Polyurethane acrylate emulsion and secondary battery

Abstract

The invention belongs to the technical field of secondary batteries, and discloses a polyurethane acrylate emulsion and a secondary battery. The polyurethane acrylate emulsion comprises water and a polymer dispersed in the water; the polymer comprises a core and a shell, the shell covers the core, the core comprises water, and the shell comprises urethane acrylate; the polyurethane acrylate comprises a polyurethane unit and a polyacrylate unit, and the polyacrylate unit comprises an organic silicon chain segment; on the basis of the mass of the polyurethane acrylate, the mass content of the organic silicon chain segment is 0.1%-10%. The polyurethane acrylate emulsion provided by the invention has the characteristics of high bonding strength, electrolyte corrosion resistance and good dispersity to a conductive agent, and when the polyurethane acrylate emulsion is used as a silicon-based negative electrode binder, the polyurethane acrylate emulsion can effectively cope with large volume change of a silicon-based material, the problem of interface failure of a silicon-based negative electrode is solved, and the cycle performance and the dynamic performance of a battery are improved.

Description

Technical Field

[0001] This application relates to the field of secondary battery technology, specifically to a polyurethane acrylate emulsion and a secondary battery. Background Technology

[0002] Silicon-based materials have attracted much attention due to their theoretical specific capacity, which is far higher than that of traditional graphite, and are expected to become the anode active materials for next-generation high-energy-density lithium-ion secondary batteries. However, silicon-based materials undergo significant volume expansion and contraction during charge and discharge, which can easily lead to interfacial stress concentration at the interface between the active material and the binder. This can cause problems such as electrode pulverization and delamination, which in turn seriously affect the cycle life and safety performance of the battery.

[0003] To mitigate the instability of the negative electrode structure caused by silicon expansion, the industry widely adopts a composite system of polyacrylic acid (PAA) and styrene-butadiene copolymer (SBR) as the negative electrode binder. This type of water-soluble binder possesses certain flexibility and adhesion capabilities; however, its linear chain structure cannot effectively buffer the internal stress concentration generated during charging and discharging, and its bond strength decreases significantly during long-term cycling. This results in significant shortcomings in the compatibility of this type of binder with the negative electrode structure. Furthermore, with the increase in silicon content in the negative electrode material, more conductive agents are needed to maintain the electron conduction pathway. In particular, the addition of novel high-efficiency conductive agents, such as single-walled carbon nanotubes (SWCNTs), can effectively improve the rate performance of the electrode, but it leads to increased viscosity, poor dispersibility, and easy agglomeration of the negative electrode slurry. This makes it difficult for the traditional PAA / SBR binder system to simultaneously meet the dual requirements of slurry dispersibility and electrode adhesion. Therefore, there is an urgent need to develop a binder material with good interfacial compatibility with silicon-based materials and that can enhance the dispersibility of conductive agents. Summary of the Invention

[0004] In view of this, the main objective of this application is to provide a polyurethane acrylate emulsion, which, as a silicon-based anode binder, not only has good interfacial compatibility with silicon-based materials and can effectively anchor the silicon-based anode and cope with its volume changes, but also enhances the dispersibility of conductive agents in the anode slurry. This solves the problem of silicon-based anode interface failure, improves slurry yield, enhances battery cycle retention, and prevents the reduction of the lithium plating window.

[0005] The objective of this application is achieved through the following technical solution:

[0006] According to an embodiment of this application, in a first aspect, a polyurethane acrylate emulsion is provided, comprising water and a polymer dispersed in the water; the polymer comprises a core and a shell, the shell covering the core, the core comprising water, and the shell comprising polyurethane acrylate; the polyurethane acrylate comprises polyurethane units and polyacrylate units, the polyacrylate units comprising silicone segments; and the silicone segments have a mass content of 0.1% to 10% based on the mass of the polyurethane acrylate.

[0007] In some alternative embodiments, the polyacrylate unit covers the polyurethane unit.

[0008] In some alternative embodiments, the polyacrylate unit and the polyurethane unit are located in the same shell.

[0009] In some alternative embodiments, the mass content of the polyacrylate unit is 2.5%-25% based on the mass of the polyurethane acrylate.

[0010] In some optional embodiments, the raw materials for the polyacrylate unit include hydroxyacrylate monomers, unsaturated carboxylic acid monomers, and organosilicon monomers, wherein the organosilicon monomers include olefin bonds.

[0011] Furthermore, in some optional embodiments, the organosilicon monomer includes at least one of allyltrimethoxysilane, 3-(trimethoxysilyl)propyl acrylate, methacryloyloxymethyltriethoxysilane, 3-(methacryloyloxy)propyltrimethoxysilane, vinyltriethoxysilane, and vinyltrimethoxysilane.

[0012] Furthermore, in some optional embodiments, the hydroxyacrylate monomer includes at least one of 2-hydroxyacrylate, hydroxypropyl acrylate, and hydroxypropyl methacrylate.

[0013] Furthermore, in some optional embodiments, the unsaturated carboxylic acid monomer includes at least one of acrylic acid, 2-methacrylic acid, 2-ethylacrylic acid, sorbic acid, linoleic acid, linolenic acid, vinylacetic acid, maleic acid, fumaric acid, and itaconic acid.

[0014] Furthermore, in some optional embodiments, the raw materials of the polyacrylate unit further include an initiator and a first neutralizing agent, wherein the mass ratio of the first neutralizing agent to the unsaturated carboxylic acid is 0.3:1-1.3:1.

[0015] In some alternative embodiments, the mass content of the polyurethane unit is 75%-97.5% based on the mass of the polyurethane acrylate.

[0016] In some optional embodiments, the raw materials of the polyurethane unit include hard segment monomers, soft segment monomers, crosslinking agents, hydrophilic chain extenders, and a second neutralizing agent; wherein:

[0017] The hard segment monomer includes a polyisocyanate, and the polyisocyanate includes at least one aliphatic ring;

[0018] The soft segment monomer includes a chain aliphatic polyols, wherein the main chain of the chain aliphatic polyol has not less than 3 carbon atoms;

[0019] The crosslinking agent comprises at least three hydroxyl groups; based on the total mass of the raw materials of the polyurethane unit, the mass content of the crosslinking agent is 0.1%-5%;

[0020] The hydrophilic chain extender comprises at least two hydroxyl groups and at least one carboxyl group, and the mass ratio of the second neutralizing agent to the hydrophilic chain extender is 0.25:1-0.85:1.

[0021] In some alternative embodiments, the mass content of the hard segment monomer is 30%-75% based on the total mass of the raw materials of the polyurethane unit.

[0022] In some alternative embodiments, the number of carbon atoms in the aliphatic ring of the hard segment monomer is 5 or 6.

[0023] In some alternative embodiments, when the polyacrylate unit and the polyurethane unit are located in the same shell, the polyisocyanate does not contain any unsaturated groups other than the isocyanate group.

[0024] Furthermore, in some optional embodiments, the hard segment monomer includes at least one of 1,4-cyclohexane diisocyanate, 1,3-cyclohexane diisocyanate, hydrogenated toluene diisocyanate, isophorone diisocyanate, norbornane diisocyanate, dicyclohexylmethane diisocyanate, and norbornane dimethylene isocyanate.

[0025] In some alternative embodiments, when the polyacrylate unit and the polyurethane unit are located in the same shell, the main chain of the chain aliphatic polyol does not contain heteroatoms or unsaturated groups, wherein the heteroatoms include at least one of oxygen, sulfur, and selenium.

[0026] In some alternative embodiments, the main chain carbon number of the chain aliphatic polyol does not exceed 200.

[0027] Furthermore, in some optional embodiments, the chain-like aliphatic polyol further includes branches, each branch comprising at least one carbon atom, and the number of branches not exceeding 40.

[0028] In some alternative embodiments, the mass content of the soft segment monomer is 15%-63% based on the total mass of the raw materials of the polyurethane unit.

[0029] Furthermore, in some optional embodiments, the soft segment monomer includes at least one of 2-methyl-1,5-pentanediol, 2,2-dimethyl-1,3-propanediol, 12-hydroxystearic acid, 6-aminohexanoic acid, dihydroxy-terminated polyethylene, and dihydroxy-terminated hydrogenated polybutadiene.

[0030] In some alternative embodiments, the molecular weight of the crosslinking agent does not exceed 300.

[0031] Furthermore, in some optional embodiments, the crosslinking agent includes at least one of trimethylolpropane, glycerol, pentaerythritol, tri(2-hydroxyethyl) isocyanurate, and trihydroxypolyoxypropylene ether.

[0032] In some alternative embodiments, the molecular weight of the hydrophilic chain extender does not exceed 200.

[0033] Furthermore, in some optional embodiments, the hydrophilic chain extender includes at least one of 2,2-dimethylolpropionic acid, 2,2-dimethylolbutyric acid, 3,5-dihydroxy-3-methylvaleric acid, dihydroxymaleic acid, 2,6-dihydroxybenzoic acid, and 3,4-diaminobenzoic acid.

[0034] Furthermore, in some optional embodiments, the raw materials for the polyurethane unit further include a catalyst, which includes at least one of an organotin compound and an organobismuth compound.

[0035] Furthermore, in some optional embodiments, the raw materials of the polyurethane unit further include an amine chain extender, wherein the amine chain extender comprises at least two amine groups.

[0036] In some alternative embodiments, the number-average molecular weight of the polyurethane acrylate is 20,000 to 40,000.

[0037] In some alternative embodiments, the glass transition temperature of the polyurethane acrylate is -40°C to -20°C.

[0038] In some optional embodiments, the particle size Dn90 of the polyurethane acrylate emulsion is 100 nm to 500 nm.

[0039] In some alternative embodiments, the polyurethane acrylate emulsion is formed into a film, which, after being immersed in a lithium-ion electrolyte at 60°C, satisfies at least one of the following conditions:

[0040] A. Swelling rate is 30-90%;

[0041] B. Dissolution rate not exceeding 5%;

[0042] C. Elongation at break is not less than 150%;

[0043] D. The fracture strength is not less than 8 MPa;

[0044] The lithium-ion electrolyte comprises an organic solvent, a lithium salt, and additives. The organic solvent comprises ethylene carbonate, propylene carbonate, propyl propionate, and diethyl carbonate. The lithium salt comprises lithium hexafluorophosphate. The additives comprise at least one of fluoroethers, fluoroethylene carbonate, fluoronitrile, and polyarylether ether nitrile.

[0045] According to an embodiment of this application, in a second aspect, a secondary battery is also provided, including a negative electrode sheet, the negative electrode sheet including a negative electrode current collector and a negative electrode active layer disposed on at least one side surface in the thickness direction of the negative electrode current collector, the negative electrode active layer including a negative electrode active material and a binder; the binder includes the polyurethane acrylate emulsion described in the first aspect of this application.

[0046] In some optional embodiments, the negative electrode active material comprises a silicon-based material; the peel strength between the negative electrode active layer and the negative electrode current collector is not less than 15 N / m.

