Polyimides, methods for their preparation, composite films and uses thereof

Abstract

The application relates to the technical field of new materials, and discloses a polyimide, a preparation method of the polyimide, a composite film and application of the polyimide. The polyimide contains repeating units formed by bonding of a polyamine and a polyacid anhydride, and the structural formula of the polyamine is shown in formula (I) and / or formula (II). The preparation method of the polyimide comprises the following steps: mixing the polyamine and the polyacid anhydride under an oxygen-free condition to perform a first-stage reaction, and then mixing the polyamine and the polyacid anhydride with an initiator to perform a second-stage reaction. The polyimide is soluble in both a low-boiling-point solvent and a high-boiling-point solvent, has the characteristics of high heat resistance, strong adhesion and high strength, and the heat performance and safety performance of the composite film formed by coating the polyimide are greatly improved.

Description

Technical Field

[0001] This invention relates to the field of new materials technology, specifically to a polyimide and its preparation method, a composite film and its application. Background Technology

[0002] Lithium-ion battery separators are a crucial component of lithium-ion batteries, and high safety is essential for long-term operation. Commercially available coated separators are mainly divided into three categories: inorganic ceramic coated separators, polyvinylidene fluoride (PVDF) coated separators, and aramid coated separators. Among these, oil-based PVDF coated separators, with their strong adhesion, are primarily used in consumer batteries. Oil-based PVDF is coated using two types of solvent systems: one is high-boiling-point organic solvent coating, with solvents including N,N-dimethylacetamide (DMAC), N-methylpyrrolidone (NMP), and N,N-dimethylformamide (DMF); the other is low-boiling-point organic solvent coating, with solvents including acetone and tetrahydrofuran, primarily acetone. PVDF separators coated with high-boiling-point solvents have lower costs and higher safety; PVDF separators coated with low-boiling-point solvents such as acetone have better porosity, higher permeability, and better air permeability. Both types of solvent-based PVDF coated separators have a broad market.

[0003] However, due to the low melting point and glass transition temperature of PVDF, its heat resistance is poor, resulting in low heat resistance in separators. With the increasing demand in the consumer battery market and the rising requirements for battery safety, the overall performance of existing PVDF-coated separators, especially in terms of heat resistance and strength, urgently needs improvement. At the same time, because the production and use of PVDF are environmentally damaging, the industry has begun to gradually reduce or even restrict PVDF production capacity and usage.

[0004] To address this, the industry has attempted to develop novel organic polymers to replace PVDF or improve the performance of PVDF membranes through coating with novel composite organic materials. Novel organic polymers must possess the following characteristics: ① Excellent solubility in organic solvents, especially for PVDF systems coated with low-boiling-point organic solvents; the new material must be soluble in low-boiling-point, low-polarity solvents such as acetone. ② Strong heat resistance, which helps reduce the membrane's thermal shrinkage rate. ③ Strong adhesiveness, ensuring that after PVDF coating, while improving heat resistance, the overall adhesion of the membrane is enhanced, reducing PVDF usage and improving overall performance. However, due to the low polarity and low boiling point of acetone used in coating, and the low drying temperature during processing, polymers, including polyimide, have poor solubility in acetone systems. Even with the addition of small amounts of high-boiling-point, highly polar solvents for solubilization, the high-boiling-point solvents are difficult to separate, leading to static electricity buildup in the membrane.

[0005] Therefore, to date, there are few reports on polymer materials that meet these characteristics and their applications in the preparation of lithium battery separators coated in acetone, DMAC, and NMP systems. Summary of the Invention

[0006] The purpose of this invention is to overcome the problems of poor solubility of polyimide in low-boiling-point solvents and weak adhesion to lithium-ion batteries in the prior art. This invention provides a polyimide, its preparation method, a composite film, and its applications. This polyimide is soluble in both low-boiling-point and high-boiling-point solvents and possesses characteristics such as high heat resistance, strong adhesion, and high strength. The composite film formed by coating with this polyimide exhibits significantly improved thermal and safety performance.

[0007] To achieve the above objectives, the first aspect of the present invention provides a polyimide containing repeating units formed by the bonding of a polyamine and a polyacid anhydride, wherein the polyamine has the structural formula shown in formula (I) and / or formula (II).

[0008]

[0009] Wherein, R1 is absent or selected from C1-C5 alkylene groups, -O-,

[0010] R2 and R3 are each independently selected from at least one of hydrogen, halogen, hydroxyl, carboxyl, and perfluoromethyl.

[0011] R4 is a C1-C5 alkylene group.

