Lithium iron manganese phosphate composite material, preparation method and application thereof

Abstract

The application relates to the technical field of lithium ion battery positive electrode materials, in particular to a lithium manganese iron phosphate composite material and a preparation method and application thereof; the lithium manganese iron phosphate composite material is provided, which comprises a lithium manganese iron phosphate core containing doped particles and a biomass-derived carbon coating layer coated on the surface of the lithium manganese iron phosphate core, wherein the doped particles comprise mixed particles of nano gold and nano silver. The obtained lithium manganese iron phosphate composite material can effectively reduce the resistance of the material, can enhance the stability of the material, can improve the conductivity of the material and the diffusion coefficient of Li ions to a certain extent, and can improve the cycle performance of the material.

Description

Technical Field

[0001] This application belongs to the field of lithium-ion battery cathode material technology, and particularly relates to a lithium manganese iron phosphate composite material, its preparation method and application. Background Technology

[0002] Lithium-ion batteries are rechargeable batteries that primarily function by the movement of lithium ions between the positive and negative electrodes. During charging and discharging, Li... + Intercalation and deintercalation between the two electrodes: During charging, Li... + The lithium is extracted from the positive electrode, inserted into the negative electrode through the electrolyte, and the negative electrode is in a lithium-rich state; the opposite occurs during discharge.

[0003] Lithium-ion batteries use carbon materials as the negative electrode and lithium-containing compounds as the positive electrode. There is no metallic lithium present, only lithium ions. The charging and discharging process of a lithium-ion battery is essentially the process of lithium ion insertion and extraction. During this process, an equivalent number of electrons are simultaneously inserted and extracted (conventionally, the positive electrode uses insertion / extraction, while the negative electrode uses insertion / extraction). During charging and discharging, lithium ions shuttle back and forth between the positive and negative electrodes, a process figuratively called a "rocking chair battery." When the battery is charged, lithium ions are generated at the positive electrode and move through the electrolyte to the negative electrode. The carbon used as the negative electrode has a layered structure with many micropores. The lithium ions that reach the negative electrode are inserted into these micropores; the more lithium ions inserted, the higher the charging capacity. Similarly, when the battery is discharged (i.e., when we use the battery), the lithium ions embedded in the carbon layer of the negative electrode are extracted and move back to the positive electrode. The more lithium ions that return to the positive electrode, the higher the discharge capacity.

[0004] Lithium iron phosphate (LFP) is a battery cathode material with an olivine structure. It is widely used due to its low cost, good safety, and long cycle life; however, its low energy density has limited its further development. Lithium manganese iron phosphate (LMFP), prepared by introducing manganese into LFP, has a higher voltage plateau (4.1V), thus exhibiting significantly better energy density than LFP. It inherits the advantages of LFP while addressing its shortcomings, becoming an important upgrade direction for LFP. However, LMFP also suffers from low conductivity and poor cycle performance, hindering its widespread application. Summary of the Invention

[0005] The purpose of this application is to provide a lithium manganese iron phosphate composite material, its preparation method and application, which aims to solve the problems of low conductivity and poor cycle performance of lithium manganese iron phosphate in the prior art.

[0006] To achieve the above-mentioned objectives, the technical solution adopted in this application is as follows:

[0007] In a first aspect, this application provides a lithium manganese iron phosphate composite material, which includes a lithium manganese iron phosphate core containing doped particles and a biomass-derived carbon coating layer covering the surface of the lithium manganese iron phosphate core, wherein the doped particles include a mixture of nano-gold and nano-silver particles.

[0008] Secondly, this application provides a method for preparing a lithium manganese iron phosphate composite material, comprising the following steps:

[0009] Precursor preparation: After mixing lithium source, phosphorus source, iron source, manganese source and doped particle source, the mixture is subjected to homogenization treatment, ball milling treatment, spray drying treatment and pulverization treatment in sequence to obtain lithium manganese iron phosphate precursor containing doped particles;

[0010] Pre-sintering: Provide biomass carbon source, pre-sinter the lithium manganese iron phosphate precursor containing doped particles and the biomass carbon source respectively, then mix them and perform the first sand milling process to obtain the first sintered product.

[0011] Secondary sintering: An inert atmosphere is provided, and the product from the first sintering is mixed with a reducing agent and then subjected to a secondary sintering process to obtain the product from the second sintering process.

[0012] Product preparation: The biomass carbon source is acid-dissolved to obtain acid-dissolved biomass carbon source. The secondary calcination product and the acid-dissolved biomass carbon source are mixed and subjected to a second sand milling process, followed by post-processing to obtain lithium manganese iron phosphate composite material.

[0013] Thirdly, this application provides a secondary battery, including a positive electrode, a negative electrode, and a separator stacked between the positive electrode and the negative electrode. The positive electrode includes the above-mentioned lithium manganese iron phosphate composite material or is prepared by the above-mentioned method for preparing lithium manganese iron phosphate composite material.

[0014] The first aspect of this application provides a lithium manganese iron phosphate composite material, comprising a lithium manganese iron phosphate core containing doped particles and a biomass-derived carbon coating layer covering the surface of the lithium manganese iron phosphate core. The doped particles include a mixture of nano-gold and nano-silver particles. Firstly, the biomass-derived carbon coating layer retains good biomass characteristics; the reinforced concrete structure composed of lignin, cellulose, and hemicellulose is preserved during sintering, forming a uniform and dense carbon coating layer on the surface of the lithium manganese iron phosphate. This can, to a certain extent, increase the specific surface area and resistivity of the material, while also reducing material costs and achieving efficient resource utilization. Secondly, the lithium manganese iron phosphate core is doped with a mixture of nano-gold and nano-silver particles. These doped particles can significantly reduce the material's resistivity and improve its conductivity, resulting in superior electrochemical performance. Furthermore, nano-gold and nano-silver are very stable metals, which can, to a certain extent, reduce the decline in electrochemical performance and structural collapse caused by the dissolution of transition metals such as manganese and iron. This improves the conductivity and cycle performance of the lithium manganese iron phosphate composite material.

