Polyimide separator and preparation method therefor, and electrochemical device

Abstract

The present invention belongs to the field of lithium-ion battery materials, and discloses a polyimide separator and a preparation method therefor, and an electrochemical device. The method for preparing a polyimide separator for a battery comprises the following steps: (1) adding a diamine monomer and a dianhydride monomer to a solvent, and reacting same to obtain a solution containing an intermediate; (2) adding a monomer containing an amide group to the solution containing the intermediate, and reacting same to obtain a slurry; and (3) coating the slurry onto a substrate having a release force of 50-500 gf / 25 mm, then soaking same in a coagulation bath solution to perform pre-curing, finally removing a pre-cured wet membrane from the surface of the substrate, and drying same to obtain a polyimide separator. The preparation method is conducive to large-scale industrial production, and the prepared polyimide separator has a tensile strength of no less than 20 MPa, a porosity of 55% or more and an electrolyte absorption rate of 150% or more, and does not melt or deform at 350°C or lower.

Description

Polyimide membrane, its preparation method and electrochemical device Technical Field

[0001] This invention relates to polyimide separators, their preparation methods, and electrochemical devices, belonging to the field of lithium-ion battery materials. Background Technology

[0002] To address the shortcomings of polyolefin separators and meet the future needs of lithium-ion batteries, researchers are actively developing novel lithium-ion battery separators—a new generation of high-performance lithium-ion battery separators with high temperature resistance, high wettability, and safety. In this field, novel high-temperature resistant polymer separators based on polyimide-based nanofiber membranes are one of the most promising options, because nanofiber membranes offer significant advantages in improving gas permeability and liquid absorption compared to polyolefin microporous membranes.

[0003] The most significant characteristic of fiber membranes is the extremely fine diameter of individual fibers, resulting in a high specific surface area and consequently, excellent electrolyte wettability. Furthermore, compared to the typical porosity of around 40% in commercial polyolefin microporous membranes, layered fiber membranes composed of these nano- or submicron-sized fibers can achieve porosity of 80% or even higher. This results in high air permeability, liquid absorption, and consequently, high ionic conductivity, which can significantly reduce battery impedance and improve battery performance. However, electrospinning is too inefficient for large-scale industrial production.

[0004] Furthermore, since the fibers in the fiber membrane are only loosely overlapped, the following two situations may occur: 1. The mechanical properties and puncture strength of the nanofiber membrane are far lower than those of the traditional polyolefin separator, making it easy for lithium dendrites to puncture the separator and cause a short circuit in the battery; 2. After prolonged immersion in electrolyte, the fiber membrane is prone to swelling and disintegration, resulting in poor long-term charge-discharge cycle life of the battery and greatly affecting the safety performance of the battery. Summary of the Invention

[0005] To address the aforementioned issues, a polyimide separator, its preparation method, and an electrochemical device are provided. The preparation method of the polyimide separator is conducive to large-scale industrial production, and the prepared polyimide separator has a tensile strength of not less than 20 MPa, a porosity of more than 55%, an electrolyte absorption rate of more than 150%, and is able to remain unmelted and undeformed below 350°C.

[0006] According to one aspect of the present invention, a method for preparing a polyimide separator is provided, comprising the following steps:

[0007] (1) Add diamine monomers and dianhydride monomers to a solvent and react to obtain a solution containing intermediates;

[0008] (2) Add a monomer with an amide group to the solution containing the intermediate, and after reaction, a slurry is obtained;

[0009] (3) The slurry is applied to a substrate with a release force of 50gf / 25mm-500gf / 25mm, then immersed in a coagulation bath solution for pre-curing, and finally the pre-cured wet film is removed from the substrate surface and dried to obtain a polyimide diaphragm.

[0010] In step (1), the amine monomer undergoes a nucleophilic attack reaction on the acid anhydride to obtain an intermediate, which is a polyamic acid. The reaction structure is as follows:

[0011] In step (2), after polyamic acid and monomers containing amide groups are uniformly mixed, the reaction structure is as follows. During this process, the C=O group of polyamic acid and the NH group of monomers containing amide groups generate hydrogen bonding forces, which enables the monomers containing amide groups to be uniformly dispersed. At the same time, the monomers containing amide groups themselves will also generate forces through hydrogen bonding.

