Lithium manganese iron phosphate, and preparation method therefor and use thereof

Abstract

Disclosed are a lithium iron manganese phosphate, and a preparation method and the use thereof, belonging to the technical field of battery materials. Manganese ferrous phosphate is synthesized by using a coprecipitation method, and oxidized with chlorine gas. During the oxidation process, some ferrous ions are oxidized into trivalent iron, which reacts with chlorine gas to generate ferric chloride. With the volatilization of ferric chloride, the trivalent iron is removed, which forms channels available for lithium ion diffusion in manganese ferrous phosphate, thereby improving the diffusion rate of lithium ions, and forming a lithium iron manganese phosphate positive electrode material with a more stable structure and excellent performance.

Description

Lithium manganese iron phosphate and its preparation method and application Technical Field

[0001] This article relates to the field of battery material technology, and specifically to lithium manganese iron phosphate and its preparation method and application. Background Art

[0002] Power lithium-ion batteries offer high specific energy and power, excellent safety, and long cycle life, making them widely used in electric vehicles and various power tools. Among lithium-ion battery cathode materials, lithium manganese iron phosphate (LMFP) has the same specific capacity as lithium iron phosphate (theoretical capacity 170 mAh / g). LMFP shares the same crystal structure as lithium iron phosphate, and because it contains manganese ions, the battery's discharge voltage is higher, reaching 4.1V. This makes LMFP's energy density approximately 20% higher than that of lithium iron phosphate. Therefore, LMFP is currently attracting attention as one of the active cathode materials for power lithium-ion batteries.

[0003] The main methods for synthesizing lithium manganese iron phosphate cathode materials include solid-phase method, sol-gel method, and co-precipitation method. The co-precipitation lithium sintering method is a widely used method in industry. It mainly involves physically mixing the precursor, lithium salt, and additives at one time. The temperature is then raised at a specific rate. After reaching the target temperature, the temperature is maintained for a period of time, and then the temperature is naturally lowered to complete the sintering process. The single-stage sintering process is relatively simple, but the chemical reaction during sintering is not sufficient, making it difficult to control the lithium ion diffusion rate. The calcination time is also long, which is detrimental to the performance of the final synthetic material.

[0004] In view of this, this article is proposed.

[0005] Summary of the Invention

[0006] The purpose of this article is to overcome the shortcomings of the existing technology and provide a lithium manganese iron phosphate and its preparation method and application. The lithium manganese iron phosphate has a stable structure and excellent performance. The lithium-ion battery prepared from the lithium manganese iron phosphate has a stable structure, high capacity and excellent cycle performance.

[0007] To achieve the above objectives, the technical solutions adopted in this paper are:

[0008] A method for preparing lithium manganese iron phosphate comprises the following steps:

[0009] Dissolving a divalent manganese salt and a divalent iron salt in water, adding a phosphorus source to obtain a mixed solution, adjusting the pH to 5-8, co-precipitating, washing, filtering, and drying to obtain ferrous manganese phosphate;

[0010] The ferrous manganese phosphate is placed in a muffle furnace, chlorine gas is introduced to carry out oxidation reaction, washed, and dried to obtain a precursor;

[0011] The precursor, lithium source and carbon source are mixed evenly and calcined to obtain lithium manganese iron phosphate;

[0012] The flow rate of the chlorine gas is 1 to 3 L / min;

[0013] The temperature during the oxidation reaction is ≥320°C.

[0014] This paper synthesizes ferrous manganese phosphate by co-precipitation, and creatively uses chlorine to oxidize ferrous manganese phosphate. During the oxidation process, ferrous ions are partially oxidized into ferric iron, and the ferric iron ions react with chlorine to produce ferric chloride. During the oxidation process, the ferric iron is volatilized and removed along with the ferric chloride (ferric chloride volatilizes above 320°C), forming a channel for lithium ion diffusion in the ferrous manganese phosphate, thereby increasing the diffusion rate of lithium ions and reducing the calcination time. In the process of the reaction between lithium ions and the precursor, the lithium ions are distributed more evenly, and a lithium iron manganese phosphate positive electrode material with a more stable structure and excellent performance is formed.

