A positive electrode slurry and a method for preparing the same, a positive electrode

Abstract

The application provides a positive electrode paste and a preparation method thereof, and a positive electrode. The positive electrode paste comprises solid substances, the solid substances comprise a positive electrode active material and a conductive agent, the solid substances further comprise a composite binder, the composite binder comprises a skeleton phase binder and a dispersed phase binder, the mass ratio of the skeleton phase binder to the dispersed phase binder is (2:1) to (6:1), and the composite binder accounts for 0.8% to 1.5% of the mass of the solid substances; and the total solid content of the positive electrode paste is not less than 70%. The positive electrode paste reduces the friction between particles by using the interfacial activity of the dispersed phase binder, realizes the slurry preparation with a solid content of more than 70%, constructs a double-layer network in which the dispersed phase binder flexibly covers the rigid bridging of the composite skeleton phase binder, has good dispersion effect and significantly improves the peel strength of the electrode sheet, reduces the solvent consumption by about 30%, greatly shortens the coating and drying time and reduces the energy consumption, simultaneously reduces the binder consumption, improves the proportion of active substances and brings the improvement of the energy density.

Description

Technical Field

[0001] This invention relates to the field of battery manufacturing, specifically to a positive electrode slurry and its preparation method, and a positive electrode. Background Technology

[0002] Currently, PVDF (polyvinylidene fluoride) is commonly used as a binder and NMP (N-methylpyrrolidone) as a solvent in the preparation of cathode slurries for lithium-ion batteries. However, cathode slurries prepared using this system still have the following problems: First, traditional cathode slurry preparation mainly relies on high molecular weight PVDF to provide binding force. However, PVDF molecular chains are semi-curled spherical in NMP solvent. When the solid content increases to over 70%, the interparticle spacing decreases sharply, and the PVDF molecular chains are prone to physical entanglement, leading to a non-Newtonian fluid "shear thickening" phenomenon in the slurry. This makes the slurry unpumpable or uncoated, and forcibly reducing the viscosity usually requires adding a large amount of NMP, severely limiting production efficiency.

[0003] Secondly, in existing technologies, the surface energies of conductive agents (such as CNTs) and active material particles differ significantly. A single PVDF system cannot balance the wettability of both. CNTs are prone to "self-aggregation," while the surface of active materials often contains residual alkali (such as LiOH / Li2CO3 on the surface of high-nickel materials), which can induce the defluorination reaction of PVDF, causing the slurry to gel during storage.

[0004] Third, in pursuit of high energy density, the compaction density of electrode sheets is constantly increasing. As a semi-crystalline polymer, PVDF has a high modulus, resulting in high internal stress in the electrode sheet after rolling. During battery charge-discharge cycles, the volume expansion / contraction of active material particles can cause fatigue cracks in the PVDF skeleton, ultimately leading to electrochemical contact failure, which is particularly severe at ultra-low binder content (<1.5%). Summary of the Invention

[0005] The first objective of this invention is to provide a positive electrode slurry that addresses the shortcomings of existing positive electrode slurries in maintaining and increasing solids content, reducing binder usage, ensuring the peel strength of the electrode sheet, the processing viscosity and long-term stability of the slurry, and improving the flexibility of high-voltage solid electrodes.

[0006] A second objective of this invention is to provide a method for preparing a positive electrode slurry, for use in preparing the aforementioned positive electrode slurry.

[0007] A third objective of the present invention is to provide a positive electrode comprising the aforementioned positive electrode slurry.

[0008] To achieve the aforementioned first objective, the present invention provides a positive electrode slurry comprising a solid substance, the solid substance including a positive electrode active material and a conductive agent, the solid substance further comprising a composite binder, the composite binder comprising a framework phase binder and a dispersed phase binder, wherein the mass ratio of the framework phase binder to the dispersed phase binder is (2:1) to (6:1), and the composite binder accounts for 0.8% to 1.5% of the mass of the solid substance; the total solid content of the positive electrode slurry is not less than 70%, preferably, the total solid content of the positive electrode slurry is 70% to 85%.

