribution may be as follows: 1.3 / 1.6 / 1.9 or 1.5 / 1.9 / 2.0; during 5-point stretching, the stretching ratio distribution may be as follows: 1.1 / 1.4 / 2.0 / 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; during 7-point stretching, the stretching ratio distribution may be as follows: 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.

[0076] Generally, the three adjacent points with increasing stretching ratios do not exist only in the last three stretching points in the stretching section. It is preferred that the three adjacent stretching points in the front and middle sections of the stretching section have a ratio increasing relationship.

[0077] The increasing longitudinal stretching ratio is more conducive to sufficient stretching and more complete orientation of the polymer, so that the formed fibrils have better mechanical properties and more uniform size distribution, and the microporous membrane separator has better elastic recovery.

[0078] According to some further preferred embodiments of the present invention, a preheating step is provided before stretching in the longitudinal direction, and the preheating temperature is 60-100°C. Preferably, the preheating is a gradient preheating temperature rise, and the gradient preheating temperature rise can be set to 2-4 stages, and preferably the temperature difference between two adjacent gradients is 7-25°C. The preheating time can be 2-100s, preferably 4-60s. In the case of gradient preheating temperature rise, the preheating time of two adjacent gradients can be the same or different, and each independently is 1-25s.

[0079] To further improve the in-plane resilience of the microporous separator, in step (c), the transverse stretching temperature is 90-125°C, preferably 90-115°C, and the stretching ratio is 4-8 times, preferably 4-6 times. Furthermore, the absolute value of the difference between the longitudinal stretching ratio and the transverse stretching ratio is preferably less than 1, more preferably 0.2-0.9, and even more preferably 0.4-0.8.

[0080] According to the present invention, in step (d), the plasticizer can be removed from the stretched body by any method, preferably by using an extractant. The extractant can be an alkane extractant, preferably a halogenated hydrocarbon extractant, with dichloromethane being most preferred.

[0081] As a method for removing the plasticizer from the stretched body using an extractant, the plasticizer in the stretched body can be removed by circulating the extractant. Preferably, the extractant circulation rate is 1-5 m³ / h. After extraction, the stretched body can be dried by heating using one or more of a heated roller, a heated plate, or hot air, preferably at a drying temperature of 20-150°C.

[0082] According to a preferred embodiment of the present invention, in the step (d), a second transverse stretching (TD2 stretching) step is further included after the drying process, and the stretching temperature of the second transverse stretching is 125-140°C, preferably 130-140°C, and the stretching ratio is 1.4-1.8 times.

[0083] In the microporous membrane prepared using the above preparation method, the polyolefin segment component having a weight-average molecular weight of 100,000 or less accounts for 15-30 mol%, preferably 15-25 mol%, and more preferably 15-21 mol%. Preferably, the polyolefin segment component having a weight-average molecular weight of 10,000 or less accounts for 0.5-2.5 mol%, preferably 0.5-1.5 mol%, of the microporous membrane. Furthermore, the longitudinal elastic recovery rate of the microporous diaphragm measured under the following conditions is greater than or equal to 14%, preferably greater than or equal to 18%, and the transverse elastic recovery rate is greater than or equal to 14%, preferably greater than or equal to 18%, and the conditions include: cutting a diaphragm spline with a width of 15 mm in one of the longitudinal direction (MD direction) or the transverse direction (TD direction), stretching the diaphragm spline from L0 = 100 mm in the direction at a speed of 50 mm / min to 50% elongation, holding it for 60 seconds, and then standing it for natural shrinkage for 3 minutes, and then measuring the length L1, and calculating the elastic recovery rate is = (1.5*L0-L1) / (0.5*L0)*100%.

[0084] The third aspect of the present invention provides a microporous separator for lithium batteries obtained by the preparation method of the second aspect of the present invention.

[0085] In the following examples and comparative examples, the testing methods of various parameters are as follows.

[0086] (1) Detailed test method for elastic recovery rate:

[0087] Cut a test strip with a width of 15 mm along the MD or TD direction, measure 100 mm in the length direction of the test strip and draw lines at the corresponding ends. This length is L0.

[0088] The test specimen was placed on the tensile device, the tensile chuck was clamped at the drawn line (i.e. the clamping distance was 100 mm), and the tensile speed was 50 mm / min.