[0012] The structural formula of polyamic acid is as follows:

[0013] In the polyamic acid structural formula, n is the number of repeating polymer units, and Ar and R are as follows:

[0014] In step (3), a substrate with a release force of 50gf / 25mm-500gf / 25mm is used for preparation. This is beneficial for the phase transformation of the slurry, reduces the adhesion between the pre-cured wet film and the substrate, which is conducive to large-scale industrial production, and reduces the shrinkage of the wet film during pre-curing, thus improving the formability of the polyimide separator. If the release force of the substrate is lower than 50gf / 25mm, the wet film will shrink severely during pre-curing, and the uniformity of the polyimide separator will decrease. If the release force is too high, it will cause the film to easily stick to the substrate during film formation, which is not conducive to large-scale industrial production.

[0015] Optionally, the substrate is made of at least one of polyethylene terephthalate, polypropylene, polyvinylidene fluoride, and polyethylene.

[0016] Optionally, the thickness of the substrate is 50μm-200μm.

[0017] Optionally, the coagulation bath solution is composed of water and an organic solvent, wherein the organic solvent accounts for 25 wt% to 75 wt% of the coagulation bath solution. In the presence of the aforementioned release force substrate, the proportion of organic solvent in the coagulation bath solution can simultaneously improve the porosity and mechanical strength of the polyimide separator. If the proportion of organic solvent is too low, the pore-forming effect of the separator during pre-curing will be poor, and blind pores will easily form within the separator, resulting in a decrease in the separator's porosity and a reduction in the uniformity of its porosity distribution. If the proportion of organic solvent is too high, the pore-forming efficiency during pre-curing will be too fast, leading to a decrease in the mechanical strength of the separator, and the separator will easily swell and rupture in the electrolyte.

[0018] Optionally, the soaking time in step (3) is 3-10 min, and the soaking temperature is 20-40℃. The above soaking time is beneficial to achieve porosity of the polyimide membrane during the pre-curing process, thereby improving the porosity and mechanical strength of the membrane. If the soaking time is too short, the perforation rate will be low and the porosity will be small; if the soaking time is too long, the perforation rate will be high and the mechanical strength of the membrane will be poor, which will be lower than 20 MPa.

[0019] For example, the soaking time can be 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min, 10 min, or any value in between.

[0020] Optionally, the thickness of the diaphragm is 5μm-50μm, and the porosity of the diaphragm is 55%-85%, preferably 65%-85%. When the diaphragm is used in wearable electronic products, the thickness of the diaphragm is 10-20μm, and when the diaphragm is used in high-power products such as electric vehicles or charging piles, the thickness of the diaphragm is 30-50μm.

[0021] A thicker separator reduces the number of winding layers, resulting in a lower battery capacity and higher internal resistance. Conversely, a thinner separator reduces its liquid retention capacity and electronic insulation, thereby decreasing battery life and safety.

[0022] For example, the thickness of the polyimide membrane can be 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, or any value in between.

[0023] For example, the porosity of the membrane can be 55%, 60%, 65%, 70%, 75%, 80%, 85%, or any value in between.

[0024] Optionally, the drying in step (3) is a segmented drying process, first drying at 50-150℃ for 10-60 min, and then drying at 180-300℃ for 10-60 min. The above segmented drying can improve the mechanical strength of the separator and its chemical resistance in the electrolyte, reduce swelling and disintegration in the electrolyte, and thus improve the battery's lifespan and safety.

[0025] Optionally, in step (2), a curing accelerator is added along with the monomer containing an amide group, and the molar ratio of the curing accelerator to the diamine monomer is 2-3:1.

[0026] Optionally, 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.

[0027] After adding the curing accelerator, the reaction in step (2) is carried out by thermal imidization and chemical imidization. The curing accelerator acts as an affinity agent to attack the carbon atoms on the carboxyl groups in polyamic acid, transferring hydrogen atoms to the carboxyl groups and promoting the catalytic mechanism of cyclization and dehydration, thereby improving the performance of the obtained membrane, reducing the reaction time, and saving production costs. If the amount of the above curing accelerator is too small, the improvement of the reaction promotion will not be obvious. If the amount is too large, it will increase the cost and reduce the molecular weight.

