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

Abstract

Disclosed in the present disclosure are a lithium manganese iron phosphate material, a preparation method therefor and the use thereof. In the present disclosure, the introduction of polyurethane polymer into the coating layer of lithium manganese iron phosphate can, on the one hand, reduce the contact between the material and the electrolyte, thereby reducing the occurrence of side reactions, which is beneficial for reducing the dissolution of manganese in the lithium manganese iron phosphate material, thus also being beneficial to prolonging the cycle stability and cycle life of the material; on the other hand, polyurethane has a certain deformability and toughness, which is beneficial for improving the structural stability of lithium manganese iron phosphate when applied to the coating layer.

Description

Lithium manganese iron phosphate material, preparation method and application

[0001] Cross-reference to Related Applications

[0002] The present disclosure claims priority to the Chinese patent application No. 202410835428.1, filed on June 26, 2024, entitled “Lithium manganese iron phosphate material, preparation method and application”, the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0003] The present disclosure relates to the technical field of positive electrode materials, in particular to a lithium manganese iron phosphate material, a preparation method and an application thereof. BACKGROUND

[0004] Lithium manganese iron phosphate (LiMn x Fe (1-x) PO4) has the same olivine structure as lithium iron phosphate (LiFePO4) and is expected to be widely used in power batteries. However, under high temperature conditions, Mn in the lithium manganese iron phosphate material is prone to leaching from the material, resulting in capacity decay and shortened cycle life during high-temperature cycling.

[0005] Existing methods for inhibiting manganese leaching in lithium manganese iron phosphate materials include improving the preparation method of lithium manganese iron phosphate material, changing the composition of electrolyte, and surface coating of lithium manganese iron phosphate, etc. However, these methods still have some problems. First, improving the preparation method of lithium manganese iron phosphate material or changing the composition of electrolyte may affect other performance of the battery, such as energy density and power density. Second, although surface coating can improve the compatibility of lithium manganese iron phosphate material and electrolyte, the existing coating materials such as aluminum oxide and titanium oxide have limited improvement effect and may increase the cost of the battery.

[0006] In summary, how to effectively improve the side reaction of lithium manganese iron phosphate material with electrolyte without changing the preparation method of lithium manganese iron phosphate material and the composition of electrolyte, and improve the cycle performance and safety performance of the battery, is an important challenge faced by current technology. SUMMARY

[0007] The present disclosure aims to provide a lithium manganese iron phosphate material, a preparation method and an application, and to improve the cycle stability and cycle life of the lithium manganese iron phosphate material.

[0008] The present disclosure is achieved in the following way:

[0009] In a first aspect, the present disclosure provides a lithium manganese iron phosphate material, comprising lithium manganese iron phosphate and a polyurethane protective layer arranged on the surface of the lithium manganese iron phosphate.

[0010] In an optional embodiment, the mass fraction of the polyurethane protective layer in the lithium manganese iron phosphate material is 0.008%-0.5%.

[0011] In a second aspect, the present disclosure provides a lithium manganese iron phosphate material, and the method comprises: mixing polyurethane, a solvent, and lithium manganese iron phosphate powder to obtain a mixed solution, and then removing the solvent in the mixed solution to obtain the lithium manganese iron phosphate material.

[0012] In an optional embodiment, the average particle size of the lithium manganese iron phosphate is 0.8 μm-1.2 μm.

[0013] And / or, the polyurethane protective layer is a nano polyurethane protective layer.

[0014] And / or, the lithium manganese iron phosphate powder is removed of surface free carbon before being mixed with the polyurethane solution.

[0015] In an optional embodiment, the mass fraction of the polyurethane in the mixed solution is 2%-8%.

[0016] And / or, the solvent is N-dimethylformamide.

[0017] And / or, removing the solvent in the mixed solution comprises: firstly, primary drying the mixed solution, and then vacuum drying at 100°C-120°C for 10h-12h to obtain the lithium manganese iron phosphate material.

[0018] And / or, the lithium manganese iron phosphate powder is removed of surface free carbon before being mixed with the polyurethane and the solvent.

