Lithium iron manganese phosphate positive electrode material coated with lithium metaaluminate and preparation method of lithium iron manganese phosphate positive electrode material

Abstract

The invention relates to a lithium metaaluminate coated lithium iron manganese phosphate positive electrode material and a preparation method thereof, and the positive electrode material is mainly prepared by the following method: (1) adding a lithium iron manganese phosphate positive electrode material into an organic aqueous solution containing an aluminum source and a lithium source, stirring and mixing to obtain a lithium iron manganese phosphate positive electrode material metal ion dispersion liquid; (2) stirring and evaporating the metal ion dispersion liquid of the lithium iron manganese phosphate positive electrode material obtained in the step (1) until the metal ion dispersion liquid is evaporated to dryness, performing vacuum drying, and grinding to obtain mixed powder of the lithium iron manganese phosphate positive electrode material; and (3) calcining the lithium manganese iron phosphate positive electrode material mixed powder obtained in the step (2) in an inert atmosphere to obtain the lithium metaaluminate coated lithium manganese iron phosphate positive electrode material. The lithium iron manganese phosphate positive electrode material coated with the lithium metaaluminate is good in coating effect, good in conductivity, good in electrochemical performance under high magnification, ultra-long in cycle stability, simple and controllable in process, short in flow and suitable for industrial production.

Description

Technical Field

[0001] The present invention relates to a lithium iron manganese phosphate cathode material and a preparation method thereof, and particularly to a lithium aluminate-coated lithium iron manganese phosphate cathode material and a preparation method thereof. Background Art

[0002] With the vigorous development of new energy technologies, especially their extensive applications in electric vehicles and energy storage systems, higher requirements are imposed on the performance of lithium batteries. LiMn x Fe 1-x PO4 cathode materials have the advantages of being non-toxic, pollution-free, good safety performance, long life, and overcharge resistance. At the same time, Mn and Fe have abundant reserves and low prices, showing good commercial value. However, when applied on a large scale, there are still some problems to be solved. For example, the structural characteristics of LiMn x Fe 1-x PO4 cathode materials determine their poor conductivity and low lithium ion diffusion rate, and the Jahn-Teller effect of Mn 3+ promotes the precipitation of manganese, resulting in the attenuation of the cycle life and the reduction of cycle stability of the material. Therefore, how to prepare a lithium iron manganese phosphate cathode material with better performance and improve the electrochemical performance problems caused by its inherent material structure defects is of great significance.

[0003] CN109560266A discloses a preparation method of a lithium aluminate-coated lithium iron manganese phosphate cathode material, which first prepares a lithium iron manganese phosphate precursor, and then adds an aluminum source (Li2CO3 + Al2O3) for solid-state mixing and calcination to obtain a lithium aluminate-coated lithium iron manganese phosphate cathode material. However, this method requires two calcination processes under the protection of an inert atmosphere. In particular, the temperature of the second calcination is as high as 900-1000 °C. High temperature not only easily causes the destruction of the LMFP structure, particle agglomeration, poor coating uniformity, but also has high energy consumption and strict equipment requirements, which may increase production costs. In addition, since it does not provide specific electrochemical performance data (such as discharge specific capacity at 1 C, 5 C, etc.), lacking quantitative indicators, it is difficult to objectively evaluate its performance advantages.

[0004] CN116825989A discloses a preparation method of a lithium metaaluminate-coated lithium iron manganese phosphate cathode material. A lithium-containing compound and an aluminum-containing compound are mixed and stirred, ground, preheated, and synthesized into rod-shaped lithium metaaluminate by hydrothermal method. After introducing PEO lithium salt system with succinonitrile, a film is formed by solution casting method to obtain the lithium metaaluminate cathode material. Subsequently, polyethylene oxide is used to mix lithium metaaluminate and lithium iron manganese phosphate, and high-temperature calcination is carried out in a calcination furnace to obtain the lithium metaaluminate-coated lithium iron manganese phosphate cathode material. Although this method has improved the conductivity and electrochemical performance of the lithium iron manganese phosphate material, however, since this method involves multiple steps such as mixing, grinding, preheating, hydrothermal method, solution casting, and calcination, the process is complex, time-consuming, and relies on high-temperature calcination.

[0005] Therefore, there is an urgent need to find a lithium metaaluminate-coated lithium iron manganese phosphate cathode material with good coating effect, good conductivity, good electrochemical performance at high rate, and ultra-long cycle stability, and a preparation method thereof that is simple, controllable, short in process flow, and suitable for industrial production, so as to solve the problems of conductivity, rate performance, and cycle life of the lithium iron manganese phosphate material in battery applications. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the above-mentioned defects existing in the prior art, and provide a lithium metaaluminate-coated lithium iron manganese phosphate cathode material with good coating effect, good conductivity, good electrochemical performance at high rate, and ultra-long cycle stability, and a preparation method thereof that is simple, controllable, short in process flow, and suitable for industrial production.

[0007] The technical solution adopted by the present invention to solve its technical problems is as follows: A lithium metaaluminate-coated lithium iron manganese phosphate cathode material is mainly prepared by the following method: (1) Add the lithium iron manganese phosphate cathode material into an organic aqueous solution containing an aluminum source and a lithium source, stir and mix to obtain a dispersion of metal ions of the lithium iron manganese phosphate cathode material; (2) Stir and evaporate the dispersion of metal ions of the lithium iron manganese phosphate cathode material obtained in step (1) until it is completely dried, vacuum dry, and grind to obtain a mixed powder of the lithium iron manganese phosphate cathode material; (3) Calcinate the mixed powder of the lithium iron manganese phosphate cathode material obtained in step (2) in an inert atmosphere to obtain the lithium metaaluminate-coated lithium iron manganese phosphate cathode material.

