Preparation method of composite lithium iron manganese phosphate positive electrode material

Abstract

The invention discloses a preparation method of a composite lithium iron manganese phosphate positive electrode material, and belongs to the technical field of lithium ion batteries. The positive electrode material provided by the invention takes lithium ferrite and lithium iron manganese phosphate as a core, the outer layer is coated with carbon and subjected to fluorination treatment, and the preparation method comprises the following steps: (1) preparing the lithium iron manganese phosphate positive electrode material by adopting a carbon thermal reduction method; (2) mixing, sanding and spraying the lithium manganese iron phosphate positive electrode material and an organic carbon source, and sintering at high temperature in an inert atmosphere to obtain carbon-coated lithium manganese iron phosphate; (3) carrying out acid pickling and drying treatment on the carbon-coated lithium manganese iron phosphate to obtain surface-oxidized lithium manganese iron phosphate; and (4) carrying out surface fluorination treatment on the obtained oxidized lithium manganese iron phosphate to obtain a final product LiMnFePO4 (at) C (at) F.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and particularly relates to a method for preparing a composite lithium manganese iron phosphate cathode material. Background Technology

[0002] Lithium manganese iron phosphate (LMFP), as an upgraded technology of lithium iron phosphate, is rapidly penetrating the energy storage and power battery markets, but faces technological bottlenecks and competitive pressure. It boasts a significant energy density advantage: LMFP offers a 15-20% increase in energy density compared to traditional lithium iron phosphate, approaching the level of ternary lithium batteries, and has already been applied in mid-to-high-end vehicles and energy storage scenarios. High-compaction technology: In Q1 2025, the shipment share of fourth-generation high-compaction LMFP (compaction density > 2.6 g / cc) doubled compared to 2024, and is expected to reach 15% for the whole year, becoming a core competitive advantage. LMFP is rapidly penetrating the low-to-mid-end energy storage market due to its cost advantage, but it is limited by manganese leaching and cycle life issues, requiring blending with ternary materials. The leading companies hold an 88% market share. The difficulty in homogenizing lithium manganese iron phosphate (LMFP) slurry processing mainly lies in poor slurry dispersibility, high binder usage, complex drying processes, and insufficient electrode flexibility.

[0003] Currently, common methods to improve the processing difficulty of lithium manganese iron phosphate (LFP) mainly include: 1. Mixing with ternary cathode materials. However, this approach is for cost reduction, reducing the use of precious metals such as nickel and cobalt. Using LFP alone also results in suboptimal cycle performance. 2. Increasing the primary particle size of LFP through technical solutions. While increasing particle size reduces agglomeration, it also decreases the intrinsic ionic and electronic conductivity of LFP, further increasing battery impedance. 3. Adding dispersants during the homogenization process to improve particle dispersion. However, dispersants themselves are not conductive, also leading to increased battery impedance. 4. Increasing the PVDF content in the formulation to reduce the stability of the LFP slurry and slow down particle sedimentation. However, increasing PVDF also increases battery impedance and reduces the proportion of cathode material, resulting in a certain degree of decrease in battery energy density. 5. To address the increased impedance caused by the above problems, it is necessary to increase the proportion of conductive agents, further increasing costs.

[0004] Furthermore, existing patent CN 120864474 A (A carbon-coated lithium manganese iron phosphate and its preparation method and application) discloses a method for sequentially subjecting carbon-coated lithium manganese iron phosphate to low-temperature fluorination sintering and defluorination sintering treatments using a fluorine source. It indicates that this method can reduce the specific surface area of ​​the material and improve its conductivity, which is beneficial for preparing high-density carbon-coated lithium manganese iron phosphate materials and lithium-ion batteries with superior electrochemical performance. However, the lithium manganese iron phosphate slurry prepared using this patent's technical solution has poor dispersibility and stability, making it difficult to meet the needs of enterprises. Moreover, this patent's technical solution requires sintering in a box furnace to 1000℃, resulting in high energy loss. However, this invention demonstrates that the lithium manganese iron phosphate slurry prepared using the technical solution of this invention possesses excellent slurry dispersibility and stability, as well as excellent discharge capacity, meeting the needs of enterprises, and with low energy consumption. Summary of the Invention

[0005] To address the aforementioned issues, this invention proposes a process of surface oxidation followed by fluorination of lithium manganese iron phosphate. By introducing CF chains, the lithium manganese iron phosphate exhibits excellent hydrophobicity and oleophobicity, reducing its agglomeration in NMP, effectively improving its homogenization and dispersion performance, and reducing the processing difficulty of lithium manganese iron phosphate.

