A high-conductivity carbon-coated lithium iron phosphate cathode material and its preparation method

Abstract

The present invention discloses a high-conductivity carbon-coated lithium iron phosphate cathode material and a preparation method thereof, which relates to the technical field of lithium iron phosphate cathode materials. It includes a spherical lithium iron phosphate matrix, modified lithium niobium vanadate 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 modified lithium niobium vanadate. Among them, the spherical lithium iron phosphate matrix has a smaller specific surface area, which helps to reduce side reactions and can reduce the interfacial impedance. The modified lithium niobium vanadate can improve the electronic conductivity of the cathode material and the diffusion rate of lithium ions. After the organic carbon source in the outermost composite carbon layer is carbonized, it forms a continuous amorphous carbon layer, uniformly coating the particle surface, providing a basic electron channel and suppressing side reactions of the electrolyte. The inorganic carbon source constructs a three-dimensional conductive skeleton, forms a long-range conductive network through the bridging action of nanowires / nanosheets, reduces the interfacial contact resistance, and improves the conductive performance of the cathode material.

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 key factors restricting its application. Lithium iron phosphate (LiFePO4), as a safe and cost-effective cathode material, is favored for 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:

[0005] How to improve the conductivity of lithium iron phosphate cathode materials.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] In the 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 niobium 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 niobium vanadate modified. Among them, the mass ratio of the spherical lithium iron phosphate matrix to the lithium niobium 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);

[0008] Preferably, the mass ratio of the spherical lithium iron phosphate matrix to the lithium niobium 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.

[0009] Furthermore, the preparation method of the spherical lithium iron phosphate matrix includes the following steps:

[0010] 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 LiFePO4 precursor with a honeycomb-like porous structure;

[0011] Step A2: Add poly-dopamine-modified magnetic silica nanoparticles to the LiFePO4 precursor and mix evenly to obtain a mixed system, where the poly-dopamine-modified magnetic silica nanoparticles account for 0.3 - 0.5% of the total mass of the mixed system; then transfer them together into supercritical CO2 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.

[0012] Further, 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.

[0013] Further, 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.

[0014] Further, the surface Zeta potential (electrokinetic potential) of the poly-dopamine-modified magnetic silica nanoparticles is -35 mV to -45 mV.

[0015] Based on this, a preferred method for preparing a spherical lithium iron phosphate matrix includes the following steps:

[0016] 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 its benzene ring), and treat it at 135 °C for 45 min in an ultrasonic-microwave synergistic reaction device to form a LiFePO4 precursor with a honeycomb-like porous structure;

[0017] Step A2: Add poly-dopamine-modified magnetic silica nanoparticles (with a surface Zeta potential of -40 mV) to the LiFePO4 precursor and mix evenly to obtain a mixed system, where the poly-dopamine-modified magnetic silica nanoparticles account for 0.4% of the total mass of the mixed system; then transfer them together into supercritical CO2 and stir and react at 50 °C and 10 MPa for 3 h, and filter to obtain a spherical lithium iron phosphate matrix.

[0018] Further, the preparation method of the modified lithium niobium vanadate includes the following steps:

[0019] 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 hydrothermal reaction to obtain the Nb-V-O precursor;

[0020] 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 modified lithium niobium vanadate after cooling.

[0021] Based on this, a preferred preparation method of modified lithium niobium vanadate includes the following steps:

[0022] 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 hydrothermal reaction to obtain the Nb-V-O precursor;

[0023] 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 modified lithium niobium vanadate after cooling.

[0024] Furthermore, the composite carbon layer comprises an organic carbon source and an inorganic carbon source with a mass ratio of 10:(2 - 5);

[0025] Preferably, the composite carbon layer comprises an organic carbon source and an inorganic carbon source with a mass ratio of 10:3.

[0026] Furthermore, the organic carbon source comprises any one or a combination of polyacrylic acid, polyaniline, glucose, and the inorganic carbon source is a composition composed of carbon nanotubes and graphene with a mass ratio of 1:1;

[0027] Preferably, the organic carbon source is glucose.

[0028] Second, 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:

[0029] 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 carry out ball milling and refinement to obtain a refined liquid;

[0030] Step 2: Adjust the pH of the refining liquid to 4 - 6, and then use the spray drying method for spray granulation to obtain precursor particles;

[0031] Step 3: Add the organic carbon source and the inorganic carbon source to anhydrous 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;

[0032] Step 4: Place the slurry in a sintering furnace for segmented sintering to obtain a high-conductivity carbon-coated lithium iron phosphate cathode material.

