Double-doped and double-coated lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The invention relates to the technical field of lithium ion battery positive electrode materials, and discloses a double-doped double-coated lithium iron phosphate positive electrode material and a preparation method thereof. The positive electrode material sequentially comprises a Mg < 2 + > and Ti < 4 + > co-doped lithium iron phosphate core, a lithium phosphate ion conductor coating layer and a graphene conducting layer from inside to outside. The preparation method comprises the following steps: mixing raw materials containing iron, phosphorus, lithium, magnesium and titanium sources, and carrying out ball milling, spray drying and two-stage sintering to obtain a doped inner core; and then sequentially carrying out high-pressure rolling pretreatment, lithium phosphate coating heat treatment and graphene chemical vapor deposition on the inner core to finally prepare the positive electrode material. Through the synergistic effect of core co-doping, high-pressure rolling morphology control and a surface double-coating structure, the prepared lithium iron phosphate positive electrode material has high compaction density, conductivity, gram volume under high magnification and cycling stability at the same time.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, specifically to a double-doped and double-coated lithium iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium-ion batteries, as rechargeable energy storage devices, have been widely used in various electronic devices and new energy vehicles. Among them, lithium iron phosphate cathode materials have become one of the key research and application directions in this field due to their stable cycle performance, high safety characteristics, and cost-effectiveness.

[0003] However, there is a significant gap between the actual compaction density and the theoretical density of current lithium iron phosphate materials. This limits the energy that batteries made with this material can hold per unit volume, thus affecting the further expansion of its applications. One technical approach to improve volumetric energy density is to increase the compaction density of the cathode material. However, applying high pressure to existing lithium iron phosphate materials to achieve a compaction density exceeding a certain threshold often leads to particle breakage. Simultaneously, the increased contact resistance between particles significantly raises the electrode resistance, directly impairing the battery's rate performance.

[0004] On the other hand, to improve the electrochemical reaction kinetics of materials, existing technologies often employ methods to reduce the particle size to the nanoscale. While this method can shorten the diffusion path of lithium ions, it also increases the porosity between particles, which contradicts the goal of achieving high compaction density and leads to a decrease in the compaction performance of the material.

[0005] Therefore, how to effectively improve the compaction density of lithium iron phosphate cathode materials without compromising the integrity of the material particle structure and electrochemical performance is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] The technical problem to be solved by this invention is that existing lithium iron phosphate cathode materials have limitations in simultaneously achieving high ionic conductivity, electronic conductivity, and high solid density, which limits the rate performance of the material and the final volumetric energy density of the battery.

[0007] To address the aforementioned technical problems, this invention provides a lithium iron phosphate cathode material with specific doping and coating structure design, and a method for preparing the same.

[0008] The first aspect of this invention provides a double-doped and double-coated lithium iron phosphate cathode material.

[0009] The cathode material comprises, from the inside out, a lithium iron phosphate core, a lithium phosphate ion conductor coating layer, and a graphene conductive layer. The mass percentages of each component in the cathode material are as follows: Lithium iron phosphate core: 92.5%~98.0%; Lithium phosphate conductor coating: 1.5%–4.5%; Graphene conductive layer: 0.5%~3.0%.

[0010] The lithium iron phosphate core is a lithium iron phosphate co-doped with magnesium and titanium ions. This core is prepared from raw materials containing iron, phosphorus, lithium, magnesium, and titanium sources.

[0011] In the raw materials, the molar ratio of iron source, phosphorus source and lithium source is (0.95-0.97):(0.97-1.01):(1.02-1.06).

[0012] The amount of magnesium source added for doping is 0.5-1.5 at%, and the amount of titanium source added is 0.5-1.0 at%. Magnesium ions enter the lithium iron phosphate lattice and replace the positions of ferrous ions, while titanium ions enter the lattice. This non-equivalent ion doping can stabilize the crystal structure and optimize the lithium ion transport channels.

[0013] The lithium phosphate conductor coating layer is uniformly coated around the lithium iron phosphate core. This coating layer is a lithium ion conductor, and its function is to construct a fast lithium ion transport channel on the particle surface, reducing the transport impedance of lithium ions at the solid-solid interface.

