A positive electrode binder, a preparation method thereof, a preparation method of a positive electrode sheet, and a lithium battery

Abstract

This invention discloses a positive electrode binder and its preparation method, a method for preparing a positive electrode sheet, and a lithium battery, belonging to the field of secondary batteries. This invention provides a positive electrode binder that does not experience the removal of HF molecules during use, effectively preventing slurry coagulation when using PVDF as the positive electrode binder, improving battery processing yield, avoiding environmental pollution from fluorine-containing substances, and also avoiding the impact of adding other components on the safety and stability of the lithium battery. The positive electrode binder has a large number of polar groups that can form a strong bond with the positive electrode active material and the current collector aluminum foil.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and in particular to a positive electrode binder and its preparation method, a method for preparing a positive electrode sheet, and a lithium battery. Background Technology

[0002] With the development of technology, lithium batteries have been widely used in various electronic devices, electric vehicles, and other fields due to their advantages such as high energy density, long cycle life, and environmental friendliness. Among these, the preparation of the positive electrode sheet is one of the key factors affecting the performance of lithium batteries. A lithium-ion positive electrode sheet includes a positive current collector, positive active material, binder, and conductive agent. Currently, polyvinylidene fluoride (PVDF) is widely used as a binder for lithium battery positive electrodes due to its good chemical and electrochemical stability. However, in lithium battery manufacturing systems, the positive active materials are mainly lithium iron phosphate or high-nickel ternary materials, most of which contain varying degrees of residual alkali. PVDF is prone to elimination reactions in alkaline environments, leading to denaturation. This not only makes the slurry gel difficult to process during production but also affects the performance of the assembled lithium-ion battery.

[0003] To address this issue, methods typically involve adding inorganic acids or other organic solvents to adjust the pH of the PVDF slurry and prevent coagulation; or altering the molecular structure of PVDF or adding other functional materials. However, these methods still present other problems. First, adding inorganic acids or other organic solvents may pollute the environment and potentially affect the safety and stability of the battery. Second, altering the molecular structure of PVDF or adding other functional materials involves high technical barriers, is difficult to modify, and may negatively impact the battery's energy density and power density. Therefore, effectively preventing the coagulation of the cathode slurry and improving battery performance without adding any harmful substances is a significant challenge currently facing lithium battery manufacturing technology.

[0004] CN117038987A discloses a high-performance, low-cost positive electrode sheet and lithium-ion battery. The positive electrode sheet uses a polyacrylonitrile binder, which is a mixture of polyacrylonitrile and polyvinylidene fluoride (PVDF). CN115763818A discloses a method for preparing lithium battery positive electrode slurry using a fluorine-free binder. This method introduces a 1:1 mass ratio of polyacrylate copolymer and PVDF as the positive electrode binder. The polyacrylate copolymer's resistance to strong alkalis and its fluorine-free and environmentally friendly properties effectively overcome the shortcomings of using PVDF alone as a binder. CN117777899A discloses a method for preparing and applying a high-alkali-resistant positive electrode binder for sodium batteries. This binder uses a polyacrylic acid / acrylamide / styrene copolymer as the main component. This copolymer provides a large number of polar groups that allow for good adhesion to the positive electrode material and aluminum foil. As a fluorine-free binder, it exhibits no gelation phenomenon and significantly improves the electrode sheet peel strength.

[0005] However, due to the use of polyacrylate copolymers, polyacrylonitrile, and polyacrylic acid / acrylamide / styrene copolymers as single binders, the resulting electrodes are brittle and difficult to process. Therefore, it is still impossible to completely abandon the use of PVDF. This results in the continued existence of the alkaline defluorination crosslinking reaction of PVDF, making it impossible to achieve true anti-gelling.

[0006] Chinese patent CN117089017A discloses a non-fluorinated binder for lithium-ion battery cathodes, obtained by copolymerizing acrylamide, acrylic acid, and acrylonitrile, followed by grafting alkyl glycidyl ethers. The introduction of alkyl glycidyl ethers improves the hardness and brittleness of the monomers used (acrylamide, acrylic acid, and acrylonitrile), enhancing the flexibility and dispersibility of the copolymer. However, excessively high carbon chain lengths and mass fractions of alkyl glycidyl ethers can lead to decreased flexibility. Furthermore, the introduction of large amounts of alkyl glycidyl ethers can reduce adhesive strength. Summary of the Invention

[0007] The present invention aims to provide a fluorine-free positive electrode binder with good flexibility and adhesion; another objective of the present invention is to provide a method for preparing a fluorine-free positive electrode binder; another objective of the present invention is to provide a method for preparing a positive electrode sheet that does not gel during homogenization and has good adhesion; and another objective of the present invention is to provide a lithium battery with excellent cycle performance.

