A method for constructing a vanadium-carbon composite coating on the surface of lithium iron phosphate based on spray drying

Abstract

A method for constructing a vanadium-carbon composite coating on the surface of lithium iron phosphate based on spray drying, which relates to a method for preparing a lithium iron phosphate positive electrode material for a lithium ion battery. The present invention aims to solve the problem of low electrochemical performance of existing carbon-coated lithium iron phosphate. The method comprises the following steps: preparing a solution using a vanadium source, a phosphorus source, and an organic acid, dispersing iron phosphate therein, and spray-drying to obtain a vanadium-coated iron phosphate precursor powder; then mixing the precursor with a lithium source and a carbon source, and calcining under an inert atmosphere to obtain a lithium iron phosphate positive electrode material having a vanadium-carbon composite coating. The coating combines the advantages of high carbon conductivity and high ion diffusion rate of vanadium to synergistically optimize charge-mass transfer. The lithium iron phosphate prepared by the present invention with a vanadium-carbon composite coating has a specific capacity of 150.8 mAh / g and 117.7 mAh / g at discharge rates of 1C and 10C, respectively, and a capacity retention rate of 94.5% after 1000 cycles. The method can be used in the field of batteries.

Description

Technical Field

[0001] The present invention relates to a method for preparing a lithium iron phosphate cathode material for a lithium ion battery, and in particular to a method for constructing a composite coating layer of the material. Background Art

[0002] As a secondary battery cathode material, lithium iron phosphate (LiFePO4) has become the most widely used cathode material for lithium-ion batteries due to its low cost, wide availability of raw materials, and excellent safety. Among the various LiFePO4 synthesis processes, carbothermal reduction is a key one-step method due to its simplicity and ease of process control. This process eliminates concerns about raw material oxidation during processing and allows for a variety of raw material treatment methods to achieve the desired dispersion.

[0003] The carbon thermal reduction method mainly includes two process routes: the red iron oxide route and the iron phosphate route. The final product prepared by the red iron oxide route has a high rate discharge performance, but its capacity performance is relatively average; the iron phosphate route has the main advantages of high capacity and the raw materials are not easy to absorb moisture and deliquesce. However, the physical parameters such as particle size distribution and particle morphology of lithium iron phosphate positive electrode materials are largely inherited from the iron phosphate precursor, and these parameters directly affect the electrochemical properties of lithium iron phosphate. Due to the inheritance of physical parameters between lithium iron phosphate and iron phosphate precursor, it is difficult to further improve the performance of lithium iron phosphate under the established carbon thermal reduction process conditions. On the other hand, traditional carbon-coated lithium iron phosphate can improve the electrical conductivity of the material, but it cannot solve the problem of low intrinsic diffusion rate of lithium ions in the material, and the discharge capacity at higher rates is not ideal. Summary of the Invention

[0004] The present invention is to solve the technical problem of low electrochemical performance of existing carbon-coated lithium iron phosphate, and provide a method for constructing a vanadium-carbon composite coating on the surface of lithium iron phosphate based on spray drying, which is a method for preparing lithium iron phosphate by optimizing the carbon thermal reduction process of the iron phosphate method, by coating the surface of the iron phosphate precursor with vanadium, thereby constructing a vanadium-carbon composite coating on the surface of the lithium iron phosphate, and improving its electrochemical performance. Specifically, the present invention adopts a spray drying method to carry out a vanadium-based phosphate coating treatment on the surface of the iron phosphate precursor, so that the modified iron phosphate precursor can generate a vanadium-carbon composite coating that synergistically optimizes the charge-mass transfer process during the carbon thermal reduction process. This method can not only effectively improve the rate discharge performance and cycle stability of lithium iron phosphate, but also can achieve performance optimization without adjusting the existing process equipment.

