Double-sided porous polyimide diaphragm as well as preparation method and application thereof

Abstract

The invention discloses a double-sided porous polyimide diaphragm as well as a preparation method and application thereof, and belongs to the technical field of polymer diaphragm materials for lithium batteries. The preparation method comprises the following steps: (1) adding a diamine monomer and a dianhydride monomer into a solvent, and reacting to obtain polyimide slurry; (2) coating a porous substrate with the polyimide slurry, and soaking the porous substrate in an aqueous solution containing ethanol to obtain a primarily cured polyimide diaphragm; and (3) taking down the primarily cured polyimide diaphragm from the surface of the porous substrate, and drying to obtain the porous polyimide diaphragm. According to the method, the polyimide slurry is coated in the porous substrate, then soaking, preliminary curing and drying are performed to obtain the double-sided porous polyimide diaphragm, the porosity of the diaphragm is remarkably improved compared with that of an existing porous polyimide diaphragm, the pore size distribution of the two sides of the diaphragm is uniform, and the porosity difference is remarkably reduced; and the wettability and the absorptivity of the diaphragm material on electrolyte are improved.

Description

Technical Field

[0001] This application relates to double-sided porous polyimide separators, their preparation methods, and applications, belonging to the technical field of polymer separator materials for lithium batteries. Background Technology

[0002] With the rapid development of high-power equipment such as new energy electric vehicles and large-scale energy storage systems, high-capacity, high-energy-density power and energy storage lithium-ion batteries have experienced explosive growth in recent years. However, the frequent spontaneous combustion and explosion incidents of new energy electric vehicles in recent years have raised serious concerns and questions about the safety of power lithium-ion batteries. One of the core reasons is that the performance of existing lithium battery separators cannot meet the application requirements of high-energy-density batteries. The separator plays two main roles in lithium-ion batteries: firstly, the separator material needs to have good insulation and a certain strength to prevent direct contact between the positive and negative electrodes within the battery, and effectively prevent short circuits caused by punctures from burrs, dendrites, etc.; secondly, it must ensure that there are no significant dimensional changes under sudden high-temperature conditions, thus guaranteeing battery safety.

[0003] Currently, most lithium-ion battery separators use polyolefin microporous membranes based on polypropylene (PP) and polyethylene (PE). Their low melting points (PP 165℃, PE 135℃) and softening temperatures make the batteries prone to thermal runaway due to separator shrinkage, especially under overcharging, over-discharging, and high-power charging and discharging conditions, which can lead to battery fires or explosions. In view of this, polyimide separators have many advantages over commercially available PP / PE separators: First, they have good high-temperature resistance (melting point >300℃), which can improve the safety performance of lithium-ion batteries; second, polyimide can be structurally designed to have a large number of polar groups, which can increase its wettability to the electrolyte, improve ionic conductivity, and make lithium-ion batteries suitable for high-rate charging and discharging, shortening charging time and extending battery life. Therefore, polyimide separators are expected to be the next-generation lithium-ion battery separator material.

[0004] However, the porous polyimide membranes currently prepared have defects such as low porosity, uneven pore size distribution, and pore closure resistance, which result in poor wettability and absorption rate of electrolyte, thus limiting the use of porous polyimide membranes. Summary of the Invention

[0005] To address the aforementioned issues, a double-sided porous polyimide membrane, its preparation method, and its applications are provided. This method involves coating a polyimide slurry onto a porous substrate, followed by soaking for preliminary curing and drying to obtain a double-sided porous polyimide membrane. Compared to existing porous polyimide membranes, this membrane exhibits significantly increased porosity, uniform pore size distribution on both sides, and a significantly reduced porosity difference, thereby improving the membrane material's wettability and absorption rate of the electrolyte.

