High-conductivity carbon-coated lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The invention discloses a high-conductivity carbon-coated lithium iron phosphate positive electrode material and a preparation method thereof, and relates to the technical field of lithium iron phosphate positive electrode materials, the high-conductivity carbon-coated lithium iron phosphate positive electrode material comprises a spherical lithium iron phosphate matrix, modified lithium niobium vanadate uniformly coating the exterior of the nanoscale spherical lithium iron phosphate matrix, and a composite carbon layer uniformly coating the exterior of the modified lithium niobium vanadate; wherein the spherical lithium iron phosphate matrix has a smaller specific surface area, so that the reduction of side reaction is facilitated, and the interface impedance can be reduced; the modified lithium niobium vanadate can improve the electronic conductivity of the positive electrode material and the diffusion rate of lithium ions; an organic carbon source in the outermost composite carbon layer is carbonized to form a continuous amorphous carbon layer which uniformly coats the surfaces of the particles, so that a basic electron channel is provided, and side reaction of an electrolyte is inhibited; the inorganic carbon source constructs a three-dimensional conductive skeleton, and a long-range conductive network is formed through the bridging effect of nanowires / sheets, so that the interface contact resistance is reduced, and the conductivity of the positive electrode material is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium iron phosphate cathode materials, and particularly relates to a highly conductive carbon-coated lithium iron phosphate cathode material and a preparation method thereof. Background Art

[0002] With the rapid development of electric vehicles and portable electronic devices, the performance requirements for lithium-ion batteries are getting higher and higher. Especially in low-temperature environments, the rate performance and stability of the battery become the key factors restricting its application. Lithium iron phosphate (LiFePO 4 ) as a safe and cost-effective cathode material is favored due to its excellent thermal stability and long cycle life. However, the electronic conductivity and ion diffusion coefficient of lithium iron phosphate are relatively low, and these problems are more prominent especially under low-temperature conditions, resulting in limited rate performance and power output.

[0003] From the perspective of cathode materials, the prior art has tried to improve the conductivity of lithium iron phosphate through means such as surface coating, particle size nanocrystallization, and element doping. Only adopting a single method often has limited effects and may bring problems of reduced stability. Therefore, developing a technology that combines multiple modification means to effectively improve the low-temperature rate performance of lithium iron phosphate while maintaining the stability and economy of the material has important practical application value and market prospects. Summary of the Invention

[0004] The purpose of the present invention is to provide a highly conductive carbon-coated lithium iron phosphate cathode material and a preparation method thereof, and solve the following technical problems: How to improve the conductivity of lithium iron phosphate cathode materials.

[0005] The purpose of the present invention can be achieved by the following technical solutions: In a first aspect, the present invention discloses a highly conductive carbon-coated lithium iron phosphate cathode material, which includes a spherical lithium iron phosphate matrix, lithium niobate vanadate modified uniformly coated on the outside of the nanoscale spherical lithium iron phosphate matrix, and a composite carbon layer uniformly coated on the outside of the lithium niobate vanadate modified. Among them, the mass ratio of the spherical lithium iron phosphate matrix to the lithium niobate vanadate modified is 100:(1 - 2), and the mass ratio of the spherical lithium iron phosphate matrix to the composite carbon layer is 100:(2.5 - 4); Preferably, the mass ratio of the spherical lithium iron phosphate matrix to the lithium niobate vanadate modified is 100:1.5, and the mass ratio of the spherical lithium iron phosphate matrix to the composite carbon layer is 100:3.

[0006] Furthermore, the preparation method of the spherical lithium iron phosphate matrix includes the following steps: Step A1: Dissolve a lithium source, an iron source, and a phosphorus source in an ethylene glycol-water mixed solvent containing 0.5 - 2 wt% sulfonated calixarene at a molar ratio of Li:Fe:P = (1.02 - 1.05):1:1, and treat it at 120 - 150 °C for 30 - 60 min in an ultrasonic-microwave synergistic reaction device to form a LiFePO 4 precursor with a honeycomb-like porous structure; Step A2: Add polydopamine-modified magnetic silica nanoparticles to the LiFePO 4 precursor and mix evenly to obtain a mixed system, where the polydopamine-modified magnetic silica nanoparticles account for 0.3 - 0.5% of the total mass of the mixed system; then transfer them together into supercritical CO 2 and stir and react at 40 - 60 °C and 8 - 12 MPa for 2 - 4 h, and filter to obtain a spherical lithium iron phosphate matrix.