[0005] The method of constructing a vanadium-carbon composite coating layer on the surface of lithium iron phosphate based on spray drying of the present invention is carried out according to the following steps:

[0006] 1. Add a vanadium source, a phosphorus source, and an organic acid to pure water at a temperature of 50-95° C. and stir at this temperature for 5-30 minutes to dissolve, obtaining a clear solution with a pH value of ≥6.5; wherein the organic acid is one or a combination of acetic acid, citric acid, oxalic acid, ascorbic acid, and tartaric acid; in this step, the pH value of the solution is controlled to be ≥6.5 to ensure that the ferric phosphate is insoluble in the solution; simultaneously, at a temperature of 50-95° C., the mixing and dissolution of the reaction materials are accelerated, and the complexation reaction and reduction reaction of the vanadium source are promoted;

[0007] 2. Add anhydrous iron phosphate crystalline powder to the mixed solution obtained in step 1 at a molar ratio of anhydrous iron phosphate to the vanadium source in step 1 of (15-500):1, and stir to uniformly disperse the iron phosphate powder in the mixed solution to obtain a dispersion;

[0008] 3. Transfer the dispersion to a spray dryer and spray dry it at an outlet air temperature of 105-130°C and a peristaltic speed of 10-30 RPM to obtain a vanadium-coated iron phosphate precursor powder. This step is a vanadium coating and rapid drying process, which coats the surface of the iron phosphate with a layer of vanadium-based phosphate while rapidly removing moisture.

[0009] 4. The molar ratio of the lithium source to the anhydrous iron phosphate in step 2 is (0.9-1.3):1, and the mass ratio of the carbon source in terms of carbon to the anhydrous iron phosphate in step 2 is (5-20):1. After the lithium source, carbon source and vanadium-coated iron phosphate precursor powder are evenly mixed, the temperature is raised to 550-800°C at a heating rate of 2-10°C / min under an inert atmosphere and maintained for 2-12 hours for calcination. Under these conditions, the carbon source is fully pyrolyzed and the carbon thermal reduction reaction is fully carried out. The temperature is cooled to room temperature to obtain a lithium iron phosphate positive electrode material with a vanadium-carbon composite coating layer on the surface.

[0010] Furthermore, the vanadium source in step 1 is one or a mixture of ammonium metavanadate, lithium metavanadate, and vanadium pentoxide.

[0011] Furthermore, the phosphorus source in step 1 is one or a mixture of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, lithium dihydrogen phosphate, dilithium hydrogen phosphate, and lithium phosphate;

[0012] Furthermore, the molar ratio of the vanadium source, phosphorus source and organic acid in step 1 is (0.5-3):(1-5):(0.5-5).

[0013] Furthermore, the concentration of the V element in the solution in step 1 is 0.4-4 mmol / L.

[0014] Furthermore, the molar ratio of the anhydrous ferric phosphate in step 2 to the vanadium source in step 1 is (35-400):1.

[0015] Furthermore, the lithium source in step 4 is one or a mixture of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium dihydrogen phosphate;

[0016] Furthermore, the carbon source in step 4 is one or a mixture of glucose, sucrose, citric acid, oxalic acid, ascorbic acid, polyacrylonitrile, and phenolic resin.

[0017] Furthermore, the precursor, lithium source, and carbon source are preferably mixed in step 4 by manual grinding with a mortar and pestle, ball milling, sand milling, or mixing using one or more of a three-dimensional mixer, a V-shaped mixer, and a ribbon mixer. This is to improve the uniformity of the mixing of the reaction materials, thereby enhancing the contact properties of the solid-phase reaction, thereby increasing the purity of the sintered product and shortening the reaction time.

[0018] Furthermore, the inert gas in step 4 is argon, nitrogen or a hydrogen-argon mixture.

