Preparation method of lithium manganese iron phosphate positive electrode material and lithium manganese iron phosphate positive electrode material

Abstract

The preparation method comprises the following steps: (1) mixing a lithium source, a manganese source, an iron source, a phosphorus source, a first carbon source, a magnesium source and a boron source, sanding, drying, and carrying out first calcination to obtain a semi-finished product; based on the total mass of the lithium source, the manganese source, the iron source, the phosphorus source, the magnesium source and the boron source, the adding proportion of the first carbon source is 9-20wt%, and the first calcining temperature is 350-600 DEG C; and (2) mixing the semi-finished product and a second carbon source, sanding, drying, and carrying out second calcination to obtain the lithium manganese iron phosphate positive electrode material, the addition proportion of the second carbon source is 2-9wt% based on the mass of the semi-finished product, the second carbon source comprises graphene, carbon nanotubes and a dispersant, and the second calcination is 600-800 DEG C. According to the prepared lithium manganese iron phosphate, the resistivity can be reduced to about 10 omega.cm, the lowest resistivity can reach 6.13 omega.cm, the highest specific discharge capacity can be higher than 155 mAh / g, and the conductivity and the specific capacity are both considered.

Description

Technical Field

[0001] The invention relates to a method for preparing a battery electrode, in particular to a method for preparing a lithium iron manganese phosphate positive electrode material, and also to the lithium iron manganese phosphate positive electrode material prepared by the preparation method. Background Art

[0002] As one of the positive electrode materials for lithium-ion batteries, lithium iron phosphate has the advantages of high energy density, good safety, and low cost. It is considered to be an ideal upgrade of lithium iron phosphate. However, the conductivity of lithium iron phosphate is + The low diffusion coefficient affects the overall performance and charge and discharge rate of the battery, limiting its commercial application.

[0003] The invention patent application with publication number CN 119118086 A discloses a manganese phosphate-based positive electrode material (Mn:Fe=6:4) and its preparation method and application. The lithium manganese iron phosphate material is synthesized by secondary sintering combined with batch addition of carbon. A low proportion of carbon is added during the first sintering, and a higher proportion of carbon is added during the second sintering to reduce the resistivity of the synthesized manganese iron phosphate product. The resistivity of the finished product is lower than 200Ω·cm and can be as low as 51.1Ω·cm. However, when the resistivity of the finished product is reduced, the first discharge specific capacity is also lower. The invention patent application with publication number CN 119503756 A discloses a method for preparing lithium manganese iron phosphate and its application. Similar to CN 119118086 A, this solution also adopts secondary sintering with batch addition of various types of carbon. The difference is that Mn:Fe≈4:6 in the finished product, the types of carbon used in the primary sintering are increased, and the amount of carbon used in the primary sintering is more than that in the secondary sintering. This solution can further reduce the resistivity of lithium manganese iron phosphate to below 10Ω·cm, but similarly, there is also the problem of low discharge specific capacity. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a method for preparing a lithium iron manganese phosphate positive electrode material with low resistivity and high first discharge specific capacity. Another purpose of the present invention is to provide a lithium iron manganese phosphate positive electrode material.

[0005] Technical solution: The method for preparing a lithium manganese iron phosphate positive electrode material according to the present invention comprises the following steps:

[0006] (1) mixing a lithium source, a manganese source, an iron source, a phosphorus source, a first carbon source, a magnesium source, and a boron source, sand grinding, drying, and performing a first calcination to obtain a semi-finished product; based on the total weight of the lithium source, the manganese source, the iron source, the phosphorus source, the magnesium source, and the boron source, the addition ratio of the first carbon source is 9 to 20 wt %, and the first calcination temperature is 350 to 600° C.;

[0007] (2) Mix the semi-finished product and the second carbon source, grind them with sand, dry them, and conduct the second calcination to obtain the lithium iron manganese phosphate cathode material. Based on the mass of the semi-finished product, the addition ratio of the second carbon source is 2-9 wt%, and the second carbon source includes graphene, carbon nanotubes, and a dispersant. The second calcination is carried out at 600-800 °C.

[0008] Preferably, in step (1), the molar ratio of the lithium source, manganese source, iron source, phosphorus source, magnesium source, and boron source is: 0.99-1.05: 0.1-0.9: 0.1-0.9: 1: 0.01-0.05: 0.02-0.1.

[0009] Preferably, in step (1), the molar ratio of the lithium source, manganese source, iron source, phosphorus source, magnesium source, and boron source is: 0.99-1.05: 0.42-0.8: 0.2-0.58: 1: 0.01-0.05: 0.02-0.1.

[0010] More preferably, the molar ratio of the lithium source, manganese source, iron source, phosphorus source, magnesium source, and boron source is: 0.99-1.05: 0.45-0.6: 0.4-0.55: 1: 0.01-0.03: 0.02-0.06.