[0008] The inventive concept of the present invention is as follows: Since lithium metaaluminate, a fast ion conductor, has high ionic conductivity and good electrochemical stability at room temperature, it can inhibit side reactions between the electrolyte and the electrode surface, and can also reduce electrode polarization during the charge and discharge process of the battery, thereby reducing the occurrence of irreversible reactions. At the same time, due to the poor conductivity of the lithium iron phosphate manganese cathode material, the coating of lithium metaaluminate can improve the overall conductivity of the electrode material, and the battery material is more likely to remain stable and reversible during the cycling process. In addition, lithium metaaluminate, which has a relatively stable crystal structure itself, can form a stable interface on the surface of the lithium iron phosphate manganese material, which can alleviate the Jahn-Teller effect of manganese in lithium iron phosphate manganese to a certain extent, inhibit the dissolution or precipitation of manganese, and reduce its migration inside the battery, thereby significantly improving the cycle life of the lithium iron phosphate manganese cathode material. In the present invention, an aluminum source and a lithium source are dissolved in an organic solution to form a homogeneous dispersion, the solvent is removed by stirring and evaporation, and a wet chemical method is used to uniformly attach the lithium metaaluminate precursor to the surface of the lithium iron phosphate manganese particles, and then calcined under an inert atmosphere to form a dense and uniform LiAlO2 coating layer, obtaining the lithium iron phosphate manganese cathode material coated with lithium metaaluminate. The method of the present invention adopts a three-step process (dispersion → evaporation → calcination), which significantly reduces the production complexity and cost, has a simple process flow, avoids complex synthesis processes, is easy to scale up production and application, and the formation process of the coating layer has strong controllability.

[0009] Preferably, in step (1), the preparation method of the lithium iron phosphate manganese cathode material is as follows: 1) grinding and mixing a manganese source, an iron(II) source, a lithium source, a phosphate source, a carbon source or also a dispersant evenly, drying in vacuum, and then grinding to obtain a lithium iron phosphate manganese precursor powder; 2) performing two-step calcination on the lithium iron phosphate manganese precursor powder obtained in step 1) under an inert atmosphere to obtain the lithium iron phosphate manganese cathode material.

[0010] Preferably, in step 1), the amounts of the manganese source, the iron(II) source, the lithium source and the phosphate source are such that the molar ratio of manganese element, iron element, lithium element, phosphate radical matches that of lithium iron phosphate manganese LiMn x Fe 1-x PO4, where 0.5 ≤ x < 0.9.

[0011] Preferably, in step 1), the amount of the carbon source is equivalent to 6-10% of the total mass of the manganese source, the iron(II) source, the lithium source and the phosphate source. Excessive carbon coating will reduce the tap density and compaction density of the material, resulting in a decrease in the volume energy density of the material. Therefore, selecting a suitable amount of carbon coating can effectively improve the overall performance of the material.

[0012] Preferably, in step 1), the mass-volume ratio g / mL of the total mass of the manganese source, the iron(II) source, the lithium source, the phosphate source and the carbon source to the dispersant is 1:0-10 (more preferably 1:2-8).

[0013] Preferably, in step 1), the manganese source includes one or more of manganese carbonate, manganese acetate, manganese oxalate, and their hydrates, etc.

[0014] Preferably, in step 1), the ferrous source includes one or more of ferrous oxalate, ferrous acetate, ferrous sulfate, and their hydrates, etc.

[0015] Preferably, in step 1), the lithium source includes one or more of lithium dihydrogen phosphate, lithium hydroxide, lithium carbonate, and their hydrates, etc. When the lithium source contains phosphate, it can also serve as a phosphate source.

[0016] Preferably, in step 1), the phosphate source includes one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, etc. When the phosphate source contains lithium, it can also serve as a lithium source.

[0017] Preferably, in step 1), the carbon source includes one or more of glucose, polyethylene glycol, citric acid, etc. Inorganic carbon sources usually form a relatively stable coating layer, while organic carbon sources can form a certain pore structure and connection function, which is more conducive to forming a uniform and continuous carbon coating.

[0018] Preferably, in step 1), the dispersant includes one or more of acetone, ethanol, water, etc.

[0019] Preferably, in step 1), the rotation speed of the grinding is 700 - 900 r / min, and the time is 7 - 9 h. The purpose of the first grinding before vacuum drying is to disperse and mix the raw materials, avoid powder caking, and ensure uniform mixing of each component; the purpose of the second grinding after vacuum drying is to further ensure the fineness and uniformity of the powder, laying a foundation for subsequent calcination. More preferably, the grinding is ball milling.

[0020] Preferably, in step 1), the temperature of the vacuum drying is 50 - 70 °C, the vacuum degree is -0.06 - -0.10 MPa, and the time is 4 - 8 h or until the dispersant is completely volatilized. The purpose of vacuum drying is to volatilize the dispersant or remove residual moisture.

[0021] Preferably, in step 2), the two-step calcination means: first, heat up to 300 - 400 °C at a rate of 5 - 10 °C / min, hold for 3 - 5 h, and then heat up to 650 - 750 °C at a rate of 5 - 10 °C / min, hold for 6 - 10 h. The purpose of the first calcination is to make each component in the precursor powder react fully to generate intermediate products or partial crystal nuclei; the purpose of the second calcination is to promote crystal growth and perfection. By optimizing the reaction process through two-step calcination, problems such as too large particle size and uneven crystal structure that may occur during direct one-time calcination at high temperature can be avoided, and the purity and consistency of the material can be improved.

[0022] Preferably, in step 2), the inert atmosphere includes one or more of nitrogen, argon, helium, etc.

[0023] Preferably, in step (1), the molar ratio of the sum of the number of moles of aluminum element and lithium element in the organic aqueous solution containing aluminum source and lithium source to the lithium iron phosphate cathode material is 0.012 - 0.092:1 (more preferably 0.02 - 0.08:1). If the coating amount of the aluminum-lithium compound formed is too small, it is difficult to achieve the purpose of protecting the matrix material; if the coating amount of the aluminum-lithium compound formed is too large, it may increase the internal resistance of the battery, reduce the charge transfer efficiency, and thus affect the battery capacity and discharge performance. Therefore, it is necessary to select an appropriate coating amount to find a balance between protecting the material and the discharge performance of the battery.

[0024] Preferably, in step (1), the method for preparing the organic aqueous solution containing aluminum source and lithium source is: adding the aluminum source into an organic solvent, stirring and dissolving it, and then adding the aqueous solution of lithium source.

