Narrowly distributed functionalized aqueous latex and method for its preparation

Abstract

This invention discloses a narrow-distribution functionalized aqueous latex and its preparation method, belonging to the field of aqueous latex preparation technology. The latex uses functionalized monomers, aromatic vinyl monomers, and aliphatic conjugated diene monomers as main raw materials, introduces silane coupling agent-modified cellulose nanocrystals, and combines them with an initiator, emulsifier, and water, undergoing one-step emulsion polymerization. The modified cellulose nanocrystals can form a physical barrier on the surface of the latex particles, reducing the content of embedded acid and free acid in the aqueous phase, and increasing the surface carboxyl group density; its rigid structure forms a three-dimensional network after film formation, effectively suppressing the volume expansion of the silicon anode. The resulting latex has a particle size of 245~255nm, a narrow particle size distribution, moderate viscosity, and high bonding strength. When used as a lithium battery anode binder, it exhibits high electrode peel strength, good slurry dispersibility, and excellent battery cycle stability. This invention features a simple process, readily available raw materials, and easy industrial scale-up, making it suitable for lithium battery anode binders, paper coating, and aqueous adhesives.

Description

Technical Field

[0001] This invention belongs to the field of waterborne latex preparation technology, specifically relating to a narrow-distribution functionalized waterborne latex and its preparation method. Background Technology

[0002] Carboxylated styrene-butadiene latex is a synthetic latex produced by emulsion polymerization of butadiene, styrene, and unsaturated carboxylic acids as the main monomers. Due to its excellent bonding properties, mechanical stability, and film-forming properties, it is widely used as a paper coating adhesive, carpet backing adhesive, water-based coating film-forming material, and lithium-ion battery negative electrode adhesive.

[0003] In the development of secondary battery technology, especially lithium-ion secondary battery technology, the negative electrode material has a crucial impact on the overall performance of the battery. Although the amount of negative electrode binder used is small, as a key material to ensure the stability of the negative electrode material's performance, its performance directly affects the battery's cycle life and safety performance. Carboxylated styrene-butadiene latex is the most commonly used battery negative electrode binder and has a large application market.

[0004] Surface acid content is a key factor affecting the adhesive properties of carboxyl-based styrene-butadiene latex. The mechanism is that the carboxyl groups in the latex can form hydrogen bonds or ionic bonds with the polar groups on the surface of the graphite anode material and the copper foil current collector, thus providing strong interfacial adhesion. The higher the surface acid content, the more carboxyl groups can participate in interfacial bonding per unit area, resulting in stronger adhesion. However, in conventional emulsion polymerization, when unsaturated carboxylic acid monomers copolymerize with butadiene and styrene, due to the randomness of the polymerization reaction and the water solubility of the carboxyl monomers, a large number of carboxyl groups are embedded inside the latex particles, forming embedded acids. Simultaneously, some unsaturated carboxylic acids fail to participate in the polymerization reaction and exist in free form in the aqueous phase, forming "aqueous free acids." Neither of these carboxyl groups can participate in interfacial bonding.

[0005] The high content of embedded acid and aqueous free acid makes it difficult to increase the surface acid content of carboxylated styrene-butadiene latex, thus failing to meet the high adhesion requirements of high-performance batteries. Furthermore, embedded acid and aqueous free acid not only fail to enhance the adhesive strength of the binder but also adversely affect the viscosity stability of the slurry, potentially posing a threat to the overall safety performance of the battery. Therefore, reducing the content of embedded acid and aqueous free acid is crucial to improving the performance of battery anode binders.

[0006] Conventional emulsion polymerization produces styrene-butadiene latex with a wide particle size distribution, leading to significant batch-to-batch performance fluctuations and making it difficult to guarantee film uniformity and mechanical properties. Furthermore, with the development of high-energy-density lithium-ion batteries, silicon-based anode materials have attracted widespread attention due to their extremely high theoretical specific capacity. However, the substantial volume expansion of silicon anodes during charge and discharge places higher demands on binders. Traditional carboxyl-based styrene-butadiene latexes have low modulus and cannot effectively suppress silicon expansion, resulting in electrode pulverization and rapid capacity decay.

