Modified polyacrylic acid binder, carbon-coated aluminum foil and preparation methods of modified polyacrylic acid binder and carbon-coated aluminum foil

Abstract

The invention discloses a modified polyacrylic acid binder, a carbon-coated aluminum foil and a preparation method of the modified polyacrylic acid binder, and the preparation method of the modified polyacrylic acid binder comprises the following steps: S1, introducing amino groups: mixing polyacrylic acid with amines, and carrying out amidation reaction to obtain the modified polyacrylic acid binder with the amino groups; amino is introduced into the polyacrylic acid binder for modification, so that after the carbon-coated aluminum foil is coated with the positive electrode active material layer, the carbon-coated layer can swell with an organic solvent N-methyl pyrrolidone (NMP) of the positive electrode active material layer, the NMP resistance of the carbon-coated layer is slightly reduced, that is, the NMP slightly dissolves the carbon-coated layer, the cohesion of the carbon-coated layer is reduced, and the performance of the positive electrode active material layer is improved. According to the present invention, the slurry of the positive electrode active material layer can more easily invade into the carbon coating layer, and can be more easily combined with the carbon coating layer by matching with the rolling so as to obtain the large interaction force, such that the stripping force of the positive electrode plate is substantially improved, and the cycle life of the lithium ion battery is further improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly relates to a modified polyacrylic acid binder, carbon-coated aluminum foil and a preparation method thereof. Background Art

[0002] In recent years, lithium-ion batteries have been widely accepted in many fields such as consumer electronics, transportation, power tools and energy storage due to their advantages of high energy density, high output power and low self-discharge. At present, carbon-coated aluminum foil is mostly used as the positive current collector in lithium-ion batteries, which is beneficial to reducing the internal resistance and interfacial contact impedance of the battery, weakening the polarization phenomenon inside the battery, and can improve the rate performance of the battery to a certain extent.

[0003] The carbon-coated aluminum foil mainly consists of aluminum foil and a carbon-coated layer. The main components of the carbon-coated layer include a conductive agent, a binder and other additives. Among them, the selection and optimization of the binder are the core links of the carbon coating process, and factors such as adhesion, conductivity, chemical stability and process compatibility need to be considered comprehensively. Commonly used binders include polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), aqueous binder styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC), etc. Among them, PAA has the following advantages compared with PVDF and SBR: ① Environmental protection and low cost: Compared with PVDF, the aqueous system of PAA is more environmentally friendly, avoiding the use of organic solvents such as NMP, and the production and treatment costs are lower, which is especially suitable for large-scale manufacturing. ② Stronger adhesion: The interfacial binding force between polar groups of PAA such as carboxyl and polar materials such as positive electrode active materials and carbon materials is stronger, and the adhesion to aluminum foil, carbon materials, etc. is better than that of SBR and PVDF. ③ Better conductivity: The insulation of PAA is lower, reducing the adverse effect on conductivity, while SBR and PVDF need to add additional conductive agents to make up for the insulation. ④ Better dispersibility: PAA has excellent dispersibility in the aqueous system, which can effectively disperse carbon materials and positive electrode material particles to form a uniform coating. Therefore, due to its strong polarity, excellent adhesion performance, good environmental protection, good conductivity and good dispersibility, PAA is often used as the binder for the carbon-coated layer slurry.

[0004] However, the adhesion performance of PAA may not be sufficient to meet the requirements of high-performance batteries. Therefore, it is necessary to modify PAA to further improve the peel strength of the positive electrode sheet. The first modification method is to compound PAA with other binders such as PVDF, SBR, CMC, etc. to improve the adhesion, but it will introduce the defects of other binders such as PVDF, SBR, CMC, etc. The second modification method is to crosslink and modify PAA to improve the adhesion, but it will cause changes in the properties of PAA itself, thus affecting the final performance of the product. Based on this, there is an urgent need to develop a new PAA modification method to improve its adhesion performance and improve the peel strength of the positive electrode sheet. Summary of the Invention

[0005] The object of the present invention is to provide a modified polyacrylic acid binder, a carbon-coated aluminum foil and a preparation method thereof in view of the deficiencies of the prior art. By modifying the polyacrylic acid binder, the carbon-coated layer can swell with the organic solvent N-methylpyrrolidone in the positive electrode active material layer, reducing the cohesive force of the carbon-coated layer, making the slurry of the positive electrode active material layer easier to penetrate into the carbon-coated layer. Combining with roll pressing can obtain a greater interaction force between the two, thereby greatly improving the peel strength of the positive electrode sheet and further improving the cycle life of the lithium-ion battery.

