A fluorine-free ion binder and its preparation method and application

Abstract

The present invention discloses a fluorine-free ionic binder and its preparation method and application, belonging to the technical field of secondary battery materials. The present invention solves the problems of poor adhesion and insufficient ionic conductivity of existing binders. The present invention copolymerizes acrylonitrile and imidazole monomers to obtain a binder polymer. The electron-rich nature of the negative nitrogen group in the polymer makes it easy for the binder to form an electrostatic effect with the positive electrode material, thereby improving its adhesion properties and making it easier to wrap the positive electrode active material; in addition, the imidazole group in the binder reacts with the benzyl group on the polybenzyl monomer to form a network structure containing ion transport groups through cross-linking, thereby improving the ion transport capacity of the binder and achieving improved capacity performance and cycle stability of the secondary battery. The binder of the present invention is fluorine-free and environmentally friendly. It can be used to prepare positive electrode slurry for secondary batteries and effectively improve the capacity retention rate of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary battery materials, and in particular relates to a fluorine-free ion binder and a preparation method and application thereof. Background Art

[0002] Secondary batteries, such as lithium-ion and sodium-ion batteries, are already widely used in consumer batteries, new energy vehicles, and energy storage. The performance of secondary batteries depends largely on the degree of optimization of the properties of their constituent materials. The positive electrode binder is a key factor in determining the overall performance of the secondary battery's positive electrode. Its primary function is to tightly bond the active material, conductive agent, and current collector together to form a stable electrode structure, ensuring good conductivity and mechanical stability during charge and discharge.

[0003] Ideal binder materials need to have excellent adhesion and ion transport capabilities, while maintaining stable chemical structures and electrochemical properties in various electrolytes. However, existing binders can only meet some of the above requirements. Taking polyvinylidene fluoride (PVDF), a common traditional secondary battery positive electrode binder, as an example, its bonding function is mainly achieved by utilizing the entanglement of its own polymer chains, so it is difficult to ensure high adhesion. At the same time, PVDF lacks intrinsic ionic groups and cannot achieve efficient ion migration. Its adhesion and conductivity are difficult to meet the current needs of high-energy density batteries, and the preparation and use of PVDF will have an impact on the environment. Therefore, there is an urgent need to find a fluorine-free binder that can be used in secondary batteries. Fluorine-free binders represented by polyacrylonitrile are difficult to cope with the volume expansion of active materials during charging and discharging and their large-scale application in soft-pack batteries due to the inherent structural brittleness of their own structure. Currently, patents for fluorine-free binders for secondary batteries have been published. Chinese patent CN116731635A discloses a conductive binder, its preparation method, and application. Polymers rich in carboxyl and hydroxyl groups, such as sodium carboxymethyl cellulose and polyacrylic acid, are mixed with ligands such as Congo red and aniline black and reacted to obtain a highly adhesive binder with polar groups. Chinese patent CN116404164A discloses a conductive binder for battery negative electrodes and its preparation method. This involves the use of ribbon-shaped graphene rich in carboxyl and hydroxyl groups and having a large aspect ratio in the binder. Its unique three-dimensional spatial extension structure provides abundant ion transport channels. Although this patent prepares a negative electrode binder, its bonding effect is also applicable to positive electrodes, which is beneficial to improving positive electrode performance. However, these binders still suffer from insufficient bonding and limited conductivity.

[0004] Therefore, developing an adhesive with good bonding ability and intrinsic ion transport ability to improve conductivity is beneficial to improving the performance of secondary batteries. Summary of the Invention

[0005] In order to solve the problems of poor bonding and insufficient ion conductivity of existing binders, the present invention provides a fluorine-free ion binder and a preparation method and application thereof.

[0006] In order to solve the above technical problems, the present invention is implemented through the following technical solutions:

[0007] One of the purposes of the present invention is to provide a method for preparing a fluorine-free ion binder. The method comprises the following steps: adding acrylonitrile and imidazole monomers to a solvent, introducing an inert gas, adding a free radical initiator, continuing to introduce the inert gas, heating the solvent after the introduction of the inert gas is completed, reacting for a period of time, heating again, continuing to react, and filtering, washing, and drying the product after the reaction is completed to obtain the binder.

[0008] It is further defined that the molar ratio of acrylonitrile to imidazole monomer is (2.3~49):1.

