Lithium ion battery negative pole piece and preparation method thereof

Abstract

The invention provides a water-soluble polyimide binder as well as a preparation method and application thereof. The water-soluble polyimide binder is prepared from the following raw materials: carboxyl-containing diamine, carboxyl-free diamine, polyol, dianhydride and a catalyst. By selecting the diamine containing carboxyl and the diamine not containing carboxyl for matching and adding the polyhydric alcohols, the finally obtained polyimide has excellent solubility in water, also has excellent mechanical properties, has excellent adhesive properties to various metals and silicon-carbon materials, and can be applied to the fields of chemical engineering, chemical engineering, chemical engineering, chemical engineering, chemical engineering, chemical engineering, chemical engineering and the like. A battery prepared from a negative pole piece prepared from the water-soluble polyimide binder can show excellent charge-discharge performance and cycling stability.

Description

[0001] This application is a divisional application. The application number of the parent application is 202310451654.5. The application date is April 20, 2023. The name of the invention is "A water-soluble polyimide adhesive, its preparation method and application". Technical Field

[0002] The present invention belongs to the technical field of lithium ion batteries, and in particular relates to a negative electrode plate of a lithium ion battery and a preparation method thereof. Background Art

[0003] In order to improve the energy density of lithium-ion batteries to meet the growing demand for portable electronic devices, electric vehicles and large-scale energy storage systems, new materials for lithium battery electrodes have received widespread attention. Among these materials, silicon-carbon materials are considered to be the most promising negative electrode materials for next-generation lithium-ion batteries because their theoretical specific capacity is more than 10 times that of currently used graphite electrodes and they are naturally abundant. However, the cycle life of silicon-carbon negative electrode materials is limited by the huge volume changes during the charge and discharge cycles, resulting in repeated formation of solid electrolyte interfaces, which in turn leads to electrode pulverization and continuous capacity decay.

[0004] The use of traditional graphite negative electrode binders can achieve higher specific capacity than traditional graphite electrodes and improve the cycle performance of silicon negative electrodes, but there is still an urgent need to develop a simple and effective method to extend the cycle life of silicon-carbon negative electrodes for further large-scale production; although the proportion of binders in silicon-carbon electrode composites is relatively small, they have a great influence on the cycle performance of silicon-carbon negative electrodes.

[0005] As a commonly used engineering material, polyimide has been applied to many fields due to its excellent comprehensive properties, including excellent mechanical properties, excellent chemical and thermal stability, and excellent solvent resistance. CN115160566A discloses a polyimide adhesive, slurry, film, preparation method thereof, and flexible display device. The raw materials for preparing the above-mentioned polyimide adhesive include diamine monomers and diamine monomers; the diamine monomers include a main monomer, the main monomer includes at least one of 9,9-bis(4-aminophenyl)fluorene and 4,4'bis(aminophenoxy)diphenylene, when the main monomer includes 9,9-bis(4-aminophenyl)fluorene, the diamine monomer also includes an auxiliary monomer, the reaction activity of the auxiliary monomer is stronger than that of the main monomer, and the molar number of the auxiliary monomer accounts for 30% of the total molar number of the diamine monomer; the diamine monomer includes at least one of cyclobutanetetracarboxylic dianhydride, cyclohexanetetracarboxylic dianhydride, and bicyclo[2.2.2]-oct-7-ene-2,3,5,6-tetracarboxylic dianhydride. The polyimide can be used to prepare a transparent polyimide film with a small phase difference in the thickness direction.

[0006] However, most traditional polyimides are insoluble or only soluble in highly polar organic solvents that are harmful to the environment, and have poor adhesion. Although the addition of amine alcohol to traditional polyamic acid can greatly improve its water solubility, the high processing temperature during imidization also limits its practical application in silicon-carbon electrodes.

[0007] Based on the above problems, developing a water-soluble polyimide adhesive with excellent water solubility, adhesion, tensile strength and ductility is a technical problem that needs to be solved urgently in this field. Summary of the invention

[0008] In view of the deficiencies in the prior art, the object of the present invention is to provide a water-soluble polyimide binder and a preparation method and application thereof. By selecting the carboxyl-containing diamine and the carboxyl-free diamine for combination and adding polyols, the molecular structure of the polyimide is successfully designed and modified, so that the obtained polyimide binder has excellent water solubility, good tensile strength and ductility, and at the same time has excellent bonding properties to metals (copper, iron, aluminum, etc.) and silicon-carbon materials, thereby helping to improve the electrochemical properties of batteries made using the water-soluble polyimide binder.

