Method for preparing lithium manganese iron phosphate by semi-solid phase method, lithium manganese iron phosphate and battery

Abstract

The invention belongs to the technical field of battery materials, and provides a method for preparing lithium manganese iron phosphate by a semi-solid phase method, lithium manganese iron phosphate and a battery, the method comprises the following steps: mixing and dissolving an iron source and phosphoric acid to form a first mixed solution; gradually mixing and dissolving a manganese source with the first mixed solution under ultrasonic assistance to form a second mixed solution; mixing and dissolving a lithium source and the second mixed solution, and reacting to obtain a third mixed solution; mixing a carbon source with the third mixed solution to obtain lithium manganese iron slurry; and drying the lithium manganese iron slurry, and sintering to obtain the carbon-coated lithium manganese iron phosphate. Through the reaction steps of optimizing the process of a traditional solid phase method, disassembling, refining and feeding, and adding an ultrasonic-assisted mixing reaction process, manganese and iron are uniformly mixed in an atomic scale in a liquid phosphorus-iron mixed solution to obtain nano-scale phosphorus-manganese-iron precursor small particles, so that the method has the advantages of simplicity and convenience of the solid phase method as well as high yield of the nano-scale phosphorus-manganese-iron precursor small particles. The advantages of uniform mixing and small particle size of a liquid phase method are realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials and relates to a method for preparing lithium manganese iron phosphate by a semi-solid phase method, lithium manganese iron phosphate and a battery. Background Art

[0002] Lithium manganese iron phosphate (LMFP) is a type of lithium-ion battery cathode material with an olivine crystal structure. In recent years, it has attracted widespread attention due to its comprehensive advantages in energy density, safety and cost control. As a manganese-doped variant of the lithium iron phosphate (LFP) system, LMFP inherits the excellent cycle stability and thermal stability of LFP, while introducing the high voltage characteristics of Mn 3 / Mn 2+ The redox pair increases its operating voltage platform to a certain extent, thus potentially achieving higher specific energy output. This characteristic gives it great application potential in power batteries, energy storage systems, and consumer electronics, especially in applications with high safety and lifespan requirements.

[0003] At present, the mainstream preparation methods of lithium manganese iron phosphate mainly include two categories: solid-phase method and liquid-phase method, with the solid-phase method occupying a dominant position. The solid-phase method is to mix and grind powdered precursors containing lithium, manganese, iron, phosphorus and carbon source in a stoichiometric ratio, and then carry out a calcination reaction at high temperature to finally obtain the target product. The biggest advantage of the solid-phase method is its simple process flow and low equipment investment. However, since the solid-phase reaction depends on the diffusion process between solid particles, its reaction rate is slow, resulting in low product crystallinity and uneven particle size distribution; secondly, the diffusion behavior of multi-component metal oxides at high temperatures varies greatly, which can easily cause uneven element distribution, especially in the regulation of the ratio of manganese and iron. There are great difficulties; thirdly, the particle size of the product obtained by the solid-phase method is often large, which is not conducive to the rapid deintercalation of lithium ions, thereby affecting the rate performance and electrochemical activity of the material.

[0004] In contrast, the liquid phase method is a more sophisticated synthesis method that can achieve uniform mixing of precursors at the atomic level, thereby obtaining lithium manganese iron phosphate materials with more uniform component distribution, smaller grain size, and controllable morphology. Common liquid phase methods include sol-gel method, hydrothermal method, co-precipitation method, microwave-assisted synthesis method, etc. However, the process flow required by the liquid phase method is relatively complex, involving multiple intermediate steps, such as solution preparation, pH adjustment, precipitation, filtration, drying, grinding, etc., which is not only time-consuming but also has high requirements for operating conditions; secondly, a large amount of wastewater containing metal ions and organic additives will be generated in the whole process, which has high treatment costs and great environmental pressure; thirdly, the equipment investment required for the liquid phase method is large, especially for large-scale continuous production equipment, which has poor economic efficiency, limiting its promotion and application in industrialization.

