Manganese ferric ammonium phosphate precursor as well as preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to an ammonium ferromanganese phosphate precursor and a preparation method and application thereof.The preparation method comprises the following steps that 1, an iron source is dissolved in a sulfuric acid solution, and a ferrous sulfate solution is prepared after dissolving and filtering; 2) respectively adding a reducing agent, a manganese source and a soluble phosphorus source into the ferrous sulfate solution, and uniformly mixing to obtain a mixed solution; and 3) adjusting the pH value of the mixed solution to 4-4.5 by using ammonia water, reacting for a period of time, continuously adding ammonia water to adjust the pH value to 5-8, continuously reacting for 0.5-3 hours at the temperature of 25-85 DEG C, filtering, washing and drying to obtain the manganese iron ammonium phosphate precursor. An iron source is dissolved by sulfuric acid to obtain a ferrous sulfate solution with high purity; after ferrous sulfate obtained by dissolving an iron source with sulfuric acid is uniformly mixed with a phosphorus source, a manganese source and a reducing agent, the phosphorus-iron ratio does not need to be adjusted, and the phosphorus-iron ratio and the reaction pH are controlled by dropwise adding ammonia water in two steps, so that the generation of an impure phase is favorably controlled, and the introduction of new impurities and the waste of resources are avoided.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a manganese iron ammonium phosphate precursor and a preparation method and application thereof. BACKGROUND

[0002] The positive electrode material is one of the key materials that determine the performance of lithium ion batteries, and directly affects the energy density, cycle life, rate performance and safety performance of the battery. Lithium manganese iron phosphate (LiMn 1-x Fe x PO4) crystal has a hexagonal close-packed structure and good stability. Even if all lithium ions are removed during charging, there will be no problem of structure collapse. At the same time, P atoms in the material form PO4 tetrahedron through P-O strong covalent bond, and O atoms are difficult to be removed from the structure, so the material has very high safety and stability. Lithium manganese iron phosphate is obtained by doping manganese on the basis of lithium iron phosphate. Manganese element has the advantage of high voltage, which raises the voltage platform from 3.4V to 4.1V, thereby improving the energy density of the battery. Therefore, lithium manganese iron phosphate is regarded as an "upgraded version of lithium iron phosphate".

[0003] However, LiMn 1-x Fe x PO4 also has the inherent defects of olivine-type positive electrode materials, such as low intrinsic electronic conductivity (10 -9 ~10 -10 S•cm -1 ) and low lithium ion diffusion rate (10 -14 ~10 -16 cm 2 •s -1 ), which greatly affects its discharge capacity and rate performance, hindering its large-scale application in high-performance lithium ion batteries. At the same time, the voltage platform of lithium manganese iron phosphate presents a two-step form, and the platform length is proportional to the manganese-iron ratio. The manganese-iron ratio directly affects the specific discharge capacity of the material. In theory, the higher the manganese content, the higher the energy density, but Mn 3+ ions are prone to cause distortion of the crystal structure of the material due to the Jahn-Teller effect, and may cause manganese dissolution in the battery cycle, react with the electrolyte, and thus affect the cycle life of the battery.

[0004] In order to realize the industrialization of lithium manganese iron phosphate, in view of its inherent performance defects, positive electrode material manufacturers and battery manufacturers have proposed corresponding modification schemes. The main modification technologies include nanocrystallization, carbon coating and ion doping.

[0005] Nanocrystallization: By reducing the particle size of the material to the nanometer level, on the one hand, the lithium ion migration path is shortened, and the lithium ion migration efficiency is improved; on the other hand, by increasing the specific surface area of the material, the material is more fully in contact with the electrolyte, the electrode interface impedance is reduced, and ultimately the conductivity is improved. In addition, improving the lithium ion migration rate can reduce electrode polarization, thereby improving the charge and discharge capacity and rate performance of the material. Nanocrystallization is the most basic and effective way to improve lithium manganese iron phosphate, but as the particle size of the material decreases, serious particle agglomeration occurs during the production and slurry preparation process, affecting the uniformity of the material, reducing the compaction density of the electrode sheet, and ultimately affecting the performance of the battery.

