Microporous separator for lithium battery and preparation method therefor

Abstract

A microporous separator for a lithium battery. The polyolefin-resin-containing microporous separator for a lithium battery has good resilience and compression resistance in the direction of thickness. Further provided is a preparation method for the high-resilience microporous separator for a lithium battery.

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] In addition, microporous membranes for lithium batteries are also required to have good resilience in the thickness direction, thereby ensuring that when the battery is impacted in the thickness direction, the microporous membrane undergoes smaller deformation in the thickness direction and can recover more quickly when the impact force is removed, thereby ensuring the safety performance of the battery.

[0004] Although the prior art has focused on compression set, there is still a need for a diaphragm with a fast rebound rate in the thickness direction. In particular, there is still a need for a diaphragm with a fast rebound rate in the thickness direction, a small compression set, and a large rebound amount.

[0005] Summary of the Invention

[0006] By controlling the proportion of high molecular weight segments in the polyolefin resin, the present invention can produce a lithium-ion battery separator with a fast rebound rate in the thickness direction. In addition, the separator of the present invention also has a low compression set rate and a large rebound amount.

[0007] The present inventors have also discovered that controlling the parameters of the extrusion step during the membrane manufacturing process can help control the compression resistance of the membrane.

[0008] A first aspect of the present invention provides a microporous separator for a lithium battery, comprising a polyolefin resin, wherein the melt index of the microporous separator is 0.04-3 g / 10 min, preferably 0.08-0.4 g / 10 min, and the amount of the polyolefin segment component with a weight average molecular weight of 2 million to 5 million in the microporous separator accounts for 5-8 mol%, preferably 5-7 mol%, and the rebound speed of the microporous separator in the thickness direction is greater than or equal to 2×10 -4 μm / s, preferably greater than or equal to 3×10 -4 μm / s, greater than or equal to 5×10 -4μm / s, greater than or equal to 7×10 -4 μm / s, greater than or equal to 8×10 -4 μm / s, most preferably greater than or equal to 10×10 -4 μm / s;

[0009] The rebound velocity of the microporous membrane in the thickness direction is calculated by the following formula: V = (D2-D1) / (T2-T1)

[0010] Where, V is the rebound velocity in μm / s.

[0011] D2——the thickness of the microporous membrane when the compressive loading force is reduced to 30mN, in μm.

[0012] D1——The thickness of the microporous diaphragm when it is compressed to the lowest point, in μm.

[0013] T2——The time when the microporous membrane compressive loading force is reduced to 30mN, in seconds.

[0014] T1——The time when the microporous diaphragm is compressed to the lowest point, in seconds.

[0015] Test conditions: A loading force of 500 mN was applied to the microporous membrane in the thickness direction. After the constant force was maintained for 300 s, the force was reduced at a rate of 30 mN / min.

[0016] Preferably, in the microporous separator, the polyolefin segment component having a weight average molecular weight greater than 5 million and less than 9 million accounts for 0-1 mol%, preferably 0.5-1 mol%.

[0017] Preferably, the microporous membrane satisfies one or more of the following combinations:

[0018] a. When measured under the test conditions, the compressive deformation rate of the microporous membrane in the thickness direction is less than or equal to 2.5%, preferably less than or equal to 1.8%, more preferably less than or equal to 1.5%, and most preferably less than or equal to 1%;

[0019] b. When measured under the test conditions, the rebound recovery rate of the microporous membrane in the thickness direction is greater than or equal to 0.5%, preferably greater than or equal to 1%, greater than or equal to 1.5%, greater than or equal to 2%, and most preferably greater than or equal to 3%;

[0020] in,

[0021] Compression deformation rate = (original thickness - thickness at the lowest point of compression deformation) / original thickness * 100%,

[0022] Rebound deformation rate = (original thickness - thickness when pressure value drops to 30mN) / original thickness * 100%,

[0023] Rebound recovery rate = |compression deformation rate - rebound deformation rate|.