[0089] The test specimen was stretched to 50% and held for 60 seconds. It was then removed and placed at room temperature for natural contraction for 3 minutes. The distance between the two ends of the test specimen was measured and recorded as L1.

[0090] (2) The test method of surface resistance is as follows:

[0091] Cut 4 pieces of diaphragm samples with a diameter of 45mm in a flat position, soak the samples in electrolyte (1.0M LiPF6 in 1:1:1 volume EC / EMC / DMC solvent) and seal for 30 minutes; pour about 15ml of 1mol / L fresh electrolyte (1.0M LiPF6 in 1:1:1 volume EC / EMC / DMC solvent) into the surface resistance test fixture; place 1, 2, 3, and 4 diaphragms in the fixture for testing respectively; use the number of diaphragm layers as the horizontal coordinate and the diaphragm resistance as the vertical coordinate for linear fitting, and calculate the slope and fit of the straight line. When the fit is greater than 0.999, the slope is the surface resistance of the diaphragm.

[0092] (3) The average pore size was measured at 25°C using a PMI instrument (Jiayun Co., Ltd., model CFP-1500AE) with galwick (surface tension at 25°C: 15.9 dynes / cm) as the immersion fluid. The pore size is expressed in nm.

[0093] (4) Thickness shall be measured in accordance with the provisions of GB-T 36363-2018.

[0094] (5) The air permeability is determined according to the provisions of GB / T 36363-2018. Under an applied pressure of 1.21 kPa, 100 ml of air passes through an area of ​​6.45 cm 2 The time required for the diaphragm.

[0095] (6) The tensile strength shall be measured in accordance with the provisions of GB / T1040.3-2006.

[0096] (7) The percentage of polyolefin segments with a weight-average molecular weight of less than 100,000 and the percentage of polyolefin segments with a weight-average molecular weight of less than 10,000 shall be measured in accordance with the provisions of GB / T 36214.4-2018.

[0097] (8) The molecular weight distribution is measured in accordance with the provisions of GB / T 36214.4-2018.

[0098] (9) The intrinsic viscosity is measured in accordance with ISO 1628-3.

[0099] Example 1

[0100] The polyethylene resin with an intrinsic viscosity of 1150 ml / g and a molecular weight distribution of 5.0 and a plasticizer (kinematic viscosity of 45 mm 2 / s paraffin oil) are mixed in a weight ratio of 25:75 and melt-kneaded in an extruder to form a melt; wherein, the number of polyethylene segments with a weight average molecular weight of less than 100,000 in the polyethylene resin accounts for 17 mol%, and the number of polyethylene segments with a weight average molecular weight of less than 10,000 accounts for 0.8 mol%, the extruder temperature is 220°C, and the extruder screw speed is 80 r / min.

[0101] The melt was cooled and solidified by a casting roller to form a thick sheet, and the casting roller temperature was 25°C; the obtained thick sheet was subjected to a three-stage preheating treatment: 60°C / 80°C / 100°C, with a total preheating time of 15s, and then stretched. The stretching in the MD direction adopted a three-point distributed stretching, the stretching temperature was a constant temperature of 95°C, the total stretching ratio was 6.29 times, and the stretching ratio distribution of each point was as follows: 1.7 / 1.85 / 2.0; the transverse stretching (TD1 stretching) temperature was 110°C, and the transverse stretching ratio was 6 times.

[0102] After removing the plasticizer (paraffin oil) and drying with hot air at 55°C, the second transverse stretching (TD2 stretching) was performed at a stretching temperature of 133°C and a stretching ratio of 1.4 times, followed by heat setting treatment at a heat setting temperature of 135°C.

[0103] Examples 2-9, Comparative Examples 1-3

[0104] The differences between the preparation methods of the battery microporous membranes involved in Examples 2-9 and Comparative Examples 1-3 and Example 1 are detailed in Table 1. The parts not shown in Table 1 are the same as those in Example 1.

[0105] Example 10

[0106] The difference from Example 1 is that the stretching in the MD direction adopts 5-point distributed stretching, the stretching temperature is a constant 95°C, the total stretching ratio is 9.3 times, and the stretching ratio distribution of each point is as follows: 1.1 / 1.6 / 2.0 / 2.4 / 1.1; the transverse stretching (TD1 stretching) temperature is 110°C, and the transverse stretching ratio is 7.5 times.

