Lithium manganese iron phosphate precursor and preparation method of lithium manganese iron phosphate

Abstract

The invention discloses a preparation method of a lithium manganese iron phosphate precursor, which comprises the following steps: mixing a manganese source and an iron source according to the molar ratio of Mn < 2 + > to Fe < 2 + > of (1.5-4): 1, adding water to prepare a manganese and iron ion mixed solution with the total concentration of Mn < 2 + > and Fe < 2 + > of 1-2mol / L, then dropwise adding the manganese and iron ion mixed solution and a precipitator into a base solution at the same time to obtain a reaction mixed solution, carrying out heat preservation and aging on the reaction mixed solution, then carrying out solid-liquid separation to obtain a filter cake, sequentially washing, drying and calcining the filter cake to obtain ferromanganese oxide, and then preparing a lithium iron manganese phosphate material by taking the obtained ferromanganese oxide as a lithium iron manganese phosphate precursor; according to the method, the soluble manganese source and the iron source are adopted for coprecipitation, and the manganese-iron ratio, the reaction rate, the pH and the reaction temperature are controlled, so that stable manganese-iron oxide with uniformly mixed manganese and iron is prepared, the stability of the further prepared lithium manganese-iron phosphate material is higher, and the electrochemical performance of the lithium manganese-iron phosphate material is effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium iron manganese phosphate preparation, and more specifically, to a lithium iron manganese phosphate precursor and a method for preparing the lithium iron manganese phosphate. Background Art

[0002] In recent years, lithium iron phosphate batteries have been widely used in UPS, power tools, electric vehicles, energy storage batteries and other fields. However, the relatively low voltage platform (3.4V) of lithium iron phosphate makes its energy density low, which limits its development and application. Lithium iron manganese phosphate has the same structure as lithium iron phosphate but its discharge platform (4.1V) is higher, thus having a higher energy density.

[0003] At present, the synthesis methods of lithium manganese iron phosphate positive electrode materials are mainly divided into solid phase method and co-precipitation method. Among them, the high temperature solid phase method is the most popular method for industrial mass production. The synthesis is simple, but the particles of the prepared material are easy to agglomerate and the particle size distribution is uneven. It is difficult for Fe and Mn to be evenly distributed in the main structure of the material, resulting in Mn 3+ The Jahn-Teller effect is serious, which affects the battery's cycle and rate performance. The co-precipitation method uses a mixed reaction of a manganese source, an iron source, and a precipitant to generate a precursor - a manganese iron complex, and then the manganese iron complex is reacted with a phosphorus source and a lithium source in a solid phase or liquid phase to generate lithium manganese iron phosphate. The co-precipitation method has mild reaction conditions, which can mix the raw materials at the molecular level or even the atomic level. By adding a precipitant and adjusting the ion precipitation conditions, a precursor precipitate with a smaller particle size and a uniform distribution can be obtained. Compared with the solid phase method, this method is more conducive to the uniform distribution of iron and manganese, and is a method that can be mass-produced.

[0004] At present, there are mainly two types of precursors, oxalic acid system and phosphoric acid system. The manganese and iron in manganese iron oxalate are evenly mixed, and lithium manganese iron phosphate positive electrode materials with better performance can be prepared. The disadvantage is that during the sintering reaction, a large amount of oxalate needs to be converted into CO2 and discharged. The production of a large amount of gas phase may affect the circulation, compaction and other properties of the material. The sintering process of the phosphoric acid system precursor will not produce a large amount of gas phase, but it is currently difficult to generate trivalent manganese iron phosphate precipitate like iron phosphate. To oxidize it to trivalent, it is usually necessary to use a nitric acid-ethanol system, or add a strong oxidant to oxidize it in a high temperature and high pressure environment. Its efficiency is extremely low and its energy consumption and pollution are high, making it difficult to apply industrially.

