Preparation method of lithium iron phosphate positive electrode material

Abstract

The invention discloses a preparation method of a lithium iron phosphate positive electrode material, three iron sources with different forms and different particle sizes participate in preparation of the lithium iron phosphate positive electrode material, and gradation is formed to improve the compaction density and rate capability of the material. The EDTMPA reduces the viscosity of the slurry, is anchored on the surface of FePO4 particles through strong adsorption, inhibits particle aggregation through steric hindrance and electrostatic repulsion effect, and introduces hydrophilic groups to enhance wettability, reduce the viscosity of the system, optimize dispersion stability, accelerate particle crushing and refining, reduce the particle size and shorten the grinding time; in the sintering process, a reducing agent is generated through pyrolysis to inhibit too fast oxalic acid decomposition to cooperate with the generated reducing gas, Fe < 3 + > is completely reduced, the reduction temperature is reduced to inhibit grain growth, and the grading effect between large and small particles is improved; in addition, an N-doped carbon coating layer is formed through high-temperature cracking, pores are filled with residual carbon generated through decomposition of ferrous oxalate, the coating density is improved, the carbon-coated graphitization degree is improved, and then the electronic conductivity and the rate capability of the material are improved.

Description

TECHNICAL FIELD

[0001] The application relates to a preparation method of a battery material, in particular to a preparation method of a lithium iron phosphate positive electrode material. BACKGROUND

[0002] With the acceleration of global energy transformation, the demand for high-performance and high-safety battery materials continues to rise in the new energy vehicle industry and the construction of energy storage systems. In the future, the global battery market will achieve a breakthrough in demand size and technical performance in the power and energy storage fields. It is estimated that by 2030, the global demand for lithium-ion batteries will reach 4.7 TWh, nearly 6 times the approximately 700 GWh in 2022, with a compound annual growth rate (CAGR) of over 30%. The market size value will also increase from 561.2 million US dollars in 2023 to 1814.5 million US dollars in 2030. In the field of power batteries, the penetration rate of lithium iron phosphate (LFP) positive electrode materials in the Chinese market has exceeded 50%, and it dominates especially in the medium and low-end vehicle market. Globally, it is estimated that by 2030, the LFP share will be more than 40%. In the field of fixed energy storage, the LFP market share is expected to increase from 20% in 2025 to 30% in 2030. By 2030, LFP will replace traditional nickel-manganese-cobalt (NCM) batteries and become the mainstream system, with a global demand for LFP materials of 2.67 million tons, of which about 1.2 million tons are for power batteries, accounting for 45%, and about 1.47 million tons are for energy storage, accounting for 55%. With the explosive growth of the global battery industry, the market's requirements for battery performance are also undergoing a structural upgrade. Safety performance has become the core bottom line, and the new national standard has raised the thermal diffusion test standard for power batteries, requiring them to be "non-flammable and non-explosive". At the same time, the commercialization of fast charging technology is reshaping the industry landscape, and the new national standard requires batteries to pass the external short circuit test after completing 300 fast charging cycles. The ultimate indicator to meet the above requirements is the energy density of the battery, and the tap density of lithium iron phosphate is a key indicator. The capacity, conductivity, and cycle performance of the current lithium iron phosphate positive electrode material are far from meeting the above requirements. The tap density of the material can be improved by mixing large and small particles, and carbon sources need to be added to meet the conductivity requirements. However, the addition of carbon sources inhibits the growth of particles during high-temperature calcination, and high calcination temperatures result in larger LiFePO4 / C particles, uneven particle size distribution, and obvious particle agglomeration, further limiting the improvement of tap density. Therefore, although the above methods can improve the tap density of the material to a certain extent, the improvement is limited, and the existing lithium iron phosphate positive electrode material cannot simultaneously have high tap density and good conductivity. SUMMARY

[0003] The application aims to improve the compaction density and rate performance of lithium iron phosphate cathode material, and provides a preparation method of lithium iron phosphate cathode material with high compaction density and good conductivity and low energy consumption.