[0047] The technical solution of this application has the following advantages:

[0048] The polyurethane acrylate emulsion provided in this application includes water and a polymer dispersed in the water; the polymer includes a core and a shell, the shell covering the core, the core including water, and the shell including polyurethane acrylate; the polyurethane acrylate includes polyurethane units and polyacrylate units, the polyacrylate units including silicone segments; based on the mass of the polyurethane acrylate, the mass content of the silicone segments is 0.1% to 10%.

[0049] In this application, the polyurethane unit can provide a certain degree of rigidity as well as high flexibility and adhesion. The acrylic functional group can form a three-dimensional network structure with other functional groups (such as hydroxyl and carboxyl groups) in the polyurethane unit through free radical polymerization or radiation curing, which further enhances mechanical properties and chemical resistance. In this way, the volume strain of silicon-based materials during charging and discharging can be effectively released, the structural stability of silicon-based negative electrode can be maintained, and the cycle performance of battery can be improved. At the same time, it can ensure that the binder is not easily corroded by electrolyte, and ensure the physical and chemical anchoring effect of the binder, so that the active material is firmly adhered to the current collector, thereby ensuring the stable performance of battery.

[0050] Based on this, this application also introduces a certain amount of organosilicon segments into the polyacrylate unit. By utilizing the interaction between the silicon-oxygen bonds and the surface defects of conductive agents such as SWCNT or the hydroxyl and carboxyl groups remaining after acid washing, the dispersion uniformity and stability of SWCNT in aqueous slurries can be greatly improved, and the viscosity and agglomeration tendency of negative electrode slurries can be reduced. This effectively alleviates the problem of increased viscosity and processing difficulties caused by the large amount of conductive agent required and poor dispersibility in high-silicon content negative electrode slurries.

[0051] Therefore, the polyurethane acrylate emulsion provided in this application possesses high bonding strength, resistance to electrolyte corrosion, and good dispersibility for conductive agents. When used as a binder for silicon-based anodes, it exhibits good interfacial compatibility with silicon-based materials, effectively anchoring them and maintaining sufficient structural stability (i.e., minimal dissolution) even after electrolyte swelling. This ensures no decrease in bonding strength during charge and discharge, effectively addressing significant volume changes in silicon-based materials, solving the problem of interface failure in silicon-based anodes, and improving battery cycle performance. Furthermore, it enhances the dispersion uniformity and stability of conductive agents in the anode slurry, improving slurry yield and thus improving battery kinetic performance and preventing a decrease in the lithium plating window. Therefore, the polyurethane acrylate emulsion of this application is particularly suitable for electrode bonding in high-strain systems.

[0052] Additional aspects and advantages of the embodiments of this application will be described and shown in part in the following description, or illustrated by practice of the embodiments of this application. Attached Figure Description

[0053] To more clearly illustrate the technical solutions in the specific embodiments of this application, the drawings used in the description of the specific embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0054] Figure 1 This is a schematic diagram of the polymer in a polyurethane acrylate emulsion in one embodiment of this application.

[0055] The reference numerals in the attached figures are explained as follows:

[0056] (a) Organosilicon segments; (b) Polyacrylate units; (c) Polyurethane units; (d) Core. Detailed Implementation

[0057] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0058] It should be noted in the description of this application that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, the technical features involved in the different embodiments of this application described below may be combined with each other as long as they do not conflict with each other.

[0059] To address the problems of poor interfacial compatibility between negative electrode binders and silicon-based materials, and insufficient dispersion of conductive agents in related technologies, this application proposes the following solutions.

[0060] According to a first aspect of this application, a polyurethane acrylate emulsion is provided, comprising water and a polymer dispersed in the water; the polymer comprises a core and a shell, the shell covering the core, the core comprising water, and the shell comprising polyurethane acrylate; the polyurethane acrylate comprises polyurethane units and polyacrylate units, the polyacrylate units comprising silicone segments; and the silicone segments have a mass content of 0.1% to 10% based on the mass of the polyurethane acrylate.

[0061] Waterborne polyurethane (WPU) is a novel polyurethane system that uses water instead of organic solvents as the dispersion medium. Based on their molecular structure, WPU can be divided into linear polyurethane emulsions and cross-linked polyurethane emulsions. Cross-linked polyurethane emulsions, in particular, possess strong structural designability and excellent mechanical properties. When used as a binder for silicon-based anodes, they can, to some extent, buffer the volume change stress of silicon-based materials during lithium intercalation / deintercalation. However, cross-linked polyurethane emulsions are inherently hydrophobic and have insufficient compatibility with waterborne conductive agent systems, especially in their poor dispersion and stabilization of carbon nanomaterials such as single-walled carbon nanotubes, resulting in poor dispersion and low slurry yield in the anode slurry.

[0062] To address this, this application introduces polyacrylate units into the chemical structure of WPU to obtain a polyurethane acrylate emulsion. The polymer dispersed in this emulsion has a core-shell structure, with the shell comprising polyurethane acrylate (PUA). The polyurethane segments provide a certain degree of rigidity, as well as high flexibility and adhesion. The acrylic functional groups can form a three-dimensional network structure with other functional groups in WPU (such as hydroxyl and carboxyl groups) through free radical polymerization or radiation curing, further enhancing mechanical properties and chemical resistance. In this way, the volumetric strain of the silicon-based material during charging and discharging can be effectively released, maintaining the structural stability of the silicon-based negative electrode and improving the cycle performance of the battery. At the same time, it can ensure that the PUA binder is not easily corroded by the electrolyte, ensuring the physical and chemical anchoring effect of the PUA binder, so that the active material adheres firmly to the current collector, thereby ensuring the stable performance of the battery.

[0063] Based on this, in order to improve the dispersibility of the conductive agent, this application also introduces a certain amount of organosilicon segments into the polyacrylate unit. By utilizing the interaction between the silicon-oxygen bonds of the organosilicon segments and the surface defects or residual hydroxyl and carboxyl groups of the conductive agent such as SWCNT after acid washing, the dispersion uniformity and stability of SWCNT in aqueous slurry can be significantly improved, and the viscosity and agglomeration tendency of the negative electrode slurry can be reduced. This effectively alleviates the problem of increased viscosity and processing difficulties caused by the large amount of conductive agent required and poor dispersibility in high-silicon content negative electrode slurry.

[0064] In summary, the PUA emulsion provided in this application possesses high bonding strength, resistance to electrolyte corrosion, and good dispersibility for conductive agents. Therefore, when used as a binder for silicon-based anodes, it exhibits good interfacial compatibility with silicon-based materials, effectively anchoring them and maintaining sufficient structural stability (i.e., minimal dissolution) even after electrolyte swelling. This ensures no decrease in bonding strength during charge and discharge, effectively addressing significant volume changes in silicon-based materials, solving the problem of interface failure in silicon-based anodes, and improving battery cycle performance. Furthermore, it enhances the dispersion uniformity and stability of conductive agents in the anode slurry, improving slurry yield and thus improving battery kinetic performance. Therefore, the PUA emulsion of this application is particularly suitable for electrode bonding in high-strain systems.

[0065] This study found that if the mass content of organosilicon segments in PUA is less than 0.1%, it cannot effectively improve the dispersion performance of conductive agents in negative electrode slurry, resulting in low slurry yield and deterioration of battery kinetic performance. When the mass content of organosilicon segments in PUA is greater than 10%, the strong hydrophobicity and low surface energy of organosilicon segments will reduce the dispersion uniformity and stability of PUA in water, resulting in poor adhesion of PUA emulsion, insufficient adhesion in some areas, and easy pulverization and detachment of active materials, thereby affecting the cycle performance of the battery.

[0066] It should be noted that the mass content of organosilicon segments in PUA can be obtained through thermogravimetric analysis. For example, the mass content of organosilicon segments can be 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, 6.0%, 7.0%, 8.0%, 9.0%, 10.0%, or values ​​within any range of two of the above values.

[0067] In this application, the terms "PUA emulsion" and "PUA adhesive" have the same meaning as "polyurethane acrylate emulsion," referring to a polyurethane acrylate system with water as the dispersion medium.

[0068] In some embodiments, the polyacrylate unit and the polyurethane unit are located in the same shell. This composite shell fully utilizes the stress-buffering effect of polyurethane and the dispersing conductive agent and bonding support function of polyacrylate, contributing to the formation of a more uniform and stable conductive network and a strong and tough bonding interface within the silicon-based anode. This, in turn, ensures the integrity of the electrode structure while improving the battery's kinetic performance and long-term cycle stability.

[0069] It should be noted that the composite shell structure of the polymer can be observed using transmission electron microscopy (TEM). Specific testing methods include: appropriately diluting the polyurethane acrylate emulsion with deionized water, then dropping it onto a copper grid supported by an ultrathin carbon film, followed by negative staining with heavy metal salts such as phosphotungstic acid (PTA) or uranyl acetate; after the sample dries, observation is performed at an accelerating voltage of 80-120 kV, which allows for direct confirmation of its water-core, polyurethane acrylate-shell encapsulation structure.

[0070] In other embodiments, the polyacrylate unit coats the polyurethane unit, such as... Figure 1 As shown, by constructing a polyurethane unit as the inner shell (such as... Figure 1 As shown in (c), the polyacrylate unit is the outer shell layer (as shown in the middle). Figure 1 The double-layer shell structure shown in (b) further ensures that acrylic functional groups participate in the formation of a three-dimensional network structure, thereby significantly enhancing the mechanical properties and chemical resistance of the PUA binder. This allows the PUA binder to maintain the structural stability of the silicon-based anode more effectively and for longer, thus significantly improving the cycle performance of the battery. On the other hand, the exposed polyacrylate units facilitate the formation of organosilicon segments (such as...) Figure 1 The introduction of (a) further enhances the dispersion performance of the conductive agent in the negative electrode slurry, improves the slurry yield, and improves battery dynamics.

[0071] It should be noted that the double-shell structure of the polymer can be obtained by combining high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with energy-dispersive X-ray spectroscopy (EDS mapping). The specific method involves: in HAADF-STEM mode, using atomic number contrast imaging to initially observe the overall morphology and contrast layering of the shell. Then, EDS mapping analysis is performed on individual polymer particles: if the characteristic X-ray signal of silicon (Si) is observed to be completely concentrated on the outermost layer of the shell, forming a closed shell distribution, while the inner region of the shell mainly shows the more representative nitrogen (N) element signal from the polyurethane unit, and the two form a clear transition from the inside to the outside (N→Si) in spatial distribution, it can be confirmed that the polyacrylate unit has successfully encapsulated the polyurethane unit, forming a clear double-shell structure.

[0072] In some embodiments, the mass content of the polyacrylic acid ester unit is 2.5%-25% based on the mass of the polyurethane acrylate. This ensures that a sufficient number of acrylic functional groups participate in the formation of the three-dimensional network structure, significantly enhancing the mechanical properties and chemical resistance of the PUA binder. This allows the PUA binder to maintain the structural stability of the silicon-based anode more effectively and for longer, thereby significantly improving the cycle performance of the battery. At the same time, it also ensures that there are enough sites for introducing organosilicon segments, thereby improving the dispersion performance of the conductive agent in the anode slurry, increasing the slurry yield, and improving the battery kinetic performance.

