A method for preparing a carbon-coated lithium iron manganese phosphate positive electrode material

Abstract

This invention provides a method for preparing carbon-coated lithium manganese iron phosphate cathode material, belonging to the field of battery cathode material technology. The method includes the following steps: co-precipitating a mixed solution of iron, manganese, and phosphorus sources with a precipitant solution, and drying to obtain a lithium manganese iron phosphate precursor; adding a lithium source, and calcining and grinding in an inert gas atmosphere to obtain lithium manganese iron phosphate powder; ball milling the lithium manganese iron phosphate powder with a flux, dispersing it in a carbon source solution, uniformly coating the carbon source, and then drying, calcining in stages, and ball milling to obtain the final product. By controlling the rate of reaction in stages with the phosphorus, manganese, and iron source solutions, the uniformity of precursor nucleation and coating is improved; the use of a mixed lithium source and staged calcination reduces lithium volatilization; and coating the lithium manganese iron phosphate powder with a modified nitrogen-doped porous carbon source generates the lithium manganese iron phosphate cathode material, enhancing its conductivity.

Description

Technical Field

[0001] This invention relates to the field of battery cathode material technology, specifically to a method for preparing carbon-coated lithium manganese iron phosphate cathode material. Background Technology

[0002] In recent years, lithium-ion batteries have been widely used in electric vehicles, portable electronic devices, and stationary energy storage due to their advantages such as high energy density, no memory effect, and long lifespan. Among them, lithium iron phosphate batteries with an olivine structure occupy half of the lithium-ion battery cathode material market due to their high safety performance, long cycle life, and low operating costs. Meanwhile, layered ternary materials occupy the other half of the lithium-ion battery cathode material market due to their high energy density and excellent low-temperature performance. With the increasing demands for range and stability from electric vehicles and various electronic devices, higher requirements are being placed on the energy density and safety of lithium-ion batteries. While lithium iron phosphate batteries have outstanding advantages in safety and stability, their energy density is slightly lower than that of ternary materials. While layered ternary batteries have an energy density advantage, their thermal stability is poor. Considering the advantages in both energy density and safety, lithium manganese iron phosphate with a high voltage platform represents one of the new directions for the development of novel cathode materials. Compared to lithium iron phosphate, it has higher voltage, higher energy density and better low-temperature performance; compared to ternary materials, it has advantages such as lower cost, higher safety and longer cycle life.

[0003] However, lithium manganese iron phosphate also suffers from 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. The hydrothermal method readily yields high-performance lithium manganese iron phosphate, but it requires a high-pressure hydrothermal reactor as the reaction vessel. During the reaction, the pressure inside the reactor is enormous, posing significant safety hazards and hindering large-scale industrial production. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing carbon-coated lithium manganese iron phosphate cathode material to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

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

[0007] S1. Add phosphorus source, iron source and manganese source to deionized water respectively to prepare phosphorus source solution, iron source solution and manganese source solution. Mix the iron source solution and manganese source solution to obtain manganese iron mixed solution.

[0008] S2. The manganese-iron mixture and the phosphorus source solution were simultaneously added dropwise to the reactor, and the dropping rate was controlled in stages. At the same time, a precipitant solution was added to the reactor, and the reaction was heated and stirred. Afterwards, the precipitate was filtered, washed, dried and sieved to obtain manganese-iron phosphate precursor powder ①.

[0009] S3. Mix the manganese iron phosphate precursor powder ① from step S2 with a lithium source, ball mill in an inert gas atmosphere, calcine in stages, cool and grind into lithium manganese iron phosphate crystal powder ②.

[0010] S4. Add the lithium manganese iron phosphate crystal powder ② from step S3 and the flux to a ball mill jar, seal it, evacuate it and fill it with inert gas, and ball mill to obtain lithium manganese iron phosphate precursor powder ③.

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

[0012] S6. Add the dispersant and the carbon source complex from step S5 to deionized water and disperse evenly to obtain a composite carbon source solution; add the lithium manganese iron phosphate precursor powder ③ obtained from step S4 to the composite carbon source solution, disperse evenly, and dry to obtain a mixed powder ④.

