Modified lithium ferric manganese phosphate material, preparation method thereof, positive electrode and lithium ion battery

Abstract

The invention provides a modified lithium ferric manganese phosphate material and a preparation method thereof, a positive electrode and a lithium ion battery, the modified lithium ferric manganese phosphate material comprises iron-based triphenylphosphine boride and a lithium ferric manganese phosphate material, and the iron-based triphenylphosphine boride is located on the surface of the lithium ferric manganese phosphate material, the chemical formula of the iron-based triphenylphosphine boride is FeCl2-x (BH4) x (PPh3) 2, wherein the value range of x is more than 0 and less than 2. The provided modified lithium ferric manganese phosphate material adopts the iron-based triphenylphosphine boride to modify the lithium ferric manganese phosphate material, so that the electrochemical stability and the electrical performance of the positive electrode material can be effectively improved, and the cycle performance and the electrochemical performance of the battery are further improved.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium batteries, and in particular relates to the modification of lithium iron manganese phosphate positive electrode materials. Background Art

[0002] As an important upgraded version of lithium iron phosphate battery, lithium manganese iron phosphate battery has always been highly expected by the market due to its advantages of high voltage, high safety and low cost. Lithium manganese iron phosphate has a higher voltage platform than lithium iron phosphate, and its theoretical energy density is 20% higher than that of lithium iron phosphate, which can break through the energy density bottleneck faced by lithium iron phosphate to a certain extent. Compared with ternary materials, lithium manganese iron phosphate batteries are also safer.

[0003] The introduction of manganese can improve the energy density of lithium iron phosphate batteries, but after the introduction of manganese, the lithium ion diffusion rate and electronic conductivity of the material will decrease. In addition, due to its dual voltage platform characteristics, lithium manganese iron phosphate batteries have challenges in management and control, which may affect the stability and reliability of the battery. Summary of the invention

[0004] In view of the above technical problems, the purpose of the present invention is to provide a modified lithium iron manganese phosphate material and a preparation method thereof, a positive electrode, and a lithium ion battery.

[0005] To achieve the above object, the present invention proposes the following solution: In a first aspect, a modified lithium iron manganese phosphate material is provided, wherein the modified lithium iron manganese phosphate material comprises iron-based triphenylphosphine boride and a lithium iron manganese phosphate material, wherein the iron-based triphenylphosphine boride is located on the surface of the lithium iron manganese phosphate material, and the chemical formula of the iron-based triphenylphosphine boride is FeCl 2-x (BH 4 ) x (PPh 3 ) 2 , where the value range of x is 0<x<2.

[0006] The present invention adopts iron-based triphenylphosphine boride to modify the iron manganese phosphate material. The iron-based triphenylphosphine boride exhibits excellent redox activity and can adsorb irreversible decomposition products in the organic electrolyte to the greatest extent, so that the main material itself is not corroded by the electrolyte, thereby effectively improving the structural stability of the positive electrode material.

[0007] Furthermore, the mass ratio of the iron-based triphenylphosphine boride to the lithium iron manganese phosphate is 0.01-0.05:1.

[0008] Furthermore, the chemical formula of the lithium iron manganese phosphate is LiMn y Fe 1-y PO 4 / C, the value range of y is 0.1≤y≤0.6.

[0009] In a second aspect, a method for preparing a modified lithium iron manganese phosphate material is provided, comprising: Dissolving ferrous chloride in an organic solvent, adding an organic phosphorus ligand, and performing a first reaction to obtain an intermediate product; Add borohydride to the intermediate product, drop an organic amine solvent into it, and carry out a second reaction. After the reaction is completed, separate the solid from the liquid and dry it to obtain an iron-based triphenylphosphine boride material. The modified lithium iron manganese phosphate material is obtained by mixing and ball-milling the lithium iron manganese phosphate and the iron-based triphenylphosphine boride material and sintering at a low temperature.

