Separator and preparation method therefor

Abstract

The present application provides a separator and a preparation method therefor. The separator comprises a substrate layer and a heat-resistant layer located on at least one surface of the substrate layer, wherein the substrate layer comprises a polyolefin, and the heat-resistant layer comprises polymer fibers and inorganic ceramic particles; and the separator satisfies the following conditions: 1≤BL50 / BW50≤6, 0.8≤[Equation]≤6, 0.1 μm≤BL50≤0.6 μm, and 0.04 μm≤TL50≤0.2 μm. According to the present application, adjusting dimensions of internal structural pores of a substrate layer and a heat-resistant layer reduces rigidity mismatch between the heat-resistant layer and the substrate layer, thereby increasing a rupture temperature of the separator, and mitigating the problem of separators easily fracturing when subjected to high-temperature treatment for an extended period of time. Moreover, this also greatly reduces the common occurrence of separator curling after heat-resistant layer coating, addresses the problem of separators being prone to powder shedding when stretched or subjected to external impact, and improves the thermal safety performance of the separator.

Description

A diaphragm and a preparation method thereof

[0001] The present application claims priority to the Chinese patent application No. 202411236974.X filed on September 4, 2024, and entitled "A diaphragm and a preparation method thereof", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the technical field of electrochemistry, in particular to a diaphragm and a preparation method thereof. BACKGROUND

[0003] The diaphragm is an important component of a secondary battery, mainly used to separate the positive and negative electrode sheets to prevent internal short circuit of the secondary battery. The traditional diaphragm mainly adopts a polyolefin porous film, such as a single-layer film or a multi-layer composite film of polyethylene (PE) or polypropylene (PP). However, the melting point of the polyolefin diaphragm is relatively low, which will cause serious thermal shrinkage or even breakage of the diaphragm at high temperature, resulting in thermal shrinkage and close contact between the positive and negative electrodes, further leading to internal short circuit, fire or even explosion. In the prior art, a polyolefin porous film coated with ceramic particles is used to solve the problem of thermal shrinkage of the polyolefin porous film, but pure ceramic particle coating, due to the small size of the ceramic particles, needs to be fixed by an acrylic adhesive, and the adhesive will dissolve during heating, causing the ceramic particles to fall off from the polyolefin porous film, resulting in fragmentation of the coated film.

[0004] In the prior art, polymer materials such as aramid and polyimide can also be used as coating materials, which can improve the heat resistance, wettability and oxidation resistance of the polyolefin diaphragm when used together with ceramic particles as a coating. However, the above-mentioned polymer materials have relatively strong molecular chain rigidity, and the thermal expansion coefficient difference between the polymer materials and the polyolefin porous film is large, so the stress is difficult to release in time during the drying process, further causing the diaphragm to curl. At the same time, the longitudinal strength of the coated diaphragm is low and the elongation is large, which is prone to overstretching, further increasing the internal stress of the finished diaphragm, causing the curing process to appear adverse phenomena such as bulging. Due to the large rigidity of the coating and the low elongation at break, when used in a battery, the coating will fall off from the polyolefin porous film upon impact of external force on the wound battery, and the ceramic particles in the coating have high hardness, which can easily pierce the diaphragm and cause short circuit of the battery.

[0005] Therefore, it is urgent to find a new solution to solve the problem of poor flatness of the diaphragm caused by overstretching during the coating process, and to inhibit the coating from falling off from the surface of the base film while improving the thermal safety performance of the diaphragm. SUMMARY

[0006] The application aims to provide a diaphragm and a preparation method thereof, so as to improve the diaphragm breaking temperature, reduce the diaphragm curling rate, improve the diaphragm powder dropping phenomenon, and improve the thermal safety performance of the diaphragm. The specific technical solutions are as follows.

[0007] The first aspect of the application provides a diaphragm, which comprises a substrate layer and a heat-resistant layer located on at least one surface of the substrate layer, the substrate layer comprises a polyolefin, and the heat-resistant layer comprises polymer fibers and inorganic ceramic particles; the diaphragm satisfies:

[0008] 1≤B L50 / B W50 ≤6, 0.1μm≤B L 50≤0.6μm, 0.04μm≤T L 50≤0.2μm;

[0009] Wherein, B W50 is the average length of the pores of the substrate layer in the longitudinal section along the cross-sectional thickness direction, B L50 is the average length of the pores of the substrate layer in the longitudinal section perpendicular to the cross-sectional thickness direction; T W50 is the average length of the pores of the heat-resistant layer in the longitudinal section along the cross-sectional thickness direction, and T L50 is the average length of the pores of the heat-resistant layer in the longitudinal section perpendicular to the cross-sectional thickness direction.

[0010] In some embodiments, 0.5≤T L50 / T W50 ≤4.

[0011] In some embodiments, the average diameter of the polymer fibers is 10-80 nm.

[0012] In some embodiments, the polymer is selected from at least one of para-aramid, meta-aramid, and polyimide.

[0013] In some embodiments, the polyolefin is selected from one or more copolymers or multiple blends of polyethylene, polypropylene, polybutylene, and poly-4-methyl-pentene, and the weight average molecular weight of the polyolefin is 100-300 million, preferably 100-240 million.

[0014] In some embodiments, the material of the inorganic ceramic particles is selected from at least one of alumina, silica, boehmite, magnesium hydroxide, titanium dioxide, barium sulfate, and barium titanate.

[0015] In some embodiments, the inorganic ceramic particles comprise at least one of nano inorganic ceramic particles having a Dv50 of D1, 0.01 μm≤D1≤0.1 μm, and micro-nano inorganic ceramic particles having a Dv50 of D2, 0.1 μm

[0016] In some embodiments, the mass percentage content M1 of the polymer fibers is 30-70%, and the mass percentage content M2 of the inorganic ceramic particles is 30-70%, based on the total mass of the heat-resistant layer; the mass percentage content of the polymer fibers on the surface of the heat-resistant layer is greater than the mass percentage content of the inorganic ceramic particles on the surface of the heat-resistant layer.

[0017] In some embodiments, the thickness of the substrate layer is 5-16 μm.

[0018] In some embodiments, the thickness of the separator is 10-18 μm.

[0019] In some embodiments, the longitudinal (MD) elastic modulus of the separator is greater than the transverse (TD) elastic modulus, and the longitudinal elastic modulus of the separator is ≥ 2500 MPa.

[0020] In some embodiments, the longitudinal breaking elongation of the separator is less than the transverse breaking elongation, the longitudinal breaking elongation of the separator is 5-100%, and the transverse breaking elongation of the separator is 25-170%.

[0021] In some embodiments, the transverse curling rate of the separator is ≤ 15%.

[0022] In some embodiments, the film breaking temperature of the separator is ≥ 250℃.

[0023] In some embodiments, the air permeability per unit thickness of the heat-resistant layer is 15-45 s / 100cc / μm.

