Gradient aperture composite diaphragm with high wettability and preparation process thereof

Abstract

The invention discloses a high-wettability gradient aperture composite diaphragm and a preparation process thereof. The diaphragm comprises a polyolefin base membrane layer and a synthetic fiber web layer prepared by a spunlace process, the overall thickness of the diaphragm is 8-12 [mu] m, the aperture of one side facing the positive electrode is 30-50 nm, and the aperture of one side facing the negative electrode is 50-100 nm; and the synthetic fiber web layer contains a tannic acid modifier. The tannic acid modified synthetic fiber web layer in the composite diaphragm can provide better wettability for the composite diaphragm, and polar functional groups on the surface of the tannic acid modified synthetic fiber web layer can homogenize lithium ion flow, guide uniform deposition of lithium and inhibit generation of lithium dendrites. The polyolefin layer mainly provides mechanical strength, and forms a gradient aperture structure with the spunlace modified polyethylene layer, so that the uniformity of current density is improved, the uniform deposition of lithium is effectively induced, and the growth of lithium dendrites is reduced.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a highly wettable gradient pore size composite separator and its preparation process. Background Technology

[0002] Lithium-ion batteries are the core power source for new energy vehicles and energy storage systems, and their safety and cycle life are crucial. As a key inner component, the battery separator's core function is to isolate the positive and negative electrodes to prevent short circuits, while simultaneously providing a transport channel for lithium ions. From an industry demand perspective, the "White Paper on the Development of China's Lithium-ion Battery Separator Industry" states that the market demand for lithium battery separators is mainly divided into two categories: power batteries and energy storage batteries. Power batteries dominate, primarily used in electric vehicles, hybrid vehicles, and other new energy vehicle fields. With the rapid development of the new energy industry, the market demand for power batteries continues to grow, and the performance requirements for separators are constantly increasing. The global lithium battery separator market size is projected to exceed US$20 billion by 2025, with high-safety and high-heat-resistant separators experiencing a compound annual growth rate exceeding 25%. The main requirements for power batteries include fast charging and safety, corresponding to properties such as battery separator pore size, porosity, thermal shrinkage, strength, and hydrophilicity. Traditional polyolefin battery separators possess uniform porosity and excellent mechanical properties. However, it has poor high-temperature resistance and high thermal shrinkage, which can easily cause contact between the positive and negative electrodes, leading to short circuits. In addition, polyolefins themselves have poor hydrophilicity, so the electrolyte wetting speed is slow and the ionic conductivity is low, which affects the cycle performance of the battery.

[0003] In recent years, dopamine biomimetic modification has been frequently used for membrane material modification. However, dopamine monomers are expensive, and dopamine is more suitable for alkaline environments, while lithium-ion batteries operate in a slightly acidic environment. To address this, researchers have discovered lower-cost plant polyphenols, natural polyphenol macromolecules extracted and purified from plant bark, leaves, and fruits. Their molecular structure contains numerous phenolic hydroxyl groups and pyrogallol structures, providing abundant binding sites for water molecules, thus exhibiting good hydrophilicity and being commonly used for hydrophilic modification of membrane materials. The dihydroxyphenyl and trihydroxyphenyl groups in tannic acid can act as hydrogen bond donors, forming strong hydrogen bonds with molecules containing hydrogen bond acceptors; while the pyrogallol structure can form coordinate bonds with various metal cations, thus interacting with a variety of materials through hydrogen bonds, electrostatics, coordinate bonds, and hydrophobic interactions. For example, in CN113629352B, polyethyleneimine and polyethylene are miscibly extruded into a cast sheet, stretched to form a film, and then impregnated in a tannic acid buffer solution. After impregnation with deionized water, oxygen anions can be generated between adjacent hydroxyl groups in the tannic acid, which can form chelates with metal ions, causing the tannic acid to polymerize and form a hydrated layer, further enhancing surface polarity, improving wettability, and increasing ionic conductivity. Although this effectively improves the wettability of the membrane, the chelates formed by tannic acid with trivalent chromium ions and metal ions firmly adhere to the membrane surface, which to some extent reduces the porosity of the membrane itself, causing pore blockage and hindering ion transport. In CN110416476B, a dispersant, lithium-conducting polymer, pore-forming agent, and tannic acid are mixed and ground to form a highly conductive slurry, which is then coated on both sides of a polyolefin-based membrane by roller coating. The high polarity of tannic acid molecules can improve the affinity of the electrolyte and has good adhesion; the lithium-conducting polymer can promote the dissociation of lithium salts, which is conducive to lithium ion migration. However, tannic acid itself is easy to combine with metal ions, so it is necessary to add metal ions to the slurry to cross-link with tannic acid first, so that its viscosity can be suitable for the roller coating process. However, this is not reflected in the slurry formulation of the patent.

[0004] In summary, traditional polyolefin lithium battery separators suffer from shortcomings in high-temperature resistance and electrolyte wettability. While natural plant polyphenols (such as tannic acid) are considered promising modification materials due to their excellent hydrophilicity and cost advantages, they also face challenges in practical applications: tannic acid readily binds to metal ions, potentially causing pore blockage in the separator and affecting lithium-ion transport efficiency. The core technical problem this invention urgently needs to solve is how to innovatively utilize the hydrophilic advantage of tannic acid while effectively mitigating the risk of pore blockage through material design or process optimization, thereby preparing a highly wettable battery separator that combines efficient ion channels with excellent electrolyte affinity. Summary of the Invention

[0005] The purpose of this invention is to provide a highly wettable gradient pore size composite membrane and its preparation process.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] In a first aspect, the present invention claims protection for a highly wettable gradient pore size composite membrane, the composite membrane comprising a polyolefin-based membrane layer and a synthetic fiber web layer prepared by a hydroentangling process; the composite membrane having a thickness of 5-15 μm, wherein the pore size on the positive electrode side is 30-50 nm and the pore size on the negative electrode side is 50-100 nm; and the synthetic fiber web layer contains a tannic acid modifier.

