Preparation methods for and use of polyamide-imide binder and positive electrode sheet

Abstract

The present invention relates to the technical field of positive electrode sheets of lithium batteries, and in particular relates to preparation methods for and the use of a polyamide-imide binder and a positive electrode sheet. The polyamide-imide binder is a high-molecular polymer prepared by polymerizing a diamine monomer and a dianhydride monomer to prepare a polyamide acid intermediate, then adding a diisocyanate thereto and performing cross-linking. The polyamide-imide binder comprises both amide groups and imide groups, and has a number-average molecular weight of 50,000-300,000. The positive electrode sheet comprises the polyamide-imide binder. In the polyamide-imide binder, some amide groups are added on the basis of polyimide, thereby retaining high tensile strength and elasticity modulus of the imide groups, improving the flexibility and impact resistance of the electrode sheet, lowering the cracking risk of the electrode sheet, enhancing the capacity retention ratio and safety of a battery and prolonging the service life; moreover, the energy density of the battery can be further improved by means of thick coating.

Description

Polyamide-imide binder, preparation method and application of positive electrode sheet TECHNICAL FIELD

[0001] The application relates to a polyamide-imide binder, a preparation method and application of a positive electrode sheet, and belongs to the technical field of lithium battery positive electrode sheets. BACKGROUND

[0002] Lithium ion batteries have the advantages of high energy density, high working voltage, light weight, no pollution and the like, are widely used in the field of power batteries, and are currently in a rapid development stage. With the continuous acceleration of the application of lithium ion batteries, the use safety of the lithium ion batteries is also concerned. In lithium ion batteries (LIBs), a binder can tightly connect active materials, conductive agents, current collectors and the like into a whole, maintains the stability of an electrode structure and the integrity of an electronic channel, and directly affects the use safety and cycle stability of the LIBs. Design and optimization of a molecular structure of the binder are of great significance to improving the safety and service life of lithium batteries.

[0003] Polyvinylidene fluoride (PVDF) plays a core role in the field of LIBs binders, and mainly relies 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 has many shortcomings, such as being prone to degradation in the charging and discharging cycle process and high-temperature work, leading to an increase in internal resistance and a decrease in the rate capability and cycle performance of the battery; gel is generated in the pulping process, leading to coating difficulty; and toxic fluorine-containing gas is generated in the high-temperature cutting process of the electrode sheet.

[0004] Polyimide has the advantages of high temperature resistance, good adhesion and low expansion rate, can be designed based on a molecular structure to achieve better matching with a positive electrode material, greatly improves the rate capability, safety and service life of lithium ion batteries, and has a broad development prospect in the field of new lithium battery materials. However, polyimide is strong in rigidity, and has a cracking risk in the coating, rolling and winding processes, and the cracking phenomenon is more significant in the case of thick coating, thereby limiting the energy density of the battery.

[0005] Chinese patent application CN111403745A discloses a high-temperature-resistant binder for lithium ion batteries and a battery electrode sheet using the binder, and proposes that polyamide acid is prepared by polymerization of a functional diamine and a dianhydride, active substances and a conductive agent are added and mixed, are coated on a current collector, and are heated to perform thermal imidization to form polyimide, and then a battery electrode sheet is obtained. The binder has high adhesion strength and good high and low temperature performance. However, the electrode sheet prepared from the polyamide acid is relatively brittle, is prone to cracking, has poor processability and poor battery stability. The addition of a functional group increases the cost of raw materials, and the binder has a large amount, and the energy density of the battery is low.

[0006] Chinese patent application CN105609780A discloses an electrode binder, positive electrode material and lithium ion battery, diamine monomer and dianhydride monomer are polymerized at 160-200℃ for 12-24h, catalyst is added in imidization process and reacted at 30-60℃ for 2-4h, and then reacted at 160-200℃ for 12-24h, finally purified in poor solvent to obtain polymer binder with molecular weight of 1000-50000 and good thermal stability. The polymer has low molecular weight, poor adhesion, easy to drop slag, poor battery stability and short cycle life; catalyst needs to be added and post-treated, which not only increases raw material cost, but also increases post-treatment steps and produces a large amount of waste liquid, which is not conducive to production.

[0007] Chinese patent CN117447960B discloses a polyimide binder, positive electrode sheet and its preparation method and application, diamine monomer and dianhydride monomer are polymerized, chemically imidized and post-treated to obtain high molecular polymer. The preparation method also needs to add catalyst and post-treatment, which is not conducive to production; although the coating layer has high peeling strength and high battery capacity retention rate, the imide ring of the binder has high rigidity, which has cracking risk in coating, rolling and winding processes, and the cracking phenomenon is more significant in the case of increasing the thickness of the coating layer, which limits the energy density of the battery.

[0008] In summary, the existing binder has problems of poor adhesion and flexibility, easy cracking of thick positive electrode sheet, low battery safety and service life.

[0009] SUMMARY

[0010] The present application provides a polyamide-imide binder, a preparation method of a positive electrode sheet and application, which increases a part of amide groups on the basis of polyimide, retains high tensile strength and elastic modulus of imide groups, improves the flexibility and impact resistance of the electrode sheet, reduces the cracking risk of thick coated electrode sheet, improves the capacity retention rate and safety of the battery, and prolongs the service life.

[0011] The technical scheme for solving the above technical problems is as follows: a polyamide-imide binder is prepared by polymerizing diamine monomer and dianhydride monomer to obtain polyamide acid intermediate A, then adding diisocyanate to crosslink to obtain polyamide-imide binder precursor, mixing the polyamide-imide binder precursor with conductive agent and positive electrode material to prepare electrode sheet by pulping and coating, and finally converting to the polyamide-imide binder during drying of the electrode sheet;

[0012] The dianhydride monomer is The diamine monomer is The diisocyanate is wherein R1-R3 are independently selected from aryl, aliphatic or alicyclic group;

[0013] The structure of the polyamide-imide adhesive is is a repeating unit structure.