[0011] Preferably, in step (1), the addition ratio of the first carbon source is 9-15 wt%, and the first carbon source is one of glucose, sucrose, fructose, lactose, citric acid, starch, ethylene glycol, polyvinylpyrrolidone, methanol, ethanol, glycine, aspartic acid, tartaric acid, β-cyclodextrin, graphene, and carbon nanotubes.

[0012] Preferably, in step (1), the first calcination temperature is 400-580 °C, and the calcination time is 4-7 hours.

[0013] Preferably, in step (2), the second calcination temperature is 650-770 °C, and the calcination time is 9-15 hours.

[0014] Preferably, in step (2), the addition ratio of graphene is 2-8‰; the addition ratio of carbon nanotubes is 0.5-3.5‰, and the addition ratio of the dispersant is 1-8%; the addition ratio of carbon nanotubes does not exceed that of graphene.

[0015] Preferably, in step (2), the addition ratio of graphene is 3-7‰; the addition ratio of carbon nanotubes is 0.8-2‰, and the addition ratio of the dispersant is 2-7%.

[0016] Preferably, the lithium source is one or more of lithium carbonate, lithium phosphate, lithium sulfate, lithium chloride, lithium hydroxide, lithium dihydrogen phosphate, lithium hydrogen phosphate, lithium nitrate, lithium oxalate, and lithium acetate; the manganese source is one or more of manganese tetroxide, manganese carbonate, manganese acetate, manganese sesquioxide, and manganese oxide; the iron source is one or more of iron phosphate, iron sesquioxide, iron acetate, iron sulfate, and iron nitrate; the phosphorus source is one or more of phosphoric acid, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, lithium phosphate, iron phosphate, and manganese phosphate; the magnesium source is one or more of magnesium metaborate, magnesium borate, magnesium boride, magnesium oxide, magnesium nitrate, magnesium chloride, magnesium carbonate, magnesium sulfate, magnesium hydroxide, magnesium acetate, magnesium phosphate, and magnesium sulfide; the boron source is one or more of magnesium metaborate, magnesium borate, magnesium boride, boric acid, and sodium borate; the dispersant is one or more of polyethylene glycol, polyvinyl alcohol, polyacrylic acid, sodium dodecylbenzenesulfonate, polyethyleneimine, polycarbonate, and polyethylene ether, wherein the polyethylene glycol can be PEG2000, PEG4000, or PEG6000.

[0017] The lithium iron manganese phosphate cathode material prepared by the method for preparing a lithium iron manganese phosphate cathode material as described above.

[0018] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages: 1. Low resistivity and high initial discharge specific capacity: The resistivity is reduced to about 10 Ω·cm, and the lowest can reach 6.13 Ω·cm. The highest discharge specific capacity can be higher than 160 mAh / g, taking into account both conductivity and specific capacity; 2. Effectively inhibit manganese dissolution: Graphene and carbon nanotubes are introduced simultaneously to form a three-dimensional conductive network with a secondary coating outside the lithium iron manganese phosphate. The three-dimensional conductive network has a certain deformation margin and improves the coating compactness, effectively inhibiting manganese dissolution during charge and discharge, improving the electrode cycle stability, and ensuring safety; 3. Wide application range: When the manganese ratio in the lithium iron manganese phosphate cathode material increases to more than 0.7, the resistivity of the lithium iron manganese phosphate cathode material can be reduced to 40 Ω·cm, while maintaining the initial discharge specific capacity of not less than 150 mAh / g, greatly expanding the applicable range of the process scheme; 4. High tap density, which helps to improve the energy density. Description of the Drawings

[0019] Figure 1 The SEM image at a low magnification of the lithium iron manganese phosphate cathode material prepared in the first embodiment of the present invention;

[0020] Figure 2 The SEM image at a high magnification of the lithium iron manganese phosphate cathode material prepared in the first embodiment of the present invention;

[0021] Figure 3 The charge-discharge curve at 0.1C of the lithium iron manganese phosphate cathode material prepared in the first embodiment of the present invention;

[0022] Figure 4SEM image of the lithium iron manganese phosphate cathode material prepared in the first comparative example of the present invention;

[0023] Figure 5 XRD pattern of the lithium iron manganese phosphate cathode material prepared in the first embodiment of the present invention. Detailed implementation manners

[0024] The technical solutions of the present invention will be further described below with reference to the accompanying drawings.

