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 thereof, and the preparation method of the modified polyacrylic acid binder comprises the following steps: S1, introducing cyano groups: dissolving polyacrylic acid in deionized water, stirring to obtain a polyacrylic acid aqueous solution, slowly dropwise adding a sodium cyanide solution into the polyacrylic acid aqueous solution, and stirring for reaction, after the reaction is finished, dropwise adding diluted hydrochloric acid to neutralize sodium cyanide and adjust the pH value to be neutral, so as to obtain a modified polyacrylic acid binder with cyano groups; and / or S2, main chain branching: dissolving polyacrylic acid in an organic solvent, stirring to obtain a polyacrylic acid solution, then adding a branching agent into the polyacrylic acid solution, stirring for reaction, and after the reaction is finished, drying to obtain the main chain branching modified polyacrylic acid binder. According to the invention, the polyacrylic acid binder is subjected to cyano group introduction and / or main chain branching modification treatment to improve the bonding performance and thermal stability, so that the stripping force of the positive plate is improved, the resistance is reduced, and the cycle life of the lithium ion battery is prolonged.

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 multiple 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 the bonding force, conductivity, chemical stability and process adaptability need to be comprehensively considered. Commonly used binders include polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), water-based 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 water-based 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 bonding force: 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 water-based system, and can effectively disperse carbon materials and positive electrode material particles to form a uniform coating. Therefore, due to its strong polarity, excellent bonding 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 bonding 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 bonding force, 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 bonding force, 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 bonding 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 through introducing cyano groups and / or main-chain branching, the bonding performance is improved, thereby increasing the peel strength of the positive electrode sheet and further enhancing 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 cyano groups: Dissolve polyacrylic acid in deionized water and stir to obtain an aqueous polyacrylic acid solution. Then, slowly dropwise add a sodium cyanide solution to the aqueous polyacrylic acid solution and stir for reaction. After the reaction is completed, dropwise add dilute hydrochloric acid to neutralize the sodium cyanide and adjust the pH to neutral to obtain a modified polyacrylic acid binder with cyano groups.

[0008] And / or, S2. Main-chain branching: Dissolve polyacrylic acid in an organic solvent and stir to obtain a polyacrylic acid solution. Then, add a branching agent to the polyacrylic acid solution and stir for reaction. After the reaction is completed, dry to obtain a modified polyacrylic acid binder with main-chain branching.

[0009] The present invention modifies the polyacrylic acid binder from two aspects. The first aspect is to introduce cyano groups into the polyacrylic acid binder, and the second aspect is to perform main-chain branching treatment on the polyacrylic acid binder. One of the first aspect and the second aspect can be selected for modification, or both can be selected and the order can be changed.

[0010] Introducing cyano groups into the polyacrylic acid binder can improve the interfacial interaction between the carbon-coated layer, the positive electrode active material and the aluminum foil current collector, thereby increasing the peel strength of the positive electrode sheet. Specifically, ① enhancing intermolecular forces, the introduction of cyano groups can enhance the van der Waals forces or hydrogen bond interactions between polymer chains. That is, after the positive electrode active material is coated, the cyano groups on the carbon-coated layer form hydrogen bonds with the groups in the positive electrode active material, thereby increasing the interaction force between the carbon-coated layer and the positive electrode active material layer. ② Improving the compatibility with electrode materials: The introduction of cyano groups can improve the compatibility between the binder and the positive electrode active material, and the polar effect of the cyano group can promote the good combination between the binder and the positive electrode active material.

