Evaluation method of water-based binder for secondary battery

Abstract

The invention discloses an evaluation method of a water-based binder for a secondary battery, and belongs to the technical field of secondary batteries. The evaluation method comprises the following steps: S1, diluting a water-based binder into a clear glue solution; measuring the viscosity and the pH value of the glue solution; s2, preparing an adhesive film; s3, testing the glass transition temperature and the infrared spectrum of the obtained adhesive film; testing the mass swelling ratio of the adhesive film; testing the mechanical property of the adhesive film; s4, preparing the aqueous binder into electrode slurry, and carrying out a thixotropic area test; and S5, coating a current collector with the electrode slurry, processing the electrode slurry into electrode slices, assembling the electrode slices into a button cell, and measuring the electrical properties of the button cell. According to the evaluation method provided by the invention, a set of method for comprehensively evaluating the performance of the adhesive film is established, the chemical stability, the mechanical stability and the thermal stability of the water-based adhesive film and the electrochemical performance of the prepared electrode plate are effectively quantified, and a systematic and effective method for screening the water-based adhesive is provided for research and development of a secondary battery.

Description

Technical Field

[0001] The invention relates to the technical field of secondary batteries, and in particular to an evaluation method for a water-based binder for secondary batteries. Background Art

[0002] In recent years, the rapid development of secondary batteries has provided important support for portable electronic devices, electric vehicles and energy storage. As a key component of secondary batteries, electrode aqueous binders have a direct impact on the overall performance of secondary batteries, such as electrode structural stability, electrical conductivity and ion conductivity, electrolyte compatibility, mechanical properties, processing adaptability, battery energy density and other methods. Therefore, the performance of the negative electrode aqueous binder directly affects the cycle stability and power output of the battery. Therefore, in battery development, how to screen aqueous binders has received widespread attention. The aqueous binder needs to show good performance in electrochemistry, thermal stability and chemical stability; in addition, it should also have a moderate swelling rate and good mechanical properties.

[0003] However, there is currently no systematic and effective evaluation method for screening high-quality aqueous binders for secondary batteries to promote the research and development of secondary batteries. Summary of the invention

[0004] The object of the present invention is to provide a systematic and effective method for evaluating aqueous binders for secondary batteries.

[0005] The present invention discloses a method for evaluating an aqueous binder for a secondary battery, comprising the following steps: S1: diluting the aqueous binder to a clear glue solution; measuring the viscosity and pH value of the glue solution; S2: preparing adhesive film; S3: Test the glass transition temperature and infrared spectrum of the obtained film; test the mass swelling rate of the film; test the mechanical properties of the film; S4: preparing the aqueous binder into an electrode slurry and conducting a thixotropic area test; S5: coating the electrode slurry on a current collector, processing it into an electrode sheet, assembling the electrode sheet into a button cell, and measuring its electrical performance.

[0006] The secondary battery includes a lithium ion battery or a sodium ion battery.

[0007] Viscosity testing of water-based binders is very important because viscosity directly affects the quality of electrode slurry preparation, coating process, and the consistency and performance of the final electrode. In order to ensure that the binder and the entire electrode slurry perform well during the manufacturing process, viscosity testing is usually required.

[0008] Viscosity testing has the following significance: Coating uniformity: Appropriate viscosity can ensure the fluidity and uniformity of the electrode slurry during the coating process, avoiding uneven coating or defects.

[0009] Stability: Appropriate viscosity helps maintain the stability of the slurry during storage and prevents sedimentation or stratification.

[0010] Processing adaptability: Different equipment and processes require different viscosity ranges to ensure good processing performance.

[0011] Furthermore, in the step S1, the viscosity of the glue solution at 25°C and 40°C is measured.

[0012] The significance of the change of viscosity of water-based binders with temperature to secondary batteries is mainly reflected in the following aspects: Improve the stability of the production process: In the manufacturing process of secondary batteries, binders are used to fix active materials, conductive agents, etc. on the current collector. If the viscosity of the binder is not sensitive to temperature changes, it can maintain a relatively constant performance under different environmental conditions, which is conducive to maintaining the stability of production process parameters and ensuring smooth production.

