Synthesis method of spherical lithium iron manganese phosphate positive electrode material

Abstract

The invention discloses a synthesis method of a spherical lithium manganese iron phosphate positive electrode material, which comprises the following steps: by taking iron lactate, manganese methoxide, phosphoric acid and battery lithium lithium carbonate as raw materials, dissolving the iron lactate and the manganese methoxide by the phosphoric acid, and adding the battery lithium lithium carbonate to regulate and control the pH value to obtain mixed slurry; the mixed slurry is subjected to mixed ball milling and natural evaporation to dryness and then subjected to nitrogen-hydrogen mixed gas protection calcination to obtain a first-firing product, a carbon source is added into the first-firing product for wet ball milling, spray drying and secondary calcination, and the carbon-coated primary-particle spherical lithium manganese iron phosphate positive electrode material is obtained. The lithium manganese iron phosphate material prepared by the preparation method disclosed by the invention is excellent in performance and high in charge and discharge gram volume, primary particles of a product are detected to be spherical-like, the 0.1 C first discharge gram volume reaches 157.5 mAh / g, the preparation process is simple, waste water, waste and solid waste are not generated, and the lithium manganese iron phosphate material is green and environment-friendly and is beneficial to large-scale production.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion battery positive electrode material preparation, and in particular to a method for synthesizing spherical lithium manganese iron phosphate positive electrode material. Background Art

[0002] Lithium iron phosphate is the main lithium battery power material at present, with advantages such as good safety, long service life and low cost. Compared with lithium iron phosphate, lithium manganese iron phosphate has advantages such as high specific capacity, high energy density, long cycle life and high voltage platform, and has gradually emerged in recent years. However, the primary particle size of lithium manganese iron phosphate is large, the particle morphology is poor, and the material conductivity is low, which limits its gram capacity; and the precursor sources of lithium manganese iron phosphate are relatively few, and a stable and reliable synthesis process route needs to be explored.

[0003] In view of this, the present invention uses iron lactate, manganese methanol, phosphoric acid and battery lithium carbonate as raw materials, dissolves iron lactate and manganese methanol by phosphoric acid, adds battery lithium carbonate to adjust the pH value to obtain a mixed slurry, and the mixed slurry is naturally evaporated by mixed ball milling and then calcined under the protection of nitrogen and hydrogen mixed gas to obtain a primary calcination product. The primary calcination product is wet-ball-milled and spray-dried by adding a carbon source, and then calcined twice to obtain a carbon-coated primary particle spherical lithium manganese iron phosphate positive electrode material. Summary of the invention

[0004] In view of the problems that the existing lithium iron manganese phosphate positive electrode materials have a large primary particle size, poor particle morphology, and low material conductivity, which limits their gram capacity, and there are few sources of lithium iron manganese phosphate precursors, the purpose of the present invention is to provide a method for synthesizing spherical lithium iron manganese phosphate positive electrode materials.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for synthesizing a spherical lithium manganese iron phosphate positive electrode material, the method for synthesizing a spherical lithium manganese iron phosphate positive electrode material comprising the following steps:

[0007] (1) adding water to 80-90% phosphoric acid to prepare an 8-20% phosphoric acid solution, adding ferric lactate, heating and stirring at 50-70° C. to dissolve, filtering, adding manganese formate to the filtrate under stirring, heating and reacting at 60-90° C. for 1-3 hours to obtain a mixed slurry;

[0008] (2) adding battery-grade lithium carbonate to the mixed slurry under stirring conditions, reacting at 70-90° C. for 1-3 hours, and the pH value of the slurry after the addition of the battery-grade lithium carbonate is 7-8 to obtain a lithium manganese iron phosphate precursor slurry;

[0009] (3) naturally evaporating the lithium manganese iron phosphate precursor slurry under mixed ball milling to obtain a precursor powder, and then calcining the precursor powder in an inert gas to obtain a calcined product;

[0010] (4) adding 800-1000 kg of water, 15-20 kg of glucose, 3-4 kg of citric acid, 6-9 kg of PEG4000 and 0.8-1.2 kg of magnesium hydroxide to 300-400 kg of the calcined product and wet-milling for 2-5 h to control the particle size and obtain a powder by spray drying;

[0011] (5) The powder is subjected to secondary calcination in an inert gas to obtain a secondary calcination product, lithium manganese iron phosphate.