[0012] The second aspect of the present invention provides a method for preparing polyimide, the method comprising: mixing a polyamine with a polyacid anhydride under anaerobic conditions to carry out a first-stage reaction, and then mixing with an initiator to carry out a second-stage reaction;

[0013] The structural formulas of the polyamines are shown in formula (I) and / or formula (II);

[0014]

[0015] Wherein, R1 is absent or selected from C1-C5 alkylene groups, -O-,

[0016] R2 and R3 are each independently selected from at least one of hydrogen, halogen, hydroxyl, carboxyl, and perfluoromethyl.

[0017] R4 is a C1-C5 alkylene group.

[0018] A third aspect of the present invention provides a composite membrane comprising a base membrane and a coating coated on the surface of the base membrane, the coating comprising a polyimide as described above or a polyimide prepared according to the method described above.

[0019] The fourth aspect of the present invention provides the use of at least one of the polyimide as described above, the polyimide prepared according to the method as described above, and the composite film as described above in a battery separator.

[0020] Through the above technical solution, the polyimide provided by the present invention has repeating units formed by the bonding of specific polyamines and polyanhydrides. It is soluble in both low-boiling-point solvents such as acetone and high-boiling-point solvents such as N,N-dimethylacetamide. It has excellent solubility in organic solvents, and has strong heat resistance, strong adhesion, and high strength. The composite film formed by coating with this polyimide has high adhesion to the electrode, strong heat resistance, and high puncture strength. When applied to batteries, it improves the thermal abuse pass rate of the battery cell, reduces cell expansion, enhances safety, and helps extend the battery's service life. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the composite film I formed using the polyimide prepared in Example 1 of the present invention in Test Example 1. Detailed Implementation

[0022] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0023] The first aspect of the present invention provides a polyimide containing repeating units formed by the bonding of a polyamine and a polyacid anhydride, wherein the polyamine has the structural formula shown in formula (I) and / or formula (II).

[0024]

[0025] Wherein, R1 is absent or selected from C1-C5 alkylene groups, -O-,

[0026] R2 and R3 are each independently selected from at least one of hydrogen, halogen, hydroxyl, carboxyl, and perfluoromethyl.

[0027] R4 is a C1-C5 alkylene group.

[0028] During their research, the inventors of this invention discovered that polyimide possesses repeating units formed by the bonding of specific polyamines and polyacid anhydrides. This polyimide material is soluble in both low-boiling-point solvents such as acetone and high-boiling-point solvents such as N,N-dimethylacetamide, exhibiting excellent solubility in organic solvents. Furthermore, it possesses strong heat resistance, strong adhesion, and high strength, resulting in high adhesion between the composite film coated with this polyimide and the electrode. Simultaneously, it was unexpectedly discovered that the composite film coated with this polyimide exhibits strong heat resistance and high puncture strength. Its application in batteries improves the thermal abuse tolerance of the cell, reduces cell expansion, enhances safety, and helps extend battery life.

[0029] In this invention, the absence of R1 in formula (I) indicates that the two benzene rings in formula (I) are directly connected. When R1 is a C1-C5 alkylene group, it can specifically be methylene, 1,1-ethylene, 1,1-propylene, 1,3-propylene, 2,2-propylene, 2-methyl-1,1-propylene, 1,2-ethylene, 1-methyl-1,2-ethylene, 1-ethyl-1,2-ethylene, 1-propyl-1,2-ethylene, 1-isopropyl-1,2-ethylene, 2 One of the following: methyl-1,2-ethylene, 2-ethyl-1,2-ethylene, 2-propyl-1,2-ethylene, 2-isopropyl-1,2-ethylene, 1-methyl-1,3-propylene, 1-ethyl-1,3-propylene, 2-methyl-1,3-propylene, 2-ethyl-1,3-propylene, 2-propyl-1,3-propylene, 1,4-butylene, 1-methyl-1,4-butylene, and 2-methyl-1,4-butylene; preferably methylene.

[0030] When R2 and R3 are each an independent halogen, the halogen can be fluorine, chlorine, or bromine, with fluorine being preferred.

[0031] R4 is a C1-C5 alkylene group, specifically 1,1-ethylene, 1,1-propylene, 1,3-propylene, 2,2-propylene, 2-methyl-1,1-propylene, 1,2-ethylene, 1-methyl-1,2-ethylene, 1-ethyl-1,2-ethylene, 1-propyl-1,2-ethylene, 1-isopropyl-1,2-ethylene, 2-methyl-1,2-ethylene, 2-ethyl-1,2- One of ethylene, 2-propyl-1,2-ethylene, 2-isopropyl-1,2-ethylene, 1-methyl-1,3-propylene, 1-ethyl-1,3-propylene, 2-methyl-1,3-propylene, 2-ethyl-1,3-propylene, 2-propyl-1,3-propylene, 1,4-butylene, 1-methyl-1,4-butylene, and 2-methyl-1,4-butylene; preferably 2,2-propylene.

