Binder for lithium ion battery as well as preparation method and application of binder

Abstract

The invention relates to the field of lithium ion batteries, in particular to a binder for a lithium ion battery as well as a preparation method and application of the binder. The binder for the lithium ion battery at least comprises the following raw materials in parts by mass: 40-75 parts of acrylonitrile, 15-40 parts of an acrylate monomer, 250-400 parts of an organic solvent and 0.2-1 part of an initiator. The finally prepared binder for the lithium ion battery not only has the characteristics of difficulty in gelling and high binding power, but also can replace the traditional positive electrode binder PVDF, solves the problem that gelling is easily caused when PVDF is applied to a high-nickel ternary lithium ion battery at present, and effectively improves the performance of a positive electrode piece and the lithium ion battery.

Description

Technical Field

[0001] The present application relates to the field of lithium-ion batteries, and more specifically to a binder for lithium-ion batteries, a preparation method thereof, and an application thereof. Background Art

[0002] Binders are an important component of electrode materials, adhering the positive electrode active material to the current collector surface. Their properties have a significant impact on the electrochemical performance of the battery. Currently, the commercialized positive electrode binder is polyvinylidene fluoride (PVDF). PVDF has a high dielectric constant, good chemical and thermal stability, and a certain viscosity, which can basically meet the positive electrode binder requirements of lithium-ion batteries. For example, patent CN119542424A provides a composite positive electrode binder and its preparation method. This uses modified polyvinylidene fluoride as a positive electrode binder to capture dissolved transition metal ions and improve electrochemical performance.

[0003] However, the above scheme also has technical difficulties, such as the use of weak van der Waals forces to bond the positive electrode components and the poor bonding performance of the electrode plates when working at high temperatures. The higher Young's modulus leads to poor flexibility of the prepared plates, and high swelling occurs when immersed in electrolyte, which is not conducive to battery safety. In addition, due to the difficulty of production technology, the total output of PVDF binders that can be actually used in lithium batteries in China is low and expensive.

[0004] With the increasing demand for lithium-ion battery energy density, high-nickel ternary cathode materials (LiNixCoyMnzO2, x ≥ 0.8) have become a mainstream development direction due to their high specific capacity (>200 mAh / g). However, the increased nickel content significantly increases the surface alkalinity of the material (pH > 12), posing a severe challenge to traditional binder systems. Oily binders, such as polyvinylidene fluoride (PVDF), are susceptible to chemical degradation in highly alkaline environments. The C-H bonds in the PVDF molecular chain undergo dehydrofluorination in the presence of strong bases, forming a continuous conjugated double bond structure. When exposed to air, these unsaturated double bonds react with oxygen to form peroxide intermediates, which further decompose to produce free radicals and trigger chain coupling reactions, ultimately leading to crosslinking and gelation of the cathode slurry. This phenomenon directly degrades the slurry's rheological properties (such as a surge in viscosity and loss of thixotropy), leading to poor electrode uniformity and abnormal porosity during the coating process, severely hindering the large-scale production of high-nickel battery systems.

[0005] Therefore, there is an urgent need to design and develop a positive electrode binder to overcome the problem of gelation of the positive electrode slurry during use, and to improve the cycle performance problem of high-nickel ternary material batteries caused by the increase in nickel content. Summary of the Invention

[0006] In summary, developing a positive electrode binder that overcomes the problem of gelation of positive electrode slurry during use has become a key issue for those skilled in the art. Through in-depth research and development on this issue, the present applicant has ultimately proposed a binder for lithium-ion batteries and a method for preparing the same.

[0007] The binder for lithium-ion batteries finally prepared in this application is not only not easy to gel and has high bonding strength, but can also replace the traditional positive electrode binder PVDF, solving the problem that PVDF is prone to gel when used in high-nickel ternary lithium-ion batteries, and effectively improving the performance of the positive electrode sheet and lithium-ion battery.