[0011] The main chain of the polyacrylic acid binder is branched. The polyacrylic acid binder has a single molecular chain structure. By introducing branches onto the main chain, the originally linear polymer chain is transformed into a polymer chain with a branched structure. Its functions are as follows: ① Increasing the interaction force between the positive electrode active material and the carbon-coated layer: The branching of the main chain of the polyacrylic acid binder can increase the intermolecular force, thereby improving the bonding strength. When the main chain of the polyacrylic acid binder is branched, its molecular structure becomes more complex, and the intermolecular force also increases accordingly, which helps to improve the bonding force between the polyacrylic acid binder and the positive electrode active material as well as the current collector. ② Increasing the intermolecular interaction area: The branching of the main chain of the polyacrylic acid binder results in a rougher molecular structure, increasing the contact area with other materials such as aluminum foil and the positive electrode active material, thereby enhancing the intermolecular interaction force and improving the peel strength of the positive electrode sheet. ③ Improving thermal stability: The branching of the main chain of the polyacrylic acid binder can improve its thermal stability. During the operation of a lithium battery, the battery undergoes multiple charge-discharge cycles, and a certain amount of heat is generated during this process. The thermal stability of the polyacrylic acid binder directly affects the long-term stability of the battery. The main chain branching can increase the thermal decomposition temperature of the polyacrylic acid binder, enabling the polyacrylic acid binder to maintain stable bonding performance within the working temperature range of the battery.

[0012] Based on this, the present invention improves the bonding performance and thermal stability of the polyacrylic acid binder by introducing cyano groups and / or modifying the main chain branching, thereby improving the peel strength of the positive electrode sheet and reducing the resistance, and further improving the cycle life of the lithium-ion battery.

[0013] Preferably, in S1, polyacrylic acid is dissolved in deionized water and stirred at a speed of 500 - 700 r / min for 20 - 40 min to obtain an aqueous polyacrylic acid solution with a viscosity of 100 - 500 mPa·S. Then, a sodium cyanide solution with a mass percentage of 0.5 - 1.5% and a concentration of 0.1 - 1 mol / L is slowly dropped into the aqueous polyacrylic acid solution, and stirred at a speed of 900 - 1100 r / min for 50 - 70 min for reaction. After the reaction is completed, a dilute hydrochloric acid with a concentration of 0.2 - 0.4 mol / L is dropped to neutralize the sodium cyanide and adjust the pH to neutral to obtain a modified polyacrylic acid binder with cyano groups.

[0014] During the reaction between the sodium cyanide solution and the aqueous polyacrylic acid solution, the reaction process can be monitored by methods such as thin-layer chromatography (TLC) or Fourier transform infrared spectroscopy (FTIR) to ensure that cyano groups are successfully introduced onto the main chain of polyacrylic acid. Generally, the introduction of cyano groups occurs at the carboxyl hydroxyl position of the polyacrylic acid molecule.

[0015] Preferably, in S1, after adjusting the pH to neutral, unreacted sodium cyanide, impurities and by-products are removed by precipitation, filtration or dialysis. The removal of unreacted sodium cyanide, impurities and by-products is beneficial to improving the purity of the modified polyacrylic acid binder with a cyano group, so that the binding performance and thermal stability of the modified polyacrylic acid binder are not affected.

[0016] Preferably, in S2, polyacrylic acid is dissolved in an organic solvent and stirred at a speed of 500-700 r / min for 20-40 min to obtain a fully dissolved polyacrylic acid solution. Then, a branching agent accounting for 0.04-0.06% of its mass is added to the polyacrylic acid solution, and the mixture is stirred at a speed of 1400-1600 r / min for 70-80 min for reaction. After the reaction is completed, it is dried to obtain a modified polyacrylic acid binder with a branched main chain.

[0017] Preferably, in S2, the reaction temperature is 55-65 °C. After reacting for 70-80 min, the mixed solution is quickly transferred to -8 to -10 °C for refrigeration to stop the branching reaction. After the reaction stops, it is taken out and restored to room temperature. Then, the organic solvent and unreacted branching agent are removed by distillation or extraction, and it is dried to obtain a modified polyacrylic acid binder with a branched main chain.

[0018] The reaction temperature of 55-65 °C can accelerate the reaction rate. Rapid cooling and refrigeration after reacting for 70-80 min can promptly stop the branching reaction and avoid over-branching from affecting the performance of the modified polyacrylic acid binder.