[0025] Example 1 The lithium iron manganese phosphate cathode material prepared in this example has the molecular formula CNT / RGO-Li 1.03 Mn 0.58 Fe 0.4 Mg 0.02 PO4@B, and the preparation method is as follows:

[0026] (1) Mix 3820.62 g of Li2CO3, 6734.44 g of MnCO3, 6180.2 g of FePO4, 6930.6 g of H3PO4, and 270.6 g of Mg(BO2)2·H2O, and then sequentially add 2390 g of glucose and 40000 g of pure water and grind in a sand mill for 4 h to obtain a slurry;

[0027] (2) Spray the slurry obtained in step (1) by two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 5.0 m 3 / h, heat it to 500 °C at a rate of 5 °C / min in a nitrogen atmosphere and calcine for 5 h, cool it to 25 °C and take it out to obtain a semi-finished product;

[0028] (3) Transfer 6‰ of the graphene solution by the mass of the finished product, 1.5‰ of the carbon nanotube solution by the mass of the finished product, 2% of the polyethylene glycol solution by the mass of the finished product, 3 times the mass of the semi-finished product of water and the semi-finished product obtained in step (2) into a sand mill for grinding for 2 h to obtain a slurry;

[0029] (4) Spray the slurry obtained in step (3) by two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 3.0 m 3 / h, heat it to 765 °C at a rate of 2 °C / min in a nitrogen atmosphere and calcine for 10 h, and naturally cool it to room temperature, and obtain the lithium iron manganese phosphate cathode material CNT / RGO-Li 1.03 Mn 0.58 Fe 0.4 Mg 0.02 PO4@B, and the particle size of this material is 50 - 300 nm.

[0030] From Figure 5 , the XRD pattern shows the characteristic peaks of lithium iron manganese phosphate, proving that the material synthesized in this example is lithium iron manganese phosphate; From Figure 1 ,Figure 2 It can be seen that the lithium iron manganese phosphate particles are evenly distributed, with particle sizes ranging from 50 to 300 nm, showing obvious nanoparticle characteristics. There is a thin layer of graphene and linear carbon nanotubes on the particle surface, indicating good material composite degree.

[0031] Figure 3 This is the charge-discharge curve of the material obtained in this example at a current of 0.1C, and it can be found that the curve fully conforms to the charge-discharge characteristics of lithium iron manganese phosphate.

[0032] Example 2 The lithium iron manganese phosphate cathode material prepared in this example has the molecular formula CNT / RGO-Li 1.03 Mn 0.48 Fe 0.5 Mg 0.02 PO4@B, and the preparation method is as follows:

[0033] (1) Mix 3820.62 g of Li2CO3, 5580.1 g of MnCO3, 7730.2 g of FePO4, 5770.6 g of H3PO4, and 270.6 g of Mg(BO2)2·H2O, then sequentially add 2815 g of sucrose and 43000 g of pure water, and grind in a sand mill for 5 h to take out the slurry;

[0034] (2) For the slurry obtained in step (1), perform two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 4.0 m 3 / h, heat it to 400 °C at a rate of 3 °C / min in a nitrogen atmosphere and calcine for 6 h, cool it to 25 °C and take it out to obtain a semi-finished product;

[0035] (3) Transfer 5‰ of the graphene solution by the mass of the finished product, 1‰ of the carbon nanotube solution by the mass of the finished product, 3% of the polyethylene glycol solution by the mass of the finished product, 3 times the mass of the semi-finished product of water, and the semi-finished product obtained in step (2) into a sand mill for grinding for 2.5 h to take out the slurry;

[0036] (4) For the slurry obtained in step (3), perform two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 3.0 m 3 / h, heat it to 720 °C at a rate of 2.5 °C / min in a nitrogen atmosphere and calcine for 10 h, and naturally cool it to room temperature, and obtain the lithium iron manganese phosphate cathode material CNT / RGO-Li 1.03 Mn 0.48 Fe 0.5 Mg0.02PO4@B, and the particle size of this material is 50 - 300 nm.

[0037] Example 3 The lithium iron manganese phosphate cathode material prepared in this example has the molecular formula CNT / RGO-Li 1.03 Mn 0.53 Fe 0.45Mg 0.02 PO4@B, The preparation method is as follows:

[0038] (1) Mix 3820.62 g of Li2CO3, 6155.7 g of MnCO3, 6955.1 g of FePO4, 6350 g of H3PO4, and 270.6 g of Mg(BO2)2·H2O. Then, successively add 3060 g of sucrose and 45000 g of pure water, and grind in a sand mill for 4.5 h. Take out the slurry;

[0039] (2) For the slurry obtained in step (1), perform two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 5.0 m 3 / h. Heat it to 430 °C at a rate of 4 °C / min under a nitrogen atmosphere and calcine for 6 h. Cool it to 25 °C and take it out to obtain a semi-finished product;

[0040] (3) Transfer a graphene solution of 4‰ of the finished product quality, a carbon nanotube solution of 1.5‰ of the finished product quality, a polyethylene glycol solution of 3% of the finished product quality, water of 3.5 times the semi-finished product quality, and the semi-finished product obtained in step (2) into a sand mill for grinding for 2.5 h. Take out the slurry;

[0041] (4) For the slurry obtained in step (3), perform two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 3.0 m3 / h. Heat it to 695 °C at a rate of 2.5 °C / min under a nitrogen atmosphere and calcine for 9 h. Naturally cool it to room temperature and crush it to obtain the lithium manganese iron phosphate cathode material CNT / RGO-Li 1.03 Mn 0.53 Fe 0.45 Mg 0.02 PO4@B, and the particle size of this material is 70 - 300 nm.