[0006] The first aspect of the present invention provides a preparation method of a modified polyacrylic acid binder, comprising the following steps:

[0007] S1. Introducing an amino group: Mixing polyacrylic acid with an amine for an amidation reaction to obtain a modified polyacrylic acid binder with an amino group.

[0008] Taking ethylenediamine as an example, the reaction formula is: PAA-COOH + H2N-(CH2)2-NH2 → PAA-CONH-(CH2)2-NH2 + H2O.

[0009] By introducing an amino group to modify the polyacrylic acid binder in the present invention, after coating the positive electrode active material layer on the carbon-coated aluminum foil, the carbon-coated layer can swell with the organic solvent N-methylpyrrolidone (NMP) in the positive electrode active material layer, causing the performance of the carbon-coated layer to resist NMP to slightly decrease, that is, NMP slightly dissolves the carbon-coated layer, reducing the cohesive force of the carbon-coated layer, making the slurry of the positive electrode active material layer easier to penetrate into the carbon-coated layer. Combining with roll pressing can make the two more easily combined and obtain a greater interaction force, thereby greatly improving the peel strength of the positive electrode sheet and further improving the cycle life of the lithium-ion battery.

[0010] Preferably, in S1, polyacrylic acid solid and an amine are mixed in a mass ratio of (8-12):1, and under the catalysis of a first catalyst, the reaction is carried out at 75-85 °C for 45-50 h. After the reaction is completed, methanol is used to precipitate the production area, filtered and washed, and dried at 45-55 °C for 22-26 h to obtain a modified polyacrylic acid binder with an amino group. The introduction of the amino group can be characterized and proved by methods such as nuclear magnetic resonance or Fourier transform infrared spectroscopy.

[0011] Preferably, in S1, the amine includes at least one of ethylenediamine, butanediamine, and propanediamine;

[0012] The first catalyst includes at least one of 4-dimethylaminopyridine, thiourea, and low-concentration phosphoric acid.

[0013] Preferably, the preparation method further includes S2. Increasing the number of carboxyl groups: The modified polyacrylic acid binder with amino groups is configured into a solution, and then mixed with carboxylic acid for carboxylation reaction to obtain a modified polyacrylic acid binder with amino groups and an increased number of carboxyl groups.

[0014] In the present invention, the polyacrylic acid binder is modified by a two-step method, which can stably introduce amino groups and increase the number of carboxyl groups, thereby improving the affinity between the modified polyacrylic acid binder and NMP, causing swelling between the modified polyacrylic acid binder and NMP. Macroscopically, it is manifested as slight dissolution of the carbon-coated layer. Furthermore, after the positive electrode active material layer and the carbon-coated layer are compounded and rolled, a large interaction force can be obtained between the two, so as to greatly improve the peel strength of the positive electrode sheet.

[0015] Specifically, ① Solvent penetration: As an organic solvent, NMP can penetrate into the polymer chain network of the modified polyacrylic acid binder, resulting in an increase in the distance between polymer chains and volume expansion; ② Hydrogen bond disruption: The carboxyl groups originally possessed by polyacrylic acid, the carboxyl groups increased after modification, and the introduced amino groups can form a network structure through hydrogen bonds. NMP can form new hydrogen bonds with these carboxyl groups and amino groups, thereby destroying the original hydrogen bond structure and promoting swelling; ③ Solvent-polymer interaction: The interaction forces between NMP and the modified polyacrylic acid chain segments, such as van der Waals forces and ionic bonds, can affect the degree of swelling to a certain extent. Thus, macroscopically, after the carbon-coated layer protrudes above the positive electrode active material layer, NMP will invade and corrode a part of the carbon-coated layer, resulting in a decrease in the cohesive force of the carbon-coated layer, and the positive electrode active material layer is more likely to combine with the carbon-coated layer.

