A lithium iron manganese phosphate positive electrode material and preparation method thereof

Abstract

The present invention discloses a lithium iron manganese phosphate cathode material and a preparation method thereof. The lithium ion battery cathode material is obtained by compounding carbon-coated lithium iron manganese phosphate with cobalt iron magnesium aluminum hydrotalcite, and comprises the following steps: S1, preparing carbon-coated lithium iron manganese phosphate; S2, preparing a solution of cobalt iron magnesium aluminum hydrotalcite; S3, adding carbon-coated lithium iron manganese phosphate to a solution of cobalt iron magnesium aluminum hydrotalcite, drying after stirring, and obtaining a cathode material through annealing. The present invention adopts CoFeMgAl LDHs and carbon-coated lithium iron manganese phosphate composite material to effectively inhibit the agglomeration and growth of lithium iron manganese phosphate material particles, significantly reduce the diffusion path of lithium ions inside the positive electrode active material particles, improve the lithium ion diffusion coefficient, efficiently realize the deintercalation of lithium ions, and improve the rate performance of the material; after using CoFeMgAl LDHs and carbon-coated lithium iron manganese phosphate composite material, the contact between active material and electrolyte can be reduced, side reactions with electrolyte are avoided, the dissolution of Mn is reduced, the crystal structure distortion is reduced, and the cycle performance of the material is improved.

Description

Technical Field

[0001] The invention relates to a lithium manganese iron phosphate positive electrode material and a preparation method thereof, and belongs to the field of preparation of positive electrode materials for lithium ion batteries. Background Art

[0002] In recent years, the Earth has been plagued by pollution and energy security issues. Achieving sustainable and healthy development requires the active development and utilization of pollution-free, sustainable new energy sources, such as solar energy, tidal energy, wind energy, hydrogen energy, and nuclear energy. However, these energy sources operate intermittently and cannot meet large-scale power supply needs, necessitating the use of complementary chemical power sources. The development of low-cost, safe, and high-energy-density chemical power sources has been included in the national medium- and long-term development plan as a key project.

[0003] Lithium-ion batteries are the most promising rechargeable secondary batteries, which have many advantages: high energy density, good safety performance, long charge and discharge cycle life, low self-discharge, etc. With the deepening of new energy technology research, the performance of lithium-ion batteries in all aspects has been improved, such as safety and cycle life, making them the first choice for power and energy storage batteries, and promoting and facilitating the rapid development of electric vehicles and intelligent energy storage industries. The explosive growth of the lithium-ion powered vehicle market has resulted in a large gap in electric vehicle power batteries and their core raw material products, an imbalance in supply and demand, and an inability to meet the needs of the new energy vehicle industry. The new olivine-type phosphate-based cathode material is superior to traditional layered structure cathode materials (such as lithium cobalt oxide, lithium nickel oxide, and ternary materials) in terms of safety performance and cycle life. Its representative material, lithium iron phosphate (LiFePO4), has been widely studied and confirmed by academia and industry, and is widely used in power and energy storage batteries and other fields. However, lithium iron phosphate 3.4V (vs. Li / Li + ) limits the improvement of battery energy density, so the development of lithium iron phosphate power battery market is limited. Compared with lithium iron phosphate, lithium manganese iron phosphate (LiMn x Fe 1-x Lithium iron phosphate (LIMnPO4) has a high potential of over -4.0 V and a nearly identical theoretical capacity. Under conditions of equal capacity utilization, the energy density of LFP batteries is approximately 15% higher than that of LFP batteries. Therefore, LFP is internationally recognized as a new generation of high-energy-density cathode material for power lithium-ion batteries.

