Lithium manganese iron phosphate material as well as preparation method and application thereof

Abstract

The invention belongs to the field of batteries, and particularly discloses a lithium manganese iron phosphate material as well as a preparation method and application thereof. The preparation method comprises the following steps: sequentially performing wet grinding and mixing, spray drying and first sintering on a lithium source, a manganese source, an iron source, a phosphorus source and a doping source to obtain an intermediate material; and carrying out ball-milling mixing and secondary sintering on the intermediate material, a forming aid and a carbon source to finally obtain the lithium manganese iron phosphate material. According to the method, the compaction density and the conductivity of the material can be improved at the same time, so that the specific discharge capacity, the efficiency and the cycle performance of the battery are improved.

Description

Technical Field

[0001] The present invention belongs to the field of batteries, and in particular relates to a lithium manganese iron phosphate material and a preparation method and application thereof. Background Art

[0002] Lithium manganese iron phosphate (LMFP) is a new type of lithium-ion battery cathode material. It combines the advantages of lithium iron phosphate (LFP) and lithium manganese phosphate (LMP), not only improving the operating voltage, but also improving the electrochemical performance and safety characteristics. However, the existing lithium manganese iron phosphate materials still face some challenges in practical applications, such as low compaction density, low conductivity, and complex synthesis process.

[0003] At present, after the sintering of lithium iron manganese phosphate materials, the large-sized lithium iron manganese phosphate materials need to be crushed through the gas breaking (air flow crushing) process. However, this process requires strict control of various influencing factors. If the gas breaking pressure is too large, the carbon layer of the material will easily detach, affecting the electrical conductivity of the material; if the gas breaking pressure is too small, the material cannot be crushed to the corresponding particle size, affecting subsequent processing.

[0004] In addition, in order to improve the compaction density of lithium iron manganese phosphate positive electrode materials, grading and secondary processes are mainly used, but the conductivity of the material is low in this way. Although the conductivity can be increased by increasing the carbon source content, other properties of the material such as compaction density are reduced due to excessive carbon content. Therefore, it is still necessary to develop a solid phase method for preparing lithium iron manganese phosphate positive electrode materials, so that the lithium iron manganese phosphate positive electrode materials can take into account both high conductivity and compaction density, thereby improving the cycle performance and specific capacity of the battery. Summary of the invention

[0005] In view of the problem that the process preparation involved in the above-mentioned prior art cannot take into account the problems such as high compaction density and conductivity of lithium manganese iron phosphate, the present invention will provide a lithium manganese iron phosphate material and its preparation method and application.

[0006] To achieve the above purpose, the following technical solutions are specifically included:

[0007] In a first aspect, the present invention provides a method for preparing a lithium manganese iron phosphate material, comprising the following steps:

[0008] (1) According to the element stoichiometric ratio of the lithium manganese iron phosphate material chemical formula, a lithium source, an iron source, a manganese source, a phosphorus source, a doping metal element M source and a solvent are sequentially ground, mixed, spray-dried and sintered for the first time to obtain an intermediate material;

[0009] (2) The intermediate material, carbon source and molding aid are sequentially subjected to ball milling, spray drying and a second sintering to obtain lithium manganese iron phosphate material.

[0010] In the present invention, a lithium source, a manganese source, an iron source, a phosphorus source, and a doping source are first subjected to wet grinding and mixing, spray drying, and first sintering in sequence to obtain a polycrystalline solid solution intermediate material in which trivalent iron has not been reduced. Then, the intermediate material, a shaping aid, and a carbon source are subjected to ball milling and mixing and second sintering. The trivalent iron in the intermediate material is reduced by the carbon source to form lithium iron manganese phosphate particles. At the same time, the shaping aid improves the fluidity and plasticity of the particles, and together with the carbon source, affects the sintering process, increases the shrinkage rate of the material during sintering, and forms a carbon coating layer while forming the lithium iron manganese phosphate particles, and at the same time improves the tap density and conductivity of the material, and finally obtains a single-crystalline lithium iron manganese phosphate material. The method of the present invention can improve the tap density and conductivity of the material at the same time, thereby improving the discharge specific capacity, efficiency, and cycle performance of the battery.