[0010] Furthermore, the organophosphorus ligand is triphenylphosphine; the borohydride is at least one of sodium borohydride and lithium borohydride; the organic amine solvent is one or more of triethanolamine, ethanolamine, and ethylenediamine; and the organic solvent is at least one of ethanol and tetrahydrofuran.

[0011] Furthermore, the molar ratio of the ferrous chloride to the organic phosphine ligand is 1:2-3.

[0012] Furthermore, the molar ratio of the ferrous chloride to the borohydride is 1:1-2.

[0013] Furthermore, the molar ratio of the borohydride to the organic amine solvent is 1-2:0.01-0.05.

[0014] Furthermore, the temperature of the first reaction is 30-50° C.; the duration of the first reaction is 6-10 hours; and the first reaction is carried out under a protective atmosphere.

[0015] Furthermore, the temperature of the second reaction is 40-60° C.; the duration of the second reaction is 10-20 hours; and the second reaction is carried out under a protective atmosphere.

[0016] Furthermore, the mixed ball milling is carried out under a protective atmosphere.

[0017] Furthermore, the temperature of the low-temperature sintering is 200-300° C.; the duration of the low-temperature sintering is 2-6 hours; and the atmosphere of the low-temperature sintering is nitrogen or argon atmosphere.

[0018] In a third aspect, a positive electrode is provided, comprising the aforementioned modified lithium iron manganese phosphate material.

[0019] In a fourth aspect, a lithium-ion battery is provided, comprising the aforementioned positive electrode.

[0020] Compared with the prior art, the present invention has the following beneficial effects: The modified lithium iron manganese phosphate material provided adopts iron-based triphenylphosphine boride to modify the lithium iron manganese phosphate material, which can effectively improve the electrochemical stability and electrical properties of the positive electrode material, thereby improving the cycle performance and electrochemical performance of the battery.

[0021] The preparation method of the modified lithium iron manganese phosphate material provided has a simple process and is easy to operate. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0023] Figure 1 This is the HRTEM image of the iron-based triphenylphosphine boride material prepared in step (2) of Example 1.

[0024] Figure 2 This is the EDS Mapping diagram of the iron-based triphenylphosphine boride material prepared in step (2) of Example 1, wherein (a), (b), (c), (d), (e), and (f) are the EDS Mapping diagrams of Fe, Cl, C, P, O, and B elements, respectively.

[0025] Figure 3 This is the TGA chart of the iron-based triphenylphosphine boride material prepared in step (2) of Example 1.

[0026] Figure 4 The cycle performance diagram of the battery assembled with the positive electrode materials prepared in each embodiment and comparative example 1.

[0027] Figure 5 TEM images of the positive electrode material prepared in Example 1 before (a) and after (b) cycling.

[0028] Figure 6 This is the SEM image of the positive electrode material prepared in Example 1 after cycling.

[0029] Figure 7 TEM images of the positive electrode material prepared in Comparative Example 1 before (a) and after (b) cycling.

[0030] Figure 8 This is the SEM image of the positive electrode material prepared in Comparative Example 1 after cycling. DETAILED DESCRIPTION

[0031] The applicant has found that the use of iron-based triphenylphosphine boride FeCl 2-x (BH 4 )x (PPh 3 ) 2 Modification of the lithium iron manganese phosphate material can significantly improve the electrochemical stability and electrical properties of the lithium iron manganese phosphate material, thereby improving the cycle performance and electrochemical performance of the battery. Based on this, the present invention is completed.

[0032] Some embodiments provide a modified lithium iron manganese phosphate material, wherein the modified lithium iron manganese phosphate material comprises iron-based triphenylphosphine boride and lithium iron manganese phosphate material, wherein the iron-based triphenylphosphine boride is located on the surface of the lithium iron manganese phosphate material, and the chemical formula of the iron-based triphenylphosphine boride is FeCl 2-x (BH 4 ) x (PPh 3 ) 2 , where the value range of x is 0<x<2.

