A mutual mixing type adhesive, a paste, and a preparation method and application thereof

Abstract

The present application belongs to the technical field of lithium ion batteries, and particularly relates to a mutual mixing type binder, slurry, and preparation method and application thereof. The mutual mixing type binder comprises a branched polymer A and a linear polymer B, and the combination of the two can fully exert the dispersion and binding effects thereof. The slurry using the mutual mixing type binder, compared with the traditional slurry using a low molecular weight binder or a high molecular weight binder or a modified binder alone, can realize the improvement of electrode sheet peeling force, ensure the dispersibility and stability of the slurry, and thus is more conducive to the production and preparation of lithium battery electrodes.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and more specifically, relates to an intermixed binder, slurry, preparation method and application thereof. Background Technology

[0002] Polyvinylidene fluoride (PVDF) was the first widely used electrode binder for lithium-ion batteries. Based on the production process, PVDF is mainly divided into two types: emulsion polymerized PVDF and suspension polymerized PVDF. Emulsion polymerized PVDF generally has a molecular weight below 1 million, is easily dispersed, has high electrode sheet compaction density, low rebound, and good flexibility, but has poor peel strength. Suspension polymerized PVDF has a large molecular weight and good peel strength, but the slurry prepared using it is difficult to disperse, easily gels, has high internal resistance in the electrode sheet, and has large rebound. Existing conventional lithium iron phosphate (LiFePO4, LFP) systems generally use emulsion polymerized PVDF alone, resulting in low electrode sheet peel strength. During electrode sheet winding and subsequent battery use, the active coating is prone to peeling off, affecting battery performance. Therefore, improving the peel strength of LFP electrode sheets is essential for improving battery performance.

[0003] Existing technologies mainly improve the peel strength of lithium battery electrode sheets by using PVDF containing polar groups and high molecular weight PVDF. Chinese patent application CN110183562A discloses a mixed PVDF copolymer for improving the peel strength of electrode sheets, which contains an ultra-high molecular weight PVDF (Mw, 1.6 million to 2 million) and a high molecular weight PVDF (Mw, 1 million to 1.3 million). The molecular weight of the PVDF mixture used in this scheme is greater than 1 million. However, the use of a larger molecular weight leads to an increase in slurry viscosity and makes it prone to gelation.

[0004] The positive electrode slurry of the LFP system consists of positive electrode active material (LFP particles, which are prepared from phosphorus source, iron source, lithium source and carbon source), conductive agent, PVDF and solvent. Its solid content is generally 55% to 65%, of which the PVDF content is 2% to 4%. For conventional LFP cathode slurries, the LFP particles have a carbon coating layer on their surface. This carbon coating layer is amorphous carbon formed after high-temperature sintering of sucrose or glucose, containing a certain amount of polar groups such as -OH and -COOH. It easily forms a strong bond with modified PVDF containing polar groups. The polar groups at different positions on the modified PVDF molecular chain interact with the surrounding LFP particles, causing "bridging" between particles and forming a gel with poor flowability. Therefore, modified PVDF containing polar groups cannot be directly applied to high-solids-content LFP systems. In addition, high molecular weight PVDF produced by suspension processes without polar groups is also unsuitable for LFP systems because its molecular chains are prone to coiling into "coil structures," which will cause local conductive carbon and LFP particles to be surrounded, forming local gels and reducing the flowability of the slurry. Furthermore, emulsion polymerization of PVDF is limited by the process, making it difficult to produce PVDF polymers with a molecular weight of over 1 million. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the existing low molecular weight PVDF alone, which leads to insufficient peel force of electrode sheets and difficulty in improving battery performance, as well as the defects and deficiencies of using high molecular weight PVDF or modified PVDF alone to improve peel force but lead to decreased slurry stability. The present invention provides a mixed binder that improves the peel force of electrode sheets while ensuring slurry stability.

[0006] The purpose of this invention is to provide a positive electrode slurry.

[0007] Another objective of this invention is to provide an application of an intermixed binder in lithium iron phosphate batteries.

[0008] The above-mentioned objective of this invention is achieved through the following technical solution:

[0009] A blendable adhesive comprising polymer A and polymer B;

[0010] Wherein, polymer A is a homopolymer of vinylidene fluoride, the molecular chain contains side chains, and the molecular weight is 200,000 to 900,000;

[0011] The polymer B is a homopolymer of vinylidene fluoride, or a copolymer of vinylidene fluoride and hexafluoropropylene, with no side chains or polar groups in its molecular chain, and a molecular weight of 1.1 million to 2 million.