[0009] It is further defined that the solvent is deionized water; the imidazole monomer is 1-vinylimidazole or N-allylimidazole; and the free radical initiator is ammonium persulfate, potassium persulfate or sodium persulfate.

[0010] It is further defined that the mass of the solvent is 2 to 10 times the total mass of the acrylonitrile and imidazole monomers; and the amount of the free radical initiator is 2% of the total mass of the acrylonitrile and imidazole monomers.

[0011] It is further defined that after the inert gas is introduced, the temperature is raised to 55-70° C., the reaction is carried out for 1-3 hours, the temperature is raised to 90-110° C. again, and the reaction is continued for 1-3 hours.

[0012] A second object of the present invention is to provide a fluorine-free ion-type binder obtained by the above preparation method.

[0013] A third object of the present invention is to provide an application of the above-mentioned fluorine-free ion binder, specifically for preparing a positive electrode sheet of a secondary battery.

[0014] It is further defined that the method for preparing a positive electrode sheet of a secondary battery comprises the following steps:

[0015] (1) Adding the above binder into N-methylpyrrolidone solvent to prepare a binder solution;

[0016] (2) adding a multi-component benzyl monomer to the binder solution, and then sequentially adding the conductive agent and the positive electrode active material according to the mass ratio of the positive electrode active material, the conductive agent and the binder of (70-95): (5-20): (3-20) to obtain a positive electrode slurry;

[0017] (3) The positive electrode slurry is scraped onto the current collector, and the secondary battery positive electrode sheet is obtained by drying and rolling.

[0018] It is further defined that the mass ratio of the binder in (1) to the multi-component benzyl monomer in (2) is (50~1000):1.

[0019] It is further defined that the polybenzyl monomer in (2) is a polybenzyl bromide monomer or a polybenzyl chloride monomer.

[0020] It is further defined that the polyvalent benzyl bromide monomer is 1,3-di(bromomethyl)benzene, 1,4-di(bromomethyl)benzene or 1,2-di(bromomethyl)benzene; and the polyvalent benzyl chloride monomer is 1,3-di(chloromethyl)benzene, 1,4-di(chloromethyl)benzene or 1,2-di(chloromethyl)benzene.

[0021] The beneficial effects of the present invention are:

[0022] Compared with traditional PVDF binders, the present invention provides a fluorine-free ion binder with higher adhesion and ion transmission capacity. Compared with the prior art, the present invention also has the following advantages:

[0023] (1) The present invention utilizes acrylonitrile and imidazole monomers to synthesize a binder. The binder polymer introduces an imidazole functional unit. Compared to a polymer made entirely of acrylonitrile, the introduction of the imidazole functional unit is equivalent to reducing the proportion of acrylonitrile monomer required, thereby reducing the crystallinity of the synthesized binder, improving the overall flexibility of the binder, and thus improving the adhesiveness of the binder. At the same time, the electron-rich nature of the negative nitrogen groups in the binder makes it easy for the binder to form an electrostatic interaction with the positive electrode material, thereby increasing the interphase interaction force, improving the adhesiveness of the binder, and making it easier to encapsulate the positive electrode active material.

[0024] (2) The present invention crosslinks the binder with the multi-benzyl monomer in situ, solving the problem that traditional binders are difficult to dissolve in common solvents, which leads to difficult processing. At the same time, nitrogen cations are formed at the imidazole group, and a large number of ionic groups are efficiently introduced. The nitrogen cations are functional groups with charges and are easy to conduct ions, thereby improving the conductivity and optimizing the migration of ions at the positive electrode material during the charge and discharge process, thereby improving the battery capacity and stabilizing the long-term cycle. At the same time, the present invention introduces imidazole monomers as comonomers into the fluorine-free ion-type secondary battery positive electrode binder system, and subsequently utilizes the imidazole group to undergo a coupling reaction with the benzyl group on the multi-benzyl monomer to achieve the ionization of the imidazole group and the construction of a cross-linked binder network. The charged quaternary ammonium structural unit obtained by the reaction of the imidazole monomer with the multi-benzyl monomer can promote ion conduction and further improve the ion transport capacity of the binder. In addition, the cross-linked network structure formed when the binder of the present invention is used can inhibit the swelling of the binder and effectively fix the positive electrode active material, ultimately achieving the improvement of the capacity performance and cycle stability of the secondary battery.

