Method for preparing separator of secondary battery, secondary battery

Abstract

The present invention provides a method for preparing a separator of a secondary battery and a secondary battery. The preparation method includes: respectively performing silane modification on a base film and inorganic particles to obtain a modified base film and modified inorganic particles; mixing and processing raw materials including an elastic polymer, the modified inorganic particles, the modified base film, a chain extender, and an organic solvent to obtain a spinning precursor solution; performing electrospinning on the spinning precursor solution to obtain a separator; wherein the base film is polyethylene and / or polypropylene; the elastic polymer is a polyurethane polymer; the thermal conductivity of the modified inorganic particles is 35 to 200 W / (m·K); the inorganic particles are alumina and / or aluminum nitride; when the inorganic particles are a combination of alumina and aluminum nitride, the mass ratio of alumina to aluminum nitride is 1:1 to 1:2; the chemical general formula of the polyurethane polymer is, where n is 10 to 35. The separator obtained by the above preparation method has high elasticity and high thermal conductivity.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and more particularly, to a method for preparing a separator of a secondary battery and a secondary battery. Background Art

[0002] Currently, the development trend of battery cell products is towards large capacity, high energy density and high safety. The methods to achieve high energy density batteries mainly include developing positive and negative electrode materials with higher compaction and higher specific capacity, and optimizing the internal space layout of the battery cell to achieve the design of lithium-ion batteries with high energy density as much as possible. However, the performance of the positive and negative electrode materials has approached the theoretical performance, and the internal space of the battery cell has been optimized reasonably. To further achieve higher energy density, new optimization directions need to be explored.

[0003] During the charge and discharge process of lithium-ion batteries, there are still problems such as excessive swelling force and bulging of the battery cell caused by volume expansion. In addition, large-capacity batteries are prone to uneven temperature distribution during normal operation, resulting in local high temperatures, which leads to attenuation of the battery cell performance; at the same time, when large-capacity batteries are out of control, higher energy will be released, making it easier to cause safety problems such as battery cell fire and explosion; for the safety problems of large-capacity battery cells, currently, it is mainly regulated by the electrolyte composition, by adding flame retardants and other components to inhibit the thermal runaway of the battery cell. However, by regulating the electrolyte composition, the electrochemical performance of the battery cell will be reduced to a certain extent, and at the same time, the problems of uneven temperature distribution and heat dissipation during the charge and discharge process of large-capacity battery cells cannot be solved. Summary of the Invention

[0004] The main object of the present invention is to provide a method for preparing a separator of a secondary battery and a secondary battery to solve the problems of excessive swelling force and bulging of the battery cell caused by volume expansion and poor heat dissipation during the overcharge process of lithium-ion batteries in the prior art.

[0005] To achieve the above object, according to one aspect of the present invention, a method for preparing a separator of a secondary battery is provided. The preparation method includes: Step S1, subjecting a base film and inorganic particles to silane modification respectively to obtain a modified base film and modified inorganic particles; Step S2, mixing and processing raw materials including an elastic polymer, modified inorganic particles, modified base film, chain extender and organic solvent to obtain a spinning precursor solution; Step S3, electrospinning the spinning precursor solution to obtain a separator; wherein, the base film is polyethylene and / or polypropylene; the elastic polymer is a polyurethane polymer; the thermal conductivity of the modified inorganic particles is 35~200W / (m·K); the inorganic particles are alumina and / or aluminum nitride; when the inorganic particles are a combination of alumina and aluminum nitride, the mass ratio of alumina to aluminum nitride is 1:1~1:2; and / or, the chemical general formula of the polyurethane polymer is , where n is from 10 to 35.

[0006] Further, in the above step S2, the mass ratio of the modified base film, the modified inorganic particles and the elastic polymer is 50-75:5-10:25-40.

[0007] Further, the mass ratio of the elastic polymer to the modified base film is 25-35:55-70; and / or, the elongation rate of the elastic polymer is 400-700%.

[0008] Further, the above step S1 further includes: ball-milling the inorganic particles to obtain ball-milled particles; mixing the ball-milled particles and water, and then sequentially performing centrifugal dispersion, washing and first drying to obtain pretreated inorganic particles; wherein, the rotation speed of the ball-milling is 500-600 r / min, and / or, the time of the ball-milling is 12-20 h; and / or, the temperature of the first drying is 80-100 °C, and / or, the time of the first drying is 24-36 h.

