PAA powder type water-based binder, its preparation method and application

Abstract

This invention belongs to the technical field of lithium-ion battery electrode materials, specifically relating to a PAA powder-type aqueous binder, its preparation method, and its application. The binder is in powder form of polyacrylic acid or its salts, with a solid content ≥99wt%, a particle size distribution D50 of 5~8μm, a span (D90-D10) / D50 ≤1.5, an average molecular weight of 30,000~100,000, and a neutralization degree of 0.3~0.7. Using acrylic acid as a monomer, a polymerization reaction is carried out in water under the action of an initiator and a chain transfer agent to obtain a PAA solution. The pH of the PAA solution is adjusted to 4.7~5.6, and after spray drying, it is sieved. This powder has a core-shell structure consisting of a binding core and a hydrophilic shell, which disperses rapidly in water, exhibits high slurry stability, high peel strength, and excellent cycle stability when used in silicon-carbon anodes, significantly superior to traditional binders.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery electrode material technology, specifically relating to a PAA powder-type aqueous binder, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, as highly efficient energy storage devices, are widely used in portable electronic devices, electric vehicles, and energy storage systems. Silicon-based anode materials, due to their theoretical specific capacity being far higher than that of traditional graphite anodes, have become a research hotspot for improving battery energy density. However, silicon exhibits a volume expansion rate of up to 300% during charge and discharge, leading to the pulverization of active materials and damage to the electrode structure, severely impacting cycle life. The binder, as a key inactive component in the electrode, is crucial for maintaining the integrity of the electrode structure.

[0003] Traditional polyvinylidene fluoride (PVDF) binders require the use of the toxic organic solvent N-methylpyrrolidone (NMP), posing environmental and safety concerns. Furthermore, PVDF is prone to swelling in electrolytes, and its rigid molecular chains make it difficult to adapt to the significant volume changes of silicon anodes. Polyacrylic acid (PAA) and its salts, rich in carboxyl groups, can form strong hydrogen bonds or covalent bonds with the hydroxyl groups on the surface of active materials, effectively anchoring particles and buffering volume expansion, making them considered one of the most promising silicon-based anode binders. However, currently available PAA solutions are mostly low-solids content (<10%), resulting in high transportation costs, high viscosity, and a tendency to separate, and making precise metering and automated batching difficult to achieve.

[0004] To overcome the problems of the aforementioned PAA solution, existing technologies have attempted to prepare PAA into solid powder. However, the prepared solid powder has poor solubility recovery and slow dispersion speed. Therefore, how to balance the rapid dispersion and high adhesion of PAA binder is a difficult problem that needs to be overcome. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a PAA powder-based aqueous binder, its preparation method, and its application. This invention proposes a PAA powder-based aqueous binder with high solids content, easy dispersibility, and high stability. The PAA solution is converted into powder through a spray drying process, while the molecular weight and neutralization degree are optimized to meet the comprehensive requirements of lithium-ion battery electrodes for binders with high strength, high stability, and easy processability.

[0006] It should be noted that the reason why traditional PAA products have long maintained a low-solids-content solution form is due to multiple technical obstacles in preparing solid powders, including thermal decomposition, high viscosity treatment, and solubility recovery. In particular, when preparing PAA into solid powder, the solubility recovery of solid powders is poor. This invention successfully overcomes these technical difficulties through innovative spray drying process, core-shell structure design, and precise process control, achieving a breakthrough from liquid to solid state. The narrow particle size distribution design significantly improves electrode processability and cycle life. The prepared powder disperses rapidly in water, exhibits excellent solubility recovery performance, high slurry stability, high peel strength when used in silicon-carbon anodes, and excellent cycle stability, significantly superior to traditional binders.

