A polyolefin microporous membrane, a method for preparing the same, and an application thereof

Abstract

The application belongs to the technical field of battery porous membrane, and particularly relates to a polyolefin microporous membrane and a preparation method and application thereof. The polyolefin resin is subjected to controllable slight crosslinking, then is mixed with 0.05%-0.3% nanoscale crystal nucleating agent and is extruded into a film, is quenched to below-80 DEG C through an ultralow-temperature gradient, so that the nucleating agent induces the polyolefin to form nanoscale crystal grains; then is heated to 20-40 DEG C to perform synchronous biaxial stretching, and the stretching ratio is 1.2:1-1.5:1, so that the interface debonding pores between the crystal region and the amorphous region are connected along the biaxial orientation; finally, the microporous structure is fixed in the state of maintaining the biaxial tension by heating to 140-155 DEG C to perform heat setting. The microporous membrane obtained by the application has the excellent stability of air permeability < 35 s / 100 mL, room temperature puncture strength > 550 gf, 90 DEG C puncture strength > 250 gf, and longitudinal and transverse heat shrinkage rates both lower than 0.8%, and a membrane breaking temperature > 200 DEG C, has the characteristics of low air permeability, high puncture strength, high tensile strength and heat resistance, and is suitable for the field of batteries.

Description

Technical Field

[0001] This invention relates to the field of battery porous membrane technology, and in particular to a polyolefin microporous membrane, its preparation method, and its application. Background Technology

[0002] Polyolefin microporous membranes are widely used in battery separators, precision filtration membranes, gas separation membranes, and medical packaging materials due to their excellent chemical stability, mechanical strength, and cost advantages. Among these, lithium-ion battery separators are one of the most important applications of polyolefin microporous membranes, as their performance directly determines the battery's safety, cycle life, and rate performance. Currently, the industrial preparation methods for polyolefin microporous membranes are mainly divided into two categories: dry methods and wet methods.

[0003] The dry process involves melting and extruding polyolefins, casting them into films, and then using high-ratio unidirectional or bidirectional cold stretching to separate the lamellar crystals and form micropores. In the existing dry unidirectional stretching process, polypropylene films are stretched 5-8 times in one direction at temperatures below their melting point, causing mechanical separation of the layered wafers and forming slit-like micropores. This method does not require solvent extraction and is relatively simple, but it has the following drawbacks: (1) The micropores are slit-shaped and oriented along the stretching direction, resulting in a wide pore size distribution; (2) High-ratio unidirectional stretching leads to highly oriented molecular chains, making it difficult to control the transverse thermal shrinkage rate, and causing longitudinal shrinkage and transverse tearing at high temperatures; (3) The puncture strength is limited by the stress at the edge of the slit holes and is difficult to further improve.

[0004] The wet process involves melting and mixing polyolefins with a large amount of liquid hydrocarbon diluents (such as paraffin oil, phthalates, etc.) at high temperature to form a homogeneous solution. After cooling, liquid-liquid or solid-liquid phase separation occurs. After biaxial stretching, the diluent is removed by extraction with an organic solvent to obtain a microporous membrane. The resulting microporous membrane has good pore connectivity and high transverse strength, but it also has obvious shortcomings: (1) A large amount of organic solvents such as dichloromethane and hexane need to be used and recycled during the production process, resulting in high energy consumption, heavy pollution, and great environmental pressure; (2) Trace amounts of solvent are easily left in the membrane after extraction, affecting the long-term safety of the battery; (3) The micropores have high tortuosity, resulting in high gas permeation resistance and difficulty in further reducing the permeability; (4) Residual stress exists in the membrane after extraction, and the longitudinal thermal shrinkage rate at high temperature is usually greater than 3%.

