A dry double-drawing composite diaphragm, a preparation method and application thereof

Abstract

The application provides a dry double-drawing composite diaphragm and a preparation method and application thereof. The dry double-drawing composite diaphragm comprises a polypropylene-polyethylene blending layer and polypropylene layers arranged on both sides of the polypropylene-polyethylene blending layer. The dry double-drawing composite diaphragm realizes complementary advantages of PP and PE materials through a sandwich type multi-layer composite structure of PP / PE+PP / PP, the PP high-strength skeleton layers on both sides can provide the diaphragm with super strong anti-penetration ability and super high film breaking temperature, and the PE+PP functional inner container layer in the middle can provide the diaphragm with consistent pore size and low closed pore temperature, so that the diaphragm has an extremely wide reliable temperature safety interval.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a dry-process biaxially stretched composite separator, its preparation method, and its application. Background Technology

[0002] The separator is a crucial component of lithium-ion batteries. As an insulating layer, it separates the positive and negative electrode materials while providing a pathway for lithium-ion migration, thus highlighting its safety and functionality. The main raw materials for separator production are PP and PE (polypropylene and polyethylene). Due to significant differences in their physical properties, their processing techniques and applications differ. PP is used in dry-process membrane fabrication because of its higher melting point and stronger puncture resistance, making it suitable for high-rate batteries with higher safety requirements. PE is used in wet-process membrane fabrication because of its consistent pore structure and thinner profile, making it suitable for high-density batteries with higher requirements for cell consistency and energy density.

[0003] The raw materials for membrane production, PP and PE (polypropylene and polyethylene), have significant differences in physical properties, resulting in different processing techniques and application areas. Due to the material limitations, there are two main problems: First, because PP and PE have different molecular structures and physicochemical properties, they have significantly different advantages in processing characteristics and functional properties. Second, their molten systems are difficult to be compatible and composited. Due to poor adhesion at the compatibility interface, the composite film may delaminate and become unusable.

[0004] CN115939660A discloses a continuous and efficient dry process for preparing large-capacity PP composite battery separators, belonging to the field of battery separator processing technology. The process includes the following steps: weighing 20-30 parts by weight of butyl acrylate, 20-30 parts by weight of 3-(methacryloyloxy)propyltrimethoxysilane, 60-90 parts by weight of toluene, and 0.5-0.7 parts by weight of initiator, and adding them to a three-necked flask for stirring. This invention not only effectively simplifies the production process of PP composite battery separators and improves production efficiency, but also improves their tensile strength and puncture strength to a certain extent. However, the highest puncture strength is only 0.41 N / μm, indicating that this method has limited effect on improving the mechanical strength of the separator. Furthermore, because the molten systems of the two components are difficult to integrate, the composite film suffers from poor adhesion at the interface, leading to separator delamination, unusable conditions, or short cycle life.

[0005] CN111180635A discloses a method for producing PP / PE / PP lithium battery power separators using a casting method, relating to the field of lithium battery separator processing technology, including the following steps: (1) raw material preparation; (2) melt extrusion; (3) co-extrusion casting; (4) stretching; (5) heat shrinking; (6) heat setting. This invention uses polypropylene and polyethylene as the main raw materials to prepare PP / PE / PP lithium battery power separators by co-extrusion casting. The raw materials are readily available, the processing method is simple, and the process repeatability and stability are good. The separator produced by this invention has good thermal stability. However, the required thickness of the separator needs to reach 20-30μm to improve the rupture temperature and needle penetration strength. However, there are many drawbacks to the battery separator being too thick. First, the PP / PE melt system itself is difficult to be compatible and composite. The thickness of the separator makes it easier for the two to composite. Second, the thickness of the separator will inevitably sacrifice some electrochemical performance.

[0006] Therefore, how to achieve the complementary advantages of PP and PE materials and the compatibility of the composite system are the two major bottleneck problems in improving the safety and composite functionality of the diaphragm.

[0007] In view of this, the present invention is hereby proposed. Summary of the Invention

[0008] One of the objectives of this invention is to provide a dry-process biaxially stretched composite separator. This invention achieves complementary functionalities of PP and PE materials through a sandwich-style multilayer composite structure of PP / PE + PP / PP, effectively solving the two major bottleneck problems of how to achieve complementary advantages of PP and PE materials and compatibility of the composite system, which are crucial for improving separator safety and composite functionality.

[0009] The second objective of this invention is to provide a method for preparing a dry biaxially stretched composite separator. The method employs a dry biaxial stretching process, resulting in a composite separator with consistent pore size and a low pore-closing temperature, thus providing the separator with an extremely wide reliable temperature safety range. It also enables the composite casting of melts from different materials, enhancing the peel strength between different layers.

[0010] The third objective of this invention is to provide an application of a dry-process biaxially stretched composite separator in the preparation of lithium-ion batteries.

[0011] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0012] In a first aspect, the present invention provides a dry biaxially stretched composite separator, the dry biaxially stretched composite separator comprising a polypropylene-polyethylene blend layer and polypropylene layers disposed on both sides of the polypropylene-polyethylene blend layer.

[0013] In this invention, the sandwich-style multilayer composite structure of PP / PE+PP / PP achieves complementary advantages of PP and PE materials. The high-strength PP skeleton layers on both sides can provide the separator with super strong resistance to needle puncture and super high membrane rupture temperature, so that the separator is not punctured by foreign objects and burrs or melted and ruptured, which would lead to short circuit between the positive and negative electrodes. The middle PE+PP functional inner liner layer can provide the separator with a consistent pore size and a low pore-closing temperature. When the battery experiences short circuit thermal runaway, the PE can melt and close the pores in time, separating the positive and negative electrodes and avoiding the safety risk of combustion or explosion caused by the cell continuing to discharge and heat up.

[0014] The raw materials for preparing the polyethylene-polypropylene blend layer include, by weight, 5-25 parts of polypropylene, 75-95 parts of polyethylene, 0.01-5 parts of β-crystal nucleating agent, 0.01-5 parts of plasticizer, and 0.01-5 parts of solubilizer.

[0015] In this invention, an appropriate amount of β-crystal nucleating agent, plasticizer and solubilizer are added to the intermediate polyethylene-polypropylene blend layer. While enhancing its plasticizing effect, this can improve the compatibility between the intermediate functional layer and the outer skeleton layer, realize the composite casting of different material melts, and enhance the peel strength between different layers.

[0016] In the raw materials for preparing the polyethylene-polypropylene blend layer, the polypropylene content is 5 to 25 parts, for example, it can be 5 parts, 6 parts, 8 parts, 10 parts, 12 parts, 14 parts, 16 parts, 18 parts, 20 parts, 22 parts, 25 parts, etc.

[0017] In the raw materials for preparing the polyethylene-polypropylene blend layer, the content of polyethylene is 75 to 95 parts, for example, it can be 75 parts, 76 parts, 78 parts, 80 parts, 82 parts, 84 parts, 86 parts, 88 parts, 90 parts, 92 parts, 94 parts, 95 parts, etc.

[0018] In the raw materials for preparing the polyethylene-polypropylene blend layer, the content of the β-crystal nucleating agent is 0.01 to 5 parts, for example, it can be 0.01 parts, 0.05 parts, 0.1 parts, 0.2 parts, 0.5 parts, 0.6 parts, 0.8 parts, 1 part, 1.2 parts, 1.5 parts, 1.8 parts, 2 parts, 2.5 parts, 3 parts, 3.5 parts, 4 parts, 4.5 parts, 5 parts, etc.

[0019] In the raw materials for preparing the polyethylene-polypropylene blend layer, the content of plasticizer is 0.01 to 5 parts, for example, it can be 0.01 parts, 0.05 parts, 0.1 parts, 0.2 parts, 0.5 parts, 0.6 parts, 0.8 parts, 1 part, 1.2 parts, 1.5 parts, 1.8 parts, 2 parts, 2.5 parts, 3 parts, 3.5 parts, 4 parts, 4.5 parts, 5 parts, etc.

[0020] In the raw materials for preparing the polyethylene-polypropylene blend layer, the content of the solubilizer is 0.01 to 5 parts, for example, it can be 0.01 parts, 0.05 parts, 0.1 parts, 0.2 parts, 0.5 parts, 0.6 parts, 0.8 parts, 1 part, 1.2 parts, 1.5 parts, 1.8 parts, 2 parts, 2.5 parts, 3 parts, 3.5 parts, 4 parts, 4.5 parts, 5 parts, etc.

[0021] The β-crystal nucleating agent includes pimelic acid and calcium stearate.

