A positive electrode material, a preparation method thereof, and a lithium ion battery

Abstract

In view of the problem that there is no positive electrode material with both high energy density and high ionic conductivity in the prior art, the embodiments of the present application provide a positive electrode material, a preparation method thereof and a lithium ion battery, so as to improve the ionic conductivity of the lithium iron manganese phosphate positive electrode material, thereby improving the capacity of the positive electrode material. The positive electrode material comprises: an inner layer material and a coating layer covering the inner layer material, the inner layer material is LiFe x Mn 1‑x PO4, the coating layer comprises Nb2C MXene and carbon; wherein 0 < x ≤ 0.5.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a positive electrode material and a preparation method thereof, and a lithium ion battery. Background Art

[0002] Lithium-ion batteries are widely used in a variety of fields due to their high operating potential, high specific energy, and environmentally friendly properties. For example, they are used in electric vehicles, computers, and other electronic products. For lithium-ion batteries, the active components, especially the composition of the cathode material, often play a decisive role in influencing their performance.

[0003] Due to its high safety, low cost and high cycle stability, olivine-structured lithium iron phosphate has been successfully industrialized. On this basis, in order to further improve the energy density of lithium iron phosphate, lithium iron manganese phosphate came into being. Compared with lithium iron phosphate, although the energy density of lithium iron manganese phosphate is improved, its capacity is difficult to volatilize due to its low ionic conductivity. In the prior art, in order to improve the ionic conductivity of lithium iron manganese phosphate materials, carbon coating is usually used to improve the ionic conductivity of lithium iron manganese phosphate materials. However, during the charge-discharge process, this carbon-coated lithium iron manganese phosphate positive electrode material has the problem of deterioration of the stability of the positive electrode material due to carbon deposition. Therefore, the prior art lacks a positive electrode material with excellent energy density and ionic conductivity. Summary of the Invention

[0004] In response to the problem that the existing technology lacks a positive electrode material with excellent energy density and ionic conductivity, the embodiments of the present application provide a positive electrode material and a preparation method thereof, and a lithium-ion battery, which are used to improve the ionic conductivity of the lithium iron manganese phosphate positive electrode material, thereby increasing the capacity of the positive electrode material.

[0005] In the first aspect, the present invention provides a positive electrode material, comprising: an inner layer material and a coating layer covering the inner layer material, wherein the inner layer material is LiFe x Mn 1-x PO4, the coating layer includes Nb2C MXene and carbon; wherein 0<x≤0.5.

[0006] The embodiment of the present application coats the lithium manganese iron phosphate material, and on the basis of utilizing the good energy density and stability of lithium manganese iron phosphate, based on the fact that the Nb2C MXene coating layer has a two-dimensional graphene-like structure and good mechanical properties, the Nb2C MXene coating layer can coat the inner layer material (i.e., lithium manganese iron phosphate material) in the form of layers, and improve the ionic conductivity of the positive electrode material, thereby improving the capacity of the above-mentioned positive electrode material. At the same time, the coating layer can inhibit the side reaction between the lithium manganese iron phosphate material and the electrolyte, thereby improving the stability of the lithium manganese iron phosphate positive electrode material. In addition, the Nb2C MXene coating layer can effectively inhibit the manganese ion (Mn) of the lithium manganese iron phosphate material in the above-mentioned positive electrode material during the charge-discharge process. 2+ ) dissolution phenomenon, thereby alleviating the problem of continuous dissolution of manganese ions during the charge-discharge process, which causes structural instability in the olivine-type lithium manganese iron phosphate material, thereby improving the stability of the lithium manganese iron phosphate cathode material. In addition, because the Nb2C MXene in the coating layer has a two-dimensional layered structure, this two-dimensional layered structure can ensure that the carbon in the coating layer is relatively flat and stably distributed in the spaces between its layers. Therefore, the problem of carbon deposition caused by carbon migration in the coating layer during the charge-discharge process is avoided, further improving the stability of the lithium manganese iron phosphate cathode material.

[0007] In a possible implementation manner, the mass of the coating layer is 15 wt%-19 wt% of the mass of the inner layer material.

[0008] In one possible implementation manner, the mass ratio between the carbon and the Nb2C MXene in the coating layer is 2.40:1-4.67:1.

[0009] In a possible implementation manner, the coating layer has a thickness of 1-5 nm.

[0010] In one possible embodiment, the compaction density of the positive electrode material is greater than 2.2 g / cm 3 .

