A large and small flake graphene composite lithium iron phosphate material, a preparation method therefor, and an application thereof

Abstract

The application provides a kind of size flake diameter graphene composite lithium iron phosphate material and its preparation method and application, belong to battery technical field.The porous amorphous carbon obtained after graphene and organic carbon source sintering in the application improves the conductivity of lithium iron phosphate composite material, and the application controls the flake diameter of small flake diameter graphene, which coats the surface of lithium iron phosphate particles and lithium iron phosphate clusters, controls the flake diameter of large flake diameter graphene, which inlays or fills in the lithium iron phosphate clusters, so that graphene does not completely block the ion diffusion channel in lithium iron phosphate composite material, thereby increasing electron migration and reducing internal resistance without affecting ion diffusion. The graphene partially coated, covered, inlaid or filled reduces the probability of graphene collision during the crushing process, so that graphene is not easy to fall off, further reducing the internal resistance.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, specifically relating to a graphene composite lithium iron phosphate material with varying sheet sizes, its preparation method, and its application. Background Technology

[0002] Lithium iron phosphate (LFP) possesses advantages such as good thermal stability, high theoretical capacity, excellent cycle performance, low raw material cost, and environmental friendliness, making it an ideal cathode material for next-generation lithium batteries. However, LFP has extremely low electronic conductivity, approaching that of an insulator, which limits its application in medium-to-large-scale energy storage batteries and electric vehicles. To address the poor electronic conductivity of LFP, existing technologies employ graphene or porous amorphous carbon to coat it. Graphene-coated LFP exhibits better conductivity than porous amorphous carbon-coated LFP, but graphene coating can block the ion diffusion channels within the LFP, leading to increased internal resistance. Furthermore, after obtaining graphene-coated LFP, it typically requires pulverization to meet particle size requirements. However, in existing graphene-coated LFP, the coating is prone to detachment during pulverization, exposing the LFP and further increasing internal resistance. Summary of the Invention

[0003] The purpose of this invention is to provide a graphene composite lithium iron phosphate material with varying sheet sizes, its preparation method, and its application. The lithium iron phosphate composite material prepared by the method of this invention has low internal resistance.

[0004] This invention provides a method for preparing lithium iron phosphate composite materials, comprising the following steps:

[0005] A mixture containing a solid phase is spray-dried to obtain a lithium iron phosphate composite material precursor. The solid phase comprises the following components: a lithium source, an iron source, an organic carbon source, a dispersant, and graphene. When the iron source does not contain phosphorus, the solid phase also includes a phosphorus source. When the iron source contains phosphorus, the solid phase may or may not include a phosphorus source. The graphene is small-diameter graphene and large-diameter graphene. The diameter of the small-diameter graphene is 20-60% of the target particle size of lithium iron phosphate, and the diameter of the large-diameter graphene is 50-100% of the square root of the target surface area of ​​lithium iron phosphate.

[0006] The lithium iron phosphate composite material precursor is sintered to obtain the lithium iron phosphate composite material.

[0007] Preferably, the mixture containing the solid phase is directly spray-dried or the mixture containing the solid phase is subjected to a hydrothermal reaction before being spray-dried.

[0008] Preferably, when the mixture containing the solid phase is directly spray-dried, the preparation method of the mixture containing the solid phase includes the following steps: wet milling the solid phase to obtain a wet milling slurry; diluting the wet milling slurry to obtain a mixture containing the solid phase;

[0009] When the mixture containing the solid phase undergoes a hydrothermal reaction and is then spray-dried, the method for preparing the mixture containing the solid phase includes the following steps: mixing the solid phase with water to obtain a mixture containing the solid phase.

[0010] Preferably, when the mixture containing the solid phase is directly spray-dried, the mass of the solid phase is 55-60% of the mass of the dilution water; in the wet abrasive slurry, the mass content of the lithium source is 8.5-11.5%, the mass content of the iron source is 30-40%, the mass content of the organic carbon source is 3-4%, the mass content of the dispersant is 3-5%, and the mass content of graphene is 1-2.5%. ， The phosphorus source has a mass content of 0-20%, with the remainder being water.

