Composite material, method for preparing the same and negative electrode sheet

Abstract

The application relates to the technical field of batteries, and discloses a composite material, a preparation method thereof and a negative electrode sheet, the preparation method of the composite material provided by the application realizes the chemical bonding of rigid polyacrylic acid and flexible styrene-butadiene rubber on a molecular scale by in-situ polymerization of the polyacrylic acid and the styrene-butadiene rubber, and solves the problems of poor physical blending compatibility and phase separation of the two; and the polyacrylic acid-styrene-butadiene rubber graft copolymer is grafted to the surface of activated carbon nanotubes through a chemical bond to form a ternary grafting composite material, which can realize the efficient and stable dispersion of a conductive agent in a negative electrode slurry, and can also synergistically improve the adhesion and flexibility of the electrode sheet, is applied to a battery, and the battery has the characteristics of high initial coulomb efficiency, good cycle stability and excellent rate performance, and the preparation efficiency and electrochemical performance of the battery are comprehensively improved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a composite material, its preparation method, and a negative electrode sheet. Background Technology

[0002] As the mainstream energy storage device, lithium-ion batteries' electrode fabrication process directly affects their electrochemical performance and manufacturing cost. In the negative electrode fabrication process, active materials (such as silicon), conductive agents (carbon nanotubes), binders, and dispersants are typically mixed in a solvent to form a uniform slurry, which is then coated onto the current collector.

[0003] However, the existing technology presents the following technical challenges: The dispersibility problem of carbon nanotubes: Due to their large specific surface area and strong van der Waals forces, carbon nanotubes are prone to aggregation. Simultaneously, their highly hydrophobic surface makes them difficult to disperse uniformly in aqueous slurry systems. Existing technologies often require the addition of large amounts of dispersants or prolonged, energy-intensive dispersion treatments, which not only increases costs but also results in poor dispersion stability, leading to sedimentation after settling and affecting coating consistency and electrode quality.

[0004] Limitations of binder performance: Polyacrylic acid (PAA), as a water-based binder, possesses high bonding strength and abundant carboxyl functional groups, but its molecular chain rigidity is relatively high, leading to high electrode brittleness and poor flexibility. This results in problems such as material shedding and cracking during coating, rolling, and cell winding. While styrene-butadiene rubber (SBR) binders offer good flexibility, they are themselves hydrophobic latexes, exhibiting poor compatibility with hydrophilic PAA and silicone materials, making it difficult to achieve both high adhesion and high flexibility in a single system.

[0005] The volume expansion problem of silicon materials: Silicon-based anode materials experience significant volume expansion during charging and discharging, which places higher demands on the adhesion, flexibility, and stability of the conductive network of the binder. A single conductive agent or binder is insufficient to simultaneously construct an electrode structure with both excellent electron transport and mechanical buffering capabilities.

[0006] Therefore, developing a composite functional material that can achieve efficient and stable dispersion of conductive agents and synergistically improve the adhesion and flexibility of the electrode is of great significance for improving the electrochemical performance of silicon-based anode electrodes. Summary of the Invention

[0007] This invention provides a method for preparing a composite material to solve the problem of poor dispersion of conductive agent in existing negative electrode sheets, which cannot simultaneously achieve high adhesion and high flexibility.

[0008] In a first aspect, the present invention provides a method for preparing a composite material, comprising the following steps: (1) In the presence of an initiator, polyacrylic acid and styrene-butadiene rubber undergo a first polymerization reaction to obtain a polyacrylic acid-styrene-butadiene rubber graft copolymer; (2) Activate the carbon nanotubes to obtain modified carbon nanotubes; (3) In the presence of a catalyst, the polyacrylic acid-styrene-butadiene rubber graft copolymer and the modified carbon nanotubes undergo a second polymerization reaction to obtain the composite material; The mass ratio of the polyacrylic acid to the styrene-butadiene rubber is 2-4:1; The mass ratio of the modified carbon nanotubes to the polyacrylic acid-styrene-butadiene rubber graft copolymer is 1:5-20; The catalyst is selected from at least one of dicyclohexylcarbodiimide, 4-dimethylaminopyridine, 1-ethyl-(3-dimethylaminopropyl)carbodiimide, N-hydroxysuccinimide, N,N'-carbonyldiimidazole, p-toluenesulfonic acid, and sulfuric acid.

