A carbon-coated cathode material, its preparation method and application

Abstract

This invention provides a carbon-coated cathode material, its preparation method, and its application. The preparation method includes the following steps: mixing a halocholine compound, a phosphate, and a solvent to obtain a carbon source solution; placing the cathode active material and the anode in the carbon source solution and performing electrodeposition to obtain the carbon-coated cathode material. This invention, by using a specific carbon source solution and performing electrochemical deposition, obtains a halogen-nitrogen dual-doped carbon-coated cathode material. This controls the grain size and internal transport characteristics of the cathode active material, suppresses lattice distortion, improves the ionic and electronic conductivity of the cathode material, ensures uniform carbon deposition, enhances the bonding strength and uniformity of the carbon coating layer, strengthens the conductivity between particles, reduces battery polarization, and inhibits grain growth, shortening the Li-Nitrogen phase transition time. + The diffusion pathway within the particles enhances the electrochemical performance of the cathode material.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to a carbon-coated cathode material, its preparation method, and its application. Background Technology

[0002] The cathode material significantly influences the performance of lithium-ion batteries. Currently, common cathode active materials suffer from low conductivity. Methods to improve the conductivity of cathode materials include preparing nano-electrode active materials, refining the grain size of electrode active materials, and surface carbon coating. However, current carbon coatings on cathode materials exhibit poor affinity, low bonding strength, poor uniformity, and low conductivity. Furthermore, they cannot prevent electrolyte erosion of cathode materials such as lithium iron phosphate. Moreover, most carbon coating methods involve high-temperature carbonization, which easily damages the crystal structure of the cathode material.

[0003] For example, CN 101494288A discloses a method for preparing lithium iron phosphate, a cathode material for lithium-ion secondary batteries, including the following steps: A. Mixing a lithium source compound, a divalent iron source compound, a phosphorus source compound, and an organic small molecule carbon source additive, ball milling, and sintering to obtain a sintering precursor; B. Mixing the sintering precursor from step A with the organic polymer carbon source additive, ball milling, sintering, and pulverizing to obtain finished lithium iron phosphate powder. The disclosed method first synthesizes an iron coordination polymer by reacting an aromatic polycarboxylic acid compound with an iron source compound under hydrothermal / solvothermal conditions; then, this polymer is mixed with a lithium source compound, a simple organic compound, and a dopant compound and sintered to obtain the lithium iron phosphate cathode material. The method improves the conductivity and electrochemical performance of the synthesized lithium iron phosphate carbon-coated material by generating SP2 hybrid carbon coating on the surface of the lithium iron phosphate through high-temperature decomposition of organic matter. However, the uniformity and bonding strength of the carbon material coated on the surface by this method are poor, which will damage the crystal structure of the cathode material and has limited improvement on the electrochemical performance of the lithium iron phosphate cathode material.

[0004] Based on the above research, there is a need to provide a method for preparing carbon-coated cathode materials. This method can avoid the destruction of the crystal structure of the cathode material by high-temperature carbonization, avoid the direct contact between the cathode material and the electrolyte to prevent side reactions, and the carbon coating layer has high bonding strength and uniformity. Therefore, the obtained cathode material has good conductivity and cycle life. Summary of the Invention

[0005] The purpose of this invention is to provide a carbon-coated cathode material, its preparation method, and its application, particularly a halogen-nitrogen dual-doped carbon-coated cathode material, its preparation method, and its application. The preparation method is carried out by electrochemical deposition, which can improve the bonding strength and uniformity of the carbon film on the surface of the cathode material, avoid the destruction of the crystal structure of the cathode material by high-temperature carbonization, effectively improve the conductivity of the material, avoid the side reaction caused by direct contact between the material and the electrolyte, and thus significantly improve the cycle life of the cathode material.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing a carbon-coated cathode material, the method comprising the following steps:

[0008] (1) Mix the halocholine compound, phosphate and solvent to obtain a carbon source solution;

[0009] (2) The positive electrode active material and the anode are placed in the carbon source solution described in step (1) and electrodeposited to obtain the carbon-coated positive electrode material.