[0024] The second aspect of the present application provides a preparation method of the separator provided in the first aspect of the present application, comprising the following steps:

[0025] 1) mixing and mixing-extruding a polyolefin and a porogen to prepare a casting sheet;

[0026] 2) stretching the casting sheet, the temperature of the casting sheet during stretching is 120-140℃, and the temperature difference between different points in the temperature field of the stretching area is less than 0.5℃, and an oil film is prepared after stretching, the stretching comprising first longitudinal stretching and first transverse stretching.

[0027] 3) extracting, drying, then expanding and heat setting the oil film to obtain the substrate layer, the expanding including a second longitudinal stretching and a second transverse stretching;

[0028] 4) coating a polymer coating liquid on at least one surface of the substrate layer, solidifying, washing with water, drying to obtain the separator;

[0029] wherein the product of the stretching ratios of the first longitudinal stretching and the second longitudinal stretching L1 is 5.5-14, the product of the stretching ratios of the first transverse stretching and the second transverse stretching L2 is 1-6.5, and L1>L2; the mass percentage of the polymer fibers based on the total mass of the polymer coating liquid is 2-10%.

[0030] In some embodiments, the maximum tension F used in the entire production line during the coating of step 4) satisfies: wherein E B is the longitudinal modulus of elasticity of the substrate layer, D is the thickness of the substrate layer, W is the width of the substrate layer, E B ≥2500MPa, D is 0.005-0.016mm, and W is 500-1100mm.

[0031] In some embodiments, the method of solidifying of step 4) is selected from solidification bath curing or constant temperature and humidity curing.

[0032] The solvent of the solidification bath curing includes a first organic solvent and a poor solvent, the mass percentage P1 of the first organic solvent based on the total mass of the solidification bath curing solvent is 45-75%, and the mass percentage P2 of the poor solvent is 25-55%; the first organic solvent is selected from at least one of N-methyl pyrrolidone (NMP), N,N-dimethylacetamide, dimethyl sulfoxide, and the poor solvent is selected from at least one of water, ethanol, methanol, ethylene glycol, and tripropylene glycol;

[0033] The constant temperature and humidity curing includes a curing temperature of 40-70℃, a curing humidity of 55-85%, and a curing time of 3-30s.

[0034] In some embodiments, the polymer is selected from at least one of para-aramid and meta-aramid, and the preparation step of the polymer coating liquid includes: mixing and stirring a polymer glue liquid and inorganic ceramic particles in a nitrogen environment at normal pressure, adjusting the temperature to 1-10℃, then adding a second organic solvent containing a cosolvent concentration of 2-10wt%, and stirring and mixing uniformly to obtain the polymer coating liquid.

[0035] The polymer glue solution comprises polymer fibers, chloride ions and a third organic solvent, the mass percentage of the polymer fibers W1 is 2-10% based on the total mass of the polymer glue solution, and the mass percentage of the chloride ions W2 is 0.5-10%; the rotational viscosity of the polymer glue solution is 3000-200000 mpa·s, and the specific viscosity is 1-3 dL / g, the mass ratio of the polymer fibers to the inorganic ceramic particles is (30-70):(70-30), and the cosolvent is selected from at least one of CaCl2, KOH, LiCl and pyridine.

[0036] Alternatively, the polymer is selected from polyimide, and the preparation step of the polymer coating solution comprises the following steps: mixing and stirring the polymer glue solution and the inorganic ceramic particles in a nitrogen environment at normal pressure, controlling the temperature to be 1-10 ℃, then adding a second organic solvent, and stirring and mixing uniformly to obtain the polymer coating solution; wherein the polymer glue solution comprises polymer fibers and a third organic solvent, the mass percentage of the polymer fibers W1 is 2-10% based on the total mass of the polymer glue solution; the rotational viscosity of the polymer glue solution is 3000-200000 mpa·s, and the specific viscosity is 1-3 dL / g, and the mass ratio of the polymer fibers to the inorganic ceramic particles is (30-70):(70-30).

[0037] The second organic solvent and the third organic solvent are each independently selected from at least one of N-methylpyrrolidone, N,N-dimethylformamide, acetone and N,N-dimethylacetamide.

[0038] In some embodiments, the mass ratio of the polyolefin to the pore-forming agent is 1:(2-5).

[0039] The application provides a diaphragm and a preparation method thereof. The diaphragm comprises a substrate layer and a heat-resistant layer on at least one surface of the substrate layer. The substrate layer comprises a polyolefin, and the heat-resistant layer comprises polymer fibers and inorganic ceramic particles. The diaphragm satisfies: 1≤B L50 / B W50 ≤6, 0.1 μm≤B L 50≤0.6 μm, 0.04 μm≤T L 50≤0.2 μm. By controlling the size of the internal structure pores of the substrate layer and the heat-resistant layer, the rigidity difference between the heat-resistant layer and the substrate layer is reduced, the film breaking temperature of the diaphragm is improved, the problem of easy cracking of the diaphragm during long-time high-temperature treatment is alleviated, the curling phenomenon of the diaphragm after coating of the heat-resistant layer is greatly reduced, the problem of easy powder falling of the diaphragm during stretching or impact from the outside is improved, and the thermal safety performance of the diaphragm is improved.

[0040] Of course, practicing either product or method of the present application need not necessarily achieve all of the above-mentioned advantages simultaneously. BRIEF DESCRIPTION OF DRAWINGS

[0041] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced as follows. Obviously, the drawings in the following description only constitute some of the embodiments of the present application, and other embodiments can also be obtained by those skilled in the art based on these drawings.

[0042] Fig. 1 is an SEM photograph of the MD-ZD section of the separator in Example 1-1 in the longitudinal direction;

[0043] Fig. 2 is an SEM photograph of the surface of the heat-resistant layer of the separator in Example 1-1. DETAILED DESCRIPTION

[0044] The technical solutions in the embodiments of the present application will be described clearly and completely in the following with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments only constitute some of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on the present application also belong to the scope of protection of the present application.

[0045] A first aspect of the present application provides a separator comprising a substrate layer and a heat-resistant layer on at least one surface of the substrate layer, the substrate layer comprising a polyolefin, and the heat-resistant layer comprising polymer fibers and inorganic ceramic particles; the separator satisfies:

[0046] 1≤B L50 / B W50 ≤6, 0.1μm≤B L 50≤0.6μm, 0.04μm≤T L 50≤0.2μm;

[0047] wherein B W50 is the average length of the pores of the substrate layer in the longitudinal section in the direction of the cross-sectional thickness, B L50 is the average length of the pores of the substrate layer in the longitudinal section in the direction perpendicular to the cross-sectional thickness; T W50 is the average length of the pores of the heat-resistant layer in the longitudinal section in the direction of the cross-sectional thickness, T L50 is the average length of the pores of the heat-resistant layer in the longitudinal section in the direction perpendicular to the cross-sectional thickness. For example, B L50 / B W50may be 1, 1.2, 1.5, 1.8, 2, 2.3, 2.5, 2.8, 3, 3.5, 4, 4.2, 4.6, 5, 5.3, 5.5, 5.8, 6, or a range defined by any two of the above values, may be 0.8, 1, 1.2, 1.5, 1.8, 2, 2.3, 2.5, 2.8, 3, 3.5, 4, 4.2, 4.6, 5, 5.3, 5.5, 5.8, 6, or a range defined by any two of the above values, B L may be 0.1 pm, 0.15 pm, 0.2 pm, 0.25 pm, 0.3 pm, 0.35 pm, 0.4 pm, 0.45 pm, 0.5 pm, 0.55 pm, 0.6 pm, or a range defined by any two of the above values, T L may be 0.04 pm, 0.07 pm, 0.1 pm, 0.12 pm, 0.13 pm, 0.15 pm, 0.16 pm, 0.18 pm, 0.2 pm, or a range defined by any two of the above values. By adjusting the size of the internal structure pores of the substrate layer and the heat-resistant layer, the rigidity difference between the heat-resistant layer and the substrate layer is reduced, the membrane breaking temperature of the separator is improved, the problem of membrane cracking during long-term high-temperature treatment is alleviated, the curling phenomenon of the separator after coating of the heat-resistant layer is greatly reduced, and the problem of powder falling of the separator during stretching or external impact is improved.