[0007] Specifically, to address the challenge of simultaneously achieving rapid dispersion and high adhesion in PAA binders, this invention reveals that during the critical spray drying process, when the neutralization degree of PAA is controlled within a specific range of 0.3 to 0.7, spontaneous micro-phase separation occurs due to the simultaneous presence of ionized hydrophilic carboxylate ions (-COO⁻) and unionized relatively hydrophobic carboxyl groups (-COOH) on the PAA molecular chain. This separation occurs during the instantaneous heating of the spray droplets and rapid solvent evaporation. Segments with higher ionization and stronger hydrophilicity tend to migrate and accumulate towards the droplet surface, ultimately forming a hydrophilic outer shell on the dried powder particles. Conversely, segments with lower ionization and higher adhesion activity remain inside the particles, forming the adhesive core.

[0008] This unique core-shell structure is not achieved through additional coating or template steps, but rather is generated in situ and spontaneously through the synergistic effect of a spray-drying environment with specific neutralization, molecular weight, and high tension. This gives the PAA powder of this invention significant advantages: on the one hand, the hydrophilic outer shell on the particle surface can rapidly hydrate and dissolve upon contact with water for slurry preparation, ensuring that the powder does not clump and achieving excellent dispersibility; on the other hand, the internal binding core gradually dissolves, fully releasing the high molecular weight PAA long chains, providing stronger anchoring force with the active material. This structure resolves the technical contradiction in existing PAA binders where "rapid dispersion" and "high adhesion" are difficult to achieve simultaneously.

[0009] This invention is specifically achieved through the following technical solutions: The first objective of this invention is to provide a PAA powder-type water-based binder, wherein the binder is a powder composed of polyacrylic acid or its salts, the powder having a solid content ≥99wt%, a particle size distribution D50 of 5~8μm, and a span = (D90-D10) / D50 ≤ 1.5; the polyacrylic acid or its salts have an average molecular weight of 30,000~100,000 and a neutralization degree of 0.3~0.7. The PAA powder-type water-based binder has a core-shell structure consisting of an adhesive core and a hydrophilic shell. The adhesive core is composed of PAA segments containing -COOH, and the hydrophilic shell is composed of PAA segments containing -COO⁻.

[0010] Preferably, the polyacrylate is lithium polyacrylate or sodium polyacrylate.

[0011] A second objective of this invention is to provide a method for preparing the above-mentioned PAA powder-type waterborne adhesive, comprising the following steps: (1) Using acrylic acid as a monomer, a polymerization reaction is carried out in water under the action of an initiator and a chain transfer agent to obtain a PAA solution; (2) Add an alkaline compound to the PAA solution to adjust the pH of the system to 4.7~5.6 to obtain a partially neutralized PAA solution; (3) Spray drying will be carried out. The particle size D50 of the powder will be controlled at 5~8μm by adjusting the pressure and feed rate, and then sieving will be performed.

[0012] Preferably, in step (1) PAA solution preparation step, the initial mass concentration of the monomer is controlled to be 15%~25%.

[0013] Preferably, the initiator is ammonium persulfate, and the amount used is 8% to 12% of the monomer mass; the chain transfer agent is isopropanol, and the amount used is 5% to 8% of the monomer mass.

[0014] In step (2), the alkaline compound is lithium hydroxide or sodium hydroxide.

[0015] Preferably, during the polymerization reaction, solution polymerization is carried out at 85~95°C for 3.5~4.5 hours; Preferably, during spray drying, an airflow spray drying method is used, with a feed viscosity ≤3000 mPa·s, an inlet air temperature of 180~210℃, an outlet air temperature of 85~105℃, and a nozzle pressure of 0.2~0.3 MPa. Under these conditions, PAA molecules undergo spontaneous micro-phase separation during the instantaneous heating of the spray droplets and the rapid evaporation of the solvent.

[0016] Preferably, during sieving, 0.1% to 0.5% of nano-silica by mass of the product to be sieved is added as an anti-caking agent.

