Polyolefin porous membrane, preparation method thereof and secondary battery

Abstract

The invention 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 range of the average pore size is 35-45nm, the value range of the most probable pore size is 35-45nm, the proportion of the most probable pore size is greater than or equal to 12%, and the proportion of the most probable pore size in the range of + / -2nm is greater than or equal to 50%. According to the preparation method provided by the invention, the prepared polyolefin porous membrane has a uniform pore diameter, and when the porous membrane with the uniform pore diameter is used as a diaphragm of a secondary battery, the cycle life of the battery can be prolonged, and the safety of the battery can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of porous membrane materials, and in particular to a polyolefin porous membrane and a preparation method thereof, and a secondary battery. Background Art

[0002] Polyolefin porous membranes are widely used in many technical fields such as battery separators, capacitor separators, fuel cell materials and various separation membranes. The most commonly used method for preparing polyolefin porous membranes is thermally induced phase separation (TIPS). The TIPS process for preparing polyolefin porous membranes is to form a homogeneous solution with a polymer and a diluent at high temperature, then cool the system to phase separate, and then select a corresponding extractant to remove the diluent to obtain a corresponding porous membrane. However, the pore size uniformity of the porous membrane affects the service life of the membrane material and components, especially when used as a separator for lithium secondary batteries. The pore size uniformity of the porous membrane affects the uniformity of lithium ion deposition during the battery charging and discharging process, and further affects the formation and growth of lithium dendrites. Therefore, the pore size uniformity of the porous membrane is crucial to the safety of lithium batteries.

[0003] For example, Toray's WO2024019069A1 patent provides a diaphragm that achieves high levels of ion permeability and voltage resistance by properly controlling the pore size of a polyolefin microporous membrane. The average flow diameter of the diaphragm is less than 32nm, and the skewness of the pore size distribution is -1.0 to 1.5. The TIPS process requires that the molecular weight at the maximum peak in the polyolefin molecular weight distribution curve must be greater than 4.0×10 5 , and the molecular weight is less than 3.0×10 4 The proportion is more than 15%. However, the above-mentioned polyolefin materials need to be pre-mixed in a certain proportion using ultra-high molecular weight polyolefin resin (UHPE) and high-density polyolefin (HDPE), and the process and equipment flow are more complicated. The patent only describes the method for regulating the pore size and pore size distribution symmetry, and does not specify the pore size uniformity index and regulation method.

[0004] Qingdao Lanketu US20230216140A1 patent provides a new type of lithium-ion battery separator with uniform microstructure and high strength. It adopts TIPS process, uses polypropylene as the main material, adds solubilizers, nucleating agents, etc., sets the stretching temperature, stretching rate and other parameters, and obtains a separator with a thickness of 3.5-30μm, a porosity of 30-80%, an adjustable pore size of 20-2000nm, a biaxial tensile strength of ≥50MPa, an air permeability of ≤400s / 100cc, and a fracture temperature of ≥160℃. The patent only gives the range of pore size, and does not specify the pore uniformity index and the control method.

[0005] Toray's JP2021105166A patent provides a microporous polyolefin membrane with output characteristics, strength and shrinkage that are superior to conventional membranes. It uses the TIPS process, with polyolefin as the main material, and selects resins with specific molecular weight and molecular weight distribution, sets the solid content range, and matches the two stretching ratios to obtain surface pore size data: the proportion of pores in the range of 20 to 40 nm is more than 28%, and the proportion of pores in the range of 0 to 20 nm is more than 16%. The patent only clarifies the proportion of two pore size intervals, and does not describe the pore size uniformity index and control method.

[0006] Therefore, there is an urgent need to provide a method for characterizing pore size uniformity and a systematic control method. Summary of the invention

[0007] The present invention aims to solve one of the technical problems in the related art to at least some extent. To this end, the present invention provides a polyolefin porous membrane and a preparation method thereof, and a secondary battery. The preparation method provided by the present invention can make the prepared polyolefin porous membrane have a uniform pore size. When the porous membrane with uniform pore size is used as a separator of a secondary battery, the cycle life and safety of the battery can be improved.