[0019] The beneficial effects of the present invention compared with the prior art are:

[0020] (1) The present invention uses a technology combining spray drying and carbon thermal reduction. First, the iron phosphate precursor is subjected to vanadium coating modification by spray drying, and then a vanadium-carbon composite coating structure is formed on the surface of the lithium iron phosphate through a carbon thermal reduction process. Compared with the conventional lithium iron phosphate coated carbon shell, the vanadium-carbon composite coating constructed by the present invention comprehensively utilizes the advantages of the high electronic conductivity of the carbon layer and the high ion diffusion rate of the vanadium-based material, synergistically promoting charge transfer and ion diffusion. The prepared material has a specific capacity of 150.8mAh / g at a 1C discharge rate and a specific capacity of 117.7mAh / g at a 10C discharge rate; while the lithium iron phosphate prepared in the comparative example has a 1C discharge specific capacity of 138.6mAh / g and a 10C discharge specific capacity of 94.0mAh / g. Thanks to the vanadium-carbon composite coating's optimized charge-mass transfer in the lithium iron phosphate cathode material, the vanadium-carbon composite-coated lithium iron phosphate cathode material synthesized in the present invention exhibited a capacity retention rate of 94.5% after 1000 cycles of 1C / 1C constant current charge-discharge; while the capacity retention rate of the control sample under the same conditions was 82.7%. Clearly, the vanadium-phosphate-coated iron phosphate precursor prepared in the present invention, after undergoing a carbothermal reduction process to construct a vanadium-carbon composite coating, significantly improves the cycling and rate performance of the lithium iron phosphate cathode material.

[0021] (2) The present invention utilizes a spray drying method to perform vanadium coating modification on an iron phosphate precursor, and the obtained vanadium-based phosphate-coated iron phosphate precursor has good process compatibility. Since the precursor after vanadium coating still has similar physical parameters such as morphology and particle size as the ordinary iron phosphate precursor, in the subsequent carbon thermal reduction process, there is no need to add new equipment or change the type and mixing method of the lithium source and carbon source. It is only necessary to appropriately adjust the amount of lithium source and carbon source added to meet the lithium matching requirements of vanadium, and make full use of the organic acid that dissolves vanadium as part of the carbon source to construct a vanadium-carbon composite coating layer, which significantly improves the electrochemical performance of the lithium iron phosphate positive electrode material. The present invention is fully compatible with the existing iron phosphate carbon thermal reduction process, and effectively improves the economic benefits of the iron phosphate precursor and the lithium iron phosphate positive electrode material.

[0022] (3) The raw materials required for the preparation method of the present invention are widely available, low in cost, and the process flow is simple. The product has high reproducibility and can be produced in large quantities at one time, meeting the needs of practical applications of lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 X-ray diffraction patterns of lithium iron phosphate, a positive electrode material for lithium-ion batteries, prepared in Example 1, Comparative Example 1, and Comparative Example 2;

[0024] Figure 2 This is a scanning electron microscope image of the vanadium-coated iron phosphate precursor powder prepared in Example 1;

[0025] Figure 3 Element distribution diagram of the vanadium-coated iron phosphate precursor powder prepared in Example 1;

[0026] Figure 4 This is a scanning electron microscope image of the iron phosphate precursor powder described in Comparative Example 1;

[0027] Figure 5 Element distribution diagram of the iron phosphate precursor powder described in Comparative Example 1;

[0028] Figure 6 This is a scanning electron microscope image of the vanadium-carbon composite coated lithium iron phosphate positive electrode material prepared in Example 1;

[0029] Figure 7 This is the element distribution diagram of the vanadium-carbon composite coated lithium iron phosphate positive electrode material prepared in Example 1;

[0030] Figure 8 This is a scanning electron microscope image of the lithium iron phosphate positive electrode material prepared in Comparative Example 1;

[0031] Figure 9 Element distribution diagram of the lithium iron phosphate positive electrode material prepared in Comparative Example 1;

[0032] Figure 10 This is a transmission electron microscope image of the lithium iron phosphate positive electrode material with a vanadium-carbon composite coating layer prepared in Example 1.

