A method for preparing a lithium-ion battery cathode slurry

Abstract

This invention provides a method for preparing a lithium-ion battery positive electrode slurry, belonging to the field of lithium-ion battery technology. The method includes the following steps: dissolving an auxiliary binder in a first solvent, adding a high-nickel positive electrode active material for a first mixing treatment to obtain a first mixture; adding a conductive agent to the first mixture for a second mixing treatment to obtain a second mixture; adding a main binder to the second mixture for a third mixing treatment to obtain the lithium-ion battery positive electrode slurry; wherein the auxiliary binder is a copolymer of trifluoroethyl methacrylate and 2-(methacryloyloxy)ethyl phosphate, and the main binder is a fluoropolymer binder. This invention can improve the dispersion stability and coating processing performance of the high-nickel positive electrode slurry.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a method for preparing a positive electrode slurry for lithium-ion batteries. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, long cycle life, and good environmental adaptability, have been widely used in consumer electronics, electric vehicles, and energy storage systems. As the demand for high energy density in power batteries continues to increase, high-nickel ternary cathode materials are gradually becoming an important development direction for lithium-ion battery cathode materials. These materials can provide higher specific capacity, which is beneficial for improving the overall energy density of the battery.

[0003] However, high-nickel cathode materials often contain residual alkaline substances such as LiOH and Li₂CO₃ on their surface. During cathode slurry preparation, these residual alkalis can easily affect the stability of the slurry system, especially when using fluoropolymer binders such as PVDF, leading to problems such as increased slurry viscosity, localized gelation, uneven dispersion, and decreased coating stability. Furthermore, existing cathode slurries are typically prepared by directly mixing the cathode active material, conductive agent, binder, and solvent. Traditional fluoropolymer binders mainly function as mechanical adhesives and film-forming agents, but their ability to regulate residual alkalis and active sites on the surface of high-nickel cathode materials is limited, making it difficult to effectively improve the processing stability of high-nickel cathode slurries. Therefore, improvements are needed. Summary of the Invention

[0004] In view of the above situation and to overcome the defects of the prior art, the purpose of this invention is to provide a method for preparing lithium-ion battery cathode slurry, so as to at least partially solve the problems mentioned in the background art.

[0005] The technical solution adopted in this invention is as follows: This invention proposes a method for preparing a positive electrode slurry for lithium-ion batteries, comprising the following steps: The auxiliary binder is dissolved in a first solvent, and a high-nickel positive electrode active material is added for a first mixing treatment to obtain a first mixture; A conductive agent is added to the first mixture, and a second mixing process is performed to obtain a second mixture; A main binder is added to the second mixture, and a third mixing treatment is performed to obtain a lithium-ion battery cathode slurry. The auxiliary binder is a copolymer of trifluoroethyl methacrylate and 2-(methacryloyloxy)ethyl phosphate, and the main binder is a fluoropolymer binder.

[0006] In some embodiments of the present invention, the preparation method of the auxiliary adhesive includes the following steps: The monomers trifluoroethyl methacrylate and 2-(methacryloyloxy)ethyl phosphate were mixed with a chain transfer agent, an initiator, and a second solvent, and the total molar number of monomers to the molar ratio of chain transfer agent were controlled to be (100-200):1 to obtain a mixed system. After deoxygenation treatment of the mixed system, polymerization is carried out at 65-75℃ for 12-20 hours. After the reaction is completed, the product is taken and vacuum dried at 50-70℃ to obtain the auxiliary binder.

[0007] In some embodiments of the present invention, the chain transfer agent is 2-cyano-2-propylbenzoic acid dithioate, the initiator is azobisisobutyronitrile, and the second solvent is selected from at least one of DMF and 1,4-dioxane.

[0008] In some embodiments of the present invention, the molar ratio of trifluoroethyl methacrylate to 2-(methacryloyloxy)ethyl phosphate is (5-10):1.

[0009] In some embodiments of the present invention, the number-average molecular weight of the auxiliary binder is 25,000-40,000 g / mol, and the molecular weight distribution coefficient is ≤1.25.

[0010] In some embodiments of the present invention, the mass percentage of the main binder is 80%-95% and the mass percentage of the auxiliary binder is 5%-20% based on the total mass of the auxiliary binder and the main binder in the positive electrode slurry.