[0019] In an optional embodiment, the method further comprises synthesis of the polyurethane:

[0020] Pre-polymerization, reacting polypropylene glycol and 2,4-toluene diisocyanate under inert atmosphere to obtain a polyurethane prepolymer;

[0021] Polymerization, mixing the polyurethane prepolymer and diethylene glycol to obtain a mixture, and reacting the mixture under vacuum condition to obtain the polyurethane.

[0022] In an optional embodiment, the mass ratio of the polypropylene glycol and 2,4-toluene diisocyanate is 1:(2-2.5).

[0023] And / or, the molecular weight of the polypropylene glycol is 1500-2500.

[0024] And / or, the temperature of the pre-polymerization reaction is 80°C-100°C, and the time is 2h-4h.

[0025] In an optional embodiment, the mass ratio of the diethylene glycol and the polypropylene glycol is 1:(0.9-1.1).

[0026] And / or, the temperature of the polymerization reaction is 100-120℃, and the time is 10-12h.

[0027] In a third aspect, the present disclosure provides a positive electrode sheet comprising the lithium manganese iron phosphate material of the preceding embodiments.

[0028] In a fourth aspect, the present disclosure provides a lithium ion battery comprising the positive electrode sheet of the preceding embodiments.

[0029] The present disclosure has the following beneficial effects:

[0030] In the present disclosure, the polyurethane polymer is introduced into the coating layer of the lithium manganese iron phosphate. On the one hand, it can reduce the contact between the material and the electrolyte, thereby reducing the occurrence of side reactions, which is conducive to reducing the dissolution of manganese in the lithium manganese iron phosphate material, thereby prolonging the cycle stability and cycle life of the material. On the other hand, polyurethane has certain deformability and toughness, which is conducive to improving the stability of the structure of the lithium manganese iron phosphate when applied to the coating layer. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following will briefly introduce the drawings needed in the embodiments. It should be understood that the following drawings only show some embodiments of the present disclosure, and therefore should not be considered as limiting the scope. For those skilled in the art, other related drawings can also be obtained without creative labor.

[0032] Figure 1 is the manganese dissolution of the positive electrode material prepared in some embodiments and comparative examples of the present disclosure;

[0033] Figure 2 is the cycle curve of the positive electrode material prepared in some embodiments and comparative examples of the present disclosure. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of the embodiments of the present disclosure more clear, the technical solutions in the embodiments of the present disclosure will be described clearly and completely. The specific conditions are not specified in the embodiments, which are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not specified by the manufacturer, which are conventional products that can be purchased on the market.

[0035] The embodiments of the present disclosure provide a lithium manganese iron phosphate material, which comprises lithium manganese iron phosphate and a polyurethane protective layer arranged on the surface of the lithium manganese iron phosphate. The mass fraction of the polyurethane protective layer in the lithium manganese iron phosphate material is 0.008%-0.5%.

[0036] In the embodiments of the present disclosure, the polyurethane polymer is introduced into the coating layer of the lithium manganese iron phosphate, which can reduce the contact between the material and the electrolyte, thereby reducing the occurrence of side reactions, and is conducive to reducing the dissolution of manganese in the lithium manganese iron phosphate material, thereby being conducive to prolonging the cycle stability and cycle life of the material. On the other hand, the polyurethane has certain deformability and toughness, and the application of the polyurethane in the coating layer is conducive to improving the stability of the lithium manganese iron phosphate structure.

[0037] In the embodiments of the present disclosure, the mass fraction of the polyurethane protective layer in the lithium manganese iron phosphate material is 0.008% to 0.5%, and specifically can be 0.01%, 0.03%, 0.05%, 0.1%, 0.01%, 0.2%, 0.3%, 0.4%, 0.5%, or any value between 0.01% and 0.5%. If the amount of polyurethane is too large, the effect of improving the cycle stability and cycle life of the lithium manganese iron phosphate material gradually decreases.

