Polyimide binder, positive electrode sheet and preparation method therefor and use thereof

Abstract

A polyimide binder, a lithium battery positive electrode sheet and a preparation method therefor and a use thereof. The polyimide binder is a high-molecular polymer obtained by polymerization, chemical imidization, and post-treatment of a diamine monomer and a dianhydride monomer, wherein the imidization rate is 65-80%; the polyimide binder contains an aromatic group, a fatty group, and a carboxyl group; and the carboxyl content of the polyimide binder is 0.4-1.5 mmol / g, and the number-average molecular weight of the polyimide binder is 100,000-400,000. The positive electrode sheet comprises the polyimide binder. The positive electrode sheet is applied to a lithium battery. The polyimide binder has both rigidity and flexibility and also has relatively high adhesion. Polyimide is not prone to degradation during a charge / discharge cycle and high-temperature operation, so that the rate and cycle performance of a battery are stable.

Description

A polyimide binder, positive electrode sheet, and preparation method and application thereof Technical Field

[0001] The present invention relates to a polyimide binder, a positive electrode plate and a preparation method and application thereof, belonging to the technical field of positive electrode plates for lithium batteries. Background Art

[0002] The role of the binder in lithium-ion batteries (LIBs) is to tightly connect the active materials, conductive agents, current collectors, etc. into one, maintain the stability of the electrode structure and the integrity of the electron channel, and directly affect the safety and cycle stability of LIBs.

[0003] Polyvinylidene fluoride (PVDF) occupies a core position in the field of LIBs binders, mainly relying on intermolecular van der Waals forces to bond active materials, conductive agents, and current collectors. PVDF has good electrochemical stability and electrolyte wettability. However, PVDF is prone to degradation during charge-discharge cycles and high-temperature operation, resulting in increased internal resistance and decreased battery rate and cycle performance. Polyimide has advantages such as high temperature resistance, good adhesion, and low expansion rate. Based on molecular structure design, it can achieve better matching with positive electrode materials, significantly improving the rate capability, safety, and service life of lithium-ion batteries. It has broad development prospects in the field of new lithium battery materials.

[0004] Chinese patent application CN111403745A discloses a high-temperature resistant adhesive for lithium-ion batteries and a battery pole piece using the adhesive. The patent proposes preparing polyamic acid by polymerizing functional diamines and dianhydrides, adding active substances and conductive agents, mixing, coating on a current collector, heating and performing thermal imidization to form polyimide, and then obtaining a battery pole piece. The adhesive has high bonding strength and good high and low temperature performance. However, polyamic acid is easily degraded and deteriorated, making it inconvenient to store and transport. In addition, the addition of functional groups increases the cost of raw materials, and the pole pieces prepared from polyamic acid are relatively brittle, prone to cracking, have poor processing performance, and poor battery stability.

[0005] Chinese patent application CN105609780A discloses an electrode binder, positive electrode material, and lithium-ion battery. The polymer reaction of diamine monomers and dianhydride monomers occurs at 160-200°C for 12-24 hours. A catalyst is added during the imidization process, reacting at 30-60°C for 2-4 hours, and then at 160-200°C for 12-24 hours. Finally, the reaction is purified in a poor solvent to obtain a polymer binder with a molecular weight of 1,000-50,000 and good thermal stability. The patent involves harsh reaction conditions that are not conducive to production. The polymer has a low molecular weight, poor adhesion, and is prone to slagging, resulting in poor battery stability and a short cycle life.

[0006] Chinese patent application CN114573812A discloses a binder for lithium-ion batteries and its preparation method. The method involves first preparing an imide diacid monomer, then an imide diamine monomer, and finally adding a catalyst for polymerization to produce a polyamide-imide binder. The preparation method proposed in this patent is complex, requires high reaction temperatures, and is time-consuming, making it unsuitable for production. The binder is a liquid that easily degrades and deteriorates when stored at room temperature, making it inconvenient to transport. While the coating layer exhibits high peel strength and battery capacity retention, it suffers from poor electrode wettability, low initial efficiency, and low energy density.

[0007] In summary, existing binders and positive electrode sheets have problems such as poor adhesion, poor high temperature resistance, complex preparation process, low battery capacity retention rate, and low initial efficiency.

[0008] Summary of the Invention

[0009] In view of the deficiencies in the prior art, the present invention provides a polyimide binder, a positive electrode plate, and a preparation method and application thereof. The polyimide binder has both rigidity and flexibility and has high adhesion. The polyimide is not easily degraded during the charge and discharge cycle process and high-temperature operation, and the battery rate and cycle performance are stable.

[0010] The technical solution of the present invention to solve the above technical problems is as follows: a polyimide binder, which is a high molecular polymer obtained by polymerization, chemical imidization and post-treatment of diamine monomers and dianhydride monomers, with an imidization rate of 65% to 80%; the polyimide binder contains aromatic groups, fatty groups and carboxyl groups;

[0011] The dianhydride monomer is Any one or several combinations thereof, wherein R1 is any one of -CH2-, -O-, bisphenol A group, and hexafluorobisphenol A group;

[0012] The diamine monomer is an aromatic diamine and / or C2-C 12 Aliphatic diamine.

[0013] Furthermore, the aromatic diamine is Any one or more combinations thereof, wherein R2 is -(CH2) n -、 -O-, bisphenol A group, hexafluorobisphenol A group, -CH2-O-CH2-, -CH(NH)-(CH2) n -、 Any one of .

[0014] Furthermore, the dianhydride monomer is one or a combination of pyromellitic dianhydride, diphenyl ether tetracarboxylic dianhydride, bisphenol A diether dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and 3,3',4,4'-benzophenone tetracarboxylic dianhydride;

[0015] The aromatic diamine is one or a combination of m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, and 4,4'-diaminodiphenyl sulfone;

[0016] The C2~C 12 The aliphatic diamine is any one or a combination of hexamethylenediamine, heptamethylenediamine, octanediamine, decanediamine, and 1,12-diaminododecane.

