Microporous separator for lithium battery, and preparation method for microporous separator

Abstract

A microporous separator for a lithium battery. The microporous separator is a polyolefin porous membrane. In the microporous separator, the molar percentage of polyolefin chain segment components having a weight-average molecular weight of not more than 100,000 is 15-30 mol%; and the microporous separator has a first heating crystallinity of greater than or equal to 60%, and a second heating crystallinity of less than or equal to 45%.

Description

A microporous separator for lithium batteries and its preparation method Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a microporous separator for lithium batteries and its preparation method. Background Technology

[0002] Polyolefin microporous membranes are widely used as separation membranes and separation materials for various substances, including those used in separation and selective permeation. For example, they are used as precision filtration membranes, separators for fuel cells, separators for capacitors, master materials for functional membranes with pores filled with functional materials to achieve new functions, and separators for batteries. Among these applications, polyolefin microporous membranes are particularly suitable as separators for lithium-ion batteries, which are widely used in laptops, mobile phones, and digital cameras.

[0003] For microporous membranes used in lithium batteries, good planar elastic recovery is required. However, currently commonly used polyolefin microporous membranes have low planar elastic recovery rates, especially wet-process lithium battery membranes, whose lateral and longitudinal elastic recovery rates are both below 14%. This can affect the irreversible deformation of the microporous membrane during battery assembly, causing deviations between the microporous membrane and the battery design dimensions, thus affecting electrochemical safety performance. Furthermore, microporous membranes for lithium batteries also require good mechanical strength and air permeability.

[0004] Currently, there is no wet-process lithium-ion battery separator that combines high mechanical strength, air permeability, and good planar elastic recovery, while also being thin and having low surface resistance. Summary of the Invention

[0005] This invention, by controlling the proportion of low molecular weight segments in the microporous membrane and the crystallinity during primary and secondary heating, achieves a lithium-ion battery separator with excellent tensile strength, planar elastic recovery, and air permeability, even when using commonly used polyolefin resins as the main raw material. Furthermore, this invention further solves the problem of high sheet resistance in the separator; the sheet resistance of the lithium-ion battery separator of this invention is reduced by more than 20% compared to existing technologies. In addition, the separator of this invention achieves a good balance between high tensile strength and planar elastic recovery, making it suitable for use in lithium-ion batteries.

[0006] The present invention also found that by using a low-temperature multi-point distributed stretching process in longitudinal (MD) stretching and a low-temperature low-ratio stretching in transverse (TD1) stretching, and by controlling the difference between the MD stretching ratio and the TD1 stretching ratio, the overall performance of the obtained microporous membrane can be further improved.

[0007] The first aspect of this invention provides a microporous separator for lithium batteries, which is a polyolefin porous membrane; in the microporous separator, the proportion of polyolefin segments with a weight average molecular weight of less than 100,000 is 15-30 mol%, preferably 15-25 mol%, more preferably 15-21 mol%; the crystallinity of the microporous separator after a first heating is ≥60%, preferably 60%-80%, and the crystallinity after a second heating is ≤45%, preferably 20%-45%.

[0008] In some embodiments, the microporous diaphragm has a longitudinal elastic recovery rate of ≥14%, preferably ≥18%, and a transverse elastic recovery rate of ≥14%, preferably ≥18%, measured under the following conditions: the conditions include cutting a diaphragm strip with a width of 15 mm from one of the longitudinal (MD) or transverse (TD) directions, stretching the diaphragm strip from L0 = 100 mm in the direction at a speed of 50 mm / min to 50% of L0 (i.e., the length after stretching is 1.5 * L0), holding it for 60 s, allowing it to naturally shrink for 3 min, measuring the length L1, and calculating the elastic recovery rate as = (1.5 * L0 - L1) / (0.5 * L0) * 100%.

[0009] In some embodiments, the difference between the crystallinity of the first heating and the crystallinity of the second heating is 15% or more, preferably 25%-45%.

[0010] In some embodiments, the proportion of polyolefin segment components with a weight average molecular weight of less than 10,000 in the microporous membrane is 0-3.5 mol%, preferably 0.5-2.5 mol%, and more preferably 0.5-1.5 mol%.

[0011] In some embodiments, the microporous membrane satisfies one or more of the following combinations:

[0012] a. Surface resistance is 0.2-0.7Ω;

[0013] b. Thickness is 1-30μm;

[0014] c. Breathability is 10-300 seconds / 100cc.

[0015] In some embodiments, the microporous membrane has a tensile strength of 4000 kgf / cm² in the MD or TD direction. 2 The above values ​​are preferably 4000-6000 kgf / cm³. 2 More preferably 4800-6000 kgf / cm³ 2 .

[0016] In some embodiments, the microporous membrane is a membrane prepared by a wet process.