[0015] This article can effectively control the lithium ion diffusion channel and iron content in lithium manganese iron phosphate by controlling the flow rate of chlorine gas, thereby effectively improving the structural stability, capacity and cycle performance of the lithium ion battery prepared by lithium manganese iron phosphate. If the chlorine gas flow rate is too small (less than 1L / min), the lithium ion diffusion channel formed will be reduced. If the chlorine gas flow rate is too large (greater than 3L / min), the iron content in the lithium manganese iron phosphate will be too low, which will also lead to a decrease in the performance of the lithium ion battery prepared by lithium manganese iron phosphate.

[0016] In one embodiment, the divalent manganese salt includes at least one of manganous sulfate, manganous nitrate, manganous chloride, and manganous acetate.

[0017] In one embodiment, the divalent iron salt includes at least one of ferrous sulfate, ferrous nitrate, and ferrous chloride.

[0018] In one embodiment, the phosphorus source includes at least one of phosphoric acid, sodium phosphate, sodium dihydrogen phosphate, and ammonium dihydrogen phosphate.

[0019] In one embodiment, the molar ratio of the divalent manganese salt, the divalent iron salt, and the phosphorus source is (0.3-0.5): (0.5-0.7): (1-1.2). The inventors of this article have found that the molar ratio of the divalent manganese salt, the divalent iron salt, and the phosphorus source has a certain influence on the effect. By controlling the molar ratio of the three within this range, the stability, capacity, and cycle performance of the secondary battery prepared by lithium iron manganese phosphate are further improved. Among them, if the content of the divalent iron salt is too low, the iron content in the ferrous manganese phosphate will be too low. After the subsequent addition of chlorine gas, the generated ferric chloride will volatilize, resulting in too little iron content in the lithium iron manganese phosphate, which will eventually lead to a decrease in the structural stability, capacity, and cycle performance of the lithium ion battery prepared by lithium iron manganese phosphate. If the iron content is too high, the content of manganese and phosphorus will be too low, which will also lead to a decrease in the structural stability, capacity, and cycle performance of the lithium ion battery prepared by lithium iron manganese phosphate. Therefore, in this article, it is necessary to control the molar ratio of the three.

[0020] In one embodiment, the coprecipitation temperature is 30-60° C. and the coprecipitation time is 2-4 hours.

[0021] In one embodiment, sodium hydroxide solution is used to adjust the pH to 5-8.

[0022] In one embodiment, the temperature of the oxidation reaction is 320-500°C, and the reaction time is 30-60 minutes. The temperature of the oxidation reaction has a significant impact on the effect. If the temperature is lower than 320°C, the ferric chloride cannot be volatilized, resulting in the inability to form lithium ion diffusion channels. If the temperature is too high, it will lead to high energy consumption. By controlling the temperature of the oxidation reaction at 320-500°C, not only can the ferric chloride be volatilized and sufficient lithium ion diffusion channels be formed, but the crystal water in the ferrous manganese phosphate can also be removed.

[0023] As for the reaction time, if the oxidation reaction time is too long (greater than 60 minutes), the iron content in the lithium manganese iron phosphate will be too low, and the performance of the lithium-ion battery prepared by the lithium manganese iron phosphate will be reduced. If the oxidation reaction time is too short (less than 30 minutes), the lithium ion diffusion channel will be reduced. Therefore, the oxidation reaction time needs to be controlled.

[0024] In one embodiment, the molar ratio of the precursor to the lithium source is 1:(1-1.25).

[0025] In one embodiment, the lithium source includes at least one of lithium carbonate, lithium oxalate, and lithium hydroxide.

[0026] In one embodiment, the mass ratio of the sum of the mass of the precursor and the lithium source to the mass of the carbon source is 1:(0.05-0.13).

[0027] In one embodiment, the carbon source includes at least one of sucrose, glucose, soluble starch, citric acid, phenolic resin, graphite, and carbon black.

[0028] In one embodiment, the calcination temperature is 600-800°C for 6-10 hours. Due to the special oxidation reaction in this invention, a lithium ion diffusion channel is formed. If the calcination temperature is too high, crystal defects are easily generated, which in turn affects the performance of lithium manganese iron phosphate. If the calcination temperature is too low, lithium ions cannot diffuse into the material.

[0029] This article also provides a lithium manganese iron phosphate, which is prepared using the above-mentioned preparation method.

