Polyolefin base film, low-light-transmission diaphragm and preparation method and application of low-light-transmission diaphragm

Abstract

The invention discloses a polyolefin-based membrane, a low-light-transmittance diaphragm and a preparation method and application thereof, and belongs to the technical field of battery diaphragms. The polyolefin-based membrane disclosed by the invention is provided with nano-scale micropores, and the number of the micropores per unit volume is 3000 / [mu] m < 3 > to 5000 / [mu] m < 3 >; the average pore size of the micropores is 35-50 nm; the haze is 15%-35%; the light transmittance is 45%-60%; the thickness is 3-12 [mu] m; the surface of the base film is provided with oxygen-containing functional groups formed through ion beam bombardment. The polyolefin base film can effectively reduce energy absorption difference caused by light penetration, improves the photo-thermal stability of the diaphragm, has a good ion transmission channel, and is beneficial to improving the wetting and liquid absorption performance of an electrolyte.

Description

Technical Field

[0001] This invention belongs to the field of battery separator technology, and more specifically, relates to polyolefin-based membranes, low-transmittance separators, their preparation methods and applications. Background Technology

[0002] Lithium-ion battery separators are porous thin-film materials located between the positive and negative electrodes. Their main function is to prevent direct contact between electrons, which could lead to a short circuit, while allowing lithium ions to migrate freely within the electrolyte carrier, thus achieving reversible storage and release of electrochemical energy. Currently, commercially available lithium-ion battery separators are primarily made of polyolefin materials such as polyethylene (PE) and polypropylene (PP), prepared through wet or dry biaxial stretching processes. Due to the inherent advantages of polyolefin materials, such as high chemical stability, good mechanical strength, and low cost, they are widely used in power batteries, energy storage batteries, and 3C electronic products.

[0003] For example, Chinese patent application publication number CN112018307A (application date: October 13, 2020, title: "A polyethylene film containing silica and its preparation method") discloses a polyethylene film containing silica and its preparation method, including polyethylene, paraffin oil, silica nanoparticles, and 4,4' Thiodi(6) (tert-butyl-m-cresol) is mixed in batches, heated, and extruded. After transverse and longitudinal stretching, laser scanning is used to obtain abnormal points in the film's transmittance distribution, thereby determining the areas to be stretched. For example, Chinese patent application publication number CN114188661A (application date: October 28, 2021, title: "A Low Transmittance Film and Its Preparation Method and Battery") discloses a low transmittance film containing a polyolefin, wherein the polyolefin includes polyethylene, polypropylene, and poly(I)... At least one of butene and polypentene. For example, Chinese patent application publication number CN116656031A (application date: July 6, 2023, title: "A Special Color Masterbatch for Lithium Battery Separator and Its Preparation Method and Application") discloses a special color masterbatch for separator, including polymer base material, mixed colorant, antioxidant, and modified nanofiller.

[0004] However, existing polyolefin separators still face a series of performance bottlenecks. First, traditional separators have high light transmittance, which easily generates photothermal effects in the battery operating environment, exacerbating local heat accumulation and potentially inducing separator shrinkage or failure, thus reducing battery safety. Second, polyolefin materials themselves have low surface energy and poor wettability with electrolytes, resulting in slow initial electrolyte absorption and high interfacial impedance, affecting battery rate performance and cycle stability. Third, to improve surface hydrophilicity, existing technologies often employ ceramic coating or polymer coating modification methods, but the coating layer suffers from insufficient adhesion and is prone to detachment or pulverization during charging, discharging, and winding, potentially causing particles to detach and enter the electrode system, leading to interfacial side reactions and reliability risks. Furthermore, the coating process is complex, increases costs, and is not conducive to continuous production line scaling.

[0005] One effective approach is to improve the base membrane. Therefore, how to achieve controllable low light transmittance of the diaphragm without relying on additional coatings has become a pressing technical problem in this field. Summary of the Invention

[0006] 1. The problem to be solved In view of the technical problem that the high light transmittance of the diaphragm in the prior art leads to poor diaphragm stability, the first objective of the present invention is to provide a polyolefin-based membrane; A second objective of this invention is to provide a method for preparing the above-mentioned polyolefin-based film; A third objective of the present invention is to provide a low-transmittance membrane comprising the above-described polyolefin-based membrane; The fourth objective of this application is to provide a lithium-ion battery comprising the aforementioned low-transmittance separator.

[0007] 2. Technical Solution The technical solution adopted in this invention is as follows: Based on the purpose of this invention, a first aspect of this invention provides a polyolefin-based membrane, wherein the polyolefin-based membrane has nanoscale micropores; The polyolefin-based membrane has a micropore count of 3000 per μm. 3 ~5000 / μm 3 (or 3000 / μm) 3 ~5000 / μm 3 Any value between 1 / μm 3 Incremental, for example, 3001 cells / μm 3 ; or 3000 / μm 3 ~5000 / μm 3 Any value within the range between, in 1 / μm 3 Incremental, for example, 3500 cells / μm 3 Up to 4500 / μm 3 ).

[0008] The average pore size of the micropores is 35 nm to 50 nm (or any value between 35 nm and 50 nm, in increments of 1 nm, such as 36 nm; or any range between 35 nm and 50 nm, in increments of 1 nm, such as 36 nm to 50 nm).

[0009] The haze of the polyolefin-based film is 15% to 35% (or any value between 15% and 35% in increments of 1%, such as 16%; or any range between 15% and 35% in increments of 1%, such as 20% to 25%).

[0010] The transmittance of the polyolefin-based film is 45% to 60% (or any value between 45% and 60% in increments of 1%, such as 46%; or any range between 45% and 60% in increments of 1%, such as 47% to 53%).

[0011] The thickness of the polyolefin-based film is 3 μm to 12 μm (or any value between 3 μm and 12 μm, in increments of 0.1 μm, for example, 8.1 μm; or any range between 3 μm and 12 μm, in increments of 0.1 μm, for example, 3.2 μm to 11.9 μm).

