A polyolefin composition, a polyolefin microporous membrane, and a lithium battery

Abstract

The present invention provides a polyolefin composition, a polyolefin microporous membrane and a lithium battery. The polyolefin composition includes: a first polyolefin with a number-average molecular weight of 2,000,000 to 2,200,000, a second polyolefin with a number-average molecular weight of 1,400,000 to 1,600,000, and a third polyolefin with a number-average molecular weight of 400,000 to 600,000, and their dosages satisfy the following formula: 1 ≤ (B - C) / A ≤ 3.5; where A represents the mass percentage of the first polyolefin, B represents the mass percentage of the second polyolefin, and C represents the mass percentage of the third polyolefin; in the present invention, a high-molecular-weight second polyolefin and a low-molecular-weight third polyolefin are introduced into the ultra-high-molecular-weight first polyolefin, and their co-plasticizing effect with small-molecule mineral oil is utilized to promote the disentanglement of ultra-high-molecular-weight polyolefin long-chain molecules; the processing of a high-molecular-weight polyolefin-based membrane is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of diaphragms and preparation methods, and particularly relates to a polyolefin composition, a polyolefin microporous membrane and a lithium battery. Background Art

[0002] The diaphragm is one of the core materials of lithium batteries. With the continuous increase in the sales volume of new energy vehicles at home and abroad, the safety of its core component, the lithium battery, has become the top priority of the industry development. The diaphragm is an important link to improve the high temperature resistance of lithium batteries, inhibit thermal failure and optimize the safety factor. Its indicators such as thinness, low cost, high heat resistance and high membrane rupture temperature determine the performance of downstream lithium battery products such as energy density, cycle life and safety. With the rapid development of the domestic power battery industry, the mid- and low-end diaphragms have basically achieved localization. However, in the field of high-end diaphragms, there are still many technical, technological and equipment barriers. There is still a large gap in the thinness and physical and chemical performance indicators of diaphragm products. China's high-end diaphragms have long relied on imports. With the continuous increase in the demand for high safety and high energy in the lithium battery industry, the requirements for high heat resistance and thinness of diaphragms are also continuously increasing. Therefore, it is of great significance to develop a new type of ultra-thin and high heat-resistant coating to enhance the comprehensive performance of the diaphragm and thus improve the safety of the battery.

[0003] The ultra-thin, high porosity and high mechanical strength of the base film are the mainstream directions of market demand. The wet polyolefin (PE) base film has more advantages than the dry diaphragm in terms of thinness, mechanical properties and air permeability. However, when the wet polyolefin base film is further thinned, there are problems of reduced strength and decreased safety of the diaphragm. The mechanical strength of the diaphragm largely depends on the molecular weight of the polyolefin used in the processing of the base film. Ultra-high molecular weight polyolefin has become the preferred main material for ultra-thin and high-strength polyolefin base films due to its advantages such as low density, high strength and high modulus. However, the long-chain molecules of high molecular weight polyolefin are highly entangled, with high melt viscosity and low flow rate, resulting in difficult processing and poor film-forming properties. Summary of the Invention

[0004] Aiming at the above problems existing in the prior art, the present invention provides a polyolefin composition, a polyolefin microporous membrane and a lithium battery. The lithium battery diaphragm prepared from the polyolefin composition has excellent mechanical properties and air permeability. The specific content of the invention is as follows:

[0005] In the first aspect, the present invention provides a polyolefin composition, comprising:

[0006] A first polyolefin with a number average molecular weight of 2 million to 2.2 million, a second polyolefin with a number average molecular weight of 1.4 million to 1.6 million, and a third polyolefin with a number average molecular weight of 0.4 million to 0.6 million, and their dosages satisfy the following formula:

[0007] 1 ≤ (B - C) / A ≤ 3.5;

[0008] Among them, A represents the mass percentage of the first polyolefin, B represents the mass percentage of the second polyolefin, and C represents the mass percentage of the third polyolefin.

[0009] Optionally, the mass percentage of the first polyolefin is 1-6%, the mass percentage of the second polyolefin is 45-55%, and the mass percentage of the third polyolefin is 40-48%.