[0025] Preferably, the molar ratio of aluminum element in the aluminum source to lithium element in the lithium source is 1:1.0 - 1.1 (more preferably 1:1.03 - 1.06). The purpose of excessive lithium element is to compensate for partial volatilization or decomposition at high temperature, which is beneficial to the formation of a high-quality lithium metaaluminate coating layer.

[0026] Preferably, the volume ratio of the organic solvent to the aqueous solution of lithium source is 1 - 3:1. The ratio of water to the organic solvent directly affects the solubility of the reactants. An appropriate ratio can promote the dissolution of the reactants, and thus is beneficial to the uniform progress of the reaction.

[0027] Preferably, the molar volume ratio mmol / L of aluminum element in the aluminum source to the organic solvent is 2 - 10:1 (more preferably 2.5 - 8.5:1). If the concentration of the reactants is too low, the reaction rate may be too slow, affecting the production efficiency; if the concentration of the reactants is too high, the reaction rate may be too fast, affecting the uniformity of the coating. Therefore, a moderate concentration of the reactants is crucial in the liquid-phase coating method, which can balance the reaction rate and the uniformity of the coating layer.

[0028] Preferably, the molar volume ratio mmol / L of lithium element in the lithium source to water is 4 - 14:1 (more preferably 5 - 12:1). If the amount of water used is too small, that is, the concentration of lithium element is too high, the reaction may be violent, affecting the coating effect; if the amount of water used is too much, the reaction will take a longer time to complete, increasing energy consumption. Therefore, selecting an appropriate concentration of lithium element is of great significance for optimizing the synthesis of the material.

[0029] Preferably, the temperature for stirring and dissolving is room temperature, the rotation speed is 300 - 600 r / min, and the time is 2 - 4 h.

[0030] Preferably, the aluminum source includes one or more of aluminum isopropoxide, aluminum nitrate, aluminum sulfate, etc.

[0031] Preferably, the lithium source includes one or more of lithium acetylacetonate, lithium hydroxide, lithium acetate, and their hydrates, etc.

[0032] Preferably, the organic solvent includes one or more of absolute ethanol, methanol, isopropanol, etc.

[0033] Preferably, in step (2), the temperature of the stirring evaporation is 50 - 70 °C (more preferably 55 - 65 °C), and the rotation speed is 300 - 600 r / min (more preferably 400 - 500 r / min). The process of stirring evaporation generates a lithium metaaluminate precursor through solvent evaporation and chemical reactions. If the temperature is too low, the evaporation rate will be too slow, affecting production efficiency; if the temperature is too high, the evaporation rate will be too fast, possibly resulting in side reactions and affecting the product quality. If the rotation speed is too low, local concentration differences may occur, affecting the reaction uniformity; if the rotation speed is too fast, splashing of the solution may occur, wasting materials and increasing equipment wear.

[0034] Preferably, in step (2), the temperature of the vacuum drying is 50 - 70 °C, the vacuum degree is -0.06 - -0.10 MPa, and the time is 4 - 8 h.

[0035] Preferably, in step (2), the rotation speed of the grinding is 100 - 200 r / min, and the time is 4 - 10 min. Grinding can ensure good bonding between the coating material and the lithium iron phosphate manganese material, promoting the reaction or coating during the subsequent calcination process. In addition, the grinding rotation speed should not be too fast, and the time should not be too long to prevent the crystal structure of the lithium iron phosphate manganese material from being damaged during grinding. More preferably, the grinding is ball milling.

[0036] Preferably, in step (3), the calcination means: heating to 550 - 650 °C (more preferably 570 - 650 °C) at a rate of 5 - 10 °C / min, and calcining for 2 - 8 h (more preferably 2 - 6 h). The purpose of the calcination is to decompose the lithium metaaluminate precursor, thereby forming a lithium metaaluminate coating layer on the surface of the lithium iron phosphate manganese material. In the said temperature range, the formation of lithium metaaluminate can be promoted, ensuring the uniformity and stability of the lithium metaaluminate coating layer, while avoiding the destruction of the crystal structure of the lithium iron phosphate manganese due to excessive temperature. The selection of the heating rate is to avoid the accumulation of thermal stress and particle rupture caused by rapid heating, ensuring the compatibility between the coating layer and the lithium iron phosphate manganese material. In addition, sufficient calcination time can ensure the complete progress of the reaction, forming a uniform and stable coating structure and improving the comprehensive performance of the lithium iron phosphate manganese material.

[0037] Preferably, in step (3), the inert atmosphere includes one or more of nitrogen, argon, helium, etc.

[0038] The inert atmosphere used in the present invention is a high-purity atmosphere with a purity ≥ 99.999%.

[0039] The beneficial effects of the present invention are as follows: (1) In the cathode material obtained by the method of the present invention, lithium metaaluminate is uniformly coated on the surface of the lithium iron phosphate cathode material, and it will not have a negative impact on the crystal structure of the lithium iron phosphate material; (2) For the battery assembled with the lithium iron phosphate cathode material coated with lithium metaaluminate of the present invention, at the current densities of 2 C, 5 C, and 10 C, the initial discharge specific capacities are as high as 134.91 mAh / g, 124.93 mAh / g, and 115.33 mAh / g respectively (the nominal specific capacity is 1 C = 170 mAh / g), indicating that the presence of the lithium metaaluminate coating layer significantly improves the electrochemical performance of the lithium iron phosphate cathode material at high rates; when the current density gradually decreases from the 10 C rate to the 2 C rate, the initial discharge specific capacity returns to 115.67 mAh / g (10 C), 125.15 mAh / g (5 C), and 135.08 mAh / g (2 C) in turn, indicating that the lithium iron phosphate cathode material coated with lithium metaaluminate of the present invention does not have serious capacity attenuation after high-rate discharge, and the lithium metaaluminate coating layer alleviates the expansion or irreversible reaction of the electrode material caused by rapid charge and discharge. In addition, at 2.0 - 4.5 V and a current density of 1 C, after 600 cycles of charge and discharge, the discharge specific capacity still remains at 130.12 mAh / g, and the capacity retention rate is as high as 93.38%, indicating that the lithium iron phosphate cathode material coated with lithium metaaluminate of the present invention has excellent ultra-long cycle stability; (3) The coating process of the method of the present invention only involves three main steps: solution mixing, evaporation drying, and calcination. The process flow is simple, easy to operate and control, with good coating effect and suitable for industrial production. Description of the Drawings

