Preparation method of lithium manganese iron phosphate positive electrode material

Abstract

The application discloses a preparation method of a lithium iron manganese phosphate positive electrode material, which comprises the following steps: mixing pre-lithiated lithium iron manganese phosphate precursors, lithium carbonate, iron phosphate, trimanganese tetraoxide and phosphoric acid according to a stoichiometric ratio, preparing a slurry, pretreating the slurry under pH 9-10, mixing the slurry with a polyethylene glycol carbon source after sand milling, and sintering the mixture under an inert atmosphere at 720 DEG C for 6-12 hours after spray drying to obtain a carbon-coated lithium iron manganese phosphate positive electrode material with a particle size D50 of 100-200 nm. The method is simple in process, the obtained material is uniform in particle size, excellent in electrochemical performance, and suitable for high-performance lithium ion batteries.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of positive electrode materials, in particular to a preparation method of a lithium manganese iron phosphate positive electrode material. BACKGROUND

[0002] With the wide application of lithium ion batteries in electric vehicle energy storage systems and portable electronic device fields, as a core component determining the energy density, cycle stability and safety of the battery, the research and development of the positive electrode material has become a hotspot in the field of material science. Lithium manganese iron phosphate LiMn x Fe 1-x PO4, abbreviated as LMFP, as a representative upgraded product of olivine structure positive electrode material, has evolved from a single iron-based system of lithium iron phosphate LiFePO4 (LFP) to a composite system coexisting with manganese and iron. This development stems from the pursuit of higher voltage platform and energy density. LMFP, by regulating the manganese-iron ratio, raises the average discharge voltage from the lower level of LFP to a higher level, thus retaining the thermal stability and low cost advantage of LFP. In terms of preparation methods, the early solid phase method has realized large-scale industrialization, but is limited by particle agglomeration and phase purity control; subsequently, the liquid phase method, especially the solvothermal method and the coprecipitation method, have been optimized. These methods realize the uniform synthesis of nanoscale particles by precisely regulating the solubility of the precursor and the crystal nucleus growth, and further improve the electronic conductivity and lithium ion diffusion coefficient by combining carbon coating or ion doping. In recent years, with the integration of multi-element doping and surface modification technology, the electrochemical performance of LMFP has been significantly improved, and progress has been made in inhibiting manganese leaching, promoting its transformation from laboratory prototype to commercial application.

[0003] For example, Chinese patent (CN118289728B) discloses a preparation method of high-performance nano lithium manganese iron phosphate positive electrode material, which comprises mixing lithium carbonate, ferrous oxalate, manganese nitrate, sodium hypophosphite and deionized water to obtain a base mixture solution, then preparing a mixed salt solution of La(NO3)3·6H2O, NiSO4·6H2O and Ti(SO4)2, dropping it into the base, adding a surfactant to obtain a precipitate, drying and sintering to obtain a cubic crystal coated material, then adding a modified graphene oxide dispersion solution, evaporating and calcining to obtain a double-layer coated material, and finally crushing to obtain a nano material. This method aims to improve the conductivity and stability through cubic crystal and carbon double coating. Although this prior art shows significant potential in energy density and economy, it still faces structural defects, especially the uniformity of ion doping and the inhibition of Jahn-Teller effect in the solvothermal method, which directly restricts the cycle life and high-low temperature stability of the material. Under solvothermal conditions, manganese nitrate Mn(NO3)2 and ferrous oxalate FeC2O4 are prone to form complexes with significantly different solubilities, resulting in a large difference in the solubility of Mn 2+ and Fe2+ uneven redox potential, Mn 2+ more prone to oxidation to Mn 3+ , thereby inducing local pH gradient fluctuations, interfering with the uniform distribution of doping ions, Mn 3+ Jahn-Teller lattice distortion caused by aggravating lattice stress accumulation, leading to an increase in volume change during charging and discharging, accelerated manganese ion leaching, and ultimately manifested as an increase in capacity decay rate and low rate performance. SUMMARY

