Preparation method of hollow spherical lithium iron phosphate

Abstract

The invention discloses a preparation method of hollow spherical lithium iron phosphate, which comprises the following steps: mixing phosphoric acid, a lithium source, an iron source, a carbon source and water, forming slurry, spray-drying the slurry, forming lithium iron phosphate precursor powder, calcining a lithium iron phosphate precursor, and obtaining the hollow spherical lithium iron phosphate, the carbon source at least comprising polyethylene glycol. According to the preparation method, phosphoric acid is used for replacing ammonium phosphate to serve as a phosphorus source, gas generation is reduced to a great extent, and lithium iron phosphate formed by combining a spray drying and pyrolysis process with polyethylene glycol has a micron-sized hollow sphere structure and has excellent electrochemical performance.

Description

Technical Field

[0001] This invention relates to a method for synthesizing electrode materials, and more particularly to a method for preparing hollow spherical lithium iron phosphate. Background Technology

[0002] LiFePO4 (LFP) is a widely used cathode material. Among them, hollow spherical lithium iron phosphate, due to its hollow structure, can fully contact the electrolyte, provide a fast lithium-ion migration channel, reduce impedance, and has high discharge specific capacity and excellent cycle stability, which can meet the market demand for high energy density, high power density and long cycle life of lithium iron phosphate products.

[0003] Based on the formation method of the precursor, the preparation methods of hollow spherical lithium iron phosphate can be mainly divided into two categories: solid-phase method and liquid-phase method. Among them, the precursor of the liquid-phase method is formed in a solvent. For example, CN 113540455 A discloses a hollow carbon-coated lithium iron phosphate particle and its preparation method and application. The hollow structure is controlled by controlling the pH of the precursor formation and the reaction rate. However, the lithium iron phosphate prepared by the liquid-phase method generally has a lower compaction density, more complex process and higher cost. The precursor of the solid-phase method is directly mixed to form. For example, CN 115602833 A discloses a solid-phase method for synthesizing hollow lithium iron phosphate spheres. Ammonium polyphosphate is used as the phosphorus source and glucose and polyethylene glycol are used as carbon sources to prepare spherical hollow lithium iron phosphate spheres with a hollow structure. However, the phosphorus source used in this scheme contains ammonium ions, which will generate a large amount of gas in the subsequent sintering process, increasing the difficulty and cost of tail gas treatment, making it unsuitable for large-scale production. In addition, the sphericity and uniformity of the particles formed by this scheme are not good. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a method for preparing hollow spherical lithium iron phosphate to reduce the gas generated during sintering.

[0005] Technical solution: The present invention provides a method for preparing hollow spherical lithium iron phosphate, comprising the following steps: mixing phosphoric acid, lithium source, iron source, carbon source and water, milling to form a slurry, spray drying the slurry to form lithium iron phosphate precursor powder, calcining the lithium iron phosphate precursor to obtain hollow spherical lithium iron phosphate, wherein the carbon source includes at least polyethylene glycol.

[0006] Preferably, the amount of polyethylene glycol added is 1% to 20% of the molar ratio of lithium iron phosphate.

[0007] Preferably, the amount of polyethylene glycol added relative to each mole of lithium iron phosphate is 3% to 9%. More preferably, the amount of polyethylene glycol added relative to each mole of lithium iron phosphate is 4% to 8%.

[0008] Preferably, to improve the uniformity of lithium iron phosphate particle size, capacity, and high-current electrical performance, the slurry viscosity is 5250~8200 mPa·s. When the amount of polyethylene glycol added is small, the viscosity of the precursor slurry is one of the key control factors determining whether perfect hollow spheres can be formed during spray drying. The slurry viscosity can be adjusted by the amount of polyethylene glycol and water added, thus obtaining lithium batteries with high rate performance. This viscosity range is most conducive to the formation of hollow microspheres with complete structure and good monodispersity. More preferably, the slurry viscosity is 5250~7000 mPa·s.

[0009] Preferably, the slurry contains 36-45 wt% solids.

[0010] Preferably, the mass ratio of polyethylene glycol to water in the slurry is 0.25 to 0.7. More preferably, the mass ratio of polyethylene glycol to water in the slurry is 0.3 to 0.5.

[0011] Preferably, the polyethylene glycol has a molecular weight of 2000-12000. More preferably, the polyethylene glycol has a molecular weight of 8000-12000.

[0012] Preferably, the calcination temperature is 690~720℃, and the duration is 6~10h. More preferably, the calcination temperature is 705~720℃.

[0013] Preferably, the abrasive particle size is D50 = 0.1 ~ 0.45μm.

[0014] Preferably, the inlet air temperature for the spray drying is 180~200℃.

