Flexible aqueous binder and preparation method thereof, negative electrode slurry, negative electrode pole piece and lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, and particularly discloses a flexible water-based binder and a preparation method thereof, negative electrode slurry, a negative electrode pole piece and a lithium ion battery, the flexible water-based binder has the advantages of high binding strength, low swelling rate in electrolyte, good graphite dispersibility, capability of supplementing lithium for a negative electrode, and the like; compared with a traditional binder SBR and polyacrylic acid and polyacrylate water-based binders, the water-based adhesive has better performance, can completely replace the traditional binder, and overcomes the limitation of the traditional binder. When the flexible water-based adhesive is applied to the lithium ion battery, the flexibility and stripping force of a pole piece and the first effect of the battery can be improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a flexible aqueous binder and its preparation method, a negative electrode slurry, a negative electrode sheet, and a lithium-ion battery. Background Technology

[0002] With the continuous development of lithium-ion battery technology, anode binders play a crucial role in improving battery performance. Anode slurries typically use water as a solvent. While existing binders such as styrene-butadiene rubber (SBR) are widely used in graphite anode systems, their poor water solubility leads to limitations in their point-to-point bonding method, such as insufficient bond strength, easy delamination, and limited contribution to lithium-ion transport. Waterborne binders based on polyacrylic acid or polyacrylates, although possessing good water solubility, suffer from insufficient bond strength, high swelling rates in electrolytes, and poor dispersion in graphite. These issues, coupled with the increasing demand for high-energy-density and high-performance batteries, are gradually becoming bottlenecks restricting battery development. Summary of the Invention

[0003] This invention aims to at least partially solve one of the technical problems in the related art. To this end, one object of this invention is to provide a flexible aqueous binder that has the advantages of high bonding strength, low swelling rate in electrolyte, good dispersibility in graphite, and the ability to replenish lithium to the negative electrode. Applying this binder to lithium-ion batteries can improve the flexibility, peel strength, and initial efficiency of the electrode.

[0004] Specifically, the first aspect of the present invention provides a flexible waterborne adhesive comprising the following raw materials in parts by weight: 20-50 parts by weight of a hydrophilic monomer, 34-75 parts by weight of a flexible monomer, 5-15 parts by weight of a functional monomer, 0.1-0.7 parts by weight of a crosslinking agent, 0.1-0.5 parts by weight of an emulsifier, 0.02-0.4 parts by weight of an initiator, and an acid-base neutralizer; The hydrophilic monomer includes unsaturated carboxylic acids; The flexible monomer includes C1-C 18 Alkyl acrylates, C1-C 18 At least one of alkyl methacrylates; The functional monomers include C1-C 22 Alkyl polyethylene glycol acrylate, C1-C 22 At least one of alkyl polyethylene glycol methacrylates; The acid-base neutralizing agent includes lithium hydroxide.

[0005] This invention uses hydrophilic monomers, flexible monomers, functional monomers, and lithium hydroxide neutralizers to prepare flexible waterborne adhesives. The hydrophilic monomers, flexible monomers, and lithium hydroxide neutralizers impart hydrophilicity, flexibility, and lithium replenishment to the adhesives. The functional monomers contain a polyethylene glycol hydrophilic segment and C1-C... 22 The alkyl lipophilic segment, being an amphoteric molecule, can increase the dispersibility of negative electrode active materials such as graphite in aqueous slurries, thereby increasing the peeling force of the electrode and the initial efficiency of the battery; additionally, C1-C 22 The alkyl hydrophilic segment, with its relatively long alkyl chain, can increase bond strength and electrode peel strength. Furthermore, the functional monomers of this invention can reduce the swelling rate of the binder, maintaining the stability of the electrode structure. In summary, the binder of this invention combines high bond strength, low swelling rate in electrolyte, good graphite dispersibility, and the ability to replenish lithium to the negative electrode. Compared to traditional binders such as SBR and aqueous binders based on polyacrylic acid and polyacrylate, it exhibits superior performance and can completely replace traditional binders, overcoming their limitations. This invention, through the innovative design and application of the binder, provides a novel solution for the high-performance development of lithium-ion batteries, meeting the future demands for high energy density and high-performance batteries.

[0006] According to some embodiments of the present invention, the functional unit includes C 12 -C 22 Alkyl polyethylene glycol acrylate, C 12 -C 22 At least one of alkyl polyethylene glycol methacrylate; preferably, the functional monomer includes at least one of octadecyl polyethylene glycol acrylate and octadecyl polyethylene glycol methacrylate.

[0007] According to some embodiments of the present invention, the amount of the functional monomer is 8-12 parts by weight.

[0008] According to some embodiments of the present invention, the C1-C 18 Alkyl acrylates include one or more of methyl acrylate, ethyl acrylate, n-butyl acrylate, n-pentyl acrylate, isoamyl acrylate, isooctyl acrylate, hydroxyethyl acrylate, and hydroxypropyl acrylate; the C1-C 18 Alkyl methacrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, n-pentyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate, octyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate.