[0017] Furthermore, the carboxyl content of the polyimide binder is 0.4 to 1.5 mmol / g, and the number average molecular weight of the polyimide binder is 100,000 to 400,000.

[0018] The present invention also discloses a method for preparing a polyimide adhesive, which comprises:

[0019] S1, a diamine monomer and a dianhydride monomer are polymerized in an organic solvent to obtain a polymerization reaction liquid;

[0020] S2, adding a cyclizing agent and a cyclization aid to the polymerization reaction solution to carry out a chemical imidization reaction;

[0021] S3. After the chemical imidization reaction is completed, a polyimide binder is obtained by post-processing. The post-processing operation can be used to obtain a dry polyimide binder product. The following post-processing operation method can be used: After the chemical imidization reaction is completed, the reaction solution is added dropwise to a precipitation solvent, and then the polyimide binder product is obtained by solid-liquid separation, elution, and drying. The precipitation solvent is one or a mixture of water, ethanol, methanol, and isopropanol, but is not limited to these precipitation solvents, nor is it limited to this post-processing operation method.

[0022] Furthermore, in step S1, the polymerization reaction temperature is 0-80° C.; the molar ratio of the diamine monomer to the dianhydride monomer is 1:(0.8-1.3);

[0023] The mass fraction of the solid content in the polymerization reaction process is 5% to 30%, and the organic solvent is any one of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide, or a combination thereof.

[0024] Furthermore, in step S2, the cyclizing agent is acetic anhydride, and the cyclization auxiliary agent is any one or a combination of pyridine, quinoline, isoquinoline, triethylamine, and 4-dimethylaminopyridine.

[0025] The chemical imidization reaction temperature is 0-80° C.; the molar ratio of the cyclizing agent to the diamine monomer is 1.5-3.5; and the molar ratio of the cyclization auxiliary to the cyclizing agent is 1:(0.5-2).

[0026] The present invention also discloses a positive electrode plate, wherein the positive electrode plate comprises the polyimide binder.

[0027] The present invention also discloses a method for preparing a positive electrode sheet. The positive electrode sheet comprises 90 to 99 parts of a positive electrode material, 0.5 to 5 parts of a conductive agent, and 0.5 to 5 parts of a polyimide binder, in parts by weight. The positive electrode sheet preparation method is as follows:

[0028] S1. Slurry preparation: adding positive electrode material, conductive agent, binder and solvent in proportion to obtain positive electrode slurry;

[0029] S2. Coating: Use a coating machine to evenly coat the obtained slurry on the current collector aluminum foil;

[0030] S3, drying: the first stage of drying removes the solvent, and the second stage of drying performs thermal imidization to obtain the positive electrode sheet;

[0031] The positive electrode material may be one or a combination of lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide;

[0032] The conductive agent may be one or a combination of carbon black, conductive polymer, acetylene black, Ketjen black, carbon fiber, carbon nanotube, and graphite.

[0033] The solvent is any one or a combination of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide;

[0034] The drying conditions of step S3 are as follows: the first stage drying temperature is 80-120° C., the drying time is 1-12 hours, the second stage drying temperature is 120-200° C., and the drying time is 1-8 hours.

[0035] The invention also discloses the application of the positive electrode plate, which is applied in a lithium battery.

[0036] The beneficial effects of the present invention are:

[0037] (1) The molecular structure of the polyimide binder of the present invention contains both aromatic groups and aliphatic groups, and has both rigidity and flexibility. The combination of diamine or dianhydride monomers improves the solubility of the product. The imide groups in the polyimide binder structure give the binder high tensile strength and elastic modulus, and the electrode has strong shrinkage and expansion capabilities, ensuring the structural integrity and chemical stability of the battery during the cycle. The polyimide binder structure contains a large number of carbonyl groups, which can form intermolecular hydrogen bonds, improve the adhesion between the current collector and the coating layer, improve the stability of the positive electrode sheet, reduce the shedding of active materials in the positive electrode sheet during the charge and discharge process, improve the capacity retention rate and safety of the battery, and extend the service life.

[0038] (2) By controlling the material ratio and reaction conditions of the chemical imidization process, the imidization rate is controlled within an appropriate range, partial imidization is achieved, and a certain amount of carboxyl groups remain, thereby improving the adhesion between the coating layer and the current collector, reducing the brittleness of the electrode, improving the processing performance, and avoiding the cracking of the electrode that leads to a decrease in battery cycle performance and reduced safety of use. The cyclization agent and the cyclization auxiliary agent are used in combination to achieve chemical imidization at a lower temperature, obtaining a polyimide with a higher imidization rate, avoiding the degradation of the polymer caused by high-temperature imidization, resulting in reduced adhesion and electrode slag.

[0039] (3) After coating, thermal imidization is carried out during the drying process, making the polyimide structure more stable, enhancing the binding force of the binder to the active material and current collector, and improving the thermal stability of the electrode. During the continuous charge and discharge process of the battery, the binder is not easily degraded, the shedding of the coating layer is reduced, and the battery capacity retention rate is further improved. The combination of chemical imidization and thermal imidization is of great significance to improving the safety and cycle stability of the battery.

[0040] (4) During the preparation of the polyimide binder, the reaction conditions are mild and the equipment requirements are low. The raw material cost is low, and after the product is precipitated, it is stored in a solid form and is not easily degraded or deteriorated, which reduces the requirements for storage and transportation conditions. The method provided by the present invention is simple and easy to produce. It can be applied to the field of lithium battery materials and has good economic benefits and broad application prospects.

