Microporous separator for lithium battery, and preparation method therefor

Abstract

The present invention relates to the technical field of lithium-ion batteries. Specifically, provided in the present invention are a polyolefin resin-containing microporous separator for a lithium battery, and a preparation method therefor. The microporous separator for a lithium battery in the present invention has good puncture strength, a long memory effect duration and a low compression deformation rate.

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. Reasons for this include the excellent mechanical strength and closed-cell properties of polyolefin microporous membranes.

[0003] However, existing diaphragms cannot achieve a balance between puncture strength, memory effect duration, and compression deformation rate. Therefore, diaphragms with high puncture strength, short memory effect duration, and low compression deformation rate are still needed. Summary of the Invention

[0004] This invention achieves lithium-ion battery separators with high puncture strength, short memory effect duration, and low compression set by appropriately controlling the crystallinity of the microporous separator and the proportion of polyolefin segments with a specified molecular weight. Furthermore, this invention also reveals that controlling the parameters of the extrusion step during separator manufacturing helps to control the separator's compression resistance.

[0005] The first aspect of the present invention provides a microporous separator for lithium batteries, wherein the microporous separator is a polyolefin porous membrane, wherein the polyolefin segment component with a weight average molecular weight of 2 million to 5 million accounts for 5-8 mol%, preferably 5-7 mol%, more preferably 6-7 mol%, and the microporous separator has a primary temperature-induced crystallinity and a secondary temperature-induced crystallinity, wherein the primary temperature-induced crystallinity is ≥65%, and the difference between the primary temperature-induced crystallinity and the secondary temperature-induced crystallinity is >15%.

[0006] In some embodiments, the proportion of polyolefin segment components with a weight-average molecular weight greater than 5 million and less than 9 million in the microporous membrane is 0.5-2 mol, preferably 0.5-1.5 mol.

[0007] In some embodiments, the crystallinity of the first heating is 65%-80%, preferably 68%-75%.

[0008] In some embodiments, the crystallinity of the secondary heating is ≤50%, preferably 35%-50%, and more preferably 45%-50%.

[0009] In some embodiments, the difference between the crystallinity of the first heating and the crystallinity of the second heating is ≥16%, preferably 16%-45%, and more preferably 18%-30%.

[0010] In some embodiments, the microporous membrane satisfies one or more of the following combinations:

[0011] a) The puncture strength per unit thickness of the microporous membrane is ≥85gf / μm, preferably ≥90gf / μm;

[0012] b) The memory effect duration of the microporous membrane in the thickness direction is 0-200s, preferably 0-100s, more preferably 0-50s, even more preferably 0-20s, and most preferably 0-10s;

[0013] c) The compressibility of the microporous membrane is ≤2.5%, preferably ≤1.5%, more preferably ≤1%, and even more preferably ≤0.7%;

[0014] d) The thickness of the microporous membrane is 1-30 μm, preferably 4-12 μm.

[0015] In some embodiments, the microporous membrane is a membrane made of polyolefin resin, wherein the molecular weight of the polyolefin resin is between 3 and 6, preferably between 3.5 and 5.5, and more preferably between 4.5 and 5.

[0016] In some embodiments, the polyolefin resin contains 5-9 mol% polyolefin segment components with a weight-average molecular weight of 2 million to 5 million, preferably 5-8 mol%, and more preferably 6-8 mol%.

[0017] In some embodiments, the amount of polyolefin segment components with a weight-average molecular weight greater than 5 million and less than 9 million in the polyolefin resin is 0-2 mol, preferably 0.2-1.5 mol.