[0008] A binder for lithium-ion batteries comprises raw materials, in parts by mass, of at least: 40-75 parts of acrylonitrile, 15-40 parts of acrylate monomers, 250-400 parts of an organic solvent, and 0.2-1 part of an initiator.

[0009] In a preferred embodiment, the acrylic acid ester monomer is at least one of methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, isobutyl acrylate, isobornyl acrylate, amyl acrylate, isoamyl acrylate, isooctyl acrylate, 2-ethylhexyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, lauryl acrylate, lauryl methacrylate, and isodecyl acrylate.

[0010] In a preferred embodiment, the acrylic acid ester monomer is at least one of methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, isobutyl acrylate, and isobornyl acrylate.

[0011] In a preferred embodiment, the acrylic acid ester monomer is at least one of butyl acrylate, isobutyl acrylate and isobornyl acrylate.

[0012] In a preferred embodiment, the acrylic acid ester monomer is butyl acrylate.

[0013] In a preferred embodiment, the mass ratio of acrylonitrile to acrylate monomer is (50-70): (15-35).

[0014] In a preferred embodiment, the mass ratio of acrylonitrile to acrylate monomer is (55-65): (25-30).

[0015] In a preferred embodiment, the organic solvent is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, ethyl acetate and dimethyl sulfoxide.

[0016] In a preferred embodiment, the organic solvent is N,N-dimethylformamide.

[0017] In a preferred embodiment, the initiator is at least one of azobisisobutyronitrile, azobisisoheptanenitrile, dimethyl azobisisobutyrate, azobispentancyanate, dicumyl peroxide and benzoyl peroxide.

[0018] In a preferred embodiment, the initiator is azobisisobutyronitrile.

[0019] In a preferred embodiment, the raw materials of the binder for lithium-ion batteries further include, by weight: 5 to 20 parts of vinyl monomers, 1.5 to 5 parts of slightly branched cross-linking monomers, 3 to 8 parts of auxiliary monomers, and 2 to 6 parts of composite additives.

[0020] In a preferred embodiment, the mass ratio of acrylonitrile, vinyl monomer, slightly branched cross-linking monomer and auxiliary monomer is (50-70): (7-20): (1.5-4): (4-6).

[0021] In a preferred embodiment, the mass ratio of acrylonitrile, vinyl monomer, slightly branched cross-linking monomer and auxiliary monomer is (55-65): (8-12): (1.5-3): (4-5).

[0022] In a preferred embodiment, the vinyl monomer is at least one of styrene, α-methylstyrene, vinyltoluene, 2,4-dimethylstyrene, ethylstyrene, 4-isopropylstyrene, 4-tert-butylstyrene, 4-phenylstyrene, and cyclohexylstyrene.

[0023] In a preferred embodiment, the vinyl monomer is at least one of styrene, α-methylstyrene, vinyltoluene, 2,4-dimethylstyrene, and ethylstyrene.

[0024] In a preferred embodiment, the vinyl monomer is styrene.

[0025] In a preferred embodiment, the micro-branched cross-linking monomer is at least one of diethylene glycol diacrylate, dipropylene glycol diacrylate, divinylbenzene, allyl diethylene glycol dicarbonate, polyethylene glycol diacrylate, diallylamine, tripropylene glycol diacrylate, phthalic acid diacrylate, 1,2,3-trivinylbenzene, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, and tris(2-hydroxyethyl)isocyanuric acid triacrylate.

[0026] In a preferred embodiment, the micro-branched cross-linking monomer is at least one of diethylene glycol diacrylate, dipropylene glycol diacrylate, divinylbenzene, allyl diethylene glycol dicarbonate, and polyethylene glycol diacrylate.

[0027] In a preferred embodiment, the micro-branched cross-linking monomer is allyl diethylene glycol dicarbonate or polyethylene glycol diacrylate.

