Preparation method of gradient distribution composite material and lithium ion battery

Abstract

The invention provides a preparation method of a gradient distribution composite material and a lithium ion battery, and relates to the technical field of lithium ion batteries. The preparation method comprises the following steps: adding slurry C into a mixed solution B, reacting to obtain slurry D, and drying the slurry D to obtain a composite material precursor; the slurry C is obtained by mixing and treating a lithium source, an iron source, a phosphorus source, a carbon source and a solvent; the mixed solution B is obtained by mixing a first precursor, a carbon source and an initiator; the content of iron ions in the mixed solution B is lower than that in the slurry C, and the concentration of the carbon source in the mixed solution B is higher than that in the slurry C; and sintering the composite material precursor to obtain the composite material: LiMnxFe (1-x) PO4Cy, wherein 0 < x < = 1, and 0.1 < y < 0.2; the surface of the LiMn < x > Fe < 1-x > PO4 is coated with the carbon layer; the content of Mn in the LiMnxFe1-xPO4 is in a decreasing trend, and the content of Fe in the LiMnxFe1-xPO4 is in an increasing trend; the content of the C element is gradually reduced in the direction from the center to the outer surface of the composite material.

Description

[0001] This application is a divisional application of the parent application with application number "CN202310354282.4", application date "2023.04.03", and title "A gradient-distributed composite material, its preparation method and its application". Technical Field

[0002] This application relates to the field of lithium-ion battery technology, and in particular to a method for preparing a gradient-distributed composite material and a lithium-ion battery. Background Technology

[0003] With the rapid development of the new energy industry both domestically and internationally, lithium iron phosphate (LFP) battery cathode materials have become the most critical raw materials for electric vehicles and energy storage. Lithium iron manganese phosphate (LMFP), as a high-voltage upgrade of LFP, has attracted widespread attention due to its high energy density, high stability, and low cost. However, it also suffers from the typical drawbacks of polyanionic materials, such as low electronic and ionic conductivity. Furthermore, the introduction of manganese (Mn) introduces problems related to the high-temperature dissolution and gas generation of trivalent Mn ions. To address these issues, methods similar to those used for LFP are commonly employed, such as carbon coating, doping, and nano-sizing. While these methods remain effective for LMFP, the traditional approaches are not ideal because LMFP's electronic and ionic conductivity are more than two orders of magnitude lower than that of LFP. Therefore, modifying LMFP is very difficult, costly, and has a low return on investment, necessitating new methods for its modification.

[0004] Currently used LMFP materials generally have very small particles, typically around 200nm or even less than 100nm. To improve conductivity, conductive carbon is mixed in, with a content exceeding 2wt%, resulting in a very large specific surface area, usually greater than 18m² / g. Such materials have poor processing performance; the viscosity of the slurry is very high, the solid content is very low, and coating and baking are difficult, making electrode processing difficult and the quality highly unstable. Furthermore, because Mn ions dissolve in the electrolyte at high temperatures, conventional dry or wet carbon coating methods always leave pores or uneven coating areas, making it difficult to completely eliminate exposed LMFP (e.g., ...). Figure 1 As shown in the figure, although deposition-based coating can achieve uniform coating, the types of materials that can be selected are very limited, and the cost is high. The material coating throughput is low, making it difficult to mass-produce on a large scale.

[0005] Therefore, the effects of traditional nano-sizing and carbon-coating modification of LMFP are not obvious, and they bring many side effects. Summary of the Invention

[0006] The technical problem solved by the present invention is to provide a composite material with a gradient distribution. The composite material provided by the present application has good material stability and good rate performance as the positive electrode material of a lithium-ion battery.

[0007] The present application also provides a composite material with a gradient distribution as shown in formula (I), LiMn

[0011] Fe 1-x PO4C y (I); where 0 < x ≤ 1, 0.1 < y < 0.2; In the composite material, a carbon layer is coated on the surface of LiMn x Fe 1-x PO4; And along the direction from the center to the outer surface of the composite material, the content of Mn element in the LiMn x Fe 1-x PO4 shows a decreasing trend, the content of Fe element shows an increasing trend, and the content of C element in the carbon layer shows a decreasing trend.