[0012] In the step (1), 85% phosphoric acid is added with water to prepare a 10-15% phosphoric acid solution, and iron lactate is added to the solution and heated and stirred at 60°C to dissolve, followed by filtration. Manganese formate is added to the filtrate under stirring, and the solution is heated and reacted at 80°C for 3 hours to obtain a mixed slurry;

[0013] The molar ratio of the ferric lactate, manganese methoxide and phosphoric acid is 0.3-5:0.5-0.7:1-1.05; the purity of the ferric lactate is greater than 98%; the purity of the manganese methoxide is greater than 98%.

[0014] In the step (2), the molar ratio of lithium and phosphorus elements of battery-grade lithium carbonate and phosphoric acid is 1-1.02:1-1.02, and the battery-grade lithium carbonate is added and reacted at 80° C. for 2 hours.

[0015] In the step (3), the calcination temperature is 500-800° C., the calcination time is 3-8 hours, the inert gas is a nitrogen-hydrogen mixed gas atmosphere, and the volume ratio of nitrogen to hydrogen in the nitrogen-hydrogen mixed gas is 9-9.5:0.5-1.

[0016] In the step (3), the calcination temperature is 600-700° C., the calcination time is 3-5 hours, the inert gas is a nitrogen-hydrogen mixed gas atmosphere, and the volume ratio of nitrogen to hydrogen in the nitrogen-hydrogen mixed gas is 9.5:0.5.

[0017] In the step (4), 350 kg of the calcined product is added with 900 kg of water, 17 kg of glucose, 3.5 kg of citric acid, 7.5 kg of PEG4000 and 1.05 kg of magnesium hydroxide, and wet ball milled for 3-4 hours to control the particle size, and a powder is obtained by spray drying.

[0018] In the step (4), the particle size is controlled at D500.2-0.6 um, the spray air inlet temperature is controlled at 180-220°C, and the air outlet temperature is controlled at 80-90°C.

[0019] In the step (4), the particle size is controlled at D500.3-0.5um, the spray inlet temperature is controlled at 200°C, and the outlet temperature is controlled at 80°C.

[0020] In the step (5), the secondary calcination temperature is 600-850° C., the secondary calcination time is 5-10 hours, the inert gas is a nitrogen-hydrogen mixed gas atmosphere, and the nitrogen-hydrogen volume ratio of the nitrogen-hydrogen mixed gas is 9-9.5:0.5.

[0021] In the step (5), the secondary calcination temperature is 700-800° C., the secondary calcination time is 8-10 hours, the inert gas is a nitrogen-hydrogen mixed gas atmosphere, and the nitrogen-hydrogen volume ratio of the nitrogen-hydrogen mixed gas is 9.5:0.5.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. The present invention synthesizes lithium manganese iron phosphate through iron lactate, manganese methanol, phosphoric acid and battery-grade lithium carbonate, accurately controls the molar ratio of iron, manganese and phosphorus, and controls the particle size by naturally evaporating and milling the precursor mixture. The particle size of the finally prepared lithium manganese iron phosphate is small, and the conductive performance is further improved.

[0024] 2. In the process of preparing lithium iron manganese phosphate material of the present invention, glucose, citric acid, PEG4000 and magnesium hydroxide are used as carbon sources. The lithium iron manganese phosphate material is coated by adding the carbon source to prevent the dissolution of manganese ions, provide a highly conductive network for the transmission of lithium ions and the transfer of electrons, further accelerate the conduction efficiency of lithium ions, optimize the transmission path of electrons, and uniformly coat the lithium iron manganese phosphate material with carbon source materials to conveniently and quickly synthesize a highly spherical material with uniform composition. The prepared lithium iron manganese phosphate has good conductivity.