[0011] In one possible implementation, the median particle size of the positive electrode material is 2-30 microns.

[0012] In a second aspect, the present invention provides a method for preparing a positive electrode material as described in the first aspect and any possible embodiment, comprising:

[0013] Sand milling a first slurry comprising an ammonium manganese iron phosphate precursor, a lithium source, a carbon source, and Nb2C MXene to a median particle size of 0.1-1.0 μm; wherein the molar ratio of the lithium source to the ammonium manganese iron phosphate is 1:1-1:1.06;

[0014] drying the first slurry to obtain a powder material;

[0015] The powder material is sintered at 680-800° C. for 5-20 hours to obtain a positive electrode material.

[0016] In a possible implementation manner, the solid content of the first slurry is 30%-80%, and the first slurry is obtained by a stirring device, wherein the rotation speed of the rotor in the stirring device is 1000-3000 rpm.

[0017] In one possible implementation manner, the added mass of the carbon source accounts for 12wt% to 14wt% of the mass of the lithium manganese iron phosphate, the added mass of the Nb2C MXene accounts for 3wt% to 5wt% of the mass of the lithium manganese iron phosphate, and the lithium manganese iron phosphate is obtained by reacting the ammonia manganese iron phosphate precursor with the lithium source.

[0018] In one possible implementation manner, the carbon source is at least one of sucrose, glucose, citric acid, polyvinyl alcohol, polyethylene glycol, carbon black, acetylene black, and graphene.

[0019] In one possible embodiment, before sand-milling the first slurry comprising the manganese iron ammonium phosphate precursor, the lithium source, the carbon source, and the Nb2C MXene, the following steps may be further performed:

[0020] Mixing soluble phosphate, ammonia, a manganese source, and an iron source in water at a pH of 5-7 to perform a coprecipitation reaction to generate a second slurry containing a manganese phosphate iron ammonia precursor;

[0021] The second slurry is filtered, washed and dried to obtain the manganese iron phosphate ammonia precursor; wherein the molecular formula of the manganese iron phosphate ammonia precursor is: NH4Mn 1-x Fe x PO4·H2O;0<x≤0.5.

[0022] In one possible implementation manner, the soluble phosphate is at least one of phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, calcium dihydrogen phosphate, calcium phosphate, ammonium phosphate, calcium monohydrogen phosphate, and ammonium pyrophosphate.

[0023] In a possible implementation manner, the manganese source is at least one of manganese monoxide, manganese dioxide, trimanganese tetraoxide, manganese sulfate, manganese carbonate, or manganese hydroxide.

[0024] In one possible implementation manner, the iron source is at least one of iron, ferrous sulfate, ferric oxide, ferric acetate, ferric phosphate, ferrous phosphate, ferric monohydrogen phosphate, ferric dihydrogen phosphate, ferric nitrate, and ferric chloride.

[0025] In a possible implementation manner, the coprecipitation reaction is carried out in the presence of an antioxidant; wherein the added mass of the antioxidant accounts for 0.4 wt% to 0.6 wt% of the mass of the iron source.

[0026] In one possible implementation manner, the antioxidant is ascorbic acid.

[0027] In one possible implementation manner, the protective atmosphere of the coprecipitation reaction is at least one of argon, nitrogen and carbon dioxide.

[0028] In one possible embodiment, before sand-milling the first slurry comprising the manganese iron ammonium phosphate precursor, the lithium source, the carbon source, and the Nb2C MXene, the following steps may be further performed:

[0029] dissolving Nb2AlC in water with hydrofluoric acid or a mixture of lithium fluoride and hydrochloric acid to perform a corrosion reaction to obtain a third slurry containing Nb2CMXene;

[0030] washing the precipitate of the third slurry until the washing solution is neutral;

[0031] The washed precipitate was dried at -60°C to -20°C to obtain Nb2C MXene.