[0028] Through the construction of the above-mentioned monomer formula, acrylonitrile, acrylate monomers, vinyl monomers, micro-branched cross-linking monomers and auxiliary monomers are used as copolymer units, and the unity of high bonding strength and anti-gelation is achieved through the coordinated design of molecular structure.

[0029] Acrylonitrile is used as the core monomer of the main chain. The highly polar cyano group (-CN) in its molecular chain can simultaneously form strong bonding interactions (including hydrogen bonds and coordination bonds) with the surface active sites of the positive electrode material and the oxide layer of the aluminum current collector, thereby giving the electrode pole piece excellent mechanical bonding properties. Acrylate monomers adjust the glass transition temperature of the polymer through long-chain alkyl structures (such as butyl), significantly improving the flexibility of the adhesive, so that it can effectively buffer the volume expansion stress of high-nickel materials during coating, rolling and other processes, and avoid cracking of the pole piece. Vinyl monomers enhance the rigidity of the molecular chain with a rigid benzene ring structure, ensuring the uniformity of slurry dispersion and the deformation resistance of the electrode structure.

[0030] Furthermore, the innovatively introduced micro-branched cross-linking monomers and auxiliary monomers form a controllable three-dimensional network structure during the polymerization process. By stabilizing covalent cross-linking points, they inhibit molecular chain slippage, enabling the binder to maintain structural integrity during electrolyte immersion and long-term cycling. Compared to traditional fluorinated binders (such as PVDF), this invention completely avoids the introduction of fluorine, fundamentally eliminating the risk of slurry gelation caused by hydrogen fluoride removal in highly alkaline environments. High-nickel cathodes prepared using this binder exhibit excellent cycling performance and are both industrially compatible and adaptable to high energy density.

[0031] In a preferred embodiment, the auxiliary monomer is a combination of ethyl methacrylate sulfonate and vinyl phosphonate.

[0032] In a preferred embodiment, the mass ratio of ethyl methacrylate sulfonate to vinyl phosphonate is (5-8):(2-4).

[0033] In a preferred embodiment, the mass ratio of ethyl methacrylate sulfonate to vinyl phosphonate is (6-7):(2.5-3).

[0034] The addition of auxiliary monomers allows the sulfonic acid groups to effectively neutralize residual alkaline substances on the surface of high-nickel cathode materials, reducing local alkalinity and blocking the chemical driving force behind the HF desorption reaction in PVDF. Furthermore, the phosphonate groups form a six-membered ring chelate structure (MOPOC) with transition metal ions such as Ni³⁺ / Co³⁺ in high-nickel materials, significantly enhancing the interfacial bonding between the binder and active particles and inhibiting the shedding of active materials during cycling. Furthermore, the phosphonate groups can partially dissociate into PO4³⁻ in the electrolyte, forming an ionic crosslinked network with Li⁺, endowing the binder with self-healing capabilities, thereby comprehensively improving the binder's overall performance.

[0035] In a preferred embodiment, the composite auxiliary agent is a composition of triphenyl phosphate and perfluoropolyether.

[0036] In a preferred embodiment, the mass ratio of triphenyl phosphate to perfluoropolyether is (3-5): (1-1.8).

[0037] In a preferred embodiment, the mass ratio of triphenyl phosphate to perfluoropolyether is (4-4.5): (1-1.5).

[0038] In a preferred embodiment, the weight average molecular weight of the perfluoropolyether is 2000-3000 Da.

[0039] The preparation method of the above-mentioned binder for lithium-ion batteries in the present application specifically includes the following steps: S1: mixing acrylonitrile, acrylate monomers, vinyl monomers, slightly branched cross-linking monomers and auxiliary monomers in an organic solvent, and stirring evenly in a reactor; S2: adding an initiator and stirring evenly, evacuating and introducing nitrogen, heating to 70-75°C, stirring at a speed of 200-250 rpm and keeping warm for 7-8 hours; S3: after the reaction is completed, naturally cooling to 45-50°C, adding a composite auxiliary agent, stirring at a speed of 150-200 rpm for 30-40 minutes, and after completion, naturally cooling the product to room temperature, and filtering through a 0.45-0.5 μm filter membrane to remove unreacted monomers and agglomerated particles to obtain the product.