[0008] Furthermore, the composite membrane has a porosity of 60-70% (e.g., 60%, 62%, 64%, 66%, 68%, or 70%), an air permeability of 105-130 sec / 100cc (e.g., 105 sec / 100cc, 110 sec / 100cc, 115 sec / 100cc, 120 sec / 100cc, 125 sec / 100cc, 130 sec / 100cc), and a puncture strength of 530-640 gf. (e.g., 530gf, 540gf, 550gf, 560gf, 570gf, 580gf, 590gf, 600gf, 610gf, 620gf, 630gf), the composite diaphragm has a MD tensile strength of 3000~3700kgf (e.g., 3000kgf, 3100kgf, 3200kgf, 3300kgf, 3400kgf, 3500kgf, 3600kgf), and the composite diaphragm has a TD tensile strength of 2700~3400kgf. The composite diaphragm has a heat shrinkage rate of less than 2.6% (e.g., 2700kgf, 2800kgf, 2900kgf, 3000kgf, 3100kgf, 3200kgf, 3300kgf, 3400kgf) per kgf (e.g., 2700kgf, 2800kgf, 2900kgf, 3000kgf, 3100kgf, 3200kgf, 3300kgf, 3400kgf), a heat shrinkage rate of less than 2.6% (e.g., 2.1~2.6%, further 2.1%, 2.2%, 2.4%, 2.6%), a heat shrinkage rate of less than 1.5% (e.g., 1.1~1.5%, further 1.1%, 1.2%, 1.4%, 1.5%), and a contact angle of less than 50° (e.g., 25~50°, further 25°, 30°, 35°, 40°, 45°, 50°).

[0009] Furthermore, the composite membrane has a pore size difference of 20~50nm between the negative electrode side and the positive electrode side (for example, it can be 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, or 50nm), and more preferably 27~42nm, which enables the composite membrane to have a smaller contact angle and lower air permeability.

[0010] Furthermore, the thickness of the composite membrane is 5~12μm, more preferably 7~12μm.

[0011] Furthermore, the tannic acid modifier is applied via a water jet in a hydroentanglement process. The water jet is a tannic acid solution containing 3-16 g / L tannic acid, 10 g / L bis-Tris buffer, and 0.1 M NaCl, with a pH of 6.5-7.0.

[0012] Furthermore, the tannic acid solution comprises: 15 g / L tannic acid, 10 g / L bis-Tris buffer, and 0.1 M NaCl, with a pH of 6.5-7.0.

[0013] Furthermore, the polyolefin-based film can be polyethylene, polypropylene, or a mixture of polyethylene and polypropylene.

[0014] Furthermore, the denier of the synthetic fibers used to prepare the synthetic fiber web layer is 0.3-3 denier (preferably 1 denier).

[0015] Furthermore, the synthetic fiber is at least one of polyethylene fiber, polypropylene fiber, and viscose fiber.

[0016] Secondly, the present invention claims protection for the preparation process of the above-mentioned highly wettable gradient pore size composite membrane, which includes the following steps:

[0017] (1) Fiber preparation and opening: The synthetic fibers are opened;

[0018] (2) Carding into a web: The opened fibers are carded and cross-laid through a carding machine to form a three-dimensional network structure;

[0019] (3) Pre-wetting and preliminary modification: The fiber web is pre-wetted using a water jet; the water jet is a tannic acid solution; the pre-needling pressure during pre-wetting is 0.8-2 MPa (preferably 0.8-1.5 MPa), and the diameter of the water jet is 0.08-0.18 mm;

[0020] (4) Hydroentangling reinforcement and modification: The pre-wetted fiber web is reinforced by main hydroentangling, wherein the front section of the hydroentangling head (e.g., the 1st to 4th hydroentangling head) uses the tannic acid solution as the water jet, and the rear section of the hydroentangling head (e.g., the 5th to 9th hydroentangling head) is rinsed with ultrapure water; the hydroentangling pressure range is 2-14MPa, the diameter of the water needle is 0.08-1.8mm, and the water needle action distance is 10-35mm;

[0021] (5) Dehydration and drying: Vacuum dehydration of the hydroentangled fiber web and drying at 50-100℃;

[0022] (6) Composite: The dried modified fiber web and polyolefin base film are composited by hot pressing at 100-130℃.

[0023] Furthermore, in step (4), the hydroentangling process parameters during the main hydroentangling reinforcement adopt a low-pressure distribution, specifically: hydroentangling head 1 pressure: 2-5MPa, hydroentangling head 2 pressure: 2-5MPa, hydroentangling head 3 pressure: 3-7MPa, hydroentangling head 4 pressure: 3-7MPa, hydroentangling head 5 pressure: 4-9MPa, hydroentangling head 6 pressure: 5-12MPa, hydroentangling head 7 pressure: 5-12MPa, hydroentangling head 8 pressure: 5-14MPa, hydroentangling head 9 pressure: 5-14MPa.

[0024] Furthermore, the process parameters of the combing machine in step (2) are set as follows: breast cylinder: 200-500 m / min, breast cylinder working roll: 20-60 m / min, main cylinder: 700-1200 m / min, main cylinder working roll: 20-60 m / min, main cylinder stripping roll: 50-100 m / min, upper doffer spacing: 10-50 mm, lower doffer spacing: 10-50 mm.

[0025] Thirdly, the present invention claims protection for a lithium-ion battery that uses the above-described highly wettable gradient pore size composite separator or the highly wettable gradient pore size composite separator prepared by the above-described preparation process.

