Dual-coated lithium iron manganese phosphate cathode material, preparation method therefor, and use thereof

Abstract

The present application belongs to the technical field of lithium-ion batteries, and specifically relates to a dual-coated lithium iron manganese phosphate cathode material, a preparation method therefor, and use thereof. Provided in the present application is a preparation method for a dual-coated lithium iron manganese phosphate cathode material, comprising the following steps: (1) mixing an iron source, a manganese source, a compound containing a doping metal, and a precipitant to prepare a manganese-iron precursor, the doping metal comprising at least one of nickel and magnesium; mixing a phosphorus source, a lithium source, and the manganese-iron precursor, and performing first sintering to obtain a lithium iron manganese phosphate cathode material; and (2) mixing the lithium iron manganese phosphate cathode material, a cobalt source, a lithium source, and an additive, performing second sintering, adding a carbon source for mixing, and performing third sintering. The dual-coated lithium iron manganese phosphate cathode material prepared in the present application has good electrical conductivity, relatively high energy density, high specific capacity, and good cycle performance and rate performance.

Description

A double-coated lithium manganese iron phosphate cathode material, its preparation method and application

[0001] Cross-reference to related applications

[0002] This application claims priority to Chinese Patent Application No. 202510382550.2, filed on March 28, 2025, entitled "A Double-Coated Lithium Manganese Iron Phosphate Cathode Material and Its Preparation Method and Application", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application belongs to the field of lithium-ion battery technology, specifically relating to a double-coated lithium manganese iron phosphate cathode material, its preparation method, and its application. Background Technology

[0004] Lithium-ion batteries offer advantages such as high energy density and safety. Ternary cathode materials have high energy density but poor safety, while lithium iron phosphate materials have high safety but low energy density. Lithium manganese iron phosphate materials have both high specific capacity and excellent safety performance, but their slow internal lithium-ion transport results in poor rate performance. Current technologies improve the conductivity and ion diffusion capabilities of materials through primary particle nano-sizing, ion doping, and carbon coating, which can effectively improve the rate performance of the battery, but the capacity and cycle performance of the material will be affected to some extent. Current technologies also combine ternary cathode materials with lithium manganese iron phosphate to improve the capacity and rate performance of the material, but the cycle performance also decreases, leading to accelerated capacity decay and deterioration of battery performance under long-term cycling. Summary of the Invention

[0005] Therefore, the technical problem to be solved by this application is to overcome the defects of poor conductivity, low energy density, poor rate performance and cycle performance of lithium manganese iron phosphate cathode materials in the prior art, thereby providing a double-coated lithium manganese iron phosphate cathode material, its preparation method and application.

[0006] Therefore, this application provides the following technical solution.

[0007] This application provides a method for preparing a double-coated lithium manganese iron phosphate cathode material, comprising the following steps:

[0008] (1) Mix an iron source, a manganese source, a compound containing a doped metal and a precipitant to prepare a manganese iron precursor, wherein the doped metal includes at least one of nickel and magnesium; mix a phosphorus source, a first lithium source and the manganese iron precursor, and perform a first sintering to obtain a lithium manganese iron phosphate cathode material.

[0009] (2) Mix lithium manganese iron phosphate cathode material, cobalt source, second lithium source and additives, sinter the mixture, add carbon source and mix the mixture, and sinter the mixture a third time.

[0010] In one optional embodiment, the particle size of the lithium manganese iron phosphate cathode material is 400-550 nm;

[0011] In one optional embodiment, the preparation steps of the manganese-iron precursor include: mixing an iron source, a manganese source, and a compound containing doped metals, adjusting the pH value with a precipitant, separating, and drying.

[0012] In one optional embodiment, the precipitant is used to adjust the pH value at a temperature of 50-55°C.

[0013] In one optional embodiment, the process further includes pressure filtration prior to drying;

[0014] In one optional embodiment, the particle size of the lithium manganese iron phosphate cathode material is 400-500 nm;

[0015] In one optional embodiment, the iron source includes at least one of ferrous sulfate, ferrous acetate, and ferrous nitrate;

[0016] In one optional embodiment, the manganese source includes at least one of manganese sulfate and manganese chloride;

[0017] In one alternative embodiment, the metal-doped compound includes at least one of a chloride, sulfate, nitrate, acetate, or acetate containing nickel and / or magnesium.