[0012] Furthermore, the weight ratio of polymer A to polymer B is 1:9 to 9:1.

[0013] Preferably, the mixing ratio of polymer A and polymer B is 3:7 to 7:3.

[0014] More preferably, the mixing ratio of polymer A and polymer B is 5:5 to 7:3.

[0015] Most preferably, the mixing ratio of polymer A and polymer B is 7:3.

[0016] Furthermore, the side chain branching degree of polymer A is 0.5% to 1.5%.

[0017] Furthermore, the polymer A is prepared by an emulsion polymerization process.

[0018] Furthermore, the polymer B is prepared by a suspension polymerization process.

[0019] The polymer A described in this invention contains side chains, i.e., a branched structure, while polymer B does not contain side chains, i.e., a linear structure. Due to the difference in their adsorption configurations, there is a difference in their adsorption energies with the substrate (the surface of the adherent material). The branched polymer A contacts the substrate with its side chains, resulting in the smallest conformational entropy loss, the largest adsorption energy, and a stable adsorption layer, primarily acting as a "dispersant." The high-molecular-weight linear polymer B exists mostly in a free state in the liquid phase, serving to adhere to the current collector (aluminum foil). By utilizing the molecular chains of polymer B to block the intermolecular interactions of the "brush-like" branched polymer A, while simultaneously preventing the effective entanglement and linkage between the molecular chains of the linear polymer B, the combination of the branched polymer A and the linear polymer B can fully exert its dispersing and adhesive effects.

[0020] This invention also provides a method for preparing an intermixable adhesive, comprising the following steps:

[0021] The polymer A and polymer B are thoroughly mixed to obtain a blendable adhesive.

[0022] Preferably, the thorough mixing is achieved by stirring.

[0023] The present invention also provides a positive electrode slurry, the positive electrode slurry comprising a positive electrode active material, a conductive agent, a binder and a solvent.

[0024] Furthermore, the positive electrode active material is lithium iron phosphate (LiFePO4) (LFP).

[0025] Furthermore, the raw materials for preparing LFP consist of a phosphorus source, an iron source, a lithium source, and a carbon source.

[0026] Further, the positive electrode slurry, by weight, comprises 55-65 parts of positive electrode active material, 0.2-0.8 parts of conductive agent, 0.8-4 parts of binder, and 34-40 parts of solvent.

[0027] Furthermore, the positive electrode slurry, by weight, comprises 60.67 parts of positive electrode active material, 0.44 parts of conductive agent, 1.89 parts of binder, and 37 parts of solvent.

[0028] Electrode slurries require stable and appropriate viscosity, which has a crucial impact on the electrode coating process. Viscosities that are too high or too low are detrimental to electrode coating. High-viscosity slurries are less prone to sedimentation, but excessive viscosity hinders leveling and coating. Conversely, low viscosity, while providing good flowability, can lead to coating cracking, floating of conductive and binder agents, and poor areal density consistency during drying. Therefore, improving peel strength without a significant increase in slurry viscosity is beneficial for process application, resulting in better electrode coating and improved battery performance.

[0029] The blended binder prepared in this invention, by adjusting the addition amounts of branched polymer A and linear polymer B to a suitable ratio, allows both branched polymer A and linear polymer B to fully exert their dispersing and adhesive effects. Simultaneously, the linear polymer B molecular chains block the intermolecular interactions of the "brush-like" branched polymer A molecules, preventing effective entanglement and linkage between linear polymer B molecular chains. Therefore, the slurry containing the blended binder of this invention, compared to traditional slurries using only low molecular weight PVDF, not only improves the peel strength of LFP electrode sheets but also achieves a lower viscosity than slurries using only low molecular weight PVDF. This is extremely beneficial for improving the coating effect of the electrode sheets and enhancing battery performance, achieving unexpected technical results. Furthermore, the slurry prepared in this invention, compared to slurries using only high molecular weight or modified PVDF, meets the slurry stability requirements (no sedimentation after 24 hours, no gelation after 24 hours) while maintaining a certain peel strength.

[0030] Preferably, the conductive agent is conductive carbon black (sp).