[0008] Preferably, the content of Mn element decreases at a rate of v1, 0.01 mol% / nm < v1 < 0.10 mol% / nm; the content of Fe element increases at a rate of v2, 0.01 mol% / nm < v2 < 0.10 mol% / nm; the content of C element decreases at a rate of v3, 0.01 mol% / nm < v3 < 0.10 mol% / nm.

[0009] Preferably, the content of Mn element decreases at a rate of v1, 0.01 mol% / nm < v1 < 0.03 mol% / nm; the content of Fe element increases at a rate of v2, 0.01 mol% / nm < v2 < 0.03 mol% / nm; the content of C element decreases at a rate of v3, 0.01 mol% / nm < v3 < 0.03 mol% / nm.

[0010] Preferably, the D50 of the composite material is 1.1 μm to 1.5 μm, and the D10 is 0.3 to 0.8 μm.

[0011] Preferably, the specific surface area of the composite material is 10 to 15 m 2 / g, and the powder resistivity of the composite material is 0 to 100 Ω·cm.

[0012] The present application also provides a preparation method of the composite material with a gradient distribution, including the following steps: A) Mix a lithium source, a manganese source, an iron source, a phosphorus source, a reducing agent and a solvent to obtain a mixed solution A; The mixed solution A is heated and subjected to a combustion reaction. The resulting combustion reaction product is crushed to obtain the first precursor. The first precursor, carbon source, and initiator are mixed to obtain mixed solution B; B) Mix lithium source, iron source, phosphorus source, carbon source and solvent, then dissociate and homogenize to obtain slurry C; C) Add slurry C to mixed solution B, react to obtain slurry D, and dry slurry D to obtain composite material precursor; D) Sinter the composite material precursor to obtain the composite material; The iron ion content in the mixed solution B is lower than the iron ion content in the slurry C; The concentration of the carbon source in the mixed solution B is higher than the concentration of the carbon source in the slurry C.

[0013] Preferably, in step A), the lithium source is selected from one or two of lithium carbonate and lithium hydroxide; the manganese source is selected from one or two of manganese nitrate and manganese carbonate; the iron source is selected from one or two of ferric nitrate and ferrous nitrate; the phosphorus source is selected from one or more of phosphoric acid, metaphosphoric acid, pyrophosphate, ammonium dihydrogen phosphate, diamine hydrogen phosphate, and phosphorus oxides; the carbon source is selected from one or more of ethylene oxide, ethylene glycol, acrylate, dopamine, and aniline; the reducing agent is selected from one or more of urea, citric acid, and glucose; and the initiator is selected from one or more of ammonium persulfate, azobisisobutyronitrile, cumene hydroperoxide, and benzoyl peroxide. The solvent is selected from one or more of water, tetrahydrofuran, acetone, ethanol, methanol, benzene, toluene, and xylene; in step B), the lithium source is selected from one or two of lithium carbonate and lithium hydroxide, the iron source is selected from one or more of ferric phosphate, ferrous oxalate, ferrous acetate, ferric oxide, and ferric hydroxide, the phosphorus source is selected from one or more of phosphoric acid, metaphosphoric acid, pyrophosphate, ammonium dihydrogen phosphate, diamine hydrogen phosphate, and phosphorus oxides, the carbon source is selected from one or more of ethylene oxide, ethylene glycol, acrylate, dopamine, and aniline, and the solvent is selected from one or more of water, tetrahydrofuran, acetone, ethanol, methanol, benzene, toluene, and xylene.

[0014] Preferably, the rate at which the slurry C is added to the mixed solution B is 0.02~0.5 mol / min; the solid content of the slurry C is 10~60 wt%, and the average particle size is 5~200 nm; the drying temperature is 100~500℃, and the time is 2~6 h; the sintering temperature is 500~1000℃, and the time is 5~15 h.

[0015] This application also provides a lithium-ion battery, including a positive electrode and a negative electrode, wherein the material of the positive electrode is the composite material described above or the composite material prepared by the preparation method described above.

[0016] The present application also provides an electrical device, including the lithium-ion battery as described above.

