Microporous separator for lithium battery and method for preparing same

Abstract

The present invention relates to a microporous separator for a lithium battery and a method for preparing the same. Specifically, the present invention provides a microporous separator for a lithium battery containing a polyolefin resin, which has excellent resilience and compression resistance in the thickness direction. The present invention also provides a method for preparing a microporous separator for a lithium battery with high resilience.

Description

[Technical Field]

[0001] The present invention relates to the field of lithium ion battery technology, and in particular to a microporous separator for lithium batteries and a method for preparing the same. [Background technology]

[0002] Polyolefin microporous membranes are widely used as separation membranes and separator materials for the separation and selective permeation of various substances. For example, polyolefin microporous membranes are used as microfiltration membranes, fuel cell separators, capacitor separators, base materials for functional membranes whose pores are filled with functional materials to exhibit new functions, battery separators, etc. Among these uses, polyolefin microporous membranes are particularly suitable for use as separators for lithium-ion batteries, which are widely used in laptop computers, mobile phones, digital cameras, etc. This is because polyolefin microporous membranes have excellent mechanical strength and pore sealing properties.

[0003] Furthermore, a microporous separator for a lithium battery must have excellent resilience in the thickness direction. In this way, when the battery is subjected to an impact in the thickness direction, the microporous membrane undergoes less deformation in the thickness direction, ensuring faster recovery after the impact force is removed, thereby ensuring the safety of the battery.

[0004] Although conventional techniques have focused on the compressive deformation rate, there is still a need for separators with a high rebound rate in the thickness direction, and in particular, a need for separators with a high rebound rate in the thickness direction, a small compressive deformation rate, and a large rebound amount. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention provides a lithium ion battery separator with a high resilience in the thickness direction by controlling the proportion of high molecular weight chain segments in the polyolefin resin. The separator according to the present invention also has a small compressive deformation rate and a large resilience.

[0006] The present inventors have further discovered that controlling the parameters of the extrusion process in the manufacture of the separator contributes to the control of the compression resistance of the separator. [Means for solving the problem]

[0007] In a first aspect, the present invention provides a microporous separator for a lithium battery containing a polyolefin resin, the microporous separator having a melt index of 0.04 g / 10 min to 3 g / 10 min, preferably 0.08 g / 10 min to 0.4 g / 10 min, a composition ratio of polyolefin chain segments having a weight average molecular weight of 2 million to 5 million in the microporous separator is 5 mol % to 8 mol %, preferably 5 mol % to 7 mol %, and a repulsion speed in the thickness direction of the microporous separator is 2×10 -4 μm / s or more, and 3×10 -4 μm / s or more, 5×10 -4 μm / s or more, 7×10 -4 μm / s or more, 8×10 -4 μm / s or more is preferable, and 10×10 -4 μm / s or more is most preferred, The rebound speed in the thickness direction of the microporous separator is calculated by the following formula. V=(D2-D1) / (T2-T1)

[0008] In the formula, V - velocity of repulsion, in μm / s, D2 - the thickness of the microporous separator when the compressive load is reduced to 30 mN, in μm; D1 - the thickness of the microporous separator when it is compressed to its lowest point, in μm; T2 - the time required for the compressive load of the microporous separator to decrease to 30 mN, in s; T1 - the time required for the microporous separator to be compressed to its lowest point, in s; Test conditions: A load of 500 mN is applied to the microporous separator in the thickness direction, and after maintaining this state for 300 seconds, the load value is reduced at a rate of 30 mN / min.

[0009] Preferably, in the microporous separator, the proportion of the number of polyolefin chain segments in the composition having a weight average molecular weight of more than 5 million and not more than 9 million is 0 mol % to 1 mol %, and more preferably 0.5 mol % to 1 mol %.

[0010] Preferably, the microporous separator is a) When measuring under the test conditions, the compressive deformation rate of the microporous separator in the thickness direction is 2.5% or less, preferably 1.8% or less, more preferably 1.5% or less, and most preferably 1% or less; and b. When measured under the test conditions, the rebound recovery rate of the microporous separator in the thickness direction satisfies one or a combination of the following conditions: 0.5% or more, preferably 1% or more, 1.5% or more, 2% or more, and most preferably 3% or more; where: Compression deformation rate = (original thickness - thickness when compressed to the lowest point) / original thickness * 100% Rebound deformation rate = (original thickness - Compression load thickness when the force is reduced to 30mN) / original thickness*100% The rebound recovery rate is set to |compression deformation rate-rebound deformation rate|.