[0005] In summary, although the solid-phase method and the liquid-phase method each have their own advantages in the synthesis of lithium manganese iron phosphate, neither fully meets the requirements of high performance, low cost, environmental protection, and large-scale industrial production. Therefore, there is an urgent need to develop a new synthetic route that combines the advantages of both methods, is simple to operate, environmentally friendly, and suitable for large-scale preparation, so as to obtain high-quality and high-performance lithium manganese iron phosphate and lay a solid foundation for its widespread application in the new energy field. Summary of the Invention

[0006] In view of the problems existing in the prior art, the purpose of the present invention is to provide a method for preparing lithium iron manganese phosphate by a semi-solid phase process, lithium iron manganese phosphate, and a battery. The method comprises mixing and dissolving an iron source with phosphoric acid to form a first mixed solution; gradually mixing and dissolving a manganese source with the first mixed solution under the assistance of ultrasound to form a second mixed solution; mixing and dissolving a lithium source with the second mixed solution, reacting to obtain a third mixed solution; mixing a carbon source with the third mixed solution to obtain a lithium iron manganese slurry; and drying the lithium iron manganese slurry and then sintering it to obtain carbon-coated lithium iron manganese phosphate. By optimizing the traditional solid-phase process, disassembling and refining the reaction steps of adding materials, and adding an ultrasonic-assisted mixing reaction process, manganese and iron are uniformly mixed at the atomic level in the liquid phosphorus-iron mixed solution, obtaining nano-sized phosphorus-manganese-iron precursor particles. This method combines the simplicity of the solid-phase method with the advantages of uniform mixing and small particle size of the liquid-phase method.

[0007] To achieve this object, the present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides a method for preparing lithium manganese iron phosphate by a semi-solid phase method, comprising the following steps:

[0009] mixing and dissolving an iron source and phosphoric acid to form a first mixed solution;

[0010] Under ultrasound assistance, gradually mixing and dissolving the manganese source with the first mixed solution to form a second mixed solution;

[0011] mixing and dissolving the lithium source with the second mixed solution, and reacting the mixture to obtain a third mixed solution;

[0012] mixing a carbon source with the third mixed solution to obtain a manganese iron lithium slurry;

[0013] The lithium iron manganese phosphate slurry is dried and then sintered to obtain carbon-coated lithium iron manganese phosphate.

[0014] Semi-solid phase method of the present invention refers to mixing in liquid phase system before solid phase sintering, so that precursor is gradually formed in liquid phase system. Specifically, one of the cores of the method is to mix and dissolve an iron source with phosphoric acid, to obtain liquid ferrophosphorus mixed solution (i.e., the first mixed solution), then under ultrasonic wave assistance, by progressively adding manganese source mixed and dissolved, so that in the process of adding manganese source precipitation, manganese and iron can reach atomic-level uniform mixing, directly forming the small particles of nanometer-scale ferrophosphorus manganese precursor in the second mixed solution. Nanometer-scale ferrophosphorus manganese precursor does not need to be filtered, dried and then sand-milled, directly mixed and dissolved with lithium source in sequence, now reacted to form the precipitated product of ferrophosphorus manganese lithium, but did not generate ferrophosphorus manganese lithium crystalline phase, then mixed with carbon source to obtain ferrophosphorus lithium slurry, then dried and sintered to obtain product, solve the problem that small particle product in traditional liquid phase method industrialization is difficult to filter and clean, and can save the cost of liquid phase precipitant, washing and drying process compared to traditional liquid phase method, method is simpler. Compared with the traditional solid-phase method, by adding only two steps in the mixing process (the iron source and phosphorus source are first mixed and dissolved, and then the manganese source is gradually dissolved under the assistance of ultrasound), the grinding process and process of grinding the particles can be omitted, while the uniformity of the raw material mixing can be increased, so that it has the high uniformity of the liquid phase method, thereby improving the performance upper limit.

