A lithium-ion battery separator with high liquid absorption rate and high heat resistance and its preparation method

Abstract

The present invention relates to the field of battery separator technology, specifically a lithium-ion battery separator with high liquid absorption rate and high heat resistance and a preparation method thereof. The present invention prepares a ZIF-8 material by adding 2-methylimidazole, Zn(NO3)2·6H2O and deionized water. A composite material is then prepared by adding the ZIF-8 material, 2-phenylimidazole and methanol. The composite material, polyvinylidene fluoride and dimethylacetamide are mixed and stirred thoroughly to obtain a slurry; the slurry is evenly applied to the surfaces of both sides of a polyethylene porous substrate, and vacuum dried to obtain a finished product. The finished product prepared by the present invention has good heat resistance and wettability, and can significantly improve the safety performance and cycle performance of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of battery separators, in particular to a lithium-ion battery separator with high liquid absorption rate and high heat resistance and a preparation method thereof. Background Art

[0002] As an energy storage system, lithium-ion batteries have advantages such as a wide operating voltage range, high energy density, and long cycle life. They have broad application prospects in portable electronic products and electric vehicles. Battery separators are one of the core factors affecting battery performance. The selection of separator materials and control of their structure have a significant impact on the electrochemical performance and safety of the battery. However, current commercial polyolefin separators are limited by the properties of the materials and have poor thermal stability. Currently, commercial polyolefin separators are mainly made of polyethylene (PE) or polypropylene (PP). The rupture temperature of PE is approximately 140°C, and that of PP is approximately 170°C. Therefore, PE and PP separators are prone to thermal deformation or even melting when heated at high temperatures. They are unable to maintain their original size to isolate the positive and negative electrodes from direct contact, resulting in battery short circuits and safety accidents.

[0003] In order to overcome the defects of the prior art, the present invention provides a lithium ion battery separator with high liquid absorption rate and high heat resistance and a preparation method thereof. Summary of the Invention

[0004] The object of the present invention is to provide a lithium ion battery separator with high liquid absorption rate and high heat resistance and a preparation method thereof, so as to solve the problems in the prior art.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0006] A method for preparing a lithium-ion battery separator with high liquid absorption rate and high heat resistance comprises the following steps:

[0007] Step 1: dissolving 2-methylimidazole in deionized water to obtain a 2-methylimidazole solution; dissolving Zn(NO3)2·6H2O in deionized water to obtain a zinc nitrate solution; adding the zinc nitrate solution to the 2-methylimidazole solution at 25-30°C, stirring thoroughly for 3-4 hours, and then standing for 12-14 hours. The milky white dispersion layer is removed and centrifuged, washed, and vacuum-dried to obtain ZIF-8 material;

[0008] Step 2: adding ZIF-8 material to methanol and sonicating for 2-5 minutes to obtain a ZIF-8 solution; dissolving 2-phenylimidazole in methanol to obtain a 2-phenylimidazole solution; adding the 2-phenylimidazole solution to the ZIF-8 solution, and then sealing the mixture and heating it at 60-70°C for 70-75 hours. After the reaction is completed, cooling to obtain a solid product, and then washing, centrifuging, and drying to obtain a composite material;

[0009] Step 3: Mix the composite material, polyvinylidene fluoride and dimethylacetamide, and stir thoroughly to obtain a slurry; evenly apply the slurry to both sides of the polyethylene porous substrate, and then vacuum dry at 50-60° C. for 45-50 hours to obtain a finished product.

[0010] More optimally, in step 1, the reaction mass ratio of 2-methylimidazole and Zn(NO3)2·6H2O is (3.4-3.5):1.5.

[0011] More optimally, in step 1, the centrifugation parameters are: rotation speed 8500-9000 r / min, time 3-5 min; vacuum drying parameters: vacuum drying at 70-80° C. for 20-24 h.

[0012] More optimally, in step 2, the reaction mass ratio of ZIF-8 material and 2-phenylimidazole is (0.9-1):3.