[0019] Preferably, in S2, the organic solvent includes at least one of benzene and toluene, and the branching agent includes at least one of allyl maleic anhydride, pentaerythritol, trimellitic anhydride, and divinylbenzene.

[0020] The second aspect of the present invention provides a modified polyacrylic acid binder, which is prepared by the above preparation method. The modified polyacrylic acid binder has high binding performance and thermal stability.

[0021] The third aspect of the present invention provides a preparation method of carbon-coated aluminum foil, including 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 and 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 a mixture of a polyacrylic acid binder and the above-mentioned modified polyacrylic acid binder.

[0023] Preferably, deionized water is used to adjust the solid content of the binder to 15-30% and the viscosity to 800-3500 mPa·s. Then, half of the carbon powder and half of the graphite powder are added, and they are highly dispersed in a double planetary stirring tank at a rotation speed of 2000-2600 rpm / min for 25-35 min. Then, the other half of the carbon powder and the other half of the graphite powder are added, and they are highly dispersed at a rotation 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 they are highly dispersed at a rotation 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 they are highly dispersed at a rotation speed of 10-15 rpm / min for 30-45 min and then discharged. The slurry is homogenized twice in a homogenizer at a pressure of 250-350 bar to obtain a conductive slurry. 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, and a carbon-coated aluminum foil is obtained;

[0024] The content of the modified polyacrylic acid binder is 50-100% of the total binder content.

[0025] The binder is preferably a modified polyacrylic acid binder, and its solid content can be 15%, 16%, 17%, 18%, 20%, 22%, 25% or 30%, and its viscosity can be 800 mPa·s, 850 mPa·s, 900 mPa·s, 950 mPa·s, 1000 mPa·s, 1050 mPa·s, 1100 mPa·s, 1200 mPa·s, 1300 mPa·s, 1400 mPa·s, 1500 mPa·s, 2000 mPa·s, 2500 mPa·s, 3000 mPa·s or 3500 mPa·s.

[0026] Preferably, the component contents of the conductive slurry are as follows: by mass fraction, 30-50% conductive agent, 20-50% binder, 1-10% pH regulator, 5-20% wetting agent, and the balance is deionized water;

[0027] The mass ratio of the carbon powder to the graphite powder is 1:1.

[0028] Preferably, the pH regulator includes at least one of sodium hydroxide, lithium hydroxide, and sodium bicarbonate, and the wetting agent includes at least one of polyether siloxane, modified polyether siloxane, and alcohol reagents.

[0029] The fourth aspect of the present invention provides a carbon-coated aluminum foil, which is prepared by the above preparation method. Description of the Drawings

[0030] Figure 1Infrared spectra of the modified polyacrylic acid binders prepared in Examples 1-2, Examples 4-6, and Comparative Example 3. Detailed implementation manners

[0031] For better understanding and implementation, the technical solutions of the present application will be clearly and completely described below in conjunction with the examples. Obviously, the described examples are only a part of the examples of the present application, rather than all of 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 the present application herein are only for the purpose of describing specific examples and are not intended to limit the present application.

[0033] Unless otherwise stated, 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 as required to obtain the desired performance.

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

[0035] As used herein, "comprising" and "including" 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 specified.

[0037] Unless otherwise stated, the words "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, and thus more than one component may be contemplated and may be employed or used in the implementation of the described embodiments.

[0038] Example 1

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

[0040] S1. Introduction of cyano group: Polyacrylic acid was dissolved in deionized water and stirred at a speed of 600 r / min for 30 min to obtain an aqueous polyacrylic acid solution with a viscosity of 300 mPa·S. Then, a sodium cyanide solution with a concentration of 0.5 mol / L and accounting for 1.0% of its mass was slowly dropped into the aqueous polyacrylic acid solution, and the mixture was stirred at a speed of 1000 r / min for 60 min for reaction. After the reaction, dilute hydrochloric acid with a concentration of 0.3 mol / L was added to neutralize sodium cyanide and adjust the pH to neutral. Unreacted sodium cyanide, impurities, and by-products were removed by precipitation, filtration, or dialysis to obtain a polyacrylic acid binder with a cyano group;