[0004] The present application provides a preparation method of a lithium manganese iron phosphate positive electrode material, comprising the following steps: Step S1. Pre-lithiated lithium manganese iron phosphate precursor is mixed with lithium carbonate, iron phosphate, trimanganese tetraoxide, and phosphoric acid in a stoichiometric ratio of LiMn 0.6 Fe 0.4 PO4, deionized water is added to prepare a uniform slurry, wherein the added mass of the pre-lithiated lithium manganese iron phosphate precursor accounts for 20% of the total mixture mass, and the slurry is pretreated in an alkaline environment with a pH of 9-10 through mechanical stirring for 2-4 hours; Step S2. The slurry obtained in Step S1 is subjected to sand milling treatment; Step S3. The sand-milled material obtained in Step S2 is mixed with a carbon source and then subjected to spray drying; Step S4. The spray-dried powder obtained in Step S3 is sintered at 720°C to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

[0005] It should be noted that in Step S1, the pre-lithiated lithium manganese iron phosphate precursor (LiMn 0.6 Fe 0.4 PO4) is added in a mass ratio of 20%, mixed with lithium carbonate, iron phosphate, trimanganese tetraoxide, and phosphoric acid in a stoichiometric ratio of LiMn 0.6 Fe 0.4 PO4, deionized water is added to prepare a uniform slurry, and the slurry is pretreated in an alkaline environment with a pH of 9-10 through mechanical stirring for 2-4 hours, the stable olivine structure of the pre-lithiated precursor and the pre-embedded Li + as a seed template reduces the redox potential difference between Mn 2+ and Fe 2+ , avoids local pH gradient fluctuations caused by differences in solubility of manganese nitrate and ferrous oxalate dihydrate, the alkaline environment promotes the reaction of phosphoric acid with lithium carbonate to form a stable lithium phosphate phase, and mechanical stirring enhances ion diffusion uniformity; the sand milling treatment in Step S2 optimizes particle size and mixing uniformity through high-energy shearing, reducing agglomeration; in Step S3, the sand-milled material is mixed with a carbon source and then subjected to spray drying, the carbon source pyrolysis forms a uniform conductive carbon layer, avoiding the generation of byproducts by early reaction with acidic phosphoric acid; Step S4 is sintered in an inert atmosphere at 720°C for 6-12 hours, reducing Li volatilization and lattice defects, optimizing the lattice structure, and significantly inhibiting Mn 3+The Jahn-Teller effect is eliminated, and excellent electrochemical performance is finally achieved.

[0006] As a preferred technical solution of the preparation method of the pre-lithiated lithium manganese iron phosphate precursor, the preparation method comprises the following steps: mixing manganese nitrate, ferrous oxalate dihydrate, ammonium hydrogen phosphate and lithium carbonate according to LiMn 0.6 Fe 0.4 PO4 in a stoichiometric ratio, adding deionized water and adjusting the pH to 9-10, performing hydrothermal reaction at 120-150 DEG C for 12-24 hours, centrifuging and washing after cooling, and pre-calcining at 300-400 DEG C for 2-4 hours, and then sintering at 650-700 DEG C in an inert atmosphere for 8-12 hours to obtain the pre-lithiated LiMn 0.6 Fe 0.4 PO4 precursor.

[0007] It should be noted that the hydrothermal reaction enhances the ion solubility and diffusion capacity of manganese nitrate (Mn(NO3)2) and ferrous oxalate dihydrate (FeC2O4·2H2O) in a high-temperature and high-pressure aqueous environment (120-150 DEG C, pressure about 1.5-5 MPa), and the alkaline environment with pH 9-10 inhibits the oxidation of Mn 2+ to Mn 3+ , PO4 3- is provided by ammonium hydrogen phosphate ((NH4)2HPO4), and the preliminary olivine structure (LiMPO4) is generated by reacting with lithium carbonate (Li2CO3), the reaction time of 12-24 hours promotes the growth of crystal nucleus and the uniformity of the crystal lattice; after cooling, centrifugation and washing remove unreacted impurities, pre-calcining at 300-400 DEG C for 2-4 hours removes water and volatile by-products, lays the foundation for the crystal lattice, and sintering at 650-700 DEG C in an inert atmosphere for 8-12 hours further optimizes the ordered embedding of Li+, stabilizes the oxidation states of Mn 2+ and Fe 2+ , reduces the accumulation of lattice defects and stress, and finally forms the pre-lithiated LiMn 0.6 Fe 0.4 PO4 precursor, and the stable nanostructure provides a high-quality template for subsequent doping and carbon coating, which significantly improves the phase purity and ion distribution uniformity compared with the traditional solid-phase method.

