A flexible water-based binder for a lithium battery negative electrode and a preparation method thereof

Abstract

A flexible water-based binder for a lithium battery negative electrode and a preparation method thereof. The flexible water-based binder is first prepared by using hydroxyl-terminated polybutadiene, isocyanate and hydroxyl acrylate as main raw materials to prepare a flexible polyurethane prepolymer containing a vinyl group. A water-soluble acrylic acid (salt) or acrylamide, an oil-soluble acrylate or acrylonitrile and other vinyl-containing compounds are used as monomers, and the flexible polyurethane prepolymer is combined to prepare a negative electrode water-based binder with flexible characteristics of crosslinking point position through free radical polymerization. The water-based binder prepared by the technical method provided in the application can realize two kinds of bonding modes of "point to point" and "point to line" with the active material of the pole piece in the microstructure, improves the bonding strength of the active layer material and the copper foil under the premise of not affecting the electrolyte resistance of the binder, and significantly improves the flexibility and subsequent processing stability of the negative electrode pole piece.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of high polymer material synthesis, and particularly relates to a flexible water-based binder for lithium battery negative electrodes and a preparation method thereof. BACKGROUND

[0002] In recent years, with the increasing demand for clean energy in China, lithium batteries, as an energy carrier with high energy storage density and good cycle life, play an important role in electric vehicles and large-scale energy storage devices. The electrode sheet, as an important component of lithium batteries, is prepared by bonding a small amount of conductive material to the metal current collector. In actual production, lithium battery positive and negative electrode binders mainly include water-based binders (such as butylphenyl emulsion, polyacrylic acid, etc.) and oil-based binders (such as solvent-based PVDF, etc.), among which water-based binders are mainly used for the bonding of battery negative active materials.

[0003] At present, butylphenyl emulsion and polyacrylic acid binder are widely used in negative electrode binders. Butylphenyl emulsion has been used in the field of battery negative electrode binders for a long time. The micro-bonding state of this binder and the active material of the electrode sheet is a "point-to-point" bonding mode. The prepared electrode sheet has excellent flexibility and good cutting processing performance, but there is still a problem of adhesion decline after long-term electrical cycling, which leads to the collapse of the electrode structure. With the increasing demand for energy density and electrical cycling performance of lithium batteries, polyacrylic acid type binders are increasingly widely used. The molecular structure of polyacrylic acid binder is linear, and the micro-bonding state of the active material of the electrode sheet is a "point-to-line" bonding mode, so it has more excellent adhesion and electrical cycling performance. For example, CN112680147B discloses a preparation method of a battery binder, which uses acrylic acid salt or acrylamide monomer and initiator to prepare by solution polymerization. This water-based binder has strong bonding capacity, and the prepared electrode sheet has stable cycle performance and high capacity. CN115050963B discloses a binder for lithium ion battery negative electrodes, a preparation method and use thereof. The invention prepares a negative electrode binder with excellent electrolyte resistance through a core-shell emulsion polymerization method.

[0004] Although the above-mentioned water-based polyacrylic acid binder can provide excellent electrolyte resistance and excellent bonding performance, it still has poor flexibility, high brittleness, and the electrode sheet is prone to cracking and powdering, which seriously affects the subsequent processing of the electrode sheet. Therefore, it is of great market and application value to develop a preparation route of a water-based polyacrylic acid binder with flexibility. SUMMARY

[0005] The purpose of the present application is to solve the problems of poor flexibility and poor subsequent processing performance of existing water-based polyacrylic acid binders, and to provide a flexible water-based polyacrylic acid binder for lithium battery negative electrodes and a preparation method thereof.

[0006] To achieve the above object, the technical scheme adopted by the present application is as follows:

[0007] A flexible water-based binder for a lithium battery negative electrode, the flexible water-based binder comprises, in mass parts, 10-80 parts of a water-based polyurethane prepolymer, 15-100 parts of a water-soluble monomer, 2-40 parts of an oil-soluble monomer, 0.1-3 parts of a functional monomer, 0.01-1.6 parts of an initiator, 5-20 parts of a pH regulator, and 80-500 parts of deionized water.