[0004] However, lithium manganese iron phosphate also has its own problems, such as poor conductivity and cycle performance. For example, its poor conductivity leads to poor rate performance. The presence of Mn causes a significant gap between its John-Teller effect cycle performance and that of lithium iron phosphate materials, and its capacity is not fully utilized, which limits its further development. Summary of the Invention

[0005] The present invention aims to provide a positive electrode material for a lithium-ion battery. The present invention composites CoFeMgAl-LDHs with a carbon-coated lithium iron manganese phosphate material to inhibit the agglomeration and growth of lithium iron manganese phosphate material particles, reduce the diffusion path of lithium ions inside the positive electrode active material particles, increase the lithium ion diffusion coefficient, efficiently realize the deintercalation of lithium ions, and improve the rate performance of the material. After the composite, the contact between the active material and the electrolyte is reduced, side reactions with the electrolyte are avoided, the dissolution of Mn is reduced, the crystal structure distortion is reduced, and the cycle performance of the material is improved.

[0006] The positive electrode material for lithium ion batteries provided by the present invention is obtained by compounding carbon-coated lithium manganese iron phosphate and cobalt iron magnesium aluminum-hydrotalcite.

[0007] Specifically, the method for preparing the positive electrode material for lithium ion batteries of the present invention comprises the following steps:

[0008] S1. preparing carbon-coated lithium manganese iron phosphate;

[0009] S2, preparing a solution of cobalt-iron-magnesium-aluminum-hydrotalcite;

[0010] S3. Adding the carbon-coated lithium manganese iron phosphate to the cobalt iron magnesium aluminum-hydrotalcite solution, stirring and drying, and annealing to obtain the positive electrode material.

[0011] In step S1, the carbon-coated lithium manganese iron phosphate is prepared according to the following steps:

[0012] A lithium source, a manganese source, an iron source, and a phosphorus source are weighed according to a molar ratio of Li, Mn, Fe, and P of 1.005-1.02:0.6:0.4:1, and a carbon source is weighed. After adding deionized water, the mixture is subjected to coarse grinding, ultrafine grinding, spray drying, calcination, and air flow milling in sequence to obtain the carbon-coated lithium manganese iron phosphate;

[0013] The carbon content of the carbon-coated lithium manganese iron phosphate is 1% to 2%.

[0014] Wherein, the lithium source is one or a mixture of two or more of lithium carbonate, lithium acetate and lithium dihydrogen phosphate;

[0015] The manganese source is one or a mixture of two or more of manganese acetate, manganese oxalate and manganese carbonate;

[0016] The iron source is one or a mixture of two or more of ferrous acetate, ferrous oxalate and ferric phosphate;

[0017] The phosphorus source is one or a mixture of two or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate and iron phosphate.

[0018] Wherein, the carbon source is one of glucose, sucrose, fructose and starch, or a mixture of two or more thereof.

[0019] Preferably, the particle size of the coarse grinding is controlled to be 1~2um, and the particle size of the ultrafine grinding is 0.3~0.9um; the air inlet temperature of the spray drying is 200~280℃, and the air outlet temperature is 110~140℃; the calcination conditions are: under a protective atmosphere, a temperature of 700~800℃, and a time of 5~15h; the particle size D50 after the air flow crushing is 1~3um.

[0020] In the preparation method of the present invention, in step S2, the cobalt-iron-magnesium-aluminum-hydrotalcite solution is prepared according to the following steps:

[0021] Cobalt source, iron source, magnesium source, and aluminum source were weighed according to the molar ratio of Co, Fe, Mg, and Al elements of 2:1:2:1, and ultrasonically dissolved in deionized water to obtain a mixed solution. Sodium carbonate and sodium hydroxide were added to adjust the pH of the mixed solution to 9.5-10.5, and then poured into a crystallization kettle. Crystallization was carried out at a temperature of 130-170°C for 10-14 hours. After cooling, centrifugation was carried out, and the mixture was washed with water until neutral. Anhydrous ethanol was added to prepare a mixed solution with a solid content of 30-60%.

