Binder composition, aqueous positive electrode slurry and positive electrode sheet

Abstract

The present invention discloses a binder composition comprising an aqueous binder and a post-crosslinking agent. The aqueous binder comprises a polymer containing a carboxyl-containing structural unit capable of cross-linking with the post-crosslinking agent, wherein the mass ratio of the carboxyl-containing structural unit to the post-crosslinking agent is (1-30):(1-5). The binder composition of the present invention is fluorine-free and uses water as a solvent. The coating process does not generate organic solvent emissions, making it environmentally friendly. The binder composition is used in positive electrode slurry and has excellent dispersibility and adhesion. The resulting positive electrode sheet has excellent adhesion and stability, suitable flexibility, good peel resistance, stable structure, and extremely low water content in the positive electrode sheet. The resulting lithium-ion battery has excellent cycle performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and in particular relates to a binder composition, an aqueous positive electrode slurry and a positive electrode sheet. Background Art

[0002] People's understanding of and requirements for chemical power sources are increasing. Lithium-ion batteries, with their advantages such as high energy density, high operating voltage, low self-discharge rate, long service life, no memory effect, environmental friendliness, and high safety, have been widely used in power supply and grid energy storage systems. In recent years, to meet the higher requirements of applications such as long-range electric vehicles and portable electronic products, high-performance, low-cost lithium-ion batteries have become a research and development hotspot.

[0003] Lithium-ion batteries are primarily composed of a positive electrode, a negative electrode, a separator, an electrolyte, and a battery casing. The positive electrode is typically made of a metal substrate, which is dissolved in an organic solvent by a binder, mixed with the positive electrode active material, and then coated on the metal surface. Currently, the commercialized positive electrode binder is polyvinylidene fluoride (PVDF). PVDF exhibits excellent electrochemical stability, adhesion, lithium ion migration, and thermal stability, and can generally meet the requirements for positive electrode binders in lithium-ion batteries.

[0004] However, the coating process requires the emission of large amounts of solvents (such as NMP), which pollutes the environment. Furthermore, PVDF, a fluorine-based chemical material, is expensive to produce and poses a significant environmental risk. Furthermore, global availability of raw materials for PVDF production has led to a tight supply of PVDF. Consequently, some battery manufacturers have adopted water-based systems for the production of positive electrodes to reduce production costs and avoid environmental pollution.

[0005] However, due to the difficulty in dispersing the conductive agent and lithium iron phosphate in the aqueous system, the prepared positive electrode slurry is prone to stratification. The solid content of the aqueous positive electrode slurry is relatively low (~50%), while the solid content of the oily positive electrode slurry is not less than 65%. The production efficiency is not as good as that of the oily system, and the moisture in the positive electrode sheets prepared by the aqueous slurry is not easy to remove. The overall battery performance is still far behind that of the oily system. Summary of the Invention

[0006] In order to solve the problems existing in the production of positive electrode sheets using an existing aqueous system, the present invention provides a binder composition, an aqueous positive electrode slurry and a positive electrode sheet.

[0007] The purpose of the present invention is achieved through the following technical solutions.

[0008] In a first aspect, the present invention provides an adhesive composition, comprising an aqueous adhesive and a post-crosslinking agent, wherein the aqueous adhesive comprises a polymer, the polymer containing an acrylic structural unit capable of undergoing cross-linking with the post-crosslinking agent, and the mass ratio of the acrylic structural unit to the post-crosslinking agent is (1-30):(1-5).

[0009] Furthermore, the Tg of the water-based binder is -60~50°C.

[0010] Furthermore, the polymer includes an alkali-soluble resin structural unit, a main monomer structural unit, an acrylic structural unit and a self-crosslinking structural unit, and the mass ratio of the alkali-soluble resin structural unit, the main monomer structural unit, the acrylic structural unit and the self-crosslinking structural unit is (10~25): (65~80): (1~5): (1~5).

[0011] Furthermore, the self-crosslinking structural unit is selected from any one or a combination of at least two of hydroxymethyl acrylamide structural units, hydroxyethyl acrylamide structural units, N-n-butoxymethyl acrylamide structural units, N-isobutoxymethyl acrylamide structural units, and hydroxyethyl acryloyl urea structural units.

[0012] Furthermore, the main monomer structural unit includes a hard monomer structural unit and a soft monomer structural unit, and the mass ratio of the hard monomer structural unit to the soft monomer structural unit is (1-20): (80-99); preferably (5-15): (85-95).

[0013] Furthermore, the post-crosslinking agent is aziridine or polycarbodiimide.

[0014] The water-based binder of the present invention is prepared by a conventional solution polymerization method. For example, the preparation method can be as follows: an alkali-soluble resin and water are added to a reactor, an alkaline substance is then added to adjust the pH to 7-9, heating and stirring are performed until the alkali-soluble resin is completely dissolved, an initiator solution is added, the temperature is controlled at 50-90° C., a mixed solution of a main monomer, a carboxyl monomer, a self-crosslinking monomer and water is dropwise added, a polymerization reaction is carried out at 50-90° C., and the temperature is lowered after the reaction is completed to obtain the water-based binder.

[0015] The alkaline substance is selected from any one or a combination of at least two of ammonia water, lithium hydroxide, and sodium hydroxide. The initiator solution is an aqueous solution containing an initiator, and the initiator is any one or a combination of at least two of ammonium persulfate, sodium persulfate, and potassium persulfate.