[0009] In one optional embodiment, the mass ratio of the polyacrylic acid to the styrene-butadiene rubber is 2.5-3.5:1.

[0010] In one alternative embodiment, the mass ratio of the modified carbon nanotubes to the polyacrylic acid-styrene-butadiene rubber graft copolymer is 1:12-18.

[0011] In one alternative embodiment, the initiator is present in a mass of 0.5 wt% to 2 wt% of the polyacrylic acid.

[0012] In one alternative embodiment, the mass of the catalyst is 0.1wt%-1wt% of the mass of the polyacrylic acid-styrene-butadiene rubber graft copolymer.

[0013] In one alternative embodiment, the carbon nanotubes are selected from at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0014] In one alternative embodiment, the initiator is selected from at least one of benzoyl peroxide and azobisisobutyronitrile.

[0015] In one alternative embodiment, the temperature of the first polymerization reaction is 70°C-100°C.

[0016] In one alternative embodiment, the first polymerization reaction takes 6-10 hours.

[0017] In one alternative embodiment, the temperature of the second polymerization reaction is 70°C-90°C.

[0018] In one alternative embodiment, the second polymerization reaction takes 4-8 hours.

[0019] In one alternative embodiment, the activation process includes reacting carbon nanotubes with an oxidant.

[0020] In one alternative embodiment, the mass molar ratio of the carbon nanotubes to the oxidant is 1:(0.1-0.5) g / mol.

[0021] In one alternative embodiment, the oxidant is selected from at least one of nitric acid, sulfuric acid, hydrogen peroxide, ammonium persulfate, potassium persulfate, and potassium permanganate.

[0022] In one alternative embodiment, the reaction temperature is 60°C-80°C.

[0023] In one alternative implementation, the reaction time is 1-3 hours.

[0024] In one alternative embodiment, the first polymerization reaction further includes the addition of a first solvent.

[0025] In one alternative embodiment, the first solvent is selected from at least one of toluene and water.

[0026] In one alternative embodiment, the second polymerization reaction further includes the addition of a second solvent.

[0027] In one alternative embodiment, the second solvent is selected from N,N-dimethylformamide.

[0028] In a second aspect, the present invention provides a composite material prepared by the preparation method described in the first aspect.

[0029] Thirdly, the present invention provides an application of the composite material described in the second aspect in a negative electrode sheet.

[0030] Fourthly, the present invention provides a silicon-based negative electrode sheet, comprising a current collector and a negative electrode slurry coated on at least one side of the current collector, wherein the negative electrode slurry comprises a conductive agent, a binder and a negative electrode material, and the negative electrode material comprises the composite material described in the second aspect.

[0031] Fifthly, the present invention provides an application of the silicon-based negative electrode sheet described in the fourth aspect in a ternary lithium-ion battery.

[0032] The technical solution of this invention has the following advantages: 1. The method for preparing the composite material provided by this invention first involves in-situ polymerization of polyacrylic acid and styrene-butadiene rubber (SBR) to achieve chemical bonding at the molecular scale between rigid polyacrylic acid and flexible SBR, solving the problems of poor compatibility and phase separation in physical blending. Then, the polyacrylic acid-SBR graft copolymer is chemically grafted onto the surface of activated carbon nanotubes to form a ternary grafted composite material. This material provides strong adhesion (the rigidity of polyacrylic acid), flexibility (the elastic buffer of SBR), and a uniform conductive network (the conductivity of carbon nanotubes). In negative electrode slurries, it enables efficient and stable dispersion of the conductive agent and synergistically enhances the adhesion and flexibility of the electrode. When applied to batteries, the batteries exhibit high initial coulombic efficiency, good cycle stability, and excellent rate performance, comprehensively improving the battery's preparation efficiency and electrochemical performance. Detailed Implementation

[0033] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0034] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0035] To address the aforementioned problems, in a first aspect, the present invention provides a method for preparing a composite material, comprising the following steps: (1) In the presence of an initiator, polyacrylic acid and styrene-butadiene rubber undergo a first polymerization reaction to obtain a polyacrylic acid-styrene-butadiene rubber graft copolymer; (2) Activate the carbon nanotubes to obtain modified carbon nanotubes; (3) In the presence of a catalyst, the polyacrylic acid-styrene-butadiene rubber graft copolymer and the modified carbon nanotubes undergo a second polymerization reaction to obtain the composite material; The mass ratio of the polyacrylic acid to the styrene-butadiene rubber is 2-4:1; The mass ratio of the modified carbon nanotubes to the polyacrylic acid-styrene-butadiene rubber graft copolymer is 1:5-20; The catalyst is selected from at least one of dicyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP), 1-ethyl-(3-dimethylaminopropyl)carbodiimide (EDC), N-hydroxysuccinimide (NHS), N,N'-carbonyldiimidazole (CDI), p-toluenesulfonic acid (TsOH), and sulfuric acid.

