A method for preparing a high-porosity polyolefin microporous membrane

Abstract

The present invention relates to a method for preparing a microporous membrane. The surface layer material is prepared using ultra-high molecular weight polyethylene (UHMWPE) through a two-stage temperature increase process. An inorganic particle powder is then blended with polyethylene terephthalate (PET) to form a granulated product, which is then mixed with polypropylene to prepare the core layer material. A three-layer co-extrusion process is then used to prepare a hot film having a surface-core-surface structure. After cooling and drying, the film is longitudinally stretched, transversely stretched, and heat-set to obtain a high-porosity polyolefin microporous membrane. The resulting microporous membrane not only exhibits high porosity and high conductivity, but also exhibits low dimensional change and balanced, reliable mechanical properties, making it particularly suitable for high-performance applications in battery separators.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and in particular to a method for preparing a high-porosity polyolefin microporous membrane that can be used as a positive and negative electrode separator for a battery. Background Art

[0002] In recent years, China's microporous membrane industry has entered a period of rapid growth, with its application in seawater desalination, water treatment, and wastewater reuse projects rapidly expanding. Several landmark large-scale membrane-based water supply, wastewater reuse, and seawater desalination projects have been completed. Furthermore, microporous membranes serve as separators in secondary batteries, separating the positive and negative electrodes and becoming one of the four key materials in battery construction. The performance of these membranes determines the battery's interface structure, internal resistance, and other properties.

[0003] The battery industry places higher demands on the performance of microporous membranes. The porosity and size distribution of microporous membranes are important factors affecting battery performance. Controlling the membrane porosity can achieve a balance between battery capacity and cycle life. Currently, the porosity of conventional commercial lithium battery separators is concentrated between 35% and 55%. Low porosity and / or small size affect electrolyte fluidity and ion shuttling, resulting in a reduced battery discharge rate and capacity. However, excessive porosity and / or size cannot prevent the flow of dissolved substances in the electrolyte, resulting in poor separation between the positive and negative electrodes, which in turn affects the battery's cycle life and makes it difficult to maintain high battery performance in practice. Excessive porosity can also cause the membrane to shrink, reducing its safety performance. Furthermore, battery manufacturing requires hot and cold deformation and winding processes, and the battery generates heat during operation. Therefore, microporous membranes must meet requirements for flexibility, temperature resistance, and deformation resistance at high porosity. Furthermore, the microporous membrane must eliminate anisotropy differences, maintaining essentially consistent performance indicators in both the transverse and longitudinal directions.

[0004] Existing microporous membrane preparation technologies struggle to achieve the aforementioned performance improvements for battery applications. Extrusion is currently the most commonly used and economical technique for producing microporous membranes. However, due to inherent processing challenges such as poor fluidity of polymer raw materials and uneven extrusion, improvements in porosity and flexibility are limited. Furthermore, the extrusion process can easily lead to increased anisotropy, all of which restrict the application of microporous membranes as battery separators. Optimized and developed microporous membrane preparation methods and products more suitable for battery separators are urgently needed.

[0005] For this purpose, this application is filed. Summary of the Invention

[0006] In view of the above problems in the prior art, the present invention proposes a method for preparing a high-porosity polyolefin microporous membrane. The obtained microporous membrane not only has high porosity and high conductivity, but also has low dimensional change and balanced and reliable mechanical properties.

[0007] The method for preparing a high-porosity polyolefin microporous membrane of the present invention comprises the following steps:

[0008] S1: Preparation of surface material: thoroughly mix the surface material including ultra-high molecular weight polyethylene (PE), antioxidant, plasticizer, and organic solvent, keep the mixture at 60-100°C, then continue to heat it to 170-200°C, vacuumize it after it is completely dissolved, and keep it warm for later use;

[0009] S2: Preparation of core layer material: Inorganic particle powder with a particle size of less than 0.05 μm is blended with polyethylene terephthalate (PET) to form a granulation masterbatch, and the granulation masterbatch is mixed with polypropylene (PP) for standby use; wherein the inorganic particles account for 0.5-5% by weight of the blended granulation raw materials, and the granulation masterbatch accounts for 10-25% of the total weight of the granulation masterbatch and PP;

[0010] S3: Using a three-layer co-extrusion method, the surface layer material is formed on both sides of the core layer material to obtain a hot film with a three-layer structure of surface layer-core layer-surface layer. After cooling and drying, it is longitudinally stretched, transversely stretched and heat-set to obtain a high-porosity polyolefin microporous membrane; wherein the core layer material accounts for 5-20% of the total mass of the core layer material and the surface layer material;

[0011] There is no particular order in which the above steps S1 and S2 are performed.