[0009] According to some embodiments of the present invention, the flexible monomer includes at least one of C4-C8 alkyl acrylate and C4-C8 alkyl methacrylate; the C4-C8 alkyl acrylate includes one or more of n-butyl acrylate, n-amyl acrylate, isoamyl acrylate, and isooctyl acrylate; the C4-C8 alkyl methacrylate includes one or more of n-butyl methacrylate, n-amyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate, and octyl methacrylate.

[0010] According to some embodiments of the present invention, the unsaturated carboxylic acid includes one or more of acrylic acid, methacrylic acid, itaconic acid, fumaric acid, crotonic acid, aconitic acid, and maleic acid, preferably acrylic acid; the pH of the flexible water-based adhesive is 6-8.

[0011] A second aspect of the present invention provides a method for preparing the flexible waterborne adhesive described in the first aspect of the present invention, comprising the following steps: Mix 20-50 parts by weight of hydrophilic monomer, 34-75 parts by weight of flexible monomer, 5-15 parts by weight of functional monomer, 0.1-0.7 parts by weight of crosslinking agent, 0.1-0.5 parts by weight of emulsifier, 0.02-0.4 parts by weight of initiator and water, react at 45℃-80℃, then add lithium hydroxide for neutralization and adjust the pH to 6-8.

[0012] The preparation method of this invention is simple, and the prepared flexible aqueous binder has the advantages of high bonding strength, low swelling rate in electrolyte, good dispersibility in graphite, and the ability to replenish lithium for the negative electrode. It provides a brand-new solution for the high-performance development of lithium-ion batteries and meets the future demand for high energy density and high-performance batteries.

[0013] A third aspect of the present invention provides a negative electrode slurry comprising a negative electrode active material, a conductive agent, lithium polyacrylate, the flexible waterborne binder described in the first aspect of the present invention, and water.

[0014] Because it employs the flexible water-based binder of the first aspect of this invention, the negative electrode slurry of this invention possesses all the advantages of the flexible water-based binder, which will not be elaborated further here.

[0015] Lithium polyacrylate is a thickener with lithium-supplementing properties developed in recent years, which can replace sodium carboxymethyl cellulose (CMC). However, lithium polyacrylate is brittle, leading to reduced flexibility of the negative electrode sheet. Combining the flexible aqueous binder of this invention with lithium polyacrylate can overcome the problem of electrode brittleness. At the same time, the combination with lithium polyacrylate significantly increases the lithium content of the negative electrode of lithium-ion batteries, which can replenish lithium ions to the negative electrode, ultimately improving the first-efficiency and rate performance of lithium-ion batteries.

[0016] According to some embodiments of the present invention, the mass ratio of the flexible waterborne adhesive to the lithium polyacrylate is 1:(0.2-0.4).

[0017] According to some embodiments of the present invention, the mass ratio of the negative electrode active material, the conductive agent, the lithium polyacrylate, and the flexible water-based binder is (95-97):1.0:(0.3-1):(1.5-3).

[0018] A fourth aspect of the present invention provides a negative electrode sheet, comprising a negative electrode active material, a conductive agent, lithium polyacrylate, and the flexible waterborne binder described in the first aspect of the present invention.

[0019] Because it employs the flexible water-based binder of the first aspect of this invention, the negative electrode sheet of this invention possesses all the advantages of the flexible water-based binder, which will not be elaborated further here.

[0020] The fifth aspect of the present invention provides a lithium-ion battery, including the negative electrode sheet described in the fourth aspect of the present invention.

[0021] Because it employs the flexible aqueous binder of the first aspect of this invention, the lithium-ion battery of this invention possesses all the advantages of the flexible aqueous binder, which will not be elaborated further here.

[0022] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Detailed Implementation

[0023] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0024] 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.

[0025] In this document, the terms "comprising" or "including" are open-ended expressions, meaning they include the contents specified in this invention but do not exclude other aspects. Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0026] With the continuous development of lithium-ion battery technology, anode binders play a crucial role in improving battery performance. Anode slurries typically use water as a solvent. While existing binders such as styrene-butadiene rubber (SBR) are widely used in graphite anode systems, their poor water solubility leads to limitations in their point-to-point bonding method, such as insufficient bond strength, easy delamination, and limited contribution to lithium-ion transport. Waterborne binders based on polyacrylic acid or polyacrylates, although possessing good water solubility, suffer from insufficient bond strength, high swelling rates in electrolytes, and poor dispersion in graphite. These issues, coupled with the increasing demand for high-energy-density and high-performance batteries, are gradually becoming bottlenecks restricting battery development.