[0032] According to the present invention, preferably, the polyamine is selected from at least one of 9,9-dimethylfluorene-2,7-diamine, 3,3'-dihydroxybenzidine, 6,6'-diamino-3,3'-methylenedibenzoic acid, 2,2'-di(trifluoromethyl)diaminobiphenyl, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 9,9-di(4-amino-3-fluorophenyl)fluorene, N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-dibiphenyl]-4,4'-diyl)bis(4-aminobenzamide), 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether, and 3,4'-diaminodiphenyl ether. The inventors have found that, under this preferred embodiment, it is advantageous to further improve the solubility of the polyimide in low-boiling-point and high-boiling-point solvents, as well as its heat resistance, adhesion, and strength.

[0033] According to the present invention, preferably, the polyimide further comprises repeating units formed by bonding a copolyamine to the polyanhydride, wherein the copolyamine is selected from at least one of the diamines shown in formulas (III) to (VIII).

[0034]

[0035] R5, R6, R7, and R8 are each independently selected from C1-C5 alkyl groups, specifically methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, 2-methylbutyl, 3-methylbutyl, etc. The inventors have found that this preferred embodiment is beneficial for further improving the solubility, heat resistance, adhesion, and strength of polyimide in both low-boiling-point and high-boiling-point solvents.

[0036] According to the present invention, preferably, the molar ratio of the polyamine to the copolyamine is 1:15-15:1, specifically 1:15, 1:12, 1:9, 1:6, 1:3, 1:1, 3:1, 6:1, 9:1, 12:1, 15:1, or any value between the above two values, more preferably 1:2-2:1.

[0037] In this invention, the polybasic acid anhydride can be a diabasic acid anhydride, a tribasic acid anhydride, a tetrabasic acid anhydride, etc. Preferably, the polybasic acid anhydride is a diabasic acid anhydride, more preferably, it is selected from at least one of p-phenylene-bis(phenyltrilate) dianhydride, hexafluorodianhydride, 1,2,4,5-cyclohexanetetracarboxylic acid dianhydride, cyclobutanetetracarboxylic acid dianhydride, 1,2,3,4-butanetetracarboxylic acid dianhydride, and bisphenol A type diether dianhydride. The inventors have found that, under this preferred embodiment, it is beneficial to further improve the solubility, heat resistance, adhesion, and strength of polyimide in low-boiling-point solvents and high-boiling-point solvents.

[0038] The second aspect of the present invention provides a method for preparing polyimide, the method comprising: mixing a polyamine with a polyacid anhydride under anaerobic conditions to carry out a first-stage reaction, and then mixing with an initiator to carry out a second-stage reaction;

[0039] The structural formulas of the polyamines are shown in formula (I) and / or formula (II);

[0040]

[0041] Wherein, R1 is absent or selected from C1-C5 alkylene groups, -O-,

[0042] R2 and R3 are each independently selected from at least one of hydrogen, halogen, hydroxyl, carboxyl, and perfluoromethyl.

[0043] R4 is a C1-C5 alkylene group.

[0044] The polyimide preparation method provided by this invention is green and environmentally friendly, with a simple and controllable process, and the prepared polyimide is easy to separate and purify with a high yield.

[0045] According to the present invention, preferably, the conditions for the first stage reaction include at least: a temperature of -10 to 15°C, specifically -10°C, -5°C, 0°C, 5°C, 10°C, 15°C, or any value between the above two values; and a time of 1-5 hours, specifically 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, or any value between the above two values. The inventors have found that under this preferred embodiment, it is beneficial to improve the reaction efficiency and yield of polyimide.

[0046] According to the present invention, preferably, the conditions for the second stage reaction include at least: a temperature of 150-200°C, specifically 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, or any value between the above two values; and a time of 3-10 hours, specifically 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, or any value between the above two values. The inventors have found that under this preferred embodiment, it is beneficial to improve the polymerization efficiency of polyimide and the product yield.

[0047] According to the present invention, preferably, the molar ratio of the polyamine to the polyanhydride is (0.98-1.1):1, specifically it can be 0.98:1, 1:1, 1.02:1, 1.04:1, 1.06:1, 1.08:1, 1.1:1, or any value between the above two values.