[0012] As some exemplary preferred solutions, the degree of untangling of the surface material prepared in step S1 is expressed in terms of motor torque, reaching a motor torque of 33.9 to 101.7 N.

[0013] As some exemplary preferred embodiments, ultra-high molecular weight polyethylene is selected from polyethylene with an average molecular weight of 0.5×10 6 ~2.0×10 6 g / mol, ultra-high molecular weight polyethylene (PE) with an intrinsic viscosity of 5.0 to 15.0 dl / g in decalin at 135°C.

[0014] As some exemplary preferred solutions, in step S1, the stirring rate during the process of fully mixing and dissolving the surface raw materials is 50 to 80 rpm, and the stirring rate during vacuuming after dissolution is 2 to 10 rpm.

[0015] As some exemplary preferred solutions, the surface raw materials are fully mixed and completely dissolved and then vacuumed to 0.0 to -0.05 MPa, which can meet good preservation requirements.

[0016] As some exemplary preferred embodiments, the antioxidant is selected from one or more of antioxidant 1010, antioxidant 1076, antioxidant 168, antioxidant 300, antioxidant GS, Weston 705, antioxidant 702, antioxidant 3114, and antioxidant 626.

[0017] As some exemplary preferred schemes, the plasticizer is selected from organic acid ester plasticizers, specifically selected from one or more of dioctyl phthalate, dibutyl phthalate, dioctyl sebacate, dioctyl sebacate, dioctyl adipate, acetyl tributyl citrate, dimethyl phthalate, tricresyl phosphate, and diisononyl phthalate.

[0018] As some exemplary preferred embodiments, the organic solvent is selected from hydronaphthalene organic solvents, specifically one or more selected from decahydronaphthalene and tetrahydronaphthalene.

[0019] As some exemplary preferred schemes, the ratio of the surface layer raw materials in S1 of the present invention is, by mass, PE 15-30%, antioxidant 0.075-0.5%, plasticizer 0.045-0.6%, organic solvent 60-90%, and the sum of the mass of the above surface layer raw materials is 100%.

[0020] As some exemplary preferred solutions, in S2 of the present invention, the mass proportion of inorganic particles in the blended granulation raw materials is 0.5-5%, and the granulation masterbatch accounts for 10-25% of the total mass of the granulation masterbatch and PP.

[0021] As some exemplary preferred embodiments, polypropylene is selected from isotactic polypropylene with an average molecular weight of 0.5×10 6 ~5.0×10 6 , the melt index is 1.0~5.0g / 10min.

[0022] As some exemplary preferred embodiments, the inorganic particles are selected from inorganic ceramic particles, specifically selected from one or more of montmorillonite, kaolinite, talc, sodium bicarbonate, sodium carbonate, calcium carbonate, barium sulfate, aluminum oxide, silicon dioxide, and magnesium oxide.

[0023] As some exemplary preferred solutions, in S3 of the present invention, the temperatures of the surface layer material and the core layer material in the extruder are 120-170° C. and 180-270° C., respectively.

[0024] As some exemplary preferred solutions, in S3 of the present invention, when the surface layer material and the core layer material are co-extruded, the temperature of the co-extrusion die is 170-270°C.

[0025] As some exemplary preferred solutions, in S3 of the present invention, the hot film cooling process adopts a rapid cooling method, and the rapid cooling temperature is 5 to 25°C.

[0026] As some exemplary preferred schemes, in S3 of the present invention, the longitudinal stretching ratio is 4 to 10 times, the temperature is 110 to 150°C, the transverse stretching ratio is 3 to 10 times, the temperature is 120 to 150°C, and the heat setting temperature is 110 to 130°C.

[0027] The present invention also provides a microporous membrane obtained by the above method, which has a porosity of ≥75%, a thickness of 6 to 100 microns, a surface layer with a uniform three-dimensional cross-fiber structure, a melon-shaped pore morphology, a pore diameter between 100 and 600 nm, and a longitudinal and transverse tensile strength greater than 480 kg f / cm 2 The thermal shrinkage in both longitudinal and transverse directions at 120℃×1h is no more than 3%.