[0040] Figure 1 is the SEM image of the cathode material LiMn 0.5 Fe 0.5 PO4 of Reference Example 1 of the present invention; Figure 2 is the XRD image of the cathode material LiMn 0.5 Fe 0.5 PO4 of Reference Example 1 of the present invention; Figure 3 is the TEM image of the cathode material LiMn 0.5 Fe 0.5 PO4 of Reference Example 1 of the present invention; Figure 4 is the rate performance graph of the battery assembled with the cathode material LiMn 0.5 Fe 0.5 PO4 in Reference Example 1 of the present invention; Figure 5 is the cycle performance graph of the battery assembled with the cathode material LiMn 0.5 Fe 0.5 PO4 in Reference Example 1 of the present invention; Figure 6 is the SEM graph of the lithium metaaluminate-coated LiMn 0.5 Fe 0.5 PO4 cathode material in Example 1 of the present invention; Figure 7 is the XRD graph of the lithium metaaluminate-coated LiMn 0.5 Fe 0.5 PO4 cathode material in Example 1 of the present invention; Figure 8 is the TEM graph of the lithium metaaluminate-coated LiMn 0.5 Fe 0.5 PO4 cathode material in Example 1 of the present invention; Figure 9 is the rate performance graph of the battery assembled with the lithium metaaluminate-coated LiMn 0.5 Fe 0.5 PO4 cathode material in Example 1 of the present invention; Figure 10 is the cycle performance graph of the battery assembled with the lithium metaaluminate-coated LiMn 0.5 Fe 0.5 PO4 cathode material in Example 1 of the present invention. Detailed implementation manners

[0041] The present invention will be further described below in conjunction with examples and drawings.

[0042] The MnCO3, FeC2O4·2H2O, LiH2PO4, LiOH·H2O, and aluminum isopropoxide used in the examples, reference examples, and comparative examples of the present invention are all purchased from Aladdin Reagent Co., Ltd.; the purity of the high-purity argon gas used is 99.999%; the raw materials or chemical reagents used in the examples, reference examples, and comparative examples of the present invention are all obtained through conventional commercial channels unless otherwise specified.

[0043] Reference Example 1 for the preparation method of lithium iron phosphate manganese cathode material 1) 0.5748 g (0.005 mol) of MnCO3, 0.8995 g (0.005 mol) of FeC2O4·2H2O, 1.0393 g (0.010 mol) of LiH2PO4, 0.1508 g (0.7610 mmol) of C6H 120.0503 g (0.0063 mmol) of polyethylene glycol (number average molecular weight of 8000) and 20 mL of acetone were placed in an agate ball milling jar. In a planetary ball mill, ball milling was carried out at a speed of 750 r / min for 8 h until evenly mixed. Then it was transferred to a vacuum drying oven and vacuum dried at 60 °C and a vacuum degree of -0.06 MPa until the acetone completely volatilized. Then it was placed in a planetary ball mill again and ball milled at a speed of 750 r / min for 8 h to obtain the lithium iron manganese phosphate precursor powder. 2) Transfer the lithium iron manganese phosphate precursor powder obtained in step 1) to a tubular furnace filled with a high-purity argon atmosphere. Under a high-purity argon atmosphere, two-step calcination was carried out: first, it was heated to 350 °C at a rate of 5 °C / min and held for 4 h, and then heated to 700 °C at a rate of 5 °C / min and held for 8 h to obtain the lithium iron manganese phosphate cathode material LiMn 0.5 Fe 0.5 PO4 (abbreviated as LMFP, carbon content is 2.89 wt.%).

[0044] As Figure 1 shown, the surface of the LMFP particles obtained in the reference example of the present invention is smooth and has no obvious pores or cracks, showing good crystallinity.

[0045] As Figure 2 shown, the XRD diffraction pattern of the LMFP obtained in the reference example of the present invention highly coincides with the PDF#83-2092 standard card, belonging to the typical orthorhombic olivine structure, indicating that the obtained LMFP has a high purity and no impurities are generated.

[0046] As Figure 3 shown, the lattice fringe spacing of the LMFP obtained in the reference example of the present invention is 4.27 Å, corresponding to the (101) crystal plane of LMFP, and a carbon layer about 1 nm thick can be seen on the particle surface, showing an amorphous structure.

[0047] The LMFP obtained in the reference example of the present invention was made into a positive electrode plate, assembled into a battery, and its electrochemical performance was tested.

[0048] Battery assembly: The LMFP, acetylene black, and polyvinylidene fluoride obtained in the reference example of the present invention were weighed according to a mass ratio of 8:1:1, placed in a mortar and mixed evenly; then 8 drops of N-methylpyrrolidone (the dropping bottle specification is 30 mL) were added and wet milled for 3 min to form a black viscous slurry, which was evenly coated on the aluminum foil with a scraper; the aluminum foil was placed in a vacuum drying oven at 120 °C and a vacuum degree of -0.06 MPa and dried for 8 h, and then sliced to obtain a circular piece with a diameter of 12 mm as the positive electrode plate; using metallic lithium as the counter electrode, a CR2025 type button battery was assembled in a vacuum glove box for electrochemical performance testing.

[0049] Performance test: Constant current charge and discharge tests were carried out at different current densities from 2.0 V to 4.5 V.

[0050] As Figure 4 shown, the LMFP obtained in the reference example of the present invention was subjected to rate performance tests at 0.2 C, 0.5 C, 1 C, 2 C, 5 C, and 10 C. Among them, at the rates of 2 C, 5 C, and 10 C, the initial discharge specific capacities of the LMFP obtained in the reference example of the present invention were 130.62 mAh / g, 113.24 mAh / g, and 98.63 mAh / g, respectively; when the current density was gradually decreased from the 10 C rate to the 2 C rate, the initial discharge specific capacities were restored to 97.60 mAh / g (10 C), 111.89 mAh / g (5 C), and 128.46 mAh / g (2 C) in turn, indicating that the cathode material was highly reversible during the charge and discharge reaction process, but the capacity attenuation was obvious after high-rate discharge.

