A lithium manganese iron phosphate material, a preparation method and application thereof

Abstract

The present invention provides a lithium iron manganese phosphate material, its preparation method, and application. The lithium iron manganese phosphate material comprises, from the inside out, a lithium iron manganese phosphate matrix core, a first coating layer, and a second coating layer. The first coating layer includes a carbon material, and the second coating layer includes a composite material comprising carbon nitride and a conductive polymer. By doping and coating the carbon material in the first coating layer and synergizing the conductive polymer and carbon nitride in the second coating layer, the present invention improves the conductivity of the lithium iron manganese phosphate material, thereby enhancing its cycle stability, capacity, and rate performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and relates to a lithium manganese iron phosphate material and a preparation method and application thereof. BACKGROUND

[0002] There are many kinds of positive electrode materials for lithium ion batteries. The lithium manganese iron phosphate (LMFP) positive electrode material has safety performance comparable to that of lithium iron phosphate, can solve many safety problems under low temperature conditions, has a capacity close to that of ternary 523 material, is 10-20% higher than that of the lithium iron phosphate positive electrode material, has a cost only 5-6% higher than that of the lithium iron phosphate positive electrode material, has many advantages such as high working voltage and excellent cycle performance, and is more likely to be used in combination with high-energy ternary materials, thereby becoming a research hotspot. However, the one-dimensional conduction of lithium ions in the olivine-type positive electrode material determines that the ionic conductivity is low. Therefore, the electronic transmission capacity of the lithium manganese iron phosphate is lower than that of the lithium iron phosphate with semiconductor properties. The conductivity of the lithium iron phosphate (LMFP) is 10 -9 S / cm, and the conductivity of the lithium manganese iron phosphate is only 10 -13 S / cm. Therefore, it is essential to improve the conductivity of the lithium manganese iron phosphate and improve the rate performance of the battery.

[0003] Current technical solutions for improving the conductivity of the LMFP are mainly carbon coating and ion doping. Carbon coating mainly improves the electronic conductivity, and ion doping mainly improves the ion diffusion coefficient and the conductivity.

[0004] CN105047922A discloses a carbon-coated lithium manganese iron phosphate positive electrode material and a preparation method. The chemical formula of the carbon-coated lithium manganese iron phosphate is C-Li 1-x M x Mn y Fe 1-y PO4, wherein C represents carbon crosslinked with the compound Li 1-x M x Mn y Fe 1-y PO4, x and y are numbers, 0

[0005] CN115332516A provides a magnesium and nickel co-doped lithium manganese iron phosphate positive electrode material and a preparation method thereof, which comprises a positive electrode active material and a coating material compounded on the surface of the positive electrode active material. The positive electrode active material comprises a lithium manganese iron phosphate matrix and magnesium and nickel co-doped in the lithium manganese iron phosphate matrix. The doping weight percentage of the magnesium is 0.5-3%, and the doping weight percentage of the nickel is 0.5-3%. The preparation method comprises the following steps: S1, dissolving a lithium source, a phosphorus source, an iron source, a manganese source, a magnesium source and a nickel source in a solvent and continuously stirring, and adding a carbon source dropwise until the stirring is gel-like; S2, drying the gel-like mixture to obtain a dry gel; and S3, calcining the dry gel in a protective atmosphere to obtain a carbon-coated magnesium and nickel co-doped lithium manganese iron phosphate positive electrode material.

[0006] Although the above-mentioned documents respectively perform carbon coating and doping treatment on the lithium manganese iron phosphate positive electrode material, the current LMFP still has problems of uneven carbon coating and low ionic conductivity. The carbon material of the coating layer is prone to defects in the preparation process, and these defects act as active sites in the lithium ion battery to aggravate the side reaction of the electrolyte on the surface of the lithium manganese iron phosphate material, resulting in the growth of the impedance of the battery in the later stage. Moreover, the LMFP material with incomplete coating has poor reversibility in the charging and discharging process and is prone to polarization, resulting in low capacity. The low ionic conductivity also leads to poor rate performance of the material.