[0073] If the mass content of polyacrylate units in PUA is less than 2.5%, it is difficult to maintain the stability of the silicon-based anode structure and improve the dispersibility of the conductive agent. Conversely, when the mass content of polyacrylate units in PUA is greater than 25%, it will lead to insufficient flexibility of the PUA adhesive, weak buffering ability when facing physical impact, and the bonding interface is easily affected by external forces, resulting in detachment or crack propagation.

[0074] In this application, the mass content of polyacrylate units can be determined by thermogravimetric-pyrolysis gas chromatography-mass spectrometry. For example, the mass content of polyacrylate units in PUA can be 2.5%, 5%, 7.5%, 10%, 12.5%, 15%, 17.5%, 20%, 22.5%, 25%, or a value within any two of the above ranges.

[0075] Furthermore, in some embodiments, the raw materials for the polyacrylate unit include hydroxyacrylate monomers, unsaturated carboxylic acid monomers, and organosilicon monomers, wherein the organosilicon monomers include olefin bonds.

[0076] By employing hydroxyl acrylate monomers, on the one hand, their carbon-carbon double bonds can copolymerize with unsaturated carboxylic acid monomers to form polyacrylate units. On the other hand, the hydroxyl functional groups in these monomers can undergo addition reactions with the isocyanate groups in the polyurethane prepolymer to form stable urethane bonds. This provides the necessary chemical anchoring points for constructing the polyurethane acrylate shell structure, ensuring a strong bond and functional synergy between the polyacrylate units and the polyurethane units.

[0077] The chemical structures of hydroxyacrylate monomers can be determined and resolved by thermogravimetric-infrared spectroscopy. For example, the hydroxyacrylate monomers include at least one of 2-hydroxyacrylate, hydroxypropyl acrylate, and hydroxypropyl methacrylate.

[0078] By employing unsaturated carboxylic acid monomers, appropriate amounts of carboxylic acid functional groups can be introduced into the chemical structure of PUA. Utilizing the strong hydration and polar characteristics after carboxyl salting, the dispersion and storage stability of PUA emulsion can be ensured. Furthermore, through strong polar interactions and hydrogen bond networks with the negative electrode active material and current collector, the electrode can be provided with durable and robust bonding strength. In addition, the lithium-ion transport rate can be improved, thereby ensuring the cycle life and kinetic performance of the battery.

[0079] The chemical structure of unsaturated carboxylic acid monomers can be obtained by thermogravimetric-infrared spectroscopy. For example, the unsaturated carboxylic acid monomers include at least one selected from acrylic acid, 2-methacrylic acid, 2-ethylacrylic acid, sorbic acid, linoleic acid, linolenic acid, vinylacetic acid, maleic acid, fumaric acid, and itaconic acid.

[0080] By employing organosilicon monomers containing olefin bonds, and utilizing the copolymerization reaction between their olefin bonds and the carbon-carbon double bonds in hydroxy acrylate monomers and / or unsaturated carboxylic acid monomers, organosilicon segments are stably bonded to polyacrylate units, thereby effectively improving the dispersion uniformity and stability of conductive agents such as SWCNT in negative electrode slurry.

[0081] The chemical structure of organosilicon monomers can be obtained by thermogravimetric-infrared spectroscopy. For example, the organosilicon monomers include at least one of allyltrimethoxysilane, 3-(trimethoxysilyl)propyl acrylate, methacryloyloxymethyltriethoxysilane, 3-(methacryloyloxy)propyltrimethoxysilane (abbreviated KH-570), vinyltriethoxysilane, and vinyltrimethoxysilane.

[0082] Furthermore, in some embodiments, the raw materials of the polyacrylate unit further include an initiator and a first neutralizing agent, wherein the mass ratio of the first neutralizing agent to the unsaturated carboxylic acid monomer is 0.3:1-1.3:1.

[0083] By adding an initiator, the reaction rate of hydroxyacrylate monomers and unsaturated carboxylic acid monomers can be increased, ensuring the smooth progress of the polymerization reaction. As a preferred embodiment, this application uses a redox initiator. Its oxidizing component initiates the formation of free radicals from carbon-carbon double bonds to start the polymerization chain reaction, while its reducing component efficiently quenches residual free radicals in the later stages of the reaction. This avoids residual carbon-carbon double bonds in the PUA adhesive, further improving the PUA adhesive's resistance to electrolyte corrosion, effectively inhibiting its dissolution in the electrolyte, and enhancing the structural stability of the bonding interface.

[0084] For example, the redox initiator may be at least one of the following: benzoyl peroxide / sucrose, tert-butyl hydroperoxide / sodium formaldehyde sulfoxylate, tert-butyl hydroperoxide / sodium metabisulfite, benzoyl peroxide / N,N-dimethylaniline, ammonium persulfate / sodium bisulfite, potassium persulfate / sodium bisulfite, hydroperoxide / tartaric acid, hydroperoxide / sodium formaldehyde sulfoxylate, ammonium persulfate / ferrous sulfate, hydroperoxide / ferrous sulfate, benzoyl peroxide / N,N-diethylaniline, benzoyl peroxide / ferrous pyrophosphate, potassium persulfate / silver nitrate, persulfate / thiol, cumene hydroperoxide / ferrous chloride, potassium persulfate / ferrous chloride, hydroperoxide / ferrous chloride, and cumene hydroperoxide / tetraethyleneimine.

[0085] By controlling the mass ratio of the first neutralizing agent to the unsaturated carboxylic acid monomer within the range of 0.3:1 to 1.3:1, appropriate salting of the carboxylic acid functional groups in the PUA emulsion can be achieved, thereby ensuring the dispersion and storage stability of the PUA emulsion. When used for negative electrode bonding in lithium-ion secondary batteries, it can not only ensure high bonding strength but also improve the lithium-ion transport rate and optimize the cycle stability and kinetic performance of the battery.

[0086] If the mass ratio of the first neutralizing agent to the unsaturated carboxylic acid monomer is less than 0.3:1, the salinization degree of the PUA emulsion will be insufficient. This will not only affect the stability of the PUA emulsion and lead to a decrease in the bonding effect, but also hinder lithium-ion transport, thereby affecting the cycle performance and kinetics of the battery. Conversely, if the mass ratio of the first neutralizing agent to the unsaturated carboxylic acid monomer is greater than 1.3:1, the salinization degree of the PUA emulsion will be too high, resulting in excessive hydrophilicity. This makes it difficult to dry during electrode preparation, and excessive residual moisture inside the battery will trigger a large number of side reactions, thus affecting the stable performance of the battery.

[0087] The mass ratio of the first neutralizing agent to the unsaturated carboxylic acid monomer can be obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES). For example, the mass ratio of the first neutralizing agent to the unsaturated carboxylic acid monomer can be 0.3:1, 0.4:1, 0.5:1, 0.55:1, 0.6:1, 0.65:1, 0.7:1, 0.75:1, 0.8:1, 0.85:1, 0.9:1, 1.0:1, 1.1:1, 1.2:1, 1.3:1, or a value within any range of two of the above values.

[0088] In some embodiments, the mass content of the polyurethane unit is 75%-97.5% based on the mass of the polyurethane acrylate. By controlling the mass content of the polyurethane unit in the PUA to be no less than 75%, the dominant phase status of the polyurethane unit is ensured, allowing its cross-linked structure rigidity and soft segment flexibility to be fully utilized. This effectively buffers the stress generated by the huge volume changes of the silicon-based material during charging and discharging, inhibits the pulverization and shedding of the active material, and maintains the structural stability of the silicon-based anode. At the same time, controlling the mass content of the polyurethane unit in the PUA to be no more than 97.5% ensures that enough acrylic functional groups (derived from polyacrylate units) participate in the formation of the three-dimensional network structure, further enhancing the mechanical properties and chemical resistance of the PUA binder. This allows the PUA binder to maintain the structural stability of the silicon-based anode more effectively and for longer, thereby significantly improving the cycle performance of the battery. Furthermore, it also ensures that there are enough sites (derived from polyacrylate units) for introducing organosilicon segments, thereby improving the dispersion performance of the conductive agent in the anode slurry, increasing the slurry yield, and improving the battery kinetic performance.

[0089] If the mass content of polyurethane units in the PUA is less than 75%, the elastic buffering capacity of the PUA binder is insufficient, making it difficult to effectively cope with the drastic volume deformation of the silicon-based material, resulting in a decrease in battery cycle stability. Conversely, if the mass content of polyurethane units in the PUA is higher than 97.5%, it means that the content of polyacrylate units is too low, which cannot improve the mechanical properties and chemical resistance of the PUA binder, nor can it improve the dispersibility of the conductive agent, leading to a deterioration in the battery's cycle performance and kinetics.

[0090] The mass content of polyurethane units in the PUA can be determined by pyrolysis gas chromatography-mass spectrometry. For example, the mass content of polyurethane units in the PUA can be 75.0%, 76.6%, 78.2%, 79.8%, 81.4%, 83.0%, 84.6%, 86.2%, 87.8%, 89.4%, 91.0%, 92.6%, 94.2%, 95.8%, 97.5%, or values ​​within any range of two of the above values.

[0091] As is well known, polyurethane, or polyurethane, is formed by the condensation reaction of polyols and polyisocyanates. This results in polyurethane molecular chains consisting of hard and soft segments. The hard segments are mainly formed by the reaction of polyisocyanates, chain extenders, and crosslinking agents. These segments possess high cohesive energy, large volume, and rigidity. The soft segments refer to the polymeric polyols on the carbon-carbon backbone, giving polyurethane its excellent flexibility. Therefore, the characteristics of both hard and soft segments jointly determine the physicochemical properties of polyurethane. Consequently, the molecular design of the hard and soft segment structures in polyurethane becomes a key factor in improving its performance.

[0092] In some embodiments, the raw materials of the polyurethane unit include hard segment monomers, soft segment monomers, crosslinking agents, hydrophilic chain extenders, and a second neutralizing agent; wherein: the hard segment monomers include polyisocyanates, the polyisocyanates including at least one aliphatic ring; the soft segment monomers include chain-like aliphatic polyols, the chain-like aliphatic polyols having a main chain carbon number of not less than 3; the crosslinking agent includes at least 3 hydroxyl groups; based on the total mass of the raw materials of the polyurethane unit, the mass content of the crosslinking agent is 0.1%-5%; the hydrophilic chain extender includes at least 2 hydroxyl groups and at least 1 carboxyl group, and the mass ratio of the second neutralizing agent to the hydrophilic chain extender is 0.25:1-0.85:1.

[0093] By employing polyisocyanates with aliphatic ring structures as hard segment monomers, their cyclic structure provides the polyurethane units with the necessary rigid support, ensuring the firm anchoring of the PUA binder to the active material and current collector. At the same time, by employing chain-like aliphatic polyols with no less than 3 carbon atoms in the main chain as soft segment monomers, their flexible segments endow the polyurethane units with excellent elastic recovery capabilities. This "rigid-flexible" microphase separation structure provides a stable mechanical support foundation for the polyacrylate units, further ensuring that the PUA binder can effectively buffer volume strain during the charging and discharging process of silicon-based materials.