[0017] The present application provides a composite material with a gradient distribution. From the center to the surface of the particles, the content of Mn decreases from high to low. The center is LiMnPO4, which gradually transitions from LiMn x Fe 1-x PO4 (0 < x < 1) to LiFePO4. The gradient design of the Mn element content makes the outer layer mainly composed of LFP. Its reaction kinetics is good, which can ensure that the outer layer reacts first. And LFP has high stability and low solubility in the electrolyte, which can maintain the stability of the outer layer structure; the inner layer is mainly composed of LMFP. LMFP has relatively high energy but poor reaction kinetics and starts to react in the later stage of the charging curve; from the center to the surface of the material, the content of C also decreases from high to low; the outer layer of LFP has relatively high conductivity and a relatively low content of mixed C, while the inner layer of LMFP has low conductivity and a relatively high content of mixed C. This combination of high and low can ensure the uniform distribution of electronic conductance inside the entire material, thereby improving the rate performance of the material.

[0018] On the outer surface of the composite material, there is also partial carbon coating for establishing the conductive network between particles; this dual-gradient composite material is a single crystal or quasi-single crystal particle, and the integrity of the olivine structure can be maintained inside a single particle. Different amounts of conductive carbon are embedded inside the olivine structure. Taking the particle center as the origin, the elements in the dual-gradient material show a linear change process. The contents of Mn and C elements gradually decrease from the center, and the content of Fe element gradually increases from the center. This gradient change can also ensure that the stress inside the material remains consistent and the material will not crack due to different thermal expansions. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0020] Figure 1 It is a schematic structural diagram of LMFP in the prior art of the present invention; Figure 2 It is a schematic structural diagram of the composite material of the present invention; Figure 3 It is a radial distribution curve diagram of the mass contents of Mn element, Fe element and C element in the composite material of the present invention; Figure 4 It is a physical and chemical parameter diagram of the composite material of the present invention; Figure 5 Bar chart of the capacity retention rate of the composite material of the present invention; Figure 6 Bar chart of the resistance value of the composite material of the present invention; Figure 7 Bar chart of the gram capacity of the composite material of the present invention. Detailed implementation manners

[0021] To further understand the present invention, the preferred implementation manners of the present invention will be described below in conjunction with embodiments. However, it should be understood that these descriptions are only for further explaining the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0022] In view of the technical problem that the effect of the modified lithium iron manganese phosphate composite material in the prior art is not obvious, the present application provides a double-gradient composite lithium iron manganese phosphate material and its preparation method. While achieving carbon coating, it also has a double gradient of Mn element, Fe element and C element, so that when the lithium iron manganese phosphate is applied to the cathode material, it has better rate performance and material stability. Specifically, the embodiment of the present invention discloses a composite material with a gradient distribution as shown in formula (I), LiMn x Fe 1-x PO4C y (I); Where, 0 < x ≤ 1, 0.1 < y < 0.2; In the composite material, the carbon layer is coated on the surface of LiMn x Fe 1-x PO4; And along the direction from the center to the outer surface of the composite material, the content of Mn element in the LiMn<00​​​​​​​​​​​​​​Specifically, in the gradient-distributed composite material provided by the present application, the content of the Mn element decreases at a rate of v1, where 0.01 mol% / nm < v1 < 0.10 mol% / nm; the content of the Fe element increases at a rate of v2, where 0.01 mol% / nm < v2 < 0.10 mol% / nm; the content of the C element decreases at a rate of v3, where 0.01 mol% / nm < v3 < 0.10 mol% / nm; More specifically, the content of the Mn element decreases at a rate of v1, where 0.01 mol% / nm < v1 < 0.037 mol% / nm; the content of the Fe element increases at a rate of v2, where 0.01 mol% / nm < v2 < 0.052 mol% / nm; the content of the C element decreases at a rate of v3, where 0.01 mol% / nm < v3 < 0.015 mol% / nm.

