Polyolefin microporous membrane and its preparation method and application

Abstract

The present application provides a polyolefin microporous membrane, a preparation method, and applications thereof. The polyolefin microporous membrane comprises a core layer and a surface layer disposed on the surface of the core layer. The surface layer has an average micropore length diameter of D1, an average pore diameter of the polyolefin microporous membrane of D0, and a ratio of D1 to D0 of ≥ 6. Because the ratio of the average length diameter of the surface layer to the average pore diameter of the polyolefin microporous membrane is ≥ 6, the surface layer has higher wettability than the core layer, facilitating the infiltration and spreading of a polar adhesive on the surface of the separator, thereby improving the peel strength of the polyolefin microporous membrane. This facilitates surface modification of the separator and avoids affecting the cycling performance of the secondary battery.

Description

Technical Field

[0001] The present application relates to the technical field of secondary batteries, and more specifically to a polyolefin microporous membrane and a preparation method and application thereof. Background Art

[0002] Lithium battery separator is one of the core components of lithium-ion batteries. Its cost accounts for about 20% to 30% of the total cost of the lithium battery. It plays an important role in isolating the positive and negative poles of the battery and allowing lithium ion current to pass during charging and discharging.

[0003] The polyolefin base film is a non-polar material. When a coating is set on its surface, the coating usually contains a polar adhesive, which has poor adhesion to the non-polar polyolefin microporous membrane. When the base film is bonded to the electrode, the active material layer of the electrode usually contains a polar adhesive, which has poor adhesion to the non-polar polyolefin microporous membrane. Therefore, the bonding strength between the coating or the electrode and the base film is low, which makes it difficult to meet the peeling strength requirements of the diaphragm, is not conducive to the surface modification of the diaphragm or affects the cycle performance of the secondary battery.

[0004] Therefore, how to improve the wettability of the surface of polyolefin-based membranes is an urgent problem to be solved.

[0005] Application Contents

[0006] The present application provides a polyolefin microporous membrane and a preparation method and application thereof, in order to solve the problems of poor wettability and low bonding strength on the surface of the polyolefin base membrane.

[0007] According to a first aspect, the present application provides a polyolefin microporous membrane, the polyolefin microporous membrane comprising a core layer and a surface layer disposed on the surface of the core layer;

[0008] The core layer and the surface layer are both provided with a plurality of micropores;

[0009] The average long diameter of the micropores of the surface layer is D1, the average pore diameter of the polyolefin microporous membrane is D0, and the ratio of D1 to D0 is ≥6.

[0010] In an optional embodiment, the average pore size of the micropores of the surface layer is larger than the average pore size of the micropores of the core layer.

[0011] In an optional embodiment, the value range of D1 is 200nm≤D1≤300nm.

[0012] In an optional embodiment, the value range of D0 is ≤50 nm.

[0013] In an optional embodiment, the polyolefin microporous membrane satisfies at least one of the conditions (1) to (3):

[0014] (1) The porosity of the polyolefin microporous membrane is ≥36%;

[0015] (2) The air permeability of the polyolefin microporous membrane is 50s / 100mL-300s / 100mL;

[0016] (3) The contact angle between the polyolefin microporous membrane and water is ≤95°.

[0017] In an optional embodiment, the core layer satisfies at least one of the conditions (1) to (2):

[0018] (1) The core layer comprises at least polypropylene A and polypropylene B;

[0019] Optionally, the isotacticity of the polypropylene A is ≥98%, and the melt index of the polypropylene A is 0.3 g / 10 min-1.5 g / 10 min;

[0020] Optionally, the isotacticity of the polypropylene B is ≥98%, and the melt index of the polypropylene B is 1.0 g / 10 min-6.5 g / 10 min;

[0021] (2) The ratio of polypropylene A to polypropylene B in the core layer is 0.1-0.5 by mass.

[0022] In an optional embodiment, the surface layer satisfies at least one of the conditions (1) to (2):

[0023] (1) The surface layer comprises at least polypropylene A and polypropylene B;

[0024] Optionally, the isotacticity of the polypropylene A is ≥98%, and the melt index of the polypropylene A is 0.3 g / 10 min-1.5 g / 10 min;

[0025] Optionally, the isotacticity of the polypropylene B is ≥98%, and the melt index of the polypropylene B is 1.0 g / 10 min-6.5 g / 10 min;

[0026] (2) The ratio of polypropylene A to polypropylene B in the surface layer is 1.6-10 by mass.