[0094] Meanwhile, by using a crosslinking agent containing at least three hydroxyl groups and controlling its mass content to 0.1%-5%, the polyurethane unit can construct a moderately crosslinked three-dimensional network. This crosslinked structure, on the one hand, allows the polyurethane unit to maintain a moderately swollen state in the electrolyte, forming a sufficient number of continuous lithium-ion transport channels, which helps to improve the ion conductivity and reaction rate of the silicon-based anode, thereby improving the kinetic performance of the battery. On the other hand, it can also significantly enhance the structural stability of the polyurethane unit after swelling (i.e., less dissolution), ensuring that the PUA maintains an intact shell structure during long-term cycling, preventing shell function failure due to excessive swelling or collapse of the polyurethane unit, and thus affecting the formation of a uniform and stable conductive network and a strong and tough bonding interface of the PUA binder inside the silicon-based anode. Thirdly, the appropriate amount of crosslinking agent can ensure that the polyurethane unit has a suitable crosslinking density, which is conducive to reducing the glass transition temperature (Tg) of the PUA, ensuring that the PUA binder maintains its flexibility within the battery operating temperature range, so as to effectively cope with the volume changes of the silicon-based material.

[0095] Based on this, this application introduces a hydrophilic chain extender containing a carboxyl group and at least two hydroxyl groups into the polyurethane unit by using a hydrophilic chain extender containing a carboxyl group and controlling the mass ratio of the second neutralizer to the hydrophilic chain extender to be 0.25:1-0.85:1. Under the action of an appropriate amount of the second neutralizer, the carboxyl group can be converted into the corresponding carboxylate (i.e., salting), thereby achieving stable co-emulsification with the polyacrylate unit. This not only endows PUA with good water dispersibility and emulsion stability, ensuring the uniformity of mixing among raw materials during the preparation of the negative electrode slurry, thus guaranteeing high bonding strength, but also enhances the compactness of the PUA adhesive film after film formation through ionic crosslinking, thereby improving the PUA adhesive's tolerance to electrolyte and long-term bonding reliability.

[0096] In summary, this application, by adjusting and designing the molecular structure of raw materials used to prepare polyurethane units, such as hard-segment monomers, soft-segment monomers, crosslinking agents, and hydrophilic chain extenders, and by strictly controlling the amount of crosslinking agents and hydrophilic chain extenders, can construct polyurethane units with appropriate ratios of rigidity and flexibility. This provides stable mechanical support for polyacrylate units, allowing the two major functional units in PUA to fully leverage their respective advantages. Ultimately, this enables the PUA emulsion in the high-silicon anode system to simultaneously meet the processing requirements of low slurry viscosity and good dispersibility of conductive agents, as well as the mechanical requirements of good structural stability of silicon-based anodes, thereby ensuring the cycle life and kinetic performance of the battery.

[0097] The chemical structure of hard segment monomers can be determined by thermogravimetric-infrared spectroscopy.

[0098] The chemical structure of the soft segment monomer can be determined and resolved by thermogravimetric-pyrolysis gas chromatography-mass spectrometry.

[0099] The chemical structure of the crosslinking agent can be determined by thermogravimetric-pyrolysis gas chromatography-mass spectrometry.

[0100] The chemical structure of hydrophilic chain extenders can be determined by combining thermogravimetric-infrared spectroscopy with thermogravimetric-pyrolysis gas chromatography-mass spectrometry.

[0101] This study found that if the polyisocyanate does not contain a cyclic structure, the polyurethane unit will lack rigidity, making it difficult to guarantee its bonding strength and dimensional stability. At the same time, it will also reduce the tolerance of the PUA binder to the electrolyte, making it impossible to prevent the PUA binder from dissolving in the electrolyte, thus affecting the stability of the electrode structure. When the cyclic structure in the polyisocyanate is an aromatic ring, although it helps to improve the structural rigidity, the good compatibility between the aromatic ring and the electrolyte will accelerate the dissolution of the PUA binder in the electrolyte, which is also detrimental to the stability of the electrode structure.

[0102] If the main chain carbon atoms of a chain aliphatic polyol are less than 3, it is difficult to form effective flexible segments, resulting in a higher glass transition temperature of PUA. The flexibility and elastic recovery ability of the film after film formation are reduced, and it cannot effectively buffer the volume change stress of silicon-based anode during charging and discharging.

[0103] If the number of hydroxyl groups in the crosslinking agent molecule is less than 3, a polyurethane system with a three-dimensional network structure cannot be constructed. This not only affects the bonding strength and bonding durability of the PUA emulsion, but also hinders the improvement of PUA's electrolyte retention capacity and the inhibition of PUA dissolution in the electrolyte, thus causing a decline in battery cycle life and kinetic performance. Furthermore, when the mass content of the crosslinking agent is less than 0.1%, the crosslinking density of PUA molecules is insufficient, making it difficult to form an effective three-dimensional network structure. This results in a significant decrease in the bonding strength, electrolyte retention capacity, and electrolyte corrosion resistance of the film after formation, failing to maintain the long-term stability of the bonding interface and hindering the improvement of battery kinetics. Conversely, when the mass content of the crosslinking agent is greater than 5%, the crosslinking density is too high, restricting the movement of PUA molecular chains and easily causing film embrittlement, decreased elasticity, and loss of deformation recovery ability. It may also block lithium-ion transport channels, weakening the kinetics of electrode reactions, which is detrimental to improving battery cycle performance and kinetic performance.

[0104] It should be noted that the mass content of the crosslinking agent can be obtained through thermogravimetric analysis. For example, the mass content of the crosslinking agent can be 0.1%, 0.3%, 0.5%, 0.7%, 0.9%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5%, or a value within any two of the above ranges.

[0105] Similar to the role of crosslinking agents, if the hydrophilic chain extender has only one hydroxyl group in its molecular structure, it is also detrimental to the construction of the three-dimensional network structure of polyurethane units. This, in turn, affects the adhesion strength, liquid retention capacity, and electrolyte corrosion resistance of the PUA emulsion film after formation, which is not conducive to improving the cycle performance and kinetic performance of the battery. Furthermore, when the mass ratio of neutralizer to hydrophilic chain extender is less than 0.25:1, the degree of carboxyl group salification is insufficient, resulting in a lower anionic charge density in the polyurethane units, weakened electrostatic repulsion, increased PUA emulsion particle size, and decreased dispersion stability. This makes it prone to particle aggregation or sedimentation, affecting the coating processability of the slurry. Simultaneously, the unneutralized residual carboxyl groups have better compatibility with the electrolyte, which may lead to the film being more easily dissolved in the electrolyte, affecting the stability of the electrode structure. Conversely, if the mass ratio of neutralizer to hydrophilic chain extender is large... A ratio of 0.85:1 results in excessive carboxyl saltification, causing the polyurethane units to carry too many anionic charges and become overly hydrophilic. This leads to excessively small PUA particle size or an excessively thick interfacial double layer, which in turn reduces the stability of the PUA emulsion. Furthermore, after film formation, the excessively high ion density can exacerbate the excessive swelling of the film in the electrolyte, resulting in decreased mechanical strength and an increased risk of polyurethane unit chain dissolution. It may also accelerate electrolyte side reactions, impair the chemical stability and long-term durability of the PUA binder, and ultimately affect the cycle life of the battery.

[0106] For example, the mass ratio of the second neutralizing agent to the hydrophilic chain extender can be, for example, 0.25:1, 0.35:1, 0.45:1, 0.55:1, 0.65:1, 0.70:1, 0.75:1, 0.80:1, 0.85:1, or a value within the range of any two of the above values.

[0107] In some embodiments, the mass content of the hard segment monomer is 30%-75% based on the total mass of the raw materials of the polyurethane unit. By strictly controlling the mass content of the hard segment monomer, the rigidity and flexibility of the polyurethane unit structure can be adjusted, ensuring that it has sufficient rigidity while also possessing a certain degree of flexibility. This not only guarantees the high bonding strength of PUA and the durability of its bonding effect, but also allows it to adapt to the volume changes of the silicon-based material, ensuring the structural integrity of the silicon-based anode and the stability of the battery's electrochemical performance. If the mass content of the hard segment monomer is less than 30%, the rigidity of the polyurethane unit is insufficient, resulting in a decrease in the mechanical strength and electrolyte swelling resistance of the PUA film after film formation, making it difficult to resist the stress generated by the volume changes of the silicon-based anode. Conversely, if the mass content of the hard segment monomer is greater than 75%, it means that the flexibility of the polyurethane unit is insufficient, significantly reducing the flexibility and elastic recovery ability of the PUA film after film formation, which can easily lead to cracking or active material shedding of the silicon-based anode during cycling, affecting the electrochemical performance of the battery.

[0108] It should be noted that the mass content of hard segment monomers can be obtained by thermogravimetric-infrared spectroscopy (TGA). For example, the mass content of hard segment monomers can be 30%, 35%, 40%, 45%, 50%, 52.5%, 55%, 57.5%, 60%, 62.5%, 65%, 67.5%, 70%, 72.5%, 75%, or values ​​within any range of two of the above values.

[0109] In some embodiments, the aliphatic rings in the hard segment monomer have 5 or 6 carbon atoms. Compared to aliphatic rings with other carbon atom numbers, pentagonal or hexagram aliphatic rings have lower ring strain and better structural stability. This ensures that the polyurethane unit has sufficient rigidity to maintain the bonding strength and dimensional stability of the PUA binder. Furthermore, pentagonal or hexagram aliphatic rings also have greater steric hindrance, which helps enhance the PUA binder's resistance to electrolytes and inhibits the dissolution of the PUA binder in the electrolyte, thereby ensuring the interfacial integrity and electrochemical performance of the battery during long-term cycling.

[0110] For example, the hard segment monomer includes at least one of 1,4-cyclohexane diisocyanate, 1,3-cyclohexane diisocyanate, hydrogenated toluene diisocyanate, isophorone diisocyanate, norbornane diisocyanate, dicyclohexylmethane diisocyanate, and norbornane dimethylene isocyanate, wherein hydrogenated toluene diisocyanate is a mixture of compounds composed of cyclohexane diisocyanates with different substitution sites (mainly 1,3- and 1,4-positions) and spatial configurations (cis and trans), and the CAS number of norbornane dimethylene isocyanate is 74091-64-8.

[0111] In some embodiments, when the polyacrylate unit and the polyurethane unit are located in the same shell, the polyisocyanate does not contain any unsaturated groups other than isocyanate groups, and / or the main chain of the chain aliphatic polyol does not contain heteroatoms or unsaturated groups. This improves the PUA adhesive's resistance to electrolyte corrosion, effectively inhibits the dissolution of the PUA adhesive in the electrolyte, and thus enhances the structural stability of the bonding interface.

[0112] It should be noted that the term "unsaturated group" is used in contrast to "saturated group." A "saturated group" refers to a group in which each carbon atom forms four single bonds (i.e., covalent bonds) and has no remaining valence electrons to form more bonds. Conversely, an "unsaturated group" refers to a group containing unsaturated bonds such as double or triple bonds, or a cyclic structure. For example, unsaturated groups include, but are not limited to, at least one of carbon-carbon double bonds, carbon-oxygen double bonds, nitrogen-nitrogen double bonds, carbon-carbon triple bonds, nitrogen-nitrogen triple bonds, carbonates, and phenyl groups. In this application, the term "heteroatom" includes, but is not limited to, at least one of oxygen, sulfur, and selenium.

