FR 3143209 A1 20240614 FR 2313933 A 20231211 The present application discloses a cathode material of lithium manganese iron phosphate and a preparation method therefor, and relates to the technical field of lithium-ion battery. The preparation method comprises: (1) weighing a lithium source, a manganese source, a phosphorus source, and an iron source according to a stoichiometric ratio of lithium, to manganese, to iron, and to phosphorus in LiMnxFe1-xPO4, wherein 0.1≤x≤0.8; (2) mixing the lithium source, the manganese source, the phosphorus source, and the iron source with a reductant, performing wet grinding, and then drying to obtain a powder, wherein the reductant comprises a reductant A which undergoes sublimation at 500-800 °C and has reducibility within this temperature range, and also comprises or does not comprise a carbon source; and (3) heating the powder to 400-500 °C in a protective atmosphere, maintaining it at this temperature for 0.5-2 h, and then heating to 500-800 °C and sintering for 5-12 h to obtain the cathode material of lithium manganese iron phosphate. The cathode material of lithium manganese iron phosphate according to the present application has a higher capacity per gram. CATHODE MATERIAL OF LITHIUM MANGANESE IRON PHOSPHATE AND PREPARATION METHOD THEREFOR The present application relates to the technical field of lithium-ion battery, and particularly relates to a cathode material of lithium manganese iron phosphate and a preparation method therefor. BACKGROUND Lithium iron phosphate, as a widely used cathode material for lithium batteries, has the advantages of low price and long cycle life. With its widespread application, its shortcomings are gradually exposed, such as low working voltage, low battery energy density, etc., which cannot meet people’s requirements for endurance of new energy vehicles. Therefore, how to further improve its energy density is the research focus of material modification for lithium iron phosphate. Lithium manganese phosphate is a material with ordered olivine structure, and its theoretical specific capacity is the same as that of lithium iron phosphate. However, its discharge plateau is higher than that of lithium iron phosphate, and has the potential advantage of high energy density and is characterized by high stability and safety. At present, there are many methods capable of being used for synthesizing cathode materials of lithium manganese iron phosphate, such as sol-gel method, liquid phase method, high-temperature solid phase method, etc. In most of the above-described methods, divalent iron is used as iron source, however, the divalent iron source is very unstable and easily oxidized, and it is easy to form a multivalent mixture in a preparation of precursor, affecting the stoichiometric ratio of the iron source. It is necessary to introduce protective gas and add a certain amount of reductant during the preparation of the precursor. In the preparation process disclosed in CN102738465B, carbon is directly used for reduction, avoiding the use of hydrogen gas, ammonia gas, or other mixed gases. Moreover, a carbon source is added during sintering, which further play a role of coating, allowing the preparation process and carbon coating to be completed in one step. However, directly using carbon as a reductant may easily lead to excessive carbon coating and affect the capacity per gram of the battery. SUMMARY The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the protection scope of the claims. The present application is to provide a cathode material of lithium manganese iron phosphate and a preparation method therefor to overcome the defects in the prior art. The cathode material of lithium manganese iron phosphate has good conductivity, and a battery prepared with the cathode material has a higher capacity per gram. To achieve the above-described objective, the present application adopts the technical solutions described below. A preparation method for the cathode material of lithium manganese iron phosphate, comprises the following steps: (1) weighing a lithium source, a manganese source, a phosphorus source, and an iron source according to a stoichiometric ratio of lithium, to manganese, to iron, and to phosphorus in LiMnxFe1-xPO4, wherein 0.1 ≤ x ≤ 0.8; (2) mixing the lithium source, the manganese source, the phosphorus source, and the iron source with a reductant, performing wet grinding, and then drying to obtain a powder material, wherein the reductant comprises a reductant A which undergoes sublimation at 500-800 °C and has reducibility within this temperature range, and also comprises or does not comprise a carbon source, and the iron in the iron source is trivalent iron; and (3) heating the powder material to 400-500 °C in a protective atmosphere, maintaining it at this temperature for 0.5-2 h, then