[0007] Therefore, how to improve the electrical conductivity of the lithium manganese iron phosphate positive electrode material and further improve its electrochemical performance is currently in urgent need of research. SUMMARY

[0008] In view of the deficiencies in the prior art, the purpose of the present application is to provide a lithium manganese iron phosphate material and a preparation method and application thereof. The present application improves the electrical conductivity of the lithium manganese iron phosphate material through double-layer coating of carbon material and composite material, and synergistically improves the cycle stability, capacity and rate performance of the lithium manganese iron phosphate material.

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

[0010] In a first aspect, the present application provides a lithium manganese iron phosphate material, which comprises a lithium manganese iron phosphate matrix core, a first coating layer and a second coating layer from inside to outside. The first coating layer comprises carbon material, and the second coating layer comprises composite material, which comprises carbon nitride and conductive polymer.

[0011] The lithium manganese iron phosphate material provided by the application improves the internal conductivity of the lithium manganese iron phosphate material through the doping and coating of the carbon material in the first coating layer, the lithium manganese iron phosphate material coated once is coated again, and the conductivity is further improved by the conductive polymer and the carbon nitride coated again, wherein the carbon nitride has a graphite-like structure, the stacking distance between the aromatic layers is low (0.319 nm), and the combination is strong, more charges are provided perpendicular to the plane than graphite, the conductivity of the material is improved by the incorporation of nitrogen and the carbon matrix, thereby realizing effective charge transmission, in addition, the conductive polymer can also adsorb the manganese sub-ion separated out due to the Jahn-Teller effect, reduce the structural instability of the lithium manganese iron phosphate material caused by the separation of manganese ions, and improve the cycle stability, capacity and rate performance of the lithium manganese iron phosphate material.

[0012] In the application, the first coating layer and the second coating layer cooperate to simultaneously improve the conductivity, cycle performance, capacity and rate performance; and the coating sequence between the first coating layer and the second coating layer cannot be exchanged, and once exchanged, the composite material of the second coating layer cannot effectively adsorb the manganese sub-ion separated out due to the Jahn-Teller effect, and the effect of maintaining the stability of the LMFP structure cannot be achieved.

[0013] The following is a preferred technical solution of the application, but is not a limitation on the technical solution provided by the application. Through the following preferred technical solution, the technical purpose and beneficial effects of the application can be better achieved and realized.

[0014] Preferably, the D50 of the lithium manganese iron phosphate matrix core is 650-1050 nm, for example, 650 nm, 700 nm, 750 nm, 800 nm, 850 nm, 900 nm, 950 nm, 1000 nm or 1050 nm, etc., but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0015] Preferably, the thickness of the first coating layer and the thickness of the second coating layer are each independently 30-50 nm, for example, 30 nm, 35 nm, 40 nm, 45 nm or 50 nm, etc., but are not limited to the listed values, and other values not listed in the value range are also applicable.

[0016] Preferably, the carbon material includes pyrolytic carbon material.

[0017] Preferably, the lithium manganese iron phosphate matrix core is also doped with carbon material.

[0018] In the application, the carbon material is doped and coated in the lithium manganese iron phosphate matrix core, that is, the internal doping and external coating are realized, the first coating layer is in close contact with the core, and the internal conductivity of the core is also improved.

[0019] Preferably, the conductive polymer comprises polypyrrole.

[0020] In the present application, the conductive polymer is selected as polypyrrole, which can better realize the adsorption of manganese ions precipitated due to the Jahn-Teller effect and reduce the structural instability of the lithium manganese iron phosphate material caused by the precipitation of manganese ions.

[0021] In the present application, the combination of carbon nitride and polypyrrole can realize good compounding of the two, without affecting the normal performance of the core in the lithium manganese iron phosphate matrix, and better improve the electrochemical performance of the final lithium manganese iron phosphate material.

[0022] Preferably, the conductive polymer is in-situ grown on the surface of the carbon nitride.

[0023] In the present application, the conductive polymer in the composite material is in-situ grown on the surface of the carbon nitride, which can adsorb manganese ions precipitated due to the Jahn-Teller effect and reduce the structural instability of LMFP caused by the precipitation of manganese ions.

[0024] In a second aspect, the present application provides a preparation method of the lithium manganese iron phosphate material according to the first aspect, which comprises the following steps:

[0025] The lithium manganese iron phosphate precursor material and the carbon source are mixed and coated, and carbonized to obtain an intermediate material;

[0026] The intermediate material is coated with the composite material to obtain the lithium manganese iron phosphate material;

[0027] The composite material comprises carbon nitride and a conductive polymer.