[0009] Further, the above step S1 further includes: respectively performing modification treatment on the base film and the pretreated inorganic particles with a silane coupling agent, and the modification treatment includes sequentially performing silane modification and second drying; wherein, the silane coupling agent is selected from any one or more of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane and γ-methacryloxypropyltrimethoxysilane; and / or, the temperature of the silane modification is 80-150 °C, the time of the silane modification is 4-6 h; and / or, the temperature of the second drying is 100-130 °C, and / or, the time of the second drying is 2-4 h.

[0010] Further, in the above step S2, the preparation method of the elastic polymer includes: sequentially performing first heating, first vacuum dehydration and cooling on polytetrahydrofuran ether glycol to obtain pretreated polytetrahydrofuran ether glycol; performing a polymerization reaction on the pretreated polytetrahydrofuran ether glycol and diphenylmethane diisocyanate to obtain an elastic polymer; wherein, the molecular weight of the polytetrahydrofuran ether glycol is 1000-3000; and / or, the temperature of the first heating is 110-120 °C; and / or, the negative pressure of the first vacuum dehydration is 0.1-0.3 MPa, the time of the first vacuum dehydration is 2-4 h; and / or, the temperature of the cooling is 60-80 °C; and / or, the temperature of the polymerization reaction is 80-100 °C, the time of the polymerization reaction is 40-100 min; and / or, the molar ratio of the pretreated polytetrahydrofuran ether glycol to the diphenylmethane diisocyanate is 2-4:1.

[0011] Further, the above step S2 further includes: sequentially subjecting a raw material including an elastic polymer, modified inorganic particles, a modified base film, and an organic solvent to second heating and second vacuum water removal to obtain a first mixture; sequentially stirring, defoaming, and centrifuging a raw material including the first mixture and a chain extender to obtain a spinning precursor solution; wherein the organic solvent is N,N-dimethylacetamide; and / or the temperature of the second heating is 75-85°C; and / or the negative pressure of the second vacuum water removal is 0.1-0.3 MPa, and the time of the second vacuum water removal is 1.5-4; and / or the mass ratio of the first mixture to the chain extender is 97-99:1-3; and / or the chain extender is a diamine compound, and the diamine compound is selected from any one or more of ethylenediamine, 3,3-dichloro-4'-diaminodiphenylmethane, and hydroxypropyl acrylate; and / or the rotation speed of the stirring is 3000-5000 rpm, and the time of the stirring is 10-20 min.

[0012] Further, in the above step S2, the ratio of the total mass of the elastic polymer, the modified inorganic particles, and the modified base film to the volume of the organic solvent is 20-40:100 g / mL.

[0013] Further, in the above step S3, the spinning precursor solution is sucked into a syringe for electrospinning; wherein the sucking speed is 0.3-0.6 mL / h; and / or the voltage of the electrospinning is 18-25 kV, and / or the rotation speed of the spinning roller of the electrospinning is 200-300 r / min.

[0014] According to another aspect of the present invention, a secondary battery is provided, including a positive electrode sheet, a negative electrode sheet, and a separator, and the separator is obtained by the above preparation method, and the thickness of the separator is 12-15 μm.

[0015] Applying the technical solution of the present invention, in step S1 of this application, the base film and inorganic particles are respectively modified with silane, which can improve the composite effect among the silane-modified base film, silane-modified inorganic particles and elastic polymer. By adding the above-mentioned types of modified base film to the raw materials in step S2, it has the characteristics of high strength and good processing performance. The above-mentioned types of elastic polymers have extremely high elasticity, with a relatively high elongation rate, and can quickly return to the original length during the stretching process. When the elastic polymer is compounded with the modified base film, it can improve the resilience of the separator while ensuring the strength of the separator, thereby alleviating the excessive swelling force and bulging of the battery core caused by volume expansion during the overcharge and discharge of lithium-ion batteries. At the same time, it can also relax the design group margin and improve the energy density of the battery core. Preferably, the elastic polymer is the above-mentioned polyurethane polymer, which can further improve the resilience of the separator and better compound with the modified base film and modified inorganic particles. The thermal conductivity of the modified inorganic particles is within the above range, which is much greater than the thermal conductivity of the base film (0.13 - 0.5 W / (m·K)). Therefore, the addition of the above-mentioned modified inorganic particles can greatly improve the thermal conductivity of the separator. Specifically, the above-mentioned modified inorganic particles can effectively conduct the heat generated by the battery core during the charge and discharge process, thereby reducing the performance attenuation of the battery core caused by temperature concentration during the battery core testing process, and at the same time, it can also reduce the risk of fire and explosion of the battery core during thermal runaway. Adding a chain extender can increase the length and cross-linking density of the polymer chain, thereby improving the performance of the separator. Through the electrospinning process in step S3, the morphology, structure and performance of the separator can be better controlled. Therefore, the separator obtained by the preparation method of this application has high elasticity and high thermal conductivity, providing a material basis for designing high-energy-density and safe lithium-ion batteries. Detailed Embodiments

