Lithium ferric manganese phosphate material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of battery materials, and particularly relates to a lithium ferric manganese phosphate material and a preparation method and application thereof.The lithium ferric manganese phosphate material is doped with sodium, the surface of the lithium ferric manganese phosphate material is of a carbon shell coating structure, the carbon shell coating structure comprises an inner layer and an outer layer, the inner layer is a sulfur-doped carbon nanotube, and the outer layer is a sulfur-doped carbon nanotube. And the outer layer is made of honeycomb-shaped carbon.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and particularly relates to a lithium iron manganese phosphate material, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium iron phosphate is widely used in power batteries due to its high stability and low cost. However, due to the olivine structure of lithium iron phosphate, Li + can only move one-dimensionally inside the material, and Li + cannot quickly deintercalate and intercalate between the positive and negative electrodes, resulting in generally poor electrochemical performance of the batteries using lithium iron phosphate materials, which limits the application of lithium iron phosphate materials in batteries. Therefore, it is urgent to improve the existing lithium iron phosphate materials to improve their electrochemical performance. Summary of the Invention

[0003] The present invention aims to solve at least one of the technical problems existing in the related art. For this purpose, the present invention provides a lithium iron manganese phosphate material, a preparation method thereof, and an application thereof, and the lithium iron manganese phosphate material has excellent conductivity and cycle performance.

[0004] The above technical object of the present invention is achieved by the following technical solutions:

[0005] A lithium iron manganese phosphate material, in which sodium is doped, and the surface of the lithium iron manganese phosphate material is provided with a carbon shell coating structure, and the carbon shell coating structure includes an inner layer and an outer layer, the inner layer is a sulfur-doped carbon nanotube, and the outer layer is a honeycomb carbon.

[0006] In one embodiment, the chemical formula of the lithium iron manganese phosphate material is: Na 1-X Li X Mn 1-Y Fe Y PO4 / C, where 0.93 ≤ X ≤ 0.99 and 0.85 ≤ Y ≤ 0.95.

[0007] A preparation method of a lithium iron manganese phosphate material, comprising the following steps:

[0008] (1) Mix a lithium source, an iron source, a sodium source, and a manganese source and dissolve them in an acidic solution, and then add a morphology control agent to the acidic solution to obtain a mixed solution;

[0009] (2) Heat the mixed solution in step (1) by microwave for a phase change reaction, and obtain a precursor after centrifugal drying;

[0010] (3) Mix the precursor obtained in step (2) with polythiophene, and heat and pyrolyze it in a sulfur-containing atmosphere to generate an S-CNTs coating layer;

[0011] (4) Activate the biomass carbon source with an alkali solution and then mix it with the precursor treated in step (3). Heat and carbonize it under a protective atmosphere to form a honeycomb-like carbon shell on the surface. After washing and drying, the finished product is obtained.

[0012] In one embodiment, it further includes step (5): Perform a lithiophilic modification on the surface of the carbon shell of the finished product prepared in step (4) by plasma etching. The plasma etching lithiophilic modification regulates the surface morphology, removes impurities or weakly bonded layers, increases the active sites of the material, promotes the infiltration of the electrolyte, and thus shows enhanced lithium ion adsorption and reduced Li + diffusion energy barrier.

[0013] In one embodiment, in step (1), the lithium source is at least one of LiOH·H2O, Li2CO3, CH3COOLi, and Li2C2O4.

[0014] In one embodiment, in step (1), the iron source is at least one of FeSO4·7H2O, FeC2O4, and Fe2O3.

[0015] In one embodiment, in step (1), the sodium source is at least one of NaH2PO4, Na2CO3, and NaOH.

[0016] In one embodiment, in step (1), the manganese source is at least one of Mn(CH3COO)2, MnSO4, and MnCl2.

[0017] In one embodiment, in step (1), the acidic solution is a solution formed by mixing ethylene glycol and citric acid at a volume ratio of (2 - 4):1.

[0018] In one embodiment, in step (1), the morphology control agent is polyvinylpyrrolidone.

[0019] In one embodiment, the concentration of the polyvinylpyrrolidone in the mixed solution is 0.05 - 0.2 mol / L.

[0020] In one embodiment, in step (2), the power of the microwave is 300 - 800 w, the heating temperature is 100 - 150 °C, and the heating time is 1 - 3 h.

