A positive electrode sheet manufacturing method, a positive electrode sheet, and a lithium ion battery manufacturing method

Abstract

The application relates to a positive plate manufacturing method, a positive plate and a lithium ion battery manufacturing method. The positive plate manufacturing method comprises the following steps: providing lithium iron phosphate primary particles; preparing lithium iron phosphate secondary particles; uniformly mixing the lithium iron phosphate primary particles, the lithium iron phosphate secondary particles, a conductive agent, an NMP solvent and a PVDF binder with a weight average molecular weight greater than 800,000 to prepare a lower layer slurry; uniformly mixing the lithium iron phosphate primary particles, the lithium iron phosphate secondary particles, the conductive agent, the NMP solvent and a PVDF binder with a weight average molecular weight of 60-800,000 to prepare an upper layer slurry; coating the lower layer slurry and the upper layer slurry respectively to prepare the positive plate; and the lithium ion battery manufacturing method comprises the step of manufacturing the lithium ion battery by using the positive plate, a negative plate and a diaphragm. The technical scheme of the application matches the weight average molecular weight of the PVDF binder with the coating structure and the slurry properties, optimizes the bonding strength between the coating structures, and avoids the local shedding or powder dropping of the positive plate.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and particularly relates to a method for manufacturing a positive electrode, a positive electrode, and a method for manufacturing a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries have core advantages such as high energy density, long cycle life, and low self-discharge rate, enabling them to be widely used in consumer electronics, electric vehicles, energy storage batteries, robots, drones and other fields, becoming the mainstream mobile energy source. Among them, lithium iron phosphate batteries have the largest market share. The internal structure of a lithium-ion battery includes a positive electrode, a negative electrode, and an electrolyte. Currently, the positive electrode is usually made by coating a positive electrode slurry onto the surface of the positive electrode current collector. Common positive electrode slurries include one or more of lithium iron phosphate primary particles and lithium iron phosphate secondary particles. Due to the large particle size of lithium iron phosphate primary particles, the electrical performance of the positive electrode is often unsatisfactory when the primary particles are coated alone on the surface of the positive electrode current collector. Lithium iron phosphate secondary particles are made by agglomerating and granulating primary lithium iron phosphate particles. The particle size of the primary particles in the secondary particles is significantly reduced, which is beneficial to improving the electrical performance of the positive electrode. However, the adhesion strength between the secondary lithium iron phosphate particles and the positive electrode current collector is low, and detachment, powder shedding, or delamination are very likely to occur, affecting the reliability and stability of the positive electrode product. In order to balance the mechanical properties, reliability, and stability of the positive electrode, existing technologies often mix primary and secondary lithium iron phosphate particles together to form a mixed positive electrode slurry.

[0003] For example, patent document CN113451548B discloses a lithium iron phosphate cathode sheet and its preparation method, as well as a lithium iron phosphate lithium-ion battery. The lithium iron phosphate cathode sheet contains lithium iron phosphate particles. Of these particles, 70-90% have a particle size in the range of 50-500 nm, 5-20% have a particle size greater than 500 nm and less than 1000 nm, and 2-10% have a particle size in the range of 1-10 μm. Using this patented technology, a lithium iron phosphate cathode sheet with ultra-high compaction density is prepared by compacting lithium iron phosphate particles within a certain particle size and proportion range, thus improving the energy density and cycle performance of the resulting lithium iron phosphate battery. However, the mixed cathode slurry composed of primary and secondary lithium iron phosphate particles usually only improves the electrical performance of the cathode sheet, but does not improve the bonding strength between the secondary lithium iron phosphate particles and the cathode current collector. The secondary lithium iron phosphate particles in the mixed cathode slurry are still prone to local detachment or powder shedding, which affects the reliability and stability of the cathode sheet. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for manufacturing a positive electrode sheet, and a method for manufacturing a positive electrode sheet and a lithium-ion battery.

[0005] This invention provides a method for manufacturing a positive electrode, comprising the following steps: Step 1: Provide primary lithium iron phosphate pellets; Step 2: Provide secondary lithium iron phosphate particles, which are obtained by agglomeration and granulation of primary lithium iron phosphate particles. Step 3: After uniformly mixing the primary lithium iron phosphate particles, secondary lithium iron phosphate particles, conductive agent, NMP solvent and PVDF binder with a weight average molecular weight greater than 800,000, the lower layer slurry is obtained. Step 4: After uniformly mixing the primary lithium iron phosphate particles, secondary lithium iron phosphate particles, conductive agent, NMP solvent and PVDF binder with a weight average molecular weight of 600,000 to 800,000, the upper slurry is obtained. Step 5: Apply the lower layer slurry to the surface of the positive electrode current collector. After the lower layer slurry solidifies, it forms the lower coating layer. Step 6: Apply the upper layer slurry to the surface of the lower layer coating. After the upper layer slurry solidifies, it forms the upper layer coating. Step 7: The positive current collector, the upper coating layer, and the lower coating layer constitute the positive electrode sheet.

[0006] In step 3, the mass percentage of the primary lithium iron phosphate particles relative to the lower slurry is 40% to 68%, and the mass percentage of the secondary lithium iron phosphate particles relative to the lower slurry is 32% to 60%.

[0007] In step 4, the mass percentage of the primary lithium iron phosphate particles relative to the upper slurry is 72% to 90%, and the mass percentage of the secondary lithium iron phosphate particles relative to the upper slurry is 10% to 28%.

[0008] In steps 5 and 6, the areal density ratio of the upper slurry to the lower slurry is 1:1 to 1:3, and the coating of the upper and lower layers can be completed simultaneously by a dual-cavity coating die.