[0014] The graphene conductive layer is a carbon layer coated outside the lithium phosphate conductor coating layer by chemical vapor deposition. This graphene conductive layer forms a three-dimensional conductive network between particles, reducing the contact resistance between cathode material particles and improving the overall electronic conductivity of the material.

[0015] In one specific embodiment, the double-doped and double-coated lithium iron phosphate cathode material is in powder form with a particle size D50 of 3.5-4.5 μm.

[0016] A second aspect of the present invention provides a method for preparing the double-doped and double-coated lithium iron phosphate cathode material.

[0017] The preparation method includes the following steps: S1. Add the raw materials of iron source, phosphorus source, lithium source, magnesium source and titanium source to anhydrous ethanol and stir at room temperature to obtain a uniform mixed solution.

[0018] S2. The mixed solution is subjected to ball milling.

[0019] S3. The solution after ball milling in step S2 is dried and granulated using a spray dryer to obtain spherical precursor powder.

[0020] S4. The precursor powder is subjected to low-temperature two-stage sintering to obtain a sintered material containing a lithium iron phosphate core co-doped with magnesium ions and titanium ions.

[0021] S5. The sintered material cooled in step S4 is subjected to high-pressure roller pressing pretreatment, followed by crushing and sieving. This step breaks up particle agglomeration through high pressure, thereby increasing the compaction density of the material.

[0022] S6. Add the sieved material from step S5 to the pre-prepared coating solution, stir until the solvent is completely evaporated, and then perform heat treatment to form a lithium phosphate conductor coating layer on the material surface.

[0023] S7. The lithium iron phosphate material obtained in step S6 is placed in a chemical vapor deposition furnace and a chemical vapor deposition reaction is carried out using methane as a carbon source to form a graphene conductive layer outside the lithium phosphate ion conductor coating layer, and finally the double-doped and double-coated lithium iron phosphate cathode material is obtained.

[0024] In one specific embodiment, the raw materials in step S1 may be: the iron source is one of ferric phosphate, ferrous sulfate and ferric oxalate; the lithium source is one of lithium hydroxide and lithium carbonate; the phosphorus source is one of ammonium dihydrogen phosphate and lithium dihydrogen phosphate; the magnesium source is magnesium acetate tetrahydrate; and the titanium source is tetrabutyl titanate.

[0025] In one specific embodiment, the ball milling process parameters in step S2 are as follows: using zirconia balls with a diameter of 5 mm as the grinding medium, setting the ball-to-material mass ratio to 8-10:1, controlling the ball mill speed at 1000-1200 rpm, and maintaining the ball milling time for 30-45 minutes.

[0026] In one specific embodiment, the spray drying process parameters in step S3 are as follows: the feed temperature of the spray dryer is set to 180-190°C, and the outlet air temperature is set to 90-110°C.

[0027] In one specific implementation, the specific conditions for the low-temperature two-stage sintering in step S4 are as follows: First stage: Under a nitrogen atmosphere, heat the material to 300-350℃ at a heating rate of 3-5℃ / min, and hold at this temperature for 2-4 hours.

[0028] The second stage involves switching the atmosphere to an argon-hydrogen mixture with a volume ratio of 95:5 for argon and hydrogen. The temperature is then increased to 600-700℃ at a rate of 3-5℃ / min and held at this temperature for 4-6 hours.

[0029] In one specific embodiment, the specific parameters for the high-pressure roller pressing pretreatment and crushing and sieving in step S5 are as follows: The sintered material is subjected to a linear pressure of 500-1000 MPa for roller pressing, with the roller rotation speed being 0.5-1 m / min, and is pressed back and forth 3 times; then the compacted material is crushed and sieved so that the particle size D50 of the final material is controlled at 3.5-4.5 μm.