[0010] Optionally, the first polyolefin is selected from polyethylene and / or polypropylene; the second polyolefin is selected from polyethylene and / or polypropylene; the third polyolefin is selected from polyethylene and / or polypropylene.

[0011] Optionally, the polyolefin composition further includes mineral oil.

[0012] Optionally, the mass ratio of the total mass of the first polyolefin, the second polyolefin and the third polyolefin to the mass of the mineral oil is (15-30):(75-82).

[0013] Optionally, the mineral oil is selected from one or a combination of white oil, petroleum ether, petroleum hydrocarbon and paraffin oil.

[0014] Optionally, the polyolefin composition further includes an antioxidant.

[0015] Optionally, the mass ratio of the total mass of the first polyolefin, the second polyolefin and the third polyolefin to the mass of the antioxidant is (15-30):(0.1-0.5).

[0016] Optionally, the antioxidant is selected from one or a combination of isooctyl β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, n-octadecyl β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate and N,N-1,6-hexamethylene-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionamide].

[0017] In a second aspect, the present invention provides a polyolefin microporous membrane, including the polyolefin composition described in the first aspect above.

[0018] Optionally, the polyolefin microporous membrane is obtained by the following preparation method:

[0019] The first polyolefin with a number average molecular weight of 200w-220w, the second polyolefin with a number average molecular weight of 140w-160w, the third polyolefin with a number average molecular weight of 40w-60w, mineral oil and an antioxidant are melt-blended, and the obtained mixed melt is formed into an oil film by a casting method;

[0020] The oil film is biaxially stretched to form a film, and then subjected to extraction treatment and heat setting treatment to obtain the polyolefin microporous membrane.

[0021] Optionally, in the mixed melt, the mass ratio of the total mass of the first polyolefin, the second polyolefin and the third polyolefin to the mineral oil and the antioxidant is (15-30):(75-82):(0.1-0.5).

[0022] Optionally, the mass of the first polyolefin accounts for 1-6% of the total mass of the first polyolefin, the second polyolefin and the third polyolefin;

[0023] the mass of the second polyolefin accounts for 45-55% of the total mass of the first polyolefin, the second polyolefin and the third polyolefin;

[0024] the mass of the third polyolefin accounts for 40-48% of the total mass of the first polyolefin, the second polyolefin and the third polyolefin.

[0025] Optionally, the temperature of the mixed melt is 150-250 °C.

[0026] Optionally, the extractant used in the extraction process is selected from one or a combination of more of n-hexane, n-heptane, dichloromethane, chloroform, isopropanol and ethanol.

[0027] Optionally, the thickness of the polyolefin microporous membrane is 5-10 μm.

[0028] Optionally, the puncture strength of the polyolefin microporous membrane is 60-90 g / μm.

[0029] Optionally, the biaxial tensile strength of the polyolefin microporous membrane is 250-350 MPa.

[0030] In a third aspect, the present invention provides a lithium battery, including the polyolefin microporous membrane described in the second aspect above.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] A polyolefin composition for preparing a polyolefin microporous membrane provided by the present invention includes: a first polyolefin with a number average molecular weight of 2,000,000-2,200,000, a second polyolefin with a number average molecular weight of 1,400,000-1,600,000, and a third polyolefin with a number average molecular weight of 400,000-600,000, and their dosages satisfy the following formula: 1≤(B-C) / A≤3.5; where A represents the mass ratio of the first polyolefin, B represents the mass ratio of the second polyolefin, and C represents the mass ratio of the third polyolefin.

[0033] In the preparation method provided by the present invention, according to the disentanglement process of high molecular weight polyolefin molecules and the change rule of the melt index of the composite melt, a high molecular weight second polyolefin (number average molecular weight of 1.4 million - 1.6 million) and a low molecular weight third polyolefin (number average molecular weight of 0.4 million - 0.6 million) are introduced into the ultra-high molecular weight first polyolefin (number average molecular weight of 2 million - 2.2 million). By using their co-plasticizing effect with small molecule mineral oil, the disentanglement of ultra-high molecular weight polyolefin long-chain molecules is promoted; the processing of a base film based on high molecular weight polyolefin (formulation molecular weight above 1 million) is realized. Compared with existing commercial diaphragms, when the polyolefin microporous film is prepared with the polyolefin composition provided by the present invention, the obtained polyolefin microporous film has obvious advantages in various performance parameters, especially being able to balance air permeability and mechanical properties. The obtained polyolefin microporous film has a thickness of 5 - 10 μm, an air permeability value of 190 - 230 s / 100cc, a diaphragm puncture strength of 60 - 90 g / μm, and a diaphragm biaxial tensile strength of 250 - 350 MPa. Detailed Embodiments