[0022] In this invention, the β-crystal nucleating agent is a mixture of pimelic acid and calcium stearate. This composite β-crystal nucleating agent can induce PP and / or PE to form β-crystals, thereby improving mechanical properties such as tensile fracture strength, needle penetration strength, and impact toughness. At the same time, it was unexpectedly found that it also has an effect on improving the compatibility of PP and PE after blending.

[0023] Preferably, the mass ratio of pimelic acid to calcium stearate is (0.8–3.8):(1.2–4.2);

[0024] Among them, "0.8~3.8" can be, for example, 0.8, 1, 1.5, 2, 2.5, 3, 3.8, etc.;

[0025] Among them, "1.2~4.2" can be, for example, 1.2, 1.5, 2, 2.5, 3, 3.8, 4, 4.2, etc.

[0026] In this invention, when the mass ratio of pimelic acid to calcium stearate is within the above range, it is more conducive to the formation of β crystal form in PP and / or PE, thereby further improving mechanical properties such as tensile fracture strength, needle penetration strength, and impact toughness.

[0027] The plasticizer includes any one or a combination of at least two of the following: white oil, dioctyl phthalate, dioctyl terephthalate, dibutyl phthalate, or diethylhexyl phthalate.

[0028] Preferably, the plasticizer comprises white oil and dioctyl terephthalate.

[0029] In this invention, the plasticizer is a mixture of white oil and dioctyl terephthalate. The white oil and dioctyl terephthalate work together to have a synergistic effect, which not only significantly enhances the plasticizing effect, but also improves the compatibility between the intermediate functional layer and the outer skeleton layer, thereby significantly improving the peel strength between different layers.

[0030] Preferably, the mass ratio of the white oil to dioctyl terephthalate is (3-4):(6-7);

[0031] Among them, "3 to 4" can be, for example, 3, 3.2, 3.4, 3.6, 3.8, 4, etc.;

[0032] Among them, "6 to 7" can be, for example, 6, 6.2, 6.4, 6.6, 6.8, 7, etc.

[0033] The solubilizer includes ethylene-propylene copolymer and / or ethylene-octene copolymer.

[0034] Preferably, the solubilizer includes ethylene-propylene copolymer and ethylene-octene copolymer.

[0035] In this invention, the solubilizer is a mixture of ethylene-propylene copolymer and ethylene-octene copolymer. The ethylene-propylene copolymer and ethylene-octene copolymer work together to have a synergistic effect, which can further improve the compatibility between PP and PE in the polyethylene-polypropylene blend layer, making the cast extrusion blend more uniform and chemically bonded to each other, thereby further improving the stability of the intermediate layer itself, indirectly improving the compatibility between the intermediate functional layer and the outer skeleton layer, and significantly improving the peel strength between different layers.

[0036] Preferably, the mass ratio of the ethylene-propylene copolymer to the ethylene-octene copolymer is (8-9):(1-2);

[0037] Among them, "8-9" can be, for example, 8.2, 8.4, 8.6, 8.8, or 9;

[0038] Among them, "1 to 2" can be, for example, 1, 1.2, 1.4, 1.6, 1.8, 2, etc.

[0039] Preferably, the raw materials for preparing the polypropylene layer include, by weight, 99-99.9 parts of polypropylene and 0.1-1 parts of β-crystal nucleating agent.

[0040] The polypropylene content in the raw materials for preparing the polypropylene layer is 99 to 99.9 parts, for example, it can be 99 parts, 99.1 parts, 99.2 parts, 99.3 parts, 99.4 parts, 99.5 parts, 99.6 parts, 99.7 parts, 99.8 parts, 99.9 parts, etc.

[0041] The content of β-crystal nucleating agent in the raw materials for preparing the polypropylene layer is 0.1 to 1 part, for example, it can be 0.1 part, 0.2 part, 0.3 part, 0.4 part, 0.5 part, 0.6 part, 0.7 part, 0.8 part, 0.9 part, 1 part, etc.

[0042] The β-crystal nucleating agent includes pimelic acid and calcium stearate.

[0043] Preferably, the polypropylene is isotactic polypropylene.

[0044] Preferably, the weight-average molecular weight of the isotactic polypropylene is 500,000 to 1,300,000, for example, it can be 500,000, 600,000, 700,000, 800,000, 900,000, 1,000,000, 1,100,000, 1,200,000, 1,300,000, etc.

[0045] Preferably, the melt index of the isotactic polypropylene is 0.5 to 2 g / 10 min, for example, it can be 0.5 g / 10 min, 0.6 g / 10 min, 0.8 g / 10 min, 1 g / 10 min, 1.2 g / 10 min, 1.4 g / 10 min, 1.6 g / 10 min, 1.8 g / 10 min, 2 g / 10 min, etc.

[0046] Preferably, the polyethylene is ultra-high molecular weight polyethylene.

[0047] Preferably, the weight-average molecular weight of the ultra-high molecular weight polyethylene is 600,000 to 1,500,000, for example, it can be 600,000, 700,000, 800,000, 900,000, 1,000,000, 1,100,000, 1,200,000, 1,300,000, 1,400,000, 1,500,000, etc.

[0048] Preferably, the melt index of the ultra-high molecular weight polyethylene is 0.1 to 1 g / 10 min, for example, it can be 0.1 g / 10 min, 0.2 g / 10 min, 0.4 g / 10 min, 0.5 g / 10 min, 0.6 g / 10 min, 0.8 g / 10 min, 1 g / 10 min, etc.

[0049] Preferably, the raw materials for preparing the polyethylene-polypropylene blend layer further include 0.01 to 5 parts of antioxidant.

[0050] The antioxidant content in the raw materials for preparing the polyethylene-polypropylene blend layer is 0.01 to 5 parts, for example, it can be 0.01 parts, 0.05 parts, 0.1 parts, 0.2 parts, 0.5 parts, 0.6 parts, 0.8 parts, 1 part, 1.2 parts, 1.5 parts, 1.8 parts, 2 parts, 2.5 parts, 3 parts, 3.5 parts, 4 parts, 4.5 parts, 5 parts, etc.

[0051] The antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].

[0052] Preferably, the mass ratio of the raw materials used to prepare the polypropylene layer and the polypropylene-polyethylene blend layer is (2-5):(5-8);

[0053] Among them, "2 to 5" can be, for example, 2, 2.5, 3, 3.5, 4, 4.5, 5, etc.;

[0054] Among them, "5 to 8" can be, for example, 5, 5.5, 6, 6.5, 7, 7.5, 8, etc.

[0055] Preferably, the thickness of the dry biaxially stretched composite diaphragm is 10-15 μm, for example, it can be 10 μm, 10.5 μm, 11 μm, 11.5 μm, 12 μm, 12.5 μm, 13 μm, 13.5 μm, 14 μm, 14.5 μm, 15 μm, etc., and is preferably 11.77-12.55 μm.

[0056] Preferably, the porosity of the dry biaxially stretched composite membrane is 45-55%, for example, it can be 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, etc., and is more preferably 45.2-51.2%.

[0057] Preferably, the air permeability of the dry-process biaxially stretched composite diaphragm is 100-160 s / 100cc, for example, it can be 100 s / 100cc, 105 s / 100cc, 110 s / 100cc, 115 s / 100cc, 120 s / 100cc, 125 s / 100cc, 130 s / 100cc, 135 s / 100cc, 140 s / 100cc, 145 s / 100cc, 150 s / 100cc, 155 s / 100cc, 160 s / 100cc, etc., and is preferably 105-155 s / 100cc.

[0058] Preferably, the needle penetration strength of the dry biaxially stretched composite diaphragm is 0.75 to 0.90 N / μm, for example, it can be 0.75 N / μm, 0.78 N / μm, 0.80 N / μm, 0.82 N / μm, 0.84 N / μm, 0.86 N / μm, 0.88 N / μm, 0.90 N / μm, etc., and is more preferably 0.79 to 0.87 N / μm.

[0059] Preferably, the reliable temperature safety range of the dry biaxially stretched composite diaphragm is 30 to 60°C, for example, it can be 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, etc., and preferably 35 to 53°C.

[0060] In a second aspect, the present invention provides a dry-process biaxially stretched composite separator as described in the first aspect, wherein the preparation method of the dry-process biaxially stretched composite separator includes the following steps:

[0061] (1) The raw materials for preparing the polypropylene-polyethylene blend layer and the raw materials for preparing the polypropylene layer are heated and plasticized to form a homogeneous melt, and then cast and extruded together. After cooling, a crystalline cast sheet is obtained.

