High-strength lithium-ion battery separator and method for preparing same

Abstract

The present invention relates to the technical field of lithium ion battery separators, and provides an ultra-thin lithium ion battery separator with high mechanical strength and excellent thermal dimensional stability, and a method for preparing the same.

Description

[Technical Field]

[0001] The present invention relates to the technical field of lithium ion battery separators, and more particularly to a high-strength lithium ion battery separator that is thin, has high mechanical strength, and is excellent in thermal dimensional stability, and to a method for preparing the same. [Background technology]

[0002] In recent years, with the widespread use of new energy vehicles, the demand for battery energy density and safety has been increasing. Separators, a key component of lithium-ion batteries, are required not only to improve battery energy density through thinning but also to have sufficient mechanical strength to prevent the separator from being damaged by external forces during battery assembly and use. Furthermore, lithium-ion batteries are subject to some degree of thermal influence or heat generation during the manufacturing and charging / discharging processes. If the separator has relatively poor thermal dimensional stability, it will deform under heat, leading to short circuits due to contact between the positive and negative electrode materials. Therefore, the thermal dimensional stability of the separator is an important safety indicator. However, generally, the thinner the separator, the more rapidly its mechanical strength will deteriorate. Meanwhile, separators with relatively high mechanical strength have relatively poor thermal dimensional stability.

[0003] Currently, conventional techniques have not been able to provide a separator that combines high mechanical strength with excellent thermal dimensional stability, particularly a separator for a lithium ion battery that is relatively thin, has high mechanical strength, and has excellent thermal dimensional stability even in a high-temperature environment. Summary of the Invention [Problem to be solved by the invention]

[0004] The present invention solves the technical problem of lithium ion battery separators in the prior art being unable to combine mechanical strength and thermal dimensional stability, and provides a lithium ion battery separator with high mechanical strength and excellent thermal dimensional stability, as well as a method for preparing the same, and in particular, provides a thin battery separator with excellent mechanical strength and thermal stability. [Means for solving the problem]

[0005] Generally, the higher the mechanical strength of a polymer product, the poorer its processability and thermal dimensional stability become, and conversely, the better its processability and thermal dimensional stability become, the poorer its mechanical strength becomes. In order to achieve both mechanical strength and thermal dimensional stability of a polymer separator, the present invention designs the separator to include a core layer with a relatively high molecular weight and a surface layer with a relatively low molecular weight, where the core layer has a high molecular weight and high shrinkage but high strength, and the surface layer has a low molecular weight and low shrinkage but low strength, thereby achieving a separator with a good balance of low shrinkage and high strength.

[0006] The present invention further found that, because the polymer melt index is related to the molecular weight of the polymer to some extent, by designing a separator to include a core layer with a relatively low melt index and surface layers with a relatively high melt index, it is possible to achieve a core layer with high shrinkage but high strength and surface layers with low shrinkage but low strength, thereby obtaining a separator with a good balance of low shrinkage and high strength.

[0007] Based on research into separator composition, the present invention further developed a separator preparation method that ensures high mechanical strength of the separator without the need for a relatively large stretching ratio, thereby contributing to better control of thermal dimensional stability and stretching uniformity. The method mainly includes the following steps: melting a surface layer material containing at least a polyolefin and a pore-forming agent to form a surface layer, melting a core layer material containing at least a polyolefin and a pore-forming agent to form a core layer, and covering the top and bottom surfaces of the core layer with the surface layers to form a laminate; biaxially stretching the laminate to obtain a stretched laminate; removing the pore-forming agent from the stretched laminate to obtain a separator precursor; and heat-setting the separator precursor to obtain the separator.

[0008] The separator according to the present invention has significantly improved mechanical strength, particularly stretching strength, and excellent thermal dimensional stability in a relatively high temperature environment compared to separators according to the prior art. Furthermore, the separator according to the present invention has excellent gas permeability and puncture strength, and in particular, even when it is relatively thin, it has high mechanical strength and excellent thermal dimensional stability.

[0009] In a specific embodiment, the present invention relates to a separator for a lithium ion battery, comprising a porous core layer and porous surface layers covering two upper and lower surfaces of the core layer, wherein the surface layers and the core layer are: (1) The weight-average molecular weight of the polymer of the core layer is 1,100,000 to 1,700,000, and is, for example, a value in a range consisting of any one or any two values of 1,100,000, 1,150,000, 1,200,000, 1,300,000, 1,400,000, 1,450,000, 1,550,000, 1,600,000, and 1,700,000, preferably 1,200,000 to 1,700,000, more preferably 1,300,000 to 1,600,000, and even more preferably 1,400,000 to 1,550,000. the weight-average molecular weight of the polymer in the surface layer is 1.01 to 2.20 times the weight-average molecular weight of the polymer in the surface layer, and is, for example, a value in a range consisting of any one or any two of 1.01, 1.05, 1.09, 1.15, 1.30, 1.45, 1.60, 1.65, 1.75, 1.80, 2.10, and 2.20 times, preferably 1.05 to 1.80 times, and more preferably 1.10 to 1.30 times; (2) The melt index of the polymer of the surface layer is 1.01 to 2.00 times, preferably 1.05 to 1.70 times, and more preferably 1.10 to 1.50 times, the melt index of the polymer of the core layer, and the melt index is measured under the conditions of 190°C and 2.16 kg / 10 min. At least one of the conditions (1) to (2) is satisfied.

[0010] In an embodiment that satisfies condition (1), both the core layer and the surface layer are porous membrane layers, and a specific core layer polymer is used. By keeping the ratio of the weight-average molecular weight of the surface layer polymer to the core layer polymer within a specific range, the separator has excellent gas permeability and can ensure a certain degree of mechanical strength.

[0011] In an embodiment that satisfies condition (2), both the core layer and the surface layer are porous membrane layers, and the melt index of the polymer in the surface layer and the melt index of the polymer in the core layer are in a specific proportional relationship. This allows the separator to have excellent gas permeability, ensure mechanical strength, and provide relatively good product processability.

[0012] In a specific embodiment, the present invention relates to a separator for a lithium ion battery, wherein a polymer of a porous membrane layer A occupying 2.5 to 10%, preferably 4.5 to 9%, more preferably 6 to 7.5% of the membrane thickness from each of the two sides of the separator toward the center of the separator has a weight average molecular weight a or a melt index a', and a polymer of a porous membrane layer B in the remaining central portion has a weight average molecular weight b or a melt index b'; (1) The weight-average molecular weight b is 1,100,000 to 1,700,000, and is, for example, a value in a range consisting of any one or any two of 1,100,000, 1,150,000, 1,200,000, 1,300,000, 1,400,000, 1,450,000, 1,550,000, 1,600,000, and 1,700,000, preferably 1,200,000 to 1,700,000, more preferably 1,300,000 to 1,600,000, and even more preferably 1,400,000 to 1,550,000; and the weight-average molecular weight b is 1.01 to 2.20 times the weight-average molecular weight a, preferably 1.05 to 1.80 times, and even more preferably 1.10 to 1.30 times. (2) The melt index a' is 1.01 to 2.00 times the melt index b', for example, a value in a range consisting of any one or any two values of 1.01, 1.05, 1.09, 1.15, 1.30, 1.45, 1.60, 1.65, 1.75, 1.80, 2.10, and 2.20 times, preferably 1.05 to 1.70 times, more preferably 1.10 to 1.50 times, and the measurement conditions for the melt index are 190°C and 2.16 kg / 10 min; At least one of the conditions (1) to (2) is satisfied.