[0022] Wherein, the cobalt source is one or a mixture of cobalt nitrate and cobalt carbonate; the iron source is one or a mixture of ferric nitrate and ferric chloride; the magnesium source is one or a mixture of magnesium nitrate and magnesium chloride; the aluminum source is one or a mixture of aluminum nitrate and aluminum chloride;

[0023] The mass ratio of the sodium carbonate to the sodium hydroxide is 1-2:1.

[0024] In the preparation method of the present invention, in step S3, the mass ratio of the carbon-coated lithium manganese iron phosphate to the cobalt iron magnesium aluminum-hydrotalcite solution is 97-99:1;

[0025] In step S3, the drying temperature is 70-100° C. and the drying time is 6-10 hours;

[0026] The annealing temperature is 100-150° C., and the annealing time is 1-4 hours.

[0027] The present invention uses CoFeMgAl-LDHs and carbon-coated lithium manganese iron phosphate materials to composite, which can effectively inhibit the agglomeration and growth of lithium manganese iron phosphate material particles, significantly reduce the diffusion path of lithium ions inside the positive electrode active material particles, improve the lithium ion diffusion coefficient, and efficiently realize the deintercalation of lithium ions, thereby improving the rate performance of the material; after using CoFeMgAl-LDHs and carbon-coated lithium manganese iron phosphate materials to composite, the contact between the active material and the electrolyte can be reduced, side reactions with the electrolyte can be avoided, the dissolution of Mn can be reduced, the crystal structure distortion can be reduced, and the cycle performance of the material can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 These are SEM images of the products prepared in Example 3 of the present invention and Comparative Example 2.

[0029] Figure 2 0.2C charge-discharge curves of the products prepared in Example 3 and Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0030] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0031] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0032] Example 1: Preparation of lithium manganese iron phosphate positive electrode material

[0033] S1. Preparation of lithium manganese iron phosphate material: lithium carbonate, manganese acetate, ferrous acetate, and lithium dihydrogen phosphate were weighed according to the molar ratio of Li / Mn / Fe / P elements of 1.005:0.6:0.4:1. Glucose was further weighed and added to deionized water. The mixture was coarsely ground to 1 μm, ultrafinely ground to 0.3 μm, spray-dried at an air inlet temperature of 200°C and an air outlet temperature of 110°C, calcined at 700°C for 5 h under high-purity nitrogen, and airflow-pulverized to a particle size D50 of 1 μm to obtain carbon-coated lithium manganese iron phosphate with a carbon content of 1%.

[0034] S2. Preparation of CoFeMgAl-LDHs mixed solution: Cobalt nitrate, ferric nitrate, magnesium nitrate, and aluminum nitrate were weighed according to the molar ratio of Co / Fe / Mg / Al elements of 2:1:2:1, and ultrasonically dissolved in deionized water to obtain a mixed solution. Sodium carbonate and sodium hydroxide were added in a mass ratio of 1:1. After adjusting the pH of the mixed solution to 9.5, the solution was poured into a crystallization kettle and crystallized at 130°C for 10 hours. After cooling, the solution was centrifuged and washed with water until neutral. Anhydrous ethanol was added to prepare a CoFeMgAl-LDHs mixed solution with a solid content of 30%.

[0035] S3. Add the carbon-coated lithium manganese iron phosphate in S1 to the CoFeMgAl-LDHs mixed solution in S2, wherein the mass ratio of the carbon-coated lithium manganese iron phosphate to the CoFeMgAl-LDHs mixed solution is 97:1. After stirring, dry at 70°C for 6h, and anneal at 100°C for 1h to obtain the CoFeMgAl-LDHs composite carbon-coated lithium manganese iron phosphate material.

