Waterborne polyurethane-acrylate composite emulsion with core-shell-shell structure as well as preparation method and application of waterborne polyurethane-acrylate composite emulsion

Abstract

The invention discloses a waterborne polyurethane-acrylate composite emulsion with a core-shell-shell structure as well as a preparation method and application of the waterborne polyurethane-acrylate composite emulsion. The composite emulsion is prepared from a component A, a component B and a component C through sequential seed emulsion polymerization, and the component A forms a core part through double-bond-terminated waterborne polyurethane; the component B comprises the following components in parts by mass: 20-30 parts of a fluorine-containing acrylate monomer, 10-20 parts of an alkyl acrylate monomer with 4-18 carbon atoms, 0.15-0.25 part of a first initiator and 0.5-3 parts of a first emulsifier which are copolymerized to form an inner shell part; and the component C comprises the following components in parts by mass: 30-60 parts of an acrylic monomer, 20-40 parts of an acrylamide monomer, 10-30 parts of a hydroxyl acrylate monomer, 0.3-0.65 part of a second initiator and 0.5-3 parts of a second emulsifier which are copolymerized to form a shell part. The composite emulsion provided by the invention can effectively overcome the defects of insufficient electrolyte swelling resistance and high-voltage oxidation resistance in the application of a high-voltage lithium ion battery, and can effectively improve the cycle performance of the battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery materials technology, specifically to an aqueous binder for lithium-ion battery electrodes, especially high-voltage positive electrodes, and its preparation method, particularly an aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure, its preparation method, and its application. Background Technology

[0002] With the rapid development of new energy vehicles and the large-scale energy storage market, higher requirements have been placed on the energy density, cycle life, and safety of lithium-ion batteries. Using high-capacity, high-operating-voltage cathode materials (such as high-nickel ternary materials NCM811 and NCM90505, and lithium-rich manganese-based materials) is one of the key pathways to improve battery energy density. However, the operating voltage of these cathode materials is typically as high as 4.4V and above, posing a severe challenge to the internal materials of the battery, especially the binder.

[0003] Currently, the binders widely used in lithium-ion battery cathode manufacturing mainly include oil-based polyvinylidene fluoride (PVDF) and traditional water-based binders. PVDF and its limitations: PVDF requires the use of toxic N-methylpyrrolidone as a solvent, resulting in high cost, significant environmental pollution, and poor ionic conductivity. More importantly, PVDF relies on weak van der Waals forces for bonding, making it easily oxidized under high voltage and exhibiting limited resistance to electrolyte swelling, which can easily lead to electrode structure damage during long-term cycling. Traditional water-based binders and their limitations: Water-based binders, such as styrene-butadiene rubber (SBR), polyacrylic acid, and their salts, have gained attention for their environmental friendliness, low cost, and strong adhesion. Polyacrylic acid binders, in particular, possess abundant carboxyl groups that can form strong hydrogen and ionic bonds with the surface of the cathode active material, exhibiting excellent bonding performance. However, they also face bottlenecks: poor resistance to electrolyte swelling; the numerous polar groups on the polymer chains easily interact with carbonate electrolyte solvents, leading to swelling and softening of the binder film, a decrease in cohesive strength and modulus, and an inability to effectively constrain the active material, thus causing electrode structure pulverization. Under high-voltage conditions, the highly oxidizing cathode material attacks and degrades the CH, CC, and other chemical bonds in the binder molecular chains, leading to binder failure, a sharp increase in interfacial impedance, and rapid capacity decay. Polyacrylic acid binders are generally hard and brittle, making it difficult to effectively buffer the stress caused by volume changes in active particles during charging and discharging.