[0033] After analysis, it was found that iron-based triphenylphosphine boride can significantly improve the electrochemical stability and electrical properties of lithium iron manganese phosphate materials. This may be because iron-based triphenylphosphine boride can effectively combine with organic groups in lithium-based organic electrolytes through molecular bonds and can achieve preferential adsorption, so that the organic groups in the electrolyte are adsorbed in the structure of iron-based triphenylphosphine boride, thereby enabling the lithium ions in the electrolyte to be rapidly released and embedded; and iron-based triphenylphosphine boride can effectively adsorb the byproducts of irreversible analysis of the electrolyte in the later stage of the electrochemical reaction, such as PF 5 The organic and inorganic fluoride by-products such as HF react with iron-based triphenylphosphine boride to form PF and BF molecular bonds, which ultimately effectively improves the irreversible corrosion of the electrolyte to the main material and exhibits excellent structural stability.

[0034] In some preferred embodiments, the mass ratio of the iron-based triphenylphosphine boride to the lithium iron manganese phosphate is 0.01-0.05:1, for example, 0.01:1, 0.02:1, 0.03:1, 0.04:1, 0.05:1, etc.

[0035] In some preferred embodiments, the chemical formula of the lithium iron manganese phosphate is LiMn y Fe 1-y PO 4 / C, the value range of y is 0.1≤y≤0.6.

[0036] It should be noted that the carbon content in the lithium iron manganese phosphate material can be the conventional carbon content in the art.

[0037] Some embodiments provide a method for preparing a modified lithium iron manganese phosphate material, comprising: Dissolving ferrous chloride in an organic solvent, adding an organic phosphorus ligand, and performing a first reaction to obtain an intermediate product; Add borohydride to the intermediate product, drop an organic amine solvent into it, and carry out a second reaction. After the reaction is completed, separate the solid from the liquid and dry it to obtain an iron-based triphenylphosphine boride material. The modified lithium iron manganese phosphate material is obtained by mixing and ball-milling the lithium iron manganese phosphate and the iron-based triphenylphosphine boride material and sintering at a low temperature.

[0038] In some preferred embodiments, the organophosphorus ligand is triphenylphosphine.

[0039] In some preferred embodiments, the borohydride is at least one of sodium borohydride and lithium borohydride.

[0040] In some preferred embodiments, the organic amine solvent is one or more of triethanolamine, ethanolamine, and ethylenediamine.

[0041] In some preferred embodiments, the organic solvent is at least one of ethanol and tetrahydrofuran.

[0042] In some preferred embodiments, the molar ratio of ferrous chloride to the organophosphine ligand is 1:2-3, for example, 1:2, 1:2.1, 1:2.2, 1:2.3, 1:2.4, 1:2.5, 1:2.6, 1:2.7, 1:2.8, 1:2.9, 1:3, etc.

[0043] In some preferred embodiments, the molar ratio of ferrous chloride to borohydride is 1:1-2, for example, 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2, etc.

[0044] In some preferred embodiments, the molar ratio of the borohydride to the organic amine solvent is 1-2:0.01-0.05, for example, 20:1, 40:1, 60:1, 80:1, 100:1, 120:1, 140:1, 160:1, 180:1, 200:1, etc.

[0045] In some preferred embodiments, the temperature of the first reaction is 30-50°C, for example, 30°C, 35°C, 40°C, 45°C, 50°C, etc.; the duration of the first reaction is 6-10h, for example, 6h, 7h, 8h, 9h, 10h, etc.; the first reaction is carried out under a protective atmosphere, for example, a nitrogen atmosphere, an inert atmosphere (such as an argon atmosphere, etc.).

