Preparation method and application of lithium manganese iron phosphate positive electrode material

Abstract

The invention belongs to the technical field of lithium ion battery positive electrode materials, and particularly relates to a preparation method and application of a lithium iron manganese phosphate positive electrode material. Through the vapor deposition technology, it is guaranteed that manganese and iron elements are evenly mixed on the atomic scale from the source, the obtained precursor is high in purity and high in activity, impurity ions introduced by a traditional liquid phase method are avoided, liquid phase grinding and a two-step sintering method are combined, even dispersion and effective coating of a carbon source are achieved, an excellent conductive network is constructed, and the high-performance lithium ion battery anode material is obtained. The phase separation and Jahn-Teller effect in the charging and discharging process are effectively avoided, the rate capability of the material is remarkably improved, and the cycle life of the material is remarkably prolonged.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material technology, specifically relating to a preparation method and application of lithium manganese iron phosphate cathode material. Background Technology

[0002] Among commonly used cathode materials, LiFePO4 (LFP) stands out due to its unique olivine structure, featuring (PO4)3-tetrahedral structures and strong PO covalent bonds. This structure contributes to its excellent cycle life and safety performance. However, its energy density is relatively low, necessitating the development of novel cathode materials with similar structures but higher energy density. Lithium manganese iron phosphate (LMFP), as an upgraded material of lithium iron phosphate, is gradually becoming a rising star in the field of lithium battery cathode materials thanks to its high voltage platform of 4.1V and the stability of its olivine structure.

[0003] However, the traditional solid-state method for preparing lithium manganese iron phosphate materials struggles to achieve uniform distribution of manganese and iron atoms. Localized manganese-rich regions are prone to Jahn-Teller distortion, leading to a precipitous decline in electrochemical performance. To address these issues, a manganese-iron coprecipitation precursor process is employed. This process precisely controls the manganese-iron element ratio and synthesis pathway, achieving atomic-level uniform distribution of manganese and iron, thus mitigating phase separation risks at the source and ensuring material structural stability. However, the coprecipitation reaction involves several key parameters, including pH, temperature, stirring speed, reactant concentration (metal salt, precipitant, complexing agent such as ammonia), feed rate, and residence time. These parameters interact and require precise control to achieve ideal chemical composition, particle size distribution (PSD), and morphology (sphericity, density). Furthermore, the production process utilizes large quantities of ammonia as a complexing agent, resulting in costly wastewater treatment and significant environmental impact. Summary of the Invention

[0004] Based on the existing technical challenges, the purpose of this invention is to overcome the shortcomings of existing liquid-phase and solid-phase methods in preparing manganese-iron precursors, such as uneven element distribution, complex process flow, and difficulty in morphology control. The invention provides a method for preparing manganese-iron precursors by gas phase method, and further synthesizes carbon-coated lithium manganese iron phosphate material, which exhibits good electrochemical performance when applied in secondary batteries.

[0005] To achieve the above objectives, the present invention provides a method for preparing lithium manganese iron phosphate cathode material, the specific scheme of which is as follows: A method for preparing lithium manganese iron phosphate cathode material includes the following steps: (1) Volatile organometallic compounds containing manganese and iron are used as precursor sources and heated and vaporized under inert gas protection to form a homogeneous gas-phase mixture; (2) The gaseous mixture is transported to the reaction zone by a carrier gas, and a gaseous chemical reaction occurs under specific temperature and pressure conditions in the presence of a reactive gas, and is deposited on a collection device to obtain a manganese-iron precursor material; (3) The manganese iron precursor material is mixed and ground with lithium source, phosphorus source and carbon source in solvent to form a slurry, which is then dried and sintered to obtain the lithium manganese iron phosphate cathode material.

[0006] In step (1), the manganese-containing volatile organometallic compound is selected from at least one of dicarbonylcyclopentadienylmanganese, tris(2,2,6,6-tetramethyl-3,5-heptadecanoic acid)manganese, acetylacetone manganese, methylcyclopentadienyltricarbonylmanganese, or hexacarbonylmanganese; the iron-containing volatile organometallic compound is selected from at least one of tricarbonylcycloheptadecyl iron, tris(2,2,6,6-tetramethyl-3,5-heptadecanoic acid) iron, acetylacetone iron, pentacarbonyl iron, or ferrocene. These precursors need to have suitable vapor pressures and similar thermal decomposition temperatures. Further, the precursor source is placed in a vaporization device and heated to 100-250°C under the protection of an inert gas (such as high-purity argon or nitrogen) to completely vaporize it, forming a homogeneous manganese-iron gaseous mixture.

[0007] In step (2), the reactive gas is ozone or a mixture of water vapor and oxygen.

[0008] In step (2), the specific temperature is 400-600℃ and the pressure is 1-50 Torr.

[0009] In step (3), the lithium source is one or a combination of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium metal, and lithium phosphate; the phosphorus source is one or a combination of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium dihydrogen phosphate, and lithium phosphate.

[0010] In the lithium manganese iron phosphate cathode material obtained in step (3), the molar ratio of Li:(Mn+Fe+M):P is (1.01~1.03):(0.97~1):1.