[0016] Preferably, in S2, the modified polyacrylic acid binder with amino groups is dissolved in an organic solvent to form a solution, and then carboxylic acid is added to the solution, stirred at a speed of 700-900 r / min for 50-70 min, and reacted in a water bath environment at 75-85 °C for 45-50 h under the catalysis of a second catalyst. Methanol is used to precipitate the product area, filtered, washed, and dried to obtain a modified polyacrylic acid binder with amino groups and an increased number of carboxyl groups.

[0017] Preferably, in S2, the organic solvent includes at least one of dimethylformamide, dimethylacetamide, and propylene carbonate;

[0018] The carboxylic acid includes at least one of acetic acid, formic acid, propionic acid, oxalic acid, and citric acid;

[0019] The second catalyst includes at least one of 1,8-bicyclo[5.4.0]undec-7-ene, p-toluenesulfonic acid, and trifluoroacetic acid.

[0020] The second aspect of the present invention provides a modified polyacrylic acid binder, which is prepared by the above preparation method.

[0021] The third aspect of the present invention provides a method for preparing carbon-coated aluminum foil, comprising the following steps: adjusting the solid content of the binder with deionized water, adding half of the conductive agent and stirring at high speed, then adding the other half of the conductive agent and stirring at high speed, then adding an appropriate amount of deionized water to adjust the slurry concentration and stirring at high speed, then adding a pH regulator to adjust the pH to weakly acidic, then adding a wetting agent and stirring at low speed before discharging, homogenizing to obtain a conductive slurry, and coating the conductive slurry on at least one surface of the aluminum foil to obtain carbon-coated aluminum foil;

[0022] The binder is the above-mentioned modified polyacrylic acid binder.

[0023] Preferably, the solid content of the binder is adjusted to 15-30% and the viscosity is 800-3500 mPa·s with deionized water, then half of the carbon powder and half of the graphite powder are added, and high-speed dispersion is carried out at a speed of 2000-2600 rpm / min in a double planetary stirring tank for 25-35 min, then the other half of the carbon powder and the other half of the graphite powder are added, and high-speed dispersion is carried out at a speed of 2000-2600 rpm / min for 55-65 min, then an appropriate amount of deionized water is added to reduce the slurry concentration and high-speed dispersion is carried out at a speed of 2000-2600 rpm / min for 25-35 min, then a pH regulator is added to adjust the pH to 5-7, then a wetting agent is added and high-speed dispersion is carried out at a speed of 10-15 rpm / min for 30-45 min before discharging, the slurry is homogenized 2 times in a homogenizer under a pressure of 250-350 bar to obtain a conductive slurry, and the conductive slurry is uniformly coated on at least one surface of the aluminum foil to form a carbon-coated layer with a single-sided thickness of 0.4-0.6 μm to obtain carbon-coated aluminum foil.

[0024] Preferably, the component contents of the conductive slurry are: by mass fraction, 20-60% conductive agent, 10-40% binder, 1-10% pH regulator, 10-20% wetting agent, and the balance is deionized water;

[0025] The mass ratio of the carbon powder to the graphite powder is (70-100):(10-30), and the mass ratio of the binder to the conductive agent is 1:(1-5).

[0026] Preferably, the mass ratio of the binder to the conductive agent is 1:(1-2), and the mass ratio of the carbon powder to the graphite powder in the conductive agent is 1:1.

[0027] Preferably, the pH regulator includes at least one of sodium hydroxide solution, lithium hydroxide solution, and sodium bicarbonate solution, and the wetting agent includes at least one of polyether siloxane, modified polyether siloxane, and alcohol reagents.

[0028] The fourth aspect of the present invention provides a carbon-coated aluminum foil, which is prepared by the above-mentioned preparation method. Description of the Drawings

[0029] Figure 1 Infrared spectrum of the modified polyacrylic acid binder prepared in Examples 1-5.

