Double-coated lithium iron manganese phosphate positive electrode material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a double-coated lithium iron manganese phosphate positive electrode material as well as a preparation method and application thereof. The invention provides a preparation method of a double-coated lithium manganese iron phosphate positive electrode material, which comprises the following steps: (1) mixing an iron source, a manganese source, a compound containing doped metal and a precipitator to prepare a manganese iron precursor, the doped metal comprising at least one of nickel and magnesium; mixing a phosphorus source, a lithium source and the ferromanganese precursor, and performing first sintering to obtain a lithium ferromanganese phosphate positive electrode material; and (2) mixing the lithium manganese iron phosphate positive electrode material, a cobalt source, a lithium source and an additive, carrying out second sintering, adding a carbon source, mixing, and carrying out third sintering. The double-coated lithium manganese iron phosphate positive electrode material prepared by the invention has relatively good conductivity, relatively high energy density, high specific capacity and good cycle performance and rate capability.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a double-coated lithium iron manganese phosphate cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium-ion batteries have advantages such as high energy density and safety. The energy density of ternary cathode materials is high but the safety is poor. The safety of lithium iron phosphate materials is high but the energy density is low. Lithium iron manganese phosphate materials have both a relatively high specific capacity and excellent safety performance. However, the internal lithium ion transmission is slow, resulting in poor rate performance. In the prior art, the conductivity and ion diffusion ability of materials are improved by primary particle nanosizing, ion doping, and carbon coating technologies. Although the rate performance of the battery can be effectively improved, the capacity and cycle performance of the materials will be affected to a certain extent. The prior art also combines ternary cathode materials and lithium iron manganese phosphate to improve the capacity and rate performance of the materials, but the cycle performance also decreases, resulting in increased capacity attenuation under long cycles and poor battery performance. Summary of the Invention

[0003] Therefore, the technical problem to be solved by the present invention is to overcome the defects of poor conductivity, low energy density, poor rate performance, and poor cycle performance of the lithium iron manganese phosphate cathode material in the prior art, so as to provide a double-coated lithium iron manganese phosphate cathode material, a preparation method thereof, and an application thereof.

[0004] To this end, the present invention provides the following technical solutions.

[0005] The present invention provides a preparation method of a double-coated lithium iron manganese phosphate cathode material, comprising the following steps:

[0006] (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 first sintering to obtain a lithium iron manganese phosphate cathode material;

[0007] (2) Mix the lithium iron manganese phosphate cathode material, a cobalt source, a second lithium source, and an additive, perform second sintering, add a carbon source and mix, and perform third sintering.

[0008] In an optional embodiment, the particle size of the lithium iron manganese phosphate cathode material is 400-550 nm;

[0009] In an optional embodiment, the preparation steps of the manganese-iron precursor include: mixing an iron source, a manganese source, and a compound containing a doped metal, adjusting the pH value with a precipitant, separating, and drying.

[0010] In an optional embodiment, the temperature for adjusting the pH value with the precipitant is 50-55 °C;

[0011] In an alternative embodiment, pressure filtration is further included before drying;

[0012] In an alternative embodiment, the particle size of the lithium iron manganese phosphate cathode material is 400 - 500 nm;

[0013] In an alternative embodiment, the iron source includes at least one of ferrous sulfate, ferrous acetate, and ferrous nitrate;

[0014] In an alternative embodiment, the manganese source includes at least one of manganese sulfate and manganese chloride;

[0015] In an alternative embodiment, the compound containing a doped metal includes at least one of chlorides, sulfates, nitrates, acetates, or acetic acid salts containing nickel and / or magnesium elements;

[0016] In an alternative embodiment, the precipitating agent includes at least one of sodium hydroxide and sodium carbonate;

[0017] In an alternative embodiment, the molar ratio of iron element in the iron source, manganese element in the manganese source, and doped metal in the compound containing a doped metal is (1 - 1.01):(0.99 - 1):(0.01 - 0.02);

[0018] Preferably, the molar ratio of iron element in the iron source, manganese element in the manganese source, and doped metal in the compound containing a doped metal is 1:1:(0.017 - 0.019);

[0019] In an alternative embodiment, the pH value is 8 - 10;

[0020] In an alternative embodiment, the rotation speed of the mixing is 500 - 1000 r / min;

[0021] In an alternative embodiment, in step (1), the phosphorus source includes at least one of phosphoric acid, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate;

[0022] In an alternative embodiment, in step (1), the first lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium oxalate;

[0023] In an alternative embodiment, in step (1), the molar ratio of phosphorus element in the phosphorus source, lithium element in the lithium source to the sum of manganese element and iron element in the manganese - iron precursor is (1.01 - 1.02):(1.02 - 1.03):1.