[0028] Optionally, the reaction temperature in step (1) is 22-25℃, and the reaction time is 60-240 min; the reaction temperature in step (2) is 25℃-80℃, and the reaction time is 6-24 h. In step (1), if the reaction temperature is too high, the molecular weight will decrease, which is not conducive to improving the mechanical strength of the membrane. If the reaction temperature is too low or the time is too short, the reaction will be insufficient, the molecular weight of the membrane will decrease, and thus affect the performance of the membrane. Optionally, inert gas is introduced for protection in both steps (1) and (2), and the inert gas includes nitrogen, argon, helium, neon, krypton, and xenon. In step (2), if the reaction temperature is too high or the reaction time is too long, the production time will increase, and the product will also be degraded. If the reaction temperature is too low or the reaction time is too short, it will not be conducive to promoting the catalytic mechanism of cyclization dehydration.

[0029] Optionally, the solvent is selected from polar solvents, low-boiling-point solvents, or low-water-absorbing solvents.

[0030] 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).

[0031] Optionally, the molar ratio of the diamine monomer to the dianhydride monomer is 1:1;

[0032] The monomer containing the amide group accounts for 10%-50% of the weight percentage of the intermediate;

[0033] The monomer containing the amide group is selected from at least one of benzoylaniline, methyl 2-acetaminophen, 2-benzoylacetaniline, and oxaloylaniline, preferably benzoylaniline;

[0034] The diamine monomer is selected from at least one of 4,4'-diaminodiphenyl ether, p-phenylenediamine, 3,5-diaminobenzoic acid, 4,4-diaminodiphenylmethane, N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-dibiphenyl]-4,4'-diyl)bis(4-aminobenzamide), 2,2'-bis(trifluoromethyl)diaminobiphenyl, 4,4'-diaminobenzoylaniline, 3,3'-dihydroxybenzidine, and 9,9-bis(4-aminophenyl)fluorene;

[0035] 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.

[0036] The content of monomers containing amide groups affects the wettability and absorption rate of the membrane to the electrolyte, as well as its mechanical strength. Within the aforementioned range, the mechanical strength gradually increases with the increase of the amount of amide groups. This is because the C=O and NH functional groups in the amide groups generate an interaction force, thereby reinforcing the mechanical strength of the membrane. In addition, the increase of the number of monomers containing amide groups also improves the wettability of the electrolyte, thereby increasing the electrolyte absorption rate. It can be seen that amide groups can attract electrolyte groups to form an ion-transfer carrier. Therefore, when the number of monomers containing amide groups is greater, the monomers containing amide groups can form a positive harmonic effect in the polyimide structure, which strengthens its overall properties. The smaller the contact angle, the better the wettability and absorption rate of the electrolyte, and the higher the tensile strength and modulus.

[0037] If the monomer containing amide groups accounts for less than 10% by weight of polyamic acid, there is no significant difference in electrolyte wettability and absorption rate compared to when no amide groups are added. If it is greater than 50%, although it is beneficial to the electrolyte absorption rate, its mechanical strength will decrease.

[0038] The aforementioned monomers containing amide groups possess hydrogen bonding capabilities, which can enhance the overall structural strength when added within a specific range. Furthermore, the amide groups can improve the absorption of the electrolyte, facilitating lithium-ion transfer within the separator and thus improving battery performance.

[0039] Optionally, the coating method in step (3) can be spin coating, bar coating, doctor blade coating, roller coating, gravure coating or other suitable coating methods.

[0040] According to another aspect of the present invention, a polyimide separator prepared by the method described in any one of the preceding claims is provided.

[0041] According to another aspect of the present invention, an electrochemical device is provided, the electrochemical device comprising a positive electrode, a negative electrode, an electrolyte, a packaging shell, and a polyimide membrane, the polyimide membrane being located between the positive electrode and the negative electrode, the electrolyte being located within the packaging shell and wetting the positive electrode, the negative electrode, and the polyimide membrane, the polyimide membrane being prepared by the method for preparing polyimide membranes according to any one of the preceding claims.

[0042] Optionally, the electrochemical device is one of a lithium secondary battery, a supercapacitor, a fuel cell, a sodium-sulfur battery, or a vanadium redox flow battery;

[0043] Preferably, the lithium secondary battery is selected from one of lithium-ion secondary batteries, lithium metal secondary batteries, lithium-air secondary batteries, and lithium-sulfur secondary batteries.

[0044] More preferably, the lithium-ion secondary battery includes a polymer lithium-ion secondary battery and an electrolyte support material for a solid-state battery.

[0045] The beneficial effects of the present invention include, but are not limited to:

[0046] 1. The method for preparing the polyimide separator of the present invention involves phase inversion molding on a substrate with a thickness of 50 gf / 25 mm to 500 gf / 25 mm, which is suitable for industrial production and processing. The prepared separator has good consistency in mechanical strength and porosity and can avoid swelling and disintegration in the electrolyte, thereby enabling long-term use in lithium batteries and extending the battery's service life.