[0062] (2) The crystalline casting obtained in step (1) is stretched to obtain a porous thin film precursor with highly oriented crystals;

[0063] The stretching is either bidirectional synchronous stretching or longitudinal stretching followed by transverse stretching.

[0064] (3) The highly oriented porous film precursor obtained in step (2) is stretched laterally, and then subjected to heat shrinkage and heat setting in sequence to obtain the dry bi-stretched composite diaphragm.

[0065] In this invention, during the preparation process of step (1), the outer layer of PP is cast and cooled to crystallize. Under the action of the β-crystal nucleating agent, it forms a loosely arranged, divergent bundle-like lamellar structure, which has high flexibility and ductility. The PP and PE blended in the middle layer crystallize and aggregate in phases due to their density difference.

[0066] In this invention, during the preparation process of step (2), the β crystals of the outer PP layer gradually transform into dense and stable α crystals under the action of heating and tensile stress. Since the α crystals are denser than the β crystals, volume shrinkage occurs during the crystal transformation, resulting in pores. Due to the crystal form difference between PE and PP, the middle layer slips during the stress stretching process, forming pores, thus obtaining a thin film with highly oriented crystals.

[0067] In this invention, during the preparation process of step (3), a small-ratio stretching is performed. Through the dual effects of heating and orientation, the crystallinity of the film is further improved, the microporous structure of the film is solidified, and the strength of the film is increased. In addition, heating and shrinking can eliminate the internal stress generated during the stretching process and improve the thermal stability of the film.

[0068] Preferably, in step (1), the heating time for the raw materials used to prepare the polypropylene layer to form a homogeneous melt is 30 to 60 seconds, for example, 30 seconds, 35 seconds, 40 seconds, 45 seconds, 50 seconds, 55 seconds, 60 seconds, etc., and the heating temperature is 200 to 240 degrees Celsius, for example, 200 degrees Celsius, 205 degrees Celsius, 210 degrees Celsius, 215 degrees Celsius, 220 degrees Celsius, 225 degrees Celsius, 230 degrees Celsius, 235 degrees Celsius, 240 degrees Celsius, etc.

[0069] Preferably, in step (1), the heating time for the raw materials used to prepare the polypropylene-polyethylene blend layer to form a homogeneous melt is 30 to 60 seconds, for example, 30 seconds, 35 seconds, 40 seconds, 45 seconds, 50 seconds, 55 seconds, 60 seconds, etc., and the heating temperature is 190 to 210 degrees Celsius, for example, 190 degrees Celsius, 195 degrees Celsius, 200 degrees Celsius, 205 degrees Celsius, 210 degrees Celsius, etc.

[0070] Preferably, in step (1), the heating and plasticizing to form a homogeneous melt is carried out using a twin-screw extruder. The screw speed of the twin-screw extruder is 100-240 rpm, for example, 100 rpm, 120 rpm, 140 rpm, 160 rpm, 180 rpm, 200 rpm, 220 rpm, 240 rpm, etc. The screw diameter is 32-56 mm, for example, 32 mm, 34 mm, 36 mm, 38 mm, 40 mm, 42 mm, 44 mm, 46 mm, 48 mm, 50 mm, 52 mm, 54 mm, 56 mm, etc. The screw length-to-diameter ratio is 56-68, for example, 56, 58, 60, 62, 64, 66, 68, etc.

[0071] Preferably, in step (1), the co-cast extrusion is performed through a T-shaped three-layer composite die head, and the heating temperature of the die head is 200-220℃, for example, 200℃, 205℃, 210℃, 215℃, 220℃, etc.

[0072] Preferably, in step (1), the cooling temperature is 15 to 25°C, for example, it can be 15°C, 16°C, 18°C, 20°C, 22°C, 25°C, etc.

[0073] Preferably, in step (2), the stretching ratio is 5 to 15 times, for example, it can be 5 times, 6 times, 8 times, 10 times, 12 times, 15 times, etc.

[0074] Preferably, in step (2), the bidirectional synchronous stretching is: bidirectional synchronous stretching at 130-160℃ (e.g., 130℃, 135℃, 140℃, 145℃, 150℃, 155℃, 160℃, etc.) at a speed of 10-150% / s (e.g., 10% / s, 20% / s, 40% / s, 60% / s, 80% / s, 100% / s, 120% / s, 150% / s, etc.).

[0075] Preferably, in step (2), the longitudinal stretching followed by the transverse stretching is performed at 110–130°C (e.g., 110°C, 115°C, 120°C, 125°C, 130°C, etc.) at a speed of 25–250% / s (e.g., 25% / s, 40% / s, 60% / s, 80% / s, 100% / s, 120% / s, 150% / s, 160% / s, 180% / s, 2...). The medium is stretched longitudinally at a speed of 00% / s, 220% / s, 250% / s, etc.; then stretched laterally at a speed of 10% / s (e.g., 10% / s, 20% / s, 40% / s, 60% / s, 80% / s, 100% / s, 120% / s, 150% / s, etc.) at 140-160℃ (e.g., 140℃, 145℃, 150℃, 155℃, 160℃, etc.).

[0076] Preferably, in step (3), the stretching ratio of the transverse stretching is 1.2 to 2 times, for example, it can be 1.2 times, 1.3 times, 1.4 times, 1.5 times, 1.6 times, 1.7 times, 1.8 times, 1.9 times, 2 times, etc.

[0077] Preferably, in step (3), the transverse stretching is: stretching laterally at a speed of 2-30% / s (e.g., 2% / s, 4% / s, 6% / s, 8% / s, 10% / s, 12% / s, 14% / s, 16% / s, 18% / s, 20% / s, 25% / s, 30% / s, etc.) at 140-160℃ (e.g., 140℃, 145℃, 150℃, 155℃, 160℃, etc.).

[0078] Preferably, in step (3), the temperature of the heat retraction is 150-170°C, for example, 150°C, 155°C, 160°C, 165°C, 170°C, etc., and the heat retraction rate is 0.5-5% / s, for example, 0.5% / s, 1% / s, 1.5% / s, 2% / s, 2.5% / s, 3% / s, 3.5% / s, 4% / s, 4.5% / s, 5% / s, etc., and the heat retraction ratio is 5-25%, for example, 5%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 25%, etc.

[0079] Preferably, in step (3), the heat setting temperature is 110-170℃, for example, it can be 110℃, 120℃, 130℃, 135℃, 140℃, 145℃, 150℃, 155℃, 160℃, 170℃, etc., and the heat setting time is 5-50s, for example, it can be 5s, 10s, 15s, 20s, 25s, 30s, 35s, 40s, 45s, 50s, etc.

[0080] As the most preferred technical solution of the present invention, the dry-process biaxially stretched composite diaphragm is prepared by the following method:

[0081] (1) The raw materials for preparing the polypropylene layer are fed into the first twin-screw extruder and heated at 200-240℃ for 30-60s at a speed of 100-240rpm to form a homogeneous PP melt. The raw materials for preparing the polypropylene-polyethylene blend layer are fed into the second twin-screw extruder and heated at 190-210℃ for 30-60s at a speed of 100-240rpm to form a homogeneous PP-PE melt.

[0082] The homogeneous PP melt is fed into a T-shaped three-layer composite die at 200-220°C through a first melt pipe at 200-240°C and the homogeneous PP-PE melt is fed into a second melt pipe at 190-210°C. After cooling, a crystalline cast sheet is obtained.

[0083] (2) The above-mentioned crystalline casting is pulled into a bidirectional synchronous stretching machine along the roller and stretched to 5 to 15 times in both the transverse and longitudinal directions at a temperature of 130 to 160°C and a speed of 10 to 150% / s to obtain a porous film with highly oriented crystals.

[0084] Alternatively, the crystalline cast sheet is first drawn along the rollers of a longitudinal stretching machine and stretched to 5-15 times its original length at a temperature of 110-130°C and a speed of 25-250% / s to obtain a longitudinally highly oriented thin film precursor; then the longitudinally highly oriented thin film is stretched to 5-15 times its original length at a temperature of 140-160°C and a speed of 10-150% / s to obtain a crystalline highly oriented porous film precursor.

[0085] (3) The above-mentioned highly oriented porous film precursor is stretched to 1.5 times at a temperature of 140-160°C at a speed of 2-30% / s, and then heated and shrunken (lateral width shrinkage) at 150-170°C at a speed of 0.5-5% / s. The heating and shrunken (lateral width shrinkage) ratio is 5-25%. Finally, it is heat-set by hot roller traction heat treatment at 110-170°C for 25-50s. It is then pulled along the roller and wound up by the winding roller to obtain the dry bi-stretched composite diaphragm.

