Preparation method and application of nanometer manganese iron lithium phosphate

Abstract

The application discloses a preparation method and application of nanometer manganese iron lithium phosphate, and comprises the following steps: mixing pre-lithiated manganese iron lithium phosphate precursor, lithium carbonate, iron phosphate, trimanganese tetroxide and ammonium phosphate into slurry according to the stoichiometric ratio of LiMn0.7Fe0.3PO4, and pretreating in an alkaline environment with pH being 8.5-9.5; wet ball milling treatment; mixing with modified glucose and then spray drying; sintering at 700-750 DEG C for 8-14 hours in an inert atmosphere to obtain carbon-coated nanometer manganese iron lithium phosphate. The pre-lithiated precursor is prepared through a hydrothermal reaction, and the modified glucose is carboxylated through citric acid. The method realizes uniform carbon coating of nanoscale particles, suppresses manganese dissolution and interface side reactions, improves lithium ion diffusion and electronic conductivity, improves specific capacity, rate performance and cycle stability, and is suitable for anode materials of 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 nano lithium manganese iron phosphate and application thereof. BACKGROUND

[0002] With the wide application of lithium ion batteries in the fields of electric vehicles, energy storage systems and portable electronic devices, the performance optimization of positive electrode materials has become the core driving force to promote the progress of battery technology. As a new type of olivine structure positive electrode material, lithium manganese iron phosphate (LiMnFePO4, LMFP) is developed on the basis of lithium iron phosphate (LiFePO4, LFP). By introducing manganese elements, the average discharge voltage of the material is significantly improved, thereby increasing the theoretical energy density and making up for the limitation of the low voltage platform of traditional LFP. Early research mainly adopts solid phase synthesis methods such as high-temperature solid phase reaction and mechanical ball milling, which depend on the mixing and calcination of cheap precursors, but the particle size of the product is large, which limits the lithium ion diffusion kinetics. Subsequently, liquid phase synthesis technologies such as co-precipitation, solvothermal method and spray drying method gradually rise, which can realize the accurate control of nano-particles, and promote the material uniformity and electrochemical activity. In recent years, with the growth of industrialization demand, relevant enterprises have increased their research and development investment in LMFP, focusing on optimizing the rate performance and cycle stability by adjusting the manganese-iron ratio and surface carbon coating. The progress of these technologies makes the production process of LMFP highly compatible with LFP, reducing the industrialization threshold, but its application in actual commercialization is still in the rapid development stage, and further breakthroughs in performance bottlenecks are needed to meet the demand for high-performance batteries.

[0003] However, the existing LMFP preparation technology has exposed significant limitations in the application of nanometerization strategy, especially in improving the lithium ion diffusion rate. The problem of side reactions caused by particle size control becomes the key to restricting long-term performance. In order to improve the one-dimensional diffusion efficiency of lithium ions in the olivine structure, the particle size is usually controlled at the nanometer level, which can effectively shorten the diffusion path and improve the rate performance, but the too small particle size leads to too high specific surface area, which significantly increases the interface contact area with the electrolyte, thereby accelerating the dissolution of manganese ions and the irreversible decomposition reaction of the electrolyte. This interface side reaction is particularly evident in high-temperature or high-voltage cycling, which leads to the decline of the structure stability of the positive electrode material, the abnormal growth of the solid electrolyte interface film on the negative electrode surface, and further accelerates the capacity decay and the decrease of coulombic efficiency, significantly shortening the cycle life. SUMMARY

[0004] The application provides a preparation method of nano lithium manganese iron phosphate, which comprises the following steps:

[0005] Step S1. Pre-lithiated lithium manganese iron phosphate precursor is mixed with lithium carbonate, iron phosphate, trimanganese tetraoxide and ammonium phosphate according to LiMn 0.7 Fe0.3 The stoichiometric ratio of PO4 is mixed, deionized water is added to prepare a uniform slurry, and the mass of the pre-lithiated lithium manganese iron phosphate precursor accounts for 15-25% of the total mixture mass, and the pretreatment is carried out in an alkaline environment with a pH of 8.5-9.5;

[0006] Step S2. The slurry obtained in step S1 is subjected to wet ball milling treatment;

[0007] Step S3. The ball milled material obtained in step S2 is mixed with modified glucose, and the amount of modified glucose added is 6-12 wt% of the total solid mass, and then spray drying is carried out to obtain dry powder;

[0008] Step S4. The spray dried powder obtained in step S3 is sintered at 700-750°C for 8-14 hours under an inert atmosphere to obtain a lithium manganese iron phosphate positive electrode material with a carbon-coated structure.