[0010] In the carbon source solution of this invention, a p-halocholine compound is used simultaneously as the carbon source, halogen source, and nitrogen source to obtain a halogen-nitrogen dual-doped carbon-coated cathode material. The doped elements affect the grain size and internal transport characteristics of the cathode active material, suppress lattice distortion, and improve the ionic and electronic conductivity of the cathode material. Simultaneously, this invention employs electrodeposition for carbon coating. To ensure carbon deposition and improve the bonding strength and uniformity of the carbon coating layer, phosphate is added to the carbon source solution to lower the voltage and promote carbon deposition, thereby introducing carbon material onto the surface of the cathode active material. The introduction of carbon material enhances the conductivity between particles, reduces battery polarization, and inhibits grain growth, thus refining the grains and shortening the Li-Ni ratio. + The diffusion path within the particles means that the specific method of this invention not only avoids the damage to the positive electrode active material caused by conventional high-temperature carbonization of carbon coating and effectively improves the conductivity of the material, but also improves the bonding strength and uniformity of the carbon coating layer.

[0011] Preferably, when mixing the halogenated choline compound, phosphate and solvent in step (1), a supercritical CO2 fluid is also added.

[0012] In this invention, supercritical carbon dioxide fluid is added to the carbon source solution. The introduction of supercritical CO2 fluid enables the resulting coated carbon layer to have good uniformity, flatness and excellent adhesion strength, and it can also serve as a carbon source. Therefore, compared with carbon source without the addition of supercritical carbon dioxide fluid, the coated carbon layer obtained by this invention is more complete and uniform, the bonding strength between the positive electrode active material and the carbon coating layer is higher, and the performance of the obtained positive electrode material is better.

[0013] Preferably, the mass ratio of the halogenated choline compound, phosphate, solvent, and supercritical CO2 fluid is (0.5-2):(2-10):(15-25):(25-35), for example, it can be 1:5:20:30, 0.5:2:15:35, or 2:10:15:25, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0014] Preferably, the halogenated choline compound in step (1) includes any one or a combination of at least two of fluorocholine compounds, chlorocholine compounds, bromocholine compounds or iodocholine compounds. Typical but non-limiting combinations include combinations of fluorocholine compounds and chlorocholine compounds, combinations of bromocholine compounds and iodocholine compounds, and preferably fluorocholine compounds.

[0015] The preferred halogenated choline compound of this invention is a fluorocholine compound. Since fluorocholine compounds contain fluorine and nitrogen elements, these substituent elements can affect the grain size and internal transport characteristics of the material, and can suppress lattice distortion, reduce triboelectric resistance, and improve the ionic conductivity and electronic conductivity of lithium iron phosphate. Therefore, the carbon-coated cathode material obtained has better performance.

[0016] Preferably, the fluorocholine compounds of the present invention include any one or a combination of at least two of fluorophosphocholine, fluoromethylcholine, fluoroethylcholine or 2-fluorocholine, and typical but non-limiting combinations include a combination of fluorophosphocholine and fluoromethylcholine, or a combination of fluoroethylcholine and 2-fluorocholine.

[0017] Preferably, the phosphate in step (1) includes sodium phosphate and / or potassium phosphate.

[0018] Preferably, the solvent in step (1) includes an alcohol solvent, which includes ethanol.

[0019] Preferably, the electrodeposition voltage in step (2) is 30-50V, for example, it can be 30V, 35V, 40V, 45V or 50V, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0020] The electrochemical deposition described in this invention needs to be carried out at a specific voltage to match the components in the carbon source solution and improve the deposition effect. If the voltage of electrochemical deposition is too low, the deposited carbon coating layer will be uneven and the side reactions between the cathode material and the electrolyte cannot be effectively avoided. If the deposition voltage is too high, it will damage the structure of the cathode active material itself.

[0021] Preferably, the electrodeposition time in step (2) is 2-5 hours, for example, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours or 5 hours, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0022] Preferably, the electrodeposition temperature in step (2) is 20-50°C, for example, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C or 50°C, and the pressure is 5-15 MPa, for example, 5 MPa, 7 MPa, 9 MPa, 11 MPa, 13 MPa or 15 MPa, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0023] The electrochemical deposition described in this invention is carried out under specific pressure, which can ensure the state of supercritical carbon dioxide fluid and enable it to function. If the pressure is too low, the state of supercritical fluid cannot be guaranteed, and if the pressure is too high, it will affect the structural stability of the cathode material.

