Preparation method of nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material

Abstract

The present invention provides a method for preparing a nitrogen-doped biochar-coated spherical lithium iron manganese phosphate composite electrode material. The method comprises the following steps: preparing a biomass-coated gel; preparing a spherical lithium iron manganese phosphate slurry; preparing a biomass-gel-coated spherical lithium iron manganese phosphate precursor; and preparing a nitrogen-doped biochar-coated spherical lithium iron manganese phosphate electrode material. The nitrogen-containing carbon source used in the above method is derived from waste biomass raw materials such as shrimp and crab shells, which not only achieves solid waste recycling but also reduces raw material costs. The biomass gel prepared through a simple reaction has an efficient adsorption effect on the lithium iron manganese phosphate precursor, achieving uniform and dense coating of the precursor particles with the biomass carbon source. Finally, through carbon thermal reduction, a nitrogen-doped carbon layer is generated, which significantly improves the conductivity of the electrode material and enhances the capacity and rate performance of the composite material.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and in particular relates to a method for preparing a nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material. Background Art

[0002] Lithium manganese iron phosphate (LMFP) is a phosphate cathode material. Its manganese ion open-circuit discharge platform is 4.1V, 20% higher than that of LFP (3.4V). Although the theoretical specific capacity (170mAh / g) of the two is the same, the actual energy density of LMFP is 15-20% higher than that of LFP. However, the olivine-structured LMFP suffers from extremely low electronic conductivity, which cannot be improved by traditional carbon coating methods, thus limiting its practical application. Researchers have tried to improve it by means such as nitrogen-doped carbon coating, but the artificially synthesized nitrogen-containing organic carbon sources described in patent CN109244391A have high addition amounts and low residual rates, which not only leads to high costs and limits its large-scale promotion and application, but also inevitably introduces impurities such as sulfur during the preparation process, affecting the battery's cycle stability. Summary of the Invention

[0003] The present invention provides a method for preparing a nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material to solve the above problems.

[0004] The present invention provides a method for preparing a nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material, comprising the steps of:

[0005] (1) Preparation of biomass-coated gel: A biomass raw material with a high chitin content is washed, crushed, dried, and then added to an alkaline solution. After heating and reacting, the mixture is cooled to room temperature, filtered through a mesh to remove the residue, and an acidic solution is added to the filtrate to obtain a gel-like substance. The solid content is dried to a range of 10% to 20%, thereby obtaining a biomass-coated gel;

[0006] (2) Preparing a spherical lithium iron manganese phosphate slurry: Weighing a lithium source, a manganese source, an iron source, and a phosphorus source respectively, adding a flux and a carbon source to obtain a slurry, adding pure water to the slurry until the solid content of the slurry is 30-50%, mixing well, and then ultrafine grinding to obtain a spherical lithium iron manganese phosphate slurry;

[0007] (3) preparing a biomass gel-coated spherical lithium manganese iron phosphate precursor: adding the biomass-coated gel to the spherical lithium manganese iron phosphate slurry, stirring at a uniform speed after standing, and spray drying by a multi-fluid spray dryer to obtain a biomass gel-coated spherical lithium manganese iron phosphate precursor;

[0008] (4) Preparation of nitrogen-doped biochar-coated spherical lithium manganese iron phosphate electrode material: The biomass gel-coated spherical lithium manganese iron phosphate precursor is subjected to carbon thermal reduction in a nitrogen atmosphere to generate a nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material.

[0009] As a preferred embodiment of the method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material according to the present invention, in step (1), the biomass raw material with a high chitin content is any one or more of the shells of arthropods, molluscs or fungi; the pH of the alkaline solution is 12-14, and the alkaline solution is selected from any one or more of sodium hydroxide solution, potassium hydroxide solution, sodium carbonate solution, sodium bicarbonate solution, potassium carbonate solution or potassium bicarbonate solution; the pH of the acidic solution is 2, and the acidic solution is any one or more of the combination of phosphoric acid solution, hydrochloric acid solution or citric acid solution.

[0010] As a preferred embodiment of the method for preparing a nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material according to the present invention, in step (1), the temperature of the heating reaction is 60-180°C, the pressure is 0.1-5 MPa, and the time is 0.1-10 h; the drying temperature for drying to a solid content range of 10%-20% is 100°C.

