Lithium iron phosphate material and preparation method and application thereof

Abstract

The invention relates to the technical field of secondary batteries, in particular to a lithium iron phosphate material and a preparation method and application thereof.The lithium iron phosphate material is prepared through combined use of a first lithium iron phosphate primary sintered product and a second lithium iron phosphate primary sintered product and control over the particle size of slurry corresponding to the first lithium iron phosphate primary sintered product and the second lithium iron phosphate primary sintered product, and large particles are used for forming a basic framework to provide support; gaps among the large particles are filled with the medium particles, residual micropores of the medium particles and the large particles are further filled with the fine particles, and the most effective filling of the finished product is realized by regulating and controlling the optimal particle size ratio of the medium particles to the large particles, so that the relatively high compaction density is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a lithium iron phosphate material, a preparation method and application thereof. BACKGROUND

[0002] Lithium iron phosphate (LiFePO4, LFP for short) has been widely used in power and energy storage battery fields, and has broad application prospects due to its high safety, long cycle life and low cost. However, on the one hand, its compaction density is low. On the other hand, due to the structural defects of LiFePO4 itself, it has low electronic conductivity and lithium ion diffusion coefficient, which leads to its underutilization at high current charge and discharge and unsatisfactory rate performance. SUMMARY

[0003] The present application provides a lithium iron phosphate material, a preparation method and application thereof to solve the problems of low compaction density and unsatisfactory rate performance of existing lithium iron phosphate materials.

[0004] In a first aspect, the present application provides a preparation method of a lithium iron phosphate material, comprising the following steps: S1 step: etching iron red particles to obtain an iron red framework; S2 step: wet grinding the iron red framework, a first phosphorus source, a first lithium source, a first carbon source A and water, spray drying and then performing first sintering to obtain a first lithium iron phosphate one-sintered product; S3 step: mixing the first lithium iron phosphate one-sintered product with water to obtain a first slurry, grinding the first slurry to a particle size Dv50 of 0.7-2.5 μm; mixing the second lithium iron phosphate one-sintered product with water to obtain a second slurry, and grinding the second slurry to a particle size Dv50 of 0.2-0.6 μm; S4 step: mixing the ground first slurry and the ground second slurry, spray drying, performing second sintering, and then crushing to obtain a lithium iron phosphate material; wherein the face fraction ρ, the pore influence coefficient δ and the specific surface area S of the first lithium iron phosphate one-sintered product obtained in the S2 step satisfy the following relationship: 0.16≤ ρ / (3.6× (1- δ × S)) ≤0.30; wherein 0.5≤ ρ≤0.9; 0.5≤ S≤5.0; 2 / g; Wherein ρ is the face fraction of the first lithium iron phosphate one-sintered product, which is a dimensionless value; S is the specific surface area of the first lithium iron phosphate one-sintered product, with a unit of m 2 / g; δ is the pore influence coefficient of the first lithium iron phosphate one-sintered product, with a unit of g / m 2 ; δ is calculated using the following formula: δ = (D × P) / φ, where D is the average pore size of the first lithium iron phosphate precipitate in nm; and P is the true density of the first lithium iron phosphate precipitate in g / cm³. 3 φ represents the open porosity of the first lithium iron phosphate product, which is a dimensionless value.

[0005] In some optional embodiments, the first phosphorus source is selected from one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, iron pyrophosphate, lithium dihydrogen phosphate, or lithium phosphate. And / or, the first lithium source is selected from one or more of lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, or lithium hydroxide; And / or, the first carbon source A is selected from one or more of sucrose, glucose, starch, PEG, ascorbic acid, cellulose, PVA, cyclodextrin, polyacrylonitrile, or PVP; And / or, in step S2, the iron oxide skeleton, the first phosphorus source, and the first lithium source are mixed in a Li:Fe:P molar ratio of 0.96–1.05: 0.96–1.05: 0.96–1.05; And / or, the amount of the first carbon source A added is 3-6 wt% of the iron oxide skeleton; And / or, the solid content of the first slurry is 40%-60%; And / or, the solid content of the second slurry is 40%-60%; And / or, the first sintering includes: in an ammonia atmosphere or a mixture of ammonia and nitrogen, activation with ammonia at 300-400°C for 1-4 hours, then switching to a nitrogen or inert atmosphere, holding at 420-500°C for 2-4 hours, and subsequently holding at 600-800°C for 3-10 hours. And / or, the second sintering includes: heating to 500-820°C at a rate of 3-10°C / min under a protective atmosphere and holding at that temperature for 4-10 hours; And / or, the inlet air temperature of the spray dryer is 180~280℃, and the outlet air temperature is 80~130℃; And / or, 0.17 ≤ ρ / (3.6 × (1 - δ × S)) ≤ 0.24.

[0006] In some optional embodiments, the method for preparing the second lithium iron phosphate product is selected from one or more of method one or method two; Method 1 includes wet grinding of iron source, second phosphorus source, second lithium source, second carbon source A and water, followed by spray drying and first sintering to obtain a second lithium iron phosphate calcined product. Method 2 involves etching the iron oxide particles to obtain the iron oxide skeleton; then wet-grinding the iron oxide skeleton, the third phosphorus source, the third lithium source, the second carbon source B, and water, followed by spray drying and a first sintering to obtain the second lithium iron phosphate sintered product. Optionally, the second phosphorus source and the third phosphorus source are independently selected from one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, iron pyrophosphate, lithium dihydrogen phosphate, or lithium phosphate. Optionally, the second lithium source and the third lithium source are independently selected from one or more of lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, or lithium hydroxide; Optionally, the second carbon source A and the second carbon source B are independently selected from one or more of sucrose, glucose, starch, PEG, ascorbic acid, cellulose, PVA, cyclodextrin, polyacrylonitrile, or PVP. Optionally, the iron source, the second phosphorus source, and the second lithium source are mixed in a Li:Fe:P molar ratio of 0.96–1.05:0.96–1.05:0.96–1.05; Optionally, the iron oxide skeleton, the third phosphorus source, and the third lithium source are mixed in a Li:Fe:P molar ratio of 0.96–1.05:0.96–1.05:0.96–1.05; Optionally, the mass of the second carbon source A is 3-6 wt% of the mass of the iron source; Optionally, the mass of the second carbon source B is 3-6 wt% of the mass of the iron oxide skeleton; Optionally, in Method 2, the first sintering includes: in an ammonia protective atmosphere, activation with ammonia at 300~400℃ for 1~4h, then switching to nitrogen or an inert atmosphere, holding at 420~500℃ for 2~4h, and subsequently holding at 600~800℃ for 5~10h. Optionally, in Method 1, the first sintering includes: heating to 450-650°C at a rate of 1-5°C / min in a nitrogen atmosphere, and holding at 450-650°C for 5-10 hours.

[0007] In some alternative implementations, at least one of the following conditions must be met: A. When preparing the first lithium iron phosphate monocalcined product or the second lithium iron phosphate monocalcined product, an acid solution is used to etch the iron oxide particles; optionally, the acid solution includes one or more of phosphoric acid solution, hydrochloric acid solution, and sulfuric acid solution; B. The iron oxide particles used in the preparation of the first or second lithium iron phosphate calcined product are composed of α-type Fe2O3, and the particle size Dv50 of the iron oxide particles is 0.5~5μm. C. In the preparation of the first or second lithium iron phosphate monocalcined product, an additive mixing step is included before wet milling; optionally, the additive is independently selected from one or more of vanadium pentoxide, ammonium metavanadate, titanium dioxide, boric acid, cerium oxide, molybdenum oxide, magnesium oxide, aluminum oxide, or strontium carbonate; optionally, the amount of dopant added is 0.2~1wt% of the theoretically synthesized lithium iron phosphate. The mixing steps D and S3 further include the step of mixing a first carbon source B into the first lithium iron phosphate calcined product; the mass of the first carbon source B is 5 to 12 wt% of the mass of the first lithium iron phosphate calcined product; optionally, the first carbon source B is selected from one or more of sucrose, glucose, starch, PEG, ascorbic acid, cellulose, PVA, cyclodextrin, polyacrylonitrile or PVP. The E and S3 mixing steps also include a step of mixing a second carbon source C into the second lithium iron phosphate calcined product; the mass of the second carbon source C is 5 to 12 wt% of the mass of the second lithium iron phosphate calcined product; optionally, the second carbon source C is selected from one or more of sucrose, glucose, starch, PEG, ascorbic acid, cellulose, PVA, cyclodextrin, polyacrylonitrile or PVP.

