Diaphragm, preparation method thereof and secondary battery

Abstract

The invention provides a diaphragm, a preparation method thereof and a secondary battery. The preparation method comprises the following steps: dipping and absorbing cellulose powder in a shear thickening fluid, then coating at least one side of a base membrane with the obtained dipping and absorbing material, and drying to obtain the diaphragm. The cellulose powder is soaked and absorbed in the shear thickening fluid, the shear thickening fluid and the cellulose are strongly combined, the distribution uniformity of the shear thickening fluid in the cellulose is improved, the shear thickening fluid has a bonding effect on cellulose molecules, and therefore the impact deformation resistance and the heat shrinkage resistance of the cellulose are improved. The diaphragm prepared by adopting the preparation method disclosed by the invention has relatively high impact resistance, puncture resistance and heat resistance, so that the damage of a pole piece can be reduced, battery thermal runaway caused by impact and puncture can be reduced, and the safety performance of a battery cell can be enhanced. In addition, the preparation process flow and the operation mode are simple and convenient, and the preparation cost is low.

Description

Technical Field

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

[0002] At present, new energy vehicles are gradually occupying the share of the automobile market due to their advantages in energy saving and environmental protection. Major manufacturers have launched lithium-ion batteries with greater energy density and energy level to meet consumers' requirements for endurance, which has also increased the incidence of electric vehicle accidents. Lithium-ion batteries have gradually become the focus of people's attention. Lithium-ion batteries will cause thermal runaway under various abuse conditions, such as high temperature, extrusion, puncture, overcharge and over-discharge, among which puncture is one of the most difficult test and evaluation conditions to pass. The separator is the main material of lithium-ion batteries. Its function is to separate the positive and negative electrodes and at the same time provide Li + If the lithium battery separator is punctured by external foreign objects or internal lithium dendrites, it will cause an internal short circuit in the lithium-ion battery, resulting in thermal runaway of the battery, and then triggering combustion and explosion. This places higher demands on the separator's heat puncture resistance.

[0003] The Chinese patent application, published as CN115064838A, achieves improved heat-resistant needle puncture resistance of the aramid-coated membrane by applying a meta-aramid coating to at least one side of the base film. The surface of the meta-aramid coating exhibits a dense honeycomb-like porous structure, with micropores forming a grid structure across the thickness. This coating improves the heat-resistant needle puncture resistance of the aramid-coated membrane without significantly increasing air permeability. While this structural design achieves needle puncture resistance, it is difficult to operate and has requirements for micropore area, placing high demands on the production process.

[0004] The Chinese patent, with patent authorization announcement number CN213459794U, uses a double-layer composite aluminum foil to increase the toughness of the current collector, significantly improving its puncture resistance. While this puncture-resistant current collector composite material improves the current collector's puncture resistance, it also fails to ensure interfacial contact between the current collector and the conductive paste, impacting battery performance. Summary of the Invention

[0005] The main purpose of the present invention is to provide a diaphragm and a preparation method thereof and a secondary battery, so as to solve the problems of poor impact resistance, puncture resistance and heat resistance of the diaphragm in the prior art.

[0006] In order to achieve the above-mentioned purpose, according to one aspect of the present invention, a method for preparing a diaphragm is provided, which comprises: impregnating and absorbing cellulose powder in a shear thickening fluid, then coating the obtained impregnated absorbent on at least one side of a base film, and obtaining a diaphragm after drying.

[0007] Furthermore, the mass ratio of the cellulose powder to the shear thickening fluid is 1:(1-2).

[0008] Furthermore, the shear thickening fluid comprises solid particles, a dispersant and a solvent; preferably, the solid particles are selected from any one or more of silicon dioxide particles, polystyrene particles and calcium carbonate particles; further preferably, the solid particles are silicon dioxide particles.

[0009] Furthermore, the mass ratio of the solid particles, dispersant and solvent is (0.5-0.65):(0.01-0.05):1.

[0010] Furthermore, the average particle size of the solid particles is 200 to 500 nm; and / or the specific surface area of ​​the solid particles is 50 to 400 m 2 / g; and / or, the dispersant is selected from any one or more of polyethylene glycol, polyvinyl pyrrolidone and sodium polyacrylate; and / or, the solvent is selected from any one or more of anhydrous ethanol, isopropanol and tetrahydrofuran.