[0006] This invention provides a method for preparing lithium manganese iron phosphate (LiMnFePO4@C@F) as a cathode material for lithium-ion batteries, the method comprising the following steps: (1) The lithium source, manganese source, iron source and carbon source are mixed in water and then milled to obtain a slurry; (2) The slurry is spray-dried to obtain a precursor, and then the precursor is heat-treated in a protective atmosphere to obtain lithium manganese iron phosphate cathode material A. (3) The above-mentioned lithium manganese iron phosphate cathode material A is mixed with a carbon source and sand-milled, then spray-dried, and then heat-treated in a protective atmosphere to obtain carbon-coated lithium manganese iron phosphate cathode material B. (4) The above-mentioned lithium manganese iron phosphate cathode material B is mixed with acid, then filtered, washed and dried to obtain surface-oxidized lithium manganese iron phosphate C. (5) The above-mentioned lithium manganese iron phosphate C is reacted with a fluorine source to obtain the final product D.

[0007] Furthermore, in step (1), the lithium source powder is one or more of LiOH, LiOH·H2O, LiH2PO4, and Li2CO3.

[0008] Furthermore, in step (1), the manganese source is one or more of manganese tetroxide, manganese trioxide, manganese dioxide, and manganese carbonate.

[0009] Furthermore, in step (1), the iron source is iron phosphate with a particle size D50 = 2-7 ​​micrometers.

[0010] Furthermore, in step (1), the carbon source is one or more of glucose, sucrose, and polyethylene glycol (PEG).

[0011] Furthermore, in step (1), the molar ratio of lithium source to manganese source is 1:0.2-0.6.

[0012] Furthermore, in step (1), the molar ratio of lithium source to iron source is 1:0.1-0.5.

[0013] Furthermore, in step (1), the molar ratio of lithium source to carbon source is 1:0.2-0.5.

[0014] Furthermore, in step (1), the amount of water used is 3-10 times the mass of the lithium source.

[0015] Furthermore, the grinding time in step (1) is 1-5 hours.

[0016] Furthermore, in step (1), the slurry has a D50 of less than 400 nm.

[0017] Furthermore, in step (2), the inlet air temperature of the spray dryer is 160-180°C, and the outlet air temperature is 60-90°C.

[0018] Furthermore, in step (2), the heat treatment temperature is 600-750℃ and the time is 5-10h.

[0019] Furthermore, the protective gas in step (2) includes one or more of nitrogen, argon, and xenon.

[0020] Furthermore, in step (3), the carbon source is one or more of glucose, sucrose, and polyethylene glycol (PEG).

[0021] Furthermore, in step (3), the mass ratio of lithium manganese iron phosphate cathode material A to carbon source is 1:0.1-0.2.

[0022] Furthermore, the grinding time in step (3) is 1-5 hours.

[0023] Furthermore, in step (3), the inlet air temperature of the spray dryer is 160-180°C, and the outlet air temperature is 60-90°C.

[0024] Furthermore, in step (3), the heat treatment temperature is 600-750℃ and the time is 5-10h.

[0025] Furthermore, the protective gas in step (3) includes one or more of nitrogen, argon, and xenon.

[0026] Furthermore, in step (4), the acid is nitric acid and / or hydrochloric acid.

[0027] Furthermore, both nitric acid and hydrochloric acid exist in aqueous solution form; in the aqueous solution, the concentration of nitric acid is 60-70 wt%, and the concentration of hydrochloric acid is 10-15 wt%.

[0028] Furthermore, the mass ratio of lithium manganese iron phosphate cathode material B to nitric acid is 1:1.5-2.

[0029] Furthermore, the mass ratio of lithium manganese iron phosphate cathode material B to hydrochloric acid is 1:0-0.5.