[0004] The preparation method of the lithium iron phosphate cathode material comprises the following steps:

[0005] S1: adding a lithium source, an iron source, a phosphorus source, a carbon source and a doping element into water, adding a chelating agent, and uniformly mixing to obtain lithium iron phosphate precursor slurry;

[0006] S2: grinding the lithium iron phosphate precursor slurry obtained in step S1, and spray drying to obtain lithium iron phosphate precursor powder;

[0007] S3: calcining the lithium iron phosphate precursor powder in step S2 in an inert atmosphere to obtain lithium iron phosphate cathode material;

[0008] The iron source in S1 is layered iron phosphate, spherical iron phosphate and ferrous oxalate, and the mass ratio is 1-5:9-5:0.01-1.

[0009] Further, the chelating agent in step S1 is ethylenediamine tetramethylene phosphonic acid, and the addition amount is 6%-8% of the total iron molar amount. 3+ The chelating agent reduces the grinding viscosity, prevents particle agglomeration, and improves the grinding efficiency in the preparation process of the precursor, and cooperates with ferrous oxalate to provide a reducing atmosphere to ensure Fe

[0010] Further, the molar ratio of the lithium source: the iron source: the phosphorus source in step S1 is 1.01-1.05:1:0.9-1.1.

[0011] Further, the doping element in step S1 is at least one of Mg, Ti, V, Al, Zr and Nb, and the doping amount is 2000-5000ppm, thereby improving the diffusion coefficient of lithium ions.

[0012] Further, the grinding particle size D50 of the lithium iron phosphate precursor slurry in step S2 is 0.3-0.5μm.

[0013] Further, the grinding time in step S2 is 5-15 min. Since the addition of the chelating agent reduces the viscosity of the system and the agglomeration between particles, the grinding time is shortened, the grinding efficiency is improved, the problems of agglomeration between particles or uneven mixing caused by long-time grinding are avoided, and the quality and uniformity of the particles after sintering are affected.

[0014] Further, the calcination temperature in step S3 is 500-800℃, and the calcination time is 6-10h. A positive electrode material with good quality is obtained at a lower sintering temperature, and the energy consumption for generation is reduced.

[0015] Further, the inert atmosphere in step S3 is nitrogen, argon or argon-hydrogen mixed gas, and the inert protective gas flow rate is 5-10L / min.

[0016] Further, the carbon source in step S1 is one or more of glucose, sucrose, polyethylene glycol, citric acid, polyvinyl alcohol, polypropylene, ascorbic acid, starch, phenolic resin, epoxy resin; the lithium source is at least one of lithium carbonate, lithium oxalate, lithium hydroxide; the phosphorus source is at least one of ammonium dihydrogen phosphate, phosphoric acid, ammonium phosphate, and the addition amount of the carbon source in step S1 is 8wt%-15wt% of the total amount of the feed.

[0017] Advantages: Compared with the prior art, the present application has the following remarkable advantages: 1. Three different forms and particle sizes of iron sources are used in the preparation of lithium iron phosphate positive electrode material, which forms a size grading of particles with different sizes and improves the compaction density of the material; 2. The addition of chelating agent EDTMPA reduces the high shear viscosity of the slurry, anchors on the surface of FePO4 particles through strong adsorption, inhibits particle agglomeration through steric hindrance and electrostatic repulsion, and introduces hydrophilic groups to enhance the wettability of particles in water phase, reduce the viscosity of the system, optimize the dispersion stability, accelerate the particle crushing and refinement, reduce the particle size, greatly improve the grinding efficiency and reduce the grinding time; 3. EDTMPA is pyrolyzed to produce NH3 and CO as a low-temperature reducing agent during sintering, which cooperates with the self-reducing gas (CO / CO2) of ferrous oxalate to reduce Fe 3+ to Fe 3+ to Fe 2+EDTMPA can reduce the energy consumption, reduce the reduction temperature, inhibit the grain growth, control the difference between the large and small particles, and improve the grading effect between the large and small particles; 4, EDTMPA can inhibit the unevenness of particles caused by the too fast decomposition of oxalate, refine the lithium iron phosphate particle size to 50-100nm to ensure the proportion of small particles, and then ensure the grading of large and small particles to improve the tap density; 5, EDTMPA high-temperature cracking forms N-doped carbon coating layer, and the residual carbon generated by the decomposition of ferrous oxalate fills the pores, improves the coating density, improves the graphitization degree of carbon coating, and then improves the electronic conductivity of the material, which is beneficial to the improvement of the rate performance of the material, and the discharge specific capacity at 0.1C, 0.5C and 1C is improved. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 SEM image of LiFePO4 material prepared by the present application;