[0005] For example, the invention patent with publication number CN106340620A provides a method for preparing lithium iron manganese phosphate / carbon composite positive electrode material for lithium battery, which comprises first weighing manganese sulfate and iron phosphate in a certain stoichiometric ratio to prepare a metal ion solution, then using ammonium oxalate as a precipitant, and adjusting the pH of the solution with dilute sulfuric acid to prepare a precursor Mn x Fe (1-x)C2O4·2H2O (0.1≤x≤0.9); then the precursor is mixed with lithium carbonate and ammonium dihydrogen phosphate, and after adding a carbon source, anhydrous ethanol is added to mix evenly, and then ball milling and drying are performed to obtain lithium iron phosphate precursor powder; finally, the obtained precursor powder is crushed and sieved, and then calcined in a tubular furnace protected by ordinary pure nitrogen atmosphere to obtain LiMn x Fe (1-x) PO4 / C positive electrode material; this invention uses oxalate as raw material. During the sintering reaction, a large amount of oxalate ions are converted into CO2 and discharged, which may affect the circulation, compaction and other properties of the material. In addition, anhydrous ethanol needs to be used as a solvent when synthesizing lithium manganese iron phosphate, which leads to increased costs. For another example, the invention patent with publication number CN115043387A discloses a method for preparing ammonium manganese iron phosphate, lithium manganese iron phosphate and its application, firstly, a metal mixed salt solution, a diammonium phosphate solution and an organic solution are mixed respectively to obtain a metal salt mixed solution and a phosphate mixed solution, wherein the metal mixed salt solution is a mixed solution of manganese salt and ferrous salt, and the organic solution is obtained by dissolving a surfactant in an organic solvent; then, under an inert atmosphere, the metal salt mixed solution, the phosphate mixed solution and ammonia water are added to the base liquid in parallel to react, and when the reaction material reaches the target particle size, the ammonium manganese iron phosphate is obtained by solid-liquid separation; then, the ammonium manganese iron phosphate is used as a precursor, mixed with a lithium source and a carbon source and calcined to obtain lithium manganese iron phosphate, and a lithium manganese iron phosphate positive electrode material with a porous structure can be obtained, but when preparing the precursor, iron salt and manganese salt are used to configure the mixed phosphate, the raw material cost is relatively high, and organic solvents such as surfactants are used, which are difficult to clean.

[0006] In addition, in order to further make the Fe / Mn distribution in the prepared precursor uniform and avoid the problem of poor performance caused by large differences in the local Fe / Mn molar ratio, some studies have first prepared a more stable manganese iron oxide as a precursor of lithium iron manganese phosphate. For example, the invention patent with publication number CN117727940A discloses a lithium iron manganese phosphate positive electrode material and a preparation method thereof, which is to first mix manganese powder and iron powder in proportion, and then heat at 1500℃-2000℃ for 5-6h in an inert gas atmosphere to obtain Mn x Fe 1-x alloy; then the obtained Mn x Fe 1-xThe alloy is mixed and ground with an ammonium salt solution, and compressed air is continuously added simultaneously, the reaction is stirred, filtered, washed, dried, and ground to obtain a manganese iron oxide precursor with a D50 of 0.5-30 μm; then the manganese iron oxide precursor, a lithium source, a phosphorus source, a doping element, and a carbon source are added to ultrapure water, mixed to form a suspension, and dried to obtain a mixed powder, which is then placed in a high-temperature tubular furnace, and a gas with a volume ratio of Ar:H2=95:5 is introduced, and the mixture is reacted at 600°C for 4 hours, and taken out after cooling to obtain a mixed solid, and finally a carbon / phosphorus composite material and a thioindigo carbamide chloride compound are added, and the lithium iron manganese phosphate positive electrode material is obtained after ball milling and sintering; the manganese iron oxide precursor prepared by the invention has uniform distribution, high compaction density, and large specific surface area, and a fluidized bed oxidation method is used to uniformly mix iron and manganese at the atomic level, thereby avoiding large differences in the local Fe / Mn molar ratio, and further improving the stability of the structure and the electrochemical performance of the lithium iron manganese phosphate positive electrode material; however, this method requires high temperature and compressed air, which is costly and not conducive to safe operation. Summary of the invention

[0007] In view of the above, the present invention provides a lithium iron manganese phosphate precursor and a method for preparing lithium iron manganese phosphate, which solves the problem of uneven distribution of manganese iron in the preparation of lithium iron manganese phosphate. At the same time, oxalic acid system and phosphoric acid system are not used as precursors of lithium iron manganese phosphate, high temperature and high pressure reactions are not used, reaction conditions are mild, and costs are low.