[0036] Example 2: Preparation of lithium manganese iron phosphate positive electrode material

[0037] S1. Preparation of lithium manganese iron phosphate material: lithium acetate, manganese oxalate, ferrous oxalate, and ammonium dihydrogen phosphate were weighed according to the molar ratio of Li / Mn / Fe / P elements of 1.007:0.6:0.4:1. Sucrose was further weighed and added to deionized water. The mixture was coarsely ground to 1.2 μm, ultrafinely ground to 0.5 μm, spray-dried at an air inlet temperature of 240°C and an air outlet temperature of 120°C, calcined at 720°C for 7 h under high-purity nitrogen, and airflow-pulverized to a particle size D50 of 1.2 μm to obtain carbon-coated lithium manganese iron phosphate with a carbon content of 1.3%.

[0038] S2. Preparation of CoFeMgAl-LDHs mixed solution: Cobalt carbonate, ferric chloride, magnesium chloride, and aluminum chloride were weighed according to the molar ratio of Co / Fe / Mg / Al elements of 2:1:2:1, and ultrasonically dissolved in deionized water to obtain a mixed solution. Sodium carbonate and sodium hydroxide were added in a mass ratio of 1.3:1. The pH of the mixed solution was adjusted to 9.8, poured into a crystallization kettle, crystallized at 140°C for 12 hours, cooled and centrifuged, washed with water to neutrality, and added with anhydrous ethanol to prepare a CoFeMgAl-LDHs mixed solution with a solid content of 40%.

[0039] S3. Add the carbon-coated lithium manganese iron phosphate in S1 to the CoFeMgAl-LDHs mixed solution in S2, wherein the mass ratio of the carbon-coated lithium manganese iron phosphate to the CoFeMgAl-LDHs mixed solution is 97.5:1. After stirring, dry at 75°C for 7h, and anneal at 110°C for 1.5h to obtain the CoFeMgAl-LDHs composite carbon-coated lithium manganese iron phosphate material.

[0040] Example 3: Preparation of lithium manganese iron phosphate positive electrode material

[0041] S1. Preparation of lithium manganese iron phosphate material: lithium carbonate, manganese acetate, iron phosphate, and lithium dihydrogen phosphate were weighed according to the molar ratio of Li / Mn / Fe / P elements of 1.01:0.6:0.4:1. Glucose was further weighed and added to deionized water. The mixture was coarsely ground to 1.5 μm, ultrafinely ground to 0.6 μm, spray-dried at an air inlet temperature of 260°C and an air outlet temperature of 120°C, calcined at 750°C for 10 h under high-purity nitrogen, and airflow-pulverized to a particle size D50 of 2 μm to obtain carbon-coated lithium manganese iron phosphate with a carbon content of 1.5%.

[0042] S2. Preparation of CoFeMgAl-LDHs mixed solution: Cobalt nitrate, ferric nitrate, magnesium chloride, and aluminum nitrate were weighed according to the molar ratio of Co / Fe / Mg / Al elements of 2:1:2:1, and ultrasonically dissolved in deionized water to obtain a mixed solution. Sodium carbonate and sodium hydroxide were added in a mass ratio of 1.5:1. After adjusting the pH of the mixed solution to 10, the solution was poured into a crystallization kettle and crystallized at 150°C for 12 hours. After cooling, the solution was centrifuged and washed with water until neutral. Anhydrous ethanol was added to prepare a CoFeMgAl-LDHs mixed solution with a solid content of 45%.

[0043] S3. Add the carbon-coated lithium manganese iron phosphate in S1 to the CoFeMgAl-LDHs mixed solution in S2, wherein the mass ratio of the carbon-coated lithium manganese iron phosphate to the CoFeMgAl-LDHs mixed solution is 98:1. After stirring, dry at 85°C for 8h, and anneal at 120°C for 2.5h to obtain the CoFeMgAl-LDHs composite carbon-coated lithium manganese iron phosphate material.