[0017] This invention provides a method for preparing a negative electrode of a lithium-ion battery, wherein the above-mentioned binder is mixed with electrode active material, conductive agent and water to form a slurry, which is coated on a current collector and then dried and rolled to form an electrode sheet; the amount of binder added is 2wt% to 4wt% of the total solid content of the negative electrode.

[0018] Preferably, the electrode active material is a silicon-carbon composite material or graphite.

[0019] The present invention also provides a lithium-ion battery comprising a negative electrode prepared by the above method.

[0020] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention first discovered and utilized the hydrophilic shell-hydrophobic core structure that PAA spontaneously forms during spray drying under a specific degree of neutralization, fundamentally solving the problem of redispersibility of powder PAA, while fully releasing the active chain segments of the bonding core, achieving a synergistic unity of rapid dispersion and high bonding strength.

[0021] (2) The PAA powder of this invention has a high solid content (≥99wt%) and a narrow particle size distribution (span ≤1.5), ensuring a consistent dissolution rate during slurry preparation and maintaining slurry viscosity for more than 24 hours. When applied to silicon-carbon anodes, it exhibits excellent peel strength and high cycle capacity retention, demonstrating excellent electrochemical stability. The preparation method of this invention is simple, easy to implement, and suitable for large-scale industrial production applications. Attached Figure Description

[0022] Figure 1 This is a particle size distribution diagram of PAA powder in Example 1.

[0023] Figure 2 The graph shows a comparison of the cycle performance of silicon-carbon anodes in Example 1, Comparative Example 4, and Comparative Example 5.

[0024] Figure 3 The graph shows the effect of PAA weight-average molecular weight on electrode peel strength. Detailed Implementation

[0025] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below with reference to specific embodiments and accompanying drawings. However, the embodiments described are not intended to limit the present invention. Unless otherwise specified, the experimental methods and detection methods described in the following embodiments are conventional methods; unless otherwise specified, the reagents and materials described are commercially available.

[0026] Example 1 A method for preparing a PAA powder-based water-based binder includes the following steps: Step 1, Polymerization: Mix 200g of acrylic acid, 16g of ammonium persulfate, 12g of isopropanol and 772g of deionized water evenly, and stir at 90℃ for 4 hours to obtain PAA solution.

[0027] Step 2, Neutralization treatment: Lithium hydroxide is slowly added to the above PAA solution with stirring to adjust the pH to 5.0 (corresponding to a neutralization degree of about 0.55) to obtain a partially neutralized lithium polyacrylate solution.

[0028] Step 3, Spray Drying: The neutralized solution is subjected to airflow spray drying with an inlet air temperature of 200℃, an outlet air temperature of 95℃, a nozzle pressure of 0.28MPa, and a feed rate of 45mL / min. After collecting the powder, it is passed through a 300-mesh sieve, and 0.2% nano-silica is added as an anti-caking agent to obtain the powder.

[0029] Example 2 The difference from Example 1 is that, in step 3, during spray drying, the nozzle pressure was adjusted to 0.22 MPa and the feed rate was adjusted to 80 mL / min. Other conditions were the same as in Example 1.

[0030] Example 3 The difference from Example 1 is that in step 1, the amount of ammonium persulfate is changed to 20g, isopropanol to 20g, and deionized water to 760g, the polymerization temperature is 85℃, and the reaction time is 3.5 hours. Steps 2 and 3 are the same as in Example 1.

[0031] Example 4 The difference from Example 1 is that in step 1, the amount of ammonium persulfate was changed to 12g, isopropanol to 6g, and deionized water to 782g, the polymerization temperature was 95℃, and the reaction time was 4.5 hours. Step 2 is the same as in Example 1. In step 3, before spray drying, an appropriate amount of water was added to adjust the solution viscosity to ≤3000 mPa·s, and then spray drying was performed according to the conditions of Example 1.

[0032] Example 5 The difference from Example 1 is that, in step 2, during the neutralization process, the pH was adjusted to 4.7 using sodium hydroxide (corresponding to a degree of neutralization of approximately 0.35). Everything else is the same as in Example 1.

