Lithium iron phosphate composite material and preparation method thereof

Abstract

The application provides a lithium iron phosphate composite material and a preparation method thereof. By introducing oxide conductive links such as nano cerium oxide to connect a framework and carbon-coated liquid before secondary sintering, the surface and the interior of lithium iron phosphate particles have complete carbon-coated conductive networks after secondary sintering, forming a double-layer carbon-coated lithium iron phosphate material, which has high conductivity, good crystallinity, high tap density, good processing performance, high discharge capacity and excellent rate performance.

Description

Technical Field

[0001] This invention relates to the field of battery materials, specifically to a nano-oxide-modified lithium iron phosphate composite material and its preparation method. Background Technology

[0002] Lithium iron phosphate (LFP), as a cathode material for lithium-ion batteries, exhibits a volume change of only 6.81% during charge and discharge. This volume change perfectly balances the volume change caused by lithium insertion / extraction on the carbon anode, supporting the internal structure. Therefore, LFP batteries demonstrate good cycle stability and long cycle life during charge and discharge. However, LFP also suffers from poor conductivity and low lithium-ion diffusion. Currently, to improve the conductivity of LFP materials, coating the particle surface with conductive materials has become the main solution. Commonly used coating materials include amorphous carbon, polyphenylene oxide, carbon nanotubes, and silver, with amorphous carbon being the most widely used. Most LFP materials used in lithium-ion batteries today have undergone amorphous carbon coating treatment. However, the performance of carbon-coated LFP from different manufacturers varies significantly, mainly due to differences in carbon coating uniformity and coating layer thickness.

[0003] To improve the processing performance of lithium iron phosphate materials, the materials usually need to be crushed. However, due to the external force, the carbon coating layer on the surface of the particles will be damaged during crushing, making them incomplete.

[0004] Currently, most lithium iron phosphate batteries on the market have a 10C discharge rate of 120mAh / g to 140mAh / g, but their compaction density is generally low at high discharge rates, mostly around 2.40g / cm³. 3 The following limitations restrict the improvement of energy density of high-rate lithium iron phosphate materials, thus hindering the application and development of lithium iron phosphate batteries in fast charging and low-temperature applications. Summary of the Invention

[0005] The main objective of this invention is to provide a lithium iron phosphate composite material and its preparation method, which has an intact carbon coating layer, high tap density, good processing performance, high discharge capacity, excellent rate performance, and high conductivity.

[0006] To achieve the above objectives, the present invention provides a lithium iron phosphate composite material and a method for preparing the same:

[0007] A lithium iron phosphate composite material, the composite material comprising lithium iron phosphate particles having a first carbon coating layer, a second carbon coating layer, and nano-oxide conductive wires; the second carbon coating layer and the nano-oxide conductive wires are located between the lithium iron phosphate particles coated by the first carbon coating layer, and the second carbon coating layer and the nano-oxide conductive wires are connected to the first carbon coating layer on the surface of the lithium iron phosphate particles to form a three-dimensional spatial network conductive structure with complete carbon coating.

[0008] The nano-oxide is one or a mixture of two or more of nano-yttrium oxide, nano-cerium oxide, nano-neodymium oxide, and nano-tungsten oxide.

[0009] The total carbon content in the lithium iron phosphate composite material is 1.3-1.5%, which means that the coated carbon accounts for 1.3-1.5% of the total mass of the lithium iron phosphate composite material (e.g., 1.3%, 1.35%, 1.4%, 1.41%, 1.42%, 1.43%, 1.45%, 1.48%, 1.5%, etc.).

[0010] In some embodiments, the total carbon content in the lithium iron phosphate composite material is 1.4-1.45%, preferably 1.41-1.45%, and more preferably 1.41-1.43%.

[0011] Lithium iron phosphate particles with a first carbon coating layer are placed in a fluidized bed reactor through which heated gas is passed, and a secondary coating treatment is performed by a coating liquid, which includes water, carbon source and nano-oxide.