[0032] In a third aspect, an embodiment of the present application further provides a lithium-ion battery, comprising:

[0033] The positive electrode material as described in the first aspect and any possible embodiment, or the positive electrode material prepared by the method as described in the second aspect and any possible embodiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a scanning electron microscope image of Synthesis Example 1 provided in the examples of this application;

[0035] Figure 2 This is a charge-discharge performance test diagram of device Example 1 provided in the embodiments of the present application;

[0036] Figure 3 This is a scanning electron microscope image of Synthesis Example 2 provided in the examples of this application;

[0037] Figure 4 This is a charge-discharge performance test diagram of device Example 2 provided in the embodiments of the present application;

[0038] Figure 5 This is a scanning electron microscope image of Synthesis Example 3 provided in the examples of this application;

[0039] Figure 6 This is a charge-discharge performance test diagram of device Example 3 provided in the embodiments of the present application;

[0040] Figure 7 The X-ray diffraction pattern of Synthesis Example 2 provided in the examples of this application;

[0041] Figure 8 This is a scanning electron microscope image of the synthetic comparative example 1 provided in the embodiments of the present application;

[0042] Figure 9 This is a comparison chart of the charge-discharge performance tests of device embodiment 2, device comparative example 1, and device comparative example 2 provided in the embodiments of the present application. DETAILED DESCRIPTION

[0043] In view of the problem that the prior art lacks a positive electrode material with good energy density and ion conductivity, the present application proposes a positive electrode material, which includes an inner layer material and a coating layer covering the inner layer material, wherein the inner layer material is LiFe x Mn 1-x PO4, the coating layer includes Nb2C MXene and carbon; wherein 0<x≤0.5. Based on the good stability and high energy density of the inner layer material, the conductivity and mechanical properties of the Nb2C MXene coating layer are used to coat the inner layer material and improve the capacity of the positive electrode material. At the same time, Nb2C MXene can also inhibit the side reaction between the inner layer material and the electrolyte and avoid the Mn 2+ The dissolution causes the olivine structure of the positive electrode material to loosen and then collapse, which effectively improves the positive electrode material of lithium manganese iron phosphate.

[0044] To further enhance the energy density of the aforementioned material, in one embodiment of the present application, the coating layer also includes carbon, thereby enhancing the electron transport capability of the positive electrode material during the charge-discharge process. Furthermore, because the Nb2C MXene in the coating layer is a two-dimensional layered structure, when carbon is present in the coating layer, it fills the interlayers of the MXene structure, resulting in a relatively uniform distribution of the carbon in the coating layer. The carbon does not migrate during the charge-discharge process, thus avoiding the problem of carbon deposition.

[0045] Furthermore, the coating layer has a thickness of 1-5 nm, a mass ratio of carbon to the Nb2C MXene in the coating layer of 2.40:1-4.67:1, and the mass of the coating layer is 15 wt%-19 wt% of the mass of the inner layer material.

[0046] In one embodiment of the present application, when the median particle size of the positive electrode material is 2-30 microns, the primary particle size of the positive electrode material is suitable and the crystal phase is fully grown, thereby exhibiting good electron transport capability during the charge-discharge process. Accordingly, the positive electrode material has a discharge specific capacity greater than 150 mAh / g for a lithium-ion battery and a first coulombic efficiency greater than 95%.

[0047] Furthermore, the compaction density of the above-mentioned positive electrode material is greater than 2.2g / cm 3 .

[0048] Based on the same inventive concept, an embodiment of the present application provides a method for preparing the above-mentioned positive electrode material, the method comprising:

[0049] Step 101: sand-milling a first slurry comprising an ammonium manganese iron phosphate precursor, a lithium source, a carbon source, and Nb2C MXene to achieve a median particle size of 0.1-1.0 μm.

[0050] The molar ratio of the lithium source to the manganese iron phosphate ammonia precursor is 1:1-1:1.06. The solvent of the first slurry is water.

[0051] The carbon source is added in an amount of 12 to 14 wt% of the lithium manganese iron phosphate, while the Nb2C MXene is added in an amount of 3 to 5 wt% of the lithium manganese iron phosphate. The lithium manganese iron phosphate corresponds to the inner layer material of the target product, which is obtained by reacting the ammonium manganese iron phosphate precursor and the lithium source in step 103.

[0052] To ensure uniform mixing and sufficient reaction of the substances in the first slurry, in one embodiment of the present application, the first slurry is prepared in a stirring apparatus (e.g., a stirred tank). The rotor speed of the stirring apparatus is 1000-3000 rpm, the temperature in the stirring apparatus is greater than 50°C, and the solids content of the first slurry is 30%-80%.

[0053] The carbon source is at least one of sucrose, glucose, citric acid, polyvinyl alcohol, polyethylene glycol, carbon black, acetylene black, and graphene.