[0015] It should be noted that in order to ensure that manganese and iron are uniformly mixed at the atomic level when the manganese source is mixed and dissolved with the first mixed solution, the manganese source needs to be added gradually. The gradual addition process needs to be controlled to be slow and / or added in batches. If the addition rate is too fast and / or the amount added each time is too large, the reaction will be violent and difficult to control. Therefore, it is necessary to reasonably control it according to the actual situation. Similarly, when the lithium source is mixed and dissolved with the second mixed solution, a reaction also occurs. Therefore, according to the actual situation, it is preferably also controlled to add the lithium source slowly and / or in batches at a fixed rate or frequency (for example, within 15 to 60 minutes).

[0016] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. Through the following technical solutions, the technical objectives and beneficial effects of the present invention can be better achieved and realized.

[0017] As a preferred technical solution of the present invention, the phosphoric acid is in excess relative to the iron source.

[0018] Preferably, when the molar ratio of the manganese element in the manganese source to the iron element in the iron source is (2-4):1, for example, it can be 2:1, 2.3:1, 2.5:1, 2.8:1, 3:1, 3.2:1, 3.5:1, 3.8:1 or 4:1, etc., and the molar ratio of the iron element to the phosphate in the first mixed solution is 1:(1.5-3.5), the amount of the iron source and the phosphoric acid is controlled, for example, 1:1.5, 1:1.8, 1:2, 1:2.2, 1:2.5, 1:2.8, 1:3, 1:3.2 or 1:3.5, etc.

[0019] In the present invention, the molar amount of manganese provided by the manganese source is greater than the molar amount of iron provided by the iron source, i.e., the molar ratio Mn:Fe>1:1. The molar amount of phosphoric acid should be greater than the molar amount of iron, and the excess amount of phosphoric acid depends on the manganese-iron ratio. For example, if Mn:Fe=6:4, 2.5 mol of phosphoric acid can be used to dissolve 1 mol of Fe.

[0020] As a preferred technical solution of the present invention, the ultrasonic frequency of the ultrasonic assistance is 15 to 45 kHz, for example, 15 kHz, 20 kHz, 25 kHz, 30 kHz, 35 kHz, 40 kHz or 45 kHz, and the output power is 200 to 600 W, for example, 200 W, 250 W, 300 W, 350 W, 400 W, 450 W, 500 W, 550 W or 600 W, etc.

[0021] The ultrasonic-assisted mixed dissolution of the present invention not only promotes manganese and iron to reach a uniform mixing state at the atomic level, but also helps to make the particles small and uniform, and has a beneficial effect on the primary particle morphology and size control of manganese iron lithium.

[0022] As a preferred technical solution of the present invention, the temperature for mixing and dissolving the iron source and phosphoric acid is 30-60°C, for example, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C or 60°C; and the treatment time is 2-10h, for example, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h or 10h.

[0023] As a preferred technical solution of the present invention, the temperature for mixing and dissolving the manganese source and the first mixed solution is 60-90°C, for example, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C or 90°C; the processing time is 30-60min, for example, 30min, 35min, 40min, 45min, 50min, 55min or 60min.

[0024] As a preferred technical solution of the present invention, the concentration of the phosphoric acid is 0.5 to 2.5 mol / L, for example, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, 2 mol / L, 2.3 mol / L or 2.5 mol / L.

[0025] As a preferred technical solution of the present invention, the iron source includes at least one of iron powder, anhydrous ferric phosphate or iron oxide.

[0026] As a preferred technical solution of the present invention, the manganese source includes at least one of manganese carbonate, manganese monoxide, manganese dioxide or trimanganese tetraoxide.

[0027] As a preferred technical solution of the present invention, the lithium source includes lithium carbonate and / or lithium hydroxide.

[0028] As a preferred technical solution of the present invention, the carbon source includes at least one of glucose, starch, citric acid or polyethylene glycol (PEG).

[0029] Preferably, the amount of the carbon source is controlled according to 8% to 15% of the mass of the theoretically generated lithium manganese iron phosphate, for example, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%.

[0030] As a preferred technical solution of the present invention, the lithium source is first prepared into a lithium source solution, and then the lithium source solution is mixed and dissolved with the second mixed solution.