[0025] Furthermore, in the double-gradient composite material provided by the present application, the particle size is also relatively large, with D50 being 1.1 μm to 1.5 μm and D10 being 0.3 to 0.8 μm, which can reach the level of general high-compaction LFP, and the specific surface area is 10 to 15 m 2 / g, thus ensuring that the material has good processing performance; the powder resistivity of the composite material is 0 to 100 Ω·cm, thus ensuring that the material has good electrical performance.

[0026] The present application also provides a method for preparing a gradient-distributed composite material, which includes the following steps: A) Mix a lithium source, a manganese source, an iron source, a phosphorus source, a reducing agent and a solvent to obtain a mixed solution A; Heat the mixed solution A and conduct a combustion reaction, and crush the obtained combustion reaction product to obtain a first precursor; Mix the first precursor, a carbon source and an initiator to obtain a mixed solution B; B) Mix a lithium source, an iron source, a phosphorus source, a carbon source and a solvent, and then conduct dissociation and homogenization to obtain a slurry C; C) Add the slurry C to the mixed solution B, react to obtain a slurry D, and dry the slurry D to obtain a composite material precursor; D) Sinter the composite material precursor to obtain a composite material; The content of iron ions in the mixed solution B is lower than the content of iron ions in the slurry C; The concentration of the carbon source in the mixed solution B is higher than the concentration of the carbon source in the slurry C.

[0027] In the above preparation process, the gradient change of element C is formed as follows: The mixed solution B (high-concentration solution) of high-concentration polymer monomers contains a polymerization initiator, which enables the polymer monomers to undergo polymerization and coat the surface of the first precursor material. The initiator is in excess. The slurry C (low-concentration solution) of low-concentration polymer monomers does not contain a polymerization initiator. When it is injected into the high-concentration solution at a certain rate, the concentration of polymer monomers decreases in a gradient. The precursor material is also injected into the high-concentration solution along with the low-concentration solution. At the same time as injection, the excess initiator initiates the polymerization reaction of polymer monomers and coats the surface of the injected precursor material. Due to the gradient change of polymer concentration during the process, the coating thickness also changes in a gradient. In the mixed slurry D, the precursors with different coating thicknesses are evenly dispersed. During sintering, polymers of different thicknesses form surface coatings with different carbon contents, resulting in a carbon gradient change.

[0028] The gradient changes in Mn and Fe elements are formed by the following reasons: the first precursor contains Mn compounds, so the Mn content is relatively high; the slurry C does not contain Mn compounds, so the Mn content is relatively low; conversely, the Fe content in the first precursor is relatively low, while the Fe content in the slurry C is relatively high; based on the above, when the slurry C is added to the mixed solution B at a certain rate, the slurry C disperses and coats the surface of the first precursor; the Mn content inside the composite precursor particles is high, while the Fe content on the surface is high. During the heat treatment process, Mn and Fe undergo thermal diffusion, with Mn diffusing from the center to the surface and Fe diffusing from the surface to the center. As the diffusion distance increases, the amount of element diffusion gradually decreases, thus the Mn and Fe elements exhibit a gradient change.

[0029] In step A), the lithium source is selected from one or two of lithium carbonate and lithium hydroxide; the manganese source is selected from one or two of manganese nitrate and manganese carbonate; the iron source is selected from one or two of ferric nitrate and ferrous nitrate; the phosphorus source is selected from one or more of phosphoric acid, metaphosphoric acid, pyrophosphate, ammonium dihydrogen phosphate, diamine hydrogen phosphate, and phosphorus oxide; the carbon source is selected from one or more of ethylene oxide, ethylene glycol, acrylate, dopamine, and aniline; the reducing agent is selected from one or more of urea, citric acid, and glucose; the initiator is selected from one or more of ammonium persulfate, azobisisobutyronitrile, cumene hydroperoxide, and benzoyl peroxide; and the solvent is selected from one or more of water, tetrahydrofuran, acetone, ethanol, methanol, benzene, toluene, and xylene. In step B), the lithium source is selected from one or both of lithium carbonate and lithium hydroxide; the iron source is selected from one or more of ferric phosphate, ferrous oxalate, ferric acetate, ferric oxide, and ferric hydroxide; the phosphorus source is selected from one or more of phosphoric acid, metaphosphoric acid, pyrophosphate, ammonium dihydrogen phosphate, diamine hydrogen phosphate, and phosphorus oxides; the carbon source is selected from one or more of ethylene oxide, ethylene glycol, acrylate, dopamine, and aniline; and the solvent is selected from one or more of water, tetrahydrofuran, acetone, ethanol, methanol, benzene, toluene, and xylene.

