Preparation method of high-compaction-density lithium manganese iron phosphate positive electrode material

Abstract

The invention relates to the technical field of lithium ion battery positive electrode materials, in particular to a preparation method of a high-compaction-density lithium iron manganese phosphate positive electrode material. The preparation method comprises the following steps: mixing a manganese source, an iron source, a phosphorus source, a lithium source, a carbon source, a doped element precursor and water to obtain slurry; grinding, spray drying and high-temperature sintering are performed on the slurry to obtain the lithium manganese iron phosphate positive electrode material; double manganese sources are adopted, the first manganese source is a soluble manganese source, and the second manganese source is an insoluble manganese source; the double-manganese-source strategy can greatly improve the compaction density of the lithium manganese iron phosphate, and the high-compaction-density lithium manganese iron phosphate is obtained; the preparation method provided by the invention is simple and easy to implement, does not change the original process route of the lithium manganese iron phosphate, is suitable for large-scale preparation of the high-compaction-density lithium manganese iron phosphate by a solid-phase method, and does not reduce the battery performance of the lithium manganese iron phosphate positive electrode material while improving the compaction density.

Description

Technical Field

[0001] The present invention relates to the technical field of cathode materials for lithium-ion batteries, and particularly to a preparation method of a high tap density lithium iron manganese phosphate cathode material. Background Art

[0002] At present, the cathode materials for power batteries are mainly divided into ternary cathodes and lithium iron phosphate cathodes; ternary materials have high voltage, large specific capacity, and high energy density, but their safety is relatively low; due to high safety, the market share of lithium iron phosphate has increased year by year in recent years and once exceeded ternary materials, but its low voltage platform and low specific capacity of 170 mAh / g determine its low energy density. In recent years, with the rapid development of electric vehicles, higher requirements for endurance have been put forward; undoubtedly, the low energy density of lithium iron phosphate is difficult to meet the growing vehicle endurance demand. Lithium iron manganese phosphate is considered an upgraded version of lithium iron phosphate; by replacing part of the iron in lithium iron phosphate with manganese, on the one hand, lithium iron manganese phosphate has the same structure as lithium iron phosphate, both having high stability and high safety, and on the other hand, lithium iron manganese phosphate has a higher voltage platform, up to 4.1 V at most, while lithium iron phosphate is 3.4 V, which can increase the energy density by 10-20%.

[0003] Compared with lithium iron phosphate, lithium iron manganese phosphate has a higher energy density, but there are still two key problems in its industrialization: (1) low conductivity: whether it is electronic conductivity or ionic conductivity, lithium iron manganese phosphate is at least 3-4 orders of magnitude lower than lithium iron phosphate; (2) dissolution of Mn 3+ ; Mn generated during the charge and discharge process 3+ will dissolve in the electrolyte, resulting in a sharp decline in capacity. Due to the extremely low conductivity of lithium iron manganese phosphate, its primary particles are generally small, resulting in a low tap density. Currently, the tap density of lithium iron manganese phosphate powder on the market is generally lower than 2.3 g / cm 3 .

[0004] The following methods are adopted in the prior art to improve the tap density of lithium iron manganese phosphate: (1) By controlling the grinding time to obtain precursor slurries with different particle sizes, and grading the slurries with different particle sizes to obtain a lithium iron manganese phosphate cathode with particle grading, so as to improve the tap density.

[0005] (2) Adopting a multi-step carbon coating strategy to solve the problem of uneven carbon layer to improve the tap density.

[0006] Although the above two methods have significant effects on improving the tap density, they both increase the complexity of the process and the uncontrollability of the process to varying degrees, resulting in an increase in the difficulty of production process control and production cost.

[0007] Therefore, when ensuring a relatively high capacity, how to improve the tap density of lithium iron manganese phosphate has become an urgent problem to be solved. Summary of the Invention

[0008] In view of the deficiencies of the prior art, the present application provides a method for preparing a lithium iron manganese phosphate cathode material with high tap density; the method is simple and low-cost, and does not change the original production process of lithium iron manganese phosphate; moreover, when improving the tap density, it will not have a negative impact on the battery performance of lithium iron manganese phosphate.