[0034] If specific experimental steps or conditions are not specified in the examples, the operations or conditions of the conventional experimental steps described in the existing technologies in this field can be followed. For reagents and other instruments whose manufacturers are not specified, they are all conventional reagent products that can be obtained through commercial purchase.

[0035] Technologies, methods, and equipment known to those of ordinary skill in the relevant fields may not be discussed in detail, but where appropriate, the said technologies, methods, and equipment should be regarded as part of the specification of the present invention.

[0036] In the description of the present invention, it should be understood that using terms such as "first" and "second" to limit components is only for facilitating the distinction of corresponding components. Without further statement, the above terms have no special meaning, and thus should not be construed as limiting the protection scope of the present invention.

[0037] In addition, the technical features involved in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0038] In a first aspect, the present invention provides a polyolefin composition, comprising:

[0039] A first polyolefin with a number average molecular weight of 2 million - 2.2 million, a second polyolefin with a number average molecular weight of 1.4 million - 1.6 million, and a third polyolefin with a number average molecular weight of 0.4 million - 0.6 million, and their dosages satisfy the following formula:

[0040] 1 ≤ (B - C) / A ≤ 3.5;

[0041] Among them, A represents the mass percentage of the first polyolefin, B represents the mass percentage of the second polyolefin, and C represents the mass percentage of the third polyolefin.

[0042] In specific implementation, due to the high entanglement of the long-chain molecules of ultra-high molecular weight polyolefins, high melt viscosity, and low flow rate, there are problems of poor film-forming property and high processing difficulty during the film-forming process; therefore, in the embodiments of the present invention, an ultra-high molecular weight first polyolefin (number average molecular weight of 2 million - 2.2 million), a high molecular weight second polyolefin (number average molecular weight of 1.4 million - 1.6 million), and a low molecular weight third polyolefin (number average molecular weight of 0.4 million - 0.6 million) are selected to form a polyolefin composition, and their dosages satisfy the following formula: 1 ≤ (B - C) / A ≤ 3.5; where A represents the mass percentage of the first polyolefin, B represents the mass percentage of the second polyolefin, and C represents the mass percentage of the third polyolefin; this makes the polyolefin composition have a wide molecular weight distribution, which helps to optimize the performance of the microporous membrane. At the same time, the co-plasticizing effect between them and mineral oil is utilized to promote the disentanglement of the long-chain molecules of ultra-high molecular weight polyolefins; the processing of a base film based on high molecular weight polyolefins (formula molecular weight above 1 million) is realized.

[0043] In specific implementation, based on performance requirements such as tensile strength, puncture strength, and air permeability, the present invention adjusts the mass percentages of the ultra-high molecular weight first polyolefin (number average molecular weight of 2 million - 2.2 million), the high molecular weight second polyolefin (number average molecular weight of 1.4 million - 1.6 million), and the low molecular weight third polyolefin (number average molecular weight of 0.4 million - 0.6 million) in the polyolefin composition; among them, the ultra-high molecular weight polyolefin has extremely high mechanical strength and can improve the tensile strength and tear resistance of the microporous separator; the high molecular weight polyolefin has good flexibility and can enable the microporous separator to withstand bending and deformation; the low molecular weight polyolefin can reduce the melt viscosity and adjust the pore size and porosity of the microporous separator.