[0028] The preparation method provided by the present application realizes uniform coating of the carbon material and the composite material on the surface of the lithium manganese iron phosphate matrix material, and also realizes uniform doping of the carbon material in the core; the lithium manganese iron phosphate material prepared has high electrical conductivity and excellent cycle performance, capacity and rate performance.

[0029] Preferably, the lithium manganese iron phosphate precursor material is prepared by a hydrothermal method.

[0030] Preferably, the method for preparing lithium manganese iron phosphate by a hydrothermal method comprises mixing a lithium source, a phosphorus source, an iron source and a manganese source, and hydrothermal reaction to obtain the lithium manganese iron phosphate precursor material.

[0031] It should be noted that the selection of the specific substance type and the adjustment of the preparation parameters of the lithium manganese iron phosphate precursor material prepared by the hydrothermal method in the present application are all conventional technical solutions, and the skilled person in the art can make adaptive selection and adjustment according to actual needs.

[0032] Exemplarily, the present application provides a specific preparation method of a lithium iron manganese phosphate precursor material:

[0033] The lithium source, the phosphorus source, the iron source and the manganese source are mixed according to the chemical formula LiMn x Fe 1-x PO4(0.5<x≤0.8, preferably x=0.6) in a proportion, and a reaction liquid to be reacted is obtained after mixing, and the reaction liquid is subjected to a hydrothermal reaction at 180-240℃ for 5-20h, and the lithium iron manganese phosphate precursor material is obtained after the hydrothermal reaction is completed.

[0034] Optionally, the lithium source includes but is not limited to one or more of lithium hydroxide, lithium carbonate, lithium nitrate, lithium sulfate, lithium chloride, lithium phosphate and lithium acetate; the phosphorus source includes but is not limited to one or more of phosphoric acid, ammonium dihydrogen phosphate, ammonium phosphate, pyrophosphoric acid, metaphosphoric acid and diaphosphorus pentoxide; the iron source includes but is not limited to one or more of ferrous oxalate, ferrous sulfate, ferrous nitrate, ferrous phosphate and ferrous carbonate; and the manganese source includes but is not limited to one or more of manganese acetate, manganese nitrate, manganese sulfate, manganese carbonate and manganese chloride.

[0035] Optionally, the temperature of the hydrothermal reaction can be 180℃, 190℃, 200℃, 210℃, 220℃, 230℃ or 240℃, etc., and the time of the hydrothermal reaction can be 5h, 8h, 10h, 13h, 15h, 18h or 20h, etc.

[0036] Preferably, the mass ratio of the precursor to the carbon source is 1:(0.05-0.3), for example, 1:0.05, 1:0.1, 1:0.13, 1:0.15, 1:0.18, 1:0.2, 1:0.25 or 1:0.3, etc., preferably 1:(0.1-0.2), but is not limited to the listed values, and other values not listed in the range are also applicable.

[0037] In the present application, the mass ratio of the precursor to the carbon source is 1:(0.05-0.3), which can realize uniform coating of the carbon material after carbonization and will not affect the capacity of the lithium iron manganese phosphate material, and is more conducive to the improvement of the electrical conductivity of the material; further, the type of the carbon source can be selected according to conventional technology, and the types of the carbon source that can realize pyrolysis and carbonization are applicable to the present application, including but not limited to at least one of phenolic resin, carbon gel or citric acid.

[0038] Preferably, the coated mixed substance is subjected to spray pyrolysis and carbonization to obtain the intermediate material.

[0039] In the present application, carbonization is carried out by spray pyrolysis, which ensures uniform distribution of carbon materials in the first coating layer, and the carbon materials can further enter the interior of the lithium iron manganese phosphate matrix core for internal doping and uniform distribution, thereby improving the internal electrical conductivity.

[0040] Preferably, the composite material is obtained by in-situ growth of the conductive polymer on the surface of carbon nitride.

[0041] In the present application, the polymerization method of the conductive polymer and the preparation method of carbon nitride (carbon nitride can also be directly purchased) are not specially limited, and conventional technical methods for preparing the above-mentioned substances are applicable.