[0021] In one embodiment, in step (3), the precursor and polythiophene are mixed at a mass ratio of 1:(0.1 - 0.2).

[0022] In one embodiment, in step (3), the sulfur-containing atmosphere is formed by mixing argon and hydrogen sulfide at a volume ratio of (92 - 98):(2 - 8).

[0023] In one embodiment, in step (3), the temperature of the thermal cracking is 600 - 1000 °C, and the time of the thermal cracking is 1 - 3 h.

[0024] In one embodiment, in step (4), the biomass carbon source is at least one of rice husk, straw and bamboo charcoal.

[0025] In one embodiment, in step (4), the alkali solution is at least one of KOH solution and NaOH solution.

[0026] In one embodiment, in step (4), the mass ratio of the biomass carbon source to the treated precursor is 1:(1 - 1.1).

[0027] In one embodiment, in step (4), the temperature of the thermal carbonization is 400 - 600 °C, and the time of the thermal carbonization is 2 - 5 h.

[0028] In one embodiment, in step (4), the washing includes first cleaning with an acid solution to remove the residual alkali solution, and then washing with water.

[0029] In one embodiment, in step (4), the temperature of the drying is 100 - 150 °C, and the time of the drying is 1 - 3 h.

[0030] In one embodiment, in step (5), the power of the plasma etching is 100 - 300 w, the time of the plasma etching is 5 - 15 min, and the atmosphere of the plasma etching is composed of argon and oxygen in a volume ratio of 1:(0.9 - 1.2).

[0031] A lithium ion battery includes the lithium iron manganese phosphate material as described above.

[0032] The beneficial effects of the present invention are:

[0033] (1) The lithium iron manganese phosphate material of the present invention adopts a gradient co-doping system. By partially replacing Li + sites with Na + to broaden the Li + diffusion channels, refine the particle size of the material, improve the lithium ion diffusion coefficient, and introduce Mn 2+ to partially replace Fe 2+ sites to inhibit the Jahn-Teller effect, improve the charge and discharge capacity, and also improve the electrical conductivity of the material;

[0034] (2) The lithium iron manganese phosphate material of the present invention includes a hetero-carbon shell coating structure:

[0035] A. Inner layer: S-doped carbon nanotube network: Sulfur-doped carbon nanotubes (S-CNTs) are generated by in-situ pyrolysis of polythiophene and a precursor at high temperature in a specific sulfur-containing atmosphere. The thickness is 50 - 80 nm, which introduces polar sites and enhances the chemical bonding force (C-O-Fe) with lithium iron manganese phosphate particles. Compared with the combination method of introducing a carbon layer and the material by traditional high-temperature physical sintering, its bonding strength is higher. The three-dimensional conductivity formed by the S-CNTs nanotubes during cycling is 5.3*10 -2 S / cm, which is significantly higher than that of traditional graphitized carbon at 1*10 -2 S / cm.

[0036] B. Outer layer: Honeycomb hard carbon: After activating the biomass carbon source with alkali solution, a honeycomb porous structure is formed. The porosity is higher and more uniform than that of traditional coatings, and at the same time, the electrolyte infiltration ability is improved. Both its material conductivity and mechanical buffering ability are enhanced;

[0037] With the synergistic effect of the high conductivity of the inner layer S-CNTs and the honeycomb hard carbon in the outer layer, the capacity retention rate of the material is >90% after 1000 cycles at a 0.5C rate, and the volume expansion rate ≤0.8%.

[0038] (3) The preparation method of the present invention is green and efficient:

[0039] A. Microwave - rheological phase method: It realizes atomic-level mixing, avoiding element segregation caused by traditional solvothermal method and high-temperature solid-phase method, and its energy consumption is reduced by about 35%;

[0040] B. The biomass carbon source is easily available. After alkali activation + plasma etching to optimize the interface, its silicon impurity removal rate >98.5%, the carbon yield can reach 40%, and the cost is also lower than that of traditional carbon sources. Description of the Drawings

[0041] Figure 1 XRD pattern of the lithium iron manganese phosphate material prepared in Examples 1 - 3 of the present invention;

[0042] Figure 2 SEM image of the lithium iron manganese phosphate material prepared in Example 1 of the present invention;

[0043] Figure 3 EDS image of the lithium iron manganese phosphate material prepared in Example 1 of the present invention;

[0044] Figure 4 SEM image of the lithium iron manganese phosphate material prepared in Comparative Example 1. Detailed Embodiments

[0045] The present invention will be further described below in conjunction with specific embodiments.