[0005] To address the aforementioned shortcomings, several improvement schemes have been proposed. For example, patent CN103390741A discloses a method for improving the thermal shrinkage of wet-process membranes by adding nano-inorganic fillers. However, the introduction of nano-fillers increases interfacial compatibility, and the fillers themselves do not participate in pore formation, thus offering limited improvement in air permeability. Patent CN102134342A discloses a silane-crosslinked polyolefin microporous membrane, which improves thermal stability through chemical crosslinking. However, the crosslinking reaction requires precise control; excessive crosslinking can lead to decreased melt flowability and difficulty in film formation. Furthermore, patent CN104681763A discloses a polyolefin microporous membrane using supercritical carbon dioxide. While environmentally friendly, the resulting pore size is relatively large and the pore size distribution is wide, making it difficult to meet the micron-level precision pore size requirements of battery separators.

[0006] In summary, existing polyolefin microporous membrane preparation technologies generally face the following technical bottlenecks: (1) it is difficult to achieve a balance between high porosity, low air permeability, low thermal shrinkage, and high puncture strength; (2) the solvent dependence problem of wet processes has not been fundamentally solved; and (3) the uniformity of pore size distribution and the controllability of pore orientation need to be improved. Therefore, developing a polyolefin microporous membrane preparation method that does not require organic solvent extraction, has a narrow pore size distribution, and combines high permeability, high mechanical strength, and excellent dimensional stability has significant application value. Summary of the Invention

[0007] This invention addresses the shortcomings of existing technologies by proposing a polyolefin microporous membrane, its preparation method, and its applications.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: a method for preparing a polyolefin microporous membrane, comprising the following steps:

[0009] (a) Controllable mild crosslinking of polyolefin resin, with the gel content controlled at 1%-3%.

[0010] (b) A mildly cross-linked polyolefin is melt-mixed with 0.05%-0.3% of a nanoscale crystal nucleating agent and extruded into a film;

[0011] (c) The obtained melt film is subjected to ultra-low temperature gradient quenching at a cooling rate of not less than 50℃ / s, with a final cooling temperature of -80℃ to -150℃, so that the nucleating agent induces the polyolefin to form a large number of nanoscale grains, and a high-stress non-porous precursor film is obtained.

[0012] (d) Heat the precursor film to 20-40℃ and perform biaxial synchronous stretching with a stretching ratio of 1.2:1-1.5:1 in both the longitudinal and transverse directions, so that the debonding voids at the interface between the crystalline and amorphous regions are uniformly pulled apart and oriented along the biaxial direction.

[0013] (e) While maintaining the biaxial tensile tension, heat the membrane to 140-155°C for heat setting, hold for 5-30 seconds to fix the oriented microporous structure, and cool it down to room temperature at a rate of 5%-20% per minute. During the cooling process, gradually release the biaxial tension to obtain a polyolefin microporous membrane.

[0014] Preferably, the polyolefin resin in (a) is selected from one or more of isotactic polypropylene, high-density polyethylene, linear low-density polyethylene, polybutene-1, and ethylene-propylene copolymer.

[0015] Preferably, the controllable mild crosslinking in (a) is achieved by electron beam irradiation of 5-15 kGy, or by melt grafting with the addition of 0.01%-0.05% dicumyl peroxide.

[0016] Preferably, the nanoscale crystal nucleating agent in (b) is a sorbitol-based crystal nucleating agent, selected from one or more of bis-3,4-dimethylbenzylmethylsorbitol and 1,3-2,4-bis(p-methylbenzylmethyl)sorbitol.

[0017] Preferably, the ultra-low temperature gradient quenching in (c) includes three temperature gradients: -20℃ to -40℃, -60℃ to -80℃, and -80℃ to -150℃, with a total dwell time of 10-30 seconds.

[0018] Preferably, the grain size of the polyolefin after quenching in (c) is 50-150 nm.

[0019] Preferably, the strain rate of the biaxial synchronous stretching in (d) is 10%-100% per minute, and the surface temperature of the membrane is controlled at 25-35℃ during the stretching process.

[0020] Preferably, the heat setting in step (e) is carried out in nitrogen, and the crystallinity of the film after heat setting is 45%-60%.

[0021] Furthermore, the present invention also provides a polyolefin microporous membrane, which is prepared by the above-described preparation method.