[0009] As can be seen from the above scheme, this scheme includes both a framework phase binder and a dispersed phase binder. The dispersed phase binder undertakes the core functions of interface coating, high solids content viscosity reduction, electrode toughening, and inhibition of slurry gelation. The framework phase binder undertakes the core functions of long-range bridging, structural bonding, and interface stabilization, achieving ultra-high solids content slurry preparation of over 70%, while reducing the total binder usage to below 1.5%, balancing high peel strength, excellent flexibility, and long-cycle stability of the electrode, and simultaneously reducing NMP solvent usage and drying energy consumption. The mass ratio of the framework phase binder to the dispersed phase binder is (2:1) to (6:1), and the composite binder accounts for 0.8% to 1.5% of the mass of the solid matter. This avoids excessive dispersed phase binder content, which would significantly reduce the film modulus after slurry drying and prevent the formation of rigid long-range physical interlocking between active material particles. The electrode is prone to excessive plastic deformation during rolling, leading to porosity collapse and severely affecting electrolyte wetting. At the same time, it is important to prevent excessively high dispersed phase binder content, which would form an excessively thick insulating layer on the particle surface, blocking electron transport channels and causing a sharp increase in the battery's DC internal resistance (DCR). Conversely, insufficient dispersed phase binder content would prevent complete coverage of the high-energy sites of the conductive agent and active material, failing to effectively shield the van der Waals forces between particles. This would result in the yield stress of high-solids-content (>70%) slurries not being reduced, leading to a recurrence of the "jelly-like" phenomenon.

[0010] A further option is that the framework phase binder is selected from at least one of PVDF, PVDF-HFP, PVDF-CTFE, and PTFE-PVDF; and the dispersed phase binder is selected from at least one of HNBR, XNBR, NBR, and PAA-PMA.

[0011] As can be seen from the above scheme, the molecular chains of the dispersed phase binder have strong polar coordination groups, which can form strong adsorption with hydroxyl groups and transition metal ions on the surface of active materials. It has excellent NMP solvent solubility, can be formulated into a low viscosity uniform solution, and has excellent electrochemical stability at high voltage (above 4.2V), is resistant to electrolyte swelling, and can improve the bending resistance of the electrode. The framework phase binder has a high molecular weight linear structure, which can form a continuous and stable solution in NMP. It has excellent electrochemical stability, high voltage oxidation resistance, and electrolyte swelling resistance. It also has excellent interfacial adhesion with the current collector (aluminum foil), does not chemically react with the dispersed phase binder, and has good compatibility.

[0012] A further proposed approach is to use PVDF as the framework phase binder and HNBR as the dispersed phase binder; wherein the weight-average molecular weight of PVDF is [missing information]. The degree of hydrogenation of HNBR is 95%~99.9%, and the acrylonitrile content of HNBR is 25%~45%. The weight average molecular weight of HNBR is... .

[0013] As can be seen from the above solutions, the combined use of PVDF and HNBR can simultaneously achieve high solids content, reduced viscosity, high adhesion, high voltage stability, electrode flexibility, industrialization cost, and adaptability to all scenarios.

[0014] A further option is that the positive electrode active material is selected from at least one of high-nickel ternary materials, lithium iron phosphate, lithium-rich manganese-based materials, or lithium manganese iron phosphate; the conductive agent is selected from one-dimensional conductive materials and / or zero-dimensional conductive materials.

[0015] A further proposed approach is that, by mass percentage, the solid material comprises 97.0% to 98.5% positive electrode active material, 0.5% to 1.5% conductive agent, and 0.8% to 1.5% composite binder.

[0016] As can be seen from the above scheme, the cathode slurry prepared with the above-mentioned proportion of solid materials utilizes the interfacial activity of the dispersed phase binder to reduce interparticle friction, achieving an ultra-high solids content of over 70%. A two-layer network is constructed, with the dispersed phase binder flexibly coating the composite framework binder and rigidly bridging it, resulting in good dispersion and significantly improved electrode peel strength. Solvent usage is reduced by approximately 30%, significantly shortening coating and drying time and reducing energy consumption. Simultaneously, reducing binder usage and increasing the proportion of active material leads to an increase in energy density.

[0017] To achieve the second objective mentioned above, the present invention provides a method for preparing a positive electrode slurry. The method comprises the following steps: S1: dissolving a dispersed phase binder in a solvent, then adding a conductive agent and a portion of the positive electrode active material to the dispersed phase binder solution, and stirring to disperse; S2: adding the remaining positive electrode active material to the mixture obtained in step S1, stirring until homogeneous, and then kneading; S3: adding a framework phase binder solution to the kneaded material obtained in step S2, and kneading in a mixer; S4: adding a solvent to the mixture obtained in step S3 until the viscosity and solid content of the mixture meet preset requirements, and then performing defoaming treatment to obtain the positive electrode slurry.