[0015] The second aspect of this application provides a method for preparing lithium manganese iron phosphate composite materials. This method involves first preparing a precursor core material. The precursor undergoes pressure homogenization to ensure uniform mixing of raw materials, effectively reducing the particle size of the precursor and promoting better formation of a lithium manganese iron phosphate eutectic, thus improving the electrochemical performance of the material. Next, a biomass carbon source and the precursor material undergo two sintering processes. The conditions of each step in the two sintering processes are controlled to manage the particle size changes during the first and second sintering stages, while also reducing secondary particle agglomeration and increasing the number of primary particles, thereby improving the material's processing performance. Carbon is added after the second sintering to finally obtain the lithium manganese iron phosphate composite material. This carbon addition after the second sintering can reduce the loss of the carbon coating layer during ball milling and sand milling, and can also control the number of secondary particles to reduce agglomeration and improve conductivity. This preparation method is simple, does not require large-scale equipment, and is conducive to large-scale applications.

[0016] The secondary battery provided in the third aspect of this application has a positive electrode comprising the aforementioned lithium manganese iron phosphate composite material or prepared by the aforementioned method for preparing lithium manganese iron phosphate composite material. Since the provided lithium manganese iron phosphate composite material has excellent conductivity and cycle performance, it improves the overall electrochemical performance of the battery and is beneficial for its widespread use. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is an electron micrograph of the lithium manganese iron phosphate composite material provided in Example 1 of this application. Detailed Implementation

[0019] To make the technical problems, technical solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0020] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0021] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0022] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0023] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms "a" and "the" as used in the embodiments of this application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0024] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass in the embodiments of this application can be a well-known unit of mass in the chemical industry, such as μg, mg, g, or kg.

[0025] The terms "first" and "second" are used for descriptive purposes only, to distinguish objects, such as substances, from one another, and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. For example, without departing from the scope of the embodiments of this application, "first XX" may also be referred to as "second XX," and similarly, "second XX" may also be referred to as "first XX." Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of that feature.

[0026] The first aspect of this application provides a lithium manganese iron phosphate composite material, which includes a lithium manganese iron phosphate core containing doped particles and a biomass-derived carbon coating layer covering the surface of the lithium manganese iron phosphate core, wherein the doped particles include a mixture of nano-gold and nano-silver particles.

[0027] The lithium manganese iron phosphate composite material provided in the first aspect of this application includes a lithium manganese iron phosphate core containing doped particles and a biomass-derived carbon coating layer covering the surface of the lithium manganese iron phosphate core. The doped particles include a mixture of nano-gold and nano-silver particles. On one hand, the provided biomass-derived carbon coating layer retains good biomass characteristics. The reinforced concrete structure composed of lignin, cellulose, and hemicellulose is preserved during sintering, forming a uniform and dense carbon coating layer on the surface of the lithium manganese iron phosphate. This can improve the specific surface area and resistivity of the material to a certain extent, while also reducing material costs and achieving efficient resource utilization. On the other hand, the lithium manganese iron phosphate core is doped with a mixture of nano-gold and nano-silver particles. These doped particles can significantly reduce the material's resistance and improve its conductivity, resulting in superior electrochemical performance. Furthermore, nano-gold and nano-silver are very stable metals, which can reduce the decline in electrochemical performance and structural collapse caused by the dissolution of transition metals such as manganese and iron to a certain extent, thereby improving the conductivity and cycle performance of the lithium manganese iron phosphate composite material.

[0028] In some embodiments, the lithium manganese iron phosphate composite material includes a lithium manganese iron phosphate core containing doped particles, wherein the doped particles include a mixture of nano-gold and nano-silver particles. The lithium manganese iron phosphate core is doped with the mixture of nano-gold and nano-silver particles. These doped particles can significantly reduce the material's resistance and improve its conductivity, thereby resulting in superior electrochemical performance. Furthermore, nano-gold and nano-silver are very stable metals, which can, to some extent, reduce the decline in electrochemical performance and structural collapse caused by the dissolution of transition metals such as manganese and iron.

[0029] In some embodiments, the doping amount of the dopant particles is 0.001 to 1%. If the doping amount of the dopant particles is too high, it will lead to increased costs, an increase in impurity phases in the product, or substitution of Li sites, which will have an adverse effect on energy efficiency and cycle life. If the doping amount of the dopant particles is too low, it will not be conducive to improving the conductivity of the material, nor will it be conducive to maintaining the electrochemical performance and structural stability of the material.

[0030] In some specific embodiments, the doping amount of the doped particles includes, but is not limited to, 0.001%, 0.005%, 0.01%, 0.05%, 0.1%, 0.5%, and 1%.

[0031] In some embodiments, the mass ratio of gold nanoparticles to silver nanoparticles is 1:0.5 to 3.5. Controlling the mass ratio of gold nanoparticles to silver nanoparticles to this ratio aims to ensure that, at a certain proportion, the structure is more stable and beneficial to improving electrochemical performance.

[0032] In some specific embodiments, the mass ratio of nano-gold to nano-silver includes, but is not limited to, 1:0.5, 1:1, 1:1.5, 1:2, 1:2.5, 1:3, and 1:3.5.

[0033] In some embodiments, the D50 particle size of the lithium manganese iron phosphate core containing doped particles is 0.5 μm-2.0 μm. If the particle size of the lithium manganese iron phosphate core containing doped particles is too large, it will lead to a decrease in compaction density and energy efficiency. If the particle size of the lithium manganese iron phosphate core containing doped particles is too small, it will lead to poor processing performance.

[0034] Furthermore, the lithium manganese iron phosphate composite material also includes a biomass-derived carbon coating layer covering the surface of the lithium manganese iron phosphate core. The provided biomass-derived carbon coating layer retains good biomass characteristics. The reinforced concrete structure composed of lignin, cellulose and hemicellulose is retained during the sintering process. A uniform and dense carbon coating layer can be formed on the surface of lithium manganese iron phosphate, which can improve the specific surface area and resistivity of the material to a certain extent, while also reducing the material cost to a certain extent, and achieving effective utilization of resources.

[0035] In some embodiments, the raw materials for the biomass-derived carbon coating include at least one of branches, sawdust, wood chips, tips, bark, cut ends, residual branches, and leaves from the timber harvesting, transportation, and processing.

[0036] In some embodiments, the thickness of the carbon coating layer is 2.5–5.0 nm. If the carbon coating layer is too thin, it is not conducive to forming a dense protective film on the core surface, which will not improve the specific surface area and resistivity of the material, and it will also be not conducive to isolating the core from moisture and carbon dioxide in the air; if the carbon coating layer is too thick, it will affect the release of lithium ions in the core material and affect the performance of the lithium manganese iron phosphate composite material.

