Oxide-reinforced high-strength high-temperature-resistant diaphragm and preparation method thereof

Abstract

The invention discloses an oxide-enhanced, high-strength and high-temperature-resistant diaphragm and a preparation method thereof, and belongs to the technical field of battery diaphragms. The diaphragm is of a three-layer co-extrusion structure, a surface layer is composed of a block olefin polymer and a styrene elastomer, the block olefin polymer comprises soft chain segment polyethylene and hard chain segment polyolefin containing rigid groups, the styrene elastomer is a block copolymer with an (S) m-(P / B) n-(S) m microphase separation structure, and the thickness of the styrene elastomer is 1-10 micrometers. And the middle layer consists of high-density polyethylene and hydrophobically modified oxide. The preparation method comprises the following steps: synthesis of the block olefin polymer, three-layer co-extrusion sheet casting, two-way stretching, extraction drying, secondary transverse stretching and heat setting. Through molecular structure design and multiphase compounding, the mechanical strength, the membrane rupture temperature and the thermal stability of the diaphragm are synergistically improved, meanwhile, good electrolyte wettability and processing consistency are kept, the problems that a traditional wet-process diaphragm is poor in temperature resistance and insufficient in strength are effectively solved, and the safety of a lithium battery is remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery separators, and in particular relates to an oxide-reinforced, high-strength and high-temperature-resistant separator and a preparation method thereof. Background Art

[0002] As a major component of liquid lithium-ion batteries, battery separators play a crucial role in the battery. Separator material is an insulating film containing a large number of microporous structures between the positive and negative electrodes of a lithium battery, primarily composed of an insulating polymer material. Separators have two main functions: first, they separate the positive and negative electrodes in the battery, preventing direct contact and short circuits. While ensuring safety, they must be as thin as possible to minimize the distance between the two electrodes and lower the battery's internal resistance. Second, they are able to store and maintain sufficient electrolyte. Their microporous structure allows Li+ in the electrolyte to pass freely, enabling rapid Li+ transfer between the positive and negative electrodes. Therefore, the performance of battery separators can directly impact key lithium battery properties such as capacity, cycle performance, and charge and discharge current density.

[0003] The mainstream production methods for battery separators are divided into three categories: dry uniaxial stretching, dry biaxial stretching, and wet biaxial stretching. Currently, dry uniaxial stretching and wet biaxial stretching are primarily used in the electric vehicle sector. Compared to wet biaxial stretching, dry uniaxial stretching suffers from lower transverse strength and puncture strength, limiting its use in wound battery cells. Currently, wet biaxial stretching accounts for 70% of the market share. With increasing demands for battery safety, higher requirements are being placed on separators for heat resistance and strength.

[0004] Traditional wet-process biaxially oriented lithium battery separators primarily use polyethylene as a raw material. While this material exhibits excellent film-forming properties and electrochemical stability, it has a low melting point and is prone to melt rupture at temperatures exceeding 130°C, leading to battery short circuits and severely limiting the safety and cycle life of lithium batteries. To improve the heat resistance of separators, existing technologies have attempted to introduce highly heat-resistant materials or adopt multi-layer composite structures. For example, patent CN 114784454 A discloses a highly heat-resistant polyolefin microporous membrane employing a three-layer design: surface layer / middle layer / surface layer. The surface layer is a block olefin polymer containing soft segments (polyethylene) and hard segments (polyolefin containing styrene-based rigid monomers), while the middle layer is high-density polyethylene. However, this type of solution still has obvious shortcomings: on the one hand, although the introduced polyolefin chain segments containing rigid groups can increase the glass transition temperature and melting point of the material, it often leads to a decrease in the toughness of the diaphragm and deterioration of the mechanical properties, and is prone to brittle fracture or uneven film formation during the stretching process; on the other hand, due to the large differences in material structure and polarity between the surface layer and the intermediate layer, the rheological behavior and compatibility do not match during the co-extrusion process, which can easily cause instability or phase separation at the interlayer interface, ultimately resulting in poor overall consistency of the diaphragm and weak interlayer bonding, making it difficult to achieve both high temperature resistance and high mechanical properties.