[0011] Preferably, in steps (1) and (2), the chemical formula of the lithium iron manganese phosphate material is LiMn x Fe y M 1-x-y PO4, where x:y = (0.1 - 0.9):(0.1 - 0.9), and 0 ≤ (1 - x - y) ≤ 0.1.

[0012] Preferably, the lithium iron manganese phosphate is of single-crystalline type.

[0013] Preferably, in step (1), the lithium source includes at least one of lithium hydroxide, lithium dihydrogen phosphate, lithium carbonate, lithium nitrate, lithium oxalate, lithium acetate, and lithium phosphate.

[0014] Preferably, in step (1), the iron source includes at least one of ferrous oxalate, iron(III) oxide, iron(III) hydroxide, iron(III) phosphate, magnetite, and iron acetate.

[0015] Preferably, in step (1), the manganese source includes at least one of manganese dioxide, manganese carbonate, manganese oxalate, manganese acetate, and manganese tetraoxide.

[0016] Preferably, in step (1), the phosphorus source includes at least one of phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate, lithium dihydrogen phosphate, and lithium hydrogen phosphate.

[0017] Preferably, in step (1), the metal element M in the doping metal element M source includes at least one of Ti, Mg, Al, Ni, Nb, Gr, V, Zr, Ta, La, Ce, Nd, Pr, and Eu.

[0018] Preferably, in step (1), the doping metal element M source includes at least one of an oxide, hydroxide, salt, and element of the metal element M.

[0019] Preferably, in step (1), the solvent includes at least one of ethanol, isopropanol, water, acetone, toluene, and hexane.

[0020] Preferably, in step (1), the average particle size of the slurry after grinding and mixing is < 1 μm. Further preferably, the average particle size of the slurry after grinding and mixing is 300 - 500 nm.

[0021] As used herein, the "slurry" refers to a mixture containing solid particles and a liquid, and the average particle size thereof is obtained by testing with a laser particle size analyzer.

[0022] Preferably, in step (1), the average particle size of the intermediate material is < 100 μm, and more preferably 20 - 50 μm.

[0023] Preferably, in step (1), the temperature of the first sintering is 450 - 900 °C, the time of the first sintering is 3 - 20 h, the heating rate for heating to the temperature of the first sintering is 1 - 10 °C / min, the atmosphere of the first sintering is an inert gas atmosphere, and the inert gas includes at least one of nitrogen, argon, and helium.

[0024] Further preferably, in step (1), the temperature of the first sintering is 680 - 740 °C, and the time of the first sintering is 4 - 6 h.

[0025] The inventors of the present invention found that the temperature and time of the first sintering affect the compaction density, conductivity, specific capacity, and cycling performance of the final LMFP particles. Under the above mass content range, LMFP has better compaction density, conductivity, specific capacity, and cycling performance.

[0026] Preferably, in step (2), the carbon source includes at least one of starch, sucrose, fructose, glucose, maltose, lactose, polyethylene, polypropylene, polyethylene glycol (PEG), polypropylene glycol (PPG), conductive carbon black, acetylene black, carbon nanotubes, graphene, citric acid, and ascorbic acid.

[0027] More preferably, the polyethylene glycol includes at least one of PEG 200, PEG 400, PEG 800, PEG 1000, and PEG 1200.

[0028] Preferably, in step (2), the molding aid includes at least one of phenolic resin, epoxy resin, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyacrylic acid (PAA), polyvinylpyrrolidone (PVP), polyethylene (PE), polyethylene oxide (PEO), polypropylene (PP), dextrin, carboxymethyl cellulose (CMC), carboxymethyl cellulose (CMC-Li), hydroxypropyl methylcellulose (HPMC), magnesium stearate, ascorbic acid (vitamin C), dibutylhydroxytoluene (BHT), and styrene-butadiene rubber.