[0018] In one optional embodiment, the precipitant includes at least one of sodium hydroxide and sodium carbonate;

[0019] In one optional embodiment, the molar ratio of iron in the iron source, manganese in the manganese source, and doped metal in the compound containing doped metal is (1-1.01):(0.99-1):(0.01-0.02).

[0020] Optionally, the molar ratio of iron in the iron source, manganese in the manganese source, and doped metal in the compound containing doped metal is 1:1:(0.017-0.019).

[0021] In one alternative implementation, the pH value is 8-10;

[0022] In one optional embodiment, the mixing speed is 500-1000 r / min;

[0023] In one optional embodiment, in step (1), the phosphorus source includes at least one of phosphoric acid, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate;

[0024] In one optional embodiment, step (1) includes the first lithium source comprising at least one of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium oxalate.

[0025] In an optional embodiment, in step (1), the molar ratio of phosphorus in the phosphorus source, lithium in the lithium source, and manganese and iron in the manganese-iron precursor is (1.01-1.02):(1.02-1.03):1.

[0026] In one optional embodiment, in step (2), the cobalt source includes at least one of cobalt oxide, carboxylated cobalt oxide, and cobalt hydroxide;

[0027] In one optional embodiment, in step (2), the second lithium source includes at least one of lithium carbonate, lithium hydroxide, and organolithium.

[0028] In an optional embodiment, in step (2), the additive includes at least one of oxides, hydroxides, chlorides, sulfates, nitrates, acetates, or acetates containing lanthanum, zirconium, or tungsten.

[0029] In an optional embodiment, in step (2), the molar ratio of the lithium manganese iron phosphate cathode material, the cobalt element in the cobalt source, the lithium element in the second lithium source, and the lanthanum, zirconium, or tungsten element in the additive is 1:(0.006-0.008):(0.004-0.006):(0.001-0.002).

[0030] Optionally, the molar ratio of the lithium manganese iron phosphate cathode material, the cobalt element in the cobalt source, the lithium element in the lithium source, and the lanthanum, zirconium, or tungsten element in the additive is 1:(0.007-0.008):(0.004-0.006):(0.001-0.002);

[0031] In one alternative embodiment, the carbon source includes at least one of glucose, sucrose, and polyvinyl alcohol;

[0032] In one alternative embodiment, the amount of carbon source added is 7-8 wt%, based on the mass of the second sintered product.

[0033] In one optional embodiment, the temperature of the first sintering is 800-900°C;

[0034] In one optional embodiment, the first sintering time is 8-10 hours;

[0035] In one alternative embodiment, the first sintering is performed in at least one of nitrogen and an inert atmosphere;

[0036] In one optional embodiment, the temperature of the second sintering is 650-750°C;

[0037] In one optional embodiment, the second sintering time is 6-8 hours;

[0038] In one alternative embodiment, the second sintering is performed in at least one of nitrogen and an inert atmosphere;

[0039] In one optional embodiment, the temperature of the third sintering is 600-700°C;

[0040] In one optional embodiment, the third sintering time is 2-5 hours;

[0041] In one alternative embodiment, the third sintering is performed in at least one of nitrogen and an inert atmosphere.

[0042] In one alternative embodiment, the inert atmosphere of the first sintering includes at least one of helium and neon.

[0043] In one alternative embodiment, the inert atmosphere of the second sintering includes at least one of helium and neon.

[0044] In one alternative embodiment, the inert atmosphere of the third sintering includes at least one of helium and neon.

[0045] In the process of mixing lithium manganese iron phosphate cathode material, cobalt source, lithium source and additives, the mixing is carried out by sand milling;

[0046] Optionally, the milling speed is 800 r / min;

[0047] Optionally, the mixing process further includes drying;

[0048] Optionally, the drying temperature is 450°C.

[0049] This application also provides a double-coated lithium manganese iron phosphate cathode material prepared by the above preparation method.