[0041] S2. Main chain branching: Polyacrylic acid was dissolved in toluene and stirred at a speed of 600 r / min for 30 min to obtain a fully dissolved polyacrylic acid solution. Then, allyl maleic anhydride accounting for 0.05% of its mass was added to the polyacrylic acid solution, and the mixture was stirred at a speed of 1500 r / min at 60 °C for 75 min for reaction. After 75 min of reaction, the mixed solution was quickly transferred to -10 °C for refrigeration to stop the branching reaction. After the reaction stopped, it was taken out and restored to room temperature. Then, the organic solvent and unreacted branching agent were removed by distillation or extraction, and dried to obtain a modified polyacrylic acid binder with a branched main chain and a cyano group;

[0042] S3. The binder used is a modified polyacrylic acid binder. Deionized water was used to adjust the solid content of the modified polyacrylic acid binder to 20% and the viscosity to 1500 mPa·s. Then, half of the conductive agent, that is, half of the carbon powder and half of the graphite powder, was added, and the mixture was highly dispersed at a speed of 2300 rpm / min in a double planetary stirring tank for 30 min. Then, the other half of the conductive agent, that is, the other half of the carbon powder and the other half of the graphite powder, was added, and the mixture was highly dispersed at a speed of 2300 rpm / min for 60 min. Then, an appropriate amount of deionized water was added to reduce the slurry concentration and the mixture was highly dispersed at a speed of 2300 rpm / min for 30 min. Then, a pH regulator was added to adjust the pH to 6, and then a wetting agent, modified polyether siloxane, was added and highly dispersed at a speed of 13 rpm / min for 40 min and then discharged. The slurry was homogenized twice in a homogenizer at a pressure of 300 bar to obtain a conductive slurry. The conductive slurry was uniformly coated 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 contents of the conductive slurry are: by mass fraction, 40% conductive agent, 30% binder, 3% pH regulator, 10% wetting agent, and the balance is deionized water.

[0044] The mass ratio of carbon powder to graphite powder is 1:1.

[0045] The pH regulator is sodium hydroxide.

[0046] Example 2

[0047] The difference between this embodiment and Embodiment 1 is that the polyacrylic acid binder is not subjected to S2. Main chain branching treatment.

[0048] Embodiment 3

[0049] The difference between this embodiment and Embodiment 1 is that the polyacrylic acid binder is not subjected to S1. Introducing cyano treatment.

[0050] Embodiment 4

[0051] The difference between this embodiment and Embodiment 1 is that in S3, the binder is a mixture of a modified polyacrylic acid binder and a polyacrylic acid binder, and the mass ratio of the two is 5:1.

[0052] Embodiment 5

[0053] The difference between this embodiment and Embodiment 1 is that in S3, the binder is a mixture of a modified polyacrylic acid binder and a polyacrylic acid binder, and the mass ratio of the two is 2:1.

[0054] Embodiment 6

[0055] The difference between this embodiment and Embodiment 2 is that in S3, the binder is a mixture of a modified polyacrylic acid binder and a polyacrylic acid binder, and the mass ratio of the two is 1:1.

[0056] Comparative Example 1

[0057] The difference between this comparative example and Embodiment 1 is that the polyacrylic acid binder is not subjected to S2. Main chain branching treatment and S1. Introducing cyano treatment, and the binder is all unmodified polyacrylic acid binder.

[0058] Comparative Example 2

[0059] The difference between this comparative example and Embodiment 1 is that in S2, after reacting for 75 min, rapid cooling and refrigeration are not carried out, but it is allowed to react naturally to end, and then the organic solvent and unreacted branching agent are removed by distillation or extraction, and dried to obtain a modified polyacrylic acid binder.

