A method for preparing a low-temperature curing polyimide binder for lithium ion batteries, and a binder and electrode pole piece prepared thereby

Abstract

The application relates to a preparation method of a low-temperature curing polyimide binder for lithium ion batteries and a prepared binder and electrode pole piece. A dianhydride, a diamine monomer and a monomer containing a special functional group are condensed and polymerized in a solvent to obtain a copolymer polyamic acid; a capping agent is added and fully stirred to obtain a capped polyamic acid; a catalyst is added and fully stirred to obtain a polyimide precursor solution. The polyimide precursor solution is mixed with an active substance and a conductive agent to obtain a slurry, which is coated on the surface of a current collector, and then is subjected to heat treatment at 100-150 DEG C to obtain a lithium ion battery pole piece using a low-temperature curing polyimide as a binder. The method can greatly reduce the process use temperature of the polyimide binder, save energy and reduce cost, and meanwhile, the influence of high-temperature treatment of traditional thermal imidization on the performance of electrode materials is avoided. The lithium ion battery assembled by using the electrode pole piece prepared by the application has excellent electrical properties and safety performance, such as higher specific capacity and capacity retention rate.

Description

Technical Field

[0001] This invention relates to the field of electrode material technology, and in particular to a method for preparing a low-temperature curing polyimide binder for lithium-ion batteries, as well as the binder and electrode sheet prepared therefrom. Background Technology

[0002] Lithium-ion batteries, with their high specific energy, low self-discharge, lack of memory effect, and environmental friendliness, have become one of the most promising next-generation energy sources. However, with the continuous improvement of their energy density and capacity, battery safety issues have become increasingly prominent. As a crucial component of lithium-ion battery electrodes, the binder, despite its relatively low mass percentage in the electrode materials, plays an indispensable role. The binder not only tightly binds the active materials, conductive agents, and current collectors but also plays a key role in electrode performance and stability. Specifically, the binder can suppress the volume expansion of the electrode during charge and discharge processes and effectively reduce side reactions between the active materials and the electrolyte, thereby improving the battery's cycle life and safety. Currently, while traditional binders have low costs, their stability at high temperatures is poor. During long-term use of lithium-ion batteries, the increase in internal battery temperature may cause thermal degradation of the binder, which not only affects the battery's mechanical integrity but may also lead to serious safety hazards. Furthermore, the bonding performance of existing binders weakens after multiple charge-discharge cycles, leading to the failure of the interfacial connection between the active materials and the current collector, thus failing to effectively suppress the volume expansion of the electrode. These issues ultimately lead to shortened cycle life and accelerated capacity decay in lithium-ion batteries, limiting their application in high-performance applications. Therefore, developing novel adhesive materials with higher thermal stability and stronger adhesion, addressing the shortcomings of existing binders in terms of heat resistance and bonding performance, has become a key research direction for improving lithium-ion battery performance. This will not only help improve battery safety and cycle life but also support the application of lithium-ion batteries under more demanding operating conditions.

[0003] Polyimide (PI) monomers are diverse and its molecular structure is highly designable. As a binder, it is considered the preferred binder for lithium-ion batteries due to its resistance to high and low temperatures, high bond strength, chemical stability, and self-extinguishing flame retardant properties. Its excellent heat resistance maintains the structural stability of electrode materials under high temperature and high pressure, improving the electrode's capacity under high-pressure cycling. Good chemical stability makes polyimide binders less prone to oxidation or reduction and prevents side reactions with other materials, ensuring the electrochemical stability of the electrode. Furthermore, the molecular structure of polyimide gives it higher tensile strength and elastic recovery ability, effectively adapting to electrode shrinkage and expansion, ensuring the structural integrity and stability of the electrode during cycling. However, the imidization process of polyimide typically requires high-temperature treatment above 300°C, which is detrimental to energy consumption reduction, and high-temperature treatment may also affect the performance of the electrode material itself. Therefore, there is an urgent need for a low-temperature curing technology to lower the imidization temperature of polyimide, making it more promising for industrial applications. Summary of the Invention

[0004] The purpose of this invention is to overcome the problems of poor safety performance of traditional adhesives in high-capacity lithium-ion batteries, and the high imidization temperature of existing polyimide binders, which leads to high energy consumption, high cost, and is not conducive to industrial application. This invention provides a method for preparing a low-temperature curing polyimide binder for lithium-ion batteries, as well as the binder and electrode sheets prepared therefrom. This preparation method, through special design of the molecular structure of polyimide and the synergistic effect of subsequent catalysts, can significantly reduce the thermal imidization temperature of the polyimide binder in the later stages, thereby reducing energy consumption and exhibiting superior electrical performance compared to traditional adhesives.