[0031] Preferably, the solid content of the positive electrode slurry is 60% to 66%.

[0032] More preferably, the solid content of the positive electrode slurry is 63%.

[0033] Preferably, the solvent is N-methylpyrrolidone.

[0034] A method for preparing the positive electrode slurry includes the following steps:

[0035] S1. Thoroughly mix the conductive agent and the positive electrode active material to obtain a mixture;

[0036] S2. Thoroughly mix the mixture obtained in step S1 with the binder to obtain the dry-mixed electrode material;

[0037] S3. Add solvent to the dry mixed electrode material obtained in step S2, mix thoroughly, and defoam. The resulting material is the positive electrode slurry.

[0038] Preferably, in steps S1, S2 and S3, the method of thorough mixing is stirring.

[0039] Specifically, in step S3, the thorough mixing operation is as follows: after adding the dry-mixed electrode material obtained in step S2 under low-speed stirring conditions, high-speed stirring is performed.

[0040] Preferably, the defoaming method is low-speed stirring.

[0041] Preferably, the low-speed stirring conditions are: stirring speed of 50-200 rpm and stirring time of 15-30 min.

[0042] In addition, the present invention also provides the application of intermixed binders or cathode slurries in the preparation of lithium iron phosphate batteries.

[0043] The present invention has the following beneficial effects:

[0044] 1. The slurry containing the blended binder of the present invention, compared with the traditional slurry using low molecular weight PVDF alone, can improve the peel force of LFP electrode sheets while further reducing the amount of binder used, thus reducing material costs. Moreover, the viscosity of this slurry is lower than that of the slurry using low molecular weight PVDF alone, which is beneficial to the coating effect of the electrode sheets and the improvement of battery performance.

[0045] 2. The slurry containing the blended binder of the present invention, compared with the slurry using high molecular weight or modified PVDF alone, can meet the slurry stability requirements (no sedimentation in 24 hours, no gelation in 24 hours) while ensuring a certain peeling force.

[0046] 3. The preparation method of the present invention is simple and easy to implement. Detailed Implementation

[0047] The following specific embodiments are used to further illustrate the present invention, but the embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in this technical field.

[0048] Unless otherwise specified, all reagents and materials used in the following examples are commercially available.

[0049] Example 1: A method for preparing an intermixed binder, slurry, and electrode sheet, and its performance testing.

[0050] 1. Preparation of a blended adhesive: Polymer A (a homopolymer of vinylidene fluoride) with a molecular weight of 700,000 and a branching degree of about 0.7% and copolymer B (a copolymer of vinylidene fluoride and hexafluoropropylene) with a molecular weight of 1.1 million are mixed at a weight ratio of 5:5 and stirred at 120 rpm for 30 min to obtain a blended adhesive.

[0051] 2. Dry mixing of electrode materials: Mix 336g of lithium iron phosphate (LFP) powder and 3.5g of conductive carbon (sp) powder, and stir at 120rpm for 120min. Then add 10.5g of the obtained intermixable binder, and stir at 120rpm for 30min to obtain electrode powder.

[0052] 3. Preparation of positive electrode slurry: Weigh 205.55g of nitrogen-methylpyrrolidone (NMP) and add electrode powder while stirring at 500rpm. After the powder is added, perform high-speed stirring at 2500rpm for 5h and low-speed defoaming at 300rpm for 0.5h to obtain the final slurry. Use a rotational viscometer to test the viscosity of the slurry.

[0053] 4. Preparation of electrode sheet: The slurry is coated onto the surface of a 13μm carbon-coated aluminum foil at a speed of 10mm / min. The coated aluminum foil is then transferred to a 100℃ forced-air drying oven and dried for 30min. After that, a second layer is coated and dried again to obtain the final electrode sheet.

[0054] 5. Peel strength test: The electrode sheet is rolled (compaction density controlled at 2.45~2.50g / cm³). 3 After rolling, the electrode sheet is cut into strips 25mm wide and 150mm long. The strips are then subjected to a 180° tensile peeling method and tested on a mechanical testing machine at a speed of 150mm / min. The average value of the travel distance from 20 to 120mm is taken as the final measured peeling force.

[0055] 6. Flowability test after standing for 24 hours: After the slurry has stood for 24 hours, pour it out in a beaker and observe whether the slurry can flow continuously. If it cannot flow continuously, it is considered to be gel.