[0030] In one specific embodiment, in step S6, the coating solution is made of deionized water, ammonium dihydrogen phosphate, and lithium hydroxide, wherein the mass ratio of ammonium dihydrogen phosphate, lithium hydroxide, and the sieved material from step S5 is (2-3):(0.5-1.5):(95-100). The conditions for the subsequent heat treatment are: heating to 300-350°C at a heating rate of 3-5°C / min under a nitrogen atmosphere, and holding at that temperature for 2-4 hours.

[0031] In one specific embodiment, the conditions for chemical vapor deposition of graphene in step S7 are as follows: the reaction chamber of the chemical vapor deposition furnace is evacuated to 10 °C. -2 Pa, followed by the introduction of a hydrogen-argon mixture with a hydrogen-to-argon volume ratio of 1:4 and a total gas flow rate of 100-120 sccm, raising the furnace temperature to 600-800℃. Then, methane is introduced at a flow rate of 20-30 sccm as a carbon source for the deposition reaction, with a reaction time of 20-30 min.

[0032] This invention provides a double-doped, double-coated lithium iron phosphate cathode material and its preparation method. It has the following beneficial effects: 1. This invention involves Mg in the lithium iron phosphate core. 2+ and Ti 4+ Co-doping stabilizes the olivine crystal structure of the material by replacing it with non-equivalent ions, while widening the migration channels of lithium ions within the crystal lattice and reducing the migration resistance of lithium ions in the bulk phase of the material, thereby improving the intrinsic ionic conductivity of the active material itself.

[0033] 2. This invention constructs a dual-coating structure combining a Li3PO4 ion-conducting coating layer and a graphene conductive layer, simultaneously building a rapid lithium-ion transport channel and an electronic conductive network on the surface of lithium iron phosphate particles. This synergistic effect effectively reduces the charge transfer resistance at the electrode-electrolyte interface and the contact resistance between particles, thereby minimizing the polarization of the electrochemical reaction.

[0034] 3. This invention employs a high-pressure rolling pretreatment process following low-temperature two-stage sintering, and controls the particle size D50 of the final material. This method effectively breaks down hard agglomerates formed during sintering and improves the compactness of secondary particles. The powder material prepared in this way has high compaction density, and when prepared into electrode sheets, it can pack more active material within a unit volume, thereby increasing the volumetric energy density of the battery. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of a double-doped and double-coated lithium iron phosphate cathode material proposed in this invention; Figure 2 This is a flowchart of a method for preparing a double-doped and double-coated lithium iron phosphate cathode material proposed in this invention; Figure 3 SEM image of the lithium iron phosphate cathode sheet in Example 4 of the preparation method of the double-doped and double-coated lithium iron phosphate cathode material proposed in this invention; Figure 4 The image shows a SEM image of a lithium iron phosphate cathode sheet, which is a comparative example 1 of the method for preparing a double-doped and double-coated lithium iron phosphate cathode material proposed in this invention. Detailed Implementation

[0036] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] Raw materials and reagents: The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0038] Ferrous oxalate dihydrate, CAS No.: 6047-25-2; Ammonium dihydrogen phosphate, CAS No.: 7722-76-1; Lithium carbonate, CAS No.: 554-13-2; Magnesium acetate tetrahydrate, CAS No.: 16674-78-5; Tetrabutyl titanate, CAS No.: 5593-70-4; Lithium hydroxide monohydrate, CAS No.: 1310-66-3; Ferric phosphate, CAS No.: 10045-86-0; Polyvinylidene fluoride, CAS No.: 24937-79-9; Glucose, CAS No.: 50-99-7; Lithium hexafluorophosphate, CAS No.: 21324-40-3; Ethylene carbonate, CAS No.: 96-49-1; Dimethyl carbonate, CAS No.: 616-38-6; Anhydrous ethanol, CAS No.: 64-17-5; N-Methylpyrrolidone, CAS No.: 872-50-4.

[0039] Examples 1-4: Example 1:

[0040] Example 1 This example provides a method for preparing lithium iron phosphate cathode material, including the following steps: Raw material mixing: The raw materials are mixed according to the molar ratio of iron, phosphorus and lithium of 0.96:1.00:1.04, and the amount of titanium source added is 0.8 at%.