[0046] In some preferred embodiments, the temperature of the second reaction is 40-60°C, for example, 40°C, 45°C, 50°C, 55°C, 60°C, etc.; the duration of the second reaction is 10-20h, for example, 10h, 12h, 14h, 16h, 18h, 20h, etc.; the second reaction is carried out under a protective atmosphere, for example, a nitrogen atmosphere, an inert atmosphere (such as an argon atmosphere, etc.).

[0047] In some preferred embodiments, the mixed ball milling is carried out under a protective atmosphere, such as a nitrogen atmosphere, an inert atmosphere (such as an argon atmosphere, etc.).

[0048] In some preferred embodiments, the low-temperature sintering temperature is 200-300°C, for example, 200°C, 220°C, 250°C, 280°C, 300°C, etc.

[0049] In some preferred embodiments, the low-temperature sintering time is 2 to 4 hours, for example, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, etc.

[0050] In some preferred embodiments, the low temperature sintering atmosphere is a nitrogen or argon atmosphere.

[0051] The lithium iron manganese phosphate material can be prepared by conventional preparation methods.

[0052] Some embodiments provide a positive electrode comprising the aforementioned modified lithium iron manganese phosphate material.

[0053] Some embodiments provide a lithium-ion battery comprising the aforementioned positive electrode.

[0054] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with the accompanying drawings and preferred embodiments of the present invention, but the protection scope of the present invention is not limited to the following specific embodiments.

[0055] Unless otherwise defined, all the professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0056] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0057] Example 1 (1) Disperse 0.2 mol manganese sulfate and 0.2 mol ferrous sulfate in 400 ml deionized water to form a metal salt solution. 4 ) 2 HPO 4Dissolve in 200 ml of deionized water to form a phosphate solution. Add the phosphate solution to the metal salt solution and stir continuously. After fully reacting for 16 hours, filter, wash and dry to obtain an iron manganese phosphate precursor. The iron manganese phosphate precursor material is ball-milled with 0.21 mol of lithium carbonate and 8 mmol of graphene, and then sintered at 900 ° C for 22 hours in a nitrogen atmosphere to prepare an iron manganese phosphate lithium positive electrode material.

[0058] (2) 0.1 molFeCl 2 Dissolve in 800 ml of ethanol, add 0.25 mol of triphenylphosphine, and react at 40 ° C for 8 hours to obtain FeCl 2 (PPh 3 ) 2 intermediate product; further add 0.15mol sodium borohydride to the above solution, drop 3mmol ethylenediamine to promote the dissolution of the boride, continue the reaction, and react for 16h under heating conditions at 45°C. After the reaction is completed, a multifunctional iron-based triphenylphosphine boride material can be prepared. The above reactions are all carried out under a nitrogen atmosphere.

[0059] The HRTEM image of the prepared iron-based triphenylphosphine boride complex material is shown in Figure 1 As shown in the figure, it can be seen that the lattice fringes of the iron-based triphenylphosphine boride prepared in step (2) are single-phase phases, proving that the prepared iron-based triphenylphosphine boride is a pure phase, not a composite. Figure 2 As shown in the figure, it can be seen that the elements in the iron-based triphenylphosphine boride phase are evenly distributed, proving that the iron-based triphenylphosphine boride has been successfully synthesized. In combination with its elements Fe, Cl, B, P and C (elements such as H cannot be measured), it can be determined that the product chemical formula is FeCl 2-x (BH 4 ) x (PPh 3 ) 2 .

[0060] TGA test parameters: the test atmosphere is nitrogen atmosphere, the test temperature range is from room temperature to 600°C, the heating rate is 1°C / min, and the sample weight is 1g, and the sample is ground into powder.

[0061] The TGA diagram of the prepared complex material is shown in Figure 3 As shown, from Figure 3 It can be seen that the complex material does not undergo obvious weight change below 350°C, indicating that the complex is stable below 350°C in an inert atmosphere.