[0012] The light transmittance of the base film described herein is affected by haze and light transmittance, which are two concepts affecting light transmittance. Light transmittance, or transmittance, refers to the proportion of light passing through the base film, usually expressed as a percentage. The higher the light transmittance, the better the transparency of the material. However, if the light transmittance of the base film is too high, such as greater than 60%, it means that the pore size of the base film is too large or the porosity is too high, which often sacrifices the mechanical strength of the base film. On the other hand, if the light transmittance is too low, such as less than 45%, it means that the pore size of the base film is too small or the porosity is too low, which often sacrifices the air permeability of the base film and leads to a decrease in ionic conductivity. Haze refers to the degree of scattering of light when it passes through the base film, which manifests as a blurry or turbid appearance of the material. The higher the haze value, the stronger the diffuse light effect. If the haze value is too high, such as above 35%, it means that the porosity of the base film is uneven, the pore size is too large, or there are microscopic defects, which can easily lead to battery short circuits or lithium dendrites. If the haze value is too low, such as below 15%, it means that the porosity is too low or the pore size is too small, which often sacrifices the air permeability of the base film and leads to a decrease in ionic conductivity.

[0013] The separator prepared in this application has both high haze value and low light transmittance. When the battery generates photothermal effect in the working environment, it can effectively avoid heat accumulation and improve battery safety.

[0014] Both transmittance and haze values ​​are used to evaluate the light transmittance that affects the appearance of the base film. The main factors influencing haze and transmittance are the surface micropore structure. For base films of the same thickness, controlling the number of micropores and the average pore size will alter light transmission and scattering, thus affecting transmittance and haze. However, it is important to note that this does not mean that more micropores or a smaller average pore size is always better. In this application, only the number of micropores per unit volume of the polyolefin base film is controlled to be 3000 per μm. 3 ~5000 / μm 3 In order to reduce the light transmittance of the base film, the average pore size of the micropores must be in the range of 35 nm to 50 nm to obtain lower light transmittance and higher haze value.

[0015] According to any embodiment of the first aspect of the present invention, the polyolefin-based film has oxygen-containing functional groups formed by ion beam bombardment on its surface.

[0016] The "oxygen-containing functional groups" mentioned herein are generated by ion beam bombardment. The ion beam bombardment takes place in the air, introducing oxygen from the air into the surface of the polyolefin base film, forming oxygen-containing functional groups on the surface of the polyolefin base film. This enhances the compatibility and wettability between the base film and the electrolyte, thereby enhancing the compatibility and wettability between the separator and the electrolyte. This results in the separator having lower interfacial contact resistance and better electrochemical stability, thus improving the cycle performance and safety reliability of the battery.

[0017] According to any embodiment of the first aspect of the invention, the polyolefin-based film is a polyethylene, polypropylene, or a mixture of both.

[0018] According to any embodiment of the first aspect of the present invention, the polyolefin-based membrane has a micropore count of 3300 per μm. 3 ~4500 / μm 3 The average pore size of the micropores is 35 nm to 49 nm; the haze of the polyolefin-based film is 15% to 28%; and the light transmittance is 45% to 60%.

[0019] A second aspect of the present invention provides a method for preparing the above-mentioned polyolefin-based film, comprising the following raw materials: Polyolefin polymers: including first polyolefin, second polyolefin and third polyolefin in a mass ratio of (80-95):(2-15):(2-10); Including the following steps: S1. Polyolefin polymers are mixed with processing aids and extruded into cast sheets, which are then biaxially stretched. S2. Surface modification treatment is carried out by bombardment with an inert gas ion beam to form a nanoscale microporous structure and oxygen-containing functional groups on the surface of the polyolefin-based film; S3. Heat setting treatment to obtain the polyolefin-based film.

[0020] Preferably, the inert gas ion is a helium ion or an argon ion.

[0021] According to any embodiment of the second aspect of the present invention, in the method for preparing a polyolefin-based film, the first polyolefin is an ultra-high molecular weight polyolefin with a molecular weight of 1.2 million to 3 million (or any value between 1.2 million and 3 million, in increments of 10,000, for example, 1.21 million; or any range between 1.2 million and 3 million, in increments of 10,000, for example, 1.22 million to 2.22 million); the second polyolefin is a high molecular weight polyolefin with a molecular weight of 500,000 to 1 million (or any value between 500,000 and 1 million, in increments of 10,000, for example, 510,000; or any range between 500,000 and 1 million, in increments of 10,000, for example, 520,000 to 920,000); the second polyolefin is a common polyolefin with a molecular weight of 100,000 to 490,000 (or any value between 100,000 and 490,000, in increments of 10,000, for example, 110,000; or any range between 100,000 and 490,000, in increments of 10,000, for example, 120,000 to 320,000).

[0022] According to any embodiment of the second aspect of the present invention, the method for preparing a polyolefin-based film, wherein the first polyolefin is polyethylene, polypropylene, or a mixture of both; The second polyolefin is polyethylene, polypropylene, or a mixture of both; The third polyolefin is polyethylene, polypropylene, or a mixture of both.

[0023] According to any embodiment of the second aspect of the present invention, in the method for preparing a polyolefin-based film, the difference between the molecular weight of the first polyolefin and the molecular weight of the second polyolefin is 200,000 to 2,500,000, preferably 1,000,000 to 2,500,000; the difference between the molecular weight of the second polyolefin and the molecular weight of the third polyolefin is 10,000 to 900,000, preferably 300,000 to 800,000.

[0024] The role of the "polyolefins of different molecular weights" mentioned here is to control the number and spatial distribution of molecular chain entanglement points in the raw materials by utilizing the molecular weight differences between the first polyolefin (ultra-high molecular weight polyethylene), the second polyolefin (high molecular weight polyethylene), and the third polyolefin (ordinary polyethylene), so that the molecular chains at the interface can fully diffuse and re-entangle during the formation of the polyolefin-based film, thereby achieving controllable construction of pore number, pore size, and pore volume.

[0025] Among them, the first polyolefin, the second polyolefin and the third polyolefin jointly regulate the number and pore size of micropores on the base film surface; the difference in molecular weight between different polyolefins is controlled to regulate the number and pore size of micropores on the base film surface.

