Preparation method of diaphragm for improving impact resistance of battery

Abstract

The invention relates to the technical field of diaphragms, and discloses a preparation method of a diaphragm for improving the impact resistance of a battery. Comprising the following operation steps: S1) respectively adding a barrier material and sodium alginate into deionized water to obtain a solution A and a solution B; (2) uniformly mixing the solution A and the solution B, adding nanocellulose fibrils and the solution A, performing ultrasonic treatment for 5-10 minutes at the frequency of 80-100 kHz, and performing hydrothermal reaction for 5-6 hours at the temperature of 70-80 DEG C to obtain nanocellulose fibrils coated with a barrier layer; s2, uniformly mixing the modified polytetrafluoroethylene filler and the nano cellulose fibril coated with the barrier layer with polyolefin particles, melting, plasticizing, extruding at low temperature, cooling, and then sizing and annealing to obtain a sheet polyolefin base film; and S3, performing two-way stretching, high-temperature shaping and cooling on the sheet polyolefin base film to obtain the diaphragm.

Description

Technical Field

[0001] The present invention relates to the technical field of battery separators, and specifically to a method for preparing a separator for improving the impact resistance of a battery. Background Art

[0002] In recent years, the new energy industry has gradually replaced traditional energy in many fields. Lithium battery vehicles with high energy density and excellent cycle performance have entered thousands of households. However, during the driving process of the battery, accidental collisions and squeezes will pose great challenges to lithium batteries. Instantaneous impacts usually cause the battery to smoke, catch fire or even explode.

[0003] When the battery undergoes large-area extrusion, when the extrusion force reaches a certain level, among the main failure reasons for the battery to catch fire and explode, an important point is that the separator is extruded; during extrusion, its micropores are pulled and enlarged, resulting in direct contact short circuit between the positive and negative active substances. And because the thickness of the separator becomes thinner, short lithium dendrites may also pierce it due to strong extrusion. Therefore, there is an urgent need to develop a battery separator that can effectively prevent the battery from catching fire and exploding due to instantaneous impact. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing a separator for improving the impact resistance of a battery to solve the problems raised in the prior art.

[0005] To achieve the above purpose, the present invention provides the following technical solutions: A method for preparing a separator for improving the impact resistance of a battery, comprising the following operating steps: S1: (1) Add a barrier material and sodium alginate to deionized water respectively to obtain solution A and solution B; (2) Uniformly mix solution A and solution B, add nanofibrillated cellulose and solution A, and perform ultrasonic treatment at a frequency of 100 kHz for 5-10 minutes, and then perform hydrothermal reaction at 70-80 °C for 5-6 hours to obtain nanofibrillated cellulose coated with a barrier layer; S2: Uniformly mix the modified polytetrafluoroethylene filler and the nanofibrillated cellulose coated with a barrier layer with polyolefin particles, melt and plasticize, extrude at low temperature, cool and then perform shaping annealing to obtain a sheet-shaped polyolefin-based film; S3: Biaxially stretch, high-temperature shape and cool the sheet-shaped polyolefin-based film to obtain a separator.

[0006] Preferably, the concentration of solution A is 1-2 wt%, and the concentration of solution B is 3 wt%; the raw materials of the nanofibrillated cellulose coated with a barrier layer include the following components: by mass, 70-120 parts of solution A, 15-30 parts of solution B, and 5-20 parts of nanofibrillated cellulose; The raw materials of the sheet-like polyolefin-based film include the following components: by mass, 5 to 20 parts of modified polytetrafluoroethylene, 5 to 30 parts of nanofibrillated cellulose encapsulated by a barrier layer, and 80 to 100 parts of polyolefin particles.

[0007] More preferably, in S2, the temperature for melt plasticization is 180 to 240 °C; the temperature for low-temperature extrusion is 190 to 220 °C; the temperature for shaping and annealing is 100 to 120 °C; in S3, the temperature for high-temperature shaping is 130 to 150 °C.

[0008] More preferably, the preparation method of the nanofibrillated cellulose is as follows: adding plant cellulose into a nitric acid solution and mixing evenly, reacting at 15 to 30 °C for 6 to 12 hours, adjusting the pH to neutral, washing with deionized water, and drying to obtain oxidized cellulose; preparing a 0.2 wt% cellulose suspension, performing high-intensity microfluidic homogenization treatment on it, and drying to obtain nanofibrillated cellulose.

[0009] More preferably, the concentration of the nitric acid solution is 1 to 2 wt%; the aspect ratio of the oxidized cellulose is 5 to 50; the pressure of the high-intensity microfluidic jet is 70 to 200 MPa; the aspect ratio of the nanofibrillated cellulose is 50 to 1000.

[0010] More preferably, the barrier material includes one of guar gum and its derivatives, and arabic gum and its derivatives.

[0011] More preferably, the preparation method of the modified polytetrafluoroethylene filler is as follows: (1) adding a coupling agent into absolute ethanol and mixing, adding polytetrafluoroethylene resin, glass fiber, and graphite and stirring for 1 to 1.5 hours to obtain a mixture; (2) sintering the mixture at 300 °C, then adding it into a zinc acetate ethanol solution, ball-milling and mixing for 50 to 60 minutes, and heat-treating at 100 to 150 °C for 4 to 5 hours to obtain a zinc-polytetrafluoroethylene filler; (3) adding the zinc-polytetrafluoroethylene filler into absolute ethanol and mixing evenly, adding an imidazole-based blend under ultrasonic conditions of 80 to 100 W, and washing with ethanol 1 to 2 times to obtain a modified polytetrafluoroethylene filler.

[0012] More preferably, the raw materials of the mixture include the following components: by mass, 85 to 95 parts of polytetrafluoroethylene, 1 to 5 parts of glass fiber, 1 to 5 parts of graphite, and 0.1 to 0.3 parts of coupling agent; the concentration of the zinc acetate ethanol solution is 0.26 to 0.42 wt%, the concentration of the zinc-polytetrafluoroethylene filler is 0.2 to 0.35 wt%, and the concentration of the imidazole-based blend is 4 to 6 wt%.