[0009] The conductive agent is one or more of carbon black, carbon nanotubes, graphene, and conductive graphite.

[0010] The positive current collector is a composite aluminum foil, carbon-coated aluminum foil, or aluminum foil made by coating aluminum onto the surface of a polymer film.

[0011] In addition, the present invention also provides a positive electrode sheet, which is prepared using the positive electrode sheet manufacturing method described above.

[0012] Furthermore, the present invention also provides a method for manufacturing a lithium-ion battery, comprising the following steps: Step 1: Prepare a positive electrode using the positive electrode manufacturing method described above; Step 2: After uniformly mixing graphite, carbon black, carboxymethyl cellulose, styrene-butadiene rubber, and deionized water in the following mass proportions, a negative electrode slurry is prepared: Graphite: 95.4 parts; Carbon black: 1.5 parts; Carboxymethyl cellulose: 1.3 parts; Styrene-butadiene rubber: 1.8 parts; Deionized water: 100 parts; Step 3: The negative electrode slurry is coated on the surface of the negative electrode current collector. The negative electrode slurry solidifies to form a negative electrode coating. The negative electrode current collector and the negative electrode coating together form a negative electrode sheet. Step 4: The positive electrode, negative electrode, and separator are used to make a lithium-ion battery.

[0013] In step three, the negative electrode current collector is copper foil.

[0014] Step four, which involves fabricating the positive electrode, negative electrode, and separator into a lithium-ion battery, refers to: Step 4a: After stacking an even number of negative electrode plates, several separators and an odd number of positive electrode plates alternately in a sequence from the outside to the inside, a bare cell is obtained; Step 4b: After encapsulating the bare battery cell with an aluminum-plastic film, a dry battery cell is obtained; Step 4c: The dry cell is sequentially subjected to baking to remove water, liquid injection, sealing, standing, formation, degassing and packaging, and capacity testing to obtain a lithium-ion battery.

[0015] The beneficial effects of this invention are as follows: By employing the technical solution of this invention, primary lithium iron phosphate particles and secondary lithium iron phosphate particles are inter-doped and layered onto the surface of the positive electrode current collector. This improves the electrical performance of the positive electrode sheet and the corresponding lithium battery. For the upper slurry, the weight-average molecular weight of the PVDF binder is controlled to be 600,000 to 800,000. The relatively small molecular weight of the PVDF binder improves the fluidity and permeability of the upper slurry, allowing it to fully wet the lower coating layer. Simultaneously, the design, combined with a component with a high mass proportion of primary lithium iron phosphate particles, enhances the density and wear resistance of the upper coating layer, effectively solving the problem of surface powder shedding. Furthermore, it avoids the weight-average molecular weight of the PVDF binder exceeding 800,000, preventing slurry problems caused by excessively large PVDF binder molecular weight. Excessive viscosity rebound in the slurry; for the lower slurry, the weight-average molecular weight of the PVDF binder is controlled to be greater than 800,000. Increasing the molecular weight of the PVDF binder results in longer molecular chains, stronger entanglement, and greater adhesive tensile strength. This strengthens the interfacial bonding between the lower slurry and the current collector, as well as the upper slurry, compensating for the poor adhesion of the higher mass fraction of lithium iron phosphate secondary particles in the lower layer, thus ensuring the overall structural stability of the cathode. By using a high molecular weight PVDF binder to enhance the adhesion strength between the coating and the current collector, the weight-average molecular weight of the PVDF binder is matched with the coating structure and material properties, optimizing the adhesion strength between the coating and the current collector as much as possible. This ensures a firm coating bond, preventing localized detachment or powder shedding from the cathode, and guaranteeing the reliability and stability of the cathode. Attached Figure Description

[0016] Figure 1 This is a flowchart of the manufacturing process of the positive electrode sheet of this invention; Figure 2 This is a flowchart of the manufacturing process of the lithium-ion battery of the present invention; Figure 3 This is a schematic diagram of the positive electrode sheet of the present invention; Figure 4 This is a schematic diagram of the negative electrode sheet of the present invention; Figure 5 This is a schematic diagram of the bare battery cell of the present invention.

[0017] In the diagram: 1-positive current collector, 2-upper coating, 3-lower coating, 4-negative current collector, 5-negative coating, 6-positive electrode sheet, 7-separator, 8-negative electrode sheet. Detailed Implementation

[0018] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but the scope of protection claimed is not limited thereto; like Figures 1 to 5 As shown, the present invention provides a method for manufacturing a positive electrode sheet, comprising the following steps: Step 1: Provide primary lithium iron phosphate pellets; Step 2: Provide secondary lithium iron phosphate particles, which are obtained by agglomeration and granulation of primary lithium iron phosphate particles. Step 3: After uniformly mixing the primary lithium iron phosphate particles, secondary lithium iron phosphate particles, conductive agent, NMP solvent and PVDF binder with a weight average molecular weight greater than 800,000, the lower layer slurry is obtained. Step 4: After uniformly mixing the primary lithium iron phosphate particles, secondary lithium iron phosphate particles, conductive agent, NMP solvent and PVDF binder with a weight average molecular weight of 600,000 to 800,000, the upper slurry is obtained. Step 5: Apply the lower layer slurry to the surface of the positive electrode current collector 1. After the lower layer slurry solidifies, it forms the lower coating layer 3. Step 6: Apply the upper layer slurry to the surface of the lower layer coating 3. After the upper layer slurry solidifies, it forms the upper layer coating 2. Step 7: The positive electrode current collector 1, the upper coating layer 2, and the lower coating layer 3 form the positive electrode sheet 6.