[0048] According to the present invention, preferably, the polyamine is selected from at least one of 9,9-dimethylfluorene-2,7-diamine, 3,3'-dihydroxybenzidine, 6,6'-diamino-3,3'-methylenedibenzoic acid, 2,2'-di(trifluoromethyl)diaminobiphenyl, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 9,9-di(4-amino-3-fluorophenyl)fluorene, N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-diphenyl]-4,4'-diyl)bis(4-aminobenzamide), 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether, and 3,4'-diaminodiphenyl ether.

[0049] According to the present invention, preferably, the raw materials for the first stage reaction further include a copolyamine, wherein the copolyamine is selected from at least one of the diamines with structural formulas as shown in formulas (III) to (VIII);

[0050]

[0051]

[0052] R5, R6, R7 and R8 are each independently selected from C1-C5 alkyl groups;

[0053] According to the present invention, preferably, the molar ratio of the polyamine to the copolyamine is 1:15-15:1.

[0054] According to the present invention, preferably, the initiator contains acetic anhydride and pyridine. The inventors have found that, under this preferred embodiment, it is advantageous to improve the polymerization efficiency of polyimide and the product yield.

[0055] More preferably, the molar ratio of the acetic anhydride to the polyamine is (0.3-0.8):1, specifically 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, or any value between the two aforementioned values; the molar ratio of the pyridine to the polyamine is (0.3-0.8):1, specifically 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, or any value between the two aforementioned values.

[0056] In this invention, the oxygen-free condition can be achieved by introducing an inert gas.

[0057] According to the present invention, preferably, the first stage reaction is carried out in the presence of a reaction solvent selected from at least one of acetone, N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF), and N-ethylpyrrolidone (NEP). In this case, the process of mixing the polyamine with the polyacid anhydride can be as follows: first, the polyamine is mixed with the reaction solvent and cooled to -10 to 15°C, and then mixed with the diacid anhydride to carry out the first stage reaction. The inventors have found that, under this preferred embodiment, it is beneficial to improve the reaction efficiency and product yield of the polyimide.

[0058] More preferably, the weight ratio of the reaction solvent to the polyamine is 5-15:1;

[0059] According to the present invention, preferably, the method further includes: mixing the reaction solution obtained from the second stage reaction with a poor solvent, performing solid-liquid separation, and drying, so as to separate and purify the polyimide from the reaction solution.

[0060] More preferably, the undesirable solvent is selected from at least one of water, methanol, ethanol, isopropanol, and petroleum ether.

[0061] Preferably, the conditions for the mixing process include at least the following: a temperature of 5-35°C, specifically 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, or any value between the two above; and a time of 1.5-3 hours, specifically 1.5 hours, 2 hours, 2.5 hours, 3 hours, or any value between the two above.

[0062] The polyimide provided by this invention exhibits excellent solubility in organic solvents, strong heat resistance, strong adhesion, and high strength, making it suitable for preparing battery separators. It demonstrates high adhesion to electrodes, strong heat resistance, and high puncture strength. A third aspect of this invention provides a composite membrane comprising a base membrane and a coating applied to the surface of the base membrane. The coating contains the polyimide described above or a polyimide prepared according to the methods described above.

[0063] In this invention, the coating is located on one or both sides of the base film, and the thickness of the coating is 1-10 μm, specifically 1 μm, 3 μm, 5 μm, 7 μm, 9 μm, 10 μm, or any value between the two values ​​mentioned above.

[0064] According to the present invention, preferably, the base film is selected from at least one of polypropylene film (PP), polyethylene film (PE), polypropylene film with a ceramic layer coated on its surface, and polyethylene film with a ceramic layer coated on its surface. The inventors have found that, under this preferred embodiment, it is beneficial to improve the adhesion, heat resistance, and puncture strength between the composite film and the electrode.

[0065] According to the present invention, preferably, the polyimide content in the coating is 1-30% by weight, specifically 1% by weight, 5% by weight, 10% by weight, 15% by weight, 20% by weight, 25% by weight, 30% by weight, or any value between the above two values. The inventors have found that, under this preferred embodiment, it is beneficial to improve the adhesion, heat resistance, and puncture strength between the composite film and the electrode, resulting in stronger battery safety and helping to extend the battery's lifespan.

[0066] According to the present invention, preferably, the coating further comprises an additive and a dispersant. The additive content in the coating is 0.5-90% by weight, specifically 0.5% by weight, 10% by weight, 20% by weight, 30% by weight, 40% by weight, 50% by weight, 60% by weight, 70% by weight, 80% by weight, or 90% by weight, or any value between the above two values; the dispersant content is 1-20% by weight, specifically 1% by weight, 5% by weight, 10% by weight, 15% by weight, or 20% by weight, or any value between the above two values. The inventors have found that, under this preferred embodiment, it is beneficial to improve the adhesion, heat resistance, and puncture strength between the composite film and the electrode, resulting in stronger battery safety and helping to extend the battery's service life.