[0030] This article also provides an application of lithium manganese iron phosphate in the preparation of secondary batteries.

[0031] Specifically, the secondary battery mentioned includes a positive electrode sheet, a negative electrode sheet and a separator.

[0032] In one embodiment, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material is the lithium manganese iron phosphate mentioned above.

[0033] The type of positive electrode current collector is not particularly limited and can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector comprises a metal material such as aluminum, stainless steel, nickel plating, titanium, tantalum, or a carbon material such as carbon cloth or carbon paper. In one embodiment, the positive electrode current collector is a metal material. In one embodiment, the positive electrode current collector is aluminum.

[0034] The form of the positive electrode current collector is not particularly limited. When the positive electrode current collector is a metal material, the positive electrode current collector may be in the form of metal foil, metal cylinder, metal strip coil, metal plate, metal foil, metal mesh, stamped metal, foamed metal, etc. When the positive electrode current collector is a carbon material, the positive electrode current collector may be in the form of, but not limited to, carbon plate, carbon film, carbon cylinder, etc.

[0035] In one embodiment, the positive electrode active material layer further includes a conductive agent, a binder, and a solvent.

[0036] In one embodiment, the secondary battery further includes a negative electrode plate, wherein the negative electrode plate includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material.

[0037] In one embodiment, there is no particular limitation on the negative electrode current collector as long as it can achieve the purpose of this document. For example, it can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, or a composite current collector.

[0038] In one embodiment, the negative electrode active material can be natural graphite, artificial graphite, mesophase microcarbon beads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel structure lithium titanate Li4Ti5O 12 , at least one of Li-Al alloy and metallic lithium.

[0039] In one embodiment, the negative electrode active material layer further includes a conductive agent, a binder, and a solvent.

[0040] In one embodiment, the type of the conductive agent mentioned herein is not limited, and any known conductive agent can be used.

[0041] In one embodiment, the conductive agent includes at least one of carbon materials such as natural graphite, artificial graphite, acetylene black, needle coke, carbon nanotubes, and graphene.

[0042] In one embodiment, the type of the binder mentioned herein is not limited, and any known positive electrode binder can be used.

[0043] In one embodiment, the binder includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, styrene-butadiene rubber, nitrile rubber, fluororubber, isoprene rubber, polybutadiene rubber, and ethylene-propylene rubber.

[0044] In one embodiment, the type of solvent is not limited, as long as it can dissolve or disperse the active material, the conductive agent, and the binder.

[0045] In one embodiment, the solvent includes any one of an aqueous solvent and an organic solvent. Aqueous solvents include, but are not limited to, water and mixed media of alcohol and water. Organic media include at least one of aliphatic hydrocarbon solvents, ketones such as benzene, toluene, xylene, methylnaphthoquinoline, pyridine, acetone, methyl ethyl ketone, and cyclohexanone; and methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, diethyl ether, propylene oxide, tetrahydrofuran, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, hexamethylphosphoramide, and dimethyl sulfoxide.

[0046] In the secondary battery mentioned herein, a separator is generally provided between the positive electrode and the negative electrode to prevent short circuits. The material and shape of the separator are not particularly limited, as long as they do not significantly impair the effects of the present invention.

[0047] In one embodiment, the separator comprises a porous sheet or non-woven fabric having excellent liquid retention. Materials of the resin or glass fiber separator include, but are not limited to, polyolefin, aromatic polyamide, polytetrafluoroethylene, polyethersulfone, and the like.

[0048] In one embodiment, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The above-mentioned materials for the diaphragm can be used alone or in any combination.

[0049] In one embodiment, the secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0050] In one embodiment, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0051] There is no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape.