[0022] Preferably, the microporous membrane has a thickness of 12-16 μm, a porosity of 50%-60%, an average pore size of 0.01-0.25 μm, a pore size distribution coefficient of <10%, an air permeability of <35s / 100mL, a room temperature puncture strength of >550gf, a puncture strength at 90℃ of >250gf, and after being placed at 105℃ for 1 hour, a longitudinal thermal shrinkage rate of <0.8%, a transverse thermal shrinkage rate of <0.8%, a pore closure temperature of >170℃, and a membrane rupture temperature of >200℃.

[0023] Preferably, the formation mechanism of the polyolefin microporous membrane of the present invention is explained as follows:

[0024] Polyolefin resins are semi-crystalline polymers. The density difference between crystalline and amorphous regions forms a natural interface. The sorbitol-based nucleating agent added in this invention self-assembles into a nanoscale fiber network in the melt, providing numerous heterogeneous nucleation sites for the molecular chains. When heterogeneous nucleation dominates, the grain size is compressed to 50-150 nm, and the number of grains per unit volume increases by 100-1000 times. Cryogenic quenching controls the melt in a non-equilibrium state, with the amorphous chain segments under high stress. When tempering is heated to above 20°C, the amorphous regions attempt to shrink back but are physically anchored by the nanocrystals. Stress concentration occurs at the amorphous interface within each grain. When the stress exceeds the interfacial bonding strength, interfacial debonding occurs, forming voids. Due to the grain size of only 50-150 nm, the size of the debonded voids is limited to 15-30 nm. The pore size is directly proportional to the grain size; the smaller the grain size, the smaller the pore size.

[0025] In this invention, biaxial stretching is performed at 20-40℃, at which point the amorphous region is in a highly elastic state and the stored quenching internal stress has not yet been released. When biaxial stretching of 1.2:1-1.5:1 is applied, the debonding voids are uniformly stretched and expanded, and the hole shape changes from a closed ellipsoid to a connected biaxial oriented channel. The tortuosity of the channel decreases from 2.5-4.0 to 1.2-1.5. The decrease in tortuosity increases the permeability, thereby reducing the air permeability.

[0026] Crosslinking points with 1%-3% gel content form sparse but uniform chemical connections in the amorphous region. Grains of 50-150nm are uniformly dispersed in the matrix. External forces are uniformly transmitted to the surrounding material through the grain-matrix interface. A large number of micro-shear bands are generated around the grains, which consume puncture energy. During puncture, radial cracks are generated around the needle tip. When the crack tip encounters a crosslinking point, the stress field is forced to redistribute, and the crack propagation energy is dissipated. The crosslinking density is controlled within a certain range, which is sufficient to prevent crack penetration without making the material brittle.

[0027] The present invention also provides a battery comprising: a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the separator is made of the above-mentioned polyolefin microporous membrane.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] (1) High permeability and low air permeability: High-density nanocrystals are induced by nanoscale crystal nucleating agents, generating a large number of debonding points at the interface of crystalline / amorphous regions; combined with biaxial synchronous stretching, the debonding pores are uniformly stretched open under high stress, reducing the tortuosity of the pores to 1.2-1.5, and the air permeability can be as low as 35s / 100mL.

[0030] (2) Ultra-high mechanical strength: The synergistic effect of the mild cross-linked network and the nanocrystal reinforced skeleton makes the room temperature puncture strength reach more than 550gf, which is 80% higher than the existing dry membrane; at the same time, the cross-linked network inhibits chain slippage at high temperature, and the puncture strength at 90℃ is still not less than 250gf, which exceeds the attenuation level of the existing technology at high temperature.

[0031] (3) Excellent dimensional stability: Biaxial synchronous stretching combined with tension heat setting fixes the molecular chains in both directions, forming a stable biaxial orientation network; at the same time, the slight cross-linking points act as physical anchors, further suppressing the springback of chain segments during heating. After being placed at 105°C for 1 hour, the longitudinal and transverse thermal shrinkage rates are both <0.8%.

[0032] (4) High safety window: The mild cross-linking allows the polyolefin to retain the physical support skeleton after melting, increasing the film breaking temperature to over 200℃ and expanding the safety temperature to 30-35℃, which is more than 40% higher than the existing technology, providing a more sufficient buffer time for battery thermal runaway.