[0030] Furthermore, in step (4), the mixing process is carried out at a temperature of 80-90℃ for 2-5 hours.

[0031] Furthermore, the fluorine source mentioned in step (5) includes a solid fluorine source or a gaseous fluorine source; the solid fluorine source includes fluorosilanes and / or fluorosilane coupling agents; the gaseous fluorine source includes one or more of F2, chlorofluorocarbons, CF4, NF3, and SF6.

[0032] Furthermore, in step (5), when the fluorine source is a solid fluorine source, the mass ratio of lithium manganese iron phosphate C to the fluorine source is 1:0.01-0.005.

[0033] Furthermore, in step (5), when the fluorine source is a gaseous fluorine source, the mass ratio of lithium manganese iron phosphate C to the fluorine source is 1:0.001-0.005.

[0034] Furthermore, the reaction in step (5) needs to be carried out in a protective gas environment; the protective gas includes one or more of nitrogen, argon, and xenon.

[0035] Furthermore, the reaction temperature in step (5) is 450-550℃.

[0036] Furthermore, the reaction time in step (5) is 8-12 hours.

[0037] The present invention provides lithium manganese iron phosphate prepared according to the above preparation method.

[0038] The present invention relates to the application of lithium manganese iron phosphate in the field of lithium batteries.

[0039] Furthermore, the application includes mixing lithium manganese iron phosphate, conductive carbon black and polyvinylidene fluoride into a slurry, then coating and drying it, and finally rolling and cutting it for use in assembling button batteries.

[0040] Beneficial effects Compared with existing technologies, this invention improves the performance of lithium manganese iron phosphate cathode materials by acid washing and surface oxidation treatment followed by fluorination treatment. This results in electrode slurries with good dispersibility, good stability, and excellent discharge capacity, meeting processing requirements.

[0041] Furthermore, the processing method used in this invention does not involve ultra-high temperatures, has low energy loss, and is easy to operate, making it a more feasible and industrially applicable processing method. Detailed Implementation

[0042] Example 1 A method for preparing lithium iron manganese phosphate (LiMnFePO4@C@F), a cathode material for lithium-ion batteries, includes: (1) Weigh 1.02 mol of battery-grade lithium carbonate, 0.40 mol of iron phosphate, 0.60 mol of manganese carbonate, and 14.5 g of glucose. First, add lithium carbonate and manganese carbonate to 300 mL of deionized water and stir to form a suspension. Then add the iron phosphate material and put it into a sand mill. After sand milling for 1 hour, a mixed slurry is obtained. Add glucose to the mixed slurry and sand mill again for 1 hour to obtain a mixed slurry.

[0043] (2) Adjust the inlet air temperature of the spray dryer to 170°C and control the outlet air temperature at 60-90°C. Spray dry and granulate the mixed slurry to obtain the precursor.

[0044] (3) The precursor was placed in a tube furnace under nitrogen atmosphere protection, and the heating rate was 2°C / min. The temperature was raised from room temperature to 720°C and held for 10 hours. After cooling, lithium manganese iron phosphate cathode material A was obtained.

[0045] (4) The above-mentioned lithium manganese iron phosphate cathode material A and carbon source PEG are fully mixed at a mass ratio of 1:0.1 and milled for 2 hours. The inlet air temperature of the spray dryer is adjusted to 170°C and the outlet air temperature is controlled at 60-90°C. After spray drying, the heating rate is 2°C / min. The temperature is raised from room temperature to 700°C and heat-treated in nitrogen atmosphere for 10 hours to obtain carbon-coated lithium manganese iron phosphate cathode material B.

[0046] (5) The above-mentioned lithium manganese iron phosphate cathode material B is mixed with nitric acid aqueous solution (65wt%) and hydrochloric acid aqueous solution (12wt%) in a mass ratio of 1:1.5:0.5, stirred at 90°C for 3.5 h, filtered, washed and dried to obtain surface-oxidized lithium manganese iron phosphate C. (6) The above 10 g of lithium manganese iron phosphate C was sintered in a mixed gas (fluorine and nitrogen, with flow rates of 100 sccm and 500 sccm). The sintering conditions were: heating from room temperature to 500℃ at a heating rate of 2℃ / min, and holding at 500℃ for 10 h, and then cooling with the furnace to obtain the final product D.