[0008] The present invention provides a method for preparing a lithium iron manganese phosphate precursor, comprising the following steps: (1) The manganese source and iron source are divided into 2+ with Fe 2+ The molar ratio of manganese and iron is 1.5 to 4:1, and water is added to prepare a manganese and iron ion mixture, wherein Mn in the manganese and iron ion mixture is 2+ with Fe 2+ The total concentration is 1-2 mol / L; (2) dropping a mixed solution of manganese and iron ions and a precipitant into the base solution simultaneously to obtain a reaction mixed solution; (3) heat-insulating and aging the reaction mixture to obtain a reaction slurry; (4) performing solid-liquid separation on the reaction slurry to obtain a filter cake, and the filter cake is washed and dried in sequence to obtain a brown powder; (5) The brown powder is calcined to obtain ferromanganese oxide, which is a precursor of lithium iron manganese phosphate.

[0009] Mn 2+ with Fe 2+ The molar ratio of Mn affects the performance of the final lithium iron manganese phosphate. When synthesizing the lithium iron manganese phosphate precursor, the ratio of manganese to iron needs to be controlled. When the ratio of manganese exceeds a certain range, the electrical properties, cycle performance and stability of the final lithium iron manganese phosphate will decrease.2+ with Fe 2+ The molar ratio of Mn is 1.5 to 4:1, which is the appropriate range. 2+ with Fe 2+ The concentration affects the nucleation rate of the lithium iron manganese phosphate precursor. Under the same other conditions, the higher the concentration, the faster the nucleation. If the concentration is too low, the nucleation efficiency is slow, affecting the production efficiency. If the concentration is too high, the nucleation rate is too fast, which can easily lead to uneven manganese iron precipitation.

[0010] The manganese source is a water-soluble divalent manganese salt, and the iron source is a water-soluble divalent iron salt. The soluble manganese source and the iron source are co-precipitated. The manganese source and the iron source are dissolved in water and mixed in molecular form to more fully achieve uniform mixing of manganese and iron elements.

[0011] The manganese source is one or more of manganese chloride, manganese sulfate, manganese carbonate and manganous nitrate, and the iron source is one or two of ferrous sulfate and ferrous chloride.

[0012] The precipitant is one or more of sodium hydroxide, potassium hydroxide, and ammonia water, and the concentration is 2.5-5 mol / L.

[0013] The base liquid is one or more of citric acid, sodium citrate, ethylenediaminetetraacetic acid, and disodium ethylenediaminetetraacetate, with a concentration of 0.01-0.10 mol / L and a pH of 10.5±0.5.

[0014] The dropping rate of the manganese and iron ion mixed solution and the precipitant into the base solution is 5-20 mL / min.

[0015] The concentration of the precipitant and the dripping rate of the manganese, iron ion mixture and the precipitant into the base liquid at the same time will affect the nucleation rate of the lithium iron manganese phosphate precursor: when the concentration of the precipitant or the dripping rate of the manganese, iron ion mixture and the precipitant into the base liquid at the same time is too low, the nucleation of the lithium iron manganese phosphate precursor is slow, affecting the production efficiency; when the concentration of the precipitant is too high, the nucleation of the lithium iron manganese phosphate precursor is fast, which can easily lead to uneven precipitation of manganese and iron; when the dripping rate of the manganese, iron ion mixture and the precipitant into the base liquid at the same time is too fast, it will also lead to uneven precipitation of manganese and iron, and the formed particle size is small.

[0016] The base liquid is used as a complexing agent. When the concentration of the base liquid is too high, ferromanganese is not easy to precipitate. When the concentration of the base liquid is too low, the uniformity of ferromanganese precipitation is affected. At the same time, the pH of the base liquid is adjusted so that the pH of the reaction system is 10.5±0.5 when the manganese and iron ion mixture and the precipitant are simultaneously dripped into the base liquid. The dripping rate, i.e., the reaction rate, is controlled to ensure that the pH of the entire reaction system is 10.5±0.5, so as to promote the complete precipitation of manganese and iron in the reaction mixture and ensure that the Mn in the obtained lithium manganese iron phosphate precursor is 2+ with Fe 2+The molar ratio is 1.5 to 4:1.