[0027] According to a second aspect, the present application provides a method for preparing a polyolefin microporous membrane, wherein the polyolefin microporous membrane is the polyolefin microporous membrane as described above, and the steps include:

[0028] Providing raw materials for preparing core layer and surface layer;

[0029] The raw materials are respectively placed into different extruders for melting and plasticizing to form a melt;

[0030] The melt is separated into layers by a splitter and then co-extruded from an extrusion die of an extruder, and the separated melt is cast using a casting roll to form a prefabricated film;

[0031] The prefabricated membrane is annealed to perfect the wafer, and stretched to form a polyolefin microporous membrane.

[0032] In an optional embodiment, the raw materials of the core layer and the surface layer include at least polypropylene A and polypropylene B, and the proportions of the polypropylene A and polypropylene B in the raw materials of the core layer and the surface layer are different.

[0033] In an optional embodiment, the method for preparing the polyolefin microporous membrane satisfies at least one of the conditions (1) to (5):

[0034] (1) The processing temperature in the extruder is 200°C-260°C;

[0035] (2) The extrusion temperature of the co-extrusion is 180°C-220°C;

[0036] (3) The speed of the casting roller is 30 m / min-100 m / min, and the temperature of the casting roller is 60° C.-100° C.;

[0037] (4) The annealing temperature during the annealing treatment is 120°C-170°C, and the heat treatment time is 5h-20h;

[0038] (5) The stretching and hole forming includes cold stretching, hot stretching and heat setting;

[0039] Optionally, the cold stretching ratio is 1.0-3.0, and the cold stretching temperature is 50° C.-100° C.;

[0040] Optionally, the thermal stretching ratio is 0.5-2.0, and the thermal stretching temperature is 140° C.-160° C.;

[0041] Optionally, the heat setting temperature is 120°C-160°C.

[0042] According to a third aspect, the present application provides a separator, comprising the polyolefin microporous membrane as described above or the polyolefin microporous membrane prepared by the preparation method described above.

[0043] The beneficial effects of the present application are as follows: the average length diameter of the surface layer of the polyolefin microporous membrane prepared in the present application is significantly larger than the pore diameter of the core layer, and the ratio of the average length diameter to the average pore diameter of the polyolefin microporous membrane is greater than or equal to 6. Under the premise of ensuring the pressure resistance and ion conductivity requirements of the polyolefin microporous membrane, the surface layer of the polyolefin microporous membrane can have better wettability, which is beneficial to the infiltration and spreading of the polar adhesive, and is beneficial to improving the peel strength of the polyolefin microporous membrane, thereby being beneficial to the surface modification of the diaphragm and avoiding affecting the cycle performance of the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a scanning electron microscope image of a polyolefin microporous membrane in an embodiment. DETAILED DESCRIPTION

[0045] The present application is further described in detail below by means of specific embodiments in conjunction with the accompanying drawings. Similar elements in different embodiments are numbered with associated similar elements. In the following embodiments, many detailed descriptions are provided to enable the present application to be better understood. However, those skilled in the art will readily appreciate that some of the features may be omitted in different circumstances, or may be replaced by other elements, materials, or methods. In some cases, some operations related to the present application are not shown or described in the specification. This is to avoid the core portion of the present application being overwhelmed by excessive descriptions. For those skilled in the art, it is not necessary to describe these related operations in detail. They can fully understand the related operations based on the description in the specification and the general technical knowledge in the art.

[0046] In addition, the features, operations, or characteristics described in the specification may be combined in any appropriate manner to form various implementations, and the operational steps involved in each embodiment may be interchanged or adjusted in a manner that is obvious to those skilled in the art. Therefore, the specification and drawings are only for the purpose of clearly describing a particular embodiment and do not imply a required composition and / or sequence.

[0047] The serial numbers assigned to components herein, such as "first," "second," etc., are used solely to distinguish the objects being described and do not convey any sequential or technical meaning. References to "connection" and "coupling" herein, unless otherwise specified, include both direct and indirect connections (couplings).

