Heat-resistant high-infiltration composite coating diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention discloses a heat-resistant high-infiltration composite coating diaphragm, a preparation method thereof and a lithium ion battery, and belongs to the technical field of lithium ion battery diaphragms. The heat-resistant high-infiltration composite coating diaphragm provided by the invention comprises a polyolefin composite base membrane, the basalt chopped fiber coating is compounded on at least one surface of the polyolefin composite base membrane; the polyphthalamide coating is compounded on the basalt chopped fiber coating; the polyolefin composite base film comprises polyolefin and boehmite; the mass ratio of polyolefin to boehmite is (4-8): 1; the thickness of the basalt chopped fiber coating is 1.2 to 8 [mu] m, and the thickness of the polyphthalamide coating is 0.5 to 2 [mu] m. Through the synergistic effect of multiple layers, the upper limit of heat resistance, wettability, needling resistance and mechanical property of the diaphragm are improved at the same time, and a lithium ion battery assembled by the diaphragm has good high-temperature cycling stability.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery separator technology, and in particular to a heat-resistant, highly wettable composite coating separator, its preparation method, and a lithium-ion battery. Background Technology

[0002] The information disclosed in the background section of this invention is intended only to enhance the understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] As one of the core components of lithium-ion batteries, the separator not only performs the basic functions of isolating the positive and negative electrodes and preventing short circuits, but also directly affects the battery's capacity, cycle life, and safety performance. With the continuous upgrading of battery energy density and rate performance, the heat generated by the cell during operation increases significantly, placing higher demands on the separator's dimensional stability, ion migration efficiency, and electrolyte affinity under high-temperature environments.

[0004] Currently, the most widely used commercially available polyolefin separators (such as polyethylene and polypropylene) possess good ion permeability, but their melting points are low (typically below 150°C). When the internal temperature of the battery rises abnormally, the separator is prone to shrinkage or even rupture, leading to a short circuit between the positive and negative electrodes and posing a serious safety hazard. To improve this issue, the industry commonly uses ceramic coating on polyolefin separators to enhance thermal stability. However, the dimensional retention of these separators at temperatures above 150°C remains insufficient, and with the widespread adoption of fast charging technology, their wettability is no longer sufficient to meet the ion migration efficiency requirements of high-rate charging and discharging. Some improvement schemes attempt to optimize performance through structural composites (such as multilayer co-extrusion) or ceramic co-coating, but these still cannot effectively solve the dimensional stability problem at temperatures above 150°C. Existing technologies employ aramid coating technology, which can improve heat resistance, but suffers from complex processes and high costs, and it is difficult to simultaneously ensure the cycle stability of lithium-ion batteries assembled with the separator.

[0005] Therefore, there is an urgent need for a simple preparation method to obtain a composite coating separator that combines high heat resistance, good cycle stability and high wettability, so as to adapt to lithium-ion battery systems with higher energy density and fast charging requirements. Summary of the Invention

[0006] In view of this, the present invention provides a heat-resistant high-wetting composite coating separator and its preparation method, and a lithium-ion battery. The heat-resistant high-wetting composite coating separator provided by the present invention has a high upper limit of heat resistance and good wettability, needle puncture resistance and mechanical properties. The lithium-ion battery assembled with it has good high-temperature cycle stability.

[0007] In a first aspect, the present invention provides a heat-resistant, highly wettable composite coating membrane, comprising: Polyolefin composite base film; A basalt chopped fiber coating laminated on at least one side of a polyolefin composite film; A polyphthalamide coating composited on a basalt chopped fiber coating; The polyolefin composite base film includes polyolefin and boehmite; the mass ratio of polyolefin to boehmite is (4~8):1; the thickness of the basalt chopped fiber coating is 1.2~8μm, and the thickness of the polyphthalamide coating is 0.5~2μm.

[0008] Preferably, in the basalt chopped fiber coating, the diameter D50 of the basalt chopped fibers is ≤1.5μm, and the length is ≤350μm.

[0009] Preferably, the basalt chopped fiber coating further includes a binder; the binder content in the basalt chopped fiber coating is 4~10wt%.

[0010] Preferably, the polyphthalamide coating further includes an adhesive; the adhesive content in the polyphthalamide coating is 10~25wt%.

[0011] Preferably, the polyolefin includes polyethylene and / or polypropylene; the polyolefin composite base membrane is a microporous membrane; and the thickness is 3~10μm.