[0008] A positive electrode binder comprising the compound of formula (I):

[0009] (I);

[0010] Where a = 1~500000, b = 0~5000000, and R is a molecular chain of alkane, alkene, or alkyne containing one or more of the following structures: carboxyl, hydroxymethyl, ether, ester, carbonyl, cyano, amide, or benzene ring.

[0011] The optimal bonding effect can be achieved by adjusting the number and type of a and b segments to change the flexibility and bonding strength of the adhesive.

[0012] Furthermore, the weight-average molecular weight of the compound is 1,000-5,000,000 Da.

[0013] Furthermore, the compound has a four-armed PEG structure.

[0014] The four-arm structure endows the binder with highly branched properties, enabling it to form a network structure, which enhances the binder's coating of the positive electrode active material and improves the adhesion between materials.

[0015] This invention also discloses a method for preparing a positive electrode binder, comprising the following steps:

[0016] (1) Under a protective gas atmosphere, the solvent, chain transfer agent, emulsifier, and first reactant four-arm PEG methacrylate are mixed evenly; the temperature is raised to 40-100℃;

[0017] (2) Add the second reactant and initiator and mix evenly; copolymerize for 3-5 h to obtain the compound.

[0018] The solvent is water; the chain transfer agent includes one of ethyl acetate, ammonium persulfate / sodium bisulfite, and dodecyl mercaptan; the emulsifier includes one of sodium dodecyl sulfate and stearyl alcohol; and the initiator includes one of ammonium persulfate and azobisisobutyronitrile.

[0019] Furthermore, the second reactant includes one or a combination of at least two of octyl acrylate, acrylamide, acrylic acid, and acrylonitrile.

[0020] The polyethylene glycol and acrylate segments give the binder good flexibility, improve the brittleness of the positive electrode binder when used as a single binder, and provide sufficient dispersibility, making it a complete replacement for PVDF binders without the need to add other functional components.

[0021] Furthermore, based on mass parts, the amount of the first reactant, tetra-arm PEG methacrylate, added is >0 parts and ≤20 parts; the amount of the second reactant added is 80-100 parts.

[0022] This invention also discloses a method for preparing a positive electrode sheet, comprising the following steps:

[0023] S1: Mix the positive electrode binder as described above with an organic solvent to obtain a glue solution;

[0024] S2: Mix the conductive agent with the adhesive solution evenly to obtain a mixture;

[0025] S3: Mix the positive electrode active material with the mixture until homogeneous;

[0026] S4: Mix the positive electrode binder with the mixture obtained in step S3 until homogeneous;

[0027] S5: Mix the positive electrode active material with the material obtained in step S4 evenly to obtain a positive electrode slurry. Coat the positive electrode slurry onto the surface of the current collector, dry it, and compact it to obtain a positive electrode sheet.

[0028] Furthermore, the amount of positive electrode active material added each time is half of the total amount.

[0029] The positive electrode sheet prepared by this invention, in addition to the positive electrode binder, conductive agent, and positive electrode active material, does not have any additional components added, and will not affect the performance of the assembled lithium battery.

[0030] The organic solvent includes one of N-methylpyrrolidone, ethyl acetate, toluene, and xylene; the conductive agent includes one or more of conductive carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotube, and graphene; the positive electrode active material includes one or more of lithium iron phosphate, nickel cobalt manganese positive electrode material, nickel cobalt aluminum positive electrode material, cobalt-free positive electrode material, or lithium iron manganese phosphate.

[0031] Furthermore, in step S5, the solid content and viscosity of the positive electrode slurry are adjusted by using an organic solvent, so that the solid content is 20-90% and the viscosity is 100-20000 mPa·s.

[0032] More preferably, its solid content is 60-75% and its viscosity is 7000-10000 mPa·s.

[0033] Furthermore, in steps S1-S5, the ratio of the total mass of the added positive electrode binder to the total mass of the conductive agent to the total mass of the positive electrode active material is 1-2:1:97-98.

[0034] The present invention also discloses a lithium battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte obtained by the preparation method described above.

[0035] The negative electrode, separator, and electrolyte are all commercially available negative electrode, separator, and electrolyte in the existing technology.