[0009] To achieve this object, the present invention adopts the following technical solutions:

[0010] In a first aspect, the present invention provides a water-soluble polyimide binder, wherein raw materials for preparing the water-soluble polyimide binder include a carboxyl-containing diamine, a carboxyl-free diamine, a polyol, a dianhydride and a catalyst.

[0011] The raw materials for preparing the water-soluble polyimide binder provided by the present invention include a combination of a carboxyl-containing diamine, a carboxyl-free diamine, a polyol, a dianhydride and a catalyst; wherein the polyol, the carboxyl-containing diamine and the residual acid after the dianhydride is esterified are all helpful to improve the water solubility of the polyimide, so that the finally obtained polyimide has excellent solubility in water; in addition, the polyol also provides sufficient flexibility, so that the obtained polyimide has excellent elongation, and the combination of the rigid dianhydride and diamine groups also ensures the thermal properties of the polyimide, and the skeleton of the polyimide also helps to make it have excellent mechanical properties; finally, the obtained water-soluble polyimide binder not only has excellent solubility in water, good tensile strength and ductility, but also has excellent bonding properties to metals (copper, iron, aluminum) and silicon-carbon materials, so that the battery further prepared by using the water-soluble polyimide as the negative electrode binder to prepare the negative electrode sheet can protect the silicon-carbon negative electrode during the charge and discharge process, so that the battery exhibits excellent cycle stability.

[0012] Preferably, the molar ratio of the non-carboxyl-containing diamine, the carboxyl-containing diamine and the polyol is 1:(1-6):(1-3), and more preferably 1:(1.5-4.5):(1.2-2).

[0013] Wherein, the molar ratio of the diamine without carboxyl group to the diamine containing carboxyl group is 1:2, 1:3, 1:4 or 1:5, etc.

[0014] The molar ratio of the diamine not containing a carboxyl group to the polyol is 1:1.2, 1:1.4, 1:1.6, 1:1.8, 1:2, 1:2.2, 1:2.4, 1:2.6 or 1:2.8, etc.

[0015] As a preferred technical solution, the present invention limits the molar ratio of the diamine containing no carboxyl group to the diamine containing carboxyl group to 1:(1-6). If the molar ratio of the diamine containing no carboxyl group to the diamine containing carboxyl group is too high, the water solubility of the obtained polyimide will be deteriorated, the peeling force will be reduced, and the cycle capacity retention rate of the battery prepared at the same amount of polyimide binder will be reduced; if the molar ratio of the diamine containing no carboxyl group to the diamine containing carboxyl group is too low, the first efficiency of the battery prepared at the same amount of polyimide binder will be reduced.

[0016] Preferably, the carboxyl-free diamine includes any one of p-phenylenediamine, m-phenylenediamine, 4,4'-diaminodiphenyl ether, 2,2'-bis(trifluoromethyl)diaminobiphenyl, 4,4'-diaminobenzanilide, 2,2'-bis[4-(4-aminophenoxyphenyl)]propane, 4,4'-diaminodiphenyl sulfone, 2-(4-aminophenyl)-5-aminobenzimidazole, 2,2-bis(4-hydroxy-3-aminophenyl)propane, 4,4'-bis(3-aminophenoxy)diphenyl sulfone, 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl, 4,4'-diamino-biphenyl or 9,9-bis(4-aminophenyl)fluorene, or a combination of at least two thereof.

[0017] Preferably, the carboxyl-containing diamine includes any one of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid, 6,6'-diamino-3,3'-methylenedibenzoic acid, 3,5-diaminobenzoic acid or benzidine disulfonic acid, or a combination of at least two thereof.

[0018] Preferably, the polyol includes a diol, more preferably any one of ethylene glycol, propylene glycol, butylene glycol or polyethylene glycol, or a combination of at least two thereof.