[0007] Therefore, there is an urgent need in the field for a method to prepare carboxyl styrene-butadiene latex that can reduce the content of embedded acid and aqueous phase free acid, increase the content of surface acid, and has a narrow particle size distribution and can suppress silicon expansion.

[0008] Therefore, this invention is proposed to solve the above-mentioned technical problems. Summary of the Invention

[0009] Therefore, this invention provides a method for preparing a narrow-distribution functionalized waterborne latex. This invention introduces silane coupling agent-modified cellulose nanocrystals as a steric barrier into the emulsion polymerization system. Utilizing their nanosize effect and hydrophobic surface properties, the hydrophobic segments on the CNC surface have good compatibility with the SBR backbone during polymerization, allowing them to firmly adsorb onto the surface of the latex particles and form a physical barrier that prevents the diffusion of carboxylic acid monomers into the particle interior. This forces the carboxylic acid monomers to polymerize on the particle surface, thereby significantly increasing the surface carboxyl content and reducing embedded and free acids. Simultaneously, the rigid framework of the modified cellulose nanocrystals forms a three-dimensional network after drying and film formation, effectively suppressing the volume expansion of the silicon anode. It also provides additional nucleation sites, enabling all latex particles to nucleate and grow synchronously, avoiding micelle aggregation or uneven nucleation. By precisely controlling the nucleation and growth process of the latex particles, a functionalized waterborne latex with a narrow particle size distribution and stable performance can be obtained.

[0010] In a first aspect, the present invention provides a narrow-distribution functionalized aqueous latex, wherein the raw materials for preparing the aqueous latex include the following components by weight: 5-8 parts of functional group monomer, 20-40 parts of aromatic vinyl monomer, 60-80 parts of aliphatic conjugated diene monomer, 6-10 parts of silane coupling agent modified cellulose nanocrystals, 3-5 parts of initiator, 1.5-3.5 parts of emulsifier and 100-120 parts of water.

[0011] Furthermore, the preparation method of the hydrophobically modified cellulose nanocrystals is as follows: silane coupling agent and 95% ethanol are mixed evenly and heated to obtain reaction solution A. At the same time, cellulose nanocrystals and 95% ethanol are mixed evenly, and the pH value is adjusted to 4-5 with acid to obtain solution B. Solution B is added to solution A, and the reaction is continued by stirring. After the reaction is completed, the solid component is obtained by centrifugation, washed with ethanol, and dried to obtain silane coupling agent modified cellulose nanocrystals.

[0012] Further, the aromatic vinyl monomer is styrene, α-methylstyrene, or vinyltoluene; the aliphatic conjugated diene monomer is 1,3-butadiene, 2-methyl-1,3-butadiene, or 2,3-dimethyl-1,3-butadiene.

[0013] Furthermore, the functional group monomer is selected from at least one of methacrylic acid, acrylic acid, itaconic acid, and maleic acid.

[0014] Furthermore, the initiator is potassium persulfate or ammonium persulfate.

[0015] Furthermore, the emulsifier is potassium oleate or potassium disproportionate.

[0016] Furthermore, the cellulose nanocrystals have a length of 50-60 nm and a diameter of 8-12 nm.

[0017] Furthermore, the silane coupling agent is selected from at least one of triethoxyoctylsilane, dodecyltrimethoxysilane, or hexadecyltrimethoxysilane.

[0018] Furthermore, the solid-liquid ratio of the cellulose nanocrystals and the silane coupling agent is 1:2.5~3.5 (g / ml).

[0019] Secondly, the present invention provides a method for preparing narrow-distribution functionalized aqueous latex according to any of the above technical solutions, characterized in that water is used as a solvent, and functional group monomers, aromatic vinyl monomers, aliphatic conjugated diene monomers, silane coupling agent modified cellulose nanocrystals, initiators, and emulsifiers are added to a reaction vessel to carry out emulsion polymerization reaction.