[0011] As a preferred embodiment of the method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material of the present invention, in step (1), the sieve is 400 mesh.

[0012] As a preferred embodiment of the method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material of the present invention, in step (2), the molar ratio of lithium in the lithium source, manganese in the manganese source, iron in the iron source and phosphorus in the phosphorus source is X Li :X Mn :X Fe :1, where 0.95≤X Li ≤1.05、0≤X Mn ≤1, X Fe =1-X Mn , the total molar concentration of all metal ions is 1-10 mol / L; the flux is selected from phosphoric acid and / or citric acid, and the flux accounts for 0.1-1% of the mass of the solid raw material; the carbon source is selected from any one or more combinations of sugars, soluble polymers or urea, and the carbon source accounts for 1-10% of the mass of the solid raw material.

[0013] As a preferred embodiment of the method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material of the present invention, in step (2), the ultrafine grinding is to control the grinding D50 to be 100-1000 nm.

[0014] As a preferred embodiment of the method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material of the present invention, in step (3), the mass ratio of the biomass-coated gel to the spherical lithium manganese iron phosphate slurry is 0.1-1:1.

[0015] As a preferred embodiment of the method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material of the present invention, in step (3), the standing time is 1-10 hours; the stirring speed is 100-1000 rpm, and the time is 10-120 minutes.

[0016] As a preferred embodiment of the method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material of the present invention, in step (3), the D50 of the biomass gel-coated spherical lithium manganese iron phosphate precursor is 1-20 μm.

[0017] As a preferred embodiment of the method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material of the present invention, in step (4), the sintering temperature of the carbon thermal reduction is 600-900°C and the time is 5-10h.

[0018] The preparation method of a nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material proposed in the present invention has the following advantages compared with the prior art:

[0019] 1. Using waste biomass with high chitin content, such as shrimp and crab shells, insect shells, and fungi, as carbon-coated raw materials for electrode materials, achieves waste utilization and reduces the use of original carbon sources such as sugars and artificial polymers;

[0020] 2. Through a simple reaction, the original chitin powder composed of tightly packed nanosheets is exfoliated into individual two-dimensional chitin nanosheets, which then form a hydrogel in an aqueous environment. By dispersing the gel-like chitin into a lithium iron manganese phosphate slurry, the chitin's adsorption function is fully utilized, and the lithium, iron, and manganese sources in the slurry are adsorbed into its pores, achieving not only nanoscale dispersion of the raw materials but also chitin coating of the raw materials.

[0021] 3. Through carbothermal reduction, the chitin coating the lithium manganese iron phosphate is reduced to a carbon material. Since chitin contains nitrogen and the sintering temperature is controlled at 600-900°C, the chitin coating will form a nitrogen-doped carbon with a high proportion of pyridinic nitrogen and pyrrolic nitrogen. Compared with traditional carbon coating, it can provide efficient electron and ion transmission channels, greatly improving the lithium ion transmission efficiency and conductivity, and improving the electrochemical performance of the electrode material at high rates;

[0022] 4. Not only can nitrogen-doped carbon coating be obtained to effectively improve the poor electrical conductivity and electrochemical properties of lithium manganese iron phosphate, but this carbon coating method, with its cheap and readily available raw materials, simple process and low cost, can promote the market application of lithium manganese iron phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a 5000-fold magnified SEM image of the nitrogen-doped biochar-coated spherical lithium iron manganese phosphate composite electrode material prepared in Example 1 according to the method for preparing a nitrogen-doped biochar-coated spherical lithium iron manganese phosphate composite electrode material of the present invention;

[0024] Figure 2 This is a 5000-fold magnified SEM image of the nitrogen-doped biochar-coated spherical lithium iron manganese phosphate composite electrode material prepared in Example 2 according to the method for preparing a nitrogen-doped biochar-coated spherical lithium iron manganese phosphate composite electrode material of the present invention;

[0025] Figure 3 This is a 5000-fold magnified SEM image of the nitrogen-doped biochar-coated spherical lithium iron manganese phosphate composite electrode material prepared in Example 3 of the method for preparing a nitrogen-doped biochar-coated spherical lithium iron manganese phosphate composite electrode material according to the present invention;

[0026] Figure 4 This is a SEM image of the electrode material prepared in the comparative example, magnified 5000 times. DETAILED DESCRIPTION

[0027] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below in conjunction with specific embodiments.