[0014] Further, the dianhydride monomer is one or a combination of the following: pyromellitic dianhydride, oxydiphthalic dianhydride, bisphenol A diether dianhydride, 3,3',4,4'-biphenyl tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 2,3,3',4'-biphenyl tetracarboxylic dianhydride, 2,3,4-cyclobutane tetracarboxylic dianhydride.

[0015] The diamine monomer is any one or a combination of the following: m-xylylenediamine, p-xylylenediamine, 2,4-toluenediamine, 2,6-toluenediamine, 4,4'-diaminodiphenyl methane, p-phenylenediamine, m-phenylenediamine, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4'-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)biphenyl, 4,4'-bis(3-aminophenoxy)biphenyl, 2,2'-bis[4-(4-aminophenoxyphenyl)]propane, 4,4'-diaminodiphenyl sulfone, 4,4'-bis(4-aminophenoxy)diphenyl sulfone, 4,4'-diaminobenzanilide, 9,9-bis(4-aminophenyl)fluorene, 1,6-diaminohexane, 1,7-diaminoheptane, 1,8-diaminooctane, 1,10-diaminodecane, 1,12-diaminododecane, 1,4-cyclohexanediamine, 4,4'-diaminodicyclohexylmethane, isophorone diamine.

[0016] The diisocyanate is any one or a combination of the following: hexamethylene diisocyanate, p-phenylene diisocyanate, 4,4'-methylenebis(isocyanate), toluene diisocyanate, isophorone diisocyanate, 3,3'-dimethylbiphenyl-4,4'-diisocyanate, 4,4-diisocyanate dicyclohexylmethane, 1,3-bis(isocyanatomethyl)cyclohexane.

[0017] Further, the number average molecular weight of the polyamide-imide adhesive is 500,000 to 3,000,000.

[0018] The present application also discloses a preparation method of the polyamide-imide adhesive, which comprises the following steps:

[0019] S1, polymerization: the diamine monomer and the dianhydride monomer are subjected to a polymerization reaction in an organic solvent to obtain a polyamic acid intermediate A;

[0020] S2, crosslinking: adding diisocyanate to the reaction system after the polymerization reaction of step S1 is completed to perform chemical crosslinking, to obtain a polyamide-imide binder precursor.

[0021] Further, in step S1, the polymerization reaction temperature is 10-100 DEG C; the molar ratio of the diamine monomer to the dianhydride monomer is 1:(0.8-1.3);

[0022] The mass fraction of the solid content in the polymerization reaction process is 10%-35%, and the organic solvent is a polar aprotic solvent.

[0023] Further, in step S2, the molar ratio of the diisocyanate to the diamine monomer is 1:(0.05-0.2); and the chemical crosslinking reaction temperature is 40-120 DEG C.

[0024] The application further discloses a positive electrode tab, which comprises the polyamide-imide binder.

[0025] Further, the positive electrode tab comprises 90-99 parts of a positive electrode material, 0.5-5 parts of a conductive agent, and 0.5-5 parts of the polyamide-imide binder precursor, according to weight parts.

[0026] S1, slurry preparation: adding the positive electrode material, the conductive agent, the binder precursor and the solvent in proportion to obtain a positive electrode slurry;

[0027] S2, coating: uniformly coating the obtained slurry on a current collector aluminum foil by using a coating machine;

[0028] S3, drying: removing the solvent and CO2 by one-stage drying, and performing thermal imidization by two-stage drying, so that the polyamide-imide binder precursor becomes a polyamide-imide, and finally, a positive electrode tab is obtained by rolling;

[0029] The positive electrode material can 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.

[0030] The conductive agent can be one or a combination of carbon black, a conductive polymer, acetylene black, ketjen black, carbon fiber, carbon nanotube, and graphite.

[0031] The solvent is a high-boiling-point polar aprotic solvent, which can be, but is not limited to, any one or a combination of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0032] The drying conditions of step S3 are as follows: the one-stage drying temperature is 80-160 DEG C, and the drying time is 1-12 h; and the two-stage drying temperature is 160-300 DEG C, and the drying time is 1-12 h.

[0033] The application also discloses application of the positive electrode sheet to a lithium battery.

[0034] The application has the following advantages:

[0035] (1) Since the imide ring of the polyimide is rigid, there is a risk of cracking during coating, rolling and winding, which reduces the use safety and capacity retention rate of the battery. In the case of increasing the thickness of the coating layer, the cracking phenomenon is more obvious, which limits the energy density of the battery. The polyamide-imide binder has both amide groups and imide groups in the molecular structure. The polyamide-imide binder increases a part of amide groups on the basis of the polyimide, retains the high tensile strength and elastic modulus of the imide group, improves the flexibility and impact resistance of the electrode sheet, reduces the risk of cracking of the electrode sheet, improves the capacity retention rate and safety of the battery, prolongs the service life, and can also increase the energy density of the battery by thick coating.

[0036] (2) The first step reaction for preparing the polyamide acid is a conventional reaction without special equipment, and the preparation method is mature and simple. Fluorine-free raw materials are used, which avoids gelation caused by generation of hydrogen fluoride in the pulping process and also avoids the generation of toxic fluorine-containing gas in the electrode sheet cutting process. The second step reaction adds isocyanate crosslinking, which not only increases a part of amide groups, but also improves the viscosity, improves the adhesion between the coating layer and the current collector, reduces the risk of cracking and falling of the coating layer, and improves the use safety and life of the battery. By controlling the amount of isocyanate added, the viscosity can be adjusted, which is suitable for different manufacturers or models of lithium battery positive materials.