[0033] Figure 11 The rate performance diagram of the lithium-ion battery positive electrode materials vanadium-carbon composite coated lithium iron phosphate and lithium iron phosphate prepared in Example 1 and Comparative Example 1;

[0034] Figure 12 This is a cycle performance diagram of the lithium-ion battery positive electrode materials vanadium-carbon composite coated lithium iron phosphate and lithium iron phosphate prepared in Example 1 and Comparative Example 1 under 1C / 1C charge and discharge cycles. DETAILED DESCRIPTION

[0035] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the scope of protection of the present invention.

[0036] Example 1: The method of constructing a vanadium-carbon composite coating layer on the surface of lithium iron phosphate based on spray drying in this embodiment is carried out according to the following steps:

[0037] 1. Weigh 0.25g of vanadium pentoxide, 0.48g of ammonium dihydrogen phosphate, and 0.37g of oxalic acid, stir and dissolve in 1L of pure water, heat to 90°C and stir until the raw materials are dissolved to obtain a clear solution; the concentration of the vanadium element in this solution is 2.8mmol / L, and the pH value of the solution is 6.5. Under these conditions, iron phosphate will not dissolve in the solution; the temperature at 90°C will accelerate the mixing and dissolution of the reaction raw materials and promote the complexation reaction and reduction reaction of the vanadium source;

[0038] 2. Weigh 8.14 g of iron phosphate powder and add it to the clear solution obtained in step 1. Stir for 15 minutes to evenly disperse the iron phosphate powder in the mixed solution to obtain a dispersion;

[0039] 3. Transfer the dispersion to the spray dryer and dry it under the following conditions: outlet air temperature of 120℃, peristaltic pump feed speed of 20RPM, wind speed of 40.0m 3 The vanadium-coated iron phosphate precursor powder was spray-dried at 400 nm / min to obtain a light yellow-green vanadium-coated iron phosphate precursor powder. This step is a vanadium-coating and rapid drying process, in which a layer of vanadium-based phosphate is coated on the surface of the iron phosphate while rapidly removing moisture.

[0040] 4. Use a planetary ball mill to grind 5.00g of vanadium-coated iron phosphate precursor powder, 1.16g of lithium carbonate and 0.72g of glucose at a ball mill speed of 1000RPM and a ball-to-material mass ratio of 20:1 for 9 hours. The grinding method is dry grinding to obtain a mixed powder; then transfer the mixed powder to a tubular furnace, and under the protection of a flowing argon atmosphere, raise the temperature to 650℃ at a heating rate of 5℃ / min and calcine for 9 hours. After natural cooling, a lithium iron phosphate positive electrode material with a vanadium-carbon composite coating layer on the surface is obtained.

[0041] Comparative Example 1: In this comparative example, lithium iron phosphate is prepared by carbon thermal reduction of an iron phosphate precursor that has not been subjected to vanadium coating treatment. The specific preparation method is as follows:

[0042] 1. Weigh 5.00 g of ferric phosphate, 1.22 g of lithium carbonate, and 0.82 g of glucose and add them to a planetary ball mill. Mill them for 9 h at a ball-to-material ratio of 20:1 and a dry grinding method to obtain a mixed powder.

[0043] Second, the mixed powder was transferred to a tube furnace and calcined under a flowing argon atmosphere at a heating rate of 5°C / min to 650°C for 9 hours. After natural cooling, the lithium iron phosphate cathode material was obtained. This was synthesized by carbothermal reduction of an unvanadium-coated iron phosphate precursor to obtain lithium iron phosphate.

[0044] The carbon-coated lithium iron phosphate cathode material obtained in Comparative Example 1 was used as the active material, Super P as the conductive agent, and PVDF as the binder. The materials were mixed with a paste at a mass ratio of 8:1:1 to obtain a cathode slurry. The slurry was coated on aluminum foil, dried, and cut into cathode sheets with a diameter of 14 mm. Finally, a half-cell was assembled and the cathode was tested at 2.5-4.2 V (vs. Li + The battery was tested for charge and discharge within a voltage range of 1 / Li. The results showed that at a 1C rate, the discharge capacity was 138.6 mAh / g, with a capacity retention of 82.7% after 1,000 cycles. At a 10C rate, the discharge capacity was 94.0 mAh / g.

