Preparation method of high-mechanical-strength high-density lithium iron phosphate agglomerate cathode material

Abstract

The application discloses a preparation method of a high-mechanical-strength high-density lithium iron phosphate agglomerate positive electrode material, and belongs to the technical field of lithium ion batteries. Different types of carbon sources are doped to form a composite carbon source in the rough and fine precision grinding process of iron phosphate and a lithium source, so that different interactions are generated among the substances in the grinding process of the composite carbon source, and the density and the mechanical strength of lithium iron phosphate agglomerate large particles are improved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology and relates to a method for preparing a large-particle cathode material of lithium iron phosphate agglomerates with high mechanical strength and high density. Background Technology

[0002] With the continuous innovation in the field of new energy vehicle technology, especially the rapid rise of pure electric vehicles, the energy density of lithium-ion batteries urgently needs to be further improved to meet the requirements of long driving range for pure electric vehicles. The energy density of lithium-ion batteries is mainly determined by the electrode materials. Therefore, developing new electrode materials with high energy density, long cycle life, and good rate performance is imperative. The main cathode materials on the market are LiCoO2, LiMn2O4, and LiCo... x Mn y Ni 1-x-y O2 and LiFePO4 are two main materials used in power batteries. LiFePO4, with its stable structure, high safety, long cycle life, and low cost, is widely used in these batteries. As research into LiFePO4 materials continues, its potential as a power electrode material has been developed, primarily for energy storage and start-stop power supplies. This has led to more stringent requirements for its rate performance and low-temperature performance. Furthermore, given the poor processing performance of current commercially available power-type LiFePO4 materials, improving the processing performance and mechanical strength of the particles is crucial for improving low-temperature performance and reducing impedance. Therefore, research has focused on improving the low-temperature and rate performance of LiFePO4, as well as the compressive strength of large particles.

[0003] Existing preparation methods for lithium iron phosphate cathode materials typically involve directly mixing multiple material sources (such as lithium, iron, phosphorus, and carbon sources), especially carbon sources. This fails to fully utilize the inherent characteristics of different types of carbon sources, resulting in materials with limited overall performance in terms of mechanical strength, density, and other properties. For example, patent application CN101948102A discloses a method for preparing lithium iron phosphate cathode materials. First, iron, lithium, and phosphoric acid sources are weighed and separately prepared into solutions. Then, the three solutions are mixed and heated in an oil bath at 120°C for 2.5 hours until a dark green precipitate appears at the bottom. The precipitate is then filtered and separated to obtain nano-sized primary lithium iron phosphate particles. These particles are then spray-granulated, calcined at 800°C for 6.5 hours, and finally furnace-cooled to room temperature to obtain the lithium iron phosphate cathode material. While this method successfully prepared nanoscale lithium iron phosphate materials with good electrical properties and uniform primary particle distribution through co-precipitation, the poor compaction density of the particles prevented the provision of high energy density and resulted in unremarkable rate performance. Furthermore, the limited synthesis process hindered large-scale production. Patent application CN102173403 A describes a method that uniformly disperses nanoscale precursor materials, then mixes them with a lithium source, carbon source, and a suitable binder, followed by dry granulation to obtain spherical micro-nano lithium iron phosphate precursor materials with micron-sized secondary particles. After drying, high-temperature heat treatment yields spherical micro-nano lithium iron phosphate materials. Although this micro-nano lithium iron phosphate material exhibits high tap density, good processability, large specific surface area, and porosity, the limitations of the dry mixing process result in poor sample uniformity, low low-temperature performance, and low rate performance, making it unsuitable for direct application as a power-type material. The porous structure also restricts electrode compaction, further hindering energy density improvement. Patent application CN102642820 A describes a process using lithium compounds, iron compounds, phosphates, doped metal compounds, and carbon black as raw materials. These are wet-mixed in a ball mill, spray-dried, and then pre-calcined in a furnace with N2 as the protective gas. A binder, polyethylene glycol, is then added and wet-mixed again. After spray drying, the mixture is placed in a furnace with N2 as the protective gas for a second calcination, resulting in a high-density spherical lithium iron phosphate material. However, the aforementioned patents either do not use a carbon source or utilize only a single carbon source. Even when multiple carbon sources are considered, the different roles and synergistic effects of different types of carbon sources are not recognized. Therefore, how to fully utilize the characteristics of different types of carbon sources and further improve comprehensive properties such as mechanical strength and density through carbon source combination design is an important research topic. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing large particles of lithium iron phosphate agglomerates with high mechanical strength and high density. By using iron phosphate and lithium source to dope different types of carbon sources to form composite carbon sources during coarse and fine grinding, the composite carbon sources generate different interactions between materials during the grinding process, thereby improving the density and mechanical strength of large particles of lithium iron phosphate agglomerates.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for preparing a high-mechanical-strength, high-density lithium iron phosphate aggregate cathode material includes the following steps:

[0007] A primary mixed system is prepared by uniformly dispersing a first type of carbon source in water. The first type of carbon source is selected from at least one of polyethylene glycol, polyvinylpyrrolidone, polyvinyl alcohol, cellulose, starch, and sodium carboxymethyl cellulose.