[0033] Example 6 The difference from Example 1 is that, in step 2, during the neutralization process, the pH was adjusted to 5.6 using sodium hydroxide (corresponding to a degree of neutralization of approximately 0.68). Everything else is the same as in Example 1.

[0034] Comparative Example 1 The spray pressure was changed to 0.5 MPa and the feed rate was changed to 30 mL / min. Other preparation and testing were the same as in Example 1.

[0035] Comparative Example 2 The preparation parameters were the same as in Example 3, but the amount of isopropanol was increased to 40g. The resulting powder had a molecular weight of 18,000, which was too low, a D50 of 6.0μm, and a span of 1.3.

[0036] Comparative Example 3 The preparation parameters were the same as in Example 4, but the amount of isopropanol was reduced to 3g. The resulting powder had a molecular weight of 130,000 (Mw = 130,000), which was too high, a D50 of 6.5μm, and a span of 1.4. The slurry viscosity was too high, the electrode flexibility was poor, the peel strength was 0.70 N / mm, and the capacity retention rate after 100 cycles was 81.0%.

[0037] Comparative Example 4 Commercially available lithium polyacrylate aqueous solution, solid content 8%, Mw≈50,000.

[0038] Comparative Example 5 PVDF adhesive is used, with NMP as the solvent.

[0039] Comparative Example 6 Compared to Example 1, adjusting the pH to 3.2 corresponds to a neutrality of approximately 0.15.

[0040] Comparative Example 7 Compared to Example 1, adjusting the pH to 7.8 corresponds to a neutrality of approximately 0.92.

[0041] The above PAA powder was mixed with silicon carbide and carbon black in a ratio of 3:94:3 to form a slurry, which was then coated onto the current collector. After drying, the slurry was tested. The capacity retention rate was tested after 100 cycles at 0.5°C. The results are shown in Table 1.

[0042] Table 1 Comparison of properties of PAA powders with different particle sizes All electrochemical test conditions were the same as before, and each example was performed in triplicate with the mean ± standard deviation.

[0043] As shown in Table 1, Comparative Example 1 had excessively large particle size and wide particle distribution, resulting in significant sedimentation of the slurry after 24 hours, a peel strength of 0.65 N / mm, and a capacity retention rate of 74.2% after 100 cycles. Comparative Example 2 had an excessively low molecular weight, and Comparative Example 3 had an excessively high molecular weight, both of which led to a decrease in peel strength and capacity retention rate after 100 cycles. Figure 1 The PAA powder obtained by spray drying is spherical in shape, with a concentrated particle size distribution and obvious narrow distribution characteristics. It can be seen that D50=5.5μm and span 1.2. Figure 2 For the comparison of the cycling performance of silicon-carbon anodes in Example 1, Comparative Example 4, and Comparative Example 5, after 100 cycles at 0.5C, it can be seen that the retention rate of Example 1 is higher than that of Comparative Example 4 (PAA solution) and Comparative Example 5 (PVDF). Figure 3The effect of PAA weight-average molecular weight on electrode peel strength was investigated. The optimal molecular weight of 78,000 resulted in a peel strength peak of 0.96 N / mm, corresponding to a particle size of D50 ≈ 5.5-6.5 μm and a neutralization degree of 0.55.

[0044] To illustrate the effect of different degrees of neutralization on the properties of PAA powder, PAA powders prepared with different degrees of neutralization in Examples 1, 5, 6, 6, and 7 were compared, and the results are shown in Table 2. Here, redispersion time refers to the time required for the powder to be reconstituted and form a homogeneous system.