[0019] Preferably, the number average molecular weight of the polyethylene glycol is 100 to 50,000, such as 1,000, 5,000, 10,000, 20,000, 30,000 or 40,000, and more preferably 200 to 3,000.

[0020] Preferably, the molar ratio of the total moles of the carboxyl-containing diamine, the non-carboxyl-containing diamine and the polyol to the dianhydride is 1:(1-1.1), for example 1:1.01, 1:1.02, 1:1.03, 1:1.04, 1:1.05, 1:1.07 or 1:1.09, etc.

[0021] Preferably, the dianhydride comprises aromatic tetracarboxylic dianhydride.

[0022] Preferably, the aromatic tetracarboxylic dianhydride includes 2,2',3,3'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-oxydiphthalic anhydride, 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride, (4-phthalic anhydride)formyloxy-4-phthalate, bis[(3,4-dianhydride)phenyl]terephthalate, 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, p-phenylene-diphenyltrimethylol dianhydride, 4,4'-phenylenedioxydiphthalic anhydride, pyromellitic dianhydride, 2,2'-bis(3,4-dicarboxylic acid )hexafluoropropane dianhydride, 2,2-bis(4-(3,4-dicarboxyphenoxy)phenyl)hexafluoropropane dianhydride, 2,2-bis(4-(3,4-dicarboxybenzoyloxy)phenyl)hexafluoropropane dianhydride or 2,2'-bis(trifluoromethyl)-4,4'-bis(3,4-dicarboxyphenoxy)biphenyl dianhydride, or a combination of at least two thereof, and further preferably is any one or a combination of at least two thereof selected from pyromellitic acid dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 2,2'-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride or 4,4'-dioxyphthalic anhydride.

[0023] Preferably, the present invention has no special requirements on the type of catalyst selected. For example, any one of isoquinoline, N-methylmorpholine, triethylenediamine, isoquinoline or triethylamine or a combination of at least two thereof can be selected.

[0024] Preferably, the raw materials for preparing the water-soluble polyimide adhesive further include a solvent.

[0025] Preferably, the solvent is a non-protonated polar solvent.

[0026] Preferably, the non-protonated polar solvent includes any one of N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, γ-butyrolactone, propylene glycol monomethyl ether, cyclopentanone, cyclohexanone, ethyl acetate, toluene or methyl ethyl ketone, or a combination of at least two thereof, and more preferably N,N-dimethylformamide and / or N,N-dimethylacetamide.

[0027] In a second aspect, the present invention provides a method for preparing the water-soluble polyimide binder as described in the first aspect, the preparation method comprising the following steps:

[0028] (1) reacting a polyol and a portion of a dianhydride in a solvent to obtain an intermediate product;

[0029] (2) reacting the intermediate product obtained in step (1), the carboxyl-containing diamine, the carboxyl-free diamine, the remaining dianhydride and a catalyst to obtain the water-soluble polyimide binder.

[0030] Preferably, the reactions in step (1) and step (2) are both carried out under the protection of protective gas.

[0031] Preferably, the protective gas comprises nitrogen or argon.

[0032] Preferably, the reaction temperature in step (1) is 50-120°C, such as 60°C, 70°C, 80°C, 90°C, 100°C or 110°C, and more preferably 60-105°C.

[0033] Preferably, the reaction time of step (1) is 1 to 24 h, for example 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, 20 h or 22 h, and more preferably 3 to 10 h.

[0034] Preferably, after the reaction in step (2) is completed, the method further comprises the steps of: settling the obtained reaction solution in ethanol, washing and drying.

[0035] As a preferred technical solution of the present invention, the preparation method of the water-soluble polyimide binder comprises the following steps:

[0036] (1) Under protective gas protection conditions, polyol and part of dianhydride are reacted in a solvent at a temperature of 50 to 120° C. for a time of 1 to 24 hours to obtain an intermediate product;

[0037] (2) reacting the intermediate product obtained in step (1), a carboxyl-containing diamine, a carboxyl-free diamine, the remaining dianhydride and a catalyst, stopping the reaction after anhydrous water is generated to obtain a reaction solution, pouring the reaction solution into ethanol for sedimentation, washing the sediment with a large amount of ethanol, and vacuum drying to obtain the water-soluble polyimide binder.