[0042] Example 4 The lithium manganese iron phosphate cathode material prepared in this example has the molecular formula CNT / RGO-Li 1.02 Mn 0.63 Fe 0.35 Mg 0.02 PO4@B, The preparation method is as follows:

[0043] (1) Mix 3785.2 g of Li2CO3, 7320 g of MnCO3, 5406.2 g of FePO4, 7502 g of H3PO4, and 270.6 g of Mg(BO2)2·H2O. Then, successively add 2702 g of sucrose and 38000 g of pure water, and grind in a sand mill for 5 h. Take out the slurry;

[0044] (2) For the slurry obtained in step (1), perform two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 5.0 m 3Perform two-fluid spraying at 3 °C / min under a nitrogen atmosphere and heat to 510 °C for 4 h, then cool to 25 °C and take out to obtain semi-finished products.

[0045] (3) Transfer 3‰ of the graphene solution by the mass of the finished product, 1‰ of the carbon nanotube solution by the mass of the finished product, 3% of the polyethylene glycol solution by the mass of the finished product, 2.5 times the mass of the semi-finished product of water, and the semi-finished product obtained in step (2) into a sand mill for sanding for 2 h, and take out the slurry.

[0046] (4) For the slurry obtained in step (3), perform two-fluid spraying at a feeding frequency of 10 Hz and an air intake frequency of 3.0 m 3 / h, heat to 670 °C at 4 °C / min under a nitrogen atmosphere for 10 h, cool naturally to room temperature, and obtain the lithium iron manganese phosphate cathode material CNT / RGO-Li 1.02 Mn 0.63 Fe 0.35 Mg 0.02 PO4@B, and the particle size of this material is 50 - 300 nm.

[0047] Example 5 The lithium iron manganese phosphate cathode material prepared in this example has the molecular formula CNT / RGO-Li 1.02 Mn 0.68 Fe 0.3 Mg 0.02 PO4@B, and the preparation method is as follows:

[0048] (1) Mix 3785.2 g of Li2CO3, 7900.42 g of MnCO3, 4636.4 g of FePO4, 8075 g of H3PO4, and 270.6 g of Mg(BO2)2·H2O, and then add 3451 g of starch and 45000 g of pure water in sequence to a sand mill for sanding for 6 h, and take out the slurry.

[0049] (2) For the slurry obtained in step (1), perform two-fluid spraying at a feeding frequency of 10 Hz and an air intake frequency of 5.0 m 3 / h, heat to 580 °C at 3 °C / min under a nitrogen atmosphere for 5 h, cool to 25 °C and take out to obtain semi-finished products.

[0050] (3) Transfer 3.5‰ of the graphene solution by the mass of the finished product, 1.5‰ of the carbon nanotube solution by the mass of the finished product, 3% of the polyethylene glycol solution by the mass of the finished product, 3.5 times the mass of the semi-finished product of water, and the semi-finished product obtained in step (2) into a sand mill for sanding for 3 h, and take out the slurry.

[0051] (4) For the slurry obtained in step (3), perform two-fluid spraying at a feeding frequency of 10 Hz and an air intake frequency of 3.0 m 3Perform two-fluid spraying at 2 °C / min heating rate to 650 °C and calcine for 12 h under a nitrogen atmosphere, then naturally cool to room temperature and obtain the lithium iron manganese phosphate cathode material CNT / RGO-Li after crushing. 1.02 Mn 0.68 Fe 0.3 Mg 0.02 PO4@B, and the particle size of this material is 70 - 250 nm.

[0052] Example 6: The lithium iron manganese phosphate cathode material prepared in this example has the molecular formula CNT / RGO-Li 1.01 Mn 0.73 Fe 0.25 Mg 0.02 PO4@B, and the preparation method is as follows:

[0053] (1) Mix 3745.1 g of Li2CO3, 8480.1 g of MnCO3, 3865.4 g of FePO4, 8655 g of H3PO4, and 270.6 g of Mg(BO2)2·H2O, then successively add 3750 g of fructose and 48000 g of pure water and grind in a sand mill for 6.5 h, and take out the slurry.

[0054] (2) For the slurry obtained in step (1), perform two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 5.0 m 3 / h, heat to 450 °C at a rate of 2 °C / min under a nitrogen atmosphere, calcine for 7 h, cool to 25 °C and take out to obtain a semi-finished product.

[0055] (3) Transfer the graphene solution at 5.5‰ of the finished product quality, the carbon nanotube solution at 2.5‰ of the finished product quality, the polyethylene glycol solution at 5% of the finished product quality, 4 times the mass of the semi-finished product of water and the semi-finished product obtained in step (2) into a sand mill for grinding for 6 h, and take out the slurry.