[0037] (3) After coating, CO2 is removed through a drying process, so that more pores are formed in the active layer, the porosity of the positive electrode sheet is improved, the migration rate of lithium ions is improved, the contact area between the positive material and the electrolyte is increased, the wettability of the electrode sheet is improved, and the capacity retention rate of the battery is improved. Through the heat imidization in the drying process, the polyamide acid is crosslinked between or within the molecules to form polyimide, which has a more stable structure. The binder not only improves the adhesion of the active material and the current collector, but also improves the thermal stability of the electrode sheet. In the process of continuous charging and discharging of the battery, the binder is not easy to degrade, the falling of the coating layer is reduced, and the capacity retention rate of the battery is further improved.

[0038] (4) The preparation process of the polyamide-imide binder has mild reaction conditions and low requirements for equipment, and is easy to industrialize. The reaction process is simple, no catalyst is used, the raw material cost is reduced, and the step of removing the catalyst after treatment is also saved, which avoids the influence of the residual catalyst on the performance of the battery. The product does not need to be precipitated and purified, which avoids solvent waste and does not produce waste gas, waste liquid and waste solid, thereby reducing the production cost. The polyamide-imide binder can be applied to the field of lithium battery materials and has a wide application prospect.

[0039] In summary, the polyamide-imide adhesive preparation method has low raw material cost, simple and reasonable preparation process, easy production, adjustable viscosity of the adhesive, strong applicability, and good economic benefits. DETAILED DESCRIPTION

[0040] The specific embodiments of the present application are described in detail below. The present application can be implemented in many different ways than described herein, and those skilled in the art can make similar improvements without departing from the spirit of the present application, and the present application is not limited by the disclosed specific embodiments.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terms used are only for describing specific embodiments and not for limiting the present application.

[0042] A polyamide-imide adhesive is prepared by polymerizing diamine monomers and dianhydride monomers to prepare a polyamide acid intermediate A, adding diisocyanate to crosslink to obtain a polyamide-imide adhesive precursor, mixing the polyamide-imide adhesive precursor with a conductive agent and a positive electrode material to prepare a slurry, coating the slurry to prepare an electrode sheet, and finally converting the polyamide-imide adhesive in the electrode sheet drying process. The specific principle is as follows:

[0043] The dianhydride monomer, diamine monomer and diisocyanate are respectively selected from aryl, aliphatic or alicyclic groups, The structure is a repeated unit structure.

[0044] Specifically, the dianhydride monomer is one or a combination of the following: pyromellitic dianhydride, diphenyl ether tetracarboxylic dianhydride, bisphenol A type diether dianhydride, 3,3',4,4'-biphenyl tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 2,3,3',4'-biphenyl tetracarboxylic dianhydride, and 1,2,3,4-cyclobutane tetracarboxylic dianhydride.

[0045] Specifically, the diamine monomer is any one or combination of m-xylylenediamine, p-xylylenediamine, 2,4-toluenediamine, 2,6-toluenediamine, 4,4'-diaminodiphenyl methane, p-phenylenediamine, m-phenylenediamine, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4'-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)biphenyl, 4,4'-bis(3-aminophenoxy)biphenyl, 2,2'-bis[4-(4-aminophenoxyphenyl)]propane, 4,4'-diaminodiphenyl sulfone, 4,4'-bis(4-aminophenoxy)diphenyl sulfone, 4,4'-diaminobenzanilide, 9,9-bis(4-aminophenyl)fluorene, 1,6-diaminohexane, 1,7-diaminoheptane, 1,8-diaminooctane, 1,10-diaminodecane, 1,12-diaminododecane, 1,4-cyclohexanediamine, 4,4'-diaminodicyclohexylmethane, isophorone diamine.

[0046] Specifically, the diisocyanate is any one or combination of hexamethylene diisocyanate, p-phenylene diisocyanate, 4,4'-methylenebis(isocyanate), toluene diisocyanate, isophorone diisocyanate, 3,3'-dimethylbiphenyl-4,4'-diisocyanate, 4,4-diisocyanate dicyclohexylmethane, 1,3-bis(isocyanatomethyl)cyclohexane.

[0047] Specifically, the polyamide-imide adhesive has a number average molecular weight of 500,000 to 3,000,000.

[0048] A preparation method of a polyamide-imide adhesive, the preparation method comprising:

[0049] S1, polymerization: the diamine monomer and the dianhydride monomer are subjected to a polymerization reaction in an organic solvent to obtain a polyamic acid intermediate A;

[0050] S2, crosslinking: a diisocyanate is added to the reaction system after the polymerization reaction of step S1 to perform chemical crosslinking, thereby obtaining a polyamide-imide adhesive precursor.

[0051] Specifically, in step S1, the polymerization reaction temperature is 10-100°C, and the reaction time is 1-24h; 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.

[0052] The mass fraction of the solid content in the polymerization process is 10% to 35%, the organic solvent is a high-boiling polar aprotic solvent, and the high-boiling polar aprotic solvent is any one or a combination of N-methyl pyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0053] Specifically, in step S2, the molar ratio of the diisocyanate to the diamine monomer is 1:(0.05-0.2), and the chemical crosslinking reaction temperature is 40-120°C.

[0054] A positive electrode sheet, wherein the positive electrode sheet comprises the polyamide-imide binder.

[0055] Specifically, the positive electrode sheet comprises 90-99 parts of a positive electrode material, 0.5-5 parts of a conductive agent, and 0.5-5 parts of a polyamide-imide binder precursor by weight.

[0056] The positive electrode material can 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.

[0057] The conductive agent can be one or a combination of carbon black, a conductive polymer, acetylene black, ketjen black, carbon fiber, carbon nanotube, and graphite.

[0058] More specifically, the preparation process of the positive electrode sheet comprises the following steps: adding the positive electrode material, the conductive agent, and the binder precursor into a solvent according to the formula proportion, adjusting the mass fraction of the solid content of the slurry to 40-60%, uniformly mixing under high-speed stirring, sieving, coating on an aluminum foil, and then performing two-stage drying and roll pressing to obtain the positive electrode sheet.