[0011] In some embodiments of the present invention, in the lithium-ion battery cathode slurry, the high-nickel cathode active material accounts for 92%-97% of the total mass of solid components, the conductive agent accounts for 1.5%-4% of the total mass, and the auxiliary binder and the main binder account for 1.5%-3.5% of the total mass.

[0012] In some embodiments of the present invention, the rotation speed of the first mixing process is 500-1000 rpm and the time is 30-60 min.

[0013] In some embodiments of the present invention, the second mixing process is carried out at a rotation speed of 2000-3000 rpm for a time of 30-90 min; the third mixing process is carried out under vacuum conditions with stirring and mixing for a time of 60-180 min.

[0014] In some embodiments of the present invention, the high-nickel cathode active material is a layered oxide cathode material with a nickel content of not less than 80 mol%, and the high-nickel cathode active material includes LiNi. x Co y Mn z O2, LiNi x Coy Al z One or more of O2, wherein x≥0.80, y>0, z>0 and x+y+z=1.

[0015] The beneficial effects achieved by this invention are as follows: This invention uses a copolymer of trifluoroethyl methacrylate and 2-(methacryloyloxy)ethyl phosphate as a co-binder. The phosphate structural units are beneficial for acting on the surface of the high-nickel cathode active material, reducing the adverse effects of residual alkaline substances and active sites on the slurry stability. The fluorinated structural units improve the compatibility between the co-binder and the main fluorinated polymer binder, thereby mitigating problems such as increased slurry viscosity, localized gelation, and uneven dispersion.

[0016] Meanwhile, the present invention adopts a stepwise approach of first adding an auxiliary binder and mixing it with the high-nickel positive electrode active material, then adding a conductive agent and mixing it, and finally adding the main binder and mixing it. This allows the auxiliary binder to be preferentially distributed on the surface of the high-nickel positive electrode active material, preventing it from being diluted or embedded too early by the conductive agent or the main binder. It also facilitates the dispersion of the conductive agent and the formation of the main binder network, thereby improving the dispersion stability and coating processing performance of the positive electrode slurry. Detailed Implementation

[0017] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those familiar to those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to this invention. The preferred embodiments and materials described herein are for illustrative purposes only and do not limit the scope of this invention.

[0019] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0020] To address the problems mentioned in the background art, this invention provides a method for preparing a lithium-ion battery cathode slurry, comprising the following steps: The auxiliary binder is dissolved in a first solvent, and then a high-nickel cathode active material is added for a first mixing treatment to obtain a first mixture. The auxiliary binder is a copolymer of trifluoroethyl methacrylate (CAS No.: 352-87-4) and 2-(methacryloyloxy)ethyl phosphate (CAS No.: 52628-03-2). The trifluoroethyl methacrylate structural unit has fluorinated segments, which is beneficial for improving the compatibility between the auxiliary binder and the subsequently added fluoropolymer binder; the 2-(methacryloyloxy)ethyl phosphate structural unit contains phosphate groups, which is beneficial for its action on the surface of the high-nickel cathode active material.

[0021] The first solvent can be an organic solvent capable of dissolving the auxiliary binder and suitable for the positive electrode slurry system. Preferably, the first solvent is N-methylpyrrolidone. By dissolving the auxiliary binder in the first solvent first, the auxiliary binder can have a better dispersion state when it comes into contact with the high-nickel positive electrode active material, avoiding the auxiliary binder from directly entering the slurry system in the form of solid agglomerates, thereby affecting its effect on the surface of the high-nickel positive electrode active material.

[0022] Then, a high-nickel cathode active material is added to the solution containing the auxiliary binder, and a first mixing treatment is performed to obtain a first mixture. The high-nickel cathode active material can be a layered oxide cathode material with a high nickel content. Since the surface of high-nickel cathode active materials usually contains residual alkaline substances such as LiOH and Li2CO3, and has certain transition metal active sites, it is easy to affect the stability of the slurry system during conventional pulping. In the early stage of pulping, the auxiliary binder comes into contact with the high-nickel cathode active material, so that the phosphate ester structural units in the auxiliary binder can preferentially act on the surface of the high-nickel cathode active material. Through this first mixing treatment, the auxiliary binder can form a pre-distributed or pre-acting state on the surface of the high-nickel cathode active material, thereby obtaining the first mixture. Compared with the method of mixing the cathode active material, conductive agent, main binder and auxiliary binder at one time, the present invention allows the auxiliary binder to contact the high-nickel cathode active material before the conductive agent and main binder are added, which is beneficial to improve the effective action of the auxiliary binder on the surface of the high-nickel cathode active material and avoid it being diluted or buried by other solid components or the main binder system too early.