[0007] Furthermore, the lithium source includes one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium phosphate, and lithium dihydrogen phosphate; the iron source includes one or more of iron oxides and iron salts; the phosphorus source includes one or more of phosphoric acid, ammonium phosphate, and ammonium dihydrogen phosphate.

[0008] Furthermore, the sulfonated calixarene has a C4 symmetry structure, and the substitution degree of sulfonic acid groups on the benzene ring of the sulfonated calixarene is 2.8 - 3.2.

[0009] Furthermore, the surface Zeta potential (electrokinetic potential) of the polydopamine-modified magnetic silica nanoparticles is -35 mV to -45 mV.

[0010] Based on this, a preferred preparation method of the spherical lithium iron phosphate matrix includes the following steps: Step A1: Dissolve lithium carbonate, iron oxide, and ammonium phosphate at a molar ratio of Li:Fe:P = 1.03:1:1 in an ethylene glycol-water mixed solvent containing 1 wt% sulfonated calixarene (with a C4 symmetry structure and a substitution degree of 3 of sulfonic acid groups on the benzene ring), and treat it at 135 °C for 45 min in an ultrasonic-microwave synergistic reaction device to form a LiFePO 4 precursor with a honeycomb-like porous structure; Step A2: Add polydopamine-modified magnetic silica nanoparticles (with a surface Zeta potential of -40 mV) to the LiFePO 4 precursor and mix evenly to obtain a mixed system, where the polydopamine-modified magnetic silica nanoparticles account for 0.4% of the total mass of the mixed system; then transfer them together into supercritical CO 2 and stir and react at 50 °C and 10 MPa for 3 h, and filter to obtain a spherical lithium iron phosphate matrix.

[0011] Further, the preparation method of the modified lithium niobium vanadate includes the following steps: Step B1: Dissolve niobium pentoxide and ammonium metavanadate in an appropriate amount of citric acid solution at a molar ratio of Nb:V = 1:(1.2 - 1.5). The dosage ratio of the citric acid solution to the total dosage of niobium pentoxide and ammonium metavanadate is 10 mL:1 g. Then add polyethylene glycol accounting for 5 - 10 wt% of the total amount of niobium pentoxide and ammonium metavanadate, and then carry out a hydrothermal reaction to obtain a Nb-V-O precursor. Step B2: After mixing the Nb-V-O precursor and lithium carbonate at a molar ratio of Nb:Li = 1:1, calcine them in an argon atmosphere containing 5 - 8 vol% hydrogen at 850 - 950 °C for 6 - 10 h, and obtain the modified lithium niobium vanadate after cooling.

[0012] Based on this, the preferred preparation method of the modified lithium niobium vanadate includes the following steps: Step B1: Dissolve niobium pentoxide and ammonium metavanadate in an appropriate amount of citric acid solution at a molar ratio of Nb:V = 1:1.3. The dosage ratio of the citric acid solution to the total dosage of niobium pentoxide and ammonium metavanadate is 10 mL:1 g. Then add polyethylene glycol accounting for 8 wt% of the total amount of niobium pentoxide and ammonium metavanadate, and then carry out a hydrothermal reaction to obtain a Nb-V-O precursor. Step B2: After mixing the Nb-V-O precursor and lithium carbonate at a molar ratio of Nb:Li = 1:1, calcine them in an argon atmosphere containing 6 vol% hydrogen at 900 °C for 8 h, and obtain the modified lithium niobium vanadate after cooling.

[0013] Further, the composite carbon layer includes an organic carbon source and an inorganic carbon source with a mass ratio of 10:(2 - 5). Preferably, the composite carbon layer includes an organic carbon source and an inorganic carbon source with a mass ratio of 10:3.

[0014] Further, the organic carbon source includes any one or a combination of polyacrylic acid, polyaniline, and glucose, and the inorganic carbon source is a composition composed of carbon nanotubes and graphene with a mass ratio of 1:1. Preferably, the organic carbon source is glucose.