[0012] Secondly, the present invention provides a method for preparing the above-mentioned heat-resistant, highly wettable composite coating membrane, comprising the following steps: Polyolefin resin, boehmite and paraffin oil are melt-extruded and then biaxially stretched. After extraction and drying, they are biaxially stretched a second time to obtain a polyolefin composite base film. Short-cut basalt fiber slurry is coated on one or both sides of a polyolefin composite base film and dried to obtain a basalt short-cut fiber coating. Then, a polyphthalamide slurry is coated on the surface of the basalt short-cut fiber coating, and after drying, a heat-resistant, highly wettable composite coating membrane is obtained.

[0013] Preferably, the chopped basalt fiber slurry comprises chopped basalt fiber, binder, wetting agent, dispersant, defoamer, and water; wherein the chopped basalt fiber slurry contains 30-50 wt% chopped basalt fiber, 2-8 wt% binder, 0.1-0.5 wt% wetting agent, 0.5-2 wt% dispersant, and 0.08-0.15 wt% defoamer.

[0014] Furthermore, the preparation method of the chopped basalt fiber slurry is as follows: wetting agent, dispersant and chopped basalt fiber are added to water and ball-milled, then binder and defoamer are added, and the slurry is obtained by sieving.

[0015] Preferably, the polyphthalamide slurry comprises polyphthalamide, a thickener, a binder, a dispersant, a surfactant, and a solvent; wherein the polyphthalamide slurry contains 20-40 wt% polyphthalamide, 0.1-1 wt% thickener, 5-15 wt% binder, 3-8 wt% dispersant, and 0.1-0.5 wt% surfactant.

[0016] Thirdly, the present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the separator is the heat-resistant high-wetting composite coating separator described above or the heat-resistant high-wetting composite coating separator prepared by the above preparation method.

[0017] Compared with the prior art, the present invention has achieved the following beneficial effects: (1) The heat-resistant, high-wetting composite coating separator provided by this invention effectively enhances the heat resistance of the substrate by incorporating boehmite into the polyolefin-based membrane; at the same time, the composite basalt chopped fiber coating forms a network structure on the separator surface, further inhibiting thermal shrinkage and crack propagation, so that the separator can still maintain excellent dimensional stability in the high-temperature range above 150°C, significantly reducing the safety hazard of short circuit between positive and negative electrodes caused by separator shrinkage, and achieving a breakthrough in heat resistance compared with traditional ceramic-coated separators. The polar amide bonds (-NH-CO-) in the polyphthalamide (PPA) coating can form hydrogen bonds or dipole-dipole interactions with the polar solvent of the electrolyte, reducing the interfacial contact angle; and the surface energy of PPA matches the surface energy of the electrolyte highly, promoting electrolyte spreading and penetration. Combined with the liquid storage function of the basalt fiber coating, the liquid retention rate of the separator is significantly improved, which not only extends the cell life, but also accelerates the wetting speed, and shortens the standing time after liquid injection, thereby improving production efficiency.

[0018] (2) The vein structure formed by boehmite and polyolefin in the substrate of the present invention enhances the tensile strength and needle punching strength of the substrate; the high tensile strength of basalt fiber further improves the overall impact resistance of the separator; and the dual coating effect of PPA and basalt reduces the performance degradation caused by thermal shrinkage or electrolyte loss during cycling by stabilizing the size and improving wettability, thereby improving the high temperature cycling stability of the battery, and the process is simpler and the cost is lower. Attached Figure Description

[0019] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0020] Figure 1 This is a scanning electron microscope image of the heat-resistant, highly wettable composite coating diaphragm of Embodiment 1 of the present invention. Detailed Implementation

[0021] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0022] This invention provides a heat-resistant, highly wettable composite coating membrane, comprising: Polyolefin composite base film; A basalt chopped fiber coating laminated on at least one side of a polyolefin composite film; A polyphthalamide coating composited on a basalt chopped fiber coating; The polyolefin composite base film includes polyolefin and boehmite; the mass ratio of polyolefin to boehmite is (4~8):1; the thickness of the basalt chopped fiber coating is 1.2~8μm, and the thickness of the polyphthalamide coating is 0.5~2μm.