[0006] Carbon coating: significantly improves conductivity, inhibits manganese dissolution, and improves cycle stability. By uniformly coating a carbon layer on the surface of the material (surface layer coating), a fast conductive network is constructed for Li + ion diffusion, while avoiding excessive growth and agglomeration of crystal particles, thereby improving the external conductivity of the material. In addition, carbon coating can also inhibit the dissolution of manganese ions and reduce the impact of the Jahn-Teller effect on battery capacity and cycle stability. Common carbon sources include glucose, sucrose, citric acid, graphene, carbon nanotubes, etc.

[0007] (3) Ion doping: improves the structural stability of the material, and doping of some ions (such as Cu 2+ , Mg 2+ , Ti 4+ , etc.) can inhibit the dissolution of manganese ions. By doping ions in the material, defects can be introduced into the original lattice, expanding the Li + diffusion channel and increasing the carrier density of the material, thereby improving the internal conductivity of the material. Among them, the doping of Mg 2+ not only improves the conductivity, but also improves the capacity and cycle stability of the material by inhibiting the Jahn-Teller effect. The method of doping Mg 2+ is currently the most widely used and researched.

[0008] The production process of lithium manganese iron phosphate includes high-temperature solid-phase method, co-precipitation method, sol-gel method, hydrothermal / solvothermal method, spray drying method, etc. Specifically, the high-temperature solid-phase method is a relatively low-difficulty process with high yield, suitable for large-scale industrial production, but the product particle size is large and the performance is relatively weak compared to the wet process. The wet process technology is relatively difficult, and the product performance is relatively good, but there are problems such as long synthesis cycle, high cost, and use of special raw materials. How to improve the electrochemical performance of LiMn 1-x Fe x PO4 is a key problem that needs to be solved at present.

[0009] The precursor of lithium manganese iron phosphate is prepared by the liquid-phase coprecipitation method, which has simple operation method, can accurately control the reaction conditions, and the obtained precursor material has good uniformity in shape. However, when the lithium manganese iron ammonium phosphate is prepared by the liquid-phase coprecipitation method, if the ferrous sulfate solution contains impurity elements (such as chloride ions, chromium ions, aluminum ions, etc.), the impurity elements will enter the lithium manganese iron phosphate material through the liquid-phase reaction. The introduction of these impurity elements may cause a large number of defects and pores in the material, prolong the embedding and migration path of lithium ions, reduce the ion migration rate, and thus cause the conductivity of the material to decrease; the impurity elements may also form soluble impurities and magnetic impurities, which will further reduce the specific capacity and energy density of the material; in addition, the impurity elements may also form unstable compounds during the charging and discharging process of the battery, causing the capacity attenuation and safety problems of the battery; secondly, the liquid-phase coprecipitation method also has certain defects, and the phosphorus-iron ratio in the liquid-phase solution needs to be controlled, because the phosphorus-iron ratio will directly affect the morphology and particle size of the lithium manganese iron ammonium phosphate precursor, and then affect the particle morphology and size of the LFMP after sintering. In the existing technical route, the phosphorus-iron ratio is generally controlled by supplementing phosphorus source and introducing trivalent metal, but this may also cause resource waste and introduce new impurities. SUMMARY

[0010] The purpose of the present application is to provide a lithium manganese iron ammonium phosphate precursor, a preparation method and application thereof, and to solve the problem that the quality of the lithium manganese iron ammonium phosphate precursor is not ideal due to the presence of many impurity elements in the prior art.

[0011] The scheme of the present application is as follows: A preparation method of a lithium manganese iron ammonium phosphate precursor, characterized in that it comprises the following steps: 1) dissolving an iron source in a sulfuric acid solution to prepare a ferrous sulfate solution after dissolution and filtration; 2) adding a reducing agent, a manganese source and a soluble phosphorus source into the ferrous sulfate solution respectively, mixing uniformly to obtain a mixed solution; 3) adjusting the pH of the mixed solution to 4-4.5 with ammonia water, continuing to add ammonia water to adjust the pH to 5-8 after reacting for a period of time, and continuing to react for 0.5-3 h at a temperature of 25-85 ℃, and then filtering, washing and drying to obtain the lithium manganese iron ammonium phosphate precursor.