[0037] In some specific embodiments, the thickness of the carbon coating layer includes, but is not limited to, 2.5nm, 2.6nm, 2.7nm, 2.8nm, 2.9nm, 3.0nm, 3.1nm, 3.2nm, 3.3nm, 3.4nm, 3.5nm, 3.6nm, 3.7nm, 3.8nm, 3.9nm, 4.0nm, 4.1nm, 4.2nm, 4.3nm, 4.4nm, 4.5nm, 4.6nm, 4.7nm, 4.8nm, 4.9nm, and 5.0nm.

[0038] In some embodiments, the particle size of the lithium manganese iron phosphate composite material is 0.5–2.5 μm. In some specific embodiments, the particle size of the lithium manganese iron phosphate composite material includes, but is not limited to, 0.5 μm, 1 μm, 1.5 μm, 2 μm, and 2.5 μm.

[0039] The second aspect of this application provides a method for preparing a lithium manganese iron phosphate composite material, comprising the following steps:

[0040] S01. Precursor preparation: After mixing lithium source, phosphorus source, iron source, manganese source and doped particle source, the mixture is subjected to homogenization treatment, ball milling treatment, spray drying treatment and pulverization treatment in sequence to obtain lithium manganese iron phosphate precursor containing doped particles;

[0041] S02. Pre-sintering: Provide biomass carbon source, pre-sinter the lithium manganese iron phosphate precursor containing doped particles and the biomass carbon source respectively, then mix them and perform the first sand milling process to obtain the first sintered product.

[0042] S03. Secondary sintering: An inert atmosphere is provided to mix the first-calcined product and a reducing agent and then perform a second calcination treatment to obtain the second-calcined product;

[0043] S04. Product preparation: The biomass carbon source is acid-dissolved to obtain acid-dissolved biomass carbon source. The secondary calcination product and the acid-dissolved biomass carbon source are mixed and subjected to a second sand milling process, followed by post-processing to obtain lithium manganese iron phosphate composite material.

[0044] The second aspect of this application provides a method for preparing lithium manganese iron phosphate composite materials. This method involves first preparing a precursor core material. The precursor undergoes pressure homogenization to ensure uniform mixing of raw materials, effectively reducing the precursor particle size and promoting better formation of a lithium manganese iron phosphate eutectic, thus improving the material's electrochemical performance. Next, a biomass carbon source and the precursor material undergo two sintering processes. The conditions of each step in the two sintering processes are controlled to manage particle size changes during the first and second sintering stages, while also reducing secondary particle agglomeration and increasing the number of primary particles, thereby improving the material's processing performance. Carbon is added after the second sintering to finally obtain the lithium manganese iron phosphate composite material. This carbon addition after the second sintering reduces the loss of the carbon coating layer during ball milling and sand milling, and also controls the number of secondary particles to reduce agglomeration, improving conductivity. This preparation method is simple, does not require large-scale equipment, and is beneficial for large-scale applications.

[0045] The components are provided according to the lithium manganese iron phosphate composite material. The selection and addition amount of each component of the provided lithium manganese iron phosphate composite material are as discussed above, and will not be repeated here to save space.

[0046] In step S01, the precursor is prepared by mixing lithium source, phosphorus source, iron source, manganese source and doped particle source, and then performing homogenization treatment, ball milling treatment, spray drying treatment and pulverization treatment in sequence to obtain lithium manganese iron phosphate precursor containing doped particles.

[0047] In some embodiments, the molar ratio of lithium source, phosphorus source, manganese source, and iron source is 0.05–1.55: 0.05–1.55: 0.3–0.8: 0.3–0.8. The molar ratio of each metal source is further determined based on the prepared lithium manganese iron phosphate precursor to ensure that the lithium manganese iron phosphate precursor can be prepared.

[0048] In some embodiments, the lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium citrate, lithium phosphate, lithium sulfate, lithium perchlorate, and lithium chloride.

[0049] In some embodiments, the iron source includes at least one of ferrous phosphate, ferric phosphate, ferric chloride, ferric sulfate, and ferrous sulfate.

[0050] In some embodiments, the manganese source includes at least one of manganese sulfate, manganese dihydrogen phosphate, manganese oxalate, manganese carbonate, manganese dioxide, manganese oxide, manganese nitrate, and manganese sulfate.

[0051] In some embodiments, the phosphorus source includes at least one of phosphoric acid, pyrophosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ferric phosphate.

[0052] In some embodiments, the dopant particle source is selected from a mixed solution of silver nanoparticles and gold nanoparticles, and the concentration of the mixed solution is 100–10000 ppm, wherein the molar ratio of silver nanoparticles to gold nanoparticles is 1:0.5–3.5. Controlling the concentration of the mixed solution of silver nanoparticles and gold nanoparticles to 100–10000 ppm ensures that the doping amount of the dopant particles in the obtained lithium manganese iron phosphate precursor is appropriate, ensuring that the obtained material has significantly reduced resistance and improved conductivity, thereby resulting in superior electrochemical performance.

[0053] In some specific embodiments, the concentration of the mixed solution of nano-silver and nano-gold includes, but is not limited to, 100ppm, 500ppm, 1000ppm, 1500ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, 5500ppm, 6000ppm, 6500ppm, 7000ppm, 7500ppm, 8000ppm, 8500ppm, 9000ppm, 9500ppm, and 10000ppm.

[0054] Furthermore, after mixing the lithium source, phosphorus source, iron source, manganese source and doped particle source, the mixture is subjected to homogenization treatment, ball milling treatment, spray drying treatment and pulverization treatment in sequence.

[0055] In some embodiments, the lithium source, phosphorus source, iron source, manganese source, and dopant particle source are mixed and then homogenized. By employing pressure homogenization, the raw materials are mixed uniformly, effectively reducing the particle size of the precursor, and simultaneously enabling better formation of the lithium manganese iron phosphate eutectic, thereby improving the electrochemical performance of the material.

[0056] In some embodiments, the homogenization process is performed under a pressure of 10–40 MPa for 30–150 minutes. Excessive homogenization time can lead to material aggregation, while insufficient homogenization time can lead to uneven dispersion.

[0057] Furthermore, after homogenization, ball milling is performed. Ball milling allows for more uniform mixing of the components and further control of the particle size of the material. In some embodiments, the ball milling time is 0.5 to 2.5 hours.

[0058] Furthermore, after ball milling, spray drying is performed. In some embodiments, the spray drying conditions are as follows: a flow rate of 0.2–1.4 mL / min, a temperature of 120–220 °C, and a pressure of 0.01–0.8 MPa.