[0013] More optimally, in step 2, the centrifugation parameters are: rotation speed of 6000-6500 r / min, time of 25-30 min; vacuum drying parameters: vacuum drying at 60-65° C. for 20-24 h.

[0014] More optimally, in step three, the contents of the slurry components are: by mass fraction, 5-30% composite material, 5-15% polyvinylidene fluoride, and the balance is dimethylacetamide.

[0015] More optimally, in step three, the coating thickness on each side is 2-3 μm, and the thickness of the polyethylene porous substrate is 7-9 μm.

[0016] Beneficial effects of the present invention:

[0017] The present invention prepares a ZIF-8 material by adding 2-methylimidazole, Zn(NO₃)₂·6H₂O, and deionized water. A composite material is then prepared by adding the ZIF-8 material, 2-phenylimidazole, and methanol. The composite material, polyvinylidene fluoride, and dimethylacetamide are mixed and stirred thoroughly to form a slurry. The slurry is evenly coated on both sides of a polyethylene porous substrate and vacuum dried to obtain a finished product.

[0018] The present invention is characterized in that the ZIF-8 material has a three-dimensional pore network, and its three-dimensional pore network structure can provide stability and thermal stability. Under high temperature conditions, the ZIF-8 material can maintain its structural integrity, thereby protecting other components in the battery from the effects of high temperature. In addition, the specific surface area of ​​the ZIF-8 material is large, so it has more active sites on its surface, which can interact with the electrolyte. The increased specific surface area helps to improve the wettability of the diaphragm. Furthermore, in the composite material, ZIF-8 forms a chemical bond with 2-phenylimidazole. Through the strong bonding between the reactive atoms or ions, the composite material can resist the chemical and physical changes that occur at high temperatures and improve the heat resistance of the diaphragm.

[0019] Therefore, the lithium-ion battery separator prepared by the present invention can overcome the disadvantage of poor heat resistance of existing PE, greatly improve the heat resistance and wettability of the separator, and improve the battery safety performance and cycle performance. DETAILED DESCRIPTION

[0020] The following will provide a clear and complete description of the technical solutions of the present invention in conjunction with the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0021] Source of raw materials:

[0022] Polyvinylidene fluoride, provided by Dongguan Xinfengyi Plastic Co., Ltd., model number is XH5270IUV7ME.

[0023] Example 1: Step 1: Dissolve 3.498 g of 2-methylimidazole in 120 mL of deionized water to obtain a 2-methylimidazole solution; dissolve 1.53 g of Zn(NO3)2·6H2O in 120 mL of deionized water to obtain a zinc nitrate solution; add the zinc nitrate solution to the 2-methylimidazole solution at 25°C, stir thoroughly for 3 hours, and then let it stand for 12 hours. Remove the milky white dispersion layer, centrifuge, wash, and vacuum dry to obtain a ZIF-8 material; centrifuge parameters: speed 8500 r / min, time 5 min; vacuum drying parameters: vacuum drying at 70°C for 24 hours;

[0024] Step 2: Add 0.1 g ZIF-8 material to 20 mL methanol and ultrasonicate for 3 minutes to obtain a ZIF-8 solution; dissolve 2-phenylimidazole in 30 mL methanol to obtain a 2-phenylimidazole solution; add the 2-phenylimidazole solution to the ZIF-8 solution, and then seal the mixture and heat it at 60°C for 72 hours. After the reaction is completed, cool it to obtain a solid product, which is then washed, centrifuged, and dried to obtain a composite material; centrifugation parameters: speed of 6000 r / min, time of 30 minutes; vacuum drying parameters: vacuum drying at 60°C for 24 hours; the reaction mass ratio of ZIF-8 material and 2-phenylimidazole is 1:3;

[0025] Step 3: Mix 8.5% of the composite material, 10% of polyvinylidene fluoride, and 81.5% of dimethylacetamide by mass and stir thoroughly to obtain a slurry; evenly apply the slurry to both sides of the polyethylene porous substrate, and then vacuum dry at 50°C for 48 hours to obtain a finished product; the coating thickness on each side is 2 μm, and the thickness of the polyethylene porous substrate is 7 μm.

