Preparation method of superfine lithium manganese iron phosphate nanoparticles embedded in three-dimensional porous carbon skeleton

Abstract

The invention provides a preparation method of superfine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional porous carbon skeleton, and belongs to the technical field of lithium ion batteries. The method comprises the following steps: uniformly mixing a lithium source, a phosphorus source, a manganese source, an iron source or a ferromanganese source according to a ratio, and carrying out heat treatment to obtain a target product. Wherein the composite carbon source is composed of high-molecular organic carbon and small-molecular organic carbon; the addition amount of the additive is 1-30 wt% of the lithium manganese iron phosphate. The particle size of the prepared superfine lithium manganese iron phosphate nano-particles is smaller than or equal to 500 nm, a short lithium ion migration path is achieved, and the three-dimensional continuous porous carbon skeleton not only establishes a continuous conductive network among nano-particles, but also can effectively prevent agglomeration of the nano-particles. In addition, the pore structure on the carbon skeleton is beneficial to electrolyte permeation and lithium ion diffusion, so that the conductivity of electrons and ions is greatly improved. The obtained material shows excellent electrochemical performance, and meanwhile, the preparation method is simple in process and easy for industrial large-scale production.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium - ion batteries, and particularly relates to a preparation method of ultrafine lithium iron manganese phosphate nanoparticles embedded in a three - dimensional porous carbon framework. Background Art

[0002] As a clean, efficient, and renewable secondary battery, lithium - ion batteries play a crucial role in the fields of modern energy storage and electric vehicles. As the core component of lithium - ion batteries, the performance of the cathode material directly affects the energy density, cycle stability, and safety of the battery. Lithium iron manganese phosphate (LMFP) has become one of the important lithium - ion cathode materials due to its high voltage platform, high energy density, and low cost. However, the further development of LMFP is limited by its low lithium - ion and electron diffusion coefficients, as well as the reduction of energy density due to voltage decay during cycling.

[0003] Carbon coating, as an effective modification method, can significantly improve the electrochemical performance of materials. Carbon coating not only improves the conductivity of materials but also prevents unnecessary oxidation and nanoparticle agglomeration during the sintering process. However, traditional dense carbon coating may limit the transport of lithium ions and increase the contact resistance between particles, thereby increasing the internal resistance of the battery and deteriorating the performance.

[0004] In previous studies, many research teams have actively explored the carbon coating of lithium iron manganese phosphate. Some focus on screening low - cost carbon sources, such as using natural biomass like lignin and cellulose. However, such biomass has complex compositions and unstable qualities, making it difficult to accurately control the structure of the coating layer. Some attempt to innovate the coating process, such as using chemical vapor deposition, sol - gel method, etc. However, the equipment and operation are complex, which is not conducive to industrial promotion. Summary of the Invention

[0005] The purpose of the present invention is to provide a preparation method of ultrafine lithium iron manganese phosphate nanoparticles embedded in a three - dimensional porous carbon framework. The obtained ultrafine lithium iron manganese phosphate nanoparticles with a particle size of ≤500 nm significantly shorten the migration path of lithium ions inside the material. The three - dimensional continuous porous carbon framework establishes a continuous conductive network between nanoparticles and can effectively prevent nanoparticle agglomeration. The pore structure on the carbon framework promotes the penetration of the electrolyte and the diffusion of lithium ions, realizing a significant improvement in the electronic and ionic conductivities. The obtained material has ultra - high rate performance, high specific capacity, energy density, and cycle stability.

[0006] To solve the above - mentioned technical problems, the technical solution adopted by the present invention is:

[0007] A preparation method of ultrafine lithium iron manganese phosphate nanoparticles embedded in a three - dimensional porous carbon framework, specifically including the following steps:

[0008] Weigh the lithium source, phosphorus source, manganese source, iron source or manganese-iron source according to the stoichiometric ratio, mix them evenly with the composite carbon source, and heat-treat the mixture to obtain ultrafine lithium iron manganese phosphate nanoparticles embedded in a three-dimensional continuous porous carbon framework.