[0008] Add iron phosphate, lithium source, and second type of carbon source to the above primary mixture system and stir evenly to prepare a secondary mixture system. The second type of carbon source is selected from at least one of sucrose, glucose, rock sugar, and activated carbon.

[0009] The secondary mixing system is coarsely ground using a coarse grinding mill, and the coarsely ground slurry is transferred to a fine grinding mill for fine grinding.

[0010] The finely ground slurry is spray-dried, and the dried powder is sintered in stages under an inert atmosphere. After cooling and sieving, large particles of lithium iron phosphate agglomerates are obtained.

[0011] Preferably, the molar ratio of lithium source to iron phosphate is Li:P = 1:(1.0~1.1), more preferably 1:(1.0~1.02); the mass of the first type of carbon source is 1%~2% of iron phosphate, and the mass of the second type of carbon source is 7%~9% of iron phosphate.

[0012] Preferably, the solid content of ferric phosphate after fine grinding is 30% to 50%.

[0013] Preferably, the lithium source is one or more of lithium phosphate, lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium dihydrogen phosphate.

[0014] Preferably, a third type of carbon source is added to the finely ground slurry and stirred evenly before spray drying. The third type of carbon source is selected from at least one of phenolic resin, asphalt, and tetrabutyl titanate, and the mass of the third type of carbon source is 1% to 2% of iron phosphate.

[0015] Preferably, the particle size of coarse grinding is controlled at 700-800 nm, and the particle size of fine grinding is controlled at 200-220 nm.

[0016] Preferably, the spray dryer used for spray drying is a centrifugal dryer, a pressure dryer, a fluid dryer, or other types of special-purpose dryers.

[0017] Preferably, the gas pressure for spray drying is 0.3–0.6 MPa, and the feed frequency of the peristaltic pump is 0–60 Hz.

[0018] Preferably, the inlet air temperature of the spray dryer is 200–280°C, and the outlet air temperature is 60–110°C; the median particle size D50 of the spray is 6–12 μm.

[0019] Preferably, the staged heating sintering conditions are as follows: heating at a rate of 1-10℃ / min from room temperature to 200-350℃ and holding for 1-5 hours; then heating to 400-500℃ and holding for 3-7 hours; and then heating to 700-800℃ and holding for 5-15 hours.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] 1. Enhanced Interactions Through Composite Carbon Sources: This invention designs a combination of different types of carbon sources, utilizing their physical and chemical properties to categorize them into two to three types. The first type of carbon source possesses interaction forces including hydrogen bonds and van der Waals forces. During the continuous contact between iron phosphate and lithium sources during grinding, strong interaction forces (hydrogen bonds and van der Waals forces) are continuously formed on the surfaces of the raw materials using the hydrogen bonds and van der Waals forces of this type of carbon source. The second type of carbon source possesses interaction forces including covalent bonds or ionic bonds. During the grinding process, this special carbon source continuously contacts lithium carbonate on the surface of iron phosphate, forming localized... The covalent and ionic bonds of the carbon source tightly bind the material together. After the primary particles are tightly wrapped with carbon, they also connect with each other, forming high-density aggregates. The third type of carbon source is added after fine grinding and before drying. During spray drying, a carbon layer composed of this third type of carbon source is coated on the surface of the particles. This mainly utilizes the "door-closing effect" formed by the condensation reaction after heating, forming a dense, rigid coating layer on the surface of the secondary particles after fine grinding. That is, a rigid carbon film is formed in the diffusion layer, binding the internal primary particles. The unique microporous structure of the carbon film can ensure the insertion and extraction of lithium ions. The addition of the carbon layer enhances the overall compressive strength and the flexibility of the spheres. It can be seen that through the synergistic effect of different types of carbon sources, strong intermolecular and interparticle interactions can be formed, which can greatly improve the mechanical strength and density of the spherical material.

