Preparation method of carbon-coated lithium iron manganese phosphate positive electrode material

Abstract

The invention provides a preparation method of a carbon-coated lithium iron manganese phosphate positive electrode material, and belongs to the technical field of battery positive electrode materials, and the preparation method comprises the following steps: co-precipitating a mixed solution of an iron source, a manganese source and a phosphorus source and a precipitant solution, and drying to obtain a manganese iron phosphate precursor; adding a lithium source, calcining in stages in an inert gas atmosphere, and grinding to obtain lithium manganese iron phosphate powder; the preparation method comprises the following steps: carrying out ball milling on lithium manganese iron phosphate powder and a fluxing agent, dispersing in a carbon source solution, uniformly wrapping a carbon source, drying, carrying out segmented calcination, and carrying out ball milling. The precursor nucleation coating uniformity is improved through segmented speed control reaction of a phosphorus source solution, a manganese source solution and an iron source solution; lithium source mixing and segmented calcination are adopted, and lithium volatilization is reduced; the modified nitrogen-doped porous carbon source is used for coating the lithium iron manganese phosphate powder to generate the lithium iron manganese phosphate positive electrode material, so that the conductivity of the lithium iron manganese phosphate positive electrode material is enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of battery positive electrode materials, and in particular to a method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material. Background Art

[0002] In recent years, lithium-ion batteries have been widely used in electric vehicles, portable electronic devices, and stationary energy storage due to their high energy density, lack of memory effect, and long service life. Olivine-structured lithium iron phosphate batteries, with their high safety, long cycle life, and low cost, have captured half of the lithium-ion battery cathode material market. Layered ternary materials, with their high energy density and excellent low-temperature performance, have dominated the other half. As electric vehicles and various electronic devices continue to demand greater range and stability, the energy density and safety of lithium-ion batteries are also being further enhanced. While lithium iron phosphate batteries offer significant advantages in safety and stability, their energy density is slightly lower than that of ternary materials. Layered ternary batteries, while offering advantages in energy density, suffer from poor thermal stability. With its combined advantages in energy density and safety, lithium manganese iron phosphate, with its high voltage platform, is a new direction for the development of new cathode materials. Compared with lithium iron phosphate, it has high voltage, high energy density and better low-temperature performance; compared with ternary materials, it has the advantages of low cost, high safety and long cycle life.

[0003] However, lithium manganese iron phosphate also has the disadvantages of poor electronic conductivity and lithium ion diffusion, making electron transfer and lithiation / delithiation processes difficult. Currently, preparing lithium manganese iron phosphate materials with small and uniform particle size is an effective solution to these two problems. Among them, the hydrothermal method can easily obtain high-performance lithium manganese iron phosphate. However, the hydrothermal method requires a high-pressure hydrothermal reactor as a reaction vessel. During the reaction, the reactor is under tremendous pressure, posing a significant safety hazard and preventing large-scale industrial production. Summary of the Invention

[0004] The object of the present invention is to provide a method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material to solve the problems raised in the above background technology.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material, characterized by comprising the following steps:

[0007] S1. A phosphorus source, an iron source, and a manganese source were added to deionized water to prepare a phosphorus source solution, an iron source solution, and a manganese source solution. The iron source and manganese source solution were mixed to obtain a manganese-iron mixture;

[0008] S2. The ferromanganese mixture and the phosphorus source solution were added dropwise to the reactor simultaneously, the addition rate was controlled in stages, and a precipitant solution was added to the reactor, the reaction was heated and stirred, and the precipitate was then filtered, washed, dried, and sieved to obtain ferromanganese phosphate precursor powder ①;

[0009] S3 step S2 ferromanganese phosphate precursor powder ① mixed with a lithium source, in an inert gas atmosphere after ball milling staged calcination, and after cooling ground into lithium iron manganese phosphate crystalline powder ②;