[0038] The embodiments of the present disclosure also provide a preparation method of a lithium manganese iron phosphate material, which comprises: mixing polyurethane, a solvent, and lithium manganese iron phosphate powder to obtain a mixed solution, and then removing the solvent in the mixed solution to obtain the lithium manganese iron phosphate material.

[0039] After the polyurethane is configured into a solution and then mixed with the lithium manganese iron phosphate powder, the polyurethane is more conducive to being uniformly formed into a film on the surface of the lithium manganese iron phosphate in the presence of the solvent.

[0040] In optional embodiments, the mass fraction of the polyurethane in the mixed solution is 2% to 8%, and specifically can be 2%, 3%, 4%, 5%, 6%, 7%, 8%, or any value between 2% and 8%. The concentration of the polyurethane solution is related to the viscosity. If the concentration of the polyurethane is too large, the polyurethane is not conducive to being uniformly formed into a film on the surface of the lithium manganese iron phosphate, and if the concentration of the polyurethane is too small, the energy consumption required for subsequent removal of the solvent is too large.

[0041] In optional embodiments, the average particle size of the lithium manganese iron phosphate is 0.8 μm to 1.2 μm.

[0042] In optional embodiments, the polyurethane protective layer is a nano-polyurethane protective layer. The thickness of the protective layer is thin, and the nano-polyurethane protective layer with a thickness of nanometers can improve the cycle performance while reducing the influence on the capacity and lithium ion migration.

[0043] In optional embodiments, the solvent is N-dimethylformamide.

[0044] In optional embodiments, removing the solvent in the mixed solution comprises: first performing preliminary drying on the mixed solution, and then vacuum drying at 100°C to 120°C for 10 h to 12 h to obtain the lithium manganese iron phosphate material.

[0045] The preliminary drying aims to remove most of the solvent, and in some embodiments, the preliminary drying is accompanied by stirring, which is beneficial to improving the uniformity of the polyurethane protective layer. The temperature of the preliminary drying can be any temperature that can evaporate the solvent, for example, 60-80°C. Vacuum drying is beneficial to improving the drying efficiency, and the vacuum environment can also avoid the influence of water and oxygen on the lithium manganese iron phosphate.

[0046] It should be noted that, since the lithium manganese iron phosphate is prone to react with water and oxygen, the introduction or contact of water and oxygen should be avoided as much as possible during the whole process. Therefore, in some embodiments, before the vacuum drying of the dried material, the vacuum oven is purged with nitrogen to avoid the influence of residual water or oxygen in the oven on the material.

[0047] In optional embodiments, the surface free carbon is removed before the lithium manganese iron phosphate powder is mixed with the polyurethane and the solvent. The presence of free carbon is beneficial to the occurrence of side reactions of the material, and thus affects the cycle performance of the material, so the free carbon is removed before coating.

[0048] In optional embodiments, the method further comprises the synthesis of the polyurethane:

[0049] Pre-polymerization, polypropylene glycol (PPG) and 2,4-toluene diisocyanate (2,4-TDI) are reacted under an inert atmosphere to obtain a polyurethane prepolymer;

[0050] Polymerization, the polyurethane prepolymer is mixed with diethylene glycol (DEG) to obtain a mixture, and the mixture is reacted under vacuum conditions to obtain a polyurethane.

[0051] In optional embodiments, the mass ratio of the polypropylene glycol and 2,4-toluene diisocyanate is 1:(2-2.5).

[0052] In optional embodiments, the molecular weight of the polypropylene glycol is 1500-2500.

[0053] In optional embodiments, the temperature of the pre-polymerization reaction is 80-100°C, and the time is 2-4h.

[0054] In optional embodiments, the mass ratio of the diethylene glycol and the polypropylene glycol is 1:(0.9-1.1).

[0055] In optional embodiments, the temperature of the polymerization reaction is 100-120°C, and the time is 10-12h.

[0056] During the synthesis of the polyurethane, the degree of polymerization of the polyurethane has a great influence on the cycle performance of the lithium manganese iron phosphate material. In the present disclosure, the polymerization time and temperature are appropriately adjusted to improve the degree of polymerization, so as to improve the cycle performance of the material.