[0062] (3) In a glove box filled with nitrogen, 10 g of the lithium iron manganese phosphate prepared in step (1) and 0.3 g of the iron-based triphenylphosphine boride material prepared in step (2) were ground and solid-phase mixed, and the mixed material was sealed and transferred into a tube furnace for sintering at 240° C. for 3 h to prepare an iron-based triphenylphosphine boride composite lithium iron manganese phosphate material.

[0063] Comparative Example 1 Disperse 0.2 mol manganese sulfate and 0.2 mol ferrous sulfate in 400 ml deionized water to form a metal salt solution. 4 ) 2 HPO 4 Dissolve in 200 ml of deionized water to form a phosphate solution. Add the phosphate solution to the metal salt solution and stir continuously. After fully reacting for 16 hours, filter, wash and dry to obtain a lithium iron manganese phosphate precursor. The iron manganese phosphate precursor material is ball-milled with 0.21 mol of lithium carbonate and 8.4 mmol of graphene, and then sintered at 900 ° C for 22 hours in a nitrogen atmosphere to prepare a lithium iron manganese phosphate positive electrode material.

[0064] Example 2 (1) Disperse 0.24 mol manganese sulfate and 0.16 mol ferrous sulfate in 400 ml deionized water to form a metal salt solution. 4 ) 2 HPO 4 Dissolve in 200 ml of deionized water to form a phosphate solution. Add the phosphate solution to the metal salt solution and stir continuously. After fully reacting for 10 hours, filter, wash and dry to obtain a lithium iron manganese phosphate precursor. The iron manganese phosphate precursor material is ball-milled with 0.204 mol of lithium carbonate and 4 mmol of layered graphite, and then sintered at 800°C for 30 hours in a nitrogen atmosphere to prepare a lithium iron manganese phosphate positive electrode material.

[0065] (2) 0.1 molFeCl 2 Dissolve in 800 ml of ethanol, add 0.2 mol of triphenylphosphine, and react at 30 ° C for 10 h to obtain FeCl 2 (PPh 3 ) 2 intermediate product; further add 0.1 mol of lithium borohydride to the above solution, drop 1 mmol of triethanolamine to promote the dissolution of the boride, continue the reaction, and react under heating conditions at 40°C for 20 hours. After the reaction is completed, a multifunctional iron-based triphenylphosphine boride material can be prepared. The above reactions are all carried out under a nitrogen atmosphere.

[0066] (3) In a glove box filled with nitrogen, 10 g of the lithium iron manganese phosphate prepared in step (1) and 0.1 g of the iron-based triphenylphosphine boride material prepared in step (2) were ground and solid-phase mixed, and the mixed material was sealed and transferred into a tube furnace for sintering at 200° C. for 4 h to prepare an iron-based triphenylphosphine boride composite lithium iron manganese phosphate material.

[0067] Example 3 (1) Disperse 0.04 mol manganese nitrate and 0.36 mol ferrous nitrate in 400 ml of deionized water to form a metal salt solution. 4 ) 2 HPO 4 Dissolve in 200 ml of deionized water to form a phosphate solution. Add the phosphate solution to the metal salt solution and stir continuously. After fully reacting for 24 hours, filter, wash and dry to obtain a lithium iron manganese phosphate precursor. The iron manganese phosphate precursor material is ball-milled with 0.21 mol of lithium carbonate and 0.012 mol of flake graphite, and then sintered at 900°C for 22 hours in a nitrogen atmosphere to prepare a lithium iron manganese phosphate positive electrode material.

[0068] (2) 0.1 molFeCl 2 Dissolve in 800 ml of ethanol, add 0.3 mol of triphenylphosphine, and react at 50 ° C for 6 hours to obtain FeCl 2 (PPh 3 ) 2 intermediate product; further add 0.2 mol sodium borohydride to the above solution, drop 5 mmol ethanolamine to promote the dissolution of the boride, continue the reaction, and react under heating conditions at 60°C for 10 hours. After the reaction is completed, a multifunctional iron-based triphenylphosphine boride material can be prepared. The above reactions are all carried out in a reaction chamber with a nitrogen atmosphere.