[0031] Preferably, the amount of the lithium source is controlled to be 1.02 to 1.05 times the molar amount of the generated lithium manganese iron phosphate, for example, 1.02 times, 1.03 times, 1.04 times or 1.05 times.

[0032] As a preferred technical solution of the present invention, the lithium source is in excess relative to the manganese source and the iron source.

[0033] As a preferred technical solution of the present invention, the dosage is controlled according to the ratio of the molar amount of lithium element in the lithium source to the total molar amount of iron element in the iron source and manganese element in the manganese source (1 to 1.08):1, for example, 1:1, 1.01:1, 1.02:1, 1.03:1, 1.04:1, 1.05:1, 1.06:1, 1.07:1 or 1.08:1, etc.

[0034] As a preferred technical solution of the present invention, the reaction time is 1 to 4 hours, for example, 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours or 4 hours.

[0035] As a preferred technical solution of the present invention, the drying method includes spray drying and / or ball milling drying.

[0036] Preferably, the spray drying temperature is 200-240°C, for example, 200°C, 210°C, 220°C, 230°C or 240°C.

[0037] Preferably, the ball milling and drying process includes first ball milling at 500-800 rpm, such as 500 rpm, 600 rpm, 700 rpm or 800 rpm, for 1-3 h, such as 1 h, 1.5 h, 2 h, 2.5 h or 3 h, and then drying at 100-120 ° C, such as 100 ° C, 105 ° C, 110 ° C, 115 ° C or 120 ° C, etc.

[0038] As a preferred technical solution of the present invention, the sintering is carried out under the protection of an inert atmosphere.

[0039] As a preferred technical solution of the present invention, the sintering temperature is 650-750°C, for example, 650°C, 660°C, 670°C, 680°C, 690°C, 700°C, 710°C, 720°C, 730°C, 740°C or 750°C.

[0040] As a preferred technical solution of the present invention, the sintering time is 4 to 8 hours, for example, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours or 8 hours.

[0041] In a second aspect, the present invention provides a lithium manganese iron phosphate obtained according to the method described in the first aspect.

[0042] In a second aspect, the present invention provides a battery comprising the lithium manganese iron phosphate described in the second aspect.

[0043] It should be noted that due to space limitations and to avoid redundancy, the present invention does not exhaustively list all point values within the above numerical range, but is not limited to the listed values. Other unlisted values within the above numerical range are also applicable.

[0044] Compared with the existing technical solutions, the present invention has at least the following beneficial effects:

[0045] The method of the present invention adopts a semi-solid phase method. By refining the mixing process, the iron source is controlled to be almost completely dissolved in phosphoric acid to form a liquid-phase phosphorus-iron mixed liquid. Then, during the gradual addition of the manganese source, an ultrasonic-assisted mixing reaction is used to promote the conversion of iron and manganese into phosphate, thereby achieving a liquid-phase manganese-iron atomic-level uniform mixing state. Nano-sized phosphorus-manganese-iron precursors can be directly generated under the action of ultrasound. This small particle precipitate does not need to be filtered, dried, and then sand-milled. It can be directly mixed with the lithium source and carbon source in liquid phase to form a slurry, and the product is obtained after drying and sintering. The method solves the problem that small particle products in the traditional liquid phase method are difficult to filter and clean in industrialization. Compared with the traditional solid phase method, only two steps are added to the mixing process, eliminating the mechanical grinding process of grinding the particles, but can effectively increase the uniformity of the raw material mixing, so that it has the high uniformity of the liquid phase method and the performance upper limit is improved. Compared with the traditional liquid phase method, the cost of liquid phase precipitant, washing and drying processes, etc. is saved. The method is simpler and more suitable for industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 This is a SEM test image of the carbon-coated lithium manganese iron phosphate material obtained by the semi-solid phase method for preparing lithium manganese iron phosphate in Example 1.

[0047] Figure 2 1 and 2 are XRD test patterns of the carbon-coated lithium manganese iron phosphate materials obtained in Example 1 and Example 2.

[0048] Figure 3 This is an SEM test image of the carbon-coated lithium manganese iron phosphate material obtained by the method for preparing lithium manganese iron phosphate in Comparative Example 3.