[0048] In some embodiments, 0.5≤T L50 / T W50 ≤4. For example, T L50 / T W50 may be 0.5, 0.8, 1, 1.2, 1.5, 1.8, 2, 2.3, 2.5, 2.8, 3, 3.5, 4, or a range defined by any two of the above values. By adjusting T L50 / T W50 Within the above range, the thermal safety performance of the separator can be further improved.

[0049] In some embodiments, the average diameter of the polymer fibers is 10-80 nm. For example, the average diameter of the polymer fibers can be 10 nm, 20 nm, 30 nm, 50 nm, 60 nm, 70 nm, 80 nm, or a range defined by any two of the above values. By adjusting the average diameter of the polymer fibers within the above range, the mechanical strength of the heat-resistant layer, the membrane breaking temperature of the separator, and the flatness of the separator can be further improved.

[0050] In some embodiments, the polymer is selected from at least one of para-aramid, meta-aramid, and polyimide. By using the above polymer materials, a separator with good thermal safety performance can be obtained.

[0051] The polyimide is not particularly limited in kind as long as the object of the present application can be achieved; in some embodiments, the polyimide can be a soluble polyimide, which can be a polymer containing a characteristic group as follows:

[0052] wherein R is a substituted or unsubstituted aromatic, alicyclic, heterocyclic, or aliphatic group. The polyimide includes at least one of an aliphatic polyimide, an aromatic polyimide, and a copolymer polyimide such as a block copolymer polyimide, a graft copolymer polyimide, containing the above characteristic group. The polyimide can also be at least one of a polyamide-imide, a polyhydrazine-imide, and a polyester-imide. In some embodiments, the polyimide is an aromatic polyimide, which is suitable for use as a high-temperature-resistant polymer for the preparation of a separator for a secondary battery such as a lithium ion battery. The aromatic polyimide includes a monomer as shown in the following general formula:

[0053] wherein each independently of one another is or a mixture thereof;

[0054] each independently of one another is

[0055] wherein each independently of one another is

[0056] X and X' are each independently selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl, fluoroalkyl, hydroxyl, alkoxy, phenoxy, cyano, nitro, amino, acetylamino, ester, acyl, halogen, carboxyl; preferably, -H, -Br, -Cl, -F, -NO2, -CN, -H, -CH3, -CH2CH3, -CH2CH2CH3, isopropyl, isobutyl, t-butyl, cyclopentyl, cyclohexyl, phenyl, naphthyl;

[0057] n is each independently 1, 2, 3, or 4;

[0058] each independently of one another is

[0059] or a mixture thereof. The polyimide dope solution according to the present application can be a polyimide / organic solvent dope solution after thermal imidization, or a dope solution obtained by dissolving a precipitated polyimide in an organic solvent.

[0060] In some embodiments, the polyolefin is selected from one or more copolymers or blends of polyethylene, polypropylene, polybutylene, poly-4-methyl-pentene, and the weight average molecular weight of the polyolefin is 1 million to 3 million, preferably 1 million to 2.4 million. For example, the weight average molecular weight of the polyolefin can be 1 million, 3 million, 5 million, 7 million, 10 million, 12 million, 15 million, 17 million, 20 million, 22 million, 24 million, 26 million, 28 million, 30 million, or a range between any two of these values.

[0061] In some embodiments, the material of the inorganic ceramic particles is selected from at least one of alumina, silica, boehmite, magnesium hydroxide, titanium dioxide, barium sulfate, barium titanate.

[0062] In some embodiments, the inorganic ceramic particles comprise at least one of nano inorganic ceramic particles having a Dv50 of D1, 0.01 μm≤D1≤0.1 μm, and micro-nano inorganic ceramic particles having a Dv50 of D2, 0.1 μm

[0063] In some embodiments, the mass percentage content M1 of the polymer fibers is 30-70%, and the mass percentage content M2 of the inorganic ceramic particles is 30-70%, based on the total mass of the heat-resistant layer; the mass percentage content of the polymer fibers on the surface of the heat-resistant layer is greater than the mass percentage content of the inorganic ceramic particles on the surface of the heat-resistant layer. For example, M1 can be 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55%, 58%, 60%, 63%, 65%, 67%, 70%, or a range defined by any two of the above values, and M2 can be 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55%, 58%, 60%, 63%, 65%, 67%, 70%, or a range defined by any two of the above values. In the present application, during the preparation of the heat-resistant layer, the inorganic ceramic particles will spontaneously deposit downward, so that the content of the polymer fibers on the surface of the heat-resistant layer is higher than the content of the inorganic ceramic particles. By adjusting the mass percentage content M1 of the polymer fibers and the mass percentage content M2 of the inorganic ceramic particles within the above range, the present application can ensure that the separator has good heat resistance, while the air permeability of the separator and the ion passability when used in a secondary battery are also ensured.

[0064] In some embodiments, the thickness of the substrate layer is 5-16 μm. For example, the thickness of the substrate layer can be 5 μm, 6 μm, 7 μm, 8 μm, 10 μm, 11 μm, 12 μm, 13 μm, 15 μm, 16 μm, or a range defined by any two of the above values. In some embodiments, the thickness of the separator is 10-18 μm. For example, the thickness of the separator can be 10 μm, 10.5 μm, 11 μm, 11.5 μm, 12 μm, 13 μm, 13.5 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, or a range defined by any two of the above values. By adjusting the thickness of the separator within the above range, the present application can ensure that the separator has excellent mechanical strength and heat resistance, and when used in a secondary battery, the secondary battery can have a lower internal resistance and a higher ion conductivity, so that the comprehensive performance of the secondary battery is better.