[0060] Comparative Example 3

[0061] The difference between this comparative example and Embodiment 1 is that in S3, the binder is a mixture of a modified polyacrylic acid binder and a polyacrylic acid binder, and the mass ratio of the two is 1:2.

[0062] The carbon-coated aluminum foils of Embodiments 1 to 6 and Comparative Examples 1 to 3 are used to prepare positive electrode sheets and lithium-ion batteries, and the preparation method is as follows:

[0063] Preparation of the positive electrode sheet: The carbon-coated aluminum foils prepared in Examples 1 to 6 and Comparative Examples 1 to 3 were used as the positive electrode current collectors, 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 to obtain the positive electrode sheet.

[0064] Preparation of the negative electrode sheet: Copper foil was used 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 to obtain the negative electrode sheet.

[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 to obtain a carbonate solvent. LiPF6 was added to the carbonate solvent to prepare a carbonate solution of LiPF6 with a concentration of 1 mol·L -1 and this carbonate solution was used as the electrolyte of the lithium-ion battery.

[0066] Preparation of the lithium-ion battery: The positive electrode sheet, separator, and negative electrode sheet prepared above were stacked in sequence to prepare a bare battery cell; the bare battery cell was placed in the outer packaging case of the lithium battery, dried, and then the electrolyte was injected. After vacuum packaging, standing, forming, shaping and other processes, the lithium-ion battery was obtained.

[0067] Testing methods

[0068] (1) Testing of the pole piece peeling force: The positive electrode sheets prepared in each example and comparative example were attached to the test board with 3M double-sided tape with a width of 2 cm. The active material on the positive electrode sheet was separated from the carbon-coated layer, and the tape paper was attached to the carbon-coated aluminum foil. The 180° pole piece peeling force was tested on a tensile machine.

[0069] (2) Testing of the through resistance: Carbon-coated aluminum foils with a size of 2*2 cm prepared in each example and comparative example were tested for resistance under a film resistance meter.

[0070] (3) Testing of the battery cycle performance: The lithium-ion batteries prepared with the carbon-coated aluminum foils in each example and comparative example were tested at room temperature (25°C). The coin-type symmetric battery was charged at a constant current and constant voltage to 3 - 4.5 V at rates of 0.2C, 0.3C, 0.5C, 1C, 2C, 3C, 5C, and the cut-off current was 0.15 - 3A. Then, the coin-type symmetric battery was discharged at a constant current to 3 - 4.5 V at rates of 0.2C, 0.3C, 0.5C, 1C, 2C, 3C, 5C. This was regarded as one charge and discharge cycle. The coin-type symmetric battery was charged and discharged in this way for 100 cycles, and the discharge capacity of the coin-type symmetric battery before and after cycling was tested. The capacity retention rate of the coin-type symmetric battery after 100 cycles was calculated 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 peeling force, penetration resistance of the carbon-coated aluminum foil of Examples 1-6 applied to the positive electrode sheet, and the capacity retention rate applied to the battery are all better than those of Comparative Examples 1-3. Moreover, the pole piece peeling force, penetration resistance of the carbon-coated aluminum foil of Example 1 applied to the positive electrode sheet, and the capacity retention rate applied to the battery are better than those of Examples 2-3 and Examples 4-6. The carbon-coated aluminum foil prepared in Example 1 has the optimal performance. The following will be combined with Figure 1 (There is an obvious cyanide group characteristic peak) for specific analysis:

[0075] Comparative Example 1 uses an unmodified common polyacrylic acid binder on the market, with poor adhesion. The pole piece peeling force of the carbon-coated aluminum foil prepared with this binder applied to the positive electrode sheet is very low, and it has a high resistance. When applied to the battery, it will cause a high internal resistance of the battery, and the positive active material layer is likely to fall off, thus greatly reducing the cycle life of the battery. Examples 1-6 all use a modified polyacrylic acid binder with introduced cyanide groups and / or main chain branching, which has high adhesion performance and thermal stability. Therefore, the carbon-coated aluminum foil prepared has a high pole piece peeling force when applied to the positive electrode sheet and a low resistance. When applied to the battery, it will result in a low internal resistance of the battery, and the positive active material layer is not likely to fall off, which is conducive to improving the cycle life of the battery.

