Preparation method and application of lithium manganese iron phosphate nanorod positive electrode material

Abstract

The present invention discloses a preparation method and application of a lithium iron manganese phosphate nanorod positive electrode material. The particle size of the lithium iron manganese phosphate nanorod positive electrode material prepared by the present invention is controlled within the range of 100 to 200 nm, has an ideal particle size, can effectively promote the electrochemical kinetics of the lithium iron manganese phosphate material, improves the conductivity and accelerates ion diffusion. The present invention provides an application of the prepared lithium iron manganese phosphate nanorod positive electrode material as a lithium ion battery positive electrode material, which exhibits high electrochemical capacity and cycle stability.

Description

Technical Field

[0001] The invention relates to the field of lithium ion battery positive electrode materials, and in particular to a preparation method of a lithium iron manganese phosphate nanorod positive electrode material and an application thereof as a lithium ion battery electrode material. Technical Background

[0002] As an important component of lithium-ion batteries, the performance of positive electrode materials directly affects the performance of batteries. The large-scale use of LiFePO4 positive electrode materials has obvious advantages in safety and cycle performance, but the material has fatal disadvantages of low electronic conductivity and slow lithium ion diffusion, which limits its further development. Compared with LiFePO4, LiMnPO4 positive electrode materials have the same olivine structure and higher electrode potential (4.1Vvs.Li + / Li), with higher energy density, but poorer electronic conductivity, which makes it impossible to commercialize on a large scale. At the same time, LiMnPO4 positive electrode materials also have Mn 3+ The John-Teller effect leads to structural instability, further reducing electronic conductivity and delaying lithium ion diffusion. x Fe 1-x PO4) combines the advantages of LiFePO4 and LiMnPO4, has relatively high electronic conductivity and higher operating voltage, and shows a broader application prospect.

[0003] At present, technical means such as ion doping, carbon coating, material nano-materials and conductive composite materials are often used to improve the electrochemical performance of lithium manganese iron phosphate positive electrode materials. Nano-materials have fine particle sizes, can achieve more sufficient contact with the electrolyte, and have better wettability. At the same time, materials with nano-particle sizes can achieve shorter lithium ion transfer paths, thereby increasing the migration rate of lithium ions in the material and improving the electrochemical performance of the material. Therefore, the present invention uses a solvothermal method to prepare a lithium manganese iron phosphate nanorod positive electrode material with a particle size of about 200nm, showing good electrochemical capacity and cycle performance. Summary of the invention

[0004] The present invention provides a method for preparing a lithium iron manganese phosphate nanorod positive electrode material. The lithium iron manganese phosphate nanorod positive electrode material with a size of about 200 nm is prepared through a simple liquid phase synthesis route, using a mixed solvent and introducing a surfactant, and the application of the lithium iron manganese phosphate nanorod positive electrode material in a lithium ion battery electrode material.

[0005] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical scheme.

[0006] A method for preparing a lithium manganese iron phosphate nanorod positive electrode material is implemented according to the following steps:

[0007] (1) weighing ascorbic acid, surfactant and lithium dihydrogen phosphate and dissolving them in deionized water, stirring evenly to completely dissolve the raw materials, to obtain a mixed solution A, wherein the mass ratio of ascorbic acid: surfactant and lithium dihydrogen phosphate is 0.2:0.2:0.005-0.01, and the concentration of lithium dihydrogen phosphate is 0.8-1.5M;

[0008] (2) Weighing a divalent iron metal salt and a divalent manganese metal salt and dissolving them in deionized water, adding an organic solvent after they are completely dissolved, and stirring and mixing to obtain a mixed solution B, wherein the volume ratio of deionized water: organic solvent is 1:3-5, the molar ratio of manganese metal salt to iron metal salt is (7-3): (3-7), and the concentration of divalent iron ions is 100-130 mM;

[0009] (3) weighing a water-soluble lithium compound and dissolving it in deionized water, adding the same organic solvent as in step (2) after the water-soluble lithium compound is completely dissolved, and stirring to obtain a mixed solution C, wherein the volume ratio of deionized water to the organic solvent is 1:1-5, and the lithium ion concentration is 500-800 mM;