[0013] More preferably, the method for preparing the imidazole-based blend is as follows: (1) Potassium carbonate is added to DMF and uniformly mixed, followed by the addition of tannic acid and 5,6-dichlorobenzimidazole. The mixture is heated at 130-140 °C for 16-18 hours, cooled to room temperature, and filtered to obtain a filtrate. The filtrate is concentrated under reduced pressure, deionized water is added, and the mixture is stirred in an ice-water bath for 1-2 hours, followed by filtration, washing, drying, and purification to obtain benzimidazole-based tannic acid; (2) The benzimidazole-based tannic acid and 2-methylimidazole are added to ethanol and uniformly mixed, and then dried to obtain the imidazole-based blend.

[0014] More preferably, the raw materials of the benzimidazole-based tannic acid include the following components: by mass, 3-4 parts of tannic acid, 0.2-0.4 parts of potassium carbonate, and 0.3-0.5 parts of 5,6-dichlorobenzimidazole; the mass ratio of the 2-methylimidazole to the benzimidazole-based tannic acid is (2-4):1.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: In the present invention, nanofibrillated cellulose with a large aspect ratio and modified polytetrafluoroethylene are used to fill the polyolefin separator base film to improve the tensile strength and impact strength of the separator, reduce the deformation of the separator in the plane direction when the battery is squeezed, and reduce the deformation of the micropores of the separator. Fluorine in polytetrafluoroethylene is a polar atom, and the arrangement density of fluorine substituents on the main chain is very high, resulting in difficult internal rotation and large rigidity. The intermolecular force of cellulose is strong, and the six-membered pyranose ring structure makes internal rotation difficult. At the same time, hydrogen bonds are formed within and between molecules, so it has large rigidity. The nanofibrillated cellulose with a large aspect ratio can be intertwined within the base film to further improve the strength of the base film. At the same time, the nanofibrillated cellulose selected in this scheme is coated with a barrier layer, which can improve the strength of the cellulose fibrils and provide good barrier performance, avoiding swelling of the separator in the electrolyte during normal use and increasing the internal resistance of the battery. When the battery is impacted or squeezed, the cross-section exposed after the cellulose fibrils are pulled and broken will quickly absorb liquid and swell, increasing the thickness of the separator, reducing the distance between the positive and negative electrodes, and reducing the risk of short circuit.

[0016] In the scheme, polytetrafluoroethylene resin, glass fiber, and graphite are uniformly mixed and then calcined at 300 °C to obtain the modified polytetrafluoroethylene filler. Among them, glass fiber can improve the tensile strength and pressure resistance of the separator, making the separator not easily deformed or broken during use. Moreover, the glass fiber has high thermal stability and can improve the high-temperature resistance of the powder, which is beneficial to improving the safety of the battery. In the solution, graphite is added to the modified polytetrafluoroethylene filler to improve the conductivity and mechanical properties of the separator, enhance the safety of the battery while reducing the internal resistance. Due to the strong hydrophobicity of polytetrafluoroethylene, it is calcined at 300 °C to make the surface of the modified polytetrafluoroethylene filler rougher or increase the pore structure, so that it has more contact points with the electrolyte, improving the wetting effect and thus optimizing the internal resistance. However, its effect is limited. To further improve the performance of the separator, in the solution, polytetrafluoroethylene resin, glass fiber, and graphite are uniformly mixed and then calcined at 300 °C, and then ball-milled and heat-treated with zinc acetate solution to obtain zinc-polytetrafluoroethylene. Zinc ions are coated on its surface, and then 2-methylimidazole is added to coat a layer of MOFs material on the surface of zinc-polytetrafluoroethylene. This MOFs material can improve the hydrophobicity of polytetrafluoroethylene and the impact resistance of the separator.

[0017] To further improve the wetting effect and impact resistance of the separator, in the solution, 2-methylimidazole and benzimidazole-based tannic acid are mixed in a certain proportion to form an imidazole mixture, and then coordinated with the zinc on zinc-polytetrafluoroethylene to obtain a modified polytetrafluoroethylene filler.

[0018] In the solution, using DMF as the solvent and potassium carbonate as the catalyst, the hydroxyl group on tannic acid undergoes a nucleophilic addition reaction with the halogen on 5,6-dichlorobenzimidazole to form an ether bond, obtaining benzimidazole tannic acid. Among them, the rigid structure of benzimidazole can act as a ligand to form a coordination bond with metal ions, thereby enhancing the mechanical strength and thermal stability of the MOFs material on the surface of zinc-polytetrafluoroethylene. The active groups on tannic acid can improve the interfacial compatibility between the modified polytetrafluoroethylene filler, polyolefin particles, and nanofibrillated cellulose filaments coated with the barrier layer, as well as the wettability of the separator, so it can improve the mechanical properties and reduce the internal resistance. Specific embodiments

[0019] In the following specific embodiments, "parts" refers to parts by mass. The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0020] The preparation method of the imidazole-based blend is as follows: (1) Add 0.3 parts of potassium carbonate to DMF and mix evenly, then add 3 parts of tannic acid and 0.4 parts of 5,6-dichlorobenzimidazole, heat at 140 °C for 18 hours, cool to room temperature, and filter to obtain a filtrate; Concentrate the filtrate under reduced pressure, add deionized water, stir for 2 hours in an ice-water bath, filter, wash, and dry to obtain a crude product; Add the crude product and activated carbon to methanol and reflux for 2 hours, filter hot with diatomite, distill the filtrate under reduced pressure, stir at room temperature for 2 hours, stir in an ice-water bath for 1 hour, filter, wash 3 times with ice methanol, and dry at 60 °C to obtain benzimidazole-based tannic acid; (2) Add benzimidazole-based tannic acid and 2-methylimidazole to ethanol, mix evenly, and dry to obtain the imidazole-based blend; The mass ratio of 2-methylimidazole to imidazole-based tannic acid is 3:1.