[0047] Example 2 A method for preparing lithium iron manganese phosphate (LiMnFePO4@C@F), a cathode material for lithium-ion batteries, includes: (1) Weigh out 0.42 mol of battery-grade lithium hydroxide monohydrate, 0.6 mol of lithium hydroxide, 0.40 mol of iron phosphate, 0.60 mol of manganese dioxide, and 14.5 g of glucose. First, add the lithium source and manganese source to 300 mL of deionized water and stir to form a suspension. Then add the iron phosphate material and put it into a sand mill. After sand milling for 1 hour, a mixed slurry is obtained. Add glucose to the mixed slurry and sand mill again for 1 hour to obtain a mixed slurry.

[0048] (2) Adjust the inlet air temperature of the spray dryer to 170°C and control the outlet air temperature at 60-90°C. Dry and granulate the mixed slurry to obtain the precursor.

[0049] (3) The precursor was placed in a tube furnace under nitrogen atmosphere protection, and the heating rate was 2°C / min. The temperature was raised from room temperature to 720°C and held for 10 hours. After cooling, lithium manganese iron phosphate cathode material A was obtained.

[0050] (4) The above-mentioned lithium manganese iron phosphate cathode material A and carbon source PEG are fully mixed at a mass ratio of 1:0.1 and milled for 2 hours. The inlet air temperature of the spray dryer is adjusted to 170°C and the outlet air temperature is controlled at 60-90°C. After spray drying, the heating rate is 2°C / min. The temperature is raised from room temperature to 700°C and heat-treated in nitrogen atmosphere for 10 hours to obtain carbon-coated lithium manganese iron phosphate cathode material B.

[0051] (5) The above-mentioned lithium manganese iron phosphate cathode material B is mixed with nitric acid aqueous solution (65wt%) and hydrochloric acid aqueous solution (12wt%) in a mass ratio of 1:1.5:0.5, stirred at 90°C for 3.5h, filtered, washed and dried to obtain surface-oxidized lithium manganese iron phosphate C.

[0052] (6) The above 10 g of lithium manganese iron phosphate C was sintered in a mixed gas (fluorine and nitrogen, with flow rates of 100 sccm and 500 sccm). The sintering conditions were: heating from room temperature to 500℃ at a heating rate of 2℃ / min, and holding at 500℃ for 10 h, and then cooling with the furnace to obtain the final product D.

[0053] Example 3 A method for preparing lithium iron manganese phosphate (LiMnFePO4@C@F), a cathode material for lithium-ion batteries, includes: (1) Weigh 1.02 mol of battery-grade lithium carbonate, 0.40 mol of iron phosphate, 0.60 mol of manganese carbonate, and 14.5 g of glucose. First, add lithium carbonate and manganese carbonate to 300 mL of deionized water and stir to form a suspension. Then add the iron phosphate material and put it into a sand mill. After sand milling for 1 hour, a mixed slurry is obtained. Add glucose to the mixed slurry and sand mill again for 1 hour to obtain a mixed slurry.

[0054] (2) Adjust the inlet air temperature of the spray dryer to 170°C and control the outlet air temperature at 60-90°C. Dry and granulate the mixed slurry to obtain the precursor.

[0055] (3) The precursor was placed in a tube furnace under nitrogen atmosphere protection, and the heating rate was 2°C / min. The temperature was raised from room temperature to 720°C and held for 10 hours. After cooling, lithium manganese iron phosphate cathode material A was obtained.

[0056] (4) The above-mentioned lithium manganese iron phosphate cathode material A and carbon source PEG are fully mixed at a mass ratio of 1:0.1 and milled for 2 hours. The inlet air temperature of the spray dryer is adjusted to 170°C and the outlet air temperature is controlled at 60-90°C. After spray drying, the heating rate is 2°C / min. The temperature is raised from room temperature to 700°C and heat-treated in nitrogen atmosphere for 10 hours to obtain carbon-coated lithium manganese iron phosphate cathode material B.