[0056] Example 2: A method for preparing an intermixed binder, slurry, and electrode sheet, and its performance testing.

[0057] 1. Preparation of a blended adhesive: Polymer A (a homopolymer of vinylidene fluoride) with a molecular weight of 700,000 and a branching degree of about 0.7% and copolymer B (a copolymer of vinylidene fluoride and hexafluoropropylene) with a molecular weight of 1.7 million are mixed at a weight ratio of 3:7 and stirred at 120 rpm for 30 min to obtain a blended adhesive.

[0058] 2. Dry mixing of electrode materials: Mix 336g of LFP powder and 3.5g of SP powder, and stir at 120rpm for 120min. Then add 10.5g of the resulting intermixable binder, and stir at 120rpm for 30min to obtain electrode powder.

[0059] 3. Preparation of positive electrode slurry: Weigh 205.55g NMP and add electrode powder while stirring at 500rpm. After the powder is added, perform high-speed stirring at 2500rpm for 5h and low-speed defoaming at 300rpm for 0.5h to obtain the final slurry. Use a rotational viscometer to test the viscosity of the slurry.

[0060] 4. Preparation of electrode sheet: The slurry is coated onto the surface of a 13μm carbon-coated aluminum foil at a speed of 10mm / min. The coated aluminum foil is then transferred to a 100℃ forced-air drying oven and dried for 30min. After that, a second layer is coated and dried again to obtain the final electrode sheet.

[0061] 5. Peel strength test: The electrode sheet is rolled (compaction density controlled at 2.45~2.50g / cm³). 3 After rolling, the electrode sheet is cut into strips 25mm wide and 150mm long. The strips are then subjected to a 180° tensile peeling method and tested on a mechanical testing machine at a speed of 150mm / min. The average value of the travel distance from 20 to 120mm is taken as the final measured peeling force.

[0062] 6. Flowability test after standing for 24 hours: After the slurry has stood for 24 hours, pour it out in a beaker and observe whether the slurry can flow continuously. If it cannot flow continuously, it is considered to be gel.

[0063] Example 3: A method for preparing and testing the performance of a mixed binder, slurry, and electrode sheet.

[0064] 1. Preparation of a blended adhesive: Polymer A (a homopolymer of vinylidene fluoride) with a molecular weight of 700,000 and a branching degree of about 0.7% and copolymer B (a copolymer of vinylidene fluoride and hexafluoropropylene) with a molecular weight of 1.7 million are mixed at a weight ratio of 7:3 and stirred at 120 rpm for 30 min to obtain a blended adhesive.

[0065] 2. Dry mixing of electrode materials: Mix 336g of LFP powder and 3.5g of SP powder, and stir at 120rpm for 120min. Then add 10.5g of the resulting intermixable binder, and stir at 120rpm for 30min to obtain electrode powder.

[0066] 3. Preparation of positive electrode slurry: Weigh 205.55g NMP and add electrode powder while stirring at 500rpm. After the powder is added, perform high-speed stirring at 2500rpm for 5h and low-speed defoaming at 300rpm for 0.5h to obtain the final slurry. Use a rotational viscometer to test the viscosity of the slurry.

[0067] 4. Preparation of electrode sheet: The slurry is coated onto the surface of a 13μm carbon-coated aluminum foil at a speed of 10mm / min. The coated aluminum foil is then transferred to a 100℃ forced-air drying oven and dried for 30min. After that, a second layer is coated and dried again to obtain the final electrode sheet.

[0068] 5. Peel strength test: The electrode sheet is rolled (compaction density controlled at 2.45~2.50g / cm³). 3 After rolling, the electrode sheet is cut into strips 25mm wide and 150mm long. The strips are then subjected to a 180° tensile peeling method and tested on a mechanical testing machine at a speed of 150mm / min. The average value of the travel distance from 20 to 120mm is taken as the final measured peeling force.

[0069] 6. Flowability test after standing for 24 hours: After the slurry has stood for 24 hours, pour it out in a beaker and observe whether the slurry can flow continuously. If it cannot flow continuously, it is considered to be gel.

[0070] Example 4: A method for preparing an intermixed binder, slurry, and electrode sheet, and its performance testing.