[0044] Furthermore, the mass percentage of the first polyolefin is 1-6%, the mass percentage of the second polyolefin is 45-55%, and the mass percentage of the third polyolefin is 40-48%. After a large number of experimental studies, it is found that the mass percentages of the first polyolefin, the second polyolefin, and the third polyolefin satisfy 1≤(B-C) / A≤3.5 (where A represents the mass percentage of the first polyolefin, B represents the mass percentage of the second polyolefin, and C represents the mass percentage of the third polyolefin), so that the obtained polyolefin microporous membrane can balance the gas permeability and mechanical properties. The thickness of the obtained polyolefin microporous membrane is 5-10 μm, the puncture strength of the diaphragm is 60-90 g / μm, and the biaxial tensile strength of the diaphragm is 250-350 MPa. When the mass percentage of the first polyolefin in the polyolefin microporous membrane does not satisfy the above mass relationship formula, situations such as inability to dissolve and process, low puncture strength of the diaphragm, and low tensile strength will occur, which cannot meet the actual application requirements. Compared with the existing commercial diaphragms, the polyolefin microporous membrane developed in the present invention has obvious advantages in various performance parameters.

[0045] In some embodiments, the first polyolefin may be selected from polyethylene or polypropylene; the second polyolefin may be selected from polyethylene or polypropylene; the third polyolefin may be selected from polyethylene or polypropylene.

[0046] In some embodiments, the polyolefin composition further includes mineral oil, and the mass ratio of the total mass of the first polyolefin, the second polyolefin, and the third polyolefin to the mass of the mineral oil is controlled at (15-30):(75-82). The addition of mineral oil can effectively regulate the phase separation process when using the thermally induced phase separation (TIPS) process to prepare the diaphragm material, which is beneficial to controlling the porosity and pore size distribution of the diaphragm. Moreover, since the number average molecular weight of the first polyolefin is relatively large, this will cause a high melt viscosity during the processing, increasing the processing difficulty. In the present invention, the co-plasticizing effect between the small molecule mineral oil and the first polyolefin, the second polyolefin, and the third polyolefin can promote the disentanglement of the ultra-high molecular weight polyolefin long-chain molecules; realizing the processing of the high molecular weight polyolefin-based membrane. In addition, the addition of mineral oil can also significantly reduce the melt viscosity, contribute to the uniform dispersion of the polyolefin molecular chains, improve the fluidity, and facilitate the process operations such as extrusion and stretching.

[0047] In some embodiments, the mineral oil may be selected from one or more combinations of white oil, petroleum ether, petroleum hydrocarbon, and paraffin oil.

[0048] In some embodiments, the polyolefin composition further includes an antioxidant. The mass ratio of the total mass of the first polyolefin, the second polyolefin, and the third polyolefin to the mass of the antioxidant is controlled to be (15 - 30):(0.1 - 0.5). The addition of the antioxidant can increase the temperature of high-temperature thermal decomposition of the polyolefin, effectively preventing the polymer from undergoing thermal-oxidative degradation during high-temperature processing, resulting in molecular chain breakage, crosslinking, or discoloration. The antioxidant can capture free radicals (such as RO·, ROO·) to interrupt the chain reaction and maintain the material properties. In addition, the addition of the antioxidant can also prevent the polyolefin from oxidizing to form chromophores (such as carbonyl groups) and keep the diaphragm appearance transparent.

[0049] In some embodiments, the antioxidant is selected from one or a combination of isooctyl β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, n-octadecyl β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, and N,N'-hexamethylene-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionamide].

[0050] In a second aspect, the present invention provides a polyolefin microporous membrane comprising the polyolefin composition described in the first aspect above.

[0051] Specifically, when implemented, the polyolefin microporous membrane is obtained through the following preparation method:

[0052] The first polyolefin with a number-average molecular weight of 2,000,000 - 2,200,000, the second polyolefin with a number-average molecular weight of 1,400,000 - 1,600,000, the third polyolefin with a number-average molecular weight of 400,000 - 600,000, mineral oil, and an antioxidant are melt-blended, and the obtained mixed melt is formed into an oil film by casting.

[0053] The oil film is biaxially stretched to form a film, and then obtained the polyolefin microporous membrane through extraction treatment and heat setting treatment.

[0054] Specifically, when implemented, the mineral oil is selected from one or a combination of white oil, petroleum ether, petroleum hydrocarbons, and paraffin oil. The antioxidant is selected from one or a combination of isooctyl β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, n-octadecyl β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, and N,N'-hexamethylene-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionamide].