[0010] S4 step S3 lithium iron manganese phosphate crystal powder ② and flux was added to the ball mill jar, sealed and evacuated and filled with inert gas, ball milled to obtain lithium iron manganese phosphate precursor powder ③;

[0011] S5. The carbon source powder is ball-milled and mixed, and then heated and carbonized to obtain a nitrogen-doped porous carbon source composite;

[0012] S6. The dispersant and the carbon source complex of step S5 were added to deionized water and dispersed uniformly to obtain a composite carbon source solution; the lithium iron manganese phosphate precursor powder obtained in step S4 was added to the composite carbon source solution ③, dispersed uniformly, and dried to obtain a mixed powder ④;

[0013] S7. The mixed powder ④ of step S6 is calcined in stages in an inert gas, and after cooling, ethanol is added and wet-grinded to obtain a lithium manganese iron phosphate positive electrode material.

[0014] Preferably, in step S1, the iron source includes one or more of ferrous sulfate, hydrated ferrous sulfate, ferrous nitrate and ferrous chloride; the manganese source includes one or more of manganous sulfate, hydrated manganous sulfate, manganous nitrate and manganous chloride; the phosphorus source includes one or more of phosphoric acid, sodium phosphate, potassium phosphate, ammonium dihydrogen phosphate and diammonium hydrogen phosphate, the concentration of the phosphorus source solution is 1-2 mol / L, the concentration of the manganese-iron mixed solution is 0.5-2 mol / L, and the molar ratio of iron to manganese is 2:8-5:5; most preferably, the manganese source is MnSO4·H2O, the iron source is FeSO4·7H2O, and the phosphorus source is H3PO4.

[0015] Preferably, in step S2, the precipitant solution includes aqueous ammonia with a mass fraction of 5%.

[0016] Preferably, in step S3, the lithium source is a mixture of LiOH·H2O and Li2CO3, the molar ratio of LiOH·H2O to Li2CO3 is (2.5-3.5):1, and the mass ratio of ferromanganese phosphate precursor powder ① to Li2CO3 is 1:(0.15-0.4).

[0017] Among them, the low-temperature reactivity of LiOH·H2O can accelerate the initial lithiation, and Li2CO3 supplements the lithium source at the high-temperature stage to reduce lithium volatilization. The use of mixed lithium sources can balance the reactivity and high-temperature stability.

[0018] Preferably, in step S4, the flux comprises boric acid, and the added mass is 0.5-2% of the mass of the lithium manganese iron phosphate crystal powder ②.

[0019] Among them, flux can reduce carbonization temperature, enhance graphitization degree and increase conductivity.

[0020] Preferably, in step S5, the carbon source powder includes tannic acid, CNTs, glucose and urea, and the mass ratio of the four is 1:1:(2-4):(1.5-2.5).

[0021] Among them, tannic acid contains a large number of phenolic hydroxyl groups, which can act as a surfactant to help disperse CNTs, prevent agglomeration, and ensure dispersibility. At the same time, it can form a porous carbon structure after high-temperature carbonization; CNTs form a continuous conductive network; glucose provides sufficient carbon source to form a dense coating layer; urea decomposes to produce NH3, form pores and introduce nitrogen doping to generate nitrogen-doped porous carbon, which improves conductivity and electrolyte wettability.

[0022] Preferably, in step S6, the concentration of the carbon source complex in the composite carbon source solution is 10-15 wt %, the dispersant includes PVP, the dispersant concentration is 0.3-0.6 wt %, and the mass ratio of the lithium manganese iron phosphate precursor powder ③ to the composite carbon source solution is 1:(2-4).

[0023] Preferably, in step S2, the procedure for segmented control of the dropping acceleration is: the flow rate in the first stage is ≤5 mL / min, the pH value is maintained between 8.3 and 8.7, and the duration is 1 hour; the flow rate in the second stage is ≤10 mL / min, the pH value is maintained at 8.5±0.2, and the duration is 2 hours; the reaction temperature is 45-55°C; the conductivity of the washing liquid at the washing end point is ≤50 μS / cm, and the pH value is neutral; the drying temperature is 80-120°C, and the drying time is 12-24 hours; the precipitate D50≈300 nm after aging, and is passed through a 200 mesh sieve after drying.