[0023] In the above-mentioned technical solution of the present invention, the polyolefin composite base membrane enhances the basic heat resistance and mechanical properties through boehmite filling, the basalt chopped fiber coating strengthens the heat resistance and liquid storage capacity through a network structure, and the polyphthalamide coating optimizes wettability through polar groups. The three are layered together to form a synergistic structure of "support-reinforcement-functionalization", giving the membrane high heat resistance, low shrinkage, high wettability, and good ion permeability. Specifically, the polyolefin composite base membrane, as the core support layer, is composed of polyolefin and boehmite in a mass ratio of (4~8):1. Polyolefins are the basic film-forming material for membranes. The flexibility and processability of their molecular chains ensure the initial film-forming properties and ion permeability of the base membrane. Boehmite, as an inorganic filler, is uniformly dispersed between polyolefin molecular chains in a specific ratio. Through its filling effect, it restricts the free movement of polyolefin molecular chains at high temperatures, thereby improving the heat resistance of the base membrane and delaying high-temperature softening or melting. At the same time, the dense network structure formed by the interfacial bonding between boehmite and polyolefin enhances the tensile strength and needle-puncture resistance of the base membrane, providing a stable substrate support for subsequent coatings. When the boehmite content is too high, the particles tend to agglomerate, leading to a decrease in the film-forming properties of the base membrane, making it prone to cracks or pores, and reducing flexibility. When the boehmite content is too low, its restriction effect on the polyolefin molecular chains is insufficient, and it cannot effectively improve heat resistance.

[0024] Building upon this, the basalt chopped fiber coating, composited on the base membrane surface, forms a physical support layer on the base membrane surface through its interwoven network structure of chopped fibers. Basalt fibers themselves possess high tensile strength and wide-temperature stability; their network structure effectively inhibits thermal shrinkage and crack propagation of the base membrane at high temperatures, further improving the overall dimensional stability of the separator. Simultaneously, the porous structure between the fibers provides electrolyte storage space, laying the foundation for subsequent wettability improvement. When the thickness of the basalt chopped fiber coating is less than 1.2 μm, the fiber network density is insufficient to form effective physical support, resulting in limited thermal shrinkage suppression and minimal improvement in heat resistance. Excessive coating thickness significantly increases the overall thickness of the separator, while overfilling the pores between fibers leads to prolonged ion migration paths and increased internal resistance, affecting the battery's fast-charging performance. A more preferable thickness for the basalt chopped fiber coating is 1.2–3 μm.

[0025] The outermost polyphthalamide (PPA) coating is specifically optimized for wettability through its molecular structure. As a semi-aromatic nylon, PPA contains polar amide bonds (-NH-CO-) in its molecular chain, which can form hydrogen bonds or dipole-dipole interactions with polar solvents in the electrolyte (such as ethylene carbonate EC and dimethyl carbonate DMC), reducing the interfacial contact angle between the membrane and the electrolyte. Simultaneously, the surface energy of PPA (40-50 mN / m) is highly compatible with the surface energy of the electrolyte (25-35 mN / m), promoting the spread and penetration of the electrolyte on the membrane surface. The thin design of this coating (0.5~2 μm, more preferably 0.5~1.5 μm) achieves highly efficient wettability while avoiding excessive obstruction of ion migration pathways, ensuring good ion permeability of the membrane.

[0026] In this invention, the basalt chopped fiber coating has a diameter D50 ≤ 1.5 μm and a length L ≤ 350 μm. Compared to long fibers, basalt chopped fibers are more likely to randomly distribute during slurry flow and drying shrinkage, resulting in more fiber intersections and denser packing, thus forming a denser three-dimensional network structure. This structure can effectively suppress the thermal shrinkage of the base membrane and store more electrolyte through denser pores, improving the wettability of the membrane. In contrast, long fibers have poor packing density, poor thermal shrinkage suppression effect, and form larger pores with less capillary effect, which is not conducive to electrolyte wetting.

[0027] In this invention, the basalt chopped fiber coating also includes a binder; the binder content in the basalt chopped fiber coating is 4-10 wt%. The binder's intermolecular forces (such as van der Waals forces and hydrogen bonds) tightly bond the chopped basalt fibers to the polyolefin composite base film, forming a stable coating structure. If the binder content is too low, the bonding force between the fibers and the base film is insufficient, making it prone to detachment during battery cycling; if the content is too high, it will overfill the pores between the fibers, reducing the coating's electrolyte storage capacity and ion permeability. The binder is preferably one or more of SBR (styrene-butadiene rubber), CMC (sodium carboxymethyl cellulose), PAA (polyacrylic acid), BM-900, LA132, LA133, etc.; the choice can be made according to specific circumstances.