[0026] In a specific embodiment of the present invention, the detailed preparation method of the highly wettable gradient pore size composite membrane includes the following steps:

[0027] (1) Fiber preparation and opening

[0028] The synthetic fibers are opened using an opening machine to break down larger fiber blocks and clusters into smaller ones, removing impurities from the fiber blocks. The synthetic fibers have a denier of 0.3-3 denier; the smaller the denier, the smaller the pores in the resulting fiber web. In addition to the polyethylene fiber exemplified in the specific embodiment, other fibers suitable for hydroentangling processes can be used, such as polypropylene and viscose fibers, but are not limited to these.

[0029] (2) Organize into a network

[0030] The opened fibers are fed into a carding machine for further opening, impurity removal, and mixing, turning the bulk fibers into bundles and ultimately forming individual fibers arranged longitudinally. Then, through cross-laying, the fiber web is evenly folded to a certain width and thickness, increasing the aspect ratio and forming an isotropic three-dimensional network structure.

[0031] Carding machine process parameters: breast cylinder: 200-500m / min, breast cylinder work roll: 20-60m / min, main cylinder: 700-1200m / min, main cylinder work roll: 20-60m / min, main cylinder stripping roll: 50-100m / min, upper doffer spacing: 10-50mm, lower doffer spacing: 10-50mm.

[0032] When carding fine denier fibers, excessively high speeds can lead to insufficient fiber carding and fiber damage. Therefore, the speeds of the main cylinder and other carding elements should be appropriately reduced. Smaller doffer spacing results in less fiber return to the main cylinder, reducing surface load and improving the uniformity of the fiber web.

[0033] (3) Pre-wetting and polyethylene modification

[0034] Pre-wetting wets and compacts the loose fiber web, expelling internal air and enhancing fiber entanglement during hydroentangling. This improves the efficiency of fiber absorption of hydroentanglement energy.

[0035] The water jet is a tannic acid solution. The tannic acid solution is immersed into the fiber web with the water jet. The tannic acid molecules can form a huge hydrogen bond network with each other through hydroxyl groups, and cross-link themselves into a strong film. In addition, the phenolic hydroxyl groups of tannic acid can form strong hydrogen bonds with the oxygen-containing groups formed by slight oxidation on the surface of synthetic fibers. It can be firmly adsorbed on the surface of synthetic fibers (such as polyethylene fibers) to achieve the purpose of modification.

[0036] The tannic acid solution contains: 3-16 g / L tannic acid, 10 g / L bis-Tris buffer and 0.1 M NaCl, pH 6.5-7.0.

[0037] In a specific embodiment of the present invention, the preparation process of the tannic acid solution is as follows: 5g of bis-Tris buffer is dissolved in 500ml of 0.1mol / L NaCl solution, and the pH is adjusted to 6.5-7 using concentrated hydrochloric acid to form a bis-Tris-NaCl solution. 1.5g-8g of tannic acid is dissolved in the bis-Tris-NaCl solution, with a tannic acid concentration range of 3g / L-16g / L.

[0038] Pre-wetting pre-puncture pressure: 0.8-2MPa (preferably 0.8-1.5MPa), water needle diameter: 0.08-0.18mm.

[0039] (4) Hydroentanglement reinforcement and polyethylene modification

[0040] After being pre-wetted, the fiber web enters the main hydroentangling process. The water jets from the hydroentangling head spray multiple fine water jets that impact the fiber web at a vertical angle. Under the dual effects of the direct impact of the water jets and the rebound and penetration of the water column from the reverse drag net curtain, the fibers in the fiber web shift, interweave, entangle, and bind together, forming countless flexible entanglement points, thereby reinforcing the fiber web and enhancing its strength.

[0041] Main hydroentangling process parameters:

[0042] The higher the hydroentanglement pressure, the more energy the fibers receive, resulting in better entanglement and higher mechanical strength of the web. However, when the web is thin, the hydroentanglement pressure should not be too high, otherwise it will damage the fibers. Hydroentanglement pressure range: 2-14 MPa; Hydroentanglement head 1 pressure: 2-5 MPa; Hydroentanglement head 2 pressure: 2-5 MPa; Hydroentanglement head 3 pressure: 3-7 MPa; Hydroentanglement head 4 pressure: 3-7 MPa; Hydroentanglement head 5 pressure: 4-9 MPa; Hydroentanglement head 6 pressure: 5-12 MPa; Hydroentanglement head 7 pressure: 5-12 MPa; Hydroentanglement head 8 pressure: 5-14 MPa; Hydroentanglement head 9 pressure: 5-14 MPa.

[0043] A smaller water needle diameter increases the water needle's energy, enhancing the impact force on the fiber web during high-speed jetting from the hydroentanglement nozzle, resulting in tighter fiber entanglement. The water needle diameter ranges from 0.08 to 1.8 mm.

[0044] Reducing the water jet's effective distance is beneficial for fiber entanglement and improves the tightness of the entanglement. Water jet effective distance: 10-35mm.

[0045] The water jet from water jet heads 1-4 is a tannic acid solution containing 3-16 g / L tannic acid, 10 g / L bis-Tris buffer, and 0.1 M NaCl, with a pH of 6.5-7.0. This is the same modification as the pre-wetting process.

[0046] The water jet from the water jet heads 5-9 is ultrapure water, which washes the modified fiber web to remove unreacted substances and poorly adhered tannic acid polymers from the water-washed surface.

[0047] The synthetic fiber web layer formed in this process has a three-dimensional network structure with a pore size ranging from 50 to 100 nm, which is larger than that of conventional polyethylene-based films.

[0048] (5) Dehydration and drying

[0049] Vacuum dehydration is performed on the hydroentangled fiber web to quickly remove excess water from the web. Excessive water residue will disperse the energy of the water jet and affect the consolidation of the entanglement.

[0050] The dehydrated fiber web is then dried to further remove moisture and ensure the dryness of the product. The drying temperature is 50-100℃.