[0009] It should be noted that the pre-lithiated precursor is pretreated in an alkaline environment by ultrasonic assisted dynamic pH regulation, which utilizes cavitation effect and surface charge change to promote crystal surface reconstruction, reduce defect sites and improve particle dispersion, and reduce side reaction activity. Wet ball milling optimizes the particle size, shortens the one-dimensional diffusion path of lithium ions, and controls the specific surface area to reduce the interface contact. Modified glucose (containing carboxyl oligosaccharides) pyrolysis forms a uniform single-layer carbon coating, enhances the interface bonding with LMFP crystals through chemical bonding, effectively shields the electrolyte contact, inhibits manganese ion dissolution and electrolyte decomposition, cooperates with the stable olivine structure optimized by the Mn / Fe ratio, improves the electronic conductivity and high temperature cycle stability, and ensures excellent capacity retention rate and coulombic efficiency.

[0010] As a preferred technical solution of the preparation method of the nano lithium manganese iron phosphate, the preparation method of the pre-lithiated lithium manganese iron phosphate precursor comprises: mixing manganese sulfate, ferrous sulfate, ammonium dihydrogen phosphate and lithium carbonate according to LiMn 0.7 Fe 0.3 The stoichiometric ratio of PO4 is mixed, deionized water is added and the pH is adjusted to 8.5-9.5, and hydrothermal reaction is carried out at 130-160°C for 10-20 hours, and after cooling, it is filtered, washed and precalcined at 350-450°C for 3-5 hours, and then sintered at 680-720°C for 6-10 hours under an inert atmosphere to obtain a pre-lithiated LiMn 0.7 Fe 0.3 PO4 precursor with high crystallinity.

[0011] It should be noted that the hydrothermal reaction is in an alkaline environment (pH 8.5-9.5), and high temperature and high pressure are used to promote the formation of olivine structure initial crystals of manganese sulfate, ferrous sulfate, ammonium phosphate and lithium carbonate in stoichiometric ratio, lithium ions are uniformly embedded in the crystal lattice, the crystal defects are reduced, and the Mn / Fe ratio is optimized to reduce the manganese oxidation activity. Pre-calcination removes surface impurities and preliminarily stabilizes the crystal structure, high-temperature sintering further enhances the lattice regularity and crystallinity in an inert atmosphere, forming high-stability nanoscale precursor particles. This precursor provides uniform crystal surfaces for subsequent carbon coating, enhances interface bonding strength, suppresses manganese ion dissolution and electrolyte side reactions, while promoting lithium ion diffusion and electronic conductivity, improving high-temperature cycle stability and electrochemical performance.

[0012] As a preferred technical solution of the preparation method of the nano manganese iron lithium phosphate, in step S1, the pretreatment is performed by ultrasonic dispersion combined with mechanical stirring for 1-3 hours, wherein the ultrasonic dispersion power is 100-300 W, and the mechanical stirring speed is 200-500 rpm.

[0013] It should be noted that the pretreatment by ultrasonic dispersion combined with mechanical stirring promotes the uniform dispersion of particles in the slurry and the reconstruction of the crystal surface through the synergistic effect of ultrasonic cavitation and mechanical shear force, significantly reduces the defect sites and enhances the interface stability of the subsequent process.

[0014] As a preferred technical solution of the preparation method of the nano manganese iron lithium phosphate, in step S2, the wet ball milling treatment uses zirconium oxide balls as the grinding medium, the ball milling time is 3-5 hours, the speed is 800-1500 rpm, and the ball-to-material ratio is 10:1 to 20:1.

[0015] It should be noted that the wet ball milling treatment precisely controls the particle size to be nanoscale by the mechanical grinding action of zirconium oxide balls under the conditions of optimized speed, time and ball-to-material ratio, shortens the lithium ion diffusion path, and at the same time avoids the destruction of the crystal structure caused by excessive grinding.

[0016] As a preferred technical solution of the preparation method of the nano manganese iron lithium phosphate, the preparation method of the modified glucose is as follows: glucose and citric acid are mixed in a mass ratio of 10:1, deionized water is added to prepare a solution with a mass concentration of 15-25 wt%, the temperature is raised to 130-160℃ at a rate of 3-6℃ / min under nitrogen protection, preheating is performed for 2-3 hours by ultrasonic auxiliary power of 100-200 W and frequency of 20-40 kHz, and then spray drying is performed at an inlet temperature of 150-170℃ and an outlet temperature of 70-90℃ to obtain modified glucose powder.