[0046] Example 1:

[0047] A lithium iron manganese phosphate material, in which sodium is doped in the lithium iron manganese phosphate material, and the surface of the lithium iron manganese phosphate material is set as a carbon shell coating structure. The inner layer of the coating structure is sulfur-doped carbon nanotubes, and the outer layer is honeycomb-shaped carbon. The chemical formula of the lithium iron manganese phosphate material is: Na 0.03 Li 0.97 Mn 0.1 Fe 0.9 PO4 / C.

[0048] The preparation method of the above lithium iron manganese phosphate material includes the following steps:

[0049] (1) Preparation of the feed solution: Mix LiOH·H2O, FeSO4·7H2O, NaH2PO4, and Mn(CH3COO)2 according to the molar ratio of Li:Fe:Na:Mn = 0.97:0.9:0.03:0.1, and dissolve them in an acidic solution formed by mixing ethylene glycol and citric acid in a volume ratio of 4:1. Add polyvinylpyrrolidone as a morphology control agent to the acidic solution to obtain a mixed solution, so that the concentration of polyvinylpyrrolidone in the mixed solution is 0.1 mol / L;

[0050] (2) Microwave flow-phase reaction: Heat the mixed solution by microwave, with a microwave power of 500 w, a reaction temperature of 120 °C, and a reaction duration of 2 h. After centrifugation and drying, a precursor is obtained;

[0051] (3) Growth of the inner layer of S-CNTs: Mix the precursor and polythiophene according to a mass ratio of 1:0.15, and heat it to 700 °C in an atmosphere of argon and hydrogen sulfide mixed in a volume ratio of 95:5 for 1 h to generate an S-CNTs coating layer;

[0052] (4) Construction of the outer layer of honeycomb hard carbon: After activating rice husk with a KOH solution, mix it with the treated precursor according to a mass ratio of 1:1.05, and carbonize it at 600 °C in nitrogen for 3 h to form a honeycomb-shaped carbon shell on the surface, and then wash it with hydrochloric acid to remove the residual alkali;

[0053] (5) Wash it with deionized water, and after washing it clean, put it in an oven at 110 °C and dry it for 2 h to obtain the finished product;

[0054] (6) Interface optimization treatment: Use plasma etching to perform lithiation modification on the surface of the carbon shell of the finished product to improve the lithium ion migration rate. The power of plasma etching is 200 w, the time of plasma etching is 10 min, and the atmosphere of plasma etching is composed of argon and oxygen in a volume ratio of 1:1. The XRD pattern of the prepared lithium iron manganese phosphate material is as shown by the yellow line in Figure 1 The SEM image of the prepared lithium iron manganese phosphate material is as shown in Figure 2 The EDS image of the prepared lithium iron manganese phosphate material is as shown in Figure 3 as shown.Figure 3 Its C / S peak comes from the inner sulfur-doped carbon nanotubes, and the Na / Mn peak is also relatively obvious, indicating a high degree of elemental doping binding, and the material meets the expected design.

[0055] Example 2:

[0056] A lithium iron manganese phosphate material, in which the lithium iron manganese phosphate material is doped with sodium, and the surface of the lithium iron manganese phosphate material is set as a carbon shell coating structure. The inner layer of the coating structure is sulfur-doped carbon nanotubes, and the outer layer is honeycomb-shaped carbon. The chemical formula of the lithium iron manganese phosphate material is: Na 0.03 Li 0.97 Mn 0.1 Fe 0.9 PO4 / C.