[0011] Preferably, when the mixture containing the solid phase undergoes a hydrothermal reaction followed by spray drying, the mixture containing the solid phase comprises, by mass fraction, 3-6% lithium source, 15-35% iron source, 0-21% phosphorus source, 0.5-2% organic carbon source, 0.08-2% dispersant, 0.3-1.5% graphene, and the balance being water.

[0012] Preferably, the mass ratio of the small-diameter graphene to the large-diameter graphene is 1:1 to 2.

[0013] Preferably, the sintering includes a first sintering and a second sintering performed sequentially; the temperature of the first sintering is 300-550℃ and the holding time is 3-10h; the temperature of the second sintering is 730-750℃ and the holding time is 8-10h.

[0014] Preferably, the hydrothermal reaction is carried out at a temperature of 100–200°C, a pressure of 1–100 MPa, and a time of 10–24 h.

[0015] The present invention also provides a lithium iron phosphate composite material prepared by the preparation method described above, comprising lithium iron phosphate particles and lithium iron phosphate clusters, porous amorphous carbon, small-diameter graphene, and large-diameter graphene; wherein the lithium iron phosphate clusters are formed by the aggregation of multiple lithium iron phosphate particles; the porous amorphous carbon completely coats the surfaces of the lithium iron phosphate particles and lithium iron phosphate clusters, and the small-diameter graphene coats part of the surfaces of the lithium iron phosphate particles and lithium iron phosphate clusters; the large-diameter graphene is embedded in or fills the lithium iron phosphate clusters.

[0016] The present invention also provides the application of the lithium iron phosphate composite material described above as a positive electrode material in lithium batteries.

[0017] This invention provides a method for preparing a lithium iron phosphate composite material, comprising the following steps: spray drying a mixture containing a solid phase to obtain a lithium iron phosphate composite material precursor; the solid phase comprises the following components: lithium source, iron source, phosphorus source, organic carbon source, dispersant, and graphene; when the iron source does not contain phosphorus, the solid phase further comprises a phosphorus source; when the iron source contains phosphorus, the solid phase may or may not include a phosphorus source; the graphene is small-diameter graphene and large-diameter graphene; the diameter of the small-diameter graphene is 20-60% of the target particle size of lithium iron phosphate, and the diameter of the large-diameter graphene is 50-100% of the square root of the target surface area of ​​lithium iron phosphate; sintering the lithium iron phosphate composite material precursor to obtain the lithium iron phosphate composite material. The porous amorphous carbon obtained by sintering graphene and organic carbon source in this invention improves the conductivity of lithium iron phosphate composite materials. Furthermore, this invention controls the size of small-diameter graphene flakes to partially coat the surface of lithium iron phosphate particles and clusters, and controls the size of large-diameter graphene flakes to embed or fill within the lithium iron phosphate clusters. This prevents the graphene from completely blocking the ion diffusion channels in the lithium iron phosphate composite material, thereby increasing electron migration and reducing internal resistance without affecting ion diffusion. The partial coating and embedding / filling of graphene reduces the probability of graphene collisions during pulverization, making it less prone to detachment and further reducing internal resistance. Example results show that the internal resistance of the lithium iron phosphate composite material prepared by this invention is reduced by 5% compared to lithium iron phosphate without composite graphene.

[0018] Furthermore, in this invention, the mixture containing the solid phase is directly spray-dried and then sintered. At this time, the coating of porous carbon and the formation of lithium iron phosphate occur simultaneously, resulting in the phenomenon of porous amorphous carbon coating multiple lithium iron phosphate particles.

[0019] This invention involves first subjecting a mixture containing a solid phase to a hydrothermal reaction, followed by spray drying and sintering. Since lithium carbonate and iron phosphate react to form lithium iron phosphate particles during the hydrothermal reaction, the porous amorphous carbon generated from the organic carbon source during sintering can achieve a more uniform coating of the lithium iron phosphate, thereby further reducing internal resistance. Example results show that the internal resistance of the lithium iron phosphate composite material prepared by the hydrothermal reaction is reduced by 5% compared to lithium iron phosphate without composite graphene. Attached Figure Description

[0020] Figure 1 SEM analysis image of the lithium iron phosphate composite material in Example 2 at a magnification of 200 nm;

[0021] Figure 2SEM analysis of the lithium iron phosphate composite material in Example 2 at a magnification of 50 nm. Detailed Implementation

