Manganese iron lithium phosphate based on molten salt assisted rapid preparation and preparation method and application thereof

Abstract

This application provides a method for the rapid preparation of lithium manganese iron phosphate (LFP) using molten salt-assisted synthesis, and its applications, relating to the field of lithium battery cathode materials technology. This invention prepares high-performance LFP cathode materials through a one-step, short-time, high-temperature calcination process by controlling the types and proportions of iron, manganese, transition metal, and carbon sources, using a multi-element inorganic salt as the liquid reaction environment for the solid-state reaction. The multi-element inorganic salt forms a molten salt liquid under high-temperature conditions, improving mass transfer kinetics, avoiding secondary calcination, shortening the solid-state sintering reaction time, and alleviating the problem of uneven mass transfer during solid-state sintering. Furthermore, the multi-element inorganic salt and solvent do not participate in the chemical reaction, ensuring the uniformity and stability of the material. This invention features simple process conditions, low equipment requirements, easy process control, and easy large-scale industrial production. The multi-element inorganic salt and solvent are recyclable, making it environmentally friendly, and it reduces the energy consumption and production costs of traditional high-temperature calcination processes, resulting in significant economic benefits.

Description

Technical Field

[0001] This invention relates to the field of lithium battery cathode material technology, and in particular to lithium manganese iron phosphate based on molten salt-assisted rapid preparation, its preparation method and application. Background Technology

[0002] With the booming development of the electric vehicle industry and the continuous increase in demand for renewable energy storage, the market has placed higher demands on the energy density of lithium-ion battery cathode materials. Lithium manganese iron phosphate, with its Mn²⁺... + / Mn³ + The corresponding redox potential of 4.1 V is significantly higher than that of lithium iron phosphate (LiFePO4) at 3.4 V, which increases its energy density by 15%-20% compared to LiFePO4. At the same time, it inherits the high stability of the olivine structure and is considered a core candidate for the next generation of lithium-ion battery cathode materials, attracting widespread research and attention from the industry.

[0003] However, during the charging and discharging process of lithium manganese iron phosphate, the trivalent manganese induces the Jan Taylor effect, leading to manganese dissolution, which severely reduces the cycle life of the battery. At the same time, the introduction of manganese further reduces the electronic conductivity and ion diffusion rate of the material, resulting in severe polarization and poor rate performance under high current. In addition, although the solid-state sintering process of lithium manganese iron phosphate material is simple and easy to scale up, it requires long-term high-temperature calcination, which not only consumes a lot of energy but also makes it difficult to achieve uniform contact and full reaction of raw materials. This results in poor particle uniformity and unstable phase purity, ultimately causing poor batch consistency of products and seriously restricting its industrialization process.

[0004] Compared to traditional solid-state sintering, which relies on contact reactions between solid particles and suffers from low mass transfer efficiency, the molten salt-assisted preparation strategy fundamentally accelerates mass transfer kinetics by introducing inorganic salts to create a liquid reaction environment during calcination. This significantly shortens the long, high-temperature calcination cycle, dramatically improving preparation efficiency while reducing energy consumption. Furthermore, the liquid environment ensures thorough mixing and uniform reaction of reactants, effectively addressing the problem of uneven phase distribution and greatly improving material uniformity and stability. However, the molten salt-assisted strategy requires precise selection of suitable inorganic salts as the molten salt medium to ensure it only functions as a reaction medium and does not chemically react with reactants or products. Simultaneously, it must avoid generating additional wastewater, waste gas, and waste materials, thus preventing environmental pollution and increased costs.

[0005] Therefore, there is an urgent need for a new preparation strategy that can overcome the intrinsic performance defects of lithium manganese iron phosphate, break through the limitations of traditional preparation processes, and meet the requirements of dielectric compatibility, phase integrity, and green low cost, so as to promote the industrial application of lithium manganese iron phosphate cathode materials. Summary of the Invention

[0006] To address the problems existing in the background technology, the purpose of this invention is to propose a rapid preparation method for lithium manganese iron phosphate based on molten salt assistance, and its application. This method is simple and efficient, requires low equipment, has low cost, and the prepared material has the performance advantages of high safety, high capacity, and long life.