[0005] Therefore, the current separator technology field urgently needs a new method that can synergistically improve temperature resistance, mechanical strength, and processing consistency. By designing block polymers with specific sequence structures and optimizing the interfacial properties of inorganic fillers, achieving multiphase compatibility and microstructural control at the molecular level has become a key approach to breaking through existing technological bottlenecks and preparing high-performance lithium battery separators. Summary of the Invention

[0006] In order to solve the above problems, the present invention provides an oxide-enhanced, high-strength and high-temperature-resistant diaphragm and its preparation method. Compared with traditional wet-process polyethylene diaphragm, the present invention adopts a three-layer co-extrusion structure, and the surface layer is composed of a block olefin polymer and a styrene elastomer, wherein the block olefin polymer comprises a soft segment polyethylene and a hard segment polyolefin containing a rigid group, and the styrene elastomer is a block copolymer with a (S)m-(P / B)n-(S)m microphase separation structure. The middle layer is composed of high-density polyethylene and a hydrophobically modified oxide. The preparation method includes: synthesis of block olefin polymer, three-layer co-extrusion casting, biaxial stretching, extraction drying, secondary transverse stretching and heat setting. The present invention, through molecular structure design and multiphase compounding, synergistically improves the mechanical strength, film breaking temperature and thermal stability of the diaphragm, while maintaining good electrolyte wettability and processing consistency, effectively solving the problems of poor temperature resistance and insufficient strength of traditional wet-process diaphragms, and significantly improving the safety of lithium batteries.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions: An oxide-reinforced, high-strength, high-temperature-resistant diaphragm is a three-layer co-extruded structure of surface layer / middle layer / surface layer; The surface layer is composed of block olefin polymer and styrene elastomer; The middle layer is composed of high-density polyethylene and hydrophobically modified oxide; The styrene elastomer is a block copolymer with a microphase separation structure, and its general structural formula is (S)m-(P / B)n-(S)m, wherein S is a styrene block, P / B is a propylene-butene copolymer block, and m and n are the degrees of polymerization; the molecular weight of the styrene elastomer is 100,000 to 200,000, of which the mass proportion of the styrene block is 40% to 50%.

[0008] The styrene elastomer in the present invention is a polymer with a microphase separation structure formed by the polymerization of styrene-polypropylene-polybutylene. Due to the addition of styrene, the polymer with the microphase separation structure and the block olefin polymer have similar processing temperatures, which reduces the processing difficulty of the diaphragm. At the same time, the styrene elastomer can form a microphase separation structure with the block olefin polymer after co-extrusion stretching, wherein the block olefin polymer serves as the continuous phase and the styrene elastomer serves as the dispersed phase. When the diaphragm is subjected to stretching or puncture tests, the styrene elastomer dispersed phase acts as a stress concentration point, inducing a large number of microcracks or local plastic deformation to absorb and disperse impact energy, thereby improving the mechanical properties of the diaphragm.

[0009] Furthermore, the block olefin polymer includes a soft segment and a hard segment, wherein the soft segment is polyethylene, the hard segment is a polyolefin containing a rigid group, the molecular weight of which is 500,000 to 1,000,000, and the mass ratio of the soft segment to the hard segment is 60-40:40-60.

[0010] The present invention introduces a hard segment containing a rigid group into polyethylene, which can effectively increase the melting temperature of the block polymer and then improve the high temperature resistance of the diaphragm. And as the content of the hard segment containing a rigid group in the block polymer increases, the melting temperature of the polymer is further improved. At the same time, the introduction of the rigid group also improves the mechanical properties of the diaphragm, and has higher strength after stretching. Moreover, due to the use of a method of copolymerizing polyethylene with a polyolefin containing a rigid group, when co-extruded with the polyethylene of the intermediate layer, the difference in processability between the surface layer and the intermediate layer is effectively avoided, so that the obtained diaphragm maintains good consistency.

[0011] Furthermore, the high-density polyethylene in the middle layer has a weight average molecular weight of 300,000 to 500,000 and a polydispersity coefficient of 4 to 6.

[0012] Furthermore, the hydrophobically modified oxide in the intermediate layer is one or more of aluminum oxide, titanium oxide, and silicon oxide; the hydrophobically modified material is one or more of silane compounds, siloxane polymers, and fluorocarbon compounds, the oxide content is not less than 95%, and the primary particle size of the hydrophobically modified oxide is 10~40 nm, and the secondary particle size is 100~400 nm.