[0019] Figure 2 XRD image of LiFePO4 material prepared by the present application;

[0020] Figure 3 0.1C, 0.5C and 1C charge-discharge curve graph of the first circle of Example 1 of the present application. DETAILED DESCRIPTION

[0021] The technical solutions of the present application will be further described below in conjunction with the drawings. The reagents used are all commercially available, and the carbon source is one or more of glucose, sucrose, polyethylene glycol, citric acid, polyvinyl alcohol, polypropylene, ascorbic acid, starch, phenolic resin, epoxy resin; the lithium source is at least one of lithium carbonate, lithium oxalate and lithium hydroxide; the phosphorus source is at least one of ammonium dihydrogen phosphate, phosphoric acid and ammonium phosphate.

[0022] Example 1

[0023] A preparation method of a lithium iron phosphate positive electrode material, comprising the following steps:

[0024] S1: lithium source, iron source and phosphorus source are weighed according to a molar ratio of 1.05:1:1, wherein the iron source is layered iron phosphate, spherical iron phosphate and ferrous oxalate with a molar ratio of 3:6.98:0.02, 10wt% of glucose is added as a carbon source, TiO2 doping element is added, the doping amount is 3500ppm, 7% of the total iron source molar amount is added EDTMPA as a chelating agent, and the mixture is stirred in water at a speed of 500rpm for 10min to obtain a lithium iron phosphate precursor slurry;

[0025] S2: the lithium iron phosphate precursor slurry obtained in step S1 is sand milled in a sand mill to a particle size of D50 of 0.40μm, and then transferred to a spray dryer for granulation to obtain a lithium iron phosphate precursor powder;

[0026] S3: The lithium iron phosphate precursor powder in step S2 is placed in a tube furnace in an inert atmosphere, the tube furnace is kept at a heating rate of 3℃ / min, the inert gas flow rate is 6L / min, the temperature is first raised to 500℃ and kept for 3h, then the temperature is raised to 780℃ and kept for 6h, to obtain the lithium iron phosphate positive electrode material.

[0027] Example 2

[0028] A preparation method of a lithium iron phosphate positive electrode material, comprising the following steps:

[0029] S1: Lithium source, iron source and phosphorus source are weighed according to a molar ratio of 1.04:1:0.9, wherein the iron source is layered iron phosphate, spherical iron phosphate and ferrous oxalate with a molar ratio of 2:7.97:0.03, 8wt% of sucrose is added as a carbon source, a doping element NH4VO3 is added with a doping amount of 3000ppm, and 6% of the total iron source molar amount of chelating agent ethylenediamine tetramethylene phosphonic acid EDTMPA is added, stirring in water at a speed of 300rpm for 20min to obtain a lithium iron phosphate precursor slurry;

[0030] S2: The lithium iron phosphate precursor slurry obtained in step S1 is sand milled in a sand mill to a particle size of D50 of 0.45μm, and then transferred to a spray dryer for granulation to obtain a lithium iron phosphate precursor powder;

[0031] S3: The lithium iron phosphate precursor powder in step S2 is placed in a tube furnace in an inert atmosphere, the tube furnace is kept at a heating rate of 3℃ / min, the inert gas flow rate is 6L / min, the temperature is first raised to 500℃ and kept for 3h, then the temperature is raised to 780℃ and kept for 6h, to obtain the lithium iron phosphate positive electrode material.