[0065] In some embodiments, the longitudinal modulus of elasticity of the separator is > the transverse modulus of elasticity of the separator, and the longitudinal modulus of elasticity of the separator is > 2500 MPa. In some embodiments, the longitudinal modulus of elasticity of the separator is 2500-6000 MPa. For example, the longitudinal modulus of elasticity of the separator can be 2500 MPa, 2800 MPa, 3000 MPa, 3300 MPa, 3500 MPa, 3700 MPa, 4000 MPa, 4500 MPa, 5000 MPa, 5200 MPa, 5500 MPa, 5800 MPa, 6000 MPa, or a range defined by any two of these values. In some embodiments, the transverse modulus of elasticity of the separator is 1500-3000 MPa. For example, the transverse modulus of elasticity of the separator can be 1500 MPa, 1700 MPa, 2000 MPa, 2200 MPa, 2500 MPa, 2800 MPa, 3000 MPa, or a range defined by any two of these values. By adjusting the longitudinal modulus of elasticity and the transverse modulus of elasticity of the separator to meet the above ranges, the present application can reduce the risk of excessive stretching during the preparation of the separator, thereby improving the flatness of the separator and reducing the curling rate of the separator.

[0066] In some embodiments, the longitudinal elongation at break of the separator is < the transverse elongation at break of the separator, and the longitudinal elongation at break of the separator is 5-100%, and the transverse elongation at break of the separator is 25-170%. For example, the longitudinal elongation at break of the separator can be 5%, 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 70%, 80%, 85%, 90%, 92%, 95%, 97%, 100%, or a range defined by any two of these values; and the transverse elongation at break of the separator can be 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, 105%, 110%, 115%, 120%, 125%, 130%, 135%, 140%, 145%, 150%, 160%, 170%, or a range defined by any two of these values. By adjusting the longitudinal elongation at break and the transverse elongation at break of the separator to meet the above ranges, the present application can reduce the risk of excessive stretching during the preparation of the separator, thereby improving the flatness of the separator and reducing the curling rate of the separator, and also reducing the risk of excessive creases, folds, and other defects in the separator during winding.

[0067] In some embodiments, the transverse curling ratio of the separator is ≤ 15%. For example, the transverse curling ratio of the separator can be 0%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 2%, 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or a range between any two of the aforementioned values. The transverse curling ratio of the separator described in the present application meets the aforementioned range, which can reduce the risk of excessive stretching of the separator during the heat-resistant layer coating process, avoid the deformation of the pores in the MD direction of the heat-resistant layer, and thus improve the flatness of the separator.

[0068] In some embodiments, the film breaking temperature of the separator is ≥ 250℃. For example, the film breaking temperature of the separator can be 250℃, 260℃, 280℃, 300℃, 320℃, 350℃, 380℃, 400℃, 420℃, 450℃, or a range between any two of the aforementioned values. The film breaking temperature of the separator described in the present application meets the aforementioned range, which can reduce the risk of easy breaking of the separator and improve the thermal safety performance of the separator.

[0069] In some embodiments, the air permeability per unit thickness of the heat-resistant layer is 15-45 s / 100cc / μm. For example, the air permeability per unit thickness of the heat-resistant layer can be 15 s / 100cc / μm, 18 s / 100cc / μm, 20 s / 100cc / μm, 22 s / 100cc / μm, 25 s / 100cc / μm, 28 s / 100cc / μm, 30 s / 100cc / μm, 33 s / 100cc / μm, 35 s / 100cc / μm, 38 s / 100cc / μm, 40 s / 100cc / μm, 42 s / 100cc / μm, 45 s / 100cc / μm, or a range between any two of the aforementioned values. By adjusting the air permeability per unit thickness of the heat-resistant layer within the aforementioned range, the secondary battery using the separator described in the present application can have a lower internal resistance and a higher ionic conductivity, and thus the comprehensive performance of the secondary battery is better.

[0070] The second aspect of the present application provides a preparation method of the separator provided in the first aspect of the present application, which comprises the following steps:

[0071] 1) mixing and mixing-extruding a polyolefin and a porogen to prepare a casting sheet;

[0072] 2) stretching the casting sheet, wherein the temperature of the casting sheet during the stretching is 120-140℃, and the temperature difference between different points in the temperature field of the stretching area is less than 0.5℃, and an oil film is prepared after the stretching, and the stretching comprises a first longitudinal stretching and a first transverse stretching;

[0073] 3) extracting, drying, then expanding and heat setting the oil film to obtain the substrate layer, the expanding including a second longitudinal stretching and a second transverse stretching;

[0074] 4) coating a polymer coating liquid on at least one surface of the substrate layer, solidifying, washing with water, drying to obtain the separator;

[0075] wherein the product of the stretching ratios of the first longitudinal stretching and the second longitudinal stretching L1 is 5.5-14, the product of the stretching ratios of the first transverse stretching and the second transverse stretching L2 is 1-6.5, and L1>L2; the mass percentage of the polymer fibers based on the total mass of the polymer coating liquid is 2-10%. For example, the temperature of the casting sheet during stretching can be 120℃, 125℃, 130℃, 135℃, 140℃, or a range defined by any two of them. L1 can be 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 10, 10.5, 11, 11.5, 12, 12.5, 13, 14, or a range defined by any two of them. L2 can be 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, or a range defined by any two of them. The mass percentage of the polymer fibers can be 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range defined by any two of them.

[0076] In step 1), the temperature of the mixing is not particularly limited as long as the purpose of the present application can be achieved, and for example, the temperature of the mixing is 220℃ or lower, preferably 190-215℃.

[0077] In step 2), the speed of the stretching is not particularly limited as long as the purpose of the present application can be achieved, and for example, the speed of the stretching is 30-50 m / min; the uniformity of the temperature field during the stretching is controlled, and the temperature difference between different points of the temperature field of the stretching area is less than 0.5℃, preferably less than 0.3℃.

[0078] In step 3), the solvent for the extraction can be selected from at least one of dichloromethane and cyclohexane; the temperature of the drying is not particularly limited as long as the purpose of the present application can be achieved, and for example, the temperature of the drying is 30-50℃.

[0079] In step 4), the temperature of the drying is not particularly limited as long as the purpose of the present application can be achieved, and for example, the temperature of the drying is 60-90℃.

[0080] In some embodiments, the viscosity of the polymer coating liquid is 1000-20000 mpa·s. For example, the viscosity of the polymer coating liquid can be 1000 mpa·s, 2000 mpa·s, 3000 mpa·s, 5000 mpa·s, 10000 mpa·s, 12000 mpa·s, 15000 mpa·s, 16000 mpa·s, 18000 mpa·s, 20000 mpa·s, or a range defined by any two of them.

[0081] In some embodiments, the maximum tension F used in the whole production line during the coating process of step 4) satisfies: wherein E B is the longitudinal modulus of elasticity of the substrate layer, D is the thickness of the substrate layer, W is the width of the substrate layer, E B ≥ 2500 MPa, D is 0.005-0.016 mm, and W is 500-1100 mm. For example, E B may be 2500 MPa, 2800 MPa, 3000 MPa, 3300 MPa, 3500 MPa, 3700 MPa, 4000 MPa, 4500 MPa, 5000 MPa, or a range defined by any two of them; D can be 0.005 mm, 0.006 mm, 0.007 mm, 0.008 mm, 0.01 mm, 0.011 mm, 0.012 mm, 0.013 mm, 0.015 mm, 0.016 mm, or a range defined by any two of them; and W can be 500 mm, 600 mm, 700 mm, 800 mm, 900 mm, 1000 mm, 1100 mm, or a range defined by any two of them. The polymer coating liquid is prone to slipping during coating, and by adjusting the maximum tension F used in the whole production line during the coating process , the risk of slipping can be greatly reduced, thereby improving the yield of the diaphragm.

