A method for preparing spherical high-density lithium manganese iron phosphate cathode material

Abstract

This invention belongs to the field of lithium manganese iron phosphate cathode material preparation, and relates to a method for preparing spherical high-density lithium manganese iron phosphate cathode material. This invention utilizes the acid solubility of phosphoric acid to spherically modify the surface of the lithium manganese iron phosphate precursor. The phosphate ester and phosphate diester bonds contained in phosphoric acid can form phosphorus-rich soluble substances with iron, manganese, and other metal elements in the precursor under heating conditions. These substances can dissolve the metal ions on the precursor surface under water bath heating, and then the metal ions are redeposited onto the precursor surface by water bath cooling. Through repeated water bath heating, cooling, and stirring, the crystal planes of the precursor are dissolved and regrown, ultimately forming a spherical precursor.

Description

Technical Field

[0001] This invention belongs to the field of lithium manganese iron phosphate cathode material preparation, and relates to a method for preparing spherical high-density lithium manganese iron phosphate cathode material. Background Technology

[0002] Among commonly used lithium-ion battery cathode materials, LiFePO4 (LFP) stands out due to its unique olivine structure (PO4). 3- The tetrahedral structure and strong PO covalent bonds give it 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 due to its high voltage plateau of 4.1V and the stability of its olivine structure. Compared to lithium iron phosphate, the high voltage characteristic of manganese gives LMFP a higher voltage plateau, which also leads to its higher energy density. However, the introduction of manganese significantly reduces the material's conductivity. Compared to the layered structure of ternary NCM materials, LMFP exhibits greater structural stability and safety during charge and discharge, and also has a lower cost.

[0003] However, due to its inherent properties, lithium manganese iron phosphate (LFP) materials require grinding to achieve a smaller particle size to improve conductivity. This results in a lower compaction density, directly impacting the battery's volumetric energy density. Currently, the compaction density of LFP materials is generally between 2.1 and 2.3 g / cm³. 3 And greater than 2.2 g / cm 3 Compacted lithium manganese iron phosphate (LFP) materials are achieved through secondary sintering, which results in a lack of significant advantages in volumetric energy density. The mainstream process for improving compaction density involves secondary sintering, but this process consumes substantial energy and generates more magnetic materials, impacting battery safety. Consequently, the commercialization of LFP materials remains lukewarm.

[0004] To address the problem of low compaction density, researchers have employed various modification methods, such as secondary sintering, particle size distribution, surface coating, and ion doping. Recently, some scholars reported improving the compaction density of materials through secondary sintering and multi-stage particle size distribution. However, after secondary sintering, the material exhibits higher levels of magnetic impurities and a 50% reduction in production capacity. Summary of the Invention

[0005] Based on the existing technical problems, this invention adopts a method of first modifying the precursor into a spherical shape and then introducing it into a lithium source and carbon source for sintering. The phosphoric acid acidification, heating and stirring process of the precursor can effectively improve the irregular morphology of the material surface, thereby increasing the compaction density of the material.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a spherical high-density lithium manganese iron phosphate cathode material includes the following steps: (1) Grind and crush the required precursor to D50 = 0.3-0.4 μm; add phosphoric acid to adjust the pH of the slurry to 0-1 for phosphoric acid acidification, and stir the slurry at high speed with a stirrer; (2) The phosphoric acid slurry is heated from 20°C to the reaction temperature zone of 60-80°C using a constant temperature water bath at a rate of 3-5°C / min, and kept at the reaction temperature zone of 60-80°C for 1-3 hours; then the acidified slurry is cooled down to 20°C in the water bath at a rate of 3-5°C / min, and kept at 20°C for 1-3 hours; in the high temperature zone, the protruding corners of the particles have a higher centrifugal speed and preferentially complex and dissolve with phosphoric acid, while in the low temperature zone, the surface tension of the particles will preferentially precipitate to the pits and depressions of the particles to reduce the surface energy of the particles; the dissolution and growth of the precursor particles are controlled by heating and cooling in the water bath, thereby achieving the spheroidization of the particles; this process of heating-cooling-cooling-cooling is repeated multiple times according to the roundness of the precursor particles; (3) Finally, after washing away excess phosphoric acid from the spheroidized precursor, it is introduced into a water-soluble lithium source and a carbon source; after spray drying, sintering and post-treatment processes, a spherical high-compacted lithium manganese iron phosphate product is obtained.

[0007] In step (1), the precursor is one or more of manganese ferric phosphate, manganese ferric ammonium phosphate, iron phosphate, and manganese phosphate.

[0008] In step (3), the water-soluble lithium source is one or more of the following: lithium acetate, lithium formate, lithium oxalate, lithium hydroxide, etc., which have high solubility.

[0009] In step (1), the mass fraction of phosphoric acid is 20%–85%.

[0010] In step (1), the high-speed stirring speed is 300-500 r / min.