[0017] The insulation temperature of the reaction mixture for insulation and aging treatment is 50-90°C for 2-6 hours, and the aging temperature is 50-90°C for 6-12 hours.

[0018] The calcination temperature of the brown powder is 600-800°C, and the calcination time is 2-6 hours.

[0019] The molecular formula of ferromanganese oxide is Mn x Fe (2-x) O3, x=1.2~1.6, the crystal structure is Mn2O3, Fe2O3, manganese iron oxide is a stable oxide and an ideal precursor of lithium manganese iron phosphate.

[0020] The present invention also provides a method for preparing lithium iron manganese phosphate using a lithium iron manganese phosphate precursor, comprising the following steps: (1) mixing a lithium iron manganese phosphate precursor, a lithium source, a phosphorus source, a carbon source, a dopant and pure water and then sand-milling the mixture to obtain a sand-milled slurry; The lithium iron manganese phosphate precursor, lithium source, phosphorus source, and dopant are calculated as Fe+Mn, Li, P, and doping element, and the molar ratio is Fe+Mn: Li: P: doping element = 1: 1.0-1.05: 1.0-1.05: 0.01-0.05; wherein Fe+Mn is the total amount of Fe element and Mn element; The carbon source accounts for 5-15wt% of the total amount of the lithium iron manganese phosphate precursor, the lithium source and the carbon source; (2) spray drying the sand-milled slurry to obtain a powder; (3) Finally, the powder is calcined, crushed and sieved under the protection of inert gas to obtain lithium manganese iron phosphate.

[0021] The D50 of the slurry after sand grinding is ≤250nm.

[0022] The lithium source is one or more of lithium dihydrogen phosphate, lithium carbonate, lithium hydroxide and lithium nitrate; the phosphorus source is one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate and phosphoric acid; and the carbon source is one or more of starch, sucrose, glucose, polyethylene glycol and citric acid.

[0023] The present invention adopts a soluble manganese source and an iron source for co-precipitation, and controls the manganese-iron ratio, reaction rate, pH and reaction temperature, so as to form a solid solution after sintering, thereby achieving uniform mixing of manganese and iron at the atomic level; at the same time, the manganese iron oxide prepared by the present invention is a stable oxide, and the further preparation of lithium manganese iron phosphate has higher stability than other preparation methods, thereby improving the electrochemical performance of lithium manganese iron phosphate; and the method of the present invention is simple and flexible to operate, low in cost, mild in reaction conditions, and suitable for large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a SEM image of the ferromanganese oxide obtained in Example 1.

[0025] Figure 2 This is the XRD pattern of the ferromanganese oxide obtained in Example 1.

[0026] Figure 3 This is the XRD diagram of the lithium manganese iron phosphate obtained in Example 1.

[0027] Figure 4 This is the 0.1C first charge and discharge curve of the lithium manganese iron phosphate obtained in Example 1. DETAILED DESCRIPTION

[0028] The embodiments of the present application will be described in more detail below. The application can be implemented in various forms, and should not be construed as being limited to the embodiments set forth herein, but rather these embodiments are provided to provide a more thorough and complete understanding of the present application. It should be understood that the embodiments of the present application are only for exemplary purposes and are not intended to limit the scope of protection of the present application.