[0016] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the embodiments.

[0017] As analyzed in the background art of this application, in the prior art, there are problems such as excessive swelling force and bulging of the battery core caused by volume expansion during the overcharge and discharge of lithium-ion batteries, and poor heat dissipation. To solve the above problems, this application provides a preparation method for a separator of a secondary battery and a secondary battery.

[0018] In a typical embodiment of the present application, a method for preparing a separator of a secondary battery is provided. The preparation method includes: Step S1, respectively performing silane modification on a base film and inorganic particles to obtain a modified base film and modified inorganic particles; Step S2, mixing and processing raw materials including an elastic polymer, modified inorganic particles, modified base film, chain extender, and organic solvent to obtain a spinning precursor solution; Step S3, performing electrospinning on the spinning precursor solution to obtain a separator; wherein, the base film is polyethylene and / or polypropylene; the elastic polymer is a polyurethane polymer; the thermal conductivity of the modified inorganic particles is 35-200 W / (m·K); the inorganic particles are alumina and / or aluminum nitride; when the inorganic particles are a combination of alumina and aluminum nitride, the mass ratio of alumina to aluminum nitride is 1:1-1:2;

[0019] In an embodiment of the present application, the chemical general formula of the polyurethane polymer is , where n is 10-35.

[0020] By respectively performing silane modification on the base film and inorganic particles in Step S1 of the present application, the composite effect among the silane-modified base film, silane-modified inorganic particles, and elastic polymer can be improved. By adding the above-mentioned type of modified base film to the raw materials in Step S2, it has the characteristics of high strength and good processing performance. The above-mentioned type of elastic polymer has extremely high elasticity, a high elongation rate, and can quickly return to its original length during the stretching process. The composite of the elastic polymer and the modified base film can improve the resilience of the separator while ensuring the strength of the separator, thereby alleviating the excessive swelling force and bulging of the battery core caused by volume expansion during the charge and discharge process of the lithium-ion battery. At the same time, it can also relax the design group margin and improve the energy density of the battery core. Preferably, the elastic polymer is the above-mentioned polyurethane polymer, which can further improve the resilience of the separator and better composite with the modified base film and modified inorganic particles. The thermal conductivity of the modified inorganic particles is within the above range, which is much greater than the thermal conductivity of the base film (0.13-0.5 W / (m·K)). Therefore, the addition of the above-mentioned modified inorganic particles can greatly improve the thermal conductivity of the separator. Specifically, the above-mentioned modified inorganic particles can effectively conduct the heat generated by the battery core during the charge and discharge process, thereby reducing the performance attenuation of the battery core caused by temperature concentration during the battery core test, and at the same time, it can also reduce the risk of fire and explosion of the battery core during thermal runaway. Adding a chain extender can increase the length and crosslinking density of the polymer chain, thereby improving the performance of the separator. Through the electrospinning process in Step S3, the morphology, structure, and performance of the separator can be better controlled. Therefore, the separator obtained by the preparation method of the present application has high elasticity and high thermal conductivity, providing a material basis for designing high-energy-density and safe lithium-ion batteries.

[0021] In addition, in the chemical general formula of the polyurethane polymer, n can be 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34 or 35. Of course, n can be any point value within 10 to 35, which will not be elaborated here.

[0022] Preferably, in the chemical general formula of the polyurethane polymer, n is 20 to 35. The longer the molecular chain of the polyurethane with a high degree of polymerization, the better its excellent properties such as higher strength and higher elasticity are manifested.