[0018] As can be seen from the above scheme, firstly, the strong adsorption of the nitrile (-CN) group on the side chain of the dispersed phase binder to the hydroxyl groups and metal ions on the surface of the positive electrode active material is utilized to form an ultra-thin protective film on the surface of the active material and conductive agent particles. This prevents particle agglomeration and lays the foundation for subsequent interfacial bonding. Subsequently, the strong kneading under the high solids content system generates a large shear force, which can effectively break up the agglomerates of the positive electrode active material, making the active material particles uniformly dispersed. At the same time, it promotes the uniform distribution of the conductive agent in the gaps between the active material particles, constructing a preliminary conductive network. Next, since the surface of the positive electrode active material and conductive agent particles has been pre-coated by the dispersed phase binder and occupies some active sites, after the addition of the framework phase binder solution, its molecular chains are more easily distributed in the gaps between particles, forming a "point-to-point" long-range bridging effect, firmly connecting the dispersed particles, and improving the overall bonding strength and network continuity. Finally, vacuum degassing is performed to avoid problems such as pinholes and defects after the electrode is dried, ensuring the quality of the electrode.

[0019] A further option is that, in step S1, the viscosity of the dispersed phase binder solution is 500 mPa·s to 1000 mPa·s; and in step S3, the solid content of the skeleton phase binder solution is 10% to 15%.

[0020] A further option is to add positive electrode active material in step S1 at 10% to 20% of the total mass of solid matter.

[0021] A further proposed solution is that the kneading process in step S2 is carried out under the condition that the solid content is above 85%, the kneading temperature is controlled at 40℃~60℃, the kneading speed is 300r / min~500r / min, and the kneading time is 60min~90min.

[0022] To achieve the third objective mentioned above, the present invention provides a positive electrode, which includes a positive electrode slurry as described in any of the above embodiments. Detailed Implementation

[0023] The present invention provides a positive electrode slurry comprising solid matter, wherein the total solid content of the positive electrode slurry is not less than 70%, preferably, the total solid content of the positive electrode slurry is 70% to 85%.

[0024] By mass percentage, the solid material comprises 97.0% to 98.5% positive electrode active material, 0.5% to 1.5% conductive agent, and 0.8% to 1.5% composite binder.

[0025] Specifically, the positive electrode active material is selected from at least one of high-nickel ternary materials (NCM), lithium iron phosphate (LFP), lithium-rich manganese-based materials, or lithium manganese iron phosphate (LMFP).

[0026] The conductive agent is selected from one-dimensional conductive materials and / or zero-dimensional conductive materials. Preferably, the conductive agent comprises one-dimensional conductive materials and zero-dimensional conductive materials, wherein the one-dimensional conductive material is preferably carbon nanotubes (CNTs) with an aspect ratio > 500, and the zero-dimensional conductive material is conductive carbon black SP.

[0027] The composite binder comprises a skeleton phase binder and a dispersed phase binder, wherein the mass ratio of the skeleton phase binder to the dispersed phase binder is (2:1) to (6:1). The skeleton phase binder is selected from at least one of PVDF (polyvinylidene fluoride), PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene copolymer), PVDF-CTFE (polyvinylidene fluoride-trifluorochloroethylene copolymer), and PTFE-PVDF (polytetrafluoroethylene-vinylidene fluoride copolymer); the dispersed phase binder is selected from at least one of HNBR (hydrogenated nitrile butadiene rubber), XNBR (carboxylated nitrile butadiene rubber), NBR (nitrile butadiene rubber), and PAA-PMA (polyacrylate copolymer). The weight-average molecular weight of PVDF is [missing information]. The weight-average molecular weight of PVDF-HFP is The hexafluoropropylene monomer content is 3%~8%; the weight average molecular weight of PVDF-CTFE is... The trifluoroethylene monomer content is 2%~5%; the weight average molecular weight of PTFE-PVDF is... The tetrafluoroethylene monomer content is 5%~10%. The hydrogenation degree of HNBR is 95%~99.9%, the acrylonitrile content is 25%~45%, and the weight average molecular weight of HNBR is... XNBR has an acrylonitrile content of 25%–45% and a carboxyl content of 1%–5%. The weight-average molecular weight of XNBR is [missing information]. The acrylonitrile content of NBR is 30%~45%, and the weight average molecular weight of NBR is... The weight-average molecular weight of PAA-PMA is Furthermore, the glass transition temperature is ≤20℃. Preferably, the framework phase binder is PVDF, and the dispersed phase binder is HNBR.