[0021] Example 1: A method for preparing a separator for improving the impact resistance of a battery, including the following operating steps; S1: (1) Add plant cellulose to a 2 wt% nitric acid solution and mix evenly, react at 25 °C for 10 hours, adjust the pH to neutral, wash with deionized water to remove residual acid and other impurities, and dry to obtain oxidized cellulose with an aspect ratio of (5 - 50); Add the oxidized cellulose to deionized water to prepare a 0.2 wt% cellulose suspension, and perform high-intensity micro-jet homogenization treatment on it under a pressure of 200 MPa, and dry to obtain nanofibrillated cellulose with an aspect ratio of 50 - 1000; (2) Add hydroxypropyl guar gum and sodium alginate to deionized water respectively to obtain solution A with a concentration of 2 wt% and solution B with a concentration of 3 wt%; (3) Mix 40 parts of solution A and 30 parts of solution B evenly, add 10 parts of nanofibrillated cellulose, 30 parts of solution A, perform ultrasonic treatment at a frequency of 100 kHz for 10 minutes, and then perform hydrothermal reaction at 80 °C for 6 hours, and dry to obtain nanofibrillated cellulose coated with a barrier layer; S2: (1) Add 0.2 parts of coupling agent to anhydrous ethanol and mix evenly, add 90 parts of polytetrafluoroethylene resin, 3 parts of glass fiber, and 3 parts of graphite and mix evenly for 1 hour, and dry to obtain a mixture; Calcinate the mixture at 300 °C for 2 hours, and then perform ball milling and mixing to obtain modified polytetrafluoroethylene filler; (2) Mix 10 parts of modified polytetrafluoroethylene filler, 15 parts of nanofibrillated cellulose coated with a barrier layer, and 80 parts of polypropylene particles (polypropylene particles, model: F5006, Yanshan Petrochemical) evenly, melt and plasticize, extrude at low temperature, cool, and then perform shaping and annealing to obtain a sheet-like polypropylene-based film; Among them, the temperature of melt plasticization is 220 °C; The temperature of low-temperature extrusion is 200 °C; The temperature of shaping and annealing is 110 °C; S3: Biaxially stretch, heat-set at high temperature, and cool the sheet-like polyolefin-based film to obtain a separator; The temperature of high-temperature heat setting is 150 °C.

[0022] Example 2 is based on Example 1, the difference is that in S1(2), the concentration of solution A is 1 wt%, and the conditions of the hydrothermal reaction are to react at 40 °C for 2 hours; S1: (1) Add plant cellulose into 1 wt% nitric acid solution and mix evenly, react at 25 °C for 10 hours, adjust the pH to neutral, wash with deionized water to remove residual acid and other impurities, dry to obtain oxidized cellulose with an aspect ratio of (5 - 50); add the oxidized cellulose into deionized water to prepare a 0.2 wt% cellulose suspension, and perform high-strength micro-jet homogenization treatment on it under a pressure of 200 MPa, then dry to obtain nanofibrillated cellulose with an aspect ratio of 50 - 1000; (2) Add hydroxypropyl guar gum and sodium alginate into deionized water respectively to obtain 1 wt% solution A and 3 wt% solution B; (3) Mix 40 parts of solution A and 30 parts of solution B evenly, add 10 parts of nanofibrillated cellulose, 30 parts of solution A, perform ultrasonic treatment at a frequency of 100 kHz for 10 minutes, then perform hydrothermal reaction at 40 °C for 2 hours, and dry to obtain nanofibrillated cellulose coated with a barrier layer; S2: (1) Add 0.2 part of coupling agent into anhydrous ethanol and mix evenly, add 90 parts of polytetrafluoroethylene resin, 3 parts of glass fiber, and 3 parts of graphite and mix evenly for 1 hour, then dry to obtain a mixture; calcine the mixture at 300 °C for 2 hours, and then perform ball milling and mixing to obtain modified polytetrafluoroethylene filler; (2) Mix 10 parts of modified polytetrafluoroethylene filler and 15 parts of nanofibrillated cellulose coated with a barrier layer with 80 parts of polyolefin particles (polypropylene particles, model: F5006, Yanshan Petrochemical) evenly, perform melt plasticization, low-temperature extrusion, cooling, and then perform shaping and annealing to obtain a sheet-like polyolefin-based film; among them, the temperature of melt plasticization is 220 °C; the temperature of low-temperature extrusion is 200 °C; the temperature of shaping and annealing is 110 °C; S3: Biaxially stretch, high-temperature shape, and cool the sheet-like polyolefin-based film to obtain a separator; the temperature of high-temperature shaping is 150 °C.

[0023] Example 3 is based on Example 1, the difference is that in S1(1), the pressure of high-strength micro-jet homogenization treatment is 5 - 20 MPa, and the aspect ratio of nanofibrillated cellulose is 5 - 100; S1: (1) Add plant cellulose to 2 wt% nitric acid solution and mix evenly. React at 25 °C for 10 hours, adjust the pH to neutral, wash with deionized water to remove residual acid and other impurities, and dry to obtain oxidized cellulose with an aspect ratio of (5 - 50); Add the oxidized cellulose to deionized water to prepare a 0.2 wt% cellulose suspension, and perform high-strength microfluidic homogenization treatment on it under a pressure of 20 MPa, and dry to obtain nanofibrillated cellulose with an aspect ratio of 5 - 100; (2) Add hydroxypropyl guar gum and sodium alginate to deionized water respectively to obtain 2 wt% solution A and 3 wt% solution B; (3) Mix 40 parts of solution A and 30 parts of solution B evenly, add 10 parts of nanofibrillated cellulose, 30 parts of solution A, and perform ultrasonic treatment at a frequency of 100 kHz for 10 minutes, then perform hydrothermal reaction at 80 °C for 6 hours, and dry to obtain nanofibrillated cellulose coated with a barrier layer; S2: (1) Add 0.2 parts of coupling agent to anhydrous ethanol and mix evenly, add 90 parts of polytetrafluoroethylene resin, 3 parts of glass fiber, and 3 parts of graphite and mix evenly for 1 hour, and dry to obtain a mixture; Calcinate the mixture at 300 °C for 2 hours, and then perform ball milling and mixing to obtain modified polytetrafluoroethylene filler; (2) Mix 10 parts of modified polytetrafluoroethylene filler and 15 parts of nanofibrillated cellulose coated with a barrier layer with 80 polyolefin particles (polypropylene particles, model: F5006, Yanshan Petrochemical) evenly, melt and plasticize, extrude at low temperature, cool, and then perform shaping and annealing to obtain a sheet-like polyolefin-based film; Among them, the temperature of melt plasticization is 220 °C; the temperature of low-temperature extrusion is 200 °C; the temperature of shaping and annealing is 110 °C; S3: Biaxially stretch, heat-set at high temperature, and cool the sheet-like polyolefin-based film to obtain a separator; the temperature of high-temperature heat setting is 150 °C.