[0026] On the one hand, micropores are formed on the surface of the base film by the molecular weight difference between polyolefins of different molecular weights: the larger the molecular weight of the polyolefin, the longer the molecular chain, and the more difficult it is to break when bombarded by ion beam. In fact, the polyolefins with relatively small molecular weights are generally broken when bombarded by ion beam. The small molecular weight polyolefins on the surface of the base film break under the bombardment of ion beam, thus forming micropores.

[0027] On the other hand, high molecular weight polyolefins are introduced to regulate the number and size of micropores on the base film surface, resulting in a greater number of micropores. Due to the large difference between ultra-high molecular weight polyolefins and ordinary polyolefins, bombardment of low molecular weight polyolefins is too easy, while bombardment of ultra-high molecular weight polyolefins is too difficult, leading to a small number of micropores and large micropore sizes on the base film surface. This makes it impossible to obtain the low-transmittance polyolefin base film of this application. Through the transitional effect of high molecular weight polyolefins, the number of micropores per unit volume of the polyolefin base film reaches 3000 per μm. 3 ~5000 / μm 3 The average pore size of the micropores is 35 nm to 50 nm.

[0028] According to any embodiment of the second aspect of the present invention, in the method for preparing a polyolefin-based film, the mass ratio of the first polyolefin, the second polyolefin, and the third polyolefin is any value between (80-95):(2-15):(2-10) or (80-100):(2-15):(2-10) in increments of 1, for example, 85:8:7, 80:10:10, 80:13:7, 90:5:5; or any range between (80-95):(2-15):(2-10) in increments of 1, for example, (80-90):(5-15):(5-10).

[0029] The ratio of different molecular weight polyolefins described herein affects light transmittance. A higher proportion of ultra-high molecular weight polyolefins reduces membrane transmittance while ensuring the polyolefin-based membrane maintains high mechanical strength and structural stability under high-speed winding and electrochemical cycling conditions. However, excessive ultra-high molecular weight polyolefin content makes ion beam bombardment difficult, hindering the formation of micropores on the membrane surface. Conversely, excessive low molecular weight polyolefin content results in excessively large micropore sizes on the membrane surface. Therefore, by introducing high molecular weight polyolefins as a transition and controlling the ratio of the three different molecular weight polyolefins, the number of micropores per unit volume of the polyolefin-based membrane is reduced to 3000 per μm. 3 ~5000 / μm 3 The average pore size of the micropores is 35 nm to 50 nm.

[0030] According to any embodiment of the second aspect of the present invention, in the method for preparing a polyolefin-based film, in step S1, the mass ratio of the polyolefin polymer to the processing aid is (8~30):(70~92), or any value between (8~30):(70~92) in increments of 1, for example 9:91, 10:90, 18:82, 20:80; or any range between (8~30):(70~92) in increments of 1, for example (10~20):(90~80), preferably, the processing aid is paraffin oil.

[0031] According to any embodiment of the second aspect of the present invention, in the method for preparing a polyolefin-based film, step S1 further includes preliminary stretching before biaxial stretching, wherein the stretching ratio of the preliminary stretching is 1.1 to 2 times (or any value between 1.1 and 2 times, in increments of 0.1 times, for example, 1.2 times; or any range between 1.1 and 2 times, in increments of 0.1 times, for example, 1.3 to 1.9 times), preferably 1.8 times; and the preliminary stretching temperature is 90°C to 110°C (or any value between 90°C and 110°C, in increments of 1°C, for example, 91°C; or any range between 90°C and 110°C, in increments of 1°C, for example, 95°C to 105°C), preferably 100°C.

[0032] According to any embodiment of the second aspect of the present invention, in the method for preparing a polyolefin-based film, in step S1, the longitudinal stretching ratio in the biaxial stretching is 5 to 11 times (or any value between 5 and 11 times, in increments of 1, for example, 6 times; or any range between 5 and 11 times, in increments of 1, for example, 7 to 10 times), preferably 7.5 times; the transverse stretching ratio is 5 to 11 times (or any value between 5 and 11 times, in increments of 1, for example, 6 times; or any range between 5 and 11 times, in increments of 1, for example, 7 to 10 times), preferably 7.5 times; the stretching temperature of the biaxial stretching is 120°C to 130°C, preferably 125°C.

[0033] According to any embodiment of the second aspect of the present invention, in the method for preparing a polyolefin-based film, in step S1, the biaxially stretched polyolefin-based film needs to be extracted and dried. The extraction temperature is 10℃~20℃ (or any value between 10℃~20℃, in increments of 1℃, for example 11℃; or any range between 10℃~20℃, in increments of 1℃, for example 13℃~17℃), preferably 15℃; the extraction time is 50s~80s (or any value between 50s~80s, in increments of 1s, for example 51s; or any range between 50s~80s, in increments of 1s, for example 60s~70s), preferably 60s; the drying temperature is 40℃~50℃ (or any value between 40℃~50℃, in increments of 1℃, for example 41℃; or any range between 40℃~50℃, in increments of 1℃, for example 43℃). The temperature is 45℃ to 47℃, preferably 45℃; the drying time is 50s to 80s (or any value between 50s and 80s, in increments of 1s, for example 51s; or any range between 50s and 80s, in increments of 1s, for example 60s to 70s), preferably 60s.

[0034] According to any embodiment of the second aspect of the present invention, in the method for preparing a polyolefin-based film, in step S1, the extrusion casting process temperature is 65~210°C, and the extrusion casting process is divided into a low-temperature to medium-temperature transition zone and a high-temperature zone: Low temperature to medium temperature transition zone: temperature ranges from 65 ℃ to 160 ℃, with the temperature gradually increasing from front to back; High-temperature zone: Temperature ranges from 200 ℃ to 210 ℃.