[0050] This application also provides an application of the double-coated lithium manganese iron phosphate cathode material prepared by the above preparation method in lithium-ion batteries.

[0051] The technical solution of this application has the following advantages:

[0052] 1. The preparation method of the double-coated lithium manganese iron phosphate cathode material provided in this application includes the following steps: (1) mixing an iron source, a manganese source, a compound containing a doped metal, and a precipitant to prepare a manganese iron precursor, wherein the doped metal includes at least one of nickel and magnesium; mixing a phosphorus source, a first lithium source, and the manganese iron precursor, and performing a first sintering to obtain the lithium manganese iron phosphate cathode material; (2) mixing the lithium manganese iron phosphate cathode material, a cobalt source, a second lithium source, and additives, performing a second sintering, adding a carbon source and mixing, and performing a third sintering. The double-coated lithium manganese iron phosphate cathode material obtained in this application has good conductivity and high energy density, high specific capacity, and good cycle performance and rate performance. This application first prepares an ion-doped manganese iron precursor with a hydroxide structure, which exhibits better ion doping uniformity compared to the phosphate structure in existing technologies. The doped ions entering the crystal lattice significantly improve the conductivity and lithium-ion diffusion of the lithium manganese iron phosphate cathode material. Then, a first sintering process is performed to obtain a small-particle-size lithium manganese iron phosphate cathode material. The lithium manganese iron phosphate cathode material, cobalt source, lithium source, and additives are mixed and subjected to a second sintering process, forming a coating layer on the substrate interface that facilitates lithium-ion transport. This not only improves the stability of the material interface but also further enhances the migration ability of lithium ions in the cathode material, thereby improving its rate performance. Finally, carbon material is coated to form a double-coating structure. The carbon coating layer further improves electronic conductivity, inhibits grain growth, and reduces internal resistance and polarization. The double-coating structure effectively improves the stability, conductivity, and energy density of the cathode material, thereby enhancing its cycle performance.

[0053] 2. The preparation method of the double-coated lithium manganese iron phosphate cathode material provided in this application allows for further improvement of the cathode material's capacity and cycle performance by selectively adding dopant ions or additives. The particle size of the lithium manganese iron phosphate cathode material is 400-550 nm, optionally 400-500 nm, which shortens the lithium-ion diffusion path, increases the lithium-ion migration rate, and further enhances the rate performance of the prepared double-coated lithium manganese iron phosphate cathode material. Detailed Implementation

[0054] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0055] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0056] Example 1

[0057] This embodiment provides a method for preparing a double-coated lithium manganese iron phosphate cathode material, including the following steps:

[0058] (1) Ferrous sulfate, manganese sulfate, nickel sulfate and magnesium sulfate are mixed, and the pH of the solution is adjusted to 8-10 with sodium hydroxide. The mixture is stirred at 800 r / min at 50℃ to precipitate. The precipitate is separated, filtered and dried to obtain the manganese-iron precursor. The molar ratio of iron, manganese, nickel and magnesium in the manganese-iron precursor is 1:1:0.006:0.013.

[0059] (2) Phosphoric acid, lithium carbonate and manganese iron precursor were mixed, milled at 800 r / min to a particle size d50 = 499 nm, dried at 450 °C, and sintered at 800 °C for 8 h under a nitrogen atmosphere to obtain lithium manganese iron phosphate cathode material; the molar ratio of phosphorus in phosphoric acid, lithium in lithium carbonate and manganese and iron in manganese iron precursor was 1.01:1.02:1; the particle size of lithium manganese iron phosphate cathode material was 500 nm.

[0060] (3) The lithium manganese iron phosphate cathode material, cobalt oxide, lithium carbonate, and additives (lanthanum oxide and zirconium oxide) were mixed evenly and sintered at 650°C for 6 hours under a nitrogen atmosphere. A protective layer that facilitates lithium-ion transport was formed on the surface of the lithium manganese iron phosphate cathode material. Glucose was added and mixed, and the mixture was sintered at 600°C for 2 hours under a nitrogen atmosphere to obtain a double-coated lithium manganese iron phosphate cathode material. The molar ratio of the lithium manganese iron phosphate cathode material, cobalt in cobalt oxide, lithium in lithium carbonate, lanthanum in lanthanum oxide, and zirconium in zirconium oxide was 1:0.008:0.004:0.001:0.001; the mass ratio of the lithium manganese iron phosphate cathode material and the surface protective layer to glucose (calculated based on carbon content) was 1:0.08.

