Preparation method of nanorod-like ferromanganese spinel oxide and lithium manganese iron phosphate

Abstract

The invention provides a preparation method of nanorod-like ferromanganese spinel oxide and lithium manganese iron phosphate, and relates to the field of lithium batteries, the method comprises two stages of preparation of a nanorod-like ferromanganese oxide material and phosphorus-doped mixed lithium, and preparation of lithium manganese iron phosphate through high-temperature calcination. The preparation of the early-stage ferromanganese oxide comprises the following steps: preparing a solution containing a catalyst and mixed slurry of manganese powder and iron powder; mixing the mixed slurry, the catalyst solution and an oxidizing agent, then carrying out a leaching dissolution-oxidation precipitation reaction until the reaction is finished, and filtering, washing and drying a reaction product to obtain the ferromanganese oxide. The preparation method of the ferromanganese oxide and the lithium manganese iron phosphate positive electrode material is simple to operate, short in process, mild in reaction, high in product purity and suitable for industrial production. The raw materials are easy to obtain, the product yield is high, the production cost is low, and large-scale production can be realized.

Description

Technical Field

[0001] The present invention relates to the field of lithium batteries, and more particularly to a method for preparing nanorod-shaped manganese iron spinel oxide and lithium manganese iron phosphate. Background Art

[0002] Lithium manganese iron phosphate (LiMn x Fe 1-x Lithium iron phosphate (LiFePO4, LFP) is an upgraded material that partially replaces iron (Fe) by introducing manganese (Mn) to form a solid solution structure. The introduction of manganese significantly improves the voltage platform of the material, thereby increasing the energy density by about 15-20%, while retaining the safety, long cycle life and low cost advantages of LFP. However, due to the 1-x Fe x The one-dimensional lithium ion diffusion channel and high charge transfer energy barrier of PO4 cause it to undergo slow lithium insertion and deintercalation behavior during the electrochemical process.

[0003] Uniform solid solution of manganese iron can suppress crystal expansion caused by localized Jahn-Teller distortion of trivalent manganese, improve the heterogeneity of the active phase distribution within the cathode material particles, and improve the conditions for lithium-ion transport kinetics. Therefore, the preparation process of lithium iron manganese phosphate (LFMPO) cathode materials places stringent requirements on uniform phase distribution, nanosized particles, and an efficient conductive shell. Due to the abundant and inexpensive raw materials, LFMPO has diverse synthesis routes. To date, there is no standardized LFMPO preparation process. Traditionally, LFMPO is obtained by doping LiFePO4 with Mn, allowing its preparation to follow the same process path as LFMPO. Conventional synthesis methods include solid-phase and liquid-phase methods. The liquid-phase method achieves homogeneity in the precursor material through atomic-level mixing of the raw materials in a liquid phase, while the solid-phase method utilizes traditional industrial techniques (ball milling, spray drying, and annealing) to enhance mass transfer through mechanical nanocrystallization or high-temperature thermal diffusion. However, the former process is complex and costly, while the latter produces poor product consistency and high energy consumption. To overcome current challenges, researchers in this field can introduce new developments, such as optimized raw materials, green synthesis methods, and appropriate modification strategies to improve the overall performance of the material. Optimizing the synthesis of LMFP by combining the advantages of solid-phase and liquid-phase methods, and exploring a simple, reliable, economical, and green technology route, will facilitate the commercial application of lithium manganese iron phosphate materials. Summary of the Invention

[0004] In view of the shortcomings of traditional preparation technology, the purpose of the present invention is to provide an economical and effective method for preparing lithium manganese iron phosphate positive electrode material that can be produced on a large scale.

[0005] On the one hand, the present invention provides an expandable manganese iron oxide precursor material, and the chemical formula of the manganese iron oxide can be (Mn 1-x Fe x )3O4, where 0 < x < 1. It is composed of spherical secondary particles aggregated by regular hexagonal nanorod-shaped primary particles.

[0006] According to the morphology characteristics adjustable by the manganese-iron ratio, the molar ratio of manganese to iron can be (1-99):1.

[0007] The preparation method of the manganese iron oxide may include the following steps: preparing a solution containing a catalyst and a mixture of manganese powder and iron powder; mixing the mixture and the catalyst solution and then carrying out an in-situ leaching dissolution-oxidation precipitation reaction until the reaction ends, filtering, washing, and drying the precipitate obtained after the reaction to obtain the manganese iron oxide.

