A lithium iron phosphate battery positive pole slurry, a positive pole piece, a preparation method thereof and a lithium iron phosphate battery

Abstract

The application discloses a lithium iron phosphate battery positive electrode slurry, a positive electrode sheet and a preparation method thereof and a lithium iron phosphate battery. The lithium iron phosphate battery positive electrode slurry comprises: lithium iron phosphate hollow large particles with a "doughnut" structure, lithium iron phosphate medium solid particles, optional lithium iron phosphate spherical small particles, single-walled carbon nanotubes, conductive graphite and polyvinylidene fluoride glue solution. The application realizes the preparation of a high-energy high-fast-charging lithium iron phosphate battery by the hierarchical matching design of the lithium iron phosphate hollow large particles with the "doughnut" structure, the lithium iron phosphate medium solid particles and the optional lithium iron phosphate spherical small particles, the compounding of the three types of particles, the mutual filling and mutual support, and the construction of a novel lithium iron phosphate positive electrode with high compaction and high power performance.

Description

Technical Field

[0001] This invention belongs to the field of lithium iron phosphate battery technology, and more specifically, relates to a lithium iron phosphate battery positive electrode slurry, a positive electrode sheet, a method for preparing the same, and a lithium iron phosphate battery. Background Technology

[0002] In recent years, the government has introduced a series of policies to encourage and support the rapid development of power batteries, such as the "Action Plan for Achieving Carbon Peak Before 2030" and the "Interim Management Standards for New Energy Storage Projects." Lithium iron phosphate batteries have extremely prominent cost and safety advantages, and with continuous breakthroughs in manufacturing technology, they have become the top-selling power battery product, holding a market share of over 50%.

[0003] The PO4 structure in lithium iron phosphate (LFP) materials exhibits strong structural stability and excellent oxygen-locking ability, resulting in superior safety and cycle performance for LFP cells. However, the inherent characteristics of the exceptionally stable olivine crystal structure of LFP materials also mean that the lithium-ion extraction and insertion channels are only one-dimensional, leading to poor ion diffusion and electronic conductivity. Consequently, LFP materials have poor conductivity, resulting in poor rate performance.

[0004] As consumers' demands for user experience continue to rise, power batteries are required to have both long driving range (i.e., high energy) and fast charging capabilities, which has become a key issue restricting the further development of lithium iron phosphate power batteries.

[0005] In the field of battery cell design, energy density and fast charging capability are contradictory; higher energy density often comes at the cost of reduced fast charging performance. At the cell level, improving the energy density of lithium iron phosphate (LFP) batteries focuses on increasing areal density and electrode compaction, with the design philosophy being to fill a limited space with as much energy as possible. Conversely, improving fast charging power requires consideration of cathode material nano-sizing, the use of composite conductive agents in the slurry formulation, and electrode design and processing (reducing coating amount or using double-layer coating), with the design mechanism being to improve the ionic and electronic conductivity of the electrodes and the overall structure. While current LFP batteries can generally achieve their energy density goals, a feasible solution that balances both energy density and fast charging capability has yet to be proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a lithium iron phosphate battery positive electrode slurry, positive electrode sheet, preparation method thereof, and lithium iron phosphate battery. This invention utilizes a hierarchical matching design of large hollow lithium iron phosphate particles with a "donut" structure, medium solid lithium iron phosphate particles, and optional small spherical lithium iron phosphate particles. The three types of particles are compounded, filled, and supported by each other, to construct a novel lithium iron phosphate positive electrode that integrates high compaction and high power performance, thereby realizing the preparation of high-energy, high-fast-charging lithium iron phosphate.

[0007] To achieve the above objectives, a first aspect of the present invention provides a lithium iron phosphate battery cathode slurry comprising: large hollow lithium iron phosphate particles having a "donut" structure, medium-sized solid lithium iron phosphate particles, optionally small spherical lithium iron phosphate particles, single-walled carbon nanotubes, conductive graphite, and polyvinylidene fluoride adhesive.

[0008] In this invention, the compound application of different lithium iron phosphate particles, the large spheres, namely "donut" spheres, can reduce the difficulty of homogenizing lithium iron phosphate materials and occupy the depth space in the electrode. The medium solid phase particles provide the strength backbone, and the optional nano homogeneous particles fill the gaps and pores. The above-mentioned compound of different particles fills and supports each other, greatly improving the compaction of the lithium iron phosphate electrode, greatly improving the space utilization rate of the active material on the electrode, and enhancing the energy carrying capacity.

[0009] According to the present invention, preferably, the sum of the masses of the hollow large particles of lithium iron phosphate with a "donut" structure, the medium solid solid particles of lithium iron phosphate, and the optional spherical small particles of lithium iron phosphate accounts for 93-98% of the total solid mass in the slurry; The mass of single-walled carbon nanotubes accounts for 0.02% to 1% of the total solid mass in the slurry; The mass of conductive graphite accounts for 0.8~1.5% of the total solid mass in the slurry; The solid content of the polyvinylidene fluoride (PVDF) adhesive is 3-7%. Preferably, the total mass percentage of the hollow lithium iron phosphate particles with a "donut" structure, the medium-sized solid lithium iron phosphate particles, and the optional spherical lithium iron phosphate particles is 10-50%, the total mass percentage of the hollow lithium iron phosphate particles with a "donut" structure is 50-90%, and the total mass percentage of the spherical lithium iron phosphate particles is 0-20%.

[0010] In this invention, the solid content of polyvinylidene fluoride (PVDF) adhesive refers to the content of PVDF in the PVDF adhesive.

[0011] According to the present invention, preferably, the primary particle D50 size of the hollow lithium iron phosphate particles with the "donut" structure is 10~100nm, the secondary particle D50 size is 15~30µm, and the compaction density is 2.8~3.8g / cm³. 3 The carbon content is 3-8%.

