Cross-linked polyimide lithium battery separator, preparation method therefor and use thereof

Abstract

The present invention relates to the technical field of lithium-ion battery materials. Disclosed are a cross-linked polyimide lithium battery separator, a preparation method therefor and a use thereof. The preparation method comprises the following steps: (1) adding a diamine monomer and a dianhydride monomer in a molar ratio of 1:(0.7-0.95) to a solvent for reaction, and then adding an accelerator to obtain a polyamic acid-containing slurry; and (2) coating a porous substrate with the slurry, then soaking in a reaction solution to obtain a pre-cured film, and drying the pre-cured film to obtain a cross-linked polyimide lithium battery separator, wherein the reaction solution comprises an end‑capping agent and an organic solvent, and the end‑capping agent is selected from at least one of phthalic anhydride, 4-phenylethynylphthalic anhydride, 4-ethynylphthalic anhydride, phenylethynyl trimellitic anhydride, ethynyl bisphthalic anhydride and methyl ethynylphthalic anhydride. The preparation method can improve the temperature resistance of the separator, ensuring that the separator does not melt and deform at 350°C or less, and improving the tensile strength and puncture resistance of the separator.

Description

Cross-linked polyimide lithium battery separator, its preparation method and application Technical Field

[0001] This invention relates to cross-linked polyimide lithium battery separators, their preparation methods, and applications, belonging to the field of lithium-ion battery material technology. Background Technology

[0002] The most common method to improve the high-temperature resistance of battery separators is to coat a porous base membrane with high-temperature resistant inorganic materials, most commonly alumina or boehmite. These materials can reduce the thermal shrinkage of the separator by 1% to 3% at 120°C for 1 hour. However, at higher temperatures, such as above 150°C, the separator coated with the inorganic high-temperature resistant material will still shrink significantly. Furthermore, because the high-temperature resistant inorganic materials are relatively heavy, they significantly increase the areal density of the separator, thereby reducing the energy density of the battery.

[0003] Furthermore, separators coated with inorganic high-temperature resistant materials do not adhere to the positive and negative electrode interfaces, leading to interface deformation and lithium plating during long-term battery cycling. Currently, separators consisting of a polyethylene microporous membrane and a heat-resistant porous layer are known, with ceramic powder incorporated into the heat-resistant porous layer primarily to improve ion permeability. However, even the addition of ceramic powder, typically metal oxides, has no effect on the separator's flame retardancy. Because ceramic particles are generally hard, such separators cause significant wear on the battery's metal casing, resulting in metal powder detaching and adhering to the separator, thus reducing battery performance.

[0004] To address the above issues, it is necessary to research a release membrane that is more heat-resistant, lightweight, safe, flame-retardant, and has a certain degree of interfacial adhesion. Summary of the Invention

[0005] To address the aforementioned issues, a cross-linked polyimide lithium battery separator, its preparation method, and its applications are provided. The preparation method of this cross-linked polyimide lithium battery separator can improve the temperature resistance of the separator, ensuring that it does not melt or deform below 350°C, while also improving the tensile strength and puncture strength of the separator.

[0006] According to one aspect of the present invention, a method for preparing a cross-linked polyimide lithium battery separator includes the following steps:

[0007] (1) Add diamine monomers and dianhydride monomers in a molar ratio of 1:(0.7-0.95) to a solvent and react them. Then add an accelerator to obtain a slurry containing polyamic acid.

[0008] (2) The slurry is coated on a porous substrate and then immersed in a reaction solution to obtain a pre-cured film. The pre-cured film is dried to obtain a cross-linked polyimide lithium battery separator.

[0009] The reaction solution includes a capping agent and an organic solvent, wherein the capping agent is selected from at least one of phthalic anhydride, 4-phenylethynyl phthalic anhydride, 4-ethynyl phthalic anhydride, phenylethynyl trimellitic anhydride, ethynyl bisphthalic anhydride and methylethynyl phthalic anhydride.