[0036] This invention provides a positive electrode binder that does not remove HF molecules during use, effectively preventing slurry coagulation when using PVDF as a positive electrode binder, improving battery processing yield, avoiding environmental pollution from fluorine-containing substances, and also avoiding the impact of adding other components on the safety and stability of lithium batteries; the positive electrode binder has a large number of polar groups that can form a strong bond with the positive electrode active material and the current collector aluminum foil. Detailed Implementation

[0037] To make the technical solution of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments. Example 1

[0038] The preparation method of the positive electrode binder is as follows:

[0039] (1) Add 1600ml of water to the reactor, then draw a vacuum and pass nitrogen to form a protective atmosphere. Add 4g of sodium dodecyl sulfate, 2g of ethyl acetate, and 3g of tetra-arm PEG methacrylate (molecular weight ~2000), and then heat to 60℃.

[0040] (2) Add 100g acrylic acid, 150g octyl acrylate, 30g acrylonitrile, and 25g 15wt% ammonium persulfate to the reaction system in step (1). The dropping rate of ammonium persulfate is 0.05g / min, and the dropping rates of 100g acrylic acid, 150g octyl acrylate, and 30g acrylonitrile are all 0.3g / min. After reacting for 5h, a positive electrode binder (a about 2000, b about 10000~100000) is obtained.

[0041] The preparation method of the positive electrode sheet is as follows:

[0042] The mass ratio of the positive electrode binder: conductive agent: positive electrode active material is 1.5:1:97.5. The positive electrode active material is LiNiCoMnO2 (NCM).

[0043] S1: Mix 50g of positive electrode binder with 1000g of N-methylpyrrolidone (NMP), stir and disperse for 3h at 1500-2000 rpm / min to obtain a gel solution, and remove air bubbles by vacuuming.

[0044] S2: Add 50g of conductive carbon black to the adhesive solution and stir and disperse at 3000 rpm / min for 1 hour to obtain conductive adhesive;

[0045] S3: Add 2437.5g of positive electrode active material to the conductive gel and stir for 1 hour at a speed of 3000rpm / min.

[0046] S4: Add 25g of positive electrode binder to the material obtained in step S3, and stir at 3000rpm / min for 0.5 hours.

[0047] S5: Add the remaining 2437.5g of positive electrode active material, stir for 3 hours at 3000rpm / min, and use NMP to adjust the solid content of the positive electrode slurry to 68% and the viscosity to 8000 mPa·s. Coat the positive electrode slurry onto the surface of the positive electrode current collector aluminum foil at a speed of 5m / min, dry it at 50-200℃, let it stand at room temperature, cool it, and roll it until the compaction density reaches 3.5g / cc to obtain the positive electrode sheet. Example 2

[0048] The difference between this embodiment and Embodiment 1 is that the mass ratio of the positive electrode binder: conductive agent: positive electrode active material is 1:1:98. Example 3

[0049] The difference between this embodiment and Embodiment 1 is that the mass ratio of the positive electrode binder: conductive agent: positive electrode active material is 2:1:97.5. Example 4

[0050] The difference between this embodiment and Embodiment 1 is that the mass ratio of the positive electrode binder: conductive agent: positive electrode active material is 2.5:1:97.5. Example 5

[0051] The difference between this embodiment and Embodiment 1 lies in the method of preparing the positive electrode binder:

[0052] (1) Add 1600ml of water to the reactor, then draw a vacuum and purge with nitrogen to form a protective atmosphere. Add 4g of sodium dodecyl sulfate, 2g of ethyl acetate, and 3g of tetra-arm PEG methacrylate (molecular weight ~4000), and then heat to 60℃.

[0053] (2) Add 100g acrylic acid, 90g octyl acrylate, 90g acrylonitrile, and 25g 15wt% ammonium persulfate to the reaction system in step (1). The dropping rate of the initiator ammonium persulfate is 0.05g / min, and the dropping rates of 100g acrylic acid, 90g octyl acrylate, and 90g acrylonitrile are all 0.3g / min. React for 5h to obtain the positive electrode binder (a about 4000, b about 20000~200000). Example 6

[0054] The difference between this embodiment and Embodiment 1 is that the positive electrode active material is lithium iron phosphate positive electrode material. The compaction density of the rolling in step S5 is 2.5 g / cc. Comparative Example 1

[0055] The difference between this comparative example and Example 1 is that commercially available PVDF is used instead of the positive electrode binder provided by this invention. Comparative Example 2

[0056] The difference from Example 1 is that:

[0057] The difference between this comparative example and Example 1 is that a commercially available polyacrylate binder is used instead of the positive electrode binder provided by this invention.