[0047] 2. The polyimide separator prepared by this invention can improve the wettability and absorption rate of the separator to the electrolyte by means of the amide-containing monomers being able to form hydrogen bonds with the intermediates, and the amide-containing monomers themselves also generating forces through hydrogen bonds. This promotes the rapid migration of lithium ions in the battery and improves the rate performance of the battery. In addition, it can also improve the mechanical strength of the separator and enhance its puncture resistance.

[0048] 3. The polyimide separator prepared by this invention has good resistance to swelling in electrolyte and can remain non-melting and non-deformed below 350°C. The separator has good durability, which is beneficial to improving the cycle life of the battery. Attached Figure Description

[0049] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings:

[0050] Figure 1 is a schematic diagram of the preparation of polyimide membranes using substrates with different release forces according to Embodiment 2 of the present invention. Detailed Implementation

[0051] The present invention will now be described in detail with reference to the embodiments, but the present invention is not limited to these embodiments.

[0052] Unless otherwise specified, the raw materials used in the embodiments of this invention were all purchased through commercial channels.

[0053] In the examples described below, the Celgard 2325 diaphragm was purchased from Celgard Inc., and the PI180 diaphragm was purchased from Jiangxi Xiancai Nanofiber Technology Co., Ltd.

[0054] Unless otherwise specified, the methods used in the embodiments of the present invention are conventional methods in the prior art.

[0055] The membrane thickness test method in the following embodiments is as follows: the thickness of different membranes is tested using a micrometer (accuracy 0.01 mm), and 5 points are randomly selected on the sample and the average value is taken.

[0056] The porosity test method is as follows: immerse the membrane in n-butanol for 2 hours, and then calculate the porosity according to the formula:

[0057] P(%) = (Mw - Md) / ρVd x100, where ρ is the density of n-butanol and Vd is the geometric volume of the membrane, and Mw and Md are the mass of n-butanol absorbed by the membrane and the mass of the membrane itself.

[0058] The method for testing electrolyte absorption rate is as follows: Immerse the diaphragm in the electrolyte (LiPF6EC:DEC (v / v = 50 / 50)) for 2 hours to saturate the diaphragm with electrolyte. Test the mass of the diaphragm before and after absorbing the electrolyte, and calculate it according to the following formula:

[0059] EU = [(W-Wo) / Wo] × 100%, where Wo and W are the masses of the diaphragm before and after absorbing the electrolyte.

[0060] The ionic conductivity was measured using electrochemical impedance spectroscopy (EIS) at 25°C in an electrochemical fixture. During the measurement, an electrolyte-impeded membrane was sandwiched between two stainless steel (SS) electrode plates, and the impedance spectrum was recorded and analyzed. The test frequency range was 0.1–105 Hz, and the amplitude was 0.2 mV. The ionic conductivity was then calculated using the following formula: σ = d / (Rb × S)

[0061] Where d and S are the film thickness and the contact area between the film and the stainless steel plate, respectively. Rb is the intercept of the Nyquist plot on the real axis, which refers to the volume resistance.

[0062] Wetting angle test method: Using a contact angle meter, cut the sample into 2*2 cm pieces and fix it flat on a glass slide with all four corners intact. Take 10 μL of EC / DEC electrolyte and submerge it below the separator membrane for analysis.

[0063] Test method for positive LFP: Press the positive and negative electrode materials and the diaphragm of the test cell onto the button cell and let it stand for 6 hours to confirm that the open critical voltage (OCV) is stable. Starting from the open loop voltage, use constant current mode to input the charging and discharging current and cutoff voltage (2.5V / 3.8V for LFP) and perform cyclic testing.

[0064] The mechanical strength testing method is to use a tensile testing machine to test the material. This method can quickly obtain test data, and the changes in the sample's resistance to force can be directly observed at the time of the test. The test curve is obtained, which is mostly represented by force and displacement curves.

[0065] The thermal shrinkage of the diaphragm electrolyte: First, a circle with a diameter of 19 mm was cut from the diaphragm to be tested and dried at 105°C for 1 hour. Then, it was placed in a glass serum bottle containing an electrolyte solution of ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 1:1. The immersion temperature was 120°C, and the immersion time was 1 hour. After that, the electrolyte on the membrane surface was gently aspirated in a drying room. To more accurately determine the shrinkage ratio, the area was compared using Autodesk mapping. The formula for calculating the thermal shrinkage rate of the diaphragm electrolyte is: (A1-A2) / A1×100%, where A1 is the area of ​​the diaphragm before the test, and A2 is the area of ​​the diaphragm after the test.