[0010] (4) under continuous stirring, dripping the mixed solution A into the mixed solution B, and after the mixed solution A is completely dripped, dripping the mixed solution C into the mixed solution B, controlling the dripping rate to be 0.6-2 mL / min, so that the molar ratio of (iron element + manganese element): phosphorus element in the final mixed system is 1:1, and the molar amount of lithium element contained in the water-soluble lithium compound is twice the molar amount of lithium dihydrogen phosphate;

[0011] (5) transferring the mixed solution obtained in step (4) into a Teflon liner, inserting the mixture into a stainless steel shell, and subjecting the mixture to high temperature treatment at 160 to 200° C. for 6 to 12 hours. After the reaction is completed, centrifuging and separating the precipitate;

[0012] (6) vacuum drying the precipitate collected in step (5) and collecting the material;

[0013] (7) The material obtained in step (6) is placed in a tubular furnace, Ar / H2 (H2 volume fraction 5%) mixed gas is introduced, the heating rate is 2-5°C / min, the temperature is controlled at 600-800°C and calcined for 2-4h, and after complete cooling, lithium manganese iron phosphate nanorod positive electrode material is obtained.

[0014] Preferably, the surfactant used in step (1) is polyvinyl pyrrolidone (PVP), SPAN-80, TWEEN-40, cetyltrimethylammonium bromide (CTAB) or sodium dodecylbenzene sulfonate (SDBS).

[0015] Preferably, in step (1), the mass ratio of ascorbic acid: surfactant and lithium dihydrogen phosphate is 0.2:0.2:0.0072, wherein the concentration of lithium dihydrogen phosphate is 1.44M.

[0016] Preferably, the divalent manganese metal salt and the divalent iron metal salt selected in step (2) are nitrates, chlorides, sulfates or acetates of divalent manganese and divalent iron, respectively.

[0017] Preferably, in step (2), the molar ratio of manganese metal salt to iron metal salt is 1:1, and the concentration of divalent iron ions is 120 mM.

[0018] Preferably, the organic solvent selected in step (2) is methanol, ethylene glycol, diethylene glycol or isopropanol.

[0019] Preferably, the water-soluble lithium compound is lithium hydroxide.

[0020] Preferably, in step (3), the volume ratio of deionized water to organic solvent is 1:4, and the lithium ion concentration is 576 mM.

[0021] Preferably, in step (4), the dripping rate is 1 mL / min.

[0022] Preferably, in step (5), the high temperature treatment temperature is 180° C. and the treatment time is 12 h.

[0023] Preferably, in step (6), the vacuum drying temperature is 60-100°C.

[0024] Preferably, in step (7), the calcination temperature is 600° C. and the calcination time is 4 h.

[0025] The present invention provides an application of a lithium iron manganese phosphate nanorod positive electrode material as a positive electrode material for a lithium ion battery.

[0026] Compared with the prior art, the present invention has the following characteristics and advantages:

[0027] (1) In the embodiment of the present invention, a lithium manganese iron phosphate nanorod positive electrode material with a special morphology is obtained by a simple and feasible solvothermal method.

[0028] (2) The particle size of the prepared lithium manganese iron phosphate nanorod positive electrode material is controlled within the range of 100 to 200 nm, which has an ideal particle size and can effectively promote the electrochemical kinetics of the lithium manganese iron phosphate material, improve the conductivity and accelerate ion diffusion.

[0029] (3) The raw materials of the preparation method of the present invention are simple and easy to obtain, the cost is low, the preparation time is controllable, the operation process is safe and reliable, no hazardous chemicals are involved, it is green and pollution-free, and large-scale batch production can be easily realized.

[0030] (4) The lithium manganese iron phosphate nanorod positive electrode material prepared by the present invention exhibits high electrochemical capacity and cycle stability as a positive electrode material for lithium ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The descriptions of the drawings herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention.

[0032] Figure 1 This is a scanning electron microscope (SEM) image of the lithium manganese iron phosphate nanorods prepared in Example 1.

[0033] Figure 2 This is a transmission electron microscope (TEM) image of the lithium manganese iron phosphate nanorods prepared in Example 1.