[0058] The viscosity of the positive electrode slurry in step S5 of Examples 1-6 and Comparative Examples 1-2 was tested from 0 to 24 hours, and the test results are shown in Table 1.

[0059] Table 1 Viscosity Test Results

[0060]

[0061] As can be seen from Table 1, Examples 1-6 do not gel rapidly within 0-24 hours, and the positive electrode binder has good anti-gelling properties during positive electrode homogenization.

[0062] The positive electrode sheets prepared in Examples 1-6 and Comparative Examples 1-2 were then assembled into pouch cells with commercially available negative electrode sheets, separators, and electrolytes. The peel strength of the positive electrode sheets was tested using a universal tensile testing machine; constant current charge-discharge cycle experiments were conducted using a charge-discharge current of 0.2C. The test results are shown in Table 1.

[0063] Table 2. Test results of positive electrode peel strength, positive electrode flexibility, and battery cycle capacity retention.

[0064]

[0065] As can be seen from Table 2, the electrode flexibility and electrode peeling force of Examples 1-6 are higher than those of Comparative Examples 1-2, indicating that the positive electrode binders prepared in Examples 1-6 have excellent bonding performance and better flexibility than PVDF and polyacrylate binders. Furthermore, increasing the cyano group and molecular weight will enhance the electrode peeling force.

[0066] Furthermore, the lithium batteries in Examples 1-6 exhibit better cycle performance than those in Comparative Examples 1-2, with Example 1 showing the highest capacity retention after 500 cycles. The lithium batteries assembled using the positive electrode binder disclosed in this invention exhibit better cycle performance than those using PVDF and polyacrylate binders. It should be noted that the amount of positive electrode binder provided in this invention should not be too high or too low; the amount of positive electrode binder needs to be adjusted according to the system.

[0067] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A positive electrode binder, characterized in that, Including the compounds described in formula (I): （I）； Where a = 1~500000, b = 0~5000000, b is not 0, and R is a molecular chain of alkane, alkene or alkyne containing one or more of carboxyl, ester bond, carbonyl, cyano or amide bond; The preparation method of the positive electrode binder includes the following steps: (1) Under a protective gas atmosphere, the solvent, chain transfer agent, emulsifier, and first reactant four-arm PEG methacrylate are mixed evenly; the temperature is raised to 40-100℃; (2) Add the second reactant and initiator and mix evenly; copolymerize for 3-5 h to obtain the positive electrode binder; The second reactant includes one or a combination of at least two of octyl acrylate, acrylamide, acrylic acid, and acrylonitrile.

2. The positive electrode binder according to claim 1, characterized in that, The weight-average molecular weight of the compound is 1,000-5,000,000 Da.

3. The positive electrode binder according to claim 1, characterized in that, In step (1), the amount of the first reactant, tetra-arm PEG methacrylate, added is >0 parts and ≤20 parts, calculated by mass parts; the amount of the second reactant added is 80-100 parts.

4. A method for preparing a positive electrode sheet, characterized in that, Includes the following steps: S1: Mix the positive electrode binder as described in any one of claims 1-3 with an organic solvent to obtain a glue solution; S2: Mix the conductive agent with the adhesive solution evenly to obtain a mixture; S3: Mix the positive electrode active material with the mixture until homogeneous; S4: Mix the positive electrode binder with the mixture obtained in step S3 until homogeneous; S5: Mix the positive electrode active material with the material obtained in step S4 evenly to obtain a positive electrode slurry. Coat the positive electrode slurry onto the surface of the current collector, dry it, and compact it to obtain a positive electrode sheet.

5. A method for preparing a positive electrode sheet according to claim 4, characterized in that, In step S5, the solid content and viscosity of the positive electrode slurry are adjusted by using an organic solvent to make the solid content 20-90wt% and the viscosity 100-20000mPa·s.

6. The method for preparing a positive electrode sheet according to claim 4, characterized in that, In steps S1-S5, the ratio of the total mass of the added positive electrode binder to the total mass of the conductive agent to the total mass of the positive electrode active material is 1-2:1:97-98.

7. A lithium battery, characterized in that, The positive electrode, negative electrode, separator, and electrolyte are obtained by the method for preparing the positive electrode as described in any one of claims 4-6.

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