[0049] Figure 4 1 is a schematic flow chart of the method for preparing lithium manganese iron phosphate by the semi-solid phase method of Example 1. DETAILED DESCRIPTION

[0050] The technical solution of the present invention is further illustrated below through specific implementation methods.

[0051] It should be apparent to those skilled in the art that the embodiments are only intended to help understand the present invention and should not be considered as specific limitations of the present invention.

[0052] Example 1

[0053] This embodiment provides a method for preparing lithium manganese iron phosphate by a semi-solid phase method. Figure 4 As shown, the following steps are included:

[0054] S1. Dissolve 604 g of anhydrous ferric phosphate, an iron source, in 5 L of 2 mol / L phosphoric acid, and react at 50° C. for 6 h to obtain a first mixed solution.

[0055] S2. Place the first mixed solution in an ultrasonic reactor, set the ultrasonic frequency to 45 kHz, the power to 400 W, and gradually add a total of 690 g of manganese carbonate, a manganese source, at 85° C., and mix and dissolve to form a second mixed solution;

[0056] S3, dissolving 441g of lithium hydroxide monohydrate (a lithium source) in 5L of pure water to prepare a lithium source solution, and slowly adding the lithium source solution to the second mixed solution at a fixed addition rate over 60min, reacting for 1h to generate a third mixed solution;

[0057] S4. Adding a carbon source, including 79 g of glucose and 95 g of PEG, to the third mixed solution to obtain a manganese iron lithium slurry;

[0058] S5, spray drying the lithium iron manganese slurry to obtain a lithium iron manganese phosphate precursor;

[0059] S6, the lithium manganese iron phosphate precursor is placed under nitrogen protection and sintered at 710 ° C for 6 hours to obtain the carbon-coated lithium manganese iron phosphate material Li (Mn 0.6 Fe 0.4 )PO4 / C.

[0060] Example 2

[0061] This embodiment provides a method for preparing lithium manganese iron phosphate by a semi-solid phase method, comprising the following steps:

[0062] S1. Dissolve 68 g of anhydrous ferric phosphate, an iron source, in 1 L of 0.55 mol / L phosphoric acid, and react at 55° C. for 4 h to obtain a first mixed solution.

[0063] S2. Place the first mixed solution in an ultrasonic reactor, set the ultrasonic frequency to 45 kHz, the power to 400 W, and gradually add a total of 63.28 g of manganese carbonate, a manganese source, at 80° C., and mix and dissolve to form a second mixed solution;

[0064] S3, dissolving 44g of lithium hydroxide monohydrate (a lithium source) in 0.5L of pure water to prepare a lithium source solution, and slowly adding the lithium source solution to the second mixed solution at a fixed addition rate over 15min, reacting for 1h to generate a third mixed solution;

[0065] S4. Adding a carbon source, including 9 g of glucose and 11 g of PEG, to the third mixed solution to obtain a manganese iron lithium slurry;

[0066] S5, ball-milling and drying the lithium iron manganese slurry to obtain a lithium iron manganese phosphate precursor;

[0067] S6, the lithium manganese iron phosphate precursor is placed under nitrogen protection and sintered at 700 ° C for 7 hours to obtain the carbon-coated lithium manganese iron phosphate material Li (Mn0.55 Fe 0.45 )PO4 / C.

[0068] Example 3

[0069] This embodiment provides a method for preparing lithium manganese iron phosphate by a semi-solid phase method, comprising the following steps:

[0070] S1. Dissolve 45.3 g of anhydrous ferric phosphate as an iron source in 0.7 L of 1 mol / L phosphoric acid, and react at 60° C. for 4 h to obtain a first mixed solution.