[0007] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: In a first aspect, the present invention provides a method for the rapid preparation of lithium manganese iron phosphate based on molten salt assistance, comprising the following steps: S1: Lithium source, manganese source, iron source, phosphorus source and transition metal source are mixed to obtain precursor raw material, carbon source is added for mechanical grinding, then multi-element inorganic salt is added for mixing, and calcined under inert conditions to obtain gray powder. The molar ratio of the lithium source, manganese source, iron source, phosphorus source and transition metal source is 1:0.2~0.8:0.2~0.8:0.8~1.2:0.01~0.08; The mass ratio of the precursor raw material, carbon source, and multi-element inorganic salt is 1:0.05~0.2:0.2~0.8; The multi-element inorganic salt is at least one of potassium carbonate, potassium chloride, potassium nitrate, sodium chloride, and sodium iodide; The transition metal source is at least one of titanium, magnesium, molybdenum, niobium, aluminum, copper, and nickel sources; The carbon source is at least one of glucose, sucrose, oxalic acid, polyvinylpyrrolidone, and polyethylene glycol; S2: The gray powder is washed and separated by an aqueous solution, and the separated solid is dried to obtain the lithium manganese iron phosphate.

[0008] Furthermore, the molar ratio of the lithium source, manganese source, iron source, phosphorus source, and transition metal source is 1:0.3~0.6:0.3~0.6:0.8~1.1:0.01~0.05; The mass ratio of the precursor raw material, carbon source, and multi-element inorganic salt is 1:0.05~0.15:0.3~0.7; The lithium source is at least one of lithium carbonate, lithium acetate, lithium oxalate, and lithium hydroxide. The manganese source is at least one of manganese acetate, manganese carbonate, and manganese oxide; The iron source is at least one of ferric oxide, ferrous oxalate, and ferrous oxide. The phosphorus source is at least one of diammonium hydrogen phosphate, lithium phosphate, and phosphorus pentoxide. The titanium source is at least one of titanium dioxide, tetrabutyl titanate, titanium carbide, and titanium fluoride; The magnesium source is at least one of magnesium carbonate, magnesium oxalate, magnesium acetate, and magnesium metaborate. The molybdenum source is at least one of molybdenum acetylacetonate, molybdenum carbide, molybdenum fluoride, and ammonium molybdate. The niobium source is at least one of niobium oxide, niobium pentachloride, and niobium oxalate. The aluminum source is at least one of aluminum hydroxide, aluminum acetylacetone, aluminum metaphosphate, aluminum nitrate, and aluminum dihydrogen phosphate. The copper source is at least one of copper citrate, copper pyrophosphate, copper acetylacetone, and copper oxide; The nickel source is at least one of nickel hydroxide, nickel acetylacetone, nickel oxide, and nickel acetate.

[0009] Furthermore, the multi-element inorganic salt is selected from one of the following combinations: potassium carbonate and potassium chloride, potassium carbonate and potassium nitrate, potassium chloride and sodium chloride, or potassium chloride and sodium iodide.

[0010] Furthermore, the two inorganic salts in the combination are mixed in a molar ratio of 1~2:1~2 to obtain the multi-element inorganic salt.

[0011] Furthermore, the mechanical grinding speed is 600~800 r / min and the time is 5~10 h; after adding the multi-element inorganic salt, the mixture is further subjected to low-speed mixing at a speed of 100~300 r / min for 10~60 min.

[0012] High-speed mechanical grinding is used to mix raw materials evenly and to increase the binding force between raw materials through ball mill shearing force. Low-speed mixing process is only used to mix inorganic salts and reaction raw materials evenly and does not participate in chemical reaction. Therefore, high-energy ball milling should not be carried out in the low-speed mixing process to increase the possibility of inorganic salts reacting with raw materials.

[0013] Furthermore, the calcination temperature is 620℃~680℃, and the time is 3~5 h.

[0014] Furthermore, the washing and separation rotation speed is 5000~8000 r / min, and the time is 3~5 min; the drying temperature is 80℃~160℃, and the time is 10~50 h.

[0015] Furthermore, the waste liquid from washing and separation is collected and evaporated and condensed to recover multi-element inorganic salts. The evaporation and condensation temperature is 80℃~120℃ and the time is 10~30 h. Cold water is used as the condensing agent in the condensation process.

[0016] If the evaporation temperature is too high, it will hinder the precipitation of inorganic salts; if the evaporation temperature is too low, the process efficiency will be compromised.