[0059] The solvent is a high-boiling polar aprotic solvent, which can be any one or a combination of N-methyl pyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0060] The two-stage drying comprises the following steps: one-stage drying at a temperature of 80-160°C for 1-12 hours, and two-stage drying at a temperature of 160-300°C for 1-12 hours.

[0061] The positive electrode sheet is applied to a lithium battery.

[0062] Example 1

[0063] Preparation of the polyamide-imide binder:

[0064] Into a 1000 mL three-necked flask, 4.334 g of 1,6-diaminohexane, 6.973 g of m-phenylenediamine and 180.101 g of N-methylpyrrolidone were sequentially added, and after dissolution, 52.258 g of bisphenol A type diether dianhydride and 180.101 g of N-methylpyrrolidone were added, and the reaction was carried out at 10-15°C for 24 h. 1.251 g of 4,4'-methylenebis(isocyanate) was added, and the reaction was continued at 50-55°C for 20 h. After the reaction was completed, a polyamide-imide binder precursor was obtained.

[0065] Preparation of positive electrode sheet:

[0066] Into a closed container, 92 parts of lithium manganese oxide, 4 parts of carbon black and 4 parts of the binder precursor were added, and N-methylpyrrolidone solvent was further added to adjust the solid content of the slurry to 52%, and then the mixture was uniformly mixed by high-speed stirring, and then sieved and coated on an aluminum foil. Vacuum drying was carried out at 100°C for 12 h, and then vacuum drying was carried out at 200°C for 5 h, and then a positive electrode sheet was prepared by rolling.

[0067] Example 2

[0068] Preparation of polyamide-imide binder:

[0069] Into a 500 mL three-necked flask, 11.919 g of 4,4'-diaminodiphenylmethane, 8.673 g of 1,8-diaminooctane and 115.934 g of N,N-dimethylformamide were sequentially added, and after dissolution, 37.375 g of diphthalic acid dianhydride and 115.934 g of N,N-dimethylformamide were added, and the reaction was carried out at 25-30°C for 16 h. 2.018 g of hexamethylene diisocyanate was added, and the reaction was continued at 70-75°C for 10 h. After the reaction was completed, a polyamide-imide binder precursor was obtained.

[0070] Preparation of positive electrode sheet:

[0071] Into a closed container, 95 parts of lithium cobalt oxide, 2.5 parts of acetylene black and 2.5 parts of the binder precursor were added, and N,N-dimethylformamide solvent was further added to adjust the solid content of the slurry to 45%, and then the mixture was uniformly mixed by high-speed stirring, and then sieved and coated on an aluminum foil. Vacuum drying was carried out at 140°C for 6 h, and then vacuum drying was carried out at 180°C for 8 h, and then a positive electrode sheet was prepared by rolling.

[0072] Example 3

[0073] Preparation of polyamide-imide binder:

[0074] Into a 1000 mL three-necked flask, 5.423 g of p-phenylenediamine, 7.227 g of 1,8-diaminooctane and 183.908 g of N-methylpyrrolidone were sequentially added, after dissolution, 52.258 g of bisphenol A type diether dianhydride and 183.908 g of N-methylpyrrolidone were added, and the reaction was carried out at 50-55°C for 8 h. 3.203 g of p-phenylene diisocyanate was added, and the reaction was continued at 110-115°C for 2 h. After the reaction was completed, a polyamide-imide binder precursor was obtained. Positive electrode sheet preparation:

[0075] 98.4 parts of lithium nickel oxide, 0.8 parts of Ketjen black and 0.8 parts of the binder precursor were added to a closed container, N-methylpyrrolidone solvent was added, the solid content of the slurry was adjusted to 58%, and high-speed stirring was carried out to mix uniformly, and then sieving was carried out, and coating was carried out on an aluminum foil. Vacuum drying at 120°C for 8 h, and then vacuum drying at 160°C for 10 h, and then roll pressing to obtain a positive electrode sheet.

[0076] Example 4

[0077] Polyamide-imide binder preparation:

[0078] Into a 500 mL three-necked flask, 25.898 g of 1,10-diaminodecane and 102.699 g of N,N-dimethylacetamide were added, after dissolution, 9.865 g of pyromellitic dianhydride, 32.703 g of diphenyl ether tetra-carboxylic dianhydride and 102.699 g of N,N-dimethylacetamide were added, and the reaction was carried out at 60-65°C for 6 h. 5.001 g of isophorone diisocyanate was added, and the reaction was continued at 60-65°C for 12 h. After the reaction was completed, a polyamide-imide binder precursor was obtained.

[0079] Positive electrode sheet preparation:

[0080] 97.6 parts of lithium iron phosphate, 1.2 parts of carbon nanotubes and 1.2 parts of the binder precursor were added to a closed container, N,N-dimethylacetamide solvent was added, the solid content of the slurry was adjusted to 57%, high-speed stirring was carried out to mix uniformly, sieving was carried out, and coating was carried out on an aluminum foil. Vacuum drying at 160°C for 4 h, and then vacuum drying at 220°C for 6 h, and then roll pressing to obtain a positive electrode sheet.

[0081] Example 5

[0082] Polyamide-imide binder preparation:

[0083] Into a 500 mL three-necked flask were added 9.933 g of 4,4'-diaminodiphenyl methane, 5.811 g of 1,6-diaminohexane, and 90.567 g of N,N-dimethylformamide in this order, and after dissolution, 29.540 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride and 90.567 g of N,N-dimethylformamide were added, and the reaction was carried out at 70-75°C for 6 hours. 1.306 g of toluene diisocyanate was added, and the reaction was continued at 80-85°C for 6 hours. After the reaction was completed, a polyamide-imide binder precursor was obtained.