heating to 500-800 °C, and sintering for 5-12 h to obtain the cathode material of lithium manganese iron phosphate. In the present application, a substance that undergoes sublimation at 500-800 °C and has reducibility within this temperature range is used as a reductant, thereby avoiding residual impurities on the surface of the cathode material caused by excessive dosage, which will affect its conductivity and reduces the capacity per gram of a battery. Trivalent iron is more stable and will not change the stoichiometric ratio of the product. Preferably, the reductant A is at least one of arsenic sulfide, molybdenum sulfide, and sulfur. Furthermore, the reductant A is arsenic sulfide. Arsenic sulfide can be slowly decomposed to produce reducing substances, ensuring the stability and uniformity of valence states of iron and manganese during the reaction process. Arsenic sulfide is converted to gaseous state after being heated, which can create a porous structure in the material, facilitating its contact with the electrolytic solution and improving its electrochemical performance. In addition, the doping of a small amount of arsenic can increase the electrical conductivity of the cathode material. Excess arsenic sulfide can be directly removed by controlling the temperature without any impurities remained. This method is simple and efficient, overcoming the problem of using carbon as a reductant in the prior art that tends to lead to low capacity per gram. Preferably, an amount of substance of the carbon source is a, an amount of substance of the reductant A is b, and an amount of substance of the iron source is c, (a+3b): c= 1: (1-1.5), and a: b= (0-0.3): (0.3-0.5). Further preferably, a: b= (0.1-0.3): (0.3-0.5). Choosing both components of carbon and arsenic sulfide as reductants can significantly improve the conductivity of the cathode material. Carbon and arsenic can form a multiple conductive network. The above-described limitation to the usage amount of the carbon and arsenic and their ratios to iron can avoid the problem of incomplete reduction, while also increase the capacity per gram of the cathode material. Preferably, the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium chloride, lithium nitrate, lithium dihydrogen phosphate, and dilithium hydrogen phosphate; the manganese source is at least one of manganese phosphate, manganese oxalate, manganese carbonate, manganese chloride, manganese sulfate, and manganese nitrate; the phosphorus source is at least one of phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium dihydrogen phosphate, and dilithium hydrogen phosphate; and the iron source is at least one of iron oxide, iron nitrate, iron chloride, iron iodide, iron bromide, iron sulfide, and iron hydroxide. Preferably, in step (2), a method for the wet grinding comprises: adding water, ethanol, or a mixture thereof to a mixture of the lithium source, the manganese source, the phosphorus source, the iron source, and the reductant to perform the wet grinding, and stopping the wet grinding when a particle size of powder is less than 5 μm; and the drying is performed under vacuum at 50-65 °C for 5-12 h. Preferably, in step (3), heating to 400-500 °C is performed at a rate of 5-10 °C/min, and heating to 500-800 °C is performed at a rate of 1-5 °C/min. Preferably, in step (3), after the sintering, a product is subjected to an airflow crushing to obtain the cathode material of lithium manganese iron phosphate with a particle size of D10 ≥ 0.4 μm, D50 of 0.5-0.8 μm, and D90 ≤ 1 μm. In addition, the present application further provides a cathode material of lithium manganese iron phosphate prepared by the above-described method. Compared with the prior art, the beneficial effects of the present application are as follows. In the present application, a substance that sublimates and has reducibility at 600-800 °C is used as a reductant, which can stabilize the valence states of iron and manganese during a preparation process of a cathode material of lithium manganese iron phosphate, and no other impurities are remained on the surface of the prepared cathode material; and a battery prepared with the cathode material has a higher capacity per gram. Other aspects can be understood upon reading and understanding the detailed description. The drawings are used to provide a further understanding of the technical solutions herein and form a part of the description and, together with the embodiments of the present application to explain the technical solutions herein and do not constitute a limitation on the technical solutions herein. is a graph showing the preparation process of the cathode material of lithium manganese iron phosphate of the present application. DETAILED DESCRIPTION OF EMBODIMENTS In order to better illustrate the objectives, technical solutions and