[0029] The embodiments and comparative examples of the present invention are implemented according to the following technical scheme, and the specific steps include: Embodiment 1: 1. Preparation of ferromanganese oxide (1) 304.236 g of manganese sulfate monohydrate and 333.612 g of ferrous sulfate heptahydrate were mixed and dissolved in 1.5 L of pure water. 2+ with Fe 2+ The molar ratio of Mn is 1.8:1.2, and Mn 2+ with Fe 2+ A mixed solution of manganese and iron ions with a total concentration of 2 mol / L; (2) Dissolve 14.4 g of citric acid in 1.5 L of pure water, and add 1 mol / L of NaOH to adjust the pH to 10.5 ± 0.5 to obtain a base solution with a concentration of 0.05 mol / L; (3) preparing 5 mol / L NaOH solution to obtain a precipitant; (4) dropping the manganese and iron ion mixture and the precipitant into the base liquid at a rate of 5 mL / min, controlling the reaction temperature at 60° C., stirring at 250 rpm, and maintaining the pH value in the reaction system at 10.5±0.5. After the dropwise addition was completed, the pH value of the system at the end point was 11, thereby obtaining a reaction mixture; (5) stirring the reaction mixture at 60° C. for 4 h, and then keeping the mixture warm for 12 h to obtain a reaction slurry; (6) performing solid-liquid separation on the reaction slurry to obtain a filter cake, and the filter cake is washed and dried in sequence to obtain a brown powder; (7) calcining the brown powder at 600° C. for 6 h to obtain manganese iron oxide, i.e., lithium manganese iron phosphate precursor; 2. Preparation of lithium manganese iron phosphate (1) Weigh 39.7 g of ferromanganese oxide, 54.6 g of lithium dihydrogen phosphate, 8 g of glucose, 0.5 g of titanium dioxide and pure water, and then sand grind until the D50 of the slurry is ≤250 nm; (2) spray drying the sand-milled slurry to obtain a powder; (3) The powder obtained in step (2) is calcined at 600° C. for 12 h under nitrogen protection, crushed and sieved to obtain a lithium manganese iron phosphate positive electrode material.

[0030] Embodiment 2: 1. Preparation of ferromanganese oxide (1) 152.118 g of manganese sulfate monohydrate and 166.806 g of ferrous sulfate heptahydrate were mixed and dissolved in 1.5 L of pure water. 2+ with Fe 2+ The molar ratio of Mn is 0.9:0.6, and Mn 2+ with Fe 2+ A mixed solution of manganese and iron ions with a total concentration of 1 mol / L; (2) Dissolve 10.08 g of EDTA-2Na in 1.5 L of pure water, and add 1 mol / L of NaOH to adjust the pH to 10.5 ± 0.5 to obtain a base solution with a concentration of 0.02 mol / L; (3) preparing 2.5 mol / L NaOH solution to obtain a precipitant; (4) dropping the manganese and iron ion mixture and the precipitant into the base liquid at a rate of 10 mL / min, controlling the reaction temperature at 60° C., stirring at 250 rpm, and maintaining the pH value in the reaction system at 10.5±0.5. After the dropwise addition is completed, the pH value of the system at the end point is 11, thereby obtaining a reaction mixture; (5) stirring the reaction mixture at 60° C. for 4 h, and then keeping the mixture warm for 8 h to obtain a reaction slurry; (6) performing solid-liquid separation on the reaction slurry to obtain a filter cake, and the filter cake is washed and dried in sequence to obtain a brown powder; (7) calcining the brown powder at 800° C. for 2 h to obtain ferromanganese oxide, i.e., lithium ferromanganese phosphate precursor; 2. The preparation of lithium manganese iron phosphate is the same as that in Example 1.

[0031] Embodiment 3: 1. Preparation of ferromanganese oxide (1) 202.824 g of manganese sulfate monohydrate and 83.403 g of ferrous sulfate heptahydrate were mixed and dissolved in 1.5 L of pure water. 2+ with Fe 2+ The molar ratio of Mn is 1.2:0.3, and Mn 2+ with Fe 2+ A mixed solution of manganese and iron ions with a total concentration of 1 mol / L; (2) Dissolve 4.4 g of citric acid in 1.5 L of pure water, and add 1 mol / L of KOH to adjust the pH to 10.5 ± 0.5 to obtain a base solution with a concentration of 0.02 mol / L; (3) preparing 2.5 mol / L KOH solution to obtain a precipitant; (4) dropping the manganese and iron ion mixture and the precipitant into the base liquid at a rate of 10 mL / min, controlling the reaction temperature at 50° C., stirring at 250 rpm, and maintaining the pH value in the reaction system at 10.5±0.5. After the dropwise addition is completed, the pH value of the system at the end point is 11, thereby obtaining a reaction mixture; (5) stirring the reaction mixture at 50° C. for 4 h, and then keeping the mixture warm for 12 h to obtain a reaction slurry; (6) performing solid-liquid separation on the reaction slurry to obtain a filter cake, and the filter cake is washed and dried in sequence to obtain a brown powder; (7) calcining the brown powder at 700° C. for 4 h to obtain ferromanganese oxide, i.e., lithium ferromanganese phosphate precursor; 2. The preparation of lithium manganese iron phosphate is the same as that in Example 1.