[0032] Example 3

[0033] A preparation method of a lithium iron phosphate positive electrode material, comprising the following steps:

[0034] S1: Lithium source, iron source and phosphorus source are weighed according to a molar ratio of 1.03:1:1.1, wherein the iron source is layered iron phosphate, spherical iron phosphate and ferrous oxalate with a molar ratio of 1:8.95:0.05, 15wt% of polyethylene glycol is added as a carbon source, a doping element MgO is added with a doping amount of 5000ppm, and 8% of the total iron source molar amount of chelating agent ethylenediamine tetramethylene phosphonic acid EDTMPA is added, stirring in water at a speed of 600rpm for 20min to obtain a lithium iron phosphate precursor slurry;

[0035] S2: The lithium iron phosphate precursor slurry obtained in step S1 is sand milled in a sand mill to a particle size of D50 of 0.42μm, and then transferred to a spray dryer for granulation to obtain a lithium iron phosphate precursor powder;

[0036] S3: The lithium iron phosphate precursor powder in step S2 is placed in a tube furnace in an inert atmosphere, the tube furnace is kept at a heating rate of 3°C / min, the inert gas flow rate is 8L / min, the temperature is first raised to 550°C and kept for 3h, then the temperature is raised to 750°C and kept for 10h, to obtain the lithium iron phosphate positive electrode material.

[0037] Example 4

[0038] A preparation method of a lithium iron phosphate positive electrode material, comprising the following steps:

[0039] S1: Lithium source, iron source and phosphorus source are weighed according to a molar ratio of 1.02:1:1, wherein the iron source is layered iron phosphate, spherical iron phosphate and ferrous oxalate in a molar ratio of 4:5:1, 15wt% of citric acid is added as a carbon source, ZnO is added with a doping amount of 2000ppm, 7% of the total iron source molar amount of chelating agent ethylenediaminetetramethylene phosphonic acid EDTMPA is added, and the mixture is stirred in water at a speed of 1000rpm for 20min to obtain a lithium iron phosphate precursor slurry;

[0040] S2: The lithium iron phosphate precursor slurry obtained in step S1 is sand milled in a sand mill to a particle size of D50 of 0.38μm, and then transferred to a spray dryer for granulation to obtain a lithium iron phosphate precursor powder;

[0041] S3: The lithium iron phosphate precursor powder in step S2 is placed in a tube furnace in an inert atmosphere, the tube furnace is kept at a heating rate of 3°C / min, the inert gas flow rate is 8L / min, the temperature is first raised to 550°C and kept for 3h, then the temperature is raised to 750°C and kept for 10h, to obtain the lithium iron phosphate positive electrode material.

[0042] Comparative Example 1

[0043] Different from Example 1, the iron source is layered iron phosphate and spherical iron phosphate in a molar ratio of 3:7.

[0044] Comparative Example 2

[0045] Different from Example 1, the iron source is all spherical iron phosphate.

[0046] Comparative Example 3

[0047] Different from Example 1, the iron source is all layered iron phosphate.

[0048] Comparative Example 4

[0049] Different from Example 1, no chelating agent ethylenediaminetetramethylene phosphonic acid EDTMPA is added in step S1.

[0050] Comparative Example 5

[0051] Different from Example 1, the iron source is layered iron phosphate, spherical iron phosphate: ferrous oxalate in a molar ratio of 7:2.98:0.02.

[0052] Comparative Example 6

[0053] Different from Example 1, the iron source is layered iron phosphate, spherical iron phosphate: ferrous oxalate in a molar ratio of 6:3.95:0.05.