[0052] The beneficial effects of this paper are as follows: (1) This paper adopts the co-precipitation method to synthesize ferrous manganese phosphate, and creatively uses chlorine to oxidize ferrous manganese phosphate. During the oxidation process, ferrous ions are partially oxidized to trivalent iron, and the trivalent iron ions react with chlorine to produce ferric chloride. During the oxidation process, the trivalent iron is volatilized and removed along with the ferric chloride (ferric chloride volatilizes above 320°C), forming a channel for lithium ion diffusion in the ferrous manganese phosphate, thereby increasing the diffusion rate of lithium ions and reducing the calcination time. In the process of the reaction between lithium ions and precursors, the lithium ions are distributed more evenly, and a lithium iron manganese phosphate positive electrode material with a more stable structure and excellent performance is formed; (2) This paper can effectively control the lithium ion diffusion channel and iron content in lithium iron manganese phosphate by controlling the flow rate of chlorine, thereby effectively improving the structural stability, capacity and cycle performance of the lithium-ion battery prepared by lithium iron manganese phosphate. DETAILED DESCRIPTION

[0053] In order to better illustrate the purpose, technical solutions and advantages of this article, this article will be further described below in conjunction with specific embodiments and comparative examples. The purpose is to understand the content of this article in detail, rather than to limit this article.

[0054] Unless otherwise specified, the components and raw materials used in the examples and comparative examples herein are all commercially available raw materials, and the components and raw materials used in each parallel experiment are all of the same kind.

[0055] Example 1

[0056] A method for preparing lithium manganese iron phosphate comprises the following steps:

[0057] Dissolve 0.5 M ferrous sulfate and 0.5 M manganese sulfate in water, and then add 1.1 M phosphoric acid solution to obtain a mixed solution;

[0058] Sodium hydroxide was added to the mixed solution to adjust the pH value of the system to 7, and the mixture was stirred and reacted at 45° C. in a nitrogen atmosphere for 3 hours to obtain ferrous manganese phosphate slurry. The ferrous manganese phosphate slurry was washed with deionized water, filtered, and dried at 100° C. for 4 hours to obtain ferrous manganese phosphate;

[0059] The ferrous manganese phosphate was placed in a muffle furnace and chlorine was introduced for oxidation reaction at a flow rate of 2 L / min. The temperature was kept at 400°C for 45 minutes. After the reaction was completed, the product was washed with deionized water and filtered to obtain ferrous manganese phosphate in which part of the divalent iron was oxidized to trivalent iron.

[0060] The manganese iron phosphate and lithium hydroxide obtained above were weighed and mixed evenly according to a molar ratio of 1:1.1, and 7wt% of sucrose was added (the mass ratio of the sum of the mass of manganese iron phosphate and lithium hydroxide to sucrose was 1:0.07). The mixture was placed in a muffle furnace under a nitrogen atmosphere and calcined at 700°C for 9h to obtain manganese iron phosphate.

[0061] Example 2

[0062] A method for preparing lithium manganese iron phosphate comprises the following steps:

[0063] Dissolve 0.3M ferrous sulfate and 0.7M manganese sulfate in water, and then add 1.1M phosphoric acid solution to obtain a mixed solution;

[0064] Sodium hydroxide was added to the mixed solution to adjust the pH value of the system to 8, and the mixture was stirred and reacted at 30° C. in a nitrogen atmosphere for 2 hours to obtain ferrous manganese phosphate slurry, and the ferrous manganese phosphate slurry was washed with deionized water, filtered, and dried at 100° C. for 4 hours to obtain ferrous manganese phosphate;

[0065] The ferrous manganese phosphate was placed in a muffle furnace, and chlorine gas was introduced at a flow rate of 1 L / min. The temperature was kept at 320°C for 60 minutes. After the reaction was completed, the product was washed with deionized water and filtered to obtain ferrous manganese phosphate in which part of the divalent iron was oxidized to trivalent iron.

[0066] The manganese iron phosphate and lithium hydroxide obtained above were weighed and mixed uniformly in a molar ratio of 1:1.25, and 13 wt% of sucrose was added (the mass ratio of the sum of the mass of manganese iron phosphate and lithium hydroxide to sucrose was 1:0.07). The mixture was placed in a muffle furnace under a nitrogen atmosphere and calcined at 600 ° C for 10 h to obtain manganese iron phosphate.

[0067] Example 3

[0068] A method for preparing lithium manganese iron phosphate comprises the following steps:

[0069] Dissolve 0.4 M ferrous sulfate and 0.6 M manganese sulfate in water, and add 1.2 M phosphoric acid solution to obtain a mixed solution;

[0070] Sodium hydroxide was added to the mixed solution to adjust the pH value of the system to 5, and the mixture was stirred and reacted at 60° C. in a nitrogen atmosphere for 4 hours to obtain ferrous manganese phosphate slurry. The phosphoric acid was washed with deionized water, filtered, and dried at 100° C. for 4 hours to obtain ferrous manganese phosphate.