[0005] To achieve the above objectives, the present invention provides a method for preparing a low-temperature curing polyimide binder for lithium-ion batteries and an electrode sheet prepared therefrom, comprising the following steps.

[0006] (1) Under nitrogen atmosphere and ice-water bath conditions, excess dianhydride monomer, diamine monomer and a third monomer containing special functional groups are subjected to condensation polymerization reaction in the reaction solvent to obtain a copolyamic acid solution. The molecular weight of polyimide can be adjusted by the relative amount of dianhydride monomer added.

[0007] (2) Add a capping agent to the polyamic acid solution obtained by copolymerization and stir thoroughly to obtain a capped polyamic acid solution;

[0008] (3) Add the catalyst to the polyamic acid solution obtained in step (2) and stir thoroughly to obtain a low-temperature curing polyimide precursor solution;

[0009] (4) The active material, conductive agent and precursor solution obtained in step (3) are mixed evenly to prepare a slurry, which is coated on the surface of the current collector and then subjected to thermal imidization treatment to prepare an electrode sheet with low-temperature curing polyimide as the binder.

[0010] Preferably, the solid content of the polyamic acid solution in step (1) is 0.5-40 wt%; the molar ratio of dianhydride monomer to diamine monomer is 1.01:1-1.3:1; the molar ratio of dianhydride monomer to diamine monomer containing special functional groups is 9-25:1; and the reaction time is 0.5-12 h.

[0011] Preferably, the dianhydride monomer in step (1) is one or more of the following in any proportion: pyromellitic dianhydride (PMDA), 4,4'-diphenyl ether dianhydride (ODPA), biphenyl dianhydride (BPDA), 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride (DSDA), hexafluoro dianhydride (6FDA), 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), 4,4'-(4,4'-isopropylidene diphenoxy) bisphthalic anhydride (BPADA), and p-phenylene bisphenyltriester dianhydride (TAHQ).

[0012] Preferably, the diamine monomer in step (1) is one or more of p-phenylenediamine (p-PDA), m-phenylenediamine (m-PDA), 4,4'-diaminodiphenyl ether (ODA), 3,4'-diaminodiphenyl ether (3,4'-ODA), 4,4'-diaminodiphenyl sulfone (DDS), 3,3'-diaminodiphenyl sulfone (MSDS), 2,2'-dimethyl-4,4'-diaminobiphenyl (MTB), 4,4'-diaminodiphenylmethane (MDA), and 1,3-bis(4'-aminophenoxy)benzene (TPE) in any proportion;

[0013] Preferably, the third monomer containing a special functional group in step (1) is one or more of 2,3-diaminopyridine, 2,4-diaminopyridine, 2,5-diaminopyridine, 3,4-diaminopyridine, 5,6-diaminobenzimidazole, 2,4-diamino-6-[2-methyl-(-imidazolyl)ethyl]-1,3,5-quinoline, and 5-amino-2-(4-aminophenyl)benzimidazole, mixed in any proportion;

[0014] Preferably, the solvent in step (1) is one or more of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), N-ethylpyrrolidone (NEP), dimethyl sulfoxide (DMSO), and hexamethylphosphoramide (HMPA) in any proportion;

[0015] Preferably, in step (2), the molar ratio of the capping agent to the dianhydride monomer is 0.01:1-5:1; and the reaction time is 0.5-12 h.

[0016] Preferably, the capping agent in step (2) is one or more of 5-aminobenzimidazole (ABZ), 2-aminoimidazole, 2-aminopyridine, and aminopyrazine in any proportion;

[0017] Preferably, in step (3), the molar ratio of the catalyst to the dianhydride monomer is 0.01:1-5:1; and the reaction time is 0.5-12 h.