[0029] When the carbon source, the molding aid and the lithium iron manganese phosphate intermediate material are mixed, the addition of the molding aid enables the carbon source to be well coated on the surface of the lithium iron manganese phosphate material, making the carbon layer coating on the sintered lithium iron manganese phosphate more uniform and having higher conductivity and tap density.

[0030] Preferably, in step (2), based on the mass of the intermediate material, the mass percentage content of the molding aid is 1-5%.

[0031] Preferably, in step (2), based on the mass of the intermediate material, the mass percentage content of the carbon source is 1-20%, and more preferably 3.5-19%.

[0032] The inventors of the present invention found that the content of the molding aid and carbon affects the tap density, conductivity, specific capacity and cycling performance of the final LMFP particles. Under the above mass content range, LMFP has better tap density, conductivity, specific capacity and cycling performance.

[0033] Preferably, in step (2), the average particle size of the mixture after ball milling is <1 μm, and more preferably, the average particle size of the mixture after grinding and mixing is 300-500 nm.

[0034] Preferably, in step (2), the temperature of the second sintering is 400-900 °C, the time of the second sintering is 3-20 h, the heating rate to the temperature of the second sintering is 1-10 °C / min, and the atmosphere of the second sintering is an inert gas atmosphere.

[0035] More preferably, in step (4), the temperature of the second sintering is 660-680 °C, and the time of the second sintering is 6-8 h.

[0036] The inventors of the present invention found that the temperature and time of the second sintering affect the tap density, conductivity, specific capacity and cycling performance of the final LMFP particles. Under the above temperature and time range of the second sintering, LMFP has better tap density, conductivity, specific capacity and cycling performance.

[0037] Preferably, in step (2), the particle size of the lithium iron manganese phosphate material is 100 - 300 mesh.

[0038] In a second aspect, the present invention provides a lithium iron manganese phosphate material prepared by the preparation method of the lithium iron manganese phosphate material described above.

[0039] The lithium iron manganese phosphate material of the present invention has high tap density, conductivity, specific capacity, efficiency and cycle performance. Among them, the tap density reaches 2.30 - 2.45 g / cm 3 , the conductivity reaches 0.03 - 0.15 S / cm, the discharge specific capacity is 150 - 156 mAh / g, the efficiency is 93% - 96%, and the capacity retention rate reaches 94% - 99% after 100 cycles.

[0040] Preferably, the lithium iron manganese phosphate material includes lithium iron manganese phosphate single crystal particles and a carbon coating layer coating the lithium iron manganese phosphate single crystal particles, and the carbon coating layer accounts for 1% - 4% of the mass of the lithium iron manganese phosphate material.

[0041] In a third aspect, the present invention provides a secondary battery, including a positive electrode sheet and a negative electrode sheet, and the positive electrode sheet includes the lithium iron manganese phosphate material described above.

[0042] Compared with the prior art, the present invention has the following beneficial effects: First, the lithium source, manganese source, iron source, phosphorus source and doping source are successively subjected to wet grinding and mixing, spray drying and the first sintering to obtain a polycrystalline solid solution intermediate material in which trivalent iron has not been reduced; then the intermediate material, a forming aid and a carbon source are subjected to ball milling and mixing and the second sintering. The trivalent iron in the intermediate material is reduced by the carbon source to form lithium iron manganese phosphate particles. At the same time, the forming aid improves the fluidity and plasticity of the particles, affects the sintering process with the carbon source, increases the shrinkage rate of the material during sintering, forms a carbon coating layer while forming the lithium iron manganese phosphate particles, and simultaneously improves the tap density and conductivity of the material, and finally obtains a single crystal type lithium iron manganese phosphate material. The method of the present invention can simultaneously improve the tap density and conductivity of the material, thereby improving the discharge specific capacity, efficiency and cycle performance of the battery. In addition, the material prepared by the method of the present invention does not need to be further processed by air jet milling, which can avoid the complicated process of the air jet milling process and the defect of a large amount of fine powder generated, resulting in more waste. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 XRD spectrum of the intermediate material in Example 1.

[0044] Figure 2 XRD spectrum of lithium iron manganese phosphate in Example 1.