[0008] In some optional embodiments, in step S4, the mass ratio of the first lithium iron phosphate calcined product in the first slurry to the second lithium iron phosphate calcined product in the second slurry during mixing is 3:7-9:1; preferably 7:3-8:2.

[0009] In some optional embodiments, the method for preparing the lithium iron phosphate material satisfies at least one of the following: A. The surface of the first lithium iron phosphate product includes a carbon coating layer, and optionally, the thickness of the carbon coating layer is 1-20 nm; B. The surface of the first lithium iron phosphate product includes grain boundaries; optionally, the width of the grain boundaries is 0.5-100 nm.

[0010] Secondly, the present invention provides a lithium iron phosphate material prepared by the method described in the first aspect.

[0011] Thirdly, the present invention provides a positive electrode sheet, comprising: a positive current collector, and a positive active material layer disposed on at least one side of the positive current collector, wherein the positive active material layer comprises the lithium iron phosphate material described in the second aspect.

[0012] Fourthly, the present invention provides a secondary battery comprising the positive electrode sheet described in the third aspect.

[0013] In some embodiments, the secondary battery further includes a negative electrode, an electrolyte, and a separator.

[0014] Fifthly, the present invention provides an electrical device comprising the secondary battery described in the fourth aspect.

[0015] The technical solution of this invention has the following advantages: 1. The method for preparing lithium iron phosphate material provided by the present invention includes the following steps: Step S1: Etching iron oxide particles to obtain an iron oxide skeleton; Step S2: Wet grinding of the iron oxide skeleton, a first phosphorus source, a first lithium source, a first carbon source A, and water, followed by spray drying and a first sintering to obtain a first lithium iron phosphate sintered product; Step S3: Mixing the first lithium iron phosphate sintered product with water to prepare a first slurry, grinding the first slurry to a particle size Dv50 of 0.7-2.5 μm; mixing the second lithium iron phosphate sintered product with water to prepare a second slurry, grinding the second slurry to a particle size Dv50 of 0.2-0.6 μm; Step S4: Mixing the ground first slurry and the ground second slurry, followed by spray drying and a second sintering and pulverization to obtain the lithium iron phosphate material; wherein, the surface fraction ρ, porosity influence coefficient δ, and specific surface area Sm of the first lithium iron phosphate sintered product obtained in Step S2 are... 2 The following relationship exists between ρ and S: 0.16 ≤ ρ / (3.6 × (1 - δ × S)) ≤ 0.30; where 0.5 ≤ ρ ≤ 0.9; 0.5 ≤ S ≤ 5.0.

[0016] First, by using the etched red iron skeleton as the iron source to prepare lithium iron phosphate sintered product, the sintering activity of the red iron particles during the sintering process can be improved, the sintering kinetics of the powder can be enhanced, giving it high specific surface energy and good compactness, thereby improving the compaction density of the finished product.

[0017] By combining first and second lithium iron phosphate monocalcined products and controlling the particle size of their corresponding slurries, large particles form the basic framework, providing support. Medium particles fill the gaps between the large particles, while fine particles further fill the micropores remaining in the medium and large particles. By adjusting the optimal particle size ratio, the finished product achieves the most effective filling, resulting in a high compaction density. Furthermore, the presence of small particles increases the number and density of contact points between particles, shortening the lithium ion diffusion path and constructing an excellent ionic and electronic conductivity network for the system, thereby effectively improving the rate performance of the material.

[0018] By controlling the ρ / (3.6×(1 - δ × S)) (referred to as "relationship 1") of the first calcined lithium iron phosphate product within the range of 0.16-0.30, the first calcined lithium iron phosphate particles have moderate particle size and strength and suitable pore structure. After sand milling and secondary calcination, the particles can undergo moderate plastic deformation or slight breakage and reorganization during rolling. At the same time, the retained pore structure will be partially retained or evolved into short-circuit channels for ion transport after secondary sintering, ultimately achieving a unity between the densely stacked macrostructure and the highly efficient ion-conducting microstructure, realizing a balance between high compaction density and excellent electrochemical performance. When ρ / (3.6×(1 - δ × S)) < 0.16, the porosity of the first calcined lithium iron phosphate product will be too low, making the product too dense and hard, resulting in irregular particle shape and sharp edges after sand milling. During rolling, these hard and irregularly shaped particles are like a pile of "pebbles" with few contact points between them, which easily leads to a bridging effect and makes it impossible to effectively fill the gaps, resulting in a low compaction density of the finished product. When ρ / (3.6×(1 - δ × S))>0.30, the porosity of the first lithium iron phosphate product is too high. After sand milling, the particles exhibit a loose, coral-like or irregular morphology with a large number of depressions. The particle strength is very low, the shape is extremely irregular, and the surface is rough. During rolling, plastic deformation occurs or even crushing occurs, resulting in extremely low compaction density. At the same time, the crushed particles hinder ion conduction and increase side reactions, leading to poor capacity performance, poor rate performance, and shortened cycle life.

[0019] In summary, by employing a first lithium iron phosphate calcined product that satisfies the above relationship 1 and whose ρ and S are within the above range, and by using the first lithium iron phosphate calcined product and the second lithium iron phosphate calcined product in combination to control the particle size of their corresponding slurries, it is possible to achieve moderate particle strength in lithium iron phosphate materials while solving the problem of ion migration within the particles. By regulating the stacking method between particles to increase compaction density, it is possible to improve both ionic and electronic conductivity, thereby achieving a balance between high compaction density and excellent electrochemical performance. Detailed Implementation

[0020] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0021] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0022] In a first aspect, the present invention provides a method for preparing lithium iron phosphate material, comprising the following steps: S1 step: etching iron oxide particles to obtain an iron oxide skeleton; S2 step: wet grinding the iron oxide skeleton, a first phosphorus source, a first lithium source, a first carbon source A, and water, spray drying, and then performing a first sintering to obtain a first lithium iron phosphate sintered product; S3 step: mixing the first lithium iron phosphate sintered product with water to prepare a first slurry, grinding the first slurry to a particle size Dv50 of 0.7-2.5 μm; mixing the second lithium iron phosphate sintered product with water to prepare a second slurry, grinding the second slurry to a particle size Dv50 of 0.2-0.6 μm; S4 step: mixing the ground first slurry and the ground second slurry, spray drying, performing a second sintering, and pulverizing to obtain the lithium iron phosphate material; wherein, the surface fraction ρ, porosity influence coefficient δ, and specific surface area Sm of the first lithium iron phosphate sintered product obtained in step S2 are specified. 2 The following relationship exists between ρ and S: 0.16 ≤ ρ / (3.6 × (1 - δ × S)) ≤ 0.30; where 0.5 ≤ ρ ≤ 0.9; 0.5 ≤ S ≤ 5.0.