[0025] 3. The lithium manganese iron phosphate prepared by the present invention has excellent performance and high charge and discharge capacity. The primary particles of the product are spherical and the 0.1C first discharge capacity reaches 157.5 mAh / g.

[0026] 4. The preparation process of the present invention is simple. Compared with the traditional lithium iron manganese phosphate prepared from ammonium dihydrogen phosphate, the present invention does not produce wastewater, waste and solid waste, is green and environmentally friendly, and is beneficial to large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a SEM image of the lithium manganese iron phosphate sample prepared in Inventive Example 1;

[0028] Figure 2 This is a SEM image of the lithium manganese iron phosphate sample prepared in Inventive Example 1;

[0029] Figure 3This is a charge and discharge curve diagram of lithium manganese iron phosphate prepared in Inventive Example 1;

[0030] Figure 4 It is a process flow chart. DETAILED DESCRIPTION

[0031] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. The present invention is further described below in conjunction with examples. The examples are only further supplements and explanations of the present invention, rather than limitations of the invention.

[0032] Embodiment 1:

[0033] Preparation process:

[0034] (1) 261 kg of 85% phosphoric acid was added with 1700 kg of water to prepare a phosphoric acid solution, 236.32 kg of ferric lactate with a purity of 99% was added to the phosphoric acid solution under stirring to dissolve, the temperature was maintained at 60° C., until the ferric lactate was completely dissolved, and a filtrate was obtained by filtering; the filtrate was stirred continuously and 221.66 kg of manganese formate with a purity of 98.5% and a fineness of 120 mesh was added, the temperature was raised to 80° C. and the mixture was reacted for 3 hours to obtain a mixed slurry with a pH of 2.5;

[0035] (2) adding 84.5 kg of battery-grade lithium carbonate to the mixed slurry under stirring conditions, and keeping the mixture at 80° C. for 2 h to obtain a lithium iron manganese phosphate precursor slurry, wherein the pH of the slurry is 7-8;

[0036] (3) naturally evaporating the lithium manganese iron phosphate precursor slurry under a mixed ball mill to obtain a precursor powder, and then sending the precursor powder into a roller furnace under the protection of a nitrogen-hydrogen mixed gas (nitrogen-hydrogen ratio 9.5:0.5) and calcining at 650° C. for 5 h to obtain a calcined product;

[0037] (4) 350 kg of the calcined product was added with 900 kg of water, 17 kg of glucose, 3.5 kg of citric acid, 7.5 kg of PEG4000 and 1.05 kg of magnesium hydroxide for wet ball milling for 3-4 h. The particle size of the slurry was measured to be 0.35 μm. The slurry was spray dried at an inlet air temperature of 200 ° C and an outlet air temperature of 85 ° C to obtain a powder;

[0038] (5) The powder was then sent into a roller furnace and calcined at 780° C. for 8 h under the protection of a nitrogen-hydrogen mixed gas (nitrogen-hydrogen ratio of 9.5:0.5) to obtain a di-calcined product, lithium manganese iron phosphate (manganese iron ratio of 6:4).

[0039] The lithium manganese iron phosphate primary particles prepared by the above method are spherical (see Figure 1 , Figure 2), a button-type half-cell was assembled with lithium manganese iron phosphate material as the positive electrode and a metal lithium sheet as the negative electrode, and charged to 4.3V at a constant current and constant voltage of 0.1C (cut off at a current of 0.02C), and then discharged to 2.5V at a constant current of 0.1C. The first discharge capacity of the button-type half-cell was 157.5mAh / g.

