Lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The invention relates to the technical field of battery positive electrode materials, and provides a lithium iron phosphate positive electrode material and a preparation method thereof. The lithium iron phosphate positive electrode material has the characteristic of high energy density, the proportion of iron phosphide impurities in the total mass of the lithium iron phosphate positive electrode material is 0.001-0.008 PPM, the compaction density under the pressure of 3T is 2.68-2.8 g / cm < 3 >, the powder resistivity is 8-15 omega.cm, the 0.1 C discharge capacity is 161-163 mAh / g, the 0.1 C charge-discharge efficiency is 98.5-100%, and the 1C discharge capacity is 145-148 mAh / g. The preparation process is simple and safe, low in cost and high in stability. The content of iron phosphide impurities in a final lithium iron phosphate product can be regulated and controlled by regulating and controlling the phosphorus-iron ratio of the iron phosphate precursor and the ratio of the carbon source to the iron phosphate precursor in two-time burdening and controlling the sintering temperature in cooperation with secondary sintering, and the high-energy-density lithium iron phosphate positive electrode material is obtained.

Description

TECHNICAL FIELD

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

[0002] Lithium iron phosphate has gradually become the main cathode material of lithium ion batteries due to its low cost, high safety and long service life. However, as the technology of lithium ion batteries is gradually popularized, the performance requirements of lithium ion batteries on cathode materials are also increasing. Impurities in lithium iron phosphate products are important factors affecting the processing performance and electrochemical performance of lithium iron phosphate, and iron phosphide is one of the main impurities in lithium iron phosphate. Therefore, the control of iron phosphide impurities in lithium iron phosphate is an important link in the preparation process of lithium iron phosphate and a key indicator for judging the processing performance and electrochemical performance of lithium iron phosphate. At present, there are few reports on the control of iron phosphide impurities in lithium iron phosphate. CN116161638A proposes to first sinter to obtain large particle lithium iron phosphate containing iron phosphide impurities, then perform secondary grinding and drying on the large particle lithium iron phosphate, and then perform secondary low-temperature sintering to obtain lithium iron phosphate cathode material with low iron phosphide content. Although this method has good product consistency, the introduction of dilute acid washing process increases the process cost and easily brings in acidic impurities. CN119461306A proposes to reduce the residence time of reducing gas in the sintering process, that is, to control the input and output flow of the protective gas to achieve extremely low content of iron phosphide impurities. However, this method has high difficulty in airflow regulation, high requirement for equipment, and difficulty in ensuring the consistency of the sintered sample.

[0003] The control of iron phosphide impurities is particularly important for the preparation of high-performance lithium iron phosphate cathode materials. However, the current process for controlling iron phosphide impurities has the disadvantages of high equipment precision requirement, no market advantage in process cost, and poor product consistency. Through the optimization of the preparation process of lithium iron phosphate cathode material, the control of iron phosphide impurities in high-performance lithium iron phosphate cathode material can be realized, and the processing performance and electrochemical performance of lithium iron phosphate cathode material can be comprehensively improved. SUMMARY

[0004] The present application relates to the technical field of battery cathode materials, in particular to a lithium iron phosphate cathode material and a preparation method thereof.

[0005] To achieve the purpose of the present application, in a first aspect, the present application provides a lithium iron phosphate cathode material comprising lithium iron phosphate and iron phosphide impurities; wherein the proportion of iron phosphide impurities in the total mass of lithium iron phosphate cathode material is 0.001-0.008 PPM; The lithium iron phosphate cathode material has high energy density, and the compaction density under 3T pressure is 2.68-2.8 g / cm 3, the powder resistivity is 8-15 Ω·cm, the 0.1C discharge capacity is 161-163 mAh / g, the 0.1C charge-discharge efficiency is 98.5-100%, and the 1C discharge capacity is 145-148 mAh / g.

[0006] In a second aspect, the application provides a preparation method of the lithium iron phosphate positive electrode material, which comprises the steps of primary batching, primary sand milling, primary spray drying, primary sintering, secondary batching, secondary sand milling, secondary spray drying, secondary sintering, and airflow crushing.

[0007] Specifically, the method comprises the following steps: S1. dispersing a lithium iron phosphate precursor iron source, a phosphorus source, a lithium source, a carbon source, a dispersing agent, and a metal dopant in a solvent, with a solid content of the slurry being 38-45%; stirring the slurry at a speed of 200-800 rpm with a cantilever stirrer for 15-45 min (preferably, stirring the slurry at a speed of 500 rpm for 30 min), and then transferring the slurry to a sand mill for grinding at a speed of 1500-1650 rpm until the D50 of the slurry is 0.300-0.400 um; S2. drying the slurry after sand milling to obtain a first grinding material; S3. placing the first grinding material in a graphite crucible, sintering at a low temperature (300-500℃) under an inert gas atmosphere, and obtaining a first lithium iron phosphate; S4. dispersing the first lithium iron phosphate, a carbon source, and a dispersing agent in a solvent, with a solid content of the slurry being 38-45%; stirring the slurry at a speed of 200-800 rpm with a cantilever stirrer for 15-45 min (preferably, stirring the slurry at a speed of 500 rpm for 30 min), and then transferring the slurry to a sand mill for grinding at a speed of 1000-1850 rpm (preferably, 1650 rpm) until the D50 of the slurry is 0.300-0.400 um, and drying the slurry after sand milling to obtain a second grinding material; S5. sintering at a high temperature of 750-800℃ under an inert gas atmosphere, and obtaining a second lithium iron phosphate; S6. obtaining the high-energy-density lithium iron phosphate positive electrode material after airflow crushing of the second lithium iron phosphate.

[0008] Further, the mass ratio of the iron source, the phosphorus source, the lithium source, the carbon source, the dispersing agent, and the metal dopant in step S1 is (1.0-1.2):(1.0-1.2):(0.20-0.28):(0.05-0.15):(0.005-0.01):(0.002-0.01).

[0009] The iron source in step S1 is selected from one or more of ferrous phosphate, ferrous chloride, ferrous oxalate, ferrous sulfate, ferrous nitrate, ferrous hydroxide, iron hydroxide, diiron trioxide, iron powder, and the like.

[0010] The lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium phosphate, lithium chloride, lithium oxalate, etc.