[0028] The positive electrode slurry also includes a solvent, preferably NMP.

[0029] This invention provides a method for preparing the above-mentioned positive electrode slurry, the method comprising the following steps: S1: Add an appropriate amount of solvent to the dispersed phase binder and stir at room temperature until the dispersed phase binder is completely dissolved to obtain a dispersed phase binder solution with a viscosity of 500 mPa·s to 1000 mPa·s. Then, add a conductive agent and a portion of positive electrode active material to the dispersed phase binder solution. The added positive electrode active material accounts for 10% to 20% of the total mass of solid matter. Disperse the solution in a high-speed disperser at a speed of 1000 r / min to 1500 r / min for 30 min to 60 min. S2: Add the remaining positive electrode active material to the mixture obtained in step S1, stir evenly, and then knead under the condition that the solid content is above 85%. The kneading temperature is controlled at 40℃~60℃, the kneading speed is 300r / min~500r / min, and the kneading time is 60min~90min. S3: Add the skeleton phase binder solution to the kneaded material obtained in step S2. The skeleton phase binder solution is obtained by adding an appropriate amount of solvent to the skeleton phase binder in advance and stirring at room temperature until the skeleton phase binder is completely dissolved. The solid content of the skeleton phase binder solution is 10%~15%. Knead in a mixer at a speed of 200r / min~300r / min for 40min~60min. S4: The mixture obtained in step S3 is added to a solvent and stirred to adjust the viscosity of the slurry to 2000 mPa·s~5000 mPa·s (25℃) and the solid content to 70%~85%. Then the slurry is transferred to a vacuum defoamer and stirred and defoamed at a speed of 50 r / min~100 r / min for 20 min~30 min under a vacuum of -0.08 MPa to -0.1 MPa to remove bubbles inside the slurry and obtain the positive electrode slurry.

[0030] The following detailed description of the solution is provided in conjunction with specific embodiments.

[0031] Example 1 The positive electrode slurry of Example 1 comprises solid matter and solvent. By mass percentage, the solid matter includes 97.5% LiNi. 0.8 Co 0.1 Mn 0.1 The composition includes O2, 1.3% CNT, 0.3% HNBR, and 0.9% PVDF. The solvent is NMP, and the solid content of the positive electrode slurry is 78%.

[0032] The preparation method of the positive electrode slurry in Example 1 includes the following steps: S1: Dissolve HNBR in NMP to form an HNBR adhesive solution with a viscosity of 800 mPa·s. Add CNTs and 15% LiNi to the HNBR adhesive solution. 0.8 Co 0.1 Mn 0.1 O2 was dispersed in a high-speed disperser at a speed of 1200 r / min for 45 min; S2: Add the remaining positive electrode active material to the mixture obtained in step S1, stir evenly, and then knead under the condition of 86% solid content. The kneading temperature is controlled at 50℃, the kneading speed is 400r / min, and the kneading time is 75min. S3: Add the skeleton phase binder solution to the kneaded material obtained in step S2. The skeleton phase binder solution is obtained by adding an appropriate amount of NMP to PVDF in advance and stirring at room temperature until PVDF is completely dissolved. The solid content of the PVDF solution is 12%. Knead it in a mixer at a speed of 250r / min for 50min. S4: NMP is added to the mixture obtained in step S3, and the mixture is stirred to adjust the viscosity of the slurry to 2000 mPa·s~5000 mPa·s (25℃) and the solid content is 78%. Then the slurry is transferred to a vacuum defoamer and stirred at 80 r / min for 25 min under a vacuum of -0.089 MPa to remove bubbles inside the slurry, thus obtaining the positive electrode slurry of Example 1.

[0033] Example 2 The positive electrode slurry of Example 2 comprises solid matter and solvent. By mass percentage, the solid matter includes 98.2% LiNi. 0.8 Co 0.1 Mn 0.1 The composition includes O2, 0.8% CNT, 0.17% HNBR, and 0.83% PVDF. The solvent is NMP, and the solid content of the positive electrode slurry is 80%.

[0034] The preparation process of Example 2 is exactly the same as that of Example 1, the only difference being the content of the added substances.

