Preparation method of low-manganese dissolution, high-compaction and long-cycle lithium manganese iron phosphate positive electrode material

Abstract

This invention discloses a method for preparing a low-manganese-leaching, high-compact, long-cycle lithium manganese iron phosphate cathode material, comprising: isothermal calcination of a manganese source containing water of crystallization to dehydrate and create pores, forming a porous manganese phosphate precursor; dispersing the porous manganese phosphate precursor in water, adding a soluble iron source and an organic complexing agent, and stirring the reaction; subsequently adding a lithium source and a phosphorus source for low-temperature sintering to obtain a uniform and stable nucleated lithium manganese iron phosphate precursor; uniformly mixing the nucleated lithium manganese iron phosphate precursor with a carbon source and a metal dopant, followed by wet milling, spray drying, high-temperature sintering, and airflow pulverization to obtain the lithium manganese iron phosphate cathode material. The lithium manganese iron phosphate cathode material prepared by this method has a compaction density ≥2.46 g / cm³. 3 The specific capacity at 1C discharge in the 2.0-4.3V voltage range is >142mAh / g, the capacity retention rate after 500 cycles is >92%, and the manganese dissolution rate after immersion in a high-temperature electrolyte at 60℃ for 7 days is <0.02%.

Description

Technical Field

[0001] This invention belongs to the field of lithium manganese iron phosphate cathode materials, and particularly relates to a method for preparing lithium manganese iron phosphate cathode materials with low manganese leaching, high compaction, and long cycle life. Background Technology

[0002] Lithium manganese iron phosphate (LMFP) is an olivine-type lithium-ion battery cathode material that combines the high safety and long lifespan of lithium iron phosphate with the high voltage and high energy density of lithium manganese phosphate, making it a core upgrade material for power batteries and large-scale energy storage batteries. However, existing commercial processes have significant technical shortcomings, severely restricting the large-scale application of this material.

[0003] Currently, the publicly disclosed synthesis processes for lithium manganese iron phosphate mainly focus on mainstream routes such as co-precipitation, hydrothermal / solvothermal methods, template-based pore-forming modification, and multi-manganese source composite solid-phase methods. However, all existing processes have inherent technical defects that are difficult to eradicate, and cannot simultaneously achieve the core industrial requirements of atomic-level uniform distribution of manganese and iron elements, effective suppression of manganese leaching attenuation, and large-scale production with uniform and stable performance. The specific defects of each process route are as follows:

[0004] In the coprecipitation system, Mn and Fe metal ions have different precipitation rates, making stepwise precipitation very likely during the synthesis process. The precursor itself contains obvious manganese and iron element segregation. The manganese-rich regions of the crystal lattice will greatly aggravate the dissolution of manganese ions during the charging and discharging process. Even with the use of complexing agents and segmented pH control, the degree of segregation can only be slightly weakened. It is impossible to achieve homogeneous dispersion of manganese and iron from the root. Moreover, the elemental uniformity of the precursor fluctuates significantly during batch preparation, making it difficult to control the consistency of mass production.

[0005] While hydrothermal / solvothermal methods can alleviate manganese-iron segregation in a closed liquid environment, production relies on high-pressure special reactors. Furthermore, this synthesis route is a batch-based production process, making it difficult to standardize and uniformly control crystal nucleation and growth conditions, resulting in inconsistent mass production. Coupled with issues such as difficult solvent recovery, limited single-batch capacity, and high equipment and maintenance costs, it is difficult to adapt to large-scale industrial production.

[0006] While template-based pore-forming method can construct porous structures to optimize lithium-ion conduction as an auxiliary modification process, it requires additional template loading and template removal processes, making the process lengthy. Template agents are prone to introducing impurities, which continuously damage the stability of the electrode interface. At the same time, a large number of through-pores will significantly reduce the compaction density of the powder, sacrificing the energy density of the battery cell. This process can only optimize mass transfer performance and cannot solve the intrinsic segregation problem of manganese and iron from the source.