[0051] The difference between this design and conventional processes is that conventional modification is carried out through dipping, roller coating, spraying, or dot coating. However, this design combines modification with hydroentangling, and the modification is completed directly through the hydroentangling process while the hydroentangled polyethylene layer is being prepared, thus saving the separate modification process.

[0052] (6) Composite diaphragm

[0053] Hydroentangled modified synthetic fiber webs and polyolefin-based membranes are hot-pressed to prepare hydroentangled modified composite membranes at a temperature of 100-130℃.

[0054] The composite membrane consists of two layers: a hydroentangled modified synthetic fiber web layer and a polyolefin-based membrane. The tannic acid-modified synthetic fiber web layer provides better wettability to the composite membrane, and its surface polar functional groups can homogenize lithium-ion flow, guide uniform lithium deposition, and suppress lithium dendrite formation. The polyolefin-based membrane layer primarily provides mechanical strength and forms a gradient pore structure with the hydroentangled modified synthetic fiber web layer (e.g., ...). Figure 1 As shown in the figure, this improves the uniformity of current density, effectively induces uniform lithium deposition, and reduces the growth of lithium dendrites.

[0055] The beneficial effects of this invention are:

[0056] (1) Significantly improves electrolyte wettability and ionic conductivity: This invention effectively overcomes the inherent defect of poor affinity between traditional polyolefin membranes and electrolytes. On the one hand, the surface of tannic acid (TA) is rich in a large number of polar phenolic hydroxyl groups (-OH). These groups can form strong hydrogen bonding forces with polar electrolyte solvent molecules (such as ethylene carbonate EC), which greatly reduces the surface energy of the membrane, making it easier for the electrolyte to spread and penetrate into all the pores of the membrane, achieving rapid and uniform wetting.

[0057] On the other hand, the unique gradient pore size design (30-50 nm on the positive electrode side and 50-100 nm on the negative electrode side, with a smaller pore size on the positive electrode side and a larger pore size on the negative electrode side) optimizes the ion transport path: the smaller pore size side provides denser channels, which is conducive to forming a tighter contact with the positive electrode material and reducing interfacial impedance. The larger pore size side provides a wider ion flow path, which is conducive to the rapid and uniform flow of lithium ions to the negative electrode. This structure can significantly optimize the ion flow field and reduce concentration polarization, thus exhibiting higher ionic conductivity and lower battery internal resistance.

[0058] (2) Lithium dendrites: The polar functional groups on the surface of the tannic acid coating can reversibly and weakly adsorb lithium ions (Li).+ This forms a uniform ion distribution layer. This avoids Li + Concentrated deposition at certain points on the negative electrode surface makes lithium deposition / stripping behavior more uniform, thereby suppressing dendrite nucleation and growth from the source.

[0059] In addition, by designing a gradient aperture, with the larger aperture side facing the negative electrode, the open structure allows for a more uniform local current density distribution, preventing lithium ions from concentrating and depositing at certain points on the negative electrode surface, thus effectively inducing uniform lithium deposition. Simultaneously, the gradient structure itself physically hinders the longitudinal growth of dendrites, greatly suppressing the formation and penetration of lithium dendrites, thereby improving battery safety.

[0060] (3) Improved rate performance: Excellent wettability ensures full contact between the diaphragm and the electrode, reducing resistance to ion transport. Uniform lithium deposition also maintains a stable electrode interface, reducing interfacial impedance; good hydrophilicity helps improve ion transport.

[0061] (4) Extends battery life: Tannic acid can effectively complex trace amounts of HF and transition metal ions (such as Mn) in the electrolyte. 2+ Co 2+ These substances, produced by lithium salt decomposition and positive electrode corrosion, accelerate battery performance degradation. Capturing them can extend battery life. Attached Figure Description

[0062] Figure 1 This is a structural diagram of a gradient pore size composite membrane with high wettability. Detailed Implementation

[0063] The present invention will be further described below with reference to embodiments. The following description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make equivalent changes to the disclosed technical content to create equivalent embodiments. Any modifications or equivalent changes made to the following embodiments based on the technical essence of the present invention without departing from the scope of the present invention fall within the protection scope of the present invention.

[0064] Example 1

[0065] Polyethylene fibers with a denier of 1 denier are opened using an opening machine.

[0066] The opened fibers are fed into a carding machine. The carding machine's process parameters are: breast cylinder: 300 m / min, breast cylinder work roll: 35 m / min, main cylinder: 800 m / min, main cylinder work roll: 35 m / min, main cylinder stripping roll: 60 m / min, upper doffer spacing: 20 mm, lower doffer spacing: 20 mm. Then, through cross-laying, the fiber web is evenly folded and laid to a certain width and thickness, increasing the aspect ratio of the fiber web and forming an isotropic three-dimensional network structure.

[0067] The fiber web formed by cross-laying is pre-wetted, and the water jet is a tannic acid solution. The pre-wetting pressure is 1.5 MPa, and the water needle diameter is 0.1 mm.

[0068] The tannic acid solution comprises: 5 g / L tannic acid, 10 g / L bis-Tris buffer, and 0.1 mol / L NaCl, with a pH of 6.5-7.0. The specific preparation method is as follows: Dissolve 5 g of bis-Tris buffer in 500 ml of 0.1 mol / L NaCl solution, and adjust the pH to 6.5-7.0 using concentrated hydrochloric acid to form a bis-Tris-NaCl solution. Dissolve 2.5 g of tannic acid in the bis-Tris-NaCl solution to obtain a tannic acid solution with a concentration of 5 g / L.

[0069] The pre-wetted fiber web enters the main hydroentangling process. Hydroentangling head pressure: 3 MPa for head 1, 3 MPa for head 2, 5 MPa for head 3, 5 MPa for head 4, 6 MPa for head 5, 7 MPa for head 6, 7 MPa for head 7, 7 MPa for head 8, and 7 MPa for head 9. Hydroentangling needle diameter range: 0.1 mm. Hydroentangling needle action distance: 25 mm.