[0030] Figure 2 Interface state diagram during the stripping of the positive electrode sheet, where 2a is the positive electrode sheet of Example 1, 2b is the positive electrode sheet of Comparative Example 1, and 2c is the positive electrode sheet of Comparative Example 2. Detailed implementation manners

[0031] For better understanding and implementation, the technical solutions of the present application will be clearly and completely described below in combination with the examples. Obviously, the described examples are only a part of the examples of the present application, rather than all the examples.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which this application belongs. The terms used in the description of this application herein are only for the purpose of describing specific examples and are not intended to limit this application.

[0033] Unless otherwise specified, all numerical values expressing amounts of ingredients, reaction conditions, etc. used in the specification and claims are understood to be modified by the term "about". Therefore, unless otherwise indicated, the numerical parameters set forth herein are approximate values that can vary according to the desired properties required.

[0034] As used herein, "and / or" refers to one or all of the recited elements.

[0035] As used herein, "comprises" and "comprising" cover the case where only the recited elements are present and the case where there are other unrecited elements in addition to the recited elements.

[0036] All percentages in this application are weight percentages unless otherwise stated.

[0037] Unless otherwise specified, the terms "a", "an", "one" and "the" used in this specification are intended to include "at least one" or "one or more". For example, "a component" means one or more components, so more than one component may be considered and may be adopted or used in the implementation of the described embodiments.

[0038] Example 1

[0039] A method for preparing carbon-coated aluminum foil, comprising the following steps:

[0040] S1. Introduction of amino group: Mix polyacrylic acid solid and ethylenediamine with a mass ratio of 10:1, and react at 80 °C for 48 h under the catalysis of 4-dimethylaminopyridine. After the reaction, precipitate the product area with methanol, filter and wash, and dry at 50 °C for 24 h to obtain a modified polyacrylic acid binder with amino groups;

[0041] S2. Increase the number of carboxyl groups: Dissolve the modified polyacrylic acid binder with amino groups in dimethylformamide to prepare a solution, then add acetic acid accounting for 2% of the mass of the polyacrylic acid solid to the solution, stir at a speed of 800 r / min for 60 min, and react in a water bath environment at 80 °C for 48 h under the catalysis of 1,8-bicyclo[5.4.0]undec-7-ene. Precipitate the product area with methanol, filter, wash and dry to obtain a modified polyacrylic acid binder with amino groups and an increased number of carboxyl groups;

[0042] S3. Use deionized water to adjust the solid content of the modified polyacrylic acid binder to 20% and the viscosity to 1500 mPa·s. Then add half of the conductive agent, that is, half of the carbon powder and half of the graphite powder, and disperse at a high speed of 2300 rpm in a double planetary stirring tank for 30 min. Then add the other half of the conductive agent, that is, the other half of the carbon powder and the other half of the graphite powder, and disperse at a high speed of 2300 rpm for 60 min. Then add an appropriate amount of deionized water to reduce the slurry concentration and disperse at a high speed of 2300 rpm for 30 min. Then add a pH regulator to adjust the pH to 6, and then add a wetting agent, modified polyether siloxane, and disperse at a high speed of 13 rpm for 40 min and then discharge. Homogenize the slurry twice in a homogenizer at a pressure of 300 bar to obtain a conductive slurry. Uniformly coat the conductive slurry on at least one surface of the aluminum foil to form a carbon-coated layer with a single-sided thickness of 0.5 μm to obtain a carbon-coated aluminum foil.

[0043] Among them, the component content of the conductive slurry is: by mass fraction, 30% conductive agent, 30% binder, 5% pH regulator, 15% wetting agent, and the balance is deionized water.

[0044] The mass ratio of the carbon powder to the graphite powder is 1:1, and the mass ratio of the modified polyacrylic acid binder to the conductive agent is 1:1.

[0045] The pH regulator is a sodium hydroxide solution.

[0046] Example 2

[0047] The difference between this example and Example 1 is that the mass ratio of the modified polyacrylic acid binder to the conductive agent is 1:5.