[0113] In some embodiments, the main chain carbon number of the chain-like aliphatic polyol does not exceed 200. This avoids the polyurethane unit being too flexible, resulting in insufficient rigidity, which would affect the bonding strength and electrolyte resistance of the PUA. Furthermore, if the main chain carbon number of the chain-like aliphatic polyol is too large, i.e., the carbon chain is too long, it may also lead to excessive entanglement of the polyurethane unit chain segments, resulting in a significant decrease in the crosslinking density and molecular chain mobility of the polyurethane unit. This would cause a decrease in the strength of the PUA emulsion film after film formation, a poorer deformation recovery ability, and difficulty in effectively suppressing the volume change of the silicon-based material.

[0114] For example, the number of carbon atoms in the main carbon chain of a chain aliphatic polyol can be 3, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, or a value within the range of any two of the above values.

[0115] Furthermore, in some embodiments, the chain-like aliphatic polyol further includes branches, each branch comprising at least one carbon atom, and the number of branches not exceeding 40. Exemplarily, the branches may be at least one of alkyl, alkoxy, alkylamine, cyano, or amide groups.

[0116] By introducing side chains into the main chain of chain-like aliphatic polyols, the steric hindrance effect is used to suppress the regular arrangement of the entire molecular chain, reduce the tendency of soft segment crystallization, and thus enhance the flexibility of polyurethane units. At the same time, the side chain structure can also adjust the hydrophobicity and spatial configuration of polyurethane units, so that the PUA binder and electrolyte have suitable compatibility. This can ensure the liquid retention capacity of PUA binder in electrolyte, and prevent excessive swelling or even dissolution of PUA binder in electrolyte, thereby balancing the cycle life and kinetic performance of the battery.

[0117] If the number of branches in a chain-like aliphatic polyol exceeds 40, it will lead to excessive entanglement of polyurethane unit segments, resulting in a significant decrease in the crosslinking density and molecular chain mobility of PUA. This causes a decrease in the strength and deformation recovery ability of the PUA emulsion film after film formation, making it difficult to effectively suppress the volume change of silicon-based materials. For example, the number of branches in a chain-like aliphatic polyol can be 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, or values ​​within any two of the above ranges.

[0118] For example, the soft segment monomer includes at least one of 2-methyl-1,5-pentanediol, 2,2-dimethyl-1,3-propanediol, 12-hydroxystearic acid, 6-aminohexanoic acid, dihydroxy-terminated polyethylene, and dihydroxy-terminated hydrogenated polybutadiene.

[0119] In some embodiments, the mass content of the soft segment monomer is 15%-63% based on the total mass of the raw materials of the polyurethane unit. By strictly controlling the mass content of the soft segment monomer, the rigidity and flexibility of the polyurethane unit structure can be adjusted, making it both sufficiently rigid and flexible. This not only ensures the high bonding strength and durability of the PUA binder, but also adapts to the volume changes of the silicon-based material, ensuring the structural integrity of the silicon-based anode and the stability of the battery's electrochemical performance. If the mass content of the soft segment monomer is less than 15%, the polyurethane unit will lack flexibility, and the flexibility and elastic recovery ability of the film after PUA emulsion film formation will be significantly reduced, easily leading to cracking of the silicon-based anode or shedding of active material during cycling, affecting the battery's electrochemical performance. Conversely, if the mass content of the soft segment monomer is higher than 63%, the rigid support of the polyurethane unit will be weakened, and the mechanical strength and electrolyte swelling resistance of the film after PUA emulsion film formation will decrease, making it difficult to resist the stress generated by the volume changes of the silicon-based anode.

[0120] It should be noted that the mass content of soft segment monomers can be obtained through thermogravimetric analysis. For example, the mass content of soft segment monomers can be 15%, 16%, 17%, 18%, 19%, 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54%, 56%, 58%, 60%, 61%, 62%, 63%, or values ​​within any two of the above ranges.

[0121] In some embodiments, the molecular weight of the crosslinking agent does not exceed 300. This allows the crosslinking agent to have a smaller volume and higher reactivity, making it easier to penetrate into the prepolymer of the polyurethane unit and react fully with the isocyanate groups to form a uniform and stable three-dimensional crosslinked network. This is beneficial for improving the adhesion strength, electrolyte retention capacity, and electrolyte corrosion resistance of the PUA emulsion film after formation, thereby improving the cycle performance and kinetic performance of the battery. If the molecular weight of the crosslinking agent is greater than 300, its steric hindrance increases, and its diffusion capacity and reaction efficiency decrease, which may lead to incomplete crosslinking reaction and thus affect the formation of the three-dimensional crosslinked structure of the polyurethane unit.

[0122] The molecular weight of the crosslinking agent can be determined by pyrolysis gas chromatography-mass spectrometry.

[0123] For example, the crosslinking agent includes at least one of trimethylolpropane, glycerol, pentaerythritol, tri(2-hydroxyethyl) isocyanurate, and trihydroxypolyoxypropylene ether.

[0124] In some embodiments, the molecular weight of the hydrophilic chain extender does not exceed 200. This allows the hydrophilic chain extender to have a small volume and high reactivity, enabling it to efficiently participate in the chain extension reaction of polyurethane units and distribute uniformly within the polyurethane unit segments. This introduces hydrophilic groups into the polyurethane units, providing a material basis for the preparation of PUA emulsions. If the molecular weight of the hydrophilic chain extender is greater than 200, its steric hindrance increases, and its reactivity decreases accordingly. This may lead to incomplete chain extension reactions or uneven distribution of hydrophilic groups, affecting the dispersion stability of the PUA emulsion and the density of the film after film formation, thus hindering the improvement of the PUA emulsion's adhesion effect.

[0125] The molecular weight of hydrophilic chain extenders can be determined by pyrolysis gas chromatography-mass spectrometry.

[0126] For example, the hydrophilic chain extender includes at least one of 2,2-dimethylolpropionic acid, 2,2-dimethylolbutyric acid, 3,5-dihydroxy-3-methylvaleric acid, dihydroxymaleic acid, 2,6-dihydroxybenzoic acid, and 3,4-diaminobenzoic acid.

[0127] Furthermore, in some embodiments, the raw materials for the polyurethane unit further include a catalyst, which includes at least one of organotin compounds and organobismuth compounds. The use of organotin and / or organobismuth catalysts can significantly reduce the activation energy of the reaction between the hard segment monomer and at least one of the soft segment monomer, crosslinking agent, hydrophilic chain extender, and amine chain extender, thereby improving the polymerization efficiency and ensuring the high regularity and high degree of polymerization of the polyurethane unit structure.

[0128] For example, the catalyst may be at least one of dibutyltin dilaurate, stannous octanoate, dibutyltin diacetate, dibutyltin thiolate, bismuth isooctanoate, and bismuth neodecanoate. The specific type of catalyst can be determined by a combination of inductively coupled plasma atomic emission spectrometry and infrared spectroscopy.

[0129] Furthermore, in some embodiments, the raw material of the polyurethane unit further includes an amine chain extender, which comprises at least two amine groups. Adding an amine chain extender containing at least two amine groups enhances the structural rigidity and cohesive strength of the polyurethane unit, as each amine group can react with the isocyanate group (-NCO) in the hard segment monomer to form a urea bond (-NH-CO-NH-). This allows the PUA adhesive to maintain excellent interfacial adhesion and long-term cycle stability during battery charging and discharging. If the amine chain extender contains only one amine group, it is difficult to effectively extend and crosslink the polyurethane unit chain segments, resulting in a lower degree of polymerization and a loose chain structure, which affects the mechanical strength and adhesion durability of the PUA film after formation.

[0130] For example, the amine chain extender includes at least one selected from ethylenediamine, isophorone diamine, diethyltoluene diamine, di-n-butylamine, diethylenetriamine, triethylamine, and 4,4'-diaminodicyclohexylmethane. The specific types of amine chain extenders can be determined by pyrolysis gas chromatography-mass spectrometry.

[0131] It should be noted that, firstly, this application employs a two-step method (i.e., the prepolymer method) to prepare polyurethane. Firstly, the soft-segment monomer reacts with the hard-segment monomer to generate an adduct with isocyanate groups at both ends, i.e., a prepolymer. Then, by adjusting the viscosity of the prepolymer and adding a neutralizing agent and water for emulsification, a chain extension reaction is carried out to increase the molecular weight of the prepolymer, resulting in a high-molecular-weight waterborne polyurethane. During the above process, the degree of crosslinking of the prepolymer or polyurethane can also be increased through a crosslinking reaction. Further, before the prepolymer is emulsified or during the chain extension stage of the prepolymer, hydroxy acrylate monomers, unsaturated carboxylic acid monomers, and organosilicon monomers are added to the reaction system. If necessary, an initiator can be added to carry out a copolymerization reaction. After a second neutralization, a polyurethane acrylate emulsion with a three-dimensional crosslinked structure is finally obtained.

[0132] In some embodiments, the number-average molecular weight of the polyurethane acrylate is 20,000 to 40,000. This ensures that the PUA emulsion film has high bonding strength, good liquid retention capacity, and resistance to electrolyte corrosion, thereby improving the cycle performance and kinetic performance of the battery. If the number-average molecular weight of PUA is lower than 20,000, the cohesive strength and adhesion of the PUA emulsion will be reduced, making it difficult to effectively resist the volume change stress of the silicon-based anode during charging and discharging. Conversely, if the number-average molecular weight of PUA is higher than 40,000, the mobility of PUA segments will be reduced, leading to decreased flexibility and deterioration of processing performance of the PUA film. It may also hinder the transport of lithium ions in the electrode sheet, thus affecting the kinetic performance of the battery.

[0133] It should be noted that the method for determining the number-average molecular weight of PUA in the PUA emulsion includes: drying the PUA emulsion to obtain a PUA film; taking a film sample, dissolving it in tetrahydrofuran, filtering it, and injecting it into a gel permeation chromatograph for molecular weight analysis, and calibrating it using a polystyrene standard sample.

[0134] For example, the number-average molecular weight of PUA can be 20,000, 22,000, 24,000, 26,000, 28,000, 30,000, 32,000, 34,000, 36,000, 38,000, 40,000, or a value within the range of any two of the above values.

[0135] Furthermore, in some embodiments, the glass transition temperature of the polyurethane acrylate is -40°C to -20°C. Thus, when the PUA emulsion is used as a binder for silicon-based anodes, it imparts excellent flexibility and elastic recovery to the anode sheet. This not only facilitates easier rolling and pressing of the electrode sheet during the manufacturing process, improving processing yield, but also allows the anode sheet to effectively adapt to significant volume changes during battery charging and discharging, alleviating stress concentration and preventing the shedding of the anode active material.

[0136] The glass transition temperature of polyurethane acrylate can be determined by differential scanning calorimetry (DSC). The specific test method includes: drying the polyurethane acrylate emulsion to obtain a polyurethane acrylate film; taking 5-10 mg of film sample, accurately weighing it and placing it in a sealed DSC-specific aluminum crucible, while using an empty crucible as a reference crucible; and performing a cyclic scan of heating-cooling-reheating at a rate of 10℃ / min and a temperature range of -80℃ to 150℃ under a nitrogen atmosphere, recording the inflection point of the glass transition region in the heat flow-temperature curve, which is the glass transition temperature Tg.