[0034] Figure 3 This is the XRD diagram of the lithium manganese iron phosphate nanorod positive electrode material prepared in Examples 1 and 2.

[0035] Figure 4 This is the charge and discharge curve of the lithium manganese iron phosphate nanorod positive electrode material prepared in Example 1.

[0036] Figure 5 These are the electrochemical cycle test diagrams of Examples 1, 2, and 3.

[0037] Figure 6 These are the electrochemical cycle test diagrams of Example 1 and Comparative Example 1.

[0038] Figure 7 Electrochemical cycle test diagram of Example 1, Comparative Examples 2 and 3 DETAILED DESCRIPTION

[0039] The technical solution of the present invention is described in detail below with reference to the accompanying drawings and embodiments, but is not limited thereto:

[0040] The CV test of the lithium manganese iron phosphate nanorod positive electrode material in the embodiment of the present invention is carried out on an electrochemical workstation, and the constant current charge and discharge test is carried out on a battery test system. The following substance concentrations correspond to the final mixed liquid volume in this step.

[0041] Embodiment 1:

[0042] (1) Weigh 0.2 g of ascorbic acid, 0.2 g of PVP and lithium dihydrogen phosphate and dissolve them in 5 mL of deionized water. Stir evenly to completely dissolve the raw materials to obtain a mixed solution A, in which the concentration of lithium dihydrogen phosphate is 1.44 M.

[0043] (2) Weigh iron (II) sulfate and manganese (II) sulfate and dissolve them in 5 mL of deionized water. After the raw materials are completely dispersed, 25 mL of ethylene glycol is added to obtain a mixed solution B, in which the concentrations of iron (II) sulfate and manganese (II) sulfate are both 120 mM.

[0044] (3) Weigh lithium hydroxide and dissolve it in 5 mL of deionized water. After it is completely dissolved, add 20 mL of ethylene glycol to obtain a mixed solution C in which the concentration of lithium hydroxide is 576 mM.

[0045] (4) under continuous stirring, the mixed solution A is dripped into the mixed solution B, and after the mixed solution A is completely dripped, the mixed solution C is dripped into the mixed solution B, and the dripping rate is controlled to be 1 mL / min. After the dripping is completed, the concentration of lithium dihydrogen phosphate in the reaction system is 0.12 M, the concentrations of iron (II) sulfate and manganese (II) sulfate are both 60 mM, and the concentration of lithium hydroxide is 0.24 M;

[0046] (5) The mixed solution obtained in step (4) was transferred to a Teflon liner, placed in a stainless steel shell, and subjected to high temperature treatment at 180° C. for 12 h. After the reaction was completed, the precipitate was separated by centrifugation.

[0047] (6) vacuum drying the precipitate collected in step (5) and collecting the material, the drying temperature being 80° C.;

[0048] (7) The material obtained in step (6) was calcined in a tube furnace, Ar / H2 (H2 volume fraction 5%) mixed gas was introduced, the temperature was controlled at 600°C, the heating rate was 5°C / min, the calcination time was 4 hours, and after complete cooling, a lithium iron manganese phosphate nanorod positive electrode material was obtained. The scanning electron microscope (SEM) image, transmission electron microscope (TEM) image, and XRD image of the lithium iron manganese phosphate nanorod positive electrode material are shown as follows: Figure 1 , Figure 2 , Figure 3 shown.

[0049] (8) Weigh 0.32g of the composite material prepared in the above embodiment, 0.04g of acetylene black as a conductive agent, and 0.04g of PVDF as a binder, place them in a mortar, grind them evenly, add 3mL of NMP solvent, stir them evenly, and apply them on the surface of 6cm×10cm aluminum foil. After vacuum drying at 120°C for 12h, take them out and assemble them into button batteries with metal lithium as the counter electrode. The button battery was tested on a lithium-ion battery charge and discharge system (LANDCT2001A). The button battery electrolyte composition is 1MLiPF6, EC:EMC:DMC=1:1:1(Vol), the test voltage range is 2.0-4.5V, and the cycle test is carried out at 1C current. Before the test, it is activated at 0.1C for 3 weeks. All batteries are tested at a constant temperature of 25°C.