[0015] In a second aspect, the present invention also discloses a preparation method of the high-conductivity carbon-coated lithium iron phosphate cathode material as described above, including the following steps: Step 1: Dissolve the spherical lithium iron phosphate matrix in a mixed solvent of ethanol and water, then add the modified lithium niobium vanadate and stir evenly, and then carry out ball milling and refinement to obtain a refined liquid. Step 2: Adjust the pH of the refined liquid to 4 - 6, and then carry out spray granulation by spray drying to obtain precursor particles. Step 3: Add the organic carbon source and the inorganic carbon source into absolute ethanol and mix evenly to form a composite carbon source. Ball-mill the precursor particles, and add the composite carbon source in small amounts and multiple times during the ball-milling process. After the ball-milling is completed, a slurry is obtained. Step 4: Place the slurry in a sintering furnace for segmented sintering to obtain a high-conductivity carbon-coated lithium iron phosphate cathode material.

[0016] Further, in Step 2, the inlet air temperature of the spray drying is 180 - 220 °C, and the outlet air temperature is 80 - 100 °C.

[0017] Further, in Step 3, the mass fraction of the solute in the composite carbon source is 30%.

[0018] Further, in Step 4, the method of the segmented sintering is as follows: Place the slurry in a sintering furnace, and first heat it to 300 - 350 °C at a rate of 10 °C / min under an argon atmosphere and keep it warm for 1 h; then heat it to 600 - 650 °C at a rate of 2 °C / min and keep it warm for 3 h, and finally cool it to 450 °C at a rate of 0.5 °C / min to finally obtain a high-conductivity carbon-coated lithium iron phosphate cathode material.

[0019] Based on this, a preferred preparation method of a high-conductivity carbon-coated lithium iron phosphate cathode material is obtained, including the following steps: Step 1: Dissolve the spherical lithium iron phosphate matrix in a mixed solvent of ethanol and water, then add modified lithium niobium vanadate and stir evenly, and then perform ball-milling and refinement to obtain a refined liquid. Step 2: Adjust the pH of the refined liquid to 5, and then use the spray drying method for spray granulation. The inlet air temperature of the spray drying is 200 °C, and the outlet air temperature is 90 °C to obtain precursor particles. Step 3: Add glucose, carbon nanotubes, and graphene into absolute ethanol in sequence according to a mass ratio of 20:1.5:1.5 and mix evenly to form a composite carbon source with a solute mass fraction of 30%. Ball-mill the precursor particles, and add the composite carbon source in small amounts and multiple times during the ball-milling process. After the ball-milling is completed, a slurry is obtained. Step 4: Place the slurry in a sintering furnace, and first heat it to 320 °C at a rate of 10 °C / min under an argon atmosphere and keep it warm for 1 h; then heat it to 620 °C at a rate of 2 °C / min and keep it warm for 3 h, and finally cool it to 450 °C at a rate of 0.5 °C / min to finally obtain a high-conductivity carbon-coated lithium iron phosphate cathode material.

[0020] The beneficial effects of the present invention: 1. In the high-conductivity carbon-coated lithium iron phosphate cathode material of the present invention, a modified lithium niobium vanadate and a composite carbon layer are sequentially and uniformly coated on the outside of the spherical lithium iron phosphate matrix. Among them, the spherical lithium iron phosphate matrix has a smaller specific surface area, which helps to reduce side reactions, can reduce the interfacial impedance, and can provide more stable charge and discharge performance under low-temperature conditions. The middle layer of modified lithium niobium vanadate can improve the electronic conductivity of the cathode material and the diffusion rate of lithium ions, thereby enhancing the low-temperature rate performance of the material, and its high-voltage characteristics also help to improve the overall energy density. The outermost composite carbon layer combines an organic carbon source and an inorganic carbon source. Among them, the organic carbon source forms a continuous amorphous carbon layer after carbonization, uniformly coats the particle surface, provides a basic electron channel, and inhibits side reactions of the electrolyte. The inorganic carbon source constructs a three-dimensional conductive framework, forms a long-range conductive network through the bridging action of nanowires / nanosheets, reduces the interfacial contact resistance, and further improves the conductivity of the cathode material.