[0013] The hydrophobically modified oxide of the present invention is prepared by a gas phase method. First, the metal salt is gasified and reacted with oxygen, and dendritic oxide agglomerates are obtained by gas-solid separation. The surface energy of the oxide is then reduced by chemical bonding or surface coating to give the oxide super-hydrophobicity. By performing hydrophobic modification on the oxide surface, the compatibility of the oxide with polyethylene can be increased. At the same time, the dendritic morphology also improves the contact with the polyethylene substrate, allowing the oxide to be better embedded in the polyethylene network to form a reinforcing phase, and then the high melting point and high strength of the oxide are used to greatly improve the temperature resistance and mechanical properties of the diaphragm. At the same time, polyethylene also has good electrolyte wettability, which can improve the diaphragm's liquid absorption and storage capacity in the intermediate layer, thereby increasing the electrical performance of the battery.

[0014] Furthermore, the oxide-reinforced, high-strength and high-temperature-resistant diaphragm has a thickness of 5 to 12 μm, a longitudinal tensile strength of not less than 2300 kgf / cm2, a transverse tensile strength of not less than 2200 kgf / cm2, a puncture strength of not less than 400 gf, a membrane rupture temperature of not less than 180°C, and a liquid absorption rate of not less than 20%.

[0015] The present invention also provides a method for preparing the block olefin polymer in the surface layer of the oxide-reinforced, high-strength, high-temperature-resistant diaphragm, comprising the following steps: (1) In a toluene solution of a rigid group vinyl monomer at a concentration of 0.1-0.5 g / ml; (2) Ethylene gas is introduced until saturation and the total pressure is 1 bar, and a catalyst, a co-catalyst and a chain transfer agent are added, and polymerization is carried out at 40-60°C for 1-3 hours; (3) Ethylene is continuously introduced to replenish the ethylene monomer consumed by polymerization, and the total pressure of the system is maintained at 1 bar. After the reaction is completed, the powdered polymerization product is dried.

[0016] Furthermore, the catalyst is a metallocene catalyst, the co-catalyst is an organic boron salt, and the chain transfer agent is trimethylaluminum.

[0017] The present invention also provides a method for preparing the styrene elastomer in the surface layer of the above-mentioned oxide-reinforced, high-strength and high-temperature-resistant diaphragm, comprising the following steps: using styrene, propylene, and butene as monomers, cyclohexane as a solvent, n-butyl lithium as an initiator, and tetrahydrofuran as an activator, anionic polymerization is carried out under an inert atmosphere, silicon tetrachloride is added as a coupling agent to form a linear block structure, and after the reaction is completed, drying is carried out to obtain the elastomer.

[0018] Furthermore, the present invention also provides a method for preparing the above-mentioned oxide-reinforced, high-strength, high-temperature-resistant diaphragm. First, the block olefin polymer, styrene elastomer, and white oil used in the surface layer, and the high-density polyethylene, hydrophobically modified oxide, and white oil used in the middle layer are mixed uniformly, melted in a three-manifold mold through a twin-screw extruder, and then extruded and pulled to obtain a thick sheet. The thick sheet is first stretched longitudinally and transversely to obtain a pore-free polyolefin film. The polyolefin film is then extracted with dichloromethane and dried, and then subjected to secondary transverse stretching, heat setting, pulling, thickness measurement, and then rolled to obtain the oxide-reinforced, high-strength, high-temperature-resistant diaphragm. The method comprises the following steps: (1) Casting sheet: The components of the surface layer and the middle layer are mixed with white oil in proportion, and then subjected to three-layer co-extrusion, and then pulled and cooled to obtain a casting sheet; (2) Stretching to form a film: The obtained cast sheet is sequentially stretched longitudinally and transversely to obtain a polyolefin film without holes; (3) Extraction and drying: extracting the obtained polyolefin membrane precursor with dichloromethane and drying it to obtain a polyolefin microporous membrane precursor; (4) Hole expansion and winding: The obtained polyolefin microporous membrane precursor is subjected to secondary transverse stretching, heat setting, and winding to obtain the oxide-reinforced, high-strength, and high-temperature-resistant diaphragm.