[0009] Furthermore, in some preferred embodiments, the lithium source, phosphorus source, manganese source, iron source or manganese-iron source are weighed and mixed evenly with the composite carbon source. During the heat treatment process, the polymer and small molecule organic carbon condense to form derivatives. Subsequently, the raw materials decompose to produce gas to form pores in the derivatives. At the same time, lithium iron manganese phosphate crystallizes and grows. At high temperatures, the derivatives decompose to form a carbon coating layer and a carbon framework connecting the active particles and inhibit the overgrowth of lithium iron manganese phosphate to obtain ultrafine nanoparticles, thus obtaining the ultrafine lithium iron manganese phosphate nanoparticles embedded in a three-dimensional continuous porous carbon framework.

[0010] Furthermore, in some preferred embodiments, the composite carbon source is composed of a polymer and small molecule organic carbon, and the addition amount of the composite carbon source is 1 wt% - 30 wt% of lithium iron manganese phosphate.

[0011] Furthermore, in some preferred embodiments, the final carbon content of lithium iron manganese phosphate is 1 wt% - 15 wt%.

[0012] Furthermore, in some preferred embodiments, the particle size of the lithium iron manganese phosphate nanoparticles is ≤500 nm.

[0013] Furthermore, in some preferred embodiments, the chemical composition of lithium iron manganese phosphate is LiMn x Fe 1-x PO 4 , 0 < x < 1, the space group is Pnma, belonging to the olivine structure.

[0014] Furthermore, in some preferred embodiments, the heat treatment conditions are as follows: in an argon or nitrogen atmosphere, the heating rate is 1 - 10 °C / min to 200 - 400 °C, pre-burn for 1 - 6 h, then heat at 1 - 10 °C / min to 500 - 800 °C, hold for 4 - 20 h, and after sintering, cool naturally to room temperature.

[0015] Furthermore, in some preferred embodiments, the lithium source is one or more of lithium carbonate, oxalate, sulfate, nitrate, phosphate, acetylacetonate, citrate, hydroxide and oxide.

[0016] Furthermore, in some preferred embodiments, the phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, iron phosphate, lithium phosphate, lithium hydrogen phosphate, lithium dihydrogen phosphate.

[0017] Further, in some preferred embodiments, the manganese source is one or more of manganese carbonate, oxalate, sulfate, nitrate, phosphate, acetylacetonate, citrate;

[0018] the iron source is one or more of iron carbonate, oxalate, sulfate, nitrate, phosphate, acetylacetonate, citrate;

[0019] the manganese-iron source is one or more of manganese-iron carbonate, oxalate, sulfate, nitrate, phosphate, acetylacetonate, citrate.

[0020] Further, in some preferred embodiments, the high molecular organic carbon is one or more of polyethylene, polypropylene, polyvinylpyrrolidone, polystyrene, polyacetylene, polyethylene glycol, polypropylene glycol, polyvinyl alcohol, poly(ethylene oxide), polyaniline, phenolic resin, starch;

[0021] the low molecular organic carbon is one or more of glucose, sucrose, citric acid, maltose, fructose, lactose, ascorbic acid, oxalic acid, oleic acid.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The raw materials used in this method are low-cost, and there is a variety in the selection of the composite carbon source. By using a combination of multiple carbon sources, lithium iron manganese phosphate materials with special morphologies can be prepared, thereby specifically improving the cycle performance and rate performance of the materials to meet the requirements of different application scenarios. Compared with the complex process in the prior art of first synthesizing porous carbon materials or porous templates and then combining them with raw materials to form a composite continuous porous structure, this solution innovatively utilizes the dehydration, cyclization, polymerization and other reactions of the composite carbon source at high temperature to achieve self-assembly and in-situ formation of a three-dimensional continuous carbon skeleton as a self-template, without the need to additionally add porous carbon or template materials. The ultrafine lithium iron manganese phosphate nanoparticles embedded in the three-dimensional continuous porous carbon skeleton obtained by the present invention exhibit extremely excellent electrochemical performance due to the unique properties of the ultrafine nanoparticles and the synergistic effect of the three-dimensional continuous carbon skeleton and the porous structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and thus should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0025] Figure 1 For LiMn prepared in Example 1 0.6 Fe0.4 PO 4 TEM image of / C.