[0012] The lithium iron phosphate particles after secondary coating were sintered to obtain a composite material of lithium iron phosphate modified with nano-oxide.

[0013] The solid content of carbon source and nano-oxide in the coating solution is controlled at 1-3% (e.g., 1%, 1.3%, 1.5%, 2%, 2.5%, 3%, etc.) and 0.05-0.20% (e.g., 0.05%, 0.1%, 0.15%, 0.16%, 0.18%, 0.20%, etc.) of the coated material, respectively.

[0014] In some embodiments, the solid content of the nano-oxides in the coating solution is controlled at 0.10-0.20% of the coated material.

[0015] The sintering process involves heating the temperature at a rate of 2-4℃ / min to 300-500℃ (e.g., 300℃, 350℃, 400℃, 450℃, 500℃, etc.), holding the temperature at that temperature for 1-3 hours, and then continuing to heat the temperature at a rate of 2-4℃ / min to 500-700℃ (e.g., 500℃, 550℃, 600℃, 650℃, 700℃, etc.) and holding the temperature at that temperature for 3-8 hours (e.g., 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, etc.).

[0016] The heating gas is heated air, and the temperature is controlled between 100-200℃.

[0017] In some embodiments, the temperature of the heating gas is 120-160°C (e.g., 120°C, 130°C, 135°C, 140°C, 145°C, 150°C, 155°C, 160°C, etc.).

[0018] Lithium iron phosphate particles with a first carbon coating layer are prepared by the following steps:

[0019] (1) Add iron phosphate, lithium source, carbon source and doping element source to water and stir and ball mill, then sand mill and spray dry granulation to obtain lithium iron phosphate precursor powder;

[0020] (2) The obtained lithium iron phosphate precursor powder was placed in a rotary kiln and sintered under an inert atmosphere. After natural cooling, the sintered powder product was subjected to air jet pulverization to obtain lithium iron phosphate composite material.

[0021] In step (1), the lithium source is one or a mixture of two or more of lithium carbonate, lithium nitrate, lithium acetate, lithium hydroxide and lithium oxalate.

[0022] In step (1), the carbon source is one or a mixture of two or more of starch, citric acid, sucrose, glucose, PEG1000, PEG2000, PEG3000 and phenolic resin.

[0023] In step (1), the dopant element source is one or a mixture of two or more of manganese acetate or manganese dioxide, titanium dioxide, magnesium acetate, niobium pentoxide, ammonium metavanadate or vanadium pentoxide.

[0024] In step (1), the iron phosphate, lithium source, and doping element source are in the molar ratio of iron, lithium, and doping element as 1.0:0.99-1.05:0.01-0.05, and carbon source is added at 5-10% of the total mass of iron phosphate, lithium source, and doping element source.

[0025] Specifically, in step (1), water is first added to the raw material, and the mixture is ball-milled for 1-5 hours and then sand-milled for 3-10 hours to obtain a slurry with a particle size D50 of 300-600 nm.

[0026] In some embodiments, in step (1), water is first added to the raw material, and the mixture is ball-milled for 1 hour and then sand-milled for 4 hours to obtain a slurry with a D50 of 400-450 nm.

[0027] In step (1), the spray drying granulation equipment is a pressure spray drying tower with an inlet temperature controlled at 220-300℃, an outlet temperature controlled at 90-115℃, and a dry powder particle size D50 controlled at 4-8μm.

[0028] In some embodiments, in step (1), the spray drying granulation equipment is a pressure spray drying tower, with the inlet temperature controlled at 250-280℃ (e.g., 250℃, 260℃, 265℃, 270℃, 275℃, 280℃, etc.), the outlet temperature controlled at 100-110℃ (e.g., 100℃, 102℃, 103℃, 105℃, 108℃, 100℃, etc.), and the dry powder particle size D50 controlled at 5-6μm.