[0023] First, by using the etched red iron skeleton as the iron source to prepare lithium iron phosphate sintered product, the sintering activity of the red iron particles during the sintering process can be improved, the sintering kinetics of the powder can be enhanced, giving it high specific surface energy and good compactness, thereby improving the compaction density of the finished product.

[0024] By combining first and second lithium iron phosphate monocalcined products and controlling the particle size of their corresponding slurries, large particles form the basic framework, providing support. Medium particles fill the gaps between the large particles, while fine particles further fill the micropores remaining in the medium and large particles. By adjusting the optimal particle size ratio, the finished product achieves the most effective filling, resulting in a high compaction density. Furthermore, the presence of small particles increases the number and density of contact points between particles, shortening the lithium ion diffusion path and constructing an excellent ionic and electronic conductivity network for the system, thereby effectively improving the rate performance of the material.

[0025] By controlling the ρ / (3.6×(1 - δ × S)) (referred to as "relationship 1") of the first calcined lithium iron phosphate product within the range of 0.16-0.30, especially within the range of 0.2-0.25, the first calcined lithium iron phosphate particles have moderate particle size and strength and suitable pore structure. After sand milling and secondary calcination, the particles can undergo moderate plastic deformation or slight breakage and reorganization during rolling. At the same time, the retained pore structure will be partially retained or evolved into short-circuit channels for ion transport after secondary sintering, ultimately achieving a unity between the densely stacked macrostructure and the highly efficient ion-conducting microstructure, realizing a balance between high compaction density and excellent electrochemical performance. When ρ / (3.6×(1 - δ × S)) < 0.16, the porosity of the first calcined lithium iron phosphate product will be too low, making the product too dense and hard, resulting in irregular particle shape and sharp edges after sand milling. During rolling, these hard and irregularly shaped particles are like a pile of "pebbles" with few contact points between them, which easily leads to a bridging effect and makes it impossible to effectively fill the gaps, resulting in a low compaction density of the finished product. When ρ / (3.6×(1 - δ × S))>0.30, the porosity of the first lithium iron phosphate product is too high. After sand milling, the particles exhibit a loose, coral-like or irregular morphology with a large number of depressions. The particle strength is very low, the shape is extremely irregular, and the surface is rough. During rolling, plastic deformation occurs or even crushing occurs, resulting in extremely low compaction density. At the same time, the crushed particles hinder ion conduction and increase side reactions, leading to poor capacity performance, poor rate performance, and shortened cycle life.

[0026] In summary, by employing a first lithium iron phosphate calcined product that satisfies the above relationship 1 and whose ρ and S are within the above range, and by using the first lithium iron phosphate calcined product and the second lithium iron phosphate calcined product in combination to control the particle size of their corresponding slurries, it is possible to achieve moderate particle strength in lithium iron phosphate materials while solving the problem of ion migration within the particles. By regulating the stacking method between particles to increase compaction density, it is possible to improve both ionic and electronic conductivity, thereby achieving a balance between high compaction density and excellent electrochemical performance.

[0027] For example, the first slurry can be ground to a particle size Dv50 of 0.7 μm, 0.9 μm, 1.0 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2.0 μm, 2.2 μm, 2.4 μm, 2.5 μm, or within any two of the above values; the second slurry can be ground to a particle size Dv50 of 0.2 μm, 0.25 μm, 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm. μm, 0.6μm, or any two of the above values; ρ can be 0.9, 0.87, 0.85, 0.8, 0.77, 0.75, 0.7, 0.65, 0.62, 0.6, 0.55, 0.53, 0.5, or any two of the above values; S can be 5, 4.8, 4.5, 4, 3.5, 3.2, 3, 2.8, 2.5, 2, 1.5, 1, 0.5, or any two of the above values. In some optional embodiments, ρ / (3.6×(1 - δ × S)) can be 0.16, 0.17, 0.18, 0.2, 0.23, 0.25, 0.28, 0.3, or any two of the above values.

[0028] In some alternative embodiments, the first phosphorus source is selected from one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, iron pyrophosphate, lithium dihydrogen phosphate, or lithium phosphate.

[0029] In some alternative embodiments, the first lithium source is selected from one or more of lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, or lithium hydroxide.

[0030] In some alternative embodiments, the first carbon source A is selected from one or more of sucrose, glucose, starch, PEG, ascorbic acid, cellulose, PVA, cyclodextrin, polyacrylonitrile, or PVP.

[0031] In some optional embodiments, in step S2, the iron oxide skeleton, the first phosphorus source, and the first lithium source are mixed in a Li:Fe:P molar ratio of 0.96–1.05:0.96–1.05:0.96–1.05. For example, the Li:Fe:P molar ratio in the iron oxide skeleton, the first phosphorus source, and the first lithium source can be 0.96:1.05:1.05, 0.99:1.02:1.03, 1:1.01:1.01, 1.01:1:1, 1.02:0.98:0.99, 1.03:0.96:0.97, 1.05:1:0.97, 1.05:0.96:0.96, or within any two of the above ratios.

[0032] In some optional embodiments, the amount of the first carbon source A added is 3 to 6 wt% of the mass of the iron oxide skeleton. For example, the amount of the first carbon source A added may be 3 wt%, 3.2 wt%, 3.5 wt%, 3.8 wt%, 4 wt%, 4.3 wt%, 4.5 wt%, 5 wt%, 5.5 wt%, 5.7 wt%, 6 wt% of the mass of the iron oxide skeleton, or within any two of the above values.

[0033] In some alternative embodiments, the solid content of the first slurry is 40%-60%. For example, the solid content of the first slurry may be 40%, 43%, 45%, 48%, 50%, 52%, 55%, 57%, 60%, or within any two of the above values.

[0034] In some alternative embodiments, the solid content of the second slurry is 40%-60%. For example, the solid content of the second slurry may be 40%, 42%, 46%, 49%, 50%, 53%, 55%, 58%, 60%, or within any two of the above values.

[0035] In some optional embodiments, the first sintering includes: in an ammonia atmosphere or a mixture of ammonia and nitrogen, activation with ammonia at 300-400°C for 1-4 hours, followed by switching to a nitrogen or inert atmosphere, holding at 420-500°C for 2-4 hours, and then holding at 600-800°C for 3-10 hours. For example, the activation temperature can be 300℃, 320℃, 350℃, 380℃, 400℃, or any two of the above values; the activation time can be 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, or any two of the above values; the first holding temperature can be 420℃, 450℃, 470℃, 490℃, 500℃, or any two of the above values; the first holding time can be 2h, 2.5h, 3h, 3.5h, 4h, or any two of the above values; the second holding temperature can be 600℃, 650℃, 700℃, 750℃, 800℃, or any two of the above values; the second holding time can be 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, or any two of the above values.

[0036] In some optional embodiments, the second sintering includes: heating to 500-820°C at a rate of 3-10°C / min under a protective atmosphere, and holding at that temperature for 4-10 hours. For example, the heating rate for the second sintering can be 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, or within any two of the above values; the temperature for the second sintering can be 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 820°C, or within any two of the above values; and the time for the second sintering can be 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, or within any two of the above values.

[0037] In some optional embodiments, the inlet air temperature of the spray dryer is 180~280℃, and the outlet air temperature is 80~130℃. For example, the inlet air temperature of the spray dryer can be 180℃, 200℃, 220℃, 240℃, 260℃, 280℃, or within any two of these values; the outlet air temperature of the spray dryer can be 80℃, 90℃, 100℃, 110℃, 120℃, 130℃, or within any two of these values.

[0038] In some alternative implementations, 0.17 ≤ ρ / (3.6 × (1 - δ × S)) ≤ 0.24. For example, ρ / (3.6 × (1 - δ × S)) can be 0.24, 0.23, 0.22, 0.21, 0.2, 0.19, 0.18, 0.17, or within any two of the above values.