[0041] In summary, the polyimide binder preparation method of the present invention offers low raw material costs, a simple and rational preparation process, ease of production, and easy transportation and storage of the binder, resulting in good economic benefits. When used, the polyimide binder exhibits excellent electrode wettability and high-temperature resistance, resulting in long cycle life and high safety for batteries, suggesting promising application prospects. DETAILED DESCRIPTION

[0042] The present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar improvements without violating the scope of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used are only for describing specific embodiments and are not intended to limit the present invention.

[0044] A polyimide binder, which is a high molecular weight polymer obtained by polymerization, chemical imidization, and post-treatment of diamine monomers and dianhydride monomers, with an imidization rate of 65% to 80%; the polyimide binder contains aromatic groups, fatty groups, and carboxyl groups;

[0045] The dianhydride monomer is Any one or several combinations thereof, wherein R1 is any one of -CH2-, -O-, bisphenol A group, and hexafluorobisphenol A group;

[0046] The diamine monomer is an aromatic diamine and / or C2-C 12 Aliphatic diamine.

[0047] Specifically, the aromatic diamine is Any one or more combinations thereof, wherein R2 is -(CH2) n -、 -O-, bisphenol A group, hexafluorobisphenol A group, -CH2-O-CH2-, -CH(NH)-(CH2) n -、 Any one of .

[0048] Specifically, the dianhydride monomer is one or a combination of pyromellitic dianhydride, diphenyl ether tetracarboxylic dianhydride, bisphenol A diether dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and 3,3',4,4'-benzophenone tetracarboxylic dianhydride;

[0049] The aromatic diamine is one or a combination of m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, and 4,4'-diaminodiphenyl sulfone;

[0050] The C2~C 12 The aliphatic diamine is any one or a combination of hexamethylenediamine, heptamethylenediamine, octanediamine, decanediamine, and 1,12-diaminododecane.

[0051] Specifically, the carboxyl content of the polyimide binder is 0.4 to 1.5 mmol / g, and the number average molecular weight of the polyimide binder is 100,000 to 400,000.

[0052] A preparation method of a polyimide adhesive, the preparation method comprising:

[0053] S1, a diamine monomer and a dianhydride monomer are polymerized in an organic solvent to obtain a polymerization reaction liquid;

[0054] S2, adding a cyclizing agent and a cyclization aid to the polymerization reaction solution to carry out a chemical imidization reaction;

[0055] S3. After the chemical imidization reaction is completed, a polyimide adhesive is obtained through post-processing.

[0056] Specifically, in step S1, the polymerization reaction temperature is 0-80°C, and the reaction time is 1-16h; the molar ratio of the diamine monomer to the dianhydride monomer is 1:(0.8-1.3). The closer the molar ratio of the diamine monomer to the dianhydride monomer is to 1, the greater the molecular weight, the greater the viscosity, and the better the adhesion.

[0057] The mass fraction of the solid content in the polymerization reaction process is 5% to 30%, and the organic solvent is any one of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide, or a combination thereof.

[0058] Specifically, in step S2, the cyclizing agent is acetic anhydride, and the cyclization auxiliary agent is any one or a combination of pyridine, quinoline, isoquinoline, triethylamine, and 4-dimethylaminopyridine.

[0059] The chemical imidization reaction temperature is 0-80° C.; the molar ratio of the cyclizing agent to the diamine monomer is 1.5-3.5; and the molar ratio of the cyclization auxiliary to the cyclizing agent is 1:(0.5-2).

[0060] Specifically, in step S3, the post-treatment process is: after the chemical imidization reaction is completed, the post-reaction solution is added dropwise to the precipitation solvent, and then solid-liquid separation, elution, and drying are performed to obtain the polyimide adhesive product.

[0061] The precipitation solvent is one of water, ethanol, methanol, and isopropanol, or a mixture thereof, but is not limited to these solvents.

[0062] A positive electrode plate comprises the polyimide binder.

[0063] Specifically, the positive electrode sheet includes 90 to 99 parts of positive electrode material, 0.5 to 5 parts of conductive agent, and 0.5 to 5 parts of polyimide binder in parts by weight;

[0064] The positive electrode material may be one or a combination of lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide;

[0065] The conductive agent may be one or a combination of carbon black, conductive polymer, acetylene black, Ketjen black, carbon fiber, carbon nanotube, and graphite;

[0066] The solvent is any one or a combination of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide;

[0067] More specifically, the positive electrode sheet preparation process is as follows: the positive electrode material, conductive agent, and polyimide binder are added to the N-methylpyrrolidone solvent according to the formula ratio, the mass fraction of the solid content of the slurry is adjusted to 40-60%, and the mixture is stirred at high speed for uniform mixing. After screening, the mixture is coated on aluminum foil, and then dried in two stages and roller pressed to obtain the positive electrode sheet.

[0068] The two-stage drying is as follows: the drying temperature of the first stage is 80-120° C. and the drying time is 1-12 hours; the drying temperature of the second stage is 120-200° C. and the drying time is 1-12 hours.

[0069] The positive electrode plate is used in a lithium battery.

[0070] Example 1

[0071] Preparation of polyimide adhesive:

[0072] To a 500mL three-necked flask, add 8.135g of hexamethylenediamine, 3.251g of m-phenylenediamine, and 149.882g of N,N-dimethylformamide in sequence. Once dissolved, add 21.922g of pyromellitic dianhydride and 149.882g of N,N-dimethylformamide and allow to react at 0-5°C for 16 hours. Add 15.820g of pyridine and 20.418g of acetic anhydride and continue the reaction at 0-5°C for 10 hours. After the reaction is complete, add the resulting solution dropwise to water to precipitate the solid, filter, rinse, and dry at 95°C to obtain 29.973g of a polyimide binder.