[0028] Compared with the prior art, the present invention has at least the following beneficial effects:

[0029] The present invention combines three-layer co-extrusion technology, uses ultra-high molecular weight PE as the main material to prepare the surface layer, and simultaneously PET+inorganic particles are blended and granulated with PP to prepare the core layer, and optimizes the material ratio and two-stage process conditions, successfully achieving a synergistic improvement in the porosity, strength, and heat resistance of the microporous membrane, and effectively eliminating anisotropy differences. The comprehensive performance is highly improved, so that the microporous membrane obtained by the method of the present invention can particularly meet the high performance requirements of battery separators. The preparation method is convenient and stable, which is conducive to industrial production and has wider application potential and value. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0031] Figure 1 The following are comparisons of the appearance of a conventional commercial wet-process diaphragm and the microporous membrane of the present invention. Figure a shows the appearance of a conventional commercial wet-process diaphragm, and figure b shows the appearance of a microporous membrane of the present invention.

[0032] Figure 2 This is a diagram of the pore structure formed by the enlarged gap between the PET and PP phases in the core layer of the present invention. DETAILED DESCRIPTION

[0033] For ease of understanding, the microporous membrane of the present invention will be described in more detail below, and examples of the present invention will be given, but the scope of the present invention is not limited thereby.

[0034] It should be pointed out that although the microporous membrane of the present invention is optimized with the application direction of battery separators as the starting point, so that it can achieve more advantageous performance effects in batteries (such as lithium-ion batteries, semi-solid batteries, etc.) than general microporous membranes, its application is not limited to this. In related fields such as seawater desalination, medical selective filtration membranes, reverse osmosis membranes, etc., if the performance index requirements for microporous membranes are basically equivalent to those of the present invention, they can also be well applied.

[0035] The method for preparing a high-porosity polyolefin microporous membrane proposed in the present invention comprises the following steps:

[0036] S1: Preparation of surface material: thoroughly mix the surface material including ultra-high molecular weight polyethylene (PE), antioxidant, plasticizer, and organic solvent, keep the mixture at 60-100°C, then continue to heat it to 170-200°C, vacuumize it after it is completely dissolved, and keep it warm for later use;

[0037] S2: Preparation of core layer material: Inorganic particle powder with a particle size of less than 0.05 μm is blended with polyethylene terephthalate (PET) to form a granulation masterbatch, and the granulation masterbatch is mixed with polypropylene (PP) for standby use;

[0038] S3: Using a three-layer co-extrusion method, the surface layer material is formed on both sides of the core layer material to obtain a hot film with a three-layer structure of surface layer-core layer-surface layer. After cooling and drying, it is longitudinally stretched, transversely stretched and heat-set to obtain a high-porosity polyolefin microporous membrane; wherein the core layer material accounts for 5-20% of the total mass of the core layer material and the surface layer material;

[0039] There is no particular order in which the above steps S1 and S2 are performed.

[0040] The microporous membrane of the present invention adopts a three-layer structure design, and the selection and proportion of the three-layer structural materials are designed. On the basis of ensuring high porosity of the surface layer, the core layer's good heat resistance and mechanical properties are utilized to ensure balanced performance of the microporous membrane. The surface layer has a relatively large proportion, ensuring high porosity and contributing the vast majority of the porosity of the entire microporous membrane, which can ensure the fluidity of the electrolyte and provide the battery with excellent ionic conductivity. The core layer is relatively thin and has a lower porosity than the surface layer, which helps to block the flow of solutes in the electrolyte and achieve a good isolation effect. Therefore, the reasonable structural differentiation design achieves a good balance between battery safety and performance indicators.

[0041] The surface layer uses ultra-high molecular weight PE as the main material. The ultra-high molecular weight ensures the flexibility and later mechanical properties of the material. Through the two-stage heating and insulation design (after insulation at 60-100℃, continue to heat up to 170-200℃ for insulation), the polymer raw material can achieve good swelling in the first stage and good dissolution in the second stage. It swells and dissolves in high-boiling point solvents and is fully disentangled, which provides a guarantee for the ultra-high molecular weight PE to have good processing fluidity in the extrusion process. In the subsequent stretching process, the excellent disentanglement between molecules ensures the generation of high porosity. At the same time, because PE does not have a branched structure, it brings good copolymer orientation, avoiding the anisotropic differences of the microporous membrane.