[0012] According to the present invention, preferably, the hollow lithium iron phosphate particles having a "donut" structure are prepared by a method comprising the following steps: (1) Mix lithium source, iron source, phosphorus source, dispersant and water evenly to obtain a mixed slurry; (2) Grind the mixed slurry until the particles are ground to 10-100 nm, then pass the slurry into a two-fluid spray dryer to prepare hollow precursor particles with a secondary particle D50 particle size of 15-30 μm. (3) The hollow precursor particles are mixed with the first carbon source and pre-fired in the first protective gas atmosphere to complete the first carbon coating; then mixed with the second carbon source and sintered in the second protective gas atmosphere to achieve the second carbon coating; finally, after cooling, they are crushed and graded to obtain the hollow large particles of lithium iron phosphate with the "donut" structure. Preferably, in step (1), the lithium source is lithium carbonate, the iron source is ferrous sulfate, and the phosphorus source is ammonium dihydrogen phosphate; the molar ratio of lithium source (calculated as lithium element), iron source (calculated as iron element), and phosphorus source (calculated as phosphorus element) is (1.02-1.05):1:1; the dispersant is polyethylene glycol; the solid content of the mixed slurry is 30-40%; in step (2), the grinding is carried out using a sand mill, and the grinding time is 80-120 min; the process parameters of the two-fluid spray dryer include: controlling the inlet temperature 2 20-250℃, outlet temperature 90-105℃, atomizing gas pressure 3.5-5MPa, flow rate 8-12L / h; in step (3), the first carbon source is glucose; the pre-calcination temperature is 380-420℃, and the time is 3-5h; the first protective gas is nitrogen; the thickness of the carbon layer in the first carbon coating is 2-6nm; the second carbon source is acetylene black; the sintering temperature is 780-820℃, and the time is 6-8h; the second protective gas is argon; the thickness of the carbon layer in the second carbon coating is 10-30nm.

[0013] According to the present invention, preferably, the medium-sized solid solid particles of lithium iron phosphate are prepared by solid-state sintering method; The lithium iron phosphate medium-sized solid solid particles have a D50 particle size of 1.5~8 μm, a carbon content of 0.1~1.0%, a specific surface area of ​​5~10 m² / g, and a 0.1C specific capacity of ≥156 mAh / g. The lithium iron phosphate nanospheres are nano / micron-sized homogeneous particles produced by hydrothermal method, with a D50 particle size of 500 nm to 2 μm, a carbon content of 1.0 to 2.0%, a specific surface area of ​​15 to 25 m² / g, and a 0.1C specific capacity of ≥158 mAh / g. The preferred polyvinylidene fluoride (PVDF) adhesive is an N-methylpyrrolidone (NMP) adhesive for PVDF.

[0014] In this invention, preferably, the preparation method of lithium iron phosphate battery cathode slurry includes the following steps: (1) mixing lithium iron phosphate hollow large particles with a "donut" structure, lithium iron phosphate medium solid phase particles and lithium iron phosphate spherical small particles evenly to obtain a lithium iron phosphate particle mixture; (2) mixing polyvinylidene fluoride adhesive, single-walled carbon nanotubes and conductive graphite evenly, and then adding the lithium iron phosphate particle mixture step by step, mixing evenly to obtain a slurry mixture, and finally dispersing the slurry mixture to obtain a lithium iron phosphate battery cathode slurry. In step (2), the step-by-step addition is added in 2-3 times; the total dispersion time is 4-6 hours, the dispersion is a double planetary dispersion, the revolution is 10-50 rpm, and the rotation is 1500-4500 rpm.

[0015] A second aspect of the present invention provides a method for preparing a positive electrode sheet, the method comprising: coating the above-mentioned lithium iron phosphate battery positive electrode slurry onto the upper and lower surfaces of a current collector, and performing electrode sheet rolling and baking to obtain the lithium iron phosphate positive electrode.

[0016] According to the present invention, preferably, the current collector is an aluminum foil current collector; The surface density of the coating on each side is 170~270 g / m². 2 ; The electrode sheets are rolled using high-temperature hot pressing at 50~100℃, with a compaction density of 2.5~2.75 g / cm³. 3 ; After the electrode sheets are crushed and before baking, a sheet-making process is also performed.

[0017] In this invention, after the positive electrode slurry is coated onto the current collector, during the electrode rolling operation, the loosely assembled "donut" structure particles of nano-sized microspheres disperse under high extrusion pressure, tightly adhering to the medium-sized solid particles, forming aggregates. This ensures sufficient contact between the particles, achieving a density of 2.7 g / cm³. 3 The above-mentioned high-pressure compaction capability and superior conductive network far exceed the current level. At the same time, the large number of nano-sized lithium iron phosphate particles greatly shorten the lithium-ion diffusion path, and the fast charging capability reaches an advanced level.

[0018] In this invention, the sheet-making operation involves cutting the sheet into electrode sheets of the corresponding size using a mold.

[0019] A third aspect of the present invention provides a positive electrode sheet prepared by the above-described preparation method.

[0020] A fourth aspect of the present invention provides a lithium iron phosphate battery comprising the above-described positive electrode.

[0021] According to the present invention, preferably, the lithium iron phosphate battery further includes a negative electrode sheet; The negative electrode slurry used to prepare the negative electrode sheet includes: graphite powder, conductive carbon black, binder and sodium carboxymethyl cellulose (CMC) adhesive; Preferably, the binder is styrene-butadiene rubber (SBR); the solid content of the sodium carboxymethyl cellulose (CMC) solution is 1.5-3%; the graphite powder accounts for 93-97.5% of the total solid mass of the negative electrode slurry, the conductive carbon black accounts for 0.5-6.5% of the total solid mass of the negative electrode slurry, and the binder accounts for 0.2-3.0% of the total solid mass of the negative electrode slurry.