[0011] The phosphorus source is selected from one or more of phosphoric acid, ferric phosphate, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, lithium monohydrogen phosphate, lithium dihydrogen phosphate, etc.

[0012] The carbon source is selected from one or more of glucose, sucrose, PEG, polyvinyl alcohol, phenolic resin, ascorbic acid, cellulose, starch, monocyamide, dicyandiamide, melamine, etc.

[0013] The metal dopant is selected from one or more of titanium dioxide, aluminum sesquioxide, magnesium carbonate, manganese carbonate, calcium carbonate, zirconium oxide, vanadium pentoxide, etc.

[0014] The dispersant is selected from one or more of sodium polyacrylate (PAAS), sodium dodecyl sulfate (SDS), polyvinylpyrrolidone (PVP), triisopropanolammonium (TIPA), sodium hexametaphosphate (SHMP), vitamins, etc.

[0015] The solvent is deionized water and / or anhydrous ethanol.

[0016] Further, the grinding in step S1 is sand grinding, and the sand grinder contains zirconium beads with a particle size of 0.1-0.4 um, and the sand grinding speed of the sand grinder is 1200-1650 rpm, and the sand grinding time is 60-90 min.

[0017] Further, the drying method in step S2 is centrifugal spray drying, and the inlet air temperature of the centrifugal spray dryer is 160-220℃, the outlet air temperature is 100-120℃, the air induction frequency is 40-60 Hz, and the centrifugal speed of the centrifugal head is 280-320 rpm.

[0018] Further, the inert gas in steps S3 and S5 is high-purity nitrogen and / or nitrogen containing 3%-5% v / v hydrogen; Further, the sintering conditions of step S3 are: heating at a rate of 1.5-4℃ / min to 300-500℃, holding sintering for 1-4 h, and the air flow rate is 2-10 L / min.

[0019] Further, the sintering conditions of step S5 are: heating at a rate of 2.5-4.5℃ / min to 750-800℃, holding sintering for 6-12 h (preferably 10 h), and the air flow rate is 2-10 L / min.

[0020] Further, the types of carbon source, dispersant and solvent used in step S4 are the same as those in step S1; wherein the mass ratio of the first lithium iron phosphate, carbon source and dispersant is (1.0-1.2):(0.01-0.05):(0.001-0.005).

[0021] The equipment process used for grinding and drying in step S4 is the same as that in step S1.

[0022] Further, the conditions for jet milling in step S6 are as follows: the feeding frequency is 10-50 Hz, the grinding gas pressure is 0.3-0.6 MPa, and the frequency of the classification wheel is 50-150 Hz.

[0023] Technical principle of the present application: Iron phosphide is one of the main impurities in lithium iron phosphate, and the formation of iron phosphide is mainly related to the sintering temperature in the sintering process, the ratio of iron and phosphorus (iron-phosphorus ratio) in the sintering raw material, and the reduction ability of the sintering atmosphere. Generally speaking, iron phosphide has better electrical conductivity than lithium iron phosphate, which can improve the electrical conductivity of lithium iron phosphate cathode material and reduce the amount of conductive agent used in the subsequent slurry preparation process of lithium iron phosphate cathode material. However, since iron phosphide itself does not have the ability to deintercalate lithium ions, it cannot provide effective capacity for lithium iron phosphate cathode material, and more importantly, iron phosphide will have adverse effects on the processing performance and electrochemical performance of lithium iron phosphate cathode material.

[0024] In terms of processing performance, iron phosphide impurities are easy to form large particles with high hardness, which can easily pierce the separator during the rolling of the battery anode sheet, causing micro-short circuits in the battery. In addition, large iron phosphide impurity particles can easily cause processing problems such as scratching and concave printing during the slurry coating process of lithium iron phosphate cathode material, affecting the processing smoothness and production efficiency of the lithium iron phosphate battery preparation process. In terms of electrochemical performance, iron phosphide impurities in lithium iron phosphate cathode material can be precipitated from the anode sheet into the entire battery electrolyte during the later lithium iron phosphate battery operation process, and Fe element can easily act as a catalyst to exacerbate the side reactions of the electrolyte and reduce the film quality of the SEI film grown on the surface of the graphite negative electrode. Therefore, controlling the content of iron phosphide impurities in lithium iron phosphate cathode material is an important link to improve its processing performance and electrochemical performance.

[0025] The process flow of the preparation method of the lithium iron phosphate cathode material of the present application is shown in Figure 1 .

[0026] By the above technical solution, the present application at least has the following advantages and beneficial effects: The present application provides a lithium iron phosphate cathode material and a preparation method thereof. The lithium iron phosphate cathode material has a high energy density, and the preparation process has the advantages of simplicity, safety, low cost and high stability. The specific process is as follows: (1) From the raw material end, by controlling the iron-phosphorus ratio of the precursor iron phosphate raw material, and the type of carbon source and the ratio of carbon source to precursor iron phosphate, the content of iron phosphide impurities in the final lithium iron phosphate positive electrode material is controlled. The iron-phosphorus ratio of the precursor iron phosphate is an important factor affecting the main product lithium iron phosphate and the generation of impurities during sintering. When the iron-phosphorus ratio is low, large lithium iron phosphate particles are easy to sinter and iron phosphide impurities are easy to generate. By adding a carbon source, the fusion and growth of particles can be alleviated to some extent, but the reduction gas generated by the pyrolysis of the carbon source during sintering promotes the generation of iron phosphide impurities, so by controlling the amount of carbon source added during sintering, the generation of iron phosphide can be controlled.

[0027] (2) From the process end, by a unique two-sintering process, that is, controlling the content of carbon source added during two sintering and the sintering temperature of two sintering, the generation of iron phosphide impurities during sintering is controlled. The carbon source pyrolyzes into graphitized carbon or amorphous carbon during sintering, which can significantly improve the conductivity of the lithium iron phosphate positive electrode material. On the other hand, the reduction gas is generated during the pyrolysis of the carbon source, which affects the generation of lithium iron phosphate and iron phosphate. When the concentration of reducing gas in the sintering atmosphere is too high, iron phosphide impurities are easy to produce. The present application adds part of the carbon source at low temperature to reduce the iron phosphate and achieve preliminary coating of the lithium iron phosphate during the first low-temperature sintering, and further adds the carbon source during the second sintering at high temperature. At this time, the lithium iron phosphate can be consolidated, and a carbon coating layer with high graphitization degree is generated on the surface of the lithium iron phosphate, which significantly improves the conductivity of the lithium iron phosphate positive electrode material. By adding the carbon source twice, the enrichment of the reducing atmosphere during sintering is avoided, and the content of iron phosphide impurities in the lithium iron phosphate positive electrode material is effectively reduced.