[0012] As a preferred technical scheme, the iron source in the step 1) is at least one of reduced iron powder and iron block.

[0013] As a preferred technical scheme, the reaction temperature for dissolving the iron source in sulfuric acid in the step 1) is 30-90 ℃, and the concentration of the ferrous sulfate solution is 4.5-5.5 wt%.

[0014] As a preferred technical scheme, the reducing agent in the step 2) is at least one of ascorbic acid and citric acid.

[0015] As a preferred technical solution, in the step 2), the manganese source is at least one of manganese sulfate, manganese carbonate, trimanganese tetroxide, manganese oxide and manganese oxalate.

[0016] As a preferred technical solution, in the step 2), the soluble phosphorus source is at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate and phosphoric acid.

[0017] As a preferred technical solution, in the step 2), the molar ratio of manganese element to iron element in the mixed solution is 5-8:2-5.

[0018] As a preferred technical solution, in the step 2), the molar ratio of phosphorus element to transition metal element in the mixed solution is 0.98-1.02:1.

[0019] A manganese iron ammonium phosphate precursor is prepared by the preparation method of the manganese iron ammonium phosphate precursor.

[0020] The application of the manganese iron ammonium phosphate precursor, the manganese iron lithium phosphate positive electrode material is obtained by mixing the manganese iron ammonium phosphate precursor with a lithium source and a carbon source, ball milling, spray drying and calcining; the molar ratio of the manganese iron ammonium phosphate precursor to the lithium source is 0.98-1.05; the carbon source is at least one of glucose, sucrose, polyethylene glycol, polyethylene and citric acid; and the calcining temperature is 600-800 DEG C.

[0021] The advantages of the present application are as follows: 1. In the present application, the iron source is dissolved in sulfuric acid to obtain a ferrous sulfate solution with high purity, and the ferrous sulfate solution is used to prepare the manganese iron ammonium phosphate, and the content of impurity elements such as Ca, Cu, Zn, Co, Cr, Cd, Al and Pb in the prepared manganese iron ammonium phosphate is less than 100 ppm. 2. The ferrous sulfate obtained by dissolving the iron source in sulfuric acid is mixed with a phosphorus source, a manganese source and a reducing agent, and the phosphorus-iron ratio does not need to be adjusted, the phosphorus-iron ratio and the pH of the reaction are controlled by two-step ammonia water dripping, which is beneficial to control the generation of impurities and avoid the introduction of new impurities and resource waste. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments will be briefly introduced below, and obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0023] Figure 1 The XRD pattern of the manganese iron ammonium phosphate precursor in Example 2 of the present application; Figure 2A SEM image of the ammonium manganese iron phosphate precursor of Example 2 of the present application; Figure 3 An XRD image of the lithium manganese iron phosphate material of Example 2 of the present application; Figure 4 A SEM image of the lithium manganese iron phosphate material of Example 2 of the present application; Figure 5 A charge-discharge curve of the lithium manganese iron phosphate material of Example 2 of the present application. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application.

[0025] Example 1: A preparation method of an ammonium manganese iron phosphate precursor, comprising the following steps: (1) dissolving an iron source in a sulfuric acid solution to prepare a ferrous sulfate solution after dissolution and filtration; (2) adding a reducing agent, a manganese source and a soluble phosphorus source into the ferrous sulfate solution respectively, mixing uniformly to obtain a mixed solution; (3) adjusting the pH of the mixed solution of step (2) to 4-4.5 with ammonia water, continuing to add ammonia water to adjust the pH to 5-8 after reacting for a period of time, continuing to react at a temperature of 25-85℃ for 0.5-3h, and then performing filtration, washing and drying to obtain the ammonium manganese iron phosphate precursor; In step (1), the iron source is at least one of reduced iron powder and iron block; In step (1), the reaction temperature for dissolving the iron source in sulfuric acid is 30-90℃, and the concentration of the ferrous sulfate solution is 4.5-5.5wt%; In step (2), the reducing agent is at least one of ascorbic acid and citric acid; In step (2), the manganese source is at least one of manganese sulfate, manganese carbonate, trimanganese tetraoxide, manganese oxide and manganese oxalate; In step (2), the soluble phosphorus source is at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate and phosphoric acid; In step (2), the molar ratio of manganese element to iron element in the mixed solution is 5-8:2-5; In step (2), the molar ratio of phosphorus element to transition metal element in the mixed solution is 0.98-1.02:1.