[0086] Thirdly, the present invention provides an application of the dry biaxially stretched composite separator as described in the first aspect in the preparation of lithium-ion batteries.

[0087] Compared with the prior art, the present invention has the following beneficial effects:

[0088] (1) The present invention achieves complementary advantages of PP and PE materials through a sandwich-style multilayer composite structure of PP / PE+PP / PP. The high-strength PP skeleton layers on both sides can provide the diaphragm with super strong anti-puncture ability and super high membrane breaking temperature; the PE+PP functional inner liner layer in the middle can provide the diaphragm with consistent pore size and low pore closing temperature, so that the diaphragm has an extremely wide reliable temperature safety range.

[0089] (2) In this invention, an appropriate amount of β-crystal nucleating agent, plasticizer and solubilizer are added to the intermediate polyethylene-polypropylene blend layer. While enhancing its plasticizing effect, it can improve the compatibility between the intermediate functional layer and the outer skeleton layer, realize the composite casting of different material melts, and enhance the peel strength between different layers.

[0090] (3) The dry-process biaxially stretched composite lithium-ion battery separator prepared by this invention has a thickness of 11.77–12.55 μm, a porosity of 45.2–51.2%, an air permeability of 105–155 s / 100cc, a needle penetration strength of 0.79–0.87 N / μm, and a reliable temperature safety range of 35–53℃. Compared with conventional separators, the needle penetration strength is increased by about 37.8%, the reliable temperature safety range is widened by about 5 times, and the peel strength is increased by more than 10 times, which significantly improves the safety of the separator and has good application prospects and economic benefits. Attached Figure Description

[0091] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0092] Figure 1 This is a flowchart illustrating the preparation process of the dry-process biaxially stretched composite lithium battery separator described in this invention. Detailed Implementation

[0093] Unless otherwise defined herein, the scientific and technical terms used in conjunction with this invention shall have the meanings commonly understood by one of ordinary skill in the art. The meaning and scope of terms shall be clear; however, in any case of potential ambiguity, the definitions provided herein shall prevail over any dictionary or foreign definitions. In this application, unless otherwise stated, the use of "or" means "and / or". Furthermore, the use of the term "comprising" and other forms is non-limiting.

[0094] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0095] The present invention will be further illustrated by the following examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or purchased directly from the market.

[0096] Figure 1 This is a flowchart illustrating the preparation process of the dry-process biaxially stretched composite lithium-ion battery separator described in this invention. Implementation Principle: This invention is a method for preparing a dry-process biaxially stretched composite lithium-ion battery separator. This method utilizes a sandwich-style multilayer composite structure of PP / PE + PP / PP to achieve complementary advantages of PP and PE materials. The high-strength PP skeleton layers on both sides provide the separator with superior resistance to needle puncture and ultra-high membrane breakage temperature, preventing the separator from being punctured by foreign objects or burrs, or from melting and breaking, which could lead to short circuits between the positive and negative electrodes. The middle PE+PP functional inner liner layer provides the separator with consistent pore size and a lower pore-closing temperature. When the battery experiences short-circuit thermal runaway, the PE can melt and close the pores in time, isolating the positive and negative electrodes and preventing the cell from continuing to discharge and generate heat, thus avoiding the safety risks of combustion or explosion. Simultaneously, the addition of PP, plasticizer, and solubilizer to the middle PE layer enhances its plasticizing effect and improves the compatibility between the middle functional layer and the outer skeleton layer, enabling composite casting of different material melts and improving the peel strength between the composite layers.

[0097] The raw materials and their manufacturers involved in the following examples and comparative examples are as follows:

[0098]

[0099]

[0100] Example 1

[0101] This embodiment provides a dry biaxially stretched composite separator, which includes a polypropylene-polyethylene blend layer and polypropylene layers disposed on both sides of the polypropylene-polyethylene blend layer;

[0102] The raw materials for preparing the dry-process biaxially stretched composite separator include raw material A and raw material B, wherein the mass ratio of raw material A to raw material B is 2:8, and specifically, by weight, the raw materials include the following components:

[0103]

[0104] The nucleating agent is a mixture of pimelic acid and calcium stearate in a mass ratio of 1:4; the plasticizer is white oil and dioctyl terephthalate in a mass ratio of 3:7; the solubilizer is ethylene-propylene copolymer and ethylene-octene copolymer in a mass ratio of 9:1; and the antioxidant is antioxidant 1010.

[0105] The above-mentioned dry-process biaxially stretched composite lithium-ion battery separator preparation method includes the following steps:

[0106] (1) Raw materials A and B are fed into co-rotating twin-screw extruders 1 and 2 with screw length-to-diameter ratios of 64 and 68 respectively at a uniform speed. They are heated at 220°C for 45s at a speed of 150 rpm in extruder 1 and at 200°C for 45s at a speed of 170 rpm in extruder 2. The heating and plasticizing process forms a homogeneous melt. The homogeneous melt is then fed into a T-shaped three-layer composite die head heated at 210°C through melt pipe 1 heated at 220°C and melt pipe 2 heated at 200°C respectively. The melt is then attached to the shaping roller and cooled to form a crystalline casting sheet.

[0107] (2) The crystallized casting is pulled into the bidirectional synchronous stretching machine along the roller, held by the stretching machine clamp, and stretched in the oven at 140℃ at a speed of 80% / s to 10 times in both the transverse and longitudinal directions (i.e., the stretching ratio is 10×10 times) to obtain a porous film with highly oriented crystals.

[0108] Alternatively, the crystalline cast sheet is drawn along the rollers of the longitudinal stretching machine and stretched to 10 times its original length at 100% / s at 120°C (i.e., the stretching ratio is 10 times), to obtain a longitudinally highly oriented thin film precursor; then the longitudinally highly oriented thin film is held by the clamps of the transverse stretching machine and stretched to 10 times its original length at 80% / s in an oven at 150°C (i.e., the stretching ratio is 10 times), to obtain a crystalline highly oriented porous film precursor.

[0109] (3) The film precursor is held by the clamp of the stretching machine and stretched to 1.5 times at a speed of 15% / s in an oven at 150°C. Then it is heated and shrunken (lateral width shrinkage) at a speed of 2% / s at 160°C. The ratio of heating and shrinkage (lateral width shrinkage) is 15%. Finally, it is heat-treated by hot roller traction at 130°C for 20s for heat setting. It is then pulled along the roller and wound up by the winding roller to obtain the dry bi-stretched composite battery separator.

[0110] Example 2

[0111] This embodiment provides a dry biaxially stretched composite separator, which includes a polypropylene-polyethylene blend layer and polypropylene layers disposed on both sides of the polypropylene-polyethylene blend layer;

[0112] The raw materials for preparing the dry-process biaxially stretched composite separator include raw material A and raw material B, wherein the mass ratio of raw material A to raw material B is 3:7, and specifically, by weight, the raw materials include the following components:

[0113]

[0114] The nucleating agent is a mixture of pimelic acid and calcium stearate in a mass ratio of 1:4; the plasticizer is white oil and dioctyl terephthalate in a mass ratio of 4:6; the solubilizer is ethylene-propylene copolymer and ethylene-octene copolymer in a mass ratio of 8:2; and the antioxidant is antioxidant 1010.

[0115] The above-mentioned dry-process biaxially stretched composite lithium-ion battery separator preparation method includes the following steps:

[0116] (1) Raw materials A and B are fed into co-rotating twin-screw extruders 1 and 2 with screw length-to-diameter ratios of 64 and 68 respectively at a uniform speed. They are heated at 220°C for 45s at a speed of 150 rpm in extruder 1 and at 200°C for 45s at a speed of 170 rpm in extruder 2. The heating and plasticizing process forms a homogeneous melt. The homogeneous melt is then fed into a T-shaped three-layer composite die head heated at 210°C through melt pipe 1 heated at 220°C and melt pipe 2 heated at 200°C respectively. The melt is then attached to the shaping roller and cooled to form a crystalline casting sheet.

[0117] (2) The crystallized casting is pulled into the bidirectional synchronous stretching machine along the roller, held by the stretching machine clamp, and stretched in the oven at 140℃ at a speed of 80% / s to 10 times in both the transverse and longitudinal directions (i.e., the stretching ratio is 10×10 times) to obtain a porous film with highly oriented crystals.