[0024] Preferably, the anode in step (2) comprises an expanded graphite plate.

[0025] Preferably, the positive electrode active material in step (2) includes, but is not limited to, lithium iron phosphate.

[0026] Preferably, the method for preparing the positive electrode active material in step (2) includes the following steps:

[0027] (i) Lithium salt, iron salt and phosphate are mixed to obtain a mixture;

[0028] (ii) The mixture described in step (i) is subjected to a microwave reaction to obtain the positive electrode active material.

[0029] Preferably, the power of the microwave reaction in step (ii) is 800-2000W, for example, 800W, 1000W, 1200W, 1400W, 1600W, 1800W or 2000W, and the time is 5-40min, for example, 5min, 10min, 15min, 20min, 25min, 30min, 35min or 40min, but not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0030] Preferably, the microwave reaction in step (ii) is carried out in a nitrogen atmosphere, and the flow rate of the nitrogen is 30-50 L / min, for example, it can be 30 L / min, 35 L / min, 40 L / min, 45 L / min or 50 L / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0031] Preferably, when mixing lithium salt, iron salt and phosphate in step (i), a dopant is also added.

[0032] Preferably, the dopant comprises a magnesium salt, which includes, but is not limited to, magnesium carbonate.

[0033] Preferably, the molar ratio of lithium ions in the lithium salt and iron ions in the iron salt in step (i) is (1.05-1.15):1, for example, it can be 1.05:1, 1.07:1, 1.09:1, 1.11:1, 1.13:1 or 1.15:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] As a preferred embodiment of the preparation method of the present invention, the preparation method includes the following steps:

[0035] (1) A carbon source solution is obtained by mixing a halocholine compound, a phosphate, a solvent and a supercritical CO2 fluid in a mass ratio of (0.5-2):(2-10):(15-25):(25-35);

[0036] (2) Place the positive electrode active material and the expanded graphite plate in the carbon source solution described in step (1), and perform electrodeposition at a temperature of 20-50°C, a pressure of 5-15 MPa, and a voltage of 30-50V for 2-5 hours to obtain the carbon-coated positive electrode material.

[0037] The method for preparing the positive electrode active material in step (2) includes the following steps:

[0038] (i) Lithium salt, iron salt, phosphate and dopant are mixed to obtain a mixture;

[0039] (ii) The mixture described in step (i) is subjected to a microwave reaction at a power of 800-2000W for 5-40 minutes in a nitrogen atmosphere with a flow rate of 30-50L / min to obtain the positive electrode active material.

[0040] In a second aspect, the present invention provides a carbon-coated cathode material, which is obtained by the preparation method described in the first aspect.

[0041] Thirdly, the present invention provides a lithium-ion battery comprising a carbon-coated positive electrode material as described in the second aspect.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] This invention employs a specific carbon source solution for electrochemical deposition to obtain a halogen-nitrogen dual-doped carbon-coated cathode material. The doped elements affect the grain size and internal transport characteristics of the cathode active material, suppress lattice distortion, and improve the ionic and electronic conductivity of the cathode material. Simultaneously, to ensure uniform carbon deposition and enhance the bonding strength and uniformity of the carbon coating layer, this invention adds phosphate and supercritical carbon dioxide fluid to the carbon source solution. This introduces carbon material onto the surface of the cathode active material, enhancing interparticle conductivity, reducing battery polarization, and suppressing grain growth, thus refining the grains and shortening the Li-Ni content. + The diffusion path within the particles allows the carbon-coated cathode material prepared by the specific method of this invention to possess excellent electrochemical performance. Attached Figure Description

[0044] Figure 1 The diagram shows the cycle performance of batteries made from the materials obtained in Embodiment 1, Comparative Example 1, and Comparative Example 2 of this invention. Detailed Implementation

[0045] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0046] Example 1

[0047] This embodiment provides a method for preparing a carbon-coated cathode material, the method comprising the following steps:

[0048] (1) A carbon source solution was obtained by mixing fluorocholine compound, sodium phosphate, ethanol and supercritical CO2 fluid in a mass ratio of 1:5:20:30.

[0049] The fluorocholine-based compound is fluorophosphocholine.

[0050] (2) Lithium iron phosphate and expanded graphite plate are placed in the carbon source solution described in step (1), and electrodeposited at 30°C, 10 MPa, and 30V for 3 hours to obtain the carbon-coated cathode material.