[0022] This invention provides a method for preparing lithium iron phosphate composite materials, comprising the following steps:

[0023] A mixture containing a solid phase is spray-dried to obtain a lithium iron phosphate composite material precursor. The solid phase comprises the following components: a lithium source, an iron source, an organic carbon source, a dispersant, and graphene. When the iron source does not contain phosphorus, the solid phase also includes a phosphorus source. When the iron source contains phosphorus, the solid phase may or may not include a phosphorus source. The graphene is small-diameter graphene and large-diameter graphene. The diameter of the small-diameter graphene is 20-60% of the target particle size of lithium iron phosphate, and the diameter of the large-diameter graphene is 50-100% of the square root of the target surface area of ​​lithium iron phosphate.

[0024] The lithium iron phosphate composite material precursor is sintered to obtain the lithium iron phosphate composite material.

[0025] In this invention, the mixture containing the solid phase is preferably spray-dried directly (referred to as solid phase process) or the mixture containing the solid phase is preferably spray-dried after a hydrothermal reaction (referred to as hydrothermal process).

[0026] When the mixture containing the solid phase is directly spray-dried, the preparation method of the mixture containing the solid phase includes the following steps: wet milling the solid phase to obtain a wet milling slurry; diluting the wet milling slurry to obtain a mixture containing the solid phase. In this invention, the solid phase is wet-milled to obtain a wet milling slurry. In this invention, the solid phase includes the following components: a lithium source, an iron source, a phosphorus source, an organic carbon source, a dispersant, and graphene. In the wet milling slurry, the mass content of the lithium source is preferably 8.5-11.5%, more preferably 9-10%; the lithium source preferably includes one or more of lithium carbonate, lithium hydroxide, and lithium acetate; the mass content of the iron source is preferably 30-40%, more preferably 35-38%; the iron source preferably includes one or more of iron phosphate, iron oxalate, iron oxide, and ferrous sulfate. In this invention, the mass content of the organic carbon source is preferably 3-4%, more preferably 3.5-3.6%. The organic carbon source preferably includes one or more of sucrose, glucose, and starch. In this invention, the organic carbon source generates porous amorphous carbon during sintering, improving the conductivity of lithium iron phosphate. In this invention, the mass content of the dispersant is preferably 3-5%, more preferably 3.5-3.6%. The dispersant preferably includes one or more of polyvinylpyrrolidone, sodium dodecyl sulfate, hexadecyltrimethylamine, polyacrylamide, and polyethylene glycol. In this invention, the mass content of graphene is preferably 1-2.5%, more preferably 1.5-2%. In this invention, the graphene comprises small-diameter graphene and large-diameter graphene; the mass ratio of the small-diameter graphene to the large-diameter graphene is preferably 1:1 to 2, more preferably 1:1.5 to 1.8; the diameter of the small-diameter graphene is 20 to 60% of the target particle size of lithium iron phosphate, preferably 30 to 50%, more preferably 40 to 45%; the diameter of the large-diameter graphene is 50 to 100% of the square root of the target surface area of ​​lithium iron phosphate, preferably 60 to 80%, more preferably 65 to 70%. The surface area is calculated using 4πr 2 The unit of the large-diameter graphene flake diameter is preferably the same as the unit of r, preferably in μm, where r is half the target particle size of lithium iron phosphate. In this invention, the median diameter of lithium iron phosphate is preferably used as the target particle size to calculate the flake diameters of small-diameter and large-diameter graphene. When the median diameter of lithium iron phosphate is within a range, the average of the maximum and minimum values ​​of the range is used to calculate the flake diameters of small-diameter and large-diameter graphene. Specific calculation methods are detailed in the embodiments. In this invention, the mass content of the phosphorus source is preferably 0–20%, more preferably 5–15%, and even more preferably 10–12%. The phosphorus source preferably includes one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. When the iron source is iron phosphate, the content of the phosphorus source is preferably 0.

[0027] In this invention, there are no special limitations on the wet grinding process; any method well-known to those skilled in the art can be used for uniform wet grinding. The preferred wet grinding time is 3-5 hours. There are no special limitations on the container used for the wet grinding; any container well-known to those skilled in the art can be used, such as a grinding jar.