[0008] Preferably, the solid content of manganese and iron in the slurry can be 0.1-0.8.

[0009] The catalyst is one or a mixture of several of ammonium chloride, sodium chloride, ammonium nitrate, ammonium sulfate, ammonium bromide, ammonium iodide, hydrochloric acid, and sulfuric acid, and the addition amount of the catalyst is 5-25 g / L.

[0010] Further, when mixing the mixed slurry, the catalyst, and the oxidant, the feeding rate of the oxidant is 1-5 ml / min; the pH value of the reaction system can be 5.0-8.0, the reaction temperature can be 50-90 °C, the reaction time can be 6-48 h, and the concentration of the catalyst can be 5-25 g / L.

[0011] Further, the iron source is one of iron powder or FeSO4·7H2O, FeCl2, FeCl3, and the manganese salt is one of manganese powder or MnSO4·H2O, MnCl2; the oxidant is one or a mixture of several of air, oxygen, ammonium nitrate, nitric acid, nitrous acid, potassium permanganate, ammonium persulfate, and hydrogen peroxide.

[0012] The present invention provides a preparation method of a lithium manganese iron phosphate cathode material, including the following steps: mixing the above manganese iron oxide with a lithium source, a phosphorus source, and a carbon source, spray drying, and then calcining in an inert atmosphere to obtain a carbon-coated lithium manganese iron phosphate cathode material.

[0013] After maintaining the temperature at 400-500°C for 4-6 hours, the mixture is calcined at 500-800°C for 4-8 hours to obtain a lithium iron manganese phosphate / carbon composite material. The lithium source is one or a mixture of LiOH, Li2CO3, LiH2PO4, and lithium oxalate; the phosphorus source is one or more of phosphoric acid, monoammonium phosphate, phytic acid, diammonium phosphate, and triammonium phosphate; and the carbon source is at least one or more of cyclodextrin, sucrose, starch, polyvinyl pyrrolidone, and citric acid. The molar ratio of Li, P, and (Fe+Mn) is (1-1.04):(1-1.03):1, and the mass of the carbon source is 1-30% of the mass of the manganese iron oxide.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] (1) The manganese iron spinel oxide and lithium manganese iron phosphate positive electrode material obtained by the present invention have a uniform distribution of transition metal elements, and the preparation method has low raw material cost, short process flow, simple operation, controllable product indicators, and is easy to mass produce;

[0016] (2) The manganese iron oxide of the present invention has a preferred primary particle with a regular hexagonal nanorod morphology, high crystallinity, and low impurity content; the prepared carbon-coated manganese iron phosphate lithium positive electrode material has the advantages of high compaction density, good processing performance, and excellent electrochemical performance;

[0017] (3) The physical and chemical indicators of manganese iron oxide and positive electrode materials can be customized by regulating the type of raw materials, manganese iron ratio or reaction conditions, so as to meet the material production and processing requirements of different product specifications. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above features and other objects of the present invention will become more apparent from the following description of the embodiments with reference to the accompanying drawings, in which:

[0019] Figure 1 This is the XRD pattern of nanorod manganese iron oxide prepared in Example 1 of the present invention;

[0020] Figure 2 This is a SEM image of nanorod manganese iron oxide prepared in Example 1 of the present invention;

[0021] Figure 3 This is the XRD pattern of the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present invention;

[0022] Figure 4 This is a SEM image of the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present invention;

[0023] Figure 5 0.1C charge-discharge curve of the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present invention;

[0024] Figure 6 This is the XRD pattern of nanorod manganese iron oxide prepared in Example 2 of the present invention;

[0025] Figure 7 This is an SEM image of nanorod manganese iron oxide prepared in Example 2 of the present invention;

[0026] Figure 8 This is the XRD pattern of the lithium manganese iron phosphate positive electrode material prepared in Example 2 of the present invention;

[0027] Figure 9 This is an SEM image of the lithium manganese iron phosphate positive electrode material prepared in Example 2 of the present invention;

[0028] Figure 10 This is the XRD pattern of the manganese iron oxide prepared in Comparative Example 3 of the present invention;

[0029] Figure 11 This is a SEM image of the manganese iron oxide prepared in Comparative Example 3 of the present invention;

[0030] Figure 12 This is the XRD pattern of the lithium manganese iron phosphate positive electrode material prepared in Comparative Example 3 of the present invention;

[0031] Figure 13 This is an SEM image of the lithium manganese iron phosphate positive electrode material prepared in Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0032] In order to make the technical means, creative features, objectives and effects achieved by the present invention easier to understand, the present invention is further described below in conjunction with specific implementation methods.