[0028] In this invention, the polyphthalamide coating also includes a binder, which enhances the interfacial bonding between the polyphthalamide (PPA) coating and the basalt chopped fiber coating. Since PPA itself has limited chemical affinity with basalt fibers, the binder can bridge the gap, tightly fixing PPA particles or molecules to the surface of the basalt fibers to form a continuous functional layer. The binder content in the polyphthalamide coating is 10-25 wt%. The binder is selected from PVDF (polyvinylidene fluoride), PVDF-HFP (vinylidene fluoride-hexafluoropropylene copolymer), polyacrylate binders S105, BM-900, LA132, LA133, etc.

[0029] In this invention, the polyolefin includes polyethylene and / or polypropylene; due to its excellent film-forming properties, chemical stability, and moderate melting point, it is suitable as a base support material for membranes. In one or more embodiments of this invention, the polyolefin is selected from polyethylene. The polyolefin composite base membrane is a microporous membrane; its thickness is 3~10 μm, more preferably 3~6 μm.

[0030] The present invention also provides a method for preparing the above-mentioned heat-resistant, highly wettable composite coating membrane, comprising the following steps: Polyolefin resin, boehmite and paraffin oil are melt-extruded and then biaxially stretched. After extraction and drying, they are biaxially stretched a second time to obtain a polyolefin composite base film. Short-cut basalt fiber slurry is coated on one or both sides of a polyolefin composite base film and dried to obtain a basalt short-cut fiber coating. Then, a polyphthalamide slurry is coated on the surface of the basalt short-cut fiber coating, and after drying, a heat-resistant, highly wettable composite coating membrane is obtained.

[0031] In the preparation process of the polyolefin composite base membrane of this invention, melt extrusion uniformly mixes polyolefin, boehmite, and paraffin oil (pore-forming agent); the first biaxial stretching induces the polyolefin molecular chains to oriented through mechanical force, while paraffin oil acts as a physical filler to form initial pores; after extraction to remove the paraffin oil, a second biaxial stretching further regulates the size and distribution of pores, optimizing the air permeability and ion permeability of the base membrane; during the stretching process, boehmite is dispersed between the polyolefin molecular chains, restricting their free movement at high temperatures and improving the heat resistance of the base membrane. This invention does not impose special limitations on the specific biaxial stretching, extraction, and drying processes; commonly used polyolefin membrane production methods in the art can be employed.

[0032] In this invention, the chopped basalt fiber slurry comprises chopped basalt fiber, binder, wetting agent, dispersant, defoamer, and water. The chopped basalt fiber slurry contains 30-50 wt% chopped basalt fiber, 2-8 wt% binder, 0.1-0.5 wt% wetting agent, 0.5-2 wt% dispersant, and 0.08-0.15 wt% defoamer. This invention does not impose any special restrictions on the types of binder, wetting agent, dispersant, and defoamer in the chopped basalt fiber slurry; commonly used types in the art can be used. For example, the binder is selected from water-based binders, such as SBR (styrene-butadiene latex), CMC (sodium carboxymethyl cellulose), PAA (polyacrylic acid), BM-900 (water-based acrylate), LA132 / LA133 (modified polyacrylate), etc.; the wetting agent is selected from one or more of nonionic surfactants (such as Triton X-100) or acetylenic diol wetting agents (such as Dynol 607); the dispersant is selected from polymeric dispersants, such as ammonium polyacrylate, Solsperse 41000, etc.; the defoamer is selected from silicone defoamers (such as BYK-024), mineral oil defoamers, polyether defoamers, etc.

[0033] Furthermore, the preparation method of the chopped basalt fiber slurry is as follows: Wetting agent, dispersant, and chopped basalt fibers are added to water and ball-milled. Then, binder and defoamer are added, and the mixture is sieved to obtain the chopped basalt fiber slurry. First, the fibers are refined by ball milling until the length L of the chopped basalt fibers is ≤350μm and uniformly dispersed. Then, binder is added to prevent premature fiber encapsulation, which would lead to uneven dispersion. Finally, defoamer is added to prevent air bubbles from being introduced during stirring, thus avoiding pore defects in the coating. Zirconia beads are used as the milling medium.