[0107] Examples 2-1 to 2-4

[0108] The differences from Example 2 are detailed in Table 2. The parts not shown in Table 2 are the same as those in Example 2.

[0109] Table 1

[0110] From Examples 1-11 and Comparative Examples 1-2 in Table 1, it can be seen that when the amount of the polyolefin segment component with a weight average molecular weight of less than 100,000 in the microporous membrane is 15-30 mol%, it has good elastic recovery rate (>14%), surface resistance and mechanical strength (MD and TD mechanical strength are both ≥2000 kgf / cm2 ), and when the proportion of polyolefin segments with a weight-average molecular weight of less than 100,000 in the microporous separator exceeds the above range, the elastic recovery and mechanical strength significantly decrease. A comparison of Examples 1 and 10 shows that when the stretch ratio in both MD and TD increases, while the MD tensile strength improves, the TD tensile strength decreases slightly, the elastic recovery decreases, and the surface resistivity increases.

[0111] From Examples 2-1 to 2-4 in Table 2, it can be seen that the proportion of polyolefin segments with a weight-average molecular weight of less than 100,000 in the microporous membrane can be controlled by matching the extruder process with the raw materials.

[0112] In this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. The methods described herein can be performed in any order that is logically possible, except for the specific order disclosed.

[0113] The representative examples are intended to help illustrate the present invention and are not intended to, and should not be construed as, limiting the scope of the present invention. Indeed, various modifications of the present invention and many other embodiments thereof, in addition to those shown and described herein, will become apparent to those skilled in the art, including the examples and the scientific and patent literature references cited herein. The examples contain important additional information, illustrations, and guidance that can be employed in the practice of the present invention in its various embodiments and equivalents.

Claims

1. A microporous separator for a lithium battery, characterized in that: It is a porous monolayer membrane containing a polyolefin resin, wherein the intrinsic viscosity index of the microporous membrane is 700-1500 ml / g, preferably 900-1200 ml / g; in the microporous membrane, the amount of the polyolefin segment component with a weight average molecular weight of less than 100,000 accounts for 15-30 mol%, preferably 15-25 mol%, and more preferably 15-21 mol%; the longitudinal elastic recovery rate of the microporous membrane measured under the following conditions is greater than or equal to 14%, preferably greater than or equal to 18%, and the transverse elastic recovery rate ...0%, preferably greater than or equal to 10%, and the transverse elastic recovery rate is greater than or equal to 10%. The recovery rate is greater than or equal to 14%, preferably greater than or equal to 18%, and the conditions include: cutting a diaphragm spline with a width of 15 mm in one of the longitudinal direction (MD direction) or the transverse direction (TD direction), stretching the diaphragm spline from L0=100 mm in the direction at a speed of 50 mm / min to 50% elongation, keeping it for 60 seconds, and then letting it stand for natural shrinkage for 3 minutes, and then measuring the length L1, and calculating the elastic recovery rate is = (1.5*L0-L1) / (0.5*L0)*100%.

2. The microporous separator for lithium battery according to claim 1, wherein: In the microporous membrane, the polyolefin segment component having a weight average molecular weight of 10,000 or less accounts for 0.5-2.5 mol %, preferably 0.5-1.5 mol %.

3. The microporous separator for lithium battery according to claim 1 or 2, wherein: The molecular weight distribution of the polyolefin resin is 3-6, preferably 3.5-5.

0.

4. The microporous separator for lithium battery according to any one of claims 1 to 3, wherein The microporous membrane satisfies one or more of the following combinations: a. Surface resistance is 0.2-0.7Ω, preferably 0.3-0.6Ω; b. The average pore size is 25-50nm, preferably 30-45nm; c. Thickness is 1-30 μm, preferably 4-12 μm; d. Air permeability is 10-300sec / 100cc, preferably 60-170sec / 100cc; e. The tensile strength in MD or TD direction is 2000-4000kgf / cm 2 , preferably 2800-4000kgf / cm 2 .

5. The microporous separator for lithium battery according to any one of claims 1 to 4, wherein The polyolefin resin is selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE), polypropylene, polybutene, polymethylpentene, copolymers thereof, and blends thereof.