[0042] The core layer first blends PET with small-size inorganic particles (particle size less than 0.05μm) to form granules. During the processing, the inorganic particles play a heterogeneous nucleation role on PET, which improves the crystallization behavior of PET and makes the particle size of the PET crystallization system smaller at a lower temperature. When the granulated masterbatch is blended with PP, PET carries the inorganic particles as a dispersed phase and is evenly dispersed in the PP phase. Due to the incompatibility between PET and PP crystal regions and amorphous regions, there is only a weak interfacial force between the two phases. Under the subsequent tensile force, the gap between PET and PP is enlarged, forming more pores around the dispersed phase with PET (such as Figure 2 ), further increasing the porosity of the core layer. Simultaneously, the PP resin forms a favorable coating with the PET during this process. After stretching, the benzene rings and inorganic particles in the PET structure contribute to the rigidity of the core layer, further enhancing the toughness and heat resistance of the microporous membrane. The core layer's PP also has a polymer structure more similar to that of the surface layer's PE, ensuring good compatibility and interlayer matching between the surface and core layers during three-layer coextrusion. Furthermore, the presence of more branched chains in PP promotes crosslinking with both PET and PP, resulting in excellent cohesion within and between layers.

[0043] On the basis of the above, the appropriate mass ratio of each layer of material ensures the good processability of each layer of material and the reasonable matching between layers, providing the premise and guarantee for obtaining a microporous membrane with improved comprehensive performance.

[0044] As some exemplary preferred schemes, the degree of disentanglement of the surface material prepared in step S1 is expressed as motor torque, and the torsion fraction is 15% to 45% under the extruder screw motor torque of 226N, that is, the motor torque reaches 33.9 to 101.7N.

[0045] The degree of disentanglement of the surface material mainly refers to the disentanglement of PE. The energy required for molecules to gather due to mutual attraction is called cohesive energy. Cohesive energy is the intermolecular force generated by van der Waals forces and hydrogen bonds, and is often used as the magnitude of intermolecular forces. The dissolution process of high molecular weight polymers can be understood as the process in which the intermolecular forces are broken up by solvent molecules. The dissolution process of high molecular weight polymers generally takes a long time, especially the dissolution process of ultra-high molecular weight PE. The small solvent molecules slowly penetrate into the interior of the resin particles at a certain temperature, which manifests as the particles swelling due to the entry of the solvent. This process is called the swelling stage. As the temperature rises, the solvent further penetrates into the molecular chains, weakening the strong interaction forces between the macromolecular chains. The more complete the solvation, the more the system enters the uniform dissolution stage. Finally, after the ultra-high molecular weight PE molecules are largely disentangled under the action of solvation, it presents a transparent state with a certain flow state. The degree of disentanglement not only affects the ease of extrusion processing but also, due to its correlation with molecular cohesion, influences whether the film forming process is affected by the extrusion force, resulting in anisotropic differences. An appropriate degree of disentanglement also facilitates crosslinking with the core PP layer while also maintaining the toughness and strength of the surface layer. This invention selects two suitable holding temperature ranges to promote the desired degree of disentanglement of PE.

[0046] As some exemplary preferred solutions, we also optimized the matching selection of ultra-high molecular weight polyethylene that can achieve the target degree of disentanglement, and selected the ultra-high molecular weight polyethylene with an average molecular weight of 0.5×10 6 ~2.0×10 6 g / mol, and an ultra-high molecular weight polyethylene (PE) with an intrinsic viscosity of 5.0 to 15.0 dl / g in decalin at 135° C. Suitable PE raw material selection is beneficial to ensure sufficient disentanglement under the conditions of the present invention that are easier to operate.

[0047] As some exemplary preferred schemes, in order to promote the thorough mixing of the raw materials during the untangling process, a certain stirring rate can be maintained. In step S1, the stirring rate can be 50 to 80 rpm during the process of sufficient mixing and dissolution of the surface raw materials, and the stirring rate during vacuuming after dissolution is 2 to 10 rpm.

[0048] The type of antioxidant in S1 of the present invention is generally not limited. Materials with good antioxidant properties for PE can be selected from existing technologies. As some exemplary preferred embodiments, one or more of antioxidant 1010, antioxidant 1076, antioxidant 168, antioxidant 300, antioxidant GS, Weston 705, antioxidant 702, antioxidant 3114, and antioxidant 626 can be selected.