[0041] Specifically, 172.7g of ferrous oxalate dihydrate was weighed as the iron source, 115.0g of ammonium dihydrogen phosphate as the phosphorus source, 38.4g of lithium carbonate as the lithium source, and 2.6g of tetrabutyl titanate as the titanium source.

[0042] Add all the above raw materials to 500 mL of anhydrous ethanol and stir until homogeneous at room temperature. (Note: No magnesium source was added in this example to verify the effect of Mg doping).

[0043] Ball milling: Place the mixed solution obtained in step 1 into a ball mill, use zirconia balls with a diameter of 5 mm, and a ball-to-material mass ratio of 8:1, and ball mill at 1200 rpm for 30 minutes.

[0044] Drying and granulation: The solution after ball milling is spray dried with the feed temperature set at 185℃ and the outlet air temperature at 105℃ to obtain precursor powder.

[0045] Sintering: The precursor powder obtained in step 3 was subjected to two-stage sintering. In the first stage, the temperature was increased to 350°C at a heating rate of 5°C / min under a nitrogen atmosphere and held for 4 hours. In the second stage, the atmosphere was switched to a mixture of argon and hydrogen in a volume ratio of 95:5, and the temperature was increased to 650°C at a heating rate of 5°C / min and held for 5 hours.

[0046] Chemical vapor deposition: The sintered material from step 4 is placed in a CVD furnace, and the furnace pressure is reduced to 10. -2 A mixture of hydrogen and argon gas at a volume ratio of 1:4 was introduced at a total flow rate of 100 sccm, and the temperature was raised to 600℃. Subsequently, methane was introduced at a flow rate of 20 sccm for 30 min to initiate the reaction. After the reaction, the methane supply was turned off, and the mixture was cooled to room temperature in an argon atmosphere. The resulting lithium iron phosphate material was designated LFP-1.

[0047] Example 2:

[0048] This embodiment provides a method for preparing lithium iron phosphate cathode material, including the following steps: The raw material mixing, ball milling, drying granulation and sintering steps in this embodiment are exactly the same as steps 1 to 4 of the adjusted embodiment 1.

[0049] High-pressure rolling: The material obtained after sintering in step 1 is subjected to high-pressure rolling treatment. The pressure is set at 900 MPa and the roller speed is 0.5 m / min, and the material is reciprocated and rolled 3 times. The compacted material is then crushed and sieved to control the particle size D50 between 3.5 μm and 4.5 μm.

[0050] Chemical vapor deposition: The material treated in step 2 was subjected to chemical vapor deposition, and the process conditions were exactly the same as those in step 5 of Example 1. The resulting lithium iron phosphate material was designated as LFP-2.

[0051] Example 3:

[0052] This embodiment provides a method for preparing lithium iron phosphate cathode material, including the following steps: The raw material mixing, ball milling, drying and granulation, sintering and high-pressure rolling steps in this embodiment are exactly the same as steps 1 to 2 of the adjusted embodiment 2.

[0053] Ion conductor coating: Prepare the coating solution by adding 4.6 g of ammonium dihydrogen phosphate and 1.68 g of lithium hydroxide monohydrate to 200 g of deionized water and stirring until dissolved. Then add 200 g of the powder material treated in step 1 and stir at 90 °C until the solvent is completely evaporated. Place the dried powder in a tube furnace and heat it to 300 °C at a heating rate of 5 °C / min under a nitrogen atmosphere, and hold at this temperature for 2 h.

[0054] Chemical vapor deposition: The material treated in step 2 was subjected to chemical vapor deposition, and the process conditions were exactly the same as those in step 5 of Example 1. The resulting lithium iron phosphate material was designated LFP-3.

[0055] Example 4:

[0056] This embodiment provides a method for preparing a double-doped, double-coated lithium iron phosphate cathode material. The material prepared by this method falls entirely within the protection scope of the claims of this invention, and specifically includes the following steps: Raw material mixing: The raw materials are mixed according to the molar ratio of iron, phosphorus and lithium of 0.96:1.00:1.04, the amount of magnesium source added is 1.0 at%, and the amount of titanium source added is 0.8 at%.