[0013] Enhance the stability of the electrode structure: The binder with stable viscosity can effectively maintain the bonding strength of the electrode material at various operating temperatures, reduce the phenomenon of electrode material shedding or delamination due to temperature changes, and thus improve the overall structural stability of the electrode.

[0014] Improve battery safety and life: During the battery charge and discharge cycle, the internal temperature will change. If the binder can maintain good viscosity stability, it will help maintain the integrity of the internal structure of the electrode, reduce the internal resistance changes and possible safety hazards caused by temperature fluctuations, and thus extend the battery life and improve its safety.

[0015] Improve the consistency of battery performance: Whether in high or low temperature environments, binders with small viscosity changes can ensure the quality consistency of electrode preparation, which is very important for the consistency of the overall battery performance, especially in large-scale production, which can effectively reduce the performance differences between products.

[0016] Measure the viscosity of the glue at 25°C and 40°C, compare the changes in the viscosity of the glue, and choose the water-based adhesive with the smaller viscosity change.

[0017] pH value is an indicator of the acidity and alkalinity of a solution. For water-based binders, pH value affects their solubility, stability and compatibility with other ingredients.

[0018] The glass transition temperature and infrared spectrum of the film are tested. The molecular structure and chemical properties of the film are determined based on the absorption peaks at different wave numbers in the infrared spectrum. The glass transition temperature of the film is used to evaluate the thermal stability and operating temperature range of the adhesive.

[0019] Furthermore, in step S1, the aqueous binder includes at least one of pure benzene latex, styrene acrylic latex, styrene-butadiene polymer, butadiene-acrylonitrile polymer, styrene-butadiene latex, polybutyl methacrylate, polyacrylate polymer, polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylonitrile, sodium carboxymethyl cellulose, sodium polyacrylate, polyacrylic acid, polyacrylate, polymethyl acrylate, polyethyl acrylate, polymethyl methacrylate, polyurethane, polyacrylic acid-styrene polymer, ethylene-vinyl acetate copolymer, polyimide and polyvinyl acetate.

[0020] Furthermore, the aqueous binder includes any one or more of PMMA (polymethyl methacrylate), PAA (polyacrylic acid), SBR (styrene-butadiene rubber), and PVDF (polyvinylidene fluoride).

[0021] Furthermore, in step S1, the solid content of the glue solution is 5-10%.

[0022] Furthermore, in step S1, the dilution operation is to dilute the aqueous binder with water and stir to form a glue solution with a solid content of 5% to 10%, the stirring speed is 1800-2000rps / min, the stirring time is 0.5-1h, until there are no visible lumps in the mixed glue solution, and a clear and transparent mixed glue solution is obtained.

[0023] Furthermore, in step S2, the method for preparing the adhesive film includes: Pour the glue into a mold with a groove, tilt the two molds so that the molds form an angle of 5-15° with the horizontal plane, rotate the molds so that the glue adheres to the periphery of the side wall of the mold, the height of the glue rising along the side wall is 5-10mm, place the mold horizontally, let the glue level naturally; remove bubbles from the glue; and dry into a glue film.

[0024] This application improves the preparation method of water-based adhesive film, including optimizing the solid content ratio of the adhesive solution, the dissolution process and the preparation conditions, so as to obtain an adhesive film with better surface flatness, uniform thickness and lower defect rate. The improved preparation method is not only simple and easy to implement, but also can obtain a film with stable quality and performance.

[0025] Furthermore, the material of the mold with the groove includes PE or polytetrafluoroethylene.

[0026] Furthermore, the operation of degassing the glue liquid is as follows: placing the mold containing the glue liquid in an environment of 45-55°C and evacuating the environment for 5-15 minutes; gently scraping the remaining bubble area at the bottom of the mold with a glass rod; and letting it stand to allow the bubbles to be discharged.