[0118] Alternatively, the crystalline cast sheet is drawn along the rollers of the longitudinal stretching machine and stretched to 10 times its original length at 100% / s at 120°C (i.e., the stretching ratio is 10 times), to obtain a longitudinally highly oriented thin film precursor; then the longitudinally highly oriented thin film is held by the clamps of the transverse stretching machine and stretched to 10 times its original length at 80% / s in an oven at 150°C (i.e., the stretching ratio is 10 times), to obtain a crystalline highly oriented porous film precursor.

[0119] (3) The film precursor is held by the clamp of the stretching machine and stretched to 1.5 times at a speed of 15% / s in an oven at 150°C. Then it is heated and shrunken (lateral width shrinkage) at a speed of 2% / s at 160°C. The ratio of heating and shrinkage (lateral width shrinkage) is 15%. Finally, it is heat-treated by hot roller traction at 130°C for 20s for heat setting. It is then pulled along the roller and wound up by the winding roller to obtain the dry bi-stretched composite battery separator.

[0120] Example 3

[0121] This embodiment provides a dry biaxially stretched composite separator, which includes a polypropylene-polyethylene blend layer and polypropylene layers disposed on both sides of the polypropylene-polyethylene blend layer;

[0122] The raw materials for preparing the dry-process biaxially stretched composite separator include raw material A and raw material B, wherein the mass ratio of raw material A to raw material B is 4:6, and specifically, by weight, the raw materials include the following components:

[0123]

[0124] The nucleating agent is a mixture of pimelic acid and calcium stearate in a mass ratio of 1:4; the plasticizer is white oil and dioctyl terephthalate in a mass ratio of 3:7; the solubilizer is ethylene-propylene copolymer and ethylene-octene copolymer in a mass ratio of 8:2; and the antioxidant is antioxidant 1010.

[0125] The above-mentioned dry-process biaxially stretched composite lithium-ion battery separator preparation method includes the following steps:

[0126] (1) Raw materials A and B are fed into co-rotating twin-screw extruders 1 and 2 with screw length-to-diameter ratios of 64 and 68 respectively at a uniform speed. They are heated at 220°C for 45s at a speed of 150 rpm in extruder 1 and at 200°C for 45s at a speed of 170 rpm in extruder 2. The heating and plasticizing process forms a homogeneous melt. The homogeneous melt is then fed into a T-shaped three-layer composite die head heated at 210°C through melt pipe 1 heated at 220°C and melt pipe 2 heated at 200°C respectively. The melt is then attached to the shaping roller and cooled to form a crystalline casting sheet.

[0127] (2) The crystallized casting is pulled into the bidirectional synchronous stretching machine along the roller, held by the stretching machine clamp, and stretched in the oven at 140℃ at a speed of 80% / s to 10 times in both the transverse and longitudinal directions (i.e., the stretching ratio is 10×10 times) to obtain a porous film with highly oriented crystals.

[0128] Alternatively, the crystalline cast sheet is drawn along the rollers of the longitudinal stretching machine and stretched to 10 times its original length at 100% / s at 120°C (i.e., the stretching ratio is 10 times), to obtain a longitudinally highly oriented thin film precursor; then the longitudinally highly oriented thin film is held by the clamps of the transverse stretching machine and stretched to 10 times its original length at 80% / s in an oven at 150°C (i.e., the stretching ratio is 10 times), to obtain a crystalline highly oriented porous film precursor.

[0129] (3) The film precursor is held by the clamp of the stretching machine and stretched to 1.5 times at a speed of 15% / s in an oven at 150°C. Then it is heated and shrunken (lateral width shrinkage) at a speed of 2% / s at 160°C. The ratio of heating and shrinkage (lateral width shrinkage) is 15%. Finally, it is heat-treated by hot roller traction at 130°C for 20s for heat setting. It is then pulled along the roller and wound up by the winding roller to obtain the dry bi-stretched composite battery separator.

[0130] Example 4

[0131] This embodiment provides a dry biaxially stretched composite separator, which includes a polypropylene-polyethylene blend layer and polypropylene layers disposed on both sides of the polypropylene-polyethylene blend layer;

[0132] The raw materials for preparing the dry-process biaxially stretched composite separator include raw material A and raw material B, wherein the mass ratio of raw material A to raw material B is 5:5, and specifically, by weight, the raw materials include the following components:

[0133]

[0134]

[0135] The nucleating agent is a mixture of pimelic acid and calcium stearate in a mass ratio of 1:4; the plasticizer is white oil and dioctyl terephthalate in a mass ratio of 4:6; the solubilizer is ethylene-propylene copolymer and ethylene-octene copolymer in a mass ratio of 9:1; and the antioxidant is antioxidant 1010.

[0136] The above-mentioned dry-process biaxially stretched composite lithium-ion battery separator preparation method includes the following steps:

[0137] (1) Raw materials A and B are fed into co-rotating twin-screw extruders 1 and 2 with screw length-to-diameter ratios of 64 and 68 respectively at a uniform speed. They are heated at 220°C for 45s at a speed of 150 rpm in extruder 1 and at 200°C for 45s at a speed of 170 rpm in extruder 2. The heating and plasticizing process forms a homogeneous melt. The homogeneous melt is then fed into a T-shaped three-layer composite die head heated at 210°C through melt pipe 1 heated at 220°C and melt pipe 2 heated at 200°C respectively. The melt is then attached to the shaping roller and cooled to form a crystalline casting sheet.

[0138] (2) The crystallized casting is pulled into the bidirectional synchronous stretching machine along the roller, held by the stretching machine clamp, and stretched in the oven at 140℃ at a speed of 80% / s to 10 times in both the transverse and longitudinal directions (i.e., the stretching ratio is 10×10 times) to obtain a porous film with highly oriented crystals.

[0139] Alternatively, the crystalline cast sheet is drawn along the rollers of the longitudinal stretching machine and stretched to 10 times its original length at 100% / s at 120°C (i.e., the stretching ratio is 10 times), to obtain a longitudinally highly oriented thin film precursor; then the longitudinally highly oriented thin film is held by the clamps of the transverse stretching machine and stretched to 10 times its original length at 80% / s in an oven at 150°C (i.e., the stretching ratio is 10 times), to obtain a crystalline highly oriented porous film precursor.

[0140] (3) The film precursor is held by the clamp of the stretching machine and stretched to 1.5 times at a speed of 15% / s in an oven at 150°C. Then it is heated and shrunken (lateral width shrinkage) at a speed of 2% / s at 160°C. The ratio of heating and shrinkage (lateral width shrinkage) is 15%. Finally, it is heat-treated by hot roller traction at 130°C for 20s for heat setting. It is then pulled along the roller and wound up by the winding roller to obtain the dry bi-stretched composite battery separator.

[0141] Example 5

[0142] This embodiment provides a dry-process biaxially stretched composite membrane, which differs from Example 1 only in that no white oil is added, the total amount of dioctyl terephthalate is increased to 0.1 parts, and the content of other components and the preparation method are the same as in Example 1.

[0143] Example 6

[0144] This embodiment provides a dry-process biaxially stretched composite membrane, which differs from Example 1 only in that the plasticizer is white oil and dioctyl phthalate in a mass ratio of 3:7, while the content of other components and the preparation method are the same as in Example 1.

[0145] Example 7

[0146] This embodiment provides a dry-process biaxially stretched composite diaphragm, which differs from Example 1 only in that it does not contain ethylene-propylene copolymer, and the content of ethylene-octene copolymer is increased to 3 parts. The content of other components and the preparation method are the same as in Example 1.

[0147] Example 8

[0148] This embodiment provides a dry biaxially stretched composite membrane, which differs from Example 1 only in that it does not contain ethylene-octene copolymer, and the content of ethylene-propylene copolymer is increased to 3 parts. The content of other components and the preparation method are the same as in Example 1.

[0149] Comparative Example 1

[0150] This comparative example provides a dry-process biaxially stretched composite diaphragm, which differs from Example 1 only in that it is prepared solely from raw material A. The raw material A system includes PP and a nucleating agent, with the ratio of PP to nucleating agent being X by mass. The nucleating agent is a β-crystal nucleating agent, specifically a compound mixture of pimelic acid and calcium stearate, with the ratio of pimelic acid to calcium stearate being 1:4 by mass.

[0151] The preparation method of the above-mentioned ultra-thin high-strength lithium-ion battery separator includes the following steps: raw material A is fed into a co-rotating twin-screw extruder 1 with a screw length-to-diameter ratio of 64 at a uniform speed, and heated at 220°C for 45s at a speed of 150rpm in the extruder 1 to form a homogeneous melt. The homogeneous melt is then fed into a T-shaped single-layer die head heated at 210°C through a melt pipe 1 heated at 220°C for casting extrusion, and then attached to a shaping roller to cool and form a crystalline casting sheet.