[0024] Example 4 is based on Example 1, the difference is that in S2(1), there are 80 parts of polytetrafluoroethylene resin, 15 parts of glass fiber, 10 parts of graphite, and 0.3 parts of coupling agent; S1: (1) Add plant cellulose to 2 wt% nitric acid solution and mix evenly. React at 25 °C for 10 hours, adjust the pH to neutral, wash with deionized water to remove residual acid and other impurities, and dry to obtain oxidized cellulose with an aspect ratio of (5 - 50); Add the oxidized cellulose to deionized water to prepare a 0.2 wt% cellulose suspension, and perform high-strength microfluidic homogenization treatment on it under a pressure of 200 MPa, and dry to obtain nanofibrillated cellulose with an aspect ratio of 50 - 1000; (2) Add hydroxypropyl guar gum and sodium alginate into deionized water respectively to obtain 2 wt% solution A and 3 wt% solution B; (3) Uniformly mix 40 parts of solution A and 30 parts of solution B, add 10 parts of nanofibrillated cellulose, and 30 parts of solution A, ultrasonically treat for 10 minutes at a frequency of 100 kHz, then hydrothermally react at 80 °C for 6 hours, and dry to obtain nanofibrillated cellulose coated with a barrier layer; S2: (1) Add 0.3 part of coupling agent into anhydrous ethanol and mix uniformly, add 80 parts of polytetrafluoroethylene resin, 15 parts of glass fiber, and 10 parts of graphite and mix uniformly for 1 hour, then dry to obtain a mixture; Calcinate the mixture at 300 °C for 2 hours, and then perform ball milling and mixing to obtain modified polytetrafluoroethylene filler; (2) Uniformly mix 10 parts of modified polytetrafluoroethylene filler and 15 parts of nanofibrillated cellulose coated with a barrier layer with 80 parts of polyolefin particles (polypropylene particles, model: F5006, Yanshan Petrochemical), melt and plasticize, extrude at low temperature, cool, and then perform shaping and annealing to obtain a sheet-like polyolefin-based film; Among them, the temperature of melt plasticization is 220 °C; the temperature of low-temperature extrusion is 200 °C; the temperature of shaping and annealing is 110 °C; S3: Biaxially stretch, high-temperature shape, and cool the sheet-like polyolefin-based film to obtain a separator; the temperature of high-temperature shaping is 150 °C.

[0025] Example 5 is based on Example 1, and the mixture is further modified after sintering; S1: (1) Add plant cellulose into 2 wt% nitric acid solution and mix uniformly, react at 25 °C for 10 hours, adjust the pH to neutral, wash with deionized water to remove residual acid and other impurities, and dry to obtain oxidized cellulose with an aspect ratio of (5 - 50); Add the oxidized cellulose into deionized water to prepare a 0.2 wt% cellulose suspension, and perform high-intensity microfluidic homogenization treatment on it under a pressure of 200 MPa, and dry to obtain nanofibrillated cellulose with an aspect ratio of 50 - 1000; (2) Add hydroxypropyl guar gum and sodium alginate into deionized water respectively to obtain 2 wt% solution A and 3 wt% solution B; (3) Uniformly mix 40 parts of solution A and 30 parts of solution B, add 10 parts of nanofibrillated cellulose, and 30 parts of solution A, ultrasonically treat for 10 minutes at a frequency of 100 kHz, then hydrothermally react at 80 °C for 6 hours, and dry to obtain nanofibrillated cellulose coated with a barrier layer; S2: (1) Add 0.2 part of coupling agent to anhydrous ethanol and mix evenly. Then add 90 parts of polytetrafluoroethylene resin, 3 parts of glass fiber, and 3 parts of graphite and mix evenly for 1 hour. Dry to obtain a mixed material. Calcinate the mixed material at 300 °C for 2 hours. After cooling, add it to a 0.28 wt% zinc acetate ethanol solution and ball-mill and mix for 60 minutes. Then perform heat treatment at 120 °C for 4 hours to obtain zinc-polytetrafluoroethylene filler. (2) Add the zinc-polytetrafluoroethylene filler to anhydrous ethanol and mix evenly. Under the ultrasonic condition of 100 W, add the imidazole-based blend and ultrasonically mix for 8 minutes. Wash with ethanol twice to obtain modified polytetrafluoroethylene filler. Among them, the concentration of the zinc-polytetrafluoroethylene filler is 0.32 wt%, and the concentration of the imidazole-based blend is 4.8 wt%. (3) Uniformly mix 10 parts of the modified polytetrafluoroethylene filler and 15 parts of nanofibrillated cellulose fibrils coated with a barrier layer with 80 parts of polyolefin particles (polypropylene particles, model: F5006, Yanshan Petrochemical). Then perform melt plasticization, low-temperature extrusion, cooling, and then shape setting and annealing to obtain a sheet-like polyolefin-based film. Among them, the temperature of melt plasticization is 220 °C; the temperature of low-temperature extrusion is 200 °C; the temperature of shape setting and annealing is 110 °C. S3: Biaxially stretch the sheet-like polyolefin-based film, perform high-temperature setting, and then cool to obtain a separator. The temperature of high-temperature setting is 150 °C.