[0013] S7. The mixed powder ④ from step S6 is calcined in stages under an inert gas, cooled, and then wet-milled with ethanol to obtain lithium manganese iron phosphate cathode 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 manganese sulfate, hydrated manganese sulfate, manganese nitrate, and manganese chloride; the phosphorus source includes one or more of phosphoric acid, sodium phosphate, potassium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate, wherein the concentration of the phosphorus source solution is 1-2 mol / L, the concentration of the manganese-iron mixture 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 ammonia water 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 manganese iron 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, while Li2CO3 can supplement the lithium source at high temperature and reduce lithium volatilization. Using a mixed lithium source can balance reactivity and high-temperature stability.

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

[0019] Fluxes can lower the carbonization temperature, enhance the degree of graphitization, and increase electrical conductivity.

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

[0021] 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, which creates pores and introduces nitrogen doping to generate nitrogen-doped porous carbon, improving 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 with a concentration of 0.3-0.6 wt%, and the mass ratio of 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 rate is as follows: 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 h; in the second stage, the flow rate is ≤10 mL / min, the pH value is maintained between 8.5 and 0.2, and the duration is 2 h; the reaction temperature is 45 to 55 °C; the conductivity of the washing solution at the washing endpoint is ≤50 μS / cm, and the pH value is neutral; the drying temperature is 80 to 120 °C, and the drying time is 12 to 24 h; after aging, the precipitate has a D50 of ≈300 nm and passes through a 200-mesh sieve after drying.

[0024] The dropping process employs a dual-channel segmented rate-controlled dropping technique. Based on the different reaction rates during co-precipitation, the dropping rate of the raw material solution is divided into an initial stage and a dispersion stage. In the initial stage, due to the slow reaction efficiency, the dropping rate of the manganese-iron mixture and the phosphorus source solution is reduced to prolong the nucleation time, making the precursor crystal nuclei more uniform and avoiding crystal agglomeration and the generation of amorphous impurities caused by local supersaturation. In the dispersion stage, the reaction efficiency increases, and increasing the flow rate can shorten the time that the crystals remain in the "growth window period," reducing the opportunity for large particles to engulf small particles through dissolution-redeposition, thereby inhibiting particle coarsening and effectively improving the uniformity of precursor nucleation and coating.

[0025] Preferably, in step S3, the segmented calcination process is as follows: the first stage calcination temperature is 300-400℃, the heating rate is 3℃ / min, and the pre-calcination time is 1.5-2.5h; the second stage calcination temperature is 700-800℃, the heating rate is 5℃ / min, and the time is 6-10h; the inert gas is N2 or Ar; the cooling process is to cool with the furnace to 200℃, and then use Ar gas quenching for rapid cooling, and after cooling, D50≤200nm.

[0026] The first stage of calcination decomposes residual water of crystallization and NH4. + To prevent rapid evaporation of moisture and volatilization of ammonia during the subsequent high-temperature stage, which could lead to material structure collapse or particle agglomeration and ultimately affect the electrochemical performance of the material; the second stage increases the temperature to form a complete olivine structure; after calcination, rapid cooling with Ar gas quenching can suppress excessive grain growth and effectively control grain size; at the same time, segmented calcination can also reduce lithium volatilization and improve the structural stability and electrochemical performance of the battery material.

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

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

[0029] The first stage employs low-temperature calcination to avoid interfacial delamination between the carbon layer and the precursor due to differences in thermal expansion coefficients. The high-temperature stage reduces the heating rate, allowing the lattice to relax slowly, reducing internal stress caused by carbon layer shrinkage or precursor lattice fine-tuning, preventing particle cracking, optimizing the interfacial bonding between the lithium manganese iron phosphate precursor and the carbon coating layer, and improving the electrochemical performance of the prepared material.

[0030] In another aspect, this invention discloses lithium manganese iron phosphate cathode materials prepared by the carbon-coated lithium manganese iron phosphate cathode material preparation method described in any of the above technical solutions.

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

[0032] 1. This invention discloses a dual-channel segmented rate-controlled dropping process. Based on the different reaction rates during co-precipitation, the dropping rate of the raw material solution is divided into an initial stage and a dispersion stage. Compared with the traditional uniform flow rate technology, this invention can avoid particle coarsening caused by local oversaturation, effectively improve the nucleation and coating uniformity of the precursor, and the prepared cathode material has better performance when applied to the battery.