[0018] A second aspect of the present invention provides a method for preparing a microporous separator for lithium batteries, characterized by comprising the following steps:

[0019] (a) Melt-blending a mixture containing a polyolefin resin and a plasticizer to form a melt;

[0020] (b) Extrude and solidify the melt obtained in step (a) into a thick sheet;

[0021] (c) The thick sheet obtained in step (b) is stretched in the longitudinal direction (MD direction) and in the transverse direction perpendicular to the longitudinal direction (TD direction) to obtain a stretched body;

[0022] (d) Remove the plasticizer from the stretched body obtained in step (c) to obtain a diaphragm precursor;

[0023] (e) Heat-set the membrane precursor obtained in step (d) to obtain the microporous membrane for the lithium battery;

[0024] The polyolefin resin has a crystallinity of ≤50%, preferably 35%-50%, and the polyolefin segment component with a weight average molecular weight of 2 million to 5 million accounts for 5-9 mol%, preferably 5-8 mol%, and more preferably 6-8 mol%.

[0025] In some embodiments, in step (a), the amount of polyolefin segment components with a weight-average molecular weight greater than 5 million and less than 9 million in the polyolefin resin is 0-2 mol%, preferably 0.2-1.5 mol.

[0026] In some embodiments, the molecular weight distribution of the polyolefin resin is between 3 and 6, preferably between 3.5 and 5.5, and more preferably between 4.5 and 5.

[0027] In some embodiments, the polyolefin resin is selected from polyethylene, polypropylene, polybutene, polymethylpentene, copolymers thereof, and blends thereof; preferably, the polyethylene is selected from LDPE, LLDPE, HDPE, and UHDPE.

[0028] In some embodiments, an extruder is used for melt mixing in step (a), the parameters of which include: extruder temperature 150-260°C and extruder screw speed 60-125 r / min.

[0029] In some embodiments, the weight ratio of the polyolefin resin to the plasticizer is between 15:85 and 35:65, preferably between 18:82 and 23:77.

[0030] In some embodiments, in step (b), the mixture is extruded through a die lip and attached to a casting roller to cool and solidify into a thick sheet, wherein the die lip opening is a, the sheet thickness is h, the expansion coefficient A is defined as h / a, and A is controlled to be greater than or equal to 1.2.

[0031] In some embodiments, the speed of the casting roller is 3-8 m / min, and the temperature of the die head is 160-240°C.

[0032] In some embodiments, in step (c), the stretching in the MD direction is performed at 80-120°C with a stretching ratio of 4-7 times, and the stretching in the TD direction is performed at 90-135°C with a stretching ratio of 4-15 times.

[0033] In some embodiments, in step (e), the heat setting is selected from at least one of oven heat treatment and roller heat treatment. Preferably, the temperature of the oven heat treatment is 120-150°C. Preferably, the temperature of the roller heat treatment is 50-70°C. Detailed Implementation

[0034] 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.

[0035] 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.

[0036] 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.

[0037] As used in this article, the term "polyolefin" can refer to polyolefin homopolymers (i.e., single-type polyolefin polymers), polyolefin copolymers, or polyolefin blends.

[0038] 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)).

[0039] The term "longitudinal direction" used in this article, also known as MD direction, refers to the direction of machine movement.

[0040] The term "lateral direction" used in this article, also known as the TD direction, refers to the direction perpendicular to the machine's running direction.

[0041] The first aspect of the present invention provides a microporous separator for lithium batteries, wherein the microporous separator is a polyolefin porous membrane, wherein the polyolefin segment component with a weight average molecular weight of 2 million to 5 million accounts for 5-8 mol%, preferably 5-7 mol%, more preferably 6-7 mol%, and the microporous separator has a primary temperature-induced crystallinity and a secondary temperature-induced crystallinity, wherein the primary temperature-induced crystallinity is ≥65%, and the difference between the primary temperature-induced crystallinity and the secondary temperature-induced crystallinity is >15%.