[0027] By rotating the mold, the glue is attached to the side wall of the mold. During the drying and molding process, the glue will be subjected to outward pulling force, which promotes adhesion and prevents it from curling up into a ball. In addition, the glue is dried by temperature gradient, which can keep the film flat, smooth, and uniform in thickness during the preparation process, thereby improving the accuracy and reproducibility of the test.

[0028] Furthermore, the operation of drying into a film is as follows: subjecting the mold to temperature gradient drying, in the first stage the mold temperature is raised to 70-80°C and then kept warm for 0.5-1h; in the second stage the mold temperature is lowered to 45-50°C and then kept warm for 0.5-1h, and demolding obtains a water-based adhesive film with uniform thickness and smooth surface, the thickness of the film is 100-200μm.

[0029] Furthermore, in step S3, the method for testing the mass swelling rate of the film is as follows: placing the film on a weighing machine and recording the mass as M1; immersing the film in an aluminum-plastic bag containing an electrolyte and sealing the aluminum-plastic bag; then placing the aluminum-plastic bag in a 50-60°C forced air drying oven and baking it for 24-48 hours to accelerate the reaction and penetration; after baking, taking out the film and wiping its surface clean to remove residual electrolyte; placing the film on the weighing machine again, recording the mass as M2, and calculating the mass swelling rate of the film; the mass swelling rate of the film = (M2 - M1) / M1×100%.

[0030] The mass swelling rate of the film cannot be too high, otherwise after being assembled into a battery, the thickness of the film will increase, which may easily cause damage to the electrode sheet.

[0031] Furthermore, in step S3, the mechanical properties of the film include tensile strength and elongation at break of the film.

[0032] The thixotropy of electrode slurry refers to the property that the viscosity of the slurry decreases when subjected to shear force, and the viscosity gradually recovers after it is stationary or the shear force is removed. The thixotropic area usually refers to the area surrounded by the thixotropic ring obtained through rheological testing, which is a quantitative indicator to measure the degree of thixotropy of the material.

[0033] The significance of thixotropic area is: Process adaptability: The size of thixotropy affects the behavior of the slurry during the coating process. A suitable thixotropy can ensure that the slurry can flow smoothly during the coating process, evenly distributed on the current collector, and quickly fix the shape after coating to prevent flowing and shrinking.

[0034] Coating quality: A slurry with appropriate thixotropy helps form a uniform, dense and defect-free electrode coating, which is critical to battery performance. If the thixotropic area is too large or too small, it may cause problems such as uneven coating and particle sedimentation, thus affecting the consistency of the electrode and the overall performance of the battery.

[0035] Storage stability: Thixotropy is also related to the storage stability of the slurry. Good thixotropy can help maintain the suspension of solid particles in the slurry and reduce the stratification or precipitation that may occur during long-term storage.

[0036] Production efficiency: The right thixotropic area can increase production speed and efficiency without sacrificing final product quality. For example, in an automated production line, the ability to quickly respond to changes in shear force can make the machine operate more smoothly and reduce downtime.

[0037] For the electrode slurry of secondary batteries, it is very important to control the thixotropic area, which is directly related to many key links in the battery manufacturing process and the quality and performance of the finished battery.

[0038] Furthermore, in step S5, the electrode sheet is folded in half to observe the crease; after being assembled into a buckle, the CV (cyclic voltammetry) is tested by an electrochemical workstation under a constant temperature state.

[0039] Judging the performance of water-based adhesives through CV testing: Electrode structure stability: High-quality binders can ensure good bonding between active materials, conductive additives and current collectors, thereby maintaining the stability of the electrode structure during charge and discharge. If the binder performance is poor, it may cause the active material to fall off or the electrode to delaminate, which will be manifested as abnormal current response or changes in redox peaks on the CV curve.

[0040] Internal resistance and electronic conductivity: Binders not only play a physical bonding role, but may also affect the electronic conductivity and ion diffusion path of the entire electrode. Certain high-performance binders can provide better electronic conductivity or ion transport channels, thereby reducing the internal resistance of the battery and helping to improve the clarity and repeatability of the redox peaks in the CV curve.