[0057] The embodiment of the present disclosure further provides a positive electrode sheet comprising the lithium manganese iron phosphate material according to the foregoing embodiment.

[0058] The embodiment of the present disclosure further provides a lithium ion battery comprising the positive electrode sheet according to the foregoing embodiment.

[0059] The features and performances of the present disclosure are further described in detail below in combination with examples.

[0060] Example 1:

[0061] The present embodiment provides a preparation method of a lithium manganese iron phosphate material, specifically comprising the following steps:

[0062] 1. LiMn 0.6 Fe 0.4 PO4 particles are added to anhydrous ethanol solution and cleaned by ultrasonic wave for 2h to remove uncoated free carbon, and then baked in a vacuum drying oven at 120°C for 12h. The obtained powder is ground in a ball mill for 6h to obtain a lithium manganese iron phosphate material.

[0063] 2. PPG-2K (average molecular weight 2000) and 2,4-TDI are added to a reaction container, filled with nitrogen, sealed, heated, and magnetically stirred (80°C, 2h) to obtain a polyurethane prepolymer after the reaction is completed.

[0064] 3. Diethylene glycol is added to the polyurethane prepolymer obtained in step 2, and then uniformly stirred and poured into a mold. A polyurethane (PU) is obtained after the reaction (100°C, 10h) is completed, wherein the molar ratio of PPG-2K, 2,4-TDI and diethylene glycol is 1:2.2:1.

[0065] 4. 0.5mg of PU is dissolved in 25mL of N-dimethylformamide to obtain a homogeneous PU solution; then, 1000mg of LMFP powder in step 1 is dispersed into the PU solution to obtain LMFP-PU with a PU nanomolecular layer. The mixture is stirred at 20°C for 2 hours. Finally, the LMFP-PU powder is collected by evaporation and vacuum dried at 100°C for 10h to obtain the PU-coated LMFP material. Label it as LMFP-PU1.

[0066] Example 2:

[0067] The present embodiment provides a preparation method of a lithium manganese iron phosphate material, which is different from example 1 only in that the PU content in step 4 is different, specifically comprising the following steps:

[0068] 1. LiMn 0.6 Fe 0.4PO4 particles were added to anhydrous ethanol solution and cleaned with ultrasonic for 2 h, and the uncoated free carbon was removed. The obtained powder was baked in a vacuum oven at 100 °C for 12 h, and then ground in a ball mill for 6 h to obtain the lithium manganese iron phosphate material.

[0069] 2. PPG-2K and 2,4-TDI were added to a reaction vessel, filled with nitrogen, sealed, heated, and magnetically stirred (80 °C, 2 h) to obtain a polyurethane prepolymer.

[0070] 3. Diethylene glycol was added to the polyurethane prepolymer obtained in step 2, and then uniformly stirred and poured into a mold. After the reaction (100 °C, 10 h) was completed, a polyurethane (PU) was obtained, wherein the molar ratio of PPG-2K, 2,4-TDI, and diethylene glycol was 1:2.2:1.

[0071] 4. 1 mg of PU was dissolved in 25 mL of N-dimethylformamide to obtain a homogeneous PU solution. Then, 1000 mg of the LMFP powder in step 1 was dispersed into the PU solution to obtain LMFP-PU with a PU nanolayer. The mixture was stirred at 20 °C for 2 hours. Finally, the LMFP-PU powder was collected by evaporation and vacuum dried at 100 °C for 10 h to obtain the PU-coated LMFP material. It is marked as LMFP-PU2.

[0072] Example 3:

[0073] This example provides a method for preparing a lithium manganese iron phosphate material, which is different from example 1 only in that the PU content in step 4 is different. It specifically comprises the following steps:

[0074] 1. LiMn 0.6 Fe 0.4 PO4 particles were added to anhydrous ethanol solution and cleaned with ultrasonic for 2 h, and the uncoated free carbon was removed. The obtained powder was baked in a vacuum oven at 100 °C for 12 h, and then ground in a ball mill for 6 h to obtain the lithium manganese iron phosphate material.