[0042] In the microporous membrane, the proportion of polyolefin segments with a weight-average molecular weight of 2 million to 5 million can be any one or any combination of 5 mol%, 5.1 mol%, 5.2 mol%, 5.3 mol%, 5.4 mol%, 5.5 mol%, 5.6 mol%, 5.7 mol%, 5.8 mol%, 5.9 mol%, 6 mol%, 6.1 mol%, 6.2 mol%, 6.3 mol%, 6.4 mol%, 6.5 mol%, 6.6 mol%, 6.7 mol%, 6.8 mol%, 6.9 mol%, 7 mol%, 7.1 mol%, 7.2 mol%, 7.3 mol%, 7.4 mol%, 7.5 mol%, 7.6 mol%, 7.7 mol%, 7.8 mol%, 7.9 mol%, or 8 mol%.

[0043] 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.

[0044] In some specific embodiments of the present invention, in order to further improve the memory duration and puncture strength, the proportion of polyolefin segment components with a weight-average molecular weight greater than 5 million and less than 9 million in the microporous membrane is 0.5-2 mol%, preferably 0.5-1.5 mol.

[0045] In the microporous membrane, the proportion of polyolefin segments with a weight-average molecular weight greater than 5 million and less than 9 million can be any one or any combination of 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%, or 2 mol%.

[0046] In some specific embodiments of the present invention, the crystallinity of the microporous membrane after the first heating is higher than that after the second heating. In some specific embodiments, the crystallinity of the microporous membrane after the first heating is 65%-80%, preferably 68%-75%. In some specific embodiments, the crystallinity of the microporous membrane after the second heating is ≤50%, preferably 35%-50%, more preferably 45%-50%. In some specific embodiments, the difference between the crystallinity after the first heating and the crystallinity after the second heating is ≥16%, preferably 16%-45%, more preferably 18%-30%.

[0047] In the microporous membrane, the crystallinity of the first heating can be, for example, a range of any one or any two of 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, or 80%; and the crystallinity of the second heating can be, for example, a range of any one or any two of 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, or 50%. The difference between the crystallinity obtained from the first heating and the crystallinity obtained from the second heating can be, for example, a range of any one or any two of the following: 16%, 17%, 20%, 22%, 25%, 27%, 30%, 32%, 35%, 37%, 40%, 42%, or 45%.

[0048] According to the present invention, any polyolefin resin used may be used to prepare the microporous separator for lithium batteries required by the present invention. To provide a microporous separator with superior puncture strength, memory effect duration, and compression set, preferably, the polyolefin resin contains a polyolefin segment component with a weight-average molecular weight of 2 million to 5 million in a proportion of 5-9 mol%, more preferably 5-8 mol%, and even more preferably 6-8 mol%. Preferably, the polyolefin segment component with a weight-average molecular weight greater than 5 million and less than 9 million in a proportion of 0-2 mol%, and more preferably 0.2-1.5 mol%.

[0049] In the polyolefin resin, the proportion of polyolefin segment components with a weight average molecular weight of 2 million to 5 million can be any one or any combination of 5 mol%, 5.5 mol%, 6 mol%, 6.5 mol%, 7 mol%, 7.3 mol%, 7.5 mol%, 8 mol%, 8.5 mol%, and 9 mol%.

[0050] In the polyolefin resin, the proportion of polyolefin segment components with a weight average molecular weight greater than 5 million and less than 9 million can be any one of 0 mol%, 0.2 mol%, 0.5 mol%, 1 mol%, 1.3 mol%, 1.5 mol%, 1.8 mol%, 2 mol%, or any combination thereof.

[0051] In some specific embodiments of the present invention, preferably, the molecular weight distribution of the polyolefin resin is between 3 and 6, more preferably between 3.5 and 5.5, and even more preferably between 4.5 and 5.

[0052] In some specific embodiments of the present invention, the microporous membrane satisfies one or more of the following combinations:

[0053] a) The puncture strength per unit thickness of the microporous membrane is ≥85gf / μm, preferably ≥90gf / μm;

[0054] b) The memory effect duration of the microporous membrane in the thickness direction is 0-200s, preferably 0-100s, more preferably 0-50s, even more preferably 0-20s, and most preferably 0-10s;

[0055] c) The compressibility of the microporous membrane is ≤2.5%, preferably ≤1.8%, ≤1.5%, ≤1%, or ≤0.7%;

[0056] d) The thickness of the microporous membrane is 1-30 μm, preferably 4-12 μm.