[0050] Embodiment 2:

[0051] (1) Weigh 0.2 g of ascorbic acid, 0.2 g of PVP and lithium dihydrogen phosphate and dissolve them in 6 mL of deionized water. Stir evenly to completely dissolve the raw materials to obtain a mixed solution A, in which the concentration of lithium dihydrogen phosphate is 1.2 M.

[0052] (2) Weigh iron (II) sulfate and manganese (II) sulfate, dissolve them in 6 mL of deionized water, and add 25 mL of ethylene glycol after the raw materials are completely dispersed to obtain a mixed solution B, in which the concentrations of iron (II) sulfate and manganese (II) sulfate are both 116.1 mM.

[0053] (3) Weigh lithium hydroxide and dissolve it in 6 mL of deionized water. After it is completely dissolved, add 17 mL of ethylene glycol to obtain a mixed solution C in which the concentration of lithium hydroxide is 626 mM.

[0054] (4) under continuous stirring, the mixed solution A is dripped into the mixed solution B, and after the mixed solution A is completely dripped, the mixed solution C is dripped into the mixed solution B at a rate of 1 mL / min. After the dripping is completed, the concentration of lithium dihydrogen phosphate in the reaction system is 0.12 M, the concentrations of iron (II) sulfate and manganese (II) sulfate are both 60 mM, and the concentration of lithium hydroxide is 0.24 M;

[0055] (5) The mixed solution obtained in step (4) was transferred to a Teflon liner, placed in a stainless steel shell, and subjected to high temperature treatment at 180° C. for 12 h. After the reaction was completed, the precipitate was separated by centrifugation.

[0056] (6) vacuum drying the precipitate collected in step (5) and collecting the material, the drying temperature being 80° C.;

[0057] (7) The material obtained in step (6) was calcined in a tube furnace, and Ar / H2 (H2 volume fraction 5%) mixed gas was introduced, the temperature was controlled at 600°C, the heating rate was 5°C / min, and the calcination time was 4 hours. After complete cooling, a lithium iron manganese phosphate nanorod positive electrode material was obtained. The XRD pattern of the lithium iron manganese phosphate nanorod positive electrode material is shown in Figure 3 .

[0058] (8) The battery assembly method and electrochemical performance test method are the same as in Example 1. The results are as follows: Figure 5 shown.

[0059] Embodiment 3:

[0060] (1) Weigh 0.2 g of ascorbic acid, 0.2 g of PVP and lithium dihydrogen phosphate and dissolve them in 8 mL of deionized water. Stir evenly to completely dissolve the raw materials to obtain a mixed solution A, in which the concentration of lithium dihydrogen phosphate is 0.9 M.

[0061] (2) Weigh iron (II) sulfate and manganese (II) sulfate, dissolve them in 8 mL of deionized water, and add 25 mL of ethylene glycol after the raw materials are completely dispersed to obtain a mixed solution B, in which the concentrations of iron (II) sulfate and manganese (II) sulfate are both 109.1 mM.

[0062] (3) Weigh lithium hydroxide and dissolve it in 8 mL of deionized water. After it is completely dissolved, add 11 mL of ethylene glycol to obtain a mixed solution C in which the concentration of lithium hydroxide is 757.9 mM.

[0063] (4) under continuous stirring, the mixed solution A is dripped into the mixed solution B, and after the mixed solution A is completely dripped, the mixed solution C is dripped into the mixed solution B at a rate of 1 mL / min. After the dripping is completed, the concentration of lithium dihydrogen phosphate in the reaction system is 0.12 M, the concentrations of iron (II) sulfate and manganese (II) sulfate are both 60 mM, and the concentration of lithium hydroxide is 0.24 M;

[0064] (5) The mixed solution obtained in step (4) was transferred to a Teflon liner, placed in a stainless steel shell, and subjected to high temperature treatment at 180° C. for 12 h. After the reaction was completed, the precipitate was separated by centrifugation.