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

[0009] The size of the pore size with the highest proportion of all pore sizes is the most probable pore size, and the ratio of the number of most probable pore sizes to the number of all pore sizes is the most probable pore size ratio. The present invention characterizes the pore size uniformity by the most probable pore size and the ratio of the most probable pore size ±2nm range. By limiting the most probable pore size ratio to ≥12% and the most probable pore size ±2nm range ratio to ≥50%, it is shown that the polyolefin porous membrane prepared by the present invention has a uniform pore size. When used as a separator material for a secondary battery, it can meet the homogeneous transmission requirements of the secondary battery and achieve the purpose of 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 an embodiment of the present invention, the polyolefin resin includes at least one of 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 400,000-1,000,000, preferably 500,000-700,000.

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

[0014] The second aspect of the present invention provides a method for preparing the polyolefin porous membrane according to the first aspect, comprising the following steps:

[0015] The polyolefin resin and the porogen are mixed, plasticized to obtain a homogeneous molten solution, and a sheet material is obtained by tape casting;

[0016] Stretching the sheet material once to obtain a first film;

[0017] removing the porogen in the first film to obtain a second film;

[0018] The second film is subjected to secondary stretching and shaping to obtain the polyolefin porous membrane.

[0019] The polyolefin porous membrane obtained by the preparation method of the present invention has a uniform pore size, and the uniform pore size can meet the homogeneous transmission requirements of 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 the polyolefin resin in the homogeneous molten solution is 20wt%-35wt%, preferably 24wt%-32wt%.

[0021] According to an embodiment of the present invention, the porogen 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 tape casting is ≥45° C. / min.

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

[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°C-130°C, preferably 100°C-125°C.

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

[0028] According to an embodiment of the present invention, the stretching temperature in the direction perpendicular to the production line is 90°C-130°C, preferably 100°C-125°C.

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

[0030] The porogen in the first film is removed by 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°C-135°C, preferably 130°C-134°C.

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

[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, which comprises 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 beneficial effects of the present invention compared to the prior art are as follows:

[0038] The preparation method provided by the present invention can make the prepared polyolefin porous membrane have uniform pore size. When the porous membrane with uniform pore size is used as a separator of a secondary battery, the cycle life and safety of the battery can be improved. In addition, the present invention provides a method for characterizing pore size uniformity, that is, characterizing the pore size uniformity by the most probable pore size and the proportion of the most probable pore size within ±2nm.

[0039] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. DETAILED DESCRIPTION

[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 understood 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 understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. Further, in the description of the present invention, unless otherwise specified, the meaning of "plurality" is two or more.

[0042] The endpoints and any values ​​of the ranges disclosed in this article 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 each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0043] In order to make the present invention more easily understood, certain technical and scientific terms are specifically defined below. Unless otherwise clearly defined elsewhere in this document, all other technical and scientific terms used herein have the meanings commonly understood by those skilled in the art to which the present invention belongs.

[0044] In this document, the terms “include” or “comprising” are open expressions, that is, including the contents specified in the present invention but not excluding other contents.

[0045] As used herein, the terms "optionally", "optional" or "optionally" generally mean that the subsequently described event or circumstance may but need not occur, and that the description includes instances where the event or circumstance occurs and instances where it does not.

[0046] In order to meet the mechanical, electrical and safety requirements of porous membranes in the prior art, the pore structure, pore size and surface state of the diaphragm are often regulated, but there is no method for characterizing the pore uniformity and no systematic regulation technology.