[0016] In a second aspect, the present invention provides an aqueous positive electrode slurry comprising a positive electrode active material, a conductive agent, water, and the binder composition described in the first aspect. The aqueous positive electrode slurry has a solid content of 50 to 60 wt %, and the weight ratio of the active material, conductive agent, and binder composition is (90 to 100):(2 to 5):(2 to 10).

[0017] The active material, conductive agent, and binder composition are thoroughly mixed in proportion and the solid content is adjusted to 50-60 wt% with water to obtain an aqueous positive electrode slurry. The active material is lithium iron phosphate, and the conductive agent is selected from at least one of conductive carbon black SP and carbon nanotubes.

[0018] In a third aspect, the present invention provides a positive electrode plate comprising a current collector and a positive electrode material layer disposed on the current collector, wherein the positive electrode material layer is coated on the current collector with the aqueous positive electrode slurry as described in the second aspect, and is baked and rolled at a temperature above 100°C; the moisture content of the positive electrode plate is <200ppm.

[0019] Furthermore, the positive electrode material layer includes a positive electrode active material, a conductive agent and a positive electrode binding material; the positive electrode binding material is obtained by reacting the aqueous binder and a post-crosslinking agent; and the crosslinking degree of the positive electrode binding material is less than 4%.

[0020] Furthermore, the swelling degree of the positive electrode binding material is less than 50%.

[0021] In a fourth aspect, the present invention provides a lithium-ion battery comprising the positive electrode sheet as described in the third aspect.

[0022] Compared with the prior art, the present invention has the following beneficial effects.

[0023] The binder composition of the present invention does not contain fluorine and the solvent is water. The coating process does not generate the emission of organic solvents and is environmentally friendly. When the binder composition of the present invention is used to prepare lithium iron phosphate positive electrode slurry, it has excellent coating performance and processability for lithium iron phosphate, and the positive electrode slurry has excellent dispersibility and adhesion. The positive electrode sheet prepared by the present invention has excellent adhesion and stability, suitable flexibility, good peeling resistance, stable structure, extremely low water content in the positive electrode sheet, and excellent cycle performance of the prepared lithium ion battery, effectively improving the performance of the positive electrode sheet and the lithium ion battery.

[0024] The present invention introduces alkali-soluble resin structural units into the polymer structure, which can effectively improve the dispersibility of the binder composition for the positive electrode active material and the conductive agent, and further enhance the binding force with the current collector (aluminum foil) after cross-linking; the Tg of the water-based binder is adjusted by adjusting the ratio of the hard monomer structural units to the soft monomer structural units in the main monomer structural units, so that the binder composition has suitable strength and excellent flexibility; the acrylic structural units can react with the post-crosslinking agent to form a cross-linked network structure, thereby improving the bonding force and reducing the moisture content and swelling degree.

[0025] The self-crosslinking structural units and post-crosslinking agents of the polymer in the binder composition respectively form crosslinked structures with other monomer structural units during the coating and drying process, thereby increasing the bonding strength of the binder composition while reducing the water absorption rate. The water-based binder still has a high hydrophilicity before crosslinking in the presence of a large amount of solvent water. During the coating and drying process, a non-hydrophilic crosslinked network structure is formed through the self-crosslinking of the self-crosslinking structural units and the post-crosslinking effect of the post-crosslinking agent, which greatly reduces the hydrophilicity of the binder and the water absorption of the positive electrode. Because the coating and drying process is a high-temperature baking process with a temperature of more than 100°C, which is higher than the boiling point of water, and is carried out in a long drying tunnel, the residual moisture in the positive electrode is easier to remove during the baking process, and the moisture content of the prepared positive electrode is less than 200ppm. DETAILED DESCRIPTION

[0026] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention clearer, the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the embodiments described herein are part of the embodiments of the present invention, rather than all of the embodiments, and are only used to explain the present invention and are not intended to limit the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0027] In the present invention, as is well known to those skilled in the art of chemical synthesis, each structural unit represents the structural moiety of the corresponding monomer present in the resulting polymer after the monomer participates in the polymerization reaction. The mass ratio of each structural unit refers to the mass ratio of the monomers that provide each structural unit.

[0028] In a first aspect, the present invention provides an adhesive composition, comprising an aqueous adhesive and a post-crosslinking agent, wherein the aqueous adhesive comprises a polymer, the polymer comprising a carboxyl-containing structural unit capable of undergoing cross-linking with the post-crosslinking agent, and the mass ratio of the carboxyl-containing structural unit to the post-crosslinking agent is (1~30):(1~5).

[0029] In some embodiments, the post-crosslinking agent is aziridine or polycarbodiimide.

[0030] In some specific embodiments, the polymer includes an alkali-soluble resin structural unit, a main monomer structural unit, an acrylic structural unit, and a self-crosslinking structural unit, wherein the mass ratio of the alkali-soluble resin structural unit, the main monomer structural unit, the acrylic structural unit, and the self-crosslinking structural unit is (10-25): (65-80): (1-5): (1-5). The carboxyl-containing structural unit includes an acrylic structural unit and an alkali-soluble resin structural unit.

[0031] Alkali-soluble resin structural unit: It is beneficial to dispersion during the pulping process and improves the bonding effect. If the dosage is too high, the system Tg is likely to increase. If the content of the alkali-soluble resin structural unit is too high, it is easy to cause excessive cross-linking in the later stage, affecting the flexibility of the electrode. If the content of the alkali-soluble resin structural unit is too low, the dispersion effect on the positive electrode main material is reduced, the slurry is easy to settle, the cross-linking degree is reduced, and the electrode is easy to absorb water.