[0040] The present application further limits the application of the prepared lithium-ion battery binder in high-nickel ternary positive electrode power batteries, high-voltage lithium metal batteries, energy storage batteries and flexible batteries.

[0041] Practical significance and beneficial effects: 1. The lithium-ion battery binder finally prepared in this application is not only not easy to gel and has high bonding strength, but can also replace the traditional positive electrode binder PVDF, solving the problem that PVDF is prone to gel when used in high-nickel ternary lithium-ion batteries, and effectively improving the performance of the positive electrode sheet and lithium-ion battery.

[0042] 2. The lithium-ion battery binder prepared in this application utilizes innovative microbranched cross-linking monomers and auxiliary monomers to form a controllable three-dimensional network structure during polymerization. This stabilizes covalent cross-linking points, inhibiting molecular chain slippage and maintaining structural integrity during electrolyte immersion and long-term circulation. Compared to traditional fluorinated binders (such as PVDF), this invention completely avoids the introduction of fluorine, fundamentally eliminating the risk of slurry gelation caused by hydrogen fluoride removal in highly alkaline environments.

[0043] 3. Furthermore, the addition of auxiliary monomers in this application allows the sulfonic acid groups to possess strong proton-releasing capabilities, effectively neutralizing residual alkaline substances on the surface of high-nickel cathode materials, reducing local alkalinity and blocking the chemical driving force of the HF-removal reaction of PVDF. Furthermore, the phosphonate groups in these monomers form a six-membered ring chelate structure (MOPOC) with transition metal ions such as Ni³⁺ and Co³⁺ in the high-nickel material, significantly enhancing the interfacial bonding between the binder and the active particles, inhibiting the shedding of active substances during cycling, and thus comprehensively improving the overall performance of the binder. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a comparison chart of the cycle capacity retention test results of the lithium ion battery binder prepared in Example 1 of the present application and Comparative Example 2 (conventional commercially available PVDF binder).

[0045] Figure 2 This is a comparison chart of the cycle capacity retention test results of the lithium-ion battery binder prepared in Example 6 of the present application and that of Comparative Example 2 (conventional commercially available PVDF binder). DETAILED DESCRIPTION

[0046] Example 1 The binder for lithium-ion batteries comprises, by weight, 60 parts of acrylonitrile, 25 parts of acrylate monomers, 10 parts of vinyl monomers, 1.5 parts of slightly branched cross-linking monomers, 4.5 parts of auxiliary monomers, 4 parts of composite additives, 300 parts of organic solvents, and 0.8 parts of initiators.

[0047] The acrylic ester monomer is butyl acrylate; the vinyl monomer is styrene; the slightly branched cross-linking monomer is allyl diglycol dicarbonate; the auxiliary monomer is a combination of ethyl methacrylate sulfonate and vinyl phosphonate, with a mass ratio of 7:3; the composite auxiliary agent is a combination of triphenyl phosphate and perfluoropolyether, with a mass ratio of 4:1; the organic solvent is N,N-dimethylformamide; and the initiator is azobisisobutyronitrile.

[0048] Perfluoropolyether has a weight-average molecular weight of 2500 Da and is of premium grade and was purchased from Wuhan Huaxiang Kejie Biotechnology Co., Ltd., China.

[0049] The preparation method of the binder for lithium-ion batteries described in this embodiment specifically includes the following steps: S1: mixing acrylonitrile, acrylate monomers, vinyl monomers, slightly branched cross-linking monomers and auxiliary monomers in an organic solvent, and stirring evenly in a reactor; S2: adding an initiator and stirring evenly, evacuating and introducing nitrogen, heating to 70°C, stirring at 220 rpm and keeping warm for 8 hours; S3: after the reaction is completed, naturally cooling to 45°C, adding a composite auxiliary agent, stirring at 200 rpm for 35 minutes, and naturally cooling the product to room temperature after completion, and filtering through a 0.5 μm filter membrane to remove unreacted monomers and agglomerated particles to obtain the product.