[0057] The preparation method of the above lithium iron manganese phosphate material includes the following steps:

[0058] (1) Preparation of the feed solution: Mix Li2CO3, FeC2O4, Na2CO3, and MnSO4 in a molar ratio of Li:Fe:Na:Mn = 0.97:0.9:0.03:0.1, dissolve them in an acidic solution formed by mixing ethylene glycol and citric acid in a volume ratio of 2:1, and add polyvinylpyrrolidone as a morphology control agent to the acidic solution to obtain a mixed solution, so that the concentration of polyvinylpyrrolidone in the mixed solution is 0.1 mol / L;

[0059] (2) Microwave flow phase reaction: Heat the mixed solution by microwave, with a microwave power of 600 w, a reaction temperature of 150 °C, and a reaction duration of 2 h. After centrifugation and drying, a precursor is obtained;

[0060] (3) Growth of inner layer S-CNTs: Mix the precursor and polythiophene in a mass ratio of 1:0.15, heat them to 700 °C in an atmosphere of argon and hydrogen sulfide mixed in a volume ratio of 95:5 and pyrolyze for 1 h to form an S-CNTs coating layer;

[0061] (4) Construction of the outer layer of honeycomb hard carbon: Activate the straw with a KOH solution and mix it with the treated precursor in a mass ratio of 1:1.05. Carbonize it at 600 °C in nitrogen for 3 h to form a honeycomb-shaped carbon shell on the surface, and then wash it with hydrochloric acid to remove the residual alkali;

[0062] (5) Wash it with deionized water, and after washing it clean, put it in an oven at 110 °C and dry it for 2 h to obtain the finished product;

[0063] (6) Interface optimization treatment: Plasma etching is used to perform lithiophilic modification on the surface of the carbon shell of the finished product to improve the lithium ion migration rate. The power of plasma etching is 200 w, the time of plasma etching is 10 min, and the atmosphere of plasma etching is composed of argon and oxygen in a volume ratio of 1:1. The XRD pattern of the prepared lithium iron manganese phosphate material is as shown by the blue line in Figure 1 the following.

[0064] Example 3:

[0065] A lithium iron manganese phosphate material, in which sodium is doped in the lithium iron manganese phosphate material, and the surface of the lithium iron manganese phosphate material is set as a carbon shell coating structure. The inner layer of the coating structure is sulfur-doped carbon nanotubes, and the outer layer is honeycomb-shaped carbon. The chemical formula of the lithium iron manganese phosphate material is: Na 0.03 Li 0.97 Mn 0.1 Fe 0.9 PO4 / C.

[0066] The preparation method of the above lithium iron manganese phosphate material includes the following steps:

[0067] (1) Preparation of the feed liquid: CH3COOLi, Fe2O3, NaOH, and MnCl2 are mixed in a molar ratio of Li:Fe:Na:Mn = 0.97:0.9:0.03:0.1, dissolved in an acidic solution formed by mixing ethylene glycol and citric acid in a volume ratio of 2:1, and polyvinylpyrrolidone is added to the acidic solution as a morphology control agent to obtain a mixed solution, so that the concentration of polyvinylpyrrolidone in the mixed solution is 0.1 mol / L;

[0068] (2) Microwave flow phase reaction: The mixed solution is heated by microwave, the microwave power is 600 w, the reaction temperature is 120 °C, the reaction duration is 2 h, and the precursor is obtained after centrifugal drying;

[0069] (3) Growth of the inner layer of S-CNTs: The precursor and polythiophene are mixed in a mass ratio of 1:0.15, and heated to 700 °C in an atmosphere of argon and hydrogen sulfide mixed in a volume ratio of 95:5 for 1 h to generate an S-CNTs coating layer;

[0070] (4) Construction of the outer layer of honeycomb hard carbon: The bamboo charcoal is activated by KOH solution and then mixed with the treated precursor in a mass ratio of 1:1.05, and carbonized at 400 °C in nitrogen for 3 h to form a honeycomb-shaped carbon shell on the surface, and then washed with hydrochloric acid to remove the residual alkali;

[0071] (5) Wash it with deionized water, and after washing it clean, put it in an oven at 110 °C and dry it for 2 h to obtain the finished product;

[0072] (6) Interface optimization treatment: Plasma etching is used to perform lithiophilic modification on the surface of the carbon shell of the finished product to improve the lithium ion migration rate. The power of plasma etching is 200 w, the time of plasma etching is 10 min, and the atmosphere of plasma etching is composed of argon and oxygen according to a volume ratio of 1:1. The XRD pattern of the prepared lithium iron manganese phosphate material is as shown by the red line in Figure 1 , and it can be seen from Figure 1 that the synthesized material corresponds to the standard pattern of lithium iron phosphate, with high diffraction intensity and good crystallinity, and no other elemental impurity peaks appear in the pattern, indicating that Na + / Mn 2+ has entered the lattice of the lithium iron phosphate material, and Na + replaces part of Li + , and Mn 2+ replaces part of Fe 2+ . In summary, the materials of Examples 1, 2, and 3 can all synthesize Na 0.03 Li 0.97 Mn 0.1 Fe 0.9 PO4 / C materials.