[0051] The method for preparing lithium iron phosphate in step (2) includes the following steps:

[0052] (i) Weigh 738.9g of lithium carbonate, 1438.6g of ferrous oxalate and 1150.3g of ammonium dihydrogen phosphate, add 1000ppm of magnesium carbonate and mix to obtain a mixture;

[0053] (ii) The mixture described in step (i) is subjected to a microwave reaction in a microwave reactor at a flow rate of 40 L / min and a power of 1000 W for 30 min to obtain the lithium iron phosphate.

[0054] The cycle performance diagram of the carbon-coated cathode material obtained in this embodiment is shown below. Figure 1 As shown.

[0055] Example 2

[0056] This embodiment provides a method for preparing a carbon-coated cathode material, the method comprising the following steps:

[0057] (1) A carbon source solution was obtained by mixing fluorocholine compound, sodium phosphate, ethanol and supercritical CO2 fluid in a mass ratio of 0.5:2:25:25.

[0058] The fluorocholine compound is fluoromethylcholine;

[0059] (2) Lithium iron phosphate and expanded graphite plate are placed in the carbon source solution described in step (1), and electrodeposited at 50°C and 15 MPa for 2 hours at 50V to obtain the carbon-coated cathode material.

[0060] The method for preparing lithium iron phosphate in step (2) includes the following steps:

[0061] (i) Weigh 738.9g of lithium carbonate, 1438.6g of ferrous oxalate and 1150.3g of ammonium dihydrogen phosphate, add 1000ppm of magnesium carbonate and mix to obtain a mixture;

[0062] (ii) The mixture described in step (i) is subjected to a microwave reaction in a microwave reactor at a flow rate of 50 L / min and a power of 2000 W for 5 min to obtain the lithium iron phosphate.

[0063] Example 3

[0064] This embodiment provides a method for preparing a carbon-coated cathode material, the method comprising the following steps:

[0065] (1) A carbon source solution was obtained by mixing fluorocholine compound, sodium phosphate, ethanol and supercritical CO2 fluid in a mass ratio of 2:10:25:35.

[0066] The fluorocholine-based compound is fluorophosphocholine.

[0067] (2) Lithium iron phosphate and expanded graphite plate are placed in the carbon source solution described in step (1), and electrodeposited at 20°C, 5 MPa, and 30V for 5 hours to obtain the carbon-coated cathode material.

[0068] The method for preparing lithium iron phosphate in step (2) includes the following steps:

[0069] (i) Weigh 738.9g of lithium carbonate, 1438.6g of ferrous oxalate and 1150.3g of ammonium dihydrogen phosphate, add 1000ppm of magnesium carbonate and mix to obtain a mixture;

[0070] (ii) The mixture described in step (i) is subjected to a microwave reaction in a microwave reactor at a flow rate of 30 L / min and a power of 800 W for 40 min to obtain the lithium iron phosphate.

[0071] Example 4

[0072] This embodiment provides a method for preparing a carbon-coated cathode material. The preparation method is the same as that in Example 1, except that supercritical CO2 fluid is not added in step (1).

[0073] Example 5

[0074] This embodiment provides a method for preparing a carbon-coated cathode material. Except for the mass ratio of fluorinated choline compound, sodium phosphate, ethanol and supercritical CO2 fluid in step (1) being 0.2:5:20:30, the preparation method is the same as in Example 1.

[0075] Example 6

[0076] This embodiment provides a method for preparing a carbon-coated cathode material. Except for the mass ratio of fluorinated choline compound, sodium phosphate, ethanol and supercritical CO2 fluid in step (1) being 3:5:20:30, the preparation method is the same as in Example 1.

[0077] Example 7

[0078] This embodiment provides a method for preparing a carbon-coated cathode material. The preparation method is the same as in Example 1, except that the fluorocholine compound in step (1) is replaced with a chlorocholine compound. The chlorocholine compound is chlorophosphocholine.

[0079] Example 8

[0080] This embodiment provides a method for preparing a carbon-coated cathode material. The preparation method is the same as in Example 1, except that the fluorocholine compound in step (1) is replaced with a bromocholine compound. The bromocholine compound is bromophosphocholine.