[0018] Preferably, the catalyst in step (3) is one or more of N-N' carbonyl diimidazole (CDI), 6-aminoquinoline (AQL), ​​benzotriazole (BTA) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in any proportion;

[0019] Preferably, the solid content of the electrode slurry in step (4) is 20-90 wt%; the non-solvent portion of the electrode slurry contains 80-99 wt% active material, 0.5-10 wt% conductive agent, and 0.5-10 wt% precursor solution;

[0020] Preferably, the positive electrode active material in step (4) is one or more of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt aluminum oxide and lithium nickel cobalt manganese oxide in any proportion;

[0021] Preferably, the negative electrode active material in step (4) is one or more of carbon materials, silicon and its oxides, tin and its oxides, silicon-carbon composite materials, silicon-oxygen-carbon composite materials and tin-carbon composite materials in any proportion;

[0022] Preferably, the conductive agent in step (4) is one or more of conductive carbon black, conductive graphite, graphene and carbon nanotubes in any proportion.

[0023] Preferably, the current collector in step (4) is a carbon-containing aluminum foil or a carbon-containing copper foil;

[0024] Preferably, the thickness of the electrode paste coating in step (4) is 75-200 μm; the thermal imidization temperature is 100-150℃, and the time is 0.5-2 h;

[0025] Another object of the present invention is to provide a low-temperature curing polyimide binder for lithium-ion batteries prepared by the method described above, and an electrode sheet prepared therefrom.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] 1. The method of the present invention, through the molecular structure design of polyimide and the synergistic effect with the added catalyst, can significantly reduce the imidization temperature of polyamic acid, avoid side reactions in the electrode material during the high-temperature imidization process, reduce energy consumption, save production costs, and the lithium-ion battery assembled from it has better electrical performance; the method of the present invention does not require changes to the original production and process flow, is simple to operate, and is easy to carry out continuous production.

[0028] 2. The capping agent and catalyst used in the method of this invention are more environmentally friendly than traditional chemical imidizing reagents, which improves the operating environment and is conducive to green production. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] The following specific embodiments further illustrate the point. It should be noted that the following embodiments are only used to illustrate and not limit the technical solutions described in this invention. All equivalent substitutions made based on the technical solutions of this application fall within the protection scope of this invention.

[0031] This invention provides a low-temperature curing polyimide binder for lithium-ion batteries and the electrode sheet prepared therefrom. Its electrochemical performance and applications are examined using conventional methods in the art, namely, assembling it into a coin cell for charge-discharge testing. The coin cell assembly process and technology are as follows:

[0032] After drying the electrode sheets, place them in a glove box and assemble the coin cell using a 2032 type coin cell casing: place the negative electrode casing, negative electrode sheet, separator, electrolyte drop, positive electrode sheet, gasket, and spring contact in sequence, cover with the positive electrode casing, and seal using a sealing machine. Test with a 0.1C current; the voltage range is 2.5–4.3V. This electrode preparation and battery assembly method is applicable to all the following examples and comparative examples.

[0033] The following specific embodiments further illustrate the point. It should be noted that the following embodiments are only used to illustrate and not limit the technical solutions described in this invention. All equivalent substitutions made based on the technical solutions of this application fall within the protection scope of this invention.

[0034] Example 1

[0035] (1) Dissolve 3.273 g of 4,4'-diaminodiphenyl ether and 0.198 g of 2,3-diaminopyridine in 50 ml of N-methylpyrrolidone solvent. Under nitrogen atmosphere and ice-water bath conditions, add 5.75 g of 4,4'-biphenyl ether dianhydride in batches and react completely to obtain a homogeneous polyamic acid solution.

[0036] (2) Add 2% by molar mass of 4,4'-biphenyl dianhydride and 5-aminobenzimidazole as a capping agent to the above polyamic acid solution and stir thoroughly to obtain a capped polyamic acid solution.

[0037] (3) Add 10% by molar mass of 4,4'-biphenyl dianhydride catalyst N-N' carbonyl diimidazole to the end-capped polyamic acid solution obtained in step (2), and stir thoroughly to obtain a precursor solution of low-temperature curing polyimide adhesive.

[0038] (4) The above precursor solution was diluted to a solid content of 5 wt%, and mixed with the positive electrode active material NCM811, conductive carbon black, and the diluted precursor solution in a mass ratio of 92:5:3. N-methylpyrrolidone solvent was added to dissolve the mixture, and the mixture was stirred thoroughly to obtain a positive electrode slurry. The above slurry was coated on aluminum foil (10cm×12cm) and dried at room temperature in a clean bench for 12 hours. It was then cut into 12mm diameter discs, rolled, and placed in an oven for thermal imidization treatment. The temperature was raised from room temperature to 150℃ and kept at that temperature for 2 hours. The discs were then removed and weighed.