[0007] The multi-manganese source compound solid-phase method relies solely on mechanical ball milling to complete the macroscopic mixing of raw materials, and there is no molecular / ionic level homogeneous dispersion condition. The interfacial bonding force of the powder raw materials is weak, and there are differences in the diffusion rates of Mn and Fe ions during the high-temperature sintering stage. The disordered growth of grains and the phenomenon of local element segregation cannot be avoided. The manganese-rich impurity phase generated by segregation continuously induces manganese dissolution.

[0008] In summary, all existing mainstream patented processes have irreconcilable technical shortcomings. A single route cannot simultaneously achieve the three key indicators of non-segregation homogeneity of manganese and iron, low manganese leaching, long circulation, and batch uniformity in large-scale mass production. The industry faces obvious technological bottlenecks. Summary of the Invention

[0009] Purpose of the invention: This invention provides a method for preparing lithium manganese iron phosphate cathode material, which simultaneously achieves homogeneous manganese iron without segregation, low manganese leaching, long cycle life, and batch uniformity in large-scale mass production.

[0010] Technical solution: The preparation method of the low-manganese leaching, high-pressure compaction, and long-cycle lithium manganese iron phosphate cathode material of the present invention includes the following steps:

[0011] (1) The manganese source containing water of crystallization is placed in an inert atmosphere and calcined at 260-330℃ to remove water of crystallization and form a pore, thereby forming a uniform and coherent porous manganese phosphate precursor in situ.

[0012] (2) The porous manganese phosphate precursor is dispersed in water to form a suspension, a soluble iron source and an organic complexing agent are added, the temperature is maintained at 45-75℃, and the reaction is stirred to achieve uniform complexation and dispersion of manganese and iron ions; then lithium source and phosphorus source are added and stirred evenly before sintering at 160-230℃ to obtain a uniform and stable nucleated lithium manganese iron phosphate precursor.

[0013] (3) The nucleus-type lithium manganese iron phosphate precursor is mixed with carbon source and metal dopant, and then subjected to wet sand milling for particle refinement, spray drying for spherical granulation, sintering at 660-730℃ under inert atmosphere, and air jet milling for classification and shaping to obtain a lithium manganese iron phosphate cathode material with low manganese leaching, high compaction and long cycle life.

[0014] Furthermore, in step (1) of the preparation method, the manganese source containing water of crystallization is selected from one or more of manganese phosphate monohydrate, manganese dihydrogen phosphate dihydrate, manganese hydrogen phosphate monohydrate, and manganese hydrogen phosphate dihydrate.

[0015] Furthermore, the manganese source containing water of crystallization is a pure phase orthorhombic crystal system with a purity ≥99.2%.

[0016] Furthermore, in step (2) of the preparation method, the number of moles of the organic complexing agent added is 0.08-0.25 times the total number of moles of Mn and Fe; the organic complexing agent is selected from one or more of citric acid, tartaric acid, and triammonium citrate.

[0017] Furthermore, in step (2) of the preparation method, the soluble iron source is selected from one or more of ferrous acetate, ferrous citrate, and ferrous oxalate.

[0018] Furthermore, in step (2) of the preparation method, the stirring reaction time is 1-3h, the pH of the stirring reaction system is maintained at 4.5-6.5, the sintering holding time is 3-7h, and 10-45nm uniform nano-lithium manganese iron phosphate crystal nuclei are generated in situ.

[0019] Furthermore, in step (2) of the preparation method, the lithium source is selected from one or more of lithium hydroxide, lithium acetate, and lithium carbonate, and the ratio of the number of Li moles to the total number of Mn and Fe moles in the lithium source is (1.01-1.05):1; the phosphorus source is selected from one or more of lithium dihydrogen phosphate, dilute phosphoric acid, and diammonium hydrogen phosphate, and the ratio of the number of P moles to the total number of Mn and Fe moles in the phosphorus source is (1-1.02):1.

[0020] Furthermore, in step (3) of the preparation method, the carbon source is selected from one or more of sucrose, glucose, citric acid, and polyethylene glycol, and its added mass is 8-10 wt% of the theoretical mass of lithium manganese iron phosphate cathode material.