[0026] The prepared manganese iron ammonium phosphate precursor is mixed with a lithium source and a carbon source, ball-milled, spray-dried, and calcined to obtain a lithium manganese iron phosphate positive electrode material, the manganese iron ammonium phosphate precursor and the lithium source are in a molar ratio of 0.98-1.05; the carbon source is at least one selected from glucose, sucrose, polyethylene glycol, polyethylene, and citric acid; and the calcination temperature is 600-800°C.

[0027] Example 2: Fe powder (99.99%) is dissolved in a sulfuric acid solution at 70°C, and after complete dissolution, a 5wt% ferrous sulfate solution is prepared by filtration.

[0028] Take 181g of the 5wt% ferrous sulfate solution, add 2.56g of ascorbic acid, and after complete dissolution, add 40.96g of manganese sulfate monohydrate and 50.62g of ammonium dihydrogen phosphate, and stir until completely dissolved.

[0029] Slowly add ammonia water with a concentration of 15% to the above solution, adjust the pH of the mixed solution to 4, continue to add ammonia water to adjust the pH of the liquid phase reaction to 7.0 after 10 minutes of reaction, and after 30 minutes of reaction at room temperature, filter, wash, and dry in a 50°C oven to obtain a manganese iron ammonium phosphate precursor. The X-ray diffraction pattern of the obtained precursor Figure 1 ) shows that the peak position of the material is highly consistent with the manganese iron ammonium phosphate standard card (JCPDS No. 50-0554) and has no other impurity peaks, indicating that the prepared precursor sample is manganese iron ammonium phosphate. The SEM characterization result Figure 2 ) shows that the precursor presents a lamellar morphology and a smooth surface. In addition, the content of impurity elements such as Ca, Cu, Zn, Co, Cr, Cd, Al, and Pb in the obtained precursor is less than 100ppm.

[0030] Take 55g of the precursor sample, and ball-mill with 14.6g of lithium carbonate, 4.3g of glucose, and 2.4g of polyethylene glycol, wherein the molar ratio of (Fe+Mn):Li in the precursor is 1:1.02, the rotation speed is 400rpm, ball-milling is performed for 4 hours, spray-drying is performed, the spray-dried sample is placed in a tube furnace, calcination is performed at 400°C for 2 hours and at 700°C for 10 hours under a nitrogen atmosphere, the heating rate is 5°C / min, and after cooling to room temperature, a lithium manganese iron phosphate sample is obtained. Figure 3 The X-ray diffraction pattern of the prepared lithium manganese iron phosphate material shows that the peak position of the material is consistent with the lithium manganese iron phosphate standard card (JCPDS No. 89-7115) and has no other impurity peaks. The SEM characterization result Figure 4 ) shows that the primary particles of the calcined lithium manganese iron phosphate are spherical and have a size of less than 200nm. The electrochemical performance test data Figure 5) indicates that the specific capacity of 0.1C discharge in the button half-cell is about 151 mAh / g.

[0031] Example 3: Fe powder (99.99%) is dissolved in a sulfuric acid solution, reacted at 70°C, filtered after complete dissolution, and configured into a 5wt% ferrous sulfate solution.

[0032] Take 181g of the 5wt% ferrous sulfate solution, add 2.56g of ascorbic acid, and after complete dissolution, add 40.96g of manganese sulfate monohydrate and 50.62g of ammonium dihydrogen phosphate, and stir until completely dissolved.