[0026] Figure 2 For the LiMn prepared in Example 1 0.6 Fe 0.4 PO 4 XRD pattern of / C.

[0027] Figure 3 For the LiMn prepared in Example 1 0.6 Fe 0.4 PO 4 Rate charge-discharge curves of / C.

[0028] Figure 4 For the LiMn prepared in Example 1 0.6 Fe 0.4 PO 4 / C at 5C (850 mAh g -1 ) room temperature (30 °C) cycle curve.

[0029] Figure 5 For the LiMn prepared in Example 1 0.6 Fe 0.4 PO 4 / C at 5C (850 mAh g -1 ) high temperature (60 °C) cycle curve.

[0030] Figure 6 SEM image of manganese iron oxalate dihydrate prepared in Example 2.

[0031] Figure 7 XRD pattern of manganese iron oxalate dihydrate prepared in Example 2.

[0032] Figure 8 For the LiMn prepared in Example 2 0.6 Fe 0.4 PO 4 SEM image of / C.

[0033] Figure 9 For the LiMn prepared in Example 2 0.6 Fe 0.4 PO 4 XRD pattern of / C.

[0034] Figure 10 For the LiMn prepared in Example 2 0.6 Fe 0.4 PO 4 Rate charge-discharge curves of / C.

[0035] Figure 11SEM images of LiMn 0.6 Fe 0.4 PO 4 / C prepared in Example 3.

[0036] Figure 12 SEM images of LiMn 0.6 Fe 0.4 PO 4 / C prepared in Comparative Example 1.

[0037] Figure 13 SEM images of LiMn 0.6 Fe 0.4 PO 4 / C prepared in Comparative Example 2. Detailed implementation manners

[0038] In the following, only some exemplary embodiments are briefly described. As those skilled in the art can recognize, the described embodiments can be modified in various different ways without departing from the spirit or scope of the embodiments of the present invention. Therefore, the drawings and the description are considered to be exemplary in nature rather than restrictive. The embodiments of the present invention will be described in detail below with reference to the drawings.

[0039] This embodiment discloses a preparation method of ultrafine lithium iron manganese phosphate nanoparticles embedded in a three-dimensional porous carbon framework, specifically a preparation method of ultrafine lithium iron manganese phosphate nanoparticles embedded in a three-dimensional continuous porous carbon framework, which specifically includes the following steps:

[0040] Weigh the lithium source, phosphorus source, manganese source, iron source or manganese-iron source according to the stoichiometric ratio, mix them evenly with the composite carbon source, and heat-treat the mixture to obtain ultrafine lithium iron manganese phosphate nanoparticles embedded in a three-dimensional continuous porous carbon framework.

[0041] Further, in some preferred embodiments, after weighing the lithium source, phosphorus source, manganese source, iron source or manganese-iron source and mixing them evenly with the composite carbon source, during the heat treatment, the polymer and small molecule organic carbon condense to form derivatives. Subsequently, the raw materials decompose to generate gases to form pores in the derivatives. At the same time, lithium iron manganese phosphate crystallizes and grows. At high temperatures, the derivatives decompose to form a carbon coating layer and a carbon framework connecting the active particles and inhibit the excessive growth of lithium iron manganese phosphate to obtain ultrafine nanoparticles, thus obtaining the ultrafine lithium iron manganese phosphate nanoparticles embedded in a three-dimensional continuous porous carbon framework.

[0042] Further, in some preferred embodiments, the composite carbon source is composed of a polymer and small molecule organic carbon, and the addition amount of the composite carbon source is 1 wt% - 30 wt% of lithium iron manganese phosphate.

[0043] Further, in some preferred embodiments, the final carbon content of lithium iron manganese phosphate is 1 wt% to 15 wt%.

[0044] Further, in some preferred embodiments, the particle size of lithium iron manganese phosphate nanoparticles is ≤500 nm.

[0045] Further, in some preferred embodiments, the chemical composition of lithium iron manganese phosphate is LiMn x Fe 1-x PO 4 , 0 < x < 1, with a space group of Pnma, belonging to the olivine structure.