[0054] Furthermore, the preparation of the manganese iron phosphate ammonia precursor is described below:

[0055] First, a soluble phosphate, ammonia, a manganese source, and an iron source are mixed in water at a pH of 5-7 to undergo a coprecipitation reaction, generating a second slurry containing a manganese phosphate iron ammonia precursor. The molar ratio of the soluble phosphate to the ammonia is 1:1 to 1.5-1. The molar ratio of the manganese source to the iron source relative to the ammonia is x:1-x, where 0 < x ≤ 0.5.

[0056] The soluble phosphate is at least one of phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, calcium dihydrogen phosphate, calcium phosphate, ammonium phosphate, calcium monohydrogen phosphate, and ammonium pyrophosphate.

[0057] When the soluble phosphate solution is added in the form of a solution, the concentration of the soluble phosphate solution is 1.0 mol / L to 1.5 mol / L. When ammonia is added in the form of a solution, the concentration of the ammonia water is 4.5 mol / L to 5.0 mol / L.

[0058] The manganese source is at least one of manganese monoxide, manganese dioxide, trimanganese tetraoxide, manganese sulfate, manganese carbonate, or manganese hydroxide.

[0059] The iron source is at least one of iron, ferrous sulfate, ferric oxide, ferric acetate, ferric phosphate, ferrous phosphate, ferrous monohydrogen phosphate, ferric dihydrogen phosphate, ferric nitrate, and ferric chloride.

[0060] Then, the second slurry is filtered, washed, and dried to obtain the manganese iron phosphate ammonia precursor; wherein the molecular formula of the manganese iron phosphate ammonia precursor is: NH4Fe x Mn 1-x PO4·H2O;0<x≤0.5.

[0061] The above washing step can wash away impurities such as sulfate ions introduced by the iron source and / or manganese source.

[0062] Furthermore, the drying temperature is 80-200° C., and the drying time is 2-12 hours.

[0063] In order to avoid the Fe 2+ The ions are oxidized to form Fe 3+ The coprecipitation reaction is carried out in the presence of an antioxidant, wherein the added amount of the antioxidant is 0.4 wt% to 0.6 wt% of the mass of the iron source, and the antioxidant can be ascorbic acid.

[0064] Furthermore, the protective atmosphere of the coprecipitation reaction is at least one of argon, nitrogen and carbon dioxide, thereby ensuring that any reactant and / or intermediate product does not undergo oxidation reaction during the coprecipitation reaction.

[0065] Furthermore, the preparation of Nb2C MXene is described below:

[0066] First, hydrofluoric acid or a mixture of lithium fluoride and hydrochloric acid is used as an etching solution and dissolved in water together with Nb2AlC to undergo a corrosion reaction to obtain a third slurry containing Nb2C MXene.

[0067] The molar ratio of the solute to Nb2AlC in the etching solution is 1:1.05 to 1.5:1.

[0068] Then, washing the precipitate of the third slurry until the washing solution is neutral;

[0069] Finally, the washed precipitate was dried at -60°C to -20°C to obtain Nb2C MXene.

[0070] Step 102: Dry the first slurry to obtain a powder material.

[0071] The first slurry is dried by a spray dryer. Corresponding drying parameters include: air inlet temperature of 150-360°C, air outlet temperature of 70-130°C, and atomizing disk rotation speed of 3000-20000 rpm.

[0072] Step 103: Sintering the powder material at 680-800° C. for 5-20 hours to obtain a positive electrode material.

[0073] Since the powder material has been ground in step 101, it has the characteristics of small and similar particle size. Therefore, the crystal phase of the powder material grows more uniformly during the sintering process, which makes the primary particle size in the positive electrode material relatively uniform, thereby improving the electron transmission ability of the positive electrode material during the charge-discharge process, so that the discharge specific capacity of the lithium-ion battery corresponding to the positive electrode material is greater than 150mAh / g, and the first coulombic efficiency is greater than 95%.

[0074] The inner layer material of the positive electrode material is LiFe x Mn 1-x PO4, the coating layer includes Nb2C MXene and carbon, wherein 0<x≤0.5.

[0075] Furthermore, since MXene is a two-dimensional layered material with high conductivity, rich surface functional groups (for example, -F, -O, -OH, etc.), and good mechanical properties, it can be adsorbed on the surface of the inner layer material in the form of deposition to form a coating layer under the action of lithium ions on the surface of the inner layer material during the sintering process, thereby promoting the release and embedding of lithium ions in the positive electrode material during the charge-discharge process.