[0033] (5) Green and environmentally friendly: The entire preparation process does not require the addition of diluents or pore-forming agents, nor does it require organic solvent extraction. Pore formation is achieved solely through temperature cycling and physical stretching, thus optimizing the solvent residue and environmental issues of wet processes. Attached Figure Description

[0034] Figure 1 The temperature change curve of the TMA stretching method in Example 1 is shown. Detailed Implementation

[0035] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0036] Example 1: A specific method for preparing a polyolefin microporous membrane, comprising the following steps:

[0037] (a) Isotactic polypropylene granules were laid into a 5 mm thick layer and crosslinked by irradiation using an electron accelerator under nitrogen protection. The irradiation dose was 8 kGy, and the gel content was measured to be 2.1% after irradiation. The mildly crosslinked polyolefin was melt-mixed with 0.05% of a nanoscale crystal nucleating agent and extruded into a film.

[0038] (b) The above-mentioned slightly cross-linked film was premixed with 0.15% bis-3,4-dimethylbenzyl methylene sorbitol in a high-speed mixer for 5 minutes, and then fed into a co-rotating twin-screw extruder for melt mixing and extrusion. The extrusion temperature was set as follows: Zone 1 180°C, Zone 2 200°C, Zone 3 215°C, Zone 4 210°C, and Die head 205°C. The screw speed was 350 rpm. The melt was extruded through a T-die (slit gap 1.0 mm) and cast onto a cooling roller. The casting-to-draw ratio was approximately 8:1.

[0039] (c) The extruded melt film immediately enters the temperature gradient cooling system. The first temperature gradient is -30°C, which is cooled by gas phase and held for 3 seconds; the second temperature gradient is -70°C, which is cooled by dry ice and held for 5 seconds; the third temperature gradient is -110°C, which is cooled by liquid nitrogen and held for 10 seconds, to obtain a high-stress non-porous precursor film with a thickness of about 110 μm.

[0040] (d) The precursor film was placed in a biaxial tensile testing machine and heated to 30°C at a strain rate of 30% per minute. Then, biaxial synchronous tensile testing was performed with a longitudinal and transverse tensile ratio of 1.3:1.

[0041] (e) While maintaining biaxial tensile tension, the temperature is increased to 148°C at a rate of 20°C per minute and held in nitrogen for 15 seconds. Then the tension is gradually released at a rate of 10% per minute and the temperature is reduced to room temperature at a rate of 15% per minute.

[0042] (f) The resulting membrane was slightly stretched along the longitudinal direction at a stretching ratio of 1.2:1, resulting in a final thickness of 16 μm.

[0043] Example 2: A specific method for preparing a polyolefin microporous membrane, comprising the following steps:

[0044] (a) Isotactic polypropylene granules were premixed with 0.03% dicumyl peroxide in a high-speed mixer for 10 minutes. The mixture was then fed into a twin-screw extruder and subjected to a melt grafting reaction at 200°C for approximately 3 minutes. After extrusion and granulation, a sample was taken and the gel content was measured to be 1.8%.

[0045] (b) The above-mentioned slightly cross-linked film was premixed with 0.15% bis-3,4-dimethylbenzyl methylene sorbitol in a high-speed mixer for 5 minutes, and then fed into a co-rotating twin-screw extruder for melt mixing and extrusion. The extrusion temperature was set as follows: Zone 1 180°C, Zone 2 200°C, Zone 3 215°C, Zone 4 210°C, and Die head 205°C. The screw speed was 350 rpm. The melt was extruded through a T-die (slit gap 1.0 mm) and cast onto a cooling roller. The casting-to-draw ratio was approximately 8:1.

[0046] (c) The extruded melt film immediately enters the temperature gradient cooling system. The first temperature gradient is -30°C, which is cooled by gas phase and held for 3 seconds; the second temperature gradient is -70°C, which is cooled by dry ice and held for 5 seconds; the third temperature gradient is -110°C, which is cooled by liquid nitrogen and held for 10 seconds, to obtain a high-stress non-porous precursor film with a thickness of about 110 μm.