[0008] The water-based polyurethane prepolymer comprises, in mass parts, 20-60 parts of a hydroxyl-terminated polybutadiene, 5-50 parts of an isocyanate, 0.2-5 parts of a hydrophilic chain extender, 0.4-10 parts of a hydroxyl acrylate, 0.002-0.1 parts of a catalyst, 0-20 parts of a diluent, 0.2-5 parts of a neutralizer, and 30-200 parts of deionized water.

[0009] Further, the water-soluble monomer comprises one or more of a carboxylic acid monomer, an amide monomer, a sulfonic acid monomer, and a vinyl pyrrolidone.

[0010] The oil-soluble monomer comprises one or more of an alkyl methacrylate, a methyl styrene, a methacrylonitrile, an alkyl acrylate, a styrene, and an acrylonitrile, preferably a styrene.

[0011] The functional monomer comprises one or more of a hydroxyethyl acrylate, a hydroxypropyl acrylate, a hydroxybutyl acrylate, a glycidyl methacrylate, a glycidyl acrylate, an N-hydroxymethyl acrylamide, an N-hydroxyethyl acrylamide, and an N-hydroxypropyl acrylamide, preferably a hydroxyethyl acrylate and a hydroxybutyl acrylate.

[0012] The initiator is a persulfate salt.

[0013] The pH regulator comprises one or more of an alkali metal hydroxide and aqueous ammonia, preferably sodium hydroxide.

[0014] Further, the carboxylic acid monomer is one or more of an acrylic acid, a methacrylic acid, an itaconic acid, a β-carboxyethyl acrylate, a maleic acid, and a corresponding salt monomer thereof; the amide monomer is a vinyl-containing amide monomer; and the sulfonic acid monomer is one or more of a methacrylate sulfonic acid, an acrylate sulfonic acid, and a corresponding salt thereof, preferably a carboxylic acid and an amide, and more preferably an acrylic acid, a methacrylic acid, and an acrylamide.

[0015] Further, the vinyl-containing amide monomer is one or more of an acrylamide, an N-methyl acrylamide, an N-ethyl acrylamide, an N,N-dimethyl acrylamide, and an N,N-diethyl acrylamide.

[0016] Further, the persulfate salt is one or more of ammonium persulfate, potassium persulfate, sodium persulfate, preferably ammonium persulfate.

[0017] Further, the hydroxyl-terminated polybutadiene includes hydrogenated hydroxyl-terminated polybutadiene and / or non-hydrogenated hydroxyl-terminated polybutadiene, preferably hydrogenated hydroxyl-terminated polybutadiene, more preferably the Mn molecular weight is between 2000 and 6000;

[0018] The isocyanate includes one or more of isophorone diisocyanate, hexamethylene diisocyanate, dicyclohexylmethane diisocyanate, preferably isophorone diisocyanate;

[0019] The hydrophilic chain extender includes one or more of dimethylol propanoic acid, dimethylol butanoic acid, ethylenediamine ethyl sulfonic acid sodium, N, N-bis-hydroxyethyl proline, 3, 4-dihydroxybutane sulfonic acid, 3, 6-dihydroxy-2-toluene sulfonic acid, preferably dimethylol propanoic acid or dimethylol butanoic acid;

[0020] The hydroxyl acrylate includes one or more of hydroxyethyl acrylate, hydroxypropyl acrylate, and hydroxybutyl acrylate, preferably hydroxyethyl acrylate, hydroxybutyl acrylate;

[0021] The catalyst is an organometallic catalyst;

[0022] The diluent includes one or more of acetone, methyl ethyl ketone, N-methyl pyrrolidone (NMP), N, N-dimethyl methylamide, preferably acetone;

[0023] The neutralizing agent includes one or more of tertiary amine compounds, N, N-dialkyl alkanol amines, trialkanol amines, inorganic alkali metal salts, and ammonia water. Preferably, the tertiary amine compound is triethylamine.