[0071] S2. Place the first mixed solution in an ultrasonic reactor, set the ultrasonic frequency to 45 kHz, the power to 400 W, and gradually add 80.5 g of manganese carbonate as a manganese source at 80° C., mix and dissolve to form a second mixed solution;

[0072] S3, dissolving 44g of lithium hydroxide monohydrate (a lithium source) in 0.5L of pure water to prepare a lithium source solution, and slowly adding the lithium source solution to the second mixed solution at a fixed addition rate within 20min, reacting for 2h to generate a third mixed solution;

[0073] S4. Adding a carbon source, including 7 g of glucose and 12 g of PEG, to the third mixed solution to obtain a manganese iron lithium slurry;

[0074] S5, spray drying the lithium iron manganese slurry to obtain a lithium iron manganese phosphate precursor;

[0075] S6, the lithium manganese iron phosphate precursor is placed under nitrogen protection and sintered at 700 ° C for 4 hours to obtain the carbon-coated lithium manganese iron phosphate material Li (Mn 0.7 Fe 0.3 )PO4 / C.

[0076] Example 4

[0077] This embodiment provides a method for preparing lithium manganese iron phosphate by a semi-solid phase method, comprising the following steps:

[0078] S1. Dissolve 22.4 g of iron source iron powder in 0.5 L of 2 mol / L phosphoric acid, and react at 40° C. for 8 h to obtain a first mixed solution.

[0079] S2. Place the first mixed solution in an ultrasonic reactor, set the ultrasonic frequency to 45 kHz, the power to 400 W, and gradually add a total of 69 g of manganese carbonate, a manganese source, at 80° C., and mix and dissolve to form a second mixed solution;

[0080] S3, dissolving 43.8 g of lithium hydroxide monohydrate (a lithium source) in 0.5 L of pure water to prepare a lithium source solution, and slowly adding the lithium source solution to the second mixed solution at a fixed addition rate within 30 min, reacting for 0.5 h to generate a third mixed solution;

[0081] S4. Adding a carbon source, including 11 g of glucose and 9 g of PEG, to the third mixed solution to obtain a manganese iron lithium slurry;

[0082] S5, ball-milling and drying the lithium iron manganese slurry to obtain a lithium iron manganese phosphate precursor;

[0083] S6, the lithium manganese iron phosphate precursor is placed under nitrogen protection and sintered at 730 ° C for 2 hours to obtain the carbon-coated lithium manganese iron phosphate material Li (Mn 0.6 Fe 0.4 )PO4 / C.

[0084] Example 5

[0085] This embodiment provides a method for preparing lithium manganese iron phosphate by a semi-solid phase method, comprising the following steps:

[0086] S1. Dissolve 56 g of iron source iron powder in 1 L of 2 mol / L phosphoric acid, and react at 40° C. for 8 h to obtain a first mixed solution.

[0087] S2. The first mixed solution was placed in an ultrasonic reactor, and the ultrasonic frequency was set to 45 kHz and the power was set to 400 W. At 90° C., a total of 76.3 g of manganese source manganese tetraoxide was gradually added, and mixed and dissolved to form a second mixed solution;

[0088] S3, dissolving 87.6 g of lithium hydroxide monohydrate (a lithium source) in 2 L of pure water to prepare a lithium source solution, and slowly adding the lithium source solution to the second mixed solution at a fixed addition rate over 15 min, reacting for 1 h to generate a third mixed solution;

[0089] S4. Adding a carbon source, including 24 g of glucose and 20 g of PEG, to the third mixed solution to obtain a manganese iron lithium slurry;

[0090] S5, spray drying the lithium iron manganese slurry to obtain a lithium iron manganese phosphate precursor;

[0091] S6, the lithium manganese iron phosphate precursor is placed under nitrogen protection and sintered at 680 ° C for 6 hours to obtain the carbon-coated lithium manganese iron phosphate material Li (Mn 0.5 Fe 0.5 )PO4 / C.

[0092] Comparative Example 1

[0093] This comparative example provides a method for preparing lithium manganese iron phosphate, comprising the following steps:

[0094] S1. Mix 604 g of anhydrous ferric phosphate (iron source), 690 g of manganese carbonate (manganese source), 441 g of lithium hydroxide monohydrate (lithium source), and 1187 g of 85% phosphoric acid in 10 L of pure water.

