Polyolefin porous membrane and method for manufacturing the same, Secondary battery

Abstract

The application relates to the technical field of porous membrane materials, in particular to a polyolefin porous membrane, a preparation method thereof and a secondary battery. The porosity of the polyolefin porous membrane is 35%-45%, the average pore diameter ranges from 35 nm to 45 nm, the most probable pore diameter ranges from 35 nm to 45 nm, the most probable pore diameter accounts for greater than or equal to 12%, and the range of the most probable pore diameter plus or minus 2 nm accounts for greater than or equal to 50%. The preparation method can make the prepared polyolefin porous membrane have uniform pore diameters, and when the porous membrane with uniform pore diameters is used as a diaphragm of a secondary battery, the cycle life and safety of the battery can be improved.

Description

Technical Field

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

[0002] Polyolefin porous membranes are widely used in various technical fields such as battery separators, capacitor separators, fuel cell materials, and various separation membranes. The most common method for preparing polyolefin porous membranes is thermally induced phase separation (TIPS). The TIPS process involves forming a homogeneous solution of polymer and diluent at high temperature, followed by cooling to induce phase separation. A suitable extractant is then used to remove the diluent, yielding the porous membrane. However, the pore size uniformity of the porous membrane affects the lifespan of the membrane material and components, especially for separators used in lithium-ion batteries. The pore size uniformity affects the uniformity of lithium-ion deposition during battery charging and discharging, thus influencing the formation and growth of lithium dendrites. Therefore, the pore size uniformity of the porous membrane is crucial for the safety of lithium-ion batteries.

[0003] For example, Toray's patent WO2024019069A1 discloses a membrane that achieves high levels of ion permeability and voltage resistance by appropriately controlling the pore size of a polyolefin microporous membrane. The membrane has an average flow diameter <32 nm and a pore size distribution skewness of -1.0 to 1.5. Using the TIPS process, the molecular weight at the maximum peak in the polyolefin molecular weight distribution curve must be greater than 4.0 × 10⁻⁶. 5 And the molecular weight is less than 3.0 × 10⁻⁶. 4 The proportion is over 15%. However, the aforementioned polyolefin materials require premixing of ultra-high molecular weight polyolefin resin (UHPE) and high-density polyolefin (HDPE) in a certain proportion, making the process and equipment flow more complex. The patent only describes the method for controlling pore size and pore size distribution symmetry, but does not specify the pore size uniformity index and control method.

[0004] Qingdao Lankotu's US20230216140A1 patent discloses a novel lithium-ion battery separator with a uniform microstructure and high strength. Employing the TIPS process, using polypropylene as the main material, and by adding solubilizers and nucleating agents, and setting parameters such as stretching temperature and stretching rate, a separator with a thickness of 3.5-30 μm, a porosity of 30-80%, an adjustable pore size of 20-2000 nm, a biaxial tensile strength ≥50 MPa, an air permeability ≤400 s / 100 cc, and a breaking temperature ≥160℃ is obtained. The patent only provides the pore size range and does not specify the pore size uniformity index or control method.

[0005] Toray's patent JP2021105166A discloses a microporous polyolefin membrane with superior output characteristics, strength, and shrinkage compared to conventional membranes. Utilizing the TIPS process, it uses polyolefin as the main material and selects resins with specific molecular weights and distributions, sets a solid content range, and matches two stretching ratios to obtain surface pore size data: a porous membrane with pore sizes of 20–40 nm accounting for more than 28% and pore sizes of 0–20 nm accounting for more than 16%. The patent only specifies the proportions of the two pore size ranges and does not describe pore size uniformity indicators or control methods.

[0006] Therefore, there is an urgent need to provide a method for characterizing pore size uniformity and a method for systematically controlling it. Summary of the Invention

[0007] This invention aims to at least partially solve one of the technical problems in related technologies. To this end, this invention provides a polyolefin porous membrane and its preparation method, as well as a secondary battery. The preparation method provided by this invention can produce a polyolefin porous membrane with uniform pore size. When this porous membrane with uniform pore size is used as a separator in a secondary battery, it can improve the battery's cycle life and safety.