[0024] Among them, the dropping process adopts a dual-channel segmented rate-controlled dropping process. According to the different reaction rates in the co-precipitation process, the dropping speed of the raw material solution is divided into the initial stage and the dispersion stage. In the initial stage, due to the slow reaction efficiency, the dropping speed of the manganese-iron mixed solution and the phosphorus source solution is reduced to extend the nucleation time, so that the precursor crystal nucleus is formed more uniformly, avoiding local oversaturation-induced grain agglomeration and the generation of amorphous impurities; in the dispersion stage, the reaction efficiency is accelerated, and increasing the flow rate can shorten the time that the crystal stays in the "growth window period", reducing the chance of large particles swallowing small particles through dissolution-redeposition, thereby inhibiting particle coarsening and effectively improving the nucleation and coating uniformity of the precursor.

[0025] Preferably, in step S3, the staged calcination process is: the first stage calcination temperature is 300-400°C, the heating rate is 3°C / min, and the pre-burning time is 1.5-2.5h; the second stage calcination temperature is 700-800°C, the heating rate is 5°C / min, and the time is 6-10h; the inert gas is N2 or Ar; the cooling process is to cool to 200°C in the furnace, and then rapidly cool by Ar gas quenching, and D50≤200nm after cooling.

[0026] Among them, the first stage of calcination decomposes the residual crystal water and NH4 + , to prevent the rapid evaporation of water and volatilization of ammonia in the subsequent high-temperature stage from causing the collapse of the material structure or particle agglomeration, which ultimately affects the electrochemical properties of the material; in the second stage, the temperature is increased to form a complete olivine structure; after the calcination is completed, rapid cooling using Ar gas quenching can inhibit excessive grain growth and effectively control the grain size; at the same time, segmented calcination can also reduce lithium volatilization and improve the structural stability and electrochemical properties of the battery material.

[0027] Preferably, in step S5, the carbonization temperature program is: 600-700°C / 6h, a heating rate of 5°C / min, an Ar / NH3 (9:1) atmosphere, an NH3 flow rate of 50sccm, and the nitrogen content in the obtained carbon source composite is ≥3at%.

[0028] Preferably, in step S7, the staged calcination process is as follows: in the first stage, the calcination temperature is increased from room temperature to 300°C at a heating rate of 5°C / min in an Ar atmosphere; in the second stage, the calcination temperature is increased from 300°C to 600-700°C at a heating rate of 2°C / min in an Ar plus 5% hydrogen mixed gas atmosphere, and the holding time is 4-6 hours; after wet grinding, D50 ≤ 1 μm.

[0029] Among them, low-temperature calcination is used in the first stage to avoid interface peeling between the carbon layer and the precursor due to the difference in thermal expansion coefficient; the heating rate is reduced in the high-temperature stage to allow the lattice to relax slowly, reduce the internal stress caused by carbon layer shrinkage or precursor lattice fine-tuning, prevent particle cracking, optimize the interface bonding between the lithium manganese iron phosphate precursor and the carbon coating layer, and improve the electrochemical properties of the prepared material.

[0030] Another aspect of the present invention discloses a lithium manganese iron phosphate positive electrode material prepared by the method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material described in any of the above technical solutions.

[0031] The beneficial effects of the above technical solution of the present invention are as follows:

[0032] 1. The present invention discloses a dual-channel segmented rate-controlled drop addition process. According to the different reaction rates in the co-precipitation process, the drop addition speed of the raw material solution is divided into an initial stage and a dispersion stage. Compared with the traditional uniform flow rate technology, the present invention can avoid particle coarsening caused by local oversaturation, effectively improve the nucleation and coating uniformity of the precursor, and the prepared positive electrode material has better performance after being used in batteries.