[0024] Further, the organometallic catalyst is dibutyl tin dilaurate (DBTL) or stannous octoate, preferably dibutyl tin dilaurate.

[0025] Further, the tertiary amine compound is one or more of trialkyl amines, trimethylamine, triethylamine, and tributylamine, the N, N-dialkyl alkanol amine is one or more of N, N-dimethyl ethanolamine, N, N-dimethyl propanolamine, N, N-dipropyl ethanolamine, 1-dimethylamino-2-methyl-2-propanol, the trialkanol amine is triethanolamine, and the inorganic alkali metal salt is potassium hydroxide or sodium hydroxide.

[0026] A preparation method of the above-mentioned flexible water-based binder for a lithium battery negative electrode, the method comprising the following steps:

[0027] (1) Take the hydroxyl-terminated polybutadiene into a reaction vessel with stirring, dehydrate at 110-120℃ for 2h, then cool to 50℃, add isocyanate and catalyst, heat to 60-80℃, and react until the -NCO group content reaches the theoretical value (determined by di-n-butylamine titration), then add a hydrophilic chain extender and react until the -NCO group content reaches the theoretical value (add diluent to adjust the viscosity of the system during the reaction), then add hydroxy acrylate and react until there is no -NCO group in the system, then slowly disperse into deionized water containing a neutralizing agent under high shear force, if diluent is added during the preparation, remove the diluent under reduced pressure, and the vinyl-terminated waterborne polyurethane prepolymer is obtained, which is stored for later use;

[0028] (2) Take part of the water-soluble monomers (70-90% of the total amount), all the oil-soluble monomers and functional monomers into dropping kettle 1 and mix uniformly for later use; prepare an initiator aqueous solution and place it in dropping kettle 2 for later use;

[0029] (3) Accurately take the remaining water-soluble monomers and deionized water into a reaction kettle, heat to 82-85℃, add the remaining initiator, and react for 20-40min, then simultaneously drop the monomers in dropping kettle 1 and the initiator aqueous solution in dropping kettle 2 in step (2), the dropping time is 0.5-4h, and after the dropping is completed, keep the temperature at 82-85℃ for 30-60min;

[0030] (4) After the heat preservation is completed, accurately take the waterborne polyurethane prepolymer prepared in step (1) and drop it into the reaction kettle, the dropping time is 15-60min, and after the dropping is completed, continue to heat for 1-5h;

[0031] (5) After the heat preservation is completed, cool to below 50℃, add a pH adjusting agent, and adjust the pH to 7-9, and the flexible waterborne adhesive is obtained.

[0032] Further, in step (2), the water-soluble monomers account for 70-90% of the total amount of water-soluble monomers, and in the initiator aqueous solution, the initiator used accounts for 60-80% of the total amount of initiator, and the water used accounts for 5-30% of the total amount.

[0033] The beneficial effects of the present application relative to the prior art are:

[0034] (1) The present application introduces flexible polyurethane spherical microstructure into the linear structure of polyacrylic acid polymer, realizes the "point-to-point" and "point-to-line" two bonding modes of the adhesive and the active substance of the pole piece in the microstructure, significantly improves the high brittleness of the polyacrylic acid adhesive and the cracking of the pole piece surface, and improves the processing performance of the pole piece.

[0035] (2) The hydroxyl-terminated polybutadiene with long-chain alkyl structure is used as the main structure in the present application, which effectively avoids the adverse effects of the introduction of a large number of ester groups and ether bond structures on the electrolyte resistance of the adhesive, and effectively balances the flexibility and high-temperature electrolyte resistance of the adhesive.