[0030] The slurry C has a solid content of 10-60 wt% and an average particle size of 5-200 nm, specifically, the solid content of slurry C is 20-40 wt% and the average particle size is 20-150 nm. The injection rate of slurry C into the mixed solution B is 0.02-0.5 mol / min, specifically, the injection rate of slurry C into the mixed solution B is 0.05-0.3 mol / min. During the injection of slurry C into the mixed solution B, the faster the injection rate, the faster the polymer monomer concentration decreases, and the faster the coating thickness decreases, thereby controlling the rate of change of the C content gradient.

[0031] During the preparation process, the sintering temperature is 500~1000℃ and the time is 5~15h; specifically, the sintering temperature is 600~900℃ and the time is 8~12h. The sintering temperature and time can control the diffusion rate and diffusion time of Mn and Fe elements to achieve a gradient change in the content of Mn and Fe elements.

[0032] This application also provides a lithium-ion battery, including a positive electrode and a negative electrode, wherein the material of the positive electrode is the composite material described in the above-mentioned scheme.

[0033] This application also provides an electrical device, including a lithium-ion battery as described above.

[0034] To further understand the present invention, the following detailed description of the gradient distribution composite material, its preparation method and its application are provided in conjunction with embodiments. The scope of protection of the present invention is not limited by the following embodiments.

[0035] Example 1 Step 1: Lithium nitrate, iron nitrate, manganese carbonate, phosphoric acid, and glucose were mixed in pure water at a molar ratio of Li:Fe:Mn:P:C = 1:0.3:0.7:1:0.6 to prepare a 10 mol / L suspension. The mixture was heated to 500°C to induce a self-propagating combustion reaction. The product was then crushed by airflow, and after 2 hours, lithium manganese iron phosphate precursor I with an average particle size D50 of less than 1 μm was obtained. Precursor I, dopamine monomer, and ammonium persulfate ((NH4)2S2O8) were mixed at a molar ratio of Li:dopamine monomer:S2O8. 2- A mixture of 1:0.3:0.1 in pure water yields a 1 mol / L high-concentration mixed solution B. Step 2: Lithium carbonate, iron phosphate, and dopamine are mixed in pure water at a molar ratio of Li:Fe:dopamine = 1:1:0.05, and then dissociated and homogenized to obtain a low-concentration slurry C with a solid content of 40%. Slurry C is added to mixed solution B at a rate of 0.1 L / min, causing the ammonium persulfate in mixed solution B to initiate dopamine polymerization and coat the lithium iron phosphate precursor. Due to the change in dopamine concentration, the thickness of the dopamine coating on the precursor surface changes, resulting in slurries D with different dopamine contents. Slurry D is dried in an oven at 100 degrees Celsius to obtain a composite cathode material precursor II with a lithium manganese iron phosphate precursor at the center and a lithium iron phosphate precursor on the outer layer, and the polydopamine coating thickness decreasing from the center to the outer layer.

[0036] Step 3: In high-purity argon gas, the precursor II obtained in step 2 is sintered at 600℃ for 10h, and then crushed to obtain the dual-gradient composite cathode material A.

[0037] The radial distribution ratios of Mn, Fe, and C elements in the dual-gradient composite cathode material A were determined, and the results are as follows: Figure 3 As shown, by Figure 3 It can be seen that the contents of Mn and C elements gradually decrease from the center to the outside, while the contents of Fe element gradually increase from the center to the outside. Furthermore, by performing cross-sectional SEM on the composite cathode material A, five locations were selected from the center to the surface, and the elemental contents were analyzed by scanning EDS. The results are shown in Table 1. Table 1: Molar ratio data of Mn, Fe, and C elements from the center to the surface

[0038] Figure 4 This is a graph showing the particle size and specific surface area data of the dual-gradient composite material A prepared in this embodiment. Figure 4 It is known that the particle size of the dual-gradient composite material A in this application is large, with a specific surface area of ​​10 m². 2 / g.