[0023] A conductive agent is added to the first mixture, followed by a second mixing process to obtain a second mixture. The conductive agent can be one or more of the electronically conductive materials commonly used in positive electrode slurries, such as conductive carbon black, Super P, carbon nanotubes, and conductive graphite. Since the surface of the high-nickel positive electrode active material in the first mixture has already undergone pre-contact with the binder, the conductive agent can disperse around the high-nickel positive electrode active material treated with the binder during the second mixing process, thereby facilitating the formation of a more uniform conductive distribution structure. In this step, the role of the second mixing process is not only to disperse the conductive agent in the slurry system but also to ensure a more stable distribution of the conductive agent around the high-nickel positive electrode active material where the binder has already acted. This reduces the risk of localized agglomeration or uneven distribution of the conductive agent and improves the uniformity of the subsequent positive electrode slurry.

[0024] A main binder is added to the second mixture, followed by a third mixing process to obtain a lithium-ion battery cathode slurry. The main binder is a fluoropolymer binder, such as polyvinylidene fluoride or other fluoropolymer binders suitable for cathode slurry systems. The main binder can be added in solid, solution, or gel form, preferably in gel form pre-dissolved in an organic solvent to facilitate uniform dispersion in the slurry system.

[0025] In this invention, the main binder is added after the conductive agent has been added and dispersed, so that it mainly undertakes the overall bonding and film-forming functions in the slurry system. Since the auxiliary binder has already preferentially acted on the surface of the high-nickel cathode active material in the aforementioned steps, and the conductive agent has already completed its initial dispersion in the second mixing treatment, the third mixing treatment enables the main binder to further connect the high-nickel cathode active material, the conductive agent, and other solid components in the slurry system, forming a stable bonding network.

[0026] Therefore, the pulping method of the present invention forms a stepwise mixing process that differs from conventional one-time mixing pulping: first, the high-nickel cathode active material is pretreated with an auxiliary binder; then, a conductive agent is added to form a conductive dispersion structure; and finally, a main binder is added to form an overall bonding network. This stepwise mixing method allows the interfacial effects of the auxiliary binder, the dispersing effect of the conductive agent, and the binding effect of the main binder to complement each other in terms of time sequence and functional division, thereby improving the dispersion stability and coating processing performance of the high-nickel cathode slurry.

[0027] In the lithium-ion battery cathode slurry prepared by the above method, the high-nickel cathode active material, conductive agent, auxiliary binder, and main binder can form a relatively reasonable distribution structure. Specifically, the auxiliary binder preferentially acts on the surface of the high-nickel cathode active material, the conductive agent is dispersed around the high-nickel cathode active material after treatment with the auxiliary binder, and the main binder forms a continuous or semi-continuous bonding network in the slurry system. This structure helps to mitigate the adverse effects of residual alkaline substances on the surface of the high-nickel cathode active material on the stability of the slurry system and improves the stability of the slurry during storage, transportation, and coating processes.

[0028] In some embodiments, the preparation method of the auxiliary adhesive includes the following steps: The monomers trifluoroethyl methacrylate and 2-(methacryloyloxy)ethyl phosphate were mixed with a chain transfer agent, an initiator, and a second solvent, and the molar ratio of the total monomer moles to the chain transfer agent was controlled at (100-200):1 to obtain a mixed system. By controlling the above molar ratio, it is beneficial to adjust the molecular weight of the resulting binder, so that it has good solubility, dispersibility, and ability to interact with the surface of high-nickel cathode active materials.

[0029] After deoxygenation treatment of the mixed system, polymerization is carried out at 65-75℃ for 12-20 hours. After the reaction is completed, the product is taken and vacuum dried at 50-70℃ to obtain the auxiliary binder. Deoxygenation treatment can reduce the influence of oxygen on the polymerization reaction. Deoxygenation treatment can be carried out by inert gas bubbling, vacuuming and then filling with inert gas, or freeze-thaw cycle.