[0010] In the phase separation process of polyamic acid slurry, this invention utilizes a reaction solution containing a capping agent for pre-curing, enabling the capping agent to migrate between the polyimide molecular chains of the polyamic acid. This allows the polyimide molecular chains to undergo self-crosslinking under the action of the capping agent, thereby achieving functional modification of the polyimide separator, improving the separator's temperature resistance, tensile strength, and puncture resistance, and increasing the battery's capacity under high-rate charge and discharge, thus enhancing the battery's operational safety.

[0011] The reaction equation in step (1) is as follows:

[0012] The equation for the reaction between the above-mentioned capping agent and polyamic acid is as follows:

[0013] Optionally, the capping agent accounts for 5-30% of the weight of polyamic acid.

[0014] The amount of this end-capping agent can control the number of crosslinking points in the diaphragm and the chain segment length between the crosslinking points, thereby simultaneously improving the diaphragm's temperature resistance, safety, mechanical strength, and flame retardancy. If the amount of this end-capping agent is less than 10%, the diaphragm is prone to pore breakage and shrinkage; if the amount is greater than 30%, the diaphragm's uniformity decreases and it is prone to cracking.

[0015] Optionally, the weight ratio of the capping agent in the reaction solution is 10-30 wt%.

[0016] The content of the end-capping agent in the above reaction solution can ensure the smooth progress of the above reaction. If the content of the end-capping agent is too low, the diaphragm is prone to pore breakage and shrinkage. If the content of the end-capping agent is too high, the uniformity of the diaphragm will decrease and cracks will easily occur.

[0017] Optionally, the organic solvent is selected from at least one of ethanol, acetone, tetrahydrofuran, dimethylacetamide, and dimethyl sulfoxide;

[0018] The diamine monomer is selected from at least one of 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, 2,2'-bis(trifluoromethyl)diaminobiphenyl, 2,2'-bis(trifluoromethyl)-4,4'-bis(4-aminophenylcarbonylamino)biphenyl, 4,4'-diaminodiphenyl ether, and 3,5-diaminobenzoic acid;

[0019] The dianhydride monomers are selected from at least one of diphenyl ether tetracarboxylic dianhydride, hexafluoro dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and pyromellitic dianhydride.

[0020] Optionally, the porous substrate is selected from at least one of polyethylene terephthalate substrate, polypropylene substrate, polyvinylidene fluoride substrate, polyamide substrate, polyacrylonitrile substrate, cellulose substrate, and polyethylene substrate.

[0021] Preferably, the polyethylene substrate comprises a high-density polyethylene substrate.

[0022] Optionally, the thickness of the porous substrate is 5μm-30μm, and the air permeability is 100s / 100ml to 700s / 100ml.

[0023] Excessive thickness of the porous substrate can lead to mechanical instability of the separator, increasing the risk of tearing or damage. In terms of battery performance, it can increase the internal resistance of the battery, resulting in decreased discharge efficiency and increased energy loss. It can also reduce the amount of positive and negative electrodes used in the battery, leading to a decrease in energy density.

[0024] Air permeability: Low air permeability (>1000s / 100ml) may affect the charging process of the separator in the battery, resulting in a longer charging time and the potential accumulation of gas inside the battery, increasing internal pressure and even causing the battery to swell or rupture.

[0025] Optionally, the reaction temperature in step (1) is 22-25℃ and the reaction time is 60-80 min;

[0026] The soaking time in step (3) is 10 minutes, and the soaking temperature is 25-50℃.

[0027] 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.

[0028] At the above immersion temperature, polyamic acid undergoes cross-linking to obtain a functionalized modified polyimide membrane, which improves the membrane's temperature resistance, safety, mechanical strength, and flame retardancy.

[0029] Optionally, the thickness of the cross-linked polyimide lithium battery separator is 10μm-50μm. This thickness can affect the charge and discharge performance of the separator. If the separator is too thick, it will cause the internal resistance of the battery to increase and the ion flow to be restricted, and the charging process may become slower, affecting the convenience of use.

[0030] For example, the thickness of the polyimide membrane can be 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, or any value in between.