[0040] Embodiment 2:

[0041] Preparation process:

[0042] (1) 261 kg of 85% phosphoric acid was added with 1700 kg of water to prepare a phosphoric acid solution, 261.64 kg of ferric lactate with a purity of 99% was added to the phosphoric acid solution under stirring to dissolve, the temperature was maintained at 60° C., until the ferric lactate was completely dissolved, and a filtrate was obtained by filtering; the filtrate was stirred continuously and 202.24 kg of manganese formate with a purity of 98.5% and a fineness of 120 mesh was added, the temperature was raised to 80° C. and the mixture was reacted for 3 hours to obtain a mixed slurry with a pH of 2.2;

[0043] (2) adding 83.9 kg of battery-grade lithium carbonate to the mixed slurry under stirring conditions, and keeping the mixture at 80° C. for 2 h to obtain a lithium iron manganese phosphate precursor slurry, wherein the pH of the slurry is 7-8;

[0044] (3) naturally evaporating the lithium manganese iron phosphate precursor slurry under a mixed ball mill to obtain a precursor powder, and then sending the precursor powder into a roller furnace under the protection of a nitrogen-hydrogen mixed gas (nitrogen-hydrogen ratio 9.5:0.5) and calcining at 700° C. for 3 h to obtain a calcined product;

[0045] (4) 300 kg of the calcined product was added with 800 kg of water, 15 kg of glucose, 6 kg of citric acid, 3.5 kg of PEG4000 and 0.9 kg of magnesium hydroxide for wet ball milling for 3-4 h. The particle size D500 of the slurry was detected to be 0.38 μm. The slurry was spray dried at an inlet air temperature of 200 ° C and an outlet air temperature of 85 ° C to obtain a powder;

[0046] (5) The powder was then sent into a roller furnace and calcined at 760° C. for 10 h under the protection of a nitrogen-hydrogen mixed gas (nitrogen-hydrogen ratio of 9.5:0.5) to obtain a di-sintered product, lithium manganese iron phosphate (manganese iron ratio of 5:5).

[0047] The lithium iron manganese phosphate primary particles prepared by the above method are spherical. The lithium iron manganese phosphate material is used as the positive electrode and the metal lithium sheet is used as the negative electrode to assemble into a button-type half-cell. The battery is charged to 4.3V at a constant current and voltage of 0.1C (cut off at a current of 0.02C), and then discharged to 2.5V at a constant current of 0.1C. The first discharge capacity of the battery at 0.1C is 155.3mAh / g.

[0048] Embodiment 3:

[0049] Preparation process:

[0050] (1) 261 kg of 85% phosphoric acid was added with 1700 kg of water to prepare a phosphoric acid solution, 156.98 kg of ferric lactate with a purity of 99% was added to the phosphoric acid solution under stirring to dissolve, the temperature was maintained at 60° C., until the ferric lactate was completely dissolved, and a filtrate was obtained by filtering; the filtrate was stirred continuously and 283.14 kg of manganese formate with a purity of 98.5% and a fineness of 120 mesh was added, the temperature was raised to 80° C. and the mixture was reacted for 3 hours to obtain a mixed slurry with a pH of 2.2;

[0051] (2) adding 83.9 kg of battery-grade lithium carbonate to the mixed slurry under stirring conditions, and keeping the mixture at 80° C. for 2 h to obtain a lithium iron manganese phosphate precursor slurry, wherein the pH of the slurry is 7-8;

[0052] (3) naturally evaporating the lithium manganese iron phosphate precursor slurry under a mixed ball mill to obtain a precursor powder, and then sending the precursor powder into a roller furnace under the protection of a nitrogen-hydrogen mixed gas (nitrogen-hydrogen ratio 9.5:0.5) and calcining at 700° C. for 3 h to obtain a calcined product;

[0053] (4) Take 400 kg of the calcined product, add 1000 kg of water, 16.5 kg of glucose, 7.5 kg of citric acid, 4.2 kg of PEG4000 and 1.1 kg of magnesium hydroxide, and wet ball mill for 3-4 hours. The slurry particle size D500 is detected to be 0.37 um. The slurry is spray dried under the conditions of an inlet air temperature of 200°C and an outlet air temperature of 85°C to obtain a powder.

[0054] (5) The powder was then sent into a roller furnace and calcined at 760° C. for 10 h under the protection of a nitrogen-hydrogen mixed gas (nitrogen-hydrogen ratio of 9.5:0.5) to obtain a di-calcined product, lithium manganese iron phosphate (manganese iron ratio of 7:3).