[0061] Example 2

[0062] This embodiment provides a method for preparing a double-coated lithium manganese iron phosphate cathode material. Compared with Example 1, the only difference is that in step (1), the molar ratio of iron, manganese, nickel and magnesium in the manganese iron precursor is 1:1:0.004:0.008 instead of the molar ratio of iron, manganese, nickel and magnesium in the manganese iron precursor of Example 1, which is 1:1:0.006:0.013.

[0063] Example 3

[0064] This embodiment provides a method for preparing a double-coated lithium manganese iron phosphate cathode material. Compared with Example 1, the only difference is that in step (2), grinding to a particle size d50 = 399 nm is used instead of grinding to a particle size d50 = 499 nm in Example 1; the particle size of the obtained lithium manganese iron phosphate cathode material is 400 nm.

[0065] Example 4

[0066] This embodiment provides a method for preparing a double-coated lithium manganese iron phosphate cathode material. Compared with Example 1, the only difference is that in step (3), the molar ratio of lithium manganese iron phosphate cathode material, cobalt in cobalt oxide, lithium in lithium carbonate, lanthanum in lanthanum oxide, and zirconium in zirconium oxide is 1:0.006:0.004:0.001:0.001 instead of the molar ratio of lithium manganese iron phosphate cathode material, cobalt in cobalt oxide, lithium in lithium carbonate, lanthanum in lanthanum oxide, and zirconium in zirconium oxide of Example 1, which is 1:0.008:0.004:0.001:0.001.

[0067] Example 5

[0068] This embodiment provides a method for preparing a double-coated lithium manganese iron phosphate cathode material. Compared with Example 1, the only difference is that in step (4), the mass ratio of the lithium manganese iron phosphate cathode material and the surface protective layer to glucose (calculated based on carbon content) is 1:0.07 instead of the mass ratio of the lithium manganese iron phosphate cathode material and the surface protective layer to glucose (calculated based on carbon content) of Example 1, which is 1:0.08.

[0069] Example 6

[0070] This embodiment provides a method for preparing a double-coated lithium manganese iron phosphate cathode material, including the following steps:

[0071] (1) Ferrous acetate, manganese chloride, nickel chloride and magnesium chloride are mixed, and the pH of the solution is adjusted to 8-10 with sodium carbonate. The mixture is stirred at 1000 r / min at 50℃ to precipitate. The precipitate is separated, filtered and dried to obtain the manganese-iron precursor. The molar ratio of iron, manganese, nickel and magnesium in the manganese-iron precursor is 1:1:0.006:0.013.

[0072] (2) Mix ammonium dihydrogen phosphate, lithium hydroxide and manganese iron precursor, grind at 800 r / min to a particle size d50 = 499 nm, dry at 450 °C, and sinter at 900 °C for 10 h under nitrogen atmosphere to obtain lithium manganese iron phosphate cathode material; the molar ratio of phosphorus in ammonium dihydrogen phosphate, lithium in lithium hydroxide and manganese and iron in manganese iron precursor is 1.02:1.03:1; the particle size of lithium manganese iron phosphate cathode material is 500 nm.

[0073] (3) The lithium manganese iron phosphate cathode material, cobalt hydroxide, lithium hydroxide, and additives (lanthanum oxide and tungsten oxide) were mixed evenly and sintered at 750°C for 8 hours under a nitrogen atmosphere. A protective layer that facilitates lithium-ion transport was formed on the surface of the lithium manganese iron phosphate cathode material. Sucrose was added and mixed, and the mixture was sintered at 700°C for 5 hours under a nitrogen atmosphere to obtain a double-coated lithium manganese iron phosphate cathode material. The molar ratio of the lithium manganese iron phosphate cathode material, cobalt in cobalt oxide, lithium in lithium carbonate, lanthanum in lanthanum oxide, and tungsten in tungsten oxide was 1:0.008:0.006:0.0005:0.0005; the mass ratio of the lithium manganese iron phosphate cathode material and the surface protective layer to glucose (calculated based on carbon content) was 1:0.08.