[0034] In this invention, the polyphthalamide slurry comprises polyphthalamide, thickener, binder, dispersant, surfactant, and solvent; in the polyphthalamide slurry, the mass content of polyphthalamide is 20-40 wt%; the mass content of thickener is 0.1-2 wt%; the mass content of binder is 5-15 wt%; the mass content of dispersant is 3-8 wt%; and the mass content of surfactant is 0.2-0.8 wt%. This invention does not impose any special restrictions on the types of thickeners, binders, dispersants, surfactants, and solvents. Preferably, the thickener is selected from one or more of sodium carboxymethyl cellulose, polyvinyl acetate, or ethylene-vinyl acetate copolymer; the binder is selected from polyacrylate binders (such as S105), PVDF (polyvinylidene fluoride), PVDF-HFP (vinylidene fluoride-hexafluoropropylene copolymer), LA-133, BM-900, etc.; the dispersant is selected from dimethylformamide (DMF), PEG, BYK-16, Solsperse 24000, etc.; the surfactant is selected from phosphate or polyether surfactants; and the solvent is selected from one or more of dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), and dimethyl phthalate (DMP).

[0035] This invention does not impose any special restrictions on the specific coating method. Spraying, roller coating, or other methods can be selected, as long as a coating of a specific thickness can be obtained.

[0036] The present invention also provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the separator is the heat-resistant high-wetting composite coating separator described above or the heat-resistant high-wetting composite coating separator prepared by the above preparation method.

[0037] This invention does not impose any special restrictions on the preparation method of lithium-ion batteries; any commonly used lithium-ion battery preparation method in the field can be used.

[0038] The technical solution of the present invention will be further described below with reference to specific embodiments. The present invention does not impose any special restrictions on the source of reagents used in the following embodiments; commercially available products well known to those skilled in the art can be used.

[0039] In the following examples, the viscosity-average molecular weight of the polyethylene resin is 1,500,000 Da; the length L of the selected short-cut basalt fiber is 505 μm, and the diameter D50 ≤ 1.5 μm.

[0040] Example 1 This embodiment provides a heat-resistant, highly wettable composite coating diaphragm and its preparation method.

[0041] (1) Mix 25wt% polyethylene resin, 5wt% boehmite and 70wt% paraffin oil and put them into an extruder for melt blending. Extrude through a die to obtain a sheet. The mixing temperature T=200℃ and the stirring speed n=95rpm.

[0042] (2) The sheet in step (1) is subjected to biaxial stretching at a temperature of 124°C and a stretching ratio of 9 times.

[0043] (3) Immerse the material from step (2) in dichloromethane for room temperature extraction, and then dry it at 38°C.

[0044] (4) The material obtained in step (3) is biaxially stretched and shaped to obtain a polyethylene composite base film. The stretching temperature is 135℃, the stretching ratio is 6 times, the shaping temperature is 128℃, and the thickness of the polyethylene composite base film is 4μm.

[0045] (5) Preparation of chopped basalt fiber slurry: 0.3wt% wetting agent Triton X-100 and 0.8wt% dispersant Solsperse 41000 were added to deionized water and stirred at 300 rpm for about 10 min. Then, 40wt% chopped basalt fiber (D50≤1.5μm) was slowly added and the speed was gradually increased to 1500 rpm. The stirring was continued for 40 min. Zirconia beads were added and ball milled (particle size 0.3 mm) for 3 h to ensure that the length L of the chopped basalt fiber was ≤350μm. 1wt% sodium carboxymethyl cellulose (CMC) and 1.5wt% styrene-butadiene rubber (SBR) were added and stirred at low speed (400 rpm) for 20 min. The pH was adjusted to 7~8. 0.1wt% defoamer BYK-024 was added and the mixture was sieved at 300 mesh.

[0046] (6) The chopped basalt fiber slurry obtained in step (5) is evenly dispersed on one side of the diaphragm by spraying with a rotary nozzle device, and finally dried in an oven at 90°C to obtain a basalt chopped fiber coating.

[0047] (7) Preparation of polyphthalamide slurry: Dimethylformamide (DMF) was added to water and stirred evenly to prepare a 5wt% dispersant solution for later use. 35wt% polyphthalamide, 1wt% sodium carboxymethyl cellulose thickener, 5wt% polyacrylate binder S105, and 0.5wt% phosphate surfactant were added to the dispersant solution and stirred to prepare polyphthalamide slurry for later use. The mixing temperature was T=55℃, the stirring speed was n=200rpm, and the stirring time was t=50min.