[0030] In some embodiments, the chain transfer agent is 2-cyano-2-propylbenzoic acid dithioate (CAS No.: 201611-85-0), the initiator is azobisisobutyronitrile, and the second solvent is selected from at least one of DMF (N,N-dimethylformamide) and 1,4-dioxane. Using the above-mentioned chain transfer agent and initiator facilitates the controlled polymerization of trifluoroethyl methacrylate with 2-(methacryloyloxy)ethyl phosphate, thereby obtaining a binder with a suitable molecular weight and narrow molecular weight distribution.

[0031] In some embodiments, the molar ratio of trifluoroethyl methacrylate to 2-(methacryloyloxy)ethyl phosphate is (5-10):1. Within this ratio range, 2-(methacryloyloxy)ethyl phosphate can provide a certain number of phosphate ester structural units to enhance the effect of the auxiliary binder on the surface of the high-nickel cathode active material; trifluoroethyl methacrylate can provide fluorine-containing structural units to improve the compatibility between the auxiliary binder and the main binder.

[0032] In some embodiments, the number-average molecular weight of the auxiliary binder is 25,000-40,000 g / mol, and the molecular weight distribution coefficient is ≤1.25. If the molecular weight of the auxiliary binder is too low, its retention capacity on the surface of the high-nickel cathode active material and its film-forming assistance may be insufficient; if the molecular weight of the auxiliary binder is too high, it may affect its solubility in the first solvent and its dispersion uniformity in the slurry system. Controlling the number-average molecular weight and molecular weight distribution coefficient within the above range is beneficial for balancing the solubility of the auxiliary binder, its interfacial interaction ability, and the stability of the slurry processing.

[0033] In some embodiments, based on the total mass of the auxiliary binder and the main binder in the cathode slurry, the main binder accounts for 80%-95% of the mass, and the auxiliary binder accounts for 5%-20% of the mass. The auxiliary binder primarily improves the compatibility with the surface of the high-nickel cathode active material, while the main binder primarily provides overall adhesion and film formation. By controlling the ratio of the two, the effect of the auxiliary binder on the surface of the high-nickel cathode active material can be improved without significantly weakening the film-forming and adhesive effects of the main binder.

[0034] In some embodiments, the main binder is a fluoropolymer binder. The main binder may be added in solid form, or it may be dissolved in an organic solvent to form a main binder solution before being added to the second mixture. Preferably, the main binder is added in solution form to facilitate its uniform dispersion during the third mixing process and to form a stable adhesive system.

[0035] In some embodiments, in the lithium-ion battery cathode slurry, the high-nickel cathode active material accounts for 92%-97% of the total solid components by mass, the conductive agent accounts for 1.5%-4% by mass, and the auxiliary binder and main binder account for 1.5%-3.5% by mass. This ratio balances the content of the high-nickel cathode active material, the conductive network construction of the conductive agent, and the bonding strength of the binder system, thereby meeting the preparation requirements of high-energy-density lithium-ion battery cathode slurry.

[0036] In some embodiments, the rotation speed of the first mixing treatment is 500-1000 rpm, and the time is 30-60 min. The first mixing treatment is mainly used to ensure sufficient contact between the auxiliary binder and the high-nickel cathode active material, so that the auxiliary binder preferentially acts on the surface of the high-nickel cathode active material. By using the above-mentioned rotation speed and time, sufficient contact can be ensured while avoiding adverse effects on the high-nickel cathode active material particles due to excessive shear strength.

[0037] In some embodiments, the second mixing process is carried out at a rotation speed of 2000-3000 rpm for 30-90 minutes. The second mixing process is mainly used to uniformly disperse the conductive agent in the first mixture. Since conductive agents typically have small particle sizes and large specific surface areas, they are prone to agglomeration. Therefore, using a relatively high rotation speed for dispersion is beneficial to improving the uniformity of the conductive agent's distribution in the slurry system.