[0057] (5) The above-mentioned lithium manganese iron phosphate cathode material B is mixed with nitric acid aqueous solution (65wt%) and hydrochloric acid aqueous solution (12wt%) in a mass ratio of 1:1.5:0.5, stirred at 90°C for 3.5h, filtered, washed and dried to obtain surface-oxidized lithium manganese iron phosphate C.

[0058] (6) The above 10 g of lithium manganese iron phosphate C was sintered in a mixed gas (fluorine and nitrogen, with flow rates of 100 sccm and 500 sccm). The sintering conditions were: heating from room temperature to 500℃ at a heating rate of 2℃ / min, and holding at 500℃ for 10 h, and then cooling with the furnace to obtain the final product D.

[0059] Example 4 A method for preparing lithium iron manganese phosphate (LiMnFePO4@C@F), a cathode material for lithium-ion batteries, includes: (1) Weigh 1.02 mol of battery-grade lithium carbonate, 0.40 mol of iron phosphate, 0.60 mol of manganese carbonate, and 14.5 g of glucose. First, add lithium carbonate and manganese carbonate to 300 mL of deionized water and stir to form a suspension. Then add the iron phosphate material and put it into a sand mill. After sand milling for 1 hour, a mixed slurry is obtained. Add glucose to the mixed slurry and sand mill again for 1 hour to obtain a mixed slurry.

[0060] (2) Adjust the inlet air temperature of the spray dryer to 170°C and control the outlet air temperature at 60-90°C. Dry and granulate the mixed slurry to obtain the precursor.

[0061] (3) The precursor was placed in a tube furnace under nitrogen atmosphere protection, and the heating rate was 2°C / min. The temperature was raised from room temperature to 720°C and held for 10 hours. After cooling, lithium manganese iron phosphate cathode material A was obtained.

[0062] (4) The above-mentioned lithium manganese iron phosphate cathode material A and carbon source PEG are fully mixed at a mass ratio of 1:0.1 and milled for 2 hours. The inlet air temperature of the spray dryer is adjusted to 170°C and the outlet air temperature is controlled at 60-90°C. After spray drying, the heating rate is 2°C / min. The temperature is raised from room temperature to 700°C and heat-treated in nitrogen atmosphere for 10 hours to obtain carbon-coated lithium manganese iron phosphate cathode material B.

[0063] (5) The above-mentioned lithium manganese iron phosphate cathode material B is mixed with nitric acid aqueous solution (65wt%) at a mass ratio of 1:2, stirred at 90°C for 3.5h, filtered, washed and dried to obtain surface-oxidized lithium manganese iron phosphate C.

[0064] (6) The above 10 g of lithium manganese iron phosphate C was sintered in a mixed gas (fluorine and nitrogen, with flow rates of 100 sccm and 500 sccm). The sintering conditions were: heating from room temperature to 500℃ at a heating rate of 2℃ / min, and holding at 500℃ for 10 h, and then cooling with the furnace to obtain the final product D.

[0065] Example 5 A method for preparing lithium iron manganese phosphate (LiMnFePO4@C@F), a cathode material for lithium-ion batteries, includes: (1) Weigh 1.02 mol of battery-grade lithium carbonate, 0.40 mol of iron phosphate, 0.60 mol of manganese carbonate, and 14.5 g of glucose. First, add lithium carbonate and manganese carbonate to 300 mL of deionized water and stir to form a suspension. Then add the iron phosphate material and put it into a sand mill. After sand milling for 1 hour, a mixed slurry is obtained. Add glucose to the mixed slurry and sand mill again for 1 hour to obtain a mixed slurry.

[0066] (2) Adjust the inlet air temperature of the spray dryer to 170°C and control the outlet air temperature at 60-90°C. Dry and granulate the mixed slurry to obtain the precursor.

[0067] (3) The precursor was placed in a tube furnace under nitrogen atmosphere protection, and the heating rate was 2°C / min. The temperature was raised from room temperature to 720°C and held for 10 hours. After cooling, lithium manganese iron phosphate cathode material A was obtained.