[0024] In an alternative embodiment, in step (2), the cobalt source includes at least one of cobalt oxide, cobalt oxyhydroxide, and cobalt hydroxide;

[0025] In an alternative embodiment, in step (2), the second lithium source includes at least one of lithium carbonate, lithium hydroxide, and organolithium;

[0026] In an alternative embodiment, in step (2), the additive includes at least one of oxides, hydroxides, chlorides, sulfates, nitrates, acetates, or acetic acid salts containing lanthanum, zirconium, or tungsten elements;

[0027] In an alternative embodiment, in step (2), the molar ratio of the lithium iron manganese phosphate cathode material, cobalt element in the cobalt source, lithium element in the second lithium source, and lanthanum, zirconium, or tungsten element in the additive is 1: (0.006 - 0.008): (0.004 - 0.006): (0.001 - 0.002).

[0028] Preferably, the molar ratio of the lithium iron manganese phosphate cathode material, cobalt element in the cobalt source, lithium element in the lithium source, and lanthanum or zirconium or tungsten element in the additive is 1: (0.007 - 0.008): (0.004 - 0.006): (0.001 - 0.002);

[0029] In an alternative embodiment, the carbon source includes at least one of glucose, sucrose, and polyvinyl alcohol;

[0030] In an alternative embodiment, based on the mass of the second sintered product, the addition amount of the carbon source is 7 - 8 wt%.

[0031] In an alternative embodiment, the temperature of the first sintering is 800 - 900 °C;

[0032] In an alternative embodiment, the time of the first sintering is 8 - 10 h;

[0033] In an alternative embodiment, the first sintering is carried out in at least one of nitrogen and an inert atmosphere;

[0034] In an alternative embodiment, the temperature of the second sintering is 650 - 750 °C;

[0035] In an alternative embodiment, the time of the second sintering is 6 - 8 h;

[0036] In an alternative embodiment, the second sintering is carried out in at least one of nitrogen and an inert atmosphere;

[0037] In an alternative embodiment, the temperature of the third sintering is 600 - 700 °C;

[0038] In an alternative embodiment, the time of the third sintering is 2 - 5 h;

[0039] In an alternative embodiment, the third sintering is carried out in at least one of nitrogen and an inert atmosphere.

[0040] In an alternative embodiment, the inert atmosphere for the first sintering includes at least one of helium and neon;

[0041] In an alternative embodiment, the inert atmosphere for the second sintering includes at least one of helium and neon;

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

[0043] In the mixing of the lithium iron manganese phosphate cathode material, the cobalt source, the lithium source and the additive, the mixing is carried out by sand milling;

[0044] Preferably, the rotation speed of the sand milling is 800 r / min;

[0045] Preferably, drying is further included after the mixing;

[0046] Preferably, the temperature of the drying is 450 °C.

[0047] The present invention also provides a double-coated lithium iron manganese phosphate cathode material prepared by the above preparation method.

[0048] The present invention also provides an application of the double-coated lithium iron manganese phosphate cathode material prepared by the above preparation method in a lithium ion battery.

[0049] The technical solution of the present invention has the following advantages:

[0050] 1. The preparation method of the double-coated lithium iron manganese phosphate cathode material provided by the present invention includes the following steps: (1) Mix an iron source, a manganese source, a compound containing a doped metal, and a precipitating agent to prepare a manganese-iron precursor, and 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 the first sintering to obtain the lithium iron manganese phosphate cathode material; (2) Mix the lithium iron manganese phosphate cathode material, a cobalt source, a second lithium source, and an additive, perform the second sintering, add a carbon source and mix, and perform the third sintering. The double-coated lithium iron manganese phosphate cathode material prepared by the present invention has good electrical conductivity and high energy density, high specific capacity, good cycle performance and rate performance. By first preparing a manganese-iron precursor with ion doping, which has a hydroxide structure, compared with the phosphate structure in the prior art, the ion doping uniformity is better, and the doped ions enter the lattice, greatly improving the electrical conductivity and lithium ion diffusivity of the lithium iron manganese phosphate cathode material; then perform the first sintering to obtain a lithium iron manganese phosphate cathode material with a small particle size; mix the lithium iron manganese phosphate cathode material, a cobalt source, a lithium source, and an additive, perform the second sintering, and form a coating layer on the substrate interface that helps lithium ion transport, which can not only improve the stability of the material interface, but also further improve the migration ability of lithium ions in the cathode material, thereby improving its rate performance; then coat with a carbon material to form a double-coated structure, and the carbon coating layer will also improve the electronic conductivity, inhibit grain growth, reduce internal resistance and polarization. The double-coated structure effectively improves the stability, electrical conductivity and energy density of the cathode material, and further improves the cycle performance of the cathode material.