[0008] As a preferred technical solution of the preparation method of the lithium manganese iron phosphate positive electrode material, the particle size D 50 of the lithium manganese iron phosphate positive electrode material is 100-200 nm.

[0009] It should be noted that the lithium ion diffusion path can be effectively shortened, the rate performance can be improved, the electrolyte contact area can be increased, the active site utilization rate can be improved, and the side reactions caused by too small particle size can be avoided, so that high capacity and cycle stability are considered.

[0010] As a preferred technical solution of the preparation method of the lithium manganese iron phosphate positive electrode material, the carbon source is polyethylene glycol, and the proportion of the carbon source added in step S3 is 5-10wt% relative to the total solid mass.

[0011] It should be noted that the carbon source is pyrolyzed to form a uniform coated conductive carbon layer during the spray drying and subsequent sintering process, significantly improving the electronic conductivity of the material, and forming a protective barrier on the particle surface, effectively inhibiting Mn dissolution and electrolyte side reactions; the addition amount is controlled within the range of 5-10wt%, which can balance the conductivity and specific capacity, and avoid the energy density decline caused by the over-thick carbon layer.

[0012] As a preferred technical solution of the preparation method of the lithium manganese iron phosphate positive electrode material, the sand milling treatment time is 2-6 hours, and the rotation speed is 1000-2000rpm.

[0013] It should be noted that the sand milling treatment can effectively refine the particles and uniformly mix the reactants through high-energy mechanical shearing. The time control within 2-6 hours and the rotation speed within 1000-2000rpm can balance the dispersibility and structural stability, avoiding both particle agglomeration and excessive pulverization-induced lattice defects, thereby improving the slurry uniformity and the formation quality of the crystal phase during subsequent sintering, and ensuring the electrochemical performance of the material.

[0014] As a preferred technical solution of the preparation method of the lithium manganese iron phosphate positive electrode material, the inlet air temperature of the spray drying is 180-220℃, and the outlet air temperature is 80-120℃.

[0015] It should be noted that the inlet air temperature of the spray drying is 180-220℃, and the outlet air temperature is 80-120℃. The high-temperature inlet air (180-220℃) rapidly evaporates the water in the sand milling material, converting the pre-lithiated lithium manganese iron phosphate precursor and other raw materials into dry powder, matching the slurry humidity and nanoparticle characteristics, and avoiding Li volatilization or lattice damage. The outlet air temperature of 80-120℃ controls the temperature of the powder leaving the drying tower, preventing premature pyrolysis or adhesion of the carbon source, ensuring uniform deposition of the carbon layer during sintering at 720℃, and improving the electrical conductivity.

[0016] As a preferred technical solution of the preparation method of the lithium manganese iron phosphate positive electrode material, the sintering is carried out in an inert atmosphere, and the sintering time is 6-12 hours.

[0017] It should be noted that the sintering is carried out in an inert atmosphere, and the sintering time is 6-12 hours, which aims to protect the spray-dried powder by inert gas (such as nitrogen or argon) to prevent oxidation or Li volatilization, and ensure the LiMn 0.6 Fe 0.4PO4 olivine structure is stable; sintering time of 6-12 hours optimizes lattice formation and carbon source pyrolysis, promotes reaction uniformity between particles, reduces Mn 2+ oxidation risk, inhibits Jahn-Teller effect. 3+

[0018] In addition, the lithium iron manganese phosphate positive electrode material prepared by the preparation method is prepared.

[0019] The present application uses soluble salts to prepare pre-lithiated LiMn 0.6 Fe 0.4 PO4 precursor, realizes uniform nucleation and stable olivine structure under hydrothermal conditions, combined with the use of stable inorganic salts and oxides to supplement raw materials in step S1, and the synergistic effect of the pre-lithiated precursor, effectively reduces the production cost and avoids the generation of impurities, significantly improves the phase purity and electrochemical performance of the lithium iron manganese phosphate positive electrode material. Its beneficial effects are: by using pre-lithiated precursor as seed template, the uniform distribution of Mn 2+ and Fe 2+ is optimized, the Jahn-Teller effect and local pH gradient fluctuation are inhibited, combined with sand milling, spray drying and inert atmosphere sintering process, nanoscale material with particle size D50 of 100-200 nm is prepared, uniform carbon coating layer is formed, thereby significantly improving the specific capacity (160.8-163.2 mAh / g at 0.1C, 150.3-152.8 mAh / g at 1C), the first coulombic efficiency (94.5-96.2%), the cycle stability (capacity retention rate of 92.3-94.1% after 1000 cycles) and the chemical stability (manganese leaching amount as low as 0.8-1.2 ppm), providing reliable technical support for the commercial application of high-performance lithium ion battery positive electrode materials. BRIEF DESCRIPTION OF DRAWINGS