[0048] (8) The polyphthalamide slurry obtained in step (7) is evenly dispersed on the surface of the basalt short fiber coating in step (6) by spraying using a rotary nozzle device, and finally dried in an oven at 90°C to obtain the polyphthalamide coating.

[0049] (9) Roll up the material from step (8).

[0050] The scanning electron microscope image of the heat-resistant, highly wettable composite coating membrane prepared in this embodiment is as follows: Figure 1 As shown, a dense basalt fiber coating and a polyphthalamide composite coating are formed on the surface of the polyethylene. The basalt fiber coating has a thickness of 1.5 μm, and the polyphthalamide composite coating has a thickness of 0.7 μm.

[0051] Example 2 This embodiment provides a heat-resistant, highly wettable composite coating diaphragm and its preparation method.

[0052] (1) Mix 25wt% polyethylene resin, 5wt% boehmite and 70wt% paraffin oil and put them into an extruder for melt blending. Extrude through a die to obtain a sheet. The mixing temperature T=200℃ and the stirring speed n=95rpm.

[0053] (2) The sheet in step (1) is subjected to biaxial stretching at a temperature of 124°C and a stretching ratio of 9 times.

[0054] (3) Immerse the material from step (2) in dichloromethane for room temperature extraction, and then dry it at 38°C.

[0055] (4) The material obtained in step (3) is biaxially stretched and shaped to obtain a polyethylene composite base film. The stretching temperature is 135℃, the stretching ratio is 6 times, the shaping temperature is 128℃, and the thickness of the polyethylene composite base film is 4μm.

[0056] (5) Preparation of chopped basalt fiber slurry: 0.3wt% wetting agent Triton X-100 and 1.3wt% dispersant Solsperse 41000 were added to deionized water and stirred at 300 rpm for about 10 min. Then, 45wt% chopped basalt fiber (D50≤1.5μm) was slowly added and the speed was gradually increased to 1500 rpm. The mixture was stirred for 40 min and then zirconia beads (0.3 mm particle size) were added and ball-milled for 3 h to ensure that the length L of the chopped basalt fiber was ≤350 μm. 1.5wt% sodium carboxymethyl cellulose (CMC) and 2wt% styrene-butadiene rubber (SBR) were added and stirred at low speed (400 rpm) for 20 min. The pH was adjusted to 7~8 and 0.1wt% defoamer BYK-024 was added and then sieved at 300 mesh.

[0057] (6) The chopped basalt fiber slurry obtained in step (5) is evenly dispersed on one side of the diaphragm by spraying using a rotary nozzle device, and finally dried in an oven at 90°C to obtain a basalt chopped fiber coating with a thickness of 2μm.

[0058] (7) Preparation of polyphthalamide slurry: Dimethylformamide (DMF) was added to water and stirred evenly to prepare a 5wt% dispersant solution for later use. 35wt% polyphthalamide, 1wt% sodium carboxymethyl cellulose thickener, 5wt% polyacrylate binder S105, and 0.5wt% phosphate surfactant were added to the dispersant solution and stirred to prepare polyphthalamide slurry for later use. The mixing temperature was T=55℃, the stirring speed was n=200rpm, and the stirring time was t=50min.

[0059] (8) The polyphthalamide slurry obtained in step (7) is evenly dispersed on the surface of the basalt short fiber coating in step (6) by spraying using a rotary nozzle device, and finally dried in an oven at 90°C to obtain a polyphthalamide coating with a thickness of 1 μm.

[0060] (9) Roll up the material from step (8).

[0061] Comparative Example 1 The difference between this comparative example and Example 1 is that this comparative example does not perform steps (5) and (6), that is, the diaphragm of this comparative example does not include the short-cut basalt fiber slurry layer.

[0062] Comparative Example 2 The difference between this comparative example and Example 1 is that in this comparative example, short-cut basalt fibers and boehmite are melt-blended together with polyethylene resin. The specific steps are as follows: (1) Mix 25wt% polyethylene resin, 5wt% boehmite, 5wt% chopped basalt fiber and 65wt% paraffin oil and put them into an extruder for melt blending. Extrude through a die to obtain sheet material; mixing temperature T=200℃, stirring speed n=95rpm.

[0063] (2) The sheet in step (1) is subjected to biaxial stretching at a temperature of 124°C and a stretching ratio of 9 times.

[0064] (3) Immerse the material from step (2) in dichloromethane for room temperature extraction, and then dry it at 38°C.