[0011] Furthermore, the number average molecular weight of the above-mentioned cellulose powder is 50,000 to 2,500,000 g / mol; and / or, the average length of the nanowhiskers of the cellulose powder is 100 to 500 nm; preferably, the cellulose powder is selected from bacterial cellulose and / or sulfonated cellulose powder; further preferably, the cellulose powder is sulfonated cellulose powder, and most preferably, the degree of substitution of sulfonic acid groups on cellulose molecules in the sulfonated cellulose powder is 2% to 10%.

[0012] Furthermore, the coating surface density in the above coating is 0.1 to 0.4 mg / cm 2 ; and / or, the thickness of the base film is 7 to 10 μm; and / or, the porosity of the base film is 40 to 60%; and / or, the drying temperature is 50 to 80°C; and / or, the drying time is 1 to 24 hours; preferably, the base film is selected from polyethylene film and / or polypropylene film.

[0013] Furthermore, the above coating is gravure coating; preferably, in the gravure coating, the speed ratio of the gravure coating roller is (0.9-1.2):1, the blade pressure is 0.2-1.0 MPa, and the vehicle speed is 50-200 m / min.

[0014] According to another aspect of the present invention, a diaphragm is provided, which is prepared by the above-mentioned preparation method; preferably, the tensile strength of the diaphragm is 1200-1500 kgf / cm 2 and / or, the puncture strength of the membrane under a needle diameter of 1.0 mm Φ is 4 to 7 N / m; and / or, the shrinkage rate of the membrane when placed at 130°C for 1 hour is 0.6 to 1.2%.

[0015] According to another aspect of the present invention, a secondary battery is provided, comprising a positive electrode sheet, an electrolyte, a separator and a negative electrode sheet, wherein the separator is the aforementioned separator.

[0016] By applying the technical solution of the present application, cellulose has high heat resistance, absorbs electrolyte at a faster rate, and has a higher liquid retention rate. The cellulose powder is immersed in a shear thickening fluid for absorption, and the shear thickening fluid and cellulose are strongly bonded, which helps to improve the uniformity of the shear thickening fluid distribution in the cellulose. The shear thickening fluid has an adhesive effect on the cellulose molecules, thereby helping to improve the cellulose's resistance to impact deformation and thermal shrinkage. The resulting mixture is coated on at least one side of the base film and dried to obtain a diaphragm. The coating on the diaphragm has shear fluid properties. When punctured or impacted, the shear fluid will suddenly increase in viscosity, forming a tight structure between the particles, so that the diaphragm is constrained and not easily deformed, thereby helping to make the diaphragm soft and tough, and significantly improving its puncture resistance. Therefore, the diaphragm prepared by the preparation method of the present application has high impact resistance, puncture resistance, and heat resistance, thereby helping to reduce damage to the pole piece, and helping to reduce battery thermal runaway caused by impact and puncture, thereby helping to enhance the safety performance of the battery cell. In addition, the preparation process and operation method of the present application are simple and the preparation cost is low. DETAILED DESCRIPTION

[0017] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below with reference to the embodiments.

[0018] Explanation of terms: The speed ratio of the gravure coating roller refers to the ratio between the linear speed of the gravure coating roller and the running speed of the coated substrate during the coating process; the vehicle speed refers to the running speed of the coated substrate on the coater.

[0019] Shear-thickening fluids are non-Newtonian fluids with unique rheological properties, which exhibit low viscosity at low shear stresses and a sharp increase in viscosity at high and low shear stresses.

[0020] As analyzed in the background technology of this application, the diaphragm in the prior art has problems with poor impact resistance, puncture resistance and heat resistance. In order to solve this problem, this application provides a diaphragm, a preparation method thereof and a secondary battery.

[0021] In a typical embodiment of the present application, a method for preparing a diaphragm is provided, which comprises: impregnating and absorbing cellulose powder in a shear thickening fluid, then coating the obtained impregnated absorbent on at least one side of a base film, and obtaining the diaphragm after drying.