[0033] In the description of the present invention, it should be noted that the terms "upper," "lower," "inner," "outer," "front end," "rear end," "both ends," "one end," "the other end," and the like, indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and therefore should not be construed as limiting the present invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0034] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "installed," "provided with," "connected," etc., should be understood in a broad sense. For example, "connected" may refer to a fixed connection, a detachable connection, or an integral connection; it may refer to a mechanical connection or an electrical connection; it may refer to a direct connection or an indirect connection through an intermediate medium; it may refer to internal communication between two components. Those skilled in the art will be able to understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0035] Example

[0036] Hereinafter, the manganese iron spinel oxide and its preparation method, and the method for preparing the lithium manganese iron phosphate positive electrode material according to the present invention will be described in detail with reference to the accompanying drawings and exemplary embodiments.

[0037] One aspect of the present invention provides a manganese iron spinel oxide. In some embodiments, the manganese iron spinel oxide is composed of larger nano primary particles, and the secondary particles are formed by agglomeration of the primary particles. The secondary particles have the advantages of uniform particle size and strong reactivity. The chemical formula of manganese iron oxide is (Mn 1-x Fe x )3O4, of which 0 <x<1。

[0038] In some embodiments, the following steps may be included:

[0039] A solution containing a catalyst is prepared.

[0040] After mixing ferromanganese powder slurry, catalyst solution, oxidant and deionized water, an in-situ electrochemical reaction occurs. The metal elements are replaced and participate in the precipitation oxidation reaction. The precipitate obtained after the reaction is filtered, washed and dried to obtain ferromanganese oxide. The orderly reaction process is conducive to nucleation and crystal nucleus growth, forming a unique grain morphology.

[0041] In some embodiments, the catalyst can be one or a mixture of ammonium chloride, sodium chloride, ammonium nitrate, ammonium sulfate, ammonium bromide, ammonium iodide, hydrochloric acid, and sulfuric acid.

[0042] Preferably, the initial particle size of the manganese and iron powders can be 30 to 100 μm. Within this particle size range, the synthesis reaction rate of the manganese iron oxide can be regulated, and the particle distribution and size can be uniform. If the initial particle size is too large, the reaction progress will be limited and the product phase will be impure.

[0043] Preferably, the pH of the reactant system (5 to 7) and the reaction rate can be adjusted by regulating the concentration of the catalyst to 5 to 25 g / L, thereby promoting efficient preparation.

[0044] Preferably, in some embodiments, the oxidant may be oxygen, and the flow rate of the oxidant may be 1 to 5 ml / min. Too low an oxidant flow rate may result in insufficient reaction, while too high an oxidant flow rate may lead to overoxidation, thereby generating an impurity phase.

[0045] In some embodiments, the reaction temperature of the replacement precipitation oxidation reaction can be 50°C to 90°C, and the reaction time can be 6h to 48h. The reasons for setting the above reaction temperature and reaction time are as follows: the temperature selection mainly considers the reaction efficiency of the preparation process and the crystallinity of the product. Below 50°C, the reaction is slow and the product crystallinity is low; above 90°C, the solvent evaporates quickly and the reaction is difficult to control; the reaction time mainly takes into account the production efficiency and crystal growth needs. Below 8h will cause incomplete reaction, and above 48h will cause unnecessary energy and time consumption.

[0046] Preferably, the washing process can be performed by water washing, using deionized water at 60°C to 90°C for 3 to 8 times. The drying temperature can be 50°C to 120°C, and the drying time can be 8 hours to 24 hours.

[0047] In some embodiments, the calcining atmosphere may be one or more mixed gases of argon, argon-hydrogen mixed gas, and nitrogen.

[0048] In some embodiments, the calcination may include maintaining the temperature at 300-500°C for 3-10 hours and then calcining at 500-700°C for 2-6 hours to obtain LiMn 1-y Fe y PO4 / C composite positive electrode material.