[0024] Preferably, the molecular weight distribution of the polyolefin resin is between 3 and 5, preferably between 3.5 and 4.5.

[0025] Preferably, in the polyolefin resin, 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%.

[0026] Preferably, in the polyolefin resin, the polyolefin segment component having a weight average molecular weight greater than 5 million and less than 9 million accounts for 0-2 mol%, preferably 1-1.5 mol%.

[0027] Preferably, the microporous membrane is a membrane prepared by a wet process.

[0028] Preferably, the polyolefin resin is selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE), polypropylene, polybutene, polymethylpentene, copolymers thereof, and blends thereof.

[0029] Preferably, the average pore size of the microporous membrane is 25-50 nm, preferably 30-45 nm, and the thickness of the microporous membrane is 1-30 μm, preferably 4-12 μm.

[0030] A second aspect of the present invention provides a method for preparing a microporous separator for a lithium battery, comprising the following steps:

[0031] (a) melt-kneading a mixture comprising a polyolefin resin and a plasticizer to form a melt;

[0032] (b) extruding the melt obtained in step (a) and solidifying it into a thick sheet;

[0033] (c) stretching the obtained thick sheet in the longitudinal direction (MD direction) and in the transverse direction (TD direction) perpendicular to the longitudinal direction to obtain a stretched body;

[0034] (d) removing the plasticizer from the stretched body to obtain a diaphragm precursor;

[0035] (e) heat-setting the separator precursor to obtain the microporous separator for lithium battery;

[0036] The polyolefin resin has a melt index of 0.01-3 g / 10 min, preferably 0.05-0.3 g / 10 min, and the polyolefin segment components with a weight average molecular weight of 2 million to 5 million account for 5-9 mol%, preferably 5-8 mol%, and more preferably 6-8 mol%.

[0037] Preferably, in step (a), in the polyolefin resin, the amount of the polyolefin segment component with a weight average molecular weight greater than 5 million and less than 9 million accounts for 0-2 mol%, preferably 1-1.5 mol%.

[0038] Preferably, in the microporous separator, the polyolefin segment component having a weight average molecular weight greater than 5 million and less than 9 million accounts for 0-1 mol%, preferably 0.5-1 mol%.

[0039] Preferably, the molecular weight distribution of the polyolefin resin is 3-5, preferably 3.5-4.5.

[0040] Preferably, in the microporous separator, 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%.

[0041] Preferably, the rebound speed of the microporous membrane in the thickness direction is greater than or equal to 2×10 -4 μm / s, preferably greater than or equal to 3×10 -4 μm / s, greater than or equal to 5×10 -4 μm / s, greater than or equal to 7×10 -4 μm / s, greater than or equal to 8×10 -4 μm / s, most preferably greater than or equal to 10×10 -4 μm / s;

[0042] The rebound velocity of the microporous membrane in the thickness direction is calculated by the following formula: V = (D2-D1) / (T2-T1)

[0043] Where, V is the rebound velocity in μm / s.

[0044] D2——the thickness of the microporous membrane when the compressive loading force is reduced to 30mN, in μm.

[0045] D1——The thickness of the microporous diaphragm when it is compressed to the lowest point, in μm.

[0046] T2——The time when the microporous membrane compressive loading force is reduced to 30mN, in seconds.

[0047] T1——The time when the microporous diaphragm is compressed to the lowest point, in seconds.

[0048] Test conditions: A loading force of 500 mN was applied to the microporous membrane in the thickness direction. After the constant force was maintained for 300 s, the force was reduced at a rate of 30 mN / min.

[0049] Preferably, the weight ratio of the polyolefin resin to the plasticizer in step (a) is between 15:85 and 35:65, preferably between 18:82 and 23:77.

[0050] Preferably, in step (a), an extruder is used for melt mixing, and the parameters of the extruder include: extruder temperature of 150-260° C., and extruder screw speed of 60-125 r / min.