[0084] Preparation of cathode electrode sheet

[0085] Into a closed container were added 93 parts of lithium cobalt oxide, 3.5 parts of carbon black, and 3.5 parts of the binder precursor, and N,N-dimethylformamide solvent was further added to adjust the solid content of the slurry to 55%, and the mixture was uniformly mixed by high-speed stirring, and then sieved and coated on an aluminum foil. Vacuum drying was carried out at 140°C for 7 hours, and then vacuum drying was carried out at 240°C for 3 hours, and a cathode electrode sheet was prepared by rolling.

[0086] Example 6

[0087] Preparation of polyamide-imide binder

[0088] Into a 500 mL three-necked flask were added 9.933 g of 4,4'-diaminodiphenyl methane, 5.811 g of 1,6-diaminohexane, and 90.567 g of N,N-dimethylformamide in this order, and after dissolution, 29.540 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride and 90.567 g of N,N-dimethylformamide were added, and the reaction was carried out at 70-75°C for 6 hours. 1.306 g of toluene diisocyanate was added, and the reaction was continued at 80-85°C for 6 hours. After the reaction was completed, a polyamide-imide binder precursor was obtained.

[0089] Preparation of cathode electrode sheet

[0090] Into a closed container were added 96 parts of lithium manganese oxide, 2 parts of graphite, and 2 parts of the binder precursor, and N,N-dimethylacetamide solvent was further added to adjust the solid content of the slurry to 55%, and the mixture was uniformly mixed by high-speed stirring, and then sieved and coated on an aluminum foil. Vacuum drying was carried out at 120°C for 8 hours, and then vacuum drying was carried out at 260°C for 2 hours, and a cathode electrode sheet was prepared by rolling.

[0091] Example 7

[0092] Preparation of polyamide-imide binder

[0093] Into a 500 mL three-necked flask, 5.209 g of 1, 12-diaminododecane, 20.660 g of 4, 4'-diaminodiphenyl methane and 101.955 g of N, N-dimethylformamide were added, after dissolution, 42.100 g of 3, 3', 4, 4'-benzophenonetetracarboxylic dianhydride and 101.955 g of N, N-dimethylformamide were added, and the reaction was carried out at 90-95°C for 4 h. 1.749 g of hexamethylene diisocyanate was added, and the reaction was continued at 90-95°C for 4 h. After the reaction was completed, a polyamide-imide binder precursor was obtained.

[0094] Preparation of positive electrode sheet:

[0095] Into a closed container, 94 parts of lithium nickel oxide, 3 parts of acetylene black and 3 parts of the binder precursor were added, and N, N-dimethylformamide solvent was added to adjust the solid content of the slurry to 50%, and then high-speed stirring was carried out to mix uniformly, and after sieving, it was coated on an aluminum foil. Vacuum drying at 150°C for 6 h, and then vacuum drying at 280°C for 1 h, and then roll pressing to obtain a positive electrode sheet.

[0096] Example 8

[0097] Preparation of polyamide-imide binder:

[0098] Into a 500 mL three-necked flask, 9.982 g of 4, 4'-diaminodiphenyl sulfone, 12.022 g of 1, 12-diaminododecane and 108.777 g of N-methyl pyrrolidone were added, after dissolution, 16.192 g of 3, 3', 4, 4'-benzophenonetetracarboxylic dianhydride and 108.777 g of N-methyl pyrrolidone were added, and the reaction was carried out at 100-105°C for 2 h. p-Phenylene diisocyanate was added, and the reaction was continued at 115-120°C for 2 h. After the reaction was completed, a polyamide-imide binder precursor was obtained.

[0099] Preparation of positive electrode sheet:

[0100] Into a closed container, 91 parts of lithium iron phosphate, 4 parts of Ketjen black and 5 parts of the binder precursor were added, and N-methyl pyrrolidone solvent was added to adjust the solid content of the slurry to 47%, and then high-speed stirring was carried out to mix uniformly, and after sieving, it was coated on an aluminum foil. Vacuum drying at 130°C for 10 h, and then vacuum drying at 300°C for 1 h, and then roll pressing to obtain a positive electrode sheet.

[0101] Comparative Example 1

[0102] The same method as in Example 1 was used to prepare a polyamide acid binder, except that no isocyanate was added, no crosslinking occurred, and only a polyamide acid intermediate A was obtained, and the specific preparation process was as follows:

[0103] Into a 1000 mL three-necked flask, 4.334 g of 1,6-diaminohexane, 6.973 g of m-phenylenediamine and 180.101 g of N-methylpyrrolidone were sequentially added, after dissolution, 52.258 g of bisphenol A diether dianhydride and 180.101 g of N-methylpyrrolidone were added, and the reaction was carried out at 10-15 °C for 24 h to obtain polyamic acid intermediate A.

[0104] Preparation of positive electrode sheet:

[0105] The positive electrode sheet was prepared by the same method as in Example 1, except that the binder precursor was replaced with the polyamic acid intermediate A of the present comparative example.

[0106] Comparative Example 2

[0107] The polyimide binder was prepared by the same method as in Example 1, except that no isocyanate was added, no crosslinking occurred, and after the polyamic acid intermediate A was obtained, a cyclization agent and a cyclization aid (pyridine and acetic anhydride) were added, and finally the solid was separated and dried, and the specific preparation process was as follows:

[0108] Into a 1000 mL three-necked flask, 4.334 g of 1,6-diaminohexane, 6.973 g of m-phenylenediamine and 180.101 g of N-methylpyrrolidone were sequentially added, after dissolution, 52.258 g of bisphenol A diether dianhydride and 180.101 g of N-methylpyrrolidone were added, and the reaction was carried out at 10-15 °C for 24 h to obtain polyamic acid intermediate A. Then 19.775 g of pyridine and 25.523 g of acetic anhydride were added, and the reaction was continued at 10-15 °C for 8 h. After the reaction was completed, the solid was separated by adding ethanol, and was filtered, washed, and dried at 90 °C to obtain a polyimide binder.