[0082] In some embodiments, the method of solidification of step 4) is selected from solidification bath curing or constant temperature and humidity curing.

[0083] The solvent solidified by the coagulation bath comprises a first organic solvent and a poor solvent, the mass percentage P1 of the first organic solvent is 45-75% and the mass percentage P2 of the poor solvent is 25-55% based on the total mass of the solvent solidified by the coagulation bath; the first organic solvent is selected from at least one of N-methyl pyrrolidone, N,N-dimethylacetamide, dimethyl sulfoxide, and the poor solvent is selected from at least one of water, ethanol, methanol, ethylene glycol, and tripropylene glycol; for example, P1 can be 45%, 50%, 55%, 60%, 65%, 70%, 75% or a range formed by any two of them, and P2 can be 25%, 30%, 35%, 40%, 45%, 50%, 55% or a range formed by any two of them.

[0084] The constant temperature and humidity curing comprises a curing temperature of 40-70℃, a curing humidity of 55-85%, and a curing time of 3-30s. For example, the curing temperature can be 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃ or a range formed by any two of them; the curing humidity can be 55%, 60%, 65%, 70%, 75%, 80%, 85% or a range formed by any two of them. The curing time can be 3s, 5s, 8s, 10s, 12s, 15s, 18s, 20s, 23s, 25s, 27s, 30s or a range formed by any two of them.

[0085] In some embodiments, the polymer is selected from at least one of para-aramid and meta-aramid, and the preparation step of the polymer coating liquid comprises: mixing and stirring the polymer glue solution and the inorganic ceramic particles in a nitrogen environment at normal pressure, controlling the temperature to be 1-10℃, then adding a second organic solvent containing a cosolvent concentration of 2-10wt%, and stirring and mixing uniformly to obtain the polymer coating liquid.

[0086] In some embodiments, the polymer is selected from at least one of para-aramid and meta-aramid, and the preparation step of the polymer coating liquid comprises: mixing and stirring the polymer glue solution and the inorganic ceramic particles in a nitrogen environment at normal pressure, controlling the temperature to be 1-10℃, then adding a second organic solvent containing a cosolvent concentration of 2-10wt%, and stirring and mixing uniformly to obtain the polymer coating liquid.

[0087] or, the polymer is selected from polyimide, the preparation step of the polymer coating liquid comprises: mixing and stirring the polymer glue and the inorganic ceramic particles in a nitrogen environment at normal pressure, the temperature is controlled at 1-10℃, then a second organic solvent is added, and the mixture is stirred and mixed uniformly to obtain the polymer coating liquid; wherein, the polymer glue comprises polymer fibers and a third organic solvent, the mass percentage content W1 of the polymer fibers in the total mass of the polymer glue is 2-10%; the rotational viscosity of the polymer glue is 3000-200000 mpa·s, and the specific logarithmic viscosity is 1-3 dL / g, and the mass ratio of the polymer fibers to the inorganic ceramic particles is (30-70):(70-30);

[0088] The second organic solvent and the third organic solvent are each independently selected from at least one of N-methylpyrrolidone, N,N-dimethylformamide, acetone, and N,N-dimethylacetamide.

[0089] For example, the controlled temperature is 1℃, 2℃, 3℃, 4℃, 5℃, 6℃, 7℃, 8℃, 9℃, 10℃, or a range formed by any two of them. The concentration of the cosolvent is 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, or a range formed by any two of them. W1 can be 2%, 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range formed by any two of them, and W2 can be 0.5%, 0.8%, 1%, 1.2%, 1.5%, 2%, 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range formed by any two of them. The rotational viscosity of the polymer glue can be 3000 mpa·s, 5000 mpa·s, 10000 mpa·s, 20000 mpa·s, 30000 mpa·s, 50000 mpa·s, 80000 mpa·s, 100000 mpa·s, 120000 mpa·s, 150000 mpa·s, 180000 mpa·s, 200000 mpa·s, or a range formed by any two of them, and the specific logarithmic viscosity of the polymer glue can be 1 dL / g, 1.2 dL / g, 1.5 dL / g, 1.8 dL / g, 2 dL / g, 2.3 dL / g, 2.5 dL / g, 2.8 dL / g, 3 dL / g, or a range formed by any two of them.

[0090] In some embodiments, the mass ratio of the polyolefin and the pore former is 1:(2-5). For example, the mass ratio of the polyolefin and the pore former can be 1:2, 1:3, 1:4, 1:5, or a range formed by any two of them.

[0091] In some embodiments, the pore forming agent is paraffin oil.

[0092] The thickness of the cast sheet is not particularly limited in the present application, as long as the object of the present application can be achieved; in some embodiments, the thickness of the cast sheet is 0.3-2.5 mm. For example, the thickness of the cast sheet can be 0.3 mm, 0.5 mm, 0.8 mm, 1 mm, 1.3 mm, 1.5 mm, 1.7 mm, 2 mm, 2.2 mm, 2.5 mm, or a range defined by any two of the aforementioned values.

[0093] In the present application, by regulating the product of the stretching ratios of the first longitudinal stretching and the second longitudinal stretching L1, the product of the stretching ratios of the first transverse stretching and the second transverse stretching L2 in the above-mentioned preparation method, the B W50 , B L50 , T W50 , T L50 of the separator of the present application can be changed. Specifically, when L1 increases, the B L50 of the separator increases, and the B W50 decreases; when L1 decreases, the B L50 of the separator decreases, and the B W50 increases; when L2 increases, the B W50 of the separator decreases; when L2 decreases, the B W50 of the separator increases. By regulating the maximum tension F used in the whole production line during the coating process in the above-mentioned preparation method, the B W50 , B L50 , T W50 , T L50 of the separator of the present application can be changed. Specifically, when F increases, the T L50 of the separator increases, and the T W50 decreases; when F decreases, the T L50 of the separator decreases, and the T W50 increases. By regulating the mass percentage content P1 of the first organic solvent in the above-mentioned solidification bath solidification method, the B W50 , B L50 , T W50 , T L50 of the separator of the present application can be changed. Specifically, when P1 increases, both the T 50 and the T W50 of the separator increase; when P1 decreases, both the T L50 and the T W50 of the separator decrease. By regulating the solidification temperature in the above-mentioned constant temperature and humidity solidification method, the B W50 , B L50 , T W50 , T L50 of the separator of the present application can be changed. Specifically, when the solidification temperature increases, the TL50 and T W50 are both decreased; when the curing temperature is decreased, the T L50 and T W50 are both increased. By adjusting the curing humidity in the above-mentioned constant-temperature and constant-humidity curing method, the B W50 , B L50 , T W50 , T L50 of the diaphragm of the present application can be changed. Specifically, when the curing humidity is increased, the T L50 and T W50 of the diaphragm are both increased; when the curing humidity is decreased, the T L50 and T W50 of the diaphragm are both decreased.