[0017] A second aspect of this invention provides a method for preparing a microporous separator for lithium batteries, comprising the following steps:

[0018] (a) Melt-blending a mixture containing a polyolefin resin and a plasticizer to form a melt;

[0019] (b) Extruding and solidifying the melt into a thick sheet;

[0020] (c) The thick sheet is stretched in the longitudinal direction (MD direction) and the transverse direction (TD direction) to obtain a stretched body;

[0021] (d) Remove the plasticizer from the stretched body, dry it, and obtain the microporous separator for lithium batteries;

[0022] In step (a), the polyolefin resin contains polyolefin segments with a weight-average molecular weight of less than 100,000, accounting for 10-30 mol%, preferably 10-25 mol%, more preferably 13-25 mol%, and even more preferably 13-20 mol%, with a crystallinity of less than 45%.

[0023] In some embodiments, in the polyolefin resin of step (a), the proportion of polyolefin segment components with a weight average molecular weight of less than 10,000 is 0-3 mol%, preferably 0.3-2 mol%, more preferably 0.3-1.5 mol%.

[0024] In some embodiments, the molecular weight distribution of the polyolefin resin is between 3 and 6.

[0025] In some embodiments, the proportion of polyolefin segment components with a weight-average molecular weight of less than 100,000 in the microporous membrane is 15-30 mol%, preferably 15-25 mol%, and more preferably 15-21 mol%.

[0026] In some embodiments, the proportion of polyolefin segment components with a weight average molecular weight of less than 10,000 in the microporous membrane is 0-3.5 mol%, preferably 0.5-2.5 mol%, and more preferably 0.5-1.5 mol%.

[0027] In some embodiments, the microporous diaphragm has a longitudinal elastic recovery rate of ≥14%, preferably ≥18%, and a transverse elastic recovery rate of ≥14%, preferably ≥18%, measured under the following conditions: the conditions include cutting a diaphragm strip with a width of 15 mm from one of the longitudinal (MD) or transverse (TD) directions, stretching the diaphragm strip from L0 = 100 mm in the direction at a speed of 50 mm / min to 50% of the elongation of L0, holding it for 60 s, allowing it to naturally retract for 3 min, measuring the length L1, and calculating the elastic recovery rate as = (1.5*L0-L1) / (0.5*L0)*100%.

[0028] In some embodiments, 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.

[0029] In some embodiments, an extruder is used for melt mixing in step (a), preferably with an extruder temperature of 160-250°C and an extruder screw speed of 60-100 r / min.

[0030] Further, in step (b), the mixture is extruded through a die and attached to a casting roller to cool and solidify into a thick sheet, with the casting roller temperature at 20-30°C.

[0031] In some embodiments, in step (c), the stretching in the longitudinal direction is carried out at a stretching temperature of 80-120°C, the stretching ratio is 6.5-9 times, preferably 7-9 times, and the stretching method is multi-point distributed stretching, preferably 3-7 points.

[0032] In some embodiments, in step (c), there is at least one set of three adjacent stretching points in the longitudinal direction whose stretching ratios are in an increasing relationship, and the absolute value of the difference in stretching ratios between the adjacent three stretching points is greater than 0 and less than or equal to 0.7, preferably 0.1-0.6.

[0033] In some embodiments, in step (c), the stretching temperature in the transverse direction is 90-125°C, preferably 90-115°C, and the stretching ratio is 6-8 times, preferably 7-8 times. Preferably, 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 1.1.

[0034] In some embodiments, step (d) further includes a second transverse stretching step after the drying process, wherein 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.

[0035] In some embodiments, the polyolefin resin is selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE), polypropylene, polybutene, polymethylpentene, copolymers thereof, and blends thereof. Detailed Implementation

[0036] Before further describing the invention, certain terms used in the specification, embodiments, and appended claims are collected in the following sections. The definitions set forth herein are intended to be read and understood by those skilled in the art in accordance with the remainder of the invention. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0037] The term “about” as used in this article, when referring to a value, means to include its variation, such as ±10%, ±5%, ±1%, or ±0.1% of a specific value.

[0038] The term "substantially identical" as used in this article, when referring to two values, means that the difference between the two values ​​is less than 10%, 5%, or 1% of the mean of the two values.

[0039] The term "polyolefin" as used in this article can refer to a polyolefin monomer (i.e., a single type of polyolefin), a polyolefin copolymer, or a polyolefin blend.

[0040] As used herein, the term "blend" refers to a physical mixture of two or more homopolymers, copolymers, or homopolymers and copolymers with different molecular structures. Specifically, a blend may include different polymers, i.e., at least two polymers with different chemical properties (e.g., polyethylene, polypropylene, and / or ethylene-propylene copolymers with different chemical properties); and / or polymers with the same chemical properties but different characteristics (e.g., two different polyethylenes with different characteristics (e.g., density, molecular weight, molecular weight distribution, rheology, additives (components and / or percentages)).

[0041] The term "longitudinal direction" used in this article, also known as MD direction, refers to the direction of equipment operation.

[0042] The term "lateral direction" used in this article, also known as the TD direction, refers to the direction perpendicular to the direction of equipment operation.

[0043] The first aspect of the present invention provides a microporous separator for lithium batteries, which is a polyolefin porous membrane; in the microporous separator, the proportion of polyolefin segments with a weight average molecular weight of less than 100,000 is 15-30 mol%, preferably 15-25 mol%, more preferably 15-21 mol%; the crystallinity of the microporous separator after one heating is ≥60%, and the crystallinity after two heatings is ≤45%.