[0008] To this end, the first aspect of the present invention provides a polyolefin porous membrane, wherein the porosity of the polyolefin porous membrane is 35%-45%, the average pore size is in the range of 35nm-45nm, the most probable pore size is in the range of 35nm-45nm, the proportion of the most probable pore size is ≥12%, and the proportion of the most probable pore size ±2nm range is ≥50%.

[0009] The pore size with the highest proportion of all pore sizes is the most probable pore size, and the ratio of the most probable pore size to the total number of pore sizes is the most probable pore size percentage. This invention characterizes pore size uniformity by defining the most probable pore size and the percentage within the range of the most probable pore size ±2 nm. By limiting the most probable pore size percentage to ≥12% and the percentage within the range of the most probable pore size ±2 nm to ≥50%, it indicates that the polyolefin porous membrane prepared by this invention has a uniform pore size. When used as a separator material for secondary batteries, it can meet the homogeneous transport requirements of secondary batteries, achieving long cycle life and high safety.

[0010] According to an embodiment of the present invention, the raw material of the polyolefin porous membrane includes polyolefin resin.

[0011] According to embodiments of the present invention, the polyolefin resin includes at least one selected from polyethylene, polypropylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene.

[0012] According to an embodiment of the present invention, the weight-average molecular weight of the polyolefin resin is 400,000 to 1,000,000, preferably 500,000 to 700,000.

[0013] According to an embodiment of the present invention, the thickness of the polyolefin porous membrane is 5 μm-25 μm.

[0014] A second aspect of the present invention provides a method for preparing the polyolefin porous membrane described in the first aspect, comprising the following steps:

[0015] Polyolefin resin and pore-forming agent are mixed and plasticized to obtain a homogeneous molten solution, which is then cast into sheet material.

[0016] The sheet material is stretched once to obtain a first film;

[0017] Remove the porogen from the first film to obtain the second film;

[0018] The second film is stretched and shaped a second time to obtain the polyolefin porous film.

[0019] The polyolefin porous membrane prepared by the method of the present invention has a uniform pore size. The uniform pore size can meet the requirements of homogeneous transport in lithium secondary batteries, inhibit the formation and growth of lithium dendrites, thereby extending the cycle life of the battery and improving safety.

[0020] According to an embodiment of the present invention, the content of polyolefin resin in the homogeneous molten solution is 20wt%-35wt%, preferably 24wt%-32wt%.

[0021] According to an embodiment of the present invention, the pore-forming agent includes one of paraffin oil, decane, dibutyl phthalate, and stearyl alcohol.

[0022] According to an embodiment of the present invention, the plasticizing temperature is 155°C-225°C.

[0023] According to an embodiment of the present invention, the cooling rate of the homogeneous molten solution in the casting process is ≥45℃ / min.

[0024] According to an embodiment of the present invention, the primary stretching includes stretching along the production line direction and stretching perpendicular to the production line direction.

[0025] According to an embodiment of the present invention, the stretching ratio along the production line direction is 6-10 times.

[0026] According to an embodiment of the present invention, the stretching temperature along the production line direction is 90℃-130℃, preferably 100℃-125℃.

[0027] According to an embodiment of the present invention, the stretching ratio along the vertical production line direction is 6-10 times.

[0028] According to an embodiment of the present invention, the stretching temperature along the vertical production line direction is 90℃-130℃, preferably 100℃-125℃.

[0029] According to an embodiment of the present invention, the preparation method further includes:

[0030] The porogen in the first film is removed using an extractant.

[0031] According to an embodiment of the present invention, the extractant comprises dichloromethane.

[0032] According to an embodiment of the present invention, the stretching ratio of the secondary stretching is 1.0-1.6 times, preferably 1.2-1.4 times.

[0033] According to an embodiment of the present invention, the stretching temperature of the secondary stretching is 125℃-135℃, preferably 130℃-134℃.

[0034] According to an embodiment of the present invention, the shaping temperature is 125℃-135℃, preferably 130℃-134℃.

[0035] According to an embodiment of the present invention, the shrinkage ratio of the secondary stretching is 5%-20%, preferably 8%-14%.

[0036] A third aspect of the present invention provides a secondary battery, the secondary battery comprising the polyolefin porous membrane described in the first aspect or the polyolefin porous membrane obtained according to the preparation method described in the second aspect.