[0017] Secondly, the present invention provides a high-performance lithium manganese iron phosphate material prepared by the above method.

[0018] Thirdly, this invention provides the application of the above-mentioned high-performance lithium manganese iron phosphate material in the preparation of lithium battery cathodes.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: 1) This invention uses multi-element inorganic salts to provide a liquid reaction environment for solid-phase reactions, improves mass transfer kinetics, reduces sintering processes, shortens solid-phase sintering reaction time, and solves the problem of uneven mass transfer in solid-phase sintering; this invention adopts a one-step short-time high-temperature calcination process to successfully achieve stable and efficient preparation of lithium manganese iron phosphate cathode materials.

[0020] 2) The multi-element inorganic salts and solvents used in this invention do not participate in chemical reactions, resulting in lithium manganese iron phosphate materials with uniform phases, high purity, and excellent electrochemical performance.

[0021] 3) The molten salt-assisted rapid preparation technology provided by this invention has simple process conditions, low equipment requirements, easy process control, and is easy to realize large-scale industrial production; the multi-element inorganic salts and solvents can be recycled and reused, which is green and environmentally friendly, and reduces the energy consumption of traditional high-temperature calcination processes, thereby reducing production costs and having great economic benefits. Attached Figure Description

[0022] Figure 1 This is a flowchart illustrating the rapid preparation of lithium manganese iron phosphate cathode material using molten salt assistance according to the present invention. Figure 2 The XRD patterns are of the molten salt-assisted lithium manganese iron phosphate cathode materials prepared in Examples 1, 1, and 2 of this invention. Figure 3 Raman diagrams of the molten salt-assisted lithium manganese iron phosphate cathode materials prepared in Examples 1, 1, and 2 of this invention; Figure 4 This is a SEM image of the molten salt-assisted lithium manganese iron phosphate cathode material prepared in Example 1 of this invention; Figure 5 The diagram shows the long-cycle performance of the molten salt-assisted lithium manganese iron phosphate cathode materials prepared in Examples 1, 1, and 2 of this invention. Figure 6 The above are charge-discharge curves of the molten salt-assisted lithium manganese iron phosphate cathode materials prepared in Examples 1, 1, and 2 of this invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0024] Unless otherwise specified, the experimental methods used in the embodiments of this invention are all conventional methods.

[0025] Unless otherwise specified, all materials and reagents used in the embodiments of this invention are commercially available.

[0026] The Jan Taylor effect and insufficient conductivity of lithium manganese iron phosphate are major obstacles to its development, while the lengthy processing time and unstable products further restrict its widespread application.

[0027] This invention uses lithium salts, phosphorus sources, transition metal sources, carbon sources, etc. as raw materials and suitable multi-element inorganic salts as reaction media to prepare high-performance lithium manganese iron phosphate through a one-step short-time high-temperature calcination process. As a positive electrode material for lithium-ion batteries, it has excellent electrochemical performance.

[0028] The technical solution is as follows: See Figure 1 , Figure 1 This is a flowchart illustrating the rapid preparation of lithium manganese iron phosphate cathode material using molten salt assistance according to the present invention. The invention provides a method for the rapid preparation of lithium manganese iron phosphate based on molten salt assistance. Lithium source, manganese source, iron source, phosphorus source, and transition metal source are added to a mixing tank according to atomic molar ratios, along with a certain amount of carbon source. The mixture is then subjected to high-speed mechanical grinding until fully mixed. Next, a certain amount of multi-element inorganic salt is added by mass percentage, and the mixture is subjected to low-speed mixing to obtain a composite precursor. The composite precursor is then subjected to one-step high-temperature calcination in an inert atmosphere to obtain a gray powder. The gray powder is then washed and separated using an aqueous solution, and dried at low temperature to obtain a high-performance lithium manganese iron phosphate cathode material. The waste liquid after washing and separation is evaporated, and the condensed steam is used to recover the solvent, which can be reused to wash the gray powder. The precipitated multi-element salt can be repeatedly mixed and calcined at high temperatures.