[0031] Optionally, the drying in step (2) is performed using a near-infrared tunnel oven with an energy setting of 40% and a drying time of 5 minutes.

[0032] Using a near-infrared tunnel oven prevents polyamic acid from shrinking on a heat-sensitive porous substrate, increasing its uniformity. Since polyimide can improve the mechanical strength, temperature resistance, safety, flame retardancy, and electrolyte absorption rate of the separator, it can affect the safety of the battery. If the drying time is too short, it will cause uneven drying within the separator, affecting its temperature resistance and mechanical strength. If the drying time is too long, it may cause the separator to shrink, thus affecting the improvement of the separator's internal stress and reducing the convenience of subsequent battery assembly.

[0033] Optionally, the molar ratio of the accelerator to 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene in step (1) is (1.5-4.5):1.

[0034] Preferably, the accelerator is selected from at least one of triethylamine, benzimidazole, 1-methylimidazole, quinoline, 1,8-diazabicycloundec-7-ene, imidazole, pyridine, and 3-methylpyridine.

[0035] Adding an accelerator facilitates the cross-linking reaction in step (2). The accelerator acts as an affinity agent, attacking the carbon atoms on the carboxyl groups in polyamic acid and transferring hydrogen atoms to the carboxyl groups, thereby promoting the catalytic mechanism of cyclization and dehydration, thus improving the performance of the obtained membrane, reducing the reaction time, and saving production costs. If the amount of the accelerator is too small, the improvement in the promotion of the reaction will not be obvious. If the amount is too large, it will increase the cost and reduce the molecular weight.

[0036] Optionally, an inert gas is introduced for protection during the reaction in step (1), including nitrogen, argon, helium, neon, krypton, and xenon.

[0037] Optionally, the solvent in step (1) is selected from polar solvents, low-boiling-point solvents, or low-water-absorbing solvents.

[0038] Optionally, the solvent in step (1) 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).

[0039] 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.

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

[0041] According to another aspect of the present invention, a lithium battery is provided, comprising a positive electrode, a negative electrode, an electrolyte, a packaging shell, and a separator, wherein the separator 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 separator, and the separator is prepared by the preparation method of the cross-linked polyimide lithium battery separator according to any one of claims 1-8.

[0042] Optionally, the lithium battery is selected from one of lithium-ion secondary batteries, lithium metal secondary batteries, lithium-air secondary batteries, and lithium-sulfur secondary batteries; and the lithium-ion secondary battery includes polymer lithium-ion secondary batteries.

[0043] The beneficial effects of the present invention include, but are not limited to:

[0044] 1. The preparation method of the cross-linked polyimide lithium battery separator of the present invention, in the phase separation process of polyamic acid slurry, the end-capping agent in the reaction solution can cross-link with the polyamic acid molecular chain to achieve functional modification of the polyimide separator, thereby obtaining a separator with high temperature resistance, light weight, high safety and high flame retardancy.

[0045] 2. The cross-linked polyimide lithium battery separator prepared by this invention has improved mechanical strength and puncture resistance, high porosity, and better wettability and absorption rate of electrolyte compared with existing polyimide separators. It can improve the ionic conductivity of lithium batteries, enable rapid lithium ion migration, and improve the rate performance of batteries.

[0046] 3. The cross-linked polyimide lithium battery separator prepared by this invention can remain non-melting and non-deformed below 350℃. The separator has good durability, which is beneficial for industrial production and application, and improves the stability of lithium-ion batteries while extending their service life. Attached Figure Description

[0047] 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:

[0048] Figure 1 is a schematic diagram of polyimide membranes prepared with different amounts of end-capping agent according to Example 1 of the present invention. Detailed Implementation

[0049] The present invention will be described in detail below with reference to the embodiments, but the present invention is not limited to these embodiments.

[0050] Unless otherwise specified, the raw materials used in the embodiments of this invention were all purchased through commercial channels.

[0051] In the examples described below, the Celgard 2325 membrane was purchased from Celgard Corporation. It is a three-layer (polypropylene / polyethylene / polypropylene) membrane with a thickness of 25 μm and a porosity of 39%.