[0057] Specifically, 172.7g of ferrous oxalate dihydrate was weighed as the iron source, 115.0g of ammonium dihydrogen phosphate as the phosphorus source, 38.4g of lithium carbonate as the lithium source, 2.1g of magnesium acetate tetrahydrate as the magnesium source, and 2.6g of tetrabutyl titanate as the titanium source. All the above raw materials were added to 500mL of anhydrous ethanol and stirred and mixed evenly at room temperature.

[0058] Subsequent steps: ball milling, drying and granulation, sintering, high-pressure rolling, ion conductor coating, and chemical vapor deposition are exactly the same as steps 1 to 3 in Example 3. The final lithium iron phosphate material is designated as LFP-4.

[0059] Testing revealed that the final LFP-4 material is a double-doped, double-coated lithium iron phosphate cathode material, consisting of a lithium iron phosphate core, a lithium phosphate ion conductor coating layer, and a graphene conductive layer. The mass percentages of each component are as follows: lithium iron phosphate core: 95.5%, lithium phosphate ion conductor coating layer: 2.5%, graphene conductive layer: 2.0%. All components and their contents fall within the protection scope of claim 1.

[0060] Comparative example: The difference between Comparative Example 1 and Example 4 is that: Raw material formula: Weigh 153.3g ferric phosphate, 78.4g lithium carbonate, 116.2g ammonium dihydrogen phosphate and 10.7g glucose, dissolve in deionized water, stir and mix, and control the solid content to 40%. No magnesium source or titanium source is added to this formula.

[0061] Sintering process: Single-stage high-temperature sintering is adopted, with the temperature raised to 800℃ at a heating rate of 3-5℃ / min under nitrogen atmosphere and held for 6 hours.

[0062] Post-processing: The sintered material is not subjected to high-pressure roller pressing, but is directly pulverized using an air jet mill at 35 Hz, with the particle size D50 controlled between 3.5 μm and 4.5 μm.

[0063] Coating process: No lithium iron phosphate conductor coating or graphene chemical vapor deposition coating is performed. The final lithium iron phosphate material is designated as LFP-5.

[0064] Test example: Test Example 1: Physical and Electrochemical Performance Testing Physical performance testing: Compacted density test: Weigh 2g of sample powder, place it in a tableting mold, press it into shape under a pressure of 10MPa, measure its thickness and diameter, and calculate the compacted density of the sample.

[0065] Conductivity test: The sample powder was pressed into a disc with a diameter of 10 mm and a thickness of 1 mm under a pressure of 20 MPa, and its conductivity at room temperature was measured using a four-probe tester.

[0066] Electrochemical performance testing: Electrode preparation: The active materials (LFP-1 to LFP-5) prepared in Examples 1-4 and Comparative Example 1, conductive carbon black, carbon nanotubes, and polyvinylidene fluoride binder were dispersed in N-methylpyrrolidone solvent at a mass ratio of 95:2:1:2 and mechanically stirred to form a uniform slurry. The slurry was uniformly coated onto a 12 μm thick aluminum foil current collector, and then the coated electrode was dried in a vacuum oven at 100°C for 12 h. The areal density of the coating after drying was controlled to be approximately 5.0 mg / cm³. 2 The coated electrode was then dried in a vacuum oven at 100°C for 12 hours.

[0067] Battery Assembly: The dried positive electrode, graphite negative electrode, 12μm thick PP ceramic separator, and electrolyte (1 mol / L lithium hexafluorophosphate dissolved in a 1:1 volume ratio of EC / DMC) are assembled into a steel-cased battery in a dry environment. The dried positive electrode, graphite negative electrode, 12μm thick PP ceramic separator, and electrolyte (1 mol / L lithium hexafluorophosphate dissolved in a 1:1 volume ratio of ethylene carbonate and dimethyl carbonate) are then assembled into a CR2032 coin cell in a dry environment. Electrochemical Testing: The assembled battery was tested at room temperature using a battery testing system. First, it underwent three charge-discharge cycles at a 0.1C rate to complete formation. Subsequently, its discharge specific capacity was tested at discharge rates of 0.5C, 1C, and 5C. Cyclic performance testing involved 2000 charge-discharge cycles at a 1C rate, and its capacity retention was calculated. Electrode resistance was measured using AC impedance spectroscopy. The test results are recorded in Table 1.