[0059] Furthermore, after spray drying, the particles are then pulverized. In some embodiments, the particle size obtained from the pulverization process is less than or equal to 200 mesh.

[0060] In step S02, pre-sintering: a biomass carbon source is provided, and the lithium manganese iron phosphate precursor containing doped particles and the biomass carbon source are pre-sintered separately, then mixed and subjected to a first sand milling process to obtain a first-sintered product.

[0061] In some embodiments, the biomass carbon source includes at least one of branches, sawdust, wood chips, tops, bark, cut ends, residual branches, and leaves from the timber harvesting, transportation, and processing.

[0062] Furthermore, the lithium manganese iron phosphate precursor containing doped particles and the biomass carbon source are pre-sintered separately. The purpose of sintering them separately is to make the lithium manganese iron phosphate precursor containing doped particles more stable, while the biomass carbon source can first form amorphous carbon, which can then better coat the surface of the lithium manganese iron phosphate precursor containing doped particles; if they are sintered together, uneven coating will occur.

[0063] In some embodiments, the pre-sintering temperature is 400–600°C. In some specific embodiments, the optimal pre-sintering temperature for the lithium manganese iron phosphate precursor containing doped particles is 550°C, and the optimal pre-sintering temperature for the biomass carbon source is also 550°C. The optimal pre-sintering temperatures for the two materials are similar, and the sintering temperature for the biomass carbon source can be slightly higher; however, if the pre-sintering temperature is too high, the particle size will be relatively large, and if the pre-sintering temperature is too low, crystals cannot be formed.

[0064] In some embodiments, the pre-sintering process is carried out in an inert atmosphere. In some embodiments, the inert atmosphere is selected from at least one of Ar, N2, and He atmospheres.

[0065] Further, after pre-sintering treatment, the products are mixed and then subjected to a first sand milling process to obtain a first-fired product.

[0066] In some embodiments, the mass ratio of the doped lithium manganese iron phosphate precursor to the biomass carbon source is 1:0.0005 to 0.025; controlling the addition ratio of the two ensures the formation of a coating layer of appropriate thickness. If the carbon source content is too low, the coating layer will be too thin or uneven, resulting in little improvement in conductivity; if the carbon source content is too high, the coating layer will affect the processing performance of the material, leading to a low compaction density and a larger particle size, which is not conducive to use.

[0067] In some embodiments, the mixture undergoes a first milling process to obtain a sintered product. The conditions for the first milling process are: 100 to 400 milling cycles using zirconium beads with a particle size of 0.1–10 mm. Insufficient first milling cycles will result in a large number of large particles before the second sintering, which will easily agglomerate after the second sintering, and the particle size distribution will be very uneven.

[0068] In step S03, secondary sintering: an inert atmosphere is provided, and the product from the first sintering is mixed with a reducing agent and subjected to a second sintering process to obtain the product from the second sintering.

[0069] In some embodiments, the inert atmosphere is selected from at least one of Ar atmosphere, N2 atmosphere, and He atmosphere.

[0070] Furthermore, the first-calcined product and a reducing agent are mixed and then subjected to a second calcination treatment to obtain a second-calcined product. The second calcination treatment primarily enables secondary crystal growth, and the reducing agent is added to prevent oxidation of the material at high temperatures. In some specific embodiments, the reducing agent includes, but is not limited to, ethanol.

[0071] In some embodiments, the temperature of the second sintering process is 600–900°C. If the temperature of the second sintering process is too low, the crystallization effect is poor, the surface carbon layer is uneven and not dense, the graphitized carbon content is too low, and the conductivity is poor. If the temperature of the second sintering process is too high, the operation is dangerous, the surface carbon layer is prone to cracking, which is not conducive to product molding.

[0072] In step S04, product preparation involves acid dissolution of the biomass carbon source to obtain acid-dissolved biomass carbon source. The calcined product and the acid-dissolved biomass carbon source are then mixed and subjected to a second sand milling process, followed by post-processing to obtain a lithium manganese iron phosphate composite material. Carbon replenishment after the second calcination process ultimately yields the lithium manganese iron phosphate composite material. This carbon replenishment after the second calcination process can, to some extent, reduce the loss of the carbon coating layer during ball milling and sand milling, and can also, to some extent, control the number of secondary particles, reduce agglomeration, and improve conductivity.

[0073] In some embodiments, the step of obtaining acid-dissolved biomass carbon source by acid dissolution treatment includes: providing a nitric acid solution with a mass percentage of 5% to 30%, mixing the biomass carbon source and the nitric acid solution and dissolving them, then heating the mixture at 80 to 150°C, and then filtering it to obtain the acid-dissolved biomass carbon source.

[0074] Furthermore, the secondary calcined product and the acid-dissolved biomass carbon source are mixed and then subjected to a second sand milling process.

[0075] In some embodiments, the mass ratio of the dicalcined product to the acid-dissolved biomass carbon source is 1:0.2 to 0.5. Controlling the mass ratio of the dicalcined product to the acid-dissolved biomass carbon source allows for control of the carbon coating thickness of the resulting lithium manganese iron phosphate composite material, ensuring a suitable carbon coating thickness, which is beneficial for improving the material's conductivity and cycle performance.

[0076] Further, a second sanding process is performed. In some embodiments, the conditions for the second sanding process are: 100 to 200 sanding cycles using zirconium beads with a particle size of 0.1 to 10 mm.

[0077] Further, after the second grinding process, a post-processing is performed to obtain the lithium manganese iron phosphate composite material. In some embodiments, the post-processing includes: spray drying followed by pulverization and passing through a 200-mesh sieve to obtain the lithium manganese iron phosphate composite material.

[0078] In some embodiments, the spray drying conditions are as follows: spray drying is performed at a flow rate of 0.2 to 1.4 mL / min, a temperature of 120 to 220 °C, and a pressure of 0.01 to 0.8 MPa.

[0079] A third aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, and a separator stacked between the positive and negative electrodes. The positive electrode includes a lithium manganese iron phosphate composite material or is prepared by a method for preparing a lithium manganese iron phosphate composite material.

[0080] The secondary battery provided in the third aspect of this application has a positive electrode comprising the above-mentioned lithium manganese iron phosphate composite material or prepared by the above-mentioned method for preparing lithium manganese iron phosphate composite material. Since the provided lithium manganese iron phosphate composite material has excellent conductivity and cycle performance, it improves the overall electrochemical performance of the battery and is beneficial for its widespread use.