[0015] Preferably, the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium acetate, and lithium phosphate.

[0016] Preferably, the carbon source is polyethylene glycol and one or a mixture of glucose, sucrose, citric acid, polyvinyl alcohol, starch, and polyvinylidene fluoride.

[0017] Preferably, the iron source is one or more of ferrous oxalate, ferrous carbonate, ferric nitrate, and ferric phosphate.

[0018] Beneficial Effects: Compared with the prior art, the present invention has the following significant advantages: 1. Reduced gas generation during the reaction process: By replacing ammonium phosphate with phosphoric acid as the phosphorus source, gas generation is greatly reduced. Using spray drying combined with pyrolysis and polyethylene glycol, the -CH2- units in the polyethylene glycol molecular chain are hydrophobic, while the -O- ether bonds and terminal -OH- hydroxyl groups are hydrophilic. These units rapidly accumulate on the outer surface of the droplet as water evaporates. At high temperatures, the polyethylene glycol molecular chains cross-link, melt, and tightly bind with other particles, forming a semi-permeable gel shell with a certain mechanical strength on the outermost layer of the droplet. Driven by internal vapor pressure, the remaining solute precursor and polyethylene glycol inside the droplet migrate to the solidified shell along with the solvent. The polyethylene glycol is decomposed and burned off (carbonized) during subsequent high-temperature calcination, resulting in lithium iron phosphate with a micron-sized hollow spherical structure and excellent electrochemical performance; 2. Polyethylene glycol not only acts as a binder... 1. It maintains the molding strength of the precursor and can also serve as a carbon source during subsequent calcination, forming a uniform carbon coating layer on the surface of lithium iron phosphate particles, significantly improving the electronic conductivity of the material; 2. Hollow spherical lithium iron phosphate can fully contact the electrolyte and provides a certain storage space for the electrolyte in the center of the material, providing a fast lithium-ion migration channel, reducing impedance, effectively improving the kinetic performance of lithium iron phosphate material, and exhibiting high discharge capacity and good cycle stability; 3. It reduces the amount of polyethylene glycol used and solves the problem of the inability to form a hollow structure due to the reduced amount of polyethylene glycol, thus improving the high-current cycle life of lithium iron phosphate. Attached Figure Description

[0019] Figure 1 Scanning electron microscope (SEM) images of cross-sections of lithium iron phosphate materials prepared in Examples 1-2, Examples 4-5, and Comparative Examples 1-3;

[0020] Figure 2 This is a scanning electron microscope image of the cross-section of the lithium iron phosphate material prepared in Example 6;

[0021] Figure 3 The image shows a scanning electron microscope (SEM) image of the lithium iron phosphate material prepared in Example 1.

[0022] Figure 4 The graph shows a comparison of the constant current charge-discharge cycle performance of Examples 1-3 and Comparative Example 1 at a rate of 0.1C.

[0023] Figure 5 The graph shows a comparison of the constant current charge-discharge cycle performance of Examples 4 and 6 at a rate of 0.1C.

[0024] Figure 6 This is a comparison chart of the constant current charge-discharge cycle performance of Example 1 and Comparative Examples 1-2 at a 1C rate;

[0025] Figure 7 The above are comparison graphs of charge-discharge curves for Examples 1-3 at different cycles at 1C rate. Detailed Implementation

[0026] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0027] Example 1: Lithium carbonate, iron phosphate, phosphoric acid, glucose, sucrose, and polyethylene glycol 10000 were mixed in a molar ratio of 0.5:1:1:0.1:0.4:0.05. Water was added and the mixture was stirred until homogeneous. The mixture was then milled for 2 hours, with the milling particle size D50 controlled at 0.3±0.05μm to form a slurry. The slurry was spray-dried at an inlet air temperature of 200℃. The dried material was then transferred to a tube furnace and calcined at 720℃ under a nitrogen atmosphere for 6 hours to obtain the hollow spherical lithium iron phosphate material.

[0028] Example 2: Lithium carbonate, iron phosphate, phosphoric acid, glucose, sucrose, and polyethylene glycol 10000 were mixed in a molar ratio of 0.5:1:1:0.1:0.2:0.1. Water was added, and the mixture was stirred evenly in a beaker. The mixture was then subjected to sand milling for 2 hours, with the sand milling particle size D50 controlled at 0.3±0.05μm. A slurry was formed, and the slurry was spray-dried at an inlet air temperature of 200℃. The dried material was then transferred to a tube furnace and calcined at 720℃ under a nitrogen atmosphere for 6 hours to obtain the hollow spherical lithium iron phosphate material.