[0044] In some specific embodiments of the present invention, the crystallinity of the microporous membrane after a single heating is preferably 60%-80%. In some specific embodiments of the present invention, the crystallinity of the microporous membrane after a second heating is preferably 20%-45%.

[0045] In the microporous membrane, the proportion of polyolefin segments with a weight-average molecular weight of less than 100,000 can be any one or a combination of 15 mol%, 20 mol%, 20.5 mol%, 21 mol%, 23 mol%, 25 mol%, or 30 mol%, or any combination of both. In the microporous membrane, the degree of crystallinity after a single heating can, for example, be any one or a combination of 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 82%, 83%, 84%, or 85%, or any combination of both. In the microporous membrane, the degree of crystallinity after the second heating can be, for example, any one of 20%, 25%, 30%, 35%, 36%, 40%, or 45%, or a combination of any two of them.

[0046] According to some preferred embodiments of the present invention, the difference between the crystallinity of the first heating and the crystallinity of the second heating is 15% or more, preferably 25%-45%, more preferably 35%-45%, and even more preferably 35%-40%. By having the above-mentioned difference between the crystallinity of the first heating and the crystallinity of the second heating, the tensile strength of the membrane can be further ensured, and the electrochemical safety can be improved.

[0047] In the microporous membrane, the difference between the crystallinity after the first heating and the crystallinity after the second heating can be, for example, any one or a combination of two of the following: 15%, 20%, 22%, 24%, 25%, 30%, 35%, 37%, 40%, 44%, 46%, 47%, or 45%.

[0048] In some specific embodiments of the present invention, preferably, the longitudinal elastic recovery rate of the microporous diaphragm 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%, measured under the following conditions. The conditions include: cutting a diaphragm strip with a width of 15 mm from one of the longitudinal direction (MD direction) or the transverse direction (TD direction), stretching the diaphragm strip from L0 = 100 mm in the direction at a speed of 50 mm / min to 50% of the elongation of L0, holding it for 60 s, allowing it to naturally shrink back for 3 min, measuring the length L1, and calculating the elastic recovery rate as = (1.5*L0-L1) / (0.5*L0)*100%.

[0049] Preferably, the upper limit of the elastic recovery rate of the microporous membrane measured under the above conditions is no more than 50%, more preferably no more than 30%, and even more preferably no more than 25%. By having the above elastic recovery rate, the processing stability of the membrane can be guaranteed, and excessive rebound of the membrane during the battery or membrane preparation process can be avoided, which would affect product manufacturing and product quality.

[0050] In some specific embodiments of the present invention, preferably, the proportion of polyolefin segment components with a weight average molecular weight of less than 10,000 in the microporous membrane is 0-3.5 mol%, preferably 0.5-2.5 mol%, and more preferably 0.5-1.5 mol%.

[0051] In the microporous membrane, the proportion of polyolefin segments with a weight average molecular weight of less than 10,000 can be any one or any combination of 0 mol%, 0.1 mol%, 0.2 mol%, 0.3 mol%, 0.4 mol%, 0.5 mol%, 0.6 mol%, 0.7 mol%, 0.8 mol%, 0.9 mol%, 1 mol%, 1.1 mol%, 1.2 mol%, 1.3 mol%, 1.4 mol%, 1.5 mol%, 1.6 mol%, 1.7 mol%, 1.8 mol%, 1.9 mol%, 2 mol%, 2.1 mol%, 2.2 mol%, 2.3 mol%, 2.4 mol%, 2.5 mol%, 2.6 mol%, 2.7 mol%, 2.8 mol%, 2.9 mol%, 3 mol%, 3.1 mol%, 3.2 mol%, 3.3 mol%, 3.4 mol%, or 3.5 mol%.

[0052] In this invention, the polyolefin porous membrane refers to a porous membrane prepared using a polyolefin resin. The polyolefin resin can be any polyolefin resin commonly used in the art to prepare microporous membranes for lithium batteries, such as polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE), polypropylene, polybutene, polymethylpentene, copolymers thereof, and blends thereof, wherein at least one of polyethylene and polypropylene is preferred.

[0053] According to the present invention, any polyolefin resin used may be used as long as it is capable of producing the microporous separator for lithium batteries required by the present invention. To provide a microporous separator with high planar elastic recovery, preferably, the polyolefin resin contains polyolefin segments with a weight average molecular weight of 100,000 or less in a proportion of 10-30 mol%, more preferably 10-25 mol%, more preferably 13-25 mol%, and even more preferably 13-20 mol%. Preferably, the polyolefin resin contains polyolefin segments with a weight average molecular weight of 10,000 or less in a proportion of 0-3 mol%, more preferably 0.3-2 mol%, and even more preferably 0.3-1.5 mol%.