[0026] Comparative Example 1 is based on Example 1, and the difference is that the plant cellulose is not fibrillated. S1: (1) Add hydroxypropyl guar gum and sodium alginate to deionized water respectively to obtain a 2 wt% solution A and a 3 wt% solution B. (2) Uniformly mix 40 parts of solution A and 30 parts of solution B, add 10 parts of plant cellulose and 30 parts of solution A, perform ultrasonic treatment at a frequency of 100 kHz for 10 minutes, and then perform hydrothermal reaction at 80 °C for 6 hours. Dry to obtain nanofibrillated cellulose fibrils coated with a barrier layer. S2: (1) Add 0.2 part of coupling agent to anhydrous ethanol and mix evenly. Then add 90 parts of polytetrafluoroethylene resin, 3 parts of glass fiber, and 3 parts of graphite and mix evenly for 1 hour. Dry to obtain a mixed material. Calcinate the mixed material at 300 °C for 2 hours, and then perform ball-mill mixing to obtain modified polytetrafluoroethylene filler. (2) Uniformly mix 10 parts of the modified polytetrafluoroethylene filler and 15 parts of nanofibrillated cellulose fibrils coated with a barrier layer with 80 parts of polyolefin particles (polypropylene particles, model: F5006, Yanshan Petrochemical). Then perform melt plasticization, low-temperature extrusion, cooling, and then shape setting and annealing to obtain a sheet-like polyolefin-based film. Among them, the temperature of melt plasticization is 220 °C; the temperature of low-temperature extrusion is 200 °C; the temperature of shape setting and annealing is 110 °C. S3: Biaxially stretch, heat-set at a high temperature, and cool the sheet-like polyolefin-based film to obtain a separator; the temperature for high-temperature heat setting is 150 °C.

[0027] Comparative Example 2 is based on Example 1, the difference being that the nanofibrillated cellulose is not coated with a barrier layer; S1: (1) Add plant cellulose to a 2 wt% nitric acid solution and mix evenly. React at 25 °C for 10 hours, adjust the pH to neutral, wash with deionized water to remove residual acid and other impurities, and dry to obtain oxidized cellulose with an aspect ratio of (5 - 50); Add the oxidized cellulose to deionized water to prepare a 0.2 wt% cellulose suspension, and perform high-intensity microfluidic homogenization treatment on it under a pressure of 200 MPa, and dry to obtain nanofibrillated cellulose with an aspect ratio of 50 - 1000; S2: (1) Add 0.2 parts of a coupling agent to anhydrous ethanol and mix evenly. Add 90 parts of polytetrafluoroethylene resin, 3 parts of glass fiber, and 3 parts of graphite and mix evenly for 1 hour, and dry to obtain a mixture; Calcinate the mixture at 300 °C for 2 hours, and then perform ball milling and mixing to obtain modified polytetrafluoroethylene filler; (2) Uniformly mix 10 parts of the modified polytetrafluoroethylene filler, 15 parts of the nanofibrillated cellulose, and 80 parts of polyolefin particles (polypropylene particles, model: F5006, Yanshan Petrochemical), melt and plasticize, extrude at a low temperature, cool, and then perform shaping and annealing to obtain a sheet-like polyolefin-based film; among them, the temperature for melt and plasticization is 220 °C; the temperature for low-temperature extrusion is 200 °C; the temperature for shaping and annealing is 110 °C; S3: Biaxially stretch, heat-set at a high temperature, and cool the sheet-like polyolefin-based film to obtain a separator; the temperature for high-temperature heat setting is 150 °C.

[0028] Comparative Example 3 is based on Example 1, the difference being that polytetrafluoroethylene is directly used as the filler; S1: (1) Add plant cellulose to a 2 wt% nitric acid solution and mix evenly. React at 25 °C for 10 hours, adjust the pH to neutral, wash with deionized water to remove residual acid and other impurities, and dry to obtain oxidized cellulose with an aspect ratio of (5 - 50); Add the oxidized cellulose to deionized water to prepare a 0.2 wt% cellulose suspension, and perform high-intensity microfluidic homogenization treatment on it under a pressure of 200 MPa, and dry to obtain nanofibrillated cellulose with an aspect ratio of 50 - 1000; (2) Add hydroxypropyl guar gum and sodium alginate to deionized water respectively to obtain 2 wt% solution A and 3 wt% solution B; (3) Uniformly mix solution A and solution B, add the nanofibrillated cellulose and solution A, perform ultrasonic treatment at a frequency of 100 kHz for 10 minutes, and then perform hydrothermal reaction at 80 °C for 6 hours, and dry to obtain nanofibrillated cellulose with a barrier layer coating; S2: Uniformly mix 10 parts of polytetrafluoroethylene filler and 15 parts of nanocellulose fibrils coated with a barrier layer with 80 parts of polyolefin particles (polypropylene particles, model: F5006, Yanshan Petrochemical), melt and plasticize, extrude at low temperature, cool, and then perform shaping and annealing to obtain a sheet-like polyolefin-based film; among them, the temperature for melt and plasticization is 220°C; the temperature for low-temperature extrusion is 200°C; the temperature for shaping and annealing is 110°C; S3: Biaxially stretch the sheet-like polyolefin-based film, perform high-temperature shaping, and cool to obtain a separator; the temperature for high-temperature shaping is 150°C.

[0029] Comparative Example 4 is based on Example 1, the difference being that no nanocellulose fibrils are added; S1: (1) Add 0.2 part of coupling agent to anhydrous ethanol and mix evenly, add 90 parts of polytetrafluoroethylene resin, 3 parts of glass fiber, and 3 parts of graphite and mix evenly for 1 hour, dry to obtain a mixture; calcine the mixture at 300°C for 2 hours, and then perform ball milling and mixing to obtain modified polytetrafluoroethylene filler; (2) Uniformly mix 10 parts of modified polytetrafluoroethylene filler with 80 parts of polyolefin particles (polypropylene particles, model: F5006, Yanshan Petrochemical), melt and plasticize, extrude at low temperature, cool, and then perform shaping and annealing to obtain a sheet-like polyolefin-based film; among them, the temperature for melt and plasticization is 220°C; the temperature for low-temperature extrusion is 200°C; the temperature for shaping and annealing is 110°C; S2: Biaxially stretch the sheet-like polyolefin-based film, perform high-temperature shaping, and cool to obtain a separator; the temperature for high-temperature shaping is 150°C.