[0068] (4) The above-mentioned lithium manganese iron phosphate cathode material A and carbon source sucrose are thoroughly mixed at a mass ratio of 1:0.2 and milled for 2 hours. The inlet air temperature of the spray dryer is adjusted to 170°C and the outlet air temperature is controlled at 60-90°C. After spray drying, the heating rate is 2°C / min. The temperature is raised from room temperature to 700°C and heat-treated in nitrogen atmosphere for 10 hours to obtain carbon-coated lithium manganese iron phosphate cathode material B.

[0069] (5) The above-mentioned lithium manganese iron phosphate cathode material B is mixed with nitric acid aqueous solution (65wt%) and hydrochloric acid aqueous solution (12wt%) in a mass ratio of 1:1.5:0.5, stirred at 90°C for 3.5h, filtered, washed and dried to obtain surface-oxidized lithium manganese iron phosphate C.

[0070] (6) The above 10 g of lithium manganese iron phosphate C was sintered in a mixed gas (fluorine and nitrogen, with flow rates of 100 sccm and 500 sccm). The sintering conditions were: heating from room temperature to 500℃ at a heating rate of 2℃ / min, and holding at 500℃ for 10 h, and then cooling with the furnace to obtain the final product D.

[0071] Comparative Example 1 Similar to Example 1, except that step (5) of Example 1 is omitted, that is, acid washing is not performed, and the fluorination treatment in step (6) is performed directly.

[0072] Comparative Example 2 Same as Example 1, except that the heat preservation time in step (6) of Example 1 is adjusted to 5h.

[0073] Comparative Example 3 Same as Example 1, except that the heat preservation time in step (6) of Example 1 is adjusted to 15h.

[0074] Comparative Example 4 Same as Example 1, except that steps (5) and (6) of Example 1 are omitted, thus obtaining carbon-coated lithium manganese iron phosphate cathode material B.

[0075] Comparative Example 5 Same as Example 1, except that only step (5) of Example 1 is adjusted: The above-mentioned lithium manganese iron phosphate cathode material B was mixed with nitric acid aqueous solution (65wt%) and hydrochloric acid aqueous solution (12wt%) in a mass ratio of 1:0.5:1.5, stirred at 90°C for 3.5 h, filtered, washed and dried to obtain surface-oxidized lithium manganese iron phosphate C. Then, the fluorination treatment in step (6) was carried out.

[0076] Comparative Example 6 Same as Example 1, except that only step (4) of Example 1 is adjusted: The above-mentioned lithium manganese iron phosphate cathode material A was thoroughly mixed with carbon source PEG at a mass ratio of 1:0.3 and milled for 2 hours. The inlet air temperature of the spray dryer was adjusted to 170°C and the outlet air temperature was controlled at 60-90°C. After spray drying, the heating rate was 2°C / min, and the temperature was increased from room temperature to 700°C. The material was then heat-treated in a nitrogen atmosphere for 10 hours to obtain carbon-coated lithium manganese iron phosphate cathode material B. Then, steps (5) and (6) were performed.

[0077] Test methods and experimental results 1. Button cell capacity test: Lithium replenishing agent, SP (conductive carbon black), and PVDF (polyvinylidene fluoride) are weighed in a ratio of 92:4:4, homogenized using a degassing machine, coated and dried on a semi-automatic coating machine, and then assembled into button cells in a glove box after rolling and cutting. The charging capacity is tested on Xinwei charging and discharging equipment at 3.0-4.5V and 0.1C to obtain the lithium replenishing capacity of the material.

[0078] 2. Slurry dispersibility test: The lithium manganese iron phosphate slurry prepared by the degassing machine is coated using a semi-automatic coating machine, and the state of the electrode after coating is observed (whether there are scratches, particles, etc.).

[0079] 3. Slurry stability test: Pour the lithium manganese iron phosphate slurry prepared by the degassing machine into a 100mL beaker and test the initial viscosity. Then, seal the slurry in the beaker with sealing film and place it in a simple glove box filled with N2 for 24 hours. Test the slurry viscosity. The stability data is expressed as the slurry viscosity growth rate. For example, a 20% increase means that the viscosity has increased by 20% from the initial value after 24 hours.

[0080] Table 1 Experimental Results

[0081] According to the data in Table 1, compared with Comparative Example 1 and Comparative Example 4, the present application greatly improved the agglomeration of the material slurry by acid washing followed by fluorination, and the stability of the slurry was also improved, as well as the discharge specific capacity.