[0009] In a first aspect, the present application provides a method for preparing a lithium iron manganese phosphate cathode material with high tap density, adopting the following technical solution: A method for preparing a lithium iron manganese phosphate cathode material with high tap density, the steps of the preparation method are as follows: mixing a manganese source, an iron source, a phosphorus source, a lithium source, a carbon source, and a doping element precursor to obtain a slurry; grinding, drying, and sintering the slurry to obtain the lithium iron manganese phosphate cathode material; the manganese source includes a soluble manganese source and an insoluble manganese source.

[0010] In the present application, a dual-manganese source is used as a raw material. On the one hand, using a dual-manganese source can lower the melting point. At the same temperature, the dual-manganese source has higher reaction activity, making the lithium iron manganese phosphate particles larger and having a higher tap density; on the other hand, after drying, the particle sizes of the two manganese sources are different, making the lithium iron manganese phosphate have a certain particle size distribution and a higher tap density.

[0011] Preferably, the soluble manganese source is at least one of manganese nitrate, manganese sulfate, manganese chloride, manganese acetate, manganese dihydrogen phosphate, manganese hydrogen phosphate, and manganese citrate.

[0012] Preferably, the insoluble manganese source is at least one of manganese tetraoxide, manganese carbonate, manganese dioxide, manganese sesquioxide, manganese oxalate, and manganese phosphate.

[0013] Preferably, the molar ratio of manganese elements in the soluble manganese source and the insoluble manganese source is x:(1 - x), where 0.1 ≤ x ≤ 0.9.

[0014] Preferably, the prepared slurry is ground, and the particle size after grinding is 0.1 - 0.4 μm.

[0015] In the present application, a dual-manganese source is used. The first manganese source is a soluble manganese source, and the second manganese source is an insoluble manganese source; through grinding, the particle size of the insoluble manganese is controlled to be 0.1 - 0.4 μm. At this time, the soluble manganese source is dissolved in water. After drying, the soluble manganese source will precipitate, resulting in different particle sizes of the two manganese sources.

[0016] Preferably, the molar ratio of the lithium source, the manganese source, the iron source, and the phosphorus source is (0.9 - 1.1):y:(1 - y):(0.9 - 1.1), where 0.5 ≤ y < 1.

[0017] Preferably, the iron source is at least one of iron(III) oxide, ferrous oxalate, and iron phosphate.

[0018] Preferably, the phosphorus source is at least one of phosphoric acid, lithium dihydrogen phosphate, iron phosphate, and ammonium dihydrogen phosphate.

[0019] Preferably, the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate, and lithium acetate.

[0020] Preferably, the carbon source is at least one of glucose, sucrose, β-cyclodextrin, starch, polyethylene glycol, citric acid, and methylcellulose, and the carbon content in the lithium iron manganese phosphate cathode material is 1-10%.

[0021] Preferably, the doping element in the doping element precursor is at least one of Zn, Zr, Cr, Mo, Pd, Si, Al, V, Ti, Mg, and La; the content of the doping element in the lithium iron manganese phosphate cathode material is 100-10,000 ppm.

[0022] Preferably, in the process of preparing the slurry, the solvent used is water, and the solid content in the slurry is 25-50%.

[0023] Preferably, the drying step is as follows: spray-dry the ground slurry to obtain precursor powder, wherein the temperature at the outlet of the spray dryer is 90-150 °C.

[0024] Preferably, the sintering step is as follows: perform high-temperature sintering on the precursor powder in an inert atmosphere to obtain the lithium iron manganese phosphate cathode material; wherein, the sintering temperature is 620-800 °C, and the sintering time is 4-48 h.

[0025] In a specific feasible embodiment, a method for preparing a lithium iron manganese phosphate cathode material with high tap density comprises the following steps: (1) Preparation of slurry: Add manganese source, iron source, phosphorus source, lithium source, carbon source, and doping element precursor into deionized water according to a solid content of 25-50%, and stir at high speed to fully mix the raw materials evenly to obtain a slurry.