[0030] Comparative Example 5 is based on Example 1, the difference being that no modified polytetrafluoroethylene filler is added; S1: (1) Add plant cellulose to 2 wt% nitric acid solution and mix evenly, react at 25°C for 10 hours, adjust the pH to neutral, wash with deionized water to remove residual acid and other impurities, and dry to obtain oxidized cellulose with an aspect ratio of (5 - 50); add the oxidized cellulose to deionized water to prepare a 0.2 wt% cellulose suspension, and perform high-intensity microfluidic homogenization treatment on it under a pressure of 200 MPa, and dry to obtain nanocellulose fibrils with an aspect ratio of 50 - 1000; (2) Add hydroxypropyl guar gum and sodium alginate to deionized water respectively to obtain 2 wt% solution A and 3 wt% solution B; (3) Uniformly mix 40 parts of solution A and 30 parts of solution B, add 10 parts of nanocellulose fibrils and 30 parts of solution A, perform ultrasonic treatment at a frequency of 100 kHz for 10 minutes, and then perform hydrothermal reaction at 80°C for 6 hours to obtain nanocellulose fibrils coated with a barrier layer; S2: 15 parts of nanofibrillated cellulose (NFC) coated with a barrier layer and 80 parts of polyolefin particles (polypropylene particles, model: F5006, Yanshan Petrochemical) are uniformly mixed, melt-plasticized, extruded at a low temperature, cooled, and then subjected to shape setting and annealing to obtain a sheet-like polyolefin-based film; among them, the temperature of melt-plasticization is 220 °C; the temperature of low-temperature extrusion is 200 °C; the temperature of shape setting and annealing is 110 °C; S3: The sheet-like polyolefin-based film is biaxially stretched, heat-set at a high temperature, and cooled to obtain a separator; the temperature of heat setting at a high temperature is 150 °C.

[0031] Comparative Example 6 is based on Example 1, the difference is that nanofibrillated cellulose and modified polytetrafluoroethylene filler are not added; S1: The polyolefin particles (polypropylene particles, model: F5006, Yanshan Petrochemical) are melt-plasticized, extruded at a low temperature, cooled, and then subjected to shape setting and annealing to obtain a sheet-like polyolefin-based film; among them, the temperature of melt-plasticization is 220 °C; the temperature of low-temperature extrusion is 200 °C; the temperature of shape setting and annealing is 110 °C; S2: The sheet-like polyolefin-based film is biaxially stretched, heat-set at a high temperature, and cooled to obtain a separator; the temperature of heat setting at a high temperature is 150 °C.

[0032] Comparative Example 7 is based on Example 5, and imidazole-based tannic acid is not added; S1: (1) Plant cellulose is added to a 2 wt% nitric acid solution and uniformly mixed, reacted at 25 °C for 10 hours, the pH is adjusted to neutral, washed with deionized water to remove residual acid and other impurities, and dried to obtain oxidized cellulose with an aspect ratio of (5 - 50); the oxidized cellulose is added to deionized water to prepare a 0.2 wt% cellulose suspension, which is subjected to high-intensity microfluidic homogenization treatment at a pressure of 200 MPa and dried to obtain nanofibrillated cellulose with an aspect ratio of 50 - 1000; (2) Hydroxypropyl guar gum and sodium alginate are respectively added to deionized water to obtain 2 wt% solution A and 3 wt% solution B; (3) 40 parts of solution A and 30 parts of solution B are uniformly mixed, 10 parts of nanofibrillated cellulose and 30 parts of solution A are added, ultrasonic treatment is carried out at a frequency of 100 kHz for 10 minutes, and then hydrothermal reaction is carried out at 80 °C for 6 hours and dried to obtain nanofibrillated cellulose coated with a barrier layer; S2: (1) Add 0.2 parts of coupling agent to anhydrous ethanol and mix evenly. Then add 90 parts of polytetrafluoroethylene resin, 3 parts of glass fiber, and 3 parts of graphite, and mix evenly for 1 hour. Dry to obtain a mixture. Calcinate the mixture at 300 °C for 2 hours. After cooling, add it to a 0.28 wt% zinc acetate ethanol solution, and ball-mill and mix for 60 minutes. Heat-treat at 120 °C for 4 hours to obtain zinc-polytetrafluoroethylene filler. (2) Add the zinc-polytetrafluoroethylene filler to anhydrous ethanol and mix evenly. Under ultrasonic conditions of 100 W, add 2-methylimidazole and ultrasonicate for 8 minutes. Wash with ethanol twice to obtain modified polytetrafluoroethylene filler. Among them, the concentration of the zinc-polytetrafluoroethylene filler is 0.32 wt%, and the concentration of 2-methylimidazole is 4.8 wt%. (3) Uniformly mix 10 parts of modified polytetrafluoroethylene filler and 15 parts of nanocellulose fibrils coated with a barrier layer with 80 parts of polyolefin particles (polypropylene particles, model: F5006, Yanshan Petrochemical). Perform melt plasticization, low-temperature extrusion, cooling, and then perform shaping and annealing to obtain a sheet-shaped polyolefin-based film. Among them, the temperature of melt plasticization is 220 °C; the temperature of low-temperature extrusion is 200 °C; the temperature of shaping and annealing is 110 °C. S3: Biaxially stretch, high-temperature shape, and cool the sheet-shaped polyolefin-based film to obtain a separator. The temperature of high-temperature shaping is 150 °C.

