Preparation method of high-capacity and high-compaction lithium iron phosphate positive electrode material and positive electrode material thereof

Abstract

The preparation method comprises the following steps: S1, uniformly mixing a lithium source, an iron source, a phosphorus source, a fast ion conductor and a rare earth compound, adding ethanol and an MOF material, and sintering at the temperature of 100-200 DEG C for 1-3 hours to obtain a lithium iron phosphate precursor A; s2, transferring the lithium iron phosphate precursor A into a tubular furnace, raising the temperature to 300-500 DEG C, introducing heteroatom gas, raising the temperature to 700-900 DEG C, and introducing carbon source gas to obtain a lithium iron phosphate precursor B; and S3, dispersing the lithium iron phosphate precursor B, a carbon source and a functional additive into a solvent, performing spray drying, and performing high-temperature sintering for 2-12 hours to obtain the high-capacity and high-compaction lithium iron phosphate positive electrode material. The diffusion path of lithium ions is shortened, and the transmission performance of the lithium ions is remarkably improved; meanwhile, the specific capacity of the material is improved; and moreover, agglomeration of iron phosphate is avoided, the coating uniformity is improved, the power performance is improved, and the compaction density of the lithium iron phosphate material is further improved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery material preparation, specifically relating to a method for preparing a high-capacity, high-pressure lithium iron phosphate cathode material, and also relating to the lithium iron phosphate cathode material obtained by the method. Background Technology

[0002] With the increasing market demand for high-energy-density batteries, the lithium iron phosphate cathode material used in lithium-ion batteries needs to have both high capacity and improved compaction density.

[0003] Currently, the commonly used lithium iron phosphate (LFP) process in the industry mainly uses iron phosphate, lithium carbonate, and carbon sources as raw materials, and deionized water as a dispersant. After wet ball milling and high-speed ultrafine milling, the precursor is obtained through centrifugal spray drying to form spherical powder. The resulting LFP material, after sintering, suffers from defects such as low compaction density. While the high-temperature solid-phase method for preparing LFP can increase the compaction density by increasing the material size, it also leads to increased polarization during charge and discharge, thereby reducing the specific capacity and compaction density of the LFP material. The liquid-phase method for preparing LFP, due to its nanostructure advantage, can improve the charge and discharge performance and ensure cycle life of the LFP material, but it is prone to agglomeration, making it difficult to form a good particle size distribution, and therefore, it is also difficult to obtain LFP with high compaction density.

[0004] To improve the compaction density and specific capacity of existing lithium iron phosphate materials, the following measures are generally adopted: increasing sintering temperature, extending holding time, reducing carbon content, changing raw materials (mainly iron phosphate), appropriate particle size distribution and element doping, surface coating, etc. However, while these technologies improve the specific capacity and compaction density of lithium iron phosphate materials, they also reduce the power performance of lithium iron phosphate materials.

[0005] A domestic invention patent application with publication number CN114314550A discloses a method of using a two-stage spraying process to improve the specific energy and compaction density of lithium iron phosphate by using large-particle lithium iron phosphate and small-particle lithium iron phosphate for particle gradation. Although the compaction density of the lithium iron phosphate material prepared by this process is improved, the improvement is limited, but the power performance of the lithium iron phosphate material is reduced.

[0006] Therefore, the applicant seeks new technical solutions to address the above-mentioned technical problems. Summary of the Invention

[0007] In view of this, the purpose of the present invention is to provide a method for preparing a high-capacity, high-compact lithium iron phosphate cathode material and the cathode material thereof, which shortens the diffusion path of lithium ions and significantly improves the lithium ion transport performance; at the same time, it increases the diffusion rate of lithium ions, reduces polarization, and improves the specific capacity of the material; moreover, the MOF material coating on the surface of lithium iron phosphate avoids the agglomeration of iron phosphate and improves the uniformity of the coating, thereby improving power performance and thus increasing the compaction density of the lithium iron phosphate material.