[0036] (3) The adhesive preparation method provided by the present application is green and environmentally friendly, does not use a large amount of organic solvent, and is easy to scale up. DETAILED DESCRIPTION

[0037] The technical solutions of the present application will be described below. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0038] The present application provides a molecular structure design and preparation method of a water-based polyacrylic acid adhesive with flexibility. The method introduces flexible polyurethane spherical microstructure into the linear structure of the polyacrylic acid macromolecule, and prepares an adhesive with flexible crosslinking point structure. The adhesive and the active substance of the pole piece realize two bonding modes of "point to point" and "point to line" in the microstructure, thereby significantly improving the problems of high brittleness, poor cutting and winding performance of the polyacrylic acid adhesive. Secondly, the main body of the flexible prepolymer structure prepared by the method is a long-chain alkyl structure, and there is no ether bond structure and fewer ester groups in the molecular structure, so the introduction of the flexible structure will not reduce the electrolyte resistance of the adhesive.

[0039] The technical features involved in the different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0040] Example 1

[0041] (1) First, 35 g of hydrogenated hydroxyl-terminated polybutadiene (Mn 2000) is accurately weighed and placed in a reaction container with stirring. After vacuum dehydration at 120℃ for 2h, the temperature is lowered to 50℃, and then 7.5 g of isophorone diisocyanate and 0.01 g of dibutyltin dilaurate are added. The temperature is raised to 80℃ for reaction, until the -NCO group content reaches the theoretical value (determined by di-n-butylamine titration method). Then 1.7 g of dimethylol propionic acid is added to react until the -NCO group content reaches the theoretical value (diluent is added in time to adjust the viscosity of the system during the reaction), and then 1.5 g of hydroxybutyl acrylate is added to react until there is no -NCO group in the system. Finally, slowly disperse into 120 g of 3wt% triethylamine aqueous solution under high-speed shearing force, and remove the diluent under reduced pressure to obtain a water-based polyurethane prepolymer capped with vinyl, which is stored for use.

[0042] (2) Accurately weigh 60 g of acrylic acid, 4 g of butyl acrylate, and 1 g of hydroxyethyl acrylate into dropping vessel 1 and mix well for use; accurately weigh 0.08 g of ammonium persulfate and 15 g of deionized water into dropping vessel 2 for use.

[0043] (3) Accurately weigh 10 g of acrylic acid and 110 g of deionized water into a reaction kettle, heat to 82°C, add 0.03 g of ammonium persulfate, and react for 20 min; then simultaneously drop the monomer in dropping kettle 1 and the initiator solution in dropping kettle 2 in step (1) into the reaction kettle, and drop for 1.5 h. After dropping is completed, keep the temperature at 85°C for 30 min.

[0044] (4) After the temperature is kept, accurately weigh 60 g of the prepolymer prepared in step (1) and drop into the reaction kettle, and drop for 40 min; after dropping is completed, keep the temperature for 2 h.

[0045] (5) After the temperature is kept, cool to below 50°C, add a pH regulator, adjust the pH to 7-9, and the flexible water-based adhesive is obtained.

[0046] Example 2

[0047] (1) First, accurately weigh 48 g of hydrogenated hydroxyl-terminated polybutadiene (Mn 3000) into a reaction vessel with stirring, dehydrate at 120°C under vacuum for 2 h, then cool to 50°C, add 7.2 g of isophorone diisocyanate and 0.03 g of dibutyltin dilaurate, heat to 80°C, and react until the -NCO group content reaches the theoretical value (determined by di-n-butylamine titration). Then add 1.7 g of dimethylolpropionic acid and react until the -NCO group content reaches the theoretical value (add a diluent in time to adjust the viscosity of the system during the reaction), then add 1.2 g of hydroxyethyl acrylate and react until there is no -NCO group in the system. Finally, slowly disperse into 120 g of 3% triethylamine aqueous solution under high shear force, and evaporate the diluent under reduced pressure to obtain a vinyl-terminated waterborne polyurethane prepolymer, which is stored for use.

[0048] (2) Accurately weigh 50 g of acrylic acid, 10 g of methacrylic acid, 2 g of styrene, and 1 g of hydroxybutyl acrylate into dropping vessel 1 and mix well for use; accurately weigh 0.06 g of ammonium persulfate and 15 g of deionized water into dropping vessel 2 for use.