[0038] In some embodiments, the third mixing process is performed under vacuum conditions with stirring for 60-180 minutes. This third mixing process primarily aims to ensure thorough mixing of the main binder with the second mixture, forming a stable adhesive system. Performing the third mixing process under vacuum conditions helps reduce air bubbles in the slurry, improving slurry uniformity and subsequent coating stability.

[0039] In some embodiments, the high-nickel cathode active material is a layered oxide cathode material with a nickel content of not less than 80 mol%, and the high-nickel cathode active material includes LiNi. x Co y Mn z O2, LiNi x Co y Al z One or more of O2, wherein x ≥ 0.80, y > 0, z > 0 and x + y + z = 1. The above-mentioned high-nickel cathode active material has a high specific capacity and is suitable for high-energy-density lithium-ion battery cathode slurry systems.

[0040] The present invention will be described below through specific embodiments. It should be noted that these embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0041] Example 1: Trifluoroethyl methacrylate, 2-(methacryloyloxy)ethyl phosphate, 2-cyano-2-propylbenzyl dithioate, azobisisobutyronitrile and DMF were added to a reaction vessel and stirred to form a homogeneous mixture.

[0042] The molar ratio of trifluoroethyl methacrylate to 2-(methacryloyloxy)ethyl phosphate is 7.3:1, and the molar ratio of the total number of monomers to 2-cyano-2-propylbenzoic acid dithioate is 150:1.

[0043] After the mixture was subjected to nitrogen bubbling deoxygenation treatment, the reaction vessel was sealed and polymerized at 70°C for 16 hours. After the reaction was completed, the product was collected, precipitated, washed, and then vacuum dried at 60°C for 18 hours to obtain the auxiliary binder.

[0044] The obtained auxiliary binder was characterized by molecular weight using gel permeation chromatography, with polystyrene as the standard sample and DMF as the mobile phase. The number-average molecular weight was determined to be approximately 3.02 × 10⁻⁶. 4 g / mol, with a molecular weight distribution coefficient of 1.20.

[0045] Dissolve 0.25 parts of the auxiliary binder in N-methylpyrrolidone to obtain an auxiliary binder solution.

[0046] 95.0 parts of high-nickel cathode active material, wherein the high-nickel cathode active material is LiNi, are added to the binder solution. 0.8 Co 0.1 Mn 0.1 O2. Then, a first mixing process is performed at a speed of 800 rpm for 45 minutes to obtain a first mixture.

[0047] A conductive agent comprising 2.0 parts SuperP and 0.5 parts carbon nanotubes was added to the first mixture. A second mixing process was then performed at 2500 rpm for 60 minutes to obtain a second mixture.

[0048] 2.25 parts of polyvinylidene fluoride were dissolved in N-methylpyrrolidone to obtain a main binder solution. The main binder solution was added to the second mixture, and a third mixing treatment was performed under vacuum for 120 minutes to obtain a lithium-ion battery cathode slurry.

[0049] Example 2: Trifluoroethyl methacrylate, 2-(methacryloyloxy)ethyl phosphate, 2-cyano-2-propylbenzoic acid dithioate, azobisisobutyronitrile and DMF were mixed to obtain a mixed system.

[0050] The molar ratio of trifluoroethyl methacrylate to 2-(methacryloyloxy)ethyl phosphate is 5.7:1, and the molar ratio of the total number of monomers to 2-cyano-2-propylbenzoic acid dithioate is 120:1.

[0051] After deoxygenation treatment of the mixture, polymerization was carried out at 68°C for 18 hours. After the reaction was complete, the product was collected and vacuum dried at 60°C to obtain the auxiliary binder. The number average molecular weight of the obtained auxiliary binder was approximately 3.47 × 10⁻⁶. 4 g / mol, with a molecular weight distribution coefficient of 1.22.

[0052] Dissolve 0.45 parts of the binder in N-methylpyrrolidone, and add 94.5 parts of LiNi 0.8 Co 0.1 Mn 0.1 O2 was subjected to a first mixing process at a speed of 700 rpm for 50 minutes to obtain a first mixture.

[0053] 2.0 parts Super P and 0.5 parts carbon nanotubes were added to the first mixture, and a second mixing treatment was carried out at a speed of 2600 rpm for 70 min to obtain the second mixture.