[0044] Example 4: Preparation of lithium manganese iron phosphate positive electrode material

[0045] S1. Preparation of lithium manganese iron phosphate material: lithium dihydrogen phosphate, manganese carbonate, iron phosphate, and iron phosphate were weighed according to the molar ratio of Li / Mn / Fe / P elements of 1.016:0.6:0.4:1. Starch was further weighed and added to deionized water. The mixture was coarsely ground to 2 μm, ultrafinely ground to 0.9 μm, spray-dried at an air inlet temperature of 260°C and an air outlet temperature of 135°C, calcined at 750°C for 10 h under high-purity nitrogen, and air flow-pulverized to a particle size D50 of 2.5 μm to obtain carbon-coated lithium manganese iron phosphate with a carbon content of 1.8%.

[0046] S2. Preparation of CoFeMgAl-LDHs mixed solution: Cobalt carbonate, ferric chloride, magnesium chloride, and aluminum chloride were weighed according to the molar ratio of Co / Fe / Mg / Al elements of 2:1:2:1, and ultrasonically dissolved in deionized water to obtain a mixed solution. Sodium carbonate and sodium hydroxide were added in a mass ratio of 1.8:1. After adjusting the pH of the mixed solution to 10.2, the solution was poured into a crystallization kettle and crystallized at 140°C for 10 h. After cooling, the solution was centrifuged and washed with water until neutral. Anhydrous ethanol was added to prepare a CoFeMgAl-LDHs mixed solution with a solid content of 55%.

[0047] S3. Add the carbon-coated lithium manganese iron phosphate in S1 to the CoFeMgAl-LDHs mixed solution in S2, wherein the mass ratio of the carbon-coated lithium manganese iron phosphate to the CoFeMgAl-LDHs mixed solution is 98.5:1. After stirring, dry at 95°C for 9 hours, and anneal at 140°C for 3 hours to obtain the CoFeMgAl-LDHs composite carbon-coated lithium manganese iron phosphate material.

[0048] Example 5: Preparation of lithium manganese iron phosphate positive electrode material

[0049] S1. Preparation of lithium manganese iron phosphate material: lithium dihydrogen phosphate, manganese carbonate, iron phosphate, and ammonium dihydrogen phosphate were weighed according to the molar ratio of Li / Mn / Fe / P elements of 1.018:0.6:0.4:1. Starch was further weighed and added to deionized water. The mixture was coarsely ground to 2 μm, ultrafinely ground to 0.3 μm, spray-dried at an air inlet temperature of 270°C and an air outlet temperature of 130°C, calcined at 780°C for 12 h under high-purity nitrogen, and airflow-pulverized to a particle size D50 of 2.5 μm to obtain carbon-coated lithium manganese iron phosphate with a carbon content of 1.8%.

[0050] S2. Preparation of CoFeMgAl-LDHs mixed solution: Cobalt carbonate, ferric chloride, magnesium chloride, and aluminum chloride were weighed according to the molar ratio of Co / Fe / Mg / Al elements of 2:1:2:1, and ultrasonically dissolved in deionized water to obtain a mixed solution. Sodium carbonate and sodium hydroxide were added in a mass ratio of 1.8:1. After adjusting the pH of the mixed solution to 9.8, the solution was poured into a crystallization kettle and crystallized at 160°C for 13 hours. After cooling, the solution was centrifuged and washed with water until neutral. Anhydrous ethanol was added to prepare a CoFeMgAl-LDHs mixed solution with a solid content of 55%.

[0051] S3. Add the carbon-coated lithium manganese iron phosphate in S1 to the CoFeMgAl-LDHs mixed solution in S2, wherein the mass ratio of the carbon-coated lithium manganese iron phosphate to the CoFeMgAl-LDHs mixed solution is 98.8:1. After stirring, dry at 85°C for 9 hours, and anneal at 120°C for 3 hours to obtain the CoFeMgAl-LDHs composite carbon-coated lithium manganese iron phosphate material.