[0022] In this invention, preferably, the method for preparing the negative electrode slurry includes: (1) mixing sodium carboxymethyl cellulose (CMC) adhesive and conductive carbon black evenly, then adding graphite powder and mixing evenly to obtain a mixture; (2) dispersing the mixture, and finally adding a binder, dispersing at 500-800 rpm for 25-35 minutes to obtain the negative electrode slurry; wherein, in step (2), the total time for dispersion is 4-6 hours, the dispersion is a double planetary dispersion, the revolution is 10-50 rpm, and the rotation is 1500-3000 rpm.

[0023] In this invention, preferably, the method for preparing the negative electrode sheet includes: uniformly coating a negative electrode slurry onto a current collector, and then rolling and baking the electrode sheet to obtain the negative electrode sheet. More preferably, the coating surface density is 1700~250 g / m². 2 The compacted density of the electrode sheets is 1.5~1.7 g / cm³. 3 A sheet-making process is also carried out after the electrode sheets are rolled and before baking.

[0024] The technical solution of the present invention has the following beneficial effects: (1) This invention constructs a novel lithium iron phosphate positive electrode that integrates high-pressure compaction and high-power performance by using a graded matching design of large hollow lithium iron phosphate particles with a "donut" structure, medium solid lithium iron phosphate particles and optional spherical small lithium iron phosphate particles. The three types of particles are compounded, filled and supported by each other, thereby realizing the preparation of high-energy high-fast-charging lithium iron phosphate.

[0025] (2) This invention achieves particle size distribution by mixing powders of different specifications. After being made into electrode sheets, the large spherical particles break apart under rolling pressure. The gap filling phenomenon allows for full contact between particles, resulting in a better conductive network and a significant reduction in internal resistance. Moreover, under specified upper limits, the more large spherical particles are mixed, the higher the power performance is, and the low-temperature performance is also improved. At the same time, a large number of nano-sized lithium iron phosphate particles greatly shorten the lithium-ion diffusion path, significantly improving the fast charging capability, which can be applied to ultra-fast charging power batteries.

[0026] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0027] The above and other objects, features and advantages of the present invention will become more apparent from the more detailed description of exemplary embodiments of the invention in conjunction with the accompanying drawings, wherein the same reference numerals generally represent the same components in the exemplary embodiments of the invention.

[0028] Figure 1 An electron microscope image of a hollow lithium iron phosphate particle with a "donut" structure is shown according to an embodiment of the present invention.

[0029] Figure 2 An electron microscope image of medium-sized solid lithium iron phosphate particles according to an embodiment of the present invention is shown.

[0030] Figure 3 An electron microscope image of spherical lithium iron phosphate particles according to an embodiment of the present invention is shown.

[0031] Figure 4 A scanning electron microscope image of the positive electrode sheet according to Embodiment 1 of the present invention is shown. Detailed Implementation

[0032] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0033] The present invention will be further illustrated by the following examples.

[0034] In the following embodiments and comparative examples: The preparation method of the hollow large particles of lithium iron phosphate with "donut" structure is as follows: (1) Weigh lithium carbonate, ferrous sulfate and ammonium dihydrogen phosphate according to the molar ratio of Li:Fe:P=1.05:1:1 (with 5% excess lithium source to compensate for loss), add deionized water and dispersant polyethylene glycol-6000, and stir to obtain a mixed slurry with a solid content of 35%; grind with a sand mill for 100 min, grind the particles to a D50 particle size of 60 nm, and then pass the slurry into a two-fluid spray dryer, control the inlet temperature of 230℃, the outlet temperature of 100℃, the atomizing pressure of 4MPa and the flow rate of 10L / h, and prepare Hollow precursor particles with a secondary particle size of 25 μm (D50) were obtained. These hollow precursor particles were mixed with glucose as the first carbon source and pre-calcined at 400°C for 4 hours under a nitrogen atmosphere to achieve primary carbon coating (carbon layer thickness 6 nm). They were then mixed with acetylene black as the second carbon source and sintered at 800°C for 7 hours under an argon atmosphere to achieve secondary carbon coating (carbon layer thickness 15 nm). After cooling, the particles were pulverized and graded to obtain hollow large-particle lithium iron phosphate cathode material with a primary particle size of 80 nm (D50), a compaction density of 3.2 g / cm³, a carbon content of 3%, and an irregular spherical shape with a concave center (such as hollow large-particle lithium iron phosphate with a "donut" structure). Figure 1 (As shown).

[0035] The medium-sized solid solid particles of lithium iron phosphate used (such as...) Figure 2 (As shown) The medium-sized solid solid lithium iron phosphate particles were prepared by solid-state sintering; the D50 particle size was 7 μm, the carbon content was 1.0%, and the specific surface area was 9 μm. 2 / g, 0.1C capacity is 156mAh / g; purchased from Changzhou Lithium Source Company; The lithium iron phosphate spherical particles used (such as...) Figure 3 (As shown) are nano / micron-sized homogeneous particles produced by hydrothermal synthesis, with a D50 particle size of 1.5 μm, a carbon content of 1.1%, and a specific surface area of ​​15 μm. 2 / g, 0.1C capacity is 158mAh / g; purchased from Changzhou Lithium Source Company; The PVDF used was purchased from Arkema, France, and its grade was HSV900. The SBR used was purchased from JSR Corporation of Japan, and its brand name is MARK 500.