[0051] 2. In the preparation method of the double-coated lithium iron manganese phosphate cathode material provided by the present invention, preferably, the addition amount of the doped ions or the additive can further improve the capacity and cycle performance of the cathode material. The particle size of the lithium iron manganese phosphate cathode material is 400-550 nm, preferably 400-500 nm, which can shorten the diffusion path of lithium ions, improve the migration rate of lithium ions, and further improve the rate performance of the prepared double-coated lithium iron manganese phosphate cathode material. Specific Embodiments

[0052] The following embodiments are provided to better understand the present invention further. They are not limited to the best embodiment, and do not limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior art features falls within the protection scope of the present invention.

[0053] For those not specifying specific experimental steps or conditions in the embodiments, the operations or conditions of the conventional experimental steps described in the literature in this field can be followed. For the reagents or instruments not indicating the manufacturer, they are all conventional reagent products that can be obtained through commercial purchase.

[0054] Example 1

[0055] This embodiment provides a method for preparing a double-coated lithium iron manganese phosphate cathode material, comprising the following steps:

[0056] (1) Mix ferrous sulfate, manganese sulfate, nickel sulfate, and magnesium sulfate, adjust the pH of the solution to 8 - 10 with sodium hydroxide, stir and precipitate at 50°C at 800 r / min; separate the precipitate, filter press and dry to obtain a manganese-iron precursor; wherein, the molar ratio of iron element, manganese element, nickel element, and magnesium element in the manganese-iron precursor is 1:1:0.006:0.013;

[0057] (2) Mix phosphoric acid, lithium carbonate, and the manganese-iron precursor, grind to a particle size d50 = 499 nm at 800 r / min, dry at 450°C, and sinter for 8 h at 800°C in a nitrogen atmosphere for the first time to obtain a lithium iron manganese phosphate cathode material; the molar ratio of phosphorus element in phosphoric acid, lithium element in lithium carbonate to the sum of manganese element and iron element in the manganese-iron precursor is 1.01:1.02:1; the particle size of the lithium iron manganese phosphate cathode material is 500 nm.

[0058] (3) Mix the lithium iron manganese phosphate cathode material, cobalt oxide, lithium carbonate, and additives (lanthanum oxide and zirconium oxide) evenly, sinter for 6 h at 650°C in a nitrogen atmosphere for the second time to form a protective layer on the surface of the lithium iron manganese phosphate cathode material that helps lithium ion transport, add glucose and mix, and sinter for 2 h at 600°C in a nitrogen atmosphere for the third time to obtain a double-coated lithium iron manganese phosphate cathode material. Among them, the molar ratio of the lithium iron manganese phosphate cathode material, cobalt element in cobalt oxide, lithium element in lithium carbonate, lanthanum element in lanthanum oxide, and zirconium element in zirconium oxide is 1:0.008:0.004:0.001:0.001; the mass ratio of the sum of the mass of the lithium iron manganese phosphate cathode material and the surface protective layer to glucose (calculated based on carbon content) is 1:0.08.

[0059] Example 2

[0060] This embodiment provides a method for preparing a double-coated lithium iron manganese phosphate cathode material. Compared with Example 1, the difference is only that in step (1), the molar ratio of iron element, manganese element, nickel element, and magnesium element in the manganese-iron precursor is 1:1:0.004:0.008 instead of 1:1:0.006:0.013 in the manganese-iron precursor of Example 1.

[0061] Example 3

[0062] This embodiment provides a method for preparing a double-coated lithium iron manganese phosphate cathode material. Compared with Example 1, the difference is only that in step (2), grind to a particle size d50 = 399 nm instead of grinding to a particle size d50 = 499 nm in Example 1; the particle size of the obtained lithium iron manganese phosphate cathode material is 400 nm.

