A lithium manganese iron phosphate positive electrode material and its preparation method and application

Abstract

The present disclosure provides a lithium iron manganese phosphate cathode material and its preparation method and application. The preparation method comprises the following steps: (1) mixing aniline, phytic acid and a solvent to obtain a mixed solution, and mixing the mixed solution with mesoporous manganese dioxide to carry out a one-step reaction; (2) adding a ferrous salt solution and an oxalic acid source solution to the mixture obtained by the one-step reaction, and carrying out a two-step reaction to obtain a manganese iron oxalate precursor; (3) mixing the manganese iron oxalate precursor with a lithium source and a phosphorus source, and sintering the precursor to obtain the lithium iron manganese phosphate cathode material. The present disclosure pre-prepares a manganese iron oxalate precursor having a polyaniline network structure inside. After sintering, an N and P-doped carbon network is formed inside the lithium iron manganese phosphate. Compared with carbon coating only on the outer layer, this method can make the material more uniformly conductive and improve the material's capacity and cycle stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries and relates to a lithium manganese iron phosphate positive electrode material and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries are currently widely used in mobile communications, digital technology, electric vehicles, and energy storage. As a key component, the cathode material of lithium-ion batteries determines various performance indicators. Therefore, developing high-performance cathode materials is key to lithium-ion battery production.

[0003] Currently, large-scale commercialized cathode materials include lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, and lithium iron manganese phosphate. Among them, lithium iron manganese phosphate offers advantages such as low cost, high safety, and good stability. Furthermore, the industrial production process for lithium iron manganese phosphate cathode materials is highly mature, and its market application is growing rapidly. To meet the requirements for higher energy density, existing technology has increased the cathode material voltage platform to 4.1V through the preparation of lithium iron manganese phosphate, which has an energy density approximately 15% higher than that of lithium iron manganese phosphate.

[0004] CN111900344A discloses a method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material. First, a transition metal salt solution A, a phosphorus solution B, and an ammonia solution C, which are configured according to the molar ratio of Mn and Fe, are simultaneously added dropwise to a reactor to prepare a lithium manganese iron phosphate positive electrode material precursor; then the precursor is prepared with a lithium source according to a molar ratio, and a coating carbon source and a doping metal compound are added, and the carbon-coated lithium manganese iron phosphate positive electrode material is calcined under inert atmosphere protection.

[0005] CN115535993A discloses a lithium iron manganese phosphate positive electrode material and a preparation method thereof, comprising: adding iron powder to a phosphoric acid solution; adding manganese carbonate to a first reaction mixture; grinding the second reaction mixture; adding lithium hydroxide and grinding it; adding a carbon source and grinding it; spray granulation to obtain carbon-coated lithium iron manganese phosphate particles; and calcining the carbon-coated lithium iron manganese phosphate particles to obtain the lithium iron manganese phosphate positive electrode material.

[0006] The lithium manganese iron phosphate prepared by the above scheme still has the disadvantages of low electronic conductivity, low rate performance of the battery, low lithium ion diffusion rate and poor cycle performance. Summary of the Invention

[0007] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0008] The purpose of the present disclosure is to provide a lithium manganese iron phosphate positive electrode material and its preparation method and application. The present disclosure pre-prepare a manganese iron oxalate precursor with a polyaniline network structure inside, and after sintering, form an N and P doped carbon network inside the lithium manganese iron phosphate. Compared with carbon coating only on the outer layer, it can make the material conductive evenly and improve the capacity and cycle stability of the material.

[0009] To achieve this purpose, the present disclosure adopts the following technical solutions:

[0010] In a first aspect, the present disclosure provides a method for preparing a lithium manganese iron phosphate positive electrode material, the preparation method comprising the following steps:

[0011] (1) mixing aniline, phytic acid and a solvent to obtain a mixed solution, and mixing the mixed solution with mesoporous manganese dioxide to perform a one-step reaction;

[0012] (2) adding a ferrous salt solution and an oxalic acid source solution to the mixture obtained by the one-step reaction simultaneously, and performing a two-step reaction to obtain a manganese ferrooxalate precursor;

[0013] (3) The manganese iron oxalate precursor is mixed with a lithium source and a phosphorus source, and the mixture is sintered to obtain the manganese iron phosphate lithium positive electrode material.