[0073] Example 4:

[0074] A lithium iron manganese phosphate material, in which sodium is doped in the lithium iron manganese phosphate material, and the surface of the lithium iron manganese phosphate material is set as a carbon shell coating structure. The inner layer of the coating structure is sulfur-doped carbon nanotubes, and the outer layer is honeycomb-shaped carbon. The chemical formula of the lithium iron manganese phosphate material is: Na 0.07 Li 0.93 Mn 0.15 Fe 0.85 PO4 / C.

[0075] The preparation method of the above lithium iron manganese phosphate material includes the following steps:

[0076] (1) Preparation of the material solution: LiOH·H2O, FeSO4·7H2O, NaH2PO4, and Mn(CH3COO)2 are mixed according to a molar ratio of Li:Fe:Na:Mn = 0.93:0.85:0.07:0.15 and dissolved in an acidic solution formed by mixing ethylene glycol and citric acid according to a volume ratio of 2:1. Polyvinylpyrrolidone is added to the acidic solution as a morphology control agent to obtain a mixed solution, so that the concentration of polyvinylpyrrolidone in the mixed solution is 0.05 mol / L;

[0077] (2) Microwave flow phase reaction: The mixed solution is heated by microwave, the microwave power is 300 w, the reaction temperature is 100 °C, the reaction duration is 3 h, and the precursor is obtained after centrifugal drying;

[0078] (3) Growth of inner layer S-CNTs: Mix the precursor and polythiophene in a mass ratio of 1:0.1, heat to 600 °C in an atmosphere of argon and hydrogen sulfide mixed in a volume ratio of 92:8 and pyrolyze for 3 h to form an S-CNTs coating layer;

[0079] (4) Construction of outer layer honeycomb hard carbon: After activating rice husk with KOH solution, mix it with the treated precursor in a mass ratio of 1:1, heat to 400 °C in nitrogen and carbonize for 5 h to form a honeycomb-like carbon shell on the surface, and then wash it with hydrochloric acid to remove the residual alkali;

[0080] (5) Wash it with deionized water, and after washing clean, put it in an oven at 100 °C and dry for 3 h to obtain the finished product;

[0081] (6) Interface optimization treatment: Use plasma etching to perform lithiophilic modification on the surface of the carbon shell of the finished product to improve the lithium ion migration rate. The power of plasma etching is 100 w, the time of plasma etching is 15 min, and the atmosphere of plasma etching is composed of argon and oxygen in a volume ratio of 1:0.9.

[0082] Example 5:

[0083] A lithium iron manganese phosphate material, in which sodium is doped in the lithium iron manganese phosphate material, the surface of the lithium iron manganese phosphate material is set as a carbon shell coating structure, the inner layer of the coating structure is sulfur-doped carbon nanotubes, and the outer layer is honeycomb-like carbon. The chemical formula of the lithium iron manganese phosphate material is: Na 0.01 Li 0.99 Mn 0.05 Fe 0.95 PO4 / C.

[0084] The preparation method of the above lithium iron manganese phosphate material includes the following steps:

[0085] (1) Preparation of the material solution: Mix LiOH·H2O, FeSO4·7H2O, NaH2PO4, and Mn(CH3COO)2 in a molar ratio of Li:Fe:Na:Mn = 0.99:0.95:0.01:0.05, dissolve them in an acidic solution formed by mixing ethylene glycol and citric acid in a volume ratio of 4:1, and add polyvinylpyrrolidone as a morphology control agent to the acidic solution to obtain a mixed solution, so that the concentration of polyvinylpyrrolidone in the mixed solution is 0.2 mol / L;

[0086] (2) Microwave flow phase reaction: Heat the mixed solution by microwave, the microwave power is 800 w, the reaction temperature is 150 °C, the reaction duration is 1 h, and the precursor is obtained after centrifugal drying;