[0051] Preferably, in step (b), the mixture is extruded through a die lip and attached to a casting roller to cool and solidify to form a thick sheet, the die lip opening is a, the thickness of the thick sheet is h, and the expansion coefficient A=h / a is defined, and A is controlled to be greater than or equal to 1.2.

[0052] Preferably, the roller speed of the casting roller is 3-8 m / min, and the temperature of the die head is 160-240°C.

[0053] Preferably, in step (c), stretching in the MD direction is performed at 80-120° C. with a stretching ratio of 4-7 times, and stretching in the TD direction is performed at 90-130° C. with a stretching ratio of 4-12 times.

[0054] Preferably, in step (e), the heat setting includes oven heat treatment and roller heat treatment, preferably the temperature of the oven heat treatment is 120-150°C, and the temperature of the roller heat treatment is 50-70°C.

[0055] 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

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] As used herein, the term "polyolefin" may be a polyolefin monomer (ie, a single type of polyolefin), a polyolefin copolymer, or a polyolefin blend.

[0061] 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.).

[0062] As used herein, the term "longitudinal direction," also referred to as the MD direction, refers to the machine direction.

[0063] The term "transverse direction" as used herein, also referred to as the TD direction, refers to the direction perpendicular to the machine direction.

[0064] A first aspect of the present invention provides a microporous separator for a lithium battery, comprising a polyolefin resin, wherein the melt index of the microporous separator is 0.04-3 g / 10 min, preferably 0.08-0.4 g / 10 min, and the amount of the polyolefin segment component with a weight average molecular weight of 2 million to 5 million in the microporous separator accounts for 5-8 mol%, preferably 5-7 mol%, and the rebound speed of the microporous separator in the thickness direction is greater than or equal to 2×10 -4 μm / s, preferably greater than or equal to 3×10 -4 μm / s, greater than or equal to 5×10 -4 μm / s, greater than or equal to 7×10 -4 μm / s, greater than or equal to 8×10 -4 μm / s, most preferably greater than or equal to 10×10 -4 μm / s,

[0065] The rebound velocity of the microporous membrane in the thickness direction is calculated by the following formula: V = (D2-D1) / (T2-T1)

[0066] Where, V is the rebound velocity in μm / s.

[0067] D2——the thickness of the microporous membrane when the compressive loading force is reduced to 30mN, in μm.

[0068] D1——The thickness of the microporous diaphragm when it is compressed to the lowest point, in μm.

[0069] T2——The time when the microporous membrane compressive loading force is reduced to 30mN, in seconds.

[0070] T1——The time when the microporous diaphragm is compressed to the lowest point, in seconds.

[0071] Test conditions: A loading force of 500 mN was applied to the microporous membrane in the thickness direction. After the constant force was maintained for 300 s, the force was reduced at a rate of 30 mN / min.

[0072] According to some preferred embodiments of the present invention, in the microporous separator, the polyolefin segment component with a weight average molecular weight greater than 5 million and less than 9 million accounts for 0-1 mol%, preferably 0.5-1 mol%.

[0073] In the present invention, the polyolefin resin can use any polyolefin resin commonly used in the art for preparing microporous diaphragms 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.

[0074] According to the present invention, the polyolefin resin used can be used to produce the microporous separator for lithium batteries required by the present invention. In order to provide a microporous separator with better rebound performance in the thickness direction, preferably, in the polyolefin resin, the number of polyolefin segment components with a weight average molecular weight greater than 5 million and less than 9 million accounts for 0-2mol%, preferably 1-1.5mol%. Preferably, in the polyolefin resin, the number of polyolefin segment components with a weight average molecular weight of 2 million to 5 million accounts for 5-9mol%, preferably 5-8mol%, more preferably 6-8mol%. Preferably, the molecular weight distribution of the polyolefin resin is between 3-5, preferably between 3.5-4.5.