[0006] According to one aspect of this application, a method for preparing a double-sided porous polyimide membrane is provided, comprising the following steps:

[0007] (1) Add diamine monomers and dianhydride monomers to a solvent and react to obtain polyimide slurry;

[0008] (2) The polyimide slurry is coated on a porous substrate and immersed in an aqueous solution containing ethanol for 10-15 minutes to obtain a pre-cured polyimide membrane. The thickness of the porous substrate is 7-25 μm and the porosity is above 39%.

[0009] (3) Remove the pre-cured polyimide membrane from the surface of the porous substrate and dry it to obtain a porous polyimide membrane.

[0010] The presence of this porous substrate allows the ethanol aqueous solution to enter the polyimide membrane surface without being blocked by the substrate, enabling the polyimide membrane to have a porous structure with uniform size on both the upper and lower surfaces, thereby increasing the overall porosity of the polyimide membrane and reducing the porosity difference between the two surfaces of the membrane.

[0011] Preferably, the porosity of the porous substrate is 39%-45%.

[0012] For example, the porosity of a porous substrate can be 39%, 40%, 41%, 42%, 43%, 44%, 45%, or any value in between.

[0013] Optionally, the porous substrate is a PE substrate.

[0014] Optionally, in step (2), the polyimide slurry is coated onto the porous substrate roll using a roll-to-roll coating method, and then immersed in an aqueous solution containing ethanol.

[0015] Compared to other porous polyimide membranes currently on the market, this porous polyimide coating uses a roll-to-roll coating method. With a width of 2.1 meters, a coating machine speed of 20 meters per minute, and a working time of 24 hours, it can produce 60,480 square meters of membrane. If electrospinning is used to produce porous polyimide membranes, the commercially available machine capacity is 2,200 square meters. In comparison, the production efficiency of the roll-to-roll coating method is more than 100 times that of the electrospinning method.

[0016] Optionally, the thickness of the porous polyimide membrane is 5-20 μm, preferably 7-16 μm. This thickness can affect the porosity and electrolyte absorption rate of the membrane, thereby affecting the overall transport performance. It has been verified that under the above preparation method, although the mechanical strength can be slightly improved by a thicker membrane, the overall porosity of the membrane decreases and the pore size difference becomes larger, which reduces the absorption rate of the electrolyte by the membrane. Therefore, a membrane thickness in the range of 7-16 μm can achieve better results.

[0017] For example, the thickness of the polyimide separator can be 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm, or any value between these values.

[0018] Optionally, the drying process is carried out in stages: first, drying at 60-100℃ for 5-10 minutes, then drying at 100-150℃ for 5-10 minutes, and finally drying at 150-300℃ for 5-10 minutes.

[0019] The segmented drying method described above can prevent rapid evaporation of liquid inside the membrane, thereby improving pore size consistency, reducing the difference in porosity between the two sides, and increasing the overall porosity and electrolyte absorption rate.

[0020] Optionally, the soaking temperature in step (2) is 20-30℃.

[0021] Optionally, the molar ratio of the diamine monomer to the dianhydride monomer is (0.8-1.2):1.

[0022] Optionally, in step (1), the diamine monomer is stirred at 22-25°C until dissolved, and then the dianhydride monomer is added and stirred evenly before the reaction is carried out.

[0023] Optionally, the solvent is selected from polar solvents, low-boiling-point solvents, or low-water-absorbing solvents.

[0024] Optionally, the solvent is selected from at least one of dimethylacetamide (DMAC), m-cresol, tetrahydrofuran (THF), N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), chloroform, 3-methoxy-N,N-dimethylpropionamide, and γ-butyrolactone (GBL).

[0025] Optionally, the diamine monomer and the dianhydride monomer in step (1) are prepolymerized under inert gas protection at 22-25°C for 50-60 min, and then a curing accelerator is added. After reacting at 90°C for at least 6 h, a polyimide slurry is obtained.

[0026] Optionally, the inert gas includes nitrogen, argon, helium, neon, krypton, and xenon.