[0052] Unless otherwise specified, the methods used in the embodiments of the present invention are conventional methods in the prior art.

[0053] 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.

[0054] 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: P(%)=(Mw-Md) / ρVdx100

[0055] Where ρ is the density of n-butanol and Vd is the geometric volume of the membrane, Mw and Md are the mass of n-butanol absorbed by the membrane and the mass of the membrane itself.

[0056] The method for testing electrolyte absorption rate is as follows: Immerse the diaphragm in the electrolyte (LiPF6 EC: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: EU = [(W-Wo) / Wo] × 100%

[0057] Among them, Wo and W are the mass of the diaphragm before and after absorbing the electrolyte.

[0058] 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) plate electrodes, 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.

[0059] σ=d / (Rb×S)

[0060] 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.

[0061] Puncture strength: The puncture strength of the cross-linked polyimide diaphragm was tested using a self-made puncture test fixture and puncture needle method according to the GB / T10004-2008 test standard.

[0062] Tensile strength: The tensile strength and elongation of the cross-linked polyimide diaphragm were tested using the tensile test method for plastics in GB1040-79.

[0063] Wetting angle test method: Using a contact angle meter, cut the sample into 2*2 cm pieces and fix the four corners flat on a glass slide. Take 10 μL of EC / DEC electrolyte and drop it onto the separator membrane for analysis.

[0064] Test method for negative LTO: Press the positive and negative electrode materials and the diaphragm to be tested onto the button battery 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 (1.0V / 2.0V for LTO) to perform rate testing.

[0065] Test method for temperature resistance (diaphragm shrinkage rate): First, the diaphragm to be tested is cut into circles with a diameter of 19mm and baked at 150℃ for 1 hour. To more accurately determine the shrinkage rate, the area is compared using Autodesk mapping. The formula for calculating the diaphragm shrinkage rate 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] The test method for flame retardancy (vertical tray burning) is as follows: the diaphragm material (≤25um) is clamped onto the tray and burned with a flame source (20mm high). After the flame source is removed, the standards for each level are formulated based on the burning speed, burning time, anti-dripping ability, and whether the drips burn.

[0067] Example 1

[0068] This embodiment relates to a method for preparing a cross-linked polyimide lithium battery separator, comprising the following steps:

[0069] (1) Under nitrogen atmosphere, 36.45 g (0.0850 mol) of 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene monomer was added to 494.36 g of dimethylacetamide solvent into a 1000 ml glass reaction flask and stirred at 22 °C for 30 min until completely dissolved. Then, 18.48 g (0.0595 mol) of diphenyl ether tetracarboxylic dianhydride was added and stirred at 22 °C for 60 min. After that, 24.18 g of quinoline was added to obtain a slurry containing polyamic acid.

[0070] (2) The slurry was cooled to room temperature and coated onto a porous PE substrate with a 9μm diameter and an air permeability of 150s / 100ml. It was then immersed in a reaction solution for 10 minutes to obtain a pre-cured film at 30℃. The pre-cured film was placed in a near-infrared tunnel oven with an energy setting of 40% and dried for 5 minutes to obtain a cross-linked polyimide lithium battery separator. The reaction solution consisted of a capping agent and ethanol. The weight ratio of the capping agent in the reaction solution was 20%, and the capping agent accounted for 10% of the weight of polyamic acid. The capping agents were phthalic anhydride, 4-phenylethynyl phthalic anhydride, and 4-ethynyl phthalic anhydride. The three separators were named 10% phthalic anhydride, 10% 4-phenylethynyl phthalic anhydride, and 10% 4-ethynyl phthalic anhydride, respectively.

[0071] The performance of the three types of membranes was tested. The commercially available Celgard 2325 membrane was used as control 1, and the membrane obtained by adding no capping agent to the reaction solution was used as control 2. The results are shown in Table 1.