[0026] Example 2: The amount of the composite material is adjusted to 16.7%, and the rest is the same as Example 1, and the specific steps are as follows: Step 1: dissolve 3.498g of 2-methylimidazole in 120mL of deionized water to obtain a 2-methylimidazole solution; dissolve 1.53g of Zn(NO3)2·6H2O in 120mL of deionized water to obtain a zinc nitrate solution; add the zinc nitrate solution to the 2-methylimidazole solution at 25°C, stir well for 3h, and then let it stand for 12h, remove the milky white dispersion layer, centrifuge, wash, and vacuum dry to obtain ZIF-8 material; centrifugation parameters: speed 8500r / min, time 5min; vacuum drying parameters: vacuum drying at 70°C for 24h;

[0027] Step 2: Add 0.1 g ZIF-8 material to 20 mL methanol and ultrasonicate for 3 minutes to obtain a ZIF-8 solution; dissolve 2-phenylimidazole in 30 mL methanol to obtain a 2-phenylimidazole solution; add the 2-phenylimidazole solution to the ZIF-8 solution, and then seal the mixture and heat it at 60°C for 72 hours. After the reaction is completed, cool it to obtain a solid product, which is then washed, centrifuged, and dried to obtain a composite material; centrifugation parameters: speed of 6000 r / min, time of 30 minutes; vacuum drying parameters: vacuum drying at 60°C for 24 hours; the reaction mass ratio of ZIF-8 material and 2-phenylimidazole is 1:3;

[0028] Step 3: Mix 16.7% of the composite material, 10% of polyvinylidene fluoride, and 73.3% of dimethylacetamide by mass and stir thoroughly to obtain a slurry; evenly apply the slurry to both sides of the polyethylene porous substrate, and then vacuum dry at 50°C for 48 hours to obtain a finished product; the coating thickness on each side is 2 μm, and the thickness of the polyethylene porous substrate is 7 μm.

[0029] Example 3: The amount of the composite material is adjusted to 25.5%, and the rest is the same as Example 1, and the specific steps are as follows: Step 1: dissolve 3.498g of 2-methylimidazole in 120mL of deionized water to obtain a 2-methylimidazole solution; dissolve 1.53g of Zn(NO3)2·6H2O in 120mL of deionized water to obtain a zinc nitrate solution; add the zinc nitrate solution to the 2-methylimidazole solution at 25°C, stir well for 3h, and then let it stand for 12h, remove the milky white dispersion layer, centrifuge, wash, and vacuum dry to obtain ZIF-8 material; centrifugation parameters: speed 8500r / min, time 5min; vacuum drying parameters: vacuum drying at 70°C for 24h;

[0030] Step 2: Add 0.1 g ZIF-8 material to 20 mL methanol and ultrasonicate for 3 minutes to obtain a ZIF-8 solution; dissolve 2-phenylimidazole in 30 mL methanol to obtain a 2-phenylimidazole solution; add the 2-phenylimidazole solution to the ZIF-8 solution, and then seal the mixture and heat it at 60°C for 72 hours. After the reaction is completed, cool it to obtain a solid product, which is then washed, centrifuged, and dried to obtain a composite material; centrifugation parameters: speed of 6000 r / min, time of 30 minutes; vacuum drying parameters: vacuum drying at 60°C for 24 hours; the reaction mass ratio of ZIF-8 material and 2-phenylimidazole is 1:3;

[0031] Step 3: Mix 25.5% of the composite material, 10% of polyvinylidene fluoride, and 64.5% of dimethylacetamide by mass and stir thoroughly to obtain a slurry; evenly apply the slurry to both sides of the polyethylene porous substrate, and then vacuum dry at 50°C for 48 hours to obtain a finished product; the coating thickness on each side is 2 μm, and the thickness of the polyethylene porous substrate is 7 μm.