[0076] On this basis, by comparing Example 1 with Examples 2-3, it can be seen that although the modified polyacrylic acid binder with separately introduced cyanide groups or separately main chain branching can significantly improve the adhesion performance and thermal stability, the best is to combine the introduction of cyanide groups with main chain branching to obtain a modified polyacrylic acid binder, which can greatly improve the adhesion performance and thermal stability. Therefore, the carbon-coated aluminum foil prepared has a greatly improved pole piece peeling force when applied to the positive electrode sheet and a significant reduction in resistance. When applied to the battery, it will result in a significant reduction in the internal resistance of the battery and a significant reduction in the shedding rate of the positive active material layer, thereby greatly improving the cycle life of the battery.

[0077] Based on this, by comparing Example 1 and Comparative Example 2, it can be seen that although Comparative Example 2 uses a modified polyacrylic acid binder with introduced cyanide groups and main chain branching, during the reaction process of main chain branching, it did not cool down in time to terminate the reaction, resulting in excessive branching. Not only did it not improve the adhesion performance, but also combined with the data of Example 2, it can be seen that it will also offset the improved adhesion performance caused by the introduction of cyanide groups, making the pole piece peeling force and resistance of the carbon-coated aluminum foil prepared when applied to the positive electrode sheet not improved at all, thus not improving the cycle life of the battery, and the entire modification operation is in vain.

[0078] In addition, by comparing Example 1 with Examples 4-6, it can be seen that when the binder in the carbon-coated aluminum foil is entirely the modified polyacrylic acid binder with a branched main chain and a cyano group, the bonding performance and thermal stability can be greatly improved. As a result, the pole piece peeling force of the carbon-coated aluminum foil prepared and applied in the positive electrode sheet is greatly increased, and the resistance is significantly reduced. When applied in a battery, the internal resistance of the battery is significantly reduced, and the shedding rate of the positive active material layer is significantly reduced, thereby greatly increasing the cycle life of the battery. If the modified polyacrylic acid binder with a branched main chain and a cyano group is used in combination with the common unmodified polyacrylic acid binder on the market, as the content of the modified polyacrylic acid binder decreases, the overall bonding performance and thermal stability of the binder gradually decrease. Consequently, the pole piece peeling force of the carbon-coated aluminum foil prepared and applied in the positive electrode sheet gradually decreases, and the resistance gradually increases. When applied in a battery, the internal resistance of the battery gradually increases, and the shedding rate of the positive active material layer gradually increases, and thus the cycle life of the battery gradually decreases. Moreover, according to Comparative Example 3, when the content of the modified polyacrylic acid binder with a branched main chain and a cyano group in the binder is less than 50%, its bonding performance, the pole piece peeling force and resistance of the carbon-coated aluminum foil prepared and applied in the positive electrode sheet, and the internal resistance and cycle life of the battery applied are basically equivalent to the performance of the binder entirely using the common unmodified polyacrylic acid binder on the market, and the entire operation of mixed use is meaningless. Therefore, the content of the modified polyacrylic acid binder in the carbon-coated aluminum foil binder should not be less than 50%, and the more the better.

[0079] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit 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 a cyano group: Dissolve polyacrylic acid in deionized water and stir to obtain an aqueous polyacrylic acid solution. Then, slowly dropwise add a sodium cyanide solution to the aqueous polyacrylic acid solution and stir for reaction. After the reaction ends, dropwise add dilute hydrochloric acid to neutralize the sodium cyanide and adjust the pH to neutral to obtain the modified polyacrylic acid binder with a cyano group; and / or, S2. Main chain branching: Dissolve polyacrylic acid in an organic solvent and stir to obtain a polyacrylic acid solution. Then, add a branching agent to the polyacrylic acid solution and stir for reaction. After the reaction ends, dry to obtain the modified polyacrylic acid binder with a branched main chain.