[0027] To address the above problems, this invention proposes a flexible waterborne adhesive, which is prepared using a hydrophilic monomer, a flexible monomer, a functional monomer, and a lithium hydroxide neutralizer. The hydrophilic monomer, flexible monomer, and lithium hydroxide neutralizer impart hydrophilicity, flexibility, and lithium replenishment to the adhesive. The functional monomer contains a polyethylene glycol hydrophilic segment and C1-C124 ... 22 The alkyl lipophilic segment, being an amphoteric molecule, can increase the dispersibility of graphite in aqueous slurries, thereby increasing the peeling force of the electrode and the initial efficiency of the battery; additionally, C1-C 22 The alkyl lipophilic segment has a relatively long alkyl chain, which can increase the bonding strength and electrode peeling force. Furthermore, the functional monomers of this invention can reduce the swelling rate of the binder and maintain the stability of the electrode structure. In summary, the binder of this invention has advantages such as high bonding strength, low swelling rate in the electrolyte, good dispersibility in graphite, and the ability to supplement lithium to the negative electrode. Compared with traditional binders SBR and waterborne binders of polyacrylic acid and polyacrylate, it has superior performance and can completely replace traditional binders.

[0028] Specifically, the first aspect of the present invention provides a flexible waterborne adhesive comprising the following raw materials in parts by weight: 20-50 parts by weight of a hydrophilic monomer, 34-75 parts by weight of a flexible monomer, 5-15 parts by weight of a functional monomer, 0.1-0.7 parts by weight of a crosslinking agent, 0.1-0.5 parts by weight of an emulsifier, 0.02-0.4 parts by weight of an initiator, and an acid-base neutralizer; The hydrophilic monomer includes unsaturated carboxylic acids; The flexible monomer includes C1-C 18 Alkyl acrylates, C1-C 18 At least one of alkyl methacrylates; The functional monomers include C1-C 22 Alkyl polyethylene glycol acrylate, C1-C 22 At least one of alkyl polyethylene glycol methacrylates; The acid-base neutralizing agent includes lithium hydroxide.

[0029] In some embodiments, the amount of the hydrophilic monomer used is 20 parts by weight, 25 parts by weight, 30 parts by weight, 35 parts by weight, 40 parts by weight, 45 parts by weight, or 50 parts by weight.

[0030] In some embodiments, the amount of the flexible monomer is 34 parts by weight, 35 parts by weight, 40 parts by weight, 45 parts by weight, 50 parts by weight, 55 parts by weight, 60 parts by weight, 65 parts by weight, 70 parts by weight, or 75 parts by weight.

[0031] In some embodiments, the amount of the functional unit is 5 parts by weight, 7 parts by weight, 9 parts by weight, 11 parts by weight, 13 parts by weight, or 15 parts by weight.

[0032] In some embodiments, the amount of the crosslinking agent is 0.1 parts by weight, 0.2 parts by weight, 0.3 parts by weight, 0.4 parts by weight, 0.5 parts by weight, 0.6 parts by weight, or 0.7 parts by weight.

[0033] In some embodiments, the amount of the emulsifier is 0.1 parts by weight, 0.2 parts by weight, 0.3 parts by weight, 0.4 parts by weight, or 0.5 parts by weight.

[0034] In some embodiments, the amount of the initiator is 0.02 parts by weight, 0.05 parts by weight, 0.1 parts by weight, 0.2 parts by weight, 0.3 parts by weight, or 0.4 parts by weight.

[0035] The binder, formed by polymerizing hydrophilic monomers, flexible monomers, and functional monomers, is acidic due to the presence of carboxylic acid groups. To reduce corrosion of the current collector, this invention uses lithium hydroxide for acid-base neutralization, resulting in a pH of 6-8 for the flexible aqueous binder. This reduces corrosion of the current collector and, more importantly, enhances the presence of lithium ions, replenishing those lost during SEI film formation, thereby improving the battery's electrochemical performance, such as its first-stage efficiency. This provides a new technological pathway for the development of high-performance lithium-ion batteries.

[0036] In some embodiments, the functional unit includes C 12 -C 22 Alkyl polyethylene glycol acrylate, C 12 -C 22At least one of alkyl polyethylene glycol methacrylate. Preferably, the functional monomer includes at least one of octadecyl polyethylene glycol acrylate and octadecyl polyethylene glycol methacrylate. The introduction of the octadecyl oleophilic segment and the polyethylene glycol hydrophilic segment improves the adhesive strength and dispersibility of the graphite, thereby enhancing the peel strength and first-efficiency of the electrode. The long octadecyl chain acts as a strong hydrophobic anchor, firmly adsorbing graphite and current collector through van der Waals forces; the polyethylene glycol chain acts as a steric hindrance to disperse the graphite and connects the particles with flexible long bridges, synergistically forming a robust and elastic three-dimensional network with the main binder to withstand stress and prevent electrode pulverization. Furthermore, the ether oxygen atoms in the polyethylene glycol chain guide the uniform distribution of lithium ions, promoting the formation of a thinner and more stable SEI film, reducing irreversible lithium consumption during formation, and improving first-efficiency; simultaneously, its coating layer inhibits electrolyte solvent co-intercalation, further protecting the graphite structure and reducing irreversible capacity loss. Its (meth)acrylate end groups ensure that the monomer is permanently grafted onto the binder network through covalent bonds, guaranteeing the durability and reliability of the function, thus performing far better than physically mixed components.