[0021] Furthermore, in step (3) of the preparation method, the metal dopant is at least one of magnesium source, aluminum source, titanium source, zirconium source, and lanthanum source, and the molar number of the doped metal element accounts for 0.2-1.0% of the total molar number of Mn and Fe.

[0022] Furthermore, in step (3) of the preparation method, wet sand milling is performed until the particle size D50 of the slurry is 0.3-0.7μm; the inlet air temperature of spray drying is 185-215℃ and the outlet air temperature is 80-95℃; sintering is carried out under a nitrogen or argon inert atmosphere and the sintering holding time is 6-11h; the secondary particle size D50 of the finished product after air jet milling is controlled to be <1.5μm.

[0023] Beneficial effects: Compared with the prior art, the present invention has the following significant effects: the lithium manganese iron phosphate cathode material prepared by this method has a porous rigid framework anchored nanocrystal nuclei + uniform carbon doping coating microstructure; the compaction density is ≥2.46 g / cm³. 3 The specific capacity at 1C discharge in the 2.0-4.3V voltage range is >142mAh / g, the capacity retention rate after 500 cycles is >92%, and the manganese dissolution rate after immersion in electrolyte at 60℃ for 7 days is <0.02%. Attached Figure Description

[0024] Figure 1 SEM image of lithium iron phosphate cathode material in Example 1;

[0025] Figure 2 SEM image of lithium iron phosphate cathode material in Example 2;

[0026] Figure 3 SEM image of lithium manganese iron phosphate cathode material in Example 3

[0027] Figure 4 SEM image of lithium iron phosphate cathode material for Comparative Example 1;

[0028] Figure 5 SEM image of lithium iron phosphate cathode material in Comparative Example 2;

[0029] Figure 6 The image shows a comparative example of lithium iron phosphate cathode material using SEM. Detailed Implementation

[0030] The technical solution of the present invention will be further described in detail below with reference to the embodiments. It should be noted that all raw materials used in the present invention are commercially available.

[0031] Example 1

[0032] The method for preparing lithium manganese iron phosphate with low manganese leaching, high pressure compaction, and long cycling in Example 1 specifically includes the following steps:

[0033] (1) Take 99.6% pure orthorhombic monohydrate manganese phosphate, place it in an inert atmosphere, heat it to 300℃ at 3℃ / min, and keep it at the temperature for 4h to controllably remove the crystal water, thus preparing a porous manganese phosphate precursor with a through-mesoporous structure and a specific surface area of ​​43m². 2 / g, pore size 15-25nm, complete framework without collapse.

[0034] (2) 1 mol of porous manganese phosphate precursor was dispersed in deionized water to prepare a 20 wt% stable suspension; 0.4 mol of ferrous acetate and 0.15 mol of citric acid were added, and the mixture was stirred at 60 °C for 2 h. The pH of the system was stable at 5.5, forming an atomically uniform Mn-Fe complex system; then 1.03 mol of lithium hydroxide and 1.01 mol of lithium dihydrogen phosphate were added, and the mixture was stirred and homogenized before being sintered at 200 °C for 5 h to generate 20-35 nm manganese iron lithium phosphate nanocrystals in situ within the porous framework.

[0035] (3) Add glucose, 0.6 mol% magnesium nitrate and titanium dioxide, accounting for 9 wt% of the theoretical mass of lithium manganese iron phosphate cathode material, to the crystal nucleus precursor. Wet mill the slurry until the D50 is 0.35 μm. Spray dry at 200℃ for inlet air and 90℃ for outlet air. Sinter at 700℃ in nitrogen atmosphere for 8 hours. After cooling, air-jet pulverize and classify the product. The secondary particle D50 of the finished product is controlled to be <1.5 μm to obtain the finished lithium manganese iron phosphate cathode material.

[0036] Example 2

[0037] The process in this embodiment is basically the same as that in embodiment 1, except that: in step (1), the dehydration temperature is 280℃ and the holding time is 5h; in step (2), the complexation temperature is 55℃, the low temperature sintering temperature is 180℃ and the holding time is 6h; in step (3), the sintering temperature is 690℃ and the holding time is 9h.