[0075] 2. PPG-2K and 2,4-TDI were added to a reaction vessel, filled with nitrogen, sealed, heated, and magnetically stirred (80 °C, 2 h) to obtain a polyurethane prepolymer.

[0076] 3. Diethylene glycol was added to the polyurethane prepolymer obtained in step 2, and then uniformly stirred and poured into a mold. After the reaction (100 °C, 10 h) was completed, a polyurethane (PU) was obtained, wherein the molar ratio of PPG-2K, 2,4-TDI, and diethylene glycol was 1:2.2:1.

[0077] 4. Dissolve 2 mg of PU in 25 mL of N-dimethylformamide, and stir to obtain a homogeneous PU solution; then, disperse 1000 mg of LMFP powder obtained in step 1 into the PU solution to obtain LMFP-PU with a PU nanolayer. Stir the mixture at 20°C for 2 hours. Finally, collect the LMFP-PU powder by evaporation, and vacuum dry at 100°C for 10 h to obtain the PU-coated LMFP material. Label it as LMFP-PU3.

[0078] Example 4

[0079] This example provides a method for preparing a lithium manganese iron phosphate material, which differs from Example 1 only in that the degree of polymerization of PU is different, and specifically includes the following steps:

[0080] 1. Add LiMn 0.6 Fe 0.4 PO4 particles into an anhydrous ethanol solution, clean the uncoated free carbon by ultrasonic cleaning for 2 h, and bake in a vacuum oven at 100°C for 12 h. Grind the obtained powder in a ball mill for 6 h to obtain the lithium manganese iron phosphate material.

[0081] 2. Add PPG-2K (average molecular weight 2000) and 2,4-TDI into a reaction container, fill with nitrogen, seal, heat, and magnetically stir (60°C, 1 h) to obtain a polyurethane prepolymer after the reaction is completed.

[0082] 3. Add diethylene glycol into the polyurethane prepolymer obtained in step 2, and then uniformly stir and pour into a mold. After the reaction (80°C, 8 h) is completed, polyurethane (PU) is obtained, wherein the molar ratio of PPG-2K, 2,4-TDI, and diethylene glycol is 1:2.2:1.

[0083] 4. Dissolve 2 mg of PU in 25 mL of N-dimethylformamide, and stir to obtain a homogeneous PU solution; then, disperse 1000 mg of LMFP powder obtained in step 1 into the PU solution to obtain LMFP-PU with a PU nanolayer. Stir the mixture at 20°C for 2 hours. Finally, collect the LMFP-PU powder by evaporation, and vacuum dry at 100°C for 10 h to obtain the PU-coated LMFP material. Label it as LMFP-PU3.

[0084] Example 5

[0085] This example provides a method for preparing a lithium manganese iron phosphate material, which differs from Example 1 only in that the degree of polymerization of PU is different, and specifically includes the following steps:

[0086] 1. Add LiMn 0.6 Fe 0.4PO4particles were added to anhydrous ethanol solution and cleaned with ultrasonic wave for 2-4h, to remove the uncoated free carbon, and baked in a vacuum oven at 100°C for 12h, the obtained powder was ground in a ball mill for 6h to obtain the lithium manganese iron phosphate material.

[0087] 2. PPG-2K (average molecular weight 2000) and 2,4-TDI were added to a reaction vessel, filled with nitrogen, sealed, heated, and magnetically stirred (120°C, 1h), to obtain a polyurethane prepolymer after the reaction was completed;

[0088] 3. The polyurethane prepolymer obtained in step 2 was added with diethylene glycol, then uniformly stirred, poured into a mold, and reacted (150°C, 14h) to obtain a polyurethane (PU) after the reaction was completed, wherein the molar ratio of PPG-2K, 2,4-TDI, and diethylene glycol was 1:2.2:1.