[0035] Example 3 The positive electrode slurry of Example 3 comprises solid matter and solvent. By mass percentage, the solid matter includes 97.5% LiNi. 0.8 Co 0.1 Mn 0.1 The composition includes O2, 1.3% CNT, 0.3% HNBR, and 0.9% PVDF-HFP. The solvent is NMP, and the solid content of the positive electrode slurry is 78%.

[0036] The preparation process of Example 3 is exactly the same as that of Example 1, except that the added substances are different and the slurry viscosity is different.

[0037] Example 4 The positive electrode slurry of Example 4 comprises solid matter and solvent. By mass percentage, the solid matter includes 97.5% LiNi. 0.8 Co 0.1 Mn 0.1 The composition includes O2, 1.3% CNT, 0.3% HNBR, and 0.9% PVDF-CTFE. The solvent is NMP, and the solid content of the positive electrode slurry is 78%.

[0038] The preparation process of Example 4 is exactly the same as that of Example 1, except that the added substances are different and the slurry viscosity is different.

[0039] Example 5 The positive electrode slurry in Example 5 contains only solid matter, which, by mass percentage, comprises 97.5% LiNi. 0.8 Co 0.1 Mn 0.1 O2, 1.3% CNT, 0.3% HNBR and 0.9% PTFE-HNBR.

[0040] The preparation process of Example 5 is a dry mixing powder, specifically including: S1 HNBR Dry Pre-coating: HNBR powder, CNT, SP and some positive electrode active material are added to a high-speed mixer, the temperature is controlled at 25℃~30℃, premixed at 300r / min for 5min, and then sheared at 1200r / min for 60min to obtain pre-coated composite powder.

[0041] S2 All-Solid Kneading and Dispersion: The pre-coated powder and the remaining positive electrode active material are added to a twin-screw kneader, heated to 50°C, and kneaded at 400 r / min for 90 min, with vacuum degassing for 5 min every 30 min during the process.

[0042] S3 PTFE-PVDF framework phase intervention: Maintain the temperature at 50℃, slowly add PTFE-PVDF powder, continue kneading at 250r / min for 40min, and then vacuum degas for 10min to obtain the dry cathode mixed powder of Example 5.

[0043] Example 6 The positive electrode slurry of Example 6 comprises solid matter and solvent. By mass percentage, the solid matter includes 97.5% LiNi. 0.8 Co 0.1 Mn 0.1The composition includes O2, 1.3% CNT, 0.3% XNBR, and 0.9% PVDF. The solvent is NMP, and the solid content of the positive electrode slurry is 78%.

[0044] The preparation process of Example 6 is exactly the same as that of Example 1, except that the added substances are different and the slurry viscosity is different.

[0045] Example 7 The positive electrode slurry of Example 7 comprises solid matter and solvent. By mass percentage, the solid matter includes 97.5% LiNi. 0.8 Co 0.1 Mn 0.1 The composition includes O2, 1.3% CNT, 0.24% NBR, and 0.96% PVDF. The solvent is NMP, and the solid content of the positive electrode slurry is 75%.

[0046] The preparation process of Example 7 is exactly the same as that of Example 1, except that the added substances are different, the content is different, and the viscosity of the slurry is different.

[0047] Example 8 The positive electrode slurry of Example 8 comprises solid matter and solvent. By mass percentage, the solid matter includes 97.5% LiNi. 0.8 Co 0.1 Mn 0.1 The composition includes O2, 1.3% CNTs, 0.24% PAA-PMA, and 0.96% PVDF. The solvent is NMP, and the solid content of the positive electrode slurry is 76%.

[0048] The preparation process of Example 8 is exactly the same as that of Example 1, except that the added substances are different, the content is different, and the viscosity of the slurry is different.

[0049] Comparative Example 1 The cathode slurry of Comparative Example 1 contains solids and solvents. By mass percentage, the solids comprise 96% LiNi. 0.8 Co 0.1 Mn 0.1 O2, 1.5% CNT and 2.5% PVDF. Solvent is NMP, and the solid content of the positive electrode slurry is 65%.

[0050] The preparation method of the positive electrode slurry in Comparative Example 1 is as follows: PVDF, CNT, and LiNi are mixed. 0.8 Co 0.1 Mn 0.1 O2 and all NMP were added to the disperser at the same time and dispersed at 1000 r / min for 120 min. Then, the mixture was defoamed under vacuum and the viscosity was adjusted to obtain the positive electrode slurry of Comparative Example 1.