[0020] Compared with the prior art, the present invention has the following beneficial effects: In this invention, hydrophobic segments are grafted onto the surface of silane coupling agent-modified cellulose nanocrystals. During emulsion polymerization, these hydrophobic segments exhibit strong hydrophobic interactions with the styrene-butadiene latex backbone, ensuring stable adsorption of the modified cellulose nanocrystals onto the particle surface and preventing desorption, thus forming a dense steric physical barrier. This barrier effectively prevents the diffusion of carboxylic acid monomers from the aqueous phase into the particle interior, forcing the carboxylic acid monomers to polymerize only on the particle surface. Consequently, the surface carboxyl group content is significantly increased, thereby solving the problems of embedded acid and free acid.

[0021] This modified cellulose nanocrystal possesses a uniform nanoscale size (50-60 nm in length and 8-12 nm in diameter). In the early stages of polymerization, it acts as a physical separator to prevent particle aggregation and provides additional nucleation sites, enabling all particles to nucleate and grow synchronously. This avoids micellar aggregation or uneven nucleation, precisely controlling the nucleation and growth process, ultimately achieving a narrow distribution of latex particle size between 245-255 nm. This results in balanced and excellent mechanical properties, improving product yield and quality. Furthermore, the modified cellulose nanocrystal possesses high rigidity and remains in the latex after polymerization as a nano-reinforcing filler. When this binder is used in silicon anodes, its nanorod-like structure forms a network structure within the binder film. This network structure effectively repairs silicon particles, significantly reducing slippage between the binder and silicon material. This alleviates silicon volume expansion during charge and discharge, reducing mechanical deformation and internal stress of the electrode. Consequently, it effectively delays volume changes in the silicon material, maintaining the electrode's structural integrity.

[0022] This invention is based on conventional emulsion polymerization equipment and conventional free radical polymerization methods, requiring no special equipment or harsh conditions. The cellulose nanocrystals are derived from renewable resources, and the silane coupling agent is a commonly used industrial reagent. The raw materials are readily available and easy to scale up industrially. Attached Figure Description

[0023] Figure 1 These are cycle curves corresponding to the button batteries assembled in Examples 1-3 and Comparative Example 1; Figure 2 These are scanning electron microscope (SEM) images of the surface morphology of the negative electrode sheets prepared in Examples 1-3 and Comparative Example 1 after 500 cycles. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Preparation Example 1 Preparation of silane coupling agent modified cellulose nanocrystals 100 mL of triethoxyoctylsilane was mixed thoroughly with 3000 mL of 95% ethanol and heated to 60 °C with stirring for 30 minutes to obtain reaction solution A. Separately, 30 g of cellulose nanocrystals (50-60 nm in length, 8-12 nm in diameter) were dispersed in 750 mL of 95% ethanol, the pH was adjusted to 4 with glacial acetic acid, and the mixture was ultrasonically dispersed for 30 minutes to obtain solution B. Solution B was added to solution A, and the reaction was continued at 60 °C with stirring for 6 hours. After the reaction was complete, the mixture was centrifuged (10000 r / min, 10 minutes), the supernatant was discarded, and the precipitate was washed three times with ethanol and three times with deionized water. The precipitate was then vacuum dried at 50 °C for 4 hours to obtain silane coupling agent modified cellulose nanocrystals.

[0026] Preparation Example 2 120 mL of dodecyltrimethoxysilane was mixed thoroughly with 3600 mL of 95% ethanol and heated to 60 °C with stirring for 30 minutes to obtain reaction solution A. Separately, 40 g of cellulose nanocrystals (50-60 nm in length, 8-12 nm in diameter) were dispersed in 1000 mL of 95% ethanol, and the pH was adjusted to 4 with glacial acetic acid. The mixture was then ultrasonically dispersed for 30 minutes to obtain solution B. Solution B was slowly added to solution A, and the reaction was continued with stirring at 60 °C for 6 hours. After the reaction was complete, the mixture was centrifuged (10000 r / min, 10 minutes), the supernatant was discarded, and the precipitate was washed three times with 95% ethanol and three times with deionized water. The precipitate was then vacuum dried at 50 °C to obtain silane coupling agent modified cellulose nanocrystals.