[0095] S2, 79g glucose and 95g PEG, and a certain amount of dispersing additives, stir and disperse evenly, and then sand grind to a particle size of D 50 ≈0.5μm;

[0096] S3, spray drying the sand-milled slurry to obtain a lithium manganese iron phosphate precursor;

[0097] S4, the precursor powder obtained by spraying is placed under nitrogen and sintered at 710 ° C for 6 hours to obtain carbon-coated lithium manganese iron phosphate material Li (Mn 0.6 Fe 0.4 )PO4 / C.

[0098] Comparative Example 2

[0099] This comparative example provides a method for preparing lithium manganese iron phosphate, wherein the method does not use ultrasonic wave-assisted mixed dissolution in step S2. Except for the above, other conditions are exactly the same as those in Example 1.

[0100] Comparative Example 3

[0101] This comparative example provides a method for preparing lithium manganese iron phosphate, in which step S1 and step S2 are adjusted as follows: 604 g of anhydrous iron phosphate as an iron source, 690 g of manganese carbonate as a manganese source, and 5 L of phosphoric acid with a concentration of 2 mol / L are placed in an ultrasonic reactor, first reacted at a temperature of 50° C. for 6 h, and then heated to 85° C. and reacted for 2 h to form a second mixed solution; except for the above, the other conditions are exactly the same as those in Example 1.

[0102] Characterization and testing:

[0103] Ⅰ. The carbon-coated lithium manganese iron phosphate obtained in Example 1 was subjected to SEM testing. Figure 1 As shown, it can be seen that the obtained lithium manganese iron phosphate has uniform particle size and good dispersion. Figure 3This is a SEM image of the lithium manganese iron phosphate obtained in Comparative Example 3. It can be seen that in the solution without sanding and the prior iron source dissolution and ultrasonic precipitation process, the product particle size will be too large, ultimately affecting the material's electrical properties. This is because, compared to Example 1, the simultaneous dissolution of the iron, manganese, and phosphorus sources results in insufficient excess phosphoric acid, which prevents the complete dissolution and reaction of the iron phosphate, leaving most of the iron phosphate as micron-sized particles. Therefore, dissolving the iron source first and then the manganese source in a step-by-step manner can ensure the complete dissolution of the iron source. On this basis, further ultrasonically assisted dissolution of the manganese source can achieve the best uniformity.

[0104] II. XRD test was performed on the carbon-coated lithium manganese iron phosphate obtained in Example 1 and Example 2. Figure 2 As shown, it can be seen that the lithium manganese iron phosphate obtained in Example 1 and Example 2 has good crystallinity.

[0105] III. Compaction density test: The compaction density of the lithium manganese iron phosphate positive electrode material prepared in each embodiment and comparative example was tested. The test method was determined in accordance with the provisions of Appendix L of GB / T24533-2019. The results are shown in Table 1.

[0106] IV. Electrical Performance Testing: The capacity performance of the lithium manganese iron phosphate obtained in each Example and Comparative Example as the positive electrode material was tested as follows: the phosphate positive electrode material, the conductive agent CNT, and the binder PVDF were mixed in a solvent (NMP) at a predetermined mass ratio of 80:15:5, stirred evenly, and then evenly coated onto a current collector aluminum foil. The mixture was pressed and dried, and then cut into 12 mm diameter discs to obtain the positive electrode sheet for the button cell. A lithium sheet (0.6 mm thick) served as the negative electrode, and a mixture of fluoroethylene carbonate, dimethyl carbonate, and ethylene carbonate (10:45:45 by volume) containing 1 mol / L LiPF6 was used as the electrolyte. A polypropylene film was cut and used as the separator. The button cells were assembled in an argon atmosphere in a glove box. The test voltage was set between 2.5 and 4.3 V, and the test temperature was strictly maintained at 25°C. The first discharge gram capacity and first efficiency of the button cells were tested at 0.1C using a battery testing system (LAND CT2001A).

[0107] The test results are recorded in Table 1.