[0025] (3) The adhesive provided by the present invention does not introduce fluorine elements from the raw materials to the preparation process, and can replace existing fluorine-containing adhesives to alleviate the fluorine pollution problem.

[0026] (4) In the acrylonitrile-based polymer system, since the acrylonitrile group contains a polar group cyano and its structure is relatively rigid, the overall acrylonitrile-based polymer is brittle. Adding an appropriate amount of an imidazole-based soft monomer containing an alkyl chain can effectively improve the brittleness of the overall structure of the acrylonitrile-based polymer. At the same time, the addition of an imidazole-based soft monomer containing an alkyl chain can also introduce post-crosslinking sites and ion transport groups, thereby improving the overall properties. Therefore, the present invention introduces an imidazole-based monomer and polymerizes it with acrylonitrile. The molar ratio of acrylonitrile to imidazole monomer is (70-98): (2-30) to ensure that the resulting polymer has high adhesion. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is the H NMR spectrum of the binder prepared in Example 1. DETAILED DESCRIPTION

[0028] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the embodiments of the specification.

[0029] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0030] Secondly, the term "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in various places throughout this specification does not necessarily refer to the same embodiment, nor does it constitute a separate or selective embodiment that is mutually exclusive of other embodiments.

[0031] The experimental methods used in the following examples are conventional methods unless otherwise specified. The materials, reagents, methods, and instruments used are conventional in the art and can be obtained commercially by those skilled in the art unless otherwise specified.

[0032] The conductive agent used in the following application examples is Super P conductive carbon black, purchased from Timcal Corporation, model number is Timcal Super P Li; the PVDF binder is purchased from Arkema Chemical Co., Ltd., model number is HSV900.

[0033] Example 1

[0034] 3000 mL of deionized water was added to the flask, followed by acrylonitrile (477.56 g) and 1-vinylimidazole monomer (94.11 g), and nitrogen was introduced while stirring at 20°C for 1 hour. Then, ammonium persulfate (11.43 g) was added, and nitrogen was introduced again while stirring at 20°C for 1 hour. The temperature was then raised to 55°C, and the reaction was continued under a nitrogen environment for 3 hours. The temperature was continued to be raised to 100°C and the reaction was continued for 2 hours. After the reaction was completed, the product was filtered from the reaction system, washed with deionized water and anhydrous ethanol in sequence, and dried to obtain a binder.

[0035] The obtained binder was characterized by hydrogen nuclear magnetic resonance spectroscopy, and the results were as follows: Figure 1 shown.

[0036] The fluorine-free secondary battery positive electrode binder prepared in this example was applied to lithium iron phosphate positive electrode sheets in lithium-ion batteries. Specifically, the binder was dissolved in N-methylpyrrolidone to a concentration of 25 mg / mL. After the binder was completely dissolved, 1,4-di(bromomethyl)benzene, a conductive agent, and lithium iron phosphate were added sequentially and stirred to obtain a positive electrode slurry. The mass ratio of binder, 1,4-di(bromomethyl)benzene, conductive agent, and lithium iron phosphate in the positive electrode slurry was 9.9:0.1:10:80. The prepared positive electrode slurry was evenly coated on the current collector, dried at 100°C to remove the solvent, and in-situ crosslinking was achieved. The binder and 1,4-di(bromomethyl)benzene were crosslinked in situ to form a network-type binder containing ion-transporting groups. The prepared positive electrode sheet was assembled with the positive and negative electrode shells, gaskets, springs, lithium sheet, separator, and electrolyte to form a button-type half-cell and allowed to stand for 12 hours.

[0037] Example 2

[0038] 6000 mL of deionized water was added to the flask, followed by acrylonitrile (371.44 g) and 1-vinylimidazole monomer (282.33 g), and nitrogen was introduced while stirring at 20°C for 1 hour. Then, ammonium persulfate (13.08 g) was added, and nitrogen was introduced again while stirring at 20°C for 1 hour. The temperature was then raised to 55°C, and the reaction was continued under a nitrogen environment for 2 hours. The temperature was continued to be raised to 100°C and the reaction was continued for 1 hour. After the reaction was completed, the product was filtered from the reaction system, washed with deionized water and ethanol in sequence, and dried to obtain a binder.