[0048] Example 3

[0049] The difference between this example and Example 1 is that the mass ratio of the modified polyacrylic acid binder to the conductive agent is 1:2.

[0050] Example 4

[0051] The difference between this example and Example 1 is that the mass ratio of the modified polyacrylic acid binder to the conductive agent is 2:3.

[0052] Example 5

[0053] The difference between this example and Example 1 is that the mass ratio of the modified polyacrylic acid binder to the conductive agent is 3:2.

[0054] Example 6

[0055] The difference between this example and Example 1 is that the polyacrylic acid binder was not modified by S2. Increasing the number of carboxyl groups.

[0056] Comparative Example 1

[0057] The difference between this Comparative Example 1 and Example 1 is that the polyacrylic acid binder was not modified, and conventional polyacrylic acid was used as the binder.

[0058] Comparative Example 2

[0059] The difference between this comparative example and Example 1 is that the order of S1 and S2 was changed, that is, the number of carboxyl groups was increased first, and then the amino group was introduced.

[0060] Comparative Example 3

[0061] The difference between this comparative example and Example 1 is that S1. Introducing an amino group was not performed on the polyacrylic acid binder.

[0062] The positive electrode sheets and lithium-ion batteries were prepared using the carbon-coated aluminum foils of Examples 1-6 and Comparative Examples 1-3. The preparation methods are as follows:

[0063] Preparation of the positive electrode sheet: Using the carbon-coated aluminum foils obtained in each example and comparative example as the positive electrode current collector, LiCoO2 was used as the positive electrode active material, and carbon black was used as the conductive agent. The positive electrode active material layer was formed on the surface of the positive electrode current collector, and thus the positive electrode sheet was obtained;

[0064] Preparation of the negative electrode sheet: Using a copper foil as the negative electrode current collector, silicon was used as the negative electrode active material, and carbon black was used as the conductive agent. The negative electrode active material layer was formed on the surface of the negative electrode current collector, and thus the negative electrode sheet was obtained;

[0065] Preparation of the electrolyte: Propylene carbonate, ethylene carbonate, and ethyl methyl carbonate were taken and fully mixed according to the mass ratio of 1:1:1 of the three to obtain a carbonate solvent. LiPF6 was added to the carbonate solvent to prepare a concentration of 1 mol·L -1A carbonate solution of LiPF6, using this carbonate solution as the electrolyte of a lithium-ion battery;

[0066] Preparation of lithium-ion battery: Stack the positive electrode sheet, separator, and negative electrode sheet prepared above in sequence to prepare a bare battery cell; Place the bare battery cell in the outer packaging case of the lithium battery, inject the electrolyte after drying, and obtain the lithium-ion battery through processes such as vacuum packaging, standing, formation, and shaping.

[0067] Testing method

[0068] (1) Pole piece peel strength test: Stick the positive electrode sheets prepared in each example and comparative example on the test board using 3M double-sided tape with a width of 2 cm, separate the active material on the positive electrode sheet from the carbon-coated layer, stick the tape on the carbon-coated aluminum foil, and test the 180° pole piece peel strength on a tensile machine.

[0069] (2) Penetration resistance test: Take 2 * 2 cm of the carbon-coated aluminum foil prepared in each example and comparative example and test the resistance under a film resistance meter.

[0070] (3) Battery cycle performance test: Test the batteries prepared in each example and comparative example at room temperature (25 °C). Constant current and constant voltage charge the coin-type symmetric battery at rates of 0.2C, 0.3C, 0.5C, 1C, 2C, 3C, and 5C to 3 - 4.5 V, with a cut-off current of 0.15 - 3 A. Then, discharge the coin-type symmetric battery at rates of 0.2C, 0.3C, 0.5C, 1C, 2C, 3C, and 5C to 3 - 4.5 V. This is taken as one charge-discharge cycle. Charge and discharge the coin-type symmetric battery 100 times in accordance with the above method, test the discharge capacity of the coin-type symmetric battery before and after cycling, and calculate the capacity retention rate of the coin-type symmetric battery after 100 cycles according to the following formula: Capacity retention rate of the coin-type symmetric battery after 100 cycles (%) = (Discharge capacity of the 100th cycle / Initial discharge capacity) × 100%.