[0070] The water jet from water jet heads 1-4 is a tannic acid solution (same as above, tannic acid concentration is 5g / L), and the water jet from water jet heads 5-9 is ultrapure water, which is the same as the pre-wetting modification.

[0071] The hydroentangled fiber web is vacuum dehydrated to quickly remove excess water from the web.

[0072] The dehydrated fiber web is dried at a temperature of 60℃.

[0073] Hydroentangled modified polyethylene fiber web and polyethylene base film were hot-pressed to prepare hydroentangled modified composite diaphragm at a temperature of 120℃.

[0074] Example 2

[0075] Polyethylene fibers with a denier of 1 denier are opened using an opening machine.

[0076] The opened fibers are fed into a carding machine. The carding machine's process parameters are: breast cylinder: 300 m / min, breast cylinder work roll: 35 m / min, main cylinder: 800 m / min, main cylinder work roll: 35 m / min, main cylinder stripping roll: 60 m / min, upper doffer spacing: 20 mm, lower doffer spacing: 20 mm. Then, through cross-laying, the fiber web is evenly folded and laid to a certain width and thickness, increasing the aspect ratio of the fiber web and forming an isotropic three-dimensional network structure.

[0077] The fiber web formed by cross-laying is pre-wetted, and the water jet is a tannic acid solution. The pre-wetting pressure is 1.5 MPa, and the water needle diameter is 0.1 mm.

[0078] The tannic acid solution contains 15 g / L tannic acid, 10 g / L bis-Tris buffer, and 0.1 M NaCl, with a pH of 6.5-7.0. The specific preparation method is as follows: Dissolve 5 g of bis-Tris buffer in 500 ml of 0.1 mol / L NaCl solution, and adjust the pH to 6.5-7.0 using concentrated hydrochloric acid to form a bis-Tris-NaCl solution. Dissolve 7.5 g of tannic acid in the bis-Tris-NaCl solution to obtain a tannic acid solution with a concentration of 15 g / L.

[0079] The pre-wetted fiber web enters the main hydroentangling process. Hydroentangling head pressure: 3 MPa for head 1, 3 MPa for head 2, 5 MPa for head 3, 5 MPa for head 4, 6 MPa for head 5, 7 MPa for head 6, 7 MPa for head 7, 7 MPa for head 8, and 7 MPa for head 9. Hydroentangling needle diameter range: 0.1 mm. Hydroentangling needle action distance: 25 mm.

[0080] The water jets from water jet heads 1-4 are tannic acid solutions (same as above, tannic acid concentration is 15g / L), and the water jets from water jet heads 5-9 are ultrapure water, which is the same as the pre-wetting modification.

[0081] The hydroentangled fiber web is vacuum dehydrated to quickly remove excess water from the web.

[0082] The dehydrated fiber web is dried at a temperature of 60℃.

[0083] Hydroentangled modified polyethylene fiber web and polyethylene base film were hot-pressed to prepare hydroentangled modified composite diaphragm at a temperature of 120℃.

[0084] Example 3

[0085] Polyethylene fibers with a denier of 3 denier are opened using an opening machine.

[0086] The opened fibers are fed into a carding machine. The carding machine's process parameters are: breast cylinder: 300 m / min, breast cylinder work roll: 35 m / min, main cylinder: 800 m / min, main cylinder work roll: 35 m / min, main cylinder stripping roll: 60 m / min, upper doffer spacing: 20 mm, lower doffer spacing: 20 mm. Then, through cross-laying, the fiber web is evenly folded and laid to a certain width and thickness, increasing the aspect ratio of the fiber web and forming an isotropic three-dimensional network structure.

[0087] The fiber web formed by cross-laying is pre-wetted, and the water jet is a tannic acid solution. The pre-wetting pressure is 2 MPa, and the water needle diameter is 0.12 mm.

[0088] The tannic acid solution contains: 15 g / L tannic acid, 10 g / L bis-Tris buffer, and 0.1 M NaCl, pH 6.5-7.0. The specific preparation method is the same as in Example 2.

[0089] The pre-wetted fiber web enters the main hydroentangling process. Hydroentangling head pressure: 5 MPa for head 1, 5 MPa for head 2, 7 MPa for head 3, 7 MPa for head 4, 9 MPa for head 5, 12 MPa for head 6, 12 MPa for head 7, 12 MPa for head 8, and 12 MPa for head 9. Hydroentangling needle diameter range: 0.12 mm. Hydroentangling needle action distance: 35 mm.

[0090] The water jets from water jet heads 1-4 are tannic acid solutions (same as above, tannic acid concentration is 15g / L), and the water jets from water jet heads 5-9 are ultrapure water, which is the same as the pre-wetting modification.

[0091] The hydroentangled fiber web is vacuum dehydrated to quickly remove excess water from the web.

[0092] The dehydrated fiber web is dried at a temperature of 60℃.

[0093] Hydroentangled modified polyethylene fiber web and polyethylene base film were hot-pressed to prepare hydroentangled modified composite diaphragm at a temperature of 120℃.

[0094] Example 4

[0095] Polyethylene fibers with a denier of 3 denier are opened using an opening machine.

[0096] The opened fibers are fed into a carding machine. The carding machine's process parameters are: breast cylinder: 300 m / min, breast cylinder work roll: 35 m / min, main cylinder: 800 m / min, main cylinder work roll: 35 m / min, main cylinder stripping roll: 60 m / min, upper doffer spacing: 20 mm, lower doffer spacing: 20 mm. Then, through cross-laying, the fiber web is evenly folded and laid to a certain width and thickness, increasing the aspect ratio of the fiber web and forming an isotropic three-dimensional network structure.

[0097] The fiber web formed by cross-laying is pre-wetted, and the water jet is a tannic acid solution. The pre-wetting pressure is 0.8 MPa, and the water needle diameter is 0.08 mm.