[0087] (3) Growth of inner layer of S-CNTs: Mix the precursor and polythiophene in a mass ratio of 1:0.2, and heat it to 1000 °C in an atmosphere of argon and hydrogen sulfide mixed in a volume ratio of 98:2 for pyrolysis for 1 h to form an S-CNTs coating layer;

[0088] (4) Construction of outer layer of honeycomb hard carbon: After activating rice husk with KOH solution, mix it with the treated precursor in a mass ratio of 1:1.1, heat it to 600 °C in nitrogen for carbonization for 2 h to form a honeycomb-like carbon shell on the surface, and then wash it with hydrochloric acid to remove the residual alkali;

[0089] (5) Wash it with deionized water, and after washing it clean, put it in an oven at 150 °C for drying for 1 h to obtain the finished product;

[0090] (6) Interface optimization treatment: Use plasma etching to perform lithiation modification on the surface of the carbon shell of the finished product to improve the lithium ion migration rate. The power of plasma etching is 300 w, the time of plasma etching is 5 min, and the atmosphere of plasma etching is composed of argon and oxygen in a volume ratio of 1:1.2.

[0091] Example 6: (The difference from Example 1 is that the interface optimization treatment is not carried out)

[0092] A lithium iron manganese phosphate material, in which sodium is doped in the lithium iron manganese phosphate material, the surface of the lithium iron manganese phosphate material is set as a carbon shell coating structure, the inner layer of the coating structure is sulfur-doped carbon nanotubes, and the outer layer is honeycomb-like carbon. The chemical formula of the lithium iron manganese phosphate material is: Na 0.03 Li 0.97 Mn 0.1 Fe 0.9 PO4 / C.

[0093] The preparation method of the above-mentioned lithium iron manganese phosphate material includes the following steps:

[0094] (1) Preparation of material solution: Mix LiOH·H2O, FeSO4·7H2O, NaH2PO4, and Mn(CH3COO)2 in a molar ratio of Li:Fe:Na:Mn = 0.97:0.9:0.03:0.1, dissolve them in an acidic solution formed by mixing ethylene glycol and citric acid in a volume ratio of 4:1, and add polyvinylpyrrolidone as a morphology control agent to the acidic solution to obtain a mixed solution, so that the concentration of polyvinylpyrrolidone in the mixed solution is 0.1 mol / L;

[0095] (2) Microwave flow phase reaction: Heat the mixed solution by microwave, the microwave power is 500 w, the reaction temperature is 120 °C, the reaction duration is 2 h, and the precursor is obtained after centrifugation and drying;

[0096] (3) Growth of inner layer S-CNTs: The precursor and polythiophene were mixed at a mass ratio of 1:0.15, and pyrolyzed at 700 °C for 1 h in an atmosphere of argon and hydrogen sulfide mixed at a volume ratio of 95:5 to form an S-CNTs coating layer;

[0097] (4) Construction of outer layer honeycomb hard carbon: The rice husk was activated with KOH solution and then mixed with the treated precursor at a mass ratio of 1:1.05, and carbonized at 600 °C for 3 h in nitrogen to form a honeycomb carbon shell on the surface, and then washed with hydrochloric acid to remove the residual alkali;

[0098] (5) Wash it with deionized water, and after washing it clean, put it in an oven at 110 °C and dry it for 2 h to obtain the finished product.

[0099] Comparative Example 1: (The difference from Example 1 is that only the first two steps are carried out)

[0100] A preparation method of lithium iron manganese phosphate material, comprising the following steps:

[0101] (1) Preparation of the material liquid: LiOH·H2O, FeSO4·7H2O, NaH2PO4, and Mn(CH3COO)2 were mixed at a molar ratio of Li:Fe:Na:Mn = 0.97:0.9:0.03:0.1, dissolved in an acidic solution formed by mixing ethylene glycol and citric acid at a volume ratio of 4:1, and polyvinylpyrrolidone was added to the acidic solution as a morphology control agent to obtain a mixed solution, so that the concentration of polyvinylpyrrolidone in the mixed solution was 0.1 mol / L;

[0102] (2) Microwave flow phase reaction: The mixed solution was heated by microwave, the microwave power was 500 w, the reaction temperature was 120 °C, and the reaction duration was 2 h. After centrifugation and drying, the lithium iron manganese phosphate material was obtained. The SEM image of the prepared lithium iron manganese phosphate material is as Figure 4 shown, compared with Figure 2 and Figure 4 it can be seen that for the lithium iron manganese phosphate material prepared by the preparation method of the lithium iron manganese phosphate material of the present invention, the carbon source is uniformly coated on the surface of the material particles, the particle size is uniform, and the consistency is good, which improves the particle agglomeration situation to a certain extent.