[0019] Furthermore, in step (1), during the three-layer co-extrusion, the ratio of the block olefin polymer, the styrene elastomer, and the white oil in the surface layer is 30-50:1-10:40-69, and the ratio of the high-density polyethylene, the hydrophobically modified oxide, and the white oil in the middle layer is 30-50:5-20:30-65; the extrusion temperature is 200-220°C; the cooling temperature is 20-40°C, and the pulling speed is 30-60 m / min; Furthermore, in step (2), the longitudinal stretching temperature is 120-140°C, and the stretching ratio is 6-10; the transverse stretching temperature is 140-170°C, and the stretching ratio is 6-10; In step (3), the extraction temperature is 20-30°C; the drying temperature is 40-60°C; In step (4), the secondary transverse stretching temperature is 140-170°C, the stretching ratio is 1.1-1.5, and the heat setting temperature is 130-150°C.

[0020] Therefore, the present invention has the following beneficial effects: By modifying the substrate raw materials and adding elastomers and hydrophobically modified oxides, the melting temperature and mechanical properties of the diaphragm are improved, avoiding the pore blockage and cost increase caused by improving temperature resistance through coating, while also improving the safety of the diaphragm in battery applications.

[0021] The present invention adopts block olefin polymer in the surface layer, and through the copolymerization of ethylene and olefin containing rigid groups, the melting temperature of polyolefin is increased, and the film breaking temperature of the diaphragm is effectively increased. The introduction of rigid groups also enhances the mechanical properties of the diaphragm. The addition of styrene elastomer can form a microphase separation structure of continuous phase / dispersed phase with the polymer, thereby improving the mechanical properties of the diaphragm by dispersing stress. At the same time, the copolymerization method can also improve the compatibility of block olefin polymer with styrene elastomer and polyethylene, reduce the stratification caused by different components of the surface layer and the difference in fluidity between the surface layer and the intermediate layer, and improve the integrity of the polyolefin diaphragm.

[0022] The present invention utilizes high-density polyethylene and a hydrophobically modified oxide in the middle layer. The dendritic structure and surface hydrophobicity of the hydrophobically modified oxide enhance compatibility with polyethylene, forming an oxide reinforcement phase dispersed within the polyethylene matrix. This in turn improves the separator's temperature resistance and mechanical properties. Furthermore, the excellent electrolyte wettability of polyethylene also enhances the separator's liquid absorption and storage capacity, improving the battery's electrical performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic structural diagram of the oxide-reinforced, high-strength and high-temperature-resistant diaphragm provided by the present invention.

[0024] 1. Diaphragm surface layer, 2. Diaphragm middle layer, 3. Microporous structure in the diaphragm surface layer, 4. Microporous structure in the diaphragm middle layer, 5. Hydrophobic modified oxide in the diaphragm middle layer.

[0025] Figure 2 This is a SEM cross-sectional view of the oxide-reinforced, high-strength and high-temperature-resistant diaphragm provided by the present invention. DETAILED DESCRIPTION

[0026] The present invention will be further described below in conjunction with specific embodiments.

[0027] In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in the industry. The methods in the following embodiments, unless otherwise specified, are all conventional methods in the art.

[0028] Overall embodiment

[0029] The present invention provides an oxide-reinforced, high-strength, high-temperature-resistant diaphragm having a three-layer co-extruded structure: surface layer / middle layer / surface layer. The surface layer comprises a block olefin polymer and a styrene elastomer, and the middle layer comprises high-density polyethylene and a hydrophobically modified oxide.

[0030] The method for preparing the oxide-reinforced, high-strength, high-temperature-resistant diaphragm provided by the present invention comprises the following steps:

[0031] 1. Raw Materials Preparation (1) Preparation of surface layer block olefin polymer: In a toluene solution of a rigid group vinyl monomer with a concentration of 0.1-0.5 g / ml, ethylene gas is introduced until saturation occurs, and the total pressure of the system is maintained at 1 bar; Add catalyst, co-catalyst and chain transfer agent, carry out polymerization reaction at 40-60°C for 1-3 hours, and continuously introduce ethylene to maintain the system pressure; After the reaction is completed, the powdered block olefin polymer is obtained by drying; (2) Preparation of surface layer styrene elastomer: Using styrene, propylene, and butene as monomers, cyclohexane as solvent, n-butyl lithium as initiator, and tetrahydrofuran as activator, anionic polymerization is carried out under an inert atmosphere. Silicon tetrachloride is added as a coupling agent to form a linear block structure. After the reaction is completed, the elastomer is dried. (3) Middle layer high-density polyethylene (HDPE): Select HDPE with a weight average molecular weight of 300,000 to 500,000 and a polydispersity index of 4 to 6; (4) Hydrophobically modified oxide in the middle layer: The hydrophobically modified oxide is selected from one or more of aluminum oxide, titanium oxide, and silicon oxide, and its surface is modified with silane, siloxane, or fluorocarbon hydrophobic groups. The primary particle size of the hydrophobically modified oxide is 10 to 40 nm, the secondary particle size is 100 to 400 nm, and the oxide content is not less than 95%; (5) Diluent: white oil.