[0050] Example 2 The only difference between this embodiment and Example 1 is that the raw materials of the binder for lithium-ion batteries include, by mass, 70 parts of acrylonitrile, 15 parts of acrylate monomers, 10 parts of vinyl monomers, 1.5 parts of micro-branched cross-linking monomers, 4.5 parts of auxiliary monomers, 4 parts of composite additives, 300 parts of organic solvents, and 0.8 parts of initiators.

[0051] The other embodiments are the same.

[0052] Example 3 The only difference between this embodiment and Example 1 is that the raw materials of the binder for lithium-ion batteries include, by mass, 50 parts of acrylonitrile, 25 parts of acrylate monomers, 20 parts of vinyl monomers, 1.5 parts of slightly branched cross-linking monomers, 4.5 parts of auxiliary monomers, 4 parts of composite additives, 300 parts of organic solvents, and 0.8 parts of initiators.

[0053] The other embodiments are the same.

[0054] Example 4 The only difference between this embodiment and Example 1 is that the raw materials of the binder for lithium-ion batteries include, by mass, 60 parts of acrylonitrile, 25 parts of acrylate monomers, 10 parts of vinyl monomers, 2 parts of slightly branched cross-linking monomers, 4.5 parts of auxiliary monomers, 4 parts of composite additives, 300 parts of organic solvents, and 0.8 parts of initiators.

[0055] The other embodiments are the same.

[0056] Example 5 The only difference between this embodiment and Example 1 is that the raw materials of the binder for lithium-ion batteries include, by mass, 60 parts of acrylonitrile, 25 parts of acrylate monomers, 10 parts of vinyl monomers, 3 parts of slightly branched cross-linking monomers, 4.5 parts of auxiliary monomers, 4 parts of composite additives, 300 parts of organic solvents, and 0.8 parts of initiators.

[0057] The other embodiments are the same.

[0058] Example 6 The only difference between this embodiment and embodiment 1 is that the micro-branched cross-linking monomer is polyethylene glycol diacrylate, and the amount used is 3 parts by mass.

[0059] Polyethylene glycol diacrylate, PEG400DA, was obtained from Wuhan Huaxiang Kejie Biotechnology Co., Ltd., China.

[0060] The other embodiments are the same.

[0061] Comparative Example 1 The only difference between this comparative example and Example 1 is that the raw materials of the binder for lithium-ion batteries include, in parts by mass, 60 parts of acrylonitrile, 25 parts of acrylate monomers, 10 parts of vinyl monomers, 0.5 parts of slightly branched cross-linking monomers, 4.5 parts of auxiliary monomers, 4 parts of composite additives, 300 parts of organic solvents and 0.8 parts of initiators.

[0062] The other embodiments are the same.

[0063] Comparative Example 2 Comparative Example 2 is a PVDF adhesive purchased from Aladdin, product number P432380.

[0064] The other embodiments are the same.

[0065] Comparative Example 3 The only difference between this comparative example and Example 1 is that the raw materials of the binder for lithium-ion batteries include, in parts by mass, 60 parts of acrylonitrile, 25 parts of acrylate monomers, 10 parts of vinyl monomers, 1.5 parts of slightly branched cross-linking monomers, 1.2 parts of auxiliary monomers, 4 parts of composite additives, 300 parts of organic solvents and 0.8 parts of initiators.

[0066] The other embodiments are the same.

[0067] Comparative Example 4 The only difference between this comparative example and Example 1 is that the raw materials of the binder for lithium-ion batteries include, in parts by mass, 60 parts of acrylonitrile, 25 parts of acrylate monomers, 10 parts of vinyl monomers, 1.5 parts of slightly branched cross-linking monomers, 4.5 parts of auxiliary monomers, 0.5 parts of composite additives, 300 parts of organic solvents and 0.8 parts of initiators.