[0013] In an embodiment that satisfies condition (1), both membrane layer A and membrane layer B are porous membrane layers, and a specific polymer is used for porous membrane layer B. By keeping the ratio of the weight-average molecular weights of the polymer for porous membrane layer A and the polymer for porous membrane layer B within a specific range, the separator has excellent gas permeability and can ensure a certain degree of mechanical strength.

[0014] In an embodiment that satisfies condition (2), both membrane layer A and membrane layer B are porous membrane layers, and the melt indexes of the polymers of porous membrane layer A and B are set in a specific proportional relationship, so that the separator has excellent gas permeability, ensures mechanical strength, and provides relatively good product processability.

[0015] In a preferred embodiment, the polymer of the core layer or the polymer of the porous membrane layer B is a polyolefin, preferably a single type of polyethylene, or a mixture of two or more types of polyethylene, or a mixture of one or more types of polyethylene and polypropylene; the polymer of the surface layer or the polymer of the porous membrane layer A is a polyolefin, preferably polyethylene, which is a single type of polyethylene or a mixture of two or more types of polyethylene; and the weight-average molecular weight of the polymer of the surface layer or the polymer of the porous membrane layer A is 600,000 to 1,500,000, preferably 700,000 to 1,400,000, more preferably 900,000 to 1,400,000, and even more preferably 1,100,000 to 1,350,000.

[0016] In a preferred embodiment, the surface layer or the porous membrane layer A comprises a first polyethylene and a second polyethylene, wherein the weight-average molecular weight of the first polyethylene is 300,000 to 600,000, and preferably 400,000 to 550,000, the weight-average molecular weight of the second polyethylene is 1,300,000 to 3,000,000, and preferably 1,300,000 to 1,800,000, and the weight ratio of the first polyethylene to the second polyethylene is 0 to 80:20 to 100, and preferably 1 to 50:50 to 99, more preferably 0 to 40:60 to 100, and most preferably 15 to 35:65 to 85.

[0017] The core layer or the porous membrane layer B comprises a third polyethylene and a fourth polyethylene, the weight average molecular weight of the third polyethylene is 300,000 to 600,000, and preferably 400,000 to 550,000, the weight average molecular weight of the fourth polyethylene is 1,300,000 to 3,000,000, and preferably 1,500,000 to 2,000,000, and the weight ratio of the third polyethylene to the fourth polyethylene is 0 to 30:70 to 100, and preferably 1 to 10:90 to 99, and more preferably 5 to 20:80 to 95.

[0018] Optionally, the core layer or the porous membrane layer B further comprises a fifth polypropylene, and the weight average molecular weight of the fifth polypropylene is 300,000 to 600,000, preferably 400,000 to 550,000, and the fifth polypropylene accounts for 0 to 20% by weight of the core layer or the porous membrane layer B, preferably 0 to 13% of the core layer or the porous membrane layer B.

[0019] In a preferred embodiment, the melt index of the surface layer polymer or the porous membrane layer A polymer is 0.35 to 0.7 g / 10 min, preferably 0.4 to 0.65 g / 10 min, and more preferably 0.42 to 0.55 g / 10 min, and the melt index of the core layer polymer or the porous membrane layer B polymer is 0.3 to 0.6 g / 10 min, preferably 0.3 to 0.5 g / 10 min, and more preferably 0.32 to 0.45 g / 10 min, and the melt index is measured under conditions of 190°C and 2.16 kg / 10 min.

[0020] In a preferred embodiment, the average pore size of the core layer or porous membrane layer B is 30 to 50 nm, for example, any one or any two of 30, 32, 35, 37, 40, 45, and 50 nm, and preferably 37 to 45 nm. The average pore size of the surface layer or porous membrane layer A is 35 to 60 nm, for example, any one or any two of 35, 40, 45, 50, 55, and 60 nm, and preferably 40 to 50 nm. Keeping the average pore sizes of the core layer, surface layer, or porous membrane layers A and B within a specific range contributes to appropriate control of air permeability, thereby improving the ion permeability of the separator.

[0021] In a preferred embodiment, the thickness of the separator is 3 to 15 μm, preferably 10.5 to 11.5 μm, which ensures air permeability and also makes it possible to achieve the effect of making the separator thinner.

[0022] In a preferred embodiment, the longitudinal heat shrinkage of the separator when heated at 120°C for 1 hour is less than 10%, preferably less than 6.5%, more preferably less than 6.0%, and most preferably less than 5.6%, and the longitudinal heat shrinkage of the separator when heated at 130°C for 30 minutes is less than 17%, preferably less than 15%, preferably less than 14%, preferably less than 13%, more preferably less than 11.5%, and most preferably less than 10%. The heat shrinkage rate of the separator in the transverse direction when heated at 20°C for 1 hour is less than 10%, preferably less than 7%, more preferably less than 5.0%, and most preferably less than 4.0%. The heat shrinkage rate of the separator in the transverse direction when heated at 130°C for 30 minutes is less than 15%, more preferably less than 12.5%, more preferably less than 8.5%, and most preferably less than 6.5%. Preferably, the separator has a stretch strength in the machine direction (longitudinal direction) of 2980 kgf / cm. 2or more, for example, 2980, 3000, 3200, 3300, 3800, 4000, 4500, 5000, 6000, 6500, 7000 kgf / cm 2 or a range consisting of any two of these values, and preferably 3000 kgf / cm 2 More preferably, it is 3300 to 6000 kgf / cm 2 The separator has a transverse (width) stretching strength of 2980 kgf / cm 2 or more, for example, 2980, 3000, 3200, 3300, 3800, 4000, 4500, 5000, 6000, 6500, 7000 kgf / cm 2 or a range consisting of any two of these values, and preferably 3000 kgf / cm 2 More preferably, it is 3400 to 6000 kgf / cm 2 is.

[0023] In a preferred embodiment, the separator has an air permeability of 50 to 250 sec / 100 ml, and preferably 90 to 140 sec / 100 ml, a porosity of 35 to 60%, and preferably 40 to 48%, and a puncture strength of 200 gf or more, and is, for example, any one or any two of 200 gf, 400 gf, 500 gf, 550 gf, 600 gf, 650 gf, 700 gf, and 800 gf. The total thickness of the two surface layers or the total thickness of the two porous membrane layers A is 5 to 20% of the thickness of the separator, preferably 9 to 18%, and more preferably 12 to 15%, and the weight average molecular weight of the separator is 1,100,000 to 1,600,000, preferably 1,200,000 to 1,500,000, and more preferably 1,300,000 to 1,450,000.

[0024] The higher the thickness ratio of the surface layer or porous membrane layer A, the better the heat shrinkage performance of the prepared separator, but the lower the overall mechanical strength. In the present application, by keeping the thickness ratio of the surface layer or porous membrane layer A within a specific range, it is possible to achieve high strength of the separator and contribute to obtaining appropriate heat shrinkage performance.