[0004] To overcome the shortcomings of single materials, researchers have developed composite adhesives, such as waterborne polyurethane-acrylate composite emulsions. Waterborne polyurethane offers good flexibility, while acrylate provides strong adhesion; their combination offers complementary advantages. However, existing composite technologies are mostly limited to simple physical blending or random copolymerization, or simple single-layer core-shell structures. These structures cannot precisely control interfacial properties or achieve functional partitioning, exhibiting significant drawbacks: simple blends have poor two-phase compatibility, are prone to phase separation, leading to uneven performance. Simple core-shell structures (such as WPU as the core and PAA as the shell), while improving two-phase compatibility, still suffer from the problem of the flexible WPU core being directly exposed to the harsh electrolyte environment, making it susceptible to swelling and oxidation. Furthermore, the rigidity of the PAA shell may lead to microcracks at the interface due to modulus mismatch.

[0005] Therefore, there is an urgent need in this field for a new type of water-based adhesive that can maintain high-strength bonding and ion conductivity while fundamentally improving its stability against electrolyte corrosion and high-pressure oxidation, thereby meeting the long life requirements of next-generation high-energy-density lithium-ion batteries. Summary of the Invention

[0006] In view of the above problems, the purpose of this invention is to provide an aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure, its preparation method and application. This aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure can effectively overcome the defects of insufficient resistance to electrolyte swelling and high-voltage oxidation in high-voltage lithium-ion battery applications, and can effectively improve the cycle performance of the battery.

[0007] To achieve the above objectives, the first aspect of the present invention provides an aqueous polyurethane-acrylate composite emulsion having a core-shell-shell structure, which is formed by sequential seed emulsion polymerization of component A, component B, and component C. Component A constitutes the core portion of a double-bond-terminated aqueous polyurethane; component B constitutes the inner shell portion by copolymerization of components comprising the following parts by mass: 20-30 parts of fluorinated acrylate monomer, 10-20 parts of alkyl acrylate monomer with 4-18 carbon atoms, 0.15-0.25 parts of a first initiator, and 0.5-3 parts of a first emulsifier; component C constitutes the outer shell portion by copolymerization of components comprising the following parts by mass: 30-60 parts of acrylic monomer, 10-40 parts of acrylamide monomer, 10-30 parts of hydroxy acrylate monomer, 0.3-0.65 parts of a second initiator, and 0.5-3 parts of a second emulsifier. The mass ratio of components A, B, and C is 100:30~55:50~135.

[0008] Furthermore, the fluorinated acrylate monomer of the present invention is selected from at least one of trifluoroethyl methacrylate, hexafluorobutyl acrylate, and dodecafluoroheptyl methacrylate.

[0009] Furthermore, the alkyl acrylate monomers of the present invention having 4 to 18 carbon atoms are selected from at least one of lauryl acrylate and octadecyl acrylate.

[0010] Furthermore, the hydroxyacrylate monomer of the present invention is selected from at least one of hydroxyethyl acrylate and hydroxyethyl methacrylate.

[0011] Furthermore, the first initiator and the second initiator of the present invention are each independently selected from at least one of ammonium persulfate, sodium persulfate and potassium persulfate.

[0012] Furthermore, the first emulsifier and the second emulsifier of the present invention are each independently selected from at least one of nonylphenol polyoxyethylene ether ammonium sulfate, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate and fatty alcohol polyoxyethylene ether sodium sulfate.

[0013] A second aspect of the present invention provides a method for preparing the aforementioned waterborne polyurethane-acrylate composite emulsion with a core-shell-shell structure, comprising: (1) The formulation amounts of fluorinated acrylate monomer, alkyl acrylate monomer with 4 to 18 carbon atoms and the first emulsifier are mixed evenly in deionized water to obtain a first pre-emulsion. The formulation amounts of acrylic monomer, acrylamide monomer, hydroxy acrylate monomer and the second emulsifier are mixed evenly in deionized water to obtain a second pre-emulsion. The formulation amounts of the first initiator are dissolved in deionized water to form a first initiator aqueous solution. The formulation amounts of the second initiator are dissolved in deionized water to form a second initiator aqueous solution, thus providing a double-bond-terminated waterborne polyurethane seed emulsion. (2) Add the first initiator aqueous solution to the double bond-terminated aqueous polyurethane seed emulsion and add the first pre-emulsion dropwise to carry out the polymerization reaction to obtain a primary composite emulsion with a core-shell structure; (3) Add a second initiator aqueous solution to the primary composite emulsion and add a second pre-emulsion dropwise to carry out a polymerization reaction to obtain an aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure.