[0103] Comparative Example 2: (Preparation of traditional double-layer carbon-coated lithium iron manganese phosphate material)

[0104] A preparation method of lithium iron manganese phosphate material, comprising the following steps:

[0105] (1) Li2CO3, FeSO4.7H2O, MnCO3, and Na3PO4 were hydrothermally reacted, washed and dried according to the molar ratio: Li:Fe:Mn:P = 1.05:0.9:0.1:1 to obtain a precursor powder;

[0106] (2) Carbon Coating:

[0107] The first carbon coating (inner layer graphitized carbon): Carbon source selection: glucose, process parameters: inert gas (N2), calcination at 750 °C for 7 h to form a graphitized carbon coating layer;

[0108] The second carbon coating (outer layer amorphous carbon): Carbon source selection: phenolic resin. Process parameters: Mix the first coated material and the carbon source in a ratio of 10:0.5, carbonize at 700 °C for 4 h under an inert atmosphere to form an amorphous carbon outer layer;

[0109] (3) Post-treatment / Optimization

[0110] Anneal at 600 °C for 2 h to eliminate internal stress and obtain the product.

[0111] Test Example:

[0112] Measure the tap density and powder resistance of commercially available lithium iron phosphate, commercially available lithium manganese phosphate, lithium iron manganese phosphate materials of Examples 1-6 and Comparative Examples 1-2 respectively, and use commercially available lithium iron phosphate, commercially available lithium manganese phosphate, lithium iron manganese phosphate materials of Examples 1-6 and Comparative Examples 1-2 as the cathode material to prepare button cells. The specific steps include: uniformly mix the cathode material, conductive agent acetylene black and adhesive polyvinylidene fluoride in a mass ratio of 94:3:3 in N-methylpyrrolidone to make a slurry, then coat it on aluminum foil and place it in a vacuum drying oven to dry to make a positive electrode plate. The negative electrode plate is a lithium sheet, the electrolyte is 1.2 mol / L LiPF6-EC:DMC (volume ratio 1:1), and a polypropylene porous membrane is used as the separator. The battery is assembled in an argon glove box.

[0113] Perform electrochemical performance tests on the prepared batteries. At room temperature of 25 °C, test the first charge capacity and first efficiency at a charge-discharge voltage of 2.0 - 3.65 V and a first charge-discharge rate of 0.1 C; at room temperature of 25 °C, test the cycle performance of 1000 cycles at a charge-discharge voltage of 2.0 - 3.65 V and a charge-discharge rate of 0.5 C. The test results are shown in Table 1. At the same time, compare the preparation method of the lithium iron manganese phosphate material of Example 1 with the preparation method of the traditional double-layer carbon-coated lithium iron manganese phosphate material, and the results are shown in Table 2.

[0114] Table 1. Performance Test Results

[0115]

[0116]

[0117] Table 2. Comparison of Indexes between Heterogeneous Carbon Coating of Example 1 and Traditional Double-Layer Carbon Coating

[0118]

[0119] As can be seen from Table 1, the tap density of the lithium iron manganese phosphate material prepared by the preparation method of the lithium iron manganese phosphate material of the present invention can reach 2.5 g / cm 3 and above (meeting the third-generation lithium iron phosphate standard), the powder resistance is less than or equal to 29 Ω / cm, the first charge capacity at 0.1C can reach 163.8 mAh / g and above, the first efficiency at 0.1C can reach 97.6% and above, and the capacity retention rate at 0.5C after 1000 cycles can reach 92.8% and above. The 0.1C charge capacity of the lithium iron manganese phosphate material prepared by the preparation method of the lithium iron manganese phosphate material of the present invention is much higher than that of the commercially available lithium iron phosphate, perfectly retaining the high-capacity characteristics of lithium manganese phosphate. At the same time, the powder resistance is greatly reduced, and the first efficiency at 0.1C can meet the index requirements of lithium iron phosphate. All kinds of index data are stable, and the capacity retention rate at 0.5C is significantly improved compared with the commercially available lithium iron phosphate and commercially available lithium manganese phosphate.