[0020] The accompanying drawings are included to provide a further understanding of the application, and constitute a part of the specification, together with the embodiments of the application, to explain the application, and do not constitute a limitation on the application. In the drawings:

[0021] Figure 1 The scanning electron microscope image of the lithium iron manganese phosphate positive electrode material prepared in Example 1 is shown in Figure 1.

[0022] Figure 2 The particle size distribution graph of the lithium iron manganese phosphate positive electrode material prepared in Examples 1-4 and Comparative Example 1 is shown in Figure 2.

[0023] Figure 3 The XRD graph of the lithium iron manganese phosphate positive electrode material prepared in Example 1 is shown in Figure 3. DETAILED DESCRIPTION

[0024] ​In order to make the above objectives, characteristics and advantages of the present application more obvious and comprehensible, the specific embodiments of the present application will be described in detail below with reference to the embodiments of the present application.

[0025] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. The present application, however, can be practiced in a variety of ways other than those specifically described herein and can employ different methods, structures, and techniques than those described herein, and thus the particular embodiments described herein are not intended to limit the scope of the present application.

[0026] Secondly, the "one embodiment" or "embodiment" referred to herein means that the specific features, structures or characteristics can be included in at least one implementation of the present application. "In one embodiment" appearing in different places in the specification does not mean the same embodiment, nor is it an embodiment that is independent or alternative to other embodiments.

[0027] Preparation Example

[0028] Preparation Example 1

[0029] According to the stoichiometric ratio of LiMn 0.6 Fe 0.4 PO4, manganese nitrate, ferrous oxalate dihydrate, ammonium hydrogen phosphate and lithium carbonate were weighed, mixed and then deionized water was added to form a suspension, and the pH was adjusted to 9 with ammonia water. The suspension was transferred to a high-pressure reaction kettle, and hydrothermal reaction was carried out at 120°C for 12 hours. After cooling, centrifugal separation was performed, and the product was washed with deionized water for 3 times to remove impurities. The obtained product was pre-calcined at 300°C for 4 hours in a muffle furnace, and then sintered at 650°C for 12 hours under a nitrogen atmosphere. After cooling, the pre-lithiated LiMn 0.6 Fe 0.4 PO4 precursor was obtained.

[0030] Preparation Example 2

[0031] According to the stoichiometric ratio of LiMn 0.6 Fe 0.4 PO4, manganese nitrate, ferrous oxalate dihydrate, ammonium hydrogen phosphate and lithium carbonate were weighed, mixed and then deionized water was added to form a suspension, and the pH was adjusted to 9.5 with ammonia water. The suspension was transferred to a high-pressure reaction kettle, and hydrothermal reaction was carried out at 135°C for 18 hours. After cooling, centrifugal separation was performed, and the product was washed with deionized water for 3 times to remove impurities. The product was pre-calcined at 350°C for 3.5 hours in a muffle furnace, and then sintered at 675°C for 10 hours under an argon atmosphere. After cooling, the pre-lithiated LiMn 0.6 Fe 0.4 PO4 precursor was obtained.

[0032] Preparation Example 3

[0033] According to the stoichiometric ratio of LiMn 0.6 Fe0.4 MnPO4precursor. 0.6 Fe 0.4 PO4precursor.

[0034] Preparation Example 4

[0035] Preparation Example 4 0.6 MnPO4precursor. 0.4 MnPO4precursor. 0.6 Fe 0.4 PO4precursor.