[0023] In an embodiment of the present application, in the above step S2, the mass ratio of the modified base film, the modified inorganic particles and the elastic polymer is 50 to 75: 5 to 10: 25 to 40.

[0024] Preferably, controlling the mass ratio of the modified base film, the modified inorganic particles and the elastic polymer within the above range helps to improve the elasticity of the separator while having sufficient strength, so as to better adapt to the volume change of the electrode material, reduce the internal stress caused by the swelling of the battery cell, avoid the bulging of the battery, and thus improve the cycle stability and life of the battery. At the same time, an appropriate amount of inorganic particles helps to further improve the thermal conductivity of the separator and reduce the risk of local overheating, thereby improving the overall safety and thermal performance of the battery.

[0025] In an embodiment of the present application, the mass ratio of the elastic polymer to the modified base film is 25 to 35: 55 to 70; and / or, the elongation rate of the elastic polymer is 400 to 700%.

[0026] Preferably, controlling the mass ratio of the elastic polymer to the modified base film and the elongation rate of the elastic polymer within the above range helps the elastic polymer to better composite with the modified base film, thereby further improving the resilience of the separator, and then alleviating the excessive swelling force and bulging of the battery cell caused by volume expansion during the charge and discharge of the lithium-ion battery.

[0027] In an embodiment of the present application, the above step S1 further includes: ball milling the inorganic particles to obtain ball-milled particles; mixing the ball-milled particles and water and then successively performing centrifugal dispersion, washing and first drying to obtain pretreated inorganic particles; wherein, the rotation speed of the ball milling is 500 to 600 r / min, and / or, the time of the ball milling is 12 to 20 h; and / or, the temperature of the first drying is 80 to 100 °C, and / or, the time of the first drying is 24 to 36 h.

[0028] Preferably, inorganic particles are ball-milled, and the rotation speed and time of ball-milling are controlled within the above ranges, which helps to reduce the particle size of the inorganic particles, thereby increasing the contact area between the inorganic particles and the base film, and further improving the thermal conductivity of the separator. In addition, the ball-milling process also helps to change the morphology of the inorganic particles, making them have a more uniform shape, which is beneficial for subsequent dispersion and compounding. Preferably, the above treatment is performed on the ball-milled particles and water, and the temperature and time of the first drying are controlled within the above ranges, which helps to remove impurities on the particle surface and at the same time make the particles more evenly dispersed in the solution, providing a better basis for subsequent surface modification and compounding.

[0029] In an embodiment of the present application, the above step S1 further includes: respectively performing modification treatment on the base film and the pretreated inorganic particles with a silane coupling agent, and the modification treatment includes silane modification and second drying performed in sequence; wherein, the silane coupling agent is selected from any one or more of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, and γ-methacryloxypropyltrimethoxysilane; and / or, the temperature of the silane modification is 80-150 °C, and the time of the silane modification is 4-6 h; and / or, the temperature of the second drying is 100-130 °C, and / or, the time of the second drying is 2-4 h.

[0030] Preferably, the base film and the pretreated inorganic particles are respectively modified with the above types of silane coupling agents, and the temperature and time of the silane modification are controlled within the above ranges, which helps to fully perform silane modification on the surfaces of the base film and the inorganic particles, thereby alleviating the problem of interfacial incompatibility, improving the distribution uniformity of each component of the separator, and further enhancing the compounding effect among the silane-modified base film, the silane-modified inorganic particles, and the elastic polymer.

[0031] In an embodiment of the present application, in the above step S2, the preparation method of the elastic polymer includes: sequentially heating, performing first vacuum dehydration, and cooling polytetrahydrofuran ether glycol to obtain pretreated polytetrahydrofuran ether glycol; performing a polymerization reaction on the pretreated polytetrahydrofuran ether glycol and diphenylmethane diisocyanate to obtain an elastic polymer; wherein, the molecular weight of the polytetrahydrofuran ether glycol is 1000-3000; and / or, the temperature of the first heating is 110-120 °C; and / or, the negative pressure of the first vacuum dehydration is 0.1-0.3 MPa, and the time of the first vacuum dehydration is 2-4 h; and / or, the temperature of the cooling is 60-80 °C; and / or, the temperature of the polymerization reaction is 80-100 °C, and the time of the polymerization reaction is 40-100 min; and / or, the molar ratio of the pretreated polytetrahydrofuran ether glycol to diphenylmethane diisocyanate is 2-4:1.