[0057] To obtain a microporous membrane with superior puncture strength, memory effect duration, and compression set, the microporous membrane of this invention is preferably prepared by a wet process. Specifically, the microporous membrane has a three-dimensional network structure with several interwoven protofibrils, forming pores. The protofibrils in the microporous membrane are obtained by stretching and oriented the polymer during film preparation, and the extractant extracts the plasticizer from the film to form pores.

[0058] A second aspect of the present invention provides a method for preparing a microporous separator for lithium batteries, the method comprising the following steps:

[0059] (a) Melt-blending a mixture containing a polyolefin resin and a plasticizer to form a melt;

[0060] (b) Extrude and solidify the melt obtained in step (a) into a thick sheet;

[0061] (c) The thick sheet obtained in step (b) is stretched in the longitudinal direction (MD direction) and in the transverse direction perpendicular to the longitudinal direction (TD direction) to obtain a stretched body;

[0062] (d) Remove the plasticizer from the stretched body obtained in step (c) to obtain a diaphragm precursor;

[0063] (e) Heat-set the membrane precursor obtained in step (d) to obtain the microporous membrane for the lithium battery;

[0064] The polyolefin resin has a crystallinity of ≤50%, preferably 35%-50%, more preferably 40%-50%, and the polyolefin segment component with a weight average molecular weight of 2 million to 5 million accounts for 5-9 mol%, preferably 5-8 mol%, more preferably 6-8 mol%.

[0065] In the polyolefin resin, the crystallinity of the polyolefin resin can be, for example, a range of any one or any combination of 20%, 25%, 30%, 35%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, or 50%.

[0066] In the polyolefin resin, the proportion of polyolefin segment components with a weight average molecular weight of 2 million to 5 million can be any one or any combination of 5 mol%, 5.5 mol%, 6 mol%, 6.5 mol%, 7 mol%, 7.3 mol%, 7.5 mol%, 8 mol%, 8.5 mol%, and 9 mol%.

[0067] According to the present invention, by processing a mixture containing a suitable polyolefin resin and a plasticizer according to the preparation method of the present invention, a microporous membrane with excellent puncture strength, memory effect duration and compression set can be obtained, which is suitable for use in lithium batteries.

[0068] 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.

[0069] 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.

[0070] 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 ≤50%.

[0071] 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 mmHg at 40°C is preferred. 2 / s of paraffinic oil. The kinematic viscosity test method is in accordance with GB / T265.

[0072] According to the present invention, in step (a), the weight ratio of the polyolefin resin to the plasticizer is preferably between 15:85 and 35:65, more preferably between 18:82 and 23:77. Specifically, the weight ratio of the polyolefin resin to the plasticizer can be any or a combination of any two of the following: 15:85, 16:84, 17:83, 18:82, 19:81, 20:80, 21:79, 22:78, or 23:77. By using the above-mentioned weight ratio of polyolefin resin to plasticizer, the overall performance of the microporous membrane can be improved.

[0073] To further improve the puncture strength, memory effect duration, and compression set of the microporous diaphragm, preferably, the proportion of polyolefin segments with a weight-average molecular weight greater than 5 million and less than 9 million in the polyolefin resin is 0-2 mol%, preferably 0.2-1.5 mol.

[0074] In some specific embodiments of the present invention, preferably, the molecular weight distribution of the polyolefin resin is between 3 and 6, more preferably between 3.5 and 5.5, and even more preferably between 4.5 and 5.

[0075] 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 150-260°C, preferably 180-250°C, and extruder screw speed 60-125 r / min, preferably 70-90 r / min.

[0076] According to the present invention, in step (b), the method of extruding the melt and solidifying it into a thick sheet can form a sheet of the desired thickness.