[0120] In addition, by comparing Example 1 and Example 6, it can be seen that when other conditions remain unchanged, the powder resistance of the lithium iron manganese phosphate material finally obtained without interface optimization treatment will increase to a large extent. At the same time, the first efficiency at 0.1C and the capacity retention rate at 0.5C (1000 cycles) will both decrease; by comparing Example 1 and Comparative Example 1, it can be seen that when no S-CNTs coating layer is formed in the lithium iron manganese phosphate material and no outer honeycomb hard carbon is constructed, the powder resistance of the finally obtained lithium iron manganese phosphate material will increase significantly, the first charge capacity at 0.1C will decrease significantly, and the first efficiency at 0.1C and the capacity retention rate at 0.5C (1000 cycles) will both decrease significantly; by comparing Example 1 and Comparative Example 2, it can be seen that the lithium iron manganese phosphate material prepared by the preparation method of the lithium iron manganese phosphate material of the present invention has more excellent electrochemical performance than the lithium iron manganese phosphate material prepared by the traditional double-layer carbon coating method.

[0121] As can be seen from Table 2, the preparation method of the lithium iron manganese phosphate material of the present invention requires a lower process temperature and a shorter process duration compared with the traditional double-layer carbon coating method. At the same time, the interlayer bonding strength, electronic conductivity, volume expansion rate and biomass utilization rate of the obtained lithium iron manganese phosphate material are all better than those of the products obtained by the traditional double-layer carbon coating method.

Claims

1. A lithium iron manganese phosphate material, characterized in that: The lithium iron manganese phosphate material is doped with sodium, and the surface of the lithium iron manganese phosphate material is provided with a carbon shell coating structure. The carbon shell coating structure includes an inner layer and an outer layer. The inner layer is sulfur-doped carbon nanotubes, and the outer layer is honeycomb carbon.

2. The lithium iron manganese phosphate material according to claim 1, wherein: The chemical formula of the lithium iron manganese phosphate material is: Na 1-X Li X Mn 1-Y Fe Y PO4 / C, where 0.93 ≤ X ≤ 0.99 and 0.85 ≤ Y ≤ 0.

95.

3. The preparation method of the lithium iron manganese phosphate material according to any one of claims 1-2, characterized in that: It includes the following steps: (1) Mix a lithium source, an iron source, a sodium source and a manganese source and dissolve them in an acidic solution, and then add a morphology control agent to the acidic solution to obtain a mixed solution; (2) Heat the mixed solution in step (1) by microwave for a phase change reaction, and obtain a precursor after centrifugal drying; (3) Mix the precursor obtained in step (2) with polythiophene, and heat and pyrolyze in a sulfur-containing atmosphere to generate an S-CNTs coating layer; (4) Mix the biomass carbon source after being activated by an alkaline solution with the precursor treated in step (3), heat and carbonize it under a protective atmosphere to form a honeycomb carbon shell on the surface, wash it, and dry it to obtain the finished product.

4. The preparation method of the lithium iron manganese phosphate material according to claim 3, characterized in that: It further includes step (5): performing lithiophilic modification on the surface of the carbon shell of the finished product prepared in step (4) by plasma etching.

5. The preparation method of the lithium iron manganese phosphate material according to claim 3, characterized in that: In step (1), the acidic solution is a solution formed by mixing ethylene glycol and citric acid in a volume ratio of (2-4):

1.

6. The preparation method of the lithium iron manganese phosphate material according to claim 3, characterized in that: In step (1), the morphology control agent is polyvinylpyrrolidone.

7. The preparation method of the lithium iron manganese phosphate material according to claim 3, wherein: In step (2), the power of the microwave is 300-800w, the heating temperature is 100-150°C, and the heating time is 1-3h.

8. The preparation method of the lithium iron manganese phosphate material according to claim 3, characterized in that: In step (3), the temperature of the heat pyrolysis is 600-1000°C, and the time of the heat pyrolysis is 1-3h.

9. The preparation method of the lithium iron manganese phosphate material according to claim 3, wherein: In step (4), the temperature of the heat carbonization is 400-600°C, and the time of the heat carbonization is 2-5h.

10. A lithium-ion battery, characterized in that: It includes the lithium iron manganese phosphate material according to any one of claims 1-2.

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