[0029] Example 3: Lithium carbonate, iron phosphate, phosphoric acid, glucose, sucrose, and polyethylene glycol 10000 were mixed in a molar ratio of 0.5:1:1:0.15:0.1:0.15. Water was added, and the mixture was stirred evenly in a beaker. The mixture was then milled for 2 hours, with the milling particle size D50 controlled at 0.15±0.02μm to form a slurry. The slurry was spray-dried at an inlet air temperature of 200℃. The dried material was then transferred to a tube furnace and calcined at 720℃ under a nitrogen atmosphere for 6 hours to obtain the hollow spherical lithium iron phosphate material.

[0030] Example 4: This example is the same as Example 1 in terms of steps, except that the raw material components and proportions are lithium carbonate, iron phosphate, phosphoric acid, glucose, sucrose and polyethylene glycol in a molar ratio of 0.5:1:1:0.2:0.4:0.03.

[0031] Example 5: This example follows the same steps as Example 1, except that the calcination temperature is 700℃ and the holding time is 6h.

[0032] Example 6: This example is the same as Example 1 in terms of steps, except that the raw material components and proportions are lithium carbonate, iron phosphate, phosphoric acid, sucrose, and polyethylene glycol in a molar ratio of 0.5:1:1:0.1:0.6.

[0033] Comparative Example 1: Lithium carbonate, iron phosphate, phosphoric acid, glucose, and sucrose were mixed in a molar ratio of 0.5:1:1:0.2:0.5 and stirred evenly in a beaker. The mixture was then subjected to sand milling for 2 hours, with the sand milling particle size D50 controlled at 0.15±0.02μm. The milled slurry was then spray-dried at an inlet air temperature of 200℃. The dried material was then transferred to a tube furnace and calcined at 720℃ under a nitrogen atmosphere for 6 hours to obtain the product, lithium iron phosphate material.

[0034] Comparative Example 2: Lithium carbonate, iron phosphate, phosphoric acid, glucose, sucrose, and polyethylene glycol were mixed in a molar ratio of 0.5:1:1:0.2:0.1:0.1 and stirred evenly in a beaker. The mixture was then subjected to sand milling for 2 hours, with the sand milling particle size controlled at D50 = 0.3 ± 0.05 μm. The milled slurry was then spray-dried at an inlet air temperature of 200℃. The dried material was then transferred to a tube furnace and calcined at 700℃ under a nitrogen atmosphere for 6 hours to obtain the hollow spherical lithium iron phosphate material.

[0035] Comparative Example 3: This comparative example follows the same steps as Example 1, except that the water content of the slurry is increased and the solid content is 30%.

[0036] Comparative Example 4: The steps of this comparative example are the same as those of Example 1, except that the sintering temperature is 680°C.

[0037] The materials prepared in Examples 1 to 6 and Comparative Examples 1 to 4 were subjected to cross-sectional scanning electron microscopy (SEM) tests. The resulting slurry was subjected to viscosity tests. The material prepared in Example 1 was subjected to SEM tests. The batteries assembled from the materials prepared in Examples 1 to 6 and Comparative Examples 1 to 2 were characterized by constant current charge and discharge.

[0038] Figure 1 , Figure 2 These are scanning electron microscope (SEM) images of the cross-sections of lithium iron phosphate materials prepared in Examples 1, 2, 4-6, Comparative Example 1, and Comparative Example 3. The cross-sectional images show that the spheres prepared in Examples 1, 2, and 4-6 have obvious hollow structures, while those in Comparative Example 1 do not, indicating that the addition of polyethylene glycol significantly affects the formation of hollow spheres.

[0039] The hollow structure ratio of the material prepared in Example 1 is higher than that in Examples 2 and 4-6, indicating that a suitable sintering temperature and an appropriate proportion of polyethylene glycol help to form a high proportion of hollow structures. This may be because the appropriate proportion of polyethylene glycol and the sufficient number of polyethylene glycol molecules form a continuous, stable, and viscoelastic three-dimensional network during the mixing process. During the drying stage, it can form an ideal shell layer with high strength and good toughness. When sintered at a suitable temperature, moisture continues to evaporate through the shell layer, and the polyethylene glycol and precursor inside continue to migrate to the shell layer and solidify, eventually forming microspheres with uniform shell walls and obvious hollow structures.

[0040] The material prepared in Comparative Example 4 has poor crystallinity, an uneven lithium charge-discharge curve, and an increased slope of the voltage plateau, indicating that further reducing the sintering temperature is not conducive to the material's performance.