[0075] In some embodiments of the present invention, the microporous membrane satisfies one or more of the following combinations:

[0076] a. When measured under the above conditions, the compressive deformation rate of the microporous membrane in the thickness direction is less than or equal to 2.5%, preferably less than or equal to 1.8%, more preferably less than or equal to 1.5%, and most preferably less than or equal to 1%;

[0077] b. When measured under the above conditions, the rebound recovery rate of the microporous membrane in the thickness direction is greater than or equal to 0.5%, preferably greater than or equal to 1%, greater than or equal to 1.5%, greater than or equal to 2%, and most preferably greater than or equal to 3%;

[0078] in,

[0079] Compression deformation rate = (original thickness - thickness at the lowest point of compression deformation) / original thickness * 100%,

[0080] Rebound deformation rate = (original thickness - thickness when pressure value drops to 30mN) / original thickness * 100%,

[0081] Rebound recovery rate = |compression deformation rate - rebound deformation rate|.

[0082] In order to obtain a microporous separator having more excellent rebound performance in the thickness direction, the microporous separator of the present invention is preferably a separator produced by a wet process.

[0083] According to the present invention, the average pore size of the microporous membrane may be 25-50 nm, preferably 30-45 nm, and the thickness of the microporous membrane may be 1-30 μm, preferably 4-12 μm.

[0084] 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:

[0085] (a) melt-kneading a mixture comprising a polyolefin resin and a plasticizer to form a melt;

[0086] (b) extruding the melt obtained in step (a) and solidifying it into a thick sheet;

[0087] (c) stretching the obtained thick sheet in the longitudinal direction (MD direction) and in the transverse direction (TD direction) perpendicular to the longitudinal direction to obtain a stretched body;

[0088] (d) removing the plasticizer from the stretched body to obtain a diaphragm precursor;

[0089] (e) heat-setting the separator precursor to obtain the microporous separator for lithium battery;

[0090] The polyolefin resin has a melt index of 0.01-3 g / 10 min, preferably 0.05-0.3 g / 10 min, and the polyolefin segment components with a weight average molecular weight of 2 million to 5 million account for 4-9 mol%, preferably 5-8 mol%.

[0091] According to the present invention, by treating a mixture containing an appropriate polyolefin resin and a plasticizer according to the preparation method of the present invention, a microporous separator having excellent resilience in the thickness direction can be obtained, which is suitable for use in lithium batteries.

[0092] 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.

[0093] 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 40 ° C kinematic viscosity 40-55mm 2 / s of liquid paraffin. The kinematic viscosity test method is GB / T 265.

[0094] 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, and more preferably between 18:82 and 23:77. Specifically, the weight ratio of the polyolefin resin to the plasticizer can be 15:85, 16:84, 17:83, 18:82, 19:81, 20:80, 21:79, 22:78, or 23:77. By using such a weight ratio of polyolefin resin to plasticizer, the resilience of the microporous separator in the thickness direction can be improved.

[0095] To further improve the resilience of the microporous separator in the thickness direction, the polyolefin segment component having a weight-average molecular weight greater than 5 million and less than 9 million accounts for 0-2 mol%, preferably 1-1.5 mol%. Furthermore, the molecular weight distribution of the polyolefin resin is 3-5, preferably 3.5-4.5.

[0096] 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 150-260°C, preferably 180-250°C, and an extruder screw speed of 60-125 rpm, preferably 70-90 rpm.

[0097] According to the present invention, in step (b), the method of extruding the melt and solidifying it into a slab can form a slab of a desired thickness.