[0046] Further, in some preferred embodiments, the heat treatment conditions are as follows: in an argon or nitrogen atmosphere, the heating rate is 1 - 10 °C / min to 200 - 400 °C, pre-burn for 1 - 6 h, then heat at 1 - 10 °C / min to 500 - 800 °C, hold for 4 - 20 h, and after sintering, cool naturally to room temperature.

[0047] Further, in some preferred embodiments, the lithium source is one or more of lithium carbonate, oxalate, sulfate, nitrate, phosphate, acetylacetonate, citrate, hydroxide, and oxide.

[0048] Further, in some preferred embodiments, the phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, iron phosphate, lithium phosphate, lithium hydrogen phosphate, and lithium dihydrogen phosphate.

[0049] Further, in some preferred embodiments, the manganese source is one or more of manganese carbonate, oxalate, sulfate, nitrate, phosphate, acetylacetonate, and citrate;

[0050] the iron source is one or more of iron carbonate, oxalate, sulfate, nitrate, phosphate, acetylacetonate, and citrate;

[0051] the manganese-iron source is one or more of manganese-iron carbonate, oxalate, sulfate, nitrate, phosphate, acetylacetonate, and citrate.

[0052] Further, in some preferred embodiments, the high molecular organic carbon is one or more of polyethylene, polypropylene, polyvinylpyrrolidone, polystyrene, polyacetylene, polyethylene glycol, polypropylene glycol, polyvinyl alcohol, poly(ethylene oxide), polyaniline, phenolic resin, and starch;

[0053] the small molecular organic carbon is one or more of glucose, sucrose, citric acid, maltose, fructose, lactose, ascorbic acid, oxalic acid, and oleic acid.

[0054] Among them, in some preferred embodiments, the composite carbon source is composed of a combination of high-molecular and low-molecular organic carbons, and the high-molecular and low-molecular organic carbons are mixed in a mass ratio of 1:9; 2:8; 3:7; 4:6; 5:5; 6:4; 7:3; 8:2 or 9:1.

[0055] In actual use, a composite carbon source formed by mixing is adopted, and then the mixed composite carbon source is added according to the mass of 1 wt% to 30 wt% as described above.

[0056] Furthermore, the material mixing method includes grinding mixing, ball milling mixing, and sand milling mixing. Using at least one of water, ethanol, and ethylene glycol as a dispersant, the grinding mixing time is 1 - 10 h, and then drying, spray drying, or vacuum drying is carried out.

[0057] This preparation method uniformly mixes a lithium source, a phosphorus source, a manganese source, an iron source, or a manganese-iron source with the composite carbon source according to the stoichiometric ratio; then the mixture is heat-treated to obtain ultrafine lithium iron manganese phosphate nanoparticles embedded in a three-dimensional continuous porous carbon framework; the obtained ultrafine lithium iron manganese phosphate nanoparticles with a particle size of ≤500 nm significantly shorten the migration path of lithium ions inside the material; at the same time, the three-dimensional continuous porous carbon framework establishes a continuous conductive network between the particles and can effectively prevent the aggregation of nanoparticles. The pore structure on the carbon framework promotes the penetration of the electrolyte and the diffusion of lithium ions, realizing a significant improvement in the electronic and ionic conductivities. The obtained material has ultra-high rate performance, relatively high specific capacity, energy density, and cycle stability, and is a potential cathode material for lithium-ion batteries.

[0058] To facilitate the further understanding of the present invention by those skilled in the art, the present invention will be further elaborated below in conjunction with specific embodiments.