[0067] According to the present invention, preferably, the dispersant contains at least one of modified polymethyl methacrylate (modified PMMA), carboxymethyl cellulose, sodium polyethylene glycol alkyl aryl ether sulfonate, alkylphenol polyethylene ether, polyoxyethylene alkylphenol ether, polyacrylic acid and polyvinyl alcohol, and the additive contains polyvinylidene fluoride and / or ceramics, more preferably polyvinylidene fluoride.

[0068] In this invention, the preparation method of the composite membrane may include: mixing polyimide, additives, dispersants, and solvents, stirring and dispersing to obtain a slurry, then rolling or dipping the slurry onto one or both sides of a base membrane, and drying. The solvent may be a high-boiling-point organic solvent, such as N,N-dimethylacetamide (DMAC), in which case the drying temperature is preferably 60-120°C; or a low-boiling-point solvent, such as acetone, in which case the drying temperature is preferably 30-80°C.

[0069] The fourth aspect of the present invention provides the use of at least one of the polyimide as described above, the polyimide prepared according to the method as described above, and the composite film as described above in a battery separator.

[0070] As a relatively preferred embodiment of the present invention, the method for preparing polyimide includes:

[0071] Under anaerobic conditions and in the presence of a reaction solvent, polyamines, copolyamines, and polyacid anhydrides are mixed and reacted at a temperature of -10 to 15°C for 1-5 hours in the first stage. Then, they are mixed with an initiator and reacted at a temperature of 150-200°C for 3-10 hours in the second stage. The reaction solution obtained from the second stage reaction is then mixed with a poor solvent, separated into solid and liquid components, and dried.

[0072] The polyamine has the structural formula shown in formula (I) and / or formula (II), and the copolyamine is selected from at least one of the diamines with structural formulas shown in formulas (III) to (VIII); the molar ratio of the polyamine to the polyanhydride is 0.98-1.1:1, the initiator contains acetic anhydride and pyridine, the molar ratio of the acetic anhydride to the polyamine is 0.3-0.8:1, and the molar ratio of the pyridine to the polyamine is 0.3-0.8:1;

[0073] The reaction solvent is selected from at least one of N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, and N-ethylpyrrolidone, and the weight ratio of the reaction solvent to the polyamine is 5-15:1; the unsuitable solvent is selected from at least one of water, methanol, ethanol, isopropanol, and petroleum ether; the mixing conditions include at least the following: temperature of 5-35°C and time of 1.5-3h.

[0074] The present invention will be described in detail below through embodiments.

[0075] In the following examples, 9,9-dimethylfluorene-2,7-diamine (CAS: 216454-90-9, hereinafter referred to as polyamine-1)

[0076] 3,3'-Dihydroxybenzidine (CAS: 2373-98-0, hereinafter referred to as polyamine-2),

[0077] 6,6'-Diamino-3,3'-methylenedibenzoic acid (CAS: 7330-46-3, hereinafter referred to as polyamine-3), 2,2'-bis(trifluoromethyl)diaminobiphenyl (CAS: 341-58-2, hereinafter referred to as polyamine-4),

[0078] 2,2-Bis(3-amino-4-hydroxyphenyl)hexafluoropropane (CAS: 83558-87-6, hereinafter referred to as polyamine-5),

[0079] 9,9-Di(4-amino-3-fluorophenyl)fluorene (CAS: 127926-65-2, hereinafter referred to as polyamine-6), N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-diphenyl]-4,4'-diyl)bis(4-aminobenzamide) (CAS: 1449757-11-2, hereinafter referred to as polyamine-7),

[0080] 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether (CAS: 344-48-9, hereinafter referred to as polyamine-8),

[0081] 3,4′-Diaminodiphenyl ether (CAS: 2657-87-6, hereinafter referred to as polyamine-9);

[0082] Polyamines with the structural formula shown in formula (III) (CAS: 2166310-88-7, hereinafter referred to as copolyamine-1),

[0083] Polyamines with the structural formula shown in formula (IV) (CAS: 2757326-19-3, hereinafter referred to as copolyamine-2),

[0084] Polyamines with the structural formula shown in formula (V) (CAS: 2457326-17-1, hereinafter referred to as copolyamine-3),

[0085] Polyamines with the structural formula shown in formula (VI) (CAS: 1467827-13-9, hereinafter referred to as copolyamine-4),

[0086] The polyamine with the structural formula shown in formula (VII) (hereinafter referred to as copolyamine-5) was purchased from Aldrich Chemicals.