[0036] Example

[0037] Example 1

[0038] The embodiment provides a preparation method of a lithium manganese iron phosphate positive electrode material, and comprises the following steps:

[0039] Step S1: a pre-lithiated lithium manganese iron phosphate precursor (Preparation Example 1) is mixed with lithium carbonate, iron phosphate, trimanganese tetraoxide and phosphoric acid in a stoichiometric ratio of LiMn 0.6 Fe 0.4 PO4, and deionized water is added to prepare a uniform slurry, wherein the added mass of the pre-lithiated lithium manganese iron phosphate precursor accounts for 20% of the total mixture mass, and the pre-treatment is performed through mechanical stirring for 2 hours in an alkaline environment with a pH value of 9.

[0040] Step S2: the slurry obtained in step S1 is subjected to sand mill treatment in a sand mill at a rotating speed of 1200 rpm for 3 hours.

[0041] Step S3: the sand mill material obtained in step S2 is mixed with polyethylene glycol (a carbon source), and the proportion of the polyethylene glycol accounts for 6% of the total solid mass, and then drying is performed in a spray drying device at an inlet air temperature of 190 DEG C and an outlet air temperature of 90 DEG C.

[0042] Step S4: The spray-dried powder obtained in step S3 is sintered at 720°C in an inert atmosphere (nitrogen) for 8 hours to obtain a carbon-coated lithium iron manganese phosphate positive electrode material with a particle size D50 of 156 nm.

[0043] Example 2

[0044] The embodiment provides a preparation method of a lithium iron manganese phosphate positive electrode material, comprising the following steps:

[0045] Step S1: A pre-lithiated lithium iron manganese phosphate precursor (Preparation Example 2) is mixed with lithium carbonate, iron phosphate, trimanganese tetraoxide and phosphoric acid in a stoichiometric ratio of LiMn 0.6 Fe 0.4 PO4, and deionized water is added to prepare a uniform slurry, wherein the added mass of the pre-lithiated lithium iron manganese phosphate precursor accounts for 20% of the total mixture mass, and the pre-treatment is carried out in an alkaline environment with a pH of 9.5 by mechanical stirring for 3 hours.

[0046] Step S2: The slurry obtained in step S1 is sand-milled in a sand mill at a speed of 1000 rpm for 6 hours.

[0047] Step S3: The sand-milled material obtained in step S2 is mixed with polyethylene glycol (a carbon source) in a proportion of 5wt% of the total solid mass, and then dried in a spray-drying device at an inlet air temperature of 180°C and an outlet air temperature of 80°C.

[0048] Step S4: The spray-dried powder obtained in step S3 is sintered at 720°C in an inert atmosphere (nitrogen) for 12 hours to obtain a carbon-coated lithium iron manganese phosphate positive electrode material with a particle size D50 of 185 nm.

[0049] Example 3

[0050] The embodiment provides a preparation method of a lithium iron manganese phosphate positive electrode material, comprising the following steps:

[0051] Step S1: A pre-lithiated lithium iron manganese phosphate precursor (Preparation Example 3) is mixed with lithium carbonate, iron phosphate, trimanganese tetraoxide and phosphoric acid in a stoichiometric ratio of LiMn 0.6 Fe 0.4 PO4, and deionized water is added to prepare a uniform slurry, wherein the added mass of the pre-lithiated lithium iron manganese phosphate precursor accounts for 20% of the total mixture mass, and the pre-treatment is carried out in an alkaline environment with a pH of 10 by mechanical stirring for 4 hours.

[0052] Step S2: The slurry obtained in step S1 is sand-milled in a sand mill at a speed of 2000 rpm for 2 hours.

[0053] Step S3: The sanding material obtained in step S2 is mixed with polyethylene glycol (carbon source) at a ratio of 10wt% of the total solid mass, and then dried in a spray drying device at an inlet air temperature of 220°C and an outlet air temperature of 120°C.

[0054] Step S4: The spray-dried powder obtained in step S3 is sintered at 720°C in an inert atmosphere (nitrogen) for 6 hours to obtain a carbon-coated lithium iron manganese phosphate positive electrode material with a particle size D50 of 100 nm.

[0055] Example 4

[0056] The example provides a preparation method of a lithium iron manganese phosphate positive electrode material, including the following steps:

[0057] Step S1: A pre-lithiated lithium iron manganese phosphate precursor (Preparation Example 4) is mixed with lithium carbonate, iron phosphate, trimanganese tetraoxide, and phosphoric acid at a stoichiometric ratio of LiMn 0.6 Fe 0.4 PO4, and deionized water is added to prepare a uniform slurry, wherein the added mass of the pre-lithiated lithium iron manganese phosphate precursor accounts for 20% of the total mixture mass, and the pretreatment is carried out by mechanical stirring in an alkaline environment at pH 9.8 for 2.5 hours.