[0014] Further, in step (1) of the present invention, providing a double-bond-terminated aqueous polyurethane seed emulsion specifically includes reacting a polyol and a diisocyanate under catalytic conditions to obtain a prepolymer, then sequentially adding a carboxyl-containing hydrophilic chain extender for chain extension, and a monomer containing hydroxyl and alkenyl groups for end-capping, followed by neutralization and emulsification to obtain the final product. Specifically, the molar ratio of diisocyanate to NCO / OH in the polyol is 1.2~2.0:1. As an example, the molar ratio of NCO / OH can be, but is not limited to, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, or 2.0:1; the amount of catalyst used is 0.01%~0.1% of the total mass of the polyol and isocyanate. As an example, the amount of catalyst used is 0.01%~0.1% of the total mass of the polyol and isocyanate. The amounts of the esters are 0.01%, 0.02%, 0.04%, 0.06%, 0.08%, and 0.1% of the total mass of the prepolymer; the amount of the hydrophilic chain extender is 3% to 8% of the total mass of the prepolymer. For example, the amount of the hydrophilic chain extender is 3%, 4%, 5%, 6%, 7%, and 8% of the total mass of the prepolymer; the molar ratio of the end-capping agent to the remaining -NCO is 1 to 1.2:1. For example, the molar ratio of the end-capping agent to the remaining -NCO is 1:1, 1.1:1, and 1.2:1. More specifically, the polyol is selected from polyester polyols, specifically poly(1,4-butanediol adipate); the diisocyanate is selected from at least one of isophorone diisocyanate (IPDI), toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), and 1,5-naphthalene diisocyanate (NDI); the catalyst is an organotin catalyst; the carboxyl-containing hydrophilic chain extender is selected from dimethylolpropionic acid (DMPA); and the monomer containing hydroxyl and alkenyl groups is selected from hydroxyethyl methacrylate (HEMA).

[0015] Further, step (2) of the present invention specifically includes: heating the double-bond-terminated aqueous polyurethane seed emulsion to 75-80°C under an inert atmosphere, adding a portion of the first initiator aqueous solution, and then simultaneously adding the first pre-emulsion and the remaining portion of the first initiator aqueous solution. After the addition is complete, a heat preservation reaction is carried out to obtain a primary composite emulsion with a core-shell structure. More specifically, the reaction temperature can be, but is not limited to, 75°C, 76°C, 77°C, 78°C, 79°C, or 80°C.

[0016] Further, step (3) of the present invention specifically includes: maintaining the temperature of the reaction system, adding a portion of the second initiator aqueous solution to the primary composite emulsion, and then simultaneously adding the second pre-emulsion and the remaining portion of the second initiator aqueous solution. After the addition is completed, the mixture is kept warm and matured to ensure that the monomers react completely, thereby obtaining an aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure.

[0017] Furthermore, in step (3) of the present invention, after the polymerization reaction, the pH value of the reaction solution is adjusted to 7.5-8.5 using a lithium base reagent.

[0018] The third aspect of the present invention provides the application of the aforementioned aqueous polyurethane-acrylate composite emulsion having a core-shell-shell structure or the composite emulsion prepared according to the aforementioned method for preparing the aqueous polyurethane-acrylate composite emulsion having a core-shell-shell structure in the preparation of lithium-ion battery electrodes.

[0019] Furthermore, the electrode of the present invention is a positive electrode, and the active material of the positive electrode is a lithium transition metal oxide with a working voltage of not less than 4.4V.

[0020] Furthermore, the lithium transition metal oxide of the present invention includes at least one of high-nickel ternary materials and lithium-rich manganese-based materials.