[0037] The advantages of this invention over the prior art are:

[0038] The preparation method provided by this invention can produce a polyolefin porous membrane with uniform pore size. When such a porous membrane with uniform pore size is used as a separator in a secondary battery, it can improve the battery's cycle life and safety. Furthermore, this invention provides a method for characterizing pore size uniformity, specifically by using the ratio of the most probable pore size to the range of the most probable pore size ±2 nm.

[0039] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Detailed Implementation

[0040] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0041] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0042] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0043] To facilitate understanding of the invention, certain technical and scientific terms are specifically defined below. Unless otherwise expressly defined elsewhere in this document, all other technical and scientific terms used herein have the meanings commonly understood by one of ordinary skill in the art to which this invention pertains.

[0044] In this document, the terms “comprising” or “including” are open-ended expressions, meaning they include the contents specified in this invention but do not exclude other aspects.

[0045] In this document, the terms “optionally,” “optionally,” or “optionally” generally refer to an event or condition that may, but may not, occur, and the description includes both cases in which the event or condition occurs and cases in which the event or condition does not occur.

[0046] In order to meet the mechanical, electrical and safety requirements of porous membranes, existing technologies often involve controlling the membrane pore structure, pore size and surface condition, but there are no methods for characterizing pore size uniformity or systematic control techniques.

[0047] According to an embodiment of the present invention, a first aspect of the present invention provides a polyolefin porous membrane, wherein the porosity of the polyolefin porous membrane is 35%-45%, the average pore size is in the range of 35nm-45nm, the most probable pore size is in the range of 35nm-45nm, the proportion of the most probable pore size is ≥12%, and the proportion of the most probable pore size ±2nm range is ≥50%.

[0048] This invention successfully prepared a high-performance, uniformly pore-sized polyolefin porous membrane by controlling the pore size, porosity, proportion of the most probable pore size, and proportion of the most probable pore size within ±2 nm. When the porosity is above 35%, permeability and electrolyte content are guaranteed, and electrical performance is improved. When the porosity is below 45%, mechanical strength and shrinkage are guaranteed, and safety is improved. When the average pore size is less than 45 nm, an ideal lithium-ion deposition rate can be obtained during battery charging, thereby suppressing short circuits caused by lithium dendrite growth. When the average pore size is greater than 35 nm, excellent ion permeability can be obtained. A proportion of the most probable pore size of over 12% and a proportion of the most probable pore size within ±2 nm of over 50% indicate that the prepared porous membrane has a uniform pore size distribution, meeting the requirements for homogeneous transport in lithium secondary batteries, suppressing the formation and growth of lithium dendrites, thereby extending battery cycle life and improving safety.

[0049] According to specific embodiments of the present invention, the type of raw material for the polyolefin porous membrane is not particularly limited. As some specific examples, the raw material for the polyolefin porous membrane includes polyolefin resin.

[0050] According to specific embodiments of the present invention, the type of polyolefin resin is not particularly limited. As some specific examples, the polyolefin resin includes at least one selected from polyethylene, polypropylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. The polyolefin resin can be a homopolymer, copolymer, or multi-segment polymer obtained from these resin monomers. Considering the uniformity of pore size distribution, the narrower the molecular weight distribution (weight-average molecular weight (Mw) / number-average molecular weight (Mn)), the more uniform the system, and the easier it is to obtain a uniform pore size distribution; a single resin is preferred.

[0051] According to specific embodiments of the present invention, the weight-average molecular weight of the polyolefin resin is 400,000 to 1,000,000, preferably 500,000 to 700,000. As some specific examples, the weight-average molecular weight of the polyolefin resin can be 400,000, 500,000, 600,000, 700,000, 800,000, 900,000, 1,000,000, etc. Using polyolefin resins within this weight-average molecular weight range can meet the requirements for mechanical and processing properties. Furthermore, appropriately increasing the molecular weight of the polyolefin resin results in a higher viscosity of the system, a shorter time for chain segments to move freely, more difficult diffusion of the pore-forming agent, unfavorable growth of pore-forming agent droplets, smaller pore size of the membrane, and a more uniform pore size distribution, thus achieving the requirement of uniform pore size distribution.