[0042] Exemplarily, the present application provides a preparation method of carbon nitride, which comprises the following steps:

[0043] Melamine is placed in a closed porcelain boat and heated to 500℃ at an air atmosphere and a temperature rising speed of 3℃ / min for 4h, the obtained sample is washed 3 times by centrifugation at a speed of 10000rpm for 10min with deionized water to remove any other residues, and then dried in an oven at 90℃ for 24h, finally, after being sufficiently cooled to room temperature, the carbon nitride (g-C3N4) is obtained by grinding with a mortar;

[0044] The present application also provides a specific preparation method of a composite material in which a conductive polymer is in-situ grown on the surface of carbon nitride, which comprises the following steps:

[0045] The conductive polymer monomer and the initiator are added to deionized water for continuous stirring for about 8-10h to obtain a uniform solution, and the carbon nitride is added to the above-mentioned solution under ultrasonic conditions, and the monomer can generate the conductive polymer in-situ on the surface of the carbon nitride (g-C3N4) under the action of the initiator, and the composite material is obtained after drying;

[0046] Optionally, the initiator comprises ammonium persulfate, and the amount of the initiator is adaptively adjusted according to the amount of the monomer;

[0047] Optionally, the conductive polymer monomer comprises at least one of an aniline monomer, a thiophene monomer, a pyrrole monomer, a furan monomer, a thiazole monomer or a pyridine monomer;

[0048] Optionally, the drying method comprises freeze-drying.

[0049] Preferably, in the process of in-situ growth, the mass ratio of the monomer of the conductive polymer to the solute of the carbon nitride in the solution is 1:(100-120), for example, 1:105, 1:110, 1:115, or 1:120, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0050] Preferably, the mass ratio of the intermediate material to the composite material is 1:(1-3), for example 1:1, 1:1.3, 1:1.5, 1:1.8, 1:2, 1:2.3, 1:2.5, 1:2.8 or 1:3, etc., but not limited to the listed values, and other values not listed in the range are also applicable.

[0051] In the present application, the mass ratio of the intermediate material to the composite material is 1:(1-3), which can better achieve uniform coating of the composite material in the second coating layer, and better play the synergistic effect of the conductive polymer and carbon nitride.

[0052] Preferably, the coating process includes mixing the intermediate material with the composite material and then heat treating, and the heat treatment temperature is 150-250℃, for example 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, 210℃, 220℃, 230℃, 240℃ or 250℃, etc., but not limited to the listed values, and other values not listed in the range are also applicable.

[0053] Preferably, the product after coating treatment is spray granulated to obtain the lithium manganese iron phosphate material.

[0054] In the present application, the heat treatment during the coating process and the subsequent spray granulation both better achieve uniform coating of the second coating layer, and the mixture of the intermediate material and the composite material is more uniform after heat treatment.

[0055] As a preferred technical solution, the preparation method comprises the following steps:

[0056] Mixing a lithium source, a phosphorus source, an iron source and a manganese source, and then hydrothermal reaction to obtain a lithium manganese iron phosphate precursor;

[0057] Coating and mixing the lithium manganese iron phosphate precursor material and a carbon source at a ratio of 1:(0.05-0.3), and then spray pyrolysis carbonization to obtain an intermediate material;

[0058] Mixing the intermediate material and a composite material at a mass ratio of 1:(1-3), heat treating at 150-250℃, and then spray granulating to obtain the lithium manganese iron phosphate material;

[0059] The composite material comprises carbon nitride and a conductive polymer, and the conductive polymer is grown in situ on the surface of the carbon nitride.

[0060] In a third aspect, the present application also provides a lithium ion battery, which comprises the lithium manganese iron phosphate material according to the first aspect or the lithium manganese iron phosphate prepared by the preparation method according to the second aspect.