[0072] Table 1

[0073] As shown in Table 1, at the same thickness, the membrane prepared using the porous PE substrate has the highest tensile strength of 36 MPa, while the sample using Celgard 2325 has a tensile strength of only 13 MPa. This demonstrates that the porous polyimide / PE membrane of the present invention is 1.8 times stronger than the existing Celgard 2325 membrane. Furthermore, the porosity of Comparative 2 is approximately 39% higher than that of Celgard 2325. After assembling the above membranes into batteries, rate performance tests were conducted. The battery composed of the Celgard 2325 membrane exhibited a lower capacity under high-rate charge-discharge (2C) conditions than the cross-linked polyimide membrane prepared in this invention. This is because the ionic conductivity difference between 10% 4-phenylethynyl phthalic anhydride-1 and Comparative 1 reached 136%.

[0074] Referring to Figure 1, the properties of polyimide membranes prepared with different amounts of end-capping agents are different. When the amount of end-capping agent is less than 10% of the weight of polyamic acid, it will cause the membrane to break and shrink. When the amount of end-capping agent is greater than 30% of the weight of polyamic acid, it will cause the membrane to lose uniformity and crack. Only when the amount of end-capping agent is between 10% and 30% of the weight of polyamic acid can a complete membrane be formed.

[0075] Example 2

[0076] This embodiment relates to a method for preparing a cross-linked polyimide lithium battery separator, comprising the following steps:

[0077] (1) Under nitrogen atmosphere, 42.83 g (0.1 mol) of 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene monomer was added to 483.92 g of dimethylacetamide solvent in a 1000 ml glass reaction flask and stirred at 25 °C for 30 min until completely dissolved. Then, 29.47 g (0.095 mol) of diphenyl ether tetracarboxylic dianhydride was added and the reaction was stirred at 25 °C for 60 min. After that, 38.75 g of quinoline was added to obtain a slurry containing polyamic acid.

[0078] (2) The slurry was cooled to room temperature and coated onto a porous PE substrate with a diameter of 12 μm and an air permeability of 235 s / 100 ml. It was then immersed in a reaction solution for 10 min to obtain a pre-cured film at 40°C. The pre-cured film was placed in a near-infrared tunnel oven with an energy setting of 40% and dried for 5 min to obtain a cross-linked polyimide lithium battery separator. The reaction solution consisted of a capping agent and tetrahydrofuran. The weight ratio of the capping agent in the reaction solution was 10%, and the capping agent accounted for 10% of the weight of polyamic acid. The capping agents were phenylacetyltriphenylamine anhydride, acetylene diphthalic anhydride, and methylacetylene phthalic anhydride. The three separators were named 10% phenylacetyltriphenylamine anhydride, 10% acetylene diphthalic anhydride, and 10% methylacetylene phthalic anhydride, respectively.

[0079] The performance of the three types of membranes was tested. The commercially available Celgard 2325 membrane was used as control 1, and the membrane obtained by not adding the end-capping agent to the reaction solution was used as control 2. The results are shown in Table 2.

[0080] Table 2

[0081] As shown in Table 2, at the same thickness, the membrane prepared using the porous PE substrate has the highest tensile strength of 42 MPa, while the sample using Celgard 2325 has a tensile strength of only 13 MPa. This demonstrates that the porous polyimide / PE membrane of the present invention is twice as strong as the existing Celgard 2325 membrane. Furthermore, the porosity of Comparative 2 is approximately 32.8% higher than that of Celgard 2325. After assembling the above membranes into batteries, rate performance tests were conducted. The battery composed of the Celgard 2325 membrane exhibited a lower capacity at high-rate charge / discharge (2C) than the cross-linked polyimide membrane prepared in this invention. This is because the ionic conductivity of 10% methylacetylene phthalic anhydride differs by 156% compared to Comparative 1.

[0082] Example 3

[0083] The difference between this embodiment and the sample code "10% phthalic anhydride" in Example 1 is that the weight percentage of the end-capping agent in the reaction solution is 5% (sample code 5% phthalic anhydride), 20% (sample code 20% phthalic anhydride), 30% (sample code 20% phthalic anhydride), and 40% (sample code 20% phthalic anhydride), respectively. The rest is the same as the 10% phthalic anhydride sample.