[0011] The sintering temperature in step (3) is 700℃-800℃, and the sintering time is 8h-10h.

[0012] The sintering process in step (3) is carried out under a protective atmosphere.

[0013] The chemical formula of the lithium manganese iron phosphate is LiFe x Mn 1-x PO4, 0 < x < 1.

[0014] After spheroidization, the precursor slurry is centrifuged to remove phosphoric acid, and washed until the filtrate pH is 3-4, at which point soluble lithium and carbon sources are introduced.

[0015] Beneficial effects of this invention: Traditional lithium manganese iron phosphate (LFP) material preparation processes involve mixing, grinding, and directly drying and sintering the lithium source, carbon source, and precursor. The resulting particles are irregularly shaped, and during sintering, carbon coating hinders grain growth, causing the LFP material to inherit the irregular surface morphology of the precursor. This irregularity directly leads to extremely low compaction density and increases the contact area with the electrolyte, reducing cycle life. Some researchers have attempted to improve compaction density by using a secondary sintering process to improve the sphericity of the material. However, secondary sintering directly reduces production capacity and increases energy consumption, and it also generates more magnetic foreign matter, affecting the cell's safe lifespan.

[0016] This invention utilizes the acid solubility of phosphoric acid to spherically modify the surface of lithium manganese iron phosphate precursors. The phosphate ester and phosphate diester bonds in phosphoric acid can react with iron, manganese, and other metal elements in the precursor to form phosphorus-rich P under heating conditions. (n) --Fe and P (n) --Mn bond soluble substances can dissolve metal ions on the precursor surface under water bath heating, and then the metal ions are redeposited onto the precursor surface by water bath cooling. Through repeated water bath heating and cooling and stirring, the precursor crystal plane dissolution and regeneration process is realized, and finally a spherical precursor is formed.

[0017] This invention utilizes phosphoric acid to spheroidize precursor materials without the need for secondary sintering. The process is simple, efficient, and low-cost, and is an effective method to improve the sphericity and compaction density of lithium manganese iron phosphate cathode materials. Attached Figure Description

[0018] 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.

[0019] Figure 1 A scanning electron microscope image of the modified lithium manganese iron phosphate cathode material product provided in Example 1; Figure 2 This is a scanning electron microscope image of the unmodified lithium manganese iron phosphate cathode material product from Example 2.

[0020] Figure 3 A scanning electron microscope image of the modified lithium manganese iron phosphate cathode material product provided in Example 3; Figure 4 A scanning electron microscope image of the modified lithium manganese iron phosphate cathode material product provided in Example 4; Figure 5A scanning electron microscope image of the modified lithium manganese iron phosphate cathode material product provided in Example 5; Figure 6 Scanning electron microscope image of the modified lithium manganese iron phosphate cathode material provided for Comparative Example 1; Figure 7 Scanning electron microscope image of the modified lithium manganese iron phosphate cathode material product provided for Comparative Example 2. Detailed Implementation

[0021] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0022] The technical solutions of the present invention will now be described in detail with reference to the accompanying drawings. Example 1

[0023] Grind 4 kg of precursor and 4 kg of pure water to D50 = 0.3-0.4 μm to form a grinding slurry; add phosphoric acid to adjust the pH of the grinding slurry to 0 to form a phosphoric acid acidified slurry; turn on the stirrer above the acidified slurry and stir at a speed of 300 r / min.

[0024] The phosphoric acidified slurry was heated from 20°C to the reaction temperature zone using a constant-temperature water bath at a rate of 3°C / min, and held at 60°C for 1 hour. The acidified slurry was then cooled to 20°C in the water bath at a rate of 3°C / min, and held at 20°C for 1 hour. This heating-holding-cooling-holding-temperature test was repeated twice for the precursor spheroidization reaction. The spheroidized acidified slurry was filtered to remove phosphoric acid and washed until pH=3. A molar ratio of Li:(Fe) was added to the washed precursor. x +Mn 1-x A homogeneous slurry was formed by mixing soluble lithium acetate (1.02:1), 0.4 kg of glucose, and 2 kg of pure water, and then spray-dried. The sintering was carried out at 750°C for 8 hours under an inert gas atmosphere. The sintered material was then subjected to post-processing such as airflow powdering to complete the material preparation. Example 2

[0025] The difference between this embodiment and Embodiment 1 is that the water bath is raised to 70°C and kept at that temperature for 1 hour before being cooled down to 20°C. Example 3

[0026] The difference between this embodiment and Embodiment 1 is that the water bath is raised to 80°C and kept at that temperature for 1 hour before being cooled down to 20°C. Example 4

[0027] The difference between this embodiment and Embodiment 1 is that the water bath temperature rise and fall test was performed 4 times. Example 5

[0028] The difference between this embodiment and Embodiment 1 is that the water bath temperature rise and fall test was performed 6 times.