[0037] In some embodiments, the amount of the functional monomer is 8-12 parts by weight, for example, 8 parts by weight, 9 parts by weight, 10 parts by weight, 11 parts by weight, or 12 parts by weight. Optimizing the amount of the functional monomer can further improve the bonding strength of the binder and its dispersibility in graphite, thereby improving the peel strength and first-efficiency of the electrode.

[0038] In some embodiments, the C1-C 18 Alkyl acrylates include one or more of methyl acrylate, ethyl acrylate, n-butyl acrylate, n-amyl acrylate, isoamyl acrylate, isooctyl acrylate, hydroxyethyl acrylate, and hydroxypropyl acrylate. The C1-C 18 Alkyl methacrylates, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, n-pentyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate, octyl methacrylate, hydroxyethyl methacrylate, and hydroxypropyl methacrylate are selected from one or more of these. (C1-C) 18 Alkyl groups form the side chains of the binder, which have certain electron-withdrawing properties. They can reduce the electron cloud density of the main chain and increase the rotational properties of the main chain, thereby improving the flexibility of the binder and enhancing the flexibility of the electrode.

[0039] In some embodiments, the flexible monomer includes at least one of C4-C8 alkyl acrylates and C4-C8 alkyl methacrylates. The C4-C8 alkyl acrylates include one or more of n-butyl acrylate, n-pentyl acrylate, isoamyl acrylate, and isooctyl acrylate. The C4-C8 alkyl methacrylates include one or more of n-butyl methacrylate, n-pentyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate, and octyl methacrylate. Optimizing the carbon number of the alkyl side chain is beneficial for improving the resistance to electrolytes (including resistance to electrolyte swelling) and the dispersibility with graphite, thereby improving the peel strength and first-time efficiency of the electrode. The core mechanism of the preferred C4-C8 alkyl (meth)acrylate lies in the optimal balance between steric hindrance and molecular flexibility achieved in its alkyl chain length. The C4-C8 alkyl side chain extends from the polymer backbone, forming a dense hydrophobic barrier that effectively blocks the penetration of polar electrolyte solvents, greatly enhancing the anti-swelling ability of the binder network and maintaining its structural strength. Secondly, this chain length provides a suitable anchoring effect for graphite particles, and simultaneously acts as an internal plasticizer, increasing polymer chain spacing and flexibility. This allows the electrode to better buffer stress during charging and discharging, thereby achieving high peel strength. Finally, the stable anti-swelling network provides a solid foundation for the formation of a uniform and stable SEI film, reducing irreversible lithium consumption caused by interfacial side reactions and improving first-efficiency. Furthermore, the C4-C8 chain length avoids the functional deficiencies of short chains and the drawbacks of long chains, such as easy crystallization and phase separation, making it key to achieving optimal overall performance.

[0040] In some embodiments, the unsaturated carboxylic acid includes one or more of acrylic acid, methacrylic acid, itaconic acid, fumaric acid, crotonic acid, aconitic acid, and maleic acid, preferably acrylic acid. This improves the hydrophilicity and dispersibility of the binder in the aqueous slurry, allowing it to disperse linearly within the slurry, achieving linear bonding with graphite and conductive carbon, rather than point-to-point bonding. This reduces electrode resistance and improves the battery's rate performance and cycle stability. Furthermore, the linear structure of the binder enables the construction of a lithium-ion transport network, promoting uniform distribution and rapid diffusion of lithium ions, reducing polarization, lowering the battery's DC internal resistance (DCR), and comprehensively enhancing the battery's electrochemical performance.

[0041] In some embodiments, the crosslinking agent includes one or more of divinylbenzene, diallyl phthalate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, dipentaerythritol hexamethacrylate, divinyl ethylene glycol, diallyl itaconic acid, diallyl maleate, tripropylene glycol diacrylate, dipropylene glycol diacrylate, and bisphenol A dimethacrylate.

[0042] In some embodiments, the initiator includes a free radical polymerization initiator.

[0043] The free radical polymerization initiator includes at least one of water-soluble polymerization initiators and redox polymerization initiators.

[0044] The water-soluble polymerization initiator includes one or more of potassium persulfate, sodium persulfate, and ammonium persulfate.

[0045] The redox polymerization initiator includes at least one of an oxidant and a reducing agent.