[0008] The technical solution adopted in this invention is as follows: Please see Figure 1 As shown, a method for preparing a high-capacity, high-pressure lithium iron phosphate cathode material includes the following steps: S1. After mixing lithium source, iron source, phosphorus source, fast ion conductor and rare earth compound evenly, add ethanol and MOF material, stir to make it evenly dispersed, filter, and sinter at 100-200℃ for 1-3 hours to obtain lithium iron phosphate precursor A. S2. Transfer the lithium iron phosphate precursor A obtained in step S1 to a tube furnace, heat it to 300-500℃, and then introduce heteroatom gas at a flow rate of 10-100 ml / min for 30-300 min. Heat it to 700-900℃, and then introduce carbon source gas at a flow rate of 10-100 ml / min for 30-300 min to obtain lithium iron phosphate precursor B. S3. The lithium iron phosphate precursor B, carbon source and functional additives obtained in step S2 are dispersed in a solvent, mixed evenly, spray-dried, and then sintered at high temperature for 2-12 hours under inert protective gas to obtain the high-capacity, high-pressure lithium iron phosphate cathode material.

[0009] Preferably, in step S1, the molar ratio of lithium, iron, and phosphorus in the lithium source, iron source, and phosphorus source respectively ranges from 1-1.2:1:0.9-1.2; the mass ratio of the sum of the lithium source, iron source, and phosphorus source, fast ion conductor, rare earth compound, ethanol, and MOF material ranges from 100:0.1-1:0.01-0.1:10-100:1-5.

[0010] Preferably, in step S1, the lithium source is at least one of lithium lactate, lithium oxalate, lithium formate, lithium acetate, lithium propionate, and lithium butyrate; the iron source is at least one of ferrous oxalate, ferric phosphate, ferrous sulfate, and ferrous nitrate; and the phosphorus source is at least one of ammonium dihydrogen phosphate, phosphoric acid, ammonium phosphate, sodium phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

[0011] Preferably, in step S1, the fast ion conductor is one of LiTi2(PO4)3, Li3V2(PO4)3, and LiZr2(PO4)3; the rare earth compound is one of oxides, nitrates, hydroxides, carbonates, or chlorides of lanthanum, cerium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, scandium, or yttrium; and the MOF material is one of ZIF-8, ZIF-5, ZIF-67, MOF-808, and ZnO@Mg-MOF.

[0012] Preferably, in step S2, the mass-to-volume ratio of the lithium iron phosphate precursor A, heteroatom gas, and carbon source gas is in the range of 100g:0.5-2ml:0.5-2ml.

[0013] Preferably, in step S2, the heteroatom gas is at least one of ammonia, hydrogen boride, phosphine, and hydrogen sulfide; and the carbon source gas is at least one of methane, ethane, ethylene, acetylene, and propyne.

[0014] Preferably, in step S3, the mass ratio of lithium iron phosphate precursor B, carbon source, functional additive, and solvent is in the range of 100:0.5-2:1-3:100-500.

[0015] Preferably, in step S3, the carbon source is at least one of glucose, sucrose, maltose, lactose, and corn syrup; and the functional additive is at least one of Li6FeO4, Li2NiO2, and Li6CoO4.

[0016] Preferably, in step S3, the temperature range of the high-temperature sintering is set at 700-900℃.

[0017] Preferably, a high-capacity, high-density lithium iron phosphate cathode material is prepared using the preparation method of the high-capacity, high-density lithium iron phosphate cathode material described above.

[0018] It should be noted that the full English name of "MOF" used throughout this application is Metal-Organic Framework, and MOF material refers to metal-organic framework material. It should be further noted that the specific MOF materials used in this application are all known and commercially available MOF materials. Specifically, ZIF-8 refers to zeolite imidazolium ester framework-8, ZIF-5 refers to zeolite imidazolium ester framework-5, ZIF-67 refers to zeolite imidazolium ester framework-67, MOF-808 refers to zirconium-based metal-organic framework material, and ZnO@Mg-MOF refers to zinc oxide@magnesium-based metal-organic framework composite material.