[0033] Slowly add ammonia water with a concentration of 15% to the above solution, adjust the pH of the mixed solution to 4, continue to add ammonia water to adjust the pH of the liquid phase reaction to 5.0 after 10min of reaction, and after 30min of reaction at room temperature, filter, wash, and dry in a 50°C oven to obtain the ammonium manganese iron phosphate precursor.

[0034] Take 55g of the precursor sample, mix with 14.6g of lithium carbonate, 4.3g of glucose, and 2.4g of polyethylene glycol, wherein the molar ratio of (Fe+Mn):Li in the precursor is 1:1.02, the rotation speed is 400rpm, ball mill for 4 hours, then spray dry, and place the spray-dried sample in a tube furnace under a nitrogen atmosphere, heat at 400°C for 2 hours, and heat at 650°C for 10 hours at a heating rate of 5°C / min, and obtain the lithium manganese iron phosphate sample after cooling to room temperature.

[0035] Example 4: Fe powder (99.99%) is dissolved in a sulfuric acid solution, reacted at 70°C, filtered after complete dissolution, and configured into a 5wt% ferrous sulfate solution.

[0036] Take 181g of the 5wt% ferrous sulfate solution, add 2.56g of ascorbic acid, and after complete dissolution, add 40.96g of manganese sulfate monohydrate and 50.62g of ammonium dihydrogen phosphate, and stir until completely dissolved.

[0037] Slowly add ammonia water with a concentration of 15% to the above solution, adjust the pH of the mixed solution to 4, continue to add ammonia water to adjust the pH of the liquid phase reaction to 6.0 after 10min of reaction, and after 30min of reaction at room temperature, filter, wash, and dry in a 50°C oven to obtain the ammonium manganese iron phosphate precursor.

[0038] Take 55 g of precursor sample, with 14.6 g of lithium carbonate, 4.3 g of glucose, 2.4 g of polyethylene glycol ball milling, wherein the molar ratio of (Fe+Mn):Li in the precursor is 1:1.02, the rotation speed is 400 rpm, ball milling for 4 hours, then spray drying, the sample after spray drying is placed in a tube furnace, under nitrogen atmosphere, 400 ℃ for 2 hours, 650 ℃ for 10 hours, the heating rate is 5 ℃ / min, after cooling to room temperature to obtain the lithium manganese iron phosphate sample.

[0039] Example 5: Dissolve Fe powder (99.99%) in sulfuric acid solution, react at 70℃, filter after complete dissolution, configure into a 5wt% ferrous sulfate solution.

[0040] Take 181 g of 5wt% ferrous sulfate solution, add 2.56 g of ascorbic acid, after complete dissolution, add manganese sulfate monohydrate 40.96 g, ammonium dihydrogen phosphate 50.62 g, stir until complete dissolution.

[0041] Slowly add ammonia water with a concentration of 15% to the above solution, adjust the pH of the mixed solution to 4, after 10 min of reaction, continue to add ammonia water to adjust the pH of the liquid phase reaction to 7.0, react at 50℃ for 30 min, then filter, wash and dry in a 50℃ oven to obtain the ammonium manganese iron phosphate precursor.

[0042] Take 55 g of precursor sample, with 14.6 g of lithium carbonate, 4.3 g of glucose, 2.4 g of polyethylene glycol ball milling, wherein the molar ratio of (Fe+Mn):Li in the precursor is 1:1.02, the rotation speed is 400 rpm, ball milling for 4 hours, then spray drying, the sample after spray drying is placed in a tube furnace, under nitrogen atmosphere, 400 ℃ for 2 hours, 650 ℃ for 10 hours, the heating rate is 5 ℃ / min, after cooling to room temperature to obtain the lithium manganese iron phosphate sample.

[0043] Example 6: Dissolve Fe powder (99.99%) in sulfuric acid solution, react at 70℃, filter after complete dissolution, configure into a 5wt% ferrous sulfate solution.

[0044] Take 181 g of 5wt% ferrous sulfate solution, add 2.56 g of ascorbic acid, after complete dissolution, add manganese sulfate monohydrate 40.96 g, ammonium dihydrogen phosphate 50.62 g, stir until complete dissolution.