[0033] 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.

[0034] Further, in Step 3, the mass fraction of the solute in the composite carbon source is 30%.

[0035] Further, in Step 4, the method of segmented sintering is as follows: Place the slurry in a sintering furnace, and under an argon atmosphere, first heat it to 300 - 350 °C at a rate of 10 °C / min and hold for 1 h; then heat it to 600 - 650 °C at a rate of 2 °C / min and hold 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.

[0036] Based on this, a preferred preparation method of a high-conductivity carbon-coated lithium iron phosphate cathode material is obtained, including the following steps:

[0037] 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 refining to obtain a refining liquid;

[0038] Step 2: Adjust the pH of the refining 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;

[0039] Step 3: Add glucose, carbon nanotubes and graphene to anhydrous ethanol in sequence according to the 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;

[0040] Step 4: Place the slurry in a sintering furnace, and 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, 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.

[0041] Advantages of the present invention:

[0042] 1. In the high-conductive 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 coating the particle surface, providing a basic electron channel, and suppressing side reactions of the electrolyte; the inorganic carbon source constructs a three-dimensional conductive framework, forming a long-range conductive network through the bridging effect of nanowires / nanosheets, reducing the interfacial contact resistance, and further enhancing the conductivity of the cathode material.

[0043] 2. In the spherical lithium iron phosphate matrix of the high-conductive 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 groups specifically coordinate 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 FePO4 precursor, inhibiting the Ostwald ripening phenomenon, and the negative Zeta potential generates electrostatic repulsion to prevent particle aggregation. 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.

[0044] 3. In the modified lithium niobium vanadate of the high-conductive 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. Description of the drawings

[0045] The present invention will be further described below with reference to the drawings.

[0046] Figure 1 It is a comparative curve graph of the charge transfer resistance of Examples 1-5 and Comparative Examples 1-4 of the present invention.

[0047] Figure 2 yes Figure 1 Enlarged view of the comparison curve in the red circle.

[0048] 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

[0049] 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.

[0050] 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:

[0051] Sulfonated calixarene: purchased from Wuhan Shuer Biotechnology Co., Ltd.;

[0052] Polydopamine modified magnetic silica nanoparticles: Xi'an Qiyue Biotechnology Co., Ltd.

[0053] Preparation Example 1

[0054] Preparation of spherical lithium iron phosphate matrix:

[0055] Step 1, dissolving 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 (the 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 treating at 135°C for 45min in an ultrasonic-microwave synergistic reaction device to form a LiFePO4 precursor with a honeycomb porous structure;

[0056] Step 2: Add polydopamine modified magnetic silica nanoparticles (surface Zeta potential of -40 mV) to the LiFePO4 precursor and mix evenly to obtain a mixed system, in which the polydopamine modified magnetic silica nanoparticles account for 0.4% of the total mass of the mixed system; then transfer them into supercritical CO2, stir and react at 50°C and 10MPa for 3h, and filter to obtain a spherical lithium iron phosphate matrix.

[0057] Preparation Example 2

[0058] Preparation of spherical lithium iron phosphate matrix:

[0059] Step 1: Dissolve lithium carbonate, iron oxide, and ammonium phosphate in a molar ratio of Li:Fe:P = 1.02:1:1 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 sulfonic acid group substitution degree of 2.8 per molecule on the benzene ring). Treat it in an ultrasonic-microwave synergistic reaction device at 120 °C for 60 min to form a LiFePO4 precursor with a honeycomb-like porous structure.

[0060] Step 2: Add polydopamine-modified magnetic silica nanoparticles (surface Zeta potential is -35 mV) to the LiFePO4 precursor and mix evenly to obtain a mixed system. The polydopamine-modified magnetic silica nanoparticles account for 0.3% of the total mass of the mixed system. Then transfer it all into supercritical CO2 and stir and react at 40 °C and 12 MPa for 2 h, and filter to obtain a spherical lithium iron phosphate matrix.