[0047] (d) The precursor film was placed in a biaxial tensile testing machine and heated to 30°C at a strain rate of 30% per minute. Then, biaxial synchronous tensile testing was performed with a longitudinal and transverse tensile ratio of 1.3:1.

[0048] (e) While maintaining biaxial tensile tension, the temperature is increased to 148°C at a rate of 20°C per minute and held in nitrogen for 15 seconds. Then the tension is gradually released at a rate of 10% per minute and the temperature is reduced to room temperature at a rate of 15% per minute.

[0049] (f) The resulting membrane was slightly stretched along the longitudinal direction at a stretching ratio of 1.2:1, resulting in a final thickness of 16 μm.

[0050] Example 3: A specific method for preparing a polyolefin microporous membrane, comprising the following steps:

[0051] (a) Isotactic polypropylene granules were laid into a 5 mm thick layer and crosslinked by irradiation using an electron accelerator under nitrogen protection at an irradiation dose of 8 kGy. The gel content was measured to be 2.1% after irradiation. The mildly crosslinked polyolefin was melt-mixed with 0.05% of a nanoscale crystal nucleating agent and extruded into a film.

[0052] (b) The above-mentioned slightly cross-linked film was premixed with 0.15% bis-3,4-dimethylbenzyl methylene sorbitol in a high-speed mixer for 5 minutes, and then fed into a co-rotating twin-screw extruder for melt mixing and extrusion. The extrusion temperature was set as follows: Zone 1 180°C, Zone 2 200°C, Zone 3 215°C, Zone 4 210°C, and Die head 205°C. The screw speed was 350 rpm. The melt was extruded through a T-die (slit gap 1.0 mm) and cast onto a cooling roller. The casting-to-draw ratio was approximately 8:1.

[0053] (c) The extruded melt film immediately enters the temperature gradient cooling system. The first temperature gradient is -30°C, which is cooled by gas phase and held for 3 seconds; the second temperature gradient is -70°C, which is cooled by dry ice and held for 5 seconds; the third temperature gradient is -110°C, which is cooled by liquid nitrogen gas phase and held for 10 seconds, to obtain a high-stress non-porous precursor film with a thickness of about 110 μm.

[0054] (d) The precursor film was placed in a biaxial tensile testing machine and heated to 30°C at a strain rate of 30% per minute. Then, biaxial synchronous tensile testing was performed with a longitudinal and transverse tensile ratio of 1.5:1.

[0055] (e) While maintaining biaxial tensile tension, the temperature is increased to 152°C at a rate of 20°C per minute and held in nitrogen for 25 seconds. Then the tension is gradually released at a rate of 10% per minute and the temperature is reduced to room temperature at a rate of 15% per minute.

[0056] (f) The resulting membrane was slightly stretched along the longitudinal direction at a stretching ratio of 1.2:1, resulting in a final thickness of 16 μm.

[0057] Example 4: A specific method for preparing a polyolefin microporous membrane, comprising the following steps:

[0058] (a) Isotactic polypropylene granules were laid into a 5 mm thick layer and crosslinked by irradiation using an electron accelerator under nitrogen protection at an irradiation dose of 8 kGy. The gel content was measured to be 2.1% after irradiation. The mildly crosslinked polyolefin was melt-mixed with 0.05% of a nanoscale crystal nucleating agent and extruded into a film.

[0059] (b) The above-mentioned slightly cross-linked film was premixed with 0.05% bis-3,4-dimethylbenzyl methylene sorbitol in a high-speed mixer for 5 minutes, and then fed into a co-rotating twin-screw extruder for melt mixing and extrusion. The extrusion temperature was set as follows: Zone 1 180°C, Zone 2 200°C, Zone 3 215°C, Zone 4 210°C, and Die Head 205°C. The screw speed was 350 rpm. The melt was extruded through a T-die (slit gap 1.0 mm) and cast onto a cooling roller. The casting-to-draw ratio was approximately 8:1.