[0081] The following description is based on specific embodiments.

[0082] Example 1

[0083] Lithium manganese iron phosphate composite materials and their preparation methods

[0084] The lithium manganese iron phosphate composite material includes a lithium manganese iron phosphate core containing doped particles and a biomass-derived carbon coating layer covering the surface of the lithium manganese iron phosphate core. The doped particles include a mixture of nano-gold and nano-silver particles. The doping amount of the doped particles is 0.3%, and the thickness of the carbon coating layer is 3.0 nm.

[0085] The preparation method of lithium manganese iron phosphate composite material includes:

[0086] Preparation of the precursor: A certain molar ratio of lithium source, phosphorus source, iron source, manganese source (1:1:0.6:0.4) and M source solution (the concentrations of nano gold and nano silver are 1000ppm and 2000ppm, respectively) are dissolved and mixed evenly to obtain a mixed solution. After homogenization for 90 min (pressure of 20 MPa), it is ball-milled for 2.0 h, and then spray-dried (flow rate of 1.2 mL / min, temperature of 180℃, pressure of 0.65 MPa) and pulverized to obtain the lithium manganese iron phosphate precursor containing doped particles.

[0087] Pre-sintering: The precursor and biomass carbon source were pre-sintered at 550°C in a nitrogen atmosphere. After being mixed in a certain ratio (carbon source ratio of 2.0%) and milled 200 times, the product was sintered at 450°C in a nitrogen atmosphere to obtain the first-burned product.

[0088] Secondary sintering: The obtained first-burned product is subjected to secondary sintering at 750℃ (600~900℃) under a nitrogen atmosphere, while a reducing agent (ethanol) is introduced. After sintering is completed, the temperature is lowered to room temperature to obtain the second-burned product.

[0089] Product preparation: The dicalcined product powder was mixed with 30% of acid-dissolved biomass carbon source, then sand-milled 150 times, spray-dried (flow rate 1.2 mL / min, temperature 180℃, pressure 0.65 MPa), and then pulverized through a 200-mesh sieve to obtain lithium manganese iron phosphate composite material.

[0090] Example 2

[0091] Lithium manganese iron phosphate composite materials and their preparation methods

[0092] The lithium manganese iron phosphate composite material includes a lithium manganese iron phosphate core containing doped particles and a biomass-derived carbon coating layer covering the surface of the lithium manganese iron phosphate core. The doped particles include a mixture of nano-gold and nano-silver particles. The doping amount of the doped particles is 0.4%, and the thickness of the carbon coating layer is 3.2 nm.

[0093] The preparation method of lithium manganese iron phosphate composite material includes:

[0094] Preparation of the precursor: A certain molar ratio of lithium source, phosphorus source, iron source, manganese source (1:1:0.6:0.4) and M source solution (the concentrations of nano gold and nano silver are 1000ppm and 3000ppm, respectively) are dissolved and mixed evenly to obtain a mixed solution. After homogenization for 90 min (pressure of 20 MPa), it is ball-milled for 2.0 h, and then spray-dried (flow rate of 1.2 mL / min, temperature of 180℃, pressure of 0.65 MPa) and pulverized to obtain the lithium manganese iron phosphate precursor containing doped particles.

[0095] Pre-sintering: The precursor and biomass carbon source were pre-sintered at 550°C in a nitrogen atmosphere. After being mixed in a certain ratio (carbon source ratio of 2.5%) and milled 200 times, the product was sintered at 450°C in a nitrogen atmosphere to obtain the first-burned product.

[0096] Secondary sintering: The product from the first sintering is subjected to secondary sintering at 750°C under a nitrogen atmosphere, while a reducing agent (ethanol) is introduced. After sintering is completed, the product is cooled to room temperature to obtain the product from the second sintering.

[0097] Product preparation: The dicalcined product powder was mixed with 30% of acid-dissolved biomass carbon source, then sand-milled 150 times, spray-dried (flow rate 1.2 mL / min, temperature 180℃, pressure 0.65 MPa), and then pulverized through a 200-mesh sieve to obtain lithium manganese iron phosphate composite material.

[0098] Example 3

[0099] Lithium manganese iron phosphate composite materials and their preparation methods

[0100] The lithium manganese iron phosphate composite material includes a lithium manganese iron phosphate core containing doped particles and a biomass-derived carbon coating layer covering the surface of the lithium manganese iron phosphate core. The doped particles include a mixture of nano-gold and nano-silver particles. The doping amount of the doped particles is 0.3%, and the thickness of the carbon coating layer is 3.0 nm.

[0101] The preparation method of lithium manganese iron phosphate composite material includes:

[0102] Preparation of precursor: A certain molar ratio of lithium source, phosphorus source, iron source, manganese source (1:1:0.6:0.4) and M source solution (the concentrations of nano gold and nano silver are 1000ppm and 2000ppm, respectively) are dissolved and mixed evenly to obtain a mixed solution. After homogenization for 90 min (pressure of 20 MPa), it is ball-milled for 2.0 h, and then spray-dried (flow rate of 1.0 mL / min, temperature of 160℃, pressure of 0.50 MPa) and pulverized to obtain lithium manganese iron phosphate precursor containing doped particles;

[0103] Pre-sintering: The precursor and biomass carbon source were pre-sintered at 550°C in a nitrogen atmosphere. After being mixed in a certain ratio (carbon source ratio of 2.0%) and milled 200 times, the product was sintered at 450°C in a nitrogen atmosphere to obtain the first-burned product.

[0104] Secondary sintering: The product from the first sintering is subjected to secondary sintering at 750°C under a nitrogen atmosphere, while a reducing agent (ethanol) is introduced. After sintering is completed, the product is cooled to room temperature to obtain the product from the second sintering.

[0105] Product preparation: The dicalcined product powder was mixed with 30% of acid-dissolved biomass carbon source, then sand-milled 150 times, spray-dried (flow rate 1.0 mL / min, temperature 160℃, pressure 0.50 MPa), and then pulverized through a 200-mesh sieve to obtain lithium manganese iron phosphate composite material.

[0106] Example 4

[0107] Lithium manganese iron phosphate composite materials and their preparation methods

[0108] The lithium manganese iron phosphate composite material includes a lithium manganese iron phosphate core containing doped particles and a biomass-derived carbon coating layer covering the surface of the lithium manganese iron phosphate core. The doped particles include a mixture of nano-gold and nano-silver particles. The doping amount of the doped particles is 0.3%, and the thickness of the carbon coating layer is 2.8 nm.