[0019] This invention dops lithium iron phosphate with rare earth atoms and fast ion conductors. The rare earth compounds provide numerous equivalent sites in the lattice of the lithium iron phosphate cathode material, thereby increasing the diffusion channels for lithium ions, shortening the diffusion path, and significantly improving lithium ion transport performance. Simultaneously, the fast ion conductors enhance the diffusion rate of lithium ions, reduce polarization, and improve the specific capacity of the material. Furthermore, the MOF material coating on the surface of lithium iron phosphate prevents the agglomeration of iron phosphate and improves the uniformity of the coating, thus improving power performance and increasing the compaction density of the lithium iron phosphate material. In step S2, this application also dops the lithium iron phosphate with heteroatom compounds, providing additional electronic conduction paths for the lithium iron phosphate cathode material, thereby improving the overall electronic conductivity of the material, matching the lithium ion diffusion rate. Moreover, heteroatom doping can introduce new active sites into the lithium iron phosphate cathode material, further enhancing its specific capacity. Attached Figure Description

[0020] Figure 1 This is a flowchart of the preparation method of the high-capacity, high-pressure lithium iron phosphate cathode material of the present invention; Figure 2 This is a SEM image of the high-capacity, high-pressure lithium iron phosphate cathode material prepared in Example 1 of this invention. Detailed Implementation Example 1:

[0021] A method for preparing a high-capacity, high-pressure lithium iron phosphate cathode material includes the following steps: S1. Weigh out 100g of lithium lactate (as lithium source), ferrous oxalate (as iron source), and ammonium dihydrogen phosphate (as phosphorus source) according to the molar ratio of Li, Fe, and P of 1.05:1:1. Mix them with 0.1g of Li3V2(PO4)3 and 0.01g of cerium nitrate. Then add 10g of ethanol and 1g of ZIF-8 MOF material, stir to disperse evenly, filter, transfer the filter to a tube furnace, heat to 150℃ and sinter for 2 hours to obtain lithium iron phosphate precursor A. S2. Transfer 100g of lithium iron phosphate precursor A obtained in step S1 above to a tube furnace. First, heat the furnace to 400°C and then introduce ammonia gas at a flow rate of 50ml / min for 180min. Then, heat the furnace to 800°C and introduce ethylene gas at a flow rate of 50ml / min for 180min to obtain lithium iron phosphate precursor B. S3. Disperse 100g of lithium iron phosphate precursor B obtained in step S2 above, 1g of glucose and 2g of Li6FeO4 into 300g of ethanol solvent, mix evenly, spray dry, and then sinter at high temperature for 6 hours under nitrogen inert protective gas and temperature set at 800℃ to obtain high-capacity, high-pressure lithium iron phosphate cathode material. This application uses SEM to test the high-capacity, high-pressure lithium iron phosphate cathode material prepared in Example 1. Please refer to the SEM image for details. Figure 2 As shown, the material exhibits a granular structure with uniform distribution and a particle size between 1 and 3 μm. Example 2:

[0022] A method for preparing a high-capacity, high-pressure lithium iron phosphate cathode material includes the following steps: S1. Weigh out 100g of lithium oxalate (as lithium source), iron phosphate (as iron source), and ammonium phosphate (as phosphorus source) in a Li:Fe:P molar ratio of 1:1:1. Mix them with 0.5g of LiTi2(PO4)3 and 0.05g of lanthanum nitrate. Then add 50g of ethanol and 3g of ZIF-5 MOF material and mix well. Stir to disperse the mixture evenly and filter it. Transfer the filter to a tube furnace and sinter at 100℃ for 3 hours to obtain lithium iron phosphate precursor A. S2. Transfer 100g of lithium iron phosphate precursor A obtained in step S1 above into a tube furnace. First, heat the furnace to 300°C and then introduce phosphine gas at a flow rate of 10ml / min for 300min. Then, heat the furnace to 700°C and introduce acetylene gas at a flow rate of 10ml / min for 300min to obtain lithium iron phosphate precursor B. S3. Add 100g of lithium iron phosphate precursor B obtained in step S2 above, 0.5g of sucrose and 1g of Li2NiO2 to 100g of ethanol solvent, mix evenly, spray dry, and then sinter at high temperature for 12 hours under nitrogen inert protective gas and a temperature set at 700℃ to obtain high-capacity, high-pressure lithium iron phosphate cathode material. Example 3:

[0023] A method for preparing a high-capacity, high-pressure lithium iron phosphate cathode material includes the following steps: S1. Weigh out 100g of lithium formate, ferrous sulfate, and sodium phosphate according to the molar ratio of Li, Fe, and P of 1.2:1:1.2. Mix them with 1g of LiZr2(PO4)3 and 0.1g of scandium oxide. Then add 100g of ethanol and 5g of MOF-808 material, mix them evenly, stir to disperse them evenly, filter them, transfer the filter to a tube furnace, heat it to 200℃ and sinter for 1 hour to obtain lithium iron phosphate precursor A. S2. Transfer 100g of lithium iron phosphate precursor A obtained in step S1 above into a tube furnace. First, heat the furnace to 500°C and then introduce hydrogen borate gas at a flow rate of 100ml / min for 30min. Then, heat the furnace to 900°C and introduce methane gas at a flow rate of 100ml / min for 30min to obtain lithium iron phosphate precursor B. S3. Add 100g of lithium iron phosphate precursor B obtained in step S2 above, 2g of maltose and 3g of Li6CoO4 to 500g of ethanol solvent, mix evenly, spray dry, and then sinter at high temperature for 2 hours under nitrogen inert protective gas and a temperature set at 900℃ to obtain high-capacity, high-pressure lithium iron phosphate cathode material.

[0024] Comparative Example 1: The remaining technical solutions of Comparative Example 1 are the same as those of Example 1, except that Li3V2(PO4)3 is not added in step S1 of Comparative Example 1.

[0025] Comparative Example 2: The remaining technical solutions of Comparative Example 2 are the same as those of Example 1, except that cerium nitrate is not added in step S1 of Comparative Example 2.

[0026] Comparative Example 3: The remaining technical solutions of Comparative Example 3 are the same as those of Example 1, except that MOF material is not added in step S1 of Comparative Example 3.

[0027] Comparative Example 4: The remaining technical solutions of Comparative Example 4 are the same as those of Example 1, except that Li6FeO4 is not added in step S3 of Comparative Example 4.

[0028] To verify the technical effects achieved in the above embodiments, this application conducted the following physicochemical properties and button cell tests on the above embodiments 1-3 and comparative examples 1-4: Among them, the physicochemical performance test: the specific surface area and compaction density of the material were tested in accordance with GB / T 30835-2014 "Carbon Composite Lithium Iron Phosphate Cathode Material for Lithium-ion Batteries". The compaction density refers to the compaction density of the cathode material under 200 MPa pressure for 10 seconds, which is tested by a powder compaction density tester; and the powder resistivity is tested by a four-probe tester. The coin cell battery test involved the following steps: The positive electrode active material, carbon nanotubes, PVDF, and NMP were mixed according to a mass ratio of 90:5:5:300. Lithium iron phosphate positive electrode materials prepared in Examples 1-3 and Comparative Examples 1-4 were weighed as the positive electrode active material, carbon nanotubes, and PVDF, respectively. NMP (N-methylpyrrolidone) organic solvent was then added and thoroughly mixed. A 140-micron thick film was coated onto aluminum foil, vacuum-dried at 120°C for 2 hours, and punched into 5mm round sheets. These sheets were then pressed at 10 MPa using a tablet press and vacuum-insulated at 120°C for 12 hours. The weight of the positive electrode sheets was measured. Coin cells were assembled in an argon-protected glove box, using lithium metal sheets as the counter electrode and an electrolyte with a volume ratio of 1:1 (EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate)). The solvent and electrolyte were LiPF6, and the separator was a Celgard 2400 microporous polyethylene membrane. Each coin cell was assembled. The electrical performance of each coin cell was then tested using a blue electric current meter under the following conditions: charge / discharge at a constant current of 0.2C within a voltage range of 2.75V-4.25V. Specific capacity and initial efficiency were tested. The rate performance (2C / 0.2C) of each coin cell was also tested. Please refer to Table 1 below for the test results:

[0029] As can be seen from Table 1 above, the discharge capacity and first-time efficiency of the batteries using the lithium iron phosphate cathode materials provided in Examples 1-3 are significantly higher than those in Comparative Examples 1-4. The main reason for this is that the cathode materials provided in this application are coated with fast ion conductors to improve the ion diffusion rate of the material and the functional additives in the outer shell release lithium ions to improve the first-time efficiency and the lithium ions, which can significantly improve the rate performance of the battery.

[0030] This application also conducted the following pouch battery tests on the above-described Examples 1-3 and Comparative Examples 1-4: Soft-pack battery testing: Using the materials prepared in Examples 1-3 and Comparative Examples 1-4 as positive electrode materials, graphite as negative electrode material, LiPF6 / EC+DEC (EC to DEC volume ratio 1:1) as electrolyte, and Celgard 2400 membrane as separator, soft-pack batteries with a specification of 5AH were prepared respectively. The rate discharge performance (0.5C, 1C, 2C, 4C) and cycle performance (2C / 2C, 500 cycles) of each soft-pack battery at different rates were tested. The test results are shown in Table 2 below.