[0045] Slowly add ammonia water with a concentration of 15% to the above solution, adjust the pH of the mixed solution to 4, continue to add ammonia water to adjust the pH of the liquid phase reaction to 7.0 after a period of reaction, carry out filtration, washing after reaction at 80℃ for 30 min, and drying in a 50℃ oven to obtain the ammonium manganese iron phosphate precursor.

[0046] Take 55 g of the precursor sample, ball mill with 14.6 g of lithium carbonate, 4.3 g of glucose, and 2.4 g of polyethylene glycol, wherein the molar ratio of (Fe+Mn):Li is 1:1.02, the rotation speed is 400 rpm, and the ball milling is carried out for 4 hours, then spray drying is carried out, and the spray-dried sample is placed in a tube furnace under a nitrogen atmosphere, heat-treated at 400℃ for 2 hours and at 700℃ for 10 hours at a heating rate of 5℃ / min, and the lithium manganese iron phosphate sample is obtained after cooling to room temperature.

[0047] Example 7: Dissolve Fe powder (99.99%) in a sulfuric acid solution and react at 70℃, filter after complete dissolution, and prepare a 5wt% ferrous sulfate solution.

[0048] Take 181 g of the 5wt% ferrous sulfate solution, add 3.15 g of citric acid, and after complete dissolution, add 27.87 g of manganese carbonate and 58.69 g of diammonium hydrogen phosphate and stir until complete dissolution.

[0049] Slowly add ammonia water with a concentration of 15% to the above solution, adjust the pH of the mixed solution to 4, continue to add ammonia water to adjust the pH of the liquid phase reaction to 7.0 after a period of reaction, carry out filtration, washing after reaction at 80℃ for 30 min, and drying in a 50℃ oven to obtain the ammonium manganese iron phosphate precursor.

[0050] Take 55 g of the precursor sample, ball mill with 14.6 g of lithium carbonate, 4.3 g of glucose, and 2.4 g of polyethylene glycol, wherein the molar ratio of (Fe+Mn):Li is 1:1.02, the rotation speed is 400 rpm, and the ball milling is carried out for 4 hours, then spray drying is carried out, and the spray-dried sample is placed in a tube furnace under a nitrogen atmosphere, heat-treated at 400℃ for 2 hours and at 700℃ for 10 hours at a heating rate of 5℃ / min, and the lithium manganese iron phosphate sample is obtained after cooling to room temperature.

[0051] Example 8: Dissolve Fe powder (99.99%) in a sulfuric acid solution and react at 70℃, filter after complete dissolution, and prepare a 5wt% ferrous sulfate solution.

[0052] Take 181 g of the 5wt% ferrous sulfate solution, add 2.56 g of ascorbic acid, and after complete dissolution, add 27.87 g of manganese carbonate and 58.69 g of diammonium hydrogen phosphate and stir until complete dissolution.

[0053] Slowly add ammonia water with a concentration of 15% to the above solution, adjust the pH of the mixed solution to 4, continue to add ammonia water to adjust the pH of the liquid phase reaction to 7.0 after a period of reaction, filter, wash, and dry in a 50°C oven after reaction at 80°C for 30 min, to obtain the ammonium manganese iron phosphate precursor.

[0054] Take 55 g of the precursor sample, ball mill with 14.6 g of lithium carbonate, 4.3 g of glucose, and 2.4 g of polyethylene glycol, wherein the molar ratio of (Fe+Mn):Li is 1:1.02, the rotation speed is 400 rpm, spray dry after ball milling for 4 hours, place the spray-dried sample in a tube furnace, heat at 400°C for 2 hours and at 700°C for 10 hours under a nitrogen atmosphere, and obtain the lithium manganese iron phosphate sample after cooling to room temperature at a heating rate of 5°C / min.

[0055] Example 9: Dissolve Fe powder (99.99%) in a sulfuric acid solution and react at 70°C, filter after complete dissolution, and prepare a 5wt% ferrous sulfate solution.

[0056] Take 181 g of the 5wt% ferrous sulfate solution, add 3.15 g of citric acid, and stir until completely dissolved, then add 34.65 g of manganese oxalate and 46.12 g of phosphoric acid and stir until completely dissolved.