[0011] Preferably, the molecular weight distribution of the polyolefin resin is in the range of 3 to 5, and more preferably in the range of 3.5 to 4.5.

[0012] Preferably, in the polyolefin resin, the proportion of the number of polyolefin chain segments in the composition having a weight average molecular weight of 2 million to 5 million is 5 mol% to 9 mol%, more preferably 5 mol% to 8 mol%, and more preferably 6 mol% to 8 mol%.

[0013] Preferably, in the polyolefin resin, the proportion of the number of polyolefin chain segments in the composition having a weight average molecular weight of more than 5 million and not more than 9 million is 0 mol % to 2 mol %, and more preferably 1 mol % to 1.5 mol %.

[0014] Preferably, the microporous separator is a separator prepared by a wet process.

[0015] Preferably, the polyolefin resin is selected from polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE), polypropylene, polybutylene, polymethylpentene, copolymers thereof, and co-blends thereof.

[0016] Preferably, the microporous separator has an average pore size of 25 nm to 50 nm, and more preferably 30 nm to 45 nm, and the microporous separator has a thickness of 1 μm to 30 μm, and more preferably 4 μm to 12 μm.

[0017] In a second aspect, the present invention provides a method for preparing a microporous separator for a lithium battery, said method comprising: Step (a) of melt-kneading a mixture containing a polyolefin resin and a plasticizer to form a melt; Step (b) of extruding the melt obtained in step (a) and hardening it into a slab; (c) stretching the obtained thick plate in the machine direction (MD direction) and the transverse direction (TD direction) perpendicular to the machine direction to obtain an elongated body; (d) removing the plasticizer from the elongated body to obtain a separator precursor; and (e) heat-setting the separator precursor to obtain the microporous separator for the lithium battery; Here, the polyolefin resin has a melt index of 0.01 g / 10 min to 3 g / 10 min, preferably 0.05 g / 10 min to 0.3 g / 10 min, and in the polyolefin resin, the proportion of the number of polyolefin chain segments having a weight average molecular weight of 2 million to 5 million is 5 mol% to 9 mol%, preferably 5 mol% to 8 mol%, and more preferably 6 mol% to 8 mol%.

[0018] Preferably, in step (a), the proportion of the number of polyolefin chain segments in the polyolefin resin composition having a weight average molecular weight of more than 5 million and not more than 9 million is 0 mol % to 2 mol %, and more preferably 1 mol % to 1.5 mol %.

[0019] Preferably, in the microporous separator, the proportion of the number of polyolefin chain segments in the composition having a weight average molecular weight of more than 5 million and not more than 9 million is 0 mol % to 1 mol %, and more preferably 0.5 mol % to 1 mol %.

[0020] Preferably, the molecular weight distribution of the polyolefin resin is in the range of 3 to 5, and more preferably in the range of 3.5 to 4.5.

[0021] Preferably, in the microporous separator, the proportion of the number of polyolefin chain segments having a weight average molecular weight of 2,000,000 to 5,000,000 in the composition is 5 mol % to 8 mol %, and more preferably 5 mol % to 7 mol %.

[0022] Preferably, the rebound velocity in the thickness direction of the microporous separator is 2×10 -4 μm / s or more, and 3×10 -4 μm / s or more, 5×10 -4 μm / s or more, 7×10 -4 μm / s or more, 8×10 -4 μm / s or more is preferable, and 10×10-4 μm / s or more is most preferred, Here, the rebound speed in the thickness direction of the microporous separator is calculated by the following formula. V=(D2-D1) / (T2-T1) In the formula, V - velocity of repulsion, in μm / s, D2 - the thickness of the microporous separator when the compressive load is reduced to 30 mN, in μm; D1 - the thickness of the microporous separator when it is compressed to its lowest point, in μm; T2 - the time required for the compressive load of the microporous separator to decrease to 30 mN, in s; T1 - the time required for the microporous separator to be compressed to its lowest point, in s; Test conditions: A load of 500 mN is applied to the microporous separator in the thickness direction, and after maintaining this state for 300 seconds, the load value is reduced at a rate of 30 mN / min.

[0023] Preferably, the weight ratio of the polyolefin resin to the plasticizer in step (a) is between 15:85 and 35:65, and more preferably between 18:82 and 23:77.