[0036] It should be noted that the mass ratio of modified carbon nanotubes to polyacrylic acid-styrene-butadiene rubber graft copolymer is controlled within the range of 1:5-20 in this invention. If the proportion of carbon nanotubes is too low, the reinforcing effect is not obvious, and if it is too high, it is prone to agglomeration.

[0037] It should be noted that the mass ratio of polyacrylic acid to styrene-butadiene rubber is 2-4:1. A higher polyacrylic acid content is beneficial for providing carboxyl groups. The addition of styrene-butadiene rubber can improve flexibility. By controlling the mass ratio of polyacrylic acid to styrene-butadiene rubber within the range of 2-4:1, chemical bonding at the molecular scale between rigid polyacrylic acid and flexible styrene-butadiene rubber can be achieved, overcoming the problem of poor compatibility when the two are physically blended.

[0038] In this invention, the carboxyl groups on the polyacrylic acid molecular chain first react chemically with the unsaturated double bonds or functional groups on the styrene-butadiene rubber molecular chain to form a block or graft copolymer with polyacrylic acid as the main chain and styrene-butadiene rubber as the branch chain. This achieves chemical bonding at the molecular scale between rigid polyacrylic acid and flexible styrene-butadiene rubber, overcoming the problem of poor compatibility when the two are physically blended. Furthermore, after grafting, the polyacrylic acid content increases significantly, allowing it to form stronger hydrogen bonds or chemical bonds with the hydroxyl groups on the surface of the silicon material and the current collector during the drying process, thus improving the electrode peel strength. The introduction of styrene-butadiene rubber endows the grafted material with good elasticity and flexibility, effectively alleviating the electrode brittleness caused by pure polyacrylic acid binder. The prepared negative electrode does not crack or shed powder during bending tests, and can better adapt to the volume changes of the silicon material during charging and discharging, maintaining the integrity of the electrode structure. Experimental data show that the electrode peel strength using this material is 30%-50% higher than that of traditional physical mixing systems.

[0039] By activating carbon nanotubes: single-walled carbon nanotubes are activated, and oxidizing active species preferentially attack the ends and defect sites of the carbon nanotubes, breaking the carbon-carbon double bonds. First, hydroxyl and carbonyl groups are generated on the surface of the carbon nanotubes. Under continuous oxidation, some oxygen-containing intermediates are further converted into carboxyl groups, thereby covalently introducing oxygen-containing active functional groups such as hydroxyl and carboxyl groups onto the surface of single-walled carbon nanotubes.

[0040] Polymerization of modified carbon nanotubes with polyacrylic acid-styrene-butadiene rubber graft copolymer: The unreacted carboxyl and hydroxyl functional groups in the modified carbon nanotubes undergo esterification or amidation reactions with the polyacrylic acid-styrene-butadiene rubber graft copolymer, allowing the polyacrylic acid-styrene-butadiene rubber graft copolymer to be firmly grafted onto the surface of the carbon nanotubes through chemical bonds, ultimately obtaining a ternary grafted carbon nanotube modified composite material, which greatly improves the hydrophilicity of carbon nanotubes; effectively solves the problems of easy agglomeration and difficult dispersion of traditional carbon nanotubes in aqueous slurries, reduces the energy consumption and time cost of slurry preparation, improves the yield of coating process, ensures that the conductive agent is uniformly and firmly distributed in the electrode, and the grafted material can achieve long-term (e.g., standing for more than 30 days) stable dispersion in water without sedimentation; not only does it improve the overall conductivity of the negative electrode sheet, but it also ensures the stability of the conductive network during long-term cycling.

[0041] Compared to the physical blending of conductive agents and binders, this composite material avoids the freeing and secondary agglomeration of the conductive agent in the slurry, ensuring a uniform and robust distribution of the conductive agent within the electrode. This not only improves the overall conductivity of the electrode but also guarantees the stability of the conductive network during long-term cycling.