[0017] It should be noted that by mixing glucose and citric acid in a mass ratio of 10:1 and dissolving in deionized water to form a 15-25wt% solution, slowly heating to 130-160°C under nitrogen protection combined with ultrasonic assisted preheating, promoting citric acid as a catalyst to induce partial hydrolysis and carboxylation of glucose, generating oligosaccharide derivatives containing carboxyl functional groups, which realize uniform chain scission and functional modification under the cavitation effect and mechanical vibration of ultrasonic waves, improve the thermal stability and surface activity. Subsequently, by spray drying (inlet air temperature 150-170°C, outlet air temperature 70-90°C) to rapidly evaporate water, a powder of modified glucose is formed, which is pyrolyzed into a uniform single-layer carbon coating layer during the subsequent sintering process of the LMFP material, the carboxyl groups form chemical bonding with the surface of the lithium manganese iron phosphate crystal, enhancing the interface affinity, effectively shielding the electrolyte contact, inhibiting the dissolution of manganese ions and the decomposition of electrolyte side reactions, while improving the electronic conductivity and structural stability, thereby optimizing the rate performance and cycle life of the material.

[0018] As a preferred technical solution of the method for preparing nano lithium manganese iron phosphate, in step S3, the inlet air temperature of the spray drying is 170-210°C, and the outlet air temperature is 90-130°C.

[0019] It should be noted that the slurry water can be rapidly evaporated to form a uniform dry powder, avoiding Li volatilization or lattice damage caused by high temperature, while controlling the powder temperature to prevent premature pyrolysis of the carbon source, ensuring uniform deposition of the carbon coating layer and improving the electronic conductivity and structural stability of the material.

[0020] As a preferred technical solution of the method for preparing nano lithium manganese iron phosphate, in step S4, the inert atmosphere is nitrogen or argon, and the sintering process adopts staged heating, with an initial stage of 2-5°C / min to 500°C for 1-2 hours, followed by 5-10°C / min to 700-750°C.

[0021] It should be noted that the interface between the carbon coating layer and the LMFP crystal is optimized, enhancing the structural stability and electrochemical performance, while preventing crystal defects and side reactions.

[0022] The method for preparing nano lithium manganese iron phosphate described in this application introduces pre-lithiated precursors and modified glucose carbon sources, achieving precise control of particle size at the nanoscale, significantly shortening the lithium ion diffusion path and improving the electronic conductivity, making the material exhibit excellent specific capacity, rate performance and cycle stability; at the same time, the carboxylated carbon coating layer enhances the interface chemical bonding, effectively inhibits the dissolution of manganese ions and electrolyte side reactions, optimizes the high-temperature cycle stability and coulombic efficiency, balances high energy density and long life, is suitable for high-performance lithium ion battery cathode materials, and promotes the industrialization application of electric vehicles and energy storage systems. BRIEF DESCRIPTION OF DRAWINGS

[0023] The accompanying drawings are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and are used to explain the application, but are not intended to limit the application. In the drawings:

[0024] Figure 1 SEM image of the lithium manganese iron phosphate positive electrode material prepared in Example 1;

[0025] Figure 2 IR spectrum of the dry powder prepared in step S3 in Example 1;

[0026] Figure 3 Particle size distribution graph of the lithium manganese iron phosphate positive electrode material prepared in Examples 1 to 4 and Comparative Examples 1 to 2;

[0027] Figure 4 XRD graph of the lithium manganese iron phosphate positive electrode material; (a) Example 1, (b) Comparative Example 1. DETAILED DESCRIPTION

[0028] In order to make the above objectives, features 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.

[0029] In the following description, a lot of specific details are set forth in order to provide a thorough understanding of the present application, but the present application can be practiced in other manners different from those described herein, and those skilled in the art can make similar generalizations without departing from the spirit of the present application, so the present application is not limited to the specific embodiments disclosed below.

[0030] Secondly, "one embodiment" or "an embodiment" referred to herein means that a specific feature, structure or characteristic 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 of or mutually exclusive with other embodiments.

[0031] Preparation Example

[0032] Preparation Example 1

[0033] Preparation of pre-lithiated lithium manganese iron phosphate precursor: manganese sulfate, ferrous sulfate, ammonium dihydrogen phosphate and lithium carbonate were mixed in stoichiometric ratio of LiMn 0.7 Fe 0.3 PO4, deionized water was added and pH was adjusted to 8.5, and hydrothermal reaction was carried out at 130°C for 20 hours, and after cooling, filtration and washing were carried out and pre-calcination was carried out at 350°C for 3 hours, and then sintering was carried out at 680°C for 10 hours under inert atmosphere, to obtain high crystallinity pre-lithiated LiMn 0.7 Fe 0.3PO4 precursor.

[0034] Preparation Example 2

[0035] Preparation of pre-lithiated lithium manganese iron phosphate precursor: manganese sulfate, ferrous sulfate, ammonium dihydrogen phosphate and lithium carbonate were mixed in stoichiometric ratio of LiMn 0.7 Fe 0.3 PO4, deionized water was added and pH was adjusted to 9, hydrothermal reaction was carried out at 145°C for 15 hours, after cooling, it was filtered, washed and pre-calcined at 400°C for 4 hours, then sintered at 700°C for 8 hours under inert atmosphere, to obtain high crystallinity pre-lithiated LiMn 0.7 Fe 0.3 PO4 precursor.