[0032] Embodiment 4: 1. Preparation of ferromanganese oxide (1) 304.236 g of manganese sulfate monohydrate and 333.612 g of ferrous sulfate heptahydrate were mixed and dissolved in 1.5 L of pure water. 2+ with Fe 2+ The molar ratio of Mn is 1.8:1.2, and Mn 2+ with Fe 2+ A mixed solution of manganese and iron ions with a total concentration of 2 mol / L; (2) Dissolve 4.4 g of EDTA in 1.5 L of pure water, and add 1 mol / L of KOH to adjust the pH to 10.5 ± 0.5 to obtain a base solution with a concentration of 0.01 mol / L; (3) preparing 2.5 mol / L KOH solution to obtain a precipitant; (4) dropping the manganese and iron ion mixture and the precipitant into the base liquid at a rate of 20 mL / min, controlling the reaction temperature at 90° C., stirring at 250 rpm, and maintaining the pH value in the reaction system at 10.5±0.5. After the dropwise addition is completed, the pH value of the system at the end point is 11, thereby obtaining a reaction mixture; (5) stirring the reaction mixture at 90° C. for 6 h, and then keeping the mixture warm for 6 h to obtain a reaction slurry; (6) performing solid-liquid separation on the reaction slurry to obtain a filter cake, and the filter cake is washed and dried in sequence to obtain a brown powder; The brown powder was calcined at 700°C for 2 h to obtain ferromanganese oxide, i.e., lithium ferromanganese phosphate precursor; 2. The preparation of lithium manganese iron phosphate is the same as that in Example 1.

[0033] Embodiment 5: 1. Preparation of ferromanganese oxide (1) Dissolve 132.13 g of manganese chloride and 89.5095 g of ferrous chloride tetrahydrate in 1.5 L of pure water, then add 7.2 g of citric acid. 2+ with Fe 2+ The molar ratio of Mn is 7:3, and Mn 2+ with Fe 2+ A mixed solution of manganese and iron ions with a total concentration of 1 mol / L; (2) Dissolve 14.4 g of citric acid in 1.5 L of pure water, and add 1 mol / L of NaOH to adjust the pH to 10.5 ± 0.5 to obtain a base solution with a concentration of 0.05 mol / L; (3) preparing 5 mol / L NaOH solution to obtain a precipitant; (4) dropping the manganese and iron ion mixture and the precipitant into the base liquid at a rate of 5 mL / min, controlling the reaction temperature at 60° C., stirring at 250 rpm, and maintaining the pH value in the reaction system at 10.5±0.5. After the dropwise addition was completed, the pH value of the system at the end point was 11, thereby obtaining a reaction mixture; (5) stirring the reaction mixture at 60° C. for 4 h, and then keeping the mixture warm for 12 h to obtain a reaction slurry; (6) performing solid-liquid separation on the reaction slurry to obtain a filter cake, and the filter cake is washed and dried in sequence to obtain a brown powder; (7) calcining the brown powder at 600° C. for 6 h to obtain manganese iron oxide, i.e., lithium manganese iron phosphate precursor; 2. The preparation of lithium manganese iron phosphate is the same as that in Example 1.