[0021] Compared with existing technologies, the waterborne polyurethane-acrylate composite emulsion of the present invention has a special core-shell-shell structure. Its core is a double-bond-terminated waterborne polyurethane, which can provide flexible buffering for the composite emulsion, providing good toughness and film-forming properties. At the same time, the double bonds it contains can react with the shell part, so that the core and shell are connected by chemical bonds, which improves the structural stability of the composite emulsion of the present invention, thereby possessing superior adhesion, resistance to electrolyte swelling, and resistance to high-pressure oxidation. The shell part is copolymerized from acrylic acid, acrylamide and hydroxy acrylate, which can give the composite emulsion good adhesion properties. At the same time, the synergistic effect of the three can also resist the penetration and swelling of electrolyte to a certain extent. Between the core and the outer shell is an inner shell portion. This core portion is copolymerized from fluorinated acrylate monomers and alkyl acrylate monomers with 4-18 carbon atoms. The fluorinated acrylate monomers provide low surface energy and high chemical stability, while the alkyl acrylate monomers provide some hydrophobicity. Therefore, the fluorinated acrylate polymer formed by the copolymerization of the two can effectively prevent carbonate electrolyte from penetrating into the internal aqueous polyurethane core, thereby effectively reducing the swelling degree of the composite emulsion. It also possesses a high oxidation potential and strong CF bonds, enabling it to withstand the strong oxidizing environment of the high-voltage cathode, thus protecting the entire adhesive network from degradation. Therefore, through the synergistic effect of the aforementioned three components, the composite emulsion of this invention possesses excellent bonding strength, toughness, electrolyte resistance, and high-voltage oxidation resistance, while effectively improving the cycle performance of the battery. This invention uses sequential seed emulsion polymerization, with each structure bonded by chemical bonds, avoiding the phase separation problems that may occur with simple blending, and ensuring the long-term stability of the composite structure. Meanwhile, the seed emulsion polymerization method used in this invention is a mature technology in the polymer industry. The process is clear, the conditions are mild, and it is easy to achieve large-scale production, thus having broad industrialization prospects. Detailed Implementation

[0022] To better illustrate the purpose, technical solution, and beneficial effects of this invention, the invention will be further described below with reference to specific embodiments. It should be noted that the methods described below are further explanations of this invention and should not be construed as limiting it.

[0023] In the embodiments and comparative examples of the present invention, the raw materials used are specifically described as follows: poly(1,4-butanediol adipate) diol (PBA1000, industrial grade), isophorone diisocyanate (IPDI, industrial grade), dimethylolpropionic acid (DMPA, industrial grade), hydroxyethyl methacrylate (HEMA, chemically pure), trifluoroethyl methacrylate (TFEMA, industrial grade), acrylic acid (AA, chemically pure), acrylamide (AM, chemically pure), lauryl acrylate (LA, industrial grade), hydroxyethyl acrylate (HEA, chemically pure), ammonium persulfate (APS, analytical grade), nonylphenol polyoxyethylene ether ammonium sulfate (CO-436, industrial grade), sodium dodecyl sulfate (chemically pure), sodium dodecylbenzene sulfonate (chemically pure), fatty alcohol polyoxyethylene ether sulfate (chemically pure), lithium hydroxide monohydrate (LiOH·H2O, analytical grade), N-methylpyrrolidone (NMP, industrial grade), PVDF (HSV900, commercial product).

[0024] In the embodiments and comparative examples of this invention, the preparation method of the double-bond-terminated aqueous polyurethane seed emulsion is as follows: In a four-necked flask equipped with a stirrer, condenser, and thermometer, 60g of PBA1000 and 18g of IPDI were added, and 2 drops of dibutyltin dilaurate were added dropwise. The mixture was reacted at 85°C for 2 hours to obtain a prepolymer. Then, 4g of DMPA (dissolved in a small amount of NMP) was added, and the reaction continued for 1 hour. The temperature was lowered to 60°C, and 2g of HEMA was added for end-capping. The reaction was continued for 2 hours until the -NCO groups were completely reacted. The temperature was lowered to 40°C, and 30g of water containing 3g of triethylamine was added for neutralization and emulsification. After high-speed shearing, the solvent was removed by vacuum distillation to obtain a double-bond-terminated aqueous polyurethane seed emulsion with a solid content of approximately 33%.