[0074] Comparative Example 1

[0075] This comparative example provides a method for preparing lithium manganese iron phosphate cathode material. The only difference from Example 1 is that in step (1), nickel sulfate and magnesium sulfate are not added.

[0076] Comparative Example 2

[0077] This comparative example provides a method for preparing a carbon-coated lithium manganese iron phosphate cathode material. Compared with Example 1, the only difference is that step (3) in Example 1 is not performed. Specifically, it includes the following steps:

[0078] (1) Mix ferrous sulfate, manganese sulfate, nickel sulfate and magnesium sulfate, adjust the pH of the solution to 8-10 with sodium hydroxide, and stir at 500 r / min at 50℃ to precipitate; separate the precipitate, filter and dry to obtain manganese-iron precursor; wherein the molar ratio of iron, manganese, nickel and magnesium in manganese-iron precursor is 1:1:0.006:0.013;

[0079] (2) Phosphoric acid, lithium carbonate and manganese iron precursor were mixed, milled at 800 r / min to a particle size d50 = 499 nm, dried at 450 °C, and sintered at 800 °C for 8 h under a nitrogen atmosphere to obtain lithium manganese iron phosphate cathode material; the molar ratio of phosphorus in phosphoric acid, lithium in lithium carbonate and manganese and iron in manganese iron precursor was 1.01:1.02:1; the particle size of lithium manganese iron phosphate cathode material was 500 nm.

[0080] (3) The lithium manganese iron phosphate cathode material and glucose were mixed and sintered at 600°C for 2 hours under a nitrogen atmosphere to obtain a carbon-coated lithium manganese iron phosphate cathode material. The mass ratio of the lithium manganese iron phosphate cathode material to glucose (calculated based on carbon content) was 1:0.08.

[0081] Comparative Example 3

[0082] This comparative example provides a method for preparing lithium manganese iron phosphate cathode material. Compared with Example 1, the only difference is that step (4) in Example 1 is not performed. Specifically, it includes the following steps:

[0083] (1) Mix ferrous sulfate, manganese sulfate, nickel sulfate and magnesium sulfate, adjust the pH of the solution to 8-10 with sodium hydroxide, and stir at 500 r / min at 50℃ to precipitate; separate the precipitate, filter and dry to obtain manganese-iron precursor; wherein the molar ratio of iron, manganese, nickel and magnesium in manganese-iron precursor is 1:1:0.006:0.013;

[0084] (2) Phosphoric acid, lithium carbonate and manganese iron precursor were mixed, milled at 800 r / min to a particle size d50 = 499 nm, dried at 450 °C, and sintered at 800 °C for 8 h under a nitrogen atmosphere to obtain lithium manganese iron phosphate cathode material; the molar ratio of phosphorus in phosphoric acid, lithium in lithium carbonate and manganese and iron in manganese iron precursor was 1.01:1.02:1; the particle size of lithium manganese iron phosphate cathode material was 500 nm.

[0085] (3) Mix the lithium manganese iron phosphate cathode material, cobalt oxide, lithium carbonate and additives (lanthanum oxide and zirconium oxide) evenly, and sinter at 650°C for 6 hours under a nitrogen atmosphere to form a protective layer on the surface of the lithium manganese iron phosphate cathode material that facilitates lithium-ion transport; wherein, the molar ratio of cobalt in the lithium manganese iron phosphate cathode material, cobalt in the cobalt oxide, lithium in the lithium carbonate, lanthanum in the lanthanum oxide and zirconium in the zirconium oxide is 1:0.008:0.004:0.001:0.001.