[0032] Main monomer structural unit: The Tg of the water-based adhesive is adjusted by adjusting the ratio of the hard monomer structural unit to the soft monomer structural unit in the main monomer structural unit, so that the adhesive composition has suitable strength and excellent flexibility; if the Tg is too high, it will easily lead to a decrease in bonding strength, poor flexibility of the electrode, and difficulty in processing; if the Tg is too low, the roller will stick during the coating or winding process.

[0033] Acrylic structural unit: mainly used to improve adhesion and regulate subsequent cross-linking degree, reduce swelling, etc.; if the content of acrylic structural unit is too much, the cross-linking degree is too large, the flexibility of the electrode becomes poor, which is not conducive to subsequent processing; if the content of acrylic structural unit is too little, the adhesion is reduced, the cross-linking degree decreases, and the electrode is easy to absorb water.

[0034] Self-crosslinking structural unit / post-crosslinking agent: Through crosslinking, it reduces the hydrophilicity of the system and improves the adhesion. If the dosage is too large and the crosslinking degree is too large, the flexibility of the electrode will deteriorate, which is not conducive to subsequent processing.

[0035] Therefore, the content of each structural unit (the amount of the corresponding monomer) must be within the mass ratio range of each structural unit disclosed in this application.

[0036] In some specific embodiments, the self-crosslinking structural unit is selected from any one or a combination of at least two of hydroxymethyl acrylamide structural units, hydroxyethyl acrylamide structural units, N-n-butoxymethyl acrylamide structural units, N-isobutoxymethyl acrylamide structural units, and hydroxyethyl acryloyl urea structural units.

[0037] In some specific embodiments, the alkali-soluble resin structural unit is selected from any one or a combination of at least two of a styrene-maleic anhydride resin structural unit, a polyester-modified acrylic resin structural unit, an ethylene-acrylic acid resin structural unit, a polyolefin-maleic anhydride copolymer resin structural unit, an acrylic resin structural unit, and a styrene-acrylic acid resin structural unit. The alkali-soluble resin contains an acidic group in its structure, which improves the bonding strength and the dispersion effect on the main material. The introduction of the alkali-soluble resin can effectively improve the dispersibility of the binder composition for the positive electrode active material and the conductive agent, as well as the bonding strength with the current collector (aluminum foil).

[0038] In some specific embodiments, the main monomer structural unit includes a hard monomer structural unit and a soft monomer structural unit, and the mass ratio of the hard monomer structural unit to the soft monomer structural unit is (1~20):(80~99). More specifically, the mass ratio of the hard monomer structural unit to the soft monomer structural unit can be 1:99, 2:98, 5:95, 8:92, 10:90, 12:88, 15:85 or 20:80, and is preferably (5~15):(85~95). By selecting a suitable main monomer and its mass ratio, a suitable Tg is formulated. A low Tg has good flexibility, while a too low Tg will cause sticking to the roller during processing, and a high Tg will make the adhesive brittle and have low flexibility. The ratio of the hard monomer to the soft monomer is adjusted so that the Tg of the water-based adhesive is between -60 and 50°C, so that the adhesive composition has suitable strength and excellent flexibility.

[0039] In some specific embodiments, the hard monomer in the main monomer structural unit is derived from any one or a combination of at least two of acrylamide, acrylonitrile, methacrylamide, N,N-dimethylacrylamide, and N,N-diethylacrylamide. The soft monomer is derived from any one or a combination of at least two of hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, methoxydiethylene glycol methacrylate, methoxypolyethylene glycol methacrylate, and allyl polyoxyethylene methyl ether.

[0040] In some specific embodiments, the acrylic acid structural unit is selected from any one or a combination of at least two of acrylic acid structural units, methacrylic acid structural units, β-acryloyloxypropionic acid structural units, hydroxy acrylic acid structural units, maleic acid structural units, itaconic acid structural units, and monobutyl itaconate structural units.

[0041] The water-based adhesive of the present invention is prepared by a conventional solution polymerization method. For example, the preparation method can be as follows: adding an alkali-soluble resin and water into a reactor, then adding an alkaline substance to adjust the pH to 7-9, heating and stirring until the alkali-soluble resin is completely dissolved, then adding an initiator solution, controlling the temperature to 50-90° C., dropwise adding a mixed solution of a main monomer, an acrylic monomer, a self-crosslinking monomer and water, performing a polymerization reaction at 50-90° C., and cooling after the reaction is completed to obtain the water-based adhesive.

[0042] During the synthesis stage, the alkali-soluble resin, main monomer, acrylic monomer, and self-crosslinking monomer undergo primarily linear polymerization in the presence of aqueous solvent, an alkaline environment, and temperatures below 90°C, to form a copolymer. In the presence of a large amount of aqueous solvent, in an alkaline environment, and at relatively low temperatures, the water-based binder remains uncrosslinked and maintains a high degree of hydrophilicity. The resulting water-based binder is an aqueous solution with a solids content of 10-20wt%.

[0043] In some specific embodiments, the alkaline substance is selected from any one or a combination of at least two of ammonia water, lithium hydroxide, and sodium hydroxide. The initiator solution is an aqueous solution containing an initiator, wherein the initiator is any one or a combination of at least two of ammonium persulfate, sodium persulfate, and potassium persulfate. The amount of the initiator used is 0.1-0.5 wt % based on the polymer content of the aqueous binder.

[0044] The aqueous binder and post-crosslinker in the binder composition of the present invention are stored separately and only need to be mixed when preparing the positive electrode slurry. The binder composition is obtained by mixing the aqueous binder with the post-crosslinker in a mass ratio of 100:(1-5) of the aqueous binder polymer to the post-crosslinker.