[0032] Preferably, the above preparation method is adopted to prepare the elastic polymer, and the molecular weight of the tetrahydrofuran ether diol is controlled within the above range, which helps to regulate the molecular weight and molecular chain length of the elastic polymer, so as to better compound with the base film and inorganic particles.

[0033] In one embodiment of the present application, the above step S2 further includes: sequentially performing second heating and second vacuum dehydration on raw materials including an elastic polymer, modified inorganic particles, a modified base film, and an organic solvent to obtain a first mixture; sequentially stirring, defoaming, and centrifuging the raw materials including the first mixture and a chain extender to obtain a spinning precursor solution; wherein the organic solvent is N,N-dimethylacetamide; and / or, the temperature of the second heating is 75-85 °C; and / or, the negative pressure of the second vacuum dehydration is 0.1-0.3 MPa, and the time of the second vacuum dehydration is 1.5-4; and / or, the mass ratio of the first mixture to the chain extender is 97-99:1-3; and / or, the chain extender is a diamine compound, and the diamine compound is selected from any one or more of ethylenediamine, 3,3-dichloro-4'-diaminodiphenylmethane (MOCA), and hydroxypropyl acrylate (HPA); and / or, the rotation speed of the stirring is 3000-5000 rpm, and the time of the stirring is 10-20 min.

[0034] In one embodiment of the present application, in the above step S2, the ratio of the total mass of the elastic polymer, modified inorganic particles, and modified base film to the volume of the organic solvent is 20-40:100 g / mL.

[0035] Preferably, controlling the ratio of the total mass of the elastic polymer, modified inorganic particles, and modified base film to the volume of the organic solvent within the above range helps the elastic polymer, modified inorganic particles, and modified base film to be better dispersed in the organic solvent, so as to better compound the three and improve the distribution uniformity of each component of the separator.

[0036] In one embodiment of the present application, in the above step S3, the spinning precursor solution is sucked into a syringe for electrospinning; wherein the sucking speed is 0.3-0.6 mL / h; and / or, the voltage of the electrospinning is 18-25 kV, and / or, the rotation speed of the spinning roller of the electrospinning is 200-300 r / min.

[0037] Preferably, the spinning precursor solution is sucked into a syringe for electrospinning, and controlling the sucking speed, the voltage of the electrospinning, and the rotation speed of the spinning roller within the above range helps to improve the uniformity of fiber distribution during the electrospinning process, so as to obtain a denser and more uniform separator structure, and further improve the stability and reliability of the separator in the battery. The adjustment of the electrospinning voltage helps to control the diameter of the fibers, thereby regulating the porosity and pore size distribution of the separator.

[0038] In another typical embodiment of the present application, a secondary battery is provided, which includes a positive electrode sheet, a negative electrode sheet, and a separator. The separator is obtained by the preparation method described above, and the thickness of the separator is 12 - 15 μm.

[0039] The secondary battery including the above-mentioned separator has good cycle performance, cycle life, and safety performance.

[0040] The beneficial effects of the present application will be further described below in conjunction with examples.

[0041] Example 1

[0042] Preparation of elastic polymer: Pour polytetrahydrofuran ether glycol into a reaction vessel and heat it for the first time to 115 °C, carry out the first vacuum dehydration for 2 h under a negative pressure of 0.1 MPa, and then cool it to 60 °C to obtain pretreated polytetrahydrofuran ether glycol. Under the atmosphere of a protective gas, heat the pretreated polytetrahydrofuran ether glycol and diphenylmethane diisocyanate to 90 °C for a polymerization reaction for 40 min, and carry out vacuum degassing for 30 min to obtain an elastic polymer. Among them, the molar ratio of the pretreated polytetrahydrofuran ether glycol to diphenylmethane diisocyanate is 2:1. The elastic polymer is , where n is 35.