[0029] In step (2), the powder sintering process is as follows: the temperature is increased to 300-500℃ at a rate of 2-4℃ / min, then kept constant for 1-3 hours, and then the temperature is increased to 600-800℃ at a rate of 2-4℃ / min, and then kept constant for 5-10 hours.

[0030] In some embodiments, in step (2), the powder sintering process is as follows: the temperature is increased to 400-450℃ at a rate of 2-4℃ / min, then held at the temperature for 1-3 hours, and then the temperature is increased to 700-750℃ (e.g., 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, etc.) at a rate of 2-4℃ / min, and then held at the temperature for 8-10 hours.

[0031] In step (2), after the airflow pulverization, the D50 particle size is controlled at 0.8-2.5μm and the D90 particle size is controlled at 3.0-5.0μm.

[0032] In some embodiments, in step (2), after the airflow pulverization, the D50 particle size is controlled at 1.0-2.0 μm and the D90 particle size is controlled at 3.0-4.5 μm.

[0033] The inert gases mentioned are any one or a mixture of two of nitrogen or argon.

[0034] The water or pure water mentioned in this invention is deionized water.

[0035] Technical effects of the present invention:

[0036] 1. The introduced nanoscale oxide material can serve as a connector in the carbon conductive network, firmly coating the carbon coating layer onto the LiFePO4 surface. Correspondingly, the carbon coating layer also serves to fix the nanoscale oxide material onto the LiFePO4 surface, while increasing the compaction density of the material. After the carbon between the broken or missing coating layers is stably connected by the conductive interconnection skeleton of the oxide, the carbon coating layer is then laid on top, resulting in a more complete carbon coating layer. This improves the incompleteness of traditional carbon coating layers and forms a three-dimensional spatial network structure carbon coating layer with strong spatial grafting, improving the rate performance of the material and further enhancing its conductivity. The double-layer carbon coating layer inhibits particle agglomeration and prevents particle growth, which is beneficial to improving rate performance and processing performance.

[0037] 2. The powder has high fluidity, which is beneficial for material pipeline transportation and slurry dispersion; it prevents the active material from being oxidized and corroded by the electrolyte, which helps to improve the structural stability and cycle life of lithium iron phosphate materials and extend the service life of the materials.

[0038] 3. After the material is pulverized, nano-oxide conductive wires are introduced into the fluidized state to connect the framework and the carbon source coating liquid. After secondary sintering, the surface and interior of the particles have a sufficient carbon-coated conductive network, resulting in high conductivity and good crystallinity. Through primary particle nano-sizing (D50 of 400-450 nm), the secondary particles are pulverized and sintered to form a uniform and complete carbon coating layer. The compaction density of this material is 2.55 g / cm³. 3 The material exhibits high specific capacity, with a 10C discharge capacity reaching 140mAh / g, and high energy density.

[0039] 4. The process of this invention is simple and can be applied to large-scale industrial production, achieving automated control. At the same time, it has good processing performance and good batch-to-batch consistency, which can meet the needs of the country and battery manufacturers for improving fast-charging / low-temperature materials. Attached Figure Description

[0040] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0041] Figure 1 This is an electron microscope image of a lithium iron phosphate composite material. Detailed Implementation

[0042] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0043] Example 1

[0044] Iron phosphate, lithium carbonate, and titanium dioxide were mixed in a molar ratio of 1:1.03:0.015. Glucose was added at 6% of the total mass of the materials (total mass of lithium carbonate, iron phosphate, and titanium dioxide). Deionized water was then added to the mixture with a solid content of 35%, and the mixture was stirred and ball-milled. After ball milling for 1 hour, it was sand-milled for 4 hours. The particle size D50 of the sand-milled material was controlled at 430±20nm. Then, it was spray-dried at an inlet temperature of 260℃ and an outlet temperature of 102℃. The particle size D50 of the dry powder after spraying was controlled at 5~6μm, resulting in material A, which is the lithium iron phosphate precursor powder.