[0048] The present application provides a polyolefin microporous membrane, which includes a core layer 100 and a surface layer 200 arranged on the surface of the core layer 100. The core layer 100 and the surface layer 200 are both provided with a plurality of micropores; the average pore size of the polyolefin microporous membrane is D0, the average length of the micropores of the surface layer 200 is D1, and the ratio of D1 to D0 is ≥6. By controlling the ratio of D1 to D0 to be ≥6, the average length of the micropores of the surface layer 200 is significantly larger than the average pore size of the polyolefin microporous membrane, which can make the surface layer 200 more wettable relative to the core layer 100. Optionally, the ratio of D1 to D0 is 6-9. For example, the average pore size of the polyolefin microporous membrane is tested using a PMI tester, and the test principle is the bubble point method principle. The measured pore size is the pore size value at the narrowest point of the polyolefin microporous membrane. The long diameter of the surface pore structure refers to the longest inner diameter of the pore structure. The long diameter of the surface pore structure can be obtained by observing the surface of the polyolefin microporous membrane using a scanning electron microscope image. For example, several pore structures are randomly selected from the scanning electron microscope image, their long diameters are measured respectively, and the average value is taken as the average long diameter of the surface pore structure.

[0049] In an optional embodiment, the average pore size of the micropores of the surface layer 200 is larger than the average pore size of the micropores of the core layer 100 .

[0050] By setting the micropores of the core layer 100 and the surface layer 200 to different pore sizes, and making the pore size of the surface layer 200 larger than the pore size of the core layer 100, the micropores of the surface layer 200 are made larger relative to the micropore size of the core layer 200. Under the premise of ensuring the pressure resistance and ion conductivity requirements of the polyolefin microporous membrane, the surface layer 200 of the polyolefin microporous membrane can further have better wettability, which is beneficial to the infiltration and spreading of the polar adhesive, and is beneficial to improving the peel strength of the polyolefin microporous membrane.

[0051] In an optional embodiment, the value range of D1 is 170nm≤D1≤300nm. For example, the value of D1 can be 200nm, 230nm, 240nm, 250nm, 270nm, 290nm, 300nm, or any range of the above values.

[0052] In an optional embodiment, the value range of D0 is ≤50nm. Optionally, the value range of D0 is 20nm-50nm, for example, the value of D0 can be 20nm, 30nm, 40nm, 50nm, or any range of the above values.

[0053] In an optional embodiment, the porosity of the polyolefin microporous membrane is ≥36%. Alternatively, the porosity of the polyolefin microporous membrane is 36%-60%. For example, the porosity of the polyolefin microporous membrane can be 36%, 40%, 50%, 60%, or any range thereof. The porosity of the polyolefin microporous membrane can be measured by the following test method: take three 100 mm x 100 mm samples, weigh them with an analytical balance, measure the thickness of the substrate layer with a thickness gauge, calculate the surface density based on the mass and area of ​​the substrate layer, and then calculate the porosity based on the raw material density, the surface density of the membrane, and the thickness of the membrane.

[0054] In an optional embodiment, the air permeability of the polyolefin microporous membrane is 50s / 100mL-300s / 100mL. Optionally, the air permeability of the polyolefin microporous membrane is 100s / 100mL-250s / 100mL. For example, the air permeability of the polyolefin microporous membrane can be 100s / 100mL, 150s / 100mL, 200s / 100mL, 250s / 100mL, or within a range composed of any of the above values. Exemplarily, the air permeability can be measured by the following test method: cut a 100mm×100mm membrane sample, use a U.S. Gurley 4110N air permeability tester, use a 100mL test gas mode to test, and record the time it takes for the test gas to completely pass through the membrane sample, which is the air permeability value.

[0055] In an optional embodiment, the contact angle between the polyolefin microporous membrane and water is ≤95°. The contact angle between the polyolefin microporous membrane and water is related to its wettability to polar solvents. A smaller contact angle means the polyolefin microporous membrane is easily wetted; a larger contact angle means the polyolefin microporous membrane has poor wettability. Optionally, the contact angle between the polyolefin microporous membrane and water is 90°-95°. For example, the contact angle between the polyolefin microporous membrane and water can be 90°, 91°, 92°, 93°, 94°, 95°, or within a range consisting of any of the above values.