[0052] According to specific embodiments of the present invention, the thickness of the polyolefin porous membrane is 5μm-25μm. As some specific examples, the thickness of the polyolefin porous membrane can be 5μm, 10μm, 15μm, 20μm, 25μm, etc.

[0053] According to embodiments of the present invention, a second aspect provides a method for preparing the polyolefin porous membrane described in the first aspect, comprising the following steps:

[0054] (1) Mix polyolefin resin and pore-forming agent, plasticize to obtain homogeneous molten solution, and obtain sheet material by casting.

[0055] According to specific embodiments of the present invention, the content of polyolefin resin in the homogeneous melt solution is 20wt%-35wt%, preferably 24wt%-32wt%. As some specific examples, the content of polyolefin resin in the homogeneous melt solution can be 20wt%, 24wt%, 28wt%, 32wt%, 35wt%, etc. A content greater than 20wt% can provide sufficient energy for melt mixing, enabling uniform mixing of the polymer winding, facilitating the continuous and stable extrusion of a uniform melt, and improving film-forming properties. A content less than 35wt% can suppress the deterioration of molecular chains caused by excessive polymer winding, reducing the problems of decreased miscibility and insufficient mixing between polymer resin and pore-forming agent due to large viscosity differences. Appropriately increasing the content of polyolefin resin can increase the overlap density of molecular chains, suppress the formation of macropores, improve the uniformity of stretching, and thus obtain a good pore size distribution.

[0056] According to specific embodiments of the present invention, the type of porogen is not particularly limited. As some specific examples, the porogen includes one of paraffin oil, decane, dibutyl phthalate, and stearyl alcohol.

[0057] According to specific embodiments of the present invention, the grade of the paraffin oil is not particularly limited. As some specific examples, the grade of the paraffin oil may be 50# to 70#.

[0058] According to specific embodiments of the present invention, the plasticizing temperature is 155℃-225℃. As some specific examples, the plasticizing temperature can be 155℃, 175℃, 195℃, 225℃, etc. If the plasticizing temperature is too low, the plasticizing effect cannot be guaranteed; if the temperature is too high, degradation and carbonization are likely.

[0059] According to a specific embodiment of the present invention, the cooling rate of the homogeneous molten solution in the casting process is ≥45℃ / min. The high cooling rate makes it easy to form fine and uniform crystals, making the gel-like sheets denser, thereby obtaining a uniform pore structure.

[0060] (2) The sheet material is stretched once to obtain a first film.

[0061] According to a specific embodiment of the present invention, the first stretching includes stretching along the production line direction and stretching perpendicular to the production line direction. When the sheet material is stretched along the production line direction, a longitudinal hole structure is formed; when the sheet material is stretched perpendicular to the production line direction, an irregular mesh structure is formed, thereby obtaining a first film.

[0062] According to specific embodiments of the present invention, the stretching ratio along the production line direction is 6-10 times. As some specific examples, the stretching ratio along the production line direction can be 6 times, 7 times, 8 times, 9 times, 10 times, etc.

[0063] According to specific embodiments of the present invention, the stretching ratio along the direction perpendicular to the production line is 6-10 times. As some specific examples, the stretching ratio along the direction perpendicular to the production line can be 6 times, 7 times, 8 times, 9 times, 10 times, etc. Too low a stretching ratio leads to insufficient stretching, easily resulting in a coarse and uneven pore structure; too high a stretching ratio easily causes tearing. By controlling the stretching ratio along the production line direction and the stretching ratio perpendicular to the production line direction, the surface stretching ratio is made 36-100 times.

[0064] According to a specific embodiment of the present invention, the stretching temperature along the production line direction is 90℃-130℃, preferably 100℃-125℃. As some specific examples, the stretching temperature along the production line direction can be 90℃, 100℃, 110℃, 125℃, 130℃, etc.

[0065] According to a specific embodiment of the present invention, the stretching temperature along the direction perpendicular to the production line is 90℃-130℃, preferably 100℃-125℃. As some specific examples, the stretching temperature along the direction perpendicular to the production line can be 90℃, 100℃, 110℃, 125℃, 130℃, etc. Selecting a stretching temperature within this range can prevent the gel-like sheet from melting and clogging the pores, while ensuring that the pores are fully opened, making it easy to obtain a film with a uniform pore structure.

[0066] (3) Remove the pore-forming agent from the first film to obtain the second film.