[0025] In one specific embodiment, the present invention relates to a method for preparing a separator for a lithium ion battery, the method comprising: a surface layer material and a core layer material are prepared, the surface layer material comprising a first polymer and a pore-forming agent, the first polymer comprising a first polyethylene and a second polyethylene, the weight average molecular weight of the first polyethylene being 300,000 to 600,000, preferably 400,000 to 550,000, the weight average molecular weight of the second polyethylene being 1,300,000 to 3,000,000, preferably 1,300,000 to 1,800,000, and the weight ratio of the first polyethylene to the second polyethylene being 0 to 80:20 to 100, preferably 1 to 50:50 to 99, more preferably 0 to 40:60 to 100, and most preferably 15 to 35:65 to 85; the core layer material comprises a second polymer and a pore-forming agent, the second polymer comprises a third polyethylene and a fourth polyethylene, the weight average molecular weight of the third polyethylene is 300,000 to 600,000, preferably 400,000 to 550,000, the weight average molecular weight of the fourth polyethylene is 1,300,000 to 3,000,000, preferably 1,500,000 to 2,000,000, and the weight ratio of the third polyethylene to the fourth polyethylene is 0 to 30:70 to 100, preferably 1 to 10:90 to 99, more preferably 5 to 20:80 to 95; Step (1) and Step (2) of melting the surface layer material to form a surface layer, melting the core layer material to form a core layer, and covering the upper and lower surfaces of the core layer with the surface layers to form a laminate; Step (3) of biaxially stretching the laminate to obtain a stretched laminate; Step (4) of removing the pore-forming agent from the stretched laminate to obtain a separator precursor; and (5) heat-setting the separator precursor to obtain the lithium-ion battery separator.

[0026] Optionally, the second polymer comprises a fifth polypropylene, and the weight average molecular weight of the fifth polypropylene is 300,000 to 600,000, preferably 400,000 to 550,000, and the fifth polypropylene accounts for 0 to 20 wt % of the second polymer, preferably 0 to 13 wt % of the second polymer.

[0027] In a preferred embodiment, the weight of the surface layer material accounts for 5 to 20% of the total weight of the surface layer material and the core layer material, preferably 9 to 18%, and more preferably 12 to 15%, and the weight ratio of the first polymer to the pore-forming agent in the surface layer material is 15 to 30:70 to 85, preferably 16 to 23:77 to 84, and / or the weight ratio of the second polymer to the pore-forming agent in the core layer material is 15 to 30:70 to 85, preferably 18 to 25:75 to 82.

[0028] In a preferred embodiment, before the laminate is biaxially stretched, the laminate is subjected to double-sided cooling in a cooling bath, preferably a water bath, and the temperature of the cooling bath is 5 to 40° C., preferably 10 to 25° C., and more preferably 10 to 20° C. Double-sided cooling in a cooling bath can increase the cooling rate of the laminate, make the cooling temperatures of the two opposing surfaces more uniform, achieve more complete phase separation, and contribute to better control of the consistency of separator performance.

[0029] In a preferred embodiment, the biaxial stretching includes longitudinal stretching and transverse stretching.

[0030] Longitudinal stretching: The laminate is longitudinally stretched at a stretching temperature of 80 to 120° C., preferably 90 to 115° C., and at a stretching ratio of 5.0 to 12.0 times, preferably 6.5 to 8.5 times.

[0031] Transverse stretching: The longitudinally stretched laminate is then transversely stretched at a stretching temperature of 100 to 140°C, preferably 105 to 125°C, and at a stretching ratio of 7.0 to 15.0 times, preferably 9.0 to 11.5 times.

[0032] In a preferred embodiment, the pore-forming agent is a low-molecular-weight solvent capable of dissolving polyolefin, and is preferably liquid paraffin having a kinematic viscosity at 40°C of 35 to 120 cps.

[0033] In a preferred embodiment, the melt molding is carried out using multiple (e.g., three), preferably two, extruders, with the core layer material being fed into one extruder (referred to as the first extruder) and the surface layer material being fed into the other extruder (referred to as the second extruder), and the laminate is then formed using a co-extrusion die.

[0034] In a preferred embodiment, the parameters of the second extruder are an extrusion temperature of 150 to 250°C and a screw rotation speed of 40 to 90 r / min, the parameters of the first extruder are an extrusion temperature of 150 to 250°C and a screw rotation speed of 60 to 100 r / min, and the parameters of the co-extrusion die are a temperature of 150 to 250°C.

[0035] In a preferred embodiment, the pore-forming agent is removed from the stretched laminate using an extractant, and the extractant is an alkane extractant, preferably dichloromethane, and the amount of the extractant circulated is preferably 1 to 5 m 3 After extraction, the stretched laminate is dried using one or more heating methods selected from the group consisting of a heating roll, a heating plate, and hot air, and the drying temperature is preferably 20 to 150°C.

[0036] In a preferred embodiment, the heat setting parameters include a relaxation rate of 5 to 30% and a heat setting temperature of 120 to 145°C. [Brief explanation of the drawings]

[0037] [Figure 1] 1 is a schematic diagram of a high-strength lithium-ion battery separator according to the present invention. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0038] The present invention will now be further described with reference to the drawings.

[0039] Before further describing the present invention, the following section will explain some terms used in the specification, examples and claims. The definitions set forth herein can be understood by those skilled in the art with reference to other parts of the specification. Unless otherwise specified, all technical and scientific terms used herein have the meanings commonly understood by those skilled in the art.

[0040] The terms "one" and "another" as used herein are for descriptive purposes only and do not express or imply any relative importance or number of technical features.

[0041] As used herein, when a value is expressed by the term "about," it also includes variations of ±10%, ±5%, ±1%, or ±0.1% of the particular value.

[0042] As used herein, the term "approximately the same," when describing the relationship between two values, means that the difference between the two values is less than 10%, 5%, or 1% of the average of the two values.

[0043] As used herein, the term "polyolefin" may be a polyolefin monomer (ie, a single type of polyolefin), a polyolefin copolymer, or a polyolefin blend.

[0044] As used herein, the term "and / or" refers to a relationship between related objects and represents a three-way relationship, such as A and / or B, where only A is present, both A and B are present, or only B is present.

[0045] As shown in FIG. 1, the separator according to the present invention includes a porous core layer 2 and porous surface layers covering the upper and lower surfaces of the core layer (i.e., upper surface layer 1 and lower surface layer 3 shown in FIG. 1). To achieve a separator with a good balance between low shrinkage and high strength, the melt index or weight-average molecular weight of the surface layer polymer and the core layer polymer are proportional to each other. The core layer and surface layer each have a porous structure, ensuring the passage of ions in the electrolyte. Alternatively, the melt index of the surface layer polymer is 1.01 to 2.00 times, preferably 1.05 to 1.70 times, and more preferably 1.10 to 1.50 times, the melt index of the core layer polymer. Alternatively, the weight-average molecular weight of the core layer polymer is 1.1 million to 1.7 million, preferably 1.2 million to 1.7 million, more preferably 1.3 million to 1.6 million, and even more preferably 1.4 million to 1.55 million. The weight average molecular weight of the polymer in the core layer is 1.01 to 2.20 times, preferably 1.05 to 1.80 times, and more preferably 1.10 to 1.30 times, the weight average molecular weight of the polymer in the surface layer.