[0036] Example 1

[0037] Preparation of positive electrode sheet: (1) According to the design formula, accurately weigh three types of lithium iron phosphate powders with "donut" structure: hollow large particles, medium solid solid particles, and spherical small particles. The total mass of the three types of particles is designed to account for 96.5% of the total solid mass in the positive electrode slurry. After mixing for 30 min, the particles are uniformly mixed to obtain a lithium iron phosphate particle mixture; (2) Prepare PVDF NMP adhesive with a solid content of 5.5%. Mix the PVDF NMP adhesive, single-walled carbon nanotubes and conductive graphite evenly, and add it to the lithium iron phosphate particle mixture prepared in step (1) in three parts. After mixing evenly, a slurry mixture is obtained. Finally, the slurry mixture is dispersed at high speed for 6.5 h. The dispersion conditions are double planetary dispersion, revolution at 40 rpm and rotation at 2500 rpm. After the slurry is dispersed, it is uniformly coated on the upper and lower surfaces of the aluminum foil current collector. The design value of the coating surface density on each side is 200 g / m². 2 The electrode compaction design employs high-temperature hot pressing, with a designed temperature of 90℃ and a designed compaction density of 2.6 g / cm³. 3 The electrodes are cut into corresponding sizes using a mold and then baked in an oven at 100°C for 24 hours. The mass of single-walled carbon nanotubes accounts for 0.7% of the total solid mass in the positive electrode slurry; the mass of conductive graphite accounts for 0.8% of the total solid mass in the positive electrode slurry.

[0038] Preparation of negative electrode sheet: (1) Sodium carboxymethyl cellulose (CMC) and deionized water were mixed to prepare a CMC adhesive solution with a solid content of 1.5%. The CMC adhesive solution with a solid content of 1.5% was mixed evenly with conductive carbon black, and then graphite powder (D50 particle size of 9.1 μm) was added and mixed evenly to obtain a mixture; (2) The mixture was dispersed at high speed for 4.5 h, wherein the dispersion conditions were double planetary dispersion, with a revolution of 40 rpm and a rotation of 3000 rpm; finally, SBR binder was added and dispersed at 600 rpm for 30 min to obtain a negative electrode slurry; wherein the mass of graphite powder accounted for 96.7% of the total solid mass of the negative electrode slurry, the mass of conductive carbon black accounted for 0.5% of the total solid mass of the negative electrode slurry, and the mass of binder accounted for 1.8% of the total solid mass of the negative electrode slurry. 。（ 3) The negative electrode paste is evenly coated on both the top and bottom surfaces of the copper foil, with a designed surface density of 94.5 g / m² on each side. 2 The electrode compaction density is designed to be 1.6 g / cm³. 3 The electrode sheets are cut into corresponding sizes using a mold and then baked in a 90℃ oven for 24 hours to obtain the negative electrode sheet.

[0039] Cell Assembly: After baking and passing the moisture test of the positive and negative electrode sheets, the assembly of soft-pack batteries begins. The moisture requirement for the positive electrode sheet is <300ppm, and the moisture requirement for the negative electrode sheet is <250ppm. A high-porosity separator (Xingyuan GM05 type) is used, with a 7µm PE base film, a 1.5µm ceramic layer on one side, and a 1µm PVDF adhesive on both sides. A fast-charging electrolyte (Xinzhoubang 4C lithium iron phosphate battery electrolyte) is used. The cell assembly method is a stacking process, and the external packaging is aluminum-plastic film. After assembly, the cells are baked again, and the overall moisture test shows ≤300ppm. After passing the test, electrolyte is injected. The cells undergo formation under 1300kg pressure with 0.05C charging for 120min, 0.1C charging for 120min, aging at 45℃ for 48h, and finally a full charge-discharge cycle at 0.33C before being removed from the production line. The final product cell has a voltage range of 2.5-3.65V.

[0040] Example 2

[0041] Preparation of positive electrode sheet: (1) According to the design formula, accurately weigh three types of lithium iron phosphate powders with "donut" structure: hollow large particles, medium solid solid particles, and spherical small particles. The total mass of the three types of particles is designed to account for 96.5% of the total solid mass in the positive electrode slurry. After mixing for 30 min, the particles are uniformly mixed to obtain a lithium iron phosphate particle mixture; (2) Prepare PVDF NMP adhesive with a solid content of 5.5%. Mix the PVDF NMP adhesive, single-walled carbon nanotubes (CNTs), and conductive graphite sp evenly. Add the lithium iron phosphate particle mixture prepared in step (1) in three batches. After mixing evenly, a slurry mixture is obtained. Finally, the slurry mixture is dispersed at high speed for 6.5 h. The dispersion conditions are double planetary dispersion, with a revolution of 40 rpm and a rotation of 2500 rpm. After the slurry is dispersed, it is uniformly coated on the upper and lower surfaces of the aluminum foil current collector. The design value of the coating surface density on each side is 200 g / m². 2 The electrode compaction design employs high-temperature hot pressing, with a designed temperature of 90℃ and a designed compaction density of 2.6 g / cm³. 3 The electrodes are cut into corresponding sizes using a mold and then baked in an oven at 100°C for 24 hours. The mass of single-walled carbon nanotubes accounts for 0.7% of the total solid mass in the positive electrode slurry; the mass of conductive graphite accounts for 0.8% of the total solid mass in the positive electrode slurry.

[0042] Negative electrode preparation: Same as in Example 1.