[0039] (5) Following the above electrode preparation, battery assembly, and testing methods, the NCM811 in this system has a 0.1C discharge specific capacity of 198.32 mAh / g and an initial coulombic efficiency of 86.9%. After 100 cycles at 0.1C, the capacity retention rate is 87.03%.

[0040] Example 2

[0041] (1) Dissolve 3.444 g of 4,4'-diaminodiphenyl ether and 0.113 g of 5,6-diaminobenzimidazolone in 50 ml of N-methylpyrrolidone solvent. Under nitrogen atmosphere and ice-water bath conditions, add 5.723 g of 4,4'-biphenyl ether dianhydride in batches and react completely to obtain a homogeneous polyamic acid solution.

[0042] (2) Add 2% by molar mass of 4,4'-biphenyl dianhydride and 5-aminobenzimidazole as a capping agent to the above polyamic acid solution and stir thoroughly to obtain a capped polyamic acid solution.

[0043] (3) Add 10% by molar mass of N-N'-carbonyl diimidazole of 4,4'-biphenyl dianhydride to the end-capped polyamic acid solution obtained in step (2), and stir thoroughly to obtain a precursor solution of low-temperature curing polyimide adhesive.

[0044] (4) The above precursor solution was diluted to a solid content of 5 wt%, and mixed with the positive electrode active material NCM811, conductive carbon black, and the diluted precursor solution in a mass ratio of 92:5:3. N-methylpyrrolidone solvent was added to dissolve the mixture, and the mixture was stirred thoroughly to obtain a positive electrode slurry. The above slurry was coated on aluminum foil (10cm×12cm) and dried at room temperature in a clean bench for 12 hours. It was then cut into 12mm diameter discs, rolled, and placed in an oven for thermal imidization treatment. The temperature was raised from room temperature to 150℃ and kept at that temperature for 2 hours. The discs were then removed and weighed.

[0045] (5) According to the above electrode preparation and battery assembly and testing methods, the NCM811 has a 0.1C discharge specific capacity of 195.01mAh / g, an initial efficiency of 85.43%, and a capacity retention of 85.37% after 100 cycles at 0.1C.

[0046] Example 3

[0047] (1) Dissolve 3.199 g of 4,4'-diaminodiphenyl ether and 3.199 g of 5-amino-2-(4-aminophenyl)benzimidazole in 50 ml of N-methylpyrrolidone solvent. Under nitrogen atmosphere and ice-water bath conditions, add 5.682 g of 4,4'-biphenyl ether dianhydride in batches and react completely to obtain a homogeneous polyamic acid solution.

[0048] (2) Add 2% by molar mass of 5-aminobenzimidazole, a capping agent, to the above polyamic acid solution and stir thoroughly to obtain a capped polyamic acid solution.

[0049] (3) Add 10% by molar mass of N-N'-carbonyl diimidazole of 4,4'-biphenyl dianhydride to the end-capped polyamic acid solution obtained in step (2), and stir thoroughly to obtain a precursor solution of low-temperature curing polyimide adhesive.

[0050] (4) The above precursor solution was diluted to a solid content of 5 wt%, and mixed with the positive electrode active material NCM811, conductive carbon black, and the diluted precursor solution in a mass ratio of 92:5:3. N-methylpyrrolidone solvent was added to dissolve the mixture, and the mixture was stirred thoroughly to obtain a positive electrode slurry. The above slurry was coated on aluminum foil (10cm×12cm) and dried at room temperature in a clean bench for 12 hours. It was then cut into 12mm diameter discs, rolled, and placed in an oven for thermal imidization treatment. The temperature was raised from room temperature to 150℃ and kept at that temperature for 2 hours. The discs were then removed and weighed.

[0051] (5) According to the above electrode preparation and battery assembly and testing methods, the NCM811 has a 0.1C discharge specific capacity of 190.87mAh / g, an initial efficiency of 83.34%, and a capacity retention of 86.22% after 100 cycles at 0.1C.

[0052] Example 4

[0053] (1) Dissolve 3.273 g of 4,4'-diaminodiphenyl ether and 0.198 g of 2,3-diaminopyridine in 50 ml of N-methylpyrrolidone solvent. Under nitrogen atmosphere and ice-water bath conditions, add 5.75 g of 4,4'-biphenyl ether dianhydride in batches and react completely to obtain a homogeneous polyamic acid solution.