[0042] As an example, the mass ratio of the polyacrylic acid to the styrene-butadiene rubber can be 2:1, 2.2:1, 2.4:1, 2.6:1, 2.8:1, 3:1, 3.2:1, 3.4:1, 3.6:1, 3.8:1, 4:1, or any range of any two of the above ratios.

[0043] As an example, the mass ratio of the modified carbon nanotubes to the polyacrylic acid-styrene-butadiene rubber graft copolymer can be 1:5, 1:7, 1:9, 1:11, 1:13, 1:15, 1:17, 1:19, 1:20, or any range of two of the above ratios.

[0044] Furthermore, the catalyst is selected from dicyclohexylcarbodiimide and 4-dimethylaminopyridine, or 1-ethyl-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide. Therefore, the carbon nanotubes exhibit the highest grafting efficiency, uniform grafting, and are easily purified.

[0045] Further, the molar ratio of the dicyclohexylcarbodiimide to the 4-dimethylaminopyridine is 1:1-2. As an example, the molar ratio of the dicyclohexylcarbodiimide to the 4-dimethylaminopyridine can be 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2, etc., or a range consisting of any two of the above ratios.

[0046] In one optional embodiment, the mass ratio of the polyacrylic acid to the styrene-butadiene rubber is 2.5-3.5:1. As an example, the mass ratio of the polyacrylic acid to the styrene-butadiene rubber can be 2.5:1, 2.6:1, 2.7:1, 2.8:1, 2.9:1, 3:1, 3.1:1, 3.2:1, 3.3:1, 3.4:1, 3.5:1, etc., or a range consisting of any two of the above ratios.

[0047] In one optional embodiment, the mass ratio of the modified carbon nanotubes to the polyacrylic acid-styrene-butadiene rubber graft copolymer is 1:12-18. As an example, the mass ratio of the modified carbon nanotubes to the polyacrylic acid-styrene-butadiene rubber graft copolymer can be 1:12, 1:12.5, 1:13, 1:13.5, 1:14, 1:14.5, 1:15, 1:15.5, 1:16, 1:16.5, 1:17, 1:17.5, 1:18, etc., or a range consisting of any two of the above ratios.

[0048] In one optional embodiment, the initiator is present in a mass of 0.5 wt% to 2 wt% of the polyacrylic acid. As an example, the mass of the initiator may be 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2 wt% of the polyacrylic acid, or a range consisting of any two of the above proportions.

[0049] In one optional embodiment, the mass of the catalyst is 0.1 wt% to 1 wt% of the mass of the polyacrylic acid-styrene-butadiene rubber graft copolymer. As an example, the mass of the catalyst can be 0.1, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt% of the mass of the polyacrylic acid-styrene-butadiene rubber graft copolymer, or a range consisting of any two of the above proportions.

[0050] It should be noted that controlling the mass of the catalyst to be 0.1wt%-1wt% of the mass of the polyacrylic acid-styrene-butadiene rubber graft copolymer can promote the grafting reaction efficiency.

[0051] In one optional embodiment, the carbon nanotubes are selected from at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes. Preferably, the carbon nanotubes include single-walled carbon nanotubes.

[0052] In one optional embodiment, the initiator is selected from at least one of benzoyl peroxide and azobisisobutyronitrile. Preferably, the initiator is selected from benzoyl peroxide. Benzoyl peroxide has good affinity for the double bonds of styrene-butadiene rubber and is compatible with polyacrylic acid. An appropriate initiator content can effectively control the degree of grafting and avoid excessive crosslinking.

[0053] In one alternative embodiment, the temperature of the first polymerization reaction is 70°C-100°C. As an example, the temperature of the first polymerization reaction can be 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, 100°C, etc., or a range consisting of any two of the above proportions.

[0054] In one optional embodiment, the first polymerization reaction takes 6-10 hours. As an example, the first polymerization reaction can take 6 hours, 6.5 hours, 7 hours, 7.5 hours, 8 hours, 8.5 hours, 9 hours, 9.5 hours, 10 hours, or a range consisting of any two of the above proportions.

[0055] In one alternative embodiment, the temperature of the second polymerization reaction is 70°C-90°C. As an example, the temperature of the second polymerization reaction can be 70°C, 75°C, 80°C, 85°C, 90°C, etc., or a range consisting of any two of the above proportions.