[0039] Figures 5-7 This is a bar graph showing the electrochemical performance of the dual-gradient composite material A prepared in this embodiment. Figure 5 It can be seen that the capacity retention of the dual-gradient composite material prepared in this embodiment after 1000 cycles is higher than that of a typical LMFP. Figure 6 It can be seen that, compared with a typical LMFP, the dual-gradient composite material prepared in this embodiment has a lower resistance at 50% SOC-DCR. Figure 7 It can be seen that, compared with a general LMFP, the dual-gradient composite material prepared in this embodiment has a higher gram discharge capacity at a 1C rate.

[0040] The LMFP shown in the above figure is specifically the Tianjin Scoland LMFP64 product.

[0041] Example 2 Steps 1 and 2 are the same as in Example 1.

[0042] Step 3: In high-purity argon gas, the precursor II obtained in step 2 is sintered at 600℃ for 12 hours, and then crushed to obtain the dual-gradient composite cathode material B.

[0043] Example 3 Steps 1 and 2 are the same as in Example 1.

[0044] Step 3: In high-purity argon gas, the precursor II obtained in step 2 is sintered at 600℃ for 8 hours, and then crushed to obtain the dual-gradient composite cathode material C.

[0045] Example 4 Slurry C was added to mixed solution B at a rate of 0.05 mol / min to reduce the gradient change rate of the dopamine coating layer. Other steps were the same as in Example 1 to obtain the dual-gradient composite cathode material D.

[0046] Example 5 Slurry C was added to mixed solution B at a rate of 0.2 mol / min to increase the gradient change rate of the dopamine coating layer. Other steps were the same as in Example 1 to obtain the dual-gradient composite cathode material E.

[0047] Example 6 Step 1: Lithium carbonate, ferric nitrate, manganese nitrate, ammonium dihydrogen phosphate, and citric acid were mixed in pure water at a molar ratio of Li:Fe:Mn:P:C = 1:0.3:0.7:1:0.6 to prepare a 10 mol / L suspension. The mixture was heated to 500°C to induce a self-propagating combustion reaction. The product was then crushed using an airflow to obtain lithium manganese iron phosphate precursor I with an average particle size D50 of less than 1 μm. Precursor I, acrylate monomer, and azobisisobutyronitrile (AlBN) were mixed in pure water at a molar ratio of Li:acrylate monomer:AlBN = 1:0.3:0.1 to obtain a 1 mol / L high-concentration mixed solution B. Step 2: Lithium hydroxide, iron acetate, phosphoric acid, and acrylate monomers are mixed in pure water at a molar ratio of Li:Fe:P:acrylate monomers = 1:1:1:0.05, and then dissociated and homogenized to obtain a low-concentration slurry C with a solid content of 40%. Slurry C is added to mixed solution B at a rate of 0.05 mol / min, causing the azobisisobutyronitrile in mixed solution B to initiate the polymerization of acrylate monomers and coat them onto the lithium iron phosphate precursor, resulting in slurry D. Slurry D is dried in an oven at 100 degrees Celsius to obtain a composite cathode material precursor II with a lithium manganese iron phosphate precursor at the center, an outer layer of lithium iron phosphate precursor, and a polyacrylate coating thickness that decreases from the center to the outer layer. Step 3 is the same as in Example 1, to obtain the dual-gradient composite cathode material F.

[0048] Example 7 The polymer monomer in Example 6 was replaced with aniline, and the same steps were performed as in Example 6 to obtain the dual-gradient composite cathode material G.