[0071] 1. Preparation of a blended adhesive: Polymer A (a homopolymer of vinylidene fluoride) with a molecular weight of 700,000 and a branching degree of about 0.7% and homopolymer B (a homopolymer of vinylidene fluoride) with a molecular weight of 1.7 million are mixed at a weight ratio of 5:5 and stirred at 120 rpm for 30 min to obtain a blended adhesive.

[0072] 2. Dry mixing of electrode materials: Mix 336g of LFP powder and 3.5g of SP powder, and stir at 120rpm for 120min. Then add 10.5g of the resulting intermixable binder, and stir at 120rpm for 30min to obtain electrode powder.

[0073] 3. Preparation of positive electrode slurry: Weigh 205.55g NMP and add electrode powder while stirring at 500rpm. After the powder is added, perform high-speed stirring at 2500rpm for 5h and low-speed defoaming at 300rpm for 0.5h to obtain the final slurry. Use a rotational viscometer to test the viscosity of the slurry.

[0074] 4. Preparation of electrode sheet: The slurry is coated onto the surface of a 13μm carbon-coated aluminum foil at a speed of 10mm / min. The coated aluminum foil is then transferred to a 100℃ forced-air drying oven and dried for 30min. After that, a second layer is coated and dried again to obtain the final electrode sheet.

[0075] 5. Peel strength test: The electrode sheet is rolled (compaction density controlled at 2.45~2.50g / cm³). 3 After rolling, the electrode sheet is cut into strips 25mm wide and 150mm long. The strips are then subjected to a 180° tensile peeling method and tested on a mechanical testing machine at a speed of 150mm / min. The average value of the travel distance from 20 to 120mm is taken as the final measured peeling force.

[0076] 6. Flowability test after standing for 24 hours: After the slurry has stood for 24 hours, pour it out in a beaker and observe whether the slurry can flow continuously. If it cannot flow continuously, it is considered to be gel.

[0077] Example 5: A method for preparing an intermixed binder, slurry, and electrode sheet, and its performance testing.

[0078] 1. Preparation of a blended adhesive: Polymer A (a homopolymer of vinylidene fluoride) with a molecular weight of 700,000 and a branching degree of about 0.5% and copolymer B (a copolymer of vinylidene fluoride and hexafluoropropylene) with a molecular weight of 1.7 million are mixed at a weight ratio of 7:3 and stirred at 120 rpm for 30 min to obtain a blended adhesive.

[0079] 2. Dry mixing of electrode materials: Mix 336g of LFP powder and 3.5g of SP powder, and stir at 120rpm for 120min. Then add 10.5g of the resulting intermixable binder, and stir at 120rpm for 30min to obtain electrode powder.

[0080] 3. Preparation of positive electrode slurry: Weigh 205.55g NMP and add electrode powder while stirring at 500rpm. After the powder is added, perform high-speed stirring at 2500rpm for 5h and low-speed defoaming at 300rpm for 0.5h to obtain the final slurry. Use a rotational viscometer to test the viscosity of the slurry.

[0081] 4. Preparation of electrode sheet: The slurry is coated onto the surface of a 13μm carbon-coated aluminum foil at a speed of 10mm / min. The coated aluminum foil is then transferred to a 100℃ forced-air drying oven and dried for 30min. After that, a second layer is coated and dried again to obtain the final electrode sheet.

[0082] 5. Peel strength test: The electrode sheet is rolled (compaction density controlled at 2.45~2.50g / cm³). 3After rolling, the electrode sheet is cut into strips 25mm wide and 150mm long. The strips are then subjected to a 180° tensile peeling method and tested on a mechanical testing machine at a speed of 150mm / min. The average value of the travel distance from 20 to 120mm is taken as the final measured peeling force.

[0083] 6. Flowability test after standing for 24 hours: After the slurry has stood for 24 hours, pour it out in a beaker and observe whether the slurry can flow continuously. If it cannot flow continuously, it is considered to be gel.

[0084] Example 6: A method for preparing an intermixed binder, slurry, and electrode sheet, and its performance testing.

[0085] 1. Preparation of a blended adhesive: Polymer A (a homopolymer of vinylidene fluoride) with a molecular weight of 700,000 and a branching degree of about 1.5% and copolymer B (a copolymer of vinylidene fluoride and hexafluoropropylene) with a molecular weight of 1.7 million are mixed at a weight ratio of 7:3 and stirred at 120 rpm for 30 min to obtain a blended adhesive.