[0014] In the method disclosed herein, mesoporous manganese dioxide balls are used as self-sacrificial templates. During the formation of polyaniline balls, divalent manganese is dissolved and reacted with divalent iron to form ferromanganese oxalate precipitation, which can form a ferromanganese oxalate precursor with an internal carbon network structure. After heat treatment, an N- and P-doped carbon network can be formed in the lithium manganese iron phosphate positive electrode material, which can greatly improve the material's electrical conductivity. At the same time, the N- and P-doped carbon network can improve the material's wettability, making it more conducive to the infiltration of the electrolyte and more conducive to the transmission of lithium ions. The carbon network structure disclosed herein can make the entire precursor particle have good electrical conductivity. Compared with carbon coating only on the outer layer, it can make the material conductive evenly, thereby improving the material's capacity and cycle stability.

[0015] In one embodiment, the mass ratio of aniline to phytic acid in step (1) is (4-10):1, for example: 4:1, 5:1, 6:1, 8:1 or 10:1.

[0016] In one embodiment, the mesoporous manganese dioxide in step (1) is prepared by the following method:

[0017] Butyric acid, n-butanol and water are mixed to obtain a mixed solution, the mixed solution is mixed with a potassium permanganate solution, and the mixture is stirred for reaction to obtain the mesoporous manganese dioxide.

[0018] The mesoporous manganese dioxide prepared using the method disclosed in the present invention has uniform pore distribution, which can significantly improve the electrical properties of the prepared lithium manganese iron phosphate positive electrode material.

[0019] In one embodiment, the volume fraction of butyric acid in the mixed solution is 7-9%, for example, 7%, 7.5%, 8%, 8.5% or 9%.

[0020] In one embodiment, the volume fraction of n-butanol in the mixed solution is 5-6%, for example, 5%, 5.2%, 5.5%, 5.8% or 6%.

[0021] In one embodiment, the molar concentration of the potassium permanganate solution is 0.02-0.05 mol / L, for example, 0.02 mol / L, 0.025 mol / L, 0.03 mol / L, 0.04 mol / L or 0.05 mol / L.

[0022] In one embodiment, the volume ratio of the potassium permanganate solution to the mixed solution is (0.8-1):1, for example: 0.8:1, 0.85:1, 0.9:1, 0.95:1 or 1:1, etc.

[0023] In one embodiment, the median particle size D50 of the mesoporous manganese dioxide is 300-1000 nm, for example, 300 nm, 500 nm, 600 nm, 800 nm or 1000 nm.

[0024] In one embodiment, the pore size of the mesoporous manganese dioxide is 5 to 30 nm, for example, 5 nm, 10 nm, 15 nm, 20 nm or 30 nm.

[0025] In one embodiment, the specific surface area of ​​the mesoporous manganese dioxide is 100 to 200 m 2 / g, for example: 100m 2 / g, 120m 2 / g, 150m 2 / g, 180m 2 / g or 200m 2 / g, etc.

[0026] In one embodiment, the molar ratio of the mesoporous manganese dioxide in step (1) to the aniline in the mixed solution is (10-50):1, for example: 10:1, 20:1, 30:1, 40:1 or 50:1.

[0027] In one embodiment, the one-step reaction time is 0.5 h to 1 h, for example, 0.5 h, 0.6 h, 0.8 h, 0.9 h or 1 h.

[0028] In one embodiment, the solute of the ferrous salt solution in step (2) includes any one of ferrous sulfate, ferrous chloride or ferrous citrate, or a combination of at least two of them.

[0029] In one embodiment, the molar concentration of the ferrous salt solution is 0.1 to 1 mol / L, for example, 0.1 mol / L, 0.2 mol / L, 0.5 mol / L, 0.8 mol / L or 1 mol / L.

[0030] In one embodiment, the solute of the oxalic acid source solution includes any one of oxalic acid, ammonium oxalate or sodium oxalate, or a combination of at least two of them.

[0031] In one embodiment, the molar concentration of the oxalic acid source solution is 0.1 to 1 mol / L, for example, 0.1 mol / L, 0.2 mol / L, 0.5 mol / L, 0.8 mol / L or 1 mol / L.

[0032] In one embodiment, the addition rates of the ferrous salt solution and the oxalic acid source solution are independently 50 to 200 mL / min, for example, 50 mL / min, 80 mL / min, 100 mL / min, 150 mL / min or 200 mL / min.

[0033] In one embodiment, during the two-step reaction of step (2), the molar ratio of iron, manganese and oxalate in the system is (1-3):1:(2-5), for example: 1:1:2, 2:1:3, 1.5:1:4, 2.5:1:2 or 3:1:5, etc.