[0021] 2. In the spherical lithium iron phosphate matrix of the high-conductivity carbon-coated lithium iron phosphate cathode material of the present invention, sulfonated calixarene is used as a macromolecular template to construct a three-dimensional confined space. Among them, the C4 symmetry structure induces isotropic growth, solves the problem of preferential orientation of crystal planes caused by traditional templates, and the sulfonic acid group specifically coordinates with Fe³⁺ to precisely control the nucleation sites of the precursor, making the spheroidization efficiency of lithium iron phosphate faster and better. At the same time, the catechol groups on the surface of the polydopamine-modified magnetic silica nanoparticles form strong coordination bonds with the FePO 4 precursor, inhibits the Ostwald ripening phenomenon, and the negative Zeta potential generates electrostatic repulsion to prevent particle agglomeration. There is no need to add a dispersant again, which can improve the dispersibility of the spherical lithium iron phosphate matrix, make the distribution of the cathode material in the battery slurry more uniform, and further improve the conductive effect of the cathode material.

[0022] 3. In the modified lithium niobium vanadate of the high-conductivity carbon-coated lithium iron phosphate cathode material of the present invention, the bond energy of the Nb-V-O bond is significantly higher than that of the Fe-O bond in lithium iron phosphate. Therefore, oxygen loss can be inhibited by strong covalent bonds during the charge and discharge process. At the same time, there is a difference in the thermal expansion coefficients between the modified lithium niobium vanadate and lithium iron phosphate. Therefore, micro-region prestress can be formed between the lithium iron phosphate and the composite carbon layer, reducing the volume change rate during the battery cycle, thereby protecting the battery and improving the conductive effect of the cathode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The present invention will be further described below with reference to the accompanying drawings.

[0024] Figure 1 is a comparative curve graph of the charge transfer resistance of Examples 1-5 and Comparative Examples 1-4 of the present invention.

[0025] Figure 2 is Figure 1Enlarged view of the comparison curve in the red circle.

[0026] Figure 3 It is a comparison curve diagram of the capacity retention rate of Examples 1-5 of the present invention and Comparative Examples 1-4. DETAILED DESCRIPTION

[0027] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] The experimental methods in the following examples and comparative examples, unless otherwise specified, are conventional methods, and are carried out according to the techniques or conditions described in the literature in the field or according to the product instructions. The materials, reagents, etc. used in the following examples and comparative examples, unless otherwise specified, can be obtained from commercial channels, for example: Sulfonated calixarene: purchased from Wuhan Shuer Biotechnology Co., Ltd.; Polydopamine modified magnetic silica nanoparticles: Xi'an Qiyue Biotechnology Co., Ltd.

[0029] Preparation Example 1 Preparation of spherical lithium iron phosphate matrix: Step 1: dissolve lithium carbonate, iron oxide, and ammonium phosphate at a molar ratio of Li:Fe:P = 1.03:1:1 in an ethylene glycol-water mixed solvent (weight ratio of ethylene glycol to water is 1:1) containing 1wt% sulfonated calixarene (having a C4 symmetric structure, and the degree of substitution of the sulfonic acid group on its benzene ring is 3 / molecule), and treat at 135°C for 45min in an ultrasonic-microwave synergistic reaction device to form LiFePO with a honeycomb porous structure. 4 Precursor; Step 2: LiFePO 4 Polydopamine-modified magnetic silica nanoparticles (surface Zeta potential of -40 mV) were added to the precursor and mixed evenly to obtain a mixed system, in which the polydopamine-modified magnetic silica nanoparticles accounted for 0.4% of the total mass of the mixed system; and then the precursors were transferred to supercritical CO 2 The reaction was stirred at 50°C and 10 MPa for 3 h, and the spherical lithium iron phosphate matrix was obtained by filtration.

[0030] Preparation Example 2 Preparation of spherical lithium iron phosphate matrix: Step 1. Lithium carbonate, iron oxide, and ammonium phosphate with a molar ratio of Li:Fe:P = 1.02:1:1 are dissolved in an ethylene glycol-water mixed solvent (weight ratio of ethylene glycol to water is 1:1) containing 0.5 wt% sulfonated calixarene (with a C4 symmetry structure and a degree of substitution of sulfonic acid groups on the benzene ring of 2.8 / molecule). The mixture is treated at 120 °C for 60 min in an ultrasonic-microwave synergistic reaction device to form a LiFePO 4 precursor with a honeycomb-like porous structure; Step 2. Poly(dopamine)-modified magnetic silica nanoparticles (surface Zeta potential is -35 mV) are added to the LiFePO 4 precursor and mixed evenly to obtain a mixed system. The poly(dopamine)-modified magnetic silica nanoparticles account for 0.3% of the total mass of the mixed system. Then, the mixture is transferred into supercritical CO 2 and stirred and reacted at 40 °C and 12 MPa for 2 h, followed by filtration to obtain a spherical lithium iron phosphate matrix.