[0043] Cell assembly: Same as in Example 1

[0044] Example 3

[0045] Preparation of positive electrode sheet: (1) According to the design formula, accurately weigh three types of lithium iron phosphate powders with "donut" structure: hollow large particles, medium solid solid particles, and spherical small particles. The total mass of the three types of particles is designed to account for 96.5% of the total solid mass in the positive electrode slurry. After mixing for 30 min, the particles are uniformly mixed to obtain a lithium iron phosphate particle mixture; (2) Prepare a PVDF NMP solution with a solid content of 5.5%. Mix the PVDF NMP solution, single-walled carbon nanotubes (CNTs), and conductive graphite sp evenly. Add the lithium iron phosphate particle mixture prepared in step (1) in three batches. After mixing evenly, a slurry mixture is obtained. Finally, the slurry mixture is dispersed at high speed for 6.5 h. The dispersion conditions are double planetary dispersion, with a revolution of 40 rpm and a rotation of 2500 rpm. After the slurry is dispersed, it is uniformly coated on the upper and lower surfaces of the aluminum foil current collector. The design value of the coating surface density on each side is 200 g / m². 2 The electrode compaction design employs high-temperature hot pressing, with a designed temperature of 90℃ and a designed compaction density of 2.6 g / cm³. 3 The electrodes are cut into corresponding sizes using a mold and then baked in an oven at 100°C for 24 hours. The mass of single-walled carbon nanotubes accounts for 0.7% of the total solid mass in the positive electrode slurry; the mass of conductive graphite accounts for 0.8% of the total solid mass in the positive electrode slurry.

[0046] Negative electrode preparation: Same as in Example 1.

[0047] Cell assembly: Same as in Example 1

[0048] Example 4

[0049] Preparation of positive electrode sheet: (1) According to the design formula, accurately weigh two types of lithium iron phosphate powders with "donut" structure, hollow large particles and medium solid solid particles, in a mass ratio of 3:7. The total mass of the two types of particles is designed to account for 96.5% of the total solid mass in the positive electrode slurry. After mixing for 30 min, the particles are uniformly mixed to obtain a lithium iron phosphate particle mixture; (2) Prepare a PVDF NMP solution with a solid content of 5.5%. Mix the PVDF NMP solution, single-walled carbon nanotubes (CNTs), and conductive graphite sp evenly. Add the lithium iron phosphate particle mixture prepared in step (1) in three batches. After mixing evenly, a slurry mixture is obtained. Finally, the slurry mixture is dispersed at high speed for 6.5 h. The dispersion conditions are double planetary dispersion, with a revolution of 40 rpm and a rotation of 2500 rpm. After the slurry is dispersed, it is uniformly coated on the upper and lower surfaces of the aluminum foil current collector. The design value of the coating surface density on each side is 400 g / m². 2 The electrode compaction design employs high-temperature hot pressing, with a designed temperature of 90℃ and a designed compaction density of 2.6 g / cm³. 3The electrodes are cut into corresponding sizes using a mold and then baked in an oven at 100°C for 24 hours. The mass of single-walled carbon nanotubes accounts for 0.7% of the total solid mass in the positive electrode slurry; the mass of conductive graphite accounts for 0.8% of the total solid mass in the positive electrode slurry.

[0050] Negative electrode preparation: Same as in Example 1.

[0051] Cell assembly: Same as in Example 1.

[0052] Example 5

[0053] (1) According to the design formula, accurately weigh three types of lithium iron phosphate powders with a "donut" structure: hollow large particles, medium solid solid particles, and spherical small particles. The total mass of the three types of particles is designed to account for 96.5% of the total solid mass in the positive electrode slurry. After mixing for 30 minutes, the particles are uniformly mixed to obtain a lithium iron phosphate particle mixture. (2) Prepare a PVDF NMP solution with a solid content of 5.5%. Mix the PVDF NMP solution, single-walled carbon nanotubes (CNTs), and conductive graphite sp evenly. Add the lithium iron phosphate particle mixture prepared in step (1) in three batches. After mixing evenly, a slurry mixture is obtained. Finally, the slurry mixture is dispersed at high speed for 6.5 hours. The dispersion conditions are double planetary dispersion, with a revolution of 40 rpm and a rotation of 2500 rpm. After the slurry is dispersed, it is uniformly coated on the upper and lower surfaces of the aluminum foil current collector. The designed coating surface density of each surface is 175 g / m². 2 The electrode compaction design employs high-temperature hot pressing, with a designed temperature of 90℃ and a designed compaction density of 2.6 g / cm³. 3 The electrodes are cut into corresponding sizes using a mold and then baked in an oven at 100°C for 24 hours. The mass of single-walled carbon nanotubes accounts for 0.7% of the total solid mass in the positive electrode slurry; the mass of conductive graphite accounts for 0.8% of the total solid mass in the positive electrode slurry.

[0054] Negative electrode preparation: Same as in Example 1.

[0055] Cell assembly: Same as in Example 1.

[0056] Example 6

[0057] (1) According to the design formula, accurately weigh three types of lithium iron phosphate powders with a "donut" structure: hollow large particles, medium solid solid particles, and spherical small particles. The total mass of the three types of particles is designed to account for 96.5% of the total solid mass in the positive electrode slurry. After mixing for 30 minutes, the particles are uniformly mixed to obtain a lithium iron phosphate particle mixture. (2) Prepare a PVDF NMP solution with a solid content of 5.5%. Mix the PVDF NMP solution, single-walled carbon nanotubes (CNTs), and conductive graphite sp evenly. Add the lithium iron phosphate particle mixture prepared in step (1) in three batches. After mixing evenly, a slurry mixture is obtained. Finally, the slurry mixture is dispersed at high speed for 6.5 hours. The dispersion conditions are double planetary dispersion, with a revolution of 40 rpm and a rotation of 2500 rpm. After the slurry is dispersed, it is uniformly coated on the upper and lower surfaces of the aluminum foil current collector. The designed coating surface density of each surface is 225 g / m². 2 The electrode compaction design employs high-temperature hot pressing, with a designed temperature of 90℃ and a designed compaction density of 2.6 g / cm³. 3 The electrodes are cut into corresponding sizes using a mold and then baked in an oven at 100°C for 24 hours. The mass of single-walled carbon nanotubes accounts for 0.7% of the total solid mass in the positive electrode slurry; the mass of conductive graphite accounts for 0.8% of the total solid mass in the positive electrode slurry.