[0022] 2. Highly Efficient Aggregate Preparation Process: This invention utilizes the granulation effect of the drying process to tightly bind and compress the above substances into spheres, forming uniformly sized spheres. By controlling the drying technology, highly controllable aggregate precursor particles are formed. This invention employs a stepwise heating sintering process, which continuously increases the bond energy of the chemical bonds formed during the grinding of the carbon layer, lithium source, and iron phosphate, enhancing the connection between primary particles. After high-temperature quenching, the carbon layer has a high degree of graphitization, the sphere surface is smoother, and the overall structure is dense and rounded, significantly improving the mechanical strength of the aggregates. This results in the preparation of high-mechanical-strength lithium iron phosphate aggregate cathode materials. Furthermore, the raw materials used in this invention are low-cost, non-toxic, and pollution-free, with a high overall yield and a simple preparation process, making it suitable for large-scale production.

[0023] 3. Advanced Fluid Drying Technology: This invention utilizes a novel fluid drying technology to dry the slurry. By controlling the amount of a special carbon source added and adjusting the drying process parameters, highly spherical particles with controllable particle size and high sphericity are obtained. The dried material, with each sphere as a unit, forms an integrated whole under stress. Combined with a unique formula, this produces an ultra-high mechanical strength lithium iron phosphate agglomerate cathode material. Attached Figure Description

[0024] Figure 1 This is a SEM image of lithium iron phosphate prepared in Example 1 of this invention.

[0025] Figure 2 This is the XRD pattern of lithium iron phosphate prepared in Example 1 of the present invention.

[0026] Figure 3 This is the capacity test curve of lithium iron phosphate prepared in Example 1 of the present invention.

[0027] Figure 4 This is a SEM image of lithium iron phosphate prepared in Comparative Example 1 of this invention. Detailed Implementation

[0028] The present invention will be further illustrated below with reference to specific embodiments. However, these embodiments are merely illustrative and not intended to limit the scope of the invention.

[0029] Example 1

[0030] 1) Using water as a solvent, weigh out 1% polyvinyl alcohol (based on the mass of ferric phosphate) to dissolve it, thus obtaining a primary mixed system.

[0031] 2) Weigh lithium carbonate and iron phosphate according to the molar ratio Li:P = 1:1.012. Add iron phosphate and stir until the mixture becomes lighter in color. Then add lithium carbonate to the mixture and add rock sugar at 8.0% of the mass of iron phosphate. Mix well to obtain a secondary mixture.

[0032] 3) The secondary mixture system was crushed using a coarse grinding mill until the particle size reached 700-800 nm, then transferred to a fine grinding mill for fine grinding until the particle size reached 200-220 nm and passed the retest. Then, 1.5% (by weight of iron phosphate) of polymer-modified asphalt was added to the system and mixed thoroughly.

[0033] 4) The slurry is spray-dried. A two-fluid dryer is selected for the spray dryer, with a gas pressure of 0.45MPa, an inlet temperature of 240℃, and an outlet air temperature of 80℃.

[0034] 5) Place the dried powder in a nitrogen-protected reactor and heat it to 280°C at a heating rate of 5°C / min, and hold it for 3 hours; then heat it to 460°C at a heating rate of 5°C / min, and hold it for 5 hours; then heat it to 750°C at a heating rate of 5°C / min, and hold it for 8 hours; then cool it naturally to room temperature, and after subsequent sieving, obtain large particles of lithium iron phosphate agglomerates.

[0035] Taking the lithium iron phosphate prepared in Example 1 as an example, its particle size and performance were characterized:

[0036] Figure 1 This is a SEM image of the lithium iron phosphate prepared in Example 1. It can be clearly observed that the obtained sample contains some large particles of 8–9 μm, while the primary particles are small particles of about 220 nm, resembling millet grains or flakes. The small particles are evenly distributed and have a dense surface, thereby improving the overall compressive strength and mechanical strength, which can reach 130 MPa.

[0037] Figure 2 The XRD pattern of lithium iron phosphate prepared in Example 1 is shown. The diffraction peaks in the figure correspond very closely to the standard peaks of lithium iron phosphate (JCPDS19-0721) in the PDF card, and there are no impurity peaks.

[0038] Figure 3 This is a capacity test curve of the lithium iron phosphate prepared in Example 1 of this invention. According to the capacity test results, the sample can reach 155 mAh / g at 0.1C discharge.

[0039] Example 2

[0040] 1) Using water as a solvent, weigh out 1% polyvinyl alcohol (based on the mass of ferric phosphate) to dissolve it, thus obtaining a primary mixed system.

[0041] 2) Weigh lithium carbonate and iron phosphate according to the molar ratio Li:P = 1:1.05. Add iron phosphate, lithium carbonate, and rock sugar (9% of the mass of iron phosphate) to the container of the above mixture and stir evenly to obtain a secondary mixture.