[0045] By introducing self-crosslinking structural units into the copolymer, during the drying process at temperatures above 100°C or the subsequent curing process, the hydroxymethyl groups on the self-crosslinking structural units can undergo esterification reactions with carboxyl groups on the polymer chain, or undergo condensation reactions with other hydroxymethyl groups, forming crosslinking structures such as ether bonds. This interconnects the polymer molecular chains and forms a non-hydrophilic three-dimensional network structure, greatly reducing the hydrophilicity of the binder and the water absorption of the positive electrode. Moreover, while meeting the functional requirements of a post-crosslinking agent, the introduction of self-crosslinking structural units into the copolymer can reduce the amount of post-crosslinking agent used, thereby reducing production costs.

[0046] Post-crosslinkers: For example, aziridine crosslinkers, through their active chemical properties, react with the carboxyl groups in alkali-soluble resin and acrylic structural units to form stable chemical bonds and a non-hydrophilic crosslinked network structure, significantly reducing the binder's hydrophilicity and the positive electrode's water absorption. Polycarbodiimide crosslinkers have a similar crosslinking mechanism to aziridine, but with a slower reaction rate.

[0047] In a second aspect, the present invention provides an aqueous positive electrode slurry, comprising a positive electrode active material, a conductive agent, water, and the binder composition according to the first aspect. The aqueous positive electrode slurry has a solid content of 50 to 65 wt %, and the weight ratio of the active material, the conductive agent, and the binder composition is (90 to 100):(2 to 5):(2 to 10).

[0048] The active material, conductive agent, and binder composition are thoroughly mixed in proportion and the solid content is adjusted to 50-65 wt% with water to obtain an aqueous positive electrode slurry. The active material is lithium iron phosphate, and the conductive agent is selected from at least one of conductive carbon black SP and carbon nanotubes.

[0049] In a third aspect, the present invention provides a positive electrode plate comprising a current collector and a positive electrode material layer disposed on the current collector, wherein the positive electrode material layer is obtained by coating the aqueous positive electrode slurry described in the second aspect on the current collector, baking at a temperature above 100°C and rolling; the moisture content of the positive electrode plate is <200ppm.

[0050] During the coating and drying process, a non-hydrophilic cross-linked network structure is formed through the self-cross-linking of the self-cross-linking structural units and the post-cross-linking effect of the post-cross-linking agent, which greatly improves the bonding strength of the binder composition and greatly reduces the hydrophilicity of the binder and the water absorption of the positive electrode. Because the coating and drying process is a high-temperature baking process, the temperature is above 100°C, preferably 110-130°C, which is higher than the boiling point of water, and the baking time is 120S-150S. For example, in actual applications, baking is carried out in a long drying tunnel, the conveyor speed during baking is 2m / min, and the drying tunnel length is 4.5 meters. During the baking process, the residual moisture in the positive electrode is easier to remove, and the moisture content of the prepared positive electrode is less than 200ppm.

[0051] In some specific embodiments, the positive electrode material layer includes a positive electrode active material, a conductive agent and a positive electrode binding material; the positive electrode binding material is obtained by reacting the aqueous binder and a post-crosslinking agent; and the crosslinking degree of the positive electrode binding material is less than 4%.

[0052] The crosslinking degree in this invention refers to the crosslinking degree of the positive electrode binder material as measured by the water swelling method. Under both room temperature and 60°C testing conditions, the crosslinking degree of the positive electrode binder material is less than 4%. At room temperature, the crosslinking degree of the positive electrode binder material is less than 0.5%. This relatively high degree of intermolecular crosslinking helps reduce the hydrophilicity of the system.

[0053] If the swelling of the positive electrode binding material is too high, the conductivity between the active material and the current collector will deteriorate, thereby affecting the capacity of the battery. When the swelling of the positive electrode binding material exceeds a certain degree, the active material will separate from the current collector, which will cause the battery capacity to decay. Moreover, the swollen binder will increase the internal resistance of the battery, resulting in a decrease in the output power of the battery. The increase in internal resistance will cause the battery to generate more heat during discharge, affecting the stability and life of the battery. Therefore, the swelling degree of the positive electrode binding material is less than 50%, and under the test conditions of room temperature, the swelling degree of the positive electrode binding material is less than 30%. Under the test conditions of 60°C, the swelling degree of the positive electrode binding material is less than 50%, which has little effect on the battery performance.

[0054] In a fourth aspect, the present invention provides a lithium-ion battery comprising the positive electrode sheet as described in the third aspect.

[0055] The specific embodiments of the present invention will be further explained below through examples and comparative examples.

[0056] Unless otherwise specified, the reagents, materials, and instruments used in the following description are all conventional reagents, materials, and instruments, all of which are commercially available. The reagents involved can also be synthesized by conventional synthetic methods. The methods in the examples, unless otherwise specified, are all conventional methods in the art. Monomers consistent with the present invention can be obtained commercially.

[0057] The glass transition temperature Tg can be measured using a differential scanning calorimeter (DSC) according to GB / T 19466.1-2004.