[0077] Through in-depth research, the inventors of this invention have also discovered that controlling the parameters of the extrusion step in the diaphragm manufacturing process helps to control the compressibility of the diaphragm. Under a fixed extrusion rate, the thick sheet in step (b) can be achieved by combining the die lip opening and the casting speed. Assuming the polymer melt expansion coefficient after leaving the die is k, the melt flow rate after leaving the die is v, the die lip opening is a, the casting speed is b, and the sheet thickness is h, then h = a*k*(v / b), kv = (h / a)*b. The expansion coefficient A is defined as h / a. The smaller A is, the worse the compressibility of the diaphragm. When the mixture is extruded through the die lip and attached to the casting roller for cooling and solidification to form a thick sheet, the die lip opening is a, the sheet thickness is h, and the expansion coefficient A is defined as h / a. Preferably, A is greater than or equal to 1.2, for example, 1.2-1.5.

[0078] In this invention, the expansion coefficient A can be controlled by the roller speed of the casting roller, the melt flow rate after leaving the die, and the die lip opening. The melt flow rate after leaving the die can be adjusted by the screw speed of the extruder and the die temperature.

[0079] Furthermore, the roll speed of the casting roll can be 3-8 m / min, preferably 5-6 m / min; the temperature of the die head can be 160-240℃, preferably 180-220℃. Preferably, the temperature of the casting roll can be 20-60℃.

[0080] Furthermore, the die lip opening can be 0.5-4mm, preferably 0.8-3mm.

[0081] Furthermore, the thickness of the sheet can be 1-5 mm, preferably 1.2-2 mm.

[0082] According to the present invention, in step (c), the thick sheet is stretched to obtain a stretched body. The stretching includes stretching in the MD direction and stretching in the TD direction. Specifically, the stretching in the MD direction can be performed first and then the stretching in the TD direction, or the stretching in the TD direction can be performed first and then the stretching in the MD direction, or the stretching in both the MD and TD directions can be performed simultaneously; preferably, the stretching in the MD direction is performed first and then the stretching in the TD direction. Furthermore, the stretching in the MD direction and the stretching in the TD direction can each be independently performed as a single-segment stretching or as multi-segment stretching of two or more segments. From the viewpoints of stretching uniformity, mechanical strength, and prevention of membrane rupture, multi-segment stretching is preferred.

[0083] In some specific embodiments of the present invention, in step (c), the stretching in the MD direction is performed at 80-120°C with a stretching ratio of 4-7 times, and the stretching in the TD direction is performed at 90-135°C with a stretching ratio of 4-15 times. Preferably, the stretching in the MD direction is performed at 95-110°C with a stretching ratio of 6-8 times, and the stretching in the TD direction is performed at 105-125°C with a stretching ratio of 6-10 times.

[0084] 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.

[0085] 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 rollers, heating plates, or hot air. The preferred drying temperature is 20-150°C.

[0086] In some preferred embodiments of the present invention, after step (d) and before the heat setting treatment in step (e), a further stretching treatment may be included, preferably a further stretching treatment in the TD direction (also known as secondary stretching in the TD direction). Preferably, this secondary stretching in the TD direction is performed at 120-140°C with a stretching ratio of 1-2 times; more preferably, the secondary stretching in the TD direction is performed at 130-140°C with a stretching ratio of 1.1-1.5 times. The above-mentioned secondary stretching in the TD direction can be a single-segment stretching or a multi-segment stretching of two or more segments. From the viewpoint of stretching uniformity, mechanical strength, and prevention of film breakage, multi-segment stretching is preferred.

[0087] According to the present invention, there are no particular limitations on the method and conditions of the heat setting treatment in step (e). For example, the heat setting can be selected from at least one of oven heat treatment and roller heat treatment. Preferably, the temperature of the oven heat treatment is 120-150°C; preferably, the temperature of the roller heat treatment is 50-70°C. Furthermore, the oven heat treatment time can be 2-30 seconds, and the roller heat treatment time can be 5-60 seconds.