[0060] (c) The extruded melt film immediately enters the temperature gradient cooling system. The first temperature gradient is -30°C, which is cooled by gas phase and held for 3 seconds; the second temperature gradient is -70°C, which is cooled by dry ice and held for 5 seconds; the third temperature gradient is -110°C, which is cooled by liquid nitrogen gas phase and held for 10 seconds, to obtain a high-stress non-porous precursor film with a thickness of about 110 μm.

[0061] (d) The precursor film was placed in a biaxial tensile testing machine and heated to 30°C at a strain rate of 30% per minute. Then, biaxial synchronous tensile testing was performed with a longitudinal and transverse tensile ratio of 1.2:1.

[0062] (e) While maintaining biaxial tensile tension, the temperature is increased to 148°C at a rate of 20°C per minute and held in nitrogen for 15 seconds. Then the tension is gradually released at a rate of 10% per minute and the temperature is reduced to room temperature at a rate of 15% per minute.

[0063] (f) The resulting membrane was slightly stretched along the longitudinal direction at a stretching ratio of 1.2:1, resulting in a final thickness of 16 μm.

[0064] Comparative Example 1: The difference between Comparative Example 1 and Example 1 is that the conventional dry method was used, and isotactic polypropylene was melt-extruded, cast into a film, and then subjected to longitudinal cold stretching at 120°C with a stretching ratio of 6.0:1. Subsequently, it was thermally relaxed and shaped at 145°C to obtain a polyolefin microporous membrane with a thickness of 20 μm.

[0065] Comparative Example 2: The difference between Comparative Example 2 and Example 1 is that the conventional wet process was used, in which high-density polyethylene and 60% paraffin oil were melt-mixed at 200°C, cooled to 120°C to cause liquid phase separation, biaxially stretched (longitudinal stretching ratio 5.0:1, transverse stretching ratio 4.5:1), paraffin oil was removed by dichloromethane extraction, and after heat setting, a polyolefin microporous membrane with a thickness of 20 μm was obtained.

[0066] Comparative Example 3: The difference between Comparative Example 3 and Example 1 is that the crosslinking treatment in step (a) is not performed, while the remaining steps and parameters are the same as in Example 1.

[0067] Comparative Example 4: The difference between Comparative Example 3 and Example 1 is that the nucleating agent in step (b) is not added, while the remaining steps and parameters are the same as in Example 1.

[0068] Performance testing:

[0069] The following systematic tests were performed on the thin film samples prepared in Examples 1-4 and Comparative Examples 1-4:

[0070] 1. Thickness: Using a Keyence non-contact thickness gauge, 10 points were randomly selected on the film surface to measure the thickness and the average value was taken.

[0071] 2. Porosity: Calculated using the n-butanol immersion method, formula: Where ε is the porosity, W1 is the mass of the n-butanol saturated wet film, W2 is the mass of the dry film, ρ is the density of n-butanol, A is the film area, and L is the average film area.

[0072] 3. Average pore size and pore size distribution coefficient: The pore size distribution coefficient was measured using a capillary flow pore size analyzer according to the ASTM F316-03 (2019) standard. The pore size distribution coefficient is the ratio of the standard deviation of the pore size to the average pore size.

[0073] 4. Air permeability: According to ISO 15105-2:2025, the time required for 100 mL of air to pass through a 6.45 cm² membrane area is measured using a Gurley 4110N air permeability meter.

[0074] 5. Puncture strength: Using a universal testing machine with a needle tip diameter of 1.0 mm and a puncture speed of 300 mm / min, the maximum puncture force was recorded. The result was converted to a standard thickness of 16 μm.

[0075] 6. 90℃ Puncture Strength: After preheating the membrane in a 90℃ constant temperature chamber for 30 minutes, perform the puncture test immediately.

[0076] 7. Heat shrinkage rate: Cut the film into 100mm×100mm pieces, place them in an oven at 105℃ for 1 hour, and measure the longitudinal and transverse dimensional changes.

[0077] 8. Closing temperature and rupture temperature: Take a 20-30mm diaphragm and test it using the TMA tensile method. Record the curve of the diaphragm length changing with temperature until the diaphragm breaks to test the closing temperature and rupture temperature.