[0058] Example 1

[0059] (1) Preparation of water-based binder

[0060] 20 parts of styrene-maleic anhydride resin and 320 parts of water were added to a reactor, and nitrogen was passed through for 30 minutes. The closed container was heated to 60°C, and ammonia water was slowly added to adjust the pH to 7-9. After stirring until the styrene-maleic anhydride resin was completely dissolved, the temperature was raised to 80°C, and an initiator aqueous solution of 0.3 parts of ammonium persulfate and 40 parts of water was added. Then, a mixed aqueous solution of 5 parts of acrylamide, 90 parts of methoxydiethylene glycol methacrylate, 5 parts of acrylic acid, 5 parts of hydroxymethyl acrylamide, and 600 parts of water was uniformly added dropwise to the reactor at 85°C. The total dropping time was 4 hours. After the addition was completed, the temperature was kept for 2 hours. After cooling, a water-based adhesive was obtained with a Tg of -35.8°C.

[0061] (2) Preparation of adhesive composition

[0062] The prepared aqueous binder was mixed with aziridine, with the mass ratio of the polymer of the aqueous binder to the aziridine being 100:2, to obtain a binder composition.

[0063] (3) Preparation of aqueous cathode slurry

[0064] 2 parts of the binder composition were mixed thoroughly with 90 parts of lithium iron phosphate and 2 parts of conductive carbon black SP, and the solid content was adjusted to 62% with water to obtain an aqueous positive electrode slurry.

[0065] (4) Preparation of positive electrode

[0066] The obtained aqueous positive electrode slurry was coated on the current collector aluminum foil and baked at a temperature of 130°C. The conveying speed during baking was 2m / min and the baking tunnel length was 4.5 meters. After baking and rolling, the positive electrode sheet was obtained.

[0067] (5) Preparation of lithium-ion batteries

[0068] The negative electrode sheet, separator and electrolyte of the lithium-ion battery prepared according to conventional methods in the art and the positive electrode sheet prepared by the present application are assembled into a lithium-ion battery according to the general process of preparing lithium-ion batteries.

[0069] Example 2

[0070] This embodiment includes most of the steps in Example 1 to prepare an aqueous binder, a binder composition, an aqueous positive electrode slurry, a positive electrode sheet, and a lithium-ion battery, except that:

[0071] When preparing a water-based adhesive, 60 parts of styrene-maleic anhydride resin and 320 parts of water are added to a reactor, nitrogen is passed through for 30 minutes, the closed container is heated to 60°C, ammonia water is slowly added to adjust the pH to 7-9, and the mixture is stirred until the styrene-maleic anhydride resin is completely dissolved, then the temperature is raised to 80°C, an initiator aqueous solution of 0.3 parts of ammonium persulfate and 40 parts of water is added, and then a mixed aqueous solution of 30 parts of acrylamide, 126 parts of methoxydiethylene glycol methacrylate, 12 parts of acrylic acid, 12 parts of hydroxymethyl acrylamide, and 600 parts of water is uniformly added dropwise to the reactor at 85°C. The total dropwise addition time is 4 hours. After the dropwise addition is completed, the mixture is kept warm for 2 hours and cooled to obtain a water-based adhesive with a Tg of -3.6°C.

[0072] Example 3

[0073] This embodiment includes most of the steps in Example 1 to prepare an aqueous binder, a binder composition, an aqueous positive electrode slurry, a positive electrode sheet, and a lithium-ion battery, except that:

[0074] When preparing a water-based adhesive, 20 parts of polyester-modified acrylic resin and 320 parts of water are added to a reactor, nitrogen is passed through for 30 minutes, the closed container is heated to 60°C, ammonia water is slowly added to adjust the pH to 7-9, and the temperature is raised to 80°C after stirring until the polyester-modified acrylic resin is completely dissolved. An initiator aqueous solution of 0.3 parts of ammonium persulfate and 40 parts of water is added, and then a mixed aqueous solution of 5 parts of acrylonitrile, 90 parts of hydroxyethyl acrylate, 5 parts of methacrylic acid, 5 parts of N-n-butoxymethyl acrylamide, and 600 parts of water is uniformly added dropwise to the reactor at 85°C. The total dropwise addition time is 4 hours. After the dropwise addition is completed, the temperature is kept for 2 hours. The water-based adhesive is obtained by cooling, and the Tg is 19.0°C.

[0075] Example 4

[0076] This embodiment includes most of the steps in Example 1 to prepare an aqueous binder, a binder composition, an aqueous positive electrode slurry, a positive electrode sheet, and a lithium-ion battery, except that:

[0077] When preparing a water-based adhesive, 20 parts of a polyolefin-maleic anhydride copolymer resin and 320 parts of water are added to a reactor, nitrogen is passed through for 30 minutes, the closed container is heated to 60°C, ammonia water is slowly added to adjust the pH to 7-9, and the mixture is stirred until the polyolefin-maleic anhydride copolymer resin is completely dissolved, then the temperature is raised to 80°C, 0.3 parts of an initiator aqueous solution of ammonium persulfate and 40 parts of water are added, and then a mixed aqueous solution of 5 parts of methacrylamide, 90 parts of allyl glycidyl ether, 5 parts of hydroxyacrylic acid, 5 parts of hydroxyethyl acryloyl urea, and 600 parts of water is uniformly added dropwise to the reactor at 85°C. The total addition time is 4 hours. After the addition is completed, the mixture is kept warm for 2 hours. The water-based adhesive is obtained by cooling, and the Tg is -36.3°C.