[0029] Example 1 1. 1.55 g lithium carbonate, 3.36 g manganese acetate, 3.49 g ferrous oxalate dihydrate, 4.6 g ammonium dihydrogen phosphate, 0.03 g magnesium carbonate, 0.22 g niobium oxalate, 0.03 g aluminum hydroxide, and 1.33 g sucrose were simultaneously added to a ball mill mixing jar and continuously ground at 800 rpm for 10 hours to obtain a uniformly mixed solid powder; wherein, the molar mass of lithium carbonate was 73.89 g / mL. g / mol, with 2 atoms per mole, approximately 0.042 moles. The number of moles of lithium carbonate is designated as 1. The molar ratio of lithium carbonate, manganese acetate, ferrous oxalate dihydrate, ammonium dihydrogen phosphate, magnesium carbonate, niobium oxalate, and aluminum hydroxide is 1:0.46:0.46:0.95:0.01:0.01:0.01. The actual molar ratio of lithium source, manganese source, iron source, phosphorus source, and transition metal source is 1:0.46:0.46:0.95:0.03. 2. Add 4.22 g of potassium carbonate and 3.09 g of potassium nitrate to the above solid powder, and grind continuously at 200 rpm for 20 minutes to obtain the composite precursor; 3. The composite precursor was calcined at 650°C for 3 hours in a nitrogen atmosphere to obtain a gray powder; 4. Place the gray powder in an aqueous solution, shake well, and then centrifuge at 8000 rpm for 5 minutes to obtain the solid-liquid separation product; 5. Place the waste liquid from the solid-liquid separation product in a flask, evaporate and condense at 100℃ for 12 hours to obtain an aqueous solution, and collect the precipitated multi-element inorganic salts for the next mixing. 6. After drying the solid in the solid-liquid separation product in an oven at 120°C for 24 hours, lithium manganese iron phosphate cathode material is obtained.

[0030] Comparative Example 1 1. Add 1.55 g lithium carbonate, 3.36 g manganese acetate, 3.49 g ferrous oxalate dihydrate, 4.6 g ammonium dihydrogen phosphate, 0.03 g magnesium carbonate, 0.22 g niobium oxalate, 0.03 g aluminum hydroxide, and 1.33 g sucrose to a ball mill mixing jar and grind continuously at 800 rpm for 10 hours to obtain a uniformly mixed solid powder. 2. The solid powder was calcined at 650°C for 3 hours in a nitrogen atmosphere to obtain lithium manganese iron phosphate cathode material.

[0031] Comparative Example 2 1. Add 1.55 g lithium carbonate, 3.36 g manganese acetate, 3.49 g ferrous oxalate dihydrate, 4.6 g ammonium dihydrogen phosphate, 0.03 g magnesium carbonate, 0.22 g niobium oxalate, 0.03 g aluminum hydroxide, and 1.33 g sucrose to a ball mill mixing jar and grind continuously at 800 rpm for 10 hours to obtain a uniformly mixed solid powder. 2. The solid powder was calcined at 650°C for 12 hours in a nitrogen atmosphere to obtain lithium manganese iron phosphate cathode material.

[0032] Test Example 1 The molten salt-assisted lithium manganese iron phosphate cathode materials prepared in Example 1, Comparative Example 1, and Comparative Example 2 were characterized by X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM), and the results were as follows. Figure 2-4 The results are shown in the figure.

[0033] Figure 2 XRD patterns of molten salt-assisted lithium manganese iron phosphate cathode materials, from Figure 2It can be seen that the Bragg diffraction peaks of the three materials are almost identical, indicating that all three materials can be classified as lithium manganese iron phosphate materials. Among them, the diffraction peak of Example 1 is completely matched with the characteristic diffraction peak of standard lithium manganese iron phosphate, and no other impurity peaks appear. This proves that the molten salt-assisted rapid preparation technology improves the process efficiency without destroying the intrinsic olivine structure, forming a single solid solution phase with good crystallinity, which is conducive to structural stability.

[0034] The diffraction peaks of Comparative Example 1 are weaker. Apart from the main peak having significant characteristics, the other diffraction peaks almost coincide with the baseline, proving that without the assistance of molten salt, a short calcination time cannot effectively promote the crystallization of lithium manganese iron phosphate. At the same time, the baseline shifts upward, which may be due to the incomplete decomposition of organic carbon in the raw materials.