[0041] Compared with Example 1, the material prepared in Comparative Example 3 did not have a significant hollow structure. Based on the viscosity test and solid content data of Examples 1 to 6 and Comparative Examples 1 to 3 in Table 1, the reason may be that when the amount of polyethylene glycol added is small, the water content of the slurry increases and the viscosity decreases, making it impossible to effectively form a hollow structure. This indicates that the appropriate slurry viscosity (or slurry water content) also plays a decisive role in the formation of a hollow structure.

[0042] Table 1:

[0043]

[0044] Figure 3 This is a scanning electron microscope (SEM) image of the lithium iron phosphate material prepared in Example 1. The image shows that the material obtained by spray drying is spherical with good sphericity and uniform surface particles.

[0045] Figure 4 , Figure 5 The graph shows a comparison of the constant current charge-discharge cycle performance of Examples 1-3, 4, and 6, and Comparative Example 1 at a 0.1C rate. It can be clearly seen from the graph that the discharge specific capacity of Example 1 at a 0.1C rate reaches 163 mAh / g, while the discharge capacities of Examples 2, 3, 4, and 6 at a 0.1C rate are 158 mAh / g, 160 mAh / g, 153 mAh / g, and 155 mAh / g, respectively. In contrast, the discharge specific capacity of Comparative Example 1 is only 143 mAh / g, indicating that the material with the hollow sphere structure has excellent electrochemical performance.

[0046] Figure 6The graph shows a comparison of the constant current charge-discharge cycle performance of Example 1, Comparative Example 1, and Comparative Example 2 at a 1C rate. At a 1C current, the material in Example 1 can provide a high discharge specific capacity of 154 mAh / g, indicating that the addition of polyethylene glycol effectively solves the problems of low electronic conductivity and slow ion diffusion of lithium iron phosphate materials. The performance of the material in Comparative Example 2 is reduced, indicating that too low a sintering temperature may lead to poor crystallization and poor carbon coating.

[0047] Figure 7 The graphs show a comparison of charge-discharge curves at different cycle counts at 1C for Examples 1 to 3. As can be seen from the graphs, the lithium iron phosphate with a distinct hollow structure prepared in Example 1 exhibits excellent high-current cycle life and capacity. This is because the hollow spherical structure and its internal pores provide channels for the full wetting of the electrolyte, ensuring unobstructed ion transport channels. At the same time, the hollow part can act as a buffer area to absorb the volume expansion during the charge-discharge process. Even after multiple high-current cycles, it can still maintain excellent performance without rapid degradation, which effectively improves the kinetic performance of lithium iron phosphate materials.

Claims

1. A method for preparing a hollow sphere-type lithium iron phosphate, characterized by, The method comprises the following steps: mixing phosphoric acid, lithium source, iron source, carbon source and water, sand grinding, forming a slurry, spray drying the slurry, forming a lithium iron phosphate precursor powder, calcining the lithium iron phosphate precursor to obtain a hollow spherical lithium iron phosphate, wherein the carbon source at least comprises polyethylene glycol.

2. The method of claim 1, wherein the hollow spherical lithium iron phosphate is prepared by the steps of: The added amount of the polyethylene glycol is 1% to 20% of the lithium iron phosphate in terms of molar ratio. ​ 3. The method for preparing hollow spherical lithium iron phosphate according to claim 2, characterized in that, The added amount of the polyethylene glycol is 3% to 9% of the lithium iron phosphate in terms of molar ratio.

4. The method for preparing hollow spherical lithium iron phosphate according to claim 1, characterized in that, The viscosity of the slurry is 5250 to 8200 mPa·s.

5. The method for preparing hollow spherical lithium iron phosphate according to claim 1 or 4, characterized in that, The mass ratio of polyethylene glycol to water in the slurry is 0.25 to 0.

7.

6. The method of claim 1 or 5, wherein the hollow spherical lithium iron phosphate is prepared by the steps of: The molecular weight of the polyethylene glycol is 2000 to 12000. ​ 7. The method for preparing hollow spherical lithium iron phosphate according to claim 1, characterized in that, The calcination temperature is 690 to 720℃, and the duration is 6 to 10 hours.

8. The method for preparing hollow spherical lithium iron phosphate according to claim 1, characterized in that, The sand grinding is to a particle size of D50 = 0.1 to 0.45μm.

9. The method for preparing hollow spherical lithium iron phosphate according to claim 1, characterized in that, The lithium source is one or a mixture of several of lithium carbonate, lithium hydroxide, lithium acetate and lithium phosphate, and the iron source is one or a mixture of several of ferrous oxalate, ferrous carbonate, ferric nitrate and ferric phosphate.

10. The method for preparing hollow spherical lithium iron phosphate according to claim 1, characterized in that, The carbon source is one or a mixture of several of polyethylene glycol and glucose, sucrose, citric acid, polyvinyl alcohol, starch and polyvinylidene fluoride.

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