[0055] The lithium iron manganese phosphate primary particles prepared by the above method are spherical. The lithium iron manganese phosphate material is used as the positive electrode and the metal lithium sheet is used as the negative electrode to assemble into a button half-cell. The battery is charged to 4.3V at a constant current and constant voltage of 0.1C (cut off at a current of 0.02C), and then discharged to 2.5V at a constant current of 0.1C. The first discharge capacity of the button battery at 0.1C is 153.3mAh / g.

[0056] Embodiment 4:

[0057] Preparation process:

[0058] (1) 261 kg of 85% phosphoric acid was added with 1700 kg of water to prepare a phosphoric acid solution, 236.32 kg of ferric lactate with a purity of 99% was added to the phosphoric acid solution under stirring to dissolve, the temperature was maintained at 60° C., until the ferric lactate was completely dissolved, and a filtrate was obtained by filtering; the filtrate was stirred continuously and 221.66 kg of manganese formate with a purity of 98.5% and a fineness of 120 mesh was added, the temperature was raised to 80° C. and the mixture was reacted for 3 hours to obtain a mixed slurry with a pH of 2.5;

[0059] (2) adding 84.5 kg of battery-grade lithium carbonate to the mixed slurry under stirring conditions, and keeping the mixture at 80° C. for 2 h to obtain a lithium iron manganese phosphate precursor slurry, wherein the pH of the slurry is 7-8;

[0060] (3) naturally evaporating the lithium manganese iron phosphate precursor slurry under a mixed ball mill to obtain a precursor powder, and then sending the precursor powder into a roller furnace under the protection of a nitrogen-hydrogen mixed gas (nitrogen-hydrogen ratio 9.5:0.5) and calcining at 650° C. for 5 h to obtain a calcined product;

[0061] (4) Take 300 kg of the calcined product, add 800 kg of water, 15 kg of glucose, 6 kg of citric acid, 3.5 kg of PEG4000 and 0.9 kg of magnesium hydroxide, and wet ball mill for 3-4 hours. The slurry particle size D500 is detected to be 0.38 um. The slurry is spray dried under the conditions of an inlet air temperature of 200°C and an outlet air temperature of 85°C to obtain a powder.

[0062] (5) The powder is then sent to a roller furnace and calcined at 780°C for 8 hours under the protection of a nitrogen-hydrogen mixed gas (nitrogen-hydrogen ratio of 9.5:0.5) to obtain a di-calcined product, lithium manganese iron phosphate. (The ratio of manganese to iron is 6:4)

[0063] The lithium iron manganese phosphate primary particles prepared by the above method are spherical. The lithium iron manganese phosphate material is used as the positive electrode and the metal lithium sheet is used as the negative electrode to assemble into a button-type half-cell. The battery is charged to 4.3V at a constant current and voltage of 0.1C (cut off at a current of 0.02C), and then discharged to 2.5V at a constant current of 0.1C. The first discharge capacity of the battery at 0.1C is 153.1mAh / g.

[0064] Embodiment 5:

[0065] Preparation process:

[0066] (1) 261 kg of 85% phosphoric acid was added with 1700 kg of water to prepare a phosphoric acid solution, 236.32 kg of ferric lactate with a purity of 99% was added to the phosphoric acid solution under stirring to dissolve, the temperature was maintained at 60° C., until the ferric lactate was completely dissolved, and a filtrate was obtained by filtering; the filtrate was stirred continuously and 221.66 kg of manganese formate with a purity of 98.5% and a fineness of 120 mesh was added, the temperature was raised to 80° C. and the mixture was reacted for 3 hours to obtain a mixed slurry with a pH of 2.5;

[0067] (2) adding 84.5 kg of battery-grade lithium carbonate to the mixed slurry under stirring conditions, and keeping the mixture at 80° C. for 2 h to obtain a lithium iron manganese phosphate precursor slurry, wherein the pH of the slurry is 7-8;