[0109] The preparation method of lithium manganese iron phosphate composite material includes:

[0110] Preparation of the precursor: A certain molar ratio of lithium source, phosphorus source, iron source, manganese source (1:1:0.6:0.4) and M source solution (the concentrations of nano gold and nano silver are 1000ppm and 2000ppm, respectively) are dissolved and mixed evenly to obtain a mixed solution. After homogenization for 120 min (pressure of 15 MPa), it is ball-milled for 2.0 h, and then spray-dried (flow rate of 1.2 mL / min, temperature of 180℃, pressure of 0.65 MPa) and pulverized to obtain lithium manganese iron phosphate precursor containing doped particles;

[0111] Pre-sintering: The precursor and biomass carbon source were pre-sintered at 550°C in a nitrogen atmosphere. After being mixed in a certain ratio (carbon source ratio of 2.0%) and milled 200 times, the product was sintered at 450°C in a nitrogen atmosphere to obtain the first-burned product.

[0112] Secondary sintering: The product from the first sintering is subjected to secondary sintering at 750°C under a nitrogen atmosphere, while a reducing agent (ethanol) is introduced. After sintering is completed, the product is cooled to room temperature to obtain the product from the second sintering.

[0113] Product preparation: The dicalcined product powder was mixed with 30% of acid-dissolved biomass carbon source, then sand-milled 150 times, spray-dried (flow rate 1.2 mL / min, temperature 180℃, pressure 0.65 MPa), and then pulverized through a 200-mesh sieve to obtain lithium manganese iron phosphate composite material.

[0114] Example 5

[0115] Lithium manganese iron phosphate composite materials and their preparation methods

[0116] The lithium manganese iron phosphate composite material includes a lithium manganese iron phosphate core containing doped particles and a biomass-derived carbon coating layer covering the surface of the lithium manganese iron phosphate core. The doped particles include a mixture of nano-gold and nano-silver particles. The doping amount of the doped particles is 0.3%, and the thickness of the carbon coating layer is 3.3 nm.

[0117] The preparation method of lithium manganese iron phosphate composite material includes:

[0118] Preparation of the precursor: A certain molar ratio of lithium source, phosphorus source, iron source, manganese source (1:1:0.6:0.4) and M source solution (the concentrations of nano gold and nano silver are 1000ppm and 2000ppm, respectively) are dissolved and mixed evenly to obtain a mixed solution. After homogenization for 90 min (pressure of 20 MPa), it is ball-milled for 2.0 h, and then spray-dried (flow rate of 1.2 mL / min, temperature of 180℃, pressure of 0.65 MPa) and pulverized to obtain the lithium manganese iron phosphate precursor containing doped particles.

[0119] Pre-sintering: The precursor and biomass carbon source were pre-sintered at 550°C in a nitrogen atmosphere. After being mixed in a certain ratio (carbon source ratio of 2.0%) and milled 200 times, the product was sintered at 450°C in a nitrogen atmosphere to obtain the first-burned product.

[0120] Secondary sintering: The obtained first-burned product is subjected to secondary sintering at 750℃ (600~900℃) under a nitrogen atmosphere, while a reducing agent (ethanol) is introduced. After sintering is completed, the temperature is lowered to room temperature to obtain the second-burned product.

[0121] Product preparation: The dicalcined product powder was mixed with 20% of acid-dissolved biomass carbon source, then sand-milled 150 times, spray-dried (flow rate 1.2 mL / min, temperature 180℃, pressure 0.65 MPa), and then pulverized through a 200-mesh sieve to obtain lithium manganese iron phosphate composite material.

[0122] Example 6

[0123] Lithium manganese iron phosphate composite materials and their preparation methods

[0124] The lithium manganese iron phosphate composite material includes a lithium manganese iron phosphate core containing doped particles and a biomass-derived carbon coating layer covering the surface of the lithium manganese iron phosphate core. The doped particles include a mixture of nano-gold and nano-silver particles. The doping amount of the doped particles is 0.3%, and the thickness of the carbon coating layer is 4.5 nm.

[0125] The preparation method of lithium manganese iron phosphate composite material includes:

[0126] Preparation of the precursor: A certain molar ratio of lithium source, phosphorus source, iron source, manganese source (1:1:0.6:0.4) and M source solution (the concentrations of nano gold and nano silver are 1000ppm and 2000ppm, respectively) are dissolved and mixed evenly to obtain a mixed solution. After homogenization for 90 min (pressure of 20 MPa), it is ball-milled for 2.0 h, and then spray-dried (flow rate of 1.2 mL / min, temperature of 180℃, pressure of 0.65 MPa) and pulverized to obtain the lithium manganese iron phosphate precursor containing doped particles.

[0127] Pre-sintering: The precursor and biomass carbon source were pre-sintered at 550°C in a nitrogen atmosphere. After being mixed in a certain ratio (carbon source ratio of 2.0%) and milled 200 times, the product was sintered at 450°C in a nitrogen atmosphere to obtain the first-burned product.

[0128] Secondary sintering: The obtained first-burned product is subjected to secondary sintering at 750℃ (600~900℃) under a nitrogen atmosphere, while a reducing agent (ethanol) is introduced. After sintering is completed, the temperature is lowered to room temperature to obtain the second-burned product.

[0129] Product preparation: The dicalcined product powder was mixed with 50% of the acid-dissolved biomass carbon source, then sand-milled 150 times, spray-dried (flow rate 1.2 mL / min, temperature 180℃, pressure 0.65 MPa), and then pulverized through a 200-mesh sieve to obtain the lithium manganese iron phosphate composite material.

[0130] Comparative Example 1

[0131] Lithium manganese iron phosphate composite materials and their preparation methods

[0132] The lithium manganese iron phosphate composite material includes a lithium manganese iron phosphate core containing doped particles and a carbon coating layer covering the surface of the lithium manganese iron phosphate core. The doped particles include a mixture of nano-gold and nano-silver particles. The doping amount of the doped particles is 0.3%. The carbon coating layer is selected from glucose and has a thickness of 3.2 nm.