[0027] Preparation Example 3 100 mL of hexadecyltrimethoxysilane was mixed thoroughly with 3000 mL of 95% ethanol and heated to 60 °C with stirring for 30 minutes to obtain reaction solution A. Separately, 40 g of cellulose nanocrystals (50-60 nm in length, 3-10 nm in diameter) were dispersed in 1000 mL of 95% ethanol, and the pH was adjusted to 5 with dilute hydrochloric acid. The mixture was then ultrasonically dispersed for 30 minutes to obtain solution B. Solution B was slowly added to solution A, and the reaction was continued at 60 °C with stirring for 6 hours. After the reaction was complete, the mixture was centrifuged (10000 r / min, 10 minutes), the supernatant was discarded, and the precipitate was washed three times with 95% ethanol and three times with deionized water. The precipitate was then vacuum dried at 50 °C to obtain silane coupling agent modified cellulose nanocrystals.

[0028] Example 1 This embodiment provides a narrow-distribution functionalized waterborne latex. In a high-pressure reactor, 110 parts deionized water, 30 parts styrene, 70 parts butadiene, 6 parts methacrylic acid, 4 parts potassium persulfate, 2.5 parts potassium oleate, and 8 parts of the silane coupling agent-modified cellulose nanocrystals prepared in Example 1 were added sequentially. Stirring was started and the mixture was heated to 70°C and reacted for 12 hours. After the reaction, the pH of the latex was adjusted to 7 using a 5% sodium hydroxide aqueous solution. The neutralized emulsion was then purified to remove residual monomers and water was added to adjust the solid content to 35%.

[0029] Example 2 In a high-pressure reactor, 100 parts deionized water, 20 parts styrene, 60 parts butadiene, 5 parts acrylic acid, 3 parts ammonium persulfate, 1.5 parts potassium disproportionated rosinate, and 6 parts of the silane coupling agent-modified cellulose nanocrystals prepared in Example 1 were added sequentially. Stirring was started and the mixture was heated to 70°C and reacted for 12 hours. After the reaction, the pH of the latex was adjusted to 7 using a 5% sodium hydroxide aqueous solution. The neutralized emulsion was then purified to remove residual monomers and water was added to adjust the solid content to 35%.

[0030] Example 3 In a high-pressure reactor, 120 parts deionized water, 40 parts styrene, 80 parts butadiene, 8 parts itaconic acid, 5 parts ammonium persulfate, 3.5 parts potassium disproportionated rosinate, and 10 parts of silane coupling agent-modified cellulose nanocrystals prepared in Example 1 were added sequentially. Stirring was started and the mixture was heated to 70°C and reacted for 12 hours. After the reaction, the pH of the latex was adjusted to 7 using a 5% sodium hydroxide aqueous solution. The neutralized emulsion was then purified to remove residual monomers and water was added to adjust the solid content to 35%.

[0031] Comparative Example 1 This embodiment provides a narrow-distribution functionalized waterborne latex. In a high-pressure reactor, 110 parts deionized water, 30 parts styrene, 70 parts butadiene, 6 parts methacrylic acid, 4 parts potassium persulfate, and 2.5 parts potassium oleate were added sequentially. Stirring was started and the mixture was heated to 70°C. The reaction was allowed to proceed for 12 hours. After the reaction, the pH of the latex was adjusted to 7 using a 5% sodium hydroxide aqueous solution. The neutralized emulsion was then purified to remove residual monomers and water was added to adjust the solid content to 35%.

[0032] Example 1 The average particle size and particle size distribution, as well as the viscosity at 20°C, of ​​the carboxylated styrene-butadiene latexes obtained in Examples 1-3 and the comparative examples were measured. The results are shown in Table 1.

[0033] The average particle size and particle size distribution were determined by transmission electron microscopy image analysis.

[0034] Viscosity was measured using an NDJ-1 rotational viscometer.