[0026] (2) Grinding: Transfer the slurry in step (1) to a sand mill for grinding, and the average particle size after grinding is 0.1-0.4 μm.

[0027] (3) Drying: Spray-dry the ground slurry in step (2), control the temperature at the outlet of the spray dryer to be 90-150 °C, and collect the precursor powder.

[0028] (4) Sintering: The precursor powder in step (3) is subjected to high-temperature sintering in an inert atmosphere of nitrogen or argon, controlling the sintering temperature at 620 - 800 °C and the sintering time at 4 - 48 h to obtain the lithium iron manganese phosphate cathode material.

[0029] In step (1), the manganese source is a dual manganese source. The first manganese source is a soluble manganese source, selected from at least one of manganese nitrate, manganese sulfate, manganese chloride, manganese acetate, manganese dihydrogen phosphate, manganese hydrogen phosphate, and manganese citrate; the second manganese source is an insoluble manganese source, selected from at least one of manganese tetraoxide, manganese carbonate, manganese dioxide, manganese sesquioxide, manganese oxalate, and manganese phosphate; and the molar ratio of manganese elements in the soluble manganese source and the insoluble manganese source is x:(1 - x), where 0.1 ≤ x ≤ 0.9.

[0030] In step (1), the molar ratio of the lithium source, manganese source, iron source, and phosphorus source is (0.9 - 1.1):y:(1 - y):(0.9 - 1.1), where 0.5 ≤ y < 1.

[0031] In step (1), the iron source is at least one of iron(III) oxide, ferrous oxalate, and iron phosphate.

[0032] In step (1), the phosphorus source is at least one of phosphoric acid, lithium dihydrogen phosphate, iron phosphate, and ammonium dihydrogen phosphate.

[0033] In step (1), the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate, and lithium acetate.

[0034] In step (1), the carbon source is at least one of glucose, sucrose, β-cyclodextrin, starch, PEG (polyethylene glycol), citric acid, and methyl cellulose, and the carbon content in the lithium iron manganese phosphate cathode material is 1 - 10%.

[0035] In step (1), the doping element in the doping element precursor is at least one of Zn, Zr, Cr, Mo, Pd, Si, Al, V, Ti, Mg, and La; and the doping element content in the lithium iron manganese phosphate cathode material is 100 - 10000 ppm.

[0036] In a second aspect, the present application provides a lithium iron manganese phosphate cathode material with high tap density, adopting the following technical solution: A lithium iron manganese phosphate cathode material with high tap density is prepared by the above method.

[0037] In summary, the present application includes at least one of the following beneficial technical effects: 1. In the present application, a dual manganese source is adopted, where the first manganese source is a soluble manganese source and the second manganese source is an insoluble manganese source; the dual manganese source can greatly improve the tap density of lithium iron manganese phosphate and obtain a lithium iron manganese phosphate with high tap density.

[0038] 2. The preparation method of this application is simple and easy to implement, without changing the original process route of lithium iron manganese phosphate, and is suitable for mass production of lithium iron manganese phosphate with high tap density by the solid-phase method; while improving the tap density, this method will not reduce the battery performance of the lithium iron manganese phosphate cathode material. Description of the Drawings

[0039] Figure 1 SEM image of lithium iron manganese phosphate prepared in Example 1; Figure 2 SEM image of lithium iron manganese phosphate prepared in Example 2; Figure 3 SEM image of lithium iron manganese phosphate prepared in Example 3; Figure 4 SEM image of lithium iron manganese phosphate prepared in Example 4; Figure 5 SEM image of lithium iron manganese phosphate prepared in Comparative Example 1; Figure 6 SEM image of lithium iron manganese phosphate prepared in Comparative Example 2; Figure 7 SEM image of lithium iron manganese phosphate prepared in Comparative Example 3; Figure 8 SEM image of lithium iron manganese phosphate prepared in Comparative Example 4. Detailed Description of the Embodiments

[0040] The raw materials used in this application can all be obtained commercially; the following further details this application in combination with examples and comparative examples.