[0088] The microporous membrane prepared using the above method comprises a polyolefin segment component with a weight average molecular weight of 2 million to 5 million, accounting for 5-8 mol%, preferably 5-7 mol%, and more preferably 6-7 mol%, and has a primary temperature rise crystallinity of ≥65% and a secondary temperature rise crystallinity of <50%. Preferably, the polyolefin segment component with a weight average molecular weight greater than 5 million and less than 9 million accounts for 0.5-2 mol%, preferably 0.5-1.5 mol%.

[0089] In some specific embodiments of the present invention, the microporous membrane satisfies one or more of the following combinations:

[0090] a) The puncture strength per unit thickness of the microporous membrane is ≥85gf / μm, preferably ≥90gf / μm, and more preferably ≥100gf / μm;

[0091] b) The memory effect duration of the microporous membrane in the thickness direction is 0-200s, preferably 0-100s, more preferably 0-50s, even more preferably 0-20s, and most preferably 0-10s;

[0092] c) The compressibility of the microporous membrane is ≤2.5%, preferably ≤1.8%, more preferably ≤1.5%, and most preferably ≤1%.

[0093] d) The thickness of the microporous membrane is 1-30 μm, preferably 4-12 μm.

[0094] 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.

[0095] The testing methods for the relevant parameters in the following examples and comparative examples are as follows.

[0096] (1) The proportion of polyolefin segments with a weight average molecular weight of 2 million to 5 million and polyolefin segments with a weight average molecular weight greater than 5 million and less than 9 million shall be measured in accordance with GB / T 36214.4-2018.

[0097] (2) Compression strain rate was measured using a TMA instrument (Waters, Q400 model).

[0098] (3) 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.

[0099] In addition, the crystallinity of the raw material was measured using the above-mentioned secondary heating crystallinity test method.

[0100] (4) Puncture strength shall be measured in accordance with GB-T 36363-2018. Puncture strength per unit thickness = puncture strength / film thickness.

[0101] (5) The memory effect duration is measured as follows: A loading force of 500 mN is applied to the microporous membrane in the thickness direction. After maintaining the constant force for 300 s, the force is reduced at a rate of 30 mN / min. The time difference between the time when the microporous membrane reaches its maximum compression loading force and the time when the microporous membrane reaches its minimum compression deformation is the memory effect duration. The memory effect duration reflects the deformation response speed of the microporous membrane in the thickness direction. Especially after battery compression or collision, a short memory effect duration (rapid recovery) allows for timely recovery, maintaining close contact with the positive and negative electrodes and preventing direct contact between the positive and negative electrodes to ensure battery safety. However, a membrane with a short memory effect duration will still deform after the external force is removed, causing the positive and negative electrodes to loosen, shift, and come into contact, leading to a short circuit and thus safety issues.

[0102] Example 1

[0103] Polyethylene resin (UHDPE, relevant properties are shown in Table 1, prepared by Daehan Oil & Chemical Co., Ltd.) and plasticizer (kinematic viscosity of 45 mm at 40°C) were mixed. 2 Paraffin oil ( / s) is mixed at a weight ratio of 20:80 and then melted and kneaded in an extruder to form a melt. The extruder temperature is 220℃ and the extruder screw speed is 80r / min.

[0104] The above melt is cooled and solidified by a casting roll to form a thick sheet. The temperature of the casting roll is 25°C. Other parameters are shown in the process section of Table 1. The obtained thick sheet is stretched in the MD direction and stretched once in the TD direction. Specific conditions are shown in Table 1.

[0105] After removing the plasticizer (paraffin oil) using dichloromethane as an extractant and drying under hot air at 55°C, the membrane is then subjected to secondary stretching in the TD direction, followed by heat setting treatment (roller heat treatment) in the MD direction. Specific conditions are shown in Table 1. This process yields a microporous separator for lithium batteries.