[0032] 2. Diaphragm Preparation (1) Casting sheet: The components of the surface layer and the middle layer are mixed with white oil in proportion, and then subjected to three-layer co-extrusion, and then pulled and cooled to obtain a casting sheet; (2) Stretching to form a film: The obtained cast sheet is sequentially stretched longitudinally and transversely to obtain a polyolefin film without holes; (3) Extraction and drying: extracting the obtained polyolefin membrane precursor with dichloromethane and drying it to obtain a polyolefin microporous membrane precursor; (4) Hole expansion and winding: The obtained polyolefin microporous membrane precursor is subjected to secondary transverse stretching, heat setting, and winding to obtain the oxide-reinforced, high-strength, and high-temperature-resistant diaphragm.

[0033] In step (1), when the three layers are co-extruded, the ratio of the block olefin polymer, the styrene elastomer and the white oil in the surface layer is (30-50): (1-10): (40-69); The ratio of high-density polyethylene, hydrophobically modified oxide and white oil in the middle layer is (30-50): (5-20): (30-65); The extrusion temperature of the three-layer co-extrusion is 200-220°C, the cooling roller temperature is 20-40°C, and the pulling speed is 30-60m / min; In step (2), the longitudinal stretching temperature is 120-140°C, and the stretching ratio is 6-10; the transverse stretching temperature is 140-170°C, and the stretching ratio is 6-10; In step (3), the extraction temperature is 20-30°C; the drying temperature is 40-60°C; In step (4), the secondary transverse stretching temperature is 140-170°C, the stretching ratio is 1.1-1.5, and the heat setting temperature is 130-150°C.

[0034] Example 1 1. Raw Materials Preparation (1) Preparation of surface layer block olefin polymer: Ethylene gas was introduced into a 0.3 g / ml toluene solution of 4-methyl-1-pentene (as a vinyl monomer containing a rigid group) until saturation, and the total pressure of the system was maintained at 1 bar; A metallocene catalyst (pentamethylcyclopentadienyl titanium(IV) dichloride), a cocatalyst (triphenylcarbonium tetrakis(pentafluorophenyl)borate), and a chain transfer agent (trimethylaluminum) were added, and polymerization was carried out at 50°C for 2 hours while continuously introducing ethylene to maintain the system pressure. After the reaction is completed, the powdered block olefin polymer is dried and has a molecular weight of 750,000, with a mass ratio of soft segments (polyethylene) to hard segments (polyolefin containing rigid groups) of 60:40. (2) Preparation of surface layer styrene elastomer: Using styrene, propylene, and butene as monomers, cyclohexane as solvent, n-butyl lithium as initiator, and tetrahydrofuran as activator, anionic polymerization was carried out under argon atmosphere, and silicon tetrachloride was added as a coupling agent to form a linear block structure (SP / BS); After the reaction is completed, the elastomer is dried to obtain a molecular weight of 150,000, of which the mass proportion of the styrene block is 45%; (3) Middle layer high-density polyethylene (HDPE): Use HDPE with a weight average molecular weight of 400,000 and a polydispersity index of 5.0; (4) Hydrophobically modified oxide in the middle layer: Alumina with a surface modified with octadecyltrichlorosilane (silane) was selected. Its primary particle size is 20nm, secondary particle size is 200nm, and oxide content is 97%; (5) Diluent: white oil.