[0025] Example 1 This embodiment provides an aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure, which is formed by sequential seed emulsion polymerization of components A, B, and C. Component A consists of 100g of double-bond-terminated aqueous polyurethane forming the core. Component B consists of the following components copolymerized to form the inner shell: 28g of fluorinated acrylate monomer, 12g of alkyl acrylate monomer with 4-18 carbon atoms, 0.2g of a first initiator, and 1g of a first emulsifier. Component C consists of the following components copolymerized to form the outer shell: 30g of acrylic monomer, 20g of acrylamide monomer, 10g of hydroxyacrylate monomer, 0.3g of a second initiator, and 1g of a second emulsifier. The fluorinated acrylate monomer is TFEMA, the alkyl acrylate monomer with 4-18 carbon atoms is LA, the hydroxyacrylate monomer is HEA, the first and second initiators are both APS, and the first and second emulsifiers are both CO-436.

[0026] This embodiment also provides a method for preparing the aforementioned composite emulsion, including the following steps: (1) Mix 28g TFEMA, 12g LA, 1g emulsifier CO-436 and 60g water evenly to obtain a first preemulsion; mix 30g AA, 20g AM, 10g HEA, 1g emulsifier CO-436 and 80g water evenly to obtain a second preemulsion; dissolve 0.2g APS in 20g deionized water to form a first APS aqueous solution (the amount of initiator is 0.5% of the sum of the amounts of TFEMA and LA); dissolve 0.3g APS in 30g deionized water to form a second initiator aqueous solution (the amount of initiator is 0.5% of the sum of the amounts of AA, AM and HEA). (2) Take 100g (based on solids) of double-bond-terminated aqueous polyurethane seed emulsion in a reactor, dilute it to a solid content of 15%, heat it to 78℃, and purge with nitrogen. Add 1% (based on the total mass of monomers (TFEMA and LA)) of the first APS aqueous solution. Then, start adding the first pre-emulsion and the remaining first APS aqueous solution dropwise simultaneously. The addition is completed within 3 hours, and the mixture is kept at 78℃ for 1 hour to obtain a primary composite emulsion with a core-shell structure; (3) Maintain the system temperature at 78°C and add 0.5% of the total mass of monomers (AA, AM, and HEA) of the second APS aqueous solution to the primary composite emulsion. Then, begin to add the second pre-emulsion and the remaining second APS aqueous solution dropwise simultaneously. Complete the dropwise addition within 3 hours and keep the solution at 78°C for 1.5 hours; cool the reaction solution to below 40°C and filter. Under slow stirring, neutralize the reaction solution to pH=8.0 with 10% LiOH·H2O aqueous solution to obtain an aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure and a solid content of about 30%, denoted as WPU@F@PAA.

[0027] Example 2 The difference between this embodiment and Embodiment 1 is that the mass of TFEMA is 20g and the mass of LA is 20g. All other aspects are the same as in Embodiment 1. The resulting aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure is denoted as WPU@F(low)@PAA.

[0028] Example 3 The difference between this embodiment and Embodiment 1 is that the mass of AA is 40g, the mass of AM is 15g, and the mass of HEA is 15g. All other aspects are the same as in Embodiment 1. The resulting aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure is denoted as WPU@F@PAA (high-AA).

[0029] Comparative Example 1 This comparative example provides a polyacrylate emulsion, the preparation of which includes dissolving 30g AA, 20g AM, 10g HEA and 1g emulsifier CO-436 in deionized water, adding dropwise an APS aqueous solution (where the mass of APS is 0.3g), carrying out a polymerization reaction at 80°C, and neutralizing the reaction solution to pH=8.0 with a 10% LiOH·H2O aqueous solution to obtain a polyacrylate emulsion with a solid content of approximately 30%, denoted as PAA-Binder.