[0039] In some optional embodiments, the preparation method of the second lithium iron phosphate mono-calcined product is selected from one or more of Method 1 or Method 2; wherein, Method 1 includes wet grinding an iron source, a second phosphorus source, a second lithium source, a second carbon source A, and water, spray drying, and then performing a first sintering to obtain the second lithium iron phosphate mono-calcined product; Method 2 includes etching iron red particles to obtain an iron red skeleton; wet grinding an iron red skeleton, a third phosphorus source, a third lithium source, a second carbon source B, and water, spray drying, and then performing a first sintering to obtain the second lithium iron phosphate mono-calcined product.

[0040] In some alternative embodiments, the second phosphorus source and the third phosphorus source are independently selected from one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, iron pyrophosphate, lithium dihydrogen phosphate, or lithium phosphate.

[0041] In some alternative embodiments, the second lithium source and the third lithium source are independently selected from one or more of lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, or lithium hydroxide.

[0042] In some alternative embodiments, the second carbon source A and the second carbon source B are independently selected from one or more of sucrose, glucose, starch, PEG, ascorbic acid, cellulose, PVA, cyclodextrin, polyacrylonitrile, or PVP.

[0043] In some optional embodiments, the iron source, the second phosphorus source, and the second lithium source are mixed in a Li:Fe:P molar ratio of 0.96–1.05:0.96–1.05:0.96–1.05. For example, the Li:Fe:P molar ratio in the iron source, the second phosphorus source, and the second lithium source can be 0.96–1.05:0.96–1.05:0.96–1.05, or 0.96:1.05:1.05, 0.99:1.02:1.03, 1:1.01:1.01, 1.01:1:1, 1.02:0.98:0.99, 1.03:0.96:0.97, 1.05:1:0.97, 1.05:0.96:0.96, or within any two of the above ratios.

[0044] In some optional embodiments, the iron source, the third phosphorus source, and the third lithium source are mixed in a Li:Fe:P molar ratio of 0.96–1.05:0.96–1.05:0.96–1.05. For example, the Li:Fe:P molar ratio in the iron source, the third phosphorus source, and the third lithium source can be 0.96–1.05:0.96–1.05:0.96–1.05, or 0.96:1.05:1.05, 0.99:1.02:1.03, 1:1.01:1.01, 1.01:1:1, 1.02:0.98:0.99, 1.03:0.96:0.97, 1.05:1:0.97, 1.05:0.96:0.96, or within any two of the above ratios.

[0045] In some alternative embodiments, the mass of the second carbon source A is 3 to 6 wt% of the mass of the iron source. For example, the mass of the second carbon source A can be 3 wt%, 3.2 wt%, 3.5 wt%, 3.8 wt%, 4 wt%, 4.3 wt%, 4.5 wt%, 5 wt%, 5.5 wt%, 5.7 wt%, 6 wt% of the mass of the iron source, or within any two of the above values.

[0046] In some alternative embodiments, the mass of the second carbon source B is 3 to 6 wt% of the mass of the iron oxide skeleton. For example, the mass of the second carbon source B can be 3 wt%, 3.2 wt%, 3.5 wt%, 3.8 wt%, 4 wt%, 4.3 wt%, 4.5 wt%, 5 wt%, 5.5 wt%, 5.7 wt%, 6 wt% of the iron oxide skeleton, or within any two of the above values.

[0047] In some optional embodiments, in method two, the first sintering includes: in an ammonia protective atmosphere, activation with ammonia at 300-400°C for 1-4 hours, then switching to a nitrogen or inert atmosphere, holding at 420-500°C for 2-4 hours, and subsequently holding at 600-800°C for 5-10 hours. For example, the activation temperature can be 300℃, 320℃, 350℃, 380℃, 400℃, or any two of the above values; the activation time can be 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, or any two of the above values; the first holding temperature can be 420℃, 450℃, 470℃, 490℃, 500℃, or any two of the above values; the first holding time can be 2h, 2.5h, 3h, 3.5h, 4h, or any two of the above values; the second holding temperature can be 600℃, 650℃, 700℃, 750℃, 800℃, or any two of the above values; the second holding time can be 5h, 6h, 7h, 8h, 9h, 10h, or any two of the above values.

[0048] In some optional embodiments, in method one, the first sintering includes: heating to 450-650°C at a rate of 1-5°C / min in a nitrogen atmosphere, and holding at 450-650°C for 5-10 hours. For example, the heating rate for the first sintering in method one can be 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min, or within any two of the above values; the temperature for the first sintering can be 450°C, 500°C, 550°C, 600°C, 650°C, or within any two of the above values; and the time for the first sintering can be 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, or within any two of the above values.

[0049] In some optional embodiments, the method for preparing the lithium iron phosphate material satisfies at least one of the following: A. In the preparation of the first lithium iron phosphate monoxide product or the second lithium iron phosphate monoxide product, an acid solution is used to etch the iron oxide particles; in some optional embodiments, the acid solution includes one or more of phosphoric acid solution, hydrochloric acid solution, and sulfuric acid solution; B. The iron oxide particles used in the preparation of the first or second lithium iron phosphate monoxide product are composed of α-type Fe2O3, and the particle size Dv50 of the iron oxide particles is 0.5~5μm. For example, the particle size Dv50 of the iron oxide particles can be 0.5μm, 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, or within any two of the above values. C. In the preparation of the first or second lithium iron phosphate monoxide product, an additive mixing step is included before wet milling; in some optional embodiments, the additive is independently selected from one or more of vanadium pentoxide, ammonium metavanadate, titanium dioxide, boric acid, cerium oxide, molybdenum oxide, magnesium oxide, aluminum oxide, or strontium carbonate; in some optional embodiments, the amount of dopant added is 0.2 to 1 wt% of the theoretically synthesized lithium iron phosphate mass; for example, the amount of dopant added can be 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt% of the theoretically synthesized lithium iron phosphate mass, or within any two of the above values. The mixing steps D and S3 further include the step of mixing a first carbon source B into the first lithium iron phosphate calcined product; the mass of the first carbon source B is 5 to 12 wt% of the mass of the first lithium iron phosphate calcined product; for example, the mass of the first carbon source B can be 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt% of the mass of the first lithium iron phosphate calcined product, or within any two of the above values; in some optional embodiments, the first carbon source B is selected from one or more of sucrose, glucose, starch, PEG, ascorbic acid, cellulose, PVA, cyclodextrin, polyacrylonitrile, or PVP. The E and S3 mixing steps further include a step of mixing a second carbon source C into the second lithium iron phosphate calcined product; the mass of the second carbon source C is 5 to 12 wt% of the mass of the second lithium iron phosphate calcined product; for example, the mass of the second carbon source C can be 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt% of the mass of the second lithium iron phosphate calcined product, or within any two of the above values; in some optional embodiments, the second carbon source C is selected from one or more of sucrose, glucose, starch, PEG, ascorbic acid, cellulose, PVA, cyclodextrin, polyacrylonitrile, or PVP.

[0050] In some optional embodiments, in step S4, the mass ratio of the first lithium iron phosphate monocalcined product in the first slurry to the second lithium iron phosphate monocalcined product in the second slurry during mixing is 3:7-9:1. For example, the mass ratio of the first lithium iron phosphate monocalcined product in the first slurry to the second lithium iron phosphate monocalcined product in the second slurry during mixing can be 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1, or within any two of the above ratios.