[0098] The tannic acid solution contains: 15 g / L tannic acid, 10 g / L bis-Tris buffer, and 0.1 M NaCl, pH 6.5-7.0. The specific preparation method is the same as in Example 2.

[0099] The pre-wetted fiber web enters the main hydroentangling process. Hydroentangling head pressure: 2 MPa for head 1, 2 MPa for head 2, 3 MPa for head 3, 3 MPa for head 4, 4 MPa for head 5, 5 MPa for head 6, 5 MPa for head 7, 5 MPa for head 8, and 5 MPa for head 9. Hydroentangling needle diameter: 0.08 mm. Hydroentangling needle action distance: 10 mm.

[0100] The water jets from water jet heads 1-4 are tannic acid solutions (same as above, tannic acid concentration is 15g / L), and the water jets from water jet heads 5-9 are ultrapure water, which is the same as the pre-wetting modification.

[0101] The hydroentangled fiber web is vacuum dehydrated to quickly remove excess water from the web.

[0102] The dehydrated fiber web is dried at a temperature of 60℃.

[0103] Hydroentangled modified polyethylene fiber web and polyethylene base film were hot-pressed to prepare hydroentangled modified composite diaphragm at a temperature of 120℃.

[0104] Example 5

[0105] Polyethylene fibers with a denier of 1 denier are opened using an opening machine.

[0106] The opened fibers are fed into a carding machine. The carding machine's process parameters are: breast cylinder: 300 m / min, breast cylinder work roll: 35 m / min, main cylinder: 800 m / min, main cylinder work roll: 35 m / min, main cylinder stripping roll: 60 m / min, upper doffer spacing: 20 mm, lower doffer spacing: 20 mm. Then, through cross-laying, the fiber web is evenly folded and laid to a certain width and thickness, increasing the aspect ratio of the fiber web and forming an isotropic three-dimensional network structure.

[0107] The fiber web formed by cross-laying is pre-wetted, and the water jet is a tannic acid solution. The pre-wetting pressure is 0.8 MPa, and the water needle diameter is 0.08 mm.

[0108] The tannic acid solution contains: 15 g / L tannic acid, 10 g / L bis-Tris buffer, and 0.1 M NaCl, pH 6.5-7.0. The specific preparation method is the same as in Example 2.

[0109] The pre-wetted fiber web enters the main hydroentangling process. Hydroentangling head pressure: 2 MPa for head 1, 2 MPa for head 2, 3 MPa for head 3, 3 MPa for head 4, 4 MPa for head 5, 5 MPa for head 6, 5 MPa for head 7, 5 MPa for head 8, and 5 MPa for head 9. Hydroentangling needle diameter: 0.08 mm. Hydroentangling needle action distance: 10 mm.

[0110] The water jets from water jet heads 1-4 are tannic acid solutions (same as above, tannic acid concentration is 15g / L), and the water jets from water jet heads 5-9 are ultrapure water, which is the same as the pre-wetting modification.

[0111] The hydroentangled fiber web is vacuum dehydrated to quickly remove excess water from the web.

[0112] The dehydrated fiber web is dried at a temperature of 60℃.

[0113] Hydroentangled modified polyethylene fiber web and polyethylene base film were hot-pressed to prepare hydroentangled modified composite diaphragm at a temperature of 120℃.

[0114] Comparative Example 1

[0115] Polyethylene fibers with a denier of 1 denier are opened using an opening machine.

[0116] The opened fibers are fed into a carding machine. The carding machine's process parameters are: breast cylinder: 300 m / min, breast cylinder work roll: 35 m / min, main cylinder: 800 m / min, main cylinder work roll: 35 m / min, main cylinder stripping roll: 60 m / min, upper doffer spacing: 20 mm, lower doffer spacing: 20 mm. Then, through cross-laying, the fiber web is evenly folded and laid to a certain width and thickness, increasing the aspect ratio of the fiber web and forming an isotropic three-dimensional network structure.

[0117] The fiber web, formed by cross-laying, is pre-wetted using ultrapure water. The pre-wetting pressure is 0.8 MPa, and the water jet diameter is 0.08 mm.

[0118] The pre-wetted fiber web enters the main hydroentangling process. The pressure of each hydroentanglement head is as follows: 1. Hydroentanglement head pressure: 2 MPa; 2. Hydroentanglement head pressure: 2 MPa; 3. Hydroentanglement head pressure: 3 MPa; 4. Hydroentanglement head pressure: 3 MPa; 5. Hydroentanglement head pressure: 4 MPa; 6. Hydroentanglement head pressure: 5 MPa; 7. Hydroentanglement head pressure: 5 MPa; 8. Hydroentanglement head pressure: 5 MPa; 9. Hydroentanglement head pressure: 5 MPa. The diameter of the hydroentanglement needles is 0.08 mm. The effective distance of the hydroentanglement needles is 10 mm. The water jet from the hydroentanglement heads is ultrapure water.

[0119] The hydroentangled fiber web is vacuum dehydrated to quickly remove excess water from the web.

[0120] The dehydrated fiber web is dried at a temperature of 60℃.

[0121] Hydroentangled polyethylene fiber web and polyethylene base film were hot-pressed to prepare hydroentangled modified composite membrane at a temperature of 120℃.

[0122] Comparative Example 2

[0123] Polyethylene fibers with a denier of 20 are opened using an opening machine.

[0124] The opened fibers are fed into a carding machine. The carding machine's process parameters are: breast cylinder: 300 m / min, breast cylinder work roll: 35 m / min, main cylinder: 800 m / min, main cylinder work roll: 35 m / min, main cylinder stripping roll: 60 m / min, upper doffer spacing: 20 mm, lower doffer spacing: 20 mm. Then, through cross-laying, the fiber web is evenly folded and laid to a certain width and thickness, increasing the aspect ratio of the fiber web and forming an isotropic three-dimensional network structure.