[0087] Monomeric polyamines with the structural formula shown in formula (VIII) (CAS: 31352-45-1, hereinafter referred to as copolyamine-6);

[0088] p-Phenylidene-bisphenyltriester dianhydride (CAS: 2770-49-2, hereinafter referred to as polybasic acid anhydride-1),

[0089] Hexafluorodianhydride (CAS: 1107-00-2, hereinafter referred to as polyacid anhydride-2),

[0090] 1,2,4,5-Cyclohexanetetracarboxylic dianhydride (CAS: 2754-41-8, hereinafter referred to as polyanhydride-3),

[0091] Cyclobutanetetracarboxylic dianhydride (CAS: 4415-87-6, hereinafter referred to as polybasic anhydride-4),

[0092] 1,2,3,4-Butanetetracarboxylic acid dianhydride (CAS: 4534-73-0, hereinafter referred to as polycarboxylic acid anhydride-5),

[0093] Bisphenol A type diether dianhydride (CAS: 38103-06-9, hereinafter referred to as polybasic acid anhydride-6),

[0094] All were obtained through commercial purchase.

[0095] Example 1

[0096] Under nitrogen protection, the organic solvent N-methylpyrrolidone (150 g) and the polyamine 9,9-dimethylfluorene-2,7-diamine (22.4 g, 0.1 mol) were added to the reaction flask. The mixture was stirred and cooled to 0 °C. Then, 1,2,3,4-butanetetracarboxylic acid dianhydride (19.8 g, 0.1 mol) was added, and the temperature was controlled at 0 °C for the first stage reaction for 3 h. Then, the initiator (acetic anhydride 5 g and pyridine 4 g) was added, and the temperature was raised to 180 °C for the second stage reaction for 5 h. The mixture was then cooled to room temperature (20 °C), and water (500 g) was added. The mixture was stirred at room temperature (20 °C) for 2 h, filtered, and dried to obtain polyimide.

[0097] Examples 2 to 15

[0098] Polyimide was prepared according to the method of Example 1, except that the types and molar ratios of polyamines, polyanhydrides, and organic solvents are shown in Table 1.

[0099] In Example 2, the temperature of the first stage reaction was -10℃ and the time was 5 hours, while the temperature of the second stage reaction was 150℃ and the time was 10 hours.

[0100] In Example 3, the temperature of the first stage reaction was 15°C and the time was 1 hour, while the temperature of the second stage reaction was 200°C and the time was 3 hours.

[0101] The reaction conditions for the remaining embodiments are the same as those for Embodiment 1.

[0102] Table 1

[0103]

[0104]

[0105] Test Example 1

[0106] Composite film I was prepared using the polyimide obtained in Examples 1-15. The specific process is as follows:

[0107] 12.5 g of polyimide prepared in Examples 1-15, 12.5 g of PVDF (purchased from Arkema, France, product model HSV900), and 0.5 g of carboxymethyl cellulose were dispersed in 100 g of acetone and stirred thoroughly to obtain a coating slurry. The coating slurry was then applied to both sides of a base film, with a coating thickness of 1 micrometer. The base film consisted of a PE layer and a single-sided ceramic coating, with the PE layer having a thickness of 4 micrometers and the single-sided ceramic coating having a thickness of 1 micrometer. The membrane was then dried in an oven at 50°C to obtain composite membrane I. Composite membrane I formed using the polyimide prepared in Example 1 is as follows... Figure 1 As shown.

[0108] Test Example 2

[0109] Composite membrane II was prepared using the polyimide obtained in Examples 1-15. The specific process was as follows: 12.5 g of polyimide prepared in Examples 1-15, 12.5 g of PVDF (purchased from Arkema, France, product model HSV900), and 0.5 g of carboxymethyl cellulose were dispersed in 100 g of DMAC and stirred thoroughly. The slurry was then dipped onto both sides of the base membrane, with a coating thickness of 1 micrometer. The base membrane consisted of a PE layer and a single-sided ceramic coating, with the PE layer having a thickness of 4 micrometers and the single-sided ceramic coating having a thickness of 1 micrometer. The membrane was then extracted in a coagulation bath to remove the high-boiling-point DMAC solvent. The coagulation bath contained 40% DMAC and 60% water. The membrane was then dried in a drying tunnel at 100°C to obtain composite membrane II.

[0110] Comparative Test Case 1-1

[0111] The composite membrane 1-1 prepared using the method in Test Example 1 differs from that polyimide was not added, and the amount of PVDF was increased from 12.5 g to 25 g.