[0049] (3) Accurately weigh 10 g of acrylic acid, 2 g of methacrylic acid, and 110 g of deionized water into a reaction kettle, heat to 82°C, add 0.02 g of ammonium persulfate, and react for 20 min; then simultaneously drop the monomer in dropping kettle 1 and the initiator solution in dropping kettle 2 in step (1) into the reaction kettle, and drop for 1.5 h. After dropping is completed, keep the temperature at 85°C for 30 min.

[0050] (4) After the heat preservation is completed, 40 g of the prepolymer prepared in step (1) is accurately weighed and added dropwise into the reaction kettle, the dropwise adding time is 40 min, and after the dropwise adding is completed, heat preservation is continued for 2 h.

[0051] (5) After the heat preservation is completed, the temperature is reduced to below 50 DEG C, the pH regulator is added, and the pH is adjusted to 7-9, and thus the flexible water-based adhesive is obtained.

[0052] Example 3

[0053] (1) First, 55 g of hydrogenated hydroxyl-terminated polybutadiene (Mn 5000) is accurately weighed and placed in a reaction container with stirring, after vacuum dehydration at 120 DEG C for 2 h, the temperature is reduced to 50 DEG C, and then 4.9 g of isophorone diisocyanate and 0.03 g of dibutyltin dilaurate are added, the temperature is increased to 80 DEG C for reaction, until the -NCO group content reaches the theoretical value (determined by di-n-butylamine titration method). Then 1.7 g of dimethylol propionic acid is added to react until the -NCO group content reaches the theoretical value (diluent is added in time to adjust the viscosity of the system during the reaction), then 1.2 g of hydroxyethyl acrylate is added to react until there is no -NCO group in the system. Finally, slowly disperse into 120 g of 3% triethylamine aqueous solution under high shear force, and remove the diluent under reduced pressure to obtain a waterborne polyurethane prepolymer capped with vinyl, which is stored for use.

[0054] (2) 37 g of acrylic acid, 12 g of methacrylic acid, 11 g of acrylamide, 5 g of acrylonitrile, and 1.2 g of hydroxybutyl acrylate are accurately weighed and placed in dropwise adding container 1 for uniform mixing; 0.06 g of ammonium persulfate and 15 g of deionized water are accurately weighed and placed in dropwise adding container 2.

[0055] (3) 8 g of acrylic acid, 2 g of methacrylic acid, 0.5 g of acrylonitrile, and 110 g of deionized water are accurately weighed and placed in a reaction kettle, the temperature is increased to 82 DEG C, 0.03 g of ammonium persulfate is added, and after reaction for 20 min, the monomers in dropwise adding container 1 and the initiator solution in dropwise adding container 2 in step (1) are simultaneously added dropwise, the dropwise adding time is 1.5 h. After the dropwise adding is completed, heat preservation is carried out at 85 DEG C for 30 min.

[0056] (4) After the heat preservation is completed, 40 g of the prepolymer prepared in step (1) is accurately weighed and added dropwise into the reaction kettle, the dropwise adding time is 40 min, and after the dropwise adding is completed, heat preservation is continued for 2 h.

[0057] (5) After the heat preservation is completed, the temperature is reduced to below 50 DEG C, the pH regulator is added, and the pH is adjusted to 7-9, and thus the flexible water-based adhesive is obtained.

[0058] Example 4

[0059] (1) First, accurately weigh 30 g of hydrogenated hydroxyl-terminated polybutadiene (Mn1000) into a reaction vessel with stirring, dehydrate at 120°C for 2 h under vacuum, then cool to 50°C, add 13.3 g of isophorone diisocyanate, 0.01 g of dibutyltin dilaurate, and heat to 80°C for reaction until the -NCO group content reaches the theoretical value (determined by titration with di-n-butylamine). Then add 1.7 g of dimethylol propionic acid to react until the -NCO group content reaches the theoretical value (add diluent in time to adjust the viscosity of the system during the reaction), then add 1.5 g of hydroxybutyl acrylate to react until there is no -NCO group in the system. Finally, slowly disperse into 120 g of 3% triethylamine aqueous solution under high shear force, and evaporate the diluent under reduced pressure to obtain a vinyl-terminated waterborne polyurethane prepolymer, which is stored for later use.