[0125] The fiber web formed by cross-laying is pre-wetted, and the water jet is a tannic acid solution. The pre-wetting pressure is 0.8 MPa, and the water needle diameter is 0.08 mm.

[0126] The tannic acid solution contains: 15 g / L tannic acid, 10 g / L bis-Tris buffer, and 0.1 M NaCl, pH 6.5-7.0. The specific preparation method is the same as in Example 2.

[0127] The pre-wetted fiber web enters the main hydroentangling process. Hydroentangling head pressure: 2 MPa for head 1, 2 MPa for head 2, 3 MPa for head 3, 3 MPa for head 4, 4 MPa for head 5, 5 MPa for head 6, 5 MPa for head 7, 5 MPa for head 8, and 5 MPa for head 9. Hydroentangling needle diameter: 0.08 mm. Hydroentangling needle action distance: 10 mm.

[0128] The water jets from water jet heads 1-4 are tannic acid solutions (same as above, tannic acid concentration is 15g / L), and the water jets from water jet heads 5-9 are ultrapure water, which is the same as the pre-wetting modification.

[0129] The hydroentangled fiber web is vacuum dehydrated to quickly remove excess water from the web.

[0130] The dehydrated fiber web is dried at a temperature of 60℃.

[0131] Hydroentangled modified polyethylene fiber web and polyethylene base film were hot-pressed to prepare hydroentangled modified composite diaphragm at a temperature of 120℃.

[0132] Comparative Example 3

[0133] Unlike Example 1, this example does not include the modified polyethylene fiber web, but only a polyethylene base film.

[0134] Comparative Example 4

[0135] Unlike Example 1, only the modified polyethylene fiber web was prepared and directly formed into a film without thermal lamination with the polyethylene base film.

[0136] Performance testing

[0137] Thickness testing was conducted in accordance with the requirements of GB / T 36363-2018 "Polyolefin Separators for Lithium-ion Batteries".

[0138] Five square samples of the base film were cut along the TD direction using a 10cm×10cm mold and tested. If the TD direction sample was less than 10cm, a 10cm sample was cut along the MD direction; in this case, the sample was not square. The four corners and the center point of the sample were measured using a Mahr thickness gauge (C1202), and the average value of these five points was taken as the thickness of a single sample. The average value of the five samples was taken as the thickness of the base film.

[0139] Porosity testing involved cutting 10cm x 10cm samples and measuring their thickness (Mahr thickness gauge, C1202) and mass (electronic balance, Mettler Toledo Instruments (Shanghai) Co., Ltd., ME204E / 02). Calculate the surface density (unit: g / cm³) 2 ), where m represents the mass of the sample (in g), L represents the length of the sample (in cm), and b represents the width of the sample (in cm). According to Calculate the porosity, where p is the porosity of the sample (in %) and d is the thickness of the sample (in cm).

[0140] The air permeability test was conducted in accordance with the requirements of GB / T 36363—2018 "Determination of Air Permeability of Polyolefin Separators for Lithium-ion Batteries". A 600mm×100mm base film sample was cut and measured using a Wangyan-type air permeability meter (ASAHI Corporation, EG01-55-1MR). The test time was 3s. The air permeability of the base film was measured at arbitrary positions at 100mm intervals along the 600mm TD direction. The average value of the above 5 test points was recorded as the air permeability of the base film.

[0141] Tensile testing was conducted according to GB / T 1040.3-2006. Under the test requirements of 23±3℃ and ≤60% humidity, a 1.5cm×20cm specimen was cut and marked with the MD / TD direction of the diaphragm. The specimen was tested using a tensile testing machine (High-speed Rail Testing Instruments (Dongguan) Co., Ltd., AI-3000-SU). The specimen was fixed between the upper and lower clamps of the tensile testing machine (the distance between the clamps was 100±5 mm), ensuring that the specimen was flat and wrinkle-free and vertically straight. The tensile speed was 500mm / min. The tensile testing machine output the tensile strength value based on the width and thickness of the specimen. The test was performed 3 times and the average value was taken as the tensile strength in the MD / TD direction.

[0142] The puncture strength test was conducted according to the method specified in GB / T 36363-2018. A 50*100 mm sample of the base membrane was cut along the TD direction and fixed onto the sample holder of the puncture testing machine (model: KES-GNDG5, KNC Technology Co., Ltd.). A 1.0 mm diameter steel needle was used to puncture the membrane at a speed of 0.1 cm / sec. The maximum load of the needle penetrating the diaphragm was recorded. The test was performed at least five times, and the arithmetic mean was taken.

[0143] Thermal stability testing was conducted according to the requirements of GB / T36363-2018. A 10cm × 10cm sample was cut, and the transverse (TD) and longitudinal (MD) widths were marked on the sample. The transverse and longitudinal widths were measured using a fully automatic image measuring projector (Kunshan Gaopin Precision Instruments Co., Ltd., GP-300C). The sample was then sandwiched between two sealed A4 sheets of paper and placed in a 105℃ oven for 1 hour. After the sample returned to room temperature, the transverse and longitudinal widths were measured again using the fully automatic image measuring projector. Three measurements were taken, and the average value was recorded.

[0144] The performance test results are shown in Table 1.

[0145] Table 1. Performance test results of the examples and comparative products.

[0146]

[0147] As shown in Table 1, the composite diaphragm of the present invention, due to the gradient pore size setting, exhibits high porosity, high puncture strength, high tensile strength, and low thermal shrinkage while having a small contact angle. Furthermore, through the design of the fiber web structure, the composite diaphragm achieves a small contact angle and low thermal shrinkage rate while maintaining a small thickness, thereby exhibiting good hydrophilicity and thermal stability.

[0148] As shown in Examples 1 and 2, Example 2 exhibits a smaller contact angle and better hydrophilicity. This is because the amount of tannic acid added is higher, thereby improving hydrophilicity. Therefore, the optimal formulation for tannic acid is a tannic acid concentration of 15 g / L.