[0028] First, the term "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in various places throughout this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive of other embodiments.

[0029] The preparation method of the novel nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material of the present invention comprises the following specific steps:

[0030] (1) preparing a biomass-coated gel: washing, crushing, and drying a biomass raw material with a high chitin content, such as an arthropod, mollusk shell, or fungus, and adding one or more combinations thereof to an alkaline solution of a combination of one or more combinations thereof of sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, potassium carbonate, and potassium bicarbonate with a pH value of 12-14, heating the solution at a temperature of 60-180° C. and a pressure of 0.1-5 MPa for a reaction of 0.1-10 h, cooling the solution to room temperature after the reaction, filtering the solution with a 400-mesh sieve to remove the residue, adding a phosphoric acid solution, a hydrochloric acid solution, or a citric acid solution with a pH value of 2 to the filtrate to obtain a gel-like substance, and drying the solution at a temperature of 100° C. until the solid content is in the range of 10%-20%, thereby obtaining a biomass-coated gel;

[0031] (2) Preparation of spherical lithium manganese iron phosphate slurry: Weigh the lithium source, manganese source, iron source and phosphorus source respectively, wherein the molar ratio of lithium in the lithium source, manganese in the manganese source, iron in the iron source and phosphorus in the phosphorus source is X Li :X Mn :X Fe :1, 0.95≤X Li ≤1.05、0≤X Mn ≤1, X Fe =1-X Mn , the total molar concentration of all metal ions is 1-10 mol / L. A flux and a carbon source are added, wherein the flux is selected from one or more combinations of inorganic acids such as phosphoric acid or citric acid, accounting for 0.1-1% of the weight of the solid raw material; the carbon source is selected from one or more combinations of soluble organic substances such as sugars, soluble polymers, and urea, accounting for 1-10% of the weight of the solid raw material (the solid raw material here refers to the sum of lithium source, manganese source, iron source, and phosphorus source). Pure water is added to the slurry to a solid content of 30-50%. After mixing evenly, ultrafine grinding is performed, and the grinding D50 is controlled at 100-1000 nm.

[0032] (3) Preparation of biomass gel-coated spherical lithium manganese iron phosphate precursor: adding the biomass-coated gel to the spherical lithium manganese iron phosphate slurry at a mass ratio of 0.1-1:1, standing for 1-10 hours, and then stirring at a constant speed to ensure that the gel-like carbon source is dispersed in the slurry, wherein the stirring speed is 100-1000 rpm and the stirring time is 10-120 minutes. Spray drying is performed in a two-fluid spray dryer or a four-fluid spray dryer to prepare a biomass-coated spherical lithium manganese iron phosphate precursor with a D50 of 1-20 μm;

[0033] (4) Preparation of nitrogen-doped biochar-coated spherical lithium manganese iron phosphate electrode material: Spherical lithium manganese iron phosphate precursor is subjected to carbon thermal reduction in a nitrogen atmosphere to generate nitrogen-doped biochar-coated spherical lithium manganese iron phosphate electrode material, wherein the sintering temperature is 600-900°C and the sintering time is 5-10h.

[0034] For specific implementation and related comparison, please refer to the following embodiments:

[0035] Example 1

[0036] The method for preparing a novel nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material according to the present invention comprises the following steps:

[0037] 1 kg of shrimp shells were washed, crushed, and dried, then added to 1 L of sodium hydroxide solution (pH 14). The reaction was hydrothermally reacted at 180°C for 10 hours. After cooling to room temperature, the residue was filtered through a 400-mesh sieve. Citric acid solution (pH 2) was added to the filtrate to produce a gel-like substance. The filtrate was then dried at 100°C for 1-10 hours to a solid content of 15%, yielding a biomass-coated gel. Lithium carbonate, manganese tetraoxide, ferric oxide, and phosphoric acid were weighed separately. The molar ratio of lithium, manganese, iron, and phosphorus in these raw materials was 1.01:0.6:0.4:1, and the total molar concentration of all metal ions was 6 mol / L. A flux and a carbon source were added: citric acid (0.5% by weight of the solid raw materials) and sucrose (5% by weight of the carbon raw materials). Pure water was added to a slurry with a solid content of 35%. After mixing, the mixture was ultrafine ground to a D50 of 200 nm to obtain a lithium manganese iron phosphate precursor slurry. The biomass-coated gel was added to a lithium iron phosphate (LFMOP) precursor slurry at a 1:1 mass ratio. After standing for 10 hours, the slurry was stirred at a constant speed of 300 rpm for 120 minutes to ensure that the gel-like carbon source was dispersed in the slurry. The slurry was then spray-dried in a two-fluid spray dryer to produce a biomass-coated spherical LFMOP precursor with a D50 of 10±5 μm. The precursor was then subjected to carbothermal reduction in a nitrogen atmosphere to produce nitrogen-doped biochar-coated spherical LFMOP electrode material. The sintering temperature was 750°C for 8 hours.