[0038] It can be seen from the above content that the water-soluble polyimide binder finally obtained by the method has the solvent added during the reaction removed and is in a dry powder form, that is, the water-soluble polyimide binder provided in the first aspect of the present invention does not contain a solvent.

[0039] Meanwhile, the present invention does not impose any special limitation on the reaction temperature of step (2), and a conventional polyimide reaction temperature may be used. In addition, xylene may be added in step (2) to azeotropically remove the water generated during the imidization process.

[0040] In a third aspect, the present invention provides an aqueous polyimide binder slurry, wherein the aqueous polyimide binder slurry comprises the water-soluble polyimide binder as described in the first aspect and water.

[0041] Preferably, the solid content of the aqueous polyimide binder slurry is 4-30%, 5%, 10%, 15%, 20% or 25%, etc., and more preferably 6-15%.

[0042] Preferably, the viscosity of the aqueous polyimide adhesive slurry is 500-20000 cp, for example, 1000 cp, 2000 cp, 5000 cp, 10000 cp, 12000 cp, 14000 cp, 16000 cp or 18000 cp, and more preferably 800-60000 cp.

[0043] In a fourth aspect, the present invention provides a negative electrode sheet, the negative electrode sheet comprising a copper foil, and a negative electrode slurry attached to the copper foil after drying;

[0044] The negative electrode slurry includes a negative electrode active material, a conductive agent and the aqueous polyimide binder as described in the first aspect.

[0045] Preferably, the mass percentage of the aqueous polyimide binder in the negative electrode slurry is 0.1-10%, such as 0.5%, 1%, 2%, 4%, 6%, 8% or 10%, and more preferably 0.5-5%.

[0046] Preferably, the negative electrode active material comprises a silicon-carbon active material.

[0047] Preferably, the mass percentage of the negative electrode active material in the negative electrode slurry is 0.1-10%, for example, 0.5%, 1%, 2%, 4%, 6% or 8%.

[0048] Preferably, the mass percentage of the conductive agent in the negative electrode slurry is 5-20%, for example, 7%, 9%, 11%, 13%, 15%, 17% or 19%.

[0049] In a fifth aspect, the present invention provides a battery, comprising the negative electrode plate as described in the fourth aspect.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] (1) The raw materials for preparing the water-soluble polyimide binder provided by the present invention include a combination of a carboxyl-containing diamine, a carboxyl-free diamine, a polyol, a dianhydride and a catalyst; by selecting the carboxyl-containing diamine and the carboxyl-free diamine for combination and adding the polyol, the finally obtained polyimide has excellent solubility in water and excellent mechanical properties, and has excellent bonding properties to a variety of metals and carbon materials, so that the battery prepared by using the negative electrode sheet made of the water-soluble polyimide binder can show excellent charge and discharge performance and cycle stability.

[0052] (2) The present invention further limits the molar ratio of the diamine containing a carboxyl group to the diamine containing a carboxyl group, so that the 180° peel force of the negative electrode sheet containing the water-soluble polyimide binder can reach 48 to 78 N / m, and the first charge capacity of the battery can reach 941 to 973 mAh / g, the first efficiency can reach 86 to 91%, and the capacity retention rate after 200 cycles can reach 79 to 85%. DETAILED DESCRIPTION

[0053] The technical solution of the present invention is further described below by specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0054] Example 1

[0055] A water-soluble polyimide adhesive, the preparation method of which comprises the following steps:

[0056] (1) Under nitrogen protection, polyethylene glycol (8 g, number average molecular weight 800) and pyromellitic anhydride (2.18 g) in a molar ratio of 1:1 were added to a reaction kettle containing NMP (57.68 g), and the mixture was reacted at 60° C. for 8 h to obtain an intermediate product;

[0057] (2) 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid (2.72 g), 4,4'-diaminodiphenyl ether (4 g) and pyromellitic anhydride (6.54 g) in a molar ratio of 1:2:3 are added to the reactor, and NMP (173 g), isoquinoline (0.129 g) and xylene (50 g) are added to react at 180° C. The reaction is stopped after anhydrous water is generated. After the reaction liquid is cooled to room temperature, the reaction liquid is poured into ethanol for sedimentation, and repeatedly washed with a large amount of ethanol three times and then vacuum dried to obtain the water-soluble polyimide binder.