[0022] Cellulose has high heat resistance, absorbs electrolyte quickly, and has a high liquid retention rate. The cellulose powder is immersed in a shear thickening fluid for absorption. The shear thickening fluid and the cellulose are strongly bonded, which helps to improve the uniformity of the shear thickening fluid distribution in the cellulose. The shear thickening fluid has an adhesive effect on the cellulose molecules, thereby helping to improve the cellulose's resistance to impact deformation and thermal shrinkage. The resulting mixture is coated on at least one side of the base film and dried to obtain a diaphragm. The coating on the diaphragm has shear fluid properties. When punctured or impacted, the shear fluid will suddenly increase in viscosity, forming a tight structure between the particles, so that the diaphragm is constrained and not easily deformed, thereby helping to make the diaphragm soft and tough, and significantly improving the puncture resistance. Therefore, the diaphragm prepared by the preparation method of the present application has high impact resistance, puncture resistance, and heat resistance, thereby helping to reduce damage to the pole piece, and helping to reduce battery thermal runaway caused by impact and puncture, thereby helping to enhance the safety performance of the battery cell. In addition, the preparation process and operation method of the present application are simple and the preparation cost is low.

[0023] In one embodiment of the present application, the resulting impregnated absorbent can be coated on one or both sides of a base film.

[0024] In one embodiment of the present application, the mass ratio of the above-mentioned cellulose powder and the shear thickening fluid is 1:(1-2), specifically any one or more of 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, and 1:2.

[0025] The porous structure of cellulose is conducive to the rapid infiltration of electrolyte and the transmission of ions, while the shear thickening fluid helps to maintain the structural stability of the diaphragm, reduce electrolyte leakage, and help improve the toughness of the diaphragm. Controlling the mass ratio of cellulose and shear thickening fluid within the above range helps to improve the liquid absorption rate and liquid retention rate of the diaphragm while making the diaphragm have higher impact resistance, puncture resistance and heat resistance.

[0026] In one embodiment of the present application, the shear thickening fluid comprises solid particles, a dispersant and a solvent; preferably, the solid particles are selected from any one or more of silica particles, polystyrene particles and calcium carbonate particles; further preferably, the solid particles are silica particles.

[0027] Compared with directly mixing silica particles with cellulose, mixing silica particles in a shear thickening fluid with cellulose helps to improve the uniformity of the distribution of silica particles in cellulose, thereby helping to further improve the cellulose's resistance to impact deformation and thermal shrinkage; and, compared with polystyrene particles and calcium carbonate particles, silica particles have higher thermal stability and chemical stability. In a high temperature environment, they can maintain the integrity of the diaphragm structure, reduce the short circuit of the positive and negative electrodes caused by thermal shrinkage, and help to improve the heat resistance of the diaphragm.

[0028] In one embodiment of the present application, the mass ratio of the solid particles, the dispersant and the solvent is (0.5-0.65):(0.01-0.05):1.

[0029] The addition of a dispersant helps improve the uniformity of solid particle distribution in the solvent and reduce agglomeration. It also helps impart excellent shear fluid properties to the diaphragm coating, thereby improving the microstructural consistency of the diaphragm and imparting excellent puncture and impact resistance across the entire diaphragm surface. Controlling the mass ratio of solid particles, dispersant, and solvent within the aforementioned range further improves the uniformity of solid particle distribution in the solvent, thereby improving the uniformity of the shear thickening fluid in the cellulose, and further enhancing the cellulose's resistance to impact deformation and thermal shrinkage.

[0030] In one embodiment of the present application, the average particle size of the solid particles is 200 to 500 nm; and / or the specific surface area of ​​the solid particles is 50 to 400 m 2 / g; and / or, the average pore size of the solid particles is 2 to 10 nm; and / or, the dispersant is selected from any one or more of polyethylene glycol, polyvinyl pyrrolidone and sodium polyacrylate; and / or, the solvent is selected from any one or more of anhydrous ethanol, isopropanol and tetrahydrofuran.

[0031] Controlling the average particle size of the solid particles within the above range helps to enhance the hardness and wear resistance of the diaphragm. In particular, when subjected to needle puncture or extrusion, solid particles of the above size can more effectively disperse stress and improve the puncture resistance of the diaphragm. In addition, solid particles of the above size have high thermal stability, which can maintain the structural integrity of the diaphragm when subjected to high temperatures, reduce the shrinkage or melting of the diaphragm, and thus help reduce the risk of short circuits between the positive and negative electrodes. Controlling the specific surface area and average pore size of the solid particles within the above range helps, on the one hand, to improve the adsorption capacity of the diaphragm for the electrolyte, thereby improving the wettability and liquid retention rate of the battery, improving the charge and discharge efficiency and cycle life of the battery; on the other hand, it helps to improve the electrochemical stability of the diaphragm and reduce the adsorption of lithium ions by the diaphragm during the charge and discharge process. Controlling the type of dispersant within the above range helps to improve the uniformity of the dispersion of the solid particles in the solvent and maintain the shear fluid properties of the coating in the diaphragm. Controlling the type of solvent within the above range helps to improve the stability of the suspension. The above solvent has a low boiling point and is easy to remove.