[0058] Negative electrode preparation: Same as in Example 1.

[0059] Cell assembly: Same as in Example 1.

[0060] Example 7

[0061] (1) According to the design formula, accurately weigh three types of lithium iron phosphate powders with a "donut" structure: hollow large particles, medium solid solid particles, and spherical small particles. The total mass of the three types of particles is designed to account for 96.5% of the total solid mass in the positive electrode slurry. After mixing for 30 minutes, the particles are uniformly mixed to obtain a lithium iron phosphate particle mixture. (2) Prepare a PVDF NMP solution with a solid content of 5.5%. Mix the PVDF NMP solution, single-walled carbon nanotubes (CNTs), and conductive graphite sp evenly. Add the lithium iron phosphate particle mixture prepared in step (1) in three batches. After mixing evenly, a slurry mixture is obtained. Finally, the slurry mixture is dispersed at high speed for 6.5 hours. The dispersion conditions are double planetary dispersion, with a revolution of 40 rpm and a rotation of 2500 rpm. After the slurry is dispersed, it is uniformly coated on the upper and lower surfaces of the aluminum foil current collector. The designed coating surface density of each surface is 250 g / m². 2 The electrode compaction design employs high-temperature hot pressing, with a designed temperature of 90℃ and a designed compaction density of 2.6 g / cm³. 3The electrodes are cut into corresponding sizes using a mold and then baked in an oven at 100°C for 24 hours. The mass of single-walled carbon nanotubes accounts for 0.7% of the total solid mass in the positive electrode slurry; the mass of conductive graphite accounts for 0.8% of the total solid mass in the positive electrode slurry.

[0062] Negative electrode preparation: Same as in Example 1.

[0063] Cell assembly: Same as in Example 1.

[0064] Example 8

[0065] (1) According to the design formula, accurately weigh three types of lithium iron phosphate powders with a "donut" structure: hollow large particles, medium solid solid particles, and spherical small particles. The total mass of the three types of particles is designed to account for 96.5% of the total solid mass in the positive electrode slurry. After mixing for 30 minutes, the particles are uniformly mixed to obtain a lithium iron phosphate particle mixture. (2) Prepare a PVDF NMP solution with a solid content of 5.5%. Mix the PVDF NMP solution, single-walled carbon nanotubes (CNTs), and conductive graphite sp evenly. Add the lithium iron phosphate particle mixture prepared in step (1) in three batches. After mixing evenly, a slurry mixture is obtained. Finally, the slurry mixture is dispersed at high speed for 6.5 hours. The dispersion conditions are double planetary dispersion, with a revolution of 40 rpm and a rotation of 2500 rpm. After the slurry is dispersed, it is uniformly coated on the upper and lower surfaces of the aluminum foil current collector. The design value of the coating surface density on each side is 200 g / m². 2 The electrode compaction design employs high-temperature hot pressing, with a designed temperature of 90℃ and a designed compaction density of 2.5 g / cm³. 3 The electrodes are cut into corresponding sizes using a mold and then baked in an oven at 100°C for 24 hours. The mass of single-walled carbon nanotubes accounts for 0.7% of the total solid mass in the positive electrode slurry; the mass of conductive graphite accounts for 0.8% of the total solid mass in the positive electrode slurry.

[0066] Negative electrode preparation: Same as in Example 1.

[0067] Cell assembly: Same as in Example 1.

[0068] Example 9

[0069] (1) According to the design formula, accurately weigh three types of lithium iron phosphate powders with a "donut" structure: hollow large particles, medium solid solid particles, and spherical small particles. The total mass of the three types of particles is designed to account for 96.5% of the total solid mass in the positive electrode slurry. After mixing for 30 minutes, the particles are uniformly mixed to obtain a lithium iron phosphate particle mixture. (2) Prepare a PVDF NMP solution with a solid content of 5.5%. Mix the PVDF NMP solution, single-walled carbon nanotubes (CNTs), and conductive graphite sp evenly. Add the lithium iron phosphate particle mixture prepared in step (1) in three batches. After mixing evenly, a slurry mixture is obtained. Finally, the slurry mixture is dispersed at high speed for 6.5 hours. The dispersion conditions are double planetary dispersion, with a revolution of 40 rpm and a rotation of 2500 rpm. After the slurry is dispersed, it is uniformly coated on the upper and lower surfaces of the aluminum foil current collector. The design value of the coating surface density on each side is 200 g / m². 2 The electrode compaction design employs high-temperature hot pressing, with a designed temperature of 90℃ and a designed compaction density of 2.7 g / cm³. 3 The electrodes are cut into corresponding sizes using a mold and then baked in an oven at 100°C for 24 hours. The mass of single-walled carbon nanotubes accounts for 0.7% of the total solid mass in the positive electrode slurry; the mass of conductive graphite accounts for 0.8% of the total solid mass in the positive electrode slurry.

[0070] Negative electrode preparation: Same as in Example 1.

[0071] Cell assembly: Same as in Example 1.

[0072] Comparative Example 1

[0073] Preparation of positive electrode sheet: (1) Prepare NMP solution of PVDF with a solid content of 5.5%. Mix the NMP solution of PVDF, single-walled carbon nanotubes (CNTs) and conductive graphite sp evenly. Add the hollow large particles of lithium iron phosphate with "donut" structure in three batches. After mixing evenly, a slurry mixture is obtained. Finally, the slurry mixture is dispersed at high speed for 6.5 hours. The dispersion conditions are double planetary dispersion, with a revolution of 40 rpm and a rotation of 2500 rpm. After the slurry is dispersed, it is evenly coated on the upper and lower surfaces of the aluminum foil current collector. The design value of the coating surface density on each side is 200 g / m². 2 The electrode compaction design employs high-temperature hot pressing, with a designed temperature of 90℃ and a designed compaction density of 2.18 g / cm³. 3(The highest compaction achievable by using this material alone), cut into corresponding size electrode sheets using a mold, and finally baked in an oven at 100℃ for 24 hours. Among them, the mass of hollow lithium iron phosphate particles with a "donut" structure accounts for 96.5% of the total solid mass in the positive electrode slurry; the mass of single-walled carbon nanotubes accounts for 0.7% of the total solid mass in the positive electrode slurry; and the mass of conductive graphite accounts for 0.8% of the total solid mass in the positive electrode slurry.