[0051] Comparative Example 2 The cathode slurry of Comparative Example 2 contains solids and solvents. By mass percentage, the solids consist of 97.5% LiNi. 0.8 Co 0.1 Mn 0.1 O2, 1.3% CNT and 1.2% PVDF. Solvent is NMP, and the solid content of the positive electrode slurry is 75%.

[0052] The method for preparing the positive electrode slurry in Comparative Example 2 is as follows: PVDF, CNT, and LiNi are mixed. 0.8 Co 0.1 Mn 0.1 O2 and all NMP were added to the disperser at the same time and dispersed at 1000 r / min for 120 min. Then, the mixture was defoamed under vacuum and the viscosity was adjusted to obtain the positive electrode slurry of Comparative Example 2.

[0053] Comparative Example 3 The cathode slurry of Comparative Example 3 contains solids and solvents. By mass percentage, the solids consist of 97.5% LiNi. 0.8 Co 0.1 Mn 0.1 The composition includes O2, 1.3% CNT, 0.3% HNBR, and 0.9% PVDF. The solvent is NMP, and the solid content of the positive electrode slurry is 78%.

[0054] The preparation method of the positive electrode slurry in Comparative Example 3 includes the following steps: HNBR adhesive, PVDF adhesive, CNT, LiNi 0.8 Co 0.1 Mn 0.1 O2 and all NMP were simultaneously added to a kneader and kneaded at 50°C and 400 r / min for 120 min. Vacuum degassing was then performed, and the viscosity was adjusted to obtain the positive electrode slurry of Comparative Example 3. The HNBR and PVDF solutions were the same as in Complete Example 1.

[0055] Comparative Example 4 The cathode slurry of Comparative Example 4 contains solids and solvents. By mass percentage, the solids consist of 97.5% LiNi. 0.8 Co 0.1 Mn 0.1 The composition includes O2, 1.3% CNT, 0.6% HNBR, and 0.6% PVDF. The solvent is NMP, and the solid content of the positive electrode slurry is 78%.

[0056] The preparation process of Comparative Example 4 is exactly the same as that of Example 1, except that the content of the added substances is different and the viscosity of the slurry is different.

[0057] Comparative Example 5 The cathode slurry of Comparative Example 5 contains solids and solvents. By mass percentage, the solids consist of 97.5% LiNi. 0.8 Co 0.1 Mn 0.1 The composition includes O2, 1.3% CNT, 0.133% HNBR, and 1.067% PVDF. The solvent is NMP, and the solid content of the positive electrode slurry is 78%.

[0058] The preparation process of Comparative Example 5 is exactly the same as that of Example 1, except that the content of the added substances is different and the viscosity of the slurry is different.

[0059] The positive electrode slurries from Examples 1-4, 6-8, and Comparative Examples 1-5 were uniformly coated onto the surface of a 12 μm carbon-coated aluminum foil. After drying, the corresponding positive electrode sheets were obtained by roll pressing. Specifically, this included: The positive electrode slurries prepared in Examples 1-4, Examples 6-8 and Comparative Examples 1-5 were filtered through a 200-mesh filter to remove large particulate impurities, and then uniformly coated onto the surface of an aluminum foil current collector using a precision coating machine.

[0060] The coating process control speed was set to 1 m / min. The coated wet electrode sheets were then placed in a three-stage hot air oven for gradient drying: the first stage was at 80℃ for 3 minutes; the second stage was at 110℃ for 4 minutes; and the third stage was at 90℃ for 3 minutes. During the drying process, the airflow velocity inside the oven was controlled at 1.5 m / s to ensure uniform evaporation of the NMP solvent and to prevent skin formation on the electrode surface or the formation of internal bubbles.

[0061] After drying, dry electrode sheets are obtained and then fed into a double-roll precision roller press for rolling. The rolling temperature is room temperature, the roller speed is set to 1.0 m / min, and the linear pressure is controlled at 180 kN / m. After rolling, the compaction density of the electrode sheet reaches 3.45 ± 0.05 g / cm³.

[0062] After rolling, the electrode sheets are left to stand for 4 hours in a constant temperature and humidity environment (temperature 60±2℃, relative humidity 45%-55%) to release internal stress, and then cut into positive electrode sheets of the appropriate size according to subsequent testing requirements.

[0063] The dry-process positive electrode powder of Example 5 was evenly spread on the surface of a 12μm carbon-coated aluminum foil, and then hot-rolled stepwise to a thickness of 110μm (compacted density 3.4g / cm³) under the conditions of 120℃, linear pressure 220kN / m and roller speed 0.8m / min. After cooling, it was cut to obtain the positive electrode sheet of Example 5.