[0032] In one embodiment of the present application, the number average molecular weight of the dispersant is 200 to 2000 g / mol.

[0033] The dispersant with the number average molecular weight mentioned above is in liquid form, which helps the coating on the diaphragm to maintain good shear fluid properties.

[0034] In one embodiment of the present application, the dispersant is polyethylene glycol.

[0035] As a dispersant, polyethylene glycol (PEG) possesses both good hydrophilicity and moderate hydrophobicity. This effectively stabilizes solid particles in a suspension, preventing them from settling and improving the uniformity of their dispersion in the solvent. The flexibility of the PEG chain allows it to wrap around the surface of solid particles, forming a protective layer. This not only helps secure the particles to the base film but also strengthens the adhesion between the coating and the base film, improving the coating's stability during battery assembly and use.

[0036] In one embodiment of the present application, the number average molecular weight of the cellulose powder is 50,000 to 2,500,000 g / mol; and / or the average length of the nanowhiskers of the cellulose powder is 100 to 500 nm; preferably, the cellulose powder is selected from bacterial cellulose and / or sulfonated cellulose powder; further preferably, the cellulose powder is sulfonated cellulose powder, and most preferably, the degree of substitution of sulfonic acid groups on cellulose molecules in the sulfonated cellulose powder is 2% to 10%.

[0037] Sulfonated cellulose, especially when the degree of substitution is within the above range, helps to improve the thermal stability and chemical stability of the diaphragm while improving the liquid retention rate of the diaphragm. This is because the sulfonation reaction increases the sulfonic acid groups on the cellulose molecular chain, making the diaphragm less likely to decompose under high temperature or strong electrolyte environments, enhancing the adsorption capacity of the electrolyte and durability in harsh environments. In addition, controlling the degree of substitution of sulfonic acid groups on cellulose molecules in the sulfonated cellulose powder within the above range helps to balance the mechanical strength and ion conductivity of the diaphragm, thereby helping to improve the rate performance and cycle stability of the battery during the charge and discharge process. Controlling the number average molecular weight of the cellulose powder and the average length of the nanowhiskers within the above range helps to form a tighter network structure, enhance the mechanical properties of the diaphragm, make it less likely to break when subjected to external forces, and help to improve the puncture resistance and impact resistance of the diaphragm.

[0038] In one embodiment of the present application, the coating surface density in the above coating is 0.1 to 0.4 mg / cm 2 ; and / or, the thickness of the base film is 7 to 10 μm; and / or, the porosity of the base film is 40 to 60%; and / or, the drying temperature is 50 to 80°C; and / or, the drying time is 1 to 24 hours; preferably, the base film is selected from polyethylene film and / or polypropylene film.

[0039] Controlling the coating surface density during coating within the above range helps to improve the puncture resistance and impact resistance of the diaphragm while improving the ion transmission efficiency. Controlling the thickness of the base membrane within the above range helps to improve the mechanical strength of the diaphragm without sacrificing the energy density and electrochemical performance of the battery. Controlling the porosity of the base membrane within the above range helps to improve the ion transmission efficiency and the liquid retention rate of the diaphragm. Controlling the drying temperature and time within the above range helps to remove the solvent in the coating without destroying the structure of the base membrane and the coating material, retaining the liquid dispersant and solid particles in the coating, so that the coating still has shear fluid properties.

[0040] In one embodiment of the present application, the above-mentioned coating is gravure coating; preferably, the gravure coating roller speed ratio in the gravure coating is (0.9-1.2):1, the scraper pressure is 0.2-1.0 MPa, and the vehicle speed is 50-200 m / min.