[0035] Specifically, the extrusion process is divided into ten zones. Zones one through three have progressively higher temperatures, forming a low-temperature to medium-temperature transition zone. Zones four through the final zone are high-temperature zones used to fully plasticize the mixture. The extrusion temperature is controlled within the range of 10°C to 40°C above the melting point of the polyolefin polymer (or any value between 10°C and 40°C, in increments of 1°C, such as 11°C; or any range between 10°C and 40°C, in increments of 1°C, such as 20°C to 30°C) to ensure melt flowability and uniform dispersion of components.

[0036] Preferably, the temperature of Zone 1 is 65℃~75℃ (or any value between 65℃~75℃, in increments of 1℃, for example 66℃; or any range between 65℃~75℃, in increments of 1℃, for example 67℃~73℃), the temperature of Zone 2 is 100℃~110℃ (or any value between 100℃~110℃, in increments of 1℃, for example 101℃; or any range between 100℃~110℃, in increments of 1℃, for example 103℃~107℃), the temperature of Zone 3 is 150℃~160℃ (or any value between 150℃~160℃, in increments of 1℃, for example 151℃; or any range between 150℃~160℃, in increments of 1℃, for example 153℃~157℃), and the temperature of Zones 4 to 10 is 200℃. The extrusion temperature is controlled at approximately 220°C to 210°C (or any value between 200°C and 210°C, in increments of 1°C, such as 201°C; or any range between 200°C and 210°C, in increments of 1°C, such as 203°C to 207°C), and the extrusion temperature is controlled at approximately 220°C to 230°C (or any value between 220°C and 230°C, in increments of 1°C, such as 221°C; or any range between 220°C and 230°C, in increments of 1°C, such as 223°C to 227°C).

[0037] According to any embodiment of the second aspect of the present invention, in the method for preparing a polyolefin-based film, in step S1, the thickness of the cast sheet is 1600 μm to 1800 μm (or any value between 1600 μm and 1800 μm, in increments of 1 μm, for example 1601 μm; or any range between 1600 μm and 1800 μm, in increments of 1 μm, for example 1650 μm to 1750 μm).

[0038] Preferably, the thickness of the cast sheet is 1700 μm.

[0039] According to any embodiment of the second aspect of the present invention, in the method for preparing a polyolefin-based film, in step S2, the parameters of the ion beam bombardment modification treatment are: ion beam energy of 1 kV to 50 kV (or any value between 1 kV and 50 kV, in increments of 1 kV, for example, 2 kV; or any range between 1 kV and 50 kV, in increments of 1 kV, for example, 3 kV to 40 kV), and flux of 10 13 ions / cm 2 ~10 14 ions / cm 2 (or 10) 13 ions / cm 2 ~10 14 ions / cm2 Any value between 1 ions / cm 2 For increments, 10 13 ions / cm 2 ~10 14 ions / cm 2 The incident angle is 30° to 65° (or any value between 30° and 65°, in increments of 1°, such as 31°; or any value between 30° and 65°, in increments of 1°, such as 40° to 50°), the processing temperature is not higher than 80°C (or any value between 80°C, in increments of 1°, such as 75°C; or any value between 80°C, in increments of 1°, such as any value between 70°C), optionally 30°C to 80°C, and the bombardment time is 60 s to 600 s (or any value between 60 s and 600 s, in increments of 1 s, such as 61 s; or any value between 60 s and 600 s, in increments of 1 s, such as 300 s to 500 s).

[0040] The parameters of the ion beam bombardment modification treatment described herein affect the surface microporous structure and surface properties of the base film.

[0041] For microporous structures, higher ion beam energy and longer bombardment time tend to form micropores with larger average pore size, resulting in a reduction in the number of micropores and thus affecting the transmittance and haze of the base film.

[0042] To address surface properties, ion beam bombardment modification is employed, which can directly generate nanoscale micropores and oxygen-containing functional groups on the membrane surface without introducing additional chemical additives. This makes the surface modification layer stable and less prone to peeling off, avoiding the particle shedding and electrochemical side reaction problems that may occur with traditional surface coating methods. It further reduces light transmittance and increases haze value without damaging the overall pore connectivity of the membrane.

[0043] According to any embodiment of the second aspect of the present invention, in the method for preparing a polyolefin-based film, in step S3, the temperature of the heat setting treatment is 130 ℃ to 140 ℃ (or any value between 130 ℃ and 140 ℃, in increments of 1 ℃, for example 131 ℃; or any range between 130 ℃ and 140 ℃, in increments of 1 ℃, for example 133 ℃ to 137 ℃), and the time is 5 s to 15 s (or any value between 5 s and 15 s, in increments of 1 s, for example 6 s; or any range between 5 s and 15 s, in increments of 1 s, for example 10 s to 12 s).

[0044] The heat setting described herein is used to stabilize the diaphragm pore structure and improve the thermal stability of the diaphragm dimensions.

[0045] According to any embodiment of the second aspect of the present invention, in the method for preparing a polyolefin-based film, step S3, before heat setting, further includes a secondary stretching process, wherein the stretching is transverse stretching, the stretching ratio is 1 to 2 times (or any value between 1 and 2 times, in increments of 0.1 times, for example 1.2 times; or any range between 1 and 2 times, in increments of 0.1 times, for example 1.3 to 1.9 times), preferably 1.4 times, and the stretching temperature is 130 ℃ to 140 ℃ (or any value between 130 ℃ and 140 ℃, in increments of 1 ℃, for example 131 ℃; or any range between 130 ℃ and 140 ℃, in increments of 1 ℃, for example 133 ℃ to 139 ℃), preferably 135 ℃.

[0046] A third aspect of the present invention provides a low-transmittance membrane, comprising: a polyolefin-based membrane of the first aspect of the present invention, or a polyolefin-based membrane prepared by the preparation method of the second aspect of the present invention, wherein at least one surface of the polyolefin-based membrane layer is provided with a composite coating layer.

[0047] According to any embodiment of the third aspect of the present invention, the low-transmittance diaphragm, the composite coating layer comprises ceramic particles, binder, wetting agent and dispersant in a dry weight ratio of (80-120):(0.5-10):(0.01-3):(0.1-10).