[0024] Preferably, an extruder is used for melt-kneading in step (a), and the parameters of the extruder are an extruder temperature of 150°C to 260°C and an extruder screw speed of 60 r / min to 125 r / min.

[0025] Preferably, in step (b), the mixture is extruded through a die head and die lip and attached to a casting roller to form a thick plate by cooling and hardening, the opening of the die lip is defined as a, the thickness of the thick plate is defined as h, and the expansion coefficient is defined as A=h / a, where A is controlled to be 1.2 or more.

[0026] Preferably, the roll speed of the casting roller is 3 m / min to 8 m / min, and the temperature of the die head is 160°C to 240°C.

[0027] Preferably, in step (c), the stretching in the MD direction is carried out at 80°C to 120°C at a stretching ratio of 4 to 7 times, and the stretching in the TD direction is carried out at 90°C to 130°C at a stretching ratio of 4 to 12 times.

[0028] Preferably, in step (e), the heat setting process includes heat treatment in an oven and heat treatment with a roller, and the temperature of the heat treatment in the oven is 120°C to 150°C, and the temperature of the heat treatment with a roller is 50°C to 70°C.

[0029] Preferably, the polyolefin resin is polyethylene (e.g., LDPE (Low-Density Polyethylene) , LLDPE (Linear Low-Density Polyethylene) , HDPE (High-Density Polyethylene) , UHDPE (Ultra High-Density Polyethylene) The polymer may be selected from the group consisting of polypropylene, polybutylene, polymethylpentene, copolymers thereof, and co-mixtures thereof. DETAILED DESCRIPTION OF THE INVENTION

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

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

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

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

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

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

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

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

[0038] In a first aspect, the present invention provides a microporous separator for a lithium battery containing a polyolefin resin, the microporous separator having a melt index of 0.04 g / 10 min to 3 g / 10 min, preferably 0.08 g / 10 min to 0.4 g / 10 min, a composition ratio of polyolefin chain segments having a weight average molecular weight of 2 million to 5 million in the microporous separator is 5 mol % to 8 mol %, preferably 5 mol % to 7 mol %, and a repulsion speed in the thickness direction of the microporous separator is 2×10 -4 μm / s or more, and 3×10-4 μm / s or more, 5×10 -4 μm / s or more, 7×10 -4 μm / s or more, 8×10 -4 μm / s or more is preferable, and 10×10 -4 μm / s or more is most preferred, The rebound speed in the thickness direction of the microporous separator is calculated by the following formula: (1) It is calculated as follows. V=(D2-D1) / (T2-T1) Formula (1) formula (1) Now, V - velocity of repulsion, in μm / s, D2 - the thickness of the microporous separator when the compressive load is reduced to 30 mN, in μm; D1 - the thickness of the microporous separator when it is compressed to its lowest point, in μm; T2 - the time required for the compressive load of the microporous separator to decrease to 30 mN, in s; T1 - the time required for the microporous separator to be compressed to its lowest point, in s; Test conditions: A load of 500 mN is applied to the microporous separator in the thickness direction, and after maintaining this state for 300 seconds, the load value is reduced at a rate of 30 mN / min.

[0039] According to some preferred embodiments of the present invention, in the microporous separator, the proportion of the number of polyolefin chain segments in the composition having a weight average molecular weight of more than 5 million and not more than 9 million is 0 mol% to 1 mol%, and preferably 0.5 mol% to 1 mol%.

[0040] In the present invention, the polyolefin resin can be any polyolefin resin commonly used in the art for preparing microporous separators for lithium batteries, such as polyethylene (e.g., LDPE). (Low-Density Polyethylene) , LLDPE (Linear Low-Density Polyethylene) , HDPE (High-Density Polyethylene) , UHDPE (Ultra High-Density Polyethylene)The polymer may be selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, copolymers thereof, and co-mixtures thereof, among which polyethylene and / or polypropylene are preferred.

[0041] According to the present invention, the polyolefin resin used may be any polyolefin resin that can be used to produce the microporous separator for lithium batteries required by the present invention. To provide a microporous separator with superior resilience in the thickness direction, the proportion of polyolefin chain segments having a weight-average molecular weight of more than 5 million and not more than 9 million in the polyolefin resin is preferably 0 mol% to 2 mol%, and more preferably 1 mol% to 1.5 mol%. The proportion of polyolefin chain segments having a weight-average molecular weight of 2 million to 5 million in the polyolefin resin is preferably 5 mol% to 9 mol%, and more preferably 5 mol% to 8 mol%, and more preferably 6 mol% to 8 mol%. The molecular weight distribution of the polyolefin resin is preferably 3 to 5, and more preferably 3.5 to 4.5.