[0056] (4) For the slurry obtained in step (3), perform two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 3.0 m 3 / h, heat to 670 °C at a rate of 2.5 °C / min under a nitrogen atmosphere, calcine for 13 h, naturally cool to room temperature, and obtain the lithium iron manganese phosphate cathode material CNT / RGO-Li 1.01 Mn 0.73 Fe 0.25 Mg 0.02 PO4@B, and the particle size of this material is 70 - 300 nm.

[0057] Example 7: The lithium iron manganese phosphate cathode material prepared in this example has the molecular formula CNT / RGO-Li 1.01 Mn 0.78 Fe 0.2 Mg0.02 PO4@B, the preparation method is as follows:

[0058] (1) Mix 3745.1 g of Li2CO3, 9060.7 g of MnCO3, 3100.3 g of FePO4, 9235 g of H3PO4, and 270.6 g of Mg(BO2)2·H2O, then successively add 3046 g of lactose and 38000 g of pure water, grind in a sand mill for 7 h, and take out the slurry;

[0059] (2) For the slurry obtained in step (1), perform two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 5.0 m 3 / h, heat up to 480 °C at a rate of 2 °C / min in a nitrogen atmosphere, calcine for 6 h, cool down to 25 °C and take out to obtain a semi-finished product;

[0060] (3) Transfer a graphene solution of 7‰ of the finished product quality, a carbon nanotube solution of 3‰ of the finished product quality, a polyethylene glycol solution of 7% of the finished product quality, water with a mass 4.5 times that of the semi-finished product, and the semi-finished product obtained in step (2) into a sand mill for grinding for 7 h, and take out the slurry;

[0061] (4) For the slurry obtained in step (3), perform two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 3.0 m 3 / h, heat up to 650 °C at a rate of 2.5 °C / min in a nitrogen atmosphere, calcine for 15 h, naturally cool to room temperature, and obtain the lithium iron manganese phosphate cathode material CNT / RGO-Li 1.01 Mn 0.78 Fe 0.2 Mg 0.02 PO4@B, and the particle size of this material is 70 - 250 nm.

[0062] Comparative Example 1 The lithium iron manganese phosphate cathode material prepared in this comparative example has the molecular formula Li 1.03 Mn 0.6 Fe 0.4 PO4, and the preparation method is as follows:

[0063] (1) Mix 3820.62 g of Li2CO3, 6970.6 g of MnCO3, 6180.2 g of FePO4, and 6930.6 g of H3PO4, then successively add 2450 g of glucose and 40000 g of pure water, grind in a sand mill for 3 h, and take out the slurry;

[0064] (2) For the slurry obtained in step (1), perform two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 5.0 m 3 / h, heat up to 500 °C at a rate of 5 °C / min in a nitrogen atmosphere, calcine for 5 h, cool down to 25 °C and take out to obtain a semi-finished product;

[0065] (3) Transfer a 2% polyethylene glycol solution of the finished product quality, 1.5 times the semi-finished product quality of water, and the semi-finished product obtained in step (2) into a sand mill for sanding for 1 h, and take out the slurry;

[0066] (4) Spray the slurry obtained in step (3) by two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 3.0 m3 / h, heat it to 765 °C at a rate of 2 °C / min in a nitrogen atmosphere and calcine for 10 h, naturally cool to room temperature, and obtain the lithium iron manganese phosphate cathode material Li 1.03 Mn 0.6 Fe 0.4 PO4. The particle size of this material is 50 - 300 nm.

[0067] Figure 4 This is the SEM image of the cathode material prepared in this comparative example. It can be seen from the figure that the morphology of the material is irregular blocky and rod-like particles, and the particle distribution is uneven.

[0068] Comparative Example 2 The lithium iron manganese phosphate cathode material prepared in this comparative example has the molecular formula CNT / RGO-Li 1.03 Mn 0.58 Fe 0.4 Mg 0.02 PO4, and the preparation method is as follows:

[0069] (1) Mix 2495.7 g of LiOH, 4550.62 g of Mn3O4, 3262.2 g of Fe2O3, 11530.6 g of H3PO4, and 82.3 g of MgO, and then successively add 2410 g of glucose and 42000 g of pure water and sand in a sand mill for 7 h, and take out the slurry;

[0070] (2) Spray the slurry obtained in step (1) by two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 5.0 m 3 / h, heat it to 500 °C at a rate of 3 °C / min in a nitrogen atmosphere and calcine for 6 h, cool it to 25 °C and take it out to obtain a semi-finished product;

[0071] (3) Transfer a 6‰ graphene solution of the finished product quality, a 1.5‰ carbon nanotube solution of the finished product quality, a 2% polyethylene glycol solution of the finished product quality, 3 times the semi-finished product quality of water, and the semi-finished product obtained in step (2) into a sand mill for sanding for 2 h, and take out the slurry;

[0072] (4) Spray the slurry obtained in step (3) by two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 3.0 m 3 / h, heat it to 740 °C at a rate of 3 °C / min in a nitrogen atmosphere and calcine for 11 h, naturally cool to room temperature, and obtain the lithium iron manganese phosphate cathode material CNT / RGO-Li1.03 Mn 0.58 Fe 0.4 Mg 0.02 PO4, and the particle size of this material is 50 - 300 nm.