[0041] Electrolyte wettability: A good binder usually has good electrolyte affinity, which helps the electrolyte to better penetrate into the electrode and promote the rapid migration of lithium ions. This is crucial for obtaining a stable CV curve, because uneven electrolyte distribution may cause local overheating or overcharging, which in turn affects the test results.

[0042] Durability and cycle life: The chemical stability and mechanical strength of the binder directly affect the long-term cycle stability of the battery. Through CV testing, the changes in the redox peaks after different cycles can be observed to evaluate the effect of the binder on the battery cycle life. High-quality binders can ensure that the battery maintains relatively stable CV characteristics after multiple cycles.

[0043] Furthermore, the steps S1 to S4 are all performed in a dehumidified environment with a relative humidity less than 1% RH to reduce the impact of moisture on the test results.

[0044] The present invention provides an evaluation method for an aqueous adhesive for secondary batteries, establishes a set of methods for comprehensively evaluating the performance of adhesive films, effectively quantifies the chemical stability, mechanical stability, thermal stability of the aqueous adhesive films, and the electrochemical properties after being prepared into electrode sheets, and provides a systematic and effective method for screening aqueous adhesives for the research and development of secondary batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is a diagram of the adhesive film prepared in Examples 1-4 of the present invention in a mold; Figure 2 It is a folding diagram of the negative electrode sheet of Examples 1-4 of the present invention; Figure 3 is a graph showing the test results of the glass transition temperature of the adhesive film of Example 1 of the present invention; Figure 4 is a graph showing the test results of the glass transition temperature of the adhesive film of Example 2 of the present invention; Figure 5 is a graph showing the test results of the glass transition temperature of the adhesive film of Example 3 of the present invention; Figure 6 This is a graph showing the test results of the glass transition temperature of the adhesive film of Comparative Example 4 of the present invention; Figure 7 is an infrared spectrum of the adhesive film of Example 1 of the present invention; Figure 8 is an infrared spectrum of the adhesive film of Example 2 of the present invention; Fig. 9 is an infrared spectrum of the adhesive film of Example 3 of the present invention; Fig.10 is an infrared spectrum of the adhesive film of Example 4 of the present invention; Fig.11 is the tensile curve of the film of Examples 1-4 of the present invention; Fig.12 It is the CV curve of the negative electrode sheet of Examples 1-3 in the present invention. DETAILED DESCRIPTION

[0046] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0047] Example 1

[0048] S1: At room temperature, dilute the polyacrylic acid A water-based adhesive with water and stir to form a glue solution with a solid content of 5%. The stirring speed is 1800rps / min and the stirring time is 0.5h, until there are no visible lumps in the mixed glue solution, and a clear and transparent glue solution is obtained. The glue solution is tested for viscosity at different temperatures and pH at room temperature. A viscometer is used to evaluate the rheology and fluidity of the adhesive, and a pH meter is used to measure the acidity and alkalinity of the adhesive solution. The results are shown in Table 1.

[0049] S2: Pour 40g of the glue prepared in step S1 into a polytetrafluoroethylene mold, which has a square groove of 80mm×80mm in length and width and 20mm in depth. Tilt the mold so that it is 5° with the horizontal plane, rotate the mold so that the glue adheres to the four sides of the inner wall of the mold, and the height of the glue rising along the side wall is about 5mm, then place the mold horizontally to allow the glue to level naturally.

[0050] Place the mold containing the glue in a vacuum box, the pressure displayed on the dial is ≤-0.1MPa, the temperature is set to 45℃, and the treatment time is 5min. After the vacuum treatment is completed, take the mold out of the vacuum box and use a glass rod to gently scrape the area with stubborn bubbles at the bottom of the mold, then let the mold stand at room temperature for 10min so that the bubbles can float and rise to the surface of the glue, ensuring that the upper and lower surfaces of the film are flat and free of bubbles.