[0048] According to any embodiment of the third aspect of the present invention, the low-transmittance membrane is provided in which the ceramic particles are selected from one or more of boehmite, alumina, magnesium oxide, zirconium oxide, titanium oxide, calcium oxide, aluminum nitride, boron nitride, barium sulfate, calcium fluoride, barium fluoride, or barium titanate.

[0049] According to any embodiment of the third aspect of the present invention, the low light transmittance membrane is provided, wherein the adhesive is selected from one or more of polyvinylidene fluoride, polyacrylic acid, polyacrylate, and polyurethane.

[0050] According to any embodiment of the third aspect of the present invention, the low light transmittance membrane is provided, wherein the wetting agent is selected from one or more of anionic surfactants, polyoxyethylene nonionic surfactants, silicone surfactants, polyol surfactants, methyl acrylate, ethanol, propylene glycol, glycerol, and dimethyl sulfoxide, for example, it may be one or more of methyl acrylate, polyether-modified polysiloxane, dodecylphenol polyoxyethylene ether, and sodium dodecyl sulfonate.

[0051] According to any embodiment of the third aspect of the present invention, the low light transmittance membrane is provided, wherein the dispersant is selected from one or more of polyvinylpyrrolidone, polyacrylamide, polyvinyl alcohol, and sodium polyacrylate.

[0052] The fourth aspect of the present invention provides the application of the above-mentioned low light transmittance separator in lithium-ion batteries.

[0053] The lithium-ion battery includes a separator, which includes the polyolefin-based membrane described in the first aspect of the present invention; or a polyolefin-based membrane obtained by the preparation method of the second aspect of the present invention. Alternatively, the diaphragm may be the low-transmittance diaphragm described in the third aspect of this invention.

[0054] 3. Beneficial effects I. Polyolefin-based film: (1) The polyolefin-based membrane of the present invention has a nanoscale microporous structure with a micropore count of 3000 per μm per unit volume. 3 ~5000 / μm 3 The average pore size is 35 nm to 50 nm; the haze of the membrane is 15% to 35% and the transmittance is 45% to 60%, which enables the membrane containing this polyolefin-based membrane to effectively reduce the energy absorption difference caused by light transmission, reduce heat accumulation, and improve the photothermal stability of the membrane. (2) The polyolefin-based membrane of the present invention contains oxygen-containing functional groups on its surface, which can enhance the compatibility and wettability of the membrane with the electrolyte, so that the membrane containing the polyolefin-based membrane has lower interfacial contact resistance and better electrochemical stability, thereby improving the cycle performance and safety reliability of the battery. (3) The polyolefin-based membrane of the present invention achieves good ion transport channels while maintaining high porosity, which is beneficial to improving the wetting and liquid absorption performance of the electrolyte, thereby improving the ionic conductivity of the battery. II. Preparation method of polyolefin-based film: (4) The preparation method of the present invention, by adding polyolefins of different molecular weights, is conducive to forming a uniform and continuous porous base film skeleton structure. After ion beam bombardment modification treatment, nanoscale micropores and oxygen-containing functional groups are generated on the base film surface, making the surface modification layer stable and not easy to fall off. (5) The preparation method of the present invention stabilizes the pore structure of the base film through the heat setting step, improves the thermal stability of the membrane size, and enhances the overall safety; III. Low-transmittance diaphragm: (6) The separator of the present invention has low light transmittance. When applied to lithium batteries, it can improve the electrolyte wetting speed during the battery start-up stage, reduce the initial internal resistance, and improve the output stability of the battery under high-rate charge and discharge conditions. The nanoscale microporous structure and oxygen-containing functional groups can effectively improve the lithium-ion migration efficiency, thereby improving the battery capacity retention rate and cycle life. At the same time, due to the excellent thermal stability of the separator, the risk of thermal runaway can be reduced, thereby improving the safety and reliability of the battery PACK system and the whole machine during use. The separator of the present invention is suitable for various types of lithium battery systems such as power batteries, energy storage batteries and 3C batteries. Detailed Implementation

[0055] All references cited in this invention are incorporated herein by reference in their entirety, and in the event of any inconsistency between the meanings expressed in these references and those expressed herein, the meanings expressed herein shall prevail. Furthermore, the various terms and phrases used in this invention have their general meanings known to those skilled in the art; however, this invention still seeks to provide a more detailed explanation and interpretation of these terms and phrases, and in the event of any inconsistency between the mentioned terms and their known meanings and those expressed herein, the meanings expressed herein shall prevail.

[0056] When an item is described using the combined terms “...and / or ...", the description should be understood to include any one of the listed items and all combinations thereof.

[0057] Generally, the use of the term "about" indicates an approximation that can vary depending on the desired characteristics obtained from the disclosed subject matter and will be interpreted in a context-dependent manner based on function. Therefore, those skilled in the art will be able to interpret a degree of difference on a case-by-case basis. In some cases, the number of significant figures used when expressing a particular value can be a representative technique for determining the difference allowed by the term "about." In other cases, a gradient within a range of values ​​can be used to determine the range of differences allowed by the term "about." Furthermore, all ranges in this disclosure are inclusive and composable, and references to values ​​described within a range include every value within that range.

[0058] In this invention, the term "comprising" or "containing" indicates that various ingredients may be used together in the composition of this invention. Therefore, the terms "consistent with..." and "composed of..." are included in the term "comprising" or "containing".

[0059] Unless otherwise defined, "molecular weight" as used in this article refers to average molecular weight.

[0060] Unless otherwise defined, 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; the terms used herein and / or include any and all combinations of one or more of the associated listed items.

[0061] Unless otherwise stated, any feature disclosed in this specification may be replaced by other equivalent or similar features. Unless otherwise stated, each feature is merely one example of a series of equivalent or similar features. The descriptions are merely to aid in understanding the invention and should not be construed as limiting the invention. Unless otherwise specified, specific conditions in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0062] The testing method for [microporous structure] is as follows: The number of pores per unit volume N and the pore size distribution nm were obtained using FIB-SEM three-dimensional reconstruction and ultra-small angle X-ray scattering (USAXS) combined characterization, which enables direct counting of the number of micropores.