[0045] Figure 3 SEM image of the intermediate material in Example 1 at a magnification of 40KX.

[0046] Figure 4 SEM image of lithium iron manganese phosphate in Example 1 at a magnification of 5KX. Detailed implementation mode

[0047] To better illustrate the purpose, technical solution and advantages of the present invention, the present invention will be further described below through specific examples. Unless otherwise specified, the test methods used in the examples and / or comparative examples are all conventional methods; the materials, reagents, etc. used, unless otherwise specified, can all be obtained from commercial channels.

[0048] Example 1

[0049] A preparation method of lithium iron manganese phosphate material includes the following steps:

[0050] (1) Add 0.975 mol of Li2CO3, 3 mol of MnCO3, 1.85 mol of FePO4, 3.3 mol of LiH2PO4, 0.15 mol of TiO2 and 4 L of pure water into a sand mill and grind for 2 h. After the particle size of the slurry is ground to about 350 nm, transfer it, and then spray-dry the ground slurry, controlling the particle size of the material after spray-drying to be about 30 μm; then place the material after spray-drying in a box furnace under nitrogen protection, heat it from room temperature to 740 °C at a heating rate of 5 °C / min, and perform the first sintering at 740 °C for 4 h to obtain a lithium iron manganese phosphate intermediate material;

[0051] (2) Mix 628 g of lithium iron manganese phosphate intermediate material and 30 g of sucrose evenly by ball milling at a rotation speed of 100 Hz for 2 h, then add 18.84 g of polyvinyl butyral (the added mass is 3% of the mass of the lithium iron manganese phosphate intermediate material), transfer the mixed powder after ball milling to about 380 nm in particle size, and place the ball-milled material in a box furnace under nitrogen protection, heat it from room temperature to 680 °C at a heating rate of 5 °C / min, and perform the second sintering at this temperature for 6 h; screen the sintered material with a 200-mesh metal screen, remove magnetism and water to obtain a single-crystalline lithium iron manganese phosphate material.

[0052] Examples 2 - 11

[0053] Compared with Example 1, the differences in specific raw materials and condition parameters in Examples 2 - 11 are shown in detail in Tables 1 and 2, and the rest of the preparation processes are the same.

[0054] Comparative Example 1

[0055] Compared with Example 1, in this Comparative Example 1, the forming aid, carbon source and the rest of the raw materials are sintered only once to prepare the lithium iron manganese phosphate material, which specifically includes the following steps:

[0056] 0.975 mol of Li2CO3, 3 mol of MnCO3, 1.85 mol of FePO4, 3.3 mol of LiH2PO4, 0.15 mol of TiO2, 37.5 g of sucrose, 23.55 g of polyvinyl butyral and 4 L of pure water were added to a sand mill and ground for 6 h. After the particle size of the slurry was ground to about 350 nm, it was transferred. Then the ground slurry was spray-dried, and the particle size of the spray-dried material was controlled to be about 30 μm. The spray-dried material was placed in a box furnace under nitrogen protection and sintered at a constant temperature of 680 °C for 10 h, and then passed through air breaking, sieving and demagnetization to obtain a lithium iron manganese phosphate composite material.

[0057] Comparative Example 2

[0058] Compared with Example 1, the difference in this comparative example is that in step (1), the particle size of the slurry is 1100 nm, and the rest are the same.

[0059] Comparative Example 3

[0060] Compared with Example 1, the difference in this comparative example is that in step (2), the particle size of the mixed powder is 1100 nm, and the rest are the same.

[0061] Comparative Example 4

[0062] Compared with Example 1, the difference in this comparative example is that in step (1) of this comparative example, the temperature of the first sintering is 950 °C and the time of the first sintering is 6 h, and the rest are the same.

[0063] Comparative Example 5

[0064] Compared with Example 1, the difference in this comparative example is that in step (2) of this comparative example, the temperature of the second sintering is 950 °C and the time of the second sintering is 8 h, and the rest are the same.