[0028] In this invention, the mass of the solid phase is 55-60% of the mass of the dilution water, more preferably 56-58%; the dilution time is preferably 1-2 hours, and the dilution is preferably carried out under stirring conditions, with the stirring speed preferably 700-1200 rpm. This invention does not have a special limitation on the container used for the dilution; any container well-known to those skilled in the art can be used. Specifically, a dispersion tank is acceptable. In this invention, when the wet grinding is carried out in a grinding tank and the dilution is carried out in a dispersion tank, it is preferable to wash the grinding tank after the wet grinding slurry has been transferred with 50-60% dilution water 2-3 times, then transfer the resulting mixed solution to the dispersion tank, and then use the remaining dilution water to dilute the wet grinding slurry. The dilution water in this invention serves to clean residual material in the grinding tank and adjust the viscosity.

[0029] When the mixture containing the solid phase undergoes a hydrothermal reaction followed by spray drying, the mixture preferably comprises, by mass fraction, 3-6% lithium source, 15-35% iron source, 0-21% phosphorus source, 0.5-2% organic carbon source, 0.08-2% dispersant, 0.3-1.5% graphene, and the balance being water; more preferably, it comprises 4-5% lithium source, 20-30% iron source, 10-15% phosphorus source, 1-1.5% organic carbon source, 1-1.5% dispersant, 0.5-1% graphene, and the balance being water. The method for preparing the mixture containing the solid phase preferably includes the following steps: mixing the solid phase and water to obtain the mixture containing the solid phase. This invention does not impose any particular limitation on the mixing process; uniform mixing is sufficient. In this invention, the temperature of the hydrothermal reaction is preferably 120–200°C, more preferably 150–180°C, and even more preferably 160–170°C; the time is preferably 10–24 h, more preferably 12–20 h, and even more preferably 15–16 h; and the pressure is preferably 1–100 MPa, more preferably 40–80 MPa, and even more preferably 50–60 MPa. During the hydrothermal reaction, lithium carbonate and iron phosphate react to form lithium iron phosphate particles. During sintering, the porous amorphous carbon generated by the organic carbon source can achieve a more uniform coating of the lithium iron phosphate, thereby further reducing the internal resistance. In this invention, when the iron source is iron phosphate, the phosphorus source content is preferably 0.

[0030] After obtaining a mixture containing a solid phase, the present invention spray-dries the mixture containing the solid phase to obtain a lithium iron phosphate composite material precursor.

[0031] In this invention, there are no special limitations on the spray drying process; any method well-known to those skilled in the art can be used. Specifically, in this embodiment of the invention: the spray drying is carried out in a spray drying device; the inlet temperature of the spray drying device is 200-250°C, the outlet temperature is 100-120°C, and the feed rate is 15 kg / h.

[0032] In this invention, during the solid-phase process, the particle size of the spray-dried product is preferably 500–2000 nm, more preferably 600–1500 nm, and even more preferably 800–1200 nm; during the hydrothermal process, the particle size of the spray-dried product is preferably 100–400 nm, more preferably 200–300 nm, and even more preferably 230–280 nm.

[0033] After obtaining the lithium iron phosphate composite material precursor, the present invention sintersulates the lithium iron phosphate composite material precursor to obtain the lithium iron phosphate composite material. In the present invention, the sintering preferably includes a first sintering and a second sintering performed sequentially; the temperature of the first sintering is preferably 300–550°C, more preferably 400–500°C, further preferably 450–480°C, and the holding time is preferably 3–10 h, more preferably 4–8 h, further preferably 5–6 h; the temperature of the second sintering is preferably 730–750°C, more preferably 740–745°C, and the holding time is preferably 8–10 h, more preferably 8.5–9 h. In the present invention, the temperature is preferably increased from room temperature to the temperature of the first sintering, and the heating rate is preferably 5–10°C / min. In the present invention, the particle size of the lithium iron phosphate is obtained by adjusting the process parameters of the hydrothermal reaction and sintering according to the particle size required by the customer. Specifically, for example, a hydrothermal reaction temperature of 120℃ for 12 hours, a first sintering temperature of 400℃ for 5 hours, and a second sintering temperature of 740℃ for 10 hours can yield lithium iron phosphate with a particle size of 300-400 nm.