[0054] (2) Add 2% by molar mass of 4,4'-biphenyl dianhydride and 2-aminopyridine as a capping agent to the above polyamic acid solution and stir thoroughly to obtain a capped polyamic acid solution.

[0055] (3) Add 10% by molar mass of catalyst N-N' to the end-capped polyamic acid solution obtained in step (2).

[0056] Carbonyl diimidazole was thoroughly stirred to obtain a precursor solution for low-temperature curing polyimide adhesive;

[0057] (4) The above precursor solution was diluted to a solid content of 5 wt%, and mixed with the positive electrode active material NCM811, conductive carbon black, and the diluted precursor solution in a mass ratio of 92:5:3. N-methylpyrrolidone solvent was added to dissolve the mixture, and the mixture was stirred thoroughly to obtain a positive electrode slurry. The above slurry was coated on aluminum foil (10cm×12cm) and dried at room temperature in a clean bench for 12 hours. It was then cut into 12mm diameter discs, rolled, and placed in an oven for thermal imidization treatment. The temperature was raised from room temperature to 150℃ and kept at that temperature for 2 hours. The discs were then removed and weighed.

[0058] (5) According to the above electrode preparation and battery assembly and testing methods, the NCM811 has a 0.1C discharge specific capacity of 193.23 mAh / g, an initial efficiency of 86.43%, and a capacity retention rate of 85.68% after 100 cycles at 0.1C.

[0059] Example 5

[0060] (1) Dissolve 3.273 g of 4,4'-diaminodiphenyl ether and 0.198 g of 2,3-diaminopyridine in 50 ml of N-methylpyrrolidone solvent. Under nitrogen atmosphere and ice-water bath conditions, add 5.75 g of 4,4'-biphenyl ether dianhydride in batches and react completely to obtain a homogeneous polyamic acid solution.

[0061] (2) Add 2% by molar mass of 4,4'-biphenyl dianhydride and 2-aminoimidazole as a capping agent to the above polyamic acid solution and stir thoroughly to obtain a capped polyamic acid solution.

[0062] (3) Add 10% by molar mass of N-N'-carbonyl diimidazole of 4,4'-biphenyl dianhydride to the end-capped polyamic acid solution obtained in step (2), and stir thoroughly to obtain a precursor solution of low-temperature curing polyimide adhesive.

[0063] (4) The above precursor solution was diluted to a solid content of 5 wt%, and mixed with the positive electrode active material NCM811, conductive carbon black, and the diluted precursor solution in a mass ratio of 92:5:3. N-methylpyrrolidone solvent was added to dissolve the mixture, and the mixture was stirred thoroughly to obtain a positive electrode slurry. The above slurry was coated on aluminum foil (10cm×12cm) and dried at room temperature in a clean bench for 12 hours. It was then cut into 12mm diameter discs, rolled, and placed in an oven for thermal imidization treatment. The temperature was raised from room temperature to 150℃ and kept at that temperature for 2 hours. The discs were then removed and weighed.

[0064] (5) Following the above electrode preparation, battery assembly, and testing methods, the NCM811 in this system has a 0.1C discharge specific capacity of 193.47 mAh / g and an initial coulombic efficiency of 86.28%. After 100 cycles at 0.1C, the capacity retention rate is 85.58%.

[0065] Example 6

[0066] (1) Dissolve 3.273 g of 4,4'-diaminodiphenyl ether and 0.198 g of 2,3-diaminopyridine in 50 ml of N-methylpyrrolidone solvent. Under nitrogen atmosphere and ice-water bath conditions, add 5.75 g of 4,4'-biphenyl ether dianhydride in batches and react completely to obtain a homogeneous polyamic acid solution.

[0067] (2) Add 2% by molar mass of 4,4'-biphenyl dianhydride and 5-aminobenzimidazole as a capping agent to the above polyamic acid solution and stir thoroughly to obtain a capped polyamic acid solution.

[0068] (3) Add 80% of the molar mass of 4,4'-diphenyl ether dianhydride catalyst 6-aminoquinoline to the end-capped polyamic acid solution obtained in step (2), and stir thoroughly to obtain a precursor solution of low-temperature curing polyimide adhesive.