[0076] On this basis, when the coating layer contains carbon, the carbon particles can combine with the Nb2C MXene structure and be embedded in the two-dimensional layered structure of Nb2C MXene, thereby promoting the electron transport ability and avoiding the phenomenon of carbon particles moving on the surface of the inner layer material to cause carbon deposition.

[0077] The following is a detailed description through Synthesis Examples 1-3, Synthesis Comparative Examples 1-2, Device Examples 1-3, and Device Comparative Examples 1-2.

[0078] Synthesis Example 1

[0079] S1. Ferrous sulfate and manganese sulfate were mixed in a molar ratio of 3:7, and 0.5wt% ascorbic acid was added based on the mass of ferrous sulfate to prepare a 1mol / L ferromanganese solution. 1.2mol / L ammonium dihydrogen phosphate solution (1L), 1.0mol / L ferromanganese solution (1L), and 4.8mol / L ammonia water (1L) were added to a stirred tank at a flow rate of 6.66ml / min, and the aforementioned solution was reacted at 60°C and 500rpm for 3h; during the reaction, the pH of the slurry in the stirred tank was controlled to 5 by controlling the flow rate of ammonia water. The obtained precipitate was filtered, washed with deionized water, and then dried at 110°C for 5h to obtain an ammoniacal manganese ferrophosphate precursor.

[0080] S2. Stir Nb2AlC, lithium fluoride, and hydrochloric acid in a stirred tank at a molar ratio of 1:0.8:1 for 12 hours at a stirring rate of 1000 rpm and a temperature of 30°C. Wash the resulting precipitate approximately eight times to neutralize the pH of the precipitate surface, and then dry the precipitate to a neutral pH. Dry at -45°C for 24 hours to obtain Nb2C MXene.

[0081] S3. Mix the ammonium manganese iron phosphate precursor with Nb2C MXene, lithium carbonate, anhydrous glucose, and polyethylene glycol, grind the mixture, dissolve it in water, and sand grind it in an oil bath at no more than 35°C and 1800 rpm to obtain a slurry with a median particle size of 0.11 μm. The molar ratio of the lithium source to the ammonium manganese iron phosphate precursor is 1.02:1. The Nb2C MXene accounts for 3 wt% of the inner layer material in the target product, the anhydrous glucose accounts for 4 wt% of the inner layer material, and the polyethylene glycol accounts for 8 wt% of the inner layer material.

[0082] S4. The slurry is placed in a spray dryer and dried at an air inlet temperature of 180° C., an air outlet temperature of 85° C., and an atomizing disk speed of 8000 rpm to obtain a powder material.

[0083] S5. Sintering at 600℃ for 8h in nitrogen atmosphere to obtain positive electrode material. The inner layer material of the positive electrode material is LiFe 0.5 Mn 0.5 PO4, coated with Nb2C MXene and carbon. Please refer to the morphology of this cathode material Figure 1 .

[0084] Synthesis Example 2

[0085] S1. Ferrous sulfate and manganese sulfate were mixed in a molar ratio of 2:3, and 0.05wt% ascorbic acid was added to the mass of ferrous sulfate to prepare a 1mol / L ferromanganese solution. 1.4mol / L ammonium dihydrogen phosphate solution (1L), 1.0mol / L ferromanganese solution (1L), and 4.8mol / L ammonia water (1L) were added to a stirred tank at a flow rate of 6.66ml / min, and the aforementioned solution was reacted at 65°C and 600rpm for 4h; during the reaction, the pH of the slurry in the stirred tank was controlled to 6 by controlling the ammonia flow rate. The obtained precipitate was filtered, washed with deionized water, and dried at 120°C for 5h to obtain an ammoniacal manganese ferrophosphate precursor.

[0086] S2. Stir Nb2AlC, lithium fluoride, and hydrochloric acid in a stirred tank at a molar ratio of 1:1:1 for 12 hours at a stirring rate of 1200 rpm and a temperature of 35°C. Wash the resulting precipitate approximately eight times to a neutral pH, and then dry the resulting precipitate to a neutral pH. Dry at -50°C for 36 hours to obtain Nb2C MXene.