[0028] (3) The lithium iron phosphate positive electrode material provided by the present application has the characteristics of high energy density. Specifically, the proportion of iron phosphide impurities in the total lithium iron phosphate positive electrode material is 0.001-0.008 PPM. The lithium iron phosphate positive electrode material has the characteristics of high energy density. The proportion of iron phosphide impurities in the total lithium iron phosphate positive electrode material is 0.001-0.008 PPM. A lithium iron phosphate positive electrode material has high energy density, the compaction density under 3T pressure is 2.68-2.8 g / cm 3 , the powder resistivity is 8-15 Ω·cm, the 0.1C discharge capacity is 161-163 mAh / g, the 0.1C charge-discharge efficiency is 98.5-100%, and the 1C discharge capacity is 145-148 mAh / g.

[0029] (Four) The preparation method of the lithium iron phosphate positive electrode material provided by the application can effectively control the iron phosphide impurities in the lithium iron phosphate positive electrode material. Specifically, through the process flow of one-time batching, one-time sand milling, one-time spray drying, one-time sintering, two-time batching, two-time sand milling, two-time spray drying, two-time sintering and airflow crushing, especially by adjusting the phosphorus-iron ratio of the iron phosphate precursor in the raw materials, the ratio of the carbon source to the iron phosphate precursor and the temperature in the sintering process during the two-time batching, and combining the process of two-time sintering, the content of the iron phosphide impurities in the final lithium iron phosphate product can be controlled, and the lithium iron phosphate positive electrode material with high energy density can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 The preparation method of the lithium iron phosphate positive electrode material provided by the application can effectively control the iron phosphide impurities in the lithium iron phosphate positive electrode material. Specifically, through the process flow of one-time batching, one-time sand milling, one-time spray drying, one-time sintering, two-time batching, two-time sand milling, two-time spray drying, two-time sintering and airflow crushing, especially by adjusting the phosphorus-iron ratio of the iron phosphate precursor in the raw materials, the ratio of the carbon source to the iron phosphate precursor and the temperature in the sintering process during the two-time batching, and combining the process of two-time sintering, the content of the iron phosphide impurities in the final lithium iron phosphate product can be controlled, and the lithium iron phosphate positive electrode material with high energy density can be obtained.

[0031] Figure 2 The SEM (left) and EDS Mapping (middle and right) of the lithium iron phosphate in Example 7 of the application.

[0032] Figure 3 The X-ray powder diffraction pattern of the lithium iron phosphate in Comparative Example 1 of the application. DETAILED DESCRIPTION

[0033] The iron phosphide impurities in the lithium iron phosphate positive electrode material affect the processing performance and electrochemical performance of the lithium iron phosphate positive electrode material, especially for the lithium iron phosphate positive electrode material with high performance and high energy density, the control requirement for impurities is more strict, therefore, the content of the iron phosphide impurities in the lithium iron phosphate needs to be controlled from the raw materials and the process.

[0034] The generation of the iron phosphide is mainly related to the sintering temperature in the sintering process, the ratio of the iron element to the phosphorus element (iron-phosphorus ratio) in the sintering raw materials and the reduction ability of the sintering atmosphere. The application provides a lithium iron phosphate positive electrode material with suitable content of iron phosphide impurities and a preparation method thereof.

[0035] The application provides a lithium iron phosphate positive electrode material and a preparation method thereof, which are as follows: The application provides a lithium iron phosphate positive electrode material, which comprises lithium iron phosphate and iron phosphide impurities. The content of the iron phosphide impurities in the total lithium iron phosphate positive electrode material is 0.001-0.008 PPM. The lithium iron phosphate positive electrode material has high energy density, the compaction density under 3T pressure is 2.68-2.8 g / cm 3 , the powder resistivity is 8-15 Ω·cm, the 0.1C discharge capacity is 161-163 mAh / g, the 0.1C charge-discharge efficiency is 98.5-100%, and the 1C discharge capacity is 145-148 mAh / g.

[0036] The application provides a preparation method of a lithium iron phosphate positive electrode material, which comprises the following steps:Figure 1 As shown, the process flow comprises primary batching, primary sand milling, primary spray drying, primary sintering, secondary batching, secondary sand milling, secondary spray drying, secondary sintering, and jet milling. In particular, by regulating the phosphorus-iron ratio of the iron phosphate precursor, the ratio of the carbon source to the iron phosphate precursor, and the temperature during the sintering process during the two batchings, and in combination with the secondary sintering process, the content of iron phosphide impurities in the final lithium iron phosphate product can be regulated, and a high-energy-density lithium iron phosphate positive electrode material can be obtained.

[0037] The present application adopts the following technical solutions: S1. Disperse the lithium iron phosphate precursor iron source, phosphorus source, lithium source, carbon source, dispersant, and metal dopant in a solvent, and the slurry solid content is 38-45%. Stir the material at a cantilever stirrer speed of 500 rpm for 30 min. After stirring, transfer to a sand mill and grind at a speed of 1500-1650 rpm until the slurry D50 is 0.300-0.400 um.

[0038] S2. After sand milling, dry the material to obtain a first grinding material.

[0039] S3. Place the first grinding material in a graphite crucible, and under an inert gas atmosphere, sinter at a low temperature (300-500℃) for 6 h to obtain a first lithium iron phosphate.

[0040] S4. Disperse the first lithium iron phosphate, carbon source, and dispersant in a solvent, and the slurry solid content is 38-45%. Stir the material at a cantilever stirrer speed of 500 rpm for 30 min. After stirring, transfer to a sand mill and grind at a speed of 1650 rpm until the slurry D50 is 0.300-0.400 um. After sand milling, dry the material to obtain a second grinding material.

[0041] S5. Under an inert gas atmosphere, sinter at a high temperature for 10 h to obtain a second lithium iron phosphate.

[0042] S6. Jet mill the second lithium iron phosphate to obtain a high-energy-density lithium iron phosphate positive electrode material.