[0036] Preparation Example 3

[0037] Preparation of pre-lithiated lithium manganese iron phosphate precursor: manganese sulfate, ferrous sulfate, ammonium dihydrogen phosphate and lithium carbonate were mixed in stoichiometric ratio of LiMn 0.7 Fe 0.3 PO4, deionized water was added and pH was adjusted to 9.5, hydrothermal reaction was carried out at 160°C for 10 hours, after cooling, it was filtered, washed and pre-calcined at 450°C for 5 hours, then sintered at 720°C for 6 hours under inert atmosphere, to obtain high crystallinity pre-lithiated LiMn 0.7 Fe 0.3 PO4 precursor.

[0038] Preparation Example 4

[0039] Preparation of pre-lithiated lithium manganese iron phosphate precursor: manganese sulfate, ferrous sulfate, ammonium dihydrogen phosphate and lithium carbonate were mixed in stoichiometric ratio of LiMn 0.7 Fe 0.3 PO4, deionized water was added and pH was adjusted to 9, hydrothermal reaction was carried out at 140°C for 12 hours, after cooling, it was filtered, washed and pre-calcined at 375°C for 3.5 hours, then sintered at 690°C for 7 hours under inert atmosphere, to obtain high crystallinity pre-lithiated LiMn 0.7 Fe 0.3 PO4 precursor.

[0040] Example

[0041] Example 1

[0042] The example provides a preparation method of nano lithium manganese iron phosphate, comprising the following technical steps:

[0043] Step S1: the pre-lithiated lithium manganese iron phosphate precursor prepared in the preparation example 1 is mixed with lithium carbonate, iron phosphate, trimanganese tetraoxide and ammonium phosphate in stoichiometric ratio of LiMn 0.7 Fe 0.3The stoichiometric ratio of PO4 is mixed, and deionized water is added to prepare a uniform slurry, wherein the mass of the pre-lithiated lithium manganese iron phosphate precursor accounts for 15% of the total mixture. The slurry is pretreated in an alkaline environment with a pH of 8.5 for 1 hour by ultrasonic dispersion power of 100 W combined with mechanical stirring at a speed of 200 rpm.

[0044] Step S2: The slurry obtained in step S1 is subjected to wet ball milling treatment, zirconia balls are used as grinding medium, the ball milling time is 3 hours, the rotating speed is 800 rpm, and the ball-to-material ratio is 10:1.

[0045] Step S3: The modified glucose is prepared by mixing glucose and citric acid at a mass ratio of 10:1, adding deionized water to prepare a solution with a mass concentration of 15 wt%, heating to 130°C at a rate of 3°C / min under nitrogen protection, preheating for 2 hours by ultrasonic auxiliary power of 100 W and frequency of 20 kHz, and then spray drying with an inlet temperature of 150°C and an outlet temperature of 70°C to obtain modified glucose powder. The ball milled material obtained in step S2 is mixed with the modified glucose, and the modified glucose accounts for 6 wt% of the total solid mass, and then spray drying is performed with an inlet temperature of 170°C and an outlet temperature of 90°C to obtain dry powder with a particle size D50 of 80 nm.

[0046] Step S4: The spray dried powder obtained in step S3 is sintered at 700°C for 8 hours under an inert atmosphere of nitrogen, and the sintering process is carried out in stages, i.e., the initial stage is to increase the temperature to 500°C at a rate of 2°C / min and keep it for 1 hour, and then increase the temperature to 700°C at a rate of 5°C / min. A carbon-coated lithium manganese iron phosphate positive electrode material is obtained.

[0047] Example 2

[0048] The embodiment provides a preparation method of nano lithium manganese iron phosphate, which comprises the following technical steps:

[0049] Step S1: The lithiumated lithium manganese iron phosphate precursor prepared in the above preparation example 2 is mixed with lithium carbonate, iron phosphate, trimanganese tetraoxide and ammonium phosphate according to LiMn 0.7 Fe 0.3 PO4 is mixed in a stoichiometric ratio, and deionized water is added to prepare a uniform slurry, wherein the mass of the pre-lithiated lithium manganese iron phosphate precursor accounts for 20% of the total mixture. The slurry is pretreated in an alkaline environment with a pH of 9 for 2 hours by ultrasonic dispersion power of 200 W combined with mechanical stirring at a speed of 350 rpm.

[0050] Step S2: The slurry obtained in step S1 is subjected to wet ball milling treatment, zirconia balls are used as grinding medium, the ball milling time is 4 hours, the rotating speed is 1150 rpm, and the ball-to-material ratio is 15:1.