[0087] S3. Mix the ammonium manganese iron phosphate precursor with Nb2C MXene, lithium carbonate, anhydrous glucose, and polyethylene glycol, grind the mixture, dissolve it in water, and sand grind it in an oil bath at no more than 40°C and 2000 rpm to obtain a slurry with a median particle size of 0.12 μm. The molar ratio of the lithium source to the ammonium manganese iron phosphate precursor is 1.03:1. Nb2C MXene constitutes 3.5 wt% of the inner layer material in the target product, anhydrous glucose constitutes 4 wt% of the inner layer material, and polyethylene glycol constitutes 7 wt% of the inner layer material.

[0088] S4. The slurry is placed in a spray dryer and dried at an air inlet temperature of 190° C., an air outlet temperature of 90° C., and an atomizing disk rotation speed of 10,000 rpm to obtain a powder material.

[0089] S5. Sinter at 650℃ for 9h in nitrogen atmosphere to obtain positive electrode material. The inner layer material of the positive electrode material is LiFe 0.4 Mn 0.6 PO4, coated with Nb2C MXene and carbon. Please refer to the morphology of this cathode material Figure 3 , please refer to the XRD pattern Figure 7 .

[0090] Synthesis Example 3

[0091] S1. Ferrous sulfate and manganese sulfate were mixed in a molar ratio of 1:1, and 0.05wt% ascorbic acid was added to the mass of ferrous sulfate to prepare a 1mol / L ferromanganese solution. 1.4mol / L ammonium dihydrogen phosphate solution (1L), 1.0mol / L ferromanganese solution (1L), and 4.8mol / L ammonia water (1L) were added to a stirred tank at a flow rate of 6.68ml / min, and the aforementioned solution was reacted at 70°C and 700rpm for 5h; during the reaction, the pH of the slurry in the stirred tank was controlled to 7 by controlling the ammonia flow rate. The obtained precipitate was filtered, washed with deionized water, and dried at 130°C for 5h to obtain an ammoniacal manganese ferrophosphate precursor.

[0092] S2. Stir Nb2AlC, lithium fluoride, and hydrochloric acid in a stirred tank at a molar ratio of 1:1.2:1 for 48 hours. The stirring rate is 1400 rpm and the temperature is 40°C. Wash the resulting precipitate approximately eight times to neutralize the pH of the precipitate surface. Dry the precipitate to a neutral pH. Dry at -55°C for 48 hours to obtain Nb2C MXene.

[0093] S3. Mix the ammonium manganese iron phosphate precursor with Nb2C MXene, lithium carbonate, anhydrous glucose, and polyethylene glycol, grind the mixture, dissolve it in water, and sand grind it in an oil bath at no more than 45°C and 2200 rpm to obtain a slurry with a median particle size of 0.14 μm. The molar ratio of the lithium source to the ammonium manganese iron phosphate precursor is 1.04:1. Nb2C MXene accounts for 4 wt% of the inner layer material in the target product, anhydrous glucose accounts for 4 wt% of the inner layer material, and polyethylene glycol accounts for 8 wt% of the inner layer material.

[0094] S4. Dry the slurry in a spray dryer with an air inlet temperature of 200° C., an air outlet temperature of 100° C., and an atomizing disk speed of 12,000 rpm to obtain a powder material.

[0095] S5. Sintering at 700℃ for 9h in nitrogen atmosphere to obtain positive electrode material. The inner layer material of the positive electrode material is LiFe 0.5 Mn 0.5 PO4, coated with Nb2C MXene and carbon. Please refer to the morphology of this cathode material Figure 5 .

[0096] Comparative Synthesis Example 1

[0097] S1. Ferrous sulfate and manganese sulfate were mixed in a molar ratio of 2:3, and 0.05wt% ascorbic acid was added to the mass of ferrous sulfate to prepare a 1mol / L ferromanganese solution. 1.4mol / L ammonium dihydrogen phosphate solution (1L), 1.0mol / L ferromanganese solution (1L), and 4.8mol / L ammonia water (1L) were added to a stirred tank at a flow rate of 6.66ml / min, and the aforementioned solution was reacted at 65°C and 600rpm for 4h; during the reaction, the pH of the slurry in the stirred tank was controlled to 6 by controlling the ammonia flow rate. The obtained precipitate was filtered, washed with deionized water, and dried at 120°C for 5h to obtain an ammoniacal manganese ferrophosphate precursor.