[0038] Example 2

[0039] The method for preparing a novel nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material according to the present invention comprises the following steps:

[0040] 1 kg of crab shells were cleaned, crushed, and dried, then added to 1 L of sodium hydroxide solution at pH 14. The reaction was hydrothermally reacted at 180°C for 10 hours. After cooling to room temperature, the residue was filtered through a 400-mesh sieve. A phosphoric acid solution at pH 2.5 was added to the filtrate to obtain a gel-like substance. The filtrate was then dried at 100°C to a solid content of 15%, yielding a biomass-coated gel. Lithium carbonate, manganese tetraoxide, ferric oxide, and phosphoric acid were weighed separately. The molar ratio of lithium, manganese, iron, and phosphorus in these raw materials was 1.01:0.6:0.4:1, and the total molar concentration of all metal ions was 6 mol / L. A flux and a carbon source were added: phosphoric acid (0.25% by weight of the solid raw materials) and glucose (5% by weight of the carbon source). Pure water was added to a slurry with a solid content of 35%. After mixing, the slurry was ultrafine ground to a D50 of 200 nm to obtain a lithium manganese iron phosphate precursor slurry. The biomass-coated gel was added to a lithium iron phosphate (LFMOP) precursor slurry at a mass ratio of 0.5:1. After standing for 10 hours, the slurry was stirred at a constant speed of 300 rpm for 120 minutes to ensure that the gel-like carbon source was dispersed in the slurry. The slurry was then spray-dried in a two-fluid spray dryer to produce a biomass-coated spherical LFMOP precursor with a D50 of 10±5 μm. The precursor was then subjected to carbothermal reduction in a nitrogen atmosphere to produce nitrogen-doped biochar-coated spherical LFMOP electrode material. The sintering temperature was 750°C for 8 hours.

[0041] Example 3

[0042] The method for preparing a novel nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material according to the present invention comprises the following steps:

[0043] 1 kg of shrimp and crab shell mixture was washed, crushed, and dried, and then added to 1 L of sodium hydroxide solution with a pH value of 14. The mixture was hydrothermally reacted at 180 ° C for 10 hours. After the reaction, it was cooled to room temperature and filtered through a 400-mesh sieve to remove the residue. A mixed solution of phosphoric acid and citric acid with a pH value of 3 was added to the filtrate to obtain a gel-like substance. The biomass-coated gel was then dried at 100 ° C to a solid content of 15%. Lithium carbonate, manganese tetraoxide, ferric oxide, and phosphoric acid were weighed separately. The molar ratio of lithium, manganese, iron, and phosphorus in the above raw materials was 1.01:0.6:0.4:1, and the total molar concentration of all metal ions was 6

[0044] mol / L. A fluxing agent and a carbon source were added, with phosphoric acid and citric acid each representing 0.25% by weight of the solid raw material, and glucose representing 5% by weight of the solid raw material. Pure water was added to a slurry with a solid content of 35%. After mixing thoroughly, the mixture was ultrafinely ground to a D50 of 200 nm to obtain a lithium iron phosphate (LFMPO) precursor slurry. The biomass gel was added to the LFMPO precursor slurry, maintaining a mass ratio of 0.5:1. After standing for 10 hours, the slurry was uniformly stirred at 300 rpm for 120 minutes to ensure that the gel-like carbon source was dispersed throughout the slurry. The mixture was then spray-dried in a two-fluid spray dryer to produce biomass-coated spherical LFMPO precursors with a D50 of 10±5 μm. The precursor was then subjected to carbothermal reduction under a nitrogen atmosphere to produce nitrogen-doped biochar-coated spherical LFMPO electrode materials. The sintering temperature was 750°C for 8 hours.