[0061] Compared with the prior art, the present invention has the following beneficial effects:

[0062] The lithium iron manganese phosphate material provided by the present invention improves the internal conductivity of the lithium iron manganese phosphate material by doping and coating the carbon material in the first coating layer, and the lithium iron manganese phosphate material coated once is subjected to a second coating. The conductive polymer and carbon nitride of the second coating further improve the conductivity, wherein the carbon nitride has a graphite-like structure, and the stacking distance between its aromatic layers is relatively low (0.319nm), which can show a stronger bond and provide more charge transfer perpendicular to the plane than graphite. The incorporation of nitrogen and the carbon matrix can improve the conductivity of the material, thereby realizing effective charge transfer. In addition, the conductive polymer polypyrrole can also adsorb manganese ions precipitated due to the Jan-Taylor effect, reducing the structural instability of the lithium iron manganese phosphate material caused by the precipitation of manganese ions. The two together improve the cycle stability, capacity and rate performance of the lithium iron manganese phosphate material. DETAILED DESCRIPTION

[0063] The technical solutions of the present invention are further illustrated by specific examples below. Those skilled in the art should understand that the examples are only for helping to understand the present invention and should not be regarded as specific limitations of the present invention.

[0064] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in this application are intended to cover non-exclusive inclusions.

[0065] In the description of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly indicate the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise clearly and specifically defined.

[0066] Example 1

[0067] This embodiment provides a lithium iron manganese phosphate material, which includes a lithium iron manganese phosphate matrix core (chemical formula LiMn 0.6 Fe 0.4 PO4, D50 is 850nm), a first coating layer and a second coating layer (the thickness range of the first coating layer and the second coating layer is 30-50nm); the first coating layer includes a pyrolytic carbon material, and the second coating layer includes a composite material, the composite material includes carbon nitride and polypyrrole (PPy), and the polypyrrole is in situ grown on the surface of the carbon nitride.

[0068] The preparation method of the lithium manganese iron phosphate is as follows:

[0069] S1 :

[0070] A lithium manganese iron phosphate precursor material is prepared: 0.92 mol of lithium carbonate (Li2CO3) and 0.34 mol of phosphoric acid (H3PO4) are added to 300 mL of deionized water under magnetic stirring for 20 min to obtain a lithium phosphate solution, 0.26 mol of manganese carbonate (MnCO3) and 0.1 mol of iron phosphate (FePO4) are added to 100 mL of deionized water under magnetic stirring for 20 min to obtain a metal salt solution, and finally the lithium phosphate solution and the metal salt solution are mixed and adjusted to pH 6.5 with a sodium hydroxide solution, then transferred to a high-pressure reaction kettle to react at 180°C for 8 h to obtain an A solution (lithium manganese iron phosphate precursor material);

[0071] The lithium manganese iron phosphate precursor material product in the A solution and the citric acid solution are mixed at a mass ratio of 1:0.15, then atomized by a 1.7 MHz ultrasonic atomizer, then the atomized aerosol is introduced into a vertical quartz reactor heated to 400°C, the air flow rate is 10 L / min, and finally the obtained powder is treated at 500°C in an air atmosphere for 1 h to obtain a lithium manganese iron phosphate LiMn 0.6 Fe 0.4 PO4 / C (intermediate material coated with a first coating layer on the surface of the lithium manganese iron phosphate substrate core);

[0072] S2:

[0073] Carbon nitride preparation: melamine is placed in a closed porcelain boat and heated to 500°C at an air atmosphere at a heating rate of 3°C / min for 4 h, the obtained sample is centrifuged at a speed of 10000 rpm for 10 min and washed 3 times with deionized water to remove any other residues, then dried in a 90°C oven for 24 h, and finally after being cooled to room temperature, it is ground with a mortar to obtain carbon nitride (g-C3N4);

[0074] Preparation of the composite material: pyrrole monomer and ammonium persulfate in solution with a solute mass ratio of 1:746 were added to 1000 ml of deionized water, continuously stirred at a speed of 500 rpm for about 8 h to obtain a uniform solution, under the condition of ultrasonic 45 Hz, pyrrole monomer and carbon nitride in solution with a solute mass ratio of 1:112, carbon nitride was added to the above solution and ultrasonic for 1 h to realize the uniform dispersion of the carbon nitride and polypyrrole (g-C3N4 / PPy) composite solution, and the pyrrole monomer could generate polypyrrole on the surface of carbon nitride (g-C3N4) under the action of ammonium persulfate initiator; the obtained product was centrifuged at a speed of 10000 rpm for 10 min and washed for 3 times to remove any other residues, and after freeze-drying for 12 h, the carbon nitride and polypyrrole composite material (g-C3N4 / PPy) was obtained.