[0065] (6) vacuum drying the precipitate collected in step (5) and collecting the material, the drying temperature being 80° C.;

[0066] (7) The material obtained in step (6) was calcined in a tubular furnace, and Ar / H2 (H2 volume fraction 5%) mixed gas was introduced. The temperature was controlled at 600°C, the heating rate was 5°C / min, and the calcination time was 4 hours. After complete cooling, the lithium manganese iron phosphate nanorod positive electrode material was obtained.

[0067] (8) The battery assembly method and electrochemical performance test method are the same as in Example 1. The results are as follows: Figure 5 shown.

[0068] Comparative Example 1:

[0069] (1) Weigh 0.2 g of ascorbic acid, 0.2 g of PVP and lithium dihydrogen phosphate and dissolve them in 5 mL of deionized water. Stir evenly to completely dissolve the raw materials to obtain a mixed solution A, in which the concentration of lithium dihydrogen phosphate is 1.44 M.

[0070] (2) Weigh iron (II) sulfate and manganese (II) sulfate, dissolve them in 5 mL of deionized water, and add 25 mL of ethylene glycol after the raw materials are completely dispersed to obtain a mixed solution B, in which the concentrations of iron (II) sulfate and manganese (II) sulfate are both 120 mM.

[0071] (3) Weigh lithium hydroxide and dissolve it in 5 mL of deionized water. After it is completely dissolved, add 20 mL of ethylene glycol to obtain a mixed solution C in which the concentration of lithium hydroxide is 576 mM.

[0072] (4) under continuous stirring, the mixed solution C is dripped into the mixed solution B, and after the mixed solution C is completely dripped, the mixed solution A is dripped into the mixed solution B at a rate of 1 mL / min. After the dripping is completed, the concentration of lithium dihydrogen phosphate in the reaction system is 0.12 M, the concentrations of iron (II) sulfate and manganese (II) sulfate are both 60 mM, and the concentration of lithium hydroxide is 0.24 M;

[0073] (5) The mixed solution obtained in step (4) was transferred to a Teflon liner, placed in a stainless steel shell, and subjected to high temperature treatment at 180° C. for 12 h. After the reaction was completed, the precipitate was separated by centrifugation.

[0074] (6) vacuum drying the precipitate collected in step (5) and collecting the material, the drying temperature being 80° C.;

[0075] (7) The material obtained in step (6) was calcined in a tubular furnace, and Ar / H2 (H2 volume fraction 5%) mixed gas was introduced. The temperature was controlled at 600°C, the heating rate was 5°C / min, and the calcination time was 4 hours. After complete cooling, the lithium manganese iron phosphate nanorod positive electrode material was obtained.

[0076] (8) The battery assembly method and electrochemical performance test method are the same as in Example 1. The results are as follows: Figure 6 shown.

[0077] pass Figure 6 From the comparison between Example 1 and Comparative Example 1, it can be seen that adjusting the order of adding lithium dihydrogen phosphate and lithium hydroxide is of great significance to the electrochemical performance of the final product. If the lithium hydroxide solution is first added to the metal solution, the nucleation will be too fast, the product particles will be too large, and the electrochemical performance will be poor. If lithium dihydrogen phosphate is added first, the nucleation rate is moderate, and fine particles with uniform particle size are obtained, and the performance is improved.

[0078] Comparative Example 2:

[0079] (1) Weigh 0.2 g of ascorbic acid, 0.2 g of PVP and lithium dihydrogen phosphate and dissolve them in 5 mL of deionized water. Stir evenly to completely dissolve the raw materials to obtain a mixed solution A, in which the concentration of lithium dihydrogen phosphate is 1.44 M.

[0080] (2) Weigh iron (II) sulfate and manganese (II) sulfate, dissolve them in 5 mL of deionized water, and add 25 mL of ethylene glycol after the raw materials are completely dispersed to obtain a mixed solution B, in which the concentrations of iron (II) sulfate and manganese (II) sulfate are both 60 mM.

[0081] (3) Weigh lithium hydroxide and dissolve it in 5 mL of deionized water. After it is completely dissolved, add 20 mL of ethylene glycol to obtain a mixed solution C in which the concentration of lithium hydroxide is 576 mM.