[0033] 2. Use a mixed lithium source, take advantage of the low-temperature reaction activity of LiOH·H2O to accelerate the initial lithiation, and use Li2CO3 to supplement the lithium source at the high-temperature stage to reduce lithium volatilization, balance the reaction activity and high-temperature stability, and improve the cycle life of the battery material.

[0034] 3. Tannic acid contains a large number of phenolic hydroxyl groups, which may act as a surfactant to help disperse CNTs and prevent agglomeration. At the same time, it can form a porous carbon structure after high-temperature carbonization; CNTs provide a conductive network; glucose provides sufficient carbon source to form a dense coating layer; urea decomposes to produce NH3, complete pore formation and introduce nitrogen doping, improving conductivity and electrolyte wettability.

[0035] 4. When the ferromanganese phosphate precursor powder is mixed with the lithium source, a staged calcination process is adopted. The first stage of calcination decomposes the residual crystal water and NH4 + , to prevent the rapid evaporation of water and volatilization of ammonia in the subsequent high-temperature stage from causing the collapse of the material structure or particle agglomeration, which ultimately affects the electrochemical properties of the material; in the second stage, the temperature is increased to form a complete olivine structure; after the calcination is completed, rapid cooling using Ar gas quenching can inhibit excessive grain growth and effectively control the grain size; at the same time, segmented calcination can also reduce lithium volatilization and improve the structural stability and electrochemical properties of the battery material.

[0036] 5. Add flux to lithium manganese iron phosphate powder during the ball milling process to reduce the carbonization temperature and enhance the graphitization degree of the carbon layer, thereby improving conductivity.

[0037] 6. After the lithium manganese iron phosphate precursor powder is coated with the carbon source, the segmented calcination process is still used. The low-temperature calcination stage can avoid the interface peeling between the carbon layer and the precursor due to the difference in thermal expansion coefficient; the high-temperature stage reduces the heating rate to allow the lattice to relax slowly, reducing the internal stress caused by the shrinkage of the carbon layer or the fine-tuning of the precursor lattice, and preventing the particles from cracking. Compared with the traditional one-step calcination process, this scheme optimizes the interface bonding between the lithium manganese iron phosphate precursor and the carbon coating layer, and improves the electrochemical properties of the prepared material. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The above and other objects, features and advantages of the exemplary embodiments of the present invention will become readily understood by reading the following detailed description with reference to the accompanying drawings. In the accompanying drawings, several embodiments of the present invention are shown in an illustrative and non-limiting manner, and the same or corresponding reference numerals represent the same or corresponding parts, wherein:

[0039] Figure 1 Flowchart of the present invention;

[0040] Figure 2 This is the SEM image of the lithium manganese iron phosphate positive electrode material obtained in Example 1; DETAILED DESCRIPTION

[0041] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0042] If specific experimental procedures or conditions are not specified in this protocol, the procedures or conditions described in the literature in this field can be followed. Reagents or instruments used without manufacturer's information are commercially available.

[0043] Example

[0044] S1. Dissolve the prepared phosphorus source (H3PO4), iron source (FeSO4·7H2O), and manganese source (MnSO4·H2O) with a purity of ≥99.5% in deionized water. The phosphorus source concentration is 1-2 mol / L, and the iron-manganese mixed solution: the total metal concentration is 0.5-2 mol / L, wherein the Fe:Mn molar ratio is 2:8-5:5.