[0078] Example 5

[0079] This embodiment includes most of the steps in Example 1 to prepare an aqueous binder, a binder composition, an aqueous positive electrode slurry, a positive electrode sheet, and a lithium-ion battery, except that:

[0080] When preparing a water-based adhesive, 20 parts of styrene-maleic anhydride resin and 320 parts of water are added to a reactor, nitrogen is passed through for 30 minutes, the closed container is heated to 60°C, ammonia water is slowly added to adjust the pH to 7-9, and the mixture is stirred until the styrene-maleic anhydride resin is completely dissolved, then the temperature is raised to 80°C, an initiator aqueous solution of 0.2 parts of ammonium persulfate and 40 parts of water is added, and then a mixed aqueous solution of 15 parts of acrylamide, 80 parts of methoxydiethylene glycol methacrylate, 5 parts of acrylic acid, 5 parts of hydroxymethyl acrylamide, and 600 parts of water is uniformly added dropwise to the reactor at 85°C. The total dropwise addition time is 4 hours. After the dropwise addition is completed, the mixture is kept warm for 2 hours and cooled to obtain a water-based adhesive with a Tg of -23.2°C.

[0081] Example 6

[0082] This embodiment includes most of the operating steps in Example 1 to prepare an aqueous binder, a binder composition, an aqueous positive electrode slurry, a positive electrode sheet and a lithium-ion battery. The difference is that: when preparing the binder composition, the post-crosslinking agent of the binder composition is polycarbodiimide, and the prepared aqueous binder is mixed with polycarbodiimide, and the mass ratio of the polymer of the aqueous binder to the polycarbodiimide is 100:5 to obtain the binder composition.

[0083] Example 7

[0084] This embodiment includes most of the operating steps in Example 1 to prepare an aqueous binder, a binder composition, an aqueous positive electrode slurry, a positive electrode sheet, and a lithium-ion battery. The difference is that when preparing the aqueous positive electrode slurry, 5 parts of the binder composition are thoroughly mixed with 95 parts of lithium iron phosphate and 5 parts of conductive carbon black SP, and the solid content is adjusted to 60% with water to obtain an aqueous positive electrode slurry.

[0085] Example 8

[0086] This embodiment includes most of the operating steps in Example 1 to prepare an aqueous binder, a binder composition, an aqueous positive electrode slurry, a positive electrode sheet, and a lithium-ion battery. The difference is that when preparing the aqueous positive electrode slurry, 10 parts of the binder composition are thoroughly mixed with 100 parts of lithium iron phosphate and 5 parts of carbon nanotubes, and the solid content is adjusted to 58% with water to obtain an aqueous positive electrode slurry.

[0087] Comparative Example 1

[0088] This comparative example includes most of the operating steps in Example 1 to prepare an aqueous binder, a binder composition, an aqueous positive electrode slurry, a positive electrode sheet and a lithium-ion battery, except that: the binder composition does not include a post-crosslinker; the aqueous binder includes a copolymer formed by polymerization of a main monomer and a carboxyl monomer in water as a solvent, and an alkali-soluble resin that at least partially wraps the surface of the copolymer; and the polymer does not include a self-crosslinking structural unit.

[0089] Comparative Example 2

[0090] This comparative example includes most of the operating steps in Example 1 to prepare an aqueous binder, a binder composition, an aqueous positive electrode slurry, a positive electrode sheet and a lithium-ion battery, except that: the binder composition does not include a post-crosslinker; the aqueous binder includes a polymer formed by solution polymerization of an alkali-soluble resin with a main monomer and a carboxyl monomer in water as a solvent; and the polymer does not include a self-crosslinking structural unit.

[0091] Comparative Example 3

[0092] This comparative example includes most of the operating steps in Example 1 to prepare an aqueous binder, a binder composition, an aqueous positive electrode slurry, a positive electrode sheet and a lithium-ion battery, except that: the binder composition includes a post-crosslinker, and the aqueous binder includes a polymer formed by solution polymerization of an alkali-soluble resin with a main monomer and a carboxyl monomer in water as a solvent; the polymer does not include a self-crosslinking structural unit.

[0093] Comparative Example 4

[0094] This comparative example includes most of the operating steps in Example 1 to prepare an aqueous binder, a binder composition, an aqueous positive electrode slurry, a positive electrode sheet and a lithium-ion battery, except that: the binder composition does not include a post-crosslinking agent, but the polymer monomer used in the aqueous binder includes a self-crosslinking monomer, and the aqueous binder includes a polymer formed by solution polymerization of an alkali-soluble resin with a main monomer, a carboxyl monomer and a self-crosslinking monomer in water as a solvent.

[0095] Comparative Example 5

[0096] This comparative example includes most of the operating steps in Example 1 to prepare an aqueous binder, a binder composition, an aqueous positive electrode slurry, a positive electrode sheet and a lithium-ion battery, except that: when preparing the binder composition, the mass ratio of the polymer of the aqueous binder to the post-crosslinking agent is 100:8.

[0097] Performance testing:

[0098] The binder compositions (or binders), positive electrode slurries, and positive electrode sheets prepared in the above examples and comparative examples were subjected to performance tests, and the test results are shown in Table 1.

[0099] Cross-linking Test: The cross-linking degree of the positive electrode binder material is tested using the water swelling method. The binder composition to be tested is added dropwise to a clean mold and dried in an oven at 70°C for 12 hours, then at 130°C for 2 hours to form a 1mm thick film. The film is then cut and weighed (M1). The film is then transferred to deionized water and soaked at room temperature / 60°C for 24 hours. The film is removed and dried (M2) is recorded. The dissolution mass change rate is calculated. Cross-linking degree = (M1-M2) / M1*100%. A lower cross-linking value indicates a higher degree of intermolecular cross-linking.