[0066] Example 1

[0067] This embodiment relates to a method for preparing a polyimide separator, comprising the following steps:

[0068] (1) Under nitrogen atmosphere, 26.8g of 4,4'-diaminodiphenyl ether and 317g of DMAc were added to a 500ml three-necked flask and stirred at 22℃ for 30min. After the 4,4'-diaminodiphenyl ether was completely dissolved, 29.19g of pyromellitic dianhydride was added and the reaction was continued at 22℃ for 60min to obtain a solution containing the intermediate polyamic acid.

[0069] (2) Add 43.21g of quinoline and 30% of benzoylaniline (by weight of intermediate polyamic acid) to a solution containing polyamic acid and stir until dissolved. Raise the temperature to 60°C and stir for 6 hours to obtain a slurry.

[0070] (3) The slurry was cooled to room temperature and coated onto a PET substrate with a release force of 120 gf / 25 mm. It was then pre-cured by immersing it in a coagulation bath solution of ethanol and water in different proportions for 10 minutes at a temperature of 30°C. Finally, the pre-cured wet film was peeled off the substrate surface and placed in a tunnel oven. It was first dried at 50°C for 60 minutes, and then dried at 180°C for 60 minutes to obtain the polyimide separator. The different proportions of ethanol and water in the coagulation bath solution refer to water ratios of 25% (sample code H25), 50% (sample code H50), and 75% (sample code H75).

[0071] The performance of the three types of diaphragms was tested. The commercially available Celgard 2325 diaphragm was used as Comparison 1 and the PI180 diaphragm was used as Comparison 2. The diaphragm obtained without adding benzoyl aniline in step (2) was used as Comparison 3. The results are shown in Table 1.

[0072] Table 1

[0073] In Table 1, the Celgard 2325 separator used in Comparison 1 is a three-layer structure (polypropylene / polyethylene / polypropylene) separator with a thickness of 25 μm, a porosity of 39%, and a liquid absorption rate of 98%. The positive electrode, separator, and negative electrode are sequentially wound to form a bare cell, which is then assembled into a lithium-ion battery. The PI180 separator used in Comparison 2 is a single-layer polyimide fiber separator with a thickness of 20 μm, a porosity of 54%, and a liquid absorption rate of 114%. The positive electrode, separator, and negative electrode are sequentially stacked to form a bare cell, which is then assembled into a button-type lithium-ion battery.

[0074] As shown in Table 1, the electrolyte absorption rate of samples (H25, H50, and H75) prepared from coagulation bath solutions of different proportions of ethanol and water increased by 1.5-2.97 times compared to commercially available separators as the water content increased, and the contact angle decreased significantly. This indicates that increasing the ethanol content can improve the porosity and thus the electrolyte absorption rate. The ionic conductivity is also about 2-3 times higher than that of commercially available separators due to the introduction of monomers with amide groups. After stacking the positive electrode, different separators, and negative electrodes to form bare cells and assembling them into button-type lithium-ion batteries, rate performance tests were conducted. The battery composed of Celgard 2325 separators showed a sharp decrease in capacity under high-rate charge and discharge (1C), which was lower than that of the polyimide separator prepared in this invention. This is due to its poor electrolyte adsorption, resulting in poorer capacity. The difference between H50 and Comparative 1 reached 23%.

[0075] In the positive electrode test of Comparison 2, the occurrence of 0 at 0.5C and 1.0C is not related to the physical properties of the PI180 separator itself, but because of its small pore size, it cannot be charged and discharged normally during the battery fast charging test. In the positive electrode test of Comparison 3, the reason for 0 at 1.0C is that the absence of benzoyl aniline leads to a worse porosity of the separator, causing it to be unable to be charged and discharged normally during the battery fast charging test.

[0076] Example 2

[0077] This embodiment relates to a method for preparing a polyimide separator, comprising the following steps:

[0078] (1) Under nitrogen atmosphere, 26.8g of 4,4'-diaminodiphenyl ether and 317g of DMAc were added to a 500ml three-necked flask and stirred at 22℃ for 30min. After the 4,4'-diaminodiphenyl ether was completely dissolved, 29.19g of pyromellitic dianhydride was added and the reaction was continued at 22℃ for 60min to obtain a solution containing the intermediate polyamic acid.