[0060] (2) Accurately weigh 40 g of acrylic acid, 20 g of acrylamide, 5 g of acrylonitrile, and 1.2 g of hydroxybutyl acrylate into dropping vessel 1 and mix well for later use; accurately weigh 0.06 g of ammonium persulfate and 15 g of deionized water into dropping vessel 2 for later use.

[0061] (3) Accurately weigh 12 g of acrylic acid, 1.5 g of acrylonitrile, and 110 g of deionized water into a reaction kettle, heat to 82°C, add 0.03 g of ammonium persulfate, and react for 20 min. Then simultaneously add the monomers in dropping vessel 1 and the initiator solution in dropping vessel 2 from step (1) dropwise, with a dropwise addition time of 1.5 h. After the dropwise addition is complete, incubate at 85°C for 30 min.

[0062] (4) After incubation is complete, accurately weigh 40 g of the prepolymer prepared in step (1) and add it dropwise into the reaction kettle, with a dropwise addition time of 40 min. After the dropwise addition is complete, continue to incubate for 2 h.

[0063] (5) After incubation is complete, cool to below 50°C, add a pH adjuster, and adjust the pH to 7-9 to obtain the flexible waterborne adhesive.

[0064] Comparative Example

[0065] The comparative example is prepared by a traditional solution polymerization method, with the following steps:

[0066] (1) Accurately weigh 30 g of acrylic acid, 10 g of methacrylic acid, 7 g of acrylamide, 8 g of acrylonitrile, and 1.2 g of hydroxybutyl acrylate into dropping vessel 1 and mix well for later use; accurately weigh 0.05 g of ammonium persulfate and 15 g of deionized water into dropping vessel 2 for later use.

[0067] (2) Accurately weigh 10g of acrylic acid, 2g of methacrylic acid, 0.5g of acrylonitrile and 110g of deionized water and place them in a reaction vessel. Heat the vessel to 82℃, add 0.03g of ammonium persulfate, and react for 20min. Then, simultaneously add the monomer from dropping vessel 1 and the initiator solution from dropping vessel 2 in step (1) for 1.5h. After the addition is complete, keep the vessel at 85℃ for 2h.

[0068] (3) After the heat preservation is completed, the temperature is lowered to below 50°C, and a pH adjuster is added to adjust the pH to 7~9 to obtain the water-based adhesive.

[0069] The water-based adhesive obtained above was subjected to electrolyte and slurry resistance tests:

[0070] ① Electrolyte Swelling and Precipitation Resistance Test: The prepared aqueous adhesive solution was dried at 120℃ for 1 hour, and its weight was recorded as W0. Then, it was placed in an electrolyte solution at 60℃ and kept at that temperature for 72 hours. After that, it was removed, the surface electrolyte was wiped clean, and its weight was recorded as W1. Finally, it was placed in a 200℃ oven and dried for 4 hours, and its weight was recorded as W2. The swelling rate and precipitation rate of the electrolyte resistance were calculated according to the following formulas. The test results are shown in Table 1.

[0071] Swelling rate (%) = (W1 - W0) / W0 × 100%

[0072] Extraction rate (%) = (W2 - W0) / W0 × 100%

[0073] ② Adhesion strength measurement

[0074] Using the binders from the above examples and comparative examples, negative electrode sheets were prepared, and their 180° peel strength was measured. The specific method is as follows: 98.5g of artificial graphite and 1.5g of binder, used as the negative electrode active material, were added to 100g of deionized water and dispersed evenly to obtain a negative electrode mixture slurry. This negative electrode mixture slurry was coated onto a 12μm thick copper foil, then dried and rolled. An areal density of 20mg / cm³ was formed. 2 Compacted density 1.60 g / cm³ 3 The negative electrode sheet was then attached to the surface of the negative electrode using 3M standard tape (Scotch 600 / 25mm), and rolled back and forth three times with a roller. After being placed under standard conditions for 20 minutes, a peel force test was performed. The test results are shown in Table 1.