[0152] The preparation method of the above-mentioned lithium-ion battery separator is basically the same as the preparation method of the dry biaxial composite lithium-ion battery separator in Example 1. The only difference is that only raw material A is added in this comparative example, raw material B is not added, only extruder 1 and melt pipeline 1 are used, and the three-layer composite die head is replaced with a single-layer die head.

[0153] Comparative Example 2

[0154] This comparative example provides a dry-process biaxially stretched composite diaphragm, which differs from Example 1 only in that it is prepared solely from raw material B. The raw material B system includes PP, PE, nucleating agent, compatibilizer, plasticizer, and antioxidant. The ratio of each component by mass is Y. The nucleating agent is a β-crystal nucleating agent, specifically a compound mixture of pimelic acid and calcium stearate, with a mass ratio of 1:4.

[0155] The above-mentioned dry-process biaxially stretched composite lithium-ion battery separator preparation method includes the following steps:

[0156] Step 1: Raw material B is fed into a co-rotating twin-screw extruder 2 with a screw length diameter of 68 mm at a uniform speed. The extruder is heated at 200°C for 45 seconds at a speed of 170 rpm to plasticize and form a homogeneous melt. The homogeneous melt is then fed into a T-shaped single-layer die head heated at 210°C through a melt pipe 2 heated at 200°C for casting and extrusion. The melt is then attached to a shaping roller and cooled to form a crystalline cast sheet.

[0157] The preparation method of the above-mentioned lithium-ion battery separator is basically the same as the preparation method of the dry biaxial composite lithium-ion battery separator in Example 1. The only difference is that only raw material B is added in this comparative example, raw material A is not added, only extruder 2 and melt pipeline 2 are used, and the three-layer composite die head is replaced with a single-layer die head.

[0158] Comparative Example 3

[0159] This comparative example provides a dry-process biaxially stretched composite diaphragm, which differs from Example 1 only in that the raw material system B includes only PE and antioxidants, excluding PP, nucleating agents, solubilizers and plasticizers.

[0160] Comparative Example 4

[0161] This comparative example provides a dry-process biaxially stretched composite diaphragm, which differs from Example 1 only in that the raw material system B includes only PE, PP and antioxidants, and does not include nucleating agents, solubilizers and plasticizers.

[0162] Comparative Example 5

[0163] This comparative example provides a dry-process biaxially stretched composite membrane, which differs from Example 1 only in that the nucleating agent is sodium 2,2'-methylene bis(4,6-di-tert-butylphenyl) phosphate.

[0164] Comparative Example 6

[0165] This comparative example provides a dry-process biaxially stretched composite diaphragm, which differs from Example 1 only in that the plasticizer is trihydroxytriethylamine.

[0166] Comparative Example 7

[0167] This comparative example provides a dry-process biaxially stretched composite membrane, which differs from Example 1 only in that the solubilizer is triallyl isocyanurate.

[0168] Comparative Example 8

[0169] This comparative example provides a dry biaxially stretched composite membrane, which includes a polypropylene-polyethylene blend layer and polypropylene layers disposed on both sides of the polypropylene-polyethylene blend layer;

[0170] The raw materials for preparing the dry-process biaxially stretched composite separator include raw material A and raw material B, wherein the mass ratio of raw material A to raw material B is 2:8, and specifically, by weight, the raw materials include the following components:

[0171]

[0172]

[0173] The nucleating agent is a mixture of pimelic acid and calcium stearate in a mass ratio of 1:4; the plasticizer is white oil and dioctyl terephthalate in a mass ratio of 3:7; the solubilizer is ethylene-propylene copolymer and ethylene-octene copolymer in a mass ratio of 9:1; and the antioxidant is antioxidant 1010.

[0174] The difference between the above-mentioned dry-process biaxially stretched composite lithium-ion battery separator and Example 1 is that after obtaining the highly oriented porous film in step (2), a second transverse stretching is not performed, but the film is directly heated and shrunken, as shown below:

[0175] (1) Raw materials A and B are fed into co-rotating twin-screw extruders 1 and 2 with screw length-to-diameter ratios of 64 and 68 respectively at a uniform speed. They are heated at 220°C for 45s at a speed of 150 rpm in extruder 1 and at 200°C for 45s at a speed of 170 rpm in extruder 2. The heating and plasticizing process forms a homogeneous melt. The homogeneous melt is then fed into a T-shaped three-layer composite die head heated at 210°C through melt pipe 1 heated at 220°C and melt pipe 2 heated at 200°C respectively. The melt is then attached to the shaping roller and cooled to form a crystalline casting sheet.

[0176] (2) The crystallized casting is pulled into the bidirectional synchronous stretching machine along the roller, held by the stretching machine clamp, and stretched in the oven at 140℃ at a speed of 80% / s to 10 times in both the transverse and longitudinal directions (i.e., the stretching ratio is 10×10 times) to obtain a porous film with highly oriented crystals.

[0177] Alternatively, the crystalline cast sheet is drawn along the rollers of the longitudinal stretching machine and stretched to 10 times its original length at 100% / s at 120°C (i.e., the stretching ratio is 10 times), to obtain a longitudinally highly oriented thin film precursor; then the longitudinally highly oriented thin film is held by the clamps of the transverse stretching machine and stretched to 10 times its original length at 80% / s in an oven at 150°C (i.e., the stretching ratio is 10 times), to obtain a crystalline highly oriented porous film precursor.

[0178] (3) The film precursor is heated and shrunken at 160°C at a speed of 2% / s (lateral width shrunken). The heating and shrunken ratio (lateral width shrunken) is 15%. Finally, it is heat-treated by hot roller traction at 130°C for 20s for heat setting. It is then traction along the roller and wound up by the winding roller to obtain the dry biaxially stretched composite battery separator.

[0179] Comparative Example 9

[0180] This comparative example provides a dry biaxially stretched composite membrane, which includes a polypropylene-polyethylene blend layer and polypropylene layers disposed on both sides of the polypropylene-polyethylene blend layer;

[0181] The raw materials for preparing the dry-process biaxially stretched composite separator include raw material A and raw material B, wherein the mass ratio of raw material A to raw material B is 2:8, and specifically, by weight, the raw materials include the following components:

[0182]

[0183]

[0184] The nucleating agent is a mixture of pimelic acid and calcium stearate in a mass ratio of 1:4; the plasticizer is white oil and dioctyl terephthalate in a mass ratio of 3:7; the solubilizer is ethylene-propylene copolymer and ethylene-octene copolymer in a mass ratio of 9:1; and the antioxidant is antioxidant 1010.

[0185] The preparation method of the above-mentioned dry biaxially stretched composite lithium-ion battery separator differs from that in Example 1 only in that the lateral stretching in step (3) is performed instead of step (2), as shown below:

[0186] (1) Raw materials A and B are fed into co-rotating twin-screw extruders 1 and 2 with screw length-to-diameter ratios of 64 and 68 respectively at a uniform speed. They are heated at 220°C for 45s at a speed of 150 rpm in extruder 1 and at 200°C for 45s at a speed of 170 rpm in extruder 2. The heating and plasticizing process forms a homogeneous melt. The homogeneous melt is then fed into a T-shaped three-layer composite die head heated at 210°C through melt pipe 1 heated at 220°C and melt pipe 2 heated at 200°C respectively. The melt is then attached to the shaping roller and cooled to form a crystalline casting sheet.

[0187] (2) The crystallized casting is held by the clamp of the horizontal stretching machine and stretched to 1.5 times at a speed of 15% / s in an oven at 150°C. Then it is heated and shrunken (lateral width shrinkage) at a speed of 2% / s at 160°C. The ratio of heating and shrinkage (lateral width shrinkage) is 15%. Finally, it is heat-treated by hot roller traction at 130°C for 20s for heat setting. It is then pulled along the roller and wound up by the winding roller to obtain the dry-stretched composite battery separator.

[0188] Comparative Example 10

[0189] This comparative example provides a dry biaxially stretched composite membrane, which includes a polypropylene-polyethylene blend layer and polypropylene layers disposed on both sides of the polypropylene-polyethylene blend layer;

[0190] The raw materials for preparing the dry-process biaxially stretched composite separator include raw material A and raw material B, wherein the mass ratio of raw material A to raw material B is 2:8, and specifically, by weight, the raw materials include the following components:

[0191]

[0192]

[0193] The nucleating agent is a mixture of pimelic acid and calcium stearate in a mass ratio of 1:4; the plasticizer is white oil and dioctyl terephthalate in a mass ratio of 3:7; the solubilizer is ethylene-propylene copolymer and ethylene-octene copolymer in a mass ratio of 9:1; and the antioxidant is antioxidant 1010.