[0112] Comparative Test Cases 1-2

[0113] An attempt was made to prepare composite membranes 1-2 using the method of Test Example 1. The difference was that the polyimide obtained in the example was replaced with commercially available polyimide. However, since the commercially available polyimide is prepared by polymerization of m-phenylenediamine and pyromellitic anhydride, this polyimide has poor solubility and is insoluble in acetone, making it unsuitable for acetone coating systems. Therefore, composite membranes 1-2 could not be prepared.

[0114] Comparative Test Example 2-1

[0115] The composite membrane 2-1 prepared using the method in Test Example 2 differs in that polyimide is not added, and the amount of PVDF is increased from 12.5 g to 25 g.

[0116] Comparative Test Example 2-2

[0117] Composite membrane 2-2 was prepared using the method of Test Example 1, except that the polyimide obtained in the example was replaced with commercially available polyimide.

[0118] Performance Test 1

[0119] The peel strength of the composite membrane I and composite membrane II prepared above was tested using the method of GB / T2790-1995 "Adhesives 180° Peel Strength Test Method Flexible Materials vs. Rigid Materials". The puncture strength, heat shrinkage rate (MD / TD) and air permeability of the composite membrane I and composite membrane II prepared above were tested using the method of GB / T36363-2018 "Polyolefin Separators for Lithium-ion Batteries". The results are shown in Tables 2 and 3.

[0120] Table 2

[0121]

[0122] Table 3

[0123]

[0124] As shown in the table above, the polyimide provided by this invention has good processability in both DMAC and acetone systems. Compared with membrane products made without polyimide or using commercially available polyimide, it has higher heat resistance, higher peel strength, and better air permeability. With the same raw materials, the membrane prepared using the DMAC system has better air permeability than the membrane prepared using the acetone system, while other properties are basically equivalent.

[0125] Performance Test 2

[0126] The composite membrane I prepared using Examples 8 and 9 in Test Example 1, and the composite membrane I prepared using Comparative Test Example 1, were respectively assembled into batteries. The cycle performance of the batteries was tested, with commercially available batteries used as control examples. The results are shown in Table 3. The battery structure is such that composite membrane I is installed between the positive and negative electrodes of the battery. The positive electrode is lithium cobalt oxide, and the negative electrode is a graphite negative electrode (Kelode SM02 model).

[0127] Charge-discharge cycle performance was tested in accordance with GB / T18287-2000.

[0128] Test methods and conditions: The first coulombic efficiency and cycle capacity retention of charge-discharge cycles were tested using the constant current method, with a potential window of 3.3-4.5V.

[0129] Capacity retention rate (%) = Discharge capacity after N cycles ÷ Initial discharge capacity × 100%;

[0130] Method for calculating cell thickness expansion rate: Measure the initial cell thickness h0, and after 100 charge-discharge cycle tests, measure the cell thickness h. The expansion rate is calculated using the following formula:

[0131] Cell thickness expansion rate = {(h-h0) / h0}×100%.

[0132] Table 3

[0133]

[0134] As shown in Table 3, when the composite membrane I provided by the present invention is used as a separator, the heat transfer rate is greatly improved, which effectively improves the battery safety performance.

[0135] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A polyimide, characterized in that, The polyimide contains repeating units formed by the bonding of polyamines and polyanhydrides, the structural formulas of which are shown in formula (I) and / or formula (II); Wherein, R1 is absent or selected from C1-C5 alkylene groups, -O-, R2 and R3 are each independently selected from at least one of hydrogen, halogen, hydroxyl, carboxyl, and perfluoromethyl. R4 is a C1-C5 alkylene group.

2. The polyimide according to claim 1, characterized in that, The polyamine is selected from at least one of 9,9-dimethylfluorene-2,7-diamine, 3,3'-dihydroxybenzidine, 6,6'-diamino-3,3'-methylenedibenzoic acid, 2,2'-di(trifluoromethyl)diaminobiphenyl, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 9,9-di(4-amino-3-fluorophenyl)fluorene, N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-dibiphenyl]-4,4'-diyl)bis(4-aminobenzamide), 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether, and 3,4'-diaminodiphenyl ether; Preferably, the polyimide further comprises repeating units formed by bonding a copolyamine to the polyanhydride, wherein the copolyamine is selected from at least one of the diamines shown in formulas (III) to (VIII). R5, R6, R7 and R8 are each independently selected from C1-C5 alkyl groups.

3. The polyimide according to claim 1 or 2, characterized in that, The polybasic acid anhydride is a dibasic acid anhydride, preferably selected from at least one of p-phenylene-bisphenyltriester dianhydride, hexafluorodianhydride, 1,2,4,5-cyclohexanetetracarboxylic acid dianhydride, cyclobutanetetracarboxylic acid dianhydride, 1,2,3,4-butanetetracarboxylic acid dianhydride, and bisphenol A type diether dianhydride.