[0011] In step (3), the carbon source is at least one of glucose, starch, citric acid or polyethylene glycol, and the amount of the carbon source is controlled according to 5% to 15% of the mass of the theoretically generated lithium manganese iron phosphate.

[0012] In step (3), the sintering is a two-step sintering process carried out under an inert atmosphere: The first step is pre-sintering: the temperature is increased to 350-450℃ at 2-5℃ / min and held for 2-5 hours to initially carbonize the carbon source and remove the water of crystallization and some decomposition products; The second step is high-temperature crystallization: the temperature is increased to 650-750℃ at a rate of 3-5℃ / min and held for 8-15 hours to complete the crystal phase formation and crystallization of lithium manganese iron phosphate. At the same time, the carbon source is in-situ and uniformly carbonized and coated, and finally carbon-coated lithium manganese iron phosphate cathode material is obtained.

[0013] In step (3), grinding is performed by ball milling and / or sand milling, and drying is performed by spray drying or vacuum drying.

[0014] A method for preparing lithium manganese iron phosphate cathode material. The prepared lithium manganese iron phosphate cathode material can be applied to the field of lithium-ion batteries or solid-state batteries.

[0015] This invention utilizes volatile organometallic compounds containing manganese and iron as precursor sources. Through precisely controlled gas-phase transport and chemical reactions, a homogeneous manganese-iron precursor material is deposited on a substrate at the molecular / atomic level. This precursor is then combined with lithium, phosphorus, and carbon sources through liquid-phase grinding and a two-step sintering process to ultimately obtain a high-performance lithium manganese iron phosphate material with extremely uniform manganese-iron distribution, a complete crystal structure, and dense carbon coating. This invention fundamentally solves the problems of manganese-iron element segregation and uneven mixing in traditional solid-phase or co-precipitation methods, significantly improving the electrochemical performance of the material, especially rate performance and cycle stability. The process exhibits good repeatability and is suitable for large-scale production.

[0016] The beneficial effects of this invention are as follows: By using vapor deposition technology, manganese and iron elements are ensured to be uniformly mixed at the atomic level from the source, resulting in a precursor with high purity and strong activity. This avoids the impurity ions introduced by traditional liquid phase methods. By combining liquid phase grinding and two-step sintering, uniform dispersion and effective coating of carbon source are achieved, constructing an excellent conductive network. This effectively avoids phase separation and the Jahn-Teller effect during charging and discharging, and significantly improves the rate performance and cycle life of the material. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly introduced below.

[0018] Figure 1 The image shows the line scan elemental distribution of the lithium manganese iron phosphate material in Example 1. Figure 2 The image shows the line scan elemental distribution of the lithium manganese iron phosphate material in Comparative Example 1. Detailed Implementation

[0019] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The technical solutions of the embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Example 1

[0020] (1) Using manganese acetylacetone and iron acetylacetone as manganese and iron sources, respectively, they were weighed and dissolved in ethanol according to the Mn:Fe molar ratio of 7:3 to form a mixed solution with a concentration of 0.1 mol / L. The above mixed solution was injected into the vaporization chamber through a micro pump. The temperature of the vaporization chamber was set to 250°C. Under nitrogen protection, the precursor solution was vaporized instantaneously to form a uniform gas phase mixture. (2) The gaseous precursor mixture is transported to a reaction tube at 500°C using high-purity nitrogen as a carrier gas. The system pressure is maintained at 10 Torr, and an appropriate amount of water vapor is introduced as an oxygen source. The precursor undergoes decomposition and oxidation reactions in the reaction tube. The residence time in the reaction tube is adjusted to 5s, and the precursor is deposited on the ceramic filter at the end of the reaction tube. The manganese iron oxide precursor is collected. (3) Weigh the above precursor powder with lithium carbonate (Li excess 5%), ammonium dihydrogen phosphate and glucose (target carbon content 10wt%) according to the stoichiometric ratio of Li:(Mn+Fe+M):P=1.02:1:1, add deionized water, control the solid content to 30%, ball mill and mix for 6 hours, spray dry the slurry, first heat to 400℃ at 3℃ / min and hold for 3 hours for pre-calcination, then heat to 700℃ at 5℃ / min and hold for 12 hours, and then cool with the furnace to obtain the final product. Example 2

[0021] The only difference between this embodiment and Embodiment 1 is that, in step (1), methylcyclopentadiene manganese tricarbonyl and iron pentacarbonyl are used as precursors, while the other steps are the same as in Embodiment 1. Example 3

[0022] The only difference between this embodiment and Embodiment 1 is that the molar ratio Mn:Fe = 0.6:0.4; the other steps are the same as in Embodiment 1. Example 4

[0023] The only difference between this embodiment and Embodiment 1 is that in step (3), there are two sintering steps: the temperature is increased to 350°C at 5°C / min and held for 5 hours; then the temperature is increased to 650°C at 5°C / min and held for 15 hours. The remaining steps are the same as in Embodiment 1. Example 5

[0024] The only difference between this embodiment and embodiment 1 is that in step (2), the temperature of the reaction zone is 600°C and the pressure is 1 Torr.