[0035] The diffraction peak intensity of Comparative Example 2 is significantly higher than that of Comparative Example 1, which again shows that the solid-state sintering process without molten salt assistance requires a longer calcination time to promote crystal crystallization. However, Comparative Example 2 shows obvious impurity peaks at around 29 degrees and 41 degrees, proving that the lithium manganese iron phosphate cathode material prepared by Comparative Example 2 is not pure. This is also a problem of poor material uniformity in traditional solid-state sintering.

[0036] Depend on Figure 2 It can be seen that the molten salt-assisted rapid preparation technology provided by the present invention can obtain pure phase, highly crystalline, and highly uniform lithium manganese iron phosphate cathode material in a short time, which fully verifies the advanced nature of the present invention.

[0037] Figure 3 Raman plot for molten salt-assisted lithium manganese iron phosphate cathode material, from Figure 3 It can be seen that all three materials are at 1350 cm. -1 and 1580 cm -1 The presence of Raman characteristic peaks resembling those of graphite nearby indicates that all three materials contain carbon. This is closely related to the addition of sucrose to the raw materials, which aims to address the issue of poor electrical conductivity. It is noteworthy that the carbon Raman characteristic peak intensity in Comparative Example 1 is weaker, indicating that short-time solid-state sintering without molten salt assistance did not promote complete carbon decomposition and the formation of a carbon coating layer. Furthermore, the three materials exhibit Raman characteristic peaks at 1000 cm⁻¹. -1 Phosphate vibration peaks appeared on both sides, proving the successful preparation of lithium manganese iron phosphate material. However, the phosphate vibration peak of Comparative Example 1 showed a significant peak envelope, while the phosphate vibration peak of Comparative Example 2 was consistent with that of Example 1, proving that the molten salt-assisted rapid sintering technology can effectively shorten the calcination time, improve the material preparation efficiency, and reduce energy consumption costs.

[0038] Figure 4 SEM images of molten salt-assisted lithium manganese iron phosphate cathode material, from Figure 4As can be seen, Example 1 is mainly composed of micron-sized polyhedral particles with clear particle boundaries and good dispersion, without obvious large-area sintering and agglomeration. The particle size is distributed from several micrometers to tens of micrometers, with some fine particles distributed between the larger particles, which is beneficial to improving the interparticle filling effect and electrode compaction density.

[0039] Test Example 2 The molten salt-assisted lithium manganese iron phosphate cathode materials prepared in Examples 1, 1, and 2 can be applied to the cathode of lithium-ion batteries. A lithium-ion battery includes a negative electrode, a positive electrode, a separator, and an electrolyte that wets the separator.

[0040] The positive electrode sheet is prepared by using the above-mentioned high-performance lithium manganese iron phosphate material as the active material, acetylene black as the conductive agent, and polyvinylidene fluoride as the binder, mixing them evenly and coating them on aluminum foil, and drying them to form a positive electrode sheet. The mass ratio of high-performance lithium manganese iron phosphate material, acetylene black and polyvinylidene fluoride is 7~8:1~2:1.

[0041] The electrochemical performance of the molten salt-assisted lithium manganese iron phosphate cathode materials prepared in Example 1, Comparative Example 1, and Comparative Example 2 was tested, and the results were as follows: Figure 5-6 The results are shown in the figure.

[0042] Figure 5 Long-cycle testing diagrams for molten salt-assisted lithium manganese iron phosphate cathode materials, from... Figure 4 As can be seen, at a current density of 1C, the initial capacities of Example 1 and Comparative Example 2 are similar, at 115 mAh / g and 116 mAh / g, respectively. After 1500 cycles, the capacity retention rate of Example 1 is 85%, while that of Comparative Example 2 is only 65% ​​after 1200 cycles. In contrast, the initial capacity of lithium manganese iron phosphate prepared by solid-state sintering without molten salt assistance is only 72 mAh / g, and the capacity retention rate is 88% after 850 cycles. This demonstrates that the molten salt-assisted rapid preparation technology not only improves the preparation efficiency but also improves the cycle life of the material.