[0034] Embodiment 6: 1. Preparation of ferromanganese oxide (1) 304.236 g of manganese sulfate monohydrate and 333.612 g of ferrous sulfate heptahydrate were mixed and dissolved in 1.5 L of pure water. 2+ with Fe 2+ The molar ratio of Mn is 1.8:1.2, and Mn 2+ with Fe 2+ A mixed solution of manganese and iron ions with a total concentration of 2 mol / L; (2) Dissolve 28.82 g of citric acid in 1.5 L of pure water, and add 1 mol / L of KOH to adjust the pH to 10.5 ± 0.5 to obtain a base solution with a concentration of 0.1 mol / L; (3) preparing 2.5 mol / L KOH solution to obtain a precipitant; (4) dropping the manganese and iron ion mixture and the precipitant into the base liquid at a rate of 20 mL / min, controlling the reaction temperature at 90° C., stirring at 250 rpm, and maintaining the pH value in the reaction system at 10.5±0.5. After the dropwise addition is completed, the pH value of the system at the end point is 11, thereby obtaining a reaction mixture; (5) stirring the reaction mixture at 90° C. for 6 h, and then keeping the mixture warm for 6 h to obtain a reaction slurry; (6) performing solid-liquid separation on the reaction slurry to obtain a filter cake, and the filter cake is washed and dried in sequence to obtain a brown powder; (7) calcining the brown powder at 700° C. for 2 h to obtain ferromanganese oxide, i.e., lithium ferromanganese phosphate precursor; 2. The preparation of lithium manganese iron phosphate is the same as that in Example 1.

[0035] Comparative Example 1: The same steps and conditions as in Example 1 were followed, with the only difference being that the manganese salt and the iron salt were not subjected to liquid phase coprecipitation, but were simply ground and mixed, and then sintered to obtain ferromanganese oxide, as follows: 1. Preparation of ferromanganese oxide 304.236 g of manganese sulfate monohydrate and 333.612 g of ferrous sulfate heptahydrate were simply ground and mixed, and then calcined in a muffle furnace at 600°C for 6 h to obtain ferromanganese oxide; 2. The preparation of lithium manganese iron phosphate is the same as that in Example 1.

[0036] Comparative Example 2: The same steps and conditions as in the example are used to prepare lithium manganese iron phosphate using ferric oxide and manganese oxide as raw materials, as follows: (1) 31.574 g of manganese trioxide, 7.99 g of ferric oxide, 54.6 g of lithium dihydrogen phosphate, 8 g of glucose, and 0.25 g of titanium dioxide were weighed and added into pure water and mixed, and then sand-milled until the D50 of the slurry was ≤ 250 nm, and the sand-milled slurry was spray-dried to obtain a powder; (2) The obtained powder was calcined at 600° C. for 12 h under nitrogen protection, crushed and sieved to obtain lithium manganese iron phosphate positive electrode material.

[0037] The ferromanganese oxide and lithium iron manganese phosphate materials obtained in the above examples and comparative examples were tested respectively, and the test results are shown in Table 1.

[0038] Attached Figure 1 and attached Figure 2 1 and 2 are SEM and XRD diagrams of the ferromanganese oxide obtained in Example 1, respectively. It can be concluded that the primary particle size of the ferromanganese oxide is 150-300 nm, presenting a single crystal form, and the ferromanganese element is evenly distributed, proving that the method of the present invention can achieve uniform mixing of manganese and iron atoms; Figure 4 This is the XRD diagram of the lithium manganese iron phosphate obtained in Example 1, which can prove that the method of the present invention can produce high-purity lithium manganese iron phosphate material.

[0039] As shown in Table 1, the discharge capacity and cycle performance of the lithium iron manganese phosphate materials prepared in Examples 1 to 6 are significantly better than those of the lithium iron manganese phosphate materials prepared in Comparative Examples 1 and 2, and the higher the discharge current value, the more significant the improvement effect of the discharge capacity, which proves that the ferromanganese oxide prepared by the co-precipitation method of the present invention is more conducive to the performance of lithium iron manganese phosphate, and effectively improves the discharge capacity and cycle performance of lithium iron manganese phosphate; at the same time, the discharge capacity and cycle performance of the lithium iron manganese phosphate material prepared in Comparative Example 1 are better than those of the lithium iron manganese phosphate material prepared in Comparative Example 2, which proves that the advantage of the method of first preparing ferromanganese oxide and then preparing lithium iron manganese phosphate material with ferromanganese oxide in the present invention can obtain lithium iron manganese phosphate material with higher discharge capacity and cycle performance.

[0040] In summary, the co-precipitation method of the present invention using a soluble manganese source and an iron source can achieve uniform mixing of manganese and iron elements at the atomic level, thereby producing stable ferromanganese oxide in which manganese and iron elements are evenly distributed, thereby making the further produced lithium manganese iron phosphate material more stable and effectively improving the electrochemical properties of the lithium manganese iron phosphate material.