[0073] Comparative Example 3 The lithium iron manganese phosphate cathode material prepared in this comparative example has the molecular formula CNT / RGO-Li 1.03 Mn 0.6 Fe 0.4 PO4@B, and the preparation method is as follows:

[0074] (1) Mix 4015.43 g of Li3PO4, 6970.6 g of MnCO3, 3262.2 g of Fe2O3, 7570.9 g of H3PO4, and 72.32 g of B2O3, then successively add 2840 g of glucose and 43000 g of pure water, and grind in a sand mill for 5 h, then take out the slurry;

[0075] (2) For the slurry obtained in step (1), perform two-fluid spraying at a feeding frequency of 10 Hz and an air intake frequency of 5.0 m 3 / h, heat up to 550 °C at a rate of 3 °C / min in a nitrogen atmosphere and calcine for 6 h, cool down to 25 °C and take out to obtain a semi-finished product;

[0076] (3) Transfer the graphene solution at 6‰ of the finished product quality, the carbon nanotube solution at 1.5‰ of the finished product quality, the polyethylene glycol solution at 2% of the finished product quality, water at 4 times the semi-finished product quality, and the semi-finished product obtained in step (2) into a sand mill for grinding for 2 h, then take out the slurry;

[0077] (4) For the slurry obtained in step (3), perform two-fluid spraying at a feeding frequency of 10 Hz and an air intake frequency of 3.0 m 3 / h, heat up to 730 °C at a rate of 2.5 °C / min in a nitrogen atmosphere and calcine for 11 h, naturally cool to room temperature, and obtain the lithium iron manganese phosphate cathode material CNT / RGO-Li 1.03 Mn 0.6 Fe 0.4 PO4@B, and the particle size of this material is 50 - 300 nm.

[0078] Comparative Example 4 The lithium iron manganese phosphate cathode material prepared in this comparative example has the molecular formula CNT-Li 1.03 Mn 0.58 Fe 0.4 Mg 0.02 PO4@B, and the preparation method is as follows:

[0079] (1) Mix 3820.62 g of Li2CO3, 6734.44 g of MnCO3, 6180.2 g of FePO4, 6930.6 g of H3PO4, and 270.6 g of Mg(BO2)2·H2O. Then, sequentially add 2390 g of glucose and 40000 g of pure water, and grind the mixture in a sand mill for 4 h. Take out the slurry.

[0080] (2) For the slurry obtained in step (1), perform two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 5.0 m 3 / h. Heat it to 450 °C at a rate of 5.5 °C / min in a nitrogen atmosphere and calcine for 4 h. Cool it to 25 °C and take it out to obtain a semi-finished product.

[0081] (3) Transfer a carbon nanotube solution of 1.5‰ of the finished product quality, a polyethylene glycol solution of 2% of the finished product quality, water with a mass 2.5 times that of the semi-finished product, and the semi-finished product obtained in step (2) into a sand mill for grinding for 2 h. Take out the slurry.

[0082] (4) For the slurry obtained in step (3), perform two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 3.0 m 3 / h. Heat it to 740 °C at a rate of 2.5 °C / min in a nitrogen atmosphere and calcine for 8 h. Naturally cool it to room temperature and crush it to obtain the lithium iron manganese phosphate cathode material CNT-Li 1.03 Mn 0.58 Fe 0.4 Mg 0.02 PO4@B, and the particle size of this material is 50 - 300 nm.

[0083] Comparative Example 5 The lithium iron manganese phosphate cathode material prepared in this comparative example has the molecular formula RGO-Li 1.03 Mn 0.58 Fe 0.4 Mg 0.02 PO4@B, and the preparation method is as follows:

[0084] (1) Mix 3820.62 g of Li2CO3, 6734.44 g of MnCO3, 6180.2 g of FePO4, 6930.6 g of H3PO4, and 270.6 g of Mg(BO2)2·H2O. Then, sequentially add 2390 g of glucose and 40000 g of pure water, and grind the mixture in a sand mill for 4 h. Take out the slurry.

[0085] (2) For the slurry obtained in step (1), perform two-fluid spraying with a feeding frequency of 10 Hz and an air intake frequency of 5.0 m 3 / h. Heat it to 450 °C at a rate of 5.5 °C / min in a nitrogen atmosphere and calcine for 4 h. Cool it to 25 °C and take it out to obtain a semi-finished product.