[0082] (4) under continuous stirring, the mixed solution A is dripped into the mixed solution B, and after the mixed solution A is completely dripped, the mixed solution C is dripped into the mixed solution B at a rate of 2 mL / min. After the dripping is completed, the concentration of lithium dihydrogen phosphate in the reaction system is 0.12 M, the concentrations of iron (II) sulfate and manganese (II) sulfate are both 60 mM, and the concentration of lithium hydroxide is 0.24 M;

[0083] (5) transferring the mixed solution obtained in step (4) into a Teflon liner, inserting it into a stainless steel shell, and treating it at a high temperature of 180° C. for 12 h. After the reaction is completed, centrifuging and separating the precipitate;

[0084] (6) vacuum drying the precipitate collected in step (5) and collecting the material, the drying temperature being 80° C.;

[0085] (7) The material obtained in step (6) was calcined in a tubular furnace, and Ar / H2 (H2 volume fraction 5%) mixed gas was introduced. The temperature was controlled at 600°C, the heating rate was 5°C / min, and the calcination time was 4 hours. After complete cooling, the lithium manganese iron phosphate nanorod positive electrode material was obtained.

[0086] (8) The battery assembly method and electrochemical performance test method are the same as in Example 1. The results are as follows: Figure 7 shown.

[0087] Comparative Example 3:

[0088] (1) Weigh 0.2 g of ascorbic acid, 0.2 g of PVP and lithium dihydrogen phosphate and dissolve them in 5 mL of deionized water. Stir evenly to completely dissolve the raw materials to obtain a mixed solution A, in which the concentration of lithium dihydrogen phosphate is 1.44 M.

[0089] (2) Weigh iron (II) sulfate and manganese (II) sulfate, dissolve them in 5 mL of deionized water, and add 25 mL of ethylene glycol after the raw materials are completely dispersed to obtain a mixed solution B, in which the concentrations of iron (II) sulfate and manganese (II) sulfate are both 120 mM.

[0090] (3) Weigh lithium hydroxide and dissolve it in 5 mL of deionized water. After it is completely dissolved, add 20 mL of ethylene glycol to obtain a mixed solution C in which the concentration of lithium hydroxide is 576 mM.

[0091] (4) under continuous stirring, the mixed solution A is dripped into the mixed solution B, and after the mixed solution A is completely dripped, the mixed solution C is dripped into the mixed solution B at a rate of 3 mL / min. After the dripping is completed, the concentration of lithium dihydrogen phosphate in the reaction system is 0.12 M, the concentrations of iron (II) sulfate and manganese (II) sulfate are both 60 mM, and the concentration of lithium hydroxide is 0.24 M;

[0092] (5) transferring the mixed solution obtained in step (4) into a Teflon liner, inserting it into a stainless steel shell, and treating it at a high temperature of 180° C. for 12 h. After the reaction is completed, centrifuging and separating the precipitate;

[0093] (6) vacuum drying the precipitate collected in step (5) and collecting the material, the drying temperature being 80° C.;

[0094] (7) The material obtained in step (6) was calcined in a tubular furnace, and Ar / H2 (H2 volume fraction 5%) mixed gas was introduced. The temperature was controlled at 600°C, the heating rate was 5°C / min, and the calcination time was 4 hours. After complete cooling, the lithium manganese iron phosphate nanorod positive electrode material was obtained.

[0095] (8) The battery assembly method and electrochemical performance test method are the same as in Example 1. The results are as follows: Figure 7 shown.

[0096] pass Figure 7 From the comparison between Example 1 and Comparative Examples 2-3, it can be seen that the instillation of lithium dihydrogen phosphate and lithium hydroxide affects the electrochemical performance of the final product. The best cycle performance is achieved under the condition of instillation rate of 1 mL / min. Too fast instillation rate leads to too fast nucleation of the precursor solution, resulting in larger particles with uneven particle size, and incomplete particle growth in the subsequent hydrothermal process.