[0098] Through in-depth research, the inventors of the present invention have also found that controlling the parameters of the extrusion step during the diaphragm manufacturing process helps control the compression resistance of the diaphragm. Under the condition of a fixed extrusion volume, the thick sheet in step (b) can be achieved by matching the die lip opening and the casting speed. Assuming that the polymer melt extrusion coefficient is k, the melt extrusion flow rate is v, the die lip opening is a, the casting speed is b, and the thickness of the thick sheet is h, then h = a*k*(v / b), kv = (h / a)*b, and the expansion coefficient A = h / a is defined. The smaller A is, the worse the compression resistance of the diaphragm. When the mixture is extruded through the die lip and attached to the casting roller to cool and solidify to form a thick sheet, the die lip opening is a, the thick sheet thickness is h, and the expansion coefficient A = h / a is defined. Preferably, A is controlled to be greater than or equal to 1.2.

[0099] In the present invention, the expansion coefficient A can be controlled by the roller speed of the casting roller, the melt flow rate after demolding, and the die lip opening. The melt flow rate after demolding can be comprehensively adjusted by the extruder screw speed and the die head temperature.

[0100] Furthermore, the roller speed of the casting roller can be 3-8 m / min, preferably 5-6 m / min; the temperature of the die head can be 160-240° C., preferably 180-220° C. Preferably, the temperature of the casting roller can be 20-60° C.

[0101] Furthermore, the die lip opening may be 0.5-4 mm, preferably 0.8-3 mm.

[0102] According to the present invention, in step (c), the thick sheet is stretched to obtain a stretched body, and the stretching includes stretching in the MD direction and stretching in the TD direction. Specifically, stretching in the MD direction can be performed first and then stretching in the TD direction, or stretching in the TD direction can be performed first and then stretching in the MD direction. The former is preferred.

[0103] According to some preferred embodiments of the present invention, in step (c), stretching in the MD direction is carried out at 80-120° C. with a stretching ratio of 4-7 times, and stretching in the TD direction is carried out at 90-130° C. with a stretching ratio of 4-12 times. Preferably, stretching in the MD direction is carried out at 95-110° C. with a stretching ratio of 6-8 times, and stretching in the TD direction is carried out at 105-125° C. with a stretching ratio of 6-10 times.

[0104] 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.

[0105] As a method for removing the plasticizer from the stretched body by using an extractant, the plasticizer in the stretched body can be removed by circulating the extractant into the liquid. Preferably, the amount of the extractant circulating into the liquid is 1-5m 3 After extraction, the stretched body is optionally dried by heating it with one or more methods including a hot roller, a heating plate, and hot air, preferably at a drying temperature of 20-150°C.

[0106] According to the present invention, there are no particular limitations on the method and conditions for the heat setting treatment in step (e). For example, the heat setting may include oven heat treatment and roller heat treatment. Preferably, the oven heat treatment temperature is 120-150° C., and the roller heat treatment temperature is 50-70° C. In addition, the oven heat treatment time may be 2-30 seconds, and the roller heat treatment time may be 5-60 seconds.

[0107] In the microporous membrane prepared by the above preparation method, 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%. Preferably, in the microporous membrane, the polyolefin segment component with a weight average molecular weight greater than 5 million and less than 9 million accounts for 0-1 mol%, preferably 0.5-1 mol%. Preferably, the rebound velocity of the microporous membrane in the thickness direction is greater than or equal to 2×10 -4 μm / s, preferably greater than or equal to 3×10 -4 μm / s, greater than or equal to 5×10 -4 μm / s, greater than or equal to 7×10 -4 μm / s, greater than or equal to 8×10 - 4 μm / s, most preferably greater than or equal to 10×10 -4 μm / s.

[0108] The rebound velocity of the microporous membrane in the thickness direction is calculated by the following formula: V = (D2-D1) / (T2-T1)

[0109] Where, V is the rebound velocity in μm / s.

[0110] D2——the thickness of the microporous membrane when the compressive loading force is reduced to 30mN, in μm.

[0111] D1——The thickness of the microporous diaphragm when it is compressed to the lowest point, in μm.

[0112] T2——The time when the microporous membrane compressive loading force is reduced to 30mN, in seconds.