[0051] After detection, the initial discharge specific capacity of the LMFP obtained in the reference example of the present invention was 159.68 mAh / g at the 0.1 C rate, and the initial Coulombic efficiency was 94.62%; after 3 cycles of cyclic activation at the 0.1 C rate, as Figure 5 shown, at the 1 C rate, the initial discharge specific capacity was 135.92 mAh / g, and after 600 cycles of charge and discharge, the Coulombic efficiency was 97.75%; although after 100 cycles and 300 cycles of charge and discharge, the discharge specific capacities were 143.58 mAh / g and 135.69 mAh / g in turn, and the capacity retention rates were 105.64% and 99.83% in turn, however, after 600 cycles of charge and discharge, the discharge specific capacity was only 89.48 mAh / g, and the capacity retention rate was only 65.83%, indicating that although the LMFP obtained in the reference example of the present invention had good stability during cyclic charge and discharge, it performed poorly during ultra-long cyclic charge and discharge.

[0052] Preparation method of lithium iron manganese phosphate cathode material - Reference Example 2 The difference between this reference example and Reference Example 1 is only that: in step 1), 20 mL of acetone was not added; the vacuum drying time was 4 h; in step 2), the lithium iron manganese phosphate cathode material LiMn 0.5 Fe 0.5 PO4 (abbreviated as LMFP, carbon content is 2.91 wt.%) was obtained. The rest is the same as in Reference Example 1.

[0053] An example of a lithium metaaluminate-coated lithium iron manganese phosphate cathode material - Example 1 It is mainly prepared by the following method: (1) Add 1 g (6.1736 mmol) of the lithium iron manganese phosphate cathode material obtained in Reference Example 1 (carbon content is 2.89 wt.%) into 60 mL of an organic solution containing an aluminum source and a lithium source, stir and mix to obtain a dispersion of metal ions of the lithium iron manganese phosphate cathode material; The preparation method of the organic solution containing an aluminum source and a lithium source is as follows: Add 0.0310 g (0.1518 mmol) of aluminum isopropoxide into 30 mL of absolute ethanol, and stir and dissolve at room temperature at a rotation speed of 400 r / min for 2 h, then add an aqueous solution formed by dissolving 0.0067 g (0.1597 mmol) of LiOH·H2O in 30 mL of deionized water to obtain 60 mL of an organic solution containing an aluminum source and a lithium source; (2) Place the dispersion of metal ions of the lithium iron manganese phosphate cathode material obtained in step (1) in a constant temperature water bath, stir and evaporate to dryness at 60 °C and a rotation speed of 400 r / min, transfer it to a vacuum drying oven, and vacuum dry at 60 °C and a vacuum degree of -0.06 MPa for 6 h, then place it in a planetary ball mill and ball mill at a rotation speed of 100 r / min for 5 min to obtain a mixed powder of the lithium iron manganese phosphate cathode material; (3) Place the mixed powder of the lithium iron manganese phosphate cathode material obtained in step (2) in a tubular furnace filled with a high-purity argon atmosphere, and under the high-purity argon atmosphere, heat it to 600 °C at a rate of 5 °C / min and calcine for 4 h to obtain lithium iron manganese phosphate cathode material LiMn 0.5 Fe 0.5 PO4@LiAlO2 (abbreviated as LMFP@LAO, the coating amount of LAO is 1 wt.%).

[0054] As Figure 6 shown, the particle morphology of LMFP@LAO obtained in the embodiment of the present invention is highly similar to that of LMFP obtained in the reference example of the present invention, and the particle surface is relatively rough, indicating that a thin lithium aluminate coating layer may be formed on the surface of the LMFP@LAO particles, and the surface morphology of the particles is not significantly affected.

[0055] As Figure 7 shown, the XRD diffraction pattern of LMFP@LAO obtained in the embodiment of the present invention is highly consistent with the PDF#83-2092 standard card, belonging to the typical orthorhombic olivine-type structure, and compared with the diffraction pattern of LMFP obtained in the reference example of the present invention, there is no obvious peak position shift, indicating that the lithium aluminate coating has no significant impact on the crystal structure of the matrix material.

[0056] As Figure 8As shown, a lithium aluminate coating layer with a thickness of about 2 nm is formed on the surface of the LMFP@LAO particles obtained in the embodiments of the present invention. The lattice fringe spacing is 2.59 Å, corresponding to the (200) crystal plane of LAO; at the same time, the lattice fringe spacing of the matrix material is 2.46 Å, corresponding to the (121) crystal plane of LMFP.

[0057] The LMFP@LAO obtained in the embodiments of the present invention is made into a positive electrode sheet, assembled into a battery, and its electrochemical performance is tested.

[0058] Battery assembly: The same as Reference Example 1.

[0059] Performance test: The same as Reference Example 1.

[0060] As Figure 9 shown, the rate performance of the LMFP@LAO obtained in the embodiments of the present invention is tested at 0.2 C, 0.5 C, 1 C, 2 C, 5 C, and 10 C. Although at low rates of 0.2 C, 0.5 C, and 1 C, the high-rate performance advantage of the LMFP@LAO obtained in the embodiments of the present invention is smaller than that of the LMFP obtained in the reference examples of the present invention, at high rates of 2 C, 5 C, and 10 C, the initial discharge specific capacities of the LMFP@LAO obtained in the embodiments of the present invention are 134.91 mAh / g, 124.93 mAh / g, and 115.33 mAh / g in sequence, showing more excellent high-rate performance compared to the LMFP obtained in the reference examples of the present invention; when the current density gradually decreases from the 10 C rate to the 2 C rate, the initial discharge specific capacities recover to 115.67 mAh / g (10 C), 125.15 mAh / g (5 C), and 135.08 mAh / g (2 C) in sequence, not only indicating that the LMFP@LAO obtained in the embodiments of the present invention is highly reversible during the charge and discharge reaction process, but also showing more excellent high-rate performance compared to the LMFP obtained in the reference examples of the present invention; it shows that there is no obvious capacity attenuation after high-rate discharge of the LMFP@LAO obtained in the embodiments of the present invention, and the lithium aluminate coating layer alleviates the expansion or irreversible reaction of the electrode material caused by rapid charge and discharge.