[0068] (3) naturally evaporating the lithium manganese iron phosphate precursor slurry under a mixed ball mill to obtain a precursor powder, and then sending the precursor powder into a roller furnace under the protection of a nitrogen-hydrogen mixed gas (nitrogen-hydrogen ratio 9.5:0.5) and calcining at 650° C. for 5 h to obtain a calcined product;

[0069] (4) Take 400 kg of the calcined product, add 1000 kg of water, 16.5 kg of glucose, 7.5 kg of citric acid, 4.2 kg of PEG4000 and 1.1 kg of magnesium hydroxide, and perform wet ball milling for 3-4 hours. The slurry particle size D500 is detected to be 0.35 um. The slurry is spray dried under the conditions of an inlet air temperature of 200°C and an outlet air temperature of 85°C to obtain a powder.

[0070] (5) The powder was then sent into a roller furnace and calcined at 780° C. for 8 h under the protection of a nitrogen-hydrogen mixed gas (nitrogen-hydrogen ratio of 9.5:0.5) to obtain a di-calcined product, lithium manganese iron phosphate (manganese iron ratio of 6:4).

[0071] The lithium iron manganese phosphate primary particles prepared by the above method are spherical. The lithium iron manganese phosphate material is used as the positive electrode and the metal lithium sheet is used as the negative electrode to assemble into a button-type half-cell. The battery is charged to 4.3V at a constant current and voltage of 0.1C (cut off at a current of 0.02C), and then discharged to 2.5V at a constant current of 0.1C. The first discharge capacity of the battery at 0.1C is 151.8mAh / g.

[0072] Comparative Example 1:

[0073] The following is the traditional process for preparing lithium manganese iron phosphate:

[0074] (1) Take 261 kg of ammonium dihydrogen phosphate (99% purity) and add 1800 kg of water to prepare a solution.

[0075] (2) Take 144.32 kg of ferrous oxalate, purity 99%; 140.2 kg of manganese carbonate, purity 98.5%, fineness 120 mesh; weigh 83.9 kg of battery-grade lithium carbonate, 18 kg of glucose, 8.2 kg of citric acid, 4.5 kg of PEG4000 and 1.2 kg of magnesium hydroxide, mix and grind them together for 3-4 hours, and measure the slurry particle size D50000.36 um;

[0076] (3) The slurry was spray dried at an inlet air temperature of 300°C and an outlet air temperature of 105°C to obtain a powder, which was then fed into a roller furnace and calcined at 760°C for 10 h under the protection of a nitrogen-hydrogen mixed gas (nitrogen-hydrogen ratio of 9.5:0.5) to obtain the product, lithium manganese iron phosphate (manganese-iron ratio of 6:4).

[0077] It was tested that the lithium iron manganese phosphate primary particles prepared by the above method were of a sheet structure, and were assembled into a button-type half-cell with lithium iron manganese phosphate material as the positive electrode and a metal lithium sheet as the negative electrode. The cell was charged to 4.3V at a constant current and voltage of 0.1C (cut off at a current of 0.02C), and then discharged to 2.5V at a constant current of 0.1C. The first discharge capacity of the cell was 145.4mAh / g.

[0078] Table 1 Test data of embodiments and comparative examples

[0079]

[0080]

[0081] According to Table 1, the primary particles of the lithium iron manganese phosphate products prepared in Examples 1-5 are all spherical, and the 0.1C first discharge gram capacity is as high as 157.5 mAh / g. Comparative Example 1 The lithium iron manganese phosphate primary particles prepared by the traditional process are sheet-packed structures, and the 0.1C first discharge gram capacity is lower than that of the present invention. The preparation process of the present invention is simple. Compared with the lithium iron manganese phosphate prepared by traditional ammonium dihydrogen phosphate, the present invention does not produce wastewater, waste and solid waste, is green and environmentally friendly, and is beneficial to large-scale production.