[0045] S2. A dual-channel peristaltic pump is used to synchronously add the manganese iron mixture and the phosphorus source. In the initial stage, the flow rate is ≤5mL / min, the pH is 8.3~8.7 (adjusted with 5% ammonia water), and the addition duration is 1h. Then, in the dispersion stage, the flow rate is ≤10mL / min, the pH is 8.5±0.2, and the addition duration is 2h. The reaction temperature is 45~55°C. After the reaction is completed, stirring is continued for 2h. After that, the precipitate is filtered, washed, dried, and sieved. The conductivity of the washing liquid at the end of washing is ≤50μS / cm, and the pH value is neutral. The drying temperature is 80~120°C, the drying time is 12~24h, and after drying, it is sieved through a 200-mesh sieve to obtain manganese iron phosphate precursor powder①.

[0046] S3. The lithium source was prepared according to LiOH·H2O:Li2CO3=(2.5~3.5):1 (molar ratio), the mass ratio of manganese iron phosphate precursor powder ① to Li2CO3 was 1:(0.15~0.4), dry ball milled for 2h under Ar atmosphere, and then calcined in stages. The first stage calcination temperature was 300~400℃, the heating rate was 3℃ / min, and the pre-calcination time was 1.5~2.5h. The second stage calcination temperature was 700~800℃, the heating rate was 5℃ / min, and the time was 6~10h; the inert gas was Ar; after calcination, the mixture was cooled to 200℃ in the furnace, and then rapidly cooled by Ar gas quenching, and ground to obtain lithium manganese iron phosphate crystal powder ②.

[0047] S4. Add lithium iron manganese phosphate crystal powder ② and 0.5-2 wt% H3BO3 in proportion to a ball mill jar, seal it, evacuate it, fill it with Ar, and ball mill it. Cooling gas is introduced to prevent overheating to obtain lithium iron manganese phosphate precursor powder ③.

[0048] S5. Tannic acid, CNTs, glucose, and urea were ball-milled in a mass ratio of 1:1:(2-4):(1.5-2.5). The carbonization procedure was: 600-700°C / 6h, a heating rate of 5°C / min, an Ar / NH3 (9:1) atmosphere, and an NH3 flow rate of 50 sccm to obtain a nitrogen-doped porous carbon source composite.

[0049] S6. Add the carbon source complex and dispersant to deionized water and stir at 45°C until completely dissolved and dispersed to obtain a composite carbon source solution, wherein the carbon source complex concentration is 10-15 wt % and the PVP concentration is 0.3-0.6 wt %. The lithium manganese iron phosphate precursor powder ③ from step S4 is mixed with the composite carbon source solution in a mass ratio of 1:(2-4), stirred evenly after ultrasonication, and dried to obtain a mixed powder ④.

[0050] S7. The mixed powder ④ is calcined in stages. In the first stage, the calcination temperature is raised from room temperature to 300°C at a heating rate of 5°C / min in an Ar atmosphere. In the second stage, the calcination temperature is raised from 300°C to 600-700°C at a heating rate of 2°C / min in an Ar plus 5% hydrogen mixed gas atmosphere for 4-6 hours. The mixture is then furnace-cooled to room temperature in an Ar atmosphere, wet-ground with the addition of ethanol, and sieved to obtain a uniform powder, which is the lithium manganese iron phosphate positive electrode material.

[0051] Example 1

[0052] The carbon-coated lithium manganese iron phosphate cathode material preparation method described in the above embodiment is adopted, wherein:

[0053] S1. Phosphorus source concentration is 1.5 mol / L, iron-manganese mixed solution: total metal concentration is 1.0 mol / L, where the Fe:Mn molar ratio is 3:7.

[0054] S2. The initial stage is at a flow rate of 4 mL / min, then the dispersion stage is at a flow rate of 8 mL / min, the reaction temperature is 50°C, the drying temperature is 80°C, and the drying time is 24 h.

[0055] S3. The lithium source was prepared by LiOH·H2O:Li2CO3=3:1 (molar ratio), the mass ratio of ferromanganese phosphate precursor powder ① to Li2CO3 was 1:0.25, the first stage calcination temperature was 350℃, the pre-calcination time was 2h, the second stage calcination temperature was 750℃, and the time was 8h

[0056] S4. Add lithium manganese iron phosphate crystal powder ② and 1 wt% H3BO3 in proportion to the ball mill.