[0065] Comparative Example 6

[0066] Compared with Example 1, the difference in this comparative example is that the carbon source is added step by step in this comparative example, and the specific steps are as follows:

[0067] (1) 0.975 mol of Li2CO3, 3 mol of MnCO3, 1.85 mol of FePO4, 3.3 mol of LiH2PO4, 0.15 mol of TiO2, 35.63 g of sucrose (just enough to reduce the iron in the iron source) and 4 L of pure water were added to a sand mill and ground for 2 h. After the particle size of the slurry was ground to about 350 nm, it was transferred. Then the ground slurry was spray-dried, and the particle size of the spray-dried material was controlled to be about 30 μm. Then the spray-dried material was placed in a box furnace under nitrogen protection and heated from room temperature to 740 °C at a heating rate of 5 °C / min, and sintered at 740 °C for 4 h for the first time to obtain an intermediate material of lithium iron manganese phosphate;

[0068] (2) 628 g of the lithium iron manganese phosphate intermediate material and 4.37 g of sucrose (the carbon source for coating) were ball-milled at a rotation speed of 100 Hz for 2 h until evenly mixed. Then, 18.84 g of polyvinyl butyral (the added mass was 3% of the mass of the lithium iron manganese phosphate intermediate material) was added. After the particle size of the mixed powder was ball-milled to about 380 nm, it was transferred. The ball-milled material was placed in a box furnace under nitrogen protection and heated from room temperature to 680 °C at a heating rate of 5 °C / min, and the second sintering was carried out at this temperature for 6 h. The sintered material was screened with a 200-mesh metal screen, and the lithium iron manganese phosphate material was obtained after demagnetization and dehydration.

[0069] Comparative Example 7

[0070] Compared with Example 1, the difference in this comparative example is that in step (1) of this comparative example, there is no doping source, and the rest of the raw materials are replaced by mixing 1.05 mol of Li2CO3, 3 mol of MnCO3, 2 mol of FePO4, 3.15 mol of LiH2PO4 and 4 L of pure water, and the rest are the same.

[0071] Comparative Example 8

[0072] Compared with Example 1, the difference in this comparative example is that in step (1) of this comparative example, the amounts of raw materials are different, and each raw material is replaced by mixing 0.95 mol of Li2CO3, 3 mol of MnCO3, 1.8 mol of FePO4, 3.35 mol of LiH2PO4, 0.2 mol of MgO and 4 L of pure water, and the rest are the same.

[0073] Comparative Example 9

[0074] Compared with Example 1, the difference in this comparative example is that in step (1) of this comparative example, the raw materials are mixed by dry method, and the specific steps are as follows:

[0075] (1) 0.975 mol of Li2CO3, 3 mol of MnCO3, 1.85 mol of FePO4, 3.3 mol of LiH2PO4 and 0.15 mol of TiO2 were added to a high-speed centrifuge and mixed for 4 h. After the particle size of the powder was ground to about 350 nm, it was transferred, and then dried in an oven. Then, the dried material was placed in a box furnace under nitrogen protection and heated from room temperature to 740 °C at a heating rate of 5 °C / min, and the first sintering was carried out at 740 °C for 4 h to obtain the lithium iron manganese phosphate intermediate material;

[0076] (2) After uniformly mixing 628 g of lithium iron manganese phosphate intermediate material and 30 g of sucrose by ball milling at a rotation speed of 100 Hz for 2 h, 18.84 g of polyvinyl butyral (the added mass is 3% of the mass of the lithium iron manganese phosphate intermediate material) is added. After the mixed powder is ball milled to a particle size of about 380 nm, it is transferred. The ball milled material is placed in a box furnace under nitrogen protection and heated from room temperature to 680 °C at a heating rate of 5 °C / min, and the second sintering is carried out at this temperature for 6 h; the sintered material is screened with a 200-mesh metal screen, and after demagnetization and dehydration, the lithium iron manganese phosphate material is obtained.

[0077] Comparative Example 10

[0078] Compared with Example 1, the difference in this comparative example is that in step (2) of this comparative example, no forming aid is added, and 73 g of sucrose is added, and the rest is the same.