advantages of the present application, the present application will be further described below with reference to specific embodiments and drawings. Example 1 An example according to the present application provided a cathode material of lithium manganese iron phosphate. The preparation process of the cathode material is shown in , and the preparation method is as follows: (1) 0.05 mol of battery-grade lithium carbonate, 0.02 mol of analytical pure-grade manganese carbonate, 0.04 mol of analytical pure-grade iron oxide, and 0.1 mol of battery-grade diammonium hydrogen phosphate were weighed according to the stoichiometric ratio of lithium, to manganese, to iron, and to phosphorus elements in LiMn0.2Fe0.8PO4; (2) the above-described components and 0.04 mol of arsenic sulfide were added with a mixture of deionised water and ethanol with a volume ratio of 1: 1 at room temperature for wet grinding, with a solid-liquid volume ratio of 1: 3, the wet grinding was stopped when the particle size of powder was (3) the precursor powder was heated up to 400 °C in a protective atmosphere at a rate of 5 °C/min, and maintained at this temperature for 1 h; then heated up to 800 °C at a rate of 5 °C/min, and sintered for 10 h to obtain a powder; and (4) the powder obtained in step (3) was subjected to airflow crushing, and the particle size was controlled at D10 ≥ 0.4 μm, D50 of 0.5-0.8 μm, D90 ≤ 1 μm to obtain the cathode material of lithium manganese iron phosphate. Example 2 An example according to the present application provided a cathode material of lithium manganese iron phosphate. The preparation method therefor differed from that of Example 1 only in that arsenic sulfide was replaced with molybdenum sulfide. Example 3 An example according to the present application provided a cathode material of lithium manganese iron phosphate. The preparation method therefor differed from that of Example 1 only in that arsenic sulfide was replaced with sulphur. Example 4 An example according to the present application provided a cathode material of lithium manganese iron phosphate. The preparation method therefor differed from that of Example 1 only in that in step (2), arsenic sulphide was replaced with a mixture of carbon and arsenic sulphide, and the molar ratio of carbon, to arsenic sulphide and to iron was 0.1: 0.3: 1. Example 5 An example according to the present application provided a cathode material of lithium manganese iron phosphate. The preparation method therefor differed from that of Example 1 only in that in step (2), arsenic sulphide was replaced with a mixture of carbon and arsenic sulphide, and the molar ratio of carbon, to arsenic sulphide and to iron was 1: 5: 16. Example 6 An example according to the present application provided a cathode material of lithium manganese iron phosphate. The preparation method therefor differs from that of Example 1 only in that in step (2), arsenic sulphide was replaced with a mixture of carbon and arsenic sulphide, and the molar ratio of carbon, to arsenic sulphide and to iron was 3: 3: 12. Example 7 An example according to the present application provided a cathode material of lithium manganese iron phosphate. The preparation method therefor differed from that of Example 1 only in that in step (2), arsenic sulphide was replaced with a mixture of carbon and arsenic sulphide, and the molar ratio of carbon, to arsenic sulphide and to iron was 0.4: 0.2: 1. Comparative Example 1 This comparative provided a cathode material of lithium manganese iron phosphate. The preparation method therefor differed from Example 1 only in that arsenic sulfide was replaced with citric acid. Comparative Example 2 This comparative provided a cathode material of lithium manganese iron phosphate. The preparation method therefor differed from Example 1 only in that arsenic sulfide was replaced with carbon. The Examples and Comparative Examples were tested for performance as follows. The test results are shown in Table 1. The cathode materials of lithium manganese iron phosphate were made into button batteries and charge/discharge cycle performance tests were performed. The charge/discharge voltage range was controlled between 2.5-4.5 V, the charge/discharge current was controlled at 0.1 C, and the first discharge capacity per gram was recorded. Items Capacity per gram (mAh/g) Example 1 160.2 Example 2 157.3 Example 3 157.5 Example 4 162.9 Example 5 162.8 Example 6 163.1 Example 7 153.2 Comparative Example 1 148.2 Comparative Example 2 149.8 As can be seen from Table 1, the capacities per gram of Examples 1-7 were 150 mAh/g or more, which indicates that the cathode material of lithium manganese iron phosphate according to the present application is conducive to improve the energy density of the battery, and is suitable for an application in the preparation for lithium-ion