[0074] Negative electrode preparation: Same as in Example 1.

[0075] Cell assembly: Same as in Example 1.

[0076] Comparative Example 2

[0077] (1) Prepare a PVDF NMP adhesive solution with a solid content of 5.5%. Mix the PVDF NMP adhesive solution, single-walled carbon nanotubes (CNTs), and conductive graphite sp evenly. Add medium-sized solid lithium iron phosphate particles in three batches and mix evenly to obtain a slurry mixture. Finally, disperse the slurry mixture at high speed for 6.5 hours. The dispersion conditions are dual planetary dispersion, with an orbital speed of 40 rpm and a rotational speed of 2500 rpm. After the slurry is dispersed, it is uniformly coated on the upper and lower surfaces of the aluminum foil current collector. The designed coating surface density for each surface is 200 g / m². 2 The electrode compaction design employs high-temperature hot pressing, with a designed temperature of 90℃ and a designed compaction density of 2.55 g / cm³. 3 (The highest compaction achievable by using this material alone), cut into electrode sheets of corresponding sizes using a mold, and finally baked in an oven at 100°C for 24 hours. The mass of medium-sized solid lithium iron phosphate particles accounts for 96.5% of the total solid mass of the cathode slurry; the mass of single-walled carbon nanotubes accounts for 0.7% of the total solid mass of the cathode slurry; and the mass of conductive graphite accounts for 0.8% of the total solid mass of the cathode slurry.

[0078] Negative electrode preparation: Same as in Example 1.

[0079] Cell assembly: Same as in Example 1.

[0080] Comparative Example 3

[0081] (1) Prepare a PVDF NMP adhesive solution with a solid content of 5.5%. Mix the PVDF NMP adhesive solution, single-walled carbon nanotubes (CNTs), and conductive graphite sp evenly. Add lithium iron phosphate spherical particles in three batches and mix evenly to obtain a slurry mixture. Finally, disperse the slurry mixture at high speed for 6.5 hours. The dispersion conditions are double planetary dispersion, with an orbital speed of 40 rpm and a rotational speed of 2500 rpm. After the slurry is dispersed, it is uniformly coated on the upper and lower surfaces of the aluminum foil current collector. The designed coating surface density for each surface is 200 g / m². 2The electrode compaction design employs high-temperature hot pressing, with a designed temperature of 90℃ and a designed compaction density of 2.43 g / cm³. 3 (The highest compaction achievable by using this material alone), cut into electrode sheets of corresponding sizes using a mold, and finally baked in an oven at 100°C for 24 hours. The mass of medium-sized solid lithium iron phosphate particles accounts for 96.5% of the total solid mass of the cathode slurry; the mass of single-walled carbon nanotubes accounts for 0.7% of the total solid mass of the cathode slurry; and the mass of conductive graphite accounts for 0.8% of the total solid mass of the cathode slurry.

[0082] Negative electrode preparation: Same as in Example 1.

[0083] Cell assembly: Same as in Example 1.

[0084] Test case

[0085] The finished battery cells prepared in the above embodiments and comparative examples were subjected to performance tests. The specific test methods are as follows, and the specific test results are shown in the table below.

[0086] Discharge capacity retention test: After each cell was calibrated by charging and discharging three times at 0.33C, it was fully charged at 0.33C, and then discharged at three different rates: 0.33C, 0.5C, and 1.0C. (Discharge capacity at different rates / charging capacity) 100% = Capacity retention rate at different discharge rates.

[0087] AC internal resistance test: Select GEIS AC impedance spectrum test mode, and set the parameters as follows: bias current 1A, frequency range: 10KHz-10mHz.

[0088] 0.33C constant current charge ratio: After charging and discharging each cell three times with a 0.33C current, the capacity is calibrated. Under constant current and constant voltage conditions, the cells are fully charged with a 0.33C current and recorded as Q0. The constant current charge capacity Q1 is recorded by checking the charging data. The constant current charge ratio = (Q1 / Q0). 100%.

[0089] 50% SOC Discharge Power: After charging and discharging each cell three times with a 0.33C current, the capacity is calibrated. The cells are then fully charged with a 0.33C current and discharged to 50% capacity, with the initial voltage V0 recorded. Next, the cells are discharged at a 5C current for 10 seconds, and the final voltage V1 is recorded. The DC internal resistance during discharge is then calculated as R = (V0 - V1) / 10C, followed by the power calculation P = 2.5. ((V1-2.5) / R) 1000.

[0090] -7℃ and -20℃ capacity retention calculation: After charging and discharging each cell three times with a 0.33C current, the capacity was calibrated. Then, the cells were fully charged with a 0.33C current, and placed in constant temperature chambers at -7℃ and -20℃ for 4 hours respectively. Afterward, they were discharged at a 0.33C current to 2.5V. (Discharge capacity / Charging capacity) 100% = Discharge capacity retention rate.

[0091] Table 1 Test Results

[0092] Table 2 Test Results II

[0093] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A lithium iron phosphate battery cathode slurry, characterized in that, The lithium iron phosphate battery cathode slurry includes: large hollow lithium iron phosphate particles with a "donut" structure, medium-sized solid lithium iron phosphate particles, optional small spherical lithium iron phosphate particles, single-walled carbon nanotubes, conductive graphite, and polyvinylidene fluoride adhesive.