[0039] The fluorine-free secondary battery positive electrode binder prepared in this example was applied to lithium iron phosphate positive electrode sheets in lithium-ion batteries. Specifically, the binder was dissolved in N-methylpyrrolidone to a concentration of 25 mg / mL. After the binder was completely dissolved, 1,2-bis(bromomethyl)benzene, a conductive agent, and lithium iron phosphate were added sequentially and stirred to obtain a positive electrode slurry. The mass ratio of binder, 1,2-bis(bromomethyl)benzene, conductive agent, and lithium iron phosphate in the positive electrode slurry was 9.95:0.05:10:70. The prepared positive electrode slurry was evenly coated on the current collector, dried at 80°C to remove the solvent, and in-situ crosslinking was achieved. The binder and 1,2-bis(bromomethyl)benzene were crosslinked in situ to form a network-type binder containing ion-transporting groups. The prepared positive electrode sheet was assembled with the positive and negative electrode shells, gaskets, springs, lithium sheet, separator, and electrolyte to form a button-type half-cell and allowed to stand for 12 hours.

[0040] Example 3

[0041] 1500 mL of deionized water was added to the flask, followed by acrylonitrile (424.50 g) and 1-vinylimidazole monomer (188.22 g), and nitrogen was introduced while stirring at 20°C for 1 hour. Then, ammonium persulfate (12.25 g) was added, and nitrogen was introduced again while stirring at 20°C for 1 hour. The temperature was then raised to 55°C, and the reaction was continued under a nitrogen environment for 1.5 hours. The temperature was continued to be raised to 100°C, and the reaction was continued for 3 hours. After the reaction was completed, the product was filtered from the reaction system, washed with deionized water and ethanol in sequence, and dried to obtain a binder.

[0042] The fluorine-free secondary battery positive electrode binder prepared in this example was applied to lithium iron phosphate positive electrode sheets in lithium-ion batteries. Specifically, the binder was dissolved in N-methylpyrrolidone to a concentration of 25 mg / mL. After the binder was completely dissolved, 1,4-di(bromomethyl)benzene, a conductive agent, and lithium iron phosphate were added sequentially and stirred to obtain a positive electrode slurry. The mass ratio of binder, 1,4-di(bromomethyl)benzene, conductive agent, and lithium iron phosphate in the positive electrode slurry was 14.97:0.03:10:75. The prepared positive electrode slurry was evenly coated on the current collector, dried at 120°C to remove the solvent, and in-situ crosslinking was achieved. The binder and 1,4-di(bromomethyl)benzene were crosslinked in situ to form a network-type binder containing ion-transporting groups. The prepared positive electrode sheet was assembled with the positive and negative electrode shells, gaskets, springs, lithium sheet, separator, and electrolyte to form a button-type half-cell and allowed to stand for 12 hours.

[0043] The fluorine-free secondary battery positive electrode binder prepared in this example was applied to sodium vanadium phosphate positive electrode sheets in sodium ion batteries. Specifically, the binder was dissolved in N-methylpyrrolidone to a solution with a binder concentration of 25 mg / mL. After the binder was completely dissolved, 1,2-bis(bromomethyl)benzene, a conductive agent, and a layered sodium oxide battery positive electrode were added sequentially and stirred to obtain a positive electrode slurry. The mass ratio of binder, 1,2-bis(bromomethyl)benzene, conductive agent, and sodium vanadium phosphate in the positive electrode slurry was 11.9:0.1:18:70. The prepared positive electrode slurry was evenly coated onto the current collector, dried at 80°C to remove the solvent, and in-situ crosslinking was achieved. The binder and 1,2-bis(bromomethyl)benzene were crosslinked in situ to form a network-type binder containing ion-transporting groups. The prepared positive electrode sheet was assembled with the positive and negative electrode shells, gaskets, springs, lithium sheet, separator, and electrolyte to form a button-type half-cell and allowed to stand for 12 hours.

[0044] Example 4

[0045] 2000 mL of deionized water was added to the flask, followed by acrylonitrile (504.10 g) and 1-vinylimidazole monomer (47.06 g), and nitrogen was introduced while stirring at 20°C for 1 hour. Then, ammonium persulfate (11.02 g) was added, and nitrogen was introduced again while stirring at 20°C for 1 hour. The temperature was then raised to 55°C, and the reaction was continued under a nitrogen environment for 3 hours. The temperature was further raised to 100°C and the reaction was continued for 3 hours. After the reaction was completed, the product was filtered from the reaction system, washed with deionized water and ethanol in sequence, and dried to obtain a binder.