[0058] Step S2: The slurry obtained in step S1 is treated by sanding in a sanding machine at a speed of 1500 rpm for 4 hours.

[0059] Step S3: The sanding material obtained in step S2 is mixed with polyethylene glycol (carbon source) at a ratio of 8wt% of the total solid mass, and then dried in a spray drying device at an inlet air temperature of 200°C and an outlet air temperature of 100°C.

[0060] Step S4: The spray-dried powder obtained in step S3 is sintered at 720°C in an inert atmosphere (nitrogen) for 10 hours to obtain a carbon-coated lithium iron manganese phosphate positive electrode material with a particle size D50 of 200 nm.

[0061] Comparative Example

[0062] Comparative Example 1

[0063] The comparative example provides a preparation method of a lithium iron manganese phosphate positive electrode material, the main difference from Example 1 is that the pre-lithiated lithium iron manganese phosphate precursor is not used, and part of the process parameters are adjusted to compare the influence of the pre-lithiated precursor and process conditions on the material performance. Including the following steps:

[0064] Step S1: LiMn 0.6 Fe 0.4The stoichiometric ratio of PO4 was used to weigh manganese nitrate, ferrous oxalate dihydrate, ammonium hydrogen phosphate, lithium carbonate, iron phosphate, trimanganese tetraoxide and phosphoric acid, and then mixed with deionized water to prepare a uniform slurry (without adding pre-lithiated manganese iron lithium phosphate precursor) and pretreated in a near neutral environment at pH 9 for 2 hours by mechanical stirring.

[0065] Step S2: The slurry obtained in step S1 was sand-milled in a sand mill at a speed of 1200 rpm for 3 hours.

[0066] Step S3: The sand-milled material obtained in step S2 was mixed with polyethylene glycol (carbon source) at a ratio of 6wt% of the total solid mass, and then dried in a spray drying device at an inlet air temperature of 190°C and an outlet air temperature of 90°C.

[0067] Step S4: The spray-dried powder obtained in step S3 was sintered at 720°C in an inert atmosphere (nitrogen) for 8 hours to obtain a carbon-coated lithium manganese iron phosphate positive electrode material with a particle size D50 of 250 nm.

[0068] Application Example

[0069] Application Example 1

[0070] First, the positive electrode sheet was prepared. The LMFP positive electrode material of Example 1-4 or Comparative Example 1 was weighed and mixed with conductive carbon black (Super P) and PVDF binder at a mass ratio of 80:10:10, and NMP solvent was added and stirred for 4 hours to form a uniform slurry with a solid content of 40wt%. The slurry was coated on an aluminum foil current collector (wet film thickness 100μm), dried in a vacuum oven at 80°C for 8 hours, and then cut into a circular electrode sheet with a diameter of 14mm, and pressed to a material loading of 2.5 mg / cm 2 , thickness 50μm using a roll press. The negative electrode was selected as a lithium metal foil (thickness 1mm) and cut to size. The electrolyte was 1M LiPF6 dissolved in a mixed solvent of EC / DMC / EMC (volume ratio 1:1:1), and the separator was a polypropylene microporous membrane (Celgard 2400). CR2032 button cells were assembled in an argon glove box (water and oxygen content <0.1ppm) by stacking the positive electrode shell, positive electrode sheet, separator (20μL of electrolyte was added), lithium foil negative electrode, gasket and negative electrode shell in order, and sealed using a press to ensure good contact.

[0071] Performance Test Method

[0072] 1. Particle size distribution test: The test was performed using a laser particle size analyzer (e.g., Malvern Mastersizer 3000). 0.5 g of the prepared LMFP cathode material powder (Example 1-4 or Comparative Example 1, after drying) was dispersed in deionized water, and a small amount of dispersant (e.g., sodium polyacrylate, 0.1 wt%) was added to prevent particle agglomeration. An ultrasonic oscillator (power 50 W, frequency 40 kHz) was used to treat the sample for 5 minutes to ensure uniform dispersion. During the test, the sample suspension was injected into the laser particle size analyzer at a constant flow rate, and the laser light source (wavelength 633 nm) measured the scattered light intensity of the particles. The particle size distribution was calculated based on the Mie scattering theory.