[0063] Example 4

[0064] This example provides a method for preparing a double-coated lithium iron manganese phosphate cathode material. Compared with Example 1, the only difference is that in step (3), the molar ratio of lithium iron manganese phosphate cathode material, cobalt element in cobalt oxide, lithium element in lithium carbonate, lanthanum element in lanthanum oxide, and zirconium element in zirconium oxide is 1:0.006:0.004:0.001:0.001, instead of the molar ratio of 1:0.008:0.004:0.001:0.001 in Example 1.

[0065] Example 5

[0066] This example provides a method for preparing a double-coated lithium iron manganese phosphate cathode material. Compared with Example 1, the only difference is that in step (4), the mass ratio of the sum of the mass of lithium iron manganese 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 1:0.08 in Example 1.

[0067] Example 6

[0068] This example provides a method for preparing a double-coated lithium iron manganese phosphate cathode material, which includes the following steps:

[0069] (1) Mix ferrous acetate, manganese chloride, nickel chloride, and magnesium chloride, adjust the pH of the solution to 8 - 10 with sodium carbonate, stir and precipitate at 50 °C at 1000 r / min; separate the precipitate, filter press and dry to obtain a manganese-iron precursor; among them, the molar ratio of iron element, manganese element, nickel element, and magnesium element in the manganese-iron precursor is 1:1:0.006:0.013;

[0070] (2) Mix ammonium dihydrogen phosphate, lithium hydroxide, and the manganese-iron precursor, grind to a particle size of d50 = 499 nm at 800 r / min, dry at 450 °C, and sinter for 10 h at 900 °C in a nitrogen atmosphere to obtain a lithium iron manganese phosphate cathode material; the molar ratio of phosphorus element in ammonium dihydrogen phosphate, lithium element in lithium hydroxide to the sum of manganese element and iron element in the manganese-iron precursor is 1.02:1.03:1; the particle size of the lithium iron manganese phosphate cathode material is 500 nm.

[0071] (3) Mix the lithium iron manganese phosphate cathode material, cobaltous hydroxide, lithium hydroxide, and additives (lanthanum oxide and tungsten oxide) evenly, and perform the second sintering at 750 °C for 8 h under a nitrogen atmosphere to form a protective layer on the surface of the lithium iron manganese phosphate cathode material that helps with lithium ion transportation. Then add sucrose and mix, and perform the third sintering at 700 °C for 5 h under a nitrogen atmosphere to obtain a double-coated lithium iron manganese phosphate cathode material. Among them, the molar ratio of the lithium iron manganese phosphate cathode material, cobalt element in cobalt oxide, lithium element in lithium carbonate, lanthanum element in lanthanum oxide, and tungsten element in tungsten oxide is 1:0.008:0.006:0.0005:0.0005; the mass ratio of the sum of the mass of the lithium iron manganese phosphate cathode material and the surface protective layer to glucose (calculated based on carbon content) is 1:0.08.

[0072] Comparative Example 1

[0073] This comparative example provides a preparation method of a lithium iron manganese phosphate cathode material. Compared with Example 1, the only difference is that in step (1), nickel sulfate and magnesium sulfate are not added.

[0074] Comparative Example 2

[0075] This comparative example provides a preparation method of a carbon-coated lithium iron manganese phosphate cathode material. Compared with Example 1, the only difference is that step (3) in Example 1 is not carried out, and it specifically includes the following steps:

[0076] (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 and precipitate at 50 °C at 500 r / min; separate the precipitate, filter press and dry to obtain a manganese-iron precursor; among them, the molar ratio of iron element, manganese element, nickel element, and magnesium element in the manganese-iron precursor is 1:1:0.006:0.013;

[0077] (2) Mix phosphoric acid, lithium carbonate, and the manganese-iron precursor, grind it to a particle size of d50 = 499 nm at 800 r / min, dry at 450 °C, and perform the first sintering at 800 °C for 8 h under a nitrogen atmosphere to obtain a lithium iron manganese phosphate cathode material; the molar ratio of phosphorus element in phosphoric acid, lithium element in lithium carbonate to the sum of manganese element and iron element in the manganese-iron precursor is 1.01:1.02:1; the particle size of the lithium iron manganese phosphate cathode material is 500 nm.

[0078] (3) Mix the lithium iron manganese phosphate cathode material and glucose, and perform the third sintering at 600 °C for 2 h under a nitrogen atmosphere to obtain a carbon-coated lithium iron manganese phosphate cathode material. Among them, the mass ratio of the lithium iron manganese phosphate cathode material and glucose (calculated based on carbon content) is 1:0.08.