batteries. A carbon source was used as the reductant in Comparative Examples 1-2, and the capacity per gram was significantly lower than that of the present application. Comparing the test results of Examples 1-3, it can be found that the selection of arsenic sulfide as the reductant can improve the capacity per gram of the battery. Comparing the test results of Example 1 and 4-6, it can be found that the selection of two components of carbon and arsenic sulfide as the reductants can synergistically improve the capacity per gram of the battery. Finally, it should be noted that the above-described examples are only used to illustrate the technical solutions of the present application and not to limit the protection scope of the present application. Although the present application is described in detail with reference to the preferred embodiments, it should be understood by those of ordinary skill in the art that the technical solutions of the present application can be modified or equivalently substituted without departing from the concept and scope of the technical solutions of the present application. A preparation method for a cathode material of lithium manganese iron phosphate, comprising the following steps: (1) weighing a lithium source, a manganese source, a phosphorus source, and an iron source according to a stoichiometric ratio of lithium, to manganese, to iron, and to phosphorus in LiMnxFe1-xPO4, wherein 0.1 ≤ x ≤ 0.8; (2) mixing the lithium source, the manganese source, the phosphorus source, and the iron source with a reductant, performing wet grinding, and then drying to obtain a powder material, wherein the reductant comprises a reductant A which undergoes sublimation at 500-800 °C and has reducibility within this temperature range, and comprises or does not comprise a carbon source; and (3) heating the powder material to 400-500 °C in a protective atmosphere, maintaining it at this temperature for 0.5-2 h, then heating to 500-800 °C, and sintering for 5-12 h to obtain the cathode material of lithium manganese iron phosphate. The preparation method for the cathode material of lithium manganese iron phosphate according to claim 1, wherein the reductant A is at least one of arsenic sulfide, molybdenum sulfide, and sulfur. The preparation method for the cathode material of lithium manganese iron phosphate according to claim 2, wherein the reductant A is arsenic sulfide. The preparation method for the cathode material of lithium manganese iron phosphate according to claim 3, wherein an amount of substance of the carbon source is a, an amount of substance of the reductant A is b, and an amount of substance of the iron source is c, (a+3b): c= 1: (1-1.5); and a: b= (0-0.3): (0.3-0.5). The preparation method for the cathode material of lithium manganese iron phosphate according to claim 4, wherein a: b= (0.1-0.3): (0.3-0.5). The preparation method for the cathode material of lithium manganese iron phosphate according to claim 1, wherein the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium chloride, lithium nitrate, lithium dihydrogen phosphate, and dilithium hydrogen phosphate; the manganese source is at least one of manganese phosphate, manganese oxalate, manganese carbonate, manganese chloride, manganese sulfate, and manganese nitrate; the phosphorus source is at least one of phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium dihydrogen phosphate, and dilithium hydrogen phosphate; and the iron source is at least one of iron oxide, iron nitrate, iron chloride, iron iodide, iron bromide, iron sulfide, and iron hydroxide. The preparation method for the cathode material of lithium manganese iron phosphate according to claim 1, wherein, in step (2), a method for the wet grinding comprises: adding water, ethanol, or a mixture thereof to a mixture of the lithium source, the manganese source, the phosphorus source, the iron source, and the reductant to perform the wet grinding, and stopping the wet grinding when a particle size of powder is less than 5 μm; and the drying is performed under vacuum at 50-65 °C for 5-12 h. The preparation method for the cathode material of lithium manganese iron phosphate according to claim 1, wherein, in step (3), heating to 400-500 °C is performed at a rate of 5-10 °C/min, and heating to 500-800 °C is performed at a rate of 1-5 °C/min. The preparation method for the cathode material of lithium manganese iron phosphate according to claim 1, wherein, in step (3), after the sintering, a product is subjected to airflow crushing to obtain the cathode material of lithium manganese iron phosphate with a particle size of D10 ≥ 0.4 μm, D50 of 0.5-0.8 μm, and D90 ≤ 1 μm. A cathode material of lithium manganese iron phosphate prepared by the method for the cathode material of lithium manganese iron phosphate according to any one of claims 1-9.