[0056] In an optional embodiment, the core layer 100 and the surface layer 200 include at least polypropylene A and polypropylene B.

[0057] In an optional embodiment, the isotacticity of the polypropylene A is ≥98%, and the melt index of the polypropylene A is 0.3 g / 10 min-1.5 g / 10 min. For example, the melt index of the polypropylene A can be 0.3 g / 10 min, 1.0 g / 10 min, 1.1 g / 10 min, 1.2 g / 10 min, 1.3 g / 10 min, 1.4 g / 10 min, 1.5 g / 10 min, or within the range of any of the above values.

[0058] In an optional embodiment, the isotacticity of the polypropylene B is ≥98%, and the melt index of the polypropylene B is 1.0 g / 10 min-6.5 g / 10 min. For example, the melt index of the polypropylene B can be 1.0 g / 10 min, 2.0 g / 10 min, 3.0 g / 10 min, 4.0 g / 10 min, 5.0 g / 10 min, 6.0 g / 10 min, 6.5 g / 10 min, or within a range consisting of any of the above values.

[0059] In an optional embodiment, the ratio of polypropylene A and polypropylene B in the core layer 100 is 0.1-0.5 in terms of mass fractions. For example, the ratio of polypropylene A and polypropylene B is 0.1, 0.2, 0.3, 0.4, 0.5, or any range of the above values.

[0060] In an optional embodiment, the ratio of polypropylene A and polypropylene B in the surface layer 200 is 1.6-10. For example, the ratio of polypropylene A and polypropylene B is 1.6, 1.7, 1.8, 1.9, 2, 3, 4, 5, 6, 7, 8, 9, 10, or any range of the above values.

[0061] For the convenience of explaining the present application, the present application further provides a method for preparing a polyolefin microporous membrane, the steps of which include:

[0062] S1: providing raw materials for preparing the core layer 100 and the surface layer 200.

[0063] During the specific implementation of this step, the core layer 100 and the surface layer 200 are made of different materials and are both made of polypropylene, which at least includes polypropylene A and polypropylene B. The isotacticity of the polypropylene A is ≥98%, and the melt index of the polypropylene A is 0.3g / 10min-1.5g / 10min. The isotacticity of the polypropylene B is ≥98%, and the melt index of the polypropylene B is 1.0g / 10min-6.5g / 10min.

[0064] Here, isotacticity refers to the percentage of isotactic polymer in the total amount of polymer.

[0065] Melt index refers to the weight of a molten polymer flowing out of a standard capillary of specified diameter and length under a certain load at a certain temperature within 10 minutes. The unit is g / 10min. The higher the melt index, the better the fluidity.

[0066] Furthermore, in this step, the polypropylene A and polypropylene B in the raw materials for preparing the core layer 100 and the surface layer 200 have different mixing ratios. Calculated by mass, the ratio of polypropylene A to polypropylene B in the core layer 100 is 0.1-0.5, and the mixing ratio of polypropylene A to polypropylene B in the surface layer 200 is 1.6-10.

[0067] By selecting two different polypropylenes and preparing the core layer 100 and the surface layer 200 with different mixing ratios of the two different polypropylenes, the core layer 100 and the surface layer 200 react differently to the same temperature. Polypropylene A has a small melt index, poor flow properties, and relatively good adhesion. Polypropylene B has a large melt index, good flow properties, and relatively poor adhesion. The core layer 100 contains a larger proportion of polypropylene B, and the surface layer 200 contains a larger proportion of polypropylene A. This makes the surface layer 200 have better adhesion properties relative to the core layer 100, thereby facilitating the improvement of the adhesion strength of the surface layer 200.

[0068] Furthermore, in this step, in order to ensure that the core layer 100 and the surface layer 200 formed ultimately have good wettability on the surface of the polyolefin microporous membrane while ensuring the pressure resistance and ion conductivity requirements of the polyolefin microporous membrane, the average pore diameter of the polyolefin microporous membrane is D0, the average length diameter of the micropores of the surface layer 200 is D1, and the ratio of D1 to D0 is ≥ 6. The average pore diameter of the micropores of the surface layer 200 is larger than the average pore diameter of the micropores of the core layer 100, so that the surface layer 100 has larger and narrower micropores, improving its wettability, thereby facilitating the infiltration and spreading of the polar adhesive and improving the peel strength of the polyolefin microporous membrane.