[0041] As a coating method that precisely controls coating thickness, gravure coating is particularly suitable for applications requiring a uniform, precisely thick coating. This technology utilizes grooves engraved on the roller surface to achieve a stable and continuous coating process, helping to improve coating uniformity and, in turn, the consistency of the physical and electrochemical properties of the separator. Controlling the gravure coating roller speed ratio, blade pressure, and vehicle speed within the aforementioned ranges during gravure coating helps improve the uniformity of the coating distribution, thereby enhancing the separator's impact resistance, puncture resistance, and heat resistance.

[0042] In one embodiment of the present application, a method for preparing the shear thickening fluid comprises: stirring a solvent, a dispersant, and particles at a speed of 1500 to 3000 rpm for 1 to 3 hours, and then placing the mixture in a vacuum drying oven at 60°C for 24 hours to remove excess bubbles, thereby obtaining the shear thickening fluid. The particles are selected from any one or more of silica particles, polystyrene particles, and calcium carbonate particles.

[0043] In another typical embodiment of the present application, a diaphragm is provided, which is prepared by the above-mentioned preparation method; preferably, the tensile strength of the diaphragm is 1200-1500 kgf / cm 2 and / or, the puncture strength of the membrane under a needle diameter of 1.0 mm Φ is 4 to 7 N / m; and / or, the shrinkage rate of the membrane when placed at 130°C for 1 hour is 0.6 to 1.2%.

[0044] Because the above-mentioned separator is prepared using the preparation method of the present application, it has high impact resistance, puncture resistance, and heat resistance. The use of the separator with the above-mentioned tensile strength, needle puncture strength, and shrinkage rate in secondary batteries helps to further improve the safety of the battery.

[0045] In another typical embodiment of the present application, a secondary battery is provided, including a positive electrode sheet, an electrolyte, a separator and a negative electrode sheet, wherein the separator is the aforementioned separator.

[0046] Since the secondary battery contains the diaphragm prepared by the preparation method of the present application, the electrode in the secondary battery is less damaged and is not prone to thermal runaway, which helps to improve the safety performance of the secondary battery.

[0047] The beneficial effects of the present application will be further illustrated below with reference to examples.

[0048] Example 1

[0049] The silica particles (average particle size of 300 nm, specific surface area of ​​200 m 2 / g, average pore size of 6 nm), polyethylene glycol (number average molecular weight of 200 g / mol) and anhydrous ethanol were stirred at 2000 rpm for 2 h, with the mass ratio of silica particles, polyethylene glycol and anhydrous ethanol being 0.6:0.03:1, and then placed in a vacuum drying oven at 60°C for 24 h to obtain a shear thickening fluid. Sulfonated cellulose powder (purchased from Nanjing Tianlu Nanotechnology Co., Ltd., with a molecular weight of 500,000 g / mol, a degree of substitution of cellulose molecules by sulfonic acid groups of 6%, and an average length of nanowhiskers of 300 nm) was immersed and absorbed in the shear thickening fluid prepared above, with a mass ratio of sulfonated cellulose powder to shear thickening fluid of 1:1.5. The obtained immersed absorbent was then coated on one side of a polyethylene film (8 μm thick, 50% porosity), wherein the coating was gravure coating, the gravure coating roller speed ratio in the gravure coating was 1.0:1, the doctor blade pressure was 0.5 MPa, the vehicle speed was 100 m / min, and the coating surface density was 0.3 mg / cm 2 , and dried at 60°C for 12 h to obtain a separator.

[0050] Example 2

[0051] The difference from Example 1 is that the mass ratio of the sulfonated cellulose powder to the shear thickening fluid is 1:1, and a diaphragm is finally obtained.

[0052] Example 3

[0053] The difference from Example 1 is that the mass ratio of the sulfonated cellulose powder to the shear thickening fluid is 1:2, and a diaphragm is finally obtained.

[0054] Example 4

[0055] The difference from Example 1 is that the mass ratio of the sulfonated cellulose powder to the shear thickening fluid is 1:3, and a diaphragm is finally obtained.

[0056] Example 5

[0057] The difference from Example 1 is that calcium carbonate is used to replace silicon dioxide to finally obtain a diaphragm.

[0058] Example 6

[0059] The difference from Example 1 is that the number average molecular weight of polyethylene glycol is 2000 g / mol, and a diaphragm is finally obtained.

[0060] Example 7

[0061] The difference from Example 1 is that the number average molecular weight of polyethylene glycol is 4000 g / mol, and a diaphragm is finally obtained.