[0079] (2) Add 43.21g of quinoline and 30% of benzoylaniline (by weight of intermediate polyamic acid) to a solution containing polyamic acid and stir until dissolved. Raise the temperature to 60°C and stir for 6 hours to obtain a slurry.

[0080] (3) The slurry was cooled to room temperature and coated onto PET substrates with different release forces. It was then pre-cured by immersing it in a coagulation bath solution of ethanol and water for 10 minutes. The ethanol content in the coagulation bath solution was 50%, and the immersion temperature was 30°C. Finally, the pre-cured wet film was peeled off the substrate surface and placed in a tunnel oven. It was first dried at 50°C for 60 minutes, and then dried at 180°C for 60 minutes to obtain the polyimide separator. The PET substrates with different release forces refer to substrates with release forces of 10gf / 25mm (sample code P10), 50gf / 25mm (sample code P50), 120gf / 25mm (sample code P120), and 500gf / 25mm (sample code P500).

[0081] The performance of the three types of diaphragms was tested. The commercially available Celgard 2325 diaphragm was used as Comparison 1 and the PI180 diaphragm was used as Comparison 2. The diaphragm obtained without adding benzoyl aniline in step (2) was used as Comparison 3. The results are shown in Table 2.

[0082] Table 2

[0083] Table 2 shows that for samples prepared from PET substrates with different release forces, as the release force increases, the pre-cured wet film easily adheres to the substrate, resulting in a smaller pore size on the back of the separator and a decrease in porosity, which in turn leads to a poorer electrolyte absorption rate of the separator. After stacking the positive electrode, different separators, and negative electrodes sequentially to form bare cells and assembling them into button-type lithium-ion batteries, rate performance tests were conducted. The results showed that at high-rate charge-discharge (1C), the battery performance of P10 was worse than that of P120. The main reason is that the separator exhibits inward shrinkage, affecting its membrane uniformity and reducing capacity. On the other hand, P500 has a slightly lower capacity due to its smaller porosity and poorer electrolyte adsorption. The difference between P500 and P120 reached 12.26%.

[0084] Figure 1 is a schematic diagram of the preparation of polyimide separators using PET substrates with different release forces in this embodiment. As shown in Figure 1, when the release force of the substrate is greater than 500 gf / 25 mm and between 50 and 500 gf / 25 mm, the wet film of the separator immersed in the coagulation bath solution is intact. However, the wet film prepared from a substrate with a release force greater than 500 gf / 25 mm is not easily peeled off from the substrate, while the wet film prepared from a substrate with a release force between 50 and 500 gf / 25 mm is easily peeled off, resulting in an intact wet film and the final polyimide separator. When the release force of the substrate is less than 50 gf / 25 mm, the wet film will develop membrane pores and shrinkage when immersed in the coagulation bath solution. These pores and shrinkage are more severe when the wet film is removed, making it impossible to obtain a truly usable polyimide separator.

[0085] Example 3

[0086] This embodiment relates to a method for preparing a polyimide separator, comprising the following steps:

[0087] (1) Under nitrogen atmosphere, 26.8g of 4,4'-diaminodiphenyl ether and 317g of DMAc were added to a 500ml three-necked flask and stirred at 22℃ for 30min. After the 4,4'-diaminodiphenyl ether was completely dissolved, 29.19g of pyromellitic dianhydride was added and the reaction was continued at 22℃ for 60min to obtain a solution containing the intermediate polyamic acid.

[0088] (2) Add 43.21g of quinoline and 30% of benzoylaniline (by weight of intermediate polyamic acid) to a solution containing polyamic acid and stir until dissolved. Raise the temperature to 60°C and stir for 6 hours to obtain a slurry.

[0089] (3) Cool the slurry to room temperature and coat it onto a PET substrate with a release force of 120 gf / 25 mm. Then, immerse it in a coagulation bath solution of ethanol and water for different pre-curing times. The proportion of ethanol in the coagulation bath solution is 50%, and the immersion temperature is 30°C. Finally, peel the pre-cured wet film off the substrate surface and put it into a tunnel oven. First, dry it at 50°C for 60 min, and then dry it at 180°C for 60 min to obtain the polyimide separator. The different pre-curing times refer to 3 min (sample code T3), 5 min (sample code T5), and 10 min (sample code T10), respectively.