[0098] S2. Mix the ammonium manganese iron phosphate precursor with lithium carbonate, anhydrous glucose, and polyethylene glycol, grind the mixture, and dissolve it in water. Sand mill the mixture at a temperature not exceeding 40°C and a rotation speed of 2000 rpm to obtain a slurry with a median particle size of 0.12 μm. The molar ratio of the lithium source to the ammonium manganese iron phosphate precursor is 1.03:1. Nb2C MXene constitutes 3.5 wt% of the inner layer material in the target product, anhydrous glucose constitutes 4 wt% of the inner layer material, and polyethylene glycol constitutes 7 wt% of the inner layer material.

[0099] S3. Dry the slurry in a spray dryer with an air inlet temperature of 190° C., an air outlet temperature of 90° C., and an atomizing disk rotation speed of 10,000 rpm to obtain a powder material.

[0100] S4, sintering at 700℃ for 9h in nitrogen atmosphere to obtain positive electrode material. The inner layer material of the positive electrode material is LiFe 0.4 Mn 0.6 PO4, the coating layer is carbon. Please refer to the morphology of this positive electrode material Figure 8 .

[0101] Comparative Synthesis Example 2

[0102] S1. Ferrous sulfate and manganese sulfate were mixed in a molar ratio of 2:3, and 0.05wt% ascorbic acid was added to the mass of ferrous sulfate to prepare a 1mol / L ferromanganese solution. 1.4mol / L ammonium dihydrogen phosphate solution (1L), 1.0mol / L ferromanganese solution (1L), and 4.8mol / L ammonia water (1L) were added to a stirred tank at a flow rate of 6.68ml / min, and the aforementioned solution was reacted at 65°C and 600rpm for 4h; during the reaction, the pH of the slurry in the stirred tank was controlled to 6 by controlling the ammonia flow rate. The obtained precipitate was filtered, washed with deionized water, and dried at 120°C for 5h to obtain an ammoniacal manganese ferrophosphate precursor.

[0103] S2. Stir Nb2AlC, lithium fluoride, and hydrochloric acid in a stirred tank at a molar ratio of 1:1:1 for 24 hours at a stirring rate of 1200 rpm and a temperature of 35°C. Wash the resulting precipitate approximately eight times to a neutral pH, and then dry the resulting precipitate to a neutral pH. Dry at -50°C for 36 hours to obtain Nb2C MXene.

[0104] S3. Mix the ammonium manganese iron phosphate precursor, Nb2C MXene, and lithium carbonate, grind them, and dissolve them in water. Sand mill them in an oil bath at a temperature not exceeding 40°C and a rotation speed of 2000 rpm to obtain a slurry with a median particle size of 0.12 μm. The molar ratio of the lithium source to the ammonium manganese iron phosphate precursor is 1.03:1. Nb2C MXene constitutes 3.5 wt% of the inner layer material of the target product, anhydrous glucose constitutes 4 wt% of the inner layer material, and polyethylene glycol constitutes 7 wt% of the inner layer material.

[0105] S4. The slurry is placed in a spray dryer and dried at an air inlet temperature of 190° C., an air outlet temperature of 90° C., and an atomizing disk rotation speed of 10,000 rpm to obtain a powder material.

[0106] S5. Sinter at 650℃ for 9h in nitrogen atmosphere to obtain positive electrode material. The inner layer material of the positive electrode material is LiFe 0.4 Mn 0.6 PO4, with Nb2C MXene coating layer.

[0107] Device Examples 1-3, Device Comparative Examples 1-2

[0108] Device Examples 1-3 and Comparative Examples 1-2 are button-type batteries prepared using Synthesis Examples 1-3 and Comparative Examples 1-2 as positive electrode materials, respectively.

[0109] The positive electrode sheet in device examples 1-3 and device comparative example 1 is coated with 8-10 mg / cm2 of the coating on one side of the negative electrode sheet. 2 The positive electrode material is a 15mm diameter circular aluminum sheet. Furthermore, the electrolyte for device examples 1-3 and device comparative example 1 consists of 1 mol / L LiPF6 mixed with a 1:1 molar ratio mixture of ethylene carbonate and dimethyl carbonate (EC / DMC). The negative electrode is a lithium sheet.

[0110] The discharge capacity and initial coulombic efficiency were tested for device examples 1-3 and device comparison examples 1-2. Figure 2 、 Figure 4 、 Figure 6 ,and Figure 9 , please refer to Table 1 for test data.