[0033] Comparative Example 8 is based on Example 5, and the mixture is not calcined at 300 °C. S1: (1) Add plant cellulose to nitric acid solution and mix evenly. React at 25 °C for 10 hours, adjust the pH to neutral, wash with deionized water to remove residual acid and other impurities, and dry to obtain oxidized cellulose with an aspect ratio of (5 - 50). Add the oxidized cellulose to deionized water to prepare a 0.2 wt% cellulose suspension, and perform high-intensity microjet homogenization treatment under a pressure of 200 MPa. Dry to obtain nanocellulose fibrils with an aspect ratio of 50 - 1000. (2) Add hydroxypropyl guar gum and sodium alginate to deionized water respectively to obtain solution A with a concentration of 2 wt% and solution B with a concentration of 3 wt%. (3) Uniformly mix 40 parts of solution A and 30 parts of solution B, add 10 parts of nanocellulose fibrils and 30 parts of solution A, ultrasonicate at a frequency of 100 kHz for 10 minutes, and then perform hydrothermal reaction at 80 °C for 6 hours. Dry to obtain nanocellulose fibrils coated with a barrier layer. S2: (1) Add 0.2 parts of coupling agent to anhydrous ethanol and mix evenly. Then add 90 parts of polytetrafluoroethylene resin, 3 parts of glass fiber, and 3 parts of graphite and mix evenly for 1 hour. Dry to obtain a mixed material. Add the mixed material to a 0.28 wt% zinc acetate ethanol solution and ball-mill and mix for 60 minutes. Heat-treat at 120 °C for 4 hours to obtain zinc-polytetrafluoroethylene filler. (2) Add the zinc-polytetrafluoroethylene filler to anhydrous ethanol and mix evenly. Under ultrasonic conditions of 100 W, add the imidazole-based blend and ultrasonically mix for 8 minutes. Wash twice with ethanol to obtain modified polytetrafluoroethylene filler. Among them, the concentration of the zinc-polytetrafluoroethylene filler is 0.32 wt%, and the concentration of the imidazole-based blend is 4.8 wt%. (3)Uniformly mix 10 parts of modified polytetrafluoroethylene filler and 15 parts of nano-cellulose fibrils coated with a barrier layer with 80 parts of polyolefin particles (polypropylene particles, model: F5006, Yanshan Petrochemical). Carry out melt plasticization, low-temperature extrusion, cooling, and then shape annealing to obtain a sheet-like polyolefin-based film. Among them, the temperature of melt plasticization is 220 °C; the temperature of low-temperature extrusion is 200 °C; the temperature of shape annealing is 110 °C. S3: Biaxially stretch, high-temperature shape, and cool the sheet-like polyolefin-based film to obtain a separator. The temperature of high-temperature shape is 150 °C.

[0034] Detection experiment: I. Preparation of battery; (1)Preparation of positive electrode sheet: Mix ternary active material NCM811, conductive carbon black, and binder PVDF in a weight ratio of 92:4:4 evenly in NMP to obtain a positive electrode slurry. Coat the positive electrode slurry on the current collector, dry, roll, and then die-cut to obtain a positive electrode sheet. The compaction density of the positive electrode active material layer is 3.4 g / cm 3 .

[0035] (2)Preparation of negative electrode sheet: Mix artificial graphite as the negative electrode active material, binder SBR, dispersant CMC, and conductive carbon black in a weight ratio of 96:2:12:0.8, add to water and stir and mix evenly to obtain a negative electrode slurry. Coat the negative electrode slurry on the current collector, dry, roll, and then die-cut to obtain a negative electrode sheet. The compaction density of the negative electrode active layer is 1.65 g / cm 3 .

[0036] (3)Preparation of electrolyte: Mix ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 4:6 as the solvent, and select lithium salt LiPF6 as the solute with a concentration of 1 mol / L.

[0037] (4) Stack the above positive electrode sheet, separator, and negative electrode sheet in sequence. After winding, a battery cell is obtained, which is placed in a battery case and immersed in the above electrolyte. After processes such as encapsulation, standing, formation, and grading, a sample lithium battery is obtained.

[0038] II. Internal resistance test method: Under normal temperature environment, charge and discharge the lithium batteries prepared in the examples and comparative examples for the first time at a current of 0.33C. The charging is constant current and constant voltage charging, the termination voltage is 4.2V, the cut-off current is 0.05C, the discharge termination voltage is 2.5V, and then leave the lithium battery standing for 12 hours; charge at a constant current and constant voltage at a current of 1C, the termination voltage is 4.2V, the cut-off current is 0.05C, discharge at a current of 1C, and the discharge termination voltage is 2.5V. Record the discharge capacity Cb during the first cycle of the battery cell; adjust the charge of the test object to 50% SOC and discharge at a current of 1C rate for 18 seconds. Record the battery voltage U2, current I before discharge termination, and the battery voltage U1 after the battery voltage stabilizes. Calculate the DC resistance R according to the formula R = (U2 - U1) / A, as shown in Table 1.

[0039] III. Extrusion test method: (1) Charge the fabricated lithium battery at a constant current of 0.5C to 4.2V, and then charge at a constant voltage to 0.05C; place the lithium battery to be tested between two extrusion plates of an extrusion testing machine, with the two large surfaces of the battery in contact with the extrusion plates; ensure that the extrusion testing machine is in a working state, and at the same time, it should be noted that there should be no external circuit in the battery during the experiment.

[0040] (2) Start the test equipment and extrude the lithium battery at an extrusion speed of 5mm / s. When the extrusion reaches 70% of the original size of the lithium battery, or when the extrusion force reaches 30kN, hold for 5 minutes, then release the force, and observe and record the changes in the lithium battery during the extrusion process, such as whether there is smoking, fire, explosion, etc.

[0041] (3) If there is no smoking, fire, explosion, etc. in the lithium battery, continue the extrusion until the battery smokes, catches fire, or explodes. Record the ultimate extrusion size change rate and ultimate applied pressure of the battery at this time, as shown in Table 1.

[0042]

[0043] Table 1 Conclusion: The research found that in Example 2, the barrier layer coated on the nanofibrillated cellulose was too thick. Although it was beneficial to increase the tensile strength in the plane direction of the separator, it was not conducive to exposing the cellulose fibrils when pulling the separator, and it was not conducive to the thickening of the separator when the battery was squeezed. In Example 3, the aspect ratio of the cellulose fibrils was relatively short, which was not conducive to increasing the tensile strength in the plane direction of the separator. In Example 4, the addition amount of polytetrafluoroethylene was slightly less, and the tensile strength in the plane direction of the separator decreased.

[0044] Based on Example 1, the mixture in Example 5 was sintered and then further modified. The research found that the performance of the battery was improved. In Comparative Examples 1-6, it was found that Comparative Example 2 was based on Example 1, and the difference was that the nanofibrillated cellulose was not coated with a barrier layer. For the battery prepared with it as the separator, although the anti-extrusion test results were good, because the separator cellulose had swelled severely after absorbing liquid under normal battery conditions, its internal resistance was too large and the battery could not be used normally.