[0019] By employing the technical solution of this invention, primary lithium iron phosphate particles and secondary lithium iron phosphate particles are inter-doped and layered onto the surface of the positive electrode current collector 1. This improves the electrical performance of the positive electrode sheet 6 and the corresponding lithium battery. For the upper slurry, the weight-average molecular weight of the PVDF binder is controlled to be 600,000 to 800,000. The relatively small molecular weight of the PVDF binder improves the fluidity and permeability of the upper slurry, allowing it to fully wet the positive electrode current collector 1 and the lower coating 3. This enhances the density and wear resistance of the upper coating 2, effectively solving the problem of surface powder shedding. Furthermore, it avoids the weight-average molecular weight of the PVDF binder exceeding 800,000, preventing excessive viscosity rebound due to excessively high PVDF binder molecular weight. For the lower slurry… By controlling the weight-average molecular weight of the PVDF binder to be greater than 1 million, the increased molecular weight of the PVDF binder results in longer molecular chains, stronger entanglement, and greater adhesive tensile strength. This strengthens the interfacial bonding between the lower slurry and the current collector, as well as between the upper slurry and the lower slurry, compensating for the poor adhesion of secondary lithium iron phosphate particles with a higher mass fraction, and ensuring the overall structural stability of the positive electrode 6. By using a high molecular weight PVDF binder to enhance the adhesion strength between the coating and the current collector, the weight-average molecular weight of the PVDF binder is matched with the coating structure and slurry properties, optimizing the adhesion strength between the slurry and the current collector as much as possible. This ensures a firm coating bond and prevents localized detachment or powder shedding from the positive electrode 6, guaranteeing its reliability and stability.

[0020] Specifically, in step 3, the mass percentage of PVDF binder with a weight average molecular weight greater than 800,000 relative to the lower slurry is 1.5% to 3%. In step 4, the mass percentage of PVDF binder with a weight average molecular weight of 600,000 to 800,000 relative to the upper slurry is 1.5% to 3%.

[0021] In addition, in step 3, the mass percentage of primary lithium iron phosphate particles relative to the lower slurry is 40%–68%, and the mass percentage of secondary lithium iron phosphate particles relative to the lower slurry is 32%–60%. In step 4, the mass percentage of primary lithium iron phosphate particles relative to the upper slurry is 72%–90%, and the mass percentage of secondary lithium iron phosphate particles relative to the upper slurry is 10%–28%.

[0022] Furthermore, in steps 5 and 6, the areal density ratio of the upper slurry to the lower slurry is 1:1 to 1:3. In step 4, the mass percentage of the conductive agent relative to the upper slurry is 1% to 3%. In step 3, the mass percentage of the conductive agent relative to the lower slurry is 1% to 3%. Preferably, the conductive agent is one or a combination of carbon black, carbon nanotubes, graphene, and conductive graphite. The positive electrode current collector 1 is a composite aluminum foil, carbon-coated aluminum foil, or aluminum foil prepared by coating aluminum onto the surface of a polymer film.

[0023] In addition, the present invention also provides a positive electrode 6, which is prepared using the positive electrode manufacturing method described above.

[0024] Furthermore, the present invention also provides a method for manufacturing a lithium-ion battery, comprising the following steps: Step 1: Prepare positive electrode 6 using the positive electrode manufacturing method described above; Step 2: After uniformly mixing graphite, carbon black, carboxymethyl cellulose, styrene-butadiene rubber, and deionized water in the following mass proportions, a negative electrode slurry is prepared: Graphite: 95.4 parts; Carbon black: 1.5 parts; Carboxymethyl cellulose: 1.3 parts; Styrene-butadiene rubber: 1.8 parts; Deionized water: 100 parts; Step 3: The negative electrode slurry is coated on the surface of the negative electrode current collector 4. The negative electrode slurry solidifies to form a negative electrode coating 5. The negative electrode current collector 4 and the negative electrode coating 5 together form a negative electrode sheet 8. Step 4: The positive electrode 6, negative electrode 8, and separator 7 are used to make a lithium-ion battery.

[0025] Specifically, in step three, the negative electrode current collector 4 is copper foil. Preferably, the thickness of the negative electrode current collector 4 is 5 μm. In step four, fabricating a lithium-ion battery from the positive electrode 6, negative electrode 8, and separator 7 refers to: Step 4a: After alternatingly stacking an even number of negative electrode plates 8, several separators 7, and an odd number of positive electrode plates 6 in a sequence from the outside to the inside, a bare cell is obtained; Step 4b: After encapsulating the bare battery cell with aluminum-plastic film, a dry battery cell is obtained; Step 4c: The dry cell is sequentially subjected to baking to remove water, liquid injection, sealing, standing, formation, degassing and encapsulation, and capacity testing to obtain a lithium-ion battery. Example 1:

[0026] like Figures 1 to 5 As shown, the present invention provides a method for manufacturing a positive electrode sheet, comprising the following steps: Step 1: Provide primary lithium iron phosphate pellets; Step 2: Provide secondary lithium iron phosphate particles, which are obtained by agglomeration and granulation of primary lithium iron phosphate particles. Step 3: After uniformly mixing carbon black and carbon nanotubes as a conductive agent, the primary lithium iron phosphate particles, secondary lithium iron phosphate particles, conductive agent, NMP solvent, and PVDF binder with a weight-average molecular weight of 1.1 million are uniformly mixed in the following mass proportions to obtain the lower slurry: Lithium iron phosphate primary granules: 40 parts; Lithium iron phosphate secondary granules: 56 parts; Carbon black: 1.5 parts; Carbon nanotubes: 0.5 parts; PVDF adhesive with a weight-average molecular weight of 1.1 million: 2 parts; NMP solvent: 154 parts; Step 4: After uniformly mixing carbon black and carbon nanotubes as a conductive agent, the primary lithium iron phosphate particles, secondary lithium iron phosphate particles, conductive agent, NMP solvent and PVDF binder with a weight average molecular weight of 890,000 are uniformly mixed in the following mass proportions to obtain the upper slurry. Lithium iron phosphate primary granules: 80 parts; Lithium iron phosphate secondary granules: 16 parts; Carbon black: 1.5 parts; Carbon nanotubes: 0.5 parts; PVDF adhesive with a weight-average molecular weight of 1.1 million: 2 parts; NMP solvent: 154 parts; Step 5: Select a 12μm thick carbon-coated aluminum foil as the positive electrode current collector 1, and mix the lower slurry at a concentration of 14mg / cm³. 2 The areal density is coated on the surface of the positive electrode current collector 1, and the lower slurry solidifies to form the lower coating layer 3; Step 6: Mix the upper slurry at a concentration of 7 mg / cm³ 2The areal density is applied to the surface of the lower coating layer 3, and the upper coating layer 2 is formed after the upper slurry solidifies. Step 7: The positive electrode current collector 1, the upper coating layer 2, and the lower coating layer 3 form the positive electrode sheet 6.

[0027] In addition, the present invention also provides a positive electrode 6, which is prepared using the positive electrode manufacturing method described above.

[0028] Furthermore, the present invention also provides a method for manufacturing a lithium-ion battery, comprising the following steps: Step 1: Prepare positive electrode 6 using the positive electrode manufacturing method described above; Step 2: After uniformly mixing graphite, carbon black, carboxymethyl cellulose, styrene-butadiene rubber, and deionized water in the following mass proportions, a negative electrode slurry is prepared: Graphite: 95.4 parts; Carbon black: 1.5 parts; Carboxymethyl cellulose: 1.3 parts; Styrene-butadiene rubber: 1.8 parts; Deionized water: 100 parts; Step 3: Select a copper foil with a thickness of 5μm as the negative electrode current collector 4, coat the negative electrode slurry on the surface of the negative electrode current collector 4, and the negative electrode slurry solidifies to form a negative electrode coating 5. The negative electrode current collector 4 and the negative electrode coating 5 form a negative electrode sheet 8. Step 4: The positive electrode 6, negative electrode 8, and separator 7 are used to make a lithium-ion battery.

[0029] Specifically, in step four, the process of fabricating a lithium-ion battery from the positive electrode 6, the negative electrode 8, and the separator 7 refers to: Step 4a: After alternatingly stacking an even number of negative electrode plates 8, several separators 7, and an odd number of positive electrode plates 6 in a sequence from the outside to the inside, a bare cell is obtained; Step 4b: After encapsulating the bare battery cell with aluminum-plastic film, a dry battery cell is obtained; Step 4c: The dry cell is sequentially subjected to baking to remove water, liquid injection, sealing, standing, formation, degassing and encapsulation, and capacity testing to obtain a lithium-ion battery. Example 2:

[0030] like Figures 1 to 5 As shown, the present invention provides a method for manufacturing a positive electrode sheet, comprising the following steps: Step 1: Provide primary lithium iron phosphate pellets; Step 2: Provide secondary lithium iron phosphate particles, which are obtained by agglomeration and granulation of primary lithium iron phosphate particles. Step 3: After uniformly mixing carbon black and carbon nanotubes as a conductive agent, the primary lithium iron phosphate particles, secondary lithium iron phosphate particles, conductive agent, NMP solvent, and PVDF binder with a weight-average molecular weight of 1.1 million are uniformly mixed in the following mass proportions to obtain the lower slurry: Lithium iron phosphate primary granules: 40 parts; Lithium iron phosphate secondary granules: 56 parts; Carbon black: 1.5 parts; Carbon nanotubes: 0.5 parts; PVDF adhesive with a weight-average molecular weight of 1.1 million: 2 parts; NMP solvent: 154 parts; Step 4: After uniformly mixing carbon black and carbon nanotubes as a conductive agent, the primary lithium iron phosphate particles, secondary lithium iron phosphate particles, conductive agent, NMP solvent and PVDF binder with a weight average molecular weight of 890,000 are uniformly mixed in the following mass proportions to obtain the upper slurry. Lithium iron phosphate primary granules: 80 parts; Lithium iron phosphate secondary granules: 16 parts; Carbon black: 1.5 parts; Carbon nanotubes: 0.5 parts; PVDF adhesive with a weight-average molecular weight of 1.1 million: 2 parts; NMP solvent: 154 parts; Step 5: Select a 12μm thick carbon-coated aluminum foil as the positive electrode current collector 1, and mix the lower slurry at a concentration of 15.75 mg / cm³. 2 The areal density is coated on the surface of the positive electrode current collector 1, and the lower slurry solidifies to form the lower coating layer 3; Step 6: Mix the upper slurry at a ratio of 5.25 mg / cm³. 2 The areal density is applied to the surface of the lower coating layer 3, and the upper coating layer 2 is formed after the upper slurry solidifies. Step 7: The positive electrode current collector 1, the upper coating layer 2, and the lower coating layer 3 form the positive electrode sheet 6.

[0031] In addition, the present invention also provides a positive electrode 6, which is prepared using the positive electrode manufacturing method described above.