[0079] Comparative Example 11

[0080] Compared with Example 1, the difference in this comparative example is that in step (2) of this comparative example, 75.36 g of polyvinyl butyral is added, and no sucrose is added, and the rest is the same.

[0081] Table 1 Raw materials and related condition parameters

[0082]

[0083]

[0084] Table 2

[0085]

[0086]

[0087] Performance test

[0088] The XRD patterns of the intermediate material and the lithium iron manganese phosphate material prepared in Example 1 were tested using an X-ray powder diffractometer (XRD). The microscopic surface morphology diagrams of the intermediate material and the lithium iron manganese phosphate material in Example 1 were tested using a scanning electron microscope (SEM). The tap density and conductivity of the lithium iron manganese phosphate material powder were measured using a PRCD3100 type powder conductivity & tap density meter. The electrochemical performance of the lithium iron manganese phosphate cathode material was measured using a blue electrochemical test system.

[0089] The prepared lithium iron manganese phosphate materials were respectively weighed with a conductive agent, conductive carbon black (SP), and a binder, polyvinylidene fluoride (PVDF), according to a mass ratio of 8:1:1. Using N-methylpyrrolidone (NMP) as a solvent, they were ball-milled in a planetary ball mill for 2 h to prepare a positive electrode paste, which was then coated on an aluminum foil current collector. After drying, it was rolled and punched into a positive electrode plate, which was placed in a vacuum drying oven for 12 h and then placed in a glove box under an inert gas protective atmosphere. The negative electrode used a lithium metal sheet, and the electrolyte was a 1M LiPF6 solution, in which the solvent of the solution was a mixed solution of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) with a volume ratio of 1:1:1. The separator was a polypropylene microporous membrane Celgard 2300, and a CR2032 type coin cell was assembled. Finally, the initial efficiency (efficiency) of charge and discharge tests was carried out on a battery test cabinet, where the charge and discharge rates were both 0.1C, and the test voltage range was 2.5 - 4.5V to obtain the discharge specific capacity and the capacity retention rate after 100 charge and discharge cycles. The test results are shown in Table 3.

[0090] Table 3

[0091]

[0092]

[0093] From Figure 1-2 the XRD spectra of the intermediate material and the lithium iron manganese phosphate material, it can be seen that the trivalent iron in the intermediate material has not been reduced, and the trivalent iron in the final lithium iron manganese phosphate material has been reduced to divalent iron, proving that the lithium iron manganese phosphate material has been successfully prepared; and from Figure 3-4 it can be seen that the intermediate material obtained after the first sintering is a polycrystalline solid solution; after the intermediate material, the forming aid, and the carbon source are sintered for the second time, while forming lithium iron manganese phosphate particles, a carbon coating layer is formed on the surface. The present invention finally obtains a spherical single-crystalline lithium iron manganese phosphate material with finer, denser surface particles, higher tap density, and a carbon-coated layer, and its particle size is about 100 - 500 nm.

[0094] From Examples 1 - 11, it can be seen that the lithium iron manganese phosphate material prepared by the present invention has high tap density, conductivity, discharge specific capacity, efficiency, and cycling performance, where the tap density reaches 2.30 - 2.45 g / cm 3 , the conductivity reaches 0.03 - 0.15 S / cm, the discharge specific capacity is 150 - 156 mAh / g, the efficiency is 93% - 96%, and the capacity retention rate after 100 cycles reaches 94% - 99%.

[0095] In Examples 1-4, the manganese-to-iron ratio in the lithium iron manganese phosphate material (where the doping element M replaces the Fe site and the amount of element M is included in the calculation of Fe) is 6:4, 7:3, 5:5, and 4:6 respectively. The manganese-to-iron ratio in the lithium iron manganese phosphate material can be selected as (4-7):(3-6). When the manganese-to-iron ratio is further preferably (4-6):(4-6), the tap density, conductivity, battery discharge specific capacity, efficiency, and cycle performance of the lithium iron manganese phosphate material are better.