[0049] Example 1

[0050] (1) Prepare 7 g / L catalyst solution: dissolve ammonium chloride in deionized water.

[0051] (2) Weigh manganese powder and iron powder separately, with a molar ratio of manganese to iron of 1.5. Heat the catalyst solvent to 70°C, maintain a stirring rate of 400 rpm, and slowly add the manganese-iron mixed powder. Pass oxygen through the mixture and keep it warm for 10 hours. Allow the in-situ electrochemical reaction to proceed until the reaction is complete. Filter, wash, and dry the resulting precipitate to obtain manganese-iron oxide. The above reaction pathway may include the loss of electrons by Mn and Fe, the reduction of oxygen by electrons, and the generation of OH-; the formation of hydroxide precipitates by transition metal ions; and the further oxidation of the hydroxides to spinel oxides.

[0052] (3) According to the chemical formula LiMn 0.6 Fe 0.4 Lithium carbonate, the spinel oxide obtained in step (2) and ammonium dihydrogen phosphate were weighed in a molar ratio of PO4. 10 wt% of starch was added to the total weight of the above raw materials. Deionized water was added to control the solid content to 0.1-0.8, and the precursor of lithium manganese iron phosphate was obtained by grinding and spraying.

[0053] (4) The lithium iron manganese phosphate precursor obtained in step (3) was kept at 400° C. for 4 h in a nitrogen atmosphere, then calcined at 600° C. for 4 h, and cooled to room temperature to obtain a lithium iron manganese phosphate positive electrode material.

[0054] The manganese iron oxide prepared in Example 1 was subjected to X-ray diffraction (XRD) characterization analysis, and the results were as follows: Figure 1 As stated by Figure 1 It can be seen that the characteristic diffraction peaks of the manganese iron oxide prepared in Example 1 all correspond to standard manganese manganese tetroxide, and there are no diffraction peaks of iron and manganese elements. The metal reaction is relatively thorough, indicating that manganese iron spinel oxide has been successfully prepared through the in situ co-precipitation reaction, and this method has a high yield.

[0055] The manganese iron oxide prepared in Example 1 was observed by scanning electron microscope (SEM), and the results were as follows: Figure 2 As shown by Figure 2 It can be seen that the manganese iron oxide prepared in Example 1 has a regular hexagonal nanorod structure, with a length of 400 to 600 nm and a diameter of 80 nm.

[0056] The lithium manganese iron phosphate / carbon composite material prepared in Example 1 was subjected to X-ray diffraction (XRD) characterization analysis, and the results were as follows: Figure 3 As shown by Figure 3 It can be seen that the characteristic diffraction peaks of the prepared lithium manganese iron phosphate / carbon material all correspond to standard lithium manganese iron phosphate. The absence of impurity peaks and the sharp peak shape indicate that the material has high purity and crystallinity.

[0057] The lithium manganese iron phosphate / carbon composite material prepared in Example 1 was observed by scanning electron microscopy (SEM). Figure 4 As shown by Figure 4 It can be seen that the prepared lithium manganese iron phosphate / carbon composite material is composed of spherical particles with a diameter of 2 to 8 μm.

[0058] The prepared lithium manganese iron phosphate cathode material was subjected to electrochemical performance testing, and the charge and discharge capacity test results at a current density of 0.1C (1C = 150mAh / g) were as follows: Figure 5 As shown in the figure, it can be seen that the discharge capacity of the material at a rate of 0.1C is 155.9mA / g;

[0059] Example 2

[0060] (1) Prepare 7 g / L catalyst solution: dissolve ammonium chloride in deionized water.

[0061] (2) Weigh iron powder with a molar ratio of manganese to iron of 1.0, heat the catalyst solvent to 70°C, maintain a stirring rate of 400 rpm, and slowly add the manganese-iron mixed powder. Aerate with oxygen and keep warm for 10 hours. An in-situ electrochemical reaction occurs until the reaction is complete. The resulting precipitate is filtered, washed, and dried to obtain manganese-iron oxide. The above reaction pathway may include Mn and Fe losing electrons, oxidizing and dissolving, oxygen gaining electrons and being reduced to produce OH-; transition metal ions forming hydroxide precipitates; and the hydroxides being further oxidized to spinel oxides.