[0051] Step S3: Preparation of modified glucose: glucose and citric acid were mixed in a mass ratio of 10:1, deionized water was added to prepare a solution with a mass concentration of 20wt%, and the solution was heated to 145℃ at a rate of 4.5℃ / min under nitrogen protection. The solution was preheated for 2.5 hours by ultrasonic auxiliary power of 150W and frequency of 30kHz. Then, the solution was spray dried at an inlet temperature of 160℃ and an outlet temperature of 80℃ to obtain modified glucose powder. The ball milled material obtained in step S2 was mixed with the modified glucose, and the modified glucose was added in an amount of 9wt% of the total solid mass. Then, the mixture was spray dried at an inlet temperature of 190℃ and an outlet temperature of 110℃ to obtain a dry powder with a particle size D50 of 115nm.

[0052] Step S4: The spray dried powder obtained in step S3 was sintered at 725℃ for 11 hours under an argon inert atmosphere. The sintering process was carried out in stages, with an initial stage of heating to 500℃ at a rate of 3.5℃ / min and holding for 1.5 hours, followed by heating to 725℃ at a rate of 7.5℃ / min. A lithium manganese iron phosphate positive electrode material with a carbon-coated structure was obtained.

[0053] Example 3

[0054] The example provides a method for preparing nano lithium manganese iron phosphate, comprising the following technical steps:

[0055] Step S1: The pre-lithiated lithium manganese iron phosphate precursor prepared in Preparation Example 3 was mixed with lithium carbonate, iron phosphate, trimanganese tetraoxide and ammonium phosphate in a stoichiometric ratio of LiMn 0.7 Fe 0.3 PO4, deionized water was added to prepare a uniform slurry, and the mass of the pre-lithiated lithium manganese iron phosphate precursor accounted for 25% of the total mixture. The slurry was pretreated in an alkaline environment with a pH of 9.5 by ultrasonic dispersion power of 300W combined with mechanical stirring speed of 500rpm for 3 hours.

[0056] Step S2: The slurry obtained in step S1 was subjected to wet ball milling treatment, using zirconia balls as grinding medium, ball milling time was 5 hours, rotation speed was 1500rpm, and ball to material ratio was 20:1.

[0057] Step S3: Preparation of modified glucose: glucose and citric acid were mixed in a mass ratio of 10:1, deionized water was added to prepare a solution with a mass concentration of 25wt%, and the temperature was raised to 160℃ at a rate of 6℃ / min under nitrogen protection. The preheating was performed for 3 hours by ultrasonic auxiliary power of 200W and frequency of 40kHz. Then, spray drying was performed with an inlet temperature of 170℃ and an outlet temperature of 90℃ to obtain modified glucose powder. The ball-milled material obtained in step S2 was mixed with the above modified glucose, and the addition amount of the modified glucose was 12wt% of the total solid mass. Then, spray drying was performed with an inlet temperature of 210℃ and an outlet temperature of 130℃ to obtain dry powder with a particle size D50 of 150nm.

[0058] Step S4: The spray-dried powder obtained in step S3 was sintered at 750℃ for 14 hours under a nitrogen inert atmosphere. The sintering process was performed in stages, with an initial stage of raising the temperature to 500℃ at a rate of 5℃ / min and holding for 2 hours, followed by raising the temperature to 750℃ at a rate of 10℃ / min. A lithium manganese iron phosphate positive electrode material with a carbon-coated structure was obtained.

[0059] Example 4

[0060] This example provides a method for preparing nano lithium manganese iron phosphate, comprising the following technical steps:

[0061] Step S1: The pre-lithiated lithium manganese iron phosphate precursor prepared in Preparation Example 4 was mixed with lithium carbonate, iron phosphate, trimanganese tetraoxide, and ammonium phosphate in a stoichiometric ratio of LiMn 0.7 Fe 0.3 PO4, deionized water was added to prepare a uniform slurry, and the addition amount of the pre-lithiated lithium manganese iron phosphate precursor was 18% of the total mixture. The slurry was pretreated in an alkaline environment with a pH of 9 by ultrasonic dispersion power of 150W combined with mechanical stirring at a speed of 300rpm for 1.5 hours.

[0062] Step S2: The slurry obtained in step S1 was subjected to wet ball milling treatment using zirconia balls as the grinding medium. The ball milling time was 3.5 hours, the rotation speed was 1000rpm, and the ball-to-material ratio was 12:1.