[0045] Comparative Example 2: The method for constructing a vanadium-carbon composite coating layer on the surface of lithium iron phosphate based on spray drying in this comparative example is carried out according to the following steps:

[0046] 1. Weigh 1.01 g of vanadium pentoxide, 1.91 g of ammonium dihydrogen phosphate, and 1.50 g of oxalic acid, stir and dissolve in 1 L of pure water, and heat at 90°C with stirring until a clear solution is obtained; the concentration of vanadium pentoxide in the solution in this step is 11.1 mmol / L;

[0047] 2. Weigh 8.14 g of ferric phosphate and add it to the clear solution obtained in step 1. Stir for 15 minutes to evenly disperse the ferric phosphate powder to obtain a dispersion;

[0048] 3. Put the dispersion into the spray dryer at an outlet temperature of 120℃, a peristaltic pump feed speed of 20RPM and a wind speed of 40.0m 3 The product was spray-dried at 1000 nm / min to obtain a light yellow-green vanadium-coated iron phosphate precursor powder.

[0049] 4. 5.00 g of vanadium-coated iron phosphate precursor powder, 1.03 g of lithium carbonate, and 0.52 g of glucose were ball-milled in a planetary ball mill at a rotation speed of 1000 RPM and a ball-to-material ratio of 20:1 for 9 hours. The grinding method was dry grinding to obtain a mixed powder; the mixed powder was transferred to a tubular furnace, and under the protection of a flowing argon atmosphere, the temperature was raised to 650 ° C at a heating rate of 5 ° C / min and calcined for 9 hours. After natural cooling, a vanadium-carbon composite coated lithium iron phosphate positive electrode material was obtained.

[0050] The lithium iron phosphate cathode material with vanadium-carbon composite coating layer obtained in Comparative Example 2 was used as active material, Super P was used as conductive agent, and PVDF was used as binder. The materials were mixed with paste at a mass ratio of 8:1:1 to obtain cathode slurry. The slurry was coated on aluminum foil, dried, and cut into cathode sheets with a diameter of 14 mm. Finally, half-cells were assembled and tested at 2.5-4.2 V (vs. Li + The results showed that at a 1C rate, the discharge capacity was 120.3 mAh / g; at a 10C rate, the discharge capacity was 51.0 mAh / g.

[0051] Figure 1 The X-ray diffraction patterns of lithium iron phosphate, a positive electrode material for lithium-ion batteries, prepared in Example 1, Comparative Example 1, and Comparative Example 2 are shown. Figure 1 It can be seen that the samples of Example 1 and Comparative Example 1 both show characteristic diffraction peaks of lithium iron phosphate LiFePO4; while Comparative Example 2 shows a mixed diffraction peak of lithium iron phosphate LiFePO4 and vanadium-based phosphate, which is due to excessive addition of vanadium in Comparative Example 2.

[0052] Figure 2 This is a scanning electron microscope image of the vanadium-coated iron phosphate precursor powder prepared in Example 1. It can be observed that after spray drying, the vanadium source, phosphorus source, and organic acid are coated on the surface of the agglomerated particles, and the spray drying step does not significantly change the size and distribution of the primary particles.

[0053] Figure 3This is the element distribution diagram of the vanadium-coated iron phosphate precursor powder prepared in step 3 of Example 1. It can be observed that the vanadium element is evenly distributed in the agglomerated particles without obvious vacancies or concentrated areas, indicating that the vanadium source is evenly coated on the particle surface.

[0054] Figure 4 This is a scanning electron microscope image of the iron phosphate precursor powder described in Comparative Example 1. The precursor is battery-grade anhydrous iron phosphate powder and serves as the substrate for vanadium coating treatment in Example 1, Example 2, and Comparative Example 1.