[0068] The other embodiments are the same.

[0069] Comparative Example 5 The only difference between this comparative example and Example 1 is that the auxiliary monomer is a combination of ethyl methacrylate sulfonate and vinyl phosphonate, with a mass ratio of 5:0.2.

[0070] The other embodiments are the same.

[0071] Comparative Example 6 The only difference between this comparative example and Example 1 is that the auxiliary monomer is a combination of ethyl methacrylate sulfonate and vinyl phosphonate, with a mass ratio of 1:2.

[0072] The other embodiments are the same.

[0073] Comparative Example 7 The only difference between this comparative example and Example 1 is that the composite auxiliary agent is a composition of triphenyl phosphate and perfluoropolyether, with a mass ratio of 9.5:0.5.

[0074] The other embodiments are the same.

[0075] Performance Testing Preparation of positive electrode sheet: The positive electrode active material NCM811, conductive carbon black, and the above-prepared positive electrode binder are prepared into a slurry in a mass ratio of 95:3:2, coated on an aluminum foil current collector, and then vacuum dried to prepare a positive electrode sheet.

[0076] Preparation of negative electrode sheet: Graphite, styrene-butadiene rubber and conductive carbon black are prepared into a slurry in a mass ratio of 94:3:3 and coated on a copper foil current collector, which is then vacuum dried to prepare a negative electrode sheet.

[0077] Preparation of lithium-ion battery: The positive electrode sheet, separator, negative electrode sheet and electrolyte prepared above are assembled into a lithium-ion button battery. The electrolyte is a mixture of ethylene carbonate, diethyl carbonate and ethyl methyl carbonate dissolved with 1 mol / L LiPF6. The volume ratio of ethylene carbonate: diethyl carbonate: ethyl methyl carbonate is 1:1:1.

[0078] 1. Electrolyte Tolerance: The adhesives prepared in the Examples and Comparative Examples were dissolved in NMP solvent to prepare a 5% solids solution. The solution was poured into a 5cm×5cm×1cm mold and dried at 80°C. An electrolyte solution was prepared with a volume ratio of ethylene carbonate: dimethyl carbonate: ethyl methyl carbonate = 1:1:1. The dried adhesive film was immersed in the electrolyte for 48 hours. The ratio of the difference in mass before and after soaking to the initial mass was calculated as the electrolyte swelling rate. The results of 10 tests were averaged and reported in Table 1.

[0079] 2. Peel strength: According to GB 2792-2014 "Test method for peel strength of adhesive tape", the peel force of the slurry on the positive electrode was tested using the 180° peel test method. The results were averaged from 10 tests and reported in Table 1.

[0080] 3. Flexibility: Using a cylindrical bending tester, cut the above-prepared electrode into 5cm rectangular strips, then wind them onto a metal cylinder with a diameter of 2cm and pull them at a constant speed of 180° to observe whether the electrode has any breakage or cracks. The quality of the electrode toughness can be expressed in order: good: no material falling, no cracks; general: slight material falling; poor: severe material falling. The test results are recorded in Table 1.

[0081] 4. Cycle performance: At 25°C, charge the lithium-ion battery at a constant current of 0.5C to a voltage of 4.2V, then charge at a constant voltage with a cut-off current of 0.05C, and discharge at a constant current of 0.5C to 3V. This is considered one cycle. After repeating 400 cycles, calculate the capacity retention rate (%) after 500 cycles = (discharge capacity after 400 cycles / initial charge capacity) × 100%. The results are the average of 10 tests and are reported in Table 1.

[0082] 5. Anti-coagulation: The adhesives prepared in the Examples and Comparative Examples were poured into a transparent glass bottle, allowed to stand, and the fluidity was observed by tilting the container. The gel slurry had difficulty flowing and exhibited a "jelly-like" appearance. Stir with a glass rod to observe whether there were obvious agglomerates or sudden changes in resistance, i.e., the presence of gel. The results are recorded in Table 1.