[0054] In the polyolefin resin, the proportion of polyolefin segment components with a weight average molecular weight of less than 100,000 can be any one or any combination of 10 mol%, 11 mol%, 12 mol%, 13 mol%, 14 mol%, 15 mol%, 16 mol%, 17 mol%, 18 mol%, 19 mol%, 20 mol%, 21 mol%, 22 mol%, 23 mol%, 24 mol%, 25 mol%, 26 mol%, 27 mol%, 28 mol%, 29 mol%, and 30 mol%.

[0055] In some specific embodiments of the present invention, preferably, the molecular weight distribution of the polyolefin resin is between 3 and 6. Considering melt quality and film-forming stability, the molecular weight distribution of the polyolefin resin is more preferably between 3.5 and 5.

[0056] In some specific embodiments of the present invention, the microporous membrane satisfies one or more of the following combinations:

[0057] a. Surface resistance is 0.2-0.7Ω;

[0058] b. Thickness is 1-30μm;

[0059] c. Breathability is 10-300 seconds / 100cc.

[0060] In some specific embodiments of the present invention, based on the consideration of further improving electrochemical performance, the sheet resistance is preferably 0.3-0.6Ω; based on the consideration of balancing battery cycle performance and electrochemical safety, the thickness of the microporous membrane is preferably 2-12μm; the air permeability of the polyolefin microporous membrane of the present invention is preferably as low as possible, but based on the consideration of balancing thickness, mechanical strength and electrochemical performance, the air permeability is preferably 60-170sec / 100cc, more preferably 90-125sec / 100cc.

[0061] In some specific embodiments of the present invention, the average pore size of the microporous membrane can be 10-100 nm; considering the significant improvement of battery characteristics such as cycle performance, the average pore size is preferably 30-45 nm.

[0062] In some specific embodiments of the present invention, the tensile strength of the microporous membrane in the MD direction or TD direction is 4000 kgf / cm. 2 The above values ​​are preferably 4000-6000 kgf / cm³. 2 More preferably 4800-6000 kgf / cm³ 2 Further preferred is 5000-6000 kgf / cm³ 2 .

[0063] To obtain a microporous membrane with high planar elastic recovery, the microporous membrane of the present invention is preferably a membrane prepared by a wet process. That is, the microporous membrane has a three-dimensional network structure in which several protofibrils are interwoven, and the interwoven protofibrils form pores. The protofibrils in the microporous membrane are obtained by stretching and oriented the polymer during the preparation of the film, and the extractant extracts the plasticizer in the film to form pores.

[0064] A second aspect of the present invention provides a method for preparing a microporous separator for lithium batteries, the method comprising the following steps:

[0065] (a) Melt-blending a mixture containing a polyolefin resin and a plasticizer to form a melt;

[0066] (b) Extruding and solidifying the melt into a thick sheet;

[0067] (c) The thick sheet is stretched in the longitudinal direction (MD direction) and the transverse direction (TD direction) to obtain a stretched body;

[0068] (d) Remove the plasticizer from the stretched body, dry it, and obtain the microporous separator for lithium batteries;

[0069] In step (a), the polyolefin resin contains polyolefin segments with a weight-average molecular weight of less than 100,000, accounting for 10-30 mol%, preferably 10-25 mol%, more preferably 13-25 mol%, and even more preferably 13-20 mol%, with a crystallinity of less than 45%.

[0070] In the polyolefin resin, the percentage of polyolefin segments with a weight-average molecular weight of less than 100,000 can be any one or any combination of 10 mol%, 11 mol%, 12 mol%, 13 mol%, 14 mol%, 15 mol%, 16 mol%, 17 mol%, 18 mol%, 19 mol%, 20 mol%, 21 mol%, 22 mol%, 23 mol%, 24 mol%, 25 mol%, 26 mol%, 27 mol%, 28 mol%, 29 mol%, and 30 mol%. The crystallinity of the polyolefin resin can be any one or any combination of 15%, 20%, 25%, 30%, 35%, 36%, 40%, 43%, and 45%.

[0071] According to the present invention, the microporous separator of the present invention, suitable for use in lithium batteries, can be obtained by processing a mixture containing a suitable polyolefin resin and a plasticizer according to the preparation method of the present invention.

[0072] In this invention, the polyolefin resin can be any polyolefin resin commonly used in the art to prepare microporous membranes for lithium batteries, such as polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE), polypropylene, polybutene, polymethylpentene, copolymers thereof, and blends thereof, wherein polyethylene and / or polypropylene are preferred.

[0073] According to the present invention, the polyolefin resin used as a raw material can be a single polyolefin resin or a mixture of multiple polyolefin resins. When using multiple polyolefin resins, each polyolefin resin must meet the physical properties required by the present invention.

[0074] From the perspective of better film uniformity and memory duration, regardless of whether a single polyolefin or a mixture of multiple polyolefins is used, the crystallinity of one or more polyolefin resins contained in the polyolefin resin is below 45%.

[0075] In this invention, the plasticizer is preferably a non-volatile solvent that can form a homogeneous solution above the melting point of the polyolefin resin when mixed with it. Examples include paraffin oil, diethyl phthalate, and palm oil. A kinematic viscosity of 35-120 mmHg at 40°C is preferred. 2 Paraffin oil with a kinematic viscosity of 40-55 mm at 40°C is preferred. 2 / s of paraffin oil. The test method for the kinematic viscosity is GB / T 265.