[0194] The preparation method of the above-mentioned dry biaxially stretched composite lithium-ion battery separator differs from that in Example 1 only in that the original step (3) is first stretched at a small ratio, and then the original step (2) is stretched, as shown below:

[0195] (1) Raw materials A and B are fed into co-rotating twin-screw extruders 1 and 2 with screw length-to-diameter ratios of 64 and 68 respectively at a uniform speed. They are heated at 220°C for 45s at a speed of 150 rpm in extruder 1 and at 200°C for 45s at a speed of 170 rpm in extruder 2. The heating and plasticizing process forms a homogeneous melt. The homogeneous melt is then fed into a T-shaped three-layer composite die head heated at 210°C through melt pipe 1 heated at 220°C and melt pipe 2 heated at 200°C respectively. The melt is then attached to the shaping roller and cooled to form a crystalline casting sheet.

[0196] (2) The above-mentioned crystalline casting is stretched to 1.5 times at a temperature of 150°C and a speed of 15% / s to obtain a porous film;

[0197] (3) The above-mentioned porous film is drawn into a biaxial synchronous stretching machine along the rollers and stretched to 10 times its original length in both the transverse and longitudinal directions at a temperature of 140°C and a speed of 80% / s.

[0198] Alternatively, the crystalline wafer is first pulled along the rollers of the longitudinal stretching machine and stretched to 10 times at a speed of 100% / s at a temperature of 120°C to obtain a longitudinally highly oriented thin film precursor; then the longitudinally highly oriented thin film is stretched to 10 times at a speed of 80% / s at a temperature of 150°C.

[0199] (4) Finally, the film is heated and shrunken at 160°C at a speed of 2% / s (lateral width shrinkage). The heating and shrinkage (lateral width shrinkage) ratio is 15%. Finally, it is heat-treated by hot roller traction at 130°C for 20s for heat setting. It is then pulled along the roller and wound up by the winding roller to obtain the dry double-stretch composite diaphragm.

[0200] Comparative Example 11

[0201] This comparative example provides a dry biaxially stretched composite diaphragm, which differs from Example 1 only in that the original step (3) of heat shrinkage is not performed, but heat setting is performed directly.

[0202] Comparative Example 12

[0203] This comparative example provides a dry biaxially stretched composite membrane, which differs from Example 1 only in that the heat setting step (3) of the original step is not performed, but the dry biaxially stretched composite membrane is obtained by direct cooling.

[0204] Test Example 1

[0205] Test samples: Dry biaxially stretched composite membranes prepared in Examples 1-8 and dry biaxially stretched composite membranes prepared in Comparative Examples 1-12;

[0206] Testing standards: The testing methods for thickness, porosity, air permeability, and needle penetration strength all refer to the national standard "Polyolefin Separator for Lithium-ion Batteries" (GB / T 36363-2018); the closed-cell temperature and membrane rupture temperature are tested using the resistance method; the peel strength testing method refers to the national standard "Peel Test Method for Flexible Composite Plastic Materials" (GB / T 8808-88).

[0207] The specific test results are shown in Table 1:

[0208] Table 1

[0209]

[0210]

[0211] Table 1 shows the thickness, porosity, air permeability, needle penetration strength, pore-closing temperature, membrane rupture temperature, and peel strength characteristics of the dry-process biaxially stretched composite lithium-ion battery separators prepared in Examples 1-4 and the single-layer or composite lithium-ion battery separators prepared in Comparative Examples 1-3. The dry-process biaxially stretched composite lithium-ion battery separators prepared in Examples 1-4 have a thickness of 11.77-12.55 μm, a porosity of 45.2-51.2%, an air permeability of 105-155 s / 100cc, a needle penetration strength of 0.79-0.87 N / μm, a pore-closing temperature of 122-135℃, a membrane rupture temperature of 170-176℃, a safe temperature window of 35-53℃ (safe temperature window = membrane rupture temperature - pore-closing temperature), and a peel strength of 321-417 N / 15 mm.

[0212] As shown in Table 1, the air permeability of the membrane gradually increases as the outer PP skeleton layer gradually thickens and the inner PE+PP functional layer gradually thins. This is because the pore permeability of the PP layer is worse than that of the heterogeneous PP+PE mixture, so the PP skeleton layer needs a certain thickness to provide strength support. However, excessive PP thickness will affect the pore formation efficiency of the membrane. As the proportion of nucleating agent in component A of the raw material increases, the needle punching strength of the membrane decreases slightly. This is because when the nucleating agent is added in excess, it becomes a foreign substance in the melt system, which disrupts the system balance of the homogeneous melt and reduces the crystallinity and strength of the film. When the proportion of nucleating agent is too small, it cannot significantly promote nucleation. Therefore, there is an optimal addition ratio for the nucleating agent.

[0213] As the proportion of PP added to component B of the raw material gradually increases, the pore-closure temperature of the membrane gradually rises. This is because when the PP content in the PP+PE hybrid functional layer increases, PP has a higher melting point and can withstand higher temperatures without exhibiting a pore-closure effect. At the same time, the composite membrane has better compatibility and higher peel strength. However, when the PP content is too low, the melt flow is closer to the melt state of PE, and the viscosity and flow rate of the outer PP melt are significantly different, making it difficult to achieve compatibility and composite casting. Consequently, the peel strength of the composite membrane decreases.

[0214] Comparative Examples 1 and 2 show that when using only raw material A to prepare a single-layer PP membrane, although the membrane has high strength and rupture temperature, the pore-closure temperature also increases accordingly, resulting in a safe temperature window of only 14℃ and a short reliable range. When using only raw material B to prepare a single-layer PP+PE membrane, the rupture temperature is only 143℃ due to the low melting point of PE, the main component of the membrane. This leads to poor temperature resistance and a safe temperature window of only 13℃, resulting in a short reliable range.

[0215] As shown in Comparative Example 3, the addition of PP, compatibilizer and plasticizer to raw material B can greatly improve the compatibility of different layers and increase the peel strength by more than 10 times.

[0216] As can be seen from the comparison between Example 1 and Comparative Example 4, when PE and PP are directly mixed without adding nucleating agents, solubilizers and plasticizers, PE and PP are completely incompatible and cannot be composited in casting extrusion, and therefore cannot be stretched into a film.

[0217] As can be seen from the comparison between Example 1 and Comparative Example 5, the use of a nucleating agent mixture that is not preferred in this application results in poor crystallization effect, leading to low membrane porosity and poor air permeability.

[0218] As can be seen from the comparison of Example 1 and Comparative Examples 6 and 7, even if additives with plasticizing and solubilizing effects are added, if the plasticizers and solubilizers are not specific to this application, PE and PP cannot be prepared into films by dry process. PE and PP are still not completely incompatible and cannot be composited in casting extrusion, and therefore cannot be stretched into films.

[0219] As can be seen from the comparison between Example 1 and Comparative Example 8, since the micropores were not further oriented and shaped, the porosity was low and the crystallinity was insufficient, resulting in insufficient needle penetration strength, film breaking temperature and peel strength.

[0220] As can be seen from the comparison between Example 1 and Comparative Example 9, due to the low stretching ratio, there are fewer pores, higher air permeability, less molecular chain orientation crystallization, insufficient needle punching, and lower film breaking temperature and peel strength.

[0221] As can be seen from the comparison between Example 1 and Comparative Example 10, the initial low-ratio stretching and shaping resulted in poor extensibility and viscoelasticity of the diaphragm, making it impossible to further stretch at a high ratio, which led to diaphragm rupture.

[0222] As can be seen from the comparison between Example 1 and Comparative Example 11, without retraction, the tensile stress of the diaphragm cannot be released, resulting in poor thermal stability of the diaphragm.

[0223] As can be seen from the comparison between Example 1 and Comparative Example 12, without heat setting, the microporous structure of the product is not solidified, the crystallinity of the diaphragm is low, and the needle penetration strength is low.

[0224] Test Example 2

[0225] Test samples: The dry biaxially stretched composite membranes prepared in Examples 1-8 and the dry biaxially stretched composite membranes prepared in Comparative Examples 1-12 were assembled into soft-pack batteries, with LiFePO4 and MCMB as the positive and negative electrodes, respectively, and LiPF6, EC and DEC as the electrolytes.