[0068] Table 1. Comparison of physical and electrochemical properties of each sample

[0069] As can be seen from the test data in Table 1, the lithium iron phosphate material prepared in Example 4 of this invention, after its core is co-doped with magnesium and titanium ions and successively coated with a lithium phosphate conductor layer and a graphene conductive layer, has significant differences in various performance indicators compared with Comparative Example 1 and Examples 1-3 which did not adopt the complete technical solution.

[0070] Compared to Comparative Example 1 and the samples of Examples 1-3, Example 4 exhibits higher values ​​in compaction density, conductivity, specific capacity, rate performance, and cycle stability. This indicates that introducing specific doping elements into the lithium iron phosphate core can effectively improve the intrinsic conductivity of the material. Simultaneously, high-pressure rolling treatment of the sintered material can increase its compaction density, which directly contributes to improving the volumetric energy density of the battery.

[0071] Based on core doping and particle morphology optimization, a double-coating structure consisting of a lithium phosphate conductor layer and a graphene conductive layer was constructed, further improving the electrochemical performance of the material. Comparing the data from Examples 2 and 3, the addition of the lithium phosphate conductor layer improved both the specific capacity and cycle stability of the material. Comparing the data from Examples 3 and 4, the addition of magnesium ion doping resulted in the highest values ​​for all performance indicators, especially the specific capacity and cycle stability at high rates. This confirms the synergistic effect between core doping, particle morphology control, and the surface double-coating structure, which collectively reduced the polarization of the material and improved the lithium-ion transport efficiency.

[0072] Specifically, compared to the incomplete technical solutions of Examples 1-3, Example 4's performance is not a simple linear superposition, but rather exhibits outstanding advantages in high-rate discharge and long-cycle stability. For example, its 5C specific capacity reaches 142.4 mAh / g, and its capacity retention rate after 2000 cycles is as high as 92.4%, which is an improvement of 20.2% and 16.2% respectively compared to Comparative Example 1 (118.5 mAh / g, 79.5%). This significant leap in performance fully demonstrates that the combination of dual doping + high-voltage rolling + dual coating proposed in this invention is not a simple combination of conventional technical means that would be easily conceived by those skilled in the art, but rather a synergistic effect of 1+1>2 achieved through the organic combination of various technical features, successfully solving the technical problem of conductivity degradation and cycle decay under high voltage density.

Claims

1. A double-doped, double-coated lithium iron phosphate cathode material, characterized in that, include: The material consists of a lithium iron phosphate core, a lithium phosphate conductor coating layer, and a graphene conductive layer, with the following mass percentages for each component: The lithium iron phosphate core comprises 92.5%–98.0% magnesium and titanium ions co-doped lithium iron phosphate, prepared from iron, phosphorus, lithium, magnesium, and titanium sources. The molar ratio of the iron, phosphorus, and lithium sources is (0.95–0.97):(0.97–1.01):(1.02–1.06). The amount of magnesium source added is 0.5–1.5 at%, and the amount of titanium source added is 0.5–1.0 at%. Lithium phosphate conductor coating layer: 1.5% to 4.5%, covering the lithium iron phosphate core; Graphene conductive layer: 0.5% to 3.0%, which is a carbon layer coated on the outside of the lithium phosphate conductor coating layer by chemical vapor deposition.

2. A double-doped, double-coated lithium iron phosphate cathode material, characterized in that, The positive electrode material is in powder form, and the particle size D50 is 3.5-4.5 μm.