[0051] In some optional embodiments, the method for preparing the lithium iron phosphate material satisfies at least one of the following: A. The surface of the first lithium iron phosphate product includes a carbon coating layer. In some optional embodiments, the thickness of the carbon coating layer is 1-20 nm. For example, the thickness of the carbon coating layer can be 1 nm, 2.5 nm, 5 nm, 8 nm, 10 nm, 13 nm, 15 nm, 20 nm or within any two of the above values. B. The surface of the first lithium iron phosphate product includes grain boundaries; in some optional embodiments, the width of the grain boundaries is 0.5-100 nm; for example, the width of the grain boundaries can be 0.5 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 80 nm, 90 nm, 100 nm or within any two of the above values.

[0052] Secondly, the present invention provides a lithium iron phosphate material prepared by the method described in the first aspect.

[0053] Thirdly, the present invention provides a positive electrode sheet, comprising: a positive current collector, and a positive active material layer disposed on at least one side of the positive current collector, wherein the positive active material layer comprises the lithium iron phosphate material described in the second aspect.

[0054] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0055] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0056] In some embodiments, the positive electrode active material layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0057] In some embodiments, the binder accounts for 0.1-3.5% of the total weight of the positive electrode active material layer, optionally 0.5-2.5%.

[0058] In some embodiments, the positive electrode active material layer may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black (e.g., conductive carbon black), Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0059] In some embodiments, the conductive agent accounts for 0.05-5% of the total weight of the positive electrode active material layer, optionally 0.5-3%.

[0060] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0061] Fourthly, the present invention provides a secondary battery comprising the positive electrode sheet described in the third aspect.

[0062] In some embodiments, the secondary battery further includes a negative electrode, an electrolyte, and a separator.

[0063] In some embodiments, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0064] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0065] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0066] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, the present invention is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0067] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0068] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0069] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0070] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0071] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This invention does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0072] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0073] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0074] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, propylene carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0075] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives capable of improving certain battery performance, such as additives improving battery overcharge performance, additives improving battery high-temperature or low-temperature performance, etc. For example, the additive may be at least one of fluoroethylene carbonate, trifluoroethyl methyl carbonate, 1,3-propanesulfonate lactone, propylene carbonate, methyl ethyl carbonate, ethylene sulfate, triarginyl phosphate, and lithium difluorophosphate.

[0076] In some embodiments, the secondary battery also includes a separator. The present invention does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0077] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0078] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0079] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0080] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0081] The present invention does not impose any particular restrictions on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape.

[0082] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0083] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0084] Fifthly, the present invention provides an electrical device comprising the secondary battery described in the fourth aspect.

[0085] In some embodiments, the aforementioned electrical device may also include a battery module or battery pack assembled from the aforementioned secondary batteries. The secondary batteries, battery modules, or battery packs can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0086] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements. An example electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this electrical device, a battery pack or battery module can be used.

[0087] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0088] The present invention will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention.

[0089] The methods for measuring the physical parameters involved in each embodiment and comparative example are as follows: (1) Specific surface area: A specific surface area meter (name: specific surface area meter; manufacturer: Guozi Measurement Instrument; model: V-Sorb-4804TP) was used, referring to GB / T 19587-2017 (determination of specific surface area of ​​solid substances by gas adsorption BET method). The dynamic volumetric method was used to calculate the specific surface area of ​​the sample by performing gas physical adsorption. The specific surface area is represented by S, and the unit is m. 2 / g.

[0090] (2) Surface fraction (ρ): The sample surface is precisely physically polished using an argon ion beam (equipment name: argon ion profiler; manufacturer: Ruize Technology; model: IMP220) in a low-voltage (6kV) and low-current (400μA) fine profile mode to obtain an etched surface without a deformation layer and with a complete crystal structure. The etched surface is then photographed using a field emission electron microscope. The image is selected at 10,000x magnification. ImageJ software is used to measure the solid area of ​​the profile (i.e., the area occupied by the actual material after removing internal pores, cracks or other non-solid parts in the particle profile) and the total area of ​​the profile (i.e., the total area enclosed by the outer contour of the particle profile, including internal pores, cracks and other non-solid parts). Surface fraction (ρ) = area of ​​solid structure / total area of ​​profile.

[0091] (3) Pore influence coefficient (δ): The pore influence coefficient is the product of the average pore diameter (D) and the true density (P) of the material divided by the open porosity (φ), i.e., δ = (D×P) / φ, where the open porosity φ = 1-P b / P, D is the average pore size in nm, and P is the true density of the material in g / cm³. 3 P b This refers to the tap density of the material, expressed in g / cm³. 3 The theoretical true density P of lithium iron phosphate is 3.60 g / cm³. 3 .

[0092] The average pore size (D) is measured and calculated in accordance with GB / T 19587-2017 "Determination of Specific Surface Area of ​​Solid Substances by Gas Adsorption BET Method". The static volumetric method is used to measure the adsorption / desorption isotherms of the material for inert gas (such as nitrogen N2) or other specific gas (such as argon Ar) at low temperature (such as liquid nitrogen temperature 77K) to calculate the average pore size distribution of the material, which is the average pore size (D).

[0093] The true density (P) of the material was tested according to the standard GB / T 5071-2016 "Test Method for True Density of Refractory Materials"; the tap density (P) of the material was tested. b The test method is based on the standard GB / T 30385-2014 "Carbon Composite Lithium Iron Phosphate Cathode Material for Lithium-ion Batteries".

[0094] (4) Carbon layer thickness and grain boundary width: A sintered product was tested using a transmission electron microscope (manufacturer: Nippon Electron; model: JEM-1400). The carbon layer thickness was directly observed and measured by high-resolution imaging. At the same time, the thickness was evaluated by analyzing the changes in the atomic arrangement near the grain boundary, and the grain boundary width was obtained.

[0095] Example 1 This embodiment provides a method for preparing lithium iron phosphate material, including the following steps: (1) Preparation of the first lithium iron phosphate sintered product: 5000g of iron oxide red (α-type Fe2O3, particle size Dv50 of 0.8μm) was soaked in 10000g of 35wt% phosphoric acid solution for 30min and then washed with pure water to obtain an iron oxide red skeleton with a porous surface. Lithium carbonate, iron oxide red skeleton and monoammonium phosphate were weighed according to the molar ratio of Li:Fe:P of 1.01:0.96:1.02. At the same time, a composite carbon source (glucose and PEG2000 in a mass ratio of 1:1.5 as a composite carbon source) was added at 3wt% of the mass of the iron oxide red skeleton, and titanium dioxide was added as a dopant. The amount of dopant added was 0.4wt% of the theoretical mass of lithium iron phosphate to obtain a mixture. Water was added to the mixture to obtain a slurry with a solid content of 50%. The product undergoes wet milling, spray drying (feed rate 100 mL / min, inlet air temperature 200℃, outlet air temperature 90℃), and a first sintering treatment. The first sintering treatment involves activation at 400℃ for 1 hour in an ammonia atmosphere (ammonia activation treatment), followed by switching to a nitrogen atmosphere and holding at 450℃ for 2 hours. The temperature is then increased to 700℃ (sintering temperature) at a rate of 3℃ / min and held at 700℃ for 5 hours to obtain the first sintered lithium iron phosphate product. (2) Preparation of the second lithium iron phosphate sintered product: Lithium carbonate, iron oxide red (α-type Fe2O3, particle size Dv50 of 0.8μm), and monoammonium phosphate were weighed according to the Li:Fe:P molar ratio of 1.01:0.98:1.02. At the same time, a composite carbon source (glucose and PEG2000 in a mass ratio of 1:1.5) was added at 3wt% of the iron oxide red mass, and titanium dioxide was added as a dopant. The amount of dopant added was 0.5wt% of the theoretically synthesized lithium iron phosphate mass. Water was added to the mixture to obtain a slurry with a solid content of 50%. The slurry was subjected to wet sand milling, spray drying (feed rate of 80mL / min, inlet air temperature of 200℃, outlet air temperature of 90℃), and a first sintering treatment. The first sintering treatment was carried out in a nitrogen atmosphere, heating to 500℃ at a rate of 3℃ / min, and holding at 500℃ for 8h to obtain the second lithium iron phosphate sintered product.