[0076] 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 can be any or a combination of two of the following: 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 the above-mentioned weight ratio of polyolefin resin to plasticizer, the overall performance of the microporous membrane can be improved.

[0077] To further improve the elastic recovery rate of the microporous membrane, the proportion of polyolefin segments with a weight average molecular weight of less than 10,000 in the polyolefin resin is 0-3 mol%, preferably 0.3-2 mol%, and more preferably 0.3-1.5 mol%, based on considerations of balancing elastic recovery rate and mechanical strength performance.

[0078] In the polyolefin resin, the percentage of polyolefin segments with a weight average molecular weight of less than 10,000 can be any one or any combination of 0 mol%, 0.1 mol%, 0.2 mol%, 0.3 mol%, 0.4 mol%, 0.5 mol%, 0.6 mol%, 0.7 mol%, 0.8 mol%, 0.9 mol%, 1 mol%, 1.1 mol%, 1.2 mol%, 1.3 mol%, 1.4 mol%, 1.5 mol%, 1.6 mol%, 1.7 mol%, 1.8 mol%, 1.9 mol%, 2 mol%, 2.1 mol%, 2.2 mol%, 2.3 mol%, 2.4 mol%, 2.5 mol%, 2.6 mol%, 2.7 mol%, 2.8 mol%, 2.9 mol%, or 3 mol%.

[0079] In some specific embodiments of the present invention, preferably, the molecular weight distribution of the polyolefin resin is between 3 and 6. Considering melt quality and film-forming stability, the molecular weight distribution of the polyolefin resin is more preferably between 3.5 and 5.

[0080] According to the present invention, in step (a), any existing method capable of forming the mixture into a melt can be used for processing, for example, melt mixing can be performed using an extruder. As some preferred embodiments, the parameters of the extruder include: extruder temperature 150-250°C, preferably 180-240°C, and extruder screw speed 60-100 r / min, preferably 70-90 r / min.

[0081] According to the present invention, in step (b), the method of extruding and solidifying the melt into a thick sheet is sufficient to form a sheet of the desired thickness. For example, the mixture can be extruded through a die and attached to a casting roller for cooling and solidification to form a thick sheet. Preferably, the temperature of the casting roller is 20-30°C. Furthermore, the rolling speed of the casting roller can be 3-8 m / min.

[0082] According to the present invention, in step (c), a stretched body is obtained by performing stretching in the longitudinal direction (also known as longitudinal stretching or MD stretching) and stretching in the transverse direction (also known as transverse stretching or TD1 stretching) respectively. Specifically, longitudinal stretching can be performed first and then transverse stretching, or transverse stretching can be performed first and then longitudinal stretching, or both longitudinal and transverse stretching can be performed simultaneously, with longitudinal stretching followed by transverse stretching being preferred.

[0083] To further improve the elastic recovery rate and tensile strength of the microporous diaphragm, in step (c), the longitudinal stretching is performed at a stretching temperature of 80-120°C, with a stretching ratio of 6.5-9 times, preferably 7-9 times, and more preferably 7-8.5 times. The stretching method is multi-point distributed stretching, preferably at 3-7 points, and more preferably at 3-4 points. In multi-point distributed stretching, the stretching ratio is the speed ratio between two stretching rollers with different adjacent linear velocities, i.e., the rear roller linear velocity / the front roller linear velocity. The stretching ratio at each stretching point is 1.1-3, preferably 1.5-2.6. When multi-point distributed stretching is used, the stretching ratio refers to the total stretching ratio at all points, and preferably, the stretching ratio at each point is independently 1.1-3, preferably 1.5-2.6. Preferably, there is at least one set of three adjacent stretching points with an increasing stretching ratio, and the absolute value of the difference in stretching ratio between adjacent stretching points is greater than 0 and less than or equal to 0.7, preferably 0.1-0.6. For example, when stretching at 3 points, the stretching ratio distribution can be as follows: 1.7 / 2.0 / 2.4, 1.7 / 2.0 / 2.6, or 1.7 / 2.0 / 2.1; when stretching at 5 points, the stretching ratio distribution can be as follows: 1.1 / 1.5 / 1.7 / 2.0 / 1.5 or 1.1 / 1.1 / 1.7 / 2.1 / 2.0; when stretching at 7 points, the stretching ratio distribution can be as follows: 1.1 / 1.2 / 1.3 / 1.4 / 1.9 / 1.5 / 1.2 or 1.1 / 1.1 / 1.5 / 1.9 / 2.1 / 1.1 / 1.1.

[0084] Generally, three adjacent points with an increasing stretch ratio are not only present in the last three stretching points of the stretching section. Preferably, the first and middle stretching sections should have three adjacent stretching points with an increasing stretch ratio.