[0071] Table 1. Performance results of each example and comparative example

[0072]

[0073]

[0074] As can be seen from Table 1, the pole piece peel strength, penetration resistance of the carbon-coated aluminum foil applied to the positive electrode sheet in Examples 1 - 6, and the capacity retention rate of the battery are all better than those in Comparative Examples 1 - 3. Moreover, the pole piece peel strength, penetration resistance of the carbon-coated aluminum foil applied to the positive electrode sheet in Example 1, and the capacity retention rate of the battery are better than those in Examples 2 - 5 and Example 6. The carbon-coated aluminum foil prepared in Example 1 has the best performance. The following combines Figure 1 and Figure 2 for specific analysis:

[0075] In Comparative Example 1, an unmodified common polyacrylic acid binder on the market was used, and the binding property was poor. The carbon-coated aluminum foil prepared with this binder had a very low pole piece peeling force when applied to the positive electrode sheet, and had a high resistance. When applied to a battery, it would cause a high internal resistance of the battery, and the positive electrode active material layer was likely to fall off, resulting in a significant reduction in the cycle life of the battery. In Examples 1-6, a modified polyacrylic acid binder with introduced amino groups or introduced amino groups and increased carboxyl groups was used, which was beneficial to improving the affinity between the modified polyacrylic acid binder and NMP in the positive electrode active material layer, causing swelling between the modified polyacrylic acid binder and NMP. Furthermore, after the positive electrode active material layer and the carbon-coated layer were compounded and rolled, a large mutual force could be obtained between the two, thereby increasing the peeling force of the positive electrode sheet and reducing the resistance. When applied to a battery, it would result in a lower internal resistance of the battery, and the positive electrode active material layer was not likely to fall off, which was beneficial to improving the cycle life of the battery.

[0076] See Figure 1 , 1552 cm -1 The spectral peak at is the bending vibration of N-H, indicating that the amino group was successfully introduced into the modified polyacrylic acid binder prepared in Examples 1-5. The spectral peak at 2997 cm -1 is the stretching vibration of O-H, indicating that the carboxyl group was successfully introduced into the modified polyacrylic acid binder prepared in Examples 1-5. On this basis, by comparing Example 1 with Examples 2-5, it can be seen that the mass ratio of the modified polyacrylic acid binder to the conductive agent is preferably 1:1 in Example 1. After being prepared into a positive electrode sheet, the affinity between the modified polyacrylic acid binder and NMP in the positive electrode active material layer can be utilized to cause slight swelling between the modified polyacrylic acid binder and NMP to increase the peeling force of the positive electrode sheet, and the carbon-coated layer will not be dissolved and damaged, thereby greatly increasing the peeling force of the positive electrode sheet and significantly reducing the resistance. When applied to a battery, it will result in a significantly lower internal resistance of the battery and a significant reduction in the shedding rate of the positive electrode active material layer, which is beneficial to greatly improving the cycle life of the battery. In Examples 2-4, as the content of the modified polyacrylic acid binder gradually decreases, the swelling between the modified polyacrylic acid binder and NMP gradually weakens. Therefore, the peeling force of the positive electrode sheet gradually decreases, and when applied to a battery, the shedding rate of the positive electrode active material layer gradually increases, resulting in a gradual decrease in the cycle life of the battery compared to Example 1. In Example 5, the amount of the modified polyacrylic acid binder used is too much, which will cause excessive swelling between the modified polyacrylic acid binder and NMP, resulting in slight dissolution and damage to the carbon-coated layer, causing a decrease in the peeling force of the positive electrode sheet, and when applied to a battery, the shedding rate of the positive electrode active material layer will increase, resulting in a decrease in the cycle life of the battery compared to Example 1.