[0086] 2. Dry mixing of electrode materials: Mix 336g of LFP powder and 3.5g of SP powder, and stir at 120rpm for 120min. Then add 10.5g of the resulting intermixable binder, and stir at 120rpm for 30min to obtain electrode powder.

[0087] 3. Preparation of positive electrode slurry: Weigh 205.55g NMP and add electrode powder while stirring at 500rpm. After the powder is added, perform high-speed stirring at 2500rpm for 5h and low-speed defoaming at 300rpm for 0.5h to obtain the final slurry. Use a rotational viscometer to test the viscosity of the slurry.

[0088] 4. Preparation of electrode sheet: The slurry is coated onto the surface of a 13μm carbon-coated aluminum foil at a speed of 10mm / min. The coated aluminum foil is then transferred to a 100℃ forced-air drying oven and dried for 30min. After that, a second layer is coated and dried again to obtain the final electrode sheet.

[0089] 5. Peel strength test: The electrode sheet is rolled (compaction density controlled at 2.45~2.50g / cm³). 3 After rolling, the electrode sheet is cut into strips 25mm wide and 150mm long. The strips are then subjected to a 180° tensile peeling method and tested on a mechanical testing machine at a speed of 150mm / min. The average value of the travel distance from 20 to 120mm is taken as the final measured peeling force.

[0090] 6. Flowability test after standing for 24 hours: After the slurry has stood for 24 hours, pour it out in a beaker and observe whether the slurry can flow continuously. If it cannot flow continuously, it is considered to be gel.

[0091] Comparative Example 1: A method for preparing an intermixed binder, slurry, and electrode sheet, and its performance testing.

[0092] 1. Preparation of a blended adhesive: Polymer A (a homopolymer of vinylidene fluoride) with a molecular weight of 700,000 and a branching degree of about 0.7% and copolymer B (a copolymer of vinylidene fluoride and hexafluoropropylene) with a molecular weight of 2.1 million are mixed at a weight ratio of 5:5 and stirred at 120 rpm for 30 min to obtain a blended adhesive.

[0093] 2. Dry mixing of electrode materials: Mix 336g of LFP powder and 3.5g of SP powder, and stir at 120rpm for 120min. Then add 10.5g of the resulting intermixable binder, and stir at 120rpm for 30min to obtain electrode powder.

[0094] 3. Preparation of positive electrode slurry: Weigh 205.55g NMP and add electrode powder while stirring at 500rpm. After the powder is added, perform high-speed stirring at 2500rpm for 5h and low-speed defoaming at 300rpm for 0.5h to obtain the final slurry. Use a rotational viscometer to test the viscosity of the slurry.

[0095] 4. Preparation of electrode sheet: The slurry is coated onto the surface of a 13μm carbon-coated aluminum foil at a speed of 10mm / min. The coated aluminum foil is then transferred to a 100℃ forced-air drying oven and dried for 30min. After that, a second layer is coated and dried again to obtain the final electrode sheet.

[0096] 5. Peel strength test: The electrode sheet is rolled (compaction density controlled at 2.45~2.50g / cm³). 3 After rolling, the electrode sheet is cut into strips 25mm wide and 150mm long. The strips are then subjected to a 180° tensile peeling method and tested on a mechanical testing machine at a speed of 150mm / min. The average value of the travel distance from 20 to 120mm is taken as the final measured peeling force.

[0097] 6. Flowability test after standing for 24 hours: After the slurry has stood for 24 hours, pour it out in a beaker and observe whether the slurry can flow continuously. If it cannot flow continuously, it is considered to be gel.

[0098] Comparative Example 2: A method for preparing an adhesive, slurry, and electrode sheet, and its performance testing.

[0099] 1. Dry mixing of electrode materials: Mix 336g of LFP powder and 3.5g of SP powder, and stir at 120rpm for 120min. Then add 10.5g of polymer A (a homopolymer of vinylidene fluoride) with a molecular weight of 700,000 and a branching degree of about 0.7%, and stir at 120rpm for 30min to obtain electrode powder.

[0100] 2. Preparation of positive electrode slurry: Weigh 205.55g NMP and add electrode powder while stirring at 500rpm. After the powder is added, perform high-speed stirring at 2500rpm for 5h and low-speed defoaming at 300rpm for 0.5h to obtain the final slurry. Use a rotational viscometer to test the viscosity of the slurry.