[0113] T1 is the time when the microporous diaphragm is compressed to the lowest point, measured in seconds;

[0114] Test conditions: A loading force of 500 mN was applied to the microporous membrane in the thickness direction. After the constant force was maintained for 300 s, the force was reduced at a rate of 30 mN / min.

[0115] 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.

[0116] In the following examples and comparative examples, the testing methods of relevant parameters are as follows.

[0117] (1) The percentage of polyolefin segments with a weight-average molecular weight of 2 to 5 million and the percentage of polyolefin segments with a weight-average molecular weight of greater than 5 million and less than 9 million shall be measured in accordance with GB / T 36214.4-2018.

[0118] (2) Melt index is measured in accordance with GBT3682.1-2018.

[0119] (3) The rebound velocity, compression set rate, and rebound set rate were measured using a TMA instrument (Waters, model Q400).

[0120] Example 1

[0121] A polyethylene resin with a melt index of 0.01 g / 10 min and a molecular weight distribution of 4.5 and a plasticizer (40 ° C kinematic viscosity of 45 mm 2 / s paraffin oil) in a weight ratio of 28:72, and melt-kneaded in an extruder to form a melt; wherein, the polyethylene segments with a weight average molecular weight of 2 to 5 million account for 6% of the polyethylene resin, and the polyethylene segments with a weight average molecular weight greater than 5 million and less than 9 million account for 0.7%, the extruder temperature is 220° C., and the extruder screw speed is 80 r / min;

[0122] The melt was cooled and solidified by a casting roller at a temperature of 25°C to form a thick sheet; the obtained thick sheet was stretched at a stretching temperature of 95°C in the MD direction and a stretching ratio of 6.3 times; the first stretching temperature in the TD direction was 110°C and a stretching ratio of 6 times;

[0123] After removing the plasticizer (paraffin oil), the second stretching in the TD direction was performed at a stretching temperature of 133°C and a second stretching ratio of 1.4 times, followed by heat setting treatment at a heat setting temperature of 133°C.

[0124] Examples 2 to 12, Comparative Examples 1 to 2

[0125] The microporous membrane was prepared by referring to the method of Example 1. The differences between the microporous membrane preparation methods used in Examples 2 to 12 and Comparative Examples 1 to 2 and Example 1 are detailed in Table 1. Parameters not shown in Table 1 are the same as those in Example 1.

[0126] Examples 2-1 to 2-3

[0127] The microporous membrane was prepared by referring to the method of Example 2. The differences between the microporous membrane preparation methods used in Examples 2-1 to 2-3 and Example 2 are detailed in Table 2. Table 2 does not reflect that the parameters are the same as those in Example 2.

[0128] It can be seen from Examples 1-4 in Table 1 that when the proportion of polyolefin segments with a weight average molecular weight of 2-5 million in the microporous membrane increases, the rebound speed, rebound recovery rate, compression deformation rate and rebound deformation rate of the microporous membrane all show a uniform trend of decreasing.

[0129] From the comparison of Examples 5-6 and 9 with Example 2, it can be seen that when the proportion of polyolefin segments with a weight average molecular weight of 5-9 million in the microporous membrane decreases (<1%), the rebound speed, rebound recovery rate and rebound deformation rate of the microporous membrane are greatly improved, but the compression deformation rate also increases (deteriorates) accordingly. When the proportion increases (>1%), the compression deformation rate decreases (improves), but the rebound speed and rebound recovery rate decrease.

[0130] From the comparison between Examples 7-8 and Example 2, it can be seen that the change in the molecular weight distribution of the raw materials also has some influence on the performance in the thickness direction of the microporous membrane, but the influence is not very significant.

[0131] From the comparison between Examples 10 and 12 and Example 2, and the comparison between Example 11 and Example 5, it can be seen that when the expansion coefficient is ≥1.2, the compression rebound performance in the thickness direction is better, and with the increase of the expansion coefficient, the compression rebound performance in the thickness direction is improved. When the expansion coefficient is less than 1.2, the compression rebound performance in the thickness direction is reduced.