[0046] The oxidizing agent includes one or more of potassium persulfate, sodium persulfate, ammonium persulfate, benzoyl peroxide, tert-butyl hydroperoxide, acetyl peroxide, and dicumyl hydroperoxide.

[0047] The reducing agent includes one or more of isoascorbic acid, ferrous salts, sodium sulfite, and sodium bisulfite.

[0048] In some embodiments, the emulsifier includes one or more of anionic emulsifiers, nonionic emulsifiers, and reactive emulsifiers.

[0049] The anionic emulsifier includes one or more of the following: higher alcohol sulfate salts, alkylbenzene sulfonates, alkyl diphenyl ether disulfonates, aliphatic sulfonates, aliphatic carboxylates, and sulfate salts of nonionic surfactants.

[0050] The nonionic emulsifier includes one or more of polyethylene glycol alkyl esters, alkyl phenyl ethers, and alkyl ethers.

[0051] The reactive emulsifier includes one or more of sodium methyl allyl sulfonate, sodium allyl sulfonate, and sodium p-styrene sulfonate.

[0052] In some embodiments, the weight-average molecular weight of the flexible waterborne adhesive is 400,000 Daltons to 1,000,000 Daltons, for example, 400,000 Daltons, 450,000 Daltons, 500,000 Daltons, 550,000 Daltons, 600,000 Daltons, 650,000 Daltons, 700,000 Daltons, 750,000 Daltons, 800,000 Daltons, 850,000 Daltons, 900,000 Daltons, or 950,000 Daltons and 1,000,000 Daltons.

[0053] A second aspect of the present invention provides a method for preparing the flexible waterborne adhesive described in the first aspect of the present invention, comprising the following steps: Mix 20-50 parts by weight of hydrophilic monomer, 34-75 parts by weight of flexible monomer, 5-15 parts by weight of functional monomer, 0.1-0.7 parts by weight of crosslinking agent, 0.1-0.5 parts by weight of emulsifier, 0.02-0.4 parts by weight of initiator and water, react at 45℃-80℃, then add lithium hydroxide for neutralization and adjust the pH to 6-8.

[0054] This invention uses emulsion polymerization to prepare binders, which is a simple process. The prepared flexible water-based binder has the advantages of high bonding strength, low swelling rate in electrolyte, good dispersibility in graphite, and the ability to replenish lithium to the negative electrode. It is a water-based binder suitable for the negative electrode of lithium-ion batteries, providing a brand-new solution for the high-performance development of lithium-ion batteries and meeting the future demand for high-energy-density and high-performance batteries.

[0055] In some embodiments, the mixing includes: stirring and mixing 20-50 parts by weight of a hydrophilic monomer, 34-75 parts by weight of a flexible monomer, 5-15 parts by weight of a functional monomer, 0.1-0.7 parts by weight of a crosslinking agent, 0.1-0.5 parts by weight of an emulsifier and water to obtain a mixture, introducing a protective gas and heating to the reaction temperature; and dissolving 0.02-0.4 parts by weight of an initiator in water and then adding it to the mixture.

[0056] In some embodiments, the reaction temperature is 45°C-80°C, for example 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, or 80°C. The reaction time is 4h-8h, for example 4h, 5h, 6h, 7h, or 8h.

[0057] After the reaction is complete, cool to room temperature, filter through a 150-200 mesh sieve, add lithium hydroxide for neutralization, and adjust the pH to 6-8. The mesh size of the sieve is, for example, 150, 160, 170, 180, 190, or 200 mesh. The pH is, for example, 6, 6.5, 7, 7.5, or 8.

[0058] A third aspect of the present invention provides a negative electrode slurry comprising a negative electrode active material, a conductive agent, lithium polyacrylate, the flexible waterborne binder described in the first aspect of the present invention, and water.

[0059] Because it employs the flexible water-based binder of the first aspect of this invention, the negative electrode slurry of this invention possesses all the advantages of the flexible water-based binder, which will not be elaborated further here.

[0060] Lithium polyacrylate is a thickener developed in recent years with lithium-supplementing properties, which can replace carboxymethyl cellulose (CMC). However, lithium polyacrylate is brittle, leading to reduced flexibility of the negative electrode sheet. Combining the flexible aqueous binder of this invention with lithium polyacrylate can overcome the problem of electrode brittleness. At the same time, the combination with lithium polyacrylate significantly increases the lithium content of the negative electrode of lithium-ion batteries, which can replenish lithium ions to the negative electrode, ultimately improving the first-efficiency and rate performance of lithium-ion batteries.

[0061] In some embodiments, the mass ratio of the flexible aqueous binder to the lithium polyacrylate is 1:(0.2-0.4), for example, 1:0.2, 1:0.3, or 1:0.4. If too little lithium polyacrylate is used, the electrode peel strength will decrease; if too much is used, the electrode will be too brittle to be made into a battery.