[0062] Example 8

[0063] The difference from Example 1 is that the average particle size of the silica particles is 200 nm, and the specific surface area of ​​the silica particles is 50 m 2 / g, the average pore size of the silica particles was 2nm, and a separator was finally obtained.

[0064] Example 9

[0065] The difference from Example 1 is that the average particle size of the silica particles is 500 nm, and the specific surface area of ​​the silica particles is 400 m 2 / g, the average pore size of the silica particles was 10 nm, and a separator was finally obtained.

[0066] Example 10

[0067] The difference from Example 1 is that the average particle size of the silica particles is 100 nm, and the specific surface area of ​​the silica particles is 30 m 2 / g, the average pore size of the silica particles was 1 nm, and a separator was finally obtained.

[0068] Example 11

[0069] The difference from Example 1 is that bacterial cellulose powder (purchased from Tianjin Cellus Biotechnology Co., Ltd.) is used to replace the sulfonated cellulose powder, and finally a diaphragm is obtained.

[0070] Example 12

[0071] The difference from Example 1 is that the molecular weight of the sulfonated cellulose powder is 50,000 g / mol, the degree of substitution of cellulose molecules by sulfonic acid groups in the sulfonated cellulose powder is 2%, and the average length of the nanowhiskers of the sulfonated cellulose powder is 100 nm. Finally, a diaphragm is obtained.

[0072] Example 13

[0073] The difference from Example 1 is that the molecular weight of the sulfonated cellulose powder is 2500000 g / mol, the degree of substitution of cellulose molecules by sulfonic acid groups in the sulfonated cellulose powder is 10%, and the average length of the nanowhiskers of the sulfonated cellulose powder is 500 nm. Finally, a diaphragm is obtained.

[0074] Example 14

[0075] The difference from Example 1 is that the molecular weight of the sulfonated cellulose powder is 3,000,000 g / mol, the degree of substitution of cellulose molecules by sulfonic acid groups in the sulfonated cellulose powder is 1%, and the average length of the nanowhiskers of the sulfonated cellulose powder is 50 nm. Finally, a diaphragm is obtained.

[0076] Example 15

[0077] The difference from Example 1 is that the silicon dioxide particles (average particle size of 300 nm, specific surface area of ​​200 m 2 / g, with an average pore size of 6 nm), polyethylene glycol (number average molecular weight of 200 g / mol) and anhydrous ethanol were stirred at 3000 rpm for 1 h, with the mass ratio of silica particles, polyethylene glycol and anhydrous ethanol being 0.5:0.01:1, and then placed in a vacuum drying oven at 60°C for 24 h to obtain a shear thickening fluid. Sulfonated cellulose powder (data molecular weight of 500,000 g / mol, degree of substitution of cellulose molecules by sulfonic acid groups of 6, and average length of nanowhiskers of 300 nm) was immersed and absorbed in the shear thickening fluid prepared above, with the mass ratio of sulfonated cellulose powder to shear thickening fluid being 1:1.5. The resulting immersed absorbent was then coated on both sides of a polyethylene film (thickness of 10 μm, porosity of 40%), wherein the coating was performed by gravure coating, wherein the gravure coating roller speed ratio was 0.9:1, the doctor blade pressure was 0.2 MPa, the vehicle speed was 50 m / min, and the coating surface density was 0.4 mg / cm 2 , and dried at 80 °C for 1 h to obtain a separator.

[0078] Example 16

[0079] The difference from Example 1 is that the silicon dioxide particles (average particle size of 300 nm, specific surface area of ​​200 m 2 / g, average pore size of 6 nm), polyethylene glycol (number average molecular weight of 200 g / mol) and anhydrous ethanol were stirred at 1500 rpm for 3 h, with the mass ratio of silica particles, polyethylene glycol and anhydrous ethanol being 0.65:0.05:1, and then placed in a vacuum drying oven at 60°C for 24 h to obtain a shear thickening fluid. Sulfonated cellulose powder (data molecular weight of 1,000,000 g / mol, degree of substitution of cellulose molecules by sulfonic acid groups of 6%, and average length of nanowhiskers of 300 nm) was immersed and absorbed in the shear thickening fluid prepared above, with the mass ratio of sulfonated cellulose powder to shear thickening fluid being 1:1.5. The obtained immersed absorbent was then coated on one side of a polyethylene film (thickness of 7 μm, porosity of 60%), wherein the coating was performed by gravure coating, wherein the gravure coating roller speed ratio was 1.2:1, the doctor blade pressure was 1.0 MPa, the vehicle speed was 200 m / min, and the coating surface density was 0.1 mg / cm 2 , and dried at 50°C for 24 h to obtain a separator.