[0034] In one embodiment, the two-step reaction time is 2 to 6 hours, for example, 2 hours, 3 hours, 4 hours, 5 hours or 6 hours.

[0035] In one embodiment, the sintering temperature in step (2) is 600-800°C, for example, 600°C, 650°C, 700°C, 750°C or 800°C.

[0036] In one embodiment, the sintering treatment time is 6 to 15 hours, for example, 6 hours, 8 hours, 10 hours, 12 hours or 15 hours.

[0037] In a second aspect, the present disclosure provides a lithium iron manganese phosphate positive electrode material, which is prepared by the method described in the first aspect.

[0038] In a third aspect, the present disclosure provides a positive electrode plate, which comprises the lithium manganese iron phosphate positive electrode material as described in the second aspect.

[0039] In a fourth aspect, the present disclosure provides a lithium-ion battery, wherein the lithium-ion battery comprises the positive electrode sheet as described in the third aspect.

[0040] Compared with the prior art, the present disclosure has the following beneficial effects:

[0041] (1) The manganese iron oxalate precursor prepared in the present invention has a polyaniline network structure, which can form an N and P doped carbon network in the lithium manganese iron phosphate positive electrode material after heat treatment, which can greatly improve the conductivity of the material. At the same time, the N and P doped carbon network can improve the wettability of the material, which is more conducive to the infiltration of the electrolyte and the transmission of lithium ions.

[0042] (2) The present disclosure forms a carbon network within the precursor, which enables the entire precursor particle to have good conductivity. Compared with carbon coating only on the outer layer, it can make the material conductive uniformly, improve the material's capacity and cycle stability. The reduction of oxalate radicals within the ferromanganese oxalate precursor can make the valence state of the resulting material more uniform. In addition, the porous structure generated by the decomposition of oxalate radicals during the reaction can adsorb lithium sources, promote the entry of lithium salts into the particles, and improve the uniformity of the material.

[0043] (3) The lithium manganese iron phosphate positive electrode material prepared by the method disclosed in the present invention has a 0.1C discharge capacity of a battery of more than 155.85 mAh / g, a 5C discharge capacity of more than 119.77 mAh / g, and a 5C / 0.1C rate of more than 76.85%.

[0044] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The accompanying drawings are used to provide a further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solution of this article and do not constitute a limitation on the technical solution of this article.

[0046] Figure 1 This is an SEM image of the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present disclosure. DETAILED DESCRIPTION

[0047] The technical solution of the present disclosure is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present disclosure and should not be regarded as specific limitations of the present disclosure.

[0048] Example 1

[0049] This embodiment provides a lithium manganese iron phosphate positive electrode material, which is prepared by the following method:

[0050] (1) KMnO4 was dissolved in distilled water at a concentration of 0.03 mol / L. Then, it was added to a mixed solution of butyric acid, n-butanol, and deionized water. The volume fraction of butyric acid in the mixed solution was 8.3%, and the volume fraction of n-butanol in the mixed solution was 5.5%. The volume ratio of potassium permanganate solution to the mixed solution was 0.9:1. The mixed solution was stirred for 30 min, filtered, and washed to obtain a D50 of 500 nm, a pore size of 15 nm, and a specific surface area of ​​140 m 2 / g of mesoporous manganese dioxide, a 0.3 mol / L aniline solution was prepared, phytic acid was added thereto so that the mass ratio of phytic acid to aniline was 1:5, and the mesoporous manganese dioxide was added to the mixed solution of aniline and phytic acid to react for 0.5h, and the molar ratio of manganese dioxide to aniline in the solution was 30:1;

[0051] (2) preparing a 0.5 mol / L ferrous sulfate solution and a 0.5 mol / L oxalic acid solution, and adding the ferrous sulfate solution and the oxalic acid solution dropwise to the mixture obtained in step (1) at a rate of 100 mL / min, respectively, so that the molar ratio of iron, manganese and oxalate in the mixed solution is 2:1:3.5. After the addition is completed, stirring and reacting for 3 hours, filtering, washing and drying after the reaction is completed to obtain a manganese ferrooxalate precursor;

[0052] (3) Mixing a manganese iron oxalate precursor, lithium carbonate, and phosphoric acid in a molar ratio of iron + manganese: lithium: phosphorus of 1:1.08:1.3, and sintering at 700° C. in an inert atmosphere for 10 h to obtain the manganese iron phosphate lithium positive electrode material.