[0069] (3) In a glove box filled with nitrogen, 10 g of the lithium iron manganese phosphate prepared in step (1) and 0.5 g of the iron-based triphenylphosphine boride material prepared in step (2) were ground and solid-phase mixed, and the mixed material was sealed and transferred into a tube furnace for sintering at 300° C. for 2 h to prepare an iron-based triphenylphosphine boride composite lithium iron manganese phosphate material.

[0070] The materials obtained in Examples 1 to 3 and Comparative Example 1 were assembled into batteries by the following method: The materials prepared in Examples 1 to 3 and Comparative Example 1 were used as positive electrode materials, respectively, and mixed with conductive agent acetylene black (AB) and binder polyvinylidene fluoride (PVDF) at a mass ratio of 8:1:1, and N-methylpyrrolidone (NMP) was used as a solvent. The mixture was placed in a small beaker and stirred at a speed of 800r / min for 2h to obtain a slurry. The slurry was coated on the current collector aluminum foil using an automatic coating machine, placed flat on tempered glass and transferred to a vacuum drying oven at 85°C for 4h, punched into a pole piece with a diameter of 12mm, and then dried at 105°C in a vacuum drying oven for 4h, placed in a glove box filled with argon atmosphere with a water content and oxygen content of less than 0.1ppm for 4h to reduce the moisture adsorbed by the pole piece during the transfer process, and then assembled into a CR2032 button battery in the glove box. The battery uses a pure metal lithium sheet with a diameter of 16 mm and a thickness of 0.5 mm as the negative electrode, and a porous polyethylene membrane of model Celgard2300 with a diameter of 18 mm as the separator.

[0071] After the battery was assembled, it was aged for 12 h, then activated for 3 cycles at a voltage of 2~4.3 V and a current density of 0.1 C, and then cycled for 100 cycles at a current density of 1 C.

[0072] Figure 4 The cycle performance diagram of the battery assembled with the positive electrode materials prepared in each embodiment and comparative example 1. Figure 4 It can be seen that the first discharge specific capacity at 0.1C and the cycle performance at 1C of the battery assembled with the positive electrode materials prepared in Examples 1 to 3 are significantly better than those of the battery assembled with the positive electrode materials prepared in Comparative Example 1. After analysis, this may be because the iron-based triphenylphosphine boride can effectively combine with the organic groups in the lithium-based organic electrolyte through molecular bonds, and can achieve preferential adsorption, so that the organic groups in the electrolyte are adsorbed in the structure of the iron-based triphenylphosphine boride, thereby enabling the lithium ions in the electrolyte to achieve rapid lithium ion extraction and embedding; and the iron-based triphenylphosphine boride can effectively adsorb the byproducts of the irreversible analysis of the electrolyte in the later stage of the electrochemical reaction, such as PF 5 The organic and inorganic fluoride by-products such as HF react with iron-based triphenylphosphine boride to form PF and BF molecular bonds, which ultimately effectively improves the irreversible corrosion of the electrolyte to the main material and exhibits excellent structural stability.

[0073] Figure 5 TEM images of the positive electrode material prepared in Example 1 before cycling (a) and after cycling 100 cycles at 1C (b). Comparing the surface of the positive electrode material before and after electrochemical cycling, it can be seen that the lattice fringes of the positive electrode material of Example 1 are still obvious after cycling, and there are fewer by-products. Figure 6 This is the SEM image of the positive electrode material prepared in Example 1 after cycling. Figure 6 It can be seen that the positive electrode material is still intact after the cycle.