[0049] The type of plasticizer in S1 of the present invention is generally not limited, and materials with good plasticizing effects on PE can be selected from the prior art. Among them, organic acid ester plasticizers belong to external plasticizers, which have the characteristics of high boiling point, low volatility, and the ability to form a solid solution with PE. They can be used as a preferred plasticizer of the present invention. As some exemplary preferred embodiments, they can be selected from one or more of dioctyl phthalate, dibutyl phthalate, dioctyl sebacate, dioctyl sebacate, dioctyl adipate, acetyl tributyl citrate, dimethyl phthalate, tricresyl phosphate, and diisononyl phthalate.

[0050] The type of organic solvent in S1 of the present invention is generally not limited. Materials that are well compatible with PE can be selected from existing technologies. Hydronaphthalene organic solvents, which also have a certain plasticizing and modifying effect on PE, can help adjust the flexibility and hardness of PE, are environmentally friendly, and are easily recyclable. Therefore, they can be used as a preferred organic solvent in the present invention. As some exemplary preferred embodiments, one or more of decalin and tetralin can be selected.

[0051] As some exemplary preferred schemes, the ratio of the surface layer raw materials in S1 of the present invention is, by mass, PE 15-30%, antioxidant 0.075-0.5%, plasticizer 0.045-0.6%, organic solvent 60-90%, and the sum of the mass of the above surface layer raw materials is 100%.

[0052] As some exemplary preferred embodiments, in S2 of the present invention, the inorganic particles comprise 0.5-5% by weight of the blended granulation raw materials, and the granulation masterbatch comprises 10-25% by weight of the total mass of the granulation masterbatch and PP. A suitable ratio of PP to inorganic particles facilitates the formation of a granulation masterbatch with a well-coated structure.

[0053] As some exemplary preferred solutions, we also optimized the PP that can better match the PE of S1, specifically isotactic polypropylene with an average molecular weight of 0.5×10 6 ~5.0×10 6 The melt index is 1.0-5.0 g / 10 min, which can achieve good interlayer bonding with the dissolved and disentangled PE during the three-layer co-extrusion process.

[0054] The type of inorganic particles in S2 of the present invention is generally not limited. Materials with good particle size and stable electrical properties can be selected from the prior art. Among them, inorganic ceramic particles can be used as a preferred material due to their excellent performance stability. As some exemplary preferred embodiments, they can be selected from one or more of montmorillonite, kaolinite, talc, sodium bicarbonate, sodium carbonate, calcium carbonate, barium sulfate, aluminum oxide, silicon dioxide, and magnesium oxide.

[0055] In order to further ensure the safe and stable formation of the microporous membrane structure during the three-layer co-extrusion process, the present invention further studies the optimized operating conditions of the three-layer co-extrusion process.

[0056] As some exemplary preferred schemes, in S3 of the present invention, the temperatures of the surface layer material and the core layer material in the extruder are maintained at 120-170°C and 180-270°C, respectively, so that the materials have good matching fluidity, formability, and interlayer bonding during extrusion.

[0057] As some exemplary preferred solutions, in S3 of the present invention, when the surface layer material and the core layer material are co-extruded, in order to balance the temperatures between the different materials, the temperature of the co-extrusion die can be maintained at 170-270°C.

[0058] As some exemplary preferred solutions, in S3 of the present invention, the hot film cooling process can adopt a rapid cooling method, and the temperature of the rapid cooling roller can be maintained at 5 to 25°C, so as to facilitate the rapid cooling and shaping of the hot film without causing cold brittleness.

[0059] As some exemplary preferred schemes, in S3 of the present invention, the longitudinal stretching, transverse stretching and heat setting processes are also processes for forming high porosity. In order to facilitate the stable formation and stable retention of high porosity, while taking into account the pore size and mechanical properties after film formation, the longitudinal stretching ratio is selected to be 4 to 10 times, the temperature is 110 to 150°C, the transverse stretching ratio is selected to be 3 to 10 times, the temperature is 120 to 150°C, and the heat setting temperature is 110 to 130°C.

[0060] The microporous membrane obtained by the present invention has been tested to achieve a porosity of ≥75%, a thickness of 6 to 100 microns, a uniform three-dimensional cross-fiber structure on the surface, a melon-shaped pore morphology, a pore diameter between 100 and 600 nm, and a longitudinal and transverse tensile strength greater than 480 kg f / cm 2 The thermal shrinkage in both longitudinal and transverse directions at 120℃×1h is no more than 3%.

[0061] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, and are not all embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0062] The chemical raw materials involved in the following examples and comparative examples are all prior art and are commercially available. The experimental devices, test devices, etc. involved in the following examples and comparative examples are all conventional devices in the art and are not particularly limited.