[0063] The test method for [haze and transmittance] is as follows: According to GB / T 2410-2008, the sample size is 50 mm × 50 mm; it needs to be placed at 23±2℃ and 50±5 %RH for ≥50 h before testing.

[0064] Test steps: Measurements were performed using an integrating sphere haze meter, and the instrument was calibrated using a standard haze plate. The diaphragm sample was completely covered by the integrating sphere entrance window. The test was conducted and repeated three times, and the average value was taken.

[0065] The test method for [diaphragm thickness] is as follows: The test was conducted in accordance with the requirements of GB / T 36363-2018 "Polyolefin Separators for Lithium-ion Batteries".

[0066] 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.

[0067] The present application will be further described below with reference to specific embodiments.

[0068] Example 1 (1) Polyethylene-based film: The polyethylene-based film has nanoscale micropores; The number of micropores per unit volume of the polyethylene-based film is 4200 per μm³; The average pore size of the micropores is 42 nm; The haze of the polyethylene-based film is 20%. The transmittance of the polyethylene-based film is 52%; The thickness of the polyethylene film is 9 μm.

[0069] (2) Preparation method of polyethylene-based film: The raw materials include: polyolefin polymers: a mixture of first polyethylene, second polyethylene, and third polyethylene in a mass ratio of 90:5:5. The first polyethylene has a molecular weight of 1.8 million, the second polyethylene has a molecular weight of 750,000, and the third polyethylene has a molecular weight of 150,000. The difference in molecular weight between the first and second polyethylene is 1.05 million, and the difference in molecular weight between the second and third polyethylene is 600,000.

[0070] Including the following steps: S1. A polyolefin polymer is mixed with paraffin oil to form a compound, which is then extruded into a casting. The mass ratio of the polyolefin polymer to the paraffin oil is 20:80. The mixture is fed into an extruder for melt mixing. The temperatures of each zone of the extruder are set as follows: Zone 1 70℃, Zone 2 105℃, Zone 3 155℃, and Zones 4 to 10 are all 205℃. The molten material is extruded through a coat hanger-type die at 228℃ and then rapidly cooled on a cooling roller to form a casting with a thickness of 1700 μm. The resulting casting was initially stretched longitudinally by 1.8 times at 100℃ to achieve preliminary orientation control of the molecular chains; the casting was then bidirectionally stretched at 125℃, with both longitudinal and transverse stretching ratios of 7.5 times. The wind speed in the bidirectional stretching machine was controlled at 5 m / s, and the temperature difference during the stretching process was controlled within the range of less than 0.5℃ to ensure uniform film thickness. The obtained membrane was placed in an extraction tank containing dichloromethane (CH2Cl2) for extraction at a temperature of 15°C for 60 s, and then dried at 45°C for 60 s to remove residual solvent.

[0071] S2. Using Ar + Ion beam bombardment for membrane surface modification: ion beam energy 10 kV, flux 10 13 ions / cm 2 The incident angle was 45°, the processing temperature was 80°C, and the bombardment time was 100 s.

[0072] S3. Perform a transverse secondary stretching at 135℃ with a stretching ratio of 1.4 times, and then heat set at 132℃ for 10 seconds.

[0073] The final thickness was 9 μm, and the number of micropores was 4200 / μm. 3 A polyethylene-based film with an average pore size of 42 nm, a haze of 20%, and a light transmittance of 52%.

[0074] Example 2 The difference between this embodiment and Embodiment 1 is that the molecular weights of the first polyethylene, the second polyethylene, and the third polyethylene are different: The mass ratio of the first polyethylene (molecular weight: 2.5 million), the second polyethylene (molecular weight: 600,000), and the third polyethylene (molecular weight: 120,000) is 90:5:5. The difference in molecular weight between the first and second polyethylene is 1.9 million, and the difference in molecular weight between the second and third polyethylene is 480,000.

[0075] Preparation method: Different ion beam bombardment times: The ion beam energy is 10 kV and the flux is 10 13 ions / cm 2 The incident angle was 45°, the processing temperature was 80℃, and the processing time was 300 s.

[0076] The remaining process conditions were the same as in Example 1, resulting in a final thickness of 9 μm and a micropore count of 3800 / μm. 3 A polyethylene-based film with an average pore size of 45 nm, a haze of 25%, and a light transmittance of 45%.

[0077] Example 3 The difference between this embodiment and Embodiment 1 is that the mass ratio of the first polyethylene, the second polyethylene, and the third polyethylene is different. The mass ratio of the first polyethylene (molecular weight: 1.8 million), the second polyethylene (molecular weight: 750,000), and the third polyethylene (molecular weight: 150,000) is 85:8:7.

[0078] Preparation methods: The ion beam bombardment energy, processing temperature, and processing time vary. The ion beam treatment energy is 20 kV, and the flux is 10. 13 ions / cm 2 The incident angle was 45°, the processing temperature was 60℃, and the processing time was 150 s.

[0079] The remaining process conditions were the same as in Example 1, resulting in a final thickness of 9 μm and a micropore count of 4100 / μm. 3 A polyethylene-based film with an average pore size of 44 nm, a haze of 18%, and a transmittance of 53%.

[0080] Example 4 The difference between this embodiment and Embodiment 1 is that, The molecular weight of the first polyethylene increases, and the mass ratio of the first polyethylene, the second polyethylene, and the third polyethylene is different: First polyethylene (molecular weight: 2.5 million), second polyethylene (molecular weight: 750,000), and third polyethylene (molecular weight: 150,000) are mixed in a mass ratio of 85:8:7.

[0081] Preparation methods: The ion beam bombardment energy, processing temperature, and processing time vary. The ion beam treatment energy is 20 kV, and the flux is 10. 13 ions / cm 2 The incident angle was 45°, the processing temperature was 60℃, and the processing time was 400 s.

[0082] The remaining process conditions are the same as in Example 1, resulting in a final thickness of 9 μm and a micropore count of 4000 / μm. 3 A polyethylene-based film with an average pore size of 44 nm, a haze of 22%, and a light transmittance of 48%.