2. The lithium iron phosphate battery cathode slurry according to claim 1, wherein, The sum of the masses of hollow large lithium iron phosphate particles with a "donut" structure, medium-sized solid lithium iron phosphate particles, and optional spherical small lithium iron phosphate particles accounts for 93-98% of the total solid mass in the slurry; The mass of single-walled carbon nanotubes accounts for 0.02% to 1% of the total solid mass in the slurry; The mass of conductive graphite accounts for 0.8~1.5% of the total solid mass in the slurry; The solid content of the polyvinylidene fluoride adhesive is 3-7%; Preferably, the total mass percentage of the hollow lithium iron phosphate particles with a "donut" structure, the medium-sized solid lithium iron phosphate particles, and the optional spherical lithium iron phosphate particles is 10-50%, the total mass percentage of the hollow lithium iron phosphate particles with a "donut" structure is 50-90%, and the total mass percentage of the spherical lithium iron phosphate particles is 0-20%.

3. The lithium iron phosphate battery cathode slurry according to claim 1, wherein, The hollow lithium iron phosphate particles with a "donut" structure have a primary particle size (D50) of 10-100 nm, a secondary particle size (D50) of 15-30 μm, and a compaction density of 2.8-3.8 g / cm³. 3 The carbon content is 3-8%.

4. The lithium iron phosphate battery cathode slurry according to claim 3, wherein, The hollow lithium iron phosphate particles with a "donut" structure are prepared by a method comprising the following steps: (1) Mix lithium source, iron source, phosphorus source, dispersant and water evenly to obtain a mixed slurry; (2) Grind the mixed slurry until the particles are ground to 10-100 nm, then pass the slurry into a two-fluid spray dryer to prepare hollow precursor particles with a secondary particle D50 particle size of 15-30 μm. (3) The hollow precursor particles are mixed with the first carbon source and pre-fired in the first protective gas atmosphere to complete the first carbon coating; then mixed with the second carbon source and sintered in the second protective gas atmosphere to achieve the second carbon coating; finally, after cooling, they are crushed and graded to obtain the hollow large particles of lithium iron phosphate with the "donut" structure. Preferably, in step (1), the lithium source is lithium carbonate, the iron source is ferrous sulfate, and the phosphorus source is ammonium dihydrogen phosphate; the molar ratio of lithium source (calculated as lithium element), iron source (calculated as iron element), and phosphorus source (calculated as phosphorus element) is (1.02-1.05):1:1; the dispersant is polyethylene glycol; the solid content of the mixed slurry is 30-40%; in step (2), the grinding is carried out using a sand mill, and the grinding time is 80-120 min; the process parameters of the two-fluid spray dryer include: controlling the inlet temperature 2 20-250℃, outlet temperature 90-105℃, atomizing gas pressure 3.5-5MPa, flow rate 8-12L / h; in step (3), the first carbon source is glucose; the pre-calcination temperature is 380-420℃, and the time is 3-5h; the first protective gas is nitrogen; the thickness of the carbon layer in the first carbon coating is 2-6nm; the second carbon source is acetylene black; the sintering temperature is 780-820℃, and the time is 6-8h; the second protective gas is argon; the thickness of the carbon layer in the second carbon coating is 10-30nm.

5. The lithium iron phosphate battery cathode slurry according to claim 1, wherein, The medium-sized solid solid particles of lithium iron phosphate were prepared by solid-state sintering. The lithium iron phosphate medium-sized solid solid particles have a D50 particle size of 1.5~8 μm, a carbon content of 0.1~1.0%, a specific surface area of ​​5~10 m² / g, and a 0.1C specific capacity of ≥156 mAh / g. The lithium iron phosphate nanospheres are nano / micron-sized homogeneous particles produced by hydrothermal method, with a D50 particle size of 500 nm to 2 μm, a carbon content of 1.0 to 2.0%, a specific surface area of ​​15 to 25 m² / g, and a 0.1C specific capacity of ≥158 mAh / g. The preferred polyvinylidene fluoride (PVDF) adhesive is an N-methylpyrrolidone (NMP) adhesive for PVDF.

6. A method for preparing a positive electrode sheet, characterized in that, The preparation method includes: coating the lithium iron phosphate battery positive electrode slurry according to any one of claims 1-5 onto the upper and lower surfaces of the current collector, and performing electrode rolling and baking to obtain the lithium iron phosphate positive electrode.

7. The preparation method according to claim 6, wherein, The current collector is an aluminum foil current collector; The surface density of the coating on each side is 170~270 g / m². 2 ; The electrode sheets are rolled using high-temperature hot pressing at 50~100℃, with a compaction density of 2.5~2.75 g / cm³. 3 ; After the electrode sheets are crushed and before baking, a sheet-making process is also performed.

8. The positive electrode sheet prepared by the preparation method according to claim 6 or 7.

9. A lithium iron phosphate battery, characterized in that, The lithium iron phosphate battery includes the positive electrode sheet as described in claim 8.

10. The lithium iron phosphate battery according to claim 9, wherein, The lithium iron phosphate battery also includes a negative electrode sheet; The negative electrode slurry used to prepare the negative electrode sheet includes: graphite powder, conductive carbon black, binder and sodium carboxymethyl cellulose (CMC) adhesive; Preferably, the adhesive is styrene-butadiene rubber (SBR); The sodium carboxymethyl cellulose (CMC) adhesive has a solid content of 1.5-3%; the graphite powder accounts for 93-97.5% of the total solid mass of the negative electrode slurry; the conductive carbon black accounts for 0.5-6.5% of the total solid mass of the negative electrode slurry; and the binder accounts for 0.2-3.0% of the total solid mass of the negative electrode slurry.

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