[0031] Preparation Example 3 Preparation of spherical lithium iron phosphate matrix: Step 1. Lithium carbonate, iron oxide, and ammonium phosphate with a molar ratio of Li:Fe:P = 1.05:1:1 are dissolved in an ethylene glycol-water mixed solvent (weight ratio of ethylene glycol to water is 1:1) containing 2 wt% sulfonated calixarene (with a C4 symmetry structure and a degree of substitution of sulfonic acid groups on the benzene ring of 3.2 / molecule). The mixture is treated at 150 °C for 30 min in an ultrasonic-microwave synergistic reaction device to form a LiFePO 4 precursor with a honeycomb-like porous structure; Step 2. Poly(dopamine)-modified magnetic silica nanoparticles (surface Zeta potential is -45 mV) are added to the LiFePO 4 precursor and mixed evenly to obtain a mixed system. The poly(dopamine)-modified magnetic silica nanoparticles account for 0.5% of the total mass of the mixed system. Then, the mixture is transferred into supercritical CO 2 and stirred and reacted at 60 °C and 8 MPa for 4 h, followed by filtration to obtain a spherical lithium iron phosphate matrix.

[0032] Comparative Preparation Example 1 Compared with Preparation Example 1, the only difference is that Step 2 is cancelled, and the LiFePO 4 precursor with a honeycomb-like porous structure obtained in Step 1 is directly used as the spherical lithium iron phosphate matrix.

[0033] Preparation Example 4 Preparation of modified lithium niobium vanadate: Step 1: Dissolve niobium pentoxide and ammonium metavanadate (30 g in total) in 300 mL of citric acid solution at a molar ratio of Nb:V = 1:1.3. Then add polyethylene glycol accounting for 8 wt% of the total amount of niobium pentoxide and ammonium metavanadate, and then carry out a hydrothermal reaction to obtain the Nb-V-O precursor; Step 2: Mix the Nb-V-O precursor and lithium carbonate at a molar ratio of Nb:Li = 1:1, and then calcine at 900 °C for 8 h in an argon atmosphere containing 6 vol% hydrogen. After cooling, modified lithium niobium vanadate is obtained.

[0034] Example 1 Prepare a high-conductivity carbon-coated lithium iron phosphate cathode material: Step 1: Dissolve 100 g of the spherical lithium iron phosphate matrix prepared in Preparation Example 1 in 500 mL of an ethanol aqueous solution with a mass fraction of 70%. Then add 1.5 g of the modified lithium niobium vanadate prepared in Preparation Example 4 and stir evenly, and then carry out ball milling and refinement to obtain a refined liquid; Step 2: Dropwise add hydrochloric acid to the refined liquid to adjust the pH to 5, and then carry out spray granulation by the spray drying method. The inlet air temperature of the spray drying is 200 °C, and the outlet air temperature is 90 °C to obtain precursor particles; Step 3: Add glucose, carbon nanotubes and graphene with a mass ratio of 20:1.5:1.5 to anhydrous ethanol and mix evenly to form a composite carbon source with a solute mass fraction of 30%. Add the precursor particles to a ball mill with a ball-to-material ratio of 20:1, and ball mill at 550 rpm for 10 h. The composite carbon source is added in five times during the ball milling process, with an interval of 1.5 h each time, and 0.67 g is added each time. After the ball milling is completed, a slurry is obtained; Step 4: Place the slurry in a sintering furnace. Under an argon atmosphere, first heat it to 320 °C at a rate of 10 °C / min and hold for 1 h; then heat it to 620 °C at a rate of 2 °C / min and hold for 3 h. Finally, cool it to 450 °C at a rate of 0.5 °C / min to finally obtain a high-conductivity carbon-coated lithium iron phosphate cathode material.