[0080] Comparative Example 1

[0081] The difference from Example 1 is that the sulfonated cellulose powder is not immersed in the shear thickening fluid prepared above for absorption. The sulfonated cellulose powder is directly mixed with anhydrous ethanol to prepare a slurry, which is coated on one side of a polyethylene film (thickness 8 μm, porosity 50%). The coating is gravure coating, and the gravure coating roller speed ratio in the gravure coating is 1.0:1, the doctor blade pressure is 0.5 MPa, the vehicle speed is 100 m / min, and the coating surface density is 0.3 mg / cm 2 , and dried at 60°C for 12 h to obtain a separator.

[0082] Comparative Example 2

[0083] The difference from Example 1 is that the immersion and absorption of the sulfonated cellulose powder in the shear thickening fluid prepared above is omitted. The shear thickening fluid is first coated on one side of a polyethylene film (8 μm thick, 50% porosity) and dried at 60° C. to form a coating. The sulfonated cellulose powder is then mixed with anhydrous ethanol to prepare a slurry, which is then coated on the surface of the coating. The coating is performed by gravure coating, in which the gravure coating roller speed ratio is 1.0:1, the blade pressure is 0.5 MPa, the vehicle speed is 100 m / min, and the coating surface density is 0.3 mg / cm 2 , and dried at 60°C for 12 h to obtain a separator.

[0084] Comparative Example 3

[0085] The difference from Example 1 is that the preparation of the shear thickening fluid is eliminated, and the silica particles, sulfonated cellulose powder and anhydrous ethanol are directly mixed to obtain a slurry, which is coated on one side of a polyethylene film (8 μm thick, 50% porosity). The coating is gravure coating, and the gravure coating roller speed ratio in the gravure coating is 1.0:1, the scraper pressure is 0.5 MPa, the vehicle speed is 100 m / min, and the coating surface density is 0.3 mg / cm 2 , and dried at 60°C for 12 h to obtain a separator.

[0086] Performance Testing

[0087] The separator was tested according to the GB / T 36363-2018 standard. ① Puncture strength: The diameter of the needle was 1.0 mm Φ. The lithium battery separator was flattened and clamped in the MTL (PC) tensile testing machine fixture. The puncture was performed at a rate of (100 + 10) mm / min. After completion, the sample was removed. According to the provisions of GB / T6672-2001, the thickness was tested at 4 points around the pinhole as required. The puncture strength was calculated after taking the average value. ② Tensile strength: The test was carried out in accordance with the provisions of GB / T1040.3-2006. A type 2 specimen with a width of (15 + 0.1) mm was used. The initial distance between the clamps of the MTL (PC) tensile testing machine was (100 + 5) mm, and the test speed was (250 + 10) mm / min. ③ Shrinkage rate: Cut the lithium battery separator sample into 15mm×130mm long strips, punch holes at both ends with a hole puncher so that they can be mounted on the instrument. The straight-line distance between the two holes is 100mm. Clamp the sample to the fixture of the thermal shrinkage rate tester to ensure that the sample is flat. The equipment starts to heat up. When the temperature in the test chamber reaches 130℃, send the sample into the test chamber and heat it for 1 hour. The sample shrinks due to heat. The equipment measures the real-time shrinkage force and shrinkage rate through force sensors and displacement sensors. The thermal shrinkage rate of the lithium battery separator sample is calculated based on the recorded data.

[0088] The diaphragms prepared in the examples and comparative examples were tested for tensile strength, needle puncture strength, and shrinkage. The diaphragms were assembled with lithium iron phosphate positive electrodes, graphite negative electrodes, and lithium hexafluorophosphate electrolytes into 96Ah square-shell batteries. The initial discharge capacity and capacity retention after 1000 cycles were tested at 0.5C. The test results are shown in Table 1.

[0089] Table 1

[0090]

[0091]

[0092] It can be seen from the results in Table 1 that the separator of the present application has higher tensile strength, needle puncture strength and lower thermal shrinkage, which helps to improve the cycle stability of the battery.