[0057] S5. Tannic acid, CNTs, glucose, and urea were ball-milled in a mass ratio of 1:1:3:2 and carbonized at a temperature of 650°C.

[0058] S6. The carbon source complex concentration in the composite carbon source solution is 15 wt %, the PVP concentration is 0.6 wt %, and the lithium manganese iron phosphate precursor powder ③ is mixed with the composite carbon source solution in a mass ratio of 1:2.

[0059] S7. Second stage of staged calcination: heat from 300°C to 700°C and keep warm for 5 hours.

[0060] Example 2

[0061] The difference between it and Example 1 is that:

[0062] S3. The lithium source was prepared according to the molar ratio of LiOH·H2O:Li2CO3=3.5:1, the mass ratio of ferromanganese phosphate precursor powder ① to Li2CO3 was 1:0.4, the first stage calcination temperature was 300℃, the pre-calcination time was 2.5h, and the second stage calcination temperature was 700℃, and the time was 8h.

[0063] Example 3

[0064] The difference between it and Example 1 is that:

[0065] S3. The lithium source was prepared according to the molar ratio of LiOH·H2O:Li2CO3=2.5:1, the mass ratio of ferromanganese phosphate precursor powder ① to Li2CO3 was 1:0.15, the first stage calcination temperature was 300℃, the pre-calcination time was 2.5h, and the second stage calcination temperature was 700℃, and the time was 10h.

[0066] Example 4

[0067] The difference between it and Example 1 is that:

[0068] S6. The carbon source complex concentration in the composite carbon source solution is 12 wt %, the PVP concentration is 0.5 wt %, and the lithium manganese iron phosphate precursor powder ③ is mixed with the composite carbon source solution in a mass ratio of 1:3.

[0069] Example 5

[0070] The difference between it and Example 1 is that:

[0071] S6. The carbon source complex concentration in the composite carbon source solution is 10 wt %, the PVP concentration is 0.3 wt %, and the lithium manganese iron phosphate precursor powder ③ is mixed with the composite carbon source solution in a mass ratio of 1:4.

[0072] Example 6

[0073] The difference between it and Example 1 is that:

[0074] S1. Phosphorus source concentration is 2 mol / L, iron-manganese mixed solution: total metal concentration is 2 mol / L, where the Fe:Mn molar ratio is 2:8.

[0075] S2. Reaction temperature 55°C, drying temperature 120°C, drying time 12h.

[0076] S3. The mass ratio of ferromanganese phosphate precursor powder ① to Li2CO3 is 1:0.4. The first stage calcination temperature is 400℃, the pre-calcination time is 1.5h, and the second stage calcination temperature is 800℃, and the time is 6h.

[0077] S4. Add lithium manganese iron phosphate crystal powder ② and 2 wt% H3BO3 in proportion to the ball mill.

[0078] S5. Tannic acid, CNTs, glucose, and urea were ball-milled in a mass ratio of 1:1:4:2.5, and the carbonization temperature was 700°C.

[0079] S7. Second stage of staged calcination: heat from 300°C to 700°C and keep at this temperature for 6 hours.

[0080] Example 7

[0081] The difference between it and Example 1 is that:

[0082] S1. Phosphorus source concentration is 1 mol / L, iron-manganese mixed solution: total metal concentration is 0.5 mol / L, where the Fe:Mn molar ratio is 5:5.

[0083] S2. Reaction temperature 45°C, drying temperature 80°C, drying time 18h.

[0084] S3. The mass ratio of the drive powder ① to Li2CO3 is 1:0.15. The first stage calcination temperature is 300℃, the pre-calcination time is 1.5h, and the second stage calcination temperature is 700℃, the time is 6h

[0085] S4. Add lithium manganese iron phosphate crystal powder ② and 0.5wt% H3BO3 into a ball mill in proportion.