[0055] In specific implementation, according to the mass ratio of the total mass of the first polyolefin, the second polyolefin and the third polyolefin to the mineral oil and the antioxidant being (15 - 30):(75 - 82):(0.1 - 0.5), the first polyolefin, the second polyolefin, the third polyolefin, the mineral oil and the antioxidant are mixed evenly, and then added to a twin-screw extruder for plasticization and blending to form a mixed melt at 150 - 250 °C; the obtained mixed melt is then cooled on a chill roll in a casting manner to form an oil film, and then the oil film is biaxially stretched, and then the mineral oil in it is extracted and removed using an extractant, and finally heat-set to obtain a polyolefin microporous membrane.

[0056] Based on the performance requirements such as tensile strength, puncture strength, air permeability, etc., the present invention combines a first polyolefin with ultra-high molecular weight (number-average molecular weight of 2 million - 2.2 million), a second polyolefin with high molecular weight (number-average molecular weight of 1.4 million - 1.6 million), and a third polyolefin with low molecular weight (number-average molecular weight of 0.4 million - 0.6 million) for preparing a polyolefin microporous membrane; wherein, the mass of the first polyolefin accounts for 1 - 6% of the total mass of the first polyolefin, the second polyolefin and the third polyolefin; the mass of the second polyolefin accounts for 45 - 55% of the total mass of the first polyolefin, the second polyolefin and the third polyolefin; the mass of the third polyolefin accounts for 40 - 48% of the total mass of the first polyolefin, the second polyolefin and the third polyolefin. The obtained polyolefin microporous membrane can balance air permeability and mechanical properties. The membrane thickness of the obtained polyolefin microporous membrane is 5 - 10 μm, the diaphragm puncture strength is 60 - 90 g / μm, and the diaphragm biaxial tensile strength is 250 - 350 MPa; compared with the existing commercial diaphragms, the polyolefin microporous membrane developed by the present invention has obvious advantages in various performance parameters.

[0057] In some embodiments, the extractant used in the extraction treatment process is selected from one or a combination of more of n-hexane, n-heptane, dichloromethane, chloroform, isopropyl alcohol and ethanol.

[0058] In a third aspect, the present invention provides a lithium battery including the polyolefin microporous membrane described in the second aspect above.

[0059] To make those skilled in the art understand the present invention more clearly, the following examples are now used to describe in detail a polyolefin composition, a polyolefin microporous membrane and a lithium battery of the present invention.

[0060] Example 1:

[0061] The polyolefin composition, the mineral oil and the antioxidant are mixed evenly according to the mass ratio of 17.5:82:0.5; added to a twin-screw extruder for plasticization and blending to form a mixed melt at 220 °C;

[0062] The mixed melt formed by the twin-screw extruder is formed into an oil film through a die head and a chill roll in a casting manner, the oil film is biaxially stretched, and then the mineral oil therein is extracted and removed using an extractant, and finally heat-set to obtain the polyolefin microporous membrane;

[0063] Among them, the mass percentage composition of the polyolefin composition used is: the mass ratio A of the first polyolefin (number average molecular weight of 2,000,000 - 2,200,000) is 5%, the mass ratio B of the second polyolefin (number average molecular weight of 1,400,000 - 1,600,000) is 50%, and the mass ratio C of the third polyolefin (number average molecular weight of 400,000 - 600,000) is 45%; the first polyolefin, the second polyolefin, and the third polyolefin used are all polypropylene; the mineral oil used is paraffin oil; the antioxidant used is isooctyl β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate. The extractant used is n-heptane. Among them, the mass ratio relationship of the polyolefin composition is (B - C) / A = 1, meeting the requirement of 1 ≤ (B - C) / A ≤ 3.5.

[0064] The performance test results are shown in Table 1.