[0096] (3) Preparation of the first slurry and the second slurry: Add a composite carbon source (glucose and PEG2000 in a mass ratio of 1:1.5 as the composite carbon source) at 10 wt% of the mass of the first lithium iron phosphate calcined product. Add water in a sand mill to obtain a slurry with a solid content of 55%. Sand mill for about 30 minutes until the slurry particle size Dv50 is 1.5 μm, and the first slurry after grinding is obtained (referred to as "large particle slurry").

[0097] Add a composite carbon source (glucose and PEG2000 in a mass ratio of 1:1.5) to the first batch of lithium iron phosphate at 10 wt% of the mass of the second batch of lithium iron phosphate. Add water to the mixture in a sand mill to obtain a slurry with a solid content of 55%. Sand mill for about 60 minutes until the slurry particle size is 500 nm. This is the second slurry after grinding (referred to as "small particle slurry").

[0098] (4) The large particle slurry and the small particle slurry were stirred and mixed for 2 hours according to the mass ratio of large particle slurry to small particle slurry of 7:3. The mixture was then mixed by liquid phase ball milling, and then spray dried at a feed rate of 150 mL / min, an inlet air temperature of 260℃ and an outlet air temperature of 100℃. The mixture was then kept at 800℃ for 5 hours under a nitrogen atmosphere and then pulverized by airflow to obtain lithium iron phosphate material.

[0099] Example 2 The process is basically the same as in Example 1, except that the temperature and time of the first sintering treatment in the preparation of the first lithium iron phosphate calcined product are different. Specifically, the activation temperature under ammonia atmosphere is 350°C, the sintering temperature under nitrogen atmosphere is adjusted from 700°C to 800°C, and the holding time is adjusted to 10h. This results in different parameters such as ρ and S of the first lithium iron phosphate calcined product, as shown in Table 1.

[0100] Example 3 The process is basically the same as in Example 1, except that the temperature of the first sintering treatment in the preparation of the first lithium iron phosphate calcined product is different. Specifically, the activation temperature under ammonia atmosphere is 350°C, and the sintering temperature under nitrogen atmosphere is adjusted from 700°C to 600°C. This results in different parameters such as ρ and S of the first lithium iron phosphate calcined product, as shown in Table 1.

[0101] Example 4 It is basically the same as Example 1, except that in step (4), the mass ratio of large particle slurry to small particle slurry is adjusted to 8:2.

[0102] Example 5 It is basically the same as Example 1, except that in step (4), the mass ratio of large particle slurry to small particle slurry is adjusted to 3:7.

[0103] Example 6 It is basically the same as Example 1, except that in step (4), the mass ratio of large particle slurry to small particle slurry is adjusted to 9:1.

[0104] Example 7 This embodiment provides a method for preparing lithium iron phosphate material, including the following steps: (1) Preparation of the first sintered lithium iron phosphate product: 5000g of iron oxide red (α-type Fe2O3, particle size Dv50 of 2μm) was soaked in 10000g of 35wt% phosphoric acid solution for 30min, and then washed with pure water to obtain an iron oxide red skeleton with a porous surface. Lithium carbonate, iron oxide red skeleton and monoammonium phosphate were weighed according to the molar ratio of Li:Fe:P of 1.05:1.05:0.96. At the same time, glucose was added at 5wt% of the mass of iron oxide red skeleton, and vanadium pentoxide was added as a dopant. The amount of dopant added was 0.4wt% of the theoretical mass of lithium iron phosphate to obtain a mixture. The mixture was then mixed with water to obtain a slurry with a solid content of 60%. The slurry was subjected to wet sand milling, spray drying (feed rate of 100mL / min, inlet air temperature of 200℃, outlet air temperature of 90℃) and first sintering treatment. The first sintering process is as follows: activation is carried out at 300°C for 2 hours in an ammonia atmosphere, then the atmosphere is switched to nitrogen atmosphere, and the temperature is maintained at 420°C for 4 hours in a nitrogen atmosphere. The temperature is then increased to 700°C at a rate of 3°C / min, and the temperature is maintained at 700°C for 8 hours to obtain the first sintered lithium iron phosphate product. (2) Preparation of the second lithium iron phosphate sintered product: Lithium carbonate, iron oxide red (α-type Fe2O3, particle size Dv50 of 2μm) and monoammonium phosphate were weighed according to the Li:Fe:P molar ratio of 0.96:0.96:1.05. At the same time, a composite carbon source (sucrose and PEG2000 in a mass ratio of 1:1.5) was added at 3wt% of the iron oxide red mass. Titanium dioxide was added as a dopant, and the amount of dopant added was 0.5wt% of the theoretically synthesized lithium iron phosphate mass. Water was added to the mixture to obtain a slurry with a solid content of 60%. The slurry was subjected to wet sand milling, spray drying (feed rate of 80mL / min, inlet air temperature of 200℃, outlet air temperature of 90℃) and a first sintering treatment. The first sintering treatment was carried out in a nitrogen atmosphere, heating to 550℃ at a rate of 3℃ / min, and holding at 550℃ for 6h to obtain the second lithium iron phosphate sintered product.

[0105] (3) Preparation of the first slurry and the second slurry: Add 3wt% of the first lithium iron phosphate calcined mass of composite carbon source (sucrose and PEG2000 in a mass ratio of 1:1.5 as composite carbon source), add water in a sand mill to obtain a slurry with a solid content of 50%, and sand mill for about 30 minutes until the slurry particle size Dv50 is 1μm, thus obtaining the first slurry after grinding (referred to as "large particle slurry").

[0106] Add 3 wt% of the second lithium iron phosphate calcined product to a composite carbon source (sucrose and PEG2000 in a mass ratio of 1:1.5 as the composite carbon source), add water to a sand mill to obtain a slurry with a solid content of 50%, and sand mill for about 60 minutes until the slurry particle size is 350 nm, which is the ground second slurry (referred to as "small particle slurry").

[0107] (4) The large particle slurry and the small particle slurry are stirred and mixed for 2 hours according to the mass ratio of large particle slurry to small particle slurry of 7:3. The materials are mixed by liquid phase ball milling, and then spray dried at a feed rate of 150 mL / min, an air inlet temperature of 260℃ and an air outlet temperature of 100℃. Then, under a nitrogen atmosphere, the temperature is raised to 700℃ at 3℃ / min and kept for 10 hours. Finally, the lithium iron phosphate material is obtained by air jet pulverization.

[0108] Example 8 This embodiment provides a method for preparing lithium iron phosphate material. The only difference from Embodiment 1 is the preparation method of the second lithium iron phosphate calcined product. The preparation of the second lithium iron phosphate calcined product in this embodiment is as follows: 5000g of iron oxide red (α-type Fe2O3, particle size Dv50 of 0.8μm) is soaked in 10000g of 35wt% phosphoric acid solution for 30min, and then washed with pure water to obtain an iron oxide red skeleton with a porous surface. Lithium carbonate, iron oxide red skeleton and monoammonium phosphate are weighed according to the molar ratio of Li:Fe:P of 1.01:0.96:1.02. At the same time, a composite carbon source (glucose and PEG2000 in a mass ratio of 1:1.5 as a composite carbon source) is added at 3wt% of the mass of the iron oxide red skeleton. Titanium dioxide is added as a dopant. The amount of dopant added is 0.5wt% of the theoretical mass of lithium iron phosphate to obtain a mixture. Water is added to the mixture to obtain a slurry with a solid content of 50%. The product undergoes wet milling, spray drying (feed rate 100 mL / min, inlet air temperature 200℃, outlet air temperature 90℃), and a first sintering treatment. The first sintering treatment involves activation at 400℃ for 1 hour in an ammonia atmosphere, followed by switching to a nitrogen atmosphere and holding at 450℃ for 2 hours. The temperature is then increased to 700℃ at a rate of 3℃ / min and held at 700℃ for 5 hours to obtain the second sintered lithium iron phosphate product.