[0032] Furthermore, the present invention also provides a method for manufacturing a lithium-ion battery, comprising the following steps: Step 1: Prepare positive electrode 6 using the positive electrode manufacturing method described above; Step 2: After uniformly mixing graphite, carbon black, carboxymethyl cellulose, styrene-butadiene rubber, and deionized water in the following mass proportions, a negative electrode slurry is prepared: Graphite: 95.4 parts; Carbon black: 1.5 parts; Carboxymethyl cellulose: 1.3 parts; Styrene-butadiene rubber: 1.8 parts; Deionized water: 100 parts; Step 3: Select a copper foil with a thickness of 5μm as the negative electrode current collector 4, coat the negative electrode slurry on the surface of the negative electrode current collector 4, and the negative electrode slurry solidifies to form a negative electrode coating 5. The negative electrode current collector 4 and the negative electrode coating 5 form a negative electrode sheet 8. Step 4: The positive electrode 6, negative electrode 8, and separator 7 are used to make a lithium-ion battery.

[0033] Specifically, in step four, the process of fabricating a lithium-ion battery from the positive electrode 6, the negative electrode 8, and the separator 7 refers to: Step 4a: After alternatingly stacking an even number of negative electrode plates 8, several separators 7, and an odd number of positive electrode plates 6 in a sequence from the outside to the inside, a bare cell is obtained; Step 4b: After encapsulating the bare battery cell with aluminum-plastic film, a dry battery cell is obtained; Step 4c: The dry cell is sequentially subjected to baking to remove water, liquid injection, sealing, standing, formation, degassing and encapsulation, and capacity testing to obtain a lithium-ion battery. Example 3:

[0034] like Figures 1 to 5 As shown, the present invention provides a method for manufacturing a positive electrode sheet, comprising the following steps: Step 1: Provide primary lithium iron phosphate pellets; Step 2: Provide secondary lithium iron phosphate particles, which are obtained by agglomeration and granulation of primary lithium iron phosphate particles. Step 3: After uniformly mixing carbon black and carbon nanotubes as a conductive agent, the primary lithium iron phosphate particles, secondary lithium iron phosphate particles, conductive agent, NMP solvent, and PVDF binder with a weight average molecular weight of 890,000 are uniformly mixed in the following mass proportions to obtain the lower slurry: Lithium iron phosphate primary granules: 40 parts; Lithium iron phosphate secondary granules: 56 parts; Carbon black: 1.5 parts; Carbon nanotubes: 0.5 parts; PVDF adhesive with a weight-average molecular weight of 890,000: 2 parts; NMP solvent: 154 parts; Step 4: After uniformly mixing carbon black and carbon nanotubes as a conductive agent, the primary lithium iron phosphate particles, secondary lithium iron phosphate particles, conductive agent, NMP solvent and PVDF binder with a weight average molecular weight of 890,000 are uniformly mixed in the following mass proportions to obtain the upper slurry. Lithium iron phosphate primary granules: 80 parts; Lithium iron phosphate secondary granules: 16 parts; Carbon black: 1.5 parts; Carbon nanotubes: 0.5 parts; PVDF adhesive with a weight-average molecular weight of 1.1 million: 2 parts; NMP solvent: 154 parts; Step 5: Select a 12μm thick carbon-coated aluminum foil as the positive electrode current collector 1, and mix the lower slurry at a concentration of 15.75 mg / cm³. 2 The areal density is coated on the surface of the positive electrode current collector 1, and the lower slurry solidifies to form the lower coating layer 3; Step 6: Mix the upper slurry at a ratio of 5.25 mg / cm³. 2 The areal density is applied to the surface of the lower coating layer 3, and the upper coating layer 2 is formed after the upper slurry solidifies. Step 7: The positive electrode current collector 1, the upper coating layer 2, and the lower coating layer 3 form the positive electrode sheet 6.

[0035] In addition, the present invention also provides a positive electrode 6, which is prepared using the positive electrode manufacturing method described above.

[0036] Furthermore, the present invention also provides a method for manufacturing a lithium-ion battery, comprising the following steps: Step 1: Prepare positive electrode 6 using the positive electrode manufacturing method described above; Step 2: After uniformly mixing graphite, carbon black, carboxymethyl cellulose, styrene-butadiene rubber, and deionized water in the following mass proportions, a negative electrode slurry is prepared: Graphite: 95.4 parts; Carbon black: 1.5 parts; Carboxymethyl cellulose: 1.3 parts; Styrene-butadiene rubber: 1.8 parts; Deionized water: 100 parts; Step 3: Select a copper foil with a thickness of 5μm as the negative electrode current collector 4, coat the negative electrode slurry on the surface of the negative electrode current collector 4, and the negative electrode slurry solidifies to form a negative electrode coating 5. The negative electrode current collector 4 and the negative electrode coating 5 form a negative electrode sheet 8. Step 4: The positive electrode 6, negative electrode 8, and separator 7 are used to make a lithium-ion battery.