[0027] Optionally, the molar ratio of the curing accelerator to the diamine monomer is (2-3):1;

[0028] Preferably, the curing accelerator is selected from at least one of triethylamine, benzimidazole, 1-methylimidazole, quinoline, 1,8-diazabicycloundec-7-ene, imidazole, pyridine, and 3-methylpyridine.

[0029] The above preparation method involves polymerization via thermal imidization and chemical imidization. The curing accelerator acts as an affinity agent, attacking the carbon atoms on the carboxyl groups of the polyamic acid and transferring hydrogen atoms to these groups. This promotes the catalytic mechanism of cyclization and dehydration, thereby improving the performance of the resulting membrane, reducing reaction time, and saving production costs. However, if the amount of curing accelerator is too small, the improvement in reaction promotion will be insignificant; if the amount is too large, it will increase costs and may reduce the molecular weight. Excessively high reaction temperatures or long reaction times after adding the curing accelerator will increase production time and cause product degradation; conversely, excessively low reaction temperatures or short reaction times will hinder the promotion of the cyclization and dehydration catalytic mechanism.

[0030] Optionally, the diamine monomer is selected from at least one of 3,4'-diaminodiphenyl ether, 4,4'-diaminobenzoyl aniline, N,N'-bis(4-aminophenyl)terephthalamide, 3,5-diaminobenzoic acid, 4,4-diaminodiphenylmethane, 2,2'-bis(trifluoromethyl)-4,4'-bis(4-aminophenylcarbonylamino)biphenyl, 2,2'-bis(trifluoromethyl)diaminobiphenyl, 3,3'-dihydroxybenzidine, and 9,9-bis(4-aminophenyl)fluorene;

[0031] The dianhydride monomer is selected from at least one of pyromellitic dianhydride, 4,4'-oxobisphthalic anhydride, 3,3',4,4'-biphenyltetracarboxylic anhydride, 3,3',4,4'-benzophenone tetracarboxylic anhydride, 3,3,4,4-diphenylsulfone tetracarboxylic anhydride, and hexafluorodianhydride.

[0032] Optionally, after the polyimide slurry obtained in step (1) is cooled to room temperature, a solvent is added to dilute it and the mixture is stirred evenly before step (2) is performed.

[0033] Optionally, the coating method in step (2) can be spin coating, bar coating, doctor blade coating, roller coating, gravure coating or other suitable coating methods.

[0034] According to another aspect of this application, a double-sided porous polyimide membrane prepared by the method described in any of the preceding claims is provided.

[0035] According to another aspect of this application, the application of the double-sided porous polyimide separator prepared by the preparation method of any of the above claims in lithium-ion batteries is provided.

[0036] The beneficial effects of this application include, but are not limited to:

[0037] 1. The preparation method of the double-sided porous polyimide membrane of this application uses simple and readily available raw materials that are pollution-free, and the preparation process is easy to operate and convenient for industrial production.

[0038] 2. According to the preparation method of the double-sided porous polyimide membrane of this application, the prepared membrane has a pore structure with uniform distribution and approximately the same pore size on both sides, the porosity difference between the two sides is small, and the overall porosity of the membrane is significantly improved.

[0039] 3. The double-sided porous polyimide separator prepared in this application has improved wettability and absorption rate of electrolyte, and has good mechanical properties. It can be used industrially in lithium-ion batteries to improve the stability of lithium-ion batteries and extend their service life. Detailed Implementation

[0040] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0041] Unless otherwise specified, the raw materials used in the embodiments and comparative examples of this application were all purchased commercially.

[0042] Unless otherwise specified, the methods used in the embodiments and comparative examples of this application are conventional methods in the prior art.

[0043] Example 1

[0044] This embodiment relates to a method for preparing a double-sided porous polyimide membrane, comprising the following steps:

[0045] (1) Under nitrogen atmosphere, 11.02 g of 3,4'-diaminodiphenyl ether, 29.17 g of 4,4'-diaminobenzoyl aniline, and 588 g of DMAc were added to a glass reaction flask and stirred at 22 °C for 30 min. After complete dissolution, 40 g of pyromellitic dianhydride was gradually added and stirred continuously at 22 °C for 60 min to obtain polyimide slurry; then 59.21 g of quinoline was added, the temperature was raised to 90 °C and stirred for 6 hours, the temperature was lowered to room temperature, 134 g of DMAc was added and stirred continuously for 30 min.