[0038] Example 3

[0039] The process in this embodiment is basically the same as that in embodiment 1, except that: step (1) dehydration temperature is 320℃ and heat preservation is 3h; step (2) complexation temperature is 65℃, low temperature sintering is 220℃ and heat preservation is 4h; step (3) sintering temperature is 710℃ and heat preservation is 7h.

[0040] Comparative Example 1

[0041] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that the precursor is prepared by co-precipitation of manganese sulfate and ferrous sulfate. The specific preparation method is as follows:

[0042] (1) Weigh out MnSO4·H2O, FeSO4·7H2O, and citric acid (10.7 mol% of total metal molar weight) according to the metal molar ratio of Mn:Fe = 1.0:0.4, dissolve them in deionized water, stir until completely dissolved, and bubble with high-purity nitrogen for 30 min to remove dissolved oxygen. Under nitrogen protection and constant temperature of 40℃, slowly add NH4H2PO4 solution (total metal to phosphorus molar ratio of 1:1) to the metal salt mixture, and adjust the pH of the system to stabilize at 3.5-3.8 with ammonia. After the addition is complete, continue stirring for 2 h, age for 4 h, filter under vacuum, and wash with hot water at 60-80℃ until the filtrate is free of SO4. 2- The light pink manganese iron phosphate precursor was obtained by drying at 120℃.

[0043] (2) The precursor was mixed with LiOH·H2O (lithium excess 5%), magnesium acetate (0.6 mol%), tetrabutyl titanate (0.3 mol%) and glucose (3 wt%), and deionized water was added to prepare a slurry. The slurry was then wet-milled to a particle size D. 50=0.35μm. The slurry, after spray drying and granulation (inlet air temperature 200℃, outlet air temperature 90℃), was placed in a tube furnace and sintered in stages under a high-purity nitrogen atmosphere: the temperature was increased to 320℃ at 3℃ / min and held for 2 h, then increased to 750℃ at 5℃ / min and held for 8 h, followed by furnace cooling to room temperature. The resulting sintered product was subjected to air jet milling and classification, with the secondary particle size (D50) controlled to <1.5μm, yielding the final LiMn. 0.7 Fe 0.3 PO4 / C cathode material.

[0044] Comparative Example 2

[0045] Comparative Example 2 describes the preparation of lithium manganese iron phosphate material using a conventional industry-standard external pore-forming agent template pore-forming process. This is intended to provide a precise comparison with the in-situ self-pore-forming process of the present invention. The specific process steps are as follows:

[0046] (1) Select the same batch and purity of undehydrated monohydrate manganese phosphate raw material as in Example 1 as the manganese source, without low-temperature controllable dehydration and pore-forming pretreatment; according to the molar ratio of manganese, iron, lithium and phosphorus elements in Example 1, ferrous acetate, citric acid complexing agent, lithium hydroxide and lithium dihydrogen phosphate are added in sequence, and starch accounting for 3wt% of the final product mass is added as a pore-forming agent. The mixture is uniformly added to deionized water to prepare a 20wt% suspension, stirred at 60℃ for 2h, and the pH of the system is maintained at 5.5 to complete the complexation and homogeneous dispersion to obtain the mixed slurry;

[0047] (2) The mixed slurry was sintered at 200℃ for 5h for pre-nucleation treatment to obtain precursor powder containing pore-forming agent;

[0048] (3) Add 9wt% glucose carbon source, 0.6mol% magnesium nitrate and titanium dioxide dopant to the precursor powder containing pore-forming agent, wet mill until the slurry D50=0.35μm, spray dry under conditions of 200℃ air inlet and 90℃ air outlet to form pellets, and finally sinter at a constant temperature of 700℃ for 8h under nitrogen atmosphere.

[0049] By utilizing the pyrolysis and volatilization of starch during high-temperature sintering to form pores, and then obtaining a conventional pore-forming agent-modified lithium manganese iron phosphate cathode material after airflow pulverization and classification, all other process parameters, equipment conditions, and proportions are completely consistent with those in Example 1.