[0051] The mold is then placed in an oven for temperature gradient drying, and the oven air volume is adjusted to the minimum. In the first stage, the temperature is raised to 70°C and kept warm for 0.5 h; in the second stage, the temperature is lowered to 45°C and kept warm for 0.5 h. After the water evaporates, the mold is demolded to obtain a water-based adhesive film with uniform thickness and smooth surface.

[0052] S3: Film infrared and glass transition temperature test: the prepared film is placed in an infrared spectrometer for scanning, and the molecular structure and chemical properties of the film are determined based on the absorption peaks at different wave numbers in the spectrum; the glass transition temperature of the film under temperature changes is recorded to evaluate the thermal stability and operating temperature range of the adhesive. The results are as follows: Figure 2-9 shown.

[0053] For the film mass swelling rate test, place the film on a weighing machine and record the mass as M1; soak the film in an aluminum-plastic bag containing electrolyte to ensure that the film is completely covered by the electrolyte; seal the aluminum-plastic bag to ensure that the electrolyte does not leak out, and then place it in a 50°C forced air drying oven for 24 hours to accelerate the reaction and penetration; after baking, take out the film and gently wipe its surface with a dust-free cloth to remove the residual electrolyte; place the film on the weighing machine again and record the mass as M2. The mass swelling rate = (M2 - M1) / M1 × 100%. The test results are shown in Table 2.

[0054] For the film tensile strength and elongation at break test, a dumbbell-shaped punching die made of cemented carbide was used to punch the film into tensile test samples of uniform size and shape, and the samples were placed on a universal tensile testing machine for tensile strength test. Fig.10 shown.

[0055] S4: The negative electrode slurry prepared with the target binder is subjected to a thixotropic area test using a rheometer.

[0056] The negative electrode slurry is composed of graphite, conductive agent, polyacrylic acid A formula water-based binder and dispersant sodium hydroxymethyl cellulose in a mass ratio of 96.5:0.5:2.6:0.4, and deionized water is used as solvent to prepare a uniform slurry with a solid content of 65%.

[0057] S5: Electrochemical stability test, prepare the negative electrode slurry in step S6 into a negative electrode sheet, punch and cut into small discs and bake, assemble into button cells according to the standard, use the film disc as the positive electrode and the lithium sheet as the negative electrode, assemble the button cells and let them stand for 8 hours, and then test the CV at a constant temperature by an electrochemical workstation. Fold the prepared negative electrode sheet in half to observe the crease.

[0058] Example 2

[0059] Replace the polyacrylic acid A formulation adhesive in step 1 with the polyacrylic acid B formulation adhesive.

[0060] Example 3

[0061] The polyacrylic acid C formulation adhesive replaces the polyacrylic acid A formulation adhesive in step 1.

[0062] Example 4

[0063] The polyacrylic acid D formulation adhesive replaces the polyacrylic acid A formulation adhesive in step 1.

[0064] like Figure 1 As shown, the film prepared in Examples 1-4 has a smooth and flat surface and uniform thickness.

[0065] Test results and analysis:

[0066] Table 1 Physical property measurement results of Examples 1-4 glue Glue properties Example 1 Example 2 Example 3 Example 4 pH 8.26 7.04 8.04 7.5 Viscosity 25℃ 8080 8700 9200 15000 Viscosity 40℃ 6380 6450 5483 2740 As shown in Table 1, the pH difference between Examples 1-4 is not large, and the performance of lithium ions is less affected within this numerical range. Solid content 5% / 25°C viscosity: Example 1-2 is lower than Example 3-4; solid content 5% / 40°C viscosity: Example 1-2 is higher than Example 3-4; it can be seen that compared with Example 3-4, the viscosity of Example 1-2 changes relatively less with temperature.

[0067] Table 2 Test results of mass swelling rate of films of Examples 1-4 Serial number Example 1 Example 2 Example 3 Example 4 Mass swelling rate 1.75% 1.01% 11.28% 15.62% As shown in Table 2, compared with Examples 3-4, the mass swelling rate of the adhesive film of Example 1-2 is smaller. Compared with Examples 3-4, the aqueous binder of Example 1-2 is less likely to increase the thickness of the electrode sheet prepared by immersing it in the electrolyte.