[0043] Pretreatment of inorganic particles: Mix 5 g of alumina and 10 g of aluminum nitride and add them to a ball mill jar, carry out solid-phase ball milling at 550 r / min for 15 h to obtain ball-milled particles. Disperse the ball-milled particles in 500 mL of deionized water, centrifuge and wash them with water multiple times, and carry out the first drying in a vacuum oven at 90 °C for 30 h to obtain pretreated inorganic particles.

[0044] Preparation of modified base film and modified inorganic particles: Use the silane coupling agent γ-aminopropyltriethoxysilane to modify the base film polyethylene and the pretreated inorganic particles respectively. The modification treatment includes carrying out silane modification at 120 °C for 5 h and the second drying at 110 °C for 3 h to obtain a modified base film and modified inorganic particles.

[0045] Preparation of the separator: An elastic polymer, modified inorganic particles, and a modified base film with a total mass of 20 g (mass ratio of 25:5:70) were added to 100 mL of N,N-dimethylacetamide, and the mixture was heated to 80 °C for the second time. Under a negative pressure of 0.1 MPa, the second vacuum dehydration was carried out for 1.5 h to obtain a first mixture. 0.5 g of pre-melted chain extender ethylenediamine was added to the first mixture, and the mixture was stirred with an electric stirrer at 4000 rpm for 10 min, followed by degassing and centrifugation to obtain a spinning precursor solution. The spinning precursor solution was sucked into a syringe, and electrospinning was carried out under the conditions of a voltage of 18 kV, a sucking speed of 0.3 mL / h, and a spinning roll rotation speed of 200 r / min to obtain a post-spinning separator. The post-spinning separator was placed in a vacuum oven for drying to obtain a separator with a thickness of 12 μm.

[0046] Example 2

[0047] The difference from Example 1 is that the elastic polymer is , n is 20, and finally a separator is obtained.

[0048] Example 3

[0049] The difference from Example 1 is that the mass ratio of the elastic polymer, modified inorganic particles, and modified base film is 40:5:55, and finally a separator is obtained.

[0050] Example 4

[0051] The difference from Example 1 is that the mass ratio of the elastic polymer, modified inorganic particles, and modified base film is 25:10:65, and finally a separator is obtained.

[0052] Example 5

[0053] The difference from Example 1 is that the mass ratio of the elastic polymer, modified inorganic particles, and modified base film is 40:10:50, and finally a separator is obtained.

[0054] Example 6

[0055] The difference from Example 1 is that the mass ratio of the elastic polymer, modified inorganic particles, and modified base film is 15:15:70, and finally a separator is obtained.

[0056] Example 7

[0057] The difference from Example 1 is that the temperature of silane modification is 80 °C and the time of silane modification is 6 h to obtain a modified base film and modified inorganic particles, and finally a separator is obtained.

[0058] Example 8

[0059] The difference from Example 1 is that the temperature of silane modification is 60 °C, and the time of silane modification is 2 h, obtaining a modified base film and modified inorganic particles, and finally obtaining a separator.

[0060] Example 9

[0061] The difference from Example 1 is that the voltage of electrospinning is 25 kV, and the rotation speed of the spinning roller for electrospinning is 300 r / min, and finally obtaining a separator.

[0062] Example 10

[0063] The difference from Example 1 is that the voltage of electrospinning is 15 kV, and the rotation speed of the spinning roller for electrospinning is 350 r / min, and finally obtaining a separator.

[0064] Comparative Example 1

[0065] Using polyethylene as the substrate, a coated separator with a thickness of 7 + 2 + 2 containing Al 2 O 3 is obtained, where the thickness of the polyethylene base film is 7 μm, the thickness of the coated layer covering one side surface of the base film is 2 μm, and the thickness of the Al 2 O 3 ceramic coating covering the other side surface is 2 μm.

[0066] Comparative Example 2

[0067] The difference from Example 1 is that the elastic polymer is , n is 6, and finally obtaining a separator.

[0068] Comparative Example 3

[0069] The difference from Example 1 is that no inorganic particles are added, 20 g in total of the elastic polymer and the modified base film (the mass ratio of the two is 30:70) are added to 100 mL of N,N-dimethylacetamide, and the second heating is carried out to 80 °C, and the second vacuum water removal is carried out for 1.5 h under a negative pressure of 0.1 MPa to obtain a first mixture, and finally obtaining a separator.