[0096] In Comparative Example 7, Example 7, Example 6, Example 1, and Comparative Example 8, the molar amounts of the doping element are 0 mol, 0.05 mol, 0.10 mol, 0.15 mol, and 0.2 mol respectively. As the content of the doping element increases, the tap density and conductivity of the material first gradually increase and then significantly decrease. It can be seen that increasing the doping element can significantly improve the tap density and conductivity of the material, but if the content of the doping element is too high, the discharge specific capacity will be reduced instead. Therefore, selecting an appropriate content of the doping element is more beneficial to improving the performance of the battery.

[0097] From Examples 1, 8-10 and Comparative Examples 4-5, it can be seen that the two sintering temperatures have an impact on the conductivity, tap density, battery discharge specific capacity, efficiency, and cycle performance of the lithium iron manganese phosphate material. The first sintering temperature is preferably 680-740 °C, and the second sintering temperature is preferably 660-680 °C. Too high a sintering temperature will instead reduce the conductivity, battery discharge specific capacity, efficiency, and cycle performance.

[0098] From Examples 1 and 11, it can be seen that the method of the present invention is suitable for various types of carbon sources and forming aids, and can achieve high conductivity, tap density, battery discharge specific capacity, efficiency, and cycle performance.

[0099] It can be seen from Example 1, Comparative Examples 1-3, Comparative Example 6, and Comparative Example 10 that in the present invention, a lithium source, a manganese source, an iron source, a phosphorus source, and a doping source are first wet-ground and mixed to obtain a slurry with an average particle size of less than 1 μm, and then spray-dried and sintered for the first time to obtain a polycrystalline solid solution intermediate material, in which the trivalent iron has not been reduced yet; then the intermediate material, a forming aid, and a carbon source are ball-milled and mixed to obtain a mixed material with an average particle size of less than 1 μm, and after the second sintering, the trivalent iron in the intermediate material is reduced by the carbon source to form lithium iron manganese phosphate particles. At the same time, the forming aid improves the fluidity and plasticity of the particles, affects the sintering process together with the carbon source, increases the shrinkage rate of the material during sintering, and forms a carbon coating layer while forming the lithium iron manganese phosphate particles, improving the tap density and conductivity of the material, and finally obtaining a single-crystalline lithium iron manganese phosphate material. The present invention adopts a two-step sintering solid-phase synthesis method. Compared with the conventional one-step sintering solid-phase synthesis method or the method of first preparing lithium iron manganese phosphate and then coating with carbon, the method of the present invention first forms a solid solution polycrystalline intermediate material by the first sintering, and then, on the basis of adding a forming aid to improve the particle forming effect and a carbon source to play a reducing and coating role, adopts the second sintering, generally forming a two-step solid-phase process, which can improve the tap density and conductivity of the material at the same time, thereby improving the discharge specific capacity, efficiency, and cycle performance of the battery. And it can be seen from Example 1 and Comparative Examples 10-11 that the forming aid can significantly improve the tap density, conductivity, and discharge specific capacity of the material, but an excessive amount thereof will instead reduce the tap density, conductivity, and discharge specific capacity. Therefore, after verification by the inventors of the present invention, the addition amount of the forming aid is preferably 1%-5%. In addition, it can be seen from Example 1 and Comparative Examples 2-3 that when mixing the raw material particles, it is more beneficial for solid-phase sintering and further improves the tap density, conductivity, battery discharge specific capacity, efficiency, and cycle performance of the lithium iron manganese phosphate material to grind and mix them to a particle size of less than 1 μm.

[0100] It can be seen from Example 1 and Comparative Example 9 that compared with the solid-phase mixing method, the wet-grinding mixing method is more conducive to the contact and combination of each raw material, improves the sintering efficiency, and can significantly improve the conductivity, specific capacity, efficiency, and cycle performance of the material.

[0101] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A preparation method of lithium iron manganese phosphate material, characterized in that, It includes the following steps: (1) According to the elemental stoichiometric ratio of the chemical formula of lithium iron manganese phosphate material, a lithium source, an iron source, a manganese source, a phosphorus source, a doping metal element M source and a solvent are successively subjected to grinding and mixing, spray drying and first sintering to obtain an intermediate material; (2) The intermediate material, a carbon source and a forming aid are successively subjected to ball milling and mixing, spray drying and second sintering to obtain a lithium iron manganese phosphate material.