[0094] In the present application, by adjusting the content ratio of the polymer fibers in the polymer coating liquid, the average diameter of the polymer fibers can be changed. Specifically, when the content of the polymer fibers in the polymer coating liquid is increased, the average diameter of the polymer fibers is increased; when the content of the polymer fibers in the polymer coating liquid is decreased, the average diameter of the polymer fibers is decreased.

[0095] Examples

[0096] Hereinafter, examples and comparative examples are given to more specifically describe the embodiments of the present application. Various tests and evaluations are performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are on a mass basis.

[0097] Test methods and equipment:

[0098] Polymer glue liquid and polymer coating liquid rotational viscosity determination: Under certain conditions (humidity ≤ 1%, temperature: 25 ± 1°C), about 250 ml of polymer glue liquid or polymer coating liquid is taken with a 250 ml beaker, the rotor is fixed on a viscometer (Brookfield, RVDV-2T, USA), the liquid is over the rotor and is flush with the test scale line on the rotor, the viscosity of the liquid is measured, wherein the rotor speed is selected at 15-100 rpm, the rotor is selected according to the actual viscosity of the liquid, and the test result is ensured to be between 30-80% of the test range, i.e. the rotational viscosity of the polymer glue liquid or polymer coating liquid is obtained.

[0099] Polymer glue liquid specific concentration logarithmic viscosity determination: 0.10 g of polymer in the polymer glue liquid is added to 20 mL of concentrated sulfuric acid (concentration of 98.3 wt%), stirred at 80°C until the polymer is completely dissolved, the same capillary viscometer (German standard Ubbelohde viscometer) is used, the efflux times t0 and t of the concentrated sulfuric acid and the dilute solution of the polymer with a concentration of 0.005 g / mL in the concentrated sulfuric acid are measured at a specified temperature of 30°C, respectively, and the specific concentration logarithmic viscosity of the polymer glue liquid is calculated by the formula δ = 2ln(t / t0).

[0100] Average particle size Dv50 measurement: The inorganic ceramic particles were measured using a particle size measuring device (Nikkiso Co., Ltd., Microtrac UPA150 Particle Size Analyzer), and as the measurement conditions, the load index = 0.15-0.3, the measurement time 300 seconds were set. The value of the 50% particle size obtained is Dv50.

[0101] B L50 , B W50 , T L50 , T W50 measurement:

[0102] Using an ion polishing means, a smooth longitudinal-normal (MD-ZD) cross section was prepared along the longitudinal direction of the separator, and the cross section was observed using a scanning electron microscope (SEM, ZEISS, ZEISS SIGMA 300). Using the scale on the electron microscope, the maximum length of a single pore of the substrate layer in the longitudinal cross section along the cross-sectional thickness direction was measured, and the average value of all pores was calculated as B L50 ; the maximum length of a single pore of the substrate layer in the longitudinal cross section perpendicular to the cross-sectional thickness direction was measured, and the average value of all pores was calculated as B W50 ; the maximum length of a single pore of the heat-resistant layer in the longitudinal cross section along the cross-sectional thickness direction was measured, and the average value of all pores was calculated as T L50 ; the maximum length of a single pore of the heat-resistant layer in the longitudinal cross section perpendicular to the cross-sectional thickness direction was measured, and the average value of all pores was calculated as T W50 .

[0103] Average diameter of polymer fibers measurement: The surface of the separator was observed using an SEM, and 5 photos of different positions at a magnification of 20,000 times were randomly taken. After the hole profile was outlined with a pen, the diameter of the polymer fibers was marked using the software on the scanning electron microscope, and then the average value was calculated as the average diameter of the polymer fibers.

[0104] Separator transverse curling rate measurement: A separator with a length of 100-150 mm was cut, with the heat-resistant layer completely covering the surface of the substrate layer. Under the action of an external force, the separator was flattened, and the length of the separator in the TD direction at this time was measured, denoted as L1 (mm). The separator was placed horizontally on the desktop, and under the action of no external force, the separator naturally curled in the TD direction, and the length of the curled separator projected onto the horizontal desktop in the TD direction was measured, denoted as L2 (mm), and the transverse curling rate of the separator was calculated as (L1-L2) / L1 x 100%.

[0105] Air permeability test: cut 100mm x 100mm substrate layer or membrane, use the air permeability tester of Gurley 4110N, use 100cc test gas mode, record the time of test gas passing through the substrate layer or membrane, then the air permeability of substrate layer or membrane is obtained, air permeability of heat resistant layer = air permeability of membrane - air permeability of substrate layer, air permeability per unit thickness of heat resistant layer = air permeability of heat resistant layer / thickness of heat resistant layer.

[0106] Thickness test: use micrometer to test the thickness of substrate layer, then test the thickness of membrane after coating heat resistant layer, thickness of heat resistant layer = thickness of membrane - thickness of substrate layer.

[0107] Elongation at break test: according to the national standard GB13022-91 "Plastic film tensile property test method", cut 200mm x 25mm membrane or substrate layer sample, use steel ruler to confirm that the distance between the tension clamps of tensile testing machine is 100±5mm, the pressure between the clamps is 0.6MPa, the test speed is 250m / min, and the elongation at break is directly read when tensile fracture occurs.

[0108] Elastic modulus test: according to the national standard GB13022-91 "Plastic film tensile property test method", cut 200mm x 25mm membrane or substrate layer sample, the distance between the tension clamps of tensile testing machine is 100±5mm, according to the requirements of 7.3 in the standard, draw stress-strain curve, and calculate tensile elastic modulus from the initial straight line part of the curve, expressed as E t (MPa), calculated by the following formula

[0109] In the formula: σ: stress, MPa; ε: strain.

[0110] Film breaking temperature test: cut the membrane to 80mm x 4mm, use thermal mechanical analyzer (Germany, TMA 402 ), under the test conditions of nitrogen atmosphere, gas flow of 100ml / min, tension of 0.03N, heating rate of 5℃ / min, and termination temperature of 400℃, the thermal mechanical analysis (TMA) curve is obtained, with temperature as abscissa and deformation as ordinate. With the increase of temperature, the deformation of the membrane changes, when the membrane breaks, the deformation reaches the maximum value in the ordinate direction of TMA curve, and the temperature corresponding to the abscissa at this time is the film breaking temperature of the membrane.

[0111] Powder dropping test: cut the membrane to 25mm x 200mm, stretch under 50N tension, and observe whether powder dropping occurs.

[0112] Example 1-1

[0113] <Preparation of the substrate layer>

[0114] Polyethylene powder (weight average molecular weight of 600-800 thousand, Ticona Celanese) 30 parts by weight, paraffin oil (60#, Zhejiang Zhengxin) 70 parts by weight, a total of 100 parts by weight were mixed, and then put into a twin-screw extruder at 200±10°C for mixing to prepare a mixture melt gel. The mixture melt gel was extruded using a T-shaped die, then rapidly cooled on a constant temperature metal pair roller with a surface temperature of 15°C, and rolled into a sheet to obtain a casting sheet with a thickness of 0.87 mm.