2. The preparation method of the modified polyacrylic acid binder according to claim 1, wherein: In S1, dissolve polyacrylic acid in deionized water and stir at a speed of 500 - 700 r / min for 20 - 40 min to obtain an aqueous polyacrylic acid solution with a viscosity of 100 - 500 mPa·S. Then, slowly dropwise add a sodium cyanide solution accounting for 0.5 - 1.5% of its mass and with a concentration of 0.1 - 1 mol / L to the aqueous polyacrylic acid solution, and stir at a speed of 900 - 1100 r / min for 50 - 70 min for reaction. After the reaction ends, dropwise add dilute hydrochloric acid with a concentration of 0.2 - 0.4 mol / L to neutralize the sodium cyanide and adjust the pH to neutral to obtain the modified polyacrylic acid binder with a cyano group; and / or, in S2, dissolve polyacrylic acid in an organic solvent, stir at a speed of 500 - 700 r / min for 20 - 40 min to obtain a fully dissolved polyacrylic acid solution. Then, add a branching agent accounting for 0.04 - 0.06% of its mass to the polyacrylic acid solution, and stir at a speed of 1400 - 1600 r / min for 70 - 80 min for reaction. After the reaction ends, dry to obtain the modified polyacrylic acid binder with a branched main chain.

3. The preparation method of the modified polyacrylic acid binder according to claim 1 or 2, characterized in that: In S1, after adjusting the pH to neutral, remove unreacted sodium cyanide, impurities, and by-products by precipitation, filtration, or dialysis.

4. The preparation method of the modified polyacrylic acid binder according to claim 1 or 2, characterized in that: In S2, the reaction temperature is 55 - 65°C. After reacting for 70 - 80 min, quickly transfer the mixed solution to -8 - -10°C for refrigeration to stop the branching reaction. After the reaction stops, take it out and restore it to room temperature. Then, remove the organic solvent and unreacted branching agent by distillation or extraction, and dry to obtain the modified polyacrylic acid binder with a branched main chain.

5. The preparation method of the modified polyacrylic acid binder according to claim 1 or 2, characterized in that: In S2, the organic solvent includes at least one of benzene and toluene, and the branching agent includes at least one of allyl maleic anhydride, pentaerythritol, trimellitic anhydride, and divinylbenzene.

6. 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 - 5.

7. A preparation method of carbon-coated aluminum foil, characterized in that It includes the following steps: Adjust the solid content of the binder with deionized water, 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. Next, add a pH regulator to adjust the pH to weakly acidic. Then, add a wetting agent and stir at low speed and 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 a mixture of a polyacrylic acid binder and the modified polyacrylic acid binder described in claim 6.

8. The preparation method of the carbon-coated aluminum foil according to claim 7, characterized in that: Use deionized water to adjust the solid content of the binder to 15-30% and the viscosity to 800-3500 mPa·s. 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. Then 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 them 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. Then add a wetting agent and disperse them at a high speed at 10-15 rpm / min for 30-45 min and then discharge. Homogenize the slurry twice in a homogenizer at a pressure of 250-350 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 0.4-0.6 μm to obtain the carbon-coated aluminum foil; The content of the modified polyacrylic acid binder is 50-100% of the total binder content.

9. The preparation method of the carbon-coated aluminum foil according to claim 8, wherein: The content of each component of the conductive slurry is: by mass fraction, 30-50% conductive agent, 20-50% binder, 1-10% pH regulator, 5-20% wetting agent, and the balance is deionized water; The mass ratio of the carbon powder to the graphite powder is 1:1; The pH regulator includes at least one of sodium hydroxide, lithium hydroxide, and sodium bicarbonate, and the wetting agent includes at least one of polyether siloxane, modified polyether siloxane, and alcohol reagents.

10. A carbon-coated aluminum foil, characterized in that: The carbon-coated aluminum foil is prepared by the preparation method according to any one of claims 7-9.

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