[0033] 2. Using a mixed lithium source, the low-temperature reactivity of LiOH·H2O is used to accelerate the initial lithiation, while Li2CO3 supplements the lithium source at high temperatures, reducing lithium volatilization, balancing reactivity and high-temperature stability, and improving the cycle life of battery materials.

[0034] 3. Tannic acid contains a large number of phenolic hydroxyl groups, which may act as a surfactant to help disperse CNTs and prevent aggregation. At the same time, high-temperature carbonization can form a porous carbon structure. CNTs provide a conductive network. Glucose provides sufficient carbon source to form a dense coating layer. Urea decomposes to produce NH3, which completes pore formation and introduces nitrogen doping to improve conductivity and electrolyte wettability.

[0035] 4. When mixing the manganese iron phosphate precursor powder with the lithium source, a staged calcination process is adopted. The first stage of calcination decomposes residual water of crystallization and NH4. + To prevent rapid evaporation of moisture and volatilization of ammonia during the subsequent high-temperature stage, which could lead to material structure collapse or particle agglomeration and ultimately affect the electrochemical performance of the material; the second stage increases the temperature to form a complete olivine structure; after calcination, rapid cooling with Ar gas quenching can suppress excessive grain growth and effectively control grain size; at the same time, segmented calcination can also reduce lithium volatilization and improve the structural stability and electrochemical performance of the battery material.

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

[0037] 6. After the lithium manganese iron phosphate precursor powder is coated with carbon source, a segmented calcination process is still adopted. The low-temperature calcination stage can avoid the interface delamination between the carbon layer and the precursor due to the difference in thermal expansion coefficients. The high-temperature stage reduces the heating rate, allows the lattice to relax slowly, reduces the internal stress caused by carbon layer shrinkage or precursor lattice fine adjustment, and prevents particle 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 performance of the prepared material. Attached Figure Description

[0038] The above and other objects, features, and advantages of exemplary embodiments of the present invention will become readily apparent upon reading the following detailed description with reference to the accompanying drawings. In the drawings, several embodiments of the invention are illustrated by way of example and not limitation, and like or corresponding reference numerals denote like or corresponding parts, wherein:

[0039] Figure 1 This is a flowchart of the present invention;

[0040] Figure 2 SEM image of the lithium iron phosphate cathode material obtained in Example 1; Detailed Implementation

[0041] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0042] For experiments not specified in this protocol, the procedures and conditions described in the literature in this field should be followed. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0043] Example

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

[0045] S2. A manganese-iron mixture and a phosphorus source were simultaneously added dropwise using a dual-channel peristaltic pump. In the initial stage, the mixture was added at a flow rate ≤ 5 mL / min and a pH of 8.3–8.7 (adjusted with 5% ammonia) for 1 hour. Then, in the dispersion stage, the mixture was added at a flow rate ≤ 10 mL / min and a pH of 8.5 ± 0.2 for 2 hours. The reaction temperature was 45–55 °C. After the reaction was completed, stirring was continued for 2 hours. The precipitate was then filtered, washed, dried, and sieved. The washing endpoint had a conductivity ≤ 50 μS / cm and a neutral pH. The drying temperature was 80–120 °C, and the drying time was 12–24 hours. After drying, the precipitate was sieved through a 200-mesh sieve to obtain manganese-iron phosphate precursor powder ①.

[0046] S3. Prepare a lithium source according to the molar ratio of LiOH·H2O:Li2CO3=(2.5~3.5):1. The mass ratio of manganese iron phosphate precursor powder ① to Li2CO3 is 1:(0.15~0.4). Dry ball mill under Ar atmosphere for 2h, and then calcine in stages. The first stage calcine temperature is 300~400℃, the heating rate is 3℃ / min, and the pre-calcine time is 1.5~2.5h. The second stage calcine temperature is 700~800℃, the heating rate is 5℃ / min, and the time is 6~10h. The inert gas is Ar. After calcine, cool to 200℃ in the furnace and then rapidly cool by Ar gas quenching. Grind to obtain lithium manganese iron phosphate crystal powder ②.