[0085] Increasing longitudinal stretch ratios are more conducive to full stretching, resulting in more complete polymer orientation. This leads to better mechanical properties and more uniform size distribution of the resulting fibrils, thus giving the microporous membrane better elastic resilience and mechanical properties.

[0086] To further improve the resilience of the microporous membrane in the planar direction, in step (c), the stretching temperature in the transverse direction is 90-125°C, preferably 90-115°C, and the stretching ratio is 6-8 times, preferably 7-8 times. Furthermore, the absolute value of the difference between the stretching ratio in the longitudinal direction and the stretching ratio in the transverse direction is preferably less than 1.1, more preferably 0.1-0.9, and even more preferably 0.1-0.7. The absolute value of the above difference can, for example, be any one or a range of any two of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, and 1.1.

[0087] 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. As the extractant, a reagent that is a poor solvent relative to the polyolefin resin and a good solvent relative to the plasticizer is preferred. Furthermore, the extractant preferably has a boiling point lower than the melting point of the porous membrane of the polyolefin resin. The extractant can be an alkane extractant, preferably a halogenated hydrocarbon extractant, with dichloromethane being the most preferred.

[0088] As a method for removing plasticizers from the drawn body using an extractant, the plasticizers in the drawn body can be removed by circulating the extractant. Preferably, the circulating extractant flow rate is 1-5 ml. 3 / h. After extraction, the stretched body is dried by heating it using one or more of the following methods: hot roller, heating plate, or hot air. The preferred drying temperature is 20-150°C.

[0089] According to a preferred embodiment of the present invention, in step (d), after the drying process, a second transverse stretching (TD2 stretching) step is further included, wherein 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.

[0090] When prepared using the above method, 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% in the obtained microporous membrane. Preferably, the polyolefin segment component with a weight average molecular weight of less than 10,000 accounts for 0-3.5 mol%, preferably 0.5-2.5 mol%, and more preferably 0.5-1.5 mol% in the microporous membrane. Further, the longitudinal elastic recovery rate of the microporous diaphragm 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%, measured under the following conditions: a diaphragm strip with a width of 15 mm is cut from one of the longitudinal direction (MD direction) or the transverse direction (TD direction), the diaphragm strip is stretched from L0 = 100 mm in the direction at a speed of 50 mm / min to 50% of the elongation of L0, held for 60 s, allowed to naturally retract for 3 min, and then the length L1 is measured. The elastic recovery rate is calculated as (1.5*L0-L1) / (0.5*L0)*100%.

[0091] 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 described above.

[0092] The testing methods for each parameter in the following examples and comparative examples are as follows.

[0093] (1) Detailed test method for elastic recovery rate:

[0094] Cut a test strip with a width of 15mm along the MD or TD direction. Measure 100mm along the length of the test strip and draw lines at the corresponding ends. This length is L0.

[0095] Place the test strip on the tensile equipment, clamp the tensile clamp at the marked line (i.e., the clamping distance is 100mm), and the tensile speed is 50mm / min.

[0096] After stretching the test strip to 50% of its length and holding it for 60 seconds, remove it and place it at room temperature for 3 minutes to allow it to naturally shrink back. Then measure the distance between the two ends of the line drawn on it and record it as L1.

[0097] The elastic recovery rate is calculated as (1.5*L0-L1) / (0.5*L0)*100%.

[0098] (2) The method for testing surface resistance is as follows:

[0099] Four diaphragm samples, each 45 mm in diameter, were cut from a flat surface. The samples were then immersed in an electrolyte solution (1:1:1 volume EC / EMC / DMC solvent containing 1.0 M LiPF6) and sealed for 30 minutes. Approximately 15 ml of a 1 mol / L fresh electrolyte solution (1:1:1 volume EC / EMC / DMC solvent containing 1.0 M LiPF6) was poured into a sheet resistance testing fixture. Diaphragms 1, 2, 3, and 4 were placed in the fixture for testing. A linear fit was performed with the number of diaphragm layers as the x-axis and the diaphragm resistance as the y-axis. The slope and degree of fit of the line were calculated. When the degree of fit was greater than 0.999, the slope was considered the sheet resistance of the diaphragm.

[0100] (3) The thickness shall be measured in accordance with the provisions of GB-T 36363-2018.

[0101] (4) The air permeability was determined according to GB / T 36363-2018. Under an applied pressure of 1.21 kPa, the area through which 100 ml of air passed was 6.45 cm². 2 The time required for the diaphragm to be in place.

[0102] (5) Tensile strength shall be measured in accordance with the provisions of GB / T1040.3-2006.

[0103] (6) The proportion of polyolefin segment components with a weight average molecular weight of less than 100,000 and the proportion of polyolefin segment components 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.

[0104] (7) Molecular weight distribution was measured in accordance with the provisions of GB / T 36214.4-2018.