[0045] Comparative Example 7 was based on Example 5 and did not add benzimidazole-based tannic acid, which reduced the impact resistance of the separator and thus led to a decrease in the performance of the battery. Comparative Example 8 was based on Example 5 and did not calcine the mixture at 300 °C, which reduced the surface roughness of the modified polytetrafluoroethylene filler, made the wetting effect worse and weakened its composite effect with zinc ions, thus affecting the performance of the battery.

[0046] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention.

Claims

1. A method for preparing a diaphragm for improving the impact resistance of a battery, characterized in that: The steps include: S1: (1) adding a barrier material and sodium alginate to deionized water to obtain solution A and solution B respectively; (2) uniformly mixing solution A and solution B, adding nanocellulose fibrils and solution A, ultrasonically treating at a frequency of 80-100 kHz for 5-10 minutes, and then hydrothermally reacting at 70-80°C for 5-6 hours to obtain nanocellulose fibrils coated with a barrier layer; S2: uniformly mixing the modified polytetrafluoroethylene filler and the barrier layer-coated nanocellulose fibrils with polyolefin particles, melt-plasticizing, extruding at low temperature, cooling, and then shaping annealing to obtain a sheet-like polyolefin base film; S3: biaxially stretching the sheet-like polyolefin base film, shaping at high temperature, and cooling to obtain a diaphragm.

2. A method for preparing a diaphragm for improving the impact resistance of a battery according to claim 1, characterized in that: The concentration of solution A is 1-2wt%, and the concentration of solution B is 3wt%; The raw materials of the nanocellulose fibrils coated with the barrier layer include the following components: 70-120 parts of solution A, 15-30 parts of solution B, and 5-20 parts of nanocellulose fibrils, by mass; The raw materials of the sheet-like polyolefin-based film include the following components: by weight, 5 to 20 parts of modified polytetrafluoroethylene filler, 5 to 30 parts of nano-cellulose fibrils coated with a barrier layer, and 80 to 100 parts of polyolefin particles.

3. The method for preparing a diaphragm for improving the impact resistance of a battery according to claim 1, characterized in that: In S2, the temperature of melt plasticization is 180-240°C; the temperature of low-temperature extrusion is 190-220°C; the temperature of shaping annealing is 100-120°C; in S3, the temperature of high-temperature shaping is 130-150°C.

4. The method for preparing a diaphragm for improving the impact resistance of a battery according to claim 1, characterized in that: The preparation method of the nanocellulose fibrils is as follows: adding plant cellulose to a nitric acid solution and mixing uniformly, reacting at 15-30° C. for 6-12 hours, adjusting the pH to neutral, washing with deionized water, and drying to obtain oxidized cellulose; A 0.2 wt% cellulose suspension was prepared, subjected to high-intensity microfluidization homogenization treatment, and dried to obtain nanocellulose fibrils.

5. A method for preparing a diaphragm for improving the impact resistance of a battery according to claim 4, characterized in that: The concentration of the nitric acid solution is 1-2wt%; the aspect ratio of the oxidized cellulose is 5-50; the pressure of the high-intensity micro jet is 70-200MPa; and the aspect ratio of the nanocellulose fibrils is 50-1000.

6. The method for preparing a diaphragm for improving the impact resistance of a battery according to claim 1, characterized in that: The barrier material includes one of guar gum and its derivatives, gum arabic and its derivatives.

7. The method for preparing a diaphragm for improving the impact resistance of a battery according to claim 1, characterized in that: The preparation method of the modified polytetrafluoroethylene filler is as follows: (1) adding a coupling agent to anhydrous ethanol and mixing, adding polytetrafluoroethylene resin, glass fiber and graphite and stirring for 1 to 1.5 hours to obtain a mixture; (2) sintering the mixture at 200 to 300° C. for 1 to 2 hours, adding the mixture to a zinc acetate ethanol solution after cooling, ball milling and mixing for 50 to 60 minutes, and heat treating at 100 to 150° C. for 4 to 5 hours to obtain a zinc-polytetrafluoroethylene filler; (3) adding the zinc-polytetrafluoroethylene filler to anhydrous ethanol and mixing uniformly, adding an imidazole-based blend under ultrasonic conditions of 80 to 100 W, and washing with ethanol for 1 to 2 times to obtain a modified polytetrafluoroethylene filler.

8. The method for preparing a diaphragm for improving the impact resistance of a battery according to claim 7, characterized in that: The raw materials of the mixture include the following components: 85-95 parts of polytetrafluoroethylene, 1-5 parts of glass fiber, 1-5 parts of graphite, and 0.1-0.3 parts of coupling agent, by weight; the concentration of the zinc acetate ethanol solution is 0.26-0.42wt%, the concentration of the zinc-polytetrafluoroethylene filler is 0.2-0.35wt%, and the concentration of the imidazole-based blend is 4-6wt%.

9. The method for preparing a diaphragm for improving the impact resistance of a battery according to claim 7, characterized in that: The preparation method of the imidazole-based blend is as follows: (1) adding potassium carbonate to DMF and mixing uniformly, adding tannic acid and 5,6-dichlorobenzimidazole, heating at 130-140°C for 16-18 hours, cooling to room temperature, and filtering to obtain a filtrate; concentrating the filtrate under reduced pressure, adding deionized water, stirring in an ice water bath for 1-2 hours, filtering, washing, drying, and purifying to obtain benzimidazole-based tannic acid; (2) adding benzimidazole-based tannic acid and 2-methylimidazole to ethanol, mixing uniformly, and drying to obtain an imidazole-based blend.

10. A method for preparing a diaphragm for improving the impact resistance of a battery according to claim 9, characterized in that: The raw materials of the benzimidazole-based tannic acid include the following components: 3-4 parts of tannic acid, 0.2-0.4 parts of potassium carbonate, and 0.3-0.5 parts of 5,6-dichlorobenzimidazole, by mass; the mass ratio of the 2-methylimidazole to the benzimidazole-based tannic acid is (2-4):1.

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