[0071] The ferrous manganese phosphate was placed in a muffle furnace, and chlorine gas was introduced at a flow rate of 3 L / min. The temperature was kept at 500°C for 30 minutes. After the reaction was completed, the product was washed with deionized water and filtered to obtain ferrous manganese phosphate in which part of the divalent iron was oxidized to trivalent iron.

[0072] The manganese iron phosphate and lithium hydroxide obtained above were weighed and mixed evenly in a molar ratio of 1:1, and 5wt% of sucrose was added (the mass ratio of the sum of the mass of manganese iron phosphate and lithium hydroxide to sucrose was 1:0.05). The mixture was placed in a muffle furnace under a nitrogen atmosphere and calcined at 800°C for 7h to obtain manganese iron phosphate.

[0073] Comparative Example 1

[0074] A method for preparing lithium manganese iron phosphate comprises the following steps:

[0075] Dissolve 0.5 M ferrous sulfate and 0.5 M manganese sulfate in water, and then add 1.1 M phosphoric acid solution to obtain a mixed solution;

[0076] Sodium hydroxide was added to the mixed solution to adjust the pH value of the system to 7, and the mixture was stirred and reacted at 45° C. in a nitrogen atmosphere for 3 hours to obtain ferrous manganese phosphate slurry. The ferrous manganese phosphate slurry was washed with deionized water, filtered, and dried at 100° C. for 4 hours to obtain ferrous manganese phosphate;

[0077] The above-obtained ferrous manganese phosphate and lithium hydroxide were weighed and mixed uniformly in a molar ratio of 1:1.1, and 7 wt% of sucrose was added (the mass ratio of the sum of the mass of ferrous manganese phosphate and lithium hydroxide to sucrose was 1:0.07). The mixture was placed in a muffle furnace under a nitrogen atmosphere and calcined at 700°C for 9 h to obtain ferrous manganese phosphate.

[0078] Comparative Example 2

[0079] The difference between Comparative Example 2 and Example 1 is that the chlorine gas flow rate is 0.5 L / min, and the other conditions are the same.

[0080] Comparative Example 3

[0081] The difference between Comparative Example 3 and Example 1 is that the chlorine gas flow rate is 4 L / min, and the other conditions are the same.

[0082] Comparative Example 4

[0083] The difference between Comparative Example 4 and Example 1 is that the temperature of the oxidation reaction is 300° C. (the temperature of the insulation reaction is 300° C.), and the other conditions are the same.

[0084] Comparative Example 5

[0085] The difference between Comparative Example 5 and Example 1 is that the holding time (ie, the oxidation reaction time) is different, and everything else is the same.

[0086] The holding time of this comparative example is 20min.

[0087] Comparative Example 6

[0088] The difference between Comparative Example 6 and Example 1 is that the holding time (ie, the oxidation reaction time) is different, and everything else is the same.

[0089] The holding time of this comparative example is 80min.

[0090] Comparative Example 7

[0091] The difference between Comparative Example 7 and Example 1 is that the calcination temperature is different. The calcination temperature of this comparative example is 500° C., and the other conditions are the same.

[0092] Comparative Example 8

[0093] The difference between Comparative Example 8 and Example 1 is that the calcination temperature is different. The calcination temperature of this comparative example is 900° C., and the other conditions are the same.

[0094] Comparative Example 9

[0095] Comparative Example 9 differs from Example 1 in that the molar ratio of ferrous sulfate, manganese sulfate and phosphoric acid solution is 0.2:0.8:1.1, and the other contents are the same.

[0096] Performance Testing

[0097] The lithium manganese iron phosphate positive electrode material obtained in the Examples and Comparative Examples was mixed with acetylene black as a conductive agent and PVDF as a binder in a mass ratio of 8:1:1. A certain amount of the organic solvent NMP was added, stirred, and coated onto aluminum foil to form a positive electrode sheet. A metallic lithium sheet was used as the negative electrode; the separator was a Celgard 2400 polypropylene porous membrane; the electrolyte consisted of a solution of EC, DMC, and EMC in a mass ratio of 1:1:1, with LiPF6 as the solute at a concentration of 1.0 mol / L. 2023 button-type cells were assembled in a glove box. The battery was subjected to charge and discharge cycle performance testing with a charge and discharge current of 1C within a cutoff voltage range of 2.2 to 4.3 V. The electrochemical performance results are shown in Table 1.