[0057] Slowly add ammonia water with a concentration of 15% to the above solution, adjust the pH of the mixed solution to 4, continue to add ammonia water to adjust the pH of the liquid phase reaction to 7.0 after a period of reaction, filter, wash, and dry in a 50°C oven after reaction at 80°C for 30 min, to obtain the ammonium manganese iron phosphate precursor.

[0058] Take 55 g of the precursor sample, ball mill with 14.6 g of lithium carbonate, 4.3 g of glucose, and 2.4 g of polyethylene glycol, wherein the molar ratio of (Fe+Mn):Li is 1:1.02, the rotation speed is 400 rpm, spray dry after ball milling for 4 hours, place the spray-dried sample in a tube furnace, heat at 400°C for 2 hours and at 700°C for 10 hours under a nitrogen atmosphere, and obtain the lithium manganese iron phosphate sample after cooling to room temperature at a heating rate of 5°C / min.

[0059] Comparative Example 1: A commercially available ferrous sulfate (Chuanyunrui Xiang Trade) was configured into a 5wt% ferrous sulfate solution, 2.56g ascorbic acid was added, after complete dissolution, 40.96g of manganese sulfate monohydrate, 50.62g of ammonium dihydrogen phosphate were added and stirred until complete dissolution. Slowly add 15wt% ammonia water to the above solution, adjust the pH of the liquid phase reaction to 4, continue to add ammonia water to adjust the pH of the liquid phase reaction to 7.0 after reacting for a period of time, after reacting at room temperature for 30min, filter, wash, and dry in a 50°C oven to obtain the ammonium manganese iron phosphate precursor. The Cl element content in the obtained precursor is greater than 100ppm, and the contents of impurity elements such as Mg and Ti are all more than 1000ppm.

[0060] Comparative Example 2: A commercially available ferrous sulfate (Chuanyunrui Xiang Trade) was configured into a 5wt% ferrous sulfate solution, 2.56g ascorbic acid was added, after complete dissolution, 40.96g of manganese sulfate monohydrate, 50.62g of ammonium dihydrogen phosphate were added and stirred until complete dissolution. Slowly add 15wt% ammonia water to the above solution, adjust the pH of the liquid phase reaction to 4, continue to add ammonia water to adjust the pH of the liquid phase reaction to 7.0 after reacting for a period of time, after reacting at room temperature for 30min, filter, wash, and dry in a 50°C oven to obtain the ammonium manganese iron phosphate precursor. The Cl element content in the obtained precursor is greater than 100ppm, and the contents of impurity elements such as Mg and Ti are all more than 1000ppm. The pH value is adjusted once, and the phenomenon of serious stacking of ammonium manganese iron phosphate precursor sheets occurs, with the stacking thickness concentrated at more than 2um.

[0061] Comparative Example 3: A commercially available ferrous sulfate (Chuanyunrui Xiang Trade) was configured into a 5wt% ferrous sulfate solution, 2.56g ascorbic acid was added, after complete dissolution, 40.96g of manganese sulfate monohydrate, 50.62g of ammonium dihydrogen phosphate were added and stirred until complete dissolution. Slowly add 15wt% ammonia water to the above solution, adjust the pH of the liquid phase reaction to 4, continue to add ammonia water to adjust the pH of the liquid phase reaction to 7.0 after reacting for a period of time, after reacting at room temperature for 30min, filter, wash, and dry in a 50°C oven to obtain the ammonium manganese iron phosphate precursor. The Cl element content in the obtained precursor is greater than 100ppm, and the contents of impurity elements such as Mg and Ti are all more than 1000ppm. The pH value is adjusted once, and the phenomenon of serious stacking of ammonium manganese iron phosphate precursor sheets occurs, with the stacking thickness concentrated at more than 2um.