[0042] In some specific embodiments of the present invention, the microporous separator comprises: a) When measuring under the above conditions, the compressive deformation rate of the microporous separator in the thickness direction is 2.5% or less, preferably 1.8% or less, more preferably 1.5% or less, and most preferably 1% or less; and b) When measured under the above conditions, the rebound recovery rate of the microporous separator in the thickness direction is 0.5% or more, preferably 1% or more, 1.5% or more, 2% or more, and most preferably 3% or more, and one or a combination of the following conditions is satisfied; where: The compression deformation rate, the rebound deformation rate, and the rebound recovery rate are calculated by the following formulas. Compression deformation rate = (original thickness - thickness when compressed to the lowest point) / original thickness * 100% Formula (2) Rebound deformation rate = (original thickness - Compression loadthickness when the force is reduced to 30mN) / original thickness*100% Formula (3) Rebound recovery rate = |Compression deformation rate - Rebound deformation rate| Formula (4) .

[0043] In order to obtain a microporous separator having better resilience in the thickness direction, the microporous separator according to the present invention is preferably a separator prepared by a wet process.

[0044] According to the present invention, the average pore size of the microporous separator may be 25 nm to 50 nm, preferably 30 nm to 45 nm, and the thickness of the microporous separator may be 1 μm to 30 μm, preferably 4 μm to 12 μm.

[0045] In a second aspect, the present invention provides a method for preparing a microporous separator for a lithium battery, said method comprising: Step (a) of melt-kneading a mixture containing a polyolefin resin and a plasticizer to form a melt; Step (b) of extruding the melt obtained in step (a) and hardening it into a slab; (c) stretching the obtained thick plate in the machine direction (MD direction) and the transverse direction (TD direction) perpendicular to the machine direction to obtain an elongated body; (d) removing the plasticizer from the elongated body to obtain a separator precursor; and (e) heat-setting the separator precursor to obtain the microporous separator for the lithium battery; Here, the polyolefin resin has a melt index of 0.01 g / 10 min to 3 g / 10 min, preferably 0.05 g / 10 min to 0.3 g / 10 min, and in the polyolefin resin, the ratio of the number of polyolefin chain segments having a weight average molecular weight of 2 million to 5 million is 5 The content is from 5 mol % to 9 mol %, and preferably from 5 mol % to 8 mol %.

[0046] According to the present invention, a microporous separator suitable for use in a lithium battery and having excellent resilience in the thickness direction can be obtained by processing a mixture containing a suitable polyolefin resin and a plasticizer according to the preparation method of the present invention.

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

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

[0049] According to the present invention, in step (a), the weight ratio of the polyolefin resin to the plasticizer is preferably between 15:85 and 35:65, more preferably between 18:82 and 23:77. Specifically, the weight ratio of the polyolefin resin to the plasticizer may be 15:85, 16:84, 17:83, 18:82, 19:81, 20:80, 21:79, 22:78, or 23:77. By applying the above weight ratio of the polyolefin resin to the plasticizer, the resilience performance of the microporous separator in the thickness direction can be improved.

[0050] To further improve the resilience of the microporous separator in the thickness direction, the polyolefin resin contains polyolefin chain segments with a weight-average molecular weight of more than 5 million and not more than 9 million in a ratio of 0 mol% to 2 mol%, preferably 1 mol% to 1.5 mol%, and the polyolefin resin has a molecular weight distribution of 3 to 5, preferably 3.5 to 4.5.

[0051] According to the present invention, in step (a), the mixture can be processed using any conventional method capable of forming a melt, for example, an extruder may be used for melt kneading. In some preferred embodiments, the extruder parameters include the extruder temperature and extruder screw speed, and the extruder temperature is 150°C to 260°C, preferably 180°C to 250°C, and the extruder screw speed is 60 r / min to 125 r / min, preferably 70 r / min to 90 r / min.

[0052] According to the present invention, the method for extruding the melt and hardening it into a slab in step (b) may be any method that will produce a slab of the desired thickness.