[0045] Material A was placed in a rotary kiln and heated to 450°C under a nitrogen atmosphere at a rate of 3.5°C / min and held at that temperature for 2.5 hours. Then, it was heated to 730°C at a rate of 3.5°C / min and sintered for 8 hours. After naturally cooling to room temperature, material B was obtained.

[0046] Material B is crushed, and the particle size is controlled to D50 of 1.5±0.5μm and D90 of 3.5±0.5μm to obtain material C, which is lithium iron phosphate particles with the first carbon coating layer.

[0047] Material C is fed into the fluidized bed reactor by a feeder. Heating gas at 130±10℃ is sent to the bottom of the fluidized bed by a blower and comes into contact with material C through a distribution plate to make the powder of material C fluidized. A coating liquid with a solid content of 40% and nano-cerium oxide and PEG2000 at 0.15% and 2.0% of the total mass of material C, respectively, is atomized and coated on material C to obtain material D.

[0048] Material D was placed in a rotary kiln and heated to 450℃ under a nitrogen atmosphere at a rate of 3.5℃ / min and held at that temperature for 2 hours. Then, it was heated to 650℃ at a rate of 3.5℃ / min and sintered for 5 hours. After naturally cooling to room temperature, lithium iron phosphate composite material was obtained. The carbon coating content increased by 0.22% in the coating solution and the total carbon content was 1.41%.

[0049] A lithium iron phosphate composite material was prepared as the positive electrode material, conductive carbon black and conductive graphite were used as conductive agents, and polyvinylidene fluoride 5130 was used as a binder to form an electrode sheet. Lithium metal was used as the negative electrode, and the electrode was assembled into a coin cell. Tests were conducted under different charge and discharge currents ranging from 2 to 4.2V. The initial charge and discharge efficiency at 0.1C reached 98.7%, with a discharge specific capacity of 162 mAh / g; the discharge specific capacity at 1C was 156 mAh / g; the discharge specific capacity at 10C was 140 mAh / g; and the powder compaction density was 2.58 g / cm³. 3 Electron micrographs of the material are attached. Figure 1 As shown, the carbon coating layer is complete and uniform, and the oxide material is fixed on the LiFePO4 surface.

[0050] Example 2

[0051] Iron phosphate, lithium hydroxide, and ammonium metavanadate were mixed at a molar ratio of 1:1.028:0.017 (iron, lithium, vanadium). PEG2000 was added at 9% of the total mass of the materials (total mass of lithium carbonate, iron phosphate, and ammonium metavanadate). Then, deionized water was added at a solid content of 35%, and the mixture was stirred and ball-milled. After ball milling for 1 hour, it was sand-milled for 4 hours. The particle size D50 after sand milling was controlled at 430±20nm. Then, it was spray-dried at an inlet temperature of 260℃ and an outlet temperature of 100℃. The particle size D50 of the dry powder after spraying was controlled at 5~6μm, resulting in material A, which is the lithium iron phosphate precursor powder.

[0052] Material A was placed in a rotary kiln and heated to 450℃ under a nitrogen atmosphere at a rate of 3.3℃ / min and held at that temperature for 3.0 hours. Then, it was heated to 730℃ at a rate of 3.3℃ / min and held at that temperature for 8 hours. After naturally cooling to room temperature, material B was obtained.

[0053] Material B is crushed, and the particle size is controlled to D50 of 1.5±0.5μm and D90 of 3.5±0.5μm to obtain material C, which is lithium iron phosphate particles with the first carbon coating layer.

[0054] Material C is fed into the fluidized bed reactor by a feeder. Heating gas at 145±10℃ is sent to the bottom of the fluidized bed by a blower and comes into contact with material C through a distribution plate to make the powder of material C fluidized. A coating liquid with a solid content of 40% and nano-cerium oxide and sucrose at 0.16% and 1.3% of the total mass of material C, respectively, is atomized and coated on material C to obtain material D.