[0093] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0094] Cellulose has high heat resistance, absorbs electrolyte quickly, and has a high liquid retention rate. The cellulose powder is immersed in a shear thickening fluid for absorption. The shear thickening fluid and the cellulose are strongly bonded, which helps to improve the uniformity of the shear thickening fluid distribution in the cellulose. The shear thickening fluid has an adhesive effect on the cellulose molecules, thereby helping to improve the cellulose's resistance to impact deformation and thermal shrinkage. The resulting mixture is coated on at least one side of the base film and dried to obtain a diaphragm. The coating on the diaphragm has shear fluid properties. When punctured or impacted, the shear fluid will suddenly increase in viscosity, forming a tight structure between the particles, so that the diaphragm is constrained and not easily deformed, thereby helping to make the diaphragm soft and tough, and significantly improving the puncture resistance. Therefore, the diaphragm prepared by the preparation method of the present application has high impact resistance, puncture resistance, and heat resistance, thereby helping to reduce damage to the pole piece, and helping to reduce battery thermal runaway caused by impact and puncture, thereby helping to enhance the safety performance of the battery cell. In addition, the preparation process and operation method of the present application are simple and the preparation cost is low.

[0095] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for preparing a diaphragm, characterized in that: The preparation method comprises: The cellulose powder is impregnated and absorbed in a shear thickening fluid, and then the obtained impregnated absorbent is coated on at least one side of a base film, and the separator is obtained after drying.

2. The preparation method according to claim 1, characterized in that The mass ratio of the cellulose powder to the shear thickening fluid is 1:(1-2).

3. The preparation method according to claim 1 or 2, characterized in that The shear thickening fluid comprises solid particles, a dispersant and a solvent; preferably, the solid particles are selected from any one or more of silicon dioxide particles, polystyrene particles and calcium carbonate particles; further preferably, the solid particles are silicon dioxide particles.

4. The preparation method according to claim 3, characterized in that The mass ratio of the solid particles, the dispersant and the solvent is (0.5-0.65):(0.01-0.05):

1.

5. The preparation method according to claim 3 or 4, characterized in that The average particle size of the solid particles is 200 to 500 nm; and / or the specific surface area of ​​the solid particles is 50 to 400 m 2 / g; and / or, the dispersant is selected from any one or more of polyethylene glycol, polyvinyl pyrrolidone and sodium polyacrylate; And / or, the solvent is selected from any one or more of anhydrous ethanol, isopropanol and tetrahydrofuran.

6. The preparation method according to any one of claims 1 to 5, characterized in that The number average molecular weight of the cellulose powder is 50,000 to 2,500,000 g / mol; and / or the average length of the nanowhiskers of the cellulose powder is 100 to 500 nm; preferably, the cellulose powder is selected from bacterial cellulose and / or sulfonated cellulose powder; further preferably, the cellulose powder is sulfonated cellulose powder, and most preferably, the degree of substitution of sulfonic acid groups on cellulose molecules in the sulfonated cellulose powder is 2% to 10%.

7. The preparation method according to any one of claims 1 to 6, characterized in that The coating surface density in the coating is 0.1 to 0.4 mg / cm 2 ; and / or, the thickness of the base film is 7 to 10 μm; and / or, the porosity of the base film is 40 to 60%; and / or, the drying temperature is 50 to 80° C.; and / or, the drying time is 1 to 24 hours; Preferably, the base film is selected from polyethylene film and / or polypropylene film.

8. The preparation method according to any one of claims 1 to 7, characterized in that The coating is gravure coating; preferably, in the gravure coating, the speed ratio of the gravure coating roller is (0.9-1.2):1, the blade pressure is 0.2-1.0 MPa, and the vehicle speed is 50-200 m / min.

9. A diaphragm, characterized in that: The diaphragm is prepared by the preparation method according to any one of claims 1 to 8; preferably, the tensile strength of the diaphragm is 1200-1500 kgf / cm 2 and / or, the puncture strength of the membrane under a needle diameter of 1.0 mm Φ is 4 to 7 N / m; and / or, the shrinkage rate of the membrane when placed at 130°C for 1 hour is 0.6 to 1.2%.

10. A secondary battery comprising a positive electrode, an electrolyte, a separator and a negative electrode, characterized in that: The diaphragm is the diaphragm according to claim 9.

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