[0106] Examples 2 to 18, Comparative Examples 1 to 4

[0107] Microporous membranes were prepared according to the method of Example 1. The differences between the microporous membrane preparation methods used in Examples 2 to 18 and Comparative Examples 1 to 4 and those in Example 1 are detailed in Table 1. Parameters not shown in Table 1 are the same as those in Example 1.

[0108] Table 1

[0109] In the table, "Percentage A" represents the percentage of polyolefin segments with a weight average molecular weight of 2 million to 5 million, and "Percentage B" represents the percentage of polyolefin segments with a weight average molecular weight greater than 5 million and less than 9 million. In Example 18, the raw material was obtained by mixing two types of polyethylene with a crystallinity of 50% and 40% in a weight ratio of 7:3.

[0110] A comparison of Examples 1-18 and Comparative Examples 1-2 in Table 1 shows that by controlling the number of polyolefin segments with a weight-average molecular weight of 2 million to 5 million 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 puncture strength, memory effect duration, and compression deformation rate can be obtained. However, the crystallinity of the first heating cycle in Comparative Example 1 does not meet the requirements of the present invention, and the resulting microporous membrane has poor puncture strength, memory effect duration, and compression deformation rate. The difference between the crystallinity of the first and second heating cycles in Comparative Example 2 does not meet the requirements of the present invention, and the resulting microporous membrane has poor memory effect duration. The proportion of polyolefin segments with a weight-average molecular weight of 2 million to 5 million in Comparative Examples 3-4 does not meet the requirements of the present invention, and the resulting microporous membrane has poor puncture strength or memory effect duration.

[0111] The comparison of Examples 3 and 12-14 shows that when the proportion of polyolefin segments with a weight average molecular weight greater than 5 million and less than 9 million in the microporous membrane is 0.5-1.5 mol%, the memory effect duration of the microporous membrane can be further reduced.

[0112] As can be seen from the comparison of Examples 3 and 15-16, when the proportion of polyolefin segments with a weight average molecular weight of 2 million to 5 million in the microporous membrane is 5-7 mol%, the memory effect duration of the microporous membrane can be further reduced.

[0113] As can be seen from the comparison of Examples 1-5, when the crystallinity of the microporous membrane is 68-75% after a single heating, it can have both good compressibility and excellent memory effect duration.

[0114] As can be seen from the comparison of Examples 3 and 6-9, when the crystallinity of the microporous membrane after secondary heating is 45-50%, it can have higher puncture strength while having both good memory effect duration and compression deformation rate.

[0115] 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.

[0116] 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 by, The microporous membrane is a polyolefin porous membrane. In the microporous membrane, the proportion of polyolefin segment components with a weight average molecular weight of 2 million to 5 million is 5-8 mol%, preferably 5-7 mol%, more preferably 6-7 mol%. The microporous membrane has a primary temperature rise crystallinity and a secondary temperature rise crystallinity. The primary temperature rise crystallinity is ≥65%, and the difference between the primary temperature rise crystallinity and the secondary temperature rise crystallinity is >15%.

2. The microporous separator for lithium batteries according to claim 1, wherein, In the microporous membrane, the proportion of polyolefin segments with a weight-average molecular weight greater than 5 million and less than 9 million is 0.5-2 mol, preferably 0.5-1.5 mol.

3. The microporous separator for lithium batteries according to claim 1 or 2, wherein, The crystallinity of the single temperature rise is 65%-80%, preferably 68%-75%; and / or The crystallinity of the secondary heating is ≤50%, preferably 35%-50%, more preferably 45%-50%; and / or The difference between the crystallinity obtained from the first heating and the crystallinity obtained from the second heating is ≥16%, preferably 16%-45%, and more preferably 18%-30%.