[0070] Comparative Example 4

[0071] The difference from Example 1 is that no elastic polymer is added, 20 g in total of the modified inorganic particles and the modified base film (the mass ratio of the two is 30:70) are added to 100 mL of N,N-dimethylacetamide, and the second heating is carried out to 80 °C, and the second vacuum water removal is carried out for 1.5 h under a negative pressure of 0.1 MPa to obtain a first mixture, and finally obtaining a separator.

[0072] Comparative Example 5

[0073] The difference from Example 1 is that the inorganic particles and the base film are not modified with silane, and finally a separator is obtained.

[0074] Test method:

[0075] The separators of the above examples and comparative examples were used to prepare soft-pack lithium-ion batteries. The positive electrode was lithium iron phosphate, the negative electrode was a graphite system, and the nominal capacity of the soft-pack lithium-ion battery was 2.3 Ah. Electrochemical tests were carried out.

[0076] Thermal conductivity test of the separator: A heat flow meter was used to test the separators prepared in different examples and comparative examples. During the test, multiple layers of the separator were stacked into a test sample with a thickness of 1-2 mm, and the test pressure was 10 psi to reduce the contact thermal resistance caused by multi-layer stacking.

[0077] Swelling force test during charge and discharge: A pressure sensor was used to monitor the change in the swelling force of the battery during charge and discharge. The initial swelling force of the battery was 100 kgf, and the maximum swelling forces of the batteries assembled with the separators prepared in each example and comparative example during charge and discharge were compared.

[0078] Highest temperature test during charge and discharge of the battery: The ambient temperature during the test was 25 °C. A temperature sensor was attached to the large surface of the battery to record the temperature rise change of the battery after charge and discharge.

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

[0080] Table 1

[0081]

[0082] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0083] By subjecting the base film and inorganic particles to silane modification respectively in step S1, the present application can enhance the composite effect among the silane-modified base film, silane-modified inorganic particles and elastic polymer. By adding the modified base film of the above type to the raw materials in step S2, it has the characteristics of high strength and good processability. The elastic polymer of the above type has extremely high elasticity, with a relatively high elongation rate, and can quickly return to the original length during the stretching process. When the elastic polymer is compounded with the modified base film, it can improve the resilience of the separator while ensuring the strength of the separator, thereby alleviating the excessive swelling force and bulging of the battery cell caused by volume expansion during overcharge and discharge of the lithium-ion battery. At the same time, it can also relax the design group margin and improve the energy density of the battery cell. Preferably, the elastic polymer is the above-mentioned polyurethane polymer, which can further improve the resilience of the separator and better compound with the modified base film and modified inorganic particles. The thermal conductivity of the modified inorganic particles is within the above range, which is much greater than the thermal conductivity of the base film (0.13 - 0.5 W / (m·K)). Therefore, the addition of the above-mentioned modified inorganic particles can greatly enhance the thermal conductivity of the separator. Specifically, the above-mentioned modified inorganic particles can effectively conduct out the heat generated by the battery cell during charge and discharge, thereby reducing the performance attenuation of the battery cell caused by temperature concentration during the battery cell testing process, and at the same time, it can also reduce the risk of fire and explosion of the battery cell during thermal runaway. Adding a chain extender can increase the length and crosslinking density of the polymer chain, thereby improving the performance of the separator. Through the electrospinning process in step S3, the morphology, structure and performance of the separator can be better regulated. Therefore, the separator obtained by the preparation method of the present application has high elasticity and high thermal conductivity, providing a material basis for designing high-energy-density and safe lithium-ion batteries.

[0084] The above are only the embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a secondary battery separator, characterized in that: The preparation method comprises: Step S1, performing silane modification on the base film and the inorganic particles respectively to obtain a modified base film and modified inorganic particles; Step S2, mixing the raw materials including the elastic polymer, the modified inorganic particles, the modified base film, the chain extender and the organic solvent to obtain a spinning precursor solution; Step S3, electrospinning the spinning precursor solution to obtain the diaphragm; Wherein, the base film is polyethylene and / or polypropylene; The elastic polymer is a polyurethane polymer; The thermal conductivity of the modified inorganic particles is 35-200 W / (m·K); The inorganic particles are aluminum oxide and / or aluminum nitride; when the inorganic particles are a combination of aluminum oxide and aluminum nitride, the mass ratio of the aluminum oxide to the aluminum nitride is 1:1 to 1:2; The general chemical formula of the polyurethane polymer is , where n is 10~35; In the step S2, the mass ratio of the modified base film, the modified inorganic particles and the elastic polymer is 50-75:5-10:25-40; The elastic polymer has an elongation of 400-700%.