[0035] Example 2 Prepare a high-conductivity carbon-coated lithium iron phosphate cathode material: Step 1: Dissolve 100 g of the spherical lithium iron phosphate matrix prepared in Preparation Example 1 in 500 mL of an ethanol aqueous solution with a mass fraction of 70%. Then add 1 g of the modified lithium niobium vanadate prepared in Preparation Example 4 and stir evenly, and then carry out ball milling and refinement to obtain a refined liquid; Step 2: Dropwise add hydrochloric acid to the refined liquid to adjust the pH to 4, and then carry out spray granulation by the spray drying method. The inlet air temperature of the spray drying is 180 °C, and the outlet air temperature is 80 °C to obtain precursor particles; Step 3: Add glucose, carbon nanotubes, and graphene with a mass ratio of 20:1.5:1.5 into absolute ethanol and mix evenly to form a composite carbon source with a solute mass fraction of 30%. Add the precursor particles into a ball mill with a ball-to-material ratio of 20:1, ball mill at 550 rpm for 10 h, and add the composite carbon source in five portions during the ball milling process, with an interval of 1.5 h each time and 0.56 g added each time. After the ball milling is completed, a slurry is obtained. Step 4: Place the slurry in a sintering furnace. Under an argon atmosphere, first heat it to 300 °C at a rate of 10 °C / min and hold for 1 h; then heat it to 600 °C at a rate of 2 °C / min and hold for 3 h. Finally, cool it to 450 °C at a rate of 0.5 °C / min to obtain a high-conductivity carbon-coated lithium iron phosphate cathode material.

[0036] Example 3 Step 1: Dissolve 100 g of the spherical lithium iron phosphate matrix prepared in Preparation Example 1 in 500 mL of an ethanol aqueous solution with a mass fraction of 70%, then add 2 g of the modified lithium niobium vanadate prepared in Preparation Example 4 and stir evenly, and then perform ball milling and refinement to obtain a refined liquid. Step 2: Add hydrochloric acid dropwise to the refined liquid to adjust the pH to 4, and then perform spray granulation by spray drying method. The inlet air temperature of the spray drying is 220 °C, and the outlet air temperature is 100 °C to obtain precursor particles. Step 3: Add glucose, carbon nanotubes, and graphene with a mass ratio of 20:1.5:1.5 into absolute ethanol and mix evenly to form a composite carbon source with a solute mass fraction of 30%. Add the precursor particles into a ball mill with a ball-to-material ratio of 20:1, ball mill at 550 rpm for 10 h, and add the composite carbon source in five portions during the ball milling process, with an interval of 1.5 h each time and 0.89 g added each time. After the ball milling is completed, a slurry is obtained. Step 4: Place the slurry in a sintering furnace. Under an argon atmosphere, first heat it to 350 °C at a rate of 10 °C / min and hold for 1 h; then heat it to 650 °C at a rate of 2 °C / min and hold for 3 h. Finally, cool it to 450 °C at a rate of 0.5 °C / min to obtain a high-conductivity carbon-coated lithium iron phosphate cathode material.

[0037] Example 4 Compared with Example 1, the difference is only that the spherical lithium iron phosphate matrix prepared in Preparation Example 1 is replaced with the spherical lithium iron phosphate matrix prepared in Preparation Example 2; other steps and conditions remain the same, and finally a high-conductivity carbon-coated lithium iron phosphate cathode material is prepared.

[0038] Example 5 Compared with Example 1, the difference is only that the spherical lithium iron phosphate matrix prepared in Preparation Example 1 is replaced with the spherical lithium iron phosphate matrix prepared in Preparation Example 3; other steps and conditions remain the same, and finally a high-conductivity carbon-coated lithium iron phosphate cathode material is prepared.

[0039] Comparative Example 1 Compared with Example 1, the only difference is that the spherical lithium iron phosphate matrix of Preparation Example 1 is replaced with the spherical lithium iron phosphate matrix of Comparative Preparation Example 1; other steps and conditions remain the same, and finally a carbon-coated lithium iron phosphate cathode material is prepared.

[0040] Comparative Example 2 Compared with Example 1, the only difference is that the spherical lithium iron phosphate matrix of Preparation Example 1 is replaced with a composition of lithium carbonate, iron oxide, and phosphoric acid in a molar ratio of Li:Fe:P = 1.03:1:1; other steps and conditions remain the same, and finally a coated lithium iron phosphate cathode material is prepared.