[0046] (2) The slurry was coated to a specific thickness on a porous PE substrate or a conventional PET substrate. The thickness of the porous PE substrate was 25 μm and the porosity was 39%. The thickness of the conventional PET substrate was 25 μm and the porosity was 0%. The wet films coated on the porous PE substrate were processed to a thickness of 5 μm, 10 μm and 20 μm, respectively, and were named ODP-5-E, ODP-10-E and ODP-20-E. The wet films coated on the conventional PET substrate were processed to a thickness of 5 μm and were named ODP-5-T. The coated wet films were then immersed in an ethanol-water solution (water:ethanol = 3 / 7 by weight) for 10 min to obtain a pre-cured polyimide membrane. The immersion temperature was 20℃.

[0047] (3) Tear the pre-cured polyimide film off the substrate surface and let the polyimide film enter the tunnel oven. First, dry it at 60°C for 5 minutes, then at 100°C for 5 minutes, and finally at 150°C for 5 minutes to obtain a porous polyimide membrane.

[0048] The prepared diaphragm was tested, and the results are shown in Table 1. The porosity and average pore diameter in Table 1 refer to the porosity and average pore diameter calculated by combining the two sides of the diaphragm. The porosity difference between the two sides refers to the difference obtained by calculating the porosity of the two sides separately and then subtracting them. The average pore diameter difference between the two sides refers to the difference obtained by calculating the average pore diameter of the two sides separately and then subtracting them.

[0049] Table 1

[0050]

[0051] As shown in Table 1, at the same thickness, the electrolyte absorption rate of the membrane prepared using a porous PE substrate (ODP-5-E) is 411%, while the electrolyte absorption rate of the membrane prepared using a conventional PET substrate (ODP-5-T) is 323%. This demonstrates that the porous PE substrate can increase the electrolyte absorption rate by 27% compared to the conventional PET substrate. Furthermore, the porosity of ODP-5-E is approximately 70% higher than that of ODP-5-T, and the wettability of the electrolyte is also significantly improved, decreasing by 26%. Observing the pore diameters of the upper and lower surfaces of the membranes made with porous PE as the substrate at different thicknesses, the upper surface directly contacts the ethanol aqueous solution, while the lower surface contacts the substrate surface, resulting in a difference in pore diameter of approximately 0.155-0.189 μm. In contrast, the membranes made with conventional PET as the substrate have a pore diameter difference of 0.362 μm between the upper and lower surfaces, which is significantly higher than that of the membranes made with PE as the substrate. This indicates that the ethanol aqueous solution cannot effectively penetrate into the membrane to form porosity.

[0052] Example 2

[0053] This embodiment relates to a method for preparing a double-sided porous polyimide membrane, comprising the following steps:

[0054] (1) Under nitrogen atmosphere, approximately 8.26 g of 3,4'-diaminodiphenyl ether, 53.77 g of 2,2'-bis(trifluoromethyl)-4,4'-bis(4-aminophenylcarbonylamino)biphenyl, and 768 g of DMAc were added to a glass reaction flask and stirred at 22°C for 30 min. After complete dissolution, 42.66 g of 4,4'-oxydiphthalic anhydride was gradually added, and the mixture was stirred continuously at 22°C for 60 min for prepolymerization. Then, 44.41 g of quinoline was added, and the temperature was raised to 90°C and stirred for 6 hours. The temperature was lowered to room temperature, 174 g of DMAc was added, and the mixture was stirred continuously for 30 min to obtain a polyimide slurry.