[0035] Table 1. Experimental results of average particle size, particle size distribution, and viscosity of carboxylated styrene-butadiene latex. Table 1 As can be seen from Table 1, the particle size distribution of the carboxylated styrene-butadiene latex prepared in Examples 1-3 is very concentrated. The percentage of carboxylated styrene-butadiene latex particles with a particle size of 240-260 nm is greater than 80% of the total number of particles. Example 2 is the best example with the most concentrated particle size distribution.

[0036] Example 2 The carboxylated styrene-butadiene latex obtained in Examples 1-3 and the comparative example was used as a binder in the preparation of the negative electrode sheet of lithium-ion battery. The specific steps are as follows: (1) Slurry preparation: At room temperature, 0.5 parts of sodium carboxymethyl cellulose (CMC) were added to 100 parts of deionized water and stirred at high speed for about 20 minutes. Then, 2 parts of carbon black conductive agent (SuperP), 20 parts of silicon-carbon composite negative electrode material (pure silicon-carbon mixed with 30% graphite) and 2 parts of carboxylated styrene-butadiene latex binder were added. The mixture was stirred evenly with a constant temperature magnetic stirrer and filtered through a 100-mesh filter to obtain the negative electrode slurry.

[0037] (2) Electrode coating: The prepared negative electrode slurry is uniformly coated on the current collector (copper foil) with a coating thickness of 100 μm. It is placed in an 80℃ oven to dry for 5 minutes, and then rolled at room temperature to obtain a negative electrode sheet with a thickness of 70 μm.

[0038] (3) Peel strength test: According to ASTM D3330, the peel strength (N / m) between the negative electrode sheet and the copper foil was tested using a computerized tensile testing machine (KJ1065). Five parallel samples were tested for each sample, and the average value was taken.

[0039] (4) Evaluation of slurry dispersibility: Observe the appearance of the prepared negative electrode sheet after baking, including the condition of electrode sheet cracking, curling, black spots and pits.

[0040] The surface acidity, aqueous acidity, and embedded acidity of each latex binder were tested; the slurry dispersibility and peel strength of the prepared negative electrode sheets were evaluated.

[0041] Surface acidity and aqueous phase free acidity testing: The surface acidity and aqueous phase acidity of the latex were tested according to the method in patent CN110061187B.

[0042] Encapsulated acid content: Total amount of carboxylic acid in moles minus surface acid content and free acid content in aqueous phase.

[0043] The test results are shown in Table 2. As can be seen from the comparison between Examples 1-3 and Comparative Example 1, the aqueous latex prepared by the present invention has significantly reduced the content of embedded acid and free acid in the aqueous phase, and significantly increased the surface acid content. The peel strength of the prepared negative electrode sheet is much higher than that of conventional latex, the slurry dispersibility is excellent, and the interfacial adhesion performance is significantly improved.

[0044] The CR2032 button cell was assembled from the negative electrode, Celgard2400 polypropylene separator, lithium metal (as counter electrode), and electrolyte.

[0045] The electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC), wherein the volume ratio of EC, DMC, and DEC was 1:1:1. FEC (fluoroethylene carbonate) was also added to the mixed solvent, with the volume fraction of FEC being 10% of the volume fraction of the mixed solvent. The concentration of lithium hexafluorophosphate (LiPF6) was 1 mol / L.

[0046] After the assembled CR2032 coin cells were left to stand for 12 hours, their long-cycle performance under constant current charge-discharge was tested using a LANDCT2001A battery testing system at room temperature (25±1℃). The test conditions were: constant current charge-discharge mode, current density 50 mA·g. -1 The voltage window is 0.02~2.0V, and the cycle count is 500. The tester automatically records the charge / discharge capacity per cycle, plotting the cycle count as the x-axis and the discharge specific capacity (mAhg). -1 Plot the cycle performance curve with y as the vertical axis. See the results below. Figure 1 The morphology and structure of the negative electrode material after 500 cycles were characterized using scanning electron microscopy. The results are shown below. Figure 2 .