[0069] (4) The above precursor solution was diluted to a solid content of 5 wt%, and mixed with the positive electrode active material NCM811, conductive carbon black, and the diluted precursor solution in a mass ratio of 92:5:3. N-methylpyrrolidone solvent was added to dissolve the mixture, and the mixture was stirred thoroughly to obtain a positive electrode slurry. The above slurry was coated on aluminum foil (10cm×12cm) and dried at room temperature in a clean bench for 12 hours. It was then cut into 12mm diameter discs, rolled, and placed in an oven for thermal imidization treatment. The temperature was raised from room temperature to 150℃ and kept at that temperature for 2 hours. The discs were then removed and weighed.

[0070] (5) Following the above electrode preparation, battery assembly, and testing methods, the NCM811 in this system has a 0.1C discharge specific capacity of 192.91 mAh / g and an initial coulombic efficiency of 85.11%. After 100 cycles at 0.1C, the capacity retention rate is 84.92%.

[0071] Comparative Example 1

[0072] (1) Weigh 2.68g of powdered PVDF and dissolve it in 30ml of NMP to obtain a PVDF solution with a solid content of 5%.

[0073] (2) The positive electrode active material NCM811, conductive carbon black, and PVDF adhesive were mixed in proportions of 94%, 3%, and 3% by mass, respectively. NMP solvent was added to dissolve the mixture, and the mixture was stirred for 120 min to obtain the positive electrode slurry. The PVDF-containing slurry was coated onto an aluminum foil (10cm × 12cm) and dried at room temperature in a clean bench for 12 h. It was then cut into 12mm round pieces, rolled, and weighed.

[0074] (3) Following the above electrode preparation, battery assembly, and testing methods, the NCM811 in this system has a 0.1C discharge specific capacity of 160.23 mAh / g and an initial efficiency of 73.2%. After 100 cycles at 0.1C, the capacity retention rate is 60.23%.

[0075] Comparative Example 2

[0076] (1) Dissolve 1.962g of ODA in 30ml of NMP solvent, add 3.060g of ODPA in batches under nitrogen atmosphere and ice-water bath conditions, and react fully to obtain a homogeneous polyamic acid solution; a precursor solution for polyimide adhesive;

[0077] (2) The above precursor solution was diluted to a solid content of 5 wt%, and mixed with the positive electrode active material NCM811, conductive carbon black, and the diluted precursor solution in a mass ratio of 92:5:3. NMP solvent was added to dissolve the mixture, and the mixture was stirred thoroughly to obtain a positive electrode slurry. The slurry was coated onto an aluminum foil (10 cm × 12 cm) and dried at room temperature in a clean bench for 12 h. It was then cut into 12 mm diameter discs, rolled, and placed in an oven for thermal imidization treatment. The temperature was raised from room temperature to 150 °C and held for 2 h. The discs were then removed and weighed.

[0078] (3) Following the above electrode preparation, battery assembly, and testing methods, the NCM811 in this system has a 0.1C discharge specific capacity of 119 mAh / g and an initial efficiency of 52.7%. After 100 cycles at 0.1C, the capacity retention rate is 55.31%.

[0079] Comparative Example 3

[0080] (1) Dissolve 3.273 g of 4,4'-diaminodiphenyl ether and 0.198 g of 2,3-diaminopyridine in 50 ml of N-methylpyrrolidone solvent. Under nitrogen atmosphere and ice-water bath conditions, add 5.75 g of 4,4'-biphenyl dianhydride in batches and react fully to obtain a precursor solution of low-temperature curing polyimide adhesive.

[0081] (2) The above precursor solution was diluted to a solid content of 5 wt%, and mixed with the positive electrode active material NCM811, conductive carbon black, and the diluted precursor solution in a mass ratio of 92:5:3. N-methylpyrrolidone solvent was added to dissolve the mixture, and the mixture was stirred thoroughly to obtain a positive electrode slurry. The slurry was coated onto an aluminum foil (10 cm × 12 cm) and dried at room temperature in a clean bench for 12 h. It was then cut into 12 mm diameter discs, rolled, and placed in an oven for thermal imidization treatment. The temperature was raised from room temperature to 150 °C and held for 2 h. The discs were then removed and weighed.

[0082] (3) Following the above electrode preparation, battery assembly, and testing methods, the NCM811 in this system has a 0.1C discharge specific capacity of 170.46 mAh / g and an initial efficiency of 84.3%. After 100 cycles at 0.1C, the capacity retention rate is 64.91%.