[0053] Example 2

[0054] This embodiment provides a lithium manganese iron phosphate positive electrode material, which is prepared by the following method:

[0055] (1) KMnO4 was dissolved in distilled water at a concentration of 0.02 mol / L. Then it was added to a mixed solution of butyric acid, n-butanol and deionized water. The volume fraction of butyric acid in the mixed solution was 7%, and the volume fraction of n-butanol in the mixed solution was 5%. The volume ratio of potassium permanganate solution to the mixed solution was 0.8:1. The mixed solution was stirred for 30 min, filtered and washed to obtain a D50 of 350 nm, a pore size of 15 nm, and a specific surface area of ​​120 m 2 / g of mesoporous manganese dioxide, a 0.3 mol / L aniline solution was prepared, phytic acid was added thereto so that the mass ratio of phytic acid to aniline was 1:4, and the mesoporous manganese dioxide was added to the mixed solution of aniline and phytic acid to react for 1 hour, and the molar ratio of manganese dioxide to aniline in the solution was 10:1;

[0056] (2) preparing a 0.1 mol / L ferrous chloride solution and a 0.1 mol / L sodium oxalate solution, and adding the ferrous chloride solution and the sodium oxalate solution dropwise to the mixture obtained in step (1) at a rate of 200 mL / min, respectively, so that the molar ratio of iron, manganese and oxalate in the mixed solution is 1:1:2. After the addition is completed, stirring and reacting for 3 hours, filtering, washing and drying after the reaction is completed to obtain a manganese ferrooxalate precursor;

[0057] (3) Mixing the manganese iron oxalate precursor, lithium carbonate, and phosphoric acid in a molar ratio of iron + manganese: lithium: phosphorus of 1:1.08:1.3, and sintering at 600° C. in an inert atmosphere for 15 h to obtain the manganese iron phosphate lithium positive electrode material.

[0058] Example 3

[0059] This embodiment provides a lithium manganese iron phosphate positive electrode material, which is prepared by the following method:

[0060] (1) KMnO4 was dissolved in distilled water at a concentration of 0.05 mol / L. Then it was added to a mixed solution of butyric acid, n-butanol and deionized water. The volume fraction of butyric acid in the mixed solution was 9%, and the volume fraction of n-butanol in the mixed solution was 6%. The volume ratio of potassium permanganate solution to the mixed solution was 1:1. The mixed solution was stirred for 30 min, filtered and washed to obtain a D50 of 700 nm, a pore size of 25 nm, and a specific surface area of ​​110 m 2 / g of mesoporous manganese dioxide, a 0.3 mol / L aniline solution was prepared, phytic acid was added thereto so that the mass ratio of phytic acid to aniline was 1:10, and the mesoporous manganese dioxide was added to the mixed solution of aniline and phytic acid to react for 1 hour, and the molar ratio of manganese dioxide to aniline in the solution was 50:1;

[0061] (2) preparing a 1 mol / L ferrous citrate solution and a 1 mol / L ammonium oxalate solution, and adding the ferrous citrate solution and the ammonium oxalate solution dropwise to the mixture obtained in step (1) at a rate of 50 mL / min, respectively, so that the molar ratio of iron, manganese and oxalate in the mixed solution is 3:1:5. After the addition is completed, stirring and reacting for 6 hours, filtering, washing and drying after the reaction is completed to obtain a manganese ferrooxalate precursor;

[0062] (3) Mixing the manganese iron oxalate precursor, lithium carbonate, and phosphoric acid in a molar ratio of iron + manganese: lithium: phosphorus of 1:1.08:1.3, and sintering at 600° C. in an inert atmosphere for 15 h to obtain the manganese iron phosphate lithium positive electrode material.

[0063] Example 4

[0064] The only difference between this embodiment and embodiment 1 is that the mass ratio of aniline to phytic acid is 2:1, and other conditions and parameters are exactly the same as those in embodiment 1.

[0065] Example 5

[0066] The only difference between this embodiment and embodiment 1 is that the mass ratio of aniline to phytic acid is 15:1, and other conditions and parameters are exactly the same as those in embodiment 1.

[0067] Example 6

[0068] The only difference between this embodiment and embodiment 1 is that commercially available mesoporous manganese dioxide is directly used, and other conditions and parameters are exactly the same as those in embodiment 1.

[0069] Example 7

[0070] The only difference between this embodiment and embodiment 1 is that the molar ratio of mesoporous manganese dioxide to aniline in the mixed solution is 5:1, and other conditions and parameters are exactly the same as those in embodiment 1.

[0071] Example 8

[0072] The only difference between this embodiment and embodiment 1 is that the molar ratio of mesoporous manganese dioxide to aniline in the mixed solution is 60:1, and other conditions and parameters are exactly the same as those in embodiment 1.