[0062] In some embodiments, the mass ratio of the negative electrode active material, conductive agent, lithium polyacrylate, and flexible aqueous binder is (95-97):1.0:(0.3-1):(1.5-3). The negative electrode sheet prepared by this formulation exhibits excellent adhesion properties and electrochemical stability.

[0063] In some specific embodiments, the mass ratio of the negative electrode active material, conductive agent, lithium polyacrylate, and flexible water-based binder is 96.5:1.0:0.5:2.0, 96:1.0:0.5:2.5, 97:1.0:0.5:1.5, 96.5:1.0:0.3:2.7, or 96.5:1.0:0.7:2.3.

[0064] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may include at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may include at least one of elemental tin, tin oxide compounds, and tin alloys.

[0065] In some embodiments, the conductive agent includes at least one of superconducting carbon, acetylene black, conductive carbon black (SP), Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0066] A fourth aspect of the present invention provides a negative electrode sheet, comprising a negative electrode active material, a conductive agent, lithium polyacrylate, and the flexible waterborne binder described in the first aspect of the present invention.

[0067] Because it employs the flexible water-based binder of the first aspect of this invention, the negative electrode sheet of this invention possesses all the advantages of the flexible water-based binder, which will not be elaborated further here.

[0068] The negative electrode sheet can be obtained by a method including the following steps: mixing the negative electrode active material, conductive agent, lithium polyacrylate and the above-mentioned flexible water-based binder to obtain a negative electrode slurry; coating the negative electrode slurry onto the current collector; and drying the current collector coated with the negative electrode slurry to obtain the negative electrode sheet.

[0069] The fifth aspect of the present invention provides a lithium-ion battery, including the negative electrode sheet described in the fourth aspect of the present invention.

[0070] Because it employs the flexible aqueous binder of the first aspect of this invention, the lithium-ion battery of this invention possesses all the advantages of the flexible aqueous binder, which will not be elaborated further here.

[0071] The present invention will be explained below with reference to embodiments. Those skilled in the art will understand that the following 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 the field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0072] Example 1 (1) Preparation of flexible waterborne adhesive: Add 45 parts by weight of acrylic acid, 44.6 parts by weight of ethyl acrylate, 10 parts by weight of octadecyl polyethylene glycol acrylate, 0.1 parts by weight of sodium p-styrene sulfonate (emulsifier), 0.1 parts by weight of diallyl phthalate (crosslinking agent), and 200 parts by weight of deionized water sequentially to the reaction vessel. Stir, install a condenser, and purge with nitrogen gas. Heat to 60°C. After the temperature stabilizes, completely dissolve 0.2 parts by weight of ammonium persulfate (initiator) in an appropriate amount of deionized water and pour it into the reaction vessel. After reacting for 8 hours, cool to room temperature, filter through a 150-mesh sieve, add lithium hydroxide to neutralize, and adjust the pH to 7.

[0073] (2) Preparation of negative electrode slurry and negative electrode sheet: Graphite, conductive carbon black SP, lithium polyacrylate, and the flexible water-based binder prepared in step 1 (graphite:SP:lithium polyacrylate:flexible water-based binder mass ratio = 96.5:1.0:0.5:2.0) were added to a mixing tank. After dispersion by low-speed stirring, an appropriate amount of deionized water was added to control the slurry solid content to 65wt%. The mixture was kneaded for 30 min, and then deionized water was added again to adjust the slurry solid content to 50wt%. The mixture was stirred at high speed for 2 h. After the fineness of the negative electrode slurry was tested and found to be qualified, the stirring speed was adjusted to low speed and defoamed for 30 min. The mixture was then coated onto copper foil and dried to obtain the negative electrode sheet. The areal density of the negative electrode sheet was 220 g / m³. 2 .

[0074] (3) Preparation of positive electrode sheet: The positive electrode active material NCM811 (lithium nickel cobalt manganese oxide, LiNi) 0.8 Co 0.1 Mn 0.1 O2), conductive carbon black SP, and binder polyvinylidene fluoride (PVDF, grade 5130) were mixed in a mass ratio of 96.75:2:1.25 and added to the solvent N-methylpyrrolidone (NMP). The mixture was stirred evenly under vacuum to obtain a positive electrode slurry with a solid content of 68 wt%. The positive electrode slurry was then uniformly coated onto aluminum foil and dried to obtain a positive electrode sheet.

[0075] (4) Electrolyte preparation The electrolyte used was purchased from Guangzhou Tinci Advanced Materials Co., Ltd., with the brand name TC-E8630N3.

[0076] (5) Preparation of diaphragm A 14μm thick polypropylene (PP) membrane was used as the separator.

[0077] (6) Lithium-ion battery preparation The positive electrode, separator, negative electrode, and electrolyte prepared in the above steps are assembled into a soft-pack lithium-ion battery.