[0069] S2: The raw materials are respectively placed into different extruders for melting and plasticizing to form a melt.

[0070] Furthermore, in this step, the processing temperature in the extruder is 200°C-260°C. For example, the processing temperature can be 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, or within the range of any of the above values.

[0071] S3: After the melt is separated into layers by a splitter, it is co-extruded from an extrusion die of an extruder, and the separated melt is cast using a casting roller to form a prefabricated film.

[0072] Furthermore, in this step, the diverter is a three-layer diverter, and the co-extrusion temperature during co-extrusion at the extrusion die head is 180°C-220°C. For example, the co-extrusion temperature can be 180°C, 190°C, 200°C, 210°C, 220°C, or within the range of any of the above values.

[0073] Furthermore, in this step, the melt passing through the extrusion die contacts the surface of a casting roll and is finally drawn and formed to form a prefabricated film. The speed of the casting roll is 30 m / min to 100 m / min. For example, the speed of the casting roll can be 30 m / min, 40 m / min, 50 m / min, 60 m / min, 70 m / min, 80 m / min, 90 m / min, 100 m / min, or any range thereof.

[0074] Furthermore, in this step, the cooling temperature of the casting roller is 60°C-100°C. For example, the cooling temperature of the casting roller can be 60°C, 70°C, 80°C, 90°C, 100°C, or any range thereof.

[0075] S4: annealing the prefabricated film to perfect the wafer.

[0076] Furthermore, in this step, the annealing temperature during the annealing treatment is 120° C.-170° C., for example, the annealing temperature during the annealing treatment can be 120° C., 130° C., 140° C., 150° C., 160° C., 170° C., or within a range consisting of any of the above values. The heat treatment time is 5 h-20 h, for example, the annealing time can be 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 15 h, 20 h, or within a range consisting of any of the above values.

[0077] S5: stretching the annealed wafer to form pores to form a polyolefin microporous membrane.

[0078] Furthermore, in this step, stretching to form holes includes cold stretching, hot stretching and heat setting. The ratio of cold stretching is 1.0-3.0, for example, the ratio of cold stretching can be 1.0, 1.5, 2.0, 2.5, 3.0, or within the range of any of the above values. The temperature of cold stretching is 50°C-100°C, for example, the temperature of cold stretching can be 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, or within the range of any of the above values. The ratio of hot stretching is 0.5-2.0, for example, the ratio of hot stretching can be 0.5, 1.0, 1.5, 2.0, or within the range of any of the above values. The temperature of hot stretching is 140°C-160°C, for example, the temperature of hot stretching can be 140°C, 145°C, 150°C, 155°C, 160°C, or within the range of any of the above values. The heat setting temperature is 120°C-160°C. For example, the heat setting temperature can be 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, 150°C, 155°C, 160°C, or any range thereof.

[0079] The polyolefin microporous membrane prepared by the method of the present application has a morphology as follows Figure 1 As shown, through Figure 1 It can be seen that the surface layer 200 of the polyolefin microporous membrane has larger, narrower micropores than the core layer 100 .

[0080] On the basis of the above, the present application further provides a diaphragm, which includes the above-mentioned polyolefin microporous membrane.

[0081] In order to illustrate the effects of this application, this application also provides the following more specific embodiments.

[0082] Example 1

[0083] This embodiment provides a polypropylene microporous membrane, which includes a core layer 100 and a surface layer 200 disposed on the surface of the core layer 100. The preparation process includes: co-extrusion, casting, annealing, stretching, and heat setting. The specific steps are as follows:

[0084] Polypropylene A and polypropylene B resins are provided. The melt indexes of polypropylene A and polypropylene B resins are 0.5 g / 10 min and 3.5 g / 10 min, respectively. Calculated by mass, the ratio of polypropylene resins A and B used in the two surface layers 200 is 5.0, and the ratio of polypropylene resins A and B used in the core layer 100 is 0.5. The two surface layers 200 have a higher porosity and a larger pore size, providing high liquid absorption and liquid retention performance, as well as excellent ion conductivity; the core layer has a smaller pore size and higher puncture strength, which can prevent lithium dendrite puncture.