[0062] The SEM characterization results of the obtained precursor show that the precursor presents irregular net-like interlaced structure and polyhedral block morphology. X-ray diffraction analysis shows that the material presents a series of broadened diffraction peaks in the range of 2θ=5-40°, and the overall crystallinity of the material is significantly low. Notably, no characteristic diffraction signal of ammonium manganese iron phosphate (JCPDS No. 50-0554) was detected in the standard JCPDS card comparison, confirming that the precursor did not form a complete ammonium salt crystal structure.

[0063] Comparative Example 4: The same as example 2, the only difference is that after the addition of ammonia, the pH of the mixed solution is adjusted to 7.0 once, and then the reaction is carried out at room temperature for 30 min, and then the filtration and washing are carried out. The obtained precursor sample has a serious laminar stacking and a thickness greater than 1 um, the laminar surface is uneven, and a small amount of particles are distributed.

[0064] Comparative example 5: The same as example 2, the only difference is that after the addition of ammonia, the pH of the mixed solution is adjusted to 4.0 once, and then the reaction is carried out at room temperature for 30 min, and then the filtration and washing are carried out. The XRD spectrum of the obtained precursor sample does not appear the characteristic peak of ammonium manganese iron phosphate, indicating that under the condition that the pH of the reaction solution is directly adjusted to 4.0, ammonium manganese iron phosphate crystal material cannot be generated.

[0065] The above shows and describes the basic principles, main features and advantages of the present application. It should be understood by those skilled in the art that the present application is not limited by the above examples, and the above examples and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a manganese iron ammonium phosphate precursor, characterized in that, The method comprises the following steps: 1) dissolving an iron source in a sulfuric acid solution to prepare a ferrous sulfate solution through dissolution and filtration; 2) adding a reducing agent, a manganese source, and a soluble phosphorus source into the ferrous sulfate solution, mixing uniformly to obtain a mixed solution; 3) adjusting the pH of the mixed solution to 4-4.5 by using ammonia water, continuing to add ammonia water to adjust the pH to 5-8 after reacting for a period of time, and continuing to react for 0.5-3 hours at a temperature of 25-85℃, and then performing filtration, washing, and drying to obtain an ammonium manganese iron phosphate precursor.

2. The method for preparing a manganese iron ammonium phosphate precursor as described in claim 1, characterized in that: The iron source in the step 1) is at least one of reduced iron powder and iron block.

3. The method for preparing a manganese iron ammonium phosphate precursor as described in claim 1, characterized in that: The reaction temperature for dissolving the iron source in sulfuric acid in the step 1) is 30-90℃, and the concentration of the ferrous sulfate solution is 4.5-5.5wt%.

4. The method for preparing a manganese iron ammonium phosphate precursor as described in claim 1, characterized in that: The reducing agent in the step 2) is at least one of ascorbic acid and citric acid.

5. The method for preparing a manganese iron ammonium phosphate precursor as described in claim 1, characterized in that: The manganese source in the step 2) is at least one of manganese sulfate, manganese carbonate, trimanganese tetraoxide, manganese oxide, and manganese oxalate.

6. The method for preparing a manganese iron ammonium phosphate precursor as described in claim 1, characterized in that: The soluble phosphorus source in the step 2) is at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid.

7. The method for preparing a manganese iron ammonium phosphate precursor as described in claim 1, characterized in that: In the mixed solution in the step 2), the molar ratio of manganese to iron is 5-8:2-5, and the molar ratio of phosphorus to transition metal is 0.98-1.02:

1.

8. A manganese iron ammonium phosphate precursor, characterized in that: The ammonium manganese iron phosphate precursor is prepared by using the preparation method of any one of claims 1-7.

9. Use of a manganese iron ammonium phosphate precursor, characterized in that: The ammonium manganese iron phosphate precursor in claim 8 is mixed with a lithium source and a carbon source, and then ball-milled, spray-dried, and calcined to obtain a lithium manganese iron phosphate positive electrode material.

10. Use of a manganese iron ammonium phosphate precursor according to claim 9, characterized in that: The molar ratio of the ammonium manganese iron phosphate precursor to the lithium source is 0.98-1.05, the carbon source is at least one of glucose, sucrose, polyethylene glycol, polyethylene, and citric acid, and the calcination temperature is 600-800℃.

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