[0054] 2.55 parts of polyvinylidene fluoride were dissolved in N-methylpyrrolidone and then added to the second mixture. A third mixing treatment was carried out under vacuum for 150 minutes to obtain the lithium-ion battery cathode slurry.

[0055] In this embodiment, the total mass ratio of the auxiliary adhesive and the main adhesive is 3.0 parts, of which the main adhesive accounts for 85% and the auxiliary adhesive accounts for 15%.

[0056] Example 3: Trifluoroethyl methacrylate, 2-(methacryloyloxy)ethyl phosphate, 2-cyano-2-propylbenzoic acid dithioate, azobisisobutyronitrile and 1,4-dioxane were mixed to obtain a mixed system.

[0057] The molar ratio of trifluoroethyl methacrylate to 2-(methacryloyloxy)ethyl phosphate is 9:1, and the molar ratio of the total number of monomers to 2-cyano-2-propylbenzoic acid dithioate is 180:1.

[0058] After deoxygenation treatment of the mixture, polymerization was carried out at 72°C for 14 hours. After the reaction was complete, the product was collected and vacuum dried at 55°C to obtain the auxiliary binder. The number average molecular weight of the obtained auxiliary binder was approximately 2.83 × 10⁻⁶. 4 g / mol, with a molecular weight distribution coefficient of 1.18.

[0059] Dissolve 0.12 parts of the binder in N-methylpyrrolidone, and add 96.0 parts of LiNi 0.8 Co 0.1 Mn 0.1 O2 was subjected to a first mixing process at a speed of 900 rpm for 30 minutes to obtain a first mixture.

[0060] 1.5 parts Super P and 0.5 parts carbon nanotubes were added to the first mixture, and a second mixing treatment was carried out at a speed of 2200 rpm for 45 min to obtain the second mixture.

[0061] 1.88 parts of polyvinylidene fluoride were dissolved in N-methylpyrrolidone and then added to the second mixture. A third mixing treatment was carried out under vacuum for 90 minutes to obtain a lithium-ion battery cathode slurry.

[0062] In this embodiment, the total mass ratio of the auxiliary adhesive and the main adhesive is 2.0 parts, of which the main adhesive accounts for 94% and the auxiliary adhesive accounts for 6%.

[0063] Comparative Example 1 This comparative example provides a method for preparing a lithium-ion battery cathode slurry. Compared with Example 1, the difference is that no auxiliary binder is added, and the amount of the main binder is adjusted to 2.5 parts.

[0064] Specifically, 95.0 parts of LiNi 0.8 Co 0.1 Mn 0.1 O2 was added to N-methylpyrrolidone and mixed at 800 rpm for 45 min to obtain the first mixture; 2.0 parts of Super P and 0.5 parts of carbon nanotubes were added to the first mixture and mixed at 2500 rpm for 60 min to obtain the second mixture; a solution containing 2.5 parts of polyvinylidene fluoride was then added to the second mixture and mixed under vacuum for 120 min to obtain the lithium-ion battery cathode slurry.

[0065] Comparative Example 2 This comparative example provides a method for preparing a lithium-ion battery cathode slurry. Compared with Example 1, the difference is that the amount of each component is the same, but instead of using the stepwise mixing method in Example 1, a one-time mixing method is used to prepare the slurry.

[0066] Specifically, 95.0 parts of LiNi 0.8 Co 0.1 Mn 0.1 O2, 2.0 parts Super P, 0.5 parts carbon nanotubes, 0.25 parts binder and 2.25 parts polyvinylidene fluoride were simultaneously added to N-methylpyrrolidone. The mixture was first mixed at 800 rpm for 45 min, then at 2500 rpm for 60 min, and finally mixed under vacuum for 120 min to obtain the lithium-ion battery cathode slurry.

[0067] Comparative Example 3 This comparative example provides a method for preparing a lithium-ion battery cathode slurry. Compared with Example 1, the difference is that the amount of each component is the same, but the order of addition of the binder and the conductive agent is changed.

[0068] Specifically, 95.0 parts of LiNi 0.8 Co 0.1 Mn 0.1 O2 was added to N-methylpyrrolidone, followed by 2.0 parts of SuperP and 0.5 parts of carbon nanotubes, and mixed at 2500 rpm for 60 min. Then, 0.25 parts of auxiliary binder were added, and mixed at 800 rpm for 45 min. Finally, a solution containing 2.25 parts of polyvinylidene fluoride was added, and the mixture was mixed under vacuum for 120 min to obtain the lithium-ion battery cathode slurry.