[0043] S7. Physicochemical property test the obtained lithium iron phosphate positive electrode material, and assemble a button cell for electrochemical performance test.

[0044] Further, the iron source in S1 comprises one or more of ferrous phosphate, ferrous chloride, ferrous oxalate, ferrous sulfate, ferrous nitrate, ferrous hydroxide, iron hydroxide, diiron trioxide, and iron powder; The lithium source in S1 comprises one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium phosphate, lithium chloride, and lithium oxalate; The phosphorus source in S1 is phosphoric acid, ferric phosphate, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, lithium monohydrogen phosphate, lithium dihydrogen phosphate; The carbon source in S1 includes one or more of glucose, sucrose, PEG, polyvinyl alcohol, phenol formaldehyde resin, ascorbic acid, cellulose, starch, monocyamine, dicyandiamide, and melamine. The metal dopant in S1 includes one or more of titanium dioxide, aluminum trioxide, magnesium carbonate, manganese carbonate, calcium carbonate, zirconium oxide, and vanadium pentoxide. The dispersant in S1 includes one or more of sodium polyacrylate (PAAS), sodium dodecyl sulfate (SDS), polyvinylpyrrolidone (PVP), triisopropanolammonium (TIPA), sodium hexametaphosphate (SHMP), and vitamins. The solvent in S1 is deionized water and / or anhydrous ethanol.

[0045] Further, the grinding in S1 is sand grinding, the sand grinder contains zirconium beads with a particle size of 0.1-0.4 um, the sand grinding speed of the sand grinder is 1200-1650 rpm, and the sand grinding time is 60-90 min.

[0046] The mass ratio of the iron source, phosphorus source, lithium source, carbon source, dispersant, and metal dopant in S1 is (1.0-1.2):(1.0-1.2):(0.20-0.28):(0.05-0.15):(0.005-0.01):(0.002-0.01).

[0047] Further, the drying method in S2 is centrifugal spray drying, the inlet air temperature of the centrifugal spray dryer is 160-220℃, the outlet air temperature is 100-120℃, the air induction frequency is 40-60 Hz, and the centrifugal speed of the centrifugal head is 280-320 rpm.

[0048] Further, the inert and / or reducing atmosphere in S3 is high-purity nitrogen and / or a reducing gas containing 3%-5% hydrogen (i.e., nitrogen containing 3%-5% v / v hydrogen), the sintered material is loaded into a graphite crucible, and the sintering conditions are as follows: heating at a rate of 1.5-4℃ / min to 300-500℃, holding for 1-4 h, and the air flow rate is 2-10 L / min.

[0049] Further, the types of carbon source, dispersant, dopant, and solvent in S4 are the same as in S1, and the mass ratio of the first lithium iron phosphate, carbon source, and dispersant is (1.0-1.2):(0.01-0.05):(0.001-0.005).

[0050] The equipment and process used for grinding and drying (spray drying) in S4 are also the same as in S1.

[0051] Further, the inert and / or reducing atmosphere in S5 is high-purity nitrogen and / or a reducing gas with a hydrogen content of 3-5% (i.e. nitrogen containing 3-5% v / v hydrogen), the sintering is high-temperature sintering, the sintering material is loaded into a graphite crucible, the heating rate is 2.5-4.5°C / min, and the ventilation rate is 2-10 L / min. The sintering temperature is 750-800°C. The sintering time is 6-12 h.

[0052] Further, the airflow powder feeding frequency in S6 is 10-50 Hz, the grinding gas pressure is 0.3-0.6 MPa, and the classification wheel frequency is 50-150 Hz.

[0053] Further, the button cell in S7 is composed of a lithium iron phosphate positive electrode sheet, a lithium negative electrode sheet, a separator, an electrolyte, and a button cell shell. The positive electrode sheet comprises an aluminum foil and a lithium iron phosphate slurry coating. The lithium iron phosphate slurry coating is prepared according to a mass ratio of lithium iron phosphate positive material: conductive carbon black: PVDF = 8:1:1.

[0054] The following examples are used to illustrate the present application, but are not intended to limit the scope of the present application. If not specifically indicated, the technical means used in the examples are conventional means known to those skilled in the art, and the raw materials used are commercially available.

[0055] Example 1 The present embodiment provides a lithium iron phosphate positive material and a preparation method thereof, comprising the following steps: S1. Disperse 2 kg of a first precursor, iron phosphate, 490 g of lithium carbonate, 110 g of anhydrous glucose, 220 g of polyethylene glycol (molecular weight 6000), 13 g of a dispersant polyacrylic acid, and 12 g of titanium dioxide in water, and the solid content of the slurry is 38%. Stir the material at a speed of 500 rpm using a cantilever stirrer for 30 min. After stirring, transfer the material to a sand mill and grind at a speed of 1650 rpm until the D50 of the slurry is 0.400 um.

[0056] S2. After sand milling, the material is subjected to centrifugal spray drying to obtain a first grinding material. The spray drying inlet temperature is 200°C, the outlet temperature is 105°C, and the centrifugal head rotation speed is 320 rpm.

[0057] S3. Place the first grinding material in a graphite crucible, heat it to 450°C at a rate of 2.3°C / min under a ventilation atmosphere of 5 L / min of high-purity nitrogen, and sinter at 450°C for 6 h to obtain a first lithium iron phosphate.

[0058] S4. Disperse 2 kg of the first lithium iron phosphate, 60 g of anhydrous glucose, 100 g of polyethylene glycol (molecular weight 6000), and 13ghA-18E in water to a slurry with a solids content of 38%. Stir the mixture at 500 rpm using an overhead mixer for 30 minutes. After stirring, transfer the mixture to a sand mill at 1650 rpm and grind it to a slurry with a D50 of 0.400 μm. After sand milling, centrifuge spray dry the mixture to obtain a second grind. The spray drying air inlet temperature was 200°C, the air outlet temperature was 105°C, and the centrifuge speed was 320 rpm.

[0059] S5. In a high-purity nitrogen atmosphere of 5 L / min, the second abrasive was heated to 775°C at a rate of 3°C / min and sintered at this temperature for 10 hours to obtain a second lithium iron phosphate.

[0060] S6. Grind the second lithium iron phosphate gas flow to obtain a high energy density lithium iron phosphate positive electrode material.