[0068] Table 3 Thixotropic area test results of the films of Examples 1-4 Slurry testing Example 1 Example 2 Example 3 Example 4 Thixotropic area Pa / (s.cm3) 474.70 568.20 5358.30 7571.50 As shown in Table 3, compared with Examples 3-4, Examples 1-2 have a smaller thixotropic area and better reversibility.

[0069] like Figure 2 As shown, the fold width of Example 1-2 is significantly smaller than that of Example 3-4, which indicates that the coating film on the positive electrode sheet of Example 3-4 is harder and more brittle. Example 1-2 has better flexibility.

[0070] like Figure 3-Figure 6 As shown, the glass transition temperature of the negative electrode binder of a secondary battery is usually in the range of 70-90°C. The glass transition temperatures of the adhesive films of Examples 1-4 are 78°C, 91°C, -7.5°C, and 95°C, respectively. The glass transition temperatures of the adhesive films of Examples 3-4 are too low or too high, and the glass transition temperature corresponding to Example 1-2 meets the requirements.

[0071] like Figure 7-Figure 10 As shown, the main difference between Example 1-2 and Example 3-4 is that the peaks below 1200 wavenumbers are different; the peak intensities above 1200 wavenumbers are different.

[0072] The infrared molecular structure and chemical properties of the adhesive film vary depending on its specific composition. The characteristic peaks of acrylic resin are: C=O stretching vibration (~1730 cm⁻¹), CO stretching vibration (~1150-1250 cm⁻¹), and CH stretching vibration (~2950 cm⁻¹). Whether the film has good flexibility is related to whether the electrode has good flexibility. At the same time, there is the same characteristic peak (1450cm⁻¹). The deformation vibration of methyl (CH) usually appears near 1450 cm⁻¹, and the deformation of methylene (-CH2-), especially twisting or wagging vibration, may also appear in this bandwidth. If the hard and brittle (poor flexibility) film has a clear peak at 1450 cm⁻¹, it may indicate that the chemical structure of the film contains abundant CH bonds, especially methyl or methylene structures, and it is speculated that this structure may reduce flexibility. The flexibility of the film of Example 1 is the best, with a characteristic peak at ~1166 cm⁻¹. The characteristic peak at 1166 cm⁻¹ is usually related to the stretching vibration of COC, especially in ether structures. This type of vibration indicates that the material contains a ring or chain structure with an oxidized ether bond, and its characteristics are as follows: Ether bond (COC): This bond is very common in polyethers or other polymer materials containing ether bonds. The presence of ether bonds usually gives the material good flexibility and elasticity because ether bonds can provide a certain flexibility between molecular chains. The flexibility and lower interaction force of the ether chain are conducive to the material showing good flexibility. If the film has good flexibility and an obvious characteristic peak is observed at 1166 cm⁻¹, it can be inferred that the film may contain more ether components or similar structures, which can be presumed to enhance flexibility. Therefore, the flexibility of the films of Examples 3 and 4 is poorer than that of Examples 1 and 2. This conclusion is consistent with Figure 2 The conclusions obtained are consistent with those

[0073] like Fig.11 As shown, compared with Examples 3-4, Examples 1-2 have higher tensile strength and elongation at break, the tensile strength of Example 1 is lower, and the elongation at break of Comparative Example 2 is lower.

[0074] like Fig.12 As shown, the peak intensity of Example 1-2 is higher than that of Example 3. The higher current peak in the CV (cyclic voltammetry) test may indicate a more active surface chemical property and better electron acceptance and supply capabilities. This ability may mean that the adhesive film can form a more stable electron-related interaction at the bonding interface, indicating that Example 1-2 can better bond the main material, facilitate lithium ion migration, and have better reaction kinetics.