[0037] Specifically, in step four, the process of fabricating a lithium-ion battery from the positive electrode 6, the negative electrode 8, and the separator 7 refers to: Step 4a: After alternatingly stacking an even number of negative electrode plates 8, several separators 7, and an odd number of positive electrode plates 6 in a sequence from the outside to the inside, a bare cell is obtained; Step 4b: After encapsulating the bare battery cell with aluminum-plastic film, a dry battery cell is obtained; Step 4c: The dry cell is sequentially subjected to baking to remove water, liquid injection, sealing, standing, formation, degassing and encapsulation, and capacity testing to obtain a lithium-ion battery. Example 4:

[0038] like Figures 1 to 5 As shown, the present invention provides a method for manufacturing a positive electrode sheet, comprising the following steps: Step 1: Provide primary lithium iron phosphate pellets; Step 2: Provide secondary lithium iron phosphate particles, which are obtained by agglomeration and granulation of primary lithium iron phosphate particles. Step 3: After uniformly mixing carbon black and carbon nanotubes as a conductive agent, the primary lithium iron phosphate particles, secondary lithium iron phosphate particles, conductive agent, NMP solvent, and PVDF binder with a weight average molecular weight of 890,000 are uniformly mixed in the following mass proportions to obtain the lower slurry: Lithium iron phosphate primary granules: 50 parts; Lithium iron phosphate secondary granules: 46 parts; Carbon black: 1.5 parts; Carbon nanotubes: 0.5 parts; PVDF adhesive with a weight-average molecular weight of 890,000: 2 parts; NMP solvent: 154 parts; Step 4: Select a 12μm thick carbon-coated aluminum foil as the positive electrode current collector 1, and mix the lower slurry at a concentration of 21mg / cm³. 2 The areal density is coated on the surface of the positive electrode current collector 1, and the lower slurry solidifies to form the lower coating layer 3; Step 5: The positive current collector 1 and the lower coating layer 3 form the positive electrode sheet 6.

[0039] In addition, the present invention also provides a positive electrode 6, which is prepared using the positive electrode manufacturing method described above.

[0040] Furthermore, the present invention also provides a method for manufacturing a lithium-ion battery, comprising the following steps: Step 1: Prepare positive electrode 6 using the same positive electrode manufacturing method as before; Step 2: After uniformly mixing graphite, carbon black, carboxymethyl cellulose, styrene-butadiene rubber, and deionized water in the following mass proportions, a negative electrode slurry is prepared: Graphite: 95.4 parts; Carbon black: 1.5 parts; Carboxymethyl cellulose: 1.3 parts; Styrene-butadiene rubber: 1.8 parts; Deionized water: 100 parts; Step 3: Select a copper foil with a thickness of 5μm as the negative electrode current collector 4, coat the negative electrode slurry on the surface of the negative electrode current collector 4, and the negative electrode slurry solidifies to form a negative electrode coating 5. The negative electrode current collector 4 and the negative electrode coating 5 form a negative electrode sheet 8. Step 4: The positive electrode 6, negative electrode 8, and separator 7 are used to make a lithium-ion battery.

[0041] Specifically, in step four, the process of fabricating a lithium-ion battery from the positive electrode 6, the negative electrode 8, and the separator 7 refers to: Step 4a: After alternatingly stacking an even number of negative electrode plates 8, several separators 7, and an odd number of positive electrode plates 6 in a sequence from the outside to the inside, a bare cell is obtained; Step 4b: After encapsulating the bare battery cell with aluminum-plastic film, a dry battery cell is obtained; Step 4c: The dry cell is sequentially subjected to baking to remove water, liquid injection, sealing, standing, formation, degassing and encapsulation, and capacity testing to obtain a lithium-ion battery. Example 5:

[0042] like Figures 1 to 5 As shown, the present invention provides a method for manufacturing a positive electrode sheet, comprising the following steps: Step 1: Provide primary lithium iron phosphate pellets; Step 2: Provide secondary lithium iron phosphate particles, which are obtained by agglomeration and granulation of primary lithium iron phosphate particles. Step 3: After uniformly mixing carbon black and carbon nanotubes as a conductive agent, lithium iron phosphate secondary particles, conductive agent, NMP solvent, and PVDF binder with a weight average molecular weight of 890,000 are uniformly mixed in the following mass proportions to obtain the lower slurry: Lithium iron phosphate secondary granules: 96 parts; Carbon black: 1.5 parts; Carbon nanotubes: 0.5 parts; PVDF adhesive with a weight-average molecular weight of 890,000: 2 parts; NMP solvent: 154 parts; Step 4: Select a 12μm thick carbon-coated aluminum foil as the positive electrode current collector 1, and mix the lower slurry at a concentration of 21mg / cm³. 2 The areal density is coated on the surface of the positive electrode current collector 1, and the lower slurry solidifies to form the lower coating layer 3; Step 5: The positive current collector 1 and the lower coating layer 3 form the positive electrode sheet 6.

[0043] In addition, the present invention also provides a positive electrode 6, which is prepared using the positive electrode manufacturing method described above.

[0044] Furthermore, the present invention also provides a method for manufacturing a lithium-ion battery, comprising the following steps: Step 1: Prepare positive electrode 6 using the positive electrode manufacturing method described above; Step 2: After uniformly mixing graphite, carbon black, carboxymethyl cellulose, styrene-butadiene rubber, and deionized water in the following mass proportions, a negative electrode slurry is prepared: Graphite: 95.4 parts; Carbon black: 1.5 parts; Carboxymethyl cellulose: 1.3 parts; Styrene-butadiene rubber: 1.8 parts; Deionized water: 100 parts; Step 3: Select a copper foil with a thickness of 5μm as the negative electrode current collector 4, coat the negative electrode slurry on the surface of the negative electrode current collector 4, and the negative electrode slurry solidifies to form a negative electrode coating 5. The negative electrode current collector 4 and the negative electrode coating 5 form a negative electrode sheet 8. Step 4: The positive electrode 6, negative electrode 8, and separator 7 are used to make a lithium-ion battery.