[0078] The experimental results are shown in Table 1:

[0079] Table 1 Membrane performance test results

[0080]

[0081] Lithium-ion batteries were fabricated using the polyolefin microporous membranes prepared in Examples 1-4 and Comparative Examples 1-4. The Hipot yield during the assembly of the batteries into bare cells was recorded. The final lithium-ion batteries were subjected to K-value testing and a 150℃ 1h hot box test. The experimental results are shown in Table 2.

[0082] Table 2 Battery Test Results

[0083]

[0084] Data Analysis:

[0085] As can be seen from the performance test data in Table 1, the four sample examples prepared using the technical solution of this invention have all shown a breakthrough improvement in comprehensive performance indicators compared with traditional methods, and can meet the performance requirements of battery separators. Among them, Example 1 has the highest comprehensive performance.

[0086] As can be seen from the test data in Table 2, the battery samples of the four embodiments prepared using the technical solution of this invention are superior to the existing technology in three key indicators: Hipot efficiency of bare cell assembly, battery self-discharge K value, and high-temperature safety performance, proving that it has significant industrial application value in the field of high-performance power battery separators.

[0087] As can be seen from Examples 1 and 2, controllable mild crosslinking can be achieved by electron beam irradiation of 5-15 kGy or by melt grafting with the addition of 0.01%-0.05% dicumyl peroxide, and both methods can achieve the technical effects of the present invention.

[0088] As can be seen from Examples 1 and 3, increasing the biaxial tensile ratio can further reduce the air permeability, but the puncture strength decreases slightly. This is because the debonding cavities that were originally closed or semi-closed at the crystalline-amorphous region interface are further physically pulled open, and the thin walls between adjacent debonding pores break, forming more through paths, making it easier for air to flow; however, the polyolefin pore walls between the pores are pulled thinner. During puncture, the needle tip is more likely to crush the thin walls, resulting in a decrease in the overall load-bearing capacity.

[0089] As can be seen from Examples 1 and 4, reducing the amount of nucleating agent increases pore size, decreases porosity, and widens pore size distribution, but slightly improves mechanical strength. This is because the nucleating agent is a heterogeneous nucleation site. When the amount added is reduced from 0.15% to 0.05%, the number of nucleation sites per unit volume decreases, the proportion of homogeneous nucleation increases relatively, the number of grains decreases, and the space for individual grains to grow is larger. The total number of pores is greatly reduced, the spacing between pores increases, a large number of amorphous regions fail to form debonding voids, and the total pore volume decreases. However, the polyolefin pore walls between pores are thicker, and during puncture, the thicker pore walls are more difficult to be crushed by the needle tip, resulting in stronger energy consumption.

[0090] As can be seen from Example 1 and Comparative Examples 1 and 2, the membrane obtained by the present invention has approximately 92% lower air permeability, approximately 59% higher puncture strength, approximately 84% lower heat shrinkage rate (MD), approximately 9% higher membrane rupture temperature, and a pore size distribution (Cv) that decreases from 28.0% to 8.5% compared to the membrane obtained by the wet process. The present invention also reduces air permeability by approximately 92%, increases puncture strength by approximately 22%, reduces heat shrinkage rate (MD) by approximately 87%, increases membrane rupture temperature by approximately 44%, and avoids the organic solvent extraction process.

[0091] As can be seen from Example 1 and Comparative Example 3, mild crosslinking increases the room temperature puncture strength by about 47%, the 90°C puncture strength by about 92%, the membrane rupture temperature by about 15%, and the thermal shrinkage rate by a significant amount, proving that mild crosslinking is a necessary means to achieve high safety.

[0092] As can be seen from Example 1 and Comparative Example 4, the nanonucleating agent reduced the pore size from 0.237 μm to 0.022 μm, the pore size distribution Cv from 26.1% to 8.5%, and the air permeability from 280 s / 100 ml to 32 s / 100 ml, proving that the nanonucleating agent is the key to realizing the precise microporous structure of polyolefins.