[0061] After testing, the initial discharge specific capacity of the LMFP@LAO obtained in the embodiments of the present invention is 160.41 mAh / g at a rate of 0.1 C, and the initial Coulomb efficiency is 99.02%; after cycling and activating 3 times at a rate of 0.1 C, as Figure 10As shown, at a current rate of 1 C, the initial discharge specific capacity is 139.35 mAh / g. After 600 cycles of charge and discharge, the Coulombic efficiency remains at 98.59%. After 100, 300, and 600 cycles of charge and discharge, the discharge specific capacities are as high as 143.28 mAh / g, 141.68 mAh / g, and 130.12 mAh / g respectively, and the capacity retention rates are as high as 102.82%, 101.67%, and 93.38% respectively. After 600 cycles, compared with the LMFP obtained in the reference example of the present invention, the capacity retention rate is significantly improved, indicating that the LMFP@LAO obtained in the examples of the present invention has excellent ultra-long cycle stability.

[0062] Example 2 of Lithium Aluminate Coated Lithium Iron Manganese Phosphate Cathode Material It is mainly prepared by the following method: (1) Add 1 g (6.1736 mmol) of the lithium iron manganese phosphate cathode material obtained in Reference Example 1 (carbon content is 2.89 wt.%) into 50 mL of an organic solution containing aluminum source and lithium source, stir and mix to obtain a dispersion of metal ions of the lithium iron manganese phosphate cathode material; The preparation method of the organic solution containing aluminum source and lithium source is as follows: Add 0.0854 g (0.2277 mmol) of Al(NO3)3·9H2O into 30 mL of absolute ethanol, stir and dissolve at room temperature at a rotation speed of 300 r / min for 4 h, and then add an aqueous solution formed by dissolving 0.0100 g (0.2383 mmol) of LiOH·H2O in 20 mL of deionized water to obtain 50 mL of an organic solution containing aluminum source and lithium source; (2) Place the dispersion of metal ions of the lithium iron manganese phosphate cathode material obtained in step (1) in a constant temperature water bath, stir and evaporate at 65 °C and a rotation speed of 450 r / min until dry, transfer it to a vacuum drying oven, vacuum dry at 60 °C and a vacuum degree of -0.06 MPa for 6 h, and then place it in a planetary ball mill and ball mill at a rotation speed of 150 r / min for 5 min to obtain a mixed powder of the lithium iron manganese phosphate cathode material; (3) Place the mixed powder of the lithium iron manganese phosphate cathode material obtained in step (2) in a tubular furnace filled with high-purity argon atmosphere, heat it to 650 °C at a rate of 8 °C / min under high-purity argon atmosphere, and calcine for 2 h to obtain the lithium aluminate coated lithium iron manganese phosphate cathode material LiMn 0.5 Fe 0.5 PO4@LiAlO2 (abbreviated as LMFP@LAO, the coating amount of LAO is 1.5 wt.%).

[0063] Make the LMFP@LAO obtained in the examples of the present invention into a positive electrode plate, assemble it into a battery, and test its electrochemical performance.

[0064] Battery assembly: same as Reference Example 1.

[0065] Performance test: same as Reference Example 1.

[0066] After testing, the LMFP@LAO obtained in the embodiment of the present invention has an initial discharge specific capacity of 159.71 mAh / g and an initial Coulomb efficiency of 97.77% at a rate of 0.1 C; after 3 cycles of cyclic activation at a rate of 0.1 C, at a rate of 1 C, the initial discharge specific capacity is 135.17 mAh / g. After 600 cycles of charge and discharge, the Coulomb efficiency still remains at 98.21%. After 100 cycles, 300 cycles, and 600 cycles of charge and discharge, the discharge specific capacities are successively as high as 137.83 mAh / g, 135.65 mAh / g, and 121.73 mAh / g, and the capacity retention rates are successively as high as 101.97%, 100.36%, and 90.06%; after 600 cycles, compared with the LMFP obtained in the reference example of the present invention, the capacity retention rate is greatly improved, indicating that the LMFP@LAO obtained in the embodiment of the present invention has excellent ultra-long cycle stability.

[0067] Example 3 of a lithium aluminate-coated lithium iron manganese phosphate cathode material It is mainly prepared by the following method: (1) Add 1.0 g (6.1723 mmol) of the lithium iron manganese phosphate cathode material obtained in Reference Example 2 (carbon content is 2.91 wt.%) to 45 mL of an organic solution containing an aluminum source and a lithium source, stir and mix to obtain a dispersion of metal ions of the lithium iron manganese phosphate cathode material. The preparation method of the organic solution containing an aluminum source and a lithium source is as follows: Add 0.0155 g (0.0759 mmol) of aluminum isopropoxide to 30 mL of absolute ethanol, stir and dissolve at room temperature at a rotation speed of 600 r / min for 2 h, and then add an aqueous solution formed by dissolving 0.0033 g (0.0786 mmol) of LiOH·H2O in 15 mL of deionized water to obtain 45 mL of an organic solution containing an aluminum source and a lithium source. (2) Place the dispersion of metal ions of the lithium iron manganese phosphate cathode material obtained in step (1) in a constant temperature water bath, stir and evaporate at 60 °C and a rotation speed of 400 r / min until it is completely dried, transfer it to a vacuum drying oven, and vacuum dry at 60 °C and a vacuum degree of -0.06 MPa for 6 h, then place it in a planetary ball mill and ball mill at a rotation speed of 100 r / min for 10 min to obtain a mixed powder of the lithium iron manganese phosphate cathode material. (3) Place the mixed powder of the lithium iron manganese phosphate cathode material obtained in step (2) into a tube furnace filled with a high-purity argon atmosphere. Under the high-purity argon atmosphere, heat it to 580 °C at a rate of 5 °C / min and calcine for 3 h to obtain lithium iron manganese phosphate cathode material LiMn 0.5 Fe 0.5 PO4@LiAlO2 (abbreviated as LMFP@LAO, the coating amount of LAO is 0.5 wt.%).

[0068] Make the LMFP@LAO obtained in the example of the present invention into a positive electrode sheet, assemble it into a battery, and test its electrochemical performance.