[0043] Figure 6 The charge-discharge curves of molten salt-assisted lithium manganese iron phosphate cathode material are shown below. Figure 6It can be seen that under constant current charge-discharge conditions at 1C current density, Example 1 and Comparative Example 2 exhibit obvious dual-plateau characteristics, indicating that both iron and manganese act as active sites and undergo redox reactions. The iron plateau in Comparative Example 1 is more obvious, while the manganese plateau is shorter, indicating that manganese hardly undergoes a redox reaction. Therefore, under the same conditions, Comparative Example 1 has a higher capacity. At the same time, the third plateau in Comparative Example 1 is more obvious at around 3.7V. This is due to the poor conductivity of the material and the polarization voltage caused by the uneven distribution of manganese ion charge. Comparative Example 2 shows a slight third plateau, while Example 1 hardly shows a third plateau. This proves that the conductivity of Example 1 is higher than that of Comparative Example 1 and Comparative Example 2, verifying the feasibility and advancement of the molten salt-assisted rapid sintering technology.

[0044] Example 2 1. 1.55 g lithium carbonate, 3.32 g manganese acetate, 3.45 g ferrous oxalate dihydrate, 4.6 g ammonium dihydrogen phosphate, 0.3 g nickel acetate, 0.03 g titanium dioxide, and 2.4 g sucrose were simultaneously added to a ball mill mixing jar and ground continuously at 800 rpm for 10 hours to obtain a uniformly mixed solid powder. Taking the molar number of lithium carbonate as 1, the molar ratio of lithium carbonate, manganese acetate, ferrous oxalate dihydrate, ammonium dihydrogen phosphate, nickel acetate, and titanium dioxide was 1:0.46:0.46:0.95:0.04:0.01. The actual molar ratio of lithium source, manganese source, iron source, phosphorus source, and transition metal source was 1:0.46:0.46:0.95:0.05. 2. Add 5.42 g of potassium carbonate and 3.97 g of potassium nitrate to the above solid powder, and grind continuously at 100 rpm for 60 minutes to obtain the composite precursor; 3. The composite precursor was calcined at 650°C for 4 hours in a nitrogen atmosphere to obtain a gray powder; 4. Place the gray powder in an aqueous solution, shake well, and then centrifuge at 5000 rpm for 5 minutes to obtain the solid-liquid separation product. 5. Place the waste liquid from the solid-liquid separation product in a flask, evaporate and condense at 120℃ for 12 hours to obtain an aqueous solution, and collect the precipitated multi-element inorganic salts for the next mixing. 6. The solid in the solid-liquid separation product is dried in an oven at 80°C for 48 hours to obtain lithium manganese iron phosphate cathode material.

[0045] The electrochemical performance of the lithium manganese iron phosphate cathode material prepared in Example 2 is comparable to that in Example 1.

[0046] Example 3 1. 1.55 g lithium carbonate, 3.32 g manganese acetate, 3.46 g ferrous oxalate dihydrate, 4.6 g ammonium dihydrogen phosphate, 0.24 g ammonium molybdate, 0.03 g copper oxide, and 2.64 g sucrose were simultaneously added to a ball mill mixing jar and ground continuously at 600 rpm for 8 hours to obtain a uniformly mixed solid powder. Taking the molar number of lithium carbonate as 1, the molar ratio of lithium carbonate, manganese acetate, ferrous oxalate dihydrate, ammonium dihydrogen phosphate, ammonium molybdate, and copper oxide was 1:0.46:0.46:0.95:0.03:0.01. The actual molar ratio of lithium source, manganese source, iron source, phosphorus source, and transition metal source was 1:0.46:0.46:0.95:0.04. 2. Add 1.58 g of potassium chloride and 3.17 g of sodium iodide to the above solid powder, and grind continuously at 300 rpm for 10 minutes to obtain the composite precursor; 3. The composite precursor was calcined at 650°C for 5 hours in a nitrogen atmosphere to obtain a gray powder; 4. Place the gray powder in an aqueous solution, shake well, and then centrifuge at 6000 rpm for 6 minutes to obtain the solid-liquid separation product; 5. Place the waste liquid from the solid-liquid separation product in a flask, evaporate and condense at 100℃ for 12 hours to obtain an aqueous solution, and collect the precipitated multi-element inorganic salts for the next mixing. 6. After drying the solid in the solid-liquid separation product in an oven at 120°C for 24 hours, lithium manganese iron phosphate cathode material is obtained.

[0047] The electrochemical performance of the lithium manganese iron phosphate cathode material prepared in Example 3 is comparable to that in Example 1.