[0061] Preparation Example 3

[0062] Preparation of spherical lithium iron phosphate matrix:

[0063] Step 1: Dissolve lithium carbonate, iron oxide, and ammonium phosphate in a molar ratio of Li:Fe:P = 1.05:1:1 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 sulfonic acid group substitution degree of 3.2 per molecule on the benzene ring). Treat it in an ultrasonic-microwave synergistic reaction device at 150 °C for 30 min to form a LiFePO4 precursor with a honeycomb-like porous structure.

[0064] Step 2: Add polydopamine-modified magnetic silica nanoparticles (surface Zeta potential is -45 mV) to the LiFePO4 precursor and mix evenly to obtain a mixed system. The polydopamine-modified magnetic silica nanoparticles account for 0.5% of the total mass of the mixed system. Then transfer it all into supercritical CO2 and stir and react at 60 °C and 8 MPa for 4 h, and filter to obtain a spherical lithium iron phosphate matrix.

[0065] Comparative Preparation Example 1

[0066] Compared with Preparation Example 1, the only difference is that: Step 2 is cancelled, and the LiFePO4 precursor with a honeycomb-like porous structure prepared in Step 1 is directly used as the spherical lithium iron phosphate matrix.

[0067] Preparation Example 4

[0068] Preparation of modified lithium niobium vanadate:

[0069] Step 1: Dissolve niobium pentoxide and ammonium metavanadate (a total of 30 g) 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 hydrothermal reaction to obtain Nb-V-O precursor;

[0070] 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.

[0071] Example 1

[0072] Prepare a high-conductivity carbon-coated lithium iron phosphate cathode material:

[0073] 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;

[0074] Step 2: Dropwise add hydrochloric acid to the refined liquid to adjust the pH to 5, and then carry out spray granulation by spray drying method. The inlet air temperature of spray drying is 200 °C, and the outlet air temperature is 90 °C to obtain precursor particles;

[0075] 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, and the ball-to-material ratio is 20:1. Ball mill at 550 rpm for 10 h. During the ball milling process, add the composite carbon source in five times at intervals of 1.5 h, and add 0.67 g each time. After the ball milling is completed, a slurry is obtained;

[0076] Step 4: Place the slurry in a sintering furnace. Under an argon atmosphere, first heat up to 320 °C at a rate of 10 °C / min and hold for 1 h; then heat up to 620 °C at a rate of 2 °C / min and hold for 3 h. Finally, cool down to 450 °C at a rate of 0.5 °C / min to finally obtain a high-conductivity carbon-coated lithium iron phosphate cathode material.

[0077] Example 2

[0078] Prepare a high-conductivity carbon-coated lithium iron phosphate cathode material:

[0079] 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;

[0080] Step 2: Add hydrochloric acid to the refined liquid to adjust the pH to 4, and then perform spray granulation by spray drying method. The inlet air temperature of spray drying is 180 °C and the outlet air temperature is 80 °C to obtain precursor particles;

[0081] 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. Add the composite carbon source in five times 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;

[0082] 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 finally obtain a high-conductivity carbon-coated lithium iron phosphate cathode material.

[0083] Example 3

[0084] Step 1: Dissolve 100 g of the spherical lithium iron phosphate matrix of 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 of Preparation Example 4 and stir evenly, and then perform ball milling refinement to obtain a refined liquid;

[0085] Step 2: Add hydrochloric acid to the refined liquid to adjust the pH to 4, and then perform spray granulation by spray drying method. The inlet air temperature of spray drying is 220 °C and the outlet air temperature is 100 °C to obtain precursor particles;

[0086] 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. Add the composite carbon source in five times 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;

[0087] 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 finally obtain a high-conductivity carbon-coated lithium iron phosphate cathode material.

[0088] Example 4

[0089] 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 Preparation Example 2; other steps and conditions remain the same, and finally a high-conductivity carbon-coated lithium iron phosphate cathode material is obtained.

[0090] Example 5

[0091] 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 Preparation Example 3; other steps and conditions remain the same, and finally a high-conductivity carbon-coated lithium iron phosphate cathode material is obtained.

[0092] Comparative Example 1

[0093] 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 obtained.

[0094] Comparative Example 2

[0095] 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 obtained.

[0096] Comparative Example 3

[0097] 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 the ball milling was completed, a slurry was obtained; the slurry was placed in a sintering furnace and heated to 320 °C at a rate of 10 °C / min under an argon atmosphere 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 to obtain a carbon-coated lithium iron phosphate cathode material.