[0063] Step S3: Preparation of modified glucose: glucose and citric acid were mixed in a mass ratio of 10:1, deionized water was added to prepare a solution with a mass concentration of 18wt%, and the solution was heated to 140℃ at a rate of 4℃ / min under nitrogen protection. The solution was preheated for 2.2 hours by ultrasonic auxiliary power of 120W and frequency of 25kHz, and then spray dried at an inlet temperature of 155℃ and an outlet temperature of 75℃ to obtain modified glucose powder. The ball milled material obtained in step S2 was mixed with the above modified glucose, and the amount of the modified glucose added was 8wt% of the total solid mass, and then spray dried at an inlet temperature of 180℃ and an outlet temperature of 100℃ to obtain a dry powder with a particle size D50 of 100nm.

[0064] Step S4: The spray dried powder obtained in step S3 was sintered at 710℃ for 10 hours under an argon inert atmosphere. The sintering process was carried out in stages, with an initial stage of heating to 500℃ at a rate of 3℃ / min and holding for 1.2 hours, followed by heating to 710℃ at a rate of 6℃ / min. A lithium manganese iron phosphate positive electrode material with a carbon-coated structure was obtained.

[0065] Comparative Example

[0066] Comparative Example 1

[0067] The difference between this comparative example 1 and example 1 is that no pre-lithiated precursor is used, but the sources of Mn, Fe, Li and phosphorus are mixed and reacted directly according to the stoichiometric ratio.

[0068] Comparative Example 2

[0069] The difference between this comparative example 1 and example 1 is that pure glucose without modification is directly used as a carbon source.

[0070] Application Example

[0071] Application Example 1

[0072] First, the positive electrode sheet was prepared. The LMFP positive electrode material of examples 1-4 or comparative examples 1-2 was weighed, mixed with conductive carbon black (Super P) and PVDF binder in 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℃ for 8 hours, cut into a circular electrode sheet with a diameter of 14mm, and pressed to a material loading of 2.5 mg / cm 2The positive electrode was prepared by coating slurry on an aluminum foil (thickness 50 pm). The slurry was prepared by mixing 90 wt% of the prepared LMFP cathode material (Example 1-4 or Comparative Example 1), 5 wt% of Super-P (Timcal), and 5 wt% of polyvinylidene fluoride (PVDF, Sigma-Aldrich) in N-methyl-2-pyrrolidone (NMP, Sigma-Aldrich). The slurry was stirred for 24 h at room temperature. The slurry was coated on the aluminum foil using a doctor blade, and then dried at 80 °C for 12 h. The lithium foil (thickness 1 mm) was cut into the appropriate size. The electrolyte was 1 M LiPF6in EC / DMC / EMC (volume ratio 1:1:1), and the separator was a polypropylene microporous membrane (Celgard 2400). The CR2032 button cell was assembled in an argon-filled glovebox (water and oxygen content <0.1 ppm) by stacking the positive electrode shell, the positive electrode, the separator (20 pL of electrolyte was added), the lithium foil negative electrode, the gasket, and the negative electrode shell in sequence. The button cell was sealed using a press machine to ensure good contact.

[0073] Performance test methods

[0074] 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, dried) 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 for 5 min 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.

[0075] 2. Specific capacity test method: A constant current-constant voltage charge-discharge test system (e.g., LANHE CT2001A) was used to perform 100 cycle tests in a 25 ± 1 °C constant temperature box. The test voltage range was 2.0-4.5 V (vs. Li+ / Li), and the specific capacity performance of the LMFP cathode material at 0.1 C and 1 C rates was recorded.

[0076] 3. Cycle stability test method: A constant current-constant voltage charge-discharge test system (e.g., LANHE CT2001A) was used to test in a 25 ± 1 °C constant temperature box, with a voltage range of 2.0-4.5 V, and 1000 cycles at a rate of 1 C to evaluate the long-term stability of the LMFP cathode material at high capacity (150 mAh / g).

[0077] 4. Manganese elution test method: The LMFP cathode electrode was soaked in 1 M LiPF6electrolyte (EC / DMC / EMC, volume ratio 1:1:1) and stored at 60 °C for 7 days. Inductively coupled plasma mass spectrometry (ICP-MS) was used to analyze the Mn ion concentration in the electrolyte to evaluate the chemical stability of the material.

[0078]

[0079] In combination with Example 1 and Figure 1It can be seen that the sample particles have a uniform spherical morphology, the particle size distribution is concentrated and the particles are tightly packed, and the overall morphology is dense without 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.

[0080] In combination with Example 1 and Figure 2 It can be seen that in the high wave number region (about 3400 cm -1 ), a broad O-H stretching vibration peak appears, indicating the presence of hydrogen-bonded hydroxyl groups derived from glucose residues and introduced carboxyl groups; the doublet at 2850-3000 cm -1 corresponds to C-H stretching vibration, confirming the retention of alkyl chains; the strong peak at 1700-1750 cm -1 is C=O stretching, proving that the carboxylation modification under the catalysis of citric acid is successful, and this functional group not only improves the surface activity of glucose, but also provides key sites for the chemical bonding of the subsequent carbon coating layer and manganese iron phosphate lithium crystal, thereby optimizing the interface stability and electrochemical performance of the material; in addition, the multi-peak region at 1050-1250 cm -1 reflects C-O and C-O-C stretching, derived from ether or ester bond structures of sugar chains, and the weak peak at 1400-1500 cm -1 is O-H bending vibration.