[0133] The preparation method of lithium manganese iron phosphate composite material includes:

[0134] A lithium source, phosphorus source, iron source, manganese source (1:1:0.6:0.4) and an M source solution (with nano-gold and nano-silver concentrations of 1000ppm and 2000ppm, respectively) in a certain molar ratio were dissolved and mixed evenly to obtain a mixed solution. After homogenization for 90 min (pressure of 20 MPa), the solution was ball-milled for 2.0 h, and then spray-dried (flow rate of 1.2 mL / min, temperature of 180℃, pressure of 0.65 MPa) and pulverized to obtain the lithium manganese iron phosphate precursor containing doped particles.

[0135] Pre-sintering: The obtained precursor and ordinary carbon source glucose are pre-sintered at 550℃ in a nitrogen atmosphere. After being mixed in a certain ratio (carbon source ratio of 2.0%) and milled 200 times, the product is sintered at 450℃ in a nitrogen atmosphere to obtain the first-burned product.

[0136] Secondary sintering: The product from the first sintering is subjected to secondary sintering at 750°C under a nitrogen atmosphere, while a reducing agent (ethanol) is introduced. After sintering is completed, the product is cooled to room temperature to obtain the product from the second sintering.

[0137] Product preparation: The dicalcined product powder was mixed with glucose dissolved in acid, milled 150 times, spray dried (flow rate 1.2 mL / min, temperature 180℃, pressure 0.65 MPa), and then pulverized through a 200-mesh sieve to obtain the lithium manganese iron phosphate composite material.

[0138] Comparative Example 2

[0139] Lithium manganese iron phosphate composite materials and their preparation methods

[0140] The lithium manganese iron phosphate composite material includes a lithium manganese iron phosphate core containing doped particles and a biomass-derived carbon coating layer covering the surface of the lithium manganese iron phosphate core. The doped particles include a mixture of nano-gold and nano-silver particles. The doping amount of the doped particles is 0.3%, and the thickness of the carbon coating layer is 1.0 nm.

[0141] The preparation method of lithium manganese iron phosphate composite material includes:

[0142] Preparation of the precursor: A certain molar ratio of lithium source, phosphorus source, iron source, manganese source (1:1:0.6:0.4) and M source solution (the concentrations of nano gold and nano silver are 1000ppm and 2000ppm, respectively) are dissolved and mixed evenly to obtain a mixed solution. After homogenization for 90 min (pressure of 20 MPa), it is ball-milled for 2.0 h, and then spray-dried (flow rate of 1.2 mL / min, temperature of 180℃, pressure of 0.65 MPa) and pulverized to obtain the lithium manganese iron phosphate precursor containing doped particles.

[0143] Pre-sintering: The precursor and biomass carbon source were pre-sintered at 550°C in a nitrogen atmosphere. After being mixed in a certain ratio (carbon source ratio of 2.0%) and milled 200 times, the product was sintered at 450°C in a nitrogen atmosphere to obtain the first-burned product.

[0144] Product preparation: The obtained first-calcined product is subjected to a second calcination at 750℃ (600~900℃) under a nitrogen atmosphere, while a reducing agent (ethanol) is introduced. After sintering is completed, the temperature is lowered to room temperature to obtain lithium manganese iron phosphate composite material.

[0145] Performance testing

[0146] The materials obtained in Examples 1-6 and Comparative Examples 1-2 were assembled into a secondary battery according to the following steps: The assembly method included coating a lithium manganese iron phosphate composite material onto an aluminum foil surface, rolling it, and then vacuum drying it overnight at 110°C to obtain a positive electrode; mixing ethylene carbonate and ethyl methyl carbonate at a volume ratio of 3:7, and adding LiPF6 to form an electrolyte with a LiPF6 concentration of 1 mol / L; and assembling the positive electrode, polypropylene microporous separator, lithium sheet, and electrolyte to obtain the secondary battery.

[0147] The obtained secondary batteries were subjected to the following performance tests:

[0148] (1) Quantity capacity test: After one cycle of 0.1C / 1C / 5C, calculate the capacity of the first charge and discharge;

[0149] (2) AC impedance test: The test frequency range is 10mHz-100kHz, and the AC amplitude is 5mV. Li + The formula for calculating the diffusion coefficient is:

[0150]

[0151] Where: T—absolute temperature (room temperature 298K in this experiment); n—number of electrons gained or lost per molecule during oxidation or reduction; F—Faraday constant 96485C / mol; R—ideal gas constant 8.314J / (Kmol); Determine the electrode active area A: using the double-layer capacitance of the mercury electrode 0.2F / M2 as the standard, A=0.2Cdl, Cdl=(ωmaxRct)-1;

[0152] Determine the lithium ion concentration C Li+ =(1-(D) full -D) / 169.8)*4 / 6.02*1023 / V;D full Let σ be the specific capacity of a half-cell in its fully charged state, D be the specific capacity at a certain depth of discharge (D = 0 when the battery is fully charged), and V be the cell volume of lithium iron manganese phosphate. The Warburg coefficient is obtained by fitting a straight line composed of Z' and ω-1 / 2 in the low-frequency region. The slope of the fitted line is the Warburg coefficient σ for that discharge state. The relationship between Z' and ω-1 / 2 is: Z' = Rct + Rs + σω-1 / 2, where ω is the angular frequency in the low-frequency region, which can be obtained by converting the perturbation frequency f.

[0153] (3) Cyclic performance test: After 100 cycles at 5C, the ratio of the discharge capacity of the last cycle to the discharge capacity of the first cycle is used to evaluate the cyclic performance.

[0154] (4) Electron microscopy analysis was performed on the lithium manganese iron phosphate composite material obtained in Example 1, and the results are as follows: Figure 1As shown.

[0155] Results Analysis

[0156] The secondary batteries were subjected to performance tests, and the results are shown in Table 1. It can be seen that the materials obtained in Examples 1-6 have lower conductivity, ranging from 8.1 to 55.4 Ω·cm, while the materials obtained in Comparative Examples 1-2 have higher conductivity, ranging from 108 to 167 Ω·cm. The Lo of the materials obtained in Examples 1-6... i+ The diffusion coefficient is relatively high, ranging from 3.54 to 5.54 cm⁻¹. 2 / s, Li of the materials obtained in Comparative Examples 1-2 + The diffusion coefficient is relatively low, ranging from 1.28 to 2.05 cm⁻¹. 2 / s; The 0.1C, 1C, and 5C discharge capacities of the materials obtained in Examples 1-6 are all higher than those of the materials obtained in Comparative Examples 1-2; The cycle performance of the materials obtained in Examples 1-6 is higher, ranging from 90.1% to 93.5%, while that of the materials obtained in Comparative Examples 1-2 is lower, ranging from 79.4% to 82.1%. It can be seen that the materials obtained in Examples 1-6 have low conductivity and excellent electrical conductivity; Li + It diffuses quickly, has a high discharge capacity, and exhibits good cycle performance.