[0046] The polymers of the surface layer and the core layer in the present application may be the same or different. Different polymers refer to polymers with different chemical properties (e.g., a PE-PP copolymer and a PE-PE copolymer are polymers with different chemical properties) and / or polymers with the same chemical properties but different properties (e.g., two types of PE with different properties (e.g., density, molecular weight, molecular weight distribution, rheology, additives (components and / or percentages))). The polymers may be the same or completely identical. The polymers of the surface layer and the core layer may each be independently selected polyolefins. The polyolefins may include ethylene homopolymers, propylene homopolymers, ethylene-propylene copolymers, or mixtures of the above polymers, such as a single type of polyethylene, a mixture of two or more types of polyethylene, or a mixture of one or more types of polyethylene and polypropylene. The polyolefins may be obtained by a single-stage or multi-stage polymerization process. Furthermore, to ensure processability, the melt index of the surface layer polymer used in the separator according to the present invention is 0.35 to 0.7 g / 10 min, preferably 0.4 to 0.65 g / 10 min, and more preferably 0.42 to 0.55 g / 10 min. The melt index of the core layer polymer is 0.3 to 0.6 g / 10 min, preferably 0.3 to 0.5 g / 10 min, and more preferably 0.32 to 0.45 g / 10 min. The melt index is measured at 190°C and 2.16 kg / 10 min.

[0047] In the present invention, the pore size of the separator is 30 to 60 nm, the average pore size of the core layer is 30 to 50 nm, preferably 37 to 45 nm, and the average pore size of the surface layer is 35 to 60 nm, preferably 40 to 50 nm, thereby ensuring good gas permeability of the separator.

[0048] The thickness of the separator according to the present invention is 3 μm or more, preferably 5 μm or more. In some possible embodiments, the thickness of the separator is 3 to 15 μm, specifically, 3 μm, 4.5 μm, 5 μm, 6 μm, 7.5 μm, 10 μm, 14.5 μm, 15 μm, etc., and may be any one or any two of these values, and is preferably 10.5 to 11.5 μm. The total thickness of the two surface layers is 5 to 20% of the thickness of the separator, preferably 9 to 18%, and more preferably 12 to 15%. The thicknesses of the two surface layers may be approximately the same or different, and preferably are approximately the same.

[0049] Regarding the preferred method for producing the separator according to the present invention, there are no limitations on the selection of polymer, type of pore-forming agent, extrusion method, stretching method, extraction method, heat setting method, etc., as long as the obtained separator satisfies the characteristics of the present invention.

[0050] Specifically, the separator according to the present invention is preferably obtained by the following preparation method.

[0051] 1. Raw material preparation A surface layer material and a core layer material are separately prepared. The surface layer material includes a first polymer and a pore-forming agent, the first polymer being a polyolefin monomer or a polyolefin mixture. Preferably, the first polymer includes a first polyethylene having a weight-average molecular weight of 300,000 to 600,000 and a second polyethylene having a weight-average molecular weight of 1,300,000 to 3,000,000. For example, the weight-average molecular weight of the first polyethylene is any value among 300,000, 350,000, 400,000, 450,000, 500,000, 550,000, and 600,000, or a range consisting of any two of these values, preferably 400,000 to 550,000. For example, the weight-average molecular weight of the second polyethylene is any value among 1,300,000, 1,350,000, 1,400,000, 1,450,000, 1,500,000, 1,550,000, 1,600,000, 1,650,000, 1,700,000, and 1,800,000, or a range consisting of any two of these values, preferably 1,300,000 to 1,800,000. The weight ratio of the first polyethylene to the second polyethylene is 0-80:20-100, preferably 1-50:50-99, more preferably 0-40:60-100, and most preferably 15-35:65-85.

[0052] The core layer material includes a second polymer and a pore-forming agent. The second polymer is a polyolefin monomer or a polyolefin mixture. Preferably, the second polymer includes a third polyethylene and a fourth polyethylene. The third polyethylene has a weight-average molecular weight of 300,000 to 600,000, preferably 400,000 to 550,000, and the fourth polyethylene has a weight-average molecular weight of 1,300,000 to 3,000,000, preferably 1,500,000 to 2,000,000. The weight ratio of the third polyethylene to the fourth polyethylene is 0-30:70-100, preferably 1-10:90-99, more preferably 5-20:80-95. Optionally, the second polymer includes a fifth polypropylene. The fifth polypropylene has a weight-average molecular weight of 300,000 to 600,000, preferably 400,000 to 550,000, and the fifth polypropylene accounts for 0-20 wt % of the second polymer, preferably 0-13% of the second polymer.

[0053] The first polyethylene and the third polyethylene may be the same or different polyethylenes, and the second polyethylene and the fourth polyethylene may be the same or different polyethylenes.

[0054] Any pore-forming agent commonly used in the art can be used, for example, a low molecular weight solvent capable of dissolving polyolefin, preferably liquid paraffin having a kinematic viscosity at 40°C of 35 to 120 cps.

[0055] 2. Melt extrusion Melt extrusion is performed using multiple extruders, preferably two (e.g., twin-screw extruders), with the core layer material fed into one extruder (referred to as the first extruder) and the surface layer material fed into the other extruder (referred to as the second extruder). The resulting mixture is sheared to form a uniform polymer melt, which is then extruded through a three-layer die to form a laminate. Preferably, nitrogen gas or other inert gas is filled into the cylinders of the second and first extruders to prevent oxidative crosslinking reactions.

[0056] Preferably, the parameters of the second extruder are an extrusion temperature of 150 to 250°C and a screw rotation speed of 40 to 90 r / min. The parameters of the first extruder are an extrusion temperature of 150 to 250°C and a screw rotation speed of 60 to 100 r / min. The parameters of the co-extrusion die are set at a temperature of 150 to 250°C.

[0057] In other possible embodiments, the melt-forming step does not employ a co-extrusion die, but rather employs, for example, a layer-stacking method, in which sheet materials extruded from different dies are each subjected to sheet forming by cooling, stretching, extraction drying, and heat setting, and then laminated, or sheet materials extruded from different dies are laminated, and then subsequent processes such as stretching, extraction, and heat setting are performed. Specific process parameters can be found in the process conditions for different processes in this application.

[0058] In a possible embodiment, the weight of the surface layer material accounts for 5 to 20% of the total weight of the surface layer material and the core layer material, preferably 9 to 18%, and more preferably 12 to 15%. In the surface layer material, the weight ratio of the first polymer to the pore-forming agent is 15 to 30:70 to 85, and preferably 16 to 23:77 to 84, and / or in the core layer material, the weight ratio of the second polymer to the pore-forming agent is 15 to 30:70 to 85, and preferably 18 to 25:75 to 82.

[0059] 3. Sheet formation by cooling The molten material extruded from the die is attached to a cooling roll to form a sheet material, and a water bath device is used to increase the cooling rate of the sheet material, with the cooling roll temperature being 10 to 40°C and the water bath temperature being 10 to 40°C, preferably 10 to 25°C, and more preferably 10 to 20°C.

[0060] The sheet forming process utilizes water bath cooling. Compared to conventional cooling using a single cooling roll, water bath cooling provides better consistency of both sides of the product due to double-sided cooling. Compared to double-sided cooling using two cooling rolls, water bath cooling provides a longer heat exchange time with the membrane, resulting in more thorough cooling. Furthermore, by optimizing the composition and using a three-layer structure, relatively high stretching strength can be achieved even under relatively low stretching ratios, preventing scratches on the separator surface due to slippage during stretching and uneven stretching due to high stretching ratios. Currently, patents related to high strength (e.g., CN112592500A) require a longitudinal stretching ratio of approximately 15-20 times and a transverse stretching ratio of approximately 11-20 times. In contrast, in the present invention, the longitudinal stretching ratio is 5.0 to 12.0 times (preferably 6.5 to 8.5 times), the transverse stretching ratio is 7.0 to 15.0 times (preferably 9.0 to 11.5 times), the separator stretching strength is generally 3000 kgf / cm2 or more (those related to the prior art are much smaller than this value), and the heat shrinkage at 120°C is 6% or less (in the case of the prior art, it is generally 10% or more).