[0053] After extensive research, the inventors of the present invention have further found that controlling the parameters related to the extrusion process during separator production contributes to controlling the compression resistance of the separator. Under the condition that the extrusion rate is constant, the thick plate in step (b) can be realized by combining the die lip opening and the casting speed. When the expansion coefficient of the Barus effect of the polymer melt is k, the flow rate of the Barus effect of the melt is v, the die lip opening is a, the casting speed is b, and the thickness of the thick plate is h, The parameters satisfy the following equation: h=a*k*(v / b) Equation (1) kv=(h / a)*b Equation (2) In equation (5), Expansion coefficient Define A=h / a Formula (5) The smaller A, the worse the compression resistance of the separator. When the mixture is extruded through a die head and die lip onto a casting roller and cooled and hardened to form a thick plate, the die lip opening is a, the thickness of the thick plate is h, and the expansion coefficient is defined as A = h / a, and it is preferable to control A to 1.2 or more.

[0054] In the present invention, the expansion coefficient A can be controlled by a combination of the roll speed of the casting roller, the flow rate of the molten material due to the Barus effect, and the opening of the die lip, and the flow rate of the molten material due to the Barus effect can be adjusted by a combination of the screw rotation speed of the extruder and the die head temperature.

[0055] In some embodiments The roll speed of the casting roller may be 3 m / min to 8 m / min, and preferably 5 m / min to 6 m / min. In some embodiments, The temperature of the die head may be 160°C to 240°C, and is preferably 180°C to 220°C. In some embodiments The temperature of the casting roller may be 20°C to 60°C.

[0056] In some embodiments The opening of the die lip may be 0.5 mm to 4 mm, and preferably 0.8 mm to 3 mm.

[0057] According to the present invention, in step (c), the thick plate is stretched to obtain an elongated body, and the stretching includes stretching in the MD direction and stretching in the TD direction. Specifically, stretching in the MD direction may be performed followed by stretching in the TD direction, or stretching in the TD direction may be performed followed by stretching in the MD direction, with the former being preferred.

[0058] According to some preferred embodiments of the present invention, in step (c), the stretching in the MD direction is carried out at 80°C to 120°C and the stretching ratio is 4 to 7, and the stretching in the TD direction is carried out at 90°C to 130°C and the stretching ratio is 4 to 12. Preferably, the stretching in the MD direction is carried out at 95°C to 110°C and the stretching ratio is 6 to 8, and the stretching in the TD direction is carried out at 105°C to 125°C and the stretching ratio is 6 to 10.

[0059] According to the invention, in step (d), the plasticizer can be removed from the elongated body by any method, preferably by using an extractant to remove the plasticizer from the elongated body. In some embodiments, The extractant may be an alkane extractant. , or Halogenated Hydrocarbon Extractants ,for example Dichloromethane It is okay .

[0060] As a method for removing the plasticizer from the elongated body using an extractant, there is a method for removing the plasticizer in the elongated body by circulating the extractant. Preferably, the amount of circulating the extractant is 1 m 3 / h~5m 3 After extraction, the elongated body is heated and dried by optionally selecting one or more of a heat roller, a heating plate, and hot air, and the drying temperature is preferably 20°C to 150°C.

[0061] According to the present invention, the method and conditions of the heat setting treatment in step (e) are not particularly limited, and for example, the heat setting treatment can include heat treatment in an oven and heat treatment with a roller, and preferably, the temperature of the heat treatment in the oven is 120° C. to 150° C., and the temperature of the heat treatment with a roller is 50° C. to 70° C. Furthermore, the time of the heat treatment in the oven may be 2 to 30 seconds, and the time of the heat treatment with a roller may be 5 to 60 seconds.

[0062] As a result of preparation by the above preparation method, in the obtained microporous separator, the proportion of the number of polyolefin chain segments having a weight-average molecular weight of 2 million to 5 million is 5 mol% to 8 mol%, preferably 5 mol% to 7 mol%. Preferably, in the microporous separator, the proportion of the number of polyolefin chain segments having a weight-average molecular weight of more than 5 million and not more than 9 million is 0 mol% to 1 mol%, preferably 0.5 mol% to 1 mol%. Preferably, the repulsion velocity in the thickness direction of the microporous separator is 2×10 -4 μm / s or more, and 3×10 -4 μm / s or more, 5×10 -4 μm / s or more, 7×10 -4 μm / s or more, 8×10 -4 μm / s or more is preferable, and 10×10 -4 It is most preferable that the speed is μm / s or more. Here, the repulsion speed in the thickness direction of the microporous separator is expressed by the following formula: (1) It is calculated as follows. V=(D2-D1) / (T2-T1) Formula (1) formula (1) Now, V - velocity of repulsion, in μm / s, D2 - the thickness of the microporous separator when the compressive load is reduced to 30 mN, in μm; D1 - the thickness of the microporous separator when it is compressed to its lowest point, in μm; T2 - the time required for the compressive load of the microporous separator to decrease to 30 mN, in s; T1 - the time required for the microporous separator to be compressed to its lowest point, in s; Test conditions: A load of 500 mN is applied to the microporous separator in the thickness direction, and after maintaining this state for 300 seconds, the load value is reduced at a rate of 30 mN / min.