[0086] (3) Transfer 6‰ of the graphene solution by the mass of the finished product, 2% of the polyethylene glycol solution by the mass of the finished product, 2.5 times the mass of the semi-finished product of water, and the semi-finished product obtained in step (2) into a sand mill for sanding for 2 h, and take out the slurry;

[0087] (4) For the slurry obtained in step (3), perform two-fluid spraying at a feeding frequency of 10 Hz and an air intake frequency of 3.0 m 3 / h, heat it up to 740 °C at a rate of 2.5 °C / min under a nitrogen atmosphere and calcine for 8 h, naturally cool to room temperature, and obtain the lithium iron manganese phosphate cathode material RGO-Li 1.03 Mn 0.58 Fe 0.4 Mg 0.02 PO4@B. The particle size of this material is 50 - 300 nm.

[0088] Performance test: Stir and mix the cathode materials of Examples 1 - 7 and Comparative Examples 1 - 3 with the conductive agent acetylene black and the binder PVDF in a mass ratio of 90:5:5, add an appropriate amount of N-methylpyrrolidone, fully grind and make a cathode sheet, and dry it at 150 °C in a vacuum dryer for 12 h for standby; use a lithium metal sheet as the anode, adopt a polyethylene composite separator, the electrolyte is 1 mol / L, and the electrolyte is a mixed solution of ethylene carbonate EC and dimethyl carbonate DMC. Assemble a half-cell in a glove box under argon protection, and use constant current and constant voltage charging, and charge and discharge at different rates of 2.5 - 4.3 V. The electrochemical test results are shown in Table 1.

[0089] Table 1 Electrochemical test results

[0090]

[0091] It can be seen from the test results in Table 1 that after the magnesium and boron co-doped lithium iron manganese phosphate-carbon composite material prepared in Examples 1 - 7 is made into a secondary battery, the compaction and specific capacity are significantly improved, the manganese dissolution is reduced, and the resistivity is significantly reduced. Especially for the material in Example 1, the compaction can reach 2.37 g / cm 3 , and the discharge capacity at 0.1C is 160.1 mAh / g.

[0092] For the lithium iron manganese phosphate obtained in Comparative Example 1, magnesium and boron were not doped in step 1, and a secondary coating layer was not further introduced in step 3. The resistivity of the synthesized lithium iron manganese phosphate is large, the capacity is low, and manganese is easily dissolved, indicating that the battery prepared from this material has poor cycle performance.

[0093] In Comparative Example 2, boron was not doped, resulting in an increase in resistivity, an increase in manganese dissolution, and a lower capacity. In Comparative Example 3, magnesium was not doped, leading to an increase in resistivity, a lower tap density and capacity. In Comparative Examples 4 and 5, graphene or carbon nanotubes were not doped in the materials, resulting in a decrease in the specific capacity of the materials, an increase in resistivity. For the lithium iron phosphate cathode materials prepared in Comparative Examples 2 to 5, compared with Comparative Example 1, the resistivity, manganese dissolution and capacity were all improved to some extent. However, the resistivity could not be reduced below 30 Ω·cm, the capacity could not exceed 155 mAh / g, and the manganese dissolution could not be further reduced.

[0094] In Examples 1 to 3, four substances, boron, magnesium, RGO, and CNT, were introduced simultaneously, and the performance of the prepared lithium iron phosphate material could be further optimized, indicating that there was a cooperative effect among the four substances of boron, magnesium, RGO, and CNT, or between boron and magnesium, and between RGO and CNT pairwise. For example, it can be seen that appropriate boron doping can improve the electrical conductivity of the material and reduce the resistivity; appropriate boron doping can modify the defect structure of the material, improve grain growth and inhibit the appearance of irregular morphology, thereby improving the electrochemical performance of lithium iron phosphate; magnesium doping can stabilize the crystal structure of the material, reduce the risk of phase change during charge and discharge, affect the ion migration path of the material, and improve the lithium ion conductivity, thereby improving the electrochemical performance of lithium iron phosphate. Graphene has a planar structure, with a high specific surface area and good electrical conductivity, providing more reaction sites. Carbon nanotubes are on the plane of graphene, forming a three-dimensional conductive network. On the one hand, the conductive network optimizes the transmission paths of electrons and ions, reduces the resistance of the material, makes it easier for electrons to migrate inside the material, and thus improves the electrical conductivity and kinetic performance of the battery; on the other hand, the conductive network has a certain deformation margin, and during charge and discharge, the volume change of lithium iron phosphate can be absorbed by it, thereby reducing the stress between phases and maintaining structural stability; further, using the secondary sintering technology, the conductive network forms a secondary coating outside the lithium iron phosphate semi-finished product, improving the coating density, inhibiting the dissolution of manganese ions, prolonging the cycle life of the battery, ensuring safety, and improving the overall performance of the lithium ion battery.

[0095] On this basis, carbon nanotubes and graphene have a certain lubricity, and during the mixing process, they can improve the particle dispersion and reduce the aggregation between particles, enabling the synthesized lithium iron phosphate material to be more evenly dispersed, thereby ensuring the tap density.