[0081] In combination with Examples 1 to 4, Comparative Examples 1 to 2, and Figure 3 It can be seen that the particle size distribution curves of the lithium manganese iron phosphate positive electrode materials of Examples 1 to 4 all exhibit a highly concentrated single peak feature, with D50 particle size values controlled in the nanometer range of 100-200 nm (e.g., 156 nm for Example 1, 185 nm for Example 2, 100 nm for Example 3, and 200 nm for Example 4), with narrow distribution width and sharp peak value, indicating uniform particle size and high consistency without obvious agglomeration, while the distribution curves of Comparative Examples 1 and 2 show a wide multi-peak or tailing feature, with D50 particle size values increased to more than 250 nm, with flat peak value and scattered distribution, large particles and easy agglomeration; the example group introduces pre-lithiated manganese iron phosphate lithium precursor as a seed template, effectively optimizes crystal nucleus growth and ion diffusion path in alkaline environment combined with mechanical stirring, sanding treatment and spray drying process, inhibits local unevenness caused by solubility difference, and promotes uniform refinement of nanoscale particles, while the comparative example group lacks the pre-lithiation step, directly mixes raw materials to cause pH gradient fluctuation and lattice stress accumulation, thereby increasing particle size, uneven distribution, and ultimately significantly reducing lithium ion diffusion efficiency, specific surface area utilization rate and overall electrochemical performance of the material, including specific capacity and cycle stability.

[0082] In combination with Example 1, Comparative Example 1, and Figure 4 As can be seen, the XRD pattern of Example 1 (top) exhibits sharp and high-intensity diffraction peaks, which are in full agreement with the standard diffraction characteristic peak positions of olivine-type LiMn 0.7 Fe 0.3 PO4(Pnma space group), indicating that the sample of Example 1 successfully formed the target lithium manganese iron phosphate crystal phase with high crystallinity and phase purity. The peaks are narrow and symmetrical, indicating that the crystal structure is regular, has few internal defects, and has small strain; in contrast, the sample of Comparative Example 1 has additional weak peaks near 2θ≈25° and 45°, and the main peaks are significantly broadened, indicating that there are a small amount of impurities (such as Fe2P or MnPO4) and amorphous components in the sample, the lattice is greatly distorted, and the crystallinity is low. By introducing a pre-lithiated lithium manganese iron phosphate precursor as a seed crystal, Example 1 effectively stabilized the oxidation states of Mn 2+ and Fe 2+ , inhibited Jahn-Teller distortion and local pH gradient fluctuations, promoted uniform crystal nucleus growth and Li + order insertion; in contrast, the direct mixing of raw materials in Comparative Example 1 led to differences in solubility, inducing redox inhomogeneity and lattice defect accumulation, thereby reducing phase purity and structural stability, ultimately significantly affecting the electrochemical performance of the material, including specific capacity, cycle stability, and chemical stability.

[0083] In combination with Examples 1 to 4 and Table 1, it can be seen that the specific capacity of the lithium manganese iron phosphate positive electrode material is 160.8-163.2 mAh / g at 0.1C rate, 150.3-152.8 mAh / g at 1C rate, the capacity retention rate is 92.3-94.1% after 1000 cycles, and the manganese leaching amount is 0.8-1.2 ppm.

[0084] In combination with Example 1, Comparative Example 1, and Table 1, it can be seen that the specific capacity of the lithium manganese iron phosphate positive electrode material of Example 1 is 165 mAh / g at 0.1C rate, 158 mAh / g at 1C rate, the capacity retention rate is as high as 92.5 after 1000 cycles, and the manganese leaching amount is as low as 10.2 ppm, while the corresponding indicators of Comparative Example 1 are only 148 mAh / g at 0.1C, 135 mAh / g at 1C, a capacity retention rate of 75.6%, and a manganese leaching amount of 32.1 ppm; this difference is due to the introduction of a pre-lithiated lithium manganese iron phosphate precursor as a seed crystal template in Example 1, which optimizes the Mn 2+ and Fe 2+The uniform distribution of the carbon layer suppresses the Jahn-Teller effect and local pH gradient fluctuation, and the uniform carbon coating formed by spray drying improves the lithium ion diffusion efficiency and electronic conductivity. The control example 1 lacks the pre-lithiation step, and the direct mixing of raw materials leads to poor solubility, lattice defects, and Mn 3+ The generated particles are large and severely agglomerated, thereby reducing phase purity, structural stability, and electrochemical performance.