[0098] Comparative Example 4

[0099] Glucose was added to absolute 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 prepared in 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 the ball milling was completed, a slurry was obtained. The slurry was placed in a sintering furnace and heated to 320 °C at a rate of 10 °C / min in an argon atmosphere 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 to finally obtain a carbon-coated lithium iron phosphate cathode material.

[0100] The carbon-coated lithium iron phosphate cathode materials prepared in Examples 1-5 and Comparative Examples 1-4 were respectively made into batteries for electrochemical performance testing. The manufacturing method was as follows: 90 g of the carbon-coated lithium iron phosphate cathode material, 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 coating machine, the black slurry was poured in, and after uniform coating, 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 substance, weighed, the electrode sheets were placed in a drying bottle, transferred to a vacuum drying oven and dried at 110 °C for 6 h, and finally assembled into a button battery in a vacuum glove box.

[0101] Then, the button batteries prepared were subjected to electrochemical performance testing. The charge transfer resistance was measured using an electrochemical workstation, and the capacity retention rate was measured using a 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:

[0102]

[0103] By analyzing the data in Table 1, it can be known that compared with Comparative Examples 1-4, the carbon-coated lithium iron phosphate cathode materials prepared in Examples 1-5 have a smaller charge transfer resistance and a higher capacity retention rate after 100 cycles. This shows that the high-conductivity carbon-coated lithium iron phosphate cathode material of the present invention has stronger conductivity.

[0104] 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 high-conductivity carbon-coated lithium iron phosphate cathode material, characterized in that, It includes a spherical lithium iron phosphate matrix, lithium niobium vanadate modified and 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. Among them, 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 includes the following steps: Step B1: Dissolve niobium pentoxide and ammonium metavanadate in a citric acid solution at a molar ratio of Nb:V = 1:(1.2-1.5), 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: Mix the Nb-V-O precursor and lithium carbonate at a molar ratio of Nb:Li = 1:1, and then calcine in an argon atmosphere containing 5-8 vol% hydrogen at 850-950 °C for 6-10 h, and cool to obtain the modified lithium niobium vanadate; 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 at a molar ratio of Li:Fe:P = (1.02-1.05):1:1 in an ethylene glycol-water mixed solvent containing 0.5-2 wt% sulfonated calixarene, and treat at 120-150 °C for 30-60 min in an ultrasonic-microwave synergistic reaction device to form a LiFePO4 precursor with a honeycomb-like porous structure; Step A2: Add polydopamine-modified magnetic silica nanoparticles to the LiFePO4 precursor and mix evenly to obtain a mixed system. The polydopamine-modified magnetic silica nanoparticles account for 0.3-0.5% of the total mass of the mixed system; then transfer them into supercritical CO2 together, and stir and react at 40-60 °C and 8-12 MPa for 2-4 h, and filter to obtain the spherical lithium iron phosphate matrix; The composite carbon layer includes 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).

2. The high-conductivity carbon-coated lithium iron phosphate cathode material according to claim 1, wherein 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; the phosphorus source includes one or more of phosphoric acid, ammonium phosphate, and ammonium dihydrogen phosphate.

3. The high-conductivity carbon-coated lithium iron phosphate cathode material according to claim 1, wherein In step A1, 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.

4. The high-conductivity carbon-coated lithium iron phosphate cathode material according to claim 1, wherein In step A2, the surface Zeta potential of the polydopamine-modified magnetic silica nanoparticles is -35 mV to -45 mV.

5. The high-conductivity carbon-coated lithium iron phosphate cathode material according to claim 1, wherein, The organic carbon source includes a composition composed of any one or more of polyacrylic acid, polyaniline, and glucose, and the inorganic carbon source is a composition composed of carbon nanotubes and graphene at a mass ratio of 1:

1.

6. A preparation method of a high-conductivity carbon-coated lithium iron phosphate cathode material, characterized in that, The method for preparing the high-conductivity carbon-coated lithium iron phosphate cathode material according to any one of claims 1-5 includes the following steps: Step one: 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 refining solution to 4 - 6, and then carry out spray granulation by spray drying method 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.

7. The preparation method of the high-conductivity carbon-coated lithium iron phosphate cathode material according to claim 6, characterized in that, In Step 4, the method of 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 hold for 1 h; then heat it to 600 - 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 finally obtain a high-conductivity carbon-coated lithium iron phosphate cathode material.

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