[0096] In Examples 4 to 7, the manganese-iron ratio in the lithium iron manganese phosphate material was adjusted / increased. As the manganese content increased to 0.63 or more, the manganese dissolution and resistivity showed an upward trend, while the discharge specific capacity showed a downward trend. However, the resistivity of the lithium iron manganese phosphate material obtained in Examples 4 to 7 was not higher than 41 Ω·cm, the manganese dissolution was not higher than 16 ppm, and the discharge specific capacity exceeded 150 mAh / g. This indicates that the simultaneous introduction of four substances, namely boron, magnesium, RGO, and CNT, can also be used to improve the performance of lithium iron manganese phosphate materials with a high manganese-iron ratio, and the preparation method of lithium iron manganese phosphate of the present invention has a wide range of applications.

Claims

1. A preparation method of a lithium iron manganese phosphate cathode material, characterized in that, It includes the following steps: (1) Mix a lithium source, a manganese source, an iron source, a phosphorus source, a first carbon source, a magnesium source, and a boron source, grind them with sand, dry them, and conduct a first calcination to obtain a semi-finished product; based on the total mass of the lithium source, manganese source, iron source, phosphorus source, magnesium source, and boron source, the addition ratio of the first carbon source is 9-20 wt%, and the first calcination temperature is 350-600 °C; (2) Mix the semi-finished product and a second carbon source, grind them with sand, dry them, and conduct a second calcination to obtain a lithium iron manganese phosphate cathode material. Based on the mass of the semi-finished product, the addition ratio of the second carbon source is 2-9 wt%. The second carbon source includes graphene, carbon nanotubes, and a dispersant, and the second calcination is at 600-800 °C.

2. The preparation method of the lithium iron manganese phosphate cathode material according to claim 1, characterized in that, In step (1), the molar ratio of the lithium source, manganese source, iron source, phosphorus source, magnesium source, and boron source is: 0.99-1.05:0.1-0.9:0.1-0.9:1:0.01-0.05:0.02-0.

1.

3. The preparation method of the lithium iron manganese phosphate cathode material according to claim 2, wherein, In step (1), the molar ratio of the lithium source, manganese source, iron source, phosphorus source, magnesium source, and boron source is: 0.99-1.05:0.42-0.8:0.2-0.58:1:0.01-0.05:0.02-0.

1.

4. The preparation method of the lithium iron manganese phosphate cathode material according to claim 1, wherein In step (1), the addition ratio of the first carbon source is 9-15 wt%, and the first carbon source is one of glucose, sucrose, fructose, lactose, citric acid, starch, ethylene glycol, polyvinylpyrrolidone, methanol, ethanol, glycine, aspartic acid, tartaric acid, β-cyclodextrin, graphene, and carbon nanotubes.

5. The preparation method of the lithium iron manganese phosphate cathode material according to claim 1, wherein In step (1), the first calcination temperature is 400-580 °C, and the calcination time is 4-7 hours.

6. The preparation method of the lithium iron manganese phosphate cathode material according to claim 1, characterized in that, In step (2), the second calcination temperature is 650-770 °C, and the calcination time is 9-15 hours.

7. The preparation method of the lithium iron manganese phosphate cathode material according to claim 1, characterized in that In step (2), the addition ratio of the graphene is 2-8‰; the addition ratio of the carbon nanotubes is 0.5-3.5‰, and the addition ratio of the dispersant is 1-8%; the addition ratio of the carbon nanotubes does not exceed that of the graphene.

8. The preparation method of the lithium iron manganese phosphate cathode material according to claim 1 or 7, characterized in that, In step (2), the addition ratio of the graphene is 3-7‰; the addition ratio of the carbon nanotubes is 0.8-2‰, and the addition ratio of the dispersant is 2-7%.

9. The preparation method of the lithium iron manganese phosphate cathode material according to claim 1, wherein, The lithium source is one or more of lithium carbonate, lithium phosphate, lithium sulfate, lithium chloride, lithium hydroxide, lithium dihydrogen phosphate, lithium hydrogen phosphate, lithium nitrate, lithium oxalate, and lithium acetate; the manganese source is one or more of manganese tetroxide, manganese carbonate, manganese acetate, manganese dioxide, and manganese oxide; the iron source is one or more of iron phosphate, iron(III) oxide, iron acetate, iron sulfate, and iron nitrate; the phosphorus source is one or more of phosphoric acid, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, lithium phosphate, iron phosphate, and manganese phosphate; the magnesium source is one or more of magnesium metaborate, magnesium borate, magnesium boride, magnesium oxide, magnesium nitrate, magnesium chloride, magnesium carbonate, magnesium sulfate, magnesium hydroxide, magnesium acetate, magnesium phosphate, and magnesium sulfide; the boron source is one or more of magnesium metaborate, magnesium borate, magnesium boride, boric acid, and sodium borate; the dispersant is one or more of polyethylene glycol, polyvinyl alcohol, polyacrylic acid, sodium dodecylbenzenesulfonate, polyethyleneimine, polycarbonate, and polyethylene ether.

10. A lithium iron manganese phosphate cathode material, characterized in that, Prepared by the preparation method according to Claim 1.

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