[0090] The performance of the three types of diaphragms was tested. The commercially available Celgard 2325 diaphragm was used as Comparison 1 and the PI180 diaphragm was used as Comparison 2. The diaphragm obtained without adding benzoyl aniline in step (2) was used as Comparison 3. The results are shown in Table 3.

[0091] Table 3

[0092] Table 3 shows that the solvent exchange time of samples (T3, T5, and T10) prepared by soaking in the coagulation bath for different times increased with longer soaking time, thus increasing the porosity of the polyimide separator by 19.29%-33.33%. After stacking the positive electrode, different separators, and negative electrodes in sequence to form a bare cell and assembling it into a button-type lithium-ion battery, rate performance testing was conducted. Under high-rate charge and discharge, the T10 battery performed best. However, if the soaking time is further increased, the mechanical properties of the separator will deteriorate, which is not conducive to large-scale industrial production.

[0093] Example 4

[0094] This embodiment relates to a method for preparing a polyimide separator, comprising the following steps:

[0095] (1) Under nitrogen atmosphere, 26.8g of 4,4'-diaminodiphenyl ether and 317g of DMAc were added to a 500ml three-necked flask and stirred at 25℃ for 30min. After the 4,4'-diaminodiphenyl ether was completely dissolved, 29.19g of pyromellitic dianhydride was added and the mixture was stirred at 25℃ for 240min to obtain a solution containing the intermediate polyamic acid.

[0096] (2) Add 43.21g of quinoline and 2-benzoylacetanilide of different weight percentages of intermediate polyamic acid to a solution containing polyamic acid and stir until dissolved. Raise the temperature to 80℃ and stir for 6 hours to obtain a slurry.

[0097] (3) Cool the slurry to room temperature and coat it onto a PET substrate with a release force of 120 gf / 25 mm. Then, immerse it in a coagulation bath solution of ethanol and water for 10 min for pre-curing. The proportion of ethanol in the coagulation bath solution is 50%, and the immersion temperature is 20 °C. Finally, peel the pre-cured wet film off the substrate surface and put it into a tunnel oven. First, dry it at 150 °C for 10 min, and then dry it at 300 °C for 10 min to obtain the polyimide separator. The different weight percentages of 2-benzoylacetanilide in the intermediate polyamic acid are 0% (sample code W0), 10% (sample code W10), 30% (sample code W30), and 50% (sample code W50).

[0098] The performance of the above-mentioned diaphragms was tested, and commercially available Celgard 2325 diaphragm was used as Comparison 1 and PI180 diaphragm as Comparison 2. The results are shown in Table 4.

[0099] Table 4

[0100] Table 4 shows that the electrolyte absorption rate of samples (W10, W30, and W50) prepared with monomers containing amide groups increased by 19.7%-34.3%, and the contact angle decreased significantly. This indicates that the polyimide separator prepared with monomers containing amide groups can effectively increase the wettability of the electrolyte, and its mechanical strength is also better than that of the separator without monomers containing amide groups. After stacking the positive electrode, different separators, and negative electrode to form a bare cell and assembling it into a button-type lithium-ion battery, the rate performance was tested. The W30 battery performed best under high-rate charge and discharge. However, if the amount of amide groups is increased to 50%, the increase in the mechanical properties of the separator is not significant, and its capacity tends to decrease under high-speed charge and discharge.

[0101] Example 5

[0102] This embodiment relates to a method for preparing a polyimide separator, comprising the following steps:

[0103] (1) Under nitrogen atmosphere, 26.8g of 4,4'-diaminodiphenyl ether and 317g of DMAc were added to a 500ml three-necked flask and stirred at 25℃ for 30min. After the 4,4'-diaminodiphenyl ether was completely dissolved, 29.19g of pyromellitic dianhydride was added and the reaction was continued at 25℃ for 100min to obtain a solution containing the intermediate polyamic acid.

[0104] (2) Add 43.21g of quinoline and 30% by weight of 2-benzoylacetanilide, which account for 30% of the weight of intermediate polyamic acid, to a solution containing polyamic acid and stir until dissolved. Raise the temperature to 25°C and stir for 24 hours to obtain a slurry.

[0105] (3) Cool the slurry to room temperature and coat it onto a PET substrate with a release force of 120 gf / 25 mm. Then, immerse it in a coagulation bath solution of ethanol and water for 10 min for pre-curing. The proportion of ethanol in the coagulation bath solution is 50%, and the immersion temperature is 40 °C. Finally, peel the pre-cured wet film off the substrate surface and put it into a tunnel oven. First, dry it at 150 °C for 10 min, and then dry it at 300 °C for 10 min to obtain the polyimide separator. The thicknesses of the wet film on the PET substrate after processing are 5 μm, 14 μm, 25 μm, and 50 μm, respectively named V5, V14, V25, and V50.