[0137] In DSC testing, the heat capacity and thermal properties of the reference crucible are provided solely by the crucible itself. Its core function is to establish a stable thermodynamic benchmark. By measuring the heat flow difference between the sample crucible and the reference crucible, the thermal effects generated by the sample during the heating process can be accurately identified and quantified.

[0138] For example, the glass transition temperature of polyurethane acrylate may be -40℃, -38℃, -36℃, -34℃, -32℃, -30℃, -29℃, -28℃, -27℃, -26℃, -25℃, -24℃, -23℃, -21℃, -20℃, or a value within any two of the above ranges.

[0139] Furthermore, in some embodiments, the particle size Dn90 of the polyurethane acrylate emulsion is 100 nm to 500 nm. This means that the ratio of hydrophilic to lipophilic segments in the PUA molecule is appropriate, resulting in good dispersibility and stability of the PUA emulsion, avoiding the impact of PUA particle aggregation or sedimentation on the uniformity of PUA binder distribution in the electrode. If the particle size Dn90 of the PUA emulsion is greater than 500 nm, the dispersion stability of the PUA emulsion is poor, and sedimentation or flocculation is likely to occur, affecting the uniformity of slurry coating. This results in structural defects in the film formed by the PUA emulsion, making it unable to maintain a long-term bonding effect. If the particle size Dn90 of the PUA emulsion is less than 100 nm, it may cause excessive swelling of the PUA film in the electrolyte, affecting the durability of the bonding effect.

[0140] The particle size Dn90 of polyurethane acrylate emulsion refers to the number of particles with a diameter less than or equal to this value when all particles in the sample are counted. The average particle size and particle size distribution (characterized by Dn90) of the polyurethane acrylate emulsion were determined using a laser particle size analyzer. Before testing, the polyurethane acrylate emulsion was diluted with deionized water at a volume ratio of 1:10 and subjected to ultrasonic degassing for 5 minutes. The test was conducted under constant temperature conditions of 25℃, and the Dn90 data were finally summarized.

[0141] For example, the particle size Dn90 of the polyurethane acrylate emulsion can be, for example, 100nm, 150nm, 180nm, 200nm, 250nm, 280nm, 300nm, 350nm, 380nm, 400nm, 450nm, 480nm, 500nm, etc., or a value within the range of any two of the above values.

[0142] Furthermore, in some embodiments, the polyurethane acrylate emulsion is formed into a film, which, after being immersed in a standard lithium-ion electrolyte at 60°C, satisfies at least one of the following conditions:

[0143] A. Swelling rate is 30-90%;

[0144] B. Dissolution rate not exceeding 5%;

[0145] C. Elongation at break is not less than 150%;

[0146] D. The fracture strength is not less than 8 MPa.

[0147] The electrolyte comprises an organic solvent, a lithium salt, and additives. The organic solvent comprises ethylene carbonate (EC), propylene carbonate (PC), propyl propionate (PP), and diethyl carbonate (DEC). The lithium salt comprises lithium hexafluorophosphate. The additives comprise at least one of fluoroethers, fluoroethylene carbonate (FEC), fluoronitrile, and polyarylene ether nitrile.

[0148] For example, in this electrolyte, the mass ratio of EC:PC:PP:DEC is 1:1:1:1, the molar concentration of lithium hexafluorophosphate is 1.05 mol / L, and the mass content of additives is 0.5%-10%; wherein, the fluorinated ether can be, for example, at least one of perfluoropolyether (PFPE) and bis(2,2,2-trifluoroethoxy)ethane (BTFEOE), and the fluorinated nitrile can be, for example, 3,5-bis(trifluoromethylbenzonitrile) (BFBN). In this application, the specific method for preparing a polyurethane acrylate emulsion into a film includes: determining the solid content of the polyurethane acrylate emulsion according to the national standard GB / T 1725-2007; pouring an appropriate amount of polyurethane acrylate emulsion into a polytetrafluoroethylene mold; allowing it to stand at room temperature, horizontally, and in a dust-free environment until surface dry; then transferring the sample to a 50°C constant temperature oven for continuous drying until the film quality is constant, finally obtaining a polyurethane acrylate film with a smooth surface and no bubbles; and storing it in a desiccator after demolding for later use.

[0149] After being immersed in electrolyte at 60°C, the swelling rate of the film is in the range of 30% to 90%. Thus, when PUA emulsion is used as a binder for silicon-based negative electrodes, the PUA binder can have good electrolyte retention capacity, thereby increasing lithium-ion diffusion channels and improving the kinetic performance of the battery.

[0150] It should be noted that the swelling rate of the adhesive film refers to the ratio of the increase in mass or volume of the film after immersion in the electrolyte for a certain period of time to the initial mass or volume, usually expressed as a percentage. For example, the specific method for determining the swelling rate of the adhesive film includes: completely immersing a well-formed adhesive film (thickness 100μm-200μm) with an initial mass of m1 into the electrolyte, allowing it to stand at a constant temperature of 60℃ for 72 hours, then removing it and gently blotting off any residual electrolyte on the surface with filter paper, weighing the film to obtain the swollen mass as m2; the formula for calculating the mass swelling rate is: Swelling rate (%) = (m2-m1) / m1 × 100%.

[0151] For example, the swelling rate of the film can be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, etc., or a value within the range of any two of the above values.

[0152] After being immersed in an electrolyte at 60°C, the dissolution rate of the adhesive film does not exceed 5%. Thus, when using PUA emulsion as a binder for silicon-based anodes, it can enhance the durability of the bonding effect and maintain the structural stability of the bonding interface.

[0153] It should be noted that the dissolution rate of the film refers to the percentage of the mass loss of soluble components (such as uncrosslinked small molecules, free additives, or degradation fragments) after the film has been immersed in the electrolyte for a certain period of time, relative to its initial mass. The specific method for determining the dissolution rate of the film includes: completely immersing a well-structured film (100μm-200μm thick) with an initial mass of m1 into the electrolyte, allowing it to stand at a constant temperature of 60℃ for 72 hours, then removing it and gently blotting off any residual electrolyte with filter paper. The film is then dried in an oven at 105℃ for 2 hours until a constant weight is achieved, and the dried mass is measured as m3. The formula for calculating the mass dissolution rate is: Dissolution rate (%) = (m1-m3) / m1 × 100%.

[0154] For example, the dissolution rate of the film may be 0.0%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.2%, 4.4%, 4.6%, 4.8%, 5.0%, or a value within the range of any two of the above values.

[0155] After being immersed in an electrolyte at 60°C, the elongation at break of the PUA film is not less than 150%. Thus, when the PUA emulsion is used as a binder for silicon-based anodes, it can maintain high elastic deformation capacity in the electrolyte environment. This effectively suppresses repeated volume changes in silicon-based materials during charging and discharging, preventing electrode cracking or active material shedding due to stress concentration, thereby maintaining the integrity of the electrode structure and extending the battery's cycle life.

[0156] It should be noted that the elongation at break of the adhesive film refers to the ratio of the elongation of the gauge length to the original gauge length when the film breaks during a tensile test, expressed as a percentage. The formula for calculating the elongation at break of the adhesive film is: Elongation at break of adhesive film = (L - L0) / L0 × 100%, where L0 represents the original gauge length of the adhesive film before stretching, and L represents the gauge length at the time of breakage. The specific method for determining the elongation at break of the adhesive film includes: removing the adhesive film sample after immersing it in the electrolyte for three days, wiping off the electrolyte on the surface, cutting it into dumbbell-shaped standard specimens (refer to GB / T 1040.3-2006), and testing it on an electronic universal testing machine at a tensile speed of 0.1 mm / min and a clamping distance of 25 mm. The elongation at break (%) is then calculated according to the formula.

[0157] After being immersed in electrolyte at 60°C, the tensile strength of the PUA film is not less than 8 MPa. Therefore, when using PUA emulsion as a binder for silicon-based anodes, the PUA binder can maintain sufficiently high mechanical strength under extreme operating conditions, resisting the stress generated by repeated volume changes in the silicon-based anode, preventing cracking of the electrode coating or shedding of active material, thereby maintaining the stability of the electrode structure and extending the cycle life of the battery.

[0158] It should be noted that the tensile strength of an adhesive film refers to the maximum tensile force per unit area that the film can withstand when it breaks in a tensile test, and the unit is megapascals (MPa). The formula for calculating the tensile strength of an adhesive film is: Tensile strength of adhesive film = F max / A, where: F max A represents the maximum tensile force the adhesive film can withstand before it breaks (unit: N), and A represents the initial cross-sectional area of ​​the adhesive film before it is stretched (unit: mm). 2 The specific method for determining the tensile strength of the adhesive film includes: taking out the adhesive film sample after immersing it in the electrolyte for three days, wiping off the electrolyte on the surface, cutting it into dumbbell-shaped standard specimens (refer to GB / T1040.3-2006), and testing it on an electronic universal testing machine at a tensile speed of 0.1 mm / min and a clamping distance of 25 mm. The tensile strength (MPa) is calculated according to the calculation formula.

[0159] According to a second aspect of this application, a secondary battery is also provided, including a negative electrode sheet, the negative electrode sheet including a negative electrode current collector and a negative electrode active layer disposed on at least one side surface in the thickness direction of the negative electrode current collector, the negative electrode active layer including a negative electrode active material and a binder; the binder includes the polyurethane acrylate emulsion described in the first aspect of this application.

[0160] The secondary battery of this application uses the polyurethane acrylate emulsion described in the first aspect of this application as a binder in its negative electrode active layer. This PUA binder has the characteristics of high bonding strength, resistance to electrolyte corrosion, and good dispersibility of conductive agents. Therefore, the PUA binder has good interfacial compatibility with silicon-based materials, which can not only effectively anchor the silicon-based materials, but also maintain sufficient structural stability (i.e., low dissolution) after swelling in the electrolyte. This ensures that the bonding strength of the PUA binder does not decrease during the charge and discharge process, thereby effectively coping with the large volume change of silicon-based materials, solving the problem of silicon-based negative electrode interface failure, and improving the cycle performance of the battery. On the other hand, it can also improve the dispersion uniformity and stability of conductive agents in the negative electrode slurry, improve the slurry yield, and thus improve the kinetic performance of the battery.

[0161] In some embodiments, the negative electrode active material comprises a silicon-based material; the peel strength between the negative electrode active layer and the negative electrode current collector is not less than 15 N / m. This ensures that the PUA binder establishes a strong interfacial bond between the negative electrode active material and the current collector, effectively resisting the internal stress generated by the expansion of the silicon-based material, thereby preventing the negative electrode active material from detaching and ensuring the structural integrity of the electrode and the long cycle life of the battery. For example, the silicon-based material includes at least one of silicon-oxygen materials, silicon-carbon materials, and silicon-oxygen-carbon materials.