[0061] 4. Film formation by stretching Longitudinal stretching: The sheet material obtained by sheet molding is stretched along the machine direction (longitudinal direction). The stretching temperature is 80 to 120°C, and the stretching ratio is 5.0 to 12.0 times, and can be any value of 5 times, 5.5 times, 6 times, 6.5 times, 7 times, 7.5 times, 8.5 times, 10 times, 11 times, and 12 times, or a range consisting of any two of these values, and is, for example, preferably 6.5 to 8.5 times.

[0062] Transverse stretching: The longitudinally stretched thick sheet is sent to a transverse stretching machine to perform transverse stretching. The stretching temperature is 100°C to 140°C, and the stretching ratio is 7.0 to 15.0 times, and can be any value among 7 times, 7.5 times, 8.5 times, 9 times, 10 times, 10.5 times, 11.5 times, 12 times, 13 times, and 15 times, or a range consisting of any two of these values, and is, for example, preferably 9.0 to 11.5 times.

[0063] 5. Extraction and drying The stretched separator is immersed in a tank containing an extractant, and the pore-forming agent is extracted by utilizing the principle that similar substances dissolve well in each other. The extractant is an alkane extractant, preferably dichloromethane. Preferably, the extraction agent is circulated at a rate of 1 to 5 m 3 / hour, and after extraction, the stretched laminate is dried using one or more heating methods selected from the group consisting of a heating roll, a heating plate, and hot air, preferably at a drying temperature of 20 to 150°C, and the extractant remaining on the surface of the separator can be volatilized, recovered, and recycled.

[0064] 6. Heat fixation The thermal setting parameters are a relaxation rate of 5 to 30% and a setting temperature of 120 to 145° C. If the setting temperature is the same, the greater the relaxation rate, the smaller the thermal shrinkage of the separator.

[0065] The present invention will now be further described with reference to the following examples, which are provided for illustrative purposes only and are not intended to limit the scope of the present invention.

[0066] Example Example 1 (a) A first polyethylene powder material (weight average molecular weight: 500,000) and a second polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 20:80 to obtain a polyethylene mixture, and then the polyethylene mixture and liquid paraffin were placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface layer polymer melt.

[0067] (b) A third polyethylene powder material (weight average molecular weight: 500,000) and a fourth polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 5:95 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0068] (c) The polymer melts in the second extruder and the first extruder are simultaneously injected into a three-layer coextrusion die at 200°C in a weight ratio of 13.9:86.1, and the surface layer polymer melt of the second extruder is distributed on both surfaces of the core layer polymer melt of the first extruder.

[0069] (d) The melt (polymer melt) extruded from the die was cooled using a cooling roll and a water bath to obtain a laminate with a three-layer structure of ABA (i.e., the structure shown in Figure 1), and the temperatures of both the cooling roll and the water bath were 25°C.

[0070] (e) The laminate obtained in step (d) was stretched longitudinally at a temperature of 103°C and a stretching ratio of 7.2, and then stretched transversely at a temperature of 117°C and a stretching ratio of 10.5.

[0071] (f) The biaxially stretched film was immersed in an extraction tank filled with dichloromethane liquid, and the liquid paraffin in the film was extracted, followed by drying at 25 to 40°C.

[0072] (g) The film-like material obtained in step (f) was stretched again at a temperature of 130°C and a stretching ratio of 1.4, and then heat-set at a temperature of 134°C and a relaxation rate of 18% to obtain a lithium-ion battery separator.

[0073] Example 2 (a) A first polyethylene powder material (weight average molecular weight: 500,000) and a second polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 20:80 to obtain a polyethylene mixture, and then the polyethylene mixture and liquid paraffin were placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface layer polymer melt.

[0074] (b) A third polyethylene powder material (weight average molecular weight: 500,000) and a fourth polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 5:95 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0075] (c) The polymer melts in the second extruder and the first extruder are simultaneously injected into a three-layer coextrusion die at 200°C in a weight ratio of 17.4:82.6, and the surface layer polymer melt of the second extruder is distributed on both surfaces of the core layer polymer melt of the first extruder.

[0076] (d) A lithium ion battery separator was then produced according to steps (d) to (g) of Example 1.

[0077] Example 3 (a) A first polyethylene powder material (weight average molecular weight: 500,000) and a second polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 20:80 to obtain a polyethylene mixture, and then the polyethylene mixture and liquid paraffin were placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface layer polymer melt.

[0078] (b) A third polyethylene powder material (weight average molecular weight: 500,000) and a fourth polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 5:95 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0079] (c) The polymer melts in the second extruder and the first extruder are simultaneously injected into a three-layer coextrusion die at 200°C in a weight ratio of 10.4:89.6, and the surface layer polymer melt of the second extruder is distributed on both surfaces of the core layer polymer melt of the first extruder.

[0080] (d) A lithium ion battery separator was then produced according to steps (d) to (g) of Example 1.

[0081] Example 4 (a) A first polyethylene powder material (weight average molecular weight: 500,000) and a second polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 80:20 to obtain a polyethylene mixture, and then the polyethylene mixture and liquid paraffin were placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface layer polymer melt.

[0082] (b) A third polyethylene powder material (weight average molecular weight: 500,000) and a fourth polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 5:95 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0083] (c) A lithium ion battery separator was then produced according to steps (c) to (g) of Example 1.

[0084] Example 5 (a) A first polyethylene powder material (weight average molecular weight: 500,000) and a second polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 40:60 to obtain a polyethylene mixture, and then the polyethylene mixture and liquid paraffin were placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface layer polymer melt.

[0085] (b) A third polyethylene powder material (weight average molecular weight: 500,000) and a fourth polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 30:70 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0086] (c) A lithium ion battery separator was then produced according to steps (c) to (g) of Example 1.

[0087] Example 6 (a) A first polyethylene powder material (weight average molecular weight: 500,000) and a second polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 20:80 to obtain a polyethylene mixture, and then the polyethylene mixture and liquid paraffin were placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface layer polymer melt.

[0088] (b) The fourth polyethylene powder material (weight average molecular weight: 1.5 million) and liquid paraffin were placed in the first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0089] (c) A lithium ion battery separator was then produced according to steps (c) to (g) of Example 1.

[0090] Example 7 (a) A first polyethylene powder material (weight average molecular weight: 500,000) and a second polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 20:80 to obtain a polyethylene mixture, and then the polyethylene mixture and liquid paraffin were placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface layer polymer melt.

[0091] (b) A third polyethylene powder material (weight average molecular weight: 500,000) and a fourth polyethylene powder material (weight average molecular weight: 2,000,000) were weighed and mixed in a weight ratio of 20:80 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0092] (c) A lithium ion battery separator was then produced according to steps (c) to (g) of Example 1.

[0093] Example 8 (a) A first polyethylene powder material (weight average molecular weight: 500,000) and a second polyethylene powder material (weight average molecular weight: 2,000,000) were weighed and mixed in a weight ratio of 30:70 to obtain a polyethylene mixture, and then the polyethylene mixture and liquid paraffin were placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface layer polymer melt.