[0047] Figure 1 The figures show the cycle curves of the button batteries assembled in Examples 1-3 and Comparative Example 1. As can be seen from the figures, the batteries in Examples 1-3 exhibit significantly higher discharge capacity retention rates within 500 cycles compared to Comparative Example 1, demonstrating excellent cycle stability. Example 2 shows the best capacity retention, maintaining a high discharge capacity even after 500 cycles, while Comparative Example 1 experiences rapid capacity decay in the early stages of cycling, and its discharge capacity after 500 cycles is far lower than that of the other examples. This fully demonstrates that the narrow-distribution functionalized aqueous latex prepared in this invention, as a binder for the negative electrode of lithium-ion batteries, can significantly improve the long-cycle stability of silicon-based negative electrode batteries and effectively alleviate the capacity decay problem caused by the volume expansion of the silicon negative electrode.

[0048] Figure 2The images are scanning electron microscope (SEM) images of the negative electrode materials prepared in Examples 1-3 and Comparative Example 1 after 500 cycles. Analysis shows that the electrodes in Examples 1-3 maintain a crack-free morphology. This is because the silane coupling agent-modified cellulose nanocrystals in the latex form a rigid three-dimensional network structure after the binder film is formed, effectively suppressing the volume expansion of the silicon negative electrode during charging and discharging. Simultaneously, the high carboxyl content on the latex surface provides strong interfacial adhesion, tightly connecting the active materials and resulting in a strong interfacial bond. The presence of long cracks on the surface of the electrode in Comparative Example 1 indicates that the negative electrode composite material prepared using traditional carboxylated styrene-butadiene latex as a binder cannot suppress silicon expansion and has the worst interfacial bonding effect. This morphological result is completely consistent with the electrochemical cycling performance test results.

[0049] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A narrow-distribution functionalized waterborne latex, characterized in that, The raw materials for preparing the aqueous latex include the following components by weight: 5-8 parts of functional group monomers, 20-40 parts of aromatic vinyl monomers, 60-80 parts of aliphatic conjugated diene monomers, 6-10 parts of silane coupling agent modified cellulose nanocrystals, 3-5 parts of initiator, 1.5-3.5 parts of emulsifier, and 100-120 parts of water.

2. The water-based latex according to claim 1, characterized in that, The preparation method of hydrophobically modified cellulose nanocrystals is as follows: The silane coupling agent and 95% ethanol were mixed evenly and heated to obtain reaction solution A. At the same time, cellulose nanocrystals and 95% ethanol were mixed evenly and the pH value was adjusted to 4-5 with acid to obtain solution B. Solution B was added to solution A and the reaction was continued with stirring. After the reaction was completed, the solid component was obtained by centrifugation. After washing with ethanol, it was dried to obtain silane coupling agent modified cellulose nanocrystals.

3. The water-based latex according to claim 1, characterized in that, The aromatic vinyl monomer is styrene, α-methylstyrene, or vinyltoluene; the aliphatic conjugated diene monomer is 1,3-butadiene, 2-methyl-1,3-butadiene, or 2,3-dimethyl-1,3-butadiene.

4. The water-based latex according to claim 1, characterized in that, The functional group monomer is selected from at least one of methacrylic acid, acrylic acid, itaconic acid, and maleic acid.

5. The water-based latex according to claim 1, characterized in that, The initiator is potassium persulfate or ammonium persulfate.

6. The water-based latex according to claim 1, characterized in that, The emulsifier is potassium oleate or potassium disproportionate.

7. The aqueous latex according to claim 2, characterized in that the cellulose nanocrystals have a length of 50-60 nm and a diameter of 8-12 nm.

8. The water-based latex according to claim 2, characterized in that, The silane coupling agent is selected from at least one of triethoxyoctylsilane, dodecyltrimethoxysilane, or hexadecyltrimethoxysilane.

9. The water-based latex according to claim 2, characterized in that, The solid-liquid ratio of the cellulose nanocrystals and the silane coupling agent is 1:2.5~3.5 (g / ml).

10. A method for preparing narrow-distribution functionalized aqueous latex according to claims 1-9, characterized in that, Using water as a solvent, functional group monomers, aromatic vinyl monomers, aliphatic conjugated diene monomers, silane coupling agent modified cellulose nanocrystals, initiators, and emulsifiers are added to a reactor to carry out emulsion polymerization.

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