[0073] Positive electrode preparation:

[0074] By weight, 95 parts LiCoO2, 2.5 parts carbon black, and 2.5 parts binder were added to a sealed container. N-methylpyrrolidone was then added to adjust the slurry solids content to 50%. The mixture was stirred at high speed to mix evenly. After sieving, the mixture was coated on aluminum foil. The positive electrode was first vacuum-dried at 80°C for 12 hours, then at 200°C for 2 hours, and then roller-pressed to obtain the positive electrode sheet.

[0075] Example 2

[0076] Preparation of polyimide adhesive:

[0077] To a 500mL three-necked flask, add 5.423g of p-phenylenediamine, 7.227g of octanediamine, and 124.090g of N-methylpyrrolidone. After dissolution, add 31.146g of diphenyl ether tetracarboxylic dianhydride and 124.090g of N-methylpyrrolidone. React at 10-15°C for 9 hours. Add 19.775g of pyridine and 25.523g of acetic anhydride. Continue reacting at 10-15°C for 8 hours. After the reaction is complete, add the solid dropwise to ethanol to precipitate. Filter, rinse, and dry at 90°C to obtain 39.854g of a polyimide binder.

[0078] Positive electrode preparation:

[0079] By weight, 92 parts LiNiO2, 4 parts acetylene black, and 4 parts binder were added to a sealed container. N-methylpyrrolidone was then added to adjust the slurry's solids content to 40%. The mixture was stirred at high speed until uniform, sieved, and coated onto aluminum foil. The mixture was vacuum-dried at 100°C for 8 hours, then at 180°C for 4 hours, and then roller-pressed to produce the positive electrode sheet.

[0080] Example 3

[0081] Preparation of polyimide adhesive:

[0082] To a 1L three-necked flask, add 12.477g of 4,4'-diaminodiphenyl sulfone, 7.227g of octanediamine, and 203.895g of N-methylpyrrolidone. Once dissolved, add 52.258g of bisphenol A diether dianhydride and 203.895g of N-methylpyrrolidone and react at 50-55°C for 4 hours. Add 25.312g of pyridine and 32.669g of acetic anhydride and continue reacting at 50-55°C for 2 hours. After the reaction is complete, add the mixture dropwise to water to precipitate the solid, filter, rinse, and dry at 100°C to obtain 67.357g of a polyimide binder.

[0083] Positive electrode preparation:

[0084] By weight, 93 parts LiMnO2, 3.5 parts Ketjen Black, and 3.5 parts binder were added to a sealed container. N-methylpyrrolidone was then added to adjust the slurry's solids content to 60%. The mixture was stirred at high speed until uniform, sieved, and coated onto aluminum foil. The mixture was vacuum-dried at 120°C for 4 hours, then at 160°C for 8 hours, and then roller-pressed to produce the positive electrode sheet.

[0085] Example 4

[0086] Preparation of polyimide adhesive:

[0087] To a 500mL three-necked flask, add 17.266g of decanediamine and 51.099g of N,N-dimethylacetamide. Once dissolved, add 10.961g of pyromellitic dianhydride, 15.573g of diphenyl ether tetracarboxylic dianhydride, and 51.099g of N,N-dimethylacetamide. React at 60-65°C for 1 hour. Add 25.312g of pyridine and 32.669g of acetic anhydride and continue reacting for 1 hour. After the reaction is complete, add the precipitated solid dropwise to ethanol. Filter, rinse, and dry at 70°C to obtain 39.419g of a polyimide binder.

[0088] Positive electrode preparation:

[0089] By weight, 94 parts LiFePO4, 3 parts carbon nanotubes, and 3 parts binder were added to a sealed container. N-methylpyrrolidone was then added to adjust the slurry's solids content to 50%. The mixture was stirred at high speed until uniform, sieved, and coated onto aluminum foil. The mixture was vacuum-dried at 80°C for 12 hours, then at 140°C for 12 hours, and then roller-pressed to produce the positive electrode sheet.

[0090] Example 5

[0091] Preparation of polyimide adhesive:

[0092] To a 500 mL three-necked flask, add 7.946 g of 4,4'-diaminodiphenylmethane, 7.814 g of heptanediamine, and 67.951 g of N,N-dimethylformamide. Once dissolved, add 29.540 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride and 67.951 g of N,N-dimethylformamide. React at 40-45°C for 6 h. Add 19.775 g of pyridine and 25.523 g of acetic anhydride, and continue reacting at 40-45°C for 3 h. After the reaction is complete, add the solid dropwise to isopropyl alcohol to precipitate. Filter, rinse, and dry at 85°C to obtain 40.235 g of a polyimide binder.

[0093] Positive electrode preparation:

[0094] By weight, 98 parts LiCoO2, 1 part graphite, and 1 part binder were added to a sealed container. N-methylpyrrolidone was then added to adjust the slurry's solids content to 50%. The mixture was stirred at high speed until uniform, sieved, and coated onto aluminum foil. The mixture was vacuum-dried at 100°C for 8 hours, then at 140°C for 8 hours, and then roller-pressed to produce the positive electrode sheet.

[0095] Example 6

[0096] Preparation of polyimide adhesive:

[0097] To a 500 mL three-necked flask, add 8.042 g of 4,4'-diaminodiphenyl ether, 10.359 g of decanediamine, and 101.572 g of N,N-dimethylacetamide. Once dissolved, add 32.385 g of 3,3',4,4'-benzophenonetetracarboxylic dianhydride and 101.572 g of N,N-dimethylacetamide. React at 25-30°C for 8 hours. Add 23.730 g of pyridine and 30.627 g of acetic anhydride, and continue reacting at 25-30°C for 6 hours. After the reaction is complete, add the mixture dropwise to methanol to precipitate the solid, filter, rinse, and dry at 80°C to obtain 46.469 g of a polyimide binder.