[0031] As can be seen from Table 2 above, the pouch batteries prepared using the materials provided in Examples 1-3 are significantly better than those in Comparative Examples 1-4 in terms of rate performance and cycle performance. The main reason for this is that the materials provided in Examples 1-3 have fast ion conductors and functional additives on their surface that enhance the lithium ion transport rate and improve rate performance; at the same time, the lithium iron phosphate is coated with multiple layers to reduce expansion and improve the structural stability of the material, thereby improving the cycle performance of the battery.

[0032] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0033] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing a high-capacity, high-pressure lithium iron phosphate cathode material, characterized in that, The following steps are included: S1. After mixing lithium source, iron source, phosphorus source, fast ion conductor and rare earth compound evenly, add ethanol and MOF material, stir to make it evenly dispersed, filter, and sinter at 100-200℃ for 1-3 hours to obtain lithium iron phosphate precursor A. S2. Transfer the lithium iron phosphate precursor A obtained in step S1 to a tube furnace, heat it to 300-500℃, and then introduce heteroatom gas at a flow rate of 10-100 ml / min for 30-300 min. Heat it to 700-900℃, and then introduce carbon source gas at a flow rate of 10-100 ml / min for 30-300 min to obtain lithium iron phosphate precursor B. S3. The lithium iron phosphate precursor B, carbon source and functional additives obtained in step S2 are dispersed in a solvent, mixed evenly, spray-dried, and then sintered at high temperature for 2-12 hours under inert protective gas to obtain the high-capacity, high-pressure lithium iron phosphate cathode material.

2. The method for preparing the high-capacity, high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that, In step S1, the molar ratio of lithium, iron, and phosphorus in the lithium source, iron source, and phosphorus source respectively ranges from 1-1.2:1:0.9-1.2; the mass ratio of the sum of the lithium source, iron source, and phosphorus source, fast ion conductor, rare earth compound, ethanol, and MOF material ranges from 100:0.1-1:0.01-0.1:10-100:1-5.

3. The method for preparing the high-capacity, high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that, In step S1, the lithium source is at least one of lithium lactate, lithium oxalate, lithium formate, lithium acetate, lithium propionate, and lithium butyrate; the iron source is at least one of ferrous oxalate, ferric phosphate, ferrous sulfate, and ferrous nitrate; and the phosphorus source is at least one of ammonium dihydrogen phosphate, phosphoric acid, ammonium phosphate, sodium phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

4. The method for preparing the high-capacity, high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that, In step S1, the fast ion conductor is one of LiTi2(PO4)3, Li3V2(PO4)3, and LiZr2(PO4)3; the rare earth compound is one of oxides, nitrates, hydroxides, carbonates, or chlorides of lanthanum, cerium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, scandium, or yttrium; and the MOF material is one of ZIF-8, ZIF-5, ZIF-67, MOF-808, and ZnO@Mg-MOF.

5. The method for preparing the high-capacity, high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that, In step S2, the mass-to-volume ratio of the lithium iron phosphate precursor A, heteroatom gas, and carbon source gas is in the range of 100g:0.5-2ml:0.5-2ml.

6. The method for preparing the high-capacity, high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that, In step S2, the heteroatom gas is at least one of ammonia, hydrogen boride, phosphine, and hydrogen sulfide; the carbon source gas is at least one of methane, ethane, ethylene, acetylene, and propyne.

7. The method for preparing the high-capacity, high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that, In step S3, the mass ratio of lithium iron phosphate precursor B, carbon source, functional additive, and solvent ranges from 100:0.5-2:1-3:100-500.

8. The method for preparing the high-capacity, high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that, In step S3, the carbon source is at least one of glucose, sucrose, maltose, lactose, and corn syrup; the functional additive is at least one of Li6FeO4, Li2NiO2, and Li6CoO4.

9. The method for preparing the high-capacity, high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that, In step S3, the temperature range for high-temperature sintering is set at 700-900℃.

10. A high-capacity, high-pressure lithium iron phosphate cathode material, characterized in that, The high-capacity, high-pressure lithium iron phosphate cathode material was prepared using the preparation method described in any one of claims 1-9.

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