[0050] Comparative Example 3

[0051] The preparation method of Comparative Example 3 is basically the same as that of Example 1, except that manganese phosphate monohydrate raw material is used directly, and the dehydration and pore-forming process is omitted. The specific steps are as follows:

[0052] (1) Take 1 mol of orthorhombic monohydrate manganese phosphate with a purity of 99.6% and disperse it in deionized water to prepare a 20 wt% stable suspension; add 0.4 mol of ferrous acetate and 0.15 mol of citric acid, stir at 60℃ for 2 h, and the pH of the system is stable at 5.5, forming an atomically uniform Mn-Fe complex system; then add 1.03 mol of lithium hydroxide and 1.01 mol of lithium dihydrogen phosphate, stir to homogenize and sinter at 200℃ for 5 h to obtain the precursor of lithium manganese iron phosphate crystal nuclei.

[0053] (2) Add 3wt% glucose, 0.6mol% magnesium nitrate and titanium dioxide to the crystal nucleus precursor, wet mill until the slurry D50 is 0.35μm; spray dry with air inlet at 200℃ and air outlet at 90℃; sinter at 700℃ in nitrogen atmosphere for 8h, cool and then air-jet crush and classify, and control the secondary particle D50 of the finished product to <1.5μm to obtain the finished lithium manganese iron phosphate cathode material.

[0054] Performance Characterization

[0055] The performance parameters of the LMFP cathode materials obtained in the embodiments and comparative examples of the present invention are shown in Table 1 below.

[0056] Meanwhile, the positive electrode materials obtained in the embodiments and comparative examples of the present invention were subjected to electrochemical performance tests according to the following steps, and the results are shown in Table 1.

[0057] A mixed slurry was prepared by uniformly mixing lithium manganese iron phosphate cathode material, Super P (conductive carbon black), and PVDF in an 8:1:1 ratio in NMP solution. The slurry was then applied to bright aluminum foil by hand and dried at 100°C. After the NMP had completely evaporated, the electrode sheet was cut into 13mm diameter electrode sheets and dried overnight at 105°C in a vacuum oven. After weighing, the electrode sheets were quickly transferred to a glove box. The assembled battery was prepared using lithium metal as the counter electrode, Celgard 2400 as the separator, and 1mol / L LiPF6 dissolved in a mixed solvent of EC / DMC / EMC (volume ratio 1:1:1). Electrochemical performance was tested using a Newway testing system, employing constant current-constant voltage charging (CC-CV) and constant current discharging (DC) methods to analyze the charge and discharge behavior of the coin cell. The test voltage range was 2.5-4.3V.

[0058] Table 1 Performance Parameters

[0059]

[0060] SEM images from Examples 1 to 3 and Comparative Examples 1-3 (attached) Figure 1-3 With appendix Figure 4-6The data comparison results show that this invention, through in-situ controllable pore formation using a manganese source containing water of crystallization, homogeneous in-situ nucleation of multi-element complexes, and post-densification gradient control, completely solves the defects of traditional co-precipitation processes, such as disordered morphology, elemental segregation, severe agglomeration, and structural instability. It also compensates for the technical shortcomings of unpore-formed samples, such as slow ion transport, poor lattice stability, and severe manganese dissolution. The lithium manganese iron phosphate material prepared by this invention achieves multiple structural advantages: "refined grains promote ion transport, rigid framework stabilizes the lattice, and spherical dense high packing." By controlling the microstructure, it achieves low manganese dissolution, high compaction density, high rate performance, and long-cycle stability, with a compaction density ≥ 2.46 g / cm³. 3 The specific capacity at 1C discharge in the 2.0-4.3V voltage range is >142mAh / g, the capacity retention rate after 500 cycles is >92%, and the manganese dissolution rate after immersion in electrolyte at 60℃ for 7 days is <0.02%.