[0084] The performance of the three types of diaphragms was tested, and the results are shown in Table 3.

[0085] Table 3

[0086] Table 3 shows that, at the same thickness, the membrane prepared using porous PE substrate has the highest tensile strength of 37 MPa. However, when the end-capping agent accounts for 40% of the weight percentage of polyamic acid, its performance decreases significantly. The main reason is that the end-capping and excessive amount cause the polyamic acid crosslinking to be less than expected, resulting in unevenness and cracking within the membrane surface. After assembling the above membranes into batteries, rate performance tests were conducted. The battery composed of 40% phthalic anhydride membrane had a lower capacity than the 20% phthalic anhydride membrane under high-rate charge-discharge (2C), with a difference in ionic conductivity of 53.2%.

[0087] Example 4

[0088] The difference between this embodiment and the 10% phthalic anhydride sample in Example 1 is that the polyethylene substrate is replaced with a cellulose substrate (sample code CMC), a polypropylene substrate (sample code PP), and a polyvinylidene fluoride vinyl board (sample code PVDF), respectively. The rest is the same as the 10% phthalic anhydride sample.

[0089] The performance of the three types of diaphragms was tested, and the results are shown in Table 4.

[0090] Table 4

[0091] Table 4 shows that, at the same thickness, the tensile strength of the separator prepared using the porous PP substrate can reach as high as 33 MPa. However, the performance of separators using the other two substrates is significantly reduced. The main reason is that the CMC substrate is easily damaged by immersion in an ethanol solution, which subsequently affects the crosslinking of polyamic acid and causes pores in the separator surface. On the other hand, coating the slurry onto the PVDF substrate is prone to shrinkage due to the hydrophobic surface. After assembling the above separators into batteries, rate performance tests were conducted. The battery composed of the PP substrate separator has a higher capacity than the polyimide separators of the other two crosslinking structures under high-rate charge and discharge (2C), with a difference of 37.9% between the PP substrate and the CMC substrate.

[0092] Example 5

[0093] The difference between this embodiment and the 10% phthalic anhydride sample in Example 1 is that the thickness of the porous PE substrate is 9 μm (sample code H9), 16 μm (sample code H16), and 20 μm (sample code H20), respectively. The rest is the same as the 10% phthalic anhydride sample.

[0094] The performance of the three types of diaphragms was tested, and the results are shown in Table 5.

[0095] Table 5

[0096] Table 5 shows that the basic physical properties do not differ significantly with different thicknesses of porous PE substrates. After assembling the above separators into batteries, rate performance tests were conducted. The battery composed of a separator made from a 20μm porous PE substrate exhibited lower capacity at high-rate charge-discharge (2C) than the other two cross-linked polyimide separators. This is mainly because increased substrate thickness leads to increased internal resistance; the conductivity difference between the separators prepared from 9μm and 20μm substrates reached 7.2%.

[0097] Example 6

[0098] The difference between this embodiment and the sample code "10% phthalic anhydride" in Example 1 is that the air permeability values ​​of the porous PE substrate are 100s / 100ml (sample code BV100), 400s / 100ml (sample code BV400), and 700s / 100ml (sample code BV700), respectively. The rest are the same as the 10% phthalic anhydride sample.

[0099] The performance of the three types of diaphragms was tested, and the results are shown in Table 6.

[0100] Table 6

[0101] Table 6 shows that the basic physical properties of porous PE substrates with different air permeability values ​​are not significantly different. The main difference lies in the substrate's air permeability. Higher air permeability leads to decreased backflow when soaked in ethanol, resulting in reduced porosity. The difference between BV100 and BV700 reaches 7.01%. After assembling the above separators into batteries, rate performance tests were conducted. Batteries composed of separators made with BV100 exhibit higher capacity at high-rate charge / discharge (2C) than the other two cross-linked polyimide separators. This is mainly because the reduced porosity decreases lithium-ion transport efficiency during fast charging, with a difference of 6.5% between BV100 and BV700.