[0055] (2) The slurry was coated to a specific thickness on a porous PE substrate or a conventional PET substrate. The thickness of the porous PE substrate was 12 μm and the porosity was 39%. The thickness of the conventional PET substrate was 12 μm and the porosity was 0%. The wet films coated on the porous PE substrate were processed to a thickness of 5 μm, 10 μm and 20 μm, respectively, and were named OAO-5-E, OAO-10-E and OAO-20-E. The wet films coated on the conventional PET substrate were processed to a thickness of 5 μm and were named OAO-5-T. The coated wet films were then immersed in an ethanol-water solution (water:ethanol = 3 / 7 by weight) for 10 min to obtain a pre-cured polyimide membrane. The immersion temperature was 30℃.

[0056] (3) Tear the pre-cured polyimide film off the substrate surface and let the polyimide film enter the tunnel oven. First, dry it at 100°C for 10 minutes, then at 150°C for 10 minutes, and finally at 200°C for 10 minutes to obtain a porous polyimide membrane.

[0057] The prepared diaphragm was tested, and the results are shown in Table 2. The porosity and average pore diameter in Table 2 refer to the porosity and average pore diameter calculated by combining the two sides of the diaphragm. The porosity difference between the two sides refers to the difference obtained by calculating the porosity of the two sides separately and then subtracting them. The average pore diameter difference between the two sides refers to the difference obtained by calculating the average pore diameter of the two sides separately and then subtracting them.

[0058] Table 2

[0059]

[0060] Table 2 shows that, at the same thickness, the electrolyte absorption rate of the membrane prepared using a porous PE substrate (OAO-5-E) is 432%, while that of the membrane prepared using a PET substrate (OAO-5-T) is 331%, demonstrating that porous PE substrates can increase electrolyte absorption rate by 30% compared to conventional PET substrates. Furthermore, the porosity of ODP-5-E is approximately 71% higher than that of ODP-5-T, and the wettability of the electrolyte is also significantly improved, decreasing by 21%. Observing the pore sizes of the membranes made with porous PE substrates, the upper surface directly contacts the ethanol aqueous solution, while the lower surface contacts the substrate surface, resulting in a slight difference in pore size of approximately 0.096-0.145 μm. In contrast, the membranes made with conventional PET substrates show a pore size difference of 0.307 μm between the upper and lower surfaces, significantly higher than that with PE substrates, indicating that the ethanol aqueous solution cannot effectively penetrate the membrane to form porosity.

[0061] Example 3

[0062] The difference between the diaphragm prepared in this embodiment and OAO-5-E is that in step (3), only the final baking temperature is increased from 200°C to 300°C, and the rest is the same as OAO-5-E.

[0063] The test results for the membrane are shown in Table 3. It can be seen that increasing the final drying temperature of the polyimide membrane increases its modulus from 1.97 GPa to 2.26 GPa, an increase of 14.72%, while decreasing its average pore size by 10.7%. This indicates that increasing the temperature causes the polyimide to shrink and improves its overall mechanical strength, while its porosity does not decrease.

[0064] Example 4

[0065] The difference between the diaphragm prepared in this embodiment and OAO-5-E is that the porosity of the porous substrate PE is 45%, while the rest is the same as OAO-5-E.

[0066] The test results for the diaphragm are shown in Table 3. The data shows that when the porosity of the substrate changes from 39% to 45%, the average pore size changes from 1.58 μm to 1.61 μm, and the difference in average pore size between the two sides decreases from 0.096 μm to 0.082 μm, a decrease of 14.58%. This indicates that the higher the porosity of the substrate, the higher the probability that the ethanol aqueous solution will contact the bottom of the coated wet film, resulting in a smaller difference in pore size between the two sides.

[0067] Example 5

[0068] The difference between the diaphragm prepared in this embodiment and OAO-5-E is that the thickness of the porous substrate PE is 7μm, while the rest is the same as OAO-5-E.

[0069] The test results of the diaphragm are shown in Table 3. The data shows that when the substrate thickness is changed from 12 μm to 7 μm, the bottom of the diaphragm comes into contact with the alcohol aqueous solution faster, and the difference in pore size between the two sides decreases from 0.096 μm to 0.079 μm, a decrease of 17.70%.