[0106] The performance of the above-mentioned diaphragms was tested, and commercially available Celgard 2325 diaphragm was used as Comparison 1 and PI180 diaphragm was used as Comparison 2. The results are shown in Table 5.

[0107] Table 5

[0108] As shown in Table 5, the porosity decreases slightly with increasing separator thickness. After stacking the positive electrode, different separators, and negative electrode sequentially to form a bare cell and assembling it into a button-type lithium-ion battery, rate performance testing was conducted. The V14 battery performed best under high-rate charge and discharge. However, further increasing the separator thickness gradually increased the battery's AC resistance. As we know, the internal resistance of a lithium-ion battery directly affects its electrical performance, a phenomenon also demonstrated in the table above.

[0109] The above description is merely an embodiment of the present invention, and the scope of protection of the present invention is not limited to these specific embodiments, but is determined by the claims of the present invention. For those skilled in the art, the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the technical concept and principle of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a polyimide separator, characterized in that, Includes the following steps: (1) Add diamine monomers and dianhydride monomers to a solvent and react to obtain a solution containing intermediates; (2) Add a monomer with an amide group to the solution containing the intermediate, and after reaction, a slurry is obtained; (3) The slurry is applied to a substrate with a release force of 50gf / 25mm-500gf / 25mm, then immersed in a coagulation bath solution for pre-curing, and finally the pre-cured wet film is removed from the substrate surface and dried to obtain a polyimide diaphragm.

2. The preparation method according to claim 1, characterized in that, The substrate is made of at least one of polyethylene terephthalate, polypropylene, polyvinylidene fluoride, and polyethylene.

3. The preparation method according to claim 1, characterized in that, The coagulation bath solution is composed of water and an organic solvent, wherein the organic solvent accounts for 25 wt% to 75 wt% of the coagulation bath solution.

4. The preparation method according to claim 1, characterized in that, The soaking time in step (3) is 3-10 minutes and the soaking temperature is 20-40℃.

5. The preparation method according to claim 4, characterized in that, The thickness of the diaphragm is 5μm-50μm, and the porosity of the diaphragm is 55%-85%.

6. The preparation method according to claim 1, characterized in that, The reaction temperature in step (1) is 22-25℃, and the reaction time is 60-240 min; The reaction temperature in step (2) is 25-80℃, and the reaction time is 6-24h.

7. The preparation method according to claim 1, characterized in that, The molar ratio of the diamine monomer to the dianhydride monomer is 1:1; The monomer containing the amide group accounts for 10%-50% of the weight percentage of the intermediate; The monomer containing the amide group is selected from at least one of benzoylaniline, methyl 2-acetaminophen, 2-benzoylacetaniline, and oxaloaniline; The diamine monomer is selected from at least one of 4,4'-diaminodiphenyl ether, p-phenylenediamine, 3,5-diaminobenzoic acid, 4,4-diaminodiphenylmethane, N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-dibiphenyl]-4,4'-diyl)bis(4-aminobenzamide), 2,2'-bis(trifluoromethyl)diaminobiphenyl, 4,4'-diaminobenzoylaniline, 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.

8. The polyimide separator prepared by the method of any one of claims 1-8.

9. An electrochemical device, characterized in that, The electrochemical device includes a positive electrode, a negative electrode, an electrolyte, a packaging shell, and a polyimide membrane. The polyimide membrane is located between the positive electrode and the negative electrode. The electrolyte is located inside the packaging shell and wets the positive electrode, the negative electrode, and the polyimide membrane. The polyimide membrane is prepared by the method for preparing polyimide membrane according to any one of claims 1-8.

10. The electrochemical device according to claim 9, characterized in that, The electrochemical device is one of the following: lithium secondary battery, supercapacitor, fuel cell, sodium-sulfur battery, and vanadium redox flow battery; Preferably, the lithium secondary battery is selected from one of lithium-ion secondary batteries, lithium metal secondary batteries, lithium-air secondary batteries, and lithium-sulfur secondary batteries. More preferably, the lithium-ion secondary battery includes a polymer lithium-ion secondary battery and an electrolyte support material for a solid-state battery.

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