[0226] (1) High temperature cycling test

[0227] The battery was cycled 600 times at 45℃ with a 1.2C charge / 0.7C discharge rate. The battery retained its capacity and expansion rate over the 600 times cycle.

[0228] 600T capacity retention rate = Capacity at 600T ÷ Capacity at 1T

[0229] 600T expansion rate = 600T full charge thickness ÷ 1T half charge thickness.

[0230] (2) Hot box test: Place the battery in an oven and heat it to 130°C at a heating rate of 5°C / min. Keep it for 1 hour and observe whether it catches fire. If it does not catch fire or explode, it passes; otherwise, it fails.

[0231] The specific test results are shown in Table 2:

[0232] Table 2

[0233]

[0234]

[0235] As shown in Table 2, the test data of the dry-process biaxially stretched composite separator assembled battery of the present invention exhibits a 600T capacity retention rate ≥89.3% and a 600T expansion rate ≤5.3%, both passing the hot box test, demonstrating significant advantages in cycle performance and heat resistance. In contrast, the comparative example suffers from poor heat resistance, low pore-forming efficiency, and weak composite adhesion, resulting in low cycle capacity retention, high expansion rate, and poor heat resistance.

[0236] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A dry-process biaxially stretched composite diaphragm, characterized in that, The dry-process biaxially stretched composite diaphragm includes a polypropylene-polyethylene blend layer and polypropylene layers disposed on both sides of the polypropylene-polyethylene blend layer; the raw materials for preparing the polyethylene-polypropylene blend layer include, by weight, 5-25 parts of polypropylene and 75-95 parts of polyethylene. β 0.01-5 parts of crystal nucleating agent, 0.01-5 parts of plasticizer, and 0.01-5 parts of solubilizer; Among them, the β The nucleating agents for the crystal form are pimelic acid and calcium stearate; The plasticizer is white oil and dioctyl terephthalate; The solubilizer is an ethylene-propylene copolymer and an ethylene-octene copolymer; The raw materials for preparing the polypropylene layer include, by weight, 99-99.9 parts of polypropylene. β 0.1 to 1 part of crystal nucleating agent; The preparation method of the dry-process biaxially stretched composite diaphragm includes the following steps: (1) The raw materials for preparing the polypropylene-polyethylene blend layer and the raw materials for preparing the polypropylene layer are heated and plasticized to form a homogeneous melt, and then cast and extruded together. After cooling, a crystalline cast sheet is obtained. (2) The crystalline casting obtained in step (1) is stretched to obtain a porous film precursor with highly oriented crystals; The stretching is either bidirectional synchronous stretching or longitudinal stretching followed by transverse stretching. (3) The porous film precursor with highly oriented crystal structure obtained in step (2) is stretched laterally, and then subjected to heat shrinkage and heat setting in sequence to obtain the dry bi-stretched composite diaphragm.

2. The dry-process biaxially stretched composite diaphragm according to claim 1, characterized in that, The mass ratio of pimelic acid to calcium stearate is (0.8~3.8):(1.2~4.2).

3. The dry-process biaxially stretched composite diaphragm according to claim 1, characterized in that, The mass ratio of the white oil to dioctyl terephthalate is (3~4):(6~7).

4. The dry-process biaxially stretched composite diaphragm according to claim 1, characterized in that, The mass ratio of the ethylene-propylene copolymer to the ethylene-octene copolymer is (8~9):(1~2).

5. The dry-process biaxially stretched composite diaphragm according to any one of claims 1 to 4, characterized in that, The polypropylene is isotactic polypropylene.

6. The dry-process biaxially stretched composite diaphragm according to claim 5, characterized in that, The isotactic polypropylene has a weight-average molecular weight of 500,000 to 1,300,000.

7. The dry-process biaxially stretched composite diaphragm according to claim 5, characterized in that, The melt index of the isotactic polypropylene is 0.5~2 g / 10 min.

8. The dry-process biaxially stretched composite diaphragm according to claim 1, characterized in that, The polyethylene is ultra-high molecular weight polyethylene.

9. The dry-process biaxially stretched composite diaphragm according to claim 8, characterized in that, The weight-average molecular weight of the ultra-high molecular weight polyethylene is 600,000 to 1,500,000.

10. The dry-process biaxially stretched composite diaphragm according to claim 8, characterized in that, The melt index of the ultra-high molecular weight polyethylene is 0.1~1 g / 10 min.

11. The dry-stretched biaxial composite diaphragm according to claim 1, characterized in that, The raw materials for preparing the polyethylene-polypropylene blend layer also include 0.01 to 5 parts of antioxidant.

12. The dry-stretched biaxial composite diaphragm according to claim 11, characterized in that, The antioxidant is tetrakis(t)[ β [3,5-Di-tert-butyl-4-hydroxyphenyl)propionate] pentaerythritol ester.

13. The dry-stretched biaxial composite diaphragm according to claim 1, characterized in that, The mass ratio of the raw materials used to prepare the polypropylene layer and the polypropylene-polyethylene blend layer is (2~5):(5~8).

14. The dry-process biaxially stretched composite diaphragm according to claim 1, characterized in that, The dry-process biaxially stretched composite diaphragm has a thickness of 10~15 μm, a porosity of 45~55%, an air permeability of 100~160 s / 100cc, a needle penetration strength of 0.75~0.90 N / μm, and a reliable temperature safety range of 30~60℃.

15. A method for preparing a dry-process biaxially stretched composite separator according to any one of claims 1 to 14, characterized in that, Includes the following steps: (1) The raw materials for preparing the polypropylene-polyethylene blend layer and the raw materials for preparing the polypropylene layer are heated and plasticized to form a homogeneous melt, and then cast and extruded together. After cooling, a crystalline cast sheet is obtained. (2) The crystalline casting obtained in step (1) is stretched to obtain a porous film precursor with highly oriented crystals; The stretching is either bidirectional synchronous stretching or longitudinal stretching followed by transverse stretching. (3) The porous film precursor with highly oriented crystal structure obtained in step (2) is stretched laterally, and then subjected to heat shrinkage and heat setting in sequence to obtain the dry bi-stretched composite diaphragm.

16. The preparation method according to claim 15, characterized in that, In step (1), the heating time for the raw materials used to prepare the polypropylene layer to form a homogeneous melt is 30-60 s, and the heating temperature is 200-240℃.

17. The preparation method according to claim 15, characterized in that, In step (1), the heating time for the raw materials used to prepare the polypropylene-polyethylene blend layer to form a homogeneous melt is 30-60 s, and the heating temperature is 190-210℃.

18. The preparation method according to claim 15, characterized in that, In step (1), the heating and plasticizing to form a homogeneous melt is carried out using a twin-screw extruder. The screw speed of the twin-screw extruder is 100~240 rpm, the screw diameter is 32-56 mm, and the screw length-to-diameter ratio is 56~68.

19. The preparation method according to claim 15, characterized in that, In step (1), the co-cast extrusion is carried out through a T-shaped three-layer composite die head, and the heating temperature of the die head is 200-220℃.

20. The preparation method according to claim 15, characterized in that, In step (1), the cooling temperature is 15~25℃.

21. The preparation method according to claim 15, characterized in that, In step (2), the stretching ratio is 5 to 15 times.

22. The preparation method according to claim 15, characterized in that, In step (2), the bidirectional synchronous stretching is: bidirectional synchronous stretching at 130~160℃ and a speed of 10~150% / s.

23. The preparation method according to claim 15, characterized in that, In step (2), the longitudinal stretching followed by the transverse stretching is performed as follows: first, longitudinal stretching is performed at 110~130℃ at a speed of 25~250% / s; then, transverse stretching is performed at 140~160℃ at a speed of 10~150% / s.

24. The preparation method according to claim 15, characterized in that, In step (3), the stretching ratio of the transverse stretching is 1.2 to 2 times.

25. The preparation method according to claim 15, characterized in that, In step (3), the transverse stretching is: stretching laterally at a speed of 2-30% / s at 140-160℃.

26. The preparation method according to claim 15, characterized in that, In step (3), the temperature of the heat shrinkage is 150~170℃, the heat shrinkage rate is 0.5~5% / s, and the heat shrinkage ratio is 5~25%.

27. The preparation method according to claim 15, characterized in that, In step (3), the heat setting temperature is 110~170℃ and the heat setting time is 5~50 s.

28. The application of a dry-process biaxially stretched composite separator according to any one of claims 1 to 14 in the preparation of lithium-ion batteries.

Images