[0035] 2. Diaphragm Preparation (1) Casting: 30 parts of the block olefin polymer, 1 part of the styrene elastomer and 69 parts of white oil were mixed and fed into a first twin-screw extruder; Mix 30 parts of the above HDPE, 5 parts of the above hydrophobically modified alumina and 65 parts of white oil and put them into a second twin-screw extruder; Adjust the temperature of the two extruders to 210°C. After melting and plasticization, the material extruded from the first extruder is used as the upper and lower surface layers (layer A), and the material extruded from the second twin-screw extruder is used as the middle layer (layer B). After being compounded through a multi-layer co-extrusion three-manifold die head, it is extruded; The melt extruded from the die was cooled on a cooling roller at 30°C and a three-layer composite casting sheet was obtained at a pulling speed of 40 m / min. (2) Stretch film forming: The obtained cast sheet was longitudinally stretched at 120°C with a stretching ratio of 8; Subsequently, the film was stretched transversely at 150°C with a stretching ratio of 8 to obtain a polyolefin film precursor without pores; (3) Extraction and drying: The stretched membrane was extracted in dichloromethane at 20 °C to completely remove the white oil; Then, the mixture was dried in an oven at 60° C. to obtain a polyolefin microporous membrane precursor; (4) Hole expansion and coiling: The dried film was subjected to secondary transverse stretching at 150 °C with a stretching ratio of 1.3; Then heat setting is carried out at 130°C to eliminate internal stress and stabilize the pore structure; Finally, after traction and online thickness measurement, the oxide-reinforced, high-strength and high-temperature-resistant diaphragm is rolled up. The corresponding structural diagram is shown in FIG. Figure 1 As shown, the SEM cross-sectional view is Figure 2 shown.

[0036] The average thickness of the membrane was measured to be 9.0 μm.

[0037] Example 2 The same method as in Example 1 was used, except that the mass ratio of the soft segment to the hard segment in step (1) was 40:60.

[0038] Example 3 The same method as Example 2 was used, except that step (1) contained 30 parts of block olefin polymer, 10 parts of styrene elastomer and 60 parts of white oil.

[0039] Example 4 The same method as Example 3 was used, except that in step (2), 40 parts of high-density polyethylene, 20 parts of hydrophobically modified alumina and 40 parts of white oil were used.

[0040] Example 5 The same method as in Example 1 was used, except that the mass ratio of the soft segment to the hard segment in step (1) was 50:50.

[0041] Comparative Example 1 The same method as in Example 1 was used, except that the block olefin polymer in the surface layer in step (1) contained only soft segments and no hard segments.

[0042] Comparative Example 2 The same method as in Example 1 was used, except that the surface layer in step (1) did not contain styrene elastomer.

[0043] Comparative Example 3 The same method as Example 1 was used, except that the middle layer in step (1) was 30 parts of high-density polyethylene and 70 parts of white oil, and did not contain hydrophobically modified oxide.

[0044] Comparative Example 4 The same method as in Example 1 was used, except that only the first twin-screw extruder was used and the thickness of the surface layer was 100%.

[0045] Comparative Example 5 The same method as in Example 1 was used, except that only the second twin-screw extruder was used and the thickness of the middle layer was 100%.

[0046] The oxide-reinforced, high-strength, high-temperature-resistant diaphragm prepared by the present invention was cut into A4 size and tested for various mechanical and thermal properties. The test items are as follows: (1) Average thickness Use a micrometer to measure the thickness of oxide-reinforced, high-strength, high-temperature resistant diaphragms at different locations and calculate the average value. (2) Tensile strength The longitudinal and transverse tensile strength of oxide-reinforced, high-strength and high-temperature-resistant diaphragms were tested using a Xieqiang CTM universal testing machine. Five specimens were tested in each direction and the average value was calculated. (3) Puncture strength The puncture strength of oxide-reinforced, high-strength, high-temperature resistant diaphragms was tested using a Xieqiang CTM universal testing machine. Five specimens were tested and the average value was calculated. (4) Membrane rupture temperature Use a hot stage microscope to test the rupture temperature of the oxide-reinforced, high-strength, high-temperature resistant diaphragm. Record the temperature when the diaphragm begins to melt. Test 5 samples and calculate the average value. (5) Liquid absorption rate The liquid absorption rate of oxide-reinforced, high-strength, high-temperature-resistant separators is tested gravimetrically. First, the separator is completely dried and its mass is recorded. Then, the dried separator is immersed in electrolyte for 24 hours. After wiping off the electrolyte, the separator's weight is recorded again. The difference between the two recorded values ​​is the separator's liquid absorption rate. Five samples are tested and the average value is calculated.

[0047] The tensile strength, puncture strength, thermal shrinkage, film rupture temperature, and liquid absorption of the high-strength and high-temperature-resistant separators prepared in Examples 1 to 5 and Comparative Examples 1 to 5 are shown in Table 1 below.