[0056] In one optional embodiment, the second polymerization reaction takes 4-8 hours. As an example, the temperature of the first polymerization reaction can be 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours, 8 hours, or a range consisting of any two of the above proportions.

[0057] It should be noted that controlling the time and temperature of the second polymerization reaction within the above range can ensure that the reaction is complete and that thermal degradation does not occur.

[0058] In one alternative embodiment, the oxidation treatment includes reacting carbon nanotubes with an oxidant.

[0059] It should be noted that by reacting carbon nanotubes with oxidants, active groups such as hydroxyl (-OH) and carboxyl (-COOH) groups can be introduced onto the surface of carbon nanotubes, which can provide chemical bonding sites for the second polymerization.

[0060] In one optional embodiment, the molar ratio of the carbon nanotubes to the oxidant is 1:(0.1-0.5) g / mol. As an example, the molar ratio of the carbon nanotubes to the oxidant can be 1:0.1 g / mol, 1:0.15 g / mol, 1:0.2 g / mol, 1:0.25 g / mol, 1:0.3 g / mol, 1:0.35 g / mol, 1:0.4 g / mol, 1:0.45 g / mol, 1:0.5 g / mol, etc., or a range consisting of any two of the above ratios.

[0061] In one alternative embodiment, the oxidant is selected from at least one of nitric acid, sulfuric acid, hydrogen peroxide, ammonium persulfate, potassium persulfate, and potassium permanganate.

[0062] Furthermore, the oxidant is selected from at least one of nitric acid and sulfuric acid, thereby obtaining more chemical bonding sites.

[0063] Further, the concentration of the nitric acid is 0.5 mol / L to 3 mol / L. As an example, the concentration of the nitric acid can be 0.5 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, etc., or a range consisting of any two of the above proportions.

[0064] Further, the concentration of the sulfuric acid is 1 mol / L to 5 mol / L. As an example, the concentration of the sulfuric acid can be 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, 4.5 mol / L, 5 mol / L, etc., or a range consisting of any two of the above proportions.

[0065] In one optional embodiment, the reaction temperature is 60°C-80°C. As an example, the reaction temperature is 60°C, 62°C, 64°C, 66°C, 68°C, 70°C, 72°C, 74°C, 76°C, 78°C, 80°C, or a range consisting of any two of the above proportions.

[0066] In one alternative embodiment, the reaction time is 1-3 hours. As an example, the reaction time is 1 hour, 1.2 hours, 1.4 hours, 1.6 hours, 1.8 hours, 2 hours, 2.2 hours, 2.4 hours, 2.6 hours, 2.8 hours, 3 hours, or a range consisting of any two of the above proportions.

[0067] In one alternative embodiment, the first polymerization reaction further includes the addition of a first solvent.

[0068] In one alternative embodiment, the first solvent is selected from at least one of toluene and water. Preferably, the first solvent comprises toluene and water in a volume ratio of 3-6:1.

[0069] In one alternative embodiment, the second polymerization reaction further includes the addition of a second solvent.

[0070] In one alternative embodiment, the second solvent is selected from N,N-dimethylformamide.

[0071] In a second aspect, the present invention provides a composite material prepared by the preparation method described in the first aspect.

[0072] Thirdly, the present invention provides an application of the composite material described in the second aspect in a negative electrode sheet.

[0073] Fourthly, the present invention provides a silicon-based negative electrode sheet, comprising a current collector and a negative electrode slurry coated on at least one side of the current collector, wherein the negative electrode slurry comprises a conductive agent, a binder, and a negative electrode material; the negative electrode material is the composite material described in the second aspect.

[0074] In this invention, due to the improved dispersibility of the negative electrode slurry and the excellent mechanical properties of the electrode sheet, the coating speed can be effectively increased and the drying energy consumption reduced. When applied to silicon-based negative electrode batteries, it exhibits the characteristics of high initial coulombic efficiency, good cycle stability, and excellent rate performance, thus comprehensively improving the battery preparation efficiency and electrochemical performance.

[0075] Fifthly, the present invention provides an application of the silicon-based negative electrode sheet described in the fourth aspect in a ternary lithium-ion battery.

[0076] It should be noted that lithium-ion batteries include positive electrode, negative electrode, electrolyte, and separator.

[0077] Furthermore, the positive electrode sheet includes a positive electrode material, which includes nickel-cobalt-manganese ternary positive electrode material (NCM).