[0063] Example 1

[0064] S1: Preparation of surface layer material: a material containing an average molecular weight of 0.5×10 6 Polyethylene (PE), antioxidant 1010, dioctyl phthalate, and tetralin were added to a premixing kettle in a ratio of 30%, 0.3%, 0.2%, and 69.5% by mass, and mixed thoroughly at a stirring rate of 50 rpm. After being kept at 70°C for 2 h, the temperature was further increased to 175°C at a heating rate of 10°C / 0.5 h for full dissolution. After complete dissolution, the stirring rate was reduced to 5 rpm, and the mixture was quickly evacuated to -0.05 MPa and kept warm for later use.

[0065] S2: Preparation of core layer material: montmorillonite granules with a particle size of less than 0.05 μm and polyethylene terephthalate (PET) are blended and granulated in a mass ratio of 1.8:98.2 to form a granulation masterbatch, and the granulation masterbatch is mixed with polypropylene (PP) in a mass ratio of 1:9 for standby use;

[0066] S3: The surface layer material obtained in step S1 and the core layer material obtained in step S2 are respectively fed into an extruder for melt extrusion. The extruder uses a three-layer die for co-extrusion. The temperature of the die is set to 180°C. The surface layer material at the three-layer die is symmetrically formed on both sides of the core layer material to obtain a hot film with a three-layer structure of surface layer-core layer-surface layer. The ratio of the surface layer material to the core layer material is 85:15. The hot film is cast into a primary sheet on a 20°C rapid cooling roller. The primary sheet after casting is fed into a hot air drying system to remove the solvent from the surface layer, and then the primary sheet is longitudinally stretched, transversely stretched and heat-set respectively. The longitudinal stretching ratio is 30 times, the temperature is 120°C, the transverse stretching ratio is 6 times, the temperature is 130°C, and the heat setting temperature is 110°C. After cooling and winding, it is slit to obtain a high-porosity polyolefin microporous membrane. The thickness of the obtained microporous membrane is 6 microns.

[0067] Examples 2 to 8

[0068] The preparation parameters of Example 1 were changed to obtain surface materials with different compositions, and the differences in the degree of disentanglement of the surface materials with different compositions were detected.

[0069] Comparative Examples 1-2

[0070] The preparation parameters of Example 1 were changed to obtain surface materials with different compositions, and the differences in the degree of disentanglement of the surface materials with different compositions were detected.

[0071] The differences in the preparation of the surface layer materials of Examples 1 to 8 and Comparative Examples 1 to 2 and the evaluation results of the degree of disentanglement are shown in Table 1.

[0072] Table 1

[0073]

[0074]

[0075] Table 1 provides the effects of different surface material compositions and preparation conditions on the degree of PE disentanglement. The full torque of the screw motor of the surface extruder used in the present invention is 226N, and the appropriate degree of disentanglement enables the surface material to be extruded and molded at an appropriate torque. As shown in Examples 1 to 8, the surface material composition selected by the present invention can obtain an appropriate degree of disentanglement by a two-stage heat preservation method, and is molded under an appropriate torque. The torque is moderate to ensure that the surface molding eliminates the orientation effect and maintains sufficient toughness and strength of the surface itself. Comparative Examples 1 to 2 show that inappropriate material composition and preparation conditions lead to excessive torque in the molding process. Although excessive force ensures the smooth molding of the surface, it easily leads to molecular reorientation in the extrusion molding direction, which is not conducive to the uniformity of tensile strength in MD and TD.

[0076] Figure 1 Shows the surface morphology of traditional commercial wet-process diaphragm ( Figure 1 a) and the apparent morphology of the microporous membrane of the present invention ( Figure 1 b) Structural differences. It can be seen that the PE surface layer produced by traditional commercial wet-process diaphragms has relatively coarse continuous polymer bundles, poor PE molecular disentanglement, and large pores after stretching and shaping, with a low porosity. The microporous membrane of the present invention has fewer and thinner continuous polymer bundles (three-dimensional cross-fiber structure), providing sufficient strength support while introducing more pores. The pore morphology is melon-like, matching good tensile elasticity, and the pore diameter is between 100-600nm.

[0077] Examples 9 to 19

[0078] The preparation parameters of Example 1 were changed to obtain core layer materials with different compositions, and the effects of the core layer materials with different compositions on the comprehensive properties of the microporous membrane were tested.