[0041] Comparative Example 3 Glucose, carbon nanotubes, and graphene with a mass ratio of 20:3:3 were added to anhydrous ethanol and mixed evenly to form a composite carbon source with a solute mass fraction of 30%. 100 g of the spherical lithium iron phosphate matrix of Preparation Example 1 was added to a ball mill with a ball-to-material ratio of 20:1 and ball milled at 550 rpm for 10 h. The composite carbon source was added in five portions during the ball milling process, with an interval of 1.5 h each time, and 0.67 g was added each time. After ball milling, a slurry was obtained; the slurry was placed in a sintering furnace, and under an argon atmosphere, it was first heated to 320 °C at a rate of 10 °C / min and held for 1 h; then heated to 620 °C at a rate of 2 °C / min and held for 3 h, and finally cooled to 450 °C at a rate of 0.5 °C / min, and finally a carbon-coated lithium iron phosphate cathode material was obtained.

[0042] Comparative Example 4 Glucose was added to anhydrous ethanol and mixed evenly to form a carbon source with a solute mass fraction of 30%. 100 g of the spherical lithium iron phosphate matrix of Preparation Example 1 was added to a ball mill with a ball-to-material ratio of 20:1 and ball milled at 550 rpm for 10 h. The carbon source was added in five portions during the ball milling process, with an interval of 1.5 h each time, and 0.67 g was added each time. After ball milling, a slurry was obtained; the slurry was placed in a sintering furnace, and under an argon atmosphere, it was first heated to 320 °C at a rate of 10 °C / min and held for 1 h; then heated to 620 °C at a rate of 2 °C / min and held for 3 h, and finally cooled to 450 °C at a rate of 0.5 °C / min, and finally a carbon-coated lithium iron phosphate cathode material was obtained.

[0043] The lithium iron phosphate cathode material coated with carbon prepared in Examples 1-5 and Comparative Examples 1-4 was made into batteries for testing the electrochemical performance. The manufacturing method is as follows: 90 g of the lithium iron phosphate cathode material coated with carbon, 5 g of Super-P, and 5 g of polyvinylidene fluoride were mixed evenly, and then 200 g of N-methylpyrrolidone solvent was added and mixed evenly to obtain a black slurry. The aluminum foil was placed in a film coater, the black slurry was poured in, and after evenly coating the film, it was transferred to an oven and dried at 40 °C for 4 h. Then, the dried aluminum foil was placed in a punching machine to punch several electrode sheets containing the active material, weighed, and the electrode sheets were placed in a drying bottle and transferred to a vacuum drying oven and dried at 110 °C for 6 h. Finally, it was assembled into a button battery in a vacuum glove box.

[0044] Then, the electrochemical performance of the obtained button battery was tested. The charge transfer resistance was tested using an electrochemical workstation, and the capacity retention rate was tested using the constant current-constant voltage charge-discharge method to obtain Figures 1-3 the test results, and the test results are listed in Table 1 as follows:

[0045] By analyzing the data in Table 1, it can be known that compared with Comparative Examples 1-4, the lithium iron phosphate cathode material coated with carbon prepared in Examples 1-5 has a smaller charge transfer resistance and a higher capacity retention rate after 100 cycles. This shows that the high-conductivity lithium iron phosphate cathode material coated with carbon of the present invention has stronger conductivity.

[0046] One embodiment of the present invention has been described in detail above, but the content described is only a preferred embodiment of the present invention and cannot be considered as limiting the scope of implementation of the present invention. All equivalent changes and improvements made according to the scope of the application of the present invention should still fall within the scope covered by the patent of the present invention.

Claims

1. A highly conductive carbon-coated lithium iron phosphate positive electrode material, characterized in that: It comprises a spherical lithium iron phosphate matrix, modified lithium niobium vanadate uniformly coated on the outside of the nano-scale spherical lithium iron phosphate matrix, and a composite carbon layer uniformly coated on the outside of the modified lithium niobium vanadate, wherein the mass ratio of the spherical lithium iron phosphate matrix to the modified lithium niobium vanadate is 100:(1-2), and the mass ratio of the spherical lithium iron phosphate matrix to the composite carbon layer is 100:(2.5-4); The preparation method of the modified lithium niobium vanadate comprises the following steps: Step B1, dissolving niobium pentoxide and ammonium metavanadate in a citric acid solution at a molar ratio of Nb:V=1:(1.2-1.5), adding polyethylene glycol accounting for 5-10wt% of the total amount of niobium pentoxide and ammonium metavanadate, and then performing a hydrothermal reaction to obtain a Nb-VO precursor; Step B2: Mix the Nb-VO precursor and lithium carbonate in a molar ratio of Nb:Li=1:1, calcine at 850-950° C. for 6-10 h in an argon atmosphere containing 5-8 vol % hydrogen, and obtain modified lithium niobium vanadate after cooling.