[0065] Example 2:

[0066] The polyolefin composition, mineral oil, and antioxidant are mixed evenly according to a mass ratio of 24.9:75:0.1; added to a twin-screw extruder for plasticization and blending to form a mixed melt at 150 °C;

[0067] The mixed melt formed by the twin-screw extruder is formed into an oil film through a die head and a chill roll in a casting manner, the oil film is biaxially stretched, and then the mineral oil therein is extracted and removed using an extractant, and finally heat-set to obtain the polyolefin microporous membrane;

[0068] The mass percentage composition of the polyolefin composition used is: the mass ratio A of the first polyolefin (number average molecular weight of 2,000,000 - 2,200,000) is 1%, the mass ratio B of the second polyolefin (number average molecular weight of 1,400,000 - 1,600,000) is 51%, and the mass ratio C of the third polyolefin (number average molecular weight of 400,000 - 600,000) is 48%; the first polyolefin, the second polyolefin, and the third polyolefin used are all polyethylene; the mineral oil used is petroleum hydrocarbon; the antioxidant used is N,N-1,6-hexamethylene-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionamide]; the extractant used is dichloromethane. Among them, the mass ratio relationship of the polyolefin composition is (B - C) / A = 3, meeting the requirement of 1 ≤ (B - C) / A ≤ 3.5.

[0069] The performance test results are shown in Table 1.

[0070] Example 3:

[0071] Mix the polyolefin composition, mineral oil, and antioxidant uniformly at a mass ratio of 19.6:80:0.4; add them to a twin-screw extruder for plasticization and blending to form a mixed melt at 180 °C.

[0072] Form an oil film from the mixed melt formed by the twin-screw extruder by means of casting, passing through a die head and a chill roll, perform biaxial stretching on the oil film, then use an extractant to extract and remove the mineral oil therein, and finally perform heat setting to obtain the polyolefin microporous membrane.

[0073] The mass percentage composition of the polyolefin composition used is as follows: the mass ratio A of the first polyolefin (number-average molecular weight of 2,000,000 - 2,200,000) is 4%, the mass ratio B of the second polyolefin (number-average molecular weight of 1,400,000 - 1,600,000) is 55%, and the mass ratio C of the third polyolefin (number-average molecular weight of 400,000 - 600,000) is 41%; the first polyolefin, the second polyolefin, and the third polyolefin used are all polypropylene; the mineral oil used is white oil; the antioxidant used is n-octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate; the extractant used is n-hexane. Among them, the mass ratio relationship of the polyolefin composition is (B - C) / A = 3.5, meeting the requirement of 1 ≤ (B - C) / A ≤ 3.5.

[0074] The performance test results are shown in Table 1.

[0075] Example 4:

[0076] Mix the polyolefin composition, mineral oil, and antioxidant uniformly at a mass ratio of 21.7:78:0.3; add them to a twin-screw extruder for plasticization and blending to form a mixed melt at 230 °C.

[0077] Form an oil film from the mixed melt formed by the twin-screw extruder by means of casting, passing through a die head and a chill roll, perform biaxial stretching on the oil film, then use an extractant to extract and remove the mineral oil therein, and finally perform heat setting to obtain the polyolefin microporous membrane.

[0078] The mass percentage composition of the polyolefin composition used is as follows: the mass ratio A of the first polyolefin (number-average molecular weight of 200,000 - 220,000) is 6%, the mass ratio B of the second polyolefin (number-average molecular weight of 140,000 - 160,000) is 54%, and the mass ratio C of the third polyolefin (number-average molecular weight of 40,000 - 60,000) is 40%; the first polyolefin, the second polyolefin, and the third polyolefin used are all polyethylene; the mineral oil used is paraffin oil; the antioxidant used is isooctyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate; the extractant used is n-hexane. Among them, the mass ratio relationship of the polyolefin composition is (B - C) / A = 2.33, meeting the requirement of 1 ≤ (B - C) / A ≤ 3.5.

[0079] The performance test results are shown in Table 1.

[0080] Example 5:

[0081] Mix the polyolefin composition, mineral oil, and antioxidant evenly according to a mass ratio of 19.8:80:0.2; add them to a twin-screw extruder for plasticization and blending to form a mixed melt at 250 °C.

[0082] The mixed melt formed by the twin-screw extruder is formed into an oil film by casting through a die head and a chill roll, the oil film is biaxially stretched, and then the mineral oil in it is extracted and removed using an extractant, and finally heat-set to obtain the polyolefin microporous membrane.