[0059] Example 1

[0060] Lithium dihydrogen phosphate is mixed with ferrous oxalate dihydrate and manganese oxalate dihydrate in a molar ratio of 1:0.4:0.6. Then, 10% glucose and 10% polyethylene glycol based on the mass of the raw materials are added, and ethanol twice the mass ratio of the raw materials is added as a dispersant. Ball milling is carried out at 400 rpm for 6 h in a planetary ball mill, and then it is placed in an oven and dried overnight at 70 °C. The ball-milled product is placed in a corundum boat and heated to 300 °C at a heating rate of 5 °C / min for pre-sintering for 5 h. This step is to enable the hydroxyl groups on the polymer and small-molecule organic carbon to react and condense at this temperature to form derivatives. Subsequently, a large amount of carbon monoxide and carbon dioxide gases are released by the decomposition of the raw materials to form pores in the derivative; then it is heated to 700 °C at 5 °C / min and held for 6 h. This step is to allow the crystallization growth of lithium manganese iron phosphate, and then the derivative decomposes at this temperature to form a porous carbon coating layer and a carbon skeleton connecting the active particles and inhibit the excessive growth of lithium manganese iron phosphate to obtain ultrafine nanoparticles. Finally, it is cooled to room temperature to obtain ultrafine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional continuous porous carbon skeleton.

[0061] See Figure 1 , it can be seen that the particle size of the final product is ≤500 nm, and the pore structure can be clearly observed. At the same time, the particles are connected by a carbon skeleton to form a three-dimensional continuous and uninterrupted conductive network.

[0062] See Figure 2 , it can be seen that the final product is in a Pnma crystal structure, with good crystallinity, no impurity peaks, and no carbon-related peaks, indicating that the carbon is in an amorphous structure.

[0063] LiMn prepared in Example 1 0.6 Fe 0.4 PO 4 / C rate charge-discharge curves are as Figure 3 shown: Using ultrafine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional continuous mesoporous carbon skeleton can achieve high rate performance, and still contribute a specific capacity of more than 120 mAh / g at a high rate of 20C.

[0064] LiMn prepared in Example 1 0.6 Fe 0.4 PO 4 / C 5C (850 mAh g -1 ) room temperature (30 °C) cycle curves are as Figure 4 shown: Ultrafine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional continuous porous carbon skeleton have good cycle stability. After 500 charge-discharge cycles at a 5C rate, they still have 99% of the initial capacity.

[0065] LiMn prepared in Example 1 0.6 Fe 0.4 PO4 5C (850 mAh g of / C -1 ) high temperature (60 °C) cycle curve is as Figure 5 shown: The ultrafine lithium iron manganese phosphate nanoparticles embedded in the three-dimensional continuous porous carbon framework have good high-temperature cycle performance. After 500 charge-discharge cycles at a 5C rate, they still have 97% of the initial capacity.

[0066] Example 2

[0067] This example is basically the same as Example 1, except that in this example, ferrous oxalate dihydrate and manganese oxalate dihydrate are replaced with manganese iron oxalate dihydrate prepared by the coprecipitation method as the iron and manganese source. Therefore, the remaining steps and the use of the composite carbon source are the same as in Example 1. The specific steps for synthesizing manganese iron oxalate by the coprecipitation method are as follows:

[0068] First, weigh manganese sulfate monohydrate and ferrous sulfate heptahydrate according to a molar ratio of 6:4 and dissolve them in deionized water to form a homogeneous solution of 1 mol / L. Dissolve sodium oxalate in deionized water to form a precipitant of 0.5 mol / L. Then, under the premise of introducing argon as a protective gas, slowly drop the sodium oxalate solution into the metal sulfate solution and stir for 30 min to form a pale yellow precipitate of manganese iron oxalate. Then, after filtration, washing, and drying, manganese iron oxalate dihydrate is obtained.

[0069] See Figures 6 - 10 , Figure 6 which is the SEM image of the manganese iron oxalate dihydrate synthesized in Example 2, showing the microscopic morphology of the material.

[0070] Figure 7 is the XRD pattern of manganese iron oxalate dihydrate, proving the successful synthesis of manganese iron oxalate dihydrate.

[0071] Figure 8 is the SEM image of the ultrafine lithium iron manganese phosphate nanoparticles embedded in the three-dimensional continuous porous carbon framework synthesized using the synthesized manganese iron oxalate dihydrate as the iron and manganese source, proving that the raw materials can also be replaced with manganese iron oxalate dihydrate.

[0072] Figure 9 is the XRD pattern of the ultrafine lithium iron manganese phosphate nanoparticles embedded in the three-dimensional continuous porous carbon framework synthesized using the synthesized manganese iron oxalate dihydrate as the iron and manganese source, proving the successful synthesis of lithium iron manganese phosphate.