[0065] (4) The material obtained in step (3) is biaxially stretched and shaped to obtain a polyethylene composite base film. The stretching temperature is 135℃, the stretching ratio is 6 times, the shaping temperature is 128℃, and the thickness of the polyethylene composite base film is 4μm.

[0066] (5) Preparation of polyphthalamide slurry: Dimethylformamide (DMF) was added to water and stirred evenly to prepare a 5wt% dispersant solution for later use. 35wt% polyphthalamide, 1wt% sodium carboxymethyl cellulose thickener, 5wt% polyacrylate binder S105, and 0.5wt% phosphate surfactant were added to the dispersant solution and stirred to prepare polyphthalamide slurry for later use. The mixing temperature was T=55℃, the stirring speed was n=200rpm, and the stirring time was t=50min.

[0067] (6) The polyphthalamide slurry obtained in step (5) is evenly dispersed on one side of the polyethylene composite base film in step (4) by spraying using a rotary nozzle device, and finally dried in an oven at 90°C to obtain a polyphthalamide coating with a thickness of 1.5 μm.

[0068] (9) Roll up the material from step (8).

[0069] Comparative Example 3 The difference between this comparative example and Example 1 is that, in step (6), the thickness of the basalt short-cut fiber coating is 1 μm.

[0070] Comparative Example 4 The difference between this comparative example and Example 1 is that, in step (6), the thickness of the basalt short-cut fiber coating is 10 μm.

[0071] Comparative Example 5 The difference between this comparative example and Example 1 is that ball milling is not performed in step (5).

[0072] Comparative Example 6 The difference between this comparative example and Example 1 is that the thickness of the polyethylene composite base film is 7 μm, and steps (5) to (9) are not performed.

[0073] Test example 1. Diaphragm performance testing: The air permeability, mechanical properties, heat resistance, liquid retention and wetting properties of the diaphragms of Examples 1 and 2 and Comparative Examples 1 to 6 were measured, and the test results are summarized in Tables 1 and 2.

[0074] Liquid retention rate test method: Weigh the diaphragm before and after liquid absorption, and calculate the percentage difference in mass: (mass after liquid absorption - initial mass) / initial mass × 100%.

[0075] Wetting angle: The contact angle of a liquid on a material surface is measured directly using a contact angle measuring instrument.

[0076] Air permeability: The time it takes for 100 mL of gas to pass through the sample is measured using a Gurley air permeability meter (unit: s / 100 mL). The smaller the value, the better the air permeability.

[0077] Tensile strength: The fracture stress in the MD (longitudinal) and TD (transverse) directions of the diaphragm test specimen was measured using a universal tensile testing machine.

[0078] Needle penetration strength: The maximum force (in gf) used to puncture the diaphragm sample is measured using a needle penetration strength tester.

[0079] 150℃ heat shrinkage rate: The sample was heated in an oven at 150℃ for 1 hour, and the dimensional change rate before and after heating was measured: heat shrinkage rate = (original length - shrinkage length) / original length × 100%.

[0080] 180℃ heat shrinkage rate: The sample was heated in an oven at 180℃ for 1 hour, and the dimensional change rate before and after heating was measured: heat shrinkage rate = (original length - shrinkage length) / original length × 100%.

[0081] Membrane rupture temperature: Using a thermomechanical analyzer (TMA), the diaphragm sample is fixed and heated at a constant rate while a small, constant tension is applied to keep the sample taut. When the instrument detects that the sample breaks (sudden drop in tension or abrupt change in displacement), the temperature at this moment is recorded as the membrane rupture temperature.

[0082] Table 1. Wetting and air permeability data of Examples 1 and 2 and Comparative Examples 1-6

[0083] Table 2 Strength and heat resistance data of Examples 1 and 2 and Comparative Examples 1 to 6

[0084] As can be seen from Tables 1 and 2, the examples with a double-layer coating of basalt chopped fiber and polyphthalamide exhibited the best wettability, mechanical properties, and high-temperature resistance. Although the air permeability decreased slightly, it still met the requirements for diaphragm air permeability and would not have a significant impact on the subsequent application of the diaphragm.