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

[0048] The present invention successfully obtains a polyolefin porous membrane with excellent performance and uniform pore size by controlling the pore size, porosity, most probable pore size ratio and most probable pore size range ratio of ±2nm of the porous membrane. When the porosity is above 35%, the permeability and electrolyte content are guaranteed, and the electrical performance is improved. When the porosity is below 45%, the mechanical strength and shrinkage are guaranteed, and the safety is improved. When the average pore size is less than 45nm, the ideal lithium ion deposition rate of the battery during charging can be obtained, thereby suppressing the short circuit caused by the growth of lithium dendrites. When the average pore size is greater than 35nm, excellent ion permeability can be obtained. The most probable pore size ratio is more than 12%, and the most probable pore size range of ±2nm accounts for more than 50%, indicating that the obtained porous membrane has a uniform pore size distribution, which can meet the requirements of homogeneous transmission of lithium secondary batteries, suppress the formation and growth of lithium dendrites, thereby extending the cycle life of the battery and improving safety.

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

[0050] According to a specific embodiment of the present invention, the type of the polyolefin resin is not particularly limited. As some specific examples, the polyolefin resin includes at least one of polyethylene, polypropylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. The polyolefin resin can be a homopolymer, copolymer, or multi-stage polymer obtained from these resin monomers. From the perspective of pore size distribution uniformity, 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, preferably a single resin.

[0051] According to a specific embodiment of the present invention, the weight average molecular weight of the polyolefin resin is 400,000-1,000,000, preferably 500,000-700,000, and 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. The use of polyolefin resins within this weight average molecular weight range can meet the requirements of mechanical properties and processing properties; in addition, the viscosity of the system increases with the appropriate increase in the molecular weight of the polyolefin resin, the time for the chain segments to move freely is short, the diffusion of the porogen becomes difficult, which is not conducive to the growth of the porogen droplets, the pore size of the diaphragm becomes smaller, and the pore size distribution becomes more uniform, meeting the requirement of uniform pore size distribution.

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

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

[0054] (1) A polyolefin resin and a porogen are mixed and plasticized to obtain a homogeneous molten solution, and a sheet material is obtained by tape casting.

[0055] According to a specific embodiment of the present invention, the content of polyolefin resin in the homogeneous molten solution is 20wt%-35wt%, preferably 24wt%-32wt%. As some specific examples, the content of polyolefin resin in the homogeneous molten solution can be 20wt%, 24wt%, 28wt%, 32wt%, 35wt%, etc. When the content is greater than 20wt%, sufficient energy can be provided for melt mixing, so that the entanglement of the polymer is uniformly mixed, it is easy to form a uniform melt for continuous and stable extrusion, and improve film forming properties. When the content is less than 35wt%, the degradation of the molecular chain caused by excessive entanglement of the polymer can be suppressed, and the problems of reduced miscibility and insufficient mixing due to the huge difference in viscosity between the polymer resin and the pore-forming agent can be reduced. When the content of polyolefin resin is appropriately increased, the overlapping density of the molecular chain can be increased, the formation of macropores can be suppressed, and the uniformity of stretching can be improved, thereby obtaining a good pore size distribution.

[0056] According to a specific embodiment of the present invention, the type of the 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 a specific embodiment of the present invention, the brand of the paraffin oil is not particularly limited. As some specific examples, the brand of the paraffin oil may be 50# to 70#.

[0058] According to a specific embodiment of the present invention, the plasticizing temperature is 155° C.-225° C. As some specific examples, the plasticizing temperature may be 155° C., 175° C., 195° C., 225° C., etc. If the plasticizing temperature is too low, the plasticizing effect cannot be guaranteed, and if the temperature is too high, degradation and carbonization are likely to occur.

[0059] According to a specific embodiment of the present invention, the cooling rate of the homogeneous molten solution in the tape casting is ≥45°C / min. The high cooling rate can easily form fine and uniform crystals, making the gel-like sheets dense, 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 primary stretching includes stretching along the production line direction and stretching along the perpendicular production line direction. When the sheet material is stretched along the production line direction, a longitudinal pore structure is formed; when the sheet material is stretched along the perpendicular production line direction, an irregular mesh structure is formed, thereby obtaining a first film.