[0069] Battery assembly: The same as Reference Example 1.

[0070] Performance test: The same as Reference Example 1.

[0071] After testing, the LMFP@LAO obtained in the example of the present invention has an initial discharge specific capacity of 159.98 mAh / g and an initial Coulomb efficiency of 99.08% at a 0.1 C rate; after 3 cycles of cyclic activation at a 0.1 C rate and then at a 1 C rate, the initial discharge specific capacity is 138.52 mAh / g. After 600 cycles of charge and discharge, the Coulomb efficiency still remains at 99.13%. After 100 cycles, 300 cycles, and 600 cycles of charge and discharge, the discharge specific capacities are successively as high as 143.39 mAh / g, 138.41 mAh / g, and 124.46 mAh / g, and the capacity retention rates are successively as high as 103.52%, 99.92%, and 89.85%; after 600 cycles, compared with the LMFP obtained in the reference example of the present invention, the capacity retention rate is improved, indicating that the LMFP@LAO obtained in the example of the present invention has excellent ultra-long cycle stability.

[0072] A comparative example 1 of lithium iron manganese phosphate cathode material coated with lithium aluminate The difference between this comparative example and Example 1 is only that: in step (1), the preparation method of the organic solution containing aluminum source and lithium source is: add 0.0310 g (0.1518 mmol) of aluminum isopropoxide to 20 mL of absolute ethanol, stir and dissolve at room temperature at a rotation speed of 400 r / min for 2 h, and then add an aqueous solution formed by dissolving 0.0067 g (0.1597 mmol) of LiOH·H2O in 5 mL of deionized water to obtain 25 mL of the organic solution containing aluminum source and lithium source; in step (3), the lithium iron manganese phosphate cathode material LiMn 0.5 Fe 0.5 PO4@LiAlO2 (abbreviated as LMFP@LAO, the coating amount of LAO is 1 wt.%). The rest is the same as Example 1 of the present invention.

[0073] The LMFP@LAO obtained from the comparative example of the present invention was made into a positive electrode sheet, assembled into a battery, and its electrochemical performance was tested.

[0074] Battery assembly: The same as Reference Example 1.

[0075] Performance test: The same as Reference Example 1.

[0076] After testing, the initial discharge specific capacity of the LMFP@LAO obtained from the comparative example of the present invention was 156.69 mAh / g at a rate of 0.1 C, and the initial Coulomb efficiency was 96.40%; after 3 cycles of cyclic activation at a rate of 0.1 C and then at a rate of 1 C, the initial discharge specific capacity was 135.54 mAh / g. After 600 cycles of charge and discharge, the Coulomb efficiency was 97.67%; although after 100 cycles and 300 cycles of charge and discharge, the discharge specific capacities were 138.62 mAh / g and 134.73 mAh / g respectively, and the capacity retention rates were 102.27% and 99.40% respectively, after 600 cycles of charge and discharge, the discharge specific capacity was only 99.28 mAh / g, and the capacity retention rate was only 73.25%; this shows that the ultra-long cycle stability of the LMFP@LAO material obtained from the comparative example of the present invention is poor, mainly because the high-concentration metal ion dispersion causes the solution viscosity to be too high, resulting in an enhanced electrostatic interaction between the lithium iron phosphate manganese positive electrode material particles, thereby causing particle aggregation or agglomeration, resulting in an insufficiently significant improvement in the electrochemical performance of the lithium iron phosphate manganese positive electrode material.

[0077] A Comparative Example 2 of Lithium Aluminate-Coated Lithium Iron Phosphate Manganese Positive Electrode Material The difference between this comparative example and Example 1 is only that: in step (2), the evaporation temperature is 85 °C. The rest is the same as Example 1 of the present invention.

[0078] The LMFP@LAO obtained from the comparative example of the present invention was made into a positive electrode sheet, assembled into a battery, and its electrochemical performance was tested.

[0079] Battery assembly: The same as Reference Example 1.

[0080] Performance test: The same as Reference Example 1.

[0081] After testing, for the LMFP@LAO obtained in the comparative example of the present invention, at a rate of 0.1 C, the initial discharge specific capacity is 155.17 mAh / g, and the initial Coulombic efficiency is 95.63%; after 3 cycles of cyclic activation at a rate of 0.1 C and then at a rate of 1 C, the initial discharge specific capacity is 132.69 mAh / g. After 600 cycles of charge and discharge, the Coulombic efficiency is 98.97%; although after 100 cycles and 300 cycles of charge and discharge, the discharge specific capacities are 133.84 mAh / g and 132.03 mAh / g respectively, and the capacity retention rates are 100.87% and 99.50% respectively, however, after 600 cycles of charge and discharge, the discharge specific capacity is only 96.22 mAh / g, and the capacity retention rate is only 72.51%; this shows that the ultra-long cycle stability of the LMFP@LAO material obtained in the comparative example of the present invention deteriorates. This is mainly because the evaporation temperature for forming the lithium metaaluminate precursor is too high, causing the organic solvent to volatilize too rapidly, resulting in the aggregation of material particles, affecting the formation and uniformity of the coating layer. At the same time, unnecessary by-products may also be generated, affecting the purity of the subsequent coating layer and being unfavorable for improving the overall performance of the matrix material.

[0082] Comparative Example 3 of Lithium Metaaluminate Coated Lithium Iron Manganese Phosphate Cathode Material The difference between this comparative example and Example 1 is only that: in step (3), the mixed powder of the lithium iron manganese phosphate cathode material obtained in step (2) is placed in a tubular furnace filled with a high-purity argon atmosphere. Under the high-purity argon atmosphere, it is heated to 800 °C at a rate of 5 °C / min and calcined for 8 h to obtain the lithium metaaluminate coated lithium iron manganese phosphate cathode material LiMn 0.5 Fe 0.5 PO4@LiAlO2 (abbreviated as LMFP@LAO, the coating amount of LAO is 1 wt.%). The rest is the same as in Example 1 of the present invention.

[0083] The LMFP@LAO obtained in the comparative example of the present invention is made into a positive electrode sheet, assembled into a battery, and its electrochemical performance is tested.

[0084] Battery assembly: The same as in Reference Example 1.