[0078] Furthermore, the electrolyte includes a lithium salt, a solvent, and a film-forming agent.

[0079] Furthermore, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, and lithium tetrafluoroborate.

[0080] Furthermore, the solvent is selected from at least one of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate.

[0081] Furthermore, the film-forming agent is selected from at least one of vinylene carbonate and fluoroethylene carbonate.

[0082] The polyacrylic acid used in this invention is a commercially available product with a weight-average molecular weight of 2000-5000; The styrene-butadiene rubber used is commercially available SBR-1502 type; The single-walled carbon nanotubes used were commercially available products with a diameter of 1nm-2nm, a length of 5-30μm, and a purity of ≥90%. The maleic anhydride used is a commercially available product with a purity of ≥99%; The natural rubber used is a commercially available product, such as SCR WF (full latex).

[0083] Example 1 This embodiment provides a method for preparing a composite material, including the following steps: (1) Add 30g polyacrylic acid, 10g styrene-butadiene rubber and 0.3g benzoyl peroxide to 200mL toluene and 50mL deionized water, polymerize at 80℃ for 8h, filter and dry to obtain polyacrylic acid-styrene-butadiene rubber graft copolymer. (2) Place 1g of single-walled carbon nanotubes in 0.1L of mixed acid solution (3mol / L sulfuric acid and 1mol / L nitric acid), sonicate for 30min, then perform surface activation treatment at 70℃ for 1h, wash with deionized water until pH>5, and dry to obtain activated single-walled carbon nanotubes. (3) 1g of activated single-walled carbon nanotubes, 15g of polyacrylic acid-styrene-butadiene rubber graft copolymer and 0.075g of catalyst (the molar ratio of dicyclohexylcarbodiimide and 4-dimethylaminopyridine is 1.5:1) were dispersed in 200mL of anhydrous DMF, reacted at 80℃ for 6h, filtered, washed with acetone for 24h, and dried to obtain the composite material.

[0084] Comparative Example 1 This comparative example provides a method for preparing a composite material, which is basically the same as the steps in Example 1, except that in step (1), the mass of styrene-butadiene rubber is 20g.

[0085] Comparative Example 2 This comparative example provides a method for preparing a composite material, which is basically the same as the steps in Example 1, except that in step (1), the mass of styrene-butadiene rubber is 5g.

[0086] Comparative Example 3 This comparative example provides a method for preparing a composite material, which is basically the same as the steps in Example 1, except that in step (1), polyacrylic acid is replaced with the same mass of maleic anhydride.

[0087] Comparative Example 4 This comparative example provides a method for preparing a composite material, which is basically the same as the steps in Example 1, except that in step (1), styrene-butadiene rubber is replaced with the same mass of natural rubber.

[0088] Comparative Example 5 This comparative example provides a method for preparing a composite material, which is basically the same as the steps in Example 1, except that in step (3), the mass of the polyacrylic acid-styrene-butadiene rubber graft copolymer is 3g.

[0089] Comparative Example 6 This comparative example provides a method for preparing a composite material, which differs from Example 1 in that it is changed to a step-by-step grafting method, including the following steps: (1) Activated single-walled carbon nanotubes were prepared using the same method as in Example 1; (2) 1g of activated single-walled carbon nanotubes were dispersed in 200mL of anhydrous DMF, 30g of polyacrylic acid and 0.075g of catalyst (molar ratio of dicyclohexylcarbodiimide and 4-dimethylaminopyridine 1.5:1) were added, and the reaction was carried out at 80℃ for 3h. After filtration, washing and drying, polyacrylic acid grafted carbon nanotubes were obtained. (3) Polyacrylic acid grafted carbon nanotubes were dispersed in 200 mL toluene and 50 mL deionized water, 10 g styrene-butadiene rubber and 0.3 g benzoyl peroxide were added, and the mixture was reacted at 80 °C for 8 h. After washing with acetone for 24 h, the composite material was obtained by drying.

[0090] Comparative Example 7 This comparative example provides a method for preparing a composite material, which is basically the same as the steps in Example 1, except that in step (3), the catalyst is replaced with the same mass of toluenesulfonic acid.

[0091] Negative electrode slurry and negative electrode sheet The composite material, silicon carbon material, and natural graphite prepared in Example 1 and Comparative Examples 1-7 were mixed with deionized water in a mass ratio of 1:2:8 to prepare a negative electrode slurry. The negative electrode slurry was then processed into a negative electrode sheet through processes such as coating, drying, and rolling.