[0094] (b) A third polyethylene powder material (weight average molecular weight: 500,000) and a fourth polyethylene powder material (weight average molecular weight: 2,000,000) were weighed and mixed in a weight ratio of 20:80 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0095] (c) A lithium ion battery separator was then produced according to steps (c) to (g) of Example 1.

[0096] Example 9 (a) A first polyethylene powder material (weight average molecular weight: 500,000) and a second polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 20:80 to obtain a polyethylene mixture, and then the polyethylene mixture and liquid paraffin were placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface layer polymer melt.

[0097] (b) A third polyethylene powder material (weight average molecular weight: 500,000), a fifth polypropylene powder material (weight average molecular weight: 500,000), and a fourth polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 5:13:82 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0098] (c) A lithium ion battery separator was then produced according to steps (c) to (g) of Example 1.

[0099] Example 10 (a) A first polyethylene powder material (weight average molecular weight: 500,000) and a second polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 20:80 to obtain a polyethylene mixture, and then the polyethylene mixture and liquid paraffin were placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface layer polymer melt.

[0100] (b) A third polyethylene powder material (weight average molecular weight: 500,000) and a fourth polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 5:95 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0101] (c) The polymer melts in the second extruder and the first extruder are simultaneously injected into a three-layer coextrusion die at 200°C in a weight ratio of 86.1:13.9, and the surface layer polymer melt of the second extruder is distributed on both surfaces of the core layer polymer melt of the first extruder.

[0102] (d) A lithium ion battery separator was then produced according to steps (d) to (g) of Example 1.

[0103] Example 11 (a) A first polyethylene powder material (weight average molecular weight: 500,000) and a second polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 20:80 to obtain a polyethylene mixture, and then the polyethylene mixture and liquid paraffin were placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface layer polymer melt.

[0104] (b) A third polyethylene powder material (weight average molecular weight: 500,000) and a fourth polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 18:82 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0105] (c) A lithium ion battery separator was then produced according to steps (c) to (g) of Example 1.

[0106] Example 12 (a) A first polyethylene powder material (weight average molecular weight: 500,000) and a second polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 20:80 to obtain a polyethylene mixture, and then the polyethylene mixture and liquid paraffin were placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface layer polymer melt.

[0107] (b) A third polyethylene powder material (weight average molecular weight: 500,000) and a fourth polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 5:95 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0108] (c) The polymer melts in the second extruder and the first extruder are simultaneously injected into a three-layer coextrusion die at 200°C in a weight ratio of 78.3:21.7, and the surface layer polymer melt of the second extruder is distributed on both surfaces of the core layer polymer melt of the first extruder.

[0109] (d) A lithium ion battery separator was then produced according to steps (d) to (g) of Example 1.

[0110] Comparative Example 1 (a) A first polyethylene powder material (weight average molecular weight: 500,000) and a second polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 70:30 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface layer polymer melt.

[0111] (b) A third polyethylene powder material (weight average molecular weight: 500,000) and a fourth polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 50:50 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0112] (c) A lithium ion battery separator was then produced according to steps (c) to (g) of Example 1.

[0113] Comparative Example 2 (a) The second polyethylene powder material (weight average molecular weight: 1.3 million) and liquid paraffin were placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface polymer melt.

[0114] (b) The fourth polyethylene powder material (weight average molecular weight: 1.3 million) and liquid paraffin were placed in the first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0115] (c) A lithium ion battery separator was then produced according to steps (c) to (g) of Example 1.

[0116] Comparative Example 3 (a) A first polyethylene powder material (weight average molecular weight: 500,000) and a second polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 70:30 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a second extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 80 rpm to obtain a surface layer polymer melt.

[0117] (b) A third polyethylene powder material (weight average molecular weight: 500,000), a fifth polypropylene powder material (weight average molecular weight: 500,000), and a fourth polyethylene powder material (weight average molecular weight: 1,500,000) were weighed and mixed in a weight ratio of 30:20:50 to obtain a polyethylene mixture, and the polyethylene mixture and liquid paraffin were then placed in a first extruder in a weight ratio of 18:82, and melted and mixed at a temperature of 200°C and a rotation speed of 90 rpm to obtain a core layer polymer melt.

[0118] (c) A lithium ion battery separator was then produced according to steps (c) to (g) of Example 1.

[0119] As long as the weight-average molecular weight or melt index of the separator membrane layer satisfies the rules specified in the present invention, the technical effects of the present invention can be achieved, and are not limited to the proportional relationship between the thickness of the core layer and the surface layer. When a clear interlayer interface exists in the thickness direction of the separator, the membrane layer closest to the center of the separator relative to the interface is the core layer, and the membrane layers from both surfaces of the separator to the interface are the surface layers on both sides. When the interlayer interface is not clear, in a preferred embodiment, the membrane layer occupying 2.5 to 10%, preferably 4.5 to 9%, and more preferably 6 to 7.5% of the membrane thickness in the direction from each of the separator's both surfaces to the separator's center in the thickness direction of the separator is the surface layer (one side) in the present invention, and the remaining central portion is the core layer.

[0120] In the present invention, measurements were carried out based on the following test methods.

[0121] Thickness The measurements were carried out using a German Mahr C1208 thickness measuring instrument in accordance with GB / T6672-2001.

[0122] Air permeability Measurements were carried out using a Japanese Asahi EG01-55-1MR air permeability tester in accordance with GB / T36363-2018.

[0123] porosity The measurement was carried out in accordance with GB / T36363-2018 using the mass method.

[0124] Puncture strength The measurements were carried out using the Chinese puncture test device EM6.102 in accordance with GB / T36363-2018.

[0125] Stretching strength The measurements were carried out using the Chinese elongation tester EM6.202 in accordance with GB / T 1040[1].3-2006.

[0126] Heat shrinkage rate The distance L0 between two points on the separator was marked and measured in a room temperature (23°C) environment, and then the separator was placed between two pieces of A4 paper. Both were then placed in a 120°C oven and kept warm for 1 hour, then removed and allowed to cool naturally to room temperature (or both were placed in a 130°C oven and kept warm for 0.5 hours, then removed and allowed to cool naturally to room temperature). The distance L1 between the two points was measured, and the thermal shrinkage rate was calculated using the equation shrinkage rate = (L0 - L1) / L0 × 100%.

[0127] Melt Index The measurements were carried out using an MTS ZRZ1452 melt flow rate tester in accordance with GB / T 3682.2-2018.

[0128] The measured data of the compositions and properties of Examples 1 to 12 and Comparative Examples 1 to 3 are shown in Tables 1-1 and 1-2.

[0129] [Table 1-1]

[0130] [Table 1-2]

[0131] Compared to conventional technologies, the present invention has researched the compounding ratio of the surface layer and core layer and developed a preparation process to produce a lithium-ion battery separator that combines thinness with high mechanical strength (especially tensile strength) and good thermal dimensional stability, making it particularly suitable for the field of power lithium-ion batteries, which have high energy density and high safety requirements. The separator also has excellent air permeability and puncture strength. Furthermore, the manufacturing process of the present invention is simple and suitable for high-speed, continuous production, contributing to separator cost reduction.

[0132] In addition to ensuring good overall performance, the separator surfaces of Examples 1 to 12 were normal and free of mottle. While the separator surfaces of Comparative Examples 1 and 2 were normal, Comparative Example 3 had slight mottle on the separator surface, which also presented problems with processability. Examples 13, 14, 15, Comparative Examples 4, and 5 used the same raw material composition as Example 1, and differed only in the cooling method or draw ratio. Property measurement data are shown in Table 2.