[0046] The fluorine-free secondary battery positive electrode binder prepared in this example was applied to lithium iron phosphate positive electrode sheets in lithium-ion batteries. Specifically, the binder was dissolved in N-methylpyrrolidone to a concentration of 25 mg / mL. After the binder was completely dissolved, 1,3-bis(bromomethyl)benzene, a conductive agent, and lithium iron phosphate were added sequentially and stirred to obtain a positive electrode slurry. The mass ratio of binder, 1,3-bis(bromomethyl)benzene, conductive agent, and lithium iron phosphate in the positive electrode slurry was 5.99:0.01:15:79. The prepared positive electrode slurry was evenly coated on the current collector, dried at 110°C to remove the solvent, and in-situ crosslinking was achieved. The binder and 1,3-bis(bromomethyl)benzene were crosslinked in situ to form a network-type binder containing ion-transporting groups. The prepared positive electrode sheet was assembled with the positive and negative electrode shells, gaskets, springs, lithium sheet, separator, and electrolyte into a button-type half-cell and allowed to stand for 12 hours.

[0047] Example 5

[0048] 1300 mL of deionized water was added to the flask, followed by acrylonitrile (520.02 g) and 1-vinylimidazole monomer (18.822 g), and nitrogen was introduced while stirring at 20°C for 1 hour. Then, ammonium persulfate (10.78 g) was added, and nitrogen was introduced again while stirring at 20°C for 1 hour. The temperature was then raised to 55°C, and the reaction was continued under a nitrogen environment for 1.5 hours. The temperature was further raised to 100°C and the reaction was continued for 1.5 hours. After the reaction was completed, the product was filtered from the reaction system, washed with deionized water and ethanol in sequence, and dried to obtain a binder.

[0049] The fluorine-free secondary battery positive electrode binder prepared in this example was applied to lithium iron phosphate positive electrode sheets in lithium-ion batteries. Specifically, the binder was dissolved in N-methylpyrrolidone to a concentration of 25 mg / mL. After the binder was completely dissolved, 1,3-bis(bromomethyl)benzene, a conductive agent, and lithium iron phosphate were added sequentially and stirred to obtain a positive electrode slurry. The mass ratio of binder, 1,3-bis(bromomethyl)benzene, conductive agent, and lithium iron phosphate in the positive electrode slurry was 11.9:0.1:18:70. The prepared positive electrode slurry was evenly coated on the current collector, dried at 85°C to remove the solvent, and in-situ crosslinking was achieved. The binder and 1,3-bis(bromomethyl)benzene were crosslinked in situ to form a network-type binder containing ion-transporting groups. The prepared positive electrode sheet was assembled with the positive and negative electrode shells, gaskets, springs, lithium sheet, separator, and electrolyte to form a button-type half-cell and allowed to stand for 12 hours.

[0050] The fluorine-free secondary battery positive electrode binder prepared in this example was applied to sodium vanadium phosphate positive electrode sheets in sodium ion batteries. Specifically, the binder was dissolved in N-methylpyrrolidone to a solution with a binder concentration of 25 mg / mL. After the binder was completely dissolved, 1,3-bis(bromomethyl)benzene, a conductive agent, and a layered sodium oxide battery positive electrode were added sequentially and stirred to obtain a positive electrode slurry. The mass ratio of binder, 1,3-bis(bromomethyl)benzene, conductive agent, and sodium vanadium phosphate in the positive electrode slurry was 11.97:0.03:10:78. The prepared positive electrode slurry was evenly coated on the current collector, dried at 95°C to remove the solvent, and in-situ crosslinking was achieved. The binder and 1,3-bis(bromomethyl)benzene were crosslinked in situ to form a network binder containing ion-transporting groups. The prepared positive electrode sheet was assembled with the positive and negative electrode shells, gaskets, springs, lithium sheet, separator, and electrolyte to form a button-type half-cell and allowed to stand for 12 hours.