[0034] This invention involves sintering the lithium iron phosphate composite material precursor, and preferably cooling and pulverizing the sintered product to obtain the lithium iron phosphate composite material. This invention does not have specific requirements for the cooling method; any method well-known to those skilled in the art can be used. The cooling time is preferably 6–8 hours, and the temperature after cooling is preferably ≤50°C. This invention does not have specific limitations on the pulverization method; any technical solution well-known to those skilled in the art can be used. Specifically, in this embodiment of the invention: air jet milling. This invention does not have specific limitations on the particle size of the lithium iron phosphate composite material; it can be adjusted according to customer requirements.

[0035] This invention also provides a lithium iron phosphate composite material prepared by the above-described preparation method, comprising lithium iron phosphate particles and lithium iron phosphate clusters, porous amorphous carbon, small-diameter graphene, and large-diameter graphene; the lithium iron phosphate clusters are formed by the aggregation of multiple lithium iron phosphate particles; the porous amorphous carbon completely coats the surfaces of the lithium iron phosphate particles and lithium iron phosphate clusters, and the small-diameter graphene coats part of the surfaces of the lithium iron phosphate particles and lithium iron phosphate clusters; the large-diameter graphene is embedded in or fills the lithium iron phosphate clusters. In this invention, the preferred particle size of the lithium iron phosphate particles in the lithium iron phosphate composite material is 100–400 nm, and the preferred particle size of the lithium iron phosphate clusters is 500–2000 nm.

[0036] The graphene and porous amorphous carbon in this invention improve the conductivity of lithium iron phosphate composite materials. Small-diameter graphene is coated on part of the surface of lithium iron phosphate particles, while large-diameter graphene is embedded or filled in multiple lithium iron phosphate particles. This allows the graphene to not completely block the pores for ion diffusion in the lithium iron phosphate composite material, thereby increasing electron migration and reducing internal resistance without affecting ion diffusion.

[0037] The present invention also provides the application of the lithium iron phosphate composite material described above as a positive electrode material in lithium batteries.

[0038] To further illustrate the present invention, the following detailed description, in conjunction with the accompanying drawings and embodiments, provides a graphene composite lithium iron phosphate material of varying sheet sizes, its preparation method, and its applications. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0039] Example 1

[0040] Preparation of lithium iron phosphate with a particle size D50 of 600–1000 nm: Based on a particle size of 800 nm, the particle size of small-diameter graphene is 0.16–0.48 μm; the particle size of large-diameter graphene is 0.71–1.4 μm (the smallest calculated value is [4 × 3.14 × (0.8 / 2)]). 2 ] 1 / 2 ×50%=0.71, the largest is [4×3.14×(0.8 / 2)2] 1 / 2 ×100%=1.4);

[0041] 11 kg of lithium carbonate, 3.85 kg of sucrose, 4.3 kg of PEG, 1.65 kg of graphene (0.65 kg of small-diameter graphene and 1.0 kg of large-diameter graphene), 38.5 kg of iron phosphate, and 41.7 kg of pure water were wet-milled for 4 hours. 50 kg of pure water was then used to wash the grinding tank in 2-3 batches, transferring the entire mixture to a dispersion tank. Another 50 kg of pure water was added and the mixture was diluted at 900 rpm for 2 hours. The resulting product was then pumped into a spray drying apparatus for spray drying to obtain a lithium iron phosphate composite material precursor with a particle size of 500-800 nm. The inlet temperature of the spray drying apparatus was adjusted to 225℃, the outlet temperature to 110℃, and the feed rate to 15 kg / h.

[0042] The lithium iron phosphate composite precursor was subjected to a first sintering and a second sintering in a nitrogen atmosphere. The first sintering temperature was 330℃, and the holding time was 5 h; the second sintering temperature was 740℃, and the holding time was 8 h. The second sintered material was cooled for 8 h to obtain the sintered material.

[0043] The sintered material is pulverized and screened by airflow to form a lithium iron phosphate composite material with a particle size of D50: 600-1000nm.