[0083] Comparative Example 4

[0084] (1) Dissolve 3.273g of 4,4'-diaminodiphenyl ether in 50ml of N-methylpyrrolidone solvent, and add 5.75g of 4,4'-biphenyl ether dianhydride in batches under nitrogen atmosphere and ice-water bath conditions to obtain a homogeneous polyamic acid solution.

[0085] (2) Add 2% by molar mass of 4,4'-biphenyl dianhydride and 5-aminobenzimidazole as a capping agent to the above polyamic acid solution and stir thoroughly to obtain a capped polyamic acid solution.

[0086] (3) Add 10% by molar mass of 4,4'-biphenyl dianhydride catalyst N-N' carbonyl diimidazole to the end-capped polyamic acid solution obtained in step (2), and stir thoroughly to obtain a precursor solution of low-temperature curing polyimide adhesive.

[0087] (4) Obtain a precursor solution for low-temperature curing polyimide adhesive; dilute the above precursor solution to a solid content of 5 wt%.

[0088] The solution was mixed with the positive electrode active material NCM811, conductive carbon black, and diluted precursor solution in a mass ratio of 92:5:3. N-methylpyrrolidone solvent was added to dissolve the mixture, and the mixture was stirred thoroughly to obtain a positive electrode slurry. The slurry was coated onto aluminum foil (10cm×12cm) and dried at room temperature in a clean bench for 12 hours. It was then cut into 12mm diameter discs, rolled, and placed in an oven for thermal imidization treatment. The temperature was raised from room temperature to 150℃ and held for 2 hours. The discs were then removed and weighed.

[0089] (5) Following the above electrode preparation, battery assembly, and testing methods, the NCM811 in this system has a 0.1C discharge specific capacity of 175.22 mAh / g and an initial efficiency of 82.22%. After 100 cycles at 0.1C, the capacity retention rate is 61.33%.

[0090] Figure 1 Image of a low-temperature cured polyimide film after immersion in NMP and electrolyte for 7 days.

[0091] Table 1. Residual weight of low-temperature cured polyimide after immersion in NMP and electrolyte for 7 days.

[0092] Example 1 Example 2 Comparative Example 2 NMP 95.3％ 94.6％ 0％ In electrolyte 99.2％ 99.1％ 69.2％

[0093] Table 2. Performance of the positive electrode sheets prepared in the examples and comparative examples.

[0094]

[0095] from Figure 1 As shown in Table 1, the film obtained by casting and imidization of the low-temperature curing polyimide at 150°C showed no significant change in surface morphology and only slight weight loss after 7 days of immersion in either solvent or electrolyte. However, the polyimide cured at 150°C without special molecular structure design and catalytic synergy completely dissolved in the solvent after one day. This indicates that the low-temperature curing polyimide in this invention can be successfully imidized at 150°C.

[0096] As shown in Table 2, the batteries using low-temperature curing polyimide binders in Comparative Example 1 generally exhibit higher specific capacity at 0.1C discharge and higher capacity retention after 100 cycles compared to lithium-ion batteries using PVDF binders. This further demonstrates that the low-temperature curing polyimide binder effectively maintains the structural stability of the cathode material, promotes cathode capacity utilization, and enhances battery cycle life. Meanwhile, compared to Comparative Example 2, lithium-ion batteries using low-temperature curing polyimide binders without special molecular structure design and synergistic catalyst effects show significant degradation in both specific capacity at 0.1C discharge and capacity retention after 100 cycles. Comparative Examples 3 and 4, lithium-ion batteries made with low-temperature curing polyimide binders without special functional group monomer copolymerization or catalysts, exhibited reduced imidization levels, thus decreasing capacity retention after 100 cycles. Comparing Examples 2 and 3 reveals that the 0.1C discharge specific capacity and capacity retention after 100 cycles show no significant changes, indicating that the monomers with specific functional groups listed can effectively reduce the curing temperature of polyimide after copolymerization. Comparing Examples 4 and 6 demonstrates that the catalysts listed can also effectively reduce the curing temperature of polyimide. Comparing Example 5 shows that the end-capping agents listed can also effectively reduce the curing temperature of polyimide.