[0030] Comparative Example 2 This comparative example provides an aqueous polyurethane-polyacrylate blend emulsion, the preparation of which includes: (1) Preparation of waterborne polyurethane emulsion: In a four-necked flask equipped with a stirrer, condenser and thermometer, add 60g PBA1000 and 18g IPDI, add 2 drops of dibutyltin dilaurate, react at 85℃ until NCO reaches the theoretical value, cool down to 40℃, add 30g water containing 3g triethylamine for neutralization and emulsification, after high-speed shearing, remove the solvent by vacuum distillation, and obtain waterborne polyurethane emulsion with a solid content of about 30%.

[0031] (2) Preparation of polyacrylate emulsion: Same as Comparative Example 1; The aqueous polyurethane emulsion obtained in step (1) and the polyacrylate emulsion obtained in step (2) are mechanically mixed at a solid mass ratio of 1:1 to obtain a blended emulsion, denoted as WPU / PAA-Blend.

[0032] Comparative Example 3 This comparative example uses a commercial PVDF adhesive. PVDF powder was dissolved in NMP to prepare an adhesive solution with a solid content of 30%, which served as the control standard. This solution is denoted as PVDF.

[0033] Comparative Example 4 The difference between this comparative example and Example 1 is that the composite emulsion in this comparative example does not contain component B (i.e., it does not have an inner shell layer), while all other aspects are the same as in Example 1. The resulting composite emulsion is denoted as WPU@PAA.

[0034] Comparative Example 5 The difference between this comparative example and Example 1 is that the composite emulsion in this comparative example does not contain component C (i.e., it does not have a shell layer), while all other aspects are the same as in Example 1. The resulting composite emulsion is denoted as WPU@F.

[0035] Comparative Example 6 The difference between this comparative example and Example 1 is that, in component C, 10g of methyl methacrylate monomer is used instead of 10g of hydroxyacrylate monomer.

[0036] The emulsions prepared in Examples 1-3 and Comparative Examples 1-6 were used as binders in lithium-ion battery electrodes, and relevant performance tests were conducted. The specific applications and test conditions are as follows.

[0037] Battery assembly: The positive electrode active material NCM811, the conductive agent Super P, and the binder (one of the emulsions from Examples 1-3 and Comparative Examples 1-6) were mixed evenly at a mass ratio of 96:2.5:1.5 to prepare a positive electrode slurry, which was then coated onto aluminum foil for current collectors with a coating weight of 324 g / m². 2 The positive electrode is prepared through processes such as drying. The negative electrode is made of lithium metal sheet. The electrolyte is a 1M LiPF6 EC / DEC / EMC solution (volume ratio 1:1:1). The battery is assembled using the aforementioned positive electrode, negative electrode, and electrolyte according to conventional lithium-ion battery manufacturing methods in the art.

[0038] Adhesion strength (peel strength) test: The positive electrode sheet prepared by the above method is cut into a specific size (e.g., 20 mm wide), fixed to the metal substrate with double-sided tape, and the electrode sheet is peeled from the aluminum foil current collector using a universal testing machine at a peel angle of 180° and a speed of 50 mm / min. The average peel force is recorded and the peel strength (N / m) is calculated.

[0039] Electrolyte swelling rate: The emulsions of Examples 1-3 and Comparative Examples 1-6 were placed in a polytetrafluoroethylene box to form an adhesive film with a dry film thickness of 2 mm. Then, the film was immersed in a 1 M LiPF6 EC / DEC / EMC (volume ratio 1:1:1) electrolyte solution at 60°C for 72 h, and the swelling rate was calculated by weighing. SR(%)=[(W t -W0) / W0]×100%, where W0 is the initial weight, W t This is the weight after soaking for 72 hours.

[0040] Linear sweep voltammetry (LSV): The oxidation initiation potential of an adhesive film with a dry film thickness of 2 mm prepared by the aforementioned method on the working electrode was measured.

[0041] Battery cycle performance: Within a voltage range of 3.0-4.5V, the battery was charged and discharged at a rate of 1C for 500 cycles. The initial coulombic efficiency and capacity retention after 500 cycles were recorded.

[0042] Table 1 Performance test results of the examples and comparative examples

[0043] As can be seen from the test results in Table 2, compared with Comparative Examples 1 to 6, Examples 1 to 3 of the present invention have superior overall bonding performance, electrolyte resistance, high voltage resistance and cycling performance.