[0132] It can be seen from Examples 2-1 to 2-3 in Table 2 that the proportion of polyolefin segments with a weight average molecular weight of 2-5 million in the microporous membrane can be regulated by matching the extruder process with the raw materials.

[0133] 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.

[0134] 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: The microporous membrane comprises a polyolefin resin, the melt index of the microporous membrane is 0.04-3 g / 10 min, preferably 0.08-0.4 g / 10 min, the weight average molecular weight of the polyolefin segment component in the microporous membrane is 2 million to 5 million, the amount of the polyolefin segment component accounts for 5-8 mol%, preferably 5-7 mol%, and the rebound speed of the microporous membrane in the thickness direction is greater than or equal to 2×10 -4 μm / s, preferably greater than or equal to 3×10 -4 μm / s, greater than or equal to 5×10 -4 μm / s, greater than or equal to 7×10 -4 μm / s, greater than or equal to 8×10 -4 μm / s, most preferably greater than or equal to 10×10 -4 μm / s; The rebound velocity of the microporous membrane in the thickness direction is calculated by the following formula: V=(D2-D1) / (T2-T1) Where, V is the rebound velocity, in μm / s. D2——The thickness of the microporous membrane when the compressive loading force is reduced to 30mN, in μm, D1——The thickness of the microporous diaphragm when it is compressed to the lowest point, in μm. T2——The time when the microporous membrane compressive loading force is reduced to 30mN, in seconds. T1——The time when the microporous diaphragm is compressed to the lowest point, in seconds. Test conditions: A loading force of 500 mN was applied to the microporous membrane in the thickness direction. After the constant force was maintained for 300 s, the force was reduced at a rate of 30 mN / min.

2. The microporous separator for lithium battery according to claim 1, wherein: In the microporous separator, the amount of the polyolefin segment component having a weight average molecular weight greater than 5 million and less than 9 million accounts for 0-1 mol %, preferably 0.5-1 mol %.

3. The microporous separator for lithium battery according to claim 1 or 2, wherein: The microporous membrane satisfies one or more of the following combinations: a. When measured under the test conditions, the compression shape of the microporous membrane in the thickness direction The variability is less than or equal to 2.5%, preferably less than or equal to 1.8%, more preferably less than or equal to 1.5%, and most preferably less than or equal to 1%; b. When measured under the test conditions, the rebound recovery rate of the microporous diaphragm in the thickness direction is greater than or equal to 0.5%, preferably greater than or equal to 1%, greater than or equal to 1.5%, greater than or equal to 2%, and most preferably greater than or equal to 3%; in, Compression deformation rate = (original thickness - thickness at the lowest point of compression deformation) / original thickness * 100%, Rebound deformation rate = (original thickness - thickness when pressure value drops to 30mN) / original thickness * 100%, Rebound recovery rate = |compression deformation rate - rebound deformation rate|.

4. The microporous separator for lithium battery according to any one of claims 1 to 3, wherein The molecular weight distribution of the polyolefin resin is between 3-5, preferably between 3.5-4.

5.

5. The microporous separator for lithium battery according to any one of claims 1 to 4, wherein In the polyolefin resin, the amount of 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%; Furthermore, in the polyolefin resin, the amount of the polyolefin segment component having a weight average molecular weight greater than 5 million and less than 9 million accounts for 0-2 mol%, preferably 1-1.5 mol%.

6. The microporous separator for lithium battery according to any one of claims 1 to 5, wherein The microporous membrane is a membrane prepared by a wet process.

7. The microporous separator for lithium battery according to any one of claims 1 to 6, wherein The polyolefin resin is selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE), polypropylene, polybutene, polymethylpentene, copolymers thereof, and blends thereof.