[0055] Figure 5 : is the element distribution diagram of the iron phosphate precursor powder described in Comparative Example 1. It can be observed that the iron phosphate precursor described in Comparative Example 1 does not contain vanadium.

[0056] Figure 6 This is a scanning electron microscope image of the lithium iron phosphate cathode material with a vanadium-carbon composite coating layer prepared in Example 1. As can be seen from the image, the vanadium-carbon composite coating layer has no significant effect on the morphology, size and particle size distribution of the lithium iron phosphate primary particles.

[0057] Figure 7 This is an element distribution diagram of the lithium iron phosphate cathode material with a vanadium-carbon composite coating layer prepared in Example 1. It can be observed that there are no vacancies or dense areas in the distribution of vanadium elements in the agglomerated particles, indicating that vanadium is evenly coated on the surface of the lithium iron phosphate cathode material.

[0058] Figure 8 This is a scanning electron microscope image of the lithium iron phosphate cathode material prepared in Comparative Example 1. It can be seen that its morphology is consistent with the typical characteristics of the carbon-coated lithium iron phosphate cathode material prepared by the iron phosphate method carbothermal reduction process.

[0059] Figure 9 This is an element distribution diagram of the lithium iron phosphate positive electrode material prepared in Comparative Example 1. It can be observed that the carbon-coated lithium iron phosphate positive electrode material prepared in Comparative Example 1 does not contain vanadium.

[0060] Figure 10 This is a transmission electron microscope image of the lithium iron phosphate cathode material with a vanadium-carbon composite coating prepared in Example 1. It can be seen that the vanadium layer Li3V2(PO4)3 and the carbon layer C are coated on the LiFePO4 particles, visually demonstrating the presence of the vanadium-carbon composite coating constructed in the lithium iron phosphate cathode material prepared in Example 1.

[0061] The lithium iron phosphate positive electrode material with a vanadium-carbon composite coating layer prepared in Example 1 and the carbon-coated lithium iron phosphate prepared in Comparative Example 1 were used as active materials, Super P was used as a conductive agent, and PVDF was used as a binder. The paste was mixed at a mass ratio of 8:1:1 to obtain a positive electrode slurry. The slurry was coated on aluminum foil, dried, and cut into positive electrode sheets with a diameter of 14 mm. Finally, a half-cell was assembled and the positive electrode was tested at 2.5-4.2 V (vs. Li + / Li) voltage range for charge and discharge tests.

[0062] Figure 11 The rate performance diagrams of the vanadium-carbon composite-coated lithium iron phosphate and carbon-coated lithium iron phosphate, lithium-ion battery positive electrode materials prepared in Example 1 and Comparative Example 1. At 1C and 10C rates, the vanadium-carbon composite-coated lithium iron phosphate positive electrode material prepared in Example 1 has a discharge capacity of 150.8 mAh / g and 117.7 mAh / g; the carbon-coated lithium iron phosphate positive electrode material prepared in Comparative Example 1 under the same carbothermal reduction process conditions has a discharge capacity of only 138.6 mAh / g and 94.0 mAh / g at 1C and 10C rates. This indicates that the vanadium-carbon composite coating constructed based on spray drying has a significant effect on improving the rate performance of the lithium iron phosphate positive electrode material.

[0063] Figure 12 The cycle performance diagram of the lithium ion battery positive electrode materials vanadium-carbon composite coated lithium iron phosphate and carbon coated lithium iron phosphate prepared in Example 1 and Comparative Example 1. + After 1000 cycles in the voltage range of 1 / Li), the capacity retention rate of the vanadium-carbon composite-coated lithium iron phosphate cathode material prepared in Example 1 was 94.5%, while the capacity retention rate of the carbon-coated lithium iron phosphate cathode material prepared in Comparative Example 1 was 82.7%. This indicates that the vanadium-carbon composite coating layer constructed by spray drying significantly improves the cycling performance of the lithium iron phosphate cathode material.