[0062] According to a specific embodiment 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 a specific embodiment of the present invention, the stretching ratio of the stretching in the direction perpendicular to the production line is 6-10 times. As some specific examples, the stretching ratio of the stretching in the direction perpendicular to the production line can be 6 times, 7 times, 8 times, 9 times, 10 times, etc. A stretching ratio that is too low leads to insufficient stretching, and a coarse and uneven pore structure is easily obtained; a stretching ratio that is too large is prone to tearing. By controlling the stretching ratio of the stretching in the production line direction and the stretching ratio of the stretching in the direction perpendicular to the production line, the surface stretching ratio is 36-100 times.

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

[0065] According to a specific embodiment of the present invention, the stretching temperature for stretching in the direction perpendicular to the production line is 90°C-130°C, preferably 100°C-125°C. As some specific examples, the stretching temperature for stretching in the direction perpendicular to the production line can be 90°C, 100°C, 110°C, 125°C, 130°C, etc. Selecting a stretching temperature in this range can prevent the gel-like sheet from melting and clogging the pores, and can ensure that the pores are fully opened, so that a film with a uniform pore structure can be easily obtained.

[0066] (3) Removing the porogen in the first film to obtain a second film.

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

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

[0069] According to a specific embodiment of the present invention, the type of the extractant is not particularly limited. As some specific examples, the extractant includes dichloromethane.

[0070] (4) The second film is subjected to secondary stretching and shaping to obtain the polyolefin porous membrane.

[0071] According to a specific embodiment 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. By controlling the stretching ratio of the secondary stretching within this range, the porosity can be increased, the coagulation of the original fibers can be suppressed, and a polyolefin porous membrane with few coarse pores and uniformity can be easily obtained.

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

[0073] According to a specific embodiment of the present invention, the shaping temperature is 125°C-135°C, preferably 130°C-134°C. As some specific examples, the shaping temperature may be 125°C, 130°C, 134°C, 135°C, etc. By controlling the secondary stretching and shaping temperatures within this range, good heat shrinkage performance can be obtained, and the melting fusion of the original fibers can be suppressed, so that a porous membrane with few coarse pores and uniform pore size can be obtained.

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

[0075] According to a specific embodiment of the present invention, the specific operation of the preparation method of the polyolefin porous membrane may be: through the TIPS process, the polyolefin resin is made into a homogeneous molten solution with a porogen at a certain concentration, and after mixing and plasticizing in an extruder, it is extruded through a die head and falls on a cooling roller to cool and solidify the film to obtain a sheet material; the sheet material is passed through multiple longitudinal pulling rollers for stretching in the MD (along the production line direction), and this process will form a longitudinal pore structure, and then the two ends of the sheet are clamped and introduced into a tentering machine for stretching in the TD (vertical to the production line) direction to form an irregular mesh structure to obtain a first film; the stretched first film is extracted to remove the porogen to obtain a second film; the second film is secondary stretched and shaped 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 according to the first aspect or the polyolefin porous membrane obtained according to the preparation method according to the second aspect.

[0077] Homogeneous transmission is very important for battery cycle life and safety. The uniform transmission of lithium ions into and out of the electrode through the diaphragm reduces the risk of incomplete lithiation and delithiation (i.e., reduced capacity) and local overcharging, which puts higher requirements on the pore size uniformity of the diaphragm. At the same time, the pore size uniformity of the diaphragm affects the interface structure and internal resistance of the battery, and then affects the overall capacity and charge and discharge current density of the battery, which is very critical for inhibiting the formation and growth of lithium dendrites. Therefore, the pore size uniformity of the diaphragm is crucial for the cycle life and safety of the battery. The polyolefin porous membrane with uniform pore size prepared by the present invention solves the problems of short battery cycle life and low safety caused by wide pore size distribution of the diaphragm.

[0078] The scheme of the present invention will be explained below in conjunction with the embodiments. It will be appreciated by those skilled in the art that the following embodiments are only used to illustrate the present invention and should not be considered as limiting the scope of the present invention. Where specific techniques or conditions are not indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. The reagents or instruments used are not indicated by the manufacturer and are all conventional products that can be obtained commercially.