[0052] Example 6: Preparation of lithium manganese iron phosphate positive electrode material

[0053] S1. Preparation of lithium manganese iron phosphate material: lithium dihydrogen phosphate, manganese carbonate, iron phosphate, and ammonium dihydrogen phosphate were weighed according to the molar ratio of Li / Mn / Fe / P elements of 1.02:0.6:0.4:1. Sucrose was further weighed and added to deionized water. The mixture was coarsely ground to 2 μm, ultrafinely ground to 0.9 μm, spray-dried at an air inlet temperature of 280°C and an air outlet temperature of 140°C, calcined at 800°C for 15 h under high-purity nitrogen, and airflow-pulverized to a particle size D50 of 3 μm to obtain carbon-coated lithium manganese iron phosphate with a carbon content of 2%.

[0054] S2. Preparation of CoFeMgAl-LDHs mixed solution: Cobalt carbonate, ferric chloride, and aluminum chloride were weighed according to the molar ratio of Co / Fe / Mg / Al elements of 2:1:2:1, and ultrasonically dissolved in deionized water to obtain a mixed solution. Sodium carbonate and sodium hydroxide were added in a mass ratio of 2:1. The pH of the mixed solution was adjusted to 10.5, poured into a crystallization kettle, and crystallized at 170°C for 14 h. After cooling, centrifugation was performed, and the mixture was washed with water until neutral. Anhydrous ethanol was added to prepare a CoFeMgAl-LDHs mixed solution with a solid content of 60%.

[0055] S3. Add the carbon-coated lithium manganese iron phosphate in S1 to the CoFeMgAl-LDHs mixed solution in S2, wherein the mass ratio of the carbon-coated lithium manganese iron phosphate to the CoFeMgAl-LDHs mixed solution is 99:1. After stirring, dry at 100°C for 10 hours, and anneal at 150°C for 4 hours to obtain the CoFeMgAl-LDHs composite carbon-coated lithium manganese iron phosphate material.

[0056] Comparative Example 1

[0057] The only difference from Example 3 is that the pure carbon-coated lithium manganese iron phosphate is used, wherein:

[0058] Preparation of lithium manganese iron phosphate material: lithium carbonate, manganese acetate, iron phosphate, and lithium dihydrogen phosphate were weighed according to the molar ratio of Li / Mn / Fe / P elements of 1.01:0.6:0.4:1. Glucose was further weighed and added to deionized water. The mixture was coarsely ground to 1.5um, ultrafinely ground to 0.6um, spray-dried at an air inlet temperature of 260°C and an air outlet temperature of 120°C, calcined at 750°C for 10h under high-purity nitrogen, and airflow-pulverized to a particle size D50 of 2um to obtain carbon-coated lithium manganese iron phosphate with a carbon content of 1.5%.

[0059] Comparative Example 2

[0060] The difference from Example 3 is that CoFeMgAl-LDHs is not added, wherein:

[0061] S1. Preparation of lithium manganese iron phosphate material: lithium carbonate, manganese acetate, iron phosphate, and lithium dihydrogen phosphate were weighed according to the molar ratio of Li / Mn / Fe / P elements of 1.01:0.6:0.4:1. Glucose was further weighed and added to deionized water. The mixture was coarsely ground to 1.5 μm, ultrafinely ground to 0.6 μm, spray-dried at an air inlet temperature of 260°C and an air outlet temperature of 120°C, calcined at 750°C for 10 h under high-purity nitrogen, and airflow-pulverized to a particle size D50 of 2 μm to obtain carbon-coated lithium manganese iron phosphate with a carbon content of 1.5%.

[0062] S3. Carbon-coated lithium manganese iron phosphate: The same mass of anhydrous ethanol as that in Example 3 was stirred, dried at 85° C. for 8 h, and annealed at 120° C. for 2.5 h to obtain a carbon-coated lithium manganese iron phosphate material.

[0063] Figure 1 From the SEM images of the materials prepared in Example 3 and Comparative Example 2, it can be seen that the primary particles of the coated samples are smaller, indicating that the modification process of the present invention effectively inhibits the agglomeration and growth of the lithium manganese iron phosphate material particles, significantly reduces the diffusion path of lithium ions in the positive electrode active material particles, and improves the performance of the material.