[0062] (3) According to the chemical formula LiMn 0.5 Fe 0.5 Lithium carbonate, the spinel oxide obtained in step (2) and ammonium dihydrogen phosphate were weighed in a molar ratio of PO4. 10 wt% of starch was added to the total weight of the above raw materials. Deionized water was added to control the solid content to 0.1-0.8, and the precursor of lithium manganese iron phosphate was obtained by grinding and spraying.

[0063] (4) The lithium iron manganese phosphate precursor obtained in step (3) was kept at 400° C. for 4 h in a nitrogen atmosphere, then calcined at 600° C. for 4 h, and cooled to room temperature to obtain a lithium iron manganese phosphate positive electrode material.

[0064] The prepared manganese iron oxide was subjected to XRD test, and the test results are as follows: Figure 6 As shown in the figure, it can be seen that the characteristic diffraction peaks all correspond to standard manganese manganese tetraoxide, and there are no diffraction peaks of iron and manganese elements. The synthesized material has good crystallinity.

[0065] The prepared manganese iron oxide was subjected to SEM test, and the test results are as follows Figure 7 As shown in the figure, it can be seen that the primary particle morphology of the synthesized material is nanorod-shaped and the size is nanometer-scale.

[0066] The lithium manganese iron phosphate / carbon composite material prepared in Example 2 was subjected to X-ray diffraction (XRD) characterization analysis, and the results were as follows: Figure 8 As shown by Figure 8 It can be seen that the characteristic diffraction peaks of the prepared lithium manganese iron phosphate / carbon material all correspond to standard lithium manganese iron phosphate. The absence of impurity peaks and the sharp peak shape indicate that the material has high purity and crystallinity.

[0067] The lithium manganese iron phosphate / carbon composite material prepared in Example 2 was observed by scanning electron microscopy (SEM). Figure 9 As shown by Figure 9 It can be seen that the prepared lithium manganese iron phosphate / carbon composite material is composed of spherical particles with a diameter of 2 to 8 μm.

[0068] Example 3

[0069] (1) Prepare a mixed salt solution with a total transition metal ion concentration of 2 mol / L and a molar ratio of manganese to iron of 1.5: dissolve 1 mol of manganese sulfate and 1 mol of ferrous sulfate in 1 L of deionized water.

[0070] (2) Prepare a solution with a catalyst concentration of 4 mol / L: dilute 538.46 g of concentrated ammonia in 462 ml of deionized water.

[0071] (3) Under the condition of introducing oxygen, the solutions in steps (1) and (2) are mixed at a rate of 5 ml / min and 2 ml / min, respectively, and the mixed solution is heated to 70° C., maintaining pH=7, and maintaining a stirring rate of 400 rpm to allow a precipitation-oxidation reaction to occur. The precipitate obtained after the reaction is filtered, washed, and dried to obtain manganese iron oxide. The above reaction pathway may include complex precipitation of manganese iron ions; the precipitate is further oxidized to spinel oxide.

[0072] (4) According to the chemical formula LiMn 0.6 Fe 0.4 Lithium carbonate, the spinel oxide obtained in step (3) and ammonium dihydrogen phosphate were weighed in a molar ratio of PO4. A carbon source accounting for 10 wt% of the total mass of the raw materials was added, and deionized water was added to control the solid content to 0.1-0.8. The precursor of lithium manganese iron phosphate was obtained by grinding and spraying.

[0073] (5) The lithium iron manganese phosphate precursor obtained in step (3) was kept at 400° C. for 4 h in a nitrogen atmosphere, then calcined at 600° C. for 4 h, and cooled to room temperature to obtain a lithium iron manganese phosphate positive electrode material.

[0074] The prepared manganese iron oxide was subjected to XRD test, and the test results are as follows: Figure 10 As shown in the figure, it can be seen that the synthesized material corresponds to the standard diffraction peak of manganese tetraoxide, has no impurity phase peak and has good crystallinity.

[0075] The prepared manganese iron oxide was subjected to SEM test, and the test results are as follows Figure 11 As shown in the figure, it can be seen that the synthesized material is composed of nano-scale primary spherical particles agglomerated, and the secondary particles are spherical in shape of about 10 μm.