[0073] Comparative Example 1

[0074] This comparative example provides a lithium manganese iron phosphate positive electrode material, which is prepared by the following method:

[0075] (1) Prepare a mixed solution of ferrous sulfate with a concentration of 0.5 mol / L and ascorbic acid with a mass fraction of 10%, an oxalic acid solution with a concentration of 0.5 mol / L, and a manganese sulfate solution with a concentration of 0.5 mol / L.

[0076] (2) Using oxalic acid solution as the base liquid, add a mixed solution of ferrous sulfate, ascorbic acid and manganese sulfate solution to it so that the oxalate: manganese: iron in the reaction solution is 3.5:1:2, and stir the reaction for 3 hours to obtain a manganese iron oxalate precursor.

[0077] (3) A manganese iron oxalate precursor, lithium carbonate, phosphoric acid, and glucose were mixed and sintered at 700°C in an inert atmosphere for 10 h to obtain a manganese iron phosphate lithium cathode material. The molar ratio of (iron + manganese): lithium: phosphorus was 1:1.08:1.3, and the mass of glucose added was 10% of the mass of the precursor.

[0078] Comparative Example 2

[0079] The only difference between this comparative example and Example 1 is that the oxalic acid solution is replaced with phosphoric acid, and the other conditions and parameters are exactly the same as Example 1. The precipitation coefficients of manganese phosphate and ferrous phosphate are quite different, and the manganese and iron elements in the synthesized positive electrode material are unevenly distributed.

[0080] Performance testing:

[0081] The lithium manganese iron phosphate positive electrode material, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) prepared in the embodiment and comparative example were mixed uniformly in a mass ratio of 90:5:5 with N-methylpyrrolidone (NMP) as solvent, and then coated on aluminum foil. After drying, the mixture was rolled to form a simulated battery positive electrode. The negative electrode was a metal lithium sheet, the separator was Celgard 2400, and the electrolyte was 1 mol L -1 A CR2025 simulated battery was constructed using LiPF6 / DMC+DEC (volume ratio 1:1). The charge and discharge voltage range was 2.9-3.7V, and electrochemical performance data for lithium manganese iron phosphate was obtained. The powder conductivity at 30 MPa was measured using a powder conductivity meter. The test results are shown in Table 1:

[0082] Table 1

[0083]

[0084] As can be seen from Table 1, from Examples 1-3, the powder conductivity of the lithium manganese iron phosphate positive electrode material prepared by the method described in the present disclosure can reach more than 0.1s / cm, the 0.1C discharge capacity of the prepared battery can reach more than 155.85mAh / g, the 5C discharge capacity can reach more than 119.77mAh / g, and the 5C / 0.1C rate can reach more than 76.85%.

[0085] By comparing Example 1 with Examples 4-5, it can be seen that in the preparation process of the lithium manganese iron phosphate positive electrode material described in the present disclosure, the mass ratio of aniline and phytic acid affects its performance. When the mass ratio of aniline and phytic acid is controlled at (4-10):1, the performance of the lithium manganese iron phosphate positive electrode material obtained is better. If the amount of phytic acid added is too large, manganese dissolves too quickly, and it is difficult to control the uniformity of manganese iron distribution. If the amount of phytic acid added is too small, it is difficult to promote the dissolution of manganese ions in the one-step reaction, which will also affect the manganese iron distribution.

[0086] From the comparison between Example 1 and Example 6, it can be seen that the pore distribution of the mesoporous manganese dioxide prepared by the method of the present disclosure is uniform, and the performance of the lithium manganese iron phosphate positive electrode material prepared is significantly improved.

[0087] By comparing Example 1 with Examples 7-8, it can be seen that in the preparation process of the lithium manganese iron phosphate positive electrode material disclosed in the present invention, the molar ratio of mesoporous manganese dioxide to aniline in the mixed solution affects its performance. The molar ratio of mesoporous manganese dioxide to aniline in the mixed solution is controlled at (10-50):1, and the performance of the lithium manganese iron phosphate positive electrode material obtained is better. If the amount of mesoporous manganese dioxide added is too large and the aniline content is low, the conductivity of the synthesized positive electrode material is not significantly improved. If the amount of mesoporous manganese dioxide added is too small, the aniline content is too high, which affects the gram capacity of the material.