[0075] Steps S1 and S2 have no sequence;

[0076] S3: The intermediate material obtained in S1 and the composite material (g-C3N4 / PPy) obtained in S2 were ball milled at a mass ratio of 1:2 for 2 h, then heat treated at 180℃ for 15 h to obtain a melt blend; finally, the melt blend was dried and granulated by spray drying, and finally ground in a mortar to obtain the lithium iron manganese phosphate material LiMn 0.6 Fe 0.4 PO4 / C@g-C3N4 / PPy.

[0077] Example 2

[0078] The difference between this embodiment and example 1 is that the pyrolytic carbon source in step S1 of this embodiment is carbon gel.

[0079] The rest of the preparation method and parameters remain the same as example 1.

[0080] Example 3

[0081] The difference between this embodiment and example 1 is that the pyrolytic carbon source in S1 of this embodiment is phenolic resin.

[0082] The rest of the preparation method and parameters remain the same as example 1.

[0083] Example 4

[0084] The difference between this embodiment and example 1 is that in step S3 of this embodiment, the mass ratio of the intermediate material obtained in S1 to the composite material obtained in S2 is 1:1.

[0085] The rest of the preparation method and parameters remain the same as example 1.

[0086] Example 5

[0087] The difference between this embodiment and embodiment 1 is that in step S3 of this embodiment, the mass ratio of the intermediate material obtained in S1 to the composite material obtained in S2 is 1:1.5.

[0088] The rest of the preparation method and parameters are consistent with those of embodiment 1.

[0089] Embodiment 6

[0090] The difference between this embodiment and embodiment 1 is that in step S3 of this embodiment, the mass ratio of the intermediate material obtained in S1 to the composite material obtained in S2 is 1:2.5.

[0091] The rest of the preparation method and parameters are consistent with those of embodiment 1.

[0092] Embodiment 7

[0093] The difference between this embodiment and embodiment 1 is that in step S3 of this embodiment, the mass ratio of the intermediate material obtained in S1 to the composite material obtained in S2 is 1:3.

[0094] The rest of the preparation method and parameters are consistent with those of embodiment 1.

[0095] Embodiment 8

[0096] The difference between this embodiment and embodiment 1 is that in step S3 of this embodiment, the mass ratio of the intermediate material obtained in S1 to the composite material obtained in S2 is 1:0.5.

[0097] The rest of the preparation method and parameters are consistent with those of embodiment 1.

[0098] Embodiment 9

[0099] The difference between this embodiment and embodiment 1 is that in step S3 of this embodiment, the mass ratio of the intermediate material obtained in S1 to the composite material obtained in S2 is 1:4.

[0100] The rest of the preparation method and parameters are consistent with those of embodiment 1.

[0101] Comparative Example 1

[0102] The difference between this comparative example and embodiment 1 is that the lithium manganese iron phosphate material provided in this comparative example does not contain a second coating layer.

[0103] In the preparation method, steps S2 and S3 are not performed, and only step S1 is performed.

[0104] The rest of the preparation method and parameters are consistent with those of embodiment 1.

[0105] Comparative Example 2

[0106] The difference between the present comparative example and Example 1 is that the lithium manganese iron phosphate material provided in the present comparative example does not contain the first coating layer.

[0107] In the preparation method, after obtaining the lithium manganese iron phosphate precursor material, the lithium manganese iron phosphate precursor material is directly used as an intermediate material to perform the preparation processes of steps S2 and S3; and the carbon coating process in step S1 is not performed.

[0108] The remaining preparation methods and parameters are consistent with those of Example 1.

[0109] Comparative Example 3

[0110] The difference between the present comparative example and Example 1 is that the lithium manganese iron phosphate material provided in the present comparative example does not contain carbon nitride in the second coating layer, i.e., the second coating layer is polypyrrole.

[0111] In the preparation method, the preparation process of carbon nitride is not performed, and the polymerization of polypyrrole is directly performed.

[0112] The remaining preparation methods and parameters are consistent with those of Example 1.

[0113] The lithium manganese iron phosphate materials provided in Examples 1-9 and Comparative Examples 1-3 were tested for powder resistivity: at 25°C, the lithium manganese iron phosphate materials prepared in Examples 1-9 and Comparative Examples 1-3 were tested using a four-probe powder resistivity meter, and the resistivity values were recorded when the pressure reached 20 MPa. The test results are shown in Table 1.