[0089] 4. 0.5mg of PU was dissolved in 25mL of N-dimethylformamide to obtain a homogeneous PU solution; then, 1000mg of the LMFP powder in step 1 was dispersed into the PU solution to obtain LMFP-PU with a PU nanolayer. The mixture was stirred at 20°C for 2 hours. Finally, the LMFP-PU powder was collected by evaporation and vacuum dried at 100°C for 10h to obtain the PU-coated LMFP material. Label it as LMFP-PU5.

[0090] Comparative Example 1

[0091] This comparative example provides a method for preparing a lithium manganese iron phosphate material, which is different from Example 1 only in that PU is not coated, and specifically includes the following steps:

[0092] 1. LiMn 0.6 Fe 0.4 PO4particles were added to anhydrous ethanol solution and cleaned with ultrasonic wave for 2h to remove the uncoated free carbon;

[0093] 2. The solution obtained in step 1 was baked in a vacuum oven at 100°C for 12h, and the obtained powder was ground in a ball mill for 6-8h to obtain the lithium manganese iron phosphate material.

[0094] Comparative Example 2

[0095] This comparative example provides a method for preparing a lithium manganese iron phosphate material, which is different from Example 1 only in that PU is not coated, and PVA is coated, and specifically includes the following steps:

[0096] 1. LiMn 0.6 Fe 0.4 PO4particles were added to anhydrous ethanol solution and cleaned with ultrasonic wave for 2h to remove the uncoated free carbon;

[0097] 2. Dissolve 0.5 mg of polyvinyl alcohol in 25 mL of N-dimethylformamide, stir to get homogeneous PVA solution; then, take 1000 mg of LMFP powder in step 1 and disperse into the PVA solution to get LMFP-PVA with PVA nanolayer. Stir the mixture at 20 °C for 2 hours. Finally, collect the LMFP-PVA powder by evaporation, vacuum dry at 100 °C for 10 h, and finally get the PVA-coated LMFP material, marked as LMFP-PVA.

[0098] Manganese dissolution test and 45 degree discharge cycle test were carried out on the above materials

[0099] Manganese dissolution test was carried out on the materials obtained in the examples and comparative examples. The powders obtained in examples 1, 2, 3 and comparative example 1 were immersed in 0.008 mol / L dilute hydrochloric acid solution for 4 h, filtered with filter paper with a pore size of 1 μm, and the filtered solution was taken to a 25 °C water bath for 2 h. The Mn dissolution amount was tested by ICP, and the test results are shown in Figure 1 and Table 1.

[0100] The materials obtained in the examples and comparative examples were assembled into batteries, and the first discharge capacity and cycle performance were tested. The test results are shown in Figure 2 and Table 1.

[0101] Battery assembly: positive shell - positive plate - electrolyte - separator - electrolyte - lithium sheet - gasket - negative shell.

[0102] The positive electrode coating surface density was 8 mg / cm2, and the electrolyte solvent was ethylene carbonate: methyl ethyl carbonate = 1:1.

[0103] Cycle performance test: at 45 °C, constant current charge to 4.35 V at 0.5 C current, constant voltage charge to 0.05 C at 4.35 V, stand for 10 minutes, discharge to 2.0 V at 0.5 C current, stand for 10 minutes.

[0104] Cycle test was carried out according to the above steps.

[0105] Table 1

[0106] According to Table 1, the comparison of examples and comparative examples shows that the PU-coated LMFP has an improved effect on the Mn dissolution of LMFP material in dilute acid solution.

[0107] According to Table 1, the comparison of examples and comparative examples shows that the PU-coated LMFP with a certain thickness can reduce the manganese dissolution of lithium manganese iron phosphate at 45 degrees and improve the cycle performance. If the thickness is too thick, it can affect the capacity of the material. Comparative example 2 is another polymer-coated LMFP material, and its performance is significantly worse than that of PU-coated material.