[0085] 2. Lithium-ion battery performance testing: Assembly of the lithium-ion battery: The positive electrode was a commercially available lithium iron phosphate cathode, and the negative electrode was a commercially available graphite anode. The electrolyte was a mixed solution of EC (ethylene carbonate), EMC (ethyl methyl carbonate), and DMC (dimethyl carbonate) containing 1 mol / L LiPF6 (EC:EMC:DMC = 3:5:2). The separator was the same as that used in Example 1 and Comparative Example 6. A pouch cell was fabricated, and full-cell performance was tested in the 2.0-3.65V range. The test conditions were 45℃ and 100 cycles at a 1C charge / discharge rate. The test results are shown in Table 3.

[0086] Table 3 Electrochemical performance of lithium-ion batteries in Example 1 and Comparative Example 6

[0087] The comparison between the heat-resistant, high-wetting composite coating membrane of Example 1 and the polyethylene-based membrane of Comparative Example 6 shows that the high wettability and electrolyte affinity of the membrane of Example 1 imparts a superior coating to the Li in the electrolyte. + Better kinetics result in better performance at high rates, while lower internal resistance and high liquid retention give the battery better performance in dealing with Li+ loss under long-cycle conditions and higher capacity retention.

[0088] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A heat-resistant, highly wettable composite coating diaphragm, characterized in that, include: Polyolefin composite base film; A basalt chopped fiber coating laminated on at least one side of a polyolefin composite film; A polyphthalamide coating composited on a basalt chopped fiber coating; The polyolefin composite base film includes polyolefin and boehmite; the mass ratio of polyolefin to boehmite is (4~8):1; the thickness of the basalt chopped fiber coating is 1.2~8μm, and the thickness of the polyphthalamide coating is 0.5~2μm.

2. The heat-resistant, high-wetting composite coating diaphragm as described in claim 1, characterized in that, In the basalt chopped fiber coating, the diameter D50 of the basalt chopped fibers is ≤1.5μm and the length is ≤350μm.

3. The heat-resistant, high-wetting composite coating diaphragm as described in claim 1, characterized in that, The basalt chopped fiber coating also includes a binder; the binder content in the basalt chopped fiber coating is 4~10wt%.

4. The heat-resistant, high-wetting composite coating diaphragm as described in claim 1, characterized in that, The polyphthalamide coating also includes an adhesive; the adhesive content in the polyphthalamide coating is 10~25wt%.

5. The heat-resistant, high-wetting composite coating diaphragm as described in claim 1, characterized in that, The polyolefin includes polyethylene and / or polypropylene; the polyolefin composite base membrane is a microporous membrane with a thickness of 3~10μm.

6. The method for preparing the heat-resistant, high-wetting composite coating diaphragm according to any one of claims 1 to 5, characterized in that, Includes the following steps: Polyolefin resin, boehmite and paraffin oil are melt-extruded and then biaxially stretched. After extraction and drying, they are biaxially stretched a second time to obtain a polyolefin composite base film. Short-cut basalt fiber slurry is coated on one or both sides of a polyolefin composite base film and dried to obtain a basalt short-cut fiber coating. Then, a polyphthalamide slurry is coated on the surface of the basalt short-cut fiber coating, and after drying, a heat-resistant, highly wettable composite coating membrane is obtained.

7. The preparation method according to claim 6, characterized in that, The chopped basalt fiber slurry comprises chopped basalt fiber, binder, wetting agent, dispersant, defoamer, and water; in the chopped basalt fiber slurry, the mass content of chopped basalt fiber is 30~50wt%, the mass content of binder is 2~8wt%, the mass content of wetting agent is 0.1~0.5wt%, the mass content of dispersant is 0.5~2wt%, and the mass content of defoamer is 0.08~0.15wt%.

8. The preparation method according to claim 7, characterized in that, The preparation method of the chopped basalt fiber slurry is as follows: wetting agent, dispersant and chopped basalt fiber are added to water and ball-milled, then binder and defoamer are added, and the slurry is obtained by sieving.

9. The preparation method according to claim 6, characterized in that, The polyphthalamide slurry comprises polyphthalamide, thickener, binder, dispersant, surfactant, and solvent; in the polyphthalamide slurry, the mass content of polyphthalamide is 20~40wt%; the mass content of thickener is 0.1~1wt%; the mass content of binder is 5~15wt%; the mass content of dispersant is 3~8wt%; and the mass content of surfactant is 0.1~0.5wt%.

10. A lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The diaphragm is the heat-resistant, highly wettable composite coating diaphragm according to any one of claims 1 to 5 or the heat-resistant, highly wettable composite coating diaphragm prepared by the preparation method according to any one of claims 6 to 9.

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