[0083] The mass percentage composition of the polyolefin composition used is as follows: the mass fraction A of the first polyolefin (number average molecular weight of 2,000,000 - 2,200,000) is 3%, the mass fraction B of the second polyolefin (number average molecular weight of 1,400,000 - 1,600,000) is 50%, and the mass fraction C of the third polyolefin (number average molecular weight of 400,000 - 600,000) is 47%; the first polyolefin, the second polyolefin, and the third polyolefin used are all polypropylene; the mineral oil used is petroleum ether; the antioxidant used is N,N'-hexamethylene-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionamide]; the extractant used is dichloromethane. Among them, the mass fraction relationship of the polyolefin composition is (B - C) / A = 1, meeting the requirement of 1 ≤ (B - C) / A ≤ 3.5.

[0084] The performance test results are shown in Table 1.

[0085] Comparative Example 1:

[0086] Compared with Example 1, the polyolefin composition does not contain the first polyolefin, only the second polyolefin and the third polyolefin, and the mass ratio of the second polyolefin to the third polyolefin is 50:45, and the others are the same as in Example 1.

[0087] Comparative Example 2:

[0088] The mass percentage composition of the polyolefin composition used is as follows: the mass fraction A of the first polyolefin (number average molecular weight of 2,000,000 - 2,200,000) is 10%, the mass fraction B of the second polyolefin (number average molecular weight of 1,400,000 - 1,600,000) is 40%, and the mass fraction C of the third polyolefin (number average molecular weight of 400,000 - 600,000) is 50% (wherein, the mass fraction relationship of the polyolefin composition is (B - C) / A = -1, not meeting the formula 1 ≤ (B - C) / A ≤ 3.5); the others are the same as in Example 1.

[0089] Comparative Example 3:

[0090] The existing commercial lithium-ion battery separator is composed of polyethylene with a number-average molecular weight of 100,000 to 120,000; model SW809I, and the manufacturer is Shenzhen Xingyuan Materials Technology Co., Ltd.

[0091] Table 1 Performance test results of polyolefin microporous membranes

[0092]

[0093] From the performance test results shown in Examples 1-5, it can be seen that the film thickness of the polyolefin microporous membrane provided by the present invention is 5-10 μm. In the polyolefin composition, the mass ratio of the first polyolefin is 1-6%, the mass ratio of the second polyolefin is 45-55%, the mass ratio of the third polyolefin is 40-48%, and the areal density is 4.5-5.3 g / m 2 , the air permeability value is 190-230 s / 100cc, the puncture strength of the separator is 60-90 g / μm, and the biaxial tensile strength of the separator is 250-350 MPa.

[0094] From the comparison of the performance test results of Example 1 and Comparative Example 1, it can be seen that since there is no first polyolefin in the polyolefin composition forming the polyolefin microporous membrane shown in Comparative Example 1, the puncture strength and tensile strength of the polyolefin microporous membrane provided by Comparative Example 1 are both less than those of the polyolefin microporous membrane provided by Example 1.

[0095] By comparing the performance test results of Example 1 and Comparative Example 2, since in the polyolefin microporous membrane given in Comparative Example 2, the mass ratios of the first polyolefin (A), the second polyolefin (B), and the third polyolefin (C) do not satisfy the relationship of 1≤(B-C) / A≤3.5, its mechanical properties are all worse than those of the polyolefin microporous membrane provided by Example 1.

[0096] In addition, by comparing the performance test results of Example 1 and Comparative Example 3, it is found that the polyolefin microporous membrane obtained by the preparation method provided by the present invention can take into account the advantages of air permeability and mechanical properties, while the commercial separator is significantly worse in terms of mechanical properties.

[0097] In the description of this specification, the description referring to terms such as "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with that 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 can be combined in a suitable manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine the different embodiments or examples described in this specification.

[0098] For the method embodiments, for the sake of simplicity of description, they are all expressed as a series of combinations of actions. However, those skilled in the art should be aware that the present invention is not limited by the described order of actions, because according to the present invention, certain steps can be performed in other orders or simultaneously. Secondly, those skilled in the art should also be aware that the embodiments described in the specification are all preferred embodiments, and the actions and components involved are not necessarily essential to the present invention.