[0044] Example 2

[0045] Lithium iron phosphate with a particle size D50 of 200–400 nm was prepared. Based on a particle size of 300 nm, the diameter of small-diameter graphene flakes is 0.06–0.18 μm; the diameter of large-diameter graphene flakes is 0.27–0.55 μm (the smallest calculated value is [4 × 3.14 × (0.3 / 2)]). 2 ] 1 / 2 ×50%=0.27, the largest is [4×3.14×(0.3 / 2)] 2 ] 1 / 2 ×100%=0.55);

[0046] 11 kg of lithium carbonate, 3.3 kg of sucrose, 3.3 kg of PEG, 2.2 kg of graphene (1 kg of small-diameter graphene and 1.2 kg of large-diameter graphene), 38.5 kg of iron phosphate, and 141.7 kg of pure water were mixed evenly and then subjected to a hydrothermal reaction. The hydrothermal reaction was carried out at a temperature of 120℃, a pressure of 1.5 MPa, and a time of 12 h. The resulting hydrothermal reaction product was pumped into a spray dryer for spray drying to obtain a lithium iron phosphate composite material precursor with a particle size of 200–400 nm. The inlet temperature of the spray dryer was adjusted to 225℃, the outlet temperature to 110℃, and the feed rate to 15 kg / h.

[0047] The lithium iron phosphate composite precursor was subjected to a first sintering and a second sintering in a nitrogen atmosphere. The first sintering temperature was 430℃, and the holding time was 5 h; the second sintering temperature was 740℃, and the holding time was 8 h. The second sintered material was cooled for 8 h to obtain the sintered material.

[0048] The sintered material is pulverized and screened by airflow to form a lithium iron phosphate composite material with a particle size of D50: 200-400nm.

[0049] Comparative Example 1

[0050] The only difference from Example 1 is that no graphene was added, and the amount of sucrose added was 5.5 kg.

[0051] Comparative Example 2

[0052] The only difference from Example 2 is that no graphene was added, and the amount of sucrose added was 5.5 kg.

[0053] The internal resistance and low-temperature experiments of the lithium iron phosphate composite materials of Example 1 and Comparative Example 1 were measured, and the experimental results are shown in Table 1.

[0054] The internal resistivity of the positive electrode sheets prepared from the lithium iron phosphate composite material in Example 1 and Comparative Example 1 was compared by testing, and the experimental results are shown in Table 1. The composition of the positive electrode sheet was: 3% carbon black conductive agent + 5% PVDF (Hsv900) + 92% lithium iron phosphate composite material (LiFePO4 composite material in Example 1 or Comparative Example 1). The solid content of the slurry used to prepare the positive electrode was 58%, and the viscosity was 2500 ± 200 cp. The slurry was then coated onto carbon-coated aluminum foil.

[0055] Using the same mass of lithium iron phosphate composite materials from Example 1 and Comparative Example 1 as positive electrode materials and artificial graphite as negative electrode, lithium-ion batteries with model number 484570 and designed capacity of 1000mAh were prepared using the same polymer soft-pack battery process. After formation and capacity testing at room temperature and full charging, the batteries were placed in a programmable constant temperature and humidity chamber with the temperature adjusted to -20℃ and humidity 45%RH for 0.1C discharge testing (to avoid local heating due to high-rate discharge affecting the test results). The discharge capacity of the lithium iron phosphate composite materials of Example 1 and Comparative Example 1 was tested, and the experimental results are shown in Table 1.

[0056] Table 1. Performance results of lithium iron phosphate composite materials in Example 1 and Comparative Example 1

[0057] Internal resistance (mohm) -20℃ 0.1C discharge capacity Comparative Example 1 22.4 55％ Example 1 21.2 66％

[0058] As shown in Table 1, using graphene-composite sucrose as a carbon source to carbon-coat lithium iron phosphate can improve its low-temperature discharge performance.

[0059] The internal resistance and performance of the lithium iron phosphate composite materials in Example 2 and Comparative Example 2 were measured. The experimental results are shown in Table 2.

[0060] Following the above preparation method, small pouch batteries prepared from lithium iron phosphate composite materials of the same mass in Example 2 and Comparative Example 2 were subjected to charge-discharge tests at 0.5C after formation and capacity grading. The results are shown in Table 2.

[0061] Table 2. Performance results of lithium iron phosphate composite materials in Example 2 and Comparative Example 2.

[0062]

[0063]

[0064] Note: Initial efficiency = Initial discharge capacity / Initial charge capacity

[0065] As shown in Table 2, the hydrothermal method for preparing lithium iron phosphate using composite graphene as a carbon source for carbon coating can improve the specific capacity, enhance its initial efficiency, and also improve the discharge voltage platform.