[0045] Comparative Example

[0046] Lithium carbonate, manganese dioxide, ferric oxide, and phosphoric acid were weighed separately. The molar ratio of lithium, manganese, iron, and phosphorus in the raw materials was 1.01:0.6:0.4:1, and the total molar concentration of all metal ions was 6 mol / L. A fluxing agent and a carbon source were added. Citric acid was the flux, accounting for 0.5% of the weight of the solid raw materials, and sucrose was the carbon source, accounting for 5% of the weight of the solid raw materials. Pure water was added to a slurry with a solid content of 35%. After mixing, the mixture was ultrafine ground to a D50 of 200 nm. A lithium iron manganese phosphate precursor with a D50 of 10±5 μm was prepared by spray drying in a two-fluid spray dryer. The precursor was then subjected to carbothermal reduction under a nitrogen atmosphere to generate the lithium iron manganese phosphate electrode material. The sintering temperature was 750°C for 8 hours.

[0047] Comparative Examples and Example test results are as follows Figures 1 to 4 , as shown in Table 1.

[0048] 1. SEM test

[0049] See also Figures 1 to 4 , Figure 1 This is a 5000-fold magnified SEM image of the nitrogen-doped biochar-coated spherical lithium iron manganese phosphate composite electrode material prepared in Example 1 according to the method for preparing a nitrogen-doped biochar-coated spherical lithium iron manganese phosphate composite electrode material of the present invention; Figure 2 This is a 5000-fold magnified SEM image of the nitrogen-doped biochar-coated spherical lithium iron manganese phosphate composite electrode material prepared in Example 2 according to the method for preparing a nitrogen-doped biochar-coated spherical lithium iron manganese phosphate composite electrode material of the present invention; Figure 3This is a 5000-fold magnified SEM image of the nitrogen-doped biochar-coated spherical lithium iron manganese phosphate composite electrode material prepared in Example 3 of the method for preparing a nitrogen-doped biochar-coated spherical lithium iron manganese phosphate composite electrode material according to the present invention; Figure 4 The SEM image of the electrode material prepared in the comparative example was magnified 5000 times. Figures 1 to 4 As shown, the nitrogen-doped biochar-coated spherical lithium iron manganese phosphate composite electrode materials of Examples 1, 2, and 3 have a distinct spherical shape, uniform particle size, and a dense carbon coating on the surface; while the lithium iron manganese phosphate composite electrode material of the comparative example has poor sphericity, a wide particle size distribution, and an uneven and non-dense carbon coating on the surface. This indicates that after adopting the method described in the present invention, the biomass gel has a good coating effect on the primary particles of the lithium iron manganese phosphate precursor, not only suppressing the agglomeration and breakage of the precursor particles during the spray drying process, but also carbonizing into a uniform and dense nitrogen-doped carbon coating during the sintering process, which has a good coating effect on the lithium iron manganese phosphate material.

[0050] 2. Resistivity test

[0051] The powder resistivity of the lithium manganese iron phosphate materials in Examples 1, 2, 3 and the comparative example was tested by a powder resistivity tester, wherein the test pressure was 8 MPa.

[0052] The results are shown in the following table:

[0053] Example Powder resistivity Ω·cm Example 1 15.4 Example 2 25.2 Example 3 27.8 Comparative Example 139.7

[0054] Table 1

[0055] As shown in Table 1, the resistivity of Example 1 is significantly lower than that of Example 2 and Example 3. This is due to the higher amount of biomass gel added during the precursor preparation process. The powder resistivity in Example 2 and Example 3 decreased by 82.0% and 80.1% compared with the control example, demonstrating that nitrogen-doped biochar coating can effectively and significantly reduce the powder resistivity of lithium manganese iron phosphate and improve its electrochemical performance.

[0056] 3. Power-off test

[0057] The positive electrode material, conductive agent, and binder were mixed in an 8:1:1 ratio using N-methylpyrrolidone solvent, coated onto aluminum foil, and dried at 120°C for 6 hours. The dried electrode sheet was cut into discs. Coin-type cells were assembled using the positive electrode discs, separator, graphite anode, and electrolyte. The battery was charged and discharged within the 2.0-3.75V voltage window, with discharge capacities measured at 0.1C and 1C, respectively. The test results are shown in Table 2.