[0045] Specifically, in step four, the process of fabricating a lithium-ion battery from the positive electrode 6, the negative electrode 8, and the separator 7 refers to: Step 4a: After alternatingly stacking an even number of negative electrode plates 8, several separators 7, and an odd number of positive electrode plates 6 in a sequence from the outside to the inside, a bare cell is obtained; Step 4b: After encapsulating the bare battery cell with aluminum-plastic film, a dry battery cell is obtained; Step 4c: The dry cell is sequentially subjected to baking to remove water, liquid injection, sealing, standing, formation, degassing and encapsulation, and capacity testing to obtain a lithium-ion battery. Example 6:

[0046] like Figures 1 to 5 As shown, the present invention provides a method for manufacturing a positive electrode sheet, comprising the following steps: Step 1: Provide primary lithium iron phosphate pellets; Step 2: After uniformly mixing carbon black and carbon nanotubes as a conductive agent, lithium iron phosphate primary particles, conductive agent, NMP solvent, and PVDF binder with a weight average molecular weight of 890,000 are uniformly mixed in the following mass proportions to obtain the lower slurry: Lithium iron phosphate primary granules: 96 parts; Carbon black: 1.5 parts; Carbon nanotubes: 0.5 parts; PVDF adhesive with a weight-average molecular weight of 890,000: 2 parts; NMP solvent: 154 parts; Step 3: Select a 12μm thick carbon-coated aluminum foil as the positive electrode current collector 1, and mix the lower slurry at a concentration of 21mg / cm³. 2 The areal density is coated on the surface of the positive electrode current collector 1, and the lower slurry solidifies to form the lower coating layer 3; Step 4: The positive current collector 1 and the lower coating layer 3 form the positive electrode sheet 6.

[0047] In addition, the present invention also provides a positive electrode 6, which is prepared using the positive electrode manufacturing method described above.

[0048] Furthermore, the present invention also provides a method for manufacturing a lithium-ion battery, comprising the following steps: Step 1: Prepare positive electrode 6 using the positive electrode manufacturing method described above; Step 2: After uniformly mixing graphite, carbon black, carboxymethyl cellulose, styrene-butadiene rubber, and deionized water in the following mass proportions, a negative electrode slurry is prepared: Graphite: 95.4 parts; Carbon black: 1.5 parts; Carboxymethyl cellulose: 1.3 parts; Styrene-butadiene rubber: 1.8 parts; Deionized water: 100 parts; Step 3: Select a copper foil with a thickness of 5μm as the negative electrode current collector 4, coat the negative electrode slurry on the surface of the negative electrode current collector 4, and the negative electrode slurry solidifies to form a negative electrode coating 5. The negative electrode current collector 4 and the negative electrode coating 5 form a negative electrode sheet 8. Step 4: The positive electrode 6, negative electrode 8, and separator 7 are used to make a lithium-ion battery.

[0049] Specifically, in step four, the process of fabricating a lithium-ion battery from the positive electrode 6, the negative electrode 8, and the separator 7 refers to: Step 4a: After alternatingly stacking an even number of negative electrode plates 8, several separators 7, and an odd number of positive electrode plates 6 in a sequence from the outside to the inside, a bare cell is obtained; Step 4b: After encapsulating the bare battery cell with aluminum-plastic film, a dry battery cell is obtained; Step 4c: The dry cell is sequentially subjected to baking to remove water, liquid injection, sealing, standing, formation, degassing and encapsulation, and capacity testing to obtain a lithium-ion battery.

[0050] Approximately 800 bare cells were prepared in each group according to Examples 1 to 6 above. Each bare cell was then subjected to an insulation withstand voltage test under the following conditions: voltage 100V, time 6s. Bare cells with an insulation resistance greater than 4MΩ were classified as good cells. The test results are shown in Table 1. Two good cells were then extracted from each group of examples, with a nominal capacity of 21Ah. The 5C capacity retention rate and -20℃ capacity retention rate of the two good cells were then tested. The test steps are as follows: (1) Environmental adaptation: Place the good quality battery cells in a 25℃ high and low temperature chamber and let them stand for 3 hours; (2) Constant current and constant voltage charging: Charge at a constant current of 7A to 3.65V, and then charge at a constant voltage to the cutoff current of 0.07A; (3) 0.33C discharge: After the good quality cells are left to cool for 1 hour, they are discharged at a constant current of 7A to 2.5V, and the discharge capacity Q is recorded. 1 / 3C @25℃; (4) Repeat step (1); (5) 5C discharge: After the good quality cells are left to cool for 1 hour, they are discharged at a constant current of 105A to 2.5V, and the discharge capacity Q is recorded. 5C @25℃.

[0051] (6) Repeat step (2); (7) Environmental adaptation: Place the good quality battery cells in a -20℃ high and low temperature chamber and let them stand for 3 hours; (8) 0.33C discharge: Discharge at a constant current of 7A to 2.5V and record the discharge capacity Q. 1 / 3C @-20℃.

[0052] The test results are shown in Table 2.

[0053] Table 1. Statistical table of yield rate of bare cell insulation withstand voltage test in Examples 1 to 6

[0054] Table 2 Performance test results of good battery cells from Examples 1 to 6

[0055] Compared with Example 6, Example 5 uses lithium iron phosphate secondary sphere particles, which significantly improves both rate performance and low-temperature performance. However, the problem of powder shedding from the positive electrode is more prominent, resulting in a significant decrease in the yield of bare cell insulation withstand voltage test.

[0056] Example 4 uses a conventional single-layer coating method to mix secondary particles with primary particles. The battery performance is between that of Example 5 and Example 6. The yield of bare cell insulation withstand voltage test is significantly improved compared with Example 5, but it is still difficult to meet the requirements of large-scale mass production.