[0064] Comparative Example 3

[0065] The difference from Example 3 is that more CoFeMgAl-LDHs are added, wherein:

[0066] S1 and S2 are the same as in Example 3,

[0067] S3. Add the carbon-coated lithium manganese iron phosphate in S1 to the CoFeMgAl-LDHs mixed solution in S2, wherein the mass ratio of the carbon-coated lithium manganese iron phosphate to the CoFeMgAl-LDHs mixed solution is 96:1. After stirring, dry at 85°C for 8h, and anneal at 120°C for 2.5h to obtain the CoFeMgAl-LDHs composite carbon-coated lithium manganese iron phosphate material.

[0068] Comparative Example 4

[0069] The difference from Example 3 is that less CoFeMgAl-LDHs is added, and the characteristics are:

[0070] S1 and S2 are the same as in Example 3,

[0071] S3. Add the carbon-coated lithium manganese iron phosphate in S1 to the CoFeMgAl-LDHs mixed solution in S2, wherein the mass ratio of the carbon-coated lithium manganese iron phosphate to the CoFeMgAl-LDHs mixed solution is 99.5:1. After stirring, dry at 85°C for 8h, and anneal at 120°C for 2.5h to obtain the CoFeMgAl-LDHs composite carbon-coated lithium manganese iron phosphate material.

[0072] A simulated battery was assembled using the lithium iron manganese phosphate material positive electrode sheet obtained in Examples 1 to 6 and Comparative Examples 1 to 4 above, a battery-grade lithium sheet as the negative electrode material, and lithium hexafluorophosphate as the main component as the electrolyte. The charge and discharge window was 2.5-4.45V, and relevant performance tests were performed. The initial charge and discharge capacity was tested at 0.2C, the discharge capacity was tested at 1C in turn, and the capacity retention rate after 200 cycles under the condition of 1C charge and discharge was tested. In addition, the Mn dissolution test method was to disassemble the 200-cycle simulated battery and test the Mn content of the material measured at the diaphragm negative electrode by inductively coupled plasma spectrometry (ICP). The test results are listed in Table 1.

[0073] Figure 2 0.2C charge-discharge curves of the products prepared in Example 3 (left figure) of the present invention and Comparative Example 2 (right figure).

[0074] From the data, the CoFeMgAl-LDHs composite carbon-coated lithium manganese iron phosphate material, especially in Example 3, the 0.2C discharge specific capacity is 158.96 mAh / g, the 1C discharge specific capacity is 154.14 mAh / g, the 200-week capacity retention rate is 95.76%, and the Mn dissolution is 7.7 ppm, which is significantly better than Comparative Examples 1-2. Comparative Example 3 uses too much CoFeMgAl-LDHs and carbon-coated lithium manganese iron phosphate material. Although it can inhibit the dissolution of Mn, the electrochemical performance is significantly worse than that of Comparative Example 3. Comparative Example 4 uses less CoFeMgAl-LDHs and carbon-coated lithium manganese iron phosphate material, the Mn dissolution is improved, and the electrochemical performance is also poor. It is shown that the composite of CoFeMgAl-LDHs and carbon-coated lithium manganese iron phosphate material inhibits the agglomeration and growth of lithium manganese iron phosphate material particles, reduces the diffusion path of lithium ions inside the positive electrode active material particles, improves the lithium ion diffusion coefficient, efficiently realizes the deintercalation of lithium ions, and improves the rate performance of the material; after the composite, the contact between the active material and the electrolyte is reduced, side reactions with the electrolyte are avoided, the dissolution of Mn is reduced, the crystal structure distortion is reduced, and the cycle performance of the material is improved.

[0075] Table 1 Comparison of test data between the embodiment and the comparative example

[0076]

[0077] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes based on the technical solution and invention of the present invention within the technical scope of the present invention, and they should be covered by the scope of protection of the present invention.