[0086] S5. Tannic acid, CNTs, glucose, and urea were ball-milled in a mass ratio of 1:1:3:1.5 and carbonized at a temperature of 600°C.

[0087] S7. Second stage of staged calcination: heat from 300°C to 650°C and keep at this temperature for 4 hours.

[0088] Comparative Example 1

[0089] The difference from Example 1 is that the lithium source in step S3 is pure LiOH·H2O.

[0090] Comparative Example 2

[0091] The difference from Example 1 is that the lithium source in step S3 is pure Li2CO3.

[0092] Comparative Example 3

[0093] The difference from Example 1 is that no flux is added in step S4.

[0094] Comparative Example 4

[0095] The difference from Example 1 is that in step S2, a uniform flow rate of 8 mL / min is used, the pH value is adjusted to 8.5±0.2 with 5% ammonia water, and the duration is 2.5 h.

[0096] Comparative Example 5

[0097] The difference from Example 1 is that: in step S3, one-step calcination is adopted, and the calcination procedure is: calcination at 750°C / 9h in Ar atmosphere, heating rate 5°C / min, and cooling to room temperature with the furnace after calcination.

[0098] Comparative Example 6

[0099] The difference from Example 1 is that in step S5, tannic acid, CNTs, and glucose are powdered and mixed by ball milling in a mass ratio of 1:1:3.

[0100] Comparative Example 7

[0101] The difference from Example 1 is that a one-step calcination is adopted in step S7, and the calcination procedure is: room temperature to 700°C, a heating rate of 5°C / min, in an atmosphere of Ar plus 5% H2 mixed gas, and a holding time of 5h.

[0102] The lithium iron manganese phosphate positive electrode material and the lithium counter electrode were assembled into button cells, and the charge and discharge cycle performance tests were carried out on Examples 1 to 7 and Comparative Examples 1 to 7. The charge and discharge specific capacities at 1C and 5C rates and the capacity retention rate after 1000 cycles were tested, respectively. The test results are shown in Table 1.

[0103] Table 1. Performance test results

[0104]

[0105]

[0106] It can be seen from the data in the table that, compared with Example 1, the performance indicators of Examples 2 to 7 are not much different. Compared with Example 1, the performance indicators of Examples 1 to 7 are significantly reduced. Comparative Examples 1 to 3 illustrate that the use of mixed lithium sources and the addition of flux can improve the performance of the battery; Comparative Example 4 illustrates that the dual-channel segmented rate-controlled droplet addition process can improve the nucleation and coating uniformity of the precursor, thereby improving the performance of the battery; Comparative Example 5 illustrates that the step-by-step calcination process in the lithium source addition stage helps to form a complete olivine structure while reducing lithium volatilization; after calcination, the use of Ar gas quenching and rapid cooling can inhibit excessive grain growth, effectively control the grain size, and improve battery performance; Comparative Example 6 illustrates that the introduction of porous nitrogen doping by urea can improve conductivity and electrolyte wettability; Comparative Example 7 illustrates that the step-by-step calcination process in the carbon source coating stage avoids the interface peeling between the carbon layer and the precursor due to the difference in thermal expansion coefficient, thereby improving the electrochemical performance of the prepared material.