[0100] [Swelling degree test] Add the binder composition to be tested dropwise into a clean mold, put it into an oven and dry it at 70℃ for 12h, then dry it at 130℃ for 2h to make a film with a thickness of 1mm. Cut it into pieces and weigh and record m1. Immerse it in electrolyte (EC:EMC:DEC=3:5:2, 1mol / L LiPF6) and test it at room temperature and 60℃ for 72h. After taking out the film, wipe the residual electrolyte on the surface of the film with a wipe and weigh and record m2. Calculate the test swelling degree change rate; swelling degree = (m2-m1) / m1*100%.

[0101] [Viscosity test of positive electrode slurry]: Use a rotary digital viscometer to test the viscosity of the positive electrode slurry. The smaller the viscosity value, the better the dispersion of the positive electrode slurry.

[0102] [Positive Electrode Peel Test]: The peel and cohesive strength of the positive electrode sheets were tested using an electronic tensile testing machine in accordance with the national standard GB / T 2792-2014, "Test Method for Peel Strength of Adhesive Tapes." Peel strength measures the peeling force between the current collector and the coating; cohesive strength measures the peeling force between the main positive electrode materials. A higher peel strength value indicates stronger adhesion.

[0103] [Steel Needle Winding Flexibility Test]: Test the flexibility of the positive electrode using the steel needle winding method. Wrap the electrode around a shaft of a certain diameter (mm) and observe the diameter at which cracks, powder loss, and peeling appear on the electrode. The smaller the value, the better the flexibility.

[0104] [Moisture content test]: The positive electrode is tested for moisture content using a Karl Fischer titrator according to GB-T 6283-2008 Determination of moisture content in chemical products. The test temperature is 150°C. The smaller the value, the less residual moisture in the positive electrode.

[0105] Table 1: Test results of positive electrode adhesive materials, positive electrode slurry, and positive electrode sheets

[0106]

[0107] The performance of the lithium-ion batteries prepared in the above examples and comparative examples was tested, and the test results are shown in Table 2.

[0108] Internal resistance test: Charge the battery at 25°C at 0.03C to 3.4V, 0.1C to 3.75V, 0.5C to 4.5V, and discharge it at 0.5C to 3.0V, recording the discharge capacity. Charge it at 0.5C to 4.5V with a cut-off current of 0.02C, and discharge it at 0.5C to 3.0V, recording the discharge capacity. Charge it at 0.5C to 50% SOC, and test the internal resistance using a Tonghui Electronics TH2523 internal resistance tester. The smaller the value, the better.

[0109] [First Efficiency]: Charge the battery at 0.5C to 4.5V at 25°C, using a constant voltage of 4.5V and a cutoff current of 0.05C, and record the charge capacity. Then discharge the battery at a constant current of 0.5C to 3V, and record the discharge capacity. Calculate the first efficiency as the percentage of the first discharge capacity to the first charge capacity. The larger the value, the better.

[0110] Cycle Performance Test: Charge the battery at 25°C at a constant current of 3000mA (1C) to 4.5V, then charge at a constant voltage with a cut-off current of 150mA, and discharge at a constant current of 3000mA to 3V. This is considered one cycle. Repeat 500 times. Calculate the capacity retention rate (%) after 500 cycles = (discharge capacity after 500 cycles / initial discharge capacity) × 100%. The larger the value, the better.

[0111] Table 2: Test results of lithium-ion batteries

[0112]

[0113] It can be seen from the test results in Table 1 and Table 2 that the present invention comprises a polymer formed by an alkali-soluble resin structural unit, a main monomer structural unit, an acrylic structural unit and a self-crosslinking structural unit, and a post-crosslinking agent as main components to form a positive electrode binder formula. When it replaces the PVdF binder for the preparation of lithium iron phosphate positive electrode slurry and positive electrode sheet, it has excellent coating performance and processability for lithium iron phosphate, excellent dispersibility and adhesion, and is not easy to cause moisture residue, effectively improving the performance of the positive electrode sheet and the lithium-ion battery, improving the adhesion of the binder composition while reducing the water absorption rate, and the moisture content of the prepared positive electrode sheet is easier to control.

[0114] From the test results of Examples 1 to 8 and Comparative Example 1, it can be seen that if the binder composition does not include a post-crosslinking agent, the polymer in the aqueous binder does not include a self-crosslinking structural unit, and the alkali-soluble resin is wrapped on the surface of the copolymer: during the processing of the aqueous positive electrode slurry, although the binder has good dispersibility for lithium iron phosphate, the alkali-soluble resin cannot form an effective interpenetrating and cross-linked network structure with the binder, resulting in relatively low peel strength and cohesive strength of the positive electrode sheet, and poor bonding performance; because the solvent used in the preparation process of the aqueous positive electrode slurry is water, and under non-crosslinking conditions, the binder still has high hydrophilicity during coating and drying, even if the baking temperature and time are increased, the moisture in the electrode sheet is difficult to remove, and the energy consumption and process generated will lead to increased processing costs; when the residual moisture in the electrode sheet is higher than 600 ppm, the residual moisture easily reacts with the lithium salt in the electrolyte to generate highly corrosive hydrofluoric acid (HF), resulting in a rapid decrease in battery capacity; the swelling degree is large at 60°C, the internal resistance is significantly increased, and the battery performance is poor.

[0115] It can be seen from the test results of Examples 1 to 8 and Comparative Example 2 that if the binder composition does not include a post-crosslinker, the polymer formed by solution polymerization in the aqueous binder, but the polymer does not include a self-crosslinking structural unit: there are similar results to Comparative Example 1. Although the adhesion is slightly better than that of Comparative Example 1, the binder is not cross-linked, and there is also a problem of high water content leading to a large attenuation of battery capacity; the swelling degree is large at 60°C, the internal resistance is significantly increased, and the battery performance is poor; compared with the emulsion polymerization process, the monomers selected for solution polymerization are mostly water-soluble monomers, and compared with oily monomers, the compatibility difference with electrolyte solvents is greater, so the prepared water-soluble binder has suitable adhesion while having lower swelling and less impact on battery performance.