[0115] The casting sheet was pulled and stretched, and rapidly stretched at a speed of 40 m / min at 120°C, with the MD direction and the TD direction being stretched by 7.5 and 4.0 times, respectively, and the temperature difference between different points of the temperature field of the stretching area being controlled to be less than 0.3°C. An oil film was prepared after stretching.

[0116] The oil film was pulled into dichloromethane for extraction, and the paraffin oil in the oil film was extracted out. Then it was dried by constant temperature hot air at 40°C, and the dichloromethane remaining in the membrane was brought out. During the extraction and drying process, the membrane TD direction was slightly shrunk under the action of the MD direction pulling, with a shrinkage rate of 15%. Then it was expanded and heat set at 131°C, without stretching in the MD direction, and the stretching ratio in the TD direction was 1.1. A polyethylene porous membrane with uniform fiber structure orientation was obtained. Then it was cut by a slitting machine to obtain a substrate layer with a width of 810 mm and a thickness of 12 μm. The tensile modulus of the substrate layer in the MD direction was measured to be 4500 MPa.

[0117] <Preparation of the substrate layer>

[0118] In a 200 kg reactor, 84 kg of NMP solvent (Henan Maiqi Chemical Co., Ltd., purity ≥ 99.5%) and 5.45 kg of anhydrous calcium chloride (Xilong Scientific Co., Ltd., analytical pure) were added, and the temperature was raised to 80°C under nitrogen atmosphere, and the calcium chloride was completely dissolved after stirring for 4 h; then 2.658 kg of p-phenylenediamine (Inner Mongolia Kailidie Chemical Co., Ltd., purity > 99.5%) was added, and the stirring was continued until the solids were completely dissolved. The air in the reactor was removed by vacuum filtration, the internal pressure was lower than -60 bar, and the internal pressure was maintained at atmospheric pressure by adding 99.99% high-purity nitrogen. The temperature of the solution was reduced to 10±0.5°C using chilled water, and 4.940 kg of terephthaloyl chloride (Ningxia Fenghua Biological Technology Co., Ltd., purity > 99.5%) was slowly added in multiple batches. A p-aramid polymer solution was obtained. Calcium oxide (Tianjin Xintaiyi Technology Co., Ltd., analytical pure) was added to adjust the pH of the glue solution to 4-8, and further diluted with NMP to obtain a p-aramid glue solution with a mass fraction of 5%. The rotational viscosity was tested to be 69000 mPa·s, and the specific logarithmic viscosity was 2.0 dL / g.

[0119] Then 80 kg of p-aramid glue solution with a mass fraction of 5% was added to a double planetary mixer, 8 kg of Al2O3 (D1 is 0.05±0.03 μm of nano Al2O3 and D2 is 0.3±0.15 μm of micro-nano Al2O3, mass ratio = 3:1) was added in three times (mass ratio of addition is 40:40:20), and stirred for 30 min. Then 80 kg of NMP / CaCl2 solution (CaCl2 solid content is 5 wt%) was added, and the slurry was stirred for 90 min to disperse and mix uniformly, and a slurry with a viscosity of 9000 mPa·s was obtained. The slurry was filtered through a 300 mesh filter before use to obtain a p-aramid coating liquid, wherein the mass percentage of p-aramid fiber in the p-aramid coating liquid is 2.38%.

[0120] The p-aramid coating liquid was uniformly coated onto one side surface of the substrate layer with a width of 810 mm prepared above using a coating machine. Using the reverse method of immersion, the membrane including a substrate layer and a heat-resistant layer was obtained through coagulation bath curing, deionized water washing, and drying, wherein the coagulation bath curing solvent is NMP and ultrapure water with a mass ratio of 2:1, the NMP content P1 is 67 wt%, the maximum tension F used in the whole production line during the coating process is 160 N, and the thickness of the membrane is 16 μm.

[0121] Examples 1-2 to 1-16

[0122] Except that the preparation parameters in <preparation of substrate layer> and <preparation of membrane> are adjusted according to Table 1, the rest is the same as Example 1-1, wherein, The curing is coagulation bath curing or constant temperature and humidity curing, and the mass of Al2O3 and NMP / CaCl2 solution in the polymer coating liquid is unchanged.

[0123] Examples 2-1 to 2-9

[0124] Except that the corresponding preparation parameters are adjusted according to Table 3 in <Preparation of substrate layer> and <Preparation of separator>, the rest is the same as Example 1-1.

[0125] Example 2-10

[0126] Except that the mass fraction of 5% para-aramid glue solution is replaced by the mass fraction of 5% polyimide glue solution, and 80 kg of NMP / CaCl2 solution (CaCl2 solid content is 5wt%) is replaced by 80 kg of NMP in <Preparation of separator>, the rest is the same as Example 1-1, wherein the preparation steps of the polyimide glue solution are: dissolving polyimide with Mw of 3-10 million in NMP to obtain a polyimide glue solution with a mass fraction of 5%, and the rotational viscosity is 30000 mpa·s, and the specific logarithmic viscosity is 2.0 dL / g, and the structural formula of the polyimide is:

[0127] Examples 3-1 to 3-5

[0128] Except that the corresponding preparation parameters are adjusted according to Table 5 in <Preparation of separator>, the rest is the same as Example 1-1, wherein the total mass of the polymer fiber and the inorganic ceramic particles is unchanged.

[0129] Comparative Examples 1-1 to 1-5

[0130] Except that the corresponding preparation parameters are adjusted according to Table 1 in <Preparation of substrate layer> and <Preparation of separator>, the rest is the same as Example 1-1, wherein the curing is coagulation bath curing or constant temperature and humidity curing, and the mass of Al2O3 and NMP / CaCl2 solution in the polymer coating liquid is unchanged.

[0131] The SEM photos of the MD-ZD section of the separator in the longitudinal direction of Example 1-1 are shown in Figure 1, and the SEM photos of the surface of the heat-resistant layer of the separator are shown in Figure 2. According to the results in Tables 2, 4, and 6, the pore size of the separator, the average diameter of the polymer fibers, and the particle size Dv50 of the inorganic ceramic particles are within the scope of the present application, which can reduce the curling rate of the separator, improve the powder dropping phenomenon of the separator, increase the membrane breaking temperature of the separator, and at the same time, can take into account the air permeability of the heat-resistant layer of the separator of the present application, and ensure the ion passing rate of the separator.

[0132] The above description is merely preferred embodiments of the present application, and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the scope of protection of the present application.

Claims

1. A separator comprising a substrate layer and a heat-resistant layer on at least one surface of the substrate layer, the substrate layer comprising a polyolefin, the heat-resistant layer comprising polymer fibers and inorganic ceramic particles; the separator satisfying: 1 < B < 6, 0.1 μm < B < 50 μm, 0.04 μm < T < 0.2 μm. L50 / B W50 ≤6, 0.1μm≤B L 50≤0.6μm,0.04μm≤T L 50≤0.2μm; wherein B W50 B is the average length of the pores of the substrate layer in the longitudinal cross section in the direction of the cross-sectional thickness, B L50 B is the average length of the pores of the substrate layer in the longitudinal cross section in the direction perpendicular to the cross-sectional thickness; T W50 T is the average length of the pores of the heat-resistant layer in the longitudinal section in the direction of the cross-sectional thickness, L50 T is the average length of the pores of the heat-resistant layer in the longitudinal section in the direction perpendicular to the cross-sectional thickness.