[0079] Comparative Example 3

[0080] This comparative example provides a method for preparing a lithium iron manganese phosphate cathode material. Compared with Example 1, the only difference is that step (4) in Example 1 is not carried out. The specific steps are as follows:

[0081] (1) Mix ferrous sulfate, manganese sulfate, nickel sulfate, and magnesium sulfate, adjust the pH of the solution to 8 - 10 with sodium hydroxide, stir and precipitate at 50°C at 500 r / min; separate the precipitate, filter press and dry to obtain a manganese-iron precursor; the molar ratio of iron element, manganese element, nickel element, and magnesium element in the manganese-iron precursor is 1:1:0.006:0.013;

[0082] (2) Mix phosphoric acid, lithium carbonate, and the manganese-iron precursor, grind to a particle size of d50 = 499 nm at 800 r / min, dry at 450°C, and sinter for 8 h at 800°C in a nitrogen atmosphere for the first time to obtain a lithium iron manganese phosphate cathode material; the molar ratio of phosphorus element in phosphoric acid, lithium element in lithium carbonate, and the sum of manganese element and iron element in the manganese-iron precursor is 1.01:1.02:1; the particle size of the lithium iron manganese phosphate cathode material is 500 nm.

[0083] (3) Mix the lithium iron manganese phosphate cathode material, cobalt oxide, lithium carbonate, and additives (lanthanum oxide and zirconium oxide) evenly, and sinter for 6 h at 650°C in a nitrogen atmosphere for the second time to form a protective layer on the surface of the lithium iron manganese phosphate cathode material that helps lithium ion transport; among them, the molar ratio of the lithium iron manganese phosphate cathode material, cobalt element in cobalt oxide, lithium element in lithium carbonate, lanthanum element in lanthanum oxide, and zirconium element in zirconium oxide is 1:0.008:0.004:0.001:0.001.

[0084] Comparative Example 4

[0085] This comparative example provides a method for preparing a lithium iron manganese phosphate cathode material. Compared with Example 1, the only differences are that steps (3) and (4) in Example 1 are not carried out. The specific steps are as follows:

[0086] (1) Mix ferrous sulfate, manganese sulfate, nickel sulfate, and magnesium sulfate, adjust the pH of the solution to 8 - 10 with sodium hydroxide, stir and precipitate at 50°C at 500 r / min; separate the precipitate, filter press and dry to obtain a manganese-iron precursor; the molar ratio of iron element, manganese element, nickel element, and magnesium element in the manganese-iron precursor is 1:1:0.006:0.013;

[0087] (2) Mix phosphoric acid, lithium carbonate and manganese-iron precursor, grind them at 800 r / min until the particle size d50 = 499 nm, dry at 450 °C, and sinter for the first time at 800 °C for 8 h under a nitrogen atmosphere to obtain the lithium iron manganese phosphate cathode material; the molar ratio of phosphorus element in phosphoric acid, lithium element in lithium carbonate to the sum of manganese element and iron element in the manganese-iron precursor is 1.01:1.02:1. The particle size of the lithium iron manganese phosphate cathode material is 500 nm.

[0088] Test Example

[0089] Perform performance tests on the lithium iron manganese phosphate cathode materials prepared in the examples and comparative examples, as follows:

[0090] Mix the lithium iron manganese phosphate cathode material (from each example and comparative example), polyvinylidene fluoride (PVDF) and acetylene black in a mass ratio of 93:2:5, add an appropriate amount of N-methylpyrrolidone (NMP) to prepare a mixed cathode slurry with certain fluidity, mix graphite, binder LA133 and carbon nanotube CNT in a mass ratio of 92:6:2 to prepare a mixed anode slurry with certain fluidity, and then prepare a finished soft-pack battery through processes such as coating - double-roll - slicing - winding - assembling - injecting electrolyte - forming - second sealing - grading.

[0091] Perform performance tests on the batteries prepared above, as follows:

[0092] (1) Test method for the first charge specific capacity and the first discharge specific capacity: At 25 °C, charge the soft-pack battery at 0.1C and discharge it at 0.1C once, with the voltage range of 2.0V - 4.5V. The results are shown in Table 1.

[0093] (2) Test method for the first efficiency: The calculation formula is as follows, and the results are shown in Table 1.