[0162] It should be noted that the peel strength test method includes: uniformly coating the prepared negative electrode slurry onto the copper foil current collector to obtain the electrode sheet, and then cutting it into standard test strips (15 mm wide); using the 180° peel method (refer to GB / T2790-1995 or ASTM D903 standard), a tensile test is performed using a tensile testing machine at a rate of 50 mm / min, the average force value during the peeling process is recorded, and the peel strength is converted into peel strength (unit: N / m) according to the following formula: Peel strength (N / m) = average peel force (N) / sample width (m).

[0163] Furthermore, in some embodiments, the negative electrode active layer further includes a conductive agent and a thickener; based on the mass of the negative electrode active layer, the mass content of the negative electrode active material is 90.2%-97.8%, the mass content of the binder is 1.2%-8%, the mass content of the conductive agent is 0.05%-10%, and the mass content of the thickener is 0.2%-1%.

[0164] For example, the mass content of the negative electrode active material can be, for example, 90.2%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 97.5%, 97.8%, etc., or values ​​within the range of any two of the above values; the mass content of the binder can be, for example, 1.2%, 2%, 2.5%, 3%, 4%, 5%, 6%, 7%, 7.5%, 8%, etc., or values ​​within the range of any two of the above values; The mass content of the conductive agent may be, for example, 0.05%, 0.1%, 0.5%, 1%, 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a value within any two of the above values; the mass content of the thickener may be, for example, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or a value within any two of the above values.

[0165] It is understood that the secondary battery further includes a positive electrode sheet, which includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector in the thickness direction. The positive active layer includes a positive active material, a conductive agent, and a binder. The positive current collector includes aluminum foil or other positive current collectors commonly used in the art, and the thickness of the positive current collector is 1μm-15μm. The mass content ratio of the positive active material, conductive agent, and binder in the positive active layer is (95%-98.5%):(0.1%-3%):(0.1%-5%).

[0166] For example, the thickness of the positive current collector can be 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, or a value within the range of any two of the above values.

[0167] For example, the positive electrode active material includes at least one of lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium iron phosphate (LFP), ternary materials (lithium nickel cobalt manganese oxide (NCM) and lithium nickel cobalt aluminum oxide (NCA)), lithium aluminate, and lithium manganese oxide; the conductive agent includes at least one of conductive carbon black, conductive graphite, graphene, and carbon nanotubes; the binder includes at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, a copolymer of styrene and acrylate, a copolymer of styrene and butadiene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinyl acetate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene.

[0168] The secondary battery further includes a separator, which comprises at least one of polyethylene, polypropylene, polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyethylene terephthalate, polyimide, and aramid, and the thickness of the separator is 3 μm-12 μm. Additionally, the surface of the separator includes a porous layer disposed on at least one side surface along the thickness direction of the separator, the porous layer comprising inorganic particles and a binder, and the pore diameter of the porous layer is 0.01 μm-1 μm.

[0169] For example, the thickness of the membrane can be 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, or a value within any two of the above values; the pore diameter of the porous layer can be 0.01μm, 0.02μm, 0.03μm, 0.05μm, 0.07μm, 0.1μm, 0.15μm, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 1μm, or a value within any two of the above values.

[0170] For example, the inorganic particles include at least one of alumina (Al2O3), silicon dioxide (SiO2), magnesium oxide (MgO), titanium oxide (TiO2), hafnium dioxide (HfO2), tin oxide (SnO2), cerium dioxide (CeO2), nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide (ZrO2), yttrium oxide (Y2O3), silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate; the binder in the porous layer includes at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, carboxymethyl cellulose salt, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene.

[0171] The present application will be further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application. Where specific experimental steps or conditions are not specified in the embodiments and comparative examples, they can be performed according to the conventional experimental steps or conditions described in the literature in the art. Unless otherwise specified, all materials used are commercially available analytical grade. The use of terms such as "parts" and "%" in the following text, unless otherwise specified, refers to mass fractions.

[0172] Example 1

[0173] This embodiment provides a method for preparing a secondary battery, including the following steps:

[0174] Step 1: Preparation of polyurethane acrylate emulsion

[0175] Add 126 parts of 1,4-cyclohexane diisocyanate, 166 parts of dihydroxy-terminated hydrogenated polybutadiene (number average molecular weight 2000), 11.6 parts of trimethylolpropane, and 55 parts of 2,2-dimethylolpropionic acid sequentially to a reactor equipped with temperature monitoring and stirring functions. Purge with nitrogen to remove oxygen, control the reaction temperature at 70-75℃, and react for 4 hours. Use di-n-butylamine titration to test the content of -NCO groups until no free -NCO residue remains. Use 150-450 parts of methyl ethyl ketone as a solvent to adjust the viscosity of the prepolymer. Lower the temperature to 60℃, add 28.4 parts of sodium hydroxide as a second neutralizing agent for neutralization, and slowly add 2714 parts of water to the reactor. Use an emulsifying disperser to... Emulsification and dispersion were performed, and 28 parts of ethylenediamine and 5 parts of dibutyltin dilaurate (diluted to 100 parts of water) were added for chain extension reaction. 15 parts of hydroxyethyl acrylate were added, and after reacting for 1 hour, 57 parts of acrylic acid and 3 parts of KH-570 were added. Under the condition of maintaining 60°C, 1.36 parts of ammonium persulfate dissolved in 191 parts of water were added dropwise over 2 hours. After reacting at this temperature for 1 hour, 32.7 parts of sodium hydroxide (the first neutralizing agent) were added for neutralization, and 1.36 parts of sodium bisulfite dissolved in 191 parts of water were added dropwise over 1 hour. After the reaction time was reached, methyl ethyl ketone was removed under reduced pressure to obtain a polyurethane acrylate emulsion with a solid content of about 20%.

[0176] Step 2: Preparation of the negative electrode

[0177] Artificial graphite, silicon carbon, the polyurethane acrylate emulsion prepared in the first step, carbon nanotubes, and lithium carboxymethyl cellulose were dissolved in deionized water at a weight ratio of 58.02:38.68:2.0:1:0.7 to form a negative electrode slurry. The negative electrode slurry was then coated onto both surfaces of a 6 μm thick negative electrode copper foil, with a coating weight of 3.5 mg / cm². 2 After drying, cold pressing, and slitting, a negative electrode sheet is obtained, wherein the silicon content in the negative electrode active layer is 19.34% by mass.

[0178] Step 3: Preparation of the positive electrode sheet

[0179] Lithium cobalt oxide, a conductive agent, and polyvinylidene fluoride (PVDF) binder were dissolved in N-methylpyrrolidone at a weight ratio of 97.2:1.1:1.7 to form a positive electrode slurry. An 8μm thick aluminum foil was used as the positive electrode current collector. The positive electrode slurry was coated onto both sides of the current collector at a coating weight of 13 mg / cm². 2 After drying, cold pressing, and slitting, positive electrode sheets are obtained.

[0180] Step 4: Preparation of the diaphragm

[0181] The diaphragm substrate is made of 7μm thick polyethylene. A 2μm thick alumina ceramic layer is coated on each of the two sides of the diaphragm substrate and then dried to obtain the diaphragm.

[0182] Step 5: Preparation of Electrolyte

[0183] In an environment with a water content of less than 10 ppm, lithium hexafluorophosphate was mixed evenly with a non-aqueous organic solvent (EC:PC:PP:DEC in a mass ratio of 1:1:1:1) and the additive fluoroethylene carbonate (FEC) to prepare an electrolyte with a lithium salt concentration of 1.05 mol / L and an FEC mass concentration of 8 wt%.

[0184] Step 6: Preparation of secondary batteries

[0185] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as a separator. The electrode assembly is then wound up to obtain the electrode assembly. The electrode assembly is placed in an outer aluminum-plastic film and dehydrated at 80°C. The electrolyte prepared in step five is then injected and the assembly is sealed. After processes such as formation, degassing, shaping, and capacity testing, a secondary battery is obtained.

[0186] Example 12

[0187] This embodiment provides a method for preparing a secondary battery, which differs from Embodiment 1 only in that:

[0188] Step 1: Preparation of polyurethane acrylate emulsion

[0189] Add 126 parts of 1,4-cyclohexane diisocyanate, 166 parts of dihydroxy-terminated hydrogenated polybutadiene (number average molecular weight 2000), 11.6 parts of trimethylolpropane, and 55 parts of 2,2-dimethylolpropionic acid sequentially to a reactor equipped with temperature monitoring and stirring functions. Purge with nitrogen to remove oxygen, control the reaction temperature at 70-75℃, and react for 4 hours. Use di-n-butylamine titration to test the content of -NCO groups until no free -NCO residue remains. Use 150-450 parts of methyl ethyl ketone as a solvent to adjust the viscosity of the prepolymer. Add 15 parts of hydroxyethyl acrylate, react for 1 hour, then add 57 parts of acrylic acid, 3 parts of KH-570, and 2 parts of sodium hydroxide as a second neutralizing agent. 8.4 parts were neutralized, the temperature was lowered to 60℃, and 2714 parts of water were slowly added to the reactor. Emulsification and dispersion were carried out using an emulsifier. 28 parts of ethylenediamine and 5 parts of dibutyltin dilaurate, diluted to 100 parts of water, were added as chain extenders to carry out chain extension reaction. Under the condition of maintaining 60℃, 1.36 parts of ammonium persulfate dissolved in 191 parts of water were added dropwise over 2 hours. After the reaction was kept at this temperature for 1 hour, 32.7 parts of sodium hydroxide, the first neutralizing agent, were added for neutralization. 1.36 parts of sodium bisulfite dissolved in 191 parts of water were added dropwise over 1 hour. After the reaction time was reached, methyl ethyl ketone was removed under reduced pressure to obtain a polyurethane acrylate emulsion with a solid content of about 20%.

[0190] The rest of the content is the same as in Example 1.

[0191] Example 13

[0192] The only difference from Example 12 is that the 1,4-cyclohexane diisocyanate is replaced with phenyl diisocyanate. Everything else is the same as in Example 12.

[0193] Example 14

[0194] The only difference from Example 12 is that the dihydroxy-terminated hydrogenated polybutadiene is replaced with a polycarbonate polyol. All other aspects are the same as in Example 12.

[0195] Example 35

[0196] The only difference from Example 1 is that the type of silicon-based material is changed, while the mass content of silicon in the negative electrode active layer remains unchanged. Everything else is the same as in Example 1.

[0197] Example 36

[0198] The only difference from Example 1 is that the type of silicon-based material is changed, and the mass content of silicon in the negative electrode active layer is 38.6%. Everything else is the same as in Example 1.

[0199] The preparation methods of Examples 2-11, Examples 15-34, and Comparative Examples 1-2 are basically the same as those of Example 1. The differences are shown in Tables 1, 2, and 3. In the table, " / " indicates that the item does not exist. "Double shell" refers to the double shell structure formed by polyacrylate units covering polyurethane units. "Composite shell" refers to the composite shell structure in which polyacrylate units and polyurethane units are located in the same shell layer.