[0083] Example 5 The difference between this embodiment and Embodiment 1 is that, The molecular weights of the second and third polyethylene decrease, and the mass ratios of the first, second, and third polyethylenes are different. The mass ratio of the first polyethylene (molecular weight: 1.8 million), the second polyethylene (molecular weight: 600,000), and the third polyethylene (120,000) is 80:10:10. The difference in molecular weight between the first and second polyethylene is 1.2 million, and the difference in molecular weight between the second and third polyethylene is 480,000.

[0084] Preparation methods: The ion beam bombardment energy, processing temperature, and processing time vary. The ion beam treatment energy is 30 kV, and the flux is 10. 13 ions / cm 2 The incident angle was 45°, the processing temperature was 40℃, and the processing time was 200 s.

[0085] The remaining process conditions were the same as in Example 1, resulting in a final thickness of 9 μm and a micropore count of 4250 / μm. 3 A polyethylene-based film with an average pore size of 40 nm, a haze of 20%, and a light transmittance of 50%.

[0086] Example 6 The difference between this embodiment and Embodiment 1 is that the molecular weight and mass ratio of the first polyethylene, the second polyethylene, and the third polyethylene are different: The mass ratio of the first polyethylene (molecular weight: 2.5 million), the second polyethylene (molecular weight: 600,000), and the third polyethylene (molecular weight: 120,000) is 80:15:5.

[0087] Preparation methods: The ion beam bombardment energy, processing temperature, and processing time vary. The ion beam treatment energy is 30 kV, and the flux is 10. 13 ions / cm 2The incident angle was 45°, the processing temperature was 40℃, and the processing time was 500 s.

[0088] The remaining process conditions were the same as in Example 1, resulting in a final thickness of 9 μm and a micropore count of 3900 / μm. 3 A polyethylene-based film with an average pore size of 43 nm, a haze of 25%, and a light transmittance of 45%.

[0089] Example 7 The difference between this embodiment and Embodiment 1 is that, The mass ratios of the first, second, and third polyethylene are different: First polyethylene (molecular weight: 1.8 million), second polyethylene (molecular weight: 750,000), and third polyethylene (150,000) are mixed in a mass ratio of 80:13:7.

[0090] Preparation methods: The ion beam bombardment energy, processing temperature, and processing time vary. The ion beam treatment energy is 40 kV, and the flux is 10. 13 ions / cm 2 The incident angle was 45°, the processing temperature was 30℃, and the processing time was 300 s.

[0091] The remaining process conditions were the same as in Example 1, resulting in a final thickness of 9 μm and a micropore count of 4300 / μm. 3 A polyethylene-based film with an average pore size of 39 nm, a haze of 18%, and a light transmittance of 52%.

[0092] Example 8 The difference between this embodiment and Embodiment 1 is that the molecular weight and mass ratio of the first polyethylene, the second polyethylene, and the third polyethylene are different: First polyethylene (molecular weight: 2.5 million), second polyethylene (molecular weight: 600,000), and third polyethylene (molecular weight: 120,000) are mixed in a mass ratio of 80:13:7.

[0093] Preparation methods: The ion beam bombardment energy, processing temperature, and processing time vary. The ion beam treatment energy is 40 kV, and the flux is 10. 13 ions / cm 2 The incident angle was 45°, the processing temperature was 30℃, and the processing time was 600 s.

[0094] The remaining process conditions were the same as in Example 1, resulting in a final thickness of 9 μm and a micropore count of 4500 / μm. 3 A polyethylene-based film with an average pore size of 35 nm, a haze of 15%, and a light transmittance of 48%.

[0095] Example 9 The difference between this embodiment and Embodiment 1 is that the molecular weight and mass ratio of the first polyethylene, the second polyethylene, and the third polyethylene are different: First polyethylene (molecular weight: 1.2 million), second polyethylene (molecular weight: 1 million), and third polyethylene (molecular weight: 200,000) are mixed in a mass ratio of 95:3:2.

[0096] Preparation methods: The ion beam bombardment energy, processing temperature, and processing time vary. The ion beam treatment energy is 50 kV, and the flux is 10. 13 ions / cm 2 The incident angle was 45°, the processing temperature was 30℃, and the processing time was 600 s.

[0097] The remaining process conditions were the same as in Example 1, resulting in a final thickness of 9 μm and a micropore count of 3300 / μm. 3 A polyethylene-based film with an average pore size of 49 nm, a haze of 28%, and a light transmittance of 45%.

[0098] Example 10 The difference between this embodiment and Embodiment 1 is that the molecular weight and mass ratio of the first polyethylene, the second polyethylene, and the third polyethylene are different; the ion beam bombardment energy, the processing temperature, and the processing time are also different.

[0099] First polyethylene (molecular weight: 3 million), second polyethylene (molecular weight: 500,000), and third polyethylene (molecular weight: 200,000) are mixed in a mass ratio of 90:5:5.

[0100] Preparation methods: The ion beam bombardment energy, processing temperature, and processing time vary. The ion beam treatment energy is 1 kV, and the flux is 10. 13 ions / cm 2 The incident angle was 45°, the processing temperature was 80℃, and the processing time was 100 s.

[0101] The remaining process conditions were the same as in Example 1, resulting in a final thickness of 9 μm and a micropore count of 4400 / μm. 3 A polyethylene-based film with an average pore size of 38 nm, a haze of 17%, and a light transmittance of 55%.

[0102] Comparative Example 1 This comparative example uses only the first polyethylene as raw material, with a molecular weight of 1.8 million, without adding the second or third polyethylene.

[0103] The remaining process conditions were the same as in Example 1, resulting in a final thickness of 9 μm and a micropore count of 5500 / μm. 3A polyethylene-based film with an average pore size of 31 nm, a haze of 2%, and a light transmittance of 45%.

[0104] Comparative Example 2 The difference between this comparative example and Example 1 is that the amount of the first polyethylene added is too small, and the molecular weights of the second and third polyethylenes are low, that is, the mass ratio of the first polyethylene (1.8 million), the second polyethylene (700,000), and the third polyethylene (100,000) is 50:25:25.