[0082] Furthermore, a comparison between Examples 1 and Examples 2-5 revealed that using fluorine gas for fluorination yielded better results, and acid washing with nitric acid and hydrochloric acid together facilitated fluorination.

[0083] Finally, comparing Example 1 with Comparative Examples 2 and 3, it was found that fluorination for 10 hours yielded the best results.

[0084] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.

Claims

1. A method for preparing lithium iron phosphate, a cathode material for lithium-ion batteries, characterized in that, The preparation method includes the following steps: (1) A slurry is obtained by mixing a lithium source, a manganese source, an iron source and a carbon source in water and then grinding them together. The molar ratio of the lithium source to the manganese source is 1:0.2-0.6; the molar ratio of the lithium source to the iron source is 1:0.1-0.5; the molar ratio of the lithium source to the carbon source is 1:0.2-0.5; the iron source is iron phosphate; and the carbon source is one or more of glucose, sucrose and polyethylene glycol. (2) The slurry is spray-dried to obtain a precursor, and then the precursor is heat-treated in a protective atmosphere to obtain lithium manganese iron phosphate cathode material A. (3) The above-mentioned lithium manganese iron phosphate cathode material A is mixed with a carbon source and sand-milled, then spray-dried, and then heat-treated in a protective atmosphere to obtain carbon-coated lithium manganese iron phosphate cathode material B; the mass ratio of lithium manganese iron phosphate cathode material A to carbon source is 1:0.1-0.2; the carbon source is one or more of glucose, sucrose, and polyethylene glycol; (4) The above-mentioned lithium manganese iron phosphate cathode material B is mixed with acid, then filtered, washed and dried to obtain surface-oxidized lithium manganese iron phosphate C; the acid is nitric acid and / or hydrochloric acid; the mass ratio of lithium manganese iron phosphate cathode material B to nitric acid is 1:1.5-2; the mass ratio of lithium manganese iron phosphate cathode material B to hydrochloric acid is 1:0-0.

5. (5) The above-mentioned lithium manganese iron phosphate C is reacted with a fluorine source to obtain the final product D; the reaction temperature is 450-550℃ and the reaction time is 8-12 hours. The fluorine source includes solid fluorine sources or gaseous fluorine sources; solid fluorine sources include fluorosilanes and / or fluorosilane coupling agents; gaseous fluorine sources include one or more of F2, chlorofluorocarbons, CF4, NF3, and SF6.

2. The preparation method according to claim 1, characterized in that, In step (1), the lithium source powder is one or more of LiOH, LiOH·H2O, LiH2PO4, and Li2CO3.

3. The preparation method according to claim 1, characterized in that, In step (1), the manganese source is one or more of manganese tetroxide, manganese trioxide, manganese dioxide, and manganese carbonate.

4. The preparation method according to claim 1, characterized in that, In steps (2) and (3), the inlet air temperature of the spray drying is 160-180°C and the outlet air temperature is 60-90°C.

5. The preparation method according to claim 1, characterized in that, In step (2), the heat treatment temperature is 600-750℃ and the time is 5-10h.

6. The preparation method according to claim 1, characterized in that, In step (3), the heat treatment temperature is 600-750℃ and the time is 5-10h.

7. The preparation method according to claim 1, characterized in that, In step (4), both nitric acid and hydrochloric acid exist in the form of aqueous solutions; in the aqueous solution, the concentration of nitric acid is 60-70 wt% and the concentration of hydrochloric acid is 10-15 wt%.

8. The preparation method according to claim 1, characterized in that, The mixing process in step (4) is carried out at a temperature of 80-90℃ for 2-5 hours.

9. The preparation method according to claim 1, characterized in that, In step (5), if the fluorine source is a solid fluorine source, the mass ratio of lithium manganese iron phosphate C to the fluorine source is 1:0.01-0.005; if the fluorine source is a gaseous fluorine source, the mass ratio of lithium manganese iron phosphate C to the fluorine source is 1:0.001-0.

005.

10. A lithium manganese iron phosphate, characterized in that, The lithium manganese iron phosphate Prepared by the preparation method according to any one of claims 1-9.

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