[0045] Table 2 Comparison of redispersion time for PAA powders with different degrees of neutralization As shown in Table 2, Examples 1, 5, and 6 exhibited shorter redispersion times and higher peel strength when the degree of neutralization was within the specific range of 0.3 to 0.7. Comparative Examples 6 and 7, with their excessively low or high neutralization, both resulted in prolonged redispersion times and reduced peel strength. This is because when the degree of neutralization of PAA is controlled within this specific range of 0.3 to 0.7, spontaneous micro-phase separation occurs during the instantaneous heating of the spray droplets and rapid evaporation of the solvent, due to the simultaneous presence of ionized hydrophilic carboxylate ions (-COO⁻) and unionized relatively hydrophobic carboxyl groups (-COOH) on the PAA molecular chain. The segments with higher ionization and stronger hydrophilicity tend to migrate and accumulate on the droplet surface, ultimately forming a hydrophilic outer shell on the dried powder particles; while the segments with lower ionization and higher bonding activity remain inside the particles, forming a bonding core. The hydrophilic outer shell on the particle surface rapidly hydrates and dissolves upon contact with water for slurry preparation, ensuring the powder does not clump and achieving excellent dispersibility. Meanwhile, the internal binding core gradually dissolves, fully releasing the high molecular weight PAA long chains and providing stronger anchoring force with the active materials. This fundamentally solves the problem of PAA redispersibility in powder, while simultaneously allowing the active segments of the binding core to be fully released, achieving a synergistic unity of rapid dispersion and high bonding strength.

[0046] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, it is intended to include any modifications and variations that fall within the scope of the claims and their equivalents.

Claims

1. A PAA powder-type water-based binder, characterized in that, The binder is a powder composed of polyacrylic acid or its salts, the powder having a solid content ≥99wt%, a particle size distribution D50 of 5~8μm, and a span (D90-D10) / D50 ≤ 1.5; the polyacrylic acid or its salts have an average molecular weight of 30,000~100,000 and a neutralization degree of 0.3~0.7; PAA powder-type water-based binders have a core-shell structure consisting of an adhesive core and a hydrophilic shell. The adhesive core is composed of PAA segments containing -COOH, and the hydrophilic shell is composed of PAA segments containing -COO⁻.

2. The PAA powder-type water-based binder according to claim 1, characterized in that, The polyacrylate is either lithium polyacrylate or sodium polyacrylate.

3. A method for preparing the PAA powder-type waterborne binder according to claim 1, characterized in that, Includes the following steps: Acrylic monomer, initiator and chain transfer agent are mixed in water and subjected to solution polymerization to obtain PAA solution; an alkaline compound is added to the PAA solution to adjust the pH of the system to 4.7-5.6 to obtain a partially neutralized PAA solution; the partially neutralized PAA solution is spray-dried and sieved to obtain the PAA powder-type waterborne binder.

4. The preparation method according to claim 3, characterized in that, In the PAA solution preparation step, the initial mass concentration of the monomer is controlled at 15%~25%; the amount of initiator is 8%~12% of the monomer mass; and the amount of chain transfer agent is 5%~8% of the monomer mass.

5. The preparation method according to claim 3, characterized in that, The alkaline compound is lithium hydroxide or sodium hydroxide.

6. The preparation method according to claim 3, characterized in that, During spray drying, an airflow spray drying method is adopted, with an inlet air temperature of 180℃~210℃, an outlet air temperature of 85℃~105℃, a nozzle pressure of 0.2MPa~0.3MPa, and a feed rate of 45mL / min~80mL / min.

7. The preparation method according to claim 3, characterized in that, During sieving, 0.1% to 0.5% of nano-silica by mass of the product to be sieved is added as an anti-caking agent.

8. A method for preparing a lithium-ion battery negative electrode, characterized in that, The binder described in claim 1 is mixed with electrode active material, conductive agent and water to form a slurry, which is then coated onto the current collector and dried and rolled to form an electrode sheet; the amount of binder added is 2wt% to 4wt% of the total solid content of the negative electrode.

9. A lithium-ion battery negative electrode, characterized in that, It was prepared by the method described in claim 8.

10. A lithium-ion battery, characterized in that, It includes the negative electrode as described in claim 9.

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