[0067] According to a specific embodiment of the present invention, the preparation method further includes:

[0068] The porogen in the first film is removed using an extractant.

[0069] According to specific embodiments of the present invention, the type of extractant is not particularly limited; as some specific examples, the extractant includes dichloromethane.

[0070] (4) The second film is stretched and shaped twice to obtain the polyolefin porous film.

[0071] According to specific embodiments of the present invention, the stretching ratio of the secondary stretching is 1.0-1.6 times, preferably 1.2-1.4 times. As some specific examples, the stretching ratio of the secondary stretching can be 1.0 times, 1.2 times, 1.4 times, 1.6 times, etc. Controlling the stretching ratio of the secondary stretching within this range can increase the porosity, inhibit the aggregation of fibrils, and easily obtain a polyolefin porous membrane with fewer and more uniform pores.

[0072] According to a specific embodiment of the present invention, the stretching temperature of the secondary stretching is 125℃-135℃, preferably 130℃-134℃. As some specific examples, the stretching temperature of the secondary stretching can be 125℃, 130℃, 134℃, 135℃, etc.

[0073] According to a specific embodiment of the present invention, the setting temperature is 125℃-135℃, preferably 130℃-134℃. As some specific examples, the setting temperature can be 125℃, 130℃, 134℃, 135℃, etc. By controlling the secondary stretching and setting temperatures within this range, good heat shrinkage properties can be obtained, while inhibiting the melting and fusion of the fibrils, thereby obtaining a porous membrane with fewer coarse pores and uniform pore size.

[0074] According to specific embodiments of the present invention, the shrinkage ratio of the secondary stretching is 5%-20%, preferably 8%-14%. As some specific examples, the shrinkage ratio of the secondary stretching can be 5%, 8%, 10%, 14%, 20%, etc. By controlling the shrinkage ratio of the secondary stretching within this range, it is easy to obtain a balance between a uniform fibrillary structure and a low heat shrinkage rate.

[0075] According to a specific embodiment of the present invention, the specific operation of the preparation method of the polyolefin porous membrane is as follows: A homogeneous molten solution of polyolefin resin and a pore-forming agent at a certain concentration is prepared using the TIPS process. After being mixed and plasticized in an extruder, the solution is extruded through a die and falls onto a cooling roller to cool and solidify, resulting in a sheet material. The sheet material is stretched in the MD (along the production line direction) direction by multiple longitudinal stretching rollers, which forms a longitudinal pore structure. Subsequently, the two ends of the sheet are clamped and introduced into a tenter frame for stretching in the TD (perpendicular to the production line) direction, forming an irregular mesh structure, resulting in a first film. The stretched first film is extracted to remove the pore-forming agent, resulting in a second film. The second film is then stretched and shaped again in the transverse direction to obtain the polyolefin porous membrane.

[0076] According to an embodiment of the present invention, a third aspect of the present invention provides a secondary battery, the secondary battery comprising the polyolefin porous membrane described in the first aspect or the polyolefin porous membrane obtained according to the preparation method described in the second aspect.

[0077] Homogeneous transport is crucial for battery cycle life and safety. Uniform transport of lithium ions through the separator to and from the electrodes reduces the risk of incomplete lithiation and delithiation (i.e., capacity reduction) and localized overcharging. This places higher demands on the pore size uniformity of the separator. Simultaneously, the pore size uniformity of the separator affects the battery's interface structure and internal resistance, thus influencing the overall battery capacity and charge / discharge current density. It is also critical for suppressing the formation and growth of lithium dendrites. Therefore, the pore size uniformity of the separator is essential for battery cycle life and safety. The polyolefin porous membrane with uniform pore size prepared in this invention solves the problem of short battery cycle life and low safety caused by wide pore size distribution in separators.

[0078] The present invention will be explained below with reference to embodiments. Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0079] Example 1

[0080] (1) Add 25 parts of polyethylene (weight average molecular weight of 500,000) and 75 parts of paraffin oil to the extruder hopper, adjust the screw and extrusion rate to fully mix and plasticize the polyethylene and paraffin oil, extrude through the die head, and then solidify through the cooling roller to obtain sheet material.