[0055] Material D was placed in a rotary kiln and heated to 450℃ under a nitrogen atmosphere at a rate of 3.0℃ / min, and held at that temperature for 2.5 hours. Then, it was heated to 600℃ at a rate of 3.0℃ / min and sintered for 6 hours. After naturally cooling to room temperature, lithium iron phosphate composite material was obtained. The carbon coating content increased by 0.20% in the coating solution and the total carbon content was 1.42%.

[0056] Coin cells were assembled using the method described in Example 1 and tested under different charge and discharge current conditions. The initial charge-discharge efficiency at 0.1C reached 98.9%, with a discharge specific capacity of 163 mAh / g; the discharge specific capacity at 1C was 157 mAh / g; the discharge specific capacity at 10C was 142 mAh / g; and the powder compaction density was 2.56 g / cm³. 3 .

[0057] Example 3

[0058] Iron phosphate, lithium carbonate, and ammonium metavanadate were mixed at a molar ratio of 1:1.03:0.017 (iron, lithium, vanadium). PEG2000 was added at 9% of the total mass of the materials (lithium carbonate, iron phosphate, and ammonium metavanadate). Then, deionized water was added at a solid content of 35%, and the mixture was stirred and ball-milled. After ball milling for 1 hour, it was sand-milled for 4 hours. The particle size D50 after sand milling was controlled at 430±20nm. Then, it was spray-dried at an inlet temperature of 265℃ and an outlet temperature of 103℃. The particle size D50 of the dry powder after spraying was controlled at 5~6μm, resulting in material A, which is the lithium iron phosphate precursor powder.

[0059] Material A was placed in a rotary kiln and heated to 450℃ under a nitrogen atmosphere at a rate of 3.3℃ / min and held at that temperature for 3.0 hours. Then, it was heated to 730℃ at a rate of 3.3℃ / min and held at that temperature for 8 hours. After naturally cooling to room temperature, material B was obtained.

[0060] Material B is crushed, and the particle size is controlled to D50 of 1.5±0.5μm and D90 of 3.5±0.5μm to obtain material C, which is lithium iron phosphate particles with the first carbon coating layer.

[0061] Material C is fed into the fluidized bed reactor by a feeder. Heating gas at 145±10℃ is sent to the bottom of the fluidized bed by a blower and comes into contact with material C through a distribution plate to make the powder of material C fluidized. A coating liquid with a solid content of 40% nano-yttrium oxide and PEG2000 at 0.15% and 2.5% of the total mass of material C, respectively, is atomized and coated on material C to obtain material D.

[0062] Material D was placed in a rotary kiln and heated to 450℃ under a nitrogen atmosphere at a rate of 3.0℃ / min, and held at that temperature for 2.5 hours. Then, it was heated to 600℃ at a rate of 3.0℃ / min and sintered for 6 hours. After naturally cooling to room temperature, lithium iron phosphate composite material was obtained. The carbon coating content increased by 0.24% in the coating solution and the total carbon content was 1.43%.

[0063] Coin cells were assembled using the method described in Example 1 and tested under different charge and discharge current conditions. The initial charge-discharge efficiency at 0.1C reached 98.5%, with a discharge specific capacity of 160 mAh / g; the discharge specific capacity at 1C was 155 mAh / g; the discharge specific capacity at 10C was 139 mAh / g; and the powder compaction density was 2.59 g / cm³. 3 .

[0064] Comparative Example 1

[0065] Iron phosphate, lithium carbonate, and titanium dioxide were mixed in a molar ratio of 1:1.03:0.015. Glucose was added at 6% of the total mass of the materials (lithium carbonate, iron phosphate, and titanium dioxide). Deionized water was then added to the mixture with a solid content of 35%, and the mixture was stirred and ball-milled. After ball milling for 1 hour, it was sand-milled for 4 hours. The D50 particle size after sand milling was controlled at 430±20nm. Then, it was spray-dried at an inlet temperature of 260℃ and an outlet temperature of 102℃. The D50 particle size of the dry powder after spraying was controlled at 5~6μm, resulting in material A, which is the lithium iron phosphate precursor powder.