[0079] Example 1

[0080] (1) adding 25 parts of polyethylene (weight average molecular weight of 500,000) and 75 parts of paraffin oil into the silo of the extruder, adjusting the screw and the extrusion amount so that the polyethylene and the paraffin oil are fully mixed and plasticized, extruding through a die head, and then solidifying through a cooling roller to obtain a sheet material;

[0081] (2) The sheet material is subjected to a first longitudinal stretching (MD) by a multi-point stretching method, the stretching temperature is 102° C., and the longitudinal stretching ratio is 6.8 times; then the sheet material is subjected to a first transverse stretching (TD), the stretching ratio is 7.2 times, and the stretching temperature is 120° C.;

[0082] (3) extracting and drying the first film obtained by stretching with dichloromethane to obtain a second film;

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

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

[0085] The raw material formulas and stretching and shaping process conditions of Examples 2-3 and Comparative Examples 1-3 are shown in Table 1, and other process conditions not mentioned are the same as those of Example 1. The performance test data are shown in Table 2.

[0086] Table 1

[0087]

[0088] Test Case

[0089] The thickness, surface density, porosity, puncture strength, thermal shrinkage, average pore size, most probable pore size ratio, most probable pore size ±2 nm range ratio, and capacity retention rate after 1000 cycles of the polyolefin porous membranes prepared in the examples and comparative examples were tested, and the test method was as follows:

[0090] (1) Thickness test:

[0091] Use a thickness tester with a resolution of not less than 1μm and follow GB / T 6672-2001.

[0092] (2) Surface density test:

[0093] Use a steel ruler with an accuracy of 0.1 cm to take samples, take 2 samples with a standard width of 200 mm, and weigh them using an analytical balance.

[0094] The surface density of the separator was calculated according to the following formula based on the measured data.

[0095] ρ1=m / (L×b)

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

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

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

[0099] b—Width of the specimen, 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] Where:

[0104] p is the porosity of the diaphragm, expressed in %;

[0105] m is the mass of the diaphragm, in grams (g);

[0106] s is the diaphragm area in square centimeters (cm 2 );

[0107] d is the thickness of the diaphragm, in centimeters (cm);

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

[0109] (4) Puncture strength test:

[0110] Use electronic tensile machine (1st grade precision), maximum test force 1KN, test machine grade 0.5. Test according to GB / T 10004-2008. In the sampled diaphragm, evenly distribute left, middle and right in width direction, cut out 2 10cm×10cm samples. The clamp diameter is 30mm, select 100N sensor and Ф=1.0mm extensometer. Puncture at a rate of 50mm / min±5mm / min.

[0111] (5) Thermal shrinkage test:

[0112] Use a blast thermostat (working temperature range: room temperature ~ 250℃), Abbe length meter and other instruments. Cut the sample to be tested into three equal parts and take 3 12cm×12cm samples. Draw two perpendicular lines of about 10cm in length in the middle of the sample and mark the MD and TD directions. Use the Abbe length meter to measure the length of the two lines and record them. Place the diaphragm in weighing paper to prevent the diaphragm from directly contacting the inner wall of the oven. Put it in the oven that reaches the set temperature for the specified time, take out the sample, leave it for 30 minutes to cool it to room temperature, and then use the Abbe length meter to measure the length of the two lines and record them. Calculate the thermal shrinkage rate based on the change in the length of the two lines before and after baking. The test conditions are (105℃±1℃) / 1h or the conditions specified by the customer.

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

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

[0115] Where: The unit of shrinkage is percentage (%);

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

[0117] L1: Marking length of the sample after heat treatment, in millimeters (mm).

[0118] (6) Average pore size test:

[0119] The 3H-2000PB bubble pressure filter membrane pore size distribution instrument and BSD16 infiltration liquid were used. The sample was cut into a circular specimen with a diameter of 20 mm. The test temperature was 20°C, the infiltration time was 3 minutes, and the displacement gas was nitrogen. The test was carried out according to the national standard GB / T 32361-2005 separation membrane pore size test method bubble point and average flow method.