[0054] The lithium iron phosphate positive electrode materials prepared in the above examples and comparative examples are characterized by scanning electron microscopy (SEM) and XRD for morphology and structure of the positive electrode materials, as shown in Figs. 1-6. Figure 1 As shown in Figs. 1-6, the obtained lithium iron phosphate positive electrode materials are spherical, and the morphology is regular spherical, and the size particles form a grading effect, so that the compaction density of the material is greatly improved. As shown in Figs. 1-6, the diffraction peak of the sample is high and sharp, and the crystallinity is good, which is consistent with the standard spectrum of LFP of ordered olivine structure in orthorhombic Pnma space group. Figure 2

[0055] Electrical performance test

[0056] The lithium iron phosphate positive electrode materials prepared in the above examples and comparative examples, Super-P, and PVDF are coated on aluminum foil in a ratio of 85:15:5, a lithium metal sheet (battery grade) is used as a negative electrode, a 1 mol / L LiPF6 solution of EC:DMC:EMC (volume ratio 1:1:1) is used as an electrolyte, and Celgard 2400 microporous polypropylene membrane is used as a separator. A button cell is assembled in an argon-filled glove box, and its electrical performance is tested, and the results are shown in Table 1.

[0057] As shown in Table 1, Examples 1-4 use three forms of iron phosphate sources to participate in the reaction according to the appropriate ratio, and the compaction density of the obtained lithium iron phosphate positive electrode material is increased by about 18%. The three forms of iron sources with different particle sizes participate in the reaction to form particles with different sizes, thereby improving the compaction density of the material. In particular, in addition to providing active Fe 2+ , the ferrous oxalate in the iron source also generates reducing gas (CO / CO2) and EDTMPA reducing gas NH3 during high-temperature sintering, and Fe 3+ is completely reduced, which avoids the generation of impurities and reduces the reduction temperature to about 500°C; in addition, EDTMPA inhibits the uneven particles caused by the too fast decomposition of oxalate, and refines the lithium iron phosphate particle size to 50-100 nm to ensure the proportion of small particles, thereby ensuring the grading of large and small particles to improve the compaction density. The addition of the chelating agent ethylenediamine tetramethylene phosphonic acid (EDTMPA) generates NH3 and CO as low-temperature reducing agents during sintering, and reduces Fe 3+ to Fe 2+ ​The EDTMPA can reduce the energy consumption of the lithium iron phosphate anode material, inhibit the grain growth while reducing the reduction temperature, control the difference between the large and small particles, and improve the grading effect between the large and small particles. The EDTMPA is high-temperature cracked to form a N-doped carbon coating layer, the residual carbon generated by the decomposition of ferrous oxalate fills the pores, improves the coating density, improves the graphitization degree of the carbon-coated material, and further improves the electronic conductivity of the material, which is beneficial to the improvement of the rate performance of the material, and the discharge specific capacity at 0.1C, 0.5C and 1C is improved.

[0058] Table 1: Test results of the electrical properties of different examples and comparative examples

[0059]

[0060] As shown in Table 1, compared with Examples 1-4, Comparative Example 4 does not add the chelating agent EDTMPA, and needs a longer grinding time to meet the particle size requirement after sanding, and the grinding efficiency is reduced by 50%, which indicates that EDTMPA can reduce the high shear viscosity of the slurry. The phosphonic acid group (-PO(OH)2) of EDTMPA is anchored on the surface of FePO4 particles through strong adsorption, and the negative anion (-PO3 2- ) generated by the dissociation of water phase inhibits the particle agglomeration through steric hindrance and electrostatic repulsion effect, and introduces a hydrophilic group (imino group, phosphonic acid group), which enhances the wettability of the particles in the water phase, optimizes the dispersion stability, reduces the system viscosity, improves the grinding efficiency, accelerates the particle crushing and refinement, reduces the particle size, greatly improves the grinding efficiency, and reduces the grinding time. In addition, the EDTMPA chelating agent in-situ N-doped carbon layer during sintering, the residual carbon generated by ferrous oxalate fills the pores, improves the coating density, improves the graphitization degree of the carbon-coated material, improves the electronic conductivity of the material, and makes the material have better electrochemical performance, excellent rate performance, and finally obtains the lithium iron phosphate anode material with uniform particle size and excellent performance, and realizes the reduction of sintering temperature, which lays a solid foundation for industrial production.