[0061] Example 2 The difference from Example 1 is that the carbon source content of the primary batching in Example 2 is lower than that in Example 1.

[0062] S1. Disperse 2 kg of the first precursor (ferric phosphate), 490 g of lithium carbonate, 100 g of anhydrous glucose, 170 g of polyethylene glycol (molecular weight 6000), 13ghA-18E, and 12 g of titanium dioxide in water to a slurry with a solids content of 38%. Stir the mixture at 500 rpm using an overhead stirrer for 30 minutes. After stirring, transfer the mixture to a sand mill at 1650 rpm and grind until the slurry has a D50 of 0.400 μm.

[0063] S2. After sand milling, the material is subjected to centrifugal spray drying to obtain a first grinding material. The spray drying air inlet temperature is 200°C, the air outlet temperature is 105°C, and the centrifugal head centrifugal speed is 320 rpm.

[0064] S3. Place the first abrasive in a graphite crucible, raise the temperature to 450°C at a rate of 2.3°C / min in a high-purity nitrogen atmosphere at a flow rate of 5 L / min, and sinter at 450°C for 6 h to obtain a first lithium iron phosphate.

[0065] S4. Disperse 2 kg of the first lithium iron phosphate, 60 g of anhydrous glucose, 100 g of polyethylene glycol (molecular weight 6000), and 13ghA-18E in water to a slurry with a solids content of 38%. Stir the mixture at 500 rpm using an overhead mixer for 30 minutes. After stirring, transfer the mixture to a sand mill at 1650 rpm and grind it to a slurry with a D50 of 0.400 μm. After sand milling, centrifuge spray dry the mixture to obtain a second grind. The spray drying air inlet temperature was 200°C, the air outlet temperature was 105°C, and the centrifuge speed was 320 rpm.

[0066] S5. Under the atmosphere of high-purity nitrogen gas 5 L / min, the second grinding material is heated to 775℃ at a rate of 3℃ / min and sintered for 10 h to obtain the second lithium iron phosphate.

[0067] S6. The high-energy-density lithium iron phosphate positive electrode material obtained after airflow crushing of the second lithium iron phosphate.

[0068] Example 3 Different from Example 1, the content of the carbon source in the one-time batching of Example 3 is higher than that of Example 1.

[0069] S1. 2 kg of precursor iron phosphate, 490 g of lithium carbonate, 120 g of anhydrous glucose, 270 g of polyethylene glycol (molecular weight 6000), 13 g of hA-18E, and 12 g of titanium dioxide are dispersed in water, and the solid content of the slurry is 38%. The material is stirred at a speed of 500 rpm by a cantilever stirrer for 30 min. After stirring, it is transferred to a sand mill for grinding at a speed of 1650 rpm until the D50 of the slurry is 0.400 um.

[0070] S2. After sand grinding, the material is centrifugally spray-dried to obtain the first grinding material, the spray-drying inlet temperature is 200℃, the outlet temperature is 105℃, and the centrifugal head rotation speed is 320 rpm.

[0071] S3. The first grinding material is placed in a graphite crucible, heated to 450℃ at a rate of 2.3℃ / min under the atmosphere of high-purity nitrogen gas 5 L / min, and sintered at 450℃ for 6 h to obtain the first lithium iron phosphate.

[0072] S4. 2 kg of the first lithium iron phosphate, 60 g of anhydrous glucose, 100 g of polyethylene glycol (molecular weight 6000), and 13 g of hA-18E are dispersed in water, and the solid content of the slurry is 38%. The material is stirred at a speed of 500 rpm by a cantilever stirrer for 30 min. After stirring, it is transferred to a sand mill for grinding at a speed of 1650 rpm until the D50 of the slurry is 0.400 um. After sand grinding, the material is centrifugally spray-dried to obtain the second grinding material, the spray-drying inlet temperature is 200℃, the outlet temperature is 105℃, and the centrifugal head rotation speed is 320 rpm.

[0073] S5. Under the atmosphere of high-purity nitrogen gas 5 L / min, the second grinding material is heated to 775℃ at a rate of 3℃ / min and sintered for 10 h to obtain the second lithium iron phosphate.

[0074] S6. The high-energy-density lithium iron phosphate positive electrode material obtained after airflow crushing of the second lithium iron phosphate.

[0075] Example 4 Example 4 differs from Example 1 in that the iron phosphate precursor of Example 4 has a lower phosphorus to iron ratio than Example 1.

[0076] S1. Disperse 2 kg of the precursor iron phosphate, 490 g of lithium carbonate, 110 g of anhydrous glucose, 220 g of polyethylene glycol (molecular weight of 6000), 13 g of hA-18E, and 12 g of titanium dioxide in water, with a slurry solid content of 38%. Stir the material at a cantilever stirrer speed of 500 rpm for 30 min. After stirring is complete, transfer the material to a sand mill and grind at a speed of 1650 rpm until the slurry D50 is 0.400 um.

[0077] S2. After sand grinding is complete, centrifugal spray dry the material to obtain a first grinding material, with a spray drying inlet temperature of 200 °C, an outlet temperature of 105 °C, and a centrifugal head centrifugal speed of 320 rpm.

[0078] S3. Place the first grinding material in a graphite crucible, and heat to 450 °C at a rate of 2.3 °C / min under a high-purity nitrogen atmosphere of 5 L / min, and sinter at 450 °C for 6 h to obtain a first lithium iron phosphate.

[0079] S4. Disperse 2 kg of the first lithium iron phosphate, 60 g of anhydrous glucose, 100 g of polyethylene glycol (molecular weight of 6000), and 13 g of hA-18E in water, with a slurry solid content of 38%. Stir the material at a cantilever stirrer speed of 500 rpm for 30 min. After stirring is complete, transfer the material to a sand mill and grind at a speed of 1650 rpm until the slurry D50 is 0.400 um. After sand grinding is complete, centrifugal spray dry the material to obtain a second grinding material, with a spray drying inlet temperature of 200 °C, an outlet temperature of 105 °C, and a centrifugal head centrifugal speed of 320 rpm.

[0080] S5. Heat the second grinding material to 775 °C at a rate of 3 °C / min under a high-purity nitrogen atmosphere of 5 L / min, and sinter at 775 °C for 10 h to obtain a second lithium iron phosphate.