[0109] Preparation of positive electrode sheet:

[0110] The positive electrode sheet was prepared by the same method as in Example 1, except that the binder precursor was replaced with the polyimide binder of the present comparative example.

[0111] Comparative Example 3

[0112] The polyamide-imide binder was prepared by the same method as in Example 1, except that the solid content was reduced during the preparation of the polyamide-imide binder, and the specific preparation process was as follows:

[0113] Into a 1000 mL three-necked flask, 4.334 g of 1,6-diaminohexane, 6.973 g of m-phenylenediamine and 180.101 g of N-methylpyrrolidone were sequentially added, and after dissolution, 52.258 g of bisphenol A type diether dianhydride and 180.101 g of N-methylpyrrolidone were added, and the reaction was carried out at 10-15°C for 24 h to obtain polyamic acid intermediate A. 1.251 g of 4,4'-methylenebis(isocyanate) and 211.883 g of N-methylpyrrolidone were added, and the reaction was continued at 50-55°C for 20 h. After the reaction was completed, a polyamide-imide binder precursor was obtained.

[0114] Preparation of positive electrode sheet

[0115] The positive electrode sheet was prepared in the same manner as in Example 1.

[0116] Comparative Example 4

[0117] The polyamide-imide binder was prepared in the same manner as in Example 2, except that the crosslinking reaction temperature was lowered during the preparation of the polyamide-imide binder, and the preparation process was as follows:

[0118] Into a 500 mL three-necked flask, 11.919 g of 4,4'-diaminodiphenylmethane, 8.673 g of 1,8-diaminooctane and 115.934 g of N,N-dimethylformamide were sequentially added, and after dissolution, 37.375 g of diphenyl ether tetracarboxylic dianhydride and 115.934 g of N,N-dimethylformamide were added, and the reaction was carried out at 25-30°C for 16 h. 2.018 g of hexamethylene diisocyanate was added, and the reaction was continued at 25-30°C for 20 h. After the reaction was completed, a polyamide-imide binder precursor was obtained.

[0119] Preparation of positive electrode sheet

[0120] The positive electrode sheet was prepared in the same manner as in Example 2.

[0121] Comparative Example 5

[0122] The polyamide-imide binder was prepared in the same manner as in Example 2, except that the crosslinking reaction temperature was increased during the preparation of the polyamide-imide binder, and the preparation process was as follows:

[0123] Into a 500 mL three-necked flask, 11.919 g of 4,4'-diaminodiphenyl methane, 8.673 g of 1,8-diaminooctane and 115.934 g of N,N-dimethylformamide were sequentially added, after dissolution, 37.375 g of diphthalic anhydride and 115.934 g of N,N-dimethylformamide were added, and the reaction was carried out at 25-30°C for 16 h. 2.018 g of hexamethylene diisocyanate was added, and the reaction was carried out at 140-150°C for 1 h, after which the gel aggregated into a jelly-like state, and the state did not change after cooling. N,N-dimethylformamide was added to a solid content of 5%, and it was still insoluble.

[0124] Comparative Example 6

[0125] The polyamide-imide binder was prepared in the same manner as in Example 2, except that the ratio of isocyanate in the crosslinking reaction was increased during the preparation of the polyamide-imide binder. The specific preparation process was as follows:

[0126] Into a 500 mL three-necked flask, 11.919 g of 4,4'-diaminodiphenyl methane, 8.673 g of 1,8-diaminooctane and 115.934 g of N,N-dimethylformamide were sequentially added, after dissolution, 37.375 g of diphthalic anhydride and 115.934 g of N,N-dimethylformamide were added, and the reaction was carried out at 25-30°C for 16 h. 6.055 g of hexamethylene diisocyanate was added, and the reaction was carried out at 70-75°C for 3 h, after which the gel aggregated into a jelly-like state, and the state did not change after cooling. N,N-dimethylformamide was added to a solid content of 5%, and it was still insoluble.

[0127] Comparative Example 7

[0128] The polyamide-imide binder was prepared in the same manner as in Example 3, except that the pole piece was not subjected to secondary drying. The specific pole piece preparation process was as follows:

[0129] Preparation of the positive pole piece:

[0130] 98.4 parts of lithium nickel oxide, 0.8 parts of Ketjen black and 0.8 parts of the binder precursor were added to a sealed container, and N-methyl pyrrolidone solvent was added to adjust the solid content of the slurry to 58%. After mixing uniformly at high speed, sieving was carried out, and the slurry was coated on an aluminum foil. Vacuum drying was carried out at 120°C for 8 h, and the positive pole piece was prepared by rolling.

[0131] Comparative Example 8

[0132] The polyamide-imide binder was prepared in the same manner as in Example 4, except that the pole piece was directly subjected to high-temperature drying. The specific pole piece preparation process was as follows:

[0133] Preparation of the positive pole piece:

[0134] Example 1: Preparation of positive electrode sheet

[0135] Comparative Example 9

[0136] Example 1: Preparation of positive electrode sheet

[0137] Performance test:

[0138] I. Performance determination method of polyamide-imide binder precursor obtained in the above examples and comparative examples:

[0139] (1) Number average molecular weight Mw was tested using gel permeation chromatography (GPC);

[0140] (2) Viscosity of 5% solid content binder was tested using a cone and plate viscometer.

[0141] II. Performance test method of positive electrode sheet:

[0142] (1) Thickness test: A Japanese Mitutoyo digital thickness gauge was used to test the total thickness of the positive electrode sheet, and the thickness of the uncoated current collector was subtracted, which was recorded as the coating layer thickness.

[0143] (2) Porosity test: More than 20 good-looking round pieces without powder falling off the edges were selected with tweezers and loaded into the sample cup. The number of pieces was recorded, the apparent volume was calculated, and then the sample cup was placed in the true density tester in a closed test system. Helium was introduced according to the program, and the gas pressure in the sample chamber and the expansion chamber was detected, and then the true volume was calculated according to the Boyle's law (PV=nRT), so as to obtain the porosity of the sample. The porosity result of the electrode sheet was not deducted from the substrate.