[0157] Table 1

[0158]

[0159]

[0160] In summary, the lithium manganese iron phosphate composite material provided in this application includes a lithium manganese iron phosphate core containing doped particles and a biomass-derived carbon coating layer covering the surface of the lithium manganese iron phosphate core. The doped particles include a mixture of nano-gold and nano-silver particles. On one hand, the provided biomass-derived carbon coating layer retains good biomass characteristics; the reinforced concrete structure composed of lignin, cellulose, and hemicellulose is preserved during sintering, forming a uniform and dense carbon coating layer on the surface of the lithium manganese iron phosphate. This can improve the specific surface area and resistivity of the material to a certain extent, while also reducing material costs to some extent.

[0161] It can also achieve the effective utilization of resources; on the other hand, the lithium manganese iron phosphate core is doped with nano-gold.

[0162] Mixed particles with nano-silver, these doped particles can significantly reduce the material's resistance and improve its performance.

[0163] The high conductivity of nanomaterials enhances their electrochemical performance, and nanomaterials also contribute to this.

[0164] Silver is a very stable metal and can reduce the effects of transition metals such as manganese and iron to some extent.

[0165] Dissolution-induced degradation of electrochemical performance and structural collapse; improved the performance of lithium manganese iron phosphate composite materials.

[0166] Its conductivity and cycling performance.

Claims

1. A method for preparing a lithium iron manganese phosphate composite material, characterized in that, comprising the following steps: The precursor preparation: after mixing the lithium source, the phosphorus source, the iron source, the manganese source and the doping particle source, homogenization treatment, ball milling treatment, spray drying treatment and crushing treatment are sequentially carried out to obtain the manganese iron lithium phosphate precursor containing doping particles; The pre-sintering: a biomass carbon source is provided, the manganese iron lithium phosphate precursor containing doping particles and the biomass carbon source are respectively subjected to pre-sintering treatment, and then after mixing, first sanding treatment is carried out to obtain a sintered product; wherein the doping particle source is selected from a mixed solution of nano-silver and nano-gold, and the concentration of the mixed solution is 100-10000 ppm, wherein the molar ratio of the nano-silver and the nano-gold is 1:0.5-3.5; the biomass carbon source includes at least one of branches, sawdust, wood chips, tips, board skin, truncated, residual branches, leaves in the process of wood harvesting and processing; The second sintering: the sintered product and a reducing agent are mixed to carry out the second sintering treatment to obtain a second sintered product; The product preparation: the biomass carbon source is subjected to acid dissolution treatment to obtain an acid-dissolved biomass carbon source, the second sintered product and the acid-dissolved biomass carbon source are mixed to carry out second sanding treatment, and then post-treatment is carried out to obtain a manganese iron lithium phosphate composite material.

2. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, characterized in that, The molar ratio of the lithium source, the phosphorus source, the manganese source and the iron source is 0.05-1.55:0.05-1.55:0.3-0.8:0.3-0.8; and / or, The mass ratio of the manganese iron lithium phosphate precursor containing doping particles and the biomass carbon source is 1:0.0005-0.025; and / or, The mass ratio of the second sintered product and the acid-dissolved biomass carbon source is 1:0.2-0.

5.

3. The preparation method of the manganese iron lithium phosphate composite material according to claim 1, wherein, The lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium citrate, lithium phosphate, lithium sulfate, lithium perchlorate and lithium chloride; and / or, The iron source includes at least one of ferrous phosphate, iron phosphate, ferric chloride, iron sulfate and ferrous sulfate; and / or, The manganese source includes at least one of manganese sulfate, manganese dihydrogen phosphate, manganese oxalate, manganese carbonate, manganese oxide and manganese nitrate; and / or, The phosphorus source includes at least one of phosphoric acid, pyrophosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, manganese dihydrogen phosphate and iron phosphate.

4. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, characterized in that, In the step of the precursor preparation, The homogenization treatment is carried out under the condition of a pressure of 10-40 MPa for 30-150 minutes; and / or, The ball milling treatment is carried out for 0.5-2.5 hours; and / or, The spray drying treatment is carried out at a flow rate of 0.2-1.4 mL / min, a temperature of 120-220℃ and a pressure of 0.01-0.8 MPa; and / or, The particle size of the particles obtained by the crushing treatment is less than or equal to 200 mesh.

5. The preparation method of the manganese iron lithium phosphate composite material according to claim 1, wherein, The pre-sintering treatment and the second sintering treatment are both carried out in an inert atmosphere; and / or, The temperature of the pre-sintering treatment is 400-600℃; and / or, The temperature of the second burning treatment is 600-900℃; and / or, The first sand milling treatment is performed 100-400 times using zirconium beads with a particle size of 0.1-10 mm; and / or, The second sand milling treatment is performed 100-200 times using zirconium beads with a particle size of 0.1-10 mm.

6. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, characterized in that, In the step of performing acid dissolution treatment on the biomass carbon source to obtain an acid-dissolved biomass carbon source, the following steps are included: A nitric acid solution with a mass percentage of 5%-30% is provided, and the biomass carbon source is mixed with the nitric acid solution and then dissolved; The mixture is heated at 80-150℃, filtered, and an acid-dissolved biomass carbon source is obtained.

7. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, characterized in that, The lithium manganese iron phosphate composite material comprises a lithium manganese iron phosphate core containing doped particles and a biomass-derived carbon coating layer coated on the surface of the lithium manganese iron phosphate core, wherein the doped particles comprise mixed particles of nano-gold and nano-silver.

8. The method for preparing the lithium manganese iron phosphate composite material according to claim 7, characterized in that, The thickness of the biomass-derived carbon coating layer is 2.5-5.0 nm.

9. The method for preparing the lithium manganese iron phosphate composite material according to claim 7, characterized in that, D of the doped particle-containing lithium iron manganese phosphate core 50 particle size of 0.5 μm to 2.0 μm; and / or, The particle size of the lithium manganese iron phosphate composite material is 0.5-2.5 μm.

10. A secondary battery comprising a positive electrode, a negative electrode, and a separator laminated between the positive electrode and the negative electrode, characterized by, The positive electrode comprises the lithium manganese iron phosphate composite material prepared by the method of any one of claims 1-9.

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