[0081] S6. A high-energy-density lithium iron phosphate positive electrode material is obtained after jet milling the second lithium iron phosphate.

[0082] Example 5 Example 5 differs from Example 1 in that the iron phosphate precursor of Example 5 has a higher phosphorus to iron ratio than Example 1.

[0083] S1. 2kg precursor of iron phosphate, 490g lithium carbonate, 110g anhydrous glucose, 220g polyethylene glycol (molecular weight of 6000), 13g hA-18E, 12g titanium dioxide were dispersed in water, the solid content of the slurry was 38%. The material was stirred at a speed of 500 rpm for 30 min with a cantilever stirrer. After stirring, it was transferred to a sand mill and ground at a speed of 1650 rpm until the D50 of the slurry was 0.400um.

[0084] S2. After sand grinding, the material was centrifugally spray dried to obtain a first grinding material. The spray drying inlet temperature was 200℃, the outlet temperature was 105℃, and the centrifugal head rotation speed was 320 rpm.

[0085] S3. The first lithium iron phosphate was obtained by placing the first grinding material in a graphite crucible, heating to 450℃ at a speed of 2.3℃ / min under a high-purity nitrogen atmosphere of 5L / min, and sintering at 450℃ for 6h.

[0086] S4. 2kg of the first lithium iron phosphate, 60g of anhydrous glucose, 100g of polyethylene glycol (molecular weight of 6000), and 13g of hA-18E were dispersed in water, and the solid content of the slurry was 38%. The material was stirred at a speed of 500 rpm for 30 min with a cantilever stirrer. After stirring, it was transferred to a sand mill and ground at a speed of 1650 rpm until the D50 of the slurry was 0.400um. After sand grinding, the material was centrifugally spray dried to obtain a second grinding material. The spray drying inlet temperature was 200℃, the outlet temperature was 105℃, and the centrifugal head rotation speed was 320 rpm.

[0087] S5. The second lithium iron phosphate was obtained by heating the second grinding material to 775℃ at a speed of 3℃ / min under a high-purity nitrogen atmosphere of 5L / min and sintering for 10h.

[0088] S6. The high-energy-density lithium iron phosphate positive electrode material was obtained by airflow crushing of the second lithium iron phosphate.

[0089] Example 6 Different from example 1, the temperature of the secondary sintering of example 6 is lower than that of example 1.

[0090] S1. 2kg of the first precursor of iron phosphate, 490g of lithium carbonate, 110g of anhydrous glucose, 220g of polyethylene glycol (molecular weight of 6000), 13g of hA-18E, and 12g of titanium dioxide were dispersed in water, and the solid content of the slurry was 38%. The material was stirred at a speed of 500 rpm for 30 min with a cantilever stirrer. After stirring, it was transferred to a sand mill and ground at a speed of 1650 rpm until the D50 of the slurry was 0.400um.

[0091] S2. After sanding is completed, the material is centrifugally spray dried to obtain a first grinding material, the spray drying inlet temperature is 200°C, the outlet temperature is 105°C, and the centrifugal head centrifugal speed is 320 rpm.

[0092] S3. The first grinding material is placed in a graphite crucible, and is heated to 450°C at a speed of 2.3°C / min under a high-purity nitrogen atmosphere of 5 L / min, and is sintered at 450°C for 6 h to obtain a first lithium iron phosphate.

[0093] S4. 2 kg of the first lithium iron phosphate, 60 g of anhydrous glucose, 100 g of polyethylene glycol (molecular weight 6000), and 13 g of hA-18E are dispersed in water, and the slurry solid content is 38%. The material is stirred at a speed of 500 rpm for 30 min by using a cantilever stirrer. After stirring is completed, the material is transferred to a sand mill and is ground at a speed of 1650 rpm until the slurry D50 is 0.400 um. After sanding is completed, the material is centrifugally spray dried to obtain a second grinding material, the spray drying inlet temperature is 200°C, the outlet temperature is 105°C, and the centrifugal head centrifugal speed is 320 rpm.

[0094] S5. The second grinding material is heated to 760°C at a speed of 3°C / min under a high-purity nitrogen atmosphere of 5 L / min, and is sintered for 10 h to obtain a second lithium iron phosphate.

[0095] S6. A high-energy-density lithium iron phosphate positive electrode material is obtained after airflow crushing of the second lithium iron phosphate.

[0096] Example 7 Different from example 1, the temperature of the secondary sintering of example 7 is higher than that of example 1.

[0097] S1. 2 kg of the first precursor iron phosphate, 490 g of lithium carbonate, 110 g of anhydrous glucose, 220 g of polyethylene glycol (molecular weight 6000), 13 g of hA-18E, and 12 g of titanium dioxide are dispersed in water, and the slurry solid content is 38%. The material is stirred at a speed of 500 rpm for 30 min by using a cantilever stirrer. After stirring is completed, the material is transferred to a sand mill and is ground at a speed of 1650 rpm until the slurry D50 is 0.400 um.

[0098] S2. After sanding is completed, the material is centrifugally spray dried to obtain a first grinding material, the spray drying inlet temperature is 200°C, the outlet temperature is 105°C, and the centrifugal head centrifugal speed is 320 rpm.

[0099] S3. The first grinding material is placed in a graphite crucible, and is heated to 450°C at a speed of 2.3°C / min under a high-purity nitrogen atmosphere of 5 L / min, and is sintered at 450°C for 6 h to obtain a first lithium iron phosphate.

[0100] S4. Disperse 2 kg of the first lithium iron phosphate, 60 g of anhydrous glucose, 100 g of polyethylene glycol (molecular weight 6000), 13 g of hA-18E in water, and the solid content of the slurry is 38%. Stir the material at a speed of 500 rpm for 30 min using a cantilever stirrer. After stirring, transfer to a sand mill and grind at a speed of 1650 rpm until the D50 of the slurry is 0.400 um. After sand grinding, centrifugal spray drying is performed on the material to obtain a second grinding material, the inlet temperature of the spray drying is 200°C, the outlet temperature is 105°C, and the centrifugal speed of the centrifugal head is 320 rpm.

[0101] S5. Under the atmosphere of high-purity nitrogen gas at a flow rate of 5 L / min, the second grinding material is heated to 790°C at a rate of 3°C / min and sintered for 10 h to obtain the second lithium iron phosphate.