[0029] Comparative Example 1 The difference from Example 1 is that the precursor is not modified by phosphoric acid.

[0030] Comparative Example 2 The difference from Example 1 is that the lithium source is replaced with water-insoluble lithium carbonate with small particles D50=0.3-0.4um.

[0031] Performance testing and results analysis: The samples obtained in Examples 1-5 and Comparative Examples 1-2 were subjected to the following performance tests: Compacted density: Measured according to national standard GB / T 5162-2021.

[0032] Specific surface area: tested according to national standard GB / T 24533-2019.

[0033] Electrochemical performance: Using the prepared material as the positive electrode active material and lithium metal sheet as the negative electrode, a CR2032 coin cell was assembled. Charge-discharge tests were conducted at a 0.2C rate, and the initial discharge specific capacity and 1C cycle test results were recorded.

[0034] The test results are shown in the table below: The active material, carbon black, and polyvinylidene fluoride (PVDF) were weighed in a mass ratio of 8:1:1. First, the PVDF was dissolved in an appropriate amount of NMP and magnetically stirred until the solution became transparent. Then, the positive active material and conductive carbon black were added separately to the above solution and stirred for 12 hours. The slurry was coated onto aluminum foil and dried in a vacuum drying oven at 120°C for 12 hours. The positive electrode sheet was cut into 12 mm diameter discs, and the lithium-ion half-cells for testing were assembled in a glove box. Electrochemical tests were then conducted after allowing the cells to stand at room temperature for 12 hours. The electrical performance test results are shown in Table 1.

[0035] Table 1

[0036] As can be seen from Example 1 and Comparative Example 1, the compaction density is greatly improved by phosphoric acid treatment of the precursor. The sphericity of the phosphoric acid-treated material is higher, which makes the particles arranged in an orderly and compact manner, thus improving the compaction density. The compaction density of Examples 2 and 4 is higher than that of Example 1 by increasing the water bath temperature and the number of water bath heating and cooling cycles. This is because increasing the water bath temperature and the number of heating and cooling cycles makes the material morphology more rounded. Although the particles are more rounded due to the highest water bath temperature and the most heating and cooling cycles in Examples 3 and 5, the small particles in the precursor are completely dissolved and the large particles are overgrown, resulting in a low specific surface area of ​​the material, which limits the material capacity.

[0037] 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 spherical high-density lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: (1) Grind and crush the required precursor to D50 = 0.3-0.4 μm; add phosphoric acid to adjust the pH of the slurry to 0-1 for phosphoric acid acidification, and stir the slurry at high speed with a stirrer; (2) Use a constant temperature water bath to raise the phosphoric acid slurry from 20°C to the reaction temperature zone of 60-80°C at a rate of 3-5°C / min, and keep it at the reaction temperature zone of 60-80°C for 1-3 hours; then cool the acidified slurry down to 20°C in the water bath at a rate of 3-5°C / min, and keep it at 20°C for 1-3 hours; repeat this water bath heating-constant temperature-cooling-constant temperature process multiple times according to the roundness of the precursor particles. (3) Finally, after washing away excess phosphoric acid from the spheroidized precursor, it is introduced into a water-soluble lithium source and a carbon source; after spray drying, sintering and post-treatment processes, a spherical high-compacted lithium manganese iron phosphate product is obtained.

2. The method for preparing a spherical high-density lithium manganese iron phosphate cathode material according to claim 1, characterized in that: In step (1), the precursor is one or more of manganese ferric phosphate, manganese ferric ammonium phosphate, iron phosphate, and manganese phosphate.

3. The method for preparing a spherical high-density lithium manganese iron phosphate cathode material according to claim 1, characterized in that: In step (3), the water-soluble lithium source is one or more of lithium acetate, lithium formate, lithium oxalate, and lithium hydroxide, which have high solubility.

4. The method for preparing a spherical high-density lithium manganese iron phosphate cathode material according to claim 1, characterized in that: In step (1), the mass fraction of phosphoric acid is 20%–85%.

5. The method for preparing a spherical high-density lithium manganese iron phosphate cathode material according to claim 1, characterized in that: In step (1), the high-speed stirring speed is 300-500 r / min.

6. The method for preparing a spherical high-density lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The sintering temperature in step (3) is 700℃-800℃, and the sintering time is 8h-10h.

7. The method for preparing a spherical high-density lithium manganese iron phosphate cathode material according to claim 6, characterized in that: The sintering process in step (3) is carried out under a protective atmosphere.

8. The method for preparing a spherical high-density lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The chemical formula of the lithium manganese iron phosphate is LiFe x Mn 1-x PO4, 0 < x < 1.

9. The method for preparing a spherical high-density lithium manganese iron phosphate cathode material according to claim 1, characterized in that: After spheroidization, the precursor slurry is centrifuged to remove phosphoric acid, and washed until the filtrate pH is 3-4, at which point soluble lithium and carbon sources are introduced.

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