[0044] Specifically, although the peel strength (35.2~40.1 N / m) of Examples 1-3 of the present invention is slightly lower than that of Comparative Example 1 (45.0 N / m), it still meets the strength requirements for electrode bonding (e.g., much higher than the 15.5 N / m of commonly used PVDF adhesives in the industry). More importantly, the present invention, through a core-shell-shell structure design, achieves superior electrolyte swelling resistance, oxidation resistance, and long-cycle performance of the battery compared to Comparative Example 1, while sacrificing only a small amount of bonding strength, thus achieving an excellent balance of comprehensive performance. Furthermore, among Examples 1-3, Example 3 exhibits the strongest bonding force due to its higher AA content, demonstrating the tunability of the components in the present invention.

[0045] Compared to Comparative Examples 1-6, Examples 1-3 of the present invention exhibit superior electrolyte resistance and high-voltage resistance. This indicates that the core-shell-shell composite emulsion structure of the present invention possesses superior barrier function. Furthermore, with an oxidation initiation potential above 4.78, it is suitable for batteries with operating voltages of 4.4V and above.

[0046] Comparative Example 3 exhibited the highest initial coulombic efficiency due to its inert polymer composition and minimal side reactions with the electrolyte. The initial coulombic efficiencies of Examples 1-3 of this invention are essentially comparable to those of PVDF and significantly higher than those of Comparative Examples 1, 2, 4, 5, and 6. This indicates that the composite emulsion of this invention can effectively suppress irreversible side reactions between a large number of carboxyl groups in the PAA and the electrolyte / positive electrode surface during the first charge, thereby improving interfacial stability.

[0047] Examples 1-3 of this invention, while maintaining high-strength adhesion and good initial efficiency, achieved excellent cycle capacity retention due to their superior solvent resistance and oxidation resistance. Example 1 showed an even higher cycle capacity retention of 92.5%. Comparative Examples 1 and 2 experienced a sharp decline in capacity retention due to severe swelling and degradation during cycling. Comparative Example 3, although exhibiting a high initial efficiency, suffered from weak adhesion and insufficient oxidation resistance, resulting in significantly inferior cycle performance compared to this invention. In Comparative Example 4, the absence of an inner shell component and the lack of an anti-swelling and anti-oxidation barrier composed of a fluorinated / alkyl acrylate copolymer significantly deteriorated its resistance to electrolyte swelling and high-voltage oxidation, leading to rapid electrode structure failure during cycling and a substantial decrease in battery capacity retention. In Comparative Example 5, due to the absence of a shell component, the lack of a strong adhesive shell rich in polar functional groups (such as carboxyl and amide groups) resulted in extremely low peel strength, failing to effectively confine the active material. Simultaneously, the absence of the shell layer weakened the bonding strength between the composite particles and the active material interface, as well as the overall film density, indirectly exacerbating electrolyte erosion. Consequently, the battery capacity retention rate was also unsatisfactory. In Comparative Example 6, because the shell component did not contain hydroxyl acrylic monomers, the absence of hydroxyl components in the resulting shell polymer weakened its hydrogen bonding interaction with the active material surface and its potential interface stabilization ability, leading to decreased adhesion performance and interface stability, thus comprehensively affecting the battery's cycle life.