[0093] In summary, this invention achieves a comprehensive breakthrough in the permeability, mechanical strength, dimensional stability, and thermal safety of polyolefin microporous membranes through the synergistic effects of mild crosslinking, grain refinement induced by nanonucleating agents, ultra-low temperature quenching, biaxial synchronous stretching, and tension heat setting, thus solving the long-standing technical bottlenecks in existing technologies.

[0094] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a polyolefin microporous membrane, characterized in that, Includes the following steps: (a) Controllable mild crosslinking of polyolefin resin, with gel content controlled at 1%-3%; (b) A mildly cross-linked polyolefin is melt-mixed with 0.05%-0.3% of a nanoscale crystal nucleating agent and extruded into a film; (c) The obtained melt film is subjected to ultra-low temperature gradient quenching at a cooling rate of not less than 50℃ / s, with a final cooling temperature of -80℃ to -150℃, so that the nucleating agent induces the polyolefin to form a large number of nanoscale grains, and a high-stress non-porous precursor film is obtained. (d) Heat the precursor film to 20-40℃ and perform biaxial synchronous stretching with a stretching ratio of 1.2:1-1.5:1 in both the longitudinal and transverse directions, so that the debonding voids at the interface between the crystalline and amorphous regions are uniformly pulled apart and oriented along the biaxial direction. (e) While maintaining the biaxial tensile tension, heat the membrane to 140-155°C for heat setting, hold for 5-30 seconds to fix the oriented microporous structure, and then cool it to room temperature at a rate of 5%-20% per minute. During the cooling process, the biaxial tension is gradually released to obtain a polyolefin microporous membrane.

2. The method for preparing a polyolefin microporous membrane according to claim 1, characterized in that, The polyolefin resin in (a) is selected from one or more of isotactic polypropylene, high-density polyethylene, linear low-density polyethylene, polybutene-1, and ethylene-propylene copolymer.

3. The method for preparing a polyolefin microporous membrane according to claim 1, characterized in that, The controllable mild crosslinking in (a) is achieved by electron beam irradiation of 5-15 kGy or by melt grafting with the addition of 0.01%-0.05% dicumyl peroxide.

4. The method for preparing a polyolefin microporous membrane according to claim 1, characterized in that, The nanoscale crystal nucleating agent in (b) is a sorbitol-based crystal nucleating agent, selected from one or more of bis-3,4-dimethylbenzylmethylsorbitol and 1,3-2,4-bis(p-methylbenzylmethyl)sorbitol.

5. The method for preparing a polyolefin microporous membrane according to claim 1, characterized in that, The ultra-low temperature gradient quenching in (c) includes three temperature gradients: -20℃ to -40℃, -60℃ to -80℃, and -80℃ to -150℃, with a total dwell time of 10-30 seconds.

6. The method for preparing a polyolefin microporous membrane according to claim 1, characterized in that, The grain size of the polyolefin after quenching in (c) is 50-150 nm.

7. The preparation method according to claim 1, characterized in that, The strain rate of biaxial synchronous stretching in (d) is 10%-100% per minute, and the surface temperature of the membrane is controlled at 25-35℃ during the stretching process.

8. The method for preparing a polyolefin microporous membrane according to claim 1, characterized in that, In step (e), the heat setting is carried out in nitrogen, and the crystallinity of the film after heat setting is 45%-60%.

9. A polyolefin microporous membrane, characterized in that, The microporous membrane is prepared by the preparation method according to any one of claims 1-8, wherein the thickness is 12-16 μm, the porosity is 50%-60%, the average pore size is 0.01-0.25 μm, the pore size distribution coefficient is <10%, the air permeability is <35s / 100mL, the room temperature puncture strength is >550gf, the puncture strength at 90℃ is >250gf, and after being placed at 105℃ for 1 hour, the longitudinal thermal shrinkage rate is <0.8%, the transverse thermal shrinkage rate is <0.8%, the pore closure temperature is >170℃, and the membrane rupture temperature is >200℃.

10. A battery, characterized in that, The battery includes: a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the separator is made of a polyolefin microporous membrane as described in any one of claims 1-9.

Images