[0098] Positive electrode preparation:

[0099] By weight, 99 parts LiNiO2, 0.5 parts carbon black, and 0.5 parts binder were added to a sealed container. N-methylpyrrolidone was then added to adjust the slurry solids content to 50%. The mixture was stirred at high speed to mix evenly. After sieving, the mixture was coated on aluminum foil. The positive electrode was first dried under vacuum at 120°C for 4 hours, then at 160°C for 6 hours, and then roller-pressed.

[0100] Example 7

[0101] Preparation of polyimide adhesive:

[0102] To a 500 mL three-necked flask, add 20.036 g of 1,12-diaminododecane and 76.498 g of N,N-dimethylformamide. After dissolution, add 16.192 g of 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 14.770 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride, and 76.498 g of N,N-dimethylformamide. React at 40-45°C for 2 h. Add 27.685 g of pyridine and 35.732 g of acetic anhydride, and continue reacting at 40-45°C for 1 h. After the reaction is complete, add the mixture dropwise to methanol to precipitate the solid, filter, rinse, and dry at 60°C to obtain 46.155 g of a polyimide binder.

[0103] Positive electrode preparation:

[0104] By weight, 92 parts LiMnO2, 4 parts acetylene black, and 4 parts binder were added to a sealed container. N-methylpyrrolidone was then added to adjust the slurry's solids content to 50%. The mixture was stirred at high speed until uniform, sieved, and coated onto aluminum foil. The mixture was vacuum-dried at 100°C for 12 hours, then at 180°C for 8 hours, and then roller-pressed to produce the positive electrode sheet.

[0105] Example 8

[0106] Preparation of polyimide adhesive:

[0107] To a 500 mL three-necked flask, add 3.251 g of m-phenylenediamine, 10.118 g of octanediamine, and 88.664 g of N-methylpyrrolidone. After dissolution, add 16.192 g of 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 14.770 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride, and 88.664 g of N-methylpyrrolidone. React at 75-80°C for 1 hour. Add 15.820 g of pyridine and 20.418 g of acetic anhydride, and continue reacting at 75-80°C for 1 hour. After the reaction is complete, add the mixture dropwise to isopropyl alcohol to precipitate the solid, filter, rinse, and dry at 70°C to obtain 41.007 g of a polyimide binder.

[0108] Positive electrode preparation:

[0109] By weight, 91 parts LiFePO4, 4 parts Ketjen Black, and 5 parts binder were added to a sealed container. N-methylpyrrolidone was then added to adjust the slurry's solids content to 50%. The mixture was stirred at high speed until uniform, sieved, and coated onto aluminum foil. The mixture was vacuum-dried at 120°C for 6 hours, then at 160°C for 12 hours, and then roller-pressed to produce the positive electrode sheet.

[0110] Example 9

[0111] Preparation of polyimide adhesive:

[0112] To a 500mL three-necked flask, add 5.423g of p-phenylenediamine, 7.227g of octanediamine, and 124.090g of N,N-dimethylformamide in sequence. After dissolution, add 31.146g of diphenyl ether tetracarboxylic dianhydride and 124.090g of N,N-dimethylformamide, and react at 10-15°C for 9 hours. Add 25.832g of quinoline and 25.523g of acetic anhydride, and continue to react at 10-15°C for 8 hours. After the reaction is complete, add the resulting solid dropwise to ethanol to precipitate. Filter, rinse, and dry at 90°C to obtain 39.734g of a polyimide binder.

[0113] Positive electrode preparation:

[0114] By weight, 92 parts LiNiO2, 4 parts acetylene black, and 4 parts binder were added to a sealed container. N-methylpyrrolidone was then added to adjust the slurry's solids content to 50%. The mixture was stirred at high speed until uniform, sieved, and coated onto aluminum foil. The mixture was vacuum-dried at 100°C for 8 hours, then at 180°C for 4 hours, and then roller-pressed to produce the positive electrode sheet.

[0115] Example 10

[0116] Preparation of polyimide adhesive:

[0117] To a 500mL three-necked flask, add 8.135g of hexamethylenediamine, 3.251g of m-phenylenediamine, and 149.882g of N,N-dimethylformamide in sequence. Once dissolved, add 21.922g of pyromellitic dianhydride and 149.882g of N,N-dimethylformamide and allow to react at 0-5°C for 16 hours. Add 20.238g of triethylamine and 20.418g of acetic anhydride and continue the reaction at 0-5°C for 10 hours. After the reaction is complete, add the resulting solution dropwise to water to precipitate the solid, filter, rinse, and dry at 90°C to obtain 29.948g of a polyimide binder.

[0118] Positive electrode preparation:

[0119] By weight, 95 parts LiCoO2, 2.5 parts carbon black, and 2.5 parts binder were added to a sealed container. N-methylpyrrolidone was then added to adjust the slurry solids content to 55%. The mixture was stirred at high speed to mix evenly. After sieving, the mixture was coated on aluminum foil. The positive electrode was dried in a vacuum at 80°C for 12 hours, then at 200°C for 4 hours, and then roller pressed.

[0120] Comparative Example 1

[0121] The polyimide binder was prepared by the same method as in Example 1, except that the polyimide binder did not contain a fatty group, i.e., hexamethylenediamine was not added during the preparation process. The specific preparation process was as follows:

[0122] To a 500mL three-necked flask, add 10.836g of m-phenylenediamine and 149.882g of N,N-dimethylformamide. Once dissolved, add 21.922g of pyromellitic dianhydride and 149.882g of N,N-dimethylformamide. React at 0-5°C for 16 hours. Add 15.820g of pyridine and 20.418g of acetic anhydride. Continue reacting at 0-5°C for 10 hours. After the reaction is complete, add the resulting solution dropwise to water to precipitate the solid. Filter, rinse, and dry at 95°C to obtain 29.809g of a polyimide binder.