[0064] The following performance tests were performed on the positive electrode slurries of Examples 1-8 and Comparative Examples 1-5: 1) Slurry viscosity test Take approximately 50 mL of the prepared positive electrode slurry and let it stand for 10 minutes to eliminate air bubbles, ensuring that the sample has no obvious agglomeration or stratification. Using a rotational viscometer, immerse the rotor in the slurry, ensuring that the rotor is completely covered by the slurry without touching the container. Start the viscometer and record the viscosity value after the reading stabilizes (approximately 30 seconds). Perform the test in triplicate for each sample and take the arithmetic mean as the final result. The test temperature is 25 ± 0.5℃, and the shear rate is 12 s. -1 .

[0065] 2) Slurry stability (viscosity change rate over 24 hours) Take approximately 100 mL of the prepared positive electrode slurry and measure its initial viscosity using a rotational viscometer, denoted as η0. Then, place the sample vial in a constant temperature environment and let it stand for 24 hours without stirring or shaking. After 24 hours, gently shake the sample (avoiding air bubbles) and measure the viscosity again, denoted as η. 24 .

[0066] A change rate of <5% indicates excellent stability, 5%~10% indicates good stability, and >10% indicates poor stability.

[0067] 3) Coating performance test When using a coating machine, observe the coating state of the slurry and record whether there are defects such as particles, pinholes, streaks, or stringiness.

[0068] Use a micrometer to randomly select 10 points on the coated electrode to measure the thickness. Electrode thickness deviation: (maximum thickness - minimum thickness) / average thickness × 100%. The smaller the electrode thickness deviation, the more uniform the electrode coating and the better the slurry processing performance. 4) Peel strength test The electrode to be tested was cut into samples with a width of 15 mm and a length of 100 mm. The active material side of the electrode was adhered to the stainless steel plate using double-sided tape, ensuring a firm and air-free bond. The free end of the sample was clamped in the upper fixture of the tensile testing machine, and the stainless steel plate was clamped in the lower fixture. The testing machine was started, and a 180° peel test was performed. The force curve during the peel process was recorded. Five samples were tested in parallel for each sample, and the average value of the stable segment of the force curve was taken as the peel strength. The test temperature was 25 ± 2℃, and the tensile speed was 50 mm / min.

[0069] 5) Electrode bending radius Cut the electrode to be tested into a sample with a width of 20 mm and a length of 100 mm. Prepare a cylindrical shaft bending tester and cylindrical shafts of different diameters (1 mm, 2 mm, 3 mm, 4 mm, 5 mm).

[0070] Starting with the cylindrical shaft with the largest diameter (5 mm), bend the sample 180° along the cylindrical shaft. Observe the electrode surface with a magnifying glass to see if cracks or active material detachment appear. If no cracks appear, repeat the test with cylindrical shafts of smaller diameters until cracks appear on the electrode. Record the smallest diameter of the cylindrical shaft when the electrode is crack-free; this is the electrode bending radius. The test temperature is 25±2℃.

[0071] 6) Battery internal resistance The electrode to be tested was cut into a circular piece with a diameter of 14 mm as the positive electrode; a lithium metal sheet was used as the negative electrode, Celgard2400 was used as the separator, and the electrolyte was 1 mol / L LiPF6 / EC+DMC+EMC (volume ratio 1:1:1). The CR2032 coin cell was assembled in an argon atmosphere glove box.

[0072] The test temperature was 25±2℃, and the battery was tested after being left to stand for 24 hours.

[0073] Charge the battery at a constant current rate of 0.2C to 4.3V, then charge at a constant voltage rate until the current is less than 0.05C. Let it stand for 30 minutes and record the battery open-circuit voltage (V0); Discharge at a constant current of 1C for 10s and record the voltage (V1) at 10s. Calculate the internal resistance of the battery .

[0074] 7) 1C cycle retention rate (500 cycles) The battery was charged and discharged three times at a rate of 0.2C, and the capacity of the third discharge was recorded as the initial capacity (C0). Charge at a constant current rate of 0.2C to 4.3V, then charge at a constant voltage rate until the current is less than 0.05C; let stand for 10 minutes; discharge at a constant current rate of 1C to 3.0V; let stand for 10 minutes. This completes one cycle. Repeat the above cycle 500 times, and record the discharge capacity (C) on the 500th cycle. 500 ) calculate ; The test temperature was 25±2℃, and the charge / discharge voltage range was 3.0V~4.3V.