[0092] The negative electrode slurries prepared in Example 1 and Comparative Examples 1-7 were subjected to slurry dispersibility tests; the negative electrode sheets were subjected to sheet peel force and softness tests, and the results are shown in Table 1; specifically: Dispersibility: Tested using a scraper fineness meter (0-50μm), the smaller the value, the better the dispersibility.

[0093] Peel force: A universal tensile testing machine was used, and the 180° peel method was followed (refer to GB / T 2790). The sample width was 20 mm, the peel speed was 50 mm / min, and 5 samples were tested in each group. The average value was taken, and the result was expressed as N / m.

[0094] Softness: A softness tester was used with a slit width of 10 mm and a sample size of 70 mm × 70 mm. Five samples were tested and the average value was taken. The softness was evaluated by the maximum bending force (mN).

[0095] Table 1. Performance test results of negative electrode slurry and negative electrode sheet prepared in each example and control group.

[0096] As can be seen from Table 1, the negative electrode sheets prepared in the embodiments of the present invention have good dispersibility, electrode sheet peeling force, and softness.

[0097] In Comparative Example 1, the amount of styrene-butadiene rubber added was too large, resulting in an excessively high proportion of the rubber phase in the graft copolymer, a significant decrease in rigidity, and a deterioration in the mechanical properties of the composite material.

[0098] In Comparative Example 2, there was too little styrene-butadiene rubber, resulting in insufficient rubber phase content, extremely poor toughening effect, significantly increased brittleness, and poor softness of the material.

[0099] In Comparative Example 3, polyacrylic acid was replaced with maleic anhydride. Because maleic anhydride could not be effectively grafted and copolymerized with styrene-butadiene rubber, the reaction system was severely phase-separated, and the dispersion ability of the resulting copolymer for carbon nanotubes was greatly reduced.

[0100] In Comparative Example 4, styrene-butadiene rubber was replaced with natural rubber. Natural rubber has excessively high molecular chain flexibility and low polarity, resulting in poor compatibility with polyacrylic acid and insufficient grafting efficiency. This led to a significant reduction in the thermal stability and interfacial bonding strength of the composite material.

[0101] In Comparative Example 5, the amount of graft copolymer was too low, which could not effectively coat the carbon nanotubes, resulting in severe agglomeration of the carbon nanotubes and a sharp decline in the performance of the composite material.

[0102] Comparative Example 6 showed that the two-step grafting resulted in a large amount of carboxyl active sites being consumed in the first step, and the grafting rate of styrene-butadiene rubber was extremely low in the second step. In addition, the intermediate products were severely lost through filtration, resulting in poor uniformity of carbon nanotube dispersion in the composite material.

[0103] Comparative Example 7 used toluenesulfonic acid as a catalyst. Toluenesulfonic acid has low catalytic efficiency in this system and is prone to inducing ester bond hydrolysis side reactions, which significantly reduces the grafting rate and results in poor reinforcement effect of the composite material.

[0104] Battery assembly and testing The positive electrode active material (NCM811), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 8:1:1. N-methylpyrrolidone (NMP) was added and stirred into a uniform slurry. The slurry was coated onto the surface of aluminum foil, dried under vacuum at 120°C for 12 hours, rolled, and cut to obtain the positive electrode sheet.

[0105] The negative electrode, separator, and positive electrode prepared in Example 1 and Comparative Examples 1-7 were stacked alternately in a Z-shape and injected with an electrolyte (lithium hexafluorophosphate (LiPF6) dissolved in a mixed solvent of ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) (volume ratio 1:1:1) to prepare an electrolyte with a concentration of 1.0 mol / L, and 2 wt% of vinylene carbonate was added as a film-forming additive), and finally a stacked soft-pack battery was prepared.

[0106] First coulombic efficiency: Using the Xinwei CT-4008-5V20A equipment, constant current charge-discharge cycles were performed in a constant temperature chamber at 25±2℃ with a current density of 0.5C / 0.5C. The discharge capacity and charge capacity of each cycle were recorded, and the coulombic efficiency was calculated as discharge capacity / charge capacity × 100%. The average value of ≥5 parallel samples was taken for each test group.