[0114] Table 1

[0115] Resistivity (Ω-cm) Example 1 22.34 Example 2 24.53 Example 3 24.12 Example 4 26.78 Example 5 25.89 Example 6 24.87 Example 7 25.93 Example 8 27.88 Example 9 26.03 Comparative Example 1 30.74 Comparative Example 2 33.67 Comparative Example 3 30.23

[0116] The lithium manganese iron phosphate materials provided in Examples 1-9 and Comparative Examples 1-3 were used as positive electrode materials to prepare batteries and perform tests:

[0117] 1. Preparation of button cells

[0118] The lithium iron manganese phosphate material of Example 1-9 and Comparative Example 1-3 was weighed in a mass ratio of 95:2:0.8:0.6, with a total weight of 10 g, and each material was poured into a ball mill tank, stirred for 10 min using a high-speed swing ball mill to obtain a uniformly stirred slurry, which was then uniformly coated on an aluminum current collector foil, dried in a vacuum drying oven at 105 DEG C for 4 h, rolled to a compacted density of 2.35 mg / cm3, and then die-cut, weighed, dried in a vacuum drying oven at 105 DEG C for 2 h, and then placed in a glove box to assemble a half battery. Metal lithium was used as the negative electrode, and the assembly process of the coin battery was completed in the order of negative electrode shell, spring sheet, gasket, lithium sheet, separator, electrolyte, positive electrode (electrode sheet), and positive electrode shell in an argon-filled glove box, and finally the battery was packaged using a battery sealing machine.

[0119] 2. Performance test

[0120] (1) Charge-discharge capacity

[0121] At 25 DEG C, the lithium ion battery was rested for 12 h, then charged to 4.2 V at a 1C rate, rested for 5 min, then discharged to 2.5 V at a 1C rate, rested for 5 min, and the first cycle charge-discharge capacity was recorded;

[0122] (2) Cycle performance

[0123] At 25 DEG C, the lithium ion battery was charged to 4.2 V at a 1C rate, rested for 5 min, then discharged to 2.5 V at a 1C rate, rested for 5 min, and the above was one charge-discharge cycle. The lithium ion battery was subjected to 200 charge-discharge cycles at 25 DEG C according to the above conditions, and the charge-discharge capacity and capacity retention rate were recorded and calculated;

[0124] The test results are shown in Table 2.

[0125] Table 2

[0126]

[0127]

[0128] From Tables 1 and 2, it can be seen that:

[0129] From the data results of Examples 1-8, it can be seen that the lithium iron manganese phosphate material provided by the application has excellent electrical conductivity, capacity, cycle performance, and rate performance.

[0130] From the data results of Example 1 and Examples 4-9, it can be seen that the mass ratio of the intermediate material to the composite material is too small, and a suitable ratio should be selected for coating, and too high or too low has an impact on the specific capacity of the active material and the cycle retention rate at a 1C rate.

[0131] From the data results of Example 1 and Comparative Examples 1-3, it can be seen that the discharge capacity and capacity retention of the lithium manganese iron phosphate positive electrode material provided by the present application, which is one-layer carbon-coated, are far less effective than those of Example 1, which is two-layer carbon-coated, and Comparative Examples 1 and 2, which is one-layer carbon-coated, indicating that the first coating layer and the second coating layer synergistically improve the electrical conductivity, improve the cycle stability of the battery, and increase the capacity. The capacity retention of Comparative Examples 2 and 3 is higher than that of Comparative Example 1, which should be that the polypyrrole in the carbon nitride and polypyrrole composite material (g-C3N4 / PPy) can adsorb the manganese ions precipitated due to the Jahn-Teller effect, reducing the structural instability of LMFP caused by the precipitation of manganese ions, thereby improving the cycle performance. In addition, the resistivity of Comparative Example 3 is lower than that of Comparative Example 2, indicating that the main improvement in electrical conductivity is the coating and doping of the pyrolysis carbon source, and the addition of carbon nitride can further improve the electrical conductivity and improve the battery performance.