[0107] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention and are not limiting. Other modifications or equivalent substitutions made to the technical solution of the present invention by ordinary technicians in this field should be included in the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material, characterized in that: The steps include: S1. A phosphorus source, an iron source, and a manganese source were added to deionized water to prepare a phosphorus source solution, an iron source solution, and a manganese source solution. The iron source and manganese source solution were mixed to obtain a manganese-iron mixture; S2. The ferromanganese mixture and the phosphorus source solution were added dropwise to the reactor simultaneously, the addition rate was controlled in stages, and a precipitant solution was added to the reactor, the reaction was heated and stirred, and the precipitate was then filtered, washed, dried, and sieved to obtain ferromanganese phosphate precursor powder ①; S3 step S2 ferromanganese phosphate precursor powder ① mixed with a lithium source, in an inert gas atmosphere after ball milling staged calcination, and after cooling ground into lithium iron manganese phosphate crystalline powder ②; S4 step S3 lithium iron manganese phosphate crystal powder ② and flux was added to the ball mill jar, sealed and evacuated and filled with inert gas, ball milled to obtain lithium iron manganese phosphate precursor powder ③; S5. The carbon source powder is ball-milled and mixed, and then heated and carbonized to obtain a nitrogen-doped porous carbon source composite; S6. The dispersant and the carbon source complex of step S5 were added to deionized water and dispersed uniformly to obtain a composite carbon source solution; the lithium iron manganese phosphate precursor powder obtained in step S4 was added to the composite carbon source solution ③, dispersed uniformly, and dried to obtain a mixed powder ④; S7. The mixed powder ④ of step S6 is calcined in stages in an inert gas, and wet-milled after cooling to obtain a lithium manganese iron phosphate positive electrode material.

2. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step S1, the iron source includes one or more of ferrous sulfate, hydrated ferrous sulfate, ferrous nitrate, and ferrous chloride; the manganese source includes one or more of manganous sulfate, hydrated manganous sulfate, manganous nitrate, and manganous chloride; the phosphorus source includes one or more of phosphoric acid, sodium phosphate, potassium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; in step S2, the precipitant solution includes aqueous ammonia; in step S4, the flux includes boric acid, and the added mass is 0.5-2% of the mass of the lithium manganese iron phosphate crystal powder ②.

3. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step S3, the lithium source is a mixture of LiOH·H2O and Li2CO3, the molar ratio of LiOH·H2O to Li2CO3 is (2.5-3.5):1, and the mass ratio of ferromanganese phosphate precursor powder ① to Li2CO3 is 1:(0.15-0.4).

4. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step S5, the carbon source powder includes tannic acid, CNTs, glucose and urea, and the mass ratio of the four is 1:1:(2-4):(1.5-2.5).

5. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step S6, the concentration of the carbon source complex in the composite carbon source solution is 10-15 wt %, the dispersant includes PVP, the dispersant concentration is 0.3-0.6 wt %, and the mass ratio of the lithium manganese iron phosphate precursor powder ③ to the composite carbon source solution is 1:(2-4).

6. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step S2, the procedure for segmented control of the dropping acceleration is: in the first stage, the flow rate is ≤5 mL / min, the pH value is maintained between 8.3 and 8.7, and the duration is 1 hour; in the second stage, the flow rate is ≤10 mL / min, the pH value is maintained at 8.5±0.2, and the duration is 2 hours.

7. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step S3, the staged calcination process is as follows: the calcination temperature of the first stage is 300-400°C, the heating rate is 3°C / min, and the pre-calcination time is 1.5-2.5h; the calcination temperature of the second stage is 700-800°C, the heating rate is 5°C / min, and the time is 6-10h; the cooling process is to cool to 200°C in the furnace, and then rapidly cool by Ar gas quenching, and D50≤200nm after cooling.

8. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step S5, the carbonization temperature program is: 600-700°C / 6h, a heating rate of 5°C / min, an Ar / NH3 (9:1) atmosphere, and the nitrogen content in the obtained carbon source composite is ≥3at%.

9. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step S7, the staged calcination process is as follows: the calcination temperature in the first stage is increased from room temperature to 300°C at a heating rate of 5°C / min in an Ar atmosphere; the calcination temperature in the second stage is increased from 300°C to 600-700°C at a heating rate of 2°C / min in an Ar plus 5% H2 mixed gas atmosphere, and the holding time is 4-6 hours; D50 after wet grinding is ≤1μm.

10. The lithium manganese iron phosphate positive electrode material prepared according to the method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 9.

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