[0063] In a third aspect, the present invention provides a microporous separator for lithium batteries obtainable by the preparation process according to the second aspect of the present invention described above.

[0064] In the following Examples and Comparative Examples, the test methods for relevant parameters are as follows: (1) The proportion of polyolefin chain segments having a weight-average molecular weight of 2 million to 5 million and the proportion of polyolefin chain segments having a weight-average molecular weight of more than 5 million and not more than 9 million is measured in accordance with GB / T 36214.4-2018. (2) The melting index is measured in accordance with GBT3682.1-2018. (3) The rebound speed, compression deformation rate, and rebound deformation rate are measured by TMA (thermodynamic analysis, Thermomechanical Analysis) It is measured using an instrument (Waters, Model Q400).

[0065] Example 1 A polyethylene resin with a melt index of 0.1 g / 10 min and a molecular weight distribution of 4.5 was mixed with a plasticizer (kinematic viscosity at 40°C of 45 mm 2 The polyethylene resin is mixed with paraffin oil (of which the weight ratio is 28:72) and melt-kneaded in an extruder to form a melt. The ratio of the number of polyethylene chain segments with a weight-average molecular weight of 2 million to 5 million in the polyethylene resin is 6. mol %, and the ratio of the number of polyethylene chain segments having a weight-average molecular weight of more than 5 million and not more than 9 million is 1.3 mol %, the temperature of the extruder is 220 ° C, and the screw rotation speed of the extruder is 80 r / min, The melt is cooled and hardened by a casting roller to form a thick plate, and the temperature of the casting roller is set to 25°C. The obtained thick plate is stretched, and the stretching temperature in the MD direction is set to 95°C, and the stretching ratio is 6.3 times. The first stretching temperature in the TD direction is set to 110°C, and the stretching ratio is 6 times. After removing the plasticizer (paraffin oil), the sheet was stretched a second time in the TD direction at a stretching temperature of 133°C and a second stretching ratio of 1.4 times. After this, the sheet was subjected to a heat setting process at a heat setting temperature of 133°C.

[0066] Regarding Examples 2 to 12 and Comparative Examples 1 and 2 The microporous separators were prepared with reference to the method of Example 1. The differences between the microporous separator preparation methods specifically used in Examples 2 to 12 and Comparative Examples 1 and 2 and the method of Example 1 are shown in detail in Table 1, and the same parameters as those in Example 1 are not shown in Table 1.

[0067] Examples 2-1 to 2-3 The microporous separators were prepared with reference to the method of Example 2. The differences between the microporous separator preparation methods specifically used in Examples 2-1 to 2-3 and the method of Example 2 are shown in detail in Table 2, and the same parameters as those in Example 2 are not shown in Table 2.

[0068] [Table 1]

[0069] [Table 2]

[0070] As is clear from Examples 1 to 4 in Table 1, as the proportion of the number of polyolefin chain segments with a weight average molecular weight of 2 million to 5 million in the microporous separator increases, the rebound speed, rebound recovery rate, compression deformation rate, and rebound deformation rate of the microporous separator decrease in a uniform manner.

[0071] As is clear from a comparison of Examples 5-6 and Example 9 with Example 2, when the proportion of polyolefin chain segments with a weight-average molecular weight of 5 million to 9 million in the microporous separator decreases (<1%), the rebound speed, rebound recovery rate, and rebound deformation rate of the microporous separator increase significantly, but the compressive deformation rate also increases (worsens) accordingly; when this proportion increases (>1%), the compressive deformation rate decreases (improves), but the rebound speed and rebound recovery rate decrease.