[0061] As can be seen from Table 1, compared with Comparative Examples 1, 2 and 3, the three different morphologies of iron source are used at the same time, the compaction density of lithium iron phosphate is higher, and any one or two of them cannot obtain lithium iron phosphate material with better compaction density, especially ferrous oxalate plays a key role in the compaction density and rate performance of the material and the sintering process, and the performance decreases after missing. This is because the layered iron phosphate generates large particles during sintering, which is beneficial to improve the compaction density of lithium iron phosphate, the spherical iron phosphate forms a uniform morphology of lithium iron phosphate, which improves the capacity of lithium iron phosphate material, the chelation of ferrous oxalate with EDTMPA reduces the reduction temperature, inhibits grain growth, controls the difference between large and small particles, improves the grading effect between large and small particles, and at the same time, the residual carbon generated by the decomposition of ferrous oxalate fills the pores, further improves the carbon-coated compactness and carbon-coated graphitization degree, and then improves the electronic conductivity of the material, which is beneficial to improve the rate performance of the material.

[0062] As can be seen from Table 1, compared with Comparative Examples 1, 2 and 3, the three different morphologies of iron source are used at the same time, the compaction density of lithium iron phosphate is higher, and any one or two of them cannot obtain lithium iron phosphate material with better compaction density, especially ferrous oxalate plays a key role in the compaction density and rate performance of the material and the sintering process, and the performance decreases after missing. This is because the layered iron phosphate generates relatively more large particles, which produces a "bridging effect", and the large particles cannot be effectively and completely filled by small particles, forming an "arch-shaped hollow", resulting in a decrease in compaction density, and a large proportion of large particles greatly reduces the electrochemical performance of the material.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, Includes the following steps: S1: Add lithium source, iron source, phosphorus source, carbon source and doping element to water, add chelating agent, and mix evenly to obtain lithium iron phosphate precursor slurry; S2: The lithium iron phosphate precursor slurry obtained by grinding S1 is spray-dried to obtain lithium iron phosphate precursor powder. S3: The lithium iron phosphate precursor powder in S2 is calcined in an inert atmosphere to obtain lithium iron phosphate cathode material; The iron source in S1 is layered ferric phosphate, spherical ferric phosphate, and ferrous oxalate, with a mass ratio of 1-5:9-5:0.01-1.

2. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, The chelating agent of S1 is ethylenediaminetetramethylenephosphonic acid, and the amount added is 6%-8% of the total iron molar amount.

3. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The molar ratio of lithium source: iron source: phosphorus source in S1 is 1.01-1.05:1:0.9-1.

1.

4. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The doping element of S1 is at least one of Mg, Ti, V, Al, Zr, and Nb, and the doping amount is 2000-5000ppm.

5. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, The grinding particle size D50 of the lithium iron phosphate precursor slurry in S2 is 0.3 to 0.5 μm.

6. The method for preparing the lithium iron phosphate cathode material according to claim 5, characterized in that, The grinding time for S2 is 5-15 minutes.

7. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The S3 is calcined at a temperature of 500-800℃ for 6-10 hours.

8. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, The inert atmosphere of S3 is nitrogen, argon, or an argon-hydrogen mixture, and the inert protective gas flow rate is 5-10 L / min.

9. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The carbon source of S1 is one or more of glucose, sucrose, polyethylene glycol, citric acid, polyvinyl alcohol, polypropylene, ascorbic acid, starch, phenolic resin, and epoxy resin; the lithium source is at least one of lithium carbonate, lithium oxalate, and lithium hydroxide; and the phosphorus source is at least one of ammonium dihydrogen phosphate, phosphoric acid, and ammonium phosphate.

10. The method for preparing the lithium iron phosphate cathode material according to claim 9, characterized in that, The amount of carbon source added in S1 is 8wt% to 15wt% of the total feed.

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