[0102] S6. The high-energy-density second lithium iron phosphate cathode material is obtained by airflow crushing of the second lithium iron phosphate.

[0103] Comparative Example 1 The phosphorus-iron ratio of the carbon source and the precursor used in Comparative Example 1 is consistent with that of Example 1, except that the carbon source is added once in Comparative Example 1 and sintered twice.

[0104] S1. Disperse 2 kg of the first precursor iron phosphate, 490 g of lithium carbonate, 170 g of anhydrous glucose, 320 g of polyethylene glycol (molecular weight 6000), 13 g of hA-18E, and 12 g of titanium dioxide in water, and the solid content of the slurry is 38%. Stir the material at a speed of 500 rpm for 30 min using a cantilever stirrer. After stirring, transfer to a sand mill and grind at a speed of 1650 rpm until the D50 of the slurry is 0.400 um.

[0105] S2. After sand grinding, centrifugal spray drying is performed on the material to obtain a first grinding material, the inlet temperature of the spray drying is 200°C, the outlet temperature is 105°C, and the centrifugal speed of the centrifugal head is 320 rpm.

[0106] S3. Place the first grinding material in a graphite crucible, heat to 450°C at a rate of 2.3°C / min under the atmosphere of high-purity nitrogen gas at a flow rate of 5 L / min, and sinter at 450°C for 6 h to obtain the first lithium iron phosphate.

[0107] S4. Disperse 2 kg of the first lithium iron phosphate in water, and the solid content of the slurry is 38%. Stir the material at a speed of 500 rpm for 30 min using a cantilever stirrer. After stirring, transfer to a sand mill and grind at a speed of 1650 rpm until the D50 of the slurry is 0.400 um. After sand grinding, centrifugal spray drying is performed on the material to obtain a second grinding material, the inlet temperature of the spray drying is 200°C, the outlet temperature is 105°C, and the centrifugal speed of the centrifugal head is 320 rpm.

[0108] S5. In a high-purity nitrogen atmosphere of 5 L / min, the second abrasive was heated to 775°C at a rate of 3°C / min and sintered at this temperature for 10 hours to obtain a second lithium iron phosphate.

[0109] S6. Grind the second lithium iron phosphate gas flow to obtain a high energy density lithium iron phosphate positive electrode material.

[0110] Comparative Example 2 The carbon source and the phosphorus-iron ratio of the precursor used in Comparative Example 2 are consistent with those in Example 1, except that Comparative Example 1 adopts one-time sintering.

[0111] S1. Disperse 2 kg of the first precursor (ferric phosphate), 490 g of lithium carbonate, 170 g of anhydrous glucose, 320 g of polyethylene glycol (molecular weight 6000), 13ghA-18E, and 12 g of titanium dioxide in water to a slurry with a solids content of 38%. Stir the mixture at 500 rpm using an overhead stirrer for 30 minutes. After stirring, transfer the mixture to a sand mill at 1650 rpm and grind until the slurry has a D50 of 0.400 μm.

[0112] S2. After sand milling, the material is subjected to centrifugal spray drying to obtain a first grinding material. The spray drying air inlet temperature is 200°C, the air outlet temperature is 105°C, and the centrifugal head centrifugal speed is 320 rpm.

[0113] S3. Place the first abrasive in a graphite crucible, raise the temperature to 450°C at a rate of 2.3°C / min in a high-purity nitrogen atmosphere at a flow rate of 5 L / min, and sinter at 450°C for 6 h to obtain a first lithium iron phosphate.

[0114] S4. Disperse 2 kg of the first lithium iron phosphate, 60 g of anhydrous glucose, 100 g of polyethylene glycol (molecular weight 6000), and 13ghA-18E in water to a slurry with a solids content of 38%. Stir the mixture at 500 rpm using an overhead mixer for 30 minutes. After stirring, transfer the mixture to a sand mill at 1650 rpm and grind it to a slurry with a D50 of 0.400 μm. After sand milling, centrifuge spray dry the mixture to obtain a second grind. The spray drying air inlet temperature was 200°C, the air outlet temperature was 105°C, and the centrifuge speed was 320 rpm.

[0115] S5. In a high-purity nitrogen atmosphere of 5 L / min, the second abrasive was heated to 775°C at a rate of 3°C / min and sintered at this temperature for 10 hours to obtain a second lithium iron phosphate.

[0116] S6. Grind the second lithium iron phosphate gas flow to obtain a high energy density lithium iron phosphate positive electrode material.

[0117] The obtained lithium iron phosphate positive electrode material is subjected to physical and chemical property testing, and a button cell is assembled for electrochemical performance testing. The physical and chemical testing and electrochemical performance comparison of the sample of the example and the comparative example are shown in Table 2.

[0118] Table 1: EDS Mapping element proportion of a lithium iron phosphate

[0119] Table 2: Physical and chemical testing and electrochemical performance comparison of the sample of the example and the comparative example

[0120] From the above experimental results, it can be seen that Examples 1, 2 and 3 show that the proportion of the carbon source in the primary ingredients has an effect on the physical and chemical properties of the final sample. When the proportion of the carbon source in the primary ingredients is too high, the carbon content in the obtained sample increases, the iron phosphide content increases, and the compaction is low. When the proportion of the carbon source in the primary ingredients is too low, the powder resistivity of the obtained sample increases, and the discharge capacity decreases. Examples 1, 4 and 5 show that the iron-to-phosphorus ratio of the iron phosphate raw material in the primary ingredients affects the performance of the final sample. When the iron phosphate used in the primary ingredients has a low iron-to-phosphorus ratio, the iron phosphide impurity in the final sample is high, and the sample discharge capacity decreases. When the iron phosphate used in the primary ingredients has a high iron-to-phosphorus ratio, the iron phosphide content in the final sample decreases, but the sample compaction density also decreases. Examples 1, 6 and 7 show that the high or low secondary sintering temperature has an effect. Increasing the sintering temperature increases the content of iron phosphide impurities in the obtained sample, and the sample discharge capacity decreases significantly. Table 1 is the scanning result corresponding to Example 1 and Example 7. The results show that the proportion of P element in the sample particles sintered at high temperature increases significantly, Figure 2 The scanning electron microscope and the corresponding EDS spectrum of the sample particles obtained in Example 7 are shown in the figure. As shown in the left scanning electron microscope image, more large particles are easily obtained by high-temperature sintering. The middle and right EDS images of the particles show that the phosphorus and iron elements are uniformly distributed, and the proportion of phosphorus element is higher. The EDS results show that high-temperature sintering leads to the generation of more iron phosphide impurities. Low-temperature sintering can effectively control the content of iron phosphide in the final product.