[0032] Comparative Example 1: Remove step 2, and the rest is the same as Example 3, and the specific steps are as follows: Step 1: dissolve 3.498g of 2-methylimidazole in 120mL of deionized water to obtain a 2-methylimidazole solution; dissolve 1.53g of Zn(NO3)2·6H2O in 120mL of deionized water to obtain a zinc nitrate solution; add the zinc nitrate solution to the 2-methylimidazole solution at 25°C, stir well for 3h, and then let it stand for 12h, remove the milky white dispersion layer, centrifuge, wash, and vacuum dry to obtain ZIF-8 material; centrifugation parameters: speed 8500r / min, time 5min; vacuum drying parameters: vacuum drying at 70°C for 24h;

[0033] Step 2: Mix 25.5% of the composite material, 10% of polyvinylidene fluoride, and 64.5% of dimethylacetamide by mass and stir thoroughly to obtain a slurry; evenly apply the slurry to both sides of the polyethylene porous substrate, and then vacuum dry at 50°C for 48 hours to obtain a finished product; the coating thickness on each side is 2 μm, and the thickness of the polyethylene porous substrate is 7 μm.

[0034] Comparative Example 2: Remove step 1 and step 2, and the rest are the same as Example 1. The specific steps are as follows: Step 1: Mix 10% polyvinylidene fluoride and 90% dimethylacetamide by mass and stir thoroughly to obtain a slurry; evenly apply the slurry to both sides of the polyethylene porous substrate, and then vacuum dry at 50°C for 48 hours to obtain a finished product; the coating thickness on each side is 2 μm, and the thickness of the polyethylene porous substrate is 7 μm.

[0035] Comparative Example 3: Remove step 1, step 2 and polyvinylidene fluoride, and the rest is the same as Example 1. The specific steps are as follows: Step 1: Immerse the polyethylene porous substrate in dimethylacetamide solvent, take it out and vacuum dry it at 50°C for 48 hours to obtain the finished product; the thickness of the polyethylene porous substrate is 7μm.

[0036] Detection test:

[0037] Film thickness test: The diaphragm thickness is measured by using a Mahr thickness gauge.

[0038] Thermal shrinkage performance test: The diaphragm was baked in an oven at 105°C for 1 hour, and then the dimensional change of the diaphragm was measured and the thermal shrinkage was calculated.

[0039] Membrane rupture temperature test: The membrane rupture temperature test is measured using thermomechanical analysis (TMA). The specific method can refer to NASATM2010-216099.

[0040] Contact angle test: The method for testing the contact angle of the diaphragm is to use a TBU100 contact angle meter (manufactured by Dataphysics, Germany) at a temperature of 25°C and a relative humidity of less than 60%. EC / DMC (v:v = 1:1) mixed solvent is slowly dripped onto the membrane surface. After dripping for 60 seconds, the wetting contact angle of the mixed solution is measured. The contact angles are measured at three points on the film, and the average of the three points can be used as the contact angle.

[0041] Liquid absorption test: The method for testing the liquid absorption rate of the membrane is to cut a circular membrane with a diameter of 19mm and fully dry it under vacuum conditions, then weigh the initial mass w0. Then, soak the membrane in a mixed solvent of EC / DMC (v:v = 1:1) for 24 hours. Remove the membrane, wipe off the electrolyte on the membrane surface, and weigh the mass wi. Then, calculate the liquid absorption rate according to the formula. The results are shown in the following table:

[0042]

[0043] Conclusion: Note: “-” means the film has shrunk at this temperature and cannot be tested.

[0044] Analysis of the above table shows that the finished products prepared by adding composite materials in Examples 1-3 are significantly better than those prepared in Comparative Examples 1-3 in terms of heat resistance and wettability. After being kept warm at 150°C for 1 hour, their heat shrinkage performance is also significantly better than that of ordinary polyethylene diaphragms.