[0105] (8) The crystallinity of the microporous membrane under primary and secondary heating was measured using a differential scanning calorimeter. The test method was as follows: the membrane was first heated at 10℃ / min to above the melting point of the polyolefin and held for 3 min to obtain the primary heating crystallinity of the polyolefin. Then, the temperature was lowered at 10℃ / min to ≤40℃ and held for 3 min. Finally, the membrane was heated a second time at 10℃ / min to above the melting point of the polyolefin to obtain the secondary heating crystallinity of the polyolefin. The primary heating crystallinity of the polyolefin membrane was obtained by dividing the enthalpy of melting of the polyolefin measured during the first heating process by the standard enthalpy of melting of the polyolefin. The secondary heating crystallinity of the polyolefin membrane was obtained by dividing the enthalpy of melting of the polyolefin measured during the second heating process by the standard enthalpy of melting of the polyolefin. The heating temperature was 200℃ in all measurements. For polyethylene, the standard enthalpy of melting is calculated as 293 J / g, and for polypropylene, the standard enthalpy of melting is calculated as 209 J / g.

[0106] In addition, the crystallinity of the raw material was measured using the above-mentioned secondary heating crystallinity test method.

[0107] Example 1

[0108] Polyethylene resin (relevant properties are shown in Table 1, prepared by Daehan Oil & Chemical Co., Ltd.) and plasticizer (kinematic viscosity 45 mm) were mixed. 2 Paraffin oil (at a weight ratio of 25:75) is mixed and melt-blended in an extruder to form a melt.

[0109] The above melt is cooled and solidified by casting rollers to form a thick sheet at a temperature of 25°C. The resulting thick sheet is then subjected to a three-stage preheating process: 60°C / 80°C / 100°C, with a total preheating time of 15 seconds. After that, MD stretching and TD1 stretching processes are performed. For specific conditions, please refer to Table 1.

[0110] After removing the plasticizer (paraffin oil) and drying with hot air at 55°C, a second transverse stretching (TD2 stretching) is performed, followed by heat setting treatment. Specific conditions are shown in Table 1; thus, a microporous separator for lithium batteries is prepared.

[0111] Examples 2-12, Comparative Examples 1-4

[0112] The differences between the battery microporous separator preparation methods involved in Examples 2-12 and Comparative Examples 1-4 and Example 1 are detailed in Table 1. The parts not shown in Table 1 are the same as those in Example 1.

[0113] In Example 12, the raw materials were obtained by mixing two types of polyethylene with crystallinity of 40% and 30% at a weight ratio of 75:25.

[0114] As can be seen from Examples 1-11 and Comparative Examples 1-4 in Table 1, by controlling the proportion of polyolefin segments with a weight-average molecular weight below 100,000 and the crystallinity of the first and second heating cycles in the microporous membrane using the method of the present invention, a microporous membrane with good elastic recovery rate, tensile strength, and air permeability can be obtained. However, the proportion of polyolefin segments with a weight-average molecular weight below 100,000 in Comparative Examples 1 and 2 does not meet the requirements of the present invention, the crystallinity of the first heating cycle in Comparative Example 3 does not meet the requirements of the present invention, and the crystallinity of the second heating cycle in Comparative Example 4 does not meet the requirements of the present invention. The resulting microporous membranes cannot simultaneously possess good elastic recovery rate and tensile strength.

[0115] As can be seen from the comparison of Examples 1-4, when the crystallinity of the microporous membrane is 60%-80% after a single heating, the elastic recovery rate and tensile strength of the microporous membrane can be improved in a balanced way.

[0116] The comparison between Examples 2 and 5-6 shows that when the proportion of polyolefin segments with a weight average molecular weight of less than 100,000 in the microporous membrane is 15-25 mol%, the elastic recovery rate and tensile strength of the microporous membrane can be improved in a balanced way.

[0117] As can be seen from the comparison of Examples 2 and 7-11, when the proportion of polyolefin segments with a weight average molecular weight of less than 10,000 in the microporous membrane is 0.5-2.5 mol%, preferably 0.5-1.5 mol%, the elastic recovery rate and tensile strength of the microporous membrane can be improved in a balanced way.

[0118] Unless otherwise expressly indicated by the context, the singular forms “a,” “an,” and “the / said” in this specification and the appended claims include the plural forms. Unless otherwise defined, 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 may be performed in any logically possible order, except for the specific order disclosed.

[0119] The representative examples are intended to help illustrate the invention and are not intended, nor should they be construed as limiting the scope of the invention. In fact, many modifications and numerous other embodiments of the invention will become apparent to those skilled in the art, in addition to those shown and described herein, including the embodiments and scientific and patent literature references cited herein. The embodiments contain important additional information, illustrations, and guidance that can be adopted in the various embodiments and equivalents of the invention.

Claims

1. A microporous separator for lithium batteries, characterized in that, It is a polyolefin porous membrane; in the microporous membrane, the proportion of polyolefin segments with a weight average molecular weight of less than 100,000 is 15-30 mol%, preferably 15-25 mol%, more preferably 15-21 mol%; the crystallinity of the microporous membrane after one heating is ≥60%, preferably 60%-80%, and the crystallinity after two heatings is ≤45%, preferably 20%-45%.