6. The microporous separator for lithium battery according to any one of claims 1 to 5, wherein In the polyolefin resin, the amount of the polyolefin segment component with a weight average molecular weight of less than 100,000 accounts for 10-30 mol%, preferably 10-25 mol%, more preferably 13-25 mol%, and further preferably 13-20 mol%; 7. The microporous separator for lithium battery according to any one of claims 1 to 6, wherein In the polyolefin resin, the amount of the polyolefin segment component with a weight average molecular weight of less than 10,000 accounts for 0-2 mol%, preferably 0.3-1.0 mol%.

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

9. A method for preparing a microporous separator for a lithium battery, characterized in that: The following steps are involved: (a) melt-kneading a mixture comprising a polyolefin resin and a plasticizer to form a melt; (b) extruding and solidifying the melt into a thick sheet; (c) stretching the thick sheet in the longitudinal direction (MD direction) and the transverse direction (TD direction) to obtain a stretched body; (d) removing the plasticizer from the stretched body and drying it to obtain the microporous separator for lithium battery; Among them, in the polyolefin resin of step (a), the amount of polyolefin segment components with a weight average molecular weight of less than 100,000 accounts for 10-30 mol%, preferably 10-25 mol%, more preferably 13-25 mol%, and further preferably 13-20 mol%; the intrinsic viscosity index of the polyolefin resin is 800-1600 ml / g, preferably 1000-1300 ml / g.

10. The preparation method according to claim 9, wherein: In the polyolefin resin of step (a), the amount of the polyolefin segment component with a weight average molecular weight of less than 10,000 accounts for 0-2 mol%, preferably 0.3-1.5 mol%; Furthermore, the molecular weight distribution of the polyolefin resin is 3-6, preferably 3.5-5.

0.

11. The preparation method according to claim 9 or 10, wherein: In the microporous membrane, the amount of the polyolefin segment component with a weight average molecular weight of less than 100,000 accounts for 15-30 mol%, preferably 15-25 mol%, and more preferably 15-21 mol%; Furthermore, in the microporous membrane, the amount of the polyolefin segment component with a weight average molecular weight of less than 10,000 accounts for 0.5-2.5 mol%, preferably 0.5-1.5 mol%; Furthermore, the longitudinal elastic recovery rate of the microporous diaphragm measured under the following conditions is greater than or equal to 14%, preferably greater than or equal to 18%, and the transverse elastic recovery rate is greater than or equal to 14%, preferably greater than or equal to 18%, and the conditions include: cutting a diaphragm spline with a width of 15 mm in one of the longitudinal direction (MD direction) or the transverse direction (TD direction), stretching the diaphragm spline from L0=100 mm in the direction at a speed of 50 mm / min to 50% elongation, holding it for 60 seconds, and then standing it for natural shrinkage for 3 minutes, and then measuring the length L1, and the elastic recovery rate is calculated to be =(1.5*L0-L1) / (0.5*L0)*100%.

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. The preparation method according to any one of claims 9 to 12, wherein In step (a), an extruder is used for melt mixing, preferably the extruder temperature is 160-250° C., and the extruder screw speed is 60-100 r / min; Furthermore, in step (b), the mixture is extruded through a die and attached to a casting roll to be cooled and solidified to form a thick sheet, and the temperature of the casting roll is 20-30°C.

14. The preparation method according to any one of claims 9 to 13, wherein In step (c), the stretching in the longitudinal direction is performed at a stretching temperature of 80-120° C., a stretching ratio of 4-9 times, preferably a stretching ratio of 4-7 times, and a stretching method of multi-point distribution stretching, preferably 3-7 points; Furthermore, a preheating step is provided before stretching in the longitudinal direction, and the preheating temperature is 60-100°C; preferably, the preheating is a gradient preheating, and the gradient preheating has 2-4 stages, and the temperature difference between two adjacent gradients is 7-25°C.

15. The preparation method according to any one of claims 9 to 14, wherein In step (c), the stretching temperature in the transverse direction is 90-125°C, preferably 90-115°C, the stretching ratio is 4-8 times, preferably 4-6 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 less than 1.

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

17. The preparation 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, polybutene, polymethylpentene, copolymers thereof, and blends thereof.