[0078] Example 2-26 Lithium-ion batteries were prepared according to the method described in Example 1, with the differences shown in Table 1.

[0079] Comparative Example 1 A lithium-ion battery was prepared according to the method described in Example 1, except that step 1 was not performed. Step 2 is performed as follows: Negative electrode slurry formulation: Graphite: Conductive carbon: Carboxymethyl cellulose: Styrene-butadiene rubber mass ratio = 96.5:1.0:1.2:1.3; Deionized water was added to a mixing tank, carboxymethyl cellulose was added and dissolved, conductive carbon black SP was added, and high-speed dispersion was carried out for 2 hours. Graphite was added, and high-speed dispersion was continued for 2 hours. The solid content of the negative electrode slurry was 50 wt%. After the fineness of the negative electrode slurry was tested and found to be qualified, it was stirred at low speed, and styrene-butadiene rubber (SBR) emulsion was added. After defoaming for 30 minutes, it was coated on copper foil and dried to obtain the negative electrode sheet.

[0080] Comparative Examples 2-5 Lithium-ion batteries were prepared according to the method described in Example 1, with the differences shown in Table 1.

[0081] Table 1

[0082]

[0083]

[0084]

[0085]

[0086] Negative electrode sheet and battery performance testing (1) Peel force test of negative electrode sheet: The negative electrode sheets prepared in the above examples and comparative examples were tested for 180° peel force using a universal tensile tester. The test specimens were made with a width of about 2 cm and a length of about 5 cm. The pulling speed was 2 mm / s. The unit of peel force is N / m. The test results are shown in Table 2.

[0087] (2) Negative electrode flexibility test: Refer to national standard GB1731-2020: 1. Cutting: Cut the electrode into 1*8CM sample strips; 2. Test: Press the sample strip tightly onto the shaft, with the electrode facing outwards, bend it 180°, and remove it for 2-3 seconds to observe; 3. Observation: Use a 4x magnifying glass to observe whether there are cracks or peeling on the surface of the sample strip. The smallest diameter shaft that passes through is the flexibility. The unit of the coiling needle diameter is mm. The test results are shown in Table 2. The smaller the coiling needle diameter, the better the electrode flexibility.

[0088] (3) Negative electrode resistance test: The resistance was tested using a Yuaneng Technology BER2500 resistance meter at a pressure of 25MPa. The test results are shown in Table 2.

[0089] (4) Battery first efficiency test: Charge the battery at a constant current of 0.1C to the cutoff voltage, let it stand for 1 hour, and then discharge it at a constant current of 0.1C to the termination voltage. Record the first charge capacity Qc and the first discharge capacity Qd. Calculate the first efficiency as Qd / Qc×100%. The test results are shown in Table 2.

[0090] (5) Swelling rate test of adhesive in electrolyte: The adhesive was placed in a 2*7*0.2cm mold, dried in an oven, and weighed to obtain the weight before immersion. The adhesive film was then immersed in the electrolyte at 60℃ for 7 days. The adhesive film was removed, gently wiped dry with a lint-free cloth, and weighed to obtain the weight after immersion. The swelling rate of the adhesive = (weight after immersion - weight before immersion) / weight before immersion * 100%. The test results are shown in Table 2. Among them, the adhesive of Comparative Example 1 is a mixture of carboxymethyl cellulose and styrene-butadiene rubber (mass ratio of 1.2:1.3).

[0091] (6) Viscosity test of negative electrode slurry: The viscosity of the negative electrode slurry prepared in the above examples and comparative examples was tested using a rotational viscometer with an RV04 or RV05 rotor at a speed of 20 rpm. The test results are shown in Table 1.

[0092] Table 2

[0093] Results and Discussion: By comparing Example 1 and Comparative Example 1, it can be seen that the binder of the present invention has a low swelling rate, the electrode using the binder of the present invention has high peel strength, good flexibility, low electrode resistance, and the battery using the binder of the present invention has high first efficiency.

[0094] By comparing Example 1 and Comparative Example 2, it can be seen that the functional monomer of the present invention can reduce the swelling rate of the binder, improve the peeling force and flexibility of the electrode, reduce the electrode resistance, and improve the first efficiency of the battery.

[0095] By comparing Example 1 and Comparative Example 3, it can be seen that the hydrophilic monomer of the present invention can reduce the swelling rate of the binder, improve the peeling force and flexibility of the electrode, reduce the electrode resistance, and improve the first efficiency of the battery.

[0096] By comparing Example 1 and Comparative Example 4, it can be seen that the flexible monomer of the present invention can reduce the swelling rate of the binder, significantly improve the peeling force and flexibility of the electrode, reduce the electrode resistance, and improve the first efficiency of the battery.