[0133] [Table 2]

[0134] As found in the present invention, if the cooling effect is not good, the cooling will be uneven, resulting in unevenness on the separator surface and poor performance consistency. Therefore, the solution of the present invention uses a cooling bath to contribute to rapid cooling of the separator and more complete phase separation during the wet membrane formation process, and the cooling time is preferably 10 seconds or more.

[0135] In the present invention, if the stretching ratio is too low, the degree of molecular chain orientation will be low, resulting in low strength of the product, whereas if the stretching ratio is too high, the stretching stress will be large, resulting in poor thermal shrinkage and direct tearing of the separator.

[0136] The melt index of a polymer is related to the molecular weight of the polymer. If the ratio of the melt index of the surface layer to the core layer exceeds 2, the molecular weight of the surface layer is relatively low, meaning that the mechanical strength of the separator is relatively low. If the ratio of the melt index of the surface layer to the core layer is less than 1, the effect of improving thermal shrinkage cannot be achieved. In the present invention, by appropriately selecting the proportion of the melt index or the ratio of the weight average molecular weight, a separator with excellent strength, improved thermal shrinkage, and thin thickness can be obtained.

[0137] In the present invention, if the temperature of the extrusion system is too high, crosslinking reactions between the raw materials will occur, resulting in black spots; if the temperature of the extrusion system is too low, the raw materials will not melt or will not be well melted, which will affect production; if the extruder speed is too low, the raw material transport efficiency will be low and the pressure will be unstable; if the extruder speed is too high, the shear force will be too strong, causing molecular chain scission and local crosslinking reactions of the polymer due to shear heat; if the extractant flow rate exceeds 5 m3 / h, it will easily lead to waste; if the extractant flow rate is less than 1 m3 / h, extraction will be incomplete and the overall performance of the membrane will be impaired.

[0138] Preferably, the present invention can provide a separator with excellent overall performance by appropriately selecting each parameter.

[0139] Unless otherwise indicated, in the specification and claims, the singular forms "a," "an," and "the" can also refer to plural elements. Unless otherwise specified, all technical and scientific terms used herein have the meanings commonly understood by those of ordinary skill in the art. The methods described herein may be carried out in any order that is logically practicable in addition to the specific order disclosed.

[0140] The representative examples are merely illustrative of the present invention and are not intended to limit the scope of the present invention. In addition to what is described herein, various modifications and many other embodiments of the present invention (including the examples and the scientific and patent literature cited herein) will be readily apparent to those skilled in the art. The examples contain important additional information, exemplification, and teachings that are useful in the actual practice of many embodiments of the present invention and equivalents thereof. [Explanation of symbols]

[0141] 1 Upper surface layer 2 Core layer 3 Lower surface layer

Claims

1. A lithium ion battery separator, The lithium ion battery separator includes a porous core layer and porous surface layers covering two upper and lower surfaces of the core layer, and the core layer and the surface layers are (1) The weight-average molecular weight of the polymer in the core layer is 1.1 million to 1.7 million, preferably 1.2 million to 1.7 million, more preferably 1.3 million to 1.6 million, and even more preferably 1.4 million to 1.55 million, and the weight-average molecular weight of the polymer in the core layer is 1.01 to 2.20 times, preferably 1.05 to 1.80 times, and even more preferably 1.10 to 1.30 times, the weight-average molecular weight of the polymer in the surface layer; (2) The melt index of the polymer of the surface layer is 1.01 to 2.00 times, preferably 1.05 to 1.70 times, and more preferably 1.10 to 1.50 times, that of the polymer of the core layer, and the melt index is measured under conditions of 190°C and 2.16 kg / 10 min. At least one of the conditions (1) to (2) is satisfied. A lithium-ion battery separator comprising:

2. A lithium ion battery separator, the polymer of porous membrane layer A occupying 2.5 to 10%, preferably 4.5 to 9%, more preferably 6 to 7.5% of the membrane thickness in a direction from each of the opposite sides of the separator toward the center of the separator has a weight average molecular weight a or a melt index a', and the polymer of porous membrane layer B in the remaining central portion has a weight average molecular weight b or a melt index b'; (1) The weight average molecular weight b is 1,100,000 to 1,700,000, preferably 1,200,000 to 1,700,000, more preferably 1,300,000 to 1,600,000, and even more preferably 1,400,000 to 1,550,000, and the weight average molecular weight b is 1.01 to 2.20 times, preferably 1.05 to 1.80 times, and even more preferably 1.10 to 1.30 times the weight average molecular weight a; (2) The melt index a' is 1.01 to 2.00 times, preferably 1.05 to 1.70 times, and more preferably 1.10 to 1.50 times, the melt index b', and the measurement conditions for the melt index are 190°C and 2.16 kg / 10 min. At least one of the conditions (1) to (2) is satisfied. A lithium-ion battery separator comprising:

3. The polymer of the core layer or the polymer of the porous membrane layer B is a polyolefin, preferably a single type of polyethylene, or a mixture of two or more types of polyethylene, or a mixture of one or more types of polyethylene and polypropylene. The polymer of the surface layer or the polymer of the porous membrane layer A is a polyolefin, preferably polyethylene, and the polyethylene is a single type of polyethylene or a mixture of two or more types of polyethylene. The weight-average molecular weight of the polymer of the surface layer or the polymer of the porous membrane layer A is 600,000 to 1,500,000, preferably 700,000 to 1,400,000, more preferably 900,000 to 1,400,000, and even more preferably 1,100,000 to 1,350,000.

3. The lithium ion battery separator according to claim 1 or 2.

4. the surface layer or the porous membrane layer A comprises a first polyethylene and a second polyethylene, the weight average molecular weight of the first polyethylene is 300,000 to 600,000, preferably 400,000 to 550,000, the weight average molecular weight of the second polyethylene is 1,300,000 to 3,000,000, preferably 1,300,000 to 1,800,000, and the weight ratio of the first polyethylene to the second polyethylene is 0 to 80:20 to 100, preferably 1 to 50:50 to 99, more preferably 0 to 40:60 to 100, and most preferably 15 to 35:65 to 85; the core layer or the porous membrane layer B comprises a third polyethylene and a fourth polyethylene, the weight average molecular weight of the third polyethylene is 300,000 to 600,000, preferably 400,000 to 550,000, the weight average molecular weight of the fourth polyethylene is 1,300,000 to 3,000,000, preferably 1,500,000 to 2,000,000, and the weight ratio of the third polyethylene to the fourth polyethylene is 0-30:70-100, preferably 1-10:90-99, more preferably 5-20:80-95; Optionally, the core layer or the porous membrane layer B further comprises a fifth polypropylene, and the weight average molecular weight of the fifth polypropylene is 300,000 to 600,000, preferably 400,000 to 550,000, and the fifth polypropylene accounts for 0 to 20 wt % of the core layer or the porous membrane layer B, preferably 0 to 13% of the core layer or the porous membrane layer B. The lithium ion battery separator according to claim 3 .

5. The melt index of the polymer of the surface layer or the polymer of the porous membrane layer A is 0.35 to 0.7 g / 10 min, preferably 0.4 to 0.65 g / 10 min, and more preferably 0.42 to 0.55 g / 10 min. The melt index of the polymer of the core layer or the polymer of the porous membrane layer B is 0.3 to 0.6 g / 10 min, preferably 0.3 to 0.5 g / 10 min, and more preferably 0.32 to 0.45 g / 10 min. The melt index is measured under conditions of 190° C. and 2.16 kg / 10 min. The lithium ion battery separator according to any one of claims 1 to 4.