[0064] Example 2: The method of constructing a vanadium-carbon composite coating layer on the surface of lithium iron phosphate based on spray drying in this embodiment is carried out according to the following steps:

[0065] 1. Weigh 0.03g of vanadium pentoxide, 0.05g of ammonium dihydrogen phosphate, and 0.04g of oxalic acid, stir and dissolve in 1L of pure water, heat to 90°C and stir until the raw materials are dissolved to obtain a clear solution; the concentration of the vanadium element in the solution is 0.3mmol / L, and the pH value of the solution is 6.5. Under these conditions, iron phosphate will not dissolve in the solution; the temperature at 90°C will accelerate the mixing and dissolution of the reaction raw materials and promote the complexation reaction and reduction reaction of the vanadium source;

[0066] 2. Weigh 8.14 g of iron phosphate powder and add it to the clear solution obtained in step 1. Stir for 15 minutes to evenly disperse the iron phosphate powder in the mixed solution to obtain a dispersion;

[0067] 3. Transfer the dispersion to the spray dryer and dry it under the following conditions: outlet air temperature of 120℃, peristaltic pump feed speed of 20RPM, wind speed of 40.0m 3 The vanadium-coated iron phosphate precursor powder was spray-dried at 400 nm / min to obtain a light yellow-green vanadium-coated iron phosphate precursor powder. This step is a vanadium-coating and rapid drying process, in which a layer of vanadium-based phosphate is coated on the surface of the iron phosphate while rapidly removing moisture.

[0068] 4. Use a planetary ball mill to grind 5.00g of vanadium-coated iron phosphate precursor powder, 1.22g of lithium carbonate and 0.80g of glucose at a ball mill speed of 1000RPM and a ball-to-material mass ratio of 20:1 for 9 hours. The grinding method is dry grinding to obtain a mixed powder; then transfer the mixed powder to a tubular furnace, and under the protection of a flowing argon atmosphere, raise the temperature to 650℃ at a heating rate of 5℃ / min and calcine for 9 hours. After natural cooling, a lithium iron phosphate positive electrode material with a vanadium-carbon composite coating layer on the surface is obtained.

[0069] The lithium iron phosphate positive electrode material with a vanadium-carbon composite coating layer prepared in Example 2 was used as the active material, Super P was used as the conductive agent, and PVDF was used as the binder. The mixture was mixed with a paste at a mass ratio of 8:1:1 to obtain a positive electrode slurry. The slurry was coated on aluminum foil, dried, and cut into positive electrode sheets with a diameter of 14 mm. Finally, a half-cell was assembled and the positive electrode was tested at 2.5-4.2 V (vs. Li + The charge and discharge tests were conducted in the voltage range of 1 / Li. The results showed that at a 1C rate, its discharge capacity was 145.8mAh / g.

[0070] The present invention combines spray drying with carbon thermal reduction, optimizes and modifies the iron phosphate precursor by coating with vanadium-based phosphate, and then constructs a vanadium-carbon composite coating layer that optimizes charge and mass transfer through a carbon thermal reduction reaction. The high electronic conductivity of the carbon layer and the high ion diffusion rate advantage of the vanadium-based material are combined to synergistically optimize the transfer process of electrons and ions, significantly improving the rate performance and cycle stability of the prepared lithium iron phosphate, thereby achieving the overall optimization of the carbon thermal reduction process of the iron phosphate method. The vanadium-carbon composite coated lithium iron phosphate positive electrode material prepared by the present invention exhibits excellent specific capacity at various discharge rates, and has a high capacity retention rate in long-term charge and discharge cycles. In addition, the method of the present invention has good process compatibility, does not require additional equipment or changes to the existing process, and only needs to appropriately regulate the amount of lithium source and carbon source added to achieve performance improvement. The raw materials used are low in cost and widely available, the process flow is simple, the product reproducibility is high, and it can be mass-produced.