[0076] The prepared lithium manganese iron phosphate cathode material was subjected to XRD test, and the test results are as follows: Figure 12 As shown in the figure, it can be seen that the synthesized material is pure phase lithium manganese iron phosphate, without impurity phase peaks and good crystallinity.

[0077] The prepared lithium manganese iron phosphate cathode material was subjected to SEM testing, and the test results are as follows: Figure 13 As shown in the figure, it can be seen that the lithium manganese iron phosphate particles are spherical particles with rough surfaces, and the secondary sphere size is micron-level.

[0078] Although the present invention has been described above with reference to the exemplary embodiments, it will be apparent to those skilled in the art that various modifications and variations may be made to the exemplary embodiments of the present invention without departing from the spirit and scope defined in the claims.

[0079] In order to better understand the present invention, the content of the present invention is further explained below with reference to specific examples, but the content of the present invention is not limited to the following examples.

[0080] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A manganese iron oxide, characterized in that The chemical formula of ferromanganese spinel oxide synthesized from manganese powder and iron powder is (Mn 1-x Fe x )3O4 Among them, When 0 < x < 1, manganese-iron oxide with secondary particle size of 5 - 10 μm composed of primary particles in the shape of nanorods with a length of 400 - 600 nm. The primary particles are hexagonal rod-shaped nanostructures, and the secondary particles are formed by the disordered combination of primary particles; The preparation steps are as follows: S1: Prepare a solution containing a catalyst and a mixed slurry of manganese and iron; S2: Mix the mixed slurry, the catalyst solution and the oxidant, and carry out a leaching dissolution - oxidation precipitation reaction by heating and stirring until the reaction ends; S3: Filter, wash and dry the reaction product to obtain manganese-iron oxide.

2. The method for preparing ferromanganese oxide according to claim 1, wherein In the mixed slurry in S1, the molar ratio of manganese to iron is 9 - 0.1, and the solid content is 0.1 - 0.

8.

3. The method for preparing ferromanganese oxide according to claim 1, wherein: In S2, the catalyst is one or a mixture of several of ammonium chloride, sodium chloride, ammonia water, ammonium nitrate, ammonium sulfate, ammonium bromide, ammonium iodide, hydrochloric acid, sulfuric acid, and the addition amount of the catalyst is 5 - 25 g / L.

4. The method for preparing ferromanganese oxide according to claim 1, wherein: In S2, the oxidant is one or a mixture of several of air, oxygen, ammonium nitrate, nitric acid, nitrous acid, potassium permanganate, ammonium persulfate, hydrogen peroxide.

5. The method for preparing ferromanganese oxide according to claim 1, wherein: When mixing the mixed slurry, the catalyst and the oxidant, the flow rate of the oxidant is 1 - 5 ml / min; The pH value of the reaction system is 5.0 - 8.0, the reaction temperature is 50 - 90 °C, and the reaction time is 6 - 48 h.

6. The method for preparing ferromanganese oxide according to claim 1, wherein: The iron source is one of iron powder or FeSO4·7H2O, FeCl2, FeCl3, and the manganese source is one of manganese powder or MnSO4·H2O, MnCl2.

7. A method for preparing lithium manganese iron phosphate / carbon, characterized in that: It includes the following steps: Mix the manganese-iron oxide, lithium source, phosphorus source, and carbon source as described in claim 1 and disperse them in a solvent. After sand grinding and spray drying, under an inert atmosphere, keep it at 400 - 500 °C for 4 - 6 h and then calcine at 500 - 800 °C for 4 - 8 h to obtain a lithium iron manganese phosphate / carbon composite material.

8. The method for preparing a lithium iron manganese phosphate / carbon composite material according to claim 7, characterized in that: The lithium source is one or a mixture of several of LiOH, Li2CO3, LiH2PO4, lithium oxalate; the phosphorus source is one or more of phosphoric acid, monoammonium phosphate, phytic acid, diammonium phosphate, triammonium phosphate; the carbon source is at least one or more of cyclodextrin, sucrose, starch, polyvinylpyrrolidone, citric acid.

9. The method for preparing a lithium iron manganese phosphate / carbon composite material according to claim 8, characterized in that: The molar ratio of Li, P, (Fe + Mn) is (1 - 1.04):(1 - 1.03):1, and the mass of the carbon source is 1 - 30% of the mass of the manganese-iron oxide.

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