[0061] Specifically, by Figure 1-3 As shown in Table 1, the samples prepared in different batches from Examples 1 to 3 exhibit good microstructure regularity, forming a uniform, dense, and spherical microstructure with consistent batches and stable, controllable morphology. Further analysis of the mechanism reveals that by first controlling the removal of uniformly distributed water of crystallization from the manganese source containing water of crystallization, a uniformly distributed and well-connected mesoporous channel can be formed in the precursor stage, without defects such as framework collapse or pore blockage. This porous framework anchors the Mn-Fe complex system; then, combined with low-temperature sintering, multi-element in-situ bonding is achieved, with uniform nucleation of manganese, iron, lithium, and phosphorus, resulting in uniformly sized lithium manganese iron phosphate nanocrystals; after high-temperature densification sintering, the precursor mesopores are completely filled by crystal nucleus growth, forming dense primary particles. Finally, through sand milling, spray pelletizing, and airflow classification, lithium manganese iron phosphate particles with high sphericity, dense packing, and clear interfaces are obtained. The mesoporous structure of the manganese iron phosphate precursor of this invention achieves atomic-level uniform elemental distribution in the early stage, eliminating elemental segregation and avoiding the formation of impurity phases, thereby improving electrochemical performance and significantly reducing Jahn-Teller distortion and compaction performance. Furthermore, the mesoporous structure restricts grain refinement, enabling abnormal growth of the crystal nuclei. The dense spherical particles after sintering further significantly increase the compaction density, while the rigid framework's genetic effect also stabilizes the crystal structure and inhibits manganese dissolution, thus comprehensively achieving a simultaneous improvement in both electrochemical performance and compaction density.

[0062] Comparative Example 1 was prepared using a traditional co-precipitation process, such as... Figure 4As shown, the sample exhibits a disordered overall particle morphology, a wide particle size distribution, significant differences in particle size, and the coexistence of ultrafine and coarse grains, with severe particle agglomeration and loose packing. Due to the inconsistent precipitation rates of manganese and iron ions, the system is highly susceptible to metal element segregation and localized phase separation. The particle surface has numerous defects, lacks uniform mesoporous channels, and exhibits extremely poor structural regularity. This disordered microstructure leads to weak lattice stability and severe Jahn-Teller distortion during discharge. Table 1 data shows that the manganese dissolution rate after 7 days of immersion in a 60℃ high-temperature electrolyte is 0.125%, far higher than the 0.016% in Example 1. This product has a prominent manganese dissolution problem. Simultaneously, the loose particle packing results in low compaction density and disordered ion transport, ultimately manifesting as low specific capacity, poor rate performance, and rapid cycle decay, perfectly corresponding to the degradation results of electrochemical testing.

[0063] Comparative Example 2: This sample used an external starch pore-forming agent for physical pore creation, but did not form an in-situ rigid framework, such as... Figure 5 As shown, the sample exhibits disordered pores and uneven pore sizes, with numerous closed pores and incomplete channels. Pyrolysis of the pore-forming agent easily leaves behind carbonaceous defects that accumulate at grain boundaries, exacerbating interfacial side reactions. Externally generated pores lack skeletal constraints, resulting in disordered grain growth, significant agglomeration, and poor particle density. This leads to significant performance drawbacks: ineffective channels increase ion transport resistance and cause severe rate decay; grain boundary defects and grain agglomeration exacerbate polarization and reduce effective capacity; the loose porous structure cannot buffer lattice distortion, leading to easy collapse of the circulating structure and high manganese leaching; and the disordered packing of porous material further reduces the compaction density (2.19 g / cm³). 3 Its overall performance has obvious shortcomings.

[0064] Comparative Example 3 did not undergo a controlled dehydration and pore-forming process for manganese phosphate monohydrate; instead, it directly used raw material mixing and reaction for molding. Because a uniform and interconnected mesoporous structure was not formed in the precursor stage, iron ions could not achieve uniform anchoring within the pores, resulting in insufficient uniformity in the mixing of manganese and iron elements. Figure 6 As shown, the sample particles are generally dense, but the grain size is uneven, with localized grain accumulation and slight agglomeration. Furthermore, due to the lack of a rigid porous framework, the material cannot effectively buffer lattice volume deformation during charging and discharging, resulting in insufficient lattice stability. As shown in Table 1, in Comparative Example 3, the manganese dissolution rate after 7 days of immersion in a 60℃ high-temperature electrolyte was only 0.098%, indicating poor manganese ion dissolution suppression. The ion diffusion efficiency and particle packing density were significantly inferior to the modified sample of this invention, demonstrating significant structural shortcomings.