4. A method for preparing polyimide, characterized in that, The method includes: mixing polyamines and polyacid anhydrides under anaerobic conditions to carry out a first-stage reaction, and then mixing them with an initiator to carry out a second-stage reaction; The structural formulas of the polyamines are shown in formula (I) and / or formula (II); Wherein, R1 is absent or selected from C1-C5 alkylene groups, -O-, R2 and R3 are each independently selected from at least one of hydrogen, halogen, hydroxyl, carboxyl, and perfluoromethyl. R4 is a C1-C5 alkylene group.

5. The preparation method according to claim 4, characterized in that, The conditions for the first stage reaction include at least: a temperature of -10 to 15°C and a time of 1 to 5 hours; Preferably, the conditions for the second stage reaction include at least: a temperature of 150-200°C and a time of 3-10 hours.

6. The preparation method according to claim 4 or 5, characterized in that, The molar ratio of the polyamine to the polyacid anhydride is (0.98-1.1):1; Preferably, the polyamine is selected from at least one of 9,9-dimethylfluorene-2,7-diamine, 3,3'-dihydroxybenzidine, 6,6'-diamino-3,3'-methylenedibenzoic acid, 2,2'-di(trifluoromethyl)diaminobiphenyl, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 9,9-di(4-amino-3-fluorophenyl)fluorene, N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-diphenyl]-4,4'-diyl)bis(4-aminobenzamide), 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether, and 3,4'-diaminodiphenyl ether; Preferably, the raw materials for the first stage reaction further include a copolyamine, wherein the copolyamine is selected from at least one of the diamines with structural formulas as shown in formulas (III) to (VIII); R5, R6, R7 and R8 are each independently selected from C1-C5 alkyl groups; Preferably, the molar ratio of the polyamine to the copolyamine is 1:15-15:1; Preferably, the polybasic acid anhydride is a dibasic acid anhydride, and is preferably selected from at least one of p-phenylene-bisphenyltriester dianhydride, hexafluorodianhydride, 1,2,4,5-cyclohexanetetracarboxylic acid dianhydride, cyclobutanetetracarboxylic acid dianhydride, 1,2,3,4-butanetetracarboxylic acid dianhydride and bisphenol A type diether dianhydride. Preferably, the initiator contains acetic anhydride and pyridine; Preferably, the molar ratio of the acetic anhydride to the polyamine is (0.3-0.8):1, and the molar ratio of the pyridine to the polyamine is (0.3-0.8):

1.

7. The preparation method according to claim 4 or 5, characterized in that, The first stage reaction is carried out in the presence of a reaction solvent, which is selected from at least one of acetone, N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide and N-ethylpyrrolidone; Preferably, the weight ratio of the reaction solvent to the polyamine is 5-15:1; Preferably, the method further includes: mixing the reaction solution obtained in the second stage reaction with a poor solvent, performing solid-liquid separation, and drying; Preferably, the unsuitable solvent is selected from at least one of water, methanol, ethanol, isopropanol, and petroleum ether; Preferably, the conditions for the mixing process include at least the following: a temperature of 5-35°C and a time of 1.5-3 hours.

8. A composite membrane, characterized in that, The composite membrane includes a base membrane and a coating applied to the surface of the base membrane, wherein the coating contains the polyimide according to any one of claims 1-3 or the polyimide prepared by the method according to any one of claims 4-7.

9. The composite membrane according to claim 8, characterized in that, The coating is located on one or both sides of the base film, and the thickness of the coating is 1-10 μm; Preferably, the base film is selected from at least one of polypropylene film, polyethylene film, polypropylene film with ceramic coating on the surface, and polyethylene film with ceramic coating on the surface; Preferably, the polyimide content in the coating is 1-30% by weight; Preferably, the coating further comprises an additive and a dispersant, wherein the content of the additive in the coating is 0.5-90% by weight and the content of the dispersant is 1-20% by weight; Preferably, the dispersant contains at least one of modified polymethyl methacrylate, carboxymethyl cellulose, sodium polyethylene glycol alkyl aryl ether sulfonate, alkylphenol polyethylene ether, polyoxyethylene alkylphenol ether, polyacrylic acid, and polyvinyl alcohol, and the additive contains polyvinylidene fluoride and / or ceramics.

10. The use of at least one of the polyimide according to any one of claims 1-3, the polyimide prepared by the method according to any one of claims 4-7, and the composite film according to claim 8 or 9 in a battery separator.

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