[0082] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for synthesizing a spherical lithium manganese iron phosphate positive electrode material, characterized in that: The synthesis method of the spherical lithium manganese iron phosphate positive electrode material comprises the following steps: (1) adding water to 80-90% phosphoric acid to prepare an 8-20% phosphoric acid solution, adding ferric lactate, heating and stirring at 50-70° C. to dissolve, filtering, adding manganese formate to the filtrate under stirring, heating and reacting at 60-90° C. for 1-3 hours to obtain a mixed slurry; (2) adding battery-grade lithium carbonate to the mixed slurry under stirring conditions, reacting at 70-90° C. for 1-3 hours, and the pH value of the slurry after the addition of the battery-grade lithium carbonate is 7-8 to obtain a lithium manganese iron phosphate precursor slurry; (3) naturally evaporating the lithium manganese iron phosphate precursor slurry under mixed ball milling to obtain a precursor powder, and then calcining the precursor powder in an inert gas to obtain a calcined product; (4) adding 800-1000 kg of water, 15-20 kg of glucose, 3-4 kg of citric acid, 6-9 kg of PEG4000 and 0.8-1.2 kg of magnesium hydroxide to 300-400 kg of the calcined product and wet-milling for 2-5 h to control the particle size and obtain a powder by spray drying; (5) The powder is subjected to secondary calcination in an inert gas to obtain a secondary calcination product, lithium manganese iron phosphate.

2. The method for synthesizing the spherical lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In the step (1), 85% phosphoric acid is added with water to prepare a 10-15% phosphoric acid solution, and iron lactate is added to the solution and heated and stirred at 60°C to dissolve, followed by filtration. Manganese formate is added to the filtrate under stirring, and the solution is heated and reacted at 80°C for 3 hours to obtain a mixed slurry; The molar ratio of the ferric lactate, manganese methoxide and phosphoric acid is 0.3-5:0.5-0.7:1-1.05; the purity of the ferric lactate is greater than 98%; the purity of the manganese methoxide is greater than 98%.

3. The method for synthesizing the spherical lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In the step (2), the molar ratio of lithium and phosphorus elements of battery-grade lithium carbonate and phosphoric acid is 1-1.02:1-1.02, and the battery-grade lithium carbonate is added and reacted at 80° C. for 2 hours.

4. The method for synthesizing the spherical lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In the step (3), the calcination temperature is 500-800° C., the calcination time is 3-8 hours, the inert gas is a nitrogen-hydrogen mixed gas atmosphere, and the volume ratio of nitrogen to hydrogen in the nitrogen-hydrogen mixed gas is 9-9.5:0.5-1.

5. The method for synthesizing the spherical lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In the step (3), the calcination temperature is 600-700° C., the calcination time is 3-5 hours, the inert gas is a nitrogen-hydrogen mixed gas atmosphere, and the volume ratio of nitrogen to hydrogen in the nitrogen-hydrogen mixed gas is 9.5:0.

5.

6. The method for synthesizing the spherical lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In the step (4), 350 kg of the calcined product is added with 900 kg of water, 17 kg of glucose, 3.5 kg of citric acid, 7.5 kg of PEG4000 and 1.05 kg of magnesium hydroxide, and wet ball milled for 3-4 hours to control the particle size, and a powder is obtained by spray drying.

7. The method for synthesizing the spherical lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In the step (4), the particle size is controlled at D500.2-0.6 um, the spray air inlet temperature is controlled at 180-220°C, and the air outlet temperature is controlled at 80-90°C.

8. The method for synthesizing the spherical lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In the step (4), the particle size is controlled at D500.3-0.5um, the spray inlet temperature is controlled at 200°C, and the outlet temperature is controlled at 80°C.

9. The method for synthesizing the spherical lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In the step (5), the secondary calcination temperature is 600-850° C., the secondary calcination time is 5-10 hours, the inert gas is a nitrogen-hydrogen mixed gas atmosphere, and the nitrogen-hydrogen volume ratio of the nitrogen-hydrogen mixed gas is 9-9.5:0.

5.

10. The method for synthesizing the spherical lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In the step (5), the secondary calcination temperature is 700-800° C., the secondary calcination time is 8-10 hours, the inert gas is a nitrogen-hydrogen mixed gas atmosphere, and the nitrogen-hydrogen volume ratio of the nitrogen-hydrogen mixed gas is 9.5:0.5.

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