[0109] Comparative Example 1 The process is basically the same as in Example 1, except that in step (4), the use of large particle slurry is omitted, and only small particle slurry is used for spray drying, sintering and pulverization according to the conditions of step (4).

[0110] Comparative Example 2 The process is basically the same as in Example 1, except that in step (4), the use of small particle slurry is omitted, and only large particle slurry is used for spray drying, sintering and pulverization under the conditions of step (4).

[0111] Comparative Example 3 The process is basically the same as in Example 1, except that in step (1), unetched iron oxide is used instead of the iron oxide skeleton during the preparation of the mixture. Also, in step (1), the ammonia activation treatment is omitted, and the spray-dried material is directly sintered under a nitrogen atmosphere according to the process conditions of step (1).

[0112] Comparative Example 4 The process is basically the same as in Example 1, except that the temperature and time of the first sintering treatment in the preparation of the first lithium iron phosphate calcined product are different. Specifically, the activation time under ammonia atmosphere is adjusted to 0.5 h, and the sintering temperature under nitrogen atmosphere is adjusted to 830 °C. This results in different parameters such as ρ and S of the first lithium iron phosphate calcined product, as shown in Table 1.

[0113] Comparative Example 5 The process is basically the same as in Example 1, except that the temperature of the first sintering treatment in the preparation of the first lithium iron phosphate calcined product is different. Specifically, the sintering temperature under nitrogen atmosphere is adjusted to 550°C and the holding time is adjusted to 10h; this results in different parameters such as ρ and S of the first lithium iron phosphate calcined product, as shown in Table 1.

[0114] Comparative Example 6 The process is basically the same as in Example 1, except that the temperature of the first sintering treatment in the preparation of the first lithium iron phosphate calcined product is different. Specifically, the activation temperature under ammonia atmosphere is 320°C; the sintering temperature under nitrogen atmosphere is adjusted from 700°C to 850°C, and the holding time is adjusted to 10h; resulting in different parameters such as ρ and S of the first lithium iron phosphate calcined product, as shown in Table 1.

[0115] Comparative Example 7 The process is basically the same as in Example 1, except that the temperature of the first sintering treatment in the preparation of the first lithium iron phosphate calcined product is different. Specifically, the activation temperature under ammonia atmosphere is 380°C, and the sintering temperature under nitrogen atmosphere is adjusted from 700°C to 500°C. This results in different parameters such as ρ and S of the first lithium iron phosphate calcined product, as shown in Table 1.

[0116] Comparative Example 8 The process is basically the same as in Example 1, except that the particle size of the first slurry after grinding in step (3) is different. In this comparative example, the first slurry with a particle size Dv50 of 0.2 μm was obtained by grinding.

[0117] Comparative Example 9 It is basically the same as Example 1, except that the particle size of the first slurry after grinding in step (3) is different. In this comparative example, the first slurry with a particle size Dv50 of 3μm was obtained by grinding.

[0118] Comparative Example 10 The process is basically the same as in Example 1, except that the particle size of the second slurry after grinding in step (3) is different. In this comparative example, the second slurry with a particle size Dv50 of 0.1 μm was obtained by grinding.

[0119] Comparative Example 11 It is basically the same as Example 1, except that the particle size of the second slurry after grinding in step (3) is different. In this comparative example, the second slurry with a particle size Dv50 of 1 μm was obtained by grinding.

[0120] Table 1 Physical parameters of lithium iron phosphate calcined product

[0121] Experimental Example 1 1. Use a powder tap density tester (manufacturer: Sansi Zongheng; model: UTM7305) or equipment conforming to GB / T30835-2014 "Carbon Composite Lithium Iron Phosphate Cathode Material for Lithium-ion Batteries" to determine the compaction density of the finished powder.

[0122] 2. Using the lithium iron phosphate materials prepared in each embodiment and comparative example as the positive electrode active material, secondary batteries were prepared according to the following method: 1) Lithium iron phosphate material, acetylene black, and polyvinylidene fluoride were dissolved in N-methyl-pyrrolidone at a weight ratio of 90:5:5. After stirring evenly, the solution was coated onto aluminum foil and dried in a forced-air drying oven at 85°C to obtain the positive electrode sheet. The dried electrode sheet was then cut into small round pieces with a diameter of 14 mm to serve as the positive electrode sheet.

[0123] 2) Using lithium metal sheets as negative electrodes, polypropylene microporous membranes as separators, and 1 mol / L LiPF6 / EC+DMC+DEC (volume ratio 1:1:1) as electrolyte, CR2032 coin cells were assembled in an argon-filled glove box.

[0124] The electrical performance of each group of secondary batteries was tested using Shenzhen Xinweier (CT-4008-5V50mA-164) testing equipment.

[0125] (1) 1C discharge specific capacity: The charge and discharge voltage is limited to 2–3.75V. The charge and discharge performance test is conducted under normal temperature conditions. First, the battery is charged to 3.75V at 0.15C and then discharged to 2V at 0.1C. The 0.1C discharge specific capacity is recorded. Then, the battery is charged to 3.8V at 1C and discharged to 2V at 1C. The 1C discharge specific capacity is recorded.

[0126] (2) Cyclic performance: Under the voltage range of 2–3.75V, after 2 cycles of charging and discharging at a rate of 0.1C, and 50 cycles of charging and discharging at a rate of 1C, the cycle performance is the discharge capacity of the 50th cycle at 1C / the discharge capacity of the 1st cycle at 1C.

[0127] (3) Rate performance: Under the voltage range of 2–3.75V, charge and discharge at a rate of 0.1C for 2 cycles; the discharge rate of the third cycle is 1C, and the discharge rate of the fourth cycle is 5C, to obtain the 1C discharge capacity and 5C discharge capacity of the material. The rate performance is 5C capacity / 1C capacity.

[0128] Table 2. Test results of compaction density and electrical properties

[0129] As can be seen from the table above, compared with the comparative examples, the embodiments of this application can significantly improve the powder compaction density of lithium iron phosphate materials and improve electrical performance.

[0130] Comparing Examples 5 and 6, Example 1 shows that by limiting the mass ratio of the first lithium iron phosphate calcined product to the second lithium iron phosphate calcined product in the second slurry within a preferred range, Example 1 can further improve the powder compaction density of lithium iron phosphate material and enhance its electrical performance.

[0131] Comparing Example 1 and Example 8, it can be seen that Example 1, by using a non-etching method (Method 2) to prepare the second lithium iron phosphate calcined product and using it in combination with the first lithium iron phosphate calcined product, can further improve the powder compaction density of lithium iron phosphate material and improve its electrical performance.