[0073] Figure 10 It shows that the ultrafine lithium iron manganese phosphate nanoparticles embedded in the three-dimensional continuous porous carbon framework can achieve high rate performance and still contribute a specific capacity of more than 125 mAh / g at a high rate of 20C.

[0074] Example 3

[0075] This example is basically similar to Example 1, and the difference lies in that: in this example, the composition of the composite carbon source is changed to 5% sucrose and 5% polyvinylpyrrolidone by mass of the raw materials, and the remaining steps are the same as those in Example 1.

[0076] See Figure 11 , Figure 11 It is proved that ultrafine lithium iron manganese phosphate nanoparticles embedded in a three-dimensional continuous porous carbon framework can also be obtained by adjusting the composition of the carbon source, which once again proves the feasibility of this method.

[0077] Example 4

[0078] In this example, lithium iron manganese phosphate is synthesized by a solid-phase method using lithium carbonate as the lithium source.

[0079] Lithium carbonate, ferrous carbonate, manganese carbonate, and ammonium dihydrogen phosphate are mixed evenly according to the stoichiometric ratio of 1:0.5:0.5:1, and then 10% glucose and 10% polyethylene glycol by mass of the raw materials are added and ball-milled at 400 rpm for 10 h. After drying, the product is placed in a corundum boat and heated to 350 °C at a rate of 10 °C / min for pre-sintering for 3 h, and then heated to 650 °C at a rate of 5 °C / min and held for 10 h. Finally, it is naturally cooled to room temperature to obtain LiMn 0.5 Fe 0.5 PO 4 / C.

[0080] Comparative Example 1

[0081] Compared with Example 1, the difference in this example is that only the composite carbon source is replaced with 20% glucose by mass of the raw materials as a single carbon source, and the remaining steps are the same.

[0082] See Figure 12 , Figure 12 It shows that only using a single small-molecule carbon source cannot synthesize ultrafine lithium iron manganese phosphate nanoparticle materials embedded in a three-dimensional continuous porous carbon framework.

[0083] Comparative Example 2

[0084] Compared with Example 1, the difference in this example is that only the composite carbon source is replaced with 20% polyethylene glycol by mass of the raw materials as a single carbon source, and the remaining steps are the same.

[0085] See Figure 13 , Figure 13 It shows that only using a single high-molecular carbon source cannot synthesize ultrafine lithium iron manganese phosphate nanoparticle materials embedded in a three-dimensional continuous porous carbon framework.

[0086] The electrochemical performance of the battery material at 2C is shown in the following table:

[0087]

[0088] As can be seen from Table 1, compared with the comparative examples, the initial discharge specific capacity, initial Coulombic efficiency, and cycle stability of all the examples are improved, indicating that using polymer and small molecule organic carbon as composite carbon sources can significantly enhance the capacity and cycle stability of the battery, and has greater advantages compared to using any single carbon source. At the same time, due to the low cost and simple process of this method, industrial scale-up can be easily achieved.

[0089] The present invention uses polymer and small molecule organic carbon as composite carbon sources, uses manganese salts and iron salts that can decompose to produce gases such as carbon dioxide as manganese-iron sources, mixes them with lithium sources and phosphorus sources, and obtains lithium iron phosphate manganese with a special carbon composite structure through heat treatment. The composite carbon source undergoes dehydration, cyclization, polymerization and other reactions during heat treatment to form derivatives. As the reaction progresses, a three-dimensional continuous carbon network is formed. Subsequently, lithium iron phosphate manganese nucleates and grows on this structure. The gases generated by the decomposition of the manganese-iron source will create pores in the carbon network, and further carbonization at high temperature results in ultrafine lithium iron phosphate manganese nanoparticles embedded in the three-dimensional continuous porous carbon framework. The ultrafine nanoparticles have a short lithium ion diffusion path, and the porous carbon network can enhance the transport ability of electrons and lithium ions, improving the rate performance and cycle stability of lithium iron phosphate manganese. The method disclosed in the present invention is simple to operate, low in cost, environmentally friendly and has good repeatability, and can meet industrial large-scale production.