3. A method for preparing a double-doped and double-coated lithium iron phosphate cathode material, characterized in that, The method for preparing a double-doped and double-coated lithium iron phosphate cathode material according to any one of claims 1-2 includes the following steps: S1. Add iron source, phosphorus source, lithium source, magnesium source and titanium source to anhydrous ethanol and stir evenly at room temperature to obtain a mixed solution; S2. The mixed solution is added to a ball mill for ball milling; S3. The ball-milled solution obtained in step S2 is dried and granulated using a spray dryer to obtain precursor powder; S4. The precursor powder is subjected to low-temperature two-stage sintering to obtain sintered material; S5. The sintered material that has been cooled in step S4 is subjected to high-pressure roller pressing pretreatment, and then crushed and sieved. S6. Add the sieved material from step S5 to the coating solution, stir until completely dry, and then perform heat treatment again to form a lithium phosphate conductor coating layer. S7. Place the heat-treated material from step S6 into a CVD furnace and perform chemical vapor deposition of graphene using methane as a carbon source to obtain the double-doped and double-coated lithium iron phosphate cathode material.

4. The method for preparing a double-doped, double-coated lithium iron phosphate cathode material according to claim 3, characterized in that, In step S1, the iron source is one of ferric phosphate, ferrous sulfate, and ferric oxalate; the lithium source is one of lithium hydroxide and lithium carbonate; the phosphorus source is one of ammonium dihydrogen phosphate and lithium dihydrogen phosphate; the magnesium source is magnesium acetate tetrahydrate; and the titanium source is tetrabutyl titanate.

5. The method for preparing a double-doped, double-coated lithium iron phosphate cathode material according to claim 3, characterized in that, The process parameters for ball milling in step S2 are as follows: Use zirconia balls with a ball-to-material mass ratio of 8-10:1, rotate at 1000-1200 rpm, and continue for 30-45 minutes.

6. The method for preparing a double-doped, double-coated lithium iron phosphate cathode material according to claim 3, characterized in that, The process parameters for spray drying in step S3 are as follows: The feed temperature is 180-190℃, and the outlet air temperature is 90-110℃.

7. The method for preparing a double-doped, double-coated lithium iron phosphate cathode material according to claim 3, characterized in that, The low-temperature two-stage sintering in step S4 specifically refers to: First stage: Under nitrogen atmosphere, heat to 300-350℃ at a heating rate of 3-5℃ / min and hold for 2-4 hours; Second stage: Switch the atmosphere to an argon-hydrogen mixture with a volume ratio of argon to hydrogen of 95:5, and then heat to 600-700℃ at a heating rate of 3-5℃ / min, and hold at that temperature for 4-6 hours.

8. The method for preparing a double-doped, double-coated lithium iron phosphate cathode material according to claim 3, characterized in that, The high-pressure roller pressing pretreatment and crushing and sieving in step S5 are specifically as follows: The sintered material is pressed by a pressure roller of 500-1000MPa at a roller speed of 0.5-1m / min, and pressed repeatedly 3 times. After crushing and sieving, the particle size D50 of the material is controlled at 3.5-4.5μm.

9. The method for preparing a double-doped, double-coated lithium iron phosphate cathode material according to claim 3, characterized in that, The coating solution in step S6 is made of deionized water, ammonium dihydrogen phosphate and lithium hydroxide, wherein the mass ratio of ammonium dihydrogen phosphate, lithium hydroxide and the sieved material in step S5 is (2-3):(0.5-1.5):(95-100). The conditions for the second heat treatment are: heating to 300-350℃ at a heating rate of 3-5℃ / min under N2 atmosphere and holding at that temperature for 2-4 hours.

10. The method for preparing a double-doped, double-coated lithium iron phosphate cathode material according to claim 3, characterized in that, The conditions for chemical vapor deposition of graphene in step S7 are as follows: Evacuate the CVD furnace to 10°C. -2 Pa, a hydrogen-argon mixture is introduced, with a hydrogen to argon volume ratio of 1:4 and a total gas flow rate of 100-120 sccm. The furnace temperature is raised to 600-800℃, and then methane is introduced at a flow rate of 20-30 sccm for 20-30 min.

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