[0066] Material A was placed in a rotary kiln and heated to 450℃ under a nitrogen atmosphere at a rate of 3.5℃ / min and held at that temperature for 2.5 hours. Then, it was heated to 730℃ at a rate of 3.5℃ / min and sintered for 8 hours. After naturally cooling to room temperature, the material was pulverized, and the particle size was controlled to be D50 of 1.5±0.5μm and D90 of 3.5±0.5μm to obtain titanium-doped lithium iron phosphate composite material.

[0067] Coin cells were assembled using the method described in Example 1 and tested under different charge and discharge currents. The initial charge-discharge efficiency at 0.1C reached 97.2%, with a discharge specific capacity of 160 mAh / g; the discharge specific capacity at 1C was 150 mAh / g; the discharge specific capacity at 10C was 128 mAh / g; and the powder compaction density was 2.51 g / cm³. 3 .

[0068] Comparative Example 2

[0069] Iron phosphate, lithium carbonate, titanium dioxide, and nano-cerium oxide were mixed in a molar ratio of 1:1.03:0.015:0.015. Glucose was added at 6% of the total mass of the materials (lithium carbonate, iron phosphate, titanium dioxide, and oxides). Deionized water was then added to achieve a solid content of 35%, followed by ball milling. After ball milling for 1 hour, the mixture was sand milled for 4 hours. The D50 particle size after sand milling was controlled at 430±20nm. The mixture was then spray dried at an inlet temperature of 260℃ and an outlet temperature of 102℃. The D50 particle size of the dry powder after spray drying was controlled at 5~6μm, yielding material A, which is the lithium iron phosphate precursor powder.

[0070] Material A was placed in a rotary kiln and heated to 450℃ under a nitrogen atmosphere at a rate of 3.5℃ / min and held at that temperature for 2.5 hours. Then, it was heated to 730℃ at a rate of 3.5℃ / min and sintered for 8 hours. After naturally cooling to room temperature, the material was pulverized, and the particle size was controlled to be D50 of 1.5±0.5μm and D90 of 3.5±0.5μm, to obtain a titanium and cerium co-doped lithium iron phosphate composite material.

[0071] Coin cells were assembled using the method described in Example 1 and tested under different charge and discharge currents. The initial charge-discharge efficiency at 0.1C reached 97.6%, with a discharge specific capacity of 160 mAh / g; the discharge specific capacity at 1C was 150 mAh / g; the discharge specific capacity at 10C was 131 mAh / g; and the powder compaction density was 2.49 g / cm³. 3 .

[0072] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A lithium iron phosphate composite material, characterized in that, The composite material includes lithium iron phosphate particles with a first carbon coating layer, a second carbon coating layer, and nano-oxide conductive wires; the second carbon coating layer and nano-oxide conductive wires are located between the lithium iron phosphate particles covered by the first carbon coating layer, and the second carbon coating layer and nano-oxide conductive wires connect the first carbon coating layer on the surface of the lithium iron phosphate particles to form a complete carbon-coated three-dimensional spatial network conductive structure. The nano-oxide is one or a mixture of two or more of nano-yttrium oxide, nano-cerium oxide, nano-neodymium oxide, and nano-tungsten oxide; The method for preparing the composite material includes: placing lithium iron phosphate particles with a first carbon coating layer in a fluidized bed reactor through which a heated gas is passed, and performing a secondary coating treatment by a coating liquid, wherein the coating liquid includes water, a carbon source and nano-oxides; sintering the lithium iron phosphate particles after secondary coating to obtain a composite material of lithium iron phosphate modified with nano-oxides.