[0132] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing lithium iron phosphate material, characterized in that, Includes the following steps: Step S1: Etch the iron oxide particles to obtain the iron oxide skeleton; Step S2: The iron oxide skeleton, the first phosphorus source, the first lithium source, the first carbon source A and water are wet-milled, spray-dried and then sintered for the first time to obtain the first lithium iron phosphate sintered product. Step S3: Mix the first lithium iron phosphate monocalcined product with water to prepare the first slurry, and grind the first slurry to a particle size Dv50 of 0.7-2.5μm; mix the second lithium iron phosphate monocalcined product with water to prepare the second slurry, and grind the second slurry to a particle size Dv50 of 0.2-0.6μm; Step S4: The first slurry after grinding is mixed with the second slurry after grinding, and after spray drying, it is sintered for the second time and then pulverized to obtain lithium iron phosphate material; The first lithium iron phosphate product obtained in step S2 satisfies the following relationship: 0.16≤ρ / (3.6×(1-δ×S))≤0.30; among them, 0.5≤ρ≤0.9; 0.5≤S≤5.0; Where ρ is the surface fraction of the first lithium iron phosphate product, which is a dimensionless value; S is the specific surface area of ​​lithium iron phosphate monocalcinate, in m². 2 / g; δ is the porosity influence coefficient of lithium iron phosphate calcined product, in g / m³. 2 ; δ is calculated using the following formula: δ = (D × P) / φ, where D is the average pore size of the first lithium iron phosphate precipitate in nm; and P is the true density of the first lithium iron phosphate precipitate in g / cm³. 3 φ represents the open porosity of the first lithium iron phosphate product, which is a dimensionless value.

2. The method for preparing lithium iron phosphate material according to claim 1, characterized in that, The first phosphorus source is selected from one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, iron pyrophosphate, lithium dihydrogen phosphate, or lithium phosphate; And / or, the first lithium source is selected from one or more of lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, or lithium hydroxide; And / or, the first carbon source A is selected from one or more of sucrose, glucose, starch, PEG, ascorbic acid, cellulose, PVA, cyclodextrin, polyacrylonitrile, or PVP; And / or, in step S2, the iron oxide skeleton, the first phosphorus source, and the first lithium source are mixed in a Li:Fe:P molar ratio of 0.96–1.05: 0.96–1.05: 0.96–1.05; And / or, the amount of the first carbon source A added is 3-6 wt% of the iron oxide skeleton; And / or, the solid content of the first slurry is 40%-60%; And / or, the solid content of the second slurry is 40%-60%; And / or, the first sintering includes: in an ammonia atmosphere or a mixture of ammonia and nitrogen, activation with ammonia at 300-400°C for 1-4 hours, then switching to a nitrogen or inert atmosphere, holding at 420-500°C for 2-4 hours, and subsequently holding at 600-800°C for 3-10 hours. And / or, the second sintering includes: heating to 500-820°C at a rate of 3-10°C / min under a protective atmosphere and holding at that temperature for 4-10 hours; And / or, the inlet air temperature of the spray dryer is 180~280℃, and the outlet air temperature is 80~130℃; And / or, 0.17 ≤ ρ / (3.6 × (1 - δ × S)) ≤ 0.

24.

3. The method for preparing lithium iron phosphate material according to claim 1, characterized in that, The preparation method of the second lithium iron phosphate product is selected from one or more of method one or method two; Method 1 includes wet grinding of iron source, second phosphorus source, second lithium source, second carbon source A and water, followed by spray drying and first sintering to obtain a second lithium iron phosphate calcined product. Method 2 involves etching the iron oxide particles to obtain the iron oxide skeleton; then wet-grinding the iron oxide skeleton, the third phosphorus source, the third lithium source, the second carbon source B, and water, followed by spray drying and a first sintering to obtain the second lithium iron phosphate sintered product. Optionally, the second phosphorus source and the third phosphorus source are independently selected from one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, iron pyrophosphate, lithium dihydrogen phosphate, or lithium phosphate. Optionally, the second lithium source and the third lithium source are independently selected from one or more of lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, or lithium hydroxide; Optionally, the second carbon source A and the second carbon source B are independently selected from one or more of sucrose, glucose, starch, PEG, ascorbic acid, cellulose, PVA, cyclodextrin, polyacrylonitrile, or PVP. Optionally, the iron source, the second phosphorus source, and the second lithium source are mixed in a Li:Fe:P molar ratio of 0.96–1.05:0.96–1.05:0.96–1.05; Optionally, the iron oxide skeleton, the third phosphorus source, and the third lithium source are mixed in a Li:Fe:P molar ratio of 0.96–1.05:0.96–1.05:0.96–1.05; Optionally, the mass of the second carbon source A is 3-6 wt% of the mass of the iron source; Optionally, the mass of the second carbon source B is 3-6 wt% of the mass of the iron oxide skeleton; Optionally, in Method 2, the first sintering includes: in an ammonia protective atmosphere, activation with ammonia at 300~400℃ for 1~4h, then switching to nitrogen or an inert atmosphere, holding at 420~500℃ for 2~4h, and subsequently holding at 600~800℃ for 5~10h. Optionally, in Method 1, the first sintering includes: heating to 450-650°C at a rate of 1-5°C / min in a nitrogen atmosphere, and holding at 450-650°C for 5-10 hours.

4. The method for preparing lithium iron phosphate material according to claim 3, characterized in that, The preparation method satisfies at least one of the following: A. When preparing the first lithium iron phosphate monocalcined product or the second lithium iron phosphate monocalcined product, an acid solution is used to etch the iron oxide particles; optionally, the acid solution includes one or more of phosphoric acid solution, hydrochloric acid solution, and sulfuric acid solution; B. The iron oxide particles used in the preparation of the first or second lithium iron phosphate calcined product are composed of α-type Fe2O3, and the particle size Dv50 of the iron oxide particles is 0.5~5μm. C. In the preparation of the first or second lithium iron phosphate monocalcined product, an additive mixing step is included before wet milling; optionally, the additive is independently selected from one or more of vanadium pentoxide, ammonium metavanadate, titanium dioxide, boric acid, cerium oxide, molybdenum oxide, magnesium oxide, aluminum oxide, or strontium carbonate; optionally, the amount of dopant added is 0.2~1wt% of the theoretically synthesized lithium iron phosphate. The mixing steps D and S3 further include the step of mixing a first carbon source B into the first lithium iron phosphate calcined product; the mass of the first carbon source B is 5 to 12 wt% of the mass of the first lithium iron phosphate calcined product; optionally, the first carbon source B is selected from one or more of sucrose, glucose, starch, PEG, ascorbic acid, cellulose, PVA, cyclodextrin, polyacrylonitrile or PVP. The E and S3 mixing steps also include a step of mixing a second carbon source C into the second lithium iron phosphate calcined product; the mass of the second carbon source C is 5 to 12 wt% of the mass of the second lithium iron phosphate calcined product; optionally, the second carbon source C is selected from one or more of sucrose, glucose, starch, PEG, ascorbic acid, cellulose, PVA, cyclodextrin, polyacrylonitrile or PVP.

5. The method for preparing lithium iron phosphate material according to any one of claims 1-4, characterized in that, In step S4, the mass ratio of the first lithium iron phosphate calcined product in the first slurry to the second lithium iron phosphate calcined product in the second slurry during mixing is 3:7-9:1; preferably 7:3-8:

2.

6. The method for preparing lithium iron phosphate material according to claim 1, characterized in that, The preparation method of the lithium iron phosphate material satisfies at least one of the following: A. The surface of the first lithium iron phosphate product includes a carbon coating layer, and optionally, the thickness of the carbon coating layer is 1-20 nm; B. The surface of the first lithium iron phosphate product includes grain boundaries; optionally, the width of the grain boundaries is 0.5-100 nm.

7. The lithium iron phosphate material prepared by the method of any one of claims 1-6.

8. A positive electrode sheet, characterized in that, include: Positive current collector, and A positive electrode active material layer disposed on at least one side of the positive electrode current collector, the positive electrode active material layer comprising the lithium iron phosphate material as described in claim 7.

9. A secondary battery, characterized in that, Includes the positive electrode sheet as described in claim 8.

10. An electrical device, characterized in that, Includes the secondary battery as described in claim 9.

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