[0083] Table 1 Performance test results

[0084] Judging from the final performance test results of the embodiments and comparative examples, compared with comparative example 2 (commercially available PVDF binder), the binder product known in the embodiments of the present application has obvious performance advantages in terms of electrochemical properties and actual application performance, and can be used as a superior alternative product for this type of PVDF binder.

[0085] However, the implementation schemes of Comparative Example 1 and Comparative Examples 3 to 7 all adopt technical solutions different from the scope defined in this application compared to the examples, which results in the inability to maximize the combined effect of the raw materials such as the micro-branched cross-linking monomer and the auxiliary monomer in this application, thereby resulting in a decline in overall performance.

Claims

1. A binder for lithium-ion batteries, characterized in that: The raw materials include at least 40-75 parts of acrylonitrile, 15-40 parts of acrylic acid ester monomer, 250-400 parts of organic solvent and 0.2-1 part of initiator in parts by mass; The acrylic acid ester monomer is at least one of methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, isobutyl acrylate, isobornyl acrylate, amyl acrylate, isoamyl acrylate, isooctyl acrylate, 2-ethylhexyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, lauryl acrylate, lauryl methacrylate, and isodecyl acrylate.

2. The binder for lithium-ion batteries according to claim 1, wherein: The mass ratio of the acrylonitrile to the acrylic ester monomer is (50-70): (15-35).

3. The binder for lithium ion batteries according to claim 2, wherein: The organic solvent is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, ethyl acetate and dimethyl sulfoxide.

4. The binder for lithium ion batteries according to claim 3, wherein: The initiator is at least one of azobisisobutyronitrile, azobisisoheptanenitrile, dimethyl azobisisobutyrate, azobispentancyanate, dicumyl peroxide and benzoyl peroxide.

5. The binder for lithium-ion batteries according to claim 1, wherein: The raw materials of the binder for lithium-ion batteries further include, by weight, 5 to 20 parts of vinyl monomers, 1.5 to 5 parts of slightly branched cross-linking monomers, 3 to 8 parts of auxiliary monomers, and 2 to 6 parts of composite additives.

6. The binder for lithium-ion batteries according to claim 5, wherein: The mass ratio of the acrylonitrile, vinyl monomer, slightly branched cross-linking monomer and auxiliary monomer is (50-70): (7-20): (1.5-4): (4-6).

7. The binder for lithium-ion batteries according to claim 6, wherein: The auxiliary monomer is a composition of ethyl methacrylate sulfonate and vinyl phosphonate, with a mass ratio of (5-8): (2-4).

8. The binder for lithium-ion batteries according to claim 7, wherein: The composite auxiliary agent is a composition of triphenyl phosphate and perfluoropolyether, with a mass ratio of (3-5): (1-1.8).

9. The method for preparing a binder for lithium ion batteries according to any one of claims 5 to 8, wherein: The specific steps include: S1: Mix acrylonitrile, acrylate monomer, vinyl monomer, slightly branched cross-linking monomer and auxiliary monomer in an organic solvent and stir evenly in a reactor; S2: Add initiator and stir evenly, evacuate and introduce nitrogen, raise the temperature to 70-75°C, stir at 200-250 rpm and keep warm for 7-8 hours; S3: After the reaction is completed, naturally cool to 45-50°C, add composite auxiliary agent, stir at 150-200 rpm for 30-40 minutes, and after completion, naturally cool the product to room temperature, filter through 0.45-0.5 μm filter membrane to remove unreacted monomers and agglomerated particles, and obtain the product.

10. Use of the binder for lithium-ion batteries according to any one of claims 5 to 8 in high-nickel ternary positive electrode power batteries, high-voltage lithium metal batteries, energy storage batteries and flexible batteries.

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