[0048] Table 1 Test results of high-strength and high-temperature-resistant diaphragms of Examples 1 to 5 and Comparative Examples 1 to 5

[0049]

[0050] As can be seen from Table 1, the oxide-reinforced, high-strength, high-temperature-resistant separators prepared using the methods of the present invention in Examples 1-5 can effectively improve the separator's rupture temperature, longitudinal and transverse tensile strength, and puncture strength. Compared to the prior art, the oxide-reinforced, high-strength, high-temperature-resistant separators provided by the present invention address the issues of insufficient heat resistance and wettability of conventional wet-process polyethylene separators. By introducing a composite system of block olefin polymers composed of soft / hard segments and a styrene elastomer with a specific microphase separation structure, and introducing a composite system of high-density polyethylene and a hydrophobically modified oxide in the middle layer, the separator's overall performance is successfully synergistically improved, avoiding the pore blockage and increased cost associated with increasing heat resistance through coating, while also improving the separator's safety in battery applications.

[0051] In contrast, the block olefin polymer in the surface layer of Comparative Example 1 contained only soft segments and no hard segments. The tensile strength of the separator was significantly lower than that of Example 1, demonstrating that the present invention utilizes a block olefin polymer in the surface layer. By copolymerizing ethylene with an olefin containing rigid groups, the polyolefin's melting point is increased, effectively raising the separator's rupture temperature. The introduction of rigid groups also enhances the separator's mechanical properties.

[0052] Comparative Example 2, in which the surface layer does not contain styrene elastomer, exhibits significantly lower puncture strength compared to Example 1, demonstrating that the addition of styrene elastomer can form a continuous / dispersed phase microphase separation structure with the polymer, thereby improving the mechanical properties of the membrane by dispersing stress. Furthermore, the copolymerization approach improves the compatibility of the block olefin polymer with the styrene elastomer and polyethylene, reducing delamination caused by different surface layer components and fluidity differences between the surface layer and the intermediate layer, thereby enhancing the integrity of the polyolefin membrane.

[0053] In Comparative Example 3, the middle layer is 30 parts of high-density polyethylene and 70 parts of white oil, and does not contain hydrophobically modified oxide. Compared with Example 1, the membrane rupture temperature is significantly reduced, indicating that the present invention uses high-density polyethylene and hydrophobically modified oxide in the middle layer, and utilizes the dendritic structure and surface hydrophobicity of the hydrophobically modified oxide itself to improve the compatibility with polyethylene, forming an oxide reinforcement phase dispersed in the polyethylene substrate, thereby improving the temperature resistance and mechanical properties of the diaphragm. At the same time, the good electrolyte wettability of polyethylene can also improve the liquid absorption and storage capacity of the diaphragm and increase the electrical performance of the battery.

[0054] Comparative Examples 4 and 5, using only the first and second twin-screw extruders, with a surface layer thickness of 100% (single-layer structure), demonstrate that a three-layer composite structure is the best choice for achieving both high strength, high temperature resistance, and good wettability, while avoiding performance imbalance.

[0055] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art to which this disclosure pertains. Unless otherwise specified, the raw materials and equipment used herein are conventional in the art and can be obtained from conventional commercial sources. The methods used herein are conventional in the art, unless otherwise specified.

[0056] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent transformation made to the above embodiment based on the technical essence of the present invention still fall within the scope of protection of the technical solution of the present invention.

Claims

1. An oxide-reinforced, high-strength, high-temperature-resistant diaphragm, characterized by: The oxide-reinforced, high-strength, high-temperature-resistant diaphragm is a three-layer co-extruded structure of surface layer / middle layer / surface layer; The surface layer is composed of block olefin polymer and styrene elastomer; The middle layer is composed of high-density polyethylene and hydrophobically modified oxide; The styrene elastomer is a block copolymer with a microphase separation structure, and its general structural formula is (S)m-(P / B)n-(S)m, wherein S is a styrene block, P / B is a propylene-butene copolymer block, and m and n are polymerization degrees.

2. The oxide-reinforced, high-strength, high-temperature-resistant diaphragm according to claim 1, characterized in that: The block olefin polymer includes a soft segment and a hard segment, the soft segment is polyethylene, the hard segment is a polyolefin containing a rigid group, the molecular weight of the block olefin polymer is 500,000 to 1,000,000, the mass ratio of the soft segment to the hard segment is (60 to 40): (40 to 60), and / or the molecular weight of the styrene elastomer is 100,000 to 200,000, of which the mass proportion of the styrene block is 40% to 50%.