[0094] First efficiency = First discharge specific capacity / First charge specific capacity;

[0095] (3) Test method for the capacity retention rate: At 45 °C, charge the soft-pack battery at 1C and discharge it at 1C once, cycle for 500 weeks, with the voltage range of 3V - 4.50V; the results are shown in Table 1; the calculation formula is as follows:

[0096] Capacity retention rate in the Xth week = (Discharge specific capacity in the Xth week / Discharge specific capacity in the 1st week) × 100%, where X = 100, 200, 300, 400 or 500;

[0097] (4) Test method for the rate performance: At 25 °C, after charging the soft-pack battery at 0.1C and discharging it at 0.1C for formation, then perform 0.5C charging / 0.5C discharging, 1C charging / 1C discharging, 3C charging / 3C discharging. The results are shown in Table 2, and the calculation formula is as follows:

[0098] nC rate performance = specific discharge capacity at nC / specific discharge capacity at 0.1C in the first cycle, where n = 0.5, 1, or 3.

[0099] Table 1 Performance test results of the batteries prepared in the examples and comparative examples

[0100]

[0101]

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

[0103]

[0104] As can be seen from Tables 1 - 2, the double-coated lithium iron manganese phosphate cathode material prepared by the present invention has a high specific capacity, good capacity retention rate and rate performance; by comparing the examples and comparative examples, it can be seen that the double-coated lithium iron manganese phosphate cathode material prepared by the present invention has doped elements in the internal bulk phase, and a double-layer protective layer and a carbon coating layer that are helpful for lithium ion transport on the outside. Its specific capacity, cycle performance and rate performance have all been significantly improved, indicating that the double-coated lithium iron manganese phosphate cathode material has good conductivity and energy density.

[0105] Obviously, the above examples are only for clear illustration and not a limitation of the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all the implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A method for preparing a double-coated lithium manganese iron phosphate positive electrode material, characterized in that: The steps include: (1) mixing an iron source, a manganese source, a compound containing a doping metal and a precipitant to prepare a manganese iron precursor, wherein the doping metal includes at least one of nickel and magnesium; mixing a phosphorus source, a first lithium source and the manganese iron precursor, and first sintering to obtain a lithium iron manganese phosphate positive electrode material; (2) mixing the lithium manganese iron phosphate positive electrode material, the cobalt source, the second lithium source and the additive, sintering for the second time, adding the carbon source and mixing, and sintering for the third time.

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

3. The double-coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: The particle size of the lithium manganese iron phosphate positive electrode material is 400-500nm; and / or, The iron source comprises at least one of ferrous sulfate, ferrous acetate and ferrous nitrate; and / or, The manganese source comprises at least one of manganese sulfate and manganese chloride; and / or, The compound containing the doped metal includes at least one of chloride, sulfate, nitrate, acetate or acetate containing nickel and / or magnesium; and / or, The precipitant comprises at least one of sodium hydroxide and sodium carbonate; and / or, The molar ratio of the iron element in the iron source, the manganese element in the manganese source, and the doping metal in the compound containing the doping 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 500-1000r / min.

4. The double-coated lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: In the step (1), the phosphorus source comprises 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 the step (1), the molar ratio of the phosphorus element in the phosphorus source, the lithium element in the first lithium source, and the molar sum of the manganese element and the iron element in the manganese-iron precursor is (1.01-1.02): (1.02-1.03):

1.

5. The double-coated lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 4, characterized in that: In step (2), the cobalt source comprises at least one of cobalt oxide, carboxylic cobalt oxide and cobaltous hydroxide; and / or, In step (2), the second lithium source includes at least one of lithium carbonate, lithium hydroxide and organic lithium; and / or, In step (2), the additive comprises at least one of oxides, hydroxides, chlorides, sulfates, nitrates, acetates or acetates containing lanthanum, zirconium or tungsten; and / or In the step (2), the molar ratio of the lithium iron manganese phosphate positive electrode 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 positive electrode material according to any one of claims 1 to 5, characterized in that: The carbon source comprises at least one of glucose, sucrose and polyvinyl alcohol; and / or, Based on the mass of the second sintered product, the added amount of the carbon source is 7-8wt%.

7. The double-coated lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 6, characterized in that: The first sintering temperature is 800-900° C.; and / or, The first sintering time is 8-10 hours; and / or, The first sintering is performed in at least one of nitrogen and inert atmosphere; and / or, The second sintering temperature is 650-750° C.; and / or, The second sintering time is 6-8h; and / or, The second sintering is performed in at least one of nitrogen and inert atmosphere; and / or, The temperature of the third sintering is 600-700° C.; and / or, The third sintering time is 2-5h; and / or, The third sintering is performed in at least one of a nitrogen atmosphere and an inert atmosphere.

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

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

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