[0149] Combining Examples 2 and 3, the strength of Example 3 is significantly lower. This is because the hydroentangling pressure in Example 3 is higher than that in Example 2, damaging the fiber raw material and reducing its strength. Furthermore, the greater hydroentangling distance, larger needle diameter, and higher fiber denier in Example 3 all resulted in fiber damage under high hydroentangling pressure. Therefore, hydroentangling pressure is more likely to cause fiber damage when applied to fine denier fibers, and the hydroentangling pressure should not be too high. Furthermore, in Example 4, a lower pressure distribution is more suitable for hydroentangling.

[0150] Comparing Examples 4 and 5 reveals that Example 5 has a smaller pore size, making it more suitable for forming a gradient pore structure with the polyethylene-based film layer. This is due to the different denier of the spunlace fiber raw materials; the finer the fiber, the smaller the pore size that can be formed, and the lower the strength of the fiber itself. However, the strength provided by the polyethylene-based film layer is sufficient to meet the requirements of the battery separator. Therefore, spunlace fiber raw materials with a denier of 1 are preferred when considering the pore structure.

[0151] Comparative Example 1 shows that the composite membrane without tannic acid modification has a significantly larger contact angle and poor hydrophilicity. Compared with Examples 1-5, the modification effect of tannic acid is significant.

[0152] In Comparative Example 2, the hydroentangling process used 20 denier polyethylene fibers, resulting in a fiber web with a pore size of 124 nm. When combined with the polyethylene base film layer, the pore size was too large.

[0153] The highly wettable gradient pore size composite membrane prepared in Example 5 exhibits the best performance.

Claims

1. A highly wettable gradient pore size composite membrane, characterized in that, The composite membrane comprises a polyolefin-based membrane layer and a synthetic fiber web layer prepared by hydroentangling; the composite membrane has a thickness of 5~15μm, wherein the pore size on the positive electrode side is 30~50nm and the pore size on the negative electrode side is 50~100nm; and the synthetic fiber web layer contains a tannic acid modifier.

2. The gradient pore size composite membrane according to claim 1, characterized in that, The composite membrane has a porosity of 60-70%, an air permeability of 105-130 sec / 100cc, a puncture strength of 530-640 gf, a maximum tensile strength (MD) of 3000-3700 kgf, a maximum tensile strength (TD) of 2700-3400 kgf, a MD heat shrinkage rate of less than 2.6%, a TD heat shrinkage rate of less than 1.5%, and a contact angle of less than 50°.

3. The gradient pore size composite membrane according to claim 2, characterized in that, The composite separator has a pore size difference of 20~50nm between the negative electrode side and the positive electrode side.

4. The gradient pore size composite membrane according to claim 1, characterized in that, The tannic acid modifier is applied by water jetting in a hydroentanglement process. The water jet is a tannic acid solution containing 3-16 g / L tannic acid, 10 g / L bis-Tris buffer, and 0.1 M NaCl, with a pH of 6.5-7.

0.

5. The gradient pore size composite membrane according to claim 1, characterized in that, The denier of the synthetic fibers used to prepare the synthetic fiber web layer is 0.3-3 denier.

6. The gradient pore size composite membrane according to claim 1, characterized in that, The synthetic fiber is at least one of polyethylene fiber, polypropylene fiber and viscose fiber, and the polyolefin-based film is polyethylene, polypropylene or a mixture of polyethylene and polypropylene.

7. The preparation process of the highly wettable gradient pore size composite membrane according to any one of claims 1 to 6, characterized in that, Includes the following steps: (1) Fiber preparation and opening: The synthetic fibers are opened; (2) Carding into a web: The opened fibers are carded and cross-laid through a carding machine to form a three-dimensional network structure; (3) Pre-wetting and preliminary modification: The fiber web is pre-wetted using a water jet; the water jet is a tannic acid solution; the pre-needling pressure during pre-wetting is 0.8-2 MPa, and the diameter of the water jet is 0.08-0.18 mm; (4) Hydroentangling reinforcement and modification: The pre-wetted fiber web is reinforced by main hydroentangling, wherein the tannic acid solution is used as the water jet in the front section of the hydroentangling head, and ultrapure water is used to rinse the rear section of the hydroentangling head; the hydroentangling pressure range is 2-14MPa, the diameter of the water needle is 0.08-1.8mm, and the water needle action distance is 10-35mm. (5) Dehydration and drying: Vacuum dehydration of the hydroentangled fiber web and drying at 50-100℃; (6) Composite: The dried modified fiber web and polyolefin base film are composited by hot pressing at 100-130℃.

8. The preparation process according to claim 7, characterized in that, In step (4), the hydroentangling process parameters during the main hydroentangling reinforcement are distributed under low pressure: hydroentangling head 1 pressure: 2-5MPa, hydroentangling head 2 pressure: 2-5MPa, hydroentangling head 3 pressure: 3-7MPa, hydroentangling head 4 pressure: 3-7MPa, hydroentangling head 5 pressure: 4-9MPa, hydroentangling head 6 pressure: 5-12MPa, hydroentangling head 7 pressure: 5-12MPa, hydroentangling head 8 pressure: 5-14MPa, hydroentangling head 9 pressure: 5-14MPa.

9. The preparation process according to claim 7, characterized in that, The process parameters of the combing machine in step (2) are set as follows: chest cylinder speed 200-500 m / min, main cylinder speed 700-1200 m / min, upper doffer spacing: 10-50 mm, lower doffer spacing: 10-50 mm.

10. A lithium-ion battery, characterized in that, The lithium-ion battery uses a highly wettable gradient pore size composite separator as described in any one of claims 1-6 or a highly wettable gradient pore size composite separator prepared by the preparation process described in any one of claims 7-9.

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