[0058]

[0059]

[0060] Table 2

[0061] As shown in Table 2, the discharge specific capacities at 0.1C and 1C for Examples 1, 2, and 3 were all higher than those for the comparative example. This is due, firstly, to the relatively uniform primary and secondary particles after the biomass gel coating the precursor, which prevents electrode polarization caused by large particles in the finished product. Secondly, the coating of the lithium manganese iron phosphate with highly conductive nitrogen-doped carbon significantly improves the material's conductivity, enhancing the transmission efficiency of electrons and lithium ions, and improving the capacity and rate capability of the electrode material.

[0062] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for preparing a nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material, characterized in that: Including steps: (1) preparing a biomass-coated gel: washing, crushing, and drying a biomass raw material with a high chitin content, adding the raw material to an alkaline solution, cooling the solution to room temperature after heating, filtering the solution with a mesh to remove the residue, adding an acidic solution to the filtrate to obtain a gel-like substance, and drying the solution to obtain a biomass-coated gel having a solid content in the range of 10% to 20%, wherein the biomass raw material with a high chitin content is any one or more of the shells of arthropods, molluscs, or fungi; (2) Preparing spherical lithium manganese iron phosphate slurry: weighing a lithium source, a manganese source, an iron source and a phosphorus source respectively, adding a flux and a carbon source to obtain a slurry, adding pure water to the slurry until the solid content of the slurry is 30-50%, mixing evenly and then ultrafine grinding to obtain a spherical lithium manganese iron phosphate slurry, wherein the flux is selected from phosphoric acid and / or citric acid, and the flux accounts for 0.1-1% of the mass of the solid raw material; the carbon source is selected from any one or more combinations of sugars, soluble polymers or urea, and the carbon source accounts for 1-10% of the mass of the solid raw material; (3) preparing a biomass gel-coated spherical lithium manganese iron phosphate precursor: adding the biomass-coated gel to the spherical lithium manganese iron phosphate slurry, stirring at a uniform speed after standing, and spray drying by a multi-fluid spray dryer to obtain a biomass gel-coated spherical lithium manganese iron phosphate precursor, wherein the mass ratio of the biomass-coated gel to the spherical lithium manganese iron phosphate slurry is 0.1-1:1; (4) Preparation of nitrogen-doped biochar-coated spherical lithium manganese iron phosphate electrode material: The biomass gel-coated spherical lithium manganese iron phosphate precursor is subjected to carbon thermal reduction in a nitrogen atmosphere to generate a nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material, wherein the sintering temperature of the carbon thermal reduction is 600-900°C and the time is 5-10h.

2. The method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material according to claim 1, characterized in that: In step (1), the heating reaction temperature is 60-180°C, the pressure is 0.1-5 MPa, and the time is 0.1-10 h; the drying temperature for the drying to a solid content range of 10%-20% is 100°C.

3. The method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material according to claim 1, characterized in that: In step (1), the pH of the alkaline solution is 12-14, and the alkaline solution is selected from any one or more combinations of sodium hydroxide solution, potassium hydroxide solution, sodium carbonate solution, sodium bicarbonate solution, potassium carbonate solution or potassium bicarbonate solution; The pH of the acidic solution is 2, and the acidic solution is any one or more combinations of phosphoric acid solution, hydrochloric acid solution or citric acid solution.

4. The method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material according to claim 1, characterized in that: In step (1), the sieve is 400 mesh.

5. The method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material according to claim 1, characterized in that: In step (2), the molar ratio of lithium in the lithium source, manganese in the manganese source, iron in the iron source and phosphorus in the phosphorus source is X Li :X Mn :X Fe :1, where 0.95≤X Li ≤1.05、0≤X Mn ≤1, X Fe =1-X Mn , the total molar concentration of all metal ions is 1-10 mol / L.

6. The method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material according to claim 1, characterized in that: In step (2), the ultrafine grinding is to control the grinding D50 to be 100-1000 nm.

7. The method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material according to claim 1, characterized in that: In step (3), the standing time is 1-10 h; the stirring speed is 100-1000 rpm, and the time is 10-120 min.

8. The method for preparing the nitrogen-doped biochar-coated spherical lithium manganese iron phosphate composite electrode material according to claim 1, characterized in that: In step (3), the D50 of the biomass gel-coated spherical lithium manganese iron phosphate precursor is 1-20 μm.

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