[0072] As is clear from a comparison between Examples 7 and 8 and Example 2, the change in molecular weight distribution of the raw material also affects the performance of the microporous separator in the thickness direction, but this effect is not large.

[0073] As is clear from the comparison of Examples 10 and 12 with Example 2, and the comparison of Example 11 with Example 5, when the expansion coefficient is 1.2 or greater, the compression and resilience performance in the thickness direction is excellent, and as the expansion coefficient increases, the compression and resilience performance in the thickness direction improves, while when the expansion coefficient is less than 1.2, the compression and resilience performance in the thickness direction decreases.

[0074] As is clear from Examples 2-1 to 2-3 in Table 2, by combining the extrusion method and raw materials, it is possible to adjust and control the proportion of the number of polyolefin chain segments with a weight average molecular weight of 2,000,000 to 5,000,000 in the microporous separator.

[0075] In this specification and the appended claims, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. All technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art, unless otherwise defined. The methods described herein can be carried out in any order that is logically possible, excluding the specific order disclosed.

[0076] The representative examples are intended to be illustrative of the invention and are not intended to, and should not be construed as, limiting the scope of the invention. Indeed, multiple modifications of the invention and many other embodiments in addition to those shown and described herein will be apparent to those skilled in the art, including the examples and the scientific and patent literature cited herein. The examples contain important additional information, examples, and guidance that can be employed by the practice of many embodiments of the invention and equivalents thereof.

Claims

1. A microporous separator for a lithium battery containing a polyolefin resin, wherein the microporous separator has a melt index of 0.04 g / 10 min to 3 g / 10 min, preferably 0.08 g / 10 min to 0.4 g / 10 min, a ratio of the number of polyolefin chain segments having a weight average molecular weight of 2 million to 5 million in the composition of the microporous separator is 5 mol % to 8 mol %, preferably 5 mol % to 7 mol %, and a rebound speed in the thickness direction of the microporous separator is 2×10 －４ μm / s or more, and －４ μm / s or more, 5×10 －４ μm / s or more, 7×10 －４ μm / s or more, 8×10 －４ μm / s or more, and 10×10 －４ μm / s or more is most preferred, The rebound velocity in the thickness direction of the microporous separator is calculated by the following formula: V=(D2-D1) / (T2-T1) In the above formula, V - velocity of repulsion, in μm / s, D2 - the thickness of the microporous separator when the compressive load is reduced to 30 mN, in μm; D1 - the thickness of the microporous separator when compressed to its lowest point, in μm; T2 - the time, in s, required for the compressive load of the microporous separator to decrease to 30 mN; T1 - the time required for the microporous separator to compressively deform to its lowest point, in s; Test conditions: A load of 500 mN was applied to the microporous separator in the thickness direction, and this load was maintained for 300 seconds, and then the load was reduced at a rate of 30 mN / min.

2. 2. The microporous separator for lithium batteries according to claim 1, wherein in the microporous separator, the proportion of the number of polyolefin chain segments in the composition having a weight average molecular weight of more than 5 million and not more than 9 million is 0 mol% to 1 mol%, preferably 0.5 mol% to 1 mol%.

3. The microporous separator is a) When measuring under the test conditions, the compressive deformation rate in the thickness direction of the microporous separator is 2.5% or less, preferably 1.8% or less, more preferably 1.5% or less, and most preferably 1% or less; and b. When measured under the test conditions, the rebound recovery rate of the microporous separator in the thickness direction is 0.5% or more, preferably 1% or more, 1.5% or more, 2% or more, and most preferably 3% or more, and one or a combination of the following conditions is satisfied; where: Compression deformation rate = (original thickness - thickness when compressed to the lowest point) / original thickness * 100% Rebound deformation rate = (original thickness - thickness when pressure value is reduced to 30 mN) / original thickness * 100% 3. The microporous separator for lithium batteries according to claim 1, wherein the rebound recovery rate is equal to |compression deformation rate−rebound deformation rate|.

4. 4. The microporous separator for lithium batteries according to claim 1, wherein the molecular weight distribution of the polyolefin resin is in the range of 3 to 5, and preferably in the range of 3.5 to 4.

5.

5. In the polyolefin resin, the proportion of the number of polyolefin chain segments in the composition having a weight average molecular weight of 2,000,000 to 5,000,000 is 5 mol% to 9 mol%, preferably 5 mol% to 8 mol%, and more preferably 6 mol% to 8 mol%, The microporous separator for lithium batteries according to any one of claims 1 to 4, wherein the polyolefin resin has a compositional number ratio of polyolefin chain segments having a weight-average molecular weight of more than 5 million and not more than 9 million of 0 mol% to 2 mol%, preferably 1 mol% to 1.5 mol%.