Claims

1. A method for preparing a positive electrode material for a lithium ion battery, comprising the following steps: S1. preparing carbon-coated lithium manganese iron phosphate; S2, preparing a solution of cobalt-iron-magnesium-aluminum-hydrotalcite; S3, adding the carbon-coated lithium manganese iron phosphate to the cobalt iron magnesium aluminum-hydrotalcite solution, stirring and drying, and annealing to obtain the positive electrode material; The mass ratio of the carbon-coated lithium manganese iron phosphate to the cobalt iron magnesium aluminum-hydrotalcite solution is 97-99:1; The annealing temperature is 100-150° C., and the annealing time is 1-4 hours.

2. The preparation method according to claim 1, wherein: In step S1, the carbon-coated lithium manganese iron phosphate is prepared according to the following steps: A lithium source, a manganese source, an iron source, and a phosphorus source are weighed according to a molar ratio of Li, Mn, Fe, and P of 1.005-1.02:0.6:0.4:1, and a carbon source is weighed. After adding deionized water, the mixture is subjected to coarse grinding, ultrafine grinding, spray drying, calcination, and air flow milling in sequence to obtain the carbon-coated lithium manganese iron phosphate; The carbon content of the carbon-coated lithium manganese iron phosphate is 1% to 2%.

3. The preparation method according to claim 2, wherein: The lithium source is one or a mixture of two or more of lithium carbonate, lithium acetate and lithium dihydrogen phosphate; The manganese source is one or a mixture of two or more of manganese acetate, manganese oxalate and manganese carbonate; The iron source is one or a mixture of two or more of ferrous acetate, ferrous oxalate and ferric phosphate; The phosphorus source is one or a mixture of two or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate and iron phosphate.

4. The preparation method according to claim 2 or 3, characterized in that: The carbon source is one of glucose, sucrose, fructose and starch, or a mixture of two or more thereof.

5. The preparation method according to claim 2 or 3, characterized in that: The particle size of the coarse grinding is controlled to be 1-2 μm, and the particle size of the ultrafine grinding is 0.3-0.9 μm; the air inlet temperature of the spray drying is 200-280°C, and the air outlet temperature is 110-140°C; the calcination conditions are: under a protective atmosphere, a temperature of 700-800°C, and a time of 5-15 hours; the particle size D50 after the air flow milling is 1-3 μm.

6. The preparation method according to claim 1 or 2, characterized in that: In step S2, the cobalt-iron-magnesium-aluminum-hydrotalcite solution is prepared according to the following steps: Cobalt source, iron source, magnesium source, and aluminum source were weighed according to the molar ratio of Co, Fe, Mg, and Al elements of 2:1:2:1, and ultrasonically dissolved in deionized water to obtain a mixed solution. Sodium carbonate and sodium hydroxide were added to adjust the pH of the mixed solution to 9.5-10.5, and then poured into a crystallization kettle. Crystallization was carried out at a temperature of 130-170°C for 10-14 hours. After cooling, centrifugation was carried out, and the mixture was washed with water until neutral. Anhydrous ethanol was added to prepare a mixed solution with a solid content of 30-60%.

7. The preparation method according to claim 6, characterized in that: The cobalt source is one or a mixture of cobalt nitrate and cobalt carbonate; the iron source is one or a mixture of ferric nitrate and ferric chloride; the magnesium source is one or a mixture of magnesium nitrate and magnesium chloride; the aluminum source is one or a mixture of aluminum nitrate and aluminum chloride; The mass ratio of the sodium carbonate to the sodium hydroxide is 1-2:

1.

8. The preparation method according to claim 1 or 2, characterized in that: In step S3, the drying temperature is 70-100° C. and the drying time is 6-10 hours.

9. A positive electrode material prepared by the method according to any one of claims 1 to 8.

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