2. The septum of claim 1, wherein, 0.5 < T L50 / T W50 ≤ 4.

3. The septum of claim 1, wherein, The average diameter of the polymer fibers is 10-80 nm.

4. The septum of claim 1, wherein, The polymer is selected from at least one of para-aramid, meta-aramid, polyimide.

5. The septum of claim 1, wherein, The polyolefin is selected from one or more copolymers or more than one blend of polyethylene, polypropylene, polybutylene, poly-4-methyl-pentene, and the weight average molecular weight of the polyolefin is 100-3000, preferably 100-2400.

6. The septum of claim 1, wherein, The material of the inorganic ceramic particles is selected from at least one of alumina, silica, boehmite, magnesium hydroxide, titanium dioxide, barium sulfate, barium titanate.

7. The septum of claim 6, wherein, The inorganic ceramic particles comprise at least one of nano inorganic ceramic particles and micro-nano inorganic ceramic particles, the Dv50 of the nano inorganic ceramic particles is D1, 0.01 μm≤D1≤0.1 μm, and the Dv50 of the micro-nano inorganic ceramic particles is D2, 0.1 μm<D2≤2 μm; preferably, the inorganic ceramic particles comprise nano inorganic ceramic particles and micro-nano inorganic ceramic particles, and the mass ratio H of the nano inorganic ceramic particles to the micro-nano inorganic ceramic particles is (1-8):

1.

8. The septum of claim 1, wherein, The mass percentage content M1 of the polymer fibers is 30-70% and the mass percentage content M2 of the inorganic ceramic particles is 30-70% based on the total mass of the heat-resistant layer; the mass percentage of the polymer fibers on the surface of the heat-resistant layer is greater than the mass percentage of the inorganic ceramic particles on the surface of the heat-resistant layer.

9. The septum of claim 1, wherein, The thickness of the substrate layer is 5-16 μm, and the thickness of the separator is 10-18 μm.

10. The separator of any one of claims 1-9, wherein, The separator satisfies at least one of the following characteristics: a) the longitudinal elastic modulus of the separator is greater than the transverse elastic modulus, and the longitudinal elastic modulus of the separator is ≥2500 MPa; b) the longitudinal breaking elongation of the separator is less than the transverse breaking elongation, the longitudinal breaking elongation of the separator is 5-100%, and the transverse breaking elongation of the separator is 25-170%; d) the transverse curling rate of the separator is ≤15%; e) the film breaking temperature of the separator is ≥250℃; f) the air permeability per unit thickness of the heat-resistant layer is 15-45 s / 100 cc / μm.

11. A method for preparing the separator according to any one of claims 1-10, comprising the following steps: 1) mixing and mixing-extruding a polyolefin and a porogen to prepare a casting sheet; 2) stretching the casting sheet, the temperature of the casting sheet during stretching is 120-140℃, and the temperature difference between different points of the temperature field of the stretching area is less than 0.5℃, and an oil film is prepared after stretching, the stretching comprising first longitudinal stretching and first transverse stretching; 3) extracting and drying the oil film, then expanding and heat setting to obtain the substrate layer, the expanding comprising second longitudinal stretching and second transverse stretching; 4) coating a polymer coating liquid on at least one surface of the substrate layer, and then solidifying, washing with water, and drying to obtain the separator. The product of the stretching ratio of the first longitudinal stretching and the second longitudinal stretching L1 is 5.5-14, the product of the stretching ratio of the first transverse stretching and the second transverse stretching L2 is 1-6.5, and L1>L2; the mass percentage content of the polymer fiber is 2-10% based on the total mass of the polymer coating solution.

12. The method of making according to claim 11, wherein, Step 4) the maximum tension F used in the process of said coating satisfies: where E B is the longitudinal modulus of elasticity of the substrate layer, D is the thickness of the substrate layer, W is the width of the substrate layer, E B ≥ 2500 MPa, D is 0.005-0.016 mm, W is 500-1100 mm.

13. The method of making according to claim 11, wherein, The method of the coagulation is selected from coagulation bath curing or constant temperature and humidity curing; The solvent of the coagulation bath curing includes a first organic solvent and a poor solvent, the mass percentage content P1 of the first organic solvent is 45-75% based on the total mass of the solvent of the coagulation bath curing, and the mass percentage content P2 of the poor solvent is 25-55%; the first organic solvent is selected from at least one of N-methyl pyrrolidone, N,N-dimethylacetamide and dimethyl sulfoxide, and the poor solvent is selected from at least one of water, ethanol, methanol, ethylene glycol and tripropylene glycol; The constant temperature and humidity curing includes a curing temperature of 40-70℃, a curing humidity of 55-85% and a curing time of 3-30s.

14. The production process according to any one of claims 11 to 13, wherein, The polymer is selected from at least one of para-aramid and meta-aramid, and the preparation step of the polymer coating solution includes: under normal pressure in a nitrogen environment, regulating the temperature to 1-10℃, mixing and stirring the polymer glue solution and inorganic ceramic particles, then adding a second organic solvent containing a cosolvent with a concentration of 2-10wt%, and stirring and mixing uniformly to obtain the polymer coating solution; wherein the polymer glue solution includes polymer fibers, chloride ions and a third organic solvent, the mass percentage content W1 of the polymer fibers is 2-10% based on the total mass of the polymer glue solution, and the mass percentage content W2 of the chloride ions is 0.5-10%; the rotational viscosity of the polymer glue solution is 3000-200000mpa·s, and the specific logarithmic viscosity is 1-3dL / g; the mass ratio of the polymer fibers to the inorganic ceramic particles is (30-70):(70-30), and the cosolvent is selected from at least one of CaCl2, KOH, LiCl and pyridine; Or, the polymer is selected from polyimide, and the preparation step of the polymer coating solution includes: under normal pressure in a nitrogen environment, regulating the temperature to 1-10℃, mixing and stirring the polymer glue solution and inorganic ceramic particles, then adding a second organic solvent, and stirring and mixing uniformly to obtain the polymer coating solution; wherein the polymer glue solution includes polymer fibers and a third organic solvent, the mass percentage content W1 of the polymer fibers is 2-10% based on the total mass of the polymer glue solution; the rotational viscosity of the polymer glue solution is 3000-200000mpa·s, and the specific logarithmic viscosity is 1-3dL / g; the mass ratio of the polymer fibers to the inorganic ceramic particles is (30-70):(70-30); The second organic solvent and the third organic solvent are each independently selected from at least one of N-methyl pyrrolidone, N,N-dimethylformamide, acetone and N,N-dimethylacetamide.

15. The method of manufacturing according to any one of claims 11-13, wherein, The mass ratio of the polyolefin to the pore former is 1:(2-5).

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