[0065] It should be noted that the parameter ranges defined by the preparation method of the low manganese leaching, high compaction, and long-cycle lithium manganese iron phosphate cathode material of the present invention can all achieve the technical effects claimed by the present invention, and therefore no further examples will be given to support this claim.

Claims

1. A method for preparing a low-manganese-leaching, high-compact, long-cycle lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: (1) A manganese source containing water of crystallization is placed in an inert atmosphere and calcined at 260-330°C to remove water of crystallization and form pores in situ, thereby forming a uniform and continuous porous manganese phosphate precursor; the manganese source containing water of crystallization is selected from one or more of manganese phosphate monohydrate, manganese dihydrogen phosphate dihydrate, manganese hydrogen phosphate monohydrate, and manganese hydrogen phosphate dihydrate. (2) The porous manganese phosphate precursor is dispersed in water to form a suspension, a soluble iron source and an organic complexing agent are added, the temperature is maintained at 45-75℃, and the reaction is stirred to achieve uniform complexation and dispersion of manganese and iron ions; then lithium source and phosphorus source are added and stirred evenly before sintering at 160-230℃ to obtain a uniform and stable nucleated lithium manganese iron phosphate precursor. (3) The nucleus-type lithium manganese iron phosphate precursor is mixed with carbon source and metal dopant, and then subjected to wet sand milling for particle refinement, spray drying for spherical granulation, sintering at 660-730℃ under inert atmosphere, and air jet milling for classification and shaping to obtain a lithium manganese iron phosphate cathode material with low manganese leaching, high compaction and long cycle life.

2. The preparation method of the low-manganese leaching, high-pressure compaction, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The manganese source containing water of crystallization is a pure phase orthorhombic crystal system with a purity ≥ 99.2%.

3. The preparation method of the low-manganese leaching, high-pressure compaction, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (2), the number of moles of the organic complexing agent added is 0.08-0.25 times the total number of moles of Mn and Fe; the organic complexing agent is selected from one or more of citric acid, tartaric acid, and triammonium citrate.

4. The preparation method of the low-manganese leaching, high-pressure compaction, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (2), the soluble iron source is selected from one or more of ferrous acetate, ferrous citrate, and ferrous oxalate.

5. The preparation method of the low-manganese leaching, high-pressure compaction, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (2), the stirring reaction time is 1-3h, and the pH of the stirring reaction system is maintained at 4.5-6.5; the sintering holding time is 3-7h, and 10-45nm uniform nano-lithium manganese iron phosphate crystal nuclei are generated in situ.

6. The preparation method of the low-manganese leaching, high-pressure compaction, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (2), the lithium source is selected from one or more of lithium hydroxide, lithium acetate, and lithium carbonate, and the ratio of the number of Li moles to the total number of Mn and Fe moles in the lithium source is (1.01-1.05):1; The phosphorus source is selected from one or more of lithium dihydrogen phosphate, dilute phosphoric acid, and diammonium hydrogen phosphate, and the ratio of the number of moles of P to the total number of moles of Mn and Fe in the phosphorus source is (1-1.02):

1.

7. The preparation method of the low-manganese leaching, high-pressure compaction, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (3), the carbon source is selected from one or more of sucrose, glucose, citric acid, and polyethylene glycol, and its added mass is 8-10 wt% of the theoretical mass of lithium manganese iron phosphate cathode material.

8. The preparation method of the low-manganese leaching, high-pressure compaction, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (3), the metal dopant is at least one of magnesium source, aluminum source, titanium source, zirconium source and lanthanum source, and the molar number of the doped metal element accounts for 0.2-1.0% of the total molar number of Mn and Fe.

9. The preparation method of the low-manganese leaching, high-pressure compaction, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (3), wet sand milling is performed until the particle size D50 of the slurry is 0.3-0.7μm; the inlet air temperature of spray drying is 185-215℃ and the outlet air temperature is 80-95℃; sintering is carried out in an inert atmosphere of nitrogen or argon, and the sintering holding time is 6-11h; the secondary particle size D50 of the finished product after air jet milling is controlled to be <1.5μm.

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