2. The microporous separator for lithium batteries according to claim 1, wherein, The microporous diaphragm has a longitudinal elastic recovery rate of ≥14%, preferably ≥18%, and a transverse elastic recovery rate of ≥14%, preferably ≥18%, measured under the following conditions. These conditions include: cutting a diaphragm strip with a width of 15 mm from one of the longitudinal (MD) or transverse (TD) directions; stretching the diaphragm strip from L0 = 100 mm in the aforementioned direction at a speed of 50 mm / min to 50% of L0; holding the stretch for 60 seconds; allowing it to naturally retract for 3 minutes; measuring the length L1; and calculating the elastic recovery rate as (1.5*L0-L1) / (0.5*L0)*100%.

3. The microporous separator for lithium batteries according to claim 1 or 2, wherein, The difference between the crystallinity obtained from the first heating and the crystallinity obtained from the second heating is 25%-45%.

4. The microporous separator for lithium batteries according to any one of claims 1 to 3, wherein, In the microporous membrane, the proportion of polyolefin segments with a weight average molecular weight of less than 10,000 is 0-3.5 mol%, preferably 0.5-2.5 mol%, and more preferably 0.5-1.5 mol%.

5. The microporous separator for lithium batteries according to any one of claims 1 to 4, wherein, The microporous membrane satisfies one or more of the following combinations: a. Surface resistance is 0.2-0.7Ω; b. Thickness is 1-30μm; c. Breathability is 10-300 seconds / 100cc.

6. The microporous separator for lithium batteries according to any one of claims 1 to 5, wherein, The microporous membrane has a tensile strength of 4000 kgf / cm² in the MD or TD direction. 2 The above values ​​are preferably 4000-6000 kgf / cm³. 2 More preferably 4800-6000 kgf / cm³ 2 .

7. The microporous separator for lithium batteries according to any one of claims 1 to 6, wherein, The microporous membrane is a membrane prepared by a wet process.

8. A method for preparing a microporous separator for lithium batteries, characterized in that, Includes the following steps: (a) Melt-blending a mixture containing a polyolefin resin and a plasticizer to form a melt; (b) Extruding and solidifying the melt into a thick sheet; (c) The thick sheet is stretched in the longitudinal direction (MD direction) and the transverse direction (TD direction) to obtain a stretched body; (d) Remove the plasticizer from the stretched body, dry it, and obtain the microporous separator for lithium batteries; In step (a), the polyolefin resin contains polyolefin segments with a weight-average molecular weight of less than 100,000, accounting for 10-30 mol%, preferably 10-25 mol%, more preferably 13-25 mol%, and even more preferably 13-20 mol%, with a crystallinity of less than 45%.

9. The preparation method according to claim 8, wherein, In the polyolefin resin of step (a), the proportion of polyolefin segment components with a weight average molecular weight of less than 10,000 is 0-3 mol%, preferably 0.3-2 mol%, and more preferably 0.3-1.5 mol%.

10. The preparation method according to claim 8 or 9, wherein, The molecular weight distribution of the polyolefin resin is between 3 and 6.

11. The preparation method according to any one of claims 8 to 10, wherein, In the microporous membrane, the proportion of polyolefin segments with a weight-average molecular weight of less than 100,000 is 15-30 mol%, preferably 15-25 mol%, more preferably 15-21 mol%. Furthermore, in the microporous membrane, the proportion of polyolefin segment components with a weight average molecular weight of less than 10,000 is 0-3.5 mol%, preferably 0.5-2.5 mol%, more preferably 0.5-1.5 mol%. Further, the longitudinal elastic recovery rate of the microporous diaphragm 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%, measured under the following conditions: a diaphragm strip with a width of 15 mm is cut from one of the longitudinal direction (MD direction) or the transverse direction (TD direction), the diaphragm strip is stretched from L0 = 100 mm in the direction at a speed of 50 mm / min to 50% of the elongation of L0, held for 60 s, allowed to naturally retract for 3 min, and then the length L1 is measured. The elastic recovery rate is calculated as (1.5*L0-L1) / (0.5*L0)*100%.

12. The preparation method according to any one of claims 8 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 8 to 12, wherein, 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; Further, in step (b), the mixture is extruded through a die and attached to a casting roller to cool and solidify into a thick sheet, with the casting roller temperature at 20-30°C.

14. The preparation method according to any one of claims 8 to 13, wherein, In step (c), the longitudinal stretching is performed at a stretching temperature of 80-120°C, with a stretching ratio of 6.5-9 times, preferably 7-9 times, and the stretching method is multi-point distributed stretching, preferably 3-7 points. Furthermore, in the longitudinal stretching, there is at least one set of three adjacent stretching points with an increasing stretching ratio, and the absolute value of the difference in stretching ratio between the adjacent three stretching points is greater than 0 and less than or equal to 0.7, preferably 0.1-0.

6.

15. The preparation method according to any one of claims 8 to 14, wherein, In step (c), the stretching temperature in the transverse direction is 90-125°C, preferably 90-115°C, and the stretching ratio is 6-8 times, preferably 7-8 times. Preferably, 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.

1.

16. The preparation method according to any one of claims 8 to 15, wherein, In step (d), after the drying process, a second transverse stretching step is also included. 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 8 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.

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