2. The method for preparing the lithium iron phosphate composite material according to claim 1, characterized in that: Lithium iron phosphate particles with a first carbon coating layer are placed in a fluidized bed reactor through which heated gas is passed, and a secondary coating treatment is performed by a coating liquid, which includes water, carbon source and nano-oxide. The lithium iron phosphate particles after secondary coating were sintered to obtain a composite material of lithium iron phosphate modified with nano-oxide.

3. The method for preparing the lithium iron phosphate composite material according to claim 2, characterized in that: The solid content of carbon source and nano-oxide in the coating solution is controlled at 1-3% and 0.05-0.20% of the coated material, respectively.

4. The method for preparing the lithium iron phosphate composite material according to claim 2, characterized in that: The sintering process involves heating the temperature to 300-500℃ at a rate of 2-4℃ / min, holding the temperature for 1-3 hours, and then continuing to heat the temperature to 500-700℃ at a rate of 2-4℃ / min and holding the temperature for 3-8 hours.

5. The method for preparing the lithium iron phosphate composite material according to claim 2, characterized in that, The heating gas is controlled at 100-200℃.

6. The method for preparing the lithium iron phosphate composite material according to claim 2, characterized in that... It lies in, The lithium iron phosphate particles with the first carbon coating layer are prepared by the following steps: (1) Add iron phosphate, lithium source, carbon source and doping element source to water and stir and ball mill, then spray dry and granulate after ultra-sand milling to obtain lithium iron phosphate precursor powder; (2) The obtained lithium iron phosphate precursor powder is placed in a rotary kiln and sintered under an inert atmosphere. After natural cooling, the sintered powder product is subjected to air jet pulverization to obtain lithium iron phosphate material with a first carbon coating layer.

7. The method for preparing the lithium iron phosphate composite material according to claim 6, characterized in that, In step (1), the lithium source is one or a mixture of two or more of lithium carbonate, lithium nitrate, lithium acetate, lithium hydroxide and lithium oxalate.

8. The method for preparing the lithium iron phosphate composite material according to claim 6, characterized in that, In step (1), the carbon source is one or a mixture of two or more of starch, citric acid, sucrose, glucose, PEG1000, PEG2000, PEG3000 and phenolic resin.

9. The method for preparing the lithium iron phosphate composite material according to claim 6, characterized in that, In step (1), the dopant source is one or a mixture of two or more of manganese acetate, manganese dioxide, titanium dioxide, magnesium acetate, niobium pentoxide, ammonium metavanadate, or vanadium pentoxide.

10. The method for preparing the lithium iron phosphate composite material according to any one of claims 6-9, characterized in that, In step (1), the iron phosphate, lithium source, and doping element source are in the molar ratio of iron, lithium, and doping element as 1.0:0.99-1.05:0.01-0.05, and carbon source is added at 5-10% of the total mass of iron phosphate, lithium source, and doping element source.

11. The method for preparing the lithium iron phosphate composite material according to claim 6, characterized in that, In step (1), the specific operation is to first add water to the raw material, ball mill for 1 to 5 hours, and then sand mill for 3 to 10 hours to obtain a slurry with a particle size D50 of 300 to 600 nm.

12. The method for preparing the lithium iron phosphate composite material according to claim 6, characterized in that, In step (1), the spray drying granulation equipment is a pressure spray drying tower with an inlet temperature controlled at 220-300℃, an outlet temperature controlled at 90-115℃, and a dry powder particle size D50 controlled at 4-8μm.

13. The method for preparing the lithium iron phosphate composite material according to claim 6, characterized in that, In step (2), the powder sintering process is as follows: the temperature is increased to 300-500℃ at a rate of 2-4℃ / min, then kept at a constant temperature for 1-3 hours, and then the temperature is increased to 600-800℃ at a rate of 2-4℃ / min, and then kept at a constant temperature for 5-10 hours.

14. The method for preparing the lithium iron phosphate composite material according to claim 6, characterized in that, In step (2), after the airflow pulverization, the D50 particle size is controlled at 0.8-2.5μm and the D90 particle size is controlled at 3.0-5.0μm.

Images