[0025] Comparative Example 1 The traditional solid-state sintering method was used as a comparison.

[0026] test The lithium manganese iron phosphate material prepared in the comparative examples and embodiments was weighed with conductive carbon black (SP) as a conductive agent and polyvinylidene fluoride (PVDF) as a binder at a mass ratio of 8:1:1. Using N-methylpyrrolidone (NMP) as a solvent, the mixture was homogenized in a homogenizer to prepare a positive electrode slurry. This slurry was then coated onto an aluminum foil current collector, dried, and rolled and punched into a positive electrode sheet. A lithium metal sheet was used as the negative electrode, and a polypropylene microporous membrane (Celgard 2300) was used as the separator. The resulting assembly was a CR2032 coin cell. Finally, charge-discharge tests were performed on a Blue Electric Test cabinet.

[0027] The compaction density of lithium manganese iron phosphate powder was measured using a PRCD3100 powder conductivity and compaction density meter.

[0028] Manganese leaching test of lithium manganese iron phosphate material: The ratio of lithium manganese iron phosphate powder to electrolyte was 1:50. The mixture was stored at room temperature (25℃) for 7 days, and the manganese content in the electrolyte was tested.

[0029] The test results are shown in Table 1. pass Figure 1 and Figure 2 The comparison shows that Figure 2 The distribution curve of Mn element fluctuates significantly. The manganese-iron intermediate synthesis process described in this application alleviates this fluctuation, demonstrating that the synthesis process promotes a more uniform distribution of manganese-iron. This improved uniformity also enhances the electrochemical performance of the material and slows down the dissolution of manganese.

[0030] The above embodiments are only for illustrating the technical concept and features of the present invention. Their purpose is to enable those skilled in the art to understand the content of the present invention and implement it. They should not be used to limit the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a lithium iron manganese phosphate cathode material, characterized in that, The method comprises the following steps: (1) heating and vaporizing volatile metal organic compounds containing manganese and iron as precursors under the protection of inert gas to form a uniform gas mixture; (2) transmitting the gas mixture to a reaction zone by a carrier gas, and allowing a gas phase chemical reaction to occur under specific temperature and pressure conditions in the presence of a reactive gas, and then depositing on a collection device to obtain a manganese-iron precursor material; (3) mixing and grinding the manganese-iron precursor material with a lithium source, a phosphorus source and a carbon source in a solvent to form a slurry, and then drying and sintering to obtain the manganese-iron lithium phosphate positive electrode material.

2. The method for preparing a lithium iron manganese phosphate cathode material according to claim 1, characterized in that: In the step (1), the volatile metal organic compound containing manganese is at least one selected from the group consisting of dicarbonylcyclopentadienyl manganese, tris(2,2,6,6-tetramethyl-3,5-heptanedionate) manganese, acetylacetone manganese, methylcyclopentadiene manganese tricarbonyl and manganese hexacarbonyl; and the volatile metal organic compound containing iron is at least one selected from the group consisting of tricarbonyl cycloheptatriene iron, tris(2,2,6,6-tetramethyl-3,5-heptanedionate) iron, acetylacetone iron, pentacarbonyl iron and ferrocene.

3. The method for preparing a lithium manganese iron phosphate cathode material according to claim 1, characterized in that: In the step (2), the reactive gas is ozone or a mixture of water vapor and oxygen.

4. The method for preparing a lithium manganese iron phosphate cathode material according to claim 1, characterized in that: In the step (2), the specific temperature is 400-600℃, and the pressure is 1-50 Torr.

5. The method for preparing a lithium manganese iron phosphate cathode material according to claim 1, characterized in that: In the step (3), the lithium source is one or a combination of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, metallic lithium and lithium phosphate; and the phosphorus source is one or a combination of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, dilithium hydrogen phosphate, lithium dihydrogen phosphate and lithium phosphate.

6. The method for preparing a lithium manganese iron phosphate cathode material according to claim 1, characterized in that: In the manganese-iron lithium phosphate positive electrode material obtained in the step (3), the molar ratio of Li:(Mn+Fe+M):P is (1.01-1.03):(0.97-1):

1.

7. The method for preparing a lithium manganese iron phosphate cathode material according to claim 1, characterized in that: In the step (3), the carbon source is at least one selected from the group consisting of glucose, starch, citric acid and polyethylene glycol, and the amount of the carbon source is controlled at 5%-15% of the theoretical mass of the manganese-iron lithium phosphate.

8. The method for preparing a lithium manganese iron phosphate cathode material according to claim 1, characterized in that: In the step (3), the sintering is a two-step sintering process performed in an inert atmosphere: the first step is pre-sintering and pre-carbonization of the carbon source at 350-450℃ for 2-5 hours; and the second step is crystallization and complete carbonization at 650-750℃ for 8-15 hours.

9. The method for preparing a lithium manganese iron phosphate cathode material according to claim 1, characterized in that: In the step (3), the grinding is performed by ball milling and / or sand milling, and the drying is performed by spray drying or vacuum drying.

10. A manganese-iron lithium phosphate positive electrode material prepared by the method of any one of claims 1-8, which is applied in the field of lithium ion batteries or solid-state batteries.

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