[0073] 2. Specific capacity test method: The specific capacity test method was performed using a constant current and constant voltage charge-discharge test system (e.g., LANHE CT2001A) in a 25 ± 1 °C constant temperature box for 100 cycles. The test voltage range was 2.0-4.5 V (vs. Li+ / Li), and the specific capacity performance of the LMFP cathode material at 0.1C and 1C rates was recorded.

[0074] 3. Cycle stability test method: The cycle stability test method was performed using a constant current and constant voltage charge-discharge test system (e.g., LANHE CT2001A) in a 25 ± 1 °C constant temperature box, with a voltage range of 2.0-4.5 V, and 1000 cycles at a 1C rate to evaluate the long-term stability of the LMFP cathode material at high capacity (150 mAh / g).

[0075] 4. Manganese leaching test method: The LMFP cathode sheet was immersed in 1M LiPF6 electrolyte (EC / DMC / EMC, volume ratio 1:1:1) and stored at 60 °C for 7 days. The concentration of Mn ions in the electrolyte was analyzed using inductively coupled plasma mass spectrometry (ICP-MS) to evaluate the chemical stability of the material.

[0076] Table 1

[0077]

[0078] In combination with Example 1 and Figure 1 As can be seen, the sample particles have a uniform spherical morphology, the particle size distribution is concentrated, and the particles are tightly packed. The overall morphology is dense and has no obvious agglomeration. The particle size is about 100-200 nm, indicating that the material has a small grain size and a large specific surface area, which is beneficial to the rapid diffusion of lithium ions and electron transmission, thereby improving the rate performance and cycle stability of the electrode.

[0079] In combination with Examples 1-4, Comparative Example 1, and Figure 2It can be seen that the D50 particle size of the lithium manganese iron phosphate positive electrode materials of Examples 1-4 is 156 nm, 185 nm, 100 nm and 200 nm, respectively, all controlled in the nanometer range of 100-200 nm, and the distribution is relatively concentrated and uniform, while the D50 particle size of Comparative Example 1 is 250 nm, the distribution is relatively wide and the particle is relatively large, indicating that the materials of the example group have smaller average particle size and better particle size consistency; the pre-lithiated lithium manganese iron phosphate precursor is introduced as a seed template in the example, which effectively inhibits particle agglomeration, promotes uniform nucleation and crystal growth in an alkaline environment through mechanical stirring and sanding treatment, and at the same time, the hydrothermal reaction and subsequent spray drying optimize the ion diffusion path, reduce the Mn 2+ and Fe 2+ redox potential difference induced local unevenness, while the comparative example lacks this pre-lithiation step, which causes the direct mixing of raw materials with significant solubility difference, which is easy to form pH gradient fluctuation and crystal lattice stress accumulation, thereby increasing the particle size and uneven distribution, ultimately affecting the lithium ion diffusion efficiency and electrochemical performance of the material.

[0080] In combination with Example 1, Comparative Example 1 and Figure 3 It can be seen that the XRD pattern of Example 1 shows sharp and high-intensity diffraction peaks, mainly located at about 20.3°, 29.5°, 34.2°, 42.1°, 50.6°, 56.8°, etc. 2θ, corresponding to the olivine structure (Pnma space group) of pure phase LiMn 0.6 Fe 0.4 PO4, and there is no obvious impurity phase peak, while the XRD pattern of Comparative Example 1 appears an additional weak peak, indicating the presence of a small amount of amorphous or impurity phase such as Fe2P or MnPO4, the crystal purity is lower and the peak width is slightly larger, and the lattice parameter is slightly deviated; the pre-lithiated lithium manganese iron phosphate precursor is introduced as a seed in Example 1, and the oxidation state of Mn 2+ and Fe 2+ is stabilized in an alkaline pH 9-10 environment through mechanical stirring and hydrothermal reaction, which inhibits Jahn-Teller distortion and local pH gradient fluctuation, promotes uniform crystal nucleus growth and Li + ordered insertion, while the direct mixing of raw materials in Comparative Example 1 causes redox unevenness and crystal defect accumulation due to solubility difference, thereby reducing the phase purity and structural stability, ultimately affecting the electrochemical performance of the material.