[0051] Comparative Example 1

[0052] A commercially available PVDF binder was used as a binder for lithium iron phosphate to prepare the positive electrode of a lithium-ion battery. Specifically, the binder was dissolved in an N-methylpyrrolidone solution to prepare a solution with a PVDF binder concentration of 25 mg / mL. After the binder was completely dissolved, a conductive agent and lithium iron phosphate were added and stirred evenly to obtain a positive electrode slurry. The mass ratio of the binder, conductive agent, and lithium iron phosphate in the positive electrode slurry was 1:1:8. The prepared positive electrode slurry was applied to the current collector, the solvent was removed in an 80°C oven, and then dried in a vacuum oven at 120°C for 8 hours. The prepared positive electrode sheet was assembled with the positive and negative electrode shells, gaskets, springs, lithium sheets, separators, and electrolytes into a button-type half-cell and allowed to stand for 12 hours.

[0053] Comparative Example 2

[0054] A sodium-ion battery positive electrode sheet was prepared using a commercially available PVDF binder as a binder for lithium iron phosphate. Specifically, the binder was dissolved in an N-methylpyrrolidone solution to prepare a solution with a PVDF binder concentration of 25 mg / mL. After the binder was completely dissolved, a conductive agent and a layered sodium oxide battery positive electrode were added and stirred evenly to obtain a positive electrode slurry. The mass ratio of the binder, conductive agent, and sodium vanadium phosphate in the positive electrode slurry was 1:1:8. The prepared positive electrode slurry was applied to the current collector, the solvent was removed in an 80°C oven, and then dried in a vacuum oven at 120°C for 8 hours. The prepared positive electrode sheet was assembled with the positive and negative electrode shells, gaskets, springs, lithium sheets, separators, and electrolyte into a button-type half-cell and allowed to stand for 12 hours.

[0055] Comparative Example 3

[0056] 1500 mL of deionized water was added to the flask, followed by acrylonitrile (265.3 g) and 1-vinylimidazole monomer (470.6 g), and nitrogen was introduced while stirring at 20°C for 1 hour. Then, ammonium persulfate (14.72 g) was added, and nitrogen was introduced again while stirring at 20°C for 1 hour. The temperature was then raised to 55°C, and the reaction was continued under a nitrogen environment for 1.5 hours. The temperature was further raised to 100°C and the reaction was continued for 3 hours. After the reaction was completed, the product was filtered from the reaction system, washed with deionized water and anhydrous ethanol in sequence, and dried to obtain a binder.

[0057] The binder prepared in this comparative example was applied to lithium iron phosphate positive electrodes in lithium-ion batteries. Specifically, the binder was dissolved in N-methylpyrrolidone to a concentration of 25 mg / mL. After the binder was completely dissolved, 1,3-bis(bromomethyl)benzene, a conductive agent, and lithium iron phosphate were added sequentially and stirred to obtain a positive electrode slurry. The mass percentages of binder, 1,3-bis(bromomethyl)benzene, conductive agent, and lithium iron phosphate in the positive electrode slurry were 11.9:0.1:18:70. The prepared positive electrode slurry was evenly coated onto the current collector, dried at 85°C to remove the solvent, and in-situ crosslinking was achieved. The binder and 1,3-bis(bromomethyl)benzene were crosslinked in situ to form a network-type binder containing ion-transporting groups. The prepared positive electrode sheet was assembled with the positive and negative electrode shells, gaskets, springs, lithium sheet, separator, and electrolyte to form a button-type half-cell and allowed to stand for 12 hours.

[0058] Comparative Example 4

[0059] The binder preparation process for this comparative example is the same as that for Example 1, but the application of the binder in the positive electrode sheet of a lithium-ion battery is different. The difference is that the binder prepared in this comparative example is applied to the lithium iron phosphate positive electrode sheet of the lithium-ion battery. Specifically, the binder is dissolved in an N-methylpyrrolidone solution to prepare a solution with a binder concentration of 25 mg / mL. After the binder is completely dissolved, 1,3-bis(bromomethyl)benzene, a conductive agent, and lithium iron phosphate are added in sequence and stirred evenly to obtain a positive electrode slurry. The mass percentages of binder, 1,3-bis(bromomethyl)benzene, conductive agent, and lithium iron phosphate in the positive electrode slurry are 10:1:19:70. The prepared positive electrode slurry is evenly coated on the current collector, dried at 85°C to remove the solvent, and in situ crosslinking is achieved. The binder and 1,3-bis(bromomethyl)benzene are in situ crosslinked to form a network binder containing ion-transporting groups. Assemble the prepared positive electrode sheet with the positive and negative electrode shells, gaskets, springs, lithium sheets, separators, and electrolyte into a button-type half-cell and let it stand for 12 hours.