[0085] The specific preparation process is as follows:

[0086] Cast extrusion: The mixed polypropylene A and polypropylene B resin materials are added to different extruders for melt plasticization and extrusion at a processing temperature of 210°C. After the melt passes through a three-layer splitter, it is extruded from a single-cavity co-extrusion die head at a co-extrusion extrusion temperature of 180°C.

[0087] Roller forming: The extruded melt contacts the surface of the casting roll, the casting roll speed is 60m / min, the casting cooling roll temperature is 70℃, and then is pulled into a film to form a prefabricated film.

[0088] Annealing crystallization: The precast film is placed in an oven for heat treatment at an annealing temperature of 150°C for 20 hours.

[0089] Stretching to form pores: The prefabricated membrane after annealing treatment is stretched with a cold drawing ratio of 1.5, a cold drawing temperature of 80°C, a hot drawing ratio of 2.0, a hot drawing temperature of 145°C, and a heat setting temperature of 140°C to prepare a polyolefin microporous membrane with a thickness of 12 μm, the surface layer 200 of which has a larger, narrow and long pore structure, and the core layer 100 has a smaller pore structure.

[0090] Example 2

[0091] This embodiment provides a polypropylene microporous membrane, which includes a core layer 100 and a surface layer 200 disposed on the surface of the core layer 100. The preparation process includes: co-extrusion, casting, annealing, stretching, and heat setting. The specific steps are as follows:

[0092] Polypropylene A and polypropylene B resins are provided, with melt indices of 0.5 g / 10 min and 3.5 g / 10 min, respectively. The ratio of polypropylene resin A to B used in the two surface layers 200 is 5.0, while the ratio of polypropylene resin A to B used in the core layer 100 is 0.5. The surface layers 200 have high porosity and large pore sizes, providing high liquid absorption and retention properties, as well as excellent ion conductivity. The core layer 100 has a smaller pore size and higher puncture strength, providing resistance to lithium dendrite penetration.

[0093] The specific preparation process is as follows:

[0094] Cast extrusion: The mixed polypropylene A and polypropylene B resin materials are added to different extruders for melt plasticization and extrusion at a processing temperature of 210°C. After the melt passes through a three-layer splitter, it is extruded from a single-cavity co-extrusion die head at a co-extrusion extrusion temperature of 180°C.

[0095] Roller forming: The extruded melt contacts the surface of the casting roll, the casting roll speed is 60m / min, the casting cooling roll temperature is 80℃, and then is pulled into a film to form a prefabricated film.

[0096] Annealing crystallization: The prefabricated film obtained by casting was placed in an oven for heat treatment at an annealing temperature of 145°C and an annealing time of 14 hours.

[0097] Stretching to form pores: The annealed cast film was stretched with a cold drawing ratio of 2.0, a cold drawing temperature of 80°C, a hot drawing ratio of 1.5, a hot drawing temperature of 145°C, and a heat setting temperature of 150°C to prepare a polyolefin microporous membrane with a thickness of 12 μm.

[0098] Example 3

[0099] This embodiment provides a polypropylene microporous membrane, which includes a core layer 100 and a surface layer 200 disposed on the surface of the core layer 100. The preparation process includes: co-extrusion, casting, annealing, stretching, and heat setting. The specific steps are as follows:

[0100] Polypropylene A and polypropylene B resins are provided, with melt indices of 0.5 g / 10 min and 3.5 g / 10 min, respectively. The ratio of polypropylene resin A to B used in the two surface layers 200 is 5.0, while the ratio of polypropylene resin A to B used in the core layer 100 is 0.5. The surface layers 200 have high porosity and large pore sizes, providing high liquid absorption and retention properties, as well as excellent ion conductivity. The core layer 100 has a smaller pore size and higher puncture strength, providing resistance to lithium dendrite penetration.