2. The preparation method according to claim 1, characterized in that: The mass ratio of the elastic polymer to the modified base film is 25-35:55-70.

3. The preparation method according to claim 1 or 2, characterized in that: The step S1 further comprises: ball-milling the inorganic particles to obtain ball-milled particles; The ball-milled particles are mixed with water and then centrifuged and dispersed, washed and dried for the first time to obtain pretreated inorganic particles; Wherein, the rotation speed of the ball mill is 500-600 r / min, and / or the time of the ball mill is 12-20 h; And / or, the first drying temperature is 80-100° C., and / or, the first drying time is 24-36 hours.

4. The preparation method according to claim 3, characterized in that: The step S1 further comprises: using a silane coupling agent to modify the base film and the pretreated inorganic particles respectively, wherein the modification treatment comprises the silane modification and the second drying performed in sequence; Wherein, the silane coupling agent is selected from any one or more of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane and γ-methacryloxypropyltrimethoxysilane; and / or, the temperature of the silane modification is 80-150° C., and the silane modification time is 4-6 hours; And / or, the temperature of the second drying is 100-130° C., and / or, the time of the second drying is 2-4 hours.

5. The preparation method according to claim 1 or 2, characterized in that: In the step S2, the method for preparing the elastic polymer comprises: The polytetrahydrofuran ether diol is sequentially subjected to a first heating, a first vacuum dehydration and a cooling to obtain a pretreated polytetrahydrofuran ether diol; The pretreated polytetramethylene ether diol and diphenylmethane diisocyanate are polymerized to obtain the elastic polymer; Wherein, the molecular weight of the polytetramethylene ether diol is 1000-3000; and / or, the temperature of the first heating is 110-120° C.; and / or, the negative pressure of the first vacuum dehydration is 0.1-0.3 MPa, and the time of the first vacuum dehydration is 2-4 h; and / or, the temperature of the cooling is 60-80° C.; And / or, the polymerization reaction temperature is 80-100° C., and the polymerization reaction time is 40-100 min; And / or, the molar ratio of the pretreated polytetramethylene ether diol to the diphenylmethane diisocyanate is 2-4:

1.

6. The preparation method according to claim 1 or 2, characterized in that: The step S2 further comprises: sequentially subjecting the raw materials including the elastic polymer, the modified inorganic particles, the modified base film and the organic solvent to a second heating and a second vacuum dehydration to obtain a first mixture; The raw materials including the first mixture and the chain extender are stirred, degassed and centrifuged in sequence to obtain the spinning precursor solution; Wherein, the organic solvent is N,N-dimethylacetamide; and / or, the temperature of the second heating is 75-85° C.; and / or, the negative pressure of the second vacuum dehydration is 0.1-0.3 MPa, and the time of the second vacuum dehydration is 1.5-4; And / or, the mass ratio of the first mixture to the chain extender is 97~99:1~3; and / or, the chain extender is a diamine compound, and the diamine compound is selected from any one or more of ethylenediamine, 3,3-dichloro-4'-diaminodiphenylmethane and hydroxypropyl acrylate; and / or, the stirring speed is 3000~5000rpm, and the stirring time is 10~20min.

7. The preparation method according to claim 1 or 2, characterized in that: In the step S2, the ratio of the total mass of the elastic polymer, the modified inorganic particles and the modified base film to the volume of the organic solvent is 20-40:100 g / mL.

8. The preparation method according to claim 1 or 2, characterized in that: In the step S3, the spinning precursor solution is sucked into a needle tube for electrospinning; wherein the suction speed is 0.3-0.6 mL / h; and / or the voltage of the electrospinning is 18-25 kV, and / or the spinning roller speed of the electrospinning is 200-300 r / min.

9. A secondary battery, comprising a positive electrode sheet, a negative electrode sheet and a separator, characterized in that: The separator is obtained by the preparation method according to any one of claims 1 to 8, and the thickness of the separator is 12 to 15 μm.

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