[0085] Performance test: The same as in Reference Example 1.

[0086] After testing, for the LMFP@LAO obtained in the comparative example of the present invention, the initial discharge specific capacity is 139.11 mAh / g and the initial Coulombic efficiency is 93.52% at a rate of 0.1 C; after 3 cycles of cyclic activation at a rate of 0.1 C and then at a rate of 1 C, the initial discharge specific capacity is 130.78 mAh / g. After 600 cycles of charge and discharge, the Coulombic efficiency is 99.03%; although after 100 cycles of charge and discharge, the discharge specific capacity is 128.01 mAh / g and the capacity retention rate is 97.88%, however, after 300 cycles and 600 cycles of charge and discharge, the discharge specific capacities are only 96.30 mAh / g and 52.44 mAh / g respectively, and the capacity retention rates are only 73.64% and 40.10% respectively; this shows that the ultra-long cycle stability of the LMFP@LAO material obtained in the comparative example of the present invention deteriorates, even lower than that of Reference Example 1. This is mainly because too high a temperature may cause changes in the crystal structure or chemical composition of lithium iron manganese phosphate, affecting the purity and electrochemical performance of the material and being unfavorable for the performance of the material.

Claims

1. A lithium aluminate coated lithium manganese iron phosphate positive electrode material, characterized in that: Mainly made by the following methods: (1) adding a lithium iron manganese phosphate positive electrode material to an organic aqueous solution containing an aluminum source and a lithium source, stirring and mixing, and obtaining a metal ion dispersion of the lithium iron manganese phosphate positive electrode material; (2) stirring and evaporating the metal ion dispersion of the lithium manganese iron phosphate positive electrode material obtained in step (1) to dryness, vacuum drying, and grinding to obtain a mixed powder of the lithium manganese iron phosphate positive electrode material; (3) The mixed powder of the lithium manganese iron phosphate positive electrode material obtained in step (2) is calcined under an inert atmosphere to obtain a lithium aluminate-coated lithium manganese iron phosphate positive electrode material.

2. The lithium aluminate coated lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step (1), the preparation method of the lithium iron manganese phosphate positive electrode material is: 1) grinding and mixing a manganese source, a ferrous source, a lithium source, a phosphate source, a carbon source or a dispersant uniformly, vacuum drying, and grinding again to obtain a lithium iron manganese phosphate precursor powder; 2) The lithium iron manganese phosphate precursor powder obtained in step 1) is subjected to two-step calcination under an inert atmosphere to obtain a lithium iron manganese phosphate positive electrode material.

3. The lithium aluminate coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: In step 1), the amount of the manganese source, ferrous source, lithium source and phosphate source is such that the molar ratio of manganese element, iron element, lithium element and phosphate is the same as that of lithium iron manganese phosphate LiMn x Fe 1-x PO4 is matched, wherein 0.5≤x<0.9; the amount of the carbon source is equivalent to 6-10% of the total mass of the manganese source, the ferrous source, the lithium source and the phosphate source; the mass volume ratio of the total mass of the manganese source, the ferrous source, the lithium source, the phosphate source and the carbon source to the dispersant is 1:0-10 g / mL; the manganese source includes manganese carbonate, manganese acetate or manganese oxalate, and one or more of their hydrates; the ferrous source includes ferrous oxalate, ferrous acetate or ferrous sulfate, and one or more of their hydrates; the lithium source includes lithium dihydrogen phosphate, lithium hydroxide or lithium carbonate, and one or more of their hydrates; the phosphate source includes one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate or phosphoric acid; the carbon source includes one or more of glucose, polyethylene glycol or citric acid; the dispersant includes one or more of acetone, ethanol or water; the grinding speed is 700-900 r / min, and the time is 7-9 h; the vacuum drying temperature is 50-70 ℃, vacuum degree is -0.06~-0.10 MPa, time is 4~8 h or until the dispersant is completely volatilized.

4. The lithium aluminate coated lithium manganese iron phosphate positive electrode material according to claim 2 or 3, characterized in that: In step 2), the two-step calcination refers to: first heating to 300-400°C at a rate of 5-10°C / min, keeping warm for 3-5 hours, and then heating to 650-750°C at a rate of 5-10°C / min, keeping warm for 6-10 hours; the inert atmosphere includes one or more of nitrogen, argon or helium.

5. The lithium aluminate coated lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 4, characterized in that: In step (1), the molar ratio of the sum of the molar numbers of aluminum and lithium in the organic aqueous solution containing aluminum source and lithium source to the lithium iron phosphate positive electrode material is 0.012-0.092:1; the preparation method of the organic aqueous solution containing aluminum source and lithium source is: adding aluminum source to an organic solvent, stirring and dissolving, and then adding lithium source aqueous solution to obtain the product; the molar ratio of aluminum in the aluminum source to lithium in the lithium source is 1:1.0-1.1; the volume ratio of the organic solvent to the lithium source aqueous solution is 1-3:1; the molar volume ratio of aluminum in the aluminum source to the organic solvent is 2-10:1 in mmol / L; the molar volume ratio of lithium in the lithium source to water is 4-14:1 in mmol / L; the temperature of the stirring and dissolving is room temperature, the rotation speed is 300-600 r / min, and the time is 2-4 h; the aluminum source includes one or more of aluminum isopropoxide, aluminum nitrate or aluminum sulfate; the lithium source includes one or more of lithium acetylacetonate, lithium hydroxide or lithium acetate, and hydrates thereof; the organic solvent includes one or more of anhydrous ethanol, methanol or isopropanol.

6. The lithium aluminate coated lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 5, characterized in that: In step (2), the temperature of the stirring evaporation is 50 to 70°C, and the rotation speed is 300 to 600 r / min; the temperature of the vacuum drying is 50 to 70°C, the vacuum degree is -0.06 to -0.10 MPa, and the time is 4 to 8 h; the rotation speed of the grinding is 100 to 200 r / min, and the time is 4 to 10 min.

7. The lithium aluminate coated lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 6, characterized in that: In step (3), the calcination refers to: heating to 550-650°C at a rate of 5-10°C / min, and calcining for 2-8 hours; the inert atmosphere includes one or more of nitrogen, argon or helium.

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