[0108] Table 1

[0109]

[0110] As can be seen from Table 1, the semi-solid phase method provided by the present invention can improve the uniformity of manganese iron. Compared with the traditional solid phase sand milling method using the same raw materials, the compaction density of the obtained product and the initial performance of the formed battery are more advantageous. In the present invention, the use of an ultrasonic reactor to provide ultrasonic-assisted mixed dissolution can effectively promote the uniform mixing of manganese and iron at the atomic level and reduce the particle size of the precursor. Compared with ordinary stirring and mixing, the particle size of the obtained lithium manganese iron phosphate product is more uniform and the electrical performance is significantly improved.

[0111] The preferred embodiments of the present invention are described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

[0112] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0113] In addition, the various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A method for preparing lithium manganese iron phosphate by a semi-solid phase method, characterized in that: The steps include: mixing and dissolving an iron source and phosphoric acid to form a first mixed solution; Under ultrasound assistance, gradually mixing and dissolving the manganese source with the first mixed solution to form a second mixed solution; mixing and dissolving the lithium source with the second mixed solution, and reacting the mixture to obtain a third mixed solution; mixing a carbon source with the third mixed solution to obtain a manganese iron lithium slurry; The lithium iron manganese phosphate slurry is dried and then sintered to obtain carbon-coated lithium iron manganese phosphate.

2. The method for preparing lithium manganese iron phosphate by a semi-solid phase method according to claim 1, characterized in that: The phosphoric acid is in excess relative to the iron source; Preferably, when the molar ratio of the manganese element in the manganese source to the iron element in the iron source is (2-4):1, the amount of the iron source and the phosphoric acid is controlled according to the molar ratio of the iron element to the phosphate in the first mixed solution being 1:(1.5-3.5).

3. The method for preparing lithium manganese iron phosphate by a semi-solid phase method according to claim 1 or 2, characterized in that: The ultrasonic frequency of the ultrasonic assistance is 15-45 kHz, and the output power is 200-600 W.

4. The method for preparing lithium manganese iron phosphate by a semi-solid phase method according to any one of claims 1 to 3, characterized in that: The temperature for mixing and dissolving the iron source and the phosphoric acid is 30 to 60° C., and the treatment time is 2 to 10 hours; Preferably, the temperature for mixing and dissolving the manganese source and the first mixed solution is 60-90° C., and the treatment time is 30-60 min.

5. The method for preparing lithium manganese iron phosphate by a semi-solid phase method according to any one of claims 1 to 4, characterized in that: The concentration of the phosphoric acid is 0.5 to 2.5 mol / L; Preferably, the iron source comprises at least one of iron powder, anhydrous iron phosphate or iron oxide; Preferably, the manganese source comprises at least one of manganese carbonate, manganese monoxide, manganese dioxide or manganese tetraoxide; Preferably, the lithium source comprises lithium carbonate and / or lithium hydroxide; Preferably, the carbon source comprises at least one of glucose, starch, citric acid or polyethylene glycol; Preferably, the amount of the carbon source is controlled to be 8% to 15% of the mass of the theoretically generated lithium manganese iron phosphate.

6. The method for preparing lithium manganese iron phosphate by a semi-solid phase method according to any one of claims 1 to 5, characterized in that: The lithium source is first prepared into a lithium source solution, and then the lithium source solution is mixed and dissolved with the second mixed solution; Preferably, the amount of the lithium source is controlled according to 1.02 to 1.05 times the molar amount of the generated lithium manganese iron phosphate; Preferably, the reaction time is 1 to 4 hours.

7. The method for preparing lithium manganese iron phosphate by a semi-solid phase method according to any one of claims 1 to 6, characterized in that: The drying method includes spray drying and / or ball mill drying.

8. The method for preparing lithium manganese iron phosphate by a semi-solid phase method according to any one of claims 1 to 7, characterized in that: The sintering is carried out under the protection of an inert atmosphere; Preferably, the sintering temperature is 650-750°C; Preferably, the sintering time is 4 to 8 hours.

9. A lithium manganese iron phosphate, characterized in that: Obtained according to the method according to any one of claims 1 to 8.

10. A battery, characterized in that: Containing the lithium manganese iron phosphate according to claim 9.

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