[0081] Example 9

[0082] This embodiment provides a method for preparing a carbon-coated cathode material. Except for the electrodeposition voltage of 20V in step (2), the preparation method is the same as in embodiment 1.

[0083] Example 10

[0084] This embodiment provides a method for preparing a carbon-coated cathode material. Except for the electrodeposition voltage of 60V in step (2), the preparation method is the same as in embodiment 1.

[0085] Example 11

[0086] This embodiment provides a method for preparing a carbon-coated cathode material. Except for the electrodeposition pressure of 2 MPa in step (2), the preparation method is the same as in Example 1.

[0087] Example 12

[0088] This embodiment provides a method for preparing a carbon-coated cathode material. Except for the electrodeposition pressure of 20 MPa in step (2), the preparation method is the same as in Example 1.

[0089] Comparative Example 1

[0090] This comparative example provides a cathode material, which is lithium iron phosphate, and the preparation method of the lithium iron phosphate is the same as that in Example 1;

[0091] The cycle performance diagram of the cathode material obtained in this comparative example is shown below. Figure 1 As shown.

[0092] Comparative Example 2

[0093] This comparative example provides a method for preparing a cathode material, the method comprising the following steps:

[0094] (1) Take 1000 mL of 6% fluorophosphocholine solution, add 500 g of lithium iron phosphate, and stir at 600 rpm for 20 min to obtain organic carbon coated lithium iron phosphate slurry.

[0095] The preparation method of the lithium iron phosphate is the same as that in Example 1;

[0096] (2) The organic carbon-coated lithium iron phosphate slurry described in step (1) is subjected to solid-liquid separation. The resulting solid is dried under vacuum at 85°C, then ground, and sintered at 600°C for 5 hours in an argon atmosphere to obtain the cathode material.

[0097] The cycle performance diagram of the cathode material obtained in this comparative example is shown below. Figure 1 As shown.

[0098] Comparative Example 3

[0099] This comparative example provides a method for preparing a cathode material. The preparation method is the same as in Example 1, except that the sodium phosphate in step (1) is replaced with acetic acid monochloride.

[0100] The carbon-coated cathode material provided in the above embodiments and the cathode material provided in the comparative examples were used to prepare coin cells with lithium anodes. Cycle performance was tested under 0.5C / 0.5C, 2.8-4.2V conditions. The capacity retention after 50 cycles is shown in Table 1.

[0101] Table 1

[0102] 50-cycle capacity retention rate (%) Example 1 99.1 Example 2 98.2 Example 3 98.4 Example 4 97.4 Example 5 96.9 Example 6 97.1 Example 7 96.8 Example 8 96.4 Example 9 95.2 Example 10 95.4 Example 11 97.6 Example 12 98.7 Comparative Example 1 92.3 Comparative Example 2 96.6 Comparative Example 3 97.8

[0103] The following points can be observed from Table 1:

[0104] (1) The preparation method provided by the present invention can obtain a carbon-coated cathode material co-doped with halogen and nitrogen, and the carbon coating layer has high bonding strength and uniformity, thus the battery has excellent performance; As can be seen from Examples 1 and 4, the introduction of supercritical CO2 fluid can make the coated carbon film have good uniformity, flatness and excellent adhesion strength, thereby further improving the performance of the battery; As can be seen from Examples 1 and 5-6, if the amount of fluorinated choline compound added is too small or too large, it will affect the doping and coating results, thereby reducing the performance of the battery; As can be seen from Examples 1 and 7-8, the preferred halinated choline compound of the present invention is a fluorinated choline compound; As can be seen from Examples 1 and 9-12, the voltage and pressure of electrodeposition will affect the performance.

[0105] (2) As can be seen from Example 1 and Comparative Example 1, the present invention can significantly improve the cycle performance compared with the uncoated material. As can be seen from Example 1 and Comparative Example 2, although Comparative Example 2 also uses fluorinated choline compounds as the coating source, it is prepared by traditional sintering method, which will damage the crystal structure of the cathode material, and the bonding strength and uniformity of the carbon coating layer are low, resulting in a decrease in the performance of the obtained material compared with Example 1. As can be seen from Example 1 and Comparative Example 3, if the phosphate that is paired with supercritical carbon dioxide fluid and halogenated choline compounds is replaced with halogenated carboxylic acids, the effect of electrodeposition coating decreases, the bonding strength and uniformity of the coating layer decrease, and the performance of the obtained battery decreases.