[0101] 3. Preparation of electrode sheet: The slurry is coated onto the surface of a 13μm carbon-coated aluminum foil at a speed of 10mm / min. The coated aluminum foil is then transferred to a 100℃ forced-air drying oven and dried for 30min. After that, a second layer is coated and dried again to obtain the final electrode sheet.

[0102] 4. Peel strength test: The electrode sheet is rolled (compaction density controlled at 2.45~2.50g / cm³). 3 After rolling, the electrode sheet is cut into strips 25mm wide and 150mm long. The strips are then subjected to a 180° tensile peeling method and tested on a mechanical testing machine at a speed of 150mm / min. The average value of the travel distance from 20 to 120mm is taken as the final measured peeling force.

[0103] 5. Flowability test after standing for 24 hours: After the slurry has stood for 24 hours, pour it out in a beaker and observe whether the slurry can flow continuously. If it cannot flow continuously, it is considered to be gel.

[0104] Comparative Example 3: A method for preparing an adhesive, slurry, and electrode sheet, and its performance testing.

[0105] 1. Dry mixing of electrode materials: Mix 336g LFP powder and 3.5g SP powder, and stir at 120rpm for 120min. Then add 10.5g copolymer B (a copolymer of vinylidene fluoride and hexafluoropropylene) with a molecular weight of 1.7 million, and stir at 120rpm for 30min to obtain electrode powder.

[0106] 2. Preparation of positive electrode slurry: Weigh 205.55g NMP and add electrode powder while stirring at 500rpm. After the powder is added, perform high-speed stirring at 2500rpm for 5h and low-speed defoaming at 300rpm for 0.5h to obtain the final slurry. Use a rotational viscometer to test the viscosity of the slurry.

[0107] 3. Preparation of electrode sheet: The slurry is coated onto the surface of a 13μm carbon-coated aluminum foil at a speed of 10mm / min. The coated aluminum foil is then transferred to a 100℃ forced-air drying oven and dried for 30min. After that, a second layer is coated and dried again to obtain the final electrode sheet.

[0108] 4. Peel strength test: The electrode sheet is rolled (compaction density controlled at 2.45~2.50g / cm³). 3After rolling, the electrode sheet is cut into strips 25mm wide and 150mm long. The strips are then subjected to a 180° tensile peeling method and tested on a mechanical testing machine at a speed of 150mm / min. The average value of the travel distance from 20 to 120mm is taken as the final measured peeling force.

[0109] 5. Flowability test after standing for 24 hours: After the slurry has stood for 24 hours, pour it out in a beaker and observe whether the slurry can flow continuously. If it cannot flow continuously, it is considered to be gel.

[0110] Comparative Example 4: A method for preparing an adhesive, slurry, and electrode sheet, and its performance testing.

[0111] 1. Dry mixing of electrode materials: Mix 336g LFP powder and 3.5g SP powder, and stir at 120rpm for 120min. Then add 10.5g copolymer B (a copolymer of vinylidene fluoride and hexafluoropropylene) with a molecular weight of 1.1 million, and stir at 120rpm for 30min to obtain electrode powder.

[0112] 2. Preparation of positive electrode slurry: Weigh 205.55g NMP and add electrode powder while stirring at 500rpm. After the powder is added, perform high-speed stirring at 2500rpm for 5h and low-speed defoaming at 300rpm for 0.5h to obtain the final slurry. Use a rotational viscometer to test the viscosity of the slurry.

[0113] 3. Preparation of electrode sheet: The slurry is coated onto the surface of a 13μm carbon-coated aluminum foil at a speed of 10mm / min. The coated aluminum foil is then transferred to a 100℃ forced-air drying oven and dried for 30min. After that, a second layer is coated and dried again to obtain the final electrode sheet.

[0114] 4. Peel strength test: The electrode sheet is rolled (compaction density controlled at 2.45~2.50g / cm³). 3 After rolling, the electrode sheet is cut into strips 25mm wide and 150mm long. The strips are then subjected to a 180° tensile peeling method and tested on a mechanical testing machine at a speed of 150mm / min. The average value of the travel distance from 20 to 120mm is taken as the final measured peeling force.

[0115] 5. Flowability test after standing for 24 hours: After the slurry has stood for 24 hours, pour it out in a beaker and observe whether the slurry can flow continuously. If it cannot flow continuously, it is considered to be gel.