[0077] See Figure 2It can be seen that Comparative Example 1 uses an unmodified common polyacrylic acid binder on the market. Not only is its adhesiveness poor, but also the affinity between the polyacrylic acid binder and NMP in the positive electrode active material layer is poor, resulting in basically no swelling between the polyacrylic acid binder and NMP. As a result, the stripping force of the electrode sheet is very low, and it has a high resistance. When applied to a battery, it will cause a high internal resistance of the battery, and the positive electrode active material layer is likely to fall off, thus greatly reducing the cycle life of the battery. Example 1 uses a modified polyacrylic acid binder with introduced amino groups and increased carboxyl groups. It can not only utilize the affinity between the modified polyacrylic acid binder and NMP in the positive electrode active material layer to cause slight swelling between the modified polyacrylic acid binder and NMP to improve the stripping force of the positive electrode sheet, but also will not dissolve and damage the carbon-coated layer. As a result, the stripping force of the positive electrode sheet is greatly increased, and the resistance is significantly reduced. When applied to a battery, it will cause a significantly lower internal resistance of the battery, and the shedding rate of the positive electrode active material layer is greatly reduced, which is conducive to greatly improving the cycle life of the battery. In Comparative Example 2, when modifying the polyacrylic acid binder, the carboxyl group is increased first and then the amino group is introduced. The reaction between the amine and the carboxyl group results in an excessive number of amino groups. The modified polyacrylic acid binder has an excessive affinity with NMP in the positive electrode active material layer, resulting in excessive swelling with NMP, which will cause dissolution and damage to the carbon-coated layer, reducing the stripping force of the positive electrode sheet. When applied to a battery, it will cause an increase in the shedding rate of the positive electrode active material layer, and thus the cycle life of the battery is lower than that of Example 1.

[0078] Comparing Example 1 with Example 6 and Comparative Examples 1-3, it can be seen that Example 6 only introduces amino groups, which is beneficial to improving the affinity between the modified polyacrylic acid binder and the positive electrode active material layer, so that swelling occurs between the modified polyacrylic acid binder and NMP. Furthermore, after the positive electrode active material layer and the carbon-coated layer are compounded and rolled, a large mutual force can be obtained between the two, thus improving the stripping force of the positive electrode sheet and reducing the resistance compared with Comparative Example 1. When applied to a battery, it will cause a lower internal resistance of the battery, and the positive electrode active material layer is not likely to fall off, which is conducive to improving the cycle life of the battery. Comparative Example 3 only simply increases the number of carboxyl groups, and basically does not improve the affinity between the polyacrylic acid binder and NMP in the positive electrode active material layer. Therefore, compared with Comparative Example 1, there is no improvement in the stripping force, resistance, and battery cycle life of the positive electrode sheet. And in Example 1, increasing the carboxyl group after introducing the amino group will greatly improve the stripping force, resistance, and battery cycle life of the positive electrode sheet. This shows that introducing amino groups alone is beneficial to improving the performance of the modified polyacrylic acid binder, the stripping force and resistance of the positive electrode sheet, and the battery cycle life. The amino group and the carboxyl group must be introduced in sequence to significantly improve the performance of the modified polyacrylic acid binder, the stripping force and resistance of the positive electrode sheet, and the battery cycle life.

[0079] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than limiting the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A preparation method of a modified polyacrylic acid binder, characterized in that, It includes the following steps: S1. Introducing amino groups: Mix polyacrylic acid with amines for amidation reaction to obtain the modified polyacrylic acid binder with amino groups.

2. The preparation method of the modified polyacrylic acid binder according to claim 1, characterized in that: The preparation method further includes S2. Increasing the number of carboxyl groups: Configure the modified polyacrylic acid binder with amino groups into a solution, and then mix it with carboxylic acid for carboxylation reaction to obtain the modified polyacrylic acid binder with amino groups and an increased number of carboxyl groups.