[0058] Example 2

[0059] A water-soluble polyimide adhesive is different from Example 1 only in that equimolar ethylene glycol is used to replace polyethylene glycol, and other substances, amounts and preparation methods are the same as those in Example 1.

[0060] Example 3

[0061] A water-soluble polyimide binder is different from Example 1 only in that 3,5-diaminobenzoic acid is used in an equal molar amount to replace 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid, and other substances, dosages and preparation methods are the same as those in Example 1.

[0062] Example 4

[0063] A water-soluble polyimide binder is different from Example 1 only in that ethylene glycol is used in an equal mole to replace polyethylene glycol, and 3,5-diaminobenzoic acid is used in an equal mole to replace 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid. Other substances, amounts and preparation methods are the same as those in Example 1.

[0064] Example 5

[0065] A water-soluble polyimide binder, which differs from Example 1 only in that, while keeping the total amount of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid and 4,4'-diaminodiphenyl ether unchanged, the molar ratio of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid and 4,4'-diaminodiphenyl ether is adjusted to 1:1.5, and other substances, amounts and preparation methods are the same as those in Example 1.

[0066] Example 6

[0067] A water-soluble polyimide binder, which differs from Example 1 only in that, while keeping the total amount of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid and 4,4'-diaminodiphenyl ether unchanged, the molar ratio of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid and 4,4'-diaminodiphenyl ether is adjusted to 1:4.5, and other substances, amounts and preparation methods are the same as those in Example 1.

[0068] Example 7

[0069] A water-soluble polyimide binder, which differs from Example 1 only in that, while keeping the total amount of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid and 4,4'-diaminodiphenyl ether unchanged, the molar ratio of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid and 4,4'-diaminodiphenyl ether is adjusted to 1:0.5, and other substances, amounts and preparation methods are the same as those in Example 1.

[0070] Example 8

[0071] A water-soluble polyimide binder is different from Example 1 only in that the molar ratio of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid to 4,4'-diaminodiphenyl ether is adjusted to 1:7 while keeping the total amount of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid and 4,4'-diaminodiphenyl ether unchanged, and other substances, amounts and preparation methods are the same as those in Example 1.

[0072] Comparative Example 1

[0073] A polyimide adhesive is different from Example 1 only in that the total amount of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid and 4,4'-diaminodiphenyl ether is kept unchanged, 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid is not added, and other substances, amounts and preparation methods are the same as those in Example 1.

[0074] Comparative Example 2

[0075] A polyimide adhesive is different from Example 1 only in that the total amount of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid and 4,4'-diaminodiphenyl ether is kept unchanged, 4,4'-diaminodiphenyl ether is not added, and other substances, amounts and preparation methods are the same as those in Example 1.

[0076] Comparative Example 3

[0077] A polyimide adhesive, the preparation method of which comprises: under nitrogen protection, adding 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid (2.72g), 4,4'-diaminodiphenyl ether (4g) and pyromellitic anhydride (6.54g) in a molar ratio of 1:2:3 into a reaction kettle, adding isoquinoline (0.129g) and xylene (50g), reacting at 180°C, stopping the reaction after anhydrous water is generated, and after the reaction liquid is cooled to room temperature, pouring the reaction liquid into ethanol for sedimentation, repeatedly washing with a large amount of ethanol three times, and vacuum drying to obtain the water-based polyimide.

[0078] Application Example 1

[0079] A negative electrode sheet, the preparation method of which comprises the following steps:

[0080] (1) Preparation of aqueous binder slurry: The water-soluble polyimide binder obtained in Example 1 was dissolved in deionized water to obtain an aqueous binder slurry having a solid content of 20% and a viscosity of 3000 cp;

[0081] (2) Preparation of negative electrode slurry: Silicon-carbon active material DXA5 (Betray New Materials Co., Ltd.), conductive additive SuperP and the aqueous binder slurry obtained in step (1) are uniformly mixed under high-speed stirring and degassed; wherein the mass ratio of silicon-carbon active material DXA5, conductive additive SuperP and water-soluble polyimide binder (solid component) is 90:8.5:1.5;

[0082] (3) Preparation of negative electrode sheet: The negative electrode slurry obtained in step (2) is coated on copper foil, heated and dried at 110° C. for 1 h to be completely dried, and then punched to prepare the negative electrode sheet.