[0048] In summary, the waterborne polyurethane-acrylate composite emulsion with a core-shell-shell structure provided by this invention possesses high-strength adhesion, high initial coulombic efficiency, excellent electrolyte resistance, and high-pressure oxidation resistance. It effectively solves the problems of numerous side reactions and poor stability of simple polyacrylic acid adhesives, and also overcomes the defects of uneven performance of simple blends and weak adhesion and poor oxidation resistance of PVDF. In terms of comprehensive performance, it achieves a significant improvement over the prior art.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, it is not limited to those listed in the embodiments. Those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. An aqueous polyurethane-acrylate composite emulsion having a core-shell-shell structure, characterized in that, The product is formed by sequential seed emulsion polymerization of components A, B, and C. Component A constitutes the core portion of a double-bond-terminated waterborne polyurethane. Component B constitutes the inner shell portion by copolymerization of the following components in parts by mass: 20-30 parts of fluorinated acrylate monomer, 10-20 parts of alkyl acrylate monomer with 4-18 carbon atoms, 0.15-0.25 parts of a first initiator, and 0.5-3 parts of a first emulsifier. Component C constitutes the outer shell portion by copolymerization of the following components in parts by mass: 30-60 parts of acrylic monomer, 10-40 parts of acrylamide monomer, 10-30 parts of hydroxy acrylate monomer, 0.3-0.65 parts of a second initiator, and 0.5-3 parts of a second emulsifier. The mass ratio of component A, component B and component C is 100:30~55:50~135.

2. The aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure according to claim 1, characterized in that, The fluorinated acrylate monomer is selected from at least one of trifluoroethyl methacrylate, hexafluorobutyl acrylate, and dodecafluoroheptyl methacrylate.

3. The aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure according to claim 1, characterized in that, The alkyl acrylate monomer having 4 to 18 carbon atoms is selected from at least one of lauryl acrylate and octadecyl acrylate.

4. The aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure according to claim 1, characterized in that, The hydroxyacrylate monomer is selected from at least one of hydroxyethyl acrylate and hydroxyethyl methacrylate.

5. The aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure according to claim 1, characterized in that, The first initiator and the second initiator are each independently selected from at least one of ammonium persulfate, sodium persulfate and potassium persulfate, and the first emulsifier and the second emulsifier are each independently selected from at least one of nonylphenol polyoxyethylene ether ammonium sulfate, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate and fatty alcohol polyoxyethylene ether sodium sulfate.

6. The method for preparing the aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure according to any one of claims 1 to 5, characterized in that, include: (1) The fluorinated acrylate monomer, the alkyl acrylate monomer with 4 to 18 carbon atoms and the first emulsifier are mixed evenly in deionized water to obtain a first pre-emulsion. The acrylic monomer, the acrylamide monomer, the hydroxy acrylate monomer and the second emulsifier are mixed evenly in deionized water to obtain a second pre-emulsion. The first initiator is dissolved in deionized water to form a first initiator aqueous solution. The second initiator is dissolved in deionized water to form a second initiator aqueous solution, thereby providing a double-bond-terminated waterborne polyurethane seed emulsion. (2) Add the first initiator aqueous solution to the double bond-terminated aqueous polyurethane seed emulsion, and add the first pre-emulsion dropwise to carry out the polymerization reaction to obtain a primary composite emulsion with a core-shell structure; (3) Add the second initiator aqueous solution to the primary composite emulsion and add the second pre-emulsion dropwise to carry out the polymerization reaction to obtain an aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure.

7. The method for preparing the aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure according to claim 6, characterized in that, In step (1), the provision of double-bond-terminated waterborne polyurethane seed emulsion specifically includes reacting polyol and diisocyanate under the condition of a catalyst to obtain a prepolymer, then sequentially adding a carboxyl-containing hydrophilic chain extender for chain extension, and monomers containing hydroxyl and alkenyl groups for end-capping, followed by neutralization and emulsification to obtain the final product.

8. The method for preparing the aqueous polyurethane-acrylate composite emulsion with a core-shell-shell structure according to claim 6, characterized in that, In step (3), after the polymerization reaction, the pH of the reaction solution is adjusted to 7.5-8.5 using a lithium base reagent.

9. The application of a composite emulsion prepared by the method of preparing an aqueous polyurethane-acrylate composite emulsion having a core-shell-shell structure according to any one of claims 1 to 5, or an aqueous polyurethane-acrylate composite emulsion having a core-shell-shell structure according to any one of claims 6 to 8, in the preparation of lithium-ion battery electrodes.

10. The application according to claim 9, characterized in that, The electrode is a positive electrode, and the active material of the positive electrode is a lithium transition metal oxide with a working voltage of not less than 4.4V.

Images