[0049] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0050] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a gradient distribution composite material, comprising: adding a slurry C into a mixed solution B to obtain a slurry D by reaction, drying the slurry D to obtain a composite material precursor; the slurry C is obtained by mixing a lithium source, an iron source, a phosphorus source, a carbon source and a solvent and then processing; the mixed solution B is obtained by mixing a first precursor, a carbon source and an initiator, the first precursor is obtained by processing a mixed solution A obtained by mixing a lithium source, a manganese source, an iron source, a phosphorus source, a reducing agent and a solvent; the content of iron ions in the mixed solution B is lower than the content of iron ions in the slurry C, and the concentration of the carbon source in the mixed solution B is higher than the concentration of the carbon source in the slurry C; sintering the composite material precursor to obtain the composite material: LiMn x Fe 1-x PO4C y ; wherein, 0 < x ≤ 1, 0.1 < y < 0.2; In the composite, the carbon layer coats the surface of LiMn x Fe 1-x PO4. Mn in LiMn x Fe 1-x The content of Mn element in PO4 presents a decreasing trend, and the content of Fe element presents an increasing trend. the content of C element decreases along the direction from the center to the outer surface of the composite material.

2. The production method according to claim 1, characterized by, The rate of adding the slurry C into the mixed solution B is 0.05 mol / min to 0.3 mol / min.

3. The preparation method according to claim 2, characterized in that, The sintering temperature is 600 to 900 ℃, and the sintering time is 8 to 12 hours.

4. The production method according to claim 3, characterized by, The content of Mn element decreases at a rate v1, 0.01 mol% / nm < v1 < 0.10 mol% / nm; the content of Fe element increases at a rate v2, 0.01 mol% / nm < v2 < 0.10 mol% / nm; and the content of C element decreases at a rate v3, 0.01 mol% / nm < v3 < 0.10 mol% / nm.

5. The preparation method according to claim 1, characterized in that, The first precursor obtained by processing the mixed solution A obtained by mixing a lithium source, a manganese source, an iron source, a phosphorus source, a reducing agent and a solvent comprises: heating and performing combustion reaction on the mixed solution A, and crushing the obtained combustion reaction product to obtain the first precursor.

6. The method of claim 1, wherein, The slurry C obtained by mixing a lithium source, an iron source, a phosphorus source, a carbon source and a solvent and then processing comprises: mixing the lithium source, the iron source, the phosphorus source, the carbon source and the solvent, and then dissociating and homogenizing to obtain the slurry C.

7. The preparation method according to claim 1, characterized in that, The mixed solution B obtained by mixing a first precursor, a carbon source and an initiator and the first precursor obtained by processing a mixed solution A obtained by mixing a lithium source, a manganese source, an iron source, a phosphorus source, a reducing agent and a solvent, wherein the lithium source is selected from one or both of lithium carbonate and lithium hydroxide, the manganese source is selected from one or both of manganese nitrate and manganese carbonate, the iron source is selected from one or both of ferric nitrate and ferrous nitrate, the phosphorus source is selected from one or more of phosphoric acid, metaphosphoric acid, pyrophosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate and phosphorus oxide, the reducing agent is selected from one or more of urea, citric acid and glucose, and the solvent is selected from one or more of water, tetrahydrofuran, acetone, ethanol, methanol, benzene, toluene and xylene; the carbon source is selected from one or more of oxirane, ethylene glycol, acrylate, dopamine and aniline, and the initiator is selected from one or more of ammonium persulfate, azobisisobutyronitrile, cumene hydroperoxide and dibenzoyl peroxide.

8. The preparation method according to claim 6, characterized in that, The lithium source is selected from one or both of lithium carbonate and lithium hydroxide, the iron source is selected from one or more of iron phosphate, ferrous oxalate, ferrous acetate, diiron trioxide and iron oxyhydroxide, the phosphorus source is selected from one or more of phosphoric acid, metaphosphoric acid, pyrophosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate and phosphorus oxide, the carbon source is selected from one or more of oxirane, ethylene glycol, acrylate, dopamine and aniline, and the solvent is selected from one or more of water, tetrahydrofuran, acetone, ethanol, methanol, benzene, toluene and xylene.

9. The production method according to claim 3, wherein The solid content of the slurry C is 10wt%-60wt%, and the average particle size is 5nm-200nm; the drying temperature is 100°C-500°C, and the time is 2h-6h.

10. A lithium-ion battery comprising a positive electrode and a negative electrode, characterized in that, The material of the positive electrode is the composite material prepared by the preparation method in any one of claims 1-9.

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