[0088] By comparison of Example 1 and Comparative Example 1, it can be seen that mesoporous manganese dioxide balls are used as self-sacrificial templates, and divalent manganese is dissolved and divalent iron is used to form manganese ferrooxalate precipitation during the formation of polyaniline balls, which can form a manganese ferrooxalate precursor with an internal carbon network structure. After heat treatment, an N and P-doped carbon network can be formed in the lithium manganese iron phosphate positive electrode material, which can greatly improve the material conductivity. At the same time, the N and P-doped carbon network can improve the wettability of the material, which is more conducive to the infiltration of the electrolyte and more conducive to the transmission of lithium ions. The carbon network structure can make the entire precursor particle have good conductivity, which can make the material conductive evenly compared to carbon coating only on the outer layer, thereby improving the capacity and cycle stability of the material.

[0089] From the comparison between Example 1 and Comparative Example 2, it can be seen that the present disclosure uses oxalic acid as a chelating agent to prepare a manganese ferrooxalate precursor. During the sintering reaction, the porous structure generated by the decomposition of oxalate ions can adsorb lithium sources and promote the entry of lithium salts into the interior of the particles, resulting in good material uniformity.

Claims

1. A method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following steps: (1) mixing aniline, phytic acid and a solvent to obtain a mixed solution, and mixing the mixed solution with mesoporous manganese dioxide to perform a one-step reaction; (2) adding a ferrous salt solution and an oxalic acid source solution to the mixture obtained by the one-step reaction simultaneously, and performing a two-step reaction to obtain a manganese ferrooxalate precursor; (3) mixing the manganese iron oxalate precursor with a lithium source and a phosphorus source, and sintering the mixture to obtain the manganese iron phosphate lithium positive electrode material; In step (1), the mass ratio of aniline to phytic acid is (4-10):1, the molar ratio of the mesoporous manganese dioxide to the aniline in the mixed solution is (10-50):1, and the time of the one-step reaction is 0.5h-1h.

2. The preparation method according to claim 1, wherein The mesoporous manganese dioxide in step (1) is prepared by the following method: Butyric acid, n-butanol and water are mixed to obtain a mixed solution, the mixed solution is mixed with a potassium permanganate solution, and the mixture is stirred for reaction to obtain the mesoporous manganese dioxide.

3. The preparation method according to claim 2, wherein The volume fraction of butyric acid in the mixed solution is 7-9%.

4. The preparation method according to claim 2, wherein The volume fraction of n-butanol in the mixed solution is 5-6%.

5. The preparation method according to claim 2, wherein The molar concentration of the potassium permanganate solution is 0.02-0.05 mol / L.

6. The preparation method according to claim 2, wherein The volume ratio of the potassium permanganate solution to the mixed solution is (0.8-1):

1.

7. The preparation method according to claim 1, wherein The median particle size D50 of the mesoporous manganese dioxide is 300-1000 nm.

8. The preparation method according to claim 1, wherein The pore diameter of the mesoporous manganese dioxide is 5 to 30 nm.

9. The preparation method according to claim 1, wherein The specific surface area of ​​the mesoporous manganese dioxide is 100 to 200 m 2 / g.

10. The preparation method according to claim 1, wherein The solute of the ferrous salt solution in step (2) includes any one of ferrous sulfate, ferrous chloride or ferrous citrate, or a combination of at least two of them. The preparation method according to claim 1 , wherein the molar concentration of the ferrous salt solution is 0.1 to 1 mol / L.

12. The preparation method according to claim 1, wherein the solute of the oxalic acid source solution comprises any one of oxalic acid, ammonium oxalate or sodium oxalate, or a combination of at least two of them.

13. The preparation method according to claim 1, wherein the molar concentration of the oxalic acid source solution is 0.1 to 1 mol / L.

14. The preparation method according to claim 1, wherein the addition rates of the ferrous salt solution and the oxalic acid source solution are independently 50 to 200 mL / min.

15. The preparation method according to claim 1, wherein During the two-step reaction of step (2), the molar ratio of iron element, manganese element and oxalate in the system is (1-3):1:(2-5). The preparation method according to claim 1 , wherein the second-step reaction time is 2 to 6 hours.

17. The preparation method according to claim 1, wherein The temperature of the sintering treatment in step (2) is 600-800°C. The preparation method according to claim 1 , wherein the sintering treatment time is 6 to 15 hours.

19. A lithium manganese iron phosphate positive electrode material prepared by the method according to any one of claims 1 to 18.

20. A positive electrode sheet comprising the lithium manganese iron phosphate positive electrode material according to claim 19.

21. A lithium ion battery comprising the positive electrode sheet according to claim 20.

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