[0048] Example 4 The difference between this embodiment and Embodiment 1 lies in the amount of lithium source, manganese source, iron source, phosphorus source, and transition metal source used, as detailed below: 1.55 g lithium carbonate, 2.33 g manganese acetate, 2.25 g ferrous oxalate dihydrate, 4.6 g ammonium dihydrogen phosphate, 0.03 g magnesium carbonate, 0.22 g niobium oxalate, 0.03 g aluminum hydroxide, and 0.88 g sucrose were simultaneously added to a ball mill mixing jar and ground continuously at 600 rpm for 8 hours to obtain a uniformly mixed solid powder. Taking the molar number of lithium carbonate as 1, the molar ratio of lithium carbonate, manganese acetate, ferrous oxalate dihydrate, ammonium dihydrogen phosphate, magnesium carbonate, niobium oxalate, and aluminum hydroxide was 1:0.32:0.30:0.95:0.01:0.01:0.01. The actual molar ratio of lithium source, manganese source, iron source, phosphorus source, and transition metal source was 1:0.32:0.30:0.95:0.03.

[0049] The subsequent steps are the same as in Example 1.

[0050] Example 5 The difference between this embodiment and Embodiment 1 lies in the amount of lithium source, manganese source, iron source, phosphorus source, and transition metal source used, as detailed below: 1.55 g lithium carbonate, 4.3 g manganese acetate, 4.2 g ferrous oxalate dihydrate, 4.6 g ammonium dihydrogen phosphate, 0.03 g magnesium carbonate, 0.22 g niobium oxalate, 0.03 g aluminum hydroxide, and 1.79 g sucrose were simultaneously added to a ball mill mixing jar and ground continuously at 600 rpm for 8 hours to obtain a uniformly mixed solid powder. Taking the molar number of lithium carbonate as 1, the molar ratio of lithium carbonate, manganese acetate, ferrous oxalate dihydrate, ammonium dihydrogen phosphate, magnesium carbonate, niobium oxalate, and aluminum hydroxide was 1:0.59:0.56:0.95:0.01:0.01:0.01. The actual molar ratio of lithium source, manganese source, iron source, phosphorus source, and transition metal source was 1:0.59:0.56:0.95:0.03.

[0051] The subsequent steps are the same as in Example 1.

[0052] Comparative Example 3 1. Add 1.55 g lithium carbonate, 3.36 g manganese acetate, 3.49 g ferrous oxalate dihydrate, 4.6 g ammonium dihydrogen phosphate, 0.03 g magnesium carbonate, 0.22 g niobium oxalate, 0.03 g aluminum hydroxide, and 1.33 g sucrose to a ball mill mixing jar and grind continuously at 800 rpm for 10 hours to obtain a uniformly mixed solid powder. 2. Add 4.22 g of potassium carbonate and 3.09 g of potassium nitrate to the above solid powder, and continue ball milling at 600 rpm for 20 minutes to obtain the composite precursor; 3. The composite precursor was calcined at 650°C for 3 hours in a nitrogen atmosphere to obtain a gray powder; 4. Place the gray powder in an aqueous solution, shake well, and then centrifuge at 8000 rpm for 5 minutes to obtain the solid-liquid separation product; 5. Place the waste liquid from the solid-liquid separation product in a flask, evaporate and condense at 100℃ for 12 hours to obtain an aqueous solution, and collect the precipitated multi-element inorganic salts for the next mixing. 6. After drying the solid in the solid-liquid separation product in an oven at 120°C for 24 hours, lithium manganese iron phosphate cathode material is obtained.

[0053] The lithium manganese iron phosphate cathode material prepared in Comparative Example 3 was applied to the cathode of a lithium-ion battery. The test showed that the initial capacity was 108 mAh / g at a current density of 1C, and the capacity retention rate was 82% after 1500 cycles. Compared with Example 1, the electrochemical performance was reduced.