[0086] Comparative Example 4

[0087] This comparative example provides a method for preparing lithium manganese iron phosphate cathode material. Compared with Example 1, the only difference is that steps (3) and (4) in Example 1 are not performed. Specifically, the method includes the following steps:

[0088] (1) Mix ferrous sulfate, manganese sulfate, nickel sulfate and magnesium sulfate, adjust the pH of the solution to 8-10 with sodium hydroxide, and stir at 500 r / min at 50℃ to precipitate; separate the precipitate, filter and dry to obtain manganese-iron precursor; wherein the molar ratio of iron, manganese, nickel and magnesium in manganese-iron precursor is 1:1:0.006:0.013;

[0089] (2) Phosphoric acid, lithium carbonate, and manganese iron precursor were mixed, milled at 800 r / min to a particle size d50 = 499 nm, dried at 450 °C, and sintered at 800 °C for 8 h under a nitrogen atmosphere to obtain lithium manganese iron phosphate cathode material; the molar ratio of phosphorus in phosphoric acid, lithium in lithium carbonate, and manganese and iron in the manganese iron precursor was 1.01:1.02:1. The particle size of the lithium manganese iron phosphate cathode material was 500 nm.

[0090] Test case

[0091] The performance of the lithium iron phosphate cathode materials prepared in the examples and comparative examples was tested as follows:

[0092] Lithium manganese iron phosphate cathode material (from various examples and comparative examples), polyvinylidene fluoride (PVDF), and acetylene black were mixed in a mass ratio of 93:2:5. An appropriate amount of N-methylpyrrolidone (NMP) was added to prepare a mixed cathode slurry with a certain degree of fluidity. Graphite, binder LA133, and carbon nanotubes (CNTs) were mixed in a mass ratio of 92:6:2 to prepare a mixed anode slurry with a certain degree of fluidity. The cathode and anode slurries were then processed through coating, roller pressing, slicing, winding, assembly, liquid injection, formation, secondary sealing, and capacity testing to produce the finished soft-pack battery.

[0093] The performance of the batteries prepared above was tested, as follows:

[0094] (1) Test method for the first charge specific capacity and the first discharge specific capacity: At 25℃, the soft pack battery is charged at 0.1C and discharged at 0.1C once, with a voltage range of 2.0V-4.5V. The results are shown in Table 1.

[0095] (2) First-effect test method: The calculation formula is as follows, and the results are shown in Table 1.

[0096] First-time efficiency = First discharge specific capacity / First charge specific capacity;

[0097] (3) Capacity retention test method: At 45℃, the soft-pack battery is charged at 1C and discharged at 1C once, cycled for 500 times, with a voltage range of 3V-4.50V; the results are shown in Table 1; the specific calculation formula is as follows:

[0098] The capacity retention rate in week X = (discharge specific capacity in week X / discharge specific capacity in week 1) × 100%, where X = 100, 200, 300, 400 or 500.

[0099] (4) Rate performance test method: At 25℃, the soft pack battery is charged at 0.1C and discharged at 0.1C until complete formation, then charged / discharged at 0.5C, discharged at 1C, and discharged at 3C. The results are shown in Table 2. The specific calculation formula is as follows:

[0100] nC rate performance = nC discharge specific capacity / first cycle 0.1C discharge specific capacity, where n = 0.5, 1 or 3.

[0101] Table 1. Performance test results of the batteries prepared in the examples and comparative examples.

[0102] Table 2. Rate performance test results of the batteries prepared in the examples and comparative examples.

[0103] As shown in Tables 1-2, the double-coated lithium manganese iron phosphate cathode material prepared in this application has high specific capacity, good capacity retention, and good rate performance. As can be seen from the comparison of the examples and comparative examples, the double-coated lithium manganese iron phosphate cathode material prepared in this application has significantly improved specific capacity, cycle performance, and rate performance due to the internal bulk doping elements and the external double-layer coating of the protective layer and carbon coating layer that facilitate lithium ion transport. This indicates that the double-coated lithium manganese iron phosphate cathode material has good conductivity and energy density.

[0104] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a double-coated lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: (1) Mix an iron source, a manganese source, a compound containing a doped metal and a precipitant to prepare a manganese iron precursor, wherein the doped metal includes at least one of nickel and magnesium; mix a phosphorus source, a first lithium source and the manganese iron precursor, and perform a first sintering to obtain a lithium manganese iron phosphate cathode material. (2) Mix lithium manganese iron phosphate cathode material, cobalt source, second lithium source and additives, sinter for the second time, add carbon source and mix, and sinter for the third time.