8. The microporous separator for lithium battery according to any one of claims 1 to 7, wherein The average pore size of the microporous membrane is 25-50 nm, preferably 30-45 nm, and the thickness of the microporous membrane is 1-30 μm, preferably 4-12 μm.

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 the melt obtained in step (a) and solidifying it into a thick sheet; (c) stretching the obtained thick sheet in the longitudinal direction (MD direction) and in the transverse direction (TD direction) perpendicular to the longitudinal direction to obtain a stretched body; (d) removing the plasticizer from the stretched body to obtain a diaphragm precursor; (e) thermally shaping the membrane precursor to obtain the microporous membrane for lithium battery; The melt index of the polyolefin resin is 0.01-3 g / 10 min, preferably 0.05-0.3 g / 10 min. In the polyolefin resin, the amount of 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%.

10. The preparation method according to claim 9, wherein: In step (a), in the polyolefin resin, the amount of the polyolefin segment component having a weight average molecular weight greater than 5 million and less than 9 million accounts for 0-2 mol%, preferably 1-1.5 mol%; Furthermore, in the microporous membrane, the amount of the polyolefin segment component having a weight average molecular weight greater than 5 million and less than 9 million accounts for 0-1 mol%, preferably 0.5-1 mol%; Further, the molecular weight distribution of the polyolefin resin is 3-5, preferably 3.5-4.5; Furthermore, in the microporous membrane, the amount of 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%; Furthermore, the rebound speed of the microporous membrane in the thickness direction is greater than or equal to 2×10 -4 μm / s, preferably greater than or equal to 3×10 -4 μm / s, greater than or equal to 5×10 -4 μm / s, greater than or equal to 7×10 -4 μm / s, greater than or equal to 8×10 -4 μm / s, most preferably greater than or equal to 10×10 -4 μm / s; The rebound velocity of the microporous membrane in the thickness direction is calculated by the following formula: V=(D2-D1) / (T2-T1) Where, V is the rebound velocity, in μm / s. D2——The thickness of the microporous membrane when the compressive loading force is reduced to 30mN, in μm, D1——The thickness of the microporous diaphragm when it is compressed to the lowest point, in μm. T2——The time when the microporous membrane compressive loading force is reduced to 30mN, in seconds. T1——The time when the microporous diaphragm is compressed to the lowest point, in seconds. Test conditions: A loading force of 500 mN was applied to the microporous membrane in the thickness direction. After the constant force was maintained for 300 s, the force was reduced at a rate of 30 mN / min. Furthermore, in step (a), the weight ratio of the polyolefin resin to the plasticizer is between 15:85 and 35:65, preferably between 18:82 and 23:

77.

11. The preparation method according to claim 9 or 10, wherein: In step (a), an extruder is used for melt mixing, and the parameters of the extruder include: an extruder temperature of 150-260° C. and an extruder screw speed of 60-125 r / min.

12. The preparation method according to claim 11, wherein In step (b), the mixture is extruded through a die lip and attached to a casting roll to be cooled and solidified to form a thick sheet, wherein the die lip opening is a, the thickness of the thick sheet 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 roller speed of the casting roller is 3-8 m / min, and the temperature of the die head is 160-240°C.

13. The preparation method according to any one of claims 9 to 12, wherein In step (c), stretching in the MD direction is performed at 80-120° C. with a stretching ratio of 4-7 times, and stretching in the TD direction is performed at 90-130° C. with a stretching ratio of 4-12 times.

14. The preparation method according to any one of claims 9 to 13, wherein In step (e), the heat setting includes oven heat treatment and roller heat treatment, preferably, the temperature of the oven heat treatment is 120-150°C, and the temperature of the roller heat treatment is 50-70°C.

15. The preparation method according to any one of claims 9 to 14, wherein The polyolefin resin is selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE), polypropylene, polybutene, polymethylpentene, copolymers thereof, and blends thereof.

Images