[0066] The lithium iron phosphate composite material prepared in Example 2 was analyzed by SEM at magnifications of 200 nm and 50 nm. The results are as follows: Figure 1 and Figure 2 As shown. By Figure 1 and Figure 2 It can be seen that graphene is well coated on the surface of lithium iron phosphate particles and well embedded between adjacent lithium iron phosphate particles.

[0067] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, not all embodiments. Other embodiments can be obtained based on these embodiments without creative intent, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate composite material, characterized in that, Includes the following steps: A mixture containing a solid phase is spray-dried to obtain a lithium iron phosphate composite material precursor. The solid phase comprises the following components: a lithium source, an iron source, an organic carbon source, a dispersant, and graphene. When the iron source does not contain phosphorus, the solid phase also includes a phosphorus source. When the iron source contains phosphorus, the solid phase may or may not include a phosphorus source. The graphene is small-diameter graphene and large-diameter graphene. The diameter of the small-diameter graphene is 20-60% of the target particle size of lithium iron phosphate, and the diameter of the large-diameter graphene is 50-100% of the square root of the target surface area of ​​lithium iron phosphate. The lithium iron phosphate composite material precursor is sintered to obtain the lithium iron phosphate composite material. The mass ratio of small-diameter graphene to large-diameter graphene is 1:1 to 2.

2. The preparation method according to claim 1, characterized in that, The mixture containing the solid phase is either directly spray-dried or subjected to a hydrothermal reaction before being spray-dried.

3. The preparation method according to claim 2, characterized in that, When the mixture containing the solid phase is directly spray-dried, the preparation method of the mixture containing the solid phase includes the following steps: wet milling the solid phase to obtain a wet milling slurry; diluting the wet milling slurry to obtain a mixture containing the solid phase; When the mixture containing the solid phase undergoes a hydrothermal reaction and is then spray-dried, the method for preparing the mixture containing the solid phase includes the following steps: mixing the solid phase with water to obtain a mixture containing the solid phase.

4. The method for preparing the lithium iron phosphate composite material according to claim 3, characterized in that, When the mixture containing the solid phase is directly spray-dried, the mass of the solid phase is 55-60% of the mass of the dilution water; in the wet abrasive slurry, the mass content of the lithium source is 8.5-11.5%, the mass content of the iron source is 30-40%, the mass content of the organic carbon source is 3-4%, the mass content of the dispersant is 3-5%, the mass content of graphene is 1-2.5%, the mass content of the phosphorus source is 0-20%, and the balance is water.

5. The method for preparing the lithium iron phosphate composite material according to claim 3, characterized in that, When the mixture containing the solid phase undergoes a hydrothermal reaction and is then spray-dried, the mixture containing the solid phase comprises, by mass fraction, 3-6% lithium source, 15-35% iron source, 0-21% phosphorus source, 0.5-2% organic carbon source, 0.08-2% dispersant, 0.3-1.5% graphene, and the balance being water.

6. The preparation method according to claim 1, characterized in that, The sintering includes a first sintering and a second sintering performed sequentially; the temperature of the first sintering is 300-550℃ and the holding time is 3-10h; the temperature of the second sintering is 730-750℃ and the holding time is 8-10h.

7. The preparation method according to claim 2, characterized in that, The hydrothermal reaction is carried out at a temperature of 100–200℃, a pressure of 1–100 MPa, and a time of 10–24 h.

8. The lithium iron phosphate composite material prepared by the preparation method according to any one of claims 1 to 7, characterized in that, It includes lithium iron phosphate particles and lithium iron phosphate clusters, porous amorphous carbon, small-diameter graphene, and large-diameter graphene; the lithium iron phosphate clusters are formed by the aggregation of multiple lithium iron phosphate particles; the porous amorphous carbon completely coats the surface of the lithium iron phosphate particles and lithium iron phosphate clusters, the small-diameter graphene coats part of the surface of the lithium iron phosphate particles and lithium iron phosphate clusters; and the large-diameter graphene is embedded in or fills the lithium iron phosphate clusters.

9. The application of the lithium iron phosphate composite material of claim 8 as a cathode material in lithium batteries.

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