[0101] The specific preparation process is as follows:

[0102] Cast extrusion: The mixed polypropylene A and polypropylene B resin materials are added to different extruders for melt plasticization and extrusion at a processing temperature of 210°C. After the melt passes through a three-layer splitter, it is extruded from a single-cavity co-extrusion die head at a co-extrusion extrusion temperature of 180°C.

[0103] Roller forming: The extruded melt contacts the surface of the casting roll, the casting roll speed is 60m / min, the casting cooling roll temperature is 70℃, and then is pulled into a film to form a prefabricated film.

[0104] Annealing crystallization: The precast film is placed in an oven for heat treatment at an annealing temperature of 150°C for 16 hours.

[0105] Stretching to form pores: The annealed cast film was stretched with a cold drawing ratio of 1.5, a cold drawing temperature of 80°C, a hot drawing ratio of 2.0, a hot drawing temperature of 145°C, and a heat setting temperature of 135°C to prepare a polyolefin microporous membrane with a thickness of 12 μm.

[0106] Comparative Example 1

[0107] This comparative example provides a polypropylene microporous membrane. The specific preparation steps are as follows:

[0108] Polypropylene A and polypropylene B resins are provided, with melt indices of 1.0 g / 10 min and 3.0 g / 10 min, respectively. The A:B ratio of the polypropylene resin used in the two surface layers 200 is 3.0, while the A:B ratio of the polypropylene resin used in the core layer 100 is 0.2. The two surface layers 200 have high porosity and large pore sizes, providing high liquid absorption and retention, as well as excellent ion conductivity. The core layer has a smaller pore size and high puncture strength, providing resistance to lithium dendrite penetration.

[0109] The specific preparation process is as follows:

[0110] Cast extrusion: The mixed polypropylene resin material is added to different extruders for melt plasticization and extrusion at a processing temperature of 220°C. After the melt passes through a three-layer splitter, it is extruded from a single-cavity co-extrusion die head at a co-extrusion extrusion temperature of 180°C.

[0111] Roller forming: The extruded melt is brought into contact with the surface of the casting roll at a speed of 80 m / min and a temperature of the casting cooling roll of 100°C, and then pulled into a film to form a prefabricated film.

[0112] Annealing crystallization: The prefabricated film obtained by casting was placed in an oven for heat treatment at an annealing temperature of 155°C and an annealing time of 20 hours.

[0113] Stretching to form pores: The annealed cast film was stretched with a cold drawing ratio of 1.0, a cold drawing temperature of 70°C, a hot drawing ratio of 1.5, a hot drawing temperature of 145°C, and a heat setting temperature of 160°C to prepare a polyolefin microporous membrane with a thickness of 12 μm.

[0114] The polyolefin microporous membranes of Examples 1 to 3 and Comparative Example 1 were prepared according to the parameters shown in Table 1 below.

[0115] Table 1

[0116]

[0117]

[0118] The polyolefin microporous membrane prepared according to the above data was subjected to peel strength and water contact angle tests.

[0119] The peel strength test method involves coating the polyolefin microporous membranes prepared in Examples 1 to 3 and Comparative Example 1 with aluminum oxide slurry to form an aluminum oxide coating. 3M tape is then applied to the coating surface and the peel strength is tested. The test method follows the national standard GB / T 2790-1995. After testing, the test data is processed in accordance with the national standard GB / T 17200-1998, "Processing of Adhesion and Peel Test Data."

[0120] The test results are shown in Table 2 below.

[0121] Table 2

[0122]

[0123] As can be seen from Table 2, the peel strength of Comparative Example 1 is lower than that of Examples 1 to 3, and the contact angle with water is greater than that of Examples 1 to 3. According to the relationship between the set pore sizes, the ratio of D1 / D0 in Comparative Example 1 is less than 6, indicating that when the ratio is greater than 6, the wettability of the diaphragm can be improved and the peel strength can be improved. Further increase in the ratio of D1 / D0 can further improve the wettability and peel strength.

[0124] The above specific examples are used to illustrate the present application, which is only used to help understand the present application and is not intended to limit the present application. For those skilled in the art of the present application, based on the concept of the present application, they can also make some simple deductions, modifications or substitutions.