3. The oxide-reinforced, high-strength, high-temperature-resistant diaphragm according to claim 1, characterized in that: The high-density polyethylene in the middle layer has a weight average molecular weight of 300,000 to 500,000 and a polydispersity coefficient of 4 to 6.

4. The oxide-reinforced, high-strength, high-temperature-resistant diaphragm according to claim 1, characterized in that: The hydrophobically modified oxide is one or more of aluminum oxide, titanium oxide, and silicon oxide, and its surface is modified with silane, siloxane or fluorocarbon hydrophobic groups. The primary particle size of the hydrophobically modified oxide is 10 to 40 nm, the secondary particle size is 100 to 400 nm, and the oxide content is not less than 95%.

5. The oxide-reinforced, high-strength, high-temperature-resistant diaphragm according to any one of claims 1 to 4, characterized in that: The thickness of the diaphragm is 5 to 12 μm, the longitudinal tensile strength is not less than 2300 kgf / cm², the transverse tensile strength is not less than 2200 kgf / cm², the puncture strength is not less than 400 gf, the membrane rupture temperature is not less than 180°C, and the liquid absorption rate is not less than 20%.

6. A method for preparing a surface layer block olefin polymer for the diaphragm according to claim 1, characterized in that: The following steps are involved: (1) Ethylene gas was introduced into a toluene solution of a rigid group vinyl monomer at a concentration of 0.1-0.5 g / ml until saturation, and the total pressure of the system was maintained at 1 bar; (2) Adding a catalyst, a co-catalyst and a chain transfer agent, and carrying out a polymerization reaction at 40-60°C for 1-3 hours, while continuously introducing ethylene to maintain the system pressure; (3) After the reaction is completed, the block olefin polymer is dried to obtain a powdery form.

7. The preparation method according to claim 6, characterized in that: The catalyst is a metallocene catalyst, the co-catalyst is an organic boron salt, and the chain transfer agent is trimethylaluminum.

8. A method for preparing the surface layer styrene elastomer for the diaphragm according to claim 1, characterized in that: The following steps are involved: With styrene, propylene and butene as monomers, cyclohexane as solvent, n-butyl lithium as initiator and tetrahydrofuran as activator, anionic polymerization is carried out under an inert atmosphere, silicon tetrachloride is added as a coupling agent to form a linear block structure, and after the reaction is completed, the elastomer is dried.

9. A method for preparing an oxide-reinforced, high-strength, high-temperature-resistant diaphragm according to any one of claims 1 to 5, characterized in that: The following steps are involved: (1) Casting sheet: The components of the surface layer and the middle layer are mixed with white oil in proportion, and then subjected to three-layer co-extrusion, and then pulled and cooled to obtain a casting sheet; (2) Stretching to form a film: The obtained cast sheet is sequentially stretched longitudinally and transversely to obtain a polyolefin film without holes; (3) Extraction and drying: extracting the obtained polyolefin membrane precursor with dichloromethane and drying it to obtain a polyolefin microporous membrane precursor; (4) Hole expansion and winding: The obtained polyolefin microporous membrane precursor is subjected to secondary transverse stretching, heat setting, and winding to obtain the oxide-reinforced, high-strength, and high-temperature-resistant diaphragm.

10. The method for preparing an oxide-reinforced, high-strength, high-temperature-resistant diaphragm according to claim 9, characterized in that: In step (1), when the three layers are co-extruded, the ratio of the block olefin polymer, the styrene elastomer and the white oil in the surface layer is (30-50): (1-10): (40-69); The ratio of high-density polyethylene, hydrophobically modified oxide and white oil in the middle layer is (30-50): (5-20): (30-65); The extrusion temperature of the three-layer co-extrusion is 200-220°C, the cooling roller temperature is 20-40°C, and the pulling speed is 30-60m / min; In step (2), the longitudinal stretching temperature is 120-140°C, and the stretching ratio is 6-10; the transverse stretching temperature is 140-170°C, and the stretching ratio is 6-10; In step (3), the extraction temperature is 20-30°C; the drying temperature is 40-60°C; In step (4), the secondary transverse stretching temperature is 140-170°C, the stretching ratio is 1.1-1.5, and the heat setting temperature is 130-150°C.

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