[0107] Cycle count: Using the Xinwei CT-4008-5V20A equipment, in a constant temperature chamber at 25±2℃, perform a cycle test according to the standard charge and discharge steps (charging: 1C constant current and constant voltage to the upper limit voltage, cutoff current 0.05C; stand for 10min; discharging: 1C constant current to the lower limit voltage; stand for 10min), and record the number of cycles until the capacity decays to 80% of the initial capacity.

[0108] Table 2. Cycle performance test results of batteries prepared in each embodiment and comparative example.

[0109] As can be seen from Table 2, the lithium-ion battery prepared in Example 1 of the present invention has a high initial coulombic efficiency and cycle life; the slurry of Comparative Examples 1-7 has poor dispersibility and cannot simultaneously take into account the peeling force and softness of the negative electrode sheet, resulting in poor initial coulombic efficiency and cycle performance of the battery.

[0110] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a composite material, characterized in that, Includes the following steps: (1) In the presence of an initiator, polyacrylic acid and styrene-butadiene rubber undergo a first polymerization reaction to obtain a polyacrylic acid-styrene-butadiene rubber graft copolymer; (2) Activate the carbon nanotubes to obtain modified carbon nanotubes; (3) In the presence of a catalyst, the polyacrylic acid-styrene-butadiene rubber graft copolymer and the modified carbon nanotubes undergo a second polymerization reaction to obtain the composite material; The mass ratio of the polyacrylic acid to the styrene-butadiene rubber is 2-4:1; The mass ratio of the modified carbon nanotubes to the polyacrylic acid-styrene-butadiene rubber graft copolymer is 1:5-20; The catalyst is selected from at least one of dicyclohexylcarbodiimide, 4-dimethylaminopyridine, 1-ethyl-(3-dimethylaminopropyl)carbodiimide, N-hydroxysuccinimide, N,N'-carbonyldiimidazole, p-toluenesulfonic acid, and sulfuric acid.

2. The method for preparing the composite material according to claim 1, characterized in that, The mass ratio of the polyacrylic acid to the styrene-butadiene rubber is 2.5-3.5:

1.

3. The method for preparing the composite material according to claim 1, characterized in that, The mass ratio of the modified carbon nanotubes to the polyacrylic acid-styrene-butadiene rubber graft copolymer is 1:12-18.

4. The method for preparing the composite material according to claim 1, characterized in that, The initiator has a mass of 0.5wt%-2wt% of the polyacrylic acid. And / or, the mass of the catalyst is 0.1wt%-1wt% of the mass of the polyacrylic acid-styrene-butadiene rubber graft copolymer.

5. The method for preparing the composite material according to claim 1 or 2, characterized in that, The carbon nanotubes are selected from at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes; And / or, the initiator is selected from at least one of benzoyl peroxide and azobisisobutyronitrile.

6. The method for preparing the composite material according to claim 1, characterized in that, The temperature of the first polymerization reaction is 70℃-100℃; And / or, the time for the first polymerization reaction is 6h-10h; And / or, the temperature of the second polymerization reaction is 70°C-90°C; And / or, the second polymerization reaction takes 4-8 hours.

7. The method for preparing the composite material according to claim 1, characterized in that, The activation process includes reacting carbon nanotubes with an oxidant.

8. The method for preparing the composite material according to claim 7, characterized in that, The mass molar ratio of the carbon nanotubes to the oxidant is 1:(0.1-0.5) g / mol; And / or, the oxidant is selected from at least one of nitric acid, sulfuric acid, hydrogen peroxide, ammonium persulfate, potassium persulfate, and potassium permanganate; And / or, the temperature of the reaction is 60°C-80°C; And / or, the reaction time is 1h-3h.

9. The method for preparing the composite material according to claim 1, characterized in that, The first polymerization reaction also includes the addition of a first solvent; Preferably, the first solvent is selected from at least one of toluene and water; And / or, in the second polymerization reaction, a second solvent is also added; Preferably, the second solvent is selected from N,N-dimethylformamide.

10. A composite material prepared by the preparation method according to any one of claims 1-9.

11. The application of the composite material of claim 10 in the negative electrode sheet.

12. A silicon-based negative electrode sheet, comprising a current collector and a negative electrode paste coated on at least one surface of the current collector, said negative electrode paste comprising a conductive agent, a binder, and a negative electrode material, characterized in that, The negative electrode material includes the composite material described in claim 10.

13. The application of the silicon-based negative electrode sheet according to claim 12 in a ternary lithium-ion battery.

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