[0069] To verify the beneficial effects of the present invention, performance tests were conducted on the products obtained in Examples 1-3 and Comparative Examples 1-3.

[0070] Test method: 1. Slurry viscosity stability test The lithium-ion battery cathode slurries prepared in Examples 1-3 and Comparative Examples 1-3 were adjusted to have the same solid content. After preparation, the slurries were allowed to stand at 25°C for 30 minutes, and the initial viscosity was measured using a rotational viscometer. Subsequently, the slurries were sealed and placed in a 25°C environment for 24 hours, and the viscosity was measured again under the same conditions. The viscosity growth rate was calculated using the following formula: Viscosity growth rate = (24-hour viscosity - initial viscosity) / initial viscosity × 100%.

[0071] 2. Observation of coating condition The lithium-ion battery positive electrode slurries prepared in Examples 1-3 and Comparative Examples 1-3 were respectively coated onto the surface of aluminum foil current collectors. After coating, they were pre-dried at 80°C and then vacuum-dried at 120°C to obtain positive electrode sheets.

[0072] Observe the surface of the coating layer for defects such as obvious streaks, particle agglomeration, pinholes, local gel marks, or uneven coating.

[0073] 3. Peel strength test The dried positive electrode sheet was cut into strips with a width of 25 mm. The peel strength between the positive electrode active material layer and the aluminum foil current collector was tested using a 180° peel test. The test speed was 50 mm / min, and 5 parallel samples were tested for each group of samples, and the average value was taken.

[0074] 4. Electrochemical Cyclic Performance Test The positive electrode slurries obtained in Examples 1-3 and Comparative Examples 1-3 were coated, dried, and rolled to obtain positive electrode sheets. A coin cell was assembled using a lithium metal sheet as the counter electrode.

[0075] The test voltage range is 2.8-4.3V. After activation at 0.1C, the battery is cycled 100 times at 1C rate, and the capacity retention rate is recorded on the 100th cycle.

[0076] Test results: Table 1. Results of slurry viscosity stability test

[0077] As shown in Table 1, the viscosity increases of the cathode slurries prepared in Examples 1-3 after standing for 24 hours were 10.7%, 8.5%, and 15.1%, respectively, all significantly lower than the 87.9% of Comparative Example 1. Furthermore, the coatings in Examples 1-3 were uniform, without obvious streaks, particle agglomeration, or gel marks. This indicates that using the copolymer of trifluoroethyl methacrylate and 2-(methacryloyloxy)ethyl phosphate as a co-binder can mitigate the adverse effects of residual alkaline substances and active sites on the surface of the high-nickel cathode active material on the stability of the slurry, thereby improving problems such as slurry thickening, localized gelation, and uneven coating.

[0078] Table 2. Coating condition, peel strength, and electrochemical cycling test results

[0079] As shown in Table 2, compared with Comparative Example 1, the peel strength of Examples 1-3 increased to 0.62 N·cm. -1 0.66 N·cm -1 and 0.57 N·cm -1 The comparative example 1 was only 0.42 N·cm. -1 This indicates that the addition of the auxiliary binder not only improves the stability of the slurry, but also enhances the bonding stability between the high-nickel cathode active material, the conductive agent, and the main binder, thereby improving the adhesion performance between the cathode active material layer and the current collector.

[0080] Further comparison of Example 1 with Comparative Examples 2 and 3 shows that, with the same amount of each solid component, Example 1, which involves first mixing the auxiliary binder with the high-nickel cathode active material, then adding the conductive agent for a second mixing process, and finally adding the main binder for a third mixing process, exhibits a 24-hour viscosity increase of 10.7%, significantly lower than the 40.4% of Comparative Example 2 and 51.8% of Comparative Example 3. Simultaneously, Example 1 shows a more uniform coating, and its peel strength and 100-cycle capacity retention are both higher than those of Comparative Examples 2 and 3. This demonstrates that the stepwise mixing method of the present invention allows the auxiliary binder to preferentially act on the surface of the high-nickel cathode active material, preventing premature dilution and embedding by the conductive agent or main binder in the event of a single mixing or improper addition sequence, thereby improving the effective utilization rate of the auxiliary binder.