[0079] Comparative Examples 3-4

[0080] The preparation parameters of Example 1 were changed to obtain core layer materials with different compositions, and the effects of the core layer materials with different compositions on the comprehensive properties of the microporous membrane were tested.

[0081] The differences in the preparation of the core layer materials of Examples 1, 9 to 19 and Comparative Examples 3 to 4 and their effects on the comprehensive properties of the microporous membrane are shown in Table 2. In the present invention, the porosity, tensile strength and thermal shrinkage of the microporous membrane are all in accordance with GB / T36363-2018.

[0082] Table 2

[0083]

[0084]

[0085] Table 2 demonstrates the impact of pre-granulating inorganic particles with PET on the comprehensive performance of the microporous membrane. Examples 9-19 demonstrate that the granulated masterbatch obtained within the control range of the present invention can achieve higher porosity, tensile strength, and thermal shrinkage, with minimal performance differences between the MD and TD directions. Comparative Example 3, in which the inorganic particles are added in a non-granulated form, uses montmorillonite, PET, and PP for direct co-extrusion. Because the PET is not pre-coated with the inorganic particles and a two-phase interface of PET and PP supporting the dispersed phase cannot be formed, the dispersion and crystallization behavior of the material are altered. More inorganic particles come into direct contact with PP, significantly reducing the ability to form and orderly expand pores. This reduces not only the porosity but also the tensile strength and thermal shrinkage of the material, and significantly increases the anisotropy difference. Comparative Example 4, in which inorganic particles are not introduced into the core layer material, eliminates the point-like dispersion and support effect of the inorganic particles in the core layer, reduces the crystallization behavior of PET, weakens the interfacial effect with PP, and the difficulty of dispersing the PP resin becomes the dominant factor. The performance indicators of the resulting membrane structure are also significantly reduced in various aspects.

[0086] Examples 20 to 27

[0087] In order to further comprehensively evaluate the influence of the surface layer and the core layer on the overall comprehensive performance within the control range of the present invention, the preparation parameters of Example 1 were further changed to obtain surface layer materials and core layer materials with different compositions, and the influence of different material compositions on the comprehensive performance of the microporous membrane was detected.

[0088] Comparative Examples 5 to 7

[0089] By changing the preparation parameters of Example 1, surface layer materials and core layer materials with different compositions were obtained, and the effects of different material compositions on the comprehensive properties of the microporous membrane were tested.

[0090] The differences in material preparation between Examples 20 to 27 and Comparative Examples 5 to 7 and their effects on the comprehensive properties of the microporous membrane are shown in Table 3.

[0091] Table 3

[0092]

[0093]

[0094] Table 3 confirms that within the control range of the present invention, a microporous membrane material with excellent comprehensive performance can be obtained, high porosity is guaranteed and improved, and as a battery separator, it exhibits excellent ionic conductivity, indicating that the stable structure and appropriate pores promote the efficient transmission of ions in the separator. Comparative Examples 5-6 respectively demonstrate the impact of excessive matching on performance. Among them, the change in molecular weight in Comparative Example 5 may cause a change in the degree of disentanglement, as well as the compatible matching between layers in the three-layer co-extrusion process. The imbalance in the proportion of the surface layer and the core layer materials in Comparative Example 6 may lead to insufficient porosity. Furthermore, Comparative Example 7 uses PE to replace PET to coat and granulate the inorganic particles, and the interface effect of PET and PP disappears. It can be seen that the intermediate interface effect of PET and its special structure and performance also play an irreplaceable role in the microporous membrane system of the present invention and cannot be easily replaced.

[0095] The various technical features of the above-described embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the various technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification. Although the embodiments of the present invention have been shown and described above, it can be understood that the above-described embodiments are exemplary and cannot be understood as limitations of the present invention. Those skilled in the art can change, modify, replace and deform the above-described embodiments within the scope of the present invention. In addition, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples, unless they are contradictory.