[0097] By comparing Example 1, Example 26 and Comparative Example 5, it can be seen that Comparative Example 5 did not use the binder of the present invention, and the amount of lithium polyacrylate was too large, resulting in brittle battery electrodes that could not be manufactured. In Example 26, the amount of lithium polyacrylate was too small, which reduced the electrode peeling force.

[0098] By comparing Example 1 and Example 3, it can be seen that the length of the alkyl chain in the functional monomer affects the electrode peel strength.

[0099] By comparing Examples 1 and Examples 4-9, it can be seen that the amount of functional monomer affects the swelling rate of the binder, the peeling force of the electrode, and the resistance.

[0100] By comparing Example 1 and Examples 11-17, it can be seen that the alkyl chain length in the flexible monomer affects the peeling force and flexibility.

[0101] By comparing Example 1 and Examples 18-19, it can be seen that acrylic acid is preferred as the hydrophilic monomer, which helps to reduce the swelling rate.

[0102] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are 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.

[0103] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A flexible water-based adhesive, characterized in that, It includes the following raw materials in parts by weight: 20-50 parts by weight of hydrophilic monomer, 34-75 parts by weight of flexible monomer, 5-15 parts by weight of functional monomer, 0.1-0.7 parts by weight of crosslinking agent, 0.1-0.5 parts by weight of emulsifier, 0.02-0.4 parts by weight of initiator, and acid-base neutralizer. The hydrophilic monomer includes unsaturated carboxylic acids; The flexible monomer includes C1-C 18 Alkyl acrylates, C1-C 18 At least one of alkyl methacrylates; The functional monomers include C1-C 22 Alkyl polyethylene glycol acrylate, C1-C 22 At least one of alkyl polyethylene glycol methacrylates; The acid-base neutralizing agent includes lithium hydroxide.

2. The flexible water-based adhesive according to claim 1, characterized in that, The functional monomer includes C 12 -C 22 Alkyl polyethylene glycol acrylate, C 12 -C 22 At least one of alkyl polyethylene glycol methacrylates.

3. The flexible water-based adhesive according to claim 2, characterized in that, The functional monomer includes at least one of octadecyl polyethylene glycol acrylate and octadecyl polyethylene glycol methacrylate.

4. The flexible water-based adhesive according to claim 1, characterized in that, The amount of the functional monomer used is 8-12 parts by weight.

5. The flexible water-based adhesive according to claim 1, characterized in that, The C1-C 18 Alkyl acrylates include one or more of methyl acrylate, ethyl acrylate, n-butyl acrylate, n-amyl acrylate, isoamyl acrylate, isooctyl acrylate, hydroxyethyl acrylate, and hydroxypropyl acrylate; The C1-C 18 Alkyl methacrylates include one or more of methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, n-pentyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate, octyl methacrylate, hydroxyethyl methacrylate, and hydroxypropyl methacrylate.

6. The flexible water-based adhesive according to claim 1, characterized in that, The flexible monomer includes at least one of C4-C8 alkyl acrylate and C4-C8 alkyl methacrylate; The C4-C8 alkyl acrylates include one or more of n-butyl acrylate, n-amyl acrylate, isoamyl acrylate, and isooctyl acrylate; The C4-C8 alkyl methacrylates include one or more of the following: n-butyl methacrylate, n-pentyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate, and octyl methacrylate.

7. The flexible water-based adhesive according to claim 1, characterized in that, The unsaturated carboxylic acids include one or more of acrylic acid, methacrylic acid, itaconic acid, fumaric acid, crotonic acid, aconitic acid, and maleic acid. The pH of the flexible water-based adhesive is 6-8.

8. A method for preparing the flexible waterborne adhesive according to any one of claims 1-7, characterized in that, Includes the following steps: Mix 20-50 parts by weight of hydrophilic monomer, 34-75 parts by weight of flexible monomer, 5-15 parts by weight of functional monomer, 0.1-0.7 parts by weight of crosslinking agent, 0.1-0.5 parts by weight of emulsifier, 0.02-0.4 parts by weight of initiator and water, react at 45℃-80℃, then add lithium hydroxide for neutralization and adjust the pH to 6-8.

9. A negative electrode slurry, characterized in that, It includes a negative electrode active material, a conductive agent, lithium polyacrylate, and a flexible waterborne binder according to any one of claims 1-7.

10. The negative electrode slurry according to claim 9, characterized in that, The mass ratio of the flexible water-based adhesive to the lithium polyacrylate is 1:(0.2-0.4).

11. The negative electrode slurry according to claim 9, characterized in that, The mass ratio of the negative electrode active material, conductive agent, lithium polyacrylate, and flexible water-based binder is (95-97):1.0:(0.3-1):(1.5-3).

12. A negative electrode sheet, characterized in that, It includes a negative electrode active material, a conductive agent, lithium polyacrylate, and a flexible waterborne binder as described in any one of claims 1-7.

13. A lithium-ion battery, characterized in that, Includes the negative electrode sheet as described in claim 12.

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