[0144] (3) Liquid absorption rate test: A capillary tube was used to absorb a certain height of electrolyte, and the capillary tube was in contact with the dried electrode sheet. The time from the beginning of the electrolyte liquid level drop to the complete absorption of the electrolyte by the electrode sheet was measured. Liquid absorption rate = (capillary tube bottom area x electrolyte height x electrolyte density) ÷ time.

[0145] (4) Flexibility test: The electrode sheet was bent 180° towards the current collector side, and the crease state of the electrode sheet was observed by optical microscope. Three times were repeated: perfect was marked as +, with cracks but no falling off was marked as o, and falling off or cracking was marked as x.

[0146] (5) Peeling strength test: The obtained electrode sheet was cut into a 10 cm x 2 cm sample, fixed to a 1 mm thick steel plate on the current collector side with double-sided tape, and a transparent tape was attached to the coating layer. A universal material testing machine was used to perform a 180° direction tensile peeling test at a peeling speed of 10 cm / min.

[0147] III. Battery performance test:

[0148] The button cell was assembled in the glove box, and then the initial efficiency at 0.1C current density and the capacity retention rate after 500 cycles at 1C current density were tested. 0.1C initial efficiency / % = 0.1C first cycle discharge specific capacity (mAh / g) ÷ 0.1C first cycle charge specific capacity (mAh / g) x 100%. 1C capacity retention rate / % = 1C 500th cycle discharge specific capacity (mAh / g) ÷ 1C first cycle discharge specific capacity (mAh / g) x 100%.

[0149] Table 1 Performance test results

[0150] Comparative Example 1 did not add isocyanate crosslinking, only prepared polyamide acid, compared with Example 1, it can be seen that the viscosity of the adhesive of Comparative Example 1 is low, resulting in poor adhesion of the coating layer to the current collector and low peeling strength; no amide group, poor flexibility of the electrode sheet, the coating layer is easy to crack and fall off, although the initial efficiency of the battery is not affected, but the capacity retention rate is greatly reduced. It shows that adding isocyanate crosslinking can further improve the adhesion, and introducing amide groups can improve the flexibility of the electrode sheet, the coating layer does not crack and fall off, ensuring the structural integrity and chemical stability of the battery during the cycling process.

[0151] Comparative Example 2 did not add isocyanate crosslinking, but added cyclization agent and dehydrating agent, and prepared polyimide after precipitation and purification. The molecular weight and viscosity of the adhesive are slightly improved compared with Comparative Example 1, but far lower than Example 1. The flexibility of the electrode sheet is improved compared with Comparative Example 1, and the coating layer does not fall off, but obvious breaking marks can still be observed by optical microscope after 180° bending three times, and the flexibility is still different from Example 1. Moreover, the capacity retention rate of the battery is 16.73% lower than Example 1. It shows that in the case of thick coating, using polyamide acid or polyimide as the adhesive, the electrode sheet cracking phenomenon exists, which has safety risk and limits the energy density of the battery.

[0152] Comparative Example 3, during the preparation of the adhesive, the solid content of the crosslinking reaction is reduced, and the molecular weight and viscosity of the adhesive, the flexibility and peeling strength of the electrode sheet, and the battery performance are all greatly different from Example 1, and similar to Comparative Example 1. It shows that when the solid content is low, the crosslinking reaction does not easily occur, and cannot improve the adhesion, the flexibility of the electrode sheet and the battery performance.

[0153] The cross-linking reaction temperature in the preparation of the binder of Comparative Example 4 is low, and the molecular weight and viscosity of the binder, the flexibility and peel strength of the pole piece, and the battery performance are all greatly different from those of Example 2, indicating that the cross-linking reaction does not easily occur when the cross-linking temperature is low, and even if the cross-linking reaction time is doubled, the adhesion cannot be improved, and the flexibility and battery performance of the pole piece also cannot be improved.

[0154] The cross-linking reaction temperature in the preparation of the binder of Comparative Example 5 is high, and the cross-linking reaction isocyanate ratio in the preparation of the binder of Comparative Example 6 is high, and the cross-linking degree of the binder in the reaction process is too high, and the gel aggregates into jelly, and the state does not change after cooling, and the solvent cannot be dissolved. It is indicated that the temperature, the ratio, and the solid content in the cross-linking reaction process have important influences on the cross-linking degree, and the cross-linking reaction does not occur when the temperature or the solid content is too low, and the cross-linking degree is too high when the temperature or the ratio is too high, and the gel aggregates easily.

[0155] The pole piece of Comparative Example 7 is not subjected to secondary drying, and compared with Example 3, the flexibility and the initial efficiency of the battery of the pole piece do not change greatly, but the peel strength is reduced by 69.5 N / m, and the capacity retention rate is reduced by 14.32%, indicating that the thermal imidization of the second drying is performed, the polyamide acid is cross-linked between molecules or within molecules, and the polyimide is formed, the structure is more stable, the adhesion between the coating layer and the current collector can be improved, and the thermal stability of the pole piece is improved, the binder is not easily degraded in the continuous charging and discharging process of the battery, the peeling of the coating layer is reduced, and the capacity retention rate of the battery is further improved.

[0156] The pole piece of Comparative Example 8 is directly subjected to high-temperature drying, and the solvent, CO2, and thermal imidization are simultaneously removed, and compared with Example 4, the porosity, the liquid absorption rate, and the peel strength of the pole piece are all reduced, the initial efficiency of the battery is reduced by a small margin, and the capacity retention rate is reduced by 12.28%, indicating that the step-by-step drying is beneficial to the formation of more pores in the active layer, the migration rate of lithium ions can be improved, the contact area between the positive electrode material and the electrolyte can be increased, the wettability of the pole piece is improved, and the capacity retention rate of the battery is improved.