[0074] Figure 7 TEM images of the positive electrode material prepared in Comparative Example 1 before cycling (a) and after cycling 100 cycles at 1C (b). Comparing the surface of the positive electrode material before and after electrochemical cycling, it can be seen that an obvious side reaction product layer is formed on the surface of the positive electrode material of Comparative Example 1 after cycling. Figure 8 This is the SEM image of the positive electrode material prepared in Comparative Example 1 after 100 cycles at 1C. Figure 8 It can be seen that the positive electrode material of Comparative Example 1 has obvious cracks in the particles after cycling, which indicates that the positive electrode material prepared in Comparative Example 1 has poor structural stability during the electrochemical cycle.

[0075] The above analysis further shows that the structural stability and interface stability of the positive electrode material prepared in Example 1 are significantly better than those of the positive electrode material prepared in Comparative Example 1, indicating that the coating modification has significantly improved the stability of the positive electrode material in an electrochemical cycle environment.

[0076] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. Modified lithium iron manganese phosphate material, characterized in that: The modified lithium iron manganese phosphate material comprises iron-based triphenylphosphine boride and lithium iron manganese phosphate material, and the iron-based triphenylphosphine boride is located on the surface of the lithium iron manganese phosphate material, and the chemical formula of the iron-based triphenylphosphine boride is FeCl 2-x (BH4) x (PPh3)2, where the value range of x is 0<x<2.

2. The modified lithium iron manganese phosphate material according to claim 1, characterized in that: The mass ratio of the iron-based triphenylphosphine boride to the lithium manganese iron phosphate is 0.01-0.05:

1.

3. The modified lithium iron manganese phosphate material according to claim 1 or 2, characterized in that: The chemical formula of the lithium iron manganese phosphate is LiMn y Fe 1-y The value range of PO4 / C,y is 0.1≤y≤0.

6.

4. A method for preparing a modified lithium iron manganese phosphate material, characterized in that: include: Dissolving ferrous chloride in an organic solvent, adding an organic phosphorus ligand, and performing a first reaction to obtain an intermediate product; Add borohydride to the intermediate product, drop an organic amine solvent into it, and carry out a second reaction. After the reaction is completed, separate the solid from the liquid and dry it to obtain an iron-based triphenylphosphine boride material. The modified lithium iron manganese phosphate material is obtained by mixing and ball-milling the lithium iron manganese phosphate and the iron-based triphenylphosphine boride material and sintering at a low temperature.

5. The method for preparing the modified lithium iron manganese phosphate material according to claim 4, characterized in that: The organic phosphorus ligand is triphenylphosphine; the borohydride is at least one of sodium borohydride and lithium borohydride; the organic amine solvent is one or more of triethanolamine, ethanolamine and ethylenediamine; and the organic solvent is at least one of ethanol and tetrahydrofuran.

6. The method for preparing the modified lithium iron manganese phosphate material according to claim 4, characterized in that: The molar ratio of ferrous chloride to organic phosphine ligand is 1:2-3; The molar ratio of ferrous chloride to borohydride is 1:1-2; The molar ratio of the borohydride to the organic amine solvent is 1-2:0.01-0.

05.

7. The method for preparing the modified lithium iron manganese phosphate material according to claim 4, characterized in that: The temperature of the first reaction is 30-50° C.; the duration of the first reaction is 6-10 hours; the first reaction is carried out under a protective atmosphere; The temperature of the second reaction is 40-60°C; the duration of the second reaction is 10-20h; the second reaction is carried out under a protective atmosphere; The mixing ball milling is carried out under a protective atmosphere.

8. The method for preparing the modified lithium iron manganese phosphate material according to claim 4, characterized in that: The temperature of the low-temperature sintering is 200-300° C.; the duration of the low-temperature sintering is 2-6 hours; and the atmosphere of the low-temperature sintering is nitrogen or argon atmosphere.

9. A positive electrode, characterized in that It comprises the modified lithium iron manganese phosphate material as described in any one of claims 1 to 3.

10. A lithium ion battery, characterized in that Comprising the positive electrode as claimed in claim 9.

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