[0041] Table 1: Comparison of indicators of ferromanganese oxide and lithium ferromanganese phosphate prepared in various embodiments and comparative examples.

[0042] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in this application, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for preparing a lithium manganese iron phosphate precursor, characterized in that: The following steps are involved: (1) mixing a manganese source and an iron source, and adding water to prepare a manganese and iron ion mixed solution; (2) dropping a mixed solution of manganese and iron ions and a precipitant into the base solution simultaneously to obtain a reaction mixed solution; (3) heat-insulating and aging the reaction mixture to obtain a reaction slurry; (4) performing solid-liquid separation on the reaction slurry to obtain a filter cake, and the filter cake is washed and dried in sequence to obtain a brown powder; (5) The brown powder is calcined to obtain ferromanganese oxide, which is a precursor of lithium iron manganese phosphate.

2. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: The Mn in the manganese and iron ion mixture 2+ with Fe 2+ The total concentration is 1~2mol / L.

3. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: The manganese source and iron source are as follows: 2+ with Fe 2+ The molar ratio is 1.5 to 4:

1.

4. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: The manganese source is a water-soluble divalent manganese salt, including one or more of manganese chloride, manganese sulfate, manganese carbonate and manganous nitrate.

5. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: The iron source is a water-soluble divalent iron salt, including one or both of ferrous sulfate and ferrous chloride.

6. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: The concentration of the precipitant is 2.5-5 mol / L.

7. The method for preparing a lithium iron manganese phosphate precursor according to any one of claims 1 or 6, characterized in that: The precipitant is one or more of sodium hydroxide, potassium hydroxide and ammonia water.

8. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: The concentration of the base solution is 0.01-0.10 mol / L, and the pH is 10.5±0.

5.

9. The method for preparing a lithium iron manganese phosphate precursor according to any one of claims 1 or 8, characterized in that: The base liquid is one or more of citric acid, sodium citrate, ethylenediaminetetraacetic acid, and disodium ethylenediaminetetraacetate.

10. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: The manganese and iron ion mixed solution and the precipitant are simultaneously dripped into the base liquid at a dropping rate of 5-20 mL / min.

11. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: The insulation temperature of the reaction mixture for insulation and aging treatment is 50-90° C., the time is 2-6 hours, the aging temperature is 50-90° C., and the aging time is 6-12 hours.

12. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: The brown powder has a calcination temperature of 600-800° C. and a calcination time of 2-6 hours.

13. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: The molecular formula of the ferromanganese oxide is Mn x Fe (2-x) O3, x=1.2~1.

6.

14. A method for preparing lithium manganese iron phosphate using the lithium manganese iron phosphate precursor according to claim 1, characterized in that: The following steps are involved: (1) mixing a lithium iron manganese phosphate precursor, a lithium source, a phosphorus source, a carbon source, a dopant and pure water and then sand-milling to obtain a sand-milled slurry; wherein the lithium iron manganese phosphate precursor, the lithium source, the phosphorus source and the dopant are respectively calculated as Fe+Mn, Li, P and the doping element, and the molar ratio thereof is Fe+Mn: Li: P: doping element = 1: 1.0-1.05: 1.0-1.05: 0.01-0.05; wherein the carbon source accounts for 5-15wt% of the total amount of the lithium iron manganese phosphate precursor, the lithium source and the carbon source; (2) spray drying the sand-milled slurry to obtain a powder; (3) Finally, the powder is calcined, crushed and sieved under the protection of inert gas to obtain lithium manganese iron phosphate.

15. The method for preparing lithium iron manganese phosphate from a lithium iron manganese phosphate precursor according to claim 14, characterized in that: The D50 of the slurry after sand grinding is ≤250nm.

16. The method for preparing lithium iron manganese phosphate from a lithium iron manganese phosphate precursor according to claim 14, characterized in that: The lithium source is one or more of lithium dihydrogen phosphate, lithium carbonate, lithium hydroxide and lithium nitrate; the phosphorus source is one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate and phosphoric acid; and the carbon source is one or more of starch, sucrose, glucose, polyethylene glycol and citric acid.

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