[0041] Example 1: Weigh 19.40 g of lithium carbonate, 28.77 g of ferrous oxalate, 11.44 g of manganese tetroxide, 26.84 g of manganese nitrate, 57.64 g of phosphoric acid (85% content), 15.13 g of sucrose and 0.39 g of rutile-type titanium dioxide, add them to 240 g of deionized water, and stir at high speed to mix the raw materials evenly; then put the slurry into a sand mill and grind it to an average particle size of 0.2 μm, and discharge; spray-dry the ground slurry, adjust the outlet air temperature to 100 °C to obtain the precursor powder; finally, sinter the precursor powder at high temperature in a nitrogen atmosphere, sinter at 720 °C for 18 h to obtain lithium iron manganese phosphate powder.

[0042] Example 2: Weigh 19.40 g of lithium carbonate, 28.77 g of iron oxalate, 17.24 g of manganese carbonate, 25.95 g of manganese acetate, 57.64 g of phosphoric acid (85% content), 15.13 g of sucrose and 0.39 g of rutile titanium dioxide, add them to 246 g of deionized water, and stir at high speed to make the raw materials evenly mixed; then put the slurry into a sand mill, grind it to an average particle size of 0.2 μm, and discharge; spray-dry the ground slurry, adjust the outlet air temperature to 100 °C to obtain the precursor powder; finally, sinter the precursor powder at high temperature in a nitrogen atmosphere, sinter at 720 °C for 18 h to obtain lithium iron manganese phosphate powder.

[0043] Example 3: Weigh 19.40 g of lithium carbonate, 28.77 g of iron oxalate, 11.84 g of manganese(III) oxide, 37.34 g of manganese dihydrogen phosphate, 40.35 g of phosphoric acid (85% content), 15.13 g of sucrose and 0.39 g of rutile titanium dioxide, add them to 230 g of deionized water, and stir at high speed to make the raw materials evenly mixed; then put the slurry into a sand mill, grind it to an average particle size of 0.2 μm, and discharge; spray-dry the ground slurry, adjust the outlet air temperature to 100 °C to obtain the precursor powder; finally, sinter the precursor powder at high temperature in a nitrogen atmosphere, sinter at 720 °C for 18 h to obtain lithium iron manganese phosphate powder.

[0044] Example 4: Weigh 19.40 lithium carbonate, 28.77 g of iron oxalate, 21.44 g of manganese oxalate, 22.65 g of manganese sulfate, 57.64 g of phosphoric acid (85% content), 15.13 g of sucrose and 0.39 g of rutile titanium dioxide, add them to 248 g of deionized water, and stir at high speed to make the raw materials evenly mixed; then put the slurry into a sand mill, grind it to an average particle size of 0.2 μm, and discharge; spray-dry the ground slurry, adjust the outlet air temperature to 100 °C to obtain the precursor powder; finally, sinter the precursor powder at high temperature in a nitrogen atmosphere, sinter at 720 °C for 18 h to obtain lithium iron manganese phosphate powder.

[0045] Example 5: Weigh 19.40 lithium carbonate, 28.77 g of iron oxalate, 3.45 g of manganese carbonate, 48.32 g of manganese nitrate, 57.64 g of phosphoric acid (85% content), 15.13 g of sucrose and 0.39 g of rutile titanium dioxide, add them to 260 g of deionized water, and stir at high speed to make the raw materials evenly mixed; then put the slurry into a sand mill, grind it to an average particle size of 0.2 μm, and discharge; spray-dry the ground slurry, adjust the outlet air temperature to 100 °C to obtain the precursor powder; finally, sinter the precursor powder at high temperature in a nitrogen atmosphere, sinter at 720 °C for 18 h to obtain lithium iron manganese phosphate powder.