[0097] The present invention illustrates the detailed structural features of the present invention through the above-mentioned embodiments. The above-mentioned embodiments are only some preferred embodiments of the present invention, not all embodiments. The present invention is not limited to the above-mentioned detailed structural features. Any improvements, replacements, etc. made under the concepts and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate nanorod positive electrode material, characterized in that: The preparation method is implemented according to the following steps: (1) weighing ascorbic acid, surfactant and lithium dihydrogen phosphate and dissolving them in deionized water, stirring evenly to completely dissolve the raw materials, to obtain a mixed solution A, wherein the mass ratio of ascorbic acid: surfactant and lithium dihydrogen phosphate is 0.2:0.2:0.005-0.01, and the concentration of lithium dihydrogen phosphate is 0.8-1.5 M; (2) Weighing a divalent iron metal salt and a divalent manganese metal salt and dissolving them in deionized water, adding an organic solvent after they are completely dissolved, and stirring and mixing to obtain a mixed solution B, wherein the volume ratio of deionized water: organic solvent is 1:3-5, the molar ratio of manganese element to iron element is (7-3): (3-7), and the concentration of divalent iron ions is 100-130 mM; (3) weighing a water-soluble lithium compound and dissolving it in deionized water, adding the same organic solvent as in step (2) after the water-soluble lithium compound is completely dissolved, and stirring to obtain a mixed solution C, wherein the volume ratio of deionized water to the organic solvent is 1:1-5, and the lithium ion concentration is 500-800 mM; (4) under continuous stirring, dripping the mixed solution A into the mixed solution B, and then dripping the mixed solution C into the mixed solution B after the mixed solution A is completely dripped, and controlling the dripping rate to be 0.6-2 mL / min, so that the molar ratio of (iron element + manganese element): phosphorus element in the final mixed system is 1:1, and the molar amount of lithium element contained in the water-soluble lithium compound is twice the molar amount of lithium dihydrogen phosphate; (5) transferring the mixed solution obtained in step (4) into a Teflon liner, inserting the mixture into a stainless steel shell, and subjecting the mixture to high temperature treatment at 160 to 200° C. for 6 to 12 hours. After the reaction is completed, centrifuging and separating the precipitate; (6) vacuum drying the precipitate collected in step (5) and collecting the material; (7) The material obtained in step (6) is placed in a tubular furnace, Ar / H2 mixed gas is introduced, the heating rate is 2-5°C / min, the temperature is controlled to be 600-800°C and calcined for 2-4h, and after complete cooling, lithium manganese iron phosphate nanorod positive electrode material is obtained.

2. The preparation method according to claim 1, characterized in that: The surfactant selected in step (1) is polyvinyl pyrrolidone, SPAN-80, TWEEN-40, hexadecyltrimethylammonium bromide or sodium dodecylbenzene sulfonate.

3. The preparation method according to claim 2, characterized in that: In step (1), the mass ratio of ascorbic acid: surfactant and lithium dihydrogen phosphate is 0.2:0.2:0.0072, wherein the concentration of lithium dihydrogen phosphate is 1.44M.

4. The preparation method according to any one of claims 1 to 3, characterized in that: In step (2), the divalent manganese metal salt is a nitrate, chloride, sulfate or acetate of divalent manganese, and the divalent iron metal salt is a nitrate, chloride, sulfate or acetate of divalent iron; and the organic solvent is methanol, ethylene glycol, diethylene glycol or isopropanol.

5. The preparation method according to claim 4, characterized in that: In step (2), the molar ratio of manganese element to iron element is 1:1, and the concentration of divalent iron ions is 120 mM.

6. The preparation method according to any one of claims 1 to 3, characterized in that: The water-soluble lithium compound is lithium hydroxide.

7. The preparation method according to claim 6, characterized in that: In step (3), the volume ratio of deionized water to organic solvent is 1:4, and the lithium ion concentration is 576 mM.

8. The preparation method according to any one of claims 1 to 3, characterized in that: The dripping rate in step (4) is 1 mL / min.

9. The preparation method according to any one of claims 1 to 3, characterized in that: In step (5), the high temperature treatment temperature is 180° C. and the treatment time is 12 h. In step (6), the vacuum drying temperature is 60-100° C.; in step (7), the calcination temperature is 600° C. and the calcination time is 4 h.

10. Use of the lithium manganese iron phosphate nanorod positive electrode material prepared according to the preparation method of claim 1 as a positive electrode material for lithium ion batteries.

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