Claims

1. A method for preparing a high-porosity polyolefin microporous membrane for a battery separator, comprising the following steps: S1: Preparation of surface material: The surface material comprising ultra-high molecular weight polyethylene, antioxidant, plasticizer and organic solvent is fully mixed, kept warm at 60-100°C, and then continued to heat to 170-200°C. After complete dissolution, vacuum is drawn and kept warm for use; the ultra-high molecular weight polyethylene is selected from polyethylene with an average molecular weight of 0.5×10 6 ~2.0×10 6 g / mol, ultra-high molecular weight polyethylene with an intrinsic viscosity of 5.0 to 15.0 dl / g in decalin at 135°C; S2: Preparation of core layer material: Inorganic particle powder with a particle size of less than 0.05µm is blended with polyethylene terephthalate to form a granulation masterbatch, which is then mixed with polypropylene for later use; the inorganic particles account for 0.5-5% by weight of the blended granulation raw materials, and the granulation masterbatch accounts for 10-25% of the total weight of the granulation masterbatch and PP; S3: Using a three-layer co-extrusion method, the surface layer material is formed on both sides of the core layer material to obtain a hot film with a surface layer-core layer-surface layer three-layer structure, and after cooling and drying, longitudinal stretching, transverse stretching and heat setting are performed to obtain a high-porosity polyolefin microporous membrane; wherein, The core material accounts for 5-20% of the total mass of the core material and the surface material; There is no particular order in which the above steps S1 and S2 are performed.

2. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 1, wherein: The degree of untangling of the surface material prepared in step S1 is expressed in terms of motor torque, reaching a motor torque of 33.9-101.7N.

3. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 1, wherein: In step S1, the stirring rate is 50-80 rpm during the process of fully mixing and dissolving the surface raw materials, and the stirring rate is 2-10 rpm during vacuuming after dissolution.

4. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 1, wherein: In step S1, the surface raw materials are fully mixed and completely dissolved, and then vacuumed to 0.0~-0.05MPa.

5. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 1, wherein: The antioxidant is selected from one or more of antioxidant 1010, antioxidant 1076, antioxidant 168, antioxidant 300, antioxidant GS, Weston 705, antioxidant 702, antioxidant 3114, and antioxidant 626.

6. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 1, characterized in that: The plasticizer is selected from organic acid ester plasticizers.

7. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 6, characterized in that: The organic acid ester plasticizer is selected from one or more of dioctyl phthalate, dibutyl phthalate, dioctyl sebacate, dioctyl sebacate, dioctyl adipate, acetyl tributyl citrate, dimethyl phthalate, tricresyl phosphate, and diisononyl phthalate.

8. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 1, characterized in that: The organic solvent is selected from hydrogenated naphthalene organic solvents.

9. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 8, characterized in that: The hydronaphthalene organic solvent is selected from one or more of decahydronaphthalene and tetrahydronaphthalene.

10. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 1, characterized in that: The ratio of the surface layer raw materials in step S1 is, by mass, 15-30% ultra-high molecular weight polyethylene, 0.075-0.5% antioxidant, 0.045-0.6% plasticizer, and 60-90% organic solvent, and the sum of the mass of the above surface layer raw materials is 100%.

11. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 1, characterized in that: The inorganic particles are selected from inorganic ceramic particles.

12. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 11, characterized in that: The inorganic ceramic particles are selected from one or more of montmorillonite, kaolinite, talc, sodium bicarbonate, sodium carbonate, calcium carbonate, barium sulfate, aluminum oxide, silicon dioxide, and magnesium oxide.

13. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 1, characterized in that: Polypropylene is isotactic polypropylene with an average molecular weight of 0.5×10 6 ~5.0×10 6 , the melt index is 1.0~5.0g / 10min.

14. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 1, characterized in that: In step S3, the temperatures of the surface layer material and the core layer material in the extruder are 120-170° C. and 180-270° C., respectively.

15. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 1, characterized in that: In step S3, when the surface layer material and the core layer material are co-extruded, the temperature of the co-extrusion die is 170-270°C.

16. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 1, characterized in that: In step S3, the hot film cooling process adopts a rapid cooling method, and the rapid cooling temperature is 5~25℃.

17. The method for preparing a high-porosity polyolefin microporous membrane for a battery separator according to claim 1, characterized in that: In step S3, the longitudinal stretching ratio is 4 to 10 times, the temperature is 110 to 150°C, the transverse stretching ratio is 3 to 10 times, the temperature is 120 to 150°C, and the heat setting temperature is 110 to 130°C.

18. A microporous membrane obtained by the preparation method according to any one of claims 1 to 17, having a porosity of ≥75%, a thickness of 6 to 100 microns, a surface layer having a uniform three-dimensional cross-fiber structure, a melon-like pore morphology, a pore size between 100 and 600 nm, and a longitudinal and transverse tensile strength greater than 480 kg f / cm 2 The thermal shrinkage in both longitudinal and transverse directions at 120℃×1h is no more than 3%.

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