[0098] Table 1

[0099] As can be seen from Table 1, lithium manganese iron phosphate with high capacity and excellent cycle performance was prepared in this paper.

[0100] By comparing Example 1 with Comparative Example 1, it can be seen that the present invention creatively adopts chlorine oxidation to significantly improve the capacity and cycle performance.

[0101] By comparing Example 1 with Comparative Examples 2 to 3, it can be seen that this paper effectively controls the lithium ion diffusion channel and iron content in lithium manganese iron phosphate by controlling the flow rate of chlorine, thereby effectively improving the structural stability, capacity and cycle performance of the lithium ion battery prepared by lithium manganese iron phosphate.

[0102] By comparing Example 1 with Comparative Example 4, it can be seen that the oxidation temperature needs to be strictly controlled to be greater than 320°C. If it is lower than 320°C, the ferric chloride cannot be volatilized, and thus the capacity and cycle performance cannot be improved.

[0103] By comparing Example 1 with Comparative Examples 5 to 6, it can be seen that the capacity and cycle performance are further improved by controlling the oxidation reaction time.

[0104] By comparing Example 1 with Comparative Examples 7 to 8, it can be seen that the capacity and cycle performance are further improved by controlling the sintering temperature.

[0105] By comparing Example 1 with Comparative Example 9, it can be seen that the capacity and cycle performance are significantly improved by controlling the molar ratio of the divalent manganese salt, the divalent iron salt, and the phosphorus source.

Claims

1. A method for preparing lithium manganese iron phosphate, characterized in that: The following steps are involved: Dissolving a divalent manganese salt and a divalent iron salt in water, adding a phosphorus source to obtain a mixed solution, adjusting the pH to 5 to 8, co-precipitating, washing, filtering, and drying to obtain ferrous manganese phosphate; The ferrous manganese phosphate is placed in a muffle furnace, chlorine gas is introduced to perform an oxidation reaction, and then washed and dried to obtain a precursor; The precursor, the lithium source and the carbon source are mixed evenly and calcined to obtain lithium iron manganese phosphate; The flow rate of the chlorine gas is 1 to 3 L / min; The temperature during the oxidation reaction is ≥320°C.

2. The method for preparing lithium iron manganese phosphate according to claim 1, characterized in that: The divalent manganese salt includes at least one of manganous sulfate, manganous nitrate, manganous chloride and manganous acetate.

3. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: The divalent iron salt includes at least one of ferrous sulfate, ferrous nitrate and ferrous chloride.

4. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: The phosphorus source includes at least one of phosphoric acid, sodium phosphate, sodium dihydrogen phosphate, and ammonium dihydrogen phosphate.

5. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: The molar ratio of the divalent manganese salt, the divalent iron salt and the phosphorus source is (0.3-0.5): (0.5-0.7): (1-1.2).

6. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: Sodium hydroxide solution was used to adjust the pH to 5-8.

7. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: The coprecipitation temperature is 30-60° C. and the coprecipitation time is 2-4 hours.

8. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: The temperature of the oxidation reaction is 320-500° C., and the reaction time is 30-60 minutes.

9. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: The molar ratio of the precursor to the lithium source is 1:(1-1.25).

10. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: The lithium source includes at least one of lithium carbonate, lithium oxalate and lithium hydroxide.

11. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: The mass ratio of the sum of the mass of the precursor and the lithium source to the mass of the carbon source is 1:(0.05-0.13).

12. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: The carbon source includes at least one of sucrose, glucose, soluble starch, citric acid, phenolic resin, graphite and carbon black.

13. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: The calcination temperature is 600-800° C. and the calcination time is 6-10 hours.

14. A lithium manganese iron phosphate, characterized in that: The invention is prepared by the preparation method according to any one of claims 1 to 13.

15. Use of the lithium manganese iron phosphate according to claim 14 in the preparation of secondary batteries.

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