[0116] The data from the examples and comparative examples are summarized in Table 1.

[0117] Table 1. Peel strength and 24-hour slurry flowability results

[0118] Group Peel force / N / m 24-hour slurry status Slurry viscosity / cp Example 1 15.5 Gel-free 11000 Example 2 15.7 Gel-free 12900 Example 3 18.8 Gel-free 11460 Example 4 18.7 Gel-free 13440 Comparative Example 1 19.0 gel 18920 Comparative Example 2 15.7 Gel-free 13200 Comparative Example 3 21.3 Gel-free 21300 Comparative Example 4 14.1 Gel-free 10000

[0119] As shown in Table 1, the blended polymers A and B have suitable molecular weights. Based on this, by adjusting the mixing ratio, the dispersing effect of branched polymer A and the adhesive effect of linear polymer B can be fully utilized. At the same time, a suitable ratio (as in Examples 3 and 4) avoids entanglement between macromolecular chains, further reducing the viscosity of the slurry and obtaining a slurry with high peel strength and low viscosity, achieving a good coating effect. The experimental results of Examples 5 and 6 are basically consistent with those of Example 3, both yielding slurries with high peel strength and low viscosity. However, in blended binders, when the molecular weight of polymer B is too large (as in Comparative Example 1), the molecular chain regions of polymer B become more disordered and coiled during the standing process, encapsulating solid particles, restricting particle movement, and causing the slurry to gel.

[0120] As shown in Comparative Example 2, when branched polymer A with a molecular weight of 700,000 is used alone as a binder, its low molecular weight results in low peel strength of the prepared slurry. Furthermore, due to the limitations of current emulsion polymerization processes, it is difficult to produce polymer products with a molecular weight of over 900,000 that are commercially viable. Therefore, it is difficult to improve the electrode peel strength by increasing the molecular weight of polymers prepared through emulsion polymerization.

[0121] As shown in Comparative Example 4, when polymer B with a molecular weight of 1.1 million is used alone as a binder, its molecular weight is not large enough, so there is no advantage in peel strength. If its molecular weight is further increased, an excessively large molecular weight (2.1 million, Comparative Example 3) can easily cause the prepared slurry to gel. When a more suitable molecular weight (1.7 million, Comparative Example 4) is selected, the viscosity of the prepared slurry is still higher than that of polymer A with a low molecular weight. Therefore, using polymers B with different molecular weights alone has limitations in increasing the solid content of the slurry in actual production.

[0122] In Examples 1-4, if the polymer B used contains polar groups, the polar groups - carboxyl groups and the carbon coating layer on the surface of lithium iron phosphate form a strong force, resulting in the gel of the prepared slurry. Therefore, polymer B containing polar groups is not suitable as a binder for high solids content lithium iron phosphate slurry.

[0123] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A positive electrode slurry, characterized in that, It includes 55-65 parts of positive electrode active material, 0.2-0.8 parts of conductive agent, 0.8-4 parts of binder, and 34-40 parts of solvent. The adhesive is a blended adhesive; The blended adhesive comprises polymer A and polymer B; Wherein, polymer A is a homopolymer of vinylidene fluoride, the molecular chain contains side chains, and the molecular weight is 200,000 to 900,000; The side chain branching degree of polymer A is 0.5% to 1.5%; The polymer A was prepared by emulsion polymerization. The polymer B is a homopolymer of vinylidene fluoride, or a copolymer of vinylidene fluoride and hexafluoropropylene, with no side chains or polar groups in the molecular chain and a molecular weight of 1.1 million to 2 million. The positive electrode active material is lithium iron phosphate (LiFePO4). The weight ratio of polymer A to polymer B is 5:5 to 7:

3.

2. The positive electrode slurry according to claim 1, characterized in that, The polymer B is prepared by suspension polymerization.

3. The method for preparing the positive electrode slurry according to any one of claims 1 or 2, characterized in that, Includes the following steps: S1. Thoroughly mix the conductive agent and the positive electrode active material to obtain a mixture; S2. Thoroughly mix the mixture obtained in step S1 with the binder to obtain the dry-mixed electrode material; S3. Add solvent to the dry mixed electrode material obtained in step S2, mix thoroughly, and defoam. The resulting material is the positive electrode slurry.

4. The application of the positive electrode slurry according to any one of claims 1 or 2 in the preparation of lithium iron phosphate batteries.