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

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

[0049] S6. Add deionized water to the carbon source complex and dispersant, and stir at 45°C until completely dissolved and dispersed to obtain a composite carbon source solution, wherein the concentration of the carbon source complex is 10-15 wt% and the concentration of PVP is 0.3-0.6 wt%. Mix the lithium manganese iron phosphate precursor powder ③ from step S4 with the composite carbon source solution at a mass ratio of 1:(2-4), sonicate and stir evenly, and dry 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℃ at a rate of 5℃ / min in an Ar atmosphere. In the second stage, the calcination temperature is raised from 300℃ to 600-700℃ at a rate of 2℃ / min in an Ar-5% hydrogen mixed gas atmosphere for 4-6 hours. The mixture is then cooled to room temperature in an Ar atmosphere, wet-milled with ethanol, and sieved to obtain a uniform powder, which is the lithium manganese iron phosphate cathode material.

[0051] Example 1

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

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

[0054] S2. The initial stage was carried out at a flow rate of 4 mL / min, followed by a dispersion stage at a flow rate of 8 mL / min, a reaction temperature of 50 °C, a drying temperature of 80 °C, and a drying time of 24 h.

[0055] S3. Prepare the lithium source according to the molar ratio of LiOH·H2O:Li2CO3 = 3:1. The mass ratio of manganese iron phosphate precursor powder ① to Li2CO3 is 1:0.25. The first stage calcination temperature is 350℃ and the pre-calcination time is 2h. The second stage calcination temperature is 750℃ and the time is 8h.

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

[0057] S5. Tannic acid, CNTs, glucose and urea are ball-milled and mixed in a mass ratio of 1:1:3:2, and the carbonization temperature is 650℃.

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

[0059] S7. Second stage of segmented calcination: Heat from 300℃ to 700℃ and hold for 5 hours.

[0060] Example 2

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

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

[0063] Example 3

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

[0065] S3. Prepare the lithium source according to the molar ratio of LiOH·H2O:Li2CO3 = 2.5:1. The mass ratio of manganese iron phosphate precursor powder ① to Li2CO3 is 1:0.15. The first stage calcination temperature is 300℃ and the pre-calcination time is 2.5h. The second stage calcination temperature is 700℃ and the time is 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 12wt%, the PVP concentration is 0.5wt%, and the lithium manganese iron phosphate precursor powder ③ is mixed with the composite carbon source solution at 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 10wt%, the PVP concentration is 0.3wt%, and the lithium manganese iron phosphate precursor powder ③ is mixed with the composite carbon source solution at 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, wherein the Fe:Mn molar ratio is 2:8.

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

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

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

[0078] S5. Tannic acid, CNTs, glucose and urea are ball-milled and mixed in a mass ratio of 1:1:4:2.5, and the carbonization temperature is 700℃.

[0079] S7. Second stage of segmented calcination: Heat from 300℃ to 700℃ and hold 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℃, drying temperature 80℃, drying time 18h.

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

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

[0086] S5. Tannic acid, CNTs, glucose and urea are ball-milled and mixed in a mass ratio of 1:1:3:1.5, and the carbonization temperature is 600℃.

[0087] S7. Second stage of segmented calcination: Heat from 300℃ to 650℃ and hold for 4 hours.

[0088] Comparative Example 1

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

[0090] Comparative Example 2

[0091] The difference between this example and Example 1 is that the lithium source in step S3 is pure Li2CO3.

[0092] Comparative Example 3

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

[0094] Comparative Example 4

[0095] The difference between this and 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, and the duration is 2.5 h.

[0096] Comparative Example 5

[0097] The difference between this and Example 1 is that step S3 uses a one-step calcination process, which is as follows: calcination is carried out in an Ar atmosphere at 750℃ / 9h with a heating rate of 5℃ / min, and the furnace is cooled to room temperature after calcination.

[0098] Comparative Example 6

[0099] The difference between this and Example 1 is that in step S5, tannic acid, CNTs, and glucose are ball-milled and mixed in a mass ratio of 1:1:3.

[0100] Comparative Example 7

[0101] The difference between this and Example 1 is that step S7 uses a one-step calcination process, which is as follows: from room temperature to 700°C, the heating rate is 5°C / min, and the process is carried out in an atmosphere of Ar plus 5% H2 mixed gas, with a holding time of 5h.