[0042] 3) The secondary mixing system is crushed in a coarse grinding mill until the particle size reaches 700-800nm, and then transferred to a fine grinding mill for fine grinding until the particle size finally reaches the range of 200-220nm and passes the retest.

[0043] 4) The slurry is spray-dried. A two-fluid dryer is selected for the spray dryer, with a gas pressure of 0.45MPa, an inlet temperature of 200℃, and an outlet air temperature of 60℃.

[0044] 5) Place the dried powder in a nitrogen-protected reactor and heat it to 350°C at a heating rate of 10°C / min, and hold it for 1 hour; then heat it to 500°C at a heating rate of 10°C / min, and hold it for 3 hours; then continue to heat it to 800°C at the same heating rate, and hold it for 5 hours; then cool it naturally to room temperature, and after subsequent sieving, obtain large particles of lithium iron phosphate agglomerates.

[0045] Example 3

[0046] 1) Using water as a solvent, weigh out 1% polyvinyl alcohol (based on the mass of ferric phosphate) and dissolve it to obtain a primary mixed system.

[0047] 2) Weigh lithium carbonate and iron phosphate according to the molar ratio Li:P = 1:1.1. Add iron phosphate, lithium carbonate, and rock sugar (8% of the mass of iron phosphate) to the container containing the above mixture and stir well.

[0048] 3) The secondary mixing system is crushed in a coarse grinding mill until the particle size reaches 700-800nm, and then transferred to a fine grinding mill for fine grinding until the particle size finally reaches the range of 200-220nm and passes the retest.

[0049] 4) The slurry is spray-dried. A small laboratory centrifugal dryer is selected for the spray dryer, with an inlet temperature of 280℃ and an outlet air temperature of 110℃.

[0050] 5) Place the dried powder in a nitrogen-protected reactor and heat it to 200℃ at a heating rate of 1℃ / min, and hold it for 5 hours; then heat it to 400℃ at a heating rate of 1℃ / min, and hold it for 7 hours; then heat it to 700℃ at a heating rate of 1℃ / min, and hold it for 15 hours; then cool it naturally to room temperature, and after subsequent sieving, obtain large particles of lithium iron phosphate agglomerates.

[0051] Example 4

[0052] 1) Using water as a solvent, weigh out 2% polyvinylpyrrolidone (PPP) based on the mass of iron phosphate to dissolve it, thus obtaining a primary mixed system.

[0053] 2) Weigh lithium carbonate and iron phosphate according to the molar ratio Li:P = 1:1.012. Add iron phosphate, lithium carbonate and glucose (7.5% of the mass of iron phosphate) to the container of the above mixture and stir evenly to obtain a secondary mixture.

[0054] 3) The secondary mixing system is crushed in a coarse grinding mill until the particle size reaches 700-800nm, then transferred to a fine grinding mill for fine grinding until the particle size finally reaches the range of 200-220nm and passes the retest.

[0055] 4) The slurry is spray-dried. A two-fluid dryer is selected for the spray dryer, with a gas pressure of 0.45MPa, an inlet temperature of 240℃, and an outlet air temperature of 80℃.

[0056] 5) Place the dried powder in a nitrogen-protected reactor and heat it to 220°C at a heating rate of 5°C / min, and hold it for 3 hours; then heat it to 460°C at a heating rate of 5°C / min, and hold it for 5 hours; then heat it to 710°C at a heating rate of 5°C / min, and hold it for 8 hours; then cool it naturally to room temperature, and after subsequent sieving, obtain large particles of lithium iron phosphate agglomerates.

[0057] Example 5

[0058] 1) Using water as a solvent, weigh out 2% polyvinylpyrrolidone (PPP) based on the mass of iron phosphate to dissolve it, thus obtaining a primary mixed system.

[0059] 2) Weigh lithium carbonate and iron phosphate according to the molar ratio Li:P = 1:1.012. Add iron phosphate, lithium carbonate and glucose (7.5% of the mass of iron phosphate) to the container of the above mixture and stir evenly to obtain a secondary mixture.

[0060] 3) The secondary mixing system is crushed in a coarse grinding mill until the particle size reaches 700-800nm, and then transferred to a fine grinding mill for fine grinding until the particle size finally reaches the range of 200-220nm and passes the retest.

[0061] 4) The slurry is spray-dried. A pressure dryer is selected for the spray dryer. The feeding pressure is controlled at 20-30 bar, the inlet temperature is 230℃, and the outlet air temperature is 80℃.