[0090] Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications to these embodiments once they know the basic creative concept. Therefore, the appended claims are intended to be construed as including the preferred embodiments and all changes and modifications falling within the scope of the present invention.

[0091] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. It should be noted that any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing ultrafine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional porous carbon framework, characterized in that: The specific steps include: The lithium source, phosphorus source, manganese source, iron source or manganese-iron source are weighed according to a stoichiometric ratio and evenly mixed with a composite carbon source, and the mixture is heat-treated to obtain ultrafine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional continuous porous carbon skeleton.

2. The method for preparing ultrafine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional porous carbon framework according to claim 1, characterized in that: The lithium source, phosphorus source, manganese source, iron source or manganese iron source are weighed and evenly mixed with the composite carbon source. During the heat treatment process, the polymer and small molecule organic carbon are condensed to form derivatives, and then the raw materials are thermally decomposed to generate gas to form pores in the derivatives. At the same time, lithium manganese iron phosphate crystals grow. At high temperature, the derivatives decompose to form a carbon coating layer and a carbon skeleton connecting the active particles and inhibiting the excessive growth of lithium manganese iron phosphate to obtain ultrafine nanoparticles, and finally the ultrafine lithium manganese iron phosphate nanoparticles embedded in the three-dimensional continuous porous carbon skeleton are obtained.

3. The method for preparing ultrafine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional porous carbon framework according to claim 1, characterized in that: The composite carbon source is composed of high molecular weight and small molecular weight organic carbon. The added amount of the composite carbon source is 1wt% to 30wt% of the lithium manganese iron phosphate, and the final carbon content of the lithium manganese iron phosphate is 1wt% to 15wt%.

4. The method for preparing ultrafine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional porous carbon framework according to claim 1 or 3, characterized in that: The particle size of the lithium manganese iron phosphate nanoparticles is ≤500nm.

5. The method for preparing ultrafine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional porous carbon framework according to claim 1, characterized in that: The chemical composition of lithium manganese iron phosphate is LiMn x Fe 1-x PO4, where 0 < x < 1, and the space group is Pnma, belonging to the olivine structure.

6. The method for preparing ultrafine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional porous carbon framework according to claim 1, characterized in that: The mixing method adopts high-energy ball milling, planetary ball milling, nano sand milling, etc. The heat treatment conditions are: in argon or nitrogen atmosphere, the heating rate is 1-10℃ / min to 200-400℃, pre-sintering for 1-6h, 1-10℃ / min to 500-800℃, heat preservation for 4-20h, and naturally cool to room temperature after sintering.

7. The method for preparing ultrafine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional porous carbon framework according to claim 1, characterized in that: The lithium source is one or more of lithium carbonate, oxalate, sulfate, nitrate, phosphate, acetylacetonate, citrate, hydroxide and oxide.

8. The method for preparing ultrafine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional porous carbon framework according to claim 1, characterized in that: The phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, iron phosphate, lithium phosphate, lithium monohydrogen phosphate, and lithium dihydrogen phosphate.

9. The method for preparing ultrafine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional porous carbon framework according to claim 1, characterized in that: The manganese source is one or more of manganese carbonate, oxalate, sulfate, nitrate, phosphate, acetylacetonate and citrate; The iron source is one or more of iron carbonate, oxalate, sulfate, nitrate, phosphate, acetylacetonate, and citrate; The ferromanganese source is one or more of ferromanganese carbonate, oxalate, sulfate, nitrate, phosphate, acetylacetonate and citrate.

10. The method for preparing ultrafine lithium manganese iron phosphate nanoparticles embedded in a three-dimensional porous carbon framework according to claim 1, characterized in that: The high molecular organic carbon is one or more of polyethylene, polypropylene, polyvinyl pyrrolidone, polystyrene, polyacetylene, polyethylene glycol, polypropylene glycol, polyvinyl alcohol, polyethylene oxide, polyaniline, phenolic resin and starch; The small molecule organic carbon is one or more of glucose, sucrose, citric acid, maltose, fructose, lactose, ascorbic acid, oxalic acid, and oleic acid.

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