[0105] Preparation method: Different ion beam bombardment energies: Ion beam treatment energy is 1 kV, flux is 10 13 ions / cm 2 The incident angle was 45°, the processing temperature was 80℃, and the time was 100 s.

[0106] The remaining process conditions were the same as in Example 1, resulting in a final thickness of 9 μm and a micropore count of 5200 / μm. 3 A polyethylene-based film with an average pore size of 33 nm, a haze of 7%, and a light transmittance of 50%.

[0107] Comparative Example 3 The difference between this comparative example and Example 1 is that it did not undergo ion beam treatment: Raw materials: The mass ratio of first polyethylene (3 million), second polyethylene (500,000), and third polyethylene (200,000) is 90:5:5.

[0108] The remaining process conditions were the same as in Example 1, resulting in a final thickness of 9 μm and a micropore count of 5300 / μm. 3 A polyethylene-based film with an average pore size of 32 nm, a haze of 5%, and a light transmittance of 50%.

[0109] Based on the test results of Examples 1-10 and Comparative Examples 1-3, it can be concluded that: Under the same thickness conditions, the light transmittance of the diaphragm of the present invention is significantly lower than that of the diaphragm without the introduction of multi-level molecular weight polyolefin polymer system or without ion beam treatment. It has lower light transmittance and higher haze value, achieving a low light transmittance effect that is invisible to the naked eye, which is beneficial to suppressing misjudgment in winding detection and internal photoinduced side reactions.

[0110] Any embodiment of any aspect of the present invention can be combined with other embodiments without contradiction. Furthermore, in any embodiment of any aspect of the present invention, any technical feature can be applied to the same technical feature in other embodiments without contradiction.

[0111] Without causing contradictions, any technical feature of any aspect or embodiment of the present invention is equally applicable to any other embodiment or embodiment of any other aspect. Of course, when applicable to each other, appropriate modifications may be made to the corresponding features as necessary. The various aspects and features of the present invention are further described below.

Claims

1. A polyolefin-based film, characterized in that, The polyolefin-based membrane has micropores; The polyolefin-based membrane has a micropore count of 3000 per μm. 3 ~5000 / μm 3 ; The average pore size of the micropores is 35 nm to 50 nm. The haze of the polyolefin-based film is 15%~35%; The transmittance of the polyolefin-based film is 45%~60%; The thickness of the polyolefin-based film is 3 μm to 12 μm.

2. The polyolefin-based film according to claim 1, characterized in that, The surface of the polyolefin-based film has oxygen-containing functional groups.

3. The polyolefin-based film according to claim 1, characterized in that, The polyolefin is polyethylene, polypropylene, or a mixture of both.

4. The method for preparing the polyolefin-based film according to any one of claims 1-3, characterized in that, Ingredients: Polyolefin polymers: including first polyolefin, second polyolefin and third polyolefin in a mass ratio of (80-95):(2-15):(2-10); The molecular weight of the first polyolefin is 1.2 million to 3 million; and / or, The second polyolefin has a molecular weight of 500,000 to 1,000,000; and / or, The molecular weight of the third polyolefin is between 100,000 and 490,000; Including the following steps: S1. Mix polyolefin polymers with processing aids, extrude into cast sheets, and then stretch them once; S2. Surface modification treatment is performed by bombardment with an inert gas ion beam; S3. Secondary stretching and heat setting to obtain the polyolefin-based film.

5. The method for preparing the polyolefin-based film according to claim 4, characterized in that, The difference between the molecular weight of the first polyolefin and the molecular weight of the second polyolefin is 200,000 to 2,500,000; and / or, The difference between the molecular weight of the second polyolefin and the molecular weight of the third polyolefin is between 10,000 and 900,000.

6. The method for preparing the polyolefin-based film according to claim 4, characterized in that, In step S2, the parameters for the ion beam bombardment modification treatment are: ion beam energy of 1 kV to 50 kV, and flux of 10 kV. 13 ions / cm 2 ~10 14 ions / cm 2 The incident angle is 30°~65°, the processing temperature is not higher than 80 ℃, and the processing time is 60 s~600 s.

7. The method for preparing the polyolefin-based film according to claim 4, characterized in that, In step S1, the mass ratio of the processing aid to the polyolefin polymer is (8~30):(70~92); and / or, The processing aid is paraffin oil; and / or, In step S1, the extrusion temperature for casting is 65~210℃; and / or, The extrusion casting process is divided into a low-temperature to medium-temperature transition zone and a high-temperature zone: Low temperature to medium temperature transition zone: temperature ranges from 65 ℃ to 160 ℃, with the temperature gradually increasing from front to back; High-temperature zone: temperature 200℃~210℃; and / or, In step S3, the temperature of the heat setting treatment is 130 ℃~140 ℃, and the time is 5 s~15 s.

8. The method for preparing the polyolefin-based film according to claim 4, characterized in that, In step S1, the thickness of the cast sheet is 1600 μm to 1800 μm; and / or, A single stretch includes initial stretching and biaxial stretching: The initial stretching ratio is 1.1 to 2 times, and the initial stretching temperature is 90℃ to 110℃; and / or, In biaxial stretching, the longitudinal stretching ratio is 5 to 11 times; the transverse stretching ratio is 5 to 11 times; the stretching temperature for biaxial stretching is 120℃ to 130℃; and / or, In step S3, the secondary stretching is a transverse stretching with a stretching ratio of 1 to 2 times and a stretching temperature of 130 ℃ to 140 ℃.

9. A low-transmittance membrane, comprising a polyolefin-based membrane, wherein at least one surface of the polyolefin-based membrane is provided with a composite coating layer, characterized in that, The polyolefin-based film is the polyolefin-based film according to any one of claims 1-3, or the polyolefin-based film prepared by the preparation method according to any one of claims 4-8.

10. A lithium-ion battery, comprising a separator, characterized in that, The diaphragm comprises the polyolefin-based membrane according to any one of claims 1-3; Or a polyolefin-based film obtained by the preparation method according to any one of claims 4-8; Alternatively, it can be the low-transmittance membrane as described in claim 9.