[0123] Positive electrode preparation:

[0124] The positive electrode sheet was prepared using the same method as in Example 1.

[0125] Comparative Example 2

[0126] The polyimide binder was prepared by the same method as in Example 2, except that the amount of the cyclizing agent and the cyclization aid (pyridine and acetic anhydride) was increased during the preparation of the polyimide binder. The specific preparation process was as follows:

[0127] To a 500mL three-necked flask, add 5.423g of p-phenylenediamine, 7.227g of octanediamine, and 124.090g of N-methylpyrrolidone. Once dissolved, add 31.146g of diphenyl ether tetracarboxylic dianhydride and 124.090g of N-methylpyrrolidone. React at 10-15°C for 9 hours. Add 39.550g of pyridine and 51.045g of acetic anhydride, and continue to react at 10-15°C for 8 hours. After the reaction is complete, add the solid dropwise to ethanol to precipitate. Filter, rinse, and dry at 90°C to obtain 39.767g of a polyimide binder.

[0128] Positive electrode preparation:

[0129] The positive electrode sheet was prepared using the same method as in Example 2.

[0130] Comparative Example 3

[0131] The polyimide binder was prepared by the same method as in Example 3, except that no chemical imidization reaction was performed during the preparation of the polyimide binder, i.e., no cyclizing agent and cyclization auxiliary agent were added. The specific preparation process was as follows:

[0132] To a 1L three-necked flask, add 12.477g of 4,4'-diaminodiphenyl sulfone, 7.227g of octanediamine, and 203.895g of N-methylpyrrolidone. Once dissolved, add 52.258g of bisphenol A diether dianhydride and 203.895g of N-methylpyrrolidone. React at 50-55°C for 4 hours. Upon completion, 479.752g of a polyamic acid binder with a 15% solids content was obtained.

[0133] Positive electrode preparation:

[0134] The positive electrode sheet was prepared using the same method as in Example 3.

[0135] Comparative Example 4

[0136] The prepared polyimide binder (Example 4) was vacuum-packed in bottles and stored at -5 to 0° C. for 90 days. A positive electrode was prepared according to Example 4 and its performance was tested.

[0137] Comparative Example 5

[0138] The prepared polyamic acid binder (Comparative Example 3) was vacuum-packed in bottles and stored at -5 to 0° C. for 90 days. A positive electrode was prepared according to Comparative Example 3 and its performance was tested.

[0139] Comparative Example 6

[0140] The purchased Solvay PVDF 5130 was used as a binder, and the positive electrode was prepared according to the steps and methods of Example 1 and the performance was tested.

[0141] Comparative Example 7

[0142] The battery assembled using the positive electrode sheet of Example 7 was placed in a battery temperature cycle tester at 55°C for 12 hours, and the battery performance at 55°C was tested to evaluate the high temperature stability of the corresponding battery using polyimide as a binder.

[0143] Comparative Example 8

[0144] The battery assembled using the positive electrode sheet of Comparative Example 6 was placed in a battery temperature cycle tester at 55°C for 12 hours, and then the battery performance at 55°C was tested to evaluate the high temperature stability of the corresponding battery using Solvay PVDF 5130 as the binder.

[0145] Comparative Example 9

[0146] The imidization reaction is achieved by heating, and the specific process is as follows:

[0147] Preparation of polyimide adhesive:

[0148] To a 500mL three-necked flask, 8.135g of hexamethylenediamine, 3.251g of m-phenylenediamine, and 149.882g of N,N-dimethylformamide were added in sequence. After dissolution, 21.922g of pyromellitic dianhydride and 149.882g of N,N-dimethylformamide were added and reacted at 0-5°C for 16 hours to obtain a polyamic acid solution. The solution was then added dropwise to water to precipitate a solid, which was filtered, rinsed, and dried at 95°C to obtain 30.412g of polyamic acid solid. After the reaction, the obtained solid material was dried at 150°C for 4 hours, then at 200°C for 3 hours, and finally at 220°C for 4 hours to obtain a polyimide binder.

[0149] Positive electrode preparation:

[0150] The positive electrode sheet was prepared using the same method as in Example 1.

[0151] Comparative Example 10

[0152] The polyimide binder was prepared by the same method as in Example 1, except that: during the preparation of the polyimide binder, no cyclization auxiliary agent was added, and only the cyclization agent was added.

[0153] Preparation of polyimide adhesive:

[0154] To a 500mL three-necked flask, add 8.135g of hexamethylenediamine, 3.251g of m-phenylenediamine, and 149.882g of N,N-dimethylformamide in sequence. Once dissolved, add 21.922g of pyromellitic dianhydride and 149.882g of N,N-dimethylformamide. React at 0-5°C for 16 hours. Add 40.836g of acetic anhydride and continue reacting at 0-5°C for 20 hours. After the reaction is complete, add the resulting solid dropwise to water to precipitate. Filter, rinse, and dry at 95°C to obtain 29.663g of a polyimide binder.

[0155] Positive electrode preparation:

[0156] The positive electrode sheet was prepared using the same method as in Example 1.

[0157] Comparative Example 11

[0158] The polyimide binder prepared in Example 1 was used to prepare a positive electrode sheet in the same manner as in Example 1, except that only one stage of drying, i.e., vacuum drying at 80° C. for 12 h, was performed, and no second stage of drying (thermal imidization) was performed.