[0108] The above merely describes preferred embodiments of the present disclosure and is not intended to limit the present disclosure. The present disclosure can have various modifications and changes for those skilled in the art. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present disclosure shall be included in the protection scope of the present disclosure. Industrial applicability

[0109] In the present disclosure, the polyurethane polymer is introduced into the coating layer of the lithium manganese iron phosphate, which can reduce the contact between the material and the electrolyte, thereby reducing the occurrence of side reactions, and is beneficial to reducing the dissolution of manganese in the lithium manganese iron phosphate material, thereby being beneficial to prolonging the cycle stability and cycle life of the material; on the other hand, the polyurethane has certain deformability and toughness, which is applied to the coating layer and is beneficial to improving the stability of the lithium manganese iron phosphate structure, and has a wide application prospect.

Claims

1. A lithium iron manganese phosphate material, characterized in that, The lithium manganese iron phosphate material comprises lithium manganese iron phosphate and a polyurethane protective layer arranged on the surface of the lithium manganese iron phosphate. 2.The lithium iron manganese phosphate material of claim 1, characterized in that, The mass fraction of the polyurethane protective layer in the lithium manganese iron phosphate material is 0.008wt%-0.5%. 3.The lithium iron manganese phosphate material of claim 1 or 2, characterized in that, The average particle size of the lithium manganese iron phosphate is 0.8μm-1.2μm.

4. The lithium iron manganese phosphate material according to any one of claims 1 to 3, characterized in that The polyurethane protective layer is a nano polyurethane protective layer.

5. A method of producing the lithium manganese iron phosphate material according to any one of claims 1 to 4, characterized in that, The method comprises: The polyurethane, a solvent and lithium manganese iron phosphate powder are mixed to obtain a mixed solution, and then the solvent in the mixed solution is removed to obtain the lithium manganese iron phosphate material.

6. The method of claim 5, wherein the lithium iron manganese phosphate material is prepared by the steps of: mixing a lithium source, a manganese source, and an iron source; and heating the mixture to a temperature of 600 to 800 °C for 1 to 10 hours. The mass fraction of the polyurethane in the mixed solution is 2%-8%; And / or, the solvent is N-dimethylformamide; And / or, removing the solvent in the mixed solution comprises: firstly, primary drying of the mixed solution, and then vacuum drying at 100℃-120℃ for 10h-12h to obtain the lithium manganese iron phosphate material.

7. The method of producing a lithium iron manganese phosphate material according to claim 5 or 6, characterized in that, The surface free carbon of the lithium manganese iron phosphate powder is removed before mixing with the polyurethane and the solvent.

8. The method of producing a lithium iron manganese phosphate material according to any one of claims 5 to 7, characterized in that, The method further comprises synthesis of the polyurethane: Pre-polymerization, polypropylene glycol and 2,4-toluene diisocyanate are reacted under inert atmosphere to obtain a polyurethane prepolymer; Polymerization, the polyurethane prepolymer and diethylene glycol are mixed to obtain a mixture, and the mixture is reacted under vacuum condition to obtain polyurethane. 9.The method of claim 8, wherein the lithium iron manganese phosphate material is prepared by a process comprising: mixing a lithium source, a manganese source, and an iron source; and heating the mixture at a temperature of 600-800 ℃ for 6-24 hours in an atmosphere of nitrogen or argon. The mass ratio of the polypropylene glycol and 2,4-toluene diisocyanate is 1:(2-2.5).

10. The method of producing a lithium iron manganese phosphate material according to claim 8 or 9, characterized in that, The molecular weight of the polypropylene glycol is 1500-2500.

11. The method of producing a lithium iron manganese phosphate material according to any one of claims 8 to 10, characterized in that, The pre-polymerization reaction is carried out at a temperature of 80℃-100℃ for 2h-4h.

12. The method of producing a lithium iron manganese phosphate material according to any one of claims 8 to 11, characterized in that, The mass ratio of the diethylene glycol and the polypropylene glycol is 1:(0.9-1.1).

13. The method of producing a lithium iron manganese phosphate material according to any one of claims 8 to 12, characterized in that, The polymerization reaction is carried out at a temperature of 100℃-120℃ for 10h-12h.

14. A positive electrode sheet characterized by comprising: The lithium manganese iron phosphate material of any one of claims 1-4.

15. A lithium-ion battery, characterized by, The positive electrode sheet of claim 14.

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