[0081] It can be seen from Examples 1 to 4, Comparative Example 1 and Table 1 that the specific capacity of the lithium manganese iron phosphate positive electrode material of Examples 1-4 at 0.1C rate is 162.5, 160.8, 163.2 and 161.4 mAh / g respectively, the specific capacity at 1C rate is 150.3-152.8 mAh / g, the capacity retention rate after 1000 cycles is 92.3-94.1%, and the manganese elution amount is as low as 0.8-1.2 ppm, while the specific capacity of Comparative Example 1 is 155.6 mAh / g, the specific capacity at 1C rate is 142.7 mAh / g, the capacity retention rate after 1000 cycles is only 85.6%, and the manganese elution amount is as high as 2.5 ppm, indicating that the Examples are significantly superior to the Comparative Example in specific capacity, cycle stability and chemical stability; it can be seen from the comparison of Example 1 and Comparative Example 1 that Example 1 uses the pre-lithiated LiMn 0.6 Fe 0.4 PO4 precursor as a seed template to optimize the uniform distribution of Mn 2+ and Fe 2+ in an alkaline environment with pH 9-10 through hydrothermal reaction and mechanical stirring, suppresses the Jahn-Teller effect and local pH gradient fluctuation, sand milling and spray drying further refines the particles and forms a uniform carbon coating layer, and improves the lithium ion diffusion efficiency and electronic conductivity, while Comparative Example 1 lacks the pre-lithiation step, the difference in solubility of raw materials leads to lattice defects and Mn 3+ generation, large particles and severe agglomeration, and uneven carbon layer distribution, thereby reducing the electrochemical performance and structural stability.

[0082] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can modify the technical solutions described in the foregoing embodiments or make equivalent replacements to some technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: Step S1. The pre-lithiated lithium manganese iron phosphate precursor is mixed with lithium carbonate, iron phosphate, manganese tetroxide and phosphoric acid in the form of LiMn. 0.6 Fe 0.4 The stoichiometric ratio of PO4 is mixed and deionized water is added to prepare a homogeneous slurry. The pre-lithiated lithium manganese iron phosphate precursor accounts for 20% of the total mass of the mixture. The mixture is pretreated in an alkaline environment of pH 9-10 by mechanical stirring for 2-4 hours. Step S2. The slurry obtained in step S1 is subjected to sand milling. Step S3. Mix the sand-milled material obtained in step S2 with a carbon source and then spray dry it; Step S4. The spray-dried powder obtained in step S3 is sintered at 720°C to obtain carbon-coated lithium manganese iron phosphate cathode material; The preparation method of the pre-lithiated lithium manganese iron phosphate precursor includes: mixing manganese nitrate, ferrous oxalate dihydrate, ammonium hydrogen phosphate, and lithium carbonate in a LiMn mixture. 0.6 Fe 0.4 The stoichiometric amounts of PO4 were mixed, deionized water was added and the pH was adjusted to 9-10, and the mixture was hydrothermally reacted at 120-150℃ for 12-24 hours. After cooling, the mixture was centrifuged, washed, and pre-calcined at 300-400℃ for 2-4 hours, followed by sintering at 650-700℃ for 8-12 hours under an inert atmosphere to obtain pre-lithiated LiMn. 0.6 Fe 0.4 PO4 precursor.

2. The preparation method according to claim 1, characterized in that, The particle size D50 of the lithium manganese iron phosphate cathode material is 100-200 nm.

3. The preparation method according to claim 1, characterized in that, The carbon source is polyethylene glycol, which is added in step S3 at a ratio of 5-10 wt% relative to the total solid mass.

4. The preparation method according to claim 1, characterized in that, The grinding process takes 2-6 hours and the rotation speed is 1000-2000 rpm.

5. The preparation method according to claim 1, characterized in that, The inlet air temperature of the spray dryer is 180-220℃, and the outlet air temperature is 80-120℃.

6. The preparation method according to claim 1, characterized in that, In step S4, the sintering is carried out under an inert atmosphere for 6-12 hours.

7. The lithium manganese iron phosphate cathode material prepared by the preparation method according to any one of claims 1-6.

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