[0081] The electrochemical cycling results show that the capacity retention rates of Examples 1-3 after 100 cycles were 92.8%, 93.6%, and 91.5%, respectively, all higher than those of Comparative Example 1 (86.4%), Comparative Example 2 (89.4%), and Comparative Example 3 (88.7%). This indicates that the positive electrode slurry prepared in this invention, when used to prepare the positive electrode sheet, facilitates the formation of a more stable synergistic structure of positive electrode active material, conductive agent, and binder, thereby improving the stability of the positive electrode sheet during cycling.

[0082] In summary, this invention, by using a copolymer of trifluoroethyl methacrylate and 2-(methacryloyloxy)ethyl phosphate as an auxiliary binder and employing a stepwise mixing method of "first mixing the auxiliary binder with the high-nickel cathode active material, then adding the conductive agent, and finally adding the main binder," can effectively improve the viscosity stability, coating uniformity, electrode adhesion performance, and cycle stability of the high-nickel cathode slurry.

[0083] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0084] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the protection scope of the present invention.

Claims

1. A method for preparing a lithium-ion battery positive electrode slurry, characterized in that, Includes the following steps: The auxiliary binder is dissolved in a first solvent, and a high-nickel positive electrode active material is added for a first mixing treatment to obtain a first mixture; A conductive agent is added to the first mixture, and a second mixing process is performed to obtain a second mixture; A main binder is added to the second mixture, and a third mixing treatment is performed to obtain a lithium-ion battery cathode slurry. The auxiliary binder is a copolymer of trifluoroethyl methacrylate and 2-(methacryloyloxy)ethyl phosphate, and the main binder is a fluoropolymer binder.

2. The method according to claim 1, characterized in that, The preparation method of the auxiliary adhesive includes the following steps: The monomers trifluoroethyl methacrylate and 2-(methacryloyloxy)ethyl phosphate were mixed with a chain transfer agent, an initiator, and a second solvent, and the total molar number of monomers to the molar ratio of chain transfer agent were controlled to be (100-200):1 to obtain a mixed system. After deoxygenation treatment of the mixed system, polymerization is carried out at 65-75℃ for 12-20 hours. After the reaction is completed, the product is taken and vacuum dried at 50-70℃ to obtain the auxiliary binder.

3. The method according to claim 2, characterized in that, The chain transfer agent is 2-cyano-2-propylbenzoic acid dithioester, the initiator is azobisisobutyronitrile, and the second solvent is selected from at least one of DMF and 1,4-dioxane.

4. The method according to claim 2, characterized in that, The molar ratio of trifluoroethyl methacrylate to 2-(methacryloyloxy)ethyl phosphate is (5-10):

1.

5. The method according to claim 1, characterized in that, The number-average molecular weight of the auxiliary binder is 25,000-40,000 g / mol, and the molecular weight distribution coefficient is ≤1.

25.

6. The method according to claim 1, characterized in that, Based on the total mass of the auxiliary binder and the main binder in the positive electrode slurry, the main binder accounts for 80%-95% of the mass, and the auxiliary binder accounts for 5%-20% of the mass.

7. The method according to claim 1, characterized in that, In the lithium-ion battery cathode slurry, based on the total mass of solid components, the high-nickel cathode active material accounts for 92%-97% of the mass, the conductive agent accounts for 1.5%-4% of the mass, and the auxiliary binder and the main binder account for 1.5%-3.5% of the total mass.

8. The method according to claim 1, characterized in that, The first mixing process is carried out at a speed of 500-1000 rpm for 30-60 min.

9. The method according to claim 1, characterized in that, The second mixing process is carried out at a speed of 2000-3000 rpm for 30-90 min; the third mixing process is carried out under vacuum conditions with stirring for 60-180 min.

10. The method according to claim 1, characterized in that, The high-nickel cathode active material is a layered oxide cathode material with a nickel content of not less than 80 mol%, and the high-nickel cathode active material includes LiNi. x Co y Mn z O2, LiNi x Co y Al z One or more of O2, wherein x≥0.80, y>0, z>0 and x+y+z=1.

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