[0062] 5) Place the dried powder in a nitrogen-protected reactor and heat it to 220°C at a heating rate of 5°C / min, and hold it for 3 hours; then heat it to 460°C at a heating rate of 5°C / min, and hold it for 5 hours; then heat it to 710°C at a heating rate of 5°C / min, and hold it for 8 hours; then cool it naturally to room temperature, and after subsequent sieving, obtain large particles of lithium iron phosphate agglomerates.

[0063] Comparative Example 1

[0064] The conditions and operating steps are basically the same as in Example 1, except that only rock sugar is used as the carbon source.

[0065] Figure 4 The image shows a SEM image of lithium iron phosphate prepared in Comparative Example 1. It can be clearly observed that the particle size of the sample is basically the same, all ranging from 8 to 10 μm. However, the fine powder on the particle surface increases, the surface density decreases, and the particles are relatively loose.

[0066] Table 1. Performance test results of samples prepared in the examples and comparative examples.

[0067] Test Project Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Mechanical strength (MPa) 130 86 73 46 54 34 First week discharge (mAh / g) 155.6 154.3 154.9 158.9 160.2 161.5 1C discharge (mAh / g) 138.7 136.4 135.2 138.4 140.2 139.8 <![CDATA[Tap density (g / cm 3 )]]> 1.61 1.52 1.34 1.37 1.21 1.02 <![CDATA[Compaction density (g / cm 3 )]]> 2.43 2.44 2.39 2.23 2.18 2.17 D50(μm) 8.045 8.566 16.043 9.414 7.807 6.167 D100(μm) 39.340 43.749 51.618 44.776 38.176 34.428

[0068] As can be seen from the test data in Table 1, compared with the comparative example, the lithium iron phosphate cathode material samples prepared by the examples have higher mechanical strength and greater density due to the use of different types of composite carbon sources; among them, Example 1, which uses three types of carbon sources, has better mechanical strength and density than the other examples.

[0069] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Those skilled in the art can modify or make equivalent substitutions to the technical solutions of the present invention. The scope of protection of the present invention is defined by the claims.

Claims

1. A method for preparing a high-mechanical-strength, high-density lithium iron phosphate agglomerate cathode material, characterized in that, Includes the following steps: A primary mixed system is prepared by uniformly dispersing a first type of carbon source in water. The first type of carbon source is selected from at least one of polyethylene glycol, polyvinylpyrrolidone, polyvinyl alcohol, cellulose, starch, and sodium carboxymethyl cellulose. Add iron phosphate, a lithium source, and a second type of carbon source to the above primary mixture system and stir until homogeneous to prepare a secondary mixture system. The second type of carbon source is selected from at least one of sucrose, glucose, and activated carbon; the molar ratio of lithium source to iron phosphate is Li:P = 1:(1.0~1.1); the mass of the first type of carbon source is 1%~2% of iron phosphate, and the mass of the second type of carbon source is 7%~9% of iron phosphate. The secondary mixing system is coarsely ground using a coarse grinding mill, and the coarsely ground slurry is transferred to a fine grinding mill for fine grinding. A third type of carbon source is added to the finely ground slurry and stirred evenly, and then spray-dried. The third type of carbon source is selected from at least one of phenolic resin, asphalt, and tetrabutyl titanate. The mass of the third type of carbon source is 1% to 2% of iron phosphate. The dried powder is sintered in stages under an inert atmosphere, and then cooled and sieved to obtain large particles of lithium iron phosphate agglomerates.

2. The preparation method according to claim 1, characterized in that, After fine grinding, the solid content of ferric phosphate is 30%~50%.

3. The preparation method according to claim 1, characterized in that, The lithium source is one or more of lithium phosphate, lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium dihydrogen phosphate.

4. The preparation method according to claim 1, characterized in that, The sucrose is rock sugar.

5. The preparation method according to claim 1, characterized in that, The particle size of coarse grinding is controlled at 700-800 nm, and the particle size of fine grinding is controlled at 200-220 nm.

6. The preparation method according to claim 1, characterized in that, The spray dryer selected for spray drying is a centrifugal dryer, a pressure dryer, or a fluid dryer.

7. The preparation method according to claim 6, characterized in that, The gas pressure for spray drying is 0.3–0.6 MPa, and the feed frequency of the peristaltic pump is 0–60 Hz, not 0.

8. The preparation method according to claim 6, characterized in that, The inlet air temperature of the spray dryer is 200–280℃, and the outlet air temperature is 60–110℃; the median particle size D50 of the spray is 6–12 μm.

9. The preparation method according to claim 1, characterized in that, The conditions for staged heating sintering are as follows: heating at a rate of 1-10℃ / min from room temperature to 200-350℃ and holding for 1-5 hours; then heating to 400-500℃ and holding for 3-7 hours; and then heating to 700-800℃ and holding for 5-15 hours.

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