[0081] (2) The sheet material is subjected to a first longitudinal stretch (MD) by multi-point stretching at a stretching temperature of 102℃ and a longitudinal stretching ratio of 6.8 times; then a first transverse stretch (TD) is performed at a stretching ratio of 7.2 times and a stretching temperature of 120℃.

[0082] (3) The first film obtained by stretching is subjected to dichloromethane extraction and drying treatment to obtain the second film;

[0083] (4) The second film is stretched a second time in the transverse direction, with a stretching ratio of 1.3 times and a stretching temperature of 132°C; then it is heat-set at a heat-setting temperature of 132°C and a shrinkage ratio of 11% to obtain the polyethylene porous film.

[0084] (5) The polyethylene porous membrane prepared above was used to prepare a soft-pack battery based on lithium iron phosphate system. The performance test data are shown in Table 2.

[0085] The raw material formulations and stretching and shaping process conditions for Examples 2-3 and Comparative Examples 1-3 are shown in Table 1. Other unmentioned process conditions are the same as those for Example 1. Performance test data are shown in Table 2.

[0086] Table 1

[0087]

[0088] Test case

[0089] The thickness, areal density, porosity, puncture strength, thermal shrinkage, average pore size, proportion of most probable pore size, proportion of most probable pore size ±2 nm range, and capacity retention after 1000 cycles of the polyolefin porous membranes prepared in the examples and comparative examples were tested using the following methods:

[0090] (1) Thickness test:

[0091] A thickness tester with a resolution of not less than 1 μm was used, and the procedure was performed in accordance with GB / T 6672-2001.

[0092] (2) Areal density test:

[0093] Use a steel ruler with an accuracy of 0.1 cm to take two samples with a width of 200 mm and weigh them using an analytical balance.

[0094] The areal density of the diaphragm is calculated using the following formula based on the measured data.

[0095] ρ1=m / (L×b)

[0096] ρ1—The surface density of the sample, in grams per square centimeter (g / cm²) 2 );

[0097] m—mass of the sample, in grams (g);

[0098] L—The length of the sample, in centimeters (cm);

[0099] b—Width of the sample, in centimeters (cm).

[0100] (3) Porosity test:

[0101] The formula for calculating the porosity of the base membrane is as follows:

[0102] p = [1 - m / (s * d * ρ0)] * 100%

[0103] In the formula:

[0104] p represents the membrane porosity, expressed as a percentage.

[0105] m represents the diaphragm mass, expressed in grams (g).

[0106] s represents the diaphragm area, expressed in square centimeters (cm²). 2 );

[0107] d represents the diaphragm thickness, in centimeters (cm).

[0108] ρ0 is the density of the raw material, expressed in grams per cubic centimeter (g / cm³). 3 ), at 0.95 g / cm 3 calculate.

[0109] (4) Puncture strength test:

[0110] An electronic tensile testing machine (grade 1 precision) was used, with a maximum test force of 1 kN and a testing machine grade of 0.5. Testing was conducted according to GB / T 10004-2008. Two 10cm × 10cm samples were cut from the sampled diaphragm, evenly distributed from left to right along the width direction. A 30mm diameter clamp was used, and a 100N sensor and a Ф = 1.0mm extensometer were selected. Puncture was performed at a rate of 50mm / min ± 5mm / min.

[0111] (5) Heat shrinkage rate test:

[0112] The test employs a forced-air constant temperature oven (operating temperature range: room temperature to 250℃), an Abbey length meter, and other instruments. Three 12cm × 12cm samples are cut from the sample to be tested, each segment being cut into thirds. Two perpendicular lines, approximately 10cm long, are drawn in the middle of each sample, and the MD and TD directions are marked. The lengths of the two lines are measured using the Abbey length meter and recorded. The diaphragm is placed in weighing paper to prevent direct contact with the oven wall. The sample is placed in the oven at the set temperature for the specified time. After the specified time, the sample is removed and allowed to cool to room temperature for 30 minutes. The lengths of the two lines are measured again using the Abbey length meter and recorded. The heat shrinkage rate is calculated based on the change in the lengths of the two lines before and after baking. The test conditions are (105℃ ± 1℃) / 1h or conditions specified by the customer.