[0121] Example 1 and Comparative Example 1 show that the carbon source is once put into stirring and dispersion. Since the carbon source contains a reducing effect, the high sintering reducing atmosphere caused by putting the iron phosphide impurity into the carbon source makes it easy to generate iron phosphide impurity phases. Figure 3 The X-ray powder diffraction pattern of the sample of Comparative Example 1 shows obvious impurity peaks belonging to iron phosphide. Example 1 and Comparative Example 2 show that the two-sintering process can optimize the compaction density and capacity of lithium iron phosphate, and can effectively improve the energy density of lithium iron phosphate material.

[0122] While the application has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various modifications or changes can be made therein without departing from the spirit and scope thereof. Accordingly, it is intended that all such modifications and changes be included within the scope of the application as claimed.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: Contains lithium iron phosphate and iron phosphide impurities; wherein the proportion of the iron phosphide impurities to the total mass of the lithium iron phosphate positive electrode material is 0.001-0.008PPM; The lithium iron phosphate cathode material has high energy density and a compaction density of 2.68-2.8 g / cm at 3T pressure. 3 The powder resistivity is 8-15Ω·cm, the 0.1C discharge capacity is 161-163mAh / g, the 0.1C charge and discharge efficiency is 98.5-100%, and the 1C discharge capacity is 145-148mAh / g.

2. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: The method comprises the steps of primary batching, primary sand milling, primary spray drying, primary sintering, secondary batching, secondary sand milling, secondary spray drying, secondary sintering and air flow pulverization.

3. The method according to claim 2, characterized in that The following steps are involved: S1. The lithium iron phosphate precursor iron source, phosphorus source, lithium source, carbon source, dispersant and metal dopant are dispersed in a solvent to a slurry solid content of 38-45%. The slurry is stirred at a speed of 200-800 rpm for 15-45 min. After stirring, the mixture is transferred to a sand mill at a speed of 1500-1650 rpm and ground to a slurry D50 of 0.300-0.400 μm. S2. After sanding, the slurry is dried to obtain a first abrasive; S3. The first abrasive is placed in a graphite crucible and sintered at 300-500 ℃ under an inert gas atmosphere to obtain a first lithium iron phosphate; S4. The first lithium iron phosphate, carbon source, and dispersant are dispersed in a solvent to a slurry solid content of 38-45%. The slurry is stirred at a speed of 200-800 rpm for 15-45 min. After stirring, the mixture is transferred to a sand mill at a speed of 1000-1850 rpm and ground to a slurry D50 of 0.300-0.400 um. After sand milling, the slurry is dried to obtain a second millbase. S5. In an inert gas atmosphere, the mixture is sintered at a high temperature of 750-800 ° C to obtain a second lithium iron phosphate; S6. Grind the second lithium iron phosphate into a high energy density lithium iron phosphate positive electrode material.

4. The method according to claim 3, characterized in that The mass ratio of the iron source, phosphorus source, lithium source, carbon source, dispersant and metal dopant in step S1 is (1.0-1.2):(1.0-1.2):(0.20-0.28):(0.05-0.15):(0.005-0.01):(0.002-0.01).

5. The method according to claim 3, characterized in that The iron source in step S1 is selected from one or more of ferrous phosphate, ferrous chloride, ferrous oxalate, ferrous sulfate, ferrous nitrate, ferrous hydroxide, ferric hydroxide, ferric oxide, and iron powder; The lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium phosphate, lithium chloride, and lithium oxalate; The phosphorus source is selected from one or more of phosphoric acid, ferric phosphate, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, lithium monohydrogen phosphate, and lithium dihydrogen phosphate; The carbon source is selected from one or more of glucose, sucrose, PEG, polypropylene glycol, phenolic resin, ascorbic acid, cellulose, starch, cyanamide, dicyandiamide, and melamine; The metal dopant is selected from one or more of titanium dioxide, aluminum oxide, magnesium carbonate, manganese carbonate, calcium carbonate, zirconium oxide, and vanadium pentoxide; The dispersant is selected from one or more of sodium polyacrylate, sodium lauryl sulfate, polyvinyl pyrrolidone, triisopropanol ammonium, sodium hexametaphosphate, and vitamins; The solvent is deionized water and / or anhydrous ethanol.

6. The method according to claim 3, characterized in that The grinding in step S1 is sand grinding, the sand grinder contains zirconium beads, the particle size of the zirconium beads is 0.1-0.4 μm, the sand grinding speed of the sand grinder is 1200-1650 rpm, and the sand grinding time is 60-90 min.

7. The method according to claim 3, characterized in that The drying method in step S2 is centrifugal spray drying, the air inlet temperature of the centrifugal spray dryer is 160-220°C, the air outlet temperature is 100-120°C, the induced air frequency is 40-60Hz, and the centrifugal speed of the centrifugal head is 280-320 rpm.

8. The method according to claim 3, characterized in that The inert gas in steps S3 and S5 is high-purity nitrogen and / or nitrogen containing 3%-5% v / v hydrogen; The sintering conditions in step S3 are: heating to 300-500°C at a rate of 1.5-4°C / min, sintering at this temperature for 1-4 hours, and a ventilation rate of 2-10 L / min; The sintering conditions of step S5 are: heating to 750-800°C at a rate of 2.5-4.5°C / min, sintering at this temperature for 6-12 hours, and a ventilation rate of 2-10 L / min.

9. The method according to claim 3, characterized in that The types of carbon source, dispersant, and solvent used in step S4 are the same as those in step S1; wherein the mass ratio of the first lithium iron phosphate, carbon source, and dispersant is (1.0-1.2):(0.01-0.05):(0.001-0.005); The equipment and process used for grinding and drying are the same as those in step S1.

10. The method according to any one of claims 3 to 7, characterized in that: The conditions for the airflow milling in step S6 are: a feed frequency of 10-50 Hz, a grinding air pressure of 0.3-0.6 MPa, and a classifying wheel frequency of 50-150 Hz.

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