[0085] As can be seen from Example 1, Control Example 2, and Table 1, the specific capacity of the nanoscale lithium iron manganese phosphate positive electrode material of Example 1 is 165 mAh / g at 0.1C, 158 mAh / g at 1C, and the capacity retention rate is as high as 92.5% after 1000 cycles, and the manganese elution amount is as low as 10.2 ppm, while the corresponding indicators of Control Example 2 are only 148 mAh / g at 0.1C, 135 mAh / g at 1C, a capacity retention rate of 75.6%, and a manganese elution amount of 32.1 ppm; the difference is due to the use of modified glucose as a carbon source in Example 1, which generates functional oligosaccharides through carboxylation induced by citric acid, forms a uniform single-layer carbon coating during sintering, and chemically bonds with the crystal surface to enhance the interfacial affinity, effectively shielding the electrolyte contact, suppressing manganese ion dissolution and side reactions, and improving electronic conductivity and structural stability, while the use of unmodified pure glucose in Control Example 2 results in an uneven carbon layer, weak adhesion, easy peeling and exposure of the crystal surface during cycling, and the initiation of interface side reactions, thereby reducing the rate performance, cycle stability, and chemical durability of the material.

[0086] 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 nano-lithium manganese iron phosphate, 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 ammonium phosphate in the order LiMn 0.7 Fe 0.3 The stoichiometric ratio of PO4 is mixed and deionized water is added to prepare a homogeneous slurry, wherein the added mass of the pre-lithiated lithium manganese iron phosphate precursor accounts for 15-25% of the total mass of the mixture, and pretreatment is carried out in an alkaline environment with a pH of 8.5-9.

5. Step S2. Perform wet ball milling on the slurry obtained in step S1; Step S3. Mix the ball-milled material obtained in step S2 with modified glucose, wherein the amount of modified glucose added is 6-12 wt% of the total solid mass, and then spray dry to obtain dried powder; Step S4. The spray-dried powder obtained in step S3 is sintered at 700-750℃ for 8-14 hours under an inert atmosphere to obtain lithium manganese iron phosphate cathode material with carbon coating structure. The preparation method of the pre-lithiated lithium manganese iron phosphate precursor includes: mixing manganese sulfate, ferrous sulfate, ammonium dihydrogen phosphate, and lithium carbonate in a LiMn mixture. 0.7 Fe 0.3 PO4 was stoichiometrically mixed, deionized water was added and the pH was adjusted to 8.5-9.5, and the mixture was hydrothermally reacted at 130-160℃ for 10-20 hours. After cooling, the mixture was filtered, washed, and pre-calcined at 350-450℃ for 3-5 hours, followed by sintering at 680-720℃ for 6-10 hours under an inert atmosphere to obtain highly crystalline pre-lithiated LiMn. 0.7 Fe 0.3 PO4 precursor; The modified glucose is prepared by mixing glucose and citric acid at a mass ratio of 10:1, adding deionized water to prepare a solution with a mass concentration of 15-25wt%, heating to 130-160℃ at 3-6℃ / min under nitrogen protection, preheating for 2-3 hours with ultrasonic assistance at a power of 100-200W and a frequency of 20-40kHz, and then spray drying at an inlet air temperature of 150-170℃ and an outlet air temperature of 70-90℃ to obtain modified glucose powder.

2. The method for preparing nano-lithium manganese iron phosphate according to claim 1, characterized in that, In step S1, the pretreatment is carried out by ultrasonic dispersion combined with mechanical stirring for 1-3 hours, wherein the ultrasonic dispersion power is 100-300W and the mechanical stirring speed is 200-500rpm.

3. The method for preparing nano-lithium manganese iron phosphate according to claim 1, characterized in that, In step S2, the wet ball milling process uses zirconia balls as the grinding medium, the ball milling time is 3-5 hours, the rotation speed is 800-1500 rpm, and the ball-to-material ratio is 10:1 to 20:

1.

4. The method for preparing nano-lithium manganese iron phosphate according to claim 1, characterized in that, In step S3, the inlet air temperature of the spray dryer is 170-210℃, and the outlet air temperature is 90-130℃.

5. The method for preparing nano-lithium manganese iron phosphate according to claim 1, characterized in that, In step S4, the inert atmosphere is nitrogen or argon. The sintering process adopts a staged heating method. In the initial stage, the temperature is raised to 500°C at 2-5°C / min and held for 1-2 hours. Then, the temperature is raised to 700-750°C at 5-10°C / min.

6. An application of lithium manganese iron phosphate prepared according to any one of the preparation methods of claims 1 to 5 in battery cathode materials.

Images