[0057] Example 3 employs a double-layer coating scheme, achieving performance comparable to Example 4, with a significant improvement in the bare cell insulation withstand voltage test yield compared to Example 4. However, it still falls short of yield requirements. Example 2 optimizes the lower layer of PVDF in the coating slurry by replacing it with a higher molecular weight PVDF, strengthening the interfacial bonding between the bottom coating and the current collector, as well as between the upper and lower coatings. This compensates for the poor adhesion of high-proportion lithium iron phosphate secondary particles, significantly improving the battery's 5C capacity retention and capacity retention at -20℃ to levels comparable to those using pure lithium iron phosphate secondary particles. Furthermore, due to the lower proportion of lithium iron phosphate secondary particles on the surface, the problem of surface powder shedding is essentially eliminated, resulting in a bare cell insulation withstand voltage test yield comparable to that of conventional lithium iron phosphate primary particle electrodes, which can meet the requirements of large-scale mass production.

[0058] Compared with Example 2, Example 1 further reduced the proportion of secondary spherical lithium iron phosphate material in the overall electrode by adjusting the density ratio of the upper and lower layers. The 5C capacity retention rate and the capacity retention rate at -20℃ were only slightly reduced. The bare cell insulation withstand voltage test yield reached the same level as that of conventional primary particle electrode, achieving a better balance between process yield and performance.

Claims

1. A method for manufacturing a positive electrode sheet, characterized in that: Includes the following steps: Step 1: Provide primary lithium iron phosphate pellets; Step 2: Provide secondary lithium iron phosphate particles, which are obtained by agglomeration and granulation of primary lithium iron phosphate particles. Step 3: After uniformly mixing the primary lithium iron phosphate particles, secondary lithium iron phosphate particles, conductive agent, NMP solvent and PVDF binder with a weight average molecular weight greater than 800,000, the lower layer slurry is obtained. Step 4: After uniformly mixing the primary lithium iron phosphate particles, secondary lithium iron phosphate particles, conductive agent, NMP solvent and PVDF binder with a weight average molecular weight of 600,000 to 800,000, the upper slurry is obtained. Step 5: Apply the lower layer slurry to the surface of the positive electrode current collector (1), and the lower layer slurry solidifies to form the lower layer coating (3). Step 6: Apply the upper slurry to the surface of the lower coating layer (3), and the upper slurry solidifies to form the upper coating layer (2). Step 7: The positive current collector (1), the upper coating (2) and the lower coating (3) form the positive electrode sheet (6).

2. The positive electrode manufacturing method as described in claim 1, characterized in that: In step 3, the mass percentage of the primary lithium iron phosphate particles relative to the lower slurry is 40% to 68%, and the mass percentage of the secondary lithium iron phosphate particles relative to the lower slurry is 32% to 60%.

3. The positive electrode manufacturing method as described in claim 1, characterized in that: In step 4, the mass percentage of the primary lithium iron phosphate particles relative to the upper slurry is 72% to 90%, and the mass percentage of the secondary lithium iron phosphate particles relative to the upper slurry is 10% to 28%.

4. The positive electrode manufacturing method as described in claim 1, characterized in that: In steps 5 and 6, the areal density ratio of the upper slurry to the lower slurry is 1:1 to 1:

3.

5. The positive electrode manufacturing method as described in claim 1, characterized in that: The conductive agent is one or more of carbon black, carbon nanotubes, graphene, and conductive graphite.

6. The method for manufacturing a positive electrode sheet as described in claim 1, characterized in that: The positive current collector (1) is a composite aluminum foil, carbon-coated aluminum foil, or aluminum foil made by coating aluminum onto the surface of a polymer film.

7. A positive electrode (6), characterized in that: It is prepared using the positive electrode manufacturing method as described in any one of claims 1 to 6.

8. A method for manufacturing a lithium-ion battery, characterized in that: Includes the following steps: Step 1: A positive electrode sheet (6) is prepared using the positive electrode sheet manufacturing method as described in any one of claims 1 to 6. Step 2: After uniformly mixing graphite, carbon black, carboxymethyl cellulose, styrene-butadiene rubber, and deionized water in the following mass proportions, a negative electrode slurry is prepared: Graphite: 95.4 parts; Carbon black: 1.5 parts; Carboxymethyl cellulose: 1.3 parts; Styrene-butadiene rubber: 1.8 parts; Deionized water: 100 parts; Step 3: The negative electrode slurry is coated on the surface of the negative electrode current collector (4), and the negative electrode slurry solidifies to form a negative electrode coating (5). The negative electrode current collector (4) and the negative electrode coating (5) together form a negative electrode sheet (8). Step 4: The positive electrode (6), negative electrode (8) and separator (7) are used to make a lithium-ion battery.

9. The method for manufacturing a lithium-ion battery as described in claim 8, characterized in that: In step three, the negative electrode current collector (4) is copper foil.

10. The method for manufacturing a lithium-ion battery as described in claim 8, characterized in that: In step four, the process of fabricating a lithium-ion battery from the positive electrode (6), negative electrode (8), and separator (7) refers to: Step 4a: After alternatingly stacking an even number of negative electrode plates (8), several separators (7) and an odd number of positive electrode plates (6) in an order from the outside to the inside, a bare cell is obtained; Step 4b: After encapsulating the bare battery cell with an aluminum-plastic film, a dry battery cell is obtained; Step 4c: The dry cell is sequentially subjected to baking to remove water, liquid injection, sealing, standing, formation, degassing and packaging, and capacity testing to obtain a lithium-ion battery.

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