2. The preparation method of the lithium iron manganese phosphate material according to claim 1, characterized in that, In steps (1) and (2), the chemical formula of the lithium manganese iron phosphate material is LiMn x Fe y M 1-x-y PO4, where x:y = (0.1 - 0.9):(0.1 - 0.9), 0 ≤ (1 - x - y) ≤ 0.1; the lithium manganese iron phosphate is of single crystal type.

3. The preparation method of the lithium iron manganese phosphate material according to claim 1, characterized in that, In step (1), the temperature of the first sintering is 450 - 900 °C, the time of the first sintering is 3 - 20 h, the heating rate for heating to the temperature of the first sintering is 1 - 10 °C / min, and the atmosphere of the first sintering is an inert gas atmosphere.

4. The preparation method of the lithium iron manganese phosphate material according to claim 1, characterized in that, (It) includes at least one of the following: In step (2), the carbon source includes at least one of starch, sucrose, fructose, glucose, maltose, lactose, polyethylene, polypropylene, polyethylene glycol, polypropylene glycol, conductive carbon black, acetylene black, carbon nanotubes, graphene, citric acid, ascorbic acid; In step (2), the forming aid includes at least one of phenolic resin, epoxy resin, polyvinyl alcohol, polyvinyl butyral, polyacrylic acid, polyvinylpyrrolidone, polyethylene, polyethylene oxide, polypropylene, dextrin, carboxymethyl cellulose, carboxymethyl cellulose, hydroxypropyl methyl cellulose, magnesium stearate, ascorbic acid, dibutylhydroxytoluene, styrene-butadiene rubber.

5. The preparation method of the lithium iron manganese phosphate material according to claim 1, characterized in that (It) includes at least one of the following: In step (2), based on the mass of the intermediate material, the mass percentage content of the forming aid is 1 - 5%; In step (2), based on the mass of the intermediate material, the mass percentage content of the carbon source is 1 - 20%.

6. The preparation method of the lithium iron manganese phosphate material according to claim 1, wherein, In step (2), the temperature of the second sintering is 400 - 900 °C, the time of the second sintering is 3 - 20 h, the heating rate for heating to the temperature of the second sintering is 1 - 10 °C / min, and the atmosphere of the second sintering is an inert gas atmosphere.

7. The preparation method of the lithium iron manganese phosphate material according to claim 1, characterized in that, (It) includes at least one of the following: In step (1), the average particle size of the slurry after grinding and mixing is < 1 μm; In step (2), the average particle size of the mixture after ball milling and mixing is < 1 μm.

8. The preparation method of the lithium iron manganese phosphate material according to claim 1, wherein (It) includes at least one of the following: In step (1), the lithium source includes at least one of lithium hydroxide, lithium dihydrogen phosphate, lithium carbonate, lithium nitrate, lithium oxalate, lithium acetate, lithium phosphate; In step (1), the iron source includes at least one of ferrous oxalate, iron(III) oxide, iron(III) hydroxide, iron(III) phosphate, magnetite, iron(III) acetate; In step (1), the manganese source includes at least one of manganese dioxide, manganese carbonate, manganese oxalate, manganese acetate, manganese tetraoxide; In step (1), the phosphorus source includes at least one of phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate, lithium dihydrogen phosphate, lithium hydrogen phosphate; In step (1), the metal element M in the doping metal element M source includes at least one of Ti, Mg, Al, Ni, Nb, Gr, V, Zr, Ta, La, Ce, Nd, Pr, Eu; In step (1), the solvent includes at least one of ethanol, isopropanol, water, acetone, toluene, hexane.

9. The lithium iron manganese phosphate material prepared by the preparation method of the lithium iron manganese phosphate material according to any one of claims 1-8.

10. A secondary battery, comprising a positive electrode plate and a negative electrode plate, characterized in that The positive electrode plate includes the lithium iron manganese phosphate material according to claim 9.

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