[0099] The above has introduced in detail a polyolefin composition, a polyolefin microporous membrane and a lithium battery provided by the present invention. Specific examples are used herein to elaborate on the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A polyolefin composition, characterized in that include: The first polyolefin with a number average molecular weight of 200w to 220w, the second polyolefin with a number average molecular weight of 140w to 160w, and the third polyolefin with a number average molecular weight of 40w to 60w, the amounts thereof satisfy the following formula: 1≤(BC) / A≤3.5; Wherein, A represents the mass proportion of the first polyolefin, B represents the mass proportion of the second polyolefin, and C represents the mass proportion of the third polyolefin; The mass of the first polyolefin accounts for 1-6% of the total mass of the first polyolefin, the second polyolefin and the third polyolefin; the mass of the second polyolefin accounts for 45-55% of the total mass of the first polyolefin, the second polyolefin and the third polyolefin; the mass of the third polyolefin accounts for 40-48% of the total mass of the first polyolefin, the second polyolefin and the third polyolefin; The first polyolefin is selected from polyethylene and / or polypropylene; the second polyolefin is selected from polyethylene and / or polypropylene; and the third polyolefin is selected from polyethylene and / or polypropylene.

2. The polyolefin composition according to claim 1, characterized in that The polyolefin composition also includes mineral oil.

3. The polyolefin composition according to claim 2, characterized in that The mass ratio of the total mass of the first polyolefin, the second polyolefin and the third polyolefin to the mass of the mineral oil is (15-30):(75-82).

4. The polyolefin composition according to claim 2, characterized in that The mineral oil is selected from one or more combinations of white oil, petroleum ether, petroleum hydrocarbon and paraffin oil.

5. The polyolefin composition according to claim 1, characterized in that The polyolefin composition also includes an antioxidant.

6. The polyolefin composition according to claim 5, characterized in that The mass ratio of the total mass of the first polyolefin, the second polyolefin and the third polyolefin to the mass of the antioxidant is (15-30):(0.1-0.5).

7. The polyolefin composition according to claim 5, characterized in that The antioxidant is selected from one or more combinations of β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, and N,N-1,6-hexylene-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionamide].

8. A polyolefin microporous membrane, characterized in that: The polyolefin composition comprises any one of claims 1 to 7.

9. The polyolefin microporous membrane according to claim 8, characterized in that: The polyolefin microporous membrane is obtained by the following preparation method: The first polyolefin with a number average molecular weight of 200w-220w, the second polyolefin with a number average molecular weight of 140w-160w, the third polyolefin with a number average molecular weight of 40w-60w, mineral oil and an antioxidant are melt-blended to form an oil film by casting the obtained mixed melt; The oil film is subjected to biaxial stretching to form a film, and then subjected to extraction treatment and heat setting treatment to obtain the polyolefin microporous membrane.

10. The polyolefin microporous membrane according to claim 9, characterized in that: In the mixed melt, the mass ratio of the total mass of the first polyolefin, the second polyolefin and the third polyolefin to the mass of the mineral oil and the antioxidant is (15-30): (75-82): (0.1-0.5).

11. The polyolefin microporous membrane according to claim 9, characterized in that: The mass of the first polyolefin accounts for 1 to 6% of the total mass of the first polyolefin, the second polyolefin and the third polyolefin; The mass of the second polyolefin accounts for 45-55% of the total mass of the first polyolefin, the second polyolefin and the third polyolefin; The mass of the third polyolefin accounts for 40-48% of the total mass of the first polyolefin, the second polyolefin and the third polyolefin.

12. The polyolefin microporous membrane according to claim 9, characterized in that: The temperature of the mixed melt is 150-250°C.

13. The polyolefin microporous membrane according to claim 9, characterized in that: The extractant used in the extraction treatment process is selected from one or more combinations of n-hexane, n-heptane, dichloromethane, chloroform, isopropanol and ethanol.

14. The polyolefin microporous membrane according to claim 8, characterized in that: The thickness of the polyolefin microporous membrane is 5-10 μm.

15. The polyolefin microporous membrane according to claim 8, characterized in that: The puncture strength of the polyolefin microporous membrane is 60 to 90 g / μm.

16. The polyolefin microporous membrane according to claim 8, characterized in that: The biaxial tensile strength of the polyolefin microporous membrane is 250-350 MPa.

17. A lithium battery, characterized in that: The invention comprises the polyolefin microporous membrane according to any one of claims 8 to 16.

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