[0045] In terms of heat resistance, the membrane rupture temperatures of Examples 1-3 reached 198°C, 214°C, and 197°C, respectively, which are much higher than the membrane rupture temperature of 143°C of the polyethylene membrane (Comparative Example 3), greatly improving the safety performance of the battery; in terms of wettability, the contact angle and liquid absorption rate performance of Examples 1-3 are greatly improved compared with Comparative Examples 1-3, mainly due to the composite material modified with 2-phenylimidazole. Compared with the ZIF-8 material, it exhibits a multi-level pore structure with a larger pore capacity and pore size. At the same time, the enhanced hydrogen bonds obtained after modification make it easier for the molecules to combine with water molecules, thereby enhancing the hydrophilicity, and being able to wet more electrolyte faster, shorten the ion transmission path, and improve the utilization efficiency of active substances, thereby improving the battery capacity, discharge rate and other characteristics.

[0046] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "include," "comprise," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus comprising a series of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0047] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and 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 lithium-ion battery separator with high liquid absorption rate and high heat resistance, characterized in that: The following steps are involved: Step 1: dissolving 2-methylimidazole in deionized water to obtain a 2-methylimidazole solution; dissolving Zn(NO3)2·6H2O in deionized water to obtain a zinc nitrate solution; adding the zinc nitrate solution to the 2-methylimidazole solution at 25-30°C, stirring thoroughly for 3-4 hours, and then standing for 12-14 hours. The milky white dispersion layer is removed and centrifuged, washed, and vacuum-dried to obtain ZIF-8 material; Step 2: Add ZIF-8 material to methanol and sonicate for 2-5 minutes to obtain a ZIF-8 solution; Dissolving 2-phenylimidazole in methanol to obtain a 2-phenylimidazole solution; The 2-phenylimidazole solution was added to the ZIF-8 solution, and the mixture was sealed and heated at 60-70°C for 70-75 hours. After the reaction, the solid product was cooled and then washed, centrifuged, and dried to obtain a composite material. The reaction mass ratio of ZIF-8 material to 2-phenylimidazole was (0.9-1):

3. Step 3: Mix the composite material, polyvinylidene fluoride and dimethylacetamide, and stir them thoroughly to obtain a slurry; evenly apply the slurry to the surfaces of both sides of the polyethylene porous substrate, and then vacuum dry it at 50-60°C for 45-50 hours to obtain a finished product; the content of each component of the slurry is: by mass fraction, 5-30% composite material, 5-15% polyvinylidene fluoride, and the balance dimethylacetamide.

2. The method for preparing a lithium-ion battery separator with high liquid absorption rate and high heat resistance according to claim 1, characterized in that: In step 1, the reaction mass ratio of 2-methylimidazole and Zn(NO3)2·6H2O is (3.4-3.5):1.

5.

3. The method for preparing a lithium-ion battery separator with high liquid absorption rate and high heat resistance according to claim 1, characterized in that: In step 1, the centrifugation parameters are: rotation speed of 8500-9000 r / min, time of 3-5 min; vacuum drying parameters: vacuum drying at 70-80° C. for 20-24 h.

4. The method for preparing a lithium-ion battery separator with high liquid absorption rate and high heat resistance according to claim 1, wherein: In step 2, the centrifugation parameters are: rotation speed of 6000-6500 r / min, time of 25-30 min; and drying parameters: vacuum drying at 60-65° C. for 20-24 h.

5. The method for preparing a lithium-ion battery separator with high liquid absorption rate and high heat resistance according to claim 1, characterized in that: In step three, the coating thickness on each side is 2-3 μm, and the thickness of the polyethylene porous substrate is 7-9 μm.

6. A lithium-ion battery separator with high liquid absorption rate and high heat resistance, characterized in that: Prepared according to the preparation method according to any one of claims 1 to 5.

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