[0075] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for evaluating an aqueous binder for a secondary battery, characterized in that: The following steps are involved: S1: diluting the aqueous binder to a clear glue solution; measuring the viscosity and pH value of the glue solution; S2: preparing adhesive film; S3: Test the glass transition temperature and infrared spectrum of the obtained film; test the mass swelling rate of the film; test the mechanical properties of the film; S4: preparing the aqueous binder into an electrode slurry and conducting a thixotropic area test; S5: coating the electrode slurry on a current collector, processing it into an electrode sheet, assembling the electrode sheet into a button cell, and measuring its electrical performance.

2. The method for evaluating an aqueous binder for a secondary battery according to claim 1, characterized in that: In the step S1, in the step S1, the aqueous binder includes at least one of pure benzene latex, styrene acrylic latex, styrene-butadiene polymer, butadiene-acrylonitrile polymer, styrene-butadiene latex, polybutyl methacrylate, polyacrylate polymer, polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylonitrile, sodium carboxymethyl cellulose, sodium polyacrylate, polyacrylic acid, polyacrylate, polymethyl acrylate, polyethyl acrylate, polymethyl methacrylate, polyurethane, polyacrylic acid-styrene polymer, ethylene-vinyl acetate copolymer, polyimide and polyvinyl acetate.

3. The method for evaluating an aqueous binder for a secondary battery according to claim 1, characterized in that: In the step S1, the solid content of the glue solution is 5-10%.

4. The method for evaluating an aqueous binder for a secondary battery according to claim 1, characterized in that: In the step S2, the method for preparing the adhesive film comprises: Pour the glue into a mold with a groove, tilt the two molds so that the molds form an angle of 5-15° with the horizontal plane, rotate the molds so that the glue adheres to the side wall of the mold, the height of the glue rising along the side wall is 5-10mm, place the mold horizontally, and let the glue level naturally; remove bubbles from the glue; and dry to form a film.

5. The method for evaluating an aqueous binder for a secondary battery according to claim 4, characterized in that: The operation of degassing the glue is as follows: place the mold containing the glue in an environment of 45-55℃ and evacuate for 5-15 minutes; gently scrape the remaining bubble area at the bottom of the mold with a glass rod; let it stand to allow the bubbles to be discharged.

6. The method for evaluating an aqueous binder for a secondary battery according to claim 4, characterized in that: The operation of drying and film formation is as follows: subject the mold to temperature gradient drying. In the first stage, the mold temperature is increased to 70-80°C and then kept warm for 0.5-1h; in the second stage, the mold temperature is reduced to 45-50°C and then kept warm for 0.5-1h. Demolding results in a water-based adhesive film with uniform thickness and smooth surface. The thickness of the film is 100-200μm.

7. The method for evaluating an aqueous binder for a secondary battery according to claim 1, characterized in that: In step S3, the method for testing the mass swelling rate of the film is as follows: placing the film on a weighing machine and recording the mass as M1; immersing the film in an aluminum-plastic bag containing an electrolyte and sealing the aluminum-plastic bag; then placing the aluminum-plastic bag in a 50-60° C. forced air drying oven and baking it for 24-48 hours to accelerate the reaction and penetration; after baking, taking out the film and wiping its surface clean to remove the residual electrolyte; Place the film on the weighing machine again, record the mass as M2, and calculate the mass swelling rate of the film.

8. The method for evaluating an aqueous binder for a secondary battery according to claim 1, characterized in that: In the step S3, the mechanical properties of the film include the tensile strength and elongation at break of the film.

9. The method for evaluating an aqueous binder for a secondary battery according to claim 1, characterized in that: In step S5, the electrode sheet is folded in half to observe the crease; after being assembled into a buckle, the CV is tested by an electrochemical workstation under a constant temperature state.

10. A method for evaluating an aqueous binder for a secondary battery according to any one of claims 1 to 9, characterized in that: The steps S1 to S4 are all performed in a dehumidified environment with a relative humidity less than 1% RH to reduce the impact of moisture on the test results.

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