[0054] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A method for preparing lithium manganese iron phosphate based on molten salt-assisted rapid preparation, characterized in that, Includes the following steps: S1: Lithium source, manganese source, iron source, phosphorus source and transition metal source are mixed to obtain precursor raw material, carbon source is added for mechanical grinding, then multi-element inorganic salt is added for mixing, and calcined under inert conditions to obtain gray powder. The molar ratio of the lithium source, manganese source, iron source, phosphorus source and transition metal source is 1:0.2~0.8:0.2~0.8:0.8~1.2:0.01~0.08; The mass ratio of the precursor raw material, carbon source, and multi-element inorganic salt is 1:0.05~0.2:0.2~0.8; The multi-element inorganic salt is at least one of potassium carbonate, potassium chloride, potassium nitrate, sodium chloride, and sodium iodide; The transition metal source is at least one of titanium, magnesium, molybdenum, niobium, aluminum, copper, and nickel sources; The carbon source is at least one of glucose, sucrose, oxalic acid, polyvinylpyrrolidone, and polyethylene glycol; S2: The gray powder is washed and separated by an aqueous solution, and the separated solid is dried to obtain the lithium manganese iron phosphate.

2. The method for preparing lithium manganese iron phosphate based on molten salt-assisted rapid preparation according to claim 1, characterized in that, The molar ratio of the lithium source, manganese source, iron source, phosphorus source and transition metal source is 1:0.3~0.6:0.3~0.6:0.8~1.1:0.01~0.05; The mass ratio of the precursor raw material, carbon source, and multi-element inorganic salt is 1:0.05~0.15:0.3~0.7; The lithium source is at least one of lithium carbonate, lithium acetate, lithium oxalate, and lithium hydroxide. The manganese source is at least one of manganese acetate, manganese carbonate, and manganese oxide; The iron source is at least one of ferric oxide, ferrous oxalate, and ferrous oxide. The phosphorus source is at least one of diammonium hydrogen phosphate, lithium phosphate, and phosphorus pentoxide. The titanium source is at least one of titanium dioxide, tetrabutyl titanate, titanium carbide, and titanium fluoride; The magnesium source is at least one of magnesium carbonate, magnesium oxalate, magnesium acetate, and magnesium metaborate. The molybdenum source is at least one of molybdenum acetylacetonate, molybdenum carbide, molybdenum fluoride, and ammonium molybdate. The niobium source is at least one of niobium oxide, niobium pentachloride, and niobium oxalate. The aluminum source is at least one of aluminum hydroxide, aluminum acetylacetone, aluminum metaphosphate, aluminum nitrate, and aluminum dihydrogen phosphate. The copper source is at least one of copper citrate, copper pyrophosphate, copper acetylacetone, and copper oxide; The nickel source is at least one of nickel hydroxide, nickel acetylacetone, nickel oxide, and nickel acetate.

3. The method for preparing lithium manganese iron phosphate based on molten salt-assisted rapid preparation according to claim 1, characterized in that, The multi-element inorganic salt is selected from one of the following combinations: potassium carbonate and potassium chloride, potassium carbonate and potassium nitrate, potassium chloride and sodium chloride, or potassium chloride and sodium iodide.

4. The method for preparing lithium manganese iron phosphate based on molten salt-assisted rapid preparation according to claim 3, characterized in that, The two inorganic salts in the combination are mixed in a molar ratio of 1~2:1~2 to obtain the multi-element inorganic salt.

5. The method for preparing lithium manganese iron phosphate based on molten salt-assisted rapid preparation according to claim 1, characterized in that, The mechanical grinding speed is 600~800 r / min and the time is 5~10 h; after adding the multi-element inorganic salt, the mixture is further mixed at a speed of 100~300 r / min for 10~60 min.

6. The method for preparing lithium manganese iron phosphate based on molten salt-assisted rapid preparation according to claim 1, characterized in that, The calcination temperature is 620℃~680℃, and the time is 3~5 h.

7. The method for preparing lithium manganese iron phosphate based on molten salt-assisted rapid preparation according to claim 1, characterized in that, The washing and separation process involves a rotation speed of 5000~8000 r / min and a time of 3~5 min; the drying process involves a temperature of 80℃~160℃ and a time of 10~50 h.

8. The method for preparing lithium manganese iron phosphate based on molten salt-assisted rapid preparation according to claim 1, characterized in that, The waste liquid from washing and separation is collected and evaporated and condensed to recover multi-element inorganic salts. The evaporation and condensation temperature is 80℃~120℃ and the time is 10~30 h.

9. Lithium manganese iron phosphate prepared by the method of rapid preparation of lithium manganese iron phosphate based on molten salt as described in any one of claims 1 to 8.

10. The application of lithium manganese iron phosphate as described in claim 9 in the preparation of cathode materials for lithium-ion batteries.

Images