4. The microporous separator for lithium batteries according to any one of claims 1 to 3, wherein, The microporous membrane satisfies one or more of the following combinations: a) The puncture strength per unit thickness of the microporous membrane is ≥85gf / μm, preferably ≥90gf / μm; b) The memory effect duration of the microporous membrane in the thickness direction is 0-200s, preferably 0-100s, more preferably 0-50s, even more preferably 0-20s, and most preferably 0-10s; c) The compressibility of the microporous membrane is ≤2.5%, preferably ≤1.5%, more preferably ≤1%, and even more preferably ≤0.7%; d) The thickness of the microporous membrane is 1-30 μm, preferably 4-12 μm.

5. The microporous separator for lithium batteries according to any one of claims 1 to 4, wherein, The microporous membrane is a membrane made of polyolefin resin, wherein the molecular weight of the polyolefin resin is between 3 and 6, preferably between 3.5 and 5.5, and more preferably between 4.5 and 5.

6. The microporous separator for lithium batteries according to claim 5, wherein, In the polyolefin resin, the proportion of polyolefin segment components with a weight average molecular weight of 2 million to 5 million is 5-9 mol%, preferably 5-8 mol%, more preferably 6-8 mol%; and / or In the polyolefin resin, the proportion of polyolefin segment components with a weight-average molecular weight greater than 5 million and less than 9 million is 0-2 mol%, preferably 0.2-1.5 mol.

7. A method for producing a microporous separator for lithium batteries, characterized by, Includes the following steps: (a) Melt-blending a mixture containing a polyolefin resin and a plasticizer to form a melt; (b) Extrude and solidify the melt obtained in step (a) into a thick sheet; (c) The thick sheet obtained in step (b) is stretched in the longitudinal direction (MD direction) and in the transverse direction perpendicular to the longitudinal direction (TD direction) to obtain a stretched body; (d) Remove the plasticizer from the stretched body obtained in step (c) to obtain a diaphragm precursor; (e) Heat-set the membrane precursor obtained in step (d) to obtain the microporous membrane for the lithium battery; The polyolefin resin has a crystallinity of ≤50%, preferably 35%-50%, more preferably 40%-50%, and the polyolefin segment component with a weight average molecular weight of 2 million to 5 million accounts for 5-9 mol%, preferably 5-8 mol%, more preferably 6-8 mol%.

8. The production method according to claim 7, wherein In step (a), the polyolefin resin contains polyolefin segments with a weight-average molecular weight greater than 5 million and less than 9 million, accounting for 0-2 mol%, preferably 0.2-1.5 mol%; and / or The polyolefin resin has a molecular weight distribution between 3 and 6, preferably between 3.5 and 5.5, and more preferably between 4.5 and 5; and / or The polyolefin resin is selected from polyethylene, polypropylene, polybutene, polymethylpentene, copolymers thereof, and blends thereof; preferably, the polyethylene is selected from LDPE, LLDPE, HDPE, and UHDPE.

9. The production method according to claim 7 or 8, wherein In step (a), melt mixing is performed using an extruder, the parameters of which include: extruder temperature 150-260°C, extruder screw speed 60-125 r / min; and / or The weight ratio of the polyolefin resin to the plasticizer is between 15:85 and 35:65, preferably between 18:82 and 23:

77.

10. The production method according to any one of claims 7 to 9, wherein, In step (b), the mixture is extruded through a die lip and attached to a casting roller to cool and solidify into a thick sheet. The die lip opening is a, the sheet thickness is h, and the expansion coefficient A = h / a is defined, and A is controlled to be greater than or equal to 1.

2. Furthermore, the speed of the casting roller is 3-8 m / min, and the temperature of the die head is 160-240℃.

11. The production method according to any one of claims 7 to 10, wherein, In step (c), the stretching in the MD direction is performed at 80-120°C with a stretching ratio of 4-7 times, and the stretching in the TD direction is performed at 90-135°C with a stretching ratio of 4-15 times.

12. The production method according to any one of claims 7 to 11, wherein, In step (e), the heat setting is selected from at least one of oven heat treatment and roller heat treatment, preferably with an oven heat treatment temperature of 120-150°C and / or a roller heat treatment temperature of 50-70°C.

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