6. The average pore size of the core layer or porous membrane layer B is 30 to 50 nm, preferably 37 to 45 nm, and the average pore size of the surface layer or porous membrane layer A is 35 to 60 nm, preferably 40 to 50 nm. The lithium ion battery separator according to any one of claims 1 to 5.

7. The thickness of the separator is 3 to 15 μm, preferably 10.5 to 11.5 μm. The lithium ion battery separator according to any one of claims 1 to 6.

8. The heat shrinkage of the separator in the longitudinal direction when heated at 120°C for 1 hour is less than 10%, preferably less than 6.5%, more preferably less than 6.0%, and most preferably less than 5.6%. The heat shrinkage of the separator in the longitudinal direction when heated at 130°C for 30 minutes is less than 17%, preferably less than 15%, preferably less than 14%, preferably less than 13%, more preferably less than 11.5%, and most preferably less than 10%. The heat shrinkage of the separator in the transverse direction when heated at 120°C for 1 hour is less than 10%, preferably less than 7%, more preferably less than 5.0%, and most preferably less than 4.0%. The heat shrinkage of the separator in the transverse direction when heated at 130°C for 30 minutes is less than 15%, more preferably less than 12.5%, more preferably less than 8.5%, and most preferably less than 6.5%. Preferably, the separator has a stretch strength in the longitudinal direction (longitudinal direction) of 2980 kgf / cm ２ or more, preferably 3000 kgf / cm ２ More preferably, it is 3300 to 6000 kgf / cm ２ The separator has a transverse (width) stretching strength of 2980 kgf / cm ２ or more, preferably 3000 kgf / cm ２ More preferably, it is 3400 to 6000 kgf / cm ２ is The lithium ion battery separator according to any one of claims 1 to 7.

9. The separator has an air permeability of 50 to 250 sec / 100 ml, and preferably 90 to 140 sec / 100 ml, a porosity of 35 to 60%, and preferably 40 to 48%, a pin puncture strength of 200 gf or more, and preferably 400 gf or more, and more preferably 400 to 1500 gf, a total thickness of the two surface layers or a total thickness of the two porous membrane layers A is 5 to 20%, and preferably 9 to 18%, and more preferably 12 to 15%, of the thickness of the separator, and a weight average molecular weight of 1,100,000 to 1,600,000, and preferably 1,200,000 to 1,500,000, and more preferably 1,300,000 to 1,450,000. The lithium ion battery separator according to any one of claims 1 to 8.

10. A method for preparing a separator for a lithium ion battery, comprising: a surface layer material and a core layer material are prepared, the surface layer material comprising a first polymer and a pore-forming agent, the first polymer comprising a first polyethylene and a second polyethylene, the weight average molecular weight of the first polyethylene being 300,000 to 600,000, preferably 400,000 to 550,000, the weight average molecular weight of the second polyethylene being 1,300,000 to 3,000,000, preferably 1,300,000 to 1,800,000, and the weight ratio of the first polyethylene to the second polyethylene being 0 to 80:20 to 100, preferably 1 to 50:50 to 99, more preferably 0 to 40:60 to 100, and most preferably 15 to 35:65 to 85; Step (1), in which the core layer material includes a second polymer and a pore-forming agent, the second polymer including a third polyethylene and a fourth polyethylene, the third polyethylene having a weight average molecular weight of 300,000 to 600,000, preferably 400,000 to 550,000, the fourth polyethylene having a weight average molecular weight of 1,300,000 to 3,000,000, preferably 1,500,000 to 2,000,000, and a weight ratio of the third polyethylene to the fourth polyethylene being 0 to 30:70 to 100, preferably 1 to 10:90 to 99, more preferably 5 to 20:80 to 95; Step (2) of melting the surface layer material to form a surface layer, melting the core layer material to form a core layer, and covering the upper and lower surfaces of the core layer with the surface layer to form a laminate; Step (3) of biaxially stretching the laminate to obtain a stretched laminate; (4) removing the pore-forming agent from the stretched laminate to obtain a separator precursor; and (5) a step of heat-setting the separator precursor to obtain the lithium ion battery separator, Optionally, the second polymer comprises a fifth polypropylene, the fifth polypropylene having a weight average molecular weight of 300,000 to 600,000, preferably 400,000 to 550,000, and the fifth polypropylene accounts for 0 to 20 wt % of the second polymer, preferably 0 to 13 wt % of the second polymer. A method for preparing a separator for a lithium ion battery, comprising:

11. The weight of the surface layer material is 5 to 20%, preferably 9 to 18%, and more preferably 12 to 15% of the total weight of the surface layer material and the core layer material; the weight ratio of the first polymer to the pore-forming agent in the surface layer material is 15 to 30:70 to 85, preferably 16 to 23:77 to 84; and / or the weight ratio of the second polymer to the pore-forming agent in the core layer material is 15 to 30:70 to 85, preferably 18 to 25:75 to 82.

11. The method of claim 10.

12. Before biaxially stretching the laminate, the laminate is cooled on both sides in a cooling bath, preferably a water bath, and the temperature of the cooling bath is 5 to 40°C, preferably 10 to 25°C, and more preferably 10 to 20°C.

11. The method of claim 10.

13. The biaxial stretching includes longitudinal stretching and transverse stretching, Longitudinal stretching: The laminate is longitudinally stretched at a stretching temperature of 80 to 120°C, preferably 90 to 115°C, and at a stretching ratio of 5.0 to 12.0 times, preferably 6.5 to 8.5 times. Transverse stretching: The longitudinally stretched laminate is subjected to transverse stretching at a stretching temperature of 100 to 140°C, preferably 105 to 125°C, and at a stretching ratio of 7.0 to 15.0 times, preferably 9.0 to 11.5 times.

11. The method of claim 10.

14. The pore-forming agent is a low molecular weight solvent capable of dissolving polyolefin, and is preferably liquid paraffin having a kinematic viscosity of 35 to 120 cps at 40°C.

12. The method of claim 11 .

15. The melt molding is carried out using a plurality of extruders, preferably two extruders, with the core layer material being fed into one extruder (referred to as the first extruder) and the surface layer material being fed into the other extruder (referred to as the second extruder), and the laminate is formed through a co-extrusion die. The preparation method according to any one of claims 10 to 14.

16. The parameters of the second extruder are an extrusion temperature of 150 to 250°C and a screw rotation speed of 40 to 90 r / min; The parameters of the first extruder are an extrusion temperature of 150 to 250°C and a screw rotation speed of 60 to 100 r / min; The co-extrusion die parameters are set to a temperature of 150 to 250°C.

16. The method of claim 15.

17. The pore-forming agent is removed from the stretched laminate using an extractant, and the extractant is an alkane extractant, preferably dichloromethane, and the amount of the extractant circulated is preferably 1 to 5 m ３ / hour, and after extraction, the stretched laminate is dried using one or more heating methods selected from the group consisting of a heating roll, a heating plate, and hot air, and the drying temperature is preferably 20 to 150°C.

17. The method according to any one of claims 10 to 16.

18. The heat setting parameters are a relaxation rate of 5 to 30% and a heat setting temperature of 120 to 145°C.

18. A method according to any one of claims 10 to 17.

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