[0200] The content of each component was calculated using the following methods: the content of organosilicon segments, polyacrylate units, and polyurethane units were all calculated based on the mass of polyurethane acrylate. Specifically, the mass of a polyacrylate unit equals the sum of the masses of hydroxyacrylate monomers, unsaturated carboxylic acid monomers, organosilicon monomers, and the first neutralizing agent; the mass of a polyurethane unit equals the sum of the masses of hard segment monomers, soft segment monomers, crosslinking agents, hydrophilic chain extenders, the second neutralizing agent, and amine chain extenders; and the mass of polyurethane acrylate equals the sum of the masses of polyurethane units and polyacrylate units. The content of hard segment monomers, soft segment monomers, crosslinking agents, and hydrophilic chain extenders were all calculated based on the total mass of the raw materials for the polyurethane units.

[0201] Table 1

[0202]

[0203]

[0204]

[0205] Table 2

[0206]

[0207]

[0208] Table 3

[0209]

[0210]

[0211] Test case

[0212] 1. Lithium plating test

[0213] At 25℃, 20 batteries from the same batch were charged at different charging rates XC (X is a multiple of 0.05, 1≤X≤6). The batteries were charged to 4.5V at the corresponding charging rate, then charged at a constant voltage of 4.5V to 0.05C, allowed to stand for 5 minutes, and then discharged at a constant current of 1C to 3.0V, allowed to stand for 5 minutes. This constituted one cycle, which was repeated 10 times. The batteries were then disassembled in a fully charged state. The minimum charging rate at which lithium plating occurred among the 20 batteries in the same batch was defined as the lithium plating window. Lithium plating was defined as the area of ​​the lithium plating region on the negative electrode being greater than or equal to 2 mm². 2 The lithium plating window (in C) characterizes the lithium plating capability of a battery. The higher the lithium plating window, the lower the degree of lithium plating at the same charging rate.

[0214] 2. Volumetric energy density test

[0215] At 25°C, the secondary batteries (the finished secondary batteries after formation) are charged at a constant current of 1C to 4.5V, left to stand for 10 minutes, and then discharged at a constant current of 1C to the cutoff voltage of 3.0V (5 batteries per group) to obtain the energy of the secondary batteries. The energy density of the secondary battery (Wh / L) = energy of the secondary battery / volume of the secondary battery.

[0216] 3. Cyclic performance test

[0217] Test the sample condition (voltage, internal resistance, thickness, DC internal resistance) at 25℃±5℃.

[0218] 1) Let stand at 25℃±2℃ for 10 minutes;

[0219] 2) Discharge at 0.2C to the lower limit voltage of 3V, and let stand for 10 minutes;

[0220] 3) Charge at 0.7C to the upper limit voltage of 4.25V, cut off at 0.025C, and let stand for 10 minutes;

[0221] 4) Discharge at 0.2C to the lower limit voltage of 3V, and record the initial discharge capacity as C0;

[0222] 5) Let stand for 10 minutes;

[0223] 6) Charge at 2C to 4.25V (cut off at 1.5C), charge at 1.5C to 4.3V (cut off at 1C), charge at 1C to 4.5V (cut off at 0.25C), test the first full charge data, voltage, internal resistance, thickness, and DC internal resistance;

[0224] 7) Let stand at 25℃±2℃ for 10 minutes;

[0225] 8) Discharge at 0.7C to the lower limit voltage of 3V; let stand for 10 minutes;

[0226] 9) Charge at 2C to 4.25V (cut off at 1.5C), charge at 1.5C to 4.3V (cut off at 1C), charge at 1C to 4.5V (cut off at 0.25C), and let stand for 10 minutes;

[0227] Repeat steps 8) to 9) 500 times and record the discharge capacity, denoted as C1.

[0228] Calculate the capacity retention rate = C1 / C0 × 100%.

[0229] The test results are shown in Table 4.

[0230] Table 4

[0231]

[0232]

[0233] As can be seen from the examples and comparative examples in Tables 1 to 4, by introducing polyacrylate units into the chemical structure of the polyurethane unit and introducing a certain amount of organosilicon segments into the polyacrylate unit, a polyurethane acrylate emulsion with high bonding strength, resistance to electrolyte corrosion, and good dispersibility of conductive agents can be obtained. When used as a silicon-based anode binder, it can effectively cope with the large volume change of silicon-based materials, solve the problem of silicon-based anode interface failure, improve the cycle performance of the battery, improve the dispersion uniformity and stability of conductive agents in the anode slurry, improve the slurry yield, improve the kinetic performance of the battery, and enhance the lithium plating window.

[0234] 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 application.

Claims

1. A polyurethane acrylate emulsion, comprising water and a polymer dispersed in the water; characterized in that, The polymer includes a core and a shell, the shell covering the core, the core comprising water, and the shell comprising polyurethane acrylate; The polyurethane acrylate comprises polyurethane units and polyacrylate units, wherein the polyacrylate units comprise silicone segments; based on the mass of the polyurethane acrylate, the mass content of the silicone segments is 0.1% to 10%.

2. The polyurethane acrylate emulsion according to claim 1, characterized in that, The polyacrylate unit encapsulates the polyurethane unit; or, the polyacrylate unit and the polyurethane unit are located in the same shell.

3. The polyurethane acrylate emulsion according to claim 1 or 2, characterized in that, The raw materials for the polyacrylate unit include hydroxy acrylate monomers, unsaturated carboxylic acid monomers, and organosilicon monomers, wherein the organosilicon monomers include olefin bonds; And / or, based on the mass of the polyurethane acrylate, the mass content of the polyacrylate unit is 2.5%-25%.

4. The polyurethane acrylate emulsion according to claim 3, characterized in that, The organosilicon monomer includes at least one of allyltrimethoxysilane, 3-(trimethoxysilyl)propyl acrylate, methacryloyloxymethyltriethoxysilane, 3-(methacryloyloxy)propyltrimethoxysilane, vinyltriethoxysilane, and vinyltrimethoxysilane. And / or, the hydroxyacrylate monomers include at least one of 2-hydroxyacrylate, hydroxypropyl acrylate, and hydroxypropyl methacrylate; And / or, the unsaturated carboxylic acid monomers include at least one of acrylic acid, 2-methacrylic acid, 2-ethylacrylic acid, sorbic acid, linoleic acid, linolenic acid, vinylacetic acid, maleic acid, fumaric acid, and itaconic acid; And / or, the raw materials of the polyacrylate unit further include an initiator and a first neutralizing agent, wherein the mass ratio of the first neutralizing agent to the unsaturated carboxylic acid is 0.3:1-1.3:

1.

5. The polyurethane acrylate emulsion according to claim 1 or 2, characterized in that, Based on the mass of the polyurethane acrylate, the mass content of the polyurethane unit is 75%-97.5%; And / or, the raw materials of the polyurethane unit include hard segment monomers, soft segment monomers, crosslinking agents, hydrophilic chain extenders, and a second neutralizing agent; wherein: The hard segment monomer includes a polyisocyanate, and the polyisocyanate includes at least one aliphatic ring; The soft segment monomer includes a chain aliphatic polyols, wherein the main chain of the chain aliphatic polyol has not less than 3 carbon atoms; The crosslinking agent comprises at least three hydroxyl groups; based on the total mass of the raw materials of the polyurethane unit, the mass content of the crosslinking agent is 0.1%-5%; The hydrophilic chain extender comprises at least two hydroxyl groups and at least one carboxyl group, and the mass ratio of the second neutralizing agent to the hydrophilic chain extender is 0.25:1-0.85:

1.

6. The polyurethane acrylate emulsion according to claim 5, characterized in that, Based on the total mass of the raw materials of the polyurethane unit, the mass content of the hard segment monomer is 30%-75%; And / or, the number of carbon atoms in the aliphatic ring of the hard segment monomer is 5 or 6; And / or, when the polyacrylate unit and the polyurethane unit are located in the same shell, the polyisocyanate does not contain any unsaturated groups other than the isocyanate group; And / or, when the polyacrylate unit and the polyurethane unit are located in the same shell, the main chain of the chain aliphatic polyol does not contain heteroatoms or unsaturated groups, wherein the heteroatoms include at least one of oxygen, sulfur, and selenium; And / or, the main chain carbon number of the chain aliphatic polyol does not exceed 200; And / or, the chain-like aliphatic polyol further includes branches, each branch comprising at least one carbon atom, and the number of branches not exceeding 40; And / or, based on the total mass of the raw materials of the polyurethane unit, the mass content of the soft segment monomer is 15%-63%; And / or, the molecular weight of the crosslinking agent does not exceed 300; And / or, the molecular weight of the hydrophilic chain extender does not exceed 200; And / or, the raw materials of the polyurethane unit further include at least one of a catalyst and an amine chain extender, wherein the catalyst includes at least one of an organotin compound and an organobismuth compound, and / or, the amine chain extender includes at least two amine groups.

7. The polyurethane acrylate emulsion according to claim 6, characterized in that, The hard segment monomer includes at least one of 1,4-cyclohexane diisocyanate, 1,3-cyclohexane diisocyanate, hydrogenated toluene diisocyanate, isophorone diisocyanate, norbornane diisocyanate, dicyclohexylmethane diisocyanate, and norbornane dimethylene isocyanate. And / or, the soft segment monomer includes at least one of 2-methyl-1,5-pentanediol, 2,2-dimethyl-1,3-propanediol, 12-hydroxystearic acid, 6-aminohexanoic acid, dihydroxy-terminated polyethylene, and dihydroxy-terminated hydrogenated polybutadiene. And / or, the crosslinking agent includes at least one of trimethylolpropane, glycerol, pentaerythritol, and tris(2-hydroxyethyl)isocyanurate; And / or, the hydrophilic chain extender includes at least one of 2,2-dimethylolpropionic acid, 2,2-dimethylolbutyric acid, 3,5-dihydroxy-3-methylvaleric acid, dihydroxymaleic acid, 2,6-dihydroxybenzoic acid, and 3,4-diaminobenzoic acid.

8. The polyurethane acrylate emulsion according to claim 1 or 2, characterized in that, The number-average molecular weight of the polyurethane acrylate is 20,000 to 40,000. And / or, the glass transition temperature of the polyurethane acrylate is -40°C to -20°C; And / or, the particle size Dn90 of the polyurethane acrylate emulsion is 100nm to 500nm; And / or, the polyurethane acrylate emulsion is formulated into a film, which, after being immersed in a lithium-ion electrolyte at 60°C, satisfies at least one of the following conditions: A. Swelling rate is 30-90%; B. Dissolution rate not exceeding 5%; C. Elongation at break is not less than 150%; D. The fracture strength is not less than 8 MPa; The lithium-ion electrolyte comprises an organic solvent, a lithium salt, and additives. The organic solvent comprises ethylene carbonate, propylene carbonate, propyl propionate, and diethyl carbonate. The lithium salt comprises lithium hexafluorophosphate. The additives comprise at least one of fluoroethers, fluoroethylene carbonate, fluoronitrile, and polyarylether ether nitrile.

9. A secondary battery, comprising a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector in the thickness direction, the negative electrode active layer comprising a negative electrode active material and a binder; characterized in that, The adhesive comprises the polyurethane acrylate emulsion according to any one of claims 1-8.

10. The secondary battery according to claim 9, characterized in that, The negative electrode active material includes silicon-based materials; the peel strength between the negative electrode active layer and the negative electrode current collector is not less than 15 N / m.

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