[0070] The membranes prepared in Examples 3-5 above were tested, and the results are shown in Table 3. The porosity and average pore diameter in Table 3 refer to the porosity and average pore diameter calculated by combining the two sides of the membrane. The porosity difference between the two sides refers to the difference obtained by calculating the porosity of the two sides separately and then subtracting them. The average pore diameter difference between the two sides refers to the difference obtained by calculating the average pore diameter of the two sides separately and then subtracting them.

[0071] Table 3

[0072]

[0073] The above description is merely an embodiment of this application, and the scope of protection of this application is not limited to these specific embodiments, but is determined by the claims of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the technical concept and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for preparing a double-sided porous polyimide membrane, characterized in that, Includes the following steps: (1) Add diamine monomers and dianhydride monomers to a solvent and react to obtain polyimide slurry; (2) The polyimide slurry is coated on a porous substrate and immersed in an aqueous solution containing ethanol for 10-15 minutes to obtain a pre-cured polyimide membrane. The thickness of the porous substrate is 7-25 μm and the porosity is above 39%. (3) Remove the pre-cured polyimide membrane from the surface of the porous substrate and dry it to obtain a porous polyimide membrane.

2. The preparation method according to claim 1, characterized in that, The porous substrate is a PE substrate; in step (2), the polyimide slurry is coated onto the porous substrate roll using a roll-to-roll coating method, and then immersed in an aqueous solution containing ethanol.

3. The preparation method according to claim 1, characterized in that, The thickness of the porous polyimide membrane is 5-20 μm, preferably 7-16 μm.

4. The preparation method according to claim 1, characterized in that, The drying process is carried out in stages: first, drying at 60-100℃ for 5-10 minutes, then drying at 100-150℃ for 5-10 minutes, and finally drying at 150-300℃ for 5-10 minutes.

5. The preparation method according to claim 1, characterized in that, The molar ratio of the diamine monomer to the dianhydride monomer is (0.8-1.2):

1.

6. The method of claim 1, wherein, In step (1), the diamine monomer and the dianhydride monomer are subjected to a prepolymerization reaction at 22-25°C for 50-60 minutes under inert gas protection. Then, a curing accelerator is added, and the reaction is carried out at 90°C for at least 6 hours to obtain polyimide slurry.

7. The preparation method according to claim 1, characterized in that, The molar ratio of the curing accelerator to the diamine monomer is (2-3):1; Preferably, the curing accelerator is selected from at least one of triethylamine, benzimidazole, 1-methylimidazole, quinoline, 1,8-diazabicycloundec-7-ene, imidazole, pyridine, and 3-methylpyridine.

8. The method of claim 1, wherein, The diamine monomer is selected from at least one of 3,4'-diaminodiphenyl ether, 4,4'-diaminobenzoyl aniline, N,N'-bis(4-aminophenyl)terephthalamide, 3,5-diaminobenzoic acid, 4,4-diaminodiphenylmethane, 2,2'-bis(trifluoromethyl)-4,4'-bis(4-aminophenylcarbonylamino)biphenyl, 2,2'-bis(trifluoromethyl)diaminobiphenyl, 3,3'-dihydroxybenzidine, and 9,9-bis(4-aminophenyl)fluorene; The dianhydride monomer is selected from at least one of pyromellitic dianhydride, 4,4'-oxobisphthalic anhydride, 3,3',4,4'-biphenyltetracarboxylic anhydride, 3,3',4,4'-benzophenone tetracarboxylic anhydride, 3,3,4,4-diphenylsulfone tetracarboxylic anhydride, and hexafluorodianhydride.

9. The double-sided porous polyimide membrane prepared by the method of any one of claims 1-8.

10. The application of the double-sided porous polyimide separator prepared by the method of any one of claims 1-8 in lithium-ion batteries.

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