[0102] The lithium manganese iron phosphate cathode material was assembled with a lithium counter electrode to form a coin cell. Charge-discharge cycle performance tests were conducted on Examples 1-7 and Comparative Examples 1-7. The charge-discharge specific capacity and capacity retention rate after 1000 cycles at 1C and 5C rates were tested respectively. The test results are shown in Table 1.

[0103] Table 1. Performance Test Results

[0104]

[0105]

[0106] As can be seen from the data in the table, the performance indicators of Examples 2-7 are not significantly different compared to Example 1. Compared to Example 1, the performance indicators of Comparative Examples 1-7 are significantly reduced. Comparative Examples 1-3 show that using a mixed lithium source and adding flux can improve battery performance; Comparative Example 4 shows that the dual-channel segmented rate-controlled dropping process can improve the uniformity of precursor nucleation and coating, thereby improving battery performance; Comparative Example 5 shows that the stepwise calcination process in the lithium source addition stage helps to form a complete olivine structure while reducing lithium volatilization; after calcination, rapid cooling with Ar gas quenching can suppress excessive grain growth, effectively control grain size, and improve battery performance; Comparative Example 6 shows that introducing porous nitrogen doping through urea can improve conductivity and electrolyte wettability; Comparative Example 7 shows that the stepwise calcination process in the carbon source coating stage avoids interface peeling between the carbon layer and the precursor due to the difference in thermal expansion coefficients, thus 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 solutions of the present invention and are not intended to limit it. Any other modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention, as long as they do not depart from the spirit and scope of the technical solutions of the present invention, should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing carbon-coated lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: S1. Add phosphorus source, iron source and manganese source to deionized water respectively to prepare phosphorus source solution, iron source solution and manganese source solution. Mix the iron source solution and manganese source solution to obtain manganese iron mixed solution. S2. The manganese-iron mixture and the phosphorus source solution were simultaneously added dropwise to the reactor, and the dropping rate was controlled in stages. At the same time, a precipitant solution was added to the reactor, and the reaction was heated and stirred. Afterwards, the precipitate was filtered, washed, dried and sieved to obtain manganese-iron phosphate precursor powder ①. The procedure for segmented control of the dripping rate is as follows: 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. S3. Mix the manganese iron phosphate precursor powder ① from step S2 with a lithium source, ball mill in an inert gas atmosphere, calcine in stages, cool and grind into lithium manganese iron phosphate crystal powder ②. S4. Add the lithium manganese iron phosphate crystal powder ② from step S3 and the flux to a ball mill jar, seal it, evacuate it and fill it with inert gas, and ball mill to obtain lithium manganese iron phosphate precursor powder ③. S5. The carbon source powder is ball-milled and mixed, then heated and carbonized to obtain a nitrogen-doped porous carbon source composite. The carbon source powder includes tannic acid, CNTs, glucose, and urea; S6. Add the dispersant and the carbon source complex from step S5 to deionized water and disperse evenly to obtain a composite carbon source solution; add the lithium manganese iron phosphate precursor powder ③ obtained from step S4 to the composite carbon source solution, disperse evenly, and dry to obtain a mixed powder ④. S7. The mixed powder ④ from step S6 is calcined in stages in an inert gas, cooled, and then wet-milled to obtain lithium manganese iron phosphate cathode material.

2. The method for preparing carbon-coated lithium manganese iron phosphate cathode 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 manganese sulfate, hydrated manganese sulfate, manganese nitrate, and manganese 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 ammonia; in step S4, the flux includes boric acid, added at a mass of 0.5-2% of the mass of lithium manganese iron phosphate crystal powder ②.

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

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

5. The method for preparing carbon-coated lithium manganese iron phosphate cathode 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 with a concentration of 0.3~0.6 wt%, and the mass ratio of lithium manganese iron phosphate precursor powder ③ to the composite carbon source solution is 1:(2~4).

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

7. The method for preparing carbon-coated lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step S5, the carbonization heating program is: 600~700℃ / 6 h, heating rate 5℃ / min, Ar / NH3 (9:1) atmosphere, and the nitrogen content in the obtained carbon source composite is ≥3at.

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

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

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