3. The preparation method of the modified polyacrylic acid binder according to claim 2, wherein: In S1, mix polyacrylic acid solid and amines with a mass ratio of (8 - 12):1, react at 75 - 85°C for 45 - 50 h under the catalysis of the first catalyst. After the reaction, precipitate the product area with methanol, filter and wash, and dry at 45 - 55°C for 22 - 26 h to obtain the modified polyacrylic acid binder with amino groups; In S2, dissolve the modified polyacrylic acid binder with amino groups in an organic solvent to form a solution, then add carboxylic acid accounting for 0.5 - 2% of the mass of polyacrylic acid solid to the solution, stir at a speed of 700 - 900 r / min for 50 - 70 min, and react in a water bath environment at 75 - 85°C for 45 - 50 h under the catalysis of the second catalyst. Precipitate the product area with methanol, filter, wash and dry to obtain the modified polyacrylic acid binder with amino groups and an increased number of carboxyl groups.

4. The preparation method of the modified polyacrylic acid binder according to claim 3, characterized in that: In S1, the amines include at least one of ethylenediamine, butanediamine, and propanediamine; The first catalyst includes at least one of 4-dimethylaminopyridine, thiourea, and low-concentration phosphoric acid; In S2, the organic solvent includes at least one of dimethylformamide, dimethylacetamide, and propylene carbonate; The carboxylic acid includes at least one of acetic acid, formic acid, propionic acid, oxalic acid, and citric acid; The second catalyst includes at least one of 1,8-dicyclo[5.4.0]undec-7-ene, p-toluenesulfonic acid, and trifluoroacetic acid.

5. A modified polyacrylic acid binder, characterized in that: The modified polyacrylic acid binder is prepared by the preparation method described in any one of claims 1 - 4.

6. A preparation method of carbon-coated aluminum foil, characterized in that It includes the following steps: Use deionized water to adjust the solid content of the binder, add half of the conductive agent and stir at high speed, then add the other half of the conductive agent and stir at high speed, then add an appropriate amount of deionized water to adjust the slurry concentration and stir at high speed, then add a pH regulator to adjust the pH to weakly acidic, then add a wetting agent and stir at low speed and then discharge, homogenize to obtain a conductive slurry, and coat the conductive slurry on at least one surface of the aluminum foil to obtain the carbon-coated aluminum foil; The binder is the modified polyacrylic acid binder described in claim 5.

7. The preparation method of the carbon-coated aluminum foil according to claim 6, characterized in that: Adjust the solid content of the binder to 15-30% and the viscosity to 800-3500 mPa·s using deionized water. Then add half of the carbon powder and half of the graphite powder, and disperse them at a high speed at 2000-2600 rpm / min in a double planetary stirring tank for 25-35 min. Next, add the other half of the carbon powder and the other half of the graphite powder, and disperse them at a high speed at 2000-2600 rpm / min for 55-65 min. Then add an appropriate amount of deionized water to reduce the slurry concentration and disperse it at a high speed at 2000-2600 rpm / min for 25-35 min. Then add a pH regulator to adjust the pH to 5-7, and then add a wetting agent and disperse it at a high speed at 10-15 rpm / min for 30-45 min before discharging. Homogenize the slurry twice in a homogenizer at a pressure of 250-350 bar to obtain the conductive slurry. Uniformly coat the conductive slurry on at least one surface of the aluminum foil to form a carbon-coated layer with a single-sided thickness of 0.4-0.6 μm, and obtain the carbon-coated aluminum foil.

8. The preparation method of the carbon-coated aluminum foil according to claim 7, characterized in that: The content of each component of the conductive slurry is: by mass fraction, 20-60% conductive agent, 10-40% binder, 1-10% pH regulator, 10-20% wetting agent, and the balance is deionized water; The mass ratio of the carbon powder to the graphite powder is (70-100):(10-30), and the mass ratio of the binder to the conductive agent is 1:(0.5-5); The pH regulator includes at least one of sodium hydroxide solution, lithium hydroxide solution, and sodium bicarbonate solution, and the wetting agent includes at least one of polyether siloxane, modified polyether siloxane, and alcohol reagents.

9. The preparation method of the carbon-coated aluminum foil according to claim 8, characterized in that: The mass ratio of the binder to the conductive agent is 1:(1-2), and the mass ratio of the carbon powder to the graphite powder in the conductive agent is 1:

1.

10. A carbon-coated aluminum foil, characterized in that: The carbon-coated aluminum foil is prepared by the preparation method described in any one of claims 6-9.

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