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

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

8. 8. The microporous separator for lithium batteries according to claim 1, wherein the microporous separator has an average pore size of 25 nm to 50 nm, preferably 30 nm to 45 nm, and a thickness of 1 μm to 30 μm, preferably 4 μm to 12 μm.

9. 1. A method for preparing a microporous separator for a lithium battery, comprising: Step (a) of melt-kneading a mixture containing a polyolefin resin and a plasticizer to form a melt; Step (b) of extruding the melt obtained in step (a) and hardening it into a slab; (c) stretching the obtained thick plate in the machine direction (MD direction) and the transverse direction (TD direction) perpendicular to the machine direction to obtain an elongated body; (d) removing the plasticizer from the elongated body to obtain a separator precursor; and step (e) heat-setting the separator precursor to obtain the microporous separator for the lithium battery; wherein the polyolefin resin has a melt index of 0.01 g / 10 min to 3 g / 10 min, and preferably 0.05 to 0.3 g / 10 min, and the proportion of polyolefin chain segments having a weight-average molecular weight of 2,000,000 to 5,000,000 in the polyolefin resin is 5 mol % to 9 mol %, and preferably 5 to 8 mol %, and more preferably 6 to 8 mol %.

10. In step (a), the proportion of the number of polyolefin chain segments in the polyolefin resin composition having a weight average molecular weight of more than 5 million and not more than 9 million is 0 mol% to 2 mol%, preferably 1 mol% to 1.5 mol%, Furthermore, in the microporous separator, the proportion of the number of polyolefin chain segments in the composition having a weight average molecular weight of more than 5 million and not more than 9 million is 0 mol% to 1 mol%, preferably 0.5 mol% to 1 mol%, Furthermore, the molecular weight distribution of the polyolefin resin is in the range of 3 to 5, preferably 3.5 to 4.5; Furthermore, in the microporous separator, the proportion of the number of polyolefin chain segments having a weight-average molecular weight of 2,000,000 to 5,000,000 in the composition is 5 mol% to 8 mol%, preferably 5 mol% to 7 mol%, Furthermore, the repulsion speed in the thickness direction of the microporous separator is 2×10 －４ μm / s or more, and －４ μm / s or more, 5×10 －４ μm / s or more, 7×10 －４ μm / s or more, 8×10 －４ μm / s or more, and 10×10 －４ μm / s or more is most preferred, The rebound speed in the thickness direction of the microporous separator is calculated by the following formula: V=(D2-D1) / (T2-T1) In the formula, V - velocity of repulsion, in μm / s, D2 - the thickness of the microporous separator when the compressive load is reduced to 30 mN, in μm; D1 - the thickness of the microporous separator when compressed to its lowest point, in μm; T2—the time, in s, required for the compressive load of the microporous separator to decrease to 30 mN; T1 - the time required for the microporous separator to compressively deform to its lowest point, in seconds; Test conditions: A load of 500 mN was applied to the microporous separator in the thickness direction, and this load was maintained for 300 seconds. The load was then reduced at a rate of 30 mN / min.

10. The method of claim 9, further comprising: 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-kneading, and the parameters of the extruder include the temperature of the extruder and the screw speed of the extruder, wherein the temperature of the extruder is 150°C to 260°C, and the screw speed of the extruder is 60 r / min to 125 r / min.

12. In step (b), the mixture is attached to an extrusion casting roller through a die head and die lip, and formed into a thick plate by cooling and hardening, the opening of the die lip is defined as a, the thickness of the thick plate is defined as h, and the expansion coefficient is defined as A = h / a, and A is controlled to be 1.2 or more; The method according to claim 11, further comprising: setting the roll speed of the casting roller to 3 m / min to 8 m / min; and setting the temperature of the die head to 160°C to 240°C.

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

14. The preparation method according to any one of claims 9 to 13, wherein in step (e), the heat setting process comprises heat treatment in an oven and heat treatment with a roller, preferably the temperature of the heat treatment in the oven is 120°C to 150°C, and the temperature of the heat treatment with a roller is 50°C to 70°C.

15. The 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, polybutylene, polymethylpentene, copolymers thereof, and co-mixtures thereof.

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