[0046] Example 6: Weigh 19.40 lithium carbonate, 28.77 g of ferrous oxalate, 38.60 g of manganese oxalate, 3.78 g of manganese chloride, 57.64 g of phosphoric acid (85% content), 15.13 g of sucrose and 0.39 g of rutile titanium dioxide, add them to 246 g of deionized water, and stir at high speed to make the raw materials evenly mixed; then put the slurry into a sand mill, grind it to an average particle size of 0.2 μm, and discharge; spray-dry the ground slurry, adjust the outlet air temperature to 100 °C to obtain the precursor powder; finally, sinter the precursor powder at high temperature in a nitrogen atmosphere, sinter at 720 °C for 18 h to obtain the lithium iron manganese phosphate powder.

[0047] Comparative Example 1: Weigh 19.40 g of lithium carbonate, 28.77 g of ferrous oxalate, 22.88 g of manganese tetroxide, 57.64 g of phosphoric acid (85% content), 15.13 g of sucrose and 0.39 g of rutile titanium dioxide, add them to 216 g of deionized water, and stir at high speed to make the raw materials evenly mixed; then put the slurry into a sand mill, grind it to an average particle size of 0.2 μm, and discharge; spray-dry the ground slurry, adjust the outlet air temperature to 100 °C to obtain the precursor powder; finally, sinter the precursor powder at high temperature in a nitrogen atmosphere, sinter at 720 °C for 18 h to obtain the lithium iron manganese phosphate powder.

[0048] Comparative Example 2: Weigh 19.40 g of lithium carbonate, 28.77 g of ferrous oxalate, 53.69 g of manganese nitrate, 57.64 g of phosphoric acid (85% content), 15.13 g of sucrose and 0.39 g of rutile titanium dioxide, add them to 263 g of deionized water, and stir at high speed to make the raw materials evenly mixed; then put the slurry into a sand mill, grind it to an average particle size of 0.2 μm, and discharge; spray-dry the ground slurry, adjust the outlet air temperature to 100 °C to obtain the precursor powder; finally, sinter the precursor powder at high temperature in a nitrogen atmosphere, sinter at 720 °C for 18 h to obtain the lithium iron manganese phosphate powder.

[0049] Comparative Example 3: Weigh 19.40 g of lithium carbonate, 28.77 g of ferrous oxalate, 34.48 g of manganese carbonate, 57.64 g of phosphoric acid (85% content), 15.13 g of sucrose and 0.39 g of rutile titanium dioxide, add them to 234 g of deionized water, and stir at high speed to make the raw materials evenly mixed; then put the slurry into a sand mill, grind it to an average particle size of 0.2 μm, and discharge; spray-dry the ground slurry, adjust the outlet air temperature to 100 °C to obtain the precursor powder; finally, sinter the precursor powder at high temperature in a nitrogen atmosphere, sinter at 720 °C for 18 h to obtain the lithium iron manganese phosphate powder.

[0050] Comparative Example 4: Weigh 19.40 g of lithium carbonate, 28.77 g of ferrous oxalate, 51.91 g of manganese acetate, 57.64 g of phosphoric acid (85% content), 15.13 g of sucrose and 0.39 g of rutile titanium dioxide, add them to 260 g of deionized water, and stir at high speed to make the raw materials evenly mixed; then put the slurry into a sand mill, grind it to an average particle size of 0.2 μm, and discharge; spray-dry the ground slurry, adjust the outlet air temperature to 100 °C to obtain a precursor powder; finally, perform high-temperature sintering on the precursor powder in a nitrogen atmosphere, sinter at 720 °C for 18 h to obtain lithium iron manganese phosphate powder.

[0051] Comparative Example 5: Weigh 19.40 lithium carbonate, 28.77 g of ferrous oxalate, 1.72 g of manganese carbonate, 51.00 g of manganese nitrate, 57.64 g of phosphoric acid (85% content), 15.13 g of sucrose and 0.39 g of rutile titanium dioxide, add them to 261 g of deionized water, and stir at high speed to make the raw materials evenly mixed; then put the slurry into a sand mill, grind it to an average particle size of 0.2 μm, and discharge; spray-dry the ground slurry, adjust the outlet air temperature to 100 °C to obtain a precursor powder; finally, perform high-temperature sintering on the precursor powder in a nitrogen atmosphere, sinter at 720 °C for 18 h to obtain lithium iron manganese phosphate powder.