[0083] Application Examples 2 to 8

[0084] A negative electrode plate, which differs from Application Example 1 only in that the water-soluble polyimide binder obtained in Example 1 is replaced by the water-soluble polyimide binder obtained in Examples 2 to 8, and other conditions and steps are the same as those in Application Example 1.

[0085] Comparative Application Examples 1 to 3

[0086] A negative electrode sheet, which differs from Application Example 1 only in that the polyimide binders obtained in Comparative Examples 1 to 3 are used to replace the water-soluble polyimide binder obtained in Example 1, and other conditions and steps are the same as those in Application Example 1.

[0087] Performance Testing:

[0088] (1) 180° peel strength: tested according to the test method provided in GB / T 2792-199;

[0089] (2) Electrical performance: The button cell was assembled in a glove box. The positive electrode active material of the button cell was LiFePO 4 The separator is Celgard 2320, the electrolyte is 1M LiFP6 (EC:DEC=1:1v / v), and the assembly order is positive electrode shell, positive electrode sheet, electrolyte, separator, electrolyte, negative electrode sheet, steel sheet, shrapnel and negative electrode shell; before the full battery assembly, the positive and negative electrodes are first assembled into half-cells for activation pretreatment, and the mass ratio of the active materials of the positive and negative electrodes is about 1:1;

[0090] Five batteries were made from each negative electrode sheet, and the button cells prepared above were connected to a potential tester for charge and discharge experiments. The potential range was 0-2V, and the cycle test was performed at a constant current of 0.5C to test the first charge capacity, first charge efficiency, and capacity retention after 200 cycles.

[0091]

[0092] According to the above test method, the corresponding use cases 1 to 8 and comparative application examples 1 to 3 were tested, and the test results are shown in Table 1:

[0093] Table 1

[0094]

[0095] According to the data in Table 1, we can see that:

[0096] The 180° peeling force of the negative electrode sheets provided in Application Examples 1 to 6 is 48 to 78 N / m, and the first charge capacity of the battery further manufactured is 941 to 973 mAh / g, the first efficiency is 86 to 91%, and the capacity retention rate after 200 cycles is 79 to 85%.

[0097] By comparing Application Example 1 and Comparative Application Example 1, it can be found that the peeling force of the negative electrode sheet obtained without adding carboxyl-containing diamine is very low, and the first charge capacity, initial efficiency and 200 cycle capacity retention rate of the battery further prepared are all reduced; by comparing Application Example 2 and Comparative Application Example 2, it can be found that although the peeling force of the negative electrode sheet prepared without adding non-carboxyl-containing diamine is higher, the 200 cycle capacity retention rate of the battery further prepared is very low; finally, by comparing Application Example 3, it can be found that the peeling force of the negative electrode sheet obtained without adding polyol is lower, and the first charge capacity, initial efficiency and 200 cycle capacity retention rate of the battery prepared are all poor.

[0098] Further comparison of the data of Application Example 1 and Application Examples 5 to 8 also reveals that the molar ratio of the diamine containing a carboxyl group to the diamine not containing a carboxyl group also affects the peeling force of the negative electrode sheet and the electrical performance of the battery.

[0099] The applicant declares that the present invention illustrates a water-soluble polyimide binder and its preparation method and application through the above-mentioned embodiments, but the present invention is not limited to the above-mentioned embodiments, that is, it does not mean that the present invention must rely on the above-mentioned embodiments to be implemented. Those skilled in the art should understand that any improvement of the present invention, equivalent replacement of various raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing a negative electrode sheet of a lithium-ion battery, It is characterized in that The following steps are involved: 1) mixing a water-soluble polyimide binder with water to obtain an aqueous binder slurry, and mixing a negative electrode active material, a conductive agent and the aqueous binder slurry to obtain a negative electrode slurry; 2) coating the negative electrode slurry on copper foil, heating and drying it, and then punching it to obtain a negative electrode sheet for a lithium-ion battery; The preparation method of the water-soluble polyimide binder comprises the following steps: reacting a polyol and a part of a dianhydride in a solvent under the protection of a protective gas to obtain an intermediate product; and reacting the intermediate product, a carboxyl-containing diamine, a carboxyl-free diamine, the remaining part of a dianhydride and a catalyst to obtain the water-soluble polyimide binder. The negative electrode slurry contains 0.1-10 wt % of the water-soluble polyimide binder, 0.1-10 wt % of the negative electrode active material, and 5-20 wt % of the conductive agent.