Claims

1. A method for constructing a vanadium-carbon composite coating on the surface of lithium iron phosphate based on spray drying, characterized in that The method proceeds as follows:

1. Add a vanadium source, a phosphorus source, and an organic acid to pure water at a temperature of 50-95°C, and stir at this temperature for 5-30 minutes to dissolve, to obtain a clear solution with a pH value ≥ 6.5; wherein the organic acid is one or a combination of acetic acid, citric acid, oxalic acid, ascorbic acid, and tartaric acid; 2. Add anhydrous iron phosphate crystalline powder to the mixed solution obtained in step 1 at a molar ratio of anhydrous iron phosphate to the vanadium source in step 1 of (15-500):1, and stir to uniformly disperse the iron phosphate powder in the mixed solution to obtain a dispersion; 3. Transfer the dispersion to a spray dryer and perform spray drying at an outlet air temperature of 105-130°C and a peristaltic speed of 10-30 rpm to obtain a vanadium-coated iron phosphate precursor powder; 4. The molar ratio of the lithium source to the anhydrous iron phosphate in step 2 is (0.9~1.3):1, and the mass ratio of the carbon source in terms of carbon to the anhydrous iron phosphate in step 2 is (5~20):

1. After uniformly mixing the lithium source, the carbon source and the vanadium-coated iron phosphate precursor powder, the mixture is heated to 550~800°C at a heating rate of 2~10°C / min under an inert atmosphere and maintained for 2~12 h for calcination, and then cooled to room temperature to obtain a lithium iron phosphate positive electrode material with a vanadium-carbon composite coating layer on the surface.

2. The method for constructing a vanadium-carbon composite coating on the surface of lithium iron phosphate based on spray drying according to claim 1, characterized in that: The vanadium source in step 1 is one or a mixture of ammonium metavanadate, lithium metavanadate, and vanadium pentoxide.

3. The method for constructing a vanadium-carbon composite coating on the surface of lithium iron phosphate based on spray drying according to claim 1 or 2, characterized in that: The phosphorus source described in step 1 is one or a mixture of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, lithium dihydrogen phosphate, dilithium hydrogen phosphate, and lithium phosphate.

4. The method for constructing a vanadium-carbon composite coating on the surface of lithium iron phosphate based on spray drying according to claim 1 or 2, characterized in that: The molar ratio of the vanadium source, phosphorus source and organic acid in step 1 is (0.5-3):(1-5):(0.5-5).

5. The method for constructing a vanadium-carbon composite coating on the surface of lithium iron phosphate based on spray drying according to claim 1 or 2, characterized in that: The concentration of element V in the solution described in step 1 is 0.4-4 mmol / L.

6. The method for constructing a vanadium-carbon composite coating on the surface of lithium iron phosphate based on spray drying according to claim 1 or 2, characterized in that: The lithium source described in step 4 is one or a mixture of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium dihydrogen phosphate.

7. The method for constructing a vanadium-carbon composite coating on the surface of lithium iron phosphate based on spray drying according to claim 1 or 2, characterized in that: The carbon source in step 4 is one or a mixture of glucose, sucrose, citric acid, oxalic acid, ascorbic acid, polyacrylonitrile, and phenolic resin.

8. The method for constructing a vanadium-carbon composite coating on the surface of lithium iron phosphate based on spray drying according to claim 1 or 2, characterized in that: The precursor, lithium source and carbon source described in step 4 are mixed by manual grinding with a mortar and pestle, ball milling with a ball mill, sand milling with a sand mill, or one or more of a three-dimensional mixer, a V-shaped mixer and a ribbon mixer.

9. The method for constructing a vanadium-carbon composite coating on the surface of lithium iron phosphate based on spray drying according to claim 1 or 2, characterized in that: The inert atmosphere in step 4 is argon, nitrogen or a mixture of hydrogen and argon.

10. The method for constructing a vanadium-carbon composite coating on the surface of lithium iron phosphate based on spray drying according to claim 1 or 2, characterized in that: The molar ratio of the anhydrous ferric phosphate in step 2 to the vanadium source in step 1 is (35-400):1.

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