[0120] (7) Test of the most probable aperture ratio:

[0121] It can be read in the average pore size test report that in the average pore size test data, the size of the pore size that accounts for the highest proportion of all pore sizes is the most probable pore size, and the ratio of the number of most probable pore sizes to the number of all pore sizes is the most probable pore size ratio.

[0122] (8) Test on the proportion of the most probable pore size ±2nm:

[0123] According to the average pore size test report data, the sum of the pore size proportions in the range of ±2nm of the most probable pore size is calculated.

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

[0125] Tested in accordance with the national standard GB / T 31484-2015.

[0126] Table 2

[0127]

[0128] According to the data in Table 2, the polyolefin porous membrane obtained by the preparation method provided by the present invention achieves the purpose of regulating the uniformity of pore size without sacrificing other physical properties, provides a solution for the homogeneous transmission of lithium secondary batteries, and thus meets the requirements of long cycle life and safety of lithium secondary batteries.

[0129] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0130] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A polyolefin porous membrane, characterized in that The porosity of the polyolefin porous membrane is 35%-45%, the average pore size ranges from 35nm to 45nm, the most probable pore size ranges from 35nm to 45nm, the most probable pore size accounts for ≥12%, and the most probable pore size range of ±2nm accounts for ≥50%.

2. The polyolefin porous membrane according to claim 1, characterized in that The raw material of the polyolefin porous membrane includes polyolefin resin; Optionally, the polyolefin resin includes at least one of polyethylene, polypropylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene; Optionally, the weight average molecular weight of the polyolefin resin is 400,000-1,000,000, preferably 500,000-700,000.

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

4. A method for preparing a polyolefin porous membrane according to any one of claims 1 to 3, characterized in that: The following steps are involved: The polyolefin resin and the porogen are mixed, plasticized to obtain a homogeneous molten solution, and a sheet material is obtained by tape casting; Stretching the sheet material once to obtain a first film; removing the porogen in the first film to obtain a second film; The second film is subjected to secondary stretching and shaping to obtain the polyolefin porous membrane.

5. The preparation method according to claim 4, characterized in that: The content of the polyolefin resin in the homogeneous molten solution is 20wt%-35wt%, preferably 24wt%-32wt%; Optionally, the porogen includes one of paraffin oil, decane, dibutyl phthalate, and stearyl alcohol.

6. The preparation method according to claim 4, characterized in that: The plasticizing temperature is 155°C-225°C; Optionally, the cooling rate of the homogeneous molten solution in the tape casting is ≥45°C / min.

7. The preparation method according to claim 4, characterized in that: The primary stretching includes stretching along the production line direction and stretching along the direction perpendicular to the production line; Optionally, the stretching ratio along the production line direction is 6-10 times; Optionally, the stretching temperature along the production line direction is 90°C-130°C, preferably 100°C-125°C; Optionally, the stretching ratio along the vertical production line direction is 6-10 times; Optionally, the stretching temperature along the vertical production line direction is 90°C-130°C, preferably 100°C-125°C.

8. The preparation method according to claim 4, characterized in that: The preparation method further comprises: removing the porogen in the first film with an extractant; Optionally, the extractant comprises dichloromethane.

9. The preparation method according to claim 4, characterized in that: The stretching ratio of the secondary stretching is 1.0-1.6 times, preferably 1.2-1.4 times; Optionally, the stretching temperature of the secondary stretching is 125°C-135°C, preferably 130°C-134°C; Optionally, the setting temperature is 125°C-135°C, preferably 130°C-134°C; Optionally, the shrinkage ratio of the secondary stretching is 5%-20%, preferably 8%-14%.

10. A secondary battery, characterized in that: The secondary battery comprises the polyolefin porous membrane according to any one of claims 1 to 3 or the polyolefin porous membrane obtained by the preparation method according to any one of claims 4 to 9.

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