2. A method for preparing a negative electrode sheet for a lithium ion battery according to claim 1, It is characterized in that The negative electrode active material in step 1) is a silicon-carbon active material; The conductive agent is a conductive auxiliary agent SuperP.

3. A method for preparing a negative electrode sheet for a lithium ion battery according to claim 2, It is characterized in that The heating and drying temperature in step 2) is 110° C. and the time is 1 hour.

4. A method for preparing a negative electrode sheet for a lithium ion battery according to any one of claims 1 to 3, It is characterized in that In the preparation method of the water-soluble polyimide binder, the molar ratio of the diamine without carboxyl group, the diamine containing carboxyl group and the polyol is 1:(1-6):(1-3); The carboxyl-free diamine includes any one of p-phenylenediamine, m-phenylenediamine, 4,4'-diaminodiphenyl ether, 2,2'-bis(trifluoromethyl)diaminobiphenyl, 4,4'-diaminobenzanilide, 2,2'-bis[4-(4-aminophenoxyphenyl)]propane, 4,4'-diaminodiphenyl sulfone, 2-(4-aminophenyl)-5-aminobenzimidazole, 2,2-bis(4-hydroxy-3-aminophenyl)propane, 4,4'-bis(3-aminophenoxy)diphenyl sulfone, 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl, 4,4'-diamino-biphenyl or 9,9-bis(4-aminophenyl)fluorene, or a combination of at least two thereof; The carboxyl-containing diamine includes any one of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid, 6,6'-diamino-3,3'-methylenedibenzoic acid, 3,5-diaminobenzoic acid or benzidine disulfonic acid or a combination of at least two thereof; The polyol includes any one of ethylene glycol, propylene glycol, butylene glycol or polyethylene glycol, or a combination of at least two thereof; The number average molecular weight of the polyethylene glycol is 100 to 50,000.

5. A method for preparing a negative electrode sheet for a lithium ion battery according to claim 4, It is characterized in that In the preparation method of the water-soluble polyimide binder, the total molar number of the diamine containing a carboxyl group, the diamine not containing a carboxyl group and the polyol and the molar ratio of the dianhydride are 1: (1 to 1.1); The dianhydride includes aromatic tetracarboxylic dianhydride; The aromatic tetracarboxylic dianhydride includes 2,2',3,3'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-oxydiphthalic anhydride, 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride, (4-phthalic anhydride)formyloxy-4-phthalate, bis[(3,4-dianhydride)phenyl]terephthalate, 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, p-phenylene-bis(triphenylene)phthalate dicarboxylic anhydride, Anhydride, 4,4'-phenylenedioxydiphthalic anhydride, pyromellitic dianhydride, 2,2'-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride, 2,2-bis(4-(3,4-dicarboxyphenoxy)phenyl)hexafluoropropane dianhydride, 2,2-bis(4-(3,4-dicarboxybenzoyloxy)phenyl)hexafluoropropane dianhydride or 2,2'-bis(trifluoromethyl)-4,4'-bis(3,4-dicarboxyphenoxy)biphenyl dianhydride or a combination of at least two thereof.

6. A method for preparing a negative electrode sheet for a lithium ion battery according to claim 5, It is characterized in that In the preparation method of the water-soluble polyimide binder, the solvent is a non-protonated polar solvent; The non-protonated polar solvent includes any one of N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, γ-butyrolactone, propylene glycol monomethyl ether, cyclopentanone, cyclohexanone, ethyl acetate, toluene or methyl ethyl ketone, or a combination of at least two thereof.

7. A method for preparing a negative electrode sheet for a lithium ion battery according to claim 6, It is characterized in that The temperature of the first step reaction is 50-120° C. and the reaction time is 1-24 hours.

8. A lithium-ion battery negative electrode sheet prepared by the method for preparing a lithium-ion battery negative electrode sheet according to any one of claims 1 to 7.

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