2. The highly conductive carbon-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: The preparation method of the spherical lithium iron phosphate matrix comprises the following steps: Step A1, dissolving a lithium source, an iron source, and a phosphorus source in a molar ratio of Li:Fe:P = (1.02-1.05):1:1 in an ethylene glycol-water mixed solvent containing 0.5-2wt% sulfonated calixarene, and treating at 120-150° C. for 30-60 min in an ultrasonic-microwave synergistic reaction device to form a LiFePO4 precursor having a honeycomb porous structure; Step A2, adding polydopamine modified magnetic silica nanoparticles to the LiFePO4 precursor and mixing evenly to obtain a mixed system, wherein the polydopamine modified magnetic silica nanoparticles account for 0.3-0.5% of the total mass of the mixed system; then transferring them into supercritical CO2, stirring and reacting at 40-60°C and 8-12MPa for 2-4h, and filtering to obtain a spherical lithium iron phosphate matrix.

3. The highly conductive carbon-coated lithium iron phosphate positive electrode material according to claim 2, characterized in that: In step A1, the lithium source includes one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium phosphate, and lithium dihydrogen phosphate; the iron source includes one or more of iron oxides and iron salts; and the phosphorus source includes one or more of phosphoric acid, ammonium phosphate, and diammonium phosphate.

4. The highly conductive carbon-coated lithium iron phosphate positive electrode material according to claim 2, characterized in that: In step A1, the sulfonated calixarene has a C4 symmetrical structure, and the degree of substitution of the sulfonic acid group on the benzene ring of the sulfonated calixarene is 2.8-3.

2.

5. The highly conductive carbon-coated lithium iron phosphate positive electrode material according to claim 2, characterized in that: In step A2, the surface Zeta potential of the polydopamine-modified magnetic silica nanoparticles is -35 mV to -45 mV.

6. The highly conductive carbon-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: The composite carbon layer comprises an organic carbon source and an inorganic carbon source, and the mass ratio of the organic carbon source to the inorganic carbon source is 10:(2-5).

7. The highly conductive carbon-coated lithium iron phosphate positive electrode material according to claim 6, characterized in that: The organic carbon source includes a composition consisting of any one or more of polyacrylic acid, polyaniline, and glucose, and the inorganic carbon source is a composition consisting of carbon nanotubes and graphene in a mass ratio of 1:

1.

8. A method for preparing a highly conductive carbon-coated lithium iron phosphate positive electrode material, characterized in that: The method for preparing the highly conductive carbon-coated lithium iron phosphate positive electrode material according to any one of claims 1 to 7 comprises the following steps: Step 1: dissolving a spherical lithium iron phosphate matrix in a mixed solvent of ethanol and water, then adding modified lithium niobium vanadate and stirring evenly, and then ball milling to obtain a refined solution; Step 2: adjusting the pH of the refinement solution to 4-6, and then spray granulating by spray drying to obtain precursor particles; Step 3: adding an organic carbon source and an inorganic carbon source into anhydrous ethanol and mixing them evenly to form a composite carbon source, ball-milling the precursor particles, adding the composite carbon source in small amounts and multiple times during the ball-milling process, and obtaining a slurry after the ball-milling is completed; Step 4: placing the slurry in a sintering furnace for segmented sintering to obtain a highly conductive carbon-coated lithium iron phosphate positive electrode material.

9. The method for preparing a highly conductive carbon-coated lithium iron phosphate positive electrode material according to claim 8, characterized in that: In step four, the segmented sintering method is as follows: placing the slurry in a sintering furnace, first heating the temperature to 300-350°C at 10°C / min under an argon atmosphere, and keeping it warm for 1 hour; then heating the temperature to 600-650°C at 2°C / min, keeping it warm for 3 hours, and finally cooling the temperature to 450°C at 0.5°C / min to finally obtain a highly conductive carbon-coated lithium iron phosphate positive electrode material.

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