[0116] The test results of Examples 1-8 and Comparative Example 3 demonstrate that if the binder composition includes a post-crosslinker, but the polymer in the aqueous binder does not contain self-crosslinking structural units, the degree of intermolecular crosslinking at 60°C is relatively low, resulting in relatively low peel and cohesive strengths of the positive electrode sheet, and weak bonding performance. The positive electrode sheet also contains slightly higher levels of residual moisture, posing a potential safety risk to the battery cell. Comparative Example 3 demonstrates, compared to Comparative Examples 1-2, that adding a post-crosslinker to the binder composition increases the degree of intermolecular crosslinking, improving the peel and cohesive strengths of the positive electrode sheet. This results in relatively low residual moisture in the positive electrode sheet, resulting in relatively low swelling, and minimal impact on battery performance.

[0117] The test results of Examples 1-8 and Comparative Example 4 show that if the binder composition does not include a post-crosslinker, but the polymer in the aqueous binder includes self-crosslinking structural units, the bonding performance is weak and the crosslinking degree is relatively low, resulting in slightly higher residual moisture in the electrode sheet, which poses a certain safety risk to the battery cell. Comparative Example 4 demonstrates, compared to Comparative Examples 1-2, that the inclusion of self-crosslinking structural units in the aqueous binder polymer can increase the degree of intermolecular crosslinking, improve the peel strength and cohesive strength of the positive electrode sheet, and result in relatively less residual moisture in the positive electrode sheet, a relatively lower degree of swelling, and minimal impact on battery performance.

[0118] It can be seen from the test results of Examples 1 to 8 and Comparative Example 5 that if too much post-crosslinking agent is added to the binder composition, the degree of crosslinking between the system molecules will be too high, the flexibility of the coated positive electrode sheet will deteriorate, the brittleness will increase, and it will be unfavorable for battery cell production.

[0119] The present invention is further described above with the aid of specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the essence and scope of the present invention. Various modifications made to the above embodiments by ordinary technicians in this field after reading this specification are all within the scope of protection of the present invention.

Claims

1. A binder composition, characterized in that The binder composition includes a water-based binder and a post-crosslinking agent, wherein the water-based binder includes a polymer, the polymer contains a carboxyl-containing structural unit capable of undergoing cross-linking with the post-crosslinking agent, and the mass ratio of the carboxyl-containing structural unit to the post-crosslinking agent is (1-30):(1-5); the Tg of the water-based binder is -60-50°C; The polymer is a copolymer comprising an alkali-soluble resin structural unit, a main monomer structural unit, an acrylic structural unit and a self-crosslinking structural unit, wherein the mass ratio of the alkali-soluble resin structural unit, the main monomer structural unit, the acrylic structural unit and the self-crosslinking structural unit is (10-25): (65-80): (1-5): (1-5); The carboxyl-containing structural unit includes an acrylic structural unit and an alkali-soluble resin structural unit; The main monomer structural unit includes a hard monomer structural unit and a soft monomer structural unit, and the mass ratio of the hard monomer structural unit to the soft monomer structural unit is (1-20): (80-99).

2. The adhesive composition according to claim 1, wherein The self-crosslinking structural unit is selected from any one or a combination of at least two of hydroxymethyl acrylamide structural units, hydroxyethyl acrylamide structural units, N-n-butoxymethyl acrylamide structural units, N-isobutoxymethyl acrylamide structural units, and hydroxyethyl acryloyl urea structural units.

3. The adhesive composition according to claim 1, characterized in that The mass ratio of the hard monomer structural unit to the soft monomer structural unit is (5-15): (85-95).

4. The adhesive composition according to claim 1, characterized in that The hard monomer in the main monomer structural unit is derived from any one of acrylamide, acrylonitrile, methacrylamide, N,N-dimethylacrylamide, and N,N-diethylacrylamide, or a combination of at least two thereof.

5. The adhesive composition according to claim 1, wherein The soft monomer in the main monomer structural unit is derived from any one of hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, methoxydiethylene glycol methacrylate, methoxypolyethylene glycol methacrylate, and allyl polyoxyethylene methyl ether, or a combination of at least two thereof.

6. The adhesive composition according to claim 1, characterized in that The post-crosslinking agent is aziridine or polycarbodiimide.

7. An aqueous positive electrode slurry, characterized in that The aqueous positive electrode slurry comprises a positive electrode active material, a conductive agent, water, and the binder composition according to any one of claims 1 to 6.

8. A positive electrode plate, characterized in that: It comprises a current collector and a positive electrode material layer arranged on the current collector, wherein the positive electrode material layer is obtained by coating the aqueous positive electrode slurry according to claim 7 on the current collector, baking at a temperature above 100°C and rolling; the moisture content of the positive electrode sheet is less than 200ppm.

9. The positive electrode sheet according to claim 8, characterized in that: The positive electrode material layer includes a positive electrode active material, a conductive agent and a positive electrode binding material; the positive electrode binding material is obtained by reacting the aqueous binder and a post-crosslinking agent; and the crosslinking degree of the positive electrode binding material is less than 4%.

10. The positive electrode sheet according to claim 9, characterized in that: The swelling degree of the positive electrode binding material is less than 50%.

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