[0102] Example 7

[0103] The difference between this embodiment and the sample code "10% phthalic anhydride" in Embodiment 1 is that the soaking temperature in step (2) is 30℃ (sample code T30), 40℃ (sample code T40), and 50℃ (sample code T50), respectively, while the rest is the same as the 10% phthalic anhydride sample.

[0104] The performance of the three types of diaphragms was tested, and the results are shown in Table 7.

[0105] Table 7

[0106] Table 7 shows that the main difference in basic physical properties at different immersion temperatures lies in porosity. As the immersion temperature increases, the solvent evaporation rate increases, leading to significant concentration changes. This results in less-than-expected crosslinking of the polyamic acid, causing some pores to form within the membrane surface, thus increasing porosity. The difference between T30 and T50 reaches 10.8%. After assembling the above membranes into batteries, rate performance tests were conducted. The battery composed of the membrane prepared with T40 exhibited higher capacity at high-rate charge-discharge (2C) than the other two crosslinked polyimide membranes. This is mainly because the stability of the crosslinked polyamic acid increases at appropriate temperatures, with a difference of 10.29% between T40 and T50.

[0107] 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 producing a cross-linked polyimide lithium battery separator, characterized by, Includes the following steps: (1) Add diamine monomers and dianhydride monomers in a molar ratio of 1:(0.7-0.95) to a solvent and react them. Then add an accelerator to obtain a slurry containing polyamic acid. (2) The slurry is coated on a porous substrate and then immersed in a reaction solution to obtain a pre-cured film. The pre-cured film is dried to obtain a cross-linked polyimide lithium battery separator. The reaction solution includes a capping agent and an organic solvent, wherein the capping agent is selected from at least one of phthalic anhydride, 4-phenylethynyl phthalic anhydride, 4-ethynyl phthalic anhydride, phenylethynyl trimellitic anhydride, ethynyl bisphthalic anhydride and methylethynyl phthalic anhydride.

2. The production method according to claim 1, characterized by, The capping agent accounts for 5-30% of the weight of polyamic acid.

3. The production method according to claim 1, characterized by, The weight ratio of the capping agent in the reaction solution is 10-30 wt%.

4. The method of claim 1, wherein, The organic solvent is selected from at least one of ethanol, acetone, tetrahydrofuran, dimethylacetamide, and dimethyl sulfoxide; The diamine monomer is selected from at least one of 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, 2,2'-bis(trifluoromethyl)diaminobiphenyl, 2,2'-bis(trifluoromethyl)-4,4'-bis(4-aminophenylcarbonylamino)biphenyl, 4,4'-diaminodiphenyl ether, and 3,5-diaminobenzoic acid; The dianhydride monomers are selected from at least one of diphenyl ether tetracarboxylic dianhydride, hexafluoro dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and pyromellitic dianhydride.

5. The production method according to claim 4, characterized by, The porous substrate is selected from at least one of polyethylene terephthalate substrate, polypropylene substrate, polyvinylidene fluoride board, polyamide substrate, polyacrylonitrile substrate, cellulose substrate, and polyethylene substrate.

6. The method of claim 1, wherein, The thickness of the porous substrate is 5μm-30μm, and the air permeability is 100s / 100ml to 700s / 100ml.

7. The preparation method according to claim 1, characterized in that, The reaction temperature in step (1) is 22-25℃, and the reaction time is 60-80 min; The soaking time in step (3) is 10 minutes, and the soaking temperature is 25-50℃.

8. The cross-linked polyimide lithium battery separator prepared by the method according to any one of claims 1-8.

9. A lithium battery, characterized by It includes a positive electrode, a negative electrode, an electrolyte, a packaging shell, and a separator. The separator 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 separator. The separator is prepared by the preparation method of the cross-linked polyimide lithium battery separator according to any one of claims 1-8.

10. The electrochemical device of claim 9, wherein, The lithium battery is selected from one of lithium-ion secondary batteries, lithium metal secondary batteries, lithium-air secondary batteries, and lithium-sulfur secondary batteries; and the lithium-ion secondary battery includes polymer lithium-ion secondary batteries.

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