[0132] The applicant declares that the above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto. It should be understood by those skilled in the art that any changes or replacements within the technical scope disclosed by the present application can be easily thought of by any person skilled in the art, and all such changes or replacements fall within the protection scope and disclosure scope of the present application.

Claims

1. A lithium manganese iron phosphate material, characterized in that: The lithium manganese iron phosphate material includes, from the inside to the outside, a lithium manganese iron phosphate matrix core, a first coating layer, and a second coating layer; the first coating layer includes a carbon material, and the second coating layer includes a composite material, the composite material includes carbon nitride and a conductive polymer; the conductive polymer includes polypyrrole.

2. The lithium manganese iron phosphate material according to claim 1, characterized in that The D50 of the core of the lithium manganese iron phosphate matrix is ​​650~1050nm.

3. The lithium manganese iron phosphate material according to claim 1, characterized in that The thickness of the first cladding layer and the thickness of the second cladding layer are each independently 30 to 50 nm.

4. The lithium iron manganese phosphate material according to claim 1, characterized in that The carbon material includes a pyrolytic carbon material.

5. The lithium iron manganese phosphate material according to claim 1, characterized in that The core of the lithium manganese iron phosphate matrix is ​​also doped with carbon material.

6. The lithium iron manganese phosphate material according to claim 1, characterized in that The conductive polymer is in-situ grown on the surface of the carbon nitride.

7. A method for preparing the lithium manganese iron phosphate material according to any one of claims 1 to 6, characterized in that: The preparation method comprises the following steps: The lithium manganese iron phosphate precursor material and the carbon source are coated and mixed, and carbonized to obtain an intermediate material; Coating the intermediate material and the composite material to obtain the lithium manganese iron phosphate material; Wherein, the composite material comprises carbon nitride and a conductive polymer.

8. The preparation method according to claim 7, characterized in that The lithium manganese iron phosphate precursor material is prepared by a hydrothermal method.

9. The preparation method according to claim 8, characterized in that The hydrothermal method for preparing lithium manganese iron phosphate comprises: mixing a lithium source, a phosphorus source, an iron source and a manganese source, and performing a hydrothermal reaction to obtain the lithium manganese iron phosphate precursor material.

10. The preparation method according to claim 7, characterized in that The mass ratio of the precursor to the carbon source is 1:(0.05-0.3).

11. The preparation method according to claim 10, characterized in that: The mass ratio of the precursor to the carbon source is 1:(0.1-0.2).

12. The preparation method according to claim 7, characterized in that The coated mixed material is subjected to spray pyrolysis and carbonization to obtain the intermediate material.

13. The preparation method according to claim 7, characterized in that The composite material is obtained by in-situ growth of a conductive polymer on the surface of carbon nitride.

14. The preparation method according to claim 13, characterized in that During the in-situ growth process, the solute mass ratio of the conductive polymer monomer to the carbon nitride in the solution is 1:(100-120).

15. The preparation method according to claim 7, characterized in that The mass ratio of the intermediate material to the composite material is 1:(1-3).

16. The preparation method according to claim 7, characterized in that The coating treatment includes mixing the intermediate material with the composite material and then performing heat treatment, and the temperature of the heat treatment is 150-250°C.

17. The preparation method according to claim 7, characterized in that The coated product is spray granulated to obtain the lithium manganese iron phosphate material.

18. The preparation method according to claim 7, characterized in that The preparation method comprises the following steps: Mixing a lithium source, a phosphorus source, an iron source, and a manganese source, and subjecting them to a hydrothermal reaction to obtain a lithium manganese iron phosphate precursor; The lithium manganese iron phosphate precursor material and the carbon source are coated and mixed at a ratio of 1: (0.05-0.3), and spray pyrolysis and carbonization are performed to obtain an intermediate material; The intermediate material and the composite material are mixed in a mass ratio of 1:(1-3), heat-treated at 150-250° C., and spray-granulated to obtain the lithium manganese iron phosphate material; The composite material comprises carbon nitride and a conductive polymer, and the conductive polymer is in-situ grown on the surface of the carbon nitride.

19. A lithium ion battery, characterized in that: The lithium-ion battery comprises the lithium iron manganese phosphate material according to any one of claims 1 to 6 or the lithium iron manganese phosphate prepared by the preparation method according to any one of claims 7 to 18.

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