[0113] The formula for calculating the thermal shrinkage rate is as follows:

[0114] Shrinkage rate = (L0 - L1) / L0

[0115] In the formula: the unit of shrinkage rate is percentage (%);

[0116] L0—Length of the sample before heat treatment, in millimeters (mm);

[0117] L1—Length of the sample marking after heat treatment, in millimeters (mm).

[0118] (6) Average pore size test:

[0119] A 3H-2000PB bubble pressure method filter membrane pore size distribution instrument was used, with BSD16 wetting solution. Samples were cut into circular specimens with a diameter of 20 mm. The test temperature was 20℃, the wetting time was 3 min, and nitrogen was used as the displacing gas. Testing was conducted according to the national standard GB / T 32361-2005, "Separation Membrane Pore Size Test Method: Bubble Point and Average Flow Rate Method".

[0120] (7) Most probable aperture ratio test:

[0121] According to the average aperture test report, the aperture with the highest proportion of all apertures in the average aperture test data is the most probable aperture. The ratio of the most probable aperture to the total number of apertures is the most probable aperture percentage.

[0122] (8) Proportion test of most probable pore size ±2nm:

[0123] Based on the average pore size test report data, the sum of the proportions of pore sizes within the range of ±2nm of the most probable pore size was calculated.

[0124] (9) Cycle life and capacity retention testing:

[0125] The test was conducted in accordance with the national standard GB / T 31484-2015.

[0126] Table 2

[0127]

[0128] As shown in Table 2, the polyolefin porous membrane prepared by the method provided by this invention achieves the goal of controlling pore size uniformity without sacrificing other physical properties, providing a solution for homogeneous transport in lithium secondary batteries, and thus meeting the requirements of long cycle life and safety of lithium secondary batteries.

[0129] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0130] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a polyolefin porous membrane, characterized in that, Includes the following steps: Polyolefin resin and pore-forming agent are mixed and plasticized to obtain a homogeneous molten solution, which is then cast into sheet material. The sheet material is stretched once to obtain a first film; Remove the porogen from the first film to obtain the second film; The second film is stretched and shaped twice to obtain the polyolefin porous film. The stretching includes stretching along the production line direction and stretching perpendicular to the production line direction; The stretching ratio along the production line direction is 6.8-8.5 times, and the stretching temperature is 100℃-125℃; The stretching ratio along the vertical production line direction is 7.0-8.1 times, and the stretching temperature is 100℃-125℃; The secondary stretching has a stretching ratio of 1.3-1.4 times and a stretching temperature of 130℃-134℃. The cooling rate of the homogeneous molten solution in the casting process is ≥45℃ / min; The shrinkage ratio of the secondary stretching is 9%-11%; The polyolefin porous membrane has a porosity of 35%-45%, an average pore size range of 35nm-45nm, a most probable pore size range of 35nm-45nm, a most probable pore size percentage of ≥12%, a most probable pore size ±2nm range percentage of ≥50%, and the size of the pore size with the highest number of pore sizes is the most probable pore size. The ratio of the number of most probable pore sizes to the number of all pore sizes is the most probable pore size percentage. The polyolefin resin is polyethylene with a weight-average molecular weight of 500,000 to 700,000. The content of polyolefin resin in the homogeneous molten solution is 20wt%-35wt%.

2. The preparation method according to claim 1, characterized in that, The thickness of the polyolefin porous membrane is 5μm-25μm.

3. The preparation method according to claim 1, characterized in that, The content of polyolefin resin in the homogeneous molten solution is 24wt%-32wt%.

4. The preparation method according to claim 1, characterized in that, The pore-forming agent includes one of paraffin oil, decane, dibutyl phthalate, and stearyl alcohol.

5. The preparation method according to claim 1, characterized in that, The plasticizing temperature is 155℃-225℃.

6. The preparation method according to claim 1, characterized in that, The removal of the porogen from the first film includes: The porogen in the first film is removed using an extractant.

7. The preparation method according to claim 6, characterized in that, The extractant includes dichloromethane.

8. The preparation method according to claim 1, characterized in that, The shaping temperature is 125℃-135℃.

9. The preparation method according to claim 1, characterized in that, The shaping temperature is 130℃-134℃.

10. A secondary battery, characterized in that, The secondary battery comprises a polyolefin porous membrane prepared by the preparation method according to any one of claims 1-9.

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