[0075] The test results are shown in Tables 1 and 2 below.

[0076] Table 1

[0077] Table 2

[0078] It should be noted that vacuum degassing involves slow stirring, which will increase the viscosity of the slurry. The data in Table 1 are the slurry viscosity after vacuum degassing, and the values ​​of 2000 mPa·s to 5000 mPa·s are normal.

[0079] As can be seen from Tables 1 and 2 above, the cathode slurry of the examples shows improved performance in all aspects compared with the cathode slurry of Comparative Example 1, demonstrating the significant optimization of interfacial bonding and flexibility by the composite binder and step-by-step process. In terms of electrochemical performance, the internal resistance of the batteries in Examples 1 / 2 is reduced by 11.7%~13.8% compared with Comparative Example 1, and the 500-cycle retention rate is improved by 2.8%~2.9%, indicating that a uniform conductive network and stable interfacial bonding can effectively improve the electrochemical stability of the battery.

[0080] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A positive electrode slurry, comprising a solid substance, said solid substance comprising a positive electrode active material and a conductive agent, characterized in that: The solid material further includes a composite binder comprising a framework phase binder and a dispersed phase binder, wherein the mass ratio of the framework phase binder to the dispersed phase binder is (2:1) to (6:1), and the composite binder accounts for 0.8% to 1.5% of the mass of the solid material. The total solids content of the positive electrode slurry is not less than 70%.

2. The positive electrode slurry as described in claim 1, characterized in that: The skeleton phase binder is selected from at least one of PVDF, PVDF-HFP, PVDF-CTFE, and PTFE-PVDF; The dispersed phase binder is selected from at least one of HNBR, XNBR, NBR, and PAA-PMA.

3. The positive electrode slurry as described in claim 1, characterized in that: The framework phase binder is PVDF, and the dispersed phase binder is HNBR; The weight-average molecular weight of PVDF is The degree of hydrogenation of HNBR is 95%~99.9%, and the acrylonitrile content of HNBR is 25%~45%. The weight average molecular weight of HNBR is... .

4. A positive electrode slurry as described in any one of claims 1 to 3, characterized in that: The positive electrode active material is selected from at least one of high-nickel ternary materials, lithium iron phosphate, lithium-rich manganese-based materials, or lithium manganese iron phosphate. The conductive agent is selected from one-dimensional conductive materials and / or zero-dimensional conductive materials.

5. A positive electrode slurry as described in any one of claims 1 to 3, characterized in that: The solid material comprises, by mass percentage, 97.0% to 98.5% of the positive electrode active material, 0.5% to 1.5% of the conductive agent, and 0.8% to 1.5% of the composite binder.

6. A method for preparing a positive electrode slurry, characterized in that, The preparation method is used to prepare a positive electrode slurry as described in any one of claims 1 to 5, and the preparation method includes the following steps: S1: Dissolve the dispersed phase binder in the solvent, then add the conductive agent and a portion of the positive electrode active material to the dispersed phase binder solution, and stir to disperse; S2: Add the remaining positive electrode active material to the mixture obtained in step S1, stir evenly, and then perform a kneading process; S3: Add the skeleton phase binder solution to the kneaded material obtained in step S2 and knead it in a mixer; S4: The mixture obtained in step S3 is added to a solvent until the viscosity and solid content of the mixture meet the preset requirements. Then, a defoaming treatment is performed to obtain the positive electrode slurry.

7. The method for preparing a positive electrode slurry as described in claim 6, characterized in that: In step S1, the viscosity of the dispersed phase binder solution is 500 mPa·s to 1000 mPa·s; In step S3, the solid content of the skeleton phase adhesive solution is 10%~15%.

8. The method for preparing a positive electrode slurry as described in claim 6, characterized in that: The positive electrode active material added in step S1 accounts for 10% to 20% of the total mass of solid matter.

9. The method for preparing a positive electrode slurry as described in claim 6, characterized in that: In step S2, the kneading process is carried out under the condition that the solid content is above 85%, the kneading temperature is controlled at 40℃~60℃, the kneading speed is 300r / min~500r / min, and the kneading time is 60min~90min.

10. A positive electrode, characterized in that: The positive electrode includes a positive electrode slurry as described in any one of claims 1 to 5.

Images