[0159] Comparative Example 12

[0160] The polyimide adhesive was prepared by the same method as in Example 1, except that the chemical imidization time was prolonged during the preparation of the polyimide adhesive. The specific preparation process was as follows:

[0161] Preparation of polyimide adhesive:

[0162] To a 500mL three-necked flask, add 8.135g of hexamethylenediamine, 3.251g of m-phenylenediamine, and 149.882g of N,N-dimethylformamide in sequence. Once dissolved, add 21.922g of pyromellitic dianhydride and 149.882g of N,N-dimethylformamide and allow to react at 0-5°C for 16 hours. Add 15.820g of pyridine and 20.418g of acetic anhydride and continue the reaction at 0-5°C for 20 hours. After the reaction is complete, add the resulting solution dropwise to water to precipitate the solid, filter, rinse, and dry at 95°C to obtain 30.307g of a polyimide binder.

[0163] Positive electrode preparation:

[0164] The positive electrode sheet was prepared using the same method as in Example 1.

[0165] Performance testing:

[0166] 1. Method for measuring the properties of the polyimide adhesives obtained in the above examples and comparative examples:

[0167] (1) Use gel permeation chromatography (GPC) to measure the number average molecular weight Mw;

[0168] (2) Using a cone-plate viscometer to test the viscosity of a 5% solid content binder dissolved in N-methylpyrrolidone solution; using nuclear magnetic resonance to test the imidization rate;

[0169] (3) The carboxyl content was tested using chemical titration. The test results are summarized in Table 1.

[0170] 2. Positive electrode performance test method:

[0171] (1) Electrode peel strength test: The obtained electrode was cut into 10 cm × 2 cm strips and fixed to a 1 mm thick steel plate with double-sided tape on the current collector side. Transparent tape was pasted on the coating layer and a 180° pulling peeling test was performed using a universal material testing machine at a peeling speed of 10 cm / min.

[0172] (2) Pole flexibility test: Bend the pole piece 180° toward the current collector side, repeat three times, and observe the crease state of the pole piece under an optical microscope. If it is intact, it is marked as ○, and if it is detached or cracked, it is marked as ×.

[0173] 3. Battery performance test:

[0174] The button cells were assembled in a glove box, and then the first efficiency at a current density of 0.1C and the capacity retention after 500 cycles at a current density of 1C were tested.

[0175] 0.1C first efficiency / % = 0.1C first cycle discharge capacity (mAh / g) ÷ 0.1C first cycle charge capacity (mAh / g) × 100%. 1C capacity retention rate / % = 1C 500th cycle discharge capacity (mAh / g) ÷ 1C first cycle discharge capacity (mAh / g) × 100%.

[0176] Table 1 Performance test results

[0177] Comparing Comparative Example 1 with Example 1 shows that, in Comparative Example 1, where no aliphatic diamine was added, the polyimide had poor solubility, the electrode sheet had poor flexibility, and the material easily fell off after bending, making it impossible to prepare a battery, and no battery data was available. This indicates that the addition of aliphatic diamines can improve the solubility of the polyimide, improve the flexibility of the electrode sheet, and increase the tensile strength and elastic modulus, thereby ensuring the structural integrity and chemical stability of the battery during cycling.

[0178] Compared with Example 2, Comparative Example 2 has an imidization rate of 100% and no carboxyl groups. The electrode peel strength of Comparative Example 2 is reduced by 105 N / m, and the capacity retention of the corresponding battery is reduced by 29.91%. This shows that retaining a certain amount of carboxyl groups can improve adhesion, thereby enhancing electrode stability, reducing active material shedding during charge and discharge, improving battery capacity retention and safety, and extending battery life.

[0179] Compared with Example 3, in Comparative Example 3, where the polyimide was not chemically imidized and polyamic acid was used as the binder, the electrode sheet had poor flexibility, a 157 N / m decrease in peel strength, and a 33.29% decrease in the capacity retention of the corresponding battery. This indicates that without chemical imidization, the electrode sheet becomes more brittle, resulting in cracking of the coating layer, poor processing performance, and reduced battery stability and safety.

[0180] Comparative Example 4 shows that after 90 days of storage, the viscosity, molecular weight, imidization rate, and carboxyl group content of the polyimide in Comparative Example 4 did not change significantly. The peel strength of the corresponding electrode and the first efficiency and capacity retention of the battery also did not change much. After 90 days of storage, the viscosity and molecular weight of the polyamic acid in Comparative Example 5 showed a significant downward trend, indicating that the polymer had degraded. The peel strength and capacity retention of the corresponding electrode also decreased. This shows that the polyimide binder prepared by the present invention is easy to store and not prone to deterioration.

[0181] Comparative Example 6 uses commercial PVDF as a binder. Although the electrode has good flexibility, the peel strength is 25% lower than that of the present invention, and the capacity retention is 20% lower than that of the present invention. This shows that the polyimide binder prepared by the present invention has good adhesion, good electrode flexibility and stability, high battery cyclability and safety, and has good application prospects.

[0182] Comparative Examples 7 and 8 evaluated the cycling stability of batteries using the polyimide binder of the present invention and a commercial PVDF binder under high-temperature conditions. With the polyimide binder, the initial efficiency at high temperature was 9.35% lower than at room temperature, and the capacity retention was 14.63% lower. With the PVDF binder, the initial efficiency at high temperature was 25.4% lower than at room temperature, and the capacity retention was 37.51% lower. This demonstrates that the electrode sheets obtained with the present invention have excellent thermal stability and the battery has good safety.

[0183] Compared with Example 1, Comparative Example 9, in which the polyimide was prepared by thermal imidization, had poor solubility, was partially insoluble, had an uneven molecular weight distribution, poor flexibility of the pole piece, easy chipping at the edge of the punch, and low peel strength. Appropriate viscosity and imidization rate ensure high adhesion. Too low molecular weight or too many carboxyl groups lead to increased brittleness of the pole piece, weakened bonding between the coating layer and the current collector, and reduced capacity retention. This shows that preparing polyimide only by thermal imidization has certain defects in battery performance.