Claims

1. A method for preparing a low-temperature curing polyimide binder for lithium-ion batteries, characterized in that, The preparation method includes the following steps: (1) Under nitrogen atmosphere and ice-water bath conditions, excess dianhydride monomer, diamine monomer and a third monomer containing special functional groups are subjected to condensation polymerization reaction in the reaction solvent to obtain a copolyamic acid solution. The molecular weight of polyimide can be adjusted by the relative amount of dianhydride monomer added. (2) Add the end-capping agent to the polyamic acid solution obtained by the above copolymerization and stir thoroughly to obtain the end-capped polyamic acid solution; (3) Add the catalyst to the end-capped polyamic acid solution obtained in step (2) and stir thoroughly to obtain a low-temperature curing polyimide precursor solution; (4) The active material, conductive agent and precursor solution obtained in step (3) are mixed evenly to prepare a slurry, which is coated on the surface of the current collector and then subjected to thermal imidization treatment to prepare an electrode sheet with low-temperature curing polyimide as the binder.

2. The preparation method according to claim 1, characterized in that, The solid content of the polyamic acid solution in step (1) is 0.5-40 wt%; the molar ratio of dianhydride monomer to diamine monomer is 1.01:1-1.3:1; the molar ratio of dianhydride monomer to diamine monomer is 9-25:1; the reaction time is 0.5-12 h; the dianhydride monomer in step (1) is pyromellitic dianhydride (PMDA), 4,4'-diphenyl ether dianhydride (ODPA), biphenyl dianhydride (BPDA), 3,3',4,4'-diphenylsulfone tetracarboxylic acid dianhydride (DSDA), hexafluorodianhydride (6FDA), 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride (BTDA), 4,4'-(4 The diamine monomer in step (1) is a mixture of one or more of the following in any proportion: 4,4'-isopropylidene diphenoxy) phthalic anhydride (BPADA) and p-phenylene-bis(phenyltrimethyl) dianhydride (TAHQ); The third monomer containing a special functional group in step (1) is a mixture of one or more of 2,3-diaminopyridine, 2,4-diaminopyridine, 2,5-diaminopyridine, 3,4-diaminopyridine, 5,6-diaminobenzimidazole, 2,4-diamino-6-[2-methyl-(-imidazolyl)ethyl]-1,3,5-quinoline, and 5-amino-2-(4-aminophenyl)benzimidazole in any proportion; the solvent in step (1) is a mixture of one or more of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), N-ethylpyrrolidone (NEP), dimethyl sulfoxide (DMSO), and hexamethylphosphoramide (HMPA) in any proportion.

3. The preparation method according to claim 1, characterized in that, The molar ratio of the end-capping agent to the dianhydride monomer in step (2) is 0.01:1-5:1; the reaction time is 0.5-12h; the end-capping agent in step (2) is one or more of 5-aminobenzimidazole (ABZ), 2-aminoimidazole, 2-aminopyridine, and aminopyrazine in any proportion.

4. The preparation method according to claim 1, characterized in that, The molar ratio of the catalyst to the dianhydride monomer in step (3) is 0.01:1-5:1; the reaction time is 0.5-12h; the catalyst in step (3) is one or more of N-N' carbonyl diimidazole (CDI), 6-aminoquinoline (AQL), ​​benzotriazole (BTA) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in any proportion.

5. The preparation method according to claim 1, characterized in that, The solid content of the electrode slurry in step (4) is 20-90 wt%; the non-solvent portion of the electrode slurry contains 80-99 wt% active material, 0.5-10 wt% conductive agent, and 0.5-10 wt% precursor solution; the positive electrode active material in step (4) is one or more of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt aluminum oxide, and lithium nickel cobalt manganese oxide in any proportion; the negative electrode active material in step (4) is one or more of carbon materials, silicon and its oxides, tin and its oxides, silicon-carbon composite materials, silicon-oxygen-carbon composite materials, and tin-carbon composite materials in any proportion; the conductive agent in step (4) is one or more of conductive carbon black, conductive graphite, graphene, and carbon nanotubes in any proportion; the current collector in step (4) is carbon-containing aluminum foil or carbon-containing copper foil; the coating thickness of the electrode slurry in step (4) is 75-200 μm; The thermal imidization temperature is 100-150℃, and the time is 0.5-2h.

6. The low-temperature curing polyimide binder for lithium-ion batteries prepared according to any one of claims 1-5, and the electrode sheet prepared therefrom.

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