[0157] The binder of Comparative Example 9 is commercial PVDF, although the pole piece has good flexibility, the peel strength is 10% to 25% lower than that of the present application, and the capacity retention rate is 9% to 14% lower than that of the present application. It is indicated that the polyamide-imide binder prepared by the present application has good adhesion, the flexibility and stability of the pole piece are good, the cycle performance and safety of the battery are high, and the binder has good application prospects.

[0158] The technical features of the above-described examples can be combined arbitrarily, and in order to make the description simple, all possible combinations of the technical features in the above-described examples are not enumerated, however, as long as the combinations of the technical features do not exist contradictory, they should be considered as the scope of the present disclosure.

[0159] For those skilled in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application, the protection scope of the present application is subject to the appended claims.

Claims

1. A polyamide-imide adhesive, characterized in that, The polyamide-imide binder is prepared by polymerizing diamine monomers and dianhydride monomers to obtain polyamic acid intermediate A, and then adding diisocyanate for crosslinking to obtain polyamide-imide binder precursor. The polyamide-imide binder precursor is mixed with conductive agent and positive electrode material to form a slurry and coated to form an electrode sheet. Finally, during the electrode sheet drying process, it is transformed into the polyamide-imide binder. The dianhydride monomer is The diamine monomer is The diisocyanate is R1 to R3 are each independently selected from aryl, aliphatic, or alicyclic groups; The structure of the polyamide-imide adhesive is as follows: It is a repeating unit structure.

2. The polyamide-imide adhesive according to claim 1, characterized in that, The dianhydride monomers are one or more of the following: pyromellitic dianhydride, diphenyl ether dianhydride, bisphenol A type diether dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 2,3,3',4'-biphenyltetracarboxylic dianhydride, and 1,2,3,4-cyclobutanetetracarboxylic dianhydride. The diamine monomers are m-phenylenediamine, p-phenylenediamine, 2,4-toluenediamine, 2,6-toluenediamine, 4,4'-diaminodiphenylmethane, p-phenylenediamine, m-phenylenediamine, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4'-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)biphenyl, 4,4'-bis(3-aminophenoxy)biphenyl, 2,2'-bis[4-(4- [Aminophenoxyphenyl]propane, 4,4'-diaminodiphenyl sulfone, 4,4'-bis(4-aminophenoxy)diphenyl sulfone, 4,4'-diaminobenzoylaniline, 9,9-bis(4-aminophenyl)fluorene, 1,6-diaminohexane, 1,7-diaminoheptane, 1,8-diaminooctane, 1,10-diaminodecane, 1,12-diaminododecane, 1,4-cyclohexanediamine, 4,4'-diaminodicyclohexylmethane, and isophoronediamine, or any combination thereof; The diisocyanate is any one or a combination of several of the following: hexamethylene diisocyanate, terephthalic diisocyanate, 4,4'-methylene bis(isocyanate), toluene diisocyanate, isophorone diisocyanate, 3,3'-dimethylbiphenyl-4,4'-diisocyanate, 4,4-diisocyanate dicyclohexylmethane, and 1,3-bis(methyl isocyanate)cyclohexane.

3. A polyamide-imide adhesive according to any one of claims 1-2, characterized in that, The number average molecular weight of the polyamide-imide binder is 50,000 to 300,000.

4. A method for preparing a polyamide-imide adhesive according to any one of claims 1-3, characterized in that, The preparation method is as follows: S1. Polymerization: Diamine monomers and dianhydride monomers undergo a polymerization reaction in an organic solvent to obtain polyamic acid intermediate A; S2, Crosslinking: Diisocyanate is added to the reaction system after the polymerization reaction in step S1 to carry out chemical crosslinking, and polyamide-imide binder precursor is obtained.

5. The method for preparing a polyamide-imide adhesive according to claim 4, characterized in that, In step S1, the polymerization reaction temperature is 10–100°C; the molar ratio of the diamine monomer and the dianhydride monomer is 1:(0.8–1.3); the mass fraction of the solid content in the polymerization reaction process is 10%–35%; and the organic solvent is a polar aprotic solvent.

6. The method for preparing a polyamide-imide adhesive according to claim 4, characterized in that, In step S2, the molar ratio of the diisocyanate to the diamine monomer is 1:(0.05-0.2); the chemical crosslinking reaction temperature is 40-120℃.

7. A positive electrode sheet, characterized in that, The positive electrode sheet includes the polyamide-imide binder as described in any one of claims 1-3.

8. The positive electrode sheet according to claim 7, characterized in that, The positive electrode sheet comprises, by weight, 90-99 parts of positive electrode material, 0.5-5 parts of conductive agent, and 0.5-5 parts of polyamide-imide binder precursor. The preparation method of the positive electrode sheet is as follows: S1. Slurry preparation: Add positive electrode material, conductive agent, polyamide-imide binder precursor and solvent in proportion to obtain positive electrode slurry; S2. Coating: Use a coating machine to evenly coat the obtained slurry onto the current collector aluminum foil; S3. Drying: The first stage of drying removes the solvent and CO2, the second stage of drying performs thermal imidization, and the polyamide-imide binder precursor is transformed into polyamide-imide. Finally, the positive electrode sheet is obtained by rolling. The cathode material is 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. The conductive agent is one or a combination of several of the following: carbon black, conductive polymer, acetylene black, Ketjen black, carbon fiber, carbon nanotubes, and graphite.

9. A positive electrode sheet according to claim 8, characterized in that, The drying conditions in step S3 of the preparation method are as follows: the first stage drying temperature is 80-160℃ and the drying time is 1-12h; the second stage drying temperature is 160-300℃ and the drying time is 1-12h.

10. An application of the positive electrode sheet according to any one of claims 7-9, characterized in that, The positive electrode sheet is used in lithium batteries.

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