[0075] ③ Electrode flexibility test

[0076] The prepared negative electrode sheet was cut into strips of 150mm × 10mm and placed at a temperature of 25±2℃ and a humidity of In a 30% environment, the device was fixed on a bending tester and bent 180° with a radius of 5mm. The number of bends at which cracks or peeling of the active layer occurred was recorded. The test results are shown in Table 1.

[0077] Table 1 Water-based binder electrolyte resistance and slurry performance test results

[0078]

[0079] As can be seen from Table 1, by introducing long alkyl chain flexible structure, the present application constructs a new micro bonding mode of "point-line", which significantly improves the problem of high brittleness and easy cracking of polyacrylic acid binder, and further improves the bonding strength of the binder to the active material and the adhesion to the pole piece; in addition, the introduction of flexible structure does not cause obvious adverse effects on the electrolyte resistance of the binder.

[0080] The above only describes the preferred embodiments of the present application, and it should be pointed out that for ordinary skilled persons in the art, several improvements and refinements can be made without departing from the principles of the present application, and these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A method for preparing a flexible aqueous binder for a lithium battery negative electrode, characterized by: The flexible water-based adhesive comprises, in mass parts, 10-80 parts of water-based polyurethane prepolymer, 15-100 parts of water-soluble monomer, 2-40 parts of oil-soluble monomer, 0.1-3 parts of functional monomer, 0.01-1.6 parts of initiator, 5-20 parts of pH regulator, 80-500 parts of deionized water; The water-based polyurethane prepolymer comprises, in mass parts, 20-60 parts of hydroxyl-terminated polybutadiene, 5-50 parts of isocyanate, 0.2-5 parts of hydrophilic chain extender, 0.4-10 parts of hydroxyl acrylate, 0.002-0.1 parts of catalyst, 0-20 parts of diluent, 0.2-5 parts of neutralizing agent, 30-200 parts of deionized water; The method comprises the following steps: (1) The hydroxyl-terminated polybutadiene is weighed and placed in a reaction container with stirring, after vacuum dehydration at 110-120℃ for 2h, the temperature is lowered to 50℃, the isocyanate and catalyst are added, the temperature is raised to 60-80℃ for reaction until the content of -NCO group reaches the theoretical value; then the hydrophilic chain extender is added for reaction until the content of -NCO group reaches the theoretical value, then the hydroxyl acrylate is added for reaction until there is no -NCO group in the system; finally, the vinyl-terminated water-based polyurethane prepolymer is prepared by slowly dispersing into the deionized water containing the neutralizing agent under high shear force, if the diluent is added during the preparation process, the diluent is removed by reduced pressure evaporation, and the product is stored for use; (2) The total amount of 70-90% of the water-soluble monomer, all the oil-soluble monomer and the functional monomer are weighed and mixed uniformly in a dropping kettle one for standby; the initiator aqueous solution is prepared and placed in a dropping kettle two for standby; (3) The remaining water-soluble monomer and deionized water are accurately weighed and placed in a reaction kettle, the temperature is raised to 82-85℃, the remaining initiator is added, after reaction for 20-40min, the monomer in the dropping kettle one in step (2) and the initiator aqueous solution in the dropping kettle two are simultaneously added dropwise, the dropwise adding time is 0.5-4h, after the dropwise adding is completed, the temperature is kept at 82-85℃ for 30-60min; (4) After the keeping is completed, the water-based polyurethane prepolymer prepared in step (1) is accurately weighed and added dropwise into the reaction kettle, the dropwise adding time is 15-60min, after the dropwise adding is completed, the keeping is continued for 1-5h; (5) After the keeping is completed, the temperature is lowered to below 50℃, the pH regulator is added, and the pH is adjusted to 7-9, thus the flexible water-based adhesive is obtained.