[0184] Comparative Example 10, which does not use a cyclization aid, exhibits a low imidization rate of only 33% even when the cyclization agent dosage is increased and the imidization reaction time is prolonged. The polyimide also suffers from issues such as poor electrode flexibility, low peel strength, and low battery capacity retention. This suggests that the cyclization aid plays an important catalytic role in the chemical imidization process, increasing the imidization rate.

[0185] In Comparative Example 11, the electrode was dried in only one stage, without the second stage drying (thermal imidization), which did not affect the flexibility of the electrode and the initial efficiency of the battery. However, compared with Example 1, the peel strength was reduced by 81 N / m and the capacity retention rate was reduced by 10.36%, indicating that the second stage drying improved the bonding strength between the coating layer and the current collector, and the thermal imidization further improved the battery performance.

[0186] Compared with Example 1, in Comparative Example 12, the chemical imidization time was doubled, the imidization rate reached 90%, the corresponding carboxyl content was reduced to 0.15 mmol / g, the viscosity was reduced by 50%, the electrode peel strength decreased by 135 N / m, and the battery capacity retention rate decreased by 19.16%. This indicates that the appropriate imidization rate affects the viscosity of the polyimide, which in turn affects the electrode peel strength and battery performance.

[0187] The technical features of the above-described embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0188] For those skilled in the art, several variations and improvements may be made without departing from the scope of the present invention, which all fall within the scope of protection of the present invention. The scope of protection of the present invention shall be based on the appended claims.

Claims

1. A polyimide binder, characterized in that, The polyimide binder is a high molecular polymer obtained by polymerizing diamine monomers and dianhydride monomers through polymerization, chemical imidization, and post-treatment, with an imidization rate of 65% to 80%; the polyimide binder contains aromatic groups, aliphatic groups, and carboxyl groups; The dianhydride monomer is Any one or a combination of several of them, where R1 is any one of -CH2-, -O-, bisphenol A group, and hexafluorobisphenol A group; The diamine monomer is an aromatic diamine and / or a C2-C 12 aliphatic diamine.

2. The polyimide binder according to claim 1, characterized in that, The aromatic diamine is any one or a combination of several of them, where R2 is -(CH2) n -, -O-, bisphenol A group, hexafluorobisphenol A group, -CH2-O-CH2-, -CH(NH)-(CH2) n -, any one of them.

3. The polyimide binder according to claim 1, wherein The dianhydride monomers are one or a combination of several of pyromellitic dianhydride, diphenyl ether tetracarboxylic dianhydride, bisphenol A type diether dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride; The aromatic diamines are one or a combination of several of m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone; The C2 to C 12 The aliphatic diamine is any one or a combination of several of hexamethylenediamine, heptamethylenediamine, octamethylenediamine, decamethylenediamine, 1,12-diaminododecane.

4. A polyimide binder according to any one of claims 1-3, characterized in that, The carboxyl group content in the polyimide binder is 0.4 to 1.5 mmol / g, and the number average molecular weight of the polyimide binder is 100,000 to 400,000.

5. A preparation method of a polyimide binder according to any one of claims 1-4, characterized in that, The preparation method is as follows: S1. The diamine monomers and dianhydride monomers are subjected to a polymerization reaction in an organic solvent to obtain a polymerization reaction solution; S2. A cyclizing agent and a cyclization assistant are added to the polymerization reaction solution for a chemical imidization reaction; S3. After the chemical imidization reaction is completed, a polyimide binder is obtained through post-treatment.

6. The preparation method of a polyimide binder according to claim 5, characterized in that, In step S1, the polymerization reaction temperature is 0 to 80°C; the molar ratio of the diamine monomers to the dianhydride monomers is 1:(0.8 to 1.3); The mass fraction of the solid content in the polymerization reaction process is 5% to 30%, and the organic solvent is any one or a combination of several of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide.

7. The preparation method of a polyimide binder according to claim 5, characterized in that, In step S2, the cyclizing agent is acetic anhydride, and the cyclization assistant is any one or a combination of several of pyridine, quinoline, isoquinoline, triethylamine, and 4-dimethylaminopyridine; The chemical imidization reaction temperature is 0 to 80°C; the molar ratio of the cyclizing agent to the diamine monomer is 1:(1.0 to 4.0), and the molar ratio of the cyclization assistant to the cyclizing agent is 1:(0.5 to 2).

8. A positive electrode sheet, characterized in that, The positive electrode sheet includes the polyimide binder according to any one of claims 1-4.

9. A method for preparing a positive electrode sheet according to claim 8, characterized in that, By weight, the positive electrode sheet includes 90 to 99 parts of a positive electrode material, 0.5 to 5 parts of a conductive agent, and 0.5 to 5 parts of a polyimide binder. The preparation method of the positive electrode sheet is as follows: S1. Pulp making: Add the positive electrode material, conductive agent, binder, and solvent in proportion to obtain a positive electrode slurry; S2. Coating: Use a coater to uniformly coat the obtained slurry on a current collector aluminum foil; S3. Drying: The solvent is removed in the first-stage drying, and thermal imidization is carried out in the second-stage drying, followed by rolling to obtain a positive electrode sheet; The positive electrode material is one or a combination of several of lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide; The conductive agent is one or a combination of several of carbon black, conductive polymer, acetylene black, Ketjen black, carbon fiber, carbon nanotube, and graphite; The solvent is any one or a combination of several of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide. One or several combinations; The drying conditions in step S3 are as follows: the temperature of the first-stage drying is 80-120°C, the drying time is 1-12 h, the temperature of the second-stage drying is 120-200°C, and the drying time is 1-8 h.

10. Use of the positive electrode sheet according to claim 8, characterized in that, The positive electrode sheet is applied to a lithium battery.

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