2. The double-coated lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The lithium manganese iron phosphate cathode material has a particle size of 400-550 nm; and / or, The preparation steps of the manganese-iron precursor include: mixing an iron source, a manganese source, and a compound containing doped metals, adjusting the pH value with a precipitant, separating, and drying.

3. The double-coated lithium manganese iron phosphate cathode material according to claim 2, characterized in that, The lithium manganese iron phosphate cathode material has a particle size of 400-500 nm; and / or, The iron source includes at least one of ferrous sulfate, ferrous acetate, and ferrous nitrate; and / or, The manganese source includes at least one of manganese sulfate and manganese chloride; and / or, The metal-doped compound includes at least one of nickel- and / or magnesium-containing chlorides, sulfates, nitrates, acetates, or acetates; and / or, The precipitant includes at least one of sodium hydroxide and sodium carbonate; and / or, The molar ratio of iron in the iron source, manganese in the manganese source, and doped metal in the compound containing the doped metal is (1-1.01):(0.99-1):(0.01-0.02); and / or, The pH value is 8-10; and / or, The mixing speed is 500-1000 r / min.

4. The double-coated lithium manganese iron phosphate cathode material according to any one of claims 1-3, characterized in that, In step (1), the phosphorus source includes at least one of phosphoric acid, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate; and / or, In step (1), the first lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium oxalate; and / or, In step (1), the molar ratio of phosphorus in the phosphorus source, lithium in the first lithium source, and manganese and iron in the manganese-iron precursor is (1.01-1.02):(1.02-1.03):

1.

5. The double-coated lithium manganese iron phosphate cathode material according to any one of claims 1-4, characterized in that, In step (2), the cobalt source includes at least one of cobalt oxide, carboxylated cobalt oxide, and cobalt hydroxide; and / or, In step (2), the second lithium source includes at least one of lithium carbonate, lithium hydroxide, and organolithium compounds; and / or, In step (2), the additive includes at least one of oxides, hydroxides, chlorides, sulfates, nitrates, acetates, or acetates containing lanthanum, zirconium, or tungsten; and / or, In step (2), the molar ratio of the lithium manganese iron phosphate cathode material, the cobalt element in the cobalt source, the lithium element in the second lithium source, and the lanthanum, zirconium, or tungsten element in the additive is 1:(0.006-0.008):(0.004-0.006):(0.001-0.002).

6. The double-coated lithium manganese iron phosphate cathode material according to any one of claims 1-5, characterized in that, The carbon source includes at least one of glucose, sucrose, and polyvinyl alcohol; and / or, Based on the mass of the second sintered product, the amount of carbon source added is 7-8 wt%.

7. The double-coated lithium manganese iron phosphate cathode material according to any one of claims 1-6, characterized in that, The first sintering temperature is 800-900℃; and / or, The first sintering time is 8-10 hours; and / or, The first sintering is carried out in at least one of nitrogen and inert atmospheres; and / or, The second sintering temperature is 650-750℃; and / or, The second sintering time is 6-8 hours; and / or, The second sintering is carried out in at least one of nitrogen and inert atmospheres; and / or, The temperature of the third sintering is 600-700℃; and / or, The third sintering time is 2-5 hours; and / or, The third sintering is carried out in at least one of nitrogen and inert atmosphere.

8. The double-coated lithium manganese iron phosphate cathode material according to claim 7, characterized in that, The inert atmosphere of the first sintering includes at least one of helium and neon; and / or, The inert atmosphere for the second sintering includes at least one of helium and neon; and / or, The inert atmosphere of the third sintering includes at least one of helium and neon.

9. The double-coated lithium manganese iron phosphate cathode material prepared by the preparation method according to any one of claims 1-8.

10. The application of the double-coated lithium manganese iron phosphate cathode material prepared by the preparation method according to any one of claims 1-8 in lithium-ion batteries.

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