2. The method for preparing a flexible aqueous binder for a lithium battery negative electrode according to claim 1, characterized in that: The water-soluble monomer comprises one or more of carboxylic acid monomer, amide monomer, sulfonic acid monomer and vinyl pyrrolidone; The oil-soluble monomer comprises one or more of alkyl methacrylate, methyl styrene, methacrylonitrile, alkyl acrylate, styrene and acrylonitrile; The functional monomer comprises one or more of hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, glycidyl methacrylate, glycidyl acrylate, N-hydroxymethyl acrylamide, N-hydroxyethyl acrylamide and N-hydroxypropyl acrylamide; The initiator is a persulfate salt; The pH regulator comprises one or more of alkali metal hydroxide and aqueous ammonia.

3. The method for preparing a flexible aqueous binder for a lithium battery negative electrode according to claim 2, characterized in that: The carboxylic acid monomer is one or more of acrylic acid, methacrylic acid, itaconic acid, beta-carboxyethyl acrylate, maleic acid and corresponding salt monomers thereof; the amide monomer is a vinyl-containing amide monomer; the sulfonic acid monomer is one or more of methacryloyl sulfonic acid, acryloyl sulfonic acid and corresponding salts thereof.

4. The method for preparing a flexible aqueous binder for a lithium battery negative electrode according to claim 3, characterized in that: The vinyl-containing amide monomer is one or more of acrylamide, N-methyl acrylamide, N-ethyl acrylamide, N,N-dimethyl acrylamide, N,N-diethyl acrylamide.

5. The method for preparing a flexible aqueous binder for a lithium battery negative electrode according to claim 2, characterized in that: The persulfate salt is one or more of ammonium persulfate, potassium persulfate and sodium persulfate.

6. The method for preparing a flexible aqueous binder for a lithium battery negative electrode according to claim 1, characterized in that: The hydroxyl-terminated polybutadiene includes hydrogenated hydroxyl-terminated polybutadiene and / or non-hydrogenated hydroxyl-terminated polybutadiene; The isocyanate includes one or more of isophorone diisocyanate, hexamethylene diisocyanate and dicyclohexylmethane diisocyanate; The hydrophilic chain extender includes one or more of dimethylol propanoic acid, dimethylol butanoic acid, ethylenediamine ethanesulfonic acid sodium, N,N-bis-hydroxyethyl proline, 3,4-dihydroxybutane sulfonic acid and 3,6-dihydroxy-2-toluene sulfonic acid; The hydroxy acrylate includes one or more of hydroxyethyl acrylate, hydroxypropyl acrylate and hydroxybutyl acrylate; The catalyst is an organometallic catalyst; The diluent includes one or more of acetone, methyl ethyl ketone, N-methyl pyrrolidone and N,N-dimethylformamide; The neutralizing agent includes one or more of a tertiary amine compound, an N,N-dialkyl alkanolamine, a trialkanolamine, an inorganic alkali metal salt and ammonia.

7. The method for preparing a flexible aqueous binder for a lithium battery negative electrode according to claim 6, characterized in that: The organometallic catalyst is dibutyl tin dilaurate or stannous octoate.

8. The method for preparing a flexible aqueous binder for a lithium battery negative electrode according to claim 6, characterized in that: The tertiary amine compound is one or more of a trialkylamine, trimethylamine, triethylamine and tributylamine; the N,N-dialkyl alkanolamine is one or more of N,N-dimethyl ethanolamine, N,N-dimethyl propanolamine, N,N-dipropyl ethanolamine and 1-dimethylamino-2-methyl-2-propanol; the trialkanolamine is triethanolamine; and the inorganic alkali metal salt is potassium hydroxide or sodium hydroxide.

9. The method of claim 1, wherein the method is characterized by: In step (2), the water-soluble monomer accounts for 70% to 90% of the total amount of water-soluble monomers, and in the initiator aqueous solution, the initiator used accounts for 60% to 80% of the total amount of initiator, and the water used accounts for 5% to 30% of the total amount.

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