[0060] Effect Examples

[0061] The electrochemical performance of the above half-cell was tested. The specific test steps and results are as follows:

[0062] The lithium-ion half-cells prepared in Examples 1-5 and Comparative Examples 1, 3, and 4 were subjected to electrochemical cycling performance testing. The cells were activated for three cycles at a current density of 0.08 C, followed by cycling performance testing at 1 C. The capacity retention over 50 cycles is shown in Table 1.

[0063] Table 1 Electrochemical performance test results of lithium ion half-cells prepared in Examples 1 to 5 and Comparative Examples 1, 3, and 4

[0064]

[0065] Comparing the data from Examples 1-5 with Comparative Examples 1, 3, and 4 reveals that the introduction of the binder provided by the present invention undergoes a coupling reaction with the benzyl units during cycling, improving the capacity retention of the lithium-ion battery. The lithium-ion half-cell discharge capacity ratios of Examples 1-5 are higher than those of batteries using a PVDF binder, demonstrating that the binder provided by the present invention can effectively improve battery performance. Furthermore, in Comparative Example 3, the introduction of excessive imidazole monomers reduced the overall rigidity of the binder, making it susceptible to volume changes during charge and discharge, reducing its restrictive effect on the positive electrode active material and leading to rapid degradation of the lithium-ion half-cell performance. In Comparative Example 4, the introduction of excessive multi-component benzyl monomer crosslinkers increased the overall rigidity of the binder network structure, making it equally difficult to cope with volume changes during charge and discharge.

[0066] Electrochemical cycling performance tests were conducted on the sodium ion half-cells prepared in Examples 3, 5, and Comparative Example 2. The cells were activated for three cycles at a current density of 0.08 C, followed by cycling performance tests at 0.1 C. The capacity retention over 50 cycles is shown in Table 2.

[0067] Table 2 Electrochemical performance test results of sodium ion half-cells prepared in Example 3, Example 5 and Comparative Example 2

[0068]

[0069] By comparing the data of Example 3, Example 5 and Comparative Example 2, it can be seen that during the cycle, the introduction of the binder provided by the present invention undergoes a coupling reaction with the benzyl unit, thereby improving the capacity retention rate of the sodium ion battery. The sodium ion half-cell discharge capacity ratio of Example 3 and Example 5 is higher than that of the battery using the PVDF binder, indicating that the binder provided by the present invention can effectively improve battery performance. At the same time, it shows that the fluorine-free ion binder provided by the present invention has good adaptability and can be used in a variety of batteries.

[0070] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for preparing a secondary battery positive electrode sheet using a fluorine-free ion binder, characterized in that: The method comprises the following steps: (1) 477.56 g of acrylonitrile and 94.11 g of 1-vinylimidazole monomer were added to 3000 mL of deionized water, and nitrogen was introduced at 20°C for 1 hour while stirring. Then 11.43 g of ammonium persulfate was added, and nitrogen was introduced again at 20°C for 1 hour while stirring. The temperature was then raised to 55°C and reacted under nitrogen for 3 hours. The temperature was further raised to 100°C and the reaction was continued for 2 hours. After the reaction was completed, the product was filtered from the reaction system, washed with deionized water and anhydrous ethanol in sequence, and dried to obtain a binder. (2) Add the binder prepared in (1) to N-methylpyrrolidone solvent to prepare a binder solution with a concentration of 25 mg / mL; (3) 1,4-di(bromomethyl)benzene, a conductive agent, and lithium iron phosphate were added to the binder solution in sequence and stirred evenly to obtain a positive electrode slurry. The mass ratio of the binder, 1,4-di(bromomethyl)benzene, a conductive agent, and lithium iron phosphate in the positive electrode slurry was 9.9:0.1:10:80; (4) The positive electrode slurry is scraped onto the current collector, and the secondary battery positive electrode sheet is obtained by drying and rolling.

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