[0052] Comparative Example 6: Weigh 19.40 lithium carbonate, 28.77 g of ferrous oxalate, 40.74 g of manganese oxalate, 1.89 g of manganese chloride, 57.64 g of phosphoric acid (85% content), 15.13 g of sucrose and 0.39 g of rutile titanium dioxide, add them to 246 g of deionized water, and stir at high speed to make the raw materials evenly mixed; then put the slurry into a sand mill, grind it to an average particle size of 0.2 μm, and discharge; spray-dry the ground slurry, adjust the outlet air temperature to 100 °C to obtain a precursor powder; finally, perform high-temperature sintering on the precursor powder in a nitrogen atmosphere, sinter at 720 °C for 18 h to obtain lithium iron manganese phosphate powder.

[0053] The test characterizations of the lithium iron manganese phosphate obtained in Examples 1-6 and Comparative Examples 1-6 in this application include: the morphology is observed by SEM, the carbon content is tested by a sulfur-carbon analyzer (NJ-HW868B), and the tap density is tested by a tap density tester (FTYS-50KN).

[0054] The electrochemical performance test steps of the lithium iron manganese phosphate obtained in Examples 1-6 and Comparative Examples 1-6 of this application are as follows: Mix lithium iron manganese phosphate, SuperP, and PVDF in a mass ratio of 95:2.5:2.5 with a certain amount of NMP (solid content 45%), homogenize with a high-speed disperser, evenly coat on carbon-coated aluminum foil, dry in an oven at 120 °C, and cut into positive electrode circular wafers with a diameter of 14 mm using a slicing machine; Assemble a 2032-type button battery with the positive electrode wafer as the positive electrode, a lithium wafer as the negative electrode, and a polypropylene film as the separator, and 1M LiPF6 EC / DMC (volume ratio of EC to DMC 1:1) as the electrolyte. After the battery is assembled, let it stand for 24 h, and then perform battery performance testing on a Neware charge and discharge tester; The charge and discharge voltage range is 2.8-4.3 V, and charging is carried out in CC / CV mode, and the cut-off current of CV is 0.04C.

[0055] The comparison results of the properties and battery capacities of the lithium iron manganese phosphate prepared in the above Examples 1-6 and Comparative Examples 1-6 are shown in Table 1.

[0056] Table 1 Comparison of the properties and battery capacities of the lithium iron manganese phosphate prepared in Examples 1-6 and Comparative Examples 1-6 As can be seen from Table 1, after using the double manganese source, the tap densities in Examples 1-6 are all above 2.4 g / cm 3 3, which is relatively high; among them, the tap densities in Examples 2 and 3 can even reach 2.47 g / cm 3 . In Comparative Example 1 and Comparative Example 2, a single insoluble manganese source (manganese tetroxide) and a soluble manganese source (manganese nitrate) are used respectively, and the tap densities are only 1.94 g / cm 3 and 1.98 g / cm 3 , and the tap densities are relatively low; while the lithium iron manganese phosphate prepared with the double manganese source in Example 1 has a tap density of 2.45 g / cm 3 , indicating that after using the double manganese source, the tap density has been greatly improved. In Comparative Example 3 and Comparative Example 4, a single insoluble manganese source (manganese carbonate) and a soluble manganese source (manganese acetate) are used respectively, and the tap densities are only 2 g / cm 3 and 1.93 g / cm 3 , and the tap densities are relatively low; while the lithium iron manganese phosphate prepared with the double manganese source in Example 2 has a tap density of 2.47 g / cm 3 , indicating that after using the double manganese source, the tap density has been greatly improved.