Claims

1. A polyolefin microporous membrane, characterized in that The polyolefin microporous membrane comprises a core layer and a surface layer arranged on the surface of the core layer; The core layer and the surface layer are both provided with a plurality of micropores; The average major diameter of the micropores in the surface layer is D1, the average pore diameter of the polyolefin microporous membrane is D0, and the ratio of D1 to D0 is ≥6; The value range of D1 is 170nm≤D1≤300nm, and the value range of D0 is ≤50nm; The surface layer comprises at least polypropylene A and polypropylene B; The isotacticity of the polypropylene A is ≥98%, and the melt index of the polypropylene A is 0.3 g / 10 min-1.5 g / 10 min; The isotacticity of the polypropylene B is ≥98%, and the melt index of the polypropylene B is 1.0 g / 10 min-6.5 g / 10 min.

2. The polyolefin microporous membrane according to claim 1, characterized in that The average pore size of the micropores of the surface layer is greater than the average pore size of the micropores of the core layer.

3. The polyolefin microporous membrane according to claim 1, characterized in that The ratio of D1 to D0 of the polyolefin microporous membrane is 6-9.

4. The polyolefin microporous membrane according to claim 1, characterized in that The polyolefin microporous membrane satisfies at least one of the conditions (1) to (3): (1) The porosity of the polyolefin microporous membrane is ≥36%; (2) The air permeability of the polyolefin microporous membrane is 50s / 100mL-300s / 100mL; (3) The contact angle between the polyolefin microporous membrane and water is ≤95°.

5. The polyolefin microporous membrane according to any one of claims 1 to 4, characterized in that The core layer satisfies at least one of the conditions (1) to (2): (1) The core layer comprises at least polypropylene A and polypropylene B; The isotacticity of the polypropylene A is ≥98%, and the melt index of the polypropylene A is 0.3 g / 10 min-1.5 g / 10 min; The isotacticity of the polypropylene B is ≥98%, and the melt index of the polypropylene B is 1.0 g / 10 min-6.5 g / 10 min; (2) The ratio of the polypropylene A to the polypropylene B in the core layer is 0.1-0.5 by mass.

6. The polyolefin microporous membrane according to any one of claims 1 to 4, characterized in that In parts by mass, the ratio of the polypropylene A to the polypropylene B in the surface layer is 1.6-10.

7. A method for preparing a polyolefin microporous membrane, characterized in that: The polyolefin microporous membrane is the polyolefin microporous membrane according to any one of claims 1 to 6, and the steps include: Providing raw materials for preparing the core layer and the surface layer, wherein the raw materials for the surface layer include at least polypropylene A and polypropylene B; The raw materials are respectively placed into different extruders for melting and plasticizing to form a melt; The melt is separated into layers by a splitter and then co-extruded from an extrusion die of an extruder, and the separated melt is cast using a casting roll to form a prefabricated film; The prefabricated membrane is annealed to perfect the wafer, and stretched to form a polyolefin microporous membrane.

8. The method for preparing a polyolefin microporous membrane according to claim 7, wherein: The raw material of the core layer includes at least polypropylene A and polypropylene B, and the ratio of the polypropylene A to the polypropylene B in the raw materials of the core layer and the surface layer is different.

9. The method for preparing a polyolefin microporous membrane according to claim 7 or 8, wherein: The method for preparing the polyolefin microporous membrane satisfies at least one of the following conditions (1) to (5): (1) The processing temperature in the extruder is 200°C-260°C; (2) The extrusion temperature of the co-extrusion is 180°C-220°C; (3) The speed of the casting roller is 30m / min-100m / min, and the temperature of the casting roller is 60℃-100℃; (4) The annealing temperature during the annealing treatment is 120°C-170°C, and the heat treatment time is 5h-20h; (5) The stretching and hole forming includes cold stretching, hot stretching and heat setting; The cold stretching ratio is 1.0-3.0, and the cold stretching temperature is 50° C.-100° C.; The thermal stretching ratio is 0.5-2.0, and the thermal stretching temperature is 140°C-160°C; The temperature of the heat setting is 120°C-160°C.

10. A diaphragm, characterized in that: The invention comprises the polyolefin microporous membrane according to any one of claims 1 to 6 or the polyolefin microporous membrane prepared by the preparation method according to any one of claims 7 to 9.

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