[0106] In summary, this invention provides a carbon-coated cathode material, its preparation method, and its application. The preparation method is carried out by electrochemical deposition, which can improve the bonding strength and uniformity of the carbon film on the surface of the cathode material, avoid the damage to the crystal structure of the cathode material caused by high-temperature carbonization, effectively improve the conductivity of the material, and avoid side reactions caused by direct contact between the material and the electrolyte, thereby significantly improving the cycle life of the cathode material.

[0107] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a carbon-coated cathode material, characterized in that, The preparation method includes the following steps: (1) Mix the halocholine compound, phosphate and solvent to obtain a carbon source solution; In step (1), when mixing the halocholine compound, phosphate and solvent, supercritical CO2 fluid was also added. The halogenated choline compounds in step (1) include fluorocholine compounds; The phosphate in step (1) includes sodium phosphate and / or potassium phosphate; (2) The positive electrode active material and the anode are placed in the carbon source solution described in step (1) and electrodeposited to obtain the carbon-coated positive electrode material.

2. The preparation method according to claim 1, characterized in that, The mass ratio of the halocholine compound, phosphate, solvent and supercritical CO2 fluid is (0.5-2):(2-10):(15-25):(25-35).

3. The preparation method according to claim 1 or 2, characterized in that, The solvent in step (1) includes alcohol solvents.

4. The preparation method according to claim 1 or 2, characterized in that, The electrodeposition voltage in step (2) is 30-50V.

5. The preparation method according to claim 1 or 2, characterized in that, The electrodeposition time in step (2) is 2-5 hours.

6. The preparation method according to claim 1 or 2, characterized in that, The electrodeposition temperature in step (2) is 20-50℃ and the pressure is 5-15 MPa.

7. The preparation method according to claim 1, characterized in that, The anode in step (2) comprises an expanded graphite plate.

8. The preparation method according to claim 1 or 2, characterized in that, The positive electrode active material mentioned in step (2) includes, but is not limited to, lithium iron phosphate.

9. The preparation method according to claim 1, characterized in that, The method for preparing the positive electrode active material in step (2) includes the following steps: (i) Lithium salt, iron salt and phosphate are mixed to obtain a mixture; (ii) The mixture described in step (i) is subjected to a microwave reaction to obtain the positive electrode active material.

10. The preparation method according to claim 9, characterized in that, The power of the microwave reaction in step (ii) is 800-2000W, and the time is 5-40min.

11. The preparation method according to claim 9, characterized in that, The microwave reaction described in step (ii) is carried out in a nitrogen atmosphere at a flow rate of 30-50 L / min.

12. The preparation method according to claim 9, characterized in that, In step (i), when mixing lithium salt, iron salt and phosphate, a dopant is also added.

13. The preparation method according to claim 12, characterized in that, The dopant includes magnesium salts.

14. The preparation method according to claim 6, characterized in that, The molar ratio of lithium ions in the lithium salt to iron ions in the iron salt in step (i) is (1.05-1.15):

1.

15. The preparation method according to claim 1, characterized in that, The preparation method includes the following steps: (1) A carbon source solution is obtained by mixing a halocholine compound, a phosphate, a solvent and a supercritical CO2 fluid in a mass ratio of (0.5-2):(2-10):(15-25):(25-35); (2) Place the positive electrode active material and the expanded graphite plate in the carbon source solution described in step (1), and perform electrodeposition at a temperature of 20-50°C, a pressure of 5-15 MPa, and a voltage of 30-50V for 2-5 hours to obtain the carbon-coated positive electrode material. The method for preparing the positive electrode active material in step (2) includes the following steps: (i) Lithium salt, iron salt, phosphate and dopant are mixed to obtain a mixture; (ii) The mixture described in step (i) is subjected to a microwave reaction at a power of 800-2000W for 5-40 minutes in a nitrogen atmosphere with a flow rate of 30-50L / min to obtain the positive electrode active material.

16. A carbon-coated cathode material, characterized in that, The carbon-coated cathode material is obtained by the preparation method described in any one of claims 1-15.

17. A lithium-ion battery, characterized in that, The lithium-ion battery includes the carbon-coated cathode material as described in claim 16.

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