[0057] In Examples 1-6, the molar ratio of manganese elements in the soluble manganese source (x) and the insoluble manganese source (1-x) all satisfy 0.1 ≤ x ≤ 0.9. At this time, the dual-manganese-source strategy has a high tap density; while in Comparative Examples 5 and 6, although the dual-manganese-source is also adopted, but x is 0.05 and 0.95 respectively, and the tap densities are 1.95 g / cm 3 , 1.99 g / cm 3 respectively, and the tap density is still relatively low, indicating that in the dual-manganese-source strategy, the addition amounts of both manganese sources cannot be too small and should be within a suitable range.

[0058] The present application adopts the dual-manganese-source strategy and has a higher tap density when the carbon content and the doping element content are the same; and while ensuring a high tap density, the battery capacity can still remain at a relatively high value. Through Figures 1-8 the SEM results show that: after adopting the dual-manganese-source strategy, the primary particle size of lithium iron manganese phosphate has a significant increase, which is consistent with the tap density results.

[0059] The above examples are only used to illustrate the technical solutions of the present application, rather than limiting it; although the present application has been described in detail with reference to the foregoing examples, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing examples, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the various examples of the present application.

Claims

1. A method for preparing a high compaction density lithium manganese iron phosphate positive electrode material, characterized in that: The steps of the preparation method are as follows: mixing a manganese source, an iron source, a phosphorus source, a lithium source, a carbon source, and a doping element precursor to obtain a slurry; grinding, drying, and sintering the slurry to obtain a lithium iron manganese phosphate positive electrode material; the manganese source includes a soluble manganese source and an insoluble manganese source.

2. The method for preparing a high compaction density lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The soluble manganese source is at least one of manganese nitrate, manganese sulfate, manganese chloride, manganese acetate, manganese dihydrogen phosphate, manganese hydrogen phosphate, and manganese citrate.

3. The method for preparing a high compaction density lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The insoluble manganese source is at least one of manganese tetraoxide, manganese carbonate, manganese dioxide, manganese trioxide, manganese oxalate and manganese phosphate.

4. The method for preparing a high compaction density lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The molar ratio of manganese element in the soluble manganese source and the insoluble manganese source is x:(1-x), 0.1≤x≤0.

9.

5. The method for preparing a high compaction density lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The prepared slurry is ground, and the particle size after grinding is 0.1-0.4 μm.

6. The method for preparing a high compaction density lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The molar ratio of the lithium source, the manganese source, the iron source and the phosphorus source is (0.9-1.1):y:(1-y):(0.9-1.1), wherein 0.5≤y<1.

7. The method for preparing a high compaction density lithium iron manganese phosphate positive electrode material according to claim 1 or 6, characterized in that: The iron source is at least one of ferric oxide, ferrous oxalate and ferric phosphate; The phosphorus source is at least one of phosphoric acid, lithium dihydrogen phosphate, iron phosphate, and ammonium dihydrogen phosphate; The lithium source is at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate, and lithium acetate.

8. The method for preparing a high compaction density lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The carbon source is at least one of glucose, sucrose, beta-cyclodextrin, starch, polyethylene glycol, citric acid, and methyl cellulose, and the carbon content of the lithium manganese iron phosphate positive electrode material is 1-10%.

9. The method for preparing a high compaction density lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The doping element in the doping element precursor is at least one of Zn, Zr, Cr, Mo, Pd, Si, Al, V, Ti, Mg, and La; the doping element content in the lithium manganese iron phosphate positive electrode material is 100-10000ppm.

10. The method for preparing a high compaction density lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: During the slurry preparation process, the solvent used is water, and the solid content in the slurry is 25-50%; The drying step is as follows: spray drying the ground slurry to obtain a precursor powder, wherein the temperature of the spray drying outlet is 90-150°C; The sintering steps are as follows: the precursor powder is sintered at a high temperature in an inert atmosphere to obtain a lithium manganese iron phosphate positive electrode material; wherein the sintering temperature is 620-800°C and the sintering time is 4-48h.

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