[0111] Table 1

[0112] raw material Discharge capacity (mAh / g) First coulombic effect (%) Device Example 1 155.52 96.12 Device Example 2 156.91 98.89 Device Example 3 151.55 97.22 Device Comparative Example 1 147.56 95.49 Device Comparative Example 2 141.1 94.38

[0113] As can be seen from Table 1, the discharge capacity and first coulombic efficiency of device examples 1-3 are better than those of device comparison examples 1-2.

[0114] Obviously, those skilled in the art may make various changes and modifications to this application without departing from the spirit and scope of this application. Thus, if these modifications and variations of this application fall within the scope of the claims of this application and their equivalents, this application is intended to include these modifications and variations.

Claims

1. A positive electrode material, characterized in that include: Inner layer material and a coating layer covering the inner layer material, wherein the inner layer material is LiFe x Mn 1-x PO4, the coating layer comprises Nb2C MXene and carbon; Where, 0<x≤0.5; The mass ratio between the carbon and the Nb2C MXene in the coating layer is (2.40-4.67):1; Nb2C MXene is a two-dimensional layered structure, and the carbon in the coating layer is distributed in the space between Nb2C MXene layers.

2. The positive electrode material according to claim 1, wherein The mass of the coating layer is 15 wt% to 19 wt% of the mass of the inner layer material.

3. The positive electrode material according to claim 1 or 2, characterized in that The thickness of the coating layer is 1-5 nm.

4. The positive electrode material according to claim 3, wherein The median particle size of the positive electrode material is 2-30 microns.

5. The positive electrode material according to claim 3, wherein The compaction density of the positive electrode material is greater than 2.2 g / cm 3 .

6. A method for preparing the positive electrode material according to any one of claims 1 to 5, characterized in that: include: Sand milling a first slurry comprising an ammonium manganese iron phosphate precursor, a lithium source, a carbon source, and Nb2C MXene to a median particle size of 0.1-1.0 μm; wherein the molar ratio of the lithium source to the ammonium manganese iron phosphate is 1:(1-1.06); drying the first slurry to obtain a powder material; The powder material is sintered at 680-800° C. for 5-20 hours to obtain a positive electrode material.

7. The method according to claim 6, wherein The solid content of the first slurry is 30%-80%. The first slurry is obtained by a stirring device, and the rotation speed of the rotor in the stirring device is 1000-3000 rpm.

8. The method according to claim 6, wherein The added mass of the carbon source accounts for 12wt% to 14wt% of the mass of the lithium manganese iron phosphate, and the added mass of the Nb2C MXene accounts for 3wt% to 5wt% of the mass of the lithium manganese iron phosphate. The lithium manganese iron phosphate is obtained by reacting the ammonia manganese iron phosphate precursor with the lithium source.

9. The method according to claim 6, wherein The carbon source is at least one of sucrose, glucose, citric acid, polyvinyl alcohol, polyethylene glycol, carbon black and graphene.

10. The method according to any one of claims 7 to 9, characterized in that: Before sand-milling the first slurry comprising the manganese iron ammonium phosphate precursor, the lithium source, the carbon source and the Nb2C MXene, the method further comprises: Mixing a soluble phosphate, ammonia water, a manganese source, and an iron source in water at a pH of 5-7 to perform a coprecipitation reaction to generate a second slurry containing a manganese phosphate iron ammonia precursor; The second slurry is filtered, washed and dried to obtain the manganese iron phosphate ammonia precursor; wherein the molecular formula of the manganese iron phosphate ammonia precursor is: NH4Mn 1-x Fe x PO4·H2O;0<x≤0.

5.

11. The method according to claim 10, wherein The coprecipitation reaction is carried out in a protective atmosphere in the presence of an antioxidant; wherein the added weight of the antioxidant accounts for 0.4 wt% to 0.6 wt% of the weight of the iron source.

12. The method according to any one of claims 7 to 9, wherein: Before sand-milling the first slurry comprising the manganese iron ammonium phosphate precursor, the lithium source, the carbon source and the Nb2C MXene, the method further comprises: dissolving Nb2AlC in water with hydrofluoric acid or a mixture of lithium fluoride and hydrochloric acid to perform a corrosion reaction to obtain a third slurry containing Nb2CMXene; washing the precipitate of the third slurry until the washing solution is neutral; The washed precipitate was dried at -60